Nanoscale Thin Films Research Articles

Sub-resolution contrast in neutral helium microscopy through facet scattering for quantitative imaging of nanoscale topographies on macroscopic surfaces

Nanoscale thin film coatings and surface treatments are ubiquitous across industry, science, and engineering; imbuing specific functional or mechanical properties (such as corrosion resistance, lubricity, catalytic activity and electronic behaviour). Non-destructive nanoscale imaging of thin film coatings across large (ca. centimetre) lateral length scales, crucial to a wide range of modern industry, remains a significant technical challenge. By harnessing the unique nature of the helium atom–surface interaction, neutral helium microscopy images these surfaces without altering the sample under investigation. Since the helium atom scatters exclusively from the outermost electronic corrugation of the sample, the technique is completely surface sensitive. Furthermore, with a cross-section that is orders of magnitude larger than that of electrons, neutrons and photons, the probe particle routinely interacts with features down to the scale of surface defects and small adsorbates (including hydrogen). Here, we highlight the capacity of neutral helium microscopy for sub-resolution contrast using an advanced facet scattering model based on nanoscale features. By replicating the observed scattered helium intensities, we demonstrate that sub-resolution contrast arises from the unique surface scattering of the incident probe. Consequently, it is now possible to extract quantitative information from the helium atom image, including localised ångström-scale variations in topography.

Thermal capillary waves on bounded nanoscale thin films.

The effect of confining walls on the fluctuation of a nanoscale thin film's free surface is studied using stochastic thin-film equations(STFEs). Two canonical boundary conditions are employed to reveal the influence of the confinement: (1) an imposed contact angle and (2) a pinned contact line. A linear stability analysis provides the wave eigenmodes, after which thermal-capillary-wave theory predicts the wave fluctuation amplitudes. Molecular dynamics (MD) simulations are performed to test the predictions, and a Langevin diffusion model is proposed to capture oscillations of the contact lines observed in MD simulations. Good agreement between the theoretical predictions and the MD simulation results is recovered, and it is discovered that confinement can influence the entire film. Notably, a constraint on the length scale of wave modes is found to affect fluctuation amplitudes from our theoretical model, especially for 3D films. This opens up challenges and future lines of inquiry.

A practical guide to pulsed laser deposition.

Nanoscale thin films are widely implemented across a plethora of technological and scientific areas, and form the basis for many advancements that have driven human progress, owing to the high degree of functional tunability based on the chemical composition. Pulsed laser deposition is one of the multiple physical vapour deposition routes to fabricate thin films, employing laser energy to eject material from a target in the form of a plasma. A substrate, commonly a single-crystal oxide, is placed in the path of the plume and acts as a template for the arriving species from the target to coalesce and self-assemble into a thin film. This technique is tremendously useful to produce crystalline films, due to the wide range of atmospheric conditions and the extent of possible chemical complexity of the target. However, this flexibility results in a high degree of complexity, oftentimes requiring rigorous optimisation of the growth parameters to achieve high quality crystalline films with desired composition. In this tutorial review, we aim to reduce the complexity and the barrier to entry for the controlled growth of complex oxides by pulsed laser deposition. We present an overview of the fundamental and practical aspects of pulsed laser deposition, discuss the consequences of tailoring the growth parameters on the thin film properties, and describe in situ monitoring techniques that are useful in gaining a deeper understanding of the properties of the resultant films. Particular emphasis is placed on the general relationships between the growth parameters and the consequent structural, chemical and functional properties of the thin films. In the final section, we discuss the open questions within the field and possible directions to further expand the utility of pulsed laser deposition.

Peeling by Pulling: Characterizing the Mechanical Behavior of Nanoscale Thin Films

Peeling by Pulling: Characterizing the Mechanical Behavior of Nanoscale Thin Films

Atomic layer engineering on resistive switching in sub-4 nm AlN resistive random access memory devices

This study demonstrates the precise tailoring of material properties of nanoscale thin films and electrical properties of AlN RRAM devices by atomic layer annealing.

Stable chemical enhancement of passivating nanolayer structures grown by atomic layer deposition on silicon.

