Studies Of Graphene On Preparation, Characterisation, Optoelectronic Properties And Applications

Graphene is a single atomic layer of sp2bonded carbon atoms arranged in a twodimensional hexagonal lattice. Charge carriers in graphene move at an ultrafast speed, similarto relativistic, massless Dirac particles. Its mobility reaches to~1/300light speed, much higherthan that of ordinary electron in conductor. In theory, the carrier mobility of graphene is morethan2×105cm2/V·s at room temperature, making graphene possesses outstanding conductivity.Graphene has high transparency, the monolayer graphene only absorb2.3%light in visible toinfrared region. Therefore, graphene is a kind of conductive and transparent material, which canbe as the transparent electrode for many devices, such as touch-screen panel, liquid crystaldisplays and solar cells. Besides unique electronic structure，graphene shows typical saturableabsorption behavior. Unlike the tranditional semiconductors, the absorption characteristics ofgraphene are little wavelength dependence due to its zero-bandgap structure. Thus graphene canmodulate many lasers working in different wavelength. Recently, the extraordinary propertiesof graphene have attracted widely investigations. Based on these studies, we did a series ofwork around the improvement of conductivity and transparency and application of graphene.In this study, we fabricated lage-area graphene films on copper foils using chemical vapordeposition (CVD) method and synthesized high-quality graphene on quartz substrates in theabsent of metal catalyst. Based on these grown graphene, we fabricated loudspeakers, biosensor,and graphene saturable absorber, etc. The main works was as follows:(1) We demonstrate that the continuous and uniform graphene films can be directlysynthesized on the substrates using a two-temperature zone assembled chemical vapordeposition system and their layers can be controlled by adjusting the precursor partial pressure.Raman spectroscopy and transmission electron microscopy confirm the formation of themonolayer graphene with a grain size of~100nm. Hall measurement shows a roomtemperature carrier mobility of above1500cm2/V·s. The optical transmittance and conductanceof the graphene films are comparable to those of transferred metal-catalyzed graphene. Themethod avoids the complicated and skilled post growth transfer process, providing a novel wayfor the preparation of graphene.(2) Based on the direct-grown graphene, we fabricated a biosensor in the configuration of field-effect transistor (FET) for the detection of energy molecules (ATP). This graphene-basedbiosensor achieves a high time resolution of a few milliseconds and a high sensitivity to ATPmolecules in a very wide range from0.002to5mM. It is found that current change is roughlylinear increase with logarithm of ATP concentration in a very wide range from0.002to5mM.The sensitivity reached up to50%. This technique provides a novel platform to detect ATPmolecules and could potentially be employed in studying some essential cellular functions inliving cells.(3) We transferred graphene on the two sides of polyvinylidene fluoride (PVDF)piezoelectric film. Based on converse piezoelectric effect, we fabricated a kind of flexible andtransparent loudspeakers. This thin film loudspeaker can generate sound with wide frequencyrange, high sound pressure level (SPL) and low total harmonic distortion (THD), and has muchless power consumption. Compared to the commercial thin film speaker, graphene-basedloudspeakers have many advantages, such as magnet-free, transparent and flexible, which isexpected to be used as an extremely thin, lightweight wearable electronic devices.(4) We report a hybrid structure employing two-dimensional graphene and networks ofone-dimensional silver nanowires as transparent and flexible electrodes. The hybrid films havesheet resistances as low as~16/□with high transmittance of91.1%at550nm, and exhibitimpressive stability against oxidation and mechanical flexibility. These properties are superior tocommercial transparent electrodes such as indium tin oxides and comparable to the best reportedresults in transparent electrodes. Our method could readily provide a route towards low-costapproaches for realizing high performance transparent electrodes.(5) We trasfered graphene on c-plane quartz and maked graphene saturable absorber. Thehigh-quality monolayer graphene film with large area was grown on Cu foils by chemical vapordeposition (CVD) technique. The graphene saturable absorbers (SAs) with the well-defined layernumber were fabricated by controlling of transfer process precisely. Using the graphene SAs, thestable Q-switched mode-locking pulse trains were realized in a diode-pumped Nd: YVO4laser.At the pump power of6.0W, the average output power reached a high value of1.6W. This worksuggests that the low-cost, broadband graphene SA could potentially be employed in thepractical high-power mode-locking laser system.