type-II Alignment Research Articles

In-situ construction of 2D direct Z-scheme g-C3N4/g-C3N4 homojunction with high photocatalytic activity

Constructing all-solid-state Z-scheme junction is a very effective strategy to design highly active photocatalysts for solar energy conversion and environmental purification. We herein firstly construct 2D g-C3N4/g-C3N4 Z-scheme homojunction by using a bottom-up approach, during which the supramolecular complex is initially formed, followed by a facile thermal polycondensation. Based on the active species trapping experiments, Mott–Schottky test and band edge position analysis, the prepared 2D nanosheet g-C3N4/g-C3N4 homojunctions are found to be Z-scheme type, different from those available reported ones with a type-II energy alignment. Benefiting from the specific 2D morphology with large exposed surface area and Z-scheme junction with efficient separation and high redox abilities of the photoinduced electrons and holes, the obtained 2D g-C3N4/g-C3N4 homojunctions are much more active than the conventional g-C3N4/g-C3N4 homojunction (CN-MT) and bulk g-C3N4 (CN-M) under visible light irradiation, validating by the high rhodamine degradation rate of 0.833 h−1, which is about 3.9 and 15.4 times higher than that of CN-MT (0.214 h−1) and CN-M (0.054 h−1), respectively. The present work sheds light on design of novel Z-scheme photocatalysts with specific morphology and thus further application in the field of environment or energy.

Theoretical design of blue phosphorene/arsenene lateral heterostructures with superior electronic properties

In-plane integration of various two-dimensional materials has recently emerged as an exciting approach to the tailoring of desired properties for novel nanoelectronic devices. Type-II lateral heterostructure (LHS) semiconductors with direct bandgap are urgently desired in photovoltaic, photocatalytic and optoelectronic devices. In this work, we design novel AsmPn LHSs with excellent stability composed of blue phosphorene and arsenene along the zigzag interline. In contrast to the wide indirect band gaps of pristine blue phosphorene and arsenene, all LHSs considered here except As2P2 possess narrower, direct gaps. As2P2 is an indirect-gap semiconductor with quasi-type-II band alignment, while the other LHSs are direct-gap semiconductors and have type-II alignment with the electrons (holes) located in monolayer blue phosphorene (arsenene). In addition, it is found these LHSs have high carrier mobilities of up to cm2 V−1 s−1. Interestingly, several attractive characteristics of BP/arsenene LHSs, including direct bandgap and high mobility, can be well preserved even if the component ratios and tensile strains are changed. As4P4 undergoes a transition from type-II to type-I-like band alignment at when we apply tensile strain along the x direction, while a transition from type-II to type-I alignment is observed at if the tensile strain is applied along the y direction. Furthermore, we have found that the energy gaps of LHSs can be effectively manipulated via the strain, width and component ratio. Our predictions highlight the potential applications of blue phosphorene/arsenene LHSs in electronics, photovoltaics, optoelectronics and photocatalysis.

Interface Engineering of Monolayer MoS2/GaN Hybrid Heterostructure: Modified Band Alignment for Photocatalytic Water Splitting Application by Nitridation Treatment.

Interface engineering is a key strategy to deal with the two-dimensional (2D)/three-dimensional (3D) hybrid heterostructure, since the properties of this atomic-layer-thick 2D material can easily be impacted by the substrate environment. In this work, the structural, electronic, and optical properties of the 2D/3D heterostructure of monolayer MoS2 on wurtzite GaN surface without and with nitridation interfacial layer are systematically investigated by first-principles calculation and experimental analysis. The nitridation interfacial layer can be introduced into the 2D/3D heterostructure by remote N2 plasma treatment to GaN sample surface prior to stacking monolayer MoS2 on top. The calculation results reveal that the 2D/3D integrated heterostructure is energetically favorable with a negative formation energy. Both interfaces demonstrate indirect band gap, which is a benefit for longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of the MoS2 side increases after nitridation treatment. The modification to band alignment is then verified by X-ray photoelectron spectroscopy measurement on MoS2/GaN heterostructures constructed by a modified wet-transfer technique, which indicates that the MoS2/GaN heterostructure without nitridation shows a type-II alignment with a conduction band offset (CBO) of only 0.07 eV. However, by the deployment of interface nitridation, the band edges of MoS2 move upward for ∼0.5 eV as a result of the nitridized substrate property. The significantly increased CBO could lead to better electron accumulation capability at the GaN side. The nitridized 2D/3D heterostructure with effective interface treatment exhibits a clean band gap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.

