A Piezotronic and Magnetic Dual-Gated Ferroelectric Semiconductor Transistor

Piezotronics is the coupling effect of the piezoelectric and semiconductor properties; however, the piezoelectric constant of the piezoelectric semiconductor is relatively small while the ferroelectric materials with large piezoelectric constant typically possess weak semiconductor properties, thus limiting the effective coupling coefficient of the piezotronic materials and devices. Here, a piezotronics and magnetic dual-gated ferroelectric semiconductor transistor (PM-FEST) is fabricated by Terfenol-D, aluminum oxide (Al₂O₃), and ferroelectric semiconductor α-In₂Se₃, which has a large piezoelectric coefficient, room-temperature ferroelectricity, and dipole locking. The charge carrier transport and corresponding drain current of the PM-FEST can be directly modulated by either the applied magnetic field or external strain. At a low magnetic field (<200 mT), the maximum current on/off ratio of α-In₂Se₃ based PM-FEST is as high as 1700%. Compared with traditional piezotronic devices, the PM-FEST demonstrates a higher gauge factor (2.3 × 10⁴) than that of the piezoelectric semiconductors. This work provides a possibility of realizing magnetism-modulated electronics in semiconductors by exploiting the coupling of piezotronics and magnetostriction.     

Enhanced Performance of a ZnO Nanowire‐Based Self‐Powered Glucose Sensor by Piezotronic Effect

A self‐powered, piezotronic effect‐enhanced glucose sensor based on metal‐semiconductor‐metal (M–S–M) structured single ZnO nanowire device is demonstrated. A triboelectrical nanogenerator (TENG) is integrated to build a self‐powered glucose monitoring system (GMS) to realize the continuously monitoring of glucose concentrations. The performance of the glucose sensor is generally enhanced by the piezotronic effect when applying a –0.79% compressive strain on the device, and magnitude of the output signal is increased by more than 200%; the sensing resolution and sensitivity of sensors are improved by more than 200% and 300%, respectively. A theoretical model using energy band diagram is proposed to explain the observed results. This work demonstrates a promising approach to raise the sensitivity, improve the sensing resolution, and generally enhance the performance of glucose sensors, also providing a possible way to build up a self‐powered GMS.     

A General Approach for Fabricating Arc‐Shaped Composite Nanowire Arrays by Pulsed Laser Deposition

Here, a new method is demonstrated that uses sideways pulsed laser deposition to deliberately bend nanowires into a desired shape after growth and fabricate arc‐shaped composite nanowire arrays of a wide range of nanomaterials. The starting nanowires can be ZnO, but the materials to be deposited can be metallic, semiconductor, or ceramic depending on the application. This method provides a general approach for rational fabrication of a wide range of side‐by‐side or “core–shell” nanowire arrays with controllable degree of bending and internal strain. Considering the ZnO is a piezoelectric and semiconductive material, its electrical properties change when deformed. This technique has potential applications in tunable electronics, optoelectronics, and piezotronics.     

Coupled Ion‐Gel Channel‐Width Gating and Piezotronic Interface Gating in ZnO Nanowire Devices

The piezotronic effect has been extensively investigated and applied to the third generation of semiconductors. However, there currently is no effective method compatible with microelectronics techniques to harness the piezotronic effect. In this work, a facile and low‐energy‐consuming method to couple the channel‐width gating effect with piezotronic devices is developed by precisely patterning ion‐gel electrolyte on ZnO NW. The ultrahigh capacitance of ion gel resulting from electrical double layers allows efficient modulation of the charge carrier density in ZnO NW at low gate voltage (2 V) to compensate for the piezotronic effect. The obtained output current variation under negative gate voltage (420%, i.e., enhanced piezotronic effect) is two times higher than that under zero or positive gate biases (200%). Through quantifying the reverse‐biased Schottky barrier height and charge carrier density, it is found that the applied negative gate voltage depletes free electrons in ZnO NW and alleviates the screening effect on piezoelectric polarization charges, leading to enhanced piezotronic effect. Based on this, an ion‐gel‐gated piezotronic strain sensor is fabricated with enhanced gauge factor and tunable logic devices. It is believed that the coupled ion‐gel and piezotronic gating effect is of great significance to the design of sophisticated and practical piezotronic devices.     

