Nanoscale Thin Films Research Articles

Phase field simulations of the poling behavior of BaTiO3 nano-scale thin films with SrRuO3 and Au electrodes

Abstract A continuum phase field model is used to investigate the poling behavior of BaTiO 3 (BTO) thin films with thicknesses ranging from 1 nm–80 nm. The finite element simulations focus on the influence of the electrode material on the electrical hysteresis curves. The 15 nm thick electrodes consist of either SrRuO 3 (SRO) or Au and are explicitly included in the simulations. Experimental evidence suggests that there is a depolarization field in SRO-electroded BTO thin films. This depolarization field, which is due to an imperfect charge compensation by the electrodes, causes a suppression of ferroelectricity for film thicknesses of a few unit cells. The depolarization field and its effect on the electrical behavior is captured by a Robin type boundary condition for the order parameter of the phase field model. This boundary condition is applied at the interface between the thin film and the top and bottom electrode. The simulation results show that the overall poling behavior strongly depends on the lattice parameter of the electrode material. Films with SRO electrodes, which have a lattice parameter smaller than the a -parameter of tetragonal BTO, observe a switchable polarization larger than the spontaneous polarization. If Au is used (which has lattice parameter larger than the a -parameter of tetragonal BTO) then switching is inhibited for films which are less than 20 nm thick. It is also shown that the Robin boundary condition induces a depolarization field which has the effect of dramatically reducing the switchable polarization in ultrathin films.

Scalable high-mobility MoS2thin films fabricated by an atmospheric pressure chemical vapor deposition process at ambient temperature

Nano-scale MoS2 thin films are successfully deposited on a variety of substrates by atmospheric pressure chemical vapor deposition (APCVD) at ambient temperature, followed by a two-step annealing process. These annealed MoS2 thin films are characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), micro-Raman, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-VIS-NIR spectrometry, photoluminescence (PL) and Hall Effect measurement. Key optical and electronic properties of APCVD grown MoS2 thin films are determined. This APCVD process is scalable and can be easily incorporated with conventional lithography as the deposition is taking place at room temperature. We also find that the substrate material plays a significant role in the crystalline structure formation during the annealing process and single crystalline MoS2 thin films can be achieved by using both c-plane ZnO and c-plane sapphire substrates. These APCVD grown nano-scale MoS2 thin films show great promise for nanoelectronic and optoelectronic applications.

X‐Ray Photoelectron Spectroscopy Investigations of the Surface Reaction Layer and its Effects on the Transformation Properties of Nanoscale Ti51Ni38Cu11 Shape Memory Thin Films

The depth‐dependent chemical constitution of Ti51Ni38Cu11 thin films of different total film thickness from 400 to 50 nm was characterized using X‐ray photoelectron spectroscopy (XPS). It was analyzed how reaction layers, which form on the surface of the film significantly change the chemical composition of the transforming phase, which leads in turn to altered phase transformation properties. For thinner films, the deviation from the nominal chemical composition increases. For a film thickness of 50 nm, a Ti loss of ≈9 at% is observed. The Ni content is increased by ≈5 at%, whereas the Cu content stays relatively constant for films of different thickness. The results are summarized in a layer model, which supports designing nanoscale shape memory thin films.

(Keynote) Atomic Scale and Interface Interactions in Redox-Based Resistive Switching Memories

Controlling electrochemical processes at interfaces or within thin films at the atomic scale is a key factor for further development of future nanoelectronics and information technology. This aspect becomes even more important considering concepts beyond conventional data storage like neuromorphic and memristive systems, demonstrating learning abilities and allowing logic operations. In redox-based resistive switching memories (ReRAMs), the solid electrolyte thickness varies in the range between few nanometers up to some ten nanometers. Due to the small dimensions, the high electric fields in the order of E ~ 108 Vm–1, and current densities of j ~ 109 Acm–2, the interface regions are dynamically changing as a function of the thermodynamic and operation conditions. The paper presents an overview on the current state-of-the-art knowledge on atomically scaled processes and interactions, focusing on the electrochemical dynamics at the metal/solid-electrolyte interfaces of nanoscaled thin films and quantum effects.

