Local Structure and Bonding of Transition Metal Dopants in Bi2Se3

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Local Structure and Bonding of Transition Metal Dopants in Bi2Se3 Topological Insulator Thin Films Adriana I. Figueroa,† Gerrit van der Laan,*,† Liam J. Collins-McIntyre,‡ Giannantonio Cibin,§ Andrew J. Dent,§ and Thorsten Hesjedal‡,§ †

Magnetic Spectroscopy Group, Diamond Light Source, Didcot, OX11 0DE, United Kingdom Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, OX1 3PU, United Kingdom § Diamond Light Source, Didcot, OX11 0DE, United Kingdom ‡

S Supporting Information *

ABSTRACT: Transition metal (TM) doped topological insulators have been the focus of many recent studies since they exhibit exotic quantum and magneto-electric effects, and offer the prospect of potential applications in spintronic devices. Here we report a systematic study of the local electronic and structural environment using X-ray absorption fine structure (XAFS) in TM (=Cr, Mn, and Fe) doped Bi2Se3 thin films grown by molecular beam epitaxy. Analysis of the TM K-edge XAFS reveals a divalent character for Cr, Mn, and Fe when substituting Bi in the films, despite the trivalent character of the Bi. All dopants occupy octahedral sites in the Bi2Se3 lattice, which agrees with substitutional incorporation onto the Bi sites. With the incorporation of TM dopants a local structural relaxation of the Bi2Se3 lattice is observed, which strengthens the covalent character of the TM−Se bond. The presence of additional phases and interstitial incorporation for the Mn and Fe dopants is also observed, even at low concentrations.

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INTRODUCTION

the magnetic dopant in the TI lattice is essential to fully understand their fascinating and intriguing physical properties. Several studies have recently investigated the incorporation of TM dopants into TIs using both theoretical10−13 and experimental approaches.14−16 Ab initio calculations have indicated the incorporation of the TM atoms onto the Bi sites of the Bi2Se3 crystal to be energetically more favorable in the case of Cr, Mn, and Fe.10−12 However, diverse experimental techniques have revealed that the Mn dopants form clusters and additional phases in the TI lattice.14,17 Similarly, Cu dopants have shown preferential intercalation into the Bi2Se3 lattice,18 but recent X-ray absorption fine structure (XAFS) studies have revealed Cu atoms entering substitutionally onto the Bi sites.15 The above examples, together with many others reported in a wide range of recent studies of TM-doped TIs, highlight the existing controversy about the precise location of the TM dopants and their incorporation into the TI lattice. Another issue of contradiction is the oxidation state of the TM dopant in the TI lattice. When replacing Bi3+ in Bi2Se3 or Bi2Te3, the early first 3d row TM atoms up to Fe are expected to have trivalent character, as calculated by Larson and Lambrecht.10 However, more recent first-principle calculations on Mn incorporated into Bi2Te3 have indicated a divalent

Topological insulators (TIs) are a novel class of materials characterized by insulating behavior in the bulk and topologically protected, fully spin polarized conducting surface states at the boundary.1−4 Intense research is currently focused toward the growth of high quality TI samples and the investigation of their electronic properties. Their characteristic that the charge carriers of opposite spin have opposite linear momentum makes TIs ideal systems for a range of possible spin-based and low-power electronic applications. The topological surface state of the TI is protected by time-reversal symmetry (TRS) from backscattering by nonmagnetic impurities. Breaking TRS, e.g., due to an applied magnetic field or magnetic impurities, opens an energy gap at the surface Dirac point and suppresses the local density of states at the Fermi level.5 Such magnetic TIs are a prerequisite for observing the quantum anomalous Hall (QAH) effect6 and other exotic magneto-electric effects.7 A straightforward way to introduce magnetism is to dope the TIs with a transition metal (TM). However, one of the problems with magnetically doped topological insulators8 is the formation of clusters.9 Cluster formation can be very detrimental to the materials performance and completely dominate the electronic properties of the material. Avoiding the formation of clusters is therefore of great importance before any potential device can be made. An accurate knowledge of the atomic site and electronic state of © 2015 American Chemical Society

