Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas

Direct Observations of Spontaneous In-Plane Electronic Polarization in 2D Te Films

Single-element polarization in low dimensions is fascinating for constructing next-generation nanoelectronics with multiple functionalities, yet remains difficult to access with satisfactory performance. Here, spectroscopic evidences are presented for the spontaneous electronic polarization in tellurium (Te) films thinned down to bilayer, characterized by low-temperature scanning tunneling microscopy/spectroscopy. The unique chiral structure and centrosymmetry-breaking character in 2D Te gives rise to sizable in-plane polarization with accumulated charges, which is demonstrated by the reversed band-bending trends at opposite polarization edges in spatially resolved spectra and conductance mappings. The polarity of charges exhibits intriguing influence on imaging the moiré superlattice at the Te-graphene interface. Moreover, the plain spontaneous polarization robustly exists for various film thicknesses, and can universally preserve against different epitaxial substrates. The experimental validations of considerable electronic polarization in Te multilayers thus provide a realistic platform for promisingly facilitating reliable applications in microelectronic devices.

Substantial Energy Band Modulation of Semiconducting Hexagonal GaTe Quantum Wells by Layer Thickness and Mirror Twin Boundaries

Exploring emerging two-dimensional (2D) van der Waals (vdW) semiconducting materials and precisely tuning their electronic properties at the atomic level have long been recognized as crucial issues for developing their high-end electronic and optoelectronic applications. As a III-VI semiconductor, ultrathin layered hexagonal GaTe (h-GaTe) remains unexplored in terms of its intrinsic electronic properties and band engineering strategies. Herein, we report the successful synthesis of ultrathin h-GaTe layers on a selected graphene/SiC(0001) substrate, via molecular beam epitaxy (MBE). The widely tunable quasiparticle band gaps (∼2.60-1.55 eV), as well as the vdW quantum well states (QWSs) that can be strictly counted by the layer numbers, are well characterized by onsite scanning tunneling microscopy/spectroscopy (STM/STS), and their origins are clearly addressed by density functional theory (DFT) calculations. More intriguingly, distinctive 8|8E and 4|4P (Ga) mirror twin boundaries (MTBs) are identified in the ultrathin h-GaTe flakes, which can induce decreased band gaps and prominent p-doping effects. This work should deepen our understanding on the electronic tunability of 2D III-VI semiconductors by thickness control and line defect engineering, which may hold promise for fabricating atomic-scale vertical and lateral homojunctions toward ultrascaled electronics and optoelectronics.

Angle-Dependent Raman Spectra of Crystal Polymorphs of GaO: A Computational Study

Two crystal polymorphs of GaO consisting of GaO-H and GaO-T monolayers are proposed in this study. Based on the density functional theory calculations, the phonon dispersion demonstrates that both GaO-H and GaO-T monolayers could be stable. The band gaps of GaO-H and GaO-T monolayers are 1.51 and 1.43 eV, respectively. When an external electric field is applied, the band gaps of GaO monolayers are reduced dramatically, down to 0.13 eV with the field of 0.7 V/Å. Because of the decreased symmetry of C3v under an external electric field, more peaks of Raman spectra can be obtained. The angle-dependent Raman spectra ofA ' 1 1 ${{\rm{A}}{{^\prime}}_1^1 }$ andA ' 1 2 ${{\rm{A}}{{^\prime}}_1^2 }$ of GaO-H monolayer, andA 1 g 1 ${{\rm{A}}_{1{\rm{g}}}^1 }$ andA 1 g 2 ${{\rm{A}}_{1{\rm{g}}}^2 }$ of GaO-T monolayer are discussed seperately, with the incident lasers of 488 and 532 nm. Additionally, the Raman intensity distribution shows that the incident light should be parallel to the plane of the GaO monolayer to obtain more comparable Raman spectra. These investigations of the crystal polymorphs of GaO monolayers may guide the experimental investigations of GaO monolayers and potential optoelectronic applications.

