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Borophene Synthesis on Au(111) Brian Kiraly, Xiaolong Liu, Luqing Wang, Zhuhua Zhang, Andrew J. Mannix, Brandon L. Fisher, Boris I. Yakobson, Mark C Hersam, and Nathan P Guisinger ACS Nano, Just Accepted Manuscript • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Borophene Synthesis on Au(111) AUTHORS Brian Kiraly1,2†, Xiaolong Liu3†, Luqing Wang4†, Zhuhua Zhang5†, Andrew J. Mannix1,2, Brandon L. Fisher1, Boris I. Yakobson4*, Mark C. Hersam2,3,6*, and Nathan P. Guisinger1* AFFILIATIONS 1Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Building 440, Argonne, IL 60439, USA 2Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208, USA 3Applied Physics Graduate Program, Northwestern University, 2220 Campus Drive, Evanston, IL 60208, USA 4Department of Materials Science and NanoEngineering and Department of Chemistry, Rice University, Houston, Texas 77005, USA 5State Key Laboratory of Mechanics and Control of Mechanical Structures and Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China 6Department of Chemistry, Northwestern University, 2220 Campus Drive, Evanston, IL 60208, USA †

Equal Contribution: Kiraly, Liu, Wang, and Zhang are co-first authors Authors: [email protected]; [email protected]; [email protected] *Corresponding

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ABSTRACT Borophene (the first two-dimensional (2D) allotrope of boron) is emerging as a groundbreaking system for boron-based chemistry and, more broadly, the field of low-dimensional materials. Exploration of the phase space for growth is critical because borophene is a synthetic 2D material that does not have a bulk layered counterpart and thus cannot be isolated via exfoliation methods. Herein, we report synthesis of borophene on Au(111) substrates. Unlike previously studied growth on Ag substrates, boron diffuses into Au at elevated temperatures and segregates to the surface to form borophene islands as the substrate cools. These observations are supported by ab-initio modeling of interstitial boron diffusion into the Au lattice. Borophene synthesis also modifies the surface reconstruction of the Au(111) substrate, resulting in a trigonal network that templates growth at low coverage. This initial growth is comprised of discrete borophene nanoclusters, whose shape and size are consistent with theoretical predictions. As the concentration of boron increases, nano-templating breaks down and larger borophene islands are observed. Spectroscopic measurements reveal that borophene grown on Au(111) possesses metallic electronic structure, suggesting potential applications in 2D plasmonics, superconductivity, interconnects, electrodes, and transparent conductors.

Keywords: borophene, two-dimensional materials, Au(111), scanning tunneling microscopy, synthesis, allotrope


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As the lightest valence-three element, boron possesses chemical properties that tend to favor complex bonding arrangements.1-8 From computational modeling to experimental synthesis, the recent discovery of two-dimensional (2D) boron polymorphs, collectively referred to as borophene,7 has further elaborated the richness of boron chemistry and ushered in a class of low-dimensional materials.916

Borophene is an elemental 2D metal with superlative theoretical properties

including a tensile strength comparable to graphene and a relatively high superconducting transition temperature.9,17 The initial breakthrough for borophene synthesis occurred using Ag(111) substrates, although theoretical modeling predicts stable low-energy structures on additional metal surfaces (e.g., Au and Cu). 11 Here, we present ultra-high vacuum (UHV) synthesis and characterization of borophene on Au(111). Borophene growth begins when boron from the bulk segregates to surface, while the sample is cooled. Growth on Au(111) is in stark contrast to boron limited to the surface on Ag(111). 9,11 Furthermore, the energy minimization and strain relief of the Au(111) surface results in nanoscale templating of borophene nucleation and growth. Increasing boron coverage results in a breakdown of the nanoscale templating mechanism, allowing the borophene to evolve from small ordered islands to larger sheets. The Au(111) surface is among the few low-energy metal surfaces that undergoes a reconstruction to minimize surface energy. This reconstruction is characterized by a 22 x √3 structure consisting of surface atoms located at facecentered cubic (fcc), hexagonal close packing (hcp), and bridge-type locations with respect to the bulk lattice.18,19 The reconstruction results in a strain-mediated 3 ACS Paragon Plus Environment

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herringbone pattern of Shockley partial dislocations with uniform spacing and domain orientations of 120°. Adsorption experiments reveal that this surface reconstruction influences the growth of weakly-interacting materials on Au(111), typically leading to nucleation at the corners of the herringbone reconstruction.2025

