Area-Selective Chemical Doping on Solution-Processed MoS2 Thin-Film for Multi-Valued Logic Gates.

Multi-valued logic gates are demonstrated on solution-processed molybdenum disulfide (MoS2) thin films. A simple chemical doping process is added to the conventional transistor fabrication procedure to locally increase the work function of MoS2 by decreasing sulfur vacancies. The resulting device exhibits pseudo-heterojunctions comprising as-processed MoS2 and chemically treated MoS2 (c-MoS2). The energy-band misalignment of MoS2 and c-MoS2 results in a sequential activation of the MoS2 and c-MoS2 channel areas under a gate voltage sweep, which generates a stable intermediate state for ternary operation. Current levels and turn-on voltages for each state can be tuned by modulating the device geometries, including the channel thickness and length. The optimized ternary transistors are incorporated to demonstrate various ternary logic gates, including the inverter, NMIN, and NMAX gates.

Low-Intensity Light Detection with a High Detectivity Using 2D-Sb2Se3 Nanoflakes on 1D-ZnO Nanorods as Heterojunction Photodetectors.

Multifunctional photodetectors (PDs) with broadband responsivity (R) and specific detectivity (D*) at low light intensities are gaining significant attention. Thus, we report a bilayer PD creatively fabricated by layering two-dimensional (2D) Sb2Se3 nanoflakes (NFs) on one-dimensional (1D) ZnO nanorods (NRs) using simple thermal transfer and hydrothermal processes. The unique coupling of these two layers of materials in a nanostructured form, such as 2D-Sb2Se3 NFs/1D-ZnO NRs, provides an effective large surface area, robust charge transport paths, and light-trapping effects that enhance light harvesting. Furthermore, the combination of both layers can effectively facilitate photoactivity owing to proper band alignment. The as-fabricated device demonstrated superior overall performance in terms of a suitable bandwidth, good R, and high D* under low-intensity light, unlike the single-layered 1D-ZnO NRs and 2D-Sb2Se3 NF structures alone, which had poor detectivity or response in the measured spectral range. The PD demonstrated a spectral photoresponse ranging from ultraviolet (UV) to visible (220-628 nm) light at intensities as low as 0.15 mW·cm-2. The PD yielded a D* value of 3.15 × 1013 Jones (220 nm), which reached up to 5.95 × 1013 Jones in the visible light region (628 nm) at a 3 V bias. This study demonstrated that the 2D-Sb2Se3 NFs/1D-ZnO NRs PD has excellent potential for low-intensity light detection with a broad bandwidth, which is useful for signal communications and optoelectronic systems.

Ambipolar operation of progressively designed symmetric bidirectional transistors fabricated using single-channel vertical transistor and electrochemically prepared copper oxide.

In this study, a symmetric bidirectional transistors (SBT) is proposed. The device simultaneously implements the "strong-inversion" and "accumulation" mechanisms of a metal-oxide semiconductor field-effect transistor and TFT, respectively, in different bias directions in a single-channel vertical transistor (V-Tr). This ideal SBT device is designed and fabricated by selecting appropriate materials exhibiting a narrow bandgap and intrinsic characteristics of Sb-doped p-type Cu2O, using a V-Tr to optimize the device structure for high-field-induced short-channel and ambipolar operation, and implementing facile electrochemical deposition for channel and plasma channel treatments. To adopt artificial conductivity control for producing the transporting path of minority electron carriers, the patterned-channel-layer sidewall is locally treated using oxygen plasma, thereby suppressing the minority-carrier self-compensation. The SBT device exhibits an excellent on-current (i.e., symmetric accumulation and strong inversion modes in the p- and n-type channel regions, respectively) and excellent midregion off-current, similar to those of ideal ambipolar transistors. Moreover, owing to multilevel signals and excellent inverter behaviors, the SBT device is suitable for application in complementary-metal-oxide-semiconductors and logic memories.

Retraction: Ambipolar operation of progressively designed symmetric bidirectional transistors fabricated using single-channel vertical transistor and electrochemically prepared copper oxide.

Retraction of 'Ambipolar operation of progressively designed symmetric bidirectional transistors fabricated using single-channel vertical transistor and electrochemically prepared copper oxide' by Sung Hyeon Jung et al., Mater. Horiz., 2023, 10, 1373-1384, https://doi.org/10.1039/D2MH01413K.

Retraction: Progressive p-channel vertical transistors fabricated using electrodeposited copper oxide designed with grain boundary tunability.

