type-II Alignment Research Articles

Mechanistic insights into the charge transportation behavior and enhanced photocatalytic performance of BaTiO3/CdS heterostructure by calculations of first-principles and experiments of tracking charge flow direction

Mechanistic insight into charge transportation behavior of the heterogeneous photocatalyst is a key issue to elucidate the photocatalytic performance. This study aims to employ first-principles calculations and tracking charge flow direction to reveal the charge transportation behavior and enhanced photocatalytic performance of CdS modified tetragonal phase BaTiO3 (BTO/CdS). The experiment of X-ray photoelectron spectroscopy and the first-principles calculations confirm the establishment of a built-in electric field and a type-II energy alignment, which are favorable for transport and separation of charge carriers in BTO/CdS heterojunction. The tracking charge flow direction demonstrates that photogenerated electrons of CdS transfer to BTO and photogenerated holes of BTO transfer to CdS. As a result, the constructed BTO/CdS heterojunction nanostructure exhibits substantially enhanced photocatalytic performance. The degradation rate of rhodamine B by BTO/CdS photocatalyst is 98.6 % under simulated light irradiation within 25 min, which is significantly higher than that (29.0 %) of the pure BTO photocatalyst. This study offers a rational strategy to understand the charge transportation behavior and enhanced photocatalytic performance of heterostructures by first-principles calculations and tracking charge flow direction for the design of high-performance photocatalysts.

A Type-II InP/MoTe2 van der waals heterostructure with adjustable electronic properties under external electric field and biaxial strain

We employed first-principles methods grounded in density functional theory to explore the geometric arrangement of the InP/MoTe2 heterojunction. Various configurations were examined to identify the stable state of the heterojunction. The findings highlight that the H1-type heterojunction, featuring an interlayer distance of 3.286 Å, shows notable stability, displaying a type-II alignment with an indirect bandgap of 0.630 eV. Under the influence of external electric fields and strain, shifts in the bandgap and charge transfer of the heterojunction are observed, as indicated by the study. The heterojunction shifts to a metal state from a semiconductor under the influence of electric fields and strain, while still retaining the intrinsic properties of each layer. InP/MoTe2 heterojunctions are harnessed to boost material absorption and expand the range of detectable wavelengths, particularly in the ultraviolet spectrum. Importantly, external electric fields and mechanical strain have the capability to adjust the optical properties of the heterojunction, expanding its applicability and future development potential in optoelectronic devices.

Direct Z-scheme ZnS/HfS2 heterojunction for photocatalytic overall water splitting with high carrier mobility

The electronic and photocatalytic properties of a type-II ZnS/HfS2 heterojunction are comprehensively analyzed through first-principles calculations in this paper. By calculating the binding energy and phonon spectrum of five potential stacking patterns, the stable structure of the ZnS/HfS2 heterojunction is determined. The electronic structure calculations indicate that the heterojunction has a type-II alignment with a 1.26 eV band gap. The heterojunction exhibits high carrier mobilities (4387.36 and 9561.39 cm2V–1S−1 for X and Y directions). The photocatalytic water splitting process of the heterojunction can be explained by the direct Z-scheme mechanism. The built-in electric field enables rapid recombination of electron-hole pairs at CBM and VBM, while the hydrogen evolution reaction and oxygen evolution reaction can separately be achieved at the ZnS layer and HfS2 layer to complete the overall water splitting. The free energy analysis indicates that the ZnS/HfS2 heterojunction has high activity for overall water splitting under illumination. Moreover, the calculated solar-to-hydrogen efficiency of the heterojunction reaches 30%, ensuring its photocatalytic performance. Remarkably, the type-II alignment is maintained and the catalytic performance can be further improved under biaxial strain because of the enhanced optical absorption in visible light and the reduced band gap. This work reveals that ZnS/HfS2 heterojunction is a potentially novel photocatalytic material for efficient overall water splitting.

Contact Geometry-Dependent Excitonic Emission in Mixed-Dimensional van der Waals Heterostructures.

