Initial Growth Mechanism of Blue Phosphorene on Au(111) Surface

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Initial Growth Mechanism of Blue Phosphorene on Au(111) Surface Nannan Han, Nan Gao, and Jijun Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04209 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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The Journal of Physical Chemistry

Initial Growth Mechanism of Blue Phosphorene on Au(111) Surface Nannan Han, Nan Gao, Jijun Zhao* Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China

Abstract Blue phosphorene (blue P), a new two-dimensional allotrope of phosphorus, has attracted great attentions due to its high carrier mobility, suitable band gap and appreciable stability comparable to black phosphorene (black P). Motivated by recent experimental success in synthesizing monolayer blue P on Au(111) surface, here we investigate the nucleation mechanism and growth behavior of PN clusters on Au(111) substrate by ab initio calculations. During the initial growth stage, PN clusters transform from dispersed atoms to zigzag chain at N = 4, and further turn to ring-based one-dimensional chain at N = 11. This peculiar behavior is ascribed to the competition between the interaction among P atoms and the attraction on P atoms by Au substrate. Based on the interaction energies between black/blue phosphorene and Au(111) surface, we further propose that monolayer blue P can be synthesized on a chemical active metal substrate, while a relatively inert metal substrate would be beneficial to the growth of monolayer black P. Our theoretical findings offer experimentalists insightful guidance to fabricate monolayer blue P or black P by choosing appropriate substrate.

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Corresponding authors. E-mail: [email protected] (J. Zhao) 1

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Introduction Two-dimensional (2D) materials such as graphene1, silicene2, h-BN3, and transition metal dichalcogenide (TMD)4, have been extensively studied in the past decade and hold great promise for the future electronic nanodevices. However, all these 2D materials have their own shortcomings for device applications; for example, graphene has no band gap, and the carrier mobility in TMDs is usually very low5. Recently, a new 2D layered material named black phosphorene6 (black P) has been fabricated through exfoliation from the bulk black phosphorus. Excitingly, black P with thickness of ~10 nm exhibits a carrier mobility up to 1000 cm2/Vs7 and its electronic band gap ranges from 0.3 eV for bulk to 2.0 eV for monolayer as predicted by GW calculation8, indicating a promising perspective in 2D electronic devices. However, it is still very difficult to fabricate large-scale monolayer black P in laboratory due to its instability. In addition to monolayer black P, some other monolayer allotropes of phosphorus9-16 have been explored, such as β-P11,16,17, γ-P10,11,16, δ-P11,16,18 and porous P9, which exhibit tunable band gap from zero (metallic) to 2.30 eV9. Among these allotropes, β-P (namely blue phosphorene, abbreviated as blue P thereafter) has attracted enormous attentions owing to its appreciable stability, with formation energy only 2 meV/atom higher than black P17. Similar to silicene2, free-standing monolayer blue P has a graphene-like honeycomb lattice with out-of-plane buckling due to sp3 hybridization. However, its buckling height of 1.23 Å is much larger than that of silicene (0.44 Å). The large indirect gap of ~2.0 eV17 for monolayer blue P calculated using Perdew-Burke-Ernzerhof (PBE) functional makes it a potential material for field effect transistors and other 2D electronic devices. Furthermore, the band gap of blue P can be tuned from ~2.0 eV to ~1.4 eV by increasing the layer number from 1 to 817. In particular, the 2


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band gap of bilayer blue P is affected by the stacking types and the twisted angles between two layers19. The band gap of blue P can be also effectively tailored by applying strain18 and electric field20 or sculpturing into finite quantum dots; the latter are potential visible-light photocatalyst for water splitting21. In addition to electronic properties, other aspects of blue P, such as thermal conductivity22, superconductivity23, optical and magnetic properties24,25, have also been theoretically investigated. To achieve real applications, many vertical heterostructures based on blue P were designed, e.g., black P/blue P26-28, blue P/graphene29, blue P/MS2 (M=Mo, Nb, Ta)30,31, g-ZnO/blue P32. Recently, blue P was also proposed to be a promising candidate for gas molecular sensors33. Despite the extensive theoretical efforts on the physical properties and potential applications of blue P, synthesis of monolayer blue P remains a big challenge. Recently, Chen’s group reported growth of monolayer blue P on Au(111) substrate by molecular beam epitaxial method using black phosphorus as a precursor34. Using first-principles calculations, Zhang et al. proposed a half layer by half layer growth method of monolayer blue P on GaN(001) substrate with sufficiently high phosphorus concentration35. Nevertheless, the nucleation mechanism of monolayer blue P on metal substrate is still unknown. Previously, the nucleation mechanism and growth behavior of monolayer graphene36-38, silicene39, borophene40 and black P41,42 on various metal substrates have been comprehensively explored by first-principles calculations. It was revealed that the passivation effect of substrate plays a significant role to stabilize the initial clusters as well as the elemental monolayer. It is thus naturally to ask whether the formation mechanism of blue P is similar to the other elemental monolayers? How about the nucleation behavior of P clusters at the initial stage? How does the 3



