Thin Film Interface Research Articles

Charge injection barrier at the pentacene thin film and electrode interface characterized by time-resolved electrostatic force microscopy

The contact resistances at the metal–organic interface often limit the performance of organic thin-film transistors. However, it is not straightforward to characterize the electrical property of the metal–organic interface of the organic thin film. This is because the conventional electrical measurement only gives the total electrical property of the metal–organic–metal system that is affected by many grain boundaries. In this study, we investigated a single pentacene grain connected to a Au electrode by time-resolved electrostatic force microscopy (tr-EFM), which can capture the time-evolving electrostatic force images at a nanometer-scale spatial resolution. Using the tr-EFM, we found the gradual and uniform potential increase in the pentacene grain following the positive step voltage applied to the Au electrode, which indicates that the resistance in the grain–electrode system is governed by the grain–electrode interfacial resistance. By assuming the equivalent circuit of the grain–electrode interface system, we reconstructed the femto-ampere-order current-to-voltage characteristic at the grain–electrode interface. The asymmetric characteristic in the hole injection regime and the ejection regime suggests the existence of a metal–organic Schottky junction at the interface.

Air/water interfacial properties and thin film drainage dynamics of compositionally diverse wheat flour water extracts

The role of water-extractable (WE) wheat flour constituents and their interactions in determining the stability of wheat-based foam-type foods remain largely unexplored. Additionally, the contribution of drainage dynamics of the thin liquid films (TLFs) between gas bubbles to food foam stabilization is often overlooked. We here investigated the air/water (A/W) interfacial, TLF drainage, and foaming properties of compositionally diverse wheat flour aqueous extracts. Such extracts were prepared and then either used as such or modified by dialyzing out (i) low molecular mass constituents or (ii) both low molecular mass constituents and enzymatically hydrolyzed carbohydrates. This approach resulted in extracts with gradually increasing protein contents and distinct carbohydrate compositions. Wheat extract constituents created highly elastic A/W interfaces, regardless of the type of extract modification, suggesting a significant contribution from WE wheat proteins. TLFs stabilized by wheat extracts from which low molecular mass constituents were removed had significantly greater stability. This was ascribed to an increased disjoining pressure caused by steric repulsive forces, which in turn were attributed to the interaction between high molecular mass arabinoxylan in the bulk and adsorbed proteins at the A/W interfaces. All extracts showed similarly good foaming properties, implying a dominant role for WE wheat proteins in determining foaming. The strongly elastic A/W interfaces formed by WE wheat proteins likely reduced liquid drainage to such extent that coalescence and disproportionation were largely delayed. The findings of this study could facilitate further investigations on the possibility of tuning protein-AX interactions toward improved foam stability.

Impact of N+ Ion Irradiation on the Electrochemical Behavior of ZnO Thin Film Electrode-Electrolyte Interfaces.

Defect engineering serves as a crucial technique for enhancing the performance of advanced next-generation devices. Ion beam irradiation stands out as a highly promising method for introducing defects in a controlled manner. This study investigates the impact of nitrogen ion (N+) irradiation-induced defects on the electrochemical behavior of ZnO thin-film electrode-electrolyte interface. The oxygen vacancy defects were introduced and tuned by varying the fluence of ion irradiation. The increased Urbach energy in the irradiated samples confirms the enhanced disorder. Photoluminescence data show the emergence of new defect states upon irradiation. The behavior of the ZnO thin-film electrode-electrolyte interface was studied using cyclic voltammetry and electrochemical impedance spectroscopy. At a fixed scan rate, the enhanced peak current was observed in cyclic voltammetry in N+ Irradiated electrodes. Furthermore, reduced charge transfer resistance was observed in the case of irradiated electrodes. To unravel the underlying mechanism, we analyze the AC conductivity, which shows varying dependency on the frequency. It shows the existence of multiple ion-ion correlations in irradiated electrodes. Furthermore, the AC conductivity in the entire frequency region is enhanced significantly. Dielectric permittivity spectra suggest low-frequency dipole interactions and increased dielectric losses after irradiation, indicating non-Debye type relaxation processes. Understanding irradiation-induced changes will help engineer thin film electrodes for batteries, supercapacitors, and other electrochemical applications.

