Similar Heterostructures Research Articles

Interfacial Stress Transfer and Fracture in van der Waals Heterostructures.

Artificially stacking 2D materials (2DMs) into vdW heterostructures creates materials with properties not present in nature that offer great potential for various applications such as flexible electronics. Properties of such stacked structures are controlled largely by the interfacial interactions and the structural integrity of the 2DMs. In spite of their crucial roles, interfacial stress transfer and the failure mechanisms of the vdW heterostructures, particularly during deformation, have not been well addressed so far. In this work, the interfacial stress transfer and failure mechanisms of a MoS2/graphene vdW heterostructure are studied, through the strain distributions both laterally in individual 2DMs and vertically across different 2DMs revealed in-situ. The fracture of the MoS2 and the associated states of stress and strain are monitored experimentally. This enables various interfacial properties, such as the interfacial shear strength and interfacial fracture energy, to be estimated. Based only on the measured strength and interfacial properties of a single vdW heterostructure, a failure criterion is proposed to predict the failure mechanisms of similar vdW heterostructures with any lateral dimensions. This work provides an insight to the deformation micromechanics of vdW heterostructures that are of great value for their miniaturization and applications, especially in flexible electronics.

Investigation of RF performance of Ku-band GaN HEMT device and an in-depth analysis of short channel effects

In this paper, we have characterized an AlGaN/GaN High Electron Mobility Transistor (HEMT) with a short gate length (Lg ≈ 0.15 μm). We have studied the effect of short gate length on the small signal parameters, linearity parameters and gm-gd ratio in GaN HEMT devices. To understand how scaling results in the variation of the above-mentioned parameters a comparative study with higher gate length devices on similar heterostructure is also presented here. We have scaled down the gate length but the barrier thickness(tbar) remained same which affects the aspect ratio (Lg/tbar) of the device and its inseparable consequences are the prominent short channel effects (SCEs) barring the optimum output performance of the device. These interesting phenomena were studied in detail and explored over a temperature range of −40 °C to 80 °C. To the best of our knowledge this paper explores temperature dependence of SCEs of GaN HEMT for the first time. With an approach to reduce the impact of SCEs a simulation study in Silvaco TCAD was carried out and it is observed that a recessed gate structure on conventional heterostructure successfully reduces SCEs and improves RF performance of the device. This work gives an overall view of gate length scaling on conventional AlGaN/GaN HEMTs.

Switching Intrinsic Magnetic Skyrmions with Controllable Magnetic Anisotropy in van der Waals Multiferroic Heterostructures.

Magnetic skyrmions, topologically nontrivial whirling spin textures at nanometer scales, have emerged as potential information carriers for spintronic devices. The ability to efficiently create and erase magnetic skyrmions is vital yet challenging for such applications. Based on first-principles studies, we find that switching between intrinsic magnetic skyrmion and high-temperature ferromagnetic states can be achieved in the two-dimensional van der Waals (vdW) multiferroic heterostructure CrSeI/In2Te3 by reversing the ferroelectric polarization of In2Te3. The core mechanism of this switching is traced to the controllable magnetic anisotropy of CrSeI influenced by the ferroelectric polarization of In2Te3. We propose a useful descriptor linking the presence of magnetic skyrmions to magnetic parameters and validate this connection through studies of a variety of similar vdW multiferroic heterostructures. Our work demonstrates that manipulating magnetic skyrmions via tunable magnetic anisotropies in vdW multiferroic heterostructures represents a highly promising and energy-efficient strategy for the future development of spintronics.

Constructing Hierarchical CoGa2O4-S@NiCo-LDH Core-Shell Heterostructures with Crystalline/Amorphous/Crystalline Heterointerfaces for Flexible Asymmetric Supercapacitors.

