J. R. Engstrom

Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface Coverage

The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order.

Supersonic molecular beam scattering as a probe of thin film deposition processes

The reactions of silane, SiH4, disilane, Si2H6, and phosphine, PH3, on single crystalline Si(100) and Si(111) surfaces, and methylsilane, SiH3CH3, on a β‐SiC surface have been examined employing supersonic molecular beam scattering. The emphasis here is not on any one experimental result, but rather on the specific experimental approaches adopted and a selected set of results that serve to demonstrate the similarities and differences between these systems and the more extensively studied reactions occurring on transition metal surfaces. All reactions have been examined at substrate temperatures characteristic of steady‐state thin film growth. Translational activation is observed to be an efficient means to promote the reactivity of the group IV species: SiH4, Si2H6, and SiH3CH3. In all cases, the reactivity increases exponentially with scaled incident kinetic energy, where the scaling analysis specifically takes into account the microcorrugation of the gas‐surface potential in terms of how incident kineti...

A kinetic Monte Carlo study of the growth of Si on Si(100) at varying angles of incident deposition

Abstract Two- and three-dimensional kinetic Monte Carlo simulations were used to model the deposition of a hyperthermal molecular beam at varying angles of incidence. The simulations incorporate realistic deposition and diffusional moves, and feature many-layer growth for large systems containing up to 80 000 atoms. Kinetic Monte Carlo simulations for the three-dimensional model mimic the deposition and growth of Si on a Si(100) substrate at close to experimental length- and timescales. At high angles of incidence for the beam, the formation of porous columnar structures seen in the two-dimensional simulations evolve into flakes in the three-dimensional model. The growth angles of the columns and flakes follow the same general trends as previous ballistic deposition and molecular dynamics simulations, but existing theories do not adequately represent the simulation data. In the two-dimensional model, the effect of an additional step-edge reflection barrier increases the porosity of the deposited films at conditions for which columnar growth would be observed in the absence of the additional barrier. Raising the substrate temperature increases the widths of the columns and flakes perpendicular to the path of the beam. Increased substrate temperature also affects the depth to which grown films remain defect-free before columnar growth begins.

Dynamics of the dissociative adsorption of disilane on Si(100) : energy scaling and the effect of corrugation

The reaction of Si2H6 with the Si(100) surface has been examined via supersonic molecular beam scattering techniques. It is found that the reaction probability is most sensitive to the incident translational energy, varying nearly linearly with increasing energy for 〈Etr〉≳1 eV. The effect of incident angle θi is described by a model that accounts explicitly for surface corrugation and assumes that the reaction probability varies with 〈Etr〉 cos2 θloc, where θloc is the incident angle with respect to the local surface normal.

Post-deposition reorganization of pentacene films deposited on low-energy surfaces

We demonstrate that small-molecule organic thin films of pentacene deposited from thermal and supersonic molecular beam sources can undergo significant reorganization under vacuum or in N2 atmosphere, beginning immediately after deposition of thin films onto SiO2 gate dielectric treated with hexamethyldisilazane (HMDS) and fluorinated octyltrichlorosilane (FOTS). Films deposited on bare SiO2 remain unchanged even after extended aging in vacuum. The changes observed on low-energy surfaces include the depletion of molecules in the interfacial monolayer resulting in the population of upper layers via upward interlayer transport of molecules, indicating a dewetting-like behavior. The morphology of pristine, as-deposited thin films was determined during growth by in situ real-time synchrotron X-ray reflectivity and was measured again, ex situ, by atomic force microscopy (AFM) following aging at room temperature in vacuum, in N2 atmosphere, and in ambient air. Important morphological changes are observed in ultra-thin films (coverage < 5 ML) kept in vacuum or in N2 atmosphere, but not in ambient air. AFM measurements conducted for a series of time intervals reveal that the rate of dewetting increases with decreasing surface energy of the gate dielectric. Films thicker than ∼5 ML remain stable under all conditions; this is attributed to the fact that the interfacial layer is buried completely for films thicker than ∼5 ML. This work highlights the propensity of small-molecule thin films to undergo significant molecular-scale reorganization at room temperature when kept in vacuum or in N2 atmosphere after the end of deposition; it should serve as a cautionary note to anyone investigating the behavior of organic electronic devices and its relationship with the initial growth of ultra-thin molecular films on low-energy surfaces.

