Thin Bulk Research Articles

Revealing Strain Effects on the Chemical Composition of Perovskite Oxide Thin Films Surface, Bulk, and Interfaces

AbstractUnderstanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is a controversial subject that is critical for developing efficient energy storage and conversion materials. In this work, high‐quality epitaxial thin films of the model perovskite La0.5Sr0.5Mn0.5Co0.5O3−δ (LSMC), under compressive or tensile strain, are characterized with a combination of in situ and ex situ bulk and surface‐sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is observed that the effect of strain on reducibility is dependent on the “effective strain” induced on the chemical bonds. In‐plain strain, and in particular the relative BO length bond, is the key factor controlling which of the B‐site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it is confirmed that tensile strain favors the stabilization of a more reduced lattice, accompanied by greater segregation of strontium secondary phases and a decrease of oxygen exchange kinetics on LSMC thin films.

Spatially resolved thermoreflectance techniques for thermal conductivity measurements from the nanoscale to the mesoscale

Thermoreflectance techniques, namely, time- and frequency-domain thermoreflectance (TDTR and FDTR, respectively), are ubiquitously used for the thermophysical characterization of thin films and bulk materials. In this perspective, we discuss several recent advancements in thermoreflectance techniques to measure the thermal conductivity of solids, with emphasis on the governing length scales and future directions in expanding these advances to different length scales and material structures. Specifically, the lateral resolution of these techniques, typically on the order of several micrometers, allows for an understanding of the spatially varying properties for various materials. Similarly, limitations of TDTR and FDTR with respect to their volumetric probing regions are discussed. With a recently developed steady-state thermoreflectance technique, these limitations are overcome as probing volumes approach spot sizes. Finally, recent pushes toward the implementation of these techniques without the use of a thin metal transducer are presented, with guidelines for future avenues in the implementation under these specimen configurations.

Numerical simulation of the hydrodynamics and mass transfer in the capping of large size Z-cut plates

Numerical simulation of the hydrodynamics and mass transfer involved in the capping of large size Z-cut KDP plates has been performed. The variations of surface supersaturation with thin layer height, bulk supersaturation and rotation rate are analyzed. The results show that keeping still is very unfavorable for the growth of thin layer, a certain rotation rate should be applied to the Z-cut plate to increase the concentration gradient across the (0 0 1) face, thus making sure that thin layers can grow from the edges while small pyramids barely grow from the face center. Given the result that end faces of thin layers nearly bear a bulk supersaturation, a relative low bulk supersaturation will be better to ensure both the growth of thin layers and the good surface uniformity brought by low supersaturation. Moreover, as thin layers grow, rotation rate should be increased to make up the insufficient solute supply resulting from the decreasing linear speed. The optimal parameters for rapid and high-quality capping of large size Z-cut plates are suggested, the relevant simulation results are also presented.

How the chemical structure of the plasma-deposited SiOx film modifies its stability and barrier properties: FTIR study

Organosilicon thin films issued from the gas mixture of oxygen and hexamethyledisiloxane (HMDSO), 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS), or fluorotriethoxysilane (FTEOS) deposited on polycarbonate (PC) substrate were obtained thanks to a RF plasma source. The chemical structure of the two types of inorganic SiOx (HMDSO/O2, TMCTS/O2) and fluorinated SiOxCyFz (FTEOS/O2, FTEOS/O2/TMCTS) coatings have been investigated by FTIR and XPS spectroscopies, and wettability measurement. Network structure and density of SiOx and SiOxCyFz thin films were discussed as a function of the νSiOSi band shifting observed in their respective FTIR spectra. The influence of various plasma parameters (such as discharge power, precursor/O2 ratio and, duration) on the film properties was studied in order to optimize the growth of both inorganic and fluorinated thin films. The ageing of such layers either at ambient atmosphere or even at reduced pressure showed a chemical alteration of the thin films surface and bulk, which stabilized after 15 days of storage. Such phenomenon is explained by the hydrophobicity recovery that may induce the observed network alteration. Water and oxygen permeation and transport properties of treated PC films were correlated with the coating structure and composition; then TMCTS was identified as the most efficient precursor for enhancing water barrier property. Furthermore, the permeation properties of these deposits seem to be more controlled by the material density than by their chemistry.

