First-principles study on the tensile strength and fracture of the Al-terminated stoichiometric α-Al2O3(0001)/Cu(111) interface

The first-principles tensile tests have been applied to the Al-terminated stoichiometric α-Al₂O₃(0001)/Cu(111) interface by using the ab initio pseudo-potential method based on the density-functional theory. Firstly, the Cu/Al and Cu/Cu interlayers have been examined by the rigid-type tensile test. The interlayer potential curves derived from the first-principles calculations are well fitted by the universal binding-energy relation (UBER) curves. The Cu–Al adhesion is weaker than the back Cu–Cu adhesion. Secondly, the relaxed-type tensile test has revealed the tensile strength and features of interfacial fracture. The ideal tensile strength is about 10?GPa, and the local Young's modulus is about 40?GPa, which means that the Cu/Al interface is quite weak and soft compared with the bulk regions and the O-terminated interface. The failure is initiated from the charge depletion region near the interfacial O atoms when the interlayer stretching exceeds about 30%, and the behaviour of electrons and ions indicates no strong Cu–O bond compared with substantial Cu–Al interactions. The present ab initio data are useful for the construction of effective interatomic potentials at the interface.     

DFT study of carbon monoxide adsorption on α-Al2O3(0001)

The adsorption of CO on α-Al₂O₃(0001) was studied using the DFT-GGA computational method and on α-Al₂O₃ powder experimentally by Infra red spectroscopy. The core and valence level regions of α-Al₂O₃(0001) single crystal surface were also studied experimentally. Ar ions sputtering of the surface results in a slight but reproducible decrease in the XPS O2p lines in the valence band regions due to preferential removal of surface (and near surface) O atoms. Core level XPS O1s and Al2p further confirmed oxygen depletion with an associated surface stoichiometry close to Al₂O_2.9. The adsorption energy of CO was computed and found equal to 0.52 eV for θ = 0.25, it decreased to 0.42 eV at θ = 1. The IR frequency of νCO was also computed and in all cases it was blue shifted with respect to gas phase CO. The shift, Δν, decreased with increasing coverage where it was found equal to 56 cm^? 1 for θ = 0.25 and decreased to 30 cm^? 1 for θ = 1. Structural analyses indicated that the change in the adsorption energy and the associated frequency shift is due to surface relaxation upon adsorption. Experimentally the adsorption of CO gave rise to one main IR peak at 2154 cm^? 1 at 0.3 Torr and above. Two far smaller peaks are also seen at lower pressures of 0.03–0.2 Torr at 2189 and 2178 cm^? 1. The isosteric heat of adsorption was computed for the IR band at 2154 cm^? 1 and was found equal to 0.2 eV which did not change with coverage in the investigated range up to θ = 0.6.     

First-principles study of the topological surface states of α-Sn(111)

α-Sn is on the boundary of a couple of distinct topological phases. It will transform into a topological insulator under a suitable strain. However, a clear picture of its topological surface states (TSSs) is still lacking. Here we perform first-principles calculations on the electronic structure of α-Sn(111) surface to identify its TSSs and reveal their properties. The results show that the presence of valence band reorganizes the TSSs in the inverted sp gap into two Dirac cones. The lower one is in the valence band continuum; the upper one resides in the gap between the valence and conduction bands. We also demonstrate the transformation of the surface states by switching on or off of strain and/or spin-orbit coupling. Without spin-orbit coupling, only the TSSs associated with the lower Dirac cone survive, and they are spin unpolarized. The results are useful for understanding and engineering the topological properties of α-Sn.     

Structure of the (0001) surface of α-Al2O3 from first principles calculations

Fully self-consistent ab initio calculations based on pseudopotentials are used to study the structure and energetics of the basal-plane surface of α-Al₂O₃. The calculated forces on the atoms are used to relax the atomic positions to equilibrium. It is shown that surface relaxations are very large and lead to a reduction of the surface energy by over a factor of two. The results support the validity of earlier work based on pair-interaction models.     

Nature of the anomalies of the thermolumenescence properties of dosimetric α-Al2O3 crystals

The results of investigations into the anomalies of the thermoluminescence properties of dosimetric corundum crystals are presented. The decisive role of deep-lying traps in the quenching of luminescence in anion-defect Al₂O₃ monocrystals is shown. The existence of deep-lying traps is demonstrated by the method of direct observations of thermoluminescence (TL) peaks associated with them. Experimental evidence for the influence of the degree of occupation of deep-lying traps on the main features of the TL dosimetric peak at 450 K is given. The results obtained are interpreted for a model of the interactive system of traps, which differs radically from the models described in the literature by a consideration of the temperature dependence of the probability of trapping of charge carriers on deep-lying traps. We believe that the heat quenching of luminescence is due to the thermal ionization of excited F-center states. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebhykh Zavedenii, Fizika, No. 3, pp. 55–65, March, 2000.     

