Design And Application Studies Of Graphene And Nano-Devices

Since their discovery, graphene have attracted great scientific attention, owning to their unique structural, mechanical and electronic properties. Graphene are expected to have potential applications in many fields, such as nanocomposite materials, reinforced structures, nanoelectronic devices and field emission displays etc. This thesis focuses on understanding the interaction mechanism between small molecules and graphene, and to design novel graphene nanodevices based on these understandings. It was found that introduction of dopant and defect into the graphene surface could significantly modify the electronic properties of graphene. By using different organic donor and acceptor as absorbants, the band structure of graphene could be effectively tuned. Introducing metal atoms could strengthen the molecule/graphene interaction and tune the magnetic properties of the graphene. Nonconvalent modification of carbon nanotube sidewall using different polycyclic aromatic molecules helps to design new gas sensors.The main results of this thesis are listed blow.1. The adsorptions of four gas molecules on different graphenes were systemically investigated, which reveals that introduction of defects and dopants into graphene could improve the sensing properties of graphene. The four probe molecules show physisorption on the pristine graphene with low adsorption energies and little charge transfer, which suggests that the un-modified graphene is not an ideal material for gas sensing. After introducing dopant and defect into the graphene, the interaction between graphene and small molecules could be strengthened. The electron transport of graphene-based junctions illustrates that the sensitivity of B doped graphene to NO2 molecule could be two orders of magnitude higher than that of the prinstine graphene.2. Using density functional theory and nonequilibrium Green's function (NEGF) formalism, we have theoretically investigated the binding of organic donor, acceptor and metal atoms on graphene sheets, and revealed the effects of the different noncovalent functionalizations on the electronic structure and transport properties of graphene. The adsorptions of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tetrathiafulvalene (TTF) induce hybridization between the molecular levels and the graphene valence bands, and transform the zero-gap semiconducting graphene into metallic one. However, the current versus voltage (I-V) simulation indicates that the noncovalent modifications by organic molecules are not sufficient to significantly alter the transport property of the graphene for sensing applications. We found that the molecule/graphene interaction could be dramatically enhanced by introducing metal atoms to construct molecule/metal/graphene sandwich structures. The results of this work could help to design novel graphene-based sensing or switching devices.3. The magnetic and electron transport properties of graphene could be tuned by adsorbing dichlorobenzene (DCB). Pristine graphene shows weak interaction with DCB molecule, hence chemically or physically modified graphene sheets are required for achieving more effective adsorption. Metal atom doped graphene structures are shown to be promising to enhance the DCB/graphene interaction. The Fe/graphene shows stronger interaction with DCB molecule and higher sensitivity to DCB compared with the pristine graphene. The magnetic properties of graphene could be affected by the adsorbed organic molecule and different metal atoms. This finding shall widen the potential applications of graphene in the fields of magnetic sensors or switching devices. It is also hoped that the finding of this work will stimulate the organometallic chemists to develop new chemistry to fabricate the proposed graphene based nanostructures.4. The adsorptions of CO molecule on the pristine and defective graphene like BN sheets (g-BN) were investigated, which revealed that introducing defects and dopants into g-BN are effective methods for improving the chemical activity of g-BN. CO shows weak interaction with pristine g-BN, but shows much stronger interactions with vacancy g-BN (V-g-BN). The significantly increased binding energies and charge transfers of CO on the modified g-BN could induce significant changes in the electrical conductivity of g-BN, and therefore are promising for electronic applications. The V-g-BN shows spontaneous magnetization and reduced band gap upon CO adsorption, which may be used in novel magnetic devices. It is expected that the findings of this work will help the current mission of finding appropriate chemical methods for modifying the originally inert g-BN sheet surface to enable their applications in electronic and magnetic devices.