Unbiased Devices Research Articles

DUDS: Diversity-aware unbiased device selection for federated learning on Non-IID and unbalanced data

Federated Learning (FL) is a distributed machine learning approach that preserves privacy by allowing numerous devices to collaboratively train a global model without sharing raw data. However, the frequent exchange of model updates between numerous devices and the central server, and some model updates are similar and redundant, resulting in a waste of communication and computation. Selecting a subset of all devices for FL training can mitigate this issue. Nevertheless, most existing device selection methods are biased, while unbiased methods often perform unstable on Non-Independent Identically Distributed (Non-IID) and unbalanced data. To address this, we propose a stable Diversity-aware Unbiased Device Selection (DUDS) method for FL on Non-IID and unbalanced data. DUDS diversifies the participation probabilities for device sampling in each FL training round, mitigating the randomness of the individual device selection process. By using a leader-based cluster adjustment mechanism to meet unbiased selection constraints, DUDS achieves stable convergence and results close to the optimal, as if all devices participated. Extensive experiments demonstrate the effectiveness of DUDS on Non-IID and unbalanced data scenarios in FL.

Efficient BiVO4 Photoanode with an Excellent Hole Transport Layer of CuSCN for Solar Water Oxidation

AbstractBismuth vanadate (BiVO4) is reported as a key material in photoelectrocatalysis owing to high theoretical efficiency, relatively narrow band gap of 2.4 eV, and favorable conduction band edge position for hydrogen evolution. However, the sluggish hole transport dynamics lead to slow photogenerated charge separation and transport efficiencies, which result in charge recombination due to aggregation. Herein, a novel hole transport layer of copper(I) thiocyanate (CuSCN) with the aim of significantly enhancing the efficiency of charge transport and stability of BiVO4 photoanodes is reported. The introduction of the hole transport layer could provide an appropriate intermediate energy level for photogenerated hole transfer and avoid charge recombination and trapping. After a photoassisted electrodeposition process of NiCoFe‐Bi catalysis, the obtained photoanode achieves a photocurrent density of 5.6 mA cm−2 at 1.23 V versus reversible hydrogen electrode under AM 1.5 G simulated solar radiation, and an applied bias photon to current efficiency of 2.31%. With the CuSCN layer, BiVO4 photoanode presented impressive stable photocurrent during 50 h continuous illumination. Meanwhile, the unbiased tandem device of the NiCoFe‐Bi/CuSCN/BiVO4 photoanode and the Si solar cell exhibit a solar‐to‐hydrogen efficiency of 5.75% and excellent stability for 14 h.

Efficiently unbiased solar-to-ammonia conversion by photoelectrochemical Cu/C/Si-TiO2 tandems

Photoelectrochemical nitrate reduction reaction (PEC NO3RR) is of interest as a promising route to directly realizing the solar-to-ammonia (NH3) conversion but the limited efficiency and high applied bias voltage hamper its commercial prospects. Here, we report a bias-free photoelectrochemical cell for PEC NO3RR in aqueous conditions, achieving a substantial NH3 yield rate of 13.1 μmol·h−1·cm−2, high faradaic efficiency of 93.8%, and recorded solar-to-NH3 conversion of ∼1.5% under 1 sun illumination. A hierarchical-structured Si-based photocathode with Cu+/Cu2+-containing Cu nanoparticles cocatalysts achieves a highly efficient PEC NO3RR with NH3 yield rate of 115.3 μmol·h−1·cm−2 in a three-electrode system. Integrating operando characterizations and systematic PEC measurements, the formation of Lewis acid sites on Cu nanoparticles by accepting the photoinduced electrons is the dominant factor for facilitating the absorption and hydrogenation of nitrate. This work will guide the development of a robust, high-performance, and unbiased PEC device for sustainable solar-to-NH3/other fuels conversion.

