Strain In Films Research Articles

Structural characterization of epitaxial ScAlN films grown on GaN by low-temperature sputtering

ScAlN has garnered substantial attention for its robust piezoelectric and ferroelectric properties, holding promise for diverse electronic device applications. However, the interplay between its structural attributes and physical properties remains poorly understood. This study systematically elucidates the structural characteristics of epitaxial ScAlN films grown on GaN by low-temperature sputtering. Correlations between Sc composition, lattice constants, and film strains were revealed utilizing high-resolution X-ray diffraction, reciprocal space mapping, and machine learning analyses. Our machine-learning model predicted c-axis lattice constants of ScAlN grown on GaN under various conditions and suggested that sputtering permits coherent growth over a wide compositional range. These findings advance the understanding of ScAlN and provide valuable insights for the research and development of novel ScAlN-based devices.

Characterization of In‐Plane Piezoelectric Strain of Ferroelectric Thin Films by the Magnetoelectric Coupling Effect

Piezoelectric response of the ferroelectric/piezoelectric thin films is crucial for actuation of micro‐electromechanical systems (MEMS) devices as well as strain‐mediated electronic devices. Among many piezoelectric modes, longitudinal and transverse piezostrain is the most widely used. Various methods have been well established for quantitative characterization of the longitudinal piezostrain, such as laser interferometer and piezoelectric force microscopy. However, it is still a great challenge to characterize transverse piezostrain. Herein, a new method is established to characterize the transverse (in‐plane) piezostrain of ferroelectric/piezoelectric thin films by the inverse magnetoelectric (ME) coupling effect. A ferromagnetic thin film is patterned onto the ferroelectric thin film to form an artificial multiferroic heterostructure. The transverse piezostrain can transfer from the ferroelectric layer to the ferromagnetic layer and shift its ferromagnetic resonance field through the ME coupling effect. By comparing with a control sample of “bulk piezoceramic/ferromagnetic thin film” whose in‐plane piezostrain can be easily measured, the in‐plane piezostrain and piezoelectric coefficient of the ferroelectric thin films are then estimated. The in‐plane piezoelectric coefficient d31 and piezoelectric strain S31 of Pb(Zr,Ti)O3 (PZT) thin films are measured by this method. This new method will promote to fully understand the piezoelectric properties of the ferroelectric/piezoelectric thin films.

Significant effects of epitaxial strain on the nonlinear transport properties in Ca2RuO4 thin films with the current-driven transition

Control of the nonlinear transport properties in strongly correlated materials with metal–insulator transitions has become an important task in the development of next-generation computing elements. Here, we demonstrate that the nonlinear transport properties in the Mott insulator Ca2RuO4, which are caused by the current-driven transition with a nonthermal-type mechanism, can be significantly affected by epitaxial strain in thin films. Ca2RuO4 epitaxial thin films were grown on various single-crystal substrates by the solid-phase epitaxy method under nonvacuum conditions, and the effects of epitaxial strain on the transport properties were investigated. While nonlinear transport properties with negative differential resistance behavior were observed in [001]-oriented Ca2RuO4/LaAlO3 (001) thin films, the current transport nonlinearity was critically diminished in the current–voltage characteristics of Ca2RuO4/NdCaAlO4 (100) thin films with the [110] orientation. Through structural characterization of the thin films, a possible correlation between the strain states and nonlinear transport properties was discussed.

Characterization of 2D Transition Metal Dichalcogenides Through Anisotropic Exciton Behaviors.

This study reports the first attempt to characterize the quality, defects, and strain of as-grown monolayer transition metal dichalcogenide (TMDC)-based 2D materials through exciton anisotropy. A standard ellipsometric parameter (Ψ) to observe anisotropic exciton behavior in monolayer 2D materials is used. According to the strong exciton effect from phonon-electron coupling processes, the change in the exciton in the Van Hove singularity is sensitive to lattice distortions such as defects and strain. In comparison with Raman spectroscopy, the variations in exciton anisotropy in Ψ are more sensitive for detecting slight changes in the quality and strain of monolayer TMDC films. Moreover, the optical power requirement for TMDC characterization through exciton anisotropy in Ψ is ≈10-5mWcm-2, which is significantly less than that of Raman spectroscopy (≈106mWcm-2). The standard deviation of the signals varies with strain (defects) in Raman spectra and exciton anisotropies in Ψ are 0.700 (0.795) and 0.033 (0.073), indicating that exciton anisotropy is more sensitive to slight changes in the quality of monolayer TMDC films.

