Xi-Bo Li

Diverse and tunable electronic structures of single-layer metal phosphorus trichalcogenides for photocatalytic water splitting

The family of bulk metal phosphorus trichalcogenides (APX3, A = M(II), M(I)(0.5)M(III)(0.5); X = S, Se; M(I), M(II), and M(III) represent Group-I, Group-II, and Group-III metals, respectively) has attracted great attentions because such materials not only own magnetic and ferroelectric properties, but also exhibit excellent properties in hydrogen storage and lithium battery because of the layered structures. Many layered materials have been exfoliated into two-dimensional (2D) materials, and they show distinct electronic properties compared with their bulks. Here we present a systematical study of single-layer metal phosphorus trichalcogenides by density functional theory calculations. The results show that the single layer metal phosphorus trichalcogenides have very low formation energies, which indicates that the exfoliation of single layer APX3 should not be difficult. The family of single layer metal phosphorus trichalcogenides exhibits a large range of band gaps from 1.77 to 3.94 eV, and the electronic structures are greatly affected by the metal or the chalcogenide atoms. The calculated band edges of metal phosphorus trichalcogenides further reveal that single-layer ZnPSe3, CdPSe3, Ag0.5Sc0.5PSe3, and Ag0.5In0.5PX3 (X = S and Se) have both suitable band gaps for visible-light driving and sufficient over-potentials for water splitting. More fascinatingly, single-layer Ag0.5Sc0.5PSe3 is a direct band gap semiconductor, and the calculated optical absorption further convinces that such materials own outstanding properties for light absorption. Such results demonstrate that the single layer metal phosphorus trichalcogenides own high stability, versatile electronic properties, and high optical absorption, thus such materials have great chances to be high efficient photocatalysts for water-splitting.

Modulating the atomic and electronic structures through alloying and heterostructure of single-layer MoS2

Among dozens of transition metal dichalcogenides (TMDs), single-layer MoS2 with a direct band gap has attracted great attention because of its potential applications. In this work, the atomic structures and electronic properties of mixed alloys or heterostructures of TMDs with single-layer MoS2 are explored based on density functional theory (DFT). The calculated quasi-binary phase diagrams reveal that different alloyed TMDs have great distinct stability and band structures, and the band gap of single-layer MoS2 can be tuned from 0.89 to 1.87 eV by either alloys or heterostructures with other TMDs. Heterostructures between TMDs can not only tune the band gap, but also modulate the band edge position to enhance the photocatalytic activity. More fascinatingly, the MoS2–WS2 heterostructure exhibits the unique electronic properties of spontaneous electron–hole separation. Such a result not only reveals that both alloyed or heterostructures can effectively tune the electronic properties of TMDs, but also it will stimulate further work to design a new type of photocatalyst.

Structures, stabilities, and electronic properties of defects in monolayer black phosphorus.

The structures, stabilities, and electronic properties of monolayer black phosphorus (M-BP) with different kinds of defects are investigated within the frame of density-functional theory. All the possible configurations of defects in M-BP are explored, and the calculated results suggest that the stabilities of the configurations with different kinds of defects are greatly related to broken bonds, structural deformation and the character of the bonding. The configurations with two or three vacancies are energetically more favorable than the ones with a single vacancy. Meanwhile, the doping of two foreign atoms, such as sulfur, silicon or aluminum, is more stable than that of the corresponding single dopant. The electronic properties of M-BP are greatly affected by the types of defects. The single S-doped M-BP not only retains the character of a direct semiconductor, but it also can enlarge the band gap by 0.24 eV relative to the perfect one. Such results reveal that the defects not only greatly affect the electronic properties, but they also can be used as an effective way to modulate the band gap for the different applications of M-BP in electronic devices.

Tunable electronic and magnetic properties of WS2 nanoribbons

Two dimensional transition metal dichalcogenides (TMDs) have attracted great attention because of the versatile electronic structures. The electronic and magnetic properties of the nanoribbons are still not fully understood, which are crucial for their applications in nanodevices. In this work, the detailed atomic structural, electronic, and magnetic properties of the one dimensional WS2 nanoribbons have been carefully explored by first-principles calculations. The results suggest that the single layer WS2 will first transform into direct band gap semiconductor from indirect band gap of bulk one. Interestingly, the properties of WS2 nanoribbons are greatly affected by the type of the edges: Armchair nanoribbons (ANRs) remain nonmagnetic and semiconducting as that of bulk, whereas zigzag nanoribbons (ZNRs) exhibit ferromagnetic and metallic. Further, the electronic properties can be tuned by applying the external strains to WS2 nanoribbons: Band gap of ANRs experiences a direct-indirect-direct transition and the magnetic moment of ZNRs can be easily tuned by the different strains. All these findings suggest that the TMDs nanoribbons may exhibit extraordinary electronic and magnetic properties, and more importantly, such fascinating characters can be precisely modulated by controlling the edge types and applied strains.

