Bandgap Modulated by Electronic Superlattice in Blue Phosphorene

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Bandgap Modulated by Electronic Superlattice in Blue Phosphorene Jincheng Zhuang, Chen Liu, Qian Gao, Yani Liu, Haifeng Feng, Xun Xu, Jiaou Wang, Jijun Zhao, Shi Xue Dou, Zhenpeng Hu, and Yi Du ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02953 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Bandgap Modulated by Electronic Superlattice in Blue Phosphorene

Jincheng Zhuang,†,‡,# Chen Liu,§,# Qian Gao,|| Yani Liu,†,‡ Haifeng Feng,†,‡ Xun Xu,† Jiaou Wang,§ Jijun Zhao,

⊥

Shi Xue Dou,†,‡ Zhenpeng Hu,|| and Yi Du*,†,‡

†

Institute for Superconducting and Electronic Materials (ISEM), Australian Institute for Innovative Materials (AIIM), University of Wollongong, Wollongong, NSW 2525, Australia

‡

BUAA-UOW Joint Centre, Department of Physics, Beihang University, Haidian District, Beijing 100091, China

§

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

||

School of Physics, Nankai University, Tianjin, 300071, China

⊥

Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China

______________ * To whom correspondence should be addressed: [email protected]

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Abstract Exploring stable two-dimensional materials with appropriate band gaps and high carrier mobility is highly desirable due to the potential applications in optoelectronic devices. Here, the electronic structures of phosphorene on Au(111) substrate are investigated by scanning tunneling spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. The substrate-induced phosphorene superstructure gives a superlattice potential, leading to a strong band folding effect of the sp band of Au(111) on the band structure. The band gap could be clearly identified in the ARPES results after examining the folded sp band. The value of the energy gap (~ 1.1 eV) and the high charge carrier mobility comparable to that of black phosphorus, which is engineered by the tensile strain, are revealed by the combination of ARPES results and DFT calculations. Furthermore, the phosphorene layer on the Au(111) surface displays high surface inertness, leading to the absence of multilayer phosphorene. All these results make it manifest that the phosphorene on Au(111) could be a promising candidate, not only for fundamental research, but also for nanoelectronic and optoelectronic applications.

Keywords: blue phosphorene, bandgap, electronic superlattice, ARPES, STM, band folding

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As the most abundant pnictogen in the Earth, elemental phosphorus has various allotropes ranging from white phosphorus1 to red phosphorus,2 violet phosphorus,3 and black phosphorus,4 where the color depends on the intrinsic energy gap. Among them, black phosphorus is of particular interest for intriguing applications in electronic and optoelectronic devices due to its attractive properties, such as its layered structure,5 tunable band gap,6 high carrier mobility,7 and anisotropic physical properties.8 Tremendous work has been devoted to exploring the two-dimensional (2D) phosphorus allotropes complementary to black phosphorus with tunable properties. The stable structural phases of layered phosphorus, besides black phosphorus, have been proposed theoretically by flipping the buckled positions of specific P atoms without changing the local bond lengths and honeycomb structures.9-13 These allotropes vary from indirect band-gap semiconductors to direct band-gap semiconductors with tunable gap sizes and high carrier mobility up to several thousand cm2V-1s-1.9,11 These 2D phosphorene could not be directly prepared by general mechanical exfoliation methods, however, due to the absence of layered bulk allotropes in nature. Recently, blue phosphorene, the so-called ߚ-P, has been successfully synthesized by molecular beam epitaxy (MBE) on Au(111) substrate, simultaneously forming a restructure with large tensile strain induced by the interaction with the underlying substrate.14,15 The superstructure could break the lattice symmetry and crystalline field of pristine free-standing blue phosphorene, providing the additional freedom to modulate its electronic properties. In fact, scanning tunneling spectroscopy (STS) of phosphorene on Au(111) shows a band gap of around 1.10 eV, which is smaller than the predicted band gap in excess of 2 eV for free-standing blue phosphorene.14 Angle-resolved photoemission spectroscopy (ARPES) measurements, however, are desirable to directly investigate the electronic structure of phosphorene and its interaction with the underlying Au(111) substrate. Furthermore, the large tensile strain residing in the phosphorene reconstruction makes its structural model of atomic arrangement is still unclear. High resolution scanning tunneling microscopy (STM) images and comparative studies of the ARPES results and simulated electronic structures based on the appropriate structural model could be beneficial to resolve this issue. 3


