Germanene on Al(111): Interface Electronic States and Charge

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Germanene on Al(111): Interface Electronic States and Charge Transfer Regis Stephan, Marie-Christine Hanf, Mickael Derivaz, Didier Dentel, MariaCarmen Asensio, José Avila, Ahmed Mehdaoui, Philippe Sonnet, and Carmelo Pirri J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10307 • Publication Date (Web): 30 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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The Journal of Physical Chemistry

Germanene on Al(111): Interface Electronic States and Charge Transfer

R. Stephan1, M.C. Hanf1, M. Derivaz1, D. Dentel1, M. C. Asensio2, J. Avila2, A. Mehdaoui1, P. Sonnet1 and C. Pirri1*

1. Institut de Science des Matériaux de Mulhouse IS2M UMR 7361 CNRS, Université de Haute Alsace, 3 bis rue Alfred Werner, 68057 Mulhouse, France

2. Synchrotron SOLEIL, Saint Aubin, BP 48 91192 Gif-sur-Yvette, France

* Corresponding author: [email protected]; Phone: +33(0)389 336 435

1 Environment ACS Paragon Plus


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Abstract

The electronic structure of germanene is investigated by high resolution photoemission and density functional theory calculations. The core level Al2p and Ge3d lines are measured on germanene grown on Al(111) by using synchrotron radiation. The Ge3d line is shifted towards the low binding energies with respect to bulk Ge, and shows three components, reflecting the sites multiplicity of the germanene atomic structure. The calculations reveal a sizeable charge localization at the germanene / Al(111) interface, a charge transfer from the Al surface atoms to the germanene, and the existence of three non-equivalent Ge sites with three different atomic Bader charges, in agreement with the photoemission measurements.


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Introduction:

2D materials are now widely studied materials, due to the predicted physical and chemical properties they should have. Some of them are the 2D counterpart the elementary bricks of the 3D crystals, such as MoS2, WS2 or hBN. In contrast, except for graphene, 2D materials of group-IV semiconductors have only recently been synthesized and their structure is not a simple replica of bulk material elementary bricks. This holds for silicene and more recently for germanene with specific physical properties.1-24 The growth of silicene and germanene is a great challenge since the sp3 hybridization of Si and Ge atoms is energetically favorable with respect to the sp2 configuration expected for hypothetical planar 2D sheet. An experimental evidence of the formation of real free-standing silicene or germanene layer has not yet been reported. To date, their formation always needs a substrate, which is claimed to act as matrix for epitaxy, although epitaxy suggests a strong interaction between the substrate and the ad-layer. Epitaxy is useful to stabilize a 2D layer on a given substrate but it modifies the 2D layer intrinsic properties, either by modifying the lattice parameter or more drastically by the formation of chemical bond. This is at odd with what is expected for the growth of a free-standing 2D layer. In order to minimize the substrate effects, noble or simple metals as well as Ge saturated substrates have been chosen, with lattice parameters close to the lattice parameters calculated for free-standing germanene. Germanene has been successfully grown on four different substrates, namely Ge2Pt, Pt(111), Au(111) and Al(111) with a honeycomb surface structure.25-28 However, this procedure does not exclude the occurrence of alloy formation. This point has been widely discussed for the Ge/Pt system work for the Ge/Au system.30,

31

25, 29

and could also

The way germanene accommodates on the substrate is of

paramount importance since it governs its properties when lying on the substrate. In that respect, a characterization of the interaction of germanene with the substrate, including charge



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transfer, charges localization and chemical bonds, has never been presented. The present work includes experimental high resolution core levels photoemission measurements and a theoretical study of charge transfer and charge localization density functional theory calculations (DFT), for the germanene / Al(111) system. DFT.

Experimental and theoretical methods:

The core level measurements have been performed in the ultra-high vacuum (base pressure 1.10-10 mbar) experimental chamber of the ANTARES beamline at synchrotron SOLEIL (Orsay), and the STM (scanning tunneling microscopy) (Omicron Nanotechnology GmbH) images in an ultra-high vacuum (base pressure 3.10-11 mbar) experimental chamber in IS2M laboratory. The Al(111) substrate was cleaned by numerous cycles of sputtering and annealing at 500°C until a sharp Al(1 x 1) LEED pattern was obtained. The surface cleanliness was controlled by X-ray photoemission and STM. We checked the absence of contaminant (O, C) by XPS. The germanene layer was grown in-situ by Ge deposition on a Al(111) substrate at a temperature of about 87°C.

