85 research outputs found
Ripple-to-dome transition: the growth evolution of Ge on vicinal Si(1 1 10) surface
We present a detailed scanning tunnelling microscopy study which describes
the morphological transition from ripple to dome islands during the growth of
Ge on the vicinal Si(1 1 10) surface . Our experimental results show that the
shape evolution of Ge islands on this surface is markedly different from that
on the flat Si(001) substrate and is accomplished by agglomeration and
coalescence of several ripples. By combining first principle calculations with
continuum elasticity theory, we provide an accurate explanation of our
experimental observations
Hug-like island growth of Ge on strained vicinal Si(111) surfaces
We examine the structure and the evolution of Ge islands epitaxially grown on
vicinal Si(111) surfaces by scanning tunneling microscopy. Contrary to what is
observed on the singular surface, three-dimensional Ge nanoislands form
directly through the elastic relaxation of step-edge protrusions during the
unstable step-flow growth. As the substrate misorientation is increased, the
islands undergo a shape transformation which is driven by surface energy
minimization and controlled by the miscut angle. Using finite element
simulations, we show that the dynamics of islanding observed in the experiment
results from the anisotropy of the strain relaxation.Comment: 4 figure
Islanding, growth mode and ordering in Si heteroepitaxy on Ge(001) substrates structured by thermal annealing
Si/Ge heteroepitaxial dots under tensile strain are grown on nanostructured
Ge substrates produced by high-temperature flash heating exploiting the
spontaneous faceting of the Ge(001) surface close to the onset of surface
melting. A very diverse growth mode is obtained depending on the specific
atomic structure and step density of nearby surface domains with different
vicinal crystallographic orientations. On highly-miscut areas of the Ge(001)
substrate, the critical thickness for islanding is lowered to about 5 ML, in
contrast to the 11 ML reported for the flat Ge(001) surface, while on
unreconstructed (1x1) domains the growth is Volmer-Weber driven. An explanation
is proposed considering the diverse relative contributions of step and surface
energies on misoriented substrates. In addition, we show that the bottom-up
pattern of the substrate naturally formed by thermal annealing determines a
spatial correlation for the dot sites
Step-step interaction on vicinal Si(001) surfaces studied by scanning tunneling microscopy
We report on measurements of step-step interaction on a flat Si(111)−(7×7) surface and on vicinal Si(001) surfaces with miscut angles ranging between 0.2° and 8°. Starting from scanning tunneling microscopy images of these surfaces and describing steps profile and interactions by the continuum step model, we measured the self-correlation function of single steps and the distribution of terrace widths. Empirical parameters, such as step stiffness and step-step interaction strength, were evaluated from the images. The present experiment allows to assess the dependence of the step-step repulsion on miscut angle, showing how parameters drawn from tunneling images can be used to interpolate between continuum mesoscopic models and atomistic calculations of vicinal surfaces
Evaluation of degree of conversion, rate of cure, microhardness, depth of cure and contraction stress of three nano hybrid composites containing pre-polymerized spherical filler
Aim: Manufacturers aim at improving filler technology to enhance the properties of the restorative materials, thus maximising the aesthetic and functional outcome of the restored tooth. The present study tested the degree of conversion (DC), rate of cure (RC), microhardness (VHN), depth of cure (VHR) and contraction stress (CS) of three new nano hybrid composites with pre-polymerized spherical filler. Methods: Three commercially available composite resin were characterised in the present study, namely the Ceram.X\uae universal shade A3 (CXUA3), Ceram.X\uae duo enamel shade E2, and Ceram.X\uae duo dentin shade D3 (CXDE2 and CXDD3). The materials were light-cured
with a LED light (SmartLite Focus, measured output 1301 mW/cm2) following the protocol recommended by the manufacturer. DC was assessed by means of Fourier-transform infrared spectroscopy, calculating RC from a second-grade polynomial fitting of the kinetic curve. A microhardness testing machine equipped with a Vickers indenter served to measure the top and bottom VHN of 2 mm-high disc-shaped specimens, using the bottom/top surface values ratio (VHR) as indirect evaluation of the depth of cure. CS vs time was evaluated by a universal testing machine provided with an extensometer as feedback system, CS was normalized for the specimen bonding area. All data sets underwent statistical analysis with dedicated software and tested for the assumptions for the use of parametric tests. Multiple analyses of variance with
Scheff\ue9 post hoc test were carried out to compare the dependent variables of interest among the tested materials. Results: All tested materials exhibited a DC lower than
50%, with CXUA3 reaching the lowest DC value after 10 s. RC of CXUA3 at 5 s was comparable to that of CXDE2, while after 10s RC of CXUA3 decreased to a value proportional to that of CXDD3. For all the tested materials, top-VHN was greater than bottom-VHN. Top-VHN of CXDE2 was lower than CXUA3 and CXDD3. CXDD3 was the only material achieving VHR>80%. The main differences in CS among the tested materials were found during the irradiation with curing-light: CXDE2 displaying the lowest CS after 10 s and CXDD3 the highest after 30 s. Conclusion: The present study proved that the light curing protocol suggested by the manufacturer for the three composites might be improved: 10 s of irradiation seemed insufficient to adequately cure CXUA3 and CXDE2. Longer curing times for these materials appear advisable
Fibrin clot adhesion to conditioned root surfaces: an in vitro study with scanning electron microscopy analysis
