Optical Pumping of Dye-Complexed and -Sensitized Porous Silicon Increasing Photoluminescence Emission Rates

Abstract

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.

Contrasting photovoltaic response and photoluminescence for distinct porous silicon pore structures

“…At longer time delays, the monitored emission, while maintaining an identical wavelength dependence, decreases precipitously in intensity. Dye sensitization experiments 35 suggest that this results as longer-lived emitters with small oscillator strengths contribute to the observed spectral features. While the data in this study clearly indicate a different development of the PS luminescence for the aqueous and hybrid etched samples, which provides evidence for the influence of the underlying PS bulk structure on surface-based transformation, the overall spectral distributions are quite similar for these etched samples.…”

“…We associate the monitored shifting spectrum, at least in part, with the products of the excited state oxidation of a long-lived triplet precursor state, corresponding to the green emitter, as they are formed and relax down an excited state manifold. 11,29,35 These histograms, while indicating a differing evolution for the PL as a function of delay time, describe the manner in which the luminescence builds to the spectral distribution typically observed for longer time scales. 29,35 In other words, the dominant characteristics of the 1.5-100 s spectra develop over the time span of the histograms.…”

“…11,29,35 These histograms, while indicating a differing evolution for the PL as a function of delay time, describe the manner in which the luminescence builds to the spectral distribution typically observed for longer time scales. 29,35 In other words, the dominant characteristics of the 1.5-100 s spectra develop over the time span of the histograms. At longer time delays, the monitored emission, while maintaining an identical wavelength dependence, decreases precipitously in intensity.…”

Nanostructure and morphology modified porous silicon sensors

“…Porous silicon (PS) formed from the HF-based etch of a silicon wafer is characterized by an extensive visible photoluminescence (PL) which when excited by uv radiation displays a time dependent behavior resulting primarily from surface-based oxidation processes. The development, enhancement, and evolution of this PL can be significantly influenced through the introduction of a variety of surface treatments with, for example, HCl 22 and select dyes 23 .…”

From nanostructures to porous silicon: sensors and photocatalytic reactors

“…The electrochemical etching of silicon in a variety of electrolytes can produce a bewildering multitude of sizes and shapes [6,7]. Pore sizes can be made to vary [8][9][10][11][12][13][14][15] from the 1-10 nm range [8] to sizes of the order of 3 µm [15] and mixtures of two pore types are possible [9,[16][17][18]. Porous silicon (PS) formed from the HF-based etch of a silicon wafer is characterized by an extensive visible photoluminescence (PL) which when excited by uv radiation displays a time dependent behavior resulting primarily from surface-based oxidation processes.…”