Investigation into the growth mode of GaN thin films on 4H–SiC substrates via plasma-enhanced atomic layer deposition

GaN ultra-thin films are deposited on 4H–SiC substrates using plasma-enhanced atomic layer deposition (PEALD). Our investigation reveals a remarkably narrow temperature window (225–275 °C) for GaN PEALD growth, characterized by substrate hindered nucleation retardation. Within this temperature window, the growth process exhibits three distinct stages, which are demarcated by the 35th and 200th cycles, each displaying unique characteristic, especially in aspects growth rate, film strain, surface topography, and crystallization. Notably, the instantaneous growth rate, which is often overlooked, provided a better representation of the growth state compared with growth per cycle (GPC) and shows better consistency with film strain and surface roughness. Additionally, we observed a step filling effect during the early stages (10 cycles) of ALD growth, which reduces surface roughness to 0.12 nm form 0.20 nm of substrate SiC. X-ray photoelectron spectroscopy (XPS) analysis reveals the formation of N–Si and N–C bonds on the sample surface following 20 cycles of plasma bombardment. Further analysis using high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) consistently confirms the presence of a hexagonal wurtzite crystalline structure in GaN films deposited under varying cycle counts.

High-performance pulse light stable perovskite indoor photovoltaics

Perovskite solar cells offer great potential as a sustainable power source for distributed electronic devices that operate indoors. However, the impact of advanced lighting technology, especially the widely used pulse width modulation (PWM) technology, on perovskite photovoltaics has been ignored. Herein, for the first time in photovoltaics, we find that the light impact emitted by the PWM lighting system caused dynamic strain in perovskite thin films, induced phase separation, and accelerated the generation of metallic lead (Pb0) defects, leading to irreversible degradation of the cell performance after 27 h (T80). To address this issue, formamidinium triiodide (FAI3) is chosen to treat the surface of the perovskite and release residual stress, resulting in reduced lattice deformation during dynamic strain processes. Meanwhile, it suppresses harmful Pb0 defects and reduces Voc loss at low light intensity. The champion device achieves impressive power conversion efficiency (PCE) of 35.14% and retains 99.5% of the initial PCE after continuous strobe light soaking for 2160 h.

Investigation of gradient band gap in Cu2ZnSnS4 thin films with residual strain

This study investigates the change in the band gap of Cu2ZnSnS4 (CZTS) thin films under a gradual change in the residual strain of the thin films. Two CZTS thin film samples are fabricated at different initial substrate temperatures before annealing. X-ray diffraction results are obtained at various incidence angles, and the residual strains at the incidence angles are obtained from the Williamson–Hall plot. The larger the mean residual strain of the CZTS thin films, the greater are the redshift and full width at half maximum of the main Raman peak position. Results of Tauc plot analysis show that the CZTS thin film samples exhibit a gradient band gap. The sample with a wider distribution of the residual strain shows a greater band gap variation.

Probing the Intrinsic Strain in Suspended Graphene Films Using Electron and Optical Microscopy.

Quantifying the intrinsic properties of 2D materials is of paramount importance for advancing their applications. Large-scale production of 2D materials merits the need for approaches that provide direct information about the role of growth substrate on 2D material properties. Transferring the 2D material from its growth substrates can modify the intrinsic properties of the asgrown 2D material. In this study, suspended chemical vapor deposition (CVD) graphene films are prepared directly on their growth substrates in a high-density grid array. The approach facilitates the quantification of intrinsic strain and doping in suspended CVD graphene films. To achieve this, transmission electron microscopy and large-area Raman mapping are employed. Remarkably, the analysis reveals consistent patterns of compressive strain (≈-0.2%) both in the diffraction patterns and Raman maps obtained from these suspended graphene films. By conducting investigations directly on the growth substrates, the potential influences introduced during the transfer process are circumvented effectively. Consequently, the methodology offers a robust and reliable means of studying the intrinsic properties of 2D materials in their authentic form, uninfluenced by the transfer-induced alterations that may skew the interpretation of their properties.

Buried Interface Optimization for Flexible Perovskite Solar Cells with High Efficiency and Mechanical Stability.

