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Synthetic Diamond Materials for Emerging Quantum Technologies

Mary De Feudis

Published Oct 30, 2025• 0 citations
OpenAlex

Abstract

Progress in controlling diamond synthesis has made it possible to produce very pure crystals and to incorporate and control atomic impurities and carbon vacancies in the crystal lattice. The appropriate impurity-vacancy combinations can give rise to special defects known as color centers, such as the nitrogen-vacancy (NV) or the silicon-vacancy (SiV), which exhibit exceptional quantum properties with spins that are optically addressable. The use of diamond containing color centers has paved the way for the development of innovative quantum technologies, such as very high-resolution magnetometry and quantum information processing. Nevertheless, the development of quantum sensors at the nanometric scale remains one of the most interesting challenges. In this context, the development of a reliable nanoparticle synthesis technique that allows the desired defects to be incorporated and their quantum optical properties to be modulated is crucial. In this seminar, an efficient and recent method to produce diamond nanoparticles containing NV, SiV and germanium-vacancy (GeV) centers is proposed through direct plasma assisted chemical vapour deposition (CVD) growth. This technique allows good control of both nanoparticle growth processes and color centers formation. The optimization of the CVD nanodiamond growth process is presented, showing the impact of different plasma parameters and gas phasecomposition on nanoparticle crystallinity, size and shape. Morphological characterizations reveal the production of small (around 100 nm) and well faceted nanodiamond particles. In addition, the SiV and GeV color centers incorporation can be tuned within this material by varying the gas phase composition which emphasizes the high flexibility of the CVD growth technique. Finally, the quantum properties of the incorporated color centers have been successfully investigated under extreme pressure conditions (up to ~ 180 GPa) showing a monotonic blue shift of their optical zero-phonon line. Since their luminescence remains stable, SiV and GeV centers emerge as high-pressure nanosensors.

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Silicon has turned from a niche laboratory-scale material into the backbone of modern electronics. Diamond functionalized with color centers could follow a similar path and grow from proof-of-concepts experiments to application-oriented quantum technologies. Quantum sensing, communication, and information processing with color centers has been proven to work in scientific experiments and enhanced sensitivities as well as novel functionalities have been reported. While the first generation of diamond quantum devices relied on color centers formed uncontrolled in natural diamond with a high statistical variation in composition and crystallinity, the situation has changed nowadays. With the emergence of high purity monocrystals and the option for isotopic purification of the carbon matrix, technological progress in materials science on the atomic-scale enabled tailoring the spin properties in diamond. But, since many color centers typically require vacancies to combine with impurities in the carbon lattice via diffusion, the generation of color centers underlies a statistical process providing only little control on the center's final position in the crystal lattice. The negatively charged nitrogen-vacancy (NV$^-$) center is the most studied optically active defect in diamond and exhibits remarkable spin properties and sensing capabilities, even on the nanoscale and at room temperature. The spin properties of NV$^-$ centers close to the surface (1\,nm - 30\,nm) have been thoroughly studied and the deeper the spins are located in diamond the less decoherence from surface defects is observed. Creating NV$^-$ centers at the surface by nitrogen ion implantation and burying them beneath a thin diamond layer via chemical vapor deposition (CVD) of diamond is reported to enhance the coherence times significantly. By separating the color center generation from controlling the resulting depth by an additional diamond layer, narrow depth profiles can be achieved by reducing the ion energy during implantation. Although the coherence properties improve, many of the implanted NV$^-$ center are reported to vanish in the hydrogen-rich plasma of growth CVD chambers. The mechanism responsible for the instability of NV$^-$ centers towards the plasma remains unclear, though. One hypothesis is the formation of nitrogen-vacancy-hydrogen centers from diffusing hydrogen and implanted NV$^-$ centers but is has not been proven, yet. Here, we demonstrate the fabrication of depth confined NV$^-$ centers by combining low energy (1\,keV - 5\,keV) ion implantation and CVD overgrowth. By increasing the thickness of the overgrown layer (6\,nm - 100\,nm) the influence of surface noise is shown to decrease leading to bulk-like coherence properties of up to $450\,\upmu$s (Hahn-echo) and $T_2^*$ dephasing times around $20\,\upmu$s. Monitoring the NV$^-$ density as a function of the overgrowth time, we analyze the kinetics of the passivation in case of implanting ion doses of 10$^{11}$ $^{15}$N $^{+}$/cm$^2$ and 10$^{12}$ $^{15}$N $^{+}$/cm$^2$. The passivation turns out to occur faster for lower doses offering insights into sub-surface hydrogen diffusion in diamond. Analyzing the spin bath before and after overgrowth reveals a drastic reduction in paramagnetic spins after the CVD process but no NVH$^-$ is found around the surviving NV$^-$ centers. Checking the chemical composition in overgrown implantation sites with secondary-ion-mass-specrometry (SIMS), however, confirms significant amounts of hydrogen in the $^{15}$N-implanted layers. Since the concentration found for hydrogen exceeds the one for nitrogen by far, we suggest that diffusing hydrogen interacts also with vacancies forming probably mostly diamagnetic defects. In the last part of this dissertation, NV$^-$ centers are generated with sub-nanometer precision below a thin $^{13}$C-enriched layer for quantum simulation. For fabricating the spin architecture starting from a $^{12}$C base layer, we overgrow implanted nitrogen with a $^{13}$C layer and bury the spins beneath a 30\,nm-thick $^{12}$C capping layer. By measuring the coupling strength between the electron and nuclear spins, we deduce a NV$^-$ placement accuracy reaching below one nanometer with respect to the $^{13}$C-layer. Regarding the spin properties of the electron spins, no reduction in $T_2$ is observed despite of the proximity to the dense $^{13}$C-bath. As a consequence, the overgrowth of NV$^-$ centers is demonstrated as a bottom-up approach to produce buried spin architectures with sharp isotopic transitions and efficient $^{13}$C-NV$^-$ coupling. Together with the potential for bulk-like coherence properties the synergy of implantation and overgrowth is a powerful tool to fabricate reproducible quantum systems with tailored properties and high depth confinement.

