Suneel Kodambaka

Pathways of atomistic processes on TiN(001) and (111) surfaces during film growth: an ab initio study

Density functional methods were used to calculate binding and diffusion energies of adatoms, molecules, and small clusters on TiN(001) and TiN(111) surfaces in order to isolate the key atomistic processes which determine texture evolution during growth of polycrystalline TiN layers. The surface energy for nonpolar TiN(001), 81 meV/A2, was found to be lower than that of both N- and Ti-terminated TiN(111) polar surfaces, 85 and 346 meV/A2. While N2 molecules are only weakly physisorbed, Ti adatoms form strong bonds with both TiN(001), 3.30 eV, and TiN(111), 10.09 eV. Ti adatom diffusion is fast on (001), but slow on (111) surfaces, with calculated energy barriers of 0.35 and 1.74 eV, respectively. The overall results show that growth of 111-oriented grains is favored under conditions typical for reactive sputter deposition. However, the presence of excess atomic N (due, for example, to collisionally induced dissociation of energetic N2+ ions) leads to a reduced Ti diffusion length, an enhanced surface islan...

Formation of Compositionally Abrupt Axial Heterojunctions in Silicon-Germanium Nanowires

Sharp Nanowires The potential for using nanowires in devices can be limited by the ability to synthesize them from two or more materials while maintaining compositional purity at the interfaces. Instead of using liquid droplets at the eutectic point when the melting point is at a minimum, Wen et al. (p. 1247) show that generating the wires at solid alloy catalysts allows fabrication of silicon germanium wires with atomically sharp interfaces. The system works well because an AlAu alloy composition was chosen in which Si and Ge have a low solubility but which have a high enough eutectic temperature so that nanowire growth is not limited by the reactivity of the Si and Ge precursors. A solid alloy catalyst is used to synthesize atomically sharp interfaces in silicon-germanium nanowires. We have formed compositionally abrupt interfaces in silicon-germanium (Si-Ge) and Si-SiGe heterostructure nanowires by using solid aluminum-gold alloy catalyst particles rather than the conventional liquid semiconductor–metal eutectic droplets. We demonstrated single interfaces that are defect-free and close to atomically abrupt, as well as quantum dots (i.e., Ge layers tens of atomic planes thick) embedded within Si wires. Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness. Solid catalysts that can form functional group IV nanowire-based structures may yield an extended range of electronic applications.

Growth of Semiconducting Graphene on Palladium

We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between 723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 A in size, bounded by Pd surface steps. Surprisingly, the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior. Scanning tunneling spectroscopy measurements confirm that the graphene islands are semiconducting, with a band gap of 0.3 +/- 0.1 eV. On the basis of density functional theory calculations, we suggest that the opening of a band gap is due to the strong interaction between graphene and the Pd substrate. Our findings point to the possibility of preparing semiconducting graphene layers for future carbon-based nanoelectronic devices via direct deposition onto strongly interacting substrates.

Tungsten carbide (WC) synthesis from novel precursors

Abstract This paper deals with the production and properties of tungsten carbide (WC) powders from novel carbon coated precursors. The process has two steps in which the oxide powders were first coated with carbon by cracking of a hydrocarbon gas, propylene (C 3 H 6 ), secondly mixed with a substantial amount of carbon black, and finally treated at temperatures in the range of 600-1400°C for 2 h in flowing Ar or 10% H 2 -Ar atmosphere to synthesize WC. The produced powders were characterized using TEM, BET surface area analyzer, X-ray diffraction and chemical analysis (oxygen and carbon). The results obtained for various types of precursors treated in different atmospheres indicated that the coated precursors produced high quality powders. Single phase, submicron WC powders were synthesized at temperatures as low as 1100°C. WC powders produced at 1400°C for 2 h in flowing 10%H 2 -Ar gas mixture were submicron (3–5 m 2 /g), single phase, and had low oxygen content (0.2–0.5 wt%). The sintering tests demonstrated that these powders can be densified to near theoretical density using 20 wt% Co binder at 1500°C for 2 h in flowing 10%H 2 -Ar atmosphere.

