Nanoscale Defects Research Articles

Nanoparticle dynamics in hydrogel networks with controlled defects.

The effect of nanoscale defects on nanoparticle dynamics in defective tetra-poly(ethylene glycol) (tetra-PEG) hydrogels is investigated using single particle tracking. In a swollen nearly homogeneous hydrogel, PEG-functionalized quantum dot (QD) probes with a similar hydrodynamic diameter (dh = 15.1 nm) to the mesh size (〈ξs〉 = 16.3 nm), are primarily immobile. As defects are introduced to the network by reaction-tuning, both the percentage of mobile QDs and the size of displacements increase as the number and size of the defects increase with hydrolysis time, although a large portion of the QDs remain immobile. To probe the effect of nanoparticle size on dynamics in defective networks, the transport of dh = 47.1 nm fluorescent polystyrene (PS) and dh = 9.6 nm PEG-functionalized QDs is investigated. The PS nanoparticles are immobile in all hydrogels, even in highly defective networks with an open structure. Conversely, the smaller QDs are more sensitive to perturbations in the network structure with an increased percentage of mobile particles and larger diffusion coefficients compared to the larger QDs and PS nanoparticles. The differences in nanoparticle mobility as a function of size suggests that particles of different sizes probe different length scales of the defects, indicating that metrics such as the confinement ratio alone cannot predict bulk dynamics in these systems. This study provides insight into designing hydrogels with controlled transport properties, with particular importance for degradable hydrogels for drug delivery applications.

Comprehensive Review on Thermoelectric Electrodeposits: Enhancing Thermoelectric Performance Through Nanoengineering.

Thermoelectric devices based power generation and cooling systemsystem have lot of advantages over conventional refrigerator and power generators, becausebecause of solid-state devicesdevices, compact size, good scalability, nono-emissions and low maintenance requirement with long operating lifetime. However, the applications of thermoelectric devices have been limited owingowing to their low energy conversion efficiency. It has drawn tremendous attention in the field of thermoelectric materials and devices in the 21st century because of the need of sustainable energy harvesting technology and the ability to develop higher performance thermoelectric materials through nanoscale science and defect engineering. Among various fabrication methods, electrodeposition is one of the most promising synthesis methods to fabricate devices because of its ability to control morphology, composition, crystallinity, and crystal structure of materials through controlling electrodeposition parameters. Additionally, it is an additive manufacturing technique with minimum waste materials that operates at near room temperature. Furthermore, its growth rate is significantly higher (i.e., a few hundred microns per hour) than the vacuum processes, which allows device fabrication in cost effective matter. In this paper, the latest development of various electrodeposited thermoelectric materials (i.e., Te, PbTe, Bi2Te3 and their derivatives, BiSe, BiS, Sb2Te3) in different forms including thin films, nanowires, and nanocomposites were comprehensively reviewed. Additionally, their thermoelectric properties are correlated to the composition, morphology, and crystal structure.

Movement and Diffusion of the Helium Atom in Nanostructures (in Pores) with Water

The influence of water-filled nanoscale defects on the total movement of helium atoms through a quartz crystal is considered. The approximation of local chains is used, the interaction constant of which characterizes the interaction of the helium atom with the environment. The appearance of water molecules in defects (pores) leads to the renormalization of this interaction. The D'Alembert principle is used to evaluate this renormalization. The effect of such a renormalization of the interaction on the diffusion coefficient of helium through a crystal with defects filled with water is considered.

DETERMINATION OF THE STRESS STATE AND THE FORCE OF DEFORMATION OF BALL-SHAPED BILLETS IN A CLOSED MATRIX

At the present stage of development of science and technology, the problem of improving the quality of metal products by integrating methods of severe plastic deformation into existing technological processes is urgent. Intense plastic deformation makes it possible to obtain nanostructured metallic materials with improved quality and an attractive set of properties due to the saturation of the metal with nanoscale defects. Spherical billets with two ends along the edges are a common form of metal products. For the manufacture of such metal products, the effect of mutual influence of all-round compression and ECAP pressing in a device for the implementation of equal-channel angular pressing in a closed matrix was used. The purpose of this study is to develop a theoretical approach to determining the stress state and deformation force of spherical blanks in a closed matrix. To achieve this goal, an integrated approach was used to determine the stress state and deformation force by the method of slip lines and computer simulation in the Deform-3D software package. Analysis of the results of the stress state of the workpieces obtained by the slip line method showed that a uniform stress state is formed with a predominance of the maximum compressive stresses. The combination of the latter with angular metal extrusion into the lateral channels of the matrix predicts the production of workpieces with sub-ultrafine-grained and/or nanostructure. By the method of joint solution of differential equations of equilibrium and plasticity conditions, as well as computer modeling, the deforming force of the blanks at the final stage of deformation, when the metal flows out into the lateral channels of the matrix, is determined. The analysis of the obtained results shows that the value of the deformation force obtained by the two methods is comparable with a difference of up to 2 %, which confirms the correctness of the obtained values.

