Study on Optical and Electrical Property Changes of Molybdenum Diselenide by Reversible Hydrogenation

Department of Energy EngineeringHydrogenation is one of the chemical functionalization methods, which has been investigated as an approach for modulate electronic structure in nanomaterials, since it was theoretically suggested that electronic structure can be modified by degree of hydrogenation of graphene like two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs). In the case of graphene, its electronic structure with zero band-gap has been expected to be opened by surface hydrogenation. Compared to wide researches for hydrogenation reactions on graphene surface, the other 2D materials, ones on TMD was not sufficiently studied. In this thesis, I have studied the surface hydrogenation reaction on TMDs. Similar to the concept of graphene functionalization, chemical functionalization is known for leading to modification of their electronic and optical properties. Theoretical calculations predicted that the electronic structure of single-layer TMDs also can be tuned by hydrogenation. However, its experimental demonstration has not been realized so far. In addition, TMDs can be used as outstanding catalyst for hydrogen evolution reaction (HER). Therefore, the systematic investigation of hydrogenation on TMDs cannot only unveil the modified electronic structures in TMDs, but also can provide the critical information to understand the interaction between hydrogen atom (or molecule) and TMDs, which is fundamentally important for improving HER efficiency. Here we show modification of electronic properties in MoSe2, one of TMD materials, which is synthesized by chemical vapor deposition (CVD) process. The photoluminescence (PL) intensity and peak position indicates a direct band gap of 1.54 eV for the single-layer MoSe2. After the hydrogenation by H2 plasma treatment, semiconducting properties of single-layer MoSe2 turn into insulator. In a step-by-step PL results, hydrogenation reaction started from edge to center. Also, we confirmed the hydrogen atoms only react with selenium atoms (Se) in X-ray photoelectron spectroscopy (XPS) analysis. This study demonstrates the great potential of controlling electronic property of single-layer MoSe2 and fundamentally understanding about hydrogenation as a surface functionalization study.ope

Growth of Single-Crystalline and Layer-Controllable Hexagonal Boron Nitride

School of Energy and Chemical Engineering (Energy Engineering)Two-dimensional (2D) materials provide great potential for their applications in electronics and photonics because they can offer opportunity for extending Moore's law in beyond-CMOS (complementary metal-oxide-semiconductor) devices. Among 2D materials, hexagonal boron nitride (hBN) is a representative 2D insulting material with bandgap (~6 eV). Owing to atomically flat surface without dangling bonds yet with excellent thermal and chemical stabilities, hBN has been introduced as a promising material for an excellent dielectric layer to efficiently reduce charge scattering and a screening layer from surroundings. A key technological challenge is the scalable manufacture of single-crystal 2D hBN film to avoid a lack of durability and a poor performance influenced by inhomogeneities and grain boundaries. In addition, the controllability of the number of layers is also highly required due to the electron tunneling properties depending on the thickness of hBN. Even though several approaches to achieve large-scale single-crystal hBN and control the number of layers have been demonstrated, a growth method for few-layer single-crystalline hBN and precise control of the number of layers is still unknown. In this thesis, I demonstrate an approach to grow large-scale single-crystal hBN by chemical vapor deposition (CVD) method. First, I show the epitaxial growth of single-crystal trilayer hBN on Ni (111) foil of 2 x 5 cm at 100 oC higher temperature than normal growth temperature for Ni substrate. The trilayer hBN grains show unidirectional alignment and seamless stitching to form single-crystal film on Ni23B6/Ni (111) where a Ni23B6 layer is formed between hBN and Ni (111) during cooling. Microscopic investigations reveal epitaxial relationship between hBN, Ni23B6, and Ni (111) and enable to understand the hBN growth mechanism, the surface-mediated growth. Furthermore, single-crystal trilayer hBN on Ni23B6/Ni (111) plays a role of a catalytic-transparent protection layer for enhanced long-term stability of hydrogen evolution reaction catalyst and a dielectric layer to prevent electron doping from SiO2 substrate in MoS2 transistors. Our results suggest that few-layer single-crystal hBN allows wide applications for 2D devices and catalytic-transparent protection layer of (electro)catalysts. Next, I demonstrate a method for controlling the number of layers of 2-inch wafer-scale single-crystal hBN film on sapphire substrate by remote inductively coupled plasma CVD, which is a temperature-dependent growth method for mono-, bi-, and trilayer hBN. The x-ray photoelectron spectroscopic and transmission electron microscopic investigations show the formation of a Al-N buffer layer between sapphire substrate and the first layer and the reduction of the interlayer spacing of hBN by the Al-N bond. However, the transferred hBN onto SiO2/Si substrate shows a typical interlayer spacing of hBN. This work takes a step towards the layer-controlled growth of wafer-scale uniform hBN films.ope

