Thin Film Sandwich Structures Research Articles

Size-dependent tensile failure of epitaxial TiN/Cu/TiN sandwich pillar structures: A combined experimentation – Atomistic simulation study

A combined experimentation - molecular dynamics simulation study was conducted to understand tensile failure of TiN/Cu/TiN interfacial regions. Tensile loading was conducted on micro-pillar specimens fabricated from TiN/Cu/TiN thin film sandwich structures. The Cu layer and the TiN layer underneath were grown epitaxially on MgO(001) substrates, with Cu[110]//TiN[001] in the growth direction and Cu<111>//TiN<100> and Cu<112>//TiN<100> within the growth plane. The Cu layer contains numerous nanotwins with the {111} twin plane parallel to the growth direction, with 2–10 nm wide twin bands rotated in-plane by 90° in different yet symmetry-equivalent epitaxial domains. Tensile loading in-situ a scanning electron microscope measured tensile fracture stress ∼1.5 GPa and revealed a surprising failure mode transition. At a larger Cu layer thickness, ductile tensile fracture occurred within the Cu layer. At smaller Cu layer thicknesses, apparently brittle fracture occurred close to or at the Cu/TiN interface. The accompanying molecular dynamics simulations illustrate a significant dependence of the failure mode on the aspect ratio of Cu pillars under tensile loading. With pillars of small height-to-diameter ratios, tensile loading leads to a significant hydrostatic tension within, as well as significant plasticity throughout the Cu pillar, in particular near the top and bottom Cu/TiN interfaces. The high degree of dislocation activities close to or at the interface, combined with dislocation pile-up, serves to create nanovoids. The high hydrostatic tension furnishes a driving force for growth of such nanovoids, leading to rapid tensile fracture. The simulation results offer an analogy to experimental observations and mechanistic understanding of tensile failure mechanisms for ceramic/metal/ceramic interfacial regions.

Ultrasensitive, Stretchable, and Fast-Response Temperature Sensors Based on Hydrogel Films for Wearable Applications.

Conductive hydrogels can be used in wearable electronics integrated with skin, but the bulk structure of existing hydrogel-based temperature sensors limits the wearing comfort, response/recovery speeds, and sensitivity. Here, stretchable and transparent temperature sensors based on a novel thin-film sandwich structure (TFSS) are designed, which display unprecedented thermal sensitivity (24.54%/°C), fast response time (0.19 s) and recovery time (0.08 s), a broad detection range (from -28 to 95.3 °C), high resolution (0.8 °C), and high stability. The thin hydrogel layer (12.15 μm) is encapsulated by two thin elastomer layers, which prevent the water evaporation and enhance the heat transfer, leading to the boosted stability and accelerated response/recovery speeds. The nondrying and antifreezing capabilities are further promoted by the hydratable lithium bromide (LiBr) incorporated in the hydrogel, enabling it to avoid dehydration in an extremely arid environment and freeze below subzero temperatures (freezing point below -120 °C). A comparative study reveals that the thermal sensitivity displayed by the TFSS sensor in capacitance mode is several times higher than that in conventional conductance/resistance mode above room temperature. Importantly, a new mechanism based on a horizontal plate capacitance model is proposed to understand the high sensitivity by considering the permittivity and geometry variations of TFSS. The thin TFSS, stretchability and transparency enable the sensor to be conformally and comfortably attached to human skin for real-time and reliable monitoring of various human motions, physical states, skin temperature, etc., without affecting the appearance.

Detection of Organic Substances by a SERS Method Using a Special Ag-Poly(Chloro-P-Xylylene)-Ag Sandwich Substrate

Spectroscopy based on surface enhanced Raman scattering (SERS) is widely used as a method with extremely high sensitivity for molecular and chemical analysis. We have developed thin-film sandwich structures, in which, when used as sensitive elements for detecting organic compounds at low concentrations, high-amplitude spectra of surface enhanced Raman scattering are observed. Using gas-phase cryochemical synthesis and thermal sputtering in vacuum, SERS active sandwich structures Ag–poly(chloro-p-xylylene)–Ag (Ag–PCPX–Ag) were obtained. In the process of creating sandwich structures, the upper silver film takes the form of a complex island topology with submicron sizes. A series of samples were made with different thicknesses of the polymer and upper silver layers. SERS spectra of the analyte chemically adsorbed on the film surface were obtained, demonstrating a significant amplification (up to 104) compared with the control sample. The dependence of the gain on the silver concentration is characterized by a maximum polymer layer thickness of 600 nm and a 30 nm thick upper silver layer. A selective amplification of the low molecular weight compound spectra with respect to proteins was observed. A semi-empirical model is proposed that is in good agreement with the experimental results.