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In this work, we have investigated the crystal structure and electronic properties of phosphorene grown on Au(111) substrate by low temperature STM, low energy electron diffraction (LEED), ARPES, and density functional theory (DFT) simulations. The structural model, 4×4 phosphorene matching the 5×5 periodicity of Au(111), has been established by high-resolution STM images and LEED patterns. The ARPES results unambiguously reveal the existence of an energy gap correlated with the deposited phosphorene combined with the superstructure-induced strong band folding effect of the sp band of the Au(111) substrate. The DFT simulations are well consistent with the ARPES results and imply that the tensile strain could strongly modulate the carrier mobility up to a value of more than ten thousand cm2V-1s-1. The appropriate band gap and superior carrier mobility indicate that the phosphorene on Au(111) could be a promising candidate for nanoelectronics and optoelectronics.

RESULTS AND DISCUSSION A compact hexagonal honeycomb structure is formed on the Au(111) surface after the deposition of phosphorus, with one dark core surrounded by six triangular bright petals, as shown in Figures 1a and b, which is the same as in previous reports.14,15 The initial formation of this structure prefers to emerge at the terrace edge of the substrate, as shown in Figure S1 in the Supporting Information, which may be correlated with the effect of the energy barrier provided by the edges against the migration of deposited phosphorus atoms. The high resolution STM image (Figure 1c) displays that the lattice constant of the honeycomb structure is around 1.45 nm, which is much larger than the periodicity of 1×1 free-standing (FS) blue phosphorene (~0.33 nm).9,13 The closest distance among different protrusions is 0.36 nm, as labelled in Figure 1c, which is almost one quarter of 1.45 nm and slightly larger than the value for FS blue phosphorene. Thus, the hexagonal honeycomb arrangement is correlated with the superstructure. The periodicity relationship between them could also be reflected by the fast Fourier transform (FFT) pattern of Figure 1b, as shown in Figure 1h, where the green circles and red circles were identified as 4


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representing the periodicity of the closest protrusions and the superstructure, respectively. The superstructure could easily be identified as the 5×5 restructure in terms of the 1×1 unit cell of Au(111) substrate marked by blue circles in the LEED pattern in Figure 1g. Based on both the STM results and the LEED patterns, it could be inferred that the 4×4 superstructure in terms of P is commensurate with the 5×5 supercell of the Au(111) surface. A similar structural model has been reported based on reflected high-energy electron diffraction (RHEED) patterns.16 To build the coperiodic lattice of the P/Au hybrid system, phosphorene is stretched by ∼8.9%, which is relatively large for the monolayer samples. Such a large tensile strain could cause disconnection between parts of two nearest P atoms, forming the quantum dot-like (zero dimensional, 0D) or nanoribbon-like (one-dimensional, 1D) structures. Although there are some grain boundaries on the surface, Figure 1b shows a typical 2D structure instead of these 0D and 1D nanostructures. Thus, it is still not clear where the large tensile strain in the 4×4 superstructure resides. On the other hand, the second layer phosphorene could not be formed by prolonging the deposition time with the same window of substrate temperature (180-350°C), indicating the high mobility of further deposited P atoms and/or the absence of dangling bonds on the monolayer phosphorene surface. Nevertheless, the topography was changed when we changed the substrate temperature down to room temperature during the further deposition process, as shown in Figure 1d. The high resolution STM images (Figures 1e and f) show that most of the P atoms/clusters are located at the hollow sites of the monolayer phosphorene without altering the previous periodicity, indicating their higher energy barrier for P atoms compared with other positions. It should be noted that these hollow sites could not be fully covered by P atoms even after applying more deposition time at room temperature, implying the adsorption characteristic of P atoms on the monolayer phosphorene surface and the possible repulsive forces among them. At a certain state of the STM tip with sharp shape, atomic-resolution STM images (Figure 1f) are obtained,17-20 where the buckled up P atoms in phosphorene could be clearly resolved to illustrate the 1×1 periodicity and the 4×4 superstructure. By annealing at 300°C or 5