ZnO Nanoplatelets with Controlled Thickness: Atomic Insight into Facet-Specific Bimodal Ligand Binding Using DNP NMR

Colloidal nanoplatelets (NPLs) and nanosheets with controlled thickness have recently emerged as an exciting new class of quantum-sized nanomaterials with substantially distinct optical properties compared to 0D quantum dots. Zn-based NPLs are an attractive heavy-metal-free alternative to the so far most widespread cadmium chalcogenide colloidal 2D semiconductor nanostructures, but their synthesis remains challenging to achieve. The authors describe herein, to the best of their knowledge, the first synthesis of highly stable ZnO NPLs with the atomically precise thickness, which for the smallest NPLs is 3.2 nm (corresponding to 12 ZnO layers). Furthermore, by means of dynamic nuclear polarization-enhanced solid-state ¹⁵N NMR, the original role of the benzamidine ligands in stabilizing the surface of these nanomaterials is revealed, which can bind to both the polar and non-polar ZnO facets, acting either as X- or L-type ligands, respectively. This bimodal stabilization allows obtaining hexagonal NPLs for which the surface energy of the facets is modulated by the presence of the ligands. Thus, in-depth study of the interactions at the organic–inorganic interfaces provides a deeper understanding of the ligand–surface interface and should facilitate the future chemistry of stable-by-design nano-objects.     

Electrical properties of zinc oxide – Tetracene heterostructures with different n-type ZnO films

Electrical properties of organic-inorganic p–n heterojunction structures with tetracene (Tc) and zinc oxide (ZnO) films were investigated. The ZnO films had different n-type carrier concentrations that varied from ∼10¹⁵ cm⁻³ to 10¹⁹ cm⁻³. Lower n-type ZnO layers resulted in decreased reverse currents in the ZnO:Al/ZnO/Tc/Au structures and in an improvement of their asymmetric properties. Experimentally determined energy level alignments at the ZnO/Tc interfaces were related to the electrical behavior of the structures. An improved rectification was associated with decreased generation-recombination currents at the ZnO/Tc interface due to an increased organic-inorganic interface energy gap. Current-voltage characteristics were analyzed by a differential approach. Electrical conduction mechanisms including bimolecular recombination as well as trap-filled limited conduction were identified in the investigated structures.     

Electric transport in gallium antimonide single crystals involving molten GaSb-Sn inclusions

Electromigration of molten tin-based inclusions in single-crystal p-GaSb:Zn(111) was studied. It was shown that molten inclusions are displaced by a current (j=(1–4)×10⁵ A/m²) toward a negative electrode in the temperature range T=750–920 K. The mechanism of this phenomenon was shown to be related to concentration changes in the bulk of molten inclusions. It was noted that the inclusion transport is initiated by two competing processes: the temperature changes at phase interfaces caused by the Peltier heat and the electric transport, leading to the redistribution of components, depending on their effective charges in the melt. The size dependence of the velocity of inclusion motion W in the bulk of a single-crystal matrix was determined: W increases with the inclusion size. The numerical values of the thermoelectric parameters of all the contacting phases were experimentally determined using independent methods. This made it possible, fitting the theory to the experimental data, not only to estimate quantitatively the effective charge Z* of a molten semiconductor, but also to explain the size dependence of the activation barrier overcome by a drifting inclusion.     

