on-wire lithography bridges the gap: nanotechnology
TRANSCRIPT
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.)