Gradient Films Research Articles

Microstructure and performance of compositionally graded Ta2O5/Ti thin films on Ti6Al4V alloy deposited by magnetron sputtering

Tantalum pentoxide (Ta2O5) ceramic is a promising material for modifying metal implants due to its superior wear resistance, chemical stability, and biocompatibility. However, the clinical application of Ta2O5 coatings may be limited by inadequate adhesion, resulting from performance mismatches between Ta2O5 coatings and metal substrates. In this study, a Ta2O5/Ti gradient film was developed on the Ti6Al4V titanium alloy substrate using magnetron sputtering, consisting of a Ti bonding layer, four Ta2O5-Ti composite interlayers with graded composition, and a Ta2O5 surface layer. The microstructure and properties of the Ta2O5/Ti gradient films were investigated, with a Ta2O5 monolayer coating as a reference. Results reveal that the gradient layers have higher density, lower surface roughness, diminished residual thermal stress, and improved adhesion, mechanical properties, and wear resistance compared to the Ta2O5 monolayer coating. However, the corrosion resistance of the Ta2O5/Ti gradient layer sample is inferior to that of the Ta2O5 monolayer coating sample, attributed to interphase corrosion between Ta2O5 and Ti within the interlayers. These significant findings offer a promising approach for enhancing the overall performance of Ta2O5 coating on Ti6Al4V titanium alloy surfaces, thereby opening avenues for widespread application in the biomedical industry.

Accelerating the screening of high-entropy Co-Mn-Fe-Ni-Zn-O gradient films for highly stable NTC materials through co-sputtering combinatorial synthesis

High-entropy negative temperature coefficient (NTC) thermistor thin films, renowned for their exceptional stability, represent a significant advancement in the field of temperature measurement, facilitating the miniaturization of next-generation information and electronic devices. However, “trial and error” approach to developing multi-component NTC oxides significantly constrains the efficiency of the design process. To accelerate development process, we prepared Co-Mn-Fe-Ni-Zn-O gradient films by co-sputtering combinatorial synthesis. The effects of stoichiometric ratios on the resistance (R25), material constant (B25/50) and stability (ΔR/R0) of gradient films were visually represented through color-coded maps for straightforward and intuitive analysis. Samples close to the MnCoFe target in gradient films showed a trend of decreasing resistance and increasing material constant, both reaching 1.71 MΩ and 4083 K, respectively. The high-entropy Co21Mn17Fe19Ni21Zn22O4±δ film exhibited the lowest resistance drift rate only 2.68 % after 500 h ageing at 125 ℃. Another delightful result was that the corresponding homogeneous films, achieved through sputtering with a composite target, had a relative deviation in resistance drift rate of merely 0.18 %. This showcased the successful translation from the screening outcome to the homogeneous film, significantly accelerating the development of NTC materials via the utilization of gradient films screening.

Enhanced water and oxygen barrier properties of deacetylated konjac glucomannan/high acyl gellan gum water gradient film for improved frozen fish fillet preservation

The use of konjac glucomannan (KGM)/high acyl gellan gum (HAGG) edible film with single-sided unsaturated water swelling, designated as a water gradient film (WGF), has been shown to effectively enhance the preservation quality of frozen fish fillets. This study investigates the potential of using partially deacetylated konjac glucomannan (DKGM)/HAGG WGFs to enhance the preservation of frozen fish fillets. The partial deacetylation of KGM improved the water vapour and oxygen barrier properties of the frozen KGM/HAGG WGF, which exhibited a combination of film and ice structural characteristics. This improvement is attributed to strengthened interactions between DKGM and HAGG, resulting in a more structured film matrix that exhibited reduced permeability to both water vapour and oxygen. Furthermore, the improved interactions between DKGM and HAGG led to the formation of smaller polysaccharide ice crystals, which in turn increased the oxygen diffusion path along the intercrystalline boundaries, further decreasing oxygen permeability. Over a 90-day freezing period, the DKGM/HAGG WGF significantly outperformed traditional KGM/HAGG WGF, ice glazing, and polyethylene film packaging in preserving the quality of frozen fish fillets. This study provides a promising strategy for the design and development of DKGM-based WGFs for frozen fish fillet preservation applications.