Voltage-dependent differential conductance (dI/dV) imaging of a polymer:fullerene bulk-heterojunction

Abstract With scanning tunneling spectroscopy (STS), we have recorded voltage-dependent differential conductance (dI/dV) images of P3HT:PCBM bulk-heterojunctions (BHJs). Since the materials are seen “energetically” in dI/dV images, the imaging process has provided opportunities to view the nano-domains of the components in the BHJ. When the images were recorded at different voltages, energy-levels at the interface region between two hetero-domains could be viewed energetically. As such, the density of states (DOS) spectra of pristine materials provided location of energy levels of the polymer and the fullerene in forming energy diagram with a type-II alignment from the view-point of charge carriers. DOS spectra recorded in the P3HT:PCBM heterojunction in addition yielded energies that deviated from those of the components in the BHJ. These energies along with bias-dependent dI/dV images evidenced that the interface region between P3HT and PCBM domains possessed energy levels intermediate to those of the components.

Position‐Controlled VLS Growth of Nanowires on Mask‐Patterned GaAs Substrates for Axial GaAsSb/InAs Heterostructures (Phys. Status Solidi A 8∕2018)

Nanowires III-V semiconductor nanowires (NWs) have attracted attention as a constituent of small electronic devices with high performance. In particular, the demand for GaAsSb/InAs NWs is increasing because the GaAsSb/InAs material system can afford structures with type-II and broken-bandgap alignment, depending on the Sb content of GaAsSb. In article number 1700429 Kenichi Kawaguchi and co-workers investigate position-controlled vaporliquid-solid (VLS) growth of nanowires (NWs) for GaAsSb/InAs system. On mask-patterned GaAs(111)B substrates, rod-like InAs NWs are obtained by the introduction of a small amount of HCl gas. The InAs NW diameter depends on the Au catalyst size and pitch of NWs, which can be explained by the VLS growth accompanied by lateral growth. A peculiar change in InAs NW shapes for additive HCl gas is revealed. The formation of axial GaAsSb/InAs heterostructure NWs is demonstrated.

Photoluminescence investigation of type-II GaSb/GaAs quantum dots grown by liquid phase epitaxy

GaSb quantum dots (QDs) with an areal density of ∼1 × 1010 cm−2 are successfully grown by the modified (rapid slider) liquid phase epitaxy technique. The morphology of the QDs has been investigated by scanning electron microscope (SEM) and atom force microscope (AFM). The power-dependence and temperature-dependence photoluminescence (PL) spectra have been studied. The bright room-temperature PL suggests a good luminescence quality of GaSb QDs/GaAs matrix system. The type-II alignment of the GaSb QDs/GaAs matrix system is verified by the blue-shift of the QDs peak with the increase of excitation power. From the temperature-dependence PL spectra, the activation energy of QDs is determined to be 111 meV.

A density functional study of the effect of hydrogen on electronic properties and band discontinuity at anatase TiO2/diamond interface

Tailoring the electronic states of the dielectric oxide/diamond interface is critical to the development of next generation semiconductor devices like high-power high-frequency field-effect transistors. In this work, we investigate the electronic states of the TiO2/diamond 2 × 1–(100) interface by using first principles total energy calculations. Based on the calculation of the chemical potentials for the TiO2/diamond interface, it is observed that the hetero-interfaces with the C-OTi configuration or with two O vacancies are the most energetically favorable structures under the O-rich condition and under Ti-rich condition, respectively. The band structure and density of states of both TiO2/diamond and TiO2/H-diamond hetero-structures are calculated. It is revealed that there are considerable interface states at the interface of the anatase TiO2/diamond hetero-structure. By introducing H on the diamond surface, the interface states are significantly suppressed. A type-II alignment band structure is disclosed at the interface of the TiO2/diamond hetero-structure. The valence band offset increases from 0.6 to 1.7 eV when H is introduced at the TiO2/diamond interface.

