Thin Bulk Research Articles

Multiferroicity and Semi‐Cylindrical Alignment in Janus Nanofiber Aggregates

Abstract1D multiferroic fibers are known to exhibit attractive characteristics, including enhanced magnetoelectric (ME) coupling compared to thin film and bulk architectures. A comprehensive understanding of composite fibers, however, has been hindered by the complexity of their structure, leading to limited reports. Here, clear and strong ME coupling is experimentally detected in a composite Janus nanofiber aggregate using second harmonic generation (SHG) polarimetry under different magnetic field orientations. The observation of such a pronounced effect using an all‐optical method has not been previously reported in multiferroic fibers. A series of global fits is performed to the SHG polarimetry results to investigate the behavior of nanofibers within an aggregate. We find the magnetically assembled fibers exhibit semi‐cylindrical alignment as well as the expected lengthwise alignment despite variations in size and composition from fiber to fiber. The ME coupling and the semi‐cylindrical alignment seen in SHG are further corroborated via X‐ray diffraction under similar magnetic field conditions. These findings contribute to the development of complex composite and multifunctional devices using multiferroic nanostructures as building blocks, even those with inhomogeneous shapes and geometries.

Stacking of charge-density waves in 2H−NbSe2 bilayers

We employ electronic-structure calculations to investigate the charge-density waves and periodic lattice distortions in bilayer 2H−NbSe2. We demonstrate that the vertical stacking can give rise to a variety of patterns that may lower the symmetry of the charge-density waves exhibited separately by the two composing 1H−NbSe2 monolayers. The general tendency to a spontaneous symmetry breaking observed in the ground state and the first excited states is shown to originate from a non-negligible interlayer coupling. Simulated images for scanning tunneling microscopy as well as geometric structure factors show signatures of the different stacking orders. This may not only be useful to reinterpret past experiments on surfaces and thin films, but it may also be exploited to devise ad hoc experiments for the investigation of the stacking order in 2H−NbSe2. We anticipate that our analysis not only applies to the 2H−NbSe2, but is also relevant for thin films and bulk, whose smallest centrosymmetric component is indeed the bilayer. Finally, our results illustrate clearly that the vertical stacking is not only important for 1T structures, as exemplified by the metal-to-insulator transition observed in 1T−TaS2, but seems to be a general feature of metallic layered transition metal dichalcogenides as well. Published by the American Physical Society 2024

Nonmonotonic Hall Effect of Weyl Semimetals under a Magnetic Field.

The Hall effect of topological quantum materials often reveals essential new physics and possesses potential for application. The magnetic Weyl semimetal is one especially interesting example that hosts an interplay between the spontaneous time-reversal symmetry-breaking topology and the external magnetic field. However, it is less known beyond the anomalous Hall effect thereof, which is unable to account for plenty of magnetotransport measurements. We propose a new Hall effect characteristically nonmonotonic with respect to the external field, intrinsic to the three-dimensional Weyl topology, and free from chemical potential fine-tuning. Two related mechanisms from the Landau level bending and chiral Landau level shifting are found, together with their relation to the Shubnikov-de Hass effect. This field-dependent Hall response, universal to thin films and bulk samples, provides a concrete physical picture for existing measurements and is promising to guide future experiments.

Exploring electrical transport in thin film and bulk thermoelectric materials with an automated Seebeck coefficient and resistivity measurement platform

Accurate assessment of electrical properties, particularly the Seebeck coefficient and electrical resistivity, is critical in thermoelectric energy harvesting research, which involves converting thermal energy into electricity. This study presents an innovative automated system designed for the simultaneous measurement of the Seebeck coefficient and electrical resistivity as a function of temperature in both film and bulk samples. The system exhibits high precision across a wide range of electrical resistance, from milli Ohm (mΩ) to giga Ohm (GΩ). Furthermore, and in contrast with commercial systems, it provides excellent flexibility in accommodating samples with diverse lateral dimensions and thicknesses, ranging from a few millimetres to several centimetres. The measurement principle is based on analysing the current–voltage (I-V) characteristic of the samples. To ensure seamless monitoring and control of the measurement process, a user-friendly and adaptable Graphical User Interface (GUI) is integrated into the system, enhancing its overall user-friendliness and convenience.

