Measuring distance is perhaps the most ancient of human scientific activities. As such, distance measurements pervade modern scientific and industrial space, including construction, engineering, and health. Now, graphene is playing a key role in improving precision of distance measurements, down to sub-nanometer level.
Single emitters of light, such as dye color molecules, semiconductor quantum dots, or NV centers in diamond nanocrystals, have been studied for decades as potential sources of single photons for quantum information processing, which is expected to bring on a paradigm change in computing, enabling superfast computers and secure telecommunication. However, electrical control of the emission (on/off) state of single emitters has been a challenge for researchers.
Interestingly, work pioneered during the last decade has shown that placing single emitters near graphene enables many new features, such as electrical control of photon emission through voltage. The mechanism behind this technologically relevant development is twofold: first, emitters placed near graphene prefer to emit their photons into the graphene, and second this preference can be modulated with changing the Fermi level in graphene, which can be controlled with external voltage. The result is that researchers finally have a “knob” to tune single photon emission, which is a key leap for quantum information processing. The same technology was used to make highly efficient photodetectors, whereby the quantum system absorbs light with high efficiency, rapidly transferring the energy to graphene where it can be harvested for detection. This has led to advances in graphene-enabled broadband CMOS cameras, operating from the UV to the IR parts of the spectrum, and hybrid graphene-quantum dot photodetectors for food safety
Images: Left: Suspended graphene membrane (blue) near diamond nanocrystals (red). Right: Motion of graphene changes nanocrystal emission frequency, which is used as a measure of position with sub-nm precision. From Reserbat-Plantey, A. et al. Electromechanical control of nitrogen-vacancy defect emission using graphene NEMS. Nat. Commun. 7:10218 doi: 10.1038/ncomms10218 (2016). CC 4.0 license.
The coupling of single emitters to graphene critically depends not only on the Fermi level in graphene, but even more so on the distance between the emitter and graphene. Thus, researchers have used the technology developed for graphene-based quantum sources of light to precisely measure the spacing between molecules and graphene, with sub-nanometer resolution. After positioning NV centers near a suspended graphene membrane, scientists could precisely move the membrane, observe emission and hence precisely determine the distance. Subsequently, the method was utilized to measure the length of DNA origami molecules and image biological model membrane systems with high resolution. Many of the described experiments require high quality graphene on a transparent substrate, such as CVD graphene on quartz.
Image: Measuring length of DNA molecules. Reprinted with permission from Kaminska et al, Nano Lett. (2019) 197, 4257-4262. Copyright 2019 American Chemical Society.
These novel methods of measuring distance with graphene near single emitters of light are not only breaking records of precise distance measurements in nanometric systems, they are also enabling new applications in biological and medical imaging.