Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Rumpling and Enhanced Covalency at the SrTiO3(001) Surface M. Saghayezhian,† S. M. Rezaei Sani,‡ Jiandi Zhang,† and E. W. Plummer*,† †
Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, United States School of Nano Science, Institute for Research in Fundamental Sciences (IPM), Tehran 19395-5531, Iran
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‡
ABSTRACT: The surface of SrTiO3 (001) is considered to be weakly polar, and in this work, we study the validity of this notion. It exhibits a surface structural distortion, quantified here using low energy electron diffraction at room temperature. Structural analysis shows the presence of strong surface rumpling in the TiO2 terminated surface with the oxygen atoms moving outward and Ti atoms moving inward. Density functional calculations confirm the measured rumpling, and experimental data show the distortion is localized at the surface. Angle-dependent core-level X-ray photoemission spectroscopy (XPS) shows that the surface rumpling strongly impacts the electronic structure of the surface. This observation is reinforced by density functional theory, which demonstrates that the valence state of Ti at the surface is reduced while O is enhanced, where we found the Ti−O bonds are more covalent near the surface. Our results show that surface rumpling is accompanied by a change in the bond hybridization of Ti−O at the surface. Changes in the XPS satellite structures at the surface are consistent with this picture of the change in bonding, indicating that the (001) surface of SrTiO3 is not polar and charge rearrangement is a consequence of surface rumpling.
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INTRODUCTION Transition metal oxides (TMO) with perovskite structure exhibit a variety of exciting physical properties such as superconductivity, magnetism, and room temperature 2D electron gas, with interesting properties at surfaces or interfaces.1−3 Among TMOs, SrTiO3 (STO) has attracted special attention. It has been predicted to be an incipient ferroelectric4,5 where the ferroelectric distortion is suppressed by quantum fluctuations.6 There are also predictions that the quantum fluctuations are weaker at the surface, but no evidence of surface ferroelectricity has been reported.7 There is a second-order phase transition at ∼105 K that is accompanied by octahedral rotation.8,9 While this transition is due to the rotation of octahedron, the anticipated surface reconstruction does not appear. It has been shown that the bare surface of STO hosts a 2D electron liquid that appears and disappears with creation of oxygen vacancies whose characteristics are governed by electron−phonon coupling and complex Rashba splitting.10−12 These properties depend on the surface structure, termination and surface preparation.1,13 The surface of STO has been extensively studied, both theoretically and experimentally (refs 14 and 15 and references therein), indicating that the surface atoms undergo rumpling normal to the surface. However, there are disagreements on the magnitude and the direction of atomic displacement.14,16,17 The discrepancy between experimental data is most likely due to the absence of a well-defined and consistent sample preparation procedure.16 The clean STO (001) surface does not show any symmetry change or faceting, as opposed to © XXXX American Chemical Society
strongly polar STO (111) surface where a variety of surface reconstructions and faceting can be observed.18−21 To address the above disagreements, a detailed study of STO surface structure and electronic properties with a well-defined preparation method is essential. In this work, we study the structure and electronic properties of TiO2 terminated STO (001), both experimentally and theoretically. Quantitative low energy electron diffraction (LEED) demonstrates that the surface is 1 × 1 and wellordered with a minimal disorder. The LEED spot intensities as a function of electron beam energy (LEED I(V)) are acquired and analyzed to obtain a detailed surface structure for STO (001).22 The LEED I(V) structural analysis shows that the surface has a structural rumpling, only in the top atomic layer, i.e., TiO2. Angle-dependent X-ray photoemission spectroscopy (XPS) is used to compare the electronic properties of the bulk and the rumpled surface. We find a strong electronic reconstruction in response to the observed surface rumpling. Using density functional theory, we have calculated the structural and electronic properties of this surface, indicating more covalent bonding at the surface with stronger correlation. Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: August 1, 2018 Revised: August 27, 2018
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DOI: 10.1021/acs.jpcc.8b07452 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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EXPERIMENTAL METHODS AND RESULTS The surface structure and composition strongly depend on the preparation method. The STO (001) has a cubic perovskite structure, consisting of alternating atomic layers of TiO2 and SrO in (001) direction (Figure 1a,b). Atomically flat TiO2
The structural and electronic properties of the STO (001) is investigated using LEED and angle-dependent XPS. Typical LEED patterns for TiO2 terminated STO (001) are shown in Figure 2a,b. The surface exhibits C4v symmetry and it is not
Figure 1. (a), (b) Top and side view of STO (001) surface. (c) STM image of STO (001) after treatment. (d) Line profile across the steps and atomically flat terraces of the surface.