Toward growth of wafer-scale single-crystal hexagonal boron nitride sheets

Hexagonal boron nitride (hBN) has a two-dimensional planar structure without dangling bonds and is considered an insulator material that can overcome the limitations of SiO2 and HfO2, which typically exhibit large densities of dangling bonds and charged impurities at the interface. However, most of the reported hBN films prepared by chemical vapor deposition (CVD) are polycrystalline with grain boundaries. The grain boundaries of a polycrystalline hBN cause current leakage and gas permeability. A recent notable study reports the growth of wafer-scale single-crystal hBN monolayer, which could mitigate the aforementioned problems caused by polycrystalline hBN films. In this perspective, we discuss the recent progress in the research on single-crystal hBN and the direction to be taken for single-crystal hBN in future. The progress is closely related to the development of a single-crystal substrate and large area of monolayer single-crystal was grown on Cu (111). In terms of the hBN growth, the next step would be to grow multilayer single-crystal hBN, which is expected to expand the scope of applications

Exciton Confinement in Two-Dimensional, In-Plane, Quantum Heterostructures

Two-dimensional (2D) semiconductors are promising candidates for optoelectronic application and quantum information processes due to their inherent out-of-plane 2D confinement. In addition, they offer the possibility of achieving low-dimensional in-plane exciton confinement, similar to zero-dimensional quantum dots, with intriguing optical and electronic properties via strain or composition engineering. However, realizing such laterally confined 2D monolayers and systematically controlling size-dependent optical properties remain significant challenges. Here, we report the observation of lateral confinement of excitons in epitaxially grown in-plane MoSe2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe2 monolayer film via a sequential epitaxial growth process. Various optical spectroscopy techniques reveal the size-dependent exciton confinement in the MoSe2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low temperature as compared to continuous monolayer MoSe2. Finally, single-photon emission was also observed from the smallest dots at 1.6 K. Our study opens the door to compositionally engineered, tunable, in-plane quantum light sources in 2D semiconductors.Comment: Main Manuscript: 29 pages, 4 figures Supplementary Information: 14 pages, 12 figure

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS2/WS2 van der Waals Heterobilayer.

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices

Distinct expression patterns of two Arabidopsis phytocystatin genes, AtCYS1 and AtCYS2, during development and abiotic stresses

The phytocystatins of plants are members of the cystatin superfamily of proteins, which are potent inhibitors of cysteine proteases. The Arabidopsis genome encodes seven phytocystatin isoforms (AtCYSs) in two distantly related AtCYS gene clusters. We selected AtCYS1 and AtCYS2 as representatives for each cluster and then generated transgenic plants expressing the GUS reporter gene under the control of each gene promoter. These plants were used to examine AtCYS expression at various stages of plant development and in response to abiotic stresses. Histochemical analysis of AtCYS1 promoter- and AtCYS2 promoter-GUS transgenic plants revealed that these genes have similar but distinct spatial and temporal expression patterns during normal development. In particular, AtCYS1 was preferentially expressed in the vascular tissue of all organs, whereas AtCYS2 was expressed in trichomes and guard cells in young leaves, caps of roots, and in connecting regions of the immature anthers and filaments and the style and stigma in flowers. In addition, each AtCYS gene has a unique expression profile during abiotic stresses. High temperature and wounding stress enhanced the expression of both AtCYS1 and AtCYS2, but the temporal and spatial patterns of induction differed. From these data, we propose that these two AtCYS genes play important, but distinct, roles in plant development and stress responses

