Solution-Processed Flexible Temperature Sensor Array for Highly Resolved Spatial Temperature and Tactile Mapping Using ESN-Based Data Interpolation.

High-performance flexible temperature sensors are crucial in various technological applications, such as monitoring environmental conditions and human healthcare. The ideal characteristics of these sensors for stable temperature monitoring include scalability, mechanical flexibility, and high sensitivity. Moreover, simplicity and low power consumption will be essential for temperature sensor arrays in future integrated systems. This study introduces a solution-based approach for creating a V2O5 nanowire network temperature sensor on a flexible film. Through optimization of the fabrication conditions, the sensor exhibits remarkable performance, sustaining long-term stability (>110 h) with minimal hysteresis and excellent sensitivity (∼-1.5%/°C). In addition, this study employs machine learning techniques for data interpolation among sensors, thereby enhancing the spatial resolution of temperature measurements and adding tactile mapping without increasing the sensor count. Introducing this methodology results in an improved understanding of temperature variations, advancing the capabilities of flexible-sensor arrays for various applications.

Unusual Selective Monitoring of N,N-Dimethylformamide in a Two-Dimensional Material Field-Effect Transistor.

N,N-Dimethylformamide (DMF) is an essential solvent in industries and pharmaceutics. Its market size range was estimated to be 2 billion U.S. dollars in 2022. Monitoring DMF in solution environments in real time is significant because of its toxicity. However, DMF is not a redox-active molecule; therefore, selective monitoring of DMF in solutions, especially in polar aqueous solutions, in real time is extremely difficult. In this paper, we propose a selective DMF sensor using a molybdenum disulfide (MoS2) field-effect transistor (FET). The sensor responds to DMF molecules but not to similar molecules of formamide, N,N-diethylformamide, and N,N-dimethylacetamide. The plausible atomic mechanism is the oxygen substitution sites on MoS2, on which the DMF molecule shows an exceptional orientation. The thin structure of MoS2-FET can be incorporated into a microfluidic chamber, which leads to DMF monitoring in real time by exchanging solutions subsequently. The designed device shows DMF monitoring in NaCl ionic solutions from 1 to 200 µL/mL. This work proposes the concept of selectively monitoring redox-inactive molecules based on the nonideal atomic affinity site on the surface of two-dimensional semiconductors.

Metallic Transport in Monolayer and Multilayer Molybdenum Disulfides by Molecular Surface Charge Transfer Doping.

Carrier modulation in transition-metal dichalcogenides (TMDCs) is of importance for applying electronic devices to tune their transport properties and controlling phases, including metallic to superconductivity. Although the surface charge transfer doping method has shown a strong modulation ability of the electronic structures in TMDCs and a degenerately doped state has been proposed, the details of the electronic states have not been elucidated, and this transport behavior should show a considerable thickness dependence in TMDCs. In this study, we characterize the metallic transport behavior in the monolayer and multilayer MoS2 under surface charge transfer doping with a strong electron dopant, benzyl viologen (BV) molecules. The metallic behavior transforms to an insulative state under a negative gate voltage. Consequently, metal-insulator transition (MIT) was observed in both monolayer and multilayer MoS2 correlating with the critical conductivity of order e2/h. In the multilayer case, the BV molecules strongly modulated the topmost surface layer in the bulk MoS2; the transfer characteristics suggested a crossover from a heterogeneously doped state with a doped topmost layer to doping in the deep layers caused by the variation in the gate voltage. The findings of this work will be useful for understanding the device characteristics of thin-layered materials and for applying them to the controlling phases via carrier modulation.

Single-layered assembly of vanadium pentoxide nanowires on graphene for nanowire-based lithography technique.

Graphene nanoribbon (GNR)-based materials are a promising device material because of their potential high carrier mobility and atomically thin structure. Various approaches have been reported for preparing the GNR-based materials, from bottom-up chemical synthetic procedures to top-down fabrication techniques using lithography of graphene. However, it is still difficult to prepare a large-scale GNR-based material. Here, we develop a procedure to prepare a large-scale GNR network using networked single-layer inorganic nanowires. Vanadium pentoxide (V2O5) nanowires were assembled on graphene with an interfacial layer of a cationic polymer via electrostatic interaction. A large-scale nanowire network can be prepared on graphene and is stable enough for applying an oxygen plasma. Using plasma etching, a networked graphene structure can be generated. Removing the nanowires results in a networked flat structure whose both surface morphology and Raman spectrum indicate a GNR networked structure. The field-effect device indicates the semiconducting character of the GNR networked structure. This work would be useful for fabricating a large-scale GNR-based material as a platform for GNR junctions for physics and electronic circuits.

Photoconductive Coordination Polymer with a Lead-Sulfur Two-Dimensional Coordination Sheet Structure.