Retraction of 'Progressive p-channel vertical transistors fabricated using electrodeposited copper oxide designed with grain boundary tunability' by Sung Hyeon Jung et al., Mater. Horiz., 2022, 9, 1010-1022, https://doi.org/10.1039/D1MH01568K.

Design of Functionally Stacked Channels of Oxide Thin-Film Transistors to Mimic Precise Ultralow-Light-Irradiated Synaptic Weight Modulation.

To utilize continuous ultralow intensity signals from oxide synaptic transistors as artificial synapses that mimic human visual perception, we propose strategic oxide channels that optimally utilize their advantageous functions by stacking two oxide semiconductors with different conductivities. The bottom amorphous indium-gallium-zinc oxide (a-IGZO) layer with a relatively low conductivity was designed for an extremely low initial postsynaptic current (PSCi) by achieving full depletion at a low negative gate voltage, and the stacked top amorphous indium-zinc oxide (a-IZO) layer improved the amplitude of the synaptic current and memory retention owing to the enhancement in the persistent photoconductivity characteristics. We demonstrated an excellent photonic synapse thin-film transistor (TFT) with a precise synaptic weight change even in the range of ultralow light intensity by adapting this stacking IGZO/IZO channel. The proposed device exhibited distinct ∆PSC values of 3.1 and 18.1 nA under ultralow ultraviolet light (350 nm, 50 ms) of 1.6 and 8.0 µW/cm2. In addition, while the lowest light input exhibited short-term plasticity characteristics similar to the "volatile-like" behavior of the human brain with a current recovery close to the initial value, the increase in light intensity caused long-term plasticity characteristics, thus achieving synaptic memory transition in the IGZO/IZO TFTs.

Progressive p-channel vertical transistors fabricated using electrodeposited copper oxide designed with grain boundary tunability.

A strategically designed electrodeposition method is proposed for the coating of p-type copper(i) oxide (Cu2O) channels for oxide thin film transistors. To date, conventional p-type oxide semiconductors have revealed a poor mobility and stability and this has obstructed the development of all oxide based logic devices. Furthermore, previous studies on p-type oxide transistors have been limited by the use of a typical planar type configuration. Our Cu2O electrodeposition method designed by incorporating Sb element promotes vertical alignment of the grain boundaries (GBs) and it perfectly coincides with the charge transport direction from the source to the drain in the vertical field effect transistors. These vertically aligned GBs are bundle type GBs and are likely to be ideal for vertical transistors with supreme electrical performances owing to the structurally suppressed grain boundary charge scattering. This alignment of the GBs in the electrodeposited Sb doped Cu2O (Sb:Cu2O) also demonstrates a superior vertical taper profile with conventional wet chemical etching owing to the extremely preferential etching rate along the GBs. Surprisingly, the sidewall formation, with a smooth and steep morphology causes the formation of abrupt and non-defective gate insulator/channel interfaces for superior spacer-free vertical transistors. Consequently, the Cu2O vertical field effect transistors exhibit extraordinary transistor performances of Vth = 0.4 V, µFE = 8 cm2 V-1 s-1, subthreshold swing = 0.24 V dec-1, on/off current ratio = 2 × 108 and qualified electrical and long-term stability characteristics under various environments. To the best of our knowledge, this is the first reported study on an electrodeposited method to design troublesome p-type oxide Cu2O as novel vertical transistors. Finally, power efficient logic inverter circuits with unprecedented performances, such as good noise margins, remarkable gain values of 15.6 (2 VDD) and 62.7 (5 VDD), and high frequency operation up to 10 kHz, are demonstrated using these p-type Cu2O transistors by interconnecting n-type IGZO transistors.

ß-like FeOOH Nanoswords Activated by Ni Foam and Encapsulated by rGO toward High Current Densities, Durability, and Efficient Oxygen Evolution.

As an alternative to the oxygen evolution reaction (OER) electrocatalyst developed by a complex bi- or multimetal ion with layered double hydroxide (LDH) structures, we design a simple, self-supported, and single-metal-ion OER electrocatalyst having lower overpotentials and high current densities in alkaline water electrolyzers. Here, ß-like FeOOH nanosword structures encapsulated by reduced graphene oxide (rGO) were cost-effectively synthesized on formable Ni foam substrates as an efficient and highly durable OER catalyst. It is revealed that the rGO uniformly covered the ß-like FeOOH nanoswords to form a porous network achieving a lower overpotential of only 210 mV at 10 mA cm-2 with a stable operation for more than 40 h in alkali media. Moreover, a high current density of ∼300 mA cm-2 was achieved at less than 1.8 V. In-depth physical and electrochemical analysis indicated that the intrinsic charge transfer through activated Ni-foam, ß-like phase, and nanosword morphology was evidently beneficial for enhancing the OER activity of the bare FeOOH, and its encapsulation by rGO further improved the conductivity and long-life durability. Our integrated OER electrocatalyst developed by a simple method (repeated soaking and quenching process) will aid in scaling up ß-like FeOOH nanoswords for preparing uniform and large-area electrodes for industrial purposes.

