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 (ν₂) at the air/water interface measured using vibrational sum-frequency generation (SFG). The blue-shift of the ν₂ frequency from the gas phase value reports on the hydrogen bonding in the interfacial region. The ν₂ 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 °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 (ν₂) at the air/water interface measured using sum-frequency generation (SFG). Here, we present experimental evidence to aid the assignment of the ν₂ 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 (SFG–​visible–​infrared). This is naturally explained as an interference effect between the negative (1630 cm^–1) and positive (1662 cm^–1) peaks corresponding to “free–OH” and “H-bonded” 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^–1

Steric Hindrance of Photoswitching in Self-Assembled Monolayers of Azobenzene and Alkane Thiols

Surface-bound azobenzenes exhibit reversible photoswitching via trans–cis 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. Trans–cis 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 C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 polymer–substrate interfaces was compared for films spin-cast on SiO₂ and AlO_X substrates, before and after thermal annealing at 145 °C. On SiO₂, annealing results in the thiophene rings adopting a more edge-on orientation, tilting away from the surface plane by Δθ = +(3–10)°. In contrast, an opposite change is observed for films deposited on AlO_<i>x</i>, Δθ = −(3–26)°, 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 μm-thick silicon solar cells incorporating a hexagonally periodic nanostructured surface relief are surface-embedded in layered multispectral luminescent media containing organic dyes and NaYF₄:Yb³⁺,Er³⁺ 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 (∼40.1 mA/cm²) and energy conversion efficiency (∼12.8%) 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 ∼13.6 mA/cm² compared with microcells in a nonluminescent medium on a plain silver reflector under a confined illumination

Field-Dependent Orientation and Free Energy of D₂O 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 46° from normal at −0.9 V vs Ag/AgCl (E = −0.02 V/Å) to 32° at −3.9 V vs Ag/AgCl (E = −0.17 V/Å). 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 molecule’s permanent dipole moment. This allows us to extract the curvature of the free energy orienting potential of interfacial water, which is (4.0 ± 0.8) × 10–20 J/rad2 (or 0.25 ± 0.05 eV/rad2 )