Sn 5s Research Articles

Electronic structure and phase stability of In-free photovoltaic semiconductors, Cu2ZnSnSe4 and Cu2ZnSnS4 by first-principles calculation

AbstractWe have theoretically evaluated the phase stability and electronic structure of Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS). The enthalpies of formation for kesterite, stannite and wurtz-stannite phases of CZTSe and CZTS were calculated using a plane-wave pseudopotential method within the density functional formalism. For CZTSe, the calculated formation enthalpy (ΔH) of the kesterite phase (−312.7 kJ/mol) is a little smaller than that of the stannite phase (−311.3 kJ/mol) and much smaller than that of the wurtz-stannite phase (−305.7 kJ/mol). For CZTS, the ΔH of the kesterite phase (−361.9 kJ/mol) is smaller than that of the stannite phase (−359.9 kJ/mol) and much smaller than that of the wurtz-stannite phase (−354.6 kJ/mol). The difference of ΔH between the kesterite and stannite phases for CZTS is greater than that for CZTSe. This indicates the kesterite phase is more stable than the stannite phase in CZTS compared with CZTSe. The valence band maximums (VBMs) of both the kesterite- and stannite-type CZTSe(CZTS) are antibonding orbitals of Cu 3d and Se 4p (S 3p). The conduction band minimums (CBMs) are antibonding orbitals of Sn 5s and Se 4p (S 3p). The Zn atom does not affect the VBM or the CBM in either CZTSe(CZTS). The theoretical band gap of the kesterite phase calculated with sX-LDA in both CZTSe and CZTS is a little wider than that of the wurtz-stannite phase and much wider than that of the stannite phase.

X-ray photoelectron spectroscopy of tin oxide nanolayers

X-ray photoelectron spectra of 30- and 100-nm nanolayers, recorded in the energy range 0–35 eV, show a strong dependence of both the distribution of the density of Sn 5s, p+ O2p valence states and the change in the intensity ratio for the Sn 4d and O 2s subvalence states on the annealing temperature and nanolayer thickness. In the nanolayers fabricated at an annealing temperature of 450°C, an unusually strong band of O 2s states of unbound oxygen is observed, which is retained for nanolayers doped with palladium and disappears for nanolayers doped with gold and silver.

Role of Sn2+ in the Band Structure of SnM2O6 and Sn2M2O7(M = Nb and Ta) and Their Photocatalytic Properties

The photocatalytic properties of metal oxides consisting of Sn2+ with a 5s2 configuration were studied to find new visible light responsive photocatalysts. The band gaps of Ca2Ta2O7, Sn2Ta2O7, and Sn2Nb2O7 were found to be 4.8, 3.0, and 2.3 eV, respectively. Density functional calculations revealed that the valence band levels formed with Sn 5s orbitals were more negative than those with O 2p orbitals. The result is that niobates and tantalates containing Sn2+ have narrow band gaps compared with typical niobates and tantalates. SnNb2O6 showed photocatalytic activity for H2 and O2 evolution from aqueous solutions containing sacrificial reagents (methanol and Ag+) under visible light irradiation (λ > 420 nm). Moreover, the photocatalytic activity of SnNb2O6 depended on the material used as the Sn source. SnNb2O6 synthesized from Sn3O2(OH)2 showed higher activity for H2 and O2 evolution than that synthesized from commercial SnO, which included a small amount of SnO2. SnNb2O6 synthesized from Sn3O2(OH)2 showe...

Anatase phase stability and doping concentration dependent refractivity in codoped transparent conducting TiO2 films

Nb0.06SnxTi0.94−xO2 (x ≤ 0.3) thin films were grown by a pulsed-laser deposition method with varying Sn concentration. Through a combinatorial technique, we find that Sn concentration can reach a maximum of about x = 0.3 while maintaining the stable anatase phase and epitaxy. A doping concentration dependence of the refractivity is revealed, in which refractivity reduction at a wavelength of λ = 500 nm is estimated to be 12.4% for Nb0.06Sn0.3 Ti0.64O2 thin film. Sn doping induced band-gap blue shift can be contributed to the mixing of extended Sn 5s orbitals with the conduction band of TiO2. Low resistivity on the order of 10−4 Ω cm at room temperature and high internal transmittance of more than 95% in the visible light region are exhibited for Nb0.06Snx Ti0.94−xO2 thin films (x ≤ 0.2). Optical and transport analyses demonstrate that doping Sn into Nb0.06 Ti0.94O2 can reduce the refractivity while maintaining low resistivity and high transparency.

