Shiwei Wu

Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals

The development of probes for single-molecule imaging has dramatically facilitated the study of individual molecules in cells and other complex environments. Single-molecule probes ideally exhibit good brightness, uninterrupted emission, resistance to photobleaching, and minimal spectral overlap with cellular autofluorescence. However, most single-molecule probes are imperfect in several of these aspects, and none have been shown to possess all of these characteristics. Here we show that individual lanthanide-doped upconverting nanoparticles (UCNPs)—specifically, hexagonal phase NaYF4 (β-NaYF4) nanocrystals with multiple Yb3+ and Er3+ dopants—emit bright anti-Stokes visible upconverted luminescence with exceptional photostability when excited by a 980-nm continuous wave laser. Individual UCNPs exhibit no on/off emission behavior, or “blinking,” down to the millisecond timescale, and no loss of intensity following an hour of continuous excitation. Amphiphilic polymer coatings permit the transfer of hydrophobic UCNPs into water, resulting in individual water-soluble nanoparticles with undiminished photophysical characteristics. These UCNPs are endocytosed by cells and show strong upconverted luminescence, with no measurable anti-Stokes background autofluorescence, suggesting that UCNPs are ideally suited for single-molecule imaging experiments.

Formation of Bandgap and Subbands in Graphene Nanomeshes with Sub-10 nm Ribbon Width Fabricated via Nanoimprint Lithography

We fabricated hexagonal graphene nanomeshes (GNMs) with sub-10 nm ribbon width. The fabrication combines nanoimprint lithography, block-copolymer self-assembly for high-resolution nanoimprint template patterning, and electrostatic printing of graphene. Graphene field-effect transistors (GFETs) made from GNMs exhibit very different electronic characteristics in comparison with unpatterned GFETs even at room temperature. We observed multiplateaus in the drain current-gate voltage dependence as well as an enhancement of ON/OFF current ratio with reduction of the average ribbon width of GNMs. These effects are attributed to the formation of electronic subbands and a bandgap in GNMs. Such mesoscopic graphene structures and the nanofabrication methods could be employed to construct future electronic devices based on graphene superlattices.

Surface Plasmon‐Enhanced Photodetection in Few Layer MoS2 Phototransistors with Au Nanostructure Arrays

2D Molybdenum disulfide (MoS2 ) is a promising candidate material for high-speed and flexible optoelectronic devices, but only with low photoresponsivity. Here, a large enhancement of photocurrent response is obtained by coupling few-layer MoS2 with Au plasmonic nanostructure arrays. Au nanoparticles or nanoplates placed onto few-layer MoS2 surface can enhance the local optical field in the MoS2 layer, due to the localized surface plasmon (LSP) resonance. After depositing 4 nm thick Au nanoparticles sparsely onto few-layer MoS2 phototransistors, a doubled increase in the photocurrent response is observed. The photocurrent of few-layer MoS2 phototransistors exhibits a threefold enhancement with periodic Au nanoarrays. The simulated optical field distribution confirms that light can be trapped and enhanced near the Au nanoplates. These findings offer an avenue for practical applications of high performance MoS2 -based optoelectronic devices or systems in the future.

Valley and band structure engineering of folded MoS2 bilayers

Artificial structures made of stacked two-dimensional crystals have recently been the focus of intense research activity. As in twisted or stacked graphene layers, these structures can show unusual behaviours and new phenomena. Among the various layered compounds that can be exfoliated, transition-metal dichalcogenides exhibit interesting properties governed by their structural symmetry and interlayer coupling, which are highly susceptible to stacking. Here, we obtain-by folding exfoliated MoS2 monolayers-MoS2 bilayers with different stacking orders, as monitored by second harmonic generation and photoluminescence. Appropriate folding can break the inversion symmetry and suppress interlayer hopping, evoking strong valley and spin polarizations that are not achieved in natural MoS2 bilayers of Bernal stacking. It can also enlarge the indirect bandgap by more than 100 meV through a decrease in the interlayer coupling. Our work provides an effective and versatile means to engineer transition-metal dichalcogenide materials with desirable electronic and optical properties.

Manipulating nanoscale light fields with the asymmetric bowtie nano-colorsorter.

We present a class of devices called Asymmetric Bowtie nano-Colorsorters. These devices have specifically engineered symmetries enabling them to capture, confine, spectrally filter and steer optical fields while maintaining nanoscale field distributions.