Incorporation of carrier-selective passivating contacts is on the critical path for approaching the theoretical power conversion efficiency limit in silicon solar cells. We have used plasma-enhanced atomic layer deposition (ALD) to create ultra-thin films at the single nanometre-scale which can be subsequently chemically enhanced to have properties suitable for high-performance contacts. Negatively charged 1 nm thick HfO2 films exhibit very promising passivation properties - exceeding those of SiO2 and Al2O3 at an equivalent thickness - providing a surface recombination velocity (SRV) of 19 cm s-1 on n-type silicon. Applying an Al2O3 capping layer to form Si/HfO2/Al2O3 stacks gives additional passivation, resulting in an SRV of 3.5 cm s-1. Passivation quality can be further improved via simple immersion in hydrofluoric acid, which results in SRVs < 2 cm s-1 that are stable over time (tested for ∼50 days). Based on corona charging analysis, Kelvin probe measurements and X-ray photoelectron spectroscopy, the chemically induced enhancement is consistent with changes at the dielectric surface and not the Si/dielectric interface, with fluorination of the Al2O3 and underlying HfO2 films occurring after just 5 s HF immersion. Our results show that passivation is enhanced when the oxides are fluorinated. The Al2O3 top layer of the stack can be thinned down by etching, offering a new route for fabrication of ultra-thin highly passivating HfO2-containing nanoscale thin films.

Thermal and Ion Treatment Effect on Nanoscale Thin Films Scratch Resistance

Thermal and Ion Treatment Effect on Nanoscale Thin Films Scratch Resistance

Nanoscale ITO Films for Plasmon Resonance-Based Optical Sensors

The developing field of plasmonics has led to the possibility of creating a new type of high-speed, highly sensitive optical sensors for the analysis of chemical and biological media. The functional conducting layers of surface plasmon resonance (SPR) optical sensors are almost always nanoscale thin films of noble metals. To enhance the plasmon resonance, nanostructured films of transparent conductive oxides are introduced into the optical sensors. However, such modified optical sensors operate in the infrared region of the spectrum. In this work, we demonstrate that the use of indium tin oxide (ITO) films with a high concentration of charge carriers makes it possible to shift the surface plasmon resonance into the visible radiation region. The work presents the results of the development of magnetron deposition technology for ITO thin films, with optimal parameters for optical sensors based on surface plasmon resonance operating in the visible range of the spectrum. Their optical and electrical characteristics are investigated. Excitation of the surface and volume plasmon resonance at the dielectric-ITO film interface, using the Kretschman configuration, is studied. It is shown that SPR is excited in the investigated ITO films with a concentration of free charge carriers of the order of 1021–1022 cm−3, when irradiated with a beam of light with TM polarization in the wavelength range of 350–950 nm. At the same time, the addition of various analytes to the surface of an ITO film changes the excitation wavelength of the SPR.

Physicochemical effects of proton implantation doping on transparent nanoscale thin films of polyaniline emeraldine base covalently fabricated on organosilanized surfaces

Physicochemical effects of proton implantation doping on transparent nanoscale thin films of non-conductive polyaniline emeraldine base covalently fabricated on organosilanized surfaces were investigated in this study. Resistance measurement using the Au-electrode array coated with an initially non-conductive thin film of polyaniline emeraldine base showed that proton irradiation could first induce a drastic effect to decrease the resistance of polyaniline emeraldine base and then continue to act as a doping process to further exponentially reduce the resistance to yield a relatively stable conductive state for the proton-irradiated PANI EB. Analysis of experimental results obtained by X-ray photoelectron spectroscopy and Raman spectroscopy also revealed that (i) the proportion of bonding structures characterized as protonated amine/imine groups related to electrical conduction for polyaniline was increased after proton irradiation, (ii) nitro groups were introduced to the polymer by the applied proton beam propagating through the ambient air and (iii) crosslinking could be induced by proton irradiation through the formation of phenazine-like structures between polymer chains. With crosslinking and nitration, the proton-irradiated polyaniline exhibited electrical conductivity similar to that of polyaniline derivatives with N-alkylaniline and nitroaniline as co-monomer units in the polymer chains.

Flexible Memristor Devices Using Hybrid Polymer/Electrodeposited GeSbTe Nanoscale Thin Films.

We report on the development of hybrid organic-inorganic material-based flexible memristor devices made by a fast and simple electrochemical fabrication method. The devices consist of a bilayer of poly(methyl methacrylate) (PMMA) and Te-rich GeSbTe chalcogenide nanoscale thin films sandwiched between Ag top and TiN bottom electrodes on both Si and flexible polyimide substrates. These hybrid memristors require no electroforming process and exhibit reliable and reproducible bipolar resistive switching at low switching voltages under both flat and bending conditions. Multistate switching behavior can also be achieved by controlling the compliance current (CC). We attribute the switching between the high resistance state (HRS) and low resistance state (LRS) in the devices to the formation and rupture of conductive Ag filaments within the hybrid PMMA/GeSbTe matrix. This work provides a promising route to fabricate flexible memory devices through an electrodeposition process for application in flexible electronics.

