Progress and Perspectives in Nanostructured Thin Films

The field of nanostructured thin films is experiencing rapid development, with materials scientists creating ever more fascinating structures at the nanoscale or even subnanoscale. These new materials no longer operate as simple protective layers but form elaborate systems, whose unique features are developed precisely for advanced applications in mechanics, acoustics, magnetism, thermodynamics, optics, electronics, electrics, quantum states, topological states, and mixed feature exchanges [1,2,3,4,5]. The sustained attention that these systems have received can be explained by the unique properties they possess, such as improved mechanical strength, variable optoelectronic features, and outstanding surface-to-volume ratios, which tend to be significantly higher than those of bulk materials. These positive characteristics have led to their successful application in numerous technological fields, particularly renewable energy systems, fine electronic parts, and special sensing platforms [6,7,8,9,10]. In addition, the development of flexible electronic-films/skins (E-films/skins) and their deployment in medical physics and oncology highlights their status as a disruptive innovation assisting the delivery of accurate and personalized treatment of intractable diseases (e.g., cancers, diabetes). Of particular note are sensor-functionalized E-films/skins, which have the capability to simultaneously measure micro-environmental biomarkers (localized micro-environmental pH, hypoxia or blood glucose) that are essential in measuring tumor reaction or stages of diabetes and adjusting therapy regimens. As one of the pillars of advanced modern technologies, the wider application of thin films and/or coatings has been the focus of recent studies and is evolving rapidly to meet the demands of academicians and industrialists over the past few decades. Despite this, thin films and coatings are an indispensable technology and will always be the driving force behind advancements for the next generation of computing and information technology, new energy, biology and life science, new medicines, astronautics and aeronautics, geology and ocean engineering, military science, etc. [1].

Advancements in nanostructured thin film technology are associated with changes in fabrication methods. Though physical vapor deposition (PVD) and chemical vapor deposition (CVD) are already established methods, more recent developments in atomic layer deposition (ALD), molecular beam epitaxy (MBE), and pulsed laser deposition (PLD) allow atomic-layer control in the growth of complex oxide films, and have enabled breakthroughs in the study of correlated electron systems and quantum materials by allowing stoichiometric transfer and the real-time growth of complex oxide films. This has enabled unprecedented control of their composition and interface properties at the atomic scale [11,12]. These technological solutions have facilitated the creation of complex nanostructures, including quantum-confined systems and more complex and layered heterostructures that exhibit new physical characteristics and enhanced performance. The utilization of two-dimensional materials, in particular, the use of graphene and transition metal dichalcogenides has opened up new possibilities for the production of ultra-thin, flexible, and transparent functional thin films. In material science, thin film and/or coatings have been optimized by the simultaneous implementation of computational techniques [13]. This ongoing innovation is based on the reflection of the changing character of the field and the importance of monitoring modern research developments. Due to their unique nanoscale property, nanostructured thin films are utilized in cutting-edge electronics, biomedicine and energy applications. However, modern technology presents severe problems. Major drawbacks include the degradation of these interfaces under operational conditions, the inability to fully scale up their quantum properties, and the thermal or mechanic instability of integrated systems. Contemporary synthesis/fabrication processes yet present challenges related to deterministic defect engineering and multi-material integration at large scale. These barriers must be surmounted to achieve stable quantum computing, efficient perovskite–silicon tandem photovoltaics, stable flexible electronics, etc. The current Special Issue presents a well-curated collection of research articles and reviews highlighting the current progress of nanostructured thin films. The papers cover a broad range of topics within the field, including new synthesis/fabrication techniques, characterization methods, and applications, all of which demonstrate the potential of these materials to provide key solutions to modern technological problems.

