In association with the journal Nanotechnology Winter 2014/2015

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Nanoscale fingerprints combat the counterfeit goods industryRandomly distributed nanowires provide a simple approach to a unique barcode.

Counterfeiting is an increasingly important problem that occurs in nearly every trade and industry. Dis-tinguishing counterfeit goods from genuine products can be very diffi-cult, and as a result new nanoscale technologies are being developed to prevent and identify this illegal practice. Researchers at the Korea Advanced Institute of Science and Technology (KAIST) in South Korea have demonstrated that randomly distributed dye-coated nanowires can generate unique and simple barcode patterns that can be read-ily used for anti-counterfeiting operations.

Reporting in Nanotechnology, the researchers generated nanoscale fingerprint patterns by simply cast-ing f luorescent dye-coated silver nanowires onto a transferable flex-ible polyethylene terephthalate (PET) film. The direction and target mark-ers (“KAIST” and “X”) are patterned by a photolithographic technique to provide positional information for identification before the nanowires are cast onto the film. Then, using an optical microscope, the result-ing unique fingerprint patterns can be visually authenticated in a sim-ple and straightforward manner (as

shown in the image).Advantages of this technique are

the simple preparation of finger-

print patterns and the authenticity verification; the technique is simple enough that anyone can prepare

these barcodes. Fingerprint patterns can be visually authenticated in a simple and straightforward man-ner by using an optical microscope, which allows customers to immedi-ately determine whether products are authentic. Although generating a fingerprint pattern is quite simple, counterfeiting the pattern is essen-tially impossible because it is based on natural randomness. Counter-feiting such a fingerprint pattern is impractical and expensive; the cost of replicating it would be higher than the value of the typical target item being protected.

While the likelihood of obtain-ing identical patterns of randomly orientated nanowires is practi-cally zero, the fluorescent dyes add another layer of complexity, as does increasing the density of nanowires. Expanding this concept further, many products such as paper docu-ments will have a profile of defects on the nanoscale. As long as this profile can be recorded, it will pro-vide a unique barcode as a result of naturally occurring randomness. The researchers hope that their con-cept of nanoscale fingerprints can be widely applicable in various anti-counterfeit activities.

As the first peer-reviewed journal in nanoscale science and technol-ogy, Nanotechnology has seen the field develop from a select community early in its inception to the huge international research enterprise it is today. The journal has grown from a quarterly to a weekly publication, and publishes several times more papers per year than most of its peers.

New territory in 1990, working at the nanoscale posed a number of challenges, such as the accentuated effects of variations in a number of system parameters. Attempts to engage in nanoscale research with-out first addressing these issues was, as David Whitehouse, first Editor-in-Chief of the journal, described “akin to performing a surgical oper-ation with a blunt instrument on an excited jelly”. The field has seen tremendous progress in the under-standing of nanosystems since 1990,

revealing a wealth of new research areas to investigate further still.

The journal has always recognized the importance of multidisciplinary approaches in nanotechnology research. Yet shifts in the general focus of research in the field were

seen as the field matured, with papers moving from largely instrument-oriented characterization studies to the detailed insights and demon-strated applications reported today.

Research in nanotechnology is now a massive global endeavour and the journal continues to house lead-ing papers that define and influence the direction of activity in the field. As

well as primary research articles, the journal publishes topical reviews to provide context to the latest research, special issues where papers on the lat-est research in a particularly topical field are collated in one issue, newsy “labtalk” articles where research papers are covered simply in brief, author interviews in our publishers picks and perspective articles cover-ing the translation of research from the lab to industry and commerce.

The journal has embraced the multimedia opportunities open to electronic publications with video abstracts, movie file figures and as a celebration of the 25th volume, the launch of Nanotechnology Discussions podcasts where a panel of eminent figures in the community discuss topical issues related to research in the journal. We look forward to future developments building on the past 25 years of Nanotechnology.

“The field has seen tremendous progress in the understanding of nanosystems since 1990”

The process of producing anti-counterfeit nano-fingerprints based on randomly distributed silver nanowires.

Mark Reed, Editor-in-Chief of Nanotechnology.

Welcome to nanotechweb review, a special supplement brought to you by the editors of nanotechweb.

This issue highlights some of the headline developments over the past few months in all of the different disciplines and applications using nanoscale science and technology. Anna DemmingEditor, nanotechweb

Access all content free at nanotechweb.org and to receive free weekly technology updates to your inbox register at nanotechweb.org/cws/sign-up.

E D I T O R I A L

N A N O T E C H N O L O G Y

Nanotechnology encompasses the understanding of the fundamental physics, chemistry, biology and technology of nanometre-scale objects. As well as primary research articles, the journal publishes topical reviews to provide context to the latest research, perspective articles covering the translation of research from the lab to industry and commerce, special issues, and a range of news editorial and multimedia to highlight papers, authors and research topics. Our 2013 impact factor is 3.672*.

Editor-in-Chief: M Reed, Yale University

iopscience.org/nano*As listed in the 2013 ISI Journal Citation Reports®

25 years of the journal Nanotechnology

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3focus on: material properties

Nanosponge defects benefit water clean-upA “sponge” made from intercon-nected carbon nanotubes could soak up water contaminants more effectively than previous efforts, as demonstrated by a collaboration of researchers in Italy and France.

Carbon nanotubes (CNTs) offer a large specific area and extraor-dinary mechanical and chemical properties, which are attractive for wastewater clean-up. However, the fine CNT powders can be dif-ficult to handle and hard to retrieve following treatment.

Reporting their results in the journal Nanotechnology, Luca Camilli and colleagues at the University of Rome, University of Nantes and the University of L’Aquila show that nanosponges – randomly intercon-nected CNT frameworks – could be an excellent candidate for solving these issues. The structure is both easier to handle than regular carbon

nanotubes and has an absorption capability three times the value of previous materials.

In their paper, the authors attrib-ute the superior uptake efficiency of their CNT sponges to the frame-work of highly defective carbon nanostructures, which are formed by adding sulphur during the CNT growth process. “Oils or solvents can easily be absorbed in the empty spaces among the CNTs, which is made easier by the rough surfaces,” explained Camilli. The resulting sponge had an average length of 20 mm.

During testing, the team observed how their CNT sponges could suc-cessfully remove a toxic organic solvent – dichlorobenzene – from water, demonstrating that the porous structure could absorb a mass that was 3.5 times higher than earlier work. The CNT sponges were

also shown to absorb vegetable oil up to 150 times its initial weight and also soaked up engine oil to a

slightly higher capacity than previ-ously reported. Once they are satu-rated with contaminants, nanotube

sponges then f loat on the water where they can easily be removed. Simply squeezing or burning them removes the oil so that they can then be reused.

The defects caused by the addi-tion of sulphur also provide pref-erential sites for immobilization and subsequent nucleation of iron atoms from the ferrocene catalyst, which was added during growth. What’s more, the presence of iron nanoparticles in the CNT walls allows the sponges to be magneti-cally controlled and driven remotely, which could also benefit targeted therapeutic applications.

“The next stage of our research is to improve the synthesis process so that the sponges can be produced on a commercial scale,” added Camilli. “We must also study the toxicity of the sponges before any real-world applications can be realized.”

Conventional perpendicular mag-netic recording is reaching the limit of its recording density, at around 1 Tb/in2. But heat-assisted mag-netic recording (HAMR) may pave the way for the next generation of hard disk drives with higher capaci-ties. Reporting in Nanotechnology, researchers have employed HAMR and doubled the traditional capacity through simulation.

A special recording medium that has good thermal stability at room temperature is used in HAMR. When a focused optical beam is delivered onto its surface, the local medium is temporarily heated up to exceed its Curie temperature. This also low-ers its coercivity to allow magnetic recording. When the local medium cools down, it becomes stable again to retain data.

Localized surface plasmon reso-nances could play an important role in HAMR by providing a highly focused optical spot for heating the recording medium within a small volume, thus facilitating the record-ing density. However, researchers are struggling with low-cost integrated optic designs that not only provide a better focused optical field, but also achieve a higher efficiency to avoid reliability issues.

An international research col-laboration between the University of Rochester in the US and Swinburne University of Technology in Australia seeks to address such a dilemma. The scholars design an aluminium near-field transducer based on a novel bow-tie structure, and engineer a highly integrated micro-nano-optics system for use in HAMR.

Using 3D finite-difference time-domain simulations, they find that

their design can generate a competi-tive optical spot size of 35 nm inside the magnetic medium at a wavelength of 450 nm. This corresponds to a recording density of up to 2 Tb/in2. At the same time it offers a much higher efficiency compared with previously reported designs that are mostly based on noble metals such as gold.

Remaining work on this project may include further optimization of the structure dimensions, device fab-rication and characterization. But the researchers have demonstrated that aluminium can be a high- efficiency inexpensive plasmonic material for the development of HAMR technol-ogy, thereby providing a solution that might double the capacity of traditional hard disk drives.

Carbyne structures can be employed in efficient water-purification applications. Usually this is within graphyne, which – with optimal per-meability, high mechanical robust-ness and a uniform distribution of pores in the membrane – is more effective than other materials. Yet the impact of environmental factors on this structure is largely unknown. A study reported in Nanotechnology pro-vides a fundamental understanding of the mechanical strength of car-byne chains with different lengths, at different temperatures and varied solvent conditions. This paves the way to realizing efficient purifica-tion devices for separating specific contaminants from water.

Graphyne is a family of single- atom-thick two-dimensional mem-branes composed of carbon atoms. They have a similar nanostructure to graphene, but can comprise various pore sizes and a portion of carbon–carbon bonds are replaced with car-byne chains. Graphyne structures can be used in a range of filtration and purification applications, such as water purification.

The successful application of gra-phyne nanoweb membranes for selective purification purposes relies

on the design and directed fabrica-tion of graphyne membranes with different pore sizes. This is achiev-able through adjusting the length of carbyne chains that link the benzene rings within the graphyne structure.

Here, molecular dynamics (MD) simulations shed light on the mechanics of carbyne chains with different lengths. In order to facili-tate the design and optimization procedure a theoretical framework is also developed. This provides an analytical explanation of the car-byne strength at any temperature and chain length.

In carbyne-based water-purifica-tion structures, the carbyne chains are in direct interaction with water molecules. However, it is largely unknown how the mechanical prop-erties of this structure are affected by environmental factors. It is found that the interaction with water mol-ecules improves the bond stability, which is reflected by higher values for bond order.

For chains in water, a residual covalent bonding is observed at higher strain values, which leads to larger deformations of the carbyne chain before breaking. As a result of improving the bond stability, both

the tensile strength and rupture strain are slightly improved by inter-action with water. This renders these structures promising candidates for fabricating nanoweb membranes for use in water purification.

Carbyne-based structures may operate in either acidic or basic solutions and the effect of these chemical environments on the mechanical properties of carbyne chains is also described in the Nano-technology report. A basic environ-ment decreases the chain strength, while the strength in an acidic solution is slightly increased. A unique self-healing phenomenon is observed in the basic solution, which strongly improves the toughness of the carbyne chains before breaking.

This computational and theoreti-cal study advances the fundamental understanding of the promising role of graphyne in water purification. However, the true potential of gra-phyne in applications has yet to be reached. This may be achievable once this material, and related structures, can be synthesized in large quanti-ties and it is hoped this work will inspire broad experimental efforts to improve synthesis methods for both improved quality and quantity.

Demonstration: CNT sponge driven by a magnet soaks up oil.

Out and about: Lingyun Miao, the principal investigator of this project, visits Swinburne University of Technology.

