Intralayer Negative Poisson's Ratio in Two-Dimensional Black Arsenic by Strain Engineering

Negative Poisson's ratio as the anomalous characteristic generally exists in artificial architectures, such as re-entrant and honeycomb structures. The structures with negative Poisson's ratio have attracted intensive attention due to their unique auxetic effect and many promising applications in shear resistant and energy absorption fields. However, experimental observation of negative Poisson's ratio in natural materials barely happened, although various two-dimensional layered materials are predicted in theory. Herein, we report the anisotropic Raman response and the intrinsic intralayer negative Poisson's ratio of two-dimensional natural black arsenic (b-As) via strain engineering strategy. The results were evident by the detailed Raman spectrum of b-As under uniaxial strain together with density functional theory calculations. It is found that b-As was softer along the armchair than zigzag direction. The anisotropic mechanical features and van der Waals interactions play essential roles in strain-dependent Raman shifts and negative Poisson's ratio in the natural b-As along zigzag direction. This work may shed a light on the mechanical properties and potential applications of two-dimensional puckered materials.Comment: 23 pages, 4 figure

Bottom-Up Growth of Graphene Nanospears and Nanoribbons

One dimensional graphene nanostructures are one of the most promising materials for next generation electronics. Here, the chemical vapor depostion growth of graphene nanoribbons (GNRs) and graphene nanospears (GNSs) on a copper surface is reported. The growth of GNRs and GNSs is enabled by a vapor-liquid-solid (VLS) mechanism guided by on-surface propagation of a liquid Cu-Si catalyst particle. The slow lateral growth and the fast VLS vertical growth give rise to spear head-shaped GNSs. In situ observations further confirm that the lateral graphene growth can be completely suppressed and thus GNRs are grown. The synthesized field effect transistor (FET) devices show that the GNRs and GNSs have high carrier mobilities of approximate to 2000 cm(2) V-1 s(-1). Both FET and Kelvin probe force microscopy measurements confirm that the Fermi levels of the synthesize GNSs shift downward from the wide part to the tip is strongly p-doped. These findings yield key insights into the growth mechanism of graphene and open a door for achieving a facile and scalable method of synthesizing free standing GNRs and GNSs and their applications, such as the Fermi-level tunable devices

