Low-temperature and ambient-pressure synthesis and shape evolution of nanocrystalline pure, La-doped and Gd-doped CeO2

Nanocrystalline cuboidal ceria has been synthesized by low-temperature hydrothermal reaction of cerium nitrate hexahydrate with hexamethylene tetramine. The particles have been doped with La and Gd by adding aqueous solution of the nitrate salts of the metals to the reaction mixture. The pure and doped particles are cubic in crystal structure and 10-25 nm in size. The pure and La-doped ceria are cuboidal in morphology, whereas the Gd-doped particles are irregular in shape. High-resolution TEM imaging and image simulation indicates that atomic level steps are present on the particle surfaces. The particles are faceted parallel to the {1 1 1} and {1 0 0} crystallographic planes and a continuous switching takes place between the two possible surface facets. It appears that the surface energies of the {1 1 1} and {1 0 0} facets are quite similar in magnitude and the interplay of surface energy determines the particle shape. Chemically sensitive imaging and spectroscopy shows that the dopants are homogeneously distributed within the particles and that the oxidation state of Ce is a mixture of +3 and +4. No preferential segregation either of the dopant or the oxidation state was observed. However, since the facet switching does depend on the chemistry of the dopant, there must be an affect on the atomic scale.     

Free energy of segregation at alloy surfaces

The free energy of segregation at semi-infinite solid binary alloy surfaces is calculated using a quasi-chemical approach. The alloy is treated to be a non-regular solution. The concentration is assumed to be different only at the surface layer. The theory is applied to AgAu alloys. The concentration and the short-range-order parameters at the surface are calculated.     

Equilibrium and kinetic surface segregation in binary alloy thin films

A general theoretical analysis of the effect of film thickness on equilibrium and kinetic surface segregation in binary alloy thin films is presented. In this analysis, a constrained condition that represents the finite size of thin film system has been introduced to the modified Darken model, which has been used to describe both equilibrium and kinetic surface segregation in bulk materials. Simulation of surface segregation for alloy thin films can be carried out for all composition ranges and all film thicknesses if only knowing the surface segregation parameters for bulk materials. Simulations of equilibrium and kinetic surface segregation in Cu(1 1 1)Ag binary alloy thin film are presented.     

In situ electron microscopy study of the thermal motion of liquid lead nanoparticles in thin aluminum foils

The kinetics of thermal motion of liquid lead nanoparticles in thin aluminum foils has been investigated in situ by transmission electron microscopy. Dependences of the diffusion coefficient of particles on temperature and particle size have been obtained. The results of the investigations can be interpreted on the assumption that the mobility of particles is controlled by the nucleation of steps on {111} faces of their surface.     

Surface thermometry in combustion diagnostics by sputtered thin films of thermographic phosphors

The accuracy and robustness of the thermographic phosphors (TP) technique relies in the use of coatings with low thickness, high-intensity luminescent emission and high adhesion to the surfaces. Sputter deposition has been evaluated as an alternative for coating preparation of TPs for surface thermometry in combustion diagnostics. Thin films of \(\hbox {Gd}_{3}\hbox {Ga}_{5}\hbox {O}_{12}{:}\hbox {Cr}^{3+}\) have been deposited on fused silica and stainless steel substrates by radio frequency magnetron sputtering. Physical, chemical, and temperature-dependent luminescence properties of the phosphor films have been evaluated using X-ray diffraction, X-ray photoelectron spectroscopy and laser-induced luminescence, respectively. The results showed that the luminescence features of the thin films must be activated by heat treatment after sputter deposition. The \(\hbox {Gd}_{3}\hbox {Ga}_{5}\hbox {O}_{12}{:}\hbox {Cr}^{3+}\) films exhibited appropriate temperature sensitivity with adequate precision of the temperature determination, proving to be suitable for pointwise (0D) surface thermometry. An evaluation of the spatial homogeneity of the luminescence properties, which has not been yet addressed in the literature for thin films of TPs, revealed that thin \(\hbox {Gd}_3\hbox {Ga}_5\hbox {O}_{12}{:}\hbox {Cr}^{3+}\) films deposited on fused silica can be used for spatially resolved surface thermometry while those deposited on stainless steel require improvements to overcome spatial inhomogeneities of the luminescence lifetimes.     

