Quantum technologies

Abstract

A plurality of molecular clusters, each cluster comprising a network of Z and D coated with stabilising ligands selected from L, X and combinations; wherein in a substantial proportion of the molecula

Description

The present invention relates to the production of a plurality of colloidal quantum dots, of which a significant proportion are singly-doped, that are useful for quantum technologies. BACKGROUND Quantum dots are of interest for applications in quantum technologies, in particular, singly-doped quantum dots are regarded as having great potential for optically-addressable spin-qubits. Quantum confinement of photo-generated carriers within the quantum dot increases their overlap with the dopant, resulting in strong coupling, thereby enhancing optical excitation and readout of spin-states. One particular application is as highly efficient photon-spin interfaces for devices such as quantum repeaters and quantum memories and so quantum dots are envisaged as being able to extend the range of quantum communications and enable distributed quantum computing. Known quantum devices use epitaxially and lithographically synthesised quantum dots. However, these quantum dots are laborious to produce and are highly limited in how they may be incorporated into devices. Colloidal quantum dots exhibit many of the useful qualities of these other quantum dots, while permitting production on a larger scale, having readily tuneable wavelengths by tuning their size, and having ready applicability to solution-based processing. Colloidal quantum dots also have a much lower incidence of dopant spin-lifetime reducing defect sites that affect other quantum dots due to the high formation energy of such defects. To date, singly-doped colloidal quantum dots have been produced using stochastic approaches to introducing the dopants, followed by isolating the singly-doped quantum dots from those that are undoped or include more than one dopant atom, and have been used solely for characterisation. A particular issue is that the inclusion of a dopant atom within a colloidal quantum dot is energetically disfavoured, leading to ejection of the dopant atom from the quantum dot during synthesis. A further issue is that dopant atoms may be located towards the edge of the quantum dot, where coupling with the exciton is less effective. A more detailed overview of the foregoing is given in a recent review by Kagan et al. [‘Colloidal Quantum Dots as Platforms for Quantum Information Science’, C. R. Kagan, L. C. Bassett, C. B. Murray, and S. M. Thompson, Chemical Reviews, 2021,121, 3186], It is the object of the present invention to address one or more of the foregoing problems associated with the synthesis of singly-doped colloidal quantum dots. SUMMARY OF THE INVENTION A first aspect of the present invention relates to a plurality of molecular clusters, each cluster comprising a network of Z and D coated with stabilising ligands selected from L, X, and combinations thereof, wherein in a substantial proportion of the molecular clusters each molecular cluster comprises a single atom of a dopant metal, M, wherein: L is a chelating ligand, D is a chalcogen, Z is Zn or Cd, and X is a halide. In embodiments, at least about 40% of the molecular clusters comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. In embodiments, substantially all of the molecular clusters comprising a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. L may be a multidentate ligand, optionally being selected from diphosphines, diols, dithiols, such as benzene-1,2-dithiol, diethers, and diamines, such as ethylene diamine, tetramethylene diamine (TMEDA), 1,2-dimethylethylenediamine (DMEDA), and 1,1-dimethylethylenediamine. M may be a transition metal, optionally being selected from Co2+, Ni2+, Fe2+, Cr2+, Cu2+, or Mn2+. D may be selected from S, Se, and Te, preferably S. X may be selected from I; Br, or Cl; preferably Cl’. In embodiments, each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal comprises a core comprising the dopant metal, optionally the core having the formula [Zi3MDi3]2+, further optionally the core having the formula [Zni3MnSi3]2+. In embodiments, each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal consists of the core and the stabilising ligands, optionally wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal has the formula l_6[Zi3MDi3]X2, further optionally wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal has the formula (TMEDA)6[Zni3MnSi3]Cl2. In alternative embodiments, each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal comprise additional Z and D between the core and the stabilising ligands. The plurality of molecular cluster may have an atomic ratio of Z to M of at least about 13:1, optionally from 13:1 to 100:1, from 13:1 to 50:1, from 13:1 to 25:1, such as about 13:1. The plurality of molecular cluster may have an atomic ratio of Z to D is from 2:1 to 1:2, such as about 1:1. A second aspect of the present invention relates to a method for producing a plurality of molecular clusters, the method comprising: a) providing a solution of a capped dopant metal, the capped dopant having the formula [AL]2[M(D(ZXL))4], b) mixing a first solution comprising LZ(D(TMS))2 with the solution of the capped dopant metal, and c) mixing the product of step b) with a second solution comprising LZX2 or LZY2 to form a solution of molecular clusters, wherein: A is a cation, L is a chelating ligand, M is a dopant metal, D is a chalcogen, Z is Zn or Cd, X is a halide, and Y is a carboxylate and wherein in a substantial proportion of the molecular clusters each molecular cluster comprises a single atom of the dopant metal, M. in embodiments, at least about 40% of the molecular clusters comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. In embodiments, substantially all of the molecular clusters comprise a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. A may be a monovalent cation or a divalent cation, optionally being an alkali metal cation, such as Li+, Na+, or K+, or an alkaline earth metal cation, such as Mg2+. L may be a multidentate ligand, optionally being selected from diphosphines, diols, dithiols, such as benzene-1,2-dithiol, diethers, and diamines, such as ethylene diamine, tetramethylene diamine (TMEDA), 1,2-dimethylethylenediamine (DMEDA), and 1,1-dimethylethylenediamine. M may be a transition metal, optionally being selected from Co2+, Ni2+, Fe2+, Cr2+, Cu2+, or Mn2+. D may be selected from S, Se, and Te, preferably S. X may be selected from I; Br, or Cl; preferably Cl’. Y may be selected from formate, acetate, propionate, butyrate, and isobutyrate. The capped dopant metal may have the formula [Li(TMEDA)]2[Mn(S(ZnCI(TMEDA))4]. The first solution may comprise (TMEDA)Zn(S(TMS))2. The second solution may comprise (TMEDA)ZnCh or (TMEDA)Zn(OAc)2. Mixing of the first solution with the solution of the capped dopant metal and / or mixing of the second solution with the intermediate solution may occur at reduced temperature. The method may further comprise enlarging the molecular clusters by: d) mixing a further volume of the first solution comprising LZ(D(TMS))2 with the solution of the molecular clusters, and e) mixing the product of step d) with a further volume of the second solution comprising LZX2 or LZY2 to form a solution of enlarged molecular clusters. Providing the solution of the capped dopant metal having the formula [AL]2[M(D(ZXL))4] may comprise: a) providing a third solution comprising LZX2, b) providing a solution of a dopant metal source, wherein the dopant metal source is a compound with formula [AL]2[M(D(TMS))4]; and c) adding the solution of the dopant metal source to the third solution to provide a solution of the capped dopant metal. The third solution may comprise (TMEDA)Zn(S(TMS))2. The dopant metal source may have the formula [Li(TMEDA)]2[Mn(S(TMS))4]. Providing the dopant metal source may comprise: a) providing AD(TMS); b) providing a dopant metal solution, the dopant metal solution comprising the dopant metal, M, and a chelating ligand, L; and c) mixing the dopant metal solution with the AD(TMS) to form the dopant metal source. The plurality of molecular clusters formed by the method of the second aspect of the present invention a plurality of molecular clusters according to the first aspect of the present invention. A third aspect of the present invention relates to a plurality of molecular clusters obtained or obtainable by the method of the second aspect of the present invention. A fourth aspect of the present invention relates to the use of a plurality of molecular clusters according to the first or third aspects of the present invetion to produce a plurality of colloidal quantum dots, wherein in a substantial proportion of the colloidal quantum dots each colloidal quantum dot comprises a single atom of a dopant metal, M. A fifth aspect of the present invention relates to a plurality of colloidal quantum dots, wherein in a substantial proportion of the colloidal quantum dots each colloidal quantum dot comprises a single atom of a dopant metal, M. In embodiments, at least about 40% of the colloidal quantum dots comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. In embodiments, substantially all of the colloidal quantum dots comprise a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. Each dopant metal atom may be in a molecular cluster, optionally the molecular clusters are as defined in the first or third aspects of the present invention. The colloidal quantum dots may comprise 11 l-V material, IIA-IIB material, 11B-VIB material, ll-V material, lll-IV material, lll-VI material, IV-VI material, or material including a first element from any transition metal group of the periodic table and a second element from group 16 of the periodic table. The colloidal quantum dots may comprise lll-V materials (such as InP) or 11 B-VI B materials (such as CdSe). Each colloidal quantum dot may have a shell, optionally each shell having a thickness of from 5 to 20 nm. The Ti spin lifetime of the dopant atom may be at least 1 ms when determined using Equation 1. The Ti spin lifetime of the dopant atom may be at least 2 ms when determined using Equation 2. The T2 spin lifetime of the dopant atom may be at least 1 ps when determined using Equation 1. The T2 spin echo intensity may decay monoexponentially. The T2 spin lifetime of the dopant atom may be at least 1 ps when determined using Equation 3. A sixth aspect of the present invention relates to a method for producing a plurality of colloidal quantum dots, the method comprising: a) providing a plurality of molecular clusters; b) providing precursors for a colloidal quantum dot; and c) combining and heating the molecular clusters and the precursors, and wherein in a substantial proportion of the colloidal quantum dots each colloidal quantum dot comprises a single atom of the dopant metal, M. In embodiments, at least about 40% of the colloidal quantum dots comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. In embodiments, substantially all of the colloidal quantum dots comprise a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. The plurality of molecular clusters may be as defined in the first or third aspects of the present invention. The plurality of colloidal quantum dots may be as defined in the fifth aspect of the present invention. The method further may further comprise a size-tuning step. The method may further comprise providing each of the colloidal quantum dots with a shell. A seventh aspect of the present invention relates to a plurality of colloidal quantum dots obtained or obtainable by the method of the sixth aspect of the present invention. An eighth aspect of the present invention relates to a quantum device comprising one or more colloidal quantum dots from the plurality of colloidal quantum dots according to the fifth and seventh aspects of the present invention, such as a quantum repeater, a quantum computer, or a quantum magnetometry device. DEFINITIONS AND ABBREVIATIONS “Molecular cluster” is a term which is widely understood in the art, but for the sake of clarity should be understood herein to relate to clusters of three or