Mechanical failure of CrN/Cu/CrN interfacial regions under tensile loading

Quantitative assessment of the adhesion strength of ceramic hard coatings on substrates is a subject of great technological and scientific relevance, the achievement of which has proven difficult over the past two decades. Here we show that a micro-pillar tension testing protocol may serve as an effective method for this purpose, and demonstrate its effectiveness through a combination of micro tension testing and crystal plasticity finite element analysis (CPFEA). Tensile loading perpendicular to the interfaces, as well as accompanying CPFEA simulations, was conducted on CrN/Cu/CrN/Si micro-pillar tensile specimens fabricated from vapor phase deposited thin film sandwich structures. All tensile loading induced fracture were observed to occur within the Cu interlayer and near one Cu/CrN interface of the CrN/Cu/CrN sandwich structure. Observations, measurements, and discussions were made regarding the fracture surfaces, fracture stresses, and primary fracture mechanisms. Fracture surface examinations indicated ductile tensile fracture, induced by the formation of voids. It is shown that, due to the geometric constraint imposed by the ceramic layers in the current testing configuration, interlayer plasticity and the critical fracture stress display a strong dependence on the Cu interlayer thickness. It is postulated as a failure criterion that cooperation of deviatoric and hydrostatic stress components is needed to induce the nucleation and growth of voids. The relevance of the present work to the interfacial mechanical integrity of other metal/ceramic interfaces is discussed.

Electrochemical and Electronic Properties of Transparent Coating from Highly Solution Processable Graphene Using Block Copolymer Supramolecular Assembly: Application toward Metal Ion Sensing and Resistive Switching Memory.

Here, we have discussed the preparation of a highly solution processable graphene from a novel supramolecular assembly consisting of block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and pyrenebutyric acid (PBA)-modified reduced graphene oxide (RGO). The PBA molecules anchored on the graphene surface form supramolecules with PS-b-P4VP through H-bonding between the carboxylic acid group of 1-pyrenebutyric acid and the pyridine ring of P4VP. The formation of a supramolecular assembly results in a highly stable solution of reduced graphene oxide in common organic solvents, such as 1,4-dioxane and chloroform. Highly transparent and mechanically stable thin films can be deposited from these supramolecular assemblies on a relatively smooth surface of different substrates such as silicon wafer, glass, indium tin oxide, and flexible polymer substrates like poly(ethylene terephthalate). The graphene surface modifier (PBA) can be selectively removed from the thin film of the hybrid material by simple dissolution, resulting in a porous structure. Hybrid thin films of around 50 nm thickness exhibit interesting electrochemical properties with an areal capacitance value of 17.73 μF/cm2 at a current density of 2.66 μA/cm2 and good electrochemical stability. The pendent P4VP chains present in the composite thin film were further exploited for electrochemical detection of metal ions. The electrical measurement of the thin film sandwich structure of the composite shows a bipolar resistive switching memory with hysteresis-like current–voltage characteristics and electrical bistability. The OFF state shows ohmic conduction at a lower voltage and trap-free space-charge-limited current (SCLC) conduction at high voltage, whereas the ON state conduction is controlled by ohmic at low bias voltage, trap-free SCLC at moderate voltage, and tarp-assisted SCLC at high voltage.

Fabrication of PZT/CuO composite films and their photovoltaic properties

The existence of the Schottky barriers at the top and bottom electrodes of the ferroelectric thin film sandwich structure makes it difficult to separate and collect electron-hole pairs, thus limiting the enhancement of the photocurrent. In this paper, Pb(Zr,Ti)O3 (PZT) and composite structure of PZT/CuO films are prepared by a sol-gel method and their photovoltaic properties have been investigated. It is found that the PZT/CuO films show a short circuit photocurrent density (JSC) enhanced by nearly 6 times and power conversion efficiency (PCE) increased by six-fold when compared to those of the PZT film. The increase of photovoltaic response is due to the internal electric field of PZT/CuO p–n junction, which plays an important role in driving the photo-generated carriers. The Ohmic contact between the interfaces of LNO/PZT and CuO/Pt also reduce the resistance of the transportation of photogenerated carriers. Furthermore, the JSC of PZT/CuO film are observed to be 0.03 and 0.013 mA/cm2 after upward poling and downward poling, respectively, indicating that the photocurrent can be modulated by the direction of the polarization electric field. The photovoltaic effect of composite films and its potential mechanism are also explored. This work provides an efficient approach to develop ferroelectric film based on photovoltaic devices.

