Letter pubs.acs.org/NanoLett
Nitrogen-Vacancy-Assisted Magnetometry of Paramagnetic Centers in an Individual Diamond Nanocrystal Abdelghani Laraoui,† Jonathan S. Hodges,‡ and Carlos A. Meriles*,† †
Department of Physics, City College of New York − CUNY, New York, New York 10031, United States MITRE Corporation, 200 Forrestal Road, Princeton, NJ 08540, United States
‡
ABSTRACT: Semiconductor nanoparticles host a number of paramagnetic point defects and impurities, many of them adjacent to the surface, whose response to external stimuli could help probe the complex dynamics of the particle and its local, nanoscale environment. Here, we use optically detected magnetic resonance in a nitrogen-vacancy (NV) center within an individual diamond nanocrystal to investigate the composition and spin dynamics of the particle-hosted spin bath. For the present sample, a ∼45 nm diamond crystal, NVassisted dark-spin spectroscopy reveals the presence of nitrogen donors and a second, yet-unidentified class of paramagnetic centers. Both groups share a common spin lifetime considerably shorter than that observed for the NV spin, suggesting some form of spatial clustering, possibly on the nanoparticle surface. Using double spin resonance and dynamical decoupling, we also demonstrate control of the combined NV center−spin bath dynamics and attain NV coherence lifetimes comparable to those reported for bulk, Type Ib samples. Extensions based on the experiments presented herein hold promise for applications in nanoscale magnetic sensing, biomedical labeling, and imaging. KEYWORDS: Diamond nanocrystals, nitrogen-vacancy centers, optically detected magnetic resonance, substitutional nitrogen, surface paramagnetic centers nanocrystal these “dark” paramagnetic centers largely outnumber NVs and give rise to a rapidly fluctuating local magnetic field, which normally leads to fast NV spin relaxation. The flipside, however, is that because many of these paramagnetic centers, such as those created by dangling bonds, reside on the particle surface, the spin bath itself may reflect on the dynamics of the nanoparticle environment. Therefore, the investigation of the interplay between an NV center and the surrounding nanoparticle-hosted spin bath is of fundamental interest in the application of nanostructured diamond to high-resolution magnetic sensing and imaging. The results we present herein provide a starting point in this direction: We use NV-based confocal microscopy and double spin resonance schemes to probe the composition and dynamics of the optically inactive spin bath within an individual diamond nanocrystal. Using the NV center as a probe spin, we indirectly reconstruct the spin bath magnetic resonance spectrum, which allows us to identify contributions from nitrogen donors and other paramagnetic centers. The spin bath exhibits a single, short coherence time indicative of strong intrabath couplings, likely resulting from preferential grouping. We also explore alternate dynamical decoupling (DD) schemes where we manipulate the NV center, the spin bath, or both to extend the NV coherence lifetime. Comparison with prior DD
D
iamond nanocrystals are presently the focus of intense research in areas spanning physics, chemistry, and biology.1 At the center of this collective effort are a broad range of applications. For example, since most diamond particles are noncytotoxic and do not bleach or blink, nanodiamond (ND) bodes well for sensing schemes that rely on tracking basic cellular processes such as in cancer diagnostics and biological imaging.2 Further, ND shows potential in energy storage,3 composites,4 and catalysis5 as well as for the fabrication of photonic structures.6 Of particular importance among the various possible ND types is the group of particles containing nitrogen-vacancy (NV) centers. Negatively charged NV centers, formed by a substitutional nitrogen adjacent to a vacancy site, are paramagnetic, spin-1 point defects that can be individually polarized and readout by optical means.7 Thanks to their superb photostability and long spin coherence times at room temperature,8 NV centers trapped within diamond nanostructures are being explored as a platform for various technologies, most notably high-resolution scanning magnetometry.9−12 Among the range of conceivable nanostructures, diamond particles free within the cytoplasm of a cell13 or attached to a scanning tip9,14 provide some of the most compelling examples of the ongoing drive in this area of research. Besides the NV center, diamond nanoparticles host a rich variety of optically inactive, paramagnetic centers resulting from impurities, vacancies, and other lattice imperfections, which we will collectively refer to as the “spin bath”. In a typical diamond © 2012 American Chemical Society
Received: March 10, 2012 Revised: May 14, 2012 Published: June 22, 2012 3477
dx.doi.org/10.1021/nl300964g | Nano Lett. 2012, 12, 3477−3482
Nano Letters
Letter
Figure 1. (Center) A diamond nanocrystal can be viewed as a rich paramagnetic structure where the NV center coexists with other nonzero spin species including nitrogen donors, surface paramagnetic centers, and nuclear spins (13C, 1H, etc). (Upper row) Since these centers are optically inactive and most nanocrystals do not host an NV, the AFM (left) and confocal (right) images of the particle ensemble differ substantially. (Bottom row) AFM and confocal images of the individual particle (45 nm ±8 nm) used for the studies reported herein.
