Application of increasing the band gap of graphene oxide
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Answer:
Exfoliation, i.e. individual separation of carbon sheets, is of great interest to produce single-layered graphene nanosheets. Chemical or thermal treatments are popular approaches to exfoliate graphite chunks. In general, these conventional methods are assisted with intercalation via covalent or non-covalent functionalization, expansion, and swelling, adsorption of organic molecules in gas phase, solid nanoparticle insertion or direct molecular exfoliation. However, direct covalent modification of graphene is challenging and the zero-band gap of graphene limits its use in field-effect transistors in nanoelectronics. Therefore, the use of a band-gap tunable p-type semiconducting reduced graphene oxide (rGO) is an alternative route. There are several approaches to tune its band gap, including tailoring the chemistry. Critical parameters include the control of oxygen amount determined by the degree and time of oxidation and reduction conditions (e. g. temperature), often leading to nonstoichiometry. This short review therefore highlights the production of rGO focusing primarily on the effect of thermal treatmenton the nature and the role of oxygen during thermal exfoliation of GO. The impact of oxygen functionalization on the modulation of the band gap is also reviewed for chemically and thermally reduced GO, as well as chemically treated rGO followed by a thermal exfoliation.
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Answer:
Energy Gap:-
The energy gap of the composite is determined by plotting the Tauc plot from the UV–Vis analysis and is presented. It is found that the material formed is a direct band gap semiconductor with an energy gap of 3.29–4.27 eV. They are in the range of organic semiconductors or compound semiconductors. With the incorporation of polymer in the graphene oxide matrix, the energy gap is found to be increasing.
Sample Optical energy gap (eV)
BC 3.29
RBC 3.82
PBC-A 4.06
PBC-B 4.27
The bandwidth is controlled by adding the graphite oxide to PANI and is found to be marginally increased with decreasing concentration of GO in the composite.
Energy gaps and related parameters are listed for zinc blende crystals and for wurtzite nitrides. Band gap reductions with higher temperature mainly arise from the change of the lattice constant. The following Varshni approximation is often employed using empirical parameters A and B.
The energy gaps listed are the principal energy gaps for the different materials, that is they are the smallest energy gaps that separate the top of the valence band from the bottom of the conduction band. The relative location of the maximum and minimum in k-space is important for determining the optical properties of semiconductors. In indirect semiconductors, the maximum and minimum occur at different points in k-space and direct transitions from the conduction band to the valence band have low probability. The reason for this is that when an electron drops from the conduction band to the valence band, and releases its excess energy in the form of a photon, the crystal momentum must be conserved. (We should remember that crystal momentum is only defined to within an arbitrary reciprocal-lattice wave vector. See Kittel 1996, for example.)
While the photon may carry a large amount of energy, its momentum relative to that of the electron is very small. Thus, the transition of the electron should leave its wave vector almost unchanged from its initial value—a requirement that cannot be satisfied in an indirect semiconductor. Here, a large change in crystal momentum is associated with a transition between the conduction-band maximum and the valence-band minimum, and this must be taken up by the simultaneous emission or absorption of a phonon. Since this photon--phonon emission has a low probability, indirect materials, such as Si, are poor choices for optical emitters.
The energy gap of semiconductors varies weakly with temperature, typically decreasing with increasing temperature. In Si, for example, the size of the energy gap decreases by about 0.07 eV when the temperature is increased from 20 to 400 K. This corresponds to a relative change of roughly 6%, compared to the value of the room-temperature gap (Madelung 1996). There are a number of processes that contribute to the temperature-dependent variation of the bandgap (Ridley 1999), one of which is thermal expansion of the crystal. This weakens the overlap integrals of the orbitals on the different atoms, and so reduces the energy hybridization resulting from the overlap. Another effect is a thermal smearing of the background periodic potential, created by the atom cores in the crystal.
These effects may be accounted for by considering the influence of temperature on the electron--phonon interaction, which provides a contribution to the total energy of electrons in the crystal. The important point is that the process of exciting electrons into the conduction band corresponds to the breaking of bonds in the crystal, which in turn “softens” the vibrational modes of the atoms, by weakening the elastic restoring forces exerted between them. By lowering the energy of the vibrational modes in this manner, it is possible to show that the size of the bandgap is reduced with increasing temperature (Ridley 1999).
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