Design of Bronze-Rich Dual-Phasic TiO2 Embedded Amorphous Carbon Nanocomposites Derived from Ti-Metal-Organic Frameworks for Improved Lithium-Ion Storage.

Dual-phasic (DP)-TiO2 -based composites are considered attractive anode materials for high lithium-ion storage because of the synergetic contribution from dual-phases in lithium-ion storage. However, a comprehensive investigation on more efficient architectures and platforms is necessary to develop lithium-storage devices with high-rate capability and long-term stability. Herein, for the first time, a rationally designed bronze-rich DP-TiO2 -embedded amorphous carbon nanoarchitecture, denoted as DP-TiO2 @C, from sacrificial Ti-metal-organic frameworks (Ti-MOFs) via a two-step pyrolysis process is proposed. The bronze/anatase DP-TiO2 @C nanocomposites are successfully synthesized using a unique pyrolysis process, which decomposes individually the metal clusters and organic linkers of Ti-MOFs. DP-TiO2 @C exhibits a significantly high density and even distribution of nanoparticles (<5 nm), enabling the formation of numerous heterointerfaces. Remarkably, the bronze-rich DP-TiO2 @C shows high specific capacities of 638 and 194 mAh g-1 at current densities of 0.1 and 5 A g-1 , respectively, owing to the contribution of the synergetic interfacial structure. In addition, reversible specific capacities are observed at a high rate (5 A g-1 ) during 6000 cycles. Thus, this study presents a new approach for the synthesis of DP-TiO2 @C nanocomposites from a sacrificial Ti-MOF and provides insights into the efficient control of the volume ratio in DP-TiO2 anode architecture.

Energy Transfer-Induced Photoelectrochemical Improvement from Porous Zeolitic Imidazolate Framework-Decorated BiVO4 Photoelectrodes.

BiVO4 , which is a representative photoanode material for photoelectrochemical water splitting, intrinsically restricts high conversion efficiency, owing to faster recombination, low electron mobility, and short electron diffusion length. While the photocurrent density of typical BiVO4 corresponds to only 21.3% of the maximum photocurrent density (4.68 mA cm-2 ), decoration of the BiVO4 photoanode with zeolitic imidazolate framework-67 (ZIF-67) exhibits a synergetic effect to raise the overall photocatalytic ability at the BiVO4 surface region to a higher level via the energy-transfer process from BiVO4 to ZIF-67. The hybrid ZIF-67/BiVO4 photoanode follows two convenient photoelectrochemical pathways: 1) energy-transfer-induced water oxidation reaction in ZIF-67 and 2) water oxidation reaction by direct contact between the BiVO4 surface and electrolytes. Compared to the moderate photocurrent density (≈1 mA cm-2 ) of single-layer BiVO4 , the proposed ZIF-67/BiVO4 photoanodes show a remarkably high photocurrent (2.25 mA cm-2 ) with high stability, despite the lack of hole scavengers in the electrolyte. Furthermore, the absorbed photon-to-current efficiency of the ZIF-67/BiVO4 photoanode is ≈2.5 times greater than that of BiVO4 . This work proposes a promising solution for efficient water oxidation that overcomes the intrinsic material limitations of BiVO4 photoelectrodes by using energy transfer-induced photon recycling and the decoration of porous ZIFs.

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.

Bionanoelectronic platform with a lipid bilayer/CVD-grown MoS2 hybrid.

We demonstrate a bionanoelectronic platform for a supported lipid bilayer formed on an MoS2 film for biosensing, biomolecule recognition, and bioelectronic applications. A large-area MoS2 film was synthesized on a sapphire substrate and treated with O2 plasma or Al2O3 deposition to change the surface from hydrophobic to hydrophilic. Measurements of fluorescence and fluorescence recovery after photobleaching confirmed the physical properties of the lipid bilayer on the treated surfaces. We fabricated an electronic device using the treated MoS2 film and characterized the influence of the lipid bilayer on its electrical properties. Furthermore, transmembrane ion channels peptide (gramicidin A) were incorporated into the lipid bilayer and modulations of the electrical properties of the device under various pH conditions and calcium ion were observed. This sensitive and stable platform has strong potential for housing artificial channels and transmembrane ion channels for advanced bioapplications.

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.

Low voltage-driven oxide phototransistors with fast recovery, high signal-to-noise ratio, and high responsivity fabricated via a simple defect-generating process.

We have demonstrated that photo-thin film transistors (photo-TFTs) fabricated via a simple defect-generating process could achieve fast recovery, a high signal to noise (S/N) ratio, and high sensitivity. The photo-TFTs are inverted-staggered bottom-gate type indium-gallium-zinc-oxide (IGZO) TFTs fabricated using atomic layer deposition (ALD)-derived Al2O3 gate insulators. The surfaces of the Al2O3 gate insulators are damaged by ion bombardment during the deposition of the IGZO channel layers by sputtering and the damage results in the hysteresis behavior of the photo-TFTs. The hysteresis loops broaden as the deposition power density increases. This implies that we can easily control the amount of the interface trap sites and/or trap sites in the gate insulator near the interface. The photo-TFTs with large hysteresis-related defects have high S/N ratio and fast recovery in spite of the low operation voltages including a drain voltage of 1 V, positive gate bias pulse voltage of 3 V, and gate voltage pulse width of 3 V (0 to 3 V). In addition, through the hysteresis-related defect-generating process, we have achieved a high responsivity since the bulk defects that can be photo-excited and eject electrons also increase with increasing deposition power density.

