Abstract
Silicon-vacancy (SiV) centers in diamonds present exceptional spectral properties, including bright zero phonon line (ZPL) at wavelengths of 736 nm 746 nm and a narrow emission linewidth. As a single photon source, SiV center is a promising candidate for quantum computing as well as biomarking applications. The SiV centers in nanodiamonds, including diamond nanowires, diamond nanoislands, and diamond particles, present high-intensity of ZPL emissions over a narrow linewidth. In this study, we investigated the fabrication of ultrananocrystalline diamond (UNCD) nanostructures (UNCD size ~ 10 nm) and the spectral characteristics of the SiV centers contained within. SiV centers are typically created via chemical vapor deposition (CVD). In this process, Si impurities are incorporated within the diamond during the growth process, at elevated temperatures under high microwave powers. In this study, we created SiV centers in UNCD using two methods: (i) in-situ Si-doping during microwave plasma-enhanced chemical vapor deposition (MPECVD) at low growth temperatures; (ii) Si-ion implantation in UNCD and single crystalline diamond (SCD). The first method begins with in-situ Si-doping of diamond films with various granular structures grown on a Si-substrate at low temperature (< 550 oC). The films include microcrystalline diamond (MCD), nanocrystalline diamond (NCD), UNCD, and nitrogen-incorporated UNCD (N-UNCD) films. We devised a simple process for the fabrication of diamond nanostructures within these films using a self-assembled mask of Au nanodots followed by reactive ion etching (RIE) under O2/CF4 plasma. Field emission scanning electron microscopic images of the diamond nanostructures revealed the formation of vertical nanostructures with high density. UV-Raman spectroscopy confirmed that RIE did not degrade the quality of the diamond nanostructures. Photoluminescence (PL) spectroscopy revealed strong NV emissions from MCD nanocones and NCD nanotips as well as the quenching of NV emissions from UNCD nanopillars. The second process involves the in-situ Si-doping of diamond using various silicon oxide (SiO2) substrates, including SiO2, soda-lime glass, and soda-lime glass fibers, for the growth of MCD, NCD, and UNCD films. The PL spectra of the resulting UNCD diamond films revealed SiV centers with bright and clear emission at 738 nm - 740 nm and suppressed NV emissions. The UNCD formed as particulates rather than as a film on soda-lime glass fibers. Transmission electron microscopy (TEM) was used to study the influence of microstructure on the spectral characteristics of SiV centers. The TEM micrographs of UNCD films reveal the presence of large aggregate, which might be the cause of the NV emission from the UNCD films. We developed a simple process for the synthesis of SiV-UNCD particulates with bright emissions, wherein SiV-UNCD nanoclusters/soda-lime glass fibers were ultrasonicated in DI water, and then the water was spread over Si inverted pyramids. Time-resolved PL spectroscopy measurements of SiV-UNCD particulates revealed that the SiV centers have a short decay time of ~ 0.20 ns (SiV decay time ~ 1-2 ns). This can be attributed to the low quality of the UNCD, which includes a large number of defects and non-diamond carbon phases. We also developed two approaches to the fabrication of bright SiV-UNCD nanostructures; i.e., top-down approach for fabrication of SiV-UNCD nano-rods and bottom-up approach for fabrication of SiV-UNCD nano-tips. The resulting SiV-UNCD nanostructures exhibit bright emission over a narrow linewidth of ~ 7 nm 10.5 nm with shorter decay time of ~ 0.2 ns. To enhance the decay time of SiV centers, UNCD has grown on a Ti/Sapphire substrate using MPECVD, followed by Si-ion implantation under the following parameters: E = 125 keV and dose = 1013 ions/cm2. The resulting SiV-UNCD nanoclusters present bright SiV emission with ZPL width of ~ 7.0 nm and = 0.43 ns. Si-ion implantation was also performed on SCD (type Ia & type IIa) with E = 350 keV and dose = 1010 ions/cm2 to facilitate a comparison of the spectral characteristics of SiV-UNCD in high-quality diamond with SiV centers. The SiV centers in type IIa SCD present bright emission, narrow ZPL width of ~ 6 nm and enhanced decay time of = 1.30 ns. The SiV-UNCD nanostructures and SiV-UNCD particulates developed in this study have considerable potential in biomarking applications, due to their strong SiV emissions. Furthermore, the Si-ion implanted type IIa SCD samples are applicable as a single photon emitter in quantum information processing and quantum computation applications.