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Dual-frequency ultrasound has advantages over conventional ultrasound, which operates at a specific frequency band, due to its additional frequency response. Moreover, a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution. This facilitates the availability of more advanced techniques that require low- and high frequency ultrasound simultaneously such as harmonic imaging and image-guided therapy. In this article, we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer (PMUT). Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging. To switch the ultrasound frequency, the only requirement is to tune a DC bias for controlling the polarization state of the ferroelectric film. Flextensional vibration of the PMUT’s membrane strongly depends on the polarization state, producing low- and high-frequency ultrasound from a single excitation frequency. This strategy for dual-frequency ultrasound dispenses with the requirement for either multi-electrode configurations or hetero-designed elements, which are integrated into an array. Consequently, this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.

In electronic devices, the work function (WF) of the electrodes must be tailored to achieve a well-aligned Ohmic or Schottky contact.

Functional neuronal computing systems that support information diversification require high-density memory with selector devices to reduce leakage current in cross-point architectures, which drives us to develop a functional switching layer that operates as three distinct devices, namely non-volatile memory, selector, and synaptic devices, using a GeTe-based single material system. In this study, amorphous Ag-GeTe switching layers are engineered by doping with Te species to achieve either resistive switching (RS) or threshold switching properties. The Ag/Ag-GeTe/Ag memory device exhibits multilevel characteristics via a tunable compliance current approach. By comparison, Ag/Ag-GeTex/Ag selector device provides excellent selectivity (>106) with a very low OFF-current (∼10−11 A). The RS mechanism for memory and selector devices is interrogated by using conductive atomic force microscopy. Moreover, the Ag/Ag-GeTe/Ag RS device mimics a cohort of basic and complex synaptic plasticity properties, including potentiation-depression and four-spike time-dependent plasticity rules that include asymmetric Hebbian, asymmetric anti-Hebbian, symmetric Hebbian, and symmetric anti-Hebbian learning rules. The capability of the synaptic devices to detect image edges is demonstrated by using a convolution neural network. The present work showcases the multi-functionality of Ag-GeTe materials, which will likely emerge as a prominent candidate for high-density cross-point architecture-based neuromorphic computing systems.

Highlights Dopant-tunable transparent conductive oxide (≤ 50 nm) fabricated via electric-field-driven metal implantation (m-TCOs; m= Ni, Ag, and Cu) is demonstrated.The m-TCOs exhibit ultrahigh transparency, low sheet resistance, and broad work function tunability, leading to outstanding performance in various optoelectronic devices.The work function change is attributed to the interstitial metal atoms that provide the empty d-orbital, resulting in the shift of the Fermi level. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00735-y., Ultrathin film-based transparent conductive oxides (TCOs) with a broad work function (WF) tunability are highly demanded for efficient energy conversion devices. However, reducing the film thickness below 50 nm is limited due to rapidly increasing resistance; furthermore, introducing dopants into TCOs such as indium tin oxide (ITO) to reduce the resistance decreases the transparency due to a trade-off between the two quantities. Herein, we demonstrate dopant-tunable ultrathin (≤ 50 nm) TCOs fabricated via electric field-driven metal implantation (m-TCOs; m = Ni, Ag, and Cu) without compromising their innate electrical and optical properties. The m-TCOs exhibit a broad WF variation (0.97 eV), high transmittance in the UV to visible range (89–93% at 365 nm), and low sheet resistance (30–60 Ω cm−2). Experimental and theoretical analyses show that interstitial metal atoms mainly affect the change in the WF without substantial losses in optical transparency. The m-ITOs are employed as anode or cathode electrodes for organic light-emitting diodes (LEDs), inorganic UV LEDs, and organic photovoltaics for their universal use, leading to outstanding performances, even without hole injection layer for OLED through the WF-tailored Ni-ITO. These results verify the proposed m-TCOs enable effective carrier transport and light extraction beyond the limits of traditional TCOs. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00735-y.

High efficiency thermoelectric (TE) materials still require high thermopower for energy harvesting applications. A simple elemental metallic semiconductor, tellurium (Te), has been considered critical to realize highly efficient TE conversion due to having a large effective band valley degeneracy. This paper demonstrates a novel approach to directly probe the out-of-plane Seebeck coefficient for one-dimensional Te quantum wires (QWs) formed locally in the aluminum oxide layer by well-controlled electrical breakdown at 300 K. Surprisingly, the out-of-plane Seebeck coefficient for these Te QWs ≈ 0.8 mV/K at 300 K. This thermopower enhancement for Te QWs is due to Te intrinsic nested band structure and enhanced energy filtering at Te/AO interfaces. Theoretical calculations support the enhanced high Seebeck coefficient for elemental Te QWs in the oxide layer. The local-probed observation and detecting methodology used here offers a novel route to designing enhanced thermoelectric materials and devices in the future.

