Chain-Length Dependence of the Dissociation Dynamics of Oriented Molecular Adsorbates: n-Alkyl Bromides on GaAs(110)

Developing 1D nanostructure arrays for future nanophotonics

There is intense and growing interest in one-dimensional (1-D) nanostructures from the perspective of their synthesis and unique properties, especially with respect to their excellent optical response and an ability to form heterostructures. This review discusses alternative approaches to preparation and organization of such structures, and their potential properties. In particular, molecular-scale printing is highlighted as a method for creating organized pre-cursor structure for locating nanowires, as well as vapor–liquid–solid (VLS) templated growth using nano-channel alumina (NCA), and deposition of 1-D structures with glancing angle deposition (GLAD). As regards novel optical properties, we discuss as an example, finite size photonic crystal cavity structures formed from such nanostructure arrays possessing highQand small mode volume, and being ideal for developing future nanolasers.

Tuning Molecule−Surface Interactions with Sub-Nanometer-Thick Covalently Bound Organic Monolayers

Measurements of the thermal desorption of methyl bromide (MeBr) from bare and RS-functionalized GaAs(110), where R = CH3 and CH3CH2, reveal marked systematic changes in molecule-surface interactions. As the thickness of the organic spacer layer is increased, the electrostatic MeBr-GaAs(110) interaction decreases, lowering the activation energy for desorption, Ed, as well as decreasing the critical coverage required for nucleation of bulklike MeBr. On the CH3CH2S-functionalized surface, Ed is lowered to a value roughly equal to that for desorption from three-dimensional (3-D) clusters; because the kinetics of desorption of isolated molecules differs from that for desorption from clusters, desorption of isolated molecules from the organic surface occurs at a lower temperature than desorption from the clusters. Thus, the "monolayer" desorption wave occurs at a lower temperature than the "multilayer" desorption wave. These results illustrate the role that organic chain length in nanometer-scale thin films can play in alteration of the delicate balance of interfacial interactions.

Photoinduced-Reaction Dynamics of Halogenated Alkanes on Iron Oxide Surfaces: CH3I on Fe3O4(111)−(2×2)

The adsorption, thermal chemistry, and photoreaction dynamics of methyl iodide on the (2x2) magnetite termination of natural single-crystal hematite have been investigated by time-of-flight quadrupole mass spectrometry (TOF-QMS), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). The methyl iodide thermal desorption spectra, taken after dosing the (2x2) surface at 100 K with CH(3)I, show a multiple-peak coverage-dependent behavior, consistent with the presence of several distinct adsorbed phases, along with defect-mediated dissociative chemisorption in the first monolayer. At >1 ML, methyl iodide forms a metastable physisorbed second layer, which desorbs at 148 K, but at higher coverage converts to a layer, which desorbs at 170 K. In the presence of low-fluence-pulse irradiation at 248 nm, angle-resolved TOF-QMS measurements show that 1.6 and 0.3 eV CH(3) fragments are ejected from the adsorbate surface; these fragments originate from direct photodissociation and dissociative photoinduced electron transfer, respectively. These energetic photoejected fragments have characteristic angular distributions peaked at approximately 0 degree with respect to the surface normal. These results and the coverage-dependent relative intensities suggest that the predominant orientation in the first monolayer of the adsorbed CH(3)I is normal to the crystal plane.

Surface-reconstruction-switched adsorbate photofragmentation dynamics

Energy-resolved angular distributions of neutral fragments ejected during photoinduced electron transfer reaction of CH3Br on GaAs(100) exhibit three distinct methyl-radical ejection channels. These undergo marked changes when the termination is switched from the Ga-rich c(8 x 2) to the As-rich c(2 x 8). Our observations are consistent with a strong adsorption-site dependence of the dynamics.