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Nano-Structured Bi Grown on Epitaxial Graphene/SiC Tingwei Hu, Xin Hui, Xiaohe Zhang, Xiangtai Liu, Dayan Ma, Ran Wei, Kewei Xu, and Fei Ma J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02246 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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The Journal of Physical Chemistry Letters

Nano-Structured Bi Grown on Epitaxial Graphene/SiC

Tingwei Hu,*,†,‡ Xin Hui,† Xiaohe Zhang,† Xiangtai Liu,† Dayan Ma,† Ran Wei,# Kewei Xu,†,§ and Fei Ma*,†,‡

†

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong

University, Xi’an 710049, Shaanxi, China ‡

Collaborative Innovation Center of Suzhou Nano Science and Technology, Xi’an

Jiaotong University, Suzhou, 215123, Jiangsu, China §

Department of Physics and Opt-electronic Engineering, Xi’an University of Arts and

Science, Xi’an 710065, Shaanxi, China #

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou,

450001, China

AUTHOR INFORMATION Corresponding Authors Email: [email protected], Phone: +029-82668319 Email: [email protected], Phone: +029-82669319 ORCID Tingwei Hu: 0000-0001-7386-5599 Fei Ma: 0000-0002-3911-7121

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ABSTRACT Controllable growth of metal nano-structures on epitaxial graphene (EG) is particularly interesting and important for the applications in electric devices. In this paper, Bi nanostructures on EG/SiC are fabricated through thermal decomposition of SiC and subsequent low-flux evaporation of Bi. The orientation, atomic structure and thickness dependent electronic states of Bi are investigated by scanning tunneling microscopy/spectroscopy (STM/S). It is found that metallic Bi nano-flakes and nano-rods prefer to grow on SiC buffer layer (BL) region with higher diffusion barrier, but Bi nano-ribbons are formed on regularly ordered EG. Although the thicker Bi nano-ribbons of 11 monolayers on EG are still metallic, the thinner ones becomes semiconducting owing to the interfacial effect. It indicates that the electronic states and physical properties of Bi are substrate dependent. The results are helpful for the design of Bi and graphene based electronic and spintronic devices.

TOC GRAPHICS

Bi nano-flakes and nano-rods are formed on SiC BL, but Bi nano-ribbons on EG. Thickness-dependent electronic states of Bi are evidenced.


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Bismuth (Bi) belongs to group-V and has an electron configuration of 6s26p3 [1], which gives rise to a rich variety of nontrivial physical properties [2-4]. Because of quantum effect and size effect, Bi nano-structures exhibit high thermoelectric efficiency [5], superconductivity [6] and a semimetal to semiconductor transition [7]. The charge carrier concentration of Bi is far less than that of normal metals, resulting in a small effective electron mass and a long Fermi wavelength (~40 nm) [8]. The strong spin-orbit coupling (SOC) on the surface of Bi results in significant splitting of surface-state bands, exhibiting possible applications in thermoelectric devices and spintronics [9-11]. The ultra-thin Bi film (bismuthene) was identified as a candidate for a new high-temperature Quantum Spin Hall (QSH) paradigm [12]. It is also worth mentioning that Bi is an environmentally friendly element and has been proposed to be used as electrodes for sensing noxious metals [13]. Thus, Bi is an ideal material for the prospective exploitation in nano-electronics [14-17]. In fact, the electronic states and physical properties of Bi nanostructures are closely related to the surface orientation, and the controllable fabrication is essential to the applications.

Thermodynamically, (110) orientated Bi with similar lattice structure as that of black phosphorus is favored for the nanostructures with a thickness of several to tens of nanometers [18, 19], while (111) orientated ones are preferred for thicker ones [3]. That is, the growth orientation of Bi is thickness dependent, and a transition from (110) orientated Bi towards (111) orientated one might take place during mild annealing [20]. Not only that, the growth orientation of nanostructures and thin films is also



dependent on the substrates, especially for thinner ones. The growth of Bi thin films and nano-islands on Si(111) [21], Ag(111) [22], Bi2Te3 [23], HOPG [24, 25] and so on was extensively studied. However, only a few researches have been done on the growth of Bi on epitaxial graphene (EG) [26], although EG is considered as a promising material for high-performance devices [27-30]. Furthermore, EG prepared through thermal decomposition of SiC usually coexists with SiC buffer layer (BL). Thus, it provides us a good platform to study the substrate effect on the growth of Bi nano-structures as well as on the electronic states [31]. In this work, Bi nanostructures are grown on EG/SiC through thermal decomposition of SiC and subsequent low-flux evaporation of Bi. The morphology, orientation, atomic structure and thickness dependent electronic states of Bi are investigated by scanning tunneling microscopy/spectroscopy (STM/S), and the growth mechanism is addressed.

