Subscriber access provided by Caltech Library
C: Surfaces, Interfaces, Porous Materials, and Catalysis
Influence of the Chemical Modification of the Nanodiamond Surface on EPR/ENDOR Spectra of Intrinsic Nitrogen Defects Boris V. Yavkin, Dmitry G. Zverev, Georgy Vladimirovich Mamin, Marat R Gafurov, and Sergei B. Orlinskii J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06329 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Influence of the Chemical Modification of the Nanodiamond Surface on EPR/ENDOR Spectra of Intrinsic Nitrogen Defects Boris V. Yavkin1,2, Dmitry G. Zverev1, Georgy V.Mamin1, Marat R. Gafurov1,* and Sergei B. Orlinskii1 1
Kazan Federal University, Kremlevskaya 18, 420008 Kazan, Russia
23rd
Institute of Physics, University of Stuttgart, 70569 Stuttgart, Germany
* e-mail:
[email protected] Abstract High frequency/high magnetic field (94 GHz/3.3 T) electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) study of monocrystalline nanodiamonds (ND) is carried out. Influence of the hydrogen and fluorine modification of ND surface on EPR and ENDOR signals from the superficial and bulk paramagnetic centers is observed. The effect shows that the ND bulk paramagnetic centers could serve as intrinsic probe of ND surface modification despite deep localization, shielding from the environment and two-layered surface of the diamond nanoparticle.
1
ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 18
Introduction In recent years, due to a large range and uniqueness of the properties of the diamond based nanosystems their development is continuously growing. Nanodiamonds (ND) of synthetic nature applied or considered to be used for the delivery of bioactive molecules, as nanomagnetometers and nanothermometers1, for local electric field measurements2, quantum information processing3. In general, NDPs (nanodiamond particles) belong to a broad family of the nanocarbon materials. Their structural diversity depends on the synthesis, post-synthesis conditions and material modifications4. Surface of the ND is usually modified by attaching different functional groups5 for subsequent linking of bioactive molecules6. High penetrating ability and stability of these systems allow using them as platforms for biomarkers7, drug delivery agents and other therapeutic media8. Biosensors based on NDs can prick through the cell membrane in a targeted manner and probe changes in realtime in the intracellular fluid. Combined with the exquisite molecular recognition of antibody ligands, NDP-based sensor could thus serve as a powerful tool for exploring multi-protein molecular machineries in a living cell. Specific types of NDP-derived sensors could also be developed as multifunctional probes (antibody as well as DNA) sensitive to the cell structural changes with the ability to influence biochemical processes in single cell9-10. As it was already mentioned, to be an effective biological cargo, the surface of ND ought to have large amounts of hydrophilic functional groups, to which other biologically active species can be further chemically linked. On the other hand, NDs containing natural and artificially created defects like nitrogen vacancies (NV centers) could be a superior biomarker over organic dyes thanks to the higher stability of their photoluminescence and their lower toxicity. The interest to these objects is promoted, on one hand, by the increasing number of different applications where exceptional mechanical properties of diamond are in demand and on the other by newly discovered opportunities arising from the transition to nanoscale level11-12. Initial theoretical predictions reported on the unsurpassable difficulties of doping NDP and thermodynamically unstable configuration of nitrogen (as a most abundant impurity in natural bulk diamond) dopants within NDs. Despite first unsuccessful experimental attempts to detect nitrogen in the ND sample13-14, multiple experimental works that followed had shown these considerations to be wrong. For the first time a presence of single substitutional nitrogen in neutral charge state (so called P1 center, or N0) inside diamond nanocrystals was shown in works15-16. In addition at about the same time numerous reports on the stable color centers (e.g. NV, SiV) in nanocrystalline diamonds appeared in literature17-19.