Ultra-Sensitive Extinction Measurements of Optically Active Defects in Monolayer MoS2.

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS2 generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below 1 × 1011 cm-2. The corresponding spectra reveal a broad subgap absorption, being consistent with theoretical predictions related to sulfur vacancy-bound excitons in MoS2. Our results highlight cavity-enhanced extinction spectroscopy as efficient means for the detection of optical transitions in nanoscale thin films with weak absorption, applicable to a broad range of materials.

Carbon black nanoparticles modified gelatin composite-based thin film for rapid-response humidity sensing

Humidity monitoring is essential for food processing, pharmaceutical, and precision electronics industries. Conventional humidity sensors based on metal oxides and ceramics show an ideal response to variation humidity. However, long response time and complex manufacturing processes pose a challenge for their more comprehensive application and further development. Resistive polymers show promise in antistatic materials and sensor applications. In this study, gelatin composite, a popular water-soluble polymer, is mixed with carbon black nanoparticles for better humidity sensitivity. Using the state-of-the-art thin film deposition method, the designed interdigitated electrode is covered by a nano-scale thin film of the prepared composite film. The relative humidity (RH) range between 20.3% and 83.2% can be well detected according to the evaluation of a customized chemical vapors measurement system. Furthermore, an extreme linear relation and high sensitivity of 0.35 mV/%RH are achieved for the range from the RH of 20%–65.2%. Highly repeatable performance can be demonstrated by repeated experiments with several humidities. Rapid response and recovery features are found during the characterization, while the response and recovery time are lower than 10 s.

Oxidative Molecular Layer Deposition of Conjugated Amine Polymer Thin Films

Conjugated amine polymers such as polypyrrole (pPy), and polyaniline (pAni) exhibit high electrochemical capacities, making them appealing as electrode materials for energy storage, electrochemical desalination, and chemical sensing. Nanoscale thin films of these polymers are attractive to provide lower weight, faster charging, and higher sensitivity in these applications. This report describes the first demonstration of molecular layer-by-layer growth of conjugated amine polymers using alternating gas-phase exposures of amine monomers and MoCl5 oxidant in a process termed oxidative molecular layer deposition (oMLD). Pyrrole (Py) and MoCl5 undergo self-limiting surface reactions by oMLD to form conformal pPy thin films, but oMLD using aniline (Ani) and p-phenylenediamine (PDA) monomers yield unexpected azo functionality. This is attributed to a MoCl5-amine surface adduct that directs polymerization near amine groups, producing azo groups for primary amines. oMLD pPy exhibits record 282 mAh/g capacity in aqueous electrolyte, and PDA/MoCl5 oMLD yields azo-polymers of interest as anode materials for alkali-ion batteries. Alternating between Py and PDA monomers during oMLD produces molecularly mixed copolymers with qualitatively different electrochemical responses from the isolated monomer structures. This work lays the foundation for the growth of conformal thin films of conjugated amine polymers with molecular-level control of composition and thickness.

NEW VAPOR DEPOSITED DIELECTRIC POLYMER THIN FILMS FOR ELECTRONIC APPLICATIONS

Dielectric materials are of great interest in a vast amount of applications ranging from cable insulation to advanced electronic devices. The emerging trend of device miniaturization is creating an increased demand for dielectric thin films that can be produced precisely on the nanometer scale. In addition, special mechanical properties are often required, for example in the field of flexible organic electronics. Polymers are first-choice materials for this purpose. However, it is extremely difficult to produce precise nanoscale thin films, which have a low defect density and are free of e.g. residual solvent, by wet chemistry approaches. Initiated chemical vapor deposition (iCVD) is a solvent-free polymer thin film deposition process which can be used to produce high quality dielectric thin films with nanoscale control and circumvents thus these problems. This work demonstrates the versatility of the iCVD process in the field of electrical applications by some new application examples of iCVD. By adding e.g. a hydrophobic organosiloxane thin film on columnar zinc oxide (ZnO:Fe) gas sensing structures there was a change in the selectivity from ethanol to hydrogen, as well as improved performance at high humidity level. The modified sensors can thus be used in humid ambient, especially for breathing tests, which can lead to the diagnosis of some diseases by cutting edge non-invasive approaches.

