No abstract

Surface Enhanced Raman Scattering (SERS) gives rise to analytical applications with much promise. In our approach three steps are necessary. We require a SERS platform of high enhancement. This has been achieved using the special technique of Island Lithography, combined with Ag deposition by galvanic exchange, yielding an enhancement factor of 10(8). Probe oligonucleotide molecules are attached to a specific area on the platform, at the optimized surface concentration, using thiolated single stranded (ss) DNA molecules. The optimum surface concentration has been determined and interpreted in the light of the polyelectrolyte behaviour of ssDNA. Finally the change in SERS produced by hybridisation of the probe molecules to a target DNA molecule is measured. Highly discernible changes have been obtained. No change in probe signal is seen when presented with one base mismatched target. From this work it is concluded that the prospects for label-free DNA detection in high-density arrays is now close to achievement.

A range of pseudo-random silver structures, where there is a choice of clustered spheres or pillars or tori, have been fabricated on silicon using the method of island lithography combined with electroless plating. Pyridine has been adsorbed on these structures and the surface-enhanced Raman scattering spectrum (SERS) measured using 633 nm laser radiation. Measurements of SERS spectra as a function of pyridine solution concentration have enabled an adsorption isotherm to be obtained and the standard free energy of adsorption to be determined (24 kJ mol-1), in good agreement with the literature. The substrates are found to give a uniform signal, as sampled over the prepared area, for both saturation coverage (±12%) and for a fraction of a monolayer, ca. 0.08, (±31%). It is concluded that, in the system studied, the effective area of the SERS “site” must be large compared with the area occupied by the adsorbed pyridine, so that the SERS signal is proportional to the surface coverage, averaged over the adsorbent. The formalism due to Tian and co-workers has been adopted to determine G, the SERS enhancement factor. G is calculated as follows:  G = (scattering intensity per adsorbed molecule)/(scattering intensity per solution molecule). G from 1.9 × 106 (pillars) to 2.5 × 107 (tori) have been estimated, and larger values are expected. These silver substrates, which are robust and reproducible, would seem to be good candidates for various analytical applications.

The electric field at the surface of a semiconductor is obtained as a function of the semiconductor bulk properties and the potential difference across the space charge region. The treatment is general enough to take into account degenerate free carrier distributions and partial ionization of impurities, either in the neutral bulk or the space charge region, or both. A one-dimensional model with a spacially homogeneous impurity distribution is assumed. The density of states in the conduction and valence bands is assumed to be that appropriate for spherical and ellipsoidal energy surfaces. The common model for simple donor and acceptor states is used in the main body of the paper. Conditions under which the equation for the field can be conveniently simplified are discussed, and a particular case is treated numerically. The validity of the free carrier distribution functions in the high fields near the semiconductor surface is discussed in Appendix I. The equation for the electric field at the semiconductor surface where the bulk impurities are described by a general model is given in Appendix II.

A new process for the deposition of thick (≈10-µm) films of silica and titania-doped silica on silicon substrates is described. Films are built up by repetitive operation of a simple process cycle in which a layer of sol-gel material is deposited by spin coating, then densified by rapid thermal annealing. Stressfree layers are obtained through careful choice of the anneal temperature. Bilayer structures suitable for waveguide fabrication may also be constructed by performing two successive deposition runs using sol-gel precursors with different titania concentrations. These bilayers may be patterned topographically into ridges by using reactive ion etching, and the ridges may be planarized by applying additional layers of sol-gel material to form buried channel waveguides.