Atomic Layer Deposition of Ultrathin Glassy Lithium Borate-Carbonate Solid Electrolytes

Solid state electrolytes could enable significant improvements in energy density, cycle life, and safety of next-generation battery chemistries. The ability to fabricate thin electrolyte films with high ionic conductivity and excellent stability on complex architectures has been a bottleneck to realizing a wide range of 3D structured thin film and bulk batteries. Recent progress in ALD for solid state electrolytes has shown great promise, and enabling even higher conductivities and stability with Li metal anodes could dramatically enhance the energy and power density of ALD-based batteries. In addition, ALD films with high conductivity and good stability could be used for interfacial engineering of bulk-type solid and liquid based batteries to improve stability and safety. This work demonstrates a novel ALD process for glassy lithium borate-carbonate thin films with ionic conductivities above 10-6 S/cm at 298K. This represents a 6x improvement over the previous best reported value for an ALD solid electrolyte.1 The composition, structure, and stability of the films are characterized with X-ray photoelectron spectroscopy and a range of electrochemical measurements. These experiments are compared with those calculated with Density Functional Theory and Molecular Dynamics to elucidate the origins of the high ionic conductivity and excellent stability. The properties are studied as a function of deposition temperature showing tradeoffs between process conditions and performance, and demonstrating the precise control afforded by the ALD process.2 The optimized film remains an ionic conductor when in contact with metallic Li, with no measurable changes even over several weeks, and displays stable cycling when paired with a thin-film cathode and Li metal anode. References (1) Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C. F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E.; Chem. Mater. 2017, 29, 3740–3753. (2) Kazyak, E.; Chen, K. H.; Davis, A. L.; Yu, S.; Sanchez, A. J.; Lasso, J.; Bielinski, A. R.; Thompson, T.; Sakamoto, J.; Siegel, D. J.; Dasgupta, N. P.; J. Mater. Chem. A 2018, 6, 19425–19437.

Giant optical activity and Kerr effect in type-I and type-II Weyl semimetals

We explore optical activity in thin films and bulk of type-I and type II Weyl semimetals (WSM), and demonstrate the existence of a giant Kerr effect in both. In time-reversal symmetry broken WSM thin films, the polarization rotation is caused by the optical Hall conductivity including the anomalous Hall term. The Kerr angle is found to be $\propto Q/\omega$, with $Q$ and $\omega$ being the Weyl node separation and the optical frequency, respectively. In contrast, the optical activity in the bulk WSM is dominated by axion electrodynamics, which persists even in the Pauli blocked regime of no optical transitions. In bulk WSM, $Q$ acts analogous to the magnetization in magnetic materials, leading to large `polar Kerr effect' (linear in $Q$) when light is incident on WSM surface without Fermi arc states, and the `Voigt effect' (quadratic in $Q$), when light is incident on surface with Fermi arc states.

Mn-doped topological insulators: a review

Topological insulators (TIs) host robust edge or surface states protected by time-reversal symmetry (TRS), which makes them prime candidates for applications in spintronic devices. A promising avenue of research for the development of functional TI devices has involved doping of three-dimensional (3D) TI thin film and bulk materials with magnetic elements. This approach aims to break the TRS and open a surface band gap near the Dirac point. Utilizing this gapped surface state allows for a wide range of novel physical effects to be observed, paving a way for applications in spintronics and quantum computation. This review focuses on the research of 3D TIs doped with manganese (Mn). We summarize major progress in the study of Mn doped chalcogenide TIs, including Bi2Se3, Bi2Te3, and Bi2(Te,Se)3. The transport properties, in particular the anomalous Hall effect, of the Mn-doped Bi2Se3 are discussed in detail. Finally, we conclude with future prospects and challenges in further studies of Mn doped TIs.

