Evolution of Physical and Electronic Structures of Bilayer Graphene upon Chemical Functionalization

The chemical behavior of bilayer graphene under strong covalent and noncovalent functionalization is relatively unknown compared to monolayer graphene, which has been far more widely studied. Bilayer graphene is significantly less chemically reactive than monolayer graphene, making it more challenging to study its chemistry in detail. However, bilayer graphene is increasingly attractive for electronic applications rather than monolayer graphene because of its electric-field-controllable band gap, and there is a need for a greater understanding of its chemical functionalization. In this paper, we study the covalent and noncovalent functionalization of bilayer graphene using an electrochemical process with aryl diazonium salts in the high conversion regime (D/G ratio >1), and we use Raman spectroscopic mapping and conductive atomic force microscopy (cAFM) to study the resulting changes in the physical and electronic structures. Covalent functionalization at high chemical conversion induces distinct changes in the Raman spectrum of bilayer graphene including the broadening and shift in position of the split G peak. Also, the D peak becomes active with four components. We report for the first time that the broadening of the 2D₂₂ and 2D₂₁ components is a distinct indicator of covalent functionalization, whereas the decrease in intensity of the 2D₁₁ and 2D₁₂ peaks corresponds to doping. Conductive AFM imaging shows physisorbed species from noncovalent functionalization can be removed by mechanical and electrical influence from the AFM tip, and that changes in conductivity due to functionalization are inhomogeneous. These results allow one to distinguish covalent from noncovalent chemistry as a guide for further studies of the chemistry of bilayer graphene

Excess Thermopower and the Theory of Thermopower Waves

Self-propagating exothermic chemical reactions can generate electrical pulses when guided along a conductive conduit such as a carbon nanotube. However, these thermopower waves are not described by an existing theory to explain the origin of power generation or why its magnitude exceeds the predictions of the Seebeck effect. In this work, we present a quantitative theory that describes the electrical dynamics of thermopower waves, showing that they produce an excess thermopower additive to the Seebeck prediction. Using synchronized, high-speed thermal, voltage, and wave velocity measurements, we link the additional power to the chemical potential gradient created by chemical reaction (up to 100 mV for picramide and sodium azide on carbon nanotubes). This theory accounts for the waves’ unipolar voltage, their ability to propagate on good thermal conductors, and their high power, which is up to 120% larger than conventional thermopower from a fiber of all-semiconducting SWNTs. These results underscore the potential to exceed conventional figures of merit for thermoelectricity and allow us to bound the maximum power and efficiency attainable for such systems