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above, the clean monolayer phosphorene surface could be fully recovered, as shown in Figure S2. The quality of monolayer phosphorene could be totally preserved after long adsorption-desorption cycles under UHV conditions, implying small binding energy of the P adatoms on phosphorene. Based on the commensurate superstructure and the surface reactivity with P adatoms, we propose the following structural model: the lattice constant of 4×4 phosphorene matches with the 5×5 unit cell of Au(111) with the large tensile strain relaxed in the hollow sites (Figure 2a), forming a buckled honeycomb arrangement (Figures 2a and b) and providing a large space for the accommodation of P adatoms at room temperature. The evolution from the atomic structure model to the simulated STM image and then to the experimental STM image in Figure 2c indicates that the proposed structural model fundamentally resembles the experimental results. In the structural model, 12 out of 32 phosphorus atoms in one unit cell (labelled by the red rhombus in Figure 2a) are buckled up, which deviates from the theoretical models of FS blue phosphorene, wherein half of the phosphorus atoms are in higher positions than the other half.9,13 Meanwhile, the average height difference ∆z between the red P atoms and the green P atoms in Figure 2b is about 1.79 Å, which is also larger than the buckled height of FS blue phosphorene (1.24 Å).12 All these results demonstrate that the structural properties of phosphorene are strongly modulated by the underlying Au(111) substrate. Apart from the structural aspect, phosphorene substrate intercalation has a significant impact on the electronic structure. Figure 3 shows the constant-energy contour (CEC) of phosphorene on the Au(111) surface at different energy levels investigated by ARPES measurements. The electronic band structure of clean Au(111) was also measured as the reference, as shown in Figures 3a-c. There are two pockets in the electronic structure of Au(111) crossing the Fermi surface, where the pocket located at around the zone center ߁ point and the large pocket around the boundary of Brillion zone (BZ) are correlated with the Shockley surface state and the sp band of Au(111), respectively. The sizes of both of the two pockets shrink with increasing electron binding energy (Eb = E – EF), which is consistent with previous reports.21 After the formation of monolayer phosphorene on the Au(111) 6


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surface, Kagome-like patterns appear in the CEC results, with the periodicity matching well with the superimposed BZs of the superstructure, as shown in Figure 3d. The triangular shape of Fermi pockets centered at the KS (KS') point of the superstructure BZs resemble the wrapped Dirac-like band in CEC with fixed Eb, where each Dirac cone interacts with its three nearest neighboring Dirac cones, forming the van Hove singularity (VHS), i.e., a divergence in the density of states (DOS).22 Nevertheless, with increasing Eb, each Fermi pocket splits into two replicas, as shown in Figures 3e and f, which is inconsistent with the trend in the changing size of the Fermi pocket in Dirac cones, where it is expected to shrink in size (when the energy level of the Dirac point, ED, is below the Fermi level) or to merge into one contour (when ED is above the Fermi level). Such a phenomenon can be clearly identified in the enlarged view of the photoemission intensity contour at Eb = 0 eV (Figure 3g) and Eb = 0.6 eV (Figure 3h) with superimposed BZs of the 4×4 superstructure. The replicas cross each other without any interaction, as shown in Figure 3h, which could be due to two possible reasons. One is that the two groups of replicas have originated from two isolated parts, for example, two isolated graphene sheets at a twisted angle will form two groups of Dirac cones without any band hybridization.23 This explanation can be excluded, as no rotation of monolayer phosphorene has been detected in numerous STM images. The other one is the band folding effect evoked by the superlattice potential.24 We marked six BZ corners of the superstructure (red honeycomb) from 1 to 6 in an anticlockwise direction, as shown in Figure 3a, to illustrate their relationship with the sp band in Au(111). The sp band in CEC crosses each corner twice in the equivalent BZs, leading to the folding of the sp band into each BZ of the superstructure, labelled by S1-S6 and S1'-S6' in Figure 3a, respectively, forming a Kagome-like pattern due to the similar position of the S band and the S' band relative to their BZ corners. With the increased Eb, the shifts of the S and S' bands with respect to the BZ corner show opposite trends, creating the band splitting phenomenon shown in Figure 3h. The band folding effect is further demonstrated by the energy-momentum dispersion results, as shown in Figure S3, where the Fermi velocity of these replicas is same as that of the sp band in Au(111). 7