TWO-DIMENSIONAL MATERIAL-SUBSTRATE INTERACTIONS FOR EPITAXIAL GROWTH

Department of Materials Science and EngineeringDue to their unique dimensionality, two-dimensional (2D) materials exhibit many excellent electronic, magnetic, mechanical and thermal properties that are absent from their 3D counterparts, which makes them hold great potential in various applications, such as electronics, optoelectronics and photovoltaics, etc. To maximize the advantages of 2D materials and realize their practical applications in 2D devices, synthesizing single crystals of the 2D materials with a wafer scale is highly required. Over the past decade, various strategies for synthesizing 2D materials have been developed, and there are mainly two routes towards the fabrication of 2D single crystals especially with a wafer scale via chemical vapor deposition (CVD): (i) nucleating only one 2D nucleus on the whole substrate and growing it to wafer scale and (ii) seamless stitching of a large number of unidirectionally aligned 2D material islands on a substrate. Although route (i) can produce wafer scale single crystalline (WSSC) 2D materials with a very high quality, the synthesis process usually requires tens of hours and delicate experimental setups. Route (ii) is much more cost-effective because the synthesis process is mediated by the simultaneous growth of all the 2D islands, while it requires substrates that can template the synthesis of unidirectionally aligned 2D islands. For route (ii), an in-depth understanding on the alignment of 2D materials on substrates is of critical importance for choosing proper substrates that can template the synthesis of unidirectionally aligned 2D islands. Experimentally, various 2D material islands with both multi- and mono- alignment orientations have been observed on different substrates, including unidirectionally aligned graphene islands on Cu(111) and Cu(110) substrates, graphene islands with two orientations on Cu(100) substrates, multi-orientations of hexagonal boron nitride (hBN) islands on all low-index Cu substrates, unidirectionally aligned hBN islands on vicinal Cu(110) surfaces and on stepped Cu(111) surfaces, unidirectionally aligned MoS2 islands on vicinal Au(111) surfaces, etc. Up to now, WSSC graphene, hBN and MoS2 have been successfully synthesized by route (ii) on Cu(111) substrates, vicinal Cu(110) or stepped Cu(111) surfaces, and vicinal Au(111) surfaces, respectively. In principle, the interactions between 2D materials and their substrates are responsible for the epitaxial growth of the 2D materials and the behaviors of the 2D materials on the substrates after growth. However, up to now, an in-depth understanding on the alignment mechanism of 2D materials on substrates at atomic scale is still lacking. In this dissertation, we carry out theoretical investigations on the alignment of 2D materials on both high-symmetric and low symmetric transition metal (TM) surfaces. Firstly, we find that a high-symmetric direction of a 2D material usually prefers to align along a high symmetric direction of the substrate, which determines the most stable configuration of a 2D material island on the substrate. Moreover, we reveal that the interplay between the symmetry of the 2D material and that of the substrate determines the alignments of 2D islands on the substrate. To grow unidirectionally aligned 2D islands, the most stable orientation of the 2D material on the substrate cannot be changed by any symmetrical operation of the substrate, i.e., the symmetry group of the substrate should be a subgroup of that of the 2D material. Therefore, low symmetric TM surfaces are more promising for templating unidirectional 2D islands. This part is presented in Chapter 4. We then further investigated the alignment of 2D materials on an arbitrary high-index low symmetric TM substrate in Chapters 5-9. The high-index low symmetric FCC TM substrates are classified into three categories according to their surface configuration, which are FCC{111}-based, FCC{100}-based and FCC{110}-based low symmetric surfaces, and the alignment of various 2D materials, including graphene, hBN and transition metal dichalcogenides (TMDCs) monolayers, on these three types of low symmetric FCC substrates are systematically explored by density functional theory (DFT) calculations. It is revealed that the interaction differences between various edges of a 2D material and a unidirectional TM step edge determines the sole orientation of the 2D island along this step edge. However, TM step edges usually present a meandering direction in real cases because of surface roughness, the alignments of 2D islands along such meandering step edges are then investigated by structural analysis. We find that the similar kink heights of substrate step edges and the edges of the 2D island are critical for the unidirectional alignment of 2D islands along meandering step edges. Besides, the orientations of hBN and TMDCs on substrates are also affected by the ambient conditions in experiments due to their binary compositions. As a promising substrate for the epitaxial growth of graphene, Cu{111} foils attract lots of attentions. By collaborating with experimental groups, a single crystalline Cu{111} foil with a size up to 32 cm2 is realized using a contact-free annealing method. We theoretically revealed the transition mechanism of single crystalline Cu{111} from polycrystalline Cu foils by classical molecular dynamic (MD) simulations combined with DFT calculations in Chapter 10. It is found that the high annealing temperature and Cu{112} grains in the raw Cu foils are essential for obtaining the single crystalline Cu{111} foil. After growth, 2D materials usually show novel moir?? structures and properties modulated by the underlying substrate, and in Chapter 11 of this dissertation, the behaviors of graphene on various TM substrates that have a large lattice mismatch with graphene are explored by the DFT calculations. Depending on the rotation angles between graphene and the substrate, two kinds of graphene moir?? structures are revealed, i.e., highly corrugated graphene moir?? superstructures under small rotation angles and ultra-flat graphene layers under large rotation angles, and we further find that such different behaviors of graphene are determined by the competition between the graphene-substrate interaction and curvature energy of the graphene/TM superstructures. Compared with the ultra-flat ones, the corrugated graphene/TM superstructures show anisotropic properties and are found to be capable of templating size-tunable metal clusters. We finally formulate the evolution of the graphene-substrate interaction and curvature energy, and the morphology of graphene on various TM substrate can be estimated effectively.clos

Mechanism of Corrugated Graphene Moiré Superstructures on Transition-Metal Surfaces

A graphene layer on a transition-metal (TM) surface can be either corrugated or flat, depending on the type of the substrate and its rotation angle with respect to the substrate. It was broadly observed that the degree of corrugation generally decreases with the increase of rotation angle or the decrease of Moiré pattern size. In contrast to a flat graphene on a TM surface, a corrugated graphene layer has an increased binding energy to the substrate and a concomitant elastic energy. Here, we developed a theoretical model about the competition between the binding energy increase and the elastic energy of corrugated graphene layers on TM surfaces in which all the parameters can be calculated by density functional theory (DFT) calculations. The agreement between the theoretical model and the experimental observations of graphene on various TM surfaces, for example, Ru(0001), Rh(111), Pt(111), and Ir(111), substantiated the applicability of this model for graphene on other TM surfaces. Moreover, the morphology of a graphene layer on an arbitrary TM surface can be theoretically predicted through simple DFT calculations based on the model. Our work thus provides a theoretical framework for the intelligent design of graphene/TM superstructures with the desired structure.11Nsciescopu