ensemble of nanocrystals firmly attached to the glass substrate. The size distribution is remarkably homogeneous and has average diameter of 50 ± 7 nm. Figure 1 shows the AFM and scanning confocal images of a representative, 45 nm diameter particle used for the studies below. In the ground state, the NV center exhibits a spin triplet with a crystal field splitting D = 2.87 GHz between states ms = 0 and ms = ±1. The application of an external magnetic field breaks the degeneracy of the ms = ±1 state and leads to a pair of transitions whose frequencies depend on the relative orientations of the external field and the NV axis. Green light illumination produces a spin-conserving transition to the first excited state, also a triplet, which in turn leads to broadband photoluminescence emission. Intersystem crossing to a metastable singlet state takes place preferentially for states ms = ±1 and ultimately translates into an almost complete population of the ms = 0 state; this same mechanism allows for optical readout after resonant microwave manipulation in the ground state.7,15 In the experiments of Figure 2, we start by aligning the field B0 of our permanent magnet (∼130 mT) along the direction of the NV axis (a condition necessary to avoid level mixing and the concomitant shortening of the spin coherence lifetime16). In practice, the latter is carried out by iteratively monitoring the NV fluorescence, maximum when the field and NV axis directions coincide, and the frequency of the fluorescence dip resulting upon resonant cw microwave irradiation in the vicinity of the ms = 0 → ms −1 transition (Figure 2a). As the precise magnetic field amplitude at the NV site is unknown a priori, we also rely on the time separation between consecutive “revivals” in the signal resulting from a Hahn-echo protocol (Figure 2d). The inverse of the interval between modulation maxima is directly proportional to the observed 13C Larmor frequency
results in bulk Type Ib samples shows similar decoupling efficiency, though we observe markedly exponential signal decay unusual for single NV spin measurements. For the present experiments, we use a purpose-built confocal microscope (inverted geometry, oil immersion objective lens 1.4 NA) with excitation at 532 nm via a cw diode pumped solid-state laser (Coherent Compass). For detection, we intercalate a system of filters to select light within the NV photoluminescence spectrum (635−800 nm). The confocal system operates in combination with a commercial atomic force microscope mounted on a platform facing the objective. The sample, which is a collection of diamond nanocrystals dispersed on a thin glass substrate, sits between the AFM tip and the objective. A 25 μm thick wire overlaid on the glass surface serves as the source of microwave excitation; we use a ringshaped permanent magnet to apply a magnetic field B0 at the NV site. All experiments are carried out under ambient conditions. The nanocrystals we use herein were obtained from Microdiamant in the form of “liquid diamond”, a homogeneous solution with high-pressure, high-temperature (HPHT) diamond nanoparticles in suspension. Prior to examining the nanocrystals, we centrifuge ∼1 mL of this solution for several minutes, after which the system separates into three different phases: a clear fluid accumulates on the upper end of the centrifuge container and a black pellet at its bottom, separated by a cloudy light-brown interface. We remove and replace the fluid with distilled water, redissolve the pellet, and centrifuge again. After several iterations, the nanocrystal pellet turns from black, to light brown, to white. We use this white pellet and 1 mL of distilled water to produce a stock nanocrystal solution, which we drop-cast on a clean glass slide. Upon evaporation of the solvent, atomic force imaging reveals an evenly dispersed 3478
dx.doi.org/10.1021/nl300964g | Nano Lett. 2012, 12, 3477−3482
Nano Letters
Letter
To gain direct information on the nanocrystal-hosted spin bath, we use several double resonance schemes as shown in Figure 3a. We start by recording the amplitude of the NV spin
Figure 2. (a) Optically detected magnetic resonance spectrum (ms = 0 → ms = 1 transition) from a single NV center hosted by the nanocrystal of Figure 1 in the presence of an external magnetic field of 131.3 mT. The field direction coincides with that of the NV axis with precision of about 1°. The solid line is a Lorentzian fit with a linewidth of 13 MHz. (b) NV spin Rabi beatings. Fitting to Rabi sin(ωRabit)exp(−t/TRabi 2,NV) yields T2,NV = 1.3 μs. (c) NV signal after a “Ramsey” protocol. Fitting to sin(ΔωLarmort)exp(−t/T*2,NV) yields T*2,NV = 238 ns. (d) NV signal after a “Hahn-echo” pulse sequence. The solid line is a single exponential decay exp(−t/T2,NV) where T2,NV = 3.2 μs. As confirmed by the insert plot, the oscillations observed in the spin echo signal are 13C-induced revivals at the applied magnetic field.
Figure 3. (a) Three alternate double resonance pulse sequences. (b) NV echo signal at fixed τ as a function of the frequency of an ancillary, weak pulse of duration t1 = 80 ns synchronous with the inversion pulse in the Hahn-echo protocol. The solid curve represents the calculated P1-induced spectrum for a magnetic field aligned with the [111] axis using the hyperfine constants of ref 20. (Insert) Frequency dependence of the central peak as a function of the applied magnetic field. (c) Coherent driven oscillations at 2.8 GHz as the duration of a strong, ancillary pulse is progressively incremented. Fitting to Rabi sin(ωRabit)exp(−t/TRabi 2,SB) yields T2,SB = 0.5 μs. (d) Ramsey protocol on the bath spins. The solid line is a single exponential decay exp(−t/ T*2,SB) where T*2,SB = 62 ns. In all experiments τ = 1 μs and B0 = 100 mT.
(insert in Figure 2d) implying that the external field is aligned with the NV pair when the B0 values derived from the cw ODMR (Figure 2a) and Hahn echo experiments coincide.17,18 We note that because the NV axis can point along four possible directions in the diamond lattice with equal probability, observing more than one dip in the ODMR spectrum is likely if several NV centers are present in the chosen nanocrystal. For a particle of 45 nm, however, NV centers are extremely rare and confocal imaging shows that only a small fraction (
