Ultrathin Films Research Articles

Formation mechanism of two-dimensional hexagonal silica on SiO2/Si substrate

Owing to their remarkable electronic properties, silica ultrathin films have been utilized as an insulating layer in nanoelectronics systems. Silica films have been epitaxially grown on different substrates using various synthesis methods. Among all fabrication approaches, chemical vapor deposition has long been an advanced method for synthesizing two-dimensional (2D) materials due to its ability to ensure precise stacking control and minimize contamination between layers. This study harnessed the potential of CVD to atomically fabricate thin layered 2D silica on a SiO2/Si substrate. Significantly, a unique combination of multiple transition metals and salt as the catalysts aided the formation of 2D silica for the first time. Salt is a crucial catalyst in promoting the evaporation of high-melting-point metal catalysts, resulting in hexagonal nucleation sites on the SiO2/Si wafer. By meticulously controlling growth parameters, a distinctive hexagonal structure was obtained. Correspondingly, this work delves into the growth mechanism of 2D silica, as evidenced by experiments involving salt alone and individual transition metals. Group VB transition metals played a prominent role in achieving the hexagonal structure compared to their group IVB counterparts. This research offers insight into the formation and growth mechanism of 2D silica, expanding the understanding of silica nanostructures.

Shear-Induced Fabrication of Cellulose Nanofibril/Liquid Metal Nanocomposite Films for Flexible Electromagnetic Interference Shielding and Thermal Management.

To address electromagnetic interference (EMI) pollution in modern society, the development of ultrathin, high-performance, and highly stable EMI shielding materials is highly desired. Liquid metal (LM) based conductive materials have received enormous amounts of attention. However, the processing approach of LM/polymer composites represents great challenges due to the high surface tension and cohesive energy of LMs. In this study, we develop a universal one-step fabrication strategy to directly process composites containing LMs and cellulose nanofibrils (CNFs) and successfully fabricate the ultrathin, flexible, and stable EMI shielding films with an average specific EMI shielding efficiency (EMI SE) value of 429 dB/mm and small thickness of only 70 μm in the wide frequency range of 8.2-18 GHz. In addition, the resulting films also exhibit excellent mechanical performance and flexibility, which endow the film with the ability to withstand repeated folding, bending, and folding into complex shapes without producing cracks or fractures. Besides, the resulting films display excellent thermal conductivity with a λ of 4.90 W/(m K) and an α of 3.17 mm2/s. Thus, the presented approach shows great potential in fabricating advanced materials for EMI shielding applications.

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets exhibits a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of ultra-thin tin films therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

Giant Spin-Orbit Induced Magnon Nonreciprocity in Ultrathin Ferromagnets.

The propagation characteristics of fermionic and bosonic quasiparticles determine the fundamental transport properties of solids and are of great technological relevance for designing logic devices. In particular, nonreciprocity, which describes that a quasiparticle flows preferably along a certain direction of a symmetry path, is an essential requirement to realize logic architectures, e.g., switches, diodes, transistors, etc. Here we introduce a mechanism, which leads to giant nonreciprocity of ultrafast terahertz magnons in ferromagnetic films with a large spin-orbit coupling. The mechanism is based on the competition between the exchange and spin-orbit scattering. We anticipate that the effect can be used to excite nonreciprocal or even unidirectional magnons in a large class of ultrathin films and nanostructures grown on substrates with a large spin-orbit coupling.

Study of domain wall dynamics in Pt/Co/Pt ultrathin films

Study of domain wall dynamics in Pt/Co/Pt ultrathin films

Improvement of electrothermal and photothermal properties of ultra-thin graphite film on oxygen plasma-treated polyimide substrate

