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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 heterostructure. The developed material system and demonstrated memory device fabrication can be significant for next generation data storage applications. [ABSTRACT FROM AUTHOR]

An electrically switchable graphene terahertz (THz) modulator with a tunable‐by‐design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation‐optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies. [ABSTRACT FROM AUTHOR]

The optimization process of the desired size and quality of graphene domains is usually very difficult due to the numerous interdependent parameters in chemical vapor deposition (CVD). Here, the method so-called 'designs of experiments' is applied to estimate the relative importance and value of some of these parameters and their interactions for the CVD growth of graphene on Cu foil using waste plastic as a solid source. We found that the growth temperature, time, rate of heating, and preannealing time of the substrate influence significantly graphene growth. In particular, the growth time and rate of increasing of the carbon source temperature appear as the main factors for the growth of graphene domains, where the manipulation of only these two parameters could dramatically change the size of crystals. Thus, our experiment shows that for synthesis of larger graphene domains using waste plastic not only do the growth parameters of Cu substrate influence graphene growth but also the carbon source rate of heating. [ABSTRACT FROM AUTHOR]

A technology for converting waste materials to high quality large-area monolayer graphene film can be significant and thereby obtaining high value-added product. Here, we revealed the transformation of waste chicken fat into uniform monolayer graphene film on re-solidified Cu by a low pressure chemical vapor deposition (LPCVD) technique. The evolve gas analyzer-gas chromatography–mass spectrometry (EGA-GC–MS) analysis of chicken fat oil showed that the free fatty acid in chicken oil decomposed into a short hydrocarbon chains which makes it favorable to use as a carbon precursor for graphene synthesis. Growth of uniform monolayer graphene film on the re-solidified Cu was confirmed by Raman mapping, where 2D to G peak intensity ratio (I 2D /I G ) is 3.0 at most of the area. Thus, the use of waste from poultry industry as a carbon source instead of commonly used hydrocarbon gas sources for graphene synthesis can be an approach for green nanotechnology. [ABSTRACT FROM AUTHOR]

Fabrication of a transparent and high performance electron emission device is the key challenge for suitable display applications. Here, we demonstrate fabrication of a transparent and efficient field emission device integrating large-area chemical vapor deposited graphene and carbon doped zinc oxide (C:ZnO) nanocones. The ZnO nanocones were obtained with ion irradiation process at room temperature, over which the graphene film was transferred without destroying nanocone tips. Significant enhancement in field emission properties were observed with the transferred graphene film on C:ZnO nanocones. The threshold field for hybrid and pristine C:ZnO nanocones film at current density of 1 μA/cm 2 was obtained as 4.3 V/μm and 6.5 V/μm, respectively. The enhanced field emission properties with low turn-on field for the graphene/C:ZnO nanocones can be attributed to locally enhance electric field. Our finding shows that a graphene/C:ZnO hybridized structure is very promising to fabricate field emission devices without compromising with high transparency. [ABSTRACT FROM AUTHOR]

Chemical vapor deposited large-area graphene transfer process to different flexible substrates is one of the most critical aspect to explore a wide range of reliable applications. In this prospect, transfer of graphene on emerging cellulose based flexible paper and other lightweight substrates for various applications is of great importance. Here, we developed a simple and effective approach of polymer-free graphene transfer process onto a moldable flexible substrate. The synthesized graphene on Cu foil was hot pressed and attached to the substrate depending on the moldable temperature without any structural deformation. Etching the base Cu foil, a clean coating of graphene was obtained on the cellulose acetate (CA) based substrate. The successful coating of conducting graphene film was confirmed by optical, scanning electron microscope (SEM), Raman, atomic force microscope (AFM) and current (I)–voltage (V) studies. The developed process can be significant for graphene transfer on various moldable substrate materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Here, we reveal the structural transformation process of as-synthesized individual graphene crystals with oxidation of a copper (Cu) foil. We found that the transformation of a graphene crystal with Cu oxidation is significantly different for the thermal annealing and room temperature long-term atmospheric oxidation. Annealing creates large cracks in an individual graphene crystal due to the thermal stress and strain created by rapid oxidation of Cu surface. The cracks are further enhanced with longer annealing duration enabling oxygen diffusion through cracks, thereby accelerating oxidization of Cu. Eventually, the graphene crystals are completely damaged, leaving behind the highly oxidized Cu surface. On the other hand, in case of room temperature long-term atmospheric oxidation, oxygen diffusion occurs underneath of a graphene crystal through the reactive edge without any large cracks formation. The graphene crystals decouple from Cu surface during the oxygen diffusion and oxidation process, however no structural deformation is observed. This finding shows the significant contrast of structural change of graphene crystal and oxidation behaviors of Cu surface with thermal annealing and room temperature atmospheric oxidation. [ABSTRACT FROM AUTHOR]

Abstract: Here, we report the synthesis of high quality single crystal graphene on polycrystalline Cu foil using solid waste plastic as carbon source in an ambient pressure (AP) chemical vapor deposition (CVD) process. Gas chromatography analysis shows that the waste plastic is rich in polyethylene and polystyrene based polymer components. The growth of individual crystal strongly influenced by the injection rate of decomposed polymeric components generated during pyrolysis of waste plastic. Synthesis of large hexagonal and round shape single crystal graphene are achieved with a controlled rate of pyrolysis for the waste plastic. Raman studies confirm high quality of the graphene crystals independent of hexagonal and round shapes. Again, nucleation of bilayer or few-layer graphene crystals are observed with a higher injection rate of decomposed polymeric components. The evolution of hexagonal graphene crystals synthesized by AP-CVD process from waste plastic can be significant to obtain large single crystal graphene and stacked bi-layer structure. Our findings shows that solid waste plastic can be directly use as a feedstock for high quality graphene growth and thereby converting to a significantly value added product. [Copyright &y& Elsevier]