Growth of Germanene on Al(111) Hindered by ... - ACS Publications

Article Cite This: J. Phys. Chem. C 2019, 123, 12910−12918

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Growth of Germanene on Al(111) Hindered by Surface Alloy Formation Emanuel A. Martínez,† Javier D. Fuhr,‡,§ Oscar Grizzi,‡,§ Esteban A. Sań chez,‡,§ and Esteban D. Cantero*,‡,§ †

Instituto Balseiro, Universidad Nacional de Cuyo, Mendoza, Argentina Centro Atómico Bariloche, Comisión Nacional de Energía Atómica (CNEA), S. C. de Bariloche, Argentina § Instituto de Nanociencia y Nanotecnología - Nodo Bariloche, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), S. C. de Bariloche, Argentina Downloaded via UNIV OF SOUTHERN INDIANA on July 25, 2019 at 01:11:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Obtaining pure group IV 2D films on well-behaved substrates is at present a major goal in materials science and of great interest for the associated industries. This goal still represents a challenge in surface science because often these materials tend to form alloys. As a consequence, some of the proposed 2D films resulted in topics of controversy regarding the top-layer elemental composition and interpretation of the honeycomb patterns measured by STM. Very recently, germanene on Al(111) was proposed to be a system having a larger gap than silicene and a quantum-spin Hall effect. This system was studied by several techniques including scanning tunnel microscopy, lowenergy electron diffraction, photoemission, and density functional theory. None of the techniques used until now have the capability to detect unambiguously the presence of substrate atoms within the ultrathin film (i.e., separated from the corresponding substrate), thus leaving open the question of the composition or purity of the layer. Here we follow previous guidelines to grow a Ge film on Al(111) with the expected 3 × 3 arrangement that was assumed to be characteristic of germanene, and then we study in situ the properties of the films with ion scattering and recoiling spectrometry, a technique particularly suited for determining the elemental composition of the last surface layer. Our results unambiguously show the formation of a mixture of well-ordered Ge and Al atoms for all of the temperatures and conditions tested, in clear disagreement with the pure single germanene layer proposed in previous works. These conclusions led us to investigate by DFT calculations other possible structures compatible with our present results and the previously reported ones. The most favorable alloyed structures obtained by DFT were then compared with new I−V low-energy electron diffraction curves, and from this comparison, a top surface model composed of five Ge atoms and three Al atoms is proposed to replace the germanene model.

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INTRODUCTION

massive application of these systems in real devices is still conjectural, waiting for a more complete understanding and experimental characterization of the basic properties (i.e., the atomic and electronic structure, the interaction with the underlying surface, and the effect of adsorbing other elements). As a sign that this field is not fully developed yet, there is an increasing number of controversies regarding the formation of alloyed surfaces, the migration of adsorbates into the bulk or from the substrate into the top layer, and the interpretation of the scanning tunneling microscopy (STM) patterns.6−8 Among the new 2D systems, germanene has received considerable attention in the past few years because according to density functional theory (DFT) calculations it should present a larger gap than silicene and some specific and useful

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Expanding the family of 2D materials beyond wellcharacterized graphene is at present a major goal in materials science, which demands the combined effort of different synthesis approaches, surface characterizations, and calculations. Free standing one monolayer sheets are readily achievable for graphene, for some layered materials (hexagonal boron nitride and dichalcogenides), and for some organic molecules,2−4 but no other group IV 2D material has been reported yet with such a highly desired property. As an alternative, there is a relatively large family of monolayer films of Si, Ge, and Sn that are grown on different substrates and for which some of the typical properties expected for 2D systems were reported (i.e., a high electron mobility, quantum spin Hall effect, Dirac cones near K points, and even the possibility of manufacturing a transistor).5 Although the number of publications proposing novel and promising systems in this field has been increasing strongly during the last 5 years, the © 2019 American Chemical Society

