Grain Boundary Barrier Research Articles

Microstructure, optical, dielectric, and nonlinear properties of Cd1-xBi2x/3Cu3Ti4O12 ceramics

Microstructure features along with optical, dielectric and nonlinear properties of Cd1-xBi2x/3Cu3Ti4O12 (x = 0, 0.3, 0.6, 0.9, and 1.0) ceramics were investigated systematically. With increase in Bi3+ doping content, Cd1-xBi2x/3Cu3Ti4O12 structure sightly distorted and eventually formed more stable configuration. Bi3+ doping also caused grain refinement and increased compactness of ceramics, whereas Bi-rich phase appeared at grain boundaries. Optical band gap (Eg) decreased due to increase in defect concentration. Dielectric loss (tanδ) dropped with increase in Bi3+ doping content while dielectric constant (ɛ′) remained high. The most optimal dielectric characteristics (the lowest tanδ of ∼0.033, giant ɛ′ of ∼10028 at 10 kHz and room temperature, the highest nonlinear coefficient (α of 4.62) and breakdown field strength (Eb of up to ∼9.23 kV/cm) were achieved at x = 0.9. Impedance spectra revealed semiconducting grains and insulating grain boundaries. Dielectric response evolution in Cd1-xBi2x/3Cu3Ti4O12 ceramics was described using internal barrier layer capacitor model. Electric modulus values indicated that low-frequency relaxation originated from grain boundaries. High energy barrier of grain boundaries was beneficial for enhancement of nonlinear properties of ceramics. Thus, Bi3+ doping improved both dielectric and nonlinear characteristics of Cd1-xBi2x/3Cu3Ti4O12 ceramics.

Enhanced optical, dielectric, and non-Ohmic properties in Ta-doped Bi2/3Cu3Ti4O12 ceramics

In this paper, Ta-doped Bi2/3Cu3Ti4O12 (BCTO) ceramics are prepared by solid-state method. Phase structure, microstructure, optical, dielectric, and non-Ohmic properties of all samples are systematically investigated. Cu/Ta-rich phases at grain boundaries (GB) together with defects affect the microstructure, causing grain size and compactness to increase first and then decrease. Moreover, Ta doping increases the optical band gap of BCTO from 4.03 to 4.25 eV as well as the GB activation energy and GB barrier. An increase in dielectric constant (Ɛr), nonlinear coefficient (α) and breakdown field (Eb), and a decrease in the dielectric loss (tanδ) are observed in Ta-doped BCTO ceramics. The improvement in dielectric and non-Ohmic properties can be attributed to the internal barrier layer caused by GB Schottky barrier structure. In particular, Bi2/3Cu3Ti3.95Ta0.05O12 ceramic exhibits the high Ɛr of ∼45997 and the low tanδ of ∼0.035 at 10 kHz along with α of ∼5.62, and Eb of ∼3.45 kV/cm. The temperature-independent (-110-210 °C) dielectric response confirms that temperature stability of specimens increases with doping. These results suggest that improved optical, dielectric, and non-Ohmic properties in Bi2/3Cu3Ti4O12 can be achieved simultaneously via defect engineering.

On the grain boundary charge transport in p-type polycrystalline nanoribbon transistors.

Grain boundaries (GB) profoundly influence charge transport, and their localized potential barrier with a high density of defect states plays a crucial role in polycrystalline materials. There are a couple of models to estimate the density of states (DoS) of nanostructured materials in field-effect transistors (FETs) that probe interface traps between the semiconductor and dielectric but not at the grain boundaries. Here, we report on utilizing Levinson's and Seto's models of grain boundary transport and correlate them with the temperature-dependent hopping transport in copper iodide (CuI) polycrystalline nanoribbon (PNR) FETs. Experimentally, PNRs are obtained by e-beam lithography and thermal evaporation of CuI. To investigate the impact of GB, the devices are fabricated with different channel aspect ratios by varying widths (80, 260, and 570 nm) and lengths (20 to 90 μm). Owing to the high hole concentration, PNR FETs operate in depletion mode at 300 K. At various low temperatures (80-300 K), the figures-of-merits of FETs are estimated to understand device performance. We determine GB barrier heights, activation energy, and density of GB trap states and find equivalence between the two models. Furthermore, we calculate temperature-dependent hopping and trap-limited transport parameters to obtain DoS at the Fermi energy, trapped and free charge carrier density, localization length, hopping distance, hopping energy, etc. at various channel lengths. Based on this quantitative analysis, we propose a channel length-dependent GB barrier height variation due to the in-plane electric field and elucidate CuI energy band levels.

