Germanene Growth on Al(111): A Case Study of Interface Effect

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C: Physical Processes in Nanomaterials and Nanostructures

Germanene Growth on Al(111): A Case Study of Interface Effect Jide Fang, Peng Zhao, and Gang Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03534 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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The Journal of Physical Chemistry

Germanene Growth on Al(111): A Case Study of Interface Effect Jide Fang, Peng Zhao, and Gang Chen* Department of Physics, University of Jinan, Shandong 250022, P. R. China

Abstract Using density functional theory calculations, we have studied the germanene growth on Al(111) in detail. According to the polygons in GeN (N = 1~12) structures on substrate, three structural growth modes are studied, which would lead to the growths of single atomicthick hexagonal lattice, Kagome lattice, and buckled hexagonal superlattice germanenes. The buckled superlattices grown on pure Al(111) could reproduce the experimental scanning tunneling microscopy images, which however do not have good energetic and thermal stabilities. Detailed energy analyses suggest the possibility for forming Al2Ge surface alloy, on which the growths of the buckled superlattices turn to be preferable. Furthermore, such superlattice configurations become energetically and thermally stable. Their adhesive energies are ~83 meV/Å2, which could be further decreased by hydrogenation to facilitate their separations from Al2Ge substrate. These studies highlight the effects of interface modification on tuning two-dimensional material growth. Also, the surface alloying could be used as an effective pretreatment method to facilitate large quantity fabrication of germanene on Al(111).

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1. INTRODUCTION Inspired by the successful fabrication of single atomic-thick two-dimensional (2D) carbon nanostructure known as graphene and its amazing unique properties1-5, intensive researches have been devoted on studying its chemical and physical properties. Besides, the other 2D nanomaterials have also aroused extraordinary research attention, for example, boron nitride sheet6,7, silicene8-10, germanene11-20, phosphorene21, borophene22-26, transition metal dichalcogenide monolayer27-30, and 2D transition metal carbides and carbonitrides called MXene31-34, etc. Correspondingly, studies on 2D alloying nanostructures and 2D allotropes of the already known sheet materials have also gain tremendous research attention. These contribute lots of new nanostructures to the 2D material family, for example, the pentagonal sheets composed of pentagons35-37, the graphene allotropes composed of 5-6-7 or 5-6-8 carbon rings38,39, the graphyne sheets40,41, the complex transition metal-boron sheets42,43, the simple metal carbide sheets44,45, the In-Ga-N alloy sheet46, and the carbon nitride sheets owning active catalytic performances47-50, etc. In the field of 2D materials, the experimental fabrications of sheet nanostructures are inevitably crucial, which pave the way for the fundamental studies of their unique properties and the application investigations of next-generation high-performance nanodevices, etc. As to the experimental techniques to form atomic layers, the deposition of atoms onto substrate is one of the prevailing fabrication methods, for which the (111) oriented metallic surfaces are usually used as substrates. With the purpose to well control the structural growth of 2D materials, to facilitate transferring of 2D sheets from substrates to substances for practical applications, and to lower the fabrication costs, efforts on searching new substrates or modifying the interface effects are thus desired. As a same column element of carbon, germanium owning same number valence electrons and similar valence orbital hybridization has also attracted lots of research attention, which may be easily incorporated into nanodevices based on the current Si and Ge based


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semiconductor industrial techniques. Unlike the perfect sp2 hybridization in graphene, the slight mixing of sp2 and sp3 hybridizations in germanene favors a low-buckled honeycomblike geometry, which consists of two sublattices with one of them being displaced vertically of ~0.69 Å with respect to the other one51,52. On theory, Chen et al. investigated the effects of BeO substrate on the electronic and optical properties of germanene53. Zhao and his coworkers investigated in detail the substrate effects of MoS2 and metal substrates on germanene54,55. On experiment, the Pt(111), Au(111), and Ag(111) were previously used as substrates to grow germanene. Li et al. reported the experimental fabrication of germanene superlattice which topologically matches Pt(111)-(√19 × √19)(23.4°) periodicity11. The low energy electron diffraction pattern characteristics reported by Dávila et al. suggest the formations of ( √7 × √7 )(19.1°), 5×5, and ( √19 × √19 )(23.4°) superlattice structures of germanene with respect to Au(111) substrate12. As to the germanium growth on Ag(111), Ge was found to react with Ag(111) to form Ag2Ge surface alloy which was previously studied in detail on experiment56-59. A very attractive experimental study by evaporating Ge atoms and depositing them onto Al(111) was recently reported by Derivaz et al. who proposed a germanene configuration on Al(111)-(3×3) surface with two Ge atoms being protruded upward13. This should be competitive for preparing germanene-based nanostructures with low cost as compared to the fabrications on Au or Pt noble metal substrates. By using the total-reflection high-energy positron diffraction technique, Fukaya et al. proposed a similar germanene configuration but with only one Ge atom being protruded in the unit cell14. In these germanene models, the protruded Ge atoms sit on top of Al atoms. Recently, Stephan et al.15 and Wang and Uhrberg16 found a different germanene model on Al(111)-(3×3) with a Ge at the three-fold hollow site being protruded upward in each unit cell. In the primitive unit cell containing 8 Ge atoms, two Ge atoms are adsorbed at the hcp and fcc stacking sites of Al(111). The fcc site Ge atom forms



