Shangjr Gwo

Plasmonic Nanolaser Using Epitaxially Grown Silver Film

Going Green with Nanophotonics Plasmons are optically induced collective electronic excitations tightly confined to the surface of a metal, with silver being the metal of choice. The subwavelength confinement offers the opportunity to shrink optoelectronic circuits to the nanometer scale. However, scattering processes within the metal lead to losses. Lu et al. (p. 450) developed a process to produce atomically smooth layers of silver, epitaxially grown on silicon substrates. A cavity in the silver layer is capped with a SiO insulating layer and an AlGaN nanorod was used to produce a low-threshold emission at green wavelengths. An atomically smooth layer of silver enhances the performance of nanophotonic devices. A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.

Green, yellow, and orange defect emission from ZnO nanostructures: Influence of excitation wavelength

ZnO commonly exhibits luminescence in the visible spectral range due to different intrinsic defects. In order to study defect emissions, photoluminescence from ZnO nanostructures prepared by different methods (needles, rods, shells) was measured as a function of excitation wavelength and temperature. Under excitation at 325nm, needles exhibited orange-red defect emission, rods exhibited yellow defect emission, while shells exhibited green defect emission. Obvious color change from orange to green was observed for needles with increasing excitation wavelengths, while nanorods (yellow) showed smaller wavelength shift and shells (green) showed no significant spectral shift. Reasons for different wavelength dependences are discussed.

InGaN/GaN nanorod array white light-emitting diode

Conventional InGaN/GaN light-emitting diodes based on planar quantum wellstructures do not allow for efficient long-wavelength operation beyond the blue region due to a strong quantum confined Stark effect in lattice-mismatched polar InGaNquantum wells. Here we overcome the limitation by using self-assembled GaNnanorod arrays as strain-free growth templates for thick InGaN nanodisks. In combination with enhanced carrier localization and high crystalline quality, this approach allows us to realize full-color InGaN nanodisk emitters. By tailoring the numbers, positions, and thicknesses of polychromatic nanodisk ensembles embedded vertically in the GaNnanorod p - n junction, we are able to demonstrate natural white (color temperature ∼ 6000 K ) electroluminescence from InGaN/GaN nanorod arrays.

Facet-Dependent and Au Nanocrystal-Enhanced Electrical and Photocatalytic Properties of Au-Cu2O Core-Shell Heterostructures

We report highly facet-dependent electrical properties of Cu(2)O nanocubes and octahedra and significant enhancement of gold nanocrystal cores to the electrical conductivity of Au-Cu(2)O core-shell octahedra. Cu(2)O nanocubes and octahedra and Au-Cu(2)O core-shell cubes and octahedra were synthesized by following our reported facile procedures at room temperature. Two oxide-free tungsten probes attached to a nanomanipulator installed inside a scanning electron microscope made contacts to a single Cu(2)O nanocrystal for the I-V measurements. Pristine Cu(2)O octahedra bounded by {111} facets are 1100 times more conductive than pristine Cu(2)O cubes enclosed by {100} faces, which are barely conductive. Core-shell cubes are only slightly more conductive than pristine cubes. A 10,000-fold increase in conductivity over a cube has been recorded for an octahedron. Remarkably, core-shell octahedra are far more conductive than pristine octahedra. The same facet-dependent electrical behavior can still be observed on a single nanocrystal exposing both {111} and {100} facets. This new fundamental property may be observable in other semiconductor nanocrystals. We also have shown that both core-shell cubes and octahedra outperform pristine cubes and octahedra in the photodegradation of methyl orange. Efficient photoinduced charge separation is attributed to this enhanced photocatalytic activity. Interestingly, facet-selective etching occurred over the {100} corners of some octahedra and core-shell octahedra during photocatalysis. The successful preparation of Au-Cu(2)O core-shell heterostructures with precise shape control has offered opportunities to discover new and exciting physical and chemical properties of nanocrystals.

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission.

Inorganic Gyroid with Exceptionally Low Refractive Index from Block Copolymer Templating

Nanoporous polymers with gyroid nanochannels can be fabricated from the self-assembly of degradable block copolymer, polystyrene-b-poly(l-lactide) (PS-PLLA), followed by the hydrolysis of PLLA blocks. A well-defined nanohybrid material with SiO2 gyroid nanostructure in a PS matrix can be obtained using the nanoporous PS as a template for sol-gel reaction. After subsequent UV degradation of the PS matrix, a highly porous inorganic gyroid network remains, yielding a single-component material with an exceptionally low refractive index (as low as 1.1).