Constructing cell-membrane-mimic grain boundaries for high-performance n-type Ag2Se using high-dielectric-constant TiO2

The coupling of charge carrier and phonon transport limits the application of Ag2Se as a low-toxic near-room-temperature thermoelectric material. Strategies that reduce the thermal conductivity via enhancing the phonon scattering usually lead to reduced carrier mobility due to high grain boundary potential barrier. In this study, we developed a cell-membrane-mimic grain boundary engineering strategy for decoupling the charge carrier and phonon scattering through decorating high-dielectric-constant rutile TiO2 at Ag2Se grain boundaries to enable the charge carrier/phonon selective permeability. The nano-sized TiO2 with high dielectric permittivity can secure the charge carrier transport by shielding the interfacial Coulomb potential to lower the energy barrier of grain boundaries, rendering an enhanced power factor. Additionally, benefited from the enhanced phonon scattering by TiO2 nanoparticles, a significantly decreased lattice thermal conductivity of ∼0.20 W m−1 K−1 and a high zT of ∼0.97 at 390 K are obtained in the Ag2Se-based nanocomposites. This work demonstrates that such cell-membrane-mimic grain boundary engineering strategy may shed light on developing high-performance thermoelectric materials.

Vacancy accumulation mechanism at iron grain boundaries: The influence of grain boundary character and its coupling with grain size

The interaction between the radiation defects of vacancies (Vs) and grain boundaries (GBs) is critical to the understanding of the radiation response of polycrystalline materials. In this work, by combing atomistic calculations with kinetic Monte Carlo methods, multiscale simulations of Vs behavior near different iron GBs were carried out to reveal the Vs accumulation mechanisms at GBs. On the one hand, the dependence of Vs accumulation on the GB character was clarified. At high temperature, new V escaping processes were revealed, including the emission from twin boundary and special coincidence site lattice (CSL) GB with a low V–GB binding strength and the leakage from the extremely low-angle GB with a bulk-like region between dislocation cores. These processes weaken the trapping efficiency of such GBs for Vs. Enhanced Vs trapping could be obtained for the general low-angle GB because of the large strain field. Due to the large V–GB binding energy and trapping site density, the general high-angle GB was found to capture Vs efficiently. On the other hand, the coupling of grain size with GB character via the combination of grain size, V formation energy at the GB and migration energy barrier within the GB was proposed, which leads to the competition between the V emission from the GB and its cruise along the GB. This accounts for the experimentally observed scattered relation of the radiation-performance to defect–GB binding strength. The present work provides some theoretical guidance for optimizing the radiation resistance of polycrystalline materials based on GB engineering.

Strong charge carrier scattering at grain boundaries of PbTe caused by the collapse of metavalent bonding

Grain boundaries (GBs) play a significant role in controlling the transport of mass, heat and charge. To unravel the mechanisms underpinning the charge carrier scattering at GBs, correlative microscopy combined with local transport measurements is realized. For the PbTe material, the strength of carrier scattering at GBs depends on its misorientation angle. A concomitant change in the barrier height is observed, significantly increasing from low- to high-angle GBs. Atom probe tomography measurements reveal a disruption of metavalent bonding (MVB) at the dislocation cores of low-angle GBs, as evidenced by the abrupt change in bond-rupture behavior. In contrast, MVB is completely destroyed at high-angle GBs, presumably due to the increased Peierls distortion. The collapse of MVB is accompanied by a breakdown of the dielectric screening, which explains the enlarged GB barrier height. These findings correlate charge carrier scattering with bonding locally, promising new avenues for the design of advanced functional materials.

Mechanism of room-temperature superplasticity in ultrafine-grained Al–Zn alloys

Superplastic deformation of polycrystalline materials is usually accommodated by diffusion-assisted grain boundary (GB) sliding at high temperatures. Lowering the temperature requirement for commercial superplastic forming enables green and cost-effective manufacturing. Recently, room-temperature (RT) superplasticity was realized in ultrafine-grained Al–Zn based alloys, but the underlying mechanism remains unclear. Here, we conducted in-situ tensile straining and post-mortem electron microscopy characterization, and atomistic density functional theory simulation to understand the RT superplasticity of an Al–15 Zn (at%) alloy. Results showed that the superplasticity is achieved by GB sliding and grain rotation, assisted by the continuous diffusion of Zn. In-situ observations showed Zn atoms diffusing from within grains to GBs, resulting in a Zn nanolayer at the GBs that acts as a solid lubricant to decrease the energy barrier of GB sliding. This research advances our understanding of diffusion-assisted deformation mechanism that is a prerequisite for the rational design of new materials with RT superplasticity.