High-Performance Non-Volatile InGaZnO Based Flash Memory Device Embedded with a Monolayer Au Nanoparticles.

Non-volatile memory (NVM) devices based on three-terminal thin-film transistors (TFTs) have gained extensive interest in memory applications due to their high retained characteristics, good scalability, and high charge storage capacity. Herein, we report a low-temperature (<100 °C) processed top-gate TFT-type NVM device using indium gallium zinc oxide (IGZO) semiconductor with monolayer gold nanoparticles (AuNPs) as a floating gate layer to obtain reliable memory operations. The proposed NVM device exhibits a high memory window (ΔVth) of 13.7 V when it sweeps from -20 V to +20 V back and forth. Additionally, the material characteristics of the monolayer AuNPs (floating gate layer) and IGZO film (semiconductor layer) are confirmed using transmission electronic microscopy (TEM), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) techniques. The memory operations in terms of endurance and retention are obtained, revealing highly stable endurance properties of the device up to 100 P/E cycles by applying pulses (±20 V, duration of 100 ms) and reliable retention time up to 104 s. The proposed NVM device, owing to the properties of large memory window, stable endurance, and high retention time, enables an excellent approach in futuristic non-volatile memory technology.

Optimal n-Type Al-Doped ZnO Overlayers for Charge Transport Enhancement in p-Type Cu2O Photocathodes.

An effective strategy for improving the charge transport efficiency of p-type Cu2O photocathodes is the use of counter n-type semiconductors with a proper band alignment, preferably using Al-doped ZnO (AZO). Atomic layer deposition (ALD)-prepared AZO films show an increase in the built-in potential at the Cu2O/AZO interface as well as an excellent conformal coating with a thin thickness on irregular Cu2O. Considering the thin thickness of the AZO overlayers, it is expected that the composition of the Al and the layer stacking sequence in the ALD process will significantly influence the charge transport behavior and the photoelectrochemical (PEC) performance. We designed various stacking orders of AZO overlayers where the stacking layers consisted of Al2O3 (or Al) and ZnO using the atomically controlled ALD process. Al doping in ZnO results in a wide bandgap and does not degrade the absorption efficiency of Cu2O. The best PEC performance was obtained for the sample with an AZO overlayer containing conductive Al layers in the bottom and top regions. The Cu2O/AZO/TiO2/Pt photoelectrode with this overlayer exhibits an open circuit potential of 0.63 V and maintains a high cathodic photocurrent value of approximately -3.2 mA cm-2 at 0 VRHE for over 100 min.

High-Intensity Ultrasound-Assisted Low-Temperature Formulation of Lanthanum Zirconium Oxide Nanodispersion for Thin-Film Transistors.

The process complexity, limited stability, and distinct synthesis and dispersion steps restrict the usage of multicomponent metal oxide nanodispersions in solution-processed electronics. Herein, sonochemistry is employed for the in situ synthesis and formulation of a colloidal nanodispersion of high-permittivity (κ) multicomponent lanthanum zirconium oxide (LZO: La2Zr2O7). The continuous propagation of intense ultrasound waves in the aqueous medium allows the generation of oxidant species which, on reaction, form nanofragments of crystalline LZO at ∼80 °C. Simultaneously, the presence of acidic byproducts in the vicinity promotes the formulation of a stable as-prepared LZO dispersion. The LZO thin film exhibits a κ of 16, and thin-film transistors (TFTs) based on LZO/indium gallium zinc oxide operate at low input voltages (≤4 V), with the maximum mobility (µ) and on/off ratio (Ion/Ioff) of 5.45 ± 0.06 cm2 V-1 s-1 and ∼105, respectively. TFTs based on the compound dielectric LZO/Al2O3 present a marginal reduction in leakage current, along with enhancement in µ (6.16 ± 0.04 cm2 V-1 s-1) and Ion/Ioff (∼105). Additionally, a 3 × 3 array of the proposed TFTs exhibits appreciable performance, with a µ of 3-6 cm2 V-1 s-1, a threshold voltage of -0.5 to 0.8 V, a subthreshold swing of 0.3-0.6 V dec-1, and an Ion/Ioff of 1-2.5 (×106).