A short review of defect superlattice formation in metals and alloys under irradiation

Irradiation damage drives complex and coupled phenomena in materials at far-from-equilibrium conditions. The self-organization of nanoscale defects in materials under irradiation shows great potential to tailor the physical properties of materials by controlling nanopatterned microstructures. Irradiation-induced gas bubble and void superlattices are two important ordered nanostructures of great scientific interest. Although both types of superlattices have been investigated extensively, a consensus has yet to be reached on their formation mechanisms. In this review article, the current research status of gas bubble and void superlattices in metals and alloys and their characterization, structural stability, and mechanistic modeling are summarized. The fundamental research goals to advance the mechanistic understanding of gas bubble and void superlattices are outlined.

Electron-beam-induced current (EBIC) imaging technique to quicken polysilicon defect localization in MOSFETs

It is demonstrated that EBIC imaging technique is superior in quick localizing polysilicon defect of metal-oxide-semiconductor field-effect transistors (MOSFETs). Once polysilicon leakage phenomena are examined, relying on EBIC imaging analysis, we can rapidly isolate nanoscale defect in tens of microns length range. Although faulty polysilicon is employed by more than one MOSFET, it turns out to be unnecessary to characterize all devices after EBIC imaging analysis. By comparative studies, it is concluded that 2-probe EBIC imaging is preferred for mega-Ohm range resistive defect localization while 1-probe configuration is only applicable for kilo-Ohm range. Nanoprobe I-V measurement of the isolated device uncovers the abnormal electrical characteristic which fairly aligns with EBIC imaging analysis. Focused ion beam (FIB) and transmission electron microscopy (TEM) findings imply the EBIC imaging has submicron level spatial resolution toward resistive defects. Therefore it overcomes traditional limits in terms of technical method and lowers failure analysis (FA) difficulty considerably. Ultimately, a simplified model is proposed to explain the physical mechanism of this application. Our modeling describes the strong dependance of EBIC imaging on hot excess carrier behavior within shallow p-n junction when electron beam (e-beam) penetrates into bulk silicon. The work improves the understanding of excess minority carriers transport properties in p-n junction under scanning e-beam irradiation. And this paper may make EBIC imaging be a more promising way to identify minute material defect of future advanced MOSFET.

Atomic Pathways of Solute Segregation in the Vicinity of Nanoscale Defects.

Using GeSn semiconductor as a model system, this work unravels the atomic-level details of the behavior of solutes in the vicinity of a dislocation prior to surface segregation in strained, metastable thin layers. The dislocations appear in the 3D atom probe tomography maps as columnar regions, 3.5-4.0 nm wide, with solute concentrations 3-4 times higher than the sounding matrix. During the initial stage of phase separation, the migration of solute atoms toward the dislocation is associated with a gradual increase in Sn concentration and in density of atomic clusters, which reach 175-190 per 103 nm3 with 12-15 atoms/cluster close to dislocations. The latter provide, at advanced stages, fast diffusive channels for Sn mass-transport to the surface, thus bringing the matrix around the dislocation to the equilibrium concentration. In parallel, an increase in solute concentration (∼0.05 at. %/nm) and in the number of atomic clusters (12-16 clusters/33 nm) is observed along the dislocation core.

High Thermoelectric Performance through Crystal Symmetry Enhancement in Triply Doped Diamondoid Compound Cu2SnSe3

AbstractThe presence of high crystallographic symmetry and nanoscale defects are favorable for thermoelectrics. With proper electronic structures, a highly symmetric crystal tends to possess multiple carrier channels and promote electrical conductivity without sacrificing Seebeck coefficient. In addition, nanoscale defects can effectively scatter acoustic phonons to suppress thermal conductivity. Here, it is reported that the triple doping of Cu2SnSe3 leads to a high ZT value of 1.6 at 823 K for Cu1.85Ag0.15(Sn0.88Ga0.1Na0.02)Se3, and a decent average ZT (ZTave) value of 0.7 is also achieved for Cu1.85Ag0.15(Sn0.93Mg0.06Na0.01)Se3 from 475 to 823 K. This study reveals: 1) Ag doping on Cu sites generates numerous point defects and greatly decreases lattice thermal conductivity. 2) Doping Mg or Ga converts the monoclinic Cu2SnSe3 into a cubic structure. This symmetry enhancing leads to an increase in the effective mass from 0.8 me to 2.6 me (me, free electron mass) and the power factor from 4.3 µW cm−1 K−2 for Cu2SnSe3 to 11.6 µW cm−1 K−2. 3) Na doping creates dense dislocation arrays and nanoprecipitates, which strengthens the phonon scattering. 4) Pair distribution function analysis shows localized symmetry breakdown in the cubic Cu1.85Ag0.15(Sn0.88Ga0.1Na0.02)Se3. This work provides a standpoint to design promising thermoelectric materials by synergistically manipulating crystal symmetry and nanoscale defects.

