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A large number of experimental results have been published on the improvement of optical properties in monolayer transition metal dichalcogenides (TMDCs) by probing with photoluminescence (PL) intensity. Remarkable improvement in PL intensity in sulfur-based compounds, such as MoS2 and WS2, has almost been established by chemical treatment methods, such as p-type dopants and superacids; in contrast, selenium-based compounds of MoSe2, especially WSe2, have a very limited strategy. One potential strategy for improving PL intensity in all compounds would be to remove the substrate effect by mechanical floating of the monolayers from the underneath substrate material. However, floating the monolayer is difficult; it is possible if the monolayer does transfer to the trenches of substrates, and a post-preparation method has not been established. In this paper, we developed a universal post-preparation method for removing the substrate effect by a spontaneous fluctuation of monolayer TMDCs via coating with a simple hydrocarbon, paraffin. Because of the large thermal coefficient of paraffin, the stain is effectively applied to the TMDCs in the coating process. The paraffin-coated monolayer TMDCs show spontaneous fluctuation; and in an extreme case, the fluctuation forms the shape of a bundle. During the fluctuation and deformation, the substrate effect was removed; as a result, PL intensity improved remarkably in both sulfur- and selenium-based compounds of MoS2, WS2, MoSe2, and WSe2. This work suggests a universal strategy for improving the optical property of monolayer TMDCs by removing the substrate effect. It also extends the design of applying TMDCs as optoelectronics material due to the transparency, flexibility, and biocompatibility of the paraffin.

N,N-Dimethylformamide (DMF) is an essential solvent in industries and pharmaceutics. Its market size range was estimated to be 2 billion US 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 DMF in solutions in real time requires an unprecedented design at the scale of atomic resolution. 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 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 L/mL. This work proposes the concept of selectively monitoring redox-inactive molecules based on the non-ideal atomic affinity site on the surface of two-dimensional semiconductors.

Mechanical exfoliation methods of two-dimensional materials have been an essential process for advanced device and fundamental sciences. However, the exfoliation method usually generates various thick flakes, and a bunch of thick bulk flakes usually covers an entire substrate. Here, we developed a method to selectively isolate mono- to quadlayers of transition metal dichalcogenides (TMDCs) by sonication in organic solvents. The analysis reveals the importance of low interface energies between solvents and TMDCs, leading to effective removal of bulk flakes under sonication. Importantly, a monolayer adjacent to bulk flakes shows cleavage at the interface, and the monolayer can be selectively isolated on the substrate. This approach can extend to preparing a monolayer device with crowded 17 electrode fingers surrounding the monolayer and for the measurement of electrostatic device performance.

The interface between conventional semiconductors and aqueous ionic solutions is an important target in chemistry and materials science. Recently, a wide variety of research has been done on transition-metal dichalcogenides (TMDCs) for use as 2D layered semiconductors, and their optoelectronic properties have been widely explored. One representative TMDC, monolayer (1L) MoS

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.

To advance the development of atomically thin optoelectronics using two-dimensional (2D) materials, engineering strong luminescence with a physicochemical basis is crucial. Semiconducting monolayer transition-metal dichalcogenides (TMDCs) are candidates for this, but their quantum yield (QY) is known to be poor. Recently, a molecular superacid treatment of bis(trifluoromethane)sulfonimide (TFSI) generated unambiguously bright monolayer TMDCs and a high QY. However, this method is highly dependent on the processing conditions and therefore has not been generalized. Here, we shed light on environmental factors to activate the photoluminescence (PL) intensity of the TFSI-treated monolayer MoS2, with a factor of more than 2 orders of magnitude greater than the original by photoactivation. The method is useful for both mechanically exfoliated and chemically deposited samples. The existence of photoirradiation larger than the band gap demonstrates enhancement of the PL of MoS2; on the other hand, activation by thermal annealing, as demonstrated in the previous report, is less effective for enhancing the PL intensity. The photoactivated monolayer MoS2 shows a long lifetime of ∼1.35 ns, more than a 30-fold improvement over the original as exfoliated. The consistent realization of the bright monolayer MoS2 reveals that air exposure is an essential factor in the process. TFSI treatment in a N2 environment was not effective for achieving a strong PL, even after the photoactivation. These findings can serve as a basis for engineering the bright atomically thin materials for 2D optoelectronics.

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 MoS

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.

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.