three Al-Ge bonds with Al(111) while the hcp site Ge is upward shifted to exclude its bonding to substrate. This model was also proposed for the germanene on Al(111)-(√7 × √7)(19.1°) reconstructed surface by Wang and Uhrberg16. In a most recent work, Endo et al. proposed a new configuration as the √3 × √3 honeycomb lattice on Al(111)-(√7 × √7)(19.1°)17. Among the 6 Ge atoms in its primitive unit cell, there is only one Ge atom adsorbed on top Al atom, which is vertically lifted with respect to the other Ge atoms. Using high resolution photoemission and density functional theory calculations, Stephan et al. confirmed the charge accumulation between germanene and Al(111)18. Marjaoui et al. reported that the interaction between germanene and Al(111) could be tailored by the means of hydrogenation19. Also, Liu et al. found the hydrogenation induced magnetism in the hydrogenated germanene on Al(111)20. Besides the germanene, a very recent experimental study show the realization of honeycomb borophene on Al(111) surface, addressing the validity of Al(111) as substrate for growing 2D nanostructures26. With the purpose to help search low cost metal substrate for 2D material fabrication, we have carried out detailed investigation on structural growth of germanene on Al(111). The effect of interface modification on growing germanene sheet has been studied in detail also, which suggests the surface alloying as an effective pretreatment of Al(111) to facilitate large quantity fabrication of germanene.

2. COMPUTATIONAL DETAILS The spin-polarized first-principles calculations were carried out by using the Vienna ab initio simulation package (VASP)60 within the framework of density functional theory (DFT). The projector augmented-wave (PAW) method was employed61. The wave functions were expanded by using planewave bases with cutoff energy of 312.3 eV. Besides the DFT calculations using generalized gradient approximation with Perdew, Burke, and Ernzerhof (PBE) functional62, the PBEsol revised to enhance surface energy63, and the meta-GGA functional of Tao-Perdew-Staroversov-Scuseria (TPSS) formalism64 were also employed in


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our studies to validate the calculated results. Considering the weak interface bonding nature between germanene and aluminum, the van der Waals density functional theory with Grimme’s D3 formalism65 was also adopted in our studies. The surface was mimicked by a repeated slab consisting of six aluminum layers with the two bottom layers being frozen to account for bulk properties. In our studies, the surface was studied by using hexagonal supercell. A vacuum space of >12 Å was placed along the z-direction of the supercell to well separate the slab with its neighboring periodic images. The structural growth modes were studied by introducing Ge atoms one by one onto Al(111)-(9×9) surface. Besides, the high symmetry planar geometries of GeN cluster were also considered as structural candidates. The ground state structure and its low-lying isomers of GeN cluster were used to design structural candidates for GeN+1 cluster. Due to the large supercell size, only Γ point was used for optimizing GeN structures on substrate. As to the 2D germanene configurations grown on substrate, the smallest unit cells are the Al(111)-(6×6) for the Kagome lattice germanene and the Al(111)-(3×3) for both the hexagonal lattice and the buckled hexagonal superlattice germanenes. Correspondingly, we adopted the 3×3×1 and 7×7×1 k-meshes66, respectively. The convergence tolerance of the electronic properties was set to 10‒5 eV. The convergences of atomic relaxation were 10 meV/Å for the structural optimization of GeN cluster on Al(111)-(9×9) surface and 5 meV/Å for the germanene on substrate.

