Monolayer transition metal dichalcogenides are materials with an atomic structure complementary to graphene but diverse properties, including direct energy bandgaps, which makes them intriguing ...candidates for optoelectronic devices. Various approaches have been demonstrated for the growth of molybdenum disulphide (MoS2) on insulating substrates, but to date, growth of isolated crystalline flakes has been demonstrated at random locations only. Here we use patterned seeds of molybdenum source material to grow flakes of MoS2 at predetermined locations with micrometre-scale resolution. MoS2 flakes are predominantly monolayers with high material quality, as confirmed by atomic force microscopy, transmission electron microscopy and Raman and photoluminescence spectroscopy. As the monolayer flakes are isolated at predetermined locations, transistor fabrication requires only a single lithographic step. Device measurements exhibit carrier mobility and on/off ratio that exceed 10 cm(2) V(-1) s(-1) and 10(6), respectively. The technique provides a path for in-depth physical analysis of monolayer MoS2 and fabrication of MoS2-based integrated circuits.
Monolayer tungsten disulfides (WS2) constitute a high quantum yield two-dimensional (2D) system, and can be synthesized on a large area using chemical vapor deposition (CVD), suggesting promising ...nanophotonics applications. However, spatially nonuniform photoluminescence (PL) intensities and peak wavelengths observed in single WS2 grains have puzzled researchers, with the origins of variation in relative contributions of excitons, trions, and biexcitons to the PL emission not well understood. Here, we present nanoscale PL and Raman spectroscopy images of triangular CVD-grown WS2 monolayers of different sizes, with these images obtained under different temperatures and values of excitation power. Intense PL emissions were observed around the edges of individual WS2 grains and the grain boundaries between partly merged WS2 grains. The predominant origin of the main PL emission from these regions changed from neutral excitons to trions and biexcitons with increasing laser excitation power, with biexcitons completely dominating the PL emission for the high-power condition. The intense PL emission and the preferential formation of biexcitons in the edges and grain boundaries of monolayer WS2 were attributed to larger population of charge carriers caused by the excessive incorporation of growth promoters during the CVD, suggesting positive roles of excessive carriers in the PL efficiency of TMD monolayers. Our comprehensive nanoscale spectroscopic investigation sheds light on the dynamic competition between exciton complexes occurring in monolayer WS2, suggesting a rich variety of ways to engineer new nanophotonic functions using 2D transition metal dichalcogenide monolayers.
Grain boundaries in monolayer transition metal dichalcogenides have unique atomic defect structures and band dispersion relations that depend on the inter-domain misorientation angle. Here, we ...explore misorientation angle-dependent electrical transport at grain boundaries in monolayer MoS2 by correlating the atomic defect structures of measured devices analysed with transmission electron microscopy and first-principles calculations. Transmission electron microscopy indicates that grain boundaries are primarily composed of 5-7 dislocation cores with periodicity and additional complex defects formed at high angles, obeying the classical low-angle theory for angles <22°. The inter-domain mobility is minimized for angles <9° and increases nonlinearly by two orders of magnitude before saturating at ∼ 16 cm(2) V(-1) s(-1) around misorientation angle ≈ 20°. This trend is explained via grain-boundary electrostatic barriers estimated from density functional calculations and experimental tunnelling barrier heights, which are ≈ 0.5 eV at low angles and ≈ 0.15 eV at high angles (≥ 20°).
Photoluminescence (PL) from monolayer MoS2 has been modulated using plasma treatment or thermal annealing. However, a systematic way of understanding the underlying PL modulation mechanism has not ...yet been achieved. By introducing PL and Raman spectroscopy, we analyze that the PL modulation by laser irradiation is associated with structural damage and associated oxygen adsorption on the sample in ambient conditions. Three distinct behaviors were observed according to the laser irradiation time: (i) slow photo-oxidation at the initial stage, where the physisorption of ambient gases gradually increases the PL intensity; (ii) fast photo-oxidation at a later stage, where chemisorption increases the PL intensity abruptly; and (iii) photoquenching, with complete reduction of PL intensity. The correlated confocal Raman spectroscopy confirms that no structural deformation is involved in slow photo-oxidation stage; however, the structural disorder is invoked during the fast photo-oxidation stage, and severe structural degradation is generated during the photoquenching stage. The effect of oxidation is further verified by repeating experiments in vacuum, where the PL intensity is simply degraded with laser irradiation in a vacuum due to a simple structural degradation without involving oxygen functional groups. The charge scattering by oxidation is further explained by the emergence/disappearance of neutral excitons and multiexcitons during each stage.