The effect of particle size on the surface composition of microcrystalline alloys

Thin films a Ni?1at%Pd alloy, ranging in thickness from 10 to 100 Å were deposited onto alumina substrates by RF sputtering. The films were subsequently heat treated to produce small particles of the alloy with average sizes in the range 300 to 1000 Å. The surface composition of aggregates of small particles was determined by Auger electron spectroscopy after equilibration of the samples at 650°C. Average particle sizes were determined by analysis of photomicrographs taken by electron microscopy techniques. The results show a strong dependence of surface composition on particle size. A decrease of a factor of five in surface concentration of palladium is observed over the range of particle sizes studied. The dependence of equilibrium surface composition on particle size has been estimated by means of a mass balance model. While this model accounts qualitatively for the effects of particle size on surface composition, discrepancies between model predictions and the experimental results suggest the intervention of other phenomena, possibly related to capillarity effects.     

Polarity effects on ZnO films grown along the nonpolar [1120] direction

The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces.     

The electric and magnetic properties of epitaxially grown A0.5-xLaxSr0.5MnO3 （A=Pr, Nd） thin film

The epitaxial （single crystal-like） Pr0.4La0.1Sr0.5MnO3 （PLSMO） and Nd0.35La0.15Sr0.5MnO3 （NLSMO） thin films are prepared and characterized, and the electric and magnetic properties are examined. We find that both PLSMO and NLSMO have their own optimum deposition temperature （To） in their growing into epitaxial thin films. When the deposition temperature is higher than To, a c-axis oriented but polycrystalline thin film grows; when the deposition temperature is lower than To, the thin film tends to be a-axis oriented and also polycrystalline. The most important point is that for the epitaxial PLSMO and NLSMO thin films the electronic phase transitions are closely consistent with the magnetic phase transitions, i.e. an antiferromagnetic phase corresponds to an insulating state, a ferromagnetic phase corresponds to a metallic state and a paramagnetic phase corresponds to a semiconducting state, while for the polycrystalline thin films the electronic phase transitions are always not consistent with the magnetic transitions.     

Surface Structure of Large Synthetic Diamonds by High Temperature and High Pressure

With NiMnCo and FeCoNi alloys as solvent metals, large single-crystal diamonds of about 3mm across are grown by temperature gradient method （TGM） under high temperature and high pressure （HPHT）. Although both {100} and {111} surfaces are developed by a layer growth mechanism, some different characteristic patterns are seen clearly on the different surfaces, no matter whether NiMnCo or FeCoNi alloys are taken as the solvent metals. For {100} surface, it seems to have been melted or etched greatly, no dendritic patterns to be found, and only a large number of growth hillocks are dispersed net-likely; while for {111} surface, it often seems to be more smooth-faced, no etched or melted traces are present even when a lot of depressed trigonal growth layers. This distinct difference between {111} and {100} surfaces is considered to be related to the difference of surface-atom distribution of different surfaces, and {111} surfaces should be more difficult to be etched and more steady than {100} surfaces.     

Research of the behaviour of O chemisorption on the (110) surface of Rhx--Pt1－x alloy

An atomic group model of the disordered binary alloy Rhx-Pt1-x has been constructed to investigate surface segregation. According to the model, we have calculated the electronic structure of the Rhx-Pt1-x alloy surface by using the recursion method when O atoms are adsorbed on the Rhx-Pt1-x （110） surface under the condition of coverage 0.5. The calculation results indicate that the chemical adsorption of O changes greatly the density of states near the Fermi level, and the surface segregation exhibits a reversal behaviour. In addition, when x 〈 0.3, the surface on which O is adsorbed displays the property of Pt; whereas when x 〉 0.3 it displays the property of Rh.     

A comparative study of YBa2Cu3O7-δ/YSZ bilayer films deposited on silicon-on-insulator substrates with and without HF pretreatment

Highly epitaxial YBa2Cu3O7-δ (YBCO) and yttria-stabilized zirconia (YSZ) bilayer thin films have been deposited on silicon-on-insulator (SOI) substrates by using in situ pulsed laser deposition (PLD) technique. In the experiment, the native amorphous SiO2 layers on some of the SOI substrates are removed by dipping them in a 10% HF solution for 15 s. Comparing several qualities of films grown on substrates with or without HF pretreatment, such as thin film crystallinity, general surface roughness, temperature dependence of resistance, surface morphology, as well as average crack spacing and crack width, naturally leads to the conclusion that preserving the native SiO2 layer on the surface of the SOI substrate can not only simplify the experimental process but can also achieve fairly high quality YSZ and YBCO thin films.     