Tuning the Carrier Confinement in GeS/Phosphorene van der Waals Heterostructures.

Van der Waals (vdW) heterostructures, which have the advantage of integrating excellent properties of the stacked 2D materials by vdW interactions, have gained increasing attention recently. In this work, within the framework of density functional theory calculations, the electronic properties of vdW heterostructure composed of phosphorene (BP) in black phosphorus phase and GeS monolayer are systematically explored. The results show that the carriers are not separated for both lattice-match and lattice-mismatch heterostructures. For the lattice-match heterostructure, it is found that changing monolayer of GeS to bilayer can increase the energy difference of valence band offsets between GeS and BP, thus realizing electron-hole separation. For the lattice-mismatch heterostructure, altering the layer distance can transform the heterostructure into a typical type-I alignment, but applying the electric field or doping with 2, 3, 5, 6-tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ) can make it display a perfect desirable type-II alignment, where holes migration and electrons transfer are revealed to account respectively for the phenomenon of carrier separation. It is believed that the work would greatly enlarge the potential application of the BP-based heterostructures in photoelectronics and further stimulate the investigation enthusiasms on other fashionable heterostructures and even unassuming heterostructures in which the charming electronic properties can be modulated to emerge by various general methods.

Type-II InSe/MoSe2(WSe2) van der Waals heterostructures: vertical strain and electric field effects

InSe/MoSe2(WSe2) vdWHs with type-II alignment, effectively tuned by E-field and vertical strain, are systematically discussed for future applications in optoelectronic devices.

Design lateral heterostructure of monolayer ZrS2 and HfS2 from first principles calculations

The successful fabrication of two-dimensional lateral heterostructures (LHS’s) has opened up unprecedented opportunities in material science and device physics. It is therefore highly desirable to search for more suitable materials to create such heterostructures for next-generation devices. Here, we investigate a novel lateral heterostructure composed of monolayer ZrS2 and HfS2 based on density functional theory. The phonon dispersion and ab initio molecular dynamics analysis indicate its good kinetic and thermodynamic stability. Remarkably, we find that these lateral heterostructures exhibit an indirect to direct bandgap transition, in contrast to the intrinsic indirect bandgap nature of ZrS2 and HfS2. The type-II alignment and chemical bonding across the interline have also been revealed. The tensile strain is proved to be an efficient way to modulate the band structure. Finally, we further discuss other three stable lateral heterostructures: (ZrSe2)2(HfSe2)2 LHS, (ZrS2)2(ZrSe2)2 LHS and (HfS2)2(HfSe2)2 LHS. Generally, the lateral heterostructures of monolayer ZrS2 and HfS2 are of excellent electrical properties, and may find potential applications for future electronic devices.

Determination of Band Alignment in the Synergistic Catalyst of Electronic Structure-Modified Graphitic Carbon Nitride-Integrated Ceria Quantum-Dot Heterojunctions for Rapid Degradation of Organic Pollutants

We engineered novel heterojunction ceria (CeO2) QDs decorated on the surfaces of graphitic carbon nitride (g-C3N4) nanosheets by a facile in situ hydrothermal synthetic route. Using core-level/valence-band X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy, and work function measurements of the materials, we constructed the energy band alignment at the heterojunction. The band alignment has a Type-II alignment between organic (g-C3N4) and inorganic (CeO2 QDs) semiconductors junction with valence/conduction band offsets (VBO/CBO) of −0.07/–0.31 eV. The calculated band alignment parameters of the heterojunction were compared with the experimental values of g-C3N4/CeO2 QD composite and a new energy band diagram was proposed for the electronic structure-modified g-C3N4/CeO2 QDs heterojunction. The newly constructed heterojunction is formed by carbon-vacancy-promoted g-C3N4 coupled to lower defect-mediated (oxygen vacancies) CeO2, as determined by high-resolution XPS analysis. Moreover, t...