Unconventional phase diagrams in an ultra-thin spin-1 Ising film with site (or bond) dilution at the surfaces

The phase diagrams of two nanoscaled thin films with bond and site dilutions at the surfaces, described by the spin-1 Ising model with single-ion anisotropy (D), are investigated by the use of the effective field theory with correlations. They are consisted of two layers. A number of characteristic phenomena have been found in the behaviors of transition temperature and tricritical point, which are heavily dependent on the ratios (r=J1/J and d=D/J) and a concentration (p) of bond or site dilution at the surfaces, where J is the exchange interaction in inner layer and J1 is the exchange interaction between the surface.

Virtual X-Ray Laboratory for Teaching Crystallography and Other Courses

Contemporary research equipment is not only extremely complex and very expensive, it is typically fully computerized and most tasks are executed without student participation. This creates a number of substantial educational drawbacks and limitations. The paper presents a virtual multifunctional X-ray laboratory (v-Lab) that helps overcome the above mentioned problems, addresses the demands of distance and blended education and meets learning habits of today's students. The v-Lab enables students to practice concepts, tasks, and equipment operations in a manner that can't be achieved on actual equipment. It employs virtual X-ray equipment that realistically imitates the functionality and design of actual equipment and also includes educational analytical software. Virtual data can be exported to popular software as well. Highly interactive online experiments using virtual equipment have been created for undergraduate students to learn fundamental principles underlying the analytical x-ray methods and become familiar with the design and operation of the X-ray equipment. A variety of visual, audio and traditional learning and assessment resources were integrated with virtual experiments to provide students with "just-in-time" learning opportunities. The virtual equipment has also been used for preparing undergraduate and graduate students to perform various research studies including but not limited to: X-Ray diffraction study of phase transitions in ferroelectric ceramics and nanoscale thin films, qualitative phase analysis of various compounds, nanostructured materials and human kidney stones, etc. Examples of virtual experiments used for teaching crystallography and other science and engineering courses at several US and Russian universities are presented and discussed. An easy-to-use authoring tool enables instructors to customize existing virtual experiments, and create new ones, as well as to add their own samples into the sample collection.

Atomic Scale and Interface Interactions in Redox-Based Resistive Switching Memories

Controlling electrochemical processes at interfaces or within thin films at the atomic scale is a key factor for further development of future nanoelectronics and information technology. This aspect becomes even more important considering concepts beyond conventional data storage like neuromorphic and memristive systems, demonstrating learning abilities and allowing logic operations.Redox-based resistive switching memories (ReRAM) are major candidates to fulfil all requirements for low power consumption, ultrafast operation, high information density and non-volatility. Their metal/solid-electrolyte/metal structure makes the preparation process simple and compatible with current CMOS technology. In ReRAMs, the solid electrolyte thickness varies in the range between few nanometers up to some ten nanometers. Due to the small dimensions, the high electric fields in the order of E ~ 108 Vm– 1, and current densities of j ~ 109 Acm– 2, the interface regions are dynamically changing as a function of the thermodynamic and operation conditions. Deposited as nanoscale films, even macroscopic insulating materials like SiO2 and Ta2O5 are able to conduct ions, and electrochemical measurements well known from the liquid electrochemistry can be performed.The talk gives an overview on the current state-of-the-art knowledge on redox-based resistive switching ReRAMs, focusing on the electrochemical dynamics at the metal/solid-electrolyte interfaces of nanoscaled thin films and quantum effects. The electrochemical kinetics during formation of nanofilaments will be discussed along with influence of the local environment and the generic relevance of the counter charges. The relation of these studies to the more fundamental issue of describing the microscopic electrochemical processes at the atomic scale will be emphasized.

Crystal Growth Mode Changes during Pulsed Laser Induced Rapid Solidification in Nanoscale Thin Films of Al-Cu Eutectic

Crystal Growth Mode Changes during Pulsed Laser Induced Rapid Solidification in Nanoscale Thin Films of Al-Cu Eutectic

Chemical stability study of nanoscale thin film yttria-doped barium cerate electrolyte for micro solid oxide fuel cells

