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Quantum confinement in nanostructured silicon can lead to efficient light emission. However, the photoluminescence (PL) lifetimes in nanostructured silicon are typically very long−approximately 3 orders of magnitude longer than those of direct band gap semiconductors. Herein, we show that organic monolayer coated silicon nanocrystals ranging from 1 to 10 nm in diameter emit with nanosecond-scale lifetimes and high quantum yields, making it possible to measure the PL spectra of single Si quantum dots. The Si quantum dots demonstrate stochastic single-step "blinking" behavior and size-dependent PL spectra with line widths approximately only three times greater than those measured for CdSe nanocrystals at room temperature.

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A self-powered temperature sensor based on Seebeck effect transduction was designed for photothermal−thermoelectric coupled immunoassay of α-fetoprotein (AFP). In this system, glucose oxidase (GOx)-conjugated detection antibody was first captured onto the microplate by target-induced sandwich-type immunoreaction. Thereafter, the as-generated hydrogen peroxide via the GOx−glucose system oxidized 3,3′,5,5′-tetrametylbenzidine (TMB) into photothermal product oxidized TMB (ox-TMB). Under near-infrared (NIR) laser irradiation, the temperature change of ox-TMB was read out in an electrical signal by the flexible thermoelectric module in a 3D-printed integrated detection device. Under optimal conditions, the photothermal−thermoelectric coupled immunoassay exhibited a limit of detection of 0.39 ng mL −1 AFP over a dynamic linear range from 0.5 to 60 ng mL −1 . Impressively, such a strategy presented herein offers tremendous potentials for applying many other high-efficiency thermoelectric materials in ultrasensitive biosensors.

Several recent studies have demonstrated the use of single and few-layer graphene as a substrate for the enhancement of Raman scattering by adsorbed molecules in a method termed graphene-enhanced Raman spectroscopy (GERS). Here we determine the resonance Raman scattering cross-section for the dye molecule rhodamine 6G (R6G) adsorbed on bilayer graphene. For the 1650 cm(-1) R6G mode, we obtain a cross-section of 5.1 × 10(-24) cm(2)·molecule(-1), a greater than 3-fold reduction from the previously reported solution value. We show that the absorption spectrum of adsorbed R6G can be measured using micro-optical contrast spectroscopy, and we find that detuning of the molecular resonance explains the decreased Raman scattering cross-section. We find no evidence for a graphene Raman enhancement process. We also study the graphene thickness dependence of the adsorbed R6G Raman signal and show that a model incorporating electromagnetic interference effects can qualitatively explain the decrease in signal with increasing graphene thickness.

Rayleigh scattering spectra and Raman spectra from single bundles of aligned single-wall carbon nanotubes (SWNTs) have been obtained with dark field optical microscopy and Raman microscopy. Rayleigh scattering spectrum reveals resonance peaks due to the optically allowed interband transitions in SWNTs. The intensity of the resonance peaks was found to depend strongly on the incident light polarization. These resonance peaks are completely suppressed when the incident light polarization is perpendicular to the nanotube axis, suggesting that the interband transition dipole in SWNTs is orientated parallel to the tube axis. Polarized Raman measurements on aligned nanotubes in a single bundle show that the Raman scattering is polarized along the nanotube axis direction, and Raman scattering signal is strongest when the incident laser is polarized parallel to the tube axis. All strong Raman active modes behave as A1g. Tangential carbon stretching mode Raman scattering from semiconducting tubes shows very little change from bundle to bundle, while that from metallic SWNTs exhibits large variations. The broadened metallic Raman scattering at 1550 cm-1 can be well fitted by a Fano line shape function. This broadened Raman scattering depends sensitively on sample processing conditions. Charge transfer due to chemical doping is proposed to explain the change in Raman scattering from oxidized metallic tubes.

Relatively narrow bandwidth fluorescence spectra were observed from isolated single molecules of the conjugated polymer, MEH-PPV (poly[2-methoxy-5-(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene]) at low temperature. The spectroscopy reveals rich spectral information in energy relaxation pathways and structure of this important electroluminescent material. From room temperature to 20 K the fluorescence of MEH-PPV molecules was found to red-shift by ∼20 nm along with a decreased electron−phonon coupling. The fluorescence spectra give clear evidence for efficient electronic energy funneling to a small number (one in some cases) of low-energy sites, each with an increased effective conjugation length compared to room temperature.

The electronic spectrum of CdS colloidal quantum dots (QDs) with a radius of 1.0−2.3 nm is studied by low-temperature photoluminescence excitation spectroscopy. CdS QDs are found to exhibit a resonant Stokes shift of ∼20−70 meV, which is ∼4 times larger than similarly sized CdSe QDs. This effect can be reproduced by an effective-mass theoretical calculation, which reveals that the hole ground state (or highest occupied molecular orbital, HOMO) in CdS QDs is a P state and the ground-state exciton is an optically passive state. Compared to CdSe, in CdS, the smaller spin−orbit splitting causes the orbital-symmetry-forbidden dark exciton in a QD and its larger resonant Stokes shift. The band-edge photoluminescence in CdS QDs exhibits a lifetime of ∼200 ns at 10 K, which is consistent with the dark exciton mechanism.