type-II Interface Research Articles

Data-Driven Design of Electroactive Spacer Molecules to Tune Charge Carrier Dynamics in Layered Halide Perovskite Heterostructures.

Crafting rational heterojunctions with nanostructured materials is instrumental in fostering effective interfacial charge separation and transport for optoelectronics. Layered halide perovskites (LHPs) that form heterojunctions between organic spacer molecules and inorganic metal halide layers exhibit tunable photophysics owing to their customizable band alignment. However, controlling photogenerated carrier dynamics by strategically designing layered perovskite heterojunctions remains largely unexplored. We combine a data-driven approach with time-domain density functional theory (TD-DFT) and non-adiabatic molecular dynamics (NAMD) to screen and select electronically active spacer dications (A') that introduce a type-II heterojunction in the lead iodide-based Dion-Jacobson phase LHPs. The composition-structure-electronic property correlations reveal that the number of nitrogens in aromatic heterocycles is the key factor in designing electron-accepting spacers in these perovskites. The detailed atomistic simulations validate the design strategy further by modeling (A')PbI4 perovskites, which incorporate three different screened electroactive A' spacers. The computed excited charge carrier dynamics illustrate the phonon-mediated ultrafast interfacial electron transfer from the inorganic conduction band edge to the lower-lying unoccupied orbitals of spacers, exhibiting photoluminescence quenching in these (A')PbI4 perovskites. The spatially separated electrons and holes at the type-II heterojunction interface prolong the excited charge carrier lifetime, boosting the carrier transport and exciton dynamics. Our work illustrates a robust in silico approach for designing LHPs with exciting optoelectronic properties originating from their fine-tuned heterojunctions.

Structural characteristics of irrational Type-II Twin interfaces

The Type-II Twin Boundary (TB) is a critical interface in functional materials whose irrational Miller-index identity has recently drawn significant research interest. This study establishes general structural characteristics of the Type-II twin interface, utilizing TBs in Shape Memory Alloys (SMAs) - TiPd, TiPt, and AuCd - as study targets. It is shown how the irrational identity of each TB is explained by the Terrace-Disconnection (T-D) structural topology. It is proposed that the terrace is the rational-plane nearest to the irrational TB in the reciprocal space, having integral Miller-indices of least magnitude. Crystallographic-registry on this terrace requires non-trivial coherence-strains. A novel kinematic-origin of the coherence-strain is proposed, coming directly from a transformation of the classical twinning deformation-gradient. This transformation revealed that the classical twinning-shear partitions into the coherence-strain and a new metric termed the “terrace-shear”. It is shown that the magnitude of shear relating the twin-structure to the matrix is the terrace-shear and not the twinning-shear, contrary to classical understanding. Furthermore, the Burgers vector of the twinning disconnection is shown to be related directly to the terrace-shear. The energy of each Type-II interface is determined from ab initio Density Functional Theory (DFT) calculations. It is shown that the energy-minimal atomic-structure on the terrace requires determination of a “lattice-offset” that is non-trivial and unknown apriori. In summary, this study expounds on T-D topological structure of Type-II twin interfaces, establishing methods to identify rational terraces, coherence strains, ab initio planar TB energies and revealing a unique partitioning of the twinning-shear exhibited by this class of interfaces.

Synergistic Engineering of Energy Band Alignment and Interfacial Electric Field Distribution over Bi-bismuth-Based Hetero-nanofibers for Boosting Visible-Light-Driven Photocatalytic Ammonia Synthesis and Antibiotic Removal.

Synergistic engineering of energy band alignment and interfacial electric field distribution is essential for photocatalyst design but is still challenging because of the limitation on refined regulation in the nanoscale. This study addresses the issue by employing surface modification and thermal-induced phase transformation in Bi2MoO6/BixOyIz hetero-nanofiber frameworks. The energy band alignment switches from a type-II interface to a Z-scheme contact with stronger redox potentials and inhibited electron traps, and the optimized built-in electric field distribution could be reached based on experimental and theoretical investigations. The engineered hetero-nanofibers exhibit outstanding visible-light-driven photocatalytic nitrogen reduction activity (605 μmol/g/h) and tetracycline hydrochloride removal rate (81.5% within 30 min), ranking them among the top-performing bismuth series materials. Furthermore, the photocatalysts show promise in activating advanced oxidants for efficient organic pollutant degradation. Moreover, the Bi2MoO6/Bi5O7I hetero-nanofibers possess good recycling stability owing to their three-dimensional network structure. This research offers valuable insights into heterojunction design for environmental remediation and industrial applications.

