Cleva W. Ow-Yang

Interfacial stability of an indium tin oxide thin film deposited on Si and Si0.85Ge0.15

The stability of the interface formed by depositing indium tin oxide (ITO) thin films on Si and Si0.85Ge0.15 substrates was investigated. Cross-sectional transmission electron microscopy combined with Fourier-transform infrared spectroscopy, energy dispersive x-ray analysis, x-ray diffraction, and capacitance–voltage measurements were used to characterize the interface immediately after rf magnetron sputter deposition as a function of annealing time in ultrahigh purity (UHP) N2 at 785 °C for 10–80 min. The In–Si–O2 ternary phase equilibrium diagram was calculated to predict the possible product layer sequences. A 2-nm-thick interfacial amorphous silicon oxide, associated with ion implantation intermixing, is present in the ITO/Si as-deposited sample, while a 3–7-nm-thick amorphous oxide interlayer is observed in the ITO/Si0.85Ge0.15 sample. During annealing in UHP N2, the interlayer oxide growth rate follows the initial stage of conventional oxidation. In the ITO/Si system, experimental observations revea...

Unidirectional broadband radiation of honeycomb plasmonic antenna array with broken symmetry

Emerging plasmonic and photovoltaic applications benefit from effective interaction between optical antennas and unidirectional incident light over a wide spectrum. Here, we propose a honeycomb array of plasmonic nanoantennas with broken symmetry to obtain a unidirectional radiation pattern over a wide spectrum. The honeycomb nanoantenna array is based on a hexagonal grid with periodically arranged nanostructure building blocks. To analyze the far-field optical distribution and spectral behavior of the plasmonic antenna honeycomb, a two-dimensional Wigner-Seitz unit cell is used together with periodic boundary conditions. As a result of the vectoral superposition of the fields produced by the Wigner-Seitz unit cells, far-zone optical fields interfere constructively or destructively in different directions. The constructive interference along the arrays normal direction engenders unidirectional radiation. Due to the broken symmetry of the Wigner-Seitz cell, multiple resonances are supported by the plasmonic antenna honeycomb array over a broad spectrum.

Parametric fluorescence in oxidized AlGaAs waveguides

Parametric fluorescence at 2.1 µm is observed in oxidized, form-birefringent AlGaAs waveguides. The signal level measured is approximately 15 pW, over the range of 1.9–2.5 µm. The possibility of realizing an optical parametric oscillator is discussed, and reduction of the internal losses is shown to require optimization of the oxidation process.

Solution-Processed LiF for Work Function Tuning in Electrode Bilayers

Although ambient processing is the key to low-cost organic solar cell production, high-vacuum thermal evaporation of LiF is often a limiting step, motivating the exploration of solution processing of LiF as an alternative electrode interlayer. Submonolayer films are realized with the assistance of polymeric micelle reactors that enable LiF particle deposition with controlled nanoscale surface coverage. Scanning Kelvin probe reveals a work function tunable with nanoparticle coverage with higher values than that of bare indium tin oxide (ITO).

Solution processed LiF anode modification for polymer solar cells

The indium-tin-oxide/active layer interface is critical to the performance of organic solar cell devices. In this study, submonolayer films of LiF nanoparticles are deposited on the electrode surface with the assistance of polymeric micelle reactors, with controlled nanoscale surface coverage. Incorporation of the solution-processed bi-layer electrodes into a conventional poly(3-hexyl-thiophene): [6,6]-phenyl C61-butyric acid methyl ester device shows significant improvement in device performance, especially when used in combination with a poly(3,4-ethylenedioxythiophene: poly(styrene sulfonate) layer. The nearly 5× improvement in the short circuit current and decrease in the contact resistance is mostly likely related to the increase in surface work function from the use of LiF nanoparticles. The results strongly indicate that engineering of the interfaces is a useful tool for future device optimization.

Determining the morphology of polystyrene-block-poly(2-vinylpyridine) micellar reactors for ZnO nanoparticle synthesis

We report the use of reverse PS-b-P2VP diblock copolymer micelles as true nanoscale-sized reactor vessels to synthesize ZnO nanoparticles. The reverse micelles were formed in toluene and then sequentially loaded with zinc acetate dihydrate and tetramethylammonium hydroxide reactants. Moreover, high spatial resolution Z-contrast imaging and EDX spectroscopy techniques were used to confirm the segregation of the Zn cation to the core of the loaded micelles. Determining the chemical distribution with high nanoscale spatial resolution is shown to complement the less direct characterization by AFM, DLS and FTIR, thus demonstrating broader implications for the characterization of hybrid nanocomposite systems.

