Superlattice Heterostructures Research Articles

Physical Modeling of Resonant Tunneling in Ferroelectric Tunnel Junctions With Multiple Quantum Well Superlattice Heterostructures

Physical Modeling of Resonant Tunneling in Ferroelectric Tunnel Junctions With Multiple Quantum Well Superlattice Heterostructures

Ferro/Nonferroelectric Vertical Heterostructure Superlattice as a Visible-Light-Responsive Photocatalyst: A DFT Prediction.

Developing narrow-band-gap ferroelectric semiconducting photocatalysts is a promising strategy for efficient photocatalytic water splitting with high energy conversion efficiency. Within this context, six ferro/nonferroelectric vertical heterostructure superlattices (VHSs) are constructed in this work by stacking ferroelectric SiS or GeS with nonferroelectric layered organic photocatalysts (C2N, g-C3N4, and melon), layer by layer. The geometry and electronic structures of these six VHSs are systematically investigated by density functional theory calculations. Consequently, four VHSs (SiS/g-C3N4, GeS/C2N, GeS/g-C3N4, and GeS/melon) are predicted to simultaneously possess several important and highly desirable features for photocatalytic water splitting, namely excellent visible-light adsorption, remarkable spontaneous polarization (0.49-0.70 C/m2), spatial charge separation, as well as suitable band-edge positions, thus serving as potential candidates for photocatalytic water splitting to produce hydrogen. This work not only provides a new strategy to use narrow-band-gap ferroelectric semiconductors for photocatalytic water splitting but also offers inspiration for developing photocatalysts with high energy conversion efficiency.

Modulating Thermoelectric Properties of the MoSe2/WSe2 Superlattice Heterostructure by Twist Angles.

In the current era of limited resources, the matter of energy conversion holds significant importance. Thermoelectric materials possess the ability to transform thermal energy into electric power. Achieving an impressive thermoelectric figure of merit (ZT) necessitates the presence of a high power factor alongside low thermal conductivity. Stimulated by recent experimental reports on the in-plane lateral MoSe2/WSe2 heterostructure in the application thermoelectric device [Zhang Y. et al., Simultaneous electrical and thermal rectification in a monolayer lateral heterojunction. Science 2022, 378, 169], in this study, the method of twisting angle is used to modulate the energy bands of van der Waals MoSe2/WSe2 superlattice heterostructures to optimize the carrier concentration, band gap, electric conductance, thermal conductivity, and ZT of the heterostructure. The 21.79° twisted heterostructures among different twisting-angle heterostructures benefit from both the high power factor and low thermal conductivity, ultimately leading to significantly improved ZT compared to the untwisted counterpart.

Well-ordered single-atomic cobalt-1T-MoS2/C superlattice heterostructure toward durable overall water splitting

Van der Waals superlattice heterostructure with different two-dimensional (2D) nanosheets provides a promising way to exploit the intrinsic properties of both 2D components. However, developing efficient methods for precisely constructing ordered/controlled layer sequence superlattice heterostructures has always been challenging. Here, we report a facile strategy for the preparation of single-atomic cobalt anchored 1T-MoS2/carbon (Co-1T-MoS2/C) heterostructure with a well-ordered superlattice of alternating Co-1T-MoS2 and C nanosheets at a molecular level. The coordination of Co atoms on the 1T-MoS2 matrix and its effect on the phase transformation of MoS2 has been systematically investigated. Importantly, the Co atoms can efficiently improve the metastable 1T phase stability of MoS2. As a proof-of-concept application, the obtained Co-1T-MoS2/C is employed as an electrocatalyst for overall water splitting in alkaline conditions. The rapid interfacial charge transfer and abundant active catalytic sites, coupled with the contribution of the single-atomic Co sites enable the Co-1T-MoS2/C superlattice heterostructure to exhibit excellent performance. The assembled Pt/C||Co-1T-MoS2/C cell needs only 1.57 V to reach the current density of 10 mA/cm2. Our research strategy provides a feasible path for the design of 2D-2D van der Waals superlattice heterostructures and further broadens their applications.