Spatiotemporal Observation of Quasi-Ballistic Transport of Electrons in Graphene.

We report spatiotemporal observations of room-temperature quasi-ballistic electron transport in graphene, which is achieved by utilizing a four-layer van der Waals heterostructure to generate free charge carriers. The heterostructure is formed by sandwiching a MoS2 and MoSe2 heterobilayer between two graphene monolayers. Transient absorption measurements reveal that the electrons and holes separated by the type-II interface between MoS2 and MoSe2 can transfer to the two graphene layers, respectively. Transient absorption microscopy measurements, with high spatial and temporal resolution, reveal that while the holes in one graphene layer undergo a classical diffusion process with a large diffusion coefficient of 65 cm2 s-1 and a charge mobility of 5000 cm2 V-1 s-1, the electrons in the other graphene layer exhibit a quasi-ballistic transport feature, with a ballistic transport time of 20 ps and a speed of 22 km s-1, respectively. The different in-plane transport properties confirm that electrons and holes move independently of each other as charge carriers. The optical generation of ballistic charge carriers suggests potential applications for such van der Waals heterostructures as optoelectronic materials.

In-situ free-standing inorganic 2D Cs2PbI2Cl2 nanosheets for efficient self-powered photodetectors with carbon electrode

Inorganic two-dimensional (2D) perovskites possess excellent thermal stability and high charge mobility, making them an attractive choice for stable optoelectronic devices such as photodetectors (PDs). The formation of an appropriate inorganic 2D perovskite structure is of great importance to efficient PDs, especially to that of planar self-powered photovoltaic PDs featuring perpendicular charge transport channels. Herein, we implemented morphological engineering on wide bandgap inorganic 2D perovskite, Cs2PbI2Cl2, demonstrating a successful preparation of in-situ free-standing nanosheets structure with proper charge channels for photovoltaic type self-powered PDs. Compared with its counterpart with a nanoblock morphology, the 2D nanosheet Cs2PbI2Cl2 film exhibits enhanced charge mobility and purified Ruddlesden-Popper phase that can withstand high-energy electron beam radiation, accelerated thermal aging and long-term shelf storage. Sandwiching Cs2PbI2Cl2 nanosheet film in between tin oxide (SnO2) and polythiophene (P3HT) as electron and hole acceptors, respectively, the constructed photovoltaic type structure exhibits effective dissociation of excitons at the cascade type-II interface. The nanosheets enable lower dark current and more efficient charge collection than the nanoblock structure. As a result, the self-powered photodetectors with 2D Cs2PbI2Cl2 nanosheets deliver an outstanding responsivity of 698 mW/cm2 and a detectivity of 8.6×1012 Jones. The stable PDs can be applied to monitor ultraviolet irradiation in real outdoor conditions. Our work demonstrates the significant role of morphology tuning of 2D inorganic perovskite in stable, cost-effective and efficient photodetectors.

Evidence of linear and cubic Rashba effect in non-magnetic heterostructure

The system serves as a prototype to study the electronic properties that emerge as a result of spin–orbit coupling (SOC). In this article, we have used first-principles calculations to systematically study two types of defect-free (0 0 1) interfaces, which are termed as Type-I and Type-II. While the Type-I heterostructure produces a two dimensional (2D) electron gas, the Type-II heterostructure hosts an oxygen-rich 2D hole gas at the interface. Furthermore, in the presence of intrinsic SOC, we have found evidence of both cubic and linear Rashba interactions in the conduction bands of the Type-I heterostructure. On the contrary, there is spin-splitting of both the valence and the conduction bands in the Type-II interface, which are found to be only linear Rashba type. Interestingly, the Type-II interface also harbors a potential photocurrent transition path, making it an excellent platform to study the circularly polarized photogalvanic effect.

