Thin Film Substrate Research Articles

Enhanced integrated acoustofluidics with printed circuit board electrodes attached to piezoelectric film coated substrate

The current key issues in applying acoustofluidics in engineering lie in the inflexibility of manufacturing processes, particularly those involving modifications to piezoelectric materials and devices. This leads to inefficient prototyping and potentially high costs. To overcome these limitations, we proposed a technique that is capable of prototyping acoustofluidic devices in a straightforward manner. This is achieved by simply clamping a printed circuit board (PCB) featuring interdigital electrodes (IDEs) onto a substrate coated with a piezoelectric thin film. By applying appropriate clamping force between the PCB and the substrate, one can effectively generate surface acoustic waves (SAWs) along the surface of the substrate. This approach simplifies the prototyping process, reducing the complexity and fabrication time. The clamping mechanism allows for easy adjustment and optimization of the SAW generation, enabling fine-tuning of the fluid and particle manipulation capabilities. Furthermore, this method allows for customizable interdigital transducers (IDTs) by ‘patterning’ IDEs on thin-film piezoelectric substrates (such as ZnO/Al and ZnO/Si) with various anisotropy orientations. This facilitates the on-demand generation of wave modes, including A0 and S0 Lamb waves, Rayleigh waves, and Sezawa waves. One notable advantage of this method is its capability to rapidly test acoustic wave patterns and performance on any substrate, offering a fast and streamlined approach to assess acoustic behaviors across diverse materials, thereby paving the way for efficient exploration of novel materials in SAW technology.

Microstructural evolution of diamond thin film grown on a silicon substrate via a surface-wave plasma system: Identification of intermediated phase

Diamond thin films are promising for diverse applications because of their exceptional properties. Understanding the interface between diamond films and substrates is crucial for optimizing the film quality and functionality. In this study, a diamond thin film was grown on a Si(100) substrate using a microwave-based surface-wave plasma system and subjected to transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The formation of a SiC interlayer was identified at the interface between the diamond thin film and Si substrate through cross-sectional TEM and EELS. Analysis of the atomic structure and crystalline properties indicated that the interlayer was β-SiC and that the orientation relationships [110]β-SiC//[110]Si and (111)β-SiC//(111)Si existed at the β-SiC/Si interface. The β-SiC interlayer exhibited the {111} extra half-planes, because they were more easily formed by the {111} planes than the {110} extra half-planes generated on {001} planes through 60° misfit dislocations. Many stacking faults and microtwins were observed in the diamond grains; such planar defects were likely attributed to strain behavior and complex contrasts at the nanoscale.

A High-Gain, High-Efficiency Transparent and Flexible Antenna for Vehicular Communication

A high-gain high-efficiency flexible and transparent film antenna in millimeter-wave is proposed in this paper. Comb structure which can be regarded as developed from the grid array structure is employed to mitigate the loss of the feeding network caused by the lossy metal mesh. The gain of the antenna has low sensitivity to the sheet resistance, which is promising in developing large-scale and high-performance transparent antenna. A large-scale ([Formula: see text]@28[Formula: see text]GHz) antenna is designed and fabricated using transparent metal mesh film with sheet resistance of 0.1[Formula: see text][Formula: see text]/sq and a thin PET film substrate. Bending measurement shows the antenna has excellent mechanical flexibility. A peak gain of 15.92[Formula: see text]dBi, radiation efficiency of 65.1%, optical transparency of 84% and 1,000-times bending are obtained. To the best of our knowledge, it is the highest gain and radiation efficiency flexible-transparent antenna reported thus far, which provides a glass-integrated antenna solution for vehicular communication.

