Intrinsic Structural Defects Research Articles

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Effects of temperature and intrinsic structural defects on mechanical properties and thermal conductivities of InSe monolayers

We conduct molecular dynamics simulations to study the mechanical and thermal properties of monolayer indium selenide (InSe) sheets. The influences of temperature, intrinsic structural defect on the tensile properties were assessed by tensile strength, fracture strain, and Young’s modulus. We found that the tensile strength, fracture strain, and Young’s modulus reduce as increasing temperature. The results also indicate that with the existence of defects, the stress is concentrated at the region around the vacancy leading to the easier destruction. Therefore, the mechanical properties were considerably decreased with intrinsic structural defects. Moreover, Young’s modulus is isotropy in both zigzag and armchair directions. The point defect almost has no influence on Young’s modulus but it strongly influences the ultimate strength and fracture strain. Besides, the effects of temperature, length size, vacancy defect on thermal conductivity (κ) of monolayer InSe sheets were also studied by using none-equilibrium molecular dynamics simulations. The κ significantly arises as increasing the length of InSe sheets. The κ of monolayer InSe with infinite length at 300 K in armchair direction is 46.18 W/m K, while in zigzag direction is 45.87 W/m K. The difference of κ values in both directions is very small, indicating the isotropic properties in thermal conduction of this material. The κ decrease as increasing the temperature. The κ goes down with the number of atoms vacancy defect increases.

Epitaxial GeSn Obtained by High Power Impulse Magnetron Sputtering and the Heterojunction with Embedded GeSn Nanocrystals for Shortwave Infrared Detection.

GeSn alloys have the potential of extending the Si photonics functionality in shortwave infrared (SWIR) light emission and detection. Epitaxial GeSn layers were deposited on a relaxed Ge buffer on Si(100) wafer by using high power impulse magnetron sputtering (HiPI-MS). Detailed X-ray reciprocal space mapping and HRTEM investigations indicate higher crystalline quality of GeSn epitaxial layers deposited by Ge HiPI-MS compared to commonly used radio frequency magnetron sputtering (RF-MS). To obtain a rectifying heterostructure for SWIR light detection, a layer of GeSn nanocrystals (NCs) embedded in oxide was deposited on the epitaxial GeSn one. Embedded GeSn NCs are obtained by cosputtering deposition of (Ge1-xSnx)1-y(SiO2)y layers and subsequent rapid thermal annealing at a low temperature of 400 °C. Intrinsic GeSn structural defects give p-type behavior, while the presence of oxygen leads to the n-character of the embedded GeSn NCs. Such an embedded NCs/epitaxial GeSn p-n heterostructure shows superior photoelectrical response up to 3 orders of magnitude increase in the 1.2-2.5 μm range, as compared to performances of diode based only on embedded NCs.

Intrinsic and Dopant-Related Luminescence of Undoped and Tb Plus Tm Double-Doped Lithium Magnesium Phosphate (LiMgPO4, LMP) Crystals.

In this work, the luminescence properties of undoped, Tm3+ doped, and Tb3+ plus Tm3+ double-doped crystals of the lithium magnesium phosphate (LiMgPO4, LMP) compound were investigated. The crystals under study were grown from melt using the micro-pulling-down method. The intrinsic and dopant-related luminescence of these crystals were studied using cathodo-, radio-, photo-, and thermoluminescence methods. Double doping with Tb3+ and Tm3+ ions was analyzed as these dopants are expected to exhibit an opposite trapping nature, namely to create the hole and electron-trapping sites, respectively. The spectra measured for the undoped samples revealed three prominent broad emission bands with maxima at around 3.50, 2.48, and 1.95 eV, which were associated with intrinsic structural defects within the studied compound. These were expected due to the anion vacancies forming F+-like centers while trapping the electrons. The spectra measured for Tb and Tm double-doped crystals showed characteristic peaks corresponding to the 4f–4f transitions of these dopants. A simplified model of a recombination mechanism was proposed to explain the temperature dependence of the measured thermally stimulated luminescence spectra. It seems that at low temperatures (below 300 °C), the charge carriers were released from 5D3-related Tb3+ trapping sites and recombination took place at Tm-related sites, giving rise to the characteristic emission of Tm3+ ions. At higher temperatures, above 300 °C, the electrons occupying the Tm3+-related trapping sites started to be released, and recombination took place at 5D4-related Tb3+ recombination centers, giving rise to the characteristic emission of Tb3+ ions. The model explains the temperature dependence observed for the luminescence emission from double-doped LiMgPO4 crystals and may be fully applicable for the consideration of emissions of other double-doped compounds.