Tetragonal-zircon BiVO4: a better polymorph for the formation of coherent type-II heterostructures for water splitting applications.

The monoclinic-scheelite (m-s) polymorph of BiVO4 has the highest photocatalytic activity, whereas tetragonal-zircon (t-z) has the lowest photocatalytic activity, which may be due to a higher band gap. However, t-z has the highest crystal symmetry, which makes it a more suitable candidate to form coherent type-II interfaces for the efficient separation of electron-hole pairs. Furthermore, the method of preparation (e.g. low temperature and moderate pH) of t-z is more facile compared to the m-s polymorph. Hence, in this report, we construct coherent isomaterial and heteromaterial type-II heterostructures by facet engineering of low index surfaces of t-z polymorph with different semiconductor materials (e.g. ZnO, TiO2, CdSe, and ZnS) by screening the band gap, band edge positions, and lattice misfit strain. On the basis of the calculated band-edge positions, the polymorphs of BiVO4 can form 212 combinations of the type-II interface, which reduces to 17 coherent interfaces with lattice misfit strain between 1.55% to 28.5% when translational symmetry, atomic positions, lattice mismatch, and bond complementarity have been imposed. Furthermore, the current study suggests that t-z polymorphs can form more coherent interfaces (4 out of 168), which may be due to its highest symmetry structure in comparison to previously formed 67 isomaterial and heteromaterial type-II heterostructure combinations of BiVO4 (1 out of 67), which suggests that t-z can be a suitable candidate for the formation of type-II coherent interfaces for PEC/PC applications.

Ultrasensitive photoelectrochemical sensing platform based on heterostructural CuO/NCDs@Au nanocomposites with the efficient photo-induced carrier separation

Heterogeneous composite is considered a valuable material to boost the photo-electrochemical (PEC) properties. Herein, the porous hollow and thin-shell CuO particles were prepared by calcining a Cu-BTC precursor. N-doped carbon dots (NCDs) as both reductor and stabilizer can reduce the Au+ to gain regularly nanoflower NCDs@Au, which further to create type-Ⅱ heterogeneous interface of CuO/NCDs@Au. PEC monitoring results showed that the CuO/NCDs@Au nanocomposite displayed markedly improved photocurrent response than the monocomponent CuO or CuO/NCDs. This outstanding photoelectric property was ascribed to the multiple reflection/scattering effects from the porous hollow structure of CuO particles which led to more photo-induced e−/h+ pairs. The obtained heterostructured CuO/NCDs@Au could magnify synergistically the photocurrent output signal. Ab (antibodies) was accurately immobilized on the CuO/NCDs@Au modified ITO electrode via a facility amidation reaction. Fabricated Ab/CuO/NCDs@Au/ITO PEC sensing platform for AFP (alpha-fetoprotein) detection manifested an enhanced sensitivity (3.32 × 10-4 ng mL−1) with wide linear range (0.001 ∼ 300 ng mL−1), which was superior compared to the electrochemical results. This works developed an excellent heterojunction nanocomposite of CuO/NCDs@Au, which provided well-synthesized strategies for structuring high-performance photocatalysts based on MOFs derivant.

Time-resolved photoluminescence studies of single interface wurtzite/zincblende heterostructured InP nanowires

The interface between wurtzite and zinc blende InP has been identified as type-II, where electrons gather on the zinc blende side and holes on the wurtzite side of the interface. The photoluminescence resulting from recombination across the interface is expected to be long-lived and to exhibit non-exponential decay of emission intensity after pulsed excitation. We verify this prediction using time-resolved photoluminescence spectroscopy on nanowires containing a single heterostructure between a single segment of wurtzite and zinc blende. We find that a significant intensity of type-II emission remains even more than 30 ns after excitation. The decay of the emission intensity is also non-exponential and considerably longer than the exponential decay of the wurtzite InP segment (260 ps). Our results are consistent with the expected photoluminescence characteristics of a type-II interface between the two polytypes. We also find that the lifetime becomes shorter if we create an electron gas at the interface by n-type doping the entire wurtzite segment of the nanowire. This is expected since there are many electrons that a given hole can recombine with, in contrast to the undoped case.

