@article{nunn_milikisiyants_torelli_monge_delord_shames_meriles_ajoy_smirnov_shenderova_2023, title={Optical and electronic spin properties of fluorescent micro- and nanodiamonds upon prolonged ultrahigh-temperature annealing}, volume={41}, ISSN={["2166-2754"]}, url={https://doi.org/10.1116/6.0002797}, DOI={10.1116/6.0002797}, abstractNote={High-temperature annealing is a promising but still mainly unexplored method for enhancing spin properties of negatively charged nitrogen-vacancy (NV) centers in diamond particles. After high-energy irradiation, the formation of NV centers in diamond particles is typically accomplished via annealing at temperatures in the range of 800-900 °C for 1-2 h to promote vacancy diffusion. Here, we investigate the effects of conventional annealing (900 °C for 2 h) against annealing at a much higher temperature of 1600 °C for the same annealing duration for particles ranging in size from 100 nm to 15 μm using electron paramagnetic resonance and optical characterization. At this high temperature, the vacancy-assisted diffusion of nitrogen can occur. Previously, the annealing of diamond particles at this temperature was performed over short time scales because of concerns of particle graphitization. Our results demonstrate that particles that survive this prolonged 1600 °C annealing show increased NV T1 and T2 electron spin relaxation times in 1 and 15 μm particles, due to the removal of fast relaxing spins. Additionally, this high-temperature annealing also boosts magnetically induced fluorescence contrast of NV centers for particle sizes ranging from 100 nm to 15 μm. At the same time, the content of NV centers is decreased fewfold and reaches a level of <0.5 ppm. The results provide guidance for future studies and the optimization of high-temperature annealing of fluorescent diamond particles for applications relying on the spin properties of NV centers in the host crystals.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Nunn, Nicholas and Milikisiyants, Sergey and Torelli, Marco D. and Monge, Richard and Delord, Tom and Shames, Alexander I. and Meriles, Carlos A. and Ajoy, Ashok and Smirnov, Alex I. and Shenderova, Olga A.}, year={2023}, month={Jul} }
 @article{seed_acharya_nunn_smirnov_krim_2023, title={Tribotronic and electrochemical properties of platinum-nanofluid interfaces formed by aqueous suspensions of 5 and 40 nm TiO<sub>2</sub> nanoparticles}, volume={159}, ISSN={["1089-7690"]}, url={https://doi.org/10.1063/5.0155504}, DOI={10.1063/5.0155504}, abstractNote={Nanoparticles (NPs) can be highly beneficial as additives to lubricating fluids, and the tribotronic response of charged NPs tuned by external fields represents an area of great technological potential. Tribotronic response, however, is expected to be highly size dependent, which represents a significant design challenge. To explore this issue, quartz crystal microbalance and cyclic voltammetry were employed to characterize nanotribological and electrochemical behavior of platinum-nanofluid interfaces formed by aqueous suspensions of different-sized negatively charged titanium dioxide (TiO2) NPs. Suspensions of 5, 40, and 100 nm NPs were all observed to reduced interfacial frictional drag forces upon introduction into pure water in zero field conditions, with reductions for the 40 nm NPs about twice those of 5 nm particles at comparable concentrations. Suspensions of 100 nm NPs produced even greater reductions, but rapidly precipitated from the suspension when left unstirred. NPs were also driven to and from Pt electrode surfaces by applying external electric fields with varying amplitudes and modulation frequencies. For electric fields of sufficient amplitude and duration, the 40 nm TiO2 nanosuspension exhibited tribological properties consistent with a reversible electrophoretic deposition of the NPs, accompanied by changes in the electrochemical attributes and increasing interfacial drag. The 5 nm NP properties were consistent with progressive reductions in interfacial drag forces at the NP-suspension interface linked to field-induced increases in concentration.}, number={11}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Seed, C. M. and Acharya, B. and Nunn, N. and Smirnov, A. I. and Krim, J.}, year={2023}, month={Sep} }
 @article{nunn_milikisiyants_danilov_torelli_dei cas_zaitsev_shenderova_smirnov_shames_2022, title={Electron irradiation-induced paramagnetic and fluorescent defects in type Ib high pressure-high temperature microcrystalline diamonds and their evolution upon annealing}, volume={132}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0103313}, DOI={10.1063/5.0103313}, abstractNote={Defects introduced to synthetic type Ib diamond micrometer-size particles by electron-beam irradiation were studied by electron paramagnetic resonance and photoluminescence (PL) spectroscopy as a function of e-beam fluence and post-irradiation thermal annealing. Increasing electron-beam fluence causes a substantial reduction of the substitutional nitrogen (P1) content, accompanied by progressively higher concentrations of paramagnetic negatively charged vacancies (V−) and triplet interstitials (R1/R2). Annealing results in a drastic decrease in the V− and R1/R2 content and an increase in the negatively charged nitrogen-vacancies (NV− or W15). Analysis of PL spectra allows for identification of color centers in the irradiated diamond samples and following their evolution after annealing. These data facilitate understanding of different factors contributing to the formation of color centers in diamond and promote efforts toward controlled engineering of optical centers in fluorescent diamond particles.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Nunn, Nicholas and Milikisiyants, Sergey and Danilov, Evgeny O. and Torelli, Marco D. and Dei Cas, Laura and Zaitsev, Alexander and Shenderova, Olga and Smirnov, Alex I. and Shames, Alexander I.}, year={2022}, month={Aug} }
