Low-dimensional Heterostructures Research Articles

Impact of Acoustic and Optical Phonons on the Anisotropic Heat Conduction in Novel C-Based Superlattices.

C-based XC binary materials and their (XC)m/(YC)n (X, Y ≡ Si, Ge and Sn) superlattices (SLs) have recently gained considerable interest as valuable alternatives to Si for designing and/or exploiting nanostructured electronic devices (NEDs) in the growing high-power application needs. In commercial NEDs, heat dissipation and thermal management have been and still are crucial issues. The concept of phonon engineering is important for manipulating thermal transport in low-dimensional heterostructures to study their lattice dynamical features. By adopting a realistic rigid-ion-model, we reported results of phonon dispersions ωjSLk→ of novel short-period XCm/(YC)n001 SLs, for m, n = 2, 3, 4 by varying phonon wavevectors k→SL along the growth k|| ([001]), and in-plane k⟂ ([100], [010]) directions. The SL phonon dispersions displayed flattening of modes, especially at high-symmetry critical points Γ, Z and M. Miniband formation and anti-crossings in ωjSLk→ lead to the reduction in phonon conductivity κz along the growth direction by an order of magnitude relative to the bulk materials. Due to zone-folding effects, the in-plane phonons in SLs exhibited a strong mixture of XC-like and YC-like low-energy ωTA, ωLA modes with the emergence of stop bands at certain k→SL. For thermal transport applications, the results demonstrate modifications in thermal conductivities via changes in group velocities, specific heat, and density of states.

Electric transport and topological properties of binary heterostructures in topological insulators

The design of devices with coupled hybrid structures offers an approach to creating synthetic topological materials. This work discusses the topological and transport properties of low-dimensional binary heterostructures of topological and trivial materials. By adjusting the parameters of each component, we control the global topological properties to enhance tunneling and optimize the transmission of the topological edge states (TES). Considering a one-dimensional tight-binding model, we build heterostructures of coupled chains employing Green’s functions (GF) formalism. We determine the topological characteristics of chains and couple them together, applying Dyson’s equation to generate the heterostructure. The intensity and decay length of the TES vary depending on the coupling parameters and the size of each chain. We investigate the topological diagrams phase using the energy bands of the periodic system and calculating the invariant from the Zak phase. Using cross-band condition, we derive analytical functions of the parameter space to get the phase topological diagram, which can be compared with the LDOS maps at zero energy. Finally, we calculate the differential conductance with the Keldysh GF technique to demonstrate the tunneling of the TES at the zero bias voltage and discuss potential design and experimental applications.

Zero-Dimensional Carbon Nanomaterials: Building Blocks for Efficient Energy Conversion

Low-dimensional carbon-based materials, including 0D fullerene, 1D carbon nanotube, and 2D graphene, have been widely used as electroactive building blocks for sustainable energy conversion heterostructure nanosystems. These materials have unique physical and chemical properties at the nanoscale, such as high conductivity, high specific surface area, and tunable electronics, making them the focus of new renewable energy technologies. In this context, 0D carbon-based nanomaterials, like fullerenes and fullertubes, have garnered substantial attention as rising star energy materials. This editorial spotlight article outlines the current advancements in the utilization of 0D-fullerene and fullertube-based nanoelectrocatalysts for energy conversion. Keywords: 0D-nanocarbons, low-dimensional heterostructures, energy conversion

Simulations of Infrared Reflectivity and Transmission Phonon Spectra for Undoped and Doped GeC/Si (001).