Mnemonic Rutile-Rutile Interfaces Triggering Spontaneous Dissociation of Water

Water interaction with mineral surfaces is a complex living system decisive for any photocatalytic process. Resolving the atomistic structure of mineral-water interfaces is thus crucial for understanding these processes. Fibrous rutile TiO2 , grown hydrothermally on twinned rutile seeds under acidic conditions, is studied in terms of interface translation, atomic structure, and surface chemistry in the presence of water, by means of advanced microscopy and spectroscopy methods combined with structure modeling and density functional theory calculations. It is shown that fibers while staying in stable separation during their growth, adopt a special crystallographic registry that is controlled by repulsion forces between fully hydroxylated and protonated (110) surfaces. During relaxation, a turbulent proton transfer and cracking of O─H bonds is observed, generating a strong acidic character via proton jump from bridge ─OHb to terminal ─OHt groups, and spontaneous dissociation of interfacial water via a transient protonation of the ─OHt groups. It is shown, that this specific interface structure can be implemented to induce acidic response in an initially neutral medium when re-immersed. This is thought to be the first demonstration of quantum-confined mineral-water interface, capable of memorizing its past and conveying its structurally encoded properties into a new environment.

Carrier and phonon transport in 2D InSe and its Janus structures

Recently, two-dimensional (2D) Indium Selenide (InSe) has been receiving much attention in the scientific community due to its reduced size, extraordinary physical properties, and potential applications in various fields. In this review, we discussed the recent research advancement in the carrier and phonon transport properties of 2D InSe and its related Janus structures. We first introduced the progress in the synthesis of 2D InSe. We summarized the recent experimental and theoretical works on the carrier mobility, thermal conductivity, and thermoelectric characteristics of 2D InSe. Based on the Boltzmann transport equation (BTE), the mechanisms underlying carrier or phonon scattering of 2D InSe were discussed in detail. Moreover, the structural and transport properties of Janus structures based on InSe were also presented, with an emphasis on the theoretical simulations. At last, we discussed the prospects for continued research of 2D InSe.

Nearly Ideal Two-Dimensional Electron Gas Hosted by Multiple Quantized Kronig-Penney States Observed in Few-Layer InSe

A theoretical ideal two-dimensional electron gas (2DEG) was characterized by a flat density of states independent of energy. Compared with conventional two-dimensional free-electron systems in semiconductor heterojunctions and noble metal surfaces, we report here the achievement of ideal 2DEG with multiple quantized states in few-layer InSe films. The multiple quantum well states (QWSs) in few-layer InSe films are found, and the number of QWSs is strictly equal to the number of atomic layers. The multiple stair-like DOS as well as multiple bands with parabolic dispersion both characterize ideal 2DEG features in these QWSs. Density functional theory calculations and numerical simulations based on quasi-bounded square potential wells described as the Kronig-Penney model provide a consistent explanation of 2DEG in the QWSs. Our work demonstrates that 2D van der Waals materials are ideal systems for realizing 2DEG hosted by multiple quantized Kronig-Penney states, and the semiconducting nature of the material provides a better chance for construction of high-performance electronic devices utilizing these states, for example, superlattice devices with negative differential resistance.

Atomic insights into the influence of Bi doping on the optical properties of two-dimensional van der Waals layered InSe

As a narrow-gap semiconductor, III-VI two-dimensional (2D) van der Waals layered indium selenide (InSe) has attracted a lot of attention due to excellent physical properties. For potential optoelectronic applications, the tunability of the optical property is challenging, e.g., the modulation of optical bandgap commonly by element doping. However, the deep understanding of the influence of element doping on the microstructure and the optical properties lacks of systematic investigation. In this work, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we investigate the influence of Bi doping on controlling of the microstructure and optical properties of InSe single crystal in detail. The results show that Bi doping can introduce additional stacking faults in InSe single crystal, and more importantly, the atomic spacing and lattice constant of Bi-doped InSe are changed a lot as compared to that of the undoped one. Further optical characterizations including photoluminescence and transmission spectra reveal that Bi-doping can broaden the transmission wavelength range of InSe and make its optical bandgap blue-shift, which can also be physically interpreted from the doping-induced structure change. Our work expands new ideas for the optical property modulation of 2D thin-layer materials and brings new possibilities for the development of thin-layer InSe optical devices.

Atomic Visualization and Switching of Ferroelectric Order in β-In2 Se3 Films at the Single Layer Limit

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In₂ Se₃ films: β'- and β*-In₂ Se₃ are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in β'-In₂ Se₃ and ferroelectric order of β*-In₂ Se₃ are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.