Electroluminescence of atoms in a graphene nanogap.

Here, we report light emission from single atoms bridging a graphene nanogap that emit bright visible light based on fluorescence of ionized atoms. Oxygen atoms in the gap shows a peak emission wavelength of 569 nm with a full width at half maximum (FWHM) of 208 nm. The energy states produced by these ionized oxygen atoms bridging carbon atoms in the gap also produce a large negative differential resistance (NDR) in the transport across the gap with the highest peak-to-valley current ratio (PVR = 45) and highest peak current density (~90 kA/cm) ever reported in a solid-state tunneling device.

Improving the Optical Quality of MoSe and WS Monolayers with Complete -BN Encapsulation by High-Temperature Annealing.

We improved the optical quality and stability of an exfoliated monolayer (ML) MoSe and chemical vapor deposition (CVD)-grown WS MLs by encapsulating and sealing them with both top and bottom few-layer -BN, as tested by subsequent high-temperature annealing up to 873 K and photoluminescence (PL) measurements. These transition-metal dichalcogenide (TMD) MLs remained stable up to this maximum temperature, as seen visually. After the heating/cooling cycle, the integrated photoluminescence (PL) intensity at 300 K in the MoSe ML was ∼4 times larger than that before heating and that from exciton and trion PL in the analogous WS ML sample was ∼14 times and ∼2.

Stabilization of Chemical-Vapor-Deposition-Grown WS Monolayers at Elevated Temperature with Hexagonal Boron Nitride Encapsulation.

Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS can be stabilized at elevated temperatures by encapsulation with several layer hexagonal boron nitride (BN), but to different degrees in the presence of ambient air, flowing N, and flowing forming gas (95% N, 5% H). The best passivation of WS at elevated temperature occurs for -BN-covered samples with flowing N (after heating to 873 K), as judged by optical microscopy and photoluminescence (PL) intensity after a heating/cooling cycle. Stability is worse for uncovered samples, but best with flowing forming gas.

Passivating 1T'-MoTe multilayers at elevated temperatures by encapsulation.

Several-layer 1T'-MoTe decomposes very little during heating up to ∼550 °C under flowing argon when encapsulated by multilayer hBN, as monitored by Raman scattering and optical microscopy, but largely decomposes at much lower temperatures in incompletely covered and uncovered regions. In covered regions there are small amounts of tellurium product above ∼250 °C.

Forming Nanoparticle Monolayers at Liquid-Air Interfaces by Using Miscible Liquids.

One standard way of forming monolayers (MLs) of nanoparticles (NPs) is to drop-cast a NP dispersion made using one solvent onto a second, immiscible solvent; after this upper solvent evaporates, the NP ML can be transferred to a solid substrate by liftoff. We show that this previously universal use of only immiscible solvent pairs can be relaxed and close-packed, hexagonally ordered NP monolayers can self-assemble at liquid-air interfaces when some miscible solvent pairs are used instead. We demonstrate this by drop-casting an iron oxide NP dispersion in toluene on a dimethyl sulfoxide (DMSO) liquid substrate.

Electronic transport in nanoparticle monolayers sandwiched between graphene electrodes.

Graphene/CdSe nanoparticle monolayer/graphene sandwich structures were fabricated to explore the interactions between these layered materials. Electrical transport across these heterostructures suggests that transport is limited by tunneling through the nanoparticle (NP) ligands but not the NP core itself. Photoconductivity suggests ligands may affect the exciton separation efficiency.

Direct measurement of the thickness-dependent electronic band structure of MoS2 using angle-resolved photoemission spectroscopy.

We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (MoS2). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from Γ to K, for the case of MoS2 having more than one layer, to the case of single-layer MoS2, as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer MoS2 had earlier been predicted to arise from quantum confinement.

Thermoelectric performance of PbSe quantum dot films.

The thermoelectric (TE) performance of films of colloidal lead selenide (PbSe) quantum dots (QDs) with metal-chalcogenide complex ligands is seen to change with QD size and temperature. Films of smaller QDs have higher Seebeck coefficient magnitudes, indicating stronger quantum confinement, and lower electrical and thermal conductivities. The thermoelectric figure of merit ZT is ∼0.