Exploring the phonon characteristics of novel group-IV binary XC (X = Si, Ge, Sn) carbides and their polymorphs has recently gained considerable scientific/technological interest as promising alternatives to Si for high-temperature, high-power, optoelectronic, gas-sensing, and photovoltaic applications. Historically, the effects of phonons on materials were considered to be a hindrance. However, modern research has confirmed that the coupling of phonons in solids initiates excitations, causing several impacts on their thermal, dielectric, and electronic properties. These studies have motivated many scientists to design low-dimensional heterostructures and investigate their lattice dynamical properties. Proper simulation/characterization of phonons in XC materials and ultrathin epilayers has been challenging. Achieving the high crystalline quality of heteroepitaxial multilayer films on different substrates with flat surfaces, intra-wafer, and wafer-to-wafer uniformity is not only inspiring but crucial for their use as functional components to boost the performance of different nano-optoelectronic devices. Despite many efforts in growing strained zinc-blende (zb) GeC/Si (001) epifilms, no IR measurements exist to monitor the effects of surface roughness on spectral interference fringes. Here, we emphasize the importance of infrared reflectivity Rω and transmission Tω spectroscopy at near normal θi = 0 and oblique θi ≠ 0 incidence (Berreman effect) for comprehending the phonon characteristics of both undoped and doped GeC/Si (001) epilayers. Methodical simulations of Rω and Tω revealing atypical fringe contrasts in ultrathin GeC/Si are linked to the conducting transition layer and/or surface roughness. This research provided strong perspectives that the Berreman effect can complement Raman scattering spectroscopy for allowing the identification of longitudinal optical ωLO phonons, transverse optical ωTO phonons, and LO-phonon-plasmon coupled ωLPP+ modes, respectively.

Solution Processable Carbon Nanotube-Perovskite Nanohybrids

Carbon nanotubes (CNTs) based nanohybrids have attracted considerable interest for their potential in the design of innovative low-dimensional heterostructures. These nanohybrids can offer unique properties, including enhanced electronic transport and tunability, with a diverse range of optoelectronic applications.Here we present a novel approach for the synthesis of solution-processable nanohybrids made of single-walled carbon nanotubes (SWCNTs) and methylammonium-lead-iodide (MAPI) perovskites, where the CNTs act as templates for the facile and tuneable formation of anisotropic perovskite nanocrystals.We will present the structural characterization and electronic properties of the developed CNT-perovskite nanohybrids and demonstrate their integration (from solution) in field-effect transistor device configurations. By harnessing the synergistic properties of carbon nanotubes and perovskite nanocrystals, we will discuss pathways for the development of photoresponsive devices displaying sensitivity to a broad range of illumination wavelengths.The strategy presented here is of general applicability for the templated synthesis of perovskite nanocrystals toward the fabrication of solution processable in nanoscale optoelectronic devices.

Photoinduced dynamics during electronic transfer from narrow to wide bandgap layers in one-dimensional heterostructured materials

Electron transfer is a fundamental energy conversion process widely present in synthetic, industrial, and natural systems. Understanding the electron transfer process is important to exploit the uniqueness of the low-dimensional van der Waals (vdW) heterostructures because interlayer electron transfer produces the function of this class of material. Here, we show the occurrence of an electron transfer process in one-dimensional layer-stacking of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). This observation makes use of femtosecond broadband optical spectroscopy, ultrafast time-resolved electron diffraction, and first-principles theoretical calculations. These results reveal that near-ultraviolet photoexcitation induces an electron transfer from the conduction bands of CNT to BNNT layers via electronic decay channels. This physical process subsequently generates radial phonons in the one-dimensional vdW heterostructure material. The gathered insights unveil the fundamentals physics of interfacial interactions in low dimensional vdW heterostructures and their photoinduced dynamics, pushing their limits for photoactive multifunctional applications.

Recent developments in low-dimensional heterostructures of halide perovskites and metal chalcogenides as emergent materials: Fundamental, implementation, and outlook

In the past decades, halide perovskites and chalcogenide materials have provided significant contributions to the vast development for optoelectronic applications. Halide perovskites are known for their tunable properties, while chalcogenides are known for their high efficiency. The combination of these types of materials as heterostructures is thought to have been able to produce a superior device/photophysical performance. A peculiar aspect to consider is an inherent weak interaction between these layers via the stacking of different materials, promoting the realization of van der Waals heterostructures with novel functional properties. In this review, we summarize the progress and foresee the prospectives of material systems obtained by combining low-dimensional (0D, 1D, and 2D) halide perovskite and chalcogenide systems. Both emergent materials share their promise in terms of energy and charge transfer consideration. In addition, several aspects that are mutually important in this context will be outlined, namely, interlayer excitons, interfacial engineering, quantum confinement effect, and light–matter interactions. Based on these fundamental approaches, we translate the current understanding by highlighting several representative heterostructures with prominent performance such as light-emitting diodes, x-ray detectors, photodetectors, and solar cells. In this review, we focus on the rich chemistry and photophysics of these heterostructures, emphasizing the open questions related to their structure–property relationship. Finally, potential research directions and outlooks based on the implementation of halide perovskite–chalcogenide heterostructures are also proposed.