Multilevel resistive switching (RS) is a key property to embrace the full potential of memristive devices for non-volatile memory and neuromorphic computing applications. In this study, we employed nanoparticulated cobaltite oxide (Co3O4) as a model material to demonstrate the multilevel RS and synaptic learning capabilities because of its multiple and stable redox state properties. The Pt/Co3O4/Pt memristive device exhibited tunable RS properties with respect to different voltages and compliance currents (CC) without the electroforming process. That is, the device showed voltage-dependent RS at a higher CC whereas CC-dependent RS was observed at lower CC. The device showed four different resistance states during endurance and retention measurements and non-volatile memory results indicated that the CC-based measurement had less variation. Besides, we investigated the basic and complex synaptic plasticity properties using the analog current-voltage characteristics of the Pt/Co3O4/Pt device. In particular, we mimicked the potentiation–depression and four-spike time-dependent plasticity (STDP) rules such as asymmetric Hebbian, asymmetric anti-Hebbian, symmetric Hebbian, and symmetric anti-Hebbian learning rules. The results of the present work indicate that the cobaltite oxide is an excellent nanomaterial for both multilevel RS and neuromorphic computing applications.

In the present investigation, compact TiO2 thin film was prepared by using an artist spray gun (ASG)-assisted pyrolysis method to use as an electron transport layer (ETL) for perovskite solar cell (PSC). The influence of the distance between the spray gun nozzle and the substrate was studied to get quality TiO2 thin film onto FTO substrate among three different distantly deposited TiO2 thin films. The formation of anatase TiO2 thin film was confirmed by XRD and Raman spectroscopy studies. The bandgap of this thin film was measured by UV–Vis analysis and it was found to be 3.19 eV. The emission spectrum and charge separation of TiO2 thin film were analysed by photoluminescence studies. The emission spectrum of TiO2 thin film exhibited at 380 nm confirmed the indirect transition from the conduction band to the valence band. The FE-SEM image confirmed that the prepared TiO2 thin film was uniformly deposited over the substrate without any pinhole, which shows a good agreement with the Raman mapping analysis. A power conversion efficiency (PCE) of 2.69% has been achieved with Pb-free PSC fabricated from uniformly deposited TiO2 ETL.

This paper presents an organic memristive device based on the soft and quasi-liquid trypsin biomaterial. Accordingly, trypsin is isolated from the bovine pancreas and deposited on a conducting fluorine-doped tin oxide (FTO) substrate. The current–voltage (I–V) measurements of the trypsin/FTO device show a hysteresis loop at multiple frequencies and voltages. The results demonstrate that the current and resistance of the device can be modulated by altering the frequency and magnitude of the applied signal, suggesting the feasibility of this material for brain-inspired computing devices. The hysteresis area of the device increases in proportion to the frequency of the signal, signifying the inverse-memristive phenomenon. The time-domain flux, time-domain charge, and charge-flux properties are calculated from the experimental I–V data to demonstrate the memristive nature of the trypsin hydrogel. Interestingly, the trypsin hydrogel memristive device mimics bio-synaptic properties such as potentiation–depression and four complex spike-time-dependent plasticity learning rules. Electrochemical studies are performed to understand the electrochemical kinetics of the device. A possible switching mechanism is illustrated to show the memristive effect of the trypsin hydrogel. The results of the present investigation suggest that trypsin can be a possible biomaterial to develop an electronic synaptic device for neuromorphic computing applications.

Parameters such as electrode work function (WF), optical reflectance, electrode morphology, and interface roughness play a crucial role in optoelectronic device design; therefore, fine-tuning these parameters is essential for efficient end-user applications. In this study, amorphous carbon-silver (C-Ag) nanocomposite hybrid electrodes are proposed and fully characterized for solar photovoltaic applications. Basically, the WF, sheet resistance, and optical reflectance of the C-Ag nanocomposite electrode are fine-tuned by varying the composition in a wide range. Experimental results suggest that irrespective of the variation in the graphite-silver composition, smaller and consistent grain size distributions offer uniform WF across the electrode surface. In addition, the strong C-Ag interaction in the nanocomposite enhances the nanomechanical properties of the hybrid electrode, such as hardness, reduced modulus, and elastic recovery parameters. Furthermore, the C-Ag nanocomposite hybrid electrode exhibits relatively lower surface roughness than the commercially available carbon paste electrode. These results suggest that the C-Ag nanocomposite electrode can be used for highly efficient photovoltaics in place of the conventional carbon-based electrodes.