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substrate affect the geometries of P clusters and monolayer blue P? In particular, since the freestanding black P monolayer is energetically more favorable than blue P, why blue P is synthesized in experiment rather than black P? To address the above questions, in this paper we investigate the nucleation and growth behavior of blue P at the initial stage using first-principles calculations. The competition among different structural motifs for PN clusters up to N = 24 is discussed in terms of bond energy and formation energy. The stabilities of monolayer blue P and black P supported on Au(111) surface are also compared. Our study presents an insightful understanding of the growth behavior of blue P and further provides a guidance for experimentalists to obtain high-quality P monolayers.

Methods All calculations are implemented in Vienna Ab Initio Simulation Package (VASP) within the density functional theory (DFT)43. The projector augmented wave (PAW)44 method is adopted with cutoff energy of 400 eV of the planewave basis. To describe the exchange-correlation interaction, the generalized gradient approximation (GGA)45 in Perdew-Burke-Ernzerhof (PBE) functional is used. To account for the van der Waals (vdW) interactions between P clusters/layers and the Au(111) substrate, the semiempirical dispersion-corrected DFT-D3 method by Grimme46 is employed. To confirm the interaction between P sheets and substrate, optB88-vdW47 method is also used. The 2D Brillouin zones are sampled by uniform k-point meshes with spacing of about 0.03/Å. The convergences of force for optimization and total energy for electron wave function self-consistency are set to be 0.02 eV/Å and 10-4 eV, respectively.

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To mimic a semi-infinite metal solid, a three-layer Au(111) slab model is used with Au atoms of the bottom layer fixed during relaxation, and the vacuum region of more than 15 Å is added. To avoid the interaction between image clusters caused by periodic boundary condition, a lateral supercell dimension of ~34 Å is used for Au(111) slab model. In the calculation of monolayer blue P on Au substrate, a 4 × 4 blue P supercell is used to match a 5 × 5 Au(111) supercell with the mismatch of 8.95%, which is in accordance with the experimental observation. For black P monolayer, a 5 × 6 supercell is used to match an 8 × 4 rectangle Au(111) supercell. The mismatch along a direction is 0.08% and 0.65% along b direction, respectively. The charge transfer between P monolayers and substrate is analyzed by the Bader method48. To explore the competition between P-P and P-Au bond, Mulliken bond population analysis49 implemented in CASTEP is performed.

Results and discussion To examine the nucleation mechanism at early stage of growth, PN clusters up to N = 24 on Au(111) surface are considered. The most stable configurations of PN clusters at selected sizes are displayed in Figure 1. The energetically preferred motif of PN clusters varies with cluster size, which can be divided into the following three stages. For N ≤ 3, P atoms favor to disperse at the hollow sites of Au surface. From N = 4 to N = 10, P atoms tend to aggregate and form zigzag-like chain adsorbed on two parallel lines of surface Au atoms with every P atom being bonded with one to three Au atoms. Further increasing number of P atoms to N ≥ 11, the favorable motif of PN cluster switches from zigzag chain to ring-based chain, namely, a straight network structure composed by 4-, 5- or 6-membered rings. Intuitively speaking, multi-ring 2D island as embryo of 5



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monolayer blue P should become most favorable at sufficiently large cluster size, as found for graphene at N = 1238. However, our calculations indicate that the P islands based on 4-, 5- or 6-membered rings prefer one-dimensional (1D) form rather than 2D growth behavior up to at least N = 24. Further exploring the critical size for 1D-to-2D transition would be of interest, but it is beyond our computational capability at present.

Figure 1. Most stable configurations of PN clusters up to N = 24 on Au(111) surface. The blue and yellow balls represent phosphorus and gold atoms, respectively. The size of PN cluster is given as inset of each picture, and the value below each picture presents its formation energy.