Thinning the Bulk into the Soft: Flexible and Stretchable Germanium Photodetectors

AbstractInorganic semiconductor‐based nano/micro‐membranes are of great interest and can be used as active materials for high‐performance and flexible electronics, due to their unique and stable physical or chemical properties. However, the creation of high‐quality and single‐crystalline semiconducting membranes, particularly germanium (Ge) membranes, remains a significant challenge. In this study, a wet etching approach is developed to thin bulk Ge into soft Ge membranes, which are further used to fabricate flexible and stretchable photodetectors. The thickness of the obtained Ge membranes can be precisely controlled, with a minimum thickness of ≈1.6 µm. Investigations on the surface morphology, surface chemical composition, and Raman shifts indicate that the prepared Ge membranes are single‐crystalline and suitable for the following device fabrication. As an example, flexible and stretchable Ge photodetectors, of which the stretchability is realized by a meshed‐structure design, are demonstrated. Bending (with a minimum radius of 4 mm and a bending cycle of 10 000 times) and stretching (with a stretching strain of up to 10%) tests result in negligible variations in the device performance, revealing good flexibility, and stretchability, as well as the significant potentials of the prepared Ge membranes as candidates for flexible electronics.

Comparative structural study of Al2O3–SiO2 glasses and amorphous thin films

AbstractWe compared the impact of alumina doping on the structure of Al2O3–SiO2 amorphous thin films and bulk glasses using Raman spectroscopy and x‐ray diffraction. In both thin films and bulk glasses, the addition of Al2O3 is accompanied by an increase in the mean Si–O–T angle and an evolution of the ring statistics with a decrease in the proportion of small rings. We evidenced structural differences between sputtered films and fused bulk glasses. Sputtered Al2O3–SiO2 thin films are about 6%–7% denser than their equivalent Al2O3–SiO2 bulk glasses. This difference is mainly due to a change in ring statistics with the formation of small rings within the sputtered thin films. These structural differences in atomic structural organization highlight the impact of the synthesis conditions and open the door to further investigation of the structure–functional property relationships in sputtered Al2O3–SiO2 thin films.

КВАНТОВЫЙ РАЗМЕРНЫЙ ЭФФЕКТ В ЧИСТЫХ ПЛЕНКАХ АЛЮМИНИЯ

In the modern world, there's a notable trend towards the active miniaturization of electronic devices. With technological advances enabling the manipulation of nanostructures, there's an increasing focus on exploring quantum effects pivotal to such designs. One distinguishing feature of nanostructures is the quantum nature of the electron's energy spectrum. This spectrum becomes discrete in directions where electrons move. Depending on the direction of this confinement, structures can be categorized as nanoplates, quantum wires, or quantum dots. The properties of such structures can significantly differ from those observed in large-scale systems. When discussing superconductivity, particular emphasis is placed on its macroscopic quantum properties.The influence on electronic wave functions is reflected in the characteristics of the superconducting state on broader scales. The Bardeen-Cooper-Schrieffer (BCS) theory is frequently utilized to analyze these nanostructures. The Gor’kov equations method serves as a potent tool for tasks related to the BCS theory. For instance, it can determine the parameters of the superconducting state, critical temperature, and current. Components of these equations, like Green's functions, are associated with various system properties. Research in the early stages of superconductivity studies revealed that the critical temperature (Tc ) – the temperature at which a material transitions to a superconducting state – can differ significantly between thin films and bulk materials. Intriguingly, reducing the film's thickness can both decrease (e.g., in niobium) and increase (e.g., in aluminum) the Tc value. This study delves into the quantum size effect in thin aluminum films, paving the way for materials with higher transition temperatures. Such advancements can simplify and make the maintenance of superconducting systems more cost-effective. In this study, a theoretical relationship between the critical temperature of a thin aluminum film and its thickness was derived. The Green's function method was chosen, which hadn't been previously employed for this computation. This approach offers greater potential compared to other superconductivity theory methods, presenting extensive avenues for theoretical exploration in this domain. The authors are confident that this work will contribute to further research on quantum dimensional effects in low-dimensional superconducting structures.