Twisted-bilayer FeSe and the Fe-based superlattices

We derive BM-like continuum models for the bands of superlattice heterostructures formed out of Fe-chalcogenide monolayers: (I) a single monolayer experiencing an external periodic potential, and (II) twisted bilayers with long-range moire tunneling. A symmetry derivation for the inter-layer moire tunnelling is provided for both the \GammaΓ and MM high-symmetry points. In this paper, we focus on moire bands formed from hole-band maxima centered on \GammaΓ, and show the possibility of moire bands with C=0C=0 or ±1±1 topological quantum numbers without breaking time-reversal symmetry. In the C=0C=0 region for \theta→0θ→0 (and similarly in the limit of large superlattice period for I), the system becomes a square lattice of 2D harmonic oscillators. We fit our model to FeSe and argue that it is a viable platform for the simulation of the square Hubbard model with tunable interaction strength.

Ultraflexible two-dimensional Janus heterostructure superlattice: a novel intrinsic wrinkled structure.

The recently reported two-dimensional Janus transition metal dichalcogenide materials present promising applications such as in transistors, photocatalysts, and thermoelectric nanodevices. In this work, using molecular dynamics simulations, the self-assembled in-plane MoSSe/WSSe heterostructure superlattice is predicted with a natural sinusoidal structure constructed by an asymmetric interface. Such a sinusoidal structure shows extraordinary mechanical behavior where the fracture strain can be enhanced up to 4.7 times than that of the symmetrical interface. Besides, the deformational structure of all these MoSSe/WSSe heterostructure superlattice are in accordance with the Fourier function curve; the fracture strength and fracture strain also demonstrate pronounced size dependence. Our investigations proposed an ultrastretchable assembled heterostructure superlattice and provided a desirable strategy to tune the mechanical properties of such an in-plane two-dimensional heterostructure.

Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis.

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

Ultralow In-Plane Thermal Conductivity in 2D Magnetic Mosaic Superlattices for Enhanced Thermoelectric Performance.

Lowering thermal conductivity via introducing heterointerfaces of heterophase fillings (HPFs) is a common strategy for optimizing thermoelectric performance, but it is always accompanied by deterioration of electrical conductivity. Here we report an ordered magnetic HPF system in a CoSe2-SnSe mosaic heterostructure superlattice synthesized by van der Waals confined epitaxial growth (vdWCEG), which realizes a maximized filling amount to decrease in-plane thermal conductivity of SnSe layers and maintain the intact in-plane carrier transport path. The in-plane thermal conductivity of CoSe2-SnSe superlattice reaches the lowest range among SnSe-based materials with a value of 0.27 W m-1 K-1 at 850 K, which can be attributed to abundant interfaces between CoSe2 nanocrystals and SnSe layers. Moreover, the CoSe2 nanocrystals show superparamagnetic behavior, by which the rotation of magnetic domains provides additional phonon scattering to further decrease in-plane thermal conductivity. By combination with the preserved in-plane electrical conductivity of SnSe layers, an enhanced in-plane ZT value of 0.62 is achieved at 850 K. This vdWCEG approach can also be generally applied to fabricate various other two-dimensional (2D) mosaic heterostructures, providing an avenue for artificial 2D heterostructures with desired functionalities.

Evidence for moiré intralayer excitons in twisted WSe2/WSe2 homobilayer superlattices

Recent advances in twisted van der Waals heterostructure superlattices have emerged as a powerful and attractive platform for exploring novel condensed matter physics due to the interplay between the moiré potential and Coulomb interactions. The moiré superlattices act as a periodic confinement potential in space to capture interlayer excitons (IXs), resulting in moiré exciton arrays, which provide opportunities for quantum emitters and many-body physics. The observation of moiré IXs in twisted transition-metal dichalcogenide (TMD) heterostructures has recently been widely reported. However, the capture and study of the moiré intralayer excitons based on TMD twisted homobilayer (T-HB) remain elusive. Here, we report the observation of moiré intralayer excitons in a WSe2/WSe2 T-HB with a small twist angle by measuring PL spectrum. The multiple split peaks with an energy range of 1.55–1.73 eV are different from that of the monolayer WSe2 exciton peaks. The split peaks were caused by the trapping of intralayer excitons via the moiré potential. The confinement effect of the moiré potential on the moiré intralayer excitons was further demonstrated by the changing of temperature, laser power, and valley polarization. Our findings provide a new avenue for exploring new correlated quantum phenomena and their applications.