28

The Ge was evaporated from a

commercial evaporation cell (MBE Komponenten GmbH) and the deposited Ge amount was controlled by a quartz microbalance and by XPS. The low Ge flux used in these experiments (0.005 nanometers / minute) guarantees a perfect and flat 2D germanene layer. Ge3d and Al2p core level photoemission spectra were obtained at photon energy of hν = 130 eV and 95 eV. The photoelectrons were collected at normal electron emission using a hemispherical electron energy analyzer (VGScienta R4000). 32 The overall analyzer + source resolution was measured at the Fermi level edge on clean Al(111) and was found to be 100 ± 10 meV. The core level spectra were analyzed using CasaXPS software version 2.3.16. 33 Voigt profiles were used for both Al 2p and Ge3d components, with a Gaussian (30%) – Lorentzian


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(70%) weight. The asymmetry of the peaks was defined as a Doniach-Sunjic shape. A Shirley type baseline with varying amounts of offset at the high binding energy end point and/or starting point was used to fit the different spectra. For the Al2p lines, the spin-orbit splitting (2p3/2 - 2p1/2) was found to be between 397 and 403 meV and the branching ratio was fixed to its theoretical value of 2. As to the Ge3d lines, the spin-orbit splitting (3d5/2 - 3d3/2) was found to be 556 meV and the branching ratio was fixed to its theoretical value of 2/3. The calculations have been performed by means of the Vienna ab-initio simulation package (VASP) 34-37, using the projector augmented-wave (PAW) 38, 39 method in the general gradient approximation (GGA), with the functional of Perdew, Burke and Ernzerhof (PBE). 40, 41

The k-points sampling (3x3x1) and the cutoff energy (high precision of VASP) are the

same as a previous study.

28

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. The charge transfers are calculated in the Bader scheme with a partial charge approach, with an error lower than 1.10-4 electrons for the total electrons number (302 e-) of the slab. The charge density maps have been generated by means of 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. 42, 43

Results and discussions:

Figure 1 shows a STM surface image at a large scale (300nm x 300 nm), on which only flat atomic Al(111) steps are observed. This image is done on a sample at room temperature. The atomically resolved image (shown as inset) reveals the honeycomb network of germanene on these terraces. The Al2p lines measured on clean Al(111) and on



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germanene-covered Al(111) at photon energy hν = 130 eV are shown in Figure 2. On clean substrate, the Al2p line (Figure 2.a) is decomposed into three (Al2p1/2-2p3/2) doublets, as suggested by Borg et al.

44

The bulk Al2p3/2 spin-orbit component is located at 72.68 eV

(main blue line) binding energy (BE), the surface Al2p3/2 component (small pink line) and the main phonon-induced replica of the bulk Al2p3/2 component (small blue line) components are shifted by less than 50 meV on both sides of the Al2p3/2 component, in very good agreement with the measurements shown in Ref.44. The full width at half maximum (FWHM) of each component of the doublet is 0.11 eV. After germanene formation on Al(111), the Al2p line (Figure 2.b) is more complex, relevant of a multiple Al contribution. A decomposition (not shown) of the line into two (Al2p1/2, Al2p3/2) doublets shows a narrow doublet with the Al2p3/2 component located at 72.69 eV BE and a full width at half maximum (FWHM) of 0.17 eV, and a broader doublet (FWHM = 0.26 eV) with the Al2p3/2 component located at 72.86 eV BE. The second doublet is rather broad with respect to the first doublet and to the energy resolution of the experimental system (100 ± 10 meV). Its FWHM reflects either statistical core level shift distribution or the presence of an additional Al2p doublet. We have then decomposed the experimental line with three doublets, with phonons contributions, as shown in Figure 2.b. The Al2p3/2 components are located at 72.73 eV (blue line), 72.81 eV (orange line) and 72.95 eV (green line) BE. The FWHM are 0.12 eV, 0.14 eV and 0.15 eV, respectively, and are more compatible with the energy resolution of the experimental system. Figure 3 shows the Ge3d line measured at a photon energy hν = 130 eV. As for the Al2p line, the Ge3d line is relevant of Ge atoms with different chemical environments. Figure 3 also shows (green arrows) the Ge3d5/2 and Ge3d3/2 binding energy of a bulk Ge(111) crystal measured within the same experimental conditions. This shows that all germanene components are shifted towards lower binding energies with respect to the bulk Ge lines. The