Recently, Petallidou et al. [14] developed a single-step process to produce nanostructured palladium films by evaporation–condensation and inertial impaction. Their method enabled the anisotropic sintering of nanoparticles in a vertical form, which facilitated stable and highly sensitive hydrogen detection at an ambient temperature over a broad concentration range, from approximately 2 ppm to several percent. It is believed that the versatility of the technique presents vast potential for its further improvement and implementation in hydrogen sensing technology. In their work, Ajayeoba et al. [15] offer a solution to the key issues of two-dimensional materials by exploring molybdenum disulfide (MoS₂) thin films. The researchers applied a means of tuning the physical and chemical strengths of adsorption through deposition parameters, presenting a flexible and scalable approach to tailor a simple two-electrode electrodeposition method, with the ability to alter the microstructure and electronic properties of the film directly. In a significant achievement, they obtained adjustable forms of conductivity, whereby the p-type MoS₂ layers were obtained at the lower potentials of the cathode (1.15–1.35 V) and the n-type layers were obtained at higher cathodic potentials. This direct control of carrier type, and the possible use of MoS₂ thin films in some optoelectronic and gas sensing devices, without loss of the stability of their microstructures, presents huge practical potential.

Ebied et al. [16] conducted an extensive theoretical and experimental study of zinc phthalocyanine thin films (ZnTTBPc) in pursuit of their implementation in advanced photonic applications. Having combined Density Functional Theory (DFT) with experimental fabrication, their results supported the presence of the molecule with high non-linear optical capabilities, particularly, high hyperpolarizability. Spin-coating was used to achieve coverage of an amorphous nature, displaying a uniform nano-structural form. Some key results included the identification of direct optical band gaps of 1.42 eV (Q-band) and 2.25 eV (B-band) with significant nonlinear optical characteristics, such as high third-order nonlinear susceptibility. The high degree of correlation between the theoretical expectations and the actual outcomes depicts how these customized ZnTTBPc thin films have the potential to be high-performance materials in next-generation opto-electronic and photonic technologies. Another development, presented by H Luo et al. [2], is that of a new method of altering magneto-optical signals through engineered surface microstructures and hybrid organic/inorganic interfaces. Building arrays of micron sizes and different shapes on CoFeB thin films, the group found that micro-arrayed organic photonic crystal layers could effectively improve the magneto-optical Kerr effect (MOKE) together with the modification of the ferromagnetic strength and magnetization velocity by the metallic layers. The longitudinal MOKE signal was found to be improved twofold due to increased electron orbital coupling at the organic–inorganic interface. Their work showed that the form of the microstructure that characterizes the area of contact with the magnetic film is a key determinant of enhancement magnitude of MOKE signal. These results indicate new directions for the alteration of magneto-optical characteristics through the intentional planning of microstructures and hybrid organic–inorganic interfaces [2].

Chellamethu et al. [17] provided a very competent surfactant-assisted approach to adjusting the properties of SnO₂ nanostructured thin films. In their study, the authors achieved precise control of the architecture of the material by thoroughly varying the concentration of CTAB during hydrothermal synthesis. The optimal sample (SnO⁻⁵, achieved with 20 percent CTAB) experienced a dramatic conversion into a high-connected flower-like form, generating a larger surface area (132.70 m²/g) and increased defect density. The film performance was improved through these structural changes, resulting in a much smaller band gap (2.80 eV), an extremely low cut-in voltage (0.071 V), and the improvement of the charge transfer properties. The associated improvement in structural and electrical properties makes CTAB-engineered SnO₂ thin films remarkably useful in high-performance applications such as gas sensing, energy storage, and optoelectronics. Hamzah et al. [18] have solved a major issue associated with perovskite photovoltaics and presented alternative aqueous, surfactant-free, and simple-to-synthesize nickel oxide (NiOx) hole transport layers (HTLs). Their detailed study reveals that the temperature of calcination is a parameter of critical importance in the adjustment of film properties, with an optimal temperature of 300 °C resulting in the achievement of fine-grained and uniformly dispersed morphology, high crystallinity, and improved electrical properties, such as high carrier concentration. Their research offers a wide-ranging and extensively scalable protocol for the generation of a high-quality NiOx thin film, a technology that has great potential to improve the performance and stability of scalable, next-generation perovskite solar cells. As a thorough investigation of the scalable synthesis of complicated 3D nanostructures in tungsten oxide (WO₃) thin films using scalable spray pyrolysis, the work of Suchea et al. [19] is a notable contribution. The authors were able to use a systematic variation in layer thickness and precursor concentration to show the regular formation of unique nano-spheres and wall-like structures. Their main investigation found that the specified parameters have a direct impact on the surface morphology, crystallinity, and optical bandgap, and different concentrations of precursors lead to different evolutionary pathways of the film structure. The resulting 3D microstructures possess an abnormally large surface volume ratio, which makes these spray-deposited WO₃ films beneficial in improving the working and responsiveness of gas sensing, electrochromic, and photocatalytic systems.

