Nanoscale Thin Films Research Articles

Nanoscale thin films of niobium oxide on platinum surfaces: creating a platform for optimizing material composition and electrochemical stability

A nanoscale thin film of niobium oxide on a platinum substrate was evaluated for its influence on the electronic and chemical properties of the underlying platinum towards the oxygen reduction reaction with applications to proton exchange membrane fuel cells. The nanoscale thin film of niobium oxide was deposited using atomic layer deposition onto the platinum substrate. A film of niobium oxide is a chemically stable and electronically insulating material that can be used to prevent corrosion and electrochemical degradation when layers are several nanometers thick. These layers can be insulating if sufficiently thick and may not be sufficient to protect the platinum from corrosion if too thin. An ∼3 nm thin film of niobium oxide was fabricated on the platinum surface to determine its influence on the electronic and chemical properties at the interface of these materials. The atomic layer deposition process enabled a precise control over the material composition, structure, and layer thickness. The niobium oxide film was evaluated using cyclic voltammetry and electrochemical impedance spectroscopy to evaluate whether a balance could be found between the inhibition of platinum degradation and electronic insulation of the platinum for use in proton exchange membrane fuel cells. The 3 nm thin niobium oxide film was found to be sufficiently thin to permit electronic conductivity while reducing the incidence of platinum dissolution.

Corrosion mechanism in PVD deposited nano-scale titanium nitride thin film with intercalated titanium for protecting the surface of silicon

In this work, thin film systems consisting of: (i) monolithic titanium nitride, (ii) monolithic titanium, and, (iii) titanium nitride with intercalated titanium, were fabricated on <100> P-type Si wafers by magnetron sputtering. The thin films were characterized using electron microscopy, glancing angle x-ray diffraction and nanoindentation. Their impedance response was studied via open circuit potential and electrochemical impedance spectroscopy in a sodium chloride aqueous solution. The influence of intercalated Ti thickness on impedance behavior was investigated in detail. Scanning electron microscopy analysis confirmed the columnar structure of TiN and aggregated structure of Ti thin films. Further microstructural analysis confirmed the presence of nano-porosities in thin films which explained their low modulus and hardness. Analysis of impedance data with equivalent circuit models indicated that incorporation of titanium as intercalated layer between titanium nitride and silicon surface, greatly altered the impedance characteristics of Si. Higher impedance values of thin films were achieved for bi-layer configuration at a much smaller total thickness as compared to monolithic counterparts. The increase in impedance was attributed to the presence of less defective and compact intercalated Ti layer that effectively interrupted the corrosive ions pathways.

Significant Enhancement of the Photoelectrochemical Activity of CuWO4 by using a Cobalt Phosphate Nanoscale Thin Film

AbstractNanostructured CuWO4 photoanodes were fabricated through a facile electrodeposition method, which was followed by annealing the sample at 500 °C for 2 h. The morphologies, crystalline structure, electronic states, optical behaviors, and photoelectrochemical characteristics of the CuWO4 nanomaterial were examined by scanning electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, UV/Vis spectroscopy, and impedance spectroscopy, showing that the formed triclinic CuWO4 nanoparticles had an indirect band gap of 2.2 eV and strong response to visible light. The cobalt phosphate (CoPi) nanoscale thin film, which was used as a co‐catalyst, was electrodeposited on the CuWO4 surface. The photocurrent of the cobalt phosphate complex‐catalyzed CuWO4 electrodes exhibited an 86 % higher photocurrent response than that of the unmodified CuWO4 nanoparticles under the irradiation of one simulated sun (100 mW cm−2). The kinetics of this photoelectrochemical water splitting process was further investigated, and it was found that a more negative flat band potential and reduced charge transfer resistance were likely the primary reasons behind the enhancement.

