Vacancy Emission Research Articles

Acceptor related photoluminescence and field emission of ZnO:P nanostructures

ZnO:P nanobelts were self-assembly synthesized by thermal evaporation of Zn power and P2O5 mixture. The temperature dependence photoluminescence of ZnO:P nanostructures was studied from 81 to 291 K. As the temperature increased from 81 to 111 K, the PL intensity of DAP emission was obviously enhanced. The abnormal PL intensities were ascribed to the acceptor vibration with local phonon and lattice phonon assistant. The PL of zinc vacancy and its replica were well resolved due to the strenuous vibration of Zn vacancy. The replica of zinc vacancy emission increased while the visible emission gradually decreased with the temperature increase. It suggested that there were intensive deep acceptor vibration. The field emission properties of the ZnO:P nanostructures have been investigated according to the acceptor-related PL spectra. The influence of space charge effect on the field emission behaviors was also discussed.

Tuning of defects in ZnO nanorod arrays used in bulk heterojunction solar cells.

With particular focus on bulk heterojunction solar cells incorporating ZnO nanorods, we study how different annealing environments (air or Zn environment) and temperatures impact on the photoluminescence response. Our work gives new insight into the complex defect landscape in ZnO, and it also shows how the different defect types can be manipulated. We have determined the emission wavelengths for the two main defects which make up the visible band, the oxygen vacancy emission wavelength at approximately 530 nm and the zinc vacancy emission wavelength at approximately 630 nm. The precise nature of the defect landscape in the bulk of the nanorods is found to be unimportant to photovoltaic cell performance although the surface structure is more critical. Annealing of the nanorods is optimum at 300°C as this is a sufficiently high temperature to decompose Zn(OH)2 formed at the surface of the nanorods during electrodeposition and sufficiently low to prevent ITO degradation.

Dislocation vs. production bias revisited with account of radiation-induced emission bias. I. Void swelling under electron and light ion irradiation

Early experimental data on void swelling in electron-irradiated materials disagree with the dislocation bias models based on the dislocation-point defect elastic interactions. Later, this became one of the factors that prompted the development of models based on production bias (PBM) as the main driver for swelling, which assumed that the dislocation bias was much lower than that predicted by theoretical analyses of dislocation bias. However, the PBM in its present form fails to account for important and common observations, namely, the indefinite void growth often observed under cascade irradiation and the swelling saturation observed under high-dose irradiation and in void lattices. In this paper, we show that these contradictions can be naturally resolved in the framework of the rate theory that accounts for the radiation-induced vacancy emission from extended defects, such as voids, dislocations and grain boundaries. This modification introduces a new bias type in the theory, namely, the emission bias. This modified rate theory agrees well with the experimental data and demonstrates that the original dislocation bias should be used in rate theory models along with the emission bias in different irradiation environments. The modified theory predictions include, but are not limited to, the radiation-induced annealing of voids, swelling saturation under high-dose irradiation, generally, and in void lattices, in particular.

Effect of oxygen vacancy on magnetism of ZnO:Co single-crystalline nanorods

Co-doped ZnO single-crystalline nanorods were prepared by the modified microemulsion route. The crystalline structure, morphology, oxygen vacancy emission, and hysteresis loop at low temperature and room temperature of as-prepared materials were characterized by XRD, TEM, PL spectra, and magnetic measurement respectively. The nanorods are 60–90nm in diameter and about 2μm in length. X-ray diffraction data, TEM images and selected area electron diffraction patterns confirm that the materials synthesized in optimal conditions are ZnO:Co single crystalline solid solution without any impurities related to Co. Magnetic measurements show that different surfactants as template in synthesis process result in ferromagnetism and paramagnetism in Zn0.95Co0.05O nanorods. The PL spectra show that the ferromagnetic samples exhibit strong oxygen vacancy emission whereas in the paramagnetic samples the oxygen vacancy emission is absent, indicating that the defects may stabilize ferromagnetic order in diluted magnetic semiconductors, resulting in high-temperature ferromagnetism.