Received: November 23, 2014 Revised: July 4, 2015 Published: July 6, 2015 17344

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The Journal of Physical Chemistry C character of Mn.13 In addition, we recently demonstrated experimentally that Cr dopants possess a 2+ oxidation state when substituting the Bi sites of Bi2Se3.16 The aim of the present study is to investigate systematically the local electronic and structural environment of Bi2Se3 doped with TM = Cr, Mn, or Fe. We have used the element specific technique of XAFS to discern the location of the dopant into the Bi2Se3 matrix and to determine the valence of the TM atoms. The Bi2Se3 family of compounds, which exhibit TI properties, have a rhombohedral crystal structure with space group D53d (R3̅m). Figure 1 illustrates the layered structure of

Table 1. Coordination and Bonding of TM Dopants in Bi2Se3a 1st shell site N R (Å)

S

TM−Se1

TM−Se2

3 2.952

3 3.021 1st shell

site

TM−Se1 N R (Å)

I1

site I2

N R (Å)

6 2.710 1st shell

2nd shell TM−Bi1

3rd shell

TM−Bi2

6 3 4.134 4.405 2nd shell TM−Bi 2 2.890 2nd shell

TM−Se3 3 4.747 3rd shell TM−Se2

6 4.950 3rd shell

TM−Se1

TM−Bi1

TM−Bi2

TM−Se2

TM−Bi3

3 2.570

3 2.570

1 2.820

9 4.870

3 4.870

a

Values given according to the crystallographic information in ICSD 61707219 and after ref 15. S, I1 and I2 are the substitutional for Bi, interstitial in the van der Waals gap, and in-between Bi and Se layers sites for TM incorporation, respectively (see Figure 1). N is the coordination number and R the interatomic distance.

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EXPERIMENTAL METHODS Sample Growth and Characterization. Bi2Se3 thin films samples, doped with the transition metals Cr, Mn, and Fe, were grown by molecular beam epitaxy on c-plane sapphire [Al2O3 (0001)]. For the growth a Se overpressure of PSe/P(Bi+TM) = 10, with P partial pressure, as measured by in situ beam flux monitoring, was maintained. Standard and high-temperature effusion cells were used for Cr, Mn, and Fe, and a cracker cell for Se. All elemental materials were of ultrahigh purity. Samples doped with Mn and Fe were deposited directly onto the substrate at a temperature of 250 °C. Cr-doped films were fabricated by first depositing a 20 nm-thick Bi2Se3 buffer layer at 250 °C, before the doped layer is grown at 300 °C. All doped layers have a thickness between 100 and 120 nm. A detailed exposition of the growth method for these samples is given in refs 21 and 22. The nominal dopant concentrations in the TMdoped Bi2Se3 (TM:Bi2Se3) films were determined using the Bi/ TM flux ratio and confirmed ex situ by Rutherford backscattering spectrometry (RBS) for Cr and Mn dopants and by energy dispersive spectroscopy (EDS) for Fe dopants. X-ray diffraction (XRD) spectra were recorded for all samples on a Bruker D8 diffractometer with incident Cu Kα1 radiation. The XRD spectra for a selection of samples with low doping concentrations are shown in Figure 2. All samples show the clearly identifiable (003l) family of peaks indicative of the R3̅m space group of Bi2Se3. Additional peaks are observed for the sample doped with Mn due to a Se capping layer. At higher doping concentrations secondary phases form incorporating the dopant, primarily by reaction with Se (not shown).21,22 c-Axis lattice constants are determined by an appropriate fit to the (003l) peaks, and a systematic variance is observed for different dopant and as a function of doping concentration. The extracted value for each sample is listed in Table 2. Compared to the c lattice parameter of 28.66 ± 0.01 Å obtained for an undoped Bi2Se3, the values for the Cr samples decrease for 4.6 at. % and 11.7 at. % Cr. A reduction of the c lattice parameter is an indication of substitutional incorporation of Cr onto the Bi sites, since the neighboring Se atoms move to accommodate the smaller radius of the TM atom compared to the Bi one (cf. Figure 1).10,12,15,16 An increase in the c lattice parameter for the sample with 17.5 at. % Cr, as well as for all Mn doped samples,

Figure 1. Crystal structure of TM-doped Bi2Se3. Quintuple layers (Se−Bi−Se−Bi−Se) are separated by a weakly bonded van der Waals gap. TM dopants are shown substituting for Bi (S), interstitially in the van der Waals gap (I1), and in-between Bi and Se layers (I2). White arrows show the direction of the bond contraction to accommodate the dopant into the lattice.