Strongly interacting adsorbates induce more significant modifications to the

herringbone reconstruction, including altering the symmetry to a highly ordered trigonal network.26-28 RESULTS AND DISCUSSION The pristine Au(111) surface, shown in the UHV scanning tunneling microscopy (STM) image of Fig. 1a, possesses the herringbone surface reconstruction that consists of Shockley partial dislocations running along the [112] directions that separate the fcc from hcp striped regions. Deposition of boron from an atomic source was studied for a series of substrate temperatures (see Supporting Fig. 1). Low temperature deposition results in boron clusters confined to the surface. At substrate temperatures of ~550 °C, boron clusters are no longer present and a transformation is observed from the conventional herringbone reconstruction to a trigonal network where small nanoscale borophene islands emerge, as seen in Fig. 1b (see also Supporting Fig. 1). Atomic-resolution imaging of the Au(111) surface confirms that the trigonal strain-relief network forms within the Au surface, while borophene islands with a calculated v1/12 structure nucleate at the nodes of the network. 11 The total dose of atomic boron needed to observe nanoscale borophene islands is an order of magnitude greater than what is needed to form a monolayer on Ag(111), which is the first indication that boron may be


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dissolving into the bulk at these substrate temperatures. Figure 1(c) illustrates the computationally predicted atomic structure model for the borophene v1/12 on a Au(111) substrate. The trigonal network has a characteristic periodicity ranging from 5.5 nm to 8.0 nm and acts as a growth template for nanoscale borophene islands. Boron is not expected to form a stable alloy with Au, and we do not observe direct evidence of alloying. However, the trigonal network has been observed for clusters that are incorporated into the surface.26-28 To minimize surface energy, this reconstruction consists of alternating triangular regions of fcc and hcp stacked Au atoms,26-28 as indicated by the triangular dashed regions in Fig. 1b. The apparent embedded nature of the boron islands is likely responsible for the transition from the normal herringbone surface reconstruction to the trigonal network. The chemical properties of the boron-modified Au(111) surface are investigated by in situ X-ray photoelectron spectroscopy (XPS) and compared to that of a clean Au(111) surface, as shown in Fig. 2 and Supporting Fig. 2. The Au 4f7/2 core-level peak after room temperature boron deposition matches that of clean Au. However, the trigonal network samples show a 0.1 eV redshift (see Supporting Fig. 2). This shift is consistent with 1-2% strain-relief induced by the incorporation of boron into the Au(111) lattice and removal of Au atoms.29-31 Indeed, the transformation of the strain relief pattern from the herringbone reconstruction to the trigonal network indicates a change of strain status of the surface. The Au surface with boron particles also shows a clear peak in the B1s core-level (Fig. 2a), which is expected given limited solubility of boron in Au at


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room temperature. However, the reduced peak height from the trigonal network samples indicate that the surface coverage of boron is much lower in these cases, which suggests that boron diffuses into the subsurface region at higher substrate temperatures. The absence of obvious peak shift of the B1s core level, albeit low in intensity, suggests minimum valence changes, in contrast to the case of boron oxides (peak shifts to 192 eV.9 Increasing the boron dose eventually results in the breakdown of the trigonal network and the formation of larger borophene islands that are embedded in the topmost layer of Au, as illustrated in Fig. 2b. The dashed hexagons in the insets of Fig. 2b illustrate the trigonal network. Borophene sheets increase in size with boron dose, leading to domains that extend at least several tens of nanometers at the highest doses in this study as seen in the bottom two panels of Fig. 2b. Both STM and XPS (Fig. 2 and Supporting Fig. 2) reveal that boron clusters form on the surface at low temperature, while boron appears to dissolve into the bulk at elevated temperatures. Thus, we suggest that the borophene growth results from surface segregation of boron from the bulk when the sample is cooled (illustrated in Fig. 3a). This surface segregation of boron to form borophene on Au(111) is contrary to the surface-only growth on Ag(111); these two schemes are analogous to the two primary growth modes observed for graphene on metal substrates.32 To gain further insight into the mechanisms underlying the apparent surface segregation and recessed growth modes of boron on Au(111), we performed density functional theory (DFT) calculations to quantify the adsorption energetics