Near-surface viscosity and complex crystal growth behavior in Se90Te10 thin films and bulk surface

Considering the resurgence of interest in selenium and tellurium-based solar cells and the inter alia related relationship between the device efficiency and post-growth annealing for crystallization, increased attention should be given to the understanding of crystallization process (formation of crystals, crystal positions, and nucleation and crystal growth kinetics). This work summarizes an extensive study of the crystal growth rates and near-surface viscosity at the Se90Te10 bulk surface and in thin films using nanoindentation and measurements of thermal grating decay. The crystal growth in Se90Te10 thin films occurs at two positions – at the thin film-substrate interface and the free surface of the thin film. Here, the crystallization of the interface crystals was quantitatively studied using microscopy down to the Tg and below. The influence of different substrates, illumination, and aging on crystal growth rates was tested in the studied thin films to understand the crystal growth properly. Regarding the crystal growth at the bulk surface, the sample size-dependent crystal growth rates were found. The values can be found in the region limited by growth rates of volume crystals (lower limit) and those found in thin films (upper limit). The growth data are combined with the near-surface viscosities measured in bulk samples and thin films to assess a suitable growth model for the description and prediction of the growth data in a wide temperature range.

Surface Modification of Bi2Te3 Nanoplates Deposited with Tin, Palladium, and Tin/Palladium Using Electroless Deposition

Surface-modified nanoplate-shaped thermoelectric materials can achieve good thermoelectric performance. Herein, single-crystalline Bi2Te3 nanoplates with regular hexagonal shapes were prepared via solvothermal techniques. Surface modification was performed to deposit different metals onto the nanoplates using electroless deposition. Nanoparticle-shaped tin (Sn) and layer-shaped palladium (Pd) formed on the Bi2Te3 nanoplates via electroless deposition. For the sequential deposition of Sn and Pd, the surface morphology was mostly the same as that of the Sn-Bi2Te3 nanoplates. To assess the thermoelectric properties of the nanoplates as closely as possible, they were compressed into thin bulk shapes at 300 K. The Sn-Bi2Te3 and Sn/Pd-Bi2Te3 nanoplates exhibited the lowest lattice thermal conductivity of 1.1 W/(m·K), indicating that nanoparticle-shaped Sn facilitated the scattering of phonons. By contrast, the Pd-Bi2Te3 nanoplates exhibited the highest electrical conductivity. Thus, the highest power factor (15 μW/(m∙K2)) and dimensionless ZT (32 × 10−3) were obtained for the Pd-Bi2Te3 nanoplates. These thermoelectric properties were not as high as those of the sintered Bi2Te3 samples; however, this study revealed the effect of different metal depositions on Bi2Te3 nanoplates for improving thermoelectric performance. These findings offer venues for improving thermoelectric performance by sintering nanoplates deposited with appropriate metals.

Impact of doping on the mechanical properties of conjugated polymers.

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

High‐throughput experimental techniques for corrosion research: A review

AbstractHigh‐throughput experimental techniques can accelerate and economize corrosion evaluation, and thus, have great potential in the development of new materials for corrosion protection such as corrosion‐resistant metals, corrosion inhibitors, and anticorrosion coatings. This concise review highlights high‐throughput experimental techniques that have been recently applied for corrosion research, including (i) the high‐throughput preparation of metal samples in the form of thin films or bulk materials, (ii) high‐throughput experiments based on corrosive solutions with independent or gradient parameters, (iii) high‐throughput evaluation of changes in physicochemical properties, and (iv) high‐throughput corrosion evaluation by electrochemical methods. To advance automated and intelligent corrosion research, future directions for the development of the high‐throughput corrosion experimental and characterization techniques are also discussed.

