Boundaries In Films Research Articles

Thermodynamic stability and kinematic accessibility of curved grain boundaries in directionally solidified bicrystals

Grain boundaries always have a positive excess free energy that drives grain growth and structural evolution during high-temperature heat treatments. Grain boundaries can, however, become trapped in a metastable configuration because of the geometry or composition of the specimens in which they are contained. For example, second phase dispersoids can pin grain boundaries in a polycrystal, and thermal grooves can pin grain boundaries in thin films and wires. In these examples of frustrated grain boundary systems, further reduction in the grain boundary area is energetically preferred, but grain boundary migration is arrested because energy is required to create additional grain boundary surface area and de-pin from the obstacle. In this work, we consider an idealized class of frustrated grain boundaries, namely curved grain boundaries in geometrically complex bicrystals. By calculating the energy landscape of the grain boundary configuration space, we show that the local stability of these curved grain boundaries is a sensitive function of the bicrystal shape. We also assess the trajectory of curved grain boundaries in bicrystals grown via directional solidification to determine which grain boundary configurations are physically realizable. By comparing these two analyses, we find that a curved grain boundary may be thermodynamically metastable but kinematically inaccessible, highlighting the general importance of thermodynamics and kinematics in descriptions of frustrated grain boundary systems encountered in a wide range of different materials.

Grain boundary anisotropy on nano-polycrystalline magnetic thin films

Grain boundaries in polycrystalline thin films with crystallite sizes at nanoscale presents regions characterized by a high degree of local structural disorder. As a consequence, great values of the associated local anisotropies are expected. On this regard, a systematic investigation of the effect of the grain boundary anisotropy on the magnetic properties in such type of nanostructured systems is addressed. For developing this work, a standard Monte Carlo simulation in the framework of classical Heisenberg spins was carried out, with a Hamiltonian involving exchange couplings, dipolar interactions, Zeeman interaction and contributions of cubic magneto-crystalline anisotropy. A quantification of local structural disorder was considered. Results revealed that i) by keeping the same number of grains, different organizations give rise to different spontaneous magnetizations, ii) the critical exponent of the magnetization differs of pure models, which is attributed to the complexity of the lattice and consistent with a distribution of critical temperatures, iii) Boundary anisotropy varies with temperature and its strength are determinant factors for blocking temperatures, and iv) Boundary anisotropy inside in the hysteretic properties where coercive field variations are observed.

Dual Functions of Crystallization Control and Defect Passivation Enabled by an Ionic Compensation Strategy for Stable and High-Efficient Perovskite Solar Cells.

The fabrication of perovskite films with high crystallization quality, less defects, and fewer grain boundaries (uncoordinated ions) is one critical step to obtain excellent power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). In this work, we develop a novel method to control the perovskite growth toward better crystallinity and less defects using iodide ions (I-) and thiourea as additives for the first time (we define ITU for I- and thiourea). Grain boundaries in the perovskite films are significantly reduced compared to the traditional method. Moreover, concentration of the defects in perovskite films is decreased by nearly one-half. Based on high-quality films, the PSCs with a champion PCE of 20.39% present a stabilized output efficiency of 19.26% under one sun illumination compared to that of the control devices (17.75%). The devices also exhibit small hysteresis and excellent long-term and light stability. The devices can retain 80% of the initial PCE after 100 h of light soaking or 30 days of aging in ambient atmosphere. This work not only demonstrates a novel approach to passivate the defects by balancing iodide ions but also offers a strategy to control the perovskite film growth, which can be widely used in photoelectric devices.

Magnetic impurities as the origin of the variability in spin relaxation rates in Cu-based spin transport devices