Pressure-Engineered Ti3C2Tx MXene with Enhanced Conductivity and Accelerated Reaction Kinetics of Lithium Storage.

We studied the structure-function relationship of compressed Ti3C2Tx MXene using high-pressure in situ synchrotron radiation, impedance spectroscopy, Hall effect measurements, and first-principles calculations. With increasing pressure, the conductivity of Ti3C2Tx MXene increases along with its continued lattice shrinkage. A pressure range of 0.4-2.2 GPa exhibits a sharp decrease in resistance, which decreases by more than one order of magnitude from 3.3 × 104 to 1.4 × 103 Ω. A pressure range of 2.2-6.6 GPa exhibits a steady resistance with a slight decrease of 0.2%. As the pressure drops to atmospheric conditions, the resistance increases slightly to 4.2 × 103 Ω. This is accompanied by a transformation of the semiconductor into metal. An irreversible increase in conductivity is observed owing to an increase in the electron concentration and a decrease in the grain-boundary potential barrier. Furthermore, abundant Ti3C2Tx undergoing prepressure treatments (0.4, 2.0, and 4.0 GPa) was first prepared using a double-anvil hydraulic press. The recycled samples retain an accordion-like layered structure with slight lattice shrinkage while the voids between the sheets contract considerably, increasing the density. Correspondingly, electrochemical results show a pressure threshold of 2.0 GPa based on the rapid quenching from the hydraulic press. This weakens the electric polarization in redox reactions and increases the ionic transport rate for the formation of a Ti3C2Tx anode owing to pressure improving the conductivity and interlaminar densification. Our study shows a new, simple, and universal way to regulate various MXenes and also promotes the application of MXene-based materials in energy storage and related fields.

Synergy effect of microwave annealing and high-pressure hydrogen annealing on Poly-Si thin-film transistor

Grain boundary (GB) is a significant factor that deteriorates the transfer characteristics of poly-Si thin-film transistors (TFTs). In this study, we utilized the synergistic effect of microwave annealing (MWA) and high-pressure hydrogen annealing (HPHA) to effectively reduce grain boundary trap (GBT) density, resulting in improved field-effect mobility (μ) and subthreshold swing (SS). To investigate the synergistic effect of MWA and HPHA, the transfer characteristics of rapid thermal annealing and forming gas annealing devices were compared and analyzed as control devices. Furthermore, the mechanism of SS and mobility enhancement can be quantitatively understood by lowering the GB barrier height. In addition, Raman spectroscopy proved that poly-Si crystallinity was improved during MWA. Our results showed that MWA and HPHA play a vital role in reducing GBT density and improving poly-Si TFT characteristics.

Carrier transport mechanism in bottom gate thin‐film transistor with SnO as active layer for CMOS displays

AbstractIn this work, we report on four tin monoxide (SnO) thin‐film transistor (TFT) grain boundary (GB) models of carrier transport considering the native defects in the thin film, interface traps, and GB deep/tail states. The changes in the activation energy and the GB barrier potential on the application of gate electric field are thoroughly investigated. The shift in Fermi level and the charge carrier transport mechanisms are examined for the two‐channel model by the application of external potential. Four models are developed to study the impact of phase transformation of SnO material on the TFT characteristics. Among the four developed models which are considered as four different cases, Case (iv) shows excellent performance and the simulation results revealed that the location of Fermi level closer to the mid gap are suggested to favor the ambipolar behavior. Also, the influence of SnO material thickness and the effect of different dielectrics on the ambipolar device characteristics are examined aiming at optimized performance of the device. The developed optimized model will help the process engineers in tuning the SnO material parameters for achieving better performance in both p‐type and n‐type TFTs when employed in CMOS based displays.

Diffusion of silver in titanium nitride: Insights from density functional theory and molecular dynamics

The use of self-lubricating titanium nitride and silver (TiN(Ag)) nanocomposite coatings is a promising way to reduce the wear of tools employed dry machining operations for hard-to-cut materials/alloys. To achieve an optimal performance, the Ag diffusion within the matrix needs to be carefully controlled, and its mechanisms clearly understood. In this paper we use density functional theory calculations to investigate Ag-related point defects, adhesion energies and Ag diffusion energy barriers in TiN bulk, surfaces and grain boundaries. Classical molecular dynamics simulations have been performed on TiN(Ag) systems using a hybrid MEAM/Mie force field, to understand the relative importance of the different diffusion processes. Our results show that the main Ag transport mechanism in TiN(Ag) nanocomposites is the surface diffusion occurring along intergranular spaces.

Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe2As2

Fe-based superconductors and in particular K-doped BaFe2As2 (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (Jc) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C. We performed specific heat characterization and Tc-distribution analysis up to 16 T and we compared them with magnetic Tc and Jc characterizations, and transmission-electron-microscopy (TEM) microstructures. We found no direct correlation between the magnetic Tc and Jc, whereas the specific heat Tc-distributions did provide valuable insights. In fact the best Jc-performing sample, heat treated first at 750 °C and then at 600 °C, has the peak of the Tc-distributions at the highest temperatures and the least field sensitivity, thus maximizing Hc2. We also observed that the magnetic Tc onset was always significantly lower than the specific heat Tc: although we partially ascribe the lower magnetization Tc to the small grain size (< λ, the penetration depth) of the K-Ba122 phase, this behaviour also implies the presence of some grain-boundary barriers to current flow. Comparing the Tc-distribution with Jc, our systematic synthesis study reveals that increasing the first heat treatment above 750 °C or the second one above 600 °C significantly compromises the connectivity and suppresses the vortex pinning properties. We conclude that high-purity precursors and clean processing are not yet enough to overcome all Jc limitations. However, our study suggests that a higher temperature Tc-distribution, a larger Hc2 and a better connectivity could be achieved by lowering the second heat treatment temperature below 600 °C thus enhancing, as a consequence, Jc.

Diffusion transport over grain-boundary barriers as the origin of N1 deep level transient spectroscopy signal in [formula omitted] solar cells

In this work, we explore the hypothesis that the N1 peak in Cu(In,Ga)Se2 (CIGS) solar cells is caused by secondary barriers. We indicate, through numerical modelling of the corresponding Deep Level Transient Spectroscopy (DLTS) signal, that these barriers may be located at grain boundaries. Moreover, the proper reconstruction of the N1 signal using the double-barrier model requires an assumption that corresponding capacitance transients are due to the diffusion limited transport of holes over grain boundaries. Only then we were able to achieve sufficiently small values of the saturation current for realistic values of material parameters. We show that the position of the N1 DLTS peak and its activation energy depends on the effective density and the effective mobility of holes which can be calculated based on grain size and the properties of grain boundaries. This interpretation opens a room for the explanation of the unusual properties of the N1 signal in CIGS solar cells.

Fabrication of in-situ grown and Pt-decorated ZnO nanoclusters on new-type FTO electrode for room-temperature detection of low-concentration H2S

In this work, an in-situ grown and Pt-decorated ZnO nanoclusters (NCs) gas sensor was fabricated by employing etched FTO glass as the growth substrate and gas-sensing electrode via facile dip-coating assisted hydrothermal route and physical mixing method, which exhibited superior gas-sensing performance for low-concentration H2S at room temperature. Benefiting by the in-situ growth, good ohmic contacts between ZnO NCs and FTO electrode are formed, which maximize the gas-sensing performance and enhancing the stability of the sensor. On this basis, ZnO NCs with top cross-linked structure can provide adequate grain-boundary potential barriers and self-assembled electron pathways, as well as larger effective surface area and gas diffusion space. What's more, Pt nanoparticles (NPs) decoration on the surface of ZnO NCs can introduce more catalytic sites and proper Schottky barriers for gas-sensing tests. Due to their synergistic effects, the in-situ grown and Pt-decorated ZnO NCs sensor exhibit excellent room-temperature gas-sensing behaviors for H2S. Among samples, 1.5 at% Pt-decorated ZnO NCs sensor possesses the highest response value, excellent selectivity, repeatability and stability, which achieves the room-temperature detection of H2S at ppb level. In addition, the effect of Pt decoration on the gas-sensing behaviors under high temperature condition and the mechanism of enhancing gas-sensing performance were also systematically investigated.

Effects of post-deposition CdCl2 annealing on electronic properties of CdTe solar cells

The effects of post-deposition CdCl2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380–430 °C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. The best improvements in device efficiency are obtained upon annealing at temperature 410 °C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.

Effect of Alloying Element Segregations on the Grain Boundary Sliding in Al–Mg and Al–Ni Alloy Bicrystals: Atomistic Modeling

Segregation and grain boundary sliding were studied using atomistic modeling methods for special Σ5{013}〈100〉 and Σ5{012}〈100〉 grain boundaries in Al–3 at % Mg and Al–3 at % Ni alloys bicrystals. To reveal the mechanisms controlling the sliding processes, the energy of generalized grain boundary stacking faults (GB-SF) was determined in the plane of grain boundaries, the crystallography of grain boundary shears was analyzed, and the preferable directions and energy barrier of grain boundary sliding were established. It was shown that alloying with Mg and Ni essentially modified the GB-SF energy, increasing the resistance to sliding.

