P. M. Baldo

Substrate effects on the structure of epitaxial PbTiO3 thin films prepared on MgO, LaAlO3, and SrTiO3 by metalorganic chemical‐vapor deposition

Epitaxial PbTiO3 films were prepared by metalorganic chemical‐vapor deposition on MgO(001)‐, SrTiO3(001)‐, and LaAlO3(001)‐oriented substrates. Four‐circle x‐ray diffraction, transmission electron microscopy, Rutherford backscattering (RBS) channeling, and optical waveguiding were performed to characterize the deposited films. Epitaxial, single‐crystal films were obtained on all three substrate materials under the same growth conditions. However, the defect structure of the films, including grain tilting, threading dislocation density, and 90° domain formation, was strongly dependent on the choice of substrate material. Films grown on MgO(001) and LaAlO3(001) (pseudocubic indices) substrates are nominally c‐axis oriented; however, the PbTiO3 grains in the film form a fourfold domain structure, with the grains tilted ∼0.6° and ∼0.7°, respectively, toward the [100] directions (cubic or pseudo‐cubic) of the substrates. In addition, these films contain a significant volume fraction of 90°‐domain (a‐axis) stru...

Grain-size-dependent thermal conductivity of nanocrystalline yttria-stabilized zirconia films grown by metal-organic chemical vapor deposition

A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia.

In situ characterization of strontium surface segregation in epitaxial La0.7Sr0.3MnO3 thin films as a function of oxygen partial pressure

Using in situ synchrotron measurements of total reflection x-ray fluorescence, we find evidence of strontium surface segregation in (001)-oriented La0.7Sr0.3MnO3 thin films over a wide range of temperatures (25–900 °C) and oxygen partial pressures (pO2=0.15–150 Torr). The strontium surface concentration is observed to increase with decreasing pO2, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from −9.5 to −2.0 kJ/mol. In contrast, the La0.7Sr0.3MnO3 film thickness and epitaxial strain state have little impact on segregation behavior.

Growth of aluminum nitride thin films on Si(111) and Si(001): Structural characteristics and development of intrinsic stresses

We have grown aluminum nitride thin films by ultrahigh vacuum reactive sputter deposition on Si(111) and Si(001) substrates. We show results of film characterization by Raman scattering, ion beam channeling, and transmission electron microscopy, which establish the occurrence of epitaxial growth of wurtzitic aluminum nitride thin films on Si(111) at temperatures above 600 °C. In contrast, microstructural characterization by transmission electron microscopy shows the formation of highly oriented polycrystalline wurtzitic aluminum nitride thin films on Si(001). Real‐time substrate curvature measurements reveal the existence of large intrinsic stresses in aluminum nitride thin films grown on both Si(111) and Si(001) substrates.

Functional links between stability and reactivity of strontium ruthenate single crystals during oxygen evolution

In developing cost-effective complex oxide materials for the oxygen evolution reaction, it is critical to establish the missing links between structure and function at the atomic level. The fundamental and practical implications of the relationship on any oxide surface are prerequisite to the design of new stable and active materials. Here we report an intimate relationship between the stability and reactivity of oxide catalysts in exploring the reaction on strontium ruthenate single-crystal thin films in alkaline environments. We determine that for strontium ruthenate films with the same conductance, the degree of stability, decreasing in the order (001)>(110)>(111), is inversely proportional to the activity. Both stability and reactivity are governed by the potential-induced transformation of stable Ru(4+) to unstable Ru(n>4+). This ordered(Ru(4+))-to-disordered(Ru(n>4+)) transition and the development of active sites for the reaction are determined by a synergy between electronic and morphological effects.

Characterization of microstructure and mechanical behavior of sputter deposited Ti-containing amorphous carbon coatings.