Understanding Polymer Structure and Conductivity in Composite Electrolytes

Lithium ion batteries are a crucial technologies for electric vehicles and portable electronics due to their high energy density. However, flammable liquid electrolytes create a safety concern and transitioning to all solid state batteries allows for the potential of higher energy density by utilizing lithium metal anodes. Two classes of solid materials have been explored to replace liquid electrolytes: ceramic and polymer electrolytes. Composite electrolytes, typically in the form of dispersed ceramic particles or nanowires in polymer matrix, combine these two materials to have good ionic conductivity while maintaining the mechanical properties of the polymer. While they have achieved fairly high conductivity (typically 10-4 S/cm), the mechanism for such ion conduction is not well understood.The polymer likely plays a crucial role as the ceramic volume fractions are typically under 20%.1,2 Four possible ways that polymers could increase in conductivity in the composite have been identified: decreased crystallinity, increased free volume, disruption of polymer alignment, and increased lithium ion dissolution due to acidic surface groups on the ceramic.1,3,4 We have developed thin film model systems to probe the interface of the polymer with and ceramic surfaces. We found that when confined to thin films, the conductivity of the polymer electrolyte increased compared to the bulk. We utilized differential scanning calorimetry to identify the crystallinity and found that the small crystallinity variation could not account for the variation in conductivity. Subsequently, we utilized surface conductivity and transference number measurements to understand the effects of acidic surface groups. X-ray Absorption measurements will be similarly utilized to probe the variation in lithium ion environments within these materials to understand any variation in the lithium salt dissolution. It was found that the variation in conductivity based on this effect is small.In order to probe the possible variation in free volume, Small Angle Neutron Scattering will be utilized to quantify the free volume within thin films, bulk polymer, and composite electrolytes. This can quantify the volume associated with each monomer and therefore how much space is between polymer chains for lithium ion movement.5 Lastly, while the polymer chain alignment has not been extensively discussed composite literature, it has been identified that when polymer electrolytes are cast on a surface, polymer electrolytes typically show anisotropy with measurements parallel to the surface having higher conductivity due to the alignment of the chain.6 Therefore, ceramic fillers in the composites may disrupt this packing and allow for higher conductivity perpendicular to the cast surface, the direction in which they are most commonly measured. In order to probe this effect, we measure the anisotropy of both our thin film electrolytes as well as composite electrolytes to compare the significance of the anisotropy and utilize small angle x-ray scattering to measure any anisotropy in the structure.

Mixed-dimensional moiré systems of twisted graphitic thin films.

Moiré patterns formed by stacking atomically thin van der Waals crystals with a relative twist angle can give rise to notable new physical properties1,2. The study of moiré materials has so far been limited to structures comprising no more than a few van der Waals sheets, because a moiré pattern localized to a single two-dimensional interface is generally assumed to be incapable of appreciably modifying the properties of a bulk three-dimensional crystal. Here, we perform transport measurements of dual-gated devices constructed by slightly rotating a monolayer graphene sheet atop a thin bulk graphite crystal. We find that the moiré potential transforms the electronic properties of the entire bulk graphitic thin film. At zero and in small magnetic fields, transport is mediated by a combination of gate-tuneable moiré and graphite surface states, as well as coexisting semimetallic bulk states that do not respond to gating. At high field, the moiré potential hybridizes with the graphitic bulk states due to the unique properties of the two lowest Landau bands of graphite. These Landau bands facilitate the formation of a single quasi-two-dimensional hybrid structure in which the moiré and bulk graphite states are inextricably mixed. Our results establish twisted graphene-graphite as the first in a new class of mixed-dimensional moiré materials.