Heat‐Transport Mechanisms in Superlattices

The heat transport mechanisms in superlattices are identified from the cross‐plane thermal conductivity Λ of (AlN)_x–(GaN)_y superlattices measured by time‐domain thermoreflectance. For (AlN)_{4.1 nm}–(GaN)_{55 nm} superlattices grown under different conditions, Λ varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest Λ, Λ of (AlN)_{4 nm}–(GaN)_y superlattices decreases monotonically as y decreases, Λ = 6.35 W m⁻¹ K⁻¹ at y = 2.2 nm, 35 times smaller than Λ of bulk GaN. For long‐period superlattices (y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G ≈ 620 MW m⁻² K⁻¹. For y < 40 nm, the apparent value of G increases with decreasing y, reaching G ≈ 2 GW m⁻² K⁻¹ at y < 3 nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)_{4.1 nm}–(GaN)_{4.9 nm} superlattice irradiated by 2.3 MeV Ar ions to a dose of 2 × 10¹⁴ ions cm⁻² is reduced by <35%, suggesting that heat transport in these short‐period superlattices is dominated by long‐wavelength acoustic phonons. Calculations using a Debye‐Callaway model and the assumption of a boundary scattering rate that varies with phonon‐wavelength successfully capture the temperature, period, and ion‐dose dependence of Λ.     

Nucleotide-Driven Molecular Sensing of Monkeypox Virus Through Hierarchical Self-Assembly of 2D Hafnium Disulfide Nanoplatelets and Gold Nanospheres

Liquid interfaces facilitate the organization of nanometer-scale biomaterials with plasmonic properties suitable for molecular diagnostics. Using hierarchical assemblage of 2D hafnium disulfide nanoplatelets and zero-dimensional spherical gold nanoparticles, the design of a multifunctional material is reported. When the target analyte is present, the nanocomposites’ self-assembling pattern changes, altering their plasmonic response. Using monkeypox virus (MPXV) as an example, the findings reveal that adding genomic DNA to the nanocomposite surface increases the agglomeration between gold nanoparticles and decreases the π-stacking distance between hafnium disulfide nanoplatelets. Further, this self-assembled nanomaterial is found to have minimal cross-reactivity toward other pathogens and a limit of detection of 7.6 pg µL⁻¹ (i.e., 3.57 × 10⁴ copies µL⁻¹) toward MPXV. Overall, this study helped to gain a better understanding of the genomic organization of MPXV to chemically design and develop targeted nucleotides. The study has been validated by UV–vis spectroscopy, X-ray diffraction, scanning transmission electron microscopy, surface-enhanced Raman microscopy and electromagnetic simulation studies. To the best knowledge, this is the first study in literature reporting selective molecular detection of MPXV within a few minutes and without the use of any high-end instrumental techniques like polymerase chain reactions.     

Toughness and Fracture Properties in Nacre‐Mimetic Clay/Polymer Nanocomposites

Nacre inspires researchers by combining stiffness with toughness by its unique microstructure of aligned aragonite platelets. This brick‐and‐mortar structure of reinforcing platelets separated with thin organic matrix has been replicated in numerous mimics that can be divided into two categories: microcomposites with aligned metal oxide microplatelets in polymer matrix, and nanocomposites with self‐assembled nanoplatelets—usually clay or graphene oxide—and polymer. While microcomposites have shown exceptional fracture toughness, current fabrication methods have limited nacre‐mimetic nanocomposites to thin films where fracture properties remained unexplored. Yet, fracture resistance is the defining property of nacre, therefore centrally important in any mimic. Furthermore, to make use of these properties in applications, bulk materials are required. Here, up to centimeter‐thick nacre‐mimetic clay/polymer nanocomposites are produced by the lamination of self‐assembled films. The aligned clay nanoplatelets are separated by poly(vinyl alcohol) matrix, with 10⁶–10⁷ nanoplatelets on top of each other in the bulk plates. Fracture testing shows crack deflection and a fracture toughness of 3.4 MPa m^1/2, not far from nacre. Flexural tests show high stiffness (25 GPa) and strength (220 MPa) that, despite the hydrophilic constituents, are not substantially affected by exposure to humidity.     

