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.

Polarization-selected vibrational sum frequency generation spectroscopy (SFG) has been used to investigate the molecular orientation of methyl groups on CH3-terminated Si(111) surfaces. The symmetric and asymmetric C–H stretch modes of the surface-bound methyl group were observed by SFG. Both methyl stretches showed a pronounced azimuthal anisotropy of the 3-fold symmetry in registry with the signal from the Si(111) substrate, indicating that the propeller-like rotation of the methyl groups was hindered at room temperature. The difference in the SFG line widths for the CH3 asymmetric stretch that was observed for different polarization combinations (SPS and PPP for SFG, visible, and IR) indicated that the rotation proceeded on a 1–2 ps time scale, as compared to the ∼100 fs rotational dephasing of a free methyl rotor at room temperature.

optoelectronics. [ 1,2 ] Advantages including reduced materials consumption, relaxed requirements of materials purity, and ability to form large-area devices on unlimited classes of module substrates make them particularly useful as building blocks for realizing high-effi ciency, low-cost photovoltaic systems. [3][4][5] The photovoltaic performance of ultrathin silicon solar cell is, however, inherently limited by incomplete absorption of longer wavelength photons near its bandgap. [ 6,7 ] While light trapping methods based on various diffractive and/ or refl ective optical elements can greatly help to improve the absorption of optically thin silicon, [8][9][10][11][12][13] complementary means to additionally capitalize on such low energy photons are desirable to further improve the performance of ultrathin silicon solar cells. In particular, spectral upconversion, a concept proposed for addressing the sub-bandgap transparency of solar cells, [14][15][16][17][18][19][20][21] is an attractive approach that provides an additional pathway to enhance the quantum effi ciency of above-bandgap longer wavelength photons by converting them into high energy photons that can be more strongly absorbed by the ultrathin silicon. One of key challenges for the practical application of spectral upconversion in photovoltaics (PVs), however, is that the intensity of natural sunlight in relevant wavelengths (i.e., near-infrared) is often too weak to yield meaningful effects of upconversion. [22][23][24] In this regard, recent advances in light manipulation using metallic nanostructures, A type of composite photovoltaic system that can improve the absorption of longer wavelength photons for ultrathin silicon solar cells is presented by synergistically exploiting spectral upconversion and plasmonic light manipulation under a reconfi gurable platform where individual module components can be independently optimized and strategically combined by printing-based deterministic materials assemblies. The ultrathin (≈8 µm) nanostructured silicon solar cells are embedded in a thin polymeric medium containingNaYF 4 :Yb 3+ ,Er 3+ nanocrystals, coated on a plasmonically engineered substrate that incorporates hybrid nanostructures of cylindrical nanoholes and truncated-cone-shaped nanoposts. Both excitation and emission processes of upconversion luminophores are signifi cantly enhanced by combined effects of surface plasmon resonance to amplify the light intensity at the excitation wavelength as well as to facilitate the far-fi eld outcoupling at the emission wavelengths, respectively. The performance of the integrated solar module is improved by ≈13% compared to devices on a nanostructured plasmonic substrate without luminophores due to collective contributions from plasmonically enhanced spectral upconversion, together with effects of waveguiding and fl uorescence of NaYF 4 :Yb 3+ ,Er 3+ . Detailed studies on optical properties of engineered plasmonic nanostructures and device performance in both experiments and numerical modeling provide quanti...

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 AlOX 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 x , 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.

Surface-selective sum frequency generation (SFG) spectroscopy has been previously shown to benefit from a finite time delay between two input laser pulses, which suppresses the nonresonant background and improves spectral resolution. Here we demonstrate another consequence of the time delay in SFG: depending on the magnitude of the delay, nearby resonances (e.g., vibrational modes) can "flip" their relative phase, i.e., appear either in-phase or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up vs down) of the vibrational chromophore. We present results and model calculations for symmetric and asymmetric CH-stretch modes of the methyl-terminated Si(111) surface, showing that the phase flip occurs when the delay matches half-cycle of the difference frequency between the two modes.