The emergence of wide and ultrawide bandgap semiconductors has revolutionized the advancement of next-generation power, radio frequency, and opto- electronics, paving the way for chargers, renewable ...energy inverters, 5G base stations, satellite communications, radars, and light-emitting diodes. However, the thermal boundary resistance at semiconductor interfaces accounts for a large portion of the near-junction thermal resistance, impeding heat dissipation and becoming a bottleneck in the devices’ development. Over the past two decades, many new ultrahigh thermal conductivity materials have emerged as potential substrates, and numerous novel growth, integration, and characterization techniques have emerged to improve the TBC, holding great promise for efficient cooling. At the same time, numerous simulation methods have been developed to advance the understanding and prediction of TBC. Despite these advancements, the existing literature reports are widely dispersed, presenting varying TBC results even on the same heterostructure, and there is a large gap between experiments and simulations. Herein, we comprehensively review the various experimental and simulation works that reported TBCs of wide and ultrawide bandgap semiconductor heterostructures, aiming to build a structure–property relationship between TBCs and interfacial nanostructures and to further boost the TBCs. The advantages and disadvantages of various experimental and theoretical methods are summarized. Future directions for experimental and theoretical research are proposed.
Two-dimensional (2D) materials provide a unique platform for spintronics and valleytronics due to the ability to combine vastly different functionalities into one vertically stacked heterostructure, ...where the strengths of each of the constituent materials can compensate for the weaknesses of the others. Graphene has been demonstrated to be an exceptional material for spin transport at room temperature; however, it lacks a coupling of the spin and optical degrees of freedom. In contrast, spin/valley polarization can be efficiently generated in monolayer transition metal dichalcogenides (TMD) such as MoS2 via absorption of circularly polarized photons, but lateral spin or valley transport has not been realized at room temperature. In this Letter, we fabricate monolayer MoS2/few-layer graphene hybrid spin valves and demonstrate, for the first time, the opto-valleytronic spin injection across a TMD/graphene interface. We observe that the magnitude and direction of spin polarization is controlled by both helicity and photon energy. In addition, Hanle spin precession measurements confirm optical spin injection, spin transport, and electrical detection up to room temperature. Finally, analysis by a one-dimensional drift-diffusion model quantifies the optically injected spin current and the spin transport parameters. Our results demonstrate a 2D spintronic/valleytronic system that achieves optical spin injection and lateral spin transport at room temperature in a single device, which paves the way for multifunctional 2D spintronic devices for memory and logic applications.
Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a ...comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on–off current ratios >106, as well as carrier mobilities up to 230 cm2/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.
The Raman spectra of large-size, single-crystal, twisted bilayer graphene (tBLG) grains grown by chemical vapor deposition (CVD) are measured as a function of the rotation angle. The rotation angle ...between the graphene layers is determined using a combination of transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The 2D and G peaks follow the same trends as found previously. The low-frequency peaks (<200 cm−1) are compared to the Γ-point modes calculated from large scale molecular dynamics simulations. In this low frequency range, the ZO′ mode and new Γ-point frequencies are observed at angles far from the resonant angle of 12°. The twist-angle dependence of the new modes is not a monotonic function of twist angle or supercell size, and this non-monotonic behavior is consistent with the zone-folded modes determined from the numerical calculations.
Display omitted
Hexagonal boron nitride (h‐BN) has emerged as a strong candidate for two‐dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal ...stability, and chemical inertness. Super‐thin h‐BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti‐corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h‐BN followed by a comprehensive account of state‐of‐the‐art synthesis strategies for 2D h‐BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal‐mechanical‐chemical stability of 2D h‐BN, various potential applications of these structures are described.
h‐BN is one of the most promising inorganic materials of this century, with possible applications ranging from aerospace to medicine. It has emerged as an exotic 2D material in the post‐graphene era, owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. An encyclopedic view of the structure, properties, synthesis, and applications of h‐BN is provided.
