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Refereed Articles |
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Photoluminescence-Based Correlation of Semiconductor Electric Field Thickness with Adsorbate Hammett Substituent Constants. Adsorption of Aniline Derivatives onto Cadmium Selenide |
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| Adsorption of ring-substituted aniline derivatives, presumably through the amino group, onto the (0001) face of
single-crystal n-CdSe or n-CdS [CdS(e)] profoundly affects the semiconductor's photoluminescence (PL) by effecting charge
transfer between surface states and the bulk semiconductor. The variations in PL intensity of etched samples are well fit by
a dead-layer model, allowing estimation of the adduct-induced changes in depletion width. The magnitude of these changes
can be molecularly tuned over nearly IO00 A for moderately doped samples by the control of electron density at the coordination
site, a parameter characterized by the Hammeft substituent constant u, In contrast, the affinity of the aniline derivatives
for the CdS(e) surface, as estimated from the fit of concentration-dependent PL changes to the Langmuir adsorption isotherm
model, is relatively insensitive to aniline substituent; equilibrium constants are - 102 M-I. From variablatemperature experiments,
thc binding reaction for the p-OMe derivative of aniline is approximately thermoneutral and appears to be entropy driven.
Temporal PL results support the presence of a broad distribution of recombination sites. PL decay curves are insensitive to
adduct formation for the pOMe and p-Me derivatives, uggesting that surface recombination velocity is not greatly affected
by aniline adsorption. Variable incident light intensity studies, using the p-OMe derivative, are consistent with the notion
of depletion width-driven PL changes. A model of orbital interactions occurring at the CdSe-aniline interface is proposed,
in which interactions of the semiconductor surface with the aniline derivatives alter the occupancy of surface states by shifting their energy distribution relative to the band edges. |
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