Arnaldo R. Laracuente

Nanoscale Tunable Reduction of Graphene Oxide for Graphene Electronics

Writing Conductive Lines with Hot Tips The interface within devices between conductors, semiconductors, and insulators is usually created by stacking patterned layers of different materials. For flexible electronics, it can be advantageous to avoid this architectural constraint. Graphene oxide, formed by chemical exfoliation of graphite, can be reduced to a more conductive form using chemical reductants. Wei et al. (p. 1373) now show that layers of graphene oxide can also be reduced using a hot atomic force microscope tip to create materials comparable to those of organic conductors. This process can create patterned regions (down to 12 nanometers in width) that differ in conductivity by up to four orders of magnitude. Conducting regions can be drawn on graphene oxide sheets with a heated atomic force microscope tip. The reduced form of graphene oxide (GO) is an attractive alternative to graphene for producing large-scale flexible conductors and for creating devices that require an electronic gap. We report on a means to tune the topographical and electrical properties of reduced GO (rGO) with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip. The rGO regions are up to four orders of magnitude more conductive than pristine GO. No sign of tip wear or sample tearing was observed. Variably conductive nanoribbons with dimensions down to 12 nanometers could be produced in oxidized epitaxial graphene films in a single step that is clean, rapid, and reliable.

Direct deposition of continuous metal nanostructures by thermal dip-pen nanolithography

We describe the deposition of continuous metal nanostructures onto glass and silicon using a heated atomic force microscope cantilever. Like a miniature soldering iron, the cantilever tip is coated with indium metal, which can be deposited onto a surface forming lines of a width less than 80 nm. Deposition is controlled using a heater integrated into the cantilever. When the cantilever is unheated, no metal is deposited from the tip, allowing the writing to be registered to existing features on the surface. We demonstrate direct-write circuit repair by writing an electrical connection between two metal electrodes separated by a submicron gap.

Conductance Anisotropy in Epitaxial Graphene Sheets Generated by Substrate Interactions

We present the first microscopic transport study of epitaxial graphene on SiC using an ultrahigh vacuum four-probe scanning tunneling microscope. Anisotropic conductivity is observed that is caused by the interaction between the graphene and the underlying substrate. These results can be explained by a model where charge buildup at the step edges leads to local scattering of charge carriers. This highlights the importance of considering substrate effects in proposed devices that utilize nanoscale patterning of graphene on electrically isolated substrates.

Nanoscale reduction of graphene fluoride via thermochemical nanolithography.

Graphene nanoribbons (GNRs) would be the ideal building blocks for all carbon electronics; however, many challenges remain in developing an appropriate nanolithography that generates high-quality ribbons in registry with other devices. Here we report direct and local fabrication of GNRs by thermochemical nanolithography, which uses a heated AFM probe to locally convert highly insulating graphene fluoride to conductive graphene. Chemically isolated GNRs as narrow as 40 nm show p-doping behavior and sheet resistances as low as 22.9 KΩ/□ in air, only approximately 10× higher than that of pristine graphene. The impact of probe temperature and speed are examined as well as the variable-temperature transport properties of the GNR.

Electronic conduction in GaN nanowires

Conductivity mechanisms in unintentionally doped GaN nanowires (NWs) are studied. Gated current-voltage measurements and threshold voltage modeling demonstrate the unique impact of device parameters on NW field-effect transistors as compared to conventional systems. Temperature-dependent resistivity results, acquired with a scanning tunneling microscope equipped with multiple tips, reveal only mild temperature dependence at higher temperatures, with temperature-independent resistivity observed below ∼100K indicating impurity band conduction. The likely origins and implications of these results are discussed.

A monohydride high-index silicon surface: Si(114):H-(2×1)

We describe the adsorption of H on Si(114)-(2×1) as characterized by scanning tunneling microscopy and first-principles calculations. Like Si(001)—and despite the relative complexity of the (114) structure—a well-ordered, low-defect-density monohydride surface forms at ∼400 °C. Surprisingly, the clean surface reconstruction is essentially maintained on the (2×1) monohydride surface, composed of dimers, rebonded double-layer steps, and nonrebonded double-layer steps, with each surface atom terminated by a single H. This H-passivated surface can also be easily and uniformly patterned by selectively desorbing the H with low-voltage electrons.

Laser printed micron-scale free standing laminate composites: Process and properties

Micron-scale free standing structures were generated via the laser decal transfer process using high viscosity Ag nanoinks without the use of any sacrificial or release layers. Both cantilevered (34×10×0.46 μm3) and doubly suspended beams (38×5×0.46 μm3) were fabricated. Laminate composites were then generated by selectively coating the underside of these structures with silicon using a focused ion beam deposition technique. The static responses of the composite structures were characterized via fitting nanoindentation induced beam deflections with a derived closed-form solution yielding Young’s modulii of the Ag and Si layers as EAg≈40 GPa and ESi≈16 GPa, respectively. The dynamic response of these structures was also characterized via laser vibrometry revealing quality factors of approximately 400 and 800 for cantilevers and microbridges, respectively. Several techniques (static and dynamic) to ascertain the residual stress state of these structures were also employed revealing an average residual stres...

Direct-write polymer nanolithography in ultra-high vacuum

Summary Polymer nanostructures were directly written onto substrates in ultra-high vacuum. The polymer ink was coated onto atomic force microscope (AFM) probes that could be heated to control the ink viscosity. Then, the ink-coated probes were placed into an ultra-high vacuum (UHV) AFM and used to write polymer nanostructures on surfaces, including surfaces cleaned in UHV. Controlling the writing speed of the tip enabled the control over the number of monolayers of the polymer ink deposited on the surface from a single to tens of monolayers, with higher writing speeds generating thinner polymer nanostructures. Deposition onto silicon oxide-terminated substrates led to polymer chains standing upright on the surface, whereas deposition onto vacuum reconstructed silicon yielded polymer chains aligned along the surface.

Reversible electron-induced conductance in polymer nanostructures

We report a mechanism for controlling conductance in polymer nanostructures. Poly(3-dodecylthiophene-2,5-diyl) (PDDT) nanostructures were directly written between gold electrodes using thermal dip pen nanolithography and then characterized in UHV. We find that the conductivity of a PDDT nanostructure can be increased by more than five orders of magnitude (from <10−4 to 10 S cm−1) by exposure to energetic electrons, and then repeatedly returned to a semi-insulating state by subsequent exposure to hydrogen. Based on systematic measurements complemented by calculations of electronic structure and electron transport in PDDT, we conclude that the conductance modulation is caused by H desorption and reabsorption. The phenomenon has potential applications in hydrogen sensing and molecular electronics.