| Color centers in diamond have attracted much attention due to their outstanding optical properties such as high brightness,perfect photo-stability without photo-bleaching or photo-blinking and robust at room temperature.With the intrinsic properties of diamond of the biocompatibility and chemical inertness,they show great potential in the application for quantum information processing,optical senser and biological imaging.Compared to extensively investigated nitrogen vacancy(NV)color centers(the negatively charged type,emitted at 637 nm with a broad band width of about 100 nm at room temperature and the fluorescence lifetime of up to 25 ns),the negatively charged silicon vacancy(SiV)color centers exhibit a narrow room temperature zero photon line(ZPL)centered at 738 nm with a width of about 5 nm and a short excited-state lifetime(1?4 ns).Another important advantage is that 70%of their photoluminescence(PL)is concentrated in the ZPL.Thus,nano-crystalline diamonds(NCD)containing high emission SiV centers are ideal candidates in biological tracking and sensing areas.In this dissertation,the Si-doped NCD films are fabricated using the introduction of tetramethylsilane(TMS)for Si doping in the microwave plasma chemical vapor deposition(MPCVD)process.The study on preparation,microstructure and photoluminescence enhancement mechanism is thoroughly carried out based on SiV centers in the NCD films.Firstly,the NCD films are deposited at a growth temperature of 870?.The as-deposited films exhibit a weak SiV PL intensity located at 738 nm,that is PL-quenching behavior.Air annealing at 600? can improve significantly SiV PL in the NCD films,the maximum PL enhancement reaches more than 1799-fold when the NCD films annealed for 60 min.In the meanwhile,the effect of three different surface oxidation approaches(air annealing,acid oxidation and oxygen plasma)on SiV PL in the NCD films is systematically investigated.All oxidation methods lead to the bonding of oxygen functional groups at diamond surface.However,the PL emission of Si V centers is not distinctly improved by acid oxidation and oxygen plasma,the PL enhancement is only about 7-fold and 2-fold,respectively.It is confirmed that such different PL behavior originates from the formed different thick oxidation layers.A thicker oxidation layer with optimized crystal quality contributes to SiV centers PL emission.When the air-annealed films are re-terminated with hydrogen,the SiV PL emission tends to drop remarkably under the same crystallinity.It indicates that hydrogen termination leads to the PL-quenching of SiV centers.In addition,the hybrid diamond/graphite(D/G)films are prepared at a higher growth temperature(>920?).The D/G films are mainly composed of diamond nanosheets and a large number of nanoparticles at grain boundaries.It is observed that the PL emission of SiV centers is completely invisible.The PL intensity of SiV centers and Raman signal ratio is only 1.1 in the D/G films obtained by one-step oxidation method(air annealing).Air annealing can effectively remove H elements,but results in deterioration of the crystal quality,which is the main reason for the weak PL emission.In order to improve the PL emission of SiV centers,two-step oxidation method(acid oxidation+air annealing)is a feasible route to solve this problem.The results show that two-step oxidation method makes the formation of a porous diamond nanosheets in the D/G films,and the PL enhancement is increased by 28-fold compared with one-step oxidation method.The diamond nanosheets is transformed into nanoparticles by H plasma treatment,and the PL intensity decreases obviously with same crystallinity and surface chemical modification,which reveals that the microstructure is closely related to the extraction efficiency of SiV centers. |
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