The power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs) are significantly reduced by defect-induced charge non-radiative recombination. Also, unexpected residual strain in perovskite films leads to an unfavorable impact on the stability and efficiency of PSCs, notably flexible PSCs (f-PSCs). Considering these problems, a thorough and effective strategy is proposed by incorporating phytic acid (PA) into SnO2 as an electron transport layer (ETL). With the addition of PA, the Sn inherent dangling bonds are passivated effectively and thus enhance the conductivity and electron mobility of SnO2 ETL. Meanwhile, the crystallization quality of perovskite is increased largely. Therefore, the interface/bulk defects are reduced. Besides, the residual strain of perovskite film is significantly reduced and the energy level alignment at the SnO2/perovskite interface becomes more matched. As a result, the champion f-PSC obtains a PCE of 21.08% and rigid PSC obtains a PCE of 21.82%, obviously surpassing the PCE of 18.82% and 19.66% of the corresponding control devices. Notably, the optimized f-PSCs exhibit outstanding mechanical durability, after 5000 cycles of bending with a 5mm bending radius, the SnO2-PA-based device preserves 80% of the initial PCE, while the SnO2-based device only remains 49% of the initial value.

Perovskite stannate La-doped BaSnO3 films for near- and mid-infrared plasmonic applications

La-doped BaSnO3 (LBSO) epitaxial thin films were grown on (001) MgO substrates by pulsed laser deposition using a La0.04Ba0.96SnO3 target. The structural, electrical, and optical properties of the LBSO films were investigated as a function of oxygen pressure during deposition. The carrier mobility can be tuned from 13 cm2V−1s−1 to 44 cm2V−1s−1 by varying oxygen partial pressure from 2 Pa to 12 Pa, respectively. This improved mobility is attributed to the reduced film strain via reducing oxygen defect concentration or reducing off-stoichiometry in the film. The optical permittivity of LBSO films can also be modified as a function of the oxygen pressure during deposition, allowing tuning of their epsilon-near-zero wavelength from 1.9 µm to 5.6 µm. These results demonstrate that LBSO thin films grown on MgO can be used for plasmonic devices at mid-wave infrared wavelengths.

Time-resolved high-energy X-ray diffraction studies of ultrathin Ni ferrite films on MgO(001)

Time-resolved high-energy X-ray diffraction was used during growth of ultrathin Ni x Fe3−x O4 films with varying Ni content (0 ≤ x ≤ 1.5) deposited on MgO(001) substrates by reactive molecular beam epitaxy, providing an insight into the growth dynamics of these films. In order to obtain structural information, reciprocal-space maps were recorded and the temporal evolution of the Bragg peaks specific to the octahedral and tetrahedral lattice sites of the inverse spinel structure of Ni x Fe3−x O4 was observed during growth of the films. A time delay, corresponding to a coverage of 1.2–1.8 nm, between the appearance of the Bragg reflections originating from octahedral sites and reflections originating exclusively from tetrahedral sites indicates that the ferrite films grow in two stages. In the initial growth phase, a rock salt interface layer is formed. Afterwards, a structural transition occurs and the films grow in an inverse spinel structure. The thickness of the initial rock salt phase was found to increase with Ni content and to be responsible for atypical strain in the thin films. Films with Ni contents x > 1 do not show a structural transition. These films remain in a (deficient) rock salt structure consisting of a mixed Ni–Fe oxide and do not form a spinel structure at all. They show an increased number of NiO clusters as detected by X-ray photoelectron spectroscopy of the valence band, accompanied by a significant roughening of the films.

Synthesis of Poly(ethylene furanoate) Based Nanocomposites by In Situ Polymerization with Enhanced Antibacterial Properties for Food Packaging Applications.

Poly(ethylene 2,5-furandicarboxylate) (PEF)-based nanocomposites containing Ce-bioglass, ZnO, and ZrO2 nanoparticles were synthesized via in situ polymerization, targeting food packaging applications. The nanocomposites were thoroughly characterized, combining a range of techniques. The successful polymerization was confirmed using attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, and the molecular weight values were determined indirectly by applying intrinsic viscosity measurements. The nanocomposites' structure was investigated by depth profiling using time-of-flight secondary ion mass spectrometry (ToF-SIMS), while color measurements showed a low-to-moderate increase in the color concentration of all the nanocomposites compared to neat PEF. The thermal properties and crystallinity behavior of the synthesized materials were also examined. The neat PEF and PEF-based nanocomposites show a crystalline fraction of 0-5%, and annealed samples of both PEF and PEF-based nanocomposites exhibit a crystallinity above 20%. Furthermore, scanning electron microscopy (SEM) micrographs revealed that active agent nanoparticles are well dispersed in the PEF matrix. Contact angle measurements showed that incorporating nanoparticles into the PEF matrix significantly reduces the wetting angle due to increased roughness and introduction of the polar -OH groups. Antimicrobial studies indicated a significant increase in inhibition of bacterial strains of about 9-22% for Gram-positive bacterial strains and 5-16% for Gram-negative bacterial strains in PEF nanocomposite films, respectively. Finally, nanoindentation tests showed that the ZnO-based nanocomposite exhibits improved hardness and elastic modulus values compared to neat PEF.