the crystal, with five atomic layers as a basic unit, known as a quintuple layer (QL). The interlayer bonding within the QLs is strong due to the dominant covalent character, while the bonding between the QLs is much weaker due to the van der Waals-type interaction. The lattice parameters of Bi2Se3 are a = b = 4.143 Å and c = 28.636 Å (ICSD 617072).19 When doping the Bi2Se3 crystal, the TM atoms can replace Bi3+ located between Se atoms (substitutional site, S, in Figure 1). The neighboring Se atoms move to accommodate the smaller radius of the TM atom compared to the Bi one.10,12,15,16 Other doping scenarios are those of TM atoms incorporating into the van der Waals gap (interstitial site 1, I1, in Figure 1), or interstitially between Bi and Se layers (interstitial site 2, I2, in Figure 1), which gives a lower oxidation state with increased n-type carrier concentration and increased c lattice parameter.20 Table 1 lists the coordination numbers and bond distances for the first three coordination shells of TM atoms in Bi2Se3 for these three doping schemes. Our study provides insight into the electronic state of TM atoms, demonstrating that the Cr, Mn, and Fe dopants have divalent character. We show that the TMs are in an octahedral environment of Se atoms, which is in agreement with the TM dopant substituting Bi3+. We are able to quantify the local lattice relaxation described above, with which the crystal accommodates the TM atoms. This contraction enhances the covalent character of the TM−Se bonds, which can have important consequences for the electronic and magnetic properties of TM-doped Bi2Se3. Thus, our findings are of paramount importance for boosting the applications of these and similar systems, such as in spintronics devices. 17345

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conversion of μ(E) to χ(k), normalization and weighting scheme; all of them performed with AUTOBK and ATHENA. EXAFS data analysis and fitting on all references and samples were performed in ARTEMIS, making use of models based on crystallographic information obtained from the ICSD database. The atomic clusters used to generate the scattering paths for fitting were generated with ATOMS.24

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RESULTS AND DISCUSSION Transition Metal K-Edge XANES. Normalized XANES spectra at the Cr, Mn, and Fe K-edges of the doped Bi2Se3 thin films, together with relevant standard compounds for each edge are plotted in Figure 3. Comparison of the energy position of Figure 2. X-ray diffraction spectra for a selection of low concentration Cr:, Mn:, and Fe:Bi2Se3 as well as an undoped reference film. Peak positions of the (003l) ordered peaks are indicated. Data have been vertically shifted for clarity. % denotes atomic concentration.

Table 2. Doping Concentration and Lattice Parameter c of TM-Doped Bi2Se3 Filmsa Cr

Mn

Fe

x (at. %)

c (Å)

x (at. %)

c (Å)

x (at. %)

c (Å)

4.6 11.7 17.5

28.64 28.61 28.68

2.3 7.5 16.1

28.62 29.23 29.38

6.7 14.8 −

28.65 28.65 −

a

x denotes the TM doping atomic concentration as obtained from RBS or EDS (see main text). c values have been extracted from fits of the (003l) peaks in the XRD data, cf., c = 28.66 ± 0.01 Å obtained for an undoped Bi2Se3 film. Uncertainty in x is ±0.5 at. % for Cr and Mn and ±10% (absolute) for Fe. Uncertainty in c is ±0.01 Å.

Figure 3. XANES spectra at the TM K-edge of (a) Cr-, (b) Mn-, and (c) Fe-doped Bi2Se3 thin films and their comparison with relevant standards for reference: CrSe, Cr2O3, CrO3, and Cr foil in part a; MnSe, MnO, Mn2O3, and Mn foil in part b; FeSe, FeO, Fe2O3, and Fe foil in part c. The oxidation state of the TM atoms in the standards is indicated by square brackets. Spectra have been vertically shifted for clarity. Dotted line marks the pre-edge position.