Hyperspectral photoluminescence and reflectance microscopy of 2D materials

Optical micro-spectroscopy is an invaluable tool for studying and characterizing samples ranging from classical semiconductors to low-dimensional materials and heterostructures. To date, most implementations are based on point-scanning techniques, which are flexible and reliable, but slow. Here, we describe a setup for highly parallel acquisition of hyperspectral reflection and photoluminescence (PL) microscope images using a push-broom technique. Spatial as well as spectral distortions are characterized and their digital corrections are presented. We demonstrate close- to diffraction-limited spatial imaging performance and a spectral resolution limited by the spectrograph. The capabilities of the setup are demonstrated by recording a hyperspectral PL map of a MoSe2–WSe2 lateral heterostructure, grown by chemical vapor deposition (CVD), from which we extract the luminescence energies, intensities and peak widths across the interface.

Stochastic simulation of electron transport in a strong electrical field in low-dimensional heterostructures

Abstract In this paper we develop a stochastic simulation algorithm for electron transport in a DA-pHEMT heterostructure. Mathematical formulation of the problem of electron gas transport in the heterostructure in the form of a coupled system of Poisson, Schrödinger and kinetic Boltzmann equations is given. A Monte Carlo model of electron transport in DA-pHEMT heterostructures which accounts for multivalley parabolic band structure, as well as relevant formulas for calculating electron scattering rates and scattering phase functions on polar optical, intervalley phonons and on impurities are developed. The results of a computational experiment involving the solution of the system of Poisson–Schrödinger–Boltzmann equations for the AlGaAs/GaAs/InGaAs/GaAs/AlGaAs heterostructure are presented. The distribution of electrons by energy subband in the main and satellite valleys and the field dependences of the electron drift velocity in each valley are calculated. It was discovered that there is no spatial transfer of electrons into wide-gap AlGaAs layers due to high barriers created by modulated-doped impurities. A comparative analysis of the electron drift velocities in the studied DA-pHEMT heterostructures and in the unstrained layer of the InGaAs is given.

Cascaded charge-transfer organic alloys for the controlled hierarchical self-assembly of low-dimensional heterostructures

Cascaded charge-transfer organic alloys for the controlled hierarchical self-assembly of low-dimensional heterostructures

Low-dimensional heterostructures of tin nanoparticle-decorated Bi2Te3 nanoplates for reducing lattice thermal conductivity

Low-dimensional materials such as nanoplates exhibit unique properties. Among them, thermoelectric materials are one of the most beneficial because they allow the application of a low-dimensional effect to reduce the lattice thermal conductivity. In this study, low-dimensional heterostructures of nanoparticle-decorated nanoplates were prepared to further reduce lattice thermal conductivity. Single-crystalline bismuth telluride (Bi2Te3) nanoplates with regular hexagonal shapes were fabricated by solvothermal synthesis. Subsequently, tin nanoparticles were decorated on the surface of the Bi2Te3 nanoplates via electroless plating with various concentrations of Sn precursor (SnCl2). Surface morphology and composition analyses indicated the presence of Sn on the nanoplate surface. To evaluate the original thermal properties of the nanoplates and not those of the fused nanoplates, polycrystalline film-formed nanoplates were prepared by pressing at 300 K under a pressure of 0.45 GPa. The in-plane lattice thermal conductivity of the tin nanoparticle-decorated nanoplates decreased with increasing SnCl2 concentration. At a SnCl2 concentration of 90 mM, the lattice thermal conductivity is 1.3 W/(m·K), which is approximately 30 % lower than that of the nanoplates without electroless plating. This indicates that the deposition of tin on the nanoplate surface reduces the phonon flow owing to phonon scattering. These findings provide a reference for controlling thermal properties of low-dimensional heterostructured materials.

Phonon-Mediated Quasiparticle Lifetime Renormalizations in Few-Layer Hexagonal Boron Nitride.