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of boron atoms on this surface. We do not compute all of the atom-state sequences, as in the nanoreactor diagram,33 but only key initial configurations. Figure 3c shows the resulting DFT predictions of the relaxed energies of boron atoms at different sites within and on top of Au(111). The calculations indicate that while both bulk solution and surface locations are energetically favorable, they are significantly higher in energy than the near-surface dissolution of boron atoms. In particular, the most stable configurations of isolated boron atoms is in subsurface sites in the topmost layer, with sites along Au(111) step edges being favored by ~0.3 eV over planar subsurface sites. These energetics differ significantly from those of boron on Ag(111).9,11 The surface strain is greater in Au, up to 5.5%,34 than in Ag so that inlayer growth is preferred on Au. Despite their similar chemical properties, the noble metal surfaces have strongly differing electronic densities near the Fermi level (Ef), which leads to the stabilization of distinct boron configurations. Importantly, subsurface boron undergo a less steric effect from surrounding Au and in turn can form a stronger Au-B interaction to adopt a significantly more energetically favorable adsorption site. Modeling of the boron diffusion energetics within the first Au(111) layer shows further consistency with the experimental observation of embedded islands. For example, Fig. 3c (right panel) illustrates the energetics of multi-atom boron clusters on the Au(111) surface. As shown with respect to the energy of a single subsurface boron atom, the calculations favor the formation of small boron clusters within the Au(111) subsurface. The increasing depth of the potential well for dimers and subsequently trimers indicates the preference for boron-boron bond formation


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within this subsurface environment. Furthermore, the most stable configuration involves a Au atom being expelled out of the surface to accommodate the boron trimer cluster, attributed largely to further relief of surface stress on Au(111). The nucleated boron trimer then extends into 2D islands upon further docking of boron atoms and the removal of redundant Au atoms. Again, this behavior marks a significant difference from the 2D surface growth motif of borophene overlayers on Ag(111), 9,11 confirming our experimental observations. To experimentally confirm that boron is dissolving into the bulk, the distribution of boron in Au is further investigated via time-of-flight secondary ion mass spectrometry (ToF-SIMS). In addition to a trigonal network sample and a boron cluster sample where boron is deposited on Au held at room temperature, an as-received and a UHV cleaned Au samples are also explored as control samples. In Fig. 3d and Supporting Fig. 3, higher boron signals are seen in the SIMS spectra for the boron cluster sample compared to the trigonal network sample, suggesting a higher surface concentration of boron. This observation is in agreement with the XPS B1s level results. Depth-profiling of the four samples reveals that the boron concentration in the trigonal network sample only decays after ~30 nm into the bulk, whereas the boron signals decay more quickly in the three other cases once the surface is sputtered (Supporting Fig. 3). In agreement with the XPS results, the SIMS data suggest that boron diffuses deeply into the Au subsurface region at elevated temperatures. The UHV cleaned Au shows no obvious differences compared to as-received Au, indicating negligible effect of UHV annealing.


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On larger borophene islands, STM faintly resolves a herringbone reconstruction in the Au directly below the 2D sheets. The zoomed-in image of a borophene island (Fig. 4a) reveals apparent atomic-scale periodicity, while herringbone stripes concurrently remain visible (black arrows) below the 2D sheet. The ability to resolve the underlying Au(111) herringbone reconstruction suggests that the boron islands are atomically thin. The STM image of Fig. 4b further reveals an apparent atomic-scale periodicity within the borophene sheet. However, this periodicity is distorted by a larger superstructure that is likely related to a Moiré pattern between borophene and Au(111). A fast Fourier transform (FFT) of the STM image in Fig. 4b shows a strong periodicity of ~0.66 nm as is illustrated in Fig. 4c. This ~0.66 nm periodicity is consistently observed in the extended borophene sheets. At the early stages of nucleation and growth, the borophene islands are templated at the nanoscale. As seen in the STM images in Fig. 1, the specific shape of the borophene islands is both compact and anisotropic. Closer inspection reveals that each island can be approximately decomposed into rhombohedral units, as highlighted in Figs. 4d and 4e. Furthermore, the coalescence of multiple rhombohedral building blocks can account for the shape of larger borophene islands. The size of an individual rhombohedral unit (Figs. 4d and 4e) ranges between 0.9 and 1.4 nm2 with an aspect ratio of 1.5 +/- 0.2, where we note that the accuracy of this measurement is limited by the gold adatoms that are often found at the edges of the borophene islands. A histogram generated from over 500 islands (Fig. 4e) reveals clear discretization of island area at integer values of