In the last decade, transition metal chalcogenides (TMDCs), e.g., MoS2 and WSe2, have gathered attention as semiconducting 2D materials. Modulation of carrier concentrations in TMDCs is a crucial matter for applying to semiconductor devices. So far, we have developed a method to gain the carrier concentrations of MoS2 by a junction with organic molecules and achieved obtaining a degenerately doped state in the MoS2. The mechanism of the doping is a surface-charge transfer interaction at the interface. Although the transfer characteristic behaviors show the degenerately doped state (small ON/OFF ratio), the details of the transport behavior have not been well understood. In this presentation, I will discuss the details of the device characteristics of the monolayer and multilayer MoS2 metal-oxide-semiconductor field-effect-transistors (MOSFETs) with a junction of redox-active molecules in a manner of the surface-charge transfer interaction. The devices were fabricated via standard lithography (e-beam and photolithography) techniques to prepare each monolayer and multilayer MoS2 MOSFETs. Transport properties in the MoS2 MOSFETs were measured in the as-prepared device and the device after the molecular doping. The dopant molecule is benzyl viologen (BV) molecule which is known as a redox-active molecule with high-reduction potential. After the doping with BV molecules, the MoS2 MOSFET showed a small ON/OFF ratio in the transfer characteristic curves, indicating a degenerately doped state. The identical device further showed the metallic behavior in the temperature dependence (the conductivity increases with decreasing the temperature). Both monolayer and multilayer MoS2 MOSFETs showed similar metallic behavior. This metallic transport behavior changes to an insulative regime when applying gate-voltage to reduce the carrier concentrations in the channel MoS2. Therefore, the gate-electrostatic induced metal-insulator-transition was suggested. At the conference, I will discuss the details of the transport behaviors, gate-potential and temperature dependence of the contact resistance. In addition, I will also discuss potential candidates of other molecules for obtaining doping to TMDCs for going forward the 2D materials-based electronics. REF: Matsuyama, et al., "Metallic transport in monolayer and multilayer molybdenum disulfides by molecular surface-charge transfer doping", ACS Appl. Mater. Interfaces, 2022, 14, 8163-8170.

Due to the direct band gap nature, extensive studies have been conducted to improve the optical behavior in monolayer transition metal dichalcogenides (TMDCs) with a formula of MX2 (M = Mo, W; X = S, Se, Te). One of the strongest modulating agents of optical behavior is a molecular superacid treatment; however, the chemical event has not been unveiled. Also, the engineering protocol for keeping the treatment is immature. In this work, we systematically study the superacid treatment procedures on monolayer molybdenum disulfide (MoS2) and propose that the interaction, a hydrophilic interaction, between the superacid molecule and MoS2 surface would be critical. As a result of the interaction, the superacid molecules spontaneously form an acidic layer with the thickness of several nanometers on the surface. The power-dependent photoluminescence (PL) measurement indicates the edge of MoS2 flake is more effective and electronically modulated by the treatment. By understanding the superacid nanolayer formation by the treatment, we succeeded in maintaining the ultrastrong PL in the superacid-treated MoS2 for more than 30 days in the ambient air by encapsulation with transparent organic polymers. This study advances the understanding and designing applications of strong luminescent properties in the superacid-treated TMDCs and paves the way toward engineering exciton dynamics and an experimental platform for treating multibody states.

Superacids are strong acids with an acidity higher than pure sulfuric acid. Recently, superacid treatment of monolayer transition metal dichalcogenide (TMDC) flakes, such as MoS2 and WS2, has shown a dramatic enhancement of optical properties, such as photoluminescence (PL) intensity. The superacid molecule is bis(trifluoromethane)sulfonimide (TFSI). In this review paper, we summarize and discuss the recent works and the current understanding of the TFSI treatment, and finally, we describe the outlook of the treatment on monolayer TMDCs.

Monolayer molybdenum disulfide (MoS2) is an atomically thin semiconducting material with a direct band gap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS2 was demonstrated via a treatment with a molecular acid, bis(trifluoromethane)sulfonimide (TFSI); however, the mechanism still remains a mystery. Here, we work on PL enhancement of monolayer MoS2 by treatment of Bronsted acids (TFSI and sulfuric acid (H2SO4)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents—1,2-dichloroethane (DCE), acetonitrile, and water—were studied, as they show quite different acidity in solution. All of the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common ...

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.

