Subash Sharma

Opening of triangular hole in triangular-shaped chemical vapor deposited hexagonal boron nitride crystal

In-plane heterostructure of monolayer hexagonal boron nitride (h-BN) and graphene is of great interest for its tunable bandgap and other unique properties. Here, we reveal a H2-induced etching process to introduce triangular hole in triangular-shaped chemical vapor deposited individual h-BN crystal. In this study, we synthesized regular triangular-shaped h-BN crystals with the sizes around 2-10 μm on Cu foil by chemical vapor deposition (CVD). The etching behavior of individual h-BN crystal was investigated by annealing at different temperature in an H2:Ar atmosphere. Annealing at 900 °C, etching of h-BN was observed from crystal edges with no visible etching at the center of individual crystals. While, annealing at a temperature ≥950 °C, highly anisotropic etching was observed, where the etched areas were equilateral triangle-shaped with same orientation as that of original h-BN crystal. The etching process and well-defined triangular hole formation can be significant platform to fabricate planar heterostructure with graphene or other two-dimensional (2D) materials.

Synthesis of hexagonal graphene on polycrystalline Cu foil from solid camphor by atmospheric pressure chemical vapor deposition

Understanding of graphene nucleation and growth on a metal substrate in chemical vapor deposition (CVD) process is critical to obtain high-quality single crystal graphene. Here, we report synthesis of individual hexagonal graphene and their large cluster on Cu foil using solid camphor as a carbon precursor in the atmospheric pressure CVD (AP-CVD) process. Optical and scanning electron microscopy studies show formation of hexagonal graphene crystals across the grain, grain boundaries and twin boundaries of polycrystalline Cu foil. Electron backscattered diffraction analysis is carried out before and after the growth to identify Cu grain orientation correlating with the graphene formation. The influence of growth conditions and Cu grain structure is explored on individual hexagonal graphene formation in the camphor-based AP-CVD process.

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.

In situ fabrication of graphene from a copper–carbon nanoneedle and its electrical properties

Herein, we present a direct observation of the formation of graphene from a single copper–carbon nanoneedle (Cu–CNN) during the measurement of current–voltage (I–V) and direct heating via in situ transmission electron microscopy (TEM). Significant structural transformation of Cu–CNN was observed with an applied potential in a two probe system. Under a high current flow between 4.9 μA to 49.0 μA, the Cu nanoparticles melted and evaporated due to Joule heating. The amorphous carbon began crystallizing and transformed into sp2 hybridized hollow graphitic carbon, which was catalyzed by the dispersed Cu nanoparticles. The temperature generated during the current flow was estimated to be 1073 K, as revealed by an in situ TEM heating experiment. The graphene nanoneedle formed exhibited a high current density of 106 A cm−2, which is comparable to Cu in normal interconnect applications. Thus, the graphene nanoneedle formed will be promising for future alternative interconnect materials.

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.

Edge controlled growth of hexagonal boron nitride crystals on copper foil by atmospheric pressure chemical vapor deposition

Most of the chemical vapor deposition (CVD) systems used for hexagonal boron nitride (h-BN) growth employ pyrolytic decomposition of a precursor molecule, such as ammonia borane (AB), at a temperature close to its melting point. So the control of its partial pressure is essential for high quality crystal growth. Here, we report on the edge controlled growth of a h-BN single crystal larger than 25 μm in edge length on purchased Cu foils. The key was the controlled supply of borazine gas generated by the decomposition of AB, and the stepwise decomposition of AB was found to be essential for the growth of regular h-BN crystals. The h-BN growth was mostly governed by the position of the nucleation point rather than Cu orientation as confirmed by electron back-scattered diffraction (EBSD) analysis. It was also demonstrated that the variation in temperature during the growth and cooling processes induced wrinkles larger than 20 nm due to the thermal straining of the Cu surface and a negative expansion coefficient of h-BN. These results provide a detailed understanding of h-BN growth, which will be applicable to other 2D materials.

Switching isotropic and anisotropic graphene growth in a solid source CVD system

Controlling the isotropic and anisotropic graphene growth in a chemical vapor deposition (CVD) process is a critical aspect to understand the growth dynamics for synthesizing large-area single crystals. Here, we reveal the effect of gas flow and controllability on isotropic and anisotropic graphene growth using a solid carbon source-based atmospheric pressure CVD method. It was found that the growth rate of round-shaped crystals (isotropic growth) was much higher than that of hexagonal crystals (anisotropic growth). The average growth speed increased from 0.276 μm min−1 to 1.89 μm min−1 by switching from hexagonal to circular domain growth in the CVD process. It was also found that there was no significant difference in the quality of graphene crystals when switching the growth from anisotropic to isotropic. Understanding the growth rate of round and hexagonal-shaped crystals can be critical to achieve faster growth of large single crystals. Again, the mixed edge structures (armchair and zigzag) in round-shaped graphene crystals without a fixed orientation unlike hexagonal crystals provide a better chance of seamless merging. Our findings can be significant in understanding the formation of isotropic and anisotropic graphene domains, their growth rate and quality for synthesizing large-area single crystals.

Room temperature fabrication of 1D carbon-copper composite nanostructures directly on Cu substrate and their field emission properties

This paper demonstrates a carbon-copper (C-Cu) composite nanostructure directly fabricated on a copper (Cu) substrate using the Ar+ ion irradiation method at room temperature. The morphology of C-Cu composite was controlled by a simultaneous carbon supply during ion irradiation. Conical protrusions formed on the surface of the Cu substrate with the low carbon supply rate (RC), whereas high RC area prominently produced nanoneedle structures. The field electron emission (FEE) tests demonstrated significant improvement between conical protrusions and nanoneedle structures, where the emission current increase from 5.70 μAcm−2 to 4.37 mAcm-2, while the turn-on field reduced from 5.90 to 2.00 Vμm−1.