Received: March 19, 2019 Revised: May 6, 2019 Published: May 6, 2019 12910

DOI: 10.1021/acs.jpcc.9b02614 J. Phys. Chem. C 2019, 123, 12910−12918

The Journal of Physical Chemistry C

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optical properties. Because monolayer germanene does not exists as free-standing, all of the work done at present is for germanene grown on a supporting substrate9−22 or hydrogenated and synthesized from CaGe2.23,24 For the first case, the role of the substrate is particularly relevant because often it determines the symmetry of the 2D system on top, the buckling of the layer, the charge transfer between film and substrate, and other properties. Obtaining a full characterization of the 2D layer normally requires a combination of techniques. Low-energy electron diffraction (LEED) and STM are probably the most frequently used techniques for studying the symmetry and the atomic structure, but they are not sensitive to element type. Other element-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) are quantitative, but they lack the surface sensitivity to unambiguously delineate the presence of substrate atoms at the very top from those of the underlying substrate (or substrate atoms that suffered displacements). Photoelectron diffraction and holography have strong potentials because they combine the crystallographic precision of LEED plus composition analysis, as has been demonstrated for 2D systems in ref 25. Ion-based techniques are less frequently used even though they are sensitive to the elements and can have a very high top-layer resolution. As a drawback and depending on the mode of operation, ions can produce surface damage and quantification can be affected by shadowing or focusing effects. Within the ion scattering techniques it has been shown that the combination of a forward scattering geometry to detect direct recoils and time-of-flight techniques (TOF-DRS) results in negligible damage and has a very high sensitivity to top-layer atoms.26−28 In this work, we combine LEED, TOF-DRS, electron energy loss spectroscopy (EELS), and DFT to study the adsorption of Ge on an Al(111) substrate. We follow the adsorption recipes suggested in previous work16−18 and use the saturation of TOF-DRS signals and the disappearance of the Al surface plasmon to determine the complete formation of the monolayer. LEED analysis and the shape of the Ge 3d XPS peak agree with previous publications.16−18 Unexpectedly, TOF-DRS used with different projectiles provides clear proof of the presence of Al atoms within the Ge layer grown on top for a broad range of film thicknesses, in clear contradiction of the expected formation of a pure germanene layer reported in refs 16−18. On the basis of these results, we performed DFT calculations using the (3 × 3) symmetry determined by LEED and a top layer composed of both Al and Ge atoms inferred from TOF-DRS. Several models, including previously proposed ones such as germanene, were used in dynamic LEED calculations and contrasted with LEED I−V curves recorded for coverages of ca. 1 monolayer. The final proposed structural model is composed of five Ge atoms and three Al atoms in the unit cell of the top layer. The dynamic LEED calculations for this model provide better agreement with the experimental I−V curves than do previous models based on a pure germanene termination. Moreover, in this model two of the Ge atoms in the cell are shifted outward, which is in agreement with previous STM measurements.16 The results presented here together with previous ones on Au(111) suggest the necessity of revisiting the systems assumed to be pure terminated Xenes that were traditionally known to form alloys with techniques capable of discerning the presence of substrate atoms in the top layer.

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EXPERIMENTAL METHODS

The experiments were performed on an ultra-high-vacuum (UHV) system working in the low 10−10 mbar range with facilities for auger electron spectroscopy (AES), EELS with monochromatized electrons, ultraviolet photoelectron spectroscopy (UPS), LEED equipped with signal amplification by a single microchannel plate, and TOF-DRS with an observation angle that can be varied continuously from 0 to 60°. For TOFDRS, the UHV chamber is connected to a low-energy (1 to 100 keV) ion accelerator equipped with a switching magnet for mass selection and pulsing optics for TOF analysis. The flight paths for emitted recoils and reflected projectiles are 0.76 m for the variable-angle detector and 1.76 m for some fixed observation angles. We used Ne+, Ar+, and Kr+ projectiles in the energy range of 4 to 10 keV, with a continuous current of 1 to 5 nA measured by a Faraday cup with a ϕ 3 mm aperture. After pulsing, the incident fluence is further reduced by almost 3 orders of magnitude, resulting in negligible damage even after acquiring several successive spectra for the same sample condition. The beam shape can be adjusted by variableaperture slits using a typical size of 1 mm. More details about the instrument can be found in ref 28. For comparison with the existing literature, some XPS spectra were acquired in a separate setup. The sample consisted of a commercial Al(111) single crystal, which was cleaned and prepared by cycles of 1.5 keV Ar+ sputtering followed by annealing at 400 °C. The Ge deposition was performed in situ by means of an evaporator consisting of a BN crucible inserted into a Ta cover that is heated by electron bombardment. The evaporation rate was maintained on the order of 3 to 4 monolayers per hour. During evaporation, the pressure increased to a few 10−9 mbar. The sample temperature Tsample during evaporations was kept in the range of 100 to 140 °C, where the LEED patterns were sharper. This temperature range lies between those reported in refs 16 and 18. The sample was mounted on a goniometer with three positional axes: rotation capacity around the azimuthal direction (θazim) and rotation of the sample normal (αinc) in the plane formed by the incoming beam direction and the detector direction. Grazing incidence corresponds to αinc = 0°, while incidence normal to the surface corresponds to αinc = 90°. The azimuthal angle was labeled as θazim = 0° when the sample was oriented in the [1̅01] direction contained in the plane of the incoming beam and the detector. A schematic representation of the experimental geometry is presented in Figure 1.

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THEORY The DFT calculations have been carried out within the slabsupercell approach by using the Vienna ab initio simulation package (VASP).29,30 The one-electron Kohn−Sham orbitals are expanded in a plane-wave basis set, and electron−ion interactions are described through the PAW_PBE pseudopotentials.31,32 Exchange and correlation (XC) are described within the PBE functional.33 The sampling of the Brillouin zone is carried out according to the Monkhorst−Pack method.34 The chosen cutoff energy is 400 eV, electron smearing is introduced following the Methfessel−Paxton technique35 with σ = 0.1 eV, and all of the energies are extrapolated to zero absolute temperature. The convergence of the energy is always kept on the order of 10−4 eV. For each 12911