Closed‐Loop Defect States in 2D Materials with Honeycomb Lattice Structure: Molybdenum Disulfide

Platelets and ribbons of monolayer molybdenum disulfide () exhibit characteristic electronic states along the edges, which tend to decouple from the extended states in the interior of the 2D extended structure. Under sulfur excess specifically triangular platelets are formed, which exhibit 1D extended electronic states with a pronounced catalytic activity along the edges. The present density‐functional‐based investigation shows that very similar electronic states occur also along the edges of holes inside an otherwise perfect, extended monolayer. For triangular defects, the edge states can delocalize around the hole. Density functional molecular dynamics simulations on 150 different nanoscale defect geometries and termination variants prove the high structural stability of hole defects in against further reconstructions and a thermodynamic propensity to participate in sulfur exchange reactions.

Quantification of the Nano-Voids in Electroless Deposited Copper Layers Using SiNx TEM Grids

A plan-view analytical method to detect nano-voids of deposited copper layers, especially for the fabrication of printed circuit boards, was investigated using transmission electron microscopy (TEM). Through electroless plating, thin copper layers were directly deposited on small silicon nitride window grids, and the presence of nano-voids within the deposited copper layers were confirmed with various TEM analytical techniques. The amount and size of the nano-voids were further quantified from the plan-view TEM images, making it possible to compare quickly multiple samples prepared under various plating conditions. This analytical method is practical as it is faster in detecting and measuring nano-scale defects when compared to typical cross-sectional analytical methods.

Observation of Defect-Assisted Magnetic Vortex Core Reversal at Ultralow Critical Velocity

The stability of structures with nontrivial topology is a subject of great interest, both from fundamental and technological perspectives. The topology of a magnetic vortex, for example, results in high stability of the vortex core polarity, which is attractive for applications. This high stability, however, greatly impedes the required deterministic polarity switching, which is typically only achieved with large magnetic fields or strong dynamic driving. Here, we show that the interaction between the vortex core and manufactured nanoscale defects in the magnetic material lowers the required driving strength for core polarity reversal by more than an order of magnitude. We excite vortex dynamics in thin permalloy disks, and map the two-dimensional (2D) vortex core trajectory using 3D time-resolved Kerr microscopy. In pristine samples, we observe normal gyrotropic motion of the vortex core. After laser-induced generation of defects, however, we observe repeated vortex core reversal at much-reduced driving strength. Micromagnetic simulations reveal how local reduction of exchange coupling and saturation magnetization can create vortex core reversal sites for deterministic vortex core switching.

Topology-Mediated Enhanced Polaron Coherence in Covalent Organic Frameworks.

We employ the Holstein model for polarons to investigate the relationship among defects, topology, Coulomb trapping, and polaron delocalization in covalent organic frameworks (COFs). We find that intrasheet topological connectivity and π-column density can override disorder-induced deep traps and significantly enhance polaron migration by several orders of magnitude in good agreement with recent experimental observations. The combination of percolation networks and micropores makes trigonal COFs ideally suited for charge transport followed by kagome/tetragonal and hexagonal structures. By comparing the polaron spectral signatures and coherence numbers of large three-dimensional frameworks having a maximum of 180 coupled chromophores, we show that controlling nanoscale defects and the location of the counteranion is critical for the design of new COF-based materials yielding higher mobilities. Our analysis establishes design strategies for enhanced conductivity in COFs that can be readily generalized to other classes of conductive materials such as metal-organic frameworks and perovskites.

Racetrack memory based on current-induced motion of topological Bloch lines

A magnetic Bloch line (BL) is a nanoscale topological defect with a nontrivial topological charge. Herein, we propose a novel memory that utilizes current-driven BL motion. We demonstrate that it is possible to write and shift BLs using a spin-transfer-torque scheme. The writing efficiency and shifting velocity of the BL-based racetrack memory are better than or comparable to those of domain wall (DW)-based racetrack memory. Moreover, the current-driven BL motion is more robust against defects than the DW motion, suggesting that BL-motion-based memory can serve as a next-generation memory beyond the DW-motion-based racetrack memory.