Fig. 1(a) displays the morphology of sample EG-1600 fabricated through annealing SiC at 1623 K under a UHV environment. Fig. 1(b), enlarged from the blue square in Fig. 1(a), exhibits the coexistence of the EG layer and SiC BL. Fig.1(c) shows the topography of Bi nano-structures grown on EG-1600, both elongated flake-like and short rod-like islands are evidenced. Because of dangling bonds, the adsorption energy on the BL is much higher than that on graphene. As a result, Bi nano-flakes and nano-rods are mainly distributed on BL highlighted by the white dashed line, as evidenced previously [11]. Fig. 1(d) displays the atomically resolved structure of a Bi nano-flake zoomed in from the blue square in Fig. 1(c). The fast


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Fourier transformation (FFT) pattern is shown in the inset, and the lattice constants are measured as a≈0.47 nm and b≈0.49 nm, corresponding to the Bi(110) plane. The densely packed plane of Bi(110) is favored at low coverage owning to the higher cohesive energy and larger surface density [18, 19].

Fig. 1 The morphology of EG-1600 (a) and coexistence of EG layer and SiC BL (b). (c) the topography of Bi(110) on SiC BL. (d) atomically resolved structure of Bi(110) zoomed in from the blue square in (c), the inset shows the FFT patterns.

Fig. 2(a) shows the enlarged image and Fig. 2(b) exhibits the height profile along the green line in Fig. 2(a) to show the thickness of several Bi nano-flakes and nano-rods. The interlayer distance of Bi(110) planes is ~0.33 nm [24, 25], accordingly, the Bi nano-flakes have uniform thickness of 3 ML (~1.0 nm), but the Bi nano-rods are 6 ML (~2.0 nm) thick. Fig. 2(c) displays the STS spectra acquired from the BL, Bi nano-flakes and nano-rods indicated by the black, red and blue dots, respectively in Fig. 2(a). The local electronic states (black curve) of BL are similar to the reported



results [32]. Both the Bi nano-flakes and nano-rods exhibit as metallic, and the 3 ML Bi nano-flakes (red curve) show stronger metallic feature, as illustrated by the sharply V-shape around Fermi level. The three peaks at -0.43 V, 0.40 V and 0.93 V are ascribed to free carriers [33]. The 6 ML nano-rods (blue curve) are also characteristic of metallic features but with very low carrier density around the Fermi level. There are also three free carrier peaks but at -1.0V, -0.74V and 0.73V. Apparently, the electronic states of Bi nano-structures on the SiC BL are thickness dependent.

Fig. 2 (a) the morphology of flake-like and rod-like Bi(110) on SiC BL. (b) the height profile along the green line in (a). (c) the STS spectra from BL, Bi nano-flakes and nano-rods as marked by the black, red and blue dots, respectively.

Fig. 3(a) shows the morphology of sample EG-1800 fabricated through the optimized annealing of SiC at 1823 K under the protection of Ar gas [34]. As compared to sample EG-1600, the surface of EG-1800 is fully covered by graphene, and only few pits emerge on the terraces, indicating that regularly ordered EG is fabricated with the atomic structure of EG shown in the inset. Fig. 3(b) shows the Bi


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nano-ribbons with a uniform height and orientation on EG-1800. Different from Bi nano-flakes and nano-rods on SiC BL, the nanostructures on EG layer are described as nano-ribbons because they are hundreds of nanometers long and are comparable to that on HOPG [25]. So the regularly ordered EG substrate affects the growth of Bi(110) [35], and may allow the morphology to be regulated.

Fig. 3(c) shows the enlarged STM image of one Bi nano-ribbon across an EG step. Fig. 3(d) shows the atomic structure of EG zoomed in from the white square in Fig. 3(c). The hexagonal graphene lattice on the top of 6×6 reconstruction is observed, indicating no wetting layer for the growth of Bi. Fig. 3(e) shows the atomic structure of a Bi nano-ribbon from the green square in Fig. 3(c). As overlaid by the red and blue dots, the atoms on Bi nano-ribbon are arranged in zigzag chains. The unit cell including two Bi atoms is depicted by the dashed black rectangle. Fig. 3(f) shows the FFT of Fig. 3(e), the lattice constants are a≈0.46 nm and b≈0.48 nm, confirming the zigzag structure. Accordingly, the Bi nano-ribbons are (110) < 1 10 > orientated on EG. As highlighted by the black arrows in Figs. 3(d) and 3(e), the Bi nano-ribbon is indeed parallel to the EG edge, that is, along the armchair direction of EG ( < 1120 > ) as well as along the basic vector of 6×6 reconstruction owing to anisotropic growth speed [36]. The atoms in the zigzag chains are covalently bonded with each other but the chains interact through the van der Waals force [24]. That is, the fast growth direction is associated with the strong bonds in zigzag chains along the Bi < 1 10 > direction, along which the lattice misfit between Bi and graphene is a minimum [18].