Manipulation of excitonic emission in two-dimensional (2D) materials via the assembly of van der Waals (vdW) heterostructures unlocks numerous opportunities for engineering their photonic and optoelectronic properties. In this work, we introduce a category of mixed-dimensional vdW heterostructures, integrating 2D materials with one-dimensional (1D) semiconductor nanowires composed of vdW layers. This configuration induces spatially distinct localized excitonic emissions through a tailored interfacial heterolayer atomic arrangement. By precisely adjusting both the axial and sidewall facet orientations of bottom-up grown PbI2 vdW nanowires and by transferring them onto 1L WSe2 flakes, we establish vdW heterointerfaces with either perpendicular or parallel interatomic arrangements. The edge-standing heterojunction, featuring perpendicular PbI2 layers atop WSe2, promotes efficient charge transfer through the edges and coupled localized states, leading to an enhanced redshifted excitonic emission. Conversely, the layer-by-layer heterointerface, where PbI2 layers are in parallel contact with WSe2, exhibits substantial quenching due to deep midgap states in a type-II alignment, as evidenced by power-dependent measurements and first-principle calculations. Our results introduce a method for actively manipulating excitonic emissions in 2D transition metal dichalcogenides (TMDs) through edge engineering, highlighting their potential in the development of various quantum devices.

Programmable Interfacial Band Configuration in WS2/Bi2O2Se Heterojunctions.

van der Waals heterojunctions based on transition-metal dichalcogenides (TMDs) offer advanced strategies for manipulating light-emitting and light-harvesting behaviors. A crucial factor determining the light-material interaction is in the band alignment at the heterojunction interface, particularly the distinctions between type-I and type-II alignments. However, altering the band alignment from one type to another without changing the constituent materials is exceptionally difficult. Here, utilizing Bi2O2Se with a thickness-dependent band gap as a bottom layer, we present an innovative strategy for engineering interfacial band configurations in WS2/Bi2O2Se heterojunctions. In particular, we achieve tuning of the band alignment from type-I (Bi2O2Se straddling WS2) to type-II and finally to type-I (WS2 straddling Bi2O2Se) by increasing the thickness of the Bi2O2Se bottom layer from monolayer to multilayer. We verified this band architecture conversion using steady-state and transient spectroscopy as well as density functional theory calculations. Using this material combination, we further design a sophisticated band architecture incorporating both type-I (WS2 straddles Bi2O2Se, fluorescence-quenched) and type-I (Bi2SeO5 straddles WS2, fluorescence-recovered) alignments in one sample through focused laser beam (FLB). By programming the FLB trajectory, we achieve a predesigned localized fluorescence micropattern on WS2 without changing its intrinsic atomic structure. This effective band architecture design strategy represents a significant leap forward in harnessing the potential of TMD heterojunctions for multifunctional photonic applications.

Carrier dynamics competition in the nanocrystal-molecule complex for triplet generation studied by transient-absorption spectroscopy

Owing to the long-lived decay of triplet excited state, extensive efforts have been devoted to efficient triplet generation for applications covering triplet–triplet annihilation for photon upconversion, photocycloaddition and photoredox catalysis. Among the candidates, nanocrystal-molecule complexes have received tremendous attention for triplet generation because of easier spin flip and negligible energy loss during intersystem crossing. However, the triplet energy transfer (TET) from nanocrystals (NCs) to molecules can be very complicated in actual situation due to intricate energy level alignment and inevitable defect states, which often involves various decay pathes of the excited state competing with TET. Understanding the detailed carrier dynamics in such complexes is strongly necessary for related applications. Here, a CdSe-TCA (5-tetracene carboxylic acid) complex with a Type-II like energy level alignment is synthesized through precisely adjusting the dimension of CdSe NC. Based on series of spectral measurements, especially the transient absorption (TA) spectroscopy, the results show various carrier dynamics including hole-transfer-mediated TET, Förster resonance energy transfer (FRET) and carrier trapping. Although the carrier trapping by defect states in CdSe NC is revealed not associated with the TET from CdSe to TCA, the FRET is proved to competing with the TET process. Both the FRET and defect states should be refrained for efficient TET in such complexes. This study could provide further insight for understanding the carrier dynamics competition in NC-molecule complexes for triplet generation and benefit related optoelectronics applications.

