Subhajit Biswas

Fabrication of ZnS nanoparticles and nanorods with cubic and hexagonal crystal structures: a simple solvothermal approach

ZnS nanoparticles and nanorods with control over their crystal structure are fabricated through a solvothermal approach by changing the solvent used for the synthesis. The synthetic approach is suitable to fabricate ZnS nanoparticles with various sizes by varying the synthesis temperatures. Quantum confinement phenomena are studied by tailoring the particle sizes for both wurtzite and sphalerite polymorphs of ZnS. Photoluminescence studies reveal that the surface states greatly influence the emissions from the nanostructures. Wurtzite nanoparticles exhibit band-edge related UV emission owing to the effective surface passivation by the ethylene glycol molecules used as the solvent for the synthesis. On the other hand, the photoluminescence spectra of the cubic nanoparticles are mainly dominated by their surface states. Some of the nanorod samples exhibited Zn-vacancy related green emissions along with the surface defect related blue emission band. It is also demonstrated that ZnS nanostructures could be easily doped with useful impurities via this synthesis approach to tailor their luminescent properties.

Catalytic growth and photoluminescence properties of ZnS nanowires

Wurtzite ZnS nanowires were produced by a vapour–liquid–solid process. The growth mechanism of the ZnS nanowires was studied by scanning electron microscopic observations at different stages of the nanowire growth. The produced nanowires were characterized by x-ray diffraction, scanning and transmission electron microscopy and photoluminescence measurements. The ZnS nanowires were perfectly single crystalline and possess a very high aspect ratio. A photoluminescence study shows UV–blue emission from the ZnS nanowires due to the sulfur vacancy and surface states.

Recent advances in the growth of germanium nanowires: synthesis, growth dynamics and morphology control

One-dimensional semiconductor nanostructures have been studied in great depth over the past number of decades as potential building blocks in electronic, thermoelectric, optoelectronic, photovoltaic and battery devices. Silicon has been the material of choice in several industries, in particular the semiconductor industry, for the last few decades due to its stable oxide and well documented properties. Recently however, Ge has been proposed as a candidate to replace Si in microelectronic devices due to its high charge carrier mobilities. A number of various ‘bottom-up’ synthetic methodologies have been employed to grow Ge nanowires, including chemical vapour deposition, thermal evaporation, template methods, supercritical fluid synthesis, molecular beam epitaxy and solution phase synthesis. These bottom-up methods afford the opportunity to produce commercial scale quantities of nanowires with controllable lengths, diameters and crystal structure. An understanding of the vapour–liquid–solid (VLS) and vapour–solid–solid (VSS) mechanism by which most Ge nanowires are produced, is key to controlling their growth rate, aspect ratio and morphology. This article highlights the various bottom-up growth methods that have been used to synthesise Ge nanowires over the past 5–6 years, with particular emphasis on the Au/Ge eutectic system and the VLS mechanism. Thermodynamic and kinetic models used to describe Ge nanowire growth and morphology control will also be discussed in detail.

Manipulating the growth kinetics of vapor-liquid-solid propagated Ge nanowires.

This article describes an innovative approach in which bimetallic alloy seeds of AuxAg1-x are used to enhance the growth kinetics of Ge nanowires, via a vapor-liquid-solid (VLS) growth technique. The decreased equilibrium concentration and increased supersaturation of Ge in the liquid alloy seeds, compared to pure Au seeds, results in favorable growth kinetics and the realization of high-aspect ratio millimeter-long Ge nanowires. Also detailed is the manifestation of the Gibbs-Thompson effect resulting in diameter-dependent nanowire growth rates as a function of the Au-Ag-Ge eutectic composition. Significantly, AuxAg1-x alloy seeds lower the critical diameter of the Ge nanowires in this liquid-seeded growth approach. In situ TEM heating experiments established the correlation between the growth kinetics and equilibrium eutectic compositions in the ternary growth systems. The fundamental insights of nanowire growth demonstrated with the ternary eutectic alloys opens up opportunities to engineer the aspect ratio and morphology of a range of semiconductor nanowires.

Non-equilibrium induction of tin in germanium: towards direct bandgap Ge1-xSnx nanowires.

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1−xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour–liquid–solid growth. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 °C) during cool-down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth.