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The formation of electronic superlattice potential could be visualized in the sequence of bias dependent the scanning tunneling spectroscopy (STS) maps. The presence of the periodic potential also leads to a spatial variation in the local density of states (LDOS), as shown in Figure 4, where the hexagonal pattern of the potential is clearly visible at the bias voltages of -1.2 V and 0.8 V. Nevertheless, the hexagonal pattern is fade at the energy of -0.6 V, as shown in Figure 4c, which will be explained below. Figure 5a shows the ARPES spectra of phosphorene on Au(111) along the MS-߁ S-KS-MS direction. Except for the duplicate sp band of Au(111) substrate (indicated by the black arrows in Figure 5a) crossing the Fermi surface, all other valence bands are below the Fermi level, indicating the semiconducting nature of the phosphorene. The valence band labelled by the red arrow in Figure 5a is located at an energy level around 1.0 eV below the Fermi level, which is absent in the ARPES spectra of bare Au(111) substrate. Thus, this band is attributed to the contribution from phosphorene. In order to understand the experimental results, we performed density functional theory (DFT) calculations for 4×4 phosphorene on the Au(111) surface based on the structural model in Figure 2. The simulation results show that the value of the band gap is 1.18 eV, as displayed in Figure 5c, which is similar to that obtained by STS measurements displayed in Figure 5d. The red and purple arrows stand for the valence band maximum (VBM) and conduction band minimum (CBM), respectively, which is defined by the deviation points of linear current-bias dispersion (red dashed line). From STS results, the value of VBM is almost the same to that obtained from ARPES spectra, and the bandgap value is also close to the simulation result. It should be noted that the FS blue phosphorene shows a wide fundamental band gap in excess of 2 eV.9,13 The band gap depends sensitively on the applied in-layer strain, with a possible decrease of up to 50% in view of the nonplanarity of the structure.9 Our simulations imply that the varied electronic properties and reduced value of the band gap are caused by the cooperative effect of the large tensile strain residing in the 4×4 superstructure and the superlattice potential due to the phosphorene-Au(111) interaction. We compared the experimentally determined bands with the DFT calculations in Figure 5b, where the various top valence bands are in close agreement with the 8


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theoretical prediction (overlay). Furthermore, the blurred hexagonal pattern in Figure 4c is caused by the absence of the DOS in blue phosphorene at the energy level of Eb = 0.6 eV. Thus, our STS maps, ARPES results, and DFT simulations are consistent with each other, indicating the correctness of the proposed structural model. Besides the electronic band gap, charge carrier mobility is another factor determining the electronic properties for potential applications of 2D semiconductors. Thus, we further estimate the room temperature carrier mobilities, µ, of phosphorene on Au(111) based on the so-called acoustic phonon limited approach by using the following expression:25,26 ߤ=௞

௘ℏయ ஼

(1)

೔ మ ∗ ಳ ்௠೐ ௠೏ (ாభ )

where e is the electronic charge, ℏ is the reduced Planck’s constant, kB is Boltzmann’s constant, ݉௘∗ is the effective mass along the transport direction and ݉ௗ is the average effective mass determined by ݉ௗ = ඥ݉∗௫ ݉௬∗ . The x and y are correlated with the armchair (߁ S-KS) direction and zigzag (߁ S-MS) direction, respectively, as shown in Figure S4. The term ‫ܧ‬ଵ௜ represents the deformation potential constant derived from the respective valence-band minimum for holes or conduction-band maximum for electrons as functions of lattice compression or dilatation of phosphorene on Au(111). The term ‫ ܥ‬is the elastic modulus of the longitudinal strain in the propagation directions (both x and y) of the longitudinal acoustic wave. We calculated the longitudinal strain from -1.0% to 1.0% with a step size of 0.5% to fit the values for ‫ ܥ‬and ‫ܧ‬ଵ௜ , as shown in Figure S5, and then to obtain the value of the carrier mobility. All these values are listed in Table 1. The electron mobility in 4×4 phosphorene on Au(111) is relatively large (more than 104 cm2V-1s-1), which is even larger than that in black phosphorus at room temperature (2300 cm2V-1s-1).25 Such a high mobility is mainly attributable to the small effective mass of electrons (0.13 m0) along both the x and y directions. Although black phosphorus also has a small electron effective mass along the armchair direction (0.14-0.18), it has a heavy carrier along the zigzag direction.25 The average effective mass ݉ௗ in black phosphorus is larger than that in 4×4 phosphorene on Au(111), leading to decreased mobility according to Equation 1. The small anisotropy for all these parameters in 4×4 phosphorene on Au(111) is correlated with its 9