Optoelectronic and photocatalytic behaviour of a type-II GaAlS2/HfS2 heterostructure: ab initio study

Theoretical examination based on first principle computation has been conducted for van der Waals heterostructure (vdwHS) GaAlS2/HfS2 including structural, optoelectronic and photocatalytic characteristics. From the adhesion energy calculation, the AB configuration of GaAlS2/HfS2 vdwHS is the most stable. A type-II GaAlS2/HfS2 vdwHS is a dynamically and thermally stable structure. The band edge position, projected band, and projected charge densities verify the type-II alignment of GaAlS2/HfS2 vdwHS. For GaAlS2/HfS2, GaAlS2 is acting as a donor and HfS2 is acting as an acceptor ensured by the charge density difference plot. The electron localized function validates the weak van der Waals interaction between GaAlS2 and HfS2. The GaAlS2/HfS2 vdwHS possess an indirect bandgap of 1.54 eV with notable absorption in the visible range. The findings assure that the GaAlS2/HfS2 vdwHS is an efficient photocatalyst for pH 4–8. The band alignment of GaAlS2/HfS2 is suitable for Z-scheme charge transfer. The strain influenced band edge suggests that the GaAlS2/HfS2 vdwHS remains photocatalytic for strain −4% to +6% in both cases of uniaxial and biaxial strains.

Design and performance of GaSb-based quantum cascade detectors.

InAs/AlSb quantum cascade detectors (QCDs) grown strain-balanced on GaSb substrates are presented. This material system offers intrinsic performance-improving properties, like a low effective electron mass of the well material of 0.026 m 0, enhancing the optical transition strength, and a high conduction band offset of 2.28 eV, reducing the noise and allowing for high optical transition energies. InAs and AlSb strain balance each other on GaSb with an InAs:AlSb ratio of 0.96:1. To regain the freedom of a lattice-matched material system regarding the optimization of a QCD design, submonolayer InSb layers are introduced. With strain engineering, four different active regions between 3.65 and 5.5 µm were designed with InAs:AlSb thickness ratios of up to 2.8:1, and subsequently grown and characterized. This includes an optimized QCD design at 4.3 µm, with a room-temperature peak responsivity of 26.12 mA/W and a detectivity of 1.41 × 108 Jones. Additionally, all QCD designs exhibit higher-energy interband signals in the mid- to near-infrared, stemming from the InAs/AlSb type-II alignment and the narrow InAs band gap.

One-dimensional core/shell radial heterojunction with cascade type-II energy-band alignment for enhanced broadband photodetection

One-dimensional core/shell radial heterojunction with cascade type-II energy-band alignment for enhanced broadband photodetection

High performance metal-semiconductor-metal ultraviolet photodetector based on mixed-dimensional TiO2/CsPbBr3 heterostructures

Zero-dimensional (0D) and one-dimensional (1D) mixed heterostructure semiconductors can bring superior electrical and optoelectronic performances due to the synergistic advantages of different dimensionalities. Here, a metal-semiconductor–metal (MSM) ultraviolet (UV) photodetector based on 1D-0D TiO2/CsPbBr3 heterostructure semiconductor is constructed, which exhibits excellent photodetection performance. A back-to-back Schottky contact is formed in the MSM (Au/TiO2/Au) structure due to the large band-energy bending resulted from the abundant surface-states at 1D-TiO2 surface. Under an applied voltage, a small saturation current flows through the device. Benefiting from the decoration of CsPbBr3 QDs, the dark current of MSM photodetectors can be further suppressed, and producing the improved on/off ratio (I light/I dark), photoresponsivity (R λ ), and detectivity (D*). PL properties study suggested that an energy transfer is occurred between the 0D-CsPbBr3 and 1D-TiO2. The TiO2/CsPbBr3 heterojunctions are beneficial for photo-induced charge transfer in hetero-interface because of the type-II energy-band alignment, but not non-radiative energy transfer from 0D-CsPbBr3 to 1D-TiO2. On the whole, this study depicts a fascinating coupling architecture of mixed-dimensional materials toward implementing low-cost and high-performance optoelectronic devices.

