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Tip-Induced Switch of Germanene Atomic Structure Regis Stephan, Mickael Derivaz, Marie-Christine Hanf, Didier Dentel, Natalia Massara, Ahmed Mehdaoui, Philippe Sonnet, and Carmelo Pirri J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02137 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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The Journal of Physical Chemistry Letters

Tip-Induced Switch of Germanene Atomic Structure

Regis Stephan, Mickael Derivaz, Marie-Christine Hanf, Didier Dentel, Natalia Massara, Ahmed Mehdaoui, Philippe Sonnet and Carmelo Pirri*

Institut de Science des Matériaux de Mulhouse IS2M UMR 7361 CNRS, Université de Haute Alsace, 3 bis rue Alfred Werner, 68093, Mulhouse, France *To whom correspondence should be addressed. E-mail: [email protected]

Abstract: A new germanene crystallographic structure is investigated by scanning tunnelling microscopy and density functional theory calculations. We found that germanene can crystallize in two stable but different structures when grown on Al(111) at the same temperature. These structures are evidenced in scanning tunnelling images by a honeycomb contrast and by a hexagonal contrast. These contrasts are relevant to a Ge network with one (hexagonal) or two (honeycomb) Ge atoms per unit cell shifted upwards with respect to the other Ge atoms. These structures appear alternatively and can be turned on and off by a tip-induced process.

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2D materials are widely studied materials, due to the extraordinary physical and chemical properties predicted. These materials are composed of one-atom thick layer and can belong to two well-separated groups. The first one is deduced from layered bulk materials, such as graphene, h-BN single layer, metal dichalcogenides or selenides single layers, and quite all other van der Walls bonded layered materials.1-9 This first group also includes new 2D materials such as antimonene and phosphorene.10-25 These layers are the 2D counterpart of the elementary bricks of the 3D crystals. They have been obtained by exfoliation and have been thoroughly studied for the last ten years. Their own physical and chemical properties are now routinely studied and those 2D materials are used to form the so-called van der Walls heterostructures or components. To the second group belong 2D materials such as silicene, germanene and stanene, whose crystallographic structure is not a simple replica of bulk material elementary bricks and which have been predicted theoretically a few years ago. These 2D materials have been synthesized recently on metallic substrates first, and then on band gap materials.26-70 Note that none of the 2D materials of the second group has been exfoliated from the substrate, and this is still challenging. This has made and still makes for some people the label “2D materials” quite questionable. We will show in this work that germanene can adopt different atomic configurations close each other, with a faint germanene-Al(111) interaction, and so that germanene exfoliation could be feasible. The growth of germanene was a great challenge since the sp3 hybridization of Ge atoms seemed energetically favourable with respect to the sp2 configuration expected for hypothetical planar 2D sheet. The germanene formation needs a substrate, metallic or not. It has been first successfully grown by Ge deposition on metallic substrates, namely Au(111)57,58, Pt(111)59, Ge2Pt60,61, Al(111)62,64, Highly Oriented Pyrolitic Graphite (HOPG)65 and Cu(111)66 with a honeycomb surface structure, and later on MoS267 and Sb.68 One key parameter is the germanene buckling since it is relevant of a band gap opening in the germanene band structure. This gap 2


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opening makes germanene very interesting with respect to novel physical properties, like the quantum Hall effect, due to a sizeable spin-orbit coupling at a temperature high enough to be measured. 72-74 The way germanene accommodates on the substrate governs its crystallographic structure and then its properties. This reflects in the different proposed germanene structural models, depending on the substrate on which they are formed, and even on the same substrate, as shown below. This is due to the delicate balance between several possible Ge accommodations on a given substrate, through a stable (or meta-stable) atomic structure associated with the faint interaction of 2D material with the substrate. This is observed for silicene on Ag(111)33 and for germanene on Au(111)57 for instance, characterized by different low energy electron diffraction (LEED) superstructure co-existing on the same substrate within the same growth conditions. A dramatic consequence is sometimes the formation of size-limited sheets. The 2D germanene growth on Al(111), observed by scanning tunnelling microscopy (STM) exhibits only one single sheet orientation and surface periodicity over large Al(111) areas.62 It forms a continuous layer, with a (3x3) lattice periodicity with respect to the Al(111) surface periodicity. DFT study has already identified an atomic structure of the buckled germanene film, with a buckling of 1.23 Å, consistent with the experimental data. The unit cell is formed by eight Ge atoms arranged in a honeycomb network, with two Ge atoms displaced upwards in a top position with respect to the other Ge atoms. The electronic structure investigated by DFT clearly shows an electrostatic interaction between the germanene layer and the Al substrate. Also found is an only weak chemical interaction (no covalent bonds) between the Ge layer and the Al surface. More recently, Fukaya et al.75 investigated the (3x3) germanene layer deposited on Al(111) using a total reflection high-energy positron diffraction (without STM measurements). Their study reveals the presence of an asymmetric buckling due to the protrusion of only one Ge atom in a top position, 3



