Tunable Band Alignment Research Articles

Polyacid-Modulated Carrier Dynamic Behavior at the Interface of 0D/2D Heterojunctions.

Heterojunctions formed by polyoxometalates and 2D materials draw attention owing to their remarkable photoelectric and catalytic properties. However, the intrinsic mechanisms of polyoxometalates regulating the heterojunction photoelectric properties are unclear. Herein, we constructed two types of heterojunctions by integrating polyoxometalates (Keggin-type H3PW12O40 and Lindqvist-type H2W6O19) on g-C3N4 monolayers, exploring photoexcited carrier dynamics in these heterojunctions by ab initio calculations combined with nonadiabatic molecular dynamics (NAMD) simulations. Our results show that electrons and holes in H3PW12O40 on g-C3N4 monolayers relax within 583 and 760 fs, respectively. The electron-hole recombination occurs at 342 fs, faster than carrier separation, aligning with the behavior of Z-type heterojunctions. Contrarily, the H2W6O19/g-C3N4 heterojunction exhibits the typical characteristics of type II heterojunctions, with a long photogenerated carrier lifetime reaching 652 fs. These findings show tunable band alignment in polyoxometalate-supported systems by modulating polyoxometalate type, influencing hot electron dynamics, and guiding 0D/2D heterojunction design.

Tunability in electronic and optical properties of GaS/PbS vdW heterostructure

A promising novel class of heterostructures has recently emerged, combining a semiconducting GaS monolayer with other 2D materials for energy-related applications. In this study, we considered the layered PbS to form the van der Waals heterostructure with GaS and investigated its properties using density functional theory. The GaS/PbS heterostructure exhibits a type-II heterostructure with an indirect bandgap of 1.65 eV, displaying enhanced light absorption across the visible spectrum. Moreover, the heterostructure's energy band gap shows tunability with an applied transverse electric field attributed to the spontaneous electric polarization within the lattice. Subsequently, it contributes to increased optical absorbance and light harvesting efficiency under ±0.2 V/Å electric field. The applied electric field also offers tunable band alignments (transition type-II and type-I), making it a potential candidate for solar cells that can optimize their efficiency based on varying light conditions.

Self‐Confined Construction of Facet Heterojunction with Tunable Band Alignment for Enhanced Photocatalytic CO2 Reduction

AbstractAlthough crystal facet engineering is extensively studied in energy and environmental technologies, the in‐depth understanding of intrinsic mechanisms governing crystal facet heterojunctions in tuning band alignments is still limited. Here, novel Bi5O7NO3 crystals exposing tailor {080} facets are synthesized via NH4+‐assisted self‐confined construction. It has been confirmed that NH4+ ions selectively adsorb on the {141} facets, inversely inducing the growth of desired {080} crystal facets, while the tailored {080} facets facilitate the generation of oxygen vacancies. The controllable concentration of oxygen vacancies, influenced by different exposed facets, can optimize the relative positions of Fermi levels and shift the photoelectron transfer route between the {141} and {080}‐OV facets from type‐II to S‐scheme, thus triggering rapid charge transport channels and effectively suppressing electron‐hole recombination. DFT calculation verifies that the energy barrier for the *COOH formation on Bi5O7NO3‐{080}‐OV is the lowest, thereby promoting the generation of CO. The well‐designed Bi5O7NO3 crystals with the optimal {080}/{141} facet ratio exhibit a 3.8‐fold enhancement in photocatalytic CO2 reduction, compared to traditional Bi5O7NO3 dominated by the {141} facets. This work offers new insights into the regulation of band alignments at heterointerfaces and the design of high‐efficiency photocatalysts.

Two-Dimensional GeS/SnSe2 Tunneling Photodiode with Bidirectional Photoresponse and High Polarization Sensitivity.

