Article pubs.acs.org/crystal
Axiotaxy of Bismuth Telluride Films on InP(111)B by HighTemperature Hot Wall Epitaxy Yukihiko Takagaki,* Bernd Jenichen, Manfred Ramsteiner, and Uwe Jahn Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany ABSTRACT: We report a case of axiotaxy that takes place in growing films of the thermoelectric and topological-insulator material Bi2Te3 on InP(111)B by hot wall epitaxy. The films become poly-crystalline with a Te deficiency when the substrate temperature is barely below the limit for film deposition. In addition to the usual epitaxial component, for which the (0001) plane of Bi2Te3 is parallel to the (111) plane of InP, fiber textures led by the Bi2Te3(112̅0) plane emerge. The fiber axes are aligned to the ⟨111̅⟩- and ⟨772̅⟩-type directions of InP. Although the d-spacing is matched at the heterointerface, it is accomplished by means of coincidence site lattices for the ⟨111̅⟩ alignment.
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INTRODUCTION Bismuth telluride is well known for outstanding thermoelectric properties.1,2 The room-temperature value of the figure of merit ZT = S2σT/κ is among the highest of all reported materials,3 where, S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. The material has recently gained additional recognition as a topological insulator (TI). TIs are a new class of matter distinguished by the existence of conducting gapless surface modes in the bulk band gap energy range.4,5 The spin-momentum-locked Dirac electron-like modes of TIs are attractive for electronics, spintronics, and quantum computation. In growing Bi2Te3 films to fabricate devices, single crystallinity is ideal for TI applications as the helical modes are localized at the surface. Bi 2 Te 3 crystallizes in a rhombohedral structure with the space group D3d5 (R3̅m). Interpreting the rhombohedral system as the hexagonal one, the crystal is described as a stack of quintuple layers (QLs). Five monatomic sheets with a sequence of Te-Bi-Te-Bi-Te in the c-axis direction are linked tightly by covalent bonds. The coupling between these QLs, on the other hand, is weak. Bi2Te3 cleaves easily, exposing the (0001) surface because of the van der Waals bonding between neighboring Te atoms. The role of the (0001) surface is, as a consequence, important in the growth of Bi2Te3 films. That is, the (0001) surface normally serves as the growth front forming (0001)-oriented islands on the substrate surface. In heteroepitaxy, the lattice mismatch for Bi2Te3 films is relatively small on InP substrates.6,7 The (0001) plane of Bi2Te3 is typically oriented to be parallel to the InP(111) plane, where the mismatch is 5.6%. Preferential inplane orientation relationships are thereby the (112̅0) and (110̅ 0) planes of Bi2Te3 being parallel to the (110̅ ) and (112)̅ planes of InP, respectively. © XXXX American Chemical Society
With respect to thermoelectric applications, the high cleavability necessitates Bi2Te3-based materials for power generation or cooling applications to be poly-crystalline. Moreover, nanostructuring can enhance the thermoelectric properties.8,9 The efficiency of Bi2Te3 structures can be improved when one or more dimensions are reduced to the nanometer scale in the form of nanowires or ultrathin films. One may utilize for this aim the self-organized generation of nano-objects, which often takes place in the growth of materials on highly lattice-mismatched substrates or at low temperatures.10 In this paper, we demonstrate a new growth mode of Bi2Te3 films on InP(111)B. When hot-wall epitaxial growth of Bi2Te3 films is carried out at high temperatures, the films are no longer single crystalline. A crystalline order exists nonetheless and turns out to be due to axiotaxy. In general, a film grown on a crystalline substrate may exhibit four types of textures: random texture, fiber texture, epitaxial alignment, or what has been termed axiotaxy, see Figure 1.11 The growth of Bi2Te3 films on InP substrates has been observed to be epitaxial (if not the random texture), i.e., all three crystal axes of the films are fixed with respect to the substrates.6,7 In the case where a polycrystalline film exhibits a fiber texture, one preferential plane is aligned parallel to the surface of the substrate while the rotation around the fiber axis is random. Axiotaxy11−20 is a case where a fiber texture is realized in the grains of a film with the fiber axis being tilted from the surface normal. Rigorous epitaxial alignments are, however, imposed for the grains with the “fiber axis” being normal to the surface. This mixture of epitaxial alignment and fiber texture arising as matching the dspacing of the overlayer to that of the substrate is the Received: December 28, 2016 Revised: February 17, 2017 Published: February 22, 2017 A
DOI: 10.1021/acs.cgd.6b01899 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 1. Types of texture in films grown on crystalline substrates: (a) random orientation, (b) epitaxial alignment, (c) fiber texture, and (d) axiotaxy. The crystal orientation is rotated around the fiber axis in (c) and (d). In (b) and (d), the in-plane lattice parameters and d-spacing, respectively, are matched at the film−substrate interface, as illustrated in the bottom graphics.
