Long-Lived Ensembles of Shallow NV– Centers in Flat and Nanostructured Diamonds by Photoconversion

Federico Gorrini,Angelo Bifone,Rakshyakar Giri,Pietro Aprà,Carla Dorigoni,Domingo Olivares-Postigo,Federico Picollo

doi:10.1021/acsami.1c09825

Abstract

Shallow, negatively charged nitrogen-vacancy centers (NV–) in diamond have been proposed for high-sensitivity magnetometry and spin-polarization transfer applications. However, surface effects tend to favor and stabilize the less useful neutral form, the NV0 centers. Here, we report the effects of green laser irradiation on ensembles of nanometer-shallow NV centers in flat and nanostructured diamond surfaces as a function of laser power in a range not previously explored (up to 150 mW/μm2). Fluorescence spectroscopy, optically detected magnetic resonance (ODMR), and charge-photoconversion detection are applied to characterize the properties and dynamics of NV– and NV0 centers. We demonstrate that high laser power strongly promotes photoconversion of NV0 to NV– centers. Surprisingly, the excess NV– population is stable over a timescale of 100 ms after switching off the laser, resulting in long-lived enrichment of shallow NV–. The beneficial effect of photoconversion is less marked in nanostructured samples. Our results are important to inform the design of samples and experimental procedures for applications relying on ensembles of shallow NV– centers in diamond.

Highlights

Centers can be used in dynamic nuclear polarization (DNP) protocols where the polarization of the NV−s is transferred to 13C nuclei, leading to hyperpolarization of 13C nuclei in the diamond lattice.[14−16] Substantial efforts are ongoing to promote polarization transfer from shallow NV centers to molecules absorbed at the diamond surface,[17] enabling hyperpolarization of high-sensitivity tracers for biomedical magnetic resonance imaging
NVs are characterized by a ground state and excited states (3E for NV− and 2A1 for NV0) positioned within the diamond band gap,[37] each accompanied by a phonon band
Moieties and acceptor states at the diamond surface decrease the stability of the NV−, even at depths of tens of nanometers,[50−52] and NV0s may become the dominant charge state[53,54] with detrimental effects for sensing and polarization transfer applications

Summary

Introduction

Charged nitrogen-vacancy (NV−) centers are solidstate defects in the diamond lattice whose properties have been exploited to detect temperature gradients,[1,2] magnetic[3,4] and electric fields[5,6] at the nanoscale, and interactions with magnetic molecules and nanoparticles.[7−10] Due to their biocompatibility, NV−-enriched fluorescent nanodiamonds represent promising sensors to investigate the cellular microenvironment in living tissues and their use in highsensitivity bioassays has been proposed.[11−13] NV centers can be used in dynamic nuclear polarization (DNP) protocols where the polarization of the NV−s is transferred to 13C nuclei, leading to hyperpolarization of 13C nuclei in the diamond lattice.[14−16] Substantial efforts are ongoing to promote polarization transfer from shallow NV centers to molecules absorbed at the diamond surface,[17] enabling hyperpolarization of high-sensitivity tracers for biomedical magnetic resonance imaging For all these applications, the proximity of NV− centers to the diamond surface, where NVs can effectively interact with spins outside the diamond lattice, is of paramount importance, as the coupling strength between magnetic dipoles decreases with increasing distance. We apply fluorescence spectroscopy, as well as optically detected magnetic resonance (ODMR), in electronic-

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