7 research outputs found
Orientational Dynamics in Sum Frequency Spectroscopic Line Shapes
We
present a general response function formalism describing the
contribution of orientational dynamics of molecules at interfaces
to spectroscopic line shapes in vibrational sum frequency generation
(SFG). When reorientation occurs on the time scale comparable to vibrational
dephasing, its dynamics can be extracted from polarization-selected
SFG spectral line shapes. Unique features of orientational motion
at interfaces are (1) the anisotropic case-specific equilibrium orientational
distribution and (2) possible dynamic anisotropy (e.g., different
in-plane versus out-of-plane relaxation rates), both of which must
be taken into account. Within the small-step rotational diffusion
model, we present solutions for two cases, the weak-confinement model,
applicable when the deviations from the isotropic case are not severe,
and the wobbling-in-a-cone model, which considers a hard-wall orienting
potential. SFG line shapes are calculated for a rod-like chromophore
as a function of the rotational diffusion rate. For certain equilibrium
orientational distributions, orientational dynamics may result in
anomalous bi-Lorentzian line shapes (two Lorentzians of different
widths centered and the same frequency)
Vibrational Sum-Frequency Spectrum of the Water Bend at the Air/Water Interface
We present the spectrum of the water bend vibrational
mode (Ī½<sub>2</sub>) at the air/water interface measured using
vibrational sum-frequency
generation (SFG). The blue-shift of the Ī½<sub>2</sub> frequency
from the gas phase value reports on the hydrogen bonding in the interfacial
region. The Ī½<sub>2</sub> line shape of surface water is inhomogeneously
broadened and structured. The dominant feature is the least blue-shifted
and relatively narrow Lorenztian, tentatively assigned to water molecules
straddling the interface, those with free OH bonds. This feature appears
at different frequencies in the SFG spectra recorded using different
polarization combinations (SSP and PPP for SFG-visible-IR), pointing
to possible orientational inhomogeneity. Weaker features are observed
at higher frequencies, tentatively assigned to fully H-bonded water
molecules. Small but measurable changes of the line shape with temperature
are observed in the 0 to +20 Ā°C range
On the Assignment of the Vibrational Spectrum of the Water Bend at the Air/Water Interface
We previously reported
the spectrum of the water bend vibrational
mode (Ī½<sub>2</sub>) at the air/water interface measured using
sum-frequency generation (SFG). Here, we present experimental evidence
to aid the assignment of the Ī½<sub>2</sub> spectral features
to H-bonded classes of interfacial water, which is in general agreement
with two recent independently published theoretical studies. The dispersive
line shape shows an apparent frequency shift between SSP and PPP polarization
combinations (SFGāāvisibleāāinfrared).
This is naturally explained as an interference effect between the
negative (1630 cm<sup>ā1</sup>) and positive (1662 cm<sup>ā1</sup>) peaks corresponding to āfreeāOHā and āH-bondedā
species, respectively, which have different orientations and thus
different amplitudes in SSP and PPP spectra. A surfactant monolayer
of sodium dodecyl sulfate (SDS) was used to suppress the free OH species
at the surface, and the corresponding SFG spectral changes indicate
that these water molecules with one of the hydrogens pointing up into
the air phase contribute to the negative peak at 1630 cm<sup>ā1</sup>
Steric Hindrance of Photoswitching in Self-Assembled Monolayers of Azobenzene and Alkane Thiols
Surface-bound azobenzenes
exhibit reversible photoswitching via
transācis photoisomerization and have been proposed for a variety
of applications such as photowritable optical media, liquid crystal
displays, molecular electronics, and smart wetting surfaces. We report
a novel synthetic route using simple protection chemistry to form
azobenzene-functionalized SAMs on gold and present a mechanistic study
of the molecular order, orientation, and conformation in these self-assembled
monolayers (SAMs). We use vibrational sum-frequency generation (VSFG)
to characterize their vibrational modes, molecular orientation, and
photoisomerization kinetics. Transācis conformational change
of azobenzene leads to the change in the orientation of the nitrile
marker group detected by VSFG. Mixed SAMs of azobenzene and alkane
thiols are used to investigate the steric hindrance effects. While
100% azobenzene SAMs do not exhibit photoisomerization due to tight
packing, we observe reversible switching (>10 cycles) in mixed
SAMs