Figure 2. ω-2θ scan XRD curves of Bi2Te3 films on InP(111)B. The growth temperature Tg was 398, 393, and 310 °C for the red, green, and blue curves, respectively. The bottom curve shows the powder diffraction pattern taken from a Bi2Te3 piece used as the source. Major reflection peaks from the Bi2Te3 films are identified as (ij.k). The (lll) reflections from the InP substrates are also indicated. The peak identification was made as Bi1Te1 for the red curve. The curves are vertically offset for clarity.
underlying mechanism for axiotaxy.11 Here, we demonstrate axiotaxy in the high-temperature grown Bi 2 Te 3 films. Interestingly, the axiotaxy is induced by the usually unimportant (112̅0) plane of Bi2Te3, and the d-spacing matching is satisfied by coincidence site lattices.
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composition variation for bismuth telluride is accomplished crystallographically by changing the numbers of QLs and Bi bilayers in the unit cell.21−23 Strictly speaking, the crystal structure thus does not remain the same as the chemical composition of Bi2Te3 deviates from the stoichiometry. A resultant structural disorder presumably caused the near disappearance of the peaks at large angles in the red curve. On the other hand, peaks associated with reflections other than (00.3i) emerged in high-temperature-grown films, making the curves look similar to the powder diffraction pattern shown by the bottom curve, which was taken from a Bi2Te3 piece used as the source. The poly-crystallinity is responsible for the roughening of the surface of the films that one finds in the scanning electron micrographs in Figure 3. We emphasize that changing Tg has significant consequences for Bi2Te3 films in the transport properties and the Seebeck coefficient, as previously investigated in similarly grown films in refs 7 and 24. The electrical conduction was of p-type in optimally grown films, whereas n-type conduction took place when the growth temperature was moved away from the optimal window.24 Free carriers are supplied in Bi2Te3 from crystalline defects, and so the dominant defect is indicated to change with Tg. In the literature, annealing approaches have been attempted for enhancing the thermoelectric efficiency as the Seebeck coefficient is strongly affected by the chemical composition of the material. For instance, Lee et al.25 reported an improvement by a factor of 3 of the Seebeck coefficient for Bi2Te3 nanowires with an optimized annealing process as the concentration of crystal defects, especially the edge dislocations, increased. We performed ϕ scan measurements to examine the in-plane alignments in the high-temperature-grown films targeting the (0001)-oriented component. Let us first focus our attention on the red curve in Figure 4. The structural symmetry in the Bi2Te3(0001) plane is 3-fold. The 60°-spaced peaks reveal the
EXPERIMENTAL SECTION
Bi2Te3 films were grown on the InP(111)B substrates using the hot wall epitaxy method. Pieces of Bi2Te3 (0.5 g) were placed in an evacuated cylindrical quartz tube having an inner diameter of 21 mm. A horizontal three zone furnace was employed in the growth to set up a temperature profile. The source temperature was nominally set to 480 °C. InP(111)B substrates were placed in a low-temperature region with various distances from the source typically in a range of 22−25 cm. The growth temperature Tg was varied by changing the distance along a temperature gradient. The InP substrates were immersed in diluted hydrochloric acid (2%) for 10 min prior to the mounting to remove native oxides. The growth duration was 5 h. The samples were cooled to room temperature immediately when the growth was terminated. The main characterization of the films was X-ray diffraction (XRD) measured using PANalytical X’Pert PRO MRD system. The Cu Kα1 radiation and a Ge(220) hybrid monochromator were used.
