Transition Metal Thin Films Research Articles

Spin-fluctuation theory of temperature-driven spin reorientations in ferromagnetic transition metal thin films

Spin-fluctuation theory of temperature-driven spin reorientations in ferromagnetic transition metal thin films

Low temperature dependence of electrical resistivity in obliquely sputter-deposited transition metal thin films

Transition metals exhibiting hcp (Ti, Zr, Hf) and bcc (V, Nb, Ta, Cr, Mo, W) crystalline structures are DC sputter-deposited by oblique angle deposition. A constant film thickness of 400 nm is prepared, whereas the deposition angle α is systematically changed from 0 to 85° A columnar structure is produced with column angle reaching β = 50° for the highest deposition angle. Crystallinity and grain size are both reduced with an increasing deposition angle, especially for α higher than 60° DC electrical resistivity vs. temperature in the range 7–300 K shows a typical metallic-like behavior with films becoming more resistive for high deposition angles. For temperatures higher than 100 K, the linear temperature dependence of resistivity is obtained for films prepared with deposition angles lower than 60° The electron-phonon is the main interaction acting on electronic transport mechanism. Oblique deposition angles give rise to an enhancement of electron-phonon interactions with a saturation effect of electrical resistivity for some metals. Resistivity measurements at low temperatures (down to 7 K) show the predominance of electron-defect interactions. Electron-phonon-defect interaction effect is particularly investigated as a function of the deposition angle and a shift of the crossover temperature is brought to the fore.

Surface effects in ferrimagnetic TbFe rare-earth–transition-metal thin films and nanoparticles

Surface effects in ferrimagnetic TbFe rare-earth–transition-metal thin films and nanoparticles

Underlying Substrate Effect on Electrochemical Activity for Hydrogen Evolution Reaction with Low‐Platinum‐Loaded Catalysts

Platinum is known as the best catalyst for the hydrogen evolution reaction (HER) but the scarcity and high cost of Pt limit its widespread applicability. Herein, the role of the underlying substrate on the HER activity of dispersed Pt atoms is uncovered. A direct current magnetron sputtering technique is utilized to deposit transition metal (TM) thin films of W, Ti, and Ta as underlying substrates for extremely low loading of Pt (<1.5 at%). The electrocatalytic performance of as‐synthesized samples for the HER is examined in both alkali and acidic media. The results show that despite the low loading of Pt, the Pt/TM catalysts produce hydrogen at a rate comparable to that of pristine bulk Pt. Pt/TM catalysts also display good stability with less than 5% decay in performance after 10 h of continuous HER operation. Based on the computational study, the excellent performance is attributed to the modified electronic properties of the Pt atoms, offering ideal binding energy for HER due to interaction with the underlying substrates. This work provides a robust and industry‐friendly route toward designing efficient catalytic systems for important electrochemical reactions such as HER and others.

Understanding the Role of Underlying Substrates on Hydrogen Evolution Reaction (HER) Catalytic Activity of Atomically Dispersed Pt Atoms

Electrochemical water splitting is an environmentally friendly way of producing hydrogen, but this process requires highly efficient catalysts. Platinum (Pt) is known the best catalyst for hydrogen evolution reaction (HER) in acidic media, but the scarcity and high cost of Pt limits its applicability in widespread technologies. In this work, we have uncovered the role of underlying substrate on the HER activity of atomically dispersed Pt atoms. A DC magnetron sputtering technique has been utilized to deposit transition metal (Cu, Mo, W, Ti and Ta) thin films as underlying substrates for atomically dispersed and extremely low loading of Pt (< 1.5 at%). As synthesized samples were characterized as electrocatalysts for the HER in both alkali and acidic media. In combination of standard electrochemical experiments (e.g., CV), the differential electrochemical mass spectrometry (DEMS) results show that despite low loading of Pt, the prepared catalysts produce hydrogen at a rate comparable with that of a pristine Pt. Based on the XPS study, the excellent performance is attributed to the modified electronic properties of the Pt atoms due to interaction with underlying substrates. Additionally, the performance of the Pt atoms is significantly governed by the physical properties of underlying substances. Our catalysts also displayed good stability for HER activity and found to be stable after 1000 cycles of continuous operation.

