Bulk Crystallites Research Articles

Structural evolution during electrocrystallization: deposition of Tl on Ag(100) from monolayer to bilayer and to bulk crystallites

The structure of Tl overlayers deposited at underpotentials and the structure of bulk deposited Tl on Ag(100) in perchloric acid have been determined using surface X-ray scattering. Thallium forms a c(p × 2) close-packed monolayer which compresses uniaxially (p decreasing from 1.185 to 1.168) with decreasing potential. Upon deposition of the second layer, the first layer expands slightly along the incommensurate direction and both layers form a c(1.2 × 2) structure. Bulk deposition at potentials negative of the Nernst potential results in the formation of well aligned hcp Tl crystallites atop the c(1.2 × 2) bilayer. The stability of the bilayer structure in the presence of bulk deposits and its pronounced effects on subsequent epitaxial growth are discussed in terms of strain and interface energies.

Variation in Conductivity of Cadmium Sulfo-Selenide Sintered Films with Vacuum Heat Treatment

The conductivity of CdS-Se sintered film increases irreversibly about 109-fold with vacuum heat treatment from 100° C to 200° C. The mechanism of such a large variation has been studied using highly sensitive Hall effect and conductivity measurements. The obtained results can be analyzed by the grain-boundary potential-barrier model with the aid of the charge neutrality condition for the grains, the maximum allowable values of surface potential and potential barrier height, and the density of interface states. Increase in conductivity of 8 orders of magnitude is brought about by increase in the free electron density, which occurs in two steps: in the low-temperature region, the release of trapped electrons from the surface state occurs due to desorption of chemisorbed oxygen; in the high-temperature region, the increase in effective doping density through the decrease in acceptor density occurs due to the release of occluded oxygen from the bulk crystallites near the surface. The mobility of free electrons contributes to an increase of one order of magnitude in conductivity, which decreases once to a minimum and then increases. The variation in mobility is attributed to the barrier height at the grain boundary which is produced by trapped electrons at the intrinsic interface state.

Isosteric heat of sorption of dihydrogen on alumina-supported palladium

Pd/Al2O3 samples of different palladium loadings were prepared by an incipient wetness technique. The palladium dispersion (D) of each sample was estimated from its isotherm of hydrogen uptake measured volumetrically at 298 K. The isosteric heat of hydrogen sorption (q, including absorption and adsorption) on palladium was determined from a series of uptake isotherms, measured at temperatures in the range 273–673 K, using the Clausius–Clapeyron equation. The value of q varied with the extent of the uptake and indicated three consecutive stages for sorption of hydrogen into palladium crystallites, i.e. chemisorption on the surface of the crystallites, penetration into sublayers of the crystallites to form the α-phase PdH and diffusion into bulk crystallites to form the β-phase PdH. The dissipated heat of chemisorption (qc) decreased with increasing coverage (θ) from qc 22 kcal mol–1 at low coverages (θ < 0.25), levelling-of momentarily at qc≈ 18 kcal mol–1 before falling to 8 kcal mol–1 at high coverages. The decrease in qc from zero coverage to the point where it levels off reflects a distribution in the size of palladium crystallites on each sample. The subsequent decrease in qc is attributed to weak chemisorption.

Annealing behavior of quench-condensed hydrogen and deuterium films

We have studied the annealing behavior of thin molecular hydrogen and deuterium films (typical thickness 10 nm) prepared by quench-condensation at 1.5 K on a silver substrate. Changes in the optical thickness and the heterogeneity of the films were monitored using optically excited surface plasmons. Upon annealing the films exhibit an irreversible change from a homogeneous to a strongly heterogeneous structure on a length scale of some hundred nm, accompanied by an apparent loss in optical thickness. This behavior is interpreted as being due to the formation of large bulk crystallites on the surface, resulting from the incomplete wetting of solid hydrogen on silver.