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This thesis is focused on the production and characterization of diamond films and diamond nanoparticles containing custom colour centres. These defects intentionally created into the diamond lattice are promising candidates for single photon sources, and are becoming more important for quantum information technologies as photons can carry quantum information over long distances. With outstanding properties such as up to 80% of the photons emitted into the zero phonon line (ZPL), single photon count rates up to several Mcps under continuous excitation and a narrow ZPL at room temperature, the SiV centre has recently attracted more attention. Although different approaches for the creation of colour centres have been reported, this thesis details the creation of SiV centres by chemical vapour deposition. Colour centres are created through the incorporation of impurities during diamond growth. While Si doping can easily be achieved due to plasma etching of Si substrates or the incorporation of a Si solid source inside the reactor vacuum chamber, controlling the exact amount of Si present in the gas phase or effectively incorporated is still a handicap. Chapters five and six show comprehensive studies performed towards the creation of diamond nanoparticles with single emitters. Control over the Si content within the gas phase is achieved using silane as gas source. The subsequent milling of the diamond films hosting the SiV centres and their inclusion into a stable solution, will facilitate SiV coupling and incorporation into cavities or emission-enhancer devices.

•Jan 1, 2018•0 citations
Progress in controlling diamond synthesis has made it possible to produce very pure crystals and to incorporate and control atomic impurities and carbon vacancies in the crystal lattice. The appropriate impurity-vacancy combinations can give rise to special defects known as color centers, such as the nitrogen-vacancy (NV) or the silicon-vacancy (SiV), which exhibit exceptional quantum properties with spins that are optically addressable. The use of diamond containing color centers has paved the way for the development of innovative quantum technologies, such as very high-resolution magnetometry and quantum information processing. Nevertheless, the development of quantum sensors at the nanometric scale remains one of the most interesting challenges. In this context, the development of a reliable nanoparticle synthesis technique that allows the desired defects to be incorporated and their quantum optical properties to be modulated is crucial. In this seminar, an efficient and recent method to produce diamond nanoparticles containing NV, SiV and germanium-vacancy (GeV) centers is proposed through direct plasma assisted chemical vapour deposition (CVD) growth. This technique allows good control of both nanoparticle growth processes and color centers formation. The optimization of the CVD nanodiamond growth process is presented, showing the impact of different plasma parameters and gas phasecomposition on nanoparticle crystallinity, size and shape. Morphological characterizations reveal the production of small (around 100 nm) and well faceted nanodiamond particles. In addition, the SiV and GeV color centers incorporation can be tuned within this material by varying the gas phase composition which emphasizes the high flexibility of the CVD growth technique. Finally, the quantum properties of the incorporated color centers have been successfully investigated under extreme pressure conditions (up to ~ 180 GPa) showing a monotonic blue shift of their optical zero-phonon line. Since their luminescence remains stable, SiV and GeV centers emerge as high-pressure nanosensors.
Silicon has turned from a niche laboratory-scale material into the backbone of modern electronics. Diamond functionalized with color centers could follow a similar path and grow from proof-of-concepts experiments to application-oriented quantum technologies. Quantum sensing, communication, and information processing with color centers has been proven to work in scientific experiments and enhanced sensitivities as well as novel functionalities have been reported. While the first generation of diamond quantum devices relied on color centers formed uncontrolled in natural diamond with a high statistical variation in composition and crystallinity, the situation has changed nowadays. With the emergence of high purity monocrystals and the option for isotopic purification of the carbon matrix, technological progress in materials science on the atomic-scale enabled tailoring the spin properties in diamond. But, since many color centers typically require vacancies to combine with impurities in the carbon lattice via diffusion, the generation of color centers underlies a statistical process providing