Coating with quantum sensors embedded therein and methods of making the same

Abstract

The invention relates to a coating of zirconium(IV) oxide, which is deposited on a substrate by a sol-gel process from a liquid phase with doped nanodiamonds distributed therein and, after drying, is converted into a crystalline state by a calcination process at elevated temperature, in which the zirconium(IV) oxide is predominantly present in its tetragonal phase.

Description

[0001] The invention relates to a method for producing a coating with nanodiamonds doped with foreign atoms embedded in the coating. Furthermore, the invention relates to such a coating and a substrate coated with such a coating. Furthermore, the invention relates to the use of such a coating.

[0002] Doping diamonds with specific foreign atoms leads to the formation of color centers, which exhibit characteristic fluorescence behavior. Interactions of the electrons involved in the color centers with the nuclear and electron spins of atoms in their immediate vicinity, as well as with external magnetic fields, lead to defined changes in this fluorescence behavior and can thus be used to measure various quantities. Since the interactions of individual electrons or their spin states with their environment are used for metrological purposes, these sensors are also referred to as quantum sensors. Doping atoms that can be used for such quantum sensing applications include nitrogen, silicon, or germanium.

[0003] The excitation of the color centers is typically achieved with laser light with a wavelength matched to the absorption behavior of the respective color center. The information sought is obtained from the intensity spectrum of the emitted fluorescent light measured in different measurement configurations, e.g., the position, width, and, if applicable, splitting of the fluorescence lines or their temporal change after short excitation pulses (pulse-probe measurement, e.g., for determining spin relaxation times).

[0004] In this way, the measurement of physical quantities such as temperature, pressure, magnetic flux density or near-surface drift velocity, as well as chemical quantities such as pH values ​​or the presence or concentration of certain chemical species in the immediate vicinity of the color centers (e.g. those bound by adsorption to a surface in the vicinity of the detecting color centers) is possible (see e.g. Yuchen Feng et al., Recent applications of fluorescent nanodiamonds containing nitrogen vacancy centers in biosensing, Functional Diamond 2022, Vol. 2 (1), 192-203 and D. Cohen et al., Utilising NV based quantum sensing for velocimetry at the nanoscale, Nature Scientific Reports (2020) 10, 5298). The quantum sensors can also be used as detectors (signal pickups) for magnetic resonance spectroscopy (NMR, ESR) (see, for example, Bucher et al., Quantum diamond spectrometer for nanoscale NMR and ESR spectroscopy.Nat Protoc 14, 2707-2747 (2019). In this case, polarization of the spins to be detected in the sample by an external magnetic field and excitation by electromagnetic waves in the radio to microwave range coupled into the sample are also required.

[0005] Due to the very short range of the spin interactions underlying these measurement methods, which are limited to length scales of a few nanometers up to several tens of nanometers, they are carried out with an extremely high spatial resolution. Furthermore, since the excitation and readout of the sensors takes place exclusively via light and possibly other electromagnetic waves or fields, and thus no physical electrical or mechanical connection of the sensors - e.g., via cables or mechanical connections - is required, the use of such quantum sensors is of particular interest in applications in which the respective quantities are to be determined in very confined and possibly physically difficult or even impossible spatial areas. These can be found, for example, in the fields of biology, biochemistry and medicine, where, for example, temperatures or chemical information are to be measured inside individual cells, narrowly confined tissue areas or tiny sample volumes, e.g.,on microtiter plates or in microfluidic systems (lab-on-a-chip).

[0006] Another attractive field of application is chemical reactors, especially microreactors. In the latter, chemical reactions take place within thin channels, through which the process media (reaction mixture) typically flow in a continuous stream. The channels typically have dimensions transverse to their longitudinal extent in the range of a few tens of micrometers to a few millimeters, which significantly complicates the implementation of macroscopic sensors, and in many cases even makes them impossible, and in most cases leads to at least a significantly limited spatial (and possibly also temporal) resolution of the respective measurement.