In order to characterize the thermodynamic stability of a PN cluster/monolayer on Au(111) surface, we define its formation energy (Eform) as: Eform = Etot − Esub − N × EP, 6


(1)

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where Etot, Esub and EP are energies of PN cluster/monolayer with Au(111) surface, Au(111) substrate and one P atom in bulk black phosphorus, respectively. In addition, the adsorption energy (Eads) of P atom or cluster is defined as: Eads = (Etot − Esub − EP)/N,

(2)

where Etot, Esub and EP are energies of the whole system, metal substrate and P atom/cluster, respectively. For adsorption of a P monomer on Au(111) surface, the most favorable position is the fcc hollow site with Eads of −3.608 eV/atom, which is 0.11 eV lower than that of the hcp hollow site. For comparison, the Eads of P monomer at the top of surface Au atom is only −1.646 eV/atom. From our calculation, the diffusion barrier of P monomer from fcc to hcp hollow site is 0.184 eV, which is a little higher than that of B monomer (0.141 eV)40 on Cu(111) surface. To describe the interaction between P atoms, we define bond energy as: Ebond = (Ecluster − Edisp)/Nbond,

(3)

where Ecluster and Edisp are energies of the system with bonded and separated P atoms, respectively; Nbond is the number of P-P bonds in the PN cluster. By definition, negative (positive) bond energy means attractive (repulsive) interaction between P atoms. The bond energy of a gas-phase P2 dimer in vacuum is −5.286 eV, while it becomes 0.033 eV when adsorbed on Au surface. This is a consequence of the strong adsorption energy of P monomer on Au(111), i.e., −3.608 eV/atom at the fcc hollow site. Hence, two P atoms prefer to disperse on Au substrate due to the positive P-P bond energy. According to our DFT calculations, in the case of two B or C atoms adsorbed on Au(111) surface, individual B or C atoms always prefer to stick together with the appreciable negative bond energy of −0.941 eV or −3.128 eV, respectively.

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For P3 cluster, the Eform of the system with three dispersed P atoms (Figure 2a) is 0.052 eV or 0.029 eV lower than the system with P atoms in the form of atomic chain (Figure 2b) or triangle (Figure 2d). The bond energy (0.015 eV) in P3 chain is still positive, while the Eads of P3 chain becomes to −2.042 eV/atom. Clearly, the interaction strength between P atoms increases with number of P atoms, while reduction of the amplitude of Eads suggests a weaker attraction between Au substrate and P atoms. Essentially, the formation of P cluster is a competition process of P-P and P-Au bonds. If the difference of bond strength between P-P and P-Au bonds is strong enough, P atoms will aggregate together; otherwise, they prefer to be dispersed. To explore this competition, Mulliken bond population analysis is used, in which a larger positive population represents a stronger covalent bonding interaction. As calculated, the population of P-P/P-Au bond is 0.63/0.44 in P3 chain. This population difference of 0.19 is not strong enough to propel P monomers overcoming the attraction by Au surface and aggregating. Hence, three P atoms on Au(111) surface prefer to disperse, mainly due to the larger number of P-Au bonds. Note that, if a triangle P3 is firstly placed to a position with three P atoms sitting at the fcc hollow sites, all P atoms would shift to the bridge sites after relaxation. Besides, the formation energy of triangle P3 at the bridge site (Figure 2c) is 0.431 eV higher than that of triangle P3 at the top site (Figure 2d). In other words, the most favorable adsorption site of P atoms transforms from fcc hollow site to top site after P atoms being bonded together, which results in an energy penalty to prevent the aggregation of these three P atoms.

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Figure 2. Several isomer configurations of PN clusters on Au(111) surface with N = 3 (a-d), N = 6 (e-h) and N = 13 (i-l). The formation energy of PN cluster is given under each picture; among them the red one corresponds to the most stable structure at a given size.

In the size range of N = 4−10, the preferred motif of PN clusters is zigzag chain. Here we take P6 as a representative, since six P atoms can form a hexagon that is the smallest unit of blue P honeycomb lattice. Here four isomer configurations of P6 are considered. The formation energy of zigzag P6 chain (Figure 2f) is lower than those of six dispersed P atoms (Figure 2e), hexagonal P6 ring (Figure 2g) and two isolated P3 triangles (Figure 2h) by 0.790 eV, 0.225 eV and 0.233 eV, respectively. Different from P3 chain, the bond energy of P6 chain is a negative value of −0.158 eV, indicating the attractive interaction among P atoms. Furthermore, the Mulliken bond population of P-P/P-Au in P6 chain is 0.60/0.38. The population difference of 0.22 is larger than that in P3 chain. The increase of population difference between P-P and P-Au in P6 chain gives rise to the stronger P-P interaction, which provides the driving force for P monomers to aggregate by overcoming the 9