Effect of annealing temperatures on the turn‐on voltage of organic Schottky diode based on DPP(BTFu) 2

The electrical properties of diketopyrrolopyrrole ethyl-hexylated (DPP(TBFu)2) thin film sandwich structure of the ITO/DPP(TBFu)2/Al configuration for different annealing temperatures ranging from 40 to 130°C is investigated. The current–voltage characteristics showed an excellent rectifying behaviour of the device, and from which the diode parameters were extracted. It is found that all parameters remain constant upon annealing temperatures, with the exception of the turn-on voltage. An increase of the turn-on voltage from an initial value of 1.6–2.08 V is noticed starting from 70 to 100°C, respectively. This fact is attributed to the effect of residual solvent induced defect states that may act as free carriers. Defect states density for the non-annealed and annealed device at 130°C derived from capacitance–voltage measurements is found to correlate well with the turn-on voltage. It is found that annealing the device at 130°C results in a smaller concentration of defect states. In addition, it is also found that the annealing process produces the flat band voltage drop, which has a direct impact on the turn-on voltage.

Switching the Localized Surface Plasmon Resonance of Single Gold Nanorods with a Phase-Change Material and the Implementation of a Cellular Automata Algorithm Using a Plasmon Particle Array

We investigate the modulation of the localized surface plasmon resonance (LSPR) of a gold nanorod (AuNR) using a GeSbTe film as an active medium. We demonstrate high-contrast switching of LSPR in an AuNR/GST/Au thin film sandwich structure upon phase change. To go beyond this single-particle switching functionality, we consider a plasmon particle system interacting with a phase-change material (PCM) to discuss the possibility of parallel processing devices with memory functionality, exploiting the plasticity and threshold behavior that are inherent characteristics of PCMs. We demonstrate that the temporal and spatial evolution of a plasmon-PCM array system can be equivalent to a cellular automata algorithm.

Ion transport-related resistive switching in film sandwich structures

Resistive switching memories based on ion transport and related electrochemical reactions have been extensively studied for years. To utilize the resistive switching memories for high-performance information storage applications, a thorough understanding of the key information of ion transport process, including the mobile ion species, the ion transport paths, as well as the electrochemical reaction behaviors of these ions should be provided for material and device optimization. Moreover, ion transport is usually accompanied by processes of microstructure modification, phase transition, and energy band structure variation that lead to further modulation of other physical properties, e.g., magnetism, optical emission/absorbance, etc., in the resistive switching materials. Therefore, novel resistive switching memories that are controlled through additional means of magnetic or optical stimulus, or demonstrate extra functionalities beyond information storage, can be made possible via well-defined ion transportation in various switching materials and devices. In this contribution, the mechanism of ion transport and related resistive switching phenomena in thin film sandwich structures is discussed first, followed by a glance at the recent progress in the development of high-performance and multifunctional resistive switching memories. A brief perspective of the ion transport-based resistive switching memories is addressed at the end of this review.

Perovskite‐Based Hybrid Solar Cells Exceeding 10% Efficiency with High Reproducibility Using a Thin Film Sandwich Approach

Organometal halide perovskites have tremendous potential as light absorbers for photovoltaic applications. In this work we demonstrate hybrid solar cells based on the mixed perovskite CH3 NH3 PbI2 Cl in a thin film sandwich structure, with unprecedented reproducibility and generating efficiencies up to 10.8%. The successfulness of our approach is corroborated by the experimental electronic structure determination of this perovskite.

