Single Spins Charge Ahead

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In nanoscale devices like a quantum dot, the presence of even a single excess electron can change the material’s electrical or optical properties. Similarly, the charge on biological macromolecules, such as DNA, affects their interactions with other molecules. Researchers are therefore searching for new ways to sense localized charges on materials in a variety of environments. In Physical Review Letters, Jorg Wrachtrup at the University of Stuttgart, Germany, and his collaborators report a prototype electrometer consisting of an atom-sized defect in diamond, called a nitrogen-vacancy center (NV), that is capable of sensing a single charge on a nearby nitrogenvacancy center. The electrometer could, in principle, work at room temperature and atmospheric pressure and requires no electrical contact with the sample [1]. The technique builds on the group’s earlier work, in which they established that the resonant frequency of a NV’s spin precessing in a magnetic field is influenced by an applied electric field [2]. They now show that this spin resonance signal is sensitive enough to detect the electric field from a single electron 25 nanometers (nm) away. Several methods for sensing individual charges already exist. For example, electrostatic force microscopy [3] and scanning single-electron transistor microscopy [4] have provided astonishing nanoscale views of the distribution of charge states in ceramics and semiconductor devices. A new sensor based on spin resonance is an appealing complement because it requires no electrical connection to the sample, can operate under ambient conditions, and can potentially tolerate environments, such as liquids or harsh chemicals, that are inhospitable to other techniques. In fact, NV center spins have already been pressed into service as nanoscale magnetic-field sensors that can detect a single spin [5]. They can also function as sensitive thermometers [6] that work inside living cells [7]. The NV center is a point defect in diamond that consists of a missing carbon atom (a vacancy) and a nitrogen atom that substitutes for a carbon atom on an adjacent lattice site. At the defect site, the dangling bonds of the surrounding atoms hybridize and form localized electronic energy levels similar to those of a molecule. When an NV center is negatively charged (denoted NV−), it has a total electronic spin of 1, giving it three possible spin projections (ms = 0, +1, −1). An exciting property of NV− is that after being optically excited to a higherenergy electronic state, its relaxation back to the lowest energy state depends on its spin projection. If the spin projection is ms = 0, it almost always relaxes by emitting a photon. Relaxation for the other two spin projections (ms = ±1) could either involve emission of a photon, or it could involve a long-lived intermediate state, with no visible photon emission. As a result, the intensity of light emitted by the NV− depends on its electronic spin projection, which in turn provides an optical method to detect the changes in the average spin projection that occur in a spin resonance experiment. This property, along with the atom-scale size of the defects and their naturally narrow spin resonance linewidths, makes NV centers attractive as optically addressed nanoscale sensors that use spin resonance to monitor the local environment. In 2011, Wrachtrup and his group showed that a NV center’s spin resonance depends on local electric field [2]. In simple terms, this dependence exists because a nearby electric field changes the symmetry of the nitrogenvacancy center. When no electric field is present, the four dangling bonds around the vacancy have roughly the shape of a camera tripod (three bonds form the legs and one bond forms the pole supporting the camera). But in an electric field, the tripod becomes slightly distorted. As a result, the nitrogen-vacancy center’s electron density and spin energies shift, which the researchers can detect as a shift in the spin resonance frequency. The researchers have now shown that the vacancy is