3. RESULTS AND DISCUSSION 3.1 . Growth Modes on Pristine Al(111) Surface 3.1.1. Growth of GeN (N = 1~12) Clusters on Al(111) We started our studies by putting Ge atoms one by one onto pristine Al(111) surface. For the adsorption of a single Ge atom, four nonequivalent adsorption sites are considered. These are the top site on top a Al atom, the bridge site upon a Al-Al bond, the hcp stacking hollow site, and the fcc stacking hollow site. The adsorptions of Ge atom at top and bridge



sites are unstable, which would evolve to hollow sites during structural optimizations. The adsorptions at hcp and fcc sites are almost degenerate with the former one of about 10 meV lower in energy. The adsorption of Ge atom at hcp (fcc) site would elongate the below Al-Al bonds from 2.83 Å to 3.08 Å (3.04 Å). The interatomic distance between Ge and its nearest Al is ~2.56 Å which is only ~2% longer than the sum of the atomic radii of Ge and Al atoms. The bonding between Ge and Al results in ~1.45 eV/bond energy and charge accumulation between them, suggesting weak covalent-like bonding nature. Similar to the adsorptions of metal atom on Si(111)-7x7 semiconductor surface67,68 and hydrogen atom on Al(111) metal surface69, the Ge atom prefers to sit at multi-coordination site to develop three weak covalentlike bonds with Al(111). Putting one more Ge atom on the Al(111), the fcc site would gain priority. The ground state adsorption configuration is that two Ge atoms sit at two neighboring fcc sites, which is ~0.11 eV lower in energy than the adsorptions at two neighboring hcp sites. The Al-Ge and Ge-Ge bonds are 2.64 and 2.68 Å in length, respectively. The Ge-Ge bonding reduces the reactivity of Ge atoms to weaken the Al-Ge bonding strength to 0.7 eV/bond, releasing the structural distortion as compared to the case of single atom adsorption. Starting from Ge3, the isomers are provided in three different structural growth modes according to the geometrical characteristics. Only Ge6 hexagons are found in Fig. 1a with all the Ge atoms adsorbed at fcc sites. In Fig. 1b, one could see the alternative arrangement of Ge6 hexagons and Ge3 triangles. Also, all the Ge atoms are adsorbed at fcc sites. Differently, the Ge5 pentagons appear in the structures shown in Fig. 1c, which help stabilize Ge adsorptions at top sites. Besides the top site Ge atoms, the other Ge atoms are adsorbed at fcc or bridge sites. To facilitate discussion, we would like to refer the structural growth modes illustrated in Fig. 1a, 1b, and 1c as the HH (only hexagons), HT (hexagons and triangles), and HP (hexagons and pentagons) modes. For the HP structures shown in Fig. 1c, the pentagon


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plays a crucial role for the structural growth. For example, a Ge atom adheres to a side of a pentagon in Ge9 structure. Putting one more Ge atom, this pentagon would evolve to a hexagon in Ge10 geometry with the Ge atom at top site being protruded upward. For the isomers of GeN cluster, we calculated the relative energies as referred to the corresponding ground state and presented them in Fig. 2. Judged from the energies, the HH and HT growth modes are dominating while the HP mode is energetically unfavorable.

3.1.2. Configurations of Germanene Sheets on Al(111) The preferred mode HH would grow an ideal flat honeycomb-like hexagonal lattice (HL) sheet on pure Al(111) surface while the HT mode would lead to a single atomic-thick Kagome lattice (KL) sheet, which are schematically illustrated in Fig. 3a. Providing the energetically unfavorable HP structures could grow under well-controlled growth conditions, the buckled hexagonal superlattice (BHS) germanene would gain presence on pure Al(111) surface. Based on the HP structures shown in Fig. 1c, a BHS configuration that topologically matching Al(111)-(3×3) could be obtained. In this configuration, two top-site Ge atoms per unit are vertically lifted with respect to the other Ge atoms. Following Stephan et al.’s studies on this germanene model15, we would like hereafter to refer it as BHS-2T to facilitate discussion. In the germanene model proposed by Fukaya et al.14, one of the top-site Ge atoms of BHS-2T would be lowered down to the basal germanene plane. Using the BHS-2T, we firstly vertically moved a top-site Ge atom down to the basal germanene plane, then carefully optimized the structure. However, this Ge would protrude up again during optimization. We noted that similar optimization results were also reported previously by Wang and Uhrgerg16. To be honest, we do not know why the lowered top-site Ge could not be kept on the basal germanene plane, which is worthy to be carefully studied in the future. As to the germanene configuration with a Ge at three-fold hollow site being protruded upward in (3×3) unit cell15,16, we would like to refer it as BHS-1H in our discussion (see the



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structural configuration in Fig. 3a). We have carefully investigated its structural motifs at the initial growth stage and schematically illustrated them in the supporting material Figure S1. Unfortunately, they are 0.2 eV on average energetically unfavorable compared to the corresponding structural motif structures of BHS-2T shown in Fig. 1c, which are ~0.6 eV on average higher in energy than the corresponding ground state structures (see Fig. 1a and Fig. 1b). The calculated total energies suggest that the BHS-1H would be suppressed at the initial growth stage. But, as found by Stephan et al.15, the BHS-1H and BHS-2T can switch to each other under a perturbation as small as 0.11 eV. Thus, the BHS-1H sheet configuration could be obtained through structural transition during the germanene growth on substrate. Providing a nanoflake owning BHS-2T structural characteristics being grown through the HP growth mode, a small perturbation such as the kinetic energy of the incoming Ge atoms, the heat energy by warming up, or the voltage pulse introduced by the STM tip, etc. could make it transit to BHS-1H nanoflake, which could be then used as structural seed to help grow BHS1H germanene. Therefore, we would like to say that the HP growth mode may also help grow BHS-1H germanene. In order to compare the energetic stabilities of the HL, KL, BHS-2T and BHS-1H sheets, we have calculated the corresponding formation energies by using the below formula. = where

!

and "!#

/

$!$!/ !