[0007] The use of (diamond-based) quantum sensors would significantly expand the possibilities for determining process-relevant variables directly in the process volume of the reactor with high spatial resolution. Since the excitation and readout of the quantum sensors takes place purely optically, simultaneous or short-time-scale serial scanning measurements at a large number of measuring points along the channel are also possible with only one excitation and readout unit. For example, the spatial temperature profile that develops along a reaction channel due to the reaction-induced heat release and which reflects the temporal heat release and thus the reaction rate during the reaction process could be recorded with high measuring point density and spatial resolution. This provides important information about the reaction process and thus allows conclusions to be drawn about the reaction kinetics.Similarly, it would be extremely useful to determine other variables, such as pressure, flow velocity, or, in particular, concentration values ​​of selected chemical species involved in the reaction along the flow / reaction path during the ongoing process. More generally, useful applications of such quantum sensors in other devices for conducting physical, chemical, or biological processes—especially in fluid media—are also conceivable.

[0008] A fundamental prerequisite for such use of diamond-based quantum sensors, especially in chemical (micro) reactors, is the stable attachment or fixation of the sensors in the device. This must offer high mechanical stability to prevent detachment of the quantum sensors due to flow effects, abrasion, or mechanical influences during handling (e.g., assembly, cleaning, maintenance) of the device. Likewise, the fixation must be highly chemically stable to prevent it from being damaged or dissolved by corrosive influences of the process media. To cover the broadest possible range of applications, the fixation must therefore also withstand long-term exposure to strong acids, alkalis, and concentrated salt solutions, as well as the broadest possible range of organic substances, especially common organic solvents.

[0009] This high chemical stability must also be maintained over a wider temperature range, which ideally is limited only by the chemical-thermal stability of the quantum sensors themselves, i.e., the doped diamond(s) or the color centers contained therein, or that of the substrate (wall material of the device) to which they are applied. At a minimum, the stable application range of the fixation should extend over a temperature window up to approximately 350 °C, which covers the majority of applications for reactions in liquid media. The stability requirements relate in particular to the application of the quantum sensors to surfaces and materials, as are common in chemical or microprocess engineering apparatus construction, such as:Glass, sapphire, silicon carbide or other oxide, carbide or nitride ceramic materials, as well as stainless steels, nickel-based materials (including nickel), titanium, tantalum, zirconium or other refractory metals and their alloys.

[0010] In particular, for the measurement of concentrations of certain species in the process medium (including the determination of pH values) or on the surface of the apparatus wall to the process volume (e.g. the process channel wall in a microreactor), the quantum sensors must be fixed in such a way that at least a significant proportion of the color centers used for the measurement are in sufficiently close contact with the process medium, i.e. are not covered by passive material or are covered only to a very small thickness (max. a few nanometers).

[0011] One way to stably apply diamond to surfaces with dopants suitable for quantum sensing applications is to deposit and grow a thin diamond film directly onto the target surface. For this purpose, known chemical vapor deposition (CVD) methods exist. In these methods, in addition to the carbon carrier (e.g., methane), additional species (e.g., hydrogen) that are highly excited thermally or by microwave plasma promote preferential deposition of diamond over graphite, which is thermodynamically more stable under the deposition conditions (e.g., flame, hot filament, plasma jet, or plasma-enhanced CVD). The dopant atoms can be introduced, for example, by using suitable precursors during diamond growth or by ion implantation into the finished diamond layer.These processes involve a high level of equipment complexity and lead to high thermal stress on the substrate at temperatures in the range of 400 °C to 1200 °C, and in the case of the plasma jet process, possibly even significantly higher.

[0012] It has proven to be simpler in terms of equipment to apply doped diamond particles to the target surfaces and fix them there. For reasons of availability and material / cost efficiency, nanodiamonds, i.e., diamond particles with dimensions in the sub-micrometer range, are preferred for this purpose. For example, GB 258 67 05 A describes a fluorescence sensor for determining various quantities, in particular the concentrations of certain analytes, based on fluorescent nanoparticles, which can be nanodiamonds, among others, and which are embedded in a "matrix material." The matrix materials can be hydrogels, e.g., cellulose-based, which preferably exhibit a certain permeability to the respective analyte to promote its contact with the fluorescent nanoparticles.However, the disclosed matrix materials do not exhibit satisfactory chemical, thermal and mechanical stability to be used permanently in a wider range of chemical processes.