Bi2S3 nanorods and BiOI nanosheets co-modified BiOIO3 nanosheets: An efficient vis-light response photocatalysts for RhB degradation

BiOIO3/BiOI/Bi2S3 (BBS-x) heterostructure composites are first synthesized via in situ reduction and chemical deposition. The BiOIO3/BiOI/Bi2S3 composites integrate the merits of BiOI and Bi2S3, with a widened light response range and improved light absorbance ability, as confirmed by the DRS test. The close type-Ⅱ interface along with carrier migrate and transfer mechanism can spatially isolate photoinduced e- and h+, hinder the recombination of photogenerated carriers and boost their efficiency transfer and separation, as proven by scanning electron microscopy, transmission electron microscopy, electrochemical impedance spectroscopy, transient photocurrent response, and photoluminescence. These properties enhance the photocatalytic activity of BiOIO3/BiOI/Bi2S3 composites when applied to the photodegradation of rhodamine B, regardless of their irradiation with simulated solar light or LED light. The BBS-0.75 composite has the highest apparent rate constant k (0.034 min−1) under simulated solar light irradiation, which is approximately 4.5 times greater than that of pure BiOIO3 (0.0076 min−1). The BBS-0.75 composite almost completely photodegrades RhB within 30 min under LED light irradiation. Free radical scavenging experiments reveal that •O2- and h+ are the main reactive species. Thus, a logical type-II carrier transfer mechanism with dye sensitization is proposed.

Enhanced photoelectrochemical water splitting using zinc selenide/graphitic carbon nitride type-II heterojunction interface

The objective of this research is to construct a type-II heterojunction interface for effective photoelectrochemical (PEC) water splitting for hydrogen generation. A series of ZnSe/g-C3N4 heterojunctions is prepared by ultrasonication procedure and tested for PEC water splitting for the first time. The successful formation of ZnSe/g-C3N4 is confirmed by phase, morphological and optical analysis. Linear sweep voltammetry of 0.05 ZG (0.05% ZnSe/g-C3N4) showed a six-fold higher photocurrent density of 500 μA than g-C3N4. These results are supported by the Tafel slopes and PL (photoluminescence spectroscopy) studies by showing the smallest slope and lesser electron-hole recombination for 0.05 ZG. Increased lifetime of 107 ms and a higher donor density of 3.6 × 1019 cm−3 for 0.05 ZG is observed. The smallest semicircle for 0.05 ZG in EIS implies the least charge transfer resistance among the prepared heterojunctions. All the results comply with each other showing the successful formation of type-II heterojunction for enhanced PEC water splitting.

Elucidating the Mechanistic Origins of Photocatalytic Hydrogen Evolution Mediated by MoS2/CdS Quantum-Dot Heterostructures.

Solar fuel generation mediated by semiconductor heterostructures represents a promising strategy for sustainable energy conversion and storage. The design of semiconductor heterostructures for photocatalytic energy conversion requires the separation of photogenerated charge carriers in real space and their delivery to active catalytic sites at the appropriate overpotentials to initiate redox reactions. Operation of the desired sequence of light harvesting, charge separation, and charge transport events within heterostructures is governed by the thermodynamic energy offsets of the two components and their photoexcited charge-transfer reactivity, which determine the extent to which desirable processes can outcompete unproductive recombination channels. Here, we map energetic offsets and track the dynamics of electron transfer in MoS2/CdS architectures, prepared by interfacing two-dimensional MoS2 nanosheets with CdS quantum dots (QDs), and correlate the observed charge separation to photocatalytic activity in the hydrogen evolution reaction. The energetic offsets between MoS2 and CdS have been determined using hard and soft X-ray photoemission spectroscopy (XPS) in conjunction with density functional theory. A staggered type-II interface is observed, which facilitates electron and hole separation across the interface. Transient absorption spectroscopy measurements demonstrate ultrafast electron injection occurring within sub-5 ps from CdS QDs to MoS2, allowing for creation of a long-lived charge-separated state. The increase of electron concentration in MoS2 is evidenced with the aid of spectroelectrochemical measurements and by identifying the distinctive signatures of electron-phonon scattering in picosecond-resolution transient absorption spectra. Ultrafast charge separation across the type-II interface of MoS2/CdS heterostructures enables a high Faradaic efficiency of ∼99.4 ± 1.2% to be achieved in the hydrogen evolution reaction (HER) and provides a 40-fold increase in the photocatalytic activity of dispersed photocatalysts for H2 generation. The accurate mapping of thermodynamic driving forces and dynamics of charge transfer in these heterostructures suggests a means of engineering ultrafast electron transfer and effective charge separation to design viable photocatalytic architectures.