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RESULTS AND DISCUSSION In Figure 2, the ω-2θ scans are compared between the films grown at the optimal and higher temperatures. The optimally grown film (Tg = 310 °C) was nearly completely (0001) oriented, as evidenced by the almost exclusive appearance in the blue curve of the (00.3i) reflections with i being integers. These reflection peaks shifted their position as Tg increased; see the red and green curves, for which Tg = 398 and 393 °C, respectively. (It should be noted that films no longer grow if Tg is further increased.6) As we have previously discussed in ref 6, the peak shift originates from the fact that the Te content decreases at high temperatures as Te easily evaporates due to the low melting point and high vapor pressure. Using energydispersive X-ray spectroscopy, the chemical composition for the red curve was, in fact, evaluated to be Bi0.49Te0.51. The B
DOI: 10.1021/acs.cgd.6b01899 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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background level) (blue curve). For the situation corresponding to the (10.11) reflection (magenta curve), the split angle even became different between the twins. (As expected for this condition, the main peak was absent apart from the tail of the (10.10) reflection.) The origin of this anomalous behavior is clear in the pole figure displayed in Figure 5. Here, the intensity of the (10.10)
Figure 3. Scanning electron micrographs of Bi2Te3 films on InP(111) B grown at temperatures of (a) 398 and (b) 393 °C. The scale bars show a length of 1 μm. The [11̅0] and [112̅] directions in the InP(111) surface are indicated. The orientations are common for (a) and (b). The dashed circle in (a) marks hexagon-shaped islands.
Figure 5. Pole figure of Bi2Te3 film on InP(111)B grown at 398 °C. The intensity of the (10.10) diffraction is shown using logarithmic gray scales. The measurement was performed for 2° ≤ χ ≤ 96°. Theoretical calculations are shown by the curves. The calculations are presented for only one of the three equivalent in-plane directions for the sake of clarity. In (a), the thick and thin curves correspond to the twin domain pairs. The solid and dashed curves belong to the alignment of the fiber axis to the ⟨111̅⟩- and ⟨772̅⟩-type directions of the substrate, respectively. Two curves exist for each alignment as the angle γ between the (112̅0)-type planes and the (1 1̅ 0 10) plane of Bi2Te3 can be 90° (red and blue curves) or 57.2° (green and orange curves). The “+” symbols indicate the location of the diffraction spots for the (0001)-oriented epitaxial component. In (b), the dotted curves show the original curves of the same colors in (a). For the solid curves, γ was modified to 93.5° for the blue curves and 49° for the green curves.
diffraction when the azimuthal angle ϕ around the surface normal of the substrate and the tilt angle χ are varied is plotted on a logarithmic gray scale. One recognizes the presence of fiber textures that appear as continuous curves superimposed on a number of discrete spots representing epitaxial alignments. Axiotaxy has been observed mostly for metal silicides.11,13−15,18,19 There are also reports in the growth of MnP16 and Ge.20 From the arrangement of the circular curves with regard to the in-plane directions of the substrate surface, the fiber texture is indicated to be associated with the (112̅0) plane. As a matter of fact, we have identified the existence of two types of fiber orientations. As demonstrated by the theoretical curves in Figure 5(a), the fiber axis is aligned to the ⟨111̅⟩- or ⟨772̅⟩-type directions of InP. Here, the solid and dashed curves correspond to each alignment. The feature in the ϕ scan that appeared as if the peak split was merely accidental. Whereas the main peak is associated with the epitaxially aligned component with Bi2Te3(0001)//InP(111), as identified by “+” in Figure 5(a), the side peaks are produced by the fiber texture with Bi2Te3(112̅0)//InP(111̅). In a ϕ-scan measurement, χ is set to the angle that the reflection plane forms with the substrate surface. The separation in ϕ between the main and side peaks thus depends on which reflection is employed for the measurement.
Figure 4. ϕ-scan XRD curves of Bi2Te3 films on InP(111)B. The red, green, magenta, and blue curves are associated with the (10.10), (10.5), (10.11), and (11.18) reflections in films grown at 398 °C. (The red curve in Figure 2 was obtained from the sample.) The top curve is associated with the (224) reflection from the InP substrate. The profile for the (10.10) reflection in a film grown at a temperature of 393 °C is shown by the cyan curve. (The green curve in Figure 2 was obtained from this sample.) The curves are vertically offset for clarity.
