R. J. D. Miller

Submicron probe of polymer adhesion with atomic force microscopy: Dependence on topography and material inhomogeneities

We have used the atomic force microscope as a nanoindenter to both doped and undoped polycarbonate to probe the dependence of adhesion on topography and material inhomogeneities. Adhesion measurements at the same position are repeatable to 2%. The magnitude of the adhesion is found to decrease as the local curvature on the surface increases. Spatial adhesion maps of doped polymers show structure that is not apparent in surface topography. The spatial resolution of the measurement is at least 300 A.

Investigation of β‐BaB2O4 as a Q switch for high power applications

A beta barium borate (β‐BaB2O4 or BBO) crystal has been used to electro‐optically Q switch both diode‐pumped and lamp‐pumped Nd:YLF laser systems, resulting in stable, high average power operation. Piezoelectric ringing was found to have negligible effects on the performance of the BBO Pockels cell at repetition rates up to 6 kHz. The high damage threshold and low insertion loss of BBO permitted operation up to average powers in excess of 6 kW/cm2, which makes a BBO Pockels cell well suited for high power, small mode volume operation, such as in diode‐pumped solid state lasers. Based on typical diode‐pumped laser parameters, BBO is expected to extend simple compact Q‐switched operation of diode‐pumped lasers to average powers of 30–100 W.

Electron Injection from Adsorbed Oxazine into SnS2

Abstract The injection of photo-generated electrons from an adsorbed dye into a SnS2 crystal has been studied using time-correlated single photon counting. The system studied was oxazine 1 on SnS2. The oxazine S1 level lies .35 eV above the conduction band edge; thus, electron injection should be essentially barrierless. The fluorescence lifetime of adsorbed oxazine was determined to be less than 40 psec, the instrument response time. The fluorescence quenching of the system was determined to be quenched by a factor of nearly 105 (relative to oxazine on an inert surface, 3M tape) using time-correlated techniques to take advantage of the improved background and signal to noise ratio. This places the fluorescence lifetime of oxazine on SnS2 at 40 ± 20 fsec and the rate constant for electron injecton at approximately 3 × 1013 sec−1. This ultrafast rate implies strong coupling between the molecular state and the electronic quasi-continuum of the conduction band, suggesting that this electron transfer takes pl...

Ultrafast electron dynamics in two dimensional layered systems: two-photon photoemission studies of SnS2

Abstract We performed femtosecond two-photon photoemission studies of SnS 2 , and measured the lifetime of electrons in the conduction band at different excitation energies. The lifetime is short (⩽ 60 fs) even at the conduction band minimum. In addition, the inverse lifetime shows a linear energy dependence. The experimental results are interpreted as due to the electron-plasmon interaction in layered two dimensional materials, which causes electron-hole recombination through the emission of a plasmon. These results are discussed in the context of the electron dephasing processes involved in the barrier crossing dynamics for electron transfer from physisorbed molecular donors at semiconductor surfaces (dye sensitization) under barrierless conditions.

Picosecond surface restricted grating studies of n-GaAs(100) liquid junctions. Evidence for interfacial charge transfer approaching adiabatic limits

Abstract In situ grating studies of n-GaAs/1 M KOH (Se 2−/1− ) aqueous liquid contacts have observed 1–2 picosecond decay components in the presence of the selenium redox couple. Based on the bias and concentration dependencies of the grating signal, and known hole scavenging properties of Se 2− , the fast initial decay is assigned to interfacial hole transfer. The similarity in time scales between charge transfer and the diffusive component to the solvent relaxation along the reaction coordinate at the Helmholtz double layer indicates the electronic coupling at the interface is near the strong coupling limit.

Femtosecond photoemission study of electron relaxation in two-dimensional layered electron systems

By using the femtosecond two photon photoemission technique, we directly measured the lifetimes of photoexcited electrons in two layered materials, graphite and SnS2. The inverse lifetimes of photoexcited electrons in these two-dimensional layered materials exhibit linear energy dependence, i.e., 1/τ∝(E−EF), instead of quadratic, i.e., 1/τ∝(E−EF)2 as predicted by Fermi liquid theory. This can be explained with the layered electron gas theory, according to which the electron-plasmon interaction is strong, even for electrons at low excitation energies, due to the formation of plasmon band in layered electron systems. Furthermore, our observations in SnS2 also suggest that in layered semiconductors, photogenerated electron-hole pairs will recombine directly by emitting plasmons, and this causes the lifetimes of the conduction band electrons to be much shorter than those in three-dimensional semiconductors.

Scanning Tunneling Microscopy and Atomic Force Microscopy of Thin Polymer Films

Abstract Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have the potential of giving the three dimensional morphology of polymer surfaces with atomic resolution. We have imaged doped and undoped polyacetylene, poly(3-hexylthiophene), and polystyrene. Both these techniques are well suited to study the fibrillar nature and growth of these films. STM shows that the fibrils of the free side of iodine doped polystyrene are flat to within less that 10 A. The fluctuations in the surface conductivity of polyacetylene and the thermally induced motion of the polymer side groups limit the potential of these techniques to give atomic resolution.