Full-switching FSF-type superconducting spin-triplet magnetic random access memory element

In the present work a superconducting Co/${\mathrm{CoO}}_{x}$/${\text{Cu}}_{41}{\text{Ni}}_{59}$/Nb/${\text{Cu}}_{41}{\text{Ni}}_{59}$ nanoscale thin film heterostructure is investigated, which exhibits a superconducting transition temperature, ${T}_{c}$, depending on the history of magnetic field applied parallel to the film plane. In more detail, around zero applied field, ${T}_{c}$ is lower when the field is changed from negative to positive polarity (with respect to the cooling field), compared to the opposite case. We interpret this finding as the result of the generation of the odd-in-frequency triplet component of superconductivity arising at noncollinear orientation of the magnetizations in the ${\text{Cu}}_{41}{\text{Ni}}_{59}$ layer adjacent to the ${\mathrm{CoO}}_{x}$ layer. This interpretation is supported by superconducting quantum interference device magnetometry, which revealed a correlation between details of the magnetic structure and the observed superconducting spin-valve effects. Readout of information is possible at zero applied field and, thus, no permanent field is required to stabilize both states. Consequently, this system represents a superconducting magnetic random access memory element for superconducting electronics. By applying increased transport currents, the system can be driven to the full switching mode between the completely superconducting and the normal state.

Anodic Oxides As Electrolytes for Resistive Switching Devices

Redox-based resistive switching memories (ReRAMs) are one of most promising candidates to be next generation nonvolatile memories. They meet all requirements for green IT e.g. low power consumption, high write, erase and read speed and can also be used for beyond von Neumann computing, alternative logic and neuromorphic operations. ReRAM devices have a simple structure, high scalability, data retention, endurance and can satisfy high-density integration criterion [1]. These systems are usually divided in two categories, VCM (Valence Change Memories) and ECM (Electrochemical Metallization cells), depending on the mechanism underlying the device operation [2]. The cell structure consists of a solid electrolyte, e.g. a metal oxide or a layered oxides structure, sandwiched between two electrodes with metallic behavior (metals, TiN or conducting transparent oxides). Despite the oxide films have macroscopic dielectric (high-k) properties, deposited as nanoscale thin films they behave as solid electrolytes [3]. Electrolyte for such devices are usually prepared by physical (e.g. sputtering, pulsed laser deposition) or chemical (atomic layer deposition, chemical vapor deposition) techniques. An alternative way to produce the bottom electrode/oxide junction is the anodizing, a low-cost and low temperature technique that allows to electrochemically prepare oxides on the surface of valve metals or valve metals alloys such as Ta, Ti, Nb, Al, Hf and so on [4]. It is possible to tailor oxides features, such as thickness, morphology, structure and composition by adjusting process parameters (e.g. growth rate, anodizing electrolyte, formation voltage). Barrier-type anodic films seem to be suitable solid electrolytes for ReRAM devices since they are smooth, uniform in thickness and composition and show perfect adhesion to the metallic substrate. In this work we want to present several valve metal/anodic oxide systems prepared by anodizing of metal thin films (namely Hf, Nb, Ta, Al and their alloys). Barrier-type anodic films were grown up to different formation voltages, in order to get oxides with different thickness and properties. An investigation based on electrochemical impedance and photoelectrochemical measurements was carried out to get information on the solid-state properties of the anodic oxides (i.e. band gap, flat band potential and dielectric constant) as a function of the formation conditions. Furthermore, we fabricated ReRAM-type devices (metal/oxide/metal junctions) by depositing Pt top electrodes on the anodic films surfaces. Electrical characterization was performed to check whether these devices exhibit resistive switching and, in addition, they are suitable for the use in redox-based memories. In particular, cyclic voltammetries, dc I-V sweeps and pulse measurements were carried out to study the performances of the devices and to establish the kind of mechanism underlying the device operation [5]. The experimental findings (I-V sweeps stability, data retention and endurance) prove that anodic oxides can be considered promising electrolytes for ReRAM devices. [1] Lee, M.-J.; Lee, C. B.; Lee, D.; Lee, S. R.; Chang, M.; Hur, J. H.; Kim, Y.-B.; Kim, C.-J.; Seo, D. H.; Seo, S.; Chung, U.-I.; Yoo, I.-K.; Kim, K. Nat. Mater. 10 (2011) 625-630. [2] Valov, I. ChemElectroChem 1 (2014) 26-36. [3] Valov, I.; Lu, W. D. Nanoscale 8 (2016) 13828-13837. [4] Zaffora, A.; Di Franco, F.; Santamaria, M.; Habazaki, H.; Di Quarto, F. Electrochim. Acta 180 (2015) 666–678. [5] Lübben, M.; Karakolis, P.; Ioannou-Sougleridis, V.; Normand, P.; Dimitrakis, P.; Valov, I. Adv. Mater. 27 (2015) 6202–6207.

