Inhomogeneous Films Research Articles

Crystallization Control to Prepare Uniform CsPbI2Br Thin Films for High-Efficiency Perovskite Solar Cells.

All-inorganic perovskite solar cells (PSCs) face challenges related to film inhomogeneity, which arises from nonideal crystallization. This issue significantly impedes the advancement of all-inorganic PSCs. In this study, we observed that during the crystallization process of CsPbI2Br, the Br-rich intermediate phase is often preferentially deposited, leading to an uneven distribution in the out-of-plane direction of the perovskite film. To address this issue, crown ether molecules (dibenzo-18-crown-6) were introduced into the perovskite precursor solution. The complexation of crown ether with Cs cations and Br anions optimizes the crystallization sequence of the perovskite, ensuring that the intermediate phase closely conforms to the standard stoichiometric ratio. This adjustment significantly mitigates the problem of uneven halogen ion distribution along the out-of-plane direction. Furthermore, the crown ether thermally decomposes during the high-temperature annealing process, thereby not affecting the composition of the final perovskite film. Following crown ether treatment, the efficiency of the PSCs reached 14.08%, and the unpackaged devices maintained 80% of their initial efficiency after 1000 h of exposure to light in an atmospheric environment.

Dynamics of magnetic inhomogeneity in La2CoMnO6 films probed by Raman spectroscopy

We report the dynamic effects of magnetic inhomogeneity on the temperature evolution of the Raman modes in polycrystalline La2CoMnO6 (LCMO) films. The LCMO films were obtained via chemical solution deposition and annealed at different temperatures, 700, 800 and 900 °C. Temperature-dependent Raman spectroscopic studies uncover anomalous phonon energy behaviors, associated with strong spin-phonon couplings revealed even at ambient conditions. This effect, which is observed to occur well above ferromagnetic ordering temperature is ascribed to short-range Mn4+/Co2+ ferromagnetic clusters. Moreover, our study has shown that spin-phonon coupling strength is governed by competing antiferromagnetic (AFM) and ferromagnetic (FM) interactions. These results significantly enhance the understanding of the complex spin-phonon coupling mechanism to provide insights into magnetic inhomogeneity in systems with two or more magnetic sublattices. These findings suggest the presence of similar effects in other double perovskites within the RE2CoMnO6 (RE = rare earths) family, which exhibit analogous magnetic sublattice and order–disorder defects.

Investigation on mechanical and tribological properties of PTFE nanocomposites reinforced by surface-modified graphene using molecular dynamics simulations

Surface-modified nanoparticles are commonly used to improve the mechanical properties and wear resistance of polytetrafluoroethylene (PTFE). However, fewer studies have been devoted to quantitatively revealing the action mechanism of graphene (Gr) modified with different functional groups on the mechanical and tribological properties of PTFE. Herein, the effects of four functional groups (−OH, −NH2, −COOH, and −COOCH3 functional groups) on the surface of Gr nanosheets on the mechanical and tribological properties of PTFE nanocomposites are studied using molecular dynamics simulations. The results indicate that the incorporation of functional groups to the Gr surface is able to significantly improve the mechanical properties and wear resistance of the nanocomposites, and the COOH-functionalized Gr nanosheet shows the best reinforcing effect due to the synergistic effect of its own high surface roughness and strong interfacial interaction between itself and the matrix. It is also found that the friction coefficient of the nanocomposites is obviously increased by the inclusion of functionalized Gr nanosheets, and the greater the surface roughness of the functionalized Gr nanosheet, the more significant the growth of the friction coefficient of the nanocomposites. The pull-out test and confined shear simulation reveal that due to the increased interfacial shear strength and the isolation of functional groups, an inhomogeneous transfer film is formed at the friction interface, leading to a decreased anti-friction property. This study provides some guidance for the future design and development of polymer nanocomposites with excellent mechanical and tribological performance for use in extreme service conditions.

Isothermal magnetization and magnetoresistive variations in trilayer (La2/3Sr1/3MnO3/γ-Fe2O3/La2/3Sr1/3MnO3) hetero-structure