Jafari et al. [20] established that the temperature of the substrate is a critical factor that determines the shape and properties of WS₂ thin films fabricated by Radio Frequency (RF) sputtering. Their interesting finding was that nanoparticles could change their morphological to nanosheet-shaped structures as the substrate temperature was increased to 200 °C. The sample (T200) had the greatest surface roughness, the largest crystallite size, the best crystallinity, and the smallest optical bandgap. In addition to the high electrical resistance of structural disorder and porosity, the synergistic achievement of high surface-to-volume nanosheet shape with high crystallinity presents a promising solution to increase the sensing performance of the gas-sensing devices as they were deposited at 200 °C. He et al. [21] made an important breakthrough in the preparation of titanium dioxide (TiO₂) films by applying a new reactive atmospheric plasma method. Through this approach, they successfully created a three-dimensional porous network of single-crystal sheets of pristine TiO₂ in a single step without templates. This accomplishment also included the generation of a crystal formation, displaying two highly reactive crystalline surfaces and a considerable concentration of in situ introduced oxygen defects. This unique arrangement produces very strong white photoluminescence with an internal quantum efficiency of 0.62, just like commercial fluorescent coatings. Their study also suggests a self-limiting mechanism of development that accounts for the process of the formation of such a well-arranged structure. Their contribution of an easy, one-step method of making modified and single-crystalline metal-oxide films presents new possibilities for its use in photocatalysis, solid-state lighting, and advanced optoelectronics. A programmed chilling microfluidic process was created by Hou et al. [3] to realize the synthesis of metal–metal compound super-atoms with atomical precision. This technique allowed the accurate control of super-atom size and the generation of a heterogeneous microstructure through the spatiotemporal control of reaction kinetics using Au as a core model and WO₃ as a shell coating model at subnanoscale. One of the stabilization mechanisms, which combined the passivation of molecular clusters of WO₃ and stabilizer protection, was also found to be very important in controlling sub-nanometer growth. Their work generates an avenue through which carrier-free super-atoms can be constructed in the future, with numerous potential applications, such as superatom medicine, superatom trnasistors or quantum processors, high efficient superatom catalysts.

Wang et al. [4] used a DFT-CE-MC or computational model to create CO-tolerant Pt-Fe alloy catalysts, clearly showing that the top three layers of atoms significantly affect catalytic performance. Through simulations, a periodical Pt₂Fe surface phase with a predicted stabilization at a Pt content of 24% was predicted, which intrinsically reduces the d-band center of Pt so as to weaken the adsorption of CO, which theoretically determines the optimized atom layers (thickness) for the future atomic scale thin films in catalytic reactions. However, strain due to defects in this stage causes orbital rehybridization, reinforcing Pt-CO bonding, as indicated by COHP. Therefore, the best approach to maximizing resistance to poisoning is to maintain the structure of the Pt₂Fe surface but fine-tune its defect density to a minimum. The developed methodology connects atomic-scale electronic properties with mesoscale structural evolution, which provides a broad base for catalyst design. Ali et al. [5] conducted a literature review of the synthesis and use of multi-functional gold-doped/coating lanthanide nano-particles (Au-Ln NPs) to enhance photo-dynamic therapy (PDT) of cancer. The nano-particles combine both the surface plasmon resonance of gold and the up-conversion luminescence of lanthanides to achieve deep tissue penetration and the efficient production of cytotoxic species under near-infrared light. The ability to overcome major challenges in synthesis, such as the attainment of a uniform distribution of gold, control of the size and shape of particles, and reproducibility to scale-up, in order to maximize the therapeutic efficiency, is one of the primary focuses of this research. Moreover, key obstacles to application, including the increase in light absorption efficiency, the stability of nanoparticles under physiological conditions, and extensive toxicity and bio-distribution analysis, are found to be crucial factors in successful clinical translation. The future of this modality is dependent on innovations in controlled synthesis and high-level surface engineering to develop uniform and stable nanoplatforms that are biocompatible.