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where E(Ge−Al(111)) and E(Al(111)) are the total energies for the corresponding Ge/Al(111) structure and the clean Al(111), respectively. NAl and NGe are the numbers of Al and Ge atoms, respectively, in the top layer. For the adsorption energy calculation, we took bulk energy E(Al bulk) as the reference for Al atoms, in the case of an alloyed surface, and the isolated Ge atom energy E(Ge atom). The LEED calculations were performed using the AQuaLEED package,36 which is based on the Barbieri/Van Hove SATLEED package.37

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RESULTS AND DISCUSSION Growth of Ge Films by Evaporation. The growth of Ge was first characterized using LEED and TOF-DRS. For all stages of Ge deposition, the obtained LEED patterns were mostly consistent with the accepted (3 × 3) structure.16 In some initial stages, we could also observe spots with very low intensity characteristic of a ( 7 × 7 )R19.1° structure as was described in ref 18. No other type of reconstruction was observed for sample temperatures of between 100 and 140 °C during the evaporation. Typical diffraction patterns as a function of increasing evaporation times are presented in Figure 2. After ca. 40 min of evaporation, the shape of the spots became less sharp and the background intensity increased. According to the calibration of the growth rate discussed below, a monolayer was completed in the range of 15 to 20 min. Additional LEED patterns observed during growth and for different energies are presented in Figure S1 of the Supporting Information, while the comparison of the experimental I−V LEED curves with calculated ones for different structural models is treated further below, at the end of the Results and Discussion section. The fact that during all evaporation stages the diffraction pattern remains (3 × 3) is on one hand beneficial in the sense that it ensures that we are dealing with the phase reported in the literature.16−18 However, it presents the difficulty of not knowing exactly when a monolayer becomes complete. XPS

Figure 1. Angle definition in TOF-DRS experiments. The incident beam direction, the detector, and the normal to the sample surface are contained in the same plane.

configuration, we performed a full relaxation, except for the two bottom layers, while the forces are assured to be lower than 10−1 eV/nm. For bulk Al, by using a cutoff energy of 400 eV and a 24 × 24 × 24 k-point mesh we obtained a lattice parameter aAl of 0.40397 nm. This result is in excellent agreement with the experimental value of 0.40495 nm at room temperature. The adsorption of Ge onto a Al(111) surface was studied by considering a slab of six layers of Al(111) in a (3 × 3) unit cell representing the substrate, on top of which we considered different possible structures of a Ge or Ge−Al layer. The vacuum-layer region between consecutive slabs is 1.3 nm, thick enough to ensure negligible interactions between periodic images normal to the surface. The adsorption energy per Ge atom was calculated as ÅÄÅ Å E(Ge−Al(111)) − E(Ag(111)) − NAlE(Al bulk) Eads = − ÅÅÅÅ ÅÅ NGe ÅÇ ÉÑ ÑÑ − E(Ge atom)ÑÑÑÑ ÑÑ ÑÖ

(1)

Figure 2. LEED patterns obtained with electrons at around 75 eV for different Ge evaporation times. 12912


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located at about 8.7 μs. The strong signal at around 6 μs corresponds to the scattering of projectiles off Al and off Ge. Along the [3̅12] azimuthal direction, as the evaporation time increases the Al peak is reduced while at the same time the Ge peak grows, reaching a stable condition at about 15 min of evaporation, which we assume to correspond to the completion of the first monolayer. In agreement with this behavior, the Ar scattering peak shifts toward lower TOF due to the scattering off (heavier) Ge atoms. Careful observation of the Al peaks shows that its intensity decreases but does not disappears completely. This point is important and will be developed further below. In Figure 3b, we show the corresponding EELS spectra, which were obtained using an electron beam of 62 eV impinging at 45° from the sample normal and with the detector collecting the electrons backscattered in the specular direction in a narrow cone of approximately 2°. For the clean surface, a clear peak appears at around 10.7 eV energy loss, which corresponds to the excitation of a surface plasmon in Al.39 During evaporation, the signal from the Al surface plasmon decays very swiftly, and the energy loss peak shape evolves toward a broader peak, which has a contribution from the excitation of the remaining Al bulk plasmon and from Ge excitation.39 The final form of the loss features seems to have components expected for bulk Ge plus some less intense contribution in the lower energy loss region, close to that calculated for germanene in ref 40. The fast attenuation of the Al surface plasmon can therefore be considered to be an indication that the first monolayer is achieved at around 15 min of evaporation. We present in Figure 4 the area (integral of the peaks after background

measurements performed in a different setup also indicated a continuous growth of the Ge intensity becoming difficult to establish the point of attaining the monolayer with precision. The comparison between the Ge 3d peak measured in this setup with those reported in refs 17 and 18 presents a good agreement (accepting the different resolution of a commercial XPS system and the synchrotron based one) (Figure S3 of the Supporting Information). To get information about the formation of a single layer, we performed a combined TOF-DRS and EELS study performed at low electron energies as a function of evaporation time. We followed the decreasing Al recoil signal coming from the top layer together with the increasing Ge recoil signal. Because of the high surface sensitivity resulting from shadowing and blocking of the ion trajectories, for a continuous layer or Frank−van der Merwe type growth we expect that the substrate signal should disappear completely38 around one monolayer and that of Ge should attain a constant value. To check this, we used the azimuthal direction θazim = 19° where the Al recoil signal is particularly high (i.e., along an open direction where focusing onto Al atoms results in a high Al recoil peak).26 At the same evaporation times, we followed the EELS signal corresponding to the Al surface plasmon which should be more affected by the evaporation of Ge than the bulk plasmon (which was also measured, Supporting Information). At the electron energies used here (62 eV), the surface plasmon is observed best under the specular condition. In Figure 3a, we present TOF-DRS spectra obtained with Ar+ (4.2 keV) projectiles. The main experimental parameters,