Using irradiation effect to study the disparate anchoring stabilities of polar-organic molecules adsorbed on bulk and thin-film metal surfaces

The behavior of polar metal organic molecules, chloroaluminum phthalocyanine (ClAlPc), upon ultraviolet (UV) irradiation was investigated to evaluate the stability of the adsorption process on the Ag(111) thin film and bulk crystal. Photoelectron spectroscopy (PES) was mainly employed to measure the molecular energy states (MES) and vacuum level (VL) shift for 1-ML ClAlPc in the Cl-down configuration. A consistent trend was observed showing that ClAlPc in the Cl-down configuration is energetically more stable on the Ag thin-film surface than on the corresponding surface of the Ag bulk crystal. The intermediate adsorption state in tilted configuration during the irradiation impinging is identified by large positive VL shifts and broad spectra line shapes to infer a flipping mechanism from Cl-down to Cl-up configuration. Strain on the Ag thin films from the underlying lattice-mismatched Ge(111) substrate is considered to cause enlarged hollow sites on the Ag(111) thin-films, that anchor the Cl-down configuration more tightly on the thin-film surfaces, as confirmed by density functional theory (DFT) calculations.

In Situ Optical Absorption Studies of Point Defect Kinetics and Thermodynamics in Oxide Thin Films

AbstractThe authors review the use of in situ optical absorption to probe point defect equilibria and kinetics in oxides, with particular attention to thin films and high‐temperature measurements evaluating changes in oxygen content. The introduction notes the impact of point defects on oxide behavior, including differences in thin films versus bulk materials, and describes methods to evaluate point defect equilibria and kinetics; advantages of optical absorption among these methods are highlighted. Section 2 provides an overview of how optical absorption and point defect concentrations are related, including various possible structural and electronic origins of defect‐mediated absorption changes corresponding to oxygen content changes. Section 3 traces the development of historical understanding of optical absorption in nonstoichiometric oxides and emergence of thin film applications and measurement approaches that resulted. Section 4 details the procedure for determining surface exchange coefficients from optical transmission relaxation measurements and provides specific examples of how this approach uniquely has enabled new insights into thin film surface exchange behavior with and without current collectors. Section 5 provides examples of optically derived diffusivity measurements and comparisons of bulk and thin film defect equilibria. Section 7 outlines potential opportunities and future developments in this area, and Section 7 concludes the manuscript.

Chalcogenide microlens arrays fabricated using hot embossing with soft PDMS stamps

This paper reports the development of a hot embossing method for fabricating chalcogenide microlens arrays using soft polydimethylsiloxane (PDMS) stamps. The stamps are formed by molding PDMS on photoresist masters fabricated with sphere segments by thermal reflow. Upon proper temperatures, the chalcogenide glass softens and is embossed using PDMS stamps with low pressure. Void defects at the center and at the side of microlenses are found, and the mechanisms for the void formation are investigated and verified. The hot embossing parameters including pressure, temperature, embossing time, and the thicknesses of the PDMS stamps are optimized. A 400 × 400 microlens array with uniform profiles, smooth surface roughness, and good roundness have been successfully fabricated on both chalcogenide thin films and bulk wafers. The preliminary results demonstrate the feasibility of using hot embossing with soft PDMS stamps as a simple and low cost method to fabricate chalcogenide microlens arrays.

Numerical investigation of an evaporating meniscus in a heated capillary slot

This paper numerically studies heat transfer and fluid flow from an evaporating meniscus of a wetting fluid within a heated capillary. A simplified steady state mathematical model is developed for predicting the wicking height of the meniscus and the evaporation mass flow rate which includes: (1) one-dimensional flow and energy equations for the liquid and vapor regions, (2) one-dimensional model for the evaporating meniscus region, and (3) two-dimensional energy equation for the capillary wall. Three parameters, namely, apparent contact angle, cumulative heat transfer, and evaporating meniscus height characterize the evaporating meniscus region. In this paper, the apparent contact angle in the evaporating meniscus is uniquely deduced from the meniscus curvature at the centre of the capillary using the thickness profile obtained from standard extended meniscus theory (which includes the evaporating thin film and bulk meniscus regions). Correlations are obtained for the cumulative heat transfer, apparent contact angle and evaporating meniscus height as a function of the difference between the wall and saturation temperatures from the evaporating thin film theory for the meniscus region, which is called as micromodel. The macroscopic model accounts for wall heat conduction and heat transfer with fluid flow in the liquid and vapor regions. The micromodel deals with heat transfer and fluid flow in the evaporating meniscus region. In this paper, a novel scheme to link the “macroscopic” momentum and energy equations in the capillary slot and the evaporating meniscus through the correlations developed above is proposed. Using this numerical model, the wicking height and the evaporation mass flow rate are estimated and the results are compared with previously conducted experiments. The trends in the numerical results of the mathematical model correlate reasonably well with the experimental data.

