Group 11 Transition-Metal Halide Monolayers: High Promises for Photocatalysis and Quantum Cutting

The Tunable Electronic and Optical Properties of Two-Dimensional Bismuth Oxyhalides

Two-dimensional (2D) bismuth oxyhalides (BiOX) have attracted much attention as potential optoelectronic materials. To explore their application diversity, we herewith systematically investigate the tunable properties of 2D BiOX using first-principles calculations. Their electronic and optical properties can be modulated by changing the number of monolayers, applying strain, and/or varying the halogen composition. The band gap shrinks monotonically and approaches the bulk value, the optical absorption coefficient increases, and the absorption spectrum redshifts as the layer number of 2D BiOX increases. The carrier transport property can be improved by applying tensile strain, and the ability of photocatalytic hydrogen evolution can be obtained by applying compressive strain. General strain engineering will be effective in linearly tuning the band gap of BiOX in a wide strain range. Strain, together with halogen composition variation, can tune the optical absorption spectrum to be on demand in the range from visible to ultraviolet. This suggests that 2D BiOX materials can potentially serve as tunable novel photodetectors, can be used to improve clean energy techniques, and have potential in the field of flexible optoelectronics.

Type-II 2D AgBr/SiH van der Waals heterostructures with tunable band edge positions and enhanced optical absorption coefficients for photocatalytic water splitting

Utilizing two-dimensional (2D) heterostructures in photocatalysis can enhance optical ab-sorption and charge separation, thereby increasing solar energy conversion efficiency and tackling environmental issues. Density functional theory (DFT) was employed in this study to investigate the structural and optoelectronic properties of the AgBr/SiH van der Waals (vdW) heterostructures. All three configurations (A1, A2, and A3) were stable, with direct bandgaps of 1.83 eV, 0.99 eV, and 1.36 eV, respectively. The type-II band alignment in these structures enables electrons to be transferred from the SiH layer to the AgBr layer, and holes to move in the opposite direction. In the ultraviolet region, the optical absorption coefficients of the A1, A2, and A3 configurations are approximately 4.0 × 105 cm-1, significantly higher than that of an isolated AgBr monolayer (2.4 × 104 cm-1). In the visible light region, the A1 configuration has an absorption coefficient of 4 × 104 cm-1, higher than that of an isolated AgBr (2.2 × 104 cm-1). The band edges of the A1 configuration satisfy the redox potential for photocatalytic water splitting at pH 0-7. When the biaxial tensile strain is 5% for the A2 configuration and 2% for the A3 configuration, it can allow photocatalytic water splitting from half-reactions without strain to photocatalytic overall water splitting at pH 0-7. With a 5% biaxial tensile strain in the visible light region, the A1 and A3 configurations experience a rise in the maximum absorption coefficient of 5.7 × 104 cm-1 and 4.6 × 104 cm-1, respectively. The findings indicate that the AgBr/SiH vdW heterostructure configurations could be utilized in photocatalytic water-splitting processes with great potential.

Recent Progress in Double-Layer Honeycomb Structure: A New Type of Two-Dimensional Material

Two-dimensional (2D) materials are no doubt the most widely studied nanomaterials in the past decade. Most recently, a new type of 2D material named the double-layer honeycomb (DLHC) structure opened a door to achieving a series of 2D materials from traditional semiconductors. However, as a newly developed material, there still lacks a timely understanding of its structure, property, applications, and underlying mechanisms. In this review, we discuss the structural stability and experimental validation of this 2D material, and systematically summarize the properties and applications including the electronic structures, topological properties, optical properties, defect engineering, and heterojunctions. It was concluded that the DLHC can be a universal configuration applying to III-V, II-VI, and I-VII semiconductors. Moreover, these DLHC materials indeed have exotic properties such as being excitonic/topological insulators. The successful fabrication of DLHC materials further demonstrates it is a promising topic. Finally, we summarize several issues to be addressed in the future, including further experimental validation, defect engineering, heterojunction engineering, and strain engineering. We hope this review can help the community to better understand the DLHC materials timely and inspire their applications in the future.

