Characteristics and Electronic Band Alignment of a Transparent p-CuI/n-SiZnSnO Heterojunction Diode with a High Rectification Ratio.

Transparent p-CuI/n-SiZnSnO (SZTO) heterojunction diodes are successfully fabricated by thermal evaporation of a (111) oriented p-CuI polycrystalline film on top of an amorphous n-SZTO film grown by the RF magnetron sputtering method. A nitrogen annealing process reduces ionized impurity scattering dominantly incurred by Cu vacancy and structural defects at the grain boundaries in the CuI film to result in improved diode performance; the current rectification ratio estimated at ±2 V is enhanced from ≈106 to ≈107. Various diode parameters, including ideality factor, reverse saturation current, offset current, series resistance, and parallel resistance, are estimated based on the Shockley diode equation. An energy band diagram exhibiting the type-II band alignment is proposed to explain the diode characteristics. The present p-CuI/n-SZTO diode can be a promising building block for constructing useful optoelectronic components such as a light-emitting diode and a UV photodetector.

Excited-state dynamics of bis(9-fluorenyl)methane and its derivative 9-(9-ethylfluorenyl)-9'-fluorenylmethane: steric effect on energetics and dynamics of ground- and excited-state conformations.

Intramolecular excimer formation of bis(9-fluorenyl)methane (BFM) and 9-(9'-ethylfluorenyl)-9-fluorenylmethane (EFFM), in which an ethyl group is substituted to a 9-H atom in BFM, was studied by means of steady-state and time-resolved fluorescence. Ab initio and DFT calculations enabled the prediction of three conformers as stable species of orthogonal, trans-gauche, and gauche-gauche. The theoretical and experimental results reveal that the substitution effect is also found to appreciably influence the energies, spectroscopy, and kinetics associated with the interconversion of various conformers of the diaryl compounds. We have not observed the rising components in the excimer fluorescence decay of BFM and EFFM in PMMA as observed in the liquid solutions probably because of the existence of the sandwich conformer responsible for the excimer fluorescence prior to the laser irradiation.

Multilayered nano-prism vertex tips for tip-enhanced Raman spectroscopy and imaging.

We presented a scalable fabrication method for the preparation of multilayered nano-prism vertex (NV)-tips whose dimensions can be controlled for tip-enhanced Raman spectroscopy (TERS). The NV-tip had sharp vertices (diameter ~20 nm) originated from the chemical lift-off process after the angle-grinding process, enabling high resolution imaging. TERS measurements were performed on brilliant cresyl blue (BCB) molecules using a Ag/Au NV-tip, revealing the enhanced field localization at the vertices of the NV-tip. Furthermore, we could observe the polarization effect of the NV-tip. Our NV-tips should be a powerful tool for basic research on TERS experiments and SPM applications.

Tuning and maximizing the single-molecule surface-enhanced Raman scattering from DNA-tethered nanodumbbells.

We extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, interparticle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. Two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various interparticle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed interparticle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm interparticle gap, an EF value of as high as 5.9 × 10(13) (average value = 1.8 × 10(13)) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 10(12) and 5.9 × 10(13). In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with < 1 nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed.

Nanotube-bridged wires with sub-10 nm gaps.

We report a simple but efficient method to synthesize carbon nanotube-bridged wires (NBWs) with gaps as small as 5 nm. In this method, we have combined a strategy for assembling carbon nanotubes (CNTs) inside anodized aluminum oxide pores and the on-wire lithography technique to fabricate CNT-bridged wires with gap sizes deliberately tailored over the 5-600 nm range. As a proof-of-concept demonstration of the utility of this architecture, we have prepared NBW-based chemical and biosensors which exhibit higher analyte sensitivity (lower limits of detection) than those based on planar CNT networks. This observation is attributed to a greater surface-to-volume ratio of CNTs in the NBWs than those in the planar CNT devices. Because of the ease of synthesis and high yield of NBWs, this technique may enable the further incorporation of CNT-based architectures into various nanoelectronic and sensor platforms.

A label-free, direct and noncompetitive FRET immunoassay for ochratoxin A based on intrinsic fluorescence of an antigen and antibody complex.