Size effect on alloying ability and phase stability of immiscible bimetallic nanoparticles

In the present paper, the surface and size effects on the alloying ability and phase stability of immiscible alloy nanoparticles have been studied with calculating the heats of formation of Au-Pt alloy nanoparticles from the single element nanoparticles of their constituents (Au and Pt) with a simple thermodynamic model and an analytic embedded atom method. The results indicated that, besides the similar compositional dependence of heat of formation as in bulk alloys, the heat of formation of alloy nanoparticles exhibits notable size-dependence, and there exists a competition between size effect and compositional effect on the heat of formation of immiscible system. Contrary to the positive heat of formation for bulk-immiscible alloys, a negative heat of formation may be obtained for the alloy nanoparticles with a small size or dilute solute component, which implies a promotion of the alloying ability and phase stability of immiscible system on a nanoscale. The surface segregation results in an extension of the size range of particles with a negative heat of formation. The molecular dynamics simulations have indicated that the structurally and compositionally homogeneous AuPt nanoparticles tend to form a core-shell structure with temperature increasing.     

ToF atom-probe fim investigation of surface segregation in dilute alloys

A time-of-flight atom-probe field ion microscope was successfully employed for the first time to investigate surface segregation in dilute alloys. We were able to achieve a single atomic layer resolution and obtained the absolute concentrations of alloy species of each surface layer. Cr atoms are found to segregate to the surface of Stainless Steel 410 whereas no segregation of the minority species was found for Pt-8% W and Pt-5% Ru. The first layer concentration of Cr in the {110} plane of Stainless Steel 410 at 500°C was found to be 38.5 ± 12.5% and 63.4 ± 10.2% respectively for samples with near surface layers Cr average concentration of 6.3 ± 2.1% and 11.9 ± 2.5%. The heat of segregation of Cr to the {110} plane of Stainless 410 was found to be 3.43 and 3.92 kcal/mole respectively from the two sets of data. The data also gives the difference in surface tensions between iron and chromium at this plane to be 269 and 282 erg/cm² respectively. Segregation studies on the {012} plane as well as on a grain boundary of Stainless Steel 410 were also done. In some cases, though the first surface layer is enriched with Cr in Stainless Steel 410, the near surface layers show a depletion of Cr. In Pt-8% W and Pt-5% Ru, our concentration depth profiles with a single atomic layer resolution show no segregation or depletion of the minority species either for the top layers or for the near surface layers.     

Electronic structure of alloy surfaces: Coherent-potential approximation

The electronic structure of alloy surfaces is investigated within the framework of the coherent-potential approximation, a scheme originally derived for infinite alloy systems and here generalized to the case of the semi-infinite cleaved alloy crystal. The theory is applied to substitutional binary alloys, described by a single-band Wannier model one-electron Hamiltonian. Numerical results for the configurationally averaged surface density of states are discussed for a variety of system parameters. Among the cases considered are alloys of uniform composition and alloys in which the surface composition differs from that of the interior.     

rf reactive co-sputtered Au-Ag alloy nanoparticles in SiO2 thin films

We have studied formation of Au-Ag alloy nanoparticles in sputtered SiO₂ thin films. Silica thin films containing Au-Ag nanoparticles were deposited on quartz substrates using rf reactive magnetron co-sputtering technique. The films heat-treated in reducing Ar + H₂ atmosphere at different temperatures. They were analyzed by using UV-vis spectrophotometry, atomic force microscopy and X-ray photoelectron spectroscopy (XPS) methods for their optical, surface morphological as well as structural and chemical properties. The optical absorption of the Au-Ag alloy nanoparticles illustrated one plasmon resonance absorption peak located at 450 nm between the absorption bands of pure Au and Ag nanoparticles at 400 and 520 nm, respectively, for the thin films annealed at 800 °C. XPS results showed that the alloys were in metallic state, and they had a greater tendency to lose electrons as compared to their corresponding monometallic state. Using lateral force microscopy analysis, we have found that the alloy particles were uniformly distributed on the surface with grain size of about 20 nm.     

Chemisorption of carbon monoxide on copper-nickel alloy surfaces

The adsorption of CO on Cu-Ni alloy surfaces has been studied at 300 and 120 K using LEED, AES, TDS, and work function measurement. The alloys have been prepared as thin (111) epitaxial films evaporated on mica, and as poly crystalline foils. At 300 K the alloy surfaces show an adsorption behavior similar to that of pure Ni: the work function increases to a saturation value which is higher for Ni-rich surfaces than for Cu-rich. The isosteric heat of adsorption (106 kJ/mole) is nearly as high as with pure Ni. At 120 K the alloys exhibit a more Cu-like adsorption behavior: the work function passes through a minimum which becomes deeper at higher Cu surface concentration. The isosteric heat of adsorption at low temperatures (50 kJ/mole) is nearly as low as for pure Cu. From TDS it can be shown, that the binding energy of the highest (Ni-like) adsorption states increases with increasing Ni surface concentration. At the (111) alloy surfaces no LEED superstructures due to CO adsorption could be observed.     