Gradient conducting polymer surfaces with netrin-1-conjugation promote axon guidance and neuron transmission of human iPSC-derived retinal ganglion cells

Major advances have been made in utilizing human-induced pluripotent stem cells (hiPSCs) for regenerative medicine. Nevertheless, the delivery and integration of hiPSCs into target tissues remain significant challenges, particularly in the context of retinal ganglion cell (RGC) restoration. In this study, we introduce a promising avenue for providing directional guidance to regenerated cells in the retina. First, we developed a technique for construction of gradient interfaces based on functionalized conductive polymers, which could be applied with various functionalized ehthylenedioxythiophene (EDOT) monomers. Using a tree-shaped channel encapsulated with a thin PDMS and a specially designed electrochemical chamber, gradient flow generation could be converted into a functionalized-PEDOT gradient film by cyclic voltammetry. The characteristics of the successfully fabricated gradient flow and surface were analyzed using fluorescent labels, time of flight secondary ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS). Remarkably, hiPSC-RGCs seeded on PEDOT exhibited improvements in neurite outgrowth, axon guidance and neuronal electrophysiology measurements. These results suggest that our novel gradient PEDOT may be used with hiPSC-based technologies as a potential biomedical engineering scaffold for functional restoration of RGCs in retinal degenerative diseases and optic neuropathies.

Assessment of Wavefront Aberrations in Children with Myopia Against the Background of Ocular Surface Disorders in Combination with Computer Vision Syndrome Digital and Eye Strain

Purpose: to assess the state of the wavefront in children with myopia, who have various manifestations of digital eye strain and symptoms of ocular surface disorders, and to evaluate the diagnostic significance of wavefront criteria for assessing the state of the ocular surface.Patients and methods. The study involved 76 children (152 eyes) with myopia aged 8 to 18 years who used gadgets and computers for more than 2 hours a day. All patients used glasses as a method of optical correction and were constantly worn. The patients were divided into 2 groups: 1st with symptoms of ocular surface disorder, 2nd — comparison group. Subjective signs were studied using the online questionnaire “State of the ocular surface”. Using the “Keratograph 5M Oculus” the following parameters were assessed: non­invasive tear film breakup time (NTBR), including the first tear film breakup time, average tear film breakup time, breakup time gradient and maximum tear film breakup zone, the same device was used to perform infrared meibography and study of the lipid layer of the tear film. All patients underwent wavefront aberrometry in a darkened room without cycloplegia.Results. It was found that in the group of patients with the presence of subjective phenomena of disturbance of the state of the ocular surface, identified using the online questionnaire “State of the ocular surface”, rotor aberrations were statistically significantly higher than in the comparison group. At the same time, the indicators of higher­order corneal aberrations have comparable values. Correlation analysis between indicators of the state of the ocular surface and parameters of the wavefront in the group of children with disturbances of the state of the ocular surface showed that a higher level of corneal aberrations corresponds to a higher number of points on the questionnaire of disturbances of the ocular surface and the state of visual comfort, in turn, when comparing data on non­invasive tear film breakup time, a negative correlation was established, which means that with a higher number of points on the questionnaire, and therefore, with a more significant degree of impairment of the ocular surface, the tear film breakup time was shorter.Conclusion. The structure of the wavefront in children with myopia against the background of a disorder of the ocular surface in combination with CVS and digital eye strain is significantly different from the structure of the wavefront in the comparison group.

