Nanometric Films Research Articles

High-Sensitivity Microthermometry Method Based on Vacuum-Deposited Thin Films Exhibiting Gradual Spin Crossover above Room Temperature.

We report on the fabrication, characterization, and microthermometry application of high-quality, nanometric thin films, with thicknesses in the range 20-200 nm, of the molecular spin-crossover complex [Fe(HB(1,2,3-triazol-1-yl)3)2]. The films were obtained by vacuum thermal evaporation and characterized by X-ray diffraction, UV spectrophotometry, and atomic force microscopy. The as-deposited films are dense and crystalline with a preferred [011] orientation of the monoclinic crystal lattice normal to the substrate surface. The films exhibit a gradual spin conversion centered at ca. 374 K spanning the 273-473 K temperature range, irrespective of their thickness. When deposited on a microelectronic device, these films can be used to enhance the UV-light thermoreflectance coefficient of reflective surfaces by more than an order of magnitude, allowing for high-sensitivity thermoreflectance thermal imaging.

Nanometric Amorphous TiO2 As an Extrinsic Pseudo-Capacitive Electrode for All-Solid-State Asymmetric Hybrid Micro-Supercapacitors: Effects on the Electrochemical Performance

On-chip solid-state micro-supercapacitors (MSCs) have gained widespread attention owing to their versatile benefits, including high power density and long cycle life1,2. Recently asymmetric hybrid microsupercapacitors (AsHMSCs) introducing a charge storage mechanism based on both faradaic reaction and electric double-layer phenomena have been considered to increase the energy density. Within this context, the selection of electrode materials and their interaction at the interface with the electrolyte is a key issue to optimize the AsHMSC electrochemical performance.In this work, nanometric (5-20 nm thick) amorphous TiO2 films have been elaborated and characterized in liquid and solid-state electrolyte (LiPON) half-cell configurations. Interestingly, TiO2 films after LiPON deposition exhibited a constant initial amount of intercalated lithium ions for all considered thicknesses and did not require a first activation process, in comparison to the liquid electrolyte configuration. For all considered configurations, the specific capacity extracted from cyclic voltammetry and galvanostatic cycling within [0.5-3V] potential range correspond to the theoretical value expected for the Lix=1TiO2 phase at low current density. Furthermore, the cooperative effects of high Li ion intercalation kinetics and Low interfacial charge transfer resistance for 5nm TiO2 electrode (extracted from electrochemical impedance spectroscopy analysis) exhibit an outstanding specific capacity of 2mC.cm-2 at 1µA.cm-2, and high rate performance with 60% capacity holding ratio at 100µA.cm-2, thus highlighting the extrinsic pseudo-capacitive behavior of the sub 10nm electrodes.A TiO2 5nm/LiPON 100nm/Pt AsHMSC is successfully fabricated, which achieves an operating voltage window of 3V and a specific capacity of 0.3mC.cm-2 at 1mA.cm-2.In addition, the device also exhibited a 100% coulombic efficiency even after cycling for 6000 continuous charge-discharge cycles. This work offers an approach to tune the Li ion storage properties of TiO2 by nano-engineering and gives insights into the enhancement of pseudocapacitance-assisted lithium-storage capacity.References Kyeremateng et al., Nature Nanotech 12, 7–15 (2017). https://doi.org/10.1038/nnano.2016.196Sallaz et al., J. Power Sources 451, 227786 (2020), doi: 10.1016/j.jpowsour.2020.227786. Figure 1

Tridimensional N, P-Codoped Carbon Sponges as Highly Selective Catalysts for Aerobic Oxidative Coupling of Benzylamine.

Two tridimensional N-doped porous carbon sponges (3DC-X) have been prepared by using cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB) as soft templates and alginate to replicate the liquid crystals formed by CTA+ in water. Alginate is a filmogenic polysaccharide of natural origin having the ability to form nanometric defectless films around objects. Subsequent pyrolysis at 900 °C under an Ar flow of the resulting CTA+–polysaccharide assemblies result in 3DC-1 and 3DC-2, with the N percentages of 0.48 and 0.36 wt % for the materials resulting from CTAC and CTAB, respectively. Another four 3DC materials were obtained via pyrolysis of the adduct of phytic acid and chitosan, rendering an amorphous, N and P-codoped carbon sample (3DC-3 to 3DC-6). The six 3DC samples exhibit a large area (>650 m2 × g–1) and porosity, as determined by Ar adsorption. The catalytic activity of these materials in promoting the aerobic oxidation of benzylamine increases with the specific surface area and doping, being the largest for 3DC-4, which is able to achieve 73% benzylamine conversion to N-benzylidene benzylamine in solventless conditions at 70 °C in 5 h. Quenching studies and hot filtration tests indicate that 3DC-4 acts as a heterogeneous catalyst rather than an initiator, triggering the formation of hydroperoxyl and hydroxyl radicals as the main reactive oxygen species involved in the aerobic oxidation.

