Monolayer 1T-VSe2 has been reported as a room-temperature ferromagnet. In this work, by using first-principles calculations, we unveil that the ferromagnetism in monolayer 1T-VSe2 is originated from its intrinsic huge Stoner instability enhanced by the confinement effect, which can eliminate the interlayer coupling, and lead to a drastic increase of the density of states at the Fermi level due to the presence of Van Hove singularity. Our calculations also demonstrate that the Stoner instability is very sensitive to the interlayer distance. These results provide a useful route to modulate the nonmagnetic to ferromagnetic transition in few-layers or bulk 1T-VSe2, which also shed light on the enhancement of its Curie temperature by enlarging the interlayer distance.
Based on first-principles calculations, we predict that the monolayer AuTe2Cl is a quantum spin Hall (QSH) insulator with a topological band gap about 10 meV. The three-dimensional (3D) AuTe2Cl is a topological semimetal that can be viewed as the monolayer stacking along b axis. By studying the energy level distribution of p orbitals of Te atoms for the bulk and the monolayer, we find that the confinement effect driven p_y^- and p_z^+ band inversion is responsible for the topological nontrivial nature of monolayer. Since 3D bulk AuTe2Cl has already been experimentally synthesized, we expect th…
The effect of structural constriction on molecular mobility is investigated by broadband dielectric spectroscopy (BDS) within three types of molecular arrangements: monomers, oligomers and self-assembled monolayers (SAMs). While disordered monomers exhibit a variety of cooperative and local relaxation processes, the constrained nanodomains of oligomers and highly ordered structure of monolayers exhibit much hindered local molecular fluctuations. Particularly, in SAMs, motions of the silane headgroups are totally prevented whereas the polar endgroups forming the monolayer canopy show only one c…
To obtain high-performance spintronic devices with high integration density, two-dimensional (2D) half-metallic materials are eagerly pursued all along. Here, we propose a stable 2D material with a honeycomb-kagome lattice, i.e., the Mg3C2 monolayer, based on first-principles calculations. This monolayer is an anti-ferromagnetic (AFM) semiconductor at its ground state. We further demonstrate that a transition from AFM semiconductor to ferromagnetic half-metal in this 2D material can be induced by carrier (electron or hole) doping. This magnetic transition can be understood by the Stoner criter…
We have employed one of the well-known many-body techniques, density functional theory plus dynamical mean-field theory (DFT + DMFT), to investigate the electronic structure of ferromagnetic monolayer CrI3 as a function of temperature and hole-doping concentration. The computed magnetic susceptibility follows the Curie's law, indicating that the ferromagnetism of monolayer CrI3 originates from localized magnetic moments of Cr atoms rather than Stoner-type itinerant ones. The DFT + DMFT calculations show a different coherent temperature for each spin component, demonstrating apparent strong spi…
Density functional theory (DFT) calculations are used to investigate the electronic and magnetic structures of a two-dimensional (2D) monolayer Li$_{2}$N. It is shown that bulk Li$_{3}$N is a non-magnetic semiconductor. The non-spinpolarized DFT calculations show that $p$ electrons of N in 2D Li$_{2}$N form a narrow band at the Fermi energy $E_{\rm{F}}$ due to a low coordination number, and the density of states at the Fermi energy ($g(E_{\rm{F}}$)) is increased as compared with bulk Li$_{3}$N. The large $g(E_{\rm{F}}$) shows instability towards magnetism in Stoner's mean field model. The spin…
The surface a thin-film is attached to and the surrounding monolayer causes geometrical confinement of a interrogated molecule; we look at the base case of a SC$_{18}H_{37}$ in a SC$_{18}H_{37}$ monolayer on Au[111]. Normal mode analysis was used to get vibrations, and these are analysed using mode character indicators which can quantify: how active an element is in a mode; the overall direction of the mode; and which chemical coordinates are relevant. We examined the 4 possible packing structures. We find that the more thermodynamically stable structures were less perturbed by the surface and…
Predictive first-principles calculations suggest graphitic-like GaN to be theoretically possible. Thus far, it has not been experimentally reported. We report on GaN monolayer in a buckled geometry obtained in confinement at graphene/SiC interface by metalorganic chemical vapor deposition (MOCVD). Conductive atomic force microscopy (C-AFM) has been employed to probe vertical current injection through the graphene/SiC interface and to establish the uniformity of the intercalated areas. Scanning transmission electron microscopy (S/TEM) has been employed for atomic resolution imaging and spectros…
