Browse Articles

Reward based optimization of resonance-enhanced piezoresponse spectroscopy

Dynamic spectroscopies in scanning probe microscopy (SPM) are critical for probing material properties, such as force interactions, mechanical properties, polarization switching, electrochemical reactions, and ionic dynamics. However, the practical implementation of these measurements is constrained by the need to balance imaging time and data quality. Signal to noise requirements favor long acquisition times and high frequencies to improve signal fidelity. However, these are limited on the low end by contact resonant frequency and photodiode sensitivity and on the high end by the time needed to acquire high-resolution spectra or the propensity for sample degradation under high field excitation over long times. The interdependence of key parameters such as instrument settings, acquisition times, and sampling rates makes manual tuning labor-intensive and highly dependent on user expertise, often yielding operator-dependent results. These limitations are prominent in techniques like dual amplitude resonance tracking in piezoresponse force microscopy that utilize multiple concurrent feedback loops for topography and resonance frequency tracking. Here, a reward-driven workflow is proposed that automates the tuning process, adapting experimental conditions in real time to optimize data quality. This approach significantly reduces the complexity and time required for manual adjustments and can be extended to other SPM spectroscopic methods, enhancing overall efficiency and reproducibility.

Low-temperature buffer layer-assisted heteroepitaxial growth of γ-CuI thin films by pulsed laser deposition: Tailoring electrical properties

As the first discovered p-type transparent conductive material, copper(I) iodide (CuI) is considered the most competitive p-type candidate in the field of transparent electronics. Herein, we introduced a low-temperature buffer-layer-assisted strategy to grow γ-CuI with significantly improved structural quality and electrical transport properties by pulsed laser deposition. By adjusting the growth temperature, we can manipulate the rotation domain structure, control the hole concentration Nh from 1014 to 1019 cm−3, and achieve mobility μh = 25 cm2 V−1 s−1 being similar to that of bulk CuI. Based on the temperature-dependent Hall-effect measurement, the ionization energy of a shallow acceptor of EI,S = 137 ± 8 meV and that of a deeper acceptor of EI,D = 262 ± 23 meV were determined. This grown strategy not only enables high-quality CuI film preparation, but also to tailor their electrical properties for integration with n-type semiconductors in transparent electronic circuits.

Quantitative analysis of leakage current in III-nitride micro-light-emitting diodes

The electrical characteristics under forward- and reverse-bias conditions of III-nitride blue and green micro-light-emitting diodes (μLEDs) are analyzed. A fitting model is proposed to determine the contributions of reverse leakage current and the effectiveness of sidewall treatments. Moreover, the forward-bias currents of the μLEDs are examined using the extracted ideality factor to examine the impacts of sidewall defects. The results show that sidewall treatments are highly effective for suppression of leakage currents. From the efficiency perspective, higher wall-plug efficiency (WPE) than external quantum efficiency (EQE) is observed when the operating voltage is lower than the photon voltage in both blue and green 20 × 20 μm2 devices. This enhancement of the WPE over the EQE is due to the suppression of Shockley–Read–Hall (SRH) nonradiative recombination. These observations indicate that μLEDs with sidewall treatments not only improve optical performance but also further enhance the electrical performance of devices by suppressing the leakage current paths due to SRH nonradiative recombination processes.

Stretchable wearable thermoelectric generator from <i>in situ</i> reaction wavy tellurium films

Recent years have witnessed substantial advancements in wearable thermoelectric generators (TEGs) leveraging 2D thin-film structures. However, the rational design of TEGs that harmonizes high output performance, stretchability, and comfort remains a challenge. Herein, p-type and n-type thermoelectric units are alternately arranged on a flexible tellurium (Te) film using a solvothermal method combined with an in situ reaction. This methodology facilitates the fabrication of a wearable wavy structure TEG with an impressive stretchability of up to 300%. The effect of the geometric parameters of the wavy structure on TEG performance is explored. The designed TEG with three p–n pairs (TEG-3pn) achieves an open-circuit voltage output of 50.46 mV at a temperature difference of 60 K. Demonstrations of thermal energy harvesting from the human wrist and a hot water cup underscored the TEG's efficient exploitation of vertical temperature gradients and its versatility for diverse applications. This work provides a straightforward and practical strategy for designing thin-film-based 3D TEGs, addressing both structural considerations and the impact of geometric parameters on performance.