Graphene and its derivatives are widely used in the field of energy conversion and management due to their excellent physical and chemical properties. In this paper, ultra-thin graphite film (GF) with thickness of 100–150 nm prepared by chemical vapor deposition was transferred to oxygen plasma-treated polyimide (PI) substrate as flexible heating film. The electrothermal and photothermal properties of GF on PI substrates with different treatment time were studied. The experimental results show that the PI substrate pretreated by oxygen plasma can change the surface morphology of GF, increase its electrical conductivity and light absorption capacity, and significantly improve the electrothermal and photothermal properties of GF heater. Under the low applied voltage of 5 V (power density of 0.81 W cm−2), the surface temperature of GF on 40 min plasma-treated PI substrate can rise to 250 °C, which is nearly 50 °C higher than that of GF on untreated PI substrate. When 100 nm thick commercial multilayer graphene film (MLG) is used, plasma-treated PI substrate can increase the electric heating temperature of MLG by 70 °C. In terms of photothermal performance, the surface temperature of GF on 50 min plasma-treated PI substrate can reach 73 °C under one Sun irradiation, which is 8 °C higher than that on untreated substrate. The experimental results are in good agreement with the simulation research. Our strategy has important implications for the development of efficient and energy-saving graphene/graphite-based heating films for advanced electrothermal and photothermal conversion devices.

Electrical Detection and Nucleation of a Magnetic Skyrmion in a Magnetic Tunnel Junction Observed via Operando Magnetic Microscopy.

Magnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

Towards transparent and durable copper-containing antimicrobial surfaces

Metallic copper is a potent antimicrobial agent with high efficacy against a range of microorganisms. Whilst it is commonplace for objects such as door handles and hospital bedrails to be made of or to be coated with copper, other high-touch surfaces such as those of screens and monitors would lose their optical transparency. Herein, we report the design of a transparent antimicrobial nanostructured copper surface obtained by dewetting ultrathin metal copper films on glass. Antimicrobial effectiveness of >99.9 % was obtained within 2 h against Staphylococcus aureus following the US Environmental Protection Agency Test method. Moreover, the proposed surfaces show an average transmittance between 70–80% with little color in the visible range. Additionally, they maintain optical and antimicrobial properties after abrasion and chemical exposure tests designed to replicate physical wear and repeated exposure to biocidal cleaning materials. With the demonstrated surfaces’ antimicrobial, colorless, and durability properties, they have potential in applications such as touch-enabled public or personal displays once higher transparency is achieved.

Enhanced control over size, areal density, and shape of substrate-supported Au and Ag nanoparticles by solid-state dewetting and alloying

The ability to manipulate the dimensions, areal density, and form of substrate-supported Au and Ag nanoparticles (NPs) is highly desirable for utilizing their plasmonic properties in biosensing, photovoltaics, and nanophotonic applications. The transformation of thin films into the substrate-supported nanostructures by solid-state dewetting (SSD), provides an avenue to manipulate the dimensional aspects of nanostructures simply and cost-effectively on a large scale. However, spontaneous agglomeration of the film produces randomly distributed and non-uniform nanostructures that must be controlled. Here, we have systematically studied the effect of annealing temperature, between 200 °C and 750 °C, on the dewetting morphology evolution of Au, Ag, and Au–Ag bilayer ultrathin films sputter deposited on the c-plane (0001) sapphire substrates. Regardless of the film thickness, Ag films dewet faster than Au films and produce spherical NPs, compared to faceted Au NPs, with broader size distribution. Whereas, by the SSD of Au–Ag bilayer ultrathin films, highly spherical and monodisperse AuAg bimetallic NPs can be fabricated. Furthermore, we have shown the possibility of fabricating the AuAg bimetallic NPs of varying compositions by adjusting the thickness of individual layers, thus enabling us to smoothly tune the spectral location of plasmonic resonance within the visible range.

Correlation between morphology and transport properties of Au/Co/Au/Si wedge ultra-thin film

We report the thermoelectric power (TEP) and electrical resistivity characterization supported by structural and surface morphological properties on a wedge shaped ultra-thin sample of Au/Co/Au/Si (100) with Co have varying thickness from 0.5 nm to 4 nm across the sample. The sample was prepared by Ion Beam Sputtering method, with an ultrathin layer of Co sandwiched between two layers of Au (3 nm thicknesses each). The observed TEP results showed that the system is semiconductor in nature and the concentration of the charge carrier is inversely proportional to the Seebeck coefficient (S) and also concentration of charge carrier modifies with the film thickness leading to a change in thermo power, which is found to be higher at lower thicknesses of Co sandwiched layer. The type of charge carrier was also determined to be electrons in this system. The observed results are in good agreement with the resistivity measurement carried out as a function of film thickness indicating a decrease in resistivity as a function of film thickness due to enhancement in charge carrier mobility. The thermal activation energy was seen to increase from ∼29.5 meV to ∼280.3 meV as the film thickness increased from the minimum to the highest. Corresponding Atomic Force Microscopy (AFM) measurement revealed island type grain growth with the introduction of a significant amount of roughness, while Magnetic Force Microscopy (MFM) experiments exhibit randomly oriented magnetic domain structures. The overall results suggest that lowest thickness film shows semiconducting nature, while variable range hopping phenomena due to some localized energy levels near the valence and conduction band is observed in the highest thickness film. This result is also well supported by XRD measurement, which revealed crystalline nature of the films. Such low cost, easy to fabricate materials may find applications in Peltier cooling or energy harvesting applications without the harmful carbon dioxide emission. For this, advance experiments will be done in future.