coexistence of twins. Unexpectedly, the peaks manifesting the usual in-plane orientation relationships were accompanied by side peaks with a separation of approximately 6°. This is ordinarily regarded as suggesting a mixture of components whose in-plane orientation is rotated by ±6° around the [0001] axis. However, this seeming in-plane rotation is not real as the split angle depended on the reflection chosen for the measurement. That is, the angle was 18° for the (10.5) reflection, as one finds in the green curve, rather than the 6° for the (10.10) reflection. Furthermore, the side peaks were absent for the (11.18) reflection (or their amplitude was below the C
DOI: 10.1021/acs.cgd.6b01899 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Matching d-spacing at the heterointerface is essential in axiotaxy, where the direction of the fiber axis is determined.11 As the matching already provides an energetically favorable condition, the in-plane alignments are of secondary importance. The rotation around the fiber axis can thus be arbitrary. Preferential in-plane alignments nevertheless exist. The contrast of the fiber texture pattern is hence not uniform, as one finds in Figure 5. The d-spacing cannot play a role in establishing an epitaxial orientation relationship if the fiber axis is perpendicular to the surface, giving rise to the coexisting (0001)-oriented epitaxial alignment. The ratio of the d-spacings is given by the a lattice parameters of Bi2Te3 and InP. For the fiber texture aligned to the InP(111)̅ plane, we obtain d Bi 2Te3 dInP
=
from the composition change may become helpful for matching d-spacing. In fact, with the lattice parameter for Bi1Te1,21 the matching in the d-spacing ratio improves to be 2/3 × 0.980 and 4 × 0.998 for the alignments to the (111̅) and (772̅) planes, respectively. The alteration in the chemical composition with Tg results in changes in the Raman spectra6 as they are given as a superposition of the contributions from QLs and Bi bilayers.27 One of these changes provides a convenient criterion for anticipating axiotaxy. The Raman spectrum of Bi2Te3 consists of peaks associated with the A1g1, Eg2, and A1g2 modes of QLs, see the bottom curve in Figure 6. When Bi bilayers are
3 a Bi 2Te3 2 = 0.6468 = × 0.970. 2 aInP 3
Although the d-spacing does not match directly, every third lattice site of Bi2Te3 approximately coincides with every second lattice site of InP. We have confirmed the alignment by directly detecting the Bi2Te3(1120̅ ) and InP(111)̅ reflections. Deviations of δχ = 1.69° and δϕ = 4.2° were thereby nonetheless found. The misalignments plausibly compensated residual dspacing mismatch. (The Bi2Te3(0001) plane in the epitaxially aligned component was also inclined by 0.13° from the InP(111) plane toward the InP[110] direction to accommodate lattice mismatch.26) d-spacing is also matched for the fiber texture aligned to the InP(772̅) plane, where dBi2Te3/dInP = 3.952 = 4 × 0.988. In Figure 5(a), the red and orange curves show good agreements with the experimental behavior, whereas considerable disagreements are recognized for the green and blue curves. Significant improvements can be achieved, however, if the angle γ between the fiber texture plane and the reflection plane is modified. The dotted curves in Figure 5(b) show the original curves in Figure 5(a). As shown by the solid curves, excellent agreements with the experimental data are obtained if γ is set to 49° rather than 57.2° for the green curves and 93.5° rather than 90° for the blue curves. These discrepancies may suggest that the hexagonal crystal structure of Bi2Te3 is distorted to, for instance, the orthorhombic structure by the strains resulting from the thermal expansion mismatch. Axiotaxy was noticed only in samples for which the (00.30)and (00.33)-reflection peaks roughly vanished. For the green curve in Figure 2, these peaks are still strong even though the film is poly-crystalline. The side peaks in the ϕ scan were negligible in this sample, as shown by the cyan curve in Figure 4, implying that axiotaxy was insignificant. Random texture is suggested to blend in first as Tg exceeds a critical value, see Figure 3(b). It then evolves to axiotaxy as the thermal energy increases by further increasing Tg. In Figure 3(a), hexagonshaped islands of the (0001)-oriented component are marked by the dashed circle. A compact film of the (0001)-oriented component that one finds in Figure 4(b) as the underlying layer breaks into hexagon-shaped islands with increasing Tg. The islands rapidly lose the hexagon shape as Tg further increases. Even the (0001)-oriented islands eventually exhibit rounded geometries, becoming indistinguishable from the grains associated with the fiber textures. It is unclear at present whether the Te deficit in the grown films plays a role in inducing axiotaxy or axiotaxy takes place when Tg is high enough where the Te deficit incidentally becomes crucial. Alteration of the lattice parameters resulting
Figure 6. Raman spectra of Bi2Te3 films on InP(111)B. The growth temperature was 398, 393, and 310 °C for the red, green, and blue curves, respectively. The bottom curve was obtained from a Bi2Te3 piece used as the source. The samples correspond to those in Figure 2 shown by the same colors. The peaks due to the A1g1, Eg2, and A1g2 modes of Bi2Te3 are indicated. A frequency-doubled Nd:vanadate continuous-wave laser was used for the excitation at a wavelength of 532.0 nm. The curves are vertically offset for clarity.