more metal atoms and their associated ligands of sufficiently well-defined chemical structure such that all molecules of the cluster compound possess approximately the same relative molecular formula. The molecular clusters act as nucleation sites and are much better defined than the nucleation sites employed in other methods. The use of a molecular cluster may provide a population of nanoparticles that are essentially monodisperse. A significant advantage of this method is that it can be more easily scaled-up to production volumes when compared to other methods of nanoparticle generation. “TMS” is an abbreviation for trimethylsilyl (i.e. -Si(CH3)3). DCM is an abbreviation for dichloromethane. EPR is electron paramagnetic resonance. References to “room temperature” refer to typical laboratory conditions, such as about 20 °C. Ti is the spin-lattice relaxation time. This may be determined as the exponential decay constant for the spin inversion signals from EPR measurements. In particular, the EPR may operate in the X-band (10 GHz) and / or Q-band (35 GHz) in continuous wave mode with a field strength of 3 T and at temperatures of 3 K. Ti may be determined using Equation 1 defined herein (i.e. Ti is Tiave). Alternatively, Ti may be defined using Equation 2 defined herein. T2 is the spin-spin relaxation time. This may be determined as the exponential decay constant for the spin echo signals from EPR measurements. In particular, the EPR may operate in the X-band (10 GHz) and / or Q-band (35 GHz) in continuous wave mode with a field strength of 3 T and at temperatures of 3 K. T2 may be determined using Equation 1 defined herein (i.e. T2 is T2ave). Alternatively, T2 may be defined using Equation 3 defined herein (i.e. when s= 1, T2 is Tm). DESCRIPTION OF THE DRAWINGS Fig. 1 shows the absorbance (solid trace) and photoluminescence (dashed trace) spectra for a capped dopant material (specifically capped manganese) produced in Stage 2. Fig. 2 shows the absorbance (solid trace) and photoluminescence (dashed trace) spectra for a plurality of molecular clusters produced in Stage 3. Fig. 3 shows an absorbance spectrum for quantum dots produced in Stage 4. Fig. 4 shows the absorbance (solid trace) and photoluminescence (dashed trace) spectra for overcoated quantum dots produced in Stage 5. Fig. 5 shows a photoluminescence spectrum for overcoated InP colloidal quantum dots. The spectrum includes the observed spectrum (black, normalised to a peak intensity of 1.0), the calculated spectrum for the colloidal quantum dot (grey, peak at 2.35 eV), and the calculated spectrum for the Mn2+ atom (grey, peak at 2.12 eV corresponding to the transition 4Ti - 6Ai indicated by 500). Fig. 6A shows a photoluminescence spectrum (left hand side) and a trace of photon counts over time at specific wavelengths (right hand side) for overcoated colloidal quantum dots. The left hand side shows the PL spectrum (including observed photoluminescence and photoluminescence calculated for the colloidal quantum dot and Mn2+ atom as in Fig. 5) overlain with the lifetimes (ti, squares, and t2, circles, indicated by 600) determined by a biexponential fit to the photon count trace for emission at each wavelength. The trace of counts shows the count at a given time for wavelengths of 480 nm (lower trace) and 640 nm (higher trace). Fig. 6B shows a PL spectrum (left hand side) and a trace of photon counts over time at specific wavelengths (right hand side) for overcoated undoped colloidal quantum dots respectively. On the left hand side, the PL spectrum (including observed photoluminescence and photoluminescence calculated for the colloidal quantum dot and Mn2+ atom as in Fig. 5, showing inconsistency between the calculated and observed PL) overlain with the lifetimes (ti, squares, and t2, circles) determined by a biexponential fit to the photon count trace for emission at each wavelength. The trace of counts shows the count at a given time for wavelengths of 560 nm (lower trace) and 710 nm (higher trace). Fig. 7 shows the Q-band EPR for overcoated colloidal quantum dots produced in Stage 5. The inset shows the peaks in greater detail. Fig. 8 shows the X-band EPR for overcoated colloidal quantum dots produced in Stage 5. The inset shows the peaks in greater detail. Fig. 9 shows the echo intensity over time for Ti for Sample A colloidal quantum dots produced using molecular clusters grown as in Stage 3 (top trace showing fast decay) and for Sample B colloidal quantum dots produced using larger molecular clusters grown in a stepwise manner (bottom trace showing slow decay). Fig. 10 shows the echo intensity over time for T2 for Sample A colloidal quantum dots produced using molecular clusters grown as in Stage 3 (red, bottom trace showing fast decay) and for Sample B colloidal quantum dots produced using larger molecular clusters grown in a stepwise manner (blue, top trace showing slow decay). Fig. 11 shows overlain absorbance spectra for quantum dots produced using a) molecular clusters as described in Stage 3 (solid trace), b) larger molecular clusters grown in a single step (dashed trace), and c) larger molecular clusters grown in a stepwise manner (dotted trace). Fig. 12 is an alternative plotting of the data in Fig. 9 and shows the echo intensity over time for Ti for Sample A colloidal quantum dots produced using molecular clusters grown as in Stage 3 (top trace showing fast decay) and for Sample B colloidal quantum dots produced using larger molecular clusters grown in a stepwise manner (bottom trace showing slow decay). Fig. 13 is an alternative plotting of the data in Fig. 10 and shows the echo intensity over time for T2 for Sample A colloidal quantum dots produced using molecular clusters grown as in Stage 3 (bottom trace showing fast decay) and for Sample B colloidal quantum dots produced using larger molecular clusters grown in a stepwise manner (top trace showing slow decay). Fig. 14 shows a series of figures for TEM, HAADF, and EELS. Figs. 14a) and b) are TEM images of the colloidal quantum dots at lower and higher magnification, showing colloidal quantum dots with a consistent size and tetrahedral morphology. The white box in Fig. 14b) indicates the region from which the diffractions patterns in Figs. 14c) and d) were obtained. The diffraction pattern is corresponds to that of a zincblende crystal structure with lattice constant of 5.57 A, which is between the values for zincblende ZnSe (5.67 A) and ZnS (5.41 A) and thus consistent with what would be expected for a ZnSeS shell. Fig. 14e) is an EELS image of several colloidal quantum dots. Figs. 14f) and g) are EELS spectra for the indicated pixels. Fig. 14f) is the EELS spectrum for a pixel containing a colloidal quantum dot, with the observed signal overlain with Gaussian fits for the Mn L2 transition (left hand) signal and Mn L3 transition (right hand) signal. The Mn L2 and L3 transition signals being at approximately three times the noise level and two times the noise level respectively. These are consistent with what would be expected for a colloidal quantum dot with a single Mn dopant. Fig. 14g) is the EELS spectrum for an empty pixel, with the observed signal overlain with Gaussian fits for the Mn L2 transition (left hand) signal and Mn L3 transition (right hand) signal. The Mn L2 and L3 transition signals are at the same level as background noise, indicating an absence of Mn within the empty pixel. DETAILED DESCRIPTION A Plurality of Molecular Clusters As is known in the art, molecular clusters may be used in the controlled synthesis of colloidal quantum dots. The molecular clusters are used to ‘seed’ the colloidal quantum dots, providing a surface on which precursors of the colloidal quantum dots can nucleate. Metal complexes are an example of molecular cluster. Clusters of spin active and spininactive metals (e.g. of Manganese and Zinc) with chalcogens are known. For example, C.B. Khadka et al (“Zinc Chalcogenolate Complexes as Precursors to ZnE and Mn / ZnE (E = S, Se) Clusters”, C. B. Khadka, A. Eichhdfer, F. Weigend, and J. F. Corrigan, Inorg. Chem. 2012, 51, 5, 2747-2756) produced manganese containing ternary clusters based on the binary clusters of (TMEDA)eZni4Ei3Cl2 (E = S or Se), wherein multiple zinc (II) are replaced with manganese (II) to provide stochastic mixtures with the empirical formulae (TMEDA)eZni4-xMnxSi3Cl2 and (TMEDA)6Zni4-xMnxSei3Cl2 wherein x is, variously, 1.7, 2.0, or 5.6. However, the approaches previous employed have been unable to provide molecular clusters that are anything other than stochastic mixtures (i.e. they have been unable to selectively produce the singly-doped ternary complex or produce clusters wherein the singly-doped ternary complex is enriched compared to the stochastic mixture). The present invention relates to a plurality of molecular clusters, each of which comprises a network of Z and D coated with stabilising ligands selected from L, X, and combinations thereof wherein L is a chelating ligand, D is a chalcogen, Z is Zn or Cd, and X is a halide. A substantial proportion of the molecular clusters that constitute the plurality of molecular clusters are singly-doped (i.e. they contain a single atom of the dopant metal, M). By a substantial proportion it is meant that at least about 40% of the molecular clusters are singly-doped, preferably at least about 50% of the molecular clusters are singly-doped, more preferably at least about 60% of the molecular clusters are singly-doped, further preferably at least about 70% of the molecular clusters are singly-doped, yet further preferably at least about 80% of the molecular clusters are singly-doped, most preferably, substantially all of the molecular clusters are singly-doped. By substantially all, it is meant that at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9% of the molecular clusters in the plurality are singly-doped. The molecular clusters that are not singly-doped will either be undoped (i.e. contain no dopant metal atom) or will contain two or more atoms of the dopant metal. Such molecular clusters are not of use for application in quantum technologies. The plurality of molecular clusters may be used to produce colloidal quantum dots (e.g. by the aforementioned ‘seeding’ approaches). The molecular clusters are incorporated into the colloidal quantum dots on a one-to-one basis. Accordingly, the proportion of the molecular clusters that are singly-doped is the same as the proportion of the colloidal quantum dots that are singly-doped. The plurality of molecular clusters of the present invention, by virtue of having a large proportion that are singly-doped, is able to produce a plurality of colloidal quantum dots in which a large proportion of the quantum dots are singly-doped. This is in contrast to both statistical mixtures of molecular clusters, of which only a relatively small proportion are singly-doped, and stochastic methods of producing singly-doped colloidal quantum dots, both of which produce a large number of undoped and multiply-doped colloidal quantum dots that are functionally inseparable from the singly-doped colloidal quantum dots. It will be understood that having a higher proportion of singly-doped molecular clusters will be preferably, but that even modest proportions represent an improvement on statistical mixtures produced by methods known in the art. The proportion of the plurality of molecular clusters that is singly-doped can be determined by conversion into quantum dots and analysis of the plurality of quantum dots using EPR as described herein. In addition to the number of dopants present in each cluster, and hence each colloidal quantum dot, the molecular clusters of the present invention advantageously locate each dopant metal towards the centre of the quantum dot. Conversely, in statistical doping efforts, the dopant metal is predisposed to be moved to the edge of the quantum dot. Being located towards the centre of the quantum dot increases improves coupling of the exciton, while reducing spin-spin interaction with surface defects and other spins external to the colloidal quantum dot and thereby extending the spin lifetime of the dopant metal atom. In embodiments, the molecular clusters comprise a core