Three-dimensional carbon fiber networks with self-orienting nano-textures enabled by femtosecond laser processing

Here, an interconnected three-dimensional (3D) network of carbon fibers possessing nano-scaled ripples, or laser-induced periodic surface structures (LIPSS), is fabricated via laser processing with an 800-nm femtosecond laser. The unique architecture of the CF network realizes the coexistence of both ∼800-nm low-spatial frequency LIPSS (LSFL) and ∼100-nm high-spatial frequency LIPSS (HSFL) overlapping on the same fiber surface. It is suggested that LSFL formed through the interference with Fresnel diffraction patterns projected by the fiber edges, while HSFL formed through LSFL-splitting assisted by surface plasmons. Moreover, the fundamentally different formation mechanisms of the two types result in distinctively different LIPSS orientations, where the LSFL is structure-dependent and self-orients according to the fiber propagation direction, while the HSFL is structure-independent and self-orients perpendicularly to the polarization direction of the incident pulses. The findings not only introduce an optical approach to prepare nano-textured carbon materials for future energy and regenerative-medicine applications but also reveal important insights into the underlying formation mechanisms of LIPSS on complex three-dimensional surfaces.

Achieving effective photoluminescence control in two-dimensional van der Waals transition metal dichalcogenide heterostructures via laser-modified ozone intercalation

The integration of two-dimensional transition metal dichalcogenide (TMD) layers into van der Waals (vdW) heterostructures offers substantial opportunities for both materials synthesis and device design. Interlayer interactions enable desirable functionalities, and manipulating these interactions is essential for optimizing device performance. In this work, we introduce ozone intercalation into vdW heterostructures and adopt laser irradiation as a manipulation tool, creating a photoluminescence (PL)-based modulation type using interface engineering. Interlayer engineering can be quantitatively achieved by precisely controlling the laser modification time, enabling a controllable PL intensity from no quenching to quenching. The mechanism behind this regulation can be attributed to the interlayer exciton suppression introduced by ozone intercalation being repaired by ozone molecule reduction during the laser treatment. This effective regulatory technique is universal and can be achieved in various type II band-aligned TMD heterostructures, providing an intriguing strategy for the design of two-dimensional vdW TMD device systems.

Determining the mechanical properties of AlN films using micromechanical membranes

The mechanical properties of thin films can vary significantly due to variations in the deposition process. Monitoring these properties is essential when, e.g., engineering micro-mechanical devices. Established methods such as wafer bending and bulge testing provide only average values for the entire wafer or impose constraints on the sample fabrication process. Therefore, a method to efficiently characterize mechanical properties locally with simple sample fabrication is desirable. In this study, we determine the stress in direct-current magnetron-sputtered aluminum nitride films on prefabricated silicon nitride membranes by measuring the fundamental eigenfrequencies of the membranes and combining it with mechanical band structure simulations. We find that the stress, which is directly related to the strain, transitions from compressive to tensile as a function of the film thickness. Our simple and robust method allows in situ measurements that are compatible with existing nanofabrication techniques of, e.g., hybrid photonic circuits and can probe the stress locally over the chip.

