Ultrasmall Grain Research Articles

A synergistic two-step optimization design enables high capacitive energy storage in lead-free Sr0.7Bi0.2TiO3-based relaxor ferroelectric ceramics

A two-step design is developed to realize multi-objective synergistic optimizations including high activity/ultrafine PNRs and ultrasmall grain size with compact grain boundaries, showing huge application potential in advanced pulsed power devices.

The effect of ultrasmall grain sizes on the thermal conductivity of nanocrystalline silicon thin films

Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm−1K−1 of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials.

Excellent fluorescence detection of Cu2+ in water system using N-acetyl-L-cysteines modified CdS quantum dots as fluorescence probe

View of the negative influence of metal ions on natural environment and human health, fast and quantitative detection of metals ions in water systems is significant. Ultra-small grain size CdS quantum dots (QDs) modified with N-acetyl-L-cysteines (NALC) (NALC-CdS QDs) are successfully prepared via a facile hydrothermal route. Based on the changes of fluorescence intensity of NALC-CdS QDs solution after adding metal ions, the fluorescence probe made from the NALC-CdS QDs is developed to detect metal ions in water systems. Among various metal ions, the fluorescence of NALC-CdS QDs effectively quenched by the addition of Cu2+, the probe shows high sensitivity and selectivity for detecting Cu2+ in other interferential metal ions coexisted system. Importantly, the fluorescence intensity of NALC-CdS QDs changes upon the concentration of Cu2+, the probe displays an excellent linear relationship between the fluorescence quenching rate and the concentration of Cu2+ in ranging from 1 to 25 μM. Besides, the detected limitation of the probe towards Cu2+ as low as 0.48 μM. The measurement of Cu2+ in real water sample is also carried out using the probe. The results indicate that NALC-CdS QDs fluorescence probe may be a promising candidate for quantitative Cu2+ detection in practical application.

Ultra-thin multilayered erosion resistant Ti/TiN coatings with stress absorbing layers

In this high speed era, several machine components are operated at high speeds and at high temperatures. Machine components such as gas turbine compressor blades and vanes are exposed to high speed dust particles. Consequently, engine performance deteriorates and in extreme cases catastrophic accidents can happen. In order to protect these engine components, ultra-thin multilayered erosion resistant Ti/TiN coatings (thickness ~9–10 μm) with stress absorbing layers were developed using magnetron sputtering. Stress absorbing metallic layers were introduced to control the internal stress and also to maintain balanced ductile and brittle nature of the coating. Ultra-thin (3–4 nm) metal/ceramic bilayers provided high number of interfaces and ultra-small grain sizes. Erosion resistance performance of the developed coatings and also bare Ti6Al4V substrates was tested according to the ASTM-G76-13 standard at 400 °C. Tests were conducted with three different erodent speeds of 30, 60 and 100 m/s at four different impinging angles 90°, 60°, 45° and 30°. The average erosion rate ratios between Ti6Al4V substrate and optimized coating were 74, 13 and 12 for the erodent speeds of 30, 60 and 100 m/s, respectively. Influence of the substrate on the coating performance was also studied. Morphological, elemental and spectral analyses of the erosion scar were also probed. Structural stability, mechanical properties and erosion resistance performance of the optimized film were tested after 100 h annealing in air and vacuum at a temperature of 400 °C. Erosion resistant performance of the optimized film was unaffected after 100 h annealing.

Ultrasensitive Detection of VOCs Using a High‐Resolution CuO/Cu2O/Ag Nanopattern Sensor

AbstractThe p‐type semiconducting copper oxides (CuO and Cu2O) are promising materials for gas sensors, owing to their characteristic oxygen adsorption properties and low operation temperature. In this study, the sensing performance of a CuO‐based chemiresistor is significantly enhanced by incorporating Ag nanoparticles on high‐resolution p‐type CuO/Cu2O nanopattern channels. The high‐resolution CuO/Cu2O/Ag nanochannel is fabricated using a unique top‐down nanolithographic approach. The gas response (ΔR/Ra) of the CuO/Cu2O/Ag gas sensor increases by a maximum factor of 7.3 for various volatile organic compounds compared with a pristine CuO/Cu2O gas sensor. The sensors exhibit remarkable sensitivity (ΔR/Ra = 8.04) at 125 parts per billion (ppb) for acetone analytes. As far as it is known, this is the highest sensitivity achieved for p‐type metal oxide semiconductor (MOS)‐based gas sensors compared to previous studies. Furthermore, the outstanding gas responses observed in this study are superior to the most of n‐type MOS‐based gas sensors. The high sensitivity of the sensor is attributed to i) the high resolution (≈30 nm), high aspect ratio (≈12), and ultrasmall grain boundaries (≈10 nm) of the CuO/Cu2O nanopatterns and ii) the electronic sensitization and chemical sensitization effects induced by incorporating Ag nanoparticles on the CuO/Cu2O channels.