A major challenge in transistor technology scaling is the formation of controlled ultrashallow junctions with nanometer-scale thickness and high spatial uniformity. Monolayer doping (MLD) is an efficient method to form such nanoscale junctions, where the self-limiting nature of semiconductor surfaces is utilized to form adsorbed monolayers of dopant-containing molecules followed by rapid thermal annealing (RTA) to diffuse the dopants to a desired depth. Unlike ion implantation, the process does not induce crystal damage, thus making it highly attractive for nanoscale transistor processing. To date, reported MLD processes have relied on solution processing for monolayer formation. Gas-phase processing, however, benefits from higher intra- and interwafer uniformity and conformal coverage of 3D structures and is more desirable for manufacturing. In this regard, we report a new approach for MLD in silicon and germanium using gas-phase monolayer formation. We call this technology gas-phase monolayer doping (GP-MLD). This method relies on sequential pulse-purge cycles of gas-phase dopant-containing molecules to form a boron- or phosphorus-containing monolayer on a target semiconductor surface. Here, we show the feasibility of our approach through the formation of ultrashallow B- and P-doped junctions on Si and Ge surfaces. The mechanism of adsorption is characterized using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Sub-5 nm junction depths with high dopant dose are obtained as characterized by secondary ion mass spectrometry and sheet resistance measurements. Additionally, we demonstrate that area selectivity can be achieved via lithographic patterning of the monolayer dopants before the diffusion step. The results demonstrate the versatility of the GP-MLD approach for formation of controlled and ultrashallow junctions.

In this paper, a post superacid (SA) treatment was proposed to enhance the performance of InAs FinFETs on SiO2/Si substrates. Typically, the subthreshold swing (SS) has reduced from 217 to 170 mV/decade and the transconductance ( ${g}_{m}$ ) has increased from 6.44 to 26.5 $\mu \text{S}/\mu \text{m}$ after SA treatment. It was found that the interfacial In2O3 at the InAs/ZrO2 interface was effectively reduced after SA treatment due to the strong protonating nature of the SA solution. As a result, the interface trap density was reduced leading to a pronounced reduction of sheet resistance after SA treatment. The modeling of transfer characteristics indicates that the carrier mobility is enhanced by 5.8~7.1 folds after SA treatment due to interfacial traps reduction. The results suggest that SA treatment can be potentially extended to other III–V MOSFETs to enhance the device performances.

The electron excitation mechanism and the spin accompanied by electron transition in a multiferroic YMnO3 epitaxial thin film were studied using photoluminescence (PL) spectroscopy. The thin film exhibits an intra-atomic transition of Mn3+ and the A1 optical coherent phonon. This study particularly focuses on the correlation between the electron transition corresponding to the on-site Coulomb energy and antiferromagnetic spin order. To clarify the complex excitation mechanism, the excitation energy and temperature dependences of the PL were analyzed. The key finding was that the intensities of the PL band at 1.43 eV increase as the excitation energy approaches the absorption peak energy corresponding to the on-site Coulomb energy and as the temperature decreases below 80 K, corresponding to the Néel temperature. These results suggest that the PL band is mediated by the spin–flip and relaxation processes.

The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

Molybdenum disulfide (MoS2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS2 because the pristine MoS2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS2 surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS2. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm(2) of HER current density.

2D materials gather attention due to their unique and quantum-confined electronic structures applicable to optoelectronics and a platform for physics. One representative 2D material is transition metal dichalcogenides (TMDCs), which consist of a three-atomic-layered structure with a thickness of only ~0.7 nm. Representative TMDCs are MoS2 and WSe2, and both materials are semiconducting materials with the direct-band gap at the monolayer limit. To apply these unique materials for atomically thin electronics and quantum devices, controlling the carrier concentration is important. In this talk, I will present our recent results to control the carrier behaviors of TMDCs using molecules, with a particular focus on the molecules in non-equilibrium environments, starting with the preparation process of a dopant molecule in one (Ref.1). We found a method to prepare a strong electron dopant, benzyl viologen (BV), in a non-equilibrium manner. In the previous works, the BV molecule is synthesized using a strong reducing agent, sodium borohydride (NaBH4), in water. However, the method is inefficient because NaBH4 reacts vigorously with water at the same time. We replaced NaBH4 with indium metal and found a synthetic method to prepare BV dopant molecules under a non-equilibrium convection flow environment. The convection flow continuously synthesizes BV dopant molecules to a high concentration; furthermore, the method eliminates the purification process to use the dopant solution. Additionally, I will show a non-equilibrium dopant molecular assembly on TMDCs (Ref. 2 and 3). BV molecules usually interact immediately with the TMDC surface and assemble uniformly. Because of the hydrophobic character of the BV molecules, it is difficult to apply photoresists to make local carrier modulation at submicron scales. We overcome the issue by using the self-pattern formation process of the BV molecular films on the surface of TMDCs. By applying hydrophobic/hydrophilic repulsive interaction, the BV film destabilizes and shows pattern formation at scales of about ~200 nm periodicity. Our analysis found that the patterns induce pn and metal/semiconductor junctions on TMDCs. This non-equilibrium process would be useful to prepare electronic device structures via the spontaneous process, which is uniformly distributed at large scale. Keywords: Molecular dopant, Transition metal dichalcogenides, TMDCs, convection flow, molecular assembly, spontaneous pattern formation References: 1K. Matsuyama, A. Fukui, et al., ChemistryOpen, 2019, 8, 908. 2H. Ichimiya, et al., ACS NANO, 2018, 12, 10123. 3H. Ichimiya, et al., ACS Appl. Mat. Int. 2019, 11,15922.