The rational design and construction of composite electrodes are crucial for overcoming the issues of poor working stability and slow ionic electron mobility of a single component. Nevertheless, it is a big challenge to construct core-shell heterostructures with crystalline/amorphous/crystalline heterointerfaces in straightforward and efficient methods. Here, we have successfully converted a portion of crystalline CoGa2O4 into the amorphous phase by employing a facile sulfidation process (denoted as CoGa2O4-S), followed by anchoring crystalline NiCo-layered double hydroxide (denoted as NiCo-LDH) nanoarrays onto hexagonal plates and nucleation points of CoGa2O4-S, synthesizing dual-type hexagonal and flower-like 3D CoGa2O4-S@NiCo-LDH core-shell heterostructures with crystalline/amorphous/crystalline heterointerfaces on carbon cloth. Furthermore, we further adjust the Ni/Co ratio in LDH, achieving precise and controllable core-shell heterostructures. Benefiting from the abundant crystalline/amorphous/crystalline heterointerfaces and synergistic effect among various components, the CoGa2O4-S@Ni2Co1-LDH electrode exhibits a specific capacity of 247.8 mAh·g-1 at 1 A·g-1 and good rate performance. A CoGa2O4-S@Ni2Co1-LDH//AC flexible asymmetric supercapacitor provides an energy density of 58.2 Wh·kg-1 at a power density of 850 W·kg-1 and exhibits an impressive capacitance retention of 105.7% after 10,000 cycles at 10 A·g-1. Our research provides profound insights into the design of other similar core-shell heterostructures.

Photoluminescence, Reflection Contrast, Raman, and Second Harmonic Generation Spectroscopies Spatially Resolve Strain, Alloying, Defects, and Electronic Characteristics of Lateral MX2 Heterostructures.

Lateral heterostructures of two-dimensional (2D) transition metal dichalcogenides offer promise as platforms for a wide variety of applications from exotic physics to environmental control. Further development and study of these heterostructures require characterization methods that assess the quality of the heterostructures. Here, we extend current characterization strategies to create photoluminescence (PL), Raman, reflection contrast, and second harmonic generation (SHG) maps of individual monolayer core-shell WS2-MoS2 lateral heterostructures that were synthesized via water vapor assisted chemical vapor transport. Together, these methods provide the correlations required to resolve the effects of excitons, trions, lattice defects, strain, and alloying. The comparisons show substantial differences, especially in the regions near and at the narrow heterointerface. Comparisons between the different spectral maps show the importance of metal alloying for understanding the electronic and spatial structures of heterostructures. The results are compared to previous work on similar lateral heterostructures created by different methods.

2D-2D Nanoheterostructure of an Exposed {001}-Facet CuO and MoS2 Based Bifunctional Catalyst Showing Excellent Surface Chemistry and Conductivity for Cathodic CO2 Reduction.

A novel CuO-MoS2 based heterostructure catalyst model system is proposed where a CuO nanosheet with exposed {001} facet with proper termination is the active surface for the catalysis and a MoS2 nanosheet is the supporting layer. Density functional theory (DFT) calculations were performed to validate the model. The MoS2 bilayer forms a stable heterostructure with {001} faceted CuO with different terminations exposing oxygen and copper atoms (active sites) on the surface. The heterostructure active sites with a low oxidation state of the copper atoms and subsurface oxygen atoms provide a suitable chemical environment for the selective production of multicarbon products from CO2 electrocatalytic reduction. Furthermore, our heterostructure model exhibits good electrical conductivity, efficient electron transport to active surface sites, and less interfacial resistance compared to similar heterostructure systems. Additionally, we propose a photoenhanced electrocatalysis mechanism due to the photoactive nature of MoS2. We suggest that the photogenerated carrier separation occurs because of the interface-induced dipole. Moreover, we utilized a machine learning model trained on a 2D DFT materials database to predict selected properties and compared them with the DFT results. Overall, our study provides insights into the structure-property relationship of a MoS2 supported 2D CuO nanosheet based bifunctional catalyst and highlights the advantages of heterostructure formation with selective morphology and properly terminated surface in tuning the catalytic performance of nanocomposite materials.

PT-Symmetric Microwave Photoconductivity in Heterostructures Based on the Hg1 −xCdxTe Topological Phase

The PT-symmetric photoconductivity has been detected for the first time in microwave-irradiated heterostructures based on thick Hg1 −xCdxTe films with the CdTe content x corresponding to the topological phase although the magnetic field symmetry (T symmetry) and the symmetry in the positions of potential contact pairs (P symmetry) are not conserved separately. The microwave photoconductivity in similar heterostructures based on the trivial Hg1 −xCdxTe phase is both P- and T-symmetric.