Microstructures and tribological properties of MoS2/Mo2N multilayer gradient films deposited by reactive magnetron sputtering

In order to compare the microstructures and tribological properties of films, pure Mo2N, pure MoS2, 3-layer and 13-layer MoS2/Mo2N gradient films were deposited by reactive magnetron sputtering. The crystalline structure, surface and cross-section morphologies, 3D surface topography, binding energy, hardness and tribological properties of films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atom force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), nanoindenter, and wear tester, respectively. The results indicate that the preferred orientation of MoS2 in gradient films has changed. The surface and cross-section structures of gradient films become denser with increasing the number of gradient layers. Average surface roughness (Ra) of pure MoS2 film is the highest, and Ra of 13-layer film is the lowest. The binding energy of MoS2 in the composite layer of gradient film has changed. The hardness of gradient film is higher than that of MoS2 and lower than that of Mo2N. The wear resistance of gradient films is much better than that of pure MoS2, and this phenomenon is more obvious under high load conditions. The improvement in the wear resistance of gradient films is attributed to the increase of dense, hardness, H/E and H3/E2. In the initial stage of friction, the 3-layer gradient film can maintain low friction coefficient (≤0.15), which is attributed to the thick MoS2 surface layer. The wear resistance of 13-layer is much better than that of 3-layer gradient film, because it has higher hardness, higher H/E and H3/E2 than 3-layer gradient film.

Combinatorial processing and evaluation of the phase evolution and oxidation behavior of Hf-Al-Si refractory complex concentrated alloys

Refractory complex concentrated alloys (RCCAs) are a unique group of materials defined by the shared principality between all alloying elements and the tailorable properties at high temperatures (>1000 °C). RCCA systems can be optimized using high-throughput processing which allows for simultaneous characterization and testing over a range of compositions. A compositionally-graded Hf-Al-Si coating was compositionally mapped using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) to identify relative compositions as a function of position on the wafer. High temperature CALPHAD phase diagrams and thermodynamic predictions for a Hf50Al50-xSix alloy and its (Hf50Al50-xSix)1-yOy oxide were coupled with experimental results. The RCCA wafer was heat treated at 1200 °C to observe any phase changes and oxidation behavior at elevated temperatures. Characterization of the RCCA was performed before and after heating using synchrotron radiation X-ray diffraction (SR-XRD) to identify the phases and oxidation states as a function of the compositional gradient and to determine an alloy composition range with the least oxidation. SR-XRD pre- and post-treating revealed an inherent amorphous Hf20Al7Si13 phase across the as-deposited coating, similarities in spectra pre- and post-treating for compositions containing less Hf and more Al and Si relative to the entire gradient film, and the presence of significant HfO2 and Al6Si2O13 oxide for compositions consisting of more Hf and less Al and Si relative to the entire gradient film. The SR-XRD and CALPHAD modeling suggests that compositions of 31–48 at.% Hf, 21–48 at.% Al, and 9–27 at.% Si display the greatest resistance to detrimental oxidation upon heating to 1200 ∘C.

Tableting Engineering in Halide Gradient Perovskite: Energy Band Regulation and Design of Carrier Dynamics for THz Sensing

AbstractComponent regulation in perovskite single crystals (SCs) sets the precondition for designing energy band and carrier dynamics, which is essential for exploring functional optoelectronic devices. However, entropy‐driven ion diffusion in SCs leads to component homogenization in a few hours, while the gradient films exhibit long‐term gradient stability. Therefore, tableting engineering is proposed to introduce appropriate boundaries, ensuring good short‐range alignment within the grains and suppressed ion diffusion in the long range. With 6–10 MPa pressure, transverse/longitudinal gradient MAPbX3 (X = Cl, Br, I) tablets are constructed. Due to the alignment of the valence band maximum from MAPbCl3 to MAPbBr3 to MAPbI3, the hole extraction is significantly promoted. As well, different metal electrodes are utilized to regulate the main carrier types. Hole‐type Au/gradient tablet/Au devices show an obvious rectification effect, while electron‐type Ag/gradient tablet/Ag devices have near‐Ohmic contact. Compared to the electron‐type devices, Au/gradient tablet/Au devices show a 3.9‐fold improvement in terahertz responsivity to 3.51 µA W−1, a 2.6‐fold reduction in noise‐equivalent power to 2.22 × 10−8 W Hz−1/ 2; and a 2.6‐fold increase in specific detectivity to 1.89 × 107 Jones. This work demonstrates the tableting engineering to realize stable compositional gradients for energy band regulation in weak‐signal THz detection.