The Elliott-Yafet spin relaxation mechanism posits linear proportionality between spin and momentum lifetimes in low spin-orbit coupling nonmagnetic metals, and is widely accepted in spintronics. Accurate experimental determination of the Elliott-Yafet proportionality constants (${\ensuremath{\beta}}_{i}$) between the spin and momentum relaxation times for individual scattering sources is challenging, however. This is apparent from the literature on nonlocal spin transport in Cu, for example, where reported phonon (${\ensuremath{\beta}}_{\mathrm{ph}}$) and defect (${\ensuremath{\beta}}_{\mathrm{def}\phantom{\rule{4pt}{0ex}}}$) Elliott-Yafet constants vary by an order of magnitude. In recent work we discovered that even part-per-million-level magnetic impurity concentrations can substantially influence spin relaxation in Cu, via a spin transport analog of the Kondo effect. To clarify whether this could explain the reported variability in ${\ensuremath{\beta}}_{i}$, here we report on a comprehensive study of spin transport in Cu-based lateral nonlocal spin valves, varying the ferromagnetic contact material, interface structure, Cu thickness, and post-fabrication annealing conditions, resulting in widely varied microstructures and magnetic impurity concentrations. Quantifying the effects of magnetic impurities on charge and spin transport we demonstrate the dramatic, even dominant, effect these can have on spin relaxation rates, and thus extracted ${\ensuremath{\beta}}_{i}$. Minimization of magnetic impurity effects is achieved via Al interlayer insertion or moderate annealing, restoring the expected temperature dependence for phonon-mediated spin relaxation, and enabling more reliable determination of ${\ensuremath{\beta}}_{i}$ for phonons (740 \ifmmode\pm\else\textpm\fi{} 200), and nonmagnetic defects (240 \ifmmode\pm\else\textpm\fi{} 50). The latter contribution is shown to be dominated by grain boundaries in these polycrystalline Cu films. Cross-sectional transmission electron microscopy measurement of grain sizes in actual nonlocal spin valve devices then establishes a useful empirical relationship between average grain size and spin diffusion length. These measurements highlight the importance of magnetic impurities in metallic spin transport, explain the wide variability in reported ${\ensuremath{\beta}}_{\mathrm{ph}}$ and ${\ensuremath{\beta}}_{\mathrm{def}}$ in Cu, and elucidate the relationship between metallic spin transport and microstructure.

Conductive AFM study of the local current in thin ferroelectric sol-gel PZT films

The local current distribution across the grains and their boundaries in ferroelectric PZT films are studied using conductive AFM. The films were formed with various grain structures and different lead excess content by chemical solution deposition. C-AFM measurements have shown the influence of the lead excess and seed sublayer crystallization conditions on the grain-boundary conductivity. PZT films with fixed 0-15 wt% Pb excess demonstrate non-conductive grain boundaries, whereas in films with fixed 30 wt% Pb excess the grain-boundary conductivity is found to be much higher than that of the grains themselves. Conductive grain-boundaries was also found in PZT films without lead excess in crystallized sublayer. A study of the transient current at varied preliminary polarization revealed the current peaks in the current-voltage curves. The appearance conditions and magnitude of these peaks depend on the grain-boundary conductivity. The correlation between the grain-boundary conduction and the current polarization dependences is confirmed.

Engineering antiphase boundaries in epitaxial SrTiO3 to achieve forming free memristive devices

We here present a method to engineer Ruddlesden-Popper-type antiphase boundaries in stoichiometric homoepitaxial SrTiO3 thin films. This is achieved by using a substrate with an intentionally high miscut, which stabilizes the growth of additional SrO at the bottom interface. We prove the success of this strategy utilizing transmission electron microscopy. We find that these antiphase boundaries significantly influence the resistive switching properties. In particular, devices based on SrTiO3 thin films with intentionally induced antiphase boundaries do not require a forming step, which is ascribed to the existence of preformed filaments.

In-Field Transport Properties at Grain Boundaries in BaHfO3-doped SmBa2Cu3O y Bicrystal Films at Low Temperatures

High-temperature superconducting REBCO tapes are attractive for their high critical current density. Jc in the REBCO film is suppressed under magnetic fields and by grain boundaries. Our group has reported that artificial pinning centers (APC) enhance the Jc at the grain boundaries in the REBCO films at the liquid nitrogen temperature of 77 K. In this study, we have extended our previous work to a lower temperature since the usage of the REBCO tapes at a lower temperature and a high magnetic field is a recent development trend. Transport characteristics at grain boundaries in the REBCO films with and without APC are investigated at temperatures down to 4.2 K and at magnetic fields up to 9 T. The results show that the introduction of the APCs is effective to enhance the Jc at the grain boundaries at low temperatures and at the magnetic fields, especially around the matching field where the density of the APC is equal to that of vortices.