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square nanometers. These rhombohedral units, both in terms of size and shape, are consistent with investigations of planar size-selected boron clusters.7, 35-38 The smaller borophene islands are often at the single nanometer length scale, where strong electronic confinement effects are likely to occur. Furthermore, their discrete rhombohedral building blocks may show quantum effects. To further understand the electronic properties of the borophene islands, a series of biasdependent images and concurrently acquired dI/dV tunneling conductance maps (proportional to the electronic density of states) are presented in Fig. 5a. The biasdependent conductance maps reveal that not only is the borophene electronically distinct from the Au but that the fcc (red dashed triangle) and hcp (yellow dashed triangle) regions are also distinct, where the contrast inverts between -0.2 V and 0.4 V sample bias. At positive sample bias, the Au surface state becomes pronounced as we observe the standing wave reflecting from the step-edge and interference patterns between the borophene islands, further confirming the embedded nature of the borophene islands. The borophene islands themselves exhibit electronic variation at the nanometer length scale (particularly at -0.2 V or +0.4 V), which could be related to electronic confinement effects. DFT calculations reveal that the previously predicted borophene structure on Au(111) is metallic, as illustrated by the band structure and density of states (DOS) in Fig. 5b.11 Scanning tunneling point spectra verify that the 2D borophene on Au(111) is indeed metallic as shown in the blue curve in Fig. 5c. It is noteworthy that the spectra for the large borophene island on Au(111) are similar to the spectra seen for borophene on Ag(111).9 In contrast, the gray curve is taken on the clean


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Au(111) surface and shows the characteristic Shockley surface state at approximately -0.52 eV.39 Spectra taken on the borophene nano-islands differ significantly from larger borophene sheets, with several additional spectral features. For example, in the black curve in Fig. 5c, a shoulder is observed at approximately +0.3 V in addition to a symmetric inflection at -0.3 V (indicated by the dashed lines). We believe that these shoulders are a result of additional electron confinement within the borophene clusters.

Electron confinement in

quantum dot structures has previously been shown to result in symmetric states around the Fermi level.40,41 CONCLUSIONS In conclusion, nanoscale borophene structures have been synthesized and characterized on Au(111) in UHV. The observed growth mode is consistent with boron dissolving into the bulk at high temperatures and segregating to the surface to form borophene as the sample is cooled. The initial stages of growth for low concentrations of boron result in nanoscale templating of the borophene islands across the surface, as the Au(111) reconstructs in a trigonal network to minimize energy. At low boron concentration, the borophene islands are comprised of 1 to 8 rhombohedral units with a characteristic area of 1 nm2, as observed in statistical analysis of the island size. The electronic properties of borophene on Au(111) is confirmed to be metallic, with the nanoscale borophene islands showing evidence of electron confinement. Overall, the nanoscale templated growth of borophene on Au(111) presents opportunities for studying and manipulating the electronic properties of 2D metals.


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METHODS The Au(111) on mica substrates were commercially purchased from Keysight Technologies. The Au(111) surface was cleaned in UHV via repeated sputter/anneal cycles until atomically pristine. Characterization was performed using a variable temperature STM (Scienta Omicron) and a home-built UHV STM system adopting a Lyding-design microscope. 42,43 STM images and spectroscopy in the main text were gathered at a sample temperature of 55 K with electrochemically etched W tips. Borophene was grown via thermal deposition from a pure boron rod using electron-beam evaporators (Focus EFM and SPECS EBE-1). The substrate temperature during borophene growth was ~550C. The evaporation rate is ~ 0.02 ML/min (based on borophene growth on Ag(111) with the same power), and an deposition time ranging from 30 min to 3 hrs. DFT calculations were performed using ultrasoft pseudo-potentials for the core region and generalized gradient approximation of Perdew–Burke–Ernzerhof (PBE) functional for describing the exchange correlation, as implemented in VASP code.44,45 A kinetic energy cutoff of 400 eV is chosen for the plane-wave expansion. In all structures, the vacuum region between two adjacent periodic images is fixed to 10 Å to eliminate spurious interaction. The Brillouin-zone integration is densely sampled according to the supercell size, ensuring approximately the same k-point density among different-sized supercells. The k-point density is approximately 45 Å-1. For example, for a supercell with a size of 10 Å×8.65 Å, the k-point sampling is 5×5×1, while “1” is for the vacuum layer.The metal substrate consists of four atomic layers, with atoms in the bottom layer being


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fixed to the bulk lattice. The positions of all other atoms are relaxed using the conjugate-gradient method until the force on each atom is less than 0.01 eV/Å. The minimum-energy path was mapped out using the climbing image-nudged elastic-band method. 46 XPS spectra were taken with an Omicron DAR 400M X-ray source, XM 500 X-ray monochromator, and EA 125 energy analyzer at a base pressure of ~4 × 1010

Torr. The XPS system is integrated with the home-built UHV STM system for in

situ measurements with an energy resolution of ~0.6 eV. ToF-SIMS spectra were acquired on a PHI TRIFT III ToF-SIMS (Physical Electronics) system with a liquid metal ion gun in the positive ion mode. The analysis chamber pressure was ~1×10-9 Torr.