Modulating light absorption and charge recombination in photoelectrochemical water oxidation with spin-coated MoS2 co-catalyst on ZnO nanorods

Efficient oxygen evolution reaction (OER) on a photoanode is until today considered a major challenge. In the case of ZnO, one of the most investigated semiconductors, the limitation is due to its low visible light absorption and severe charge recombination. Combining ZnO with catalytically active 2D materials such as MoS2 has been proven to increase its water oxidation performance, but they are still often produced using complex methods. Here, we present a simple and low-cost method of spin-coating to fabricate ZnO nanorods/MoS2 nanosheets (ZRM) heterostructures for OER photoanode. Using this method, we successfully produce heterostructures with improved performance, that is, measured photocurrent density (Jph) of 0.61 mA cm−2 at 1.23 V vs. RHE, about threefold than that of pure ZnO, with excellent photostability, which is on par with similar heterostructures synthesized using more complex methods. Beyond benchmarking the performance, we also unravel the mechanism by which MoS2 improves the water oxidation process, that is, through the efficient light-harvesting (ηLHE), charge separation (ηsep), and charge injection (ηinj). Our findings provide a straightforward route to produce metal oxide/2D materials heterostructure-based photoanodes for photoelectrochemical water splitting.

Band Gap Opening in Borophene/GaN and Borophene/ZnO Van der Waals Heterostructures Using Axial Deformation: First-Principles Study.

One of the topical problems of materials science is the production of van der Waals heterostructures with the desired properties. Borophene is considered to be among the promising 2D materials for the design of van der Waals heterostructures and their application in electronic nanodevices. In this paper, we considered new atomic configurations of van der Waals heterostructures for a potential application in nano- and optoelectronics: (1) a configuration based on buckled triangular borophene and gallium nitride (GaN) 2D monolayers; and (2) a configuration based on buckled triangular borophene and zinc oxide (ZnO) 2D monolayers. The influence of mechanical deformations on the electronic structure of borophene/GaN and borophene/ZnO van der Waals heterostructures are studied using the first-principles calculations based on density functional theory (DFT) within a double zeta plus polarization (DZP) basis set. Four types of deformation are considered: uniaxial (along the Y axis)/biaxial (along the X and Y axes) stretching and uniaxial (along the Y axis)/biaxial (along the X and Y axes) compression. The main objective of this study is to identify the most effective types of deformation from the standpoint of tuning the electronic properties of the material, namely the possibility of opening the energy gap in the band structure. For each case of deformation, the band structure and density of the electronic states (DOS) are calculated. It is found that the borophene/GaN heterostructure is more sensitive to axial compression while the borophene/ZnO heterostructure is more sensitive to axial stretching. The energy gap appears in the band structure of borophene/GaN heterostructure at uniaxial compression by 14% (gap size of 0.028 eV) and at biaxial compression by 4% (gap size of 0.018 eV). The energy gap appears in the band structure of a borophene/ZnO heterostructure at uniaxial stretching by 10% (gap size 0.063 eV) and at biaxial compression by 6% (0.012 eV). It is predicted that similar heterostructures with an emerging energy gap can be used for various nano- and optoelectronic applications, including Schottky barrier photodetectors.

Mechanisms of Interface Cleaning in Heterostructures Made from Polymer-Contaminated Graphene.

Heterostructures obtained from layered assembly of 2D materials such as graphene and hexagonal boron nitride have potential in the development of new electronic devices. Whereas various materials techniques can now produce macroscopic scale graphene, the construction of similar size heterostructures with atomically clean interfaces is still unrealized. A primary barrier has been the inability to remove polymeric residues from the interfaces that arise between layers when fabricating heterostructures. Here, the interface cleaning problem of polymer-contaminated heterostructures is experimentally studied from an energy viewpoint. With this approach, it is established that the interface cleaning mechanism involves a combination of thermally activated polymer residue mobilization and their mechanical actuation. This framework allows a systematic approach for fabricating record large-area clean heterostructures from polymer-contaminated graphene. These heterostructures provide state-of-the-art electronic performance. This study opens new strategies for the scalable production of layered materials heterostructures.