Correlation between structural and electrical properties of sprayed tin oxide films with and without fluorine doping

The structural and electrical properties of polycrystalline sprayed SnO 2 films formed at 500 °C have been investigated in a wide carrier concentration range (7 × 10 17 cm −3< N a<4 × 10 20 cm −3). Using electron diffraction, transmission electron microscopy (TEM) and high resolution TEM observations, we find that (i) the grain size is in the 20–80 nm range, (ii) there are a number of planar defects (twins) which are thought to reduce the bulk mobility of crystallites and (iii) flourine doping increases the density of twins. Only the films formed from concentrated SnCl 4 solutions present a columnar structure with a 〈100〉 preferred orientation. The dependence of resistivity at room temperature on the carrier concentration N a determined by Hall measurements can be accounted for within the frame of the grain boundary model developed by Seto (J.Y.W. Seto, J. Appl. Phys., 46 (1975) 5247) for polycrystalline silicon films, using the structural data and considering the contribution of the bulk of crystallites and the role of film degeneracy. For N a⩽5×10 18 cm −3, the resistivity is thermally activated and mainly governed by the barrier effect of grain boundaries which act as carrier traps. The mobility drops for N a≈5×10 17 cm −3, i.e. for a total depletion of each grain. The grain boundary trap density Q t is evaluated as about 4.0 × 10 12 cm −2. For N a⩾5 × 10 18 cm −3, the film resistivity is governed indeed by the properties of the bulk material, the mobility of which is limited at 25 cm 2V −1 s −1 because of the high density of structural defects.

Layer-by-layer growth of solid argon films on graphite as studied by neutron diffraction.

The layer-by-layer growth of solid argon films on graphite at T=10 K is studied using elastic neutron diffraction. The growth is characterized by individual layers with commensurate in-plane lattice constants. As the coverage is increased beyond two layers, evidence of the coexistence of ABC and ABA stacking is apparent, with the ABC sequence dominating as the film thickens. A continuous decrease in the Debye-Waller factor also occurs as the film thickness grows, indicating a crossover from two-dimensional to three-dimensional behavior. As the coverage is increased beyond about four nominal layers, there is evidence of bulk crystallite formation. The diffraction results are compared with equivalent measurements for methane films and with the recent computer simulations of Hruska and Phillips.

The effects of controlled crystallization and oxidation on the magnetic properties of Fe 40Ni 40B 20, Fe 78Si 9B 13 and Co 58Ni 10Fe 5Si 11B 16 metallic glasses

By careful control of thermal and surface treatment it has been possible to produce samples of metallic glass with a known volume fraction of surface or bulk crystallites, or a known oxide structure on the surface. The dc magnetic properties have been measured, and the effects of the crystallinity and oxidation are discussed in terms of the stress-magnetostriction coupling and simple models of domain wall pinning and moment rotation.

Multilayer adsorption of oxygen on graphite near the triple point.

Using an ellipsometric technique, we have observed layer-by-layer adsorption isotherms of oxygen on a small patch of a single cleaved surface of highly oriented pyrolitic graphite. As many as nine steps are seen. The overall features are characteristic of wetting at the triple point (${T}_{3}$=54.35 K): Below ${T}_{3}$, bulk crystallite formation preempts film growth after a limited number of layers. Above ${T}_{3}$, the film grows to a thickness equivalent to about 40 layers. However, indications of bulk droplets appear above about 12 layers. Our results suggest not quite complete wetting even by the liquid phase.

Multilayer adsorption of ethylene on graphite: Layering, prewetting, and wetting

We report the results of a comprehensive x-ray-scattering study of multilayer ethylene physisorbed on graphite. At temperatures well below the triple point of bulk ethylene (103.8 K), adsorption only occurs up to a finite number of layers; any further addition of material results in the nucleation of bulk crystallites. The number of layers that coexist with bulk increases at successively increasing transition temperatures. Reversible, first-order transitions between the total number of layers absorbed and the coexisting bulk are observed at $T=75$ K (oneto two layers) and at $T=98$ K (two to three layers). Bulk particle-size information is obtained as a function of both temperature and the amount of material in the cell. We present evidence that suggests that ethylene undergoes a thin-film to thick-film transition immediately below the triple point and that the triple point is the wetting temperature of ethylene on graphite. Pronounced hysteresis effects are observed both at the layering transitions and in the neighborhood of the triple point.

