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Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene.

Chen HH, Su SH, Chang SL, Cheng BY, Chen SW, Chen HY, Lin MF, Huang JC - Sci Rep (2015)

Bottom Line: Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms.When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size.The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, National Cheng Kung University, Tainan, Taiwan 701, Taiwan.

ABSTRACT
To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

No MeSH data available.


Room-in FFT, atomic structure and electronic structure analysis.Room-in FFT of (a) image in bottom left panel (20 × 20 nm2 atomically resolved MEG image) and (b) Fig. 2d. Top right panel corresponds to zooming-in the FFT of inset in Fig. 1a. (c) The theoretically atomic structure of the 1D chain and 2D triangular structures. (d) dI/dV spectra of as-grown graphene and Bi adatom in hexagonal array.
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f3: Room-in FFT, atomic structure and electronic structure analysis.Room-in FFT of (a) image in bottom left panel (20 × 20 nm2 atomically resolved MEG image) and (b) Fig. 2d. Top right panel corresponds to zooming-in the FFT of inset in Fig. 1a. (c) The theoretically atomic structure of the 1D chain and 2D triangular structures. (d) dI/dV spectra of as-grown graphene and Bi adatom in hexagonal array.

Mentions: The MEG and the Bi hexagonal array are then analyzed by zooming in the FFT of two STM topographic images with same size, as respectively depicted in Fig. 3(a,b). The six outer spots originate from the hexagonal graphene lattice and the six inner spots correspond to the observed 6 × 6-SiC moiré pattern, as shown in Fig. 3(a). The moiré pattern of the MEG is due to the ( × ) R30° reconstruction of the buffer layer, which occurs as a result of the large difference between lattice parameters of SiC (3.08 Å) and graphene (2.46 Å). Figure 3(b), which zooms in the FFT of Fig. 2(d), shows a similar sixfold symmetry pattern to that of the inner spots in Fig. 3(a). The upper inset, which zooms in the FFT of the inset in Fig. 1(a), shows clearer pattern. That is, the large-scale hexagonal array of Bi adatoms evidences a moiré-like superstructure and such arrangement is strongly affected by the corrugated substrate. As stated above , the interatomic distance between Bi atoms changes from 1.8 nm for the 1D linear chain to 1.6 nm for the 2D triangular island structure, equivalently from four times into times the lattice spacing of the Bi nanoribbon, indicating a coverage-dependent 1D → 2D growth mode transition in this system. According to the DFT calculations, combined with previous literature262728, Bi adatom is preferentially adsorbed at the bridge (B) site, as shown in Fig. S1 and Table S1. The theoretical arrangements of 1D and 2D structures are then illustrated in Fig. 3(c). Left and right panels reveal the atomic arrangements of Bi adatoms at different coverage, respectively. From the DFT calculations, the interatomic distances between Bi atoms, 1.8 nm and 1.6 nm, coexist at 1D structure dominated configuration while only one characteristic distance of 1.6 nm appears at 2D structure. The DFT results are consistent with the STM observations. This structural transition could be related with coverage-dependent phenomenon and the effect of corrugated substrate. Correspondingly, the electronic properties of Bi adatom in hexagonal array are probed using tunneling spectroscopy, as shown in Fig. 3(d). The dI/dV spectrum of the as-grown MEG exhibits a characteristic minimum at −0.37 eV. This is attributed to Dirac point, indicative of n-type doping by the SiC substrate, while some packets exist around Fermi level (EF)21. Upon the deposition of Bi adatoms on MEG, the Dirac point shifts to near EF, appearing at −0.32 eV [Fig. 3(d)], indicating a little charge transfer (~50 meV) from MEG to Bi adatom. The observed coverage-dependent structural transition and small charge transfer are basically consistent with the STM and synchrotron-based photoemission spectroscopy (PES) measurements22. The tunneling spectrum also exhibits one peak at −0.72 eV [black arrow in Fig. 3(d)), which is similar to one of the four characteristic peaks of the Bi (110) nanoribbon, based on rhombohedral indexing72930. As recorded in the literature3031, the peak at ~−0.75 eV is mainly attributable to the p-states that are localized at the topmost layer of Bi (110) nanoribbon. Thus, we speculate that the peak in Fig. 3(d) results from the contribution of the p band of 2D Bi hexagonal array. Additionally, the characteristic feature (−0.72 eV) suggests that Bi adatom form a bound state at B site on MEG due likely to the Bi adatom-MEG interaction.


Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene.

Chen HH, Su SH, Chang SL, Cheng BY, Chen SW, Chen HY, Lin MF, Huang JC - Sci Rep (2015)

Room-in FFT, atomic structure and electronic structure analysis.Room-in FFT of (a) image in bottom left panel (20 × 20 nm2 atomically resolved MEG image) and (b) Fig. 2d. Top right panel corresponds to zooming-in the FFT of inset in Fig. 1a. (c) The theoretically atomic structure of the 1D chain and 2D triangular structures. (d) dI/dV spectra of as-grown graphene and Bi adatom in hexagonal array.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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f3: Room-in FFT, atomic structure and electronic structure analysis.Room-in FFT of (a) image in bottom left panel (20 × 20 nm2 atomically resolved MEG image) and (b) Fig. 2d. Top right panel corresponds to zooming-in the FFT of inset in Fig. 1a. (c) The theoretically atomic structure of the 1D chain and 2D triangular structures. (d) dI/dV spectra of as-grown graphene and Bi adatom in hexagonal array.
Mentions: The MEG and the Bi hexagonal array are then analyzed by zooming in the FFT of two STM topographic images with same size, as respectively depicted in Fig. 3(a,b). The six outer spots originate from the hexagonal graphene lattice and the six inner spots correspond to the observed 6 × 6-SiC moiré pattern, as shown in Fig. 3(a). The moiré pattern of the MEG is due to the ( × ) R30° reconstruction of the buffer layer, which occurs as a result of the large difference between lattice parameters of SiC (3.08 Å) and graphene (2.46 Å). Figure 3(b), which zooms in the FFT of Fig. 2(d), shows a similar sixfold symmetry pattern to that of the inner spots in Fig. 3(a). The upper inset, which zooms in the FFT of the inset in Fig. 1(a), shows clearer pattern. That is, the large-scale hexagonal array of Bi adatoms evidences a moiré-like superstructure and such arrangement is strongly affected by the corrugated substrate. As stated above , the interatomic distance between Bi atoms changes from 1.8 nm for the 1D linear chain to 1.6 nm for the 2D triangular island structure, equivalently from four times into times the lattice spacing of the Bi nanoribbon, indicating a coverage-dependent 1D → 2D growth mode transition in this system. According to the DFT calculations, combined with previous literature262728, Bi adatom is preferentially adsorbed at the bridge (B) site, as shown in Fig. S1 and Table S1. The theoretical arrangements of 1D and 2D structures are then illustrated in Fig. 3(c). Left and right panels reveal the atomic arrangements of Bi adatoms at different coverage, respectively. From the DFT calculations, the interatomic distances between Bi atoms, 1.8 nm and 1.6 nm, coexist at 1D structure dominated configuration while only one characteristic distance of 1.6 nm appears at 2D structure. The DFT results are consistent with the STM observations. This structural transition could be related with coverage-dependent phenomenon and the effect of corrugated substrate. Correspondingly, the electronic properties of Bi adatom in hexagonal array are probed using tunneling spectroscopy, as shown in Fig. 3(d). The dI/dV spectrum of the as-grown MEG exhibits a characteristic minimum at −0.37 eV. This is attributed to Dirac point, indicative of n-type doping by the SiC substrate, while some packets exist around Fermi level (EF)21. Upon the deposition of Bi adatoms on MEG, the Dirac point shifts to near EF, appearing at −0.32 eV [Fig. 3(d)], indicating a little charge transfer (~50 meV) from MEG to Bi adatom. The observed coverage-dependent structural transition and small charge transfer are basically consistent with the STM and synchrotron-based photoemission spectroscopy (PES) measurements22. The tunneling spectrum also exhibits one peak at −0.72 eV [black arrow in Fig. 3(d)), which is similar to one of the four characteristic peaks of the Bi (110) nanoribbon, based on rhombohedral indexing72930. As recorded in the literature3031, the peak at ~−0.75 eV is mainly attributable to the p-states that are localized at the topmost layer of Bi (110) nanoribbon. Thus, we speculate that the peak in Fig. 3(d) results from the contribution of the p band of 2D Bi hexagonal array. Additionally, the characteristic feature (−0.72 eV) suggests that Bi adatom form a bound state at B site on MEG due likely to the Bi adatom-MEG interaction.

Bottom Line: Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms.When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size.The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, National Cheng Kung University, Tainan, Taiwan 701, Taiwan.

ABSTRACT
To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

No MeSH data available.