Ferroelectric film-based stretch sensor with successive stretch and release motions characterized by a combined piezoelectric and electrostrictive effect

A stretch sensor fabricated from a hydrothermally grown ferroelectric PZT film on titanium foil was developed. The unique sensor structure effectively converts the stretching motion of the PZT film to an out-of-plane deformation with a linear strain variation. An experiment with a strain-stretch displacement of more than 10% was conducted using the sensor anchored on a silicone rubber plate. The induced polarization of the PZT film was linearly converted into a voltage output through the sensing circuit. The combined piezoelectric and electrostrictive effects are proposed to characterize the experimental results. Based on both the elastic Gibbs energy and piezoelectric theory, we derive a mathematical equation relating the strain and polarization of the PZT film. The measured stretch displacement and voltage data exhibited an excellent fit with the derived equation. An identical repeated motion can be sequenced by the memorized function of the sensor using the constant term of the fitting curve. With the characterized results, the computed displacement matched well with the measured displacement for repeated cyclic motions, with a correlation coefficient greater than 0.99. Compared with the simple piezoelectric effect, the more accurate displacement obtained via the detected voltage will significantly benefit wearable stretch sensors based on ferroelectric films.

Temporal and spatial tracking of ultrafast light-induced strain and polarization modulation in a ferroelectric thin film.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

Unveiling the polar properties on barium bismuthate perovskite thin films with distinct Ba/Bi ratios

This research aims to investigate the domain structure of BaBiO3 (80:20), BaBiO3 (50:50), and BaBiO3 (30:70) thin films obtained by the polymeric precursor method. For better sample designations, the following codes were employed: (80:20) (BBO82), (50:50) (BBO55), and (30:70) (BBO37). The piezo-ferroelectric coupled behavior observed for barium bismuthates thin films was elucidated in terms of bismuth and oxygen vacancies as well as grain morphology, which can be controlled by stoichiometric changes (Ba/Bi ratios). The films crystallize in a rhombohedral BaBiO3 structure with an R-3R space group without any deleterious secondary phase. Additionally, the hysteretic strain, dielectric, ferroelectric, and piezoelectric properties of BaBiO3 thin films are strongly dependent on the film texture. Sample BBO55 showed a distinguished preferred orientation along the (341) plane with a low associated coercive field (66.87 kV/cm), free of imprint and with no polarizations gap compared to samples BBO37 and BBO82. Sample BBO55 presented the highest piezoelectric (d33-eff ≈ 45.7 pm/V) and ferroelectric (Pr≈19.87 μC/cm2) responses. This behavior can be ascribed to a highly textured film, which generates less strain throughout the crystal lattice, facilitating the polarization switching mechanism and improving piezo/ferroelectricity. These findings are significant for developing novel, high-performance, lead-free piezoelectric materials to be applied in different electronic components such as magnetic field sensors, switches, actuators, and other types of memory devices.

Strain and atomic stacking of bismuth thin film in its quasi-van der Waals epitaxy on (111) Si substrate.

We report on the structural properties of Bi thin films grown on (111) Si substrates with a thickness of 22-30 BL. HRXRD and EBSD measurements show that these Bi films are mainly composed of twinning grains in the (0003) direction. The grain size can be as large as tens of microns. From a double-peak (01[Formula: see text]4) φ-scan, we found two pairs of twinning phases coexisting with a rotation angle of ~ 3.6°. We proposed a coincidence site lattice model based on preferential close-packed sites for Bi atoms on Si (111) surface to explain the coexistence of the rotation phases in the quasi-van der Waals epitaxy. From the measured lattice constants c and a of our samples, along with the data from the literature, we derived a c-a relation: (c-c0) = - 2.038(a-a0), where c0 and a0 are the values of bulk Bi. The normalized position of the second basis atom in the unit cell x, in these strained Bi films is found very close to that of bulk Bi, indicating that the strain does not disturb the Peierls distortion of the lattice. The fixed ratio of bilayer thickness to lattice constant c, reveals that the elastic properties of covalent-bonded bilayer dominate those of Bi crystal.