Light-induced hexatic state in a layered quantum material

The tunability of materials properties by light promises a wealth of future applications in energy conversion and information technology. Strongly correlated materials such as transition metal dichalcogenides offer optical control of electronic phases, charge ordering and interlayer correlations by photodoping. Here, we find the emergence of a transient hexatic state during the laser-induced transformation between two charge-density wave phases in a thin-film transition metal dichalcogenide, 1T-type tantalum disulfide (1T-TaS2). Introducing tilt-series ultrafast nanobeam electron diffraction, we reconstruct charge-density wave rocking curves at high momentum resolution. An intermittent suppression of three-dimensional structural correlations promotes a loss of in-plane translational order caused by a high density of unbound topological defects, characteristic of a hexatic intermediate. Our results demonstrate the merit of tomographic ultrafast structural probing in tracing coupled order parameters, heralding universal nanoscale access to laser-induced dimensionality control in functional heterostructures and devices.

Thickness and composition effects on atomic moments and magnetic compensation point in rare-earth transition-metal thin films

Rare-earth (RE) transition-metal (TM) thin films have exhibited great promise for spintronic applications; however, the variation of their magnetic properties with composition and thickness has yet to be adequately quantified. In this paper, we investigate the atomic magnetic moments of few-nanometer-thick GdCo and TbCo via x-ray magnetic circular dichroism and find a significant decrease in both RE and Co average moments with increasing RE concentration, consistent with a local Co environment model description. We observe a shift in compensation composition to higher RE concentrations as the thickness of GdCo and TbCo films decreases $&lt;10$ nm. Based on the dependence of the saturation magnetization $({M}_{s})$ on temperature and thickness, we posit the existence of an intrinsic unalloyed RE dead layer at room temperature that reduces the effective RE concentration in the remaining alloyed region, resulting in significant, nonzero ${M}_{s}$ in thin films at higher RE concentrations than is observed in thick films. We demonstrate a simple model for RE-TM films that accurately describes ${M}_{s}$ as a function of both RE content and film thickness. In addition, the atomic $g$ factors of GdCo and TbCo are found to vary significantly with RE concentration, largely deviating from their elemental values over the bulk of composition space.

Electrochemical Oxidation of Methane to Methanol on Electrodeposited Transition Metal Oxides

Electrochemical partial oxidation of methane to methanol is a promising approach to the transformation of stranded methane resources into a high-value, easy-to-transport fuel or chemical. Transition metal oxides are potential electrocatalysts for this transformation. However, a comprehensive and systematic study of the dependence of methane activation rates and methanol selectivity on catalyst morphology and experimental operating parameters has not been realized. Here, we describe an electrochemical method for the deposition of a family of thin-film transition metal (oxy)hydroxides as catalysts for the partial oxidation of methane. CoOx, NiOx, MnOx, and CuOx are discovered to be active for the partial oxidation of methane to methanol. Taking CoOx as a prototypical methane partial oxidation electrocatalyst, we systematically study the dependence of activity and methanol selectivity on catalyst film thickness, overpotential, temperature, and electrochemical cell hydrodynamics. Optimal conditions of low catalyst film thickness, intermediate overpotentials, intermediate temperatures, and fast methanol transport are identified to favor methanol selectivity. Through a combination of control experiments and DFT calculations, we show that the oxidized form of the as-deposited (oxy)hydroxide catalyst films are active for the thermal oxidation of methane to methanol even without the application of bias potential, demonstrating that high valence transition metal oxides are intrinsically active for the activation and oxidation of methane to methanol at ambient temperatures. Calculations uncover that electrocatalytic oxidation enables reaching an optimum potential window in which methane activation forming methanol and methanol desorption are both thermodynamically favorable, methanol desorption being favored by competitive adsorption with hydroxide anion.

Observation of Moiré Patterns in Twisted Stacks of Bilayer Perovskite Oxide Nanomembranes with Various Lattice Symmetries.

The design and fabrication of novel quantum devices in which exotic phenomena arise from moiré physics have sparked a new race of conceptualization and creation of artificial lattice structures. This interest is further extended to the research on thin-film transition metal oxides, with the goal of synthesizing twisted layers of perovskite oxides concurrently revealing moiré landscapes. By utilizing a sacrificial-layer-based approach, we show that such high-quality twisted bilayer oxide nanomembrane structures can be achieved. We observe atomic-scale distinct moiré patterns directly formed with different twist angles, and the symmetry-inequivalent nanomembranes can be stacked together to constitute new complex moiré configurations. This study paves the way to the construction of higher-order artificial oxide heterostructures based on different materials/symmetries and provides the materials foundation for investigating moiré-related electronic effects in an expanded selection of twisted oxide thin films.