The chemical stability of a nanoscale yttria-doped barium cerate (Y–BaCeO3 or BCY) thin film electrolyte for low temperature solid oxide fuel cells (SOFCs) are investigated in the temperature range of 300–400 °C. The micro-SOFCs using 300 nm-thick BCY electrolyte show a poor fuel cell performance and continuous decrease in open circuit voltage (OCV) with higher operating temperatures. Characterization results from X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed that the observed degrading fuel cell performance is attributed to the rapid decomposition of BaCeO3 into BaCO3, Ba(OH)2, and CeO2 from reactions with CO2 and H2O during fuel cell test. The lattice parameters of BCY expanded during fuel cell test, resulting in the formation of micro cracks along the electrolyte that possibly induced the OCV drop. The reactions of nano thin film BCY with CO2 and H2O are both active at temperature between 300 and 400 °C, which is unlike bulk BCY material where only the reaction with CO2 is significant. The reduction of Ce4+ species into Ce3+ in the BCY electrolyte has also lead to an electronic conduction in BCY that lowered OCVs.

Impact of the Counter‐Electrode Material on Redox Processes in Resistive Switching Memories

AbstractCation‐based resistive‐switching memories rely on the injection and drift of metal ions in nanoscale thin films. In insulators that do not initially contain mobile cations, such as SiO2, Ta2O5, and so forth, water redox reactions occurring at the counter electrode (CE) were found to be essential in enabling the dissolution of the active electrode and to keep electroneutrality. In this study, we report on the impact of the CE on redox processes prior to resistive switching. Potentiodynamic measurements for various electrode materials revealed that the catalytic activity of the CE towards the water redox process determines the concentration of dissolved ions within the oxide and influences the rate of the total cell reaction. This trend can be used as an indicator for the design of both cation‐ and anion‐conducting oxide‐based resistive‐switching random‐access memories.

Nanostructure- and Orientation-Controlled Digital Memory Behaviors of Linear-Brush Diblock Copolymers in Nanoscale Thin Films

Linear-brush diblock copolymers bearing carbazole moieties in the brush block were synthesized. Various phase-separated nanostructures were found to develop in nanoscale thin films of the copolymers, depending on the fabrication conditions including selective solvent-annealing. This variety of morphologies and orientations means that these block copolymers exhibit digital memory versatility in their devices. Overall, the relationship between the morphology and digital memory performance of these copolymers has several important features. In particular, the carbazole moieties in the vertical cylinder phase with a radius of 8 nm or less can trap charges and also form local hopping paths for charge transport, which opens the mass production of advanced digital memory devices with ultrahigh memory density. Charges can be transported through the layer when the dielectric linear block phase has a thickness of 10.6 nm; however, charge transport is not possible for a dielectric phase with a thickness of 15.9 nm. ...

Magnetocaloric effect in nanoscale thin films and heterostructures

This review focuses on the magnetocaloric effect with special attention to nanoscale thin films and heterostructures. The authors outline the general phenomenon of the magnetocaloric effect and discuss how using materials in reduced dimensions can impact this emerging area. The authors note works of significance to date and highlight general features emanating from the community. They provide important details related to sample fabrication, relevant metrology, and discuss advanced data analyses, all of which are done in a tutorial fashion. Finally, the authors provide an outlook for the application of nanoscience to magnetocalorics.

Performance Analysis of Short Journal Bearing under Thin Film Lubrication

The steady state performance analysis of short circular journal bearing is conducted using the viscosity correction model under thin film lubrication conditions. The thickness of adsorbed molecular layers is the most critical factor in studying thin film lubrication, and is the most essential parameter that distinguishes thin film from thick film lubrication analysis. The interaction between the lubricant and the surface within a very narrow gap has been considered. The general Reynolds equation has been derived for calculating thin film lubrication parameters affecting the performance of short circular journal bearing. Investigation for the load carrying capacity, friction force, torque, and power loss for the short circular journal bearing under the consideration of adsorbed layer thickness (2δ) has been carried out. The analysis is carried out for the short bearing approximation (L/D=0.5) using Gumbel’s boundary condition. It has been found that the steady state performance parameters are comparatively higher for short circular journal bearing under the consideration of adsorbed layer thickness than for plain circular journal bearing. The load carrying capability of adsorbed layer thickness considered bearing is observed to be high for the specified operating conditions. This work could promote the understanding and research for the mechanism of the nanoscale thin film lubrication.