Cuprous oxide photocathodes for solar water splitting

Solar water splitting is a promising technique for harvesting solar energy and converting abundant sunlight into storable hydrogen fuel. The cuprous oxide photocathode, one of the best-performing oxide photocathodes, possesses a theoretical photocurrent density of up to 14.7 mA cm−2 and a photovoltage as large as 1.6 V, making it possible to convert solar energy into hydrogen energy in a low-cost way. Herein, a comprehensive review of improving the solar water splitting performance of the cuprous oxide photocathode is presented with a focus on the crucial issues of increasing photocurrent density, photovoltage, and durability from the aspects of solving the incompatibility between the electron diffusion length and optical absorption distances, improving interfacial band alignment, revealing the impact of deficiencies, and introducing protective overlayers. We also outline the development of unassisted solar water splitting tandem devices with the cuprous oxide photocathode as a component, emphasizing the critical strategies to enhance the transmittance of the cuprous oxide photocathode, laying a solid foundation to further boost solar to hydrogen conversion efficiency. Finally, a perspective regarding the future directions for further optimizing the solar water splitting performance of the cuprous oxide photocathode and boosting solar to hydrogen conversion efficiency of the unbiased tandem device is also presented.

Sensitivity and Detection Limit of Spectroscopic‐Grade Perovskite CsPbBr3 Crystal for Hard X‐Ray Detection

AbstractSpectroscopic‐grade single crystal detectors can register the energies of individual X‐ray interactions enabling photon‐counting systems with superior resolution over traditional photoconductive X‐ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy‐discrimination X‐ray photon‐counting detection. Here, this work reports the deployment of a spectroscopic‐grade CsPbBr3 Schottky detector under reverse bias for continuum hard X‐ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of ≈1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X‐ray source is assessed in resolving the characteristic X‐ray peak. The methodology in enhancing X‐ray sensitivity by controlling the X‐ray energies and flux, and voltage, is described. The X‐ray sensitivity varies between a few tens to over 8000 μC Gyair−1 cm−2. The detectable dose rate of the CsPbBr3 detectors is as low as 0.02 nGyair s−1 in the energy discrimination configuration. Finally, the unbiased CsPbBr3 device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr3 single crystals to operate in “passive” self‐powered X‐ray detection mode and the X‐ray sensitivity is estimated as 14 μC Gyair−1 cm−2. The great potential of spectroscopic‐grade CsPbBr3 devices for X‐ray photon‐counting systems is anticipated in this work.

Digital study of interocclusal contact points in patients with partial tooth loss in case of immediate and nearest implantation prosthetics

The main purpose of this study was to study the qualitative and quantitative characteristics of occlusal ratios of dentitions in patients with partial loss of teeth replaced by preliminary direct or proximal dental implantsprostheses to prevent complications on the masticatory muscles and the temporomandibular joint. The article presents a detailed analysis of the interocclusal relations of dentitions in patients with partial tooth loss before and after prosthetics, including an assessment of the interocclusal contact points, their multiplicity, uniformity and strength of masticatory pressure, the presence of premature contacts, the trajectory of the overall direction of occlusal load, occlusal balance between the left and the right sides of dentition. The assessment of interocclusal relationships in the control and main groups was carried out using an available, simple, and unbiased digital device "T-Scan III" (Tekscan, USA).

Effects of total ionizing dose on single event effect sensitivity of FRAMs

Reliability of ferroelectric random access memories (FRAMs) is an important issue in Space application. In this work, we investigated the impact of total ionizing dose (TID) on single event effect (SEE) sensitivity of FRAMs. Five types of errors were detected. The event cross section detected in dynamic tests decreased after irradiation. Biased devices during TID irradiation had a greater decrease in event cross section than unbiased devices. TCAD simulations were performed to investigate the effects of heavy ions on memory cells of FRAMs. The collected charges at the drain of NMOS transistors induced by incident ions had a negative impact on polarized state of the ferroelectric capacitors.

