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.

Ultralarge Photoluminescence Enhancement of Monolayer Molybdenum Disulfide by Spontaneous Superacid Nanolayer Formation.

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.

Photoactivation of Strong Photoluminescence in Superacid-Treated Monolayer Molybdenum Disulfide.

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.

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.

Systematic Study of Photoluminescence Enhancement in Monolayer Molybdenum Disulfide by Acid Treatment.

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 Brønsted 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 solvent. Both TFSI and H2SO4 showed similar PL enhancement (∼4-8 enhancement) at the same proton concentration, indicating that the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li+ from Li2SO4, instead of H2SO4, in water. Although Li and H atoms showed similar binding energy on MoS2 from theoretical calculations, Li2SO4 treatment showed little PL enhancement; only coexisting H2SO4 reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS2. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.