Alloyed Crystalline CdSe1-xSx Semiconductive Nanomaterials - A Solid State 113Cd NMR Study.

Solid-state NMR analysis on wurtzite alloyed CdSe1-xSx crystalline nanoparticles and nanobelts provides evidence that the 113Cd NMR chemical shift is not affected by the varying sizes of nanoparticles, but is sensitive to the S/Se anion molar ratios. A linear correlation is observed between 113Cd NMR chemical shifts and the sulfur component for the alloyed CdSe1-xSx (0<x<1) system both in nanoparticles and nanobelts (δCd=169.71⋅XS+529.21). Based on this correlation, a rapid and applied approach has been developed to determine the composition of the alloyed nanoscalar materials utilizing 113Cd NMR spectroscopy. The observed results from this system confirm that one can use 113Cd NMR spectroscopy not only to determine the composition but also the phase separation of nanomaterial semiconductors without destruction of the sample structures. In addition, some observed correlations are discussed in detail.

Introduction

High‐quality colloidal, single crystalline semiconductor nanoparticles (well known as quantum dots, QD) exhibit size‐dependent optoelectronic properties, and have generated interest from both a fundamental science and a technological perspective.1, 2, 3, 4, 5 Therefore, their properties are currently under intensive study for diverse applications such as nanoelectronic devices, QD lasers, biosensing, and biolabeling.6, 7, 8, 9, 10, 11, 12 Recent advances have led to the development of core‐shell QDs,13, 14, 15 doped magnetic nanoparticles,16, 17, 18 and QD quantum‐well heterostructures.19, 20 As an alternative to tuning the electronic, optical, and magnetic properties by varying the size of the nanoparticles it is also possible to alter the composition of the nanoparticles. Many studies have focused on tuning nanoparticle properties by changing the composition. The ternary alloyed semiconductor nanoparticles, such as CdSeTe,21 CdSeS,22 ZnCdSe,23 and ZnCdS,24 have been synthesized and the dependence of properties on composition has been demonstrated.

A typical synthesis method used to generate QDs in solution is a colloidal nucleation process. As is known from the thermodynamic equilibrium constants, the nucleus formation order differs vastly between differing cation and anion pairs (e. g. K CdS

NMR techniques have previously been successfully applied in the investigation of semiconductor nanomaterials.25 Douglass et al. have measured three sizes of CdSe nanoparticles by 77Se NMR,26 and Vega et al. have studied the surface properties of precipitated nanoparticles of CdS by 113Cd NMR.27 From the available literature, a couple of research groups, e. g. Alivisatos and Strouse, have studied the nanoparticles of InP by 31P NMR spectroscopy and of CdSe by 113Cd and 77Se NMR spectroscopy.28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 Very recently, M. V. Kovalenko et al. have applied dynamic nuclear polarization (DNP) surface enhanced NMR spectroscopy to study the structures of InP and CdSe colloidal quantum dots (QDs); this technique makes it more efficient to study their cores, surfaces and capping ligands.41, 42 In this study, we have systemically synthesized different compositions of alloyed ternary CdSe1−xSx nanoparticles and nanobelts and used these systems for the indirect quantitative determination of alloyed nanoparticle compositions via 113Cd NMR spectroscopy.

Results and Discussion

CdSe1−xSx Nanoparticles

Four samples of a wurtzite hexagonal structure of the alloyed ternary nanocrystals of CdSe1−xSx (0

Figure 1

XRD patterns for CdSe1−xSx (0

Figure 2

TEM images of (a) CdSe0.41S0.59, (b) CdSe0.31S0.69, (c), CdSe0.22S0.78 and (d) CdSe0.15S0.85.

CdSe1‐xSx Nanobelts

Hydrothermal synthesis of alloyed CdSe1−xSx nanomaterials in neat ethylenediamine is straightforward and the formed products show different colors for different molar fractions of sulfur and selenium, from yellow to black (Figure 3). The synthesis reaction is near quantitative with yields of all four preparations reaching 93–98 %; elemental analysis showed that the compositions agree with our expected results (Table 1). The XRD patterns are similar to those for the CdSe1−xSx nanoparticles; all are in a wurtzite hexagonal structure (SI, Figure S1). SEM shows they are all nanobelts with lengths of a few micrometers (Figure 3).