Process: The rupture of a carbyne chain in a basic solution and the steps involved in healing the broken chain via an oxygen atom from the hydroxide group.

Doubling disk-drive capacity

Understanding the mechanical strength of carbyne

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“Heat-assisted magnetic recording may pave the way for the next generation of hard disk drives with higher capacities”

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5focus on: electronics and photonics

Low-cost printable integrated devicesBy directly printing devices onto a functional resist with a high refrac-tive index, optical components can be created without the use of any etch-ing steps. This new low-cost fabrica-tion technique can produce printable integrated circuits, as reported by researchers at aBeam Technologies, the Molecular Foundry and NanoO-ptic Devices. In a recent Nanotechnol-ogy article the team presented several optical components, including ridge waveguides, light splitters and digi-tal planar holo grams that operate in the visible wavelength range. The approach could revolution-ize the development of integrated photonic devices.

The nanoimprint lithography (NIL) of functional materials is a powerful approach for developing low-cost photonic devices with high optical performance. Supported by the US Air Force STTR Program,

researchers at aBeam Technologies, the Molecular Foundry (a Depart-ment of Energy (DOE)-funded user facility for nanoscience at Lawrence Berkeley National Laboratory) and NanoOptic Devices collaborated. They demonstrated a novel approach for fabricating low-cost, printable photonic integrated devices that work in the visible wavelength range.

The devices are printed directly onto a functional NIL resist and their optical properties (transparency and refractive index) are tuned by post-annealing. “We wanted to simplify the fabrication of photonic devices as much as possible by printing them onto a functional material in one sin-gle step, thereby avoiding any etch-ing steps,” says Christophe Peroz.

The method combines the advan-tages of top-down (NIL) and bottom-up (sol-gel chemistry) approaches. After annealing at high tempera-

tures, the photonic structures shrink and reach a refractive index up to 2.1. “Conventional lithography is very challenging for high-refractive-

index materials, particularly at few-nanometre resolution, but now we have shown that it can be easy,” said lead author Carlos Pina-Hernandez.

The printed structures on the top of a waveguide core are used to create planar lightwave circuits. To demon-strate the practicality of this process, elementary optical structures like multi-mode ridge waveguides, light splitters and more complex devices like digital planar holograms (DPH) are fabricated. Printable wavelength demultiplexer-on-chips were also created and their performance was comparable to current devices fabri-cated by standard and more expen-sive technologies.

This work demonstrates the potential of nanoimprint technol-ogy for fabricating novel photonic devices at low cost and high through-put. According to the Molecular Foundry’s Stefano Cabrini, “Direct nanoimprinting of functional mate-rials is just beginning and I have no doubt that a lot of applications will emerge in the near future.”

Researchers at the University of Illi-nois at Urbana-Champaign in the US, and Seoul National University and Samsung Display Co. in Korea have made the first transient field-effect transistors (FETs) from carbon nano-tubes. The devices, which “disappear” once they have served their purpose, might be used in a wide range of new technologies, such as green RFID tags, biodegradable medical implants, temporary environmental sensors and hardware for secure data systems.

Transient electronics is an up-and-coming new technology in which the components making up a device physically disappear or disintegrate after a certain time. “Vanishing” semiconductors, for example, have already been made from nanomem-branes of silicon, as well as zinc oxide and organic/bioorganic polymers.

Now, a joint team led by John Rog-ers in Illinois, Jong-Ho Lee and Sung Hun Jin in Seoul has made transient FETs from networks of purified single-walled carbon nanotubes (SWNTs). Such networks, as well as having excellent electrical and mechanical properties, can disperse once their supporting substrate has disappeared thanks to the fact that they disintegrate into individual tubes or small bundles. The research-ers also made transient bootstrapped inverters from the SWNT networks.

The fabrication procedure involves depositing transient materials such as molybdenum for electrodes and interconnects, and silicon oxide and silicon nitride for gate and interlayer dielectrics, on a temporary substrate followed by transfer to a water- soluble film of poly(vinyl alcohol) (PVA). This surface can be coated with a uniform layer of SWNTs.

The PVA substrate, which is around 30 µm thick, slowly disap-pears within 30 minutes as it simply dissolves in water. This causes the entire device and circuit to physi-cally disintegrate too. Each remain-ing constituent material, including the silicon oxide, silicon nitride and molybdenum, hydrolyses and disap-pears, and the SWNTs disperse and aggregate into small bundles.

“These systems have outstanding electronic characteristics for devices based on semiconductors deposited by solution processing,” explains Rogers. “And they are the best per-forming yet for any solution-pro-cessed transient electronic system.”

“Transient electronic devices have a well defined, finite lifetime,” he tells nanotechweb.org. “Examples of poten-tial applications for this technology include disposable RFID tags, bio-medical devices that do not place an unnecessary burden on the patient once they have performed their intended function, and hardware secure data systems that cannot be intercepted by hackers.”

The team is now busy working on further improving its devices and looking at integrating them into real-world applications. “Transient RFID technology is one of our particular focus areas,” adds Rogers.

Researchers at Harvard University and the MITRE Corporation have built an ultra-tiny computer from an assembly of nanowires, which is the densest, most complex nano-electronic system ever fabricated from the bottom up. The tech-niques employed to develop the nanoelectronic finite-state machine (nanoFSM), as it is called, might help extend the life of Moore’s Law – which has driven the semiconduc-tor industry for decades. Transistors in the nanoFSM measure less than 20 nm across. In comparison, the transistor channel, by itself, is 22 nm wide in standard industry devices.

In 1965, Gordon Moore, the co-founder of Intel, predicted that the number of transistors per square inch on integrated circuits would double every year and this came to be known as Moore’s Law. The silicon industry has succeeded in following this law and transistors have been exponen-tially decreasing in size since the 1970s. Things are coming to a head, however, and it will be difficult to continue miniaturizing circuits based on top-down, lithographically fabricated bulk silicon transistors.

“A nanocomputer, for its part, could overcome the ultimate scaling limitations of conventional semi-conductor electronics,” says Harvard team leader Charles Lieber. Indeed, a nanoFSM is the perfect model for programmable logic circuits and contains key arithmetic and memory logic elements. An ideal FSM should be able to maintain its internal logic state, modify this state in response to external stimuli and then output commands based on this state to the outside world.

“A basic-state transition diagram for a two-bit four-state nanoFSM we looked at in our work shows the four

binary representations 00, 01, 10 and 11, and the transition from one state to another is triggered by a binary input signal, 0 or 1,” he explained. “Larger and more complex nano-FSMs can be made using longer binary representations.”

The Harvard researchers fabricated the nanoFSM according to a design developed at MITRE, using a new technique called “deterministic nano-combing”. They assembled nano wires into six highly ordered crossbar arrays in which each neighbouring pair of nanowire arrays forms a logic “tile”. Three logic tiles were programmed so that they performed distinct logic functions and integrated to form a computing architecture. The finished system is able to compute, register data and even update the internal logi-cal state of the nanochip while com-municating with the outside world via a digital interface.

“The self-assembled nanowire devices we have demonstrated could be made much smaller than those possible using standard, top-down

lithographically defined bulk-silicon transistors,” Lieber told nanotechweb.org. “Until now, it was difficult to organize nanowires at will into dense and efficient nanocircuits, but the new bottom-up technique we used allows us to assemble dense arrays of the many devices required with extreme precision in a pre-defined design,” he said.

The technique employed could be a new way to continue miniaturiz-ing a number of electronic systems, he adds. Some examples include biomedical sensors, bio-interface controllers, environmental moni-tors, drug-delivery vehicles or even insect-sized robots.

According to MITRE’s Shamik Das, who was responsible for design-ing the nanocomputer, another key feature of the nanoFSM is that it requires very little power to run.

The team says that it is now busy perfecting its technology and graft-ing it onto different materials, sen-sors and actuators to make more complex nanoelectronics.

Printable photonic chip: a) optical picture of a photonic chip, b) scanning electron microscopy picture of a planar hologram, c) optical picture of an integrated chip excited with a red laser at the input.

A real (but false coloured) image of the nanowire nanocomputer and chip.

Transistors go transient

Nanowire nanocomputer in new complexity record

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6 focus on: electronics and photonics

Carbon and terahertz: a promising allianceThe equations that first linked elec-tric and magnetic interactions to explain light celebrate their 150th anniversary this year. Now known as the Maxwell equations, their con-tribution to the theory of relativity led Albert Einstein to remark, “One scientific epoch ended and another began with James Clerk Maxwell.” These iconic equations have since laid the bedrock for modern-day electronics, photonics and mag-netism, and have been integral to a number of studies of nanoscale systems over the years – as high-lighted by Satoshi Kawata, Adarsh Sandhu and Jennifer Dionne in a recent Nanotechnology Discussion podcast on Maxwell’s equations at the nanoscale. One region of the electromagnetic spectrum where a comparatively new alliance has been forged with nanoscale research is the terahertz regime.

Although terahertz wavelengths – measuring 0.1–100 mm – exceed the dimensions of nanostructures, terahertz radiation can be useful for probing these systems because many of their dynamical processes operate around terahertz timescales. Exam-ples can be found in the recent Nano-technology special section dedicated to terahertz nanotechnology. Here Hannah Joyce, Michael Johnston and colleagues at the University of Oxford in the UK and the Austral-

ian National University explain how an optical pump-terahertz probe approach can be used to study carrier dynamics in semiconductor nanow-ires made from Group III and V compounds. The technique has the advantages of being non-contact and operating at room temperature, and provides information on the carrier lifetime that may prove valuable for matching different nanowires to dif-ferent applications.

The same issue includes a number of approaches for harnessing nano-systems for terahertz detection, such as nanowire-based field-effect tran-sistors. Meanwhile, Joo-Hiuk Son provides a review of the principle, characteristics and applications of molecular imaging with terahertz electromagnetic waves. According to Son, terahertz radiation is a rela-tively safe imaging tool (compared with, for example, X-rays) and the sensitivity to vibrational modes of water molecules can be enhanced using nanoparticles for more effec-tive medical imaging and diagnosis.

The emergence of carbon-based nanoelectronics has already greatly modified the outlook for next- generation devices. In a recent issue of Nanotechnology Richard Hartmann from De La Salle University in the Philippines, Junichiro Kono from Rice University in the US, and Mikhail Portnoi from the University of Exeter

in the UK and Universidade Federal do Rio Grande do Norte in Brazil present an overview of what carbon

nanomaterials offer for the future of terahertz science and technology.

As explained in their review, “In this frequency range, electronic transport and optical phenomena merge with one another, and classi-cal waves (in the microwave region) make the transition to quantum mechanical photons (in the optical regime).” It is easy to imagine that the extraordinary optoelectronic properties of carbon nanotubes and graphene might make them ideally suited to applications exploiting these frequencies.

In the review the researchers point out that both carbon nano-tubes and graphene are expected to

show exotic terahertz dynamics and that these “can lead to innovative optoelectronic applications”. They attribute these characteristics to the unique low-dimensional band struc-ture of carbon nanomaterials and the nature of the quantum-confined interactions between charge carri-ers. Electric and magnetic fields, and gating can also trigger activity in the terahertz regime.

The review highlights the progress made in understanding and optimiz-ing carbon nanotube transistors, antennae and polarizers, as well as nonlinear processes, plasmonics and detectors based on graphene at tera-hertz frequencies. These potential avenues for applications join a long list as research activity into carbon nanomaterials attracts ever increas-ing interest, yet the unique charac-teristics of both terahertz radiation and carbon nanostructures suggest a disarmingly promising alliance.