Tuning the magnetic properties of Fe thin films with RF-sputtered amorphous carbon

RF-sputtered amorphous carbon (a-C) offers a simple and cheap pathway to tune the magnetic properties of transition metal thin films for magnetic memories and different spintronic applications. This paper describes changes in the magnetic properties of iron thin films with a-C overlayers. In as-deposited samples, hybridisation and intermixing at the Fe/a-C interface leads to magnetic softening (Liu et al., 2006) [1], with a reduction in the coercive field (Hc) up to a factor of five for Fe/a-C/Fe trilayers, and a 10–30% lower saturation magnetization as a function of the metal film thickness. After annealing at 500 °C, inter-diffusion and graphitization of the carbon layer results in up to a factor five increased coercivity due to increased pinning as shown via Kerr microscopy. Therefore, RF-sputtered carbon overlayers and post-processing can tune the anisotropy and domain configuration of metallic thin films in a synthesis methodology that is simple, cheap and sustainable.

Microscopic theory of light-induced ultrafast skyrmion excitation in transition metal films

Magnetic skyrmions are topological excitations of great promise for compact and efficient memory storage. However, to interface skyrmionics with electronic devices requires efficient and reliable ways of creating and destroying such excitations. In this work, we unravel the microscopic mechanism behind ultrafast skyrmion generation by femtosecond laser pulses in transition metal thin films. We employ a theoretical approach based on a two-band electronic model, and show that by exciting the itinerant electronic subsystem with a femtosecond laser ultrafast skyrmion nucleation can occur on a 100 fs timescale. By combining numerical simulations with an analytical treatment of the strong s–d exchange limit, we identify the coupling between electronic currents and the localized d-orbital spins, mediated via Rashba spin–orbit interactions among the itinerant electrons, as the microscopic and central mechanism leading to ultrafast skyrmion generation. Our results show that an explicit treatment of itinerant electron dynamics is crucial to understand optical skyrmion generation.

Enhancing the Electrocatalytic Oxygen Evolution Reaction Performance of PBA Core-Shell Nanocubes By Phosphorized WS2 Coating

As the world's population continues to grow, increasing energy consumption has raised significant demands on the availability of non-renewable fossil fuel reserves. The day is not too far when there will be an acute shortage of energy. With less dependency on fossil fuels, green energy production is the primary aim of the research community. One of the cheap and reliable energy sources is the electrochemical water-splitting (EWS) reaction. However, the oxygen evolution reaction (OER) performance needs to be improved by developing cheap and highly efficient electrocatalysts, lowering the high activation energy barrier, and boosting the EWS reaction kinetics.1 Porous coordination polymer compounds and mainly Prussian blue analogs (PBAs) receive extensive attention in electrocatalyst designing owing to their tuneable architecture.2 Overcoming structural and compositional complexity challenges, we report the successful synthesis of trimetallic core-shell PBA nanocubes. We developed a multi-component composite catalyst, Co-Co@Ni-Fe PBA@WS2, which was subsequently transformed into a porous architecture by a low-temperature gas phosphorization process. We present here a unique impressive structure of phosphorized porous woolen balls, Co-Co@Ni-Fe PBA@WS2-P.These phosphorized woolen balls exhibit a superior electrocatalytic OER performance in alkaline medium, requiring a low overpotential of only 280 mV@10 mA cm-2 with a Tafel slope value of 70 mV dec-1, and have excellent stability under the operating conditions. The work highlights the synergism between the WS2 and the core-shell structure due to the d-band electronic structure modulation and aligned arrangement of the WS2 planes. The in-situ oxidation of the transition metal phosphides during the OER process results in amorphous metal hydroxides/oxides formation, responsible for their high performance.3–5 The loose arrangement of the multi-components in the hollow frame increases the contact area between the electrolyte and the catalyst, thus, favors the rapid release of oxygen gas bubbles produced at the electrode site. Hence, the present work suggests a way to improve the electrocatalytic OER performance by utilizing the vast potential of PBA-based materials for electrocatalytic applications. References Morales-Guio, C. G., Liardet, L. & Hu, X. Oxidatively Electrodeposited Thin-Film Transition Metal (Oxy)hydroxides as Oxygen Evolution Catalysts. J. Am. Chem. Soc. 138, 8946–8957 (2016).Zakaria, M. B. & Chikyow, T. Recent advances in Prussian blue and Prussian blue analogues: synthesis and thermal treatments. Coord. Chem. Rev. 352, 328–345 (2017).Singh, B. & Indra, A. Designing Self‐Supported Metal‐Organic Framework Derived Catalysts for Electrochemical Water Splitting. Chem. – An Asian J. 15, 607–623 (2020).Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2, 1937–1938 (2017).Jamesh, M.I. & Harb, M. Tuning the electronic structure of the earth-abundant electrocatalysts for oxygen evolution reaction (OER) to achieve efficient alkaline water splitting – A review. J. Energy Chem. 56, 299–342 (2021). Figure 1

Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.

Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures.

Voltage control of magnetic order is desirable for spintronic device applications, but 180° magnetization switching is not straightforward because electric fields do not break time-reversal symmetry. Ferrimagnets are promising candidates for 180° switching owing to a multi-sublattice configuration with opposing magnetic moments of different magnitudes. In this study we used solid-state hydrogen gating to control the ferrimagnetic order in rare earth-transition metal thin films dynamically. Electric field-induced hydrogen loading/unloading in GdCo can shift the magnetic compensation temperature by more than 100 K, which enables control of the dominant magnetic sublattice. X-ray magnetic circular dichroism measurements and ab initio calculations indicate that the magnetization control originates from the weakening of antiferromagnetic exchange coupling that reduces the magnetization of Gd more than that of Co upon hydrogenation. We observed reversible, gate voltage-induced net magnetization switching and full 180° Néel vector reversal in the absence of external magnetic fields. Furthermore, we generated ferrimagnetic spin textures, such as chiral domain walls and skyrmions, in racetrack devices through hydrogen gating. With gating times as short as 50 μs and endurance of more than 10,000 cycles, our method provides a powerful means to tune ferrimagnetic spin textures and dynamics, with broad applicability in the rapidly emerging field of ferrimagnetic spintronics.

STUDY OF FILMS OF TRANSITION METALS AS MATERIALS FOR LASER NANOSTRUCTURING

This paper presents the results of a study of direct laser writing on thin films of transition metals (Hf, Ti, Zr, Ta, V). The films were deposited on fused silica substrates. A comparison of laser writing on the indicated films is carried out from the point of view of the presence of contour writing. As it was proved earlier, when writing on zirconium films, contour writing leads to formation of periodic nanostructures with a period equal to the writing step (250-500 nm). Materials were identified that are promising from the point of view of writing oxide nanostructures for the further formation of the diffraction phase microrelief of DOEs.

Molecular Mechanism of Thermal Dry Etching of Iron in a Two-Step Atomic Layer Etching Process: Chlorination Followed by Exposure to Acetylacetone

Controlling thickness and morphology of transition-metal thin films used in magnetic random-access memory fabrication is a major challenge. Thermal dry etching has emerged as a method of choice for preparing the desired films; however, the mechanisms of the underlying surface processes are very difficult to assess. In this work, thermal dry etching of metallic iron by using sequential exposure to chlorine gas and 2,4-pentanedione (acetylacetone, acacH) was investigated. The mechanism of reacting acacH with chlorine-modified iron surface was studied by following the fragments desorbing from the surface in temperature-programmed desorption experiments in half-cycle processes that compared the chlorinated, oxidized, mixed (Cl and O), and clean (sputtered) iron films. In situ Auger electron spectroscopy and ex situ X-ray photoelectron spectroscopy of each sample after each etching step confirmed that the surface is activated by chlorine and then the chlorine-containing iron species are removed from the top layer of the sample, resulting in a metallic iron surface. No etching of clean (sputtered) surface was observed with acacH. The complete mechanism of acacH reaction with chlorinated iron samples is complicated, and etch products can contain both Fe2+ and Fe3+. However, a number of major conclusions, including the formation of surface intermediates and the final products of etching, primarily removal of Fe(acac)xCly, are inferred by comparing the results of these experiments with the computational investigation of selected surface processes using density functional theory calculations.

Possibility of Benzene Exposure in Workers of a Semiconductor Industry Based on the Patent Resources, 1990-2010.

BackgroundThis study aimed to assess the possibility of benzene exposure in workers of a Korean semiconductor manufacturing company by reviewing the issued patents.MethodsA systematic patent search was conducted with the Google “Advanced Patent Search” engine using the keywords “semiconductor” and “benzene” combined with all of the words accessed on January 24, 2016.ResultsAs a result of the search, we reviewed 75 patent documents filed by a Korean semiconductor manufacturing company from 1994 to 2010. From 22 patents, we found that benzene could have been used as one of the carbon sources in chemical vapor deposition for capacitor; as diamond-like carbon for solar cell, graphene formation, or etching for transition metal thin film; and as a solvent for dielectric film, silicon oxide layer, nanomaterials, photoresist, rise for immersion lithography, electrophotography, and quantum dot ink.ConclusionConsidering the date of patent filing, it is possible that workers in the chemical vapor deposition, immersion lithography, and graphene formation processes could be exposed to benzene from 1996 to 2010.