Fig. 3 (a) the morphology of EG-1800, the inset shows the atomic structure of EG. (b) the topography of Bi(110) on EG layer. (c) the enlarged image of single Bi nano-ribbon across EG step. (d) the atomic structure of EG zoomed in from the white square in (c). (e) the atomic structure of Bi nano-ribbon zoomed in from the green square in (c). the armchair direction of EG (EG < 11 2 0 > ) and Bi < 1 10 > are marked by the black arrows, the unit cell of Bi(110) is marked by the dashed black rectangle, and the zigzag chain structures are overlaid by the blue and red dots. (f) FFT transferred from (e), and the measured sizes are marked.

Fig. 4(a) shows the morphology of nano-ribbons on EG and Fig. 4(b) shows the atomically resolved structure zoomed in from the edge as marked by the green square in Fig. 4(a). Fig. 4(c) depicts the STS spectra of Bi nano-ribbon acquired respectively from the center and edge as marked by the black and blue dots in Fig. 4(b). Typical


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semiconducting characteristics of Bi with an asymmetric band gap of ~0.5 eV is evidenced at the center of Bi nano-ribbon (black curve). As marked by the black triangles, there are five peaks of free carriers at -0.92 V, -0.61 V, -0.39 V, 0.48 V and 0.74 V [33]. Meanwhile, the STS spectrum measured at the edge of a Bi nano-ribbon (blue curve) is different from that at the center. The peaks of free carriers disappear at the edge, and are replaced by an energy gap, indicating the influence of edge structures on the electronic states of Bi nano-ribbons. The origin of the energy gap can be attributed to the relaxation of edge strain energy facilitated by the edge reconstruction [26, 37]. Moreover, the peak positions and band gap of the 6 ML Bi nano-ribbons on EG are also different from those of the 6 ML Bi nano-rods on the SiC BL. As compared to Bi on EG, more charge transfer from the SiC BL to the Bi film takes place [26]. Thus, the discrepancy can be ascribed to the influence of the underlying SiC BL substrate interaction.

Fig. 4 (a) the morphology of nano-ribbons on EG, and (b) the atomically resolved



structure zoomed in from the edge as marked by the green square in (a). (c) STS spectra from the center (black) and edge (blue) of Bi nano-ribbon as marked by the black and blue dots in (b), respectively.

Fig. 5(a) shows the morphology of thick Bi stripes on EG-1800, and Fig. 5(b) shows the enlarged image from the green square. The wide stripes on the EG substrate have multi-layered structure. Fig. 5(c) displays the height profile along the green line in Fig. 5(b). According to the height profile, the basic height of the Bi films is 6 ML (A, ~2.0 nm). Above the base, 9 ML (B, ~3.0 nm), 10 ML (C, ~3.3 nm) and 11 ML (D, ~3.7 nm) of Bi stripes are observed. As schematically illustrated in Fig. 6, the stable thickness of the additional tier on the 6 ML base is commonly 3 ML [24, 25], and then layer-by-layer growth mode takes place. Therefore, no 7 and 8 ML Bi stripes are evidenced. The layered structure of Bi stripes on EG is similar to the paired growth of Pb on silicon, which is well understood by quantum size effects (QSEs) because of the coincidence of the de Broglie wavelength and interlayer distance [38, 39]. The preferred heights coincide with “magic” quantum well widths for that system [40, 41]. However, the electronic states of layered Bi nano-ribbons are thickness dependent, resulting in different de Broglie wavelength if the thickness is changed [42]. Fig. 5(d) displays the STS spectra measured on the four terraces of Bi stripes (A, B, C and D). The energy gaps and peak positions coincide in these measurements. Similar to Fig. 4, the 6 ML base exhibits the same semiconducting characteristics (black). An energy gap of ~0.5 eV and five peaks of free carriers at -0.94 V, -0.59 V, -0.34 V, 0.51 V and 0.83 V are identified. As the thickness is increased from 9 ML to 10 ML and then to


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11 ML, the band gap disappears gradually and the metallic features are evidenced. The thicker Bi nano-ribbons (11 ML) exhibit the similar properties as that of bulk Bi because of weakened EG substrate effect [31]. That is, the physical properties of nano-structured Bi(110) on EG significantly depend on the thickness.

Fig. 5 (a) the morphology of thick Bi stripes on EG-1800. (b) the enlarged image from the green square in (a). (c) the height profile along the green line in (b). (d) the STS spectra measured on the four terraces of Bi stripes (A, B, C and D).