Remarkable Activity of 002 Facet of Ruthenium Nanoparticles Grown on Graphene Films on the Photocatalytic CO2 Methanation

AbstractIn the context of diminishing atmospheric CO2 emissions, there is an urgent need to develop processes that can be carried out at a scale commensurate with appropriate CO2 volumes. One possible reaction is the transformation of CO2 to methane (Sabatier reaction). Due to its chemical stability, catalytic CO2 hydrogenation to methane is carried out at temperatures of 450 °C or higher and pressures above 5 bars, thus, requiring a significant energy input. One alternative possibility to conventional thermal catalysis is the use of solar light as the primary energy, performing the photocatalytic CO2 hydrogenation. In this broad context, the present study shows the photocatalytic activity of nanometric films of oriented Ru nanoparticles (NPs) strongly grafted on defective graphene. These graphene films (thinner than 20 nm) containing Ru NPs nanoplatelets (less than 2 nanomolRu/cm2) are among the most active photocatalysts ever prepared for CO2 hydrogenation and operate through photoinduced charge separation.

Self-Referenced Optical Fiber Sensor Based on LSPR Generated by Gold and Silver Nanoparticles Embedded in Layer-by-Layer Nanostructured Coatings

In this work, an optical fiber sensor based on the localized surface plasmon resonance (LSPR) phenomenon has been designed for the detection of two different chemical species (mercury and hydrogen peroxide) by using Layer-by-Layer Embedding (LbL-E) as a nanofabrication technique. In the first step, silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) have been synthesized by using a chemical protocol as a function of the strict control of three main parameters, which were polyelectrolyte concentration, a loading agent, and a reducing agent. In the second step, their incorporation into nanometric thin films have been demonstrated as a function of the number of bilayers, which shows two well-located absorption peaks associated to their LSPR in the visible region at 420 nm (AgNPs) and 530 nm (AuNPs). Finally, both plasmonic peaks provide a stable real-time reference measurement, which can be extracted from the spectral response of the optical fiber sensor, which shows a specific sensing mechanism as a function of the analyte of study.

Enhanced Surface Passivation of Subnanometer Silicon Dioxide Films by Superacidic Treatments

Subnanometer-scale silicon dioxide (SiO2) films are frequently present before, during, and after silicon device processing, yet they offer minimal surface passivation and can detrimentally impact subsequent processing steps. Here we develop a process whereby the surface passivation of nanometer and subnanometer SiO2 films is enhanced by up to 2 orders of magnitude by a simple room temperature treatment using the superacid bis(trifluoromethane)sulfonimide (TFSA, sometimes TFSI). By accurately modeling the effective lifetime curves corresponding to the superacid treated SiO2 samples, we have determined that the enhanced passivation is mainly due to a reduction in the interface defect density (Dit) at the Si/SiO2 interface, with a minor contribution also arising from the presence of negative charge. X-ray photoelectron spectroscopy of the treated SiO2 films reveals the presence of fluorine, and this, along with hydrogen, is a strong candidate for the chemical passivation of defects at the Si/SiO2 interface. Post treatment, the SiO2 films show short time scale electronic instability, whereby a degradation and then recovery are observed over a period of 1–10 h which is attributed to variations in the Dit, as determined from our analysis of the injection-dependent lifetime data. Following the instability period, the surface passivation remains relatively stable for days. Nuclear magnetic resonance measurements of superacid-based solutions reveal that electron-donating solvents should be avoided, as they exacerbate surface passivation instabilities. The results presented demonstrate that simple strategies can be used to enhance the passivation properties of ultrathin films greatly, which in the age of nanotechnology could offer benefits to device performance in a range of applications including solar cells and batteries.

