Nb Thin Films Research Articles

Spectroscopic, kinetics and molecular structure investigations of acriflavine hydrochloride dye onto thiobarbituric acid nanoblend TD-DFT calculations: Removal of dyes from wastewater applications

The goal of this study is to use an ultraviolet–visible spectroscopic method at maximum absorption λmax = 450 nm to explore the elimination of poisonous acriflavine hydrochloride (ACF) utilizing antibacterial thiobarbituric acid (TBA) as an adsorbent. We looked at the effects of a number of variables, including [dye], adsorbent quantity, time, and temperature on the adsorption process. At 450 nm, thiobarbituric acid had a maximum percentage removal of ACF is 62 % at 20 °C and the adsorbent dose increased from 0.01 to 0.1 g/10 ml, so, the percentage removal efficiency of acriflavine dye increases steadily but the adsorption capacity of acriflavine dye suddenly decreases. The nanoblend (ACF + TBA)NB thin film is studied using various methods, including FT-IR, kientic, isotherms of liquid-phase adsorption and SEM methods. In FT-IR spectra the changes appears in the absorption peak suggested that the ACF molecules were interacting with the functional groups of the bio-sorbent. Again, the surface of TBA morphology and textural characteristics was carried out using SEM technique. TBA absorbed a substantial amount of ACF and developed an ACF material covering their surface. The physicochemical interactions between TBA and adsorbate molecules change the microstructure of TBA somewhat. (TBA)TF and (ACF + TBA)NB thin films with a thickness of 200 ± 5 nm are manufactured by PVD technique at a low deposition rate with basic pressure of the chamber was 5 × 10−5 mbar. Time dependent Density Functional theory (TD-DFT) was also utilised tooptimizeisolated molecule nanoblend (ACF + TBA)NB/Iso employing TD-DFTD/Mol3 and Cambridge Serial Total Energy Bundle (TD-FDT/CASTEP). Using the DFT-B3LYP method with a 6-31G(d,p) basis, the optimised geometries, vibrational wavenumbers, intensity of vibrational bands, and numerous atomic charges of 9VC have been studied. The adsorption equilibrium were evaluated using Langmuir and Freundlich isotherms, and a suitable reaction mechanism was proposed and argued. From the experimental results obtained, the temperature rises from 293 to 323 oK, and the dye's capacity of adsorption and percentage of removal decrease with rising temperature. The exothermic nature of the adsorption of ACF on TBA is demonstrated by the obtained value of ΔH° as −15.3809 kJ mol−1. The assumed ΔG° value (−13.672 kJmol−1) establishes the spontaneous type of the adsorption model. Additionally, the value of ΔS° (−5.83 J mol K−1) provides information about TBA's lack of affinity for ACF.

Electrochemical polishing of chemical vapor deposited niobium thin films

Combining chemical vapor deposition (CVD) with electrochemical polish (EP) operation is a promising route to producing performance-capable superconducting films for use in the fabrication of cost-effective components for superconducting radiofrequency (SRF) particle accelerators and superconducting quantum computers. The post-deposition EP process enables a critically necessary reduction in surface roughness of niobium (Nb) thin films to promote optimal superconducting surface conditions. This work investigated surface morphology, roughness, and crystal orientation of the CVD-grown and EP-polished Nb films. The CVD films were found to comprise steps and pyramidal features, resulting in undesirable large peak-to-valley distances. EP was demonstrated to significantly diminish the height of pyramids and effectively minimize the overall surface roughness. EP results showed a probable dependence on the crystal orientation. These understandings identify the EP principles tied to CVD-grown Nb films that allow further refinement of surface profiles for film-based SRF applications.

