S. A. Safron

Effect of temperature on the growth of ultrathin films of p-sexiphenyl on KCl(001)

Ultrathin films of p-sexiphenyl, formed by vapor-deposition onto KCl(001) substrates, have been investigated by x-ray diffraction (XRD) and atomic force microscopy (AFM). Analysis of the XRD data shows that the temperature of the alkali halide substrate during deposition affects the orientations of the molecules within the adsorbed films. AFM images contribute independent evidence consistent with the XRD results. The results are reproducible and suggest that ultrathin films of p-6P molecules can be grown with desired molecular orientations by carefully selecting the appropriate substrate temperature during deposition.

Investigation of the morphology of the initial growth of the aromatic molecule p-quaterphenyl on NaCl (001)

Investigations of the structural properties of the initial layers of p-quaterphenyl (p-4P) vapor deposited onto NaCl (001) have been carried out using atomic force microscopy (AFM), grazing incidence x-ray diffraction (GIXRD), and helium atom scattering (HAS). A series of AFM images reveal accumulations of p-4P molecules around surface defects. Film thicknesses determined from height analyses of these images are in reasonable agreement with those found using GIXRD. The GIXRD studies indicate that p-4P films vapor-deposited at ambient temperature are composed of crystallites oriented with the long molecular axis nearly perpendicular to the NaCl (001) surface. For a nominally three-monolayer film, the b axis of the crystallites appears preferentially oriented along the substrate’s [110] azimuth, but with increasing thickness the x-ray features resemble those from two-dimensional (2D) powders. The diffraction peaks found in the HAS investigation at ∼50 K for a thick film grown at ∼200 K are similarly consist...

Surface dynamics of NaCl(001) by inelastic He atom scattering

The relatively simple structure of alkali halide crystals has made these materials the paradigm for the understanding of the dynamics of ionic insulators. We have employed a time‐of‐flight (TOF) technique to determine the energy loss/gain due to single phonon creation/annihilation events in the scattering of He atoms from the NaCl(001) surface. From these data the surface dispersion curves over the ΓM region of the surface Brillouin zone have been constructed. The results compare favorably with both the slab dynamics and the Green’s function calculations reported for this crystal, and with previous measurements in the 〈100〉 direction. Of particular interest, we report the first evidence for the crossing mode in the 〈100〉 direction, which had been predicted for NaCl. Additionally, we have employed the TOF technique to examine the Debye‐Waller attenuation of the elastic (specular) intensity and the relative height of an inelastic multiphonon ‘‘foot’’ at the base of the specular beam as a function of the c...

Epitaxial growth of KBr onto NaCl(001) by high-resolution He atom scattering

Abstract The vapor deposition and subsequent growth of KBr onto the cleaved (001) surface of single crystal NaCl were studied by measuring the scattering behavior of a nearly monoenergetic (Δ E / E ⋍ 2%) He atom beam. Oscillations in the intensity of the specular beam due to layer-by-layer growth were observed for the first several layers. The structure of the surface for each deposited layer was determined by He diffraction via angular distribution measurements. The surface lattice spacings for NaCl and KBr have a mismatch of about 17% (3.99Afor NaCl and 4.66Afor KBr) and, hence, 6 KBrs can fit very nearly onto 7 NaCls (∼ 28A). The measured diffraction patterns showed peaks corresponding to a surface corrugation with this length. For the first adlayer, peaks only for NaCl and this “superstructure” were observed. As additional layers were deposited, the NaCl peaks disappeared and the superstructure peaks at first increased and then diminished as the Bragg peaks corresponding to the KBr lattice spacing grew in. By 6 monolayers only the diffraction peaks due to the KBr were observed. Additionally, time-of-flight energy analysis of the inelastically scattered He atoms was employed to determine the surface phonon dispersion curves of the adlayer. After only 4 monolayers, the Rayleigh and crossing modes were found to be almost identical to those measured from a cleaved KBr(001) surface with the exception of a non-zero value for the Rayleigh mode at the zone center. However, possible evidence for strain was seen in the S 2 mode, a surface optical mode lying in the gap between the acoustic and optical bulk bands. Similar behavior in the measured diffraction has been found in current complementary experiments for NaCl deposited onto KBr(001).

Surface lattice dynamics of NiO (001) by inelastic He atom scattering

High‐resolution inelastic He atom scattering experiments, employing a time‐of‐flight (TOF) technique, have been carried out on NiO (001) in the 〈100〉 and 〈110〉 high symmetry directions. The targets were prepared by cleaving single‐crystal NiO in air and then baking in vacuum. Nearly all of the experiments reported here were carried out at a target temperature of 575 K, which is above the Neel point (TN = 523 K), and with He atom wave vectors in the range of 7–9 A−1 (25–45 meV). Because of these high temperatures, the peaks in the TOF spectra were found to be somewhat broader and the multiphonon background somewhat higher than in reported spectra from this instrument for alklai halide surfaces. Nonetheless, sufficient data were obtained to allow the determination of the Rayleigh wave almost to the Brillouin zone boundary. The results for this mode are found to be in good agreement with recently published, room‐temperature electron energy loss spectroscopy experiments. In contrast, there is a substantial di...