Polythiophene -viologen bilayer for electro-trichromic device

An electrochromic device, capable of three-level color switching, has been envisaged by exploiting different color switching properties of Ethyl viologen (EV) and Poly (3-hexylthiophene) (P3HT) to make them switch in congruence with each other. The device, consisting of an EV/P3HT bilayer in poly-ethylene oxide matrix, appears magenta, blue or transparent under an appropriate bias. The unbiased device shows magenta colored appearance and turns transparent at a bias of 1 V when P3HT layer is used as anode. The device turns blue on increasing the bias to 1.4 V without changing the polarity. The mechanism of multilevel switching has been identified using UV–Vis and Raman spectroscopic investigations which reveal that the 1 V bias converts the P3HT into polarons and 1.4 V converts the EV dications (EV2+) into free radical. The electrochromic device demonstrates better color contrast (~ 50%) in both the optical domains. The fabricated device shows better switching speeds of ~ 1.5 s and good stability and coloration efficiency along with three-level color switching. However the reported device uses ITO coated glass as substrate, it can be used for all-organic flexible electrochromic device.

Polarized heat current generated by quantum pumping in two-dimensional topological insulators

We consider transport properties of a two dimensional topological insulator in a double quantum point contact geometry in presence of a time-dependent external field. In the proposed setup an external gate is placed above a single constriction and it couples only with electrons belonging to the top edge. This asymmetric configuration and the presence of an ac signal allow for a quantum pumping mechanism, which, in turn, can generate finite heat and charge currents in an unbiased device configuration. A microscopic model for the coupling with the external time-dependent gate potential is developed and the induced finite heat and charge currents are investigated. We demonstrate that in the non-interacting case, heat flow is associated with a single spin component, due to the helical nature of the edge states, and therefore a finite and polarized heat current is obtained in this configuration. The presence of e-e interchannel interactions strongly affects the current signal, lowering the degree of polarization of the system. Finally, we also show that separate heat and charge flows can be achieved, varying the amplitude of the external gate.

Scanning Voltage Microscopy for Emerging Electronic and Photonic Devices: Integrating nanotips with a single atom end for SVM

Scanning probe microscopy (SPM) techniques have been developed and deployed to delineate the internal characteristics of electronic and photonic devices such as doping profiles, electric potential profile, electric field profile, and charge carrier profile, as they play a crucial role in the function and performance of biased and unbiased devices. Two of the most promising techniques, scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM), are now commercially available for two-dimensional (2-D) dopant profiling by the microelectronic and optoelectronic industries, but the techniques delineate only devices at equilibrium. Therefore, to directly observe the internal behavior of operating quantum optoelectronic devices, a new SPM technique, scanning voltage microscopy (SVM), has been developed.

THz-circuits driven by photo-thermoelectric, gate-tunable graphene-junctions.

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches.

Harmonic multiplication based on Ge–Sb–Te resistive‐switching devices

Harmonic resistive multipliers that exploit the unique current–voltage (I–V) characteristics of Ag/Ge–(Sb)–Te/Pt resistive-switching devices are demonstrated. For a Ge17Sb29Te54-based device, an antisymmetric non-linear I–V curve with a hump-like structure at ±0.4 V is obtained, whereas for a Ge51Te49-based device, an asymmetric non-linear I–V curve with SET switching at +0.4 V and RESET switching at –0.1 V is observed. The Ge17Sb29Te54-based device performs third harmonic multiplication for a 160 MHz input at 0 dBm, and sixth harmonic multiplication at 5 dBm under unbiased conditions without any matching circuit. For the latter device, biasing at a voltage of ±0.4 V leads to fifth harmonic multiplication, which is absent for a 0 dBm input under unbiased conditions. No harmonic multiplication is observed for an unbiased Ge51Te49-based device due to its high resistance, but biasing at the switching voltage of 0.4 V leads to fourth harmonic multiplication for a 0 dBm input. The unique non-linear characteristics of these devices suggest their potential for radio frequency applications.

