Preparation And Optical/electrical Properties Of Graphene/Si Nanoporous Pillar Array

Single-layer graphene bonded with sp²-hybridized carbon atoms possesses two-dimensional honeycomb lattice structure,and thus exhibits excellent physical properties,such as extremely high specific surface area,light transmittance,electronic mobility,exciton binding energy and thermal stability,etc.These make it have great potential application in the field of transparent electrodes,field effect transistors,photovoltaic devices,gas sensors,and so on.However,it had no fluorescence because the band gap of graphene is zero,which greatly limits its application in optoelectronic device,so it is prerequisite to open the band gap of graphene for the application in the semiconductor device.The optical properties of carbon materials are determined by the symmetry.Therefore,the wavelength of the luminescence could be adjusted by introducing unordered structure that can be induced by changing the size of the sp²region.Si has been used in the super-large-scale integration in the past decades,and the silicon-based photonics have been become an important development direction of the next generation of information technology.However,as an indirect bandgap semiconductor,Si is an inefficient light emitter,so other direct-bandgap materials are usually hired to fabricate optical or photoelectric devices on the Si chip to shorten the length of the photoelectric interconnection,and thereby improve the information processing speed.The integration of graphene with Si not only forms graphene/Si devices,but also makes silicon dope into graphene to improve exciton binding energy of graphene and enhance the temperature of exciton emission.And the devices could also directly integrate onto the Si chip and shorten the distance of signal transmission path.Though lattice mismatch and thermal mismatch is existence in the integration of graphene with Si,growing graphene on the silicon nanostructure could solve this problem.We successfullcy prepared graphene on silicon nanoporous pillar array?Si-NPA?by chemical vapor deposition?CVD?method,realized heterogeneous contact with Si-NPA,and obtained a new type of graphene/Si-NPA nano-heterostructure device.The structure exhibits excitonic emitting characteristics and stable field emission properties at room temperature,though the photoconductivity is poor.The mean researches conducted are list as follows.?1?Prepartion of the graphene/Si-NPA nanoheterostructure through CVD and spin-coating methods.Graphene was grown on Si-NPA via CVD method,using a thin layer of pre-deposited Ni nanocrystallites as catalyst.The effects of precursors,growth time and solution pH on catalyst coverage and particle uniformity were investigated.The results showed that Ni nanocrystals prepared by Ni?CH₃COO?₂·4H₂O had suitable thickness and uniform particle size distribution.The optimized growth time of the Ni catalyst was 15 min and the pH was 8.Graphene was grown on the Si-NPA using CVD method.The growth times of graphene were 5 and 10 minutes,and got graphene with 3 and 8carbon atomic layers respectively.Furthermore,in order to compared with the photoelectric properties prepared by CVD,graphene with a size of 5⁷ nm was successfully prepared by Redox method,and spin-coated on Si-NPA to form graphene/Si-NPA nanoheterostructure.?2?Room-temperature excitonic emission with phonon replica from graphene deposited on Si-NPA.Based on the graphene/Si-NPA prepared above,XRD,SEM,TEM,HRTEM,Raman,etc.were used to characterize the morphology,structure and composition of graphene/Si-NPA.Graphene were determined to be of high quality and well-dispersed,with a diameter size of smaller than 17 nm.Light absorption measurements showed that graphene had an absorption band edge at 3.3 eV.They also showed regular and sharp excitonic emitting peaks in the ultraviolet and visible region?2.06–3.6 eV?at room temperature.Moreover,phonon replicas with long-term stability appeared with the excitonic peaks at room temperature.Temperature-dependent photoluminescence from the graphene revealed that the excitonic emission derived from free and bound excitonic recombination and defect state emission of graphene.A physical model based on band energy theory was constructed to analyze the carrier transport of the graphene.In addition to the transition between graphene bands,the doping of Si into graphene and Ni nanocrystallites on Si-NPA,which acted as a metal-enhanced fluorescence substrate,were supposed to play important roles in the room temperature exciton luminescence of graphene.Results of this study would be valuable in determining the luminescence mechanism of graphene.The quantum yield of the graphene/Si-NPA nanoheterostructure was tested to be3.09%and the average fluorescence lifetime was 1.08 ns.The lifetime analysis of