Eric Borguet

Second harmonic generation from the surface of centrosymmetric particles in bulk solution

Second harmonic generation (SHG) is reported for the first time from the surfaces of centrosymmetric particles in bulk isotropic solution. Although SHG is generally described as electric dipole forbidden in centrosymmetric systems, we show that this requires the system to be centrosymmetric on length scales much less than the coherence length of the process. This condition is not satisfied for micron-size particles, and accordingly we have observed a strong SH signal from various particles of this length scale. This promising discovery provides a powerful spectroscopic method for the investigation of physical and chemical processes on the surfaces of microscopic centrosymmetric particles.

Ambient stability of chemically passivated germanium interfaces

The stability of any semiconductor surface passivating layer is key to applications. Second harmonic generation (SHG) can be used to probe the chemical state of semiconductor interfaces, as well as investigate the mechanisms of chemical transformation. While the SHG rotational anisotropy changes upon sulfidation or alkylation of Ge surfaces, SHG appears far less sensitive to H and Cl passivation of germanium surfaces than to silicon surfaces. Investigation of the stability of chemically modified germanium surfaces using a number of additional techniques, including atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), reveals that H- and Cl-terminated Ge(1 1 1) rapidly re-oxidize in ambient. S- and alkyl-terminations are more robust, showing little sign of oxide formation after a month in ambient. 2003 Elsevier B.V. All rights reserved.

Effect of Hydrogen-Bond Strength on the Vibrational Relaxation of Interfacial Water

Time-resolved sum frequency generation (tr-SFG) reveals that the vibrational energy relaxation rate of O-H stretching of dilute HDO in D(2)O at the silica interface is markedly different from that of bulk water. As compared to the bulk liquid, the vibrational lifetime (T(1)) of HDO is shorter at the charged surface than in the bulk, but longer at the neutral surface. The vibrational decoupling of the O-H of the HDO species leads to the observation of a frequency-dependent T(1) of the O-H stretch, which is shorter at the red than the blue side of the hydrogen-bonded OH spectral region. This correlates with the red-shift of the SFG spectra with increasing surface charge and is consistent with a theoretical model that relates the vibrational lifetime to the strength of the hydrogen-bond network.

Effect of surface charge on the vibrational dynamics of interfacial water.

The effect of the structuring of interfacial water, induced by surface charge, on the ultrafast vibrational dynamics of the O-H stretch in the hydrogen bonded spectral region was studied at the H(2)O/fused silica interface. At high pH, where the electric field resulting from deprotonation of silanol groups polarizes several layers of water molecules, fast vibrational dynamics similar to the dynamics of bulk water is observed. At the neutral surface, where the structural ordering of interfacial water and the thickness of interfacial water are smaller than those at the charged surface, the vibrational lifetime of the O-H stretch becomes more than two times longer (T(1) approximately 570 fs). The longer vibrational lifetime is a result of reduced intermolecular coupling resulting from incomplete solvation of the interfacial water species.

Optimizing Single-Molecule Conductivity of Conjugated Organic Oligomers with Carbodithioate Linkers

In molecular electronics, the linker group, which attaches the functional molecular core to the electrode, plays a crucial role in determining the overall conductivity of the molecular junction. While much focus has been placed on optimizing molecular core conductivity, there have been relatively few attempts at designing optimal linker groups to metallic or semiconducting electrodes. The vast majority of molecular electronic studies use thiol linker groups; work probing alternative amine linker systems has only recently been explored. Here, we probe single-molecule conductances in phenylene-ethynylene molecules terminated with thiol and carbodithioate linkers, experimentally using STM break-junction methods and theoretically using a nonequilibrium Greens function approach. Experimental studies demonstrate that the carbodithioate linker augments electronic coupling to the metal electrode and lowers the effective barrier for charge transport relative to the conventional thiol linker, thus enhancing the conductance of the linker-phenylene-ethynylene-linker unit; these data underscore that phenylene-ethynylene-based structures are more highly conductive than originally appreciated in molecular electronics applications. The theoretical analysis shows that the nature of sulfur hybridization in these species is responsible for the order-of-magnitude increased conductance in carbodithioate-terminated systems relative to identical conjugated structures that feature classic thiol linkers, independent of the mechanism of charge transport. Interestingly, in these systems, the tunneling current is not dominated by the frontier molecular orbitals. While barriers >k(B)T to produce the low beta values seen in our experiments. Taken together, these experimental and theoretical studies indicate a promising role for carbodithioate-based connectivity in molecular-scale electronics applications involving metallic and semiconducting electrodes.