Impact of Fin Width and Back Bias Under Hot Carrier Injection on Double-Gate FinFETs

This study compares the effects of n-channel double-gate FinFETs with fin widths $(W_{\rm fin} ) $ of 10 and 25 nm with those of such devices with back biases and hot carrier injection (HCI. Compared with the device with a wide $W_{\rm fin}$ , the device with a narrow $W_{\rm fin} $ exhibits a larger current tuning range but a more off-state current leakage at a positive back bias because of the forward-biased p-n junction and the higher degradation under HCI. The gate-induced drain leakage significantly deteriorates with a positive back bias after HCI because of the generation of interfacial charges. However, a negative back bias causing hot hole injection during HCI can alleviate the degradation compared with unbiased and positive-biased devices.

Evaluation of epitaxial silicon diodes as dosimeters in X-ray mammography

In this work we report on results obtained with 2 rad-hard n-type epitaxial silicon diodes in mammography X-ray dosimetry. One sample was not irradiated before using as a dosimeter, while the other received a 60Co gamma-ray pre-dose of 200 kGy. Both unbiased devices operated in a short-circuit mode as on-line radiation dosimeters for quality assurance in medical imaging dosimetry. The irradiation was performed using 28 kV and 35 kV X-ray beams from a Pantak/Seifert generator, previously calibrated by standardized ionization chamber. The dosimetric response of these devices was investigated with respect to the repeatability, long term stability, sensitivity dependence on energy and dose-rate, charge-dose linearity and directional response. The calibration coefficients of each diode, in terms of air kerma, were also determined. These dosimetric parameters of both diodes fully meet the requirements of IEC 61674 norm, confirming their use as a reliable alternative choice for mammography photon dosimetry within the dose range of 60 μGy-10 Gy (unirradiated EPI diode); for the pre-irradiated EPI diode upper limit of dose was not reached up to now. Nevertheless, it still remains to be investigated whether or not the pre-irradiation procedure influences on the response long-term stability of EPI devices. These studies are under way.

Charge Yield at Low Electric Fields: Considerations for Bipolar Integrated Circuits

A significant reduction in total dose damage is observed when bipolar integrated circuits are irradiated at low temperature. This can be partially explained by the Onsager theory of recombination, which predicts a strong temperature dependence for charge yield under low-field conditions. Reduced damage occurs for biased as well as unbiased devices because the weak fringing field in thick bipolar oxides only affects charge yield near the Si/SiO2 interface, a relatively small fraction of the total oxide thickness. Lowering the temperature of bipolar ICs-either continuously, or for time periods when they are exposed to high radiation levels-provides an additional degree of freedom to improve total dose performance of bipolar circuits, particularly in space applications.

Photoluminescence characteristics of organic molecules in the accelerated aging organic light-emitting diodes

We have employed the photoluminescent (PL) spectrum of an accelerated aging organic light-emitting diode (OLED) to investigate the mechanisms of molecular decomposition in the whole and each organic functional layer using a bevel-cutting and peel-off method. The devices were operated at a constant current density of 37 mA/cm2 and 70 °C until the luminance reached a half of the initial luminance. The results show a slight or even negligible degradation in PL intensity compared to unbiased devices, indicating that little cracked molecular structure and destroyed host–guest transfer mechanism occurred in organic materials of the biased device Scanning electron microscopy images of devices with bias demonstrated that the delamination at the interfaces of organic layers/Al and light-emitting layer/hole transport layer occurs. We concluded that the charges accumulated at the interface of the heterojunction formed between adjacent organic layers with different energy barriers in or near the emissive layer, as well as the cathode/organic layer, should be mainly responsible for the degradation of OLEDs.

Terahertz responsivity and low-frequency noise in biased silicon field-effect transistors

We report on a comprehensive study of terahertz responsivity and low-frequency noise of current-biased antenna-coupled silicon field-effect transistors. Our measurements corroborate the findings of earlier studies that the current bias dramatically enhances the responsivity by at least an order of magnitude. However, this impressive improvement is accompanied by an equally drastic increase of the spectral density of the voltage fluctuations with the applied current. Therefore, the resulting signal-to-noise ratio (SNR) increases at most by a small amount and only for sub-threshold conditions. We do not register an absolute improvement beyond the best SNR value of the unbiased device. These findings are compared with theoretical absolute-SNR estimates derived using a hydrodynamic transport description. It predicts the maximal enhancement to be limited by a factor of about 1.35. In practical devices, the onset of excess noise suppresses even this little enhancement and limits the applicability of current biasing scheme to applications which exploit modulation frequencies above excess noise corner frequency.