Optical properties of CdSe/ZnTe type II core shell nanostructures

CdSe/ZnTe core shell nanoparticles present original optical and electronic properties because of the spatial separation of electrons and holes caused by the type-II alignment of energy states. Such structure permits access to optical transition energies that are not limited to band gap energies and hence can provide narrow emission spectra and a broad absorption range.In this study, we have investigated the nanostructure size effect on the electronic and optical properties of type II CdSe/ZnTe core shell quantum dot. The carriers wave functions and eigenenergies have been achieved by solving the Schrödinger equation, in the framework of the effective-mass envelope-function theory. The impact of core radius R1 and the shell radius R2 on electron and hole energies has been studied for the fundamental and the first excited states. The Coulomb interaction has been calculated and the exciton energy has been deduced. Then, the absorption coefficient has been examined by means of the compact density matrix, the pick of the absorption increases with R2 and is red shifted. Good excitonic radiative life time in this type II core shell quantum dot has been obtained. The photoluminescence from the CdSe/ZnTe core shell can be adjusted to covers practically all the visible wavelength, which is very important for biomedical and photovoltaics.

Layered material GeSe and vertical GeSe/MoS2 p-n heterojunctions

Group-IV monochalcogenides are emerging as a new class of layered materials beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP). In this paper, we report experimental and theoretical investigations of the band structure and transport properties of GeSe and its heterostructures. We find that GeSe exhibits a markedly anisotropic electronic transport, with maximum conductance along the armchair direction. Density functional theory calculations reveal that the effective mass is 2.7 times larger along the zigzag direction than the armchair direction; this mass anisotropy explains the observed anisotropic conductance. The crystallographic orientation of GeSe is confirmed by angleresolved polarized Raman measurements, which are further supported by calculated Raman tensors for the orthorhombic structure. Novel GeSe/MoS2 p-n heterojunctions are fabricated, combining the natural p-type doping in GeSe and n-type doping in MoS2. The temperature dependence of the measured junction current reveals that GeSe and MoS2 have a type-II band alignment with a conduction band offset of ~0.234 eV. The anisotropic conductance of GeSe may enable the development of new electronic and optoelectronic devices, such as high-efficiency thermoelectric devices and plasmonic devices with resonance frequency continuously tunable through light polarization direction. The unique GeSe/MoS2 p-n junctions with type-II alignment may become essential building blocks of vertical tunneling field-effect transistors for low-power applications. The novel p-type layered material GeSe can also be combined with n-type TMDCs to form heterogeneous complementary metal oxide semiconductor (CMOS) circuits.

Effect of strain on band alignment of GaAsSb/GaAs quantum wells

GaAsSb/GaAs quantum wells are of great interest for optical communications; however, their band alignment properties are not fully understood, particularly at 35% Sb alloy concentration used for emission at 1.3 μm. We use device simulation methods based on the 8 × 8 k·p theory to explore the effects of GaAsSb/GaAs quantum well composition, width, and strain on the band alignment. Strain-relaxed wells demonstrate type-I alignment and pseudomorphic wells demonstrate type-II alignment, regardless of quantum-well composition or thickness for wells wider than 3 nm. For partially strain-relaxed wells, we determine the band alignment as a function of the interplay of composition, width, and strain. Our calculated results at various strain conditions agree well with published experimental data. This work provides insight on band alignment of GaAsSb/GaAs quantum wells, as well as of embedded quantum dots with strong confinement along the out-of-plane direction.

Excitons in core-only, core-shell and core-crown CdSe nanoplatelets: Interplay between in-plane electron-hole correlation, spatial confinement, and dielectric confinement

Using semi-analytical models we calculate the energy, effective Bohr radius and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size $L \gtrsim 20$ nm the exciton behavior is essentially that in a quantum well, with superradiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for $L < 20$ nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, different response is observed for core/shell and core/crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g. in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching $\mu s$.