A new type of ferroelectricity, originating from charge order and coupled to magnetism, occurs in YbFe2O4 containing triangular Fe/O double layers, which has recently generated great interest in this material. YbFe2O4 tolerates more than 10% iron deficiency even in bulk single crystals with space group R m. Even though a large number of Fe deficiencies are introduced in the thin film form due to vaporization of Fe ions during deposition at high temperatures, the crystal structure with the space group of R m in the films never changes within the Fe/Yb composition ratio from 1.3 to 2.2. In the previous research, the effects of the Fe/Yb composition ratio—especially in the large Fe deficient area (1.31–1.86)—on the lattice distortion and chemical bonding state of non-stoichiometric (0001)-oriented YbFe2O4 epitaxial thin films on (111) YSZ and (0001) sapphire substrates were examined. The variation of the lattice constant, Raman spectra, and optical absorption coefficient indicated that the existence of the stacking fault and the antisite Yb play an important role in the significant iron deficiency without changing the crystal structure of non-stoichiometric YbFe2O4 thin films.

Wearable sensors play a crucial role in realizing personalized medicine, as they can continuously collect data from the human body to capture meaningful health status changes in time for preventive intervention. However, motion artifacts and mechanical mismatches between conventional rigid electronic materials and soft skin often lead to substantial sensor errors during epidermal measurement. Because of its unique properties such as high flexibility and conformability, flexible electronics enables a natural interaction between electronics and the human body. In this Account, we summarize our recent studies on the design of flexible electronic devices and systems for physical and chemical monitoring. Material innovation, sensor design, device fabrication, system integration, and human studies employed toward continuous and noninvasive wearable sensing are discussed. A flexible electronic device typically contains several key components, including the substrate, the active layer, and the interface layer. The inorganic-nanomaterials-based active layer (prepared by a physical transfer or solution process) is shown to have good physicochemical properties, electron/hole mobility, and mechanical strength. Flexible electronics based on the printed and transferred active materials has shown great promise for physical sensing. For example, integrating a nanowire transistor array for the active matrix and a conductive pressure-sensitive rubber enables tactile pressure mapping; tactile-pressure-sensitive e-skin and organic light-emitting diodes can be integrated for instantaneous pressure visualization. Such printed sensors have been applied as wearable patches to monitor skin temperature, electrocardiograms, and human activities. In addition, liquid metals could serve as an attractive candidate for flexible electronics because of their excellent conductivity, flexibility, and stretchability. Liquid-metal-enabled electronics (based on liquid-liquid heterojunctions and embedded microchannels) have been utilized to monitor a wide range of physiological parameters (e.g., pulse and temperature). Despite the rapid growth in wearable sensing technologies, there is an urgent need for the development of flexible devices that can capture molecular data from the human body to retrieve more insightful health information. We have developed a wearable and flexible sweat-sensing platform toward real-time multiplexed perspiration analysis. An integrated iontophoresis module on a wearable sweat sensor could enable autonomous and programmed sweat extraction. A microfluidics-based sensing system was demonstrated for sweat sampling, sensing, and sweat rate analysis. Roll-to-roll gravure printing allows for mass production of high-performance flexible chemical sensors at low cost. These wearable and flexible sweat sensors have shown great promise in dehydration monitoring, cystic fibrosis diagnosis, drug monitoring, and noninvasive glucose monitoring. Future work in this field should focus on designing robust wearable sensing systems to accurately collect data from the human body and on large-scale human studies to determine how the measured physical and chemical information relates to the individual's specific health conditions. Further research in these directions, along with the large sets of data collected via these wearable and flexible sensing technologies, will have a significant impact on future personalized healthcare.