Synthesis and characterizations of copper cadmium sulphide (CuCdS2) as potential absorber for thin film photovoltaics

Synthesis and characterizations of copper cadmium sulphide (CCS) thin films as potential absorber layer for low-cost thin films solar cells has been reported. Thin films of CCS were dip-coated from clear molecular ink consists of copper acetate, cadmium acetate and thiourea dissolved in methanol. Films were heated at 200 °C in air for 15 min. The effect of film thickness on the compositional, morphological, optical and electrical properties were investigated using x-ray diffraction, Raman spectroscopy, scanning electron microscopy, UV–vis spectrophotometer and Hall measurement system. Optical band gap of the films estimated from the Tauc relation was found to be ~2.3 eV. Films are compact and void free having interconnected spherical grains. XRD and Raman spectroscopy confirmed the formation of hexagonal CCS without the formation of any secondary phases. The films are p-type and showed photoconductivity. Temperature variation of electrical conductivity of films measured in the temperature range from 300 to 475 K reveals nearest neighbour hopping (NNH) and thermionic emission (TE) over grain boundary (GB) barriers (GBB) below and above 400 K, respectively. Dip-coated CCS thin films found suitable for the fabrication of thin film solar cells at low cost.

Study of the nanoscale electrical performance of NiO thin films by C-AFM and KPFM techniques: The effect of grain boundary barrier

Conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM) are applied to visualize the nanoscale current imaging and potential imaging of the NiO thin film on fluorine tin oxide (FTO) substrate. The results show that the grain boundary (GB) has a significant impact on the nanoscale current and potential of NiO film. The electronic conductivity and work function of NiO in grain face is higher than that of the NiO film on GB. The width of the conductive zone in the NiO film over GBs is ∼60 nm. The higher nanoscale current and work function in grain face is probably attributed to the presence of nickel vacancy (VNi) defects and excess oxygen (O2−) defects. The GB barrier in single-layer and bilayer NiO film was ∼0.0169 eV and ∼0.0226 eV, respectively. The nanoscale current is consistent to Schottky thermionic emission model.

Improving carrier mobility of polycrystalline Ge by Sn doping

To improve the performance of electronic devices, extensive research efforts have recently focused on the effect of incorporating Sn into Ge. In the present work, we investigate how Sn composition x (0 ≤ x ≤ 0.12) and deposition temperature Td (50 ≤ Td ≤ 200 °C) of the Ge1−xSnx precursor affect subsequent solid-phase crystallization. Upon incorporating 3.2% Sn, which is slightly above the solubility limit of Sn in Ge, the crystal grain size increases and the grain-boundary barrier decreases, which increases the hole mobility from 80 to 250 cm2/V s. Furthermore, at Td = 125 °C, the hole mobility reaches 380 cm2/V s, which is tentatively attributed to the formation of a dense amorphous GeSn precursor. This is the highest hole mobility for semiconductor thin films on insulators formed below 500 °C. These results thus demonstrate the usefulness of Sn doping of polycrystalline Ge and the importance of temperature while incorporating Sn. These findings make it possible to fabricate advanced Ge-based devices including high-speed thin-film transistors.

HUMIDITY SENSORS BASED ON SnO2 -Co3 O4 -Nb2 O5 -Cr2 O3 SEMICONDUCTOR VARISTOR CERAMICS

The varistor effect in SnO2 -Co3 O4 -Nb2 O5 -Cr2 O3 ceramics with different additions is accompanied by a strong humidity-sensitivity effect in the low-field electrical conductivity. The low-field electrical conductivity and dielectric permittivity of ceramics are increased when air relative humidity rises from 10 to 93%. Such effects are observed both in the samples prepared by solid-phase sintering without or with CaO addition and in the samples prepared by liquid-phase sintering with CuO or V2 O5 additions. The lowest humidity-sensitivity effect was found in the liquid-phase sintering samples due to the more solid structures and the secondary phases which are located between the SnO2 grains. For the studied samples the values of humidity sensitivity coefficient 1.8·103 -3.2·105 at low electric fields and the nonlinearity coefficient 30-57 at high electric fields (E1 = 5.3-7 kV·cm-1) were calculated. The properties of oxide ceramics are explained by electrical conductivity which is controlled by the grain-boundary potential barriers. The barrier height decreases with the increase both of air relative humidity at low electric fields and of voltage at high electric fields. Strong humidity sensitivity is a key factor for sensor application of SnO2 - based ceramics.