The surface structure and vibrational dynamics of CH 3 -Si͑111͒ and CD 3 -Si͑111͒ surfaces were measured using helium atom scattering. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The excellent quality of the observed diffraction patterns, along with minimal diffuse background, indicated a high degree of long-range ordering and a low defect density for this interface. The vibrational dynamics were investigated by measurement of the Debye-Waller attenuation of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular ͑ i = f ͒ decay revealed perpendicular mean-square displacements of 1.0ϫ 10 −5 Å 2 K −1 for the CH 3 -Si͑111͒ surface and 1.2ϫ 10 −5 Å 2 K −1 for the CD 3 -Si͑111͒ surface, and a He-surface attractive well depth of ϳ7 meV. The effective surface Debye temperatures were calculated to be 983 K for the CH 3 -Si͑111͒ surface and 824 K for the CD 3 -Si͑111͒ surface. These relatively large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through the Si-C local molecular modes. The parallel mean-square displacements were 7.1ϫ 10 −4 and 7.2ϫ 10 −4 Å 2 K −1 for the CH 3 -Si͑111͒ and CD 3 -Si͑111͒ surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and Si-CH 3 bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces.
The electronic structure of single InSb quantum dots (QDs) with diameters between 3 and 7 nm was investigated using atomic force microscopy (AFM) and scanning tunneling spectroscopy (STS). In this size regime, InSb QDs show strong quantum confinement effects which lead to discrete energy levels on both valence and conduction band states. Decrease of the QD size increases the measured band gap and the spacing between energy levels. Multiplets of equally spaced resonance peaks are observed in the tunneling spectra. There, multiplets originate from degeneracy lifting induced by QD charging. The tunneling spectra of InSb QDs are qualitatively different from those observed in the STS of other III-V materials, for example, InAs QDs, with similar band gap energy. Theoretical calculations suggest the electron tunneling occurs through the states connected with L-valley of InSb QDs rather than through states of the Γ-valley. This observation calls for better understanding of the role of indirect valleys in strongly quantum-confined III-V nanomaterials.
The effect of chain length on the low-energy vibrations of alkanethiol striped phase self-assembled monolayers on Au(111) was studied. We have examined the low-energy vibrational structure of well-ordered, low-density 1-decanethiol (C10), 1-octanethiol (C8), and 1-hexanethiol (C6) to further understand the interaction between adsorbate and substrate. Dispersionless Einstein mode phonons, polarized perpendicularly to the surface, were observed for the striped phases of C10, C8, and C6 at 8.0, 7.3, and 7.3 meV, respectively. An overtone at 12.3 meV was also observed for C6/Au(111). These results, in concert with molecular dynamics simulations, indicate that the forces between the adsorbate and substrate can be described using simple van der Waals forces between the hydrocarbon chains and the Au substrate with the sulfur chemisorbed in the threefold hollow site.
Collisional energy transfer at the surface of poly͑methyl methacrylate͒ thin films on SiO x / Si was investigated using low-energy neutral helium atom scattering. Analysis of spectra in two scattering regimes yields results consistent with the hypothesis that thinner films are stiffer, suggesting that for highly nanoconfined films, polymer-substrate interactions influence vibrational dynamics at the polymer-vacuum interface. Specifically, thinner films are found to have lower mean-square displacements and decreased annihilation events as compared to thicker films. The scattering spectra are fit well by a semiclassical scattering model, though deviations arise at sample temperatures near the bulk glass transition. We have found helium atom scattering to be a sensitive probe of the vibrational dynamics of the polymer thin film surface. This technique holds promise for the exploration of glassy dynamics of polymer thin films.
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