Tin Vacancies Research Articles

Chemical Bonding and Polarons in Sn2P2S(Se)6 Ferroelectrics

For ferroelectric-semiconductor Sn2P2Se6 the low temperature dielectric relaxation was found to be similar to that observed in Sn2P2S6 analog. This relaxation behavior was explained as an evidence of small polaron dynamics in the lattices with different covalence of bonds between atoms at presence of tin and sulfur (selenium) vacancies. The calculations in LDA approach of the electron band structure and the energy levels of intrinsic defects together with analysis of the Raman spectra and photoluminescence temperature behavior give evidences of polaron dynamics joining with donor-acceptor compensation processes.

Light emission from tin-dioxide crystals

The luminescence spectra of SnO2 crystals are studied in the temperature range from 5 to 40 K. Broad luminescence bands in the visible spectral region are attributed to transitions from the donor levels of oxygen vacancies and interstitial tin atoms to the acceptor levels of another type of intrinsic defects that are apparently formed from tin vacancies and interstitial oxygen atoms. It is found that emission related to one of the exciton-defect complexes with the minimum binding energy is enhanced, as the crystal is heated from 5 to 30 K. The distinguishing feature of this complex is its anomalously weak coupling with crystal-lattice vibrations.

Electron spin resonance parameters of cation vacancies in tin dioxide doped with fluorine and hydrogen

The spin Hamiltonian parameters of intrinsic defects in tin dioxide (SnO2) doped with fluorine or hydrogen are examined through the first-principles electronic structure calculations based on density functional theory (DFT). The electron paramagnetic resonance signals with g-tensor value in the range of 1.89–1.94 were found for tin vacancy (VSn) and its complex with oxygen vacancy (VSn-VO) associated with a donor like interstitial hydrogen or fluorine, substituting oxygen. The calculated parameters are consistent with experimental observations, which indicate that Sn vacancies may be present in SnO2 at essentially higher concentration than it is predicted by DFT based on the formation energies calculations. Within the second coordination shell of the tin vacancy, the VO may stabilize in the singly ionized charge state, which is otherwise considered to be unstable for isolated oxygen vacancy in the bulk of SnO2.

Stability of Pristine and Defective SnTe Surfaces from First Principles

T(h)in telluride films: Density functional theory reveals that the relative stability of topologically insulating tin telluride (SnTe) surfaces may strongly depend on the chemical environment. Hence, different crystallite morphologies are expected. Tin vacancies are predicted to be abundant, not only in the bulk material but also below the (001) surface. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Density functional theory description of origin of ferromagnetism in Cu doped SnO2

The defect formation and ferromagnetic stability in Cu doped SnO2 are studied using the first-principle density functional method within the local density approximation (LDA) and LDA+U schemes. Firstly, we investigate formation energies of oxygen vacancy (VO), tin vacancy (VSn), oxygen interstitial (Oi), and tin interstitial (Sni), Cu replacing Sn (CuSn) and Cu replacing O (CuO). The formation energy calculation by LDA and LDA+U shows that CuSn formation is more beneficial than CuO. Secondly, ferromagnetic (FM) and antiferromagnetic (AFM) couplings in Cu doped SnO2 are studied. The result by LDA and LDA+U shows that FM state in energy is lower than AFM for all configurations, which indicates that Cu doped SnO2 possesses FM stability. The FM stability can be explained by the interaction of Cu 3d level. Thirdly, impact of VO and VSn on magnetic moment is also analyzed. It is found that magnetic moment can be mediated by the defects.

Room temperature ferromagnetism in Sm-doped SnO2 PLD film

The effects of rare-earth-element Sm doping on the room-temperature magnetic properties of the pure SnO2 thin films fabricated by pulsed laser deposition are investigated. Only the Sm-doped SnO2 film at 600°C exhibits ferromagnetism at room temperature. XRD and XPS measurements reveal that no magnetic impurities or second phases are present in the film. As the SnO2 thin films were prepared via laser ablation and oxygen-deficient atmosphere, a high density of oxygen vacancies and tin vacancies may be expected. Ab inito calculations on SnO2 (200) plane with LDA+USIC method indicates that the Sm dopant could have localized magnetic moment, and the introduction of nearest-neighbored oxygen vacancy to Sm dopant in the Sm-doped SnO2 (200) plane cannot only increase the magnetic moment of the system, but makes FM coupling energetically more favorable for the system. The studies demonstrate that the observed ferromagnetism in our Sm-doped SnO2 film should originate from the coexistence of the substitution of Sn4+ ions by Sm3+ ions and nearest-neighbored oxygen vacancy.