Figure 3

SEM images of CdSe0.5S0.5 nanobelts (top) and their colors (bottom). a, b. c, d, e and f represents the color of different compositions of nanobelts shown in Figure 6. a: CdSe/CdSe1.0S0.0, b: CdSe0.8S0.2, c: CdSe0.6S0.4, d: CdSe0.5S0.5, e: CdSe0.2S0.8, and f: CdS/CdSe0.0S1.0.

Table 1

Experimental details of synthesis and elemental analysis results for the CdSe1‐xSx (0

Nanoparticles (mixed millimoles of chemical precursors)
S	4.8	5.0	5.2	4.8
Se	0.6	0.5	0.4	0.3
CdO	1.2	1.2	1.2	1.2
E.A. results	CdSe0.41S0.59	CdSe0.31S0.69	CdSe0.22S0.78	CdSe0.15S0.85

Figure 6

113Cd NMR spectra of different composition of alloyed CdSe1−xSx nanobelts in the solid state (each sample shows a different color; see Figure 3).

Nanobelts (mixed millimoles of chemical precursors)

0.2

0.8

0.8

0.2

Cd(ClO4)2

1.0

1.0

1.0

1.0

Yield (%)

94.5

92.5

93.3

97.4

Supposed

CdSe0.8S0.2

CdSe0.6S0.4

CdSe0.5S0.5

CdSe0.2S0.8

CdSe0.80S0.19

CdSe0.58S0.39

CdSe0.48S0.50

CdSe0.20S0.80

T 1 Measurements

To obtain quantitative information from the 113Cd NMR spectra, the recycle delay should be at least five times greater than the longest 113Cd T 1 (i. e., t>5T 1). Therefore, the 113Cd NMR T 1 values for CdS, CdSe, and CdTe bulk powders, as well as for one nanobelt sample, CdSe0.5S0.5 (i. e., x=0.5), were measured using the saturation recovery technique (data see Table 2). The results show that the 113Cd T 1 of bulk powders dramatically increases through the series S, Se, Te with that for the CdSe0.5S0.5 nanobelts falling in between those for CdSe and CdTe. These results show that they are all have very long T 1 values in solid bulk phases and even longer in the nano phase such as in nanobelt materials (data in Table 2 and details in SI, Figure S2).

Table 2

113Cd chemical shifts and T 1 values for alloyed CdSe1−xSx (0

Chemicals	113Cd CS [ppm]	T1 [s]	Literature data46
Individual phase			CS (ppm)
Cd‐Octadecene	−60.5 (0.1)
CdO	382.0 (0.1)
CdTe	273.0 (0.1)	385.4 (0.1)	283.0
CdSe	532.0 (0.1)	185.6 (0.1)	550.0
CdS	697.6 (0.1)	36.0 (0.1)	692.0

Nanobelts

CdSe0.8S0.2

558.0 (0.2)

CdSe0.6S0.4

591.0 (0.2)

238.8 (0.1)

CdSe0.5S0.5

624.0 (0.2)

CdSe0.2S0.8

662.0 (0.2)

Nanoparticles

630.0 (0.2)

646.0 (0.2)

658.0 (0.2)

674.0 (0.2)

The magnetically active isotopes of cadmium, 111Cd and 113Cd, have nuclear spin (I= ) and thus do not experience electric quadrupole interactions. All the possible relaxation mechanisms for these nuclei are therefore of magnetic origin; i. e., the spin systems achieve thermal equilibrium with the lattice as a result of transitions (spin flips) induced by local, fluctuating magnetic fields at the nuclear sites. The energy gaps of the semiconductors increase in the order of CdTe < CdSe < CdS, which is the same order as our measured longitudinal relaxation rates. This clearly indicates that the short 113Cd T 1 values correlate with a high energy gap. A linear correlation was found between the measured 113Cd T 1 of each CdS, CdSe and CdTe individual bulk semiconductor material and its corresponding chemical shift (SI, Figure S3).