Van der Waals Epitaxy of Organic Semiconductor Thin Films on Atomically Thin Graphene Templates for Optoelectronic Applications.

ConspectusOrganic semiconductors (OSCs) offer unique advantages with respect to mechanical flexibility, low-cost processing, and tunable properties. The optical and electrical properties of devices based on OSCs can be greatly improved when an OSC is coupled with graphene in a certain manner. Our research group has focused on using graphene as a growth template for OSCs and incorporating such high-quality heterostructures into optoelectronic devices. The idea is that graphene's atomically flat surface with a uniform sp2 carbon network can serve as a perfect quasi-epitaxial template for the growth of OSCs. In addition, OSC-graphene heterostructures benefit from graphene's unique characteristics, such as its high charge-carrier mobility, excellent optical transparency, and fascinating mechanical durability and flexibility.However, we have often found that OSC molecules assemble on graphene in unpredictable manners that vary from batch to batch. From observations of numerous research systems, we elucidated the mechanism underlying such poor repeatability and set out a framework to actually control the template effect of graphene on OSCs. In this Account, we not only present our scientific findings in this spectrum of areas but also convey our research scheme to the readers so that similar heterostructure complexes can be systematically studied.We began with experiments showing that the growth of OSCs on a graphene surface was driven by van der Waals interactions and is therefore sensitive to the cleanliness of the graphene surface. Nonetheless, we noted that, even on similarly clean graphene surfaces, the OSC thin film still varied with the underlying substrate. Thanks to the graphene-transfer method and in situ gating methods that we developed, we discovered that the decisive parameter for molecule-graphene interaction (and, hence, for the growth of OSCs on graphene) is the charge density in the graphene. Thus, to prepare a graphene template for high-quality graphene-OSC heterostructures, we controlled the charge density in the graphene to minimize the molecule-graphene interaction. Moreover, the possible charge transfer between OSC molecules and graphene, which induces additional molecule-graphene interactions, should also be taken into account. Eventually, we demonstrated a wide range of optoelectronic applications that benefitted from high-quality OSC-graphene heterostructures fabricated using our proof-of-concept systems.

Enhanced NH3 sensing performance at ppb level derived from Ti3C2Tx-supported ZnTi-LDHs nanocomposite with similar metal-semiconductor heterostructure

Layered double hydroxides (LDHs) are attracting increased attention in the field of gas sensor owing to their high specific surface and interlayer modifiability. However, the general tendency to restacking and poor conductivity significantly affect the performance of the gas sensor based on LDHs towards rarefied gas. To resolve these problems, we successfully constructed the MXene (Ti3C2Tx)-supported ZnTi-LDHs (LDHs/Ti3C2Tx) nanocomposite with loose heterostructure by one-step hydrothermal method based on electrostatic ordered self-assembly. The LDHs uniformly wrapped on Ti3C2Tx nanosheets form a highly synergistic similar metal-semiconductor contact with Ti3C2Tx, and the formed anti-barrier layer (ABL) has a trapping effect on the electrons generated by NH3-sensing. Meanwhile, the formation of tiny LDHs might be ascribed to the effect of Ti atoms in Ti3C2Tx, which is confirmed by first-principles calculations. Benefiting from similar metal-semiconductor heterostructure and the effect of Ti atoms in Ti3C2Tx, the electrical conductivity and the adsorption active sites are prominently improved, which are essential for the enhancement of sensitivity to rarefied gas. The gas sensor based on ZnTi-LDHs/Ti3C2Tx nanocomposite exhibits about 5 and 9-fold enhancement in response value to 50 ppm NH3 at room temperature (RT) compared with the pure LDHs and Ti3C2Tx, respectively. Notably, the as-fabricated gas sensor could detect as low as 100 ppb NH3 with a response of 1.26 at RT. Furthermore, the LDHs/Ti3C2Tx sensor exhibits fast response/recovery time, high selectivity, acceptable humidity tolerance and outstanding long-term stability towards NH3. Above all, this work provides a feasible route to develop excellent gas-sensing materials of sensor via interfacial design.