Techno-economic analysis of the direct solar conversion of carbon dioxide into renewable fuels

Direct conversion of carbon dioxide (CO2) using sunlight into commercially viable renewable fuels will be one key solution for decarbonization and storing renewable solar energy. However, the direct conversion of CO2 using sunlight faces uphill challenges, especially the techno-economic viability (TEV), as the produced fuels must compete with fossil fuels or at least with fossil fuel alternatives, which are cheap. This work proposes an innovative structure design for photoelectrochemical reduction of CO2 into renewable fuels and performs a detailed techno-economic analysis (TEA) using a generalized gross margin (GM) model. The proposed structure uses low-cost and earth-abundant crystalline silicon-based photoanode with triangle nano-strips on a thin-film substrate to efficiently convert solar energy into renewable fuels. The proposed structure shows 20.01% of power-conversion efficiency (ηPEC). The techno-economic GM model considers all relevant cost parameters to assess the TEV of the produced hydrogen (H2) and hydrocarbon (C1–C3) fuels from CO2 reduction processes. The TEV of the produced renewable fuels is analyzed and presented against critical device parameters, such as the photocurrent density (J), catalyst’s durability (tdur), Faradaic efficiency (FE), and catalyst cost (Ccat). We also analyzed how sensitively the TEV of the produced fuels depends on the uncertainty of different cost parameters assuming base-, worst-, and optimistic-case scenarios. We find that carbon monoxide (CO) and formic acid (HCOOH) fuels will be commercially viable in the base case, while H2 and propanol (C3H7OH) will be only in the optimistic case.

Carbon Thin-Film Electrodes as High-Performing Substrates for Correlative Single Entity Electrochemistry.

Correlative methods to characterize single entities by electrochemistry and microscopy/spectroscopy are increasingly needed to elucidate structure-function relationships of nanomaterials. However, the technical constraints often differ depending on the characterization techniques to be applied in combination. One of the cornerstones of correlative single-entity electrochemistry (SEE) is the substrate, which needs to achieve a high conductivity, low roughness, and electrochemical inertness. This work shows that graphitized sputtered carbon thin films constitute excellent electrodes for SEE while enabling characterization with scanning probe, optical, electron, and X-ray microscopies. Three different correlative SEE experiments using nanoparticles, nanocubes, and 2D Ti3C2Tx MXene materials are reported to illustrate the potential of using carbon thin film substrates for SEE characterization. The advantages and unique capabilities of SEE correlative strategies are further demonstrated by showing that electrochemically oxidized Ti3C2Tx MXene display changes in chemical bonding and electrolyte ion distribution.

Bending Effects on Polyvinyl Alcohol Thin Film for Flexible Wearable Antenna Substrate

Polyvinyl Alcohol (PVA) has been used in various applications, including the medical health industry and electronics. It is a synthetic polymer with advantages such as being transparent, flexible, biocompatible, biodegradable, and a simpler synthesis process. These advantages make PVA a very promising material for human wearable antennae. In this research, the bending effect of an antenna using a PVA substrate is studied to analyze its durability in the wearable application. Firstly, the thin film substrate synthesis is performed using PVA 2488 with the measured average dielectric constant and tangent loss of 1.24 and 0.066, respectively, across S-Band frequency. Later, a 5G antenna is designed and fabricated using the PVA substrate. Finally, the bending effects of the fabricated antenna are measured at different bending radii. Four different antenna-bending radii are selected to represent different curvatures of human body parts. Results show that bending does not have a significant effect on the reflection coefficient of the antenna, where the frequency shifts from 2.2% up to 7.4% only for all bending conditions. Hence, in that aspect of finding, the PVA thin film is a potential candidate for flexible and wearable antenna material in various human body parts in biomedical applications.

Room-temperature DBR-free VCSEL operation on an InGaN/GaN thin-film platform with a monolithic surface grating.