Mechanically Strong Globular‐Protein‐Based Fibers Obtained Using a Microfluidic Spinning Technique

Proteins used for the formation of light weight and mechanically strong biological fibers are typically composed of folded rigid and unfolded flexible units. In contrast to fibrous proteins, globular proteins are generally not regarded as a good candidate for fiber production due to their intrinsic structural defects. Thus, it is challenging to develop an efficient strategy for the construction of mechanically strong fibers using spherical proteins. Herein, we demonstrate the production of robust protein fibers from bovine serum albumin (BSA) using a microfluidic technique. Remarkably, the toughness of the fibers was up to 143 MJ m-3 , and after post-stretching treatment, their breaking strength increased to almost 300 MPa due to the induced long-range ordered structure in the fibers. The performance is comparable to or even higher than that of many recombinant spider silks or regenerated silkworm fibers. Thus, this work opens a new way for making biological fibers with high performance.

Mechanically Strong Globular‐Protein‐Based Fibers Obtained Using a Microfluidic Spinning Technique

AbstractProteins used for the formation of light weight and mechanically strong biological fibers are typically composed of folded rigid and unfolded flexible units. In contrast to fibrous proteins, globular proteins are generally not regarded as a good candidate for fiber production due to their intrinsic structural defects. Thus, it is challenging to develop an efficient strategy for the construction of mechanically strong fibers using spherical proteins. Herein, we demonstrate the production of robust protein fibers from bovine serum albumin (BSA) using a microfluidic technique. Remarkably, the toughness of the fibers was up to 143 MJ m−3, and after post‐stretching treatment, their breaking strength increased to almost 300 MPa due to the induced long‐range ordered structure in the fibers. The performance is comparable to or even higher than that of many recombinant spider silks or regenerated silkworm fibers. Thus, this work opens a new way for making biological fibers with high performance.

First-principles prediction of a two-dimensional vanadium carbide (MXene) as the anode for lithium ion batteries.

Exploring two-dimensional anode materials that can utilize the storage capacity and diffusion mobility of Li ions is at the heart of lithium ion battery (LIB) research. Herein, we report the results of ab initio electronic structure calculations on the storage capacity and diffusion mobility affinities of Li ions adsorbed onto nondefective and defective MXene V2C monolayers. It is found that Li ions strongly chemisorb on the two sides of the V2C surface with a preferential adsorption site at the hollow center of the honeycomb structure. The binding profile and open-circuit voltage calculations reveal that the Li/V2C structure exhibits a specific capacity as high as 472 mA h g-1 at the Li2V2C stoichiometry, a value relatively high compared with those of the typical anode materials including graphite (372 mA h g-1). Furthermore, the diffusion barrier of a Li ion over the V2C surface is identified to be no more than 0.1 eV, which is a few times smaller than that of graphene and graphitic anodes. In addition, during the lithiation and delithiation processes, the change in the lateral lattice is quite small, only about a 2% increase at the full lithiation of Li2V2C, implying a good cycling performance. Importantly, these intriguing findings are very robust against the intrinsic structural and atomic defects including local point vacancies and biaxial compressive and tensile strains. More specifically, the presence of a monovanadium vacancy enhances the binding energy up to 3.1 eV per Li ion, which is about a 30% enhancement compared with the defect-free Li/V2C structure, and reduces the activation barrier by about 2 meV; meanwhile, these binding and diffusion mobility features can be improved even more when the lattice constant of the V2C monolayer is expanded. These results thus suggest that MXene V2C could be a promising anode material with high capacity and high rate capabilities for next generation high-performance LIBs.