PH-shifting treatment improved the emulsifying ability of gelatin under low-energy emulsification

The effects of pH-shifting treatments (pH 3, 5, 7, 9, and 11) on the stability of gelatin emulsions made by low-energy stirring were investigated. pH-shifting treatments significantly enhanced the ESI and EAI of the emulsion (P < 0.05) and reduced its particle size (P < 0.05) under low-energy emulsifying conditions. The pH11–7 shifting treatment significantly increased the degree of depolymerization and the level of ordered structure of gelatin (P < 0.05). These transformations resulted in a significant increase in the exposure of hydrophobic and negatively charged residues (P < 0.05) on the surface of gelatin, facilitating a faster adsorption rate of gelatin onto the oil-water interface as well as an increase in the amount of gelatin adsorbed at the interface. Moreover, the alkali-shifting treatment promoted the formation of a thin viscoelastic interfacial film, which contributed to the enhanced stability of the emulsion.

Poly-(aminoethyl piperazine) membranes with ultra-thin selective layers prepared via a rate-retarded interfacial polymerization method

New monomers have been exploring to fabricate interfacial polymerized thin film composite membranes for decades. N-aminoethyl piperazine (AEP), an aliphatic amine with a heterocyclic structure, was used in this work as an amine monomer to synthesize nanofiltration membranes. Due to the high reactivity nature of AEP, a rate-retarded interfacial polymerization method was applied, which was realized by applying ultra-low AEP concentration (0.07 wt%), low reaction temperature (4 °C monomer solutions), and ice-water quench. The resulting AEP membrane had much better performance (3–4 times permeance and closed rejections) compared with the membranes prepared by the hot water post-treatment and the oven heat treatment. At lower monomer concentrations, an NF membrane with a selective layer thickness of only around 20 nm was prepared by the rate-retarded interfacial polymerization method. The membranes had a negatively charged surface with "willow leaf" like morphology. The MWCO was around 200 g mol−1, and the water permeance was up to 22.8 L m−2 h−1 bar−1, which is comparable with the commercial polyamide membrane NF270. Therefore, AEP is a very competitive alternative monomer for NF membrane synthesis.

Redox reaction at buried ZnO/Ti thin film interface as seen by hard x-ray photoemission and thermal desorption spectroscopy

In the context of low-emissivity glazing, the redox reaction at a buried ZnO/Ti interface is studied in model nanometer-thick film stacks synthesized by magnetron sputtering deposition. For a given amount of Ti, the roles of annealing temperature up to 550 °C, of ZnO layer thickness, and of its crystalline quality are explored. The main originality of the approach is the use of hard x-ray photoemission spectroscopy to probe in situ chemical reactions at a buried interface in a non-destructive way. The detailed analysis of relevant core levels reveals the formation of a ZnxTiyOz ternary compound and the nearly complete oxidation of Ti into TiO2. Thanks to complementary measurements of thermal desorption spectroscopy and electron probe microanalysis, unexpected diffusion of the Zn redox reaction by-product through the upper part of the stack and its desorption in vacuum are clearly evidenced. The reaction and mass transport pathways are rationalized through thermodynamic simulations.

Tunable Ferroionic Properties in CeO2/BaTiO3 Heterostructures.