[0013] CN 101 585 534 B describes a process for producing thin layers containing nanodiamonds, which is incorrectly referred to here as a "sol-gel process" because only ethylcellulose, graphite, and terpineol are mentioned as starting materials for the matrix material. These materials are dried at temperatures of up to 620 K and apparently carbonize at least partially in the process. Due to its opacity, the disclosed layer is not suitable, or at best only very limited, for sensor applications using light.

[0014] EP 230 34 71 B1 relates to a method for applying a thin layer of nanodiamonds to gemstones in order to improve their optical appearance and surface hardness. The nanodiamonds are first deposited on the substrate surface from a dispersion, possibly containing a sol, but are not fixed in place with satisfactory chemical stability. To improve the stability of the fixation, EP 230 34 71 B1 proposes the application of a covering layer made of a particularly hard material as an optional subsequent step, which, however, requires significantly increased equipment complexity. Furthermore, sensory applications are not possible, or at best only to a limited extent, due to the covering layer completely covering the near-surface nanodiamonds.

[0015] A sol-gel process using a tetraethyl orthosilicate-based sol is described in CN 112 146 782 B for fixing nitrogen-doped nanodiamonds for use as quantum sensors. However, satisfactory chemical resistance cannot be achieved with this process either.

[0016] JP 2009 102 188 A discloses a silicate-based glass material infused with nanodiamonds, which can also be used for coating purposes, as well as the production of the glass material using sol-gel methods. The nanodiamonds primarily serve to improve the mechanical properties of the resulting composite. Due to the limited chemical resistance of the layer containing the nanodiamonds, they are also unsuitable, or only partially suitable, for use as quantum sensors in chemical applications.

[0017] The present invention is therefore based on the object of designing and developing the method, the coating, the coated substrate and the use of the type mentioned at the outset and explained in more detail above in such a way that a fixation of doped nanodiamonds on solid surfaces can be provided which has a high mechanical stability and a high resistance to a wide selection of chemicals over a wide temperature range, can be applied using simple and cost-effective methods and enables the nanodiamonds to have as little restricted contact as possible with fluid media in the environment.

[0018] The object is achieved according to claim 1 by a method for producing a coating with nanodiamonds doped with foreign atoms embedded in the coating, in which a zirconium-containing sol with doped nanodiamonds dispersed in the sol is applied to a substrate and dried, and in which zirconium(IV) oxide is formed and crystallized in the tetragonal phase by heat treatment of the dried sol at temperatures of at least 400 °C.

[0019] The stated object is further achieved according to claim 6 by a coating comprising zirconium(IV) oxide crystallized in the tetragonal phase with doped nanodiamonds embedded therein, preferably produced by a method according to one of claims 1 to 5. The above-mentioned object is also achieved according to claim 9 by a coated substrate with a coating according to one of claims 6 to 8, characterized in that the coating is applied to a substrate made of glass, sapphire, an oxide, carbide or nitride ceramic material, stainless steel, nickel, a refractory metal or a base alloy of nickel or a refractory metal and / or that the substrate is part of an apparatus for carrying out physical, chemical or biological processes, in particular a chemical or biological reactor.

[0020] In addition, the above-mentioned object according to claim 10 is achieved by using a coating according to one of claims 6 to 8 or a coated substrate according to claim 9 for measuring physical or chemical quantities such as temperature, pressure, fluid velocity, magnetic flux density, pH value or concentration of individual or multiple substances.

[0021] The coating is deposited on the substrate using a sol-gel process from a liquid phase containing doped nanodiamonds suspended or dispersed therein and dried. The coating is then converted into a crystalline state at elevated temperature, in which the zirconium is present as zirconium(IV) oxide, at least partially in its tetragonal phase.