A two-dimensional (2D) broken-gap (type-III) p-n heterojunction has a unique charge transport mechanism because of nonoverlapping energy bands. In light of this, type-III band alignment can be used in tunneling field-effect transistors (TFETs) and Esaki diodes with tunable operation and low consumption by highlighting the advantages of tunneling mechanisms. In recent years, 2D tunneling photodiodes have gradually attracted attention for novel optoelectronic performance with a combination of strong light-matter interaction and tunable band alignment. However, an in-depth understanding of the tunneling mechanisms should be further investigated, especially for developing electronic and optoelectronic applications. Here, we report a type-III tunneling photodiode based on a 2D multilayered p-GeS/n+-SnSe2 heterostructure, which is first fabricated by the mechanical exfoliation and dry transfer method. Through the Simmons approximation, its various tunneling transport mechanisms dependent on bias and light are demonstrated as the origin of excellent bidirectional photoresponse performance. Moreover, compared to the traditional p-n photodiode, the device enables bidirectional photoresponse capability, including maximum responsivity values of 43 and 8.7 A/W at Vds = 1 and -1 V, respectively, with distinctive photoactive regions from the scanning photocurrent mapping. Noticeably, benefiting from the in-plane anisotropic structure of GeS, the device exhibits an enhanced photocurrent anisotropic ratio of 9, driven by the broader depletion region at Vds = -3 V under 635 nm irradiation. Above all, the results suggest that our designed architecture can be potentially applied to CMOS imaging sensors and polarization-sensitive photodetectors.

Band Alignment Engineering in 2D Ferroelectric Van der Waals Heterostructures for All‐In‐One Optoelectronic Architecture

Abstract2D van der Waals (vdW) heterostructures consisting of vertically stacking atomically thin semiconductors with different band structures provide a flexible platform to design integrated electronic and optoelectronic devices with multi‐functionalities. However, the realization of device multifunctionality requires the heterostructures with tunable band alignments. Here an efficient strategy is proposed by constructing 2D vdW ferroelectric semiconductor heterostructures composed of atomically thin ferroelectrics and semiconductors to achieve this goal. These calculated results indicate that the local built‐in electric field derived from the ferroelectric polarization can effectively modulate the band alignment of the heterostructures, leading to 36 potential band‐alignment transition pathways. Using SnS/In2Se3 vdW heterostructure as a prototype example, a reversible switching from high‐resistance to low‐resistance state is demonstrated by the band‐alignment transition from type‐II to type‐III driven by ferroelecric polarization switching, consequently leading to giant tunneling electroresistance (TER) ratio as high as 1012%. Moreover, the heterostructure with the momentum‐space matching band structure and in‐plane anisotropy exhibits broadband photoresponse from near‐infrared to ultraviolet regions and excellent polarization sensitivity with the dichroic ratio up to 10.3. The ferroelectric polarization‐dependent conductance state and photoresponse in the heterostructures make them large potential for the realization of all‐in‐one optoelectronic architecture in artificial vision system.

Tunable band alignment and optical properties in van der Waals heterostructures based on two-dimensional materials Janus-MoSSe and C3N4

Van der Waals heterostructures with tunable band alignments are the promising candidates for the fabrication of high-performance multifunctional nano-optoelectronic devices. In this work, we investigate the band alignments and optical properties of two-dimensional MoSSe/C3N4 and C3N4/MoSSe heterostructures using first-principles methods. The two most stable MoSSe/C3N4 (C3N4-Se) and C3N4/MoSSe (C3N4-S) heterostructures (labeled as A2 and B2, respectively) out of the twelve possible heterostructures are selected for the corresponding properties research. It is found that the A2 exhibits type-I band alignment, making it suitable for light-emitting applications, while the B2 exhibits typical type-II band alignment, which is favorable for carrier separation. Moreover, the band alignment of the two heterostructures can be modulated by the external electric fields, that is, band alignment transition between type-I and type-II. In addition, the main absorption peaks of both heterostructures in their pristine state are located in the visible light region (approximately 2.9 eV), and the peak values of the absorption peaks can be enhanced (weaken) via applying positive (negative) external electric fields. Our findings demonstrate that the C3N4/C3N4 and C3N4/MoSSe heterostructures hold significant potential for applications in multifunctional electronic devices including light-emitting, carrier separation, optical modulators, etc.

Unveiling Electric Field Induced Ultrafast Charge Transfer Dynamics and Photoresponse Enhancement in Gold‐Interlayered Bi2Se3

AbstractIn the realm of optoelectronic applications, the incorporation of a metal interlayer between layers of a topological material holds significant potential. The interlayer introduces benefits such as improved charge transfer efficiency, tunable band alignment, plasmonics effect, and enhanced light‐matter interactions, paving the way for more efficient and versatile optoelectronic devices. This work explores the incorporation of gold between Bi2Se3, resulting in an enhancement in the response time of photoresponse of interlayered systems (Bi2Se3‐Au‐Bi2Se3). The introduction of a gold interlayer between Bi2Se3 significantly influences the overlayer structure, crystallinity, crystallite size, and even phonon behavior. Moreover, the responsivity of the interlayered device at a bias (2 V) is found to have high responsivity with fast response and decay time (τr = 0.1 ms, and τd = 24 ms of interlayered systems) as compared to intrinsic Bi2Se3(τr = 0.6 ms, and τd = 28 ms). This change is attributed to the bandgap alteration of the Bi2Se3/Au interface, which is monitored through transient spectroscopy performed at different electric biases. This study suggests that incorporating a gold interlayer holds potential benefits for various optoelectronic applications.