A magneto-plasmonic resonator was made by Wang et al. [22] using hexagonally arrayed Au/bismuth iron garnet (BIG) bilayer nano-disks on a Au film. Simulations based on multi-physics showed that a strong interaction between waveguide and surface plasmon modes led to the significant enhancement of the transverse magneto-optical Kerr effect (TMOKE). Most importantly, a new circular oscillating plasmon wave was excited in the BIG layer (22 nm thick), and a nanoscale near-field interference effect was found to have enhanced the TMOKE. The unique magneto-plasmonic response of the structure afforded it very high sensitivity in terms of calculated refractive index; its figure of merit (FOM) of 7527 RIU⁻¹ attests to its great potential in higher optical sensing sensibilities. A major challenge in nanotechnology that Alshehri et al. [23] discussed in their review article is the scalable and systematic assembly of nanoparticles into nanoparticle doped thin films for larger applications. The literature review concentrates on two promising methods, blown bubble films (BBFs) and the bubble deposition method (BDM), with emphasis on their capability in the production of highly aligned and dense nanomaterial thin films on solid and flexible surfaces. Their analysis highlights that such processes are not only scalable but also cost-effective and address critical requirements of device manufacturing, such as homogeneity and operational speed. These bubble-based assembly strategies offer a simple combination of nanomaterials that have unique characteristics, which can offer a means of achieving enhancements in electronics, energy, and sensing technologies. In their work, Pan et al. [24] present an innovative hydrogen-bonded organic framework (HOF), designed to solve historical difficulties in addressing polyamine metabolism with cancer therapy via surface ligand modification of nano-entities. Synthesized through microfluidics, the nano-platform is tumor-targetable, modified with a cancer cell membrane, and co-expresses polyamine-depleting enzyme plasma amine oxidase (PAO) and CRISPR-Cas9 machinery. The internalization of PAO by tumor cells is an efficient way of depleting polyamines to reorganize the immunosuppressive tumor micro-environment and to release T-cell inhibition. Moreover, polyamine degradation byproducts, acrolein and H₂O₂, cause a two-fold reaction; carbonyl stress enhances mutational load, whereas a CRET-mediated response triggers the elimination of CRISPR-Cas9 DNA repair gene, increasing the production of neoantigens. This synchronized approach activates the dendritic cells to present antigens in a synergistic manner and increases T-cell cytotoxicity, ultimately triggering the cancer-immunity cycle. The platform of the HOF exhibited more stability and reactivity than traditional frameworks and became a new paradigm in immunotherapy, incorporating metabolic regulation and genomic editing to create powerful and accurate anti-tumor activity.

Rahman et al. [25] overcome the constraints of conventional characterization techniques to uncover defects in CVD-grown hexagonal boron nitride (hBN) by adopting a YOLOv8n deep neural network to identify issues. The primary result of the study is that the model has great potential to detect problems of different complexities, proving the possibility of the application of machine learning to simplify quality assessment. One important observation, however, was the existence of a precision–recall trade-off, especially between fine cracks and multilayer regions, which means that more refinement is required by either augmenting the data or evenly distributing the classes (i.e., by class balancing). This work was conducted on the basis that automated ML methods can be used to speed up the reliable incorporation of 2D materials, such as hBN, into high-tech devices. DuDek et al. [26] were able to functionalize a NiTi alloy surface by making hybrid TiO₂-SiO₂ nanostructured layers, which were deposited through electrophoretic deposition (EPD). The study determined a stable colloidal suspension of EPD at a pH level of 6, with the best layer uniformity obtained at 40 V of deposition in 3 min. One discovery was that further heat treatment at 800 °C for 2 h produced a significant structural change that yielded a new material, where strong Si-O-Ti bonds and a distinct interlayer were formed. This procedure illustrates a practical approach to the production of complex multi-functional coatings on NiTi alloys, and the final annealed structure was notably distinct from the materials deposited on it.