We report the magnetic field dependent magnetization and magnetoresistance (MR) properties of La2/3Sr1/3MnO3 (LSMO)/ γ -Fe2O3/LSMO trilayer heterostructure and single layer LSMO film grown on SiO2/Si (100) substrates utilizing Pulsed Laser Deposition technique. The metal- insulator-metal configuration is a magnetic tunnel junction topology, which is a parallel network of two metallic layers (LSMO) and one insulating layer ( γ -Fe2O3) in current-in-plane (CIP) geometry. The intrinsically inhomogeneous polycrystalline trilayer film shows much lower (almost half ) coercivity, compared to single layer LSMO film. The MR-H [MR = (ρ (H)-ρ (0))/ρ (0)] behavior of the films is studied under two regimes namely, Low Field Magnetoresistance (LFMR) and High Field Magnetoresistance (HFMR) at 5 K and 300 K in the field range of 0–7 T. Several equations were developed to simulate the experimental MR-H data of the studied samples. For both the films, in the LFMR region (0 T < H ≤ 1 T @ 5 K), the variation in MR exhibits a linear increase with H, followed by a logarithmic behavior in the HFMR region (1 T < H ≤ 7 T @ 5 K). For the trilayer film in CIP geometry, a negative MR of 16% is achieved at 5 K and 1 T field conditions, while LSMO single layer shows a negative MR of 13% at same temperature and field conditions. The MR obtained at high field (7 T) for both the films is quantitatively equal with a value of 38% @ 5 K and 15% @ 300 K. We obtained significantly better MR-H results for the trilayer in current-perpendicular-to-plane (CPP) configuration, where the two metallic layers and an insulating layer are in series. In the CPP configuration, MR is 21% @ 5 K and 1 T field, while it reached to 44% at 5 K and 7 T field. At room temperature, the trilayer heterostructure shows MR of 17% at 7 T field condition. At a higher field of 9 T, trilayer film in CPP mode exhibits MR value of 48% @ 5 K and 23% @ 300 K. Further, the anti-parallel and parallel magnetization states of the MIM device are well manifested in CIP and CPP geometries.

Enhanced cation interaction in perovskites for efficient tandem solar cells with silicon.

To achieve the full potential of monolithic perovskite/silicon tandem solar cells, crystal defects and film inhomogeneities in the perovskite top cell must be minimized. We discuss the use of methylenediammonium dichloride as an additive to the perovskite precursor solution, resulting in the incorporation of in situ-formed tetrahydrotriazinium (THTZ-H+) into the perovskite lattice upon film crystallization. The cyclic nature of the THTZ-H+ cation enables a strong interaction with the lead octahedra of the perovskite lattice through the formation of hydrogen bonds with iodide in multiple directions. This structure improves the device power conversion efficiency (PCE) and phase stability of 1.68 electron volts perovskites under prolonged light and heat exposure under 1-sun illumination at 85°C. Monolithic perovskite/silicon tandems incorporating THTZ-H+ in the perovskite photo absorber reached a 33.7% independently certified PCE for a device area of 1 square centimeter.

Unveiling nano-scale chemical inhomogeneity in surface oxide films formed on V- and N-containing martensite stainless steel by synchrotron X-ray photoelectron emission spectroscopy/microscopy and microscopic X-ray absorption spectroscopy

Nano-scale chemical inhomogeneity in surface oxide films formed on a V- and N-containing martensite stainless steel and tempering heating induced changes are investigated by a combination of synchrotron- based hard X-ray Photoelectron emission spectroscopy (HAXPES) and microscopy (HAXPEEM) as well as microscopic X-ray absorption spectroscopy (µ-XAS) techniques. The results reveal the inhomogeneity in the oxide films on the micron-sized Cr2N- and VN-type particles, while the inhomogeneity on the martensite matrix phase exists due to localised formation of nano-sized tempering nitride particles at 600 °C. The oxide film formed on Cr2N-type particles is rich in Cr2O3 compared with that on the martensite matrix and VN-type particles. With the increase of tempering temperature, Cr2O3 formation is faster for the oxidation of Cr in the martensite matrix than the oxidation of Cr nitride-rich particles.

Local strain induced structural inhomogeneity in Fe thin films on Cu(001)

We investigate atomic-scale surface structures of 7 monolayer Fe thin films on a Cu(001) substrate by scanning tunneling microscopy. Near the step edges, the epitaxial fcc(001) lattice is stabilized on the upper terrace. In contrast, on the lower terrace, the bulk stable bcc(110) lattice and several surface reconstructions with high-density adsorbates are observed. The changes of electronic structures on the latter region from fcc Fe to bcc Fe is verified by spectroscopic measurements after the desorption of adsorbates, suggesting the local strain effect as the dominant origin of observed structural inhomogeneity.

