arXiv subjects

Vladimir Verkhovlyuk

Publications and source records attributed to Vladimir Verkhovlyuk.

4 recordsLinked to original sources

arXiv2026

Solution-phase fluorination of nanodiamond: near-surface NV$^-$ activation and spin relaxation

Nanodiamonds hosting luminescent point defects, known as fluorescent nanodiamonds (FND), are a leading platform for quantum technology. The nitrogen-vacancy (NV) centre is the most intensively studied of these; its negatively charged state (NV$^-$) can serve as a qubit at room temperature, and its stability is governed by surface functional groups. We present two solution-phase fluorination routes for stabilising NV$^-$: direct C-F bond formation by decarboxylation with xenon difluoride via a radical mechanism, and the Balz-Schiemann reaction, which replaces surface amino groups with fluorine. The two routes were compared by infrared, X-ray photoelectron, energy-dispersive X-ray, Raman and photoluminescence spectroscopy. Both gave a high NV$^-$ fraction, up to $\sim$90% on average and approaching 100% in fluorine-rich regions, which to our knowledge is among the highest reported for surface-terminated nanodiamonds of this size and, in particular, for fluorine termination. Frequency-domain relaxometry shows that the fluorinated particles retain a long spin-lattice relaxation time, $733\pm56$ and $712\pm20$ $μ$s for the XeF$_2$ and Balz-Schiemann routes, several times the values reported for commercial HPHT nanodiamonds, although shorter than the $1173\pm123$ $μ$s of the as-received material. Charge-state stability and spin lifetime therefore do not improve together: fluorination activates near-surface NV$^-$ centres, which are the most exposed to surface noise but also the ones that dominate relaxometric sensing.

quant-ph↗

arXiv2026

Materials and Spin Characteristics of Nanodiamonds Partially Covered with Amino Groups and Embedded with Nitrogen-Vacancy Color Centers

Fluorescent nanodiamonds (FNDs) with optically read qubits hold great potential for detecting electric and magnetic fields, temperature, and other nanoscale physicochemical quantities relevant to chemistry and biology. Proper surface functionalization is essential for their application as probes, but surface modifications can impact qubit sensor properties. We systematically study nitrogen-vacancy (NV) color centers in FNDs as a function of size and surface termination. FNDs were produced from high-pressure, high-temperature diamonds, with NV centers introduced via electron irradiation and annealing. The initial oxygen-covered FNDs were homogenized with hydroxyl (-OH) groups as reference samples, while the noninvasive Hofmann degradation introduced amino (-NH2) groups for potential direct biomolecule attachment. Amino groups may not cover the nanodiamonds homogeneously, but we label them as -NH2 terminated throughout. We monitored charge state stability and the zero-field splitting parameters of the embedded NV centers. First, we resolve the size dependence of the NV(-) zero-field splitting parameters across the 10-140 nm range and show that the symmetry-breaking E parameter decreases monotonically from about 8 to about 5 MHz with increasing size while the axial D parameter is shifted only in the smallest (<= 30 nm) particles, thereby disentangling the static-strain and fluctuating electric-field contributions to the spin levels. Second, while NV charge state stabilization was observed in both -OH- and -NH2-terminated FNDs above a certain size, we demonstrate that a remarkably high and laser-power-independent NV(-) content (f_NV(-) of about 0.8) is achieved by wet-chemical Hofmann amino termination only in 140 nm particles, an effect we link through electron spin resonance to the degradation of surface paramagnetic defects rather than to the introduction of new ones.

quant-ph↗

arXiv2026

Quantum Relaxometry Under Continuous Wave Excitation

Quantum relaxometry is one of the most successful applications of nitrogen-vacancy (NV) centers in diamond and, more broadly, solid-state spin qubits, enabling ultrasensitive detection of magnetic noise and paramagnetic species via measurements of the spin-lattice relaxation time $T_1$. Conventional pulsed protocols, however, probe $T_1$ efficiently only over a limited temporal range, which restricts the scope and throughput of the technique. Here we introduce a continuous-wave quantum relaxometry protocol that operates in the frequency domain. By measuring the frequency response of the optically detected magnetic resonance signal under low-frequency microwave amplitude modulation, we extract $T_1$ from the characteristic response time of the spin system. The method enables efficient $T_1$ measurements spanning more than three orders of magnitude -- directly demonstrated from 60 $μ$s to 200 ms in our experiments -- across a broad temperature range and under substantial ensemble inhomogeneity. We further show that this protocol enables quantitative relaxometry-based sensing in nanodiamonds, achieving a substantial speed-up over the pulsed methods and offering a practical approach to optimizing nanodiamond size for enhanced sensitivity.

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arXiv2025

A Coherence-Protection Scheme for Quantum Sensors Based on Ultra-Shallow Single Nitrogen-Vacancy Centers in Diamond

Recent advances in the engineering of diamond surfaces make it possible to stabilize the charge state of 7-30 nanometers deep nitrogen-vacancy (NV) quantum sensors in diamond and to remove the charge noise at the surface principally. However, it is still a challenge to simultaneously increase the action volume of the quantum sensor by placing NV centers 0.5-2 nanometers deep and to maintain their favorable spin coherence properties which are limited by the magnetic noise from the fluctuating nuclear spins of the surface termination of diamond. Here we show by means of first principles simulations that leveraging the interplay of the surface-induced strain and small constant magnetic fields, the spin coherence times of the ultra-shallow 1-nanometer deep NV center can be significantly enhanced near the spin-phonon limited regime at room temperature in $^{12}$C enriched diamonds. We demonstrate that our protocol is beneficial to $\sim$10-nanometers deep NV centers in natural diamond too where the variable coherence properties of the center to the direction of the small constant magnetic fields establish vector magnetometry at the nanoscale.

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