An electron-counting rule to determine the interlayer magnetic coupling of the van der Waals materials

In layered magnetic materials, the magnetic coupling between neighboring van der Waals layers is challenging to understand and anticipate, although the exchange interaction inside a layer can be well rationalized for example by the superexchange mechanism. In this work, we elucidate the interlayer exchange mechanism and propose an electron-counting rule to determine the interlayer magnetic order between van der Waals layers, based on counting the d-orbital occupation (dn, where n is the number of d-electrons at the magnetic cation). With this rule, we classify magnetic monolayers into two groups, type-I () and type-II (n ≥ 5), and derive three types of interlayer magnetic coupling for both insulators and metals. The coupling between two type-II layers prefers the antiferromagnetic (AFM) order, while type-I and type-II interface favors the ferromagnetic (FM) way. However, for two type-I layers, they display a competition between FM and AFM orders and even lead to the stacking dependent magnetism. Additionally, metallic layers can also be incorporated into this rule with a minor correction from the free carrier hopping. Therefore, this rule provides a simple guidance to understand the interlayer exchange and further design van der Waals junctions with desired magnetic orders.

Type-II Interface Band Alignment in the vdW PbI2-MoSe2 Heterostructure.

Energy band alignments at heterostructure interfaces play key roles in device performance, especially between two-dimensional atomically thin materials. Herein, van der Waals PbI2-MoSe2 heterostructures fabricated by in situ PbI2 deposition on monolayer MoSe2 are comprehensively studied using scanning tunneling microscopy/spectroscopy, atomic force microscopy, photoemission spectroscopy, and Raman and photoluminescence (PL) spectroscopy. PbI2 grows on MoSe2 in a quasi layer-by-layer epitaxial mode. A type-II interface band alignment is proposed between PbI2 and MoSe2 with the conduction band minimum (valence band maximum) located at PbI2 (MoSe2), which is confirmed by first-principles calculations and the existence of interfacial excitons revealed using temperature-dependent PL. Our findings provide a scalable method to fabricate PbI2-MoSe2 heterostructures and new insights into the electronic structures for future device design.

Microstructural characteristics and crack formation in additively manufactured bimetal material of 316L stainless steel and Inconel 625

This research illustrates the rationale of adopting a preferred printing sequence by examining crack generation predominated by resultant interfaces and microstructural inhomogeneity, through underlying governing mechanisms in directed energy deposition of 316L stainless steel/Inconel 625 (SS316L/IN625) bimetals. For this purpose, microstructural and crystallographic characterizations augmented by numerical simulations were employed on additively manufactured two distinct interfaces, i.e. Type-I (IN625 deposition on SS316L) and Type-II (SS316L deposition on IN625). Changing the printing sequence generated these two types of interfaces with unique morphologies, which was found attributable to the compositional variations and mismatch in thermal properties. Type-I interface was typified by gradual-change composition in the transition zone, causing the IN625 grains to grow epitaxially on the grains of SS316L. Type-II interface was characterized as a compositional sudden-change zone (CSCZ) adjacent to SS316L, leading to merging bidirectional nucleation and grain growth from the bottom IN625 and upper CSCZ, and lack of epitaxial growth. Additionally, high cracking susceptibility occurred near the Type-II interface rather than the Type-I interface, which was related to solidification and liquidation cracking, and further promoted ductility dip cracking. This research will provide a guideline for the additive manufacturing of bimetals with the consideration of printing sequence to control interface formation for a crack-free structure.