Highly Repeatable and Recoverable Phototransistors Based on Multifunctional Channels of Photoactive CdS, Fast Charge Transporting ZnO, and Chemically Durable Al2O3 Layers.

Highly repeatable and recoverable phototransistors were explored using a "multifunctional channels" structure with multistacked chalcogenide and oxide semiconductors. These devices were made of (i) photoactive CdS (with a visible band gap), (ii) fast charge transporting ZnO (with a high field-effect mobility), and (iii) a protection layer of Al2O3 (with high chemical durability). The CdS TFT without the Al2O3 protection layer did not show a transfer curve due to the chemical damage that occurred on the ZnO layer during the chemical bath deposition (CBD) process used for CdS deposition. Alternatively, compared to CdS phototransistors with long recovery time and high hysteresis (ΔVth = 19.5 V), our "multi-functional channels" phototransistors showed an extremely low hysteresis loop (ΔVth = 0.5V) and superior photosensitivity with repeatable high photoresponsivity (52.9 A/W at 400 nm). These improvements are likely caused by the physical isolation of the sensing region and charge transport region by the insertion of the ultrathin Al2O3 layer. This approach successfully addresses some of the existing problems in CdS phototransistors, such as the high gate-interface trap site density and high absorption of molecular oxygen, which originate from the polycrystalline CdS.

Periodically pulsed wet annealing approach for low-temperature processable amorphous InGaZnO thin film transistors with high electrical performance and ultrathin thickness.

In this paper, a simple and controllable "wet pulse annealing" technique for the fabrication of flexible amorphous InGaZnO thin film transistors (a-IGZO TFTs) processed at low temperature (150 °C) by using scalable vacuum deposition is proposed. This method entailed the quick injection of water vapor for 0.1 s and purge treatment in dry ambient in one cycle; the supply content of water vapor was simply controlled by the number of pulse repetitions. The electrical transport characteristics revealed a remarkable performance of the a-IGZO TFTs prepared at the maximum process temperature of 150 °C (field-effect mobility of 13.3 cm(2) V(-1) s(-1); Ion/Ioff ratio ≈ 10(8); reduced I-V hysteresis), comparable to that of a-IGZO TFTs annealed at 350 °C in dry ambient. Upon analysis of the angle-resolved x-ray photoelectron spectroscopy, the good performance was attributed to the effective suppression of the formation of hydroxide and oxygen-related defects. Finally, by using the wet pulse annealing process, we fabricated, on a plastic substrate, an ultrathin flexible a-IGZO TFT with good electrical and bending performances.

Dual electrical behavior of multivalent metal cation-based oxide and its application to thin-film transistors with high mobility and excellent photobias stability.

The effect of multivalent metal cations, including vanadium(V) and tin (Sn), on the electrical properties of vanadium-doped zinc tin oxide (VZTO) was investigated in the context of the fabrication of thin-film transistors (TFTs) using a single VZTO film and VZTO/ZTO bilayer as channel layers. The single VZTO TFT did not show any response to the gate voltage (insulator-like behavior). On the other hand, the VZTO/ZTO bilayer TFT exhibited a typical TFT transfer characteristic (semiconducting behavior). X-ray photoelectron spectroscopy revealed that, in contrast to what is commonly true in many oxides, oxygen vacancies (V(O)) in VZTO did not provide a dominant contribution to the total carrier concentration, because the V(O) peak area in the single VZTO film was 5.4% and reduced to 4.5% in VZTO/ZTO bilayer. Instead, Sn 3d5/2 and V 2p3/2 spectra suggest that the significant reduction in Sn and V ions is strongly related to the insulator-like behavior of the VZTO film. In negative-bias illumination tests and illumination tests with various photon energies, the VZTO/ZTO bilayer TFT had much better stability than the ZTO TFT. This result is attributed to the reduction of donor-like states ([Formula: see text]O) that can be positively ionized by blue and green illumination.

Correlation between nano-scale microstructural behavior and the performance of ZnO thin-film transistors.

Binary ZnO active layers possessing a polycrystalline structure were deposited with various argon/oxygen flow ratios at 250 degrees C via sputtering. Then ZnO thin-film-transistors (TFTs) were fabricated without additional thermal treatments. As the oxygen content increased during the deposition, the preferred orientation along the (0002) was weakened and the rotation of the grains increased, and furthermore, less conducting films were observed. On the other hand, the reduced oxygen flow rate induced the formation of amorphous-like transition layers during the initial growth due to a high growth rate and high energetic bombardment of the adatoms. As a result, the amorphous phases at the gate dielectric/channel interface were responsible for the formation of a hump shape in the subthreshold region of the TFT transfer curve. In addition, the relationship between the crystal properties and the shift in the threshold voltage was experimentally confirmed by a hysteresis test.

