Location of paramagnetic defects in detonation nanodiamond from proton spin-lattice relaxation data

Highlights

• Intrinsic paramagnetic defects in diamond nanoparticles.

• Nuclear spin-lattice relaxation.

• Location of intrinsic paramagnetic defects in nanodiamonds from proton spin-lattice relaxation of surface hydrogen atoms.

Abstract

We developed an approach for determining location of intrinsic paramagnetic defects in nanodiamonds from the data of proton spin-lattice relaxation of the surface hydrogen atoms. The approach was applied to the detonation nanodiamond (DND) of the diameter of 5 nm. We found that dangling bonds with unpaired electron spins are located within the near-surface belt at the distance of 0.3–0.9 nm from the DND surface. The NMR data are compared with the results of EPR measurements.

Introduction

In the last decades, low-dimensional carbon nanomaterials such as fullerenes, nanotubes, nanodiamonds, onions and graphene have attracted significant attention of the scientific community due to their unique electronic, optical, thermal, mechanical and chemical properties. Particularly high biocompatibility and low toxicity of nanodiamonds assume various biomedical applications in drug and vaccine delivery, cancer therapeutics, as biosensors, fluorescence probes and biomarkers for medical imaging [[1], [2], [3], [4], [5], [6]]. Nanodiamonds are important physical systems for various nanotechnologies such as quantum computing and metrology, information processing and communications [7,8]. Many of these applications stem from the properties of intrinsic paramagnetic defects in the nanodiamonds. Therefore information about the defect localization in these materials is of significant importance.

From the point of view of nuclear magnetic resonance (NMR), a system of spin 1/2 nuclei placed in an external magnetic field, which is strong enough to consider the magnetic dipolar coupling as a small perturbation on the Zeeman energy, relaxes due to the contact between the nuclei and the lattice that acts as a heat reservoir. In insulators, the above-mentioned contact is often provided by paramagnetic defects, which are coupled to the various modes of the lattice vibrations. Detonation nanodiamonds (DND) discussed in this paper show two kinds of the intrinsic paramagnetic defects [[9], [10], [11], [12]]: (i) single nitrogen substitutional P1 (or N⁰) centers, in which a nitrogen atom takes the place of one of the carbons in the diamond lattice and acts as a paramagnetic defect having one more electron than carbon, and (ii) dangling bonds (DB) with unpaired electron spins. The former are distributed over the whole nanodiamond particle, while the latter are located within the near-surface region of the DND particle [11]. This is supported by the measurements of ¹³C spin-lattice relaxation mediated by the nuclear spins interaction with paramagnetic defects, which show slower relaxation of the carbon spins belonging to the diamond core and faster relaxation of carbon spins in the near-surface region [11,13,14], reflecting that paramagnetic defects are in the main positioned in this area.

Direct observation of paramagnetic defects is usually made using electron paramagnetic resonance (EPR) technique, which in most cases allows distinguishing between different paramagnetic defects. However, in small DND the spins of the aforementioned P1 and DB defects are coupled by exchange interaction resulting in the collapse of their EPR signals into a badly resolved spectrum [[9], [10], [11], [12], [13], [14], [15], [16]], which restricts the information obtained. To overcome this obstacle, Shames et al. have recently studied defects’ location in nanodiamond particles using EPR measurements of DND with Cu²⁺ ions grafted to the nanodiamond surface [15]. The ions have been used for probing the location of the defects. The authors analyzed dependence of the EPR line width and the second moment of the truncated Lorentzian EPR lines of the defects on Cu²⁺ concentration and estimated the average depths of two types of intrinsic paramagnetic defects from the DND surface as ~0.8 nm and ~1.5 nm for dangling bonds and P1 centers, respectively. Numerical simulation of NMR relaxation data [17], obtained on a poorly characterized DND sample and wrong number of paramagnetic defects, led to a depth of 0.4–1 nm from the surface. Details of the calculations were not reported, which makes their use and comparison with other data difficult.

We have recently developed an effective approach for calculating distances between the surface of diamond and graphene nanoparticles and paramagnetic ions grafted to the surface [16,18]. This approach is based on the analysis of ¹³C nuclear spin-lattice relaxation rates, which are strongly affected by the interaction between nuclear spins and electron spins of paramagnetic ions. The question is whether positioning of the intrinsic paramagnetic defects, such as dangling bonds with unpaired electron spins and nitrogen paramagnetic centers in diamonds can be determined by NMR. We found that the answer is positive for the nanodiamonds, whose surface comprises a number of the hydrogen-containing (mainly hydrocarbon and hydroxyl) groups. This is shown in the present paper, in which we developed an approach to determine location of the intrinsic paramagnetic defects inside the nanodiamond using the data of the spin-lattice relaxation of the aforementioned surface hydrogen atoms. Surface proton spins interact with the unpaired electron spins of intrinsic paramagnetic defects of the diamond, which results in the increase in the ¹H relaxation rate. Herewith the spin-lattice relaxation time T₁ is proportional to the sixth power of the distance between the electron and nuclear spins, making the surface protons good probes for studying the localization of paramagnetic defects. Our approach is applied to the DND particles with the characteristic diameter of 5 nm. The NMR data obtained are compared with the results of the EPR measurements.