Point Defect States Research Articles

Low-frequency band gap and defect state characteristics in a multi-stub phononic crystal plate with slit structure

In this paper, we investigated the low-frequency characteristics of band gaps and defect state in a two-dimensional phononic crystal structure consisting of multiple square stubs deposited on both sides of a thin slotted plate. Using the finite element method, we calculated the dispersion relationships and power transmission spectra of this structure, which agree well with each other. In contrast to a system of homogeneous plate, system of slotted plate show wide band gaps and an increased quantity of band gaps at lower frequencies. In addition, based on the finding that acoustic band gaps are very sensitive to the distance between the two adjacent square stubs in our earlier works, we investigated the point defect state in the multi-stub phononic crystal plate with slit structure. Through changing the distribution of stubs in specific scatterers, the defect phononic crystals are formed. We designed and fabricated the defect phononic crystal experimental platform to verify the existence of defect bands in the phononic crystal structure. Results show that defect bands can be induced by creating defects inside the original complete band gaps. These results will help in fabricating devices, such as acoustic filters whose band frequency can be modulated.

Ion‐beam synthesis and thermal behaviour of luminescent Zn2SiO4 nanoparticles in silica glasses and films

In the present study, the temperature‐dependent photoluminescence (PL) spectra of Zn2SiO4 nano‐crystalline phases formed in glassy and thin‐film SiO2 matrices by means of ion implantation and annealing are studied. Pure willemite ceramics were used as a reference. The green and yellow light emission observed is associated with point defects inside α‐ and β‐Zn2SiO4 nanoparticles. Different PL excitation mechanisms were distinguished for implanted silica. The electronic states of point defects are localised to dimensions much smaller than the nanocrystal size, so the PL band positions are similar to those of bulk material. Conversely, the vibrational states are extended and therefore subjected to the effects of confinement, surface defects and disorder of the host matrix. Hence, the PL intensity quenching (different laws for α‐phase and β‐phase) and line broadening shed light to the PL emission mechanism and reveal the influence of the amorphous silica matrix. The results obtained may be used to tailor the optical properties of the nanocomposite over a wide temperature range.

Localized defect states in MoS2monolayers: Electronic and optical properties

Defects usually play an important role in tuning and modifying various properties of semiconducting or insulating materials. Therefore we study the impact of point and line defects on the electronic structure and optical properties of MoS2 monolayers using density-functional methods. The different types of defects form electronic states that are spatially localized on the defect. The strongly localized nature is reflected in weak electronic interactions between individual point or line defect and a weak dependence of the defect formation energy on the defect concentration or line defect separation. In the electronic energy spectrum the defect states occur as deep levels in the band gap, as shallow levels very close to the band edges, as well as levels in-between the bulk states. Due to their strongly localized nature, all states of point defects are sharply peaked in energy. Periodic line defects form nearly dispersionless one-dimensional band structures and the related spectral features are also strongly peaked. The electronic structure of the monolayer system is quite robust and it is well preserved for point defect concentrations of up to 6%. The impact of point defects on the optical absorption for concentrations of 1% and below is found to be very small. For higher defect concentrations molybdenum vacancies were found to quench the overall absorption and sulfur defects lead to sharp absorption peaks below the absorption edge of the ideal monolayer. For line defects, we did not find a considerable impact on the absorption spectrum. These results support recent experiments on defective transition metal chalcogenides.

In-situ structural investigations of ferroelasticity in soft and hard rhombohedral and tetragonal PZT

Despite the technological importance of hard and soft PZT, Pb(Zr,Ti)O3, ceramics, the mechanisms of ferroelectric hardening and softening remain widely discussed in the literature. The hardening and softening phenomena have traditionally been investigated in relation with dielectric manifestations such as aging of the dielectric susceptibility and constriction of the polarization-electric field hysteresis loop. Here, we present a systematic investigation of the ferroelectric and ferroelastic properties of soft and hard PZT in both the tetragonal and rhombohedral phases. A particular focus has been devoted to ferroelastic domain switching by characterizing the macroscopic mechanical constitutive behavior and in-situ synchrotron X-ray diffraction during compression. It is demonstrated that variation of the ordering state of point defects in PZT ceramics affects the switching behavior of both ferroelectric and ferroelastic domains under mechanical or electrical fields. Softening of the mechanical and electrical properties of originally hard PZT ceramics was conferred by quenching the materials from above the Curie temperature. The present findings are discussed with respect to the current understanding of hardening-softening transitions in ferroelectric materials.