Constructing titanium carbide MXene/reduced graphene oxide superlattice heterostructure via electrostatic self-assembly for high-performance capacitive deionization

Capacitive deionization has attracted wide concern on accountof its high energy efficiency, low manufacturing cost and environmental friendliness. Nevertheless, the development of capacitive deionization is still impeded because of the scarcity of suitable electrode materials with superior performance. Herein, we successfully prepared the two-dimensional (2D) titanium carbide (Ti3C2Tx) MXene/ reduced graphene oxide (rGO) superlattice heterostructure by a facile electrostatic self-assembly strategy and systematically investigated its performance as capacitive deionized electrode materials. The unique 2D/2D superlattice heterostructure not only effectively alleviates the self-stacking problem of Ti3C2Tx MXene nanosheets, but also endows the heterostructure with superior conductivity and fast ion diffusion rate. As a result, the MXene/rGO superlattice heterostructure exhibits an outstanding salt (Na+) adsorption capacity (48 mg g−1) at 1.2 V significantly superior to pristine Ti3C2Tx MXene nanosheets, along with outstanding long-term cycling performance. Furthermore, the mechanism involved was elucidated through comprehensive characterizations. Therefore, this study offers a new pathway for designing high-performance electrode materials for capacitive deionization.

Atomically Sharp Lateral Superlattice Heterojunctions Built‐In Nitrogen‐Doped Nanoporous Graphene (Adv. Mater. 20/2022)

Superlattice Heterojunctions In article number 2110099, Cesar Moreno, Diego Peña, Miguel Pruneda, Aitor Mugarza, and co-workers report the synthesis of an atomically precise nitrogen-doped nanoporous graphene that electronically behaves as a nanometer-scale 2D lateral superlattice heterostructure with unprecedented band discontinuities down to the single-bond limit. The atomically sharp nanoporous heterojunctions endow this nanomaterial with a multifunctionality that can be very relevant for photodetection, excitonic solar cells, water splitting, or selective nanosieving.

Ultrathin ferroic HfO2-ZrO2 superlattice gate stack for advanced transistors.

With the scaling of lateral dimensions in advanced transistors, an increased gate capacitance is desirable both to retain the control of the gate electrode over the channel and to reduce the operating voltage1. This led to a fundamental changein the gate stack in 2008,the incorporation of high-dielectric-constant HfO2(ref.2), which remains the material of choiceto date. Here we report HfO2-ZrO2 superlattice heterostructures as a gate stack, stabilized with mixed ferroelectric-antiferroelectric order, directly integrated onto Si transistors, and scaled down to approximately 20 ångströms, the same gate oxide thickness required for high-performance transistors. The overall equivalent oxide thickness in metal-oxide-semiconductor capacitors is equivalent to an effective SiO2 thickness of approximately 6.5 ångströms. Such a low effective oxide thickness and the resulting large capacitance cannot be achieved in conventional HfO2-based high-dielectric-constant gate stacks without scavenging the interfacial SiO2, which has adverse effects on the electron transport and gate leakage current3. Accordingly, our gate stacks, which do not require such scavenging, provide substantially lower leakage current and no mobility degradation. This work demonstrates that ultrathin ferroic HfO2-ZrO2 multilayers, stabilized with competing ferroelectric-antiferroelectric order in the two-nanometre-thickness regime, provide a path towards advanced gate oxide stacks in electronic devices beyond conventional HfO2-based high-dielectric-constant materials.