While Hartmann, Kono and Port-noi point out that many challenges lie ahead, they conclude that nano-carbon terahertz technology offers serious potential for development in the coming years. Progress to date certainly makes a convincing case for the future of nanocarbon tera-hertz technology, but in the words of James Clerk Maxwell himself as he contemplated his theory of the colour of light, “time will show”.

Young’s double-slit experiment played a pivotal role in the develop-ment of classical, as well as quantum, physics. It proved the wave nature of light, while its modern version using electron beams provides a solid foundation for quantum mechan-ics. It also stimulated long debates over the paradoxical structure and philosophical implications of quan-tum mechanics. Today, interference and diffraction are routine phenom-ena observed in material structure characterizations using X-ray or electron beams. Reporting in Nano-technology, researchers from the Uni-versity of Houston have observed a similar quantum interference effect in Raman scattering in twisted bilayer graphene.

When an excitation laser beam moves from a single-layer region to a bilayer region of twisted bilayer graphene islands the Raman inten-sity of the G line doubles. In con-trast, the Raman intensity of the 2D band becomes about four times as strong. The four-fold enhancement of the 2D Raman band is a direct manifestation of the constructive interference between two Raman paths. The intensity of the G line rep-resents an excellent example where

there is no interference between two Raman paths.

There is a striking difference in the Raman spectra between twisted bilayer graphene and conventional AB-stacking bilayer graphene; in particular, between the G line and 2D band. This reveals the unique underlying electronic band struc-ture of twisted bilayer graphene as well as the unique condition for quantum interference. This find-ing will help to distinguish among different types of bilayer graphene, and accelerate the exploration of new quantum phenomena in two-dimensional nanomaterials.

Lead halide perovskites can be used as light-emitting semiconductors in an LED structure, according to new experiments by researchers at the universities of Cambridge and Oxford in the UK and at the Ludwig-Maximil-ians-University in Munich, Germany. The light emitted by the perovskites can be easily tuned – something that could make the devices ideal for col-our displays and lighting, and in opti-cal communication applications.

The best solid-state LEDs available today are based on direct-bandgap semiconductors, but making these devices is no easy task because they need to be processed at high temper-atures and in vacuum, which makes them rather expensive to produce in large quantities. Now, a team of researchers led by Richard Friend of Cambridge University’s Cavendish Laboratory has made the first bright LEDs from semiconducting materi-als known as organometal halide perovskites. The devices can be made to emit light in the infrared, green and red parts of the electromagnetic spectrum by tuning the composition of the halide in the perovskite.

For example, to make an infra-red-emitting device, the team sandwiched a thin, 15 nm layer of CH3NH3PbI3 -xCl x perovskite emitter between large-bandgap

titanium dioxide and poly(9,9’-dioc-tylfluorene) (F8) layers. This struc-ture effectively confines electrons and holes in the perovskite so that they can “recombine” to emit light.

The perovskite device emits light with an infrared radiance of 13.2 W/s/r/m2 at current densities of 363 mA/cm2 and boasts external and internal quantum efficiencies of 0.76% and 3.4%, respectively.

The green LED made by Friend and colleagues contains an ITO/PEDOT/PSS/CH3NH3PbBr3/F8/Ca/Ag structure and has a luminance of 364 cd/m2 at a current density of 123 mA/cm2, and external and inter-nal quantum efficiencies of 0.1% and 0.4%, respectively.

Organometal halide-based per-ovskites could be ideal for making optoelectronics devices, thanks to the fact that they can be processed in solution and do not need to be heated to high temperatures. This means that large-area films of the materials can be deposited easily onto a wide range of flexible or rigid substrates. The perovskites also have an optical bandgap that can be tuned in the vis-ible to infrared regions, which makes them very promising for a range of optoelectronics applications. They also strongly emit light and thus are ideal for making LEDs.

“These organometal halide per-ovskites are remarkable semicon-ductors,” says Zhi-Kuang Tan, a PhD student in the Cavendish Lab and the lead author of the paper describing the research. “We have designed the diode structure to confine electrical charges into a very thin layer of the perovskite, which sets up the condi-tions for electrons and holes to be captured and produce light.”

“The big surprise for the semicon-ductor community was to find that such simple process methods can still produce very clean semiconduc-tor properties, without the need for the complex purification procedures required for traditional semiconduc-tors such as silicon,” Friend says.

“It is also remarkable that perov-skites can easily be tuned to emit light of a variety of colours, which makes these materials extremely useful for colour displays, lighting and optical communication applications,” adds Tan. “This technology could benefit the ever-expanding f lat-panel dis-play industry.”

The team says that it is now look-ing into the photophysical proper-ties of these materials and hopes to increase the efficiencies of the LED devices it has made. “We also plan to make diode lasers using these highly emissive materials,” says Tan.

Carbon nanostructures: (clockwise from top) graphene, a graphene nanoribbon and a carbon nanotube.

Raman interference can help distinguish different types of bilayer graphene.

A graphene double-slit test

Perovskites make bright LEDs in many colours

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“In this frequency range, electronic transport and optical phenomena merge with one another”

7focus on: energy

Catering for global energy little by littleResearch on thermoelectric mate-rials was rescued from a state of stasis by work on quantum-well nanostructures in the early 1990s. Heightened interest in alternative energy sources and energy efficiency has further promoted research in this field. Now, thermoelectric gen-erators join solar cells, lithium bat-teries, piezoelectric devices and a host of other potential alternative-energy technologies that owe their credibility to the quirky character-istics that emerge at the nanoscale, as Mildred Dresselhaus, Zhong Lin Wang, George Grüner and Andrew Maynard discussed in a recent Nano-technology Discussions podcast. Yet the question remains whether ther-moelectric devices can one day make a significant contribution to the world’s energy needs, and if so how.

David Sanchez from the Universi-dad de les Illes Balears in Spain and Heiner Linke from Lund University in Sweden, the guest editors of “Focus on Thermoelectric Effects in Nano-structures” recently published by New Journal of Physics, describe some of the advantages offered by nano-structures. In their editorial they point out that nanostructures have more to offer than improvements to the thermoelectric efficiency of a material quantified by the ZT factor and emphasize the benefits of “novel thermoelectric functionalities due to their lower dimensionality and the large variety of well characterized

nanosystems at our disposal”.That said, the improvements to ZT

are significant. In 1993, when Lyn-don Hicks and Mildred Dresselhaus published their pioneering research on the thermoelectric figure of merit for quantum-well superlattice struc-tures, progress in enhancing the ZT factor of thermoelectric materials had been slow since the 1960s.

It was the interdependence of the various contributing factors that stalled efforts to improve the ther-moelectric function of materials. A high figure of merit requires a high electrical conductivity, a high See-beck coefficient (which quantifies the thermoelectric power) and low thermal conductivity, and improv-ing one factor tended to adversely affect another. The genius of the work by Hicks and Dresselhaus was to recognize that the confinement of electrons in low-dimensional nano-structures and the increased phonon scattering at interfaces in a superlat-tice would provide a route around this impasse.

The work catalysed activity in thermoelectric studies of nanostruc-tures that persists to this day. In fact a recent study reported in the New Jour-nal of Physics focus collection shows that despite the advantages of quan-tum dots due to their sharp spectral features, quantum-well structures still compare favourably for energy harvesting. Rafael Sánchez at the Instituto de Ciencia de Materiales de

Madrid, Andrew N Jordan from the University of Rochester, and Björn Sothmann and Markus Büttiker from the Université de Genéve demon-strate how their quantum-well device generates 0.18 W cm–2 for a tempera-ture difference of 1K – double the energy extracted from a similar heat engine based on quantum dots.

All the same, there are good rea-sons to expect that devices based on quantum dots will remain firmly rooted in the research interests of thermoelectric scientists for some time yet. In the same special issue, an international collaboration of researchers led by Sofia Fahlvik Sven-sson at Lund University in Sweden and Eric Hoffmann at the Technische Universität München in Germany explores the nonlinear responses of quantum-dot systems and shows how controlling these effects “expands the range in which quan-tum thermoelectric effects may be used for efficient energy conversion”. Sánchez, Sothmann, Jordan and Büt-tiker also probe the performance of a quantum-dot-based thermoelectric engine by investigating the noise properties of electric charge, heat and currents, and analysing the cor-relations between them.

The reports in the special issue are among the last pieces of research by Markus Büttiker, who sadly died in November 2013. His active career brought several significant contributions to the understand-

ing of conductivity, scattering and quantum-transport behaviour, and the contributions of his life’s work will continue to inform the studies of researchers in these fields for many years to come.

Thermoelectric devices may not yet be central to the world’s energy resources, but early devices highlight their potential. Reporting in Nano-technology, Fang Fang Song, Liming Wu and Shengde Liang at Renmin University in China used their ther-moelectric material to power an LED. The material was made from MnO2 powder and demonstrated a Seebeck coefficient 100 times greater than the “state-of-the-art of Bi2Te3”.

Other examples exist, such as a wearable glass fabric generator that can use naturally produced body heat to power small devices, reported by researchers at the Korean Advanced Institute of Science and Technology (KAIST) in Korea. Another tempt-ing heat source for thermoelectric devices is the waste heat generated by computers, which would reduce the need for cooling systems while also powering the devices. With the improving thermoelectric perfor-mance of nanomaterials, the ready application of thermoelectric power for small devices and the current pro-liferation of energy-hungry mobile devices, it is possible that the contri-bution of thermoelectric devices may little by little provide a significant portion of the world’s energy needs.

A study of how light is absorbed by silicon nanowires of different shapes and composition – both singly and in arrays – has provided insights for designing more efficient solar cells. Contrary to current thinking, the results suggest that it may be possi-ble to exploit the enhanced absorp-tion of single nanowires when scaled up into arrays.

How well a solar cell converts light into electrical energy depends on how well it absorbs light in the first place. And while the manufacture of photovoltaic devices based on silicon can benefit from decades of research and development in CMOS electron-ics, silicon’s absorption properties are far from ideal.

Hong-Gyu Park of Korea Univer-sity explains how silicon in the form of a nanowire offers much better absorption because the nanowire acts as an optical cavity that confines light – “the antenna effect”. Harness-ing this enhanced absorption could lead to low-cost, ultrathin solar-cell devices. “We wanted to under-stand the underlying principles and maximize light absorption in these nanowires,” he adds.

In collaboration with Charles Lieber at Harvard University in the

US, and researchers at Kyung Hee University and Korea University in Korea, Park and his team embarked on a series of simulations to study how silicon nanowires absorb light and how the absorption is affected by the nanowire shape and com-position, and when nanowires are packed into an array arranged either vertically or horizontally.

The work helped the research-ers understand how the light was trapped in the nanowires, allowing them to determine which shapes and compositions would optimize the antenna effect. They were also intrigued to discover that the absorp-tion continues to be enhanced when the system is scaled up from a single nanowire to an array.

Many researchers had thought that the absorption efficiency of the array should be lower because the

antenna effect of a single nanowire vanishes on scaling up into an array. The surprising result opens up the possibility of developing nanowire-array solar cells in next-generation devices. To this end the authors plan to develop a nanowire alignment technique to scale up fabrication of a single nanowire to an assembly over a wide area.

The researchers had noted from previous experimental work that pre-cise control of nanowire composition and shape could dramatically affect the absorption properties. By map-ping the calculated absorption across the nanowires for different shapes and compositions they were also able to identify the light localization and antenna effects responsible for enhancing the absorption efficiency.