Engineering of electronic and optical properties of monolayer gallium sulfide/selenide in presence of intrinsic atomic defects

In this paper, the electronic and optical properties of various point defects in gallium sulfide (GaS) and gallium selenide (GaSe) are studied. Various vacancy defects in each monolayer GaX (X = S, Se) include VX, VGa, 2VX, 2VGa, 1VGa1VX, 1VGa2VX, 2VGa1VX, 2VGa2VX. We compute the band structure, zero-bias transmission spectrum, and dielectric function for all considered structures. The calculations are carried out by the first-principles method. The calculation results indicate that the absence of S/Se atom in these semiconductors leads to the transition from an indirect band gap for the pristine materials to a direct band gap in their defective systems and the band gap energies change from 2.3 eV/2.11 eV to 1.33 eV/0.98 eV, respectively. Also, 2VX causes that the semiconductor band gap changes from indirect to direct. Furthermore, GaX monolayer is converted to a p-type semiconductor in the presence of VGa. Moreover, these findings represent that some of the point defects in this system lead to magnetic states which can be employed in spintronic devices. In addition, for the defective GaX monolayers with the direct band gap, the first peak of imaginary part of the dielectric function occurs around their band gap energy. The study of intrinsic structural defects in monolayer GaX provides new opportunities for optimizing the electronic and optical properties of these materials via defect engineering.

Composition dependent intrinsic defect structures in ASnO3 (A = Ca, Sr, Ba)

Perovskite stannates are promising semiconductors that are widely used in optoelectronic devices. Here, the composition dependent intrinsic point defects of stannate perovskites ASnO3 (A = Ca, Sr, Ba) are studied by first-principles calculations. The preferences of defects under stoichiometric and nonstoichiometric conditions are unsealed, meanwhile the charge states of each intrinsic defect varying with the change of electron Fermi energy are clarified. For stoichiometric BaSnO3 and SrSnO3, the Schottky defect complexes are predicted as the most stable defect structure, while the antisite defect complexes are the most stable one in CaSnO3. In nonstoichiometric ASnO3, excessive AO is beneficial to the formation of oxygen vacancies and A-Sn antisite defects in all ASnO3; while the Ca interstitial is another major defect existing in CaSnO3. In the case of SnO2 excess, the predominant defects are the Sn-A antisite defects and A vacancies. Since the functionality of these perovskite oxides is closely related to the types and concentrations of their point defects, the present results are expected to guide the future experiments to optimize the function of the perovskite oxides by tailoring the intrinsic defects through controlling the composition of AO and SnO2.

Structure defects promoted exciton dissociation and carrier separation for enhancing photocatalytic hydrogen evolution

Structure defect poor and rich graphitic carbon nitrides (g-C3N4) were successfully prepared to disclose the relationships between structure defects and the exciton/carrier behaviors. The partial loss of the heptazine units in the matrix was the origin of the intrinsic structure defects in the g-C3N4, evidenced by the X-ray photoelectron spectroscopic analysis. Both the fluorescence and ultrafast transient absorption analyses demonstrated that the presence of the intrinsic structure defects within g-C3N4 promoted the dissociation of excitons and the separation of photogenerated carriers, reflected by the accelerated relaxation of the excited electrons from conduction band to the trap states and the prolonged relaxation of trapped electrons to the valence band. Thus, the structure defect rich g-C3N4 performed better in photocatalytic hydrogen production than structure defect poor g-C3N4. This study not only disclosed the influences of the intrinsic structure defects on exciton/carrier behaviors, but also provide an alternative perspective to modify the semiconductors for photocatalytic applications.

Surface and interfacial study of atomic layer deposited Al2O3 on MoTe2 and WTe2

The atomic layer deposition (ALD) of high-k dielectrics could build an efficient barrier against moisture and O2 adsorption. Such a barrier is highly needed for MoTe2 and WTe2 transition metal dichalcogenides because of the poor structural stability and the fast oxidization in ambient air. In situ x-ray photoelectron spectroscopy and ex situ atomic force microscopy and scanning transmission electron microscopy were employed to report a comparative study between the growth of Al2O3 on MoTe2 and WTe2 by means of traditional thermal ALD and plasma-enhanced ALD (PEALD). Similar to what has been observed on other 2D materials such as MoS2 and Graphene, the thermal ALD results in an islanding growth of Al2O3 on MoTe2 due to the dearth of dangling bonds, whereas, a uniform coverage of Al2O3 on WTe2 is observed and likely contributed to the high concentration of intrinsic structural defects. The PEALD behavior is consistent between MoTe2 and WTe2 providing a conformal and linear growth rate (∼0.08 nm/cycle), which correlates with the creation of Te–O and metal-O nucleation sites. However, a thin layer of interfacial Mo or W oxides gradually forms, resulting from the plasma-induced damage in the topmost (1–2) layers. Attempts to enhance the Al2O3/MoTe2 interfacial quality by physically evaporating an Al2O3 seed layer are investigated as well. However, the evaporated Al2O3 process causes thermal damage on MoTe2, necessitating a more ‘gentle’ ALD technique for the surface passivation.

