Formation of Two-Dimensional AgTe Monolayer Atomic Crystal on Ag(111) Substrate

We report on the formation of two-dimensional monolayer AgTe crystal on Ag(111) substrates. The samples are prepared in ultrahigh vacuum by deposition of Te on Ag(111) followed by annealing. Using a scanning tunneling microscope(STM) and low electron energy diffraction(LEED), we investigate the atomic structure of the samples.The STM images and the LEED pattern show that monolayer AgTe crystal is formed on Ag(111). Four kinds of atomic structures of AgTe and Ag(111) are observed:(i) flat honeycomb structure,(ii) bulked honeycomb,(iii)stripe structure,(iv) hexagonal structure. The structural analysis indicates that the formation of the different atomic structures is due to the lattice mismatch and relief of the intrinsic strain in the AgTe layer. Our results provide a simple and convenient method to produce monolayer AgTe atomic crystal on Ag(111) and a template for study of novel physical properties and for future quantum devices.     

Prediction of 2D Li2X (X=Se,Te) monolayer semiconductors by first principles calculations

Two dimensional monolayer materials play important roles in new generation of electronic and optical devices in nano scale. In this paper, by using first principles calculations, the existence of 2D Li₂X (X=Se, Te) monolayer materials are theoretically predicted. Through cohesive energy calculation and phonon dispersion simulation, it is proved that the proposed 2D Li₂Se and Li₂Te monolayer materials are energetically and dynamically stable suggests their potential experimental realization. Our study shows that these newly predicted compounds are direct semiconductors and have strain tunable wide band gaps. As direct semiconductors, these new monolayers may have many applications in electronics and optoelectronics devices.     

GeSb_2Te_4相变光存储薄膜材料的短波长静态记录特性的研究

研究了单层ＧｅＳｂ２Ｔｅ４真空射频溅射薄膜在４００ｎｍ～８３０ｎｍ区域的吸收、反射光谱和光学常数（ｎ，ｋ），发现ＧｅＳｂ２Ｔｅ４薄膜在４００ｎｍ～６００ｎｍ波长范围内具有较强的吸收。在短波长静态测试仪上测试了ＧｅＳｂ２Ｔｅ４薄膜的光存储记录特性，发现在５１４．５ｎｍ波长用较低功率的激光辐照样品时薄膜在写入前后的反射率变化较大，擦除前后的反射率对比度较低，可通过膜层设计来提高     

Molecular beam epitaxy growth of monolayer hexagonal MnTe2 on Si(111) substrate

Monolayer MnTe₂ stabilized as 1T structure has been theoretically predicted to be a two-dimensional (2D) ferromagnetic metal and can be tuned via strain engineering. There is no naturally van der Waals (vdW) layered MnTe₂ bulk, leaving mechanical exfoliation impossible to prepare monolayer MnTe₂. Herein, by means of molecular beam epitaxy (MBE), we successfully prepared monolayer hexagonal MnTe₂ on Si(111) under Te rich condition. Sharp reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) patterns suggest the monolayer is atomically flat without surface reconstruction. The valence state of Mn⁴⁺ and the atom ratio of ([Te]:[Mn]) further confirm the MnTe₂ compound. Scanning tunneling spectroscopy (STS) shows the hexagonal MnTe₂ monolayer is a semiconductor with a large bandgap of ~2.78 eV. The valence-band maximum (VBM) locates at the Γ point, as illustrated by angle-resolved photoemission spectroscopy (ARPES), below which three hole-type bands with parabolic dispersion can be identified. The successful synthesis of monolayer MnTe₂ film provides a new platform to investigate the 2D magnetism.     