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more isotropic crystal structure. It should be noted that a subtle structural difference could result in different electronic properties in 2D materials, and the large tensile strain residing in 4×4 phosphorene significantly modulates the electronic properties, as discussed above. Thus, the carrier mobility of 4×4 phosphorene without tensile strain was also calculated for a comparative study, as shown in Table S1. The electron effective masses for both directions are obviously enhanced in this case, resulting in electron mobility that is reduced by one or two orders of magnitude. Consequently, the interaction between 4×4 phosphorene and the Au(111) substrate plays a crucial role in the improvement of electron mobility. The combination of a moderate band gap and high carrier mobility would make phosphorene an excellent contender for the next generation of optoelectronic devices.

CONCLUSIONS In summary, 4×4 phosphorene matching the 5×5 unit cell of Au(111) has been identified by STM and LEED results. A large tensile strain has been found in the hollow sites of 4×4 phosphorene due to the interaction with the underlying substrate, as demonstrated by both high-resolution STM images and the simulated structural model. The ARPES results show a strong band folding effect induced by the superstructure and the energy gap in 4×4 phosphorene. Utilizing first-principles calculations, we have shown that the tensile strain residing in 4×4 phosphorene modulates the electronic structure, leading to a reduced energy gap (~ 1.1 eV), small effective mass, and high carrier mobility, even higher than that in black phosphorus. Our results provide a possible means to the engineering of the electronic structure of phosphorene, which is promising for the applications in both of nanoelectronics and optoelectronics.

METHODS Synthesis of phosphorene on Au(111). The phosphorene samples were fabricated by the deposition of phosphorus atoms on Au(111) substrate by evaporation from a crucible containing bulk black phosphorus in a molecular beam 10


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epitaxy (MBE) chamber attached to an in situ STM or ARRPES system under ultrahigh vacuum (UHV). A clean Au(111) substrate was prepared by argon ion sputtering and subsequently annealed at 580°C for several cycles. Characterization of structural and electronic properties. The STM measurements were carried out using a low-temperature UHV STM/scanning near-field optical microscopy system (SNOM1400, Unisoku Co.), where the bias voltages were applied to the substrate. The differential conductance, dI/dV, spectra were acquired by using a standard lock-in technique with a 20 mV modulation at 613 Hz. In situ ARPES and LEED characterizations were performed at the Photoelectron Spectroscopy Station in the Beijing Synchrotron Radiation Facility (BSRF) using a SCIENTA R4000 analyzer, where the Regional Distribution Center connects with the MBE chamber, LEED chamber, and APRES chamber ensure the sample transferring process under in UHV condition. A monochromatized He I light source (21.2 eV) was used for the band dispersion measurements. The total energy resolution was set to 15 meV, and the angular resolution was set to ~0.3°, which gives a momentum resolution of ~0.01 π/a. DFT calculations. The calculations were carried out based on density functional theory (DFT), as implemented in the Vienna ab initio simulation package (VASP) code.27-28 The interaction between electrons and ions was described by the projector-augmented wave method.29 The Perdew-Burke-Ernzerhof (PBE) functional was used to describe the exchange-correlation (XC) interaction.30 The hybrid functional of Heyd, Scuseria and Ernzerhof (HSE06) was also used to obtain a better description of the energy gap and band structure for the relaxed configuration.31,32 The wave functions were expanded in plane waves to a cutoff of 300 eV for all calculations. A vacuum layer larger than 15 Å was applied to avoid interactions between the periodic images, and Monkhorst−Pack k-point grids of 5×5×1 were used to sample the Brillouin zone. In the process of geometry relaxation, the convergence criteria of total energy and residual forces was set to 10-5 eV per unit cell and 0.05 eV/Å for each ion. The STM images were obtained using the constant height mode by Tersoff−Hamann approach and visualized using the VESTA program.33,34 11


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figures of growth dynamics of phosphorene on Au(111); figures of ordered and reversible P adatoms; figures of band dispersion of bare Au(111) and P/Au(111); calculations of carrier effective masses, deformation potentials, elastic moduli, and carrier mobilities.