A label-free, direct and noncompetitive homogeneous immunoassay, in which ochratoxin A (OTA) coupled with the anti-OTA antibody participates in fluorescence resonance energy transfer (FRET), was developed for the detection of OTA with great specificity and a detection limit of 1 ng mL(-1).

Highly uniform and reproducible surface-enhanced Raman scattering from DNA-tailorable nanoparticles with 1-nm interior gap.

An ideal surface-enhanced Raman scattering (SERS) nanostructure for sensing and imaging applications should induce a high signal enhancement, generate a reproducible and uniform response, and should be easy to synthesize. Many SERS-active nanostructures have been investigated, but they suffer from poor reproducibility of the SERS-active sites, and the wide distribution of their enhancement factor values results in an unquantifiable SERS signal. Here, we show that DNA on gold nanoparticles facilitates the formation of well-defined gold nanobridged nanogap particles (Au-NNP) that generate a highly stable and reproducible SERS signal. The uniform and hollow gap (∼1 nm) between the gold core and gold shell can be precisely loaded with a quantifiable amount of Raman dyes. SERS signals generated by Au-NNPs showed a linear dependence on probe concentration (R(2) > 0.98) and were sensitive down to 10 fM concentrations. Single-particle nano-Raman mapping analysis revealed that >90% of Au-NNPs had enhancement factors greater than 1.0 × 10(8), which is sufficient for single-molecule detection, and the values were narrowly distributed between 1.0 × 10(8) and 5.0 × 10(9).

Nanogap-engineerable Raman-active nanodumbbells for single-molecule detection.

Surface-enhanced Raman scattering (SERS)-based signal amplification and detection methods using plasmonic nanostructures have been widely investigated for imaging and sensing applications. However, SERS-based molecule detection strategies have not been practically useful because there is no straightforward method to synthesize and characterize highly sensitive SERS-active nanostructures with sufficiently high yield and efficiency, which results in an extremely low cross-section area in Raman sensing. Here, we report a high-yield synthetic method for SERS-active gold-silver core-shell nanodumbbells, where the gap between two nanoparticles and the Raman-dye position and environment can be engineered on the nanoscale. Atomic-force-microscope-correlated nano-Raman measurements of individual dumbbell structures demonstrate that Raman signals can be repeatedly detected from single-DNA-tethered nanodumbbells. These programmed nanostructure fabrication and single-DNA detection strategies open avenues for the high-yield synthesis of optically active smart nanoparticles and structurally reproducible nanostructure-based single-molecule detection and bioassays.

Blinking photoluminescence properties of single TiO2 nanodiscs: interfacial electron transfer dynamics.

Blinking photoluminescence was observed in single TiO2 nanodiscs (NDs) by using a laser scanning confocal microscope (LSCM)-coupled steady-state and ps-time-resolved photoluminescence (PL) spectroscopic system, while it was not significantly observed for TiO2 quantum dots (QDs). Analysis of the PL blinking time trajectories revealed single-exponential kinetics with the average lifetimes of on-state (approximately 286 ms) and off-state (approximately 58 ms), implying the existence of inherent surface-trap sites which can be filled by photogenerated electron or hole. The PL spectra of single TiO2 NDs exhibited broad surface emissions with four decay times, which may be due to diffusion of the energies of electron or hole trap states related to surface structural changes by modification of TiO2 QDs. These results and the surface structural analysis (IR and XPS) suggests a simple model for the PL blinking of single TiO2 NDs that is based on repetitive interfacial electron transfer to the inherent surface trap sites (4Ti4+-OH) with Auger-assisted hole trapping in the multiple surface states as modified by the diffusive coordinate model and the surface-trap-filling model. Based on this blinking mechanism and kinetics, the rates of the interfacial electron transfer and the back electron transfer in TiO2 NDs were determined to be 18 ns and 58 ms, respectively, which are slow enough to keep the polarization of e-h pairs at the surface for efficient photocatalysis and photovoltaic activities. The present methodology and results may be applicable to obtain surface exciton dynamics of various photoelectronic semiconductor nanostructures.