Twin boundary migration in an individual platinum nanocrystal during catalytic CO oxidation

At the nanoscale, elastic strain and crystal defects largely influence the properties and functionalities of materials. The ability to predict the structural evolution of catalytic nanocrystals during the reaction is of primary importance for catalyst design. However, to date, imaging and characterising the structure of defects inside a nanocrystal in three-dimensions and in situ during reaction has remained a challenge. We report here an unusual twin boundary migration process in a single platinum nanoparticle during CO oxidation using Bragg coherent diffraction imaging as the characterisation tool. Density functional theory calculations show that twin migration can be correlated with the relative change in the interfacial energies of the free surfaces exposed to CO. The x-ray technique also reveals particle reshaping during the reaction. In situ and non-invasive structural characterisation of defects during reaction opens new avenues for understanding defect behaviour in confined crystals and paves the way for strain and defect engineering.

Machining Characteristics of Electrochemical Polishing Using Ultrasonic Vibration for Nanoscale High Surface Quality.

This study demonstrates a method to improve the surface quality by adding artificial vibration to the electrolyte in electrochemical polishing (ECP, electropolishing). ECP is a typical non-contact surface polishing process that has been used to improve surface quality without leaving any of the mechanical scratch marks that can arise when applying mechanical processes. ECP can polish work material via electrochemical dissolution between the surfaces of an anode and a cathode, and irregular defects are generated on the surface by impurities and bubbles generated during machining. This study confirms that our novel ECP method yields improved results over conventional ECP based on experiments using vibration electrochemical polishing (VECP) with ultrasonic vibrations. VECP minimizes nanoscale surface defects, improves surface roughness, makes it possible to quickly remove materials at nanoscale by increasing the material removal rate (MRR). Under high current density, where the electrochemical relatively reaction is active, value of the current is increased when ultrasonic vibration is added. The localized roughness of the work material was measured by atomic force microscopy (AFM) according to various electrical conditions. In addition, we also compared the overall surface quality and productivity to those obtained by conventional ECP.

Exploring the anti-site disorder and oxygen vacancies in Sr[formula omitted]FeMoO[formula omitted] thin films

To address the importance of nanoscale defects in complex magnetic oxides, we present an effective tool, variable energy positron annihilation spectroscopy, for probing the relatively small changes in anti-site disorder and oxygen vacancies of the in situ annealed double perovskite Sr2FeMoO6 thin films. By controlling the annealing conditions in wide pressure and temperature ranges and thus affecting the amount of nanoscale defects, we show that the magnetic properties of Sr2FeMoO6 thin films can be modified, particularly with the oxygen nonstoichiometry, and hence their spintronic functionality can be improved. On the basis of our findings together with proposed mechanism, we suggest that the annealing treatments can also be scaled to other complex magnetic perovskites to engineer nanoscale defects and thus improve their usability in future spintronic applications.

Monitoring Electrical Biasing of Pb(Zr0.2Ti0.8)O3 Ferroelectric Thin Films In Situ by DPC-STEM Imaging.

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr0.2Ti0.8)O3 thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions.

High thermoelectric performance enabled by convergence of nested conduction bands in Pb7Bi4Se13 with low thermal conductivity

Thermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450–800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moiré fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.

Defect and strain engineering of monolayer WSe2 enables site-controlled single-photon emission up to 150\u2009K

In recent years, quantum-dot-like single-photon emitters in atomically thin van der Waals materials have become a promising platform for future on-chip scalable quantum light sources with unique advantages over existing technologies, notably the potential for site-specific engineering. However, the required cryogenic temperatures for the functionality of these sources has been an inhibitor of their full potential. Existing methods to create emitters in 2D materials face fundamental challenges in extending the working temperature while maintaining the emitter’s fabrication yield and purity. In this work, we demonstrate a method of creating site-controlled single-photon emitters in atomically thin WSe2 with high yield utilizing independent and simultaneous strain engineering via nanoscale stressors and defect engineering via electron-beam irradiation. Many of the emitters exhibit biexciton cascaded emission, single-photon purities above 95%, and working temperatures up to 150 K. This methodology, coupled with possible plasmonic or optical micro-cavity integration, furthers the realization of scalable, room-temperature, and high-quality 2D single- and entangled-photon sources.

Effect of 1.5 MeV Proton Irradiation on Superconductivity in FeSe0.5Te0.5 Thin Films

Raising the critical current density Jc in magnetic fields is crucial to applications such as rotation machines, generators for wind turbines and magnet use in medical imaging machines. The increase in Jc has been achieved by introducing structural defects including precipitates and vacancies. Recently, a low-energy ion irradiation has been revisited as a practically feasible approach to create nanoscale defects, resulting in an increase in Jc in magnetic fields. In this paper, we report the effect of proton irradiation with 1.5 MeV on superconducting properties of iron–chalcogenide FeSe0.5Te0.5 films through the transport and magnetization measurements. The 1.5 MeV proton irradiation with 1 × 1016 p/cm2 yields the highest Jc increase, approximately 30% at 5–10 K and below 1 T without any reduction in Tc. These results indicate that 1.5 MeV proton irradiations could be a practical tool to enhance the performance of iron-based superconducting tapes under magnetic fields.