Structure and photoluminescence properties of epitaxially oriented gan nanorods grown on si(111) by plasma-assisted molecular-beam epitaxy

The authors show that vertically c-axis-aligned GaN nanorod arrays grown by plasma-assisted molecular-beam epitaxy are epitaxially oriented on Si(111) substrates and their crystal structure corresponds to a fully relaxed wurtzite lattice. At later growth stage, these GaN nanorods exhibit the tendency to coalesce into nanorod bundles. Low-temperature photoluminescence spectrum from 1-μm-long GaN nanorods consists of intense exciton lines of strain-free bulk GaN and additional lines at ∼3.21 and ∼3.42eV (Y7 and Y2). The Y7 line is attributed to the excitons trapped along the dislocations at the boundaries of coalesced GaN nanorods, while the Y2 line has its origin in the interface defects at the GaN∕Si(111) interfaces.

The influence of shell thickness of Au@TiO2 core–shell nanoparticles on the plasmonic enhancement effect in dye-sensitized solar cells

Plasmonic core-shell nanoparticles (PCSNPs) can function as nanoantennas and improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve maximum enhancement, the morphology of PCSNPs needs to be optimized. Here we precisely control the morphology of Au@TiO2 PCSNPs and systematically study its influence on the plasmonic enhancement effect. The enhancement mechanism was found to vary with the thickness of the TiO2 shell. PCSNPs with a thinner shell mainly enhance the current, whereas particles with a thicker shell improve the voltage. While pronounced plasmonic enhancement was found in the near infrared regime, wavelength-independent enhancement in the visible range was observed and attributed to the plasmonic heating effect. Emission lifetime measurement confirms that N719 molecules neighboring nanoparticles with TiO2 shells exhibit a longer lifetime than those in contact with metal cores. Overall, PCSNPs with a 5 nm shell give the highest efficiency enhancement of 23%. Our work provides a new synthesis route for well-controlled Au@TiO2 core-shell nanoparticles and gains insight into the plasmonic enhancement in DSSCs.

Layer-by-Layer Assembly of Three-Dimensional Colloidal Supercrystals with Tunable Plasmonic Properties

We present a simple and efficient method for synthesizing large-area (>1 cm(2)), three-dimensional (3D) gold and silver nanoparticle supercrystal films. In this approach, Janus nanoparticle (top face solvent-phobic and bottom face solvent-philic) films with an arbitrary number of close-packed nanoparticle monolayers can be formed using layer-by-layer (LbL) assembly from suspensions of thiolate-passivated gold or silver colloids. Furthermore, we demonstrate that these films can act as true 3D plasmonic crystals with strong transverse (intralayer) and longitudinal (interlayer) near-field coupling. In contrast to conventional polyelectrolyte-mediated LbL assembly processes, this approach allows multiple longitudinal coupling modes with a conspicuous spectral dependence on the layer number. We have found a universal scaling relation between the spectral position of the reflectance dips related to the longitudinal modes and the layer number. This relation can be understood in terms of the presence of a plasmonic Fabry-Perot nanocavity along the longitudinal direction that allows the formation of standing plasmon waves under plasmon resonance conditions. The realization of 3D plasmonic coupling enables broadband tuning of the collective plasmon response over a wide spectral range (visible and near-IR) and provides a pathway to designer plasmonic metamaterials.

NANOMACHINING OF (110)-ORIENTED SILICON BY SCANNING PROBE LITHOGRAPHY AND ANISOTROPIC WET ETCHING

We have demonstrated that silicon nanostructures with high aspect ratios, having ∼400 nm structural height and ∼55 nm lateral dimension, may be fabricated by scanning probe lithography and aqueous KOH orientation-dependent etching on the H-passivated (110) Si wafer. The high spatial resolution of fabricated features is achieved by using the atomic force microscope based nano-oxidation process in ambient. Due to the large (110)/(111) anisotropic ratio of etch rate and the large Si/SiO2 etch selectivity at a relatively low etching temperature and an optimal KOH concentration, high-aspect-ratio gratings with (111)-oriented structural sidewalls as well as hexagonal etch pit structures determined by the terminal etch geometry can be obtained.