Molecular beam epitaxy growth of iodide thin films

Study of two-dimensional(2D)magnetic materials is important for both fundamental research and application.Here we report molecular beam epitaxy growth of iodides,candidates for exhibiting 2D magnetism.Decomposition of CrI₃is utilized to produce stable gaseous I₂flux.Growth of MnI₂,GdI₃,and CrI₂down to monolayer is successful achieved by co-depositing I2 and corresponding metal atoms.The thin films of the three materials are characterized by scanning tunneling microscope and found to be insulators with bandgaps of 4.4 e V,0.6 e V,and 3.0 e V,respectively.The film growth paves the way for further study of magnetic properties at the 2 D limit.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Metal-to-insulator transition in two-dimensional ferromagnetic monolayer induced by substrate

Two-dimensional(2D) ferromagnetic(FM) materials have great potential for applications in next-generation spintronic devices. Since most 2D FM materials come from van der Waals crystals, stabilizing them on a certain substrate without killing the ferromagnetism is still a challenge. Through systematic first-principles calculations, we proposed a new family of 2D FM materials which combines TaX(X = S, Se or Te) monolayer and Al_2O_3(0001) substrate. The TaX monolayers provide magnetic states and the Al_2O_3(0001) substrate stabilizes the former. Interestingly, the Al_2O_3(0001)substrate leads to a metal-to-insulator transition in the Ta X monolayers and induces a band gap up to 303 meV. Our study paves the way to explore promising 2D FM materials for practical applications in spintronics devices.     

Controlled vapor growth of 2D magnetic Cr_2Se_3 and its magnetic proximity effect in heterostructures

Two-dimensional(2 D) magnetic materials have aroused tremendous interest due to the 2 D confinement of magnetism and potential applications in spintronic and valleytronic devices. However, most of the currently 2 D magnetic materials are achieved by the exfoliation from their bulks, of which the thickness and domain size are difficult to control, limiting the practical device applications. Here, we demonstrate the realization of thickness-tunable rhombohedral Cr_2Se_3 nanosheets on different substrates via the chemical vapor deposition route. The magnetic transition temperature at about 75 K is observed. Furthermore, van der Waals heterostructures consisting of Cr_2Se_3 nanosheets and monolayer WS_2 are constructed.We observe the magnetic proximity effect in the heterostructures, which manifests the manipulation of the valley polarization in monolayer WS_2. Our work contributes to the vapor growth and applications of 2 D magnetic materials.     

Theoretical prediction of topological insulators in two-dimensional ternary transition metal chalcogenides (MM'X4, M = Ta,Nb, or V; M'= Ir,Rh, or Co; X = Se or Te)

Ternary transition metal chalcogenides (TTMCs) have attracted interest due to the discovery of their Weyl semimetallic property and the recent synthesis of layered TTMCs which are regarded as potential candidates for two-dimensional (2D) topological insulators. Here, employing first-principles calculations, we predicted the emergence of non-trivial band topologies in the monolayer MM'X₄ family (M= V, Nb, or Ta; M' = Co, Rh, or Ir; and X = Se or Te) within hybrid functional calculations. Five of eighteen 2D materials were found to be topological insulators, while four of them are magnetic thin films. The nontrivial topologies were verified via the calculated Z₂ topological invariant and topologically protected edge states. Further calculations showed a strain-induced phase transition in VCoTe₄ from a magnetic phase to a nonmagnetic topological insulating phase. Our comprehensive study revealed a diverse family of monolayer ternary transition metal chalcogenides adding new members to the current catalog of 2D topological insulators and 2D magnetic materials.     

Electronic conduction through 2D arrays of nanometer diameter metal clusters

An experimental study of electrical conduction through arrays of nanometer-diameter metallic clusters linked by organic molecules is presented. Gold clusters, having diameters of ∼4 nm and encapsulated by a monolayer of dodecanethiol, are deposited from solution on to specially prepared substrates to form a close-packed cluster monolayer. Nearest-neighbors in this 2D array of encapsulated clusters are then covalently linked using a conjugated organic molecule approximately 2.2 nm in length having isocyanide groups at both ends. In order to allow both electrical characterization and TEM imaging, the cluster arrays are deposited in 500 nm wide gaps between gold contacts on a free standing, insulating SiO₂ film. Electronic conduction through linked 2D arrays approximately 80 clusters in length has been observed at room temperature. The structure of the arrays and current-voltage relationships for the linked arrays are presented.     