The electrocaloric (EC) effect of ferroelectric polymer film was investigated at a high frequency. The analysis used the finite element method and revealed that the extractive heat density per unit of time increases and the thermal diffusion length decreases with an increase in the frequency of the driving electric field. Given the small thermal diffusion coefficient of ferroelectric polymers, the high-frequency drive of the EC effect for ferroelectric polymer film is promising for cooling devices. To discuss the EC effect under an alternating current (AC) electric field, the EC temperature change of the vinylidene fluoride-trifluoroethylene copolymer 75/25 P(VDF-TrFE) film with a thickness of 2 μm was characterized through direct measurement using a thin-film thermocouple. The EC temperature change increases with an increase in the electric field and frequency and reached up to 0.23 °C at 1.5 MV cm−1 and 10 kHz. The extractive heat density was estimated to be as high as 0.69 W cm−2.

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.

Gold-mediated exfoliation of ultralarge optoelectronically perfect monolayers with lateral dimensions up to ≈500 μm is reported. Electrical, optical, and X-ray photo-electron spectroscopy characterization show that the quality of the gold-exfoliated flakes is similar to that of tape-exfoliated flakes. Large-area flakes allow manufacturing of large-area mono-layer transition metal dichalcogenide electronics.

Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.

Layered 2D materials serve as a new class of substrates for templated synthesis of various nanomaterials even with highly dissimilar crystal structures; thus overcoming the lattice constraints of conventional epitaxial processes. Here, molybdenum disulfide (MoS2) is used as a prototypical model substrate for oriented growth of in-plane Au nanowires (NWs) despite the nearly 8% lattice mismatch between MoS2 and Au. Au NWs on the MoS2 surface are oriented along three symmetrically equivalent directions within the substrate arising from the strong Au–S binding that templates the oriented growth. The kinetics of the growth process are explored through experiments and modeling. Strong charge transfer is observed between Au NWs and MoS2, resulting in degenerate p-doping of MoS2.

A 20 × 20 pixel pressure sensor array based on a printed active-matrix single-wall carbon-nanotube thin-film transistor backplane is presented. Using a gravure printing process that is compatible with fully printed large-area roll-to-roll processing, a 97% device yield is obtained on the 400-transistor backplane. As a proof of concept, pressure sensors are integrated to map the applied tactile pressure across the array.

Understanding edge effects and quantifying their impact on the carrier properties of two-dimensional (2D) semiconductors is an essential step toward utilizing these materials for high performance electronic and optoelectronic devices.1–5 WS 2 monolayers patterned into disks of varying diameters are used to experimentally determine the influence of edges on their optical properties. Carrier lifetime measurements show a decrease in the effective lifetime, τ effective , as a function of decreasing diameter, suggesting that the edges are active sites for carrier recombination. Accordingly, we introduce a metric called edge recombination velocity (ERV) to characterize the impact of 2D material edges on non-radiative recombination. The unpassivated WS 2 monolayer disks yield an ERV ∼ 4 × 104 cm/s. This work quantifies the non-radiative recombination edge effects in monolayer semiconductors, while simultaneously establishing a practical characterization technique towards experimental explorations of edge passivation methods for 2D materials.

Air-stable n-doping of carbon nanotubes is presented by utilizing SiN(x) thin films deposited by plasma-enhanced chemical vapor deposition. The fixed positive charges in SiN(x), arising from (+)Si ≡ N3 dangling bonds induce strong field-effect doping of underlying nanotubes. Specifically, an electron doping density of ∼ 10(20) cm(-3) is estimated from capacitance voltage measurements of the fixed charge within the SiN(x). This high doping concentration results in thinning of the Schottky barrier widths at the nanotube/metal contacts, thus allowing for efficient injection of electrons by tunnelling. As a proof-of-concept, n-type thin-film transistors using random networks of semiconductor-enriched nanotubes are presented with an electron mobility of ∼ 10 cm(2)/V s, which is comparable to the hole mobility of as-made p-type devices. The devices are highly stable without any noticeable change in the electrical properties upon exposure to ambient air for 30 days. Furthermore, the devices exhibit high uniformity over large areas, which is an important requirement for use in practical applications. The work presents a robust approach for physicochemical doping of carbon nanotubes by relying on field-effect rather than a charge transfer mechanism.

