Mehdi M. Yazdanpanah

Selective self-assembly at room temperature of individual freestanding Ag2Ga alloy nanoneedles

Liquid gallium drops placed on thick Ag films at room temperature spontaneously form faceted nanoneedles of Ag2Ga alloy oriented nearly normal to the surface. This observation suggests that single nanoneedles can be selectively grown by drawing silver-coated microcantilevers from gallium. Needles from 25 nm to microns in diameter and up to 33μm long were grown by this method. These metal-tipped cantilevers have been used to perform atomic force microscopy (AFM) and AFM voltage lithography.

Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping

We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET) using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the response of graphene resistance to the tip voltage shows significant variation with tip position, and SGM imaging displays mesoscopic domains of electron-doped and hole-doped regions. Our measurements reveal substantial spatial fluctuation in the carrier density in graphene due to extrinsic local doping from sources such as metal contacts, graphene edges, structural defects and resist residues. Our scanning gate measurements also demonstrate graphenes excellent capability to sense the local electric field and charges.

Oriented nanomaterial air bridges formed from suspended polymer-composite nanofibers.

In a two-step method, carbon nanotubes, inorganic nanowires, or graphene sheets are connected between two anchor points to form nanomaterial air bridges. First, a recently developed method of forming directionally oriented polymer nanofibers by hand-application is used to form suspended composite polymer-nanomaterial fibers. Then, the polymer is sacrificed by thermally induced depolymerization and vaporization, leaving air bridges of the various materials. Composite fibers and bundles of nanotubes as thin as 10 nm that span 1 microm gaps have been formed by this method. Comparable bridges are observed by electrospinning solutions of the same nanomaterial-polymer composites onto micrometer-scale corrugated surfaces. This method for assembling nanomaterial air-bridges provides a convenient way to suspend nanomaterials for mechanical and other property determinations, and for subsequent device fabrication built up from the suspended nanosubstrates.

Characterization of silver–gallium nanowires for force and mass sensing applications

We investigate the mechanical properties of cantilevered silver-gallium (Ag(2)Ga) nanowires using laser Doppler vibrometry. From measurements of the resonant frequencies and associated operating deflection shapes, we demonstrate that these Ag(2)Ga nanowires behave as ideal Euler-Bernoulli beams. Furthermore, radial asymmetries in these nanowires are detected through high resolution measurements of the vibration spectra. These crystalline nanowires possess many ideal characteristics for nanoscale force and mass sensing, including small spring constants (as low as 10(-4) N m(-1)), high frequency bandwidth with resonance frequencies in the 0.02-10 MHz range, small suspended mass (picograms), and relatively high Q-factors (approximately 2-50) under ambient conditions. We evaluate the utility of Ag(2)Ga nanowires for nanocantilever applications, including ultrasmall mass and high frequency bandwidth piconewton force detection.

Formation of highly transmissive liquid metal contacts to carbon nanotubes

We have developed a method to produce liquid metal contacts to carbon nanotubes that allows direct measurement of the influence of the contact on the nanotube conductance. Gallium is deposited onto standard gold nanotube contacts, where it gradually spreads to coat the contact region. The two-terminal multiwall nanotube conductance increases by as much as 1.2e2∕h during the transition from gold to gallium contacts, and approaches 2e2∕h at room temperature, with a current density of 2×108A∕cm2. Surprisingly, the conductance is independent of the contact area or contact separation, providing evidence that transport is ballistic in multiwall nanotubes.

Gallium-driven assembly of gold nanowire networks

Nanowire networks of Au–Ga alloy are fabricated at temperatures between 220 and 300°C by application of small drops of liquid gallium to 10- to 100-nm-thick gold films. As the liquid gallium drop spreads and reacts with the gold film, lamellar segregation of gold-rich and gallium-rich regions form fractal-like networks of Au–Ga nanowires connected between gold-rich islands in specific zones concentric to the gallium droplet. The wires are subsequently suspended by wet chemical etching that undercuts the ∼10-nm-thick chromium adhesion layer and the silicon substrate. Suspended nanowires as long as 6μm and as narrow as 35nm diameter have been produced using this method.

Toward wafer-scale patterning of freestanding intermetallic nanowires

Individual metal alloy nanowires of constant diameter and high aspect ratio have previously been self-assembled at selected locations on atomic force microscope (AFM) probes by the method reported in Yazdanpanah et al (2005 J. Appl. Phys. 98 073510). This process relies on the room temperature crystallization of an ordered phase of silver-gallium. A parallel version of this method has been implemented in which a substrate, either an array of micromachined tips (similar to tips on AFM probes) or a lithographically patterned planar substrate, is brought into contact with a continuous, nearly planar film of melted gallium. In several runs, freestanding wires are fabricated with diameters of 40-400 nm, lengths of 4-80 µm, growth rates of 80-170 nm s( - 1) and, most significantly, with yields of up to 97% in an array of 422 growth sites. These results demonstrate the feasibility of developing a batch manufacturing process for the decoration of wafers of AFM tips and other structures with selectively patterned freestanding nanowires.

High aspect ratio etching of atomic force microscope-patterned nitrided silicon

Silicon that is nitrided in a pure nitrogen plasma is patterned with voltage applied by an atomic force microscope (AFM). Wet chemical etching into AFM-patterned (110) silicon produced vertical trenches as narrow as 91 nm (for one 757 nm deep trench) and with aspect ratios as large as 8.9:1 (for a 95 nm by 849 nm trench). Compared to the ridge patterns resulting from AFM oxidation and wet etching of hydrogen-passivated silicon, a substantially higher applied voltage is required to pattern nitrided silicon.