Vertical-cavity surface-emitting lasers (VCSELs) are pivotal in various applications ranging from data communication to sensing technologies. This study introduces a VCSEL design featuring a monolithic top surface high contrast grating (HCG) reflector on a thin-film substrate, aimed at improving lasing performance while reducing fabrication costs by omitting the use of distributed Bragg reflectors (DBRs). We fabricated the proposed VCSEL with the surface grating and characterized its performance through micro-photoluminescence measurements. The laser demonstrated room-temperature lasing at 436.2 nm with a Q factor of 4600 and a lasing threshold of 5.5 kW/cm2 under optical pumping. The implementation of the surface grating reflector was instrumental in facilitating vertical lasing, significantly improving surface reflectivity compared to conventional flat GaN/air interfaces. This innovative design holds significant promise for the development of cost-effective, DBR-free VCSELs, with potential applications extending to photonic integrated circuits and light detection and ranging (LiDAR) systems.

Study the Effect of Ion Doping on ZnO Nanostructures for Room Temperature NH3 Gas Sensor

We investigated the impact of doping ion type on the performance of a ZnO-based ammonia gas sensor to show the capability of these ions to achieve high-performance gas sensing at room temperature. A sol-gel method was used to synthesize both doped and undoped ZnO nanostructures, while the gas sensor device was made by casting ZnO onto a glass substrate for a uniform thin film. Then Al electrodes were attached to the film. The characterization was carried out via field-emission scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, UV–vis, Pl luminescence, Brunnauer-Emmett-Teller, I-V characteristic, and gas sensor setup device. PL measurement shows an increase in green emission spectra with Ba ion shifting the peaks from VO to VO+ and VO+ to VO++ states. The gas sensor test at room temperature greatly enhances performance for certain ions. The Ba ions greatly influence gas sensor performance, increasing the response to 24 compared to 5 for undoped ZnO. The room-temperature enhancement achieved by the Ba ions could open the way to investigate more effective dopants for NH3 gas sensors.

Near space detection technology combining van der Waals heterojunction photodetectors with long-range infrared target detection algorithms

To test the target detection accuracy of infrared detectors in practical applications, this study proposes a near space detection (NSD) technology that integrates van der Waals heterojunction photodetectors with long-range infrared target detection algorithms (ITDAs). A van der Waals heterojunction infrared photodetector (MoS2/CdTe) is prepared by combining molybdenum disulfide thin film and cadmium telluride substrate, and depositing gold electrodes on both substrates. A fusion algorithm of spatiotemporal target detection network and local contrast method is proposed for long-distance detection of weak infrared targets, and a near-space infrared detection system is designed. The data showed that the MoS2/CdTe heterojunction photodetector had a detection range of 200–1700 nm and a response of 36.7 mA/W, demonstrating excellent optoelectronic performance. The false alarm rates of the long-distance ITDA were 0.006% and 0.009%, respectively, which are significantly better than the target detection algorithm based on local contrast. In the testing of the near-space infrared detection system, the errors in the shortwave and longwave field of view tests were 2.5% and 5.2%, respectively, and the resolution test errors were 3.6% and 3.7%. The infrared detection system meets the performance standards, has high detection ability and stability, and provides a more effective solution for NSD.

Adsorption isotherm and kinetics of diffusion of water accumulated between polypropylene thin film and Si substrate: Neutron reflectivity investigation

Isotactic polypropylene (PP) thin films deposited on Si substrates were subjected to neutron reflectivity (NR) measurements under various humidity conditions to evaluate the isothermal adsorption of water accumulated between the PP layer and Si substrate, with focus on the temperature-dependence of the adsorption isotherm. The adsorption of accumulated water followed a single type II isotherm according to the Brunauer-Emmett-Teller (BET) classification, regardless of temperature. This can be attributed to the weak interactions between the PP layer and Si substrate, and between the PP layer and water molecules. Hydrophobic PP does not significantly interact with water molecules and the hydrophilic Si surface. Therefore, the interfacial water molecules are simply adsorbed on the native oxide layer on Si via simple interaction between the water molecules and the silicon oxide surface without being affected by the PP layer. Consequently, the adsorption isotherm of the accumulated water follows the single type II adsorption isotherm regardless of temperature, similar to the adsorption isotherm of water simply adsorbed on exposed silica surfaces. These weak interactions of the PP layer with the Si surface and water molecules also lead to fast diffusion kinetics for the accumulated water along the interface.

