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Direction-controlled chemical doping for reversible G-phonon mixing in ABC trilayer graphene.

Park K, Ryu S - Sci Rep (2015)

Bottom Line: Not only the apparent atomic arrangement but the charge distribution also defines the crystalline symmetry that dictates the electronic and vibrational structures.Alternatively, the symmetry could be regained by double-side charge injection, which eliminated G(-) and formed an additional peak, G(o), originating from the barely doped interior layer.Chemical modification of crystalline symmetry as demonstrated in the current study can be applied to other low dimensional crystals in tuning their various material properties.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea.

ABSTRACT
Not only the apparent atomic arrangement but the charge distribution also defines the crystalline symmetry that dictates the electronic and vibrational structures. In this work, we report reversible and direction-controlled chemical doping that modifies the inversion symmetry of AB-bilayer and ABC-trilayer graphene. For the "top-down" and "bottom-up" hole injection into graphene sheets, we employed molecular adsorption of electronegative I2 and annealing-induced interfacial hole doping, respectively. The chemical breakdown of the inversion symmetry led to the mixing of the G phonons, Raman active Eg and Raman-inactive Eu modes, which was manifested as the two split G peaks, G(-) and G(+). The broken inversion symmetry could be recovered by removing the hole dopants by simple rinsing or interfacial molecular replacement. Alternatively, the symmetry could be regained by double-side charge injection, which eliminated G(-) and formed an additional peak, G(o), originating from the barely doped interior layer. Chemical modification of crystalline symmetry as demonstrated in the current study can be applied to other low dimensional crystals in tuning their various material properties.

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Raman characterization of pristine trilayer (3L) with ABA and ABC domains.(a) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 514 nm. (b) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 633 nm. The asterisked peaks at ~2710 cm−1 originated from the excitation laser. (c) Optical micrograph of 3L graphene with attached 1L. (d~g) The Raman maps obtained with excitation wavelength of 514 nm from the dashed rectangle in (c): (d) the peak area of G (AG), (e) the peak height of G (HG), (f) the peak frequency of G (ωG), (g) the linewidth of 2D (Γ2D). Whereas the step size of mapping was 1 micron, the map images were refined by bilinear interpolation. The dotted ABA-ABC boundary in (c~g) was determined from the Γ2D-Raman map shown in (g).
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f1: Raman characterization of pristine trilayer (3L) with ABA and ABC domains.(a) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 514 nm. (b) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 633 nm. The asterisked peaks at ~2710 cm−1 originated from the excitation laser. (c) Optical micrograph of 3L graphene with attached 1L. (d~g) The Raman maps obtained with excitation wavelength of 514 nm from the dashed rectangle in (c): (d) the peak area of G (AG), (e) the peak height of G (HG), (f) the peak frequency of G (ωG), (g) the linewidth of 2D (Γ2D). Whereas the step size of mapping was 1 micron, the map images were refined by bilinear interpolation. The dotted ABA-ABC boundary in (c~g) was determined from the Γ2D-Raman map shown in (g).

Mentions: As shown in Fig. 1a, the overall Raman spectra of pristine ABA and ABC trilayers supported on SiO2/Si substrates (see Methods for preparation of samples and measurements) are very similar to each other. Nonetheless, there are a few distinctive spectral features that can be used in characterizing their stacking order. First, the G peak for the C-C stretching mode exhibits ~1 cm−1 downshift for ABC with respect to ABA, presumably due to the different phonon band structures23. The G-peak lineshape of ABC is slightly narrower than that of ABA, since the former has weaker electron-phonon coupling than the latter consequently having longer G-phonon lifetime41. In addition, the 2D peak shows a more significant difference in its lineshape for both polytypes as shown in Fig. 1a. According to the double-resonance (DR) scattering model13, an electron and a hole excited by a Raman excitation photon scatter with two D phonons with wave vectors that match the intervalley resonant electronic transitions. Consequently polytypes of different electronic band structures would exhibit different 2D spectra even if their phonon band structures are identical. Since trilayer graphene has three sets of valence and conduction bands, there can be as many as 15 scattering processes that satisfy the requirement for DR42. As shown in Supplementary Fig. S1, however, 2D peaks of both polytypes can be satisfactorily fitted with 6 Lorentzian functions due to non-distinguishability and insufficient scattering probability of some among the 15 processes2342. Whereas these and other spectral features can be used to characterize stacking domains by Raman mapping, the frequency of G peak (ωG) and the effective linewidth of 2D peak () were chosen for the simplicity and shorter analysis time. As shown in the Raman maps (Figs. 1d ~ 1f), the trilayer sheet consists of the two domains that are indistinguishable in the optical micrograph (Fig. 1c) or the intensity map (Fig. 1d). The domain denoted ABC indeed shows ~1 cm−1 smaller ωG and ~10 cm−1 larger than the ABA-domain in agreement with previous reports2341. Although defined as a single Lorentzian linewidth does not represent the lineshape of 2D peak very well, it was proven highly efficient in differentiating the stacking domains of 3L23.


Direction-controlled chemical doping for reversible G-phonon mixing in ABC trilayer graphene.