terminated STO (001) (Figure 1b) was prepared using an exsitu method described elsewhere.23 After treatment, the sample was transferred to a UHV system with the base pressure of 2 × 10−10 Torr. High emission angle XPS on ex-situ treated samples showed a considerable amount of carbon-based absorbates and some excess Sr on the surface, the latter showing that the exsitu preparation was not adequate in etching away all the SrO layer. Vacuum annealing the sample at 400 °C for 20 min effectively removed absorbates and reduced the Sr at the surface.21 After the described processing, scanning tunneling microscopy (STM) showed atomically flat surfaces with steps and terraces (Figure 1c). The height of each step is 0.3904 nm (Figure 1d), corresponding to a one-unit cell of STO. STM and XPS showed that the combination of ex-situ and in-situ preparation creates an atomically flat and clean surface. Quantitative LEED I(V) analysis is the most widely used technique for surface structure determination.24 The short mean-free-path of the electrons limits the penetration depth of the electrons, making LEED very sensitive to out-of-plane displacement of atoms near the surface.24 As the energy (wavelength) of the incident electrons (eV) changes, the intensity of diffraction spots (I) changes. Comparing experimental I(V) data with theoretically calculated I(V) for a given structure, using multiple scattering theory, enables us to accurately determine the surface structure, using a grid searching procedure.24 The goodness of the comparison is determined by Pendry reliability factor (RP)25 which ranges from zero to one. RP = 0 means experimental and theoretical I(V) curves are perfectly correlated and RP = 1 implies that theory and experiment are completely uncorrelated. Given a large enough experimental data set (energy range of I(V)), a value of Rp ≤ 0.3 indicates that the structural determination is reliable.
Figure 2. (a), (b) LEED pattern at 62 and 99 eV at room temperature. (c) Experimental and theoretical LEED I(V) curves labeled in cubic notation. The respective RP is shown next to each curve. (d) Schematic of atomic positions from LEED I(V) analysis. The distortions in the ball model are exaggerated.
reconstructed. LEED I(V) data are collected by recording the LEED pattern in the energy intervals of 1 eV, from 45 to 420 eV, for six integer spots at room temperature. Spot intensities of symmetrically equivalent spots were averaged for (1,0), (1,1), (2,0), (2,1), (3,0), and (2,2), yielding a total energy range of 1198 eV. This data set is shown in Figure 2b. The LEED I(V) calculation was performed using the symmetrized automated tensor LEED package.26 The partial wave phase shifts were calculated using elastic-atom scattering in solids and solid surfaces code with optimized muffin-tin potential method.27 In the optimized muffin-tin method, the real part of inner potential, instead of being constant, depends on the energy of the incoming electron, which has been shown to B
DOI: 10.1021/acs.jpcc.8b07452 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Table 1. Structural Parameters Where Positive Direction Is Defined Perpendicular to the Surface and toward the Vacuum. Bader’s Effective Charges Are Compared for Sr, Ti, and O in the Bulk and the Surface. Positive (Negative) Corresponds to Losing (Gaining) Charges. effective charge Q (e)
surface structure (Å) LEED this work Ti O1 O2 Sr
−0.06 +0.10 +0.01 +0.02
± ± ± ±
0.03 0.05 0.04 0.03
DFT other
16
−0.04 +0.04
this work −0.074 +0.023 +0.034 +0.159
this work (Bader) 14
other
−0.088 −0.005 +0.022 +0.139
other (Mulliken)14
bulk
surface
bulk
surface
2.23 −1.27 −1.27 1.60
2.19 −1.21 −1.23 1.59
2.35 −1.41 −1.41 1.87
2.29 −1.30 −1.36 1.85
Figure 3. XPS spectra of O 1s, Ti 2p, and Sr 3d for normal emission, bulk sensitive (a)−(c) and high angle emission, surface sensitive (f)−(h). The orange line marks the core level shift between bulk and surface of STO (001). (d) and (i) show the bulk and surface contribution to the Ti 2p3/2 satellite peak at normal and high emission angles, respectively. The surface component is at lower binding energy. Similarly, (e) and (j) show the bulk and surface component of the Ti 2p3/2 main peak at normal and high angle emission, respectively.