Resolving the growth of 3D colloidal nanoparticle superlattices by real-time small-angle X-ray scattering.

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and in situ, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant.

Reducing strain and fracture of electrophoretically deposited CdSe nanocrystal films. II. Postdeposition infusion of monomers.

Thick electrophoretically deposited (EPD) films of ligand-capped colloidal nanocrystals (NCs) typically crack when removed from the deposition solvent due to the loss of residual solvent. We report the suppression of fracture in several micrometers thick EPD films of CdSe NCs by treating the wet, as-deposited films with solutions of polymer precursor monomers, followed by UV-initiated polymerization. The monomers diffuse into voids and, for several monomers, dissolve the NCs to form a uniform dispersion in the film.

Reducing strain and fracture of electrophoretically deposited CdSe nanocrystal films. I. Postdeposition infusion of capping ligands.

Thick electrophoretically deposited (EPD) films of ligand-capped colloidal nanocrystals that adhere to the substrate typically crack after they are removed from the deposition solvent due to the loss of residual solvent. We report the suppression of fracture in several micrometers thick EPD films of CdSe nanocrystals by treating the wet, as-deposited films with solutions containing the NC core-capping ligand, trioctylphosphine oxide (TOPO). The increase in TOPO ligand density increases photoluminescence of the dried film and leads to a decrease in elastic modulus.

Raman analysis of mode softening in nanoparticle CeO(2-δ) and Au-CeO(2-δ) during CO oxidation.

Oxygen vacancy levels are monitored during the oxidation of CO by CeO(2-δ) nanorods and Au-CeO(2-δ) nanorods, nanocubes, and nanopolyhedra by using Raman scattering. The first-order CeO(2) F(2g) peak near 460 cm(-1) decreases when this reaction is fast (fast reduction and relatively slow reoxidation of the surface), because of the lattice expansion that occurs when Ce(3+) replaces Ce(4+) during oxygen vacancy creation. This shift correlates with reactivity for CO oxidation.

Formation of thick, large-area nanoparticle superlattices in lithographically defined geometries.

Superlattices of colloidal nanocrystals hold the promise of new nanomaterials with tunable properties. The positioning and size of these structures are often poorly controlled after self-assembly from the solution phase, making studies of their properties difficult. We report the fabrication of approximately 100 layer thick, three-dimensional superlattices on a substrate with controlled lateral placement.

Versatile formation of CdSe nanoparticle-single walled carbon nanotube hybrid structures.

Pyridine-protected CdSe nanoparticles decorated SWNTs spontaneously, producing a large loading of CdSe nanoparticles on the SWNTs. The absorption spectrum of this hybrid material reflects those of the components. CdSe nanoparticles of different diameters, core-shell nanoparticles, and nanorods were shown to decorate SWNTs this way, showing the versatility of this technique.

Viscoplastic and granular behavior in films of colloidal nanocrystals.

Nanoindentation measurements of electrophoretically deposited films of colloidal CdSe nanocrystals, capped by organic ligands, show the films have an elastic stiffness modulus of approximately 10 GPa and exhibit viscoplasticity. This mechanical response suggests polymeric features that are attributable to the ligands. After particle cross-linking and partial ligand removal, the films exhibit more features of granularity.

Exciton polarizability in semiconductor nanocrystals.

The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.

Raman microprobe analysis of elastic strain and fracture in electrophoretically deposited CdSe nanocrystal films.

The mechanical stability of nanocrystal films is critical for applications, yet largely unexplored. Raman microprobe analysis used here to probe the nanocrystal cores of thick, fractured electrophoretically deposited films of 3.2 nm diameter CdSe nanocrystals measures approximately 2.

Optical diagnostics for thin film processing.

Optical diagnostics are used to probe the plasma or neutral gas above the substrate, particles in the gas or on the surface, the film surface and reactor walls, the film itself, and the substrate during thin film processing. The development and application of optical probes are highlighted, in particular for analyzing plasma/gas phase intermediates and products and film composition, and performing metrology, thermometry, and endpoint detection and control. Probing etching (particularly plasma etching) and deposition (particularly epitaxy) are emphasized.