Stress-induced structural changes in superconducting Nb thin films

Stress-induced structural changes in superconducting Nb thin films

Investigating the Superconducting Properties and Surface Morphology of Sputtered Nb Films on Cu Due to Laser Treatment

Bulk niobium is currently the material of choice for superconducting radio frequency (SRF) cavities and is a well matured process. However, it is possible that SRF cavities could be further improved beyond bulk Nb by sputtering thin Nb films onto Cu cavities. Copper has a greater thermal conductivity than Nb and is also easier to machine, whilst sputtering films on the surface reduces the amount of Nb used to fabricate the whole cavity. However, sputtering Nb on Cu produces other issues, for example, the surface quality of the Cu affects the quality of the Nb deposited on the surface and therefore the superconducting parameters. As the Nb on the surface is not perfect, the magnetic field produced by the RF can enter the cavity earlier than expected, producing RF losses, which can in turn lead to a quench. One approach is to treat the Nb post deposition by irradiating the surface using a laser to polish the surface of the Nb and increase the surface magnetic field that the cavity can maintain whilst remaining in the Meissner state. A magnetic field penetration experiment designed and built at Daresbury Laboratory has been used to measure the field of full flux penetration to characterise the effect of the laser treatment on the superconducting properties of the Nb. Surface characterisation and the response of the Nb in a DC magnetic field have also been performed to try and provide an explanation for the change in the superconducting properties. The results demonstrate that the laser treatment can lead to an increase in the magnetic field at which the flux penetrates from one side of the sample to the other, thus it could potentially improve the performance of Nb coated RF cavities.

Supercurrent diode effect in thin film Nb tracks

We demonstrate nonreciprocal critical current in 65 nm thick polycrystalline and epitaxial Nb thin films patterned into tracks. The nonreciprocal behavior gives a supercurrent diode effect, where the current passed in one direction is a supercurrent and the other direction is a normal state (resistive) current. We attribute fabrication artifacts to creating the supercurrent diode effect in our tracks. We study the variation of the diode effect with temperature and the magnetic field and find a dependence with the width of the Nb tracks from 2 to 10 μm. For both polycrystalline and epitaxial samples, we find that tracks of width 4 μm provide the largest supercurrent diode efficiency of up to ≈30%, with the effect reducing or disappearing in the widest tracks of 10 μm. We propose a model based on the limiting contributions to the critical current density to explain the track width dependence of the induced supercurrent diode effect. It is anticipated that the supercurrent diode will become a ubiquitous component of the superconducting computer.

Growth of hard metal films by deep oscillation magnetron sputtering

ABSTRACT Pulsed vapour fluxes with proper controlled time duration were proved to be an important controlling dimension for the deposition of a desired thin film. In this work, deep oscillation magnetron sputtering was employed to tailor the ionized pulsed flux for Nb and Cr thin film grown on AISI 304 stainless steel. Through tuning the micropulse oscillating voltage-off time from 10 to 40 μs, Nb and Cr thin films showed an obvious variation on microstructure owing to the deposition flux transited from continuous to chopped ones. The deposited Nb thin films showed Nb(110) preferred orientation experienced a nanocrystallization process with compressive residual stress showing a slightly decrease. While Cr thin films showed Cr(110) preferred orientation without an obvious nancrystallization trend with its compressive stress turning to be tensile. The tailoring of the microstructure and properties relied on interval time between the two interrupted ionized deposition flux.

Interface structure between Nb thin film and MgO(112) substrate: A first-principles prediction

The crystal orientation of ceramic substrates is an important factor affecting the interface structure of metal/ceramic composite materials. However, there is little information about the interface composed of metal films and ceramic substrates with a high-index plane. In this work, we predicted the interface structure between a Nb film and a MgO(112) substrate by calculating the interface separation works of different interface models by using the first-principles calculation method. The results showed that the preferred growth direction is Nb [120], and that the value of the interface separation work is 0.35 eV/Å2. The lattice mismatch between the film and substrate is less than 3%, implying that a coherent interface type is highly realizable in Nb/MgO(112). Furthermore, we analyzed the interface structures of Nb/MgO(100), Nb/MgO(110), Nb/MgO(111), and Nb/MgO(112) and found that the unique atomic configuration of the MgO substrate is the main factor determining the preferred interface structure of Nb/MgO.