The results from these simulations will now allow the team to identify design rules for strategically tailor-ing nanowire absorption properties, and the next stage will be to demon-strate this approach experimentally. This should give insights into the feasibility of nano “tandem” solar cells that use several photovoltaic material systems together, with each optimized to operate most efficiently over different bandwidths.

The triboelectric effect is one of a few effects that have been known for thousands of years. Triboelectric nanogenerators (TENGs) are a new and powerful approach for energy harvesting. Nevertheless, the output of a TENG has the common charac-teristics of a high voltage but low cur-rent and total transported charges. Therefore, it needs transformation before being applied to drive conven-tional electronics.

The power transformation of a TENG involves the lowering of the output voltage, while increas-ing the output charges and current. This is different from the traditional method of using a transformer for a sinusoidal-type AC signal because the output of a TENG can be a short pulse at variable frequency.

A research group led by Zhong Lin Wang, with members includ-ing Wei Tang and colleagues from the Beijing Institute of Nanoenergy and Nanosystems, Chinese Acad-emy of Sciences, has developed a power-transformed-and-managed TENG (PTM-TENG). They inte-grated a contact-separation-mode TENG with a self-connection-

switching capacitor array. These are connected in serial when they are being charged and then in parallel during discharging.

By using an eight-capacitor array, the PTM-TENG’s output charges are enhanced eight times. In addi-tion, the PTM-TENG’s charging/discharging mode is changed from continuous to instantaneous, which can hugely enhance the instanta-neous output current and power. Furthermore, the PTM-TENG is suc-cessfully applied in a wireless touch sensor. Without any power supply this sensor will not only detect the touch stimulation, but also convert the mechanical energy caused by the stimulation into electric power for infrared communication.

An InAs nanowire (diameter 60 nm) contacted by one cold and one heated metallic contact for thermopower measurements. The wire contains one quantum dot defined by an InP double barrier.

A nanowire absorbs a photon, generating an electron and hole pair.

Array of hope for more efficient silicon solar cellsManaging power in triboelectric nanogenerators for sensing

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“Nanostructures offer not only promising enhancements of ZT factor but also novel thermoelectric functionalities”

“Silicon’s absorption properties are far from ideal”

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9focus on: patterning and fabrication

2D transfers are made easierResearchers at the Kavli Institute of Nanoscience in the Netherlands have come up with the first all-dry technique for transferring 2D mate-rials, like graphene or molybdenum sulphide. The new technique, which is quick, efficient and clean, makes use of viscoelastic stamps. As well as being much simpler than traditional wet transfer techniques, it could also be used to fabricate freely suspended 2D structures thanks to the fact that the samples are not subject to any capillary forces during the process.

2D materials are creating a flurry of interest in labs around the world because they have dramatically dif-ferent electronic and mechanical properties from their 3D counter-parts. This means that they could find use in a host of novel device applications, such as low-power electronics circuits, low-cost or flex-ible displays, sensors and even flex-ible electronics that can be coated onto a wide variety of surfaces.

The most well known 2D materi-als are graphene (which is a sheet of carbon just one atom thick) and the transition metal dichalcogenides – which have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalco-gen (such as S, Se and Te).

For real-world applications, the materials need to be transferred onto substrates to make heterostructures based on the artificial stacking of the 2D layers. Most techniques involve wet chemistry, but the problem here is that the chemicals employed often contaminate the 2D materials, which are fragile because they are so thin, and adversely affect their pristine electronic and physical proper-ties. Moreover, the capillary forces between the chemicals and the mate-rial being transferred can cause the 2D structure to simply collapse.

A team led by Herre van der Zant

and Gary Steele used a thin layer of a commercially available visco-elastic material called Gelfilm. The researchers transferred 2D crystals, such as graphene and MoS2, onto the film by mechanically shaving off layers from the bulk, or parent 3D, material using the now famous Scotch-tape technique (first used to isolate graphene from graphite in 2004). They selected only the thin-nest flakes (by looking at them under an optical microscope) and fixed these onto the XYZ sample stage in the microscope. They then attached a stamp to the sample.

“As the stamp is transparent, we can see the sample through it and can align the flake wherever we want on a 2D substrate surface with sub-micron resolution,” said team mem-ber Andres Castellanos-Gomez. “To transfer the flake, we press the stamp against the sample surface and peel it off very slowly.”

The transfer works thanks to vis-coelasticity, he told nanotechweb.org. “Our stamps are made of a silicone rubber very similar to the ones found in the ‘stretchy sticky hands’ toys for kids.” This silicone rubber behaves like an elastic solid over short time periods, but can f low over longer timescales. “If we contact this rub-ber with a surface for a long time it will flow until it becomes intimately joined to it. This is why the sticky-hands toys can adhere to anything without any glue. We exploited this phenomenon to adhere flakes to our viscoelastic stamps without employ-ing an adhesive. By then slowly peeling the sample off the stamp surface, the viscoelastic material detaches, releas-ing the flakes that then preferentially stick to a substrate surface instead.”

The team has already proved that its technique works by transferring graphene f lakes onto hexagonal boron nitride (a 2D material that

is a good substrate for graphene). Thanks to optical microscope images, the researchers were able to confirm that nearly half of the gra-phene flakes lie flat on the h-BN with-out any bubbles or wrinkles. And the good thing is that the whole process takes just 15 minutes or less.

“Our technique could be applied to any kind of exfoliated layered crys-tal, so allowing for an infinite combi-nation of material heterostructures,” said Castellanos-Gomez. “For exam-ple, as well as depositing graphene on h-BN, we have also already managed to ‘sandwich’ a MoS2 bilayer between two h-BN flakes.”

The Kavli team has also succeeded in transferring a single-layer MoS2 crystal onto a SiO2/Si substrate pre-patterned with holes of different diameters. The single-layer MoS2

is freely suspended over the holes, forming “drumheads” – which might be used in mechanical resonator applications. Indeed, the technique might also be employed to trans-fer 2D crystals onto pre-fabricated devices with trenches and electrodes.

And that is not all. Because the stamping technique is so gentle, it can be used to deposit 2D crystals onto even the most fragile of sub-strates. For example, the team says that it has succeeded in transferring few-layer MoS2 crystals onto the cantilever of an atomic force micro-scope without damaging the cantile-ver at all. “We have also transferred 2D materials onto silicon nitride membranes and holey carbon films, which are typically employed in transmission electron microscopy,” said Castellanos-Gomez.

Solid-state nanopores in thin mem-branes could be ideal for DNA sequencing, and focused ion beams (FIBs) can be used to fabricate them with fine control over diameter. Using molecular dynamics simula-tions, researchers at the University of Illinois in the US have calculated a threshold beam flux above which nanopores form orders of magni-tude faster through a thermally dom-inated explosive boiling mechanism.

Nanopore fabrication by low-flux FIB is driven by sputter erosion. On average, each ion impact from a FIB sputters between two and three tar-get atoms from a typical silicon spec-imen, eventually thinning the target membrane and forming a through-thickness nanopore. At low flux, the formation rate is proportional to the ion flux. However, detailed simula-

tion results suggest that multiple ions, if delivered at a fast enough rate, can significantly accelerate hole for-mation via a new mechanism.

At slightly higher fluxes than typi-cally employed in experiments, the target material heats up faster than it cools via thermal conduction. This leads to local melting and explosive

boiling of the target material. Mass is rapidly rearranged via bubble growth and coalescence, leading to material removal that is orders of magnitude faster than that occur-ring by sputter erosion. The high-flux beam thus provides a shortcut to overcoming the energy barrier asso-ciated with nanopore formation.

By this new mechanism, sputter-ing no longer limits the nano pore formation rate, something that could provide a way of greatly accel-erating this process. The necessary beam fluxes can be produced using modern FIB systems, and hopefully experiments will be performed soon to study this new mechanism.

Researchers at the University of Aus-tin at Texas in the US and Konkuk University in Seoul, Korea, have fabricated a new type of transpar-ent, flexible dielectric film using gra-phene and a ferroelectric polymer. The film is easy to make, has a high dielectric constant and might be ideal in plastic electronics applications.

Graphene is a single, f lat sheet made up of sp2-bonded carbon atoms arranged in a honeycombed lattice. Its extremely high carrier mobility means that it can func-tion as both interconnect and high-mobility channel material in ultrafast transistors. The material also absorbs light from the visible to mid-infrared and is transparent to light while being mechanically flex-ible and incredibly strong.

The researchers, led by Rodney Ruoff, made their dielectric film by sandwiching together the ferroelec-tric polymer cyanoethyl pullulan (CEP) and graphene. This is the first time that CEP has been combined with graphene in such a study – the polymer of choice until now had been PMMA. The graphene, which makes up the middle of the sand-wich, was grown using a technique called chemical vapour deposition. The ensemble was then simply trans-ferred onto another CEP-coated plas-tic substrate, PET.

“Our graphene dielectric film is highly transparent, flexible and has a high dielectric constant of 51,” Ruoff told nanotechweb.org. “What is more, we are able to control the film’s di electric constant by varying the level of oxidation of the graphene interlayer.” To compare, the dielec-tric constant of ferroelectric poly-mers and high-k thin films such as hafnium oxide are around 15 and 20, respectively. A high dielectric con-stant of the film employed in plastic electronics is important because it directly affects device performance.

The graphene forms a space charge layer – that is, an accumulation of polarized charge carriers (electrons and holes) near the graphene, which polarizes small areas of the sample and thus enhances the dielectric constant of the entire film, he contin-ues. The film may be ideal in plastic electronics applications, such as flex-ible inorganic electroluminescence devices and embedded capacitors.

The team previously made a com-posite film from CEP and chlorinated reduced graphene oxide, which also enhances the dielectric constant of the film. This is thanks to increased interfacial polarization, which, in part, comes from the highly polariz-able carbon–chlorine bonds. “Based on these results, and our new work, we would like to look at how chlo-rinated graphene interlayers affect graphene-CEP films in an effort to increase the dielectric constant of this hybrid material even further,” said team member Jin Young Kim.

The viscoeleastic stamp used in the all-dry transfer method.

Transfer of atomically thin crystals onto different substrates.

Two alternative routes for nanopore formation.

Nanopores form more quickly through focused ion beam boiling

Graphene for dielectric films

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10 focuson:biologyandmedicine

Nanoparticle suicide genes tackle cancer Gene therapy can offer an effec-tive treatment for drug-resistant radio-insensitive cancer. However, progress has been hampered by the difficulties in developing an appro-priate delivery mechanism. Now researchers have demonstrated for the first time that magnetic nano-particles provide safe, effective and targeted “suicide-gene” delivery to cells of a particularly prevalent and highly resilient type of liver can-cer. Because the nanoparticles are magnetic they can also be used for hyperthermia treatments, where magnetic energy is converted into heat to elevate the temperature of the surrounding cancerous tissue, increasing the overall therapeutic effect of the gene therapy.

“Our in vivo and in vitro experiments showed that the gene therapy com-bined with the heating treatment was very effective,” explains Chen-yan Yuan, a researcher from South-east University in China. “In mice, we saw that the tumour growth rate, volume and mass were significantly less in the combined treatment group compared with gene therapy and hyperthermia therapy alone.”

Yuan and colleagues from the Affiliated Zhong Da Hospital of Southeast University and Jiangsu Key Laboratory for Biomaterials and Devices in China equipped the mag-netic nanoparticles with a tumour-specific promoter gene to specifically tackle hepatocellular carcinoma,

which is the most common form of liver cancer and causes more than 600,000 deaths worldwide each year. The promoter gene could easily be replaced to track down and treat other cancers in the body.