Plasmonic Metasurfaces for Controlling the Spectrum of Terahertz Radiation Sources

We report on the development and fabrication of a plasmonic metasurfaces for PCA based on InAlAs/InGaAs superlattice heterostructures. As a result of THz time-domain measurements, it was found that for PCA emitters with a 150 nm thick plasmonic metasurface, the maximum recorded overall THz power was 5.3 (the optical-THz conversion efficiency was 0.19 %) under laser pumping with an average power of 2.8 mW and a bias voltage of 8.6 V. The increase in the signal-to-noise ratio due to the excitation of plasmon modes exceeds ~80 dB. The experiments have shown that the increase of metallization thickness provides the increase of the emitted power. The results could open a pathway towards the development of the Russian THz spec-troscopic and imaging systems.

Probing the polar-nonpolar oxide interfaces using resonant x-ray standing wave techniques

Transition metal (TM) oxide heterostructure superlattices have attracted great attention in research communities because of their emergent interfacial phenomena that do not exist in the bulk form. In order to understand the mechanisms that cause these phenomena, it is important to use depth-resolved spectroscopies to study the electronic structure across the buried oxide interfaces. In this review, we focus on the recent applications of standing wave (SW) photoemission (SW-XPS) and resonant inelastic x-ray scattering (SW-RIXS) spectroscopy to study the depth profiles of an electronic structure or carriers around the polar-nonpolar oxide interfaces. Using the incident photon energies near the TM x-ray absorption resonance, the created SW excitation can enhance the spectral response and certain electronic transitions, providing important insight into the interfacial electronic structure in the energy and real space regimes. Following the background introductions, we describe two SW experiments and demonstrate that the combination of SW-XPS and SW-RIXS has the potential to obtain the depth distribution of electronic/orbital states around the buried interfaces with Angstrom precision.

Conversion of 2D MXene to Multi-Low-Dimensional GerMXene Superlattice Heterostructure.

Integration of 2D structures into other low‐dimensional materials results in the development of distinct van der Waals heterostructures (vdWHSs) with enhanced properties. However, obtaining 2D–1D–0D vdWHSs of technologically useful next generation materials, transition‐metal carbide MXene and monoelemental Xene nanosheets in a single superlattice heterostructure is still challenging. Here, the fabrication of a new multidimensional superlattice heterostructure “GerMXene” from exfoliated M3X2T x MXene and hydrogenated germanane (GeH) crystals, is reported. Direct experimental evidence for conversion of hydrothermally activated titanium carbide MXene (A‐MXene) to GerMXene heterostructure through the rapid and spontaneous formation of titanium germanide (TiGe2 and Ti6Ge5) bonds, is provided. The obtained GerMXene heterostructure possesses enhanced surface properties, aqueous dispersibility, and Dirac signature of embedded GeH nanosheets as well as quantum dots. GerMXene exhibits functional bioactivity, electrical conductivity, and negative surface charge, paving ways for its applications in biomedical field, electronics, and energy storage.

Construction of Multilayer Films and Superlattice- and Mosaic-like Heterostructures of 2D Metal Oxide Nanosheets via a Facile Spin-Coating Process.

This study reports a design of a variety of nanostructured films of 2D oxide nanosheets. We systematically examined the deposition of perovskite-type Ca2Nb3O10- nanosheets by spin-coating their dimethyl sulfoxide dispersion. Neat and homogeneous monolayer tiling was attained on various substrates by selecting an optimum rotation speed, which was dependent on the nanosheet concentration. Repeating the optimized spin-coating process allowed for layer-by-layer deposition of the nanosheets into multilayer films with a designed layer number. Vertical superlattice heterostructures could also be assembled by alternately spin-coating the suspensions of Ca2Nb3O10- and Ti0.87O20.52- nanosheets. Furthermore, spin-coating of a mixed suspension of Ca2Nb3O10- and Ti0.87O20.52- nanosheets led to a mixed mosaic-like monolayer of these two nanosheets. The present study thus demonstrated spin-coating as a facile and powerful route to construct various nanostructures based on 2D oxide nanosheets.