[0022] The zirconium-based sol required for the coating can be produced according to recipes known to those skilled in the art. For example, a zirconium alkoxide, such as zirconium(IV) butoxide or zirconium(IV) propoxide, is first mixed with an alcohol, e.g. ethanol or 2-methoxyethanol, and a complexing agent, such as acetylacetone or 1-benzoylacetone, and then acidified water is added. Nitric acid, for example, can be used to acidify the water. The mixture is stirred at room temperature for an extended period of time - e.g. several hours to days - during which the sol forms by hydrolysis. Other known processes, for example, are in which the sol is formed by adding aqueous ammonia solution to an aqueous or aqueous-alcoholic solution of zirconyl chloride, optionally mixed with complexing agents such as citric acid.

[0023] To promote the formation of the tetragonal crystal phase in the subsequent coating at the lowest possible calcination temperature, it is advantageous to add a certain amount of a soluble yttrium salt, for example, yttrium(III) nitrate hexahydrate, yttrium(III) chloride, yttrium(III) bromide, or yttrium(III) iodide, to the sol. The amount of this yttrium salt in the sol can preferably be such that between 0.01 mol and 0.1 mol, preferably between 0.03 mol and 0.06 mol, of yttrium is used per mole of zirconium. Before adding the doped nanodiamonds to the sol, it is advantageous to filter it to remove any impurities and flocculation that may have formed during preparation and would interfere with the subsequent layer application.

[0024] Nanodiamonds are diamonds whose individual particle dimensions are less than one micrometer in at least one spatial direction. Nanodiamonds doped with foreign atoms, which have a dimension between 10 nm and 100 nm in at least one spatial direction, are preferably used as quantum sensors. In this context, doping means the introduction of foreign atoms into the diamond lattice with the aim of generating color centers—local irregularities in the diamond lattice that can absorb light. Color centers that also exhibit fluorescence in the visible light wavelength range or in the infrared range are preferred. This is achieved, for example, by doping with nitrogen, silicon, or germanium.

[0025] The doping strength, i.e. the concentration of doping atoms in the lattice of the nanodiamonds, depends on the application and, where applicable, on the spatial distribution of the doping atoms within the diamond particles. Typical values ​​are in the range of approximately 1 ppm to 10 ppm, but significantly lower or significantly higher values ​​are also possible. Sometimes it is desirable to have only one active color center in each nanoparticle, e.g. to avoid statistical superposition of fluorescence signals caused by different influences on different color centers by targeted observation of individual color centers. In other cases it can be useful to achieve the highest possible density of color centers, e.g. to achieve a high signal strength by simultaneously recording the fluorescence signals of several color centers or to average over a spectrum of local environmental influences.In addition to doping, the nanodiamonds can be provided with chemical surface functionalization, e.g. to change their wetting properties towards the sol, their dispersion behavior or their adhesion properties on the substrate or towards the coating in which they are embedded, and to adapt them to the conditions of the coating process or the subsequent applications.

[0026] The doped nanodiamonds can be added directly to the sol in powder form, or they can first be pre-dispersed in a suitable solvent, e.g., an alcohol. Pre-dispersion can facilitate the handling of the small amounts of doped nanodiamonds required for the process, particularly in laboratory or small-scale production. Their concentration in the sol is typically in the range of 0.01 µg / mL to 100 µg / mL, depending on the size of the doped nanodiamonds, layer thickness, and desired areal density of diamond particles in the coating. To disperse the nanodiamonds in the sol or in the pre-dispersion, it can be advantageous to subject the dispersion to ultrasound for a certain period of time, e.g., one hour or longer, in order to break up any aggregates that may be present.

[0027] The sol can be applied to the substrate using methods known to those skilled in the art, which allow liquid media to be applied to solid substrates with as uniform a thickness as possible. These include, for example, dip coating, spin coating, doctor blade coating, spraying, roller coating, or screen printing. Dip and spin coating have proven particularly effective for producing coatings with a uniform thickness within the desired range, particularly for planar or slightly curved substrates with an otherwise smooth surface. The thickness of the coating in its final crystalline state should be of a similar order of magnitude to the average minimum dimension of the nanodiamonds embedded therein. In particular, the layer thickness should not be greater than five times, and preferably less than three times, the average dimension of the nanodiamonds embedded therein.The latter can be determined, for example, by dynamic light scattering (DLS) or by measuring suitably sized samples with a scanning or transmission electron microscope (SEM / TEM). In the case of dip coating, the process parameters required to achieve a specific thickness of the (still liquid or wet) sol layer can be estimated using the following equation: . d 0 = 0,8 ⋅ η ⋅ U 0 ρ ⋅ g