For several years, gene therapy has been acknowledged as a promising candidate to treat a wide range of diseases and genetic disorders. The concept of gene therapy is fairly straightforward, tackling disease at the DNA level by replacing defec-tive, disease-causing genes with healthy genes, but it has proved to be very difficult in practice, with one of the main issues being the choice of a suitable vehicle, or vector, to trans-port and introduce healthy genes into cells.

Scientists have traditionally used genetically engineered viruses as a vector because they are naturally programmed to insert their DNA into a foreign cell. However, the viruses have been known to ran-domly integrate themselves onto chromosomes and also provoke an immune response in the host, caus-ing major complications. As a result, scientists have proposed using func-tional nano particles as a vector to avoid these issues and enhance the therapeutic effect of the delivered genes.

As Yuan points out: “Magnetic nanoparticles have proven to be an extremely effective alternative to tra-ditionally used vectors. They are very

efficient when it comes to delivering DNA into a cell and do not provoke an immune response in the host. They are also safe, simple to use and easy to produce on a large scale.”

In their study, the researchers fabri-cated iron-oxide magnetic nanopar-ticles, which were around 20–30 nm wide and coated them with a positive charge so that the negatively charged DNA molecules could bind strongly to them.

The DNA that they attached to the magnetic nanoparticles included the “suicide gene”, which stops the cells in the tissue proliferating and promotes cell death, as well as a tumour-specific “promoter” gene that acts as the driver of the vehicle, directing the magnetic nanoparticle to the specific tissue.

The magnetic nanoparticles were assessed to see how well the differ-ent genes combined, and then tested in vitro on human liver-cancer cells and in vivo on healthy female mice. During the tests, the magnetic nano-particles were exposed to magnetic energy through an alternating mag-netic field, which they were able to convert into heat, raising the tem-perature of the surrounding cancer-ous tissue to 42–44 °C.

“Our results showed that the mag-netic nanoparticles could elevate the temperature of the selected tissue into an effective therapeutic range, and avoid unwanted cell death and heating to normal tissues,” says Yuan.

A plasmonic chip to diagnose type-1 diabetes? That is what researchers at Stanford University have invented. The chip, capable of detecting bio-markers such as insulin-specific autoantibodies, might be used in hospitals and doctors’ surgeries as a quick and simple way to detect early-stage type-1 diabetes (T1D).

Diabetes could affect nearly 370 million people worldwide by 2030, according to the World Health Organization. Even more worry-ing, diabetes is now the second most chronic disease in children. For rea-sons that are still unclear, the rate of T1D (also known as autoimmune diabetes) in children is increasing by about 3% every year, with a projected increase of a staggering 70% between 2004 and 2020.

Although T1D was once thought of as being exclusively a paediatric disease, around a quarter of individ-uals now contract it as adults. The rate of type-2 diabetes (T2D) (also called metabolic or diet-induced dia-betes), normally seen in overweight adults, has also alarmingly escalated in children since the early 1990s, in part because of the global obesity

epidemic. Until quite recently, it was fairly simple to distinguish between T1D and T2D populations, but this is becoming more and more difficult as the two are beginning to overlap. The main problem is that existing diagnostic tests are slow and expen-sive, and it would be better to detect diabetes as early as possible to ensure the best possible treatment.

T1D is different from T2D in that patients with the disorder have a much higher concentra-tion of autoantibodies against one or more so-called pancreatic islet antigens (such as insulin, glutamic acid decarboxylase and/or tyros-ine phosphatase). Detecting these autoantibodies, and especially those

against insulin (which are the first to appear), is thus a good way to detect T1D. Again, standard tests are not very efficient here and even the most widely used technique, radio-immunoassay (RIA) with targeted antigens, is far from ideal because it is rather slow and, of course, relies on toxic radioisotopes.

In an attempt to try and overcome these problems, researchers at Stan-ford have now developed a simple point-of-care autoantibody test that is more reliable, simpler and faster than RIA and similar tests. It works thanks to an islet antigen microarray on a plasmonic gold (pGOLD) chip and can be used to diagnose T1D by detecting autoantibodies against insulin, GAD65 and IA-2, and poten-tially new biomarkers of the disease. It is able to detect different types of autoantibodies in just 2 µL of whole human blood (from a finger-prick sample, for example) and results can

be obtained in the same day.The team, led by Hongjie Dai,

made its pGOLD chip by uniformly coating glass slides with gold nano-particles that have a surface plas-mon resonance in the near-infrared part of the electromagnetic spec-trum. The pGOLD chip is capable of enhancing the fluorescence emis-sion of near-infrared tags of biologi-cal molecules by around 100 times. Together with Brian Feldman’s group, the researchers robotically printed the islet antigens in triplicate spots onto the plasmonic gold slide to create a chip containing a micro-array of antigens.

“We tested our device by apply-ing 2 µL of human serum or blood (diluted by 10 or 100 times) to it,” explains Dai. “If the sample contains autoantibodies that match one or more of the islet antigens on the chip, those antibodies bind to the specific antigens, which are then tagged by a secondary antibody with a near-infrared dye to make the islet spots brightly fluoresce.”

The samples came from Feld-man’s patients who had new-onset diabetes. They were tested against non- diabetic controls at Stanford University Medical Center. The anti-gen spots fluoresce 100 times more brightly thanks to the plasmonic gold substrate, which allows the antibody

to be detected at much lower concen-trations (down to just 1 femtomolar) than if ordinary gold were employed in the microarray platform.

Plasmonics is a relatively new branch of photonics and is begin-ning to prove itself in the field of medical research with the discovery that specifically engineered metal-lic nanoparticles can be made to resonate in the infrared. The nano-particles interact strongly with light via localized surface plasmons (col-lective oscillations of electrons on a metal’s surface) and so act as efficient optical nanoantennas that capture more light. Such plasmonic reso-nances can be used as probes that can be tuned to and away from vari-ous vibrational modes in biological molecules, which themselves vibrate when excited with infrared light.

“We believe that our technology will be able to address the current clinical need for improved diabetes diagnostics,” Dai tells nanotechweb.org. “The pGOLD platform is also being commercialized by a new start-up company Nirmidas Bio-tech, based in San Francisco, aimed at better detecting proteins for a wide range of research and diag-nostic applications. It might even be able to detect biomarkers for other diseases, such as heart disease, with ultrahigh sensitivity.”

In vivo bioluminescence image after magnetic nanoparticle gene delivery.

Tumours from mice in four different treatment groups.

Schematic depicting the platform’s PEG layer, the islet-specific antigens, the primary autoantibodies from diluted human serum or blood and the detection of antibodies conjugated with a fluorophore signal.

Plasmonic chip detects diabetes

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11focus on: biology and medicine

Arrays in the future prospects of bionanotechnologyNanotechnology’s prominence in modern medicine is indisputable, but establishing exactly how bio-logical systems will interact with nanostructures can be far from clean cut. Biological systems can seem messy and difficult to reproduce with precision, so different reports of similar studies seem to contra-dict each other. Yet progress is being made to identify the relationships between factors that contribute to the response of biological systems to nanostructures.

A r rays of one - dimensional nanostructures serve as a case in point. The nanostructure dimen-sions – roughly the size of a cell in length but 100–1000 times smaller in diameter – can allow access deep into the cell without causing exces-sive damage, making them poten-tially very useful. However, studies of these systems have highlighted how sensitive they can be to appar-ently insignificant differences in the experimental details. Karen Mar-tinez and colleagues at the Univer-sity of Copenhagen describe some of the advances that have been made in cellular applications of arrays of one-dimensional nanostructures. They highlight the progress in the field so far and trends that are emerging from the literature.

In fact, although the attention of the early microscopists was primar-ily absorbed by biological species, when nanoscale imaging first arrived it was largely motivated by electron-ics. As co-inventor of the atomic force microscope, Christophe Ger-ber highlights in our Nanotechnology discussion podcast looking back over the first 25 years of the journal, “In the early 1980s the semiconduc-tor industry was approaching the nanoscale but there were no instru-ments that could measure at that scale.” The scanning tunnelling microscope and subsequently the atomic force microscope provided the tools the industry lacked at the

time, but it was not long before the potential to apply the same cantilever systems for detecting interactions with molecular-scale biological sub-stances became apparent.

Molecular recognition based on atomic force microscopy technology detects the presence of molecules by the change in mass of the cantile-ver system when molecules bind to it, either by a change in the deflec-tion (static mode) or a change in the resonance frequency of the canti-lever oscillations (dynamic mode). The surface of the cantilever can be treated to encourage interactions with the chemical of interest.

As with the original work to cre-ate high-resolution images with tips and cantilevers, applying these sys-tems to biosensing posed significant challenges at first, such as mislead-ing readings from the cantilever deflection. This issue was resolved by integrating a sensor cantilever and at least one reference cantilever into an array. Further details of the technique are explained in a tutorial by Christophe Gerber and colleagues from IBM Research and the Univer-sity of Basel in Switzerland.

Another aspect of nanostructures that can be invaluable for detecting trace substances is the enhancements

to electromagnetic fields that occur nearby when light of a well tuned wavelength excites a plasmon reso-nance in a nanostructure. As well as enhancing Raman signals from the chemicals present, these resonances also indicate the presence of other substances by shifts in the resonant wavelength. Lars Gunnarsson and Alexandre Dmitriev from Chalmers University in Sweden demonstrated an ultralow limit of detection of just several pg cm–2 (or several tens of attomoles cm–2) using the localized surface plasmon resonances of an array of nanodisks.

Compared with nanoparticles that have points or sharp corners, such as rice, star and crescent shapes, sens-ing from plasmons in nanodisks can be more difficult. Yet the team devel-oped a cost-effective approach using nanodisks in an array with low peri-odicity so that the spectral charac-teristics reflect those of a single disc. As they point out in their report on the work, “These experiments pave the way towards an ultra- sensitive yet compact biodetection plat-form for point-of-care diagnostics applications.”

An alternative use of nanostruc-tures for biosensing is by electronic detection, where DNA sequencing

through nanopores is a prime exam-ple. Molecules in saline solution can be detected as they pass through the pore by the change in the impedance. New fabrication protocols have also provided the means for producing chips designed around this nano-pore-type sensor with additional insulating layers to reduce noise.

Rahim Esfandyarpour and col-leagues at Stanford University and Stanford Genome Technol-ogy Center in the US modified the approach by using arrays of nano-needles. Detection occurs as mole-cules pass through the eye of the needle in a similar manner to the use of nanopores, but by adopting a needle-type structure the research-ers take advantage of 3D diffusion of the molecules for a higher hit rate, easier integration into a handheld device and for use in vivo, parallel processing using an individual on-chip amplifier and read-out system on the nanoneedles, and a lower sus-ceptibility to fluidic noise.

Martinez and colleagues provide an overview not only of applications of one-dimensional nanostructure arrays for detecting cellular activ-ity with unprecedented spatiotem-poral accuracy but also modifying this activity. They describe how the

nanostructures can be used for prob-ing cells, drug delivery, as electrodes to monitor cell activity and to guide cells to form a network or even an artificial neural network. They note trends in the way different cells respond to nanostructure arrays of various densities and geometries, and how this can be affected by the cell-handling techniques used in each experiment.

While the authors emphasize the challenge of managing the maze of variables in these systems the out-look seems optimistic. “Despite the fact that many different types of nanostructure platforms have been characterized so far, often unique in their combination of variables (mate-rial, topography, cells, methods) the obtained results are encouragingly in many cases similar despite the different experimental conditions,” they conclude.

