Laser Micronanoprocessing Of Graphene Oxide And Its Devices Fabrication

Graphene, a single-atom-thick two-dimensional carbon crystal, has be consideredas a rising star on the horizon of material science, modern physics and relateddisciplines due to its unique and excellent electrical, optical, thermal, magnetic andmechanical properties. However, some problems with respect to their applications inthe field of electronics, such as mass production, band gap opening, device fabricationand integration are still challenging. Up to now, there have been four typical methodsthat could be used to prepare graphene, including mechanical exfoliation, chemicalvapour deposition(CVD) on metal surface, epitaxial growth on SiC substrate and thereduction of graphene oxide. In comparison, chemical oxidation of graphite (thepreparation of graphene oxide, GO), shows some unique advantages over otherapproaches, such as large-scale preparation, solution processing compatibility andtractable modification. However, the presence of abundant oxygen groups makes GOa isolator, and significantly restricts its applications, and therefore, the demands onthe methods used for GO reduction become critical. In the beginning, methods for GOreduction mainly include chemical and thermal treatments, the former approachmakes use of toxic reducing reagent (e.g., hydrazine), whereas the later methodresorts to high temperature annealing in inert gases (e.g.,>1000°C). However, thesereduction processes suffer from poor compatibility with the device fabrication. Tosolve these problems, in this paper, femtosecond laser direct writing(FsLDW) andnanosecond laser interference have been adopted for the reduction and patterning ofGO towards the development of graphene-based micro-devices. With the help of laserprocessing, exquisite control of GO films over the oxygen content, conductivity,bandgap, N-doping concentration, micropatterns and even the formation ofhierarchical could be achieved easily. In this regard, new methods that could be used for the fabrication and integration of graphene-based devices have been successfullydeveloped. The main research contents are listed as follows:1. Graphene microcircuits have been successfully created on graphene oxide filmsby femtosecond laser direct writing (FsLDW) induced reduction of GO according tothe preprogrammed patterns. Any desired patterns could be directly created bycontrolling the trajectory of the laser focus; the resistivity of these microcircuits canbe precisely controlled by tuning the laser power. This micro-nanofabricationtechnology laid the foundation for the preparation and integration of graphene-basedmicro-electronic devices.2. We have carefully studied the influence of femtosecond laser power on thereduction degree of GO. Since the bandgap of GO depends on the residual oxygencontents, in our experiments, by tuning the reduction degree of GO through thecareful control of laser power, band gap of the the reduced GO (RGO) could bemodulated in a certain range. Through the first-principle study, the origin of GO bandgap tailoring is explained. Theoretically, the variation of bandgap from0to100%oxygen coverage in GO is in the range of0to2.74eV. Experimentally, we measuredthe diffuse reflectance spectra (DRS) of RGO reduced by different laser powers，which confirmed that the band gap of GO has been modulated in the range of2.4to0.9eV by tuning the femtosecond laser power from0to23mW. Combined withFsLDW technology, a RGO channel could be post-fabricated between two pre-coatedelectrodes for the fabrication of bottom-gate graphene FETs, and p type FETsbehavior with a certain on-off ratio are obtained.3. We have demonstrated the N doping and simultaneous reduction of GO byFsLDW in NH3atmosphere. We investigated the effect of laser power on theN-doping efficiency and N-bonding types by XPS. Experimental results showed that atotal nitrogen concentration as high as10.3%in the form of pyridinic, pyrrolic andgraphitic has been achieved. The ratio of different N-bonding types changes withdifferent laser power. We confirmed that doping can change the conductive type ofchannels from p to n, and finally, n type FETs behavior has been observed based onour N-doped RGO.. 4. We have fabricated humidity sensing device on flexible substrate bytwo-beam-laser interference (TBLI) reduction and patterning of GO. HierarchicalRGO nanostructures were formed after laser interference treatment of GO, whichholds great promise for gaseous molecular adsorption, and thereby significantlyincreases their sensing performance. By tuning the laser power, the content of oxygenfunctional groups could be changed within a certain range. The modulation ofoxygen-group content gives the feasibility for controlling response/recovery time. Toget further insight into the mechanism of the tunable response/recovery property ofour humidity sensors, first principle study has been carried out to give an essentialexplanation. This method is a surfactant-free, mask-free and facile approach to theproduction of large-area hierarchical micro-nanostructures on RGO films, and thusshows great potential for fabrication of future graphene-based microdevices.In summary, we have successfully developed a series of laser-related methods forthe reduction, patterning and nanostructuring of GO towards the fabrication andintegration of graphene-based devices. FS laser has been used to fabricate graphenemicrocircuits by direct reduction and patterning of GO films. Band gaps of reducedGO could be precisely modulated by controlling the laser power. N-doping can beachieved by FsLDW of GO in ammonia atmosphere, caused the transition from thep-type to n-type of RGO. Hierarchical nanostructures were formed after nanosecondlaser interference treatment of GO. We fabricated graphene-based devices using theabove-mentioned methods, they hold great promise for the wide application of GO inmicro-electronic devices.