We report the observation of surface-enhanced Raman scattering (SERS) from a chemically etched ZnSe surface using 4-mercaptopyridine (4-MPy) as probe molecules. A thin film of ZnSe is grown by molecular beam epitaxy (MBE) and then etched using a strong acid. Protrusions of hemi-ellipsoidal nanoparticles are observed on the surface. Using the results of the Mie theory, we controlled the size of the nanoparticles to overlap significantly with maximum efficiency of near-field plasmon enhancement. In the Raman spectrum, we observe large enhancements of the a1, b1, and b2 modes when 4-MPy molecules are adsorbed on the surface using a 514.5 nm laser for excitation, indicating strong charge-transfer contributions. An enhancement factor of (2×106) is observed comparable to that of silver nanoparticles. We believe this large enhancement factor is an indication of the coupled contribution of several resonances. We propose that some combination of surface plasmon, charge transfer, band gap resonances are most likely the contributing factors in the observed Raman signal enhancement, since all three of these resonances lie close to the excitation wavelength.
We present a technique for the tunable synthesis of a variety of monodisperse silver nanoparticles. Utilizing different optical wavelengths to irradiate initially grown seed crystals, the size and shape of the products can be controlled. Monitoring the absorption spectrum during growth, we observe that initially the absorption maximum shifts to longer wavelengths and broadens, indicating increasing particle size and dispersion. Remarkably, this effect gradually comes to a halt and reverses, displaying a shift to shorter wavelengths and simultaneously narrower bandwidths, until on completion, a final size and relatively narrow distribution is reached. The final morphology is found to depend on control of the laser wavelength and power. Discs, triangular prisms as well as pyramidal and pentagonal prisms may be produced. A process based on a wavelength dependent self-limiting mechanism governed by the surface plasmon resonance controlling the photochemical reduction of particles is suggested. By a similar mechanism, we show that by using a sodium lamp instead of a laser as an excitation source, a monodisperse sample of nanotetrahedra can be produced.
Using surface enhanced Raman spectroscopy (SERS), we have observed bio-molecules at extremely low concentration, adsorbed on self-organized semiconductor quantum dots, grown by molecular beam epitaxy. Quantum dots have found application in the field of biosensors, and the performance of these devices depends critically on the properties of the surface features. It is therefore of interest to explore useful and versatile spectroscopic sensing technique such as SERS to determine these properties. The SERS technique employs rough substrates with structures in the nanometer range to enhance Raman signals from adsorbed species. This spectroscopy has a number of important advantages: sensitivity, selectivity and non-destructive detection. In addition to this, SERS enables the determination of detailed information about adsorbed species such as molecular structure and orientation, while greatly increasing the Raman cross section and suppressing fluorescence. We show that the Raman signal observed from various biologically important molecules can be enhanced by up to six orders of magnitude by means of surface enhancement caused by adhesion to self-organized CdSe/CdZnSeMg quantum dots grown by molecular beam epitaxy.
We report the detection of short-lived species on electrode surfaces utilizing the enhancement of the Raman intensity afforded by proximity to silver electrodes. Using gated multiplexed detection we identify an photoproduct of flavinmononucleotide after excitation with 337nm radiation from a pulsed nitrogen laser. The product decays on the 700nsec time scale and is identified as the radical anion of the nucleotide.
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