Coordination polymers with metal-sulfur (M-S) bonds in their nodes have interesting optical properties and can be used as photocatalysts for water splitting. A wide range of inorganic-organic hybrid materials with M-S bonds have been prepared in recent years. However, there is a dearth of structural information because of their low crystallinity, which has hampered the understanding of their underlying chemistry and physics. Thus, we conducted a structural study of a novel, highly crystalline coordination polymer with M-S bonds. Theoretical calculations were performed to elucidate its photoconductivity mechanism. The photoconductive, three-dimensional coordination polymer [Pb(tadt)]n (denoted as KGF-9; tadt = 1,3,4-thiadiazole-2,5-dithiolate) was synthesized and confirmed to have a three-dimensional structure containing a two-dimensional Pb-S framework by single-crystal X-ray diffraction. We also performed diffuse-reflectance ultraviolet-visible-near-infrared spectroscopy, time-resolved microwave conductivity, and photoelectron yield spectroscopy measurements on the bulk powder samples, as well as first-principles calculations. Additionally, direct-current photoconductivity measurements were conducted on a single-crystal sample.

Convection-Flow-Assisted Preparation of a Strong Electron Dopant, Benzyl Viologen, for Surface-Charge Transfer Doping of Molybdenum Disulfide.

Transition metal dichalcogenides (TMDCs) have received attention as atomically thin post-silicon semiconducting materials. Tuning the carrier concentrations of the TMDCs is important, but their thin structure requires a non-destructive modulation method. Recently, a surface-charge transfer doping method was developed based on contacting molecules on TMDCs, and the method succeeded in achieving a large modulation of the electronic structures. The successful dopant is a neutral benzyl viologen (BV0); however, the problem remains of how to effectively prepare the BV0 molecules. A reduction process with NaBH4 in water has been proposed as a preparation method, but the NaBH4 simultaneously reacts vigorously with the water. Here, a simple method is developed, in which the reaction vial is placed on a hotplate and a fragment of air-stable metal is used instead of NaBH4 to prepare the BV0 dopant molecules. The prepared BV0 molecules show a strong doping ability in terms of achieving a degenerate situation of a TMDC, MoS2. A key finding in this preparation method is that a convection flow in the vial effectively transports the produced BV0 to a collection solvent. This method is simple and safe and facilitates the tuning of the optoelectronic properties of nanomaterials by the easily-handled dopant molecules.

Electronic Structure Mosaicity of Monolayer Transition Metal Dichalcogenides by Spontaneous Pattern Formation of Donor Molecules.

Modulating the electronic structure of semiconducting materials is critical to developing high-performance electronic and optical devices. Transition metal dichalcogenides (TMDCs) are atomically thin semiconducting materials. However, before they can be used successfully in electronic and optical devices, modulation of their carrier concentration at the nanometer scale must be achieved. Molecular doping has been successful in modulating the carrier concentration; however, the scientific approach for selective and local carrier doping at the nanometer scale is still missing. Here, we demonstrate a proof-of-concept of modulating the carrier concentration of TMDCs laterally on a scale of around 100 nm using spontaneous pattern formation of an ultrathin film consisting of molecular electron dopants. When the water made contact with the molecular film (∼10 nm), a spontaneous pattern formation was observed on both the monolayer and bulk TMDCs. We revealed that the pattern-formation dynamics and nanoscopic flow rate of the molecules were highly dependent on the thickness of the TMDCs, since the band gap varies based on the number of layers. Analyses of topographic images of the molecular patterns and photoluminescence spectra of the TMDCs indicated that the spontaneously patterned molecular films induced a local carrier doping. Our results demonstrate a spontaneous formation of a mosaic electronic structure. This work is useful for making tiny-scale electronic junctions on TMDCs and semiconducting materials to make numerous p/n junctions simultaneously for optoelectronic devices.

Tuning Transition-Metal Dichalcogenide Field-Effect Transistors by Spontaneous Pattern Formation of an Ultrathin Molecular Dopant Film.

Spontaneous pattern formation is an energetically favorable process and is shown in nature in molecular-scale assembly, biological association, and soft material organizations. The opposite regime, the artificial process, which is widely applied to the fabrication of semiconducting devices, such as lithographic techniques, requires enormous amounts of energy. Here, we propose a concept of tuning the properties of semiconducting MoS2 and WSe2 devices using the spontaneous pattern formation of adjacent molecular films. The film used was a 10 nm thick ultrathin film of a molecular electron dopant, which exhibited spontaneous pattern formation and dynamically transformed the morphology of tiny holes, a network, a maze, and dots on substrates, including SiO2, MoS2, and WSe2. These patterns were exhibited only when the film came in contact with water and was tuned with temperature and time. The specific lengths of the patterns were less than 200 nm, which is sufficiently smaller than the exfoliated ∼10 µm semiconducting MoS2 and WSe2 flakes. The properties of the field-effect devices of MoS2 and WSe2 were found to be modified according to the pattern formation process of the ultrathin molecular film on the device. This concept applies the spontaneous patterning phenomena shown in nature to the fabrication and optimization of electronic devices by using molecular films and their responses to the external environment.