S. R. Polaki

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.

Scalable transfer of vertical graphene nanosheets for flexible supercapacitor applications

Vertical graphene nanosheets (VGNs) are the material of choice for next-generation electronic device applications. The growing demand for flexible devices in electronic industry brings in restriction on growth temperature of the material of interest. However, VGNs with better structural quality is usually achieved at high growth temperatures. The difficulty associated with the direct growth on flexible substrates can overcome by adopting an effective strategy of transferring the well grown VGNs onto arbitrary flexible substrates through soft chemistry route. Hence, we demonstrated a simple, inexpensive and scalable technique for the transfer of VGNs onto arbitrary substrates without disrupting its morphology and structural properties. After transfer, the morphology, chemical structure and electronic properties are analyzed by scanning electron microscopy, Raman spectroscopy and four probe resistive methods, respectively. Associated characterization investigation indicates the retention of morphological, structural and electrical properties of transferred VGNs compared to as-grown one. Furthermore the storage capacity of the VGNs transferred onto flexible substrates is also examined. A very lower sheet resistance of 0.67 kOhm/sq. and excellent supercapacitance of 158 micro-Farrad/cm2 with 91.4% retention after 2000 cycles confirms the great prospective of this damage-free transfer approach of VGNs for flexible nanoelectronic device applicationsVertical graphene nanosheets (VGN) are the material of choice for application in next-generation electronic devices. The growing demand for VGN-based flexible devices for the electronics industry brings in restriction on VGN growth temperature. The difficulty associated with the direct growth of VGN on flexible substrates can be overcome by adopting an effective strategy of transferring the well-grown VGN onto arbitrary flexible substrates through a soft chemistry route. In the present study, we report an inexpensive and scalable technique for the polymer-free transfer of VGN onto arbitrary substrates without disrupting its morphology, structure, and properties. After transfer, the morphology, chemical structure, and electrical properties are analyzed by scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and four-probe resistive methods, respectively. The wetting properties are studied from the water contact angle measurements. The observed results indicate the retention of morphology, surface chemistry, structure, and electronic properties. Furthermore, the storage capacity of the transferred VGN-based binder-free and current collector-free flexible symmetric supercapacitor device is studied. A very low sheet resistance of 670 Ω/□ and excellent supercapacitance of 158 μF cm-2 with 86% retention after 10 000 cycles show the prospect of the damage-free VGN transfer approach for the fabrication of flexible nanoelectronic devices.

Enhanced supercapacitance of activated vertical graphene nanosheets in hybrid electrolyte

Supercapacitors are becoming the workhorse for emerging energy storage applications due to their higher power density and superior cycle life compared to conventional batteries. The performance of supercapacitors depends on the electrode material, type of electrolyte, and interaction between them. Owing to the beneficial interconnected porous structure with multiple conducting channels, vertical graphene nanosheets (VGN) have proved to be leading supercapacitor electrode materials. Herein, we demonstrate a novel approach based on the combination of surface activation and a new organo-aqueous hybrid electrolyte, tetraethylammonium tetrafluoroborate in H2SO4, to achieve significant enhancement in supercapacitor performance of VGN. As-synthesized VGN exhibits an excellent supercapacitance of 0.64 mF/cm2 in H2SO4. However, identification of a novel electrolyte for performance enhancement is the subject of current research. The present manuscript demonstrates the potential of the hybrid electrolyte in enhancin...

On the scaling behavior of hardness with ligament diameter of nanoporous-Au: Constrained motion of dislocations along the ligaments

The scaling behavior of hardness with ligament diameter and vacancy defect concentration in nanoporous Au (np-Au) has been investigated using a combination of Vickers Hardness, Scanning electron microscopy, and positron lifetime measurements. It is shown that for np-Au, the hardness scales with the ligament diameter with an exponent of −0.3, that is, at variance with the conventional Hall-Petch exponent of −0.5 for bulk systems, as seen in the controlled experiments on cold worked Au with varying grain size. The hardness of np-Au correlates with the vacancy concentration CV within the ligaments, as estimated from positron lifetime experiments, and scales as CV1/2, pointing to the interaction of dislocations with vacancies. The distinctive Hall-Petch exponent of −0.3 seen for np-Au, with ligament diameters in the range of 5–150 nm, is rationalized by invoking the constrained motion of dislocations along the ligaments.

