Luminescent Amorphous Silicon Oxynitride Systems: High Quantum Efficiencies in the Visible Range.

In recent years, researchers have placed great importance on the use of silicon (Si)-related materials as efficient light sources for the purpose of realizing Si-based monolithic optoelectronic integration. Previous works were mostly focused on Si nanostructured materials, and, so far, exciting results from Si-based compounds are still lacking. In this paper, we have systematically demonstrated the high photoluminescence external quantum efficiency (PL EQE) and internal quantum efficiency (PL IQE) of amorphous silicon oxynitride (a-SiNxOy) systems. Within an integration sphere, we directly measured the PL EQE values of a-SiNxOy, which ranged from approximately 2% to 10% in the visible range at room temperature. Then, we calculated the related PL IQE through temperature-dependent PL measurements. The obtained PL IQE values (~84% at 480 nm emission peak wavelength) were very high compared with those of reported Si-based luminescent thin films. We also calculated the temperature-dependent PL EQE values of a-SiNxOy systems, and discussed the related PL mechanisms.

Valley-polarized and enhanced transmission in graphene with a smooth strain profile.

We explore the influence of strain on the valley-polarized transmission of graphene by employing the wave-function matching and the non-equilibrium Green's function technique. When the transmission is along the armchair direction, we show that the valley polarization and transmission can be improved by increasing the width of the strained region and increasing (decreasing) the extensional strain in the armchair (zigzag) direction. It is noted that the shear strain does not affect transmission and valley polarization. Furthermore, when we consider the smooth strain barrier, the valley-polarized transmission can be enhanced by increasing the smoothness of the strain barrier. We hope that our finding can shed new light on constructing graphene-based valleytronic and quantum computing devices by solely employing strain.

High photoluminescence quantum yields generated from N-Si-O bonding states in amorphous silicon oxynitride films: erratum.

We present an erratum to our paper [Opt. Express2631617 (2018)10.1364/OE.26.031617OPEXFF1094-4087]. Due to the statistical requirements of the postdoctoral outbound assessment, the first organization of the research articles must be "Nanjing University". As all our experimental data in this article is done in the laboratory of Nanjing University, so "Nanjing University" should be used as the first organization to meet the appraisal requirements of the postdoctoral workstation.

Valleytronics in two-dimensional materials with line defect.

The concept of valley originates from two degenerate but nonequivalent energy bands at the local minimum in the conduction band or local maximum in the valence band. Manipulating the valley states for information storage and processing develops a brand-new electronics-valleytronics. Broken inversion symmetry is a necessary condition to produce pure valley currents. The polycrystalline two-dimensional materials (graphene, silicene, monolayer group-VI transition metal dichalcogenides, etc) with pristine grains stitched together by disordered grain boundaries (GBs) are the natural inversion-symmetry-broken systems and the candidates in the field of valleytronics. Different from their pristine forms, the Dirac valleys on both sides of GBs are mismatched in the momentum space and induce peculiar valley transport properties across the GBs. In this review, we systematically demonstrate the fundamental properties of valley degree of freedom across mostly studied and experimentally feasible polycrystalline structure-the line defect, and the manipulation strategies with electrical, magnetic and mechanical methods to realize the valley polarization. We also introduce an effective numerical method, the non-equilibrium Green's function technique, to tackle the valley transport issues in the line defect systems. The present challenges and the perspective on the further investigations of the line defect in valleytronics are also summarized.

A first principles investigation on the structural, mechanical, electronic, and catalytic properties of biphenylene.

Recently, a new two-dimensional allotrope of carbon (biphenylene) was experimentally synthesized. Using first-principles calculations, we systematically investigated the structural, mechanical, electronic, and HER properties of biphenylene. A large cohesive energy, absence of imaginary phonon frequencies, and an ultrahigh melting point up to 4500 K demonstrate its high stability. Biphenylene exhibits a maximum Young's modulus of 259.7 N/m, manifesting its robust mechanical performance. Furthermore, biphenylene was found to be metallic with a n-type Dirac cone, and it exhibited improved HER performance over that of graphene. Our findings suggest that biphenylene is a promising material with potential applications in many important fields, such as chemical catalysis.

Selection rule for Raman spectra of two-dimensional materials using circularly-polarized vortex light.

Conservation of spin and orbital angular momenta of circularly-polarized vortex light is discussed for Raman spectra of two-dimensional materials. We first show the selection rule for optical absorption of two-dimensional materials as a function of the spin and orbital angular momentum of incident vortex light. In the case of two-dimensional materials, the Raman tensor for the incident vortex light does not change the symmetry of the phonon mode. Furthermore, the Raman active modes are classified by either "helicity-changing" or "helicity-conserved" Raman modes, in which the scattered photon of circularly polarized light either changes or does not change the helicity of the light, respectively. We show tables of selection rules for the Raman active modes of two-dimensional materials with 2, 3, 4, and 6 rotational symmetry for vortex light.