(1)

are the total energies of the Al(111) substrate and

the 2D Ge sheet on Al(111) surface, respectively. The µGe is the chemical potential of a freestanding germanene. N is the number of Ge atoms contained in the studied germanene configuration. The calculated data are presented in Table 1. The PBE calculations suggest the HL lattice to be the ground state structure. The KL lattice is the first low lying isomer with a 48 meV/atom lower formation energy. The BHS-2T and BHS-1H are 80 and 88 meV/atom lower in formation energy, respectively. Besides the PBE calculations, we have also carried


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out calculations by using the surface energy enhanced functional PBEsol and the advanced meta-GGA functional TPSS. As shown in Table 1, both of them support the PBE results. Considering the weak bonding nature between germanene and Al(111) substrate, we have also calculated the formation energies by including the dispersive contributions with Grimme’s D3 vdW-DFT65. As shown in Table 1, the vdW-DFT studies agree with the semilocal functional calculations also, though

the formation energies are increased

correspondingly. Besides, we have also calculated the formations energies for the BHS-1H configuration and the √3 × √3 honeycomb lattice on Al(111)-(√7 × √7)(19.1°) reconstructed surface, which are >80 meV/atom lower than the value of the HL ground state geometry on Al(111). In addition, our optimized low-lying isomers of the small size GeN (N=1~12) clusters on Al(111) do not show structural characteristics to match the reconstructed Al(111) surface. In these sheet configurations on reconstructed surface, the dominating adsorption sites of Ge atoms are the low-coordination sites. Also, most of them are the slightly distorted unfavorable top-like and bridge-like sites as compared to the single Ge atom adsorption on Al(111). In comparison, the Ge atoms are found to be at the preferred fcc multi-coordination adsorption sites in both the energetically favorable GeN (N=1~12) structures and the HL ground state sheet configuration on Al(111). Besides, we have simulated the STM images of the HL, KL, BHS-2T, and BHS-1H germanene configuratioins by using Tersoff and Hamann approach70. As shown in Fig. 3b, only BHS-2T and BHS-1H reproduce the experimental STM images13,15,16 though they are ~84 meV/atom unfavorable in energy. In addition, we carried out constant temperature firstprinciples molecular dynamics simulations (FPMD) after heating the BHS-2T and BHS-1H germanenes up to 360 K (around the experimental temperature for growing germanene on Al(111) surface13,15). Instead of the (3×3) periodic unit, a (6×6) unit cell was used to minimize the effects from periodic boundary conditions to explore possible structural reconstruction.



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Both of them are however found to break after 1 ps (see the supporting material Figure S2). As to be discussed later in this paper, same conclusions are also found in our 5 ps long FPMD simulations carried out with reduced accuracy to save computational time. So, due to the lower formation energies and the instabilities found in FPMD simulations, the large quantity fabrication of BHS-2T and BHS-1H sheets on pure Al(111) surface may be challenged. In addition, we have also estimated the thermal stabilities of the previously proposed germanene models supported on Al(111) reconstructed surface by performing 360 K FPMD simulations with a (2√7 × 2√7) larger supercell. The corresponding structural characteristics do not remain during our 1 ps long simulations.

3.2. Tuning Growth Modes by Surface Alloying 3.2.1. Surface Alloying of Al(111) The properties of aluminum surface can be altered by the methodology of near-surface alloying69,71, which could modify the interface effects to tune the germanene growth on Al(111). Previous experimental study of low coverage of Ti atoms on a clean Al(111) surface show the possibility of a guest atom to substituionally dope in top and subsurface layers72. At room temperature the Ti atom would be trapped in the first-layer of Al(111), which could however penetrate into subsurface at elevated temperature. So, in our calculations, we started our studies by estimating the possibility for Ge to form surface alloy with Al(111). Following the method used by Oughaddou et al. in studying the Ag2Ge surface alloy56, we calculate the adsorption energy

&

and the substitution energy by

= '!/()(+++) − ()(+++) − '!

(2)

= '!@()(+++) + 0() − ()(+++) − '!