HiPIMS deposition of superconducting Nb thin films onto Cu substrates

The use of energetic condensation techniques, such as HiPIMS, has the potential to significantly improve the superconducting performance of coated Cu cavities by producing bulk-like Nb thin films. However, in order to benefit from these techniques, a deeper understanding of the effects of, and the relationship between, the different deposition parameters, the physical properties of the deposited films and the superconducting performance of the films, is still required. This contribution details the effects of different HiPIMS deposition parameters on the morphological, crystallographic and superconducting properties of Nb thin films deposited onto electropolished Cu substrates. Higher duty cycles (8 – 20 %) than previous HiPIMS investigations were used in the majority of the coatings. The films displayed more bulk-like crystallographic properties, indicated here by the lattice parameter, with a “transition zone” apparent in the early stages of film growth. A reduction in the surface roughness as well as interfacial voids, compared to typical DC MS Nb films, was also observed. The superconducting performance of the films, specifically the first magnetic flux entry field, showed a marked reliance on the substrate bias and the film thickness. Conversely, the HiPIMS duty cycle appears to have a comparatively insignificant effect.

Removal of Toxic Basic Fuchsin Dye from Liquids by Antibiotic Azithromycin Using Adsorption, TD-DFT Calculations, Kinetic, and Equilibrium Studies

This work aims to study the removal of new fuchsin dye (NFD) using azithromycin (Az) by a spectrophotometric technique at two different wavelengths (547 and 286 nm), which formed the blend [Az+NFD]. Afterward, the [Az]TF, [NFD]TF, and nanoblend [Az+NFD]NB thin films were fabricated using the spin-coating technique. The models of Langmuir and Freundlich isotherms were applied to examine the experimental data of adsorption equilibrium and activation parameters. At 547 and 286 nm, the sorption capacity (qm) was determined to be 0.0095 and 0.01 mg g–1, correspondingly. The kinetic data showed that the reaction was fitted with a first-order reaction and the maximum adsorption capacity of Az at 547 nm was 0.0198 mg g–1 at 313 and 323 K, whereas at 286 nm, it was 0.0071 mg g–1 at 303 K. The thermodynamic parameters (ΔH°, ΔS°, and ΔG°) were calculated, and the experimental results showing negative values of ΔH° at 547 and 286 nm (−33.67 and −11.22 kJ mol–1, respectively) indicate the adsorption of NFD using Az is an exothermic reaction. The negative values of ΔG° at 547 and 286 nm (−19.67 and −13.01 kJ mol–1, respectively) indicate a spontaneous reaction. The calculated time-dependent density functional theory (TD-DFT) spectra matched the measured IR spectra precisely and corroborated the molecular structure of the tested materials. The experimental values and TD-DFT/CASTEP predictions for these properties are in great agreement. The optical (TD-DFT/CASTEP) characteristics of the [Az]Iso, [NFD]Iso, and [Az+NFD]NB gaseous phases are in close agreement with the experimental results. The greatest absorption bands for [Az]TF, [NFD]TF, and [Az+NFD]NB correspond to the π → π* electronic transition at 268, 547, and 625 nm, respectively, and the Gaussian program corresponds well with the experimental analysis for synthetic IR and molecular electrostatic potential. Optical illumination studies suggest that the nanoblend device films investigated may be employed in solar cell applications. The quantity of dye removed by Az decreases with an increase in the contact time and optimum adsorption was achieved in 4 min at 547 nm, whereas it increases with time at 286 nm and an optimum adsorption capacity was achieved in 6 min. The isolated molecule of [Az+NFD]Iso has a band gap of 2.415 eV, as determined by TD-DFT/DMol3. Thin films of thickness 100 ± 3 nm of [Az]TF, [NFD]TF, and [Az+NFD]NB via a spin-coating technique at room temperature were fabricated.

Electrochromic Performance of Sputtered NbTi-Based Mixed Metal Oxide Thin Films with a Metallic Seed Layer.