Electrostatic gating dependent multiple-band alignments in a high-temperature ferromagnetic Mg(OH) 2/VS2 heterobilayer

Searching for ferromagnetic (FM) van der Waals (vdW) heterostructures with high Curie temperature and multiple-band alignments is crucial to develop next-generation spintronic and optoelectronic nanodevices. Here, through first-principles methods, the FM vdW heterostructure is found for the first time in the Mg(${\mathrm{OH})}_{2}/{\mathrm{VS}}_{2}$ heterobilayer with high Curie temperature (385 K) and type-II alignment. The negative electric field induces the transition from type-II to type-III alignment in the spin-up channel. However, under the positive electric field, the type-II to type-I alignment transition occurs in the spin-up and spin-down channels. This work provides the possibilities of realizing the high-temperature FM vdW heterostructures and electrostatic gating dependent multiple-band alignments in practice.

Atmospheric pressure-MOVPE growth of GaSb/GaAs quantum dots

This study focuses on the growth of GaSb/GaAs quantum dots (QD) using an atmospheric pressure MOVPE system. For the best uncapped dots, the average dot height, base diameter and density are 5nm, 45nm and 4.5×1010cm−2, respectively. Capping of GaSb QDs at high temperatures caused flattening and formation of thin inhomogeneous GaSb layer inside GaAs resulting in no obvious QD PL peak. Capping at low temperatures lead to the formation of dot-like features and a wetting layer (WL) with distinct PL peaks for QD and WL at 1097nm and 983nm respectively. Some of the dot-like features had voids. An increase in excitation power caused the QD and WL peaks to shift to higher energies. This is attributed to electrostatic band bending leading to triangular potential wells, typical of type-II alignment between GaAs and strained GaSb. Variable temperature PL measurements of the QD sample showed the decrease in the intensity of the WL peak to be faster than that of the QD peak as the temperature increased.

Band alignments at strained Ge1−xSnx/relaxed Ge1−ySny heterointerfaces

Type-I, type-II, reverse type-I, and reverse type-II band alignments are found theoretically in strained Ge1−xSnx (0 ⩽ x ⩽ 0.3) grown on relaxed Ge1−ySny substrates (0 ⩽ y ⩽ 0.3) using the model-solid theory. The prerequisite bandgaps, and energy difference between the top valence band edge and the average valence band position of GeSn are obtained by the nonlocal empirical pseudopotential method. For the indirect-gap (L valleys) Ge1−xSnx on relaxed Ge1−ySny, the band alignments are type-I and reverse type-I under biaxial compressive strain (x > y) and biaxial tensile strain (x < y), respectively. For the direct-gap (Γ valley) Ge1−xSnx on relaxed Ge1−ySny, the biaxial compressive strain yields type-I and type-II alignment, while the biaxial tensile strain yields reverse type-I and reverse type-II alignments.

How sulfidation of ZnO powders enhances visible fluorescence

The type-II alignment of ZnS domains in sulfidated ZnO phosphors scavenges free holes, dramatically enhancing white fluorescence from oxygen vacancies.

Energy spectrum of an exciton in a CdSe/ZnTe type-II core/shell spherical quantum dot

The binding energy of an exciton inside a CdSe/ZnTe core/shell spherical quantum dot was theoretically examined taking into account the dependence of the dielectric constant and charge carriers effective mass on radius, and using the envelope function approximation. Such a structure presents original optical and electronic properties because of the spatial separation of electrons and holes caused by the type-II alignment of energy states. The mean distance between the electron and hole was calculated variationally using a trial function taking into account the coulomb interaction between charge carriers. Our numerical results provide a description to the size dependence of the binding energy of an exciton inside a core/shell nanoheterostructure type-II. Indeed, by controlling the inner and outer radii, we can precisely control the energy spectrum of the exciton.