Interfaces analysis by impedance spectroscopy and transient current spectroscopy on semiconducting polymers based metal–insulator–semiconductor capacitors

Impedance and transient current measurements on metal–insulator–semiconductor (MIS) capacitors are used as tools to thoroughly investigate the bulk and interface electronic transport properties of semiconducting polymers, i.e. poly(3-hexylthiophene) (P3HT). Distinct features were observed at both interfaces, i.e. metal–semiconductor and semiconductor–insulator. The results revealed a dispersive transport in the bulk due to the band tail of the localized states, presence of interface states at the interface between the insulator and the semiconductor and formation of a less conductive small layer at the interface semiconductor–metal contact due to intrusions of sputtered Au particles. Effects of self-assembled monolayers (SAMs) treatments of the gate insulating dielectric were investigated showing that treating the gate dielectric with either ozone or hexamethyldisilazane (HMDS) or octyltrichlorosilane (OTS) alter not only the interface semiconductor–insulator but the bulk properties as well. An exponential density of states with a width parameter of 38–58 meV depending on the surface treatment was found to be representative of the band tail of P3HT. Though both OTS and HMDS treatments slightly increase the density of interface states, only OTS treated samples showed a decrease in disorder parameter of the bulk. The latter fact can be attributed to an increase of the grain size due to a favored π-π stacking film growth. An outcome explaining the already reported increase of the lateral mobility and decrease of the vertical mobility observed upon OTS treatment of the gate insulating dielectric in poly(3-hexylthiophene) based devices.     

Piezophototronic Effect Enhanced Photoresponse of the Flexible Cu(In,Ga)Se2 (CIGS) Heterojunction Photodetectors

The Cu(In,Ga)Se₂ (CIGS) heterojunction, as a mature and high efficiency thin‐film solar cell, is rarely studied as a photodetector, especially in flexible substrates. In this paper, the structure of an ITO/ZnO/CdS/CIGS/Mo heterojunction is grown on the polyimide (PI) substrate to form a flexible CIGS heterojunction photodetector. The photodetector can work in a very wide band ranging from 350 to 1200 nm with responsivity up to 1.18 A W^?1 (808 nm), detectivity up to 6.56 × 10¹⁰ Jones (cmHz^1/2 W^?1), and response time of 70 (/88) ms, respectively. Moreover, the piezophototronic effect is first used to investigate performance modulation of this device by effectively controlling the separation and transport of carriers at the interface of CdS/ZnO. Interestingly, by externally applying a 0.763% tensile strain, the photoresponsivity and detectivity of the photodetector exhibit a decrease from 1.18 to 0.88 A W^?1, and from 6.56 × 10¹⁰ to 4.81 × 10¹⁰ Jones, respectively, while under a –0.749% externally static compressive strain, the photoresponsivity could be enhanced by ≈75.4% with a maximum of 2.07 A W^?1, and the detectivity is improved by ≈66.1% with its peak value up to 10.9 × 10¹⁰ Jones. Meanwhile, the response time can be modulated from 99(/116) to 41.3(/42.6) ms. This work suggests that the CIGS heterojunction has great potential in novel applications for piezophototronic sensors and also gives a hint to modulate the performance of other multilayer heterostructures via the piezotronic effect.     

Ultra-Low Dark Current Organic–Inorganic Hybrid X-Ray Detectors

Organic-inorganic hybrid semiconductors are an emerging class of materials for direct conversion X-ray detection due to attractive characteristics such as high sensitivity and the potential to form conformal detectors. However, existing hybrid semiconductor X-ray detectors display dark currents that are 1000–10 000× higher than industrially relevant values of 1–10 pA mm⁻². Herein, ultra-low dark currents of <10 pA mm⁻², under electric fields as high as ≈4 V µm⁻¹, for hybrid X-ray detectors consisting of bismuth oxide nanoparticles (for enhanced X-ray attenuation) incorporated into an organic bulk heterojunction consisting of p-type Poly(3-hexylthiophene-2,5-diyl) (P3HT) and n-type [6,6]-Phenyl C71 butyric acid methyl ester (PC₇₀BM) are reported. Such ultra-low dark currents are realized through the enrichment of the hole selective p-type organic semiconductor near the anode contact. The resulting detectors demonstrate broadband X-ray response including an exceptionally high sensitivity of ≈1.5 mC Gy⁻¹ cm⁻² and <6% variation in angular dependence response under 6 MV hard X-rays. The above characteristics in combination with excellent dose linearity, dose rate linearity, and reproducibility over a broad energy range enable these detectors to be developed for medical and industrial applications.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