CeO2 quantum dots modified electrode for detecting hydrogen peroxide

This study describes the fabrication of a sensitive modified electrode for detecting hydrogen peroxide. The glassy carbon surface was modified with a composite containing CeO2 quantum dots and carbon sheets. The ultra-small grain size of nanoparticle endows the electrode with excellent electrocatalytic activity. As a result, the electrode shows electrocatalytic reduction activity toward H2O2 at a potential of −0.5 V. The CeO2 quantum dots sensor shows a satisfactory performance for detecting hydrogen peroxide with a linear range of 2 ppm–1000 ppm, and a limit of detection (LOD) of 900 ppb. These findings show that CeO2 quantum dots have a great potential for applications in electrochemical sensing.

Selective Functionalization of High-Resolution Cu₂O Nanopatterns via Galvanic Replacement for Highly Enhanced Gas Sensing Performance.

Recently, high-resolution patterned metal oxide semiconductors (MOS) have gained considerable attention for enhanced gas sensing performance due to their polycrystalline nature, ultrasmall grain size (~5 nm), patternable properties, and high surface-to-volume ratio. Herein, we significantly enhanced the sensing performance of that patterned MOS by galvanic replacement, which allows for selective functionalization on ultrathin Cu2O nanopatterns. Based on the reduction potential energy difference between the base channel material (Cu2O) and the decorated metal ion (Pt2+), Pt could be selectively and precisely decorated onto the desired area of the Cu2O nanochannel array. Overall, the Pt-decorated Cu2O exhibited 11-fold higher NO2 (100 ppm) sensing sensitivity as compared to the non-decorated sensing channel, the while the channel device with excessive Pt doping showed complete loss of sensing properties.

Ultrasmall Grained Pd Nanopattern H2 Sensor.

Precise control of the size and interfaces of Pd grains is very important for designing a high-performance H2 sensing channel because the transition of the Pd phase from α to β occurs through units of single grains. However, unfortunately, the grain controllability of previous approaches has been limited to grains exceeding 10 nm in size and simple macroscopic channel structures have only shown monotonic response behavior for a wide concentration range of H2. In this work, for the first time, we found that Pd channels that are precisely grain-controlled show very different H2 sensing behavior. They display dual-switching response behavior with simultaneous variation of the positive and negative response direction within single sensor. The Pd nanopattern channel having smallest grain size/interface among previous works could be fabricated via unique lithographic approaches involving low-energy plasma (Ar+) bombardment. The ultrasmall grain size (5 nm) and narrow interface gap (<2 nm) controlled by Ar+ plasma bombardment enabled both the hydrogen-induced lattice expansion (HILE) (Δ RH2 < 0) and surface electron scattering (Δ RH2 > 0) mechanisms to be simultaneously applied to the single Pd channel, thereby inducing dual-switching response according to the H2 concentration range. In addition, the unique high-aspect-ratio high-resolution morphological characteristics made it possible to achieve highly sensitive H2 detecting performance (limit of detection: 2.5 ppm) without any hysteresis and irreversible performance degradation. These noteworthy new insights are attributed to high-resolution control of the grain size and the interfaces with the Pd nanostructure channel.

Synthesis of Nanosized SAPO-34 via an Azeotrope Evaporation and Dry Gel Conversion Route and Its Catalytic Performance in Chloromethane Conversion

Nanosized SAPO-34 has been successfully synthesized by a facile and low-cost method involving an azeotrope evaporation and steam-assisted crystallization route without either expensive poregens or homemade starting materials. The as-synthesized nanosized SAPO-34 shows ultrasmall grain sizes and an impressively high total pore volume, as confirmed by X-ray diffraction, nitrogen physisorption, scanning electron microscopy, and transmission electron microscopy techniques. The formation process and the pivotal role of supersaturation in the synthesis of nanosized SAPO-34 have been revealed by a series of control experiments, on the basis of which a nonclassic oriented attachment mechanism under high supersaturation ratio conditions is proposed. When used as the catalyst for the conversion of chloromethane to olefins, the nanosized SAPO-34 exhibits acidity comparable to that of the conventional SAPO-34 but double the catalytic lifetime and a slightly lower selectivity for light olefins.