Shape Memory Micro- and Nanowire Libraries for the High-Throughput Investigation of Scaling Effects.

The scaling behavior of Ti-Ni-Cu shape memory thin-film micro- and nanowires of different geometry is investigated with respect to its influence on the martensitic transformation properties. Two processes for the high-throughput fabrication of Ti-Ni-Cu micro- to nanoscale thin film wire libraries and the subsequent investigation of the transformation properties are reported. The libraries are fabricated with compositional and geometrical (wire width) variations to investigate the influence of these parameters on the transformation properties. Interesting behaviors were observed: Phase transformation temperatures change in the range from 1 to 72 °C (austenite finish, (Af), 13 to 66 °C (martensite start, Ms) and the thermal hysteresis from -3.5 to 20 K. It is shown that a vanishing hysteresis can be achieved for special combinations of sample geometry and composition.

An Advanced Characterization Method for the Elastic Modulus of Nanoscale Thin-Films Using a High-Frequency Micromechanical Resonator.

Nanoscale materials have properties that frequently differ from those of their bulk form due to the scale effect, and therefore a measurement technique that can take account of such material characteristics with high accuracy and sensitivity is required. In the present study, advanced nanomechanical metrology was developed for evaluation of elastic properties of thin-film materials. A 52 nm thick chromium (Cr) film was deposited on a high-speed micromechanical resonator using an e-beam evaporator, and the structure was excited to resonate using an ultrasonic platform. The resonant frequencies for the first and second flexural vibration modes were measured using laser interferometry, and they were compared to analytical estimation from the classical beam theory. Results show that the experimental data are in excellent agreement with the theory, within 1% of the relative error, and a mass sensitivity up to 10.5 Hz/fg was achieved. Thus, the scale effect that reduced the Young’s modulus of Cr by 49.8% compared to its bulk property was correctly recognized by the proposed method.

Spatial Manipulation of Thermal Flux in Nanoscale Films

ABSTRACTIncreased surface-to-volume ratio is the key thermal transport determinant in nanoscale thin films and yet the impact of surface features on thermal conduction remains largely unexplored. In this work, we aim to uncover the energy flux spatial distribution in thin-film thermal conduction by developing a formulation that accounts for the existence of distinct surface conditions (roughness and correlation length) at the two interfaces. We show that designing the physical properties of thin-film surfaces provides control over the preferential spatial location of thermal flux, which could hold significant potential for rational design of thermal materials. The study highlights the importance of a rigorous description of boundary scattering mechanisms to bring in a new wave of understanding of the nature of heat at the nanoscale.

Spontaneous delamination via compressive buckling facilitates large‐scale β‐Ga2O3 thin film transfer from reusable GaAs substrates

The fabrication and transfer via spontaneous delamination of large‐area, high‐quality β‐Ga2O3 nanoscale thin films with centimeter‐scale dimensions is demonstrated via thermal oxidation of reusable GaAs substrates. The films are characterized by using X‐ray diffraction, energy dispersive spectroscopy, Raman spectroscopy, and scanning electron microscopy. The film demonstrates good mechanical flexibility facilitating reliable transfer. The β‐Ga2O3 film's optical band gap and Schottky barrier height with gold are 4.8 and 1.03 eV, respectively. Electrical and optical properties of the transferable β‐Ga2O3 thin films exhibit a potential for application in solar‐blind photodetectors. The scale of the β‐Ga2O3 films transferred herein exceeds what the research community has reported to‐date by more than four orders of magnitude. The macroscopic dimensions of the transferred films may offer a remedy for the low thermal conductance of β‐Ga2O3 via transfer onto substrates with high thermal conductivity.