Single-layer Group-IVB nitride halides as promising photocatalysts

In order to construct efficient solar-driven devices, many potential materials have been explored in search of desirable photocatalyts for water splitting. Layered structure nitride halides have received significant attention from different fields because of their unusual electronic properties. In this work, we have systematically studied the electronic structures and potential photocatalytic properties of single-layer Group-IVB nitride halides (MNX, M = Ti, Zr, Hf; X = Cl, Br, I) in different forms using first-principles calculations. The results show that the single-layer nitride halides have very low formation energies, which indicates that the isolation of these single-layer MNX materials should not be difficult. The calculated band structures reveal that all of the single-layer MNX are semiconductors, while each of them shows a distinct type of electronic properties. Among these semiconducting nitride halides, ten members of the single-layer MNX family are feasible photocatalysts for splitting water. Interestingly, single-layer α-ZrNX (X = Cl, Br, I) and α-HfNI are direct band gap semiconductors with desirable band gaps (2.23–2.83 eV), and the calculated optical adsorption spectra further confirm their excellent light absorption in visible light region. Finally, the electronic properties and optical absorption in visible light region of single-layer MNX can be easily tuned through hybridisation or doping between them because of the similarity of the MNXs. Their high stability, versatile electronic properties, and high optical absorption make single-layer Group-IVB nitride halides promising candidates for application in photocatalytic water splitting.

Electronic structures and optical properties of two-dimensional ScN and YN nanosheets

Two-dimensional (2D) materials exhibit different electronic properties than their bulk materials. Here, we present a systematic study of 2D tetragonal materials of ScN and YN using density functional theory calculations. Several thermodynamically stable 2D tetragonal structures were determined, and such novel tetragonal structures have good electronic and optical properties. Both bulk ScN and YN are indirect band gap semiconductors while the electronic structures of 2D ScN and YN are indirect gap semiconductors, with band gaps of 0.62–2.21 eV. The calculated optical spectra suggest that 2D tetragonal ScN and YN nanosheets have high visible light absorption efficiency. These electronic properties indicate that 2D ScN and YN have great potential for applications in photovoltaics and photocatalysis.

Effect of water on gas explosions: combined ReaxFF and ab initio MD calculations

In order to understand the intrinsic effect mechanism of water addition on gas explosions, the methane explosion systems with water addition of different mole fractions were systematically studied by reactive force field and first-principles molecular dynamics (MD) simulations. The results show that the effects of water addition on a gas explosion process greatly depend on the system temperature at different reaction stages. Although the water can effectively suppress the methane oxidation process at the initial reaction stage, the same amount of water addition will obviously promote the gas explosion at the later reaction process. The ab initio MD simulations reveal that the water molecules can induce the reactions between ˙HO2 and ˙H with ˙OH radicals at the initial reaction stage. These reactions consume the reactive radicals, causing the reaction activity of the methane oxidation system to decrease. However, at a higher temperature (about 3000 K), water molecules react with ˙O and ˙H radicals to form extra ˙OH free radicals, and these ˙OH free radicals can be transferred rapidly to interact with the methane molecules by the water molecules. All these processes lead to a better reactive performance at the later reaction stage. These results not only identify the intrinsic interaction mechanism of water addition on the gas explosion system, but also provide a significant theoretical guide for the development of a highly efficient suppression method for gas explosions.

Two-dimensional square-pyramidal VO2 with tunable electronic properties

In order to design the high-performance spintronics, it is rather critical to develop new materials, which can easily regulate the magnetism of nanostructures. In this work, the electronic properties of two dimensional (2D) square-pyramidal vanadium dioxide (S-VO2) are explored based on first-principles calculations. The results reveal that the monolayer S-VO2 is an ideal flexible platform to manipulate the magnetic properties by either biaxial compressive strain or surface modification. Although the ground state of the pristine S-VO2 is a direct semiconductor with antiferromagnetic (AFM) coupling between two nearest V atoms, the monolayer S-VO2 becomes ferromagnetic (FM) under a biaxial compressive strain. Furthermore, the monolayer S-VO2 can be tuned from a nonmagnetic semiconductor to a magnetic semiconductor and even to a half-metal through surface modification. The tunable magnetic properties of the monolayer S-VO2 make it a promising candidate for applications in spin-devices.

High carrier mobility of few-layer PbX (X = S, Se, Te)

Two-dimensional materials with a higher carrier mobility are promising materials for applications in nanoelectronics and photocatalysis. In this paper, we have explored the stabilities, structures, electronic properties, carrier mobility and optical properties of few-layer PbX (X = S, Se, Te) by first-principles calculations. Theoretical results show that the band gaps of PbX could be modulated by the thickness, changing from 1.65 eV (1.26 eV, 1.26 eV) for a monolayer to 0.98 eV (0.76 eV, 0.97 eV) for a tri-layer for PbS (PbSe, PbTe). Most importantly, the bi-layer PbS has an extremely high electron carrier mobility of 252 000 cm2 V−1 s−1 and the hole carrier mobility of mono- or tri-layer PbTe could possess a value of 16 000 cm2 V−1 s−1; thus, few-layer PbXs can have possible wide applications in novel electronic devices. The strong adsorptions of light of the PbX species also shows their potential implications in solar cells.

Adaptive cluster expansion approach for predicting the structure evolution of graphene oxide

An adaptive cluster expansion (CE) method is used to explore surface adsorption and growth processes. Unlike the traditional CE method, suitable effective cluster interaction (ECI) parameters are determined, and then the selected fixed number of ECIs is continually optimized to predict the stable configurations with gradual increase of adatom coverage. Comparing with traditional CE method, the efficiency of the adaptive CE method could be greatly enhanced. As an application, the adsorption and growth of oxygen atoms on one side of pristine graphene was carefully investigated using this method in combination with first-principles calculations. The calculated results successfully uncover the structural evolution of graphene oxide for the different numbers of oxygen adatoms on graphene. The aggregation behavior of the stable configurations for different oxygen adatom coverages is revealed for increasing coverages of oxygen atoms. As a targeted method, adaptive CE can also be applied to understand the evolution of other surface adsorption and growth processes.