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attraction of substrate. For the hexagonal ring of P6, the amplitude of bond energy (−0.094 eV) is less than that of the zigzag chain and the Mulliken bond population of 0.57 in P-P is smaller than P6 chain. Besides, all P atoms in the P6 ring sit at the top sites of Au surface, while three P atoms in the P6 chain locate at the fcc hollow sites. As mentioned before, Eads of P monomer at the fcc hollow site is −1.962 eV/atom, much lower than that of the top site. Hence, P atoms favor to bond together and form into a zigzag chain at N = 6. As cluster size further increases (N ≥ 11), the ring-based 1D motif prevails, which has the same buckling as monolayer blue P from the side view. For example, at N = 13, the formation energy of ring-based 1D configuration (Figure 2g) is the lowest, followed by zigzag chain (Figure 2i, 0.322 eV higher) and curved triple-ring chain (Figure 2k, 0.860 eV higher). Clearly, P13 cluster still prefers the 1D growth pattern, but adopts the ring-based 1D motif so that some P atoms are able to form three strong P-P bonds to enhance the stability. Note that the 2D island composed of three pentagons and three triangles (Figure 2l) reported in previous work34 is energetically less favorable by 1.61 eV. Figure 3 shows the evolution of Eform as function of cluster size N for different structural motifs. With increasing cluster size, the formation energy of Au-supported PN cluster reduces monotonically, indicating that formation of PN cluster becomes easier. This behavior is analogous to the initial growth stage of boron monolayer40, although the structural motifs of PN and BN clusters are entirely different.

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Figure 3. Formation energy as function of cluster size N. The black, red, green, blue, purple and orange lines represent structural motif of dispersed, zigzag chain, ring-based 1D chain, multi-ring 2D island, black P and 3D structure, respectively. The insets are several atomic configurations of the corresponding PN clusters.

Furthermore, we consider four PN clusters (N = 10, 13, 19, 24), which are directly truncated from monolayer black P structure and named as black P cluster, as shown in Figure 4a,. After relaxation, these black P clusters basically retain the same atomic configuration as monolayer black P. The Eform of black P clusters on Au surface are plotted in Figure 3. Clearly, Eform of black PN clusters (N = 10, 13, 19, 24) of −0.609~−2.276 eV are much higher than those of blue PN clusters, indicating the less preference of black P clusters.

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Figure 4. Top and side views of (a) four black PN clusters on Au(111) surface with N = 10, 13, 19 and 24; (b) five 3D PN clusters with N = 6, 10, 13, 16 and 19, respectively. The formation energy of PN cluster is given under each picture.

In addition to 1D and 2D structural motifs, we also consider some three-dimensional (3D) PN clusters on Au(111) surface. As displayed in Figure 4b, five 3D structures of PN clusters at N = 6, 10, 13, 16, 19 are taken from the previously reported structures for gas-phase PN50 or AsN51 clusters. Similar to black P clusters, the formation energies of Au(111)-supported 3D PN clusters at each cluster size are 1.566~4.576 eV higher than those of the most stable configurations. This can be understood by the fact that either zigzag chain or ring-based network are relative flat and have more P atoms interacted with Au substrate than black P clusters or their 3D counterparts. This situation is similar to the case of silicene clusters, in which 2D islands of SiN cluster (N = 6−24) are notably more stable than the 3D geometries39. Extending from clusters to monolayer black P or 12


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blue P, one can anticipate that blue P should possess higher stability on Au surface than black P due to the smaller distance between two layers of P atoms.

From our calculation, Eform of freestanding monolayer blue P and black P are 0.110 eV/atom and 0.089 eV/atom, respectively. The energy difference of 21 meV/atom is larger than that between bulk blue P and black P (2 meV/atom) from previous DFT calculations17. However, the experimentally synthesized phosphorus monolayer on Au(111) surface is blue P rather than black P34. As aforementioned, Eform of blue P clusters is much lower than that of black P clusters, which might partially account for the prepared blue P monolayer. To further reveal the origin of this fact, the stabilities of monolayer blue P and black P on Au(111) are explored, and the corresponding structures are shown in Figure 5. For freestanding blue P, the equilibrium lattice parameter is 3.277 Å and the buckling height is 1.233 Å. For monolayer black P, the lattice parameters are a = 4.603 Å and b = 3.303 Å, and the vertical distance between the two atomic layers is 2.120 Å. After relaxation, the buckling height of Au(111)-supported blue P increases to 1.580 Å, which is 0.347 Å larger than the freestanding blue P. Similarly, the interlayer distance of 2.460 Å in Au(111)-supported black P is 0.340 Å larger than the freestanding black P. The average vertical distance between blue P (black P) monolayer and Au(111) surface is 2.419 Å (2.627 Å), suggesting a slightly stronger interaction between blue P and Au(111) surface. Furthermore, the charge density difference between monolayer blue P/black P and Au substrate is plotted in Figure 5. One can see that charge transfer occurs mainly on the half-layer of bottom atoms in black P, while most P atoms in blue P interact with the substrate. According to Bader analysis, blue P monolayer accept 0.039 electrons per atom from Au substrate, while black P monolayer only 13



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accepts 0.019 electrons per atom, which further illustrates the stronger interaction between blue P and Au(111) surface.

