Big Squeeze: Highly sensitive NV-DAC sensor stands up to enormous pressure
Squeezing a material between a pair of diamonds in the sample chamber of a diamond anvil cell (DAC) generates crushing pressures that can alter the material’s structure and create valuable new properties for a wide range of advanced technologies. DACs are also used to replicate conditions in the earth’s mantle to provide better understanding of seismic activity. But extracting critical information from within a DAC’s sample chamber demands sensors that can withstand pressures millions of times greater than what we experience on our planet’s surface.
In recent years scientists have turned a natural atomic flaw that exists in all diamonds into a versatile sensor technology. The flaw - called a “nitrogen-vacancy” or NV center - is created when two adjacent carbon atoms in the diamond crystal are replaced by a nitrogen atom and a vacant lattice site. The NV center can be utilized as a sensor because its energy levels and associated spectra are sensitive to strain, electric and magnetic fields. One of the critical advantages an NV center provides as a quantum sensor is its ability to measure fields at nanometer length scales. This is in stark contrast with conventional DAC technologies, which are limited to measuring bulk properties averaged over the entire DAC geometry.
Bakar Fellow Norman Yao, Assistant Professor of Physics, has overcome these conventional limitations with the invention of the NV-DAC, which directly integrates a thin layer of NV center sensors into a diamond anvil tip. With this invention, Yao and his group have been able to obtain highly sensitive and localized DAC measurements of a sample material’s properties under enormous pressure over a wide range of temperatures.
Q: What is unique about your use of NV centers in DACs?
A: Previously, DACs have been used to explore the effect of high pressures on the behavior of NV centers. In those studies, the main goal was to understand how the spectra of NV centers became modified at high strains. In our work, we use NV centers as a sensor to study the behavior of other materials as they undergo pressure-driven changes.
Q: What types of studies are made possible by the NV-DAC sensor?
A: One of the most promising directions for our NV-DAC sensor is to image magnetic fields emanating from materials within the high-pressure chamber. In order to do this, we have to carefully disentangle two effects: changes to the NV center as a result of being subjected to the high-pressure environment within the DAC; and the magnetic field signal coming from the sample of interest. Our ability to carefully account for the first effect precisely owes to previous measurements on NV center behavior as a function of pressure.
Q: Briefly, how does one make a DAC-NV sensor?
A: We start with a diamond anvil tip that has an intrinsically high density of nitrogen impurities within its crystal lattice. Then we bombard the diamond anvil using ion implantation of carbon atoms. We tune the energy of the carbon atoms so that they only penetrate a very short distance into the diamond lattice. This creates a high density of vacancies near the diamond’s tip to go along with the high density of nitrogen impurities. The final step is to get the nitrogen atoms and the vacancies adjacent to one another so that they form NV centers. This we do via an annealing recipe at temperatures up to 1,200 degrees Celsius.
Q: You’ve already used your DAC-NV sensor to study magnetic phases in iron and gadolinium, and to image stresses with a DAC’s sample chamber. These successes point to a wide range of possibilities for your NV-DAC sensor. What do you see as the most immediate applications?
A: On the magnetometry side of things, it would be exciting to use NV-DAC sensors to study the recently discovered room temperature superconductors, which are stabilized by extremely high pressures. Studying the phases of magnetic matter under such high pressures can, for instance, reveal new pathways to smaller, faster, and cheaper ways of storing and processing data. On the stress/strain imaging front, I would be thrilled to explore questions about how materials behave in extreme conditions. For example, how and why does fracturing occur? What is the microscopic nature of the glass transition?
Q: How will you use your Bakar Fellowship to advance the development of NV-DAC sensor technology?
A: The Bakar Fellowship will enable us to further develop key technical capabilities, such as demonstrating NV-DAC operation at pressures greater than 100 gigapascals, which is the pressure under which materials deep below Earth’s surface are formed. The Fellowship will also provide a tremendous opportunity to learn from a broad network of scientific, innovation and business development experts.
The Bakar Fellows Program supports innovative research by early career faculty at UC Berkeley with a special focus on projects that hold commercial promise. For more information, see http://bakarfellows.berkeley.edu.