Thickness dependent strain effect on ferromagnetic insulating properties of LaCoO3 thin films

Epitaxial tensile-strain is an important factor to determine the superior ferromagnetic insulating property of LaCoO3 (LCO) epitaxial films. In this paper, the whole development of LCO ferromagnetic insulating properties modulated by thickness-dependent strain effect were studied and we found that the film strain decreased and the ferromagnetic property weakened with increasing film thickness. The measurements of x-ray magnetic circular dichroism confirm that the spin moment of Co3+ ions is significantly larger than its orbital magnetic moment and it also reduces with increasing thickness. The x-ray absorption spectroscopy revealed that the increasing film thickness is unfavorable for the presence of eg electrons, as well as for the intermediate-spin and high-spin states. Overall, the enhancement of film thickness leads to the decrease of the Co-O bond length and the increase of crystal field splitting energy, unfavorable for the electron transfer from t2g to eg orbitals, and in turn for the stability of intermediate-spin and high-spin states. The electronic transport behavior of LCO film was proved to be a good insulating properties and the temperature dependence of the resistivity is consistent with the 3D-Mott’s variable range hopping model. Our results not only demonstrated the feasibility of thickness-induced strain modulating the properties of LCO films, but also strengthen the understanding of the ferromagnetic insulating mechanism in LCO films.

High-quality VO2 films synthesized on polymer substrates using room-temperature pulsed laser deposition and annealing

With the growing trend towards portable and adaptable devices in various fields, the demand for flexible electronics and coatings utilizing phase transition materials like VO2 as a functional layer is rising. However, depositing high-quality crystalline VO2 films directly onto polymer substrates remains challenging due to the thermosensitivity of such materials being barely compatible with usual VO2 deposition methods. In this study, we present a simple method for directly growing VO2 on Kapton substrates. VOx films were first deposited onto the Kapton substrate at room temperature using pulsed laser deposition and subsequently annealed in an oxygen atmosphere at 390 °C. The electrical and optical properties of the VO2/Kapton samples with different film thicknesses were investigated. The 200 nm-thick vanadium dioxide films exhibit the best resistivity contrast with 3.1 orders of magnitude (OOM), while the 50 nm-thick films show the best transmittance change of 54 % at 2500 nm. The switching performances are comparable or even superior to those achieved previously using more complicated synthesis processes such as film transfer, introduction of buffer layers, and ink injection. Additionally, by coupling W elemental doping and thin film strain, we successfully tuned the VO2 transition temperature to room temperature with a dopant concentration of only 1.1 at.% while maintaining a high electrical contrast (2 OOM). The impressive electrical and optical transition performance of the VO2/Kapton samples, combined with their tunable transition temperature, makes them promising candidates for flexible electronic devices and thermochromic smart coatings.

The estimation of crack onset strain for brittle films with residual stress on ductile substrates

In this paper, a method predicting crack onset strain in thin film/substrate system with residual stress is proposed. For thin film materials with size effect, the strength criterion is improved by considering the variation of film cohesive strength. The modified strength criterion is more accurate in fitting the crack density than energy criterion. According to the modified strength criterion, the crack onset strains of three kinds SiNx films prepared in our work are 1.21 %, 1.25 % and 1.10 %, respectively. It is proved that the crack onset strain predicted by the modified strength criterion is accurate by bending test. Our work provides a new method for determining the crack onset strain of various composite films more accurately.

Nacre‐Inspired Mechanically Robust Films for Osmotic Energy Conversion

AbstractMXene, a new class of 2D layered materials, is considered a candidate for the reverse electrodialysis (RED) film because the laminated MXene nanochannels with charged functional groups provide an excellent ionic selectivity and conductivity. However, the MXene‐based ion‐exchange films suffer from weak mechanical properties and instability in water due to the poor binding force between the nanosheets, which limits their RED performance. Here, inspired by natural nacre structure, a class of cation‐MXene/cation holocellulose nanofibers (C‐MXene/C‐HNF) composite film with high performance for harvesting nanofluidic osmotic is developed. C‐HNF acts as interlocking agent to connect the MXene nanosheets by hydrogen bonding, resulting a mechanically robust and water stable film. The introduction of C‐HNF not only increases the interlayer interaction force, but also enhances the ion transport flux. When the content of C‐HNF is 20%, the tensile stress, fracture strain, and toughness of the composite film reach up to 127.75 ± 1.61 MPa, 3.50 ± 0.1%, and 2.95 ± 0.52 MJ m−3, respectively. Additionally, the composite film can produce an impressive output power density of up to 1.09 W m−2 under a 50‐fold salinity gradient (0.01/0.5 m NaCl), and only decreases by 4.58% after 50 days, demonstrating good stability and durability.