Point defect states in Sb-doped germanium

Defect states in n-type Sb-doped germanium were investigated by deep-level transient spectroscopy. Cobalt-60 gamma rays were used to generate isolated vacancies and interstitials which diffuse and react with impurities in the material to form four defect states (E37, E30, E22, and E21) in the upper half of the bandgap. Irradiations at 77 K and 300 K as well as isothermal anneals were performed to characterize the relationships between the four observable defects. E37 is assigned to the Sb donor-vacancy associate (E-center) and is the only vacancy containing defect giving an estimate of 2 × 1011 cm−3 Mrad−1 for the uncorrelated vacancy-interstitial pair introduction rate. The remaining three defect states are interstitial associates and transform among one another. Conversion ratios between E22, E21, and E30 indicate that E22 likely contains two interstitials.

Charge states of point defects in plutonium oxide: A first-principles study

The stability of charged defects in PuO2 are systematically investigated by quasi-annealing simulations based on local spin density approximation (LSDA) + U calculations. The formation energies of intrinsic (Frenkel and Schottky pairs) and extrinsic (single vacancy or interstitial) defects at different charged states are calculated and presented. According to our results, the noninteracting oxygen Frenkel pairs made of the association of doubly charged defects are the most favorable form of intrinsic defects, the formation energy of which is in good agreement with experimental values. The negative charged oxygen interstitial (IO2−) and positive charged oxygen vacancy (VO2+) defects have the lowest formation energies at oxygen-rich and oxygen-poor environments, respectively, and thus are the most commonly seen extrinsic defects. Both the existences of oxygen vacancy and oxygen interstitial induce new electronic states in the band gap of stoichiometric PuO2, showing doping effects.

Collective Diffusion of Gold Clusters and F-Centers at MgO(100) and CaO(100) Surfaces

Few-atom gold clusters get trapped at the F-centers of MgO(100) and CaO(100) surfaces quickly upon deposition due to much stronger binding at the defects compared to the O5c sites of the ideal terraces. Yet, our density functional theory (DFT) calculations reveal that their mobility is not fully suppressed since the Au dimers and trimers can diffuse at CaO(100) surface together with the F-centers they are bound to, along paths with activation energies not higher than 1.0 eV. The low energy paths are enabled by combined effects of high electron affinity of Au, the modest strength of the bonds within Au clusters, and a favorable topology of the point-defect electronic states along the paths. For other metals, the same diffusion mechanism is less effective than for gold.

Acoustic confinement and waveguiding in two-dimensional phononic crystals with material defect states

In this paper, we present the theoretical investigation of acoustic confinement and waveguiding in two-dimensional phononic crystals with material defect states. In contrast to the typical formation pattern of defect states, the proposed material defect states are created by replacing single cylinder in the core center or one row of cylinders in the perfect phononic crystal with different material cylinders. The dispersion relations and the power transmission spectra are studied by using the finite element method in combination with a supercell technique. With the introduction of material defects into perfect phononic crystals, defect states and bands appear, elastic waves of specific frequencies are localized in the point defects or along the line defects, respectively. The displacement fields of the eigenmodes are carried out to further intuitively illustrate the acoustic confinement and waveguiding in the material defect states phononic crystals. Furthermore, the effects of the material parameters of defects on the point defect states and waveguiding are further explored numerically. Numerical results show that, the location and number of defect states can be efficiently modulated by the material parameters of defects. The material defect states are more sensitive to the density parameters than the elastic constants when the elastic modulus of the defects is far higher than that of matrix.