Interface engineered MBE grown InAs/GaSb based type-II superlattice heterostructures

Strain balanced InAs/GaSb type-II superlattice structures have been grown using molecular beam epitaxy. InSb like interfaces have been introduced at both InAs on GaSb and GaSb on InAs surfaces using migration enhanced epitaxy to compensate the tensile strain between the constituent binaries of the superlattice. The superlattice layers have been characterized using high resolution X-ray diffraction, atomic force microscopy, transmission electron microscopy and absorption spectroscopy. It is shown that the substrate temperature influences the InSb thickness at the interface and therefore the net strain in the superlattice. The dependence of the surface morphology on the strained or relaxed state of the superlattice is discussed. The optimized InAs(10 ML)/InSb(0.5 ML)/GaSb(12 ML)/InSb(0.5 ML) superlattice epilayers up to 200 periods have been grown at temperature of 450 °C and have a small net strain of 0.0013%. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis in the strain compensated superlattice layers reveal InSb like interfaces. Excellent crystalline quality and morphology with an absorption edge at 5.4 µm in the mid-wave infra-red (MWIR) region are observed for the optimized epilayers.

Proposal for energy efficient spin transfer torque-magnetoresistive random access memory device

Utilizing the electronic analogs of optical phenomena such as anti-reflection coating and resonance for spintronic devices, we propose and theoretically analyze the design of a spin transfer torque-magnetoresistive random access memory (STT-MRAM) device. The proposed device consists of a superlattice heterostructure terminated with the anti-reflective regions sandwiched between the fixed and free ferromagnetic layers. Employing Green’s function spin-transport formalism coupled self-consistently with the stochastic Landau–Lifshitz–Gilbert–Slonczewski equation, we design an STT-MRAM based on the anti-reflective superlattice magnetic tunnel junction (AR-SLMTJ) device having an ultrahigh tunnel magnetoresistance (≈3.5×104%) and large spin current. We demonstrate that the STT-MRAM based on the AR-SLMTJ structure owing to the physics of bandpass spin filtering is nearly 1100% more energy efficient than trilayer magnetic tunnel junction (MTJ) based STT-MRAM. We also present detailed probabilistic switching and energy analysis to find out the optimal point of operation of a trilayer MTJ and AR-SLMTJ based STT-MRAM. Our predictions serve as a template to consider the heterostructures for next-generation spintronic device applications.

Hot carrier relaxation and inhibited thermalization in superlattice heterostructures: The potential for phonon management

One of the main loss mechanisms in photovoltaic solar cells is the thermalization of photogenerated hot carriers via phonon-mediated relaxation. By inhibiting these relaxation mechanisms and reducing thermalization losses, it may be possible to improve the power conversion efficiency of solar cells beyond the single gap limit. Here, type-II InAs/AlAsSb multi-quantum well (MQW) structures are investigated to study the impact of the phononic properties of the AlAsSb barrier material in hot carrier thermalization. Experimental and theoretical results show that by increasing the barrier thickness (increasing the relative contribution of AlAsSb content in the superlattices), the relaxation of hot carriers is reduced as observed in power-dependent photoluminescence and thermalization analysis. This is attributed to an increase in the phononic bandgap of the MQW with increasing AlAsSb composition reducing the efficiency of the dominant Klemens mechanism as the phononic properties shift toward a more AlSb-like behavior.

Synthesis of superlattice heterostructure of germanium quantum dots in silicon spacer layers and its application in photovoltaic solar cells

Superlattice heterostructure of dislocation-free Ge quantum dots buried in Si spacer layers has been synthesized using ultra high vacuum-chemical vapor deposition technique. The heterostructure has been used as active layer in Si solar cells based upon vertical p-i-n junction configuration. Experiment shows that such solar cells exhibit strong repose to infrared radiation in contrast with the negligible response of solar cells with active layers of pure Si having the same thickness. A solar cell with 300 nm thick superlattice is shown to have thermodynamic efficiency of about 11.2%. This is correlated with the rather high quality of the heterostructure used in the present study as judged from photoluminescence spectroscopy and electron microscopy as well as strain and dark current measurements. It is concluded that such type of heterostructure may provide a pathway to develop efficient and cost effective solar cells. • Superlattice heterostructure of Ge quantum dots on Si has been synthesized. • Various techniques have been used to characterize the nanostructures. • The heterostructure has been used as active layer in Si (p-i-n)-type solar cells. • The thermodynamic efficiency of respective solar cell is determined.