Figure 5. Top and side views of charge density difference between Au surface and monolayer (a) blue P and (b) black P, respectively. The blue and red parts represent the aggregation and depletion of electrons, respectively. The isosurface is 0.004 e/Å3.

Similar to the other 2D materials, Eform of P monolayers is greatly reduced due to the passivation of substrate. From our calculations, formation energies of Au(111)-supported P monolayers are −0.213 eV/atom for blue P and −0.134 eV/atom for black P, respectively, which are much smaller than the freestanding blue P (Eform = 0.110 eV/atom) and black P (Eform = 0.089 eV/atom). Especially, the reduction of Eform of blue P is 0.1 eV/atom larger than that of black P, leading to lower Eform of blue P with regard to black P. As a consequence, formation of monolayer blue P on Au(111) surface is easier than black P. We further use optB88-vdW dispersion correction method to check their relative stability and the Eform are calculated to be −0.119 eV/atom and 14


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−0.052 eV/atom for blue P and black P, respectively. The energy difference (0.067 eV) is just 0.012 eV less than that calculated by DFT-D3 method (0.079 eV). Considering the effect of strains (i.e., lattice mismatch), several black P structures on Au surface with different mismatches are explored. The Eform are −0.134 eV/atom, −0.136 eV/atom, −0.125 eV/atom and −0.081 eV/atom for black P with lattice mismatch of 0.08% (lattice a) & 0.65% (lattice b), 6.58% (lattice a) & 0.64% (lattice b), 0.64% (lattice a) & 4.08% (lattice b), and −7.69% (lattice a) & 8.23% (lattice b), respectively. All these Eform of black P structures are larger than blue P (−0.213 eV/atom) with an 8.95% lattice mismatch. We also calculated blue P with a small lattice mismatch of 1.44% and the corresponding Eform is −0.198 eV, which is still smaller than those values for the black P. The above results further indicate the harder synthesis of black P on Au(111) surface. Note that the mismatch of blue P grown on Au(111) in experiment is 8.95%.35 Hence, the comparison between blue P with an 8.95% lattice mismatch and black P with different mismatches should be more meaningful. As known, Au is a chemical active substrate for 2D materials and has strong impact on the stability of P monolayer. If a more inert metal substrate is used, would blue P still be synthesized? Bear this question in mind, we explore Al(111) substrate, since it was proposed as a moderate-strength substrate to grow stanene52. From our calculations, Eform of monolayer blue P on Al(111) surface is −0.064 eV/atom, while it is −0.081 eV/atom for black P. Interestingly, the formation energy of monolayer black P becomes smaller on Al(111) surface. On the contrary, a chemical active substrate Ag is explored to verify our proposal. The formation energy of blue P on Ag(111) surface is −0.135 eV/atom, which is 0.016 eV/atom lower than black P (−0.119 eV/atom). The lower Eform of blue P indicates that Ag(111) surface is more proper for the growth of blue P 15



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sheet. In brief, a substrate with low chemical activity is helpful for the growth of black P, whereas monolayer blue P can be synthesized on a relative chemical active substrate. This finding offers experimentalists a key guidance to synthesize the desired P monolayers by choosing proper metal substrate.

Conclusions In this paper, we explore the nucleation and growth behaviors of blue P on Au(111) surface. At the beginning stage of nucleation, P atoms favor to disperse than forming small PN clusters with N ≤ 3. Starting from N = 4, P atoms tend to aggregate into a zigzag chain up to N = 10, and then PN cluster adopts ring-based 1D motif for N ≥ 11. The unique growth behavior of PN clusters is caused by the competition of interaction among P atoms and the attraction on P atoms by Au substrate. From the comparative calculations of monolayer blue P and black P on Au(111) surface, we suggest that an active metal substrate can help stabilize monolayer blue P over black P. On the contrary, an inert metal substrate such as Al(111) surface could be beneficial to the fabrication of black P.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (11574040), the Fundamental Research Funds for the Central Universities of China (DUT16-LAB01), and the Supercomputing Center of Dalian University of Technology.

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Initial Growth Mechanism of Blue Phosphorene on Au(111) Surface

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