Continuous protein-density gradients: A new approach to correlate physical cues with cell response.

To assess cellular behavior within heterogeneous tissues, such as bone, skin, and nerves, scaffolds with biophysical gradients are required to adequately replicate the in vivo interaction between cells and their native microenvironment. In this study, we introduce a strategy for depositing ultrathin films comprised of laminin-111 with precisely controlled biophysical gradients onto planar substrates using the Langmuir-Blodgett (LB) technique. The gradient is created by controlled desynchronization of the barrier compression and substrate withdrawal speed during the LB deposition process. Characterization of the films was performed using techniques such as atomic force microscopy and confocal fluorescence microscopy, enabling the comprehensive analysis of biophysical parameters along the gradient direction. Furthermore, human adipose-derived stem cells were seeded onto the gradient films to investigate the influence of protein density on cell attachment, showing that the distribution of the cells can be modulated by the arrangement of the laminin at the air-water interface. The presented approach not only allowed us to gain insights into the intricate interplay between biophysical cues and cell behavior within complex tissue environments, but it is also suited as a screening approach to determine optimal protein concentrations to achieve a target cellular output.

The use of He buffer gas for moderating the plume kinetic energy during Nd:YAG-PLD growth of EuxY2−xO3 phosphor films

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition–gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition.

Microstructure and mechanical properties of Ti-Nb alloys: comparing conventional powder metallurgy, mechanical alloying, and high power impulse magnetron sputtering processes for supporting materials screening

At the interface of thin film development and powder metallurgy technologies, this study aims to characterise the mechanical properties, lattice constants and phase formation of Ti-Nb alloys (8–30 at.%) produced by different manufacturing methods, including conventional powder metallurgy (PM), mechanical alloying (MA) and high power impulse magnetron sputtering (HiPIMS). A central aspect of this research was to investigate the different energy states achievable by each synthesis method. The findings revealed that as the Nb content increased, both the hardness and Young’s modulus of the PM samples decreased (from 4 to 1.5 and 125 to 85 GPa, respectively). For the MA alloys, the hardness and Young’s modulus varied between 3.2 and 3.9 and 100 to 116 GPa, respectively, with the lowest values recorded for 20% Nb (3.2 and 96 GPa). The Young’s modulus of the HiPIMS thin film samples did not follow a specific trend and varied between 110 and 138 GPa. However, an increase in hardness (from 3.6 to 4.8 GPa) coincided with an increase in the β2 phase contribution for films with the same chemical composition (23 at.% of Nb). This study highlights the potential of using HiPIMS gradient films for high throughput analysis for PM and MA techniques. This discovery is important as it provides a way to reduce the development time for complex alloy systems in biomaterials as well as other areas of materials engineering.Graphical abstract

Development of a water gradient film through film and ice-glazing approach utilizing konjac glucomannan and high acyl gellan gum for enhanced preservation of frozen snakehead (Channa argus) fillets

Fish fillets are perishable, and several commonly used preservation methods, such as freezing, ice-glazing, and plastic film packaging, aim to maintain their quality during frozen storage. This study presents a novel approach by developing a water gradient edible film using konjac glucomannan (KGM) and high acyl gellan gum (HAGG), and evaluates its effectiveness in preserving frozen snakehead (Channa argus) fillets. First, KGM/HAGG films with different weight ratios of KGM and HAGG were prepared and their properties were characterized. H-bond interactions occurred between KGM and HAGG, resulting in their compatibility and the formation of a homogeneous structure. The properties of the KGM/HAGG films, including tensile strength, elongation at break, water vapour permeability, oxygen permeability, and water swelling ratio, could be modified by changing the weight ratios of KGM and HAGG. The water gradient film was then prepared by adding water to one surface of the film, resulting in controlled swelling. This water gradient film exhibited a water gradient property before freezing and a combined film and ice-glazing property after freezing. It acted as both a moisture and oxygen barrier, while also acting as a moisture-sacrificing agent during frozen storage. When compared to unpackaged fillets, ice-glazed fillets, and fillets packaged with polyethylene film, the frozen water gradient film proved to be the most effective in preserving snakehead fillets during a 150-day frozen storage period. These findings indicate that the KGM/HAGG water gradient film could be a valuable alternative to conventional preservation methods for maintaining the quality of fish fillets during frozen storage.