suggests incorporation of the dopant into the van der Waals gap.20 In the case of Fe doping, the c lattice parameter is rather constant and very close to that of the undoped Bi2Se3 film. XAFS Measurements. To characterize the electronic and structural environment of the TM atoms, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were measured at the TM and Se K-edges (Cr ∼5989 eV, Mn ∼6539 eV, Fe ∼7112 eV, and Se ∼12658 eV) at room temperature on beamline B18 at the Diamond Light Source. A nine-element solid-state Ge detector with digital signal processing for fluorescence XAFS, high energy resolution, and high count rate was used to measure with the beam at 45° incidence with respect to the sample plane. All spectra were acquired in quick-EXAFS mode, using the Pt- and Cr-coated branch of collimating and focusing mirrors for the TM and Se K-edges, respectively, a Si(111) double-crystal monochromator and a pair of harmonic rejection mirrors. The energy range for each scan allowed us to extract information in the extended region up to k = (12−14) Å−1. An undoped Bi2Se3 thin film and polycrystalline Cr2O3, CrO3, CrSe, MnO, Mn2O3, MnSe, FeO, Fe2O3, and FeSe standards in powder form as well as Cr, Mn, and Fe foils of 5 μm thickness were measured as reference. The powdered standards were pressed into pellets with the optimized quantity for measurements in transmission at their respective absorption edge. XAFS analysis. EXAFS spectra were processed and analyzed using different tools of the IFFEFIT XAFS package.23 This involved preliminary reduction of the EXAFS raw data, background removal of the X-ray absorption data μ(E),

the absorption jump for the thin films with the references suggests that the Cr, Mn, and Fe dopants in Bi2Se3 have a valence state close to that of the CrSe, MnSe, and FeSe standards, respectively, which means 2+. In the case of Fe and Mn, the energy of the absorption jump is also close to that of MnO and FeO, respectively, which have an oxidation state of 2+. The divalent character of Cr and Mn dopants in Bi2Se3 films has been previously revealed by X-ray absorption spectroscopy and X-ray magnetic circular dichroism at the Cr and Mn L2,3 edges.16,22 The K-edge structure is mainly governed by the 1s → ϵp electric-dipole interactions, hence the main absorption peak of the TM K-edge reflects the 4p unoccupied density of states of the TM. The pre-edge structure is usually associated with 1s → 3d transitions. While being formally electric-quadrupole transitions that are usually very weak, a stronger dipole allowed character can mix in due to both inter- and intra-atomic p−d hybridization.25,26 Such prepeaks will be characteristic for a noncentrosymmetric local environment of the TM, as seen in the CrO3 spectrum (Figure 3(a)), which has tetrahedrally coordinated Cr6+ atoms.27 Only a very small pre-edge structure is visible in Cr2O3, since the local Cr3+ site symmetry is slightly distorted octahedral with three Cr−O bond distances of 1.97 Å and three of 2.02 Å.28 The Cr:Bi2Se3 shows a small prepeak (see Figure 4a), suggesting a slightly distorted octahedral 17346

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Figure 5. Spectra for the undoped Bi2Se3 film and the CrSe, MnSe, and FeSe standards are included as reference. The shape

environment, as is also supported by the analysis of the extended region in the EXAFS section.

Figure 5. Se K XANES of (a) Cr:, (b) Mn:, and (c) Fe:Bi2Se3 samples. The undoped Bi2Se3 thin film is included in each panel, together with the CrSe, MnSe, and FeSe standards in parts a−c, respectively, for reference. The three main peaks in the absorption spectrum are denoted as A, B, and C. Panels d−f show the first derivative of the curves in parts a−c, respectively. Peak D marks the energy position of the maximum in the derivative of the XANES for the undoped Bi2Se3 thin film. % denotes atomic concentration.

of the latter is typical for Se2− compounds.29,31 Changes in the Se K-edge XANES reflects variations in the unoccupied Se p states (1s → ϵp electric-dipole transitions). Peak A is associated with hybridized Se p−TM d states,31 while peak B (about 7.5 eV above peak A) can arise from the eg antibonding orbital at the Se site, but it is more likely to originate from multiple scattering effects and is therefore related to the local structure of the Se site.29 Similarly, the origin of peak C (about 17.5 eV above peak A) is multiple scattering from the symmetrical Se 4p states in the coordination sphere. Peak C is more evident in the undoped Bi2Se3 than in the TM selenide standards. A reduction in the intensity of peak A is observed with Cr doping, compared to the spectrum for the undoped sample (Figure 5a). This reveals a decrease in the unoccupied p states at the Se site in the Bi2Se3 film with increasing Cr concentration, consistent with a larger hybridization. As the Cr concentration in the sample increases, both peaks A and B in the spectrum evolve toward those of the CrSe. In fact, the Se K spectra of the Cr doped samples can be modeled by a linear combination of the undoped Bi2Se3 and the CrSe spectrum (c.f., Figure 5a). The fitting results are summarized in Table 3. The contribution of CrSe to the curves scales linearly with the

Figure 4. Details of the pre-edge (left) and main edge (right) regions of the (a) Cr-, (b) Mn-, and (c) Fe-doped samples. % denotes atomic concentration.