De-vitrification of nanoscale phase-separated amorphous thin films in the immiscible copper–niobium system

Copper and niobium are mutually immiscible in the solid state and exhibit a large positive enthalpy of mixing in the liquid state. Using vapour quenching via magnetron co-sputter deposition, far-from equilibrium amorphous Cu–Nb films have been deposited which exhibit a nanoscale phase separation. Annealing these amorphous films at low temperatures (~200 °C) initiates crystallization via the nucleation and growth of primary nanocrystals of a face-centred cubic Cu-rich phase separated by the amorphous matrix. Interestingly, subsequent annealing at a higher temperature (>300 °C) leads to the polymorphic nucleation and growth of large spherulitic grains of a body-centred cubic Nb-rich phase within the retained amorphous matrix of the partially crystallized film. This sequential two-stage crystallization process has been investigated in detail by combining transmission electron microscopy [TEM] (including high-resolution TEM) and atom probe tomography studies. These results provide new insights into the crystallization behaviour of such unusual far-from equilibrium phase-separated metallic glasses in immiscible systems.

Nanoscale thin film transfer using elastomer-covered roll with buffer cavities

In this study, we present plate-to-roll transfer of nanoscale thin films using an elastomer-covered roll with buffer cavities. During the roll transfer process, defects such as cracks and wrinkles in the transferred film are easily generated. To suppress the defects, the roll’s mechanical behavior should be analyzed before any improvements can be made to the roll and subsequently the transfer quality. Finite element analysis was adopted to analyze the roll’s mechanical behavior and it was shown that the hoop and axial strain can be reduced as compared to the conventional roll by using the proposed roll. 46-nm gold thin films (9×9 mm2) were transferred using the proposed roll, and the quality of the transferred film was improved. The proposed roll in this study can be used in roll transfer processes for improving the quality of the transferred thin film.

The Use of Feature Parameters to Asses Barrier Properties of ALD coatings for Flexible PV Substrates

This paper reports on the recent work carried out as part of the EU funded NanoMend project. The project seeks to develop integrated process inspection, cleaning, repair and control systems for nano-scale thin films on large area substrates.In the present study flexible photovoltaic films have been the substrate of interest. Flexible PV films are the subject of significant development at present and the latest films have efficiencies at or beyond the level of Si based rigid PV modules. These flexible devices are fabricated on polymer film by the repeated deposition, and patterning, of thin layer materials using roll-to-roll processes, where the whole film is approximately 3um thick prior to encapsulation. Whilst flexible films offer significant advantages in terms of mass and the possibility of building integration (BIPV) they are at present susceptible to long term environmental degradation as a result of water vapor transmission through the barrier layers to the CIGS (Copper Indium Gallium Selenide CuInxGa(1-x)Se2) PV cells thus causing electrical shorts and efficiency drops. Environmental protection of the GIGS cell is provided by a thin (40nm) barrier coating of Al2O3. The highly conformal aluminium oxide barrier layer is produced by atomic layer deposition (ALD) where, the ultra-thin Al2O3 layer is deposited onto polymer thin films before these films encapsulate the PV cell. The surface of the starting polymer film must be of very high quality in order to avoid creating defects in the device layers. Since these defects reduce manufacturing yield, in order to prevent them, a further thin polymer coating (planarization layer) is generally applied to the polymer film prior to deposition.The presence of surface irregularities on the uncoated film can create defects within the nanometre-scale, aluminium oxide, barrier layer and these are measured and characterised. This paper begins by reporting the results of early stage measurements conducted to characterise the uncoated and coated polymer film surface topography using feature parameter analysis. The measurements are carried out using a Taylor Hobson Coherence Correlation Interferometer an optical microscope and SEM. Feature parameter analysis allows the efficient separation of small insignificant defects from large defects. The presence of both large and insignificant defects is then correlated with the water vapour transmission rate as measured on representative sets of films using at standard MOCON test. The paper finishes by drawing conclusions based on analysis of WVTR and defect size, where it is postulated that small numbers of large defects play a significant role in higher levels of WVTR.