Numerical analysis of Mg2Si/Si heterojunction DG-TFET for low power/high performance applications: Impact of non- idealities

Abstract In the advanced technology nodes, conventional MOSFETs are being replaced by tunnel field effect transistors (TFETs), due to its potential of achieving subthreshold swing (SS) less than 60 mV/decade. However, certain constraints are to be met to improve the performance of TFET in terms of higher ON current (ION) and lower threshold voltage (Vth). Here, in this paper, magnesium silicide/silicon (Mg2Si/Si) heterojunction double gate TFET (Mg2Si/Si HDG-TFET) is explored and is simultaneously compared with conventional silicon double gate TFET (Si DG-TFET). Results depict superior performance of Mg2Si/Si HDG-TFET as compared to conventional Si DG-TFET, in terms of dc characteristics, i.e., ION, Vth, SS, and ION/IOFF ratio. Obtained Vth (0.26 V), SS (10.05mV/decade) and ION/IOFF ratio (1013) for the case of Mg2Si HDG-TFET shows an improvement of 77%, 49% and 10 decades respectively compared to counterpart i.e., Si DG-TFET. In particular, this improvement in the performance of Mg2Si/Si HDG-TFET over Si DG-TFET is attributed to staggered type-II heterojunction interface at the source-channel junction, which leads to reduction of the width for interband tunneling barrier and hence, improves ION. This viability of the device is also determined by analyzing the impact of non-idealities present in the device. In this respect, the Gaussian and tail defects are considered in the bulk of Mg2Si. The results reveal that the Gaussian defects alter the device characteristics mainly in the subthreshold regime, whereas, in the ON state, the impact of defects is minimal. Further, it is obtained that the device is much immune for tail defects in comparison with the Gaussian defects. The CV analysis reveals a marginal degradation in parasitic capacitances for Mg2Si/Si HDG-TFET as compared to Si DG-TFET. However, this degradation can be overlooked against the remarkably enhanced drain current. Thus, the device overcomes the bottleneck of TFET and provides high ION and low Vth without degrading the other performance parameters and hence is suitable for low power analog and digital applications.

Enhancement in speed and responsivity of uni-traveling carrier photodiodes with GaAs0.5Sb0.5/In0.53Ga0.47As type-II hybrid absorbers.

We demonstrate a top-illuminated high-speed uni-traveling carrier photodiode (UTC-PD) with a novel design in the p-type absorber, which can effectively shorten the photon absorption depth at telecommunication wavelengths (1.31~1.55 μm) and further enhance the bandwidth-efficiency product of UTC-PD. In our proposed new UTC-PD structure, the p-type In0.53Ga0.47As absorption layer is replaced by the type-II GaAs0.5Sb0.5 (p)/In0.53Ga0.47As (i) hybrid absorber. Due to the narrowing of the bandgap and enhancement of the photo-absorption process at the type-II interface between the GaAs0.5Sb0.5 and In0.53Ga0.47As layers, our device shows an over 16.7% improvement in the responsivity compared with that of UTC-PD with the same thickness of pure In0.53Ga0.47As absorber (0.7 μm) and a zero optical coupling loss. Our demonstrated device with a simple top-illuminated structure offers a large active mesa (25 μm), a wide optical-to-electrical (O-E) bandwidth (33 GHz), a high responsivity (0.7 A/W), and a high saturation current (>5 mA) under 1.31 µm optical wavelength. These promising results suggest that our proposed PD structure can fundamentally overcome the trade-off among bandwidth, efficiency, and device active diameter of high-speed PDs.