(3)

&

where ()(+++), '!/()(+++) , '!@()(+++) , and '! are the total energies of the Al(111) surface, a Ge atom adsorbed on surface, a Ge atom substituionally doped in surface or subsurface, and a free-standing Ge atom. The 0() is the chemical potential of Al bulk. Negative values of 10 ACS Paragon Plus Environment

&

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and mean energetically favorable. Compared to the adsorption of a Ge atom at hcp site, the substitution doping in the top surface is 62 meV preferable in energy while the one in the subsurface is 219 meV unfavorable. Furthermore, the substitution doping of Ge atom in Al(111) results in its next-nearest hcp site to be preferable for another Ge atom adsorption, which is 51 meV lower in energy as compared to a faraway hcp site adsorption. The l1 bond connecting the first and second neighboring Al atoms (marked by A and B in Fig. 4a) changes to 3.10 Å while the l2 bond between the second and third nearest neighboring Al atoms (marked by B and C in Fig. 4a) is only slightly elongated (2.88 Å). This facilitates the substitution doping of another Ge atom at the next nearest site by ~0.1 eV lower activation energy than the adsorption at the nearest site, showing the possibility to form Al2Ge surface alloy configuration (see Fig. 4b). Here, we would like to mention again that such kind of surface alloy configuration was previously studied in detail on experiment56-59 for Ag2Ge surface alloy. In order to check whether it is favorable in energy to form Al2Ge surface alloy, ))12

we have calculated the substitution energy by following Oughaddou’s idea56 as well as the formation energy

))12

by ))12

where ()(+++) ,

))12 ,

))12

= =

3345 63 63(777)

63(777) 3345 63

(4) (5)

and '! are the total energies of the pristine Al(111), the Al2Ge surface

alloy, and a free-standing Ge atom, respectively. The 0'! and 0() are the chemical potentials of a free-standing germanene and the Al bulk, respectively. N is the number of Ge atoms in Al2Ge surface alloy. In addition, if the substitution doping resulting in Al2Ge surface alloy and gas-phase Al atoms, the energy () of a free-standing Al atom should be used in formula 4 instead of the chemical potential 0() . Thus, we also considered such situation by calculating ))12,∗

the substitution energy

as defined below



))12,∗

Positive (negative) values of

))12

=

3345 63 63(777)

))12

(

))12,∗

and

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(6)

) mean energetically favorable. The

data calculated with the PBE, PBEsol, meta-GGA TPSS, and vdW corrected functionals are tabulated in Table. 2. Judged from the data, the formation of the Al2Ge surface alloy is energetically favorable, which could be used to modify the interface interaction between substrate and germanene to tune the germanene growth.

3.2.2. Germanene Growth on Al2Ge Surface Alloy Again, by putting Ge atoms one by one onto Al2Ge alloy surface, we have carefully studied the structural growth of Ge sheet. The optimized structures for the HH, HT, and HP growth modes are presented in Fig. 5. Except the Ge3 in Fig. 5b and the Ge9, Ge10, and Ge12 structures shown in Fig. 6, all the ground state structures are now in the HP growth mode shown in Fig. 5c, which help grow BHS-2T germanene. Actually, the ground states of Ge9, Ge10, and Ge12 shown in Fig. 6 can be used as structural seeds to grow the BHS-2T germanene, which could be regarded as structural motifs of HP growth mode also. As shown in Fig. 7, the energies of the HP structures are close to those of HH and HT structures for small size GeN clusters. Starting from Ge8, the energy differences begin to be significantly enlarged. The energetic superiorities for growing germanenes follow the sequence as BHS2T > HL > KL. Based on our careful studies of the small size GeN clusters on Al2Ge surface alloy, the structural motifs for BHS-1H germanene sheet are provided in the supporting material Figure S3. These structures contain Ge6 hexagon and Ge5 pentagon units also, owning HP configuration characteristics. However, their total energies are ~0.32 eV on average higher than the corresponding BHS-2T motif species, respectively. According to Boltzmann distribution, the ~0.32 eV energy difference would significantly suppress the growth of BHS-1H at the initial growth stage. In comparison, the nanoflake owning BHS-2T germanene characteristics should have priority for growing due to that their structural motifs