Buff/wipe effects on the physicochemical properties of perfluoropolyether nanoscale thin films

Buff/Wipe (B/W) process is commonly used in disk drive manufacturing to remove the particles and asperities on the lubricated disk surface. In this paper, we investigated how B/W process impacts the physicochemical properties of perfluoropolyethers (PFPE) nano-films through the study of surface energy and bonded ratio. Two-liquid geometric method was used to analyze the surface energy of nonfunctional PFPE, i.e., Z03, and functional PFPE, i.e., Zdol, lubricated media before and after B/W process. It was found that the dispersive surface energy of Z03 films greatly decreased after B/W, which was more significant in the submonolayer regime. In addition, the bonded ratio slightly increased. However, B/W effect on the surface energy and bonded ratio was not detected for Zdol films. It is hypothesized that nonfunctional PFPE behaves liquid-like on the carbon overcoat due to the weak interaction between lubricant and overcoat. External mechanical stress as applied with B/W can change the conformation and increase the surface coverage for nonfunctional PFPE. On the other hand, functional PFPEs behave solid-like due to the strong attraction between lubricant and overcoat; therefore, it is difficult to change the conformation by external stress from B/W process.

Terahertz absorption characteristics of nickel–chromium film in a microbolometer focal plane array

Nickel-chromium (NiCr) thin film acted as a terahertz (THz) absorption layer in uncooled infrared (IR) microbolometers operated in the THz spectral range. Nanoscale NiCr thin films with different thicknesses were deposited on the diaphragms of 320 × 240 IR focal plane arrays (IRFPAs). The experiments suggested that the THz absorption of NiCr film increased with the increased frequency. Absorption for a lower frequency decreased whereas that for a higher frequency increased with the decreased film thickness. Optimised absorption for different spectral ranges was achieved by adjusting the NiCr film thickness. A 30 nm NiCr film provided the largest absorption in 0.5-1.5 THz whereas an 8 nm NiCr film showed the greatest contribution in 2-2.5 THz. The THz absorption characteristics of the pixel membrane structure in the IRFPAs coupled with a thin metallic film were modelled and analysed. A good agreement was obtained between the calculations and the measurements.

Effect of heated zone size on micro and nanoscale convective heat transfer

Recent studies on free convective heat transfer from micro and nanoscale structures show that the heat transfer coefficient is size dependent. The heat transfer coefficient is found to increase at smaller scales, which is ascribed to the higher surface area to volume ratio. Also, the mode is changed from advection to conduction due to the decaying influence of the gravitational field. Interestingly, it is tacitly assumed that the size effect is due to the specimen, since not a single study on the effect of the size of heat source on the convective heat transfer exists in the literature. In this study, we provide unambiguous experimental evidence of the predominance of heater size on the heat transfer coefficient. For micro-heaters, the heat transfer coefficient is measured to about 3200W/m2K. This value drops to about 110W/m2K for a millimeter scale heater and to 10W/m2K for a macroscopic heater; all for a nanoscale thin film specimen. This finding is particularly significant in microelectronic applications where localized heating (hot spots) in small areas is very common.

Cooperative and selective self-assembly behaviors of diblock copolypeptides in nanoscale thin films

A series of diblock copolypeptides with various compositions (PBLGm-b-PBCLn) was synthesized through the living ring-opening polymerizations of γ-benzyl-L-glutamate and ε-(benzyloxycarbonyl)-L-lysine N-carboxyanhydrides with the aid of a nickel catalyst system. They were found to be stable up to around 150 °C and easily processable. Their chain conformations and morphologies in nanoscale thin films were characterized in detail by using infrared spectroscopy, atomic force microscopy, and in situ synchrotron grazing incidence X-ray scattering. In particular, quantitative X-ray scattering analysis was used to provide for the first time the morphological structures and orientation details of the diblock copolypeptides in thin films. Fibrils are present in the thin films of the copolypeptides; interestingly, the films are composed of two different rotationally isomeric hexagonally (HEX) packed cylinder structures that are preferentially oriented in the film plane. Further, the HEX structures consist of two substructural block units: one consisting of PBLG block chain cylinders and the other consisting of PBCL block chain cylinders. The block chains in the substructural units were found to interdigitate partially via the side groups. Thus the cylinders' interdigitation takes place selectively between block chains of the same kind rather than between different kinds of block chains. It was also confirmed that this high interdigitation selectivity occurs in the blend films of the homopolypeptides. These results show that in diblock copolypeptide films such selective interdigitation can override any thermodynamic penalties associated with the high chain rigidity due to the α-helical conformation and the effects of confinement in the connected diblock architecture, which leads to phase separation and the formation of well-defined, integrated HEX cylinder structures. These cooperatively and selectively formed HEX cylinder structures were found to be stable up to the degradation temperature. Molecular structure models are presented for the copolypeptide thin films as well as for the homopolypeptide blend films.