AUTHOR CONTRIBUTIONS #

J.Z. and C.L. contributed equally to this work.

ACKNOWLEDGEMENTS This work was financially supported by Australian Research Council (ARC) Discovery Projects (DP160102627 and DP170101467), the National Natural Science Foundation of China (Grant Nos. 11575227 and 21773124), the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) (Grant No. U1501501), and a BUAA-UOW Joint Research Centre Small Grant. The work was partially supported by the University of Wollongong through an AIIM FOR GOLD grant.

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3. Thurn, V. H.; Krebs, H. Crystal Structure of Violet Phosphorus. Angew. Chem. Int. Ed. 1966, 5, 1047-1048. 4. Hultgren, R.; Gingrich, N. S.; Warren, B. E. The Atomic Distribution in Red and Black Phosphorus and The Crystal Structure of Black Phosphorus. J. Chem. Phys. 1935, 3, 351. 5. Brown, A.; Rundqvist, S. Refinement of the Crystal Structure of Black Phosphorus. Acta Crystallogr. 1965, 19, 684-685. 6. Li, L. K.; Kim, J.; Jin, C.; Ye, G. J.; Qiu, D. Y.; da Jornada, F. H.; Shi, Z.; Chen, L.; Zhang, Z.; Yang, F.; Watanabe, K.; Taniguchi, T.; Ren, W.; Louie, S. G.; Chen, X. H.; Zhang, Y.; Wang F. Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene. Nat. Nanotech. 2017, 12, 21-25. 7. Li, L. K.; Yang, F.; Ye, G. J.; Zhang, Z.; Zhu, Z.; Lou, W.; Zhou, X.; Li, L.; Watanabe, K.; Taniguchi, T.; Chang, K.; Wang, Y.; Chen, X. H.; Zhang, Y. Quantum Hall Effect in Black Phosphorus Two-Dimensional Electron System. Nat. Nanotech. 2016, 11, 593-597. 8. Xia, F.; Wang, H.; Jia, Y. Rediscovering Black Phosphorus as An Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 2014, 5, 4458. 9. Zhu, Z.; Tománek, D. Semiconducting Layered Blue Phosphorus: A Computational Study. Phys. Rev. Lett. 2014, 112, 176802. 10. Guan, J.; Zhu, Z.; Tománek, D. Phase Coexistence and Metal-Insulator Transition in Few-Layer Phosphorene: A Computational Study. Phys. Rev. Lett. 2014, 113, 046804. 11. Zhang, S.; Xie, M.; Li, F.; Yan, Z.; Li, Y.; Kan, E.; Liu, W.; Chen, Z.; Zeng, H. Semiconducting Group 15 Monolayers: A Broad Range of Band Gaps and High Carrier Mobilities. Angew. Chem. Int. Ed. 2016, 55, 1666-1669. 12. Ghosh, B.; Nahas, S.; Bhowmick, S.; Agarwal, A. Electric Field Induced Gap Modification in Ultrathin Blue Phosphorus. Phys. Rev. B 2015, 91, 115433. 13. Zeng, J.; Cui, P.; Zhang, Z. Half Layer by Half Layer Growth of A Blue Phosphorene Monolayer on A 13