Guided-wave-coupled nitrogen vacancies in nanodiamond-doped photonic-crystal fibers

Zero-phonon-line (ZPL) emission of nitrogen vacancies (NVs) is coupled to the guided modes of solid- and hollow-core nanodiamond-doped photonic-crystal fibers (PCFs). Both types of PCFs are tailored toward enhancing ZPL emission coupling to the fiber modes. In solid-core PCFs, this involves enhancing the evanescent field of the waveguide modes supported by an ultrasmall fiber core. In hollow-core PCFs, the NV emission spectrum is matched with the transmission band of the fiber, controlled by the photonic bands of the fiber cladding.

Synthesis of Luminescent Silica Crystals via a Sonochemical Reduction Route

Reduction of silica (SiO2) in the bulk form is quite challenging because the bulk silica in both the amorphous and crystalline states is a highly stoichiometric material. In this work, a solution-based synthesis route is developed to produce off-stoichiometric amorphous silica (SiOx with x < 2) by using sonochemical reduction of silica-based materials in the presence of sodium. The resulting amorphous SiOx materials are found to crystallize after annealing at temperatures higher than ∼800 °C under Ar or vacuum environment, resulting in off-stoichiometric silica crystals. The main crystal phase is cristobalite, and a small amount of tridymite was also included. The thus obtained silica crystals exhibit a broad photoluminescence (PL) emission peaking at ∼500 nm, characterized by the PL decay constant on the time scale of microseconds. The spin forbidden emission of oxygen vacancies in a silica crystal lattice is likely to be responsible for the observed broad visible PL emission. The present sonochemical re...

Controllable synthesis of corrugated CdS nanoribbons of high quality by vapor–liquid–solid method

High-quality corrugated CdS nanoribbons were synthesized through thermal evaporation of CdS powder (1.44 g). The length of periodicity could be tuned simply by varying the deposition temperature. Also, when the CdS source was reduced to 0.144 g, the product was slick nanobelts. The hexagonal wurtzite phase and single crystallinity of products were verified by XRD and SAED, respectively. The mechanism for the growth of nanoribbons has been described. In photoluminescence studies, a sharp peak due to near-band-edge emission and a broad band due to surface-trapped states or Cd vacancy emission were observed. CdS nanoribbons with corrugated structures have not been reported before and their presence makes the investigation of CdS nanostructures more interesting and intriguing.

Stability and Nonlinear Optical Properties of ZnO Nanoparticles

Photoluminescence (PL) of ZnO nanoparticles of different surface states and sizes grown by several methods has been measured. The origin of luminescence and dependence of the luminescence spectrum shape and intensity on 325 nm excitation laser power are studied. Strong ultraviolet emission at 3.26 eV, weak violet emission around 3.12 eV and weak green emission at 2.40 eV have been observed in 16 nm nanoparticles capped by octylamine grown by non-hydrolytic method. The nanoparticles are stable under high power laser radiation and their PL intensity increases nonlinearly with an increasing laser power. As the nanoparticle size decreases to 12 nm, high power laser produces nonradiative centers which may quench the luminescence in a degree. Nanoparticles of 8 nm capped by PVP and uncapped nanoparticles of 14 nm are unstable and their luminescence depends on the excitation laser power. High power laser can quench O vacancy emission and enhance ultraviolet emission in PVP capped nanoparticles while vacancy emission can not be quenched in uncapped nanoparticles.