Effect of Molecular Orientation to Proton Conductivity in Sulfonated Polyimides with bent backbones

Two organized sulfonated polyimides (ASPIs) with bent polymer backbones were synthesized to study a relation between structure and proton conductivity. Proton conductivity exhibited different values between the thin film and bulk pelletized forms. Results from small-angle X-ray scattering (SAXS) and attenuated total reflection measurements suggest that the orientation of backbones is different between the thin film and bulk forms. SAXS results suggest excess amount of water uptake in the bulk samples compared to the thin film form. These structure and water uptake property could contribute the difference of the proton conductivity between the thin film and bulk pelletized forms.

Manipulation of Conductive Domain Walls in Confined Ferroelectric Nanoislands

AbstractConductive ferroelectric domain walls—ultranarrow configurable conduction paths—have been considered as essential building blocks for future programmable domain wall electronics. For applications in high‐density devices, it is imperative to explore the conductive domain walls in small confined systems, while earlier investigations have hitherto focused on thin films or bulk single. Here, an observation and manipulation of conductive domain walls confined within small BiFeO3 nanoislands aligned in high‐density arrays are demonstrated. Using conductive atomic force microscopy, various types of conductive domain walls, including the head‐to‐head charged domain walls (CDWs), zigzag domain walls, and typical 71° head‐to‐tail neutral domain walls (NDWs), are distinctly visualized. The CDWs exhibit remarkably enhanced metallic conductivity with current of ≈nA order in magnitude and 104 times larger than that inside domains (0.01–0.1 pA), while the semiconducting NDWs allow much smaller current (≈10 pA) than the CDWs. The substantial difference in conductivity for dissimilar walls enables manipulations of various wall conduction states for individual addressable nanoislands via electrical tuning of domain structures. A controllable writing of four distinctive states in individual nanoislands can be achieved, showing application potentials for developing multilevel high‐density memories.

Numerical investigation of terminal residual thickness of the water film and surface flow in the entrainment region in Marangoni drying

Marangoni drying is widely employed in integrated circuit manufacturing to remove water from the wafer surface. The organic vapor blown at the meniscus induces a strong Marangoni effect, which strips the entrained water film induced by the wafer’s withdrawal from a deionization water bath, and realizes ultra-clean drying of the wafer. The present work implements a numerical investigation of the terminal residual film and free surface flow in the water entrainment region during Marangoni drying based on a trans-scale model containing the thin water film, meniscus, and bulk region. The results show that the terminal thickness of the water film is 30 nm, which approximately agrees with the previous experimental and numerical results. In addition, the stagnation point moves away from the thin film region compared with that of wafer withdrawn in the absence of the Marangoni effect. The magnitude of free surface velocity in the thin film region after drying depends only on the withdrawal velocity, and is irrelevant to the Marangoni number. The analyses of the entrained water film thinning and surface flow provide useful guidance about controlling the drying process.

Micro-machining of PMN-PT Crystals with Ultrashort Laser Pulses

Lead–magnesium niobate lead titanate (PMN-PT) has been proven as an excellent material for sensing and actuating applications. The fabrication of advanced ultra-small PMN-PT-based devices relies on the availability of sophisticated procedures for the micro-machining of PMN-PT thin films or bulk substrates. Approaches reported up to date include chemical etching, excimer laser ablation, and ion milling. To ensure an excellent device performance, a key mandatory feature for a micro-machining process is to preserve as far as possible the crystalline quality of the substrates; in other words, the fabrication method must induce a low density of cracks and other kind of defects. In this work, we demonstrate a relatively fast procedure for the fabrication of high-quality PMN-PT micro-machined actuators employing green femtosecond laser pulses. The fabricated devices feature the absence of extended cracks and well-defined edges with relatively low roughness, which is advantageous for the further integration of nanomaterials onto the piezoelectric actuators.