[0028] Herein refers d 0 the thickness of the still moist sol layer immediately after its application, n the dynamic viscosity of the sol, ρ its density and U 0 is the speed at which the substrate is pulled from the sol. g = 9.81 m / s 2< is the acceleration due to gravity. The thickness of the crystalline coating in the final state, for a pure zirconium oxide layer, is calculated as follows: d = M ZrO 2 ρ ZrO 2 ⋅ c Zr 4 + ⋅ d 0

[0029] Herein designate M(ZrO 2 ) = 0.123 kg / mol is the molar mass and ρ (ZrO 2 ) = 6.1 kg / L is the density of the (tetragonal) zirconium oxide and c (Zr 4+< ) is the concentration of zirconium in the sol (in mol / L).

[0030] After the sol layer has been applied, it must first be dried. This can take place at room temperature or - followed by - a further drying step at elevated temperature. The choice of drying time and temperature depends on the composition of the sol, in particular the volatile components it contains. Gel formation also occurs during the drying process. After drying, the coating must be calcined, i.e. subjected to a thermal treatment at temperatures above approximately 400°C. Preferred temperatures are in the range from 500°C to 800°C, particularly preferably in the range from 550°C to 650°C. The duration of the heat treatment is between 30 minutes and several hours, preferably 1 hour to 3 hours. Calcination is preferably carried out under a protective gas, e.g. nitrogen or argon.

[0031] Since the zirconium(IV) oxide determines the essential mechanical and chemical properties of the coating and in particular contributes significantly to the chemical resistance of the coating, it is preferred if at least 70 mass percent, preferably at least 80 mass percent, in particular at least 90 mass percent of the coating is formed by zirconium(IV) oxide.

[0032] The substrate can preferably be a workpiece made of glass, sapphire, an oxide, carbide, or nitride ceramic material, stainless steel, nickel, a refractory metal, or a base alloy of nickel or a refractory metal. The coating can form a permanent bond with these substrates. Such substrates are also suitable for forming at least part of an apparatus for conducting physical, chemical, or biological processes in a fluid medium, in particular a chemical reactor. The advantages of the coating are particularly evident in connection with such components.

[0033] In order to provide a sufficiently chemically resistant coating for embedding the nanodiamonds, it is advisable that at least 30%, preferably at least 50%, in particular at least 70%, of the zirconium(IV) oxide of the coating is crystallized in the tetragonal phase.

[0034] The crystallization of zirconium(IV) oxide in the tetragonal phase can be promoted if up to 10 mol percent of the zirconium contained in the coating is replaced by yttrium. Regardless of the extent of zirconium(IV) oxide replacement, it may be advantageous to use a sol containing a soluble yttrium compound, preferably yttrium(III) nitrate hexahydrate, yttrium(III) chloride, yttrium(III) bromide, and / or yttrium(III) iodide, for deposition on the substrate. In this case, sufficient replacement of zirconium(IV) oxide by yttrium(III) can occur during crystallization. For the reasons mentioned above, sols are generally suitable for deposition on the substrate, in which a ratio of yttrium(III) to zirconium(IV) between 0.01 and 0.11, preferably between 0.03 and 0.06, is previously set.

[0035] Effective coatings with efficient use of nanodiamonds can be achieved if the coating thickness is no more than five times, preferably no more than three times, the average minimum dimension of the embedded nanodiamonds. If the nanodiamonds are approximately spherical in shape, their minimum dimension can be considered their respective diameter. If the nanodiamonds are elongated or flattened rather than spherical, i.e., have different dimensions in different directions, the smallest dimension of the individual nanodiamonds is used to determine the average minimum dimension. This is then preferably greater than one-fifth, particularly preferably greater than one-third, of the average coating thickness.