Radiation-induced formation, annealing and ordering of voids in crystals: Theory and experiment

Void ordering has been observed in very different radiation environments ranging from metals to ionic crystals bombarded with energetic particles. The void ordering is often accompanied by a saturation of the void swelling with increasing irradiation dose, which makes an understanding of the underlying mechanisms to be both of scientific significance and of practical importance for nuclear engineering. We show that both phenomena can be explained by the original mechanism based on the anisotropic energy transfer provided by self-focusing discrete breathers or quodons (energetic, mobile, highly localized lattice solitons that propagate great distances along close-packed crystal directions). The interaction of quodons with voids can result in radiation-induced “annealing” of selected voids, which results in the void ordering under special irradiation conditions. We observe experimentally radiation-induced void annealing by lowering the irradiation temperature of nickel and copper samples pre-irradiated to produce voids or gas bubbles. The bulk recombination of Frenkel pairs increases with decreasing temperature resulting in suppression of the production of freely migrating vacancies (the driving force of the void growth). On the other hand, the rate of radiation-induced vacancy emission from voids due to the void interaction with quodons remains essentially unchanged, which results in void dissolution. The experimental data on the void shrinkage and void lattice formation obtained for different metals and irradiating particles are explained by the present model assuming the quodon propagation length to be in the micron range, which is consistent with independent data on the irradiation-induced diffusion of interstitial ions in austenitic stainless steel.

Investigations on cobalt doped GaN for spintronic applications

(Ga1−xCox)N was synthesized at 1223K using pure gallium and cobalt metal as precursors in a reactor specially designed for this purpose with ammonia as the nitrogen source. The structural confirmation of wurtzite phase of GaN is done using powder X-ray diffractometer. Photoluminescence shows red shift in band edge emission as the cobalt concentration increases from 1 to 5 atomic percentage together with nitrogen related vacancy emission at 3.24eV and cobalt related emission at 3.13eV. The magnetic properties were studied at 10K using superconducting quantum interface device (SQUID). The obtained maximum magnetic moments for 1%, 3% and 5% cobalt doped GaN are 1.63, 0.21 and 0.14μB, respectively. The decrease in magnetic moment with increase in dopant concentration may be due to the formation of secondary phases or cobalt cluster, which contributes to anti-ferromagnetism. From temperature dependent magnetization measurements it is observed that there is no drastic change in the magnetic moment upto room temperature for 1% cobalt doped GaN. Theoretical calculations based on the density functional theory within Tight Binding Linear Muffin-tin Orbital (TBLMTO) method has been carried out to study the magnetic properties of cobalt doped GaN. The obtained magnetic moment values per Co atom are 2.56μB for 1.56% and 2.77% and 1.50μB for 4.20%. This is found to be higher when compared with experimental values and this difference is explained based on structural defects and percentage of dopant used during synthesis. The half metallic behavior is observed for 1.56% and 2.77% Co dopants, but for higher percentage of dopant the system becomes metallic. Hence, it is concluded that only lower percentage of cobalt doped GaN is suitable for spintronic applications.

Porous fission fragment tracks in fluorapatite

Fission tracks caused by the spontaneous fission of $^{238}\text{U}$ in minerals, as revealed by chemical etching, are extensively used to determine the age and thermal history of Earth's crust. Details of the structure and annealing of tracks at the atomic scale have remained elusive, as the original track is destroyed during chemical etching. By combining transmission electron microscopy with in situ heating, we demonstrate that fission tracks in fluorapatite are actually porous tubes, instead of having an amorphous core, as generally assumed. Direct observation shows thermally induced track fragmentation in fluoapatite, in clear contrast to the amorphous tracks in zircon, which gradually ``fade'' without fragmentation. Rayleigh instability and the thermal emission of vacancies control the annealing of porous fission tracks in fluorapatite.