113Cd NMR Chemical Shifts of Nanoparticles

In a typical colloidal synthesis, one often controls nanoparticle sizes by varying the nucleation times. We have measured the 113Cd NMR chemical shifts of four alloyed CdSe1−xSx nanoparticle samples as a function of reaction time or nanoparticle diameter, all of which had the same initial composition (molar ratio of Cd : S : Se=1.0 : 4.2 : 0.3), but prepared with different nucleation times (10, 30, 60 and 240 minutes). Identical 113Cd NMR chemical shifts were obtained for all four samples (Figure 4), indicating that the chemical shift is not affected by varying the diameters of the nanoparticles. This behavior was also proved by measuring the NMR chemical shifts of three different sizes of CdSe nanoparticles. 113Cd NMR spectra of three different diameters of CdSe nanoparticles (3.6 nm, absorption at 532 nm; 4.5 nm, absorption at 562 nm; 8.0 nm, absorption at 642 nm) have been measured (SI, Figure S4 and Figure S4 (continue)). The spectra show that the chemical shifts are not sensitive to the nanoparticle sizes. The results clearly show that 113Cd NMR chemical shifts do not change with changing CdSe nanoparticle diameters. However, the 113Cd chemical shifts did change as a function of composition. For the four different compositions of alloyed CdSe1−xSx nanoparticles, the 113Cd NMR spectra show a broad symmetric signal for each individual, and it shifted to higher ppm values with increased sulfur content. 113Cd NMR chemical shifts (δCd) correlate (Figure 5) to the sulfur molar fraction (XS).

Figure 4

113Cd NMR spectra of CdSe0.22S0.78 nanoparticles prepared with different nucleation times (a: 10 minutes, b: 30 minutes, c: 60 minutes and d: 4 hours).

Figure 5

113Cd NMR spectra for different compositions of alloyed CdSe1−xSx nanoparticles in the solid state. These four samples XRD (as shown in Figure 1) and TEM images (as shown in Figure 2). XRD show the similar line widths and the diffraction peaks are slightly shift to CdS peak position and TEM images measured these four samples of nanoparticles diameters are a): 8.76±1.27 nm; b): 7.49±1.09 nm; c): 9.48±2.06 nm; and d): 9.39±1.71 nm (details in Figure S14 in SI).

To further elucidate this correlation, alloyed CdSe1−xSx nanobelts of precise compositions were synthesized using the hydrothermal method in a neat ethylenediamine solution. 113Cd NMR spectra of all the samples were obtained (Figure 6) and they present a very good linear correlation with the sulfur mole fraction (Figure 7). Equation (1) was obtained through a linear fit of all the experimental data from both nanoparticles and nanobelts.

Figure 7

Linear correlations between 113Cd NMR chemical shifts and the sulfur molar fraction (x) for CdSe1‐xSx (0

Thus, based on Equation (1), the composition formulas (x) for any synthesized type of CdSe1−xSx alloyed semiconductor material, including alloyed nanoparticles, can be determined by measuring their 113Cd NMR chemical shifts.