Single grain growth of Si thin film on insulating substrate by limited region aluminum induced crystallization

Polycrystalline Si thin films on insulating substrates have been widely used to fabricate thin film transistors. However, since randomly induced grain boundaries in the polycrystalline thin films have been degrading transistor performance, application of thin film transistors has been limited to low speed processing units such as display drivers. Here, in this letter, we have tried to realize single grain growth of Si thin film at controlled position. Aluminum induced layer exchange crystallization of Si thin film was performed at limited region to suppress spontaneous nucleation during the crystal growth, and thus, single grain Si thin films have been successfully realized on quartz glass substrates. This technique will open up a way to realize high speed processing thin film transistors.

Conjugated Polymer–Assisted Grain Boundary Passivation for Efficient Inverted Planar Perovskite Solar Cells

AbstractGrain boundaries in lead halide perovskite films lead to increased recombination losses and decreased device stability under illumination due to defect‐mediated ion migration. The effect of a conjugated polymer additive, poly(bithiophene imide) (PBTI), is investigated in the antisolvent treatment step in the perovskite film deposition by comprehensive characterization of perovskite film properties and the performance of inverted planar perovskite solar cells (PSCs). PBTI is found to be incorporated within grain boundaries, which results in an improvement in perovskite film crystallinity and reduced defects. The successful defect passivation by PBTI yields reduces recombination losses and consequently increases power conversion efficiency (PCE). In addition, it gives rise to improved photoluminescence stability and improved PSC stability under illumination which can be attributed to reduced ion migration. The optimal devices exhibit a PCE of 20.67% compared to 18.89% of control devices without PBTI, while they retain over 70% of the initial efficiency after 600 h under 1 sun illumination compared to 56% for the control devices.

Formation of antiphase boundaries in CuFe2O4 films induced by rough MgAl2O4 (001) substrates

Antiphase boundaries in spinel ferrites have been considered as the functional elements with great potential for applications in spintronic devices. Engineering the antiphase boundaries is a big challenge in experiments. Here, we report on that a high density of antiphase boundaries in CuFe2O4 films have been prepared on rough surface of MgAl2O4 (001) substrates. By means of advanced electron microscopy these antiphase boundaries are found to be of {011}14〈011〉-type and bound by interfacial dislocations with Burgers vector of a/4〈011〉 that contribute to relaxation of in-plane and out-of-plane lattice strain in the heterostructure. Our results provide evidence that the rough substrate surface can be applied for the formation of antiphase boundaries in the CuFe2O4/MgAl2O4 system to tune the magnetic and electrical transport properties of epitaxial spinel ferrite films.

All-inorganic quantum dot assisted enhanced charge extraction across the interfaces of bulk organo-halide perovskites for efficient and stable pin-hole free perovskite solar cells.

In spite of achieving high power conversion efficiency (PCE), organo-halide perovskites suffer from long term stability issues. Especially the grain boundaries of polycrystalline perovskite films are considered as giant trapping sites for photo-generated carriers and therefore play an important role in charge transportation dynamics. Surface engineering via grain boundary modification is the most promising way to resolve this issue. A unique antisolvent-cum-quantum dot (QD) assisted grain boundary modification approach has been employed for creating monolithically grained, pin-hole free perovskite films, wherein the choice of all-inorganic CsPbBr x I3-x (x = 1-2) QDs is significant. The grain boundary filling by QDs facilitates the formation of compact films with 1-2 μm perovskite grains as compared to 300-500 nm grains in the unmodified films. The solar cells fabricated by CsPbBr1.5I1.5 QD modification yield a PCE of ∼16.5% as compared to ∼13% for the unmodified devices. X-ray photoelectron spectral analyses reveal that the sharing of electrons between the PbI6 - framework in the bulk perovskite and Br- ions in CsPbBr1.5I1.5 QDs facilitates the charge transfer process while femtosecond transient absorption spectroscopy (fs-TAS) suggests quicker trap filling and enhanced charge carrier recombination lifetime. Considerable ambient stability up to ∼720 h with <20% PCE degradation firmly establishes the strategic QD modification of bulk perovskite films.