Fabrication of 3D rotor-like ZnO nanostructure from 1D ZnO nanorods and their morphology dependent photoluminescence property

A facile and eco-friendly sonochemical route to fabricate well-defined dentritic (rotor-like) ZnO nanostructures from 1D ZnO nanorods without alloying elements, templates and surfactants has been reported. Phase and structural analysis has been carried out by X-ray diffraction (XRD) and Fourier Transform Infra-Red (FTIR) spectroscopy, showed the formation of hexagonal wurtzite structure of ZnO. Scanning electron microscopic (SEM) study showed the formation of rotor-like ZnO nanostructure having a central core which is surrounded by side branches nanocones. Transmission electron microscopic (TEM) study showed that these nanocones grow along [0001] direction on the six {01–10} planes of central core ZnO nanorods. A plausible formation mechanism of rotor-like ZnO nanostructures was studied by SEM which indicates that the size and morphology of side branches can be controlled by adjusting the concentration of OH − ions and time duration of growth. The photoluminescence (PL) spectrum of the synthesized rotor-like ZnO nanostructures exhibited a weak ultraviolet emission at 400 nm and a strong green emission at 532 nm recorded at room temperature. The influence of morphology on the origin of green emission was discussed in detail. The results suggested a positive relationship among polar plane, oxygen vacancy and green emission.

Photoluminescence and X-Ray Photoelectron Spectroscopy of p-Type Phosphorus-Doped ZnO Films Prepared by MOCVD

Reproducible p-type phosphorus-doped ZnO (p-ZnO:P) films are prepared on semi-insulating InP substrates by metal-organic chemical vapour deposition technology. The electrical properties of these films show a hole concentration of 9.02 × 10 17 cm−3, a mobility of 1.05 cm2/Vs, and a resistivity of 6.6 Ω · cm. Obvious acceptor-bound-exciton-related emission and P-induced zinc vacancy (VZn) emission are observed by low-temperature photoluminescence spectra of the films, and the acceptor binding energy is estimated to be about 125 meV. The local chemical bonding environments of the phosphorus atoms in the ZnO are also identified by x-ray photoelectron spectra. Our results show direct experimental evidence that PZn–2VZn shallow acceptor complex most likely contributes to the p-type conductivity of ZnO:P films.

Breather mechanism of the void ordering in crystals under irradiation

The void ordering has been observed in very different radiation environments ranging from metals to ionic crystals. In the present paper the ordering phenomenon is considered as a consequence of the energy transfer along the close packed directions provided by self-focusing discrete breathers. The self-focusing breathers are energetic, mobile and highly localized lattice excitations that propagate great distances in atomic-chain directions in crystals. This points to the possibility of atoms being ejected from the void surface by the breather-induced mechanism, which is similar to the focuson-induced mechanism of vacancy emission from voids proposed in our previous paper. The main difference between focusons and breathers is that the latter are stable against thermal motion. There is evidence that breathers can occur in various crystals, with path lengths ranging from 104 to 107 unit cells. Since the breather propagating range can be larger than the void spacing, the voids can shield each other from breather fluxes along the close packed directions, which provides a driving force for the void ordering. Namely, the vacancy emission rate for “locally ordered” voids (which have more immediate neighbors along the close packed directions) is smaller than that for the “interstitial” ones, and so they have some advantage in growth. If the void number density is sufficiently high, the competition between them makes the “interstitial” voids shrink away resulting in the void lattice formation. The void ordering is intrinsically connected with a saturation of the void swelling, which is shown to be another important consequence of the breather-induced vacancy emission from voids.

Room-temperature dislocation climb in metallic interfaces

Using atomistic simulations, we show that dislocations efficiently climb in metallic interfaces, such as Cu–Nb, through absorption and emission of vacancies and a counter diffusion of Cu atoms in the interfacial plane. The efficiency of dislocation climb is ascribed to the high vacancy concentration of 0.05 in the interfacial plane, the low formation energy of 0.12 eV with respect to removal or insertion of Cu atoms, and the low kinetic barrier of 0.10 eV for vacancy migration. Dislocation climb facilitates reactions of interfacial dislocations and enables interfaces to be in the equilibrium state with respect to concentrations of point defects.