only little control on the center's final position in the crystal lattice. The negatively charged nitrogen-vacancy (NV−^-−) center is the most studied optically active defect in diamond and exhibits remarkable spin properties and sensing capabilities, even on the nanoscale and at room temperature. The spin properties of NV−^-− centers close to the surface (1\,nm - 30\,nm) have been thoroughly studied and the deeper the spins are located in diamond the less decoherence from surface defects is observed. Creating NV−^-− centers at the surface by nitrogen ion implantation and burying them beneath a thin diamond layer via chemical vapor deposition (CVD) of diamond is reported to enhance the coherence times significantly. By separating the color center generation from controlling the resulting depth by an additional diamond layer, narrow depth profiles can be achieved by reducing the ion energy during implantation. Although the coherence properties improve, many of the implanted NV−^-− center are reported to vanish in the hydrogen-rich plasma of growth CVD chambers. The mechanism responsible for the instability of NV−^-− centers towards the plasma remains unclear, though. One hypothesis is the formation of nitrogen-vacancy-hydrogen centers from diffusing hydrogen and implanted NV−^-− centers but is has not been proven, yet. Here, we demonstrate the fabrication of depth confined NV−^-− centers by combining low energy (1\,keV - 5\,keV) ion implantation and CVD overgrowth. By increasing the thickness of the overgrown layer (6\,nm - 100\,nm) the influence of surface noise is shown to decrease leading to bulk-like coherence properties of up to $450\,\upmu$s (Hahn-echo) and T2∗T_2^*T2∗​ dephasing times around $20\,\upmu$s. Monitoring the NV−^-− density as a function of the overgrowth time, we analyze the kinetics of the passivation in case of implanting ion doses of 1011^{11}11 15^{15}15N +^{+}+/cm2^22 and 1012^{12}12 15^{15}15N +^{+}+/cm2^22. The passivation turns out to occur faster for lower doses offering insights into sub-surface hydrogen diffusion in diamond. Analyzing the spin bath before and after overgrowth reveals a drastic reduction in paramagnetic spins after the CVD process but no NVH−^-− is found around the surviving NV−^-− centers. Checking the chemical composition in overgrown implantation sites with secondary-ion-mass-specrometry (SIMS), however, confirms significant amounts of hydrogen in the 15^{15}15N-implanted layers. Since the concentration found for hydrogen exceeds the one for nitrogen by far, we suggest that diffusing hydrogen interacts also with vacancies forming probably mostly diamagnetic defects. In the last part of this dissertation, NV−^-− centers are generated with sub-nanometer precision below a thin 13^{13}13C-enriched layer for quantum simulation. For fabricating the spin architecture starting from a 12^{12}12C base layer, we overgrow implanted nitrogen with a 13^{13}13C layer and bury the spins beneath a 30\,nm-thick 12^{12}12C capping layer. By measuring the coupling strength between the electron and nuclear spins, we deduce a NV−^-− placement accuracy reaching below one nanometer with respect to the 13^{13}13C-layer. Regarding the spin properties of the electron spins, no reduction in T2T_2T2​ is observed despite of the proximity to the dense 13^{13}13C-bath. As a consequence, the overgrowth of NV−^-− centers is demonstrated as a bottom-up approach to produce buried spin architectures with sharp isotopic transitions and efficient 13^{13}13C-NV−^-− coupling. Together with the potential for bulk-like coherence properties the synergy of implantation and overgrowth is a powerful tool to fabricate reproducible quantum systems with tailored properties and high depth confinement.
Abstract Diamond hosts a wide variety of colour centres that have demonstrated outstanding optical and spin properties. Among them, the nitrogen-vacancy (NV) centre is by far the most investigated owing to its superior characteristics, which promise the development of highly sophisticated quantum devices, in particular for sensing applications. Nevertheless, harnessing the potential of these centres mainly relies on the availability of high quality and purity diamond single crystals that need to be specially designed and engineered for this purpose. Plasma assisted chemical vapour deposition (CVD) has become a key enabling technology in this field of research. Nitrogen can indeed be directly doped in situ into a high crystalline quality diamond matrix in a controlled way, allowing the production of single isolated centres or ensembles that can potentially be integrated into a device. In this paper we will provide an overview of the requirements for synthesizing ‘quantum-grade’ diamond films using CVD. These include the reduction of impurities and surrounding spins that limit coherence times, the control of NV density in a wide range of concentrations as well as their spatial localization within the diamond. Enhancing the charge state and preferential orientation of the colour centres is also discussed. These improvements in material fabrication have contributed to positioning diamond as one of the most promising solid-state quantum systems and the first industrial applications in sensing are just starting to emerge.