Low loss ridge waveguides on the hybrid single crystalline silicon and lithium niobate thin films

The hybridization of silicon (Si) thin film and lithium niobate (LN) thin film with remarkable properties, including the excellent optical properties of LN, excellent electrical properties and mature processing advantages of Si, can provide an important material platform for integrated photonic applications. In this work, a 4-inch single crystalline Si thin film wafer was prepared using wafer bonding, grinding and polishing techniques on LN thin film substrate. The films were characterized by using high-resolution transmission electron microscopy (HRTEM). The interplanar spacings and strain of Si film were calculated by analysis of high-resolution X-ray diffraction (HRXRD). Si ridge straight waveguides were fabricated on the hybrid thin films, and the transmission loss was 1.2 dB/cm. The near-field mode distribution of the waveguide was investigated by combining FDTD (Finite-Difference Time-Domain) simulation and end coupling measurement.

Double-Sided Bonding Process Enables X-ray Flat Panel Detectors.

Metal halide perovskites have demonstrated superior sensitivity, lower detection limits, stability, and exceptional photoelectric properties in comparison to existing commercially available X-ray detector materials, showing their potential for shaping the next generation of X-ray detectors. Nevertheless, significant challenges persist in the seamless integration of these materials into pixelated array sensors for large-area X-ray direct detection imaging. In this article, we propose a strategy for fabricating large-scale array devices using a double-sided bonding process. The approach involves depositing a wet film on the surface of a thin-film transistor substrate to establish a robust bond between the substrate and δ-CsPbI3 wafer via van der Waals force, thereby facilitating area-array imaging. Additionally, the freestanding polycrystalline δ-CsPbI3 wafer demonstrated a competitive ultralow detection limit of 3.46 nGyair s-1 under 50 kVP X-ray irradiation, and the δ-CsPbI3 wafer still maintains a stable signal output (signal current drift is 3.5 × 10-5 pA cm-1 s-1 V-1) under the accumulated radiation dose of 234.9 mGyair. This strategy provides a novel perspective for the industrial production of large-area X-ray flat panel detectors utilizing perovskites and their derivatives.

Colloidal carbon soot templated TiO2/Ag surface functionalized 3D printed metal brushes as new generation surface enhanced Raman scattering substrates

This present work demonstrated the functional transformation of 3D printed metal substrates into a new family of Surface-enhanced Raman Scattering substrates, a promising approach in developing SERS-based Point-of-care (PoC) analytical platforms. l-Powder Bed Fusion (l-PBF, Additive manufacturing or 3D printing technique) printed metal substrates have rough surfaces, and exhibit high thermal stability and intrinsic chemical inertness, necessitating a suitable surface functionalization approach. This present work demonstrated a unique multi-stage approach to transform l-PBF printed metal structures as recyclable SERS substrates by colloidal carbon templating, chemical vapor deposition, and electroless plating methods sequentially. The surface of the printed metal structures was functionalized using the colloidal carbon soot particles, that were formed by the eucalyptus oil flame deposition method. These carbon particles were shown to interact with the metals present in the printed structures by forming metal carbides and function as an adlayer on the surface. Subsequent deposition of TiO2 onto these templates led to strong grafting of TiO2 and retaining the fractal structure of the soot template onto the metal surface. Electroless deposition of silver nanoparticles resulted in the formation of fractally structured TiO2/Ag nanostructures and these functionalized printed metal structures were shown as excellent SERS substrates in enhancing the vibrational spectral features of Rhodamine B (RhB). The presence of TiO2 photocatalyst on the surface was shown to remove the RhB analyte from the surface under photochemical conditions, which enables the regeneration of SERS activity, and the substrate can be recycled. The migration of metals from the printed metal structures into the fractally ordered TiO2/Ag nanostructures was found to enhance the photocatalytic activity and increase the recyclability of these substrates. This study demonstrates the potential of 3D-printed Inconel metal substrates as next-generation recyclable SERS platforms, offering a substantial advancement over traditional colloidal, thin-film, flexible, and hard SERS substrates.

