Gary W. Leach

Surface vibrational coherence at the CaF2/air interface: Vibrational wave packet dynamics as a probe of interface inhomogeneity

Vibrational wave packet dynamics from a monolayer-covered surface are reported. These dynamics reflect surface vibrational coherence in a monolayer of amphiphilic molecules deposited at the CaF2/air interface. The induced macroscopic polarization following coherent excitation of adsorbate CH3 vibrational modes displays quantum interference effects (quantum beats) and decays on a time scale dependent on the nature of the interfacial environment. These observations provide a link between the degree of interfacial order and the vibrational coherence lifetime and demonstrate that monitoring interfacial wave packet dynamics represents a new method for characterizing these important chemical regions.

Azines possessing strong push–pull donors/acceptors

Azines (R(2)C[double bond, length as m-dash]N-N[double bond, length as m-dash]CR(2)) are 2,3-diaza analogues of 1,3-butadiene. In this report we show that strong polarisation of the azine imparts structural features consistent with delocalization within the azine fragment; NLO properties for the azines are also reported.

Large scale crystallinity in kinetically stable polythiophene-based Langmuir-Blodgett films.

The first direct observation of large scale molecular level ordering in a complex poly(3-alkylthiophene)-based thin film is reported. The fabrication and characterization of highly ordered thin films made from the regioregular, amphiphilic polythiophene derivative poly(3-(11-(2-tetrahydropyranyloxy)undecyl)thiophene) (PTHPUDT) are described. PTHPUDT affords kinetically stable, high optical quality films deposited layer-by-layer using the Langmuir-Blodgett (LB) method. X-ray diffraction studies confirm the deposition of a film with long-range order normal to the surface, characterized by bilayer separations of approximately 30 A. The films display in-plane anisotropy, associated with the preferential alignment of the polymer main chain in the dipping direction. Molecular resolution atomic force microscopy shows the presence of highly ordered crystalline domains within the plane, consistent with an ordered array of parallel, closed-packed, polythiophene chains. Polarized optical microscopy confirms the formation of large scale domains that display uniform optical retardation across macroscopic length scales. FeCl(3)-doped films yield anisotropic conduction behavior, suggesting higher rates of transport perpendicular to the main chain direction. The kinetic stability of these films is distinct from films of other polythiophenes, other deposition methods, and simpler amphiphiles, all of which tend toward their bulk, thermodynamically stable structures. This stability and long-range order are attributed to the amphiphilic nature of the polymer and the dimensional restrictions of the polymers main chain, which limits the extent of structural defect-based reorganization, and the limited number of local structures of the alkyl chains. The degree and nature of the ordering in these semicrystalline films make them an ideal model system in which to elucidate the connection between morphology and physical property in complex pi-conjugated polymers.

Room temperature deposition of highly sensitive vanadium oxide films for infrared light sensing applications

Uncooled infrared micro bolometer detectors have gained increasing attention for military and commercial imaging applications. Due to their light weight, low power, response in the infrared region, and compatibility with readout integrated circuits, traditional thermo-resistive vanadium oxide based micro-bolometers have become popular compared to pyro-electric and photo-detectors. Traditionally, VOx films are fabricated using ion beam or magnetron sputtering 1, 2. We report on using low cost dip coating deposition of an alkoxide sol gel precursor to fabricate uniform single and triple layers of vanadium oxide to a thickness of 150nm. Experimental results demonstrate the highest reported high temperature coefficient of resistance values for these films at -2.5%/°K, making the dip coating method a low cost alternative for the fabrication of IR sensing applications.

Immunizing quantum information against decoherence via decoherence-free environments

We construct decoherence-free environments (DFEs) within the Hamiltonian formalism and derive an exact equation of motion for a quantum subsystem interacting with a DFE. We discuss the feasibility of DFEs as a means to protect the quantum information in subsystem degrees of freedom. By studying a number of examples we show how to avoid entanglement-induced decoherence as well as decoherence induced by incoherence dynamics in a quantum subsystem coupled to a DFE.

A low temperature phase transition in Langmuir-Blodgett films.

The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of approximately 8 cm (-1) in the r+, r- and rfermi modes of the terminal methyl groups at approximately 150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphiles headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems.

Flexible near infrared photoresistors based on recrystallized amorphous germanium thin films

In this paper we report on recrystallized amorphous hydrogenated germanium (a-Ge:H) thin film on polyimide to fabricate flexible near infrared (NIR) light sensitive resistors that can be used in flexible electronics. In this work we investigate the effect of pulsed excimer laser annealing of a-Ge:H thin films for fabrication of low temperature photo-detectors operating in the NIR.

A Comparative Study of the Structure/Property Relationships of Langmuir-Blodgett and Spin-Cast Films of an Amphiphilic, Regioregular Polythiophene Polymer

The effects of structural conformation and order on materials properties and reactivity in thin films of an amphiphilic, regioregular polythiophene π-conjugated polymer (πCP) derivative are presented. Thin films deposited by the Langmuir Blodgett technique provide highly ordered anisotropic films compared to those deposited by spin cast methods. Large differences in both the thermal reactivity and physical properties of the films are observed.