High-conductivity polarization-induced 2D hole gases in undoped GaN/AlN heterojunctions enabled by impurity blocking layers

High-conductivity undoped GaN/AlN 2D hole gases (2DHGs), the p-type dual of the AlGaN/GaN 2D electron gases (2DEGs), have offered valuable insights into hole transport in GaN and enabled the first GaN GHz RF p-channel FETs. They are an important step toward high-speed and high-power complementary electronics with wide-bandgap semiconductors. These technologically and scientifically relevant 2D hole gases are perceived to be not as robust as the 2DEGs because structurally similar heterostructures exhibit wide variations of the hole density over Δps>7×1013 cm−2, and low mobilities. In this work, we uncover that the variations are tied to undesired dopant impurities such as silicon and oxygen floating up from the nucleation interface. By introducing impurity blocking layers (IBLs) in the AlN buffer layer, we eliminate the variability in 2D hole gas densities and transport properties, resulting in a much tighter control over the 2DHG density variations to Δps≤1×1013 cm−2 across growths, and a 3× boost in the Hall mobilities. These changes result in a 2–3× increase in hole conductivity when compared to GaN/AlN structures without IBLs.

Hybrid ZnO/MoS2 Core/Sheath Heterostructures for Photoelectrochemical Water Splitting

The rational synthesis of semiconducting materials with enhanced photoelectrocatalytic efficiency under visible light illumination is a long-standing issue. ZnO has been systematically explored in this field, as it offers the feasibility to grow a wide range of nanocrystal morphology; however, its wide band gap precludes visible light absorption. We report on a novel method for the controlled growth of semiconductor heterostructures and, in particular, core/sheath ZnO/MoS2 nanowire arrays and the evaluation of their photoelectrochemical efficiency in oxygen evolution reaction. ZnO nanowire arrays, with a narrow distribution of nanowire diameters, were grown on FTO substrates by chemical bath deposition. Layers of Mo metal at various thicknesses were sputtered on the nanowire surface, and the Mo layers were sulfurized at low temperature, providing in a controlled way few layers of MoS2, in the range from one to three monolayers. The heterostructures were characterized by electron microscopy (SEM, TEM) and spectroscopy (XPS, Raman, PL). The photoelectrochemical properties of the heterostructures were found to depend on the thickness of the pre-deposited Mo film, exhibiting maximum efficiency for moderate values of Mo film thickness. Long-term stability, in relation to similar heterostructures in the literature, has been observed.

Roughness induced wettability amplification of novel copper molybdate-branched CuO nanorod arrays by non-aqueous solution method

A CuO/copper molybdate (labeled as CMO) dendritic structure with amplified wettability is successfully synthesized by a non-aqueous solution method. CuO nanorods are firstly prepared by thermal oxidation of Cu film on Si substrate. By thermal evaporation of Mo foils, monocrystalline CMO branches are formed via the deposited MoOx vapor on CuO nanorods. This morphology induced roughness amplifies the wettability of CuO nanorods, realizing reversible superwicking and almost superhydrophobic properties with increased reversible water contact angle ranges. This facile method may promote the preparation and application of similar metal molybdates or heterostructures.

Electric-polarization-driven magnetic phase transition in a ferroelectric–ferromagnetic heterostructure

Magnetoelectric coupling is of great interest recently to both understand the fundamental physics and device applications. Materials with strong magnetoelectric coupling, high Curie temperature, and large electric polarization are still rare. We suggest a heterostructure that combines the known memory effect of the switchable ferroelectric In2Se3 [Adv. Funct. Mater. 2019, 29, 1808606] with a van der Waals bonded two-dimensional (2D) metal-organic framework (MOF) film. The magnetic ground state of this MOF can be changed from an antiferromagnetic state to a ferromagnetic through hole-doping. We use first-principles calculations to show that in such a heterostructure, adequate doping differences to cause this phase transition are expected from the changes in the interfacial charge transfer between the MOF and In2Se3 when the polarization direction of the In2Se3 is reversed. This and similar 2D heterostructures may, therefore, provide both a fascinating material platform for understanding the fundamental physics of magnetoelectric coupling and a strategy for designing spin-current-based nonvolatile memory structures.