Facile fabrication of Au thin film substrates with crack-like nanogaps for surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has the advantage of nondestructive, quick, and accurate detection of harmful substances. Among the methods used to fabricate SERS substrates, a conventional evaporation equipment can achieve nanostructure of various shapes and sizes by adjusting the process pressure. In this study, general thermal evaporation equipment was utilized to fabricate SERS substrates. Au thin films with nanogaps were deposited on bare Si wafers with different process pressures and film thicknesses to vary the shape and size of the nanogaps. The microstructure was observed through scanning electron microscopy and transmission electron microscopy. In addition, the contact angle and reflectance of the Au thin films were measured. As the process pressure increased, the light reflectance decreased and the contact angle of the film increased. The SERS effect was found to be influenced by the width, depth, and number of nanogaps, reflectivity, and contact angle of the thin films. Optimal performance was observed under a process pressure of 13.3 Pa and film thickness of 215 nm. Under these conditions, the 2 µM rhodamine 6 G, allura red AC, and sunset yellow FCF were detected, and the uniformity and reproducibility of the thin film were also confirmed over a 6-inch substrate.

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.

Refining laser-induced dewetting for bimetallic Au–Pd nanoparticle synthesis on ZnO thin films: Optimizing fluence for substrate integrity

We report the fabrication of metal alloy Au–Pd nanoparticles on semiconductor thin film substrates (ZnO) by laser-induced dewetting. Employing a UV excimer laser, a single pulse was directed onto a three-layer film stack on a glass substrate: glass/ZnO/Au/Pd and glass/ZnO/Pd/Au. We simulated the temperature attained by the thin films enabling the prediction of energy thresholds required for melting the metal films but avoiding modifying the ZnO film. A specific range is reported of the pulse energy conducive to nanoparticle formation and the energy threshold required to modify the ZnO film beneath them. Depending on the pulse energy applied, the mean diameter of the nanoparticles varied from approximately 150 to around 70 nm. Notably, higher fluences resulted in smaller particles but also induced surface cracks in the ZnO film. Additionally, we observed a reduction in nanoparticle size with increased Pd content. Our results show that laser-induced dewetting can produce bimetallic alloy nanoparticles and, at the same time, ensure the preservation of the optical properties of the ZnO film. This approach opens avenues for tailoring material characteristics and expanding the range of applications of metal nanoparticles on semiconductor-based systems.

The growth and characterization of Au-catalyzed gallium oxide nanowires

Gallium oxide is a powerful and versatile wide band gap semiconductor, which has been gaining scientific interest in recent years due to its applications in power electronics. In this work, the growth of β-gallium oxide (Ga2O3) nanowires has been achieved by the vapor–solid–liquid method on silicon wafer, fused quartz, and gallium oxide thin-film substrates. In a chamber purged of ambient atmosphere, pure gallium was heated to produce gallium vapor, which was then oxidized by minute oxygen impurities in the backing gas to form gallium oxide nanowires on a substrate, inside a vertical tube furnace. On the silicon wafer substrates, β-Ga2O3 nanowires were observed to grow in a specific crystallographic direction via transmission electron microscopy. The resulting nanowires were highly faceted and had a unique polyhedral structure on the gold catalyst ‘cap’ of the nanowire. Time of growth and oxygen-level studies were conducted on nanowires grown on various substrates.Graphical abstract

The Influence of the Mechanical Compliance of a Substrate on the Morphology of Nanoporous Gold Thin Films.