with formula [Zi4-xMxDi3]2+, wherein x is from 0.4 to 1.6, preferably from 0.5 to 1.5, more preferably from 0.6 to 1.4, further preferably from 0.7 to 1.3, yet further preferably from 0.8 to 1.2, most preferably from 0.9 to 1.1, such as about 1. The core is stabilised by surrounding ligands selected from chelating ligands (L), halides (X), and combinations thereof. In particular embodiments, the molecular clusters may consist of said core and the stabilising ligands, such that the molecular clusters have the formula Le[Zi4-xMxDi3]X2, such as (TMEDA)6[Zni4-xMnxSi3]Cl2, wherein x is from 0.4 to 1.6, preferably from 0.5 to 1.5, more preferably from 0.6 to 1.4, further preferably from 0.7 to 1.3, yet further preferably from 0.8 to 1.2, most preferably from 0.9 to 1.1, such as about 1. In alternative embodiments, the molecular clusters comprise said core and the stabilising ligands and are provided with further atoms of Z and D between the core and the stabilising ligands. In alternative embodiments, each of the molecular clusters of the substantial proportion of the plurality of molecular clusters that comprises a single atom of dopant metal is provided with further atoms of Z and D between the core and the stabilising ligands. This provides larger molecular clusters that are nevertheless singly-doped. Without wishing to be bound by theory, these larger clusters are thought to be advantageous in that they are easier to isolate, having lower solubility. Each of the molecular clusters of the substantial proportion of the plurality of molecular clusters that comprises a single atom of dopant metal comprises a single atom of the dopant metal within a network of Zn or Cd atoms (Z) and chalcogen (D) atoms. This network may be varied in size and is stabilised with surrounding ligands selected from chelating ligands (L), halides (X), and combinations thereof. The size of the network, and hence the cluster, may be varied depending on the reaction conditions used to form the cluster. In embodiments, each of the molecular clusters of the substantial proportion of the plurality of molecular clusters that comprises a single atom of dopant metal comprises a core in which the dopant metal is located. The core may have a formula of [Zi3MDi3]2+, such as [Zni3MnSi3]2+. Without wishing to be bound by theory, it is thought that the core is the minimum network of Z, D and the single atom of the dopant metal, M, required for formation of a stable molecular cluster (in combination with the stabilising ligands). In embodiments, the core is directly coated with stabilising ligands, such that each molecular cluster consists of the core and the stabilising ligands. In such embodiments, each of the molecular clusters has the formula Le[Zi3MDi3]X2, such as (TMEDA)6[Zni3MnSi3]Cl2. Without wishing to be bound by theory, this is thought to be the smallest possible molecular cluster with its stabilising ligands. In alternative embodiments, each of the molecular clusters of the substantial proportion of the plurality of molecular clusters that comprises a single atom of dopant metal is provided with further atoms of Z and D between the core and the stabilising ligands. This provides larger molecular clusters that are nevertheless singly-doped. Without wishing to be bound by theory, these larger clusters are thought to be advantageous in that they are easier to isolate, having lower solubility. The substantial proportion of the plurality of molecular clusters that comprise a single atom of dopant metal may include molecular clusters of different sizes. For example, the substantial proportion of the plurality of molecular clusters that are singly-doped may be a mixture of molecular clusters consisting of the core and stabilising ligands as described herein and of molecular clusters further comprising further atoms of Z and D between the core and the stabilising ligands as described herein. The chelating ligand, L, is a multidentate ligand, typically bidentate. The chelating ligand is required to both stabilise the cation and the molecular cluster. For example, diphosphines, diols, dithiols, such as benzene-1,2-dithiol, diethers, and diamines, such as ethylene diamine, tetramethylene diamine (TMEDA), 1,2-dimethylethylenediamine (DMEDA), and 1,1-dimethylethylenediamine. A preferred chelating ligand is TMEDA. In embodiments, a mixture of chelating ligands is used. The dopant metal may be any suitable transition metal. The dopant metal has an unpaired spin For example, the dopant metal may be Co2+, Ni2+, Fe2+, Cr2+, Cu2+, or Mn2+. A preferred dopant metal is Mn2+. The chalcogen, D, is typically S, Se, or Te. A preferred chalcogen is S. Z may be Zn or Cd. Preferably, Z is Zn. The halide, X, is typically I; Br, or Cl’. A preferred halide is Cl’. The plurality of molecular clusters may have an atomic ratio of Z to M of at least about 13:1, optionally from 13:1 to 100:1, from 13:1 to 50:1, from 13:1 to 25:1, such as about 13:1. The plurality of molecular clusters may have an atomic ratio of D to M of at least about 13:1, optionally from 13:1 to 100:1, from 13:1 to 50:1, from 13:1 to 25:1, such as about 13:1. The plurality of molecular clusters may have an atomic ratio of Z to D of from 2:1 to 1:2, such as about 1:1. Preparation of a Plurality of Molecular Clusters The plurality of molecular clusters may be prepared by sequentially reacting a solution of capped metal dopant having the formula [AL]2[M(D(ZXL))4] with a first solution comprising LZ(D(TMS))2 and then a second solution comprising LZX2 or LZY2. The capped dopant metal provides the dopant metal in a form suitable for conversion to a molecular cluster. By ‘capped’ it is meant that the dopant metal is made stable by the surrounding compounds (e.g. by bonding to the chalcogen). The capped dopant metal has the formula [AL]2[M(D(ZXL))4], wherein: A is a cation, M is a dopant metal, D is a chalcogen, Z is Zn or Cd, X is a halide, and L is a chelating ligand The cation, A, of the capped dopant metal is a monovalent or divalent cation, typically a monovalent cation. In embodiments the cation is an alkali metal cation such as Li+, Na+, or K+. Alternatively, the cation is an alkaline earth metal cation, such as Mg2+. Typically the cation is Li+. The dopant metal, M, is as described herein and is the same as the dopant metal in the molecular clusters. The chalcogen, D, of the capped dopant metal is as described herein. The chalcogen of the capped dopant metal may be the same or different to the chalcogen in the molecular cluster. Preferably it is the same chalcogen in both the capped dopant metal and the molecular cluster. A preferred chalcogen is S. Z may be Zn or Cd in the capped dopant metal. Z of the capped dopant metal may be the same or different to Z in the molecular cluster. Preferably Z is the same in both the capped dopant metal and the molecular cluster. Z is preferably Zn. The halide, X, of the capped dopant metal is as described herein. The halide of the capped dopant metal may be the same or different to the halide in the molecular cluster. Preferably it is the same halide in both the capped dopant metal and the molecular cluster. A preferred halide is Cl. The chelating ligand, L, of the capped dopant metal is as described herein. The chelating ligand of the capped dopant metal may be the same or different to the chelating ligand in the molecular cluster. A preferred chelating ligand is TMEDA. In embodiments, a mixture of chelating ligands is used. In one embodiment, the capped dopant metal has the formula [Li(TMEDA)]2[Mn(S(ZnCI(TMEDA))4]. The solution of capped dopant metal may have a concentration of from 0.01 to 1 M, preferably from 0.05 to 0.5 M, more preferably from 0.08 to 0.2 M, such as about 0.1 M. Any suitable solvent may be used, such as DCM. The first solution comprises LZ(D(TMS))2 dissolved in a solvent. L is a chelating ligand, as described herein. L may be a different chelating ligand than is present in the capped dopant metal. Preferably, however, L is the same chelating ligand as is used in the capped dopant metal. Z may be Zn or Cd. Z may be different for each of the first solution and the capped dopant metal. Preferably, however, Z is the same for each of the first solution and the capped dopant metal. D is a chalcogen, as described herein. D may be a different chalcogen than is present in the capped dopant metal. Preferably, however, D is the same chalcogen as is used in the capped dopant metal. In embodiments, each of L, D, and Z of the first solution is the same as for the capped dopant metal. In one embodiment, LZ(D(TMS))2 is (TMEDA)Zn(S(TMS))2. Any suitable aprotic solvent may be used for the first solution, such as tetrahydrofuran (THF). The concentration of LZ(D(TMS))2 may be from 0.05 to 0.5 M, such as 0.1 to 0.3 M, 0.15 to 0.25 M, for example about 0.2 M. Mixing the solution of capped dopant metal with the first solution may comprise adding the first solution to the capped dopant metal. Such addition is preferably gradual (for example, dropwise). Mixing of the solution of capped dopant material with the first solution may take place at a reduced temperature to discourage side reactions. For example, one or both of the solutions may be cooled to between -100 and 0 °C prior to mixing, such as between -90 and -30 °C, between -80 and -50 °C, for example about -75 °C. The molar ratio of LZ(D(TMS))2 to the capped dopant metal may be from 20:1 to 1:1, optionally from 10:1 to 2:1, optionally from 8:1 to 3:1, or optionally from 5:1 to 4:1, such as about 4.5:1. After addition is complete, the mixture is stirred for a period, such as between 5 and 60 minutes, between 10 and 30 minutes, for example about 20 minutes. If cooled, the mixture is allowed to warm to room temperature during this process. The second solution comprises LZX2 or LZY2 dissolved in a solvent. L is a chelating ligand as described herein. L may be a different chelating ligand than is present in the capped dopant metal. Preferably, however, L is the same chelating ligand as is used in the capped dopant metal. Z may be Zn or Cd. Z may be different for each of the second solution and the capped dopant metal. Preferably, however, Z is the same for each of the second solution and the capped dopant metal. In embodiments, each of L and Z of the second solution is the same as for the capped dopant metal. In one embodiment. Y is a carboxylate. For example, Y may be selected from formate, acetate, propionate, butyrate, isobutyrate, and so on. A preferred carboxylate is acetate. In embodiments, the second solution comprises LZX2, which is preferably (TMEDA)ZnCh. In alternative embodiments, the second solution comprises LZY2, which is preferably (TMEDA)Zn(OAc)2. Any suitable aprotic solvent may be used for the second solution, such as THF. The concentration of LZX2 or LZY2 may be from 0.05 to 0.5 M, such as 0.1 to 0.3 M, 0.15 to 0.25 M, such as about 0.2 M. Mixing the product of mixing the first solution and the solution of capped dopant metal with the second solution may comprise adding the second solution to the intermediate solution. Such addition is preferably gradual (for example, dropwise). Mixing of the product of mixing the first solution and the solution of capped dopant metal with the second solution may take place at a reduced temperature to discourage side reactions. For example, one or both of the solutions may be cooled to between -100 and 0 °C prior to mixing, such as between -90 and -30 °C, between -80 and -50 °C, for example about -75 °C. The molar ratio of LZX2 or LZY2 to the capped dopant metal used to form the solution of molecular clusters (i.e. the molar ratio of LZX2 or LZY2 to the dopant metal) may be from 20:1 to 1:1, optionally from 10:1 to 2:1, optionally from 8:1 to 3:1, or optionally from 5:1 to 4:1, such as about 4.5:1. Preferably, the molar ratio of LZ(D(TMS))2 to the capped dopant metal and the molar ratio of LZX2 or LZY2 to the capped dopant metal is about the same. After addition is complete, the mixture is stirred for a period, such as between 5 and 60 minutes, between 10 and 30 minutes, for example about 20 minutes to form the solution of molecular clusters. If cooled, the mixture is allowed to warm to room temperature during this