Vibrational Sum Frequency Generation Spectroscopy of Surface Hydroxyls on Nickel Phyllosilicate Nanoscrolls.

Phyllosilicate clays are layered structures with diverse nanoscale morphology depending on the composition. Size mismatch between the sheets can cause them to form nanoscrolls, a spiral structure with different inner and outer surface charges. The hydroxyls on the exposed surface of the nanoscrolls determine the adsorption properties and hydrophilicity of the surface. Vibrational sum frequency generation (VSFG) spectroscopy was applied to study the surface hydroxyls of nickel phyllosilicate (Ni3Si2O5(OH)4), revealing three distinct in-phase OH-stretch modes: 3642, 3645, and 3653 cm-1. The relative signs of the peaks allow their assignment as "outward" and "inward" pointing hydroxyls on the opposite sides of the scrolled sheet, consistent with the crystal structure. Orientational analysis of polarization-selected VSFG spectra is consistent with a broad (140-164°) step-function distribution of the OH-stretch tilt angles, which we attribute to the uncompensated portion of the scroll rolled more than a whole number of full turns.

Asymmetric response of interfacial water to applied electric fields.

Our understanding of the dielectric response of interfacial water, which underlies the solvation properties and reaction rates at aqueous interfaces, relies on the linear response approximation: an external electric field induces a linearly proportional polarization. This implies antisymmetry with respect to the sign of the field. Atomistic simulations have suggested, however, that the polarization of interfacial water may deviate considerably from the linear response. Here we present an experimental study addressing this issue. We measured vibrational sum-frequency generation spectra of heavy water (D2O) near a monolayer graphene electrode, to study its response to an external electric field under controlled electrochemical conditions. The spectra of the OD stretch show a pronounced asymmetry for positive versus negative electrode charge. At negative charge below 5 × 1012 electrons per square centimetre, a peak of the non-hydrogen-bonded OD groups pointing towards the graphene surface is observed at a frequency of 2,700 per centimetre. At neutral or positive electrode potentials, this 'free-OD' peak disappears abruptly, and the spectra display broad peaks of hydrogen-bonded OD species (at 2,300-2,650 per centimetre). Miller's rule1 connects the vibrational sum-frequency generation response to the dielectric constant. The observed deviation from the linear response for electric fields of about ±3 × 108 volts per metre calls into question the validity of treating interfacial water as a simple dielectric medium.

Vibrational Sum Frequency Generation Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces.

The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120° jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 °C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 ± 360 cm-1 and an attempt frequency of (2.9 ± 4.2) × 1013 s-1 for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation.

Molecular Orientation of Poly-3-hexylthiophene at the Buried Interface with Fullerene.

Molecular orientation at the donor-acceptor interface plays a crucial role in determining the efficiency of organic semiconductor materials. We have used vibrational sum frequency generation spectroscopy to determine the orientation of poly-3-hexylthiophene (P3HT) at the planar buried interface with fullerene (C60). The thiophene rings of P3HT have been found to tilt significantly toward C60, making an average angle θ ≈ 49° ± 10° between the plane of the ring and the interface. Such tilt may be attributed to π-π stacking interactions between P3HT and C60 and may facilitate efficient charge transfer between donor and acceptor. Upon annealing, the thiophene rings tilt away from the interface by Δθ = 12-19°. This may be attributed to higher crystallinity of annealed P3HT that propagates all the way to the interface, resulting in more "edge-on" orientation, which is consistent with the observed red-shift by ∼6 cm-1 and spectral narrowing of the C=C stretch bands.

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, via 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.