Figure 4. Integral of the Al and Ge direct recoil and plasmon peaks from the spectra in Figure 3 during Ge evaporation. The areas of the Al DR peak and the surface plasmon peak were normalized to 1 at zero evaporation time. The inset shows an expansion of the DR intensities for long evaporation times.

Figure 3. (a) TOF-DRS spectra obtained with Ar+ (4.2 keV) along 19° azimuthal direction [3̅ 12] for 13° incidence during Ge evaporation. (b) Energy loss spectra for 62 eV electrons during Ge evaporation. Top spectra correspond to the clean surface; numbers indicate the evaporation time in minutes.

subtraction) of the recoils and plasmon signals as a function of evaporation time, where the behavior described above can be clearly observed. EELS spectra for the evaporated sequence recorded with the electron detector placed along a nonspecular direction to see the bulk Al plasmon better are presented in Figure S4 of the Supporting Information. Coexistence of Ge and Al Atoms at the Top Layer. We mentioned above that after 29 min of evaporation a certain amount of Al recoil signal is still present, without decaying in time (inset of Figure 4), which means that a fraction of Al atoms from the top layer are detected after the completion of the first monolayer and beyond. In TOF-DRS, the impinging

using the notation described in the previous section, were αinc = 13°, θazim = 19° (recoils scattered in the [3̅12]direction), ψscat = 30°, Ldet = 0.75 m, and Tsample = 120 °C. A low scattering angle of 30° was chosen here because with Ar projectiles the Al recoil peak appears on the left side of the high Ar scattering peak where there is no other contribution and can thus be easily separated. The spectra, from top to bottom, correspond to the scattering from the clean Al(111) surface until 29 min of Ge evaporation. The peak in the 5−5.5 μs region corresponds to direct recoils of Al, while the Ge recoils produce a peak 12913


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The Journal of Physical Chemistry C and departing atom trajectories are governed by a combination of shadowing and blocking effects respectively, with some focusing effects at the edges of shadowing and blocking cones which have a radius on the order of 0.1 nm. As a result, the particles that reach the detector must come mainly from the outermost surface layer.26 This condition is enhanced by choosing heavy projectiles, such as Ar or Kr at relatively low energy. In this section, based on these effects, we present additional proofs of the coexistence of Al and Ge atoms at the top layer and discuss their ratio. First and in order to illustrate the high sensitivity of TOFDRS to the crystallographic structure, we show in Figure 5a

Figure 6. (a) TOF-DRS spectra obtained with Ar+ (4.2 keV) along the 0° azimuthal direction at 13° incidence for the clean Al(111) substrate and after 10 min of Ge evaporation. (b) Integral of the recoil peak areas divided by the recoiling cross section plotted vs azimuthal angle.

Very often and depending on the combination of projectile type and geometry chosen for the TOF-DRS spectra, recoil peaks can be very small in comparison with the contribution of the projectile scattering from the substrate because of the higher cross section and the many multiple paths that the latter can undergo. This can be worse when the masses involved are such that the recoil peak appears at the tail of the high TOF side, as happens for Ge recoils excited by Ar. In this case, one can opt to use a heavier projectile, such as Kr where the contribution from Kr−Al scattering will not be present at 45° or higher scattering angles. In this case, the spectrum is composed of Al and Ge recoils plus some Kr scattering from Ge. Typical spectra for 5 keV Kr+ ions, for two azimuthal orientations of the sample, are shown in Figure 7. Here the

Figure 5. (a) TOF-DRS spectra obtained with Ar+ (4.2 keV) along three azimuthal directions for 13° incidence on the clean Al(111) substrate. The inset shows an expanded view around the Al recoil peak. (b) Area of the Al recoil peak versus azimuthal angle (in polar coordinates) superposed on the schematics of the top Al(111) surface.