Self-assembly of copper-free maltoheptaose-block-polystyrene nanostructured thin films in real and reciprocal space

The promising carbohydrate-based block copolymer maltoheptaose-block-polystyrene (MH-b-PS) has been used for high-performance memory transistors and next generation nanolithography. In order to realize the potential of MH-b-PS especially in microelectronic applications, we firstly improved its synthetic method for obtaining large amount of copper-free MH-b-PS. The main improvement relies on the removal of the residual copper catalyst by using a chelating resin. Then, the microphase separation of copper-free MH-b-PS in both thin film and bulk states under different solvent vapor annealing conditions were investigated comprehensively and compared with our previous report by using both real-space and reciprocal-space techniques. A phase transition of MH-b-PS from hexagonal close-packed horizontal cylinders to face-centered cubic were observed when increasing the amount of tetrahydrofuran in the mixture annealing solvent of tetrahydrofuran and H2O. More details about self-assembled MH-b-PS nanostructures were analyzed by comparing grazing incidence small angle X-ray scattering patterns with corresponding atomic force microscopy phase images.

Failure mode in first-principles computational tensile tests of grain boundaries: effects of a bulk-region size, dominant factors, and local-energy and local-stress analysis

A first-principles computational tensile test (FPCTT) is a powerful tool to investigate intrinsic strength and failure processes of grain boundaries (GBs), according to atomic and electronic behaviors based on density-functional theory, while careful interpretation is required in comparison with experiments, because of ideal conditions used in FPCTTs. We observed serious effects of a bulk-region size in FPCTTs of the {0 0 1} 5 GB in Al. For a GB supercell with enough thick bulk regions, the energy–strain curve shows spontaneous failure with catastrophic energy release just after the maximum stress point, which we name Type-A failure. For a GB supercell with thin bulk regions, the energy increases gradually even after the maximum stress and continuously becomes that of relaxed fracture surfaces, which we name Type-B failure, although the stress–strain curves are almost common until the maximum stress point in both the supercells. The peculiar failure of Type B occurs by the lack of accumulated strain energies for creating fracture surfaces even after the maximum stress point, because the accumulated strain energy is nearly proportional to the bulk-region size. We clarified that the failure mode in a FPCTT depends on the relationship among the three factors; the accumulated strain energy depending on the bulk-region size, the work of separation (the formation energy of fractured surfaces into a GB), and the maximum stress of the GB (the GB strength). We showed that the failure mode of previous FPCTTs of Al tilt GBs with segregated impurities can be reinterpreted from this viewpoint, by considering the changes of the work of separation and the GB strength by impurities. We should be aware of the distinction of the failure mode in FPCTTs, because experimentally Type-B failure does not occur except for special cases. Finally, we applied ab initio local-energy and local-stress analysis to the FPCTT of the {0 0 1} 5 GB in Al, and discussed how to extract local energy–strain or energy–separation relations independent of the bulk-region size to be combined with meso- or macroscopic simulations.

Anomalous Antiferromagnetism in Metallic RuO_{2} Determined by Resonant X-ray Scattering.

We studied the magnetic ordering of thin films and bulk crystals of rutile RuO_{2} using resonant x-ray scattering across the Ru L_{2} absorption edge. Combining polarization analysis and azimuthal angle dependence of the magnetic Bragg signal, we have established the presence and characteristic of collinear antiferromagnetism in RuO_{2} with T_{N}>300 K. In addition to revealing a spin-ordered ground state in the simplest ruthenium oxide compound, the persistence of magnetic order even in nanometer-thick films lays the ground for potential applications of RuO_{2} in antiferromagnetic spintronics.

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ) thin film and bulk model systems during iron exsolution.

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-δ). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(iv) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(iii). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data.

Sound Absorption and Transmission in Nanoengineered Polymers

In this study we use vapor deposition to incorporate carbon nanotubes (CNTs) and silicon dioxide nanopowder (nanosilica) in Polyurethane (PU) as well as to engineer Parylene C (PC) in a microfibrous thin film (µFTF) and bulk forms in an attempt to examine acoustic absorption and transmission properties in the low frequency (50 to 200 Hz) range. Incorporating CNTs or/and nanosilica into PU significantly increases its acoustic transmission, presumably due to increased rigidity caused by nanoparticle (CNTs and nanosilica) addition. This is explained as due to adding nanosilica or/and CNTs to PU increases its rigidity and, hence, its sound transmission. On the other hand, engineering the PC in the µFTF form enhances acoustic absorption in the film, especially at the resonant frequencies associated with oscillations of the microfibers making up the PC µFTF.