We report an ultrahigh vacuum scanning tunneling microscopy study of thermally driven interface rearrangement in binary films of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and zinc phthalocyanine (ZnPc), a model electron acceptor electron donor system for organic solar cells. Neat PCBM films have been previously shown to undergo a transition from a disordered (glassy) phase to a crystalline hexagonal close-packed (hcp) arrangement above a critical packing density of 0.9 molecules/nm(2). We now show how local PCBM density has a critical impact on binary film structure evolution. Bilayer films of PCBM and ZnPc undergo a spontaneous vertical phase separation to PCBM/ZnPc/Au(111) stacking at lower (<0.9 molecules/nm(2)) PCBM densities. This vertical phase separation is shown to be electrostatically preferred, with the PCBM/ZnPc/Au(111) dimer stacking lower in energy by 0.16 eV/pair relative to ZnPc/PCBM/Au(111) stacking. At higher local PCBM densities, sufficient to nucleate hcp PCBM domains, ZnPc molecules do not displace PCBM to the second layer. PCBM density variations in binary films thus give rise to heterogeneous interface structures.

Thin films of metal phthalocyanine (MPc) are grown on an Au(111) support with a newly developed aerosol molecular beam deposition source and characterized in situ via ultrahigh vacuum scanning tunneling microscopy. MPcs are delivered to Au(111) in a series of N2-entrained microsized solvent droplets of variable surface residence time. Phthalocyanine film registration to the herringbone reconstruction of the Au(111) surface, indicative of thermodynamically favored structure, is observed at submonolayer coverages for aromatic solvents with long residence times. Aerosol-deposited monolayer film structures are noncrystalline with tilted MPc orientations and vacancy nanocavities. Upon annealing, MPc molecules adopt flat-lying orientations with respect to the substrate and vacancies are eliminated. Film morphologies indicate solvation-mediated film nucleation and growth, with less long-range ordering that in vapor-generated films.

Structure evolution in monolayer films of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) deposited on Au(111) was determined via ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Molecular organization was monitored from a glassy phase, produced via a pulsed microaersol molecular beam deposition source, through ordered arrangements, following thermal annealing. At lower PCBM densities, two double-row structures arise, involving distinct PCBM hydrogen-bonding motifs, reminiscent of patterns produced by solvent-free deposition. At higher PCBM monolayer densities, hexagonal close-packed islands of PCBM form, with a 0.98 nm nearest-neighbor spacing in good agreement with structure predictions [Napoles-Duarte et al. Phys. Rev. B 2008, 78, 035425]. Under UHV conditions, solvent molecules are retained in the organic layer to temperatures up to 200 C, inhibiting PCBM ordering. Following complete solvent removal, nanosized hexagonal close-packed PCBM islands show arrested development, indicating a kinetic barrier to island growth, attributed to reorientational energy costs.

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(100) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(100) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4eV above EF and is interpreted as the CO 2π∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220K and ∼265K. These TPD peak temperatures are correlated with different C–O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.

Submitted for the MAR08 Meeting of The American Physical Society Modifying the Adsorption of Molecules at Metal Surfaces by Quantum Confinement of Electrons LEVAN TSKIPURI, ROBERT BARTYNSKI, Rutgers University — We have studied the bonding of CO on several ultrathin Cu and Co films that exhibit metallic quantum well (MQW) states, whose energies change as a function of overlayer thickness, using inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). For Co system, which has unoccupied MQW states that do not cross the Fermi level, a CO 2π-induced feature is observed in IPE at 3.8 eV above EF . CO desorbs at 375 K ( 30 K lower than for hcp Co surfaces) and a second TPD feature at 230 K appears upon low temperature (∼ 100K) dosing. These TPD peak temperatues change as a function of film thickness and are correlated with two different C-O stretch vibrational frequencies observed in the IR spectra. The intensity of the C-O stretch feature in IRAS spectra, and the peak CO desorption temperature in TPD from CO on Cu MQWs both show modulations that are correlated with MQW states crossing the EF. We have also studied the influence of MQW states on the adsorption properties of the dimethyl disulfide (thiol) molecule (CH3S)2, which forms a self-assembled monolayer when adsorbed on the Cu(100) surface. Levan Tskipuri Rutgers University Date submitted: 27 Nov 2007 Electronic form version 1.4

In the present paper detailed coordinate free description of the classical and quantum dynamics of free particle on SU(2) group manifold is carried out.