process. The molecular clusters may be isolated from the reaction mixture. For example, by filtration. It will be understood that the size of the molecular cluster may be increased by increasing the molar ratio of LZ(D(TMS))2 to the capped dopant metal when forming the intermediate solution and / or by increasing the molar ratio of LZX2 or LZY2 to the capped dopant metal when forming the solution of molecular clusters. In embodiments, the solution of molecular clusters is sequentially mixed with further volumes of the first solution and the second solution. These further additions introduce additional Z and D to the molecular cluster and increase the size of the molecular cluster. The mixing of these further volumes of the first solution and the second solution may be as described herein for the initial mixing of the first solution with the solution of the capped dopant metal and for the initial mixing of the second solution with the product of the first mixing step respectively. In embodiments, one further volume of each of the first solution and the second solution is mixed. Alternatively, two or more further volumes of each of the first solution and the second solution may be mixed. For the avoidance of doubt, the additional volumes of the first solution and the second solution are mixed with the growing molecular clusters alternately. Preparation of a Capped Dopant Metal The capped dopant metal having the formula [AL]2[M(D(ZXL))4] is prepared by reaction of a dopant metal source with a third solution, the third solution comprising LZX2. Performing the reaction may comprise the steps of a) providing the third solution, b) providing a solution of a dopant metal source, wherein the dopant metal source is a compound with formula [AL]2[M(D(TMS))4]; and c) adding the solution of the dopant metal source to the third solution to provide a solution of the capped dopant metal. Each of A, L, M, D, Z, and X is as described herein. The third solution comprises LZX2. Lis a chelating ligand as described herein and may be the same chelating ligand as for the capped dopant metal (e.g. both TMEDA). Z is Zn or Cd and may be the same as for the capped dopant metal (e.g. both Zn). X is a halide as described herein and may be the same halide as for the capped dopant metal (e.g. both Cl). In embodiments, the third solution comprises (TMEDA)ZnCh. Any suitable aprotic solvent may be used for the third solution, such as THF. The concentration of LZX2 may be from 0.05 to 0.5 M, such as 0.1 to 0.3 M, 0.15 to 0.25 M, such as about 0.2 M. The dopant metal source comprises a compound with formula [AL]2[M(D(ZXL))4]. A is a cation as described herein and may be the same cation as for the capped dopant metal. Lis a chelating ligand as described herein and may be the same chelating ligand as for the capped dopant metal (e.g. both TMEDA). M is the dopant metal and is the same dopant metal as for the capped dopant metal (e.g. Mn2+). D is the chalcogen as described herein and may be the same as a chalcogen for the capped dopant metal (e.g. S). Z is Zn or Cd and may be the same as for the capped dopant metal (e.g. both Zn). X is a halide as described herein and may be the same halide as for the capped dopant metal (e.g. both Cl). In embodiments, the dopant metal source has the formula [Li(TMEDA)]2[Mn(S(TMS))4]. The solution of the dopant metal source comprises the dopant meal source dissolved in an aprotic solvent, such as DCM. The concentration of the dopant metal source may be from 0.01 to 0.5 M, optionally from 0.02 to 0.2 M, optionally from 0.03 to 0.1 M, or optionally from 0.04 to 0.06, such as about 0.05 M. Mixing the solution of the dopant metal source with the third solution may comprise adding the solution of the dopant metal source to the third solution. Such addition is preferably gradual (for example, dropwise). The dopant metal source may be added at a rate from 1 to 40 mL / hr, such as from 4 to 20 mL / hr, from 6 to 10 mL / hr, for example about 8 mL / hr. The dopant metal source may be added over a period of from 5 to 120 minutes, such as 10 to 60 minutes, 15 to 45 minutes or 20 to 40 minutes, for example about 30 minutes. Mixing of the solution of the dopant metal source with the third solution may take place at a reduced temperature to discourage side reactions. For example, one or both of the solutions may be cooled to between -100 and 15 °C prior to mixing, such as between -50 and 10 °C, between -20 and 5 °C, for example about 0 °C. The molar ratio of LZX2 to the dopant metal source used to form the capped dopant metal may be from 10:1 to 1:1, optionally from 8:1 to 2:1, optionally from 6:1 to 3:1, or optionally from 5:1 to 4:1, such as about 4:1. After addition is complete, the mixture is stirred for a period, such as between 5 and 60 minutes, between 10 and 30 minutes, for example about 20 minutes to form the solution of capped dopant metal. If cooled, the mixture is allowed to warm to room temperature during this process. The capped dopant metal may be isolated from the reaction mixture. For example, by crystallisation. In a preferred method, a third solution comprising (TMEDA)ZnCh and a dopant metal source comprising [Li(TMEDA)]2[Mn(S(TMS))4] are provided separately and the dopant metal source is then added to the third solution to provide a solution of the capped dopant metal comprising [Li(TMEDA)]2[Mn(S(ZnCI(TMEDA))4]. Preparation of a Dopant Metal Source The dopant metal source provides the dopant metal in a form suitable for conversion to a capped dopant metal. The dopant metal source is prepared by reaction of a dopant metal with AD(TMS). The dopant metal source typically has the formula: [AL]2[M(D(TMS))4] Each of A, M, D, and L is as described herein. In one embodiment, the dopant metal source has a formula [Li(TMEDA)]2[Mn(S(TMS))4]. For the AD(TMS), A is a cation as described herein and may be the same cation as for the dopant metal source. Preferably A is Li. For the AD(TMS), D is a chalcogen as described herein and may be the same chalcogen as for the dopant metal source. Preferably D is S. The AD(TMS) may be obtained by reaction of D(TMS)2 with a suitable organometallic, such as an organolithium, an organosodium, an organopotassium, or an organomagnesium (e.g. a Grignard reagent). Suitable organolithium compounds include alkyl lithiums, such as n-butyllithium. In embodiments, the reacting the D(TMS)2 with the organometallic comprises mixing a solution of the organometallic with a solution of the D(TMS)2. For example, by adding the solution of the nucleophilic organometallic to the solution of D(TMS)2. It is preferable for the addition to be gradual. Preferably, the reaction mixture is cooled to a lower temperature, such as about 0°C. Any suitable organic solvent may be used. Following the reaction, the solvent may be removed to provide the AD(TMS) as a solid. Solvents with lower boiling points (such as DCM, chloroform, and hexane) are preferred to enable easier isolation of the resulting AD(TMS). Preferably, the AD(TMS) is LiS(TMS). The dopant metal solution comprises a compound of a dopant metal and a chelating ligand in a solvent. The molar ratio of chelating ligand to dopant metal may be from 3:1 to 1:1, optionally from 2:1 to 1:1, such as about 1.25:1. The dopant metal may be present in the dopant metal solution in a concentration of from 0.01 to 1 M, optionally from 0.02 to 0.5 M, further optionally from 0.03 to 0.2 M, yet further optionally from 0.04 to 0.1 M, such as about 0.06 M. The compound of the dopant metal is preferably a dopant metal salt, for example a halide, such as an iodide, a bromide or a chloride, or a carboxylate. For example, if the dopant metal is Mn2+, the dopant metal salt may be MnCh, MnBr2, Mnl2, or Mn(acetate)2. In embodiments, MnCh is a preferred dopant metal salt. Any suitable solvent may be used, for example DCM. The dopant metal solution is provided by dissolving the compound of the dopant metal in the solvent in the presence of the chelating ligand, L. The dopant metal solution is mixed with the AD(TMS). In embodiments, the dopant metal solution is added to the AD(TMS), preferably gradually. For example, the dopant metal solution may be added at a rate from 10 to 200 mL / hr, such as from 40 to 150 mL / hr, from 60 to 100 mL / hr, for example about 80 mL / hr. The dopant metal source may be added over a period of from 5 to 120 minutes, such as 10 to 60 minutes, 15 to 45 minutes or 20 to 40 minutes, for example about 30 minutes. Mixing of the solution of the dopant metal solution with the AD(TMS) may take place ata reduced temperature. For example, one or both of the solutions may be cooled to between -100 and 15 °C prior to mixing, such as between -50 and 10 °C, between -20 and 5 °C, for example about 0 °C. The resulting dopant metal source may then be isolated. Any aprotic solvent able to dissolve the dopant metal salt and AD(TMS) may be used, for example, dichloromethane (DCM), chloroform, tetra hydrofuran (THF), dimethylformamide (DMF), alkanes (such as pentane, hexane, heptane, and so on). Isolating the dopant metal source may be achieved by any suitable technique. In embodiments, the solvent is removed and the resulting solids dissolved in hexane and filtered. The filtrate is collected, evaporated and then washed with cold hexane (for example, at -75°C) to afford a solid product. In a preferred embodiment, S(TMS)2 is reacted with n-BuLi to form LiS(TMS), while MnCh and TMEDA are dissolved to form a dopant metal solution, the dopant metal solution is then added to the LiS(TMS) and the resulting [Li(TMEDA)]2[Mn(S(TMS))4] is isolated. A Plurality of Colloidal Quantum Dots The plurality of colloidal quantum dots of the present invention, wherein a substantial proportion of the colloidal quantum dots that form the plurality are singly-doped (i.e. they contain a single atom of the dopant metal, M). By a substantial proportion it is meant that at least about 40% of the colloidal quantum dots are singly-doped, preferably at least about 50% of the colloidal quantum dots are singly-doped, more preferably at least about 60% of the colloidal quantum dots are singly-doped, further preferably at least about 70% of the colloidal quantum dots are singly-doped, yet further preferably at least about 80% of the colloidal quantum dots are singly-doped, most preferably, substantially all of the colloidal quantum dots are singly-doped. By substantially all, it is meant that at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9% of the colloidal quantum dots in the plurality are singly-doped. The colloidal quantum dots that are not singly-doped will either be undoped (i.e. contain no dopant metal atom) or will contain two or more atoms of the dopant metal. Such colloidal quantum dots are not of use for application in quantum technologies. The proportion of the quantum dots that are singly-doped may be determined using EPR (preferably in the Q-band (35 GHz) with magnetic fields swept at 12 T and the X-band (10 GHz) swept at 3 T respectively, each at temperatures of 3K). Firstly, the proportion of quantum dots that are doped can be determined by comparison of the lower detection limit of the EPR instrument to the concentration of quantum dots (and hence the expected concentration of spins) within the detection volume of the EPR instrument. Secondly, that the doping is single-doping can be determined qualitatively from the breadth of the peaks in the EPR spectra, with narrow peaks indicating a lack of coupling (i.e. singly-doped quantum dots). By virtue of a substantial proportion (or substantially all) of the colloidal quantum dots of the plurality being singly-doped, the plurality of singly-doped colloidal quantum dots is more useful for the scalable fabrication of quantum devices. In contrast, statistical mixtures of colloidal quantum dots, of which only a relatively small proportion are singly-doped, are less useful for this purpose as a large number of the colloidal quantum dots used will be absent of the dopant metal or comprise multiple dopant metals and will be functionally inseparable from the singly-doped colloidal quantum dots. It will be understood that having a higher proportion of singly-doped colloidal quantum dots will be