The shape of the TM K-edge spectra in all cases is similar to that of the corresponding TM-selenide (i.e., CrSe, MnSe, and FeSe). Small changes in intensity of the features near the absorption edge as a function of the TM concentration are observed. The main peak reduces in intensity for increasing Cr and Fe doping (see Figure 4, parts a and c), which can be ascribed to a smaller mixing of the TM 4p−Se ϵd orbitals.29 For increasing Cr doping, the main peak structure also shifts toward higher energy. Since the main peak is sensitive to the TM-Se orbitals mixing, a decrease in the TM-Se bond length, d, is expected to increase the peak position according to the relation ΔE ∝ 1/d2.30 At the same time the reduced p-d mixing leads to a lower prepeak intensity. This is consistent with a decrease in Cr−Se bond length for higher Cr doping, as also found from the EXAFS analysis (see Table 4). In the Mn case, the intensities of all samples are very close, except for the 2.3 at. % Mn, which shows a different shape of the features near the edge (see Figure 4b), suggesting changes in the unoccupied density of p states at the Mn site. The Mn Kedge shows no prepeak, suggesting that the Mn atoms have an undistorted octahedral symmetry, with six equidistant nearest neighbors. Se K-Edge XANES. Normalized XANES spectra measured at the Se K-edge for the TM-doped samples are shown in

Table 3. Results of the Linear Combination Fit of the Se KEdge XANES Spectra of the Cr:Bi2Se3a at. % Cr

% CrSe

% Bi2Se3

4.6 11.7 17.5

9.0 ± 0.2 22.0 ± 0.7 55.2 ± 0.8

91.0 ± 0.2 78.0 ± 0.7 44.8 ± 0.8

a

Fits were performed using undoped Bi2Se3 and CrSe as standards (cf. Figure 5a). 17347

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The Journal of Physical Chemistry C nominal concentration of Cr in the samples. This result demonstrates that the local electronic environment of the Se atoms is evolving toward that of the CrSe sample, but, as we will show in the next section, this does not imply the presence of CrSe compound in all samples. In fact, there is no shift of the threshold energy at the Se K-edge with respect to that of the undoped sample, as can be clearly observed from the position of the derivative maxima of the curves (peak D in Figure 5(d)). The Se K-edge results for the Mn-doped thin films need to be interpreted taking into account the presence of a protective Se capping layer, which has a Se0 valence, lower than the nominal Se2− of the undoped film. However, the Se0 shows up at a similar energy position as Se2−,32 since the additional two 4 s electrons do not contribute to the 4p density of states. This gives that the curves for the Se capped Mn-doped samples (shown in Figure 5b) are dominated by the Se0, with an enhanced intensity of peak A, compared to MnSe and undoped Bi2Se3. It is important to mention that the Se capping layer is absent for the Cr- and Fe-doped samples. Comparison of the first derivatives in Figure 5e reveals an energy shift of the absorption jump toward higher energies (ΔE ≈ 0.5 eV) with respect to the same features for the undoped film. In the case of Fe doping, the samples measured at the Se Kedge (6.7 at. % and 14.8 at. % Fe dopant) show a very distinct behavior, and no clear trend in peak A with Fe concentration is observed (Figure 5(c)). The intensity of peak A for the sample with 6.7 at. % Fe decreases with respect to the undoped film (similar to the case of Cr); for 14.8 at. % Fe, however, the intensity is higher than that of the undoped film. The latter sample also shows a slight shift of the absorption jump toward higher energies (ΔE ≈ 0.5 eV), being closer to the position observed for the FeSe standard (Figure 5f). Peak B for both 6.7 at. % and 14.8 at. % Fe samples reveals similarities to the same feature in the FeSe reference. These results suggest that the electronic environment at the Se site in the Fe-doped samples, in particular for the 14.8 at. % Fe sample, is close to that of the FeSe compound. Summarizing the XANES results at the TM and Se K-edges, they reveal a divalent character of the Cr, Mn, and Fe dopants in the Bi2Se3 thin films. In general, the local electronic structure at the TM and Se sites is comparable to that of the TMselenides (CrSe, MnSe, and FeSe). This evidences a covalent bonding between the TM and Se atoms upon incorporation of the dopants into the lattice. Transition Metal K-Edge EXAFS. Fits of the TM K-edge EXAFS signal were performed in order to extract information about bond distances, coordination, and disorder level of the TM dopants in the Bi2Se3 thin film. Figures 6, 7, and 8 show the module of the Fourier transform (FT) and the raw χ(k) EXAFS signal for Cr-, Mn-, and Fe-doped samples performed over [3.3−12], [2.8−11], and [3.3−12.2] Å−1k-ranges using a Kaiser-Bessel window function, and Δk = 1.2, 2, and 1.5 Å−1, respectively. All plots are performed using a k2 weight. The strong amplitude reduction of the second and third shell peaks in the module of the FT can be explained with a phenomenon of destructive interference among some of the main EXAFS scattering paths involved.33 For more details see the Supporting Information. Different models were tested to fit the EXAFS signal at the TM K-edge. Fits were performed on the R-space in a [1.2−4.3], [1.5−4.6], and [1.2−4.5] Å range using a Kaiser-Bessel window function, so that it covered the first three coordination shells of the TM atoms (see Figures 6a, 7a, and 8a). The parameters