AN XPS STUDY OF AMORPHOUS THIN FILMS OF MIXED OXIDES In2O3–SnO2 SYSTEM DEPOSITED BY CO-EVAPORATION

XPS core level measurements are used to observe the surface changes in amorphous thin films of mixed oxides In 2 O 3– SnO 2 system deposited by co-evaporation. The effects of changes in composition (in mol%), film thickness, substrate temperature, post-deposition annealing, and etching with Ar+ ions on the binding energies of In (3 d ) and Sn (3 d ) doublets in mixed oxides In 2 O 3– SnO 2 system are presented. XPS core level In (3 d ) and Sn (3 d ) spectra at various compositions exhibit the characteristic 3 d 5 / 2 and 3 d 3 / 2 doublets. The positions of the In (3 d ) and Sn (3 d ) lines are those as expected for In 3+ ions in In 2 O 3 and Sn 4+ ions in SnO 2. The initial decrease in binding energy with an increase in Sn content in In 2 O 3 lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The increase in binding energy above the critical Sn content (10 mol% SnO 2) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The decrease in binding energy with film thickness is caused by the increase in free-carriers density, which is generated by oxygen vacancy acting as two electrons donor. The decrease in binding energy with a substrate and annealing temperatures is due either to the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and the formation of indium and tin species of lower valence states so that the In 3+ and the Sn 4+ oxidation states may be changed to the In 2+ and the Sn 2+ oxidation states respectively. The new oxidation states, In 2+ and Sn 2+, formed due to ion etching and annealing the samples can be attributed to the internal electron transfer from oxygen 2p to the In 5s and Sn 5s levels both in In 2 O 3 and SnO 2.

Synthetic, Structural, Mechanistic, and Computational Studies on Single-Site β-Diketiminate Tin(II) Initiators for the Polymerization of rac-Lactide

A family of tin(II) complexes supported by beta-diketiminate ligands has been investigated as initiators for the polymerization of rac-lactide. Kinetic studies reveal a first-order dependence on [lactide], but with a significant induction period. Linear plots of M(n) versus conversion and [M](o)/[I](o) versus conversion, along with narrow molecular weight distributions (typically 1.07-1.10), are indicative of well-controlled, "living" polymerizations. Less sterically hindered derivatives promote faster propagation than their bulky analogues, in accord with a more accessible active site. Enhanced rates of polymerization are observed for ligands bearing halogenated N-aryl substituents, a consequence of the more Lewis acidic nature of the Sn(II) centers. All of the initiators exhibit a similar bias toward heterotactic polylactide, which is attributed to a chain-end control mechanism influenced predominantly by the presence of the Sn 5s(2) lone pair of electrons rather than the steric or electronic properties of the beta-diketiminate ligand. The tin(II) isopropyl-(S)-lactate complex, ((Me)BDI(DIPP))SnOCH(Me)COO(i)Pr (14), has been synthesized as a model compound for the propagating species by treatment of ((Me)BDI(DIPP))Sn(NMe(2)) with isopropyl-(S)-lactate. An X-ray structure determination showed that the lactate ligand forms a five-membered chelate ring with a weak donor bond from the carbonyl oxygen atom to the tin center. A B3LYP density functional computational study indicates that insertion of the first lactide monomer into the tin(II) alkoxide bond is facile, with the induction period arising from a slower insertion of the second (and possibly third and fourth) monomer units.

First-principles calculations of the electronic band structure of In4Sn3O12 and In5SnSbO12

The electronic band structures of M7O12 rhombohedral In4Sn3O12 and In5SnSbO12 are studied using the projector-augmented-wave method based upon the density-functional theory within the generalized gradient approximation. The cation ordering in both materials, as determined by means of ab initio self-consistent total-energy calculations, confirms the results of previous structural studies. It is found that the valence-band maximum states of these two materials are mainly formed by O 2p states hybridized with In 5d characters whereas the bottoms of conduction bands are due primarily to Sn 5s electrons at M2 sites. The present study shows that the introduction of antimony into In4Sn3O12 broadens the splitting of the conduction band, which is presumably related to the substantial change in electrical conductivity observed in In5SnSbO12.