Pulsed laser deposition of Eu:Y2O3 thin films on (0001) α-Al2O3

This paper focuses on the preparation and characterization of crystalline thin films of rare-earth-doped sesquioxides (Y₂O₃, Lu₂O₃) grown by pulsed laser deposition on single-crystal (0001) sapphire substrates. X-ray diffraction measurements show that the films with thicknesses between 1 nm and 500 nm were highly textured along the 111 direction. Using Rutherford backscattering analysis, the correct stoichiometric composition of the films was established. The emission and excitation spectra of europium-doped films with a thickness 100 nm look similar to those of the corresponding crystalline bulk material, whereas films with a thickness 20 nm show a completely different emission behavior. PACS 68.55.Jk; 78.66.Nk; 81.15.F     

A Comparative Study of the Growth of Cr on (110)TiO2 Rutile, (0001) α-Al2O3 and (100)SrTiO3 Surfaces

Thin Cr films were deposited on single crystal -Al₂O₃, SrTiO₃ and TiO₂ (rutile) substrates under ultrahigh vacuum conditions using molecular beam epitaxy (MBE). The growth behavior and thermal stability of the films were investigated with scanning tunneling microscopy (STM), X-ray phototelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and transmission electron microscopy (TEM). Cr grew as 3D clusters on all substrates. For all three Cr/oxide systems a strong temperature dependent interfacial reaction was observed. The results suggested that these reactions depended greatly on thermodynamics and on transport properties in the oxide substrates.     

Photochemistry of methyl bromide on the α-Cr2O3(0001) surface

The photochemical properties of the Cr-terminated α-Cr₂O₃(0001) surface were explored using methyl bromide (CH₃Br) as a probe molecule. CH₃Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr₂O₃(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH₃Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH₃Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr³⁺ sites and near complete removal of CH₃Br TPD states above 150 K. The photochemistry of chemisorbed CH₃Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH₃Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10^? 22 cm². Photodissociation of the CH₃–Br bond was evidenced by both CH₃ detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr₂O₃, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH₃Br. It is unclear to what extent band-to-band versus direct CH₃Br photolysis play in CH₃–Br bond dissociation initiated by more energetic photons.     

Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.     

Visible light induced photodesorption of NO from the α-Cr2O3(0001) surface

Nitric oxide chemistry and photochemistry on the Cr-terminated surface of α-Cr₂O₃(0001) were examined using temperature programmed desorption (TPD), sticking coefficient measurements and photodesorption. NO exposed to α-Cr₂O₃(0001) at 100 K binds at surface Cr cation sites forming a strongly bound surface species that thermally desorbs at 320–340 K, depending on coverage. No thermal decomposition was detected in TPD in agreement with previous results in the literature. Sticking probability measurements at 100 K indicated near unity sticking for NO up to coverages of ~ 1.3 ML, with additional adsorption with higher exposures at decreased sticking probability. These results suggest that some Cr cation sites on the α-Cr₂O₃(0001) surface were capable of binding more than one NO molecule, although it is unclear whether this was as separate NO molecules or as dimers. Photodesorption of adsorbed NO was examined for surface coverages below the 1 ML point. Both visible and UV light were shown to photodesorb NO without detectable NO photodecomposition. Visible light photodesorption of NO occurred with a greater cross section than estimated using UV light. The visible light photodesorption event was not associated with bandgap excitation in α-Cr₂O₃(0001), but instead was linked to excitation of a surface Cr^3 +–NO^? charge transfer complex. These results illustrate that localized photoabsorption events at surface sites with unique optical properties (relative to the bulk) can result in unexpected surface photochemistry.     

First-principles calculations of interfacial and segregation energies in α-Cr2O3

The interfacial energies of three twin boundaries with low-index boundary planes: prismatic (101?0), basal O-terminated (0001), and basal Cr-terminated (0001), and the segregation energies of five doping elements (Ce, Hf, La, Y and Zr) have been calculated as a function of temperature. The static energies at 0 K were obtained through first-principles calculations and the energies at finite temperatures were derived based on the Debye model. The calculation results show that both the interfacial and segregation energies decrease as temperature increases and the segregation energies are found to be proportional to the ionic size mismatch and the interfacial energy. Our combined approaches suggest an efficient and less computationally intensive way to derive grain boundary energetics at finite temperatures.     

Study of the Defect Structure and Crystal-Field Parameters of α-Al2O3:Yb3+

The methods currently used for studying the defect structure of laser host crystals doped with transition metal or rare-earth ions have several drawbacks or limitations. This study proposes an alternative approach for obtaining optimized impurity structures using molecular dynamics calculation in conjunction with the superposition model. This new approach is specifically applied to a system named α-Al₂O₃:Yb³⁺, in which the simulated defect structure is used to fit the superposition model parameters directly onto the observed energy levels. Such an approach provides predicted values of crystal-field parameters, Zeeman splitting g-factor, and hyperfine structure constants. Moreover, the C_3v site symmetry is found to be a good approximation for the actual C₃ site of Yb³⁺, as doped in an α-Al₂O₃ crystal.     