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Ge3d line centroïd is shifted by about 0.8 eV with respect to bulk Ge, in line with what is observed for silicene and graphene with respect to bulk Si and diamond respectively.10, 45-47 Germanene Ge3d line shows five clearly identified intensity maxima. It is built by the superposition of three (Ge3d3/2 – Ge3d5/2) doublets, at least. We have first decomposed the Ge3d line with three doublets. This procedure was not able to fit the experimental line perfectly. An additional doublet had to be considered at low binding energy but with a small intensity with respect to the others. By doing so, a very good fit is obtained, with one large doublet with Ge3d5/2 at 28.57 eV BE (dark yellow line), two doublets with Ge3d5/2 at 28.35 (red line) and 28.88 eV (dark cyan line) BE and a small doublet with Ge3d5/2 at 28.18 eV BE (olive). Note that all the Ge3d components FWHM are larger than the energy resolution of the experimental setup but these values are close to that measured for germanene on Au(111) 27, suggesting that they reflect the natural Ge3d lines broadening rather than a statistical chemical disorder. The identification of the Al2p and Ge3d components requires the reference to the germanene model which we published recently.28 The germanene structure presents a very regular Ge network, with a (3 × 3) surface periodicity with respect to the Al(111) one. The germanene layer consists in a lattice whose unit mesh exhibits two Ge atoms, each atom located on top of an Al atom, and six Ge atoms located at an Al−Al bridge position, as shown in Figure 4. The Ge atoms located on top of Al atoms are shifted upwards (Getop), with respect to the other six Ge atoms (Gedown), by ∆z = 0.123 nm. This makes the germanene buckling. In addition, the Al atoms (2 over the 9 atoms in the unit cell) below the Getop atoms are also shifted upwards with respect to the other Al atoms, giving rise to a buckling of the Al surface itself, by ∆z’ = 0.061 nm. Note that this model also shows that the Ge-Ge bond angles are consistent with a mixed sp3 and sp2 configuration. Indeed, Getop – Gedown and Gedown – Gedown form bond angles of 99°- 100° and 116° and 120°, respectively. According to the germanene



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structure shown in Figure 4a and 4.b, the Al and Ge atoms exhibit very different chemical environments, in line with the core level photoemission spectra presented above. Indeed, there are four different environments for the Al atoms and three for the Ge atoms. Three types of non-equivalent Al atoms are located in the first Al layer below the germanene layer, labeled Al1, Al2 and Al4. There is only one Al environment in the second and third Al layers, labeled Al3. Three different environments are found for the Ge atoms. Ge atoms labeled Ge1 are Getop while Ge2 and Ge2’ atoms are Gedown. Ge2 and Ge2’ atoms are both on bridge site (nearest Al neighbors) but their next-nearest Al neighbors are slightly differently distributed. To correlate the measured core level photoemission line multiplicity with Al or Ge sites, a calculation of the electron distribution has been performed. Figure 4.c displays a charge density map of the system at 0.24 electrons per Å3. It appears that the charge is mainly concentrated around and between the Ge atoms, but tends to extend towards the interface between the germanene layer and the Al(111) substrate. The charge between the Ge atoms shows the covalent bonds within the germanene layer, as expected for a 2D crystal. We have also calculated the difference between the electronic charge of the overall germanene/Al(111) system, on one hand, and that of the germanene layer and the Al(111) crystal considered independently (without interaction) but with the atomic configuration found in the complete system, on the other hand. This allows highlighting how the charges reorganize when the germanene layer and the substrate are brought together. Figure 4.d reveals a net electronic charge gain (light green areas) at the interface between the germanene layer and the Al(111), and on top of the Gedown atoms. In contrast, a charge loss is predicted below the Gedown atoms, and at the topmost Al atoms. It has been shown previously that there are no covalent bonds between the Ge and Al atoms.