spectra taken with Ar+ projectiles along three azimuths of the clean (before evaporation) Al surface. Ar scattering is observed with high intensity at the three azimuths, and the Al recoil has a lower intensity and presents a strong dependence on the azimuth. Along the azimuth corresponding to the shortest interatomic distance (θazim = 0°), no Al recoils are produced because of blocking, while they can be observed along θazim = 30° and θazim = 19°. In particular, for the latest there is strong focusing by lateral atoms which generates a very high intensity, as evidenced in panel b of Figure 5 upon plotting the integral of the Al recoil peaks (green line) measured as a function of azimuthal angle (here plotted in polar coordinates). In this case, the variations are easily interpreted from the known crystallography of the clean surface. After 10 min of Ge evaporation, the spectrum along 0° shows a clear increase in the Al recoil intensity (Figure 6a) that suggests the existence of Al atoms at the top layer. On the contrary, if the layer were composed of bulk Al termination with Ge on top, then this increase in the Al signal could not happen. A full azimuthal angle scan, obtained as the areas of Al and Ge recoil peaks in TOF-DRS spectra recorded at different azimuths, is shown in Figure 6b for approximately full monolayer coverage. Here we observe that the Al recoil signal is present over the entire studied angular range and, in particular, in the region between −10° and 10° azimuthal angles. From this figure, we conclude that at full coverage (a) there are Al and Ge atoms exposed at the top surface, (b) the surface is well ordered, (c) Al atoms are located in different positions from those of the clean surface, and (d) the Al atoms at the top are not coming from defects (they are well-ordered) and their amount is comparable to that of Ge. These results are more consistent with the formation of an alloyed surface.

Figure 7. TOF-DRS spectra obtained with Kr+ (5 keV) along the [1̅01] (azimuthal angle = 0°) and [2̅11] (azimuthal angle = 30°) directions at 20° incidence for (a) the clean surface and (b) after 30 min of evaporation time. (Inset) Expanded view around the position of the true Al direct recoil.

geometry was αinc = 20°, θazim = 0 and 30° (recoils scattered along the [1̅01] and [2̅11] directions), ψscat = 45°, and Ldet = 1.76 m. Under this condition, the cross sections are similar. Before evaporation, the spectra for both azimuthal directions have a clearly different shape due to the crystallographic order of the sample. At 0°, the Al contribution appears broader and shifted to lower TOF and is composed mainly of multiple collisions (i.e., no single or true direct recoiling should be present along this direction, inset Figure 7a). At 30°, it appears narrow and at the TOF corresponding to the true direct recoil position. As the evaporation takes place, the Ge recoil peak and the peak corresponding to Kr scattered by Ge are seen clearly (Figure 7b). For long evaporation times (30 min), the spectra 12914


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Also, the Al peak now appears at the tail of the very strong Ne scattering peak, making the integration of the Al recoil more difficult. The areas of the recoil peaks weighted by recoil cross sections give Ge/Al ratios within the range of 0.6 < Ge/Al < 1.5. Similar ratios were obtained for larger incident angles and for a broad range of Ge evaporation times ranging from submonolayer to the equivalent of several layers, thus revealing a strong relocation of Al atoms within the topmost Ge layer. Possible Ge/Al Crystallographic Models Inferred from DFT Calculations. We explored by means of DFT relaxations possible structures compatible with the experimental data presented above. We restrict the calculations to a (3 × 3) unit cell compatible with the LEED pattern. We first calculated the previously proposed model13−15 with 8 Ge atoms in the top layer corresponding to a (2 × 2) germanene. The relaxed configuration is shown in Figure 9 (structure 1) and has an

still show a strong contribution of Al recoils, and more importantly, along 0° it shifts to the position of the true direct recoil, a condition that can be obtained only if the surface is reordered, having Al atoms exposed at the top layer, confirming the above discussion resulting from Ar excitation. The peak of Al recoils can be followed beyond 50 min of evaporation time and also at grazing incidence angles (Supporting Information, Figures S5 and S6), which confirms the assumption of a top surface composed of Al and Ge atoms. Ratio of Ge and Al Atoms at the Top Layer. We mentioned above that interpretation or identification of the recoil intensities versus the angles is relatively easy when the crystallography of the surface is known (as in the case of the clean surface), but the reverse and more interesting problem of generating a surface crystallographic model from recoil intensities alone can be extremely complex, particularly if there is more than one type of atom located at different heights. Instead of this approach, we performed calculations based on density functional theory to generate possible models that afterward were contrasted with all of the experimental evidence. This is described in the next section. The models to be considered should be consistent with (3 × 3) symmetry (obtained from LEED) and contain both Al and Ge atoms. One issue that requires more analysis is the relative amounts of Ge and Al at the top layer. TOF-DRS has enough sensitivity to delineate top-layer contributions from subsurface contributions, but this property comes from the strong shadowing and focusing effects which make quantitative analysis less precise. Note, for example, that with Ar projectiles along the 19° azimuth the Ge recoil intensity is higher than that of Al recoils while with Kr along 0 and 30° azimuths we observe the opposite. In both cases, the ratio is within a factor of 2. If we choose a lighter projectile, we get less shadowing and focusing but we can lose surface sensitivity. A compromise is obtained by using Ne projectiles at somewhat higher energies, around 10 keV. A scan for different azimuthal directions of the sample using 10 keV Ne+ is presented in Figure 8 for the experimental

Figure 9. Relaxed configuration for the highest-adsorption-energy structures. Structures 1 and 2 correspond to configurations with pure Ge in the top layer, while structures 3 and 4 correspond to Ge−Al alloy in the top layer. Ge atoms are colored green, cyan, and orange depending on the height. Al atoms are colored dark gray for the underlying layers and light gray for Al atoms in the top layer.