Dissociative adsorption of Si2H6 on silicon at hyperthermal energies : the influence of surface structure

The reactions of Si2H6 with the (100) and (111) surfaces of silicon have been investigated employing supersonic molecular beam scattering techniques. Incident translational energy has been found to influence strongly the probability of dissociative adsorption (SR) on both surfaces. The reaction on the Si(111) surface is distinct from that observed on Si(100) concerning the dependence of SR on the substrate temperature and the incident angle. In particular, the reaction on the (111) surface shows sensitivity to the surface phase transformation (7×7)→(1×1) that occurs above 825 °C. Both the dangling bond density and the effective corrugation of the surface are important in determining the reaction probability.

Dissociation and pyrolysis of Si2H6 on Si surfaces: The influence of surface structure and adlayer composition

The reaction of disilane, Si2H6, with the Si(100) and Si(111) surfaces has been examined with supersonic molecular beam scattering techniques. The emphasis has been on elucidating the reaction mechanism operative under conditions leading to steady‐state Si epitaxial growth. Two reaction mechanisms have been identified: (i) complete pyrolysis to form two adsorbed Si atoms and gas phase hydrogen; and (ii) a reaction forming one adsorbed Si atom, gas phase hydrogen, and silane, SiH4, as a gas phase product. The relative predominance of these two channels is sensitive to surface structure, adlayer composition, and incident kinetic energy. In particular, only complete pyrolysis is observed on the clean Si(100)‐(2×1) and Si(111)‐‘‘(1×1)’’ surfaces. The silane production channel, on the other hand, is observed on the Si(111)‐(7×7) surface, and on the Si(100)‐(2×1) surface in the presence of a finite coverage of either adsorbed hydrogen or phosphorus atoms. Examination of the reaction dynamics reveals that the pr...

THE ROLE OF SURFACE CORRUGATION IN DIRECT TRANSLATIONALLY ACTIVATED DISSOCIATIVE ADSORPTION

Recent experimental results concerning reactive scattering in the hyperthermal kinetic energy regime can be described by energy scaling relationships Ei cosn θi, where n<2, and Ei and θi are the incident kinetic energy and incident angle, respectively. Such power law scaling arguments are empirical, the results of which cannot easily be related to the fundamental parameters that describe the gas–surface interaction. We present a detailed and thorough analysis where the role of surface corrugation in determining the coupling between incident kinetic energy and incident angle in these translationally activated systems is considered explicitly. The key features of the analysis involve the assumption that the kinetic energy directed along the local surface normal (E⊥) controls the reaction probability (SR), and that by averaging this quantity over the unit cell, one obtains the appropriate energy scaling relationship. The major advantage associated with the proposed analysis is that one need not assume a func...

Thermal and plasma‐assisted nitridation of GaAs(100) using NH3

The thermal and plasma‐assisted nitridation of GaAs(100) using NH3 has been examined employing x‐ray diffraction, Auger electron spectroscopy, and atomic force microscopy to characterize the nitrided films. All thermally nitrided films were composed of a mixture of hexagonal and cubic GaN, whereas the addition of plasma excitation produced films purely of the cubic structure. Thicknesses of the thermally nitrided films, up to 7000 A, increased with both increasing temperature and nitridation time. The plasma‐assisted process holds promise for the formation of templates for homoepitaxial growth of cubic GaN.

Quantitative modeling ofin situx-ray reflectivity during organic molecule thin film growth

Synchrotron-based x-ray reflectivity is increasingly employed as an \textit{in situ} probe of surface morphology during thin film growth, but complete interpretation of the results requires modeling the growth process. Many models have been developed and employed for this purpose, yet no detailed, comparative studies of their scope and accuracy exists in the literature. Using experimental data obtained from hyperthermal deposition of pentane and diindenoperylene (DIP) on SiO