Investigation on tensile properties of nanocrystalline titanium with ultra-small grain size

Molecular dynamics was used to simulate tensile behavior of nanocrystalline titanium with ultra-small grain size ranging from 2.8 nm to 10.2 nm at the strain rate ranging from 108 s−1 to 1010 s−1. Three dimensional samples with random oriented grains containing no textures were developed by Voronoi tessellation. For all the samples, the yielding is solely controlled by GB mediated process. The inverse Hall-Petch relation between grain size and flow stress was found. The strain rate sensitivity shows increase with the decrease of grain size due to the enhancement of grain boundary (GB) mediated process. Meanwhile, it increases with the applied strain rate due to the local disorder around GBs. Remakeable grain coarsening observed in the 2.8 nm sample causes the slight increase of flow stress. Both GB mediated process and partial dislocation slips play an important role in the plastic deformation of nanocrystalline Ti. With the increase of grain size, rare twins initiated from GBs can be observed. From the size dependent dislocation density analysis, it is concluded that with the increase of grain size, dislocation related deformation contributes more to the plastic strain.

"Size-Independent" Single-Electron Tunneling.

Incorporating single-electron tunneling (SET) of metallic nanoparticles (NPs) into modern electronic devices offers great promise to enable new properties; however, it is technically very challenging due to the necessity to integrate ultrasmall (<10 nm) particles into the devices. The nanosize requirements are intrinsic for NPs to exhibit quantum or SET behaviors, for example, 10 nm or smaller, at room temperature. This work represents the first observation of SET that defies the well-known size restriction. Using polycrystalline Au NPs synthesized via our newly developed solid-state glycine matrices method, a Coulomb Blockade was observed for particles as large as tens of nanometers, and the blockade voltage exhibited little dependence on the size of the NPs. These observations are counterintuitive at first glance. Further investigations reveal that each observed SET arises from the ultrasmall single crystalline grain(s) within the polycrystal NP, which is (are) sufficiently isolated from the nearest neighbor grains. This work demonstrates the concept and feasibility to overcome orthodox spatial confinement requirements to achieve quantum effects.

Nanoparticle cluster gas sensor: Pt activated SnO2 nanoparticles for NH3 detection with ultrahigh sensitivity.

Pt activated SnO2 nanoparticle clusters were synthesized by a simple solvothermal method. The structure, morphology, chemical state and specific surface area were analyzed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and N2-sorption studies, respectively. The SnO2 nanoparticle cluster matrix consists of tens of thousands of SnO2 nanoparticles with an ultra-small grain size estimated to be 3.0 nm. And there are abundant random-packed wormhole-like pores, caused by the inter-connection of the SnO2 nanoparticles, throughout each cluster. The platinum element is present in two forms including metal (Pt) and tetravalent metal oxide (PtO2) in the Pt activated SnO2 nanoparticle clusters. The as-synthesized pure and Pt activated SnO2 nanoparticle clusters were used to fabricate gas sensor devices. It was found that the gas response toward 500 ppm of ammonia was improved from 6.48 to 203.44 through the activation by Pt. And the results indicate that the sensor based on Pt activated SnO2 not only has ultrahigh sensitivity but also possesses good response-recovery properties, linear dependence, repeatability, selectivity and long-term stability, demonstrating the potential to use Pt activated SnO2 nanoparticle clusters as ammonia gas sensors. At the same time, the formation mechanisms of the unique nanoparticle clusters and highly enhanced sensitivity are also discussed.

Development of diamond cathode materials for enhancing the electron field emission and plasma characteristics using two-step microwave plasma enhanced chemical vapor deposition process

The enhancement on the plasma illumination characteristics of a cylindrical microplasma device due to the utilization of hybrid-diamond (HiD) films as cathode was systematically investigated. The improved plasma illumination behavior was closely related to the enhanced electron field emission (EFE) properties of the diamond films. The HiD films, which possessed better EFE properties, including lower turn-on field for inducing the EFE process [(E0)efe = 2.7 V/μm] and higher EFE current density [(Je)efe = 2.8 mA/cm2, at 10.6 V/μm], resulted in superior illumination performance for the microplasma devices. The plasma can be triggered at a low threshold field of (Eth)pl. = 0.166 V/μm, attaining a large plasma current density of (Je)pl. = 9.6 mA/cm2 at an applied field of 0.266 V/μm (plasma density of ne = 1.70 × 1015 cm−3). The better EFE for the HiD films is ascribed to the unique granular structure of the films. Transmission electron microscopic studies revealed that the HiD films contained large diamond aggregates evenly distributed among the ultrasmall grain matrix. There presents a-few-layer graphite, surrounding the large aggregates that formed electron transport networks and improved the EFE properties for the HiD films. The superior EFE properties for the HiD cathode materials are the prime factor for improving plasma illumination characteristics for the cylindrical microplasma devices.