First-principles study of the magnetic properties of Zn-doped SnO2

First-principle calculations were used to study the magnetic properties of Zn-doped SnO2. We first investigated the energy of formation for oxygen vacancies (VO), tin vacancies (VSn), interstitial oxygen (Oi), interstitial tin (Sni), Zn substitution of Sn (ZnSn), and Zn substitution of O (ZnO). We then studied ferromagnetic (FM) and antiferromagnetic (AFM) coupling in Zn-doped SnO2. The results show that the FM state has lower energy than the AFM state for all configurations, indicating that Zn-doped SnO2 has FM stability. This stability can be explained by interaction of the O 2p level. We also analyzed the impact of VO and VSn on FM coupling. VSn can enhance FM coupling, but VO is unfavorable for FM coupling.

The synthesis and afterglow luminescence properties of a novel red afterglow phosphor: SnO2:Sm3+,Zr4+

The novel SnO2:Sm3+,Zr4+ afterglow phosphor is synthesized by solid state reaction. The SnO2:Sm3+ doesn’t show afterglow. But, after co-doping Zr4+ in SnO2:Sm3+, the red afterglow luminescence of Sm3+ is firstly observed and the important role of Zr4+ on the afterglow is investigated. The photoluminescence shows that the doping of Zr4+ increases the emission lines of Sm3+ and decreases the emission band due to trapped hole center. The thermoluminescence reveals that the co-doping of Zr4+ induces the increase of shallow traps corresponding to the thermoluminescence peak at 48 °C. The depth of the traps is calculated to be 0.61 eV. The positron annihilation indicates that the traps are due to negatively charged tin vacancies and they play the important role on the afterglow of Sm3+ as hole traps.

Origin of ferromagnetism in Zn-doped SnO2 from first-principles study

Density-functional calculations are carried out to study the magnetic properties and electronic structures of Zinc-doped SnO2 systems with and without oxygen (tin) vacancies. The magnetic moment increases almost linearly with the increase of impurity concentration (from 4% to 25%) in the cases without oxygen and tin vacancies. The induced magnetic moments mainly come from the first shell oxygen atoms surrounding the doped Zn atom. The magnetic moments become smaller (larger) when oxygen (tin) vacancy is introduced, which is not as seen in Co and Ni doped SnO2 systems. Oxygen (tin) vacancy defects are easily formed at the nearest distance from the doped Zn atom and the largest possible magnetic moments will appear at the concentration of Zn atom between 6.25% and 12.5%. The calculated results also show that Zn–Zn are ferromagnetically coupled, and how the magnetic moments vary with Zn–Zn distance when only two Zn atoms were doped into a SnO2 supercell. However, Zn–Zn show anti-ferromagnetic coupling when three Zn atoms are doped in the same SnO2 supercell. The results may be helpful for further study on TMs doped SnO2 systems.

Physicochemical characterization of point defects in fluorine doped tin oxide films

The physical and chemical properties of spray deposited FTO films are studied using FESEM, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), electrical and optical measurements. The results of XRD measurements showed that the films are polycrystalline (grain size 20–50 nm) with Rutile structure and mixed preferred orientation along the (200) and (110) planes. An angular shift of the XRD peaks after F-doping is observed and interpreted as being due to the formation of substitutional fluorine defects (FO) in presence of high concentration of oxygen vacancies (VO) that are electrically neutral. The electrical neutrality of oxygen vacancies is supported by the observation that the electron concentration n is two orders of magnitude lower than the VO concentration calculated from chemical analyses using XPS measurements. It is shown that an agreement between XPS, XRD, and Hall effect results is possible provided that the degree of deviation from stoichiometry is calculated with the assumption that the major part of the bulk carbon content is involved in O-C bonds. High temperature thermal annealing is found to cause an increase in the FO concentration and a decrease in both n and VO concentrations with the increase of the annealing temperature. These results could be interpreted in terms of a high temperature chemical exchange reaction between the SnO2 matrix and a precipitated fluoride phase. In this reaction, fluorine is released to the matrix and Sn is trapped by the fluoride phase, thus creating substitutional fluorine FO and tin vacancy VSn defects. The enthalpy of this reaction is determined to be approximately 2.4 eV while the energy of formation of a VSn through the migration of SnSn host atom to the fluoride phase is approximately 0.45 eV.