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in contrast with the model proposed in ref

62

with two Ge atoms in top positions. Although the

germanene growth seems to be made in the same experimental conditions, the model proposed by Fukaya et al. is no longer consistent with the STM images shown in ref.62 The aim of this paper is to show that Ge atoms can accommodate two stable long-range atomic structures with different registry on Al(111). The (3x3) germanene periodicity with respect to the Al(111) surface periodicity is preserved for both registries, but two different sets of STM images are measured in the atomic resolution mode. Each STM image set corresponds to a given germanene atomic structure as predicted by DFT calculations. DFT calculations have been performed using the Vienna ab initio simulation package (VASP)76-79 within the projector augmented plane-wave (PAW) method.80,81 The generalized gradient approximation (GGA) has been taken to describe the electron-electron exchange correlation interactions, using the functional of Perdew, Burke, and Ernzerhof (PBE)82,83. The Brillouin zone is sampled using (5x5x1) k-points. The cut-off energy value for plane-wave is 312.3 eV, and the system is relaxed until the components of the forces are lower than 0.005 eV. Å-1. The system is described by means of a slab constituted of 10 layers of 9 Al atoms, one Ge layer of 8 atoms, and 1.7 nm vacuum spacing. The bottom Al layer is kept fixed during the relaxation procedure. Electronic charges are obtained using the Bader charge approach, with a precision, for a total of 302 electrons, of 2x10-4 e-.84,85 The energy barriers have been calculated by means of the climbing image nudged elastic band (cNEB) method.86 The WSXM software has been used to process the STM experimental images,87 while the atomic models and charge density map are presented with the visual molecular dynamics software developed by the Theoretical and Computational Biophysics Group in the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana-Champaign.88 Sample preparation and STM measurements have been carried out in ultrahigh vacuum 4


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(base pressure: 3 × 10−11 mbar). A clean Al(111) substrate was obtained after numerous sputtering / 500°C annealing cycles. Surface order was checked by the presence of a sharp Al (1 × 1) LEED pattern. We looked for possible contaminants such as oxygen or carbon by means of X-ray photoemission (XPS) as well as STM (Omicron Nanotechnology@ GmbH). Ge was evaporated on the Al(111) substrate set at 85°C, with a commercial evaporation cell (MBE Komponenten@ GmbH), and a rate of about 0.005 nm/min. The Ge amount, measured by means of a quartz microbalance and by XPS, equals that of one germanene monolayer (ML).

Figure 1: STM images of a germanene layer deposited on Al(111) for V = +1.0V: (a) large scale (267 x 267 nm) image, (b) honeycomb contrast (8.5 x 8.1 nm) and (c) hexagonal

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contrast (8.5 x 8.1 nm). STM images displaying the contrast change from the hexagonal (lower part of the figure) to the honeycomb (upper part) contrast (5.4 x 4.3 nm) (d) and from the honeycomb (lower part) to the hexagonal (upper part) one (10.0 x 7.9 nm) (e). The black rhombuses indicate the germanene unit mesh. The vertical dashed line on (e) emphasizes the registry difference between the two structures.

For all growth conditions in this work, LEED shows a (3x3) superstructure, as shown in ref.62. Figure 1a displays a large scale STM image and exhibits a flat and uniform germanene layer. Both (3 x 3) LEED pattern and large scale STM image are relevant of germanene formation within the Al(111) sample temperature range during deposition (between about 65°C and 110°C, with an accuracy of about 20°C). Figure 1b and c show high-resolution STM images of a germanene layer at a sample bias U = + 1 V obtained within exactly the same experimental conditions as already published in ref.