Design and energy analysis of tunable nanophotonic infrared filter based on thermochromic vanadium dioxide

In this work, a mid-infrared tunable nanophotonic filter is designed by placing a silicon spacer layer between two nanometric vanadium dioxide films on a thick infrared-transparent substrate. Transfer matrix formulation incorporated with the ray-tracing method is used for the optical modeling, and a transmittance peak near 4 µm wavelength is achieved based on Fabry–Perot resonance effect. Thermochromic vanadium dioxide allows the filter to be switched between a semi-transparent state when cooled below its phase transition temperature and an opaque state when heated above its phase transition temperature. The underlying mechanism is elucidated by calculating the total phase shift of electromagnetic waves through the spacer layer, and the effect of layer thicknesses on the spectral transmittance peak is discussed. The filter can find potential applications in energy dissipation where it can actively promote or suppress radiative cooling, and in thermal camouflage where the apparent temperature of a hot surface through the filter can be greatly reduced. Detailed energy analysis is conducted for both applications with the proposed tunable filter, whose structure is further optimized through the thicknesses of the thin-film layers to achieve maximum performance.

Cathodoluminescence of intrinsic defects in films La : HfZrO

Lanthanum-doped (La:(HfZr)O2) nanometer films of a solid solution of hafnium oxide and zirconium oxide are of great interest for the development of a universal memory that combines an unlimited number of RAM reprogramming cycles and nonvolatile flash memory. This work is devoted to studying the cathodoluminescent properties of La : HfZrO thin films with different contents of lanthanum. It is shown that the cathodoluminescence spectra are dominated by two emission bands with intensity maxima at 2.7 and 2.2 eV. The blue band with an energy of 2.7 eV is due to an oxygen vacancy in La : HfZrO. The study of the influence of the lanthanum impurity and annealing of the samples in argon suggests that the yellow band with the emission maximum at 2.2 eV is related to the oxygen divacancy. Keywords: luminescence, hafnium oxide, zirconium oxide, oxygen vacancy.

Катодолюминесценция собственных дефектов в пленках La : HfZrO

Lanthanum-doped (La:(HfZr)O2) nanometer films of a solid solution of hafnium oxide and zirconium oxide are of great interest for the development of a universal memory that combines an unlimited number of RAM reprogramming cycles and nonvolatile flash memory. This work is devoted to studying the cathodoluminescent properties of La:HfZrO thin films with different contents of lanthanum. It is shown that the cathodoluminescence spectra are dominated by two emission bands with intensity maxima at 2.7 and 2.2 eV. The blue band with an energy of 2.7 eV is due to an oxygen vacancy in La:HfZrO. The study of the influence of the lanthanum impurity and annealing of the samples in argon suggests that the yellow band with the emission maximum at 2.2 eV is related to the oxygen divacancy.

Ultrathin atomic layer deposited niobium oxide as a passivation layer in silicon based photovoltaics

Atomic layer deposited (ALD) niobium oxide (NbOx) films were investigated for their passivation properties through minority carrier lifetime measurements and compared to a well-known passivating material, aluminum oxide. ALD alumina is known to passivate by a combination of field-effect passivation from fixed charges and chemical passivation from hydrogenation of dangling bonds. It was hypothesized that niobium oxide films passivate by varying degrees of the same mechanisms found in alumina. The effects of ALD oxygen source (water or ozone) and varying anneal temperatures were correlated to passivation quality. Lifetimes of sub-1 nanometer films were specifically investigated. It was found that water is a superior oxidant for passivation relative to O3. Thermally activated NbOx films deposited with water have near equivalent or superior lifetimes to alumina of the same thickness after equivalent annealing at temperatures up to 350 °C. Chemical analyses by x-ray photoelectron spectroscopy (XPS) were used to investigate the suspected mechanisms of passivation. It was suggested that field-effect passivation is the dominating mechanism in NbOx films based on correlations between band movement as probed by XPS and the lifetime data presented in this work. This work provides new insights into the applicability of niobium oxide as a passivating selective contact for silicon photovoltaics with the goal of reaching new record efficiencies in solar cells.

