Selective Engineering of Chalcogen Defects in MoS2 by Low-Energy Helium Plasma.

Structural defects in two-dimensional transition-metal dichalcogenides can significantly modify the material properties. Previous studies have shown that chalcogen defects can be created by physical sputtering, but the energetic ions can potentially displace transition-metal atoms at the same time, leading to ambiguous results and in some cases, degradation of material quality. In this work, selective sputtering of S atoms in monolayer MoS2 without damaging the Mo sublattice is demonstrated with low-energy helium plasma treatment. Based on X-ray photoelectron spectroscopy analysis, wide-range tuning of S defect concentration is achieved by controlling the ion energy and sputtering time. Furthermore, characterization with scanning transmission electron microscopy confirms that by keeping the ion energy low, the Mo sublattice remains intact. The properties of MoS2 at different defect concentrations are also characterized. In situ device measurement shows that the flake can be tuned from a semiconducting to metallic-like behavior by introducing S defects due to the creation of mid-gap states. When the defective MoS2 is exposed to air, the S defects are soon passivated, with oxygen atoms filling the defect sites.

Vapour-liquid-solid growth of monolayer MoS2 nanoribbons.

Chemical vapour deposition of two-dimensional materials typically involves the conversion of vapour precursors to solid products in a vapour-solid-solid mode. Here, we report the vapour-liquid-solid growth of monolayer MoS2, yielding highly crystalline ribbons with a width of few tens to thousands of nanometres. This vapour-liquid-solid growth is triggered by the reaction between MoO3 and NaCl, which results in the formation of molten Na-Mo-O droplets. These droplets mediate the growth of MoS2 ribbons in the 'crawling mode' when saturated with sulfur. The locally well-defined orientations of the ribbons reveal the regular horizontal motion of the droplets during growth. Using atomic-resolution scanning transmission electron microscopy and second harmonic generation microscopy, we show that the ribbons are grown homoepitaxially on monolayer MoS2 with predominantly 2H- or 3R-type stacking. Our findings highlight the prospects for the controlled growth of atomically thin nanostructure arrays for nanoelectronic devices and the development of unique mixed-dimensional structures.

Vacuum level dependent photoluminescence in chemical vapor deposition-grown monolayer MoS 2.

The stronger photoluminescence (PL) in chemical vapor deposition (CVD) grown monolayer MoS2 has been attributed to its high crystal quality compared with that in mechanically exfoliated (ME) crystal, which is contrary to the cognition that the ME crystal usually have better crystal quality than that of CVD grown one and it is expected with a better optical quality. In this report, the reason of abnormally strong PL spectra in CVD grown monolayer crystal is systematically investigated by studying the in-situ opto-electrical exploration at various environments for both of CVD and ME samples. High resolution transmission electron microscopy is used to investigate their crystal qualities. The stronger PL in CVD grown crystal is due to the high p-doping effect of adsorbates induced rebalance of exciton/trion emission. The first principle calculations are carried out to explore the interaction between adsorbates in ambient and defects sites in MoS2, which is consistent to the experimental phenomenon and further confirm our proposed mechanisms.

Enhanced light-matter interaction in atomically thin MoS2 coupled with 1D photonic crystal nanocavity.

Engineering the surrounding electromagnetic environment of light emitters by photonic engineering, e.g. photonic crystal cavity, can dramatically enhance its spontaneous emission rate through the Purcell effect. Here we report an enhanced spontaneous emission rate of monolayer molybdenum disulfide (MoS2) by coupling it to a 1D silicon nitride photonic crystal. A four times stronger photoluminescence (PL) intensity of MoS2 in a 1D photonic crystal cavity than un-coupled emission is observed. Considering the relative ease of fabrication and the natural integration with a silicon-based system, the high Purcell factor renders this device as a highly promising platform for applications such as visible solid-state cavity quantum electrodynamics (QED).

Solution Growth of Ultralong Gold Nanohelices.

Metallic nanohelices are extremely rare and, to date, have never been synthesized by a direct solution method. In this work, we report ultralong Au nanohelices grown in solution under ambient conditions. They are ultralong with several tens of micrometers in length, with extraordinary aspect ratio (length/diameter greater than 22 300) and the number of pitches (more than 22 000 pitches). The pitch and width are uniform within each helix but vary widely among the helices. Crystal analyses showed that the facets, twin boundaries, grain sizes, and orientations are aperiodic along the helices. The apparent smooth curving is only possible with a large number of surface steps, suggesting that these structural features are the mere consequence of the helix formation rather than the cause. We propose that the nanowires are formed by the active surface growth mechanism and that the helicity originates from the random and asymmetrical blocking of nuclei embedded within the floccules of ligand complexes, in the form of either asymmetric binding of ligands or asymmetric diffusion of growth materials through the floccules. The separate growth environment of these nuclei causes constant helicity within each helix but differing helicity among the individuals. The embedding also provides a robust environment for the sustained growth of the nanohelices, leading to their record length and consistency.