The interfacial trapped charge of quintuple-layers Al2O3 induced by point defect

For two-dimensional (2D) polar semiconductors with out-of-plane polarization, as the number of stacked layers increases, the electronic properties could change from single-layer semiconductor to multi-layer metal. The metallic characteristic manifests as the formation of 2D conductive electron and hole gas on the surfaces. In this study, we investigate the mechanism behind the formation of a two-dimensional electron gas (hole gas) on the surface and interlayer directional charge transfer process of polar quintuple-layers (QLs)-Al2O3 through first-principles calculations. The accumulated polarized electric field acts as the thermodynamic driving force for directional charge transfer between QL-Al2O3 layers, involving all Al and O atoms in the charge transfer process. However, the point defects in 2QLs-Al2O3, including different effective charge, significantly influence the dynamic process of directional interlayer charge transfer. The presence of unsaturated oxygen atoms at the interface contributes to the formation of interface trapped charges in 2QLs-Al2O3.

Transferable machine learning approach for predicting electronic structures of charged defects

The study of electronic properties of charged defects plays a crucial role in advancing our understanding of how defects influence conductivity, magnetism, and optical behavior in various materials. However, despite its significance, research on large-scale defective systems has been hindered by the high computational cost associated with density functional theory (DFT). In this study, we propose HamGNN-Q, an E(3) equivariant graph neural network framework capable of accurately predicting DFT Hamiltonian matrices for diverse point defects with varying charges, utilizing a unified set of network weights. By incorporating background charge features into the element representation, HamGNN-Q facilitates a direct mapping from structural characteristics and background charges to the electronic Hamiltonian matrix of charged defect systems, obviating the need for DFT calculations. We showcase the model's high precision and transferability by evaluating its performance on GaAs systems encompassing diverse charged defect configurations. Furthermore, we predicted the wave function distribution of polarons induced by defects. We analyzed the node features through principal component analysis, providing physical insights for the interpretability of the HamGNN-Q model. Our approach provides a practical solution for accelerating electronic structure calculations of neutral and charged defects and advancing the design of materials with tailored electronic properties.

Implementation of scalable suspended superinductors

Superinductors have become a crucial component in the superconducting circuit toolbox, playing a key role in the development of more robust qubits. Enhancing the performance of these devices can be achieved by suspending the superinductors from the substrate, thereby reducing stray capacitance. Here, we present a fabrication framework for constructing superconducting circuits with suspended superinductors in planar architectures. To validate the effectiveness of this process, we systematically characterize both resonators and qubits with suspended arrays of Josephson junctions, ultimately confirming the high quality of the superinductive elements. In addition, this process is broadly compatible with other types of superinductors and circuit designs. Our results not only pave the way for scalable superconducting architectures utilizing superinductors but also provide the primitive for future investigation of loss mechanisms associated with the device substrate.

Transmission of photonic entangled states encoded via eigenstates of photon-number parity operator

The transfer of quantum entangled states is of fundamental interest in quantum physics and plays an important role in quantum information processing, quantum communication, and quantum technology. Here, we propose a scheme to transfer quantum entangled states of two photonic qubits by utilizing four microwave cavities coupled to a superconducting qutrit (a three-level quantum system). The photonic qubits are encoded using two orthogonal eigenstates of the photon-number parity operator with eigenvalues ± 1, which allows for various encodings for the photonic qubits. The employment of four cavities at distinct frequencies effectively reduces the inter-cavity crosstalk. The utilization of only a single superconducting qutrit as the coupler significantly reduces the circuit resources. The entanglement transfer can be completed in just one step, making this scheme remarkably efficient. During the state transfer process, the third energy level of the coupler qutrit remains unoccupied, and thus decoherence from this level is diminished. Our numerical simulations demonstrate that within current circuit quantum electrodynamics technology, one can achieve high-fidelity transfer of the entangled states of two photonic qubits encoded via squeezed vacuum states and cat states. Our scheme possesses generality and can be applied to accomplish the same task in a variety of physical systems.