A theoretical and experimental study of the distorted pyrochlore Bi2Sn2O7

The pyrochlore based bismuth stannate, Bi2Sn2O7, is a material with important applications in catalysis and gas sensing. The thermodynamically stable α phase has 352 atoms in the unit cell and is one of the most complex oxide crystal structures to have been solved by powder diffraction. We have performed a full atomic relaxation and calculated the electronic structure, using gradient corrected density functional theory, with the resulting structure in very good agreement with the previous experimentally determined unit cell. The computed density of states is in excellent agreement with our valence band X-ray photoelectron spectra. The combined results shed new light on the bonding and lone pair activity in this material. A mixture of Bi 6s and O 2p states are found to dominate the top of the valence band while Sn 5s, O 2p and Bi 6p states dominate the bottom of the conduction band. The differing contributions of Sn 5s and Bi 6s states to the valence and conduction bands reflect both differences in atomic binding energies and differences in the strength of the metal s interactions with O 2p. The preference of this material for a distorted structure and its unique catalytic and gas sensing activity are discussed.

Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study

The electronic structure of SnO, SnS, SnSe, and SnTe in the rocksalt, litharge, and herzenbergite structures has been calculated using density functional theory. Comparison of the distorted and undistorted structures allows for an explanation of the unusual experimentally observed structural transitions seen along the Sn(II) monochalcogenides. Analysis of the electronic structure shows a strong anion dependence of the Sn(II) lone pair, with the Sn(5s) and Sn(5p) states too far apart to couple directly. However, the interaction of Sn(5s) with anion states of appropriate energy produce a filled antibonding Sn(5s)-anion p combination which allows coupling of Sn(5s) and Sn(5p) to occur, resulting in a sterically active asymmetric density on Sn. While the interaction between Sn(5s) and O(2p) is strong, interactions of Sn with S, Se, and Te become gradually weaker, resulting in less high energy 5s states and hence weaker lone pairs. The stability of the distorted structures relative to the symmetric structures of higher coordination is thereby reduced, which induces the change from highly distorted litharge SnO to highly symmetric rocksalt SnTe seen along the series.

An ab-initio study of the rôle of lone pairs in the structure and insulator–metal transition in SnO and PbO

We have performed density functional calculations on tetragonal SnO and PbO (litharge) in the space group P4/nmm with the specific intention of examining the rôle played by Sn 5s and Pb 6s lone pairs in stabilizing the structure, and in giving rise to semi-metallic behavior (of SnO at ambient pressure and of PbO in the γ phase). Use of the electron localization function has permitted real-space visualization of the lone pair in these structures. We also discuss the electronic structure of the orthorhombic PbO (massicot, space group Pbma) which again has localized lone pairs, contrary to some earlier expectation. Graphic

Possible Ferroelectricity in SnTiO3 by First-Principles Calculations

ABSTRACTThe prospect of lattice structure and ferroelectricity of SnTiO3 have been studied by first-principles calculations within local density approximation. The results showed that the SnTiO3 has the minimum total energy within almost tetragonal perovskite structure of a=b=3.80 Å, c=4.09 Å. The calculated electronic structure of SnTiO3 resembles that of PbTiO3 because the Ti 3d states, Sn 5s and 5p states hybridize with the O 2p orbitals. The moment of spontaneous polarization of SnTiO3 was estimated as 73 μ C/cm2, which is as large as that of PbTiO3.

The origin of the electron distribution in SnO

Gradient corrected density functional theory calculations have been performed on SnO in the litharge and idealized CsCl structures with the litharge structure in good agreement with experiment. The CsCl structured SnO has a spherical electron density whereas the litharge structured SnO has a nonspherical electron distribution. Such asymmetry is often attributed to a sterically active lone pair formed by the 5s2 electrons which does not take part in chemical bonding. However, analysis of the density of states and band structures indicates that the situation is more complicated. In CsCl structured SnO mixing of the Sn 5s with the oxygen 2p electronic states results in filled bonding and antibonding combinations. The antibonding combinations, at the top of valence band, can interact with Sn 5p to stabilize the structure, only when in the distorted litharge structure resulting in the asymmetric electron density. This is in contrast to the classical theory of hybridization of the tin 5s and 5p orbitals to form a “lone pair” as the asymmetric electron distribution is the result of the tin–oxygen covalent interactions.