Smart Bifunctional Sb2 Se3 Nanorods for Integrated Water Purification: Insoluble Liquid Separation and Photoelectrochemical Degradation.

Antimony selenide (Sb2 Se3 ) nanostructures enable bifunctional water purification by a single membrane through i) physical separation of water-insoluble oil and ii) photoelectrocatalytic degradation of water-soluble organic compounds. Sb2 Se3 nanorods with exposed surfaces of {h 0 0} and {h 0 l} planes exhibit superhydrophobicity (water contact angle of ≈159°) owing to extremely low surface energy of those dangling-bond-free van der Waals planes. Based on crystallographic understanding, superhydrophobic Sb2 Se3 nanorods were produced on a mesh-type substrate for utilization as a membrane for physical water/oil separation. Sb2 Se3 exhibited an optimal photocathodic response with p-type electrical conductivity under visible light along the longitudinal crystal direction. This indicated that the nanorods could be used as photoelectrocatalytic material for chemical water purification. A smart membrane with Sb2 Se3 nanostructures was proposed as a candidate for integrated water purification that can simultaneously accomplish water/oil separation and photoelectrocatalytic degradation of organic compounds in wastewater. Linear sweep voltammetry measurements of the Sb2 Se3 -membrane showed cathodic photocurrent generation (up to approximately 10 mA cm-2 at 0 V vs. reversible hydrogen electrode), which was enough to reduce O2 to an oxygen radical (O2 .- ) for degradation of methyl orange. Consequently, solar-driven integrated water purification was demonstrated for the first time by using a single material with a dual function of superhydrophobicity and photoactivity.

Highly Improved Quasi-Two-Dimensional Oxide Transistors via Non-centrosymmetric Nitrogen Dioxide Treatment, toward Extremely Low Process Temperature and Operant Self-Aligned Coplanar Structure.

Rapid degradations are typically encountered in low-temperature processed oxide thin-film transistors (TFTs) with a high indium composition and quasi-two-dimensional (Q2D) thin channel, owing to the breaking of numerous surface bonds of the Q2D oxide and the ineffectiveness of oxidation treatment. Strategically, a novel approach is proposed for the effective use of non-centrosymmetric nitrous oxide (NO2) as a reactive oxidizer gas for realizing the highly robust and rapid field-effect mobility properties of low-temperature-processed Q2D amorphous indium zinc oxide (a-IZO) TFTs. From the surface chemical analysis, it is found that NO2 stably reconstructs surface chemical bonding with NO3- ions by capturing the charged electrons and oxygen and the regions with and without NO2 treatment display extreme differences in their electrical conductivity. Thus, a new process design can be suggested for the fabrication of self-aligned coplanar Q2D transistors, with the aim of scaling down and replacing conventional hydrogen treatment or ultraviolet irradiation. This concept is tactically designed considering the problematic aging effect and impact of the NO2 treatment. The self-aligned coplanar top-gate Q2D a-IZO TFTs exhibit outstanding device performance with a field-effect mobility of 30.1 cm2 V-1 s-1 and a relatively low positive bias stress shift of 1.3 V at an extremely low process temperature of 80 °C.

Toward Robust Photoelectrochemical Operation of Cuprous Oxide Nanowire Photocathodes Using a Strategically Designed Solution-Processed Titanium Oxide Passivation Coating.

To date, TiO2 films prepared by atomic layer deposition are widely used to prepare Cu2O nanowire (NW)-based photocathodes with photoelectrochemical (PEC) durability as this approach enables conformal coating and furnishes chemical robustness. However, this common approach requires complicated interlayers and makes the fabrication of photocathodes with reproducible performance and long-term stability difficult. Although sol-gel-based approaches have been well established for coating surfaces with oxide thin films, these techniques have rarely been studied for oxide passivation in PEC applications, because the sol-gel coating methods are strongly influenced by surface chemical bonding and have been mainly demonstrated on flat substrates. As a unique strategy based on solution processing, herein, we suggest a creative solution for two problems encountered in the conformal coating of surfaces with oxide layers: (i) how to effectively prevent corrosion of materials with hydrophilic surfaces by simply using a single TiO2 surface protection layer instead of a complex multilayer structure and (ii) guaranteeing perfect chemical durability. A Cu(OH)2 NW can be easily prepared as an intermediate phase by anodization of a Cu metal, where the former inherently possesses a hydrophilic hydroxylated surface and thus, enables thorough coating with TiO2 precursor solutions. Chemically robust nanowires are then generated as the final product via the phase transformation of Cu(OH)2 to Cu2O via sintering at 600 °C. The coated NWs exhibit excellent PEC properties and a stable performance. Consequently, the perfect chemical isolation of the Cu2O NWs from the electrolyte allows a remarkable PEC operation with the maintenance of the initial photocurrent for more than one day.