Thin film and bulk morphology of PI-PS-PMMA miktoarm star terpolymers with both weakly and strongly segregated arm pairs

A series of three homologous ABC miktoarm star terpolymers having a polyisoprene (PI), a polystyrene (PS) and a poly(methyl methacrylate) (PMMA) arm, is synthesized by living anionic polymerization. While the volume fraction of the PI and the PS blocks are kept equal and constant, the volume fraction of the PMMA block is varied. The room temperature bulk structure is characterized using small-angle X-ray scattering and transmission electron microscopy, while the structure of thin films is investigated using scanning electron microscopy, atomic force microscopy and X-ray reflectometry. For both bulk and thin films, it is found that the sample with the lowest volume fraction of PMMA has a morphology distinctly different from the two samples with larger PMMA volume fraction. All data for both bulk and thin film samples are consistent with an alternating lamellar structure for the lowest PMMA volume fraction sample. The two higher PMMA volume fraction samples show standing rod structure, however the details of the packing in the PMMA matrix differ depending on sample thickness. Overall, a structure with PI domains screened from interacting with PMMA by a PS shell is seen for both samples. In bulk, core-shell cylinders pack hexagonally, while a 100 nm thin film of the sample with the largest volume fraction of PMMA shows a square-lattice packing. The structuring is driven by two factors: i) the strong segregation of the PI-PMMA arm pair together with the weak segregation of the two arm pairs with a PS block and ii) the geometrical constraints resulting from the star architecture. No wetting layers are observed for any of the thin film samples, neither at the air-polymner surface or at the polymer-substrate interface.

Superelastic NiTi Functional Components by High-Precision Laser Powder Bed Fusion Process: The Critical Roles of Energy Density and Minimal Feature Size.

Additive manufacturing (AM) was recently developed for building intricate devices in many fields. Especially for laser powder bed fusion (LPBF), its high-precision manufacturing capability and adjustable process parameters are involved in tailoring the performance of functional components. NiTi is well-known as smart material utilized widely in biomedical fields thanks to its unique superelastic and shape-memory performance. However, the properties of NiTi are extremely sensitive to material microstructure, which is mainly determined by process parameters in LPBF. In this work, we choose a unique NiTi intricate component: a robotic cannula tip, in which material superelasticity is a crucial requirement as the optimal object. First, the process window was confirmed by printing thin walls and bulk structures. Then, for optimizing parameters precisely, a Gyroid-type sheet triply periodic minimal-surface (G-TPMS) structure was proposed as the standard test sample. Finally, we verified that when the wall thickness of the G-TPMS structure is smaller than 130 μm, the optimal energy density changes from 167 J/m3 to 140 J/m3 owing to the lower cooling rate of thinner walls. To sum up, this work puts forward a novel process optimization methodology and provides the processing guidelines for intricate NiTi components by LPBF.

Hybrid homogenization theory with surface effects: Application to columnar nanoporous materials

Nanocomposites, including nanoporous ceramics, metals and polymers, are being widely used in various engineering applications. At the nanoscale, surface/interface effects have significant influence on the homogenized properties and local stresses. To account for these nanoscale effects, the well-established Gurtin–Murdoch theory of surface elasticity is typically incorporated into classical elasticity-based micromechanics schemes and numerical approaches based on the finite-element and most recently finite-volume methods. Whereas the classical micromechanics models either neglect the interactions of adjacent fibers/pores or account for the interactions in an average sense, the finite-element and finite-volume based approaches require high discretization to capture the large stress/strain gradients around the interface/surface. In contrast, the recently proposed hybrid homogenization theory (HHT) combines elements of locally-exact elasticity and finite-volume approaches and is both accurate and efficient, enabling rapid generation and analysis of ordered and quasi-random fiber or pore arrays. Herein, a continuous bulk matrix layer of arbitrary size endowed with a two-dimensional energetic surface is introduced between fiber/pore and discretized matrix region that enables exact calculation of stress jump conditions in the Young–Laplace equations of the Gurtin–Murdoch model. The enhanced HHT with interface/surface effects is validated by comparison with the stress concentration factors obtained using a classical elasticity solution and point-wise stress fields predicted by the locally-exact and finite-volume homogenization theories, exhibiting stable solutions even for very thin bulk matrix layers. The enhanced theory is then employed to study the influence of random nanopore microstructures with random pore radii on the complete set of homogenized moduli of porous silicon used in the electronics industry.