Direct Observation of Layer-by-Layer Wetting of Graphite by Ethylene

It has been demonstrated directly by use of x-ray diffraction that at temperatures below the triple point of bulk ethylene (103.8 K) adsorption of ethylene onto the basal planes of graphite only occurs up to a finite number of molecular layers. Further addition of molecules results in the formation of bulk crystallites. Reversible transitions between the total number of layers adsorbed and the coexisting bulk are observed.

X-ray study of molecular oxygen adsorbed on graphite

We have performed a detailed X-ray diffraction study of O 2 adsorbed on UCAR-ZYX and Le Carbon Lorraine vermicular exfoliated graphite between 15 and 50 K. At least four phases of physisorbed oxygen are found. The monolayer δ phase consists of a centered parallelogram lattice, with the molecular axes parallel to the graphite surface. The data are consistent with a triple point at 26 K. The melting transition at a coverage of one monolayer appears to be first order. At higher coverages the molecules undergo a lying-down to standing-up transition; the higher coverage ζ phase froms an approximately triangular lattice with the molecular axes perpendicular to the graphite surface. Satellite peaks around the (1, 0) Bragg peak indicate, however, that this cannot be a simple triangular lattice; possible explanations include successively incommensurate layers or a sinusoidal density modulation. For coverages in the two-layer region the ζ phase modulation peaks disappear at 37 K, and at 40 K the adsorbed oxygen appears to undergo a first order melting transition into a fluid phase. With increasing coverage, the 2D X-ray diffraction profiles and phase boundaries do not connect smoothly onto those of the 3D α and β phases. At low temperatures ( T < 30 K) the ζ phase always coexists with bulk crystallites; for temperatures near the 2D melting transition the 3D peaks are not observable. These data, taken together with the heat capacity results, suggest a wetting transition with only the bilayer lamellar phase or bulk O 2 being stable at low temperatures.

The application of reflection infrared and surface enhanced Raman spectroscopy to the characterization of chemisorbed organic disulfides on Au

We have found that a variety of organic molecules containing the SS linkage strongly adsorb on freshly evaporated or electrochemically cleaned gold surfaces. Adsorption stops at monolayer coverage and the packing density of the molecules can be close to the limit set by the size of the molecules. Reflection infrared spectra show that the films are strongly oriented and in the case of long alkyl-chain-substituted disulfides the films have structures similar to those of Langmuir-Blodgett type films. Most of the films were prepared by freshly evaporating gold onto polished single crystal silicon substrates. Immediately after the deposition the films were removed from the vacuum system (under N 2) and placed in solutions of the disulfides. After rinsing the films were examined by ellipsometry at 6328Å and 4420Å and in general, the results are consistent with monolayer structures with close packing similar to bulk crystallites. Infrared spectra were obtained using a Digilab 15B Fourier transform infrared spectrometer with a high f/number optical system and small beam spot size for glancing reflection. Spectra were obtained between 800 and 3200cm −1 using a mercury cadmium telluride detector. The spectrum of a monolayer of the di-p-nitrobenzoate ester of trans-o-Dithiane-4,5-diol is shown in Figure 1. The structure of the diol is given below. The spectrum of the monolayer of the di-ester of the C 16 acid (n-C 15H 31CO 2H) is given in Figure 2. Notice the alkyl chain mode band series in the monolayer spectrum which indicates crystalline packing of the chains. The infrared spectra are consistent with oriented structures which maximize packing densities. No direct evidence is yet available for defining a AuS bond. However, a sample in which the monolayer of the p-nitrobenzoate derivative has been covered with 400Å average thickness of CaF 2 and 200Å of smooth silver has yielded an enhanced Raman spectrum (λ(excite)=4880Å ∼50mW power) in which the normally strong S-S stretching mode observed at 504cm −1 in bulk samples is absent in the monolayer. These studies demonstrate an application of surface vibrational spectroscopy to defining molecular structures at surfaces. Work is presently underway to quantitatively calculate molecular orientation from the infrared spectra and to more carefully determine the extent of any ordering in appropriate monolayers.

