Dong Min Sim

Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption

MoS2 is considered a promising two-dimensional active channel material for future nanoelectronics. However, the development of a facile, reliable, and controllable doping methodology is still critical for extending the applicability of MoS2. Here, we report surface charge transfer doping via thiol-based binding chemistry for modulating the electrical properties of vacancy-containing MoS2 (v-MoS2). Although vacancies present in 2D materials are generally regarded as undesirable components, we show that the electrical properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol group and sulfur vacancies and by choosing different functional groups. For example, we demonstrate that NH2-containing thiol molecules with lone electron pairs can serve as an n-dopant and achieve a substantial increase of electron density (Δn = 3.7 × 10(12) cm(-2)). On the other hand, fluorine-rich molecules can provide a p-doping effect (Δn = -7.0 × 10(11) cm(-2)) due to its high electronegativity. Moreover, the n- and p-doping effects were systematically evaluated by photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electrical measurement results. The excellent binding stability of thiol molecules and recovery properties by thermal annealing will enable broader applicability of ultrathin MoS2 to various devices.

Tailoring of the PbS/metal interface in colloidal quantum dot solar cells for improvements of performance and air stability

Despite the outstanding advantages of a simple structure and cost-effectiveness of solution-based fabrication, Schottky junction quantum dot solar cells (QDSCs) often demonstrate low open-circuit voltage and power conversion efficiency (PCE) due to insufficient band bending at the QD/metal Schottky junction. Generally, this undesirable result stems from the presence of many defects at the QD/metal interface and the consequent Fermi-level pinning effect. Here, we show how the simple oxidation of PbS QDs at the PbS/metal interface can greatly improve the open-circuit voltage, fill factor, and PCE of Schottky junction QDSCs. On the basis of systematic analysis results using current–voltage characterization, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and light-soaking tests, we reveal that this enhancement originates from reduced interface states at the PbS/metal Schottky junction. Moreover, a significant enhancement of stability of the device is confirmed by the maintenance of >55% of its initial PCE even after 500 hours exposure in air without additional passivation.

Porous silicon nanowires for lithium rechargeable batteries

Porous silicon nanowire is fabricated by a simple electrospinning process combined with a magnesium reduction; this material is investigated for use as an anode material for lithium rechargeable batteries. We find that the porous silicon nanowire electrode from the simple and scalable method can deliver a high reversible capacity with an excellent cycle stability. The enhanced performance in terms of cycling stability is attributed to the facile accommodation of the volume change by the pores in the interconnect and the increased electronic conductivity due to a multi-level carbon coating during the fabrication process.

Highly Asymmetric n+–p Heterojunction Quantum‐Dot Solar Cells with Significantly Improved Charge‐Collection Efficiencies

The depletion region width of metal-oxide/quantum-dot (QD) heterojunction solar cells is increased by a new method in which heavily boron-doped n(+)-ZnO is employed. It is effectively increased in the QD layer by 30% compared to the counterpart with conventional n-ZnO, and provides 41% and 37% improvement of J(sc) (16.7 mA cm(-2) to 23.5 mA cm(-2) ) and power conversion efficiency (5.52% to 7.55%), respectively.

Interfacial band-edge engineered TiO2 protection layer on Cu2O photocathodes for efficient water reduction reaction

Photoelectrochemical (PEC) water splitting has emerged as a potential pathway to produce sustainable and renewable chemical fuels. Here, we present a highly active Cu2O/TiO2 photocathode for H2 production by enhancing the interfacial band-edge energetics of the TiO2 layer, which is realized by controlling the fixed charge density of the TiO2 protection layer. The band-edge engineered Cu2O/TiO2 (where TiO2 was grown at 80 °C via atomic layer deposition) enhances the photocurrent density up to −2.04 mA/cm2 at 0 V vs. RHE under 1 sun illumination, corresponding to about a 1,200% enhancement compared to the photocurrent density of the photocathode protected with TiO2 grown at 150 °C. Moreover, band-edge engineering of the TiO2 protection layer prevents electron accumulation at the TiO2 layer and enhances both the Faraday efficiency and the stability for hydrogen production during the PEC water reduction reaction. This facile control over the TiO2/electrolyte interface will also provide new insight for designing highly efficient and stable protection layers for various other photoelectrodes such as Si, InP, and GaAs.

