In-situ High Pressure Raman Study Of Graphdiyne And Twist Bilayer Graphene

Due to unique structure and outstanding properties,graphene has spawn plentiful scientific interests in recent years.Graphene is considered as building block of sp~2-hybridized carbon materials,composing novel carbon allotropes with versatile structures and excellent properties.Replacing one third carbon bonds with 1,3-butadiyne linkages,graphene transforms into graphdiyne.Rotating bilayer graphene by a certain angle,Moirésuperlattice blooms in twisted bilayer graphene.Within outstanding properties inheriting from graphene,characteristic capacities have been discovered in both graphdiyne and twisted bilayer graphene as well.On the other hand,high pressure can efficiently shorten atom spacings,distinctly affect interactions,even dramatically creating new structures with reconstituted bonds.Raman spectroscopy is an integral part of graphene research and is widely used to determine the number and orientation of layers,the quality and types of edge,and the effects of perturbations,such as strain,doping,disorder and functional groups.In turn,in-situ high pressure Raman spectroscopy provides reliable evidences of phase transition and traceable clues of changes in physical and chemical properties.The study of pure sp-hybridized carbon chains by experimental means remained problematic due to the fact that the chains are chemically unstable at ambient conditions.From this perspective,it is valuable to study bonding changes of sp-hybridized carbons in the graphdiyne network under pressure.Misaligned stacking affects degree of interlayer coupling in bilayer graphene system,resulting in unexpected properties such as superconductivity.To the best of author’s knowledge,there is not any high-pressure study on the influence of stacking order on mechanical properties of bilayer graphene.Thus,it is necessary to conduct high-pressure Raman study on graphdiyne and twisted bilayer graphene.1.High pressure Raman spectra of graphdiyne film have been studied up to34.63 GPa,aiming at exploring the mechanism of pressure-induced sp-to sp~2-rehybridization.We found that sp-hybridized carbons in graphdiyne are highly active and start to undergo a bonding change at around 5.2 GPa.Such a bonding change affects the C-C stretching vibration of sp~2-hybridized hexagon rings in graphdiyne,leading to an anomaly in the corresponding G-band pressure coefficient.These spectral changes could be considered as criterions of pressure-induced sp-hybridized to sp~2-hybridized bonding change.A three-dimensional sp~2 structure is proposed to form via pressure-induced interlayer cross-linking of sp carbons in graphdiyne and is stable up to at least34.63 GPa.Comparing Raman signals of samples decompressed from different pressures in sp-carbon specific spectral region,the irreversibility of pressure-induced sp to sp~2 bonding change has been revealed.The characteristic compression behaviors in graphdiyne present unique insights into the transition of graphyne family under pressure,demonstrating the potential application of using graphdiyne as a precursor for creating three dimensional sp~2 carbon allotropes.2.In-situ high-pressure Raman spectroscopy was used to investigate the influence of stacking order on the two-dimensional(2D)in-plane stiffness of bilayer graphene,and an optimized model was established to characterize the variation of 2D in-plane stiffness of bilayer graphene under high pressure.2D in-plane stiffness of graphene is the highest in the two-dimensional material family.Attempts have been made to increase this by various methods,such as applying stress,doping and intercalation,but with little success.High pressure Raman spectra show that twist angle has exceptional ability to regulate the 2D in-plane stiffness of bilayer graphene.Pressure coefficient of2D in-plane stiffness of 30°quasicrystal bilayers(26.3±4.7)was about three times as that of AB stacked bilayers(8.3±6.8).Under high pressure,the deformation ofπbonds in AA-regions results in the increase ofπelectron density,which assists sp~2 bonds to resist the in-plane compressive strain,leading to the increase of 2D in-plane stiffness of 30°quasicrystalline bilayer graphene.In addition,high-pressure in-situ Raman spectra revealed that 30°quasicrystalline bilayer graphene shows an ultra-high 2D in-plane stiffness of approximately 578 N∕m at 9 GPa,which is the largest 2D in-plane stiffness of known materials and is twice as much as that of monolayer graphene.Compressed 30°quasicrystalline bilayer graphene material with ultra-high2D in-plane stiffness has potential applications in the field of nanodevices.3.Theoretical simulations and high-pressure experiments have been studies on large angle twisted bilayer graphene to investigate the influence of pressure on van Hove singularities.Based on the result of simulations of 21.79°twisted bilayer graphene under pressure,the twist angle remains unchanged and interlayer spacing decreased upon compression,enhancing interlayer coupling and resulting in the reduce of energy difference of van Hove singularities.High-pressure Raman experiments were carried out on bilayer graphene samples with random twist angle,using a 473 nm or532 nm exciting light.Under 2.62 e V exciting energy,a resonant enhanced G-peak was observed at 2.36 GPa.Correspondingly,at 2.67 GPa,another resonant enhanced phenomenon on G-peak appeared using a 2.33e V incident laser.Such enhancement is attributed to the coincidence of energy difference of van Hove singularities and energy of incident laser,which is revealed by Theoretical simulations.When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons,the photocurrent generated can be enhanced.Thus,pressure reduces interlayer spacings and regulates the band structure of twist bilayer graphene.Our results proves that interlayer coupling intensity decreases while the twist angle between bilayer graphene increases.Our results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.4.In this thesis,all experimental data processing methods have been verified by F-TEST using MATLAB(?),proving our results statistically significant.This method should be widely used in data processing to ensure the reasonability of the conclusions.