Oil boundary approach for sublimation enabled camphor mediated graphene transfer.

HYPOTHESIS: Transfer of chemical-vapor-deposition (CVD) grown monolayer graphene from one substrate to another requires a transfer agent. The transfer agent usually needs to be removed by washing with organic solvent such as acetone or high temperature annealing, which is harmful to the structure integrity and intrinsic property of a graphene film. Here, we propose the use of camphor as a transfer agent to transfer monolayer graphene onto a target dielectric substrate, which bypasses these demanding steps and only needs the common alcohol solvent rinsing. EXPERIMENTS: To facilitate a crack-free graphene transfer, the proposed approach allows the camphor supported polycrystalline graphene to be rationally fastened with a thickened and solidified edge bead (i.e. camphor oil-filled boundary). A layer of camphor was first deposited onto a graphene/copper surface. The backside copper substrate was then etched away, whilst the camphor/graphene bilayer was placed onto a SiO2/Si substrate. Finally, the camphor remaining on the camphor/graphene/SiO2/Si sublimed into a vapor. The graphene/SiO2 stack was then examined by microscopic, spectral and electrical characterization. FINDINGS: The results of our examination suggest that the proposed method can guarantee a clean and damage-free graphene transfer. This method is particularly attractive in the application area for nano/micro-electronics, where it provides CVD-grown graphene the ability to be used on wide varieties of substrates that are sensitive to organic solvents and high temperature.

A Universal Stamping Method of Graphene Transfer for Conducting Flexible and Transparent Polymers.

Transfer method of chemically vapor deposition graphene is an appealing issue to realize its application as flexible and transparent electrodes. A universal stamping method to transfer as grown graphene from copper onto different flexible and transparent polymers (FTPs) reported here ensures simple, robust, rapid, clean and low-cost. This method relies on coating ethylene vinyl acetate (EVA) onto the as grown graphene, binding EVA coated graphene/Cu with FTPs and delamination by hydrogen bubbling process, which is analogous to the method used by stamping process where ink carries the imprint of the object onto any materials. The fate of the stamping method depends on how strongly the adhesion of EVA coated graphene/Cu with target FTPs. Interestingly, we have found that the thin film of EVA/graphene/Cu can only bind strongly with the FTPs of less than 25 µm in thickness and lower glass transition temperature value to the EVA while wide range of other FTPs are considered upon surface engineering to enhance the binding strength between FTPs and EVA. What's more, the electrical performance was investigated with a demonstration of triboelectric nanogenerators which confirmed the reliability of graphene transfer onto the FTPs and prospect for the development of flexible and transparent electronics.

Novel Green Biomimetic Approach for Synthesis of ZnO-Ag Nanocomposite; Antimicrobial Activity against Food-borne Pathogen, Biocompatibility and Solar Photocatalysis.

A simple, eco-friendly, and biomimetic approach using Thymus vulgaris (T. vulgaris) leaf extract was developed for the formation of ZnO-Ag nanocomposites (NCs) without employing any stabilizer and a chemical surfactant. T. vulgaris leaf extract was used for the first time, in a novel approach, for green fabrication of ZnO-Ag NCs as a size based reducing agent via the hydrothermal method in a single step. Presence of phenols in T. vulgaris leaf extract has served as both reducing and capping agents that play a critical role in the production of ZnO-Ag NCs. The effect of silver nitrate concentration in the formation of ZnO-Ag NCs was studied. The in-vitro Antimicrobial activity of NCs displayed high antimicrobial potency on selective gram negative and positive foodborne pathogens. Antioxidant activity of ZnO-Ag NCs was evaluated via (2,2-diphenyl-1-picrylhydrazyl) DPPH method. Photocatalytic performance of ZnO-Ag NCs was appraised by degradation of phenol under natural sunlight, which exhibited efficient photocatalytic activity on phenol. Cytotoxicity of the NCs was evaluated using the haemolysis assay. Results of this study reveal that T. vulgaris leaf extract, containing phytochemicals, possess reducing property for ZnO-Ag NCs fabrication and the obtained ZnO-Ag NCs could be employed effectively for biological applications in food science. Therefore, the present study offers a promising way to achieve high-efficiency photocatalysis based on the hybrid structure of semiconductor/metal.

Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites.

Morphology management for tailoring the properties of monolayer transition-metal dichalcogenides (TMDCs), that is, molybdenum disulfide (MoS2), has attracted great interest for promising applications such as in electrocatalysis and optoelectronics. Nevertheless, little progress has been made in engineering the shape of MoS2. Herein, we introduce a modified chemical vapor deposition method to grow monolayer MoS2 dendrites by pretreating substrates with adhesive tapes. The as-grown MoS2 crystals are featured with hexagonal backbones with fractal shapes and tunable degrees. By characterizing the atomic structure, it is found that these morphologies are mainly initiated from the twin defect derived growth and controlled by the S:Mo vapor ratio. Due to the accumulated sulfur vacancies in the cyclic twin regions, strong enhancement of photoluminescence emission is localized, which determines the shape dependency of optical property. This work not only enriches the understanding of the twin defects derived crystal growth mechanism and extends its applications from nanomaterials to two-dimensional crystals, but also offers a robust and controllable protocol for shape-engineered monolayer TMDCs in electrochemical and optoelectronic applications.

Shape-Dependent Defect Structures of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition.

Monolayer MoS2 crystals with tailored morphologies have been shown to exhibit shape-dependent properties and thus have potential applications in building nanodevices. However, a deep understanding of the relationship between the shape and defect structures in monolayer MoS2 is yet elusive. Monolayer MoS2 crystals in polygonal shapes, including triangle, tetragon, pentagon, and hexagon, are grown using the chemical vapor deposition technique. Compared with other shapes, the hexagon MoS2 crystal contains more electron-donor defects that are mainly due to sulfur vacancies. In the triangular shapes, the defects are mainly distributed at the vertices of the shapes while they are located at the center of hexagonal shapes. On the basis of the Coulomb interaction of exciton and trion, quantitative calculations demonstrate a high electron density (∼1012/cm2) and high Fermi level (EC - EF = 15 meV) for hexagonal shape at room temperature, compared to triangular shapes (∼1011/cm2, EC - EF ≈ 30 meV). These findings verify that a much higher number of donor-like sulfur vacancies are formed in hexagonal MoS2 shapes. This property allows more electrons or trions to localize in such sites through the physical/chemical adsorption of O2/H2O, which results in a strong enhancement of the light emission efficiency in the hexagonal crystal. The findings provide a better understanding of the formation of shape-dependent defect structures of monolayer MoS2 crystals and are inspiring for applications in fabricating nanoelectronic and optoelectronic devices through defect engineering.

Roll-to-Roll Green Transfer of CVD Graphene onto Plastic for a Transparent and Flexible Triboelectric Nanogenerator.

A novel roll-to-roll, etching-free, clean transfer of CVD-grown graphene from copper to plastic using surface-energy-assisted delamination in hot deionized water is reported. The delamination process is realized by water penetration between the hydrophobic graphene and a hydrophilic native oxide layer on a copper foil.The transferred graphene on plastic is used as a high-output flexible and transparent triboelectric nanogenerator.