with only 34% and 50% of alkane thiol spacers
Annealing-Induced Changes in the Molecular Orientation of Poly-3-hexylthiophene at Buried Interfaces
The molecular organization at interfaces
of organic semiconducting
materials plays a crucial role in the performance of organic photovoltaics
and field effect transistors. Vibrational sum-frequency generation
(VSFG) was used to characterize the molecular orientation at interfaces
of regioregular poly-3-hexylthiophene (rrP3HT). Polarization-selected
VSFG spectra of the Cī»C stretch of the thiophene ring yield
the orientation of the conjugated backbone of P3HT, which is directly
relevant to the electronic properties at the interface. The molecular
orientation at buried polymerāsubstrate interfaces was compared
for films spin-cast on SiO<sub>2</sub> and AlO<sub>X</sub> substrates,
before and after thermal annealing at 145 Ā°C. On SiO<sub>2</sub>, annealing results in the thiophene rings adopting a more edge-on
orientation, tilting away from the surface plane by ĪĪø
= +(3ā10)Ā°. In contrast, an opposite change is observed
for films deposited on AlO<sub><i>x</i></sub>, ĪĪø
= ā(3ā26)Ā°, where annealing leads to a more face-on
orientation of the thiophene rings of the polymer. Although subtle,
such orientational changes may significantly affect the electron transfer
rates across interfaces and hence the overall photovoltaic efficiency
Synergistically Enhanced Performance of Ultrathin Nanostructured Silicon Solar Cells Embedded in Plasmonically Assisted, Multispectral Luminescent Waveguides
Ultrathin silicon
solar cells fabricated by anisotropic wet chemical
etching of single-crystalline wafer materials represent an attractive
materials platform that could provide many advantages for realizing
high-performance, low-cost photovoltaics. However, their intrinsically
limited photovoltaic performance arising from insufficient absorption
of low-energy photons demands careful design of light management to
maximize the efficiency and preserve the cost-effectiveness of solar
cells. Herein we present an integrated flexible solar module of ultrathin,
nanostructured silicon solar cells capable of simultaneously exploiting
spectral upconversion and downshifting in conjunction with multispectral
luminescent waveguides and a nanostructured plasmonic reflector to
compensate for their weak optical absorption and enhance their performance.
The 8 Ī¼m-thick silicon solar cells incorporating a hexagonally
periodic nanostructured surface relief are surface-embedded in layered
multispectral luminescent media containing organic dyes and NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> nanocrystals as downshifting
and upconverting luminophores, respectively, <i>via</i> printing-enabled
deterministic materials assembly. The ultrathin nanostructured silicon
microcells in the composite luminescent waveguide exhibit strongly
augmented photocurrent (ā¼40.1 mA/cm<sup>2</sup>) and energy
conversion efficiency (ā¼12.8%) than devices with only a single
type of luminescent species, owing to the synergistic contributions
from optical downshifting, plasmonically enhanced upconversion, and
waveguided photon flux for optical concentration, where the short-circuit
current density increased by ā¼13.6 mA/cm<sup>2</sup> compared
with microcells in a nonluminescent medium on a plain silver reflector
under a confined illumination
Field-Dependent Orientation and Free Energy of D<sub>2</sub>O at an Electrode Surface Observed via SFG Spectroscopy
Polarization-selected vibrational sum frequency generation
(SFG)
spectroscopy of D2O is used to obtain the orientation of
the free OD bond at a monolayer graphene electrode. We modulate the
interfacial field by varying the applied electrochemical potential,
and we measure the resulting change in the orientation. A hyperpolarizability
model is used for the orientational analysis, which assumes a quadratic
free energy orienting potential in the absence of the field, whose
minimum and curvature determine the average tilt angle and the Gaussian
width of the orientational distribution. The average free OD tilt
angle changes in an approximately linear fashion with the applied
field, from 46Ā° from normal at ā0.9 V vs Ag/AgCl (E = ā0.02 V/Ć
) to 32Ā° at ā3.9 V
vs Ag/AgCl (E = ā0.17 V/Ć
). Using this
approach, we map the free energy profile for the molecular orientation
of interfacial water by measuring the reversible response to an external
perturbation, i.e., a torque applied by an electric field acting on
the moleculeās permanent dipole moment. This allows us to extract
the curvature of the free energy orienting potential of interfacial
water, which is (4.0 Ā± 0.8) Ć 10ā20 J/rad2 (or 0.25 Ā± 0.05 eV/rad2 )