adsorption energy of 4.32 eV per Ge atom. To explore other configurations with only Ge atoms in the top layer, we performed an ab initio molecular dynamics (AIMD) simulation at 80 °C starting from structure 1 and identified possible local minima in the energy landscape. In this way, we found structure 2 shown in Figure 9 which has a higher adsorption energy of 4.36 eV per Ge atom. This configuration can be seen as large 12-atom Ge rings, with a Ge-trimer inside each of them. Starting from these two configurations, structures 1 and 2, we considered alloyed structures in which some Ge atoms were replaced by Al atoms. With this procedure, we found structures 3 and 4 of Figure 9 which have almost the same adsorption energy of 4.43 eV per Ge atom (i.e., 70 meV higher than the best configuration with only Ge atoms). Structure 3 corresponds to a (2 × 2) honeycomb structure with Ge and

Figure 8. TOF-DRS spectra obtained with Ne+ (10 keV) at 13° incidence for different azimuthal angles after 30 min of Ge evaporation.

conditions: αinc = 13°, ψscat = 45°, Ldet = 1.76 m, and 30 min of evaporation. We observe that the dependence of the recoiling peaks on the azimuthal angle is lower than with heavier projectiles (as in Figure 6b), indicating that the influence of shadowing and focusing effects is less important. However, that influence cannot be ruled out to obtain a precise Ge/Al ratio. 12915


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other two spots could have contributions from clean Al(111) areas if the Ge overlayer does not cover all of the sample. First, there is a clear disagreement with the experimental I− V curves of the simulated ones for the structures which have only Ge atoms in the top layer (structures 1 and 2). This is particularly evident in the (2/3 −1/3) and (2/3 −2/3) spots, where the simulated curves show prominent peaks at high energy (>100 eV) not observed in the experimental curves, and also the experimental broad peak below 100 eV in the (2/ 3 −2/3) spot is not reproduced by either of these two structures. The simulated I−V curves of both alloyed structures, structures 3 and 4, improve the agreement with the experimental ones. Structure 4 is the one with the best agreement, particularly in the (2/3 −2/3) spot where the I−V curve corresponding to structure 3 presents a peak above 100 eV not shown experimentally. We can conclude that the simulated I−V curves corresponding to structure 4 show all of the main features present in the experimental curves for all of the fractional spots.

Al atoms intercalated in a 1:1 ratio. On the other hand, structure 4 corresponds to an equal mixture of Ge and Al atoms in the large rings of structure 2 while maintaining the Ge trimer inside them, resulting in a Ge/Al ratio of 5:3. We tried many other configurations starting from structures 1 and 2 and considering other possible Al replacement of Ge atoms. We also performed several AIMDs starting from structures 3 and 4 to identify other possible local minimum configurations. None of the obtained relaxed structures had higher adsorption energies. Structure 3 has one Ge atom shifted outward, while structure 4 has two higher Ge atoms. This is consistent with reported STM images of this phase16 where there are only two brilliant spots in the (3 × 3) unit cell. Comparison with I−V LEED Curves. We measured I−V curves for six spots present in the (3 × 3) phase: two of them are also present in the clean Al(111) surface ((1 −1) and (−1 1)), while the other four are fractional ones ((1/3 −1/3), (2/3 −1/3), (−2/3 2/3), and (2/3 −2/3)) that appear only when the (3 × 3) pattern develops. The identification of fractional spots in the LEED patterns is provided in Figure S1 of the Supporting Information We also simulated corresponding I−V curves for the four configurations described above. We note that the LEED simulations were performed with the atomic configurations obtained after the DFT relaxations and that no optimization of the structure was done to fit the experimental I−V curves. The same procedure was previously applied to spots (−1 1) and (1 −1) of the clean surface to check the consistency of the method. This is shown in Figure S2 of the Supporting Information We show in Figure 10 the comparison between experimental and simulated curves only for the fractional spots because the

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CONCLUSIONS We studied the adsorption of Ge on Al(111) under UHV conditions and keeping the sample temperature during the evaporation in the range of 100 to 140 °C. LEED patterns obtained for different evaporation times show a dominant (3 × 3) symmetry consistent with previous reports.16,18 The use of ion scattering at forward angles in combination with the measurement of the Al surface plasmon allows an accurate determination of one monolayer coverage. Independent TOFDRS measurements carried on with Ne+, Ar+, and Kr+ projectiles indicate the presence of Al substrate atoms within the topmost Ge layer. The Al atoms are positioned at wellordered sites, and their quantity is comparable to that of Ge, suggesting the formation of an alloyed surface. This observation was verified from the submonolayer to a few layers and for the entire sample temperatures investigated. In agreement with TOF-DRS results, the highest adsorption energy obtained from DFT calculations occurs for a (3 × 3) cell composed of three Al atoms and five Ge atoms, where two of the later are located slightly above the rest, in agreement with previous STM observations. The proposed DFT model used, as is in dynamic LEED I−V curves (i.e., without the optimization of atomic positions in the LEED calculation), presents much better agreement with experimental curves than other models based on a pure germanene layer and is consistent with the main features of TOF-DRS. From this analysis, we conclude that the current germanene model proposed for the Ge/Al(111) system should be replaced by another one based on an alloyed surface, such as the model obtained in this work. This and our previous results for Ge on Au(111)27 show the necessity of using techniques with high elemental sensitivity to better characterize the role of the substrate in recent studies of 2D materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02614.

