The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations. Unexpectedly, we have first found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle GW approach. We demonstrate that the connected network of hexagonal vacancies dominates the gap opening for both the in-plane s+p and p orbitals, with which various semiconducting boron monolayers are designed to realize the band gap engineering for the potential applications in electronic devices. The semiconducting boron monolayers in our predictions are expected to be synthesized on the proper substrates, due to the similar stabilities to the ones observed experimentally.

Screening unique two-dimensional (2D) materials with high mobility and applicable band gaps is motivated by not only the interest in basic science but also the practical applications for photoelectric materials. In this work, we have systematically studied a new family of 2D ternary quintuple layers (QLs), named ABC (A = Na, K, and Rb; B = Cu, Ag, and Au; C = S, Se, and Te).Our results indicate that the QLs of KCuTe, KAgS, KAgSe, KAuTe, RbCuTe, RbAgSe, and RbAgTe host direct band gaps. Moreover, KCuTe, RbCuTe, and RbAgTe QLs show extremely high mobilities of ∼10 4 cm 2 V −1 s −1 . Interestingly, the linear scaling between exciton binding energy and quasiparticle band gap for ABC QLs exhibits an unexpected deviation with the 1/4 law. In addition, KAgSe, KAgS, RbAgSe, and RbAgTe show outstanding power energy conversion efficiencies of up to 21.5%, suggesting that they are good potential donor materials. Our results provide many potential candidates for applications in photoelectric materials, which may be realized in experiments due to the possible exfoliation from their parent compounds.

Semiconducting molecules have been employed to passivate traps extant in the perovskite film for enhancement of perovskite solar cells (PSCs) efficiency and stability. A molecular design strategy to passivate the defects both on the surface and interior of the CH3NH3PbI3 perovskite layer, using two phthalocyanine (Pc) molecules (NP‐SC6‐ZnPc and NP‐SC6‐TiOPc) is demonstrated. The presence of lone electron pairs on S, N, and O atoms of the Pc molecular structures provides the opportunity for Lewis acid–base interactions with under‐coordinated Pb2+ sites, leading to efficient defect passivation of the perovskite layer. The tendency of both NP‐SC6‐ZnPc and NP‐SC6‐TiOPc to relax on the PbI2 terminated surface of the perovskite layer is also studied using density functional theory (DFT) calculations. The morphology of the perovskite layer is improved due to employing the Pc passivation strategy, resulting in high‐quality thin films with a dense and compact structure and lower surface roughness. Using NP‐SC6‐ZnPc and NP‐SC6‐TiOPc as passivating agents, it is observed considerably enhanced power conversion efficiencies (PCEs), from 17.67% for the PSCs based on the pristine perovskite film to 19.39% for NP‐SC6‐TiOPc passivated devices. Moreover, PSCs fabricated based on the Pc passivation method present a remarkable stability under conditions of high moisture and temperature levels.

No abstract

Monolayer (ML) tungsten ditelluride (WTe) is a well-known quantum spin Hall (QSH) insulator with topologically protected gapless edge states, thus promising dissipationless electronic devices. However, experimental findings exhibit the fast oxidation of ML WTe in ambient conditions. To reveal the changes of topological properties of WTe arising from oxidation, we systematically study the surface oxidation reaction of ML 1T'-WTe using first-principles calculations. The calculated results indicate that the fast oxidation of WTe originates from the existence of HO in air, which significantly promotes the oxidation of ML 1T'-WTe. More importantly, this low-coverage oxidized WTe loses its topological features and is changed into a trivial insulator. Furthermore, we propose a fully oxidized ML WTe that can still possess the QSH insulator states. The topological phase transition induced by oxidation provides exotic insight into understanding the topological features of layered transition-metal dichalcogenide materials.

Motivated by fundamental interest and practical applications, the investigations of two-dimensional photocatalysts are fascinating subjects in clean energy. Herein, we propose that two-dimensional Li-based ternary chalcogenides LiXY2 (X = Al, Ga, In; Y = S, Se, Te) have intrinsic polarization and direct band gaps. Our results show that LiXY2 materials possess optical absorption spectra covering the visible and ultraviolet range. We show that these materials possess extremely high electron mobility (∼103 cm2 V–1 s–1), providing great potential in overall water splitting. Furthermore, LiAlS2 and LiGaS2 can facilitate overall water splitting regardless of their energy gaps because of the large differences of surface electronic potentials of LiXY2. Importantly, it is feasible to exfoliate the layered LiAlTe2 from its bulk counterpart in experiments. Our findings open an exotic pathway to realizing promising photocatalytic applications in two-dimensional ternary materials.

The emerging two-dimensional tellurene has been demonstrated to be a promising candidate for photoelectronic devices. However, there is a lack of comprehensive insight into the effects of vacancies and common adsorbates (i.e., O 2 and H 2 O) in ambient conditions, which play a crucial role in semiconducting devices. In this work, with the aid of firstprinciples calculations, we demonstrate that H 2 O and O 2 molecules behave qualitatively differently on tellurene, while water adsorption can be remarkably promoted by adjacent preadsorbed O 2 . Upon the formation of Te vacancies, the adsorption of both O 2 and H 2 O molecules is enhanced. More importantly, the existence of H 2 O and Te vacancies can dramatically facilitate the dissociation of O 2 , suggesting that tellurene may be readily oxidized in humid conditions. In addition, it is found that the electronic properties of tellurene are well preserved upon either H 2 O or O 2 adsorption on the surface. In sharp contrast, vacancies enable significant modification on the band structure. Specifically, an indirect-to-direct band gap transition is found at a vacancy concentration of 5.3%.

Exploring high performance and excellent ambient stability in two-dimensional (2D) monolayer photoelectric materials is motivated by not only practical applications but also scientific interest. Here, a new 2D monolayer W8Se12 structure is synthesized via in situ electron-beam irradiation on 2D WSe2. Moreover, we systematically studied the photoelectric properties of the class of monolayer M8X12 (M = Mo, W; X = S, Se, and Te) materials by first principles. The results indicated that Mo8S12, Mo8Se12, W8S12, and W8Se12 monolayers possess desirable direct band gaps and remarkable anisotropic optical absorption in visible light, while Mo8Te12 and W8Te12 monolayers are metals. Impressively, the monolayer W8Se12 can result in a direct–indirect-metal transition under uniaxial strain. In addition, they show high anisotropic carrier mobilities (up to 104 cm2 V–1 s–1), significantly over those of transition-metal dichalcogenides. These new binary monolayer M8X12 structures can effectively broaden the 2D material family and may provide four potential candidates in photoelectric applications.