S. Amirthapandian

Flipping growth orientation of nanographitic structures by plasma enhanced chemical vapor deposition

Nanographitic structures (NGSs) with a multitude of morphological features are grown on SiO2/Si substrates by electron cyclotron resonance-plasma enhanced chemical vapor deposition (ECR-PECVD). CH4 is used as a source gas with Ar and H2 used as diluents. Field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy are used to study the structural and morphological features of the grown films. Herein we demonstrate how the morphology of these structures can be tuned from a planar to a vertical structure using a single control parameter, namely the level of dilution of CH4 with Ar and/or H2. Our results show that competitive growth and etching processes dictate the morphology of the NGSs. While an Ar-rich composition favors vertically oriented graphene nanosheets, an H2-rich composition aids the growth of planar films. Raman analysis reveals the dilution of CH4 with either Ar or H2 or with the two in combination helps to improve the structural quality of the films. Line shape analysis of the Raman 2D bands shows a nearly symmetrical Lorentzian profile which confirms the turbostratic nature of the grown NGSs. This aspect is further elucidated by HRTEM studies where an elliptical diffraction pattern is observed. Based on these experiments, a comprehensive understanding is obtained of the growth and the structural properties of NGSs grown over a wide range of feedstock compositions.

Growth of InN quantum dots to nanorods: a competition between nucleation and growth rates

Growth evolution of InN nanostructures via a chemical vapor deposition technique is reported using In2O3 as a precursor material and NH3 as reactive gas in the temperature range of 550–700 °C. Morphology of the nanostructures solely depends on the growth temperature, evolving from quantum dot sized nanoparticles to nanorods. It is found that 630 °C is the threshold temperature for nanorod growth. At 630 °C, nucleation starts with multifaceted particles having {10–12} surface planes. Subsequently, hexagonal polyhedral NRs are grown along the [0001] direction with non-polar surfaces of m-planes {10–10}. A comprehensive study is carried out to understand the evolution of nanorods as a function of growth parameters like temperature, time and gas flow rate. The change in the morphology of nanostructures is explained based on the nucleation and the growth rates during the phase formation. Raman studies of these nanostructures show that a biaxial strain is developed because of unintentional impurity doping with the increase in growth temperature.

Evolution of GaN nanowire morphology during catalyst-induced growth process

We report a very generic methodology to control the crystallographic orientation of GaN nanowires (NWs) in a chemical vapor deposition technique employing a standard vapor–liquid–solid mechanism. Incubation time was considered as a critical parameter to control the nanowire morphology. It was found that nanowires of a particular geometry, such as hexagonal, triangular, wurtzite/zinc-blende biphase, and square shaped forms could be obtained by varying the length of incubation time. The change in the diameter of the nanowires with respect to the size of the catalyst droplet was corroborated by a simple steady state model. Luminescence spectra recorded from the GaN NWs revealed the presence of a dominating wurtzite phase in all the as-grown samples. However, temperature independent behavior of two luminescence peaks, recorded especially from the biphase homostructure, was believed to originate from the radiative recombination of carriers localized at potential fluctuations in the zinc-blende and wurtzite phases discretely.

A facile green synthesis of reduced graphene oxide by using pollen grains of Peltophorum pterocarpum and study of its electrochemical behavior

Reduced graphene oxide (RGO) was prepared from graphite oxide (GO) by using pollen grains (Pgs) of Peltophorum pterocarpum as a reducing agent, and was then studied for its electrochemical behavior. The RGOs were also prepared without and with hydrazine hydrate as the reducing agent for comparison. All the RGOs were studied by X-ray diffraction, and by Raman and FTIR spectroscopies. The microstructure was studied by scanning and transmission electron microscopies, and the phase formation was further confirmed by electron energy loss spectroscopy. Reduction by the Pgs was comparable to that by hydrazine hydrate. Cyclic voltammetry studies showed the good electrochemical performance of RGO with a maximum specific capacitance of 27.1 F g−1 (at a scan rate of 5 mV s−1). This synthesis method is reported to be a green method, due to the non-hazardous nature of Pgs to the environment and cost effective.

Indium assisted growth of GaN nanowires at low temperatures

We report the growth of corrugated GaN nanowires (NWs) at low temperatures using chemical vapor deposition growth technique following the vapor-solid-liquid (VLS) mechanism with the assistance of In metal. Usually GaN nanostructures are grown at ∼900 °C in the VLS process. However, we have observed the growth of stoichiometric GaN NWs at 700 °C with In incorporation in Ga metal source. Morphologies of these NWs depend on the growth temperature. Optical properties using Raman scattering and photoluminescence spectroscopies have also been reported for the grown NWs. Incorporation of trace amount of In which, however has not degrade the optical and structural quality of GaN NWs, is discussed for its role for the low temperature growth of these nanostructures.

Morphological investigations on the growth of defect-rich Bi2Te3 nanorods and their thermoelectric properties

Bi2Te3 nanorods (NRs) have been successfully synthesized at different reaction temperatures via a surfactant-assisted hydrothermal method. The experimental observations revealed that the reaction temperature, SDBS surfactant (sodium dodecylbenzene sulphonate) and its concentration affect the morphology of Bi2Te3. It is identified that an imperfect oriented attachment mechanism is the dominant growth mechanism for the evolution of Bi2Te3 NRs. Moreover, a subsidiary growth mechanism namely the Oswald ripening process could also affect the Bi2Te3 NR growth process at higher reaction temperatures. Thermoelectric property measurements revealed that flake-decorated Bi2Te3 NRs achieved the maximum power factor value of ∼1.32 μW cm−1 K−2 at 410 K, which was higher than those of porous and smooth Bi2Te3 NRs. Such an enhancement in the power factor of flake-decorated Bi2Te3 NRs is primarily attributed to their outstanding electrical conductivity at 410 K.