Ferroionic materials combine ferroelectric properties and spontaneous polarization with ionic phenomena of fast charge recombination and electrodic functionalities. In this paper, we propose the concept of tunable polarization in CeO2-δ (ceria) thin (5 nm) films induced by built-in remnant polarization of a BaTiO3 (BTO) ferroelectric thin film interface, which is buried under the ceria layer. Upward and downward fixed polarizations at the BTO thin film (10 nm) are achieved by the lattice termination engineering of the SrO or TiO2 terminated Nb:SrTiO3 (NSTO or STN) substrate. We find that the ceria layer punctually replicates the polarization of the BTO interface via a dynamic reconfiguration of its intrinsic defects, i.e., oxygen vacancies and small polarons. Tunable oxidative or reducing properties (redox) also arise at the surface from the built-in polarization. Opposite polarities at the ceria termination tune the chemo-physical dynamics toward water molecule adsorbates. The inversion of the surface potential leads to a modulation of the water adsorption-desorption equilibrium and water ionization (splitting) redox overpotentials within ±400 mV at room temperature, depending on the ceria termination's charges. Such tunability opens up the perspectives of using ferroionics for wireless electrochemically enhanced catalysis.

Influence of the thermal reduction process on the tribological and conductive properties of single-layer graphene oxide

Graphene oxide (GO) can be intentionally modified by the reduction process to tune some of its physical properties. This allows the application of GO as a thin film interface in various technologies, especially MEMS devices. One of the key factors when considering GO as a nanocoating in MEMS is its ability to reduce friction and increase electrical conductivity. Here, we exemplify a simple adjustment of the friction and conductance of a single GO flake at the nanoscale by the thermal reduction process, which results in morphological, chemical, and structural changes. The core of this experimental work is based on atomic force microscopy (AFM), in particular AFM topography imaging, force spectroscopy, lateral force microscopy, and conductive AFM. Additional measurements were performed using optical microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, and wetting angle measurements. The physical properties were measured as a function of the reduction temperature for isolated single-layer GO flakes. In our research, we have identified a temperature threshold at which structural and morphological changes occur. The conversion of the properties of GO as conductivity, thickness, adhesive, and friction force, as well as defect level was shown to occur at most at 130 °C. However, some effects start even at 100 °C. We have found a non-monotonic friction dependence as a function of the sliding velocity, close to our published results for fluoroalkylsilanes [1]:Weiss M. et al. (2021), Tribology International, 162, 107133], which exhibits a characteristic friction minimum. It corresponds to the lowest possible energy dissipation in the system for a given sliding velocity and load. To extend the range of accessible sliding velocities and gain insight into the molecular details of the friction process, molecular dynamics simulations were performed. In conclusion, by controlling the thermal reduction process of GO, it is possible to influence the properties of the GO system, such as adhesion, friction, and electrical conductivity, which can enhance the performance of materials in various applications, including MEMS devices.

Elucidating the formation mechanisms of the parasitic channel with buffer-free GaN/Si hetero-bonding structures

Driven by the increasing demand for 5G communication, GaN radio frequency (RF) device on Si technology has been flourishing attributable to the large size, low cost, and compatibility with complementary metal–oxide–semiconductor technology. However, a significant challenge is that a high-conductance parasitic channel forms at the interface between the III-N epitaxial layers and the Si substrate, leading to severe RF loss, which has been considerably impairing both the performance and advancement of RF GaN-on-Si technologies. Despite continuing controversies concerning the physical mechanisms engendering the parasitic channel, clarification is critically needed. Standing apart from traditional studies on RF loss in III-N epilayers grown on Si, this article comprehensively investigates the bonding interface of GaN thin film and Si(100) substrate realized via direct surface activated bonding and ion-cutting technologies. It was clearly determined that substantial diffusion of gallium (Ga) atoms into the Si substrate at the bonding interface occurred even at an annealing temperature as low as 350 °C. Subsequent high-temperature post-annealing at 800 °C intensified this diffusion, activating Ga atoms to form a p-type highly conductive parasitic channel. Simultaneously, it triggered Ga atoms aggregation and incited melt-back etching within the Si substrate at the interface. Contrasting with the conventional hetero-epitaxy, this study presents a compelling view based on the bonding technique. It conclusively elucidates the physical mechanisms of the formation of the primary source of RF loss—the p-type highly conductive parasitic channel.