62

. The honeycomb contrast in Figure 1b is relevant of a

buckled germanene layer with two topmost Ge atoms per unit cell, located on top of the underlying Al atoms62. This model will be called Germanene-2T. The germanene layer has been also sometimes post-annealed and it is found to be stable up to about 170°C, with the same (3 x 3) LEED superstructure and the same atomic scale STM images, and thus effects described below work for annealing temperatures up to 170°C. Figure 1c displays a high resolution image of the same sample but with a different contrast. The honeycomb contrast is lost and germanene exhibits now a hexagonal periodicity. This contrast, at the same sample bias, can be turned on and off with some voltage pulses applied to the tip. Contrast modifications from that in Figure 1b to that in Figure 1c is even observed during image collection without applying a voltage pulse. Then, we can alternatively image the germanene surface with one or the other of the contrasts. Such a contrast change is generally 6


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attributed to tip apex termination modification during the scan.

Nevertheless, this contrast

change in STM images is observed for all germanene layers prepared on Al(111), and for different STM tips. Thus this effect is completely reproducible, and it seems not associated with a specific tip. Alternatively, this contrast change could be due to sample – tip interaction, and then germanene structure modification during the scan. This last comment suggests that germanene could have two distinct configurations on Al(111), at least. High resolution images showing the STM image contrast change during STM image measurement are shown in Figure 1d and 1e. The most striking feature is that the bright protrusions (Ge top atoms) of the honeycomb contrast are not located on the same registry than those of the hexagonal one. This suggests that germanene could accommodate in various ways on Al(111), with different adsorption sites for instance, as shown below. Further investigations of germanene formation on Al(111) give very surprising features, accordingly. Note although that the discovery of these two stable germanene structures on Al(111) would be in line with the choice of Al as substrate for germanene formation, since the Al(111) surface does not reconstruct, and presents an electronic structure mainly characterized by s-type orbitals. As a result, the interaction between germanene and Al(111) should be weak, and small structural germanene changes could be expected. DFT calculations have been performed for adsorption registry of germanene on Al(111) compatible with the (3x3) surface superstructure. To explain the contrast observed in Figure 1c, new germanene structural configurations have been considered, with only one upwards shifted Ge atom per (3x3) unit cell, located on top, bridge, and hollow sites of the Al(111) topmost layer. The latter configurations will be called Germanene-1T, 1B and 1H respectively, and are displayed in Figure 2a, 2b and 2c before atomic relaxation, respectively. After structural relaxation, the adsorption energy of the 1H and 1T models are -0.48 eV 7



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and -0.47 eV, respectively, while the 1B model converged towards an atomic structure close to that of the 1H one, with the same adsorption energy. It appears that the 1T system is less stable than the 1H one. Moreover, there is no registry change between the 2T and the 1T structure that conflicts with the experimental observations presented in Figures 1 d and 1e. As a result, only the 1H model, displayed in Figures 2d and 2e, will be considered to interpret the hexagonal periodicity of the STM image in Figure 1c.

Figure 2: Top views of the atomic structure before relaxation for the 1T (a), 1B (b) and 1H (c) models. Top (d) and side (e) view of the relaxed 1H model of germanene on Al(111). Only the first Al plane is displayed in (d). (f) Electron localization function (ELF) iso-surface at a value of 0.75. (g) Experimental and calculated STM images of the 1H configuration.

In the 1H asymmetrical buckled unit cell model, the Ge-Ge distances are lying in the

8


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0.258 – 0.261 nm range, and the vertical distance between the up-lifted Ge atom and the first Al plane is 0.406 nm. Six Ge atoms are located at a distance of 0.262 nm from an Al atom, which is smaller than the smallest Ge-Al distance in the 2T model (0.279 nm62). So, there is a substantial effect on the electronic charge distribution between Ge and Al, as shown below. The predicted buckling is of ∆z = 0.153 nm in the 1H model, compared to ∆z = 0.123 nm62 in the 2T model, and the distance ∆z’ between the highest and the lowest Al atoms is 0.013 nm, smaller than that in 2T model.

Adsorption energy Interaction energy Germanene deformation energy Al deformation energy Total Bader charge: germanene Total Bader charge: Al

2T

1H

-0.48 [From Ref.62] -0.75

-0.48 -0.69

0.23

0.20

0.04

0.01

-2.43 [From Ref.64]

-1.94

+2.43 [From Ref.64]

+1.94

Table 1: Adsorption energy, interaction energy, deformation energy (in eV per Ge atom), Bader charges (in |e|), for the 2T from ref. 62,64 and the 1H germanene/Al(111) models