Evaluation of Adhesion Properties of Thin Film Structure through Surface Acoustic Wave Dispersion Simulation.

A theoretical simulation study of the dispersion characteristic of the surface acoustic wave (Rayleigh wave) was conducted by modeling the adhesion interlayer with stiffness coefficients to evaluate the bonding properties of nano-scale thin film structures. For experimental validation, a set of thin film specimens were fabricated—637 nm, 628 nm, 637 nm, 600 nm, and 600 nm thick titanium (Ti) films were deposited on silicon (Si) (100) substrate using a DC Magnetron sputtering process with DC power from 28.8 W, 57.6 W, 86.4 W, 115.2 W, and 144 W. The thicknesses of the Ti films were measured using a scanning electron microscope (SEM). Surface acoustic wave velocity for each of the manufactured thin film specimens was measured by using a V(z) curve technique of a Scanning Acoustic Microscope. The measured velocity, transducer frequency, and thickness of the film were applied to dispersion characteristic simulation for a given stiffness coefficient to calculate adhesion strength of each specimen. To verify the simulation result, the adhesion force of each specimen was measured using a nano-scratch test and then compared with the calculated values from the dispersion characteristic simulation. The value of adhesion strength from the dispersion characteristic simulation and the value of adhesion force of the nano-scratch test were found to have a similar tendency according to the process variable of the thin film. The results demonstrated that the adhesion strength of a thin film could be evaluated quantitatively by calculating the dispersion characteristics with the adhesion interlayer stiffness model.

Post-buckling evolution of compressed thin films adhered to rigid substrates

Compressing a thin film adhered to a substrate can yield delaminated blisters, if the interface adhesion is relatively weak. In this work, through the combination of theoretical modeling and numerical simulations, we investigate the mechanics governing the buckle-delamination and post-buckling evolution of compressed thin films adhered to rigid substrate, where the interface adhesion is attributed by van der Waals interactions. The deformed configurations of thin films can be categorized into five deformation modes depending on the material, geometric and interfacial parameters and the in-plane compressive strain. Through the energy-variational approach, large-deformation beam models are constructed to describe the deformed configurations and energy variations of the arc-shape and self-contact buckling modes. The phase diagrams related to the equilibrium buckling state are constructed by energy analysis, based on which the evolution of the post-buckling behavior is analyzed. Our results may find potential applications in the controlled compressive-buckling assembly of micro-/nano-scale thin films.

Multiscale characterization of damage tolerance in barium titanate thin films

Barium titanate is a brittle, lead free ferroelectric and piezoelectric ceramic used in patterned and thin film forms in micro- and nano-scale electronic devices. Both during deposition and eventually during service, this material system develops stresses due to different loads acting on the system, which can lead to its failure due to cracking in the films and/or interface delamination. In situ microcantilever bending based fracture experiments and tensile tests based on shear lag tests in combination with digital image correlation were used to understand the cracking behavior of barium titanate films when deposited on flexible substrates. For the first time, the fracture behavior of these nanocrystalline barium titanate films has been quantified in terms of fracture toughness, fracture strength, and interface shear stresses for different film thicknesses. Critical defect size is estimated using the above information as a function of film thickness. It is found that damage tolerance in terms of fracture strength depends on film thickness. Furthermore, compared to a bulk single crystal, barium titanate fracture resistance of the nanocrystalline thin films is reduced. Both effects need to be considered in engineering design of reliable devices employing micro- and nano-scale barium titanate thin film structures.

Niobium Oxide Coated Mesoporous Platinum Nanoparticles for the Oxygen Reduction Reaction

