The Study Of Optical And Optoelectronic Properties Of Graphene And Graphene-based Hybrid Structures

Graphene is a two dimensional(2D)crystal consisting of a single monolayer of carbon atoms arranged in a honeycomb lattice with sp2-hybridization.It is semimetal with zero bandgap at room temperature and has ultrahigh carrier mobility(2.5×105 cm2/V s),ultrabroad absorption spectrum(from far-infrared to the entire ultraviolet)and gate-tunable carrier density,which has been considered to be the most promising candidate material for electronic and optoelectronic applications.However,there also exist obvious disadvantages of graphene for photodetection:weak optical absorption of the atomically thin graphene layer(?2.3%for one layer),and the absence of a gain mechanism,resulting in its low photodetection efficiency(photoresponsivity of intrinsic graphene is only?6 mA/W);and the ultrashort lifetime(of only picoseconds timescale)of photoexcited carriers due to its zero bandgap structure,resulting in its low collection efficiency and low photoresponsivity.Light absorbing materials such as semiconductor nanostructures,organic dye and 2D layered crystal have size tunable optical properties and ultrahigh optical absorption and emission quantum efficiency,as well as carrier multiplication.By combining graphene and these light absorbing materials to form hybrid structures,which can utilize both the superior electrical properties of graphene and the exceptional optical properties and photoconductive gain mechanism of light absorbing materials,hence could greatly enhance the optoelectronic properties.The detecting working principle based on the hybrid structures named photogating effect,that is,electron-hole pair generation takes place in the light absorbing materis under illumination condition.Subsequently,one type of photo generated charges can transfer to graphene,another type of carriers remain trapped in the light absorbing materis,where they affect the conductance of graphene layer though capacitive coupling,while the charges in the channel are recirculated during the lifetime of these trapped carriers between source and drain,giving rise to ultrahigh photodetetion gain.Thus,it can be seen clearly that the charge transfer plays significant role in enhancement of photoresponsivity of hybrid structures.Consider this,study how to effectively control the charge transfer and interaction between graphene and light absorbing materials,which will be of great help on the future development of high efficiency optoelectronic devices based on graphene and its hybrid structures.In the meanwhile,the bottleneck of photogating is also due to the slow charge transfer and/or charge trapping process,the price for achieving ultrahigh sensitivity is to greatly sacrifice response time.Hence,to fully study the factors that affect the charge transfer and/or charge trapping process,for example the oxygenous groups and defects,or to find a charge-transfer-free gain mechanism in hybrid system,which will be benifical to bridge the gap between ultrafast response and ultrahigh sensitivity in a graphene-based photodetector.To this end,we mainly carried out the following research work:systematical study of fluorescence quenching effect due to the charge/energy transfer between graphene and light absorbing materials(semiconductor quantum dot and organic dye),by using different types of graphene and graphene with different types and amounts of defects,we effectively modulated the charge/ennegy transfer between these two materials;by gradually controlling the amount of oxygenous groups and defects in graphene oxide(GO)though thermal reduction under vacuum,we greatly enhanced the optical and optoelectronic properties,revealing clearly that the oxygenous groups and defects play a significant role in optical properties of GO and related devices performances;we firstly proposed a new concept of the interfacial gating effect from a lightly doped silicon/silicon dioxide interface,we successfully bridge the gap between ultrafast response and ultrasensitivity in a graphene-based photodetector.This dissertation contains six chapters and the contents are outlined as following.In chapter one,we first briefly reviewed the history,properties and synthesis strategies of graphene.Then we introduced the principle of photodetector,as well as the research status and trends with the graphene.We also presented the background and motivation of our study.In chapter two,we studied the fluorescence quenching of CdSe quantum dots(QDs)on graphene and its multilayers,as well as GO and reduced graphene oxide(rGO).Raman intensity of QDs was used as a quantitatively measurement of its concentration in order to achieve a reliable quenching factor(QF).It was found that the QF of graphene(-13.1)and its multilayers is much larger than rGO(?4.4),while GO(?1.5)has the lowest quenching efficiency,which suggests that the graphitic structure is an important factor for quenching the fluorescence of QDs.It was also revealed that the QF of graphene is not strongly dependent on its thicknesses.In chapter three,we presented on the manipulation of the fluorescence quenching of Rhodamine 6G on graphene by defect engineering via hydrogen and Ar+ plasma treatments.The amount and nature of defects in graphene were estimated on the basis of the Raman intensity ratios ID/IG and ID/ID' of graphene.Results showed that the QF gradually decreases from?40 to?4 and?12 for hydrogenated graphene(sp3 defects)and Ar+-plasma-treated graphene(vacancy-like defects),respectively,with different amounts of defects.Our results indicated that the fluorescence quenching efficiency of graphene is strongly dependent on the amount and nature of defects.In chapter four,we systematically studied the influences of oxygenous goups and defects on the optical and optoelectronic properties of GO sheet though thermal reduction.By raising the annealing temperature,GO is expected to undergo a partial restoration of sp2-hybridized carbon due to the progressive removal of oxygen containing functional groups.The visible absorption strongly increases while the main absorbance peak red-shifts from?230 nm to?270 nm.More importantly,it is found that the optical contrast values of thermally reduced GO sheet gradually approach to those of intrinsic graphene,which allows the use of recovered optical contrast to identify the numbers of layers of GO.In addition,we also demonstrated that by controlling the amount of oxygenous defects,it is possible to effectively tune the electrical transport and photoresponse characteristics of reduced GO devices.In chapter five,we designed a monlayer graphene photodtector based on a lightly p-doped silicon/SiO2 substrate.By taking advantage of interfacial gating effect from lightly doped silicon/SiO2 interface,we demonstrated a simple approach to graphene photodetection with high performance,fast response and a broad spectral response that extends from visible to near-infrared.The proposed graphene photodetector exhibits high responsivity of?1000 A/W for weak signal of<1 nW,the photoresponse time of?400 ns,showing a great potential in ultra-fast weak signal detection.More importantly,in comparison with the previous graphene devices based heterostructures and/or hybrid structures,our device possesses the advantages of a more simple fabrication technique and is fully compatible with the silicon technology.This work therefore not only opens up a route to further development the design of simple graphene-based high performance optoelectronic devices,but also a potential to access an even wider spectral range than that achieved in our current devices by combing graphene with other oxide-semiconductor system.In chapter six,we concluded the main results of this paper as well as prospected some future research work.