The recognition of the similari-ties in results from different experi-mental conditions would seem to herald an exciting epoch in the field. As Alfred Nobel, founder of the Nobel prize, once said, “One can state, without exaggeration, that the observation of and the search for similarities and differences are the basis of all human knowledge.”

A team of researchers in Korea and the US has unveiled a new multitask-ing wearable electronic device that can monitor muscle motion, store data and deliver therapeutic drugs all at the same time. The device contains strain sensors, memory components and thermal actua-tors made of various nanomaterials printed onto a thin, flexible plastic patch that can be placed directly on the skin, just like a temporary tattoo. It could be used to monitor and treat patients with neurological “move-ment disorders” like Parkinson’s disease or epilepsy.

Portable devices that can be worn by patients have come a long way in the last few years thanks, in part, to important advances in flexible elec-tronics and devices that can mould

themselves to the soft structure of skin and tissue. Indeed, a host of novel biocompatible applications, such as patch-like thermometers that stick to the skin like temporary tattoos, and electronic circuits that interface with internal organs such as the heart and brain, have already seen the light of day. Sophisticated though such structures are, most of them are unable to independently store recorded data in memory modules during long-term patient monitoring. Also missing is the fact that they cannot deliver drugs in response to diagnoses made by ana-lysing the data patterns collected.

A team led by Dae-Hyeong Kim of Seoul National University has now developed a new system that addresses these shortcomings. The

device, which measures around 4 × 2 cm and is 0.3 mm thick, con-tains movement sensors made from single-crystal silicon nanomem-

branes that detect strain – generated, for example, when muscles expand and contract. The data obtained by these sensors is stored in ultrathin oxide nanomembranes in which uniformly sized gold nanoparti-cles are embedded. This part of the device also analyses the data col-lected and triggers the release of therapeutic drugs if needed, which are contained inside porous silica nanoparticles.

“All of these multifunctional arrays of sensors and memory, which are fabricated using conventional semiconductor-industry techniques, are then transfer-printed onto a com-mercially available plastic hydrocol-loid patch that can be placed directly onto the patient’s skin – for example, on their wrist,” explained Kim.

And that is not all: thanks to a ther-mal actuator made of resistive heat-ers, the drugs can be made to diffuse

into the skin at optimal rates. Finally, a sensor made of thin, patterned metal films monitors skin tempera-ture during drug delivery to make sure that the skin does not overheat.

“We have shown that our wear-able device can monitor and record body movements from muscle con-tractions, such as tremors, and if a patient twists his arms or legs,” Kim told nanotechweb.org. These types of movement disorders occur in patients with diseases like Parkin-son’s or epilepsy. “The device is also robust and continues to function just as well even if repeatedly bent, twisted or stretched on the skin.”

The team, which includes research-ers from the University of Texas at Austin and MC10 in Massachusetts (co-founded by John Rogers of the University of Illinois), says that it is now looking to improve its device by making it completely wireless.

Left: Applications of arrays of a vertical 1D nanostructure. Right: The universe of AFM-based microscopy and local sensor techniques.

High-performance multifunctional wearable electronics integrated with nanoparticles are conformally attached to the skin for diagnosing and treating motion-related neurological disorders.

Skin patch monitors movement

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13focus on: materials synthesis

Ring-like assembly of gold nanoparticles Self-assembled gold nanoparticles can be used as an effective and low-cost substrate for surface-enhanced Raman scattering (SERS) ultra-detection. The simple clustering of colloidal gold allows the enhance-ment of the Raman signal by up to 1010 in nanoscale gaps between the particles. Nevertheless, the synthe-sis of well ordered and reproducible nanoparticle arrays at a large scale still remains a challenge. Report-ing in Nanotechnology, researchers have investigated a simple and ver-satile dip-drawing technique to achieve this.

A multidisciplinary team devel-oped a novel method for producing high-quality silica colloidal crystals (CCs) at the centimetre scale. The team included researchers from the Institute of Biochemistry and Physi-

ology of Plants and Microorganisms, and the Institute of Laser and Infor-mation Technologies, Saratov State University in Russia. They worked in collaboration with the School of Engineering and Materials Science, Queen Mary University of London, in the UK.

The best quality two- dimensional CCs are obtained with aqueous monodisperse silica colloids at room temperature and with a time of several minutes. The decoration of CCs with gold nanoparticles and nanorods results in the formation of ring-like chains. These have a preferential tail-to-tail orientation along the hexagonal boundaries. Such nano particle packaging leads to a strong field enhancement in the nanoscale gaps between the nanoparticles.

Owing to the unique packaging, the nanostructures demonstrate an analytical SERS enhancement fac-tor at least 10 times higher than that for a random nanoparticle assembly.

This advantage, together with the highly reproducible SERS response, provides an opportunity for using such materials for quantitative SERS analytics.

Graphene has been touted as the material to usurp silicon in next-generation electronics, but it remains unlikely without an eco-nomic approach to fabricating high-quality, wafer-scale graphene layers. Now, researchers in Korea have demonstrated a way of growing several patches of oriented graphene until they coalesce seamlessly, pro-viding a route to producing high-quality single-crystal graphene over large areas.

“We are now quite close to mass production of wafer-scale single-crystal graphene, like silicon,” says Dongmok Whang at Sungkyunk-wan University, one of the research-ers behind this latest work. “I think these single-crystal graphene mono-layers can be cheaper than silicon wafers eventually.”

Whang’s team, together with researchers from the Samsung Advanced Institute of Technol-ogy, has fabricated simple transis-tor devices from the material it has grown, and shown that they do indeed deliver the enhanced properties expected for single-crystal silicon.

Large-scale graphene layers have typically been produced using chemical vapour deposition (CVD). However, this process usually cre-ates polycrystalline graphene, with single-crystal grains separated by grain boundaries that act as defects and compromise the material’s elec-trical and mechanical properties.

Dongmok Whang and colleagues at Sungkyunkwan University and Samsung Advanced Institute of Technology in Korea tried growing graphene on a germanium surface and noticed that the grains of gra-phene seemed to grow with a pre-ferred direction of orientation.

“We thought the structure and symmetry of the substrate surface might be related to the grain ori-

entation, which made us think of using the germanium surface for large-area single-crystal graphene growth,” explains Whang.

Whang points out that most researchers have grown graphene on metal surfaces. However, since he and his colleagues have been studying nanomaterials of group-IV materials, such as carbon, silicon and germanium, for the past decade, he explains: “Studying synergetic effects of carbon and germanium for obtaining new nanomaterials was a natural choice for us.”

In fact, germanium has a num-ber of advantages as a substrate for graphene growth. First, it is highly catalytic, which lowers the energy barrier for forming graphitic carbon. Second, well defined germanium crystal surfaces with atoms arranged along one direction are readily avail-able for growing aligned graphene grains. Carbon is extremely insolu-ble in germanium, which means that carbon grains can grow into a com-plete monolayer, while graphene and germanium share very similar ther-mal expansion coefficients to help to prevent the formation of wrinkles.

To grow the graphene, the researchers first deposited a single-crystal germanium surface on a

silicon wafer, and then flowed low-pressure methane (CH4) over the germanium layer at 900–930 °C in a CVD chamber. Years of previous research with CVD growth helped them to optimize the conditions.

High-resolution electron micros-copy images and electron diffraction data confirmed the presence of high-quality monolayer graphene with no defects, no wrinkles and a single orientation for the crystal lattice throughout. Experimenting with a different face of the germanium crystal relative to its lattice structure still yielded good-quality graphene, but in this case it was polycrystalline.

After lifting off the graphene the germanium substrate could be re-used five times, with no apparent deterioration in the quality of the gra-phene grown – with the potential for recycling the substrate and no tech-nological limit for mass production. Whang believes that the approach could in time be more cost effective than silicon-based electronics.

The researchers have not yet tried using the approach to produce other materials, but point out further opportunities to explore. “Hexago-nal crystalline boron hydride might be another possible choice we can try, because it has a structure that is quite similar to graphene,” says Whang.

The researchers are also planning to use the approach to develop appli-cations that require orientation-dependent graphene properties, such as in graphene nanoribbon devices.

Researchers at the University of Texas at Austin in the US and Nanjing University in China have succeeded in synthesizing a superhydropho-bic thin film that can be coated onto virtually any substrate. The mate-rial, produced using a 3D nanotex-tured hydrogel matrix, is strong, very flexible and optically transpar-ent. It might be used as a waterproof coating in applications such as self- cleaning windows, antifouling sur-faces, and as a filter and sponge for separating out oil from water (for example, after an industrial oil spill).

Superhydrophobic surfaces effi-ciently repel water in a phenomenon that is also known as the lotus effect. Indeed, the lotus leaf is a symbol of purity in many cultures because of its ability to remain clean; when rain falls onto the leaf, the drops of water that form on the surface roll off, tak-ing any dirt with them.

Now, a team led by Guihua Yu and Yi Shi has made a new type of superhydrophobic surface compris-ing a 3D silica nanostructure repli-cated from a hydrogel template. The resulting hybrid coating consists of 3D interconnected nanofibres with uniform diameters of about 100 nm. Its morphology is very much like that of the lower surface of a lotus leaf, which contains micron-sized bumps that, in turn, are covered with nanoscale hair-like tubes. The nanofibres trap air under any water drops falling on them, creating a sur-face that repels water.

The films produced by these inher-ently 3D nanotextured hydrogel templates remain superhydrophobic even when stretched to their limit – and after more than 5000 stretching cycles at 100% strain. This is a first because most superhydrophobic sur-faces made to date lose their hydro-phobic properties when exposed to a strain of more than 30%.

The films can be coated onto virtu-ally any substrate, including metals, cement, wood, fabrics and plastics, thanks to their good wettability. They are also optically transparent (letting through 98% of light fall-ing on them) and can absorb up to 40 times their weight in oil.

The researchers made their super-hydrophobic films using a polyani-line (PAni) hydrogel template. The PAni hydrogel polymerizes and gels out fairly fast, and forms into a 3D structure within three minutes.

Thanks to the high acidic, high-water-content hydrogel matrix, the silica layer preferentially coats onto the PAni nanostructured template. Next, the silica layer is chemically modified, or “silanized”, by deposit-ing trichloro(octadecyl)silane onto the template to produce a superhy-drophobic surface. The overall pro-cess is simple and can be scaled up to produce large amounts of superhy-drophobic film, team member Lijia Pan tells nanotechweb.org.

Overview (left) and magnified (right) scanning electron microscope images of 282 nm silica colloidal crystals decorated with 11 x 44 nm gold nanorods.

A single-crystal graphene monolayer can be made by allowing several aligned islands to grow and coalesce.

Graphene synthesis: Joining the dots

Hydrophobic templates

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“Graphene has been touted as the material to usurp silicon in next-generation electronics, but it remains unlikely without an economic approach to fabricating high-quality, wafer-scale graphene layers”

Untitled-1 1 21/10/2014 08:50Ad.indd 1 03/11/2014 16:13

15focus on: sensing and actuating

Spintronics offer a low-cost alternative to opticsPosition sensing with subnanometre resolution has long been the exclu-sive domain of optical interferom-etry. It operates over bandwidths as high as a few hundred kHz and progress on alternative non-optical, non-contact sensors has been mini-mal. Reporting in Nanotechnology, scientists at IBM Research – Zurich introduce a new non-optical, non-contact position sensor. This is based on detecting changes in a high- gradient magnetic field of a micro-scale magnetic dipole by means of spintronic sensors.