Effect of titanium in LiNbO3 on domain growth during e-beam writing

We report the features of domain growth during e-beam recording on non-polar Y-surfaces of LiNbO3 crystals doped with titanium at various concentrations (CTi). The nominally pure LiNbO3 of the congruent composition (CLN), LiNbO3 doped with 0.5 mol% TiO2 (CLN-0.5Ti), and the Ti:LiNbO3 planar waveguide formed as a result of high-temperature diffusion of titanium were comparatively studied. The titanium concentration at the surface of Ti: LiNbO3 was ∼7.6 at%, then CTi gradually decreased to 0.5–0.8 at% аt a depth of ∼4–5 μm. The combination of the etching technique with non-destructive methods of observation of the created domain structures of a planar type (low-voltage SEM and SHG microscopy) made it possible to detect the effect of titanium concentration on the features of the domain growth. It has revealed that the domain sizes and the average rate of their frontal growth (Vf) in CLN and CLN-0,5Ti are different. The observed differences are discussed in the framework of the current model of the intrinsic defect structure of LiNbO3. A peculiar three-dimensional structure of domain gratings in the Ti:LiNbO3 over the waveguide depth was found. The differences in the formation of the upper and lower parts of periodic gratings in Ti:LiNbO3 are explained by a significant increase in conductivity near the surface of the waveguide in comparison with deeper layers. The obtained results are useful to match the position and design of planar domain gratings in Ti:LiNbO3 optical waveguides on non-polar surfaces for better conversion efficiency of integrated nonlinear devices based on quasi-phase-matching.

Enhanced Optoelectronic and Thermoelectric Properties by Intrinsic Structural Defects in Monolayer HfS2

In the present work, we have studied the electronic, optical, and thermoelectric properties of a monolayer of pristine HfS2 and two types of vacancy and two types of dopant by using first-principle...

Composition‐dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La, Nd, Gd; B = Sn, Hf, Zr)

AbstractPoint defects are closely correlated with various properties of pyrochlore oxides and therefore play a key role on their engineering applications. Here, the native point defect complexes in RE2B2O7 (RE = La, Nd, Gd; B = Sn, Hf, Zr) under stoichiometric and nonstoichiometric compositions are studied by first‐principles calculations. The O Frenkel defect complex is predicted to be the predominant defect structure in stoichiometric zirconates and hafnates, whereas the cation antisite defect complex is the predominant one in stannates. In the case of BO2 excess, the formation of the B‐RE antisite defect together with the RE vacancy and the oxygen interstitial is energetically favorable, whereas the RE‐B antisite defect together with the oxygen vacancy and the RE interstitial is preferable under the RE2O3 excess environments. Additionally, the formation energies of the native defect complexes are greatly affected by the B‐site and/or RE‐site cations. The strategy on tailoring the intrinsic defect structures of these pyrochlore oxides is proposed. It is expected to guide the experiments on the defect‐related property optimization through stoichiometric and nonstoichiometric compositions, so as to meet the specific engineering requirements and promote their commercial applications.

Effects of microstructure and growth conditions on quantum emitters in gallium nitride

Single-photon emitters in gallium nitride (GaN) are gaining interest as attractive quantum systems due to the well-established techniques for growth and nanofabrication of the host material, as well as its remarkable chemical stability and optoelectronic properties. We investigate the nature of such single-photon emitters in GaN with a systematic analysis of various samples produced under different growth conditions. We explore the effect that intrinsic structural defects (dislocations and stacking faults), doping, and crystal orientation in GaN have on the formation of quantum emitters. We investigate the relationship between the position of the emitters—determined via spectroscopy and photoluminescence measurements—and the location of threading dislocations—characterized both via atomic force microscopy and cathodoluminescence. We find that quantum emitters do not correlate with stacking faults or dislocations; instead, they are more likely to originate from point defects or impurities whose density is modulated by the local extended defect density.