Yang et al. [27] address an important gap in the research on fundamental physics of topological quantum materials and their application in terahertz (THz) technology. Although the characteristic excitations of materials such as the noncollinear antiferromagnetic Weyl semimetal Mn₃Sn have been known to exist at the THz regime, the nanoscale architectures of the same materials are not well investigated. This is achieved in this study through the pioneering use of Ostwald ripening to prepare oriented Mn₃kSn nanostructures on sapphire. The main result is a giant, anisotropic THz response which is directly actuated by the designed spin texture, with the perpendicular-oriented films showing a large magnetic field modulation. More importantly, the topological characteristics are maintained at the nanoscale. These findings provide nanostructured Mn₃Sn with a promising platform, which proves that the combination of surface spin states and kagome-formed topology is a new channel through which to create active, subwavelength THz devices. In their contribution, Hou et al. [28] illustrate the mechanical breakdown of conventional flexible electronic thin films subjected to large strain as a major constraint to state-of-the-art flexible electronic thin films. The review assumes that the next important step in their development involves the creation of flexible stretchable electrodes that do not lose their conductivity during deformation. It integrates developments through a three-pronged focus on materials, fabrication, and structural designs, and links these developments to the use of electronic skin and wearable biomedical devices. The three fundamental challenges identified in the analysis are that, despite a solid underpinning, this area still needs proper environmental and tensile stability, device interfaces that are reliable, and the ability to scale up the range of integrated systems. The future development of the field thus depends on the convergence of disciplines in order to co-optimize materials, electronics, and mechanics to integrate functional devices.

Si et al. [29] describe the actual performance gap in microwave-absorbing materials (MAMs). The critical performance gap is their low-frequency regime (1–8 GHz), and metal–organic framework (MOF)-derived composites are seen as one of the best solutions due to their ability to be structurally (as well as compositionally) tuned. It is demonstrated that the necessary condition to achieve effective broadband absorption at these frequencies is a good match of impedance, which is typically achieved by a considered combination of magnetic loss elements to counteract the low permeability usually observed. Three key design principles are proposed based on the current advancements. The first is the engineering of nanoscale materials to achieve the requirements of a single-domain resonance and higher polarization of interfaces. The second is the utilization of multi-metal ion and material hybrids to produce a superposition of absorption peaks. The final recommendation is the fine-tuning of synthesis parameters such as sintering temperature. The discussion concludes with the assertion that even though MOF-based systems provide an obvious route to the achievement of high-efficiency MAMs of low frequencies, the optimization of their fabrication and interfaces should still be systematic in order to transform these principles into high-performance, reliable materials. Zhang et al. [30] fabricate the magneto-optical Kerr effect (MOKE) in nanoporous thin films using layered structures. CoFeB and W/CoFeB/W sandwich films and nanoporous CoFeB and W sandwich films were produced through sputtering on anodic aluminum oxide templates. The experiment shows that by adding non-ferromagnetic tungsten (W) layers, it is possible to experimentally control Kerr null points, the inversion points of Kerr rotation, which are otherwise determined by the intrinsic magnetization of the CoFeB layer. The CoFeB/W structure was shown to be able to increase the maximum susceptibility and saturated Kerr rotation by a factor of 2.5 and 2.8, respectively, compared to pure CoFeB, due to the high spin-orbital coupling effects caused by W layers. Theoretical modeling shows that these null points are based on the interaction of the in-plane magnetization and the optical interference, while the W layers control the latter. These findings present the potential for the creation of a technique of tunable MOKE and phase modulation, leading to the development of nanoporous film design to use in ultrasensitive magneto-optical sensing and high-resolution magnetometry.

Our Special Issue provides a summary of developments regarding nanostructured thin-film technology, with particular focus on their synthesis, characterization, and application. The featured studies demonstrate that the design of customizable nanoscale structures can be used to enhance functionality in energy systems, sensors, and optoelectronics. Some notable trends in the research include a shift in the fabrication approaches to considerations of sustainability, a blend of both computational and experimental approaches and a growth in the application of multifunctional systems, and the development of atomically precise, particularly atomically resolved, coatings of nanoscale or even subnanoscale entities for peculiar characteristics [1,2,3,4,5]. Such developments seek to resolve existing problems and generate new directions of innovation in flexible electronics and quantum technology. These contributions form a foundation for subsequent progress in this rapidly developing field. The emerging growth of nanostructured thin films is bound to have a tremendous impact on a diverse range of technical applications, assisting the generation of innovative solutions to global problems in the energy and manufacturing sectors, as well as those of the environment.