In the sensor, the mechanical motion of a micromagnetic dipole is transduced into a change in electri-cal resistance of a spintronic-based magnetic-field sensing element – for example, a giant magnetoresistance (GMR) sensor. Owing to the intrin-sically high bandwidth and low measurement noise of these spin-

tronic sensors, sensitivities of up to 40 Ohm/µm could be experimen-tally demonstrated. This leads to a noise floor of 0.5 pm/sqrt(Hz) over more than a megahertz bandwidth.

Magnetoresistance-based position

sensors have long been known in industrial mechatronics. However, their use in nanotechnology was lim-ited due to a relatively low sensitivity and a large amount of hysteresis. In this new position-sensing concept,

these issues are resolved by operat-ing the spintronic sensor close to the pole of a micromagnetic dipole. This is where the magnetic field has an extremely high gradient, which increases as the dimensions of the

micromagnet are scaled down.At the same time, saturation of

the spintronic sensor is avoided due to the shape anisotropy effect. The fact that the positioning resolution improves as the sensor dimensions are scaled down sets the spintronic position sensor apart from most of the position sensors available today.

The position sensor has a low cost, a small form factor and low- complexity read-out electronics. This means that it can readily be used in a multitude of high- precision scientific tools and commercial instruments that require high-speed position sensing, such as atomic force microscopes. Its prop-erties also make it highly suitable for multi-sensor configurations and applications where an easily adjustable trade-off between sens-ing resolution and range of opera-tion is required.

Optical sensors coated with carbon nanotube (CNT) clusters can dis-tinguish between carbon monox-ide (CO) and carbon dioxide (CO2). This is due to a subtle difference in how well the two molecules stick to the CNTs.

As reported in Nanotechnology, a sensor platform – which directly interfaces the CNT clusters with the optical sensor – is able to detect this very small difference under normal atmospheric conditions. This approach combines chemically selective nanomaterials with ultra-sensitive devices and could enable a wide range of experiments and diag-nostic methods.

The detection and analysis of vapours and gases in air is particu-larly challenging because there are very few ways to selectively identify a gas. However, many gases exhibit preferential absorption to nano-materials. Therefore, a vapour or gas sensor can be created by depositing a chemically selective nanomaterial directly on a sensor surface.

Prev ious gas-sensor dev ices have identified gases saturated on a substrate in a vacuum chamber by a process termed “temperature-programmed desorption”. As the temperature is increased the gas

molecules desorb and can be identi-fied by mass spectroscopy. However, the complexity of the instruments used for this approach makes it dif-ficult to miniaturize.

Directly combining substrate and sensor could help towards smaller devices, but for this there are several material requirements for the device components. CNTs offer advantages because they are phenomenally resilient to heating. However, the electrics in conven-tional temperature-programmed desorption sensors are not, prompt-ing the researchers to try an optical sensor instead.

The researchers used optical cavi-ties as the read out for the CNT sen-sors. Optical cavities can confine light at a resonant wavelength that depends on their refractive index and geometry. If CNTs are deposited on the cavity surface, desorption and adsorption of gases on these CNTs will then change the resonant wavelength of the cavity.

To demonstrate this type of hybrid sensor system, Maria Chis-tiakova and Andrea Armani from the University of Southern Califor-nia coat a silica resonant cavity opti-cal sensor with CNT clusters. After regenerating the CNT clusters with

argon, the CNTs are saturated with either CO or CO2. The researchers then measure the optical sensor signal as gas de sorbs from the CNT cluster at different temperatures.

Because of the strong attraction between CO2 and the CNT clusters, it was necessary to heat the sensor to high temperatures for very long periods of time to fully remove all of the CO2. In contrast, the CO des-orbed at moderate temperatures.

The researchers point out that given its immunity to system leaks and the ability to selectively differentiate between different types of gases, the approach has advantages over commonly used volumetric methods of detec-tion. Additionally, unlike the temperature- programmed des-orption approach, which requires vacuum testing chambers, the method is compatible with ambient environments.

“This work improves our abil-ity to measure the temperature-dependent interaction of gases with surfaces in real-world conditions,” says Armani. “This could result in advances in a range of fields; for example, the design of more robust coatings for aircraft or cars or new materials for filters.”

The application of carbon nanotubes to polymer composites remains pro-hibitively expensive. This is because of the processing needed to align the nanotubes inside the composites to access their extraordinary physical properties. Reporting in Nanotech-nology, scientists in the US and UK have been successful in making free-standing self-aligned nanotube thin films in a vacuum filtration process. This enables the synthesis of low-cost layered polymer composites and infrared triggered actuators based on self-aligned nanotubes.

In the past, randomly oriented nanotube thin films known as “bucky paper” have been used for making electronic and mechanical devices. However, there remains the tricky issue of aligning the nano-tubes to access their properties. The team takes advantage of the concen-tration gradients in a simple vacuum filtration process, which leads to the formation of nanotube liquid-crystal thin films. These consist of millions of self-aligned nanotubes. Scal-ing up such self-aligned nanotubes may enable low cost per unit area of nanotube thin films – not only for composites but also for electronic thin-film transistor applications.

The team designed a layered poly-mer composite consisting of top and bottom layers of poly-dimethyl siloxane (a low-cost elastomer) with the self-aligned nanotube thin film in between. Schlierien textures are noted suggesting there are highly aligned nanotube domains inside the composites. Mechanical prop-erty testing suggests around a 95% change in the elastic modulus at ~0.01 wt% . This suggests that an amount of nanotubes 100 times lower than in competing techniques

is used. Near-infrared light-driven actuators are also demonstrated with optomechanical conversion factors of ~0.5 MPa W–1. This is greater than the stress generated by human skel-etal muscles.

Self-aligned nanotube/polymer composites could potentially be fabricated in strands and assem-bled in such a way to bio-mimic the motion of muscle fibres or create soft photo-origami structures. The team suggests that such self-aligned nanotube thin films with optimized nanotube amounts could potentially make remote controlled actuators at ~80 cents a piece.

Schematic of an experimental realization of the spintronic sensor and an experimentally obtained AFM image of a lithographically patterned sample.

Left: rendering of the optical sensor. Middle and right: SEM image of the optical sensor surface showing the carbon nanotube clusters.

Scanning electron microscope image of liquid crystal-carbon nanotubes.

Carbon nanotube cluster sensors differentiate between carbon monoxide and carbon dioxide

Low-cost layered polymers for infrared triggered actuators

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“The composites could potentially be assembled to bio-mimic the motion of muscle fibres”

16 businessupdate

Brazil funds nanotechnologyThe Ministry of Science, Technol-ogy and Innovation in Brazil has outlined a strategy for promoting the application of nanotechnology in industry through the SisNANO project. The aim is to put a system in place that provides the necessary expertise, personnel, state-of-the-art research equipment and con-nections between researchers in academia and industry so that nano-technology can achieve its potential in various industrial applications.

“We want SisNANO labs to pro-vide a good interface between indus-try and academic researchers,” says Flavio Plentz, a professor of con-densed matter physics and the lead co-ordinator for micro- and nano-technology in Brazil’s science min-istry. “We want people working in industry to feel that if they want to make a development involving nano-technology they have the infrastruc-ture, the staff and the equipment.”

Plentz points out that the last 10 years have seen significant invest-ment in nanotechnology in Brazil, and as a result a number of labs offer the best infrastructure for pursu-ing advanced research in the field. Several networks have also sprung up between nanoscientists working in public universities. “Research-ers were already connected,” he explains, “but industry was not.”

SisNANO helps companies who lack the necessary equipment to develop nanotechnologies by making existing facilities available at a highly subsidized rate. As Plentz points out, “Building labs takes lots of money.”

SisNANO is an important part of

a wider programme called the Bra-zil Nanotechnology Initiative. The first main action of the initiative is to provide a better infrastructure for experimental work.

During 2013–2014, the Ministry of Science, Technology and Inno-vation is investing R$50 m directly into SisNANO labs. Further funding amounting to R$29 m in 2014–2015 is being used to engage the SisNANO labs in another Brazilian programme called SIBRATEC (Brazilian System of Technology), which is focused on providing services and funding innovation projects with industry. The goal is for the total investment in the Brazilian Nanotechnology Initia-tive to reach R$300 m per year, with R$150 m per year being invested directly into SisNANO.

A key point in the SisNANO strat-egy is to focus federal investments on those labs that already have the necessary equipment and expertise because this should deliver the best return on investment. A public call in 2012 invited labs throughout Bra-zil to apply to be part of SisNANO, promising successful applicants pri-ority federal funding in return for making their facilities available to other organizations. External users might include other public and pri-vate universities in Brazil or abroad and, in particular, scientists and engineers working in industry.

In response to the public call, 50 labs applied and 26 were selected to be part of SisNANO. These labs also demonstrated clearly focused research, such as agricultural appli-

cations of nanotechnology or elec-tron microscopy characterization of nanomaterials.

Of the 26 labs selected to take part in the project eight were designated “strategic labs” and the remaining 18 “associated labs”. Plentz explains that the strategic labs already had a direct link with the government, list-ing as examples Embrapa, a research organization run by Brazil’s Ministry of Agriculture; the National Centre for Nanotechnology Research, which has links with the science ministry; and the Brazilian Metrology Institute, which is linked with the Institute of Commerce. Whether the lab’s status is strategic or associated determines the minimum percentage of time that their equipment must be available for external users: 50% for strategic labs and 15% for associated labs.

When it comes to research equip-ment it’s not just a case of what you have but also what you do with it; state-of-the-art equipment generally requires a high level of expertise to be used effectively. As Plentz empha-sizes, “The staff of the lab must also make a commitment to train and help external users.”

“Today we already have 190 com-panies using labs from SisNANO, and our aim is to increase this to 1000 companies in five years,” says Plentz. He points out that increasing the usage by such a degree will require a significant level of administrative investment. “Our main challenge is good management and connections between people in the project, so that users know what equipment is avail-able and what the capabilities are.”

The commercialization of carbon nanotube technology seemed to be stepping up a gear in June, as a lead-ing manufacturer of carbon nano-tubes teamed up with a company known for developing real-world applications for carbon nanoma-terials. The merger, which OCSiAl and Zyvex claimed would create “the largest nanotechnology com-pany in the world”, was intended to harness the power of carbon nano-tubes to create truly disruptive com-mercial technologies. Although a termination to the acquisition was announced in September, it was claimed that this decision would not affect future plans for cooperation between the companies.

At the time of the merger, Lance Criscuolo, president of Zy vex, emphasized the complementary strengths of the two companies: “Our focus is R&D – we outsourced a lot of our manufacturing years ago. What we enjoy from joining forces is access to high-quality carbon nano-tubes at low cost.”

With its proprietary technology, OCSiAl offers mass production of a material trademarked TUBALL, which the company claims consists of 75% pure carbon nanotubes. Max Atanassov, CEO of OCSiAl in the US, stated the company’s produc-tion capacity at 10 tonnes of carbon nanotubes per year, with costs start-ing at $2000 per kilogram. “This is approximately 50 times cheaper than the closest competitor for com-parable quality,” he added.

According to Atanassov, single-walled carbon nanotubes are more effective than multi-walled versions for modifying a material’s proper-ties, reducing the amount of addi-tive required. While Zyvex expected future activities to be “business as usual”, Criscuolo admitted there was room to expand their scope. Applications they had developed so far had exploited mechanical prop-erties of carbon nanotubes, working with companies such as Airbus and Easton sporting goods. But OCSiAl had worked in other areas, with commercial products including an additive for lithium-ion batter-ies that improves the cycle life and charge/discharge speed, as well as an additive that improves the prop-erties of tyres.