Orbital rearrangement mechanism and half-metallicity transition in strained Fe3O4/BaTiO3 interfaces

Biaxial-strain effect emerges as one of leading topics for the strong influence on interfacial properties of nanoscale thin films. The tunable electronic characteristics in strained Fe3O4/BaTiO3 heterostructure have been investigated by first-principles calculation. Two models (OTi-FeBO and TiO-FeBO) are considered in the calculation, which is different in polarization directions of ferroelectric BaTiO3. Tunable half-metallicity (HM) and orbital rearrangement of interfacial layers are found in strained OTi-FeBO. The HM to insulator transition can be attributed to the orbital rearrangement of interfacial interaction. The interfacial properties are modulated by different biaxial-strains and the HM to insulator transition mechanism at different strain is proposed. The research is useful in ferroelectric memories application.

Development of a conformable electronic skin based on silver nanowires and PDMS

This paper presented the designed and tested a flexible and stretchable pressure sensor array that could be used to cover 3D surface to measure contact pressure. The sensor array is laminated into a thin film with 1 mm in thickness and can easily be stretched without losing its functionality. The fabricated sensor array contained 8×8 sensing elements, each could measure the pressure up to 180 kPa. An improved sandwich structure is used to build the sensor array. The upper and lower layers were PDMS thin films embedded with conductor strips formed by PDMS-based silver nanowires (AgNWs) networks covered with nano-scale thin metal film. The middle layer was formed a porous PDMS film inserted with circular conductive rubber. The sensor array could detect the contact pressure within 30% stretching rate. In this paper, the performance of the pressure sensor array was systematically studied. With the corresponding scanning power-supply circuit and data acquisition system, it is demonstrated that the system can successfully capture the tactile images induced by objects of different shapes. Such sensor system could be applied on complex surfaces in robots or medical devices for contact pressure detection and feedback.

Construction of phase diagrams for nanoscaled Ising thin films on the honeycomb lattice using cellular automata simulation approach

ABSTRACTThe phase diagrams of the three-layer Ising model on the honeycomb lattice with a diluted surface have been constructed using the probabilistic cellular automata based on Glauber algorithm. The effects of the exchange interactions on the phase diagrams have been investigated. A general mathematical expression for the critical temperature is obtained in terms of relative coupling r = J1/J and Δs = (Js/J) − 1, where J and Js represent the nearest neighbor coupling within inner- and surface-layers, respectively, and each magnetic site in the surface-layer is coupled with the nearest neighbor site in the inner-layer via the exchange coupling J1. In the case of antiferromagnetic coupling between surface-layer and inner-layer, system reveals many interesting phenomena, such as the possibility of existence of compensation line before the critical temperature.

Combinatorial screening of halide perovskite thin films and solar cells by mask-defined IR laser molecular beam epitaxy

As an extension of combinatorial molecular layer epitaxy via ablation of perovskite oxides by a pulsed excimer laser, we have developed a laser molecular beam epitaxy (MBE) system for parallel integration of nano-scaled thin films of organic–inorganic hybrid materials. A pulsed infrared (IR) semiconductor laser was adopted for thermal evaporation of organic halide (A-site: CH3NH3I) and inorganic halide (B-site: PbI2) powder targets to deposit repeated A/B bilayer films where the thickness of each layer was controlled on molecular layer scale by programming the evaporation IR laser pulse number, length, or power. The layer thickness was monitored with an in situ quartz crystal microbalance and calibrated against ex situ stylus profilometer measurements. A computer-controlled movable mask system enabled the deposition of combinatorial thin film libraries, where each library contains a vertically homogeneous film with spatially programmable A- and B-layer thicknesses. On the composition gradient film, a hole transport Spiro-OMeTAD layer was spin-coated and dried followed by the vacuum evaporation of Ag electrodes to form the solar cell. The preliminary cell performance was evaluated by measuring I-V characteristics at seven different positions on the 12.5 mm × 12.5 mm combinatorial library sample with seven 2 mm × 4 mm slits under a solar simulator irradiation. The combinatorial solar cell library clearly demonstrated that the energy conversion efficiency sharply changes from nearly zero to 10.2% as a function of the illumination area in the library. The exploration of deposition parameters for obtaining optimum performance could thus be greatly accelerated. Since the thickness ratio of PbI2 and CH3NH3I can be freely chosen along the shadow mask movement, these experiments show the potential of this system for high-throughput screening of optimum chemical composition in the binary film library and application to halide perovskite solar cell.