Broadband Van-der-Waals Photodetector Driven by Ferroelectric Polarization.

The potential for various future industrial applications has made broadband photodetectors beyond visible light an area of great interest. Although most 2D van-der-Waals (vdW) semiconductors have a relatively large energy bandgap (>1.2eV), which limits their use in short-wave infrared detection, they have recently been considered as a replacement for ternary alloys in high-performance photodetectors due to their strong light-matter interaction. In this study, a ferroelectric gating ReS2 /WSe2 vdW heterojunction-channel photodetector is presented that successfully achieves broadband light detection (>1300nm, expandable up to 2700nm). The staggered type-II bandgap alignment creates an interlayer gap of 0.46eV between the valence band maximum (VBMAX ) of WSe2 and the conduction band minimum (CBMIN ) of ReS2 . Especially, the control of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric dipole polarity for a specific wavelength allows a high photoresponsivity of up to 6.9 × 103 A W-1 and a low dark current below 0.26 nA under the laser illumination with a wavelength of 405nm in P-up mode. The achieved high photoresponsivity, low dark current, and full-range near infrared (NIR) detection capability open the door for next-generation photodetectors beyond traditional ternary alloy photodetectors.

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe2-WS2-WSe2 trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe2 valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe2 layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

Prediction of direct Z-scheme H and H́-phase of MoSi2N4/MoSX (X = S, Se) van der Waals heterostructures: A promising candidate for photocatalysis

Photocatalysis has emerged as an appealing strategy for environmental remediation and sustainable energy production. The density functional theory (DFT) approximations were employed to investigate the heterostructures (HTS) of 2H and 2H́ polymorphs of MoSi2N4/ MoSX (X = S, Se) for photocatalytic applications. To ensure their applicability for water-splitting photocatalysis, we examined the heterostructures for desirable electronic properties. The structures (except 2H́-MoSi2N4/MoSSe) have a direct Z-scheme type-II alignment which further favors the carrier separation causing hydrogen and oxygen production at distant positions. The bandgaps of structures lie within the desirable range of 1.23 to 3 eV, favoring visible light absorption. The difference in the work function of monolayers (of about 0.5 to 1 eV) develops an electric field at the interface of HTS, causing the separation of carriers. The reduction potential lies between −3.8 to −4.6 eV, while the oxidation potential lies between −5.7 to −6.2 eV. It indicates that although oxidation potentials incorporate water redox levels, reduction potentials of some HTS do not. To ensure HER, Gibbs free energies were calculated. The vacancy defects at S/Se site gave ΔGH∗ less than the catalytic threshold of 0.2 eV. Due to ultrafast charge transfer, broad spectral response, and improved photocatalytic capability due to the formation of radicals at the surface, heterostructures can act as a competitive choice over monolayers for water-splitting photocatalysis.

Gate-Induced Trans-Dimensionality of Carrier Distribution in Bilayer Lateral Heterosheet of MoS2 and WS2 for Semiconductor Devices with Tunable Functionality

Using the metal–organic chemical vapor deposition technique, we synthesized a bilayer lateral heterostucture of MoS2 and WS2, each of which layers consist of alternately arranged nanostrips of MoS2 and WS2. Transmission electron microscope images exhibit a checked pattern contrast, reflecting the three distinct interlayer metal arrangements of Mo/Mo, W/Mo (Mo/W), and W/W stacking. Theoretical calculations based on the density functional theory elucidated that the bilayer lateral heterostructure of MoS2 and WS2 exhibits complexed type-II band edge alignments depending on the interlayer metal arrangement: The conduction band edge is located at the Mo atom in a Mo/Mo sector, while the valence band edge is located at the W atom in a W/W sector. According to the complexed band edge alignment, the field-induced carrier injection behavior in the bilayer heterosheet composed of MoS2 and WS2 strips shows gate-induced trans-dimensionality, where the accumulated carrier distributions continuously vary from zero- to two-dimensional based on the applied gate voltage. Tunable carrier dimensionality can be applicable for wide areas of electronics, such as quantum dot arrays with tunable dot–dot interaction.