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces II: Results on Pt and Pt-Rh

The 3-dimensional atom probe (3DAP) is a unique instrument providing chemical analysis at the atomic scale for a wide range of materials. A dedicated 3DAP has been built specifically for analysing reactions at metal surfaces, called the catalytic atom probe (CAP). This paper presents an overview of results from the CAP on structural and chemical transformations to surface layers of Pt and Pt-17.4 at.%Rh catalysts following exposure to a number of gases typically emitted by vehicle engine exhausts, normally for 15 min at pressures of 10 mbar. Following exposure to the oxidising gases NO on Pt, and NO, O₂ or N₂O on Pt-Rh, both surfaces appear disrupted, while for Pt-Rh, Rh enrichment of the surface atomic layer is noted over the entire specimen apex for exposure temperatures up to 523 K. However, for oxidising exposures at 573-773 K relatively clean, Rh-depleted surfaces are observed on {0 0 1}, {0 1 1} and {0 1 2} crystallographic regions of Pt-Rh. It is suggested that this result is due to surface diffusion of oxide species over the specimen apex, towards the {1 1 1}-orientated areas where the oxides appear to be stabilised. In contrast, CO exposure appears to have little effect on the either the surface structure or composition of the Pt-Rh alloy. Finally, combinations of two gases (NO + CO, O₂ + NO) were also dosed onto Pt-Rh alloys in the same exposure. These revealed that while NO and CO can co-adsorb without interference, CO prevents the build up of oxide layers and reduces the extent of Rh segregation seen under NO exposure alone. On exposing Pt-Rh to NO after an oxygen exposure, heavily oxidised surfaces, Rh segregation and no intact NO molecules were seen, confirming the ability of oxidised Pt-Rh to dissociate nitric oxide.     

Gas-phase Crystallization of Titanium Dioxide Nanoparticles

We have investigated the development of crystal morphology and phase in ultrafine titanium dioxide particles. The particles were produced by a droplet-to-particle method starting from propanolic titanium tetraisopropoxide solution, and calcined in a vertical aerosol reactor in air. Mobility size classified 40-nm diameter particles were conveyed to the aerosol reactor to investigate particle size changes at 20–1200°C with 5–1-s residence time. In addition, polydisperse particles were used to study morphology and phase formation by electron microscopy. According to differential mobility analysis, the particle diameter was reduced to 21–23-nm at 600°C and above. Precursor decomposition occurred between 20°C and 500°C. The increased mobility particle size at 700°C and above was observed to coincide with irregular particles at 700°C and 800°C and faceted particles between 900°C and 1200°C, according to transmission electron microscopy. The faceted anatase particles were observed to approach a minimized surface energy by forming {101} and {001} crystallographic surfaces. Anatase phase was observed at 500–1200°C and above 600°C the particles were single crystals. Indications of minor rutile formation were observed at 1200°C. The relatively stable anatase phase vs. temperature is attributed to the defect free structure of the observed particles and a lack of crystal–crystal attachment points.     

Equilibrium segregation to terraces and steps

A method based on empirical interatomic potentials has been developed to calculate energies of segregation to low index (001) surfaces as well as to [110] and [100] steps on a (001) surface in simple (AB) binary alloys. The Baskes and Melius potentials have been used to model the A-A and B-B atomic interactions and a procedure has been outlined to derive an A-B interaction from the pure metal potentials. This approach has been used to compute the energy of segregation of gold, in an infinitely dilute nickel-gold alloy, to both terraces and steps. The results obtained are consistent with previous measurements of the anisotropy of surface segregation in that alloy.     

Landau quantization and the thickness limit of topological insulator thin films of Sb2Te3

We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb{2}Te{3} thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in a Sb{2}Te{3} topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as a 2D massless Dirac fermion system is achieved. We demonstrate that four quintuple layers are the thickness limit for a Sb{2}Te{3} thin film being a 3D topological insulator. The mechanism of the Landau-level broadening is discussed in terms of enhanced quasiparticle lifetime.