Type-I and type-II interfaces in a MoSe2/WS2 van der Waals heterostructure

We report experimental evidence that MoSe2 and WS2 allow the formation of type-I and type-II interfaces, according to the thickness of the former. Heterostructure samples are obtained by stacking a monolayer WS2 flake on top of a MoSe2 flake that contains regions of thickness from one to four layers. Photoluminescence spectroscopy and transient absorption measurements reveal a type-II interface in the regions of monolayer MoSe2 in contact with monolayer WS2. In other regions of the heterostructure formed by multilayer MoSe2 and monolayer WS2, features of type-I interface are observed, including the absence of charge transfer and dominance of intralayer excitons in MoSe2. The coexistence of type-I and type-II interfaces in a single heterostructure offers opportunities to design sophisticated two-dimensional materials with finely controlled photocarrier behaviors.

Visible-spectrum (405–505 nm) low-temperature-deposited deuterated (D) SiNx-SiOy waveguides

Deuterated silicon nitride (SiNx:D)–silicon oxide (SiOy:D) waveguides grown by low-temperature (300 °C) plasma-enhanced chemical vapor deposition (PECVD) operating in the violet (405 nm) to cyan (505 nm) visible spectrum are demonstrated. The waveguides exhibit low insertion losses ranging from 3.2 dB/cm (405 nm) to 0.8 dB/cm (505 nm). The performance of these waveguides is competitive to conventional SiNx waveguides that require significantly higher processing temperatures (≥800 °C). The low-temperature deposition and low loss of these waveguides enable advanced heterogeneous integration schemes for visible-spectrum photonic integrated circuits.

Optimized perovskite photodetector achieved through optical manipulation via biomimetic nanostructure

Photodetectors with high integration and detectivity are in great demand for the development of wearable and flexible electronic devices. However, the limited photoelectric conversion efficiency at low film thickness has significantly hindered further applications for both traditional semiconductors and newly emerged perovskite materials. In this study, we have incorporated a biomimetic nanostructure inspired by the lotus leaf onto the perovskite active layer using a modified two-step room temperature low-pressure nanoimprinting technique. This approach has greatly improved the quality of the perovskite material during the spatially limited crystallization process. By incorporating a biomimetic optical nanostructure, we have achieved a synergistic effect between enhanced scattering and local surface plasmon resonances, leading to a significant improvement in incident light utilization efficiency. With the incorporation of this biomimetic nanostructure, we observed a remarkable enhancement of 1100% in photogenerated current and 39.6% in response speed. This strategy provides a viable approach for designing high-efficiency ultra-thin photodetectors.

Wavelength-insensitive and fast photothermoelectric response in magnetic-Weyl-semimetal Co3Sn2S2 crystals

We systematically characterize the photothermoelectric (PTE) effect of pristine and Se-doped (4%, 8%, and 11%) Co3Sn2S2 crystals, a magnetic-Weyl-semimetal, on electromagnetic-wave wavelength and temperature. The photovoltages of these Co3Sn2S2 crystals are insensitive to the electromagnetic-wave wavelength, with 23% variance, in bands ranging from 444 to 1550 nm. Remarkably, the 4%-Se doped Co3Sn2S2 demonstrates the best PTE performance among these samples, yielding a 22.4 mV/W optical responsivity and a short response time (∼100 ms) under 1550 nm illumination. The photovoltage of Co3Sn2S2 nearly monotonically increased with increasing temperature. The theoretical analyses verify that the wavelength-insensitive PTE response of Co3Sn2S2 comes from the nearly same trend of its wavelength-dependent optical refractive index and extinction coefficient, and the best PTE performance of 4%-Se doped Co3Sn2S2 is attributed to the largest product of Seebeck coefficient and electrical conductivity. This work demonstrates that magnetic Weyl semimetals may be promising for infrared photodetection.