Spectroscopic features of nonlinear AgGaSe2 crystals

When producing large AgGaSe2 single crystals of high optical quality, the shadow and light scattering techniques were found effective. Annealing in proper atmosphere allows one to affect the crystal composition and the state of submicron-sized inclusions and point defects. Stoichiometric crystals demonstrate mainly photoluminescence (PL) of free excitons. New PL bands, a 727/745nm doublet and a 950nm broad band, are associated with selenium vacancy, VSe, and cation antisite defect, GaAg, respectively. Using the thermoactivation spectroscopy, the traps of charge carriers were revealed in AgGaSe2: those with thermal activation energy ET>0.4eV were found responsible for photoinduced absorption on the 0.1cm−1 level.

Quantum computing with defects

Abstract

Catalytic nature of under- and hetero-coordinated atoms resolved using zone-selective photoelectron spectroscopy (ZPS)

We present an approach for distillation of the coordination-resolved electronic binding energy and bond relaxation dynamics, which has enabled clarification of the catalytic nature of under- and hetero-coordinated atoms. It is classified that Pt adatoms and CuPd alloy serve as acceptor-type catalysts because of the bond-order-deficiency induced quantum entrapment while Rh adatoms and AgPd alloy serve as donor-type catalysts because of the polarization of the conduction electrons of the adatoms by the densely, locally entrapped core electrons. This technique also enables the distillation of the polarized electronic states of graphite point defects and the entrapped states of the monolayer skin of graphite, which may serve, respectively, donor- and acceptor-type catalyst. The developed approach complements scanning tunneling microscopy/spectroscopy and photoelectron spectroscopy for the atomistic, local, and quantitative information of bond and electronic relaxation dynamics.

Localised point defect states in asymptotic models of discrete lattices

An asymptotic theory for localised defect states created by altering a single, or finite number, of point masses within a periodic lattice is developed. Asymptotic partial differential equations based upon a high frequency homogenisation methodology are found in one and two dimensions and the frequencies associated with the localised defect states are then compared with exact solutions. In two dimensions, attention is focussed upon a special class of separable lattices that degenerate into coupled one-dimensional systems and upon flat bands in the dispersion diagram. In this latter situation local asymptotic solutions occur which switch from elliptic to hyperbolic behaviour.

Influence of Point Defects on Band Gaps of Fe-Epoxy in Two-Dimensional Phononic Crystal

The band gap of a new two-dimensional phononic crystal was studied by the plane-wave expansion method. The two-dimensional phononic crystal is formed by square-shape array geometry of iron cylinders with square cross section inserted in an epoxy resin. The band gaps of different structures were calculated such as defect-free, single cavity crystal point defect states, crystal point defect states with (10) direction coupling, crystal point defect states with (10) direction next-nearest-neighbor coupling, and crystal point defect states with (11) direction next-nearest-neighbor coupling. Compared with that of defect-free, it is conclude that point defect is beneficial to the production of band gaps. The bandwidth of point defect is about 5 times larger than that of the defect-free crystal with the filling fraction F=0.4. In addition, the maximum number of band gap is in the crystal point defect states with (10) direction next-nearest-neighbor coupling. It will provide a theoretical reference for the manufacture of phononic crystal.

Vibration energy harvesting using a phononic crystal with point defect states

A vibration energy harvesting generator was studied in the present research using point-defect phononic crystal with piezoelectric material. By removing a rod from a perfect phononic crystal, a resonant cavity was formed. The elastic waves in the range of gap frequencies were all forbidden in any direction, while the waves with resonant frequency were localized and enhanced in the resonant cavity. The collected vibration energy was converted into electric energy by putting a polyvinylidene fluoride film in the middle of the defect. This structure can be used to simultaneously realize both vibration damping and broad-distributed vibration energy harvesting.