ReS2/h‐BN/Graphene Heterostructure Based Multifunctional Devices: Tunneling Diodes, FETs, Logic Gates, and Memory

AbstractA 2D heterostructure consisting of few‐layer direct bandgap ReS2, a thin h‐BN layer, and a monolayer graphene (Gr) for application to various electronic devices is investigated. Metal‐insulator‐semiconductor (MIS)‐type devices with 2D van‐der‐Waals (vdW) heterostructures are recently studied as important components to realize various multifunctional device applications in analogue and digital electronics. The tunnel diodes of ReS2/h‐BN/Gr exhibit light tunable rectifying behaviors with low ideality factors and nearly temperature independent electrical characteristics. The devices behave like conventional MIS‐type tunnel diodes for logic gate applications. Furthermore, similar vertical heterostructures are shown to operate in field‐effect transistors with a low threshold voltage and a memory device with a large memory gate for future multifunctional device applications.

Phonon‐Enhanced Near‐Field Spectroscopy to Extract the Local Electronic Properties of Buried 2D Electron Systems in Oxide Heterostructures

AbstractIn the family of functional oxide materials, the interface between LaAlO3 and SrTiO3 (LAO/STO) is an interesting example, as both materials are large‐bandgap insulators in their bulk state but give rise to a confined 2D electron gas (2DEG) when combined through thin‐film deposition. While this 2DEG exhibits remarkable properties, its experimental investigation is mostly limited to destructive or non‐local (i.e. averaging over larger areas) methods until recently. Scanning near‐field optical microscopy is shown to overcome this limitation, detecting buried 2DEGs by using highly confined optical near‐fields. Here, a full spectroscopic approach with phonon‐enhancement and simulations based on the finite dipole model is combined to extract quantitative electronic properties of the interfacial LAO/STO 2DEG. This threefold improvement compared to previous work will enable the quantitative nanoscale, non‐destructive, sub‐surface analysis of complex oxide thin films and interfaces, as well as similar heterostructures.

Monolithic growth of GaAs/Al0.3Ga0.7As multiple quantum well structure on Ge substrate with low defects: theoretical and experimental correlation of the structural and optical properties

High quality III–V multiple quantum well (MQW) heterostructure on germanium (Ge) substrate is grown by molecular beam epitaxy. The propagation of defects and dislocations from the Ge/GaAs interface towards the active layer is suppressed via the adapted novel growth strategy. The cross-sectional transmission electron microscopy images showed the active layer of the MQW structure with reduced anti-phase domains and dislocations due to the introduction of migration enhanced epitaxy (MEE) and three-step annealed GaAs buffer layer. The optical properties are compared with another sample having similar heterostructure on GaAs substrate. The variation in full width half maxima with the well thickness has been analyzed via the correlation between photoluminescence (PL) result and calculated penetration depth of electron wave functions into the barrier material. The effect of substrate on the hetero-interface of MQW structure is also investigated. Rapid thermal annealing (RTA) is carried out on the sample with Ge substrate in order to explore the change in optical properties of the MQW structure. PL study indicates insignificant effect of low temperature RTA treatment on the thinner QWs. However, improvement in the optical property such as increment in activation energy and three order enhancement in the PL intensity was observed for the thicker QWs, owing to the amelioration in their structural property.

Pascal conductance series in ballistic one-dimensional LaAlO3/SrTiO3 channels.

One-dimensional electronic systems can support exotic collective phases because of the enhanced role of electron correlations. We describe the experimental observation of a series of quantized conductance steps within strongly interacting electron waveguides formed at the lanthanum aluminate-strontium titanate (LaAlO3/SrTiO3) interface. The waveguide conductance follows a characteristic sequence within Pascal's triangle: (1, 3, 6, 10, 15, …) ⋅ e 2 /h, where e is the electron charge and h is the Planck constant. This behavior is consistent with the existence of a family of degenerate quantum liquids formed from bound states of n = 2, 3, 4, … electrons. Our experimental setup could provide a setting for solid-state analogs of a wide range of composite fermionic phases.