Nonresonant Raman spectroscopy analysis of bithiophene and quater-thiophene chains confined in graphene bilayer: A theoretical study
Introduction
Since its first fabrication [1], graphene has attracted huge attention due to its two-dimensional crystalline structure and special optical and electronic properties [2], [3]. It is intensively explored in many applications. The two-dimensional structure of graphene offers the possibility to create layer-by-layer assembly. This method exploits the availability of materials with insulating, semiconducting and metallic behaviors.
Graphene/conjugated polymer hybrids stands as supermolecular assemblies of oligothiophene between graphene layers and generated a lot of interest for future solar cells applications. The organic solar cells are a promising low-cost alternative to silicon solar ones [4], Unfortunately the main handicap is the low power conversion efficiency of these devices. The integration of carbon nanotubes (CNTs) or graphene presents One of the alternatives to solve this handicap to form an interpenetrating blend with the polymer. The discovery of photoinduced charge transfer between nanotubes or graphene (as acceptor) and organic conjugated polymers (as donor) offers the possibility to increase the efficiency of the solar cell. This discovery gives many possibilities to fabricate photovoltaic devices with carbon nanotubes and graphene in combination with different polymers [5], [6], [7].
Raman spectroscopy provides the best signature for characterizing graphene and graphene-oligomers. The ratio of intensities of the Raman G-band of single-layer graphene and several bands of oligomers is sensitive to the concentration of oligomers intercalated in graphene. We considered it important to investigate this effect through molecular charge transfer by the interaction of graphene with molecules. We have three methods to combine oligomers with graphene to create a new hybrid system. The functionalization of oligomers at the surface of the sheet is the first method [8]. The second one is the noncovalent adsorption of various molecules [9]. The method used in this work which is the third method consists to intercalate oligomers between two sheets of graphene [10], [11], [12]. The advantage of the last method is that the graphene protects the internal organic molecules from attacks by external reactive species and photo degradation [13].
Oligomers are extensively used in organic electronic devices and recently in novel hybrid carbon materials for solar cells. In fact, new hybrid nano-systems are elaborated with oligomers. 1D systems obtained by encapsulated into the single-wall carbon nanotubes or 2D systems created by non-covalently functionalized between graphene sheets. Due to the van der Waals interaction between graphene sheets, the dispersion of graphene in the polymer matrix presents the huge challenge for the applications of graphene/polymer nanocomposites at present [14]. Several experimental works dealt with the encapsulation of oligothiophene derivatives (oTh) into CNTs(oTh@NT) [10], [12], [15]. Indeed, these systems are particularly interesting as the -conjugation of the thiophene molecules provides nonlinear optical properties and intrinsic electrical conductivity [16], [17], [18], [19], [20].
In our first work [21], we have studied the optimized structure and Raman spectra of a single oligothiophene (nT) confined between two sheets of graphene. The geometry optimization was performed using Lennard-Jones potential. We found that the optimum graphene-nT distance is close to 0.349 nm. The interlayer graphene seperation shift from 0.34 nm in unfilled graphene bilayer to 0.688 nm in single 2T and 4T molecules confined between the two graphene layers. For the obtained configurations, the nonresonant Raman spectra have been calculated using the bond-polarizability model combined with the spectral moments method. In this work, in order to complete our study about the effect of the confinement of 2T and 4T molecules inside graphene bilayer, we investigate the effect of polymerization of 2T and 4T molecules on the Raman spectra bithiophene (resp. quater-thiophene) confined between two sheets of graphene. After a brief description of the computational models, the Raman spectra of 2T and 4T conjugated polymer chains either free or confined between two graphene sheets are reported and discussed.
Section snippets
Structure and dynamics of 2T and 4T confined graphene bilayer
The structure of 2T and 4T oligomers between two graphene layers are derived from minimization energy calculations performed as follow. We encapsulate a number of oligomer molecules between two graphene layers, where the bottom layer is fixed. The top layer which is totally aligned with the bottom layer (Fig. 1-B) and the inserted molecules positions can vary during our structural relaxation procedure. We found that a favored parallel configuration of 2T and 4T oligomers inside the graphene
Results and discussions
The calculations of the Raman spectra are performed on infinite crystal of 2T@G and 4T@G. In order to reach a 100% factor filling, 24[12] 2T[4T] molecules are located between two sheet of graphene, the number of carbon atoms is close of 624 [672]atoms. That corresponds to a concentration of 39%[35%] (concentration is the ratio between atoms in 2T[4T] molecules and carbons in the graphene sheets).
Conclusion
In this article, we have investigated The confinement of 2T and 4T oligomer chains in graphene, which gives rise to nT@G layers, using a hybrid approach based on density of functional theory and molecular dynamics. The optimized configurations of 2T@G and 4T@G are derived using a Lenard-Jones potential. We found that the molecules adopt a linear chains parallel to graphene layers. Then, for the obtained geometries, the nonresonant Raman spectra have been calculated as a function of the degree
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgment
The work was supported by Moulay Ismail University Research Support (13-16).
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