Figure 10. Comparison between experimental and simulated LEED I−V curves. For each structure, the simulated I−V curves (red lines) corresponding to four fractional spots ((1/3 −1/3), (2/3 −1/3), (−2/3 2/3), and (2/3 −2/3)) are compared to the experimental data (black circles).

Additional LEED patterns, the XPS Ge 3d peak, additional electron energy loss spectra, and TOF-DRS spectra acquired at grazing incidence (PDF) 12916


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(15) Matusalem, F.; Koda, D. S.; Bechstedt, F.; Marques, M.; Teles, L. K. Deposition of Topological Silicene, Germanene and Stanene on Graphene-covered SiC Substrates. Sci. Rep. 2017, 7, 15700. (16) Derivaz, M.; Dentel, D.; Stephan, R.; Hanf, M.; Mehdaoui, A.; Sonnet, P.; Pirri, C. Continuous Germanene Layer on Al(111). Nano Lett. 2015, 15, 2510−2516. (17) Stephan, R.; Hanf, M. C.; Derivaz, M.; Dentel, D.; Asensio, M. C.; Avila, J.; Mehdaoui, A.; Sonnet, P.; Pirri, C. Germanene on Al(111): Interface Electronic States and Charge Transfer. J. Phys. Chem. C 2016, 120, 1580−1585. (18) Wang, W.; Uhrberg, R. I. G. Coexistence of strongly buckled germanene phases on Al(111). Beilstein J. Nanotechnol. 2017, 8, 1946−1951. (19) Fukaya, Y.; Matsuda, I.; Feng, B.; Mochizuki, I.; Hyodo, T.; Shamoto, S. Asymmetric Structure of Germanene on an Al(111) Surface Studied by Total-Reflection High-Energy Positron Diffraction. 2D Mater. 2016, 3, No. 035019. (20) Fang, J.; Zhao, P.; Chen, G. Germanene Growth on Al(111): A Case Study of Interface Effect. J. Phys. Chem. C 2018, 122, 18669− 18681. (21) Marjaoui, A.; Stephan, R.; Hanf, M. C.; Diani, M.; Sonnet, P. Tailoring the Germanene−Substrate Interactions by Means of Hydrogenation. Phys. Chem. Chem. Phys. 2016, 18, 15667−15672. (22) Zhuang, J.; Liu, C.; Zhou, Z.; Casillas, G.; Feng, H.; Xu, X.; Wang, J.; Hao, W.; Wang, X.; Dou, S. X.; Hu, Z. Dirac Signature in Germanene on Semiconducting Substrate. Advanced Science 2018, 5, 1800207. (23) Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Stability and Exfoliation of Germanane: a Germanium Graphane Analogue. ACS Nano 2013, 7, 4414−4421. (24) Hanks, A.; Jiang, S.; Esser, B. D.; Goldberger, J. E.; McComb, D. W. Electron Diffraction of Germanane. Microsc. Microanal. 2017, 23, 1744−1745. (25) Kuznetsov, M. V.; Ogorodnikov, I. I.; Usachov, D. Y.; Laubschat, C.; Vyalikh, D. V.; Matsui, F.; Yashina, L. V. Photoelectron Diffraction and Holography Studies of 2D Materials and Interfaces. J. Phys. Soc. Jpn. 2018, 87, No. 061005. (26) Rabalais, J. W. Scattering and Recoiling Spectrometry: An Ion’s Eye View of Surface Structure. Science 1990, 250, 521−527. (27) Cantero, E. D.; Solis, L. M.; Tong, Y.; Fuhr, J. D.; Martiarena, M. L.; Grizzi, O.; Sánchez, E. A. Growth of Germanium on Au(111): Formation of Germanene or Intermixing of Au and Ge Atoms? Phys. Chem. Chem. Phys. 2017, 19, 18580−18586. (28) Salazar Alarcón, L.; Jia, J.; Carrera, A.; Esaulov, V. A.; Ascolani, H.; Gayone, J. E.; Sánchez, E. A.; Grizzi, O. Direct Recoil Spectroscopy of Adsorbed Atoms and Self-Assembled Monolayers on Cu(001). Vacuum 2014, 105, 80−87. (29) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (30) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal−Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251−14269. (31) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (32) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13244−13249. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−2868. (34) Monkhorst, H. J.; Pack, D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (35) Methfessel, M.; Paxton, A. T. High-precision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 40, 3616−3621. (36) Lachnitt, J. AQuaLEED software; https://physics.mff.cuni.cz/ kfpp/povrchy/software. (37) Barbieri, A.; Van Hove, M. A. http://www.icts.hkbu.edu.hk/ vanhove/.