Ferroelectric Control of Band Alignments in In2Se3/h‐BN and CuInP2S6/h‐BN van der Waals Heterostructures

Recently, 2D ferroelectric (FE) heterostructures have become a subject of great interest due to their potential device applications and the underlying physics involved. In this study, the first‐principles calculations are employed to examine the FE control of electronic structures in 2D FE heterostructures, specifically In2Se3/h‐BN and CuInP2S6 (CIPS)/h‐BN. In these results, it is demonstrated that by reversing the polarization of the FE layers, the band alignment of the heterostructures can be interconverted between type II and type I. For In2Se3/h‐BN, the variation of out‐of‐plane polarization can be attributed to the hindrance and facilitation of charge transfer from h‐BN to In2Se3 by the intrinsic electric field of the In2Se3 monolayer. For CIPS/h‐BN heterostructures, the higher transferred charge in the Cdn configuration due to the presence of built‐in electric fields and the stronger interfacial interaction in the Cdn configuration result in a higher polarization value compared to the Cdn configuration. Moreover, the carrier mobility of the heterostructures can also be effectively modulated by the FE polarization. In these findings, the potential significance of FE heterostructures with tunable band alignment and bandgap is highlighted in the development of nanoscale optoelectronic devices.

2D Lateral Heterojunction Arrays with Tailored Interface Band Bending.

Two-dimensional (2D) lateral heterojunction arrays, characterized by well-defined electronic interfaces, hold significant promise for advancing next-generation electronic devices. Despite this potential, the efficient synthesis of high-density lateral heterojunctions with tunable interfacial band alignment remains a challenging. Here, a novel strategy is reported for the fabrication of lateral heterojunction arrays between monolayer Si2Te2 grown on Sb2Te3 (ML-Si2Te2@Sb2Te3) and one-quintuple-layer Sb2Te3 grown on monolayer Si2Te2 (1QL-Sb2Te3@ML-Si2Te2) on a p-doped Sb2Te3 substrate. The site-specific formation of numerous periodically arranged 2D ML-Si2Te2@Sb2Te3/1QL-Sb2Te3@ML-Si2Te2 lateral heterojunctions is realized solely through three epitaxial growth steps of thick-Sb2Te3, ML-Si2Te2, and 1QL-Sb2Te3 films, sequentially. More importantly, the precisely engineering of the interfacial band alignment is realized, by manipulating the substrate's p-doping effect with lateral spatial dependency, on each ML-Si2Te2@Sb2Te3/1QL-Sb2Te3@ML-Si2Te2 junction. Atomically sharp interfaces of the junctions with continuous lattices are observed by scanning tunneling microscopy. Scanning tunneling spectroscopy measurements directly reveal the tailored type-II band bending at the interface. This reported strategy opens avenues for advancing lateral epitaxy technology, facilitating practical applications of 2D in-plane heterojunctions.

(Invited) Metal Halide Perovskite Heterostructures: Synthesis and Fundamental Charge Transfer Studies

Metal halide perovskites are inexpensive semiconductor materials promising for high performance solar cells and light emitting diodes (LEDs) because they are easy to synthesize and tolerant of defects. Fundamental understanding of the factors controlling the carrier transfer mechanisms in heterostructures of halide perovskites is crucial for guiding the synthetic strategies to improve properties and device applications. We developed new methods for synthesizing nanostructures of both three-dimensional (3D) perovskites and two-dimensional (2D) Ruddlesden–Popper (RP) layered perovskites, and using them to create novel and arbitrary heterostructures, such as 2D/3D perovskite, vertical and lateral 2D heterostructures, with high quality interface and tunable band alignments. Various structural characterization and time-resolved spectroscopic methods have been employed to collaboratively study the carrier transfer mechanisms between these well-defined heterostructures of 2D and 3D perovskites. These diverse families of perovskite materials and their nanostructures and heterostructures can enable high performance solar cells, lasers, LEDs, and spintronic applications.