 @article{mikheev_vanyukov_mogileva_mikheev_aleksandrovich_nunn_shenderova_2021, title={Femtosecond Optical Nonlinearity of Nanodiamond Suspensions}, volume={11}, ISSN={["2076-3417"]}, DOI={10.3390/app11125455}, abstractNote={High pressure-high temperature (HP-HT) nanodiamonds and detonation nanodiamonds have unique optical properties and are promising materials for various applications in photonics. In this work, for the first time, comparative studies of the nonlinear optical properties of aqueous suspensions of HP-HT and detonation nanodiamonds under femtosecond laser excitation are performed. Using the z-scan technique, it was found that for the same laser pulse parameters HP-HT nanodiamonds exhibited optical limiting due to two-photon absorption while detonation nanodiamonds exhibited saturable absorption accompanied by short-term optical bleaching, revealing the different electronic-gap structures of the two types of nanodiamonds. The saturable absorption properties of detonation nanodiamonds are characterized by determining the saturable and non-saturable absorption coefficients, the saturation intensity, and the ratio of saturable to non-saturable losses. The nonlinear absorption in HP-HT nanodiamonds is described with the nonlinear absorption coefficient that decreases with decreasing concentration of nanoparticles linearly. The results obtained show the possibility of using aqueous suspensions of nanodiamonds for saturable absorption and optical limiting applications.}, number={12}, journal={APPLIED SCIENCES-BASEL}, author={Mikheev, Gennady M. and Vanyukov, Viatcheslav V. and Mogileva, Tatyana N. and Mikheev, Konstantin G. and Aleksandrovich, Alexander N. and Nunn, Nicholas A. and Shenderova, Olga A.}, year={2021}, month={Jun} }
 @article{mikheev_mogileva_fateev_nunn_shenderova_mikheev_2020, title={Low-Power Laser Graphitization of High Pressure-High Temperature Nanodiamond Films}, volume={10}, ISSN={["2076-3417"]}, DOI={10.3390/app10093329}, abstractNote={Laser-induced graphitization of 100 nm monocrystals of diamond particles synthesized by high-pressure high-temperature (HP-HT) methods is not typically observed. The current study demonstrates the graphitization of 150 nm HP-HT nanodiamond particles in ca. 20-μm-thick thin films formed on a glass substrate when the intensity of a focused 633 nm He-Ne laser exceeds a threshold of ~ 33 kW/cm2. Graphitization is accompanied by green luminescence. The structure and morphology of the samples were investigated before and after laser excitation while using X-ray diffraction (XRD), Raman spectroscopy, atomic force (AFM), and scanning electron microscopy (SEM). These observations are explained by photoionization of [Ni-N]- and [N]-centers, leading to the excitation of electrons to the conduction band of the HP-HT nanodiamond films and an increase of the local temperature of the sample, causing the transformation of sp3 HP-HT nanodiamonds to sp2-carbon.}, number={9}, journal={APPLIED SCIENCES-BASEL}, author={Mikheev, Konstantin G. and Mogileva, Tatyana N. and Fateev, Arseniy E. and Nunn, Nicholas A. and Shenderova, Olga A. and Mikheev, Gennady M.}, year={2020}, month={May} }
 @article{farias_brown_hearn_nunn_shenderova_khan_2020, title={Nanodiamond-stabilized Pickering emulsions: Microstructure and rheology}, volume={580}, ISSN={["1095-7103"]}, DOI={10.1016/j.jcis.2020.07.030}, abstractNote={We envisage the use of hydroxylated detonation nanodiamonds (ND-OH), a relatively novel carbonaceous filler with high adsorption activity, small size, and large surface area to create Pickering emulsions. The emulsion behavior under shear and the extent to which the microstructure can rebuild after breakdown is dependent on its yield stress. Using a model system consisting of isopropyl palmitate and water stabilized by ND-OH particles, we investigate the stability of these emulsions, their microstructure and rheological behavior as a function of ND-OH concentration. Confocal microscopy reveals that increasing ND-OH concentration results in smaller droplet sizes in the emulsions. This behavior is consistent with our rheological results of higher elastic modulus G′ and yield stress of the emulsion with increased ND-OH, as the presence of smaller droplets facilitates the formation of a densely packed network. We find the rheological behavior of these emulsions to be a hybrid of colloidal gels and surfactant-stabilized emulsions, with interparticle interactions and droplets deformability dictating their elasticity and yield stress behavior. Structure recovery following large shear reveals the degree of microstructure recovery to depend on the applied stress, with the recovered modulus collapsing into a single master-curve when the applied stress is scaled by the yield stress.}, journal={JOURNAL OF COLLOID AND INTERFACE SCIENCE}, author={Farias, Barbara V and Brown, Derek and Hearn, Allison and Nunn, Nicholas and Shenderova, Olga and Khan, Saad A.}, year={2020}, month={Nov}, pages={180–191} }