We report a robust method for engineering the optoelectronic properties of many‐layer MoS2 using low‐energy oxygen plasma treatment. Gas phase treatment of MoS2 with oxygen radicals generated in an ...upstream N2–O2 plasma is shown to enhance the photoluminescence (PL) of many‐layer, mechanically exfoliated MoS2 flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS2 from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS2. Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many‐layer MoS2 flakes (2–15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS2, while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS2, without reducing sample thickness, and hence optical density.
An indirect‐gap‐to‐direct‐gap transition in many‐layer MoS2 is demonstrated. The transition is an outcome of the decoupling of electronic states in individual layers, driven by partial intercalation of the van der Waals gap in MoS2 through exposure to low‐power, remotely generated oxygen plasma.
Misorientation of two layers of bilayer graphene leaves distinct signatures in the electronic properties and the phonon modes. The effect on the thermal conductivity has received the least attention ...and is the least well understood. In this work, the in-plane thermal conductivity of misoriented bilayer graphene (m-BLG) is investigated as a function of temperature and interlayer misorientation angle using nonequilibrium molecular dynamics (NEMD). The central result is that the calculated thermal conductivities decrease approximately linearly with the increasing lattice constant of the commensurate m-BLG unit cell. Comparisons of the phonon dispersions show that misorientation has negligible affect on the low-energy phonon frequencies and velocities. However, the larger periodicity of m-BLG reduces the Brillouin zone size to the extent that the zone edge acoustic phonons are thermally populated. This allows Umklapp scattering to reduce the lifetimes of the phonons contributing to the thermal transport, and consequently, to reduce the thermal conductivity. This explanation is supported by direct calculation of reduced phonon lifetimes in m-BLG based on density functional theory (DFT).
Display omitted
Despite the huge expansion of GO/rGO market, there is a clear lack of experimental studies on getting high quality, large scale GO and rGO nanosheets/thin films, which is a critical requirement for ...electronic applications. In this work, a detailed experimental study on the effect of lateral sheet size on properties of GO/rGO, supported by density functional theory (DFT) calculations, is presented for the first time, to help prepare pristine graphene-like rGO. Furthermore, we investigated the effect of thermal reduction at low temperature (200 °C), under ambient pressure, on the corresponding electronic properties of rGO. Current-voltage (I–V) analysis, optical and electron microscopy, atomic force microscopy, Raman, XPS, and quantitative 13C NMR spectroscopy were used to study and optimize rGO. The optimized rGO-field-effect transistor (rGO-FET) device exhibited the highest charge carrier mobilities, i.e. 2,962 (holes) and 2,183 (electrons) cm2/V.s. Furthermore, the transconductance characteristic curve of rGO-FET showed the ambipolar behavior of high-quality graphene, with Dirac point around zero. In addition, the optical band gap of rGO nanosheets (∼0.4 eV), prepared in this work, is among the smallest reported band gaps for rGO. These findings highlight the significance of our study for synthesizing large-scale graphene-like rGO thin film, for ultra-fast, low-power transistor applications.
Display omitted
Abstract
Strong quantum confinement effects lead to striking new physics in two-dimensional materials such as graphene or transition metal dichalcogenides. While spectroscopic fingerprints of such ...quantum confinement have been demonstrated widely, the consequences for carrier dynamics are at present less clear, particularly on ultrafast timescales. This is important for tailoring, probing, and understanding spin and electron dynamics in layered and two-dimensional materials even in cases where the desired bandgap engineering has been achieved. Here we show by means of core–hole clock spectroscopy that SnS
2
exhibits spin-dependent attosecond charge delocalization times (
τ
deloc
) for carriers confined within a layer,
τ
deloc
< 400 as, whereas interlayer charge delocalization is dynamically quenched in excess of a factor of 10,
τ
deloc
> 2.7 fs. These layer decoupling dynamics are a direct consequence of strongly anisotropic screening established within attoseconds, and demonstrate that important two-dimensional characteristics are also present in bulk crystals of van der Waals-layered materials, at least on ultrafast timescales.
PDF
