The strong interest in graphene has motivated the scalable production of high-quality graphene and graphene devices. As the large-scale graphene films synthesized so far are typically polycrystalline, it is important to characterize and control grain boundaries, generally believed to degrade graphene quality. Here we study single-crystal graphene grains synthesized by ambient chemical vapour deposition on polycrystalline Cu, and show how individual boundaries between coalescing grains affect graphene's electronic properties. The graphene grains show no definite epitaxial relationship with the Cu substrate, and can cross Cu grain boundaries. The edges of these grains are found to be predominantly parallel to zigzag directions. We show that grain boundaries give a significant Raman 'D' peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. Finally, we demonstrate an approach using pre-patterned growth seeds to control graphene nucleation, opening a route towards scalable fabrication of single-crystal graphene devices without grain boundaries.

Excitons in semiconductors, bound pairs of excited electrons and holes, can form the basis for new classes of quantum optoelectronic devices. A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. Employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate the transport of neutral interlayer excitons across the whole sample that can be controlled by excitation power and gate electrodes. We also realize the drift motion of charged interlayer excitons using Ohmic-contacted devices. The electrical generation and control of excitons provides a new route for realizing quantum manipulation of bosonic composite particles with complete electrical tunability.As bosonic composite particles, long-lived excitons can be potentially utilized for the realization of coherent quantum many-body systems (1, 2) or as quantum information carriers (3,4). In conventional semiconductors, the exciton lifetime can be increased by constructing double quantum well (DQW) heterostructures, where spatially separated electrons and holes form interlayer excitons (IEs) across the quantum wells (5-10). Strongly bound IEs can also be formed in atomically thin DQW. By stacking two

When an optical dipole is in proximity to a metallic substrate, it can emit light into both far field photons and SPPs. Far-field emission can be measured directly via top-down optical microscopy, whereas SPP emission can be detected by converting SPPs into far-field light via engineered out-coupling structures (Fig. 1a). On a single-crystal silver film, our metal of choice due to its low loss 11 , SPPs are strongly polarized in the out-of-plane (z) direction in the visible frequency range (see Supplementary Information). Consequently, the emission rate into SPPs for an out-of-plane dipole can be as high as 30 times larger than that of an in-plane dipole (Figs. 1b-d, for details of the analysis see Supplementary Information). At the same time, far-field emission of an in-plane dipole is strongly suppressed (Figs. 1b and d) because the in-plane electric field is close to zero near the silver surface. We note that when a point dipole is close to a metal 12 , non-radiative recombination due to ohmic loss can be the dominant decay mechanism.Remarkably, for delocalized excitons in quantum wells and 2D materials, quenching of exciton luminescence by ohmic loss is significantly reduced, even when they are placed 10 nm above a silver surface ( see [ 13 ] and Supplementary Fig. 1 and discussion). Combined together, the net effect of a nearby silver surface is significantly enhanced (suppressed) emission of an out-ofplane (in-plane) dipole into SPPs (far field). (Fig. 1a). The spacing between the monolayer TMD and the silver surface is determined by the bottom hBN thickness, and can easily be controlled by varying hBN thickness. In our devices, the typical spacing is on the order of ten nanometers.Excitons are created using off-resonant 660-nm laser excitation, and the PL spectra are voltages. We normalize both FF and SPP-PL spectra using the intensity of a charged exciton peak X T because it is known to involve a purely in-plane transition dipole moment 16 . The ratio of SPP-PL intensity to the FF-PL intensity after the normalization provides a direct measure of the orientation of the transition dipole for each luminescent species: the unity ratio represents a purely in-plane dipole, while a value larger than one indicates that the transition dipole has some out-of-plane components. Based on our theoretical calculations presented in Fig. 1d and Supplementary Fig. 4, an optical transition with a purely out-of-plane transition dipole should have a normalized coupling ratio of 7 in our device geometry. The experimental results for X D yield a value of 16: this discrepancy between theory and experiment is likely due to small, yet non-negligible absorption of SPPs by charged excitons as they propagate through the WSe 2 , which increases the apparent coupling ratio of X D after normalization (see Supplementary Fig. 5). Indeed, when SPPs propagate through a minimal distance within WSe 2 ( Supplementary Fig. 6), the normalized coupling ratio determined by experiment is close to 7, in good agreement with the theoretical calc...

Monolayer MoS, among many other transition metal dichalcogenides, holds great promise for future applications in nanoelectronics and optoelectronics due to its ultrathin nature, flexibility, sizable band gap, and unique spin-valley coupled physics. However, careful study of these properties at low temperature has been hindered by an inability to achieve low-temperature Ohmic contacts to monolayer MoS, particularly at low carrier densities. In this work, we report a new contact scheme that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kΩ.μm at a carrier density of 5.3 × 10/cm. This further allows us to observe Shubnikov-de Haas oscillations in monolayer MoS at much lower carrier densities compared to previous work.