Local electronic structure of Tl–Sn–Te compounds

We show that the combined application of Mössbauer spectroscopy and X-ray photoelectron spectroscopy (XPS) provides a consistent picture of the local electronic structure in Tl 5Te 3, TlTe, Tl 2Te 3, Tl 4SnTe 3 and Tl 2SnTe 5. The results are discussed from a tight-binding calculation of the electronic populations. We show that values of the Tl 4f 7/2 core-level binding energy do not vary noticeably for the different compounds in agreement with the close values of the calculated Tl average charges. The results obtained by both the XPS and 125Te Mössbauer spectroscopy are consistent with the existence of two types of Te atoms with very different atomic charges due to the differences in the number of Te 5p electrons. The variations of the Te charge are explained from changes in the nature of the Te nearest neighbors: Tl, Sn and Te as a function of the stoichiometry. Finally, we show that values of the 119Sn Mössbauer isomer shift and the Sn 4d 5/2 core-level binding energy both increase from Tl 2SnTe 5 to Tl 4SnTe 3 in agreement with the increase of the calculated number of Sn 5s electrons and the decrease of the calculated number of Sn 5p electrons, respectively. These changes are related to the differences between the Sn local environments of the two ternary compounds.

Reaction of S2 and H2S with Sn/Pt(111) surface alloys: Effects of metal–metal bonding on reactivity towards sulfur

The surface chemistry of S2 and H2S on polycrystalline Sn, Pt(111), and a (∛×∛)R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. At 100–300 K, S2 chemisorbs and reacts on polycrystalline tin to form metal sulfides. The reactivity of pure tin toward sulfur is large even at a temperature as low as 100 K. In contrast, tin atoms in contact with Pt(111) interact weakly with S2 or H2S. Tin does not prevent the bonding of S to Pt in a (∛×∛)R30°-Sn/Pt(111) surface alloy, but the alloy is less reactive toward H2S than polycrystalline Sn or pure Pt(111). At room temperature, S2 and H2S adsorb dissociatively on Pt sites of (∛×∛)R30°-Sn/Pt(111). Upon the dosing of S2 and H2S to (∛×∛)R30°-Sn/Pt(111), one sees the formation of only a chemisorbed layer of sulfur (i.e., no sulfides of tin or platinum are formed). The Pt–Sn bond is complex, involving a Sn(5s,5p)→Pt(6s,6p) charge transfer and a Pt(5d)→Pt(6s,6p) rehybridization that localize electrons in the region between the metal centers. These phenomena reduce the electron donor ability of Pt and Sn, and the metals are not able to respond in an effective way to the presence of species that are strong electron acceptors like S2, HS, and S. The redistribution of charge produces surfaces that have a remarkable low reactivity toward sulfur. When compared to other admetals (Cu, Zn, Ag, Au), tin is the best choice as a site blocker that can enhance the tolerance of Pt reforming catalysts to sulfur poisoning. The Sn/Pt system illustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that are less sensitive to the presence of S-containing molecules.

Synchrotron radiation photoelectron spectroscopy of the O(2s) core level as a tool for monitoring the reducing effects of ion bombardment on SnO 2 thin films

Synchrotron radiation ultraviolet photoemission spectroscopy (UPS) of O(2s) core levels, valence band (VB) and Sn(4d) spectroscopy was used to monitor the effect of Ar + sputtering on SnO 2 films. The decrease of the O(2s) peak intensity and the increase of the Sn 2+ component in the Sn(4d) peak could be satisfactorily correlated with the enhancement of the band gap feature, occurring at 2.9 eV in the VB spectrum, never been due to the formation of Sn(5s–5p) states.