Photoluminescence of combinatorically sputtered Al2O3–Y2O3 thin films with a Cr3+ and Nd3+ co-doping concentration gradient

The characterization of a wide range of luminescent thin films can be a long and tedious endeavor. With reactive combinatorial sputtering of multiple metal targets, it possible to fabricate thin films with a gradient in composition simply by not rotating the substrate. In this work, combinatorically sputtered thin films of Cr3+ and Nd3+ doped in the Al2O3–Y2O3 system (YAlO) are studied for thin film based luminescent solar concentrators (TFLSCs) application. Contrary to mm's thick plate type LSC's, TFLSCs of just several 100 nm thick require much higher Cr3+ concentration to achieve 40% absorption which can enable several 10's of W/m2 LSC power efficiencies. Our transmission measurements on a Cr2O3 film with a thickness gradient result in an absorption cross section at 460 nm of 1.3 ± 0.7 × 10−19 cm2 showing that the TFLSC absorption requirement can be fulfilled provided that the Cr3+ concentration is in the order of 1022 ions/cm3. The Y:Al ratio of the YAlO host in our films ranged between 0.5 and 3.5, thereby including the monoclinic (Y4Al2O9), perovskite (YAlO3) and garnet (Y3Al5O12) stoichiometry's on a single film. Position dependent XRD, EDX, excitation, emission and lifetime measurements of Cr3+ and Nd3+ show that the unique gradient film sputtering method is able to characterize thin films as a function of host composition and doping concentration. Energy transfer between Cr3+ and Nd3+ in co-doped YAlO films is concluded from Cr3+ excitation bands observed while monitoring Nd3+ emission and from the matching of the rise-time of Nd3+ 1340 nm emission (4F3/2 ->4I11/2) and the decay time of Cr3+ 840 nm emission (4T2 ->4A2). Nd3+ lifetime systematically decreases from 0.24 to 0.05 ms with increasing Cr3+ concentration in Y3Al5-xCrxO12:Nd (0.05 < x < 2). The constraints of heavily doped Cr3+ thin films for enabling adequate absorption and energy transfer to Nd3+ in TFLSC applications are the subjects of the discussion.

Enhancement of photoelectrochemical performance for nitrogen-doped titanium dioxide-based thin films produced by a facile air-based sputtering deposition

This work explores the use of air as a reactive gas in the sputtering deposition of N-doped TiO2-based bilayer, gradient, and multilayer thin films for photoelectrochemical evaluations. By merely adjusting the air/Ar flow ratio, crystalline phases, nitrogen doping contents, electrical properties, optical bandgaps, and valence band maximum (VBM) positions of N–TiO2 films were tailored. The N–TiO2/TiN(O) bilayer on the glass substrates exhibited a peak photocurrent density of 776 μA cm−2 at an air/Ar ratio of 1.60, which may be due to suitable bandgap and carrier mobility of the TiO2 overlayer as the bandgap was affected by the nitrogen content and rutile fraction in the TiO2 overlayer. To further improve the performance of N–TiO2, stepwise and sequential gradient structures with gradient bandgaps and nitrogen contents were produced, which could reach photocurrent densities of 805 ± 4 and 837 ± 3 μA cm−2, respectively. The gradient structures lead to enhanced light absorption efficiency and reduce carrier recombination. The N–TiO2/TiN(O) multilayer structures were also designed and revealed the highest photocurrent density of 865 ± 7 μA cm−2. Such evident enhancement is mainly due to the thin conductive while translucent TiN(O) layers between the photoelectrochemical-active N–TiO2 layers provide multiple low-impedance channels in enhancing carrier transport.