Efficient Photoelectrochemical Hydrogen Generation Based on Core Size Effect of Heterostructured Quantum Dots.

Colloidal quantum dots (QDs) are shown to be effective as light-harvesting sensitizers of metal oxide semiconductor (MOS) photoelectrodes for photoelectrochemical (PEC) hydrogen (H2) generation. The CdSe/CdS core/shell architecture is widely studied due to their tunable absorption range and band alignment via engineering the size of each composition, leading to efficient carrier separation/transfer with proper core/shell band types. However, until now the effect of core size on the PEC performance along with tailoring the core/shell band alignment is not well understood. Here, by regulating four types of CdSe/CdS core/shell QDs with different core sizes (diameter of 2.8, 3.1, 3.5, and 4.8 nm) while the thickness of CdS shell remains the same (thickness of 2.0 ± 0.1 nm), the Type II, Quasi-Type II, and Type I core/shell architecture are successfully formed. Among these, the optimized CdSe/CdS/TiO2 photoelectrode with core size of 3.5 nm can achieve the saturated photocurrent density (Jph) of 17.4 mA cm-2 under standard one sun irradiation. When such cores are further optimized by capping alloyed shells, the Jph can reach values of 22 mA cm2 which is among the best-performed electrodes based on colloidal QDs.

Tunable Band Alignment of Phase-Engineered Two-Dimensional MoS2 Monolayers

Tunable Band Alignment of Phase-Engineered Two-Dimensional MoS₂ Monolayers

Lateral Composite Structures of Graphene/Graphane/Graphone: Electronic Confinement, Heterostructures with Tunable Band Alignment, and Magnetic State

Lateral Composite Structures of Graphene/Graphane/Graphone: Electronic Confinement, Heterostructures with Tunable Band Alignment, and Magnetic State

Reversible Diode with Tunable Band Alignment for Photoelectricity‐Induced Artificial Synapse (Small 34/2023)

Reversible Diode with Tunable Band Alignment for Photoelectricity‐Induced Artificial Synapse (Small 34/2023)

Controllable Photocurrent Generation in Lateral Bilayer MoS2–WS2 Heterostructure

AbstractLaterally epitaxial two‐dimensional (2D) transition metal dichalcogenides (TMDs) heterojunctions are of considerable interest in recent years due to atomically sharp interfaces and tunable band alignment, which have potential applications in novel functional electronics and optoelectronics. However, the studies of 2D lateral heterostructures have mainly focused on the synthesis methods and pristine performance of the fabricated heterojunctions. Herein, the controllable photocurrent generation and enhanced performances of the lateral bilayer (LBL) MoS2–WS2 heterojunctions are reported. A tunable and abrupt built‐in field forms at the interface, and the gate‐tunable rectifying behavior and photovoltaic effect are realized. It is demonstrated that the generation of the photocurrent in the device can be highly controlled to realize ultrahigh responsivity of 1.06 × 104 A W−1 and detectivity of 1.14 × 1013 Jones at a 638 nm incident light. The lateral TMDs heterostructures are expected to constitute the ultimate functional elements of novel electronics and optoelectronics.

Reversible Diode with Tunable Band Alignment for Photoelectricity-Induced Artificial Synapse.

The advent of big data era has put forward higher requirements for electronic nanodevices that have low energy consumption for their application in analog computing with memory and logic circuit to address attendant energy efficiency issues. Here, a miniaturized diode with a reversible switching state based on N-n MoS2 homojunction used a bandgap renormalization effect through the band alignment type regulated by both dielectric and polarization, controllably switched between type-I and type-II, which can be simulated as artificial synapse for sensing memory processing because of its rectification, nonvolatile characteristic and high optical responsiveness. The device demonstrates a rectification ratio of 103 . When served as memory retention time, it can attain at least 7000s. For the synapse simulation, it has an ultralow-level energy consumption because of the pA-level operation current with 5pJ for long-term potentiation and 7.8fJ for long-term depression. Furthermore, the paired pulse facilitation index reaches up to 230%, and it realizes the function of optical storage that can be applied to simulate visual cells.