Reconstruction of a soft tissue defect in the toe using a serratus anterior fascia free flap: a case report

Toe injuries frequently occur as traumatic or oncologic defects. Compared with finger reconstruction, toe reconstruction has been rarely reported in the literature, because of the difficulty of toes’ relatively thin soft tissue envelope, their requirement for relatively challenging surgical techniques, and the slight improvements in gait. Therefore, toe reconstruction can be challenging for plastic surgeons, especially in cases with exposure of a tendon, bone, or joint. Herein, we present a case study of a 54-year-old woman with squamous cell carcinoma in situ (Bowen disease) that subsequently resulted in a defect on her second toe. A serratus anterior fascia free flap could be a good option for toe reconstruction due to its large caliber, lengthy pedicle, relatively easy dissection, and thin muscle bulk. We present our unique experience using the serratus anterior fascia free flap for the reconstruction of an oncologic toe defect.

Interaction of irradiation defects with precipitates in CuCrZr and Cu-1Fe alloys

In this work, we employ transmission electron microscopy and helium ion irradiation to study the interactions between the radiation-induced point defects and the incoherent/semi-coherent precipitates in CuCrZr and Cu-1Fe alloys. Both thin foil and bulk implantation were used to explore the interactions of radiation defects with interfaces. The irradiated defects show a strong interface character dependence in both implanted CuCrZr and Cu-1Fe alloys after similar level of radiation damage. Large-sized voids appear around the incoherent Cr precipitates, while no visible cavities are formed around precipitates in Cu-1Fe alloy. Interestingly, three types of semi-coherent Fe precipitates exhibit various irradiation damage mitigation mechanisms, such as twin boundaries migration, short range elements redistribution and precipitate abnormal growth, which are caused by radiation defect-mediated dislocation glide, element diffusion and mass transportation. The high sink efficiency of the incoherent precipitates trap vacancies and leads to the generation of submicron-sized voids. In contrast, the semi-coherent precipitates can regulate the motion of point defects and effectively enhance the recombination rate of vacancies and interstitials.

The Performance of V2O5:Ag Nanoparticles as Thin Film and Bulk Pellet Sensor for NO2 and NH3 Detection

A comparison of gas sensing performance of V2O5:Ag nanoparticles as thin film and as bulk pellet toward NO2 and NH3 is presented. V2O5:Ag nanoparticles thin films were deposited by vacuum thermal evaporation method on glass substrates while the pellets were prepared by powder technology. XRD patterns of thin film and pellet were polycrystalline with an orthorhombic structure. The value of average grain size is about 60 nm. The morphological properties of the samples have been distinguished by atomic force microscopy (AFM) and field effect scanning electron microscopy (FESEM) which indicated that the films showed homogeneous surfaces morphology and contained pores between the grains. Sensing results showed a various response to NO2 and NH3 gases. It was found that the sensitivity of thin films sensor is superior to that of the pellets sensor.

A comprehensive study on the metallographic preparation of W-Re samples for high resolution electron-based microscopy

Abstract Modern day rotating X-ray anodes utilize a conversion layer comprised of a tungstenrhenium alloy. The thermomechanical loading of this layer during computed tomography operation introduces various signs of fatigue like cracking, roughening, melting, or microstructural changes. Previous work on preparing tungsten samples primarily considered intact thin films or bulk material. This work focuses on the metallographic preparation of the conversion layer surface, which represents the sample edge in polished cross-sectional cuts. The main goals were minimizing preparation artefacts and maximizing obtainable image quality during electron backscatter diffraction. Twelve preparation methods were compared with regard to edge rounding, chipping, and obtainable image quality. Coating the samples with a thin layer of molybdenum and adding a tungsten sheet for edge stabilization led to vastly improved results. Chemical-mechanical polishing of such a sample gave the most balanced set of considered benchmarks.