Inherent Control of Growth, Morphology, and Defect Formation in Germanium Nanowires

The use of bimetallic alloy seeds for growing one-dimensional nanostructures has recently gained momentum among researchers. The compositional flexibility of alloys provides the opportunity to manipulate the chemical environment, reaction kinetics, and thermodynamic behavior of nanowire growth, in both the eutectic and the subeutectic regimes. This Letter describes for the first time the role of Au(x)Ag(1-x) alloy nanoparticles in defining the growth characteristics and crystal quality of solid-seeded Ge nanowires via a supercritical fluid growth process. The enhanced diffusivity of Ge in the alloy seeds, compared to pure Ag seeds, and slow interparticle diffusion of the alloy nanoparticles allows the realization of high-aspect ratio nanowires with diameters below 10 nm, via a seeded bottom-up approach. Also detailed is the influence the alloyed seeds have on the crystalline features of nanowires synthesized from them, that is, planar defects. The distinctive stacking fault energies, formation enthalpies, and diffusion chemistries of the nanocrystals result in different magnitudes of {111} stacking faults in the seed particles and the subsequent growth of <112>-oriented nanowires with radial twins through a defect transfer mechanism, with the highest number twinned Ge nanowires obtained using Ag(0.75)Au(0.25) growth seeds. Employing alloy nanocrystals for intrinsically dictating the growth behavior and crystallinity of nanowires could open up the possibility of engineering nanowires with tunable structural and physical properties.

Positron annihilation spectroscopic studies of solvothermally synthesized ZnO nanobipyramids and nanoparticles

Zinc oxide (ZnO) samples in the form of hexagonal-based bipyramids and particles of nanometer dimensions were synthesized through solvothermal route and characterized by x-ray diffraction and transmission electron microscopy. Positron annihilation experiments were performed to study the structural defects such as vacancies and surfaces in these nanosystems. From coincidence Doppler broadening measurements, the positron trapping sites were identified as Zn vacancies or Zn-O-Zn trivacancy clusters. The positron lifetimes, their relative intensities, and the Doppler broadened lineshape parameter S all showed characteristic changes across the nanobipyramid size corresponding to the thermal diffusion length of positrons. In large nanobipyramids, vacancies within the crystallites also trapped positrons and the effects of agglomeration of such vacancies due to increased temperatures of synthesis were reflected in the variation of the annihilation parameters with their base diameters. The sizes of the nanoparticles used were all in the limit of thermal diffusion length of positrons and the annihilation characteristics were in accordance with the decreasing contribution from surfaces with increasing particle size.

Substitution-induced structural transformation in Mn-doped ZnS nanorods studied by positron annihilation spectroscopy

Zinc sulphide nanorods were doped with manganese ions and the structural changes were monitored through positron annihilation measurements. X-ray diffraction and high-resolution transmission electron microscopy revealed a transformation from hexagonal to cubic structure, and we observed characteristic changes in the measured positron lifetimes, their intensities and Doppler broadened lineshapes. These features were explained on the basis of positron trapping and subsequent annihilation at the nanorod surfaces. Further incorporation of manganese resulted in an increase of the diameter of the nanorods, and the effect was reflected in the decreasing lifetime of positrons. At sufficiently high concentration of manganese ions, Mn–Mn clusters were formed. The results are compared with those obtained for ZnS nanoparticles where vacancies within the grains trapped a significant fraction of positrons.

Nanometre to micrometre wide ZnS nanoribbons

ZnS nanoribbons with variable widths ranging from a few nanometres to micrometre order have been fabricated in a controlled way. The ultra-long nanoribbons were single crystalline, smooth and uniform throughout their lengths. Growth of the nanoribbons was initiated by the vapour–liquid–solid process and subsequently their widths were regulated by the influence of the vapour–solid process. These nanoribbons possessed a bulk-like bandgap as determined from the optical absorption spectra. Room temperature photoluminescence was observed in the UV–visible region from the ZnS nanoribbons with the emission peaks at 398 and 458 nm attributed to the transitions from the sulfur vacancy related defect states and surface states respectively.

Positron annihilation studies of defects and interfaces in ZnS nanostructures of different crystalline and morphological features

Nanostructures of ZnS, both particles and rods, were synthesized through solvothermal processes and characterized by x-ray diffraction and high resolution transmission electron microscopy. Positron lifetime and Doppler broadening measurements were made to study the features related to the defect nanostructures present in the samples. The nanocrystalline grain surfaces and interfaces, which trapped significant fractions of positrons, gradually disappeared during grain growth, as indicated by the decreasing fraction of orthopositronium atoms. The crystal vacancies present within the grains also trapped positrons. These vacancies further agglomerated into clusters during the thermal treatment given to effect grain growth. The positron lifetime was remarkably large at extremely small grain sizes (approximately 1.5 nm) and this was attributed to the occurrence of quantum confinement effects, as verified through optical absorption measurements. Positron lifetimes in ZnS nanorods increased with increasing content of cubic phase in the samples and this observation is assigned to the annihilation of positrons in sites with increased cubic unit cell volume. The Doppler broadened spectra also indicated qualitative changes consistent with these observations.