High‐Responsivity Graphene/InAs Nanowire Heterojunction Near‐Infrared Photodetectors with Distinct Photocurrent On/Off Ratios

Graphene is a promising candidate material for high-speed and ultra-broadband photodetectors. However, graphene-based photodetectors suffer from low photoreponsivity and I(light)/I(dark) ratios due to their negligible-gap nature and small optical absorption. Here, a new type of graphene/InAs nanowire (NW) vertically stacked heterojunction infrared photodetector is reported, with a large photoresponsivity of 0.5 AW(-1) and I(light)/I(dark) ratio of 5 × 10(2), while the photoresponsivity and I(light)/I(dark) ratio of graphene infrared photodetectors are 0.1 mAW(-1) and 1, respectively. The Fermi level (E(F)) of graphene can be widely tuned by the gate voltage owing to its 2D nature. As a result, the back-gated bias can modulate the Schottky barrier (SB) height at the interface between graphene and InAs NWs. Simulations further demonstrate the rectification behavior of graphene/InAs NW heterojunctions and the tunable SB controls charge transport across the vertically stacked heterostructure. The results address key challenges for graphene-based infrared detectors, and are promising for the development of graphene electronic and optoelectronic applications.

Anomalous and Highly Efficient InAs Nanowire Phototransistors Based on Majority Carrier Transport at Room Temperature

Core/shell-like n-type InAs nanowire phototransistors based on majority-carrier-dominated photodetection are investigated. Under optical illumination, electrons generated from the core are excited into the self-assembled near-surface photogating layer, forming a built-in electric field to significantly regulate the core conductance. Anomalous high photoconductive gain and fast response time are obtained at room temperature.

Strong Second-Harmonic Generation in Atomic Layered GaSe.

Nonlinear effects in two-dimensional (2D) atomic layered materials have recently attracted increasing interest. Phenomena such as nonlinear optical edge response, chiral electroluminescence, and valley and spin currents beyond linear orders have opened up a great opportunity to expand the functionalities and potential applications of 2D materials. Here we report the first observation of strong optical second-harmonic generation (SHG) in monolayer GaSe under nonresonant excitation and emission condition. Our experiments show that the nonresonant SHG intensity of GaSe is the strongest among all the 2D atomic crystals measured up to day. At the excitation wavelength of 1600 nm, the SHG signal from monolayer GaSe is around 1-2 orders of magnitude larger than that from monolayer MoS2 under the same excitation power. Such a strong nonlinear signal facilitates the use of polarization-dependent SHG intensity and SHG mapping to investigate the symmetry properties of this material: the monolayer GaSe shows 3-fold lattice symmetry with an intrinsic correspondence to its geometric triangular shape in our growth condition; whereas the bilayer GaSe exhibits two dominant stacking orders: AA and AB stacking. The correlation between the stacking orders and the interlayer twist angles in GaSe bilayer indicates that different triangular GaSe atomic layers have the same dominant edge configuration. Our results provide a route toward exploring the structural information and the possibility to observe other nonlinear effects in GaSe atomic layers.

Kinetic Nature of Grain Boundary Formation in As‐Grown MoS2 Monolayers

Grain boundaries in as-grown polycrystalline MoS2 monolayers are revealed by second-harmonic-generation microscopy. Through the anisotropic polarization pattern and phase interference at the grain boundary, grain edge termination and boundary types are identified. Statistical analysis on hundreds of grains shows that grain-boundary formation is driven by kinetics and can be nicely described by the edge attachment growth model.

Two-photon-induced hot-electron transfer to a single molecule in a scanning tunneling microscope

The junction of a scanning tunneling microscope (STM) operating in the tunneling regime was irradiated with femtosecond laser pulses. A photoexcited hot electron in the STM tip resonantly tunnels into an excited state of a single molecule on the surface, converting it from the neutral to the anion. The electron-transfer rate depends quadratically on the incident laser power, suggesting a two-photon excitation process. This nonlinear optical process is further confirmed by the polarization measurement. Spatial dependence of the electron-transfer rate exhibits atomic-scale variations. A two-pulse correlation experiment reveals the ultrafast dynamic nature of photoinduced charging process in the STM junction. Results from these experiments are important for understanding photoinduced interfacial charge transfer in many nanoscale inorganic-organic structures.