First-principles investigation on electronic properties and band alignment of group III monochalcogenides.

Using first-principles calculations, we investigated the electronic properties and band alignment of monolayered group III monochalcogenides. First, we calculated the structural and electronic properties of six group III monochalcogenides (GaS, GaSe, GaTe, InS, InSe, and InTe). We then investigated their band alignment and analysed the possibilities of forming type-I and type-II heterostructures by combining these compounds with recently developed two-dimensional (2D) semiconducting materials, as well as forming Schottky contacts by combining the compounds with 2D Dirac materials. We aim to provide solid theoretical support for the future application of group III monochalcogenides in nanoelectronics, photocatalysis, and photovoltaics.

Tunable Schottky barrier in graphene/graphene-like germanium carbide van der Waals heterostructure.

The structural and electronic properties of van der Waals (vdW) heterostructrue constructed by graphene and graphene-like germanium carbide were investigated by computations based on density functional theory with vdW correction. The results showed that the Dirac cone in graphene can be quite well-preserved in the vdW heterostructure. The graphene/graphene-like germanium carbide interface forms a p-type Schottky contact. The p-type Schottky barrier height decreases as the interlayer distance decreases and finally the contact transforms into a p-type Ohmic contact, suggesting that the Schottky barrier can be effectively tuned by changing the interlayer distance in the vdW heterostructure. In addition, it is also possible to modulate the Schottky barrier in the graphene/graphene-like germanium carbide vdW heterostructure by applying a perpendicular electric field. In particular, the positive electric field induces a p-type Ohmic contact, while the negative electric field results in the transition from a p-type to an n-type Schottky contact. Our results demonstrate that controlling the interlayer distance and applying a perpendicular electric field are two promising methods for tuning the electronic properties of the graphene/graphene-like germanium carbide vdW heterostructure, and they can yield dynamic switching among p-type Ohmic contact, p-type Schottky contact, and n-type Schottky contact in a single graphene-based nanoelectronics device.

Transition-metal dichalcogenides/Mg(OH)2 van der Waals heterostructures as promising water-splitting photocatalysts: a first-principles study.

We performed first-principles calculations of the structural, electronic, and optical properties of heterostructures which consist of transition metal dichalcogenides MX2 (M = Mo, W; X = S) stacked with Mg(OH)2. All the heterostructures are formed by van der Waals forces. The MoS2/Mg(OH)2 and WS2/Mg(OH)2 vdW heterostructures were found to be semiconductors with indirect bandgaps and possess intrinsic type-II band alignment. In particular, a comparison of the band edge positions with the redox potential of water indicates that the heterostructures are potential photocatalysts for water splitting, enabling water reduction on the MX2 layer and water oxidation on the Mg(OH)2 layer. Moreover, the photogenerated charges will be effectively separated in the presence of a large built-in electric field across the interface. In addition, all of the MX2/Mg(OH)2 heterostructures show strong optical absorption in the visible and infrared regions, indicating their promise for application in photocatalytic water splitting.

Measuring the nonlocality of different types of Majorana bound states in a topological superconducting wire.

According to the degree of topological protection, Majorana bound states (MBSs) can be divided into three types: ideal zero-energy MBSs (IZMs), finite-energy MBSs (FEMs) and zero-energy MBSs at parity crossing points (PZMs). Herein, we investigate the nonlocality of these three types of MBSs by comparing the conductance spectra of a normal lead-topological superconducting wire-normal lead (NSN) junction and an NS junction. We find that for the FEM-related tunnelling process, the decrease in the nonlocal processes is trivially accompanied by an increase in the local processes, whereas for the IZM-related tunnelling process, the left and right tunnelling processes are completely independent. Remarkably, PZMs induce a nonlocal electron-blocking effect in which incoming electrons from the left lead cannot participate in local Andreev reflection unless the right lead is present, even though no nonlocal tunnelling processes occur in the right lead of an NSN junction. We show that this PZM-mediated nonlocal electron-blocking effect is due to the nonlocal coupling of the left lead to the more distant PZM and that the phase difference between the two end PZMs is [Formula: see text]. Our findings provide an experimentally accessible method for characterizing MBSs by probing their different nonlocal signatures.

A van der Waals Heterostructure Based on Graphene-like Gallium Nitride and Boron Selenide: A High-Efficiency Photocatalyst for Water Splitting.