Electronic and Optical Properties of BP, InSe Monolayer and BP/InSe Heterojunction with Promising Photoelectronic Performance

Two-dimensional (2D) materials provide a new strategy for developing photodetectors at the nanoscale. The electronic and optical properties of black phosphorus (BP), indium selenide (InSe) monolayer and BP/InSe heterojunction were investigated via first-principles calculations. The geometric characteristic shows that the BP, InSe monolayer and BP/InSe heterojunction have high structural symmetry, and the band gap values are 1.592, 2.139, and 1.136 eV, respectively. The results of band offset, band decomposed charge and electrostatic potential imply that the heterojunction structure can effectively inhibit the recombination of electron--hole pairs, which is beneficial for carrier mobility of photoelectric devices. Moreover, the optical properties, including refractive index, reflectivity, electron energy loss, extinction coefficient, absorption coefficient and photon optical conductivity, show excellent performance. These findings reveal the optimistic application potential for future photoelectric devices. The results of the present study provide new insight into challenges related to the peculiar behavior of the aforementioned materials with applications.

Adsorption of molecular iodine on the Ag(111) surface: Phase transitions, silver reconstruction, and iodide growth

Room temperature adsorption of molecular iodine on Ag(111) has been studied by scanning tunneling microscopy (STM), low energy electron diffraction, Auger electron spectroscopy with factor analysis, and density functional theory (DFT). At the chemisorption stage, iodine first forms a (3×3)R30° structure. Further iodine dosing leads to continuous commensurate-incommensurate phase transition, taking place via the formation of striped superheavy domain walls. As a result, the uniaxially compressed (13 ×3-R30°) phase is formed at an iodine coverage (θ) of 0.38 ML. At θ > 0.38 ML, first-order phase transition begins, leading to the formation of hexagonal moiré-like phases, which exhibit an anomalously large corrugation in STM (0.8-2.3 Å). In the range of 0.40-0.43 ML, the compression of hexagonal phases occurs, which ends at the formation of the (7 × 7)R21.8° structure at saturation. The DFT calculations allow us to explain the anomalous atomic corrugation of the hexagonal phases by the strong violation of the atomic structure of the substrate including up to ten layers of silver. Iodine dosing above 0.43 ML leads to the growth of 2D islands of silver iodide. The STM images of the silver iodide surface demonstrate a clear visible hexagonal superstructure with a periodicity of 25 Å superimposed with a quasi-hexagonal atomic modulation. DFT calculations of the atomic structure of AgI islands point to the formation of a sandwich-like double layer honeycomb structure similar to the case of I/Ag(100).

Toward Exotic Layered Materials: 2D Cuprous Iodide

Heterostructures composed of 2D materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials were increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist at other temperatures and pressures. This work demonstrates how such structures can be stabilized in 2D van der Waals (vdw) stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K, is produced. The results establish a simple route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.

Single-Layer Zirconium Dihalides ZrX2 (X = Cl, Br, and I) with Abnormal Ferroelastic Behavior and Strong Anisotropic Light Absorption Ability

Recently, two-dimensional (2D) metal halides have brought out an intensive interest for their unique mechanical, electronic, magnetic, and topological properties. Here, we theoretically report the existence of the single-layer (SL) zirconium dihalide materials ZrX₂ (X = Cl, Br, and I) using first-principles calculations. SL ZrX₂, which can be obtained from its bulk phase through simple mechanical exfoliation, shows the dynamic, thermodynamic, and mechanical stability. Halogen atoms can effectively tune the electronic structure, dipole moment transition, band alignment, and light absorption. Specifically, ZrX₂ monolayers intrinsically exhibit a ferroelasticity with an abnormal 120° orientation rotation, possessing a moderate switching barrier of 24-39 meV/atom. Importantly, we observe superior anisotropic light absorption responses on SL ZrX₂ in the visible region. Besides, a series of ZrX₂-based excitonic solar cells have been proposed, which hold a large power conversion efficiency limit of 12.4-18.7%.