Optical characterization of inhomogeneity of polymer-like thin films arising in the initial phase of plasma-enhanced chemical vapor deposition

In this study, an optical investigation in a wide spectral range of polymer-like (SiOxCyHz) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) is presented. The primary focus is on assessing the homogeneity of the grown films. Within the PECVD, it is possible to alter the properties of the deposited material by continually adjusting deposition process parameters and hence allow for the growth of inhomogeneous layers. However, as shown in this study, the growth of homogeneous layers could be similarly challenging. This challenge is especially pronounced at the beginning of the deposition process, where it is necessary to consider the influence of the substrate among other factors, as even slight variations in the deposition conditions can lead to the formation of inhomogeneous layers. Several series of polymer-like thin films were deposited onto silicon substrates with the goal of producing homogeneous layers, i.e. all deposition parameters were held constant. These samples were optically characterized with a special interest in homogeneity, especially at the beginning of the growth. It was found that initial inhomogeneous growth is always present. The thickness of the initial inhomogeneous part was found to be surprisingly large.

Unusual anomalous Hall effect in SrRuO3 films with linear out-of-plane Ru vacancies gradient

The manipulation of magnetic transport properties has been one of the central problems in spintronics. However, the modulation of Hall signals in thin films has stringent requirements on their thickness and means of growth. Here, a series of inhomogeneous SrRuO3 thin films with different paths of linear out-of-plane Ru vacancies gradient was designed to generate an unusual anomalous Hall effect (UAHE) under broad growth conditions. Combining x-ray diffraction and magnetic data, it was concluded that the appearance of UAHE was not a simple superposition of AHE caused by multiple magnetic phases. The interaction between these magnetic phases in the linear-vacancies-gradient SrRuO3 films was analyzed by the first-order reversal curve (FORC) method, and it was found that the change trend of FORC was the same as that of UAHE. Such out-of-plane linear-vacancies-gradient thin film provides a way to regulate the different phases by introducing the cation vacancies distribution in an orderly way to control their magnetic and transport properties in oxide films. Furthermore, a distinctive perspective on the origin of UAHE was obtained by combining FORC with UAHE.

Morphological Homogeneity and Interface Modification as Determinant Factors of the Efficiency and Stability for Upscaling Organic Solar Cell.

Morphological homogeneity and interfacial traps are essential issues to achieve high-efficiency and stable large-area organic solar cells (OSCs). Herein, by the investigation of three quinoxaline-based acceptors, i.e., PM6:Qx-1, PM6:Qx-2, and PM6:Qx-p-4Cl, the performance degradation in up-scaling OSCs is explored. The inhomogeneous morphology in PM6:Qx-2 induces a nonuniform spatial distribution of charge generation, showing a rapid decline in efficiency and stability in large-area OSCs. In comparison, the homogeneous morphology in PM6:Qx-1 and PM6:Qx-p-4Cl alleviates the stability drop. When utilizing 2-phenylethylmercaptan to fill the interfacial traps, the stability drop disappears for PM6:Qx-1 and PM6:Qx-p-4Cl, while it persists for PM6:Qx-2. The PM6:Qx-1 large-are device yields a high efficiency of 13.47% and superior thermal stability (T80 = 2888h). Consequently, the interface modification dominates the performance degradation of large-area devices with homogeneous morphology, while it cannot eliminate the traps in inhomogeneous film. These results provide a clear understanding of degradation mechanisms in upscaling devices.

Post-hot-cast annealing deposition of perovskite films with infused multifunctional organic molecules to enhance the performance of large-area light-emitting devices.

All-inorganic perovskites show great promise as an emission layer in perovskite light-emitting diodes (PeLEDs) owing to their easy solution processing, low manufacturing cost, and excellent optoelectronic properties. However, there is still an immense performance gap from small-area devices to large-area PeLED devices. The inhomogeneity of large-area high-quality perovskite films inevitably leads to vast defects and electroluminescence performance losses. Herein, a post-hot-cast annealing deposition scheme and the introduction of the multifunctional molecule 2-amino-1,3-propanediol (APDO) were proposed to regulate the crystallization of the perovskite film. As a result, uniform APDO:CsPbBr2.5Cl0.5 perovskite films with high crystallinity and lower defect density were deposited by post-hot-cast annealing. A decent maximum brightness of 2659 cd m-2 was achieved for the large-area cyan PeLEDs with an emitting area of 400 mm2.