Ferroelectricity and Polarons in Sn2P2S6 Crystals

For Sn2P2S6 ferroelectric-semiconductor the relaxation processes at low temperatures in the ferroelectric phase were investigated by low frequency dielectric and Raman spectroscopies. Observed anomalies in dielectric losses and their change under illumination together with lattice anharmonicity appearance in Raman spectral bands shape are compared with previous data on photo- and thermoluminescence. The experimental data are analyzed with consideration of local three-well potential in ground state and explained by creation and annihilation of small hole and electron polarons with participation of tin vacancies acceptor and sulfur vacancies donor in gap energy states.

Ferromagnetism induced by intrinsic defects and nitrogen substitution in SnO2 nanotube

The possibilities of magnetism induced by intrinsic defects and nitrogen substitution in (5,5) single-wall SnO2 nanotube are investigated by ab initio calculations. The calculated results indicate that a stoichiometric SnO2 nanotube is nonmagnetic. The tin (Sn) vacancy can induce the magnetic moments rather than oxygen vacancy, which is originated from the polarization of O 2p electrons. A couple of tin vacancies can lead to the ferromagnetic coupling. A nitrogen substitution for oxygen also produces magnetic moments. When substituting two nitrogen atoms, the characteristics of exchange coupling depend upon the distance of two nitrogen atoms. The longer distance of two nitrogen atoms prefers the ferromagnetic coupling, whereas the short distance leads to the antiferromagnetic coupling.

Ferroelectric and Semiconducting Properties of Sn2P2S6 Crystals with Intrinsic Vacancies

Thermoelectric properties of Sn2P2S6 (SPS) ferroelectric-semiconductor crystals confirm their hole-type conductivity at room temperature which inverses to the electron-type conductivity at heating to 500 K. Their dielectric susceptibility temperature anomaly near second order phase transition at 337 K and dielectric losses in paraelectric phase both strongly depend on the deviation from the stoichiometric content. Calculated in LDA approach donor energy level of the sulfur vacancies and acceptor level of the tin vacancies are placed near the top of valence band. Such position in the energy gap of donor states mostly determines the semiconductive properties of SPS crystals, and they is related to strong hybridization between sp2 hybrid orbitals of phosphorus atoms and p orbitals of sulfur atoms inside anions (P2S5)4− that contain sulfur vacancy.

Charge Transfer and Anharmonicity in Sn2P2S6 Ferroelectrics

The first-principles calculations in LDA approach were performed for Sn2P2S6 ferroelectrics. The evolution of energy band structure and charge density distribution clearly illustrate the stereoactivity of the tin atoms lone pair electrons and their leading role in the phase transition nature which is described by three-well potential. Energy levels and charge density distribution were calculated for the tin vacancies acceptor states and for the sulfur vacancies donor states in relaxed structure of the paraelectric phase. Taking into account the named donors and acceptors, small electron polarons at Sn+ cations and small hole polarons at S− anions, the relationship of thermally induced lattice multistability with light induced optical bistability for Sn2P2S6 ferroelectrics-semiconductors was established.

Density functional study of magnetic properties in Zn-doped SnO2

Magnetic properties in Zn-doped SnO2 are examined through the first-principles electronic structure calculations based on density functional theory. Our results reveal that Zn-doping induces localized spin magnetic moments primarily on the first coordination shell of O atoms surrounding the Zn atom as well on Zn atom with total magnetic moment of 1.47 μB per supercell. Holes localized on O atoms in ZnO6 are polarized with the same spin orientation as that of the dopant. Ferromagnetic coupling between Zn ions in Zn-doped SnO2 is attributed to the hole-mediated p-d exchange coupling interaction. With respect to native defects in Zinc-doped SnO2, formation of oxygen vacancy (VO) is suppressed whereas formation of tin vacancy (VSn) is facilitated due to Zn-doping. It is found that the observed ferromagnetism in Zn-doped SnO2 mainly originates from the Zn dopant rather than exclusively the formation of VSn.