Nanoscale composition mapping of segregation in micelles with tapping mode atomic force microscopy

Under energy-dissipative cantilevered tip-sample interaction, phase imaging using tapping-mode atomic force microscopy enables compositional mapping of composites containing a harder inorganic phase at the nanometer scale, embedded in a polymer matrix. The contrast in the phase images is shown to be dependent on the variation in the elastic properties of the diblock copolymer reverse micelles loaded with zinc acetate. Tapping conditions are also shown to determine whether the contrast is positive or negative for the harder core of the loaded micelles, based on the competition between attractive and repulsive tip-sample interaction forces. The broader implications are significant for scanning probe microscopy of other soft materials systems containing the segregation of a harder phase.

Nanoscale elastic modulus variation in loaded polymeric micelle reactors

Tapping mode atomic force microscopy (TM-AFM) enables mapping of chemical composition at the nanoscale by taking advantage of the variation in phase angle shift arising from an embedded second phase. We demonstrate that phase contrast can be attributed to the variation in elastic modulus during the imaging of zinc acetate (ZnAc)-loaded reverse polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock co-polymer micelles less than 100 nm in diameter. Three sample configurations were characterized: (i) a 31.6 μm thick polystyrene (PS) support film for eliminating the substrate contribution, (ii) an unfilled PS-b-P2VP micelle supported by the same PS film, and (iii) a ZnAc-loaded PS-b-P2VP micelle supported by the same PS film. Force-indentation (F-I) curves were measured over unloaded micelles on the PS film and over loaded micelles on the PS film, using standard tapping mode probes of three different spring constants, the same cantilevers used for imaging of the samples before and after loading. For calibration of the tip geometry, nanoindentation was performed on the bare PS film. The resulting elastic modulus values extracted by applying the Hertz model were 8.26 ± 3.43 GPa over the loaded micelles and 4.17 ± 1.65 GPa over the unloaded micelles, confirming that phase contrast images of a monolayer of loaded micelles represent maps of the nanoscale chemical and mechanical variation. By calibrating the tip geometry indirectly using a known soft material, we are able to use the same standard tapping mode cantilevers for both imaging and indentation.

Stokes-Shift-Engineered Indium Phosphide Quantum Dots for Efficient Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) show promise because of their potential for low-cost, large-area, and high-efficiency energy harvesting. Stokes shift engineering of luminescent quantum dots (QDs) is a favorable approach to suppress reabsorption losses in LSCs; however, the use of highly toxic heavy metals in QDs constitutes a serious concern for environmental sustainability. Here, we report LSCs based on cadmium-free InP/ZnO core/shell QDs with type-II band alignment that allow for the suppression of reabsorption by Stokes shift engineering. The spectral emission and absorption overlap was controlled by the growth of a ZnO shell on an InP core. At the same time, the ZnO layer also facilitates the photostability of the QDs within the host matrix. We analyzed the optical performance of indium-based LSCs and identified the optical efficiency as 1.45%. The transparency, flexibility, and cadmium-free content of the LSCs hold promise for solar window applications.

Conformation-mediated Förster resonance energy transfer (FRET) in blue-emitting polyvinylpyrrolidone (PVP)-passivated zinc oxide (ZnO) nanoparticles

Homopolymers, such as polyvinylpyrrolidone (PVP), are commonly used to passivate the surface of blue-light emitting ZnO nanoparticles during colloid nucleation and growth. However, although PVP is known to auto-fluoresce at 400nm, which is near the absorption edge of ZnO, the impact of PVP adsorption characteristics on the surface of ZnO and the surface-related photophysics of PVP-capped ZnO nanoparticles is not well understood. To investigate, we have synthesized ZnO nanoparticles in solvents containing PVP of 3 concentrations-0.5, 0.7, and 0.11gmL-1. Using time-domain NMR, we show that the adsorbed polymer conformation differs with polymer concentration-head-to-tail under low concentration (e.g., 0.05gmL-1) and looping, then train-like, with increasing concentration (e.g., 0.07gmL-1 and 0.11gmL-1, respectively). When the surface-adsorbed PVP is entrained, the surface states of ZnO are passivated and radiative emission from surface trap states is suppressed, allowing emission to be dominated by exciton transitions in the UV (ca. 310nm). Moreover, the reduced proximity between the PVP molecule and the ZnO gives rise to increased efficiency of energy transfer between the exciton emission of ZnO and the HOMO-LUMO absorption of PVP (ca. 400nm). As a result, light emission in the blue is enhanced in the PVP-capped ZnO nanoparticles. We thus show that the emission properties of ZnO can be tuned by controlling the adsorbed PVP conformation on the ZnO surface via the PVP concentration in the ZnO precipitation medium.