Sachin M. Shinde

Fabrication of poly(methyl methacrylate)-MoS2/graphene heterostructure for memory device application

Combination of two dimensional graphene and semi-conducting molybdenum disulfide (MoS2) is of great interest for various electronic device applications. Here, we demonstrate fabrication of a hybridized structure with the chemical vapor deposited graphene and MoS2 crystals to configure a memory device. Elongated hexagonal and rhombus shaped MoS2 crystals are synthesized by sulfurization of thermally evaporated molybdenum oxide (MoO3) thin film. Scanning transmission electron microscope studies reveal atomic level structure of the synthesized high quality MoS2 crystals. In the prospect of a memory device fabrication, poly(methyl methacrylate) (PMMA) is used as an insulating dielectric material as well as a supporting layer to transfer the MoS2 crystals. In the fabricated device, PMMA-MoS2 and graphene layers act as the functional and electrode materials, respectively. Distinctive bistable electrical switching and nonvolatile rewritable memory effect is observed in the fabricated PMMA-MoS2/graphene heterostr...

Structure dependent hydrogen induced etching features of graphene crystals

H2 induced etching of graphene is of significant interest to understand graphene growth process as well as to fabricate nanoribbons and various other structures. Here, we demonstrate the structure dependent H2 induced etching behavior of graphene crystals. We synthesized graphene crystals on electro-polished Cu foil by an atmospheric pressure chemical vapor deposition process, where some of the crystals showed hexagonal shaped snowflake-dendritic morphology. Significant differences in H2 induced etching behavior were observed for the snowflake-dendritic and regular graphene crystals by annealing in a gas mixture of H2 and Ar. The regular graphene crystals were etched anisotropically creating hexagonal holes with pronounced edges, while etching of all the dendritic crystals occurred from the branches of lobs creating symmetrical fractal structures. The etching behavior provides important clue of graphene nucleation and growth as well as their selective etching to fabricate well-defined structures for nanoelectronics.

Formation of graphene nanoribbons and Y-junctions by hydrogen induced anisotropic etching

Metal nanoparticles and H2 induced etching of graphene are of significant interest to synthesise graphene nanoribbons and various other structures with crystallographically defined edges. Here, we demonstrate a controllable H2-induced etching process of graphene crystals to fabricate nanoribbons, and Y-junction structures with pronounced edges. Individual graphene crystals and continuous films were grown on Cu foil by the solid source chemical vapor deposition (CVD) technique. The etching behavior of the synthesized graphene was investigated by annealing at 1000 °C in a gas mixture of H2 and Ar. A highly anisotropic etching creates hexagonal holes, nanoribbons and Y-junction graphene with clear edge structures. The distinct graphene edges of individual ribbons create a 120° angle to form a Y-shaped structure. The finding may be significant for fabricating well-defined graphene structures with controlled edges for electronic device applications as well as creating in-plane heterostructures with other two dimensional (2D) materials.

Controlling single and few-layer graphene crystals growth in a solid carbon source based chemical vapor deposition

Here, we reveal the growth process of single and few-layer graphene crystals in the solid carbon source based chemical vapor deposition (CVD) technique. Nucleation and growth of graphene crystals on a polycrystalline Cu foil are significantly affected by the injection of carbon atoms with pyrolysis rate of the carbon source. We observe micron length ribbons like growth front as well as saturated growth edges of graphene crystals depending on growth conditions. Controlling the pyrolysis rate of carbon source, monolayer and few-layer crystals and corresponding continuous films are obtained. In a controlled process, we observed growth of large monolayer graphene crystals, which interconnect and merge together to form a continuous film. On the other hand, adlayer growth is observed with an increased pyrolysis rate, resulting few-layer graphene crystal structure and merged continuous film. The understanding of monolayer and few-layer crystals growth in the developed CVD process can be significant to grow graphene with controlled layer numbers.

Influence of oxygen on nitrogen-doped carbon nanofiber growth directly on nichrome foil

The synthesis of various nitrogen-doped (N-doped) carbon nanostructures has been significantly explored as an alternative material for energy storage and metal-free catalytic applications. Here, we reveal a direct growth technique of N-doped carbon nanofibers (CNFs) on flexible nichrome (NiCr) foil using melamine as a solid precursor. Highly reactive Cr plays a critical role in the nanofiber growth process on the metal alloy foil in an atmospheric pressure chemical vapor deposition (APCVD) process. Oxidation of Cr occurs in the presence of oxygen impurities, where Ni nanoparticles are formed on the surface and assist the growth of nanofibers. Energy-dispersive x-ray spectroscopy (EDXS) and x-ray photoelectron spectroscopy (XPS) clearly show the transformation process of the NiCr foil surface with annealing in the presence of oxygen impurities. The structural change of NiCr foil assists one-dimensional (1D) CNF growth, rather than the lateral two-dimensional (2D) growth. The incorporation of distinctive graphitic and pyridinic nitrogen in the graphene lattice are observed in the synthesized nanofiber, owing to better nitrogen solubility. Our finding shows an effective approach for the synthesis of highly N-doped carbon nanostructures directly on Cr-based metal alloys for various applications.