Artificial semiconductor/insulator superlattice channel structure for high-performance oxide thin-film transistors.

High-performance thin-film transistors (TFTs) are the fundamental building blocks in realizing the potential applications of the next-generation displays. Atomically controlled superlattice structures are expected to induce advanced electric and optical performance due to two-dimensional electron gas system, resulting in high-electron mobility transistors. Here, we have utilized a semiconductor/insulator superlattice channel structure comprising of ZnO/Al2O3 layers to realize high-performance TFTs. The TFT with ZnO (5 nm)/Al2O3 (3.6 nm) superlattice channel structure exhibited high field effect mobility of 27.8 cm(2)/Vs, and threshold voltage shift of only < 0.5 V under positive/negative gate bias stress test during 2 hours. These properties showed extremely improved TFT performance, compared to ZnO TFTs. The enhanced field effect mobility and stability obtained for the superlattice TFT devices were explained on the basis of layer-by-layer growth mode, improved crystalline nature of the channel layers, and passivation effect of Al2O3 layers.

p-Channel oxide thin film transistors using solution-processed copper oxide.

Cu2O thin films were synthesized on Si (100) substrate with thermally grown 200-nm SiO2 by sol-gel spin coating method and postannealing under different oxygen partial pressure (0.04, 0.2, and 0.9 Torr). The morphology of Cu2O thin films was improved through N2 postannealing before O2 annealing. Under relatively high oxygen partial pressure of 0.9 Torr, the roughness of synthesized films was increased with the formation of CuO phase. Bottom-gated copper oxide (CuxO) thin film transistors (TFTs) were fabricated via conventional photolithography, and the electrical properties of the fabricated TFTs were measured. The resulting Cu2O TFTs exhibited p-channel operation, and field effect mobility of 0.16 cm2/(V s) and on-to-off drain current ratio of ∼1×10(2) were observed in the TFT device annealed at PO2 of 0.04 Torr. This study presented the potential of the solution-based process of the Cu2O TFT with p-channel characteristics for the first time.

Donor and acceptor dynamics of phosphorous doped ZnO nanorods with stable p-type conduction: photoluminescence and junction characteristics.

We employed temperature-dependent photoluminescence (PL) to explain the donor and acceptor dynamics in phosphorus doped stable p-type P:ZnO nanorods. The room temperature PL revealed good crystalline and optical quality of P:ZnO nanorods. The 10 K PL spectrum exhibited a dominant acceptor bound exciton (A0X) or donor bound exciton (D0X) emission corresponding to p- and n-type P:ZnO nanorods, respectively. The donor-acceptor-pair (DAP) transitions exhibited different thermal dissociation energies for the p- and n-type P:ZnO nanorods, suggesting their different quenching channels. The quenching of the DAP transitions of the p-type ZnO:P nanorods was associated with the thermal dissociation of the DAP into free excitons, while the DAP transition of the n-type ZnO:P nanorods was quenched through the thermal dissociation of the shallow donor into free electrons. The rectifying behavior of a p-n homojunction diode formed by the p-type P:ZnO nanorods on n-type ZnO film confirmed the p-type conduction of the P:ZnO nanorods.

Influence of synthesis temperature on the properties of Ga-doped ZnO nanorods grown by thermal evaporation.

This study examined the effect of the synthesis temperatures on the characteristics of vertically aligned Ga-doped ZnO (GZO) nanorods grown on a ZnO template by thermal evaporation using Zn and Ga sources. The increase in synthesis temperature at less than 700 degrees C induced stress relaxation relative to the ZnO template due to the suppression of defect generation by the formation of nanorods, while a further increase resulted in an increase in compressive strain due to dominant Ga doping. The increase in Ga concentration in the GZO nanorods with increasing synthesis temperature was also confirmed by X-ray photoelectron spectroscopy and photoluminescence. The best conductivity was observed in the GZO nanorods grown at 800 degrees C. On the other hand, the GZO nanorods synthesized at 900 degrees C showed less conductivity and weak near-band-edge emission properties due to the generation of defects from the excess Ga.

Ga-doped ZnO nanorod arrays grown by thermal evaporation and their electrical behavior.

Vertically well-aligned Ga-doped ZnO nanorods with different Ga content were grown by thermal evaporation on a ZnO template. The Ga-doped ZnO nanorods synthesized with 50 wt% Ga with respect to the Zn content showed minimum compressive stress relative to the ZnO template, which led to a rapid growth rate along the c-axis due to the rapid release of stored strain energy. A further increase in the Ga content improved the conductivity of the nanorods due to the substitutional incorporation of Ga atoms in the Zn sites based on a decrease in lattice spacing. A p-n diode structure with Ga-doped ZnO nanorods as an n-type layer displayed a distinct white light luminescence from the side view of the device, showing weak ultraviolet and various deep-level emissions.