Effect of neutron irradiation on the hydrogen state in CVD diamond films

The effects of high temperature, up to 1680 °C, and annealing in vacuum on the optical properties of fast neutron irradiated chemical vapour (CVD) deposited polycrystalline diamond films (CVD DFs) are examined. It is shown that at least fifteen relatively narrow (full width at half-height from 10 to 25 cm−1) CHx stretching vibration bands with maxima close to 2800, 2818, 2834, 2849, 2855, 2875, 2899, 2906, 2924, 2936, 2950, 2966, 2985, 3012 and 3034 cm−1, as well as a 3123 cm−1 band due to the NVH0 centers, are observed in the IR absorption spectra of the films. The transformation of the CHx stretching vibration bands in the absorption spectra of fast neutron irradiated (with a fluence of F = 2×1019 cm−2) diamond films is investigated with successive isochronous annealing runs. It is found that the neutron irradiation significantly (by ∼200°) increases the temperature above which the graphitization (darkening) of the samples starts. That threshold is determined by the temperature of graphitization of intercrystallite (grain) boundaries due to the break of C-H bonds. The phenomena observed are explained by the effect of radiation damage on the structure of grain boundaries in the diamond films.

Chemical sintering reduced grain boundary defects for stable planar perovskite solar cells

Organic–inorganic hybrid perovskite solar cells have attracted tremendous attention in photovoltaic research, but the presence of massive parasitic traps at the grain boundaries in perovskite films discourages efficient bimolecular recombination and carrier dynamics, which limits device performance and stability. Here we report a simple and fast chemical sintering protocol to substantially reduce grain boundary defects for as-formed films for different PbI2-templated perovskite materials. With this method parasitic traps on grain boundaries are intrinsically reduced, featuring the Urbach energy reduced by 1.5 meV. For optimal photovoltaic devices, we demonstrate a planar solar cell with power conversion efficiency (PCE) of 19.68% which retains 80% of this efficiency over 720 h in ambient air without any encapsulation. These findings offer a widely applicable, versatile process to efficiently reduce grain boundary defects and will be of interest to many other film material systems afflicted by polydispersity and grain boundary disturbances.

Subgrain Special Boundaries in Halide Perovskite Thin Films Restrict Carrier Diffusion

Photoluminescence microscopy reveals subgrain special boundaries in FAPbI3 perosvkite thin films that restrict carrier diffusion and serve as nonradiative recombination sites. These subgrain boundaries, which cannot be observed by conventional AFM or SEM measurements of grain-boundary grooves, may explain why the performance of perovskite solar cells has not scaled with apparent grain size.

Improvement mechanism of sputtered AlN films by high-temperature annealing

The improvement mechanism of sputtered AlN films by high temperature annealing in nitrogen ambient has been investigated. Sputtered AlN films were annealed at 1100–1700 °C and their microstructures were observed by scanning transmission electron microscopy. A two-layer structure consisting of columnar domains with different sizes was observed in the as-grown sputtered AlN films. The first layer with 10 nm thickness located at the AlN/sapphire interface, comprised columnar domains with diameters of nm order and was compressively strained owing to the lattice mismatch between AlN and sapphire. The diameter of columnar domains in the other layer was approximately 25 nm. The columnar domains split into irregularly shaped and coalesced at an elevated annealing temperature, resulting in improved crystal quality. When the annealing temperature was 1700 °C, the domain boundaries in AlN films were almost annihilated and the full width at half maximum of the (0 0 0 2)- and (1 0 −1 2)-plane X-ray rocking curves were improved to as low as 49 and 310 arcsec, respectively. The polarity switched from N-polar to Al-polar after about 4–10 AlN layers in the growth direction. The oxygen element content slightly increased at the polarity inversion boundary, which may have caused the polarity to switch.

Effects of Moisture-Based Grain Boundary Passivation on Cell Performance and Ionic Migration in Organic-Inorganic Halide Perovskite Solar Cells.

Because of the polycrystalline nature, grain boundaries (GBs) in hybrid perovskite thin films play critical roles in determining the charge collection efficiency of perovskite solar cells (PSCs), material stability, and in particular the ion migration, considering their relatively soft ionic bonds with low formation energy. Different GB passivation methods are being studied, and introducing PbI2-rich phase at GBs in methylammonium lead iodide (MAPbI3) has been found to be useful. In this study, combining macroscale measurements with tip-based microscopic probing that includes scanning Kelvin probe microscopy for surface potential mapping and conductive atomic force microscopy for charge transport mapping, we revealed the effects of PbI2-rich phase at GBs, which was introduced in moisture-assisted synthesis of MAPbI3 thin films. It was found that PbI2 passivation of GBs could change the surface potential and charge carrier screening and significantly retard current conduction at the GB while enhancing conduction through the grain interior. Inhibition of ion migration at GBs, as well as enhanced PSC device performance, is reported.