We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO2 deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO2 film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm2 and a high power conversion efficiency of 19.2%.

Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here we demonstrate the use of benzyl viologen (BV), which has one of the highest reduction potentials of all electron-donor organic compounds, as a surface charge transfer donor for MoS2 flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained a high electron sheet density of ∼1.2 × 1013 cm–2, which corresponds to the degenerate doping limit for MoS2. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS2 at the metal junctions, the contact resistances are shown to be reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-doped n+ source/drain contacts...

Molecular interaction is an essential process in various fields such as chemistry, biology and materials science. The interaction of molecules includes: 1) the local electron transfer process, 2) local chemical bonding and 3) assembling to form a structure. All three occur essentially from each molecule’s “recognition” ability. We are trying to extend the molecular interaction system onto inorganic electronic materials to harness the highest-performance of inorganic systems. In this presentation, I will present our recent results of molecular interaction on a semiconducting 2D material, transition metal dichalcogenides (TMDCs).1,2,3 TMDCs are constructed by stacking atomically-thin layers (thickness ~0.7 nm) with van der Waals interaction, and they are expected to generate tiny optoelectronic devices due to their thin nature. Since monolayer TMDC has a direct bandgap, it is a good platform for realizing atomically thin optoelectronic devices. However, an issue is their low luminescence brightness. For example, a representative TMDC, MoS­2, shows photoluminescence (PL) quantum yield of ~1% or less. In this presentation, I will show our recent results of improving the brightness of monolayer MoS2 dramatically (more than 100 times enhancement from the original) via molecular treatments. One method is using a superacid (bis(trifluoromethane)sulfonimide; TFSI) molecule.3 The superacid molecular treatment onto a monolayer MoS2 changes the PL quantum yield of MoS2 from 1% to near-unity. I will discuss the PL enhancement factor in the TFSI treatment on MoS2. The second method is using a redox-active molecule, fluoranil. The fluoranil molecule can reduce the electron concentration of monolayer MoS2, resulting in enhancement of the PL intensity.4 Furthermore, we found that the solvent used for the fluoranil treatment is critical to obtain bright monolayer MoS2. Solvent molecules would screen the charges on fluoranil molecules, as a result, which stabilizes the electron transfer-interaction between fluoranil and MoS2. Keywords: Superacid, redox-active molecules, Transition metal dichalcogenides, Photoluminescence, Transistors References: 1D. Kiriya et al., J. Am. Chem. Soc. 2014, 136, 7853. 2H. Ichimiya, et al., ACS NANO, 2018, 12, 10123. 3M. Amani*, D.H. Lien*, D. Kiriya* et al., Science, 2015, 350, 1065. (* equal contribution) 4H. Ichimiya, et al. Appl. Phys. Express, 2019, in press.

The crystal growth of YbFe2O4 requires oxidant-poor conditions, thus YbFe2O4 usually contains a large number of iron deficiencies even in the bulk single crystal. The use of an ArF laser for laser ablation is effective to reduce the amount of active oxygen species and a wide process window to form the YbFe2O4 epitaxial films becomes available. By using the widened process window and an iron-rich target, the chemical composition of the YbFe2O4 epitaxial thin films is successfully controlled. The effect of the iron composition on the charge ordering transition can be discussed using the nonlinear I–V behavior. The threshold electric field changes depending on the iron composition owing to the broadening of the 3-dimensional (3D) charge order region, which affects the robustness of the 3D charge order against an electric field in YbFe2O4 thin films.

We report visible light and X-ray imagers on lightweight and mechanically flexible plastic substrates. The process involves solution processing of organic photodetectors on top of an active-matrix backplane consisting of carbon nanotube thin-film transistors. The system takes advantage of the high mobility of nanotube transistors for low operating voltages and efficient light absorption of organic bulk-heterojunctions for high imaging sensitivity. With this highly scalable process scheme, 18 × 18 pixel-array flexible imagers (physical size of 2 cm × 1.5 cm) with high performance are successfully demonstrated. In addition, as the absorption peak of the adopted organic photodiodes covers the green band of the light spectrum, X-ray imaging is readily demonstrated by placing a scintillator film on top of the flexible imagers.