Figure 6. (a) Fourier transform of the EXAFS signal at the Cr K-edge of the thin film (symbols) together with the best fit to the first three coordination shells (solid line). (b) EXAFS signal (symbols) for the Cr-doped samples together with their best fit (solid lines). The curves have been vertically shifted for clarity. % denotes atomic concentration.

Figure 7. (a) Fourier transform of EXAFS signal at the Mn K-edge on the thin film (symbols) together with the best fit to the first three coordination shells (solid line). (b) EXAFS signal (symbols) for the Mn-doped samples together with their best fit (solid line). Curves have been vertically shifted for clarity. % denotes atomic concentration.

Figure 8. (a) Fourier transform of EXAFS signal at the Fe K-edge on the thin film (symbols) together with the best fit to the first three coordination shells (solid line). (b) EXAFS signal (symbols) for the Fe-doped samples together with their best fit (solid lines). Curves have been vertically shifted for clarity. % denotes atomic concentration.

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The Journal of Physical Chemistry C Table 4. Structural Parameters Obtained from Cr K-Edge EXAFS Fits for the Cr:Bi2Se3 Samplesa at. % Cr 4.6

11.7

17.5

N R (Å) ± 0.01 σ2 (Å2) ± 15% N R (Å) ± 0.02 σ2 (Å2) ± 20% N R (Å) ± 0.02 σ2 (Å2) ± 20%

Cr−Se1

Cr−Se2

Cr−Bi1

Cr−Bi2

Cr−Cr1

Cr−Se3

3 2.54 0.003 6 2.56 0.007 6 2.52 0.005

3 2.65 0.003 − − − − − −

6 3.44 0.030 6 3.56 0.046 1 3.70 0.005

3 3.80 0.012 3 4.03 0.013 − − −

− − − − − − 6 3.62 0.019

3 4.25 0.010 3 4.10 0.014 3 4.09 0.011

Coordination number, N, interatomic distance, R, and Debye−Waller factor, σ2, for each path. Values of ΔE0 ≈2.9 ± 0.3 eV were obtained from the fit for all Cr samples. a

fitted were the interatomic distance (R) and the Debye−Waller factor (σ2) for each scattering path. The shift in the threshold energy (ΔE0) was allowed to vary slightly, but was taken to be the same for all samples within each TM series. The amplitude reduction factor S02 was set to that obtained for the fit of the respective TM standards (S02 = 0.72, 0.8, and 0.75 for Cr, Mn, and Fe, respectively). Values of the structural parameters obtained from the best fits for the Cr-doped samples are listed in Table 4. It shows that Cr atoms are in an octahedral environment of Se atoms in all samples. For the 4.6 at. % Cr sample, two different Cr−Se bonds in the first coordination shell were used since the fit with a single Cr−Se scattering path of Cr in an octahedral symmetry was slightly worse (higher misfit and reduced χ2) and the σ2 values larger (suggesting a dispersion of bond distances). For the 11.7% and 17.5 at. % Cr sample, however, the splitting of this Cr−Se path at the first coordination shell did not provide any improvement of the fit. Attempts to fit the coordination number for these paths were consistent with the octahedral symmetry found for the Cr atoms in all samples. In addition, other attempts revealed no contribution of Cr−Cr scattering paths in the first coordination shell, which rules out the presence of Cr clusters. Both XANES and EXAFS at the Cr Kedge showed no clear oxygen environment (i.e., no Cr−O bonds). Comparison of the distances obtained for outer coordination shells with those bond lengths listed on Table 1 shows a Cr neighborhood consistent with Cr atoms substituting Bi sites (see S site in Figure 1) but with a strong local contraction of the bonds. This suggests that not only the neighboring Se atoms displace to accommodate the smaller radius of the TM atom compared to the Bi one, but also the second and third coordination shells relax. This result is consistent with the reduction in the c-axis parameter observed for these samples by XRD characterization (c.f. Table 2). As the amount of Cr in the film increases, σ2 for the Cr−Bi distances is large, which suggests a high dispersion of bond distances and an increasing structural disorder in the crystal with Cr concentration. For the sample with 17.5 at. %, an additional Cr−Cr bond was needed to improve the quality of the fit. This is consistent with the incorporation of Cr in other Bi sites, or with the formation of CrSe in this sample. σ2 for those Cr−Cr bonds is large, which again suggests a bond distance dispersion and disorder. A decrease in the Cr−Se3 bond distance observed for samples with 11.7 at. % and 17.5 at. % might be an indication of CrSe being formed. The local environment obtained for the Mn dopants in Bi2Se3 is listed in Table 5. These results show Mn in a