Theoretical study of the interface engineering for H-diamond field effect transistors with h-BN gate dielectric and graphite gate

Diamond has compelling advantages in power devices as an ultrawide-bandgap semiconductor. Using first-principles calculations, we systematically investigate the structural and electronic properties of hydrogen-terminated diamond (H-diamond) (111) van der Waals (vdW) heterostructures with graphite and hexagonal boron nitride (h-BN) layers. The graphite/H-diamond heterostructure forms a p-type ohmic contact and the p-type Schottky barrier decreases as the number of graphite layers increases. In contrast, the h-BN/H-diamond heterostructure exhibits semiconducting properties and a tunable type-II band alignment. Moreover, the charge transfer is concentrated at the interface with a large amount of charge accumulating on the C–H bonds on the H-diamond (111) surface, indicating the formation of a highly conductive two-dimensional hole gas (2DHG) layer. In a similar vein, the promising structural and electronic properties of graphite, h-BN, and H-diamond (111) in the graphite/h-BN/H-diamond (111) vdW heterostructure are well preserved upon their contact, while such heterostructure exhibits flexible band offset and Schottky contacts. These studies of interface engineering for H-diamond heterostructures are expected to advance the application of 2D materials in H-diamond field effect transistors, which is an important development in the design of surface transfer doping for 2DHG H-diamond devices.

Defect Control via Compositional Engineering of Zn-Cu-In-S Alloyed QDs for Photocatalytic H2O2 Generation and Micropollutant Degradation: Affecting Parameters, Kinetics, and Insightful Mechanism.

Photocatalytic H2O2 production and recalcitrant pollutant degradation are regarded as promising clean technology toward achieving sustainable solar-to-chemical energy conversion. Herein, nonstoichiometric Zn-Cu-In-S (ZCIS) quaternary alloyed quantum dots (QDs) are rationally fabricated via a reflux method toward H2O2 generation and ciprofloxacin degradation under visible light irradiation. The optimum catalyst (ZCIS-2) exhibits a notable H2O2 production of 1685.2 μmole h-1 g-1 (solar-to-chemical conversion efficiency (SCC), 0.19%), which is 5.3 times higher than that of CuInS2 (CIS), and a ciprofloxacin (CIP) degradation efficiency of 96% in 2 h. The observed improvement in activity corresponds to optimized exciton separation/transfer, broad photon absorption, tunable band alignment, and effective adsorption/activation. In addition, oxygen reduction goes through both direct two-electron single-step reduction and single-electron two-step superoxide radical pathways, whereas CIP degradation proceeds via direct •O2- and indirect •OH radical pathways, as confirmed by scavenger experiments. An appropriate amount of defects improves the adsorption/activation of O2 toward H2O2 and active oxygen species generation that facilitates CIP degradation. The effect of operational parameters, such as pH, surrounding environment, presence of ions, sacrificial agent, etc., on both H2O2 formation and CIP removal is vividly studied. Hence, the current study will provide an in-depth insight into O2 photoreduction and micropollutant removal, which encourages further advancement of potent alloyed quantum dot-oriented photocatalytic systems.

Programmable Photovoltaics of Metal‐Oxide‐Semiconductor Junction

AbstractProgrammability, fundamental to the computing of electronic systems, is traditionally achieved in processing and memory modules. Recently, the emergence of in‐sensor computing allows the programmability of the sensing modules and the process of raw data upon the sensor edge, which reduces data transport and energy consumption. Here, a programmable photodetector module with a tunable photoelectric response based on molybdenum sulfide (MoS2) metal‐oxide‐semiconductor (MOS) junction for visible detection is reported. The sensor takes advantage of the tunable band alignment at the MOS junction and generates an adjustable or even reversible out‐of‐plane photovoltaic effect with high efficiency. This work provides new device configurations to architect the programmability of the photodetectors, and pave the way toward in‐sensor computing at the application edge.

Tunable Band Alignment in the Arsenene/WS2 Heterostructure by Applying Electric Field and Strain

Arsenene has received considerable attention because of its unique optoelectronic and nanoelectronic properties. Nevertheless, the research on van der Waals (vdW) heterojunctions based on arsenene has just begun, which hinders the application of arsenene in the optoelectronic and nanoelectronic fields. Here, we systemically predict the stability and electronic structures of the arsenene/WS2 vdW heterojunction based on first-principles calculations, considering the stacking pattern, electric field, and strain effects. We found that the arsenene/WS2 heterostructure possesses a type-II band alignment. Moreover, the electric field can effectively tune both the band gap and the band alignment type. Additionally, the band gap could be tuned effectively by strain, while the band alignment type is robust under strain. Our study opens up a new avenue for the application of ultrathin arsenene-based vdW heterostructures in future nano- and optoelectronics applications. Our study demonstrates that the arsenene/WS2 heterostructure offers a candidate material for optoelectronic and nanoelectronic devices.