Synthesis and optical behavior of PLED devices based on (PMMA)/(PAA)/Er(AP)6Cl3 complex and N,N′-didodecyl-3,4,9,10-perylene tetracarboxylic diimide composites

The realization of efficient polymeric light emitting diode (PLEDs) in a double-layered configuration was investigated. The devices are composed by transparent conductive oxide (ITO)/MoO3/organic layers/aluminum/selenium, conformed by thin film sandwich structures obtained by vacuum evaporation. Two organic layers were developed. First a n-type organic layer of composite based on polymethylmethacrylate (PMMA)/polyacrilic acid (PAA)/Er(AP)6Cl3 complex and second a n-type organic semiconductor, N,N′-didodecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C12H25). The rare earth complex composites and the perilenic compound were synthesized and characterized using UV–Visible absorption, XPS, and NMR techniques, respectively. Driving voltage of devices was lowered by applying MoO3 thin film as buffer layer and high current intensity efficiency was obtained applying a perilenic film. The effect of MoO3 and PTCDI-C12H25 thin films, on the optical and the physical properties of the electroluminescent devices were discussed. I–V measurements have shown that the structures exhibit diode characteristics and the electroluminescent signal increases when PTCDI-C12H25 thin layer is introduced between the anode and the holes transporting layer. The morphology of the thin films with and without buffer layer indicates that introduction of this layer allows to obtain a homogeneous surface morphology. The results indicate that carrier injection ability and optimized charge balance is obtained to the lowest driving voltage and highest intensities efficiency among the referenced devices.

Electrical characterization of aluminium oxide–aluminium thin film composites by impedance spectroscopy

The impedance spectroscopy method has been used for diagnostics of the thin film sandwich structures containing two aluminium electrodes with the composite deposited between them. The aluminium oxide–aluminium composite was deposited in the high effective reactive pulsed magnetron sputtering process. The aim of the investigation was to obtain the metal–oxide composite layer in single magnetron sputtering process. The main purpose of presented research was to characterize the impedance of presented structure in the wide range of frequency. The electrical equivalent circuit of measured structures was described and led to the identification of the dielectric relaxation processes.

Electrical conductivity of vanadium dioxide switching channel

The electrical conductivity of the switching channel of vanadium dioxide thin-film sandwich structures is studied over a wide temperature range (15-300 K). It is shown that the electrical resistance of the channel varies with temperature as R~exp(aT-b/T) in the high-temperature region (above 70 K). The experimental results are discussed from the viewpoint of the small polaron hopping conduction theory which takes into account the influence of thermal lattice vibrations onto the resonance integral.

Structure and Electronic Properties of Evaporated Thin Films of Lead Sulfide

Lead sulfide (PbS) thin films were prepared by thermal evaporation onto glass substrates from PbS powder. The structure and DC electrical properties of evaporated PbS thin film sandwich structures with thicknesses (d) up to 600 nm have been investigated. X-ray diffraction studies showed that the films were crystalline, with a preferred orientation in the [111] direction. Capacitance measurements indicated that the films had a relative permittivity of 5.7. Room-temperature current density-voltage (J–V) characteristics revealed ohmic conduction below a transition voltage (Vt) and a power–law dependence with an exponent of ≈ 2 at higher voltages. This behaviour was interpreted in terms of space–charge limited conductivity controlled by an exponential distribution of traps below the conduction band edge. Further evidence for this conduction process was provided by a linear dependence of Vt upon d2. Analysis of the results yielded a room temperature electron concentration no of ≈ (3.9 – 5.4) x 109 m-3.

Thermovoltaic effect in thin-film samarium-sulfide-based structures

We report the first observation of a thermovoltaic effect in thin films of samarium sulfide (SmS). According to this, a voltage of ∼1.1 V appeared in a thin-film SmS-based sandwich structure heated to temperatures in the 360–428 K interval.

Electrical properties of iron phthalocyanine thin film device using gold and aluminium electrodes

The electrical conductivity of FePc thin film sandwich structures using gold and aluminium electrodes has been investigated for the freshly prepared devices and device after exposure to oxygen for 30 days. Current density-voltage characteristics of the devices in the forward bias showed an ohmic conduction in lower voltages and a space charge limited conduction (SCLC) controlled by a single and an exponential trapping levels at two different ranges of applied voltages. The hole concentrations are obtained as Po = 3.92 × 1016 m−3 with a hole mobility μ = 5.81 × 10−6 m−1 V−1 s−1. In the SCLC region a discrete trap level of 1.88 × 1021 m−3 is found at 0.66 eV followed by an exponential trap distribution of Pe = 4.63 × 1046 J −1 m−3 at Nt(e) = 2.23 × 1026 m−3. From the current limitations in the reverse bias, the conduction is identified as an electrode limited Schottky type of conduction. In the oxygen-doped samples, both in the forward and reverse bias the order of currents are much enhanced and a transition from the ohmic conduction to a space charged conduction is observed.

