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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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Single-sided vs double-sided hole injection in FLG/SiO2/Si substrates by I2.The Raman spectra of AB (a), ABA (b), and ABC (c) obtained in an optical cell as a function of the exposure time (t) to I2 vapor. The spectra were vertically offset for clarity after the broad fluorescence from iodine species was subtracted from each spectrum.
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f6: Single-sided vs double-sided hole injection in FLG/SiO2/Si substrates by I2.The Raman spectra of AB (a), ABA (b), and ABC (c) obtained in an optical cell as a function of the exposure time (t) to I2 vapor. The spectra were vertically offset for clarity after the broad fluorescence from iodine species was subtracted from each spectrum.

Mentions: To induce bottom-up and top-down doping simultaneously, samples were monitored in situ in an optical cell containing a small piece of I2 crystal, which gradually sublimed to reach the vapor pressure of I2 of ~0.3 Torr at room temperature. According to Jung et al.37, iodine molecules intercalate through the graphene-silica interface at a partial pressure of ~0.1 Torr and form two saturated dopant layers underneath and on top of 2L graphene, respectively. As shown in Fig. 6, the G peak of AB layer split into G− and G+ because of the top-down hole doping (Fig. 4) for short exposure time (t ≤ 1.5 hours). For t ≥ 2.0 hours, however, G+ further upshifted to 1604 cm−1 whereas G− disappeared, which indicates the double-sided charge doping by the adsorbed and the intercalated I2 layers as shown by the previous studies3756. The G peak of ABC 3L also exhibited the splitting into G− and G+ for short exposure (t ≤ 1.5 hours) and the upshift of G+ to 1600 cm−1 at the expense of G− for extended exposure (t ≥ 2.0 hours). We note that a new peak (denoted as Go for its non-dispersive character regardless of t unlike the other G-related peaks) appeared at 1586 cm−1 when ABC was double-side doped (t ≥ 1.5 hours). In a similar measurement, the G peak of ABA 3L showed a monotonous upshift to 1600 cm−1 without the splitting and Go also appeared at 1586 cm−1 (t ≥ 1.5 hours).


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

Park K, Ryu S - Sci Rep (2015)

Single-sided vs double-sided hole injection in FLG/SiO2/Si substrates by I2.The Raman spectra of AB (a), ABA (b), and ABC (c) obtained in an optical cell as a function of the exposure time (t) to I2 vapor. The spectra were vertically offset for clarity after the broad fluorescence from iodine species was subtracted from each spectrum.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6: Single-sided vs double-sided hole injection in FLG/SiO2/Si substrates by I2.The Raman spectra of AB (a), ABA (b), and ABC (c) obtained in an optical cell as a function of the exposure time (t) to I2 vapor. The spectra were vertically offset for clarity after the broad fluorescence from iodine species was subtracted from each spectrum.
Mentions: To induce bottom-up and top-down doping simultaneously, samples were monitored in situ in an optical cell containing a small piece of I2 crystal, which gradually sublimed to reach the vapor pressure of I2 of ~0.3 Torr at room temperature. According to Jung et al.37, iodine molecules intercalate through the graphene-silica interface at a partial pressure of ~0.1 Torr and form two saturated dopant layers underneath and on top of 2L graphene, respectively. As shown in Fig. 6, the G peak of AB layer split into G− and G+ because of the top-down hole doping (Fig. 4) for short exposure time (t ≤ 1.5 hours). For t ≥ 2.0 hours, however, G+ further upshifted to 1604 cm−1 whereas G− disappeared, which indicates the double-sided charge doping by the adsorbed and the intercalated I2 layers as shown by the previous studies3756. The G peak of ABC 3L also exhibited the splitting into G− and G+ for short exposure (t ≤ 1.5 hours) and the upshift of G+ to 1600 cm−1 at the expense of G− for extended exposure (t ≥ 2.0 hours). We note that a new peak (denoted as Go for its non-dispersive character regardless of t unlike the other G-related peaks) appeared at 1586 cm−1 when ABC was double-side doped (t ≥ 1.5 hours). In a similar measurement, the G peak of ABA 3L showed a monotonous upshift to 1600 cm−1 without the splitting and Go also appeared at 1586 cm−1 (t ≥ 1.5 hours).

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