Luminescent diamond of technological importance primarily originates from synthetic material as opposed to natural diamond. Detonation synthesis is an ideal method for bottom-up synthesis of nanoscale particulate diamond at large scale; it is unfortunately not currently suitable for the production of luminescent diamond hosting optical centers. Electron beam irradiation is a widely used approach for introducing vacancies in diamond particles. Optical centers in diamond are exceptional candidates for next generation solid-state quantum sensors. Techniques to produce synthetic diamond can be grouped into two broad categories: static synthesis and dynamic synthesis. High pressure high temperature synthesis attempts to mimic the growth conditions of natural diamonds formed deep within the Earth’s mantle. Chemical vapor deposition diamond is produced via plasma assisted growth on a heated substrate at either atmospheric or, reduced pressure using a gas phase carbon precursor in hydrogen. Quantum sensing enabled by optical centers in diamond primarily relies on magnetic interactions associated with the optical center and their surrounding environments.
Ensembles of nitrogen-vacancy (NV) centers in diamond are a leading platform for practical quantum sensors. Reproducible and scalable fabrication of NV-ensembles with desired properties is crucial. This work addresses these challenges by developing a chemical vapor deposition (CVD) synthesis process to produce diamond material at scale with improved NV-ensemble properties for a target NV density. The material reported in this work enables immediate sensitivity improvements for current devices. In addition, techniques established in this work for material and sensor characterization at different stages of the CVD synthesis process provide metrics for future efforts targeting other NV densities or sample geometries.
The performance of diamond color centers for quantum information and sensing depends strongly on the quality of the material. Specifically, ultra-pure, low-strain diamond is preferred. However, in order to create color centers, non-carbon atoms must be incorporated into the lattice either by rapid growth or implantation. Both these techniques cause defects and lattice damage that degrades ultimate performance. To overcome these problems, we show a molecule-seeded growth technique that decouples the doping and growth processes. The result is near-deterministic creation of specific color centers in chemically pure, low-strain diamond. Data showing selective growth of H3 and NV centers will be presented, where the NV is used to verify that the crystals are low-strain. Future application to other color centers like silicon, germanium, and tin vacancies will be discussed. Also the possibility of direct growth of more complex quantum registers.
This thesis is focused on the production and characterization of diamond films and diamond nanoparticles containing custom colour centres. These defects intentionally created into the diamond lattice are promising candidates for single photon sources, and are becoming more important for quantum information technologies as photons can carry quantum information over long distances. With outstanding properties such as up to 80% of the photons emitted into the zero phonon line (ZPL), single photon count rates up to several Mcps under continuous excitation and a narrow ZPL at room temperature, the SiV centre has recently attracted more attention. Although different approaches for the creation of colour centres have been reported, this thesis details the creation of SiV centres by chemical vapour deposition. Colour centres are created through the incorporation of impurities during diamond growth. While Si doping can easily be achieved due to plasma etching of Si substrates or the incorporation of a Si solid source inside the reactor vacuum chamber, controlling the exact amount of Si present in the gas phase or effectively incorporated is still a handicap. Chapters five and six show comprehensive studies performed towards the creation of diamond nanoparticles with single emitters. Control over the Si content within the gas phase is achieved using silane as gas source. The subsequent milling of the diamond films hosting the SiV centres and their inclusion into a stable solution, will facilitate SiV coupling and incorporation into cavities or emission-enhancer devices.