ASSOCIATED CONTENT Supporting information contains boron dosing information as a function of temperature, additional XPS data, and ToF-SIMS data.


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ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. MCH and XL acknowledge funding from the Office for Naval Research (N00014-17-1-2993). Work at Rice (ZZ and BIY) was supported by the DOE grant DE-SC0012547. COMPETING INTERESTS The authors declare no conflict of interest. DATA AVAILABILITY STATEMENT The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. CONTRIBUTIONS BK, AJM, MCH, and NPG conceived the project; BK, XL, and AJM carried out the experiment and data analysis. ZZ, LW, and BIY provided theoretical support and computational modeling. BLF provided engineering support for the experiment. NPG, BK, ZZ, XL, and MCH wrote and revised the manuscript. All authors contributed to the manuscript.


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FIGURE 1

Figure 1. (a) STM image of a clean Au(111) surface that shows the herringbone reconstruction (V = -1.5 V, I = 100 pA). (b) Following boron deposition at 550 °C, the herringbone reconstruction is modified to a trigonal network, where nanoscale borophene islands (one highlighted by a white dashed line) emerge at the nodes resulting in templated growth across the surface (V = -0.1 V, I = 200 pA). (c) The atomic structure of the borophene v1/12 computationally modeled and predicted for Au(111) (left – top down, right – cross-sectional view).


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FIGURE 2

Figure 2. (a) B1s core-level spectra for room temperature B deposition (orange), trigonal network with low-dose (green), trigonal network with high-dose (red), and clean Au(111) (blue). (b) Increasing boron dose results in the breakdown of the trigonal network and growth of larger borophene islands (top panels: V = -2 V, I = 100 pA; bottom panels: V= 3.4 V, I = 60 pA).


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FIGURE 3

Figure 3: (a) Schematic illustration of borophene growth dynamics. At low substrate temperature, the boron is predominately on the surface and forms boron clusters. For higher temperatures, the boron dissolves into the bulk and then segregates to the surface upon cooling to form 2D borophene sheets. (b) Minimum energy path for boron diffusion on Au(111) (red), penetration from the fcc site on the surface into the subsurface (gray), and diffusion in the subsurface (blue). (c) DFT calculations of the free energies of single boron atoms at various locations on the Au(111) surface and in the Au(111) bulk, as well as boron dimer and trimer in the subsurface. The gold and boron atoms are displayed in yellow and red, respectively. (d) SIMS spectra show that boron is limited to the surface when deposited at room temperature, but is present ~30 nm into the subsurface region in the trigonal network sample that was prepared at high temperature.


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FIGURE 4

Figure 4: (a) STM image of a larger borophene island that shows the herringbone reconstruction from the underlying Au(111) substrate and atomic-scale periodicity (V = 1 V, I = 100 pA). (b) Atomic-scale periodicity is apparent in larger borophene sheets (V = -0.4 V, I = 80 pA). (c) An FFT of the STM image in (b) reveals that the periodic length scale is ~0.66 nm. (d) STM image following the initial stages of growth of the templated borophene nanostructures (V = -0.4 V, I = 200 pA). (e) Histogram of the island sizes with observable peaks corresponding to integer values of the basic rhombohedral unit.


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FIGURE 5

Figure 5: (a) Series of bias-dependent STM images showing topography (top panels) and dI/dV maps (bottom panels) of the same region at the biases indicated above each panel. Red and yellow triangles represent fcc and hcp regions of the Au(111) surface, as noted in Fig. 1. The borophene islands appear as depressions in topography at all biases (I = 200 pA). (b) Calculated band structure and electronic density of states (DOS) for monolayer boron on Au(111) showing metallic characteristics. The red and grey lines represent electronic bands contributed by borophene and Au, respectively. The thickness represents the weight of the contribution. (c) dI/dV point spectra taken on (gray) Au(111) for reference, (red) large borophene sheets on Au(111) for comparison, and (dark blue) borophene nano-islands on Au(111). Dashed lines at +/- 300 mV highlight the boron island spectral features that are discussed in the text.


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TABLE OF CONTENTS FIGURE


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