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GaN(001) Substrate. Phys. Rev. Lett. 2017, 118, 046101. 14. Zhang, J. L.; Zhao, S.; Han, C.; Wang, Z.; Zhong, S.; Sun, S.; Guo, R.; Zhou, X.; Gu, C. D.; Yuan, K. D.; Li, Z.; Chen, W. Epitaxial Growth of Single Layer Blue Phosphorus: A New Phase of Two-Dimensional Phosphorus. Nano Lett. 2016, 16, 4903-4908. 15. Gu, C.; Zhao, S.; Zhang, J. L.; Sun, S.; Yuan, K.; Hu, Z.; Han, C.; Ma, Z.; Wang, L.; Huo, F.; Huang, W.; Li, Z.; Chen, W. Growth of Quasi-Free-Standing Single-Layer Blue Phosphorus on Tellurium Monolayer Functionalized Au(111). ACS Nano 2017, 11, 4943-4949. 16. Xu, J. P.; Zhang, J. Q.; Tian, H.; Xu, H.; Ho, W.; Xie, M. One-Dimensional Phosphorus Chain and Two-Dimensional Blue Phosphorene Grown on Au(111) by Molecular-Beam Epitaxy. Phys. Rev. Mater. 2017, 1, 061002. 17. Andryushechkin, B. V.; Shevlyuga, V. M.; Pavlova, T. V.; Zhidomirov, G. M.; Eltsov, K. N. Adsorption of O2 on Ag(111): Evidence of Local Oxide Formation. Phys. Rev. Lett. 2016, 117, 056101. 18. Zhuang, J. C.; Gao, N.; Li, Z.; Xu, X.; Wang, J.; Zhao, J.; Dou, S. X.; Du, Y. Cooperative Electron−Phonon Coupling and Buckled Structure in Germanene on Au(111). ACS Nano 2017, 11, 3553-3559. 19. Wang, Y. L.; Gao, H. J.; Guo, H. M.; Liu, H. W.; Batyrev, I. G.; McMahon, W. E.; Zhang, S. B. Tip Size Effect on the Appearance of A STM Image for Complex Surfaces: Theory versus Experiment for Si(111)-(7×7). Phys. Rev. B 2004, 70, 073312. 20. Zhuang, J. C.; Xu, X.; Peleckis, G.; Hao, W.; Dou, S. X.; Du, Y. Silicene: A Promising Anode for Lithium-Ion Batteries. Adv. Mater. 2017, 29, 1606716. 21. Dávila, M. E.; Le Lay, G. Few Layer Epitaxial Germanene: A Novel Two-Dimensional Dirac Material. Sci. Rep. 2016, 6, 20714. 22. McChesney, J. L.; Bostwick, A.; Ohta, T.; Seyller, T.; Horn, K.; González, J.; Rotenberg, E. Extended van Hove Singularity and Superconducting Instability in Doped Graphene. Phys. Rev. Lett. 2010, 104, 136803. 14


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23. Hicks, J.; Sprinkle, M.; Shepperd, K.; Wang, F.; Tejeda, A.; Taleb-Ibrahimi, A.; Bertran, F.; Le Fèvre, P.; de Heer, W. A.; Berger, C.; Conrad, E. H. Symmetry Breaking in Commensurate Graphene Rotational Stacking: Comparison of Theory and Experiment. Phys. Rev. B 2011, 83, 205403. 24. Winkelmann, A.; Ünal, A. A.; Tusche, C.; Ellguth, M.; Chiang C. T.; Kirschner J. Direct k-Space Imaging of Mahan Cones at Clean and Bi-Covered Cu(111) Surfaces. New J. Phys. 2012, 14, 083027. 25. Qiao, J. S.; Kong, X. H.; Hu, Z. X.; Yang, F.; Ji, W. High-Mobility Transport Anisotropy and Linear Dichroism in Few-Layer Black Phosphorus. Nat. Commun. 2014, 5, 4475. 26. Fei, R.; Faghaninia, A.; Soklaski, R.; Yan, J. A.; Lo, C.; Yang, L. Enhanced Thermoelectric Efficiency via Orthogonal Electrical and Thermal Conductances in Phosphorene. Nano Lett. 2014, 14, 6393-6399. 27. Kresse, G.; J. Furthmüller, Efficient Iterative Schemes for ab initio Total-Energy Calculations Using A Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 28. Kresse, G.; Furthmüller, J. Efficiency of ab-initio Total Energy Calculations for Metals and Semiconductors Using A Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. 29. Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. 30. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 31. Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on A Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207-8215. 32. Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: “Hybrid Functionals Based on A Screened Coulomb Potential” [J. Chem. Phys.118, 8207 (2003)]. J. Chem. Phys. 2006, 124, 219906. 33. Tersoff, J.; Hamann, D. R. Theory of the Scanning Tunneling Microscope. Phys. Rev. B 1985, 31, 805-813. 34. Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272-1276. 15


