A study of the nucleation and growth mechanism of graphene on copper

Two approaches for the large scale synthesis of graphene were investigated with the objective of achieving high-quality graphene for practical applications: (1) non-covalent solution based exfoliation of graphite, and (2) chemical vapour deposition of graphene on copper. As the processing conditions, structure, and properties of graphene are inherently connected, the aim of the study was to gain fundamental insights on the critical mechanisms that govern the properties and device performance of graphene. The first part of this thesis describes the efficient non-covalent exfoliation of graphite to produce few-layer-graphene dispersion in N-methylpyrrolidone and large-scale thin film deposition using the Langmuir-Blodgett assembly. In the second part, the chemical vapour deposition of polycrystalline graphene films on copper was investigated as it has emerged as the most promising route toward large scale synthesis of monolayer graphene for optoelectronics applications due to its properties approaching that of ideal graphene and the relatively low cost of copper. An extensive range of growth parameters was employed to develop a model of two-dimensional nucleation and self-limited growth of graphene on the surface of copper. The analysis of the nucleation and growth kinetics has revealed the relationship between atomic processes that impart two distinct temperature regimes of nucleation, whereas the growth of the individual nuclei was shown to be rate-limited by the carbon attachment at the nuclei edges. Moreover, the growth on high index copper surfaces has shown that graphene nanostructures of controlled shapes, density, and dimensions can be produced, depending on the Cu crystal orientation. Interestingly, few-layer-graphene grown on Cu frequently exhibits AA stacking with interlayer spacing of ~3.6 Å , with a preserved linear dispersion relationship. This work thus provides practical guidelines for achieving wafer scale single crystal graphene as well as the control over the mesoscale structure by the careful selection of the growth parameters

Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS2

Toward the large-area deposition of MoS2 layers, we employ metal–organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS2 grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm2 V–1 s–1 at cryogenic temperatures

Large-grain MBE-grown GaSe on GaAs with a Mexican hat-like valence band dispersion

Molecular beam epitaxy: two-dimensional GaSe with non-parabolic electronic dispersion Molecular beam epitaxy enables growth of high-quality, atomically thin GaSe on a GaAs substrate. A team led by Andras Kis at EPFL successfully demonstrated the synthesis of large-grain GaSe van der Waals epitaxial films using a two-step growth approach. The quality and spatial uniformity of the as-grown films were probed by various means of characterization, including scanning transmission electron microscopy, in-situ reflection high energy electron diffraction, and photoemission electron momentum microscopy. The results indicate a uniform distribution of Ga and Se in the GaSe film; at the atomically thin limit, the electronic band structure was found to exhibit inverted band dispersion at the Γ point, leading to a Mexican Hat-like valence band dispersion. These finding may pave the way to potential applications of GaSe in large-area electronics and spintronics

Development of a multiplex reverse transcription qPCR assay for identification of saliva, blood, and semen

The identification of body fluids can provide important information for case processing in forensic investigations. Accurate identification through messenger RNA (mRNA) is effective, especially in cases of sexual assault, where body fluids from one or more contributors are mixed. In this study, a saliva, blood, and semen simultaneous identification reverse transcription-quantitative polymerase chain reaction (SBS RT-qPCR) assay using mRNA from a DNA/RNA co-extraction method was developed. The body fluid-specific mRNA markers histatin 3 (HTN3), haemoglobin subunit beta (HBB), and protamine 1 (PRM1) were selected for identification of saliva, blood, and sperm cells, respectively. The assay accurately identified saliva, blood, and semen without cross-reaction with 10 body fluids. The limits of detection (LoD) were 1/102-, 1/105-, and 1/103-fold dilutions of total RNA in saliva, blood, and semen, respectively. Furthermore, the assay accurately identified trace amounts of body fluids in mixed samples at a 58:1:1 ratio. Finally, it was possible to analyse the origin of body fluids by obtaining an STR profile using the DNA/RNA co-extraction method. The SBS RT-qPCR assay was able to simultaneously identify each body fluid with efficiency in sample consumption. Therefore, it is advisable to use the DNA/RNA co-extraction method to identify mixed body fluids in sexual assault cases.</p

Wafer-scale MOCVD growth of monolayer MoS2 on sapphire and SiO2

High-quality and large-scale growth of monolayer molybdenum disulfide (MoS2) has caught intensive attention because of its potential in many applications due to unique electronic properties. Here, we report the wafer-scale growth of high-quality monolayer MoS2 on singlecrystalline sapphire and also on SiO2 substrates by a facile metal-organic chemical vapor deposition (MOCVD) method. Prior to growth, an aqueous solution of sodium molybdate (Na2MoO4) is spun onto the substrates as the molybdenum precursor and diethyl sulfide ((C2H5)(2)S) is used as the sulfur precursor during the growth. The grown MoS2 films exhibit crystallinity, good electrical performance (electron mobility of 22 cm(2)center dot V-1 center dot s(-1)) and structural continuity maintained over the entire wafer. The sapphire substrates are reusable for subsequent growth. The same method is applied for the synthesis of tungsten disulfide (WS2). Our work provides a facile, reproducible and cost-efficient method for the scalable fabrication of high-quality monolayer MoS2 for versatile applications, such as electronic and optoelectronic devices as well as the membranes for desalination and power generation

Free-standing electronic character of monolayer MoS2 in van der Waals epitaxy

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