Ionic layer epitaxy (ILE) has recently been developed as an effective strategy to synthesize nanometer-thick 2D materials with a non-layered crystal structure, such as ZnO. The packing density of the amphiphilic monolayer is believed to be a key parameter that controls the nanosheet nucleation and growth. In this work, we systematically investigated the growth behavior of single-crystalline ZnO nanosheets templated at the water-air interface by an anionic oleylsulfate monolayer with different packing densities. The thicknesses of ZnO nanosheets were tuned from one unit cell to four unit cells,…
We propose a two-dimensional crystal which possesses low indirect band gaps of 0.55 eV (monolayer) and 0.43 eV (bilayer) and high carrier mobilities similar to those of phosphorene: GeP3. GeP3 has a stable three-dimensional layered bulk counterpart which is metallic and is known from experiment since 1970. It has a small cleavage energy, which suggests exfoliation of bulk material as viable means for the preparation of mono- and few-layer materials. The material shows strong interlayer quantum confinement effects, resulting in a band gap reduction from mono- to bilayer, and then to a semicondu…
Atomically-precise synthesis and simultaneous integration of 2D transition metal dichalcogenides enabled by nano-confinement
Beijing National Center for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Suzhou Laboratory（苏州实验室）；Institute of Technology for Carbon Neutrality, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences（中国科学院深圳先进技术研究院碳中和技术研究所）；Frontier Institute of Science and Technology, Xi’an Jiaotong University（西安交通大学前沿科学中心）；Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University（清华大学工程力学系微纳力学与微系统教育部重点实验室）；Faculty of Materials Science and Energy Engineering, Shenzhen University of Advanced Technology（深圳先进材料能源学院）
Phase change memory (PCM) is an emerging data storage technology, however its programming is thermal in nature and typically not energy-efficient. Here we reduce the switching power of PCM through the combined approaches of filamentary contacts and thermal confinement. The filamentary contact is formed through an oxidized TiN layer on the bottom electrode, and thermal confinement is achieved using a monolayer semiconductor interface, three-atom thick MoS2. The former reduces the switching volume of the phase change material and yields a 70% reduction in reset current versus typical 150 nm diam…
In monolayer transition metal dichalcogenides, quantum emitters are associated with localized strain that can be deterministically applied to create designer nano-arrays of single photon sources. Despite an overwhelming empirical correlation with local strain, the nanoscale interplay between strain, excitons, defects and local crystalline structure that gives rise to these quantum emitters is poorly understood. Here, we combine room-temperature nano-optical imaging and spectroscopy of excitons in nanobubbles of localized strain in monolayer WSe2 with atomistic structural models to elucidate ho…
Electronic properties of quantum dots (QDs) depend sensitively on their parent materials. Therefore, confined electronic states in graphene QDs (GQDs) of monolayer and Bernal-stacked bilayer graphene are quite different. Twisted bilayer graphene (TBG) is distinct from monolayer and Bernal-stacked bilayer graphene because of the new degree of freedom: twist angle. In the past few years, numerous efforts have been made to realize the GQDs of monolayer and Bernal-stacked bilayer graphene and achieved great success. Thus far, however, strategies for realizing GQDs of TBG have been elusive. Here, w…
Materials with van der Waals-bonding are known to exhibit quantum confinement effect, in which the electronic bandgap of the three-dimensional (3D) realization of a material is lower than that of its two-dimensional (2D) counterpart. However, the possibility of an anomalous quantum confinement effect (AQCE) exists, where the bandgap trend is reversed. In this work, we computationally identify materials for which such AQCE occurs. Using density functional theory (DFT), we compute ~1000 OptB88vdW (semi-local functional), ~50 HSE06 and ~50 PBE0 (hybrid functional) bandgaps for bulk and their corr…
Advanced hydrogen lithography techniques and low-temperature epitaxial overgrowth enable patterning of highly phosphorus-doped silicon (Si:P) monolayers (ML) with atomic precision. This approach to device fabrication has made Si:P monolayer systems a testbed for multiqubit quantum computing architectures and atomically precise 2-D superlattice designs whose behaviors are directly tied to the deterministic placement of single dopants. However, dopant segregation, diffusion, surface roughening, and defect formation during the encapsulation overgrowth introduce large uncertainties to the exact do…