TiO2/LiCl-Based Nanostructured Thin Film for Humidity Sensor Applications

A simple and straightforward method of depositing nanostructured thin films, based on LiCl-doped TiO(2), on glass and LiNbO(3) sensor substrates is demonstrated. A spin-coating technique is employed to transfer a polymer-assisted precursor solution onto substrate surfaces, followed by annealing at 520°C to remove organic components and drive nanostructure formation. The sensor material obtained consists of coin-shaped nanoparticles several hundred nanometers in diameter and less than 50 nm thick. The average thickness of the film was estimated by atomic force microscopy (AFM) to be 140 nm. Humidity sensing properties of the nanostructured material and sensor response times were studied using conductometric and surface acoustic wave (SAW) sensor techniques, revealing reversible signals with good reproducibility and fast response times of about 0.75 s. The applicability of this nanostructured film for construction of rapid humidity sensors was demonstrated. Compared with known complex and expensive methods of synthesizing sophisticated nanostructures for sensor applications, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), this work presents a relatively simple and inexpensive technique to produce SAW humidity sensor devices with competitive performance characteristics.

Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni(2+) ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomic %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA cm(-2) . This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst β-Ni(OH)2 (η≈550 mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.

Ultrafast dynamics and structure at aqueous interfaces by second harmonic generation

Abstract Femtosecond time-resolved second harmonic generation studies of the barrierless isomerization of an organic dye, malachite green (MG), have been carried out at several aqueous interfaces. A comparison of the dynamics at the air/aqeous, alkane/aqueous and silica/aqueous interafces, indicates increased friction and increased water structure at the aqueous interfaces relative to bulk water, in support of molecular simulations, with the silica/aqueous interface being the most structured. The dynamics are slower at all of these interfaces than in bulk water, by a factor of three of five in the case of the air/aqueous and alkane/aqueous interfaces, and almost an order of magnitude in the case of the silica/aqueous interface. These investigations also indicate that the generally accepted isomerization model of twisting of the three aromatic rings about the central carbon atom requires modification in that the synchronous twisting of all three aromatic rings is not necessary for rapid internal conversion from the excited to ground electronic state. In contrast to MG, the dynamics of the activated photoisomerization of the cyanine dye, 3,3′-diethyloxadicarbocyanine iodide (DODCI), is faster at the air/aqueous interface tha in bulk aqueous solution. The different dynamics of MG and DODCI suggest that the interface friction must be described in terms of the orientation and solvent structure in the vicinity of the chromophores involved in the isomerization process.

Experimental Correlation Between Interfacial Water Structure and Mineral Reactivity

We present an experimental demonstration of the effect of solvent structure on the interfacial reactivity of the silica/water interface using in situ vibrational Sum-frequency Generation (vSFG) spectroscopy. The response of the molecular arrangement of the interfacial solvent to the presence of cations is pH dependent with the highest sensitivity at neutral pH, relevant to geochemical and biological environments. The pH-dependent changes in vSFG spectra are in excellent correlation with the enhancement of quartz dissolution in salt water, which was hypothesized by Dove et al. to be due to changes of the interfacial solvent structure at the silica surface. vSFG provides mechanistic insights into silica dissolution and sheds light on the role of ions in altering interfacial solvent ordering, which has implications in fields ranging from protein-water interactions to oil recovery.

Fluorescence Labeling and Quantification of Oxygen-Containing Functionalities on the Surface of Single-Walled Carbon Nanotubes

Fluorescence labeling of surface species (FLOSS) was applied to identify and determine the concentration of oxygen-containing functionalities on single-walled carbon nanotubes (SWCNTs), subjected to two different purification processes (air/HCl and nitric acid treatments) and compared to as-received (nonpurified) SWCNTs. The fluorophores were selected for their ability to covalently bind, with high specificity, to specific types of functionalities (OH, COOH, and CHO). FLOSS revealed that even as-received SWCNTs are not pristine and contain approximately 0.6 atomic % oxygen functionalities. FLOSS showed that, after nitric acid treatment, SWCNTs are approximately 5 times more functionalized than SWCNTs after air/HCl purification (5 versus 1 atomic % oxygen functionalities), supporting the idea that the former purification process is more aggressive than the latter. FLOSS demonstrated that carbonyls are the major functionalities on nitric-acid-purified SWCNTs, suggesting that chemical derivatization strategies might consider exploiting aldehyde or ketone chemistry.