Ahin Roy

Wrinkling of atomic planes in ultrathin Au nanowires.

A detailed understanding of structure and stability of nanowires is critical for applications. Atomic resolution imaging of ultrathin single crystalline Au nanowires using aberration-corrected microscopy reveals an intriguing relaxation whereby the atoms in the close-packed atomic planes normal to the growth direction are displaced in the axial direction leading to wrinkling of the (111) atomic plane normal to the wire axis. First-principles calculations of the structure of such nanowires confirm this wrinkling phenomenon, whereby the close-packed planes relax to form saddle-like surfaces. Molecular dynamics studies of wires with varying diameters and different bounding surfaces point to the key role of surface stress on the relaxation process. Using continuum mechanics arguments, we show that the wrinkling arises due to anisotropy in the surface stresses and in the elastic response, along with the divergence of surface-induced bulk stress near the edges of a faceted structure. The observations provide new understanding on the equilibrium structure of nanoscale systems and could have important implications for applications in sensing and actuation.

Single crystalline ultrathin gold nanowires: Promising nanoscale interconnects

Using first principles based density functional calculation we study the mechanical, electronic and transport properties of single crystalline gold nanowires. While nanowires with the diameter less than 2 nm retain hexagonal cross-section, the larger diameter wires show a structural smoothening leading to circular cross-section. These structural changes significantly affect the mechanical properties of the wires, however, strength remains comparable to the bulk. The transport calculations reveal that the conductivity of these wires are in good agreement with experiments. The combination of good mechanical, electronic and transport properties make these wires promising as interconnects for nano devices.

Synthesis of Hollow Nanotubes of Zn2SiO4 or SiO2: Mechanistic Understanding and Uranium Adsorption Behavior

We report a facile synthesis of Zn2SiO4 nanotubes using a two-step process consisting of a wet-chemical synthesis of core-shell ZnO@SiO2 nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn2SiO4, annealing under reducing atmosphere leads to the formation of SiO2 nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management.

Negative differential resistance in armchair silicene nanoribbons

Due to dimensional confinement of carriers and non-trivial changes in the electronic structure, novel tunable transport properties manifest in nanoscale materials. Here, we report using first-principles density functional theory and non-equilibrium Greens function formalism, the occurrence of negative differential resistance (NDR) in armchair silicene nanoribbons (ASNRs). Interestingly, NDR manifests only in pristine [Formula: see text] ASNRs, where [Formula: see text]. We show that the origin of such a novel transport phenomenon lies in the bias-induced changes in the density of states of this particular family of nanoribbons. With increasing width of the nanoribbons belonging to this family, the peak-to-valley ratios of current decrease due to the increase in the number of sub-bands leading to a reduction in NDR. NDR is possible not only in [Formula: see text] ASNRs, but also in mixed configurations of armchair and zigzag silicene nanoribbons. This universality of NDR along with its unprecedented width-induced tunability can be useful for silicene-based low-power logic and memory applications.

Insights into nucleation, growth and phase selection of WO3: morphology control and electrochromic properties

Electrochromic application of nanoscale WO3 demands stringent control in terms of phase-purity and morphology. Here, we show that two different phases of WO3 with distinct morphologies (viz. 2D plates of orthorhombic phase and 1D rods of hexagonal phase) can be obtained by tuning the solvothermal reaction conditions. Control experiments along with density functional theory based ab initio calculations show that the reaction pathway critically depends on the capping agent used in the reaction. Using this concept in conjunction with crystallographic arguments, we rationalize the morphology evolution of the two phases. Furthermore, the synthesized phases exhibit very different electrochromic properties in terms of H+ diffusion, which can be rationalized by the calculated trend in the H+-intercalation energies.

Manipulation of Optoelectronic Properties and Band Structure Engineering of Ultrathin Te Nanowires by Chemical Adsorption.

Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultrathin Te nanowires due to NO2 adsorption. Measurements of the temperature dependence of resistivity of single ultrathin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO2 reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO2 supports the appearance of a metallic state in NO2 adsorbed Te. Ab initio simulations attribute these observations to the appearance of midgap states in NO2 adsorbed Te nanowires. Our results provide fundamental insights into the effects of ambient on the electronic structures of low-dimensional materials and can be exploited for designing novel chemical sensors.

Wet-chemical Synthesis of Electrochromic WO3 and WxMo1-xO3 Nanomaterials with Phase and Morphology Control

Nanoscale WO3 has emerged as a multifunctional material as it has found various applications in electrochromic devices [1], gas sensing [2] and photocatalysis [3]. A wealth of literature is available on synthesis of different phases of WO3 with distinct morphologies. However, a thorough understanding of the growth mechanism of the material is still lacking. Furthermore, owing to the comparable ionic radii of W and Mo, WO3 phases can be alloyed with MoO3 under same synthetic conditions, leading to new mixed oxide phases. The electrochomic efficiency depends on the ability to control the phases and the morphology in these systems.

Electrochromic tungsten molybdenum oxide: synthesis with phase and morphology control

Presence of a myriad of WO3 phases demands a stringent control over the microstructure and phase to attain the desired tailoring of the properties. Among several applications, such as electrochromicity, photocatalysis and gas sensing, the electrochromic behaviour of this material has gained significant attention. Here, we thoroughly investigate the growth mechanism of the different WO3 phases under solvothermal reaction conditions, along with their electrochromic behaviour. Under the same synthetic conditions, we explore the effect of Mo-doping into the WO3 lattice, leading to the formation of new mixed oxide phases. Experiments involving the growth mechanism of different phases for WO3 and Wx Mo1-x O3 show that WO3 can form two different phases, i.e. hexagonal and orthorhombic, directed by the presence of oxalic acid in the reaction medium. Interestingly, when oxalic acid is used as a capping reagent, a plate-like 2-D morphology of the orthorhombic WO3 phase is obtained, whereas absence of capping agent yields to a 1-D rod-like morphology corresponding to a hexagonal phase of WO3 . DFTbased calculation of oxalate binding strength on different WO3 surfaces clearly shows that binding energy of the oxalate is higher on the orthorhombic {002} surface than on hexagonal {11-20} surface. Moreover, our experiments show that when a Mo precursor is introduced in the same reaction medium, formation of a 2-D plate-like morphology was observed, but the structure is closely related to the hexagonal WO3 phase. STEM-EDS profiling of the elemental composition shows that the flakes have a W0.5 Mo0.5 O3 composition. In terms of electronic properties, the orthorhombic WO3 shows significant presence of reduced W5+ species, indicating a difference in the reducibility. All these factors, viz. phase, morphology and electronic property affect the electrochromic efficiency of the material. In this spirit, we explored the intercalation kinetics of H+ in the two phases of WO3 . Our electrochromicity experiments show that the hexagonal phase has a faster kinetics of H+ diffusion. This trend is also supported by ab initio calculations, which shows a higher intercalation energy in the orthorhombic phase compared to that in hexagonal one, indicating towards a slower proton intercalation. Electrochemically measured diffusion coefficient values also reinforce this observation. We have further investigated the electrochromic property of the mixed oxide phase, illustrating the effect of Mo incorporation into the WO3 lattice.

Effect of ambient on electrical transport properties of ultra-thin Au nanowires

In this letter we present systematic studies of the dynamics of surface adsorption of various chemicals on ultra-thin single crystalline gold nanowires (AuNW) through sensitive resistance fluctuation spectroscopy measurements coupled with ab initio simulations. We show that, contrary to expectations, the adsorption of common chemicals like methanol and acetone has a profound impact on the electrical transport properties of the AuNW. Our measurements and subsequent calculations establish conclusively that in AuNW, semiconductor-like sensitivity to the ambient arises because of changes induced in its local density of states by the surface adsorbed molecules. The extreme sensitivity of the resistance fluctuations of the AuNW to ambient suggests their possible use as solid-state sensors. Published by AIP Publishing.