Regulation of static and dynamic magnetic properties of amorphous FeCoZr composition gradient films by Zr doping

A series of amorphous FeCoZr composition gradient monolayer films with varying Zr contents was prepared on the Si(100) substrate using RF magnetron sputtering. The effects of the Zr sputtering power PZr on the static and dynamic magnetic properties of FeCoZr films were systematically investigated. The results demonstrate that the introduction of the Zr element as a composition gradient into FeCo films not only improves the soft magnetic properties of the films but also enhances their in-plane uniaxial magnetic anisotropy. In particular, the doping of Zr elements leads to the destruction of FeCo lattice, inducing a transformation of the films from polycrystalline to amorphous state, resulting in a significant decrease in coercivity (Hc reduced by 82%) and surface roughness (Ra reduced by 78%). In addition, as PZr increases from 30 to 70 W, the anisotropy fields Hk of the films increase from 128 to 340 Oe, and the resonance frequency fr increases from 4.24 to 5.23 GHz. By fitting the permeability spectrum using the LLG equation, it is found that FeCoZr composition gradient films exhibit a lower damping coefficient α of around 0.011–0.014, indicating the reduction of energy loss during magnetization dynamics. These findings highlight the potential applications of FeCoZr composition gradient films in the field of high-frequency microwaves.

The oxidation resistance and mechanical properties of MoNx/AlN multilayer gradient films prepared by magnetron sputtering

In order to investigate the properties of MoNx/AlN multilayer gradient films, MoNx/AlN gradient films with 4, 8, 12, 16 and 20 gradient layers were prepared by double-target co-sputtering. The phase structure, morphology, and element composition of films before and after oxidation were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and energy dispersive spectrometer (EDS). The mechanical properties of films were characterized by nanoindenter. The results indicate that the MoNx/AlN multilayer gradient films are composed of face-centered cubic Mo2N and hexagonal AlN. With increasing the number of gradient layers, and the columnar structure of films gradually becomes uniform and dense. The average grain size of MoNx in gradient films decreases first, then increases slightly and finally decreases. The oxidation resistance of gradient films is better than that of pure MoNx film. And with increasing the number of gradient layers, the oxidation resistance of MoNx/AlN gradient films is gradually enhanced. The hardness of MoNx/AlN gradient films first decreases from 23.68 ± 1.18 GPa of 4-layer film to 16.68 ± 0.83 GPa of 12-layer film and then increases to 21.12 ± 1.06 GPa of 20-layer film.

Temperature-dependence of anomalous Hall effect in Ta-(GdFeCo)δ-Ta films with vertical composition gradients

Ferrimagnetic rare-earth (RE) – transition-metal (TM) alloys are considered small ferromagnets with antiferromagnetic exchange in sublattices, which makes them a preferable choice for future ultrafast memory-related applications. Here, we explore the magnetic properties of two GdFeCo thin films with tailored vertical composition gradients, achieved by gradually varying Gd concentration across the film thickness. The temperature-dependent magnetic behavior of these films is investigated using anomalous Hall effect (AHE) measurements and SQUID magnetometry. Both films exhibit perpendicular magnetic anisotropy with distinct coercivities, indicating variations in their magnetic properties. The films show RE-dominant behavior at 300 K. Moreover, for the positive composition gradient (+δ) film, the compensation temperature is around 350 K, while the film with a negative gradient (−δ) exhibits a compensation temperature above 400 K. Furthermore, the spin-flop behavior and AHE sign reversal provide insights into the sublattice dominance and exchange interactions within the films. The magnitude of ΔRXY (the difference in AHE resistance at different magnetic fields) is shown to be sensitive to the composition gradient, indicating stronger perpendicular magnetic anisotropy for the “−δ” sample. This study demonstrates the potential of composition engineering to tailor the magnetic characteristics of ferrimagnetic materials for specific spintronics applications.