Enhanced CO2 Reduction Performance of BiCuSeO-Based Hybrid Catalysts by Synergetic Photo-Thermoelectric Effect

Gaseous CO₂ reduction driven by solar energy is a promising solution to the current energy crisis and environmental problems. Although thermocatalysts, electrocatalysts, and photocatalysts are developed as classical strategies for CO₂ reduction, it remains a challenge for high efficiency and CO₂ net reduction during this process. Here, a multi-field driven hybrid catalyst, Pt/ZnO nanorod arrays/Bi_1-xEr_xCuSeO, is designed using the photo-thermoelectric effect, which can take advantage of both photocatalysis and thermocatalysis. The results indicate that the maximum CO production rate of 2.91 µmol g⁻¹ h⁻¹ at 423 K can be realized in such Pt/ZnONR/Bi_0.9Er_0.1CuSeO hybrid catalyst, as can be ascribed to a synergetic photo-thermoelectric effect (i.e., light irradiation can provide heat, photo-excited carriers, and the concomitant Seebeck voltage). The band alignment of ZnO/BiCuSeO heterojunction and carriers transport are proposed to be optimized by the Er doped BiCuSeO thermoelectric supports, greatly enhancing the catalytic performance. The application of thermoelectric support could be promising in the structure design of multi-field driven hybrid catalysts, and such a photo-thermoelectric catalytic process demonstrates a desirable way of solar energy utilization in CO₂ transformation.     

Synthesis of Ternary and Quaternary Au and Pt Decorated CdSe/CdS Heteronanoplatelets with Controllable Morphology

A variety of new ternary and quaternary metal–semiconductor inorganic nanostructures with unprecedented structural morphologies is achieved by the decoration of five monolayer‐thick CdSe/CdS core/crown nanoplatelets with Au and Pt domains. Significant differences in metal growth behavior are observed by varying the CdSe core and the CdS crown dimensions. Depending on the core size, Au growth can be directed only to the CdS edges, or both at the edges and at the center of the nanoplatelets. In contrast, the nucleation of Pt domains always happens at the CdS edges independently of the core and crown dimensions. Furthermore, quaternary structures are obtained by additional Au growth on Pt‐decorated CdSe/CdS nanoplatelets, where the effect of steric hindrance of the existing Pt domains results in the Au nucleation to occur only at the CdSe core. Instead, a change in the order of growth of the two noble metals results in Pt‐Au alloys present only at the surrounding edges of the nanoplatelets. Additionally, the metal‐decorated nanoplatelets are found to be efficient catalysts for H₂ fuel generation under white light irradiation. The highest apparent quantum efficiency measured is 19.3% ± 1.4% with a turnover frequency of ≈10⁵ molecules of H₂ per hour per nanoplatelet.     

Copper phthalocyanine based vertical organic field effect transistor with naturally patterned tin intermediate grid electrode

We report on low voltage vertical organic field effect transistors using crosslinked poly(vinyl alcohol) (cr-PVA) as gate insulator and copper phthalocyanine (CuPc) as channel semiconductor. Al is used as gate and drain electrode. Sn thin films deposited under proper conditions are used as intermediate grid electrode (source), since the Sn film morphology simultaneously shows pinholes and lateral intergrain connectivity, allowing in-plane charge transport. Our Al/cr-PVA/Sn/CuPc/Al VOFET operates at low voltages, presents specific transconductance of ∼0.45 S m⁻² and a linear source-drain current on gate voltage dependence.     