Multigas sensors on a chip with chemical-sensitive nanoscale silicon transistors., There is great interest in developing a low-power gas sensing technology that can sensitively and selectively quantify the chemical composition of a target atmosphere. Nanomaterials have emerged as extremely promising candidates for this technology due to their inherent low-dimensional nature and high surface-to-volume ratio. Among these, nanoscale silicon is of great interest because pristine silicon is largely inert on its own in the context of gas sensing, unless functionalized with an appropriate gas-sensitive material. We report a chemical-sensitive field-effect transistor (CS-FET) platform based on 3.5-nm-thin silicon channel transistors. Using industry-compatible processing techniques, the conventional electrically active gate stack is replaced by an ultrathin chemical-sensitive layer that is electrically nonconducting and coupled to the 3.5-nm-thin silicon channel. We demonstrate a low-power, sensitive, and selective multiplexed gas sensing technology using this platform by detecting H2S, H2, and NO2 at room temperature for environment, health, and safety in the oil and gas industry, offering significant advantages over existing technology. Moreover, the system described here can be readily integrated with mobile electronics for distributed sensor networks in environmental pollution mapping and personal air-quality monitors.

Artificial reconstruction of fibre-shaped cellular constructs could greatly contribute to tissue assembly in vitro. Here we show that, by using a microfluidic device with double-coaxial laminar flow, metre-long core-shell hydrogel microfibres encapsulating ECM proteins and differentiated cells or somatic stem cells can be fabricated, and that the microfibres reconstitute intrinsic morphologies and functions of living tissues. We also show that these functional fibres can be assembled, by weaving and reeling, into macroscopic cellular structures with various spatial patterns. Moreover, fibres encapsulating primary pancreatic islet cells and transplanted through a microcatheter into the subrenal capsular space of diabetic mice normalized blood glucose concentrations for about two weeks. These microfibres may find use as templates for the reconstruction of fibre-shaped functional tissues that mimic muscle fibres, blood vessels or nerve networks in vivo.

In bottom-up tissue engineering, a method to integrate a pathway of nutrition and oxygen into the resulting macroscopic tissue has been highly desired, but yet to be established. This paper presents a cellular building unit made from microstrand-shaped bacterial cellulose (BC microstrand) covered with mammalian cells. The BC microstrands are fabricated by encapsulating Acetobacter xylinum with a calcium alginate hydrogel microtube using a double co-axial microfluidic device. The mechanical strength and porous property of the BC microstrands can be regulated by changing the initial density of the bacteria. By folding or reeling the building unit, we demonstrated the multiple shapes of millimeter-scale cellular constructs such as coiled and ball-of-yarn-shaped structures. Histological analysis of the cellular constructs indicated that the BC microstrand served as a pathway of nutrition and oxygen to feed the cells in the central region. These findings suggest that our approach facilitates creating functional macroscopic tissue used in various fields such as drug screening, wound healing, and plastic surgery.

Printed electronics and sensors enable new applications ranging from low-cost disposable analytical devices to large-area sensor networks. Recent progress in printed carbon nanotube electronics in terms of materials, processing, devices, and applications is discussed on page 4397 by A. Javey and co-workers. The research challenges and opportunities regarding the processing and system-level integration are also discussed for enabling of practical applications.

In recent years a significant amount of effort has been devoted towards the development of dopant-free carrier selective contacts for crystalline silicon (c-Si) solar cells. This short manuscript surveys a range of materials which have the potential to induce carrier-selectivity when applied to c-Si, including metals, metal oxides, alkali / alkaline earth metal salts, and organic conductors. Simple Ohmic test structures are used to assess the selectivity of these materials, that is, hole contacts are tested on p-type c-Si and electron contacts on n-type c-Si. Among these alternatives, a number of systems with exceptional potential are identified.

Valence tautomerism (VT) refers to equilibria between two or more different tautomers differing in their charge distributions. This contribution shows how molecular VT occurring in metal–dioxolene complexes (metal = cobalt, manganese) functionalized with long alkyl chains can be coupled with several types of macroscopic phase transformations, such as solid-to-solid and solid-to-liquid transitions and the double-melting phenomenon. Observations of simultaneous transformation of molecular and macroscopic states have led to the concept of synchronic bistability. The central focus in this microreview is on molecular design, thermodynamic basis, and kinetic control of the synchronic bistability. It is demonstrated that a key factor in designing a synchronic transformation is to build up strong interdependent correlations between molecules and their assembled states.