An unusually large bandgap modulation of 1.23–2.65 eV in monolayer MoS2 on a SiO2/Si substrate is found due to the inherent local bending strain induced by the surface roughness of the substrate, ...reaching the direct‐to‐indirect bandgap transition. Approximately 80% of the surface area reveals an indirect bandgap, which is confirmed further by the degraded photoluminescence compared to that from suspended MoS2.
We report that highly crystalline graphene can be obtained from well-controlled surface morphology of the copper substrate. Flat copper surface was prepared by using a chemical mechanical polishing ...method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. We also found that grain boundaries and intentionally formed scratched area play an important role for nucleation seeds. Although the best monolayer graphene was grown from polished Cu with a low sheet resistance of 260 Ω/sq, a small portion of multilayers were also formed near the impurity particles or locally protruded parts.
Layered hexagonal boron nitride (h-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and ...transition metal dichalcogenides. However, the h-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of h-BN thin film as a substrate for monolayer MoS2 can induce ∼6.5 × 1011 cm–2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS2 on h-BN was found to be ∼5 × 1013 cm–2 at high temperature and was significantly reduced at low temperature. Further, the inserted h-BN enables us to reduce the Coulombic charge scattering in MoS2/h-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal–insulator transition at a much lower charge density of ∼1.0 × 1012 cm–2 (T = 25 K). The reduced effective Schottky barrier height in MoS2/h-BN is attributed to the decreased effective work function of MoS2 arisen from h-BN induced n-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO2.
In spite of recent successful demonstrations of flexible and transparent graphene heaters, the underlying heat‐transfer mechanism is not understood due to the complexity of the heating system. Here, ...graphene/glass defoggers are fabricated and the dynamic response of the temperature as a function of input electrical power is measured. The graphene/glass defoggers reveal shorter response times than Cr/glass defoggers. Furthermore, the saturated temperature of the graphene/glass defoggers is higher than for Cr/glass defoggers at a given input electrical power. The observed dynamic response to temperature is well‐fitted to the power‐balance model. The response time of graphene/glass defogger is shorter by 44% than that of the Cr/glass defogger. The convective heat‐transfer coefficient of graphene is 12.4 × 10−4 W cm−2 °C−1, similar to that of glass (11.1 × 10−4 W cm−2 °C−1) but smaller than that of chromium (17.1 × 10−4 W cm−2 °C−1). The graphene‐based system reveals the lowest convective heat‐transfer coefficient due to its ideal flat surface compared to its counterparts of carbon nanotubes (CNTs) and reduced graphene oxide (RGO)‐based systems.
A graphene/glass defogger is fabricated and the dynamic response of the temperature as a function of input electrical power is measured. The response time of the graphene/glass defogger is shorter by 44% than that of a Cr/glass defogger. The convective heat‐transfer coefficient of graphene is 12.4 × 10−4 W cm−2 °C−1, similar to that of glass (11.1 × 10−4 W cm−2 °C−1) but smaller by 27% than that of chromium (17.1 × 10−4 W cm−2 °C−1).
Abstract
The conversion of chalcogen atoms to other types in transition metal dichalcogenides has significant advantages for tuning bandgaps and constructing in-plane heterojunctions; however, ...difficulty arises from the conversion of sulfur or selenium to tellurium atoms owing to the low decomposition temperature of tellurides. Here, we propose the use of sodium for converting monolayer molybdenum disulfide (MoS
2
) to molybdenum ditelluride (MoTe
2
) under Te-rich vapors. Sodium easily anchors tellurium and reduces the exchange barrier energy by scooting the tellurium to replace sulfur. The conversion was initiated at the edges and grain boundaries of MoS
2
, followed by complete conversion in the entire region. By controlling sodium concentration and reaction temperature of monolayer MoS
2
, we tailored various phases such as semiconducting 2H-MoTe
2
, metallic 1T′-MoTe
2
, and 2H-MoS
2−
x
Te
x
alloys. This concept was further extended to WS
2
. A high valley polarization of ~37% in circularly polarized photoluminescence was obtained in the monolayer WS
2−
x
Te
x
alloy at room temperature.
We report the synthesis of centimeter-scale monolayer WS2 on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS2 film growth on gold ...surface. To ensure the coverage uniformity of monolayer WS2 film, the tungsten source-coated substrate was placed in parallel with Au foil under hydrogen sulfide atmosphere. The high growth temperature near 935 °C helps to increase a domain size up to 420 μm. Gold foil is reused for the repeatable growth after bubbling transfer. The WS2-based field effect transistor reveals an electron mobility of 20 cm2 V–1 s–1 with high on–off ratio of ∼108 at room temperature, which is the highest reported value from previous reports of CVD-grown WS2 samples. The on–off ratio of integrated multiple FETs on the large area WS2 film on SiO2 (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS2 FET device characteristics over a large area of 3 × 1.5 cm2.
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