The development of a sensitive porous silicon (PS) gas sensor which utilizes photoluminescence induced electroless metallization as a means of obtaining a highly efficient electrical contact has been demonstrated for the detection of HCl, NH3, and NO at the 10 ppm level. The problem of spreading resistance (kΩ–MΩ) is overcome as low resistance contacts ∼20–100 Ω are made to the mesoporous PS structure through electroless gold plating. The response of this device, which operates at a bias voltage of 1–10 mV, is rapid and reversible.

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Porous silicon samples were prepared by anodizing p-doped Si(100) substrates in both aqueous (HF/H20, HF/CHOH, HF/CH3OH/H20, HF/C2H5OH/H20) and nonaqueous (MeCN/HF) media. The time-dependent porous silicon photoluminescence (PL) was monitored during the etch (in situ) and after removal from the etch solution (ex situ). Correlation of the ex situ and in situ PL indicates that the composition of the etchant solution plays an extremely important role in the onset, time-dependent intensity, and lifetime of the emission, both in and out of solution. The effect of etchant solution additives (ethylene glycol, CH3OH, C2H5OH, NaF, HC1, and NaC1) on the porous silicon PL both during arid following the etching cycle, was also determined. The distinct and different correlations found between aqueous and nonaqueous etchants provide insights into the mechanism of PL. These results, when considered in the context of quantum chemical modeling, strongly suggest surface-bound silicon oxyhydride moieties as the source of the porous silicon PL. * Electrochemical Society Active Member. select environments, we also consider the net effect of these in situ treatments on ex situ behavior. We have carried out an extensive series of experi-ments5'79'12 in both aqueous (HF/H20, HF/CHOH, HF! CH3OH/HZO, HF/C2H5OH/H20) and nonaqueous (MeCNf HF, anhydrous) etching media. We monitor the time-dependent PL, in situ, during the etching cycle and before the PS sample is removed from the etching solution, including a study of the effect of the additives ethylene glycol, CH3OH, C2H5OH, NaF, HC1, and NaCI on the in situ luminescence process. We also consider the effect of these treatments on the ex situ behavior of the PL after removal of the PS from the etching solution. By correlating the ex situ with the in situ behavior of the PS, we find that the aqueous electrolyte composition plays an extremely important role in the onset, time-dependent intensity, and lifetime (solution) of the PL emitters.7-912 Comparable studies on nonaqueous electrolytes are found to demonstrate distinctly different correlations between the in situ and ex situ behavior and we use these resuilts to provide possible insights into the mechanism of the PL process. Further, in complement to the experimental studies, we introduce a quantum chemical modeling which suggests that silicon oxyhydride-like fluorophors9 strongly bound to the PS surface are the source of the PS PL. The silicon oxyhydride moiety can be viewed as a highly reactive intermediate in the oxidative pathway between the hydrophobic Si-H surface generated in the HF etching process and the hydrophilic Si02 surface.

Distinctly structured nanoporous and combined hybrid macroporous−nanoporous porous silicon (PS) structures have been fabricated and treated with the dyes 3,3‘-diethyloxadicarbocyanine iodide (DODCI) and Rhodamine 700, both of which have negligible absorption at the wavelengths of maximum absorption for porous silicon. After an extended period of aging in darkness (air) these dye-treated samples are pumped (PLE) at 337.1 nm (nitrogen laser) near the maximum in the PS absorption spectrum (far from the major absorption regions of the impregnating dye) and the subsequent photoluminescence from these treated samples is monitored and compared to that obtained for the untreated PS structures. The first time-dependent photoluminescence (PL) histograms obtained for these systems indicate that the resulting luminescence, initiated through the pumping of a modified PS surface, displays the manifestation of a significant interaction between the surface-bound fluorophors which characterize PS and the dye (DODCI). It is suggested that this interaction results in the creation of a distribution of PS−dye complexes which greatly enhance the nominal PL emission rate from the untreated PS surface. This enhancement not only facilitates the observation of luminescence from a formed photoluminescing PS “green” precursor state but also the mapping of its time-dependent oxidation to form the more commonly observed “orange-red” emitter. Both emitters are attributed to surface-bound silanone-based silicon oxyhydride fluorophors. The degree of PS−dye interaction, in addition to being dye dependent, is found to be PS-structure dependent. Thus the photoluminescent histograms obtained in this study also appear to provide evidence for the influence of PS morphology on the surface interaction.