preferably, but that even modest proportions represent an improvement on statistical mixtures produced by methods known in the art. In embodiments, the single atom of the dopant metal present in a given singly-doped colloidal quantum dot is within a molecular cluster (such as the singly-doped molecular clusters described herein) embedded within the colloidal quantum dot. Location of the dopant metal atom within the molecular cluster ensures that the dopant metal atom is located towards the centre of the quantum dot, improving coupling with the exciton and reducing spin-spin interactions with surface defects and other spins external to the colloidal quantum dot. The colloidal quantum dots may be formed of any material known to form quantum dots. Particularly suitable materials are those that are known to be compatible with molecular seeding approaches, such as: lll-V (i.e., 13-15) material including a first element from group 13 of the periodic table and a second element from group 15 of the periodic table, including but not limited to boron phosphide (BP), aluminium phosphide (AIP), aluminium arsenide (AlAs), aluminium antimonide (AlSb), gallium nitride (GaN), GaP, gallium arsenide (GaAs), gallium antimonide (GaSb), indium nitride (InN), InP, indium arsenide (InAs), indium antimonide (InSb), aluminium nitride (AIN), boron nitride (BN), and / or ternary or quaternary alloys of these materials. IIA-VIB (i.e., 2-16) material including a first element from group 2 of the periodic table and a second element from group 16 of the periodic table, including but not limited to magnesium sulfide (MgS), magnesium selenide (MgSe), magnesium telluride (MgTe), calcium sulfide (CaS), calcium selenide (CaSe), calcium telluride (CaTe), strontium sulfide (SrS), strontium selenide (SrSe), strontium telluride (SrTe), barium sulfide (BaS), barium selenide (BaSe), barium telluride (BaTe), and / or ternary or quaternary alloys of these materials. 11B-VIB (i.e., 12-16) material including a first element from group 12 of the periodic table and a second element from group 16 of the periodic table, including but not limited to ZnS, ZnSe, zinc telluride (ZnTe), CdS, CdSe, CdTe, HgS, mercury selenide (HgSe), mercury telluride (HgTe), and / or ternary or quaternary alloys of these materials. Il-V (i.e., 12-15) material including a first element from group 12 of the periodic table and a second element from group 15 of the periodic table, including but not limited to zinc phosphide (ZnsP2), zinc arsenide (ZnsAs2), cadmium phosphide (Cd3P2), cadmium arsenide (Cd3As2), cadmium nitride (Cd3N2), zinc nitride (Zn3N2), and / or ternary or quaternary alloys of these materials. III-IV (i.e., 13-14) material including a first element from group 13 of the periodic table and a second element from group 14 of the periodic table, including but not limited to boron carbide (B4C), aluminium carbide (AI4C3), gallium carbide (Ga4C), and / or ternary or quaternary alloys of these materials. The material may be doped with at least one suitable dopant. III-VI (i.e., 13-16) material, which includes a first element from group 13 of the periodic table and a second element from group 16 of the periodic table, including but not limited aluminium sulfide (AI2S3), aluminium selenide (AhSes), aluminium telluride (AhTes), gallium sulfide (Ga2Ss), gallium selenide (Ga2Se3), indium sulfide (^283), indium selenide (ln2Se3), gallium telluride (Ga2Te3), indium telluride (ln2Te3), and / or ternary or quaternary alloys of these materials. IV-VI (i.e., 14-16) material, which includes a first element from group 14 of the periodic table and a second element from group 16 of the periodic table, including but not limited to lead sulfide (PbS), lead selenide (PbSe), lead telluride (PbTe), tin sulfide (SnS), tin selenide (SnSe), tin telluride (SnTe), and / or ternary or quaternary alloys of these materials. Material including a first element from any transition metal group of the periodic table and a second element from group 16 of the periodic table, including but not limited to nickel sulfide (NiS), chromium sulfide (CrS), copper indium sulfide (CulnS2), copper indium diselenide (CulnSe2), copper gallium disulfide (CuGaS2), copper gallium diselenide (CuGaSe2), silver indium disulfide (AglnS2), silver indium diselenide (AglnSe2), and / or ternary or quaternary alloys of these materials. In embodiments, the colloidal quantum dots are formed from a material that will not interfere with the spin of the dopant metal (i.e. the material consists of, or consists essentially of, spin 0 atoms). In embodiments, the colloidal quantum dots are formed from a material that comprises non-spin-0 atoms. A further advantage of the molecular cluster based approach of forming colloidal quantum dots is that atoms with spins other than 0 may be used. Ordinarily, these would couple with the spin of the dopant metal reduce the spin lifetime. However, without wishing to be bound by theory, embedding the dopant metal within the molecular cluster, which comprises a network of Zn and S having 96% and >99% spin-0 isotopes respectively, is thought to isolate the dopant metal from the material of the colloidal quantum dot. This effect is enhanced for larger molecular clusters. Preferred materials are lll-V materials (such as InP) and IIB-VIB materials (such as CdSe). The material of the colloidal quantum dot may be selected and / or the size of the colloidal quantum dots may be tuned so as to provide a desirable peak absorption and / or emission wavelength. For example, these wavelengths may be tuned to be similar to the wavelengths at which the dopant metal absorbs and / or emits in order to promote interaction, such that the spin of the dopant metal is optically addressable. In particular, the emission wavelength of the colloidal quantum dots may be tuned to be larger than that of the dopant atom to prevent excitation of the dopant. The colloidal quantum dots may be provided with a shell. The shell may be formed from one or more shell materials. The material used on any shell or subsequent numbers of shells grown onto the colloidal quantum dot preferably has a crystal phase compatible with that of the colloidal quantum dot. Compatible crystal phases may be the same, e.g., a hexagonal or cubic material formed on a hexagonal or cubic colloidal quantum dot. Compatible crystal phases may alternatively be different phases, wherein a lattice spacing of the colloidal quantum dot material is close enough to a lattice spacing of the shell material such that deleterious lattice strain and / or relaxation (and concomitant defect generation) does not occur. In some embodiments, the shell material is closely lattice-matched to (i.e., has approximately the same lattice constant as) the colloidal quantum dot material. In other embodiments, a buffer layer is formed on the colloidal quantum dot to ameliorate the effects of lattice mismatch between the colloidal quantum dot material and a subsequently formed shell material. The material of a buffer layer and / or a shell material formed on the colloidal quantum dot may include at least one of the following: IIA-VIB (i.e., 2-16) material, which includes a first element from group 2 of the periodic table and a second element from group 16 of the periodic table, including but not limited to MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, and / or ternary or quaternary alloys of these materials. 11B-VIB (i.e., 12-16) material, which includes a first element from group 12 of the periodic table and a second element from group 16 of the periodic table, including but not limited to ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, and / or ternary or quaternary alloys of these materials. Il-V (i.e., 12-15) material, which includes a first element from group 12 of the periodic table and a second element from group 15 of the periodic table, including but not limited to Zn3P2, Zn3As2, Cd3P2, Cd3As2, Cd3N2, Zn3N2, and / or ternary or quaternary alloys of these materials. Ill-V (i.e., 13-15) material, which includes a first element from group 13 of the periodic table and a second element from group 15 of the periodic table, including but not limited to BP, AIP, AlAs, AlSb; GaN, GaP, GaAs, GaSb; InN, InP, InAs, InSb, AIN, BN, and / or ternary or quaternary alloys of these materials. III-IV (i.e., 13-14) material, which includes a first element from group 13 of the periodic table and a second element from group 14 of the periodic table, including but not limited to B4C, AI4C3, Ga4C, and / or ternary or quaternary alloys of these materials. III-VI (i.e., 13-16) material, which includes a first element from group 13 of the periodic table and a second element from group 16 of the periodic table, including but not limited AI2S3, AhSes, AhTes, Ga2S3, Ga2Se3, 10283, ln2Te3, and / or ternary or quaternary alloys of these materials. IV-VI (i.e., 14-16) material, which includes a first element from group 14 of the periodic table and a second element from group 16 of the periodic table, including but not limited to PbS, PbSe, PbTe, SnS, SnSe, SnTe, and / or ternary or quaternary alloys of these materials. In particular embodiments, the shell material may be formed of ZnSe / ZnS or CdSe / CdS, preferably ZnSe / ZnS. Preferably, the shell has a thickness of from 5 to 20 nm, such as from 6 to 15 nm, such as about 8 nm. Conventionally, shells have thicknesses up to 4 nm. However, it has been found that providing the core of the colloidal quantum dot with a thicker shell reduces the interaction of the atom of dopant metal with any surface defects, thereby increasing the spin lifetime of the atom of dopant metal. The dopant atoms within the plurality of colloidal quantum dots may have a T1 spin lifetime of at least 1 ms when determined using Equation 1. Preferably, the T1 spin lifetime is at least 1.5 ms, more preferably at least 3 ms, further preferably at least 5 ms, yet further preferably at least 7 ms, still further preferably at least 8 ms, most preferably at least 9 ms when determined using Equation 1. The dopant atoms within the plurality of colloidal quantum dots may have a T1 spin lifetime of at least 2 ms when determined using Equation 2. Preferably, the T1 spin lifetime is at leasts ms, more preferably at least 5 ms, further preferably at least 7 ms, yet further preferably at least 10 ms, still further preferably at least 15 ms, most preferably at least 20 ms when determined using Equation 2. The dopant atoms within the plurality of colloidal quantum dots may have a T2 spin lifetime of at least 1 ps when determined using Equation 1. Preferably, theT2 spin lifetime is at least 1.2 ps, more preferably at least 1.4 ps, further preferably at least 1.6 ps, yet further preferably at least 2 ps, still further preferably at least 2.5 ps, most preferably at least 3 ps when determined using Equation 1. In embodiments, the T2 spin echo intensity decays monoexponentially. The dopant atoms within the plurality of colloidal quantum dots may have a T2 spin lifetime of at least 1 ps when determined using Equation 3. Preferably, the T2 spin lifetime is at least 1.2 ps, more preferably at least 1.4 ps, further preferably at least 1.6 ps, yet further preferably at least 2 ps, still further preferably at least 2.5 ps, most preferably at least 3 ps when determined using Equation 3. Preparation of a Plurality Colloidal Quantum Dots A plurality of molecular clusters may be used to produce a plurality of colloidal quantum dots. Each of the plurality of molecular clusters and the plurality of colloidal quantum dots may be as described herein. The use of a plurality of molecular clusters that are predominantly singly-doped (as described herein) produces a plurality of colloidal quantum dots that are also predominantly singly-doped. The method comprises the steps of a) providing a plurality of molecular clusters; b) providing precursors for the colloidal quantum dot; and c) combining and heating the molecular clusters and the precursors. The plurality of molecular clusters and the precursors for the colloidal quantum dot are in solution. Preferably, the molecular clusters and the precursors for the colloidal quantum dots are combined in the same solution