Enable Multi-Stimuli-Responsive Biomimetic Actuation with Asymmetric Design of Graphene-Conjugated Conductive Polymer Gradient Film.

In this paper, multiresponsive actuators based on asymmetric design of graphene-conjugated poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) gradient films have been developed by a simple drop casting method. The biomimetic actuation is attributed to the hygroscopic expansion property of PEDOT:PSS and the gradient distribution of graphene sheets within the film, which resembles the hierarchical swelling tissues of some plants in nature. Graphene-conjugated PEDOT:PSS (GCP) actuators exhibit reversible bending behavior under multistimuli such as moisture, organic vapor, electrothermal, and photothermal heating. Noticeably, the bending curvature reaches 2.15 cm-1 under applied voltage as low as 1.5 V owing to the high electrical conductivity of GCP actuator. To mimic the motions of nyctinastic plants, a GCP artificial flower that spreads its petals under sunlight illumination has been fabricated. GCP actuators have been also demonstrated as intelligent light-controlled switches for light-emitting diodes and smart curtains for thermal management. Not only do the GCP gradient films exhibit potential applications in flexible electronics and energy harvesting/storage devices but also the facile fabrication of multiresponsive GCP actuators may shed light on the development of soft robotics, artificial muscles, wearable electronics, and smart sensors.

Fabrication of an Organic-Inorganic Hybrid Hard Coat with a Gradient Structure Controlled by Photoirradiation.

Organic-inorganic materials have attracted attention because of the advantages of both organic and inorganic resins. Among their disadvantages, hard coating films made of organic-inorganic mixtures of resins have opacity and interface peeling problems because of organic-inorganic phase separation and surface segregation of inorganic resins. Although an organic-inorganic gradient-structured material comprising an inorganic-rich domain at the air interface and an organic-rich domain at the organic substrate has the potential to solve these problems, the fabrication of a gradient-structured material has not yet been achieved. Here, we describe the fabrication of an organic-inorganic gradient film by impeding the movement of organic and inorganic resins through radical photopolymerization of organic and inorganic oligomers. Moreover, we successfully enhanced gouge hardness by cross-linking with photobase-catalyzed sol-gel reactions of inorganic resins at the air interface. As a result, the organic-inorganic gradient coating contributed excellent gouge hardness (pencil hardness >9H), adhesion to an organic substrate such as polycarbonate, and transparency (visible light transmittance >99%T). In addition, we demonstrated that the formation of organic-inorganic gradient structures is dominated by the surface free energy and viscosity of each resin. Achieving a gradient structure required a significant difference in surface free energy (>20 mJ/m2) and high mixture viscosity (>65 mPa·s).

Reverse Engineering Analysis of Optical Properties of (Ti,Cu)Ox Gradient Thin Film Coating

Analysis of the optical properties of a gradient (Ti,Cu)Ox thin film is presented in this paper. The thin film was prepared using reactive co-sputtering of Ti and Cu targets. The desired elemental concentration profiles of Cu and Ti versus the thin film thickness were obtained by changing the power delivered to the magnetron equipped with Cu, while the powering of the magnetron equipped with the Ti target was maintained at a constant level during the film deposition. Optical properties were analysed using the reverse engineering method, based on simultaneously measured optical transmittance and reflectance. Detailed microstructure analysis performed using transmission electron microscopy investigations revealed that the thin film consisted of at least four areas with different structural properties. Finding a satisfying fit of theoretical to experimental data required taking into account the heterogeneity in the material composition and microstructure in relation to the depth in the prepared gradient thin film. On the basis of the built equivalent layer stack model, the composition profile and porosity at the cross-section of the prepared gradient film were evaluated, which agreed well with the obtained elemental and microscopy studies.