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The frequency-domain sum frequency generation (SFG) spectroscopy and the time-domain SFG-free induction decay technique are combined for characterization of more complex molecular surfaces under ambient conditions. Relative amplitudes change and additional transitions emerge at lower surface pressure indicating changes in molecular orientation and onset of disorder.

The effect of a positively charged dodecyl trimethylammonium bromide (DTAB) surfactant on the aqueous solvation of coumarin 314 (C314), a solvatochromic molecule, at the air/water interface was investigated. Steady-state and femtosecond time-resolved second harmonic generation (SHG) spectroscopy was employed as a surface-selective tool to characterize the solvation of C314.

The effect of lipid surfactants on the solvation dynamics at air/water interface is examined using time-resolved second harmonic generation spectroscopy for this purpose. It is found that the carboxylate and sulfate surfactants induce similar dynamics, suggesting that the solation is not sensitive to the chemical composition of the headgroup but rather to the total negative surface charge density.

A study was conducted to investigate the effect of the negatively charged carboxylate group of the stearic acid surfactant CH(sub 3)(CH(sub 2))(sub 16)COO(super -) on the solvation at the surfactant air/water interface. It was found that the static solvation energetics appears to be relatively insensitive to the ionization state of the stearic acid surfactant.

A new method of broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy is developed and its high sensitivity is demonstrated, allowing measurements of vibrational spectra of submonolayers. The HD-SFG spectroscopy has provided the phase as well the amplitude of the signal and has allowed accurate subtraction of the non-resonant background.

Nonlinear surface-selective vibrational sum frequency generation (SFG) spectroscopy is applied to characterize the conformation of dodecanethiol on gold nanoparticle surfaces. The results have shown the relation between the nanoscale geometry, such as the surface curvature of spherical gold nanoparticles and the molecular conformation of the chemisorbed dodecanethiol ligand, and have illustrated the utility of the second-order nonlinear SFG spectroscopy as a sensitive probe of the molecular conformation, particularly for surfaces of metallic nanostructures.

The effect of a Langmuir monolayer of fatty acid on static solvation properties as well as femtosecond solvation dynamics at aplanar air-water interface is investigated using second harmonic generation as a surface-specific probe. The results demonstrate that the stearic acid monolayer induces significant changes in both static and dynamic solvation at the air-water interface.

Wepresent a general response function formalism describing thecontribution of orientational dynamics of molecules at interfacesto spectroscopic line shapes in vibrational sum frequency generation(SFG). When reorientation occurs on the time scale comparable to vibrationaldephasing, its dynamics can be extracted from polarization-selectedSFG spectral line shapes. Unique features of orientational motionat interfaces are (1) the anisotropic case-specific equilibrium orientationaldistribution and (2) possible dynamic anisotropy (e.g., differentin-plane versus out-of-plane relaxation rates), both of which mustbe taken into account. Within the small-step rotational diffusionmodel, 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 orientingpotential. SFG line shapes are calculated for a rod-like chromophoreas a function of the rotational diffusion rate. For certain equilibriumorientational distributions, orientational dynamics may result inanomalous bi-Lorentzian line shapes (two Lorentzians of differentwidths centered and the same frequency). [ABSTRACT FROM AUTHOR]

Molecular organization of vapor-deposited organic molecules in the active layer of organic light-emitting diodes (OLEDs) has been a matter of great interest as it directly influences various optoelectronic properties and the overall performance of the devices. Contrary to the general assumption of isotropic molecular orientation in vacuum-deposited thin-film OLEDs, it is possible to achieve an anisotropic molecular distribution at or near the surface under controlled experimental conditions. In this study, we have used interface-specific vibrational sum frequency generation (VSFG) spectroscopy to determine the orientation of a low-molecular weight OLED material, 4,4'-bis( N -carbazolyl)-1,1'-biphenyl (CBP), at free (air) and buried (CaF 2 ) interfaces. VSFG spectra were measured at four different polarization combinations for five different thicknesses of the CBP film. The spectral shift and VSFG intensity changes with the film thickness can be accurately modeled by considering the optical interference effect of the signals coming from the CBP/air and CBP/CaF 2 interfaces. A global fitting of the experimental spectra for all thicknesses along with theoretical simulations reveal that the long molecular axis of CBP is oriented at an angle of ∼58° (47-70°) from the surface normal at the air/CBP interface, whereas at the CBP/CaF 2 interface, the angle is ∼48° (43-52°). Such a change in the angle (∼10°) suggests that the CBP molecule tends to orient more vertically (edge-on) at the buried CaF 2 interface, which may be attributed to the intermolecular π-π stacking interaction between adjacent CBP molecules.

