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/cm2) ever reported in a solid-state tunneling device. While tunneling transport has been previously observed in graphene nanogaps, the bridging of ionized oxygen observed here shows a low excess current, leading to the observed PVR. On the basis of the highly reproducible light emission and NDR from these structures, we demonstrate a 65,536-pixel light-emitting nanogap array.

Improving the Optical Quality of MoSe2 and WS2 Monolayers with Complete h-BN Encapsulation by High-Temperature Annealing.

We improved the optical quality and stability of an exfoliated monolayer (ML) MoSe2 and chemical vapor deposition (CVD)-grown WS2 MLs by encapsulating and sealing them with both top and bottom few-layer h-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 MoSe2 ML was ∼4 times larger than that before heating and that from exciton and trion PL in the analogous WS2 ML sample was ∼14 times and ∼2.5 times larger at 77 K and the exciton peak was ∼9.5 times larger at 300 K. This is attributed to the reduction of impurities, the lateral expulsion of contamination leading to clean and atomically flat surfaces, and the sealing provided by the h-BN layers that prevents the diffusion of molecules such as trace O2 and H2O to the TMD ML. Stability and optical performance are much improved compared to that in earlier work using top h-BN only, in which the WS2 ML PL intensity decreased even for an optimal gas environment. This complete encapsulation is particularly promising for CVD-grown TMD MLs because they have relatively more charge and other impurities than do exfoliated MLs. These results open a new route for improving the optical properties of TMD MLs and their performance and applications both at room and higher temperatures.

Stabilization of Chemical-Vapor-Deposition-Grown WS2 Monolayers at Elevated Temperature with Hexagonal Boron Nitride Encapsulation.

Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS2 can be stabilized at elevated temperatures by encapsulation with several layer hexagonal boron nitride (h-BN), but to different degrees in the presence of ambient air, flowing N2, and flowing forming gas (95% N2, 5% H2). The best passivation of WS2 at elevated temperature occurs for h-BN-covered samples with flowing N2 (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. PL from trions, in addition to that from excitons, is seen for covered WS2 only for forming gas, during cooling below ∼323 K; the trion has an estimated binding energy of ∼28 meV. It might occur because of doping level changes caused by charge defect generation by H2 molecules diffusing between the h-BN and the SiO2/Si substrate. The decomposition of uncovered WS2 flakes in air suggests a dissociation and chemisorption energy barrier of O2 on the WS2 surface of ∼1.6 eV. Fitting the high-temperature PL intensities in air gives a binding energy of a free exciton of ∼229 meV.

Passivating 1T'-MoTe2 multilayers at elevated temperatures by encapsulation.

Several-layer 1T'-MoTe2 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. The NPs are energetically stable at the DMSO surface and remain there even with solvent mixing. Excess NPs coagulate and precipitate in the DMSO, and this limits NPs at the surface to approximately 1 ML. The ML domains at the surface nucleate independently, which is in contrast to ML growth at the receding edge of the drying drop, as is common in immiscible solvent pair systems and seen here for the toluene/diethylene glycol immiscible solvent pair system. This new use of miscible solvent pairs can enable the formation of MLs for a wider range of NPs.

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.

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 ∼0.5 at room temperature and increases with temperature to 1.0-1.37 at ∼400 K, where it is larger for smaller QD films. This is better than previous results for solution-prepared QD TE materials at these elevated temperatures.

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.

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.

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. Furthermore, one of the consequences of this progression in the electronic structure is the dramatic increase in the hole effective mass, in going from bulk to monolayer MoS2 at its Brillouin zone center, which is known as the cause for the decreased carrier mobility of the monolayer form compared to that of bulk MoS2.

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. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion.

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. Increases in the oxygen deficit δ as large as ~0.04 are measured relative to conditions when the ceria is not reduced.

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. This novel fabrication technique generates long-range order over the micrometer scale and controlled placement by employing lithographic patterning and microfluidic flow.

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 (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.

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 2.5% in-plane tensile strain in cores of unfractured films. The crack dimensions determine the overall in-plane film strain, approximately 11.7%, and the film biaxial modulus, approximately 13.8 GPa, from which the biaxial modulus of the trioctylphosphine oxide ligand matrix is inferred, approximately 5.1 GPa.

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.