Novel CdIn2S4 nano-octahedra/TiO2 hollow hybrid heterostructure: In-situ synthesis, synergistic effect and enhanced dual-functional photocatalytic activities

Abstract The development of highly efficient and multifunctional composite photocatalysts for both energy conversion and environmental governance has obtained great concerns. Here, a novel CdIn2S4/TiO2 (CIS/THS) hollow composite photocatalyst was firstly designed and synthesized via a facile in-situ growth process, where the CdIn2S4 nano-octahedra densely attached on the surface of TiO2 hollow spheres to form the unique hybrid heterostructure. The as-synthesized CIS/THS heterojunctions exhibit much superior photocatalytic activities for hydrogen evolution and Methyl Orange (MO) decomposition in comparison to pure CdIn2S4 and TiO2 hollow spheres. The experimental results display that the CIS/THS-3 sample with the 30 wt% of TiO2 presents the optimal photocatalytic H2 production efficiency and its generation rate is 3.38 and 2.56 times as high as those of pure TiO2 and CdIn2S4. Besides, the as-synthesized CIS/THS-3 hybrid also possesses the best MO photodegradation performance and its rate constant is 11.43 and 8.34 times higher than those of pure TiO2 and CdIn2S4. The enhanced photocatalytic activities can be assigned to the synergistic effect, optimized light-harvesting capacity and the formation of hybrid heterostructure for boosting interfacial charge transfer and separation. Furthermore, based on the trapping experiments and ESR analysis, the possible type-Ⅱ interface charge transport mechanism was also proposed. Our study may provide the direct guidance for constructing other hollow TiO2-based composite photocatalysts with superior photocatalytic water splitting and degradation performances.

Computational simulations of ZnO@GaN and GaN@ZnO core@shell nanotubes

The structural, electronic, and mechanical properties of armchair and zigzag chiralities double-walled and core@shell ZnO@GaN and GaN@ZnO nanotubes were investigated by periodic DFT/B3LYP calculations with an all-electron basis set. For both chiralities, GaN@ZnO presents minor strain and deposition energies, which predict that this nanotube can be easier formed and the GaN is the most favorable substrate (the core) than ZnO. On the other hand, the zigzag GaN@ZnO did not exhibit the major piezoelectric response, which is three times smaller than the ZnO@GaN nanotube, showing that the compression of the core@shell nanotube length is not favorable to this property. However, the piezoelectricity can be improved when the zigzag GaN@ZnO is under elongation. The elastic constants showed that the core@shell nanotubes are more rigid than the homogenous nanotubes and present higher piezoelectric constants. In addition, the projected DOS shows that GaN@ZnO has a type-II interface and ZnO@GaN has a type-I interface. Based on the results obtained from our theoretical models, the nanotubes have great potential for the experimental development of new electronic devices.

Ultrafast Uni-Traveling Carrier Photodiodes With GaAs0.5Sb0.5/In0.53 Ga0.47As Type-II Hybrid Absorbers for High-Power Operation at THz Frequencies

We demonstrate a novel type of ultrafast photodiode (PD), which offers high-power performance in the THz regime. The incorporation of a type-II GaAs0.5Sb0.5 (p)/In0.53Ga0.47As (i) hybrid absorber in the InP-based uni-traveling carrier photodiode (UTC-PD) structure leads to an improvement in the responsivity, due to the narrowing of the bandgap and enhancement of the photo-absorption process at the type-II interface between the GaAs0.5Sb0.5 and In0.53Ga0.47As absorption layers. Furthermore, the current blocking effect, which is usually one of the major factors limiting the output power of UTC-PDs, can be minimized due to the high-excess energy of the photo-generated electrons injected from the GaAs 0.5Sb0.5 layer to the InP-based collector layer. The flip-chip bonding packaged device with an active diameter of 3 μm shows a moderate responsivity (0.11 A/W) along with a record wide 3-dB optical-to-electrical bandwidth at 0.33 THz, among all those reported for long wavelength (1.3–1.55 μm) PDs. A 13-mA saturation current and a continuous wave output power as high as –3 dBm are successfully demonstrated at an operating frequency of 0.32 THz under an optical signal with a sinusoidal envelope and a ∼63% modulation depth for PD excitation.