prior to heating. The plurality of molecular clusters may be as described herein. The precursors for the colloidal quantum dot may be selected from those known to be suitable for the formation of colloidal quantum dots using molecular cluster based approaches. The precursors for the colloidal quantum dots are those suitable for forming colloidal quantum dots of the materials described herein under the heading “A Plurality of Colloidal Quantum Dots”. For example, an In source and a P source may be used to provide InP colloidal quantum dots, such as indium carboxylate and fr / s(trimethylsilyl)phosphine (TMSP). The method may further comprise a size-tuning step, wherein the colloidal quantum dots are grown further while retaining good control over particle size. The size-tuning step may comprise: a) Providing the previously formed plurality of the quantum dots; b) Providing precursors for the colloidal quantum dots; and c) Combining and heating the previously formed plurality of the quantum dots and the precursors for the colloidal quantum dots. The precursors for the colloidal quantum dot may be selected from those known to be suitable for size-tuning colloidal quantum dots. For example, an In source and a P source may be used to provide InP colloidal quantum dots, such as indium carboxylate and ZnP. The previously formed plurality of the quantum dots and the precursors for the colloidal quantum dot are in solution. Any suitable high-boiling solvent that is able to dissolve the precursors may be used. Typical solvents include Lewis base type coordinating solvents, such as a phosphine (e.g. TOP), a phosphine oxide (e.g. TOPO), an amine (e.g. oleylamine, hexadecylamine, and dodecylamine), non-coordinating organic solvents (e.g. alkanes and alkenes, such as squalane, squalene, and 1-octadecene), or heat transfer fluids (such as hydrogenated terphenyl (e.g. Therminol® 66), mixtures of biphenyl and diphenyl oxide (e.g. Dowtherm™), ethers, and xylenes). The solvent is preferably anhydrous and degassed prior to heating to the reaction temperature. Degassing may be performed to remove volatiles (such as low boiling solvents or dissolved gases) before or after addition of the precursors, molecular clusters and / or other components by heating the solvent to an intermediate temperature, such as 120°C, under a reduced pressure. Preferably, the previously formed plurality of the quantum dots and the first precursor for the colloidal quantum dots are combined in the same solution and heated together, followed by addition of the second precursor in one or more portions. For example, for InP colloidal quantum dots, the previously formed plurality of colloidal quantum dots is combined with the In precursor (such as indium carboxylate) and heated, followed by addition of P precursor (such as ZnP) in one or more portions. Any reaction temperature suitable for decomposition of the precursors may be used. In The reaction temperature may be at least 150°C, preferably at least 180°C, more preferably at least 200°C, further preferably at least 220°C, yet further preferably at least 240°C, or most preferably at least 280°C. The reaction temperature may be at most 300°C, preferably at most 280°C, more preferably at most 240°C, further preferably at most 220°C, yet further preferably at most 200°C, most preferably at most 180°C. In embodiments, the reaction temperature is between 150 and 300°C, preferably between 180 and 280°C, more preferably between 200 and 240°C, such as about 220°C. The colloidal quantum dots may have a size range of from 2 to 100 nm. This may be determined using dynamic light scattering to find an average hydrodynamic diameter. The method may further comprise providing each of the colloidal quantum dots with a shell. The shell material may be as described herein under the heading “A Plurality of Colloidal Quantum Dots”. In particular embodiments, the shell material may be formed of ZnSe / ZnS or CdSe / CdS, preferably ZnSe / ZnS. Devices Comprising Colloidal Quantum Dots The colloidal quantum dots of the present invention are suitable for incorporating into devices for quantum technologies. For example, the colloidal quantum dots may be incorporated into quantum repeaters, quantum computers, or quantum magnetometry devices. EXAMPLES Anhydrous solvents are used for all reactions and are stored over activated molecular sieves. Preparation of Indium Carboxylate A mixed indium carboxylate is used in the following syntheses of colloidal quantum dots. The mixed indium carboxylate comprising mixed indium acetates and myristates, with a molecular weight of 541.1 g / mol and an empirical stoichiometry of ln(Ma)i.48(Ac)i.52 (i.e. comprising 21% indium by mass). The mixed indium carboxylate is converted to indium myristate in situ during the degassing step at elevated temperature. Degassed Therminol 66 (7.594 kg) was charged to a 20 L reactor and heated to 35 °C, followed by addition of myristic acid (4.050 kg). Indium acetate (2.352 kg) was added and the mixture degassed at 200 rpm for 2 hours. The agitation was increased to 250 rpm and the temperature increased to 140 °C for 2 hours under vacuum, with acetic acid being collected in a distillation head. The reaction temperature was maintained for a further 4 hours under vacuum, after which the reaction mixture being golden-coloured and turbid. The reaction mixture was allowed to settle before half of the mixture was transferred to a 30 L filter and stirred with acetone (14.004 kg) to induce precipitation. A further charge of acetone (14.004 kg) is added before the second half of the reaction mixture is transferred to the filter. The precipitates were collected and washed with acetone (6 by 4.86 kg), with resuspension between filtrations. The precipitate is dried to constant mass via nitrogen purge (at 0.5 bar for 1 hour), under vacuum for 8 hours, and finally in a vacuum oven at 50 °C and 10 mbar. General Procedure for Obtaining Photoluminescence and Absorbance Spectra In general, absorbance spectra are obtained by diluting small amounts of the relevant solution or dissolving small amounts of the relevant solid in dry DCM. Photoluminescence spectra are typically measured at an excitation wavelength of 255 nm, scanning a range from 300 - 800 nm. For the photoluminescence spectra, solutions are diluted to optical densities of between 0.1 - 0.2 at 255 nm. Stage 1 - Manganese Source 6.75 mL of b / s(trimethylsilyl)sulphide (TMSS) was mixed with 25.25 mL of THF to prepare a 1 M TMSS solution. A dry three-necked flask was placed under a nitrogen atmosphere, charged with TMSS solution (1 M; 10 mL) and THF (84 mL) and cooled to 0°C. n-BuLi (1.6 M in THF; 6.25 mL) was then added ata rate of 1.25 mL / min, after which the reaction was maintained at a temperature of 0°C for 30 minutes. The solvent was then removed under vacuum to obtain LiS(TMS) as a white solid. In a second dry flask MnCh (0.31 g) was dissolved in DCM (40 mL), TMEDA (0.95 mL) was added and the mixture stirred for 1 to 2 hours. The LiS(TMS) was cooled to 0°C and the mixture added over a period of 30 minutes, the temperature of 0°C was maintained for 30 minutes following completion of the addition. The solvent was then removed under vacuum to leave a white precipitate. The white precipitate was dissolved in 20 mL of hexane and filtered over Celite, with the reaction vessel and solids being washed with hexane (3x10 mL). The filtrate was collected and the solvent evaporated under vacuum to form a white precipitate. The white precipitate is washed with cold hexane (2 x 1.5 mL) at a temperature of -75°C and the residual solvent extracted by syringe. The resulting manganese source has the formula [Li(TMEDA)]2[Mn(S(TMS))4] and the structure: Stage 2 - Capped Manganese ZnCh(TMEDA) (0.21 g) is dissolved in THF (5 mL), placed in a dry three necked flask under an atmosphere of nitrogen, and cooled to 0°C. The manganese source from Stage 1 (0.15 g) is dissolved in DCM (4 mL) and then added to the cooled solution of ZnCh(TMEDA) over a period of 30 minutes, after which the mixture is allowed to return to room temperature. The resulting solution is filtered over Celite before being layered with pentane and left to crystallise. The resulting capped dopant metal has the formula [Li(TMEDA)]2Mn[SZnCI(TMEDA)]4 and the structure: The absorbance spectrum showed no peak, but a steep onset at around 300 nm and the photoluminescence spectrum showed clear Mn2+ emission at around 620 nm (see Fig. 1). Stage 3 - Molecular Cluster The capped manganese from Stage 2 (0.5 mmol) was dissolved in DCM (5 mL), charged to a dry three necked flask, and cooled to -75°C. (TMEDA)Zn(S(TMS))2 (0.17 M in THF; 13 mL) was added dropwise while maintaining the temperature at-75°C. After 5 minutes, the mixture was allowed to warm to room temperature and stirred for a further 15 minutes. The mixture was then cooled to -75°C and (TMEDA)ZnCh (0.2 M in THF; 11.25 mL) was added dropwise while maintaining the temperature at -75°C. After 5 minutes, the mixture was allowed to warm to room temperature. Solids were removed by filtration to provide a clear, colourless liquid. As an alternative, the same procedure may be followed, replacing (TMEDA)ZnCh with (TMEDA)Zn(OAc)2. The absorbance spectrum showed a peak at 257 nm, corresponding to a Zn - S ligand-to-metal charge transfer transition and the photoluminescence spectrum showed clear Mn2+ emission at around 600 nm (see Fig. 2). Furthermore, the photoluminescence peak for the Stage 3 molecular cluster is blue shifted compared to the Stage 2 capped manganese, indicative of increased shielding due to the additional size of the molecular surrounding the single-Mn atoms. It should be noted that production of the cluster yields product both as a solid and in a solution. Without wishing to be bound by theory, it is believed that the solubility of the cluster is inversely proportional to its size (i.e. the solution tends towards smaller molecular clusters, whereas the solid tends towards larger molecular clusters). Stage 4 - Synthesis of Colloidal Quantum Dots The molecular cluster of Stage 3 (100 mg; 0.05 mmol) was dissolved in DCM (5 mL) and charged to a dry three-necked flask under a nitrogen atmosphere. Tr / s(trimethylsilyl)phosphine (TMSP) (1 M in Dowtherm® RP; 1 mL) was added over a period of 5 minutes and the resulting mixture stirred for 15 minutes at room temperature. A mixture of In carboxylate (2.71 g; 5 mmol), oleic acid (1.7 mL; 5.5 mmol), octadecylamine (0.67g, 2.5 mmol), and octadecene (23 mL) was degassed at 110°C for 1 hour and then cooled. The In-containing mixture (7.5 mL) was added to the threenecked flask containing the molecular cluster and TMSP over a period of 5 minutes and the resulting mixture stirred for 30 minutes at room temperature. The DCM was removed under vacuum and the remaining mixture was heated to 220°C and stirred at this temperature for 30 minutes before cooling to room temperature. It should be noted that the synthesis of the quantum dots may be ceased here. The smaller molecular clusters (i.e. from the solution in Stage 3) were used to produce quantum dots, the synthesis of which was stopped here. These quantum dots are referred to herein as “Sample A”. The quantum dots may be grown further by adding a further portion of the In-containing mixture (15 mL) was added to the reaction mixture, degassed at 110°C for 30 minutes, and placed under a nitrogen atmosphere. The reaction mixture was then heated to 250°C and a first portion of ZnP (0.46 M in Dowtherm® RP; 1 mL) was added at a rate of 4 mL / hour. The reaction mixture was then heated to 250°C and a further portion of ZnP (0.46 M in Dowtherm® RP; 1 mL) was added. The reaction mixture was then allowed to cool to room temperature. The larger molecular clusters (i.e. from the solid in Stage 3) were used to produce quantum dots, the synthesis of which was stopped here. These quantum dots are referred to herein as “Sample B”. The absorbance spectrum is shown in Fig. 3, with an absorption peak centred at 508 nm. Stage 5 - Overcoating of Colloidal Quantum Dots A dry three-necked flask was charged with colloidal quantum dots from Stage 4 and 1-octadecene (15 mL), degassed for 10 minutes, and placed under a nitrogen atmosphere. Zinc (II) acetate (1.92 g; 10.46 mmol), zinc (II) isostearate (1.31 g; 2.08 mmol), and ammonium bifluoride (0.10g; 1.75 mmol) were added and the mixture degassed at 110°C for 50 minutes. Trioctylphosphine (TOP; 1 mL) was added and the mixture degassed at 110°C for a further 10 minutes. The reaction mixture was then heated to 190°C and held at this temperature for 10 minutes, followed by heating at 210°C for 20 minutes, and heating at 230°C for 10 minutes. Trioctylphosphine selenide (TOPSe; 2 M in TOP; 0.92 mL) was added over a period of 40 minutes. 