on-wire lithography bridges the gap: nanotechnology

RESEARCH NEWS

September 200510

Researchers at Delft University of

Technology and Philips Research

Laboratories in The Netherlands have

fabricated the first superconducting

transistors based on semiconductor

nanowires [Doh et al., Science (2005)

309, 272]. Such devices promise to

combine superconductivity (i.e.

resistance-free current flow below a

characteristic temperature) with the

tunability and quantum properties of

nanoscale semiconductors.

Progress in hybrid superconductor-

semiconductor devices has been

limited because of the difficulty in

forming interfaces with low electrical

contact resistance. But InAs nanowires

individually contacted by two Al-based

superconductor electrodes yield

surprisingly low contact resistances.

Below 1 K, the high transparency of

the contacts to the superconducting

electrodes causes proximity-induced

superconductivity in the nanowire.

This supercurrent can be switched on

and off by a gate voltage acting on the

electron density in the nanowire. A

variation in gate voltage induces

universal fluctuations in the normal-

state conductance, which are

correlated to critical current

fluctuations.

The high proportion of devices showing

superconductivity should allow scaling

to superconducting circuits

incorporating multiple nanowire

devices. For example, two nanowire

devices could be used to build an

electrically tunable superconducting

quantum interference device, which

can act as a switchable coupling

element between superconducting

quantum bits in solid-state quantum

computers. The researchers also

believe that the extension of their work

to heterostructured nanowires could

open up further opportunities.

Mark Telford

Tunablesupercurrents NANOTECHNOLOGY

To create molecular electronic devices, researchersneed to insert molecules into a nanoscale gapbetween electrodes. However, lithography strugglesto create gaps <20 nm wide controllably. Now Chad A. Mirkin and coworkers at NorthwesternUniversity have developed ‘on-wire lithography’,which can produce gaps in nanowires just 5 nmwide [Qin et al., Science (2005) 309, 113]. Using porous alumina templates, the teamelectrochemically deposits a nanowire comprising,alternately, segments of Au then short segments ofAg or Ni (with length tailored by the depositioncharge). The template is then dissolved and anaqueous suspension of nanowires cast onto a slide. Au-Ni nanowires are coated with a layer of silica 50 nm thick then removed from the slide. The silicaacts as a gutter-shaped support. Ni segments arethen etched away using nitric acid, leaving gaps.

Au-Ag nanowires are coated with a Au/Ti bilayer, andAg etched away with a solution of CH3OH, NH4OH,and H2O2. The latest work has achieved 1 nm gaps. Using dip-pen nanolithography, the team has placeda conductive polymer into a 13 nm gap and studiedits electrical transport properties. It has also madearrays of face-to-face Au disks separated by 40 nmgaps that could act as plasmon waveguides.Mark Telford

On-wire lithography bridges the gap NANOTECHNOLOGY

Doping semiconductor nanocrystals NANOTECHNOLOGY

Dopant impurities should have more effect ona semiconductor in nanocrystal form than inbulk form. But incorporating certain dopantsis problematic (e.g. Mn can dope CdS andZnSe but not CdSe). Now, researchers knowwhy [Erwin et al., Nature (2005) 436, 91]. There is a belief that the growth mechanismsfor pure nanocrystals make it hard to dopethem because they ‘self-purify’ by expellingimpurities. But University of Minnesota

experimentalist David Norris says that“doping difficulties are not intrinsic”. “If the impurity binds to the nanocrystalsurface too weakly, or if the strongly bindingsurfaces are only a small fraction of thetotal, then doping is difficult,” adds NavalResearch Laboratory theorist Steven Erwin.“But if an impurity atom can adsorb on thesurface strongly enough, it can beincorporated.” Such adsorption is determinedby surface morphology, nanocrystal shape,and surfactants in the growth solution. Calculated binding energies are much largerfor (001) surfaces of zinc blende crystals(e.g. ZnSe and CdS) than for (110) and (111)surfaces, as well as all surfaces of wurtzite(CdSe) and rock-salt (PbS, PbSe) crystals.These predictions have been confirmed bycoating a ZnSe core with a CdSe shell, whichadopt zinc blende structure and incorporateMn. Also, wurtzite CdSe grown under thesame surfactant conditions used for ZnSeshow Mn incorporation, but three times lessso owing to smaller binding energy and ashape with less adsorbing surface area. Future effort will focus on impurities chosenfor specific applications, such as solar cells,lasers, and spin electronic devices. Mark Telford

CdSe nanoparticles in colloidal solution. Inset: particle surface

interacting with surfactants and impurity (purple). (Courtesy of

Giulia Galli, Lawrence Livermore National Laboratory.)

Scanning electron micrographs showing gaps in nanowires down

to 5 nm wide. The latest work has reduced gap width to just 1 nm.

(Courtesy of Chad A. Mirkin, Northwestern University.)