(Invited) Near-Field Optics and Its Applications in Nanophotonic Devices: A Review

In our presentation, we will offer an overview of aperture-type scanning near field optical microscopy (SNOM) – a family of nano-optical imaging techniques derived from scanning probe microscopy which are capable of subwavelength resolution, and the development of three dimensional (3D) SNOM methods undertaken by our group to locally image the distribution of the electromagnetic radiation in the proximity of nanoparticles and nano-objects.[1] We will discuss a few applications in which we took advantage of 3D-SNOM to design specific optical nanosystems for light harvesting device applications. Specific case studies that will be presented include the design of plasmonic thin-film solar cells enhanced by random arrays of copper nanoparticles,[2] and the use of 3D-SNOM for characterizing evanescent waveguides self-assembled from of copper nanoparticles assembled on thin films of graphene.[3] In the final part of our talk, we will we present near-field scanning thermoreflectance imaging (NeSTRI), a new pump-probe technique invented in our group (see Figure 1) in which an aperture-type SNOM is used to contactlessly determine the thermal conductivity of inhomogeneous thin films and low-dimensional systems at the nanoscale for heat-spreading and thermoelectric applications.[4,5] These examples well represent the versatility of SNOM imaging and its potential for designing an even wider family of nano-optical devices.[1] P Bazylewski, S Ezugwu, G Fanchini, Applied Sciences 7 (2017) 973[2] S Ezugwu, H Ye, G Fanchini, Nanoscale 7 (2016) 252-260[3] T Ouyang, A Akbari-Sharbaf, J Park, R Bauld, MG Cottam, G Fanchini, RSC Advances 5 (2015) 98814-98821[4] S Ezugwu, S Kazemian, DYW Choi, G Fanchini, Nanoscale 9 (2017) 4097-4106[5] S Kazemian, S Ezugwu, G Fanchini, Proc. SPIE 10926, Quantum Sensing and Nano Electronics and Photonics XVI, 109260L (1 February 2019); doi: 10.1117/12.2509828 Figure 1

Highly conductive RuO2 thin films from novel facile aqueous chemical solution deposition

Ruthenium dioxide (RuO2) thin films were synthesized by Chemical Solution Deposition (CSD) on silicon substrates using only water and acetic acid as solvents. The microstructure, phase purity, electrical and optical properties as well as the thermal stability of the thin films have been characterized. The microstructure of the thin films strongly depends on the annealing temperature: A smooth thin film was achieved at an annealing temperature of 600 °C. Higher annealing temperatures (800 °C) led to radial grain growth and an inhomogeneous thin film. A very low resistivity of 0.89 µΩm was measured for a 220 nm-thick thin film prepared at 600 °C. The resistivity of the thin films increases with temperature, which indicates metallic behavior. Phase purity of the thin films was confirmed with X-ray Diffraction (XRD) measurements, X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. Transmission and reflectivity measurements indicate that RuO2 efficiently blocks the UV-VIS and IR wavelengths. The optical constants determined via spectroscopic ellipsometry show high absorption in the near-IR region as well as a lower one in the UV-VIS region. The thermal stability was investigated by post-annealing, confirming that the thin films are stable up to 750 °C in synthetic air.Graphical

Automated software and hardware complex based on an eddy current sensor for studying the properties of thin films

The variety of existing thin-film structures used in modern science and technology is described. The design of a miniature eddy current transducer was developed and improved. Methods for determining the properties of films were presented. The main design parameters of the developed transducer are described and a scheme of operation complex is presented. The results of scanning electrically homogeneous and inhomogeneous films are presented. It was possible to establish the degree of uniformity of the conductivity of the film by constructing histograms that display the number of points with different electrical conductivity.

Study on the Adsorption Deformation of a Substrate via Spin Coating Based on the 3D-DIC Method and Its Effect on the Homogeneity of Perovskite Films.

The physical and chemical stability of perovskite films has always been a key issue for their industrialization, which has been extensively studied in terms of materials, environment, and encapsulation. Spin coating is one of the most commonly used methods for the preparation of perovskite films in research. However, little attention has been paid to the deformation state of the substrate when it is fixed by means of adsorption and its impact. In this work, the three-dimensional digital image correlation (3D-DIC) method and hyperspectral technology are used to acquire and analyze the adsorption deformation characteristics of the substrate during spin coating, as well as the resulting inhomogeneity. Plastic and four different thicknesses of float glass (0.2, 0.5, 0.7, 1.1 mm) were selected as substrates, and they were tested separately on two suction cups with different structures. The results show that the plastic and 0.2 mm specimens exhibit obvious strain localization behavior. The distribution and magnitude of the strain are affected by the size of the sucker structure, especially the width of the groove. For glass specimens, this effect shows a nonlinear decrease with increasing substrate thickness. Compared to the strain value, the irregularity of local deformation has a greater impact on the non-uniform distribution of materials. Finally, inhomogeneities in the perovskite films were observed through optical lens and hyperspectral data. Obviously, the deformation of the substrate caused by adsorption should attract the attention of researchers, especially for flexible or rigid substrates with low thickness. This may affect the centrifugal diffusion path of the precursor, causing microstructure inhomogeneity and residual stress, etc.

