Direct Growth of Graphene on Insulator Using Liquid Precursor Via an Intermediate Nanostructured State Carbon Nanotube

Abstract Synthesis of high-quality graphene layers on insulating substrates is highly desirable for future graphene-based high-speed electronics. Besides the use of gaseous hydrocarbon sources, solid and liquid hydrocarbon sources have recently shown great promises for high-quality graphene growth. Here, I report chemical vapor deposition growth of mono- to few-layer graphene directly on SiO2 substrate using ethanol as liquid hydrocarbon feedstock. The growth process of graphene has been systematically investigated as a function of annealing temperature as well as different seed layers. Interestingly, it was found that the carbon atoms produced by thermal decomposition of ethanol form sp2 carbon network on SiO2 surface thereby forming nanographene flakes via an intermediate carbon-based nanostructured state carbon nanotube. This work might pave the way to an understanding for economical and catalyst-free graphene growth compatible with current silicon-processing techniques, and it can be applied on a variety of insulating surfaces including quartz, sapphire, and fused silica

Thickness Dependent Spectroscopic Studies in 2D PtSe₂

Transition metal dichalcogenides (TMDCs) are emerging to be an exciting class of 2D materials apart from graphene or hexagonal boron nitride (h-BN). They are a class of layered materials that exhibit inspiring properties which are worth exploring, among them PtSe2 is fairly a new addition. Although bulk PtSe2 was first synthesized more than a century ago, the study of its layer-dependent properties is still at a nascent stage. The monolayer of PtSe2 exhibits a band gap between 1.2 and 1.8 eV, the band gap starts to decrease with an increase in the number of layers thus transforming into semimetal type. Among all other 2D materials it shows the highest electron mobility of about 3000 cm2 V−1 s−1 and unlike other TMDCs, it is strikingly stable in ambient conditions. Owing to its stability and tunable properties, it has great potential in the fields of optoelectronics, spintronics, sensorics, and many more. In this book chapter, we report the thickness dependent spectroscopic properties of mechanically exfoliated PtSe2. We have explored low temperature Raman spectroscopy as well as polarized Raman spectroscopy to study in detail the vibrational properties of PtSe2. Raman spectroscopy is also employed to determine its thermal conductivity. We hope that this work will provide a fresh overview of PtSe2 from a spectroscopic perspective

Salinity gradient induced blue energy generation using two-dimensional membranes

Abstract Salinity gradient energy (SGE), known as blue energy is harvested from mixing seawater with river water in a controlled way using ion exchange membranes (IEMs). Using 2D materials as IEMs improves the output power density from a few Wm−2 to a few thousands of Wm−2 over conventional membranes. In this review, we survey the efforts taken to employ the different 2D materials as nanoporous or lamellar membranes for SGE and provide a comprehensive analysis of the fundamental principles behind the SGE. Overall, this review is anticipated to explain how the 2D materials can make SGE a viable source of energy

Recent advances in two-dimensional perovskite materials for light-emitting diodes

Abstract Light-emitting diodes (LEDs) are an indispensable part of our daily life. After being studied for a few decades, this field still has some room for improvement. In this regard, perovskite materials may take the leading role. In recent years, LEDs have become a most explored topic, owing to their various applications in photodetectors, solar cells, lasers, and so on. Noticeably, they exhibit significant characteristics in developing LEDs. The luminous efficiency of LEDs can be significantly enhanced by the combination of a poor illumination LED with low-dimensional perovskite. In 2014, the first perovskite-based LED was illuminated at room temperature. Furthermore, two-dimensional (2D) perovskites have enriched this field because of their optical and electronic properties and comparatively high stability in ambient conditions. Recent and relevant advancements in LEDs using low-dimensional perovskites including zero-dimensional to three-dimensional materials is reported. The major focus of this article is based on the 2D perovskites and their heterostructures (i.e., a combination of 2D perovskites with transition metal dichalcogenides, graphene, and hexagonal boron nitride). In comparison to 2D perovskites, heterostructures exhibit more potential for application in LEDs. State-of-the-art perovskite-based LEDs, current challenges, and prospects are also discussed. Graphical Abstrac

Robust room temperature valley polarization in monolayer and bilayer WS2

We report robust room temperature valley polarization in chemical-vapor-deposition (CVD) grown monolayer and bilayer WS2via polarization-resolved photoluminescence measurements using excitation below the bandgap. We show that excitation with energy slightly below the bandgap of the multi-valleyed transition metal chalcogenides can effectively suppress the random redistribution of excited electrons and, thereby, greatly enhance the efficiency of valley polarization at room temperature. Compared to mechanically exfoliated WS2, our CVD grown WS2 films also show enhancement in the coupling of spin, layer and valley degree of freedom and, therefore, provide improved valley polarization. At room temperature, using below-bandgap excitation and CVD grown monolayer and bilayer WS2, we have reached a record-high valley polarization of 35% and 80%, respectively, exceeding the previously reported values of 10% and 65% for mechanically exfoliated WS2 layers using resonant excitation. This observation provides a new direction to enhance valley control at room temperature. © 2016 The Royal Society of Chemistryclose

Stacking angle dependent multiple excitonic resonances in bilayer tungsten diselenide

We report on multiple excitonic resonances in bilayer tungsten diselenide (BL-WSe2) stacked at different angles and demonstrate the use of the stacking angle to control the occurrence of these excitations. BL-WSe2 with different stacking angles were fabricated by stacking chemical vapour deposited monolayers and analysed using photoluminescence measurements in the temperature range 300–100 K. At reduced temperatures, several excitonic features were observed and the occurrences of these exitonic resonances were found to be stacking angle dependent. Our results indicate that by controlling the stacking angle, it is possible to excite or quench higher order excitations to tune the excitonic flux in optoelectronic devices. We attribute the presence/absence of multiple higher order excitons to the strength of interlayer coupling and doping effect from SiO2/Si substrate. Understanding interlayer excitations will help in engineering excitonic devices and give an insight into the physics of many-body dynamics

Prevention of Transition Metal Dichalcogenide Photodegradation by Encapsulation with h-BN Layers

Transition metal dichalcogenides (TMDs) have recently received increasing attention because of their potential applications in semiconducting and optoelectronic devices exhibiting large optical absorptions in the visible range. However, some studies have reported that the grain boundaries of TMDs can be easily degraded by the presence of oxygen in water and by UV irradiation, ozone, and heating under ambient conditions. We herein demonstrate the photodegradation of WSe2 and MoSe2 by laser exposure (532 nm) and the subsequent prevention of this photodegradation by encapsulation with hexagonal boron nitride (h-BN) layers. The photodegradation was monitored by variation in peak intensities in the Raman and photoluminescence spectra. The rapid photodegradation of WSe2 under air occurred at a laser power of >0.5 mW and was not observed to any extent at <= 0.1 mW. However, in the presence of a water droplet, the photodegradation of WSe2 was accelerated and took place even at 0.1 mW. We examined the encapsulation of WSe2 with h-BN and found that this prevented photodegradation. However, a single layer of h-BN was not sufficient to fully prevent this photodegradation, and so a triple layer of h-BN was employed. We also demonstrated that the photodegradation of MoSe2 was prevented by encapsulation with h-BN layers. On the basis of X-ray photoelectron spectroscopy and scanning photoemission microscopy data, we determined that this degradation was caused by the photoinduced oxidation of TMDs. These results can be used to develop a general strategy for improving the stability of 2D materials in practical applications.close0