Interpretation of friction and wear in DLC film: role of surface chemistry and test environment

In spite of the large amount of tribological work carried out to explain the friction and wear mechanism in diamond-like carbon (DLC) films, some of the core issues relating to the evolution of reactive species across sliding interfaces and their role on the friction and wear mechanism remain unclear. The phase composition, film density and hydrogen content present in a DLC film can be tailored by substrate biasing during film deposition to achieve a nearly vanishing friction coefficient. Furthermore, nitrogen doping in DLC films significantly improves wear resistance, and sliding occurs in a nearly wearless regime. Undoped and nitrogen-doped DLC films exhibit a nearly frictionless value with ultra-low wear behavior when tests are performed in argon, nitrogen and methane atmospheres. The antifriction and antiwear properties of the DLC films were improved with the reduction of adsorbed oxygen impurities on the film surface. This behavior was understood by correlating the oxygen impurities present at the surface/subsurface region of the DLC film while using x-ray photoelectron spectroscopy and depth-resolved Auger electron spectroscopy.

A review on metal nitrides/oxynitrides as an emerging supercapacitor electrode beyond oxide

Electrode materials design is the most significant aspect in constructing a supercapacitor device. The evolution of metal nitrides/oxynitrides as supercapacitor electrode is strikingly noticeable today besides prevailing carbon or 2D materials, metal oxides/hydroxides and conducting polymers electrode materials. The theoretically estimated specific capacitance of a nitride-based supercapacitor is 1,560 F g-1. These nanostructures exhibit an excellent capacitive behavior with a specific capacitance of 15–951.3 mF cm-2 or 82–990 F g-1, high energy density (16.5-162 Wh Kg-1) and power density (7.3-54,000 W Kg-1). On this account, supercapacitor performance of metal nitrides/oxynitrides is reviewed exclusively. The major focus of the present review is directed towards state-of-art progress in supercapacitor performance of nitrides/oxynitrides, underlying charge-storage mechanism, important outcomes and their limitations. Finally, we conclude with challenges and prospects of metal nitrides/oxynitrides for supercapacitor electrodes.

Formation of nanocrystalline SiGe in Polycrystalline-Ge/Si thin film without any metal induced crystallization

The formation of nanocrystalline SiGe without the aid of metal induced crystallization is reported. Re-crystallization of the as-deposited poly-Ge film (deposited at 450 °C) leads to development of regions with depleted Ge concentration upon annealing at 500 °C. Clusters with crystalline facet containing both nanocrystalline SiGe and crystalline Ge phase starts appearing at 600 °C. The structural phase characteristics were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The stoichiometry of the SiGe phase was estimated from the positions of the Raman spectral peaks.

Aluminum induced crystallization of amorphous Ge thin films on insulating substrate

Aluminium (metal) induced crystallization of amorphous Ge in bilayer and multilayer Ge/Al thin films deposited on quartz substrate at temperature well below the crystallization temperature of bulk Ge is reported. The crystallization of poly-Ge proceeds via formations of dendritic crystalline Ge grains in the Al matrix. The observed phases were characterized by Raman spectroscopy and X-ray diffraction. The microstructure of Al thin film layer was found to have a profound influence on such crystallization process and formation of dendritic grains.

Determination of surface area of nanoporous metals: Insights from double layer charging

Double layer capacitive charging method has been used in the present study for the determination of specific surface area As of nanoporous Au, obtained from the dealloying of Ag75Au25 alloy in 1 M HClO4. The results obtained were compared with the geometrical surface area Ag, estimated using the “ball-stick geometrical model”. The experiments were performed in the temperature intervals 25 - 90°C to verify the thermal stability of the porous-ligament networks in np-Au.