Chemical Shifts

In principle, the Ramsey equation describes the chemical shift for an atom A (δA) as composed of two terms, diamagnetic (δdia) and paramagnetic ((δpara).43 The diamagnetic contribution (δdia) is mainly due to the core electrons and can determine the chemical shifts of the lightest nuclei. For heavier nuclei, this diamagnetic contribution remains essentially constant and the paramagnetic term determines the chemical shifts.44 For the late transition elements, an ab initio study of metal nuclei chemical shifts in complexes of Cu, Ag, Cd and Zn concluded that for contributions to δpara the d mechanism is predominant for Cu, the shielding decreasing with increase in π(M→L) charge transfer as ClCl>CH3.45 Very recently, M. V. Kovalenko et al.42 applied DFT calculations to elucidate the magnetic shielding of cadmium in [Cd(XH2)4−n(YH2)n]2+ complexes (X, Y=O, S, Se and Te). Four different contributions (diamagnetic (σdia), paramagnetic (σpara), spin‐orbital (σso) and isotropic (σiso)) to the magnetic shielding of cadmium are explored individually and their sum contributions relative to the 113Cd shielding of [Cd(OH2)4]2+. The calculated results also indicate that the shielding to the bonding central Cd nuclei is in the atomic order O>S>Se>Te which is also consistent with this electronegativity model.42 Therefore, we can conclude that metal chemical shifts are primarily due to the metal p and d orbital contributions by the donation of electrons from the ligands to the metal outer p orbitals and by the back‐donation of electrons from the metal p orbital to the ligands. In other words, the electrons in the outer p orbitals and the holes in the valence d orbitals produce the metal chemical shifts. In principle, this model can be used here to explain the observed 113Cd chemical shifts. The electronegativity of S, Se and Te is 2.589, 2.424 and 2.158 respectively. The ability of donating electrons to the Cd(II) outer p orbitals and withdrawing electrons from the Cd(II) valence d orbitals is proportional to the bonding elemental electronegativity. 113Cd chemical shifts for CdS, CdSe and CdTe were reported early in the literature,46 and we have measured the 113Cd NMR chemical shifts of CdS, CdSe and CdTe under the same conditions (Table 2); the experimental data is very close to that reported in the literature. The 113Cd NMR chemical shifts of CdS, CdSe and CdTe fit very well to the electronegativity model. A linear plot was obtained between the electronegativity of S, Se and Te elements and their 113Cd chemical shifts (SI, Figure S5). This model is applied to evaluate our measured chemical shifts of alloyed CdSe1−xSx nanomaterials. The plot of 113Cd chemical shifts vs. summation of all anion elemental electronegativity contributions around the Cd nuclei (x⋅ENS+(1−x)ENSe, where x is the sulfur fraction of the alloyed nanoparticles) exhibits a good linear correlation and fit this model very well (Figure 8). Such a correlations has been observed in ZnX4 2−, CdX4 2−,47, 48, 49, 50, 51 PtX6 2− and PtX4 2−[52-59] halide complexes (X=F, Cl, Br and I) and a similar linear dependence has also found in the Cd(II)60 and Tl(III) nitrogen and oxygen donor ligand complexes in solution.61, 62 In the DFT calculations on [Cd(XH2)4]2+ (X=O, S, Se and Te) by M. V. Kovalenko et al. discussed above, a linear behavior when gradually exchanging of chalcogenide ions around cadmium cations was found.42 The chemical shift of cadmium coordinating to sulfur and selenium atoms thus approximately corresponds to a weighted linear combination of the pure CdS and pure CdSe chemical shifts. This is in good agreement with our studied alloyed CdSe1−xSx system as exchanging replacement of S/Se bonding atomic ratio in the Cd coordination sphere along with the linear correlation chemical shift change trend.

Figure 8

Linear correlation between EN of alloyed CdSe1−xSx nanoparticles and 113Cd NMR chemical shifts.

As shown in the literature, optical properties such as absorption and photoluminescence vary with the sizes of nanoparticles, due to quantum confinement. It is well known that the NMR chemical shift is very sensitive to the chemical environment of the observed nucleus. Considering the data reported in the literature, the deshielding has been observed as the nanoparticle diameter decreases for InP29 and CdSe26 by 31P and 77Se NMR respectively. To the best of our knowledge, there have been no reports of corresponding 113Cd NMR measurements for CdSe nanoparticles. Very surprising to us, we could not detect any obviously changes of 113Cd NMR chemical shifts when the nanoparticle diameters change for both CdSe and alloyed CdSe1−xSx systems. We do not fully understand why the 77Se chemical shifts are much sensitive to size than those for 113Cd, but we note that 77Se has a much greater chemical shift range (approximately 3000 ppm) than does 113Cd (approximately 650 ppm).29 One can consider that the ionic radius of Se2− is much larger than that for Cd2+ inside the CdSe crystal lattices. It has indicated that the 77Se NMR chemical shifts are much sensitive than 113Cd NMR chemical shifts in the previous study.