A combination of logical judging circuit and water-resistant ultrathin film PEDOT: PSS electrode for noninvasive ECG measurement

Heart disease-related deaths have increased in recent decades, with most patients dying of sudden cardiac arrest. In such instances, the effect of regular electrocardiogram (ECG) measurements is minimal. Therefore, long-term ECG monitoring has become increasingly important. In this paper, we report a non-adhesive high accuracy ECG monitoring system that can be used in various scenarios without interfering with daily activities. The ECG ultra-thin film electrode is made by water-resistant material based on poly(3,4-ethylenedioxythiophene) poly(4-styrenesulfonate) (PEDOT: PSS) electrode doped with ethylene glycol (EG) and xylitol, to improve the noise signal caused by sweat. The optimal ratio of the three ingredients of PEDOT: PSS/xylitol/EG was determined experimentally to accommodate the ECG monitoring. By using the proposed selectively closed multi-channel single-lead logic circuit, the noise of ECG signal received from the proposed film electrode can be successfully reduced during broad-area electrode measurements, thus to improve ECG measurement accuracy.

Wafer-Scale epitaxial molybdenum disulfide ultrathin film on sapphire prepared by low-energy reactive magnetron sputtering

Molybdenum disulfide, a two-dimensional semiconductor, has found broad applications in various functional devices. Currently, developing a reliable method for growing wafer-scale monolayer molybdenum disulfide presents a significant challenge in its integration into the contemporary semiconductor industry. This study employed low-energy reactive magnetron sputtering with hydrogen disulfide gas to deposit wafer-scale epitaxial few-layer and monolayer molybdenum disulfide on c-sapphire substrates. The epitaxial molybdenum disulfide nano-thin film was examined using scanning nano-probe X-ray diffraction technique, revealing a single grain size of no less than 50 × 50 μm2. Furthermore, the growth of monolayer molybdenum disulfide on both c-sapphire and thermal silicon oxide substrates was verified through Raman microscope. Measurements of photoluminescence and X-ray photoelectron spectroscopy also confirmed the monolayer state of the sputtered nano-thin films. This study suggests that reactive magnetron sputtering with hydrogen disulfide gas offers a cost-effective, scalable method for growing large-area molybdenum disulfide nano-thin films, thereby holding significant potential for practical semiconductor industrial applications.

Pushing the thinness limit of silver films for flexible optoelectronic devices via ion-beam thinning-back process

Reducing the silver film to 10 nm theoretically allows higher transparency but in practice leads to degraded transparency and electrical conductivity because the ultrathin film tends to be discontinuous. Herein, we developed a thinning-back process to address this dilemma, in which silver film is first deposited to a larger thickness with high continuity and then thinned back to a reduced thickness with an ultrasmooth surface, both implemented by a flood ion beam. Contributed by the shallow implantation of silver atoms into the substrate during deposition, the thinness of silver films down to 4.5 nm can be obtained, thinner than ever before. The atomic-level surface smooth permits excellent visible transparency, electrical conductivity, and the lowest haze among all existing transparent conductors. Moreover, the ultrathin silver film exhibits the unique robustness of mechanical flexibility. Therefore, the ion-beam thinning-back process presents a promising solution towards the excellent transparent conductor for flexible optoelectronic devices.

Precisely Controlled Polymerization of Two-Dimensional Conducting Polymers in Quasi-Liquid Layer Enables Ultrahigh Sensing Performance.