Tough Nano‐Architectured Conductive Textile Made by Capillary Splicing of Carbon Nanotubes

Flexible electronics require electrically conductive and mechanically reliable nanoscale thin films. However, thin metal films have low fracture energy, which limits the performance of flexible devices. We demonstrate the design and synthesis of highly conductive, strong and tough nano‐architectured textile by capillary splicing of aligned carbon nanotubes (CNT). Owing to the strong van der Waals forces among CNTs, the pristine CNT network has average strength of 170 MPa. The average fracture energy of the textile is 16 kJ/m2, 50 folds higher than metal nanofilms. The high toughness results from crack bifurcations and friction hysteresis in a dissipation zone propagating several millimeters ahead of the crack tip. This material is suitable for applications ranging from smart skin and flexible sensors.

New photopatternable polyimide and programmable nonvolatile memory performances

We report the first photopatternable, nonvolatile memory consisting of high-temperature polyimide (PI), poly(hexafluoroisopropylidenediphthalimide-4-cinnamoyloxytri-phenylamine) (6F-HTPA-CI), and we demonstrate the successful fabrication and programmable operation of ‘write-read-erase’ memory devices based on nanoscale thin films of 6F-HTPA-CI. The PI thin film enables scalable fine patternability, providing lines and spaces with excellent pattern fidelity. Isolated individual memory devices were successfully fabricated on a bottom electrode via a sequential process of coating, photopatterning, top electrode deposition, developing, rinsing and drying. The 6F-HTPA-CI cells exhibited excellent nonvolatile memory performances in three different modes (unipolar permanent, unipolar flash and bipolar flash memories), regardless of photo-exposure doses. The switching-ON (writing) voltage was in the range of ±1.5 to ±2.0 V, and the switching-OFF (erasing) voltage was in the range of ±0.3 to ±0.8 V; these voltages are quite low, indicating that power consumption by the devices during operation is low. The ON/OFF current ratio of the devices was in the range of 104–109. Overall, the photopatternable PI 6F-HTPA-CI opens up the possibility of low-cost mass production of high-performance, high-speed, energy-efficient, permanent or rewritable high-density nonvolatile polymer memory devices suitable for future advanced electronics in highly integrated systems.

Shape anisotropy and hybridization enhanced magnetization in nanowires of Fe/MgO/Fe encapsulated in carbon nanotubes

Arrays of tunneling magnetoresistance (TMR) nanowires were synthesized for the first time by filling Fe/MgO/Fe inside vertically grown and substrate supported carbon nanotubes. The magnetic properties of nanowires and planar nanoscale thin films of Fe/MgO/Fe showed several similarities, such as two-fold magnetic symmetry and ratio of orbital moment to spin moment. Nanowires of Fe/MgO/Fe showed higher saturation magnetization by a factor of 2.7 compared to planar thin films of Fe/MgO/Fe at 1.5kOe. The enhanced magnetic properties likely resulted from shape anisotropy of the nanowires and as well as the hybridization that occur between the π- electronic states of carbon and 3d-bands of the Fe-surface.