Type-II band alignment for atomic layer deposited HfSiO4 on α-Ga2O3

There is increasing interest in α-polytype Ga2O3 for power device applications, but there are few published reports on dielectrics for this material. Finding a dielectric with large band offsets for both valence and conduction bands is especially challenging given its large bandgap of 5.1 eV. One option is HfSiO4 deposited by atomic layer deposition (ALD), which provides conformal, low damage deposition and has a bandgap of 7 eV. The valence band offset of the HfSiO4/Ga2O3 heterointerface was measured using x-ray photoelectron spectroscopy. The single-crystal α-Ga2O3 was grown by halide vapor phase epitaxy on sapphire substrates. The valence band offset was 0.82 ± 0.20 eV (staggered gap, type-II alignment) for ALD HfSiO4 on α-Ga0.2O3. The corresponding conduction band offset was −2.72 ± 0.45 eV, providing no barrier to electrons moving into Ga2O3.

Unambiguous spectral characterization on triplet energy transfer from quantum dots mediated by hole transfer competing with other carrier dynamics.

Triplet generation by quantum dots (QDs)-sensitized molecules emerges great potential in many applications. However, the mechanism of triplet energy transfer (TET) is still fuzzy especially due to the complicated energy level alignment of QDs and molecules or trap states in QDs. Here, CdSe QDs and 5-tetracene carboxylic acid (TCA) molecules are selected as the triplet donor and acceptor, respectively, to form a TET system. By tuning the band gap of CdSe, the CdSe-TCA complex is exactly designed to present a Type-II like alignment of relative energetics. Coupling the transient absorption and time-resolved fluorescence spectra, all carrier dynamics is distinctly elucidated. Quantitative analysis demonstrates that hole transfer persisting for ∼ 2 ps outcompetes all other carrier dynamics such as electron trapping (∼100 ps level), charge recombination (∼ 5 ns) and the so-called "back transfer charge recombination" (∼50 ns), and thus leads to a hole-transfer-mediated TET process. The low TET yield (∼34.0%) ascribed to electron behavior can be further improved if electron trapping and charge recombination are efficiently suppressed. The observation on distinguishable carrier dynamics attributed to legitimate design of energy level alignment facilitates a better understanding of the TET mechanism from QDs to molecules as well as further development of photoelectronic devices based on such TET systems.

Growth behaviors and emission properties of Co-deposited MAPbI3 ultrathin films on MoS2

Hybrid organic–inorganic perovskite thin films have attracted much attention in optoelectronic and information fields because of their intriguing properties. Due to quantum confinement effects, ultrathin films in nm scale usually show special properties. Here, we report on the growth of methylammonium lead iodide (MAPbI3) ultrathin films via co-deposition of PbI2 and CH3NH3I (MAI) on chemical-vapor-deposition-grown monolayer MoS2 as well as the corresponding photoluminescence (PL) properties at different growing stages. Atomic force microscopy and scanning electron microscopy measurements reveal the MoS2 tuned growth of MAPbI3 in a Stranski–Krastanov mode. PL and Kelvin probe force microscopy results confirm that MAPbI3/MoS2 heterostructures have a type-II energy level alignment at the interface. Temperaturedependent PL measurements on layered MAPbI3 (at the initial stage) and on MAPbI3 crystals in averaged size of 500 nm (at the later stage) show rather different temperature dependence as well as the phase transitions from tetragonal to orthorhombic at 120 and 150 K, respectively. Our findings are useful in fabricating MAPbI3/transition-metal dichalcogenide based innovative devices for wider optoelectronic applications.