Hydrogen generation by photocatalytic water splitting has attained more and more research interests in the recent years since the solar energy can be directly transferred and stored as hydrogen. However, the search for a high-efficiency photocatalyst for water splitting is a really challenge. In this paper, we designed a novel 2D material-based van der Waals heterostructure (vdWH) composed by g-GaN and BSe, which is thermally stable at room temperature. The g-GaN/BSe vdWH has suitable band-edge positions for the oxidation and reduction reactions of water splitting at pH 0 and 7. The carrier mobility of this heterostructure is high, indicating the effective occurrence of reactions for water splitting. The g-GaN/BSe vdWH also possesses a type-II band alignment, which can promote the separation of the photogenerated electron-hole pairs constantly. Moreover, a large built-in electric field can be established at the interface, which will further prevent the recombination of photogenerated charges. In addition, the g-GaN/BSe vdWH also exhibits outstanding sunlight-absorption ability, and the biaxial strain can further enhance this ability. Thus, we conclude that the g-GaN/BSe vdWH can act as a high-efficiency photocatalyst for water splitting.

Strain-enhanced properties of van der Waals heterostructure based on blue phosphorus and g-GaN as a visible-light-driven photocatalyst for water splitting.

Many strategies have been developed to overcome the critical obstacles of fast recombination of photogenerated charges and the limited ability of semiconductor photocatalysts to absorb visible light. Considering all the novel properties of monolayered g-GaN and blue phosphorus (BlueP) which were revealed in recent studies, first-principles calculations were used to systematically investigate the structural stability, electronic energy, band alignment, band bending, and charge difference in the heterostructure formed by these two layered materials. The g-GaN/BlueP heterostructure is constructed by van der Waals (vdW) forces, and it possess a staggered band structure which induces electron transformation because of the different Fermi levels of the two layered materials. By aligning the Fermi levels, an interfacial electric field is built and it causes band bending, which can promote effective separation of photoexcited holes and electrons; the band-bending phenomenon was also calculated according to density functional theory (DFT). Moreover, effects of in-plane strain on the tuned bandgap, energy, and band edge were investigated, and the results show that the optical-absorption performance in the visible-light range can be improved. The findings reported in this paper are expected to provide theoretical support for the use of the g-GaN/BlueP vdW heterostructure as a photocatalyst for water splitting.

Tunable Luminescent A-SiNxOy Films with High Internal Quantum Efficiency and Fast Radiative Recombination Rates.

In this work, we systematically investigated the Nx bonding defects that induced high photoluminescence internal quantum efficiencies (PL IQEs) and very fast radiative recombination processes in amorphous silicon oxynitride (a-SiNxOy) systems. The luminescent N‒Si‒O bonding-related defect states were checked for the XPS, EPR, and temperature-dependent steady-state PL (TD-SSPL) properties. The PL IQEs were calculated from PL quantum yields through the principle of planar geometry optics, and then confirmed by the TD-SSPL properties. The radiative recombination rates [kr(R)] were determined by combining the PL IQE values and ns-PL lifetimes obtained from time-resolved PL measurements. Both the PL IQE, exceeding 72%, and the fast kr(R) (~108 s-1) are proportional to the concentration of Nx defects, which can be explained by N‒Si‒O bonding states related to the quasi-three-level model, suggesting the possible realization of stimulated light emission in a-SiNxOy systems.

Adsorption of Transition Metals on Black Phosphorene: a First-Principles Study.

Black phosphorene is a novel two-dimensional material which has unique properties and wide applications. Using first-principles calculations, we investigated the adsorption behavior of 12 different transition metals (TMs; Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) on phosphorene. Our results showed that all of the adsorption systems have a large binding energy. The Fe-, Co-, and Au-phosphorene systems display magnetic states with magnetic moments of 2, 1, and 0.96 µB, respectively, which means that these systems are magnetic semiconductors. Adsorption of oxygen molecules on TM-phosphorene was also investigated. Interestingly, all the O2-(TM-phosphorene) systems, except O2-(Pd-phosphorene), can elongate the O-O bond, which is critical to their application as catalysts in the oxidation of CO. We also found that the adsorption of O2 molecules enables the O2-(Fe-, Ni-, Cu-, Ir-, Rh-, Ag-, and Au-phosphorene) systems to become magnetic semiconductors, and it allows O2-(Co-phosphorene) to display half-metallic state. Our results are expected to have important implications for phosphorene-based catalysis and spintronics.

Electronic and optical properties of heterostructures based on transition metal dichalcogenides and graphene-like zinc oxide.