Interfacial carbon quantum dot thin film impacts on morphological and chemical structures of fluorine-doped tin oxide films

Morphology and chemical bonding states, including surface roughness, film density, F-doping, and oxygen vacancy (VO) concentration directly affect the electrical and optical properties of fluorine-doped tin oxide (FTO) films. In this study, morphology and chemical bonding states of FTO films are engineered simultaneously by introducing a carbon quantum dot (CQD) thin film as an interface layer during ultrasonic spray pyrolysis deposition. The abundant oxygen-containing surface functional groups and large surface free energy of the CQD thin film promoted (200) oriented crystal growth and accelerated nucleation of FTO, resulting in smooth and dense FTO film morphology. The abundant carboxyl surface functional groups on the CQDs generate active F− species that are effectively doped into SnO2. Formic acid, formed by dissociation of the carboxyl group, can serve as a strong reducing agent for extracting oxygen atoms from the SnO2 lattice, thereby increasing the VO concentration. The accelerated F-doping and VO concentration result in the generation of additional free electrons. The effects of the optimized CQD on the morphological and chemical structures of FTO films are demonstrated by their superior electrical (sheet resistance of ∼5.7 Ω/□) and optical properties (visible transmittance of ∼85.9 %) compared with bare FTO. Moreover, CQD/FTO used as transparent conducting electrodes in electrochromic (EC) application exhibited improved EC performances, including fast switching speeds (5.3 s and 2.3 s for bleaching and coloration, respectively) and high coloration efficiency (81.1 cm2/C) compared to that of bare FTO.

Reversible Optical Control of Polarization in Epitaxial Ferroelectric Thin Films.

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstratedin prototypical epitaxial PbZrx Ti1-x O3 (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thinfilms.

High-efficiency fiber-cladding power stripper based on all-dielectric optical thin films.

Although conventional fiber-cladding power strippers (CPSs) based on the techniques of high-index adhesive or corrosive liquids onto fiber inner cladding have been well developed, they are still facing challenges in special applications such as spaceborne or radiation-environment fiber lasers and amplifiers. In this paper, we propose and fabricate high-efficiency CPSs based on all-dielectric optical thin films. By numerically analyzing the propagation characteristics of cladding light at the thin film interface, we design a high-index T a 2 O 5 CPS and A l 2 O 3 CPS with single- and cascaded-layer films coated onto the fiber inner cladding, respectively. In our experiment, the CPSs are successfully fabricated onto the inner-cladding surface of 10/125 double-clad fiber based on ion-beam-assisted deposition technology. The stripping efficiency for the 976nm residual cladding power was measured up to 99.38%, and the stripping power of the fiber CPS without active cooling can be 24W at least. Such CPS could be advantageous for applications in spaceborne-based fiber lasers or amplifiers (e.g.,gravitational wave detection, spaceborne lidar).

Enhanced Memristive Performance via a Vertically Heterointerface in Nanocomposite Thin Films for Artificial Synapses.

Memristors can be used to mimic synaptic behavior in artificial neural networks, which makes them a key component in neuromorphic computing and holds promise for advancing the field. In this study, a memory artificial synaptic device based on ZnO-BaTiO3 (ZnO-BTO) vertically aligned nanocomposite thin films was prepared. The vertical interface between the two phases can be used as a conduit for oxygen vacancy (OV) accumulation and a channel for OV movement, which greatly optimizes the resistive switching performance of the device and has the potential for multistage storage. By applying different pulse sequences to the device, the conductance of the device is adjusted from multiple angles, and a variety of synaptic functions are simulated, such as paired-pulse facilitation, spike-timing-dependent plasticity, short-term plasticity to long-term plasticity (STP-LTP), and long-term potentiation/depression (LTP/LTD). Finally, we construct a neural network for image recognition, and the recognition accuracy can reach 91%. Our study demonstrates the feasibility of using composite thin-film vertical interface to regulate the resistive performance of memristors and its great potential in artificial synaptic simulation and neuromorphic computing.