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GeN (N = 4~12) are the ground state structures (see Fig. 5c and Fig. 6). Using the (3×3) periodic unit for both BHS-2T and BHS-1H germanenes, we estimated the structural transition by using the nudged elastic band (NEB) method73. The converged 16-image minimum energy reaction path shows an activation energy of 0.22 eV for transition from BHS-2T to BHS-1H (see the supporting material Figure S4). Such an activation energy could be overcome by the perturbation such as the kinetic energy of the incoming Ge atoms, the warming up, or the voltage pulse introduced by the STM tip etc. Considering the experimental fact of the tip-induced structural transitions between BHS-2T and BHS-1H germanenes, we think that the BHS-1H nanoflake on Al2Ge could also be formed through structural transition from the BHS-2T nanoflake. Besides the studies of the initial structural growth stage, we have also studied the energetic stabilities of the corresponding germanene configurations on Al2Ge surface alloy. The calculated data are presented in Table 1. The Al2Ge surface alloy favors the BHS-type germanenes. Judged from the calculated formation energies, the BHS-1H is the ground state germanene configuration on Al2Ge surface alloy, which has ~15 meV/atom higher formation energy than the BHS-2T germanene. The HL and KL germanenes are found to be ~38 and ~71 meV/atom lower in formation energy. For the fact that the HP structures owning BHS-2T structural characteristics are the ground states for small GeN clusters, the BHS-2T germanene should have the priority to be formed on Al2Ge alloy. Then, a BHS-1H like nanoflake could be obtained through structural transition from the BHS-2T like nanoflake under small perturbation, which could eventually grow the BHS-1H germanene. The relation between reaction rate for structural transition with the activation energy and temperature may be understood by Arrhenius equation. We wish further experimental studies to be carried out to determine the reaction rate prefactor of Arrhenius equation. Then, the perturbation for activating the structural transition could be discussed in detail. Here, we would like only to



conclude that both BHS-2T and BHS-1H could be synthesized under well controlled experimental conditions. In addition, we have also simulated STM images of the HL, KL, BHS-2T, and BHS-1H germanenes grown on Al2Ge. The images of HL and KL germanenes disagree with the experimental STM images (see Fig. 8). Again, the images of the BHS-type germanenes agree with the experimental observations13,15,16. Compared to the pristine Al(111) surface, the Al2Ge surface alloy tunes the structural growth to enhance the possibility for fabricating BHS-type germanenes. Using the BHS-type germanenes on both pristine Al(111) and Al2Ge surface alloy, we have estimated their thermal stabilities by using the constant energy FPMD simulations after heating the structures up to 800 K. Due to the intensive computer loading, we used a 4-layer slab contained in (6×6) hexagonal supercell to model the surface with the bottom layer being frozen to mimic Al bulk. The PBE functional was used. The simulations lasted for 5 ps with the time step of 0.5 fs. After the simulations, the structures were then fully optimized at 0 K. In Fig. 9, the dotted lines mark the experimental fabrication temperature used for germanene growing on aluminum13,15. As shown by the insets, the BHS-type germanenes on pristine Al(111) collapse. On Al2Ge surface alloy, the structures obtained at the end of simulations only have slight structural distortions, which could be then relaxed to the ideal BHS-type geometries under few meV activation energies. So, we could conclude that the thermal stabilities of BHS-type germanenes on Al2Ge surface alloy are significantly enhanced, which are expected to facilitate the large quantity fabrication of germanene.

3.3. Separation of Ge Sheet from Substrate Now, we could reach that Al2Ge surface alloy is a kind of appropriate supporting material for growing BHS-type germanenes. In order to facilitate further experimental and application studies, we would like to investigate how difficult the germanenes on Al2Ge could be isolated to get free-standing sheets. In Fig. 10, the charge transfer has been studied. For the


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configuration of BHS-type germanene grown on Al2Ge surface alloy (BHS/Al2Ge), we first calculated the charge densities of the structural fragments as the BHS sheet itself and the Al2Ge substrate at the geometries as those in BHS/Al2Ge. Then, we compared them with the total charge density of BHS/Al2Ge to estimate the charge transfer. The charge is transferred from both the adsorbed Ge atoms and the substrate to their interior space to be simultaneously attracted by both germanene and substrate, which is in the line with the experimental results17 to suggest covalent-like bonding nature between them. In the BHS-type germanenes, the Ge atoms could be classified into two groups. One is for the upward protruded (UP) Ge atoms and the other one is for the rest atoms lying on the basal plane (BP) of germanene sheet. In Fig. 10, no charge accumulation is between UP Ge and Al2Ge. In addition, the shortest interatomic distance between a UP Ge and its nearest atom in Al2Ge is 3.83 and 4.35 Å in BHS-2T and BHS-1H configurations, respectively. They are 53% and 74% longer than the sum of corresponding atomic radii, respectively. Therefore, the UP Ge atoms would not develop any bonds with substrate. The BP Ge atoms develop twelve 2.74 Å Al-Ge bonds with Al2Ge per unit cell of BHS-2T germanene, while they have eight 2.75 Å Al-Ge bonds and two 2.65 Ge-Ge bonds with substrate per unit cell of BHS-1H germanene. Compared to the bond lengths of 2.43 and 2.49 Å for the free-standing AlGe and Ge2 dimers, the elongated bonds indicate the weakened bonding strength accordingly. With the purpose to study the effects of surface alloy on the interaction between germanene and substrate, we have estimated the adhesive energies by where "!# ? and "!#

&:!;
$!$!/ !