Recrystallization effect on surface passivation of Hastelloy X alloy fabricated by laser powder bed fusion

Post-heat treatment is generally adopted in the additive manufacturing field due to its alleviation of high residual stress and modification of rapid-solidified multilevel heterogeneous microstructure, and the related performance of the heat-treated counterparts calls for a systemic investigation to build a criterion of the heat treatment procedure. In this work, we focus on the heat treatment effects on the recrystallization of the Hastelloy X alloy produced by the laser powder bed fusion (LPBF) method, and the related surface passivation of the heat-treated counterparts is meticulously assessed as well. Results show that the multilevel heterostructure for LPBF Hastelloy X alloy consists of sub-micro dislocation cell substructures with Cr/Mo elemental segregation, fine columnar grains, and periodically-distributed molten pools. After heat treatment, partially and fully recrystallized structures for LPBF Hastelloy X alloys were achieved at 1100 and 1200 °C for 1 h, respectively. Furthermore, the as-built LPBF Hastelloy X alloy shows superior corrosion resistance while the heat-treated one (1100 °C) exhibits the worst in the borate buffer solution. The growth of passive film exhibited a highly linear correlation with the nucleation process controlled by diffusion, and high dislocation density and low angle grain boundary decreased the diffusion coefficient of cation vacancies, augmenting the nucleation sites of the passive film and enhancing its growth rate. Moreover, the micro-galvanic effect resulting from the partially recrystallized microstructure actively facilitated the formation of inhomogeneous porous passive films, leading to the worst corrosion resistance.

Nonrelaxational FMR peak broadening in spatially inhomogeneous films

The modification of magnetic properties in spatially inhomogeneous epitaxial films of magnetic shape memory alloys in martensitic state with the temperature variation has been studied. The proposed theoretical model is based on Landau theory of martensitic transformation and statistical model of martensitic state. It was shown that that spatial inhomogeneity of the material leads to the dispersion of local martensitic transformation temperatures resulting in the variation of local magnetic anisotropy values. This model allows describing the dramatic ferromagnetic resonance line broadening observed in the experiments in epitaxial films of magnetic shape memory alloys at low temperatures.

Engineering structural homogeneity and magnetotransport in strained Nd2Ir2O7 films

The 5d pyrochlore iridate family (R2Ir2O7, where R is a rare earth ion) has garnered significant attention due to its topological properties, such as Weyl semimetallic phases and axion insulator. However, the investigation of these properties has been impeded by severe iridium loss during growth, which results in the formation of defects and impurities. Herein, we demonstrate a method for controlling impurities and defects in strained Nd2Ir2O7 (NIO-227) films by compensating for iridium loss during growth. By increasing the amount of IrO2 target ablated, we enhance the morphological quality and electrical transport properties of the fabricated films. Furthermore, our results show that the anomalous Hall effects of the films have a strong dependency on the amount of IrO2 target ablated, which is attributed to the structural inhomogeneity in the NIO-227 films. Our work provides a way to control defects and impurities and would promote the investigation of topological phases in the family R2Ir2O7.

Influences of the inhomogeneity of the ferroelectric thin films on switching current

Influences of the inhomogeneity of the ferroelectric thin films on switching current

Helical polymers for dissymmetric circularly polarized light imaging.

Control of the spin angular momentum (SAM) carried in a photon provides a technologically attractive element for next-generation quantum networks and spintronics1-5. However, the weak optical activity and inhomogeneity of thin films from chiral molecular crystals result in high noise and uncertainty in SAM detection. Brittleness of thin molecular crystals represents a further problem for device integration and practical realization of chiroptical quantum devices6-10. Despite considerable successes with highly dissymmetric optical materials based on chiral nanostructures11-13, the problem of integration of nanochiral materials with optical device platforms remains acute14-16. Here we report a simple yet powerful method to fabricate chiroptical flexible layers via supramolecular helical ordering of conjugated polymer chains. Their multiscale chirality and optical activity can be varied across the broad spectral range by chiral templating with volatile enantiomers. After template removal, chromophores remain stacked in one-dimensional helical nanofibrils producing a homogeneous chiroptical layer with drastically enhanced polarization-dependent absorbance, leading to well-resolved detection and visualization of SAM. This study provides a direct path to scalable realization of on-chip detection of the spin degree of freedom of photons necessary for encoded quantum information processing and high-resolution polarization imaging.