Research Seminar - Daniel Kenneth Angell

The remarkable optical properties of minimal phonon coupling and optical linewidths close to the lifetime-limit render silicon-vacancy (SiV) centers in diamond promising candidates as sources of single-photons in quantum photonics and as optically-addressable reporters of intracellular dynamics.  Previous studies of SiVs have mostly used optical microscopy, with insufficient spatial resolution to delineate the relationships of emission and diamond structure on length-scales relevant to crystal defects.  In order to study how SiV defects behave when close to boundaries, crystal defects, or other potential impairments, it is imperative to study their optical properties at the defect’s native lengthscales. Here, we use optically-coupled scanning transmission electron microscopy to disentangle multiple factors that contribute to optical heterogeneity of SiVs in nanodiamonds, with nanometer scale resolution. Spectral analysis of the cathodoluminescence (CL) spatial maps demonstrates that emission centers associated with Si implantation are spatially located at 2D defects, for example, grain boundaries within the nanodiamond. We show that individual sub crystallites within a single nanodiamond have distinct optical properties, such as spectrally shifted  zero phonon lines (ZPL) (changes up to 2.6nm) as well as differences in brightness (as much as 50%). These changes can occur within an area as small as 6nm, highlighting the importance of using sub-diffraction limited spectroscopy. The changes between crystallites accounts for heterogeneity in SiV emission more so than surface structure and grain boundaries within the diamond. In fact, we see little change in emitter brightness at the surface of the nanodiamond compared to the bulk, once normalized for thickness. Finally, we show for every diamond studied, that a change in quantum yield (QY) is associated with a ZPL redshift; this effect occurs in nanoscale spatial locations within a single particle, and we correlate this affect with a 1% lattice expansion, by mapping strain at the nanoscale. This lattice strain is likely altering the electronic relaxation pathways, simultaneously affecting emitter brightness and zero phonon line. We hypothesize that crystal distortion is arising from multiple factors, including low angle grain boundaries, as well as variations in defect incorporation.

Daniel Angell is a 6th year graduate student in Materials Science and Engineering, advised by Jennifer Dionne. He received his undergraduate degree from UC Berkeley in Energy Engineering Science, where he studied perovskite solar cells via conductive AFM, as well the synthesis of 2D materials. His current research interests include improving catalytic processes for renewable fuels, as well developing new electron and optical spectroscopies for biological sensing, and quantum technologies characterization. In recognition of his research, he has received an NSF graduate fellowship, as well as a GRC best poster award. He has also presented his work at the Materials Research Society conference, and the Electron Beam Spectroscopy for Nanophotonics (EBSN) conference. When not making waves in the lab, Daniel enjoys surfing up and down the California coast, as well as playing basketball and soccer