Limits...
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.

Show MeSH

Related in: MedlinePlus

Effects of the bottom-up hole injection in FLG/SiO2/Si substrates by thermal annealing: Raman spectra of (a) AB, (b) ABA and (c) ABC FLG, annealed in a vacuum at elevated temperatures (Tanneal).Upon annealing, G peak of AB and ABC splits into G− and G+ unlike that of ABA.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4352872&req=5

f2: Effects of the bottom-up hole injection in FLG/SiO2/Si substrates by thermal annealing: Raman spectra of (a) AB, (b) ABA and (c) ABC FLG, annealed in a vacuum at elevated temperatures (Tanneal).Upon annealing, G peak of AB and ABC splits into G− and G+ unlike that of ABA.

Mentions: Figure 2 presents the Raman spectra of AB, ABA and ABC samples the symmetries of which were being progressively modified by the bottom-up hole injection using the annealing-induced chemical doping194344. (see Methods for details.) When ambient O2 intercalates through the annealed graphene/SiO2 interface4044, it undergoes a redox reaction involving the O2/H2O couple with an electrochemical potential sufficiently lower than the Fermi level of graphene4546. Thus, the interfacial reaction consumes electrons in graphene above and essentially leads to bottom-up hole doping with a maximum hole density of ~2 × 1013/cm2 in 1L for the annealing temperature (Tanneal) of 600°C19. As increasing Tanneal or essentially the hole density, the G peak of AB upshifts from its intrinsic frequency of ~1581 cm−1 to ~1598 cm−1. At 400°C, a shoulder at lower frequency appears and essentially develops into a separate peak at higher temperatures. Both peaks were fitted with a double Lorentzian function and the low and high frequency peaks were labelled respectively as G− and G+ according to Yan et al.34 Whereas a similar upshift and splitting can be seen for ABC, ABA shows only upshift without splitting. The difference between ABA and ABC in the response to the bottom-up hole doping can be seen more clearly in Fig. 3 which shows the variation of the peak frequencies as a function of Tanneal. As the degree of the doping increases, G (or G+) increases in its peak frequency by 10~15 cm−1 for the three FLG systems. For AB and ABC, the new G− peak gradually downshifts with increasing charge density. Interestingly, the splitting in ωG (ΔωG) reaches ~15 cm−1 at 700°C for both of AB and ABC despite the slight difference in their frequencies. Whereas the phonon frequencies are also subject to the lattice deformation of native or thermally induced origin43, ΔωG is not significantly influenced by such effects to a first order approximation.


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

Park K, Ryu S - Sci Rep (2015)

Effects of the bottom-up hole injection in FLG/SiO2/Si substrates by thermal annealing: Raman spectra of (a) AB, (b) ABA and (c) ABC FLG, annealed in a vacuum at elevated temperatures (Tanneal).Upon annealing, G peak of AB and ABC splits into G− and G+ unlike that of ABA.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Effects of the bottom-up hole injection in FLG/SiO2/Si substrates by thermal annealing: Raman spectra of (a) AB, (b) ABA and (c) ABC FLG, annealed in a vacuum at elevated temperatures (Tanneal).Upon annealing, G peak of AB and ABC splits into G− and G+ unlike that of ABA.
Mentions: Figure 2 presents the Raman spectra of AB, ABA and ABC samples the symmetries of which were being progressively modified by the bottom-up hole injection using the annealing-induced chemical doping194344. (see Methods for details.) When ambient O2 intercalates through the annealed graphene/SiO2 interface4044, it undergoes a redox reaction involving the O2/H2O couple with an electrochemical potential sufficiently lower than the Fermi level of graphene4546. Thus, the interfacial reaction consumes electrons in graphene above and essentially leads to bottom-up hole doping with a maximum hole density of ~2 × 1013/cm2 in 1L for the annealing temperature (Tanneal) of 600°C19. As increasing Tanneal or essentially the hole density, the G peak of AB upshifts from its intrinsic frequency of ~1581 cm−1 to ~1598 cm−1. At 400°C, a shoulder at lower frequency appears and essentially develops into a separate peak at higher temperatures. Both peaks were fitted with a double Lorentzian function and the low and high frequency peaks were labelled respectively as G− and G+ according to Yan et al.34 Whereas a similar upshift and splitting can be seen for ABC, ABA shows only upshift without splitting. The difference between ABA and ABC in the response to the bottom-up hole doping can be seen more clearly in Fig. 3 which shows the variation of the peak frequencies as a function of Tanneal. As the degree of the doping increases, G (or G+) increases in its peak frequency by 10~15 cm−1 for the three FLG systems. For AB and ABC, the new G− peak gradually downshifts with increasing charge density. Interestingly, the splitting in ωG (ΔωG) reaches ~15 cm−1 at 700°C for both of AB and ABC despite the slight difference in their frequencies. Whereas the phonon frequencies are also subject to the lattice deformation of native or thermally induced origin43, ΔωG is not significantly influenced by such effects to a first order approximation.

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