[0036] To use the coating to form a sensor, it is advisable, depending on the sensory application, if at least some of the doped nanodiamonds are partially uncovered by the surrounding matrix material of the coating. In principle, it is preferable if as large a proportion of the doped nanodiamonds as possible are partially uncovered by the surrounding matrix material, without them losing contact with the matrix material and thus its fixing effect. The nanodiamonds can then form part of the surface of the coating and thus come into direct contact with the adjacent medium. The nanoparticles can partially protrude from the surrounding coating, but this is not absolutely necessary. The matrix material of the surrounding coating should therefore not fully cover all or almost all of the nanodiamonds.because such covered nanodiamonds provide no or at best reduced sensory properties depending on the application.

[0037] A preferred use for a coating of the aforementioned type is for measuring physical or chemical quantities such as temperature, pressure, fluid velocity, magnetic flux density, pH value, or the concentration of individual or multiple substances. These quantities can be determined with the coating, particularly with very high spatial resolution. Example:

[0038] The following example describes a proven, possible procedure for producing a coating according to the invention on planar substrates made of (soda-lime) glass or sapphire, without limiting the process to specific process steps or parameter values: Preparation of the sol:

[0039] 15 g of an 80% solution of zirconium(IV) butoxide in 1-butanol, along with 35.69 g of 2-methoxyethanol and 2.19 g of acetylacetone, are placed in a glass vessel and stirred at room temperature for approximately half an hour. Then, 4.51 g of deionized water acidified to pH 2 with nitric acid are added, and the mixture is stirred at room temperature for another 2 hours. Finally, 4.35 g of a solution of 0.766% wt. yttrium(III) nitrate in 2-methoxyethanol is added, and the mixture is stirred overnight. Immediately before further processing, the required amount of the resulting sol is filtered through a PTFE filter with a pore size of 0.45 µm. Incorporation of doped nanodiamonds into the sol:

[0040] 3.5 mg of nanodiamonds with nitrogen vacancies and an average particle size (hydrodynamic diameter determined by dynamic light scattering) of approximately 100 nm are placed in a beaker containing 3 mL of 1-butanol and dispersed in an ultrasonic bath for one hour until a slightly milky-opalescent suspension is formed. 2.42 mL of the suspension is added to 75 mL of the sol prepared according to the recipe described in the previous section and stirred for approximately 30 minutes. Preparation of the substrate and application of the sol layer:

[0041] The test substrates used included microscope slides and cover glasses made of soda-lime glass for microscopy and round, polished sapphire disks with a diameter of 25.4 mm and a thickness of 0.5 mm. The substrate is first cleaned in water with a little household detergent in an ultrasonic bath for 10 minutes, then rinsed successively with deionized water and acetone and finally treated in ethanol for a further 15 minutes in the ultrasonic bath. Immediately after removal from the bath, adhering ethanol residues are blown off using oil-free compressed air and the substrate is dried in this way. The substrate prepared in this way is attached in a vertically hanging position to the lifting unit of the dip coater and immersed in the sol. After remaining in the lower immersion position for approximately one minute, the substrate is pulled completely vertically upwards out of the sol at a uniform speed of approximately 0.1 to 0.3 mm / s. Drying and calcination of the coating:

[0042] After removal from the sol, the coating is first dried for approximately 1.5 hours at room temperature and then for 30 minutes in a drying cabinet at 70 °C. Further heat treatment takes place in a tube furnace under a gentle nitrogen stream. The temperature is initially raised at 300 K / h to 250 °C, where a dwell time of 15 minutes occurs. The temperature is then further increased at 300 K / h to the calcination temperature of 600 °C, which is held for 2 hours. This is followed by uncontrolled cooling to room temperature at the natural cooling rate of the furnace.