Experimental Synthesis of Strained Monolayer Silver Arsenide on Ag(111)Substrates

Two-dimensional(2 D) materials are playing more and more important roles in both basic sciences and industrial applications. For 2 D materials, strain could tune the properties and enlarge applications. Since the growth of 2 D materials on substrates is often accompanied by strain, the interaction between 2 D materials and substrates is worthy of careful attention. Here we demonstrate the fabrication of strained monolayer silver arsenide(AgAs) on Ag(111) by molecular beam epitaxy, which shows one-dimensional stripe structures arising from uniaxial strain.The atomic geometric structure and electronic band structure are investigated by low energy electron diffraction,scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and first-principle calculations. Monolayer AgAs synthesized on Ag(111) provides a platform to study the physical properties of strained 2 D materials.     

高质量单层二硫化钼薄膜的研究进展

作为一种新型的二维半导体材料,单层二硫化钼薄膜由于其优异的特性,在电子学与光电子学等众多领域具有潜在的应用价值.本文综述了我们课题组在过去几年中针对单层二硫化钼薄膜的研究所取得的进展,具体包括:在二硫化钼薄膜的制备方面,通过氧辅助化学气相沉积方法,实现了大尺寸单层二硫化钼单晶的可控生长和晶圆级单层二硫化钼薄膜的高定向外延生长;在二硫化钼薄膜的加工方面,发展了单层二硫化钼薄膜的无损转移、洁净图案化加工、可控结构相变与局域相调控的方法,为场效应晶体管等电子学器件的制备与性能优化提供了基础;在二硫化钼异质结方面,研究了二硫化钼薄膜与其他二维材料形成的异质结的电学以及光电性质,为二维材料异质结的构筑和器件特性研究提供了实验参考;在二硫化钼薄膜功能化器件与应用方面,构筑了全二维材料、亚5 nm超短沟道场效应晶体管器件,验证了单层二硫化钼对短沟道效应的有效抑制及其在5 nm工艺节点器件中的应用优势;此外,利用制备的高质量单层二硫化钼和发展的器件洁净加工技术,实现了高性能柔性薄膜晶体管的集成,获得了超高灵敏度与稳定性的非接触型湿度传感器.我们在二硫化钼薄膜的制备、加工以及器件特性研究方面所取得的进展对于二硫化钼及其他二维过渡金属硫属化合物的基础和应用研究均具有指导意义.     

Electronic properties of monolayer copper selenide with one-dimensional moiré patterns

Strain engineering is a vital way to manipulate the electronic properties of two-dimensional (2D) materials. As a typical representative of transition metal mono-chalcogenides (TMMs), a honeycomb CuSe monolayer features with one-dimensional (1D) moiré patterns owing to the uniaxial strain along one of three equivalent orientations of Cu(111) substrates. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/S) experiments and density functional theory (DFT) calculations, we systematically investigate the electronic properties of the strained CuSe monolayer on the Cu(111) substrate. Our results show the semiconducting feature of CuSe monolayer with a band gap of 1.28 eV and the 1D periodical modulation of electronic properties by the 1D moiré patterns. Except for the uniaxially strained CuSe monolayer, we observed domain boundary and line defects in the CuSe monolayer, where the biaxial-strain and strain-free conditions can be investigated respectively. STS measurements for the three different strain regions show that the first peak in conduction band will move downward with the increasing strain. DFT calculations based on the three CuSe atomic models with different strain inside reproduced the peak movement. The present findings not only enrich the fundamental comprehension toward the influence of strain on electronic properties at 2D limit, but also offer the benchmark for the development of 2D semiconductor materials.     

Multilayer adsorption and wetting of acetone on graphite

Adsorption/desorption isotherms of acetone on highly oriented pyrolytic graphite have been measured by ellipsometry for temperatures above the bulk triple point. The behavior in the monolayer and submonolayer regime is conventional, with 2D gas-liquid and 2D liquid-solid coexistence regions. Further liquid monolayers grow on top of the completed monolayer. The growth is basically layer-by-layer. For temperatures between 190 K and the triple point a prewetting-type transition occurs with a thin-thick jump of the layer thickness on adsorption but a layer-wise removal of the film on desorption. In this temperature regime the first monolayer is solid and its molecules are oriented perpendicular to the substrate whereas the higher layers are orientationally disordered polar liquid.     