An analysis of optimal structural features in the peacock tail feather

This paper presents an analysis of the optimal structural features in the peacock tail feather. Peacock tail feathers are highly optimized in three respects. The feathers are highly efficient structures that can support their own weight in a deployed position. The feathers are also optimized for producing thin-film colours with a high degree of lustre. The feathers also have a structure which produces intricate patterns with a high degree of aesthetic merit. There are structural features that allow simultaneous optimization in the different areas of performance. In particular, the thin-film sandwich structure is ideal for producing optimal structural performance and optimal optical performance.

Electrical conductivity and dynamics of electroforming in AlSiO xAl thin film sandwich structures

Recent work has established that in evaporated SiO x thin film sandwich structures with gold electrodes, which show the Poole–Frenkel effect (enhanced conductivity at high electric fields due to the lowering of the potential barrier at donor-like centres), the value of the Poole–Frenkel field-lowering coefficient β increases with voltage cycling prior to the onset of electroforming. These enhanced β values were associated with the establishment of a high-field region during the electroforming process. In the present work, aluminium electrodes were used in order to explore the characteristics of contrasting system. Electroforming occurred in some samples, but with a maximum current value of less than 1 mA, considerably less than with the gold electrodes. Poole–Frenkel conductivity was observed in the initial voltage cycles, with a β value of typically 4.5×10 −5 eV m 1/2 V −1/2, moderately exceeding the theoretical value. Electroforming normally took place after several voltage cycles, but was not permanent, with a reversion to Poole–Frenkel conduction during some cycles. As for the case with gold electrodes, the value of β increased in the first few cycles, but after further voltage cycling, the behaviour became less stable with varying values of β. After further voltage cycling, electroforming disappeared and β reduced to the order of 2×10 −5 eV m 1/2 V −1/2. This behaviour can be understood in terms of a filamentary conduction model, in which the stability of samples with aluminium electrodes is inferior to those with gold electrodes, as a consequence of the lower melting point.

Monte Carlo simulation of current–voltage characteristics in metal–insulator–metal thin film structures

Many metal–insulator–metal (MIM) thin film sandwich structures exhibit an electroforming process, following which the resulting direct current (DC) current–voltage I–V characteristic exhibits ohmic conduction and voltage-controlled differential negative resistance (VCNR) at low and high voltages, respectively. One of the more successful models of these phenomena is the filamentary model, in which it is assumed that the electroforming process establishes a population of ohmic filaments within the insulating matrix, which span the metal contacts. The VCNR behaviour results from the progressive cessation of conduction in individual filaments owing to Joule heating effects. Early work showed that by postulating plausible probability distributions of filament resistances, I–V characteristics typical of those obtained experimentally could be derived. More recently more realistic normal distributions of filament resistances, radii and cross-sectional areas have been considered, and these too have yielded characteristics in accordance with experiment. In the present work normal distributions of filament resistances and radii are considered, but in contrast to previous work a novel method using a Monte Carlo simulation was employed. Furthermore, the effects of using a combination of two different filament resistivity values were explored. Variations in conductance and shifts in the current peak were explicable using existing theory. The approach adopted in this work lends itself to the exploration of more complex filament distributions, and therefore to a better representation of the underlying filament population, including those having a multivariate distribution.

“In-Situ Raman Spectroscopy of Electronic Processes in Fullerene Thin Films

AbstractThe current voltage characteristics of C60 thin film sandwich structures fabricated by vacuum deposition on indium tin oxide (ITO) with an aluminium top electrode are presented and discussed. A strongly non-linear behavior and a sharp increase in the device conductivity was observed at relatively low voltages (∼2V), at both room and low temperatures (20K). At room temperature the system is seen to collapse, and in situ Raman measurements indicate a solid state reduction of the fullerene thin film to form a polymeric state. The high conductivity state was seen to be stable at elevated voltages and low temperatures. This state is seen to be reversible with the application of high voltages. At these high voltages the C60 film was seen to sporadically emit white light at randomly localized points analogous to the much documented Electroluminescence in single crystals.