Erratum: “Production of 13.5 nm light with 5% conversion efficiency from 2 μm laser-driven tin microdroplet plasma” [Appl. Phys. Lett. <b>23</b> , 234101 (2023)]

Enhanced breakdown strength and reduced polarization hysteresis in relaxor ferroelectric polymers with increased gamma phase content for energy storage capacitors

Polymer dielectric energy storage capacitors play a vital role in modern electronic and electrical power systems, particularly in high-voltage environments. However, achieving both high energy density and charge–discharge efficiency presents a significant challenge for next-generation applications that demand miniaturization and compact design. In this study, we present relaxor ferroelectric terpolymers with an increased gamma (γ)-phase content, prepared through a facile and scalable interfacial engineering approach that incorporates ultra-low amounts of graphene oxide. The γ-phase crystals in the terpolymer reduce hysteresis losses and generate numerous deep traps, resulting in enhanced performance. These terpolymers achieve a high energy density of up to 15.2 J/cm3 and an improved breakdown strength of 562 MV/m, representing enhancements of 62% and 39.8%, respectively, compared to the pristine terpolymer. The results suggest that tuning the phase structure of relaxor ferroelectric terpolymers offers a pathway to developing ferroelectric polymers with enhanced energy density and charge–discharge efficiency for energy storage capacitors.

Nonlinear field dependence of Hall effect and high-mobility multi-carrier transport in an altermagnet CrSb

As a promising candidate for altermagnet, CrSb possesses a distinctive compensated spin split band structure that could lead to groundbreaking concepts in the field of spintronics. In this work, we have grown high-quality CrSb single crystals and comprehensively investigated their electronic and magneto-transport properties. We have observed large, positive, and non-saturated magnetoresistance (MR) in CrSb, which obeys Kohler's rule, indicating its classic Lorentz scattering origins. The largest MR can reach 27% at 10 K and 9 T. Remarkably, a nonlinear magnetic field dependence of Hall effect resembling the spontaneous anomalous Hall is identified over a wide temperature range. It was found that the nonlinearity mainly stems from the incorporation of different carriers in the magnetoconductivity. According to the Fermi surface analyses of CrSb, we applied the three-carrier model to fit the conductivity data, yielding good agreement. The extracted carrier concentration ranges from 1018 to 1021 cm−3, with a high mobility up to ∼3 × 103 cm2/V s below 50 K. These indicate that CrSb behaves more like a semimetal. Furthermore, calculations using the semiclassical Boltzmann transport theory have reproduced the main features of the experimental MR and Hall effect in CrSb. These exceptional transport properties make CrSb unique for applications in spintronics as an altermagnet.

Magnetic field tunable spectral response of kinetic inductance detectors

We tune the onset of optical response in aluminum kinetic inductance detectors from a natural cutoff frequency of 90 GHz to 60 GHz by applying an external magnetic field. The change in the spectral response is due to the decrease in the superconducting gap, from 90 GHz at zero magnetic field to 60 GHz at a magnetic field of around 3 mT. We characterize the variation of the superconducting gap, the detector frequency shift, and the internal quality factor as a function of the applied field. In principle, the magnetic field tunable response could be used to make spectroscopic measurements. In practice, the internal quality factor behaves hysterically with the magnetic field due to the presence of vortices in the thin superconducting film. We conclude by discussing possible solutions to achieve spectroscopy measurements using kinetic inductance detectors and magnetic field.

Unipolar neuromorphic memtransistors with reconfigurable inhibition–excitation switching based on antagonistic dual mechanisms

Implementing reconfigurable excitatory–inhibitory bidirectional weight updates in a unipolar transistor is highly desirable for developing compact neuromorphic hardware but remains a significant challenge. In this study, we employ a dual-mechanism approach by designing an organic electrolyte gate-selected memtransistor (OESmT) to reliably and rapidly update synaptic weights in both directions. The switching between synaptic inhibition and excitation depends on the connection state of the gate (modulation terminal), which can be either grounded or floating. This switching mechanism is driven by the dynamics between the electric-double-layer (EDL) and charge-trapping effects, resulting from the antagonistic effect of the two mechanisms. Long-term synaptic plasticity in both directions is demonstrated. The reconfigurable characteristic of the OESmT remains stable for 5000 pulses applied. Besides, the device has been applied to the autonomously guided vehicle. Our research highlights the importance of coupling of EDL and charge-trapping effects in reconfigurable neuromorphic systems.