Table 5. Fitting Results of the EXAFS Analysis at the Mn KEdge for the Mn:Bi2Se3 Samplesa at. % Mn 7.5

12.0

16.1

N R (Å) ± 0.01 σ2 (Å2) ± 15% N R (Å) ± 0.02 σ2 (Å2) ± 20% N R (Å) ± 0.02 σ2 (Å2) ± 20%

Mn−Se1

Mn−Bi1

Mn−Mn1

Mn−Se2

6 2.72 0.014 6 2.70 0.014 6 2.69 0.014

3 4.15 0.010 3 4.05 0.014 3 4.08 0.031

3 3.97 0.027 3 3.88 0.026 6 3.91 0.031

3 4.58 0.014 3 4.49 0.014 3 4.43 0.022

a

Coordination number, N, interatomic distance, R, and Debye−Waller factor, σ2, for each path. Values of ΔE0 around −0.6 ± 0.3 eV were obtained from the fit for all Mn samples.

octahedral environment of Se atoms with a distance around 2.70 Å for all samples, which is consistent with Mn substitutional in the Bi sites (S site), interstitial in the van der Waals gap (I1 site), as well as part of the MnSe compound.15 It is very difficult to disentangle each of these different locations by considering only the first coordination shell, so that it is important to analyze the results for outer shells. For the sample with 7.5 at. % Mn the distances found for outer shells are larger than those expected for substitutional Mn. This suggests that, contrary to the Cr case, the local neighborhood of the Mn atoms, beyond the first Se neighboring atoms, does not contract to accommodate the smaller ionic radius of Mn (compared to that of Bi); another possibility is that Mn enters into the van der Waals gap (I1 site), where the outer coordination shells are located at larger distances.15 This latter scenario would explain the c lattice expansion observed by XRD (c.f. Table 2). As the concentration of Mn in the sample increases, the Mn−Mn and Mn−Se distances of the outer shells decrease, suggesting the formation of MnSe compound. σ2 values are high in all cases, which can be understood as a high dispersion of bond distances and an increasing structural disorder in the crystal with incorporation of Mn. The latter result is consistent with recently reported XRD results in these samples.22 The results of the EXAFS fits for Fe:Bi2Se3 films are listed in Table 6. The first coordination shell for the Fe-doped samples reveals the presence of Se neighbors, similar to the Cr and Mn cases, although the octahedral symmetry has reduced (we found five Se nearest neighbors instead of six). A short Bi scattering path (around 2.7 Å) was also included in the fit model since it showed to improve the quality of the fit. This 17349

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The Journal of Physical Chemistry C Table 6. Fitting Results of EXAFS Analysis at the Fe K-Edge for Fe:Bi2Se3 Samplesa at. % Fe 6.7

14.8

N R (Å) ± 0.02 σ2 (Å2) ± 20% N R (Å) ± 0.02 σ2 (Å2) ± 20%

Fe−O1

Fe−Se1

Fe−Bi1

Fe−Fe1

Fe−Se2

2 1.91 0.010 2 1.95 0.024

5 2.41 0.010 5 2.37 0.007

3 2.74 0.018 3 2.67 0.032

2 3.36 0.018 2 3.53 0.022

3 4.34 0.013 3 4.30 0.007

a Coordination number, N, interatomic distance, R, and Debye−Waller factor, σ2, for each path. Values of ΔE0 around −4.0 ± 0.3 eV were obtained from the fit for all Fe samples.