Surface lattice dynamics of the Rbl (001) crystal surface via inelastic he atom scattering

Abstract High-resolution He atom scattering experiments (HAS) are reported, which were carried out on a cleaved Rbl (001) surface (T s = 120K). The nearly monoenergetic ( Δv v ≈ 1%) He beam, produced from a high pressure nozzle source, was chopped into pulses for the analysis of the inelastic scattering by a time-of-flight (TOF) technique. The incident He atom energies employed in these studies ranged from 20 to 24 meV, corresponding to wavevectors of 6.2 to 6.8 A −1 . Data from single phonon annihilation and creation events were obtained in both the and surface high symmetry directions and used to construct the surface phonon dispersion curves. The experimental results are compared with previous theoretical calculations for this surface. In general, the observed Rayleigh wave is found to be in very good agreement. However, a crossing mode is found which was not predicted initially, but which has now been seen in the (001) surfaces of several other alkali halides. Perhaps most interesting is that the data support the existence of the predicted high-lying surface optical branch, S 2 , pushed above the optical bulk bands because of the surface relaxation. In addition, another newly discovered branch, extending about a third of the way from the M point to the \ gG point near the top of the acoustic band is probably also due to the surface relaxation. It is possibly associated with second layer vibrations.

Homoepitaxial growth investigated by high‐resolution He atom scattering: NaCl onto NaCl(001)

The homoepitaxial growth of NaCl vapor deposited onto the cleaved (001) surface of NaCl was studied with a nearly monoenergetic (ΔE/E≂2%) He atom beam. In comparison with reflection high‐energy electron diffraction, the He atom is unique for epitaxial studies in that it is uncharged, nondestructive, and sensitive only to the outer surface layer. The oscillations in the scattered intensities of the specular and Bragg peaks due to the layer‐by‐layer growth were examined by measuring the angular distributions as a function of coverage, surface temperature (150–400 K) and incident He atom wave vector (6–8 A−1). The periodic variation in the relative heights of the specular and Bragg peaks with Δkz=‖kfz−kiz‖ showed that under these conditions the growth is two‐dimensional with a terrace height of 2.8 A or half the bulk face‐centered‐cubic lattice spacing. In addition, scattering cross sections for adsorbed NaCl at low coverages were determined with respect to these same parameters. The cross sections for subst...

Surface and bulk phonon‐assisted resonances with surface bound states in the inelastic scattering of He atoms on NaCl (001)

High‐resolution He atom scattering experiments have been carried out on the NaCl (001) surface in both the 〈100〉 and 〈110〉 surface high symmetry directions. Time‐of‐flight (TOF) arrival spectra of the inelastically scattered He atoms were analyzed to obtain the surface phonon dispersion curves. Several of the TOF peaks corresponding to Rayleigh mode phonons were found to have relatively large intensities and there appeared to be a number of well‐resolved peaks in the time‐of‐flight which did not correspond to surface‐localized modes when the data were compared with previous calculations for NaCl. These data appear to be associated with surface and bulk phonon‐assisted resonances with surface bound states. Analysis of these resonances yields results consistent with the four bound states of He on NaCl (001) which had been previously reported.

Surface phonon modes of the NaI(001) crystal surface by inelastic He atom scattering

The present theoretical treatment of the surface dynamics of ionic insulators employs the shell model with parameters obtained from bulk materials. The approach has been generally very successful in comparisons with experiment. However, most of the experimental surface dynamics work has been on the low‐mass alkali halides with very little effort on higher energy modes or on the heavier alkali halides, where effects from relaxation might be important. The work of this paper explores these latter two conditions. Inelastic scattering of He atoms from the 〈110〉NaI(001) surface has been used to obtain the acoustic S1 Rayleigh mode, the S6 longitudinal mode, and the S8 crossing mode, however, no gap S4 optical mode was seen. The results compare favorably with reported theoretical models employing both slab calculations and the Green’s function method thus indicating that bulk parameters and the shell model go a long way in explaining most of the observations.

Surface phonon dispersion curves via thermal energy atom scattering: He atoms on RbCl(001) surface

A thermal energy atom scattering instrument, employing a time‐of‐flight technique to observe inelastically scattered He atoms, has been used to measure the surface phonon dispersion curves of the RbCl(001) surface. Data were collected over the entire Brillouin zone for a target temperature of 115 K and incident He wave vector of ki ≊7 A−1. The results for the 〈100〉 and 〈110〉 high‐symmetry directions are in good agreement with theoretical predictions based on a slab dynamics model calculation. Relaxation effects of the surface are negligible for this crystal.