$!$!

=

= =

/

>

? is the total energy of the germanene sheet on substrate,

(7) !

are the energies calculated for the substrate and the germanene sheet

fragments at the geometries of those in germanene/substrate structure, S is the germanene area. For the HL, KL, BHS-2T, and BHS-1H sheets grown on pristine Al(111) an Al2Ge



surface alloy, the mean bond lengths among Ge atoms in the sheets are about 2.86, 2.86, 2.62 Å, and 2.60 Å, respectively. Compared to the 2.44 Å of a free-standing germanene sheet, the bonding strength in Ge sheets on substrate is weakened. The ones in HL and KL sheets are weaker than those in BHS sheets, which could be compensated by the energy gain with the bonding between Ge sheet and substrate. In HL and KL germanenes, Ge atoms are adsorbed at fcc sites forming three Al-Ge bonds per atom. In BHS-2T, all the BP Ge atoms sit at bridge sites forming two Al-Ge bonds per Ge atom with substrate. While, the distorted Ge6 hexagons in BHS-1H germanene make the adsorption sites to shift away bridge sites (see the structure shown in Fig. 3a and Fig. 5a), resulting in ~1.4 coordination number of BP Ge atom. This slightly weaken the binding strength to substrate of BHS-1H compared to BHS-2T. The adhesive energies are calculated to be 115, 107, 91, and 85 meV/Å2 for the HL, KL, BHS-2T, and BHS-1H sheets on Al(111). On the Al2Ge surface alloy, they are changed to 103, 93, 84, and 82 meV/Å2. These show that the surface alloy reduces the activity of Al(111) surface. For the energetically stable BHS-type germanenes on Al2Ge surface alloy, the weak adhesive energies of about 83 meV/Å2 show the possibility for separating them from substrate. Previously, Marjaoui et al. proposed the hydrogenation as an effective method to help decouple germanene from Al(111)19. This is interesting because the hydrogenation and dehydrogenation could be well controlled on experiment. Using the full hydrogen coverage configuration, we have estimated the adhesive energies for the energetically stable BHS-2T and BHS-1H on Al2Ge surface alloy, which are found to be reduced to around 14 meV/Å2, confirming the hydrogenation effects to help separate germanenes from substrate. After the separations from substrate, the BHS-type geometries would quickly relax to the well-known slightly buckled honeycomb structure consisting of two sublattice planes with one being vertically displaced by 0.69 Å with respect to the other one51,52.

4. CONCLUSIONS


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In summary, we have carried out detailed first-principles studies on the structural growths of Ge sheets on both pristine and alloyed Al(111) surfaces. Three structural growth modes as the HH, HT, and HP modes for GeN (N = 1~12) growth are discussed, which would lead to the growths of HL, KL, and BHS-type sheets, respectively. The BHS-type germanenes can reproduce the experimental STM images. However, the HP growth mode would be suppressed at the initial growth stage of GeN clusters on pristine Al(111). Also, the BHS-type germanenes on pure Al(111) are less favorable in energy and could not keep their structural skeletons in our FPMD simulations carried out around the experimental temperature. These challenge their large quantity fabrications on experiment. Judged from the formation and substitution doping energies, the Al2Ge surface alloy is favorable in energy to form, on which the HP growth mode turns to be preferable. Furthermore, the BHS-type germanenes become energetically favorable, whose thermal stabilities at experimental temperature are also supported by our FPMD simulations. As to the preferred BHS-type germanenes on Al2Ge surface alloy, the adhesive energies are ~83 meV/Å2 showing the possibility for separating them from substrate. Hydrogenation could reduce the adhesive energies to help separate them from Al2Ge. These results show the effects of interface modification on tuning the 2D material growth, which also suggest the surface alloying as an effective pretreatment method to facilitate large quantity fabrication of germanene on aluminum.

ASSOCIATED CONTENT Supporting information The GeN structural motifs of BHS-1H germanene on pristine Al(111) with corresponding relative energies are provided in Figure S1. The illustrations of the collapsed BHS-2T and BHS-1H germanenes are shown in Figure S2. The initial and final structures of the 1 ps long FPMD simulations performed at 360 K are shown. The structures obtained by



fully optimizing the final geometries at 0 K are presented in Figure S2 also. The GeN structural motifs of BHS-1H germanene on Al2Ge surface alloy with corresponding relative energies are provided in Figure S3. The NEB minimum energy reaction path for the structural transition from BHS-2T to BHS-1H is presented in Figure S4. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial supports from the National Natural Science Foundation of China (NSFC) (Grants 11674129 and 11374128). Prof. Haiying Liu, Dr. Hongbo Wang, and Dr. Jinxiang Liu are highly appreciated for carefully reading through our manuscript.