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Figures and figure captions

Figure 1. (a)-(c) Large-scale and close-up STM images of monolayer phosphorene on Au(111) substrate (Vbias = 2.0 V, I = 30 pA, 100 nm × 100 nm; Vbias = 1 mV, I = 1 nA, 20 nm × 20 nm; Vbias = 100 mV, I = 30 pA, 4 nm × 4 nm). (d)-(f) Large-scale and atomic resolution STM images of P adatoms on monolayer phosphorene (Vbias = 2.0 V, I = 30 pA, 100 nm × 100 nm; Vbias = 100 mV, I = 30 pA, 20nm × 20 nm; Vbias = 1 mV, I = 5 nA, 4 nm × 4 nm). The red rhombus in panel (c) stands for the unit cell of the 4×4 superstructure. (g) LEED pattern taken at 60 eV on monolayer phosphorene on Au(111) substrate. The blue circles represent 1×1 Au(111) spots. (h) Fast Fourier transform (FFT) image of panel (b). The green circles and red circles represent the 1×1 phosphorene and the 4×4 phosphorene superstructure, respectively. (i) First Brillouin zone (BZ) of Au(111) (green solid line) and the corresponding BZs of monolayer phosphorene (1×1) (blue solid line) and (4×4) supercell (red solid line). 16


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Figure 2 Top (a) and side (b) views along the [11̅0] Au direction of the relaxed model of atomic structure of the 4×4 phosphorene/5×5 Au(111) configuration. P atoms with different heights are labeled by red (P2) and blue (P1), respectively. The red rectangle represents the unit cell of 4×4 germanene. ∆z is the buckling height of 4×4 germanene. (c) Schematic diagram of the evolution from the relaxed atomic structure model to the simulated STM image and then to the experimental STM image (from left to right).

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Figure 3 (a)-(c) Constant energy contours for the Au(111) surface with superposed scheme of the BZs, with the high symmetry points obtained by integrating the photoemission spectral weight over a small energy window (± 20 meV) with respect to the binding energy of 0 meV (a), 300 meV (b), and 600 meV (c). S1-S6 and S1'-S6' represent the sp band folded into the BZs of the 4×4 superstructure. (d)-(f) Constant energy contours of 4×4 phosphorene/Au(111) at three different binding energies of 0 meV (d), 300 meV (e), and 600 meV (f). The images are obtained by symmetrizing the original data based on the threefold symmetry. KAu' and KS' stand for the K points in the BZ of Au(111) and the 4×4 superstructure, respectively. Enlarged view of CEC of phosphorene at 0 meV (g) and 600 meV (h). S6 and S6' replicas are labelled to show the split with increased Eb.

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Figure 4 Topography of electronic superlattice detected by STS mapping in 4×4 blue phosphorene. (a) STM image of 4×4 blue phosphorene with exposed Au(111) substrate. (Vbias = 0.1 V, I = 400 pA) The red solid rhombus stands for the unit cell of 4×4 superstructure. (b)-(d) STS maps of the same area as panel (a), collected with the bias at -1.2 V, -0.6 V, and 0.8 V at 4.2 K, respectively.

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Figure 5 (a) ARPES spectra of phosphorene on Au(111) along the MS-߁ S-KS-MS direction. P and sp label the signals from deposited phosphorus and the replicas of the sp band of the Au(111) substrate, respectively. (b) Comparison of the second derivative of the bands in panel (a) with the DFT calculation of 4×4 phosphorene. The red lines stand for the simulated top valence bands. (c) DFT band structure calculation (using a Heyd-Scuseria-Ernzerhof (HSE) exchange functional) of 4×4 phosphorene, showing the wide band gap around 1.18 eV. (d) Current-bias (IV) (black line) and dI/dV (blue line) curves for 4×4 phosphorene on Au(111) surface taken at 4.2 K (set point: Vbias = 1.5 V, I = 500 pA). The red and purple arrows stand for the valence band maximum (VBM) and conduction band minimum (CBM), respectively, which is defined by the deviation points of linear IV dispersion (red dashed line).

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m*x/m0

m*y/m0

E1x (eV)

E1y (eV)

Cx (eV/Å2)

Cy (eV/Å2)

ࣆx (cm2/Vs)

ࣆy (cm2/Vs)

Electron

0.130

0.132

-2.289

-2.261

2.673

2.949

1.0263× 104

1.1429× 104

Hole

2.231

1.587

-1.007

-1.005

2.673

2.949

2.151× 102

3.350× 102

TABLE 1. Calculated carrier effective masses, deformation potentials, elastic moduli, and carrier mobilities along the x (߁S-KS) and y (߁ S-MS) directions at T = 300 K.

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TOC

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Bandgap Modulated by Electronic Superlattice in Blue Phosphorene

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