MXene, a new state-of-the-art two-dimensional (2D) nanomaterial, has attracted considerable interest from both industry and academia because of its excellent electrical, mechanical, and chemical properties. However, MXene-based device engineering has rarely been reported. In this study, we explored Ti3C2 MXene for digital and analog computing applications by engineering the top electrode. For this purpose, Ti3C2 MXene was synthesized by a simple chemical process, and its structural, compositional, and morphological properties were studied using various analytical tools. Finally, we explored its potential application in bipolar resistive switching (RS) and synaptic learning devices. In particular, the effect of the top electrode (Ag, Pt, and Al) on the RS properties of the Ti3C2 MXene-based memory devices was thoroughly investigated. Compared with the Ag and Pt top electrode-based devices, the Al/Ti3C2/Pt device exhibited better RS and operated more reliably, as determined by the evaluation of the charge-magnetic property and memory endurance and retention. Thus, we selected the Al/Ti3C2/Pt memristive device to mimic the potentiation and depression synaptic properties and spike-timing-dependent plasticity-based Hebbian learning rules. Furthermore, the electron transport in this device was found to occur by a filamentary RS mechanism (based on oxidized Ti3C2 MXene), as determined by analyzing the electrical fitting curves. The results suggest that the 2D Ti3C2 MXene is an excellent nanomaterial for non-volatile memory and synaptic learning applications.

The fine control of doping levels in graphene materials such as reduced graphene oxide (RGO) is important to properly manipulate their ambipolar transport characteristics for various device applications. However, conventional doping methods involve complex chemical reactions, large-scale doping processes, and poor stability. Herein, a simple and controllable electrochemical doping treatment (EDT), performed via the conductive channels created at the RGO surface by the application of an electric field, is introduced to tailor the electrical properties of RGO films. X-ray photoelectron spectroscopy and Raman spectroscopy measurements are performed to detect the presence of Ni atoms in RGO films after the EDT (EDT-RGO). Then, EDT-RGO field-effect transistors (FETs) are fabricated with different doping areas (0 to 100% fractional area) on the RGO active channel to investigate the effect and selective-area doping capability of the EDT. Owing to p-type doping compensation by the intercalated Ni atoms, the electron mobility of the EDT-RGO FET decreases from 1.40 to 0.12 cm2 V−1 s−1 compared with that of the undoped RGO-FET, leading to the conversion from ambipolar to unipolar p-type transfer characteristics.

We successfully synthesized a crystalline main chain conjugated copolymer, P(BDBT-co-NDI2T), having BDBT donor-NDI2T acceptor heterojunctions in the film state. Interestingly, the polymer exhibited a strong absorption band from 500 to 650 nm, which overlapped well with the emission spectrum of white light-emitting diodes (LEDs) used as an indoor light source. The P(BDBT-co-NDI2T) films had a relatively smooth surface and internal morphology compared to the blend films of P(BDBT) and P(NDI2T). In particular, the Kelvin probe force microscopy data explained the increased uniformity in the distribution of the surface charge in the annealed P(BDBT-co-NDI2T) films as compared to those of the annealed blend films of P(BDBT) and P(NDI2T). Indoor photovoltaics (IPVs) produced by using the P(BDBT-co-NDI2T) synthesized in this study as an active layer showed a high power conversion efficiency of 12.70% (@500 lux) with a short circuit current density of 49.89 μA cm−2. The PCE of the P(BDBT-co-NDI2T) film-based IPVs was twice as high as that of the blend film-based IPVs and showed excellent operational stability. This study is the first time that the P(BDBT-co-NDI2T) has been applied in IPVs, which suggests the potential for their widespread use in the development of indoor organic PVs in the future.

Although two-dimensional (2D) nanomaterials are promising candidates for use in memory and synaptic devices owing to their unique physical, chemical, and electrical properties, the process compatibility, synthetic reliability, and cost-effectiveness of 2D materials must be enhanced. In this context, amorphous boron nitride (a-BN) has emerged as a potential material for future 2D nanoelectronics. Therefore, we explored the use of a-BN for multilevel resistive switching (MRS) and synaptic learning applications by fabricating a complementary metal-oxide-semiconductor (CMOS)-compatible Ag/a-BN/Pt memory device. The redox-active Ag and boron vacancies enhance the mixed electrochemical metallization and valence change conduction mechanism. The synthesized a-BN switching layer was characterized using several analyses. The fabricated memory devices exhibited bipolar resistive switching with low set and reset voltages (+0.8 and -2 V, respectively) and a small operating voltage distribution. In addition, the switching voltages of the device were modeled using a time-series analysis, for which the Holt's exponential smoothing technique provided good modeling and prediction results. According to the analytical calculations, the fabricated Ag/a-BN/Pt device was found to be memristive, and its MRS ability was investigated by varying the compliance current. The multilevel states demonstrated a uniform resistance distribution with a high endurance of up to 10