incorporated in the layered structures of the BixTe1−x films grown at high temperatures, a new peak associated with the bilayers appears at approximately 92 cm−1. The contribution from the bilayers increases with increasing Tg in comparison to that from the QLs. (Another peak originating from the bilayers emerges in the vicinity of the A1g2 peak;26 the shift in the position of the A1g2 peak with Tg is actually a consequence of this peak dwarfing the A1g2 peak for high Tg.) Axiotaxy takes place in the sample shown as the top curve. One can thus anticipate axiotaxy for the growth conditions we employed in the present work when the peak at 92 cm−1 is stronger than the Eg2 peak. Before concluding, we want to point out that, when Bi2Se3 films are grown on InP(111) at high Tg, single-crystalline (0001) orientation is maintained as long as the growth rate is nonzero.28 Axiotaxy, therefore, does not take place for the most intensively studied TI material Bi2Se3. The lattice mismatch at the Bi2Se3(0001)-InP(111) interface is negligible (0.3%), and so epitaxial alignment should provide the lowest energy state. The Bi2Te3(0001)-InP(111) alignment is energetically less stable as the lattice mismatch is not negligible. This may yield a possibility for axiotaxy to realize a favorable state. If the Te deficiency in Bi2Te3 plays a role for axiotaxy, the composition alteration at high Tg is insignificant for Bi2Se3 in this regard.
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CONCLUSIONS In conclusion, we have demonstrated axiotaxy when Bi2Te3 films are grown by hot-wall epitaxy on InP at high D
DOI: 10.1021/acs.cgd.6b01899 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
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(21) Kim, Y.; Cho, S.; DiVenere, A.; Wong, G. K. L.; Ketterson, J. B. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 155306. (22) Kifune, K.; Kubota, Y.; Matsunaga, T.; Yamada, N. Acta Crystallogr., Sect. B: Struct. Sci. 2005, 61, 492−497. (23) Steiner, H.; Volobuev, V.; Caha, O.; Bauer, G.; Springholz, G.; Holý, V. J. Appl. Crystallogr. 2014, 47, 1889−1900. (24) Takagaki, Y.; Papadogianni, A.; Bierwagen, O. Adv. Elec. Mater. 2015, 1, 1400007. (25) Lee, J.; Kim, J.; Moon, W.; Berger, A.; Lee, J. J. Phys. Chem. C 2012, 116, 19512−19516. (26) Richardella, A.; Kandala, A.; Lee, J. S.; Samarth, N. APL Mater. 2015, 3, 083303. (27) Xu, H.; Song, Y.; Pan, W.; Chen, Q.; Wu, X.; Lu, P.; Gong, Q.; Wang, S. AIP Adv. 2015, 5, 087103. (28) Takagaki, Y.; Jenichen, B. Semicond. Sci. Technol. 2012, 27, 035015.
temperatures. The high growth temperature leads to polycrystallinity in the films, where fiber textures produced by the (112̅0) plane are interwoven. It is atypical for Bi2Te3 that the (112̅0) plane rather than the (0001) plane establishes the growth alignment. The dominant fiber texture is aligned to the (111̅)-type planes of the substrate, where coincidence site lattices enable the d-spacing matching. An additional fiber texture aligned to the (772̅)-type planes has also been identified. These orientation relationships imply that the bonding at the heterointerface cannot be based on van der Waals forces, in contrast to when (0001)-oriented Bi2Te3 films are grown by van der Waals epitaxy. Comparing the fiber texture pattern with theoretical calculations, large distortions of the Bi2Te3 crystal by thermal expansion mismatch have been suggested.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yukihiko Takagaki: 0000-0002-6691-1005 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.cgd.6b01899 Cryst. Growth Des. XXXX, XXX, XXX−XXX