Glass transition of nanometric polymer films probed by picosecond ultrasonics

The possibility to measure the glass transition temperature in poly(methyl methacrylate) (PMMA) films by picosecond ultrasonics with thicknesses ranging from 458 nm to 32 nm is demonstrated. A shift of the longitudinal acoustic eigenmodes towards lower frequencies with temperature is observed accompanied by a change in the temperature-frequency slopes at the glass transition temperature. The contributions to the frequency shift from changes in film thickness and sound velocity are discussed and the latter is extracted below the glass transition temperature. Finally, the advantages and disadvantages of the current approach in a comparison to other methods based on acoustic measurements in the GHz regime are reviewed.

Modification of Steel Surfaces with Nanometer Films of Al2O3 and TiO2 Decreases Interfacial Adhesion to Polymers: Implications for Demolding Shape-Engineered Polymer Products

Alumina (Al2O3) and titanium dioxide (TiO2) thin films were deposited by the atomic layer deposition technique on steel substrates used in the polymer injection molding industry. The modified steel surfaces were systematically characterized by SEM, EDS, AFM, XPS, and OCA. Surface energy values were obtained to predict the demolding behavior of modified steel surfaces when in contact with different polymers. The lowest surface energy was obtained using Al2O3 and H2O as an oxygen source on 1.2311 steel (1.2311-Al-H), representing a 30% decrease when compared to 1.2311 bare ground steel. Results were confirmed by the simulation of polymer–mold interaction. The produced polycarbonate surface presents defects with an average diameter of 0.5 μm that result from mimicking of the Al2O3 textured surface (contrasting with the defects with a diameter of 1.85 μm using bare 1.2311 steel). The best compromise between surface energy and roughness was obtained using the TiO2-H2O film (1.2311-Ti-H). The modified steel with TiO2 ALD coating produced the smoothest polymeric surface. The modification of steel surfaces with nanometer films of Al2O3 and TiO2 decreased the interfacial adhesion to polymers, which had implications for demolding shape-engineered polymers, a requirement for high-aspect-ratio parts.

Electrochemical Potential‐Driven High‐Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

AbstractMolecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current‐voltage (I‐V) analysis, which is often termed “molecular electronics”. Recently, there has been also an upsurge of interest in spin‐based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin‐films on suitable electrodes, the electrochemical potential‐driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non‐magnetic and magnetic electrodes to investigate the stimuli‐responsive charge and spin transport phenomena.

Electrochemical Potential-Driven High-Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications.

Molecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current-voltage (I-V) analysis, which is often termed "molecular electronics". Recently, there has been also an upsurge of interest in spin-based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin-films on suitable electrodes, the electrochemical potential-driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non-magnetic and magnetic electrodes to investigate the stimuli-responsive charge and spin transport phenomena.

Investigating the Ubiquitous Presence of Nanometric Water Films on Surfaces

When we speak of nanometric water films on surfaces we are speaking about a truly ubiquitous phenomenon in nature. All surfaces exposed to ambient conditions are covered by a thin film of water that affects or mediates surface chemistry, general physical-chemical processes on surfaces, and even solid–solid interactions. We have investigated this phenomenon for over a decade by exploiting dynamic atomic force microscopy and have (1) described how these layers affect apparent height measurements, (2) analyzed the excitation of subharmonics, (3) investigated its effects on surface functionality over time (“aging”), (4) monitored and quantified the time-dependent wettability of several relevant surfaces such as highly oriented pyrolytic graphite and monolayer systems, and (5) developed high-resolution and highly stable modes of imaging. Here, we discuss these findings to elucidate the present and future of the field. We further provide a brief but general discussion of solvation and hydration layers in vacuum, liquid, and air that center around current controversies and discuss open possibilities in the field.

TEOS thin films obtained by plasma polymerization on Ti6Al4V alloys: Influence of the deposition pressure on surface properties and cellular response

The modification of surfaces by the application of thin films has been used in the regenerative medicine area to increase the biocompatibility of metal implants. Titanium alloy has been recently used as substrate in polymerization for biomedical application. In this context, silane films were obtained by plasma polymerization in favor of the sol-gel method and the influence of different pressures in obtaining these films by argon plasma polymerization was evaluated from the alkoxysilane precursor tetraethoxysilane (TEOS) on the Ti6Al4V alloy. The morphological characterization of the films was performed by AFM, Profilometry and Spectral Ellipsometry and the chemical composition was analyzed by XPS. The biological behavior was evaluated by analyzing the mitochondrial activity and cellular viability of mesenchymal stem cells. The plasma polymerization process resulted in the deposition of a nanometric Si-based film formed, predominantly, by Si-O and organosilane bonds. The films that were applied on a sanded surface, with lower pressures in the plasma polymerization process, presented a lower layer thickness and wettability than the films obtained on nanotextured surfaces. Considering absorbance values, the Ti6Al4V samples mechanically sanded and deposited by plasma polymerization at 230 µatm presented better cell viability than samples with nanotextured surfaces coated with plasma polymerized film, indicating this material has potential to biomedical application.