the luminescence peaks at 387 and 473 nm showed that the 387 nm luminescence was from the free exciton transition,while the luminescence at 473 nm came from the emission of bound excitons and edge-defect state of graphene.For comparison,the excitation and photoluminescence spectra of graphene/Si-NPA prepared by spin-coating were tested.The results showed that there was no electron and energy transfer between the graphene and Si-NPA under optical excitation.Graphene and Si-NPA emitted their own intrinsic fluorescence respectively and graphene has no room temperature exciton emission phenomenon.In addition,as an optical window of Si-NPA,graphene increases the light absorption of Si-NPA,and thereby enhancing the luminescence intensity of Si-NPA slightly.?3?Visible photodetection realized by graphene/Si-NPA and graphite nanostructure?nano-graphite?/Si-NPA.The exciton motion process propagates momentum and energy,but does not propagate charge,and does not produce photoconductivity during motion.By this theory,the excitonic emission of graphene/SiNPA system was strong and should give birth to poor photocurrent.To verify this theory,Ag wires were used as electrodes to constructure 1.0 x 1.0 cm Si-NPA,Ni/Si-NPA and graphene/Si-NPA photoconductive detectors.The results show that the Ag electrodes were ohmic contact with Si-NPA,Ni/Si-NPA,and graphene/Si-NPA.Because the resistance of graphene/Si-NPA was lower than that of Si-NPA and Ni/Si-NPA,and the photoconductive response was slightly higher than the latters,though the photocurrent is weaker in the order of mA or even?A.Absorption of graphene/Si-NPA was low and was supposed to be due to the fewer layers of graphene and the strong exciton transitions caused by the weak electrostatic shielding of fewer-layer graphene.Therefore,in order to get better optical absorption and lower exciton emission,we obtained thick graphite nanostructures by increasing the deposition time of carbon in the subsequent experiments.Nano-graphite shown as nanoparticles and nanowires,which composed of graphite nanocrystallites?nc-graphite?was grown on Si-NPA by CVD through prolonging growth time to 15 min.The spectral measurements show that nano-graphite/Si-NPA possesses strong light absorption in the visible region of 400-800 nm.Driven by an ultralow bias of 0.1 mV,a switching ratio of 75,a photoresponsivity of⁰.16 AW^-1 and a rise/fall time of 12.24/5.66 s were obtained.Nano-graphite/Si-NPA existed no spectral response.The high switching ratio and responsivity were ascribed to the formation of a thick and compact graphite nanofilm,the low excitonic absorption and square resistance.?4?Electron field emission from graphene grown on Si nanoporous pillar array.Graphene/Si-NPA prepared by CVD has high carrier concentration(5.45×10²⁴cm^-3),lower resistivity(².5?10^-8?cm)and smaller size?¹1.1 nm?,and thus it has a large number of edge points as emitting sites.Moreover,the Si-NPA as substrate not only increases the growth area of graphene,but also reduces the electrostatic shielding of field-emitting electrons.Therefore,we believed that graphene/Si-NPA should have a promising application in the electron field emission.The electron field emission of graphene/Si-NPA prepared by the CVD method was measured in a high vacuum chamber using a diode structure.For the graphene/Si-NPA with 5 min graphene growth time,the J-E curve could keep in a normal emission state when it was applied voltages and the transferred F-N plots?ln?J/E²?-1/E relation?was approximately linear,which indicated it was cold cathode electron emission by typical quantum tunneling effect.The turn-on field was².85 V/?m,and an emission current density of⁵3.9?A/cm² was obtained at an electric field of 4.2 V/?m.The planar resistance and resistivity of graphene/Si-NPA were measured by Hall effect using Van De Pauw method.Based on the experimental data,the enhancement factor was calculated to be²700 according to the Fowler–Nordheim theory.The cold cathode also showed higher emission stability than vertically standing graphene at low operating voltages.For graphene/Si-NPA with 10 min graphene growth time,the turn-on field was twice as much as the graphene/Si-NPA with 5 min graphene growth time.The emission current density is also lower than the latter.Moreover,the F-N curves showed a two-slope behavior in the high and low voltage range.This might be due to the mainly emitting sites change in the low and high applied voltage rang.At low applied voltage range,direct F–N tunneling occurred at graphene-vacuum barrier,while F–N tunneling occurred through both Ni-graphene and graphene-vacuum barriers at a high electric field range.The experimental results show that graphene/Si-NPA with 5 min graphene growth time is more suitable for fabricating Si-based low-voltage cold cathodes with high device performances.