2
ACS Paragon Plus Environment
Page 3 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
For the efficient utilization of NDs its surface and core admixtures must be controlled. It has been shown that the properties of the famous NV defects are strongly affected by their proximal neighborhood. For example, termination/coverage of ND particles by specifically or nonspecifically bounded groups change photophysical behavior of NV centers20. For the surface characterization NV centers have to be located relatively close to the outer shell which means a large degradation in its established optically assisted readout procedure. A role of a probe could be played by P1 center which is paramagnetic and can be well studied by electron paramagnetic resonance (EPR). As for NV centers, direct observation of influence of the surface modification on EPR of intrinsic probes in nanosized unstructured materials is greatly hindered by powder averaging of the EPR spectrum although few successful attempts are known21. For the long time only X-band (microwave frequency of 9-10 GHz) EPR was used to study NDs and paramagnetic defects in diamond but its relatively low spectral resolution was complicating enormously the decomposition of a complex multicomponent EPR signal typically observed in NDs. Nevertheless, first assignments were done and parts originating from surface and from core of the nanoparticles were identified. The actual origin and structure of those paramagnetic centers depends on the synthesis route and surface modification process after the synthesis. Moreover, a significant role of the nanoparticle size was shown in papers22-25 because the relative contribution of the surface becomes larger with size decreasing. The importance of surface modification to ‘core-belonging’ impurities and defects in diamond for instance was demonstrated in paper26. The quest for the predictable surface and bulk surface-induced properties is of great significance for virtually all possible nanodiamond applications. In this work pulsed high-frequency (94 GHz) EPR and electron-nuclear double resonance (ENDOR) are used to reveal specific magnetic properties of nanodiamond particles functionalized by hydrogen (fluorine) atoms. We show a utilization of intrinsic probe to trace the changes in the core and on the surface. The results can be used for reliable control of ND modification and modification type. 1. Materials and Methods The synthesis of ND particles and their characterization by various analytical techniques are described in details in paper26. Briefly, the commercial solution of HPHT type Ib ND (Microdiamant AG, Switzerland) containing about 100–120 ppm of nitrogen was chosen as starting material. The quartz plate with highly fluorescent ND grains (with the size of 20–25 nm from AFM) was exposed 3
ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 18
to microwave-excited hydrogen plasma for 30 minutes at a temperature of 500◦C and at pressure of 1 mbar to produce an H-terminated surface (NDH samples). The H terminated samples were fluorinated by fluorine gas (NDF samples). EPR measurements were done by using commercial Bruker ElexSys E680 spectrometer operating in the W-band microwave (mw) range (94 GHz) equipped with standard cylindrical cavity (TE011 mode). The sample temperature was controlled by ER4118CF continuous flow helium manufactured by Oxford Instruments. Most experiments were carried out at T = 50 K for compromise between signal intensity and measurement time. EPR spectra were recorded in pulsed mode by using Hahn echo mw pulse sequence π/2 -- π and monitoring spin echo intensity while sweeping magnetic field. Pulse length was set to π/2 = 36 ns with the interpulse separation = 280348 ns and sequence repetition rate set to 100 Hz or 1 kHz (depending on the sample and experiment type) to avoid saturation effects. Electron-nuclear double resonance (ENDOR) spectra were measured using Mims27 and Davies28 pulse sequences. The length of the radiofrequency (RF) pulse was 18 μs or 72 μs with RF power of ca. 150 W while mw pulses with 36 ns length were applied. Mathematical modeling of EPR and ENDOR signals were done in EasySpin29 toolbox for MATLAB. 2. Results and Discussion Typical EPR spectra of both NDF and NDH samples are presented in figure 1. Following the results of papers 22 and 23, the spectra can be decomposed onto three different ingredients. Two structureless lines of lorenz and gauss-like shapes (named SC1 and SC2 hereafter) belong to the paramagnetic centers that are located at the (near) surface of nanoparticle. Third spectral component has a structure that corresponds to the substitutional nitrogen paramagnetic center N0 (P1) located inside the diamond core of a nanoparticle. Since the aim of this work was not to determine the absolute and relative concentrations of the corresponding paramagnetic centers (in contrast to paper 22, for example) the EPR registration parameters for figure 1 were chosen to demonstrate the presence of all the mentioned paramagnetic species. The precise definition of EPR parameters for (near) surface centers SC1 and SC2 also does not influence the main message of the paper. Comparing to NDH, EPR spectrum of NDF sample shows additional pair of broad lines separated by approx. 11 mT that can be ascribed to the paramagnetic center of one or few CHF complexes on the nanoparticle surface. Paramagnetic centers with similar spectroscopic properties were observed before in the study of fluorinated methyl radicals30. 4
ACS Paragon Plus Environment
Page 5 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Precise identification of those CHF centers is very complicated due to number of reasons, for instance: there is not only one, but a few CHF complexes are formed whose hyperfine interaction with 1H and 19F is different. Secondly, as hyperfine interaction of those paramagnetic complexes is strongly anisotropic, a powder-induced broadening of the spectrum will mix individual components and their separate analysis would become impossible. Using EasySpin the simulation of EPR spectra was carried out, and spectroscopic parameters of N0-center were obtained as g=2.0023, A=2.99 mT and A=4.38 mT that are well correspond to the parameters of the N0 center observed in the bulk diamond31. Further analysis of the paramagnetic centers located at the surface and in the core of the nanoparticle was performed by ENDOR spectroscopy. ENDOR spectra of only NDF sample are presented in the work because 1H ENDOR signal of NDH sample is identical to 1H part of ENDOR signal observed in NDF sample. In figure 2(b) the peak positions of the ENDOR spectra (presented in the bottom panel of Figure 2a) are plotted versus external magnetic field magnitude. This allows to distinct between 1H and
19F
nuclei due to their difference in gyromagnetic ratios. The correspondence of observed
ENDOR signals to 1H and 19F nuclei in NDF sample confirms the surface modification. The presence of narrow lines on the ENDOR spectrum (Figure 3c) in the magnetic field corresponding to N0 indicates small hyperfine interaction between surface nuclei of fluorine and hydrogen and paramagnetic nitrogen center N0 in the crystalline core of the nanoparticle. One can estimate the strength of this interaction from the width of the narrow ENDOR line as Adip