AUTHOR INFORMATION

Corresponding Author

*Phone: +54 294 4445100 5392. E-mail: [email protected]. gov.ar. ORCID

Esteban D. Cantero: 0000-0002-3667-5830 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge fruitful discussions with Dr. Marı ́a Luz Martiarena concerning DFT models and ion scattering trajectories and with Dr. Guillermo Zampieri in relation to XPS analysis and partial support from Universidad Nacional de Cuyo (06/C517), CONICET (PIP 112 201501 00274 CO), and ANPCyT (PICT-2015-2589).

(1) Cahangirov, S.; Sahin, H.; Le Lay, G.; Rubio, A. Introduction to the Physics of Silicene and Other 2D Materials. Lectures Notes in Physics; Springer, 2017; Vol. 930, DOI: 10.1007/978-3-319-46572-2. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (3) Wang, J.; Deng, S.; Liu, Z.; Liu, Z. The Rare Two-Dimensional Materials with Dirac Cones. National Science Review 2015, 2, 22−39. (4) Hamoudi, H. Bottom-up Nanoarchitectonics of Two-Dimensional Freestanding Metal Doped Carbon Nanosheet. RSC Adv. 2014, 4, 22035−22041. (5) Tao, L.; Cinquanta, E.; Chiappe, D.; Grazianetti, C.; Fanciulli, M.; Dubey, M.; Molle, A.; Akinwande, D. Silicene Field-Effect Transistors Operating at Room Temperature. Nat. Nanotechnol. 2015, 10, 227−231. (6) Svec, M.; Hapala, P.; Ondracek, M.; Merino, P.; Blanco-Rey, M.; Mutombo, P.; Vondracek, M.; Polyak, Y.; Chab, V.; Martin Gago, J. A.; Jelinek, P. Silicene Versus Two-Dimensional Ordered Silicide: Atomic and Electronic Structure of Si-(√19×√19)R23.4°/Pt(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 201412. (7) Wang, W.; Uhrberg, R. I. G. Investigation of the Atomic and Electronic Structures of Highly Ordered Two-Dimensional Germanium on Au(111). Phys. Rev. Mater. 2017, 1, No. 074002. (8) Peng, W. B.; Xu, T.; Diener, P.; Biadala, L.; Berthe, M.; Pi, X. D.; Borensztein, Y.; Curcella, A.; Bernard, R.; Prevot, G.; Grandidier, B. Resolving the Controversial Existence of Silicene and Germanene Nanosheets Grown on Graphite. ACS Nano 2018, 12, 4754−4760. (9) Acun, A.; Zhang, L.; Bampoulis, P.; Farmanbar, M.; van Houselt, A.; Rudenko, A. N.; Lingenfelder, M.; Brocks, G.; Poelsema, B.; Katsnelson, M. I.; Zandvliet, H. J. W. Germanene: the Germanium Analogue of Graphene. J. Phys.: Condens. Matter 2015, 27, 443002. (10) Wang, Y.; Li, J.; Xiong, J.; Pan, Y.; Ye, M.; Guo, Y.; Zhang, H.; Quhe, R.; Lu, J. Does the Dirac Cone of Germanene Exist on Metal Substrates? Phys. Chem. Chem. Phys. 2016, 18, 19451−19456. (11) Li, L.; Lu, S.; Pan, J.; Qin, Z.; Wang, Y.; Wang, Y.; Cao, G.; Du, S.; Gao, H. Buckled Germanene Formation on Pt(111). Adv. Mater. 2014, 26, 4820−4824. (12) Zhang, L.; Bampoulis, P.; Rudenko, A. N.; Yao, Q.; van Houselt, A.; Poelsema, B.; Katsnelson, M. I.; Zandvliet, Zandvliet H. J. W. Structural and Electronic Properties of Germanene on MoS2. Phys. Rev. Lett. 2016, 116, 256804. (13) Dávila, M. E.; Le Lay, G. Few Layer Epitaxial Germanene: a Novel Two-Dimensional Dirac Material. Sci. Rep. 2016, 6, 20714. (14) Dávila, M. E.; Xian, L.; Cahangirov, S.; Rubio, A.; Le Lay, G. Germanene: a Novel Two-Dimensional Germanium Allotrope Akin to Graphene and Silicene. New J. Phys. 2014, 16, No. 095002. 12917


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The Journal of Physical Chemistry C (38) Diebold, U.; Pan, J. M.; Madey, T. E. Ultrathin Metal Film Growth on TiO2(110): An Overview. Surf. Sci. 1995, 331−333, 845− 854. (39) Thirlwell, J. The Characteristic Energy Losses of Slow Electrons Reflected fromAluminium, Germanium, Copper and Gold. Proc. Phys. Soc., London 1967, 91, 552−564. (40) Rast, L.; Tewary, V. K. Electron Energy Loss Function of Silicene and Germanene Multilayers on Silver. 2013, arXiv:1311.0838 [cond-mat.mtrl-sci].

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