Electrochemically Assembled Cu2O Nanoparticles Using Crystallographically Anisotropic Functional Metal Ions and Highly Expeditious Resistive Switching via Nanoparticle Coarsening.

We have developed an artificially controllable strategy of an electrodeposition process adequate for resistive random-access memory (ReRAM) applications of binary Cu2O. Typically, the precise control of OH- ion concentration (the intermediate supplier of oxygen ions) at the electrode's surface decides the overall reaction rate of the Cu2O. Here, the suggested Pb and Sb metal additives preferentially contribute to the consumption of OH- ions and the supply of OH- ions, respectively, during the Cu2O electrochemical reaction so that the final products are the (200) preferential quadrangular pyramids and the (111) preferential triangular pyramids. Interestingly, the coexistence of Sb/Pb precursors in the Cu electrolytes results in extraordinarily decreased reaction rate from the opposite action of OH- ion utilization as well as intense progressive growth behavior, and the resultant Cu2O films consist of crystallized small-size nanoparticles (NPs) in an amorphous-like matrix. In the case of ReRAM applications, while the polycrystalline film induces irregular device performance and the amorphous layer shows an easily irreparable electrical breakdown, our NP-assembled Cu2O films from Pb/Sb metal ions reveal the formation of a conduction bridge via phase change to a crystalline filament with no need for forming voltage and with superior electrical stability. It is attributed to the coalescence of crystal NPs into large grains during the set/reset cycle process for the heat dissipation of Joule heating. The Cu2O sample prepared with a 3 mM Sb + 3 mM Pb mixture solution exhibits forming-free ReRAM devices with high on/off resistance ratios of 1.2 × 104 and long-term electrical/thermal stability.

Toward Adequate Operation of Amorphous Oxide Thin-Film Transistors for Low-Concentration Gas Detection.

We suggest the use of a thin-film transistor (TFT) composed of amorphous InGaZnO (a-IGZO) as a channel and a sensing layer for low-concentration NO2 gas detection. Although amorphous oxide layers have a restricted surface area when reacting with NO2 gas, such TFT sensors have incomparable advantages in the aspects of electrical stability, large-scale uniformity, and the possibility of miniaturization. The a-IGZO thin films do not possess typical reactive sites and grain boundaries, so that the variation in drain current of the TFTs strictly originates from oxidation reaction between channel surface and NO2 gas. Especially, the sensing data obtained from the variation rate of drain current makes it possible to monitor efficiently and quickly the variation of the NO2 concentration. Interestingly, we found that enhancement-mode TFT (EM-TFT) allows discrimination of the drain current variation rate at NO2 concentrations ≤10 ppm, whereas a depletion-mode TFT is adequate for discriminating NO2 concentrations ≥10 ppm. This discrepancy is attributed to the ratio of charge carriers contributing to gas capture with respect to total carriers. This capacity for the excellent detection of low-concentration NO2 gas can be realized through (i) three-terminal TFT gas sensors using amorphous oxide, (ii) measurement of the drain current variation rate for high selectivity, and (iii) an EM mode driven by tuning the electrical conductivity of channel layers.

An All Oxide-Based Imperceptible Thin-Film Transistor with Humidity Sensing Properties.

We have examined the effects of oxygen content and thickness in sputtered InSnO (ITO) electrodes, especially for the application of imperceptible amorphous-InGaZnO (a-IGZO) thin-film transistors (TFTs) in humidity sensors. The imperceptible a-IGZO TFT with 50-nm ITO electrodes deposited at Ar:O2 = 29:0.3 exhibited good electrical performances with Vth of -0.23 V, SS of 0.34 V/dec, µFE of 7.86 cm²/V∙s, on/off ratio of 8.8 × 107, and has no degradation for bending stress up to a 3.5-mm curvature. The imperceptible oxide TFT sensors showed the highest sensitivity for the low and wide gate bias of -1~2 V under a wide range of relative humidity (40-90%) at drain voltage 1 V, resulting in low power consumption by the sensors. Exposure to water vapor led to a negative shift in the threshold voltage (or current enhancement), and an increase in relative humidity induced continuous threshold voltage shift. In particular, compared to conventional resistor-type sensors, the imperceptible oxide TFT sensors exhibited extremely high sensitivity from a current amplification of >10³.