Understanding the collective behavior of the quasiparticles in solid-state systems underpins the field of nonvolatile electronics, including the opportunity to control many-body effects for well-desired physical phenomena and their applications. Hexagonal boron nitride (hBN) is a wide-energy-bandgap semiconductor, showing immense potential as a platform for low-dimensional device heterostructures. It is an inert dielectric used for gated devices, having a negligible orbital hybridization when placed in contact with other systems. Despite its inertness, we discover a large electron mass enhancement in few-layer hBN affecting the lifetime of the π-band states. We show that the renormalization is phonon-mediated and consistent with both single- and multiple-phonon scattering events. Our findings thus unveil a so-far unknown many-body state in a wide-bandgap insulator, having important implications for devices using hBN as one of their building blocks.

Launching and Manipulation of Higher-Order In-Plane Hyperbolic Phonon Polaritons in Low-Dimensional Heterostructures.

Hyperbolic phonon polaritons (HPhPs) are stimulated by coupling infrared (IR) photons with the polar lattice vibrations. Such HPhPs offer low-loss, highly confined light propagation at subwavelength scales with out-of-plane or in-plane hyperbolic wavefronts. For HPhPs, while a hyperbolic dispersion implies multiple propagating modes with a distribution of wavevectors at a given frequency, so far it has been challenging to experimentally launch and probe the higher-order modes that offer stronger wavelength compression, especially for in-plane HPhPs. In this work, the experimental observation of higher-order in-plane HPhP modes stimulated on a 3C-SiC nanowire (NW)/α-MoO3 heterostructure is reported where leveraging both the low-dimensionality and low-loss nature of the polar NWs, higher-order HPhPs modes within 2D α-MoO3 crystal are launched by the 1D 3C-SiC NW. The launching mechanism is further studied and the requirements for efficiently launching of such higher-order modes are determined. In addition, by altering the geometric orientation between the 3C-SiC NW and α-MoO3 crystal, the manipulation of higher-order HPhP dispersions as a method of tuning is demonstrated. This work illustrates an extremely anisotropic low dimensional heterostructure platform to confine and configure electromagnetic waves at the deep-subwavelength scales for a range of IR applications including sensing, nano-imaging, and on-chip photonics.

Electric Field and Strain Tuning of 2D Semiconductor van der Waals Heterostructures for Tunnel Field-Effect Transistors.

Heterostacks consisting of low-dimensional materials are attractive candidates for future electronic nanodevices in the post-silicon era. In this paper, using first-principles calculations based on density functional theory (DFT), we explore the structural and electronic properties of MoTe2/ZrS2 heterostructures with various stacking patterns and thicknesses. Our simulations show that the valence band (VB) edge of MoTe2 is almost aligned with the conduction band (CB) edge of ZrS2, and (MoTe2)m/(ZrS2)m (m = 1, 2) heterostructures exhibit the long-sought broken gap band alignment, which is pivotal for realizing tunneling transistors. Electrons are found to spontaneously flow from MoTe2 to ZrS2, and the system resembles an ultrascaled parallel plate capacitor with an intrinsic electric field pointed from MoTe2 to ZrS2. The effects of strain and external electric fields on the electronic properties are also investigated. For vertical compressive strains, the charge transfer increases due to the decreased coupling between the layers, whereas tensile strains lead to the opposite behavior. For negative electric fields a transition from the type-III to the type-II band alignment is induced. In contrast, by increasing the positive electric fields, a larger overlap between the valence and conduction bands is observed, leading to a larger band-to-band tunneling (BTBT) current. Low-strained heterostructures with various rotation angles between the constituent layers are also considered. We find only small variations in the energies of the VB and CB edges with respect to the Fermi level, for different rotation angles up to 30°. Overall, our simulations offer insights into the fundamental properties of low-dimensional heterostructures and pave the way for their future application in energy-efficient electronic nanodevices.

Dynamic Epitaxial Growth of Organic Heterostructures for Polarized Exciton Conversion.

Highly spatial and angular precision in epitaxial-growth process is crucial for constructing organic low-dimensional heterostructures (OLDHs) with the desired substructures, which remains significant challenge owing to the unpredicted location of complex heterogeneous nucleation. Herein, a dynamic epitaxial-growth approach is developed along the tailored longitudinal/horizontal directions to create diverse OLDHs with hierarchical architectures. The controlled morphology evolution of seed crystals from kinetic to thermodynamic species is achieved via incrementally increasing the crystallization time from 0 to 600s. Accordingly, the kinetic and thermodynamic seed crystals respectively present the specific lattice-matching crystal-planes of (100) and (011), which facilitates the longitudinal epitaxial-growth (LG) process for triblock heterostructures, and the horizontal epitaxial-growth (HG) process for axial-branch heterostructures. The dominant core/shell heterostructures are prepared via both LG and HG processes with a crystallization time of ≈30s. Significantly, these prepared OLDHs realize the rationally polarized exciton conversion for optical logic gate application through the exciton conversion and photon propagation at the heterojunction. This strategy provides an avenue for the precise synthesis of OLDHs with anisotropy optical characters for integrated optoelectronics.