Joining of 14YWT and F82H by friction stir welding

Friction stir welding was investigated for joining specimens of the ODS 14YWT ferritic alloy together and to an F82H tempered martensitic steel plate. The FSW run was performed using a polycrystalline boron nitride tool and resulted in good bonding between 14YWT/14YWT and 14YWT/F82H. Joints and interfaces were observed by light microscopy and SEM analysis to be narrow in width. The ultra-small grain size of 14YWT increased by a factor up to 4 while that of F82H decreased by a considerable amount in the weld zones. The TEM analysis showed no significant changes in the size of the oxygen-enriched nanoclusters in the weld zone of 14YWT. However, defects such as a wormhole on the advancing side of the weld zone in 14YWT and small pores associated with joints and interfaces were observed in the FSW sample. The hardness measurements from unaffected zone into weld zones showed ∼20% decrease in hardness for 14YWT (from ∼500VH to ∼380VH) and ∼100% increase in hardness of F82H (from ∼220VH to ∼440VH).

Nucleation-promoted L10 ordering in FePt thin films with ultrasmall grain size

FePt thin films with an ultrasmall grain size of ∼3.4 nm were successfully prepared by employing a relatively high sputtering pressure of 4 Pa that produces deposited target atoms or clusters with low energy. The L10 ordering in the FePt thin films is dominantly dependent on the nucleation of ordered domains rather than their growth which is usually observed in the films with relatively large grain size. The nucleation-promoted L10 ordering makes a reduction in grain growth and thus yields the L10-FePt thin films with a uniform and small grain size below 10 nm.

Ultrasmall grain size control in longitudinal recording media for ultrahigh areal densities

In this study, we propose a new “atomic wall” approach to underlayer design. In this design, a CrX underlayer is used where X and Cr are not miscible in the bulk phase, resulting in a reduced grain size. We have studied the addition of Zr to Cr layers and found that CrZr gives magnetic properties comparable to those of CrMo underlayers. In addition, a CrZr underlayer gives rise to a smaller grain size of 5.5 nm, in comparison to a grain size of 6.2 nm in the case of CrMo underlayers. To our knowledge, this is the smallest grain size reported for Co-alloy recording media. The recording measurements also show a higher signal to noise ratio for CrZr underlayers than for CrMo underlayers, suggesting that CrZr underlayers have potential for use in ultrahigh areal densities.

Model for ferromagnetic nanograins with discrete electronic states

We propose a simple phenomenological model for an ultrasmall ferromagnetic grain, formulated in terms of the grain's discrete energy levels. We compare the model's predictions with recent measurements of the discrete tunneling spectrum through such a grain. The model can qualitatively account for the observed features if we assume (i) that the anisotropy energy varies among different eigenstates of one grain, and (ii) that nonequilibrium spin accumulation occurs.

Microstructure study of CoCrPt/Ti/NiAl perpendicular media

Perpendicular medium with an average grain size of 8.2 nm with 20 nm thickness has been produced by using a-CoCrPt/Ti/NiAl tri-layer structure. The signal-to-noise ratio improves with the addition of a NiAl seed layer. From the TEM image analysis, the introduction of the NiAl seed layer may enhance the separation between the CoCrPt grains. The ultra-small grain size will improve the performance of perpendicular media for future high-density recording applications.

Medium noise and grain size analysis of CoCrPt/Ti perpendicular media with NiAl seed layer

Perpendicular medium with an average grain size of 8.2 nm have been produced by using a-CoCrPt/Ti/NiAl tri-layer structure. The signal-to-noise ratio for the tri-layer medium is 2-7 dB higher than CoCrPt/Ti bi-layer medium at low deposition temperatures. The medium noise is dominated by dc noise and exhibits little transition noise for densities less than 500 kFCI. The ultra-small grain size will improve the performance of perpendicular media for future high-density recording applications.

Percolation model of a nanocrystalline gas sensitive layer

A model for the nanocrystalline layer of a semiconducting gas sensor is developed that takes into account the possibility of having ultrasmall compensated grains that are insulating. The probability that an ultrasmall grain is insulating depends on the diameter D and the number of next neighbors (the coordination number k). The occurrence of insulating grains leads to specific percolation type effects on the conductivity of the layer, by which its sensitivity can be improved.