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Table 1. The formation energies per atom (meV) calculated for the 2D Ge sheets grown on pristine and alloyed Al(111) surfaces. Without vdW Correction Substrate

With vdW Correction

Structure PBE

PBEsol

TPSS

PBE

PBEsol

TPSS

KL

432

557

526

629

780

748

HL

480

583

555

677

814

777

BHS-2T

400

513

465

612

748

700

BHS-1H

392

512

472

588

745

704

KL

297

430

387

495

651

608

HL

336

450

415

541

684

641

BHS-2T

362

477

427

572

712

656

BHS-1H

378

493

441

585

726

670

))12

))12,∗

Pristine

Alloyed

Table 2. The substitution energies per Ge atom ( and ))12 per Ge atom ( ) of the Al2Ge surface alloy in eV. Without vdW Correction

) and the formation energy

With vdW Correction

Parameter ))12

))12,∗

))12

PBE

PBEsol

TPSS

PBE

PBEsol

TPSS

-3.67

-4.13

-3.71

-4.03

-4.48

-4.05

-0.13

-0.24

-0.23

-0.15

-0.26

-0.24

0.41

0.57

0.52

0.62

0.80

0.74



Figure 1 The (a), (b), and (c) show HH, HT, and HP structures of GeN on pristine Al(111), respectively. The GS in brackets accounts for the ground state. The grey and blue balls are for


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Al and Ge atoms, respectively. The Ge atoms being protruded upward in (c) are highlighted by red color.

Figure 2 The relative energies Er in eV of GeN structures grown on pristine Al(111) as referred to the corresponding ground states. The red balls, black squares, and blue triangles are for the HH, HT, and HP growth modes, respectively. The grey dotted line is added at 0 eV for eye guiding.



Figure 3 The configurations of HL, KL, BHS-2T, and BHS-1H germanenes on Al(111) (a) and the corresponding STM images simulated at +1.3 V bias (b). The depiction of the color balls is same as that of Fig. 1.


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Figure 4 The adsorption configuration of a successive Ge atom after substitution doping one in Al(111) (a) and the geometrical configuration of Al2Ge surface alloy (b). The grey, yellow, and blue balls are for the Al atoms, the Ge atoms substituionally doped in Al(111), and the Ge atom adsorbed on the nearby hcp site. The A, B, and C mark the first, second, and third nearest neighboring Al atoms. The l1 and l2 stands for the A-B and B-C bonds, respectively.



Figure 5 The (a), (b), and (c) show the HH, HT, and HP structures of GeN clusters on Al2Ge surface alloy, respectively. The GS in brackets accounts for the ground state. The grey and blue balls are for the Al and Ge atoms, respectively. The Ge atoms being protruded upward in


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(c) are highlighted by red color. The Ge atoms substituionally doped in surface are accounted by yellow balls.

Figure 6 The ground state structures of Ge9, Ge10, and Ge12 grown on Al2Ge surface alloy. The depiction of the color balls is same as that of Fig. 5.



Figure 7 The relative energies Er in eV of GeN structures on Al2Ge surface alloy referred to the corresponding ground states. The red balls, black squares, and blue triangles are for the HH, HT, and HP structures, respectively. The empty blue squares are for the ground state structures of Ge9, Ge10, and Ge12. The grey dotted line is added at 0 eV for eye guiding.


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Figure 8 The configurations of HL, KL, BHS-2T, and BHS-1H germanenes on Al2Ge surface alloy (a) and the corresponding STM images simulated at +1.3 V bias (b). The depiction of the color balls is same as that of Fig. 5.



Figure 9 The evolution of temperature versus the simulation time. The (a) and (b) are for the BHS-2T germanenes on Al(111) and Al2Ge surface alloy, respectively. The (c) and (d) are for the BHS-1H germanenes on Al(111) and Al2Ge surface alloy, respectively. The insets are the geometries obtained at the end of the simulations (the left) and the after then optimized ones at 0 K (the right).


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Figure 10 The isosurfaces at 0.01 e/Å3 for charge accumulation (the left) and depletion (the right) for BHS-2T (a) and BHS-1H (b) grown on Al2Ge surface alloy. The charge accumulation and depletion densities are highlighted by green and orange colors, respectively. The depiction of the color balls is same as that of Fig. 5.



TOC Graphic


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Germanene Growth on Al(111): A Case Study of Interface Effect

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