聚苯乙烯芪盐L—B膜的二阶非线性光学性质研究

首次合成了一种称之为聚苯乙烯芪盐的有机高分子材料.此材料可以在水面上形成稳定的单分子层膜,并且所形成的单分子层膜可以转移到固体衬底上去.对聚苯乙烯芪盐单分子层膜进行光学二次谐波产生测量,得到聚苯乙烯芪盐对应于每个发色团的二阶非线性极化率为1.2×10~(-28)esu.制备了该材料两种不同类型的多层膜,讨论了多层膜二次谐波产生信号不随层数增加而增加的可能原因.     

Electronic structures and topological properties of TeSe2 monolayers

The successfully experimental fabrication of two-dimensional Te monolayer films [Phys. Rev. Lett. 119 106101 (2017)] has promoted the researches on the group-VI monolayer materials. In this work, the electronic structures and topological properties of a group-VI binary compound of TeSe₂ monolayers are studied based on the density functional theory and Wannier function method. Three types of structures, namely, α-TeSe₂, β-TeSe₂, and γ-TeSe₂, are proposed for the TeSe₂ monolayer among which the α-TeSe₂ is found being the most stable. All the three structures are semiconductors with indirect band gaps. Very interestingly, the γ-TeSe₂ monolayer becomes a quantum spin Hall (QSH) insulator with a global nontrivial energy gap of 0.14 eV when a 3.5% compressive strain is applied. The opening of the global band gap is understood by the competition between the decrease of the local band dispersion and the weakening of the interactions between the Se p_x, p_y orbitals and Te p_x, p_y orbitals during the process. Our work realizes topological states in the group-VI monolayers and promotes the potential applications of the materials in spintronics and quantum computations.     

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge₂Sb₂Te₅ (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In₂Te₃ crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \) . Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.     

Photoemission studies of the interactions of CdTe and Te with Si(100)

The interactions between CdTe, and in particular Te, and the (100) surface of Si have been probed using photoemission and low energy electron diffraction with a view to investigating the mechanisms responsible for (100) and (111) growth orientations for CdTe on Si(100). The interfacial reactions have been studied both on room temperature deposition followed by annealing and on depositions at typical epitaxial growth temperatures. In both cases the same precursor stage of an ordered submonolayer of Te on the Si(100) surface has been identified. Line shape analysis of the Si 2p core level has suggested a structural model in which Te adatoms make up an incomplete monolayer bound in bridge sites. This model is in excellent agreement both with the (1 × 1) LEED pattern and recent SEXAFS studies of this surface. The implications of the cubic symmetry of this surface in terms of the subsequent growth orientation of CdTe are discussed. Termination of the surface by Te was also seen to induce band bending suggestive of Fermi level pinning at around midgap, in contrast to the passivating behaviour of other group VI elements on this surface. The Si 2p core level line shape analysis on termination by Te has also provided evidence to support the “covalent dimer” interpretation of the clean dimerised Si(100) surface.     

The rise of two-dimensional tellurium for next-generation electronics and optoelectronics

Single-element two-dimensional (2D) tellurium (Te) which possesses an unusual quasi-one-dimensional atomic chain structure is a new member in 2D materials family. 2D Te possesses high carrier mobility, wide tunable bandgap, strong light-matter interaction, better environmental stability, and strong anisotropy, making Te exhibit tremendous application potential in next-generation electronic and optoelectronic devices. However, as an emerging 2D material, the research on fundamental property and device application of Te is still in its infancy. Hence, this review summarizes the most recent research progresses about the new star 2D Te and discusses its future development direction. Firstly, the structural features, basic physical properties, and various preparation methods of 2D Te are systemically introduced. Then, we emphatically summarize the booming development of 2D Te-based electronic and optoelectronic devices including field effect transistors, photodetectors and van der Waals heterostructure photodiodes. Finally, the future challenges, opportunities, and development directions of 2D Te-based electronic and optoelectronic devices are prospected.