OCSiAl was also excited by what Zyvex would bring in addition to its R&D expertise. “Zyvex has a strong commercial focus,” said Atanassov. “There are a huge number of teams working in this field but Zyvex was unique in this aspect and we were impressed by their commercial focus.”

When the merger was cancelled a few months later, Atanassov com-mented, “Cancelling the deal was our mutual decision.” He added, “What is essential is that we continue to coop-erate and see prospective opportuni-ties in our partnership.”

Directa Plus in Lomazzo, Como, inaugurated its graphene factory on 23 June. The new industrial plant will produce pristine graphene nano-platelets and will be the largest Euro-pean production unit of its kind. It will produce four types of graphene-based material, each with its own specific applications and markets.

Directa Plus, based in the ComoNext Technological Park, was founded in 2005 and has been devel-oping a technological platform able to produce graphene-based materi-als in large volumes. “We are paying specific attention to product engi-neering,” explains company presi-dent and CEO Giulio Cesareo, “All our products are aimed at offering different ‘degrees’ of graphene to sat-isfy the needs of specific applications. The first module to be inaugurated in ComoNext is able to produce 30 tons of graphene materials per year.”

Cesareo made the announce-ment during the launch of the Ital-ian Council for Eco Innovation, and

Directa Plus is currently one of the 10 leading SMEs to stand out for innovation in cleantech.

“The company has developed a patented, scalable and sustainable process from an environmental and economic point of view to gener-ate graphene-based products,” says Richard Youngman, who is manag-ing director Europe/Asia of Clean-tech Group Inc.

The four types of graphene-based materials that will be produced at Directa Plus are: Basic G+, which is a super-expanded graphite; Ultra G+, a fine nanographite pow-der; Liquid G+, a highly concentrated water-based pristine dispersion of graphene nanoplatelets (GNPs); and

Pure G+, a powder of pristine GNPs.“Each of these products has its

own applications and markets,” says Cesareo. “The company’s produc-tion process comprises several steps that transform 3D graphite micro-structures into 2D graphene. Prod-ucts ready for commercialization are made at each step of the process.”

Directa Plus’ pristine GNPs con-tain only a small amount of lattice defects, he explains. They are up to 10 µm long and less than 3–5 nm thick. They are also highly crystal-line, and this, together with their high aspect ratio, makes them ideal for use in high-performance composites.

The G+ manufacturing process is continuous, simple and low cost. In

brief, it involves chemical intercala-tion of natural graphite followed by thermal plasma expansion. “This method increases the interlayer dis-tance in graphite to its extreme lim-its so that it forms super-expanded graphite f lakes from which GNPs can then be obtained by a mild post-plasma treatment,” Directa Plus sci-entific committee member Roman Sordan told nanotechweb.org. “We carry out a final exfoliation step, designed to fine tune the morphol-ogy of the GNPs in liquid media, such as water or organic solvents. The liquid dispersion can also be dried to produce an anhydrous form of GNP powder. The entire process is patented.”

Directa Plus’ objectives are to add value to its potential customer prod-ucts by keeping costs low, improving product performance and incorpo-rating new functionalities into the finished materials. The first products powered by G+ were revealed during the Graphene Factory opening event, including a CF bike wheel with a tyre containing pristine GNPs presented by Vittoria Group – an industrial partner and the largest manufacturer of bicycle tyres.

Some of the strategic and associated labs in the SisNano project.

Graphene is the main building block of all graphite materials. It is a single sheet of carbon atoms arranged in a 2D honeycomb lattice.

Italy graphene factory opens

CNT companies join forces

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18 TwentyfiveyearsofNanotechnology

Highlights from the past 25 years

C S Lent, P D Tougaw, W Porod and G H Bernstein (1993) Quantum cellular automata Nanotechnology 4 49

Who are you? I’m Craig Lent, a Professor of Electri-cal Engineering at the University of Notre Dame. My colleagues on the paper were Wolfgang Porod and Gary Bernstein, also professors in the Elec-trical Engineering Department.

What prompted you to go into this field of research?In the very early days of nanosci-ence, Rolf Landauer made the point that interference-based quantum devices were unlikely to be robust. In a talk he said that what was really required was a bistable saturating response. We were convinced by his argument and were on the lookout for such a characteristic.

What does your paper report and what impact has it had? While doing some calculations on

the basics of Coulomb blockade the-ory, I realized that the two-electron non-linear response I was seeing there was just what was needed. It could be adapted to a quantum-dot situation and be the basis of a new kind of device – the QCA cell. By placing cells near each other, one could construct useful circuits.

The question of how to think about QCA circuit design and cir-cuit architecture has produced a lot of interesting work by many groups around the world. I think we’re just scratching the surface of how to reconceive computer architecture using this post-transistor device.

What direction has your research taken since?My own work focused first on the metal-dot devices because my col-leagues Greg Snider, Gary Bernstein and Alexei Orloff were able to assem-ble several metal-dot cells and create the actual circuits. More recently, I’ve been working with several Notre Dame chemists on making QCA cells out of single molecules – that would be the ultimate in low power and high device density.

Markku Leskelä and Mikko Ritala (1999) Atomic layer epitaxy – a valuable tool for nanotechnology? Nanotechnology 10 19

Who are you?Markku Leskelä: I am a Professor of Inorganic Chemistry at the Univer-sity of Helsinki (1990–).Mikko Ritala: I am a Professor of Inor-ganic Materials Chemistry at the University of Helsinki (2003–).

What prompted you into this field of research?Markku Leskelä: I started ALD (at that time called ALE) related stud-ies in 1980. The ALD studies were focused on films needed in electro-luminescent displays which were the only commercial application of ALD in the 1980s. In 1990 we started ALD research with Mikko Ritala at the Uni-versity of Helsinki, with a broad focus

on the basic process development. During the 1990s it became clear that ALD was needed in microelectron-ics and also that our research was focused for high dielectric constant oxides, metal and barrier films. From the very beginning it was obvious to us that ALD has a lot to contribute.

What does your paper report and what impact has it had?We described the basic properties of ALD in the paper, showed how very thin films had been deposited on dif-ferent substrates and speculated on the suitability to nanotechnology. In the 1990s nanotechnology was quite a new area and no one had discussed the connection with ALD. In that sense the paper was visionary.

What direction has your research taken since?Now it is clear that ALD is a very suitable tool for growing films on nanomaterials and tuning their properties. The literature contains 1200+ papers, 80+ reviews and 2 books on ALD in nanotechnology. Our review can be considered as a trigger for this field.

Y Arntz, J D Seelig, H P Lang, J Zhang, P Hunziker, J P Ramseyer, E Meyer, M Hegner and Ch Gerber (2003) Label-

free protein assay based on a nanomechanical cantilever array Nanotechnology 14 1

Who are you? My name is Hans Peter Lang and I’m working at the Swiss Nanoscience Institute in Basel. I’m an experimen-tal physicist in solid-state physics.

What prompted you to go into this field of research? After using STM and AFM to study the properties of nanomaterials – such as high-temperature supercon-ductors and various forms of carbon, like graphite, diamond, fullerenes and nanotubes – I worked on the use of AFM cantilevers as nanomechani-cal sensors for the detection of volatile molecules. After the first success with hybridization of DNA oligonucleo-tides, which was published in Science, the next challenge was the detection

of proteins for medical applications.

What does your paper report and what impact has it had? The paper reports on our first obser-vation of protein detection using cantilever array sensors. This was our first collaboration with a hos-pital and our first application of the sensors in the medical field. We demonstrated the rapid detection of cardiac biomarkers within minutes. This can be an essential advantage in comparison with classical antibody detection assays.

What direction has your research taken since?This article was the first in a series of papers in which the applicability of the cantilever array sensing technique was explored. We studied antibod-ies, single-chain fragment antibod-ies, proteins, whole cells, phages, fungi and bacteria. Medical studies included detection of biomarkers and efficiency testing of therapy interfer-ons and melanoma drug personalized medicine. So this article actually had a large impact on the development of our research.

Th Stelzner, M Pietsch, G Andrä, F Falk, E Ose and S Christiansen (2008) Silicon nanowire-based

solar cells Nanotechnology 19 295203

Who are you?I lead a Helmholtz Institute for ‘Nano-architectures for Energy Conversion’ at the Helmholtz Center for Materi-als and Energy in Berlin, as well as a Scientific Research and Technol-ogy Development Unit for ‘Photonic Nanostructures’ at the Max Planck Institute for the Science of Light (MPL) in Erlangen, Germany.

What prompted you into this field of research, what does your paper report, what impact has it had? In 2006 when our paper was pub-lished, VLS [vapour liquid solid] growth of nanowires was a topic of increasing scientific activity but only a few papers dealing with device integration of these nanowires were published. Devices of these early days of nanowire research were more con-

cerned with biological sensing using field-effect-transistors with nanow-ire channels, rather than showing minority carrier devices such as solar cells based on nanowires. As such, our paper was one of the first ones show-ing an admittedly lousy efficiency (<<1%).Still, we dared to publish an activity which serves now as a refer-ence from the early days.

What direction has your research taken since?Starting with the Nanotechnology arti-cle, we realized that prior to showing good solar cell activities we needed to improve our understanding of electri-cal, optical and structural properties of the material and needed to develop proper technologies for device inte-gration that permit reliable device designs with nanowires. The initial work also fuelled the need to hook up better deposition and electrical, structural and optical characteriza-tion labs. Meanwhile the field moved quickly - efficiencies >10% were pub-lished and it was shown that catalyst poisoning of the semiconductor wire is detrimental to device performance so that etching became more popular.

1986Gerd Binnig and Heinrich Rohrer are awarded the Nobel Prize in Physics for the development of the scanning tunnelling microscope. Along with the atomic force microscope, this quickly becomes one of the founda-tions of modern nanotechnology.

1988Nanotechnology’s current Editor-in-Chief Mark Reed coins the phrase “quantum dot”.

1990The first issue of Nanotechnology is published. As the founding Editor-in-Chief, David Whitehouse plays a key role in establishing the world’s first ever peer-reviewed journal in nanoscale science and technology.

1991Sumio Iijima’s paper on multi-walled carbon nanotubes sparks an interest in carbon nanostructures.

1993Lyndon Hicks and Mildred Dressel-haus publish two papers that show that reducing the dimensionality of thermoelectric materials can dra-matically improve their performance.

2000sThe next decade sees nanostructures being increasingly incorporated into solar cells. Neil C Greenham and Michael Grätzel are guest editors for Nanotechnology 19 42 special issue “Nanostructured solar cells”.

2002nanotechweb.org is launched. Provid-ing news, research events, products and jobs, nanotechweb.org has become a key information source.

2008Stanley Williams’ group at HP Labs finds that Chua’s “lost circuit ele-ment”, the memristor, can be dem-onstrated with nanoscale systems.

2010The Nobel Prize in Physics is awarded to Andre Geim and Konstantin Novo-selov for their work on graphene.

2012The impact of nanotechnology on the environment continues to be of concern. Nanotechnology publishes a special issue highlighting green syn-thesis methods.

Key events in nanotechnology

Craig Lent

Markku Leskelä and Mikko Ritala

Hans Peter Lang

Silke Christiansen

nanotechwebreviewWinter 2014/2015 Sign up as a member at nanotechweb.org

As the first academic publication wholly dedicated to disseminating research in nanoscale science and technology, Nanotechnology has had a significant influence on the shape of the field. We speak to authors of some of the papers that have had the greatest impact.

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