Crystal structure and band structure calculations of [formula omitted] and [formula omitted

The crystal structures of Pb 1 3 TaS 2 and Sn 1 3 NbS 2 were determined using single-crystal X-ray diffraction. The space group is P6 322 and the unit cell dimensions are: a = 5.759(1), c = 14.813(1) A ̊ and a = 5.778(1), c = 14.394(1) A ̊ , for the Pb and Sn compounds, respectively. The post-transition metal atoms occupy one-third of the trigonal-antiprismatic holes between sandwiches NbS 2 and TaS 2. The Nb and Ta atoms are in trigonal-prismatic coordination by sulfur atoms. The stacking of sandwiches is the same as in 2H disulfides. The arrangement of the post-transition metal atoms is different for the two compounds. A bond valence calculation showed Sn and Pb to be divalent. Ab initio band structure calculations were performed for Sn 1 3 NbS 2 using the localized spherical wave method, and for Pb 1 3 TaS 2 with the augmented spherical wave method with spin-orbital interactions included. The calculations show that the rigid band model is approximately valid for the electronic structures; the main difference with those of 2HNbS 2 and 2HTaS 2 being the presence of Sn 5s and 5p (Pb 6s and 6p) bands and a larger S 3p/Nb(Ta) 4d (5d) gap in the intercalates (1.0 eV for Sn 1 3 NbS 2 , 1.3 eV for Pb 1 3 TaS 2 . The Sn 5s (Pb 6s) bands are at the bottom (bonding) and top (antibonding) of the valence bands which range from about −7 to about 0 eV. The conduction bands are composed of Nb 4d z 2 or Ta 5d z 2 orbitals hybridized with S 3p. These bands are filled to about 0.3 holes per Nb (Ta), corresponding to a donation of two electrons per Sn (Pb).

An investigation of doping of SnO 2 by ion implantation and application of ion-implanted films as gas sensors

Thin films of SnO 2 were doped with 90 keV ions of Nb, Sb and Bi at a dose of 3 × 10 16 ions cm −2. The implanted films were characterized by X-ray and ultraviolet photoemission spectroscopy, Auger depth profiling and sheet resistance measurements. Sb produces the most dramatic reduction in sheet resistance and is unique in introducing a sufficient concentration of electrons into the Sn 5s conduction band to be observable by photoemission. The response of the resistance of Sb-doped films to pulses of Ch 4, CO and H 2O in a flow of synthetic air was investigated over a range of concentrations and temperatures. The films display relatively poor selectivity in their sensor response. By contrast, the more highly resistive Bi-doped films show marked selectivity towards CO relative to CH 4 in sensor applications.

A cluster approach for the SnO 2 (110) face

Some results are given from a cluster approach for the electronic structure of the SnO 2 (110) face together with some oxygen vacancies and ‘adsorbates’. Computations are based on ab initio methods, the local-density approximation and atomic orbitals as a basis set. Solutions were calculated self-consistently, but also using a composition of atomic potentials (for some smaller clusters). The atomic-orbital nature (origin) of the cluster levels was traced by projection onto the atomic bases set. The results here refer to a basic cluster [SnO 2 13 with 17 surface atoms moedelling the SnO 2 (110) face and the other 22 atoms in the next five suface layers. The effect of oxyggen ’adsrobates‘ and oxygen vacancies in the few uppermost subsurface layers on the electronic structure was considered. In particular, the focus was on the levels related to oxygen vacancies and originating from Sn 5s orbitals, which are well-known donor levels in the deep bulk, making SnO 2 an n-type semiconductor. The results support some other theoretical and experimental predictions that oxygen vacancies behave as neutral defects at or near SnO 2 surfaces.

Sb and Bi implanted SnO 2 thin films: photoemission studies and application as gas sensors

Thin films of SnO 2 have been implanted with 90 keV 121Sb and 209Bi up to a dose of 3 x 10 16 ions/cm 2. Sb causes a four orders of magnitude reduction in sheet resistance and introduces electrons into the Sn 5s conduction band: these electrons can be seen directly in UV photoemission spectra. Bi is a less effective dopant but improves the selectivity of the films toward CO relative to CH 4 in gas-sensor applications.

The influence of oxygen deficiency and Sb doping on inverse photoemission spectra of SnO 2

The changes in the empty electronic states in SnO 2 produced by ion-beam induced oxygen deficiency and by Sb doping have been studied by inverse photoemission spectroscopy. Inverse photoemission in SnO 2 itself is dominated by peaks 4 and 12 eV above the Fermi level, the former associated with empty states of dominant Sn 5p atomic character. Sb doping populates states in the Sn 5s conduction band, shifting the empty state structure closer to the Fermi energy. By contrast oxygen deficiency introduces new states above the main Sn 5p peak. These are tentatively described as 5s-5p hybrids pushed up in energy from the 5p band by mixing between atomic orbitais of different parity in the non-centrosymmetric cation environment of oxygen deficient SnO 2.