Near Room-temperature Synthesis of Transfer-free Graphene Films

Large-area graphene films are best synthesized via chemical vapour and/or solid deposition methods at elevated temperatures (~1,000 °C) on polycrystalline metal surfaces and later transferred onto other substrates for device applications. Here we report a new method for the synthesis of graphene films directly on SiO(2)/Si substrates, even plastics and glass at close to room temperature (25-160 °C). In contrast to other approaches, where graphene is deposited on top of a metal substrate, our method invokes diffusion of carbon through a diffusion couple made up of carbon-nickel/substrate to form graphene underneath the nickel film at the nickel-substrate interface. The resulting graphene layers exhibit tunable structural and optoelectronic properties by nickel grain boundary engineering and show micrometre-sized grains on SiO(2) surfaces and nanometre-sized grains on plastic and glass surfaces. The ability to synthesize graphene directly on non-conducting substrates at low temperatures opens up new possibilities for the fabrication of multiple nanoelectronic devices.

Orientation-dependent work function of graphene on Pd(111)

Selected-area diffraction establishes that at least six different in-plane orientations of monolayer graphene on Pd(111) can form during graphene growth. From the intensities of low-energy electron microscopy images as a function of incident electron energy, we find that the work functions of the different rotational domains vary by up to 0.15 eV. Density functional theory calculations show that these significant variations result from orientation-dependent charge transfer from Pd to graphene. These findings suggest that graphene electronics will require precise control over the relative orientation of the graphene and metal contacts.

Tunable MoS2 bandgap in MoS2-graphene heterostructures

Using density functional theory calculations with van der Waals corrections, we investigated how the interlayer orientation affects the structure and electronic properties of MoS2-graphene bilayer heterostructures. Changing the orientation of graphene with respect to MoS2 strongly influences the type and the value of the electronic bandgap in MoS2, while not significantly altering the binding energy between the layers or the interlayer spacing. We show that the physical origin of this tunable bandgap arises from variations in the S–S interplanar distance (MoS2 thickness) with the interlayer orientation, variations which are caused by electron transfer away from the Mo–S bonds.

Moiré superstructures of graphene on faceted nickel islands.

Using scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations, we investigated the morphology and electronic structure of monolayer graphene grown on the (111) and (110) facets of three-dimensional nickel islands on highly oriented pyrolytic graphite substrate. We observed graphene domains exhibiting hexagonal and striped moiré patterns with periodicities of 22 and 12 Å, respectively, on (111) and (110) facets of the Ni islands. Graphene domains are also observed to grow, as single crystals, across adjacent facets and over facet boundaries. Scanning tunneling spectroscopy data indicate that the graphene layers are metallic on both Ni(111) and Ni(110), in agreement with the calculations. We attribute this behavior to a strong hybridization between the d-bands on Ni and the π-bands of carbon. Our findings point to the possibility of preparing large-area epitaxial graphene layers even on polycrystalline Ni substrates.

Self-Catalyzed Epitaxial Growth of Vertical Indium Phosphide Nanowires on Silicon

Vertical indium phosphide nanowires have been grown epitaxially on silicon (111) by metalorganic vapor-phase epitaxy. Liquid indium droplets were formed in situ and used to catalyze deposition. For growth at 350 degrees C, about 70% of the wires were vertical, while the remaining ones were distributed in the 3 other <111> directions. The vertical fraction, growth rate, and tapering of the wires increased with temperature and V/III ratio. At 370 degrees C and V/III equal to 200, 100% of the wires were vertical with a density of approximately 1.0 x 10(9) cm(-2) and average dimensions of 3.9 mum in length, 45 nm in base width, and 15 nm in tip width. X-ray diffraction and transmission electron microscopy revealed that the wires were single-crystal zinc blende, although they contained a high density of rotational twins perpendicular to the <111> growth direction. The room temperature photoluminescence spectrum exhibited one peak centered at 912 +/- 10 nm with a FWHM of approximately 60 nm.

Kinetic Control of Self-Catalyzed Indium Phosphide Nanowires, Nanocones, and Nanopillars

The morphological phase diagram is reported for InP nanostructures grown on InP (111)B as a function of temperature and V/III ratio. Indium droplets were used as the catalyst and were generated in situ in the metalorganic vapor-phase epitaxy reactor. Three distinct nanostructures were observed: wires, cones, and pillars. It is proposed that the shape depends on the relative rates of indium phosphide deposition via the vapor-liquid-solid (VLS) and vapor-phase epitaxy (VPE) processes. The rate of VLS is relatively insensitive to temperature and results in vertical wire growth starting at 350 degrees C. By contrast, the rate of VPE accelerates with temperature and drives the lateral growth of cones at 385 degrees C and then pillars at 400 degrees C.