Formation of Mn-rich interfacial phases in Co2FexMn1-xSi thin films

We report the formation of Mn-rich regions at the interface of Co2FexMn1-xSi thin films grown on GaAs substrates by molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) with electron energy loss (EEL) spectrum imaging reveals that each interfacial region: (1) is 1–2 nm wide, (2) occurs irrespective of the Fe/Mn composition ratio and in both Co-rich and Co-poor films, and (3) displaces both Co and Fe indiscriminately. We also observe a Mn-depleted region in each film directly above each Mn-rich interfacial layer, roughly 3 nm in width in the x = 0 and x = 0.3 films, and 1 nm in the x = 0.7 (less Mn) film. We posit that growth energetics favor Mn diffusion to the interface even when there is no significant Ga interdiffusion into the epitaxial film. Element-specific X-ray magnetic circular dichroism (XMCD) measurements show larger Co, Fe, and Mn orbital to spin magnetic moment ratios compared to bulk values across the Co2FexMn1-xSi compositional range. The values lie between reported values for pure bulk and nanostructured Co, Fe, and Mn materials, corroborating the non-uniform, layered nature of the material on the nanoscale. Finally, SQUID magnetometry demonstrates that the films deviate from the Slater-Pauling rule for uniform films of both the expected and the measured composition. The results inform a need for care and increased scrutiny when forming Mn-based magnetic thin films on III-V semiconductors like GaAs, particularly when films are on the order of 5 nm or when interface composition is critical to spin transport or other device applications.

A Hybrid Pulsed Laser Deposition Approach to Grow Thin Films of Chalcogenides.

Vapor-pressure mismatched materials such as transition metal chalcogenides have emerged as electronic, photonic, and quantum materials with scientific and technological importance. However, epitaxial growth of vapor-pressure mismatched materials are challenging due to differences in the reactivity, sticking coefficient, and surface adatom mobility of the mismatched species constituting the material, especially sulfur containing compounds. Here, a novel approach is reported to grow chalcogenides-hybrid pulsed laser deposition-wherein an organosulfur precursor is used as a sulfur source in conjunction with pulsed laser deposition to regulate the stoichiometry of the deposited films. Epitaxial or textured thin films of sulfides with variety of structure and chemistry such as alkaline metal chalcogenides, main group chalcogenides, transition metal chalcogenides, and chalcogenide perovskites are demonstrated, and structural characterization reveal improvement in thin film crystallinity, and surface and interface roughness compared to the state-of-the-art. The growth method can be broadened to other vapor-pressure mismatched chalcogenides such as selenides and tellurides. This work opens up opportunities for broader epitaxial growth of chalcogenides, especially sulfide-based thin film technological applications.

Multilayered elastic medium reinforced with interfacial thin film: A theoretical model for geogrid reinforced HIR asphalt pavement

Aiming at modeling the reinforcement behavior of geogrid reinforced HIR asphalt pavement structure accurately and efficiently, a mathematical formulation using the Generalized Kelvin Solution (GKS) based method for multi-layer three-dimensional elastic system with the consideration of elastic membrane without thickness is put forward in this thesis to simulate the geogrid reinforcement for HIR pavement. Solutions in transform and physical domain, solutions for z = 0, and specific solutions of surface displacements are determined. Four special cases including homogeneous half space with buried and surface thin film reinforcement, Boussinesq's solution, unreinforced multilayered half space are introduced to validate the solution. Moreover, the mechanical effect of geogrid on the deformation, stress state and fatigue life of the pavement structure is investigated using the proposed model and solution. The geogrid stiffness has significant effect on the vertical and radial strain. After the reinforcement, both the radial stress and hoop stress near the interface reduce to a very low level, the stress concentration is effectively alleviated. Meanwhile, geogrid reinforcement plays a positive role on enhancing the fatigue damage resistance of pavement stricture Our proposed method provides a novel way to model reinforcement mechanism of the geogrid on pavement structure, which is helpful to engineering practice.

Fluorine-Free Plasma Polymers to Obtain Water-Repellent Cotton Fabrics: How to Control Their Durability?