below them fixed to the bulk structure of STO, as shown schematically in Figure 1b. To preserve the C4v symmetry of the surface, the atoms were only allowed to move in the direction normal to the surface. Figure 2d shows the relative displacement of Ti and O1 at the surface and Sr and O2 in the second atomic layer. The final positions of atoms in the first and second atomic layers are referenced to the bulk interlayer distance to better illustrate the absolute displacement of atoms (Table 1). Ti in the first layer moves slightly inward (toward bulk) while O1 shows a relatively large outward (toward vacuum) displacement, resulting in a surface rumpling of 0.16 Å or 8% of interlayer spacing. The Ti−O1 bond elongates while the Ti−O2 bond length decreases. Within the error bar,
prevent systematic error in the determination of surface structure in oxide materials.24 For LEED I(V) structure analysis, two initial models based on bulk STO with two surface terminations were assumed, SrO and TiO2 terminations. SrO termination produces an Rp consistently above 0.7, which means that this termination model does not correlate with experimental data. Therefore, we focused on TiO2 termination. The experimental and theoretical I(V) curves of the TiO2 terminated surface for six integer diffraction spots are presented in Figure 2c with individual RP values indicated. The total RP factor for the TiO2 terminated surface is 0.28 ± 0.05. In LEED I(V) analysis, the first two monolayers were allowed to relax with one unit cell C
DOI: 10.1021/acs.jpcc.8b07452 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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and energy accuracy of better than 0.001 mRy/fu (formula unit). The two outmost layers in the slab were relaxed normal to the surface, restricted by symmetry. The resulting atomic displacements for TiO2-terminated (001) surface are summarized in Table 1. The Ti atom in the outermost layer relaxes inward (i.e., toward the bulk shown as a negative value) while O atoms in the same plane relax outward (shown as a positive value). In the second layer, all atoms experience an outward relaxation with a small rumpling. The direction of displacements found theoretically are in an excellent agreement with the nature of the rumbling found experimentally. The theoretically found magnitude of the displacements also has reasonable agreement with the experiment. The surface rumpling in our calculation is 0.097 Å, which is larger than other calculations (0.092 Å)14 and closer to the experimental value (0.16 ± 0.06 Å). To understand the electronic response to the surface reconstruction, we performed electron charge density analysis using Bader’s theory, where the partitioning of charge density on each atom is well-defined.31,32 The charge within each atomic partition (volume of each atom) was calculated for bulk and the first two atomic layers of the slab. Table 1 summarizes our findings and compares them to other calculations where Mulliken population analysis was used.14 The comparison shows that our data have lower valence states for Sr and Ti, and a higher valence state for O. At the surface, the valence state of Ti decreases from 2.23 to 2.19 e, while surface O (O1) increases −1.27 to −1.21 e. Similar but weaker behavior is seen for the SrO layer near the surface where the Sr valence state slightly decreases from 1.60 to 1.59 e and the O2 valence state increases from −1.27 to −1.23 e. As shown schematically in Figure 4a, the charge density distribution in (020) and (010) planes are plotted in Figure 4b,c. It is easy to see that the bonding of Ti to O2 is much more covalent than any other Ti−O bond. Surprisingly, the bond between O1 and the Sr is also more covalent than any other Sr bond (Figure 4c).