Gas sensors based on conducting polymers offer great potential for high-performance room temperature applications due to their cost-effectiveness, high-sensitivity, and operational advantage. However, their current performance is limited by the deficiency of control in conventional polymerization methods, leading to poor crystallinity and inconsistent material properties. Here, the quasi-liquid layer (QLL) on the ice surface acts as a self-regulating nano-reactor for precise control of thermodynamics and kinetics in the polymerization, resulting in a 7.62nm thick two-dimensional (2D) polyaniline (PANI) film matching the QLL thickness. The ultra-thin film optimizes the exposure of active sites, enhancing the detection of analyte gases at low concentrations. It is validated by fabricating a chemiresistive gas sensor with the 2D PANI film, demonstrating stable room-temperature detection of ammonia down to 10 ppt in ambient air with an impressive 10% response. This achievement represents the highest sensitivity among sensors of this kind while maintaining excellent selectivity and repeatability. Moreover, the QLL-controlled polymerization strategy offers an alternative route for precise control of the polymerization process for conducting polymers, enabling the creation of advanced materials with enhanced properties.

Multispectral stealth multilayer etched film structure based on ultrathin silver.

Spontaneous infrared radiation dissipation is a critical factor in facilitating object cooling, which influences the thermal stability and stealth efficacy of infrared stealth devices. Furthermore, the compatibility between efficient visible, infrared, and radar stealth is challenging due to different camouflage principles in different bands. This Letter presents a five-layer etched film structure to achieve multispectral stealth, and the utilization of the high-quality ultrathin silver films enables highly efficient infrared selective emission. This etched film structure with few layers demonstrates potential applications in diverse domains, including multi-band anti-detection and multispectral manipulation.

Establishing Quartz Crystal Microbalance with Dissipation (QCM-D) Coupled with Spectroscopic Ellipsometry (SE) as an Advantageous Technique for the Characterization of Ultra-Thin Film Hydrogels.

Despite the considerable significance of utilizing ultra-thin film (utf) hydrogels as multipurpose platforms for biomedical applications, there is still an important lack of adequate characterization techniques suitable for such materials. In this Perspective, the use of quartz crystal microbalance with dissipation (QCM-D) coupled with spectral ellipsometry (SE) is presented as a potential tool for the complete characterization of utf-hydrogels due to its nanometric sensitivity and high versatility. Herein, the fundaments for utf-hydrogel characterization are settled down as far as the QCM-D/SE response is explored under a wide range of different in operando wet working conditions measurements such as temperature or liquid media, among others. Therefore, the design of measuring protocols capable to perform is proposed and compiled, for the first time, complete and precise characterization of the cross-link density, thickness variations (swelling ratio determination), stability analyses, and mechanical studies (including the simultaneous generation of stress-strain curves and the evaluation of the viscoelastic; leading to the final determination of the Poisson's ratio) under different in operando conditions. Finally, the future challenges and implications for the general characterization of soft-thin films are discussed.

Transformation of a skyrmionium to a skyrmion through the thermal annihilation of the inner skyrmion

Skyrmioniums are doughnutlike topological spin textures in chiral magnets, which are able to move with zero skyrmion Hall effect. The annihilation of a skyrmionium involves the topological transition through singularity formation at the continuum level. Here we analytically and numerically study a type of thermal annihilation of a skyrmionium under the framework of micromagnetism. The skyrmionium initially exists in an ultrathin ferromagnetic film in the exchange-dominated regime and is stabilized under a uniform perpendicular magnetic field. The thermal annihilation of the “skyrmion” inside the skyrmionium results in the transformation of the skyrmionium to a skyrmion. We derive an analytical formula in the Arrhenius form describing the annihilation rate of this process. We find that the analytical solutions agree well with the micromagnetic simulation results. Besides, from the large deviation point of view, we derive the dynamical path for the collapse of a skyrmionium. Our results are useful for understanding the thermal stability of skyrmioniums and may lead to spintronic applications based on the thermal manipulation of skyrmioniums. Published by the American Physical Society 2024

Detection of Pseudomonas aeruginosa infection using a sustainable and selective polydopamine-based molecularly imprinted electrochemical sensor