Block Copolymer Directed Nanostructured Surfaces as Templates for Confined Surface Reactions

Despite advances in nanomaterials synthesis, the bottom-up preparation of nanopatterned films as templates for spatially confined surface reactions remains a challenge. We report an approach to fabricating nanoscale thin film surface structures with periodicities on the order of 20 nm and with the capacity to localize reactions with small molecules and nanoparticles. A block copolymer (BCP) of polystyrene-block-poly[(allyl glycidyl ether)-co-(ethylene oxide)] (PS-b-P(AGE-co-EO)) is used to prepare periodically ordered, reactive thin films. As proof-of-principle demonstrations of the versatility of the chemical functionalization, a small organic molecule, an amino acid, and ultrasmall silica nanoparticles are selectively attached via thiol–ene click chemistry to the exposed P(AGE-co-EO) domains of the BCP thin film. Our approach employing click chemistry on the spatially confined reactive surfaces of a BCP thin film overcomes solvent incompatibilities typically encountered when synthetic polymers are funct...

Stress-Induced Failure Predictions of Flexible Electronics with Nano-Scaled Thin-Films

Stress-Induced Failure Predictions of Flexible Electronics with Nano-Scaled Thin-Films

Sustainable Fabrication of Protective Nanoscale TiN Thin Film on a Metal Substrate by Using Automotive Waste Plastics

Automotive plastics are heterogeneous and complex mixtures of various plastics incorporated with significant amounts of additives and fillers. Automotive shredded residue (ASR) waste plastics are difficult to recycle and hence contribute significantly to environmental problems. In this study, a sustainable approach to fabricate protective nanoscale TiN thin film on a metal surface by using automotive waste plastics as titanium and carbon sources is investigated. The synthesis of nanoscale thin film of titanium nitride on steel substrate was carried out by using carbothermal reduction and nitridation reaction in a nitrogen atmosphere. The formation of TiN nanofilm on steel substrate was confirmed by XPS, XRD, and EDS techniques. The TiN film was characterized by HRTEM, and thickness was found to be in the range of 18 nm. The uniform and highly crystalline TiN thin film could provide good oxidation resistance to steel substrate. This innovative approach of using automotive waste plastics as titanium and reductant source for fabricating TiN film on metal could be an alternative to conventional techniques. This novel route reduces the manufacturing cost and also provides a sustainable recycling solution for automotive waste plastic.

Process control model for growth rate of molecular beam epitaxy of MgO (111) nanoscale thin films on 6H-SiC (0001) substrates

Magnesium oxide (MgO) is a good candidate for an interface layer in multifunctional metal-oxide nanoscale thin-film heterostructures due to its high breakdown field and compatibility with complex oxides through O bonding. In this research, molecular beam epitaxy (MBE) is used to deposit 10 nm to 15 nm MgO single-crystal films on silicon carbide with hexagonal polytype 6H (6H-SiC) to serve as an interface layer for effective integration of functional oxides. In this work, the effect of MBE process control variables on the growth rate of the MgO film measured in nanometers per minute is investigated. Experiments are conducted at various process conditions and the resulting MgO film growth rate at each combination of process conditions is measured. The process control variables studied are the substrate temperature (100 °C – 300 °C), magnesium source temperature (328 °C – 350 °C), plasma intensity (0 mV – 550 mV), and percentage oxygen on the starting surface of 6H-SiC substrate (9 % – 13 %) after the substrate is prepared by high-temperature hydrogen etching. The film thickness is computed using the effective attenuation length (EAL) of silicon photoelectron peak intensity as measured by x-ray photoelectron spectroscopy (XPS). The film thickness is converted to growth rate by dividing it with the duration of film growth. Using the experimental data, a neural network model is developed to estimate growth rate for any given process variable combination. From this neural network model, multiple replications of data were generated to conduct a 3-level full factorial design of experiments and response surface-based analysis. The study reveals that the plasma intensity has the most significant influence on growth rate. The results indicate that growth rate is relatively low on high-quality substrates with √3 × √3 R30° reconstructed 6H-SiC (0001) surface with optimum oxygen content (approximately 10 %); in contrast, the growth rate is relatively high on substrates with high surface roughness and excessive oxygen on the starting substrate surface.