Holes and electrons are indispensable charge carriers in various electronic and optoelectronic devices. However, holes are quasiparticles described as the absence of electrons, and their signals are often buried under strong electron-derived signals, making observation challenging. In this study, we propose a highly sensitive imaging method based on photoemission electron microscopy with a photon-energy-tunable laser source to visualize hole accumulation at the organic semiconductor–insulator interface in an organic field-effect transistor. By tuning the photon energy of the excitation source to the energy level corresponding to the top of the highest occupied molecular orbital, hole accumulation in an organic semiconductor could be detected as a decrease in photoemission intensity under an applied gate voltage. The proposed method has the potential to unveil the carrier transport mechanism in a wide range of semiconductor-based devices with real-time imaging of hole transport and accumulation. It thus paves the way for the development of state-of-the-art electronic devices.
Interlayer coupling offers a symmetry-sensitive pathway for engineering the electronic structure and optical response of atomically thin two-dimensional semiconductors. Here, we combine temperature-dependent spectroscopic ellipsometry with first-principles density functional theory (DFT) calculations to investigate large-area monolayer and bilayer 3R-phase MoS2 grown on Al2O3 substrates. Unlike monolayer-MoS2, bilayer 3R-MoS2 exhibits a fourfold valence-band fine structure at the K/K′ points. DFT calculations show that this splitting originates from interlayer-coupling-induced band separation that is further resolved by spin–orbit coupling under broken inversion symmetry. By comparison, bilayer-2H MoS2 preserves spin degeneracy because of inversion symmetry, highlighting the decisive role of stacking symmetry in controlling the valley-resolved band structure. Spectral-weight analysis further indicates an enhanced optical response in bilayer-3R MoS2, consistent with strengthened band nesting in the Γ–M region. These findings establish interlayer coupling and stacking symmetry as key parameters for tuning excitonic transitions, valley electronic structure, and light–matter interactions in two-dimensional transition metal dichalcogenides.
Since the advent of ultrasonic testing, piezoelectric ultrasonic transducers have been widely applied in nonintrusive inspection across aerospace, nuclear energy, and petrochemical industries, owing to their compact structure, low cost, and broad applicability. However, as service temperatures increase, issues such as the degradation of piezoelectric material properties and mechanical failure of transducer components become increasingly pronounced, highlighting the need for further optimization to enhance transducer performance under high-temperature conditions. This paper reviews the recent research progress on high-temperature piezoelectric ultrasonic transducers (HTPUTs) from the perspectives of materials, structures, and applications. Specifically, commonly used piezoelectric material systems for high-temperature environments and their performance characteristics are systematically summarized, with particular emphasis on degradation mechanisms at elevated temperatures. Design optimization strategies for transducers operating under high-temperature conditions are discussed in detail, including the selection of matching layers and backing materials, acoustic coupling optimization, and the development of novel structural configurations. Representative applications of HTPUTs in the energy and manufacturing sectors are also highlighted, together with the key challenges encountered. Finally, future development directions for HTPUT research are outlined.
Amorphous hydrogenated silicon (a-Si:H) based lumped elements are one of the promising ways for large-scale integration of superconducting quantum devices. We present a detailed study devoted to the plasma-enhanced chemical vapor deposition process parameters' influence on the structure and morphology of a-Si:H dielectric thin films. We experimentally demonstrate a microcrystalline-to-amorphous growth transition by changing the H2/SiH4 ratio and establish a relationship between the film's structure, thickness, and dielectric loss tangent (tan δ). It is shown that the best dielectric loss tangents of tan δLP = 4.5 × 10−5 (average 9.2 × 10−5) and tan δHP = 1.0 × 10−6 (average 1.2 × 10−6) for superconducting lumped elements with 350 nm-thick amorphous films can be achieved in single- and multi-photon regimes at sub-20 mK, correspondingly. These values are extracted by characterizing superconducting resonators with integrated parallel-plate capacitors (C = 0.2…6.0 pF) for a frequency range from 4.0 to 7.0 GHz. The presented low-loss a-Si:H films can be useful for various superconducting quantum applications, which is illustrated by the fabrication of lumped-element bandpass filters with a well-predicted amplitude–frequency response characteristic.
Miniaturized spectrometers that can be seamlessly integrated into on-chip, portable, and wearable systems are essential for next-generation sensing and information-processing technologies. Computational spectrometers, which reconstruct incident spectra by combining device-dependent spectral responses with reconstruction algorithms, offer a unique miniaturization route beyond conventional dispersive or interferometric architectures. Their ultimate performance is jointly governed by spectral encoding capability and reconstruction robustness. In the detector-encoding route, the choice of photodetector materials and tuning strategies directly determines the dimensionality, diversity, and stability of spectral encoding, thereby setting the upper limit of reconstruction accuracy and the attainable degree of system miniaturization. Two-dimensional (2D) van der Waals (vdW) materials have recently emerged as exceptional candidates for realizing high-performance ultraminiaturized computational spectrometers, owing to their atomic thickness, strong light-matter interactions, continuously tunable band structures, and vdW heterostructures compatibility. These properties make 2D photodetectors particularly attractive for approaching the ultimate limit of device-level miniaturization while maintaining rich spectral encoding capabilities. This review provides a comprehensive perspective on 2D photodetector-based computational spectrometers. We first analyze the theoretical foundations and operational schemes of linear and nonlinear computational spectrometers, highlighting the distinct roles of device physics in spectral encoding. We then discuss spectral response modulation strategies and the evaluation of spectral encoding effectiveness, followed by reconstruction algorithms with particular emphasis on compressive sensing and deep learning. Key performance metrics are summarized and compared across representative demonstrations. Emerging proof-of-concept applications are subsequently reviewed. Finally, we outline future challenges and opportunities that may advance this field from current lab-scale demonstrations toward practical edge-device deployment.