Pyocyanin, a redox-active secondary metabolite produced by Pseudomonas aeruginosa, serves as a crucial virulence factor. Detection and quantification of pyocyanin can aid early diagnosis of infection. A selective and sensitive molecularly imprinted electrochemical sensor was constructed by using a green polymerization technique to deposit an ultrathin polydopamine film on an electrode modified with gold nanoparticles and chitosan. Target recognition was facilitated by specific binding sites within the imprinted polymer matrix that are complementary to the structure of pyocyanin. Various techniques, including cyclic voltammetry, electrochemical impedance spectroscopy, and square wave voltammetry, were employed to characterize the electrochemical behavior of the sensor. We investigated the influence of fabrication components, including chitosan concentration, monomer concentration, electro-polymerization conditions, pH, and rebinding time. Demonstrating a high degree of specificity and sensitivity, the sensor showcased a broad linear detection range of 1–100 µM and a low detection limit of 0.74 µM for pyocyanin. Moreover, the sensor successfully detected pyocyanin in real bacterial culture samples, exhibiting a recovery of the spiked standard ranging from 93 to 103 %. The electrochemical sensor displayed satisfactory stability lasting for at least 5 weeks. We demonstrated the sensor's applicability for clinical measurements by detecting pyocyanin in infected burn wounds using an ex vivo porcine skin model. Leveraging the synergistic advantages of molecularly imprinted polymer and the specific redox window for pyocyanin detection, the electrochemical sensor presents a promising approach for early-stage infection identification, thus contributing to enhanced treatment and more effective healthcare for patients.

Polarized-SERS of non-isotropic molecules on thermally-induced corrugated plasmonic surface supporting a NIR-SPP mode

Thermally-induced surface reconstruction of α-Al2O3 was used to fabricate corrugated templates of different periods to support percolated ultrathin metal film. Reflectance analysis and simulations revealed the coupling of a hybrid NIR-SPP mode along its corrugation vector (kcorrugation). The fabricated samples enhanced Raman signals of phenyl thiolates (PT) with reduced photo-induced disadvantages at λexc = 785 nm. Polarized SERS were performed to study adsorbed molecule – polariton interaction on these corrugated thin film surface. A twofold SERS anisotropy with a cos2(σ) dependency (σ = azimuthal angle) was observed. SERS dichroism of PT Raman modes of different polarizability tensors were analysed. A more pronounced dependence on corrugation period was observed for totally symmetric a1 modes than the non-totally symmetric b1 and b2 modes. Apart from the relative SERS intensity, their SERS dichroism was determined mostly by the excitation of SPP along the kcorrugation rather than the orientation of PT on sample surface. Edge-on orientation of adsorbed PT was predicted due to the absence of a2 modes with both tensor components parallel to the surface. Shorter corrugation periods fabricated were reproducible and ample for uniform, large-area sensing due to effective SPP coupling at σ = 0° resulting in a spatially extended SERS excitation volume.

Large-Area Conductive MOF Ultrathin Film Controllably Integrating Dinuclear-Metal Sites and Photosensitizers to Boost Photocatalytic CO2 Reduction with H2O as an Electron Donor.

Owing to the electrical conductivity and periodic porosity, conductive metal-organic framework (cMOF) ultrathin films open new perspectives to photocatalysis. The space-selective assembly of catalytic sites and photosensitizers in/on cMOF is favorable for promoting the separation of photogenerated carriers and mass transfer. However, the controllable integration of functional units into the cMOF film is rarely reported. Herein, via the synergistic effect of steric hindrance and an electrostatic-driven strategy, the dinuclear-metal molecular catalysts (DMC) and perovskite (PVK) quantum dot photosensitizers were immobilized into channels and onto the surface of cMOF ultrathin films, respectively, affording [DMC@cMOF]-PVK film photocatalysts. In this unique heterostructure, cMOF not only facilitated the charge transfer from PVK to DMC but also guaranteed mass transfer. Using H2O as an electron donor, [DMC@cMOF]-PVK realized a 133.36 μmol·g-1·h-1 CO yield in photocatalytic CO2 reduction, much higher than PVK and DMC-PVK. Owing to the excellent light transmission of films, multilayers of [DMC@cMOF]-PVK were integrated to increase the CO yield per unit area, and the 10-layer device realized a 1115.92 μmol·m-2 CO yield in 4 h, which was 8-fold higher than that of powder counterpart. This work not only lightens the development of cMOF-based composite films but also paves a novel avenue for an ultrathin film photocatalyst.