16 minutes after addition of TOPSe was commenced, the addition of 1-dodecanethiol (DDT; 3.07 mL) began for a period of 135 minutes. After all of the DDT was added, the reaction mixture was cooled to below 100°C. Zinc (II) chloride (1.00 g; 7.33 mmol) was then added and the mixture heated to 180°C for 60 minutes. 1-octanol (3 mL) was then added over a period of 60 minutes. The reaction was cooled to 75°C, toluene (15 mL) was added, and samples were taken for photoluminescence measurements. The reaction mixture was then cooled further to 40°C and acetone (70 mL) was added. The resulting precipitate was collected by centrifugation (7500 ref, 5 minutes) and the supernatant discarded, before being redissolved in toluene (15 mL) and re-precipitated (20 mL). The precipitate was again collected by centrifugation (7500 ref, 5 minutes) and the supernatant discarded. The solid was dissolved in toluene (10 mL), (7500 ref, 5 minutes) and filtered into a vial. The absorption and photoluminescence (PL) spectra of the isolated core / shell QDs are shown in Figure 4. The absorption peak is centred around 544 nm. The PL peak is centred around 582.0 nm, with a full-width-at-half-maximum (FWHM) of 58.1 nm and a quantum yield of 68 %. Characterisation of Colloidal Quantum Dots by Photoluminescence The colloidal quantum dots were subjected to photoluminescence measurements using a Horiba Jobin Yvon Fluorolog-3 (model FL3-22iHR) spectrofluorometer with a monochromated Xe lamp, with the resulting spectrum shown in Fig. 5 for the overcoated InP colloidal quantum dots. The curve fittings demonstrate that the spectrum for the InP colloidal quantum dots is consistent with the combined photoluminescence spectra for InP-based emission at 2.35 eV (526 nm) and Mn2+ 4Ti -► 6Ai transition at 2.12 eV (585 nm). Therefore it is confirmed that the InP colloidal quantum dots are doped with Mn2+. Characterisation of Colloidal Quantum Dots by Time-Resolved Photoluminescence (TRPL) The colloidal quantum dots were subjected to TRPL measurements, using a time correlated single photon counting (TCSPC) system. The system employed a Ti:sapphire laser (Mai-Tai HP, Spectra-Physics mode-locked) to produce 100 fs pulses at a repetition rate of 80 MHz. The repetition rate was reduced to 4 MHz by an acousto-optic pulse picker (APE Select) and the initial wavelength halved via second harmonic generation (APE Harmonic Generator). These pulses were used to excite the samples. The PL emission of the samples was collected and focused into a monochromator (Spex 1870c) and detected using a multi-channel plate (Hamamatsu R3809U-50). Figs. 6A and 6B show the PL spectra for overcoated colloidal quantum dots and overcoated undoped colloidal quantum dots respectively. In this example, the colloidal quantum dots are smaller than the undoped colloidal quantum dots, and so would be expected to have shorter emission times. However, they have longer emission times indicative of coupling between the InP colloidal quantum dots and the Mn2+ dopants within them, thereby providing further evidence that the InP colloidal quantum dots comprise the Mn2+ dopant. Characterisation of Colloidal Quantum Dots by Electron Paramagnetic Resonance Spectroscopy (EPR) The colloidal quantum dots from Stage 5 were subjected to EPR measurements using Bruker EMX Plus and Bruker E580X Spectrometers operating at X-band in continuous wave (CW) and Pulse mode respectively at 3 K. The results are shown in Figs. 7 and 8 for the Q-band (35 GHz) with magnetic fields swept at 12 T and the X-band (10 GHz) swept at 3 T respectively. Each band shows six narrow and well-defined peaks, consistent with the six spin states of Mn2+ (i.e. +5 / 2, +3 / 2, +1 / 2, -1 / 2, -3 / 2, and -5 / 2), which is consistent with what would be expected from InP colloidal quantum dots doped with a single Mn2+ atom. A first additional observation is that the narrowness of the bands indicates the Mn2+ ions are not interacting and so are isolated from one another, strongly implying that the doped quantum dots are predominantly singly-doped. A second additional observation is that the EPR signal became detectable when the number of quantum dots within the detection volume reached 5x1015 quantum dots. The EPR instrument used has a lower limit of detection of 1016 to 1017 spins in the detection volume, corresponding to 2x1015 to 2x1016 Mn2+ ions (each of which has 5 spins). This means that there are between at least 0.4 and 4 Mn2+ ions for each quantum dot. The first additional observation places an upper bound of 1 Mn2+ ion per quantum dot. The second additional observation places a lower bound of between 0.4 and 4 Mn2+ ions per quantum dot. Taken together, these two additional observations demonstrate that a substantial proportion of the quantum dots are singly-doped. Determination of Spin Lifetimes The Ti and T2 spin lifetimes were determined for the colloidal quantum dots using the exponential decay constants for the spin inversion and spin echo signals in the EPR measurements, respectively. Two samples of colloidal quantum dots were tested: Sample A and Sample B. For reference, Sample A were produced using the smaller, solution phase molecular clusters from Stage 3 with a single-step quantum dot growth in Stage 4, while Sample B were produced using the larger, solid phase molecular clusters from Stage 4 with two-step quantum dot growth in Stage 4. Both samples were overcoated as set out in Stage 5. Figs. 9 and 10 show the echo intensity over time for T1 and T2 respectively for each of the samples of colloidal quantum dots. It can be seen that both T1 and T2 are greatly extended for Sample B compared to Sample A. The biexponential form indicates the presence of two relaxation mechanisms: defect-related (e.g. surface defects) and nuclear spin-related. It is notable that T2 becomes monoexponential for the larger quantum dots, indicating that one of these pathways is, in effect, blocked. Without wishing to be bound by theory, it is thought that the use of larger quantum dots allows for greater separation between the Mn2+ atom and surface defects, thereby reducing their interaction and increasing spin lifetime. It is also thought that the use of a larger cluster allows for greater separation between the Mn2+ atom and the In and P in the colloidal quantum dot, thereby reducing their interaction and increasing spin lifetime. Each of T1 and T2 may be determined using Equation 1: (~T~--) (~T~I ) or 2 — ashorte short + on9 And Tave is an average of TShort and Tiong weighted by aShort / (ashort+aiOng) and aiong / (aShort+aiong) respectively. A-short (fl-short + Along') T1 short (ms) T 1long (ms) T lave (ms) Sample A 0.52 ±0.01 0.41 ±0.01 2.91 ±0.02 1.6 Sample B 0.56 ±0.01 3.51 ±0.08 16.9 ±0.3 9.4 fl-short (fl-short + Along) T2short (MS) T2long (MS) T2ave (MS) Sample A 0.49 ±0.01 0.52 ±0.01 2.73 ±0.02 1.6 Sample B N / A 2.98 ±0.01 N / A 3.0 Alternatively, Ti can be determined using Equation 2: Ainv(t) = ^(00) + Ajexpt- t / Tj + ASDexp(- t / Tso) The spin inversion decay, 4^(0, observed is Fig. 12 is characterised by a biexponential decay, where TSD is the spectral diffusion lifetime, and A± and ASD are amplitude parameters. Tsd (ms) T1 (ms) Sample A 0.37 ±0.02 2.3 ±0.3 Sample B 4.4 ±0.4 22 ±2 And T2 can be determined using Equation 3: leched = lecho(0)exp(-(t / TM)s) The decay of the echo intensity, ZechoCO. observed in Fig. 13 is well-described by a stretched exponential, where T2 is TM (the phase memory lifetime) and s is the stretch parameter. Tm (MS) s Sample A 2.3±0.1 0.89±0.01 Sample B 2.7±0.1 ps 1.01±0.02 Effect of Cluster Size Larger clusters were produced by two approaches. Firstly, the scale of the reaction was increased so as to add more ZnS to the capped dopant material in a single step. Secondly, sequential cycles of growth were applied to add more ZnS in a stepwise fashion. These larger clusters were used to synthesise overcoated InP colloidal quantum dots as described herein. The colloidal quantum dots were analysed by photoluminescence and absorbance spectroscopy, with the overlain absorbance spectra shown in Fig. 11. Cluster Photoluminescence (nm) FWHM (nm) QY (%) Smaller Cluster (as described above) 585 68 64 Larger Cluster -single step 580 58 70 Larger Cluster -stepwise growth 583 53 72 The use of larger clusters results in improved colloidal quantum dots. Without wishing to be bound by theory, it is believed that increasing the size of the clusters reduces their solubility, thereby facilitating their purification and improving the result processing steps (such as colloidal quantum dot formation). Greater improvement was observed for the clusters formed using stepwise growth, as compared to growth in a single step. Without wishing to be bound by theory, it is believed that better control over cluster size is retained using stepwise growth, resulting in reduced polydispersity in the resulting molecular cluster, as well as in the subsequent colloidal quantum dots made therefrom. High Angle Annular Dark Field (HAADF) Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) The colloidal quantum dots grown from larger clusters with stepwise growth were imaged using HAADF / TEM and were observed to have consistent size and tetrahedral morphology (as is seen in Figs. 14a) and b). The diffraction pattern observed from the area of Fig. 14b) within the white box is displayed in Figs. 14c) and d), and corresponds to a zincblende crystal structure with a lattice constant of 5.57 A. This value is between the lattice constant values of zincblende ZnSe (5.67 A) and ZnS (5.41 A) and is therefore what would be expected for a ZnSe / ZnS shell. Fig. 14e) is an EELS image of a small number of the colloidal quantum dots, with Figs 14f) and g) showing the EELS spectra for the indicated pixels containing a colloidal quantum dot and absent of a colloidal quantum dot respectively. In each spectrum Gaussian fits have been added for the Mn L2 transition (left hand curve) and L3 transition (right hand curve). Fig. 14f) has an Mn L2 transition peak at approximately three times the background and an Mn L3 transition peak at approximately double the background, which is consistent with what would be expected for a colloidal quantum dot comprising a single Mn atom. In contrast, each of the Mn L2 and Mn L3 transition peaks in Fig. 14g), for the control pixel absent of a colloidal quantum dot, are at the level of background noise, indicating an absence of Mn. This EELS data is consistent with what would be expected for singly-doped colloidal quantum dots. Effect of Shell Thickness Ordinarily, ZnSe / ZnS shells are provided in thicknesses of approximately 2 to 5 nm. ‘Giant’ shells (approx. 8 nm) were provided to a plurality of colloidal quantum dots and the sizes compared with unshelled colloidal quantum dots and those with typical ZnSe / ZnS shells (approx. 