Plasma-assisted atomic layer deposition of transition metals and their carbides

<p indent=0mm>Thin films of transition metals and their carbides are widely used in the fields of microelectronics, energy, and catalysis. Plasma-assisted atomic layer deposition (P-ALD) can generate highly reactive species and is characteristic of low deposition temperature and completeness of reactions; therefore, P-ALD is an important technique to prepare continuous, conformal, high-quality nanofilms of transition metals and their carbides on complex 3D-structured substrates. This study first introduces the basic principles of the P-ALD technology. Then, it reviews the current research progress of P-ALD for transition metals and their carbides, with particular highlights on the advantages of reducing deposition temperature, shortening nucleation period, enhancing reaction activity, and improving deposition rate and film purity. Lastly, it provides perspectives on the future directions of the P-ALD technology.

Low-cost inkjet printing of metal–organic frameworks patterns on different substrates and their applications in ammonia sensing

Due to their vivid material properties, metal-organic frameworks (MOFs) have been successfully proven useful in a wide variety of applications. In recent years, MOFs have been explored in electrical, electronic, and optical devices as well. The fabrication of such devices requires a controlled growth of thin films of MOFs on diverse substrates. In the present work, we demonstrate a simple way of inkjet printing for the patterning of thin films of transition metals and Terbium (Tb) MOFs on substrates like paper and flexible PET (polyethylene terephthalate). In particular, inkjet printing of Cr-MIL-101, Mn-BDC, Fe-MIL-101, Co-MOF-71, and Ni-BDC MOFs is one of the novel aspects of this work. The printed patterns have been characterized with different instrumental techniques. The studies have confirmed that the inkjet printing technique can conveniently facilitate the growth of homogeneous MOF thin film patterns with intact structural and functional characteristics. The experiments have also proven that the basic characteristics of the patterned MOFs remains stable for months. Two printed patterns, i.e., one Mn- and one Tb- based MOF films, have been demonstrated as colorimetric/fluorescent sensing strips for easy detection of ammonia vapor with reasonable sensitivity. The films have facilitated the detection of ammonia over 5–80 ppm with a low limit of detection of 0.3 ppm.

Unexpected dependence of the anomalous Hall angle on the Hall conductivity in amorphous transition metal thin films

The anomalous Hall effect (AHE), and magnetic and electronic transport properties were investigated in a series of amorphous transition metal thin films---${\mathrm{Fe}}_{x}{\mathrm{Si}}_{1--x}, {\mathrm{Fe}}_{x}{\mathrm{Ge}}_{1--x}, {\mathrm{Co}}_{x}{\mathrm{Ge}}_{1--x}, {\mathrm{Co}}_{x}{\mathrm{Si}}_{1--x}$, and ${\mathrm{Fe}}_{1--y}{\mathrm{Co}}_{y}\mathrm{Si}$. The experimental results are compared with density functional theory calculations of the density of Berry curvature and intrinsic anomalous Hall conductivity. In all samples, the longitudinal conductivity (${\ensuremath{\sigma}}_{xx}$), magnetization ($M$), and Hall resistivity (${\ensuremath{\rho}}_{xy}$) increase with increasing transition metal concentration; due to the structural disorder ${\ensuremath{\sigma}}_{xx}$ is lower in all samples than a typical crystalline metal. In the systems with Fe as the transition metal (including ${\mathrm{Fe}}_{1--y}{\mathrm{Co}}_{y}\mathrm{Si}$), the magnetization and AHE are large and in some cases greater than the crystalline analog. In all samples, the AHE is dominated by the intrinsic mechanism, arising from a nonzero, locally derived Berry curvature. The anomalous Hall angle (AHA) $(={\ensuremath{\sigma}}_{xy}/{\ensuremath{\sigma}}_{xx})$ is as large as 5% at low temperature. These results are compared with the AHAs reported in a broad range of crystalline and amorphous materials. Previous work has shown that in a typical crystalline ferromagnet the Hall conductivity (${\ensuremath{\sigma}}_{xy}$) and ${\ensuremath{\sigma}}_{xx}$ are correlated and are usually either both large or both small, resulting in an AHA that decreases with increasing ${\ensuremath{\sigma}}_{xy}$. By contrast, the AHA increases linearly with increasing ${\ensuremath{\sigma}}_{xy}$ in the amorphous systems. This trend is attributed to a generally low ${\ensuremath{\sigma}}_{xx}$, while ${\ensuremath{\sigma}}_{xy}$ varies and can be large. In the amorphous systems, ${\ensuremath{\sigma}}_{xx}$ and ${\ensuremath{\sigma}}_{xy}$ are not coupled, and there may thus exist the potential to further increase the AHA by increasing ${\ensuremath{\sigma}}_{xy}$.