Final Research Abstract, by Hunter Ocker

“Developing a Nanoscale Chemical Imaging System using Optically Detected Magnetic
Resonance Enhanced by Nitrogen-Vacancy Centers”

Introduction & Background:

Magnetic resonance spectroscopy of nuclei and electrons has become one of the most widely
used analytical methods of molecular imaging in both chemical and biological research. Nuclear
Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) are capable of
obtaining valuable information about the structure and dynamics of many chemical species.
One major drawback of these methods however is their relative insensitivity when compared to
more modern analytical methods, such as mass spectroscopy. The low sensitivity of magnetic
resonance is particularly challenging for life science applications, in which biomolecules of
interest commonly occur in small absolute quantities or concentrations. Thus, there has been
great interest in the development of techniques that have the potential to greatly improve their
sensitivity. One of the most promising avenues uses a magnetic sensor based on a common
fluorescent quantum defect found in diamonds, nitrogen-vacancy color centers.
Nitrogen-vacancy (NV) centers are formed when the two neighboring carbon atoms within a
crystal lattice are replaced by a nitrogen atom and an atomic vacancy. The negatively-charged
NV- center is of particular interest as it possesses distinct spin states in an applied magnetic field
due to Zeeman effect, which can be manipulated and read out. Owing to their small atomic size
and optical readout capabilities, they are incredibly useful for the implementation of a method of
optically detected magnetic resonance (ODMR) for nuclear magnetic resonance (OD-NMR) and
electron paramagnetic resonance (OD-EPR). An assembly of NV centers packed into the surface
of a diamond chip can bolster the signal of substrates neighboring the diamond, allowing for the
interrogation of the chemistry in nanoscale sample volumes, including 2D materials or single
proteins.

Methodology:

Optically detected magnetic resonance (ODMR) relies primarily upon a quantum sensor whose
quantum states can be manipulated and read out with optical methods, in our case, an ensemble
of NV centers. To begin, a sample we seek to analyze is placed upon the surface of a diamond
with enriched NV- centers. We then subject the sample and diamond to a predetermined magnetic
field strength. This will cause the energy levels of the NV- centers (and potential energy levels of
the sample) to lose their degeneracy. An excitation source is now needed to produce fluorescence
from the sample. For our purposes, a 200 mW 532 nm laser is used to excite the diamond. With a
baseline for fluorescence now set, a range of microwave frequencies are applied to the diamond
and sample. Both the NV- centers and sample possess resonant frequencies, and when the applied
microwave frequency matches this, there will be a measurable drop in the overall fluorescence.
This fluorescence dip is how we characterize our sample and its corresponding energy levels. We
then perform this once again, but now at a new magnetic field strength. Once the resonant
frequencies / fluorescence drops of the diamond and sample are recorded for a given amount of
magnetic field strengths, they can be plotted and analyzed. This is where the molecular structure
and dynamics of the sample can be characterized and identified. Every molecular structure,
similar to traditional NMR, has a corresponding energy level behavior as the magnet field is
altered, corresponding to a resonant frequency. By outlining all present behaviors, the potential
identities of a sample can be narrowed down and eventually determined. The range of
microwave frequencies used is, in most cases, centered at approximately 2.87 GHz, or the
zero-field resonance frequency of NV- centers.

Discoveries:

In my time at the lab, our main focus was the initial characterization of multiple potential
diamonds through analysis of sensitivity and relaxation time. Many factors go into the sensitivity
and applicability of any given diamond and determine if it can potentially be used for
optically-detected EPR (OD-EPR). Some of these factors include NV concentration, depth of NV
centers, presence of transition metal ions, diamond thickness & surface area, and much more. We
gained a lot of insight into our current diamond inventory through testing over the summer, even
mapping the energy level transition of a metal ion that previously had not been seen.
In addition to this, I wrote a design and manufacturing protocol for broadband microwave
antennas meant for use in our OD-EPR setup. The antennas we were originally using had one
major issue associated with them, that being their narrowband microwave delivery. This meant
that during analysis, the further the frequency differed from the antenna’s resonant frequency
(2.8 GHz), the less sensitive data collection would be. This would cause large deformation in our
data that would have to be removed prior to analysis. Another issue was the limit on the voltage
the antenna could handle, running the risk of burning above 700 mW. With the new antenna
design, we were able to deliver a more consistent microwave emission over the desired
frequency range. We were also able to put more voltage into the antenna for even better
emission, going all the way up to around 10 W without failure.

Personal Reflection:

I can safely say that this has been one of the most insightful and awe-inspiring experiences of my
life. Before my time at LBNL, I had a lot of uncertainties about my future and what I wanted to
pursue. I knew I was passionate about renewable energy and wanted to devote myself to the
development of a cleaner world, but I was unsure of how to accomplish this task. With this
experience now behind me however, I not only feel so much more equipped and confident in my
abilities, but know where I need to improve as I move forward. There were many times over the
course of my internship where I needed to learn or revisit topics I was unfamiliar with. That
experience of having to pick up skills and knowledge that were outside of my speciality has
made me feel so much more adaptable to whatever I am working on. Revisiting many of these
topics I had forgotten and seeing the practical application of them made me realize why I learned
them in the first place. I have to thank my supervisor, Zhao Hao, for pushing me out of my
comfort zone and encouraging me to improve my problem solving abilities.
This experience also helped me discover a passion for research I never knew I had. I have grown
to love the process of scientific discovery, of hypothesising an answer to a problem, and when
the outcome is not what you were expecting, you go back to the drawing board and keep
pursuing that answer. I have held a lot of jobs throughout my life, but this was the first time I
woke up in the morning and was excited to go to work. I cannot imagine not having an
experience like this ever again, and this is why I now want to attend graduate school and further
pursue research.

I want to again thank Cal Energy Corps for the opportunity to be a participant in this year’s
program. I also want to thank my supervisor, Dr. Zhao Hao, and principal investigator, Dr.
Benjamin Gilbert, for the opportunity to assist in their research. I am truly grateful for all of the
work and assistance I received during the course of this summer. I will never forget what this
experience has done for me and my professional development.

Thank you so much for reading my final blog post, and as always, Go Bears!