2D‐Material‐Integrated Micromachines: Competing Propulsion Strategy and Enhanced Bacterial Disinfection

2Dtransition-metal-dichalcogenidematerials, such as molybdenum disulfide (MoS2 ) have received immense interest owing to their remarkable structure-endowed electronic, catalytic, and mechanical properties for applications in optoelectronics, energy storage, and wearable devices. However, 2D materials have been rarely explored in the field of micro/nanomachines, motors, and robots. Here, MoS2 with anatase TiO2 is successfully integrated into an original one-side-open hollow micromachine, which demonstrates increased light absorption of TiO2 -based micromachines to the visible region and the first observed motion acceleration in response to ionic media. Both experimentation and theoretical analysis suggest the unique type-II bandgap alignment of MoS2 /TiO2 heterojunction that accounts for the observed unique locomotion owing to a competing propulsion mechanism. Furthermore, by leveraging the chemical properties of MoS2 /TiO2 , the micromachines achieve sunlight-powered water disinfection with 99.999%Escherichia colilysed in anhour. This research suggests abundant opportunities offered by 2D materials in the creation of a new class of micro/nanomachines and robots.

Tailoring the electronic and optical properties of ZrS2/ZrSe2 vdW heterostructure by strain engineering

Under framework of first-principles calculation, we construct ZrS2/ZrSe2 heterostructure and explore the effects of the uniaxial and biaxial strains on its electronic and optical properties. Band structure shows the unstrained ZrS2/ZrSe2 heterostructure has a type-II band alignment. The large binding energy and small interface distance indicate there is a chemical combination between ZrS2 and ZrSe2 layers beyond the van der Waals interaction. Due to the redistribution of charge, a built-in electric field from ZrSe2 to ZrS2 is formed on the interface of the ZrS2/ZrSe2 heterostructure. In addition, constructing the ZrS2/ZrSe2 heterostructure can enhance light absorption intensity. Then we investigate the effects of uniaxial and biaxial strains in the range of −8%∼8% on the electronic and optical properties of the ZrS2/ZrSe2 heterostructure. The band gap of the ZrS2/ZrSe2 heterostructure increases (decreases) with the increase of tensile (compressive) strain, this tunable band gap shows its potential applications in electronic devices. The band type is very sensitive to strain, because the type-II alignment can be converted to type-I alignment. In addition, under uniaxial (−4% and −2%) and biaxial (−2%) compressive strains, a red-shift phenomenon is occurred compared to the unstrained ZrS2/ZrSe2 heterostructure, indicating the better optical gas sensitivity and can be used in infrared light detectors. These findings make possible the application of two-dimensional ZrS2/ZrSe2 heterostructure in multifunctional electronic and optoelectronic devices.

Insights into controllable electronic properties of 2D type-II Twin-Graphene/g-C3N4 and type-I Twin-Graphene/hBN vertical heterojunctions via external electric field and strain engineering

Twin-graphene (Twin-G), an accessible carbon allotrope composed of sp2 hybridized carbon atoms, has attracted considerable interest due to its intrinsic bandgap and tunable electronic properties. This work evaluates the near-edge electronic structures and the carrier mobility of the Twin-G/g-C3N4 and Twin-G/hBN vdW heterojunctions employing the first-principles calculations. The results demonstrate that Twin-G/g-C3N4 retains a staggered type-II alignment, which may benefit the highly-efficient photogenerated carrier separation. Whereas the Twin-G/hBN exhibits type-I alignment. An external electrical field induces a semiconductor-to-metal and indirect-to-direct gap transition in Twin-G/g-C3N4 heterostructure. The type-II-to-type-I transition in Twin-G/g-C3N4 under tensile strain can be understood from the near-gap wave functions morphology. G-C3N4 substrate has a more significant effect to enhance the carrier mobility of Twin-G. The electron mobility along the zigzag-direction in Twin-G/g-C3N4 is ∼1751 cm2V−1s−1, far surpassing that of Twin-G monolayer. These results open up promising opportunities for the development of optoelectronic devices based on the Twin-G heterostructures.