Large-Area Hexagonal Boron Nitride Layers by Chemical Vapor Deposition: Growth and Applications for Substrates, Encapsulation, and Membranes

ConspectusHexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material because of its unique optical properties in the deep-UV region, mechanical robustness, thermal stability, and chemical inertness. Ultrathin hBN layers have gained significant scientific attention for various applications, including nanoelectronics, photonics, single photon emission, anticorrosion, and membranes. For example, the carrier mobility of graphene- and MoS2-based transistors can be improved by using an hBN encapsulation layer, which protects graphene or MoS2from air and/or screens the charge trap site of a substrate. Moreover, deep-UV emitters and detectors have been developed on the basis of the large bandgap of hBN (???6 eV), which exhibits a sharp absorption at approximately 200 nm. Additionally, oxidation of metal surfaces can be prevented by hBN encapsulation, and proton transport can be facilitated by hBN membranes with low gas permeability. Wafer-scale growth of hBN films is crucial to enable their industrial-scale applications. In this regard, chemical vapor deposition (CVD) is a promising method in which scalable high-quality films can be grown at reasonable cost. To date, considerable efforts have been made to develop continuous hBN thin films with high crystallinity, from those with large grains to single-crystal ones, and to realize thickness control of hBN films by CVD. However, the growth of wafer-scale high-crystallinity hBN films with precise thickness control has not been reported yet. The hBN growth is significantly affected by the substrate, in particular the type of metals, because the intrinsic solubilities of boron and nitrogen depend on the type of metal; moreover, control of the grain size and thickness of hBN is difficult. Although growth mechanisms for various substrates have been proposed using various control experiments, a precise growth mechanism has not yet been established through systematic studies. Thus, a deeper understanding of the CVD-based growth of hBN is critical.In this Account, state-of-the-art strategies adopted for growing wafer-scale, highly crystalline hBN are summarized, followed by the proposed mechanisms of hBN growth on catalytic substrates. Furthermore, various applications of the hBN thin films fabricated in our laboratory are demonstrated, including a dielectric layer, an encapsulation layer, a wrapping layer of gold nanoparticles for surface enhanced Raman scattering, a proton-exchange membrane, a template for growth of other 2D materials or nanomaterials, and a platform of fabricating in-plane heterostructures. Finally, the inherent challenges are summarized, and future research directions for the facile CVD-based growth of single-crystal hBN are proposed, including the development of a transfer method that is effective for various applications

Tailoring exciton dynamics in TMDC heterobilayers in the ultranarrow gap-plasmon regime

Abstract Control of excitons in transition metal dichalcogenides (TMDCs) and their heterostructures is fundamentally interesting for tailoring light-matter interactions and exploring their potential applications in high-efficiency optoelectronic and nonlinear photonic devices. While both intra- and interlayer excitons in TMDCs have been heavily studied, their behavior in the quantum tunneling regime, in which the TMDC or its heterostructure is optically excited and concurrently serves as a tunnel junction barrier, remains unexplored. Here, using the degree of freedom of a metallic probe in an atomic force microscope, we investigated both intralayer and interlayer excitons dynamics in TMDC heterobilayers via locally controlled junction current in a finely tuned sub-nanometer tip-sample cavity. Our tip-enhanced photoluminescence measurements reveal a significantly different exciton-quantum plasmon coupling for intralayer and interlayer excitons due to different orientation of the dipoles of the respective e-h pairs. Using a steady-state rate equation fit, we extracted field gradients, radiative and nonradiative relaxation rates for excitons in the quantum tunneling regime with and without junction current. Our results show that tip-induced radiative (nonradiative) relaxation of intralayer (interlayer) excitons becomes dominant in the quantum tunneling regime due to the Purcell effect. These findings have important implications for near-field probing of excitonic materials in the strong-coupling regime