High power factor and mobility of single crystals of Bi2Se3 induced by Mo doping

Few binary compounds attract as much attention as Bi2Se3. In addition to its use as thermoelectric, it has become one of the prototype 3D topological insulators, and it has been demonstrated that doping in appropriate levels can induce magnetism and even superconductivity in bulk Bi2Se3. In this article, we continue the examination of doping with sixth group transition elements, namely, Mo. The solubility of Mo in the host matrix is very low; however, it induces remarkable changes in the material properties. For low doping levels, Mo changes the intrinsic defect structure of Bi2Se3 featuring the Se vacancies toward a structure characterized by embedded Bi2 double layers. Counterintuitively, Mo doping markedly enhances the mobility of free charge carriers in Bi2Se3 and drives its thermoelectric power factor to its maximum value over a broad temperature range. We provide evidence that the rather disordered structure must not be necessarily detrimental to charge transport in Bi2Se3.

Effect of intrinsic structural defects on mechanical properties of single layer MoS2

In this paper, the mechanical properties of single layer Molybdenum disulfide (SLMoS2) containing point defects, such as vacancies, or extra Sulfur (S) or Molybdenum (Mo) atoms are investigated by molecular dynamics (MD) simulations. It was found that the single S vacancy is most probable as the formation energy is the lowest among all point defects. Other antisite defects having a Mo atom replaced by two S atoms are the least possible ones. The structures of antisite defects are also unstable. The Young’s modulus of planner SLMoS2 with defect is found to be within the scatter margin of pristine MoS2 sheet (without defects). The effect of temperature on each type of defect has been studied and it is found that strength is more affected by vacancies than it is by phase transition.

Shallow and deep trap levels in X-ray irradiated β-Ga2O3: Mg

The results of the investigation of thermostimulated luminescence (TSL) and photoconductivity (PC) of the X-ray irradiated undoped and Mg2+ doped β-Ga2O3 single crystals are presented. Three low-temperature peaks at 116 K, 147 K and 165 K are observed on the TSL glow curves of undoped crystals. The high-temperature TSL peaks at 354 K and 385 K are dominant in Mg2+ doped crystals. The correlation between doping with Mg2+ ions and the local energy levels of the intrinsic structural defects of β-Ga2O3, which are responsible for the TSL peaks and PC, is established. The nature of TSL peaks and the appropriate photoconductivity excitation bands are discussed.

Work function, carrier type, and conductivity of nitrogen-doped single-walled carbon nanotube catalysts prepared by annealing via defluorination and efficient oxygen reduction reaction

Pyridinic and graphitic nitrogen atoms can be doped into carbon nanomaterials to create intrinsic structural defects and improve catalytic activity for the oxygen reduction reaction (ORR). An understanding of the relationship between the electronic properties and ORR activity of nitrogen-doped carbon nanomaterials could help design catalysts with better ORR performance. Here we report on the synthesis of nitrogen-doped single-walled carbon nanotubes (SWCNTs) by a combination of defluorination-assisted nanotube-substitution reaction and high-temperature annealing treatment using edge-free highly crystalline SWCNTs. The electronic properties of the prepared samples, such as the work function, carrier type, and conductivity were measured and correlated with their ORR activity in an acid electrolyte. Graphitic nitrogen-rich SWCNTs with n-type carrier, low work function, and high conductivity exhibited efficient ORR activity. The work function, carrier type, conductivity, and O2 dissociative adsorption sites are dependent on both the species/number of doping nitrogen atoms and the structural defects. The tuning of these two structural factors is necessary for achieving high ORR activity.

Varying growth stress to study how H2O affect chromia scaling in high pO2 gas at 900 °C

Thin and thick specimens were isothermally oxidized. The use of different thick specimen allowed the change of the intrinsic defect structure in thermally grown chromia scales independently from the test gas. Oxidation kinetics and micro-structure of scales formed were followed. Oxide cross-sections were analysed to determine oxide thickness and grain width. Chromia scales formed on the various substrates grew at different rates and oxide microstructures differed in grain shape and size. The chromia growth kinetics was found to be depending on the thickness of the specimens used, the gas H2O content, and the Ni alloy content.