[0043] The resulting layers of yttrium-stabilized zirconium oxide mixed with nitrogen-doped nanodiamonds are characterized by a homogeneous thickness of approximately 70 nm to 150 nm, high hardness, and good substrate adhesion, thus offering scratch resistance, high chemical resistance, optical transparency, and a virtually crack-free surface. Their ceramic matrix is ​​essentially in the tetragonal crystal phase of zirconium oxide, as demonstrated by X-ray diffraction measurements (see figures). A coating produced in this way showed no visible changes after two days of aging in 80% sulfuric acid at room temperature. The doped nanodiamonds embedded in the layer still exhibited the typical fluorescence behavior of the nitrogen vacancy centers they contained, even after undergoing the thermal treatment during the calcination process.An aqueous solution of gadolinium(III) chloride applied to the layer surface led to measurable changes in the spin relaxation behavior of the nitrogen vacancy centers, which demonstrates a high sensitivity of the embedded quantum sensors and thus their close proximity to the applied sample liquid.

[0044] The invention is explained by way of example with reference to the following figures, but is not limited thereto.

[0045] They show: Figure 1 shows a schematic section of a coating according to the invention with doped nanodiamonds embedded therein on a substrate, Figure 2 shows the result of the X-ray diffractometric examination (theta-2-theta plot) of a coating made of zirconium(IV) oxide (without nanodiamonds) on a glass substrate, produced analogously to the process according to the invention, Figures 3 and 4 show scanning electron micrographs of a coating according to the invention made of zirconium(IV) oxide with doped nanodiamonds embedded therein, Figure 5 shows an image taken with a confocal laser fluorescence microscope and a photoluminescence spectrum (inserted diagram) of a coating according to the invention made of zirconium(IV) oxide with doped nanodiamonds embedded therein,nitrogen-doped nanodiamonds on a glass substrate, and Figure 6 shows the temporal fluorescence intensity profile of an ensemble of nitrogen vacancy centers in a coating according to the invention made of zirconium(IV) oxide with doped nanodiamonds embedded therein on a glass substrate with and without gadolinium(III) chloride solution applied thereto, measured by means of the pump-probe technique.

[0046] Figure 1 shows a schematic cross-section through the substrate (1) and a coating (2) according to the invention applied thereon, made of crystalline zirconium(IV) oxide in the tetragonal crystal phase with doped nanodiamonds (3) embedded therein. In this example, some of the doped nanodiamonds (3a) are not completely covered by the coating material and thus have direct contact with the medium adjacent to the coating.

[0047] In Figure 2The result of an X-ray diffraction theta-2theta scan of a zirconium(IV) oxide layer approximately 120 nm thick, produced using a sol-gel process and dip coating and calcined at 600 °C, on a soda-lime glass substrate is shown. The resulting diffraction reflections clearly indicate the presence of the tetragonal phase of the layer material, the expected positions of which are marked here by diamond symbols above the intensity maxima. No evidence of the presence of other crystalline phases can be found.

[0048] The Figures 3 and 4Scanning electron micrographs of a zirconium(IV) oxide layer according to the invention, produced using a sol-gel process and dip coating and calcined at 600 °C, with embedded (doped) nanodiamonds are shown. White arrows mark positions where carbon-containing particles were detected by EDX analysis (energy-dispersive X-ray spectroscopy). These particles can largely be assumed to be the doped nanodiamonds introduced during layer production, several of which protrude from the layer surface. Comparative EDX scans for carbon (Kα1.2) and zirconium (Lα1.2) at various locations on such samples (not shown here) also show that at least some of the diamond particles in this sample are not covered by zirconium oxide.

[0049] In Figure 5A 100 × 100 µm 2< section of the surface of a zirconium(IV) oxide layer according to the invention, produced using a sol-gel process and dip coating and calcined at 600 °C, with doped nanodiamonds embedded therein, is shown, as viewed under a confocal laser fluorescence microscope. The sample was illuminated in a raster pattern using a green laser (λ = 532 nm), and the intensity of the fluorescence radiation induced in the nitrogen vacancy centers was measured using a long-pass filter (λ c = 650 nm) to suppress the excitation light. The fluorescence centers are thus displayed as bright spots in the raster image.The diagram in the lower right quadrant of the image shows the photoluminescence spectrum of one of the luminous points in the image field, which identifies it as a nitrogen vacancy center and thus provides evidence that the doped nanodiamonds at least partially retain their quantum optical properties during the layer production according to the invention.