Nanoporous gold (np-Au) has found its use in applications ranging from catalysis to biosensing, where pore morphology plays a critical role in performance. While the morphology evolution of bulk np-Au has been widely studied, knowledge about its thin-film form is limited. This work hypothesizes that the mechanical compliance of the thin film substrate can play a critical role in the morphology evolution. Via experimental and finite-element-analysis approaches, we investigate the morphological variation in np-Au thin films deposited on compliant silicone (PDMS) substrates of a range of thicknesses anchored on rigid glass supports and compare those to the morphology of np-Au deposited on glass. More macroscopic (10 s to 100 s of microns) cracks and discrete islands form in the np-Au films on PDMS compared to on glass. Conversely, uniformly distributed microscopic (100 s of nanometers) cracks form in greater numbers in the np-Au films on glass than those on PDMS, with the cracks located within the discrete islands. The np-Au films on glass also show larger ligament and pore sizes, possibly due to higher residual stresses compared to the np-Au/PDMS films. The effective elastic modulus of the substrate layers decreases with increasing PDMS thickness, resulting in secondary np-Au morphology effects, including a reduction in macroscopic crack-to-crack distance, an increase in microscopic crack coverage, and a widening of the microscopic cracks. However, changes in the ligament/pore widths with PDMS thickness are negligible, allowing for independent optimization for cracking. We expect these results to inform the integration of functional np-Au films on compliant substrates into emerging applications, including flexible electronics.

Analysis of Experimental Biaxial Surface Wrinkling Pattern Based on Direct 3D Numerical Simulation.

This paper presents a direct 3D numerical simulation of biaxial surface wrinkling of thin metal film on a compliant substrate. The selected compliant substrate is a commercial Scotch tape on which a gold metal thin film has been transferred by using low adhesion between the thin metal film and polyimide substrate. Compared with the previous fabrication of a cylindrical thin-film wrinkling pattern, an undulated wrinkling pattern has been implemented by increasing the width of the thin metal film in order to create biaxial straining in the thin film. To understand the wrinkling behavior due to biaxial loading, a simple direct numerical simulation based on material imperfections defined in the compliant substrate has been conducted. Through modeling and simulation, it was found that the wrinkling mode is determined by the biaxiality ratio (BR), the ratio between transversal strain and longitudinal strain. Depending on the BR, the wrinkling mode belongs to one of the cylindrical, undulated (or herringbone), checkerboard, or labyrinth modes as a function of applied strain. The cylindrical wrinkling is dominant at the input of BR less than 0.5, while the undulated (or herringbone) ones become dominant just after the onset of the wrinkling pattern at BR greater than 0.9. Through the comparison of the wrinkling patterns between simulation and experiment, the applied BR of the fabricated thin film has been successfully estimated.

Flexible Polycarbonate and Copoly(Imide-Carbonate)s-Based Frequency Selective Surface for Electromagnetic Shielding Application

Optically transparent polycarbonates (PCs) and Copoly(Imide-Carbonate)s (Co-PICs) were synthesized by the melt polycondenzation method. Rigid (imide) and flexible (-O- and –C(CH3)2−) moieties were incorporated in the structure of bisimide diol comonomer using 4-aminophenol and 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride). The structural properties of synthesized comonomers and polymers were confirmed by 1H, 13C-NMR and FT-IR spectra. Thermal properties of polycarbonates and copolycarbonates were examined using DSC and TG analysis. Thermal properties (glass transition Tg and thermal decomposition (Td) temperature) of copolymers were enhanced without sacrificing properties of BPA-based PC (high transparency, ductility, and processability) by the incorporation of active functional bisimide diol comonomer (5–10 mole %) in the polycarbonate backbone. Different sets of PCs and Co-PICs thin film substrates were prepared by the solvent casting method and used to design frequency selective surface. The proposed flexible FSS offers shielding of 20 dB at 8.8 GHz. In addition, the FSS offers polarization independent operation with its symmetrical unit cell geometry.