Effect of annealing atmosphere on the photoluminescence of ZnO nanospheres

ZnO nanospheres were synthesized by a wet-chemical method. X-ray diffraction and field-emission scanning electron microscopy confirmed the formation of wurtzite-structured ZnO with regular sphere shape. Two Raman modes located at 333 cm −1 and 437 cm −1 with two additional Raman humps centered at 577 cm −1 and 1077 cm −1 were observed. Photoluminescence spectra showed ultraviolet, green, orange and red emissions, which changed significantly after the samples were annealed in air, oxygen, argon and forming gas four different ambiences. All the evidence indicates that surface states are responsible to orange and red emissions in addition to excitonic recombination (3.18 eV) and oxygen vacancy (2.25 eV) emission.

Time scale for point-defect equilibration in nanostructures

Molecular dynamics simulations of high-temperature annealing are performed on nanostructured materials enabling direct observation of vacancy emission from planar defects (i.e., grain boundaries and free surfaces) to populate the initially vacancy-free grain interiors on a subnanosecond time scale. We demonstrate a universal time-length scale correlation that governs these re-equilibration processes, suggesting that nanostructures are particularly stable against perturbations in their point-defect concentrations, caused for example by particle irradiation or temperature fluctuations.

Non-linear dynamics of microstructure evolution and hyper void-lattice formation

Irradiation damage accumulation in metals is studied via a dynamical description of the evolution of a system of interacting crystal defects, focusing on the complex behavior caused by system instability and symmetry-breaking. The case of a supercritical void ensemble in a temperature range where void growth is significantly affected by vacancy emission is specifically considered. Conditions of instability are found in the growth dynamics of the void system, the resulting bifurcation of which causes the shrinkage of some voids and the growth of others, resulting in coarsening of the ensemble. The presence of a small amount of one-dimensionally migrating self-interstitials with mean-free path comparable to the average distance between voids can bias the void coarsening process, such that the non-aligned voids have a much larger probability to shrink than the aligned ones. The post-bifurcation evolution leaves voids aligned along the crystallographic directions to form an imperfect lattice with empty lattice sites eventually filled by preferred nucleation. For this process to occur the irradiation temperatures must be higher than 0.4 of the melting temperature. The typically low number densities of voids at these temperatures necessarily entail a void lattice parameter much larger than when vacancy emission is negligible. The implication of the formation of the hyper void-lattice, an appellation adopted from earlier studies, on properties of one-dimensionally migrating self-interstitials is also discussed.

Atomistic simulations of diffusional creep in a nanocrystalline body-centered cubic material

Molecular dynamics (MD) simulations are used to study diffusion-accommodated creep deformation in nanocrystalline molybdenum, a body-centered cubic metal. In our simulations, the microstructures are subjected to constant-stress loading at levels below the dislocation nucleation threshold and at high temperatures (i.e., T > 0.75 T melt), thereby ensuring that the overall deformation is indeed attributable to atomic self-diffusion. The initial microstructures were designed to consist of hexagonally shaped columnar grains bounded by high-energy asymmetric tilt grain boundaries (GBs). Remarkably the creep rates, which exhibit a double-exponential dependence on temperature and a double power-law dependence on grain size, indicate that both GB diffusion in the form of Coble creep and lattice diffusion in the form of Nabarro–Herring creep contribute to the overall deformation. For the first time in an MD simulation, we observe the formation and emission of vacancies from high-angle GBs into the grain interiors, thus enabling bulk diffusion.

Introducing dislocation climb by bulk diffusion in discrete dislocation dynamics

We report a method to incorporate dislocation climb controlled by bulk diffusion in a three-dimensional discrete dislocation dynamics (DDD) simulation for fcc metals. In this model we couple the vacancy diffusion theory to the DDD in order to obtain the climb rate of the dislocation segments. The capability of the model to reproduce the motion of climbing dislocations is examined by calculating several test-cases of pure climb-related phenomena and comparing the results with existing analytical predictions and experimental observations. As test-cases, the DDD is used to study the activation of Bardeen–Herring sources upon the application of an external stress or under vacancy supersaturation. Loop shrinkage and expansion due to vacancy emission or absorption is shown to be well described by our model. In particular, the model naturally describes the coarsening of a population of loops having different sizes.