Halide-hydrogen vapor transport for growth of ZnO single crystals with controllable electrical parameters

The advantages of HCl+H₂ gas mixture as a chemical vapor transport agent for ZnO single crystals growth in the closed growth chambers are shown in comparison with Cl₂, HCl and H₂ by the thermodynamic analysis. The influence of the growth temperature, density of HCl+H₂ transport agent and undercooling were investigated experimentally on the rate of ZnO mass transport. It was shown that HCl+H₂ gas mixture provides (i) a rather high growth rate (up to 1 mm per day), (ii) a minimization of wall adhesion effect and deformations during a post-growth cooling, (iii) stable and reproduced seeded growth of the void-free single crystals with controllable conductivity and charge carrier concentration varied in the range of 2–22 (Ω cm)⁻¹ and (1–31)·10¹⁷ cm⁻³, respectively. The characterization by the photoluminescence spectra, the transmission spectra and the electrical properties, as well as energy spectra of stable Cl-containing defects are analyzed.     

Millimeter-range Induced Flexo-Pyrophotronic Effect in Centrosymmetric Heterojunction for Ultrafast Night-Photomonitoring

Unlike the structure-specific piezoelectric effect, flexoelectricity is a universal phenomenon that can offer a wide range of energy-efficient, cost-effective, mechano-opto-electro-coupled applications. Even though the flexoelectric effect has been extensively studied at nanoscale, a fundamental, yet unresolved, the issue is how it can be exploited at larger scales for potential applications. Herein, the long-range (>millimeter) stimulated and regulated impact of the localized inhomogeneous strain-induced flexoelectric potential on centrosymmetric metal/titanium oxide heterojunction with nanoscale precision (≈5.8 nm) is demonstrated. The noticed phenomenon is attributed to the long-range interaction between flexoelectric and build-in potentials, which is further utilized to develop mechanically regulated (enhancement > 10⁴%), self-powered (i.e., 0 V), ultrafast (>10 million bits per second), and broadband (λ = 365–1720 nm) pyro-photosensors having high responsivity (≈1.18 mA W⁻¹). As prospective applications, proof-of-concept ultrafast night movement monitors (>720 km h⁻¹), high-performing stationery, and dynamic obstacle sensors with possible impact alerts are developed. These findings lay the groundwork for the micro-to-millimeter-range flexo-opto-electrical coupling in centrosymmetric materials, which can have a wide variety of practical applications.     

Tailoring the Optical Properties of Epitaxially Grown Biaxial ZnO/Ge,and Coaxial ZnO/Ge/ZnO and Ge/ZnO/Ge Heterostructures

Heterostructures of epitaxially grown biaxial ZnO/Ge, and coaxial ZnO/Ge/ZnO and Ge/ZnO/Ge heterostructured nanowires with ideal epitaxial interfaces between the semiconductor ZnO sublayer and the Ge sublayer have been fabricated via a two‐stage chemical vapor–solid process. Structural characterization by high‐resolution transmission electron microscopy and electron diffraction indicates that both the ZnO and Ge sublayers in the heterostructures are single crystalline. A good epitaxial relationship of (100)_ZnO∥(2 0)_Ge exists at the interface between ZnO and Ge in the ZnO/Ge biaxial heterostructure. There is also an epitaxial relationship of (0 0)_ZnO∥(020)_Ge at the interface between the ZnO and Ge substructures in the coaxial ZnO/Ge/ZnO heterostructures, and a good epitaxial relationship of (0 0)_ZnO∥(0 0)_Ge at the interface between ZnO and Ge in the Ge/ZnO/Ge coaxial heterostructure. Structural models for the crystallographic relationship between the wurtzite‐ZnO and diamond‐like cubic‐Ge subcomponents in the heterostructures are given. The optical properties for the synthesized heterostructures are studied by spatially resolved cathodoluminescence spectra at low temperature (20 K). Excitingly, the unique biaxial and coaxial heterostructures display unique new luminescence properties. It is concluded that the ideal epitaxial interface between ZnO and Ge in the prepared heterostructures induces new optical properties. The group II–VI Ge‐based nanometer‐scale heterostructures and their interesting optical properties may inspire great interest in exploring related epitaxial heterostructures and their potential applications in lasers, gas sensors, solar energy conversion, and nanodevices in the future.