Abstract We report on the characterization of microstructure and mechanical properties of sputter deposited Ti-containing amorphous carbon (Ti-aC) coatings as a function of Ti composition. Ti-aC coatings have been deposited by unbalanced magnetron sputter deposition, in an industrial-scale four-target coating deposition system. The composition and microstructure of the Ti-aC coatings have been characterized in detail by combining the techniques of Rutherford backscattering spectrometry (RBS) and hydrogen elastic recoil detection (ERD), transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. At Ti compositions 8 at.%, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti-aC coatings resembles that in cubic B1-TiC, consistent with TEM observation of precipitation of TiC nanocrystallites in the a-C matrix. Beyond the Ti dissolution limit, the Ti-aC coatings are nanocomposites with nanocrystalline TiC clusters embedded in an a-C matrix. A large scale, quasi one-dimensional composition modulation in the Ti-aC coatings was observed due to the particular coating deposition geometry. Elastic stiffness and hardness of the Ti-aC coatings were measured by instrumented nanoindentation and found to vary systematically as a function of Ti composition. Unlubricated friction coefficient of Ti-aC coatings against WC–Co balls was found to increase as the Ti composition increases. As Ti composition increases, the overall mechanical behavior of the Ti-aC coatings becomes more TiC-like.

Microstructure and mechanical properties of Ti–Si–N coatings

A series of Ti-Si-N coatings with 0 < Si < 20 at.% were synthesized by inductively coupled plasma assisted vapor deposition. Coating composition, structure, atomic short-range order, and mechanical response were characterized by Rutherford backscattering spectrometry, transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and instrumented nanoindentation. These experiments show that the present series of Ti-Si-N coatings consists of a mixture of nanocrystalline titanium nitride (TiN) and amorphous silicon nitride (a-Si:N); i.e., they are TiN/a-Si:N ceramic/ceramic nanocomposites. The hardness of the present series of coatings was found to be less than 32 GPa and to vary smoothly with the Si composition.

Friction and wear characteristics of ceramic nanocomposite coatings: Titanium carbide/amorphous hydrocarbon

Friction and wear characteristics of titanium-containing amorphous hydrocarbon (Ti–C:H) coatings were measured during unlubricated sliding against WC–Co. These Ti–C:H coatings consist of nanocrystalline TiC clusters embedded in an amorphous hydrocarbon (a-C:H) matrix, i.e., they are TiC/a-C:H nanocomposites. The elastic modulus and hardness of the coatings exhibit smooth variations with increasing Ti composition. In contrast, a relatively abrupt transition occurs in the friction coefficient and wear rate of the coatings over a relatively narrow (20–30 at. %) Ti composition range. Our results reveal bimodal friction and wear behaviors for the TiC/a-C:H nanocomposites, a-C:H like at Ti compositions below 20%, and TiC like at Ti compositions above 30%. The two different wear mechanisms that operate as the volume fraction of nanocrystalline TiC clusters changes are discussed.

GROWTH OF HIGH QUALITY SINGLE-DOMAIN SINGLE-CRYSTAL FILMS OF PBTIO3

Single‐crystal films of PbTiO3 have been deposited on (100) SrTiO3 substrates by metalorganic chemical vapor deposition. X‐ray diffraction spectra and transmission electron microscopy images revealed that the development of 90° domains was directly related to the thickness of the film. Single‐domain, single‐crystal films were obtained for film thickness less than 150 nm. The minimum yields of the films in ion‐channeling experiments were only 3 %, similar to that of a perfect single crystal. The ordinary refractive index of the film measured by optical waveguiding experiment was also the same as that of PbTiO3 single crystals.

Inductively coupled plasma assisted deposition and mechanical properties of metal-free and Ti-containing hydrocarbon coatings

Metal-free amorphous hydrocarbon (a-C:H) and Ti-containing hydrocarbon (Ti–C:H) coatings have been synthesized in a hybrid chemical vapor deposition (CVD)/physical vapor deposition (PVD) system which combines inductively coupled plasma (ICP) and sputter deposition. a-C:H coatings have been fabricated by ICP assisted CVD in inert/hydrocarbon gas mixtures while Ti–C:H coatings have been fabricated by ICP assisted magnetron sputtering of Ti in inert/hydrocarbon gas mixtures. We present results of structural characterization and mechanical property measurements on these a-C:H and Ti–C:H coatings. In particular, the influence of hydrogen on the coating mechanical properties is probed experimentally. We show that hydrogen significantly influences the mechanical properties of a-C:H and Ti–C:H coatings and needs to be considered for a full understanding of the mechanical properties of Ti–C:H coatings. Our results demonstrate that combining ICP with sputter deposition makes a versatile CVD/PVD tool capable of depo...