Contact Transport and Field Emission Properties of Low-Dimensional 2D Carbon Heterostructures

Contact Transport and Field Emission Properties of Low-Dimensional 2D Carbon Heterostructures

Rational Design of a Low-Dimensional and Metal-free Heterostructure for Efficient Water Oxidation: DFT and Experimental Studies.

A nitrogen-doped fullerene dimer is synthesized and compounded with multi-walled carbon nanotubes (MWCNTs) to construct a low-dimensional and metal-free 0D-1D heterostructure for electrocatalytic water oxidation. The (C59N)2/MWCNTs heterostructure exhibits a highly efficient performance, as verified by both first-principles density functional theory and experimental studies. The *O → *OOH process is confirmed as the rate-determining step of water oxidation. The negatively charged N-doping leads to electronic redistribution and intermolecular charge transfer and thus reduces the uphill free energies of intermediates on the (C59N)2/MWCNTs interface. Therefore, the (C59N)2/MWCNTs heterostructure has great potential to emit light and heat in metal-free-based electrocatalytic water oxidation.

The progress of fabricating the 2D materials and heterostructure devices

Contemporarily, the superior performance of low-dimensional heterostructure devices has attracted extensive attention of scientists. In order to offer a clearer understanding of low-dimensional materials and heterostructure devices, this paper introduces the basic concepts of low-dimensional materials and heterostructure fabrication and arranges the devices constructed by two-dimensional heterostructure materials. Primarily, the background information of state-of-art low-dimensional materials is demonstrated. Moreover, starting from the preparation of low dimensional materials, the mainstream methods of fabricating the two-dimensional materials are discussed. Then, the fabrication methods of two-dimensional material heterostructure and the general classification of two-dimensional material heterostructure devices are summarized. Last but not least, the full text is summarized and prospected. This paper aims to provide a more specific reference and guidance for the development of two-dimensional heterostructure devices in the future.

Predicting Van der Waals Heterostructures by a Combined Machine Learning and Density Functional Theory Approach.

Van der Waals (vdW) heterostructures are constructed by different two-dimensional (2D) monolayers vertically stacked and weakly coupled by van der Waals interactions. VdW heterostructures often possess rich physical and chemical properties that are unique to their constituent monolayers. As many 2D materials have been recently identified, the combinatorial configuration space of vdW-stacked heterostructures grows exceedingly large, making it difficult to explore through traditional experimental or computational approaches in a trial-and-error manner. Here, we present a computational framework that combines first-principles electronic structure calculations, 2D material database, and supervised machine learning methods to construct efficient data-driven models capable of predicting electronic and structural properties of vdW heterostructures from their constituent monolayer properties. We apply this approach to predict the band gap, band edges, interlayer distance, and interlayer binding energy of vdW heterostructures. Our data-driven model will open avenues for efficient screening and discovery of low-dimensional vdW heterostructures and moiré superlattices with desired electronic and optical properties for targeted device applications.

Spectroscopic ellipsometry for low-dimensional materials and heterostructures

Abstract Discovery of low-dimensional materials has been of great interest in physics and material science. Optical permittivity is an optical fingerprint of material electronic structures, and thus it is an important parameter in the study of the properties of materials. Spectroscopic ellipsometry provides a fast, robust, and noninvasive method for obtaining the optical permittivity spectra of newly discovered materials. Atomically thin low-dimensional materials have an extremely short vertical optical path length inside them, making the spectroscopic ellipsometry of low-dimensional materials unique, compared to traditional ellipsometry. Here, we introduce the fundamentals of spectroscopic ellipsometry for two-dimensional (2D) materials and review recent progress. We also discuss technical challenges and future directions in spectroscopic ellipsometry for low-dimensional materials.