Area-Selective Lift-Off Mechanism Based on Dual-Triggered Interfacial Adhesion Switching: Highly Facile Fabrication of Flexible Nanomesh Electrode

With the recent emergence of flexible and wearable optoelectronic devices, the achievement of sufficient bendability and stretchability of transparent and conducting electrodes (TCEs) has become an important requirement. Although metal-mesh-based structures have been investigated for TCEs because of their excellent performances, the fabrication of mesh or grid structures with a submicron line width is still complex due to the requirements of laborious lithography and pattern transfer steps. Here, we introduce an extremely facile fabrication technique for metal patterns embedded in a flexible substrate based on submicron replication and an area-selective delamination (ASD) pattern. The high-yield, area-specific lift-off process is based on the principle of solvent-assisted delamination of deposited metal thin films and a mechanical triggering effect by soft wiping or ultrasonication. Our fabrication process is very simple, convenient, and cost-effective in that it does not require any lithography/etching steps or sophisticated facilities. Moreover, their outstanding optical and electrical properties (e.g., sheet resistances of 0.43 Ω sq-1 at 94% transmittance), which are markedly superior to those of other flexible TCEs, are demonstrated. Furthermore, there is no significant change of resistance over 1000 repeated bending cycles, with a bending radius of 5 mm, and immersion in various solvents such as salt water and organic solvents. Finally, we demonstrate high-performance transparent heaters and flexible touch panels fabricated using the nanomesh electrode, confirming the long-range electrical conduction and reliability of the electrode.

Long-Term Stable 2H-MoS2 Dispersion: Critical Role of Solvent for Simultaneous Phase Restoration and Surface Functionalization of Liquid-Exfoliated MoS2

Chemical exfoliation approaches such as Li-intercalation for the production of two-dimensional MoS2 are highly attractive due to their high yield of monolayer forms, cost-effectiveness, and mass-scalability. However, the loss of the semiconducting property and poor dispersion stability in solvent have limited the extent of their potential applications. Here, we report simultaneous phase recovery and surface functionalization for the preparation of a highly stable 2H-MoS2 dispersion in water. This study shows that high-yield restoration of the semiconducting 2H phase from a chemically exfoliated MoS2 (ce-MoS2) can be induced by a mild-temperature (180 °C) solvent thermal treatment in N-methyl-2-pyrrolidone (NMP). In addition to a phase transition, this solvent thermal treatment in NMP realizes concurrent surface functionalization of the 2H-MoS2 surface, which provides an outstanding dispersion stability to 2H-MoS2 in water for more than 10 months. Finally, we report the humidity sensor based on the functionalized 2H-MoS2, which shows a substantial response enhancement compared with a nonfunctionalized 2H-MoS2 or ce-MoS2.

Nanopatterned High-Frequency Supporting Structures Stably Eliminate Substrate Effects Imposed on Two-Dimensional Semiconductors

Despite the outstanding physical and chemical properties of two-dimensional (2D) materials, due to their extremely thin nature, eliminating detrimental substrate effects such as serious degradation of charge-carrier mobility or light-emission yield remains a major challenge. However, previous approaches have suffered from limitations such as structural instability or the need of costly and high-temperature deposition processes. Herein, we propose a new strategy based on the insertion of high-density topographic nanopatterns as a nanogap-containing supporter between 2D materials and substrate to minimize their contact and to block the substrate-induced undesirable effects. We show that well-controlled high-frequency SiO x nanopillar structures derived from the self-assembly of Si-containing block copolymer securely prevent the collapse or deformation of transferred MoS2 and guarantee excellent mechanical stability. The nanogap supporters formed below monolayer MoS2 lead to dramatic enhancement of the photoluminescence emission intensity (8.7-fold), field-effect mobility (2.0-fold, with a maximum of 4.3-fold), and photoresponsivity (12.1-fold) compared to the sample on flat SiO2. Similar favorable effects observed for graphene strongly suggest that this simple but powerful nanogap-supporting method can be extensively applicable to a variety of low-dimensional materials and contribute to improved device performance.