Ultrasensitive Heterojunctions of Graphene and 2D Perovskites Reveal Spontaneous Iodide Loss

Summary Despite the demonstrated high efficiency of perovskite solar cells and light-emitting devices, the understanding of the intrinsic stability of perovskites is far from complete. In this work, using an ultrasensitive, exfoliated 2D perovskite single-crystal sheet/graphene heterostructure device, we reveal spontaneous iodide loss as an important degradation pathway of 2D perovskite single crystals, which n-dopes the perovskite semiconductor by generating positively charged iodide vacancies. Furthermore, we show that covering perovskites with graphene can suppress the iodide loss, significantly improving perovskite stability. A perovskite phototransistor is demonstrated with a graphene/2D perovskite/graphene structure, which shows no degradation after 75 days. Our work not only provides important insights for future stable perovskite optoelectronic device development, but also demonstrates the potential of graphene as a promising sensitive diagnostic tool for device and material degradation studies.

Influence of Defects on the Reactivity of Organic Surfaces

Molecular orientation within organic solids limits the range of applicable surface reactions, with reactive functionalities often recessed and inaccessible to adsorbates. To induce reactivity in heretofore inert orientations of acenes, a defect-mediated mechanism is utilized to functionalize thin-film phase pentacene. This mechanism was demonstrated via correlation of reaction data to defect density, determined via polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and atomic force microscopy (AFM). By controllably varying the amount of grain boundaries in the acene films, the reaction can be varied from near zero to coverage exceeding a monolayer. The extensive coverage suggests that the reaction propagates from the defects throughout the grains, a prediction borne out via direct observation of reaction progression along the surface from a single dislocation (via scanning electron microscopy). The results support a mechanism whereby the reaction is initialized at the defect si...

Carbon nanotube mediated miscibility of polyhydroxyalkanoate blends and chemical imaging using deuterium-labelled poly(3-hydroxyoctanoate)

Biopolymers have potential as scaffolds supporting regrowth of damaged tissues, however their material properties may limit the range of applications. Blending polymers with different thermomechanical properties has been demonstrated to extend the range of possible applications for polyhydroxyalkanoate (PHA) polymers, while the addition of nanoparticles can be used to modulate miscibility which influences strength and flexibility of the blend. Here we report on the blending of Poly(3-hydroxybutyrate) and Poly(3-hydroxyoctanoate) which possess different thermomechanical properties, and the effect of single wall carbon nanotubes (SWCNT) on their miscibility, electrical conductivity and thermomechanical performance. The apparent perturbation of phase boundaries in nanocomposite films observed by Scanning Electron Microscopy (SEM) was complemented by chemical mapping of film cross sections containing a deuterium-labelled poly(3-hydroxyoctanoate) phase in the blend using Infrared Microspectroscopy (IRM), suggesting increased miscibility due to nanoparticle addition. The electrical percolation threshold in nanocomposite films was observed between 0.5 and 1 wt% SWCNT, where the surface resistivity was reduced by eight orders of magnitude compared to the insulating polymer blend. Addition of SWCNT did not impact significantly on mechanical properties of films containing up to 2.5 wt% SWCNT. A solvent cast bionanocomposite film containing optimally 1 wt% SWCNT yielded a material with improved electrical conductivity compared to the SWCNT-free blend and which supported growth of Olfactory Ensheathing Cells, providing a basis for developing biopolymer scaffolds capable of conducting electrical stimulation.

Ultrafast Imaging of Carrier Transport across Grain Boundaries in Hybrid Perovskite Thin Films

For optoelectronic devices based on polycrystalline semiconducting thin films, carrier transport across grain boundaries is an important process in defining efficiency. Here we employ transient absorption microscopy (TAM) to directly measure carrier transport within and across the boundaries in hybrid organic–inorganic perovskite thin films for solar cell applications with 50 nm spatial precision and 300 fs temporal resolution. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as we...