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 NaYF 4 :Yb 3+ ,Er 3+ 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/cm 2 ) 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/cm 2 compared with microcells in a nonluminescent medium on a plain silver reflector under a confined illumination.

We previously reported the spectrum of the water bend vibrational mode (ν 2 ) at the air/water interface measured using sum-frequency generation (SFG). Here, we present experimental evidence to aid the assignment of the ν 2 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 .

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.

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.

Molecular organization of Langmuir-Blodgett (LB) monolayers of novel copper-containing metal-ligand surfactants was characterized by the surface-selective vibrational sum frequency generation (SFG) spectroscopy. The orientational and conformational order inferred from the SFG peak amplitudes and line shapes were correlated with the two-dimensional phases of the monolayers observed in the compression isotherms. The octadecyl-pyridin-2-ylmethyl-amine (L(PyC18)) ligand by itself shows good amphiphilic properties, as indicated by the high monolayer collapse pressure at the air/water interface, but its LB films transferred onto fused silica exhibit a high degree of trans-gauche conformational disorder in the alkyl tails. Coordination of copper(II) ions to the chelating head group enhances the molecular alignment and reduces the fraction of gauche defects of the alkyl chains. Monolayers of single-tail (L(PyC18)Cu(II)Cl(2)) and double-tail [(L(PyC18))(2)Cu(II)]Cl(2) metallosurfactants show distinctly different behavior of their molecular organization as a function of the area per molecule. Our observations suggest metal-ligand interactions as a pathway to induce molecular order in LB monolayer films.

Covalent surface attachment of carbohydrate moieties using maleimide-sulfhydril reaction was characterized by surface-selective vibrational sum-frequency generation (VSFG) spectroscopy. The comparative VSFG spectra of the precursor maleimide-terminated SAM and the product glucose adlayer reveal the high efficiency of the surface coupling reaction (>90%) and the details of the molecular organization of the formed carbohydrate adlayer. The glucose groups are orientationally well ordered, as judged by their sharp C-H stretch bands. The chemical structure of the linker can significantly affect the orientation of the carbohydrate moiety at the surface. Two alkanethiol linkers of different chain lengths (11 and 16 carbons) yield similar orientations of the glucose in the adlayer whereas the cysteine-containing linker produces markedly different relative peak intensities of the glucose C-H stretch bands in the VSFG spectra, suggesting a significantly different orientation with respect to the surface plane.

The attachment of cells onto solid supports is fundamental in the development of advanced biosensors or biochips. In this work, we characterize cortical neuron adhesion, growth, and distribution of an adhesive layer, depending on the molecular structure and composition . Neuronal networks are successfully grown on amino-terminated alkanethiol self-assembled monolayer (SAM) on a gold substrate without adhesion protein interfaces. Neuron adhesion efficiency was studied for amino-terminated, carboxy-terminated, and 1:1 mixed alkanethiol SAMs deposited on gold substrates. Atomic force microscopy and X-ray photoelectron spectroscopy were used to measure the roughness of gold substrate and thickness of SAM monolayers. Conformational ordering and ionic content of SAMs were characterized by vibrational sum frequency generation (VSFG) spectroscopy. Only pure amino-terminated SAMs provide efficient neuronal cell attachment. Ordering of the terminal amino groups does not affect efficiency of neuron adhesion. VSFG analysis shows that ordering of the terminal groups improves with decreasing surface roughness; however the number of gauche defects in alkane chains is independent of surface roughness. We monitor partial dissociation of carboxy groups in mixed SAMs that implies formation of NH3+ neighbors and appearance of catanionic structure. Such catanionic environment proved inefficient for neuron adhesion. Surface roughness of metal within the 0.7-2 nm range has little effect on the efficiency of neuron adhesion. This approach can be used to create new methods that help map structure-property relationships of biohybrid systems.