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Synthesis And Field Electron Emission Properties Of Graphene And Graphene-based Copper/Carbon Core-shell Structure

Posted on:2014-01-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:S M WangFull Text:PDF
GTID:1221330395996536Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Graphene is a two-dimensional honeycomb lattice material bonded with a singlelayer of sp2-hybridized carbon atoms, exhibiting excellent physical properties,including extremely high electronic mobility, thermodynamic stability, great elasticity,etc. Since graphene was obtained by Geim and Novoselov using a mechanicalexfoliation method in2004, great interest has been attracted to synthesize and exploreits potential applications in the fields of field-effect transistors, vapor sensors,batteries, supercapacitors and biosensors. In recent years, although great process hasbeen made on graphene by theoretical and experimental studies, further investigationsneed to be done on both preparation and application of graphene, such as how toreduce the cost of the preparation of graphene, what is the growth mechanism ofgraphene, how to modulate the morphology and structure of graphene, whether theproperties of graphene-based composite materials could be developed or improved?Based on these issues, the main research contents and the important results of thisdissertation are summarized as follows:In Chapter2, one to five layer graphene have been successfully prepared on apolycrystalline Co film at a relatively low temperature (800℃) and a slow total gasflow rate (78sccm) for40s by RF-PECVD technique, which has efficiently reducedthe preparation cost of graphene. Furthermore, the graphene exhibits a high opticaltransmittance of more than70%in the wavelength range500–1200nm and has asheet resistivity of2.661kΩ/sq. thickness of Co film, substrate temperature and deposition time on the growth ofgraphene. When the thickness of Co film is increased from300nm to600nm, the Cofilm becomes more continuous at800℃, which is favor to form graphene with highquality. When the substrate temperature is changed from800℃to700℃, the activityof polycrystalline Co surface decreases and the diffusion rate of carbon atoms on Cosurface becomes slower, which will cause a decline of growth rate of graphene. Thus,the quality of graphene decreases greatly. When the deposition time is increased, thequality of graphene changes greatly. Finally, we conclude that the graphenesynthesized by40s are most uniform and has the largest in-plane crystallite sizeaccording to the I2D/IGand FWHM2Dvalue. Based on the first-principles densityfunctional theorical (DFT) results, the barriers for carbon atoms penetrating from thesubsurface octahedral sites of Co (111) to the surface is extremely higher than thatdiffusing on the surface, suggesting that the graphene is more easily formed by asurface direct growth process.In Chapter4, a novel hedgehog-like core/shell structure, consisting of a highdensity of vertically aligned graphene sheets and a thin graphene shell/a copper core(VGs-GS/CC), has been synthesized using RF-PECVD. A Cu film with a thickness ofabout120nm deposited on Si (100) wafer by direct current magnetron sputtering wasused as Cu source. The growth mechanism of VGs-GS/CC is discussed. It is knownthat Cu has a relative low carbon solubility (0.04at%at1085℃) and the carbondissolved in the Cu particles will become supersaturated under a relative longdeposition time. Then, graphene sheets begin to nucleate and grow up on Cu surface.At an initial state, nucleation sites are enlarged along two-dimensional direction onCu surface. When two growing graphene sheets meet each other, graphene sheets areforced to rise upwards, causing the formation of vertically aligned graphene sheets(VGs). In the process of cooling, carbon atoms segregate from the interior of Cuparticles to form graphene layers at the interlayer between Cu and VGs. Consequently,VGs-GS/CC material is obtained. In addition, we find that VGs-GS/CC materialexhibits an improved field emission property, compared to VGs grown on Si substrate. This can be attributed to the geometrical factors such as a high density of emitters(more sharp edges) on Cu particles and the randomly distribution of VGs-GS/CC onSi substrate. Hence,‘field shielding’ effect among VGs is weakened and VGs-GS/CCmaterial exhibits an improved FE property.In Chapter5, Cu films with a thickness of6,12,36,60and120nm are depositedon Si (100) wafers by using direct current magnetron sputtering. They are used as Cusource to synthesize VGs-GS/CC6, VGs-GS/CC12, VGs-GS/CC36, VGs-GS/CC60andVGs-GS/CC120via RF-PECVD. Field emission measurements show that theVGs-GS/CC6hybrid material has the lowest turn-on field (2.4V/μm) at a currentdensity of1μA/cm2and exhibits the largest field enhancement factor (6450), whichis about4.9times higher than that of VGs on Si. The enhanced field emissionproperties of VGs-GS/CC are attributed to not only the high conduction of Cu, butalso the geometrical factors of Cu particles such as particle size and inter-particledistance:(1) In the high-field region where E>5.6V/μm, the current density is directlyproportional to the percentage of Cu particles within a size range of75–125nm forthe VGs-GS/CC samples. VGs on Cu particles within a small diameter of75–125nm like convex films, causing the area density of VGs on Cu particles decreased.Under a less screening from neighboring VGs, the emitting efficiency becomesgreatly enhanced.(2) In the low-field region where2.1<E <5.6V/μm, the Cu particle size (d) andinter-particle distance (l) will significantly affect the field emission property of VGson Cu particles. When l is larger than d, the local field strength is increased due to thehigh density of equipotential line. Thus, the field emission property is enhanced. Witha too short inter-particle distance, an electrostatic screening effect leads to an apparentdecrease in the local field strength, which will cause a damage of FE property.In conclusion, high-quality, few-layer graphene has been successfully preparedvia RF-PECVD on a polycrystalline Co film. The thickness of Co film, the substratetemperature and the deposition time play important roles in the graphene growth.Based on the first-principles density functional theorical (DFT) results, we find that graphene is more easily formed by a surface direct growth process. Theseinvestigations will give a helpful guidance in synthesis and practical application ofgraphene. In addition, we have successfully synthesized a novel hedgehog-likecore/shell structure (VGs-GS/CC) using RF-PECVD. And field emissionmeasurement shows that the VGs-GS/CC exhibits better field emission performancethan VGs/Si, which suggests that the VGs-GS/CC materials are good candidates forfield emitters.
Keywords/Search Tags:RF-PECVD, polycrystalline Co film, graphene, Cu, core-shell structure, field emission
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