The structural, electronic, and optical properties of heterostructures formed by transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se) and graphene-like zinc oxide (ZnO) were investigated using first-principles calculations. The interlayer interaction in all heterostructures was characterized by van der Waals forces. Type-II band alignment occurs at the MoS2/ZnO and WS2/ZnO interfaces, together with the large built-in electric field across the interface, suggesting effective photogenerated-charge separation. Meanwhile, type-I band alignment occurs at the MoSe2/ZnO and WSe2/ZnO interfaces. Moreover, all heterostructures exhibit excellent optical absorption in the visible and infrared regions, which is vital for optical applications.

Manifestation of topological transitions in a multi-terminal Josephson junction.

We study nonequilibrium (NE) transport in four-terminal (three-terminal) topological superconductor (SC)-quantum dot (QD) topological superconductor junctions, where the QD is connected via tunneling barriers to the two TS leads and two (one) normal leads (N), respectively. For the four-terminal junction, we find that when increasing the Zeeman field from 0 to a critical value, the supercurrent profile evolves from a typical s-wave pattern to a pure p-wave pattern. In addition, by analyzing the zero-phase difference supercurrent as a function of voltage [Formula: see text] applied to the normal leads and the Zeeman field h applied to the SC, the low-momentum gap [Formula: see text] can be inferred by utilizing the fact that the emergence of a tunneling-induced current should be satisfied under the condition [Formula: see text]. For the three-terminal junction, the NE supercurrent can reveal the quasi-Andreev bound state by exploiting the Andreev reflection process-induced current occurring between the N and SC. Our findings provide an arguably easier route for manifesting the topological phase transition by observing the gap collapse and then reopening as the Zeeman field increases through multi-terminal NE transport.

MoS2/ZnO van der Waals heterostructure as a high-efficiency water splitting photocatalyst: a first-principles study.

Previous investigations [H. L. Zhuang and R. G. Hennig, J. Phys. Chem. C, 2013, 117, 20440-20445; J. Kang, S. Tongay, J. Zhou, J. Li and J. Wu, Appl. Phys. Lett., 2013, 102, 012111] demonstrated that molybdenum disulfide (MoS2) is a potential photocatalyst for water splitting. However, the photogenerated electron-hole pairs in MoS2 remain in the same spatial regions, resulting in a high rate of recombination. Using first-principles calculations, we designed a MoS2-based heterostructure by stacking MoS2 on two-dimensional zinc oxide (ZnO) and investigated its structural, electronic, and optical properties. The interaction at the MoS2/ZnO interface was found to be dominated by van der Waals (vdW) forces. The energy levels of both water oxidation and reduction lie within the bandgap of the MoS2/ZnO vdW heterostructure, which guarantee their occurrence for water splitting. Moreover, a type-II band alignment and a large built-in electric field are formed at the MoS2/ZnO interface, which ensure the enhanced separation of the photogenerated electron-hole pairs. In addition, strong optical absorption in the visible region was also found in the MoS2/ZnO vdW heterostructure, indicating that it has potential for application in photovoltaic and photocatalytic devices.

High photoluminescence quantum yields generated from N-Si-O bonding states in amorphous silicon oxynitride films.

We investigated the high absolute photoluminescence quantum yields (PL QYs) from tunable luminescent amorphous silicon oxynitride (a-SiNxOy) films. The PL QY of 8.38 percent has been achieved at PL peak energy of 2.55 eV in a-SiNxOy systems, which is higher than those of reported nanocrystal-Si embedded silicon nitride films. The existence of N-Si-O bonding states was confirmed by performing FTIR, XPS and EPR measurements. The PL QY is proportional to the concentration of Nx defects, indicating the dominant contribution of luminescent N-Si-O bonding states in radiative recombination processes. Particularly, we precisely monitored the ns-PL lifetimes evolution profile versus detected emission wavelengths, and further verified that the N-Si-O bonding states are responsible for highly efficient PL.

The chirality dependent spin filter design in the graphene-like junction.

We investigate the chirality-dependent spin transport in a graphene-like topological insulator (TI) TI/n junction, where a perpendicular magnetic field or an off-resonant circularly- polarized light field is applied to the normal (n) region. It is found that the coupling between the helical edge states of the TI and chiral edge states from the magnetic/light field results in a perfect spin filtering effect and only one spin species can tunnel through the junction interface. The origin is ascribed to the chirality-conservation requirement, since the two spin species have the opposite chiralities in the TI region and in the n region both of them have the same chiralities. For a TI/n superlattice structure, the spin filtering effect is enhanced and even survives in a fairly strong disorder environment.