Effective and economic methods of converting chemical energy into electricity or vice versa (i.e., through the use of fuel cells) must be developed to aid in the transition to renewable energy sources. One of the major barriers in the commercialization of fuel cells is the disparity in speed and efficiency between the two fuel cell reactions: the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) of which the latter is orders of magnitude slower.1 Mesoporous Pt prepared by electrodeposition has been shown to have increased activity towards the ORR than the industry standard Pt nanoparticles (NPs) supported on carbon.2 Platinum based catalysts, however, lack long term stability under acidic fuel cell conditions. One method that has been found to mitigate the long-term loss of Pt catalyst stability and activity is coating platinum with nanoscale thin films of niobium oxide.3 In this work, niobium oxide was investigated as a stabilizing coating for mesoporous Pt NP catalysts.Mesoporous Pt NPs were synthesized via electrodeposition to form ~50 nm diameter NPs. The Pt NPs were then coated with niobium oxide films with a varying nanoscale thickness that ranged from 3 to 50 nm. The niobium oxide films were fabricated using atomic layer deposition (ALD) or electrodeposition.3,4 Scanning electron microscopy and transmission electron tomography were used to evaluate the porosity and surface morphology of the Pt NPs before and after coating with niobium oxide. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were used to confirm the presence and composition of the niobium oxide films. Electrochemical characterization by cyclic voltammetry and electrochemical impedance spectroscopy under acidic conditions was used to determine an optimal thickness of the niobium oxide film that stabilizes the size and morphology of the Pt NPs while maintaining catalyst activity. The Pt NPs coated with <10-nm thick niobium oxide films were observed to have an increased long-term performance in comparison to their uncoated counterparts. Stabilizing NP catalysts for improving their long-term activity is a vital step towards creating affordable alternatives to fossil fuels by maximizing renewable energy resources. Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43–51.Paul, M. T. Y.; Gates, B. D. Mesoporous Platinum Prepared by Electrodeposition for Ultralow Loading Proton Exchange Membrane Fuel Cells. Sci. Rep. 2019, 9, 4161.Eastcott, J.; Gates, B. D. Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability. Can. J. Chem. 2017, 96, 260–266.Zhitomirsky, I. Electrolytic Deposition of Niobium Oxide Films. Mater. Lett. 1998, 35, 188–193.

Enhancement of thermoelectric power factor by modulation doping of bulk polycrystalline SnS / thin film PEDOT:PSS bilayer

Modulation doping occurs in a heterojunction where a charge carrier-rich material transfers charge to a carrier-deficient material. The modulation-doped material is intentionally selected to have higher charge carrier mobility than the modulation dopant material, so that the overall electrical conductivity can be boosted. Although this modulation doping strategy has proven effective in enhancing power factor in thermoelectrics, selection criteria for such semiconductor couples have not been explicitly clarified, resulting in only a few discovered semiconductor couples available for modulation doping-driven thermoelectric systems [1-4]. Here, we (i) report an electronic band structure-based guideline to actualize modulation doping, (ii) reveal that hole-rich PEDOT:PSS can modulation dope otherwise undoped tin monosulfide (SnS) in their bilayered structure, (iii) prove that modulation doping is responsible for thermoelectric power factor enhancement by comparing computational and experimental Seebeck coefficient and electrical conductivity values. The optimized PEDOT:PSS thin film / SnS pellet bilayered structure had a 134.7 fold improvement in electrical conductivity and a 93.6 fold power factor enhancement over those of undoped SnS, with only a ~ 20 % decrease in Seebeck coefficient. The modulation doping effect can result in further power factor improvement when SnS becomes a nanoscale thin film or nanoparticles in the future.

Pulsed laser deposition and structural evolution of BaF2 nanolayers in Eu-doped BaF2/Al2O3 layered optical nanocomposite thin films

We have developed a pulsed laser deposition (PLD) geometry for the growth of uniform BaF2 nanoscale thin films, through control of the deposition conditions. Our goal is to use the BaF2 layers with controllable structure as component layers in layered optical nanocomposites. The structure of the BaF2 nanolayer evolves as a function of the layer thickness: BaF2 grows via a layer-by-layer growth mode on Al2O3; the layers are amorphous for a thickness < 3 nm, and then become nanocrystalline as the layer thickness increases. The BaF2 nanocrystals have an FCC crystal structure with a weak <111> texture that becomes stronger for thicker films. We then demonstrate that our BaF2 films can be introduced into layered Al2O3/BaF2/EuOx nanocomposite films, which allows for control of the relative position of the Eu ions and the BaF2 layer. Cross-section samples of the multilayered films show interfacial intermixing between the layers, which is related to implantation during the PLD process. This intermixing enables the incorporation of Eu ions into BaF2 layers and form a thin Eu-doped BaF2 nanolayer at the interface. The layered nanocomposite films show photoluminescence (PL) emission from Eu3+ ions, and the PL intensity changes can be correlated with the crystallinity and crystal size changes in the BaF2 layer. Our results provide guidance for achieving thin film nanocomposite materials with controllable structure and photoluminescence behavior for light emitting diodes, photovoltaics and other optical applications.