Finite Element Method for Analysis of Flexural Vibration Propagating in Ternary Phononic Crystal Thin Plates

Propagation of flexural vibration in a ternary phononic crystal thin plate with a point defect are explored using finite element method. The thin concrete plate is composed of steel cylinders hemmed around by rubber with a square lattice. Absolute band gaps, point defect bands and transmission response curves with low frequency are investigated. Comparing the results of finite element method with that of improved plane wave expansion method, precise identifications are obtained to identify the point defect states. The results show that the finite element method is suitable for the exploring of flexural vibration propagating in ternary phononic crystal thin plates.

Point Defect States of Phononic Crystal Thin Plates – An Application of Finite Element Method

In this paper, propagation of flexural vibration in phononic crystal thin plates with a point defect are explored using finite element method. The plate is composed of an array of circular crystalline Al2O3 cylinders embedded periodically in the epoxy matrix with a square lattice. The point defect is introduced by changing one of the cylinders’ radii. Comparing the results of finite element method with that of improved plane wave expansion method, complete and accurate band structures and transmission response curves are obtained using the former method to identify the point defect eigenmodes and band gaps. The results show that the finite element method is efficient and suitable for the exploring of point defect states of phononic crystal thin plates.

Point defect states of a hollow cylinder in two-dimensional phononic crystal

Point defect states in two-dimensional phononic crystal of a hollow mercury cylinder in a water host are studied. An improved plane expansion method combined with the supercell technique is used to calculate the band gaps and the pressure distribution at the defect position. The sonic pressure of defect modes shows that the waves are localized at or near the defect. As the filing fraction increases, more defect modes appear in the band gaps.

Analytic theory of defects in periodically structured elastic plates

We consider the problem of localized flexural waves in thin plates that have periodic structure, consisting of a two-dimensional array of pins or point masses. Changing the properties of the structure at a single point results in a localized mode within the band-gap that is confined to the vicinity of the defect, while changing the properties along an entire line of points results in a waveguide mode. We develop here an analytic theory of these modes and provide semi-analytic expressions for the eigenfrequencies and fields of the point defect states, as well as the dispersion curves of the defect waveguide modes. The theory is based on a derivation of Green's function for the structure, which we present here for the first time. We also consider defects in finite arrays of point masses, and demonstrate the connection between the finite and infinite systems.

Point defect states of exchange spin waves in all-ferromagnetic two-dimensional magnonic crystals

Using the plane-wave expansion method under supercell approximation, band structures of exchange spin waves propagating in all-ferromagnetic two-dimensional magnonic crystals with point defects are calculated. The results indicate that the point defects in these structures can create localized states inside the bandgap. The characteristics and the magnetization distributions of these localized states are studied. The results show that the group velocities of the localized states almostequal zero in the whole first Brillouin zone, and numbers of localized states and their frequency-positions in the bandgap are related to the size of the point defect. Magnonic crystalswith such defect states can be used as fabricating materials of narrow bandpass spin-wave filters.

Effect of ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te\!:\!In}$ Crystals Annealing on Their High-Temperature Electrical Properties

We studied the electrical properties of Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Te:In (CZT) single crystals with [In]=3*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> at/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at its high-temperature point-defect (PD) equilibrium state under a Cd overpressure (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cd</sub> ). We detailed the influence of thermal treatment and the deviation of stoichiometry on the electron concentration, observing unexpectedly high conductivity and an increase in free-electron density ( ~ 1.5-2 orders of magnitude) when annealing the sample at 770 K under a Cd vapor pressure (0.01 atm.). Prolonged exposure of the samples under these conditions lowered the electron density by two approximately orders-of-magnitude until it approached the intrinsic value. The electron mobility after such treatment increased to CZT's maximal values at ~ 460 K (650-700 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V*s)). Therefore, such annealing can be effective in assuring high-resistive CZT detectors after crystal growth, or by special treatment, thereby eliminating the inclusions. We analyzed these data in the framework of Kröger's theory of quasi-chemical reactions, and compared the findings to those obtained for undoped CdTe.