The combined effects of defect-defect interaction and of uniaxial or biaxial strains of up to 10\% on the development of magnetic states on the defect-core-localized quasi-one-dimensional electronic states generated by the so-called 558 linear extended defect in graphene monolayers are investigated by means of {\it ab initio} calculations. Results are analyzed on the basis of the heuristics of the Stoner criterion. We find that conditions for the emergence of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel strains (along the defect direction) at both limits of isola…
Two-dimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IV-VI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap below a critical thickness, turning TCIs into normal insulators. Surprisingly, based on first-principles calculations, here we demonstrate that (001) monolayers of rocksalt IV-VI semiconductors XY (X=Ge, Sn, Pb and Y= S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in mo…
It is shown that for monolayer graphene electrons are confined on a perfect two dimensional surface. The implications for the electronic properties of corrugated graphene are discussed in view of a derivation of the constrained relativistic dynamics for the massless carriers in two dimensions. Surface curvature is related to a series of phenomena with practical applications such as curvature induced p-n junctions, band gap opening and decoherence. We also establish a bending free energy by treating graphene as a soft electronic membrane.
Quantum confinement profoundly affects the properties and interactions of electrons, holes, and excitons in nanomaterials. We apply first-principles calculations to study the effects of extreme quantum confinement on the electronic, excitonic, and radiative properties of atomically thin GaN quantum wells with a thickness of 1 to 4 atomic monolayers embedded in AlN. We determine the quasiparticle band gaps, exciton energies and wave functions, radiative lifetimes, and Mott critical densities as a function of well and barrier thickness. Our results show that quantum confinement in GaN monolayers…
Atomically thin MoS2 layer is a direct bandgap semiconductor exhibiting strong electron-hole interaction due to the extreme quantum confinement and reduced screening of Coulomb interactions, which results in the formation of stable excitons at room temperature. Therefore, various excitonic properties of MoS2 monolayer are extremely important in determining the strength of light matter interactions including their radiative recombination lifetime and optoelectronic response. In this paper, we report a comprehensive study of the underlying annihilation mechanism of various types of exciton in Mo…
We analyze the dielectric-function spectra of low dimensional transition metal dichalcogenides (TMDCs) using a fully analytical model of the complex dielectric function that is applicable in fractional dimensional space. We extract the dimensionalities of the $A$ and $B$ excitons as well as their Lorentzian broadening widths by fitting the model to experimental data in the spectral range of photon energies (1.5 - 3 eV). Our results show the significant contribution of the lowest ground exciton state to the dielectric properties of exemplary monolayer materials (MoS$_2$, MoSe$_2$ and WSe$_2$). …
One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials, as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene. Transition-metal dichalcogenides (TMDs) provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change, since they exhibit a variety of electronic/magnetic states owing to quantum confinement. Here we report an anomalous metal-insulator transition induced by …
In the present work, we investigate how structural defects in graphene can change its transport properties. In particular, we show that breaking of the sublattice symmetry in a graphene monolayer overcomes the Klein effect, leading to confined states of massless Dirac fermions. Experimentally, this corresponds to chemical bonding of foreign atoms to carbon atoms, which attach themselves to preferential positions on one of the two sublattices. In addition, we consider the scattering off a tensor barrier, which describes the rotation of the honeycomb cells of a given region around an axis perpen…
We report intrinsic ferromagnetism in monolayer electrides or electrenes, in which excess electrons act as anions. Our first-principles calculations demonstrate that magnetism in such electron-rich two-dimensional (2D) materials originates from the anionic electrons rather than partially filled d orbitals, which is fundamentally different from ferromagnetism found in other 2D intrinsic magnetic materials. Taking the honeycomb LaBr$_2$ (La$^{3+}$Br$^{-}_{2}\cdot e^{-}$) as an example, our calculations reveal that the excess electron is localized at the center of the hexagon, which leads to stro…