An effective approach to synthesize monolayer tungsten disulphide crystals using tungsten halide precursor

The synthesis of large-area monolayer tungsten disulphide (WS2) single crystal is critical for realistic application in electronic and optical devices. Here, we demonstrate an effective approach to synthesize monolayer WS2 crystals using tungsten hexachloride (WCl6) as a solid precursor in atmospheric chemical vapor deposition process. In this technique, 0.05M solution of WCl6 in ethanol was drop-casted on SiO2/Si substrate to create an even distribution of the precursor, which was reduced and sulfurized at 750 °C in Ar atmosphere. We observed growth of triangular, star-shaped, as well as dendritic WS2 crystals on the substrate. The crystal geometry evolves with the shape and size of the nuclei as observed from the dendritic structures. These results show that controlling the initial nucleation and growth process, large WS2 single crystalline monolayer can be grown using the WCl6 precursor. Our finding shows an easier and effective approach to grow WS2 monolayer using tungsten halide solution-casting, rat...

Synthesis of a three dimensional structure of vertically aligned carbon nanotubes and graphene from a single solid carbon source

Here, we demonstrate the synthesis of a three dimensional (3D) structure of vertically aligned carbon nanotubes (VACNTs) and graphene from a single solid carbon source. Graphene growth on Cu foil is achieved using solid camphor as the carbon source, whereas the VACNTs are obtained by adding a small amount of ferrocene in the camphor feedstock with minimum contamination from the iron catalyst. Highly dense VACNTs are grown on a transferred graphene film to fabricate the hybrid structure. Raman spectroscopy, optical and scanning electron microcopy studies confirm out of plane growth of the carbon nanotubes (CNTs) from the graphene film. Current–voltage (I–V) measurements are performed to investigate the in plane and out of plane electrical characteristics of the 3D structure. Contact resistance of the VACNTs–graphene is explored taking into account the other resistive contacts in the 3D material system. Achieving a seamless contact of VACNTs–graphene film is significant for low contact resistance and thereby practical device application.

MWCNTs Synthesized From Waste Polypropylene Plastics and its Application in Super-capacitors

The Multiwall Carbon Nanotubes (MWCNTs) were synthesized at 800 °C by single stage chemical vapor deposition (CVD) from the carbonaceous source of waste polypropylene plastic (WPP) in the presence of a Ni catalyst. The fabrication of capacitor cell is very simple and does not require any binders. The electrochemical performances of the carbon nanotubes electrode were investigated by use of the cyclic voltammetry and galvanostatic charge/discharge for its application in super capacitors. The specific capacitance of 59 F/g of the electrode was achieved with scan rate of 5 mV/s in the solution of 1N KOH.

Effect of annealing in hydrogen atmosphere on ZnO films for field emission display

Surface morphology, crystallinity, conductivity and optical transmittance of ZnO films can be modified by annealing process. Hydrogen is one of the popular annealing gases as well as nitrogen, argon, oxygen and air which are commonly used for thin film cleaning or the removal of native oxide. In general, annealing is done at high temperatures (> 600degC) to improve the film properties. From a view point of environment, however, lower annealing temperature is preferable. In this work, low annealing process was challenged to understand the effect of annealing temperature on properties of ZnO thin films and nanostructured film grown on glass substrates for transparent field emission device applications. The annealing temperature employed was 100, 200 and 450°C at 100 sccm hydrogen flow rate. ZnO thin films were deposited by RF magnetron sputtering. The ZnO thin films were characterized by X-ray diffraction analysis (XRD), Atomic Force Microscopy (AFM), UV-VIS and Raman spectroscopy. The sheet resistances reduced about 15 kohm/sq at low annealing temperature. By contrast, the optical transmittance did not show any significant changes after annealing. The FE current density increased after the ZnO nanostructures film was annealed in 100°C. The results obtained could motivate a surface treatment for flexible ZnO thin film since the substrate is always suffered by heat.