7 research outputs found
A Mechanistic Study of the Oxidative Reaction of Hydrogen-Terminated Si(111) Surfaces with Liquid Methanol
H–SiÂ(111)
surfaces have been reacted with liquid methanol
(CH<sub>3</sub>OH) in the absence or presence of a series of oxidants
and/or illumination. Oxidant-activated methoxylation of H–Si(111)
surfaces was observed in the dark after exposure to CH<sub>3</sub>OH solutions that contained the one-electron oxidants acetylferrocenium,
ferrocenium, or 1,1′-dimethylferrocenium. The oxidant-activated
reactivity toward CH<sub>3</sub>OH of intrinsic and n-type H–Si(111)
surfaces increased upon exposure to ambient light. The results suggest
that oxidant-activated methoxylation requires that two conditions
be met: (1) the position of the quasi-Fermi levels must energetically
favor oxidation of the H–Si(111) surface and (2) the position
of the quasi-Fermi levels must energetically favor reduction of an
oxidant in solution. Consistently, illuminated n-type H–Si(111)
surfaces underwent methoxylation under applied external bias more
rapidly and at more negative potentials than p-type H–SiÂ(111)
surfaces. The results under potentiostatic control indicate that only
conditions that favor oxidation of the H–Si(111) surface need
be met, with charge balance at the surface maintained by current flow
at the back of the electrode. The results are described by a mechanistic
framework that analyzes the positions of the quasi-Fermi levels relative
to the energy levels relevant for each system
Profiling Photoinduced Carrier Generation in Semiconductor Microwire Arrays via Photoelectrochemical Metal Deposition
Au
was photoelectrochemically deposited onto cylindrical or tapered p-Si
microwires on Si substrates to profile the photoinduced charge-carrier
generation in individual wires in a photoactive semiconductor wire
array. Similar experiments were repeated for otherwise identical Si
microwires doped to be n-type. The metal plating profile was conformal
for n-type wires, but for p-type wires was a function of distance
from the substrate and was dependent on the illumination wavelength.
Spatially resolved charge-carrier generation profiles were computed
using full-wave electromagnetic simulations, and the localization
of the deposition at the p-type wire surfaces observed experimentally
correlated well with the regions of enhanced calculated carrier generation
in the volumes of the microwires. This technique could potentially be extended to
determine the spatially resolved carrier generation profiles in a
variety of mesostructured, photoactive semiconductors
Revisiting an Ongoing Debate: What Role Do Surface Groups Play in Silicon Nanocrystal Photoluminescence?
The origin of photoluminescence (PL)
in silicon nanocrystals (SiNCs) remains a subject of considerable
debate. Size-dependent PL that supports the quantum confinement model
has been proposed by several researchers. On the other hand, SiNC
PL arising from surface states that are independent of nanocrystal
size has also been shown. This work addresses the origin of surface-functionalized
SiNC PL as relating to surface states and the NC size. SiNCs of different
sizes (3 and 5 nm diameters) were prepared with three distinct surface
chemistries. Steady-state and time-resolved PL measurements were performed
at temperatures ranging from 37 to 377 K. Temperature-dependent luminescence
consistent with core emission was observed for alkyl-terminated SiNCs,
while alkylamine-functionalized SiNCs displayed minimal temperature-dependent
luminescence, consistent with a charge-transfer mechanism. Lightly
oxidized alkyl SiNCs had similar emission profiles to alkyl SiNCs;
however, they showed longer luminescence lifetimes and their luminescence
spectrum was shifted to shorter wavelengths than their nonoxidized
counterparts. A general mechanism is proposed to explain all three
phenomena, suggesting that surface groups play a crucial role in SiNC
optical response
Size <i>vs</i> Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum <i>via</i> Surface Groups
The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3–4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies
Red States versus Blue States in Colloidal Silicon Nanocrystals: Exciton Sequestration into Low-Density Traps
The ultrafast exciton photodynamics
of red-emitting and blue-emitting
colloidal Si nanocrystals are contrasted under low (1.5 mJ/cm<sup>2</sup>) and high (9.1 mJ/cm<sup>2</sup>) excitation powers with
broadband transient absorption spectroscopy. While the low-power initiated
transient signals differ strongly for the two samples, the high-power
signals exhibit similar nonmonotonic kinetics, resulting in a new
population formed on a 10 to 30-ps time scale with a sample independent
spectrum and decay kinetics. This phenomenon is ascribed to the saturation
of low-density red-emitting and blue-emitting traps via a state-filling
mechanism to populate new meta-stable states at higher excitation
powers. The states responsible for blue emission and high-power populations
are ascribed to traps from low-density nitrogen and oxygen impurities,
respectively, and share similar charge-transfer character with the
silicon nanocrystal core
Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry
The
role of surface species in the optical properties of silicon nanocrystals
(SiNCs) is the subject of intense debate. Changes in photoluminescence
(PL) energy following hydrosilylation of SiNCs with alkyl-terminated
surfaces are most often ascribed to enhanced quantum confinement in
the smaller cores of oxidized NCs or to oxygen-induced defect emission.
We have investigated the PL properties of alkyl-functionalized SiNCs
prepared using two related methods: thermal and photochemical hydrosilylation.
Photochemically functionalized SiNCs exhibit higher emission energies
than the thermally functionalized equivalent. While microsecond lifetime
emission attributed to carrier recombination within the NC core was
observed from all samples, much faster, size-independent nanosecond
lifetime components were only observed in samples prepared using photochemical
hydrosilylation that possessed substantial surface oxidation. In addition,
photochemically modified SiNCs exhibit higher absolute photoluminescent
quantum yields (AQY), consistent with radiative recombination processes
occurring at the oxygen-based defects. Correlating spectrally- and
time-resolved PL measurements and XPS-derived relative surface oxidation
for NCs prepared using different photoassisted hydrosilylation reaction
times provides evidence the PL blue-shift as well as the short-lived
PL emission observed for photochemically functionalized SiNCs are
related to the relative concentration of oxygen surface defects
Chemical Insight into the Origin of Red and Blue Photoluminescence Arising from Freestanding Silicon Nanocrystals
Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms and are biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared <i>via</i> solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue emitters upon exposure to nitrogen-containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the parts per million level in Si NCs gives rise to the blue emission