Fig. 6(a) shows one 6 ML Bi nano-ribbon grown and extended across two EG terraces, and Fig. 6(b) shows another 6 ML Bi nano-ribbon with a 3 ML tier on the top. Figs. 6(c) and 6(d) depict the height profiles along the green lines in Figs. 6(a)



and 6(b). The height of the EG step in Fig. 6(a) is ~0.3 nm, and the Bi nano-ribbon exhibits a uniform height of ~2.0 nm (6 ML) even across two EG steps. The additional straight tier with a height of ~1.0 nm (3 ML) in Fig. 6(b) is aligned along the same direction as the base Bi nano-ribbon, which is indeed similar to the growth mode of thick Bi stripes [Fig. 5]. Fig. 6(e) schematically depicts the carpet-like growth model of Bi nano-ribbons [43]. A and A’ depict the upper and lower EG terrace, respectively, and the Bi nano-ribbon grows across the EG step without changing thickness. It demonstrates the continuous and uniform 2D growth of Bi nano-ribbons across the EG steps [24]. Two factors are important: defect sites for the effective aggregation and large EG terrace for the fast diffusion of Bi atoms [25]. Although it is hard for Bi atoms to be adsorbed on the inert and smooth EG, edges and defects can provide the nucleation sites [16]. The diffusion energy is low because of weak Bi-C coupling [3]. The elongated morphology results from the enhanced diffusion of the directly impinged Bi atoms on EG substrates [18], and subsequent aggregation along the zigzag direction (Bi < 1 10 > ), forming long and straight Bi nano-ribbons.


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Fig. 6 The mechanism of Bi(110) grown on EG layer. (a) single Bi nano-ribbon grown over EG steps. (b) enlarged image of Bi nano-ribbon with a tier on the top. (c) and (d) the height profiles along the green lines in (a) and (b). (e) the schematic model of the growth mechanism of Bi nano-ribbon across EG steps.

In summary, Bi nanostructures are grown on EG/SiC substrates through thermal decomposition of SiC and subsequent evaporation of Bi, and the orientation, atomic structure and thickness dependent electronic states are investigated by STM/S. STM characterizations show that Bi nano-flakes and nano-rods prefer to form on the SiC BL region because of higher diffusion barrier, but Bi nano-ribbons are formed on regularly ordered EG with Bi(110) < 1 10 > paralleling to EG(0001) < 1120 > . As indicated by STS, the Bi nano-flakes and nano-rods on SiC BL exhibit metallic features, and the electronic states are thickness dependent. However, for the thinner Bi



nano-ribbons on EG, semiconducting characteristics with a gap of ~0.5 eV are evidenced due to interfacial effect. If the thickness is increased up to 11 ML, the nano-ribbons on EG will become metallic because of substantially weakened substrate effect. The controllable growth and thickness dependent electronic states of Bi on SiC/EG substrate are interesting and important for the development of electronics and spintronics based on Bi and graphene.

EXPERIMENTAL METHODS The fabrication and structural characterization were conducted in a commercial USM-1400 system (Unisoku) equipped with ultra-high vacuum (UHV) molecular beam epitaxy (MBE) and STM. MBE chamber was used for thermal decomposition of SiC and subsequent low-flux evaporation of Bi. In-situ STM/S was adopted to characterize the atomic structure and electronic states of Bi nano-ribbons. MBE and STM chambers were always kept in UHV of 10-10 Torr and 10-11 Torr, respectively. 6H-SiC(0001) wafers were used to produce EG. The annealing temperature was controlled by direct current (DC) power supply, and monitored by optical pyrometer. SiC substrates were degassed at ~823 K for at least 8 h in UHV to thoroughly remove the surface adsorbates. The sample of EG fabricated through annealing SiC at ~1623 K (~1823 K) under UHV (Ar gas) environment is marked as EG-1600 (EG-1800). Bi-atom-flux was evaporated by K-cell for the growth of Bi under the same condition. STM images and STS spectra were acquired with electrochemically etched W tips at ~77.2 K. The topographic images were taken at constant current (100 pA) mode. STS


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spectra, in which the tunneling differential conductance map of the dI/dV-V curve is proportional to the density of states, provide an accurate and efficient way to examine the local electronic states of Bi. STS were taken at constant height mode, typically the height is optimized at a tip bias of 0.5 V and a set-point current at 200 pA. An AC modulation to the tunneling current with a frequency of 1.03 KHz and an amplitude of 20 mV was added by an internal lock-in of Nanonis electronics. Each dI/dV-V curve is averaged from more than 10 times measurements. All the image processing and data analysis were performed in latest version of WsXM [44].

ACKNOWLEDGMENTS This work was jointly supported by National Natural Science Foundation of China (Grant Nos. 51601142, 51771144, 51471130), China Postdoctoral Science Foundation (2016M592785), Natural Science Foundation of Shaanxi Province (No. 2017JZ015), Fundamental Research Funds for the Central Universities and Collaborative Innovation Center of Suzhou Nano Science and Technology.



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