α-Al2O3(0001)基片表面结构与能量研究

对α-Al2O3(0001)晶体表层三种不同终止原子结构的计算模型,在三维周期边界条件下的κ空间中,采用超软赝势平面波函数描述多电子体系.应用基于密度泛函理论的局域密度近似,计算了不同表层结构的体系能量,表明最表层终止原子为单层Al的表面结构最稳定.对由10个原子组成的菱形原胞进行了结构优化,得到晶胞参数值(a0＝0.48178nm)与实验报道值误差小于1.3%.进一步计算了超晶胞(2×2)表面弛豫,弛豫后原第2层O原子层成为最表层; 对不同表层O,Al原子最外层电子进行了布居分析,表面电子有更大的概率被定域在O原子的周围,表面明显地表现出O原子的电子表面态.     

Oxygen plasma activation of Cr(CO)6 on α-Fe2O3(0001)

The chemistry of Cr(CO)₆ on the Fe₃O₄(111) surface termination of α-Fe₂O₃(0001) was explored using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED) both with and without activation from an oxygen plasma source. No thermal decomposition of Cr(CO)₆ was detected on the surface in the absence of O₂ plasma treatment, with first layer molecules desorbing in TPD at 215 K from a close-packed overlayer. The interaction of first layer Cr(CO)₆ with the Fe₃O₄(111)-termination was weak, desorbing only ～ 30 K above the leading edge of the multilayer state. Activation of multilayer coverages of Cr(CO)₆ with the O₂ plasma source at 100 K resulted in complete conversion of the outer Cr(CO)₆ layers, presumably to a disordered Cr oxide film, with Cr(CO)₆ molecules near the surface left unaffected. Absence of CO or CO₂ desorption states suggests that all carbonyl ligands are liberated for each Cr(CO)₆ molecule activated by the plasma. AES and SSIMS both show that O₂ plasma activation of Cr(CO)₆ results in a carbon-free surface (after desorption of unreacted Cr(CO)₆). LEED, however, shows that the Cr oxide film was disordered at 600 K and likely O-terminated based on subsequent water TPD. Attempts to order the film at temperatures above 650 K resulted in dissolution of Cr into the α-Fe₂O₃(0001) crystal based on SSIMS, an observation linked to the Fe₃O₄(111) termination of the surface and not to the properties of α-Cr₂O₃/α-Fe₂O₃ corundum interface. Nevertheless, this study shows that O₂ plasma activation of Cr(CO)₆ is an effective means of depositing Cr oxide films on surfaces without accompanying carbon contamination.     

First-principles investigation of the valley and electrical properties of carbon-doped α-graphyne-like BN sheet

Based on ab initio density functional theory calculations, we demonstrate that two carbon-doped boron nitride analog of α-graphyne structures, B_3C_2N_3 and BC_6 N monolayers, are two-dimensional direct wide band gap semiconductors, and there are two inequivalent valleys in the vicinities of the vertices of their hexagonal Brillouin zones. Besides, B_3C_2N_3 and BC_6 N monolayers exhibit relatively high carrier mobilities, and their direct band gap feature is robust against the biaxial strain. More importantly, the energetically most favorable B_3C_2N_3 and BC_6 N bilayers also have direct wide band gaps, and valley polarization could be achieved by optical helicity. Finally, we show that BC_6 N monolayer might have high efficiency in photo-splitting reactions of water, and a vertical van der Waals heterostructure with a type-II energy band alignment could be designed using B_3C_2N_3 and BC_6 N monolayers. All the above-mentioned characteristics make B_3C_2N_3 and BC_6 N monolayers, bilayers, and their heterostructures recommendable candidates for applications in valleytronic devices,metal-free photocatalysts, and photovoltaic cells.     

First-principles study on the doping effects of Al in α-MnO2

In this paper, first-principles calculations have been implemented to study the structural relaxation, formation energies and electronic structure of Al doped α-MnO₂. Both Al insertion and Al substitution reactions in the α-MnO₂ were considered. Calculated formation energies indicate that Mn atom is easier to be displaced by Al atom under the O-rich growth condition compared with Al insertion reaction. Besides, it can be found that Al doping can afford acceptor impurity level which can accommodate electrons, thus contributing to the improvement of conductivity of α-MnO₂. The conductivity of α-MnO₂ is gradually improved with the increasing doping concentration of Al, and Al_0.0417Mn_0.9583O₂ exhibits the best conductivity. Lastly, the electronic structure of Al_0.0417Mn_0.9583O₂ was further investigated by analysis of total charge density and Bader charge. It is clear that Al doping can afford more electrons for α-MnO₂, which also contributes to improvement of its conductivity.