Aim: Periodontal regeneration is contingent on the adhesion and maturation of fibrin clot to a root surface exposed to periodontal disease. Root surface demineralization in vitro improves the formation of a stable union between the fibrin clot and the root surface. In scientific literature there are not studies of comparison that stand which demineralizing agent is the best in promoting fibrin clot adhesion. The aim of this study was to evaluate and compare the efficacy of six root conditionings in removing the smear layer and developing the fibrin clot in static and dynamic conditions. Methods: 36 single-root teeth extracted for periodontal disease were cut with a microtome in order to obtain 72 specimens that were divided in three groups: 24 samples not covered with blood, 24 covered with fresh human whole blood, 24 covered with blood and
rinsed in a rotary shaker table (Vortex\uae). 4 specimens from each group were conditioned for 3 minutes with: physiological saline solution (FISIO) as the control group, saturated solution of citric acid 25% (AC), ethylenediaminetetraacetic acid 24% (EDTA), a solution of tetracycline 200mg/mL (TETRA), a solution of tetracycline and citric acid (TETRA+AC), Prefgel\uae and successively Emdogain\uae (EMD). Then the 48 samples from group 2 and 3 were covered with blood, which was allowed to coagulate for 20 minutes in a 37\ub0C chamber. The blocks were rinsed and dehydrated under standardized conditions; specimens of group 3 were vortexed (100rpm). All the blocks were then sputtered with gold and analyzed with SEM. SEM images were evaluated by two blinded examiners, starting from the cementoenamel junction (CEJ), at 5 standardized points 2 mm distant from each other. A statistical analysis was performed. Results: EMD samples showed a more disorganized smear layer, in which is probably present the residual vehicle (propylene glycol alginate). However, in the 90% of all the specimens smear layer was found. AC treated samples showed a firmly adherent fibrin clot that covered the surfaces for the 70% of the specimens (the data was statistically relevant). Same results were found in TETRA+AC samples. The
conditioning with EDTA, TETRA and EMD resulted in a sparsely organized clot worsened by the application of tensile forces, especially in TETRA samples. Only few
blood cells without any clot organization were found in the control group, confirming that conditioning root surfaces improves the fibrin clot adhesion. Conclusion: The best formation of fibrin clot was observed for AC treated samples; this is probably due to the increase of root surface wettability caused by AC. The other root conditioning agents, even if lead to results better than the control group, are more susceptible to external forces and do not promote a stable fibrin clot adhesion
Early stage of CVD graphene synthesis on Ge(001) substrate
In this work we shed light on the early stage of the chemical vapor
deposition of graphene on Ge(001) surfaces. By a combined use of microRaman and
x-ray photoelectron spectroscopies, and scanning tunneling microscopy and
spectroscopy, we were able to individuate a carbon precursor phase to graphene
nucleation which coexists with small graphene domains. This precursor phase is
made of C aggregates with different size, shape and local ordering which are
not fully sp2 hybridized. In some atomic size regions these aggregates show a
linear arrangement of atoms as well as the first signature of the hexagonal
structure of graphene. The carbon precursor phase evolves in graphene domains
through an ordering process, associated to a re-arrangement of the Ge surface
morphology. This surface structuring represents the embryo stage of the
hills-and-valleys faceting featured by the Ge(001) surface for longer
deposition times, when the graphene domains coalesce to form a single layer
graphene film
Abrupt changes in the graphene on Ge(001) system at the onset of surface melting
By combining scanning probe microscopy with Raman and x-ray photoelectron
spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a
function of the deposition temperature in close proximity to the Ge melting
point, highlighting an abrupt change of the graphene's quality, morphology,
electronic properties and growth mode at 930 degrees. We attribute this
discontinuity to the incomplete surface melting of the Ge substrate and show
how incomplete melting explains a variety of diverse and long-debated peculiar
features of the graphene/Ge(001), including the characteristic nanostructuring
of the Ge substrate induced by graphene overgrowth. We find that the
quasi-liquid Ge layer formed close to 930 degrees is fundamental to obtain
high-quality graphene, while a temperature decrease of 10 degrees already
results in a wrinkled and defective graphene film.Comment: in pres
Driving with temperature the synthesis of graphene films on Ge(110)
We systematically investigate the chemical vapor deposition growth of
graphene on Ge(110) as a function of the deposition temperature close to the Ge
melting point. By merging spectroscopic and morphological information, we find
that the quality of graphene films depends critically on the growth temperature
improving significantly by increasing this temperature in the 910-930 {\deg}C
range. We correlate the abrupt improvement of the graphene quality to the
formation of a quasi-liquid Ge surface occurring in the same temperature range,
which determines increased atom diffusivity and sublimation rate. Being
observed for diverse Ge orientations, this process is of general relevance for
graphene synthesis on Ge
Modified strain and elastic energy behavior of Ge islands formed on high-miscut Si(0 0 1) substrates
Abstract We investigate here the influence of Si substrate miscut on the strain and elastic energy of Ge islands. We show how the morphology, composition and the elastic energy for 4 and 13 monolayers (ML) Ge islands grown at 600 °C and 730 °C on vicinal Si(0 0 1) surfaces change with miscut angles ranging between 0° and 10°. Scanning Tunneling Microscopy is used to determine the island morphology. Resonant x-ray diffraction near the Ge-K absorption edge allows the determination of the Ge concentration as well as the elastic energy stored on such structures from their dependency on the lattice parameter. Simulations using the Finite Elements Method indicate that the enlargement of the SiGe broad peak retrieved from the x-ray diffraction measurements is actually caused by the asymmetrical faceting induced by large miscut angles. Such faceting has a strong effect on island density and elastic energy, producing differences that are proportional to those observed in conditions with distinct SiGe content
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