An attempt has been made to promote the efficiency of the electrochromic (EC) windows to perform at a faster switching rate with good coloration and easy recyclability. In this work, ion-assisted pulsed DC unbalanced confocal magnetron sputtering is used to fabricate mixed metal oxide thin films of Nb and Ti (which are termed as NTO) for EC applications. Further, to increase the EC efficiency of this film, a very thin metallic seed layer is incorporated between the substrate and the film using the layer-by-layer (LBL) coating strategy. The film was prepared by two steps: (i) very thin metallic seed layer coating and (ii) mixed metal oxide NTO layer coating. The metal layer was made of a Nb metal, Ti metal, and NbTi mixed metal, which was coated with the substrate and NTO layer, resulting in three different films, namely, Nb-NTO, Ti-NTO, and NbTi-NTO. The EC properties of these three films were studied in 1 M LiClO4 dissolved in propylene carbonate and compared with the as-prepared NTO film. The results showed that the Nb-NTO film showed better EC properties, fast switching, better stability, and good recyclability. To check the stability, the film was subjected to prolonged cycling of 500 cycles with a harsh anodic and cathodic sweep at the scan rate of 100 mV s-1. The UV-Vis spectrum confirmed the Li+ trapping in the films after prolonged cycling. To detrap the ions from the host surface, galvanostatic detrapping is carried out called rejuvenation studies. The rejuvenation rate of films is studied at a constant current loading of ∼2 × 10-5 A cm-2. To check the commercialization of the EC window, we successfully fabricated the Nb-NTO device with a PEO-LiClO4-based polymer gel electrolyte.

Anisotropic superconductivity of niobium based on its response to nonmagnetic disorder

Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, ${T}_{c}=9.32$ K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magnetosensing, radio-frequency cavities, and, more recently, as circuit metallization layers in superconducting qubits. A detailed understanding of its electronic and superconducting structure, especially its normal and superconducting state anisotropies, is crucial for mitigating the loss of quantum coherence in such devices. Recently, a microscopic theory of the anisotropic properties of niobium with the disorder was put forward. To verify theoretical predictions, we studied the effect of disorder produced by 3.5 MeV proton irradiation of thin Nb films grown by the same team and using the same protocols as those used in transmon qubits. By measuring the superconducting transition temperature and upper critical fields, we show a clear suppression of ${T}_{c}$ by potential (nonmagnetic) scattering, which is directly related to the anisotropic order parameter. We obtain a very close quantitative agreement between the theory and the experiment.

Reverse coating technique for the production of Nb thin films on copper for superconducting radio-frequency applications

In the framework of the Future Circular Collider Study, the development of thin-film coated superconducting radio-frequency copper cavities capable of providing higher accelerating fields (10–20 MV m−1 against 5 MV m−1 for the Large Hadron Collider) represents a major challenge. The method investigated here for the production of seamless niobium-coated copper cavities is based on the electroforming of the copper structure around a sacrificial aluminium mandrel that is pre-coated with a niobium thin film. The first feasibility study, applied to a flat aluminium disk mandrel, is presented. Protective precautions are taken towards the functional niobium film during the production process and it is shown that this technique can deliver well performing niobium films on a seamless copper substrate. This way, the non-trivial chemical treatments foreseen by the standard procedures (e.g. SUBU, EP) for the preparation of the copper surface to achieve the proper adhesion of the niobium layer are also avoided. The only major chemical treatment involved in the reverse-coating method is represented by the chemical dissolution of the aluminium mandrel, which has the advantage of not affecting the copper substrate and therefore the copper-niobium interface.

Influence mechanism of RF bias on microstructure and superconducting properties of sputtered niobium thin films

In this study, radio frequency (RF) biased direct current (DC) magnetron sputtering was used to simultaneously improve the surface roughness and superconducting properties of Nb thin films for Josephson circuits application. The surface roughness of the Nb films decreased with an increase in RF powers, whereas the transition temperature, transition width, and residual resistance ratio of the Nb film were optimum at an RF power of 20 W but will deteriorate at higher RF powers. The film resistivities at 10 K obviously increased when the RF power was increased to 40 W and 60 W, which indicates that electron-impurity scattering and scattering on lattice defects are severer in these Nb films. The microstructures, impurities (mainly Ar) and dislocation states were correlated with the electrical and superconducting properties of Nb films. In summary, an optimal process for depositing Nb films with simultaneously improved surface roughness and superconducting properties was found, the Nb film deposited at a DC power of 500 W and an RF bias power of 20 W with a substrate bias voltage of approximately −100 V had the best overall quality. The method is prospective to be applicated in multilayer superconducting electronic devices fabrication based on Nb and other superconductors.

