The adsorption of a C60 molecule on the graphene revealed the contribution of a conductor-semiconductor transition, based on a theoretical calculation. A stress or a strain was predicted as a possible reason to tune the transition backwards.
Graphene has attracted more attention since Andre Geim and Konstantin Novoselov were rewarded the Nobel Prize in 2010 [
Fullerene is another important carbon-based material as it has been synthesized and studied for decades [
When a fullerene was close to or even bonded with a graphene, a button-like structure would be obtained. Based on this configuration, the density of states (DOS) has been studied in this work. Our results showed it a semiconductor. That is, the effect of the adsorption of the C60 molecule on the graphene was similar to the effect of other atoms such as hydrogen. The sp2 hybridization was destroyed locally so that the sp2-sp3 hybridization led to a band gap in the system. Moreover, when a stress or a strain was applied along the fullerene-graphene direction, the band structure would be modified accordingly. A conductor would be obtained when the strain was equal or larger than 20%. Such a finding suggested a potential nanobutton or a force/pressure sensor based on the fullerene-graphene bonded structure (FGBS), and the ON/OFF state could be controlled by a stress or a strain externally.
First principle calculation was employed in this work, and the configurations were all relaxed before the calculation of physical properties. Generalized gradient approximation parameterized by Perdew Burk Ernzerhof [
This scheme of calculation had been tested on graphene and graphane. The former was shown as a semimetal and the latter was shown as a semiconductor. C–C bond length in both of them was 1.46
Fullerene C60 was arranged near a graphene to a distance of 1.40
Atomic structure of C60-graphene bonded structure, as the bond length was indicated near the bonds. The graphene sheet was shown as an inset.
The electronic properties such as the density of states (DOS) of the bonded structure were calculated and compared with those of the graphene in Figure
DOS of a C60-graphene bonded structure (red line) and the DOS of a graphene (black line), with the Gaussian broadening of 0.20 eV. The differentiation of the DOS of the C60-graphene bonded structure was shown as an inset and the gap width was indicated as well. The Fermi energy level was used as the energy reference.
Next we studied the modification of the electronic properties of the FGBS. When the button-like structure was pressed along the fullerene-graphene direction, the DOS was calculated accordingly. As indicated in Figure
DOS of C60-graphene bonded structure with the strains applied along the fullerene-graphene direction. The value of the strain was labeled and the different colors showed the DOS under different strains, namely, black for no strain, red for the strain of 7%, green for the strain of 13%, blue for the strain of 20%, light blue for the strain of 27%, and purple for the strain of 33%. The local density of states (LDOS) according to the strains of 7%, 13%, and 20% were shown as an inset to show the transition between the semiconductor and the conductor.
It was worth noting that the electronic property of the material was modified through the applied stress or strain effectively. The electronic DOS was tightly related to the external force field. When the adsorbates were bonded with the graphene, the metallic-semiconductor transition had been revealed in some systems. But due to the fact that adsorption was usually not reversible, the metallic-semiconductor transition was not reversible either. Thus, the real application of the saturated graphene was limited. The C60-graphene bonded structure was different, as the bonded structure showed metallic or semiconductor character in the same system. The atomic structure need not be destroyed, and the metallic-semiconductor transition only relied on the stress or the strain applied externally. It means that the FGBS was desired to be used as a button in nanometer scale. If the metallic state is labeled as ON while the semiconductor state is labeled as OFF, the ON/OFF switch could be controlled through changing the stress or strain directly.
Such a finding also suggested the FGBS, a force/pressure sensor in nanoscale. When the force or pressure applied on it was large enough, there should be some electric signal output based on a necessary external circuit. As the synthesis of fullerene and the graphene was mature and the bonding between them could be realized in experiments [
In the further study of the C60-graphene bonded structure, it was found that the bonds were the main reasons of the piezoelectric effect. When the stress was larger than 0.02 eV/
As follows the local density of states (LDOS) was calculated and shown as an inset in Figure
It was noted that the strain making the semiconductor-metallic transition was large so that the sensitivity was not satisfying. But it was believed that the molecule adsorption was a new method to decorate the graphene, and some other adsorbates or adsorption details should improve the sensitivity definitely.
In this work, the C60-graphene bonded structure was studied with numerical calculation. The chemical bonds connecting them were found close to those in diamond, so that the energy gap was shown. But when the strain along the C60-graphene direction was applied, a semiconductor-conductor transition could be realized. Our results evidenced that the molecule adsorption on graphene could be used as a nanobutton or force/pressure sensor, due to the interesting piezoelectric phenomenon.
The author declares that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Natural Science Foundation of China under Grants nos. 11204030, 11174048, 11174049, and 11175045.