Phase Study

The appearance of a single resonance in the 113Cd NMR spectra is evidence of homogeneity and not phase separation in these samples. However, we investigated the possibility of phase separation by quenching the samples with a cold ethanol and toluene solution (1 : 1 volume ratio). The 113Cd NMR spectra of the quenched samples obviously differed from the spectra obtained for the samples prepared by slowly cooling process. Two separate peaks with small shoulders were observed for each sample, one at greater than 580 ppm and another at less than 360 ppm (Figure 9). This indicates at least two different Cd(II) nuclear chemical environments, or separate phases, present inside the quenched alloyed nanoparticles. One can speculate that the quenching process may cause a phase separation between the nanoparticle core and surface or that it may bring about surface oxidation during fast quenching in air. 113Cd NMR spectra of CdO and Cd‐octadecene complexes show a single resonance at 382 and –60.5 ppm, respectively (SI, Figure S6), which rule out the possibility of surface oxidation. Compared to the 113Cd NMR spectra of the slow‐cooled samples, the signal above 580 ppm is assigned to Cd(II) nuclei inside the core of the nanoparticles, and the other below 360 ppm, is probably a formation of a defective phase on the particle surface or shell as a result of quick quenching. Core represents by blue and shell by light blue as shown in Figure 9. The chemical shift of greater than 580 ppm (low field peak) is assigned to the core of nanoparticles and less than 360 ppm (up field peak) assigned to the shell of nanoparticles in each quenching sample (Figure 9). Because all the chemical shifts of quenching samples greater than 580 ppm signal is well fit the equation of (1) that means the 113Cd NMR signals come from the same phases as these in no quenching synthesis samples (SI, Figure S7). XRD data indicate a cubic phase arises from the quenched samples rather than the hexagonal phase obtained for the slow‐cooled samples (SI, Figure S8). Thus, quenching CdSe1−xSx changes the lattice pattern and causes a phase separation as well. To further demonstrate the phase separation, two separate phases of bulk CdS (yellow‐brown) and bulk CdSe (black) were mixed (1 : 1 molar ratio) and grinded by agate until the phase separation could not distinguish by visual inspection. The 113Cd NMR spectrum (top spectrum in Figure 9 or SI, Figure S9) clearly shows two separate symmetric signals corresponding to those for pure CdS and CdSe individual, respectively. This result encourages us to believe that NMR spectra can be used to discern phase separation, and identify whether the phase is either homogeneous or heterogeneous.

Figure 9

113Cd NMR spectra of CdSe1−xSx (0

Correlations

XRD data show that the unit cell volumes of different alloyed CdSe1−xSx nanomaterials shrink when the sulfur molar fraction increases. This is expected since the sulfur radius is less than that for selenium. A plot between the unit cell volume, determined by X‐ray powder diffraction, and the corresponding 113Cd NMR chemical shift is both for CdSe1‐xSx nanoparticles and nanobelts (SI, Figures S10–S11). This observation provides further evidence that the phase of the alloyed nanomaterials is homogeneous. It has found that this phenomenon also perfectly fit the bulk powder phases of CdS, CdSe and CdTe (SI, Figure S12). Thus, 113Cd NMR measurements provide another possible way to determine the unit cell volume for the alloyed CdSe1−xSx nanoparticle system.

Discussions

All 113Cd NMR spectra were measured in stationary solid state using solid state NMR facility. There is no systemic data available for series of different compositions of alloyed CdSe1−xSx nanomaterials.27 However some data related to pure individual phase for bulk and nanoparticles, e. g., CdS, CdSe and CdTe were available and two types of chemical shift references were used, either solid Cd(NO3)2 4H2O35, 36, 38, 39 or 0.1 M Cd(ClO4)2 6H2O in water.40, 46 Because there is a 100 ppm difference in the chemical shifts of these two reference compounds, one must exam literature data carefully when comparing obtained values with reported values.50, 64 All our chemical shift data measured here were all referenced to 0.1 M Cd(ClO4)2 6H2O in water.

CdS and CdSe crystalline in hexagonal structure with ZnO arrangement, cadmium anisotropic nuclear magnetic shielding were observed and the order of magnitude are very similar and in small values.46 CdTe is cubic lattice and thus no anisotropic magnetic shielding has been observed.46 The 113Cd NMR spectra of these three powder samples (CdS, CdSe and CdTe) all yielded symmetric signals with the line‐widths increasing according to CdS>CdSe>CdTe) (SI, Figure S5) without any indication of anisotropic shielding. Our 113Cd NMR spectra are similar to those reported for sulfur‐rich CdS nanoparticles and obviously different from those for the cadmium‐rich CdS nanoparticles which displays a broad 113Cd spectrum pattern with many signals.27 Indeed our alloyed CdSe1−xSx nanomaterials were all anion‐rich in the initial reactants so the alloyed nanoparticle phase is a sulfur‐rich system.