Contactless excitation of acoustic resonance in insulating wafers

Contactless excitation and detection of high harmonic acoustic overtones in a thin insulator single crystal are described using radio frequency spectroscopy techniques. Single crystal [001] silicon wafer samples were investigated, one side covered with a Nb thin film, the common starting point for the fabrication of quantum devices. The coupling between electromagnetic signals and mechanical oscillation is achieved from the Lorentz force generated by an external magnetic field. This method is suitable for any sample with a metallic surface or covered with a thin metal film. High resolution measurements of the temperature dependence of the sound velocity and elastic constants of silicon are reported and compared with known results.

A facility for the characterisation of planar multilayer structures with preliminary niobium results

The maximum accelerating gradient of superconducting radio frequency cavities are currently reaching their theoretical limits, due to the magnetic field entering the superconductor in the form of vortices. To overcome these limits, thin film coated superconducting materials are required, however these need to be tested to optimise their properties. A system has been designed, built, and commissioned at Daresbury Laboratory that applies a local DC magnetic field parallel to the surface, from one side of a sample, similar to that in cavity operation. A magnetic flux density (up to 600 mT) is generated parallel to the sample surface in the 2 mm gap of a C-shaped ferrite yoke. Two Hall probe sensors are used to measure both the applied and penetrated magnetic field. The system operates in a cryogen free environment, with a minimum temperature of approximately 2.6 K. A Pb foil has been used to characterise the system, and determine how the sample size affects the results. Nb thin film samples have been tested for varying thickness to determine how the depth effects the field of full flux penetration, . The design, operation, methods of analysis and first results of this facility will be reported in this paper.

Superconducting properties of Nb, NbN, and MoN thin films grown on topological insulator Bi1.95Sb0.05Se3: a comparative study

The superconducting properties of DC magnetron sputtered Nb, NbN, and MoN films deposited on Sb-doped bismuth selenide (BSS) topological insulator films have been studied. In this two-fold study, firstly, thick superconducting films of ∼100 nm have been studied to find the modifications in the superconducting properties of the films caused by the surface morphology of pulsed laser deposition-grown BSS substrates. A drastic suppression in the critical current density has been observed in the bilayers, which has been ascribed to the strong influence of the substrate on the morphology of the superconducting film and the consequent enhancement in the density of superconducting weak links. The temperature dependence of the critical magnetic field has been carefully analyzed within the purview of the Werthamer–Hohenberg–Helfand theory, taking into account the effects of spin paramagnetism and spin–orbit scattering. In the second part of the study, ultra-thin bilayers of NbN-BSS, with thickness of the order of the coherence length (∼5 nm), have been grown to study the superconducting properties within the proximity regime. A ∼3.5 K suppression in T c is seen in the NbN(5 nm)-BSS bilayer, due to a combination of interface roughness and superconducting proximity, while a suppression of 0.6 K has been observed in the BSS(6 nm)-NbN(5 nm) bilayer, which is surmised to be solely due to the superconducting proximity effect.

Surface quality characterization of thin Nb films for superconducting radiofrequency cavities

Superconducting radiofrequency (SRF) cavities are vital components of particle accelerators nowadays. In order to minimise the energy dissipation, a perfect inner surface of the cavity, hindering the penetration of magnetic field, is required. In this work, we investigated ten planar samples differing in the surface quality of Nb film deposited on Cu substrate, and as a consequence exhibiting various levels of the first entry field, H en, at which the magnetic field starts to enter the film. The observed surface defects are categorised as hills, pits and cracks. For a practical range of dimensions of these features, the factor β, characterising the local magnetic field enhancement, was calculated by the numerical finite-element simulations. It is expected that the local field enhancement causes a premature penetration of the magnetic field, thus lowering H en. Then, for each investigated sample, the range of β values characterising defect type that cause the highest field enhancement, is identified and compared with the H en fields. We have found that the H en of the samples that contain multiple types of the surface features is indeed limited by those defects that cause the highest field enhancement. The H en vs β dependence has shown a good match with linear fit for the set of investigated samples. Thus, the main result is that the local magnetic field enhancement, computed in a straightforward way for the most significant defects, is a strong indicator of the surface quality that is relevant for the superconducting film intended for SRF cavity application.