2.85 nm). Particle sizes were measured using Dynamic Light Scattering and will be understood to be hydrodynamic diameters. Shell Diameter (nm) PDI Shell thickness (nm) No Shell 4.4 0.181 - Typical Shell 10.1 0.049 2.85 Giant Shell 20.4 0.014 8 Clauses of the Invention Clause 1. A plurality of molecular clusters, each cluster comprising a network of Z and D coated with stabilising ligands selected from L, X, and combinations thereof, wherein in a substantial proportion of the molecular clusters each molecular cluster comprises a single atom of a dopant metal, M, wherein: L is a chelating ligand, D is a chalcogen, Z is Zn or Cd, and X is a halide. Clause 2. The plurality of quantum dots of clause 1, wherein at least about 40% of the molecular clusters comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. Clause 3. The plurality of quantum dots of clause 1 or clause 2, wherein substantially all of the molecular clusters comprising a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. Clause 4. The plurality of molecular clusters of any one of the preceding clauses, wherein L is a multidentate ligand, optionally being selected from diphosphines, diols, dithiols, such as benzene-1,2-dithiol, diethers, and diamines, such as ethylene diamine, tetramethylene diamine (TMEDA), 1,2-dimethylethylenediamine (DMEDA), and 1,1-dimethylethylenediamine. Clause 5. The plurality of molecular clusters of clause 4, wherein L is tetramethylene diamine (TMEDA). Clause 6. The plurality of molecular clusters of any one of the preceding clauses, wherein M is a transition metal, optionally being selected from Co2+, Ni2+, Fe2+, Cr2+, Cu2+, or Mn2+. Clause 7, the plurality of molecular clusters of clause 6, wherein M is Mn2+. Clause 8. The plurality of molecular clusters of any one of the preceding clauses, wherein D is selected from S, Se, and Te. Clause 9. The plurality of molecular clusters of clause 8, wherein D is S. Clause 10. The plurality of molecular clusters of any one of the preceding clauses, wherein X is selected from I; Br, or Cl’. Clause 11. The plurality of molecular clusters of clause 10, wherein X is Cl’. Clause 12. The plurality of molecular clusters of any one of the preceding clauses, wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal comprises a core comprising the dopant metal. Clause 13. The plurality of molecular clusters of clause 12, wherein the core has the formula [ZisMDi3]2+. Clause 14. The plurality of molecular clusters of clause 13, wherein the core having the formula [ZniaMnSi3]2+. Clause 15. The plurality of molecular clusters of any one of clauses 12 to 14, wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal consists of the core and the stabilising ligands. Clause 16. The plurality of molecular clusters clause 15, wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal has the formula l_6[Zi3MDi3]X2, Clause 17. The plurality of molecular clusters of clause 16, wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal has the formula (TMEDA)6[Zni3MnSi3]Cl2. Clause 18. The plurality of molecular clusters of any one of clauses 12 to 14, wherein each molecular cluster of the substantial proportion of the molecular clusters that each comprise a single atom of dopant metal comprise additional Z and D between the core and the stabilising ligands. Clause 19. The plurality of molecular clusters of any one of the preceding clauses, wherein the plurality of molecular clusters has an atomic ratio of Z to M of at least about 13:1, optionally from 13:1 to 100:1, from 13:1 to 50:1, from 13:1 to 25:1, such as about 13:1. Clause 20. The plurality of molecular clusters of any one of the preceding clauses, wherein the plurality of molecular clusters has an atomic ratio of Z to D is from 2:1 to 1:2, such as about 1:1. Clause 21. A method for producing a plurality of molecular clusters, the method comprising: a) providing a solution of a capped dopant metal, the capped dopant having the formula [AL]2[M(D(ZXL))4], b) mixing a first solution comprising LZ(D(TMS))2 with the solution of the capped dopant metal, and c) mixing the product of step b) with a second solution comprising LZX2 or LZY2 to form a solution of molecular clusters, wherein: A is a cation, L is a chelating ligand, M is a dopant metal, D is a chalcogen, Z is Zn or Cd, X is a halide, and Y is a carboxylate and wherein in a substantial proportion of the molecular clusters each molecular cluster comprises a single atom of the dopant metal, M. Clause 22. The method of clause 21, wherein at least about 40% of the molecular clusters comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. Clause 23. The method of clause 21 or clause 22, wherein substantially all of the molecular clusters comprise a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. Clause 24. The method of any one of clauses 21 to 23, wherein A is a monovalent cation or a divalent cation, optionally being an alkali metal cation, such as Li+, Na+, or K+, or an alkaline earth metal cation, such as Mg2+. Clause 25. The method of clause 24, wherein A is Li+. Clause 26. The method of any one of clauses 21 to 25, wherein L is a multidentate ligand, optionally being selected from diphosphines, diols, dithiols, such as benzene-1,2-dithiol, diethers, and diamines, such as ethylene diamine, tetramethylene diamine (TMEDA), 1,2-dimethylethylenediamine (DMEDA), and 1,1-dimethylethylenediamine. Clause 27. The method of clause 26, wherein L is tetramethylene diamine (TMEDA). Clause 28. The method of any one of clauses 21 to 27, wherein M is a transition metal, optionally being selected from Co2+, Ni2+, Fe2+, Cr2+, Cu2+, or Mn2+. Clause 29. The method of clause 28, wherein M is Mn2+. Clause 30. The method of any one of clauses 21 to 29, wherein D is selected from S, Se, and Te. Clause 31. The method of clause 30, wherein D is S. Clause 32. The method of any one of clauses 21 to 31, wherein X is selected from I’ , Br, or Ch. Clause 33. The method of clause 32, wherein X is Cl’. Clause 34. The method of any one of clauses 21 to 33, wherein Y is selected from formate, acetate, propionate, butyrate, and isobutyrate. Clause 35. The method of any one of clauses 21 to 34, wherein the capped dopant metal has the formula [Li(TMEDA)]2[Mn(S(ZnCI(TMEDA))4]. Clause 36. The method of any one of clauses 21 to 35, wherein the first solution comprises (TMEDA)Zn(S(TMS))2. Clause 37. The method of any one of clauses 21 to 36, wherein the second solution comprises (TMEDA)ZnCh or (TMEDA)Zn(OAc)2. Clause 38. The method of any one of clauses 21 to 37, wherein the mixing of the first solution with the solution of the capped dopant metal and / or mixing of the second solution with the intermediate solution occurs at reduced temperature. Clause 39. The method of any one of clauses 21 to 38, further comprising enlarging the molecular clusters by: d) mixing a further volume of the first solution comprising LZ(D(TMS))2 with the solution of the molecular clusters, and e) mixing the product of step d) with a further volume of the second solution comprising LZX2 or LZY2 to form a solution of enlarged molecular clusters. Clause 40. The method of any one of clauses 21 to 39, wherein providing the solution of the capped dopant metal having the formula [AL]2[M(D(ZXL))4] comprises: a) providing a third solution comprising LZX2, b) providing a solution of a dopant metal source, wherein the dopant metal source is a compound with formula [AL]2[M(D(TMS))4]; and c) adding the solution of the dopant metal source to the third solution to provide a solution of the capped dopant metal. Clause 41. The method of clause 40, wherein the third solution comprises (TMEDA)Zn(S(TMS))2. Clause 42. The method of clause 40 or clause 41, wherein the dopant metal source has the formula [Li(TMEDA)]2[Mn(S(TMS))4]. Clause 43. The method of any one of clauses 40 to 42, wherein providing the dopant metal source comprises: a) providing AD(TMS); b) providing a dopant metal solution, the dopant metal solution comprising the dopant metal, M, and a chelating ligand, L; and c) mixing the dopant metal solution with the AD(TMS) to form the dopant metal source. Clause 44. The method of any one of clauses 21 to 43, wherein the plurality of molecular clusters is a plurality of molecular clusters according to any one of claims 1 to 12. Clause 45. A plurality of molecular clusters obtained or obtainable by the method of any one of clauses 21 to 44. Clause 45. The use of a plurality of molecular clusters according to any one of clauses 1 to 20 and 45 to produce a plurality of colloidal quantum dots, wherein in a substantial proportion of the colloidal quantum dots each colloidal quantum dot comprises a single atom of a dopant metal, M. Clause 46. A plurality of colloidal quantum dots, wherein in a substantial proportion of the colloidal quantum dots each colloidal quantum dot comprises a single atom of a dopant metal, M. Clause 47. The plurality of colloidal quantum dots of clause 46, wherein at least about 40% of the colloidal quantum dots comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. Clause 48. The plurality of colloidal quantum dots of claim 34 or claim 35, wherein substantially all of the colloidal quantum dots comprise a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. Clause 49. The plurality of colloidal quantum dots of any one of clauses 46 to 48, wherein each dopant metal atom is in a molecular cluster, optionally the molecular clusters are as defined in any one of claims 1 to 12 or 29. Clause 50. The plurality of colloidal quantum dots of any one of clauses 46 to 49, wherein the colloidal quantum dots comprise lll-V material, IIA-IIB material, IIB-VIB material, ll-V material, lll-IV material, lll-VI material, IV-VI material, or material including a first element from any transition metal group of the periodic table and a second element from group 16 of the periodic table. Clause 51. The plurality of colloidal quantum dots of any one of clauses 46 to 50, wherein the colloidal quantum dots comprise lll-V materials (such as InP) or IIB-VIB materials (such as CdSe). Clause 52. The plurality of colloidal quantum dots of any one of clauses 46 to 51, wherein each colloidal quantum dot has a shell, optionally each shell having a thickness of from 5 to 20 nm. Clause 53. The plurality of colloidal quantum dots of any one of clauses 46 to 52, wherein the Ti spin lifetime of the dopant atom is at least 1 ms when determined using Equation 1 and / or the Ti spin lifetime of the dopant atom is at least 2 ms when determined using Equation 2. Clause 54. The plurality of colloidal quantum dots of any one of clauses 46 to 53, wherein the T2 spin lifetime of the dopant atom is at least 1 ps when determined using Equation 1 and / or the T2 spin lifetime of the dopant atom is at least 1 ps when determined using Equation 3. Clause 55. The plurality of colloidal quantum dots of any one of clauses 46 to 54, wherein the T2 spin echo intensity decays monoexponentially. Clause 56. A method for producing a plurality of colloidal quantum dots, the method comprising: a) providing a plurality of molecular clusters; b) providing precursors for a colloidal quantum dot; and c) combining and heating the molecular clusters and the precursors, and wherein in a substantial proportion of the colloidal quantum dots each colloidal quantum dot comprises a single atom of the dopant metal, M. Clause 57. The method of clause 56, wherein at least about 40% of the colloidal quantum dots comprise a single atom of the dopant metal, preferably at least about 50%, more preferably at least about 60%, further preferably at least about 70%, yet further preferably at least about 80%. Clause 58. The method of clause 56 or clause 57, wherein substantially all of the colloidal quantum dots comprise a single atom of the dopant metal, optionally wherein at least about 90%, preferably at least about 95%, more preferably at least about 98%, further preferably at least about 99%, yet further preferably at least about 99.5%, and most preferably at least about 99.9%. Clause 59. The method of any one of clauses 56 to 58, wherein the plurality of molecular clusters is as defined in any one of claims 1 to 12 and 32. Clause 60. The method of any one of clauses 56 to 59, wherein the plurality of colloidal quantum dots is as defined in any one of claims 34 to 43. Clause 61. The method of any one of clauses 56 to 60, wherein the method further comprises a size-tuning step. Clause 62. The method of any one of clauses 56 to 61, wherein the method further comprises providing each of the colloidal quantum dots with a shell. Clause 63. A plurality of colloidal quantum dots obtained or obtainable by the method of any one of clauses 56 to 62. Clause 64. A quantum device comprising one or more colloidal quantum dots from the plurality of colloidal quantum dots according to any one of clauses 46 to 55 and 63, such as a quantum repeater, a quantum computer, or a quantum magnetometry device.

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