[0050] Figure 6shows the intensity curve of the fluorescence signal of an ensemble of nitrogen vacancy centers in a zirconium(IV) oxide layer according to the invention, produced by a sol-gel process and dip coating and calcined at 600 °C, with doped nanodiamonds embedded therein, as a function of the time interval between a preceding laser pulse for spin polarization and the actual readout pulse for excitation of the nitrogen vacancy centers ("pump-probe measurement"). From this signal curve, the spin relaxation time of the fluorescence centers can be determined, which in the case shown here consists of two components ( τ 1,long and τ 1,short ). To test the sensitivity of the quantum sensors to spin interactions from their immediate environment, the measurement was first performed with the bare sample and then repeated. In the case of the second series of measurements, the sample was wetted with an aqueous solution of gadolinium(III) chloride. Gd(III) ions are strongly paramagnetic due to their seven unpaired electrons in the f-shell and, with sufficiently close contact (over a few nanometers) to the nitrogen vacancy centers, cause accelerated spin-lattice relaxation and consequently a shortening of the relaxation time. τ 1 , which in the Figure 6This is clearly evident from the measurement results presented here. These therefore confirm that the doped nanodiamonds embedded in the coating according to the invention investigated here have at least partially very close or even direct contact with the surface of the coating or adjacent media and can thus interact with them as quantum sensors. List of reference symbols:

[0051] 1. Substrate 2. Layer of tetragonal crystalline zirconium(IV) oxide 3. Doped nanodiamonds 3.a Doped nanodiamonds protruding from the coating and uncovered by layer material

Claims

1. A method for producing a coating with nanodiamonds doped with foreign atoms embedded in the coating, in which a zirconium-containing sol with doped nanodiamonds dispersed in the sol is applied to a substrate and dried, and in which zirconium(IV) oxide is formed and crystallized in the tetragonal phase by heat treatment of the dried sol at temperatures of at least 400 °C.

2. The method according to claim 1, wherein the coating is formed of at least 70 mass percent, preferably at least 80 mass percent, in particular at least 90 mass percent of zirconium(IV) oxide.

3. A process according to claim 1 or 2, wherein at least 30%, preferably at least 50%, in particular at least 70%, of the zirconium(IV) oxide of the coating in the final state is crystallized in the tetragonal phase.

4. Process according to one of claims 1 to 3, in which the sol contains, in addition to zirconium, a sol-soluble yttrium compound, preferably yttrium(III) nitrate hexahydrate, yttrium(III) chloride, yttrium(III) bromide and / or yttrium(III) iodide, and / or in which a molar ratio of yttrium(III) oxide to zirconium(IV) oxide between 0.01 and 0.11, preferably between 0.03 and 0.06, is set in the coating.

5. Method according to one of claims 1 to 4, characterized in that the calcination of the dried sol-gel layer takes place at temperatures between 500 °C and 800 °C, preferably between 550 °C and 650 °C.

6. A coating comprising zirconium(IV) oxide crystallized in the tetragonal phase with doped nanodiamonds embedded therein, preferably produced by a process according to any one of claims 1 to 5.

7. Coating according to claim 6, characterized in thatthe thickness of the coating is not more than five times, preferably not more than three times, the average minimum dimension of the nanodiamonds embedded therein.

8. Coating according to claim 6 or 7, characterized in that at least a portion of the doped nanodiamonds embedded therein are partially uncovered by the surrounding matrix material of the coating.

9. Coated substrate with a coating according to any one of claims 6 to 8, characterized in that the coating is applied to a substrate made of glass, sapphire, an oxide, carbide or nitride ceramic material, stainless steel, nickel, a refractory metal or a base alloy of nickel or a refractory metal and / or that the substrate is part of an apparatus for carrying out physical, chemical or biological processes, in particular a chemical or biological reactor.

10. Use of a coating according to any one of claims 6 to 8 or of a coated substrate according to claim 9 for measuring physical or chemical quantities such as temperature, pressure, fluid velocity, magnetic flux density, pH value or concentration of one or more substances.

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