Park K, Ryu S - Sci Rep (2015)

Raman characterization of pristine trilayer (3L) with ABA and ABC domains.(a) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 514 nm. (b) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 633 nm. The asterisked peaks at ~2710 cm−1 originated from the excitation laser. (c) Optical micrograph of 3L graphene with attached 1L. (d~g) The Raman maps obtained with excitation wavelength of 514 nm from the dashed rectangle in (c): (d) the peak area of G (AG), (e) the peak height of G (HG), (f) the peak frequency of G (ωG), (g) the linewidth of 2D (Γ2D). Whereas the step size of mapping was 1 micron, the map images were refined by bilinear interpolation. The dotted ABA-ABC boundary in (c~g) was determined from the Γ2D-Raman map shown in (g).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4352872&req=5

f1: Raman characterization of pristine trilayer (3L) with ABA and ABC domains.(a) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 514 nm. (b) Raman spectra of ABA and ABC trilayers obtained with excitation wavelength of 633 nm. The asterisked peaks at ~2710 cm−1 originated from the excitation laser. (c) Optical micrograph of 3L graphene with attached 1L. (d~g) The Raman maps obtained with excitation wavelength of 514 nm from the dashed rectangle in (c): (d) the peak area of G (AG), (e) the peak height of G (HG), (f) the peak frequency of G (ωG), (g) the linewidth of 2D (Γ2D). Whereas the step size of mapping was 1 micron, the map images were refined by bilinear interpolation. The dotted ABA-ABC boundary in (c~g) was determined from the Γ2D-Raman map shown in (g).
Mentions: As shown in Fig. 1a, the overall Raman spectra of pristine ABA and ABC trilayers supported on SiO2/Si substrates (see Methods for preparation of samples and measurements) are very similar to each other. Nonetheless, there are a few distinctive spectral features that can be used in characterizing their stacking order. First, the G peak for the C-C stretching mode exhibits ~1 cm−1 downshift for ABC with respect to ABA, presumably due to the different phonon band structures23. The G-peak lineshape of ABC is slightly narrower than that of ABA, since the former has weaker electron-phonon coupling than the latter consequently having longer G-phonon lifetime41. In addition, the 2D peak shows a more significant difference in its lineshape for both polytypes as shown in Fig. 1a. According to the double-resonance (DR) scattering model13, an electron and a hole excited by a Raman excitation photon scatter with two D phonons with wave vectors that match the intervalley resonant electronic transitions. Consequently polytypes of different electronic band structures would exhibit different 2D spectra even if their phonon band structures are identical. Since trilayer graphene has three sets of valence and conduction bands, there can be as many as 15 scattering processes that satisfy the requirement for DR42. As shown in Supplementary Fig. S1, however, 2D peaks of both polytypes can be satisfactorily fitted with 6 Lorentzian functions due to non-distinguishability and insufficient scattering probability of some among the 15 processes2342. Whereas these and other spectral features can be used to characterize stacking domains by Raman mapping, the frequency of G peak (ωG) and the effective linewidth of 2D peak () were chosen for the simplicity and shorter analysis time. As shown in the Raman maps (Figs. 1d ~ 1f), the trilayer sheet consists of the two domains that are indistinguishable in the optical micrograph (Fig. 1c) or the intensity map (Fig. 1d). The domain denoted ABC indeed shows ~1 cm−1 smaller ωG and ~10 cm−1 larger than the ABA-domain in agreement with previous reports2341. Although defined as a single Lorentzian linewidth does not represent the lineshape of 2D peak very well, it was proven highly efficient in differentiating the stacking domains of 3L23.

Bottom Line: Not only the apparent atomic arrangement but the charge distribution also defines the crystalline symmetry that dictates the electronic and vibrational structures.Alternatively, the symmetry could be regained by double-side charge injection, which eliminated G(-) and formed an additional peak, G(o), originating from the barely doped interior layer.Chemical modification of crystalline symmetry as demonstrated in the current study can be applied to other low dimensional crystals in tuning their various material properties.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea.

ABSTRACT
Not only the apparent atomic arrangement but the charge distribution also defines the crystalline symmetry that dictates the electronic and vibrational structures. In this work, we report reversible and direction-controlled chemical doping that modifies the inversion symmetry of AB-bilayer and ABC-trilayer graphene. For the "top-down" and "bottom-up" hole injection into graphene sheets, we employed molecular adsorption of electronegative I2 and annealing-induced interfacial hole doping, respectively. The chemical breakdown of the inversion symmetry led to the mixing of the G phonons, Raman active Eg and Raman-inactive Eu modes, which was manifested as the two split G peaks, G(-) and G(+). The broken inversion symmetry could be recovered by removing the hole dopants by simple rinsing or interfacial molecular replacement. Alternatively, the symmetry could be regained by double-side charge injection, which eliminated G(-) and formed an additional peak, G(o), originating from the barely doped interior layer. Chemical modification of crystalline symmetry as demonstrated in the current study can be applied to other low dimensional crystals in tuning their various material properties.

Show MeSH
Related in: MedlinePlus