We report a study of graphene and graphene field effect devices after exposure to a series of short pulses of oxygen plasma. We present data from Raman spectroscopy, back-gated field-effect and magneto-transport measurements.The intensity ratio between Raman "D" and "G" peaks, ID/IG (commonly used to characterize disorder in graphene) is observed to increase approximately linearly with the number (Ne) of plasma etching pulses initially, but then decreases at higher Ne. We also discuss implications of our data for extracting graphene crystalline domain sizes from ID/IG. At the highest Ne measured, the "2D" peak is found to be nearly suppressed while the "D" peak is still prominent. Electronic transport measurements in plasma-etched graphene show an up-shifting of the Dirac point, indicating hole doping. We also characterize mobility, quantum Hall states, weak localization and various scattering lengths in a moderately etched sample.Our findings are valuable for understanding the effects of plasma etching on graphene and the physics of disordered graphene through artificially generated defects.

A rational yet scalable solution phase method has been established, for the first time, to obtain n-type Bi(2)Te(3) ultrathin nanowires with an average diameter of 8 nm in high yield (up to 93%). Thermoelectric properties of bulk pellets fabricated by compressing the nanowire powder through spark plasma sintering have been investigated. Compared to the current commercial n-type Bi(2)Te(3)-based bulk materials, our nanowire devices exhibit an enhanced ZT of 0.96 peaked at 380 K due to a significant reduction of thermal conductivity derived from phonon scattering at the nanoscale interfaces in the bulk pellets, which corresponds to a 13% enhancement compared to that of the best n-type commercial Bi(2)Te(2.7)Se(0.3) single crystals (~0.85) and is comparable to the best reported result of n-type Bi(2)Te(2.7)Se(0.3) sample (ZT = 1.04) fabricated by the hot pressing of ball-milled powder. The uniformity and high yield of the nanowires provide a promising route to make significant contributions to the manufacture of nanotechnology-based thermoelectric power generation and solid-state cooling devices with superior performance in a reliable and a reproducible way.

scattering has emerged as a method for overcoming these limitations and for controlling light at the atomic scale (3-11). For instance, highly reflective mirrors based on individual quantum emitters have been demonstrated by coupling them to optical cavities and nanophotonic waveguides (3-8). Such resonant mirrors feature very unusual properties due to their extraordinary nonlinearity down to the single-photon level (3-8). A two-dimensional (2D) layer of emitters, such as atomic lattices or excitons (9-11), has also been predicted to act as an efficient mirror when the incident light is resonant with the resonance frequency of the system. Such atomically thin mirrors represent the ultimate miniaturization limit of a reflective surface, and could enable unique applications ranging from quantum nonlinear optics (9-11) to topological photonics (12,13).This Report demonstrates that transition metal dichalcogenide (TMD) monolayers can act as atomically thin, electrically switchable, resonant mirrors. These materials are direct-bandgap semiconductors that support tightly bound excitons. Excitonic transitions in TMD monolayers exhibit large oscillator strengths (14-16), resulting in large radiative linewidths compared to excitons in other semiconductor systems. In addition, the excitonic response in monolayers can be controlled electrically via gate-induced doping and by shifting the chemical potential (17)(18)(19).Importantly, these monolayers can be easily integrated with other 2D materials via Van der Waals stacking to improve their quality or add new functionalities. One of the most studied amongst such heterostructures is a TMD monolayer encapsulated by two hexagonal boron nitride 4 (hBN) flakes: this "passivated" monolayer exhibits enhanced carrier mobility (19,20) and reduced photoluminescence linewidth (21,22).Our experiments make use of a device that consists of an hBN-passivated molybdenum diselenide (MoSe 2 ) monolayer placed on an oxide-covered silicon (Si) substrate, and we measure its reflectivity with a normally incident laser beam (Figs. 1A and 1B). The doped Si substrate is used as a gate electrode: by applying a gate voltage (V g ), MoSe 2 monolayers can be made intrinsic or n-doped. When a monochromatic laser beam is tuned to the exciton resonance, we observe substantial reflection from a monolayer device (M1) at V g < 10 V at T = 4 K (Fig. 1C).The reflection contrast between the monolayer region and the substrate disappears at V g > 20 V ( Fig. 1D), indicating that the reflection can be turned off electrically. When we illuminate another monolayer device (M2) with a supercontinuum laser and spectrally resolve the reflection, we find that both the magnitudes and wavelength positions of the reflectance peaks change with V g (Fig. 1E). When the monolayer is intrinsic (V g < 10 V), the reflection is dominated by a peak at the wavelength of the neutral exciton transition. When MoSe 2 is n-doped (V g > 20 V), however, the reflection by the neutral exciton disappears, and a new, weaker, reflectance peak ap...

Improving energy/fuel efficiency by converting waste heat into electricity using thermoelectric materials is of great interest due to its simplicity and reliability. However, many thermoelectric materials are composed of either toxic or scarce elements. Here, we report the experimental realization of using nontoxic and abundant copper zinc tin sulfide (CZTS) nanocrystals for potential thermoelectric applications. The CZTS nanocrystals can be synthesized in large quantities from solution phase reaction and compressed into robust bulk pellets through spark plasma sintering and hot press while still maintaining nanoscale grain size inside. Electrical and thermal measurements have been performed from 300 to 700 K to understand the electron and phonon transports. Extra copper doping during the nanocrystal synthesis introduces a significant improvement in the performance.