The plasma polymerization of hexamethyldisiloxane (HMDSO) leads to the environmentally friendly fabrication of water-repellent coatings through a vapor-phase surface functionalization process using alternatives to the controversial perfluoroacrylate precursors. However, the durability of these coatings is their Achilles’ heel, which requires an in-depth study of the relationship between the structure and properties of these thin films in order to propose concrete solutions for the fabrication of fluorine-free water-repellent textiles. In this context, HMDSO plasma polymers have been deposited on cotton fabrics in an original reactor that allows easy tuning of temporal and spatial parameters of the glow discharge. The functionalized fabrics were characterized to gain insights into the chemical composition of the coatings, their morphology and, above all, their adhesion properties. Interestingly, the results after washing tests revealed a significant dependence of the durability of the superhydrophobic property on the elastic modulus of the deposited polymer. The formation of some radicals at the substrate–thin film interface in the early stages of deposition also correlates with some results. These relationships between the operating conditions of the plasma polymerization, the interfacial properties and the performances of the functionalized fabrics, but also the characterization methodology developed in this work, can undoubtedly serve the engineering of water-repellent fluorine-free coatings on fabrics with optimal durability.

Post Deposition Interfacial Néel Temperature Tuning in Magnetoelectric B:Cr2O3

AbstractBoron (B) alloying transforms the magnetoelectric antiferromagnet Cr2O3 into a multifunctional single‐phase material which enables electric field driven π/2 rotation of the Néel vector. Nonvolatile, voltage‐controlled Néel vector rotation is a much‐desired material property in the context of antiferromagnetic spintronics enabling ultralow power, ultrafast, nonvolatile memory, and logic device applications. Néel vector rotation is detected with the help of heavy metal (Pt) Hall‐bars in proximity of pulsed laser deposited B:Cr2O3 films. To facilitate operation of B:Cr2O3‐based devices in CMOS (compementary metal‐oxide semiconductor) environments, the Néel temperature, TN, of the functional film must be tunable to values significantly above room temperature. Cold neutron depth profiling and X‐ray photoemission spectroscopy depth profiling reveal thermally activated B‐accumulation at the B:Cr2O3/ vacuum interface in thin films deposited on Al2O3 substrates. The B‐enrichment is attributed to surface segregation. Magnetotransport data confirm B‐accumulation at the interface within a layer of ≈50 nm thick where the device properties reside. Here TN enhances from 334 K prior to annealing, to 477 K after annealing for several hours. Scaling analysis determines TN as a function of the annealing temperature. Stability of post‐annealing device properties is evident from reproducible Néel vector rotation at 370 K performed over the course of weeks.

Interfacial Dynamics Pioneer Stephen H. Davis (1939–2021)

Stephen H. Davis (1939–2021) was an applied mathematician, fluid dynamicist, and materials scientist who lead the field in his contributions to interfacial dynamics, thermal convection, thin films, and solidification for over 50 years. Here, we briefly review his personal and professional life and some of his most significant contributions to the field.

Observation of directional leaky polaritons at anisotropic crystal interfaces

Extreme anisotropy in some polaritonic materials enables light propagation with a hyperbolic dispersion, leading to enhanced light-matter interactions and directional transport. However, these features are typically associated with large momenta that make them sensitive to loss and poorly accessible from far-field, being bound to the material interface or volume-confined in thin films. Here, we demonstrate a new form of directional polaritons, leaky in nature and featuring lenticular dispersion contours that are neither elliptical nor hyperbolic. We show that these interface modes are strongly hybridized with propagating bulk states, sustaining directional, long-range, sub-diffractive propagation at the interface. We observe these features using polariton spectroscopy, far-field probing and near-field imaging, revealing their peculiar dispersion, and – despite their leaky nature – long modal lifetime. Our leaky polaritons (LPs) nontrivially merge sub-diffractive polaritonics with diffractive photonics onto a unified platform, unveiling opportunities that stem from the interplay of extreme anisotropic responses and radiation leakage.