Low temperature thermal aging of ethylene vinyl alcohol copolymers

AbstractThe crystallites of semicrystalline polymers formed at low temperatures are subject to partial partial melting and recrystallization resulting in a systematic increase in the size and perfection of the nascent crystallites. Differential scanning calorimetry (DSC) and x‐ray studies suggest that ethylene‐vinyl alcohol copolymers (EVOH), in particular, may undergo these secondary crystallization processes upon storage at temperatures far below Tm. DSC scans have been run on “as received” and quenched samples; the mole percent of vinyl alcohol in the copolymers studied was 0, 62, 70, 72, 82 and 100 percent. Tests on secondary crystallization kinetics, performed at different temperatures, further confirm the hypothesis of an increased degree of perfection (lamellar thickening) of the bulk crystallites achieved upon long term storage at temperature between Tg and Tm.

Structure and dynamics of ammonia adsorbed on graphitized carbon black. Part 2.—Neutron diffraction

Neutron diffraction experiments determining the structure of the adsorbate in the system ammonia on Graphon are described. The experiments were done over a range of temperatures from 80 K to just above the melting point of bulk ammonia at ≈ 200 K, and a range of adsorbed volumes of vapour equivalent to surface coverages between 0.25 and 6 statistical layers. These ranges overlap those used for the isotherm measurements described in the previous paper.The results show that, on cooling to temperatures well below its bulk melting point, most of the ammonia desorbs from the surface to form bulk crystallites. Formation of the crystallites takes place over a wide range of temperature and is very sensitive to the amount of ammonia present. Melting point curves determined by neutron diffraction are shown to be closely connected with the capacity of the surface to adsorb ammonia. The results are interpreted in terms of an equilibrium between bulk solid and adsorbed fluid. Analysis of the shapes of the melting point curves and comparison of these with the isotherm measurements establishes a clear link between adsorption and heterogeneous nucleation.Addition of methane to the surface makes it less able to adsorb ammonia and the ammonia crystallites are stable at correspondingly higher temperatures.

Piezoresistive properties of polycrystalline silicon

The piezoresistive gage factor of boron-doped CVD polysilicon films deposited with a boron-to-silicon ratio of 2×10−4–1.2 ×10−2 on an aluminum-oxide-insulated molybdenum substrate is found to be between 15 and 27. Annealing increases the gage factor. The higher the doping, the lower is the gage factor. Over the range 20–140 °C, the gage factor is not temperature sensitive if the boron-to-silicon ratio is higher than 2×10−3 during deposition. The temperature dependence increases as the doping concentration is decreased. Our analysis shows that the piezoresistive properties in polysilicon is mainly due to the bulk crystallites.

On the Physical Characteristics and Chemical Composition of Electroluminescent Phosphors

Through controlled removal of surface layers, it was found that surface chemical barriers are not responsible for electroluminescence in zinc sulflde phosphors. Polycrystalline phosphors are shown to be almost uniform with respect to chemical, physical, and electroluminescent properties as one passes from the surface into the crystallite bulk. Electron micrographs show the character of the particles as successive layers of phosphor surfaces are removed by acid etching. The experimental results at progressive stages of etching are interpreted in terms of an inefficient surface layer and of decreased particle size after etching. Regions capable of serving as the site of field intensification are held to exist throughout the volume of the particles.