the Sr and O2 in the second atomic layer do not move, but the theory will indicate a small rumpling (in the following). The surface rumpling reported here (0.16 ± 0.05 Å, Rp = 0.28) proves there is a distortion at the surface while previous study (0.08 ± 0.08 Å, RP = 0.53)16 was not conclusive. Using angle-dependent XPS, we studied the electronic properties of the STO (001) surface at two emission angles: (i) normal emission (θ = 0° bulk sensitive) and (ii) high emission angle (θ = 78° surface sensitive), where the probe depth is about 25 and 5 Å, respectively. The unit cell step height (Figure 1d) is 3.9 Å. The measurements were made using a monochromatized Al Kα X-ray source and an electron energy analyzer calibrated to Au 4f7/2. The inherent instrument resolution was 0.25 eV. Figure 3 displays our data: parts a−c of Figure 3 show O 1s, Ti 2p, and Sr 3d core levels in normal emission, and parts g and h, at high emission angle, respectively. All shown spectra were subjected to Shirley background subtraction before analysis. The line shape of O 1s remains almost unchanged for θ = 78° and θ = 0°, suggesting there is no appreciable amount of oxygen vacancies or absorbates at the surface. The absence of oxygen vacancies is further confirmed by the absence of Ti3+ components at lower binding energy side of Ti 2p spectra.28 The O 1s at θ = 78° exhibits a 0.5 eV shift toward higher binding energy compared to the normal emission spectra (Figure 3a). A similar but smaller shift (0.3 eV) is seen in spectra of Ti 2p and Sr 3d. Table 2 summarizes the XPS data analysis. We will return to discuss the origin of these shifts after presenting the theory. Table 2. Peak Parameters of All Core Levels θ = 0°
O 1s Ti 2p3/2 main
Ti 2p3/2 satellite Sr 3d5/2
total bulk surface surface bulk
peak position 531.0 459.8 459.77 460.35 472.58 474.34 134.3
θ = 78° fwhm
peak position
fwhm
1.33 1.19 1.00 1.20 2.56 2.60 1.03
531.5 460.1 459.79 460.30 472.01 474.05 134.6
1.39 1.24 0.95 1.13 2.44 2.51 1.05
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DISCUSSION Our structure analysis, DFT calculations, and XPS measurement show that surface rumpling impacts the electronic properties at the surface. Therefore, it is informative to further examine the surface and bulk components of Ti, extracted from XPS spectra, focusing on the Ti 2p3/2 shown in Figure 3. Parts d and e of Figure 4 show these spectra using the deconvolution/fitting procedure outlined above. In the sudden approximation, satellites are due to the creation of two hole, single particle states, which for this system has been associated with charge redistribution between O and Ti.33,34 Experimentally, the main peak-satellite separation (Δ) of Ti 2p3/2 is reduced at the surface compared to in the bulk (Figure 4d.e). This behavior is opposite to what one would normally expect at the surface of materials.35 The surface has reduced coordination and naively smaller bandwidth, leading to more correlation, which would move the satellite structure to higher binding energy. A systematic way of analysis of core-level spectra was demonstrated by Freund et al.,36,37 where they showed that the sum rule of the first moment of spectral weight function represents the single particle energy level. Since the spectral weight at θ = 0° is dominated by Ti atoms in the bulk, the θ = 78° spectra are used to obtain the first moment. As the peak positions of the main peak and the satellite for bulk and surface change, their relative intensities change as well. The first moments of bulk and surface are 462.4
We have separated out the signal from the Ti 2p3/2 and then decomposed the spectra into a surface and bulk components. The decomposition of the main peak and its satellites is shown in Figure 3d,e for θ = 0° and in Figure 3i,j for θ = 78°. Using the mean free path of the photoelectron, it was estimated that about 10% of spectral weight at θ = 0° is from the surface which increases to 70% for θ = 78°. As expected, θ = 78° spectra are primarily from Ti atoms at the surface. Comparing the relative change in the spectral weight of Ti 2p3/2 satellite bulk and surface components, it is seen that the bulk satellite is at higher binding energy, the opposite is true for Ti 2p3/2 main peak.
