Atomic force microscopy measurements of topography and friction on dotriacontane films adsorbed on a SiO2 surface

doi:10.1063/1.2060707 (8 pages)We report comprehensive atomic force microscopy (AFM) measurements at room temperature of the nanoscale topography and lateral friction on the surface of thin solid films of an intermediate-length normal alkane, dotriacontane (n-C32H66), adsorbed onto a SiO2 surface. Our topographic and frictional images, recorded simultaneously in the contact mode, reveal a multilayer structure in which one to two layers of molecules adsorb adjacent to the SiO2 surface oriented with their long axis parallel to the interface followed by partial layers of molecules oriented perpendicular to the surface. The thicknesses of the parallel and perpendicular layers that we measured with the AFM agree with those inferred from previous x-ray specular reflectivity measurements on similarly prepared samples. We also observe bulk dotriacontane particles and, in contrast with our previous measurements, are able to determine their location. Above a minimum size, the bulk particles are separated from islands of perpendicularly oriented molecules by regions of exposed parallel layers that most likely extend underneath the particles. We find that the lateral friction is sensitive to the molecular orientation in the underlying crystalline film and can be used effectively with topographic measurements to resolve uncertainties in the film structure. We measure the same lateral friction on top of the bulk particles as on the perpendicular layers, a value that is about 2.5 times smaller than on a parallel layer. Scans on top of parallel layers indicate a constant height but reveal domains having different sublevels of friction. We explain this by the domains having different azimuthal orientations of the molecules.This work was supported by U.S. National Science Foundation under Grant Nos. DMR-0109057 and DMR-0411748, by the Chilean government under FONDECYTGrant Nos. 1010548 and 7010548, and by the Fundacion Andes Grant No. C-13768

Kinetic model of II-VI(001) semiconductor surfaces: Growth rates in atomic layer epitaxy

We present a zinc-blende lattice gas model of II-VI(001) surfaces, which is investigated by means of Kinetic Monte Carlo (KMC) simulations. Anisotropic effective interactions between surface metal atoms allow for the description of, e.g., the sublimation of CdTe(001), including the reconstruction of Cd-terminated surfaces and its dependence on the substrate temperature T. Our model also includes Te-dimerization and the potential presence of excess Te in a reservoir of weakly bound atoms at the surface. We study the self-regulation of atomic layer epitaxy (ALE) and demonstrate how the interplay of the reservoir occupation with the surface kinetics results in two different regimes: at high T the growth rate is limited to 0.5 layers per ALE cycle, whereas at low enough T each cycle adds a complete layer of CdTe. The transition between the two regimes occurs at a characteristic temperature and its dependence on external parameters is studied. Comparing the temperature dependence of the ALE growth rate in our model with experimental results for CdTe we find qualitative agreement.Comment: 9 pages (REVTeX), 8 figures (EPS). Content revised, references added, typos correcte

Nanoscale Observation of Alkane Delayering

Noncontact Atomic Force Microscopy and synchrotron x-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point Tb in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above Tb and to a solid 3D phase on cooling below Tb. An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.Comment: 13 pages, 3 figure

A lattice gas model of II-VI(001) semiconductor surfaces

We introduce an anisotropic two-dimensional lattice gas model of metal terminated II-IV(001) seminconductor surfaces. Important properties of this class of materials are represented by effective NN and NNN interactions, which result in the competition of two vacancy structures on the surface. We demonstrate that the experimentally observed c(2x2)-(2x1) transition of the CdTe(001) surface can be understood as a phase transition in thermal equilbrium. The model is studied by means of transfer matrix and Monte Carlo techniques. The analysis shows that the small energy difference of the competing reconstructions determines to a large extent the nature of the different phases. Possible implications for further experimental research are discussed.Comment: 7 pages, 2 figure

The R403Q Myosin Mutation Implicated in Familial Hypertrophic Cardiomyopathy Causes Disorder at the Actomyosin Interface

Mutations in virtually all of the proteins comprising the cardiac muscle sarcomere have been implicated in causing Familial Hypertrophic Cardiomyopathy (FHC). Mutations in the beta-myosin heavy chain (MHC) remain among the most common causes of FHC, with the widely studied R403Q mutation resulting in an especially severe clinical prognosis. In vitro functional studies of cardiac myosin containing the R403Q mutation have revealed significant changes in enzymatic and mechanical properties compared to wild-type myosin. It has been proposed that these molecular changes must trigger events that ultimately lead to the clinical phenotype.Here we examine the structural consequences of the R403Q mutation in a recombinant smooth muscle myosin subfragment (S1), whose kinetic features have much in common with slow beta-MHC. We obtained three-dimensional reconstructions of wild-type and R403Q smooth muscle S1 bound to actin filaments in the presence (ADP) and absence (apo) of nucleotide by electron cryomicroscopy and image analysis. We observed that the mutant S1 was attached to actin at highly variable angles compared to wild-type reconstructions, suggesting a severe disruption of the actin-myosin interaction at the interface.These results provide structural evidence that disarray at the molecular level may be linked to the histopathological myocyte disarray characteristic of the diseased state

Where do graduates Develop their Enterprise Skills? The Value of the Contribution of Higher Education Institutions’ Context

This study investigates the value of the contribution of HEIs’ context in developing graduates enterprise skills. HEIs are under pressure to develop more enterprising graduates, particularly with the increasing numbers of graduates seeking employment and the growing dissatisfaction of employers. This study explores where graduates develop enterprise skills through investigating the impact of HE and employment contexts on their development. The paper draws on a qualitative study in the social constructionist paradigm within the pharmacy context, where interviews were conducted with pharmacy academics and employers. Results show that ability to demonstrate skills in one context does not necessarily mean ability to demonstrate them in another since the development and demonstration of enterprise skills is impacted by the contexts in which they are developed and demonstrated. The study adds value by highlighting the significant role of both HE and employment contexts in developing enterprise skills, while emphasising that these skills become more transferable through exposure to more contexts

High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface

doi:10.1063/1.1429645Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C32H66) films adsorbed from a heptane (n-C7H16) solution onto SiO2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this "parallel film" phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at Tb = 341 K, solid bulk particles coexist on top of the "perpendicular film." For higher temperatures in the range TbTs, a uniformly thick fluid film wets to the parallel film phase. This structure of the alkane/SiO2 interfacial region differs qualitatively from that which occurs in the surface freezing effect at the bulk alkane fluid/vapor interface. In that case, there is again a perpendicular film phase adjacent to the air interface but no parallel film phase intervenes between it and the bulk alkane fluid. Similarities and differences between our model of the alkane/SiO2 interface and one proposed recently will be discussed. Our ellipsometric measurements also show evidence of a crystalline-to-plastic transition in the perpendicular film phase similar to that occurring in the solid bulk particles present at higher coverages. In addition, we have performed high-resolution ellipsometry and stray-light measurements on dotriacontane films deposited from solution onto highly oriented pyrolytic graphite substrates. After film deposition, these substrates proved to be less stable in air than SiO2.This work was supported by the Chilean government under CONICYT Grant No. 018/AT/005NSF and FONDECYT Grant No. 1980586 and by the U.S. National Science Foundation under Grant Nos. INT-9605227, DMR-9802476, and DMR-0109057