There is only one study of alloyed gradient CdSe1−xSx nanoparticles with varying composition from core to a few different outside shells that included 113Cd NMR spectroscopy, but the authors observed many 113Cd NMR peaks.38 Unfortunately the compositions of the core and shells were not determined accurately. Nevertheless, allowing for the different referencing, our NMR data are reasonably close to that reported by these authors.38

There has not been a careful 113Cd NMR investigation of spin‐lattice relaxation times (T 1) or relaxation rates (1/T 1) for these CdS, CdSe and CdTe semiconductive materials. We have measured 113Cd T 1 times for these three bulk materials and for some alloyed CdSe1−xSx nanobelts (Table 2). There are some correlations between energy gaps and their corresponding T 1s warranting further consideration. Plots energy gaps (E g) vs T 1 are all linear with coefficients of determination R 2 value 0.88223, E g vs 1/T 1 with R 2=0.97665 and E g vs log(1/T 1), R 2=0.99749 (See SI, Figure S13). This data suggests that the materials’ spin‐lattice relaxation times may be related to the energy gaps.65, 66 However, little literature data is available and here only three samples were studied; more research as well as theoretical calculations are needed to ascertain whether this is a true correlation or merely coincidental.

Conclusions

In conclusion, we have demonstrated that the 113Cd NMR chemical shifts of alloyed CdSe1−xSx materials are not affected by the nanoparticle size, but do change depending on the bound anion about the Cd(II) cation. A linear correlation has been observed between 113Cd NMR chemical shifts and the sulfur content (i. e., x) for CdSe1−xSx, both in nanoparticles and nanobelts. Based on this correlation, we have developed a rapid, easy approach to determine the composition of the alloyed nanoscale materials utilizing 113Cd NMR spectroscopy. The results for the selected system confirmed that one may determine the compositions and phase separations of semiconductor nanomaterials without destruction of the sample structures. This study expands the methodology for determining alloyed nanoparticle composition using a well‐known NMR technique and it can be further applied to study semiconductor alloyed nanofilms, nanorods and nanowires as well as various types of alloyed bulk materials. In addition, the 113Cd spin‐lattice relaxation rate may be related to the energy gap; more examples are needed to verify this correlation. If there is indeed a correlation, then one may measure the relaxation rate and readily predict the material's energy gap.

Experimental Section

Materials and Methods

Materials

Cadmium oxide (CdO, 99.99 %), selenium powder (Se, 99.9 %), sulfur powder (99.98 %), Cadmium perchlorate hydrate (99.999 %), tri‐n‐octylphosphine (TOP, 90 %), oleic acid (OA, 90 %) and 1‐octadecene (90 %) were purchased from Aldrich. Hexylphosphonic acid (HPA) was purchased from the PolyCarbon Company. Carbon‐coated copper grids (200 mesh) for preparing the TEM specimens were purchased from Electron Microscopy Sciences.

Colloidal Synthesis of Alloyed CdSe1‐xSx Nanoparticles in Solution

The alloyed CdSe1−xSx (0

Hydrothermal Synthesis of Alloyed CdSe1−xSx Nanobelts in Neat Ethylenediamine

The calculated amounts of sulfur, selenium and Cd(ClO4)2 as shown in the Table 1 were loaded into an autoclave filled with 6 ml pure neat ethylenediamine liquid, sealed and then heated for 20 hours under magnetic stirring (the heater capacity was set to 300 °C). After the reaction finished, the heater was switched off and stirring continued until the solution slowly cooled to room temperature. The products were isolated and purified by several cycles of washing with ethanol and water with centrifugation and decantation. The purified products were dried under vacuum, collected then weighed to calculate the yields. The details of the preparations are listed in Table 1 together with the elemental analysis results.

Characterization

The final isolated products were characterized by X‐ray powder diffraction (XRD) (Rigaku Multiflex X‐ray diffractometer with Cu‐Kα radiation, λ=0.154178 nm at 35 kV and 35 mA) and by transmission electron microscopy (TEM) (JEOL 2000 FX, with an accelerating voltage of 100 kV). In addition, UV‐vis absorption spectra were recorded on a Varian