We have demonstrated that Cr doping is more effective in substituting Bi in the Bi2Se3 lattice than Mn or Fe doping. The latter two dopants form other phases and incorporate into interstitial sites, such as in the van der Waals gap as well as in between the Se−Bi layers, even for low dopant concentrations (e.g., 7.5 at. % Mn and 6.7 at. % Fe). For high concentrations (>14.8 at. %), the EXAFS results evidence the formation of TM selenides in all cases (i.e., CrSe, MnSe, and FeSe, respectively). Our results are of paramount significance for the magnetic properties of these thin films.16,21,22 For instance, we have shown that Cr doped Bi2Se3 films are ferromagnetic below 12.5 K,21 while Mn doped samples shows a soft ferromagnetic response below 1.5 K.22 This suggests that an effective incorporation of the magnetic dopants into the lattice is important to improve the magnetic properties of the films. Introducing such a ferromagnetic long-range order by TM doping has been shown to break TRS, which is a prerequisite for unlocking exotic physical states.

distance is consistent with Fe entering into an interstitial site of the lattice,15 in between Se and Bi layers (I2 site in Figure 1). σ2 values are high in all cases, which, similar to the Mn case, can be understood as a high dispersion of bond distances and an increasing structural disorder in the crystal with incorporation of Fe. The Fe samples reveal some degree of oxidation, evident from the peaks at low R in the FT curves (Figure 8a) and given that a Fe−O scattering path around 1.95 Å had to be included to model the EXAFS signal (Table 6). The EXAFS analysis indicates that the Cr, Mn, and Fe dopants occupy octahedral sites in the Bi2Se3 crystal in agreement with the ab initio calculations for the TM:Bi2Se3(111) surface by Abdalla et al.12 In the latter study, a favorable energy configuration of substitutional Cr, Mn, and Fe on the Bi sites of the Bi2Se3 lattice was found. The Cr− Se distances obtained for the Cr-doped samples in our study, in particular for the 4.6 at. % Cr sample, are remarkably close to those reported therein for substitutional on the Bi sites (S site). Those obtained for the Mn doped samples are also in fair agreement with their calculated ones. In the case of Fe, however, our Fe−Se distances are shorter, suggesting the formation of other Fe−Se phases and agree with the incorporation of Fe into interstitial sites between the Bi and Se layers (I2 site).15 These results indicate that the Cr and Mn atoms in the Bi2Se3 system are substitutional on the Bi sites (S site), in which case the Se atoms need to contract locally toward the TM, compared to the original Bi−Se distance of the undoped film. In the case of Cr, the contraction is around Δd ≈ −0.36 Å, well in agreement with recent ab initio calculations.11,12 For the Mn, the contraction is around Δd ≈ −0.32 Å. This contraction increases the hybridization between the TM and neighboring Se and thus the impurity bands broaden.34

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ASSOCIATED CONTENT

S Supporting Information *

Details of the imaginary component of the Fourier Transform of the EXAFS data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jp511713s.

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AUTHOR INFORMATION

Corresponding Author

*(G.v.d.L) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Diamond Light Source for time on B18 under proposals SP9178, SP11060, and SP11119. We acknowledge the help of A. A. Baker, R. Boada, D. Gianolio, and P. Schönherr during the beamtime preparation and measurements. T.H. acknowledges the John Fell Oxford University Press (OUP) Research Fund.

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SUMMARY AND CONCLUSIONS X-ray absorption spectroscopy at the TM and Se K-edges has enabled us to study the local electronic and structural environment of TM (=Cr, Mn, and Fe) dopants in Bi2Se3 thin films. We demonstrate that the Cr-, Mn-, and Fe- dopants have a 2+ oxidation state when substituting trivalent Bi atoms in the Bi2Se3 matrix. This behavior can be correlated with the covalent character of the TM−Se bond identified in the TMdoped samples, evident from the direct comparison of the XANES at the TM and Se K-edges with relevant standards such as TM-selenides. With incorporation of TM dopants a local structural relaxation of the Bi2Se3 lattice is observed. The resulting contraction of the TM−Se distances strengthens the covalent character of the bond. The crystal relaxes beyond the first coordination shell for the Cr and Fe cases.

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REFERENCES

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