Stability, metallicity, and magnetism in niobium silicide nanofilms

Modern superconducting qubits based on two-dimensional (2D) transmons typically involve the growth of Nb thin films on high-resistivity Si substrates. Since imperfections at the Nb-Si heterointerface have been implicated as a source of two-level systems that limit quantum coherence times, detailed characterization and understanding of niobium silicide interfacial layers are critical to improving superconducting qubit technology. While bulk binary intermetallic niobium silicide phases are well understood, the thermodynamic phase stability and properties of ultrathin niobium silicides, such as those found at the Nb-Si heterointerface in 2D transmons, have not yet been explored. Here, we report finite-sized effects for ultrathin niobium silicide films using density functional theory calculations and predict nanoscale stabilization of ${\mathrm{Nb}}_{6}{\mathrm{Si}}_{5}$ over the bulk $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Nb}}_{5}{\mathrm{Si}}_{3}$ phase. This result is consistent with our experimental observations of a niobium silicide interfacial layer between a sputtered Nb thin film and the underlying Si substrate. Furthermore, our calculations show that ${\mathrm{Nb}}_{6}{\mathrm{Si}}_{5}$ nanofilms are nonmagnetic, making them superior to nanofilms of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Nb}}_{5}{\mathrm{Si}}_{3}$ that exhibit antiferromagnetic correlations detrimental to long coherence times in superconducting qubits. By providing atomic-scale insight into niobium silicide nanofilms, this paper can help guide ongoing efforts to optimize Nb-Si heterointerfaces for long coherence times in superconducting qubits.

Electron beam evaporation of superconductor-ferromagnet heterostructures

We report on the electronic and magnetic properties of superconductor-ferromagnet heterostructures fabricated by electron beam evaporation on to unheated thermally oxidised Si substrates. Polycrystalline Nb thin films (5 to 50 nm thick) were shown to possess reliably high superconducting critical temperatures (T_{c}), which correlate well with the residual resistivity ratio (RRR) of the film. These properties improved during ex-situ annealing, resulting in {Delta }T_{c} and {Delta }RRR increases of up 2.2 K (sim 40% of the pre-annealed T_{c}) and 0.8 (sim 60% of the pre-annealed RRR) respectively. Nb/Pt/Co/Pt heterostructures showed substantial perpendicular anisotropy in the ultrathin limit (≤ 2.5 nm), even in the extreme limit of Pt(0.8 nm)/Co(1 nm)/Pt(0.6 nm). These results point to the use of electron beam evaporation as route to line-of-sight deposited, low-thickness, high quality Nb-based superspintronic multilayers.

Fabrication of Josephson junctions by single line etching of Nb thin films utilizing nitrogen-gas-field ion-source focused ion beam

We focused on a focused ion beam (FIB) technology, called nitrogen gas field ion source FIB (N2 GFIS-FIB), which can etch directly at the 10 nm level or finer. We performed single line etching of Nb thin film microbridges deposited by N2 GFIS-FIB and fabricated Josephson junction (JJ) devices. The microbridge area was separated into two parts by the processing line, whose width was around 20 nm. We performed electrical characterizations of the devices at low temperature and observed typical behaviors of JJ devices, such as a superconducting current region and current jumping to the normal current region in some of the deeply etched devices. We also observed an AC resistance oscillation in some of the shallowly etched devices. The oscillation may be due to a sub-harmonic gap structure that originates from Andreev reflection. These results indicate that simple single line etching of Nb by N2 GFIS-FIB can form JJ devices.