Effects of oxygen vacancy on the magnetic properties of Cr-doped SnO 2: Density functional investigation

Magnetic properties of Cr-doped SnO 2 and the effects of oxygen vacancy (V O) on the magnetic properties are examined to understand the origin of ferromagnetism in Cr-doped SnO 2 and explain the observed discrepant phenomena. Cr-doped SnO 2 shows a p– d hopping ferromagnetic interaction that decreases gradually with the increase in Cr–Cr distance. In Cr-doped SnO 2 with V O, ferromagnetic interaction is obviously weakened. Exchange constants indicate that the ferromagnetic exchange interaction in Cr-doped SnO 2 is similar to that in Co-doped SnO 2, while it is similar to that in Fe-doped SnO 2 and/or undoped SnO 2 with tin vacancy (V Sn) in the presence of V O.

Influence of the concentration of Sb2O3 on the electrical properties of SnO2 varistors

Varistors are electronic materials with nonohmic behavior. In traditional SnO2 varistors, CoO acts as a densifying agent, Nb2O5 increases the electrical conductivity of SnO2 grains, and Cr2O3 produces a more uniform microstructure and acts as an oxygen retaining agent at the grain boundaries. The present work involved a systematic study of the substitution of Nb2O5 for Sb2O3 in the composition of a ternary varistor system. The compositions were prepared by conventional wet ceramic processing using deionized water, and the resulting slips were dried by spray-drying. Pellets were produced under a pressure of 330 MPa and sintered at 1,350 °C for 2 h. Similar to the behavior of Nb2O5, increasing the concentration of Sb2O3 reduced the nonlinear behavior of the ceramic and its breakdown electric field while increasing its leakage current. The samples’ microstructure showed greater porosity, suggesting that higher concentrations of Sb2O3 reduce the sintering rate, probably in response to the higher concentration of tin vacancies in the structure.

First-principles calculations of native defects in tin monoxide

The formation energies and electronic structure of native defects in tin monoxide are investigated by first-principles calculations. Equilibrium defect concentrations, which are obtained using the calculated formation energies and charge neutrality, indicate that the tin vacancy is the dominant defect under oxygen-rich conditions. It forms shallow acceptor states, suggesting that the $p$-type conductivity of tin monoxide originates from the tin vacancy. The equilibrium concentration of the oxygen interstitial is comparable with the tin vacancy at elevated temperatures. However, it is hardly ionized and therefore not expected to contribute to the conductivity. The concentrations of donorlike defects such as the tin interstitial and the oxygen vacancy are low enough not to compensate holes generated by the tin vacancy.

Determination of absolute orientation-dependent TiN(001) and TiN(111) step energies

We employ in situ high-temperature (1030–1250K) scanning tunneling microscopy to measure adatom island coarsening/decay kinetics and temporal fluctuations around the equilibrium shapes of two-dimensional TiN vacancy islands on atomically-smooth (0 0 1) and (1 1 1) TiN terraces. Using inverse Legendre transformations of the equilibrium island shapes, we obtain relative step energies as a function of step orientation within an orientation-independent scale factor λ, the equilibrium chemical potential of the island per unit TiN molecular area. We then use either quantitative island coarsening measurements [in the case of TiN(0 0 1)] or an exact approach for the analysis of shape fluctuations, applicable to highly anisotropic TiN(1 1 1) islands, to determine λ and, hence, absolute orientation-dependent step energies and step stiffnesses.

Determination of absolute orientation-dependent TiN(0 0 1) and TiN(1 1 1) step energies

We employ in situ high-temperature (1030– 1250 K ) scanning tunneling microscopy to measure adatom island coarsening/decay kinetics and temporal fluctuations around the equilibrium shapes of two-dimensional TiN vacancy islands on atomically-smooth (0 0 1) and (1 1 1) TiN terraces. Using inverse Legendre transformations of the equilibrium island shapes, we obtain relative step energies as a function of step orientation within an orientation-independent scale factor λ, the equilibrium chemical potential of the island per unit TiN molecular area. We then use either quantitative island coarsening measurements [in the case of TiN(0 0 1)] or an exact approach for the analysis of shape fluctuations, applicable to highly anisotropic TiN(1 1 1) islands, to determine λ and, hence, absolute orientation-dependent step energies and step stiffnesses.