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THEORETICAL METHODS AND RESULTS First-principles calculations were conducted on the basis of density functional theory (DFT) using QUANTUM ESPRESSO package.29 Seven atomic layers with TiO2 termination on both sides was chosen since the energy difference of seven and nine layer slabs was less than 0.01 eV.30 Atomic relaxation of the structures with internal parameters was accurately performed to achieve residual forces less than 0.1 mRy/Bohr D
DOI: 10.1021/acs.jpcc.8b07452 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. (a) Schematic of relaxed STO (001) structure. The top and bottom show the plane of charge density for (b) and (c), respectively. (b) and (c) display the electron charge density of the STO (001) surface and layers below for the highlighted plane shown in (a). (d), (e) Main peak and satellite of Ti 2p3/2 for surface and bulk, extracted from their respective XPS specras. The insets show the satellite peaks.
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CONCLUSION In summary, we have studied the structure and electronic properties of STO (001). Our LEED I(V) analysis shows strong surface rumpling while preserving the 1 × 1 surface structure. The surface rumpling leads to electronic reconstruction, which is reflected in the binding energy shift and change in satellite features of the core level spectra from XPS. Using density functional theory, we have shown that under such surface rumpling, the valence states of Sr, Ti, and O change in a way that enhances bond covalency at the surface. Our study shows that even in an insulator such as STO, the simple ionic picture fails to capture the underlying physics and nominal charge assignments do not represent the correct valence states of the elements.
and 462.0 eV, respectively (see Figure 4d,e). The small difference between the first moments shows that the energy states of Ti at the surface and bulk are renormalized, leading to more covalent bonds at the surface, which is further confirmed by charge redistribution between O and Ti. This observation is consistent with previous experimental and theoretical studies where it was established that STO bonds in the bulk and surface are far from being ionic but covalent in nature, stronger at the surface.14,33,34,38 The octahedral bond geometry strongly influences the electronic properties of perovskite transition metal oxides.39 Surface rumpling results in Ti−O1−Ti bond angle decrease from 180 (in the bulk) to 176° (at the surface), while the Ti− O1 bond length increases. Obviously, two effects in transition metal oxides should result in a reduction in the bandwidth, which has been empirically demonstrated.40 Our theoretical calculations show that the bandwidth for the closest band to the Fermi energy reduces from 1.88 eV (bulk) to 1.32 eV (surface). Intriguingly, as predicted theoretically, the surface bandwidth is reduced compared to that of the bulk by a factor of √2.41 All of this is still a puzzle because parts b and c of Figure 4 show that the major change in the charge density occurs between the first TiO2 plane and the second SrO plane. The STO (001) surface exhibits two features, rumpling and charge redistribution. The surface structure undergoes strong rumpling, which in an ionic picture should lead to surface polarity, which requires the transfer of charge from layers below to the surface to minimize surface energy.42 However, our data show that the surface rumpling is due to the absence of top octahedral oxygen, which is accompanied by the shortening of Ti−O2 bond. Therefore, we conclude that the notion of surface polarity is not responsible for the observed phenomena. Additionally, our charge analysis shows that fully ionic picture is inadequate in the description of the surface phenomena since it ignores the bond covalency of STO (001). Our XPS and DFT charge analysis along with surface structure determination reveal that change in the bond length and angle at the surface is responsible for charge redistribution.
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AUTHOR INFORMATION
Corresponding Author
*E. W. Plummer. E-mail:
[email protected]. ORCID
M. Saghayezhian: 0000-0001-7332-6296 S. M. Rezaei Sani: 0000-0003-1607-5341 E. W. Plummer: 0000-0003-1714-3676 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the US Department of Energy (DOE) under Grant No. DOE DE-SC0002136. REFERENCES
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DOI: 10.1021/acs.jpcc.8b07452 J. Phys. Chem. C XXXX, XXX, XXX−XXX