Sensing Performance of Fiber-Optic Combs Tuned by Nanometric Films: New Insights and Limits

The deposition of high refractive index (HRI) nanocoatings has become one of the most effective techniques used to underpin light-matter interaction, and hence to optimize sensing performance of fiber optic devices towards single molecule detection. However, it is not always that true. Herein, we report on the general polarization-dependent sensing performance of nanocoated tilted fiber Bragg gratings (TFBGs), with the results demonstrating for the first time that the sensitivity of both P- and S-polarizations can be improved, but also greatly reduced after depositing HRI nanocoating. The investigations were performed for both high- and low-order modes excited in 10° and 4° TFBGs, respectively, coated with nanometer scale metal oxide film using the full-vector complex coupled mode theory (CCMT). The results reveal that the general sensing features significantly stem from the enhanced polarized dispersion properties tuned by the nanocoating through the mode transition. The analysis of evanescent field, dispersion characteristics, and spectral response clearly confirms the improvement and deterioration of both local and general sensing performances. Due to the enhanced polarization difference, the P-polarization usually presents higher sensitivity to bulk/volume RI changes, while the S-polarization could become more sensitive to surface changes, which enables the P-/S-polarization preferable for bulk/surface refractometric sensing applications. This thorough study reveals a new insight on nanocoated fiber optic sensors and turns out to be decisive in the performance optimization, which paves the way to applications in both bulk and surface RI-based sensing.

Dithiol Self-Assembled Monolayer Based Electrochemical Surface Plasmon Resonance Optical Fiber Sensor for Selective Heavy Metal Ions Detection

Herein, we report the simultaneously application of surface plasmon resonance (SPR) spectroscopy and electrochemistry (EC) technology for the real-time quantitative investigation of heavy metal ions, with high selectivity and sensitivity. A surface of optical fiber coated over with nanometric gold film and then modified with a dithiol self-assembled monolayer of 1,6-hexanedithiol (HDT) was developed as the dual mode electrochemical-surface plasmon resonance (EC-SPR) optical fiber sensor. The limit of detection of the proposed sensor exposed in Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions solution without and with the multiple cyclic voltammetry (CV) scans to activate the HDT monolayer can be improved from 0.47 μM to 0.22 μM. More importantly, the CV scans can activate the redox reaction of Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions, and then easily strip the Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions from the HDT monolayer to restore the active sites, and endow the designed sensor the characteristics of repeated use. Under simultaneous SPR monitoring, the differential pulse voltammetry (DPV) currents of HDT-functionalized optical fiber can also act as a microelectrode to develop electrochemical sensor platform for monitoring Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , with a limit of detection of 94 nM and a dynamic range up to 90 μM. As a comparison, the EC-SPR sensor still has good analysis performance for both Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and Cu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions in presence of Ni <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Co <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , among others. We therefore demonstrated that the proposed fiber-based EC-SPR sensor has the advantages of real-time, miniaturized, and cost-effective sensing, paving the way for environmental monitoring applications for on site inspection.

A new route for caesium lead halide perovskite deposition

Inorganic metal halide perovskites are relevant semiconductors for optoelectronic devices. The successful deposition of thin films of CsPbBr3 and CsPbCl3 has recently been obtained by Radio-Frequency magnetron sputtering. In this work we compare the morphological, structural and optical characteristics of the two materials obtained with this deposition technique. A detailed photoluminescence (PL) spectroscopy study of the as-grown samples was conducted at the macro and micro scale in a wide temperature range (10-300 K) to fully characterize the PL on sample areas of square centimeters, to assess the origin of the inhomogeneous broadening and to quantify the PL quantum yield quenching. Our results prove that this technique allows for the realization of high quality nanometric films with controlled thickness of relevance for optoelectronic applications.

Plumbing the depths of the graphene wetting controversy

Plumbing the depths of the graphene wetting controversy