The Application Of Plasma Related Deposition Techniques On Fabrication Of Nanostructured Carbon Nitride Films

Since the theoretical predication of a superhard compoundβ-C₃N₄ by Cohen and Liu at University of California Berkeley in 1989,the study on the synthesis of this material has become one of foci of relational research and its synthetical fabrications have been influential in both the relevant theoretical methods and materials applications.On the other hand,one dimentional nanostructured materials like nanotubes(NTs),nanocone/tip(NCs),nanofibers(NFs) have also aroused great interests among scientists due to their unique elctrical, mechanicl and chemical properties,etc.In particular,if simultaneously having stable structures and able to be vertically aligned on substrate, they are especially attractive as being considerably suitable for field elcetron emitters,as in field emission flat-panel display,probe of scanning elctron microscope,etc.,as well as being ideal materials for supercapacitors,gas sensors,etc.In this thesis,the studies on preparation and characterization of nanostructured carbon nitride films generally comprise three sections as follows:(1) In order to acquire the properties of the discharge plasma generated from the plasma source,we had taken the in-situ analysis of plasma species with an inlet of CH₄ and N₂ mixture for growth of CN_x films through OES.The presence of dominant CN as well as concomitant C₂,CH, NH molecules in a dc-glow CH₄/N₂ plasma has been evidenced and their variation vs.ratio of CH₄/N₂ and polarity of exerted discharging electrical field have been investigated and discussed.A pronounced phenomenon is that,as methane is introduced into the pure nitrogen gas for glow discharge,main lines due to nitrogen related species such as N₂ or N₂⁺—although they were also promoted,to some extent,due to the effect of methane related species as collisional agents on emissive excitation promotion—are supplanted by those originating from CN radical at 388 and 421 nm,which require low activation energy relatively, even if the percentage of methane is as less as 1/100 CH₄/N₂ ratio.On the other hand,the line of atomic N in the range of 400nm disappeared due to promoted collisions with species induced by methane,nitrogen molecules,as well as with themselves Another interesting phenomenon is that,in spite of polarity,most detected lines including CN,N₂,C₂ (Δv=+1),C₂(Δv=0 with positive discharg) abate somewhat(compared to the linear trend as a whole),flatten or decline to a valley at the CH₄/N₂ ratio of 1/50-1/25,which should be caused by some kinds of resonance in plasma relevant to the relatively higher electronic energy there.Moreover,It can be found that there is a maximum CN generation rate with a negative top needle at 1/100-1/50 CH₄/N₂ ratio,due to the decrease of the absolute quantity of CH₄ before the 1/100 CH₄/N₂ ratio and the decrease of E_a after 1/50.With a negative top needle,the ratio of 1/100-1/50 CH₄/N₂ ratio should be an appropriate one for synthesizing graphitelike CNx films with a high nitrogen content.Otherwise,with a positive top needle,the CN generation rate is low at the 1/100-1/25 CH₄/N₂ ratio due to relatively high E_a,which is unfavorable to the deposition of fullerenelike or graphitelike films with a high nitrogen content,but nonetheless favorable for that of diamondlike films with relatively good crystalline features,for instance,β-C₃N₄.Besides,the C₂ species can provide a considerable fraction of sp² hybridization as opposed to sp³ hybridization, which is one of the important factors to induce the formation of graphitelike(or fullerenelike) structures in the deposited CN_x films. The critical point at the1/50- 1/25 CH₄/N₂ ratio with a positive top-needle polarity,where both C₂ and CN generation rates are locally minimum,is practically demonstrated to be the most favorable one for synthesis ofβ-C₃N₄.(2) Vertically aligned binary CN nanocone(CNNC),nanofiber(CNNF) and nanotube(CNNT) arrays were prepared by the direct-current abnormal glow discharge plasma reactive vapor deposition(PRVD).The CNNCs/CNNFs/CNNTs were grown on the smooth and scratched surfaces of original and n-type silicon(100) wafers,respectively,covered by Co/Ni intermediate layers.The experiments were carried out in the ambience of mixtures of methane and nitrogen,with CH₄/N₂ pressure ratios of 1/100-1/10.Scanning electron microscopy,Transmission electron microscopy(TEM),X-ray energy dispersion spectroscopy(EDS),electron diffraction,XRD,XPS and Raman spectroscopy were carried out to investigate morphology,composition,structure and crystallinity of the nanostructures in the as-deposited films.The as-grown CNNCs can be of the tip curvature diameter of only 2-3nm,and are mainly of crystallineβ-C₃N₄ structure for short ones and amorphous with dispersive crystalline domains for long ones.Finally,based on the experimental results and analyses of the growth conditions,four types of growth modes embracing sputtering-base-mode,tip-mode,multi-tip-mode and base-mode are proposed.In a sense,through manipulating the discharge parameters and substrate pretreatment,we can control the aspect-ratio, distribution density and growth sites,and could also achieve some measure of controllability on crystallity and prevailing growth modes. In a word,the dc abnormal glow discharge PRVD technique provides us with an efficient way to synthesize the binary carbon nitride nanocone and other nanostructure arrays with high nitrogen content,an extremely small tip curvature radius,controllable sizes,aspect-ratios and distribution,etc.and having large potential in applications.(3) During the last part of studies,first CN_x films were deposited on Co/Ni-covered Si(100) wafers by a self developed dc abnormal glow discharge plasma beam deposition method(GPBD) making use of available setups.The glow discharge had been carried out in a mixed gas of nitrogen and methane in different relative proportions.The as-deposited films consisted of crystallites with an average size of about 20-30 nm,and a cauliflower structure is present in the center of a prepared sample. [N]/[C]ratios in the films are 0.34-0.46,containingβ- and graphitic-C₃N₄ crystallites simultaneously.The Raman peak at 1323 cm^-1 indicates the presence of C₃N₄ components,meanwhile the seperation between D and G bands increases with the amount of nitrogen in the mixture gas increasing,suggesting a transition to crystalline C₃N₄ from graphitic structures.Then,we used the atomic beam assisted excimer pulsed laser deposition(ABPLD) technique to deposite the CN_x films still on Co/Ni-covered Si(100) wafers.The resulting films are smooth,compact and have a good adhesion to the substrate.XRD results show that the as-deposited films consisted ofβ- and graphitic-C₃N₄ crystallites simultaneously with an average size of about 9-40 nm.It is revealed in N_1s and C_1s core-level electron spectra that there are cerain amount of C-N bonds present in the films.