The stable interfaces between various edges of hBN and step edges of Cu surface in hBN epitaxial growth: a comprehensive theoretical exploration

High-index Cu surfaces were broadly shown to be substrates capable for templating the epitaxial growth of uniformly aligned hexagonal boron nitride (hBN) islands whereas the mechanism of hBN growth on high-index Cu surfaces is still missing. Since hBN nucleation prefers step edges on a high-index Cu surface, the understanding of the interfaces between the hBN edges and the step edges of Cu substrates is critical for revealing the mechanism of hBN epitaxial growth on high-index Cu surfaces. Our extensive theoretical study reveals that both types of zigzag edges and armchair edge tend to retain their pristine structures on a Cu surface due to the effective passivation of the dangling bonds of hBN edges. This study paves a way to explore the growth kinetics of hBN on high-index Cu surfaces and also sheds light on the growth mechanisms of various two-dimensional materials on active metal substrates

Epitaxial Growth of 2D Materials on High-Index Substrate Surfaces

Recently, the successful synthesis of wafer-scale single-crystal graphene, hexagonal boron nitride (hBN), and MoS2 on transition metal surfaces with step edges boosted the research interests in synthesizing wafer-scale 2D single crystals on high-index substrate surfaces. Here, using hBN growth on high-index Cu surfaces as an example, a systematic theoretical study to understand the epitaxial growth of 2D materials on various high-index surfaces is performed. It is revealed that hBN orientation on a high-index surface is highly dependent on the alignment of the step edges of the surface as well as the surface roughness. On an ideal high-index surface, well-aligned hBN islands can be easily achieved, whereas curved step edges on a rough surface can lead to the alignment of hBN along with different directions. This study shows that high-index surfaces with a large step density are robust for templating the epitaxial growth of 2D single crystals due to their large tolerance for surface roughness and provides a general guideline for the epitaxial growth of various 2D single crystals

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS2 have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials

The alignment-dependent properties and applications of graphene moire superstructures on the Ru(0001) surface

The moire superstructure of graphene on a lattice-mismatched metal substrate has profound effects on the electronic properties of graphene and can be used for many applications. Here, we propose to systematically tune the moire superstructure of graphene on the Ru(0001) surface by rotating the graphene layer. Our study reveals two kinds of graphene moire superstructures: (i) the ultra-flat graphene layers with height variations of less than 0.1 angstrom for rotation angles greater than 20 degrees that have the same structural and electronic properties everywhere, and (ii) the highly corrugated graphene moire superstructures with height variations from 0.4 to 1.6 angstrom for rotation angles less than 20 degrees, whose electronic properties are highly modulated by the interaction with the substrate. Moreover, these rotated graphene moire superstructures can serve as templates to produce matrices of size-tunable metal clusters from a few to similar to 100 atoms. This study reveals the causes of the structural fluctuation of moire superstructures of graphene on the transition metal surface and suggests a pathway to tune graphene's electronic properties for various applications

Theoretical Study of Chemical Vapor Deposition Synthesis of Graphene and Beyond: Challenges and Perspectives

Two-dimensional (2D) materials have attracted great attention in recent years because of their unique dimensionality and related properties. Chemical vapor deposition (CVD), a crucial technique for thin-film epitaxial growth, has become the most promising method of synthesizing 2D materials. Different from traditional thin-film growth, where strong chemical bonds are involved in both thin films and substrates, the interaction in 2D materials and substrates involves the van der Waals force and is highly anisotropic, and therefore, traditional thin-film growth theories cannot be applied to 2D material CVD synthesis. During the last 15 years, extensive theoretical studies were devoted to the CVD synthesis of 2D materials. This Perspective attempts to present a theoretical framework for 2D material CVD synthesis as well as the challenges and opportunities in exploring CVD mechanisms. We hope that this Perspective can provide an in-depth understanding of 2D material CVD synthesis and can further stimulate 2D material synthesis