According to Table 1, the interaction energy (in absolute value) and the deformation energies of the Ge and Al lattices are also smaller for the 1H model than for the 2T one. As to the charge transfer, the germanene Bader charge is -1.94 |e| for the 1H model, also lower than for the 2T model. These results (atomic structure, energies, Bader charges) suggest that the interaction between the germanene layer and the Al substrate is different for both models. Indeed, in contrast to the the electron localization function (ELF) calculated for the 2T model62, the ELF 9



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of the 1H model displayed in Figure 2f indicates a stronger electronic charge localization between Ge atoms and Al atoms. Finally, the 2T model is characterized by a large electrostatic interaction with delocalized charges at the interface62, whereas for the 1H model, the electrostatic interaction is weaker, but the charge is more localized along Ge-Al couples, though the adsorption energy is the same for both germanene crystals. The adsorption energy involves several energetic contributions such as electrostatic interaction and network deformation energy. The electronic charge redistribution is counterbalanced by a change in elastic energy, for the overall germanene/Al(111) system, when switching from the 2T to the 1H germanene structure. Note that by using the optB86b-van der Waals density functional89-92 for the calculations, the 1H model also displays the same adsorption energy as the 2T one. We performed STM image simulations within the Tersoff-Hamann approximation for the 1H model. The calculated image is shown in Figure 2g, and is in perfect agreement with the experimental STM image of Figure 1c. In conclusion, the honeycomb structure is related to the 2T model, as already published, while the single stable configuration with only one protruding Ge top atom per unit cell is the 1H model, with a hexagonal symmetry on the STM images. This suggests that experimental STM images are relevant of these most stable models, which have almost the same adsorption energy. But, this cannot explain the experimental contrast change in STM images. Indeed, germanene crystals with the same adsorption energy would grow concomitantly, but with different contrasts in STM images, related to domains with different structures, at odds with experimental observations. We never observed STM images with the two contrasts at the same time, except when a switch between 2T and 1H occurs, as in Figure 1d and 1e. After the switch, the STM contrast spreads over the whole image during STM scans, whatever the image size (with atomic resolution). An activation barrier must exist when going from one to another germanene structure, to guarantee the relative stability of the selected 10


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germanene structure. We thus calculated the activation barrier height for jumping from one to another registry. The barrier height is of about 0.11 eV / unit cell in the 2T to 1H or in the 1H to 2T configurations change. Note that the barrier height is small enough to be counterbalanced by either a small applied voltage pulse (about 1 volt), or even by electric field variation at room temperature during the image scan without bias pulse. In summary, DFT calculations predict that germanene can crystallize with different atomic structures, in agreement with our experimental data and in line with experimental work of Fukaya et al.75 The predicted structures are then Germanene-2T, evidenced by STM images with honeycomb contrast, and Germanene-1H, with hexagonal contrast. Figure 3 shows for both models the predicted top-view Ge atomic positions on germanene structures, with respect to the same Al(111) surface network, along with the superimposition of the calculated network and experimental STM image, in order to evidence the 2T-1H germanene structural change. The nice agreement between calculated and measured network is amazing and shows a possible germanene configuration before and after the switch between two stable germanene structures on Al(111). Note that contrast modifications in experimental STM images is strong and seems associated with important Ge atoms lateral displacements in germanene with respect to Al(111) plane underneath. However, the structural germanene modifications from Germanene-2T to Germanene-1H needs only small displacements -essentially vertical ones- of the low-lying as well as the up-lifted Ge atoms , without significant Ge-Ge distance changes, and then only small germanene lattice deformation.

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Figure 3: Superimposition of the calculated network and experimental (V = +1V) STM image, showing the structural change from the 2T configuration to the 1H one. Al, Ge, and uplifted Ge atoms are represented by beige, green, and red balls, respectively. The black rhombuses indicate the germanene unit mesh; for clarity, only the first Al plane is presented. The black circles emphasize the correspondence between the up-lifted Ge atoms in the 1H model and the bright spots in the experimental STM image.

In conclusion, the nice agreement between experimental topmost Ge positions with respect to Al(111) before and after contrast change, and DFT predictions in Figure 3 definitively confirms the germanene structural modification and the tip-induced switch between atomic structures of germanene on Al(111). The multiplicity of possible stable or meta-stable germanene structures on Al(111) confirms that the interaction between the Ge atoms and the Al(111) surface is faint. The Al(111) surface could be a model surface for other nontrivial 2D systems.

Acknowledgement This work was performed using HCP resources from GENCI-IDRIS (Grant N° 2017-092042) 12


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and the supercomputer facilities of the Mésocentre of Strasbourg.

Notes The authors declare no competing financial interest.

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