Gibbs-Thomson Effect in Planar Nanowires: Orientation and Doping Modulated Growth.

Epitaxy-enabled bottom-up synthesis of self-assembled planar nanowires via the vapor-liquid-solid mechanism is an emerging and promising approach toward large-scale direct integration of nanowire-based devices without postgrowth alignment. Here, by examining large assemblies of indium tin oxide nanowires on yttria-stabilized zirconia substrate, we demonstrate for the first time that the growth dynamics of planar nanowires follows a modified version of the Gibbs-Thomson mechanism, which has been known for the past decades to govern the correlations between thermodynamic supersaturation, growth speed, and nanowire morphology. Furthermore, the substrate orientation strongly influences the growth characteristics of epitaxial planar nanowires as opposed to impact at only the initial nucleation stage in the growth of vertical nanowires. The rich nanowire morphology can be described by a surface-energy-dependent growth model within the Gibbs-Thomson framework, which is further modulated by the tin doping concentration. Our experiments also reveal that the cutoff nanowire diameter depends on the substrate orientation and decreases with increasing tin doping concentration. These results enable a deeper understanding and control over the growth of planar nanowires, and the insights will help advance the fabrication of self-assembled nanowire devices.

Photocurrent generation in lateral graphene p-n junction created by electron-beam irradiation.

Graphene has been considered as an attractive material for optoelectronic applications such as photodetectors owing to its extraordinary properties, e.g. broadband absorption and ultrahigh mobility. However, challenges still remain in fundamental and practical aspects of the conventional graphene photodetectors which normally rely on the photoconductive mode of operation which has the drawback of e.g. high dark current. Here, we demonstrated the photovoltaic mode operation in graphene p-n junctions fabricated by a simple but effective electron irradiation method that induces n-type doping in intrinsic p-type graphene. The physical mechanism of the junction formation is owing to the substrate gating effect caused by electron irradiation. Photoresponse was obtained for this type of photodetector because the photoexcited electron-hole pairs can be separated in the graphene p-n junction by the built-in potential. The fabricated graphene p-n junction photodetectors exhibit a high detectivity up to ~3 × 10(10) Jones (cm Hz(1/2) W(-1)) at room temperature, which is on a par with that of the traditional III-V photodetectors. The demonstrated novel and simple scheme for obtaining graphene p-n junctions can be used for other optoelectronic devices such as solar cells and be applied to other two dimensional materials based devices.

A metal-dielectric-graphene sandwich for surface enhanced Raman spectroscopy.

The Raman intensity of Rhodamine B (RhB) is enhanced by inserting a thin high κ dielectric layer which reduces the surface plasmon damping at the gold-graphene interface. The results indicate that the Raman intensity increases sharply by plasmonic resonance enhancement while maintaining efficient fluorescence quenching with optimized dielectric layer thickness.

Epitaxy-enabled vapor-liquid-solid growth of tin-doped indium oxide nanowires with controlled orientations.

Controlling the morphology of nanowires in bottom-up synthesis and assembling them on planar substrates is of tremendous importance for device applications in electronics, photonics, sensing and energy conversion. To date, however, there remain challenges in reliably achieving these goals of orientation-controlled nanowire synthesis and assembly. Here we report that growth of planar, vertical and randomly oriented tin-doped indium oxide (ITO) nanowires can be realized on yttria-stabilized zirconia (YSZ) substrates via the epitaxy-assisted vapor-liquid-solid (VLS) mechanism, by simply regulating the growth conditions, in particular the growth temperature. This robust control on nanowire orientation is facilitated by the small lattice mismatch of 1.6% between ITO and YSZ. Further control of the orientation, symmetry and shape of the nanowires can be achieved by using YSZ substrates with (110) and (111), in addition to (100) surfaces. Based on these insights, we succeed in growing regular arrays of planar ITO nanowires from patterned catalyst nanoparticles. Overall, our discovery of unprecedented orientation control in ITO nanowires advances the general VLS synthesis, providing a robust epitaxy-based approach toward rational synthesis of nanowires.

Polymer pen lithography using dual-elastomer tip arrays.

Dual-elastomer tip arrays are developed as a simple and cost-effective approach to significantly improve the uniformity and precision of polymer pen lithography (PPL). Both experiment and mechanical simulation demonstrate that the hard-apex, soft-base tip structure of the dual-elastomer tip array leads to precise control of feature size and reduced variation among different tips over large areas through fine control of the tip deformation. The dual-elastomer tip array is believed to be readily applied to fabricate nano- and microstructures for fundamental study and applications such as bioassays, sensors, optical and electronic devices.