Design and Characterization of a Room Temperature All-Solid-State Electronic Source Tunable From 2.48 to 2.75 THz

We report on the design, fabrication and test of an all-solid-state, frequency agile source that produces over across the 2.48-2.75 THz band at room temperature. This frequency-multiplied source is driven by a W-band synthesizer followed by a power amplifier that delivers 350-450 mW (25.5-26.5 dBm) and a cascade of three balanced frequency triplers. The first stage tripler is based on four power-combined six-anode GaAs Schottky diode devices, and the second stage tripler is based on two four-anode GaAs devices. The output tripler uses a single unbiased device featuring two anodes monolithically integrated onto a thin GaAs membrane. The source delivers a record at 2.58 THz at room temperature. This frequency multiplied source is analyzed with a Fourier transform spectrometer (FTS) and the unwanted harmonics are found to be at least 29 dB below the desired signal. This source, when used as the local oscillator for a hot-electron bolometer mixer, will enable heterodyne instruments for future space missions to map the cosmologically-important 2.675 THz HD molecular line.

An Electrically Tuned Solid‐State Thermal Memory Based on Metal–Insulator Transition of Single‐Crystalline VO2 Nanobeams

AbstractA solid‐state thermal memory that can store and retain thermal information with temperature states as input and output is demonstrated experimentally. A single‐crystal VO2 nanobeam is used, undergoing a metal–insulator transition at ∼340 K, to obtain a nonlinear and hysteresis response in temperature. It is shown that the application of a voltage bias can substantially tune the characteristics of the thermal memory, to an extent that the heat conduction can be increased ∼60%, and the output HIGH/LOW temperature difference can be amplified over two orders of magnitude compared to an unbiased device. The realization of a solid‐state thermal memory combined with an effective electrical control thus allows the development of practical thermal devices for nano‐ to macroscale thermal management.

Inverse quantum spin Hall effect generated by spin pumping from precessing magnetization into a graphene-based two-dimensional topological insulator

We present a multiterminal nanostructure for electrical probing of the quantum spin Hall effect (QSHE) in two-dimensional (2D) topological insulators (TIs). The device consists of a ferromagnetic (FM) island with precessing magnetization that pumps (in the absence of any bias voltage) pure spin current symmetrically into the left and right adjacent 2D TIs modeled as graphene nanoribbons with the intrinsic spin-orbit (SO) coupling. In the reference frame rotating with magnetization, the device is mapped onto a dc circuit with twice as many terminals whose effectively half-metallic ferromagnetic electrodes are biased by the frequency $\ensuremath{\hbar}\ensuremath{\omega}/e$ of the microwave radiation driving the magnetization precession at the ferromagnetic resonance conditions. The QSH regime of the six-terminal $\text{TI}|\text{FM}|\text{TI}$ nanodevice, attached to two longitudinal and four transverse normal metal electrodes, is characterized by the SO-coupling-induced energy gap, chiral spin-filtered edge states within finite length TI regions, and quantized spin Hall conductance when longitudinal bias voltage is applied, despite the presence of the FM island. The same unbiased device, but with precessing magnetization of the central FM island, blocks completely pumping of total spin and charge currents into the longitudinal electrodes while generating dc transverse charge Hall currents. Although these transverse charge currents are not quantized, their induction together with zero longitudinal charge current is a unique electrical response of TIs to pumped pure spin current that cannot be mimicked by SO coupled but topologically trivial systems. In the corresponding two-terminal inhomogeneous $\text{TI}|\text{FM}|\text{TI}$ nanostructures, we image spatial profiles of local spin and charge currents within TIs which illustrate transport confined to chiral spin-filtered edges states while revealing concomitantly the existence of interfacial spin and charge currents flowing around $\text{TI}\ensuremath{\mid}\text{FM}$ interfaces and penetrating into the bulk of TIs over some short distance.