28

However, the charge density (Figure 4.c) and charge difference (Figure 4.d) calculations clearly show that the germanene layer interacts with the Al surface. We have then carried on


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Bader charge calculations to visualize charge transfers between the different parts of the system. It appears that, in each unit mesh, the germanene layer gains 2.43 electrons, while the Al substrate loses 2.43 electrons. Electronic charge is transferred from Al to Ge, in agreement with the Al and Ge electronegativity, namely 2.0 for Ge and 1.6 for Al. The Al top layer is the main supplier of electrons, since 2.30 electrons are transferred to the germanium atoms from the first Al atomic plane (atoms Al1 and Al2). This indicates that the Al top layer is positively charged, while the germanene layer is negatively charged. If we look now at the charge transfer for each atom separately, we find that the unit mesh must be divided into two submeshes. In one unit sub-mesh, each of the the three Gedown atoms receive 0.45 electrons, and in the other, each of the three Gedown atoms receive 0.37 electrons. As stated above, the difference originates in the position of the Ge atoms in relation with the second and third Al plane. The three Gedown atoms on the left in the unit mesh (Ge2 in Figure 4a), each one carrying an extra charge of 0.37 electrons, are located above Al atoms of the second plane (yellow balls in Figure 4a and 4b), which form a 2.80 Å equilateral triangle. In the right side of the mesh, the Gedown atoms (Ge2’) have an extra charge of 0.45 electrons, and lie above a 5.69 Å equilateral triangle of Al atoms. This structural difference may induce two different charges for the Gedown atoms. In contrast, the charge of the Getop atoms (Ge1) is only slightly modified. They only lose 0.02 electrons. The large difference between the charge on Getop (Ge1) and Gedown (Ge2 and Ge2’) may be related to the distance between the Ge atoms and their nearest Al neighbor. For the Gedown atoms, the mean Ge-Al distance is 2.79 Å, while it is larger (3.02 Å) for the Getop atoms. As regards the Al atoms, six Al atoms (Al2) give 0.25 electrons, while the two protruding Al (Al1) lose 0.41 electrons. The lower-lying Al (Al4) atom surrounded by a germanene hexagon only gains 0.02 electrons. Here again there is a correlation between the charge and the Al-Ge distance, namely 3.65 Å for the isolated Al



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atom (Al4), 2.89 Å for the up-lifted Al ones (Al1), and 2.79 Å for the other Al atoms (Al2). The charge on the Al atoms (Al3) of the second and third planes is not modified. The calculated charge transfer between Al and Ge atoms reflects in the experimental core Al2p and Ge3d lines. The calculations suggest that Al atoms labeled Al1, Al2 and Al3 atoms in Figure 2.b contribute as three separated doublets to the Al2p spectrum. Nevertheless, identification of Al2p doublets is not easy since there is not a strict correspondence between charge transfer and core level shifts for an adsorbate on metallic surfaces. Furthermore, Nilsson et al. has recently shown that there is no strict relationship between the charge transfer sign and the core level shift sign.

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To overcome this problem, we have performed

photoemission measurements at a different photon energy hν. Figure 2.c shows the Al2p line measured with photon energy hν = 95 eV. We have also decomposed the experimental line with three doublets, with phonons replica (smallest peaks) shifted by about 50 meV toward higher BE. The Al2p photoelectron mean free path is about 1.45 nm at photon energy of 95 eV while it is 0.35 nm at photon energy of 130 eV.

49

In other words the contribution of

deeper Al atoms is enhanced at hν =95 eV. Figure 2.c shows that the components lying at 72.73 eV (blue line) and 72.95 eV (green line) BE are relevant of deep Al planes, and then associated with atoms labeled Al2, Al3. The component lying at 72.81 eV BE (orange line) is thus associated with positively charged atoms labeled Al1. Discrimination between Al2 and Al3 contributions to the Al2p spectrum can also be done. Indeed, the component at 72.73 eV BE has the same energy as the Al2p3/2 core level of bulk Al atoms in clean Al(111). It is then relevant of Al atoms lying in the second Al plane and Al atoms in planes below (Al3) which are almost neutral atoms. Note that Al4 atoms have an atomic configuration close to that of deeper Al atoms, does not transfer charge to Ge and then could sign as Al3. Finally, the Al2p3/2 component at 72.95 eV BE is relevant of positively charged Al2 atoms at the Al/germanene interface.


