Controllable Synthesis Of Graphene With High Quality

Graphene is a single layer of sp2-hybridizd carbon atom arranged in2-dimensional honeycomb crystal lattice which is a newly discovered allotrope of carbon after nanotubes and fullerenes, and have filled the blank of2D carbon crystals. After its first experimental discovery in2004, graphene has attracted enormous research interests. Graphene’s conical valence and conduction bands give rise to charge carriers that have photon-like linear energy dispersion near the Dirac points. Electrons in graphene are like massless Dirac particles, making graphene exhibit fantastic physical properties. For instance, graphene’s conductivity never falls below a minimum value corresponding to the quantum unit of conductance, even when the concentrations of charge carriers tend to zero; The filling factor of graphene’s integer Quantum Hall Effect (QHE) appears in the half integer position, and QHE could also be observed at room temperature; Also, scientists have found the Factional QHE in graphene. Graphene has shown wide potential application prospects, such as high-speed transistors, transparent electrodes, detectors and sensitive sensors. In2010, Andre Geim and Knostantin Novoselov were awarded Nobel Prize owing to the original contribution to the field of graphene.High-quality graphene was first obtained by mechanical exfoliation from graphite, this technique however limited by the small size, low yield and can’t be repeated. Recently, lots of methods have been developed for fabrication large scale graphene. Among them, chemical vapor deposition (CVD) from gaseous carbon source, mainly methane, on cooper substrate has already shown great promises and attracted particular attention, since it can grow wager-scale graphene films with uniform single-layer thickness, which is attributed to the low solubility of carbon in Cu crystals. However, many details in the CVD synthesis graphene remain unclear and require thorough examination for ultimate control of the quality. One of the mysterious components is the important role of gas-phase dynamics in CVD growth graphene. On the other hand, the current CVD is limited to the use of gaseous carbon sources, which usually require high growth temperature, typically1000℃Low-temperature growth technique is highly desirable, since it is more convenient, economical, environment-friendly, and feasible for industrial application. In this thesis. I mainly investigate the controllable synthesis of graphene with high quality, mainly include two main aspects:(1) The important role of gas-phase dynamics in the CVD growth of graphene;(2) Low temperature growth of high quality graphene by CVD using solid and liquid carbon sources. This thesis also includes my work on the construction of periodic three-dimensional dimensional metallic nanostructures and their novel plasmonic properties. This dissertation is composed of6chapters:In the first chapter, I firstly give a brief introduction to graphene, including its structure, electronic structure, synthesis methods, characterization methods, as well as its potential applications. This is followed by a brief overview to the chemical vapor deposition for graphene synthesis and its current progresses. The chapter is ended with some statement of the content and significance of this dissertation.In the second chapter:By delicate design and control of the CVD conditions, here we demonstrate that a nonequilibrium steady state can be achieved in the gas phase along the CVD tube, i.e. the active species from methane cracking increase in quantity, which results in thickness increase continually for graphene grown independently at different positions downstream. In contrast, uniform monolayer graphene is achieved everywhere if Cu foils are distributed simultaneously with equal distance in the tube, which is attributed to the tremendous density shrink of the active species in the gas phase due to the sink effect of the Cu substrates. Our results suggest that the gas-phase reactions and dynamics are critical for the CVD growth of graphene, and further demonstrate that the graphene thickness from the CVD growth can be fine tuned by controlling the gas-phase dynamics. Similar strategy is expected to be feasible to control the growth of other nanostructures from gas phases as well.In the third chapter, we demonstrate a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid hydrocarbon feedstocks that can be easily taken from industry and much cheaper. For solid PMMA and polystyrene precursors, high quality monolayer graphene films are obtained at the growth temperature above800℃. Centimeter-scale monolayer graphene films are also synthesized even at a growth temperature down to400℃, at the expense of a little downgrade in film quality. For comparison, we attempted to grow graphene also from gaseous source (methane) at various temperatures, respectively. The Raman spectrum and SEM image clearly show that monolayer graphene with high quality is synthesized at1000℃. However for methane derived graphene grown at800℃, the quality of graphene from methane source is much degraded than that of the graphene from solid carbon source at800℃. When the growth temperature is lowered to600℃, no graphene signals can be picked up in the Raman spectrum and SEM image.600℃is thus too low for graphene formation at Cu surfaces from methane. Therefore, the CVD growth route using solid carbon sources is superior than using gaseous ones at low temperatures. This advantage renders it a simpler and more convenient choice for industrial application.In the fourth chapter:We use the liquid benzene as the hydrocarbon source, high quality and large graphene flakes are obtained at the growth temperature500℃. It is amazing to see that uniform monolayer graphene flakes with excellent quality can still be achieved at a growth temperature as low as300℃, although the flake size and density is a little smaller than that grown at500℃. The flakes usually have two well-defined shapes:slender strip and hexagon. The mechanism for this kind of shape formation is unknown and may be the result of the crystallization of Cu substrate. The successful low-temperature growth can be qualitatively understood from the first principles calculations. Our work might pave a way to undemanding route for economical and convenient graphene growth.In the fifth chapter:When a periodic potential applied by suitably patterned contacts on graphene surface, the propagation of charge carries through such a graphene superlattice is highly anisotropic, and in extreme cases results in group velocities that are reduced to zero in one direction but are unchanged in another. To better tune the charge carrier behavior, here we use the nanosphere lithography to fabricate various periodic substrates and further reveal the strong dependence of the optical response on their in-plane symmetry. For the concentrically stacked ring-cap array, its optical absorption behavior is similar to that of a ring array with the same dimension, since they have the same in-plane symmetry. However, for the non-concentrically stacked hole-cap array, the in-plane symmetry breaking results in crescent-shaped nanogaps appearing at the interfaces. And thus leads to a novel strong plasmon resonance mode. The finite-difference time-domain simulation shows that the charge mainly assembles to the sharp edges of the nanogaps at the resonant wavelength and remarkable electric field enhancement is achieved around the sharp edges. Furthermore, the strongest resonance modes of the ring-cap array and hole-cap array show large redshift as the nanostructure size increases. The presented periodic substrate may offer an idea platform for tuning the charge carries behavior of graphene.The sixth chapter is a brief summary of my whole work.