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As stated above, the germane/Al(111) lattice model in Figure 4 shows three nonequivalent Ge sites (Ge1, Ge2, Ge2’), in agreement with the number of main components (three) in the Ge3d core level spectrum. The additional Ge3d doublet at lower binding energy is very small with respect to the others (1 or 2 %). It could be assigned to germanene domains boundaries, on the germanene plane or at the step edges. The intensity of the core level relevant of the three Gedown atoms of each half-unit mesh (Ge2 and Ge2’) should contribute equally in the Ge3d spectrum since the number of atoms per unit sub-mesh is the same and the atomic environment is quite similar, however they carry different charges. Thus they reflect in the two components on each side of the main line located at 28.57 eV BE. The latter is relevant of Getop (Ge1) atoms, which have a completely different atomic environment and electronic charge. However, one intriguing feature is the large ratio between the two Ge3d lines of Gedown (Ge2 and Ge2’) and that of Getop (Ge1). This ratio does not fit the ratio between the number of Ge2 or Ge2’ sites and the number of Ge1 sites. However, this effect is often observed in complex spectra as that in Figure 3 and is well understood. This is due to interference effect in normal photoelectron diffraction.

50

At low kinetic energy, the Ge3d

photoelectron experiences a sizeable backscattering diffusion on Al atoms located beneath the Ge emitter. Interferences between the primary Ge3d wave and backscattered part of the wave modulate the photoelectron line intensities, depending on the photon energy and the optical path of the electronic wave. It depends then on the distance between the emitter (Getop or Gedown) and Al plane below. This effect can produce photoelectron intensity modulations up to 100%, which can reflect in a variation of a factor 2 of the line intensity. In the present work, we could have constructive interferences for the Getop and destructive interferences for the Gedown. This effect is large enough to explain the lines intensity in Figure 3.

Conclusions:



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In conclusion, DFT calculations and high-resolution photoemission experiments show that a significant charge transfer occurs from the Al to the Ge atoms, resulting in an electrostatic interaction between the germanene layer and the Al substrate. According to the charge difference calculations, electrons are present at the interface, indicating the existence of an only weak chemical interaction (no covalent bonds) between the Ge layer and the Al surface. Germanene interaction with Al(111) has a similarity with that of silicene with Ag(111) as to the occurrence of a charge transfer between silicene and metal. 51, although for the silicene/Ag(111) system the charge transfer occurs from the silicene to the substrate, in line with the difference of electronegativity. Yet for both germanene and silicene on metallic substrates, there is an electron accumulation at the interface which guaranties germanene and silicene adhesion on the substrate. This could be an electronic configuration for group-IV elements Si, Ge and Sn on noble or simple metals.

Acknowledgments

This work was performed using HCP resources from GENCI-IDRIS and the supercomputer facilities of the Mésocentre of Strasbourg. The authors declare no competing financial interest.


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Figure captions (Color online)

Figure 1. Large scale STM image of germanene taken at room temperature. Inset: atomicallyresolved empty-state (V = 1.3 V) STM image of germanene, taken with a current I = 0.3 nA.

Figure 2. (a) Core level Al2p line measured at normal emission on clean Al(111) taken at a photon energy hν = 130 eV , (b) Surface-sensitive Al2p line measured on germanene-covered Al(111) taken at a photon energy hν = 130 eV. (c) Bulk-sensitive Al2p line measured on germanene-covered Al(111) taken at a photon energy hν = 95 eV. All spectra were recorded at normal photoelectron detection. These spectra are decomposed into Al2p3/2-Al2p1/2 doublets. In (a) and (b), the small doublets are the phonon contributions associated with each Al2p doublets.

Figure 3. Core level Ge3d line measured on germanene-covered Al(111) taken at a photon energy hν = 130 eV. This spectrum was recorded at normal photoelectron detection. This spectrum is decomposed into Ge3d5/2-Ge3d3/2 doublets. Also indicated by the green arrows are the Ge3d5/2-Ge3d3/2 doublets binding energy for bulk Ge(111) crystal.

Figure 4. (a) Top-view and (b) side-view of the Ge/Al atomic structure. Side-view of: (c) the charge density map at 0.24 electron per Å3 and (d) the electron density difference map at 0.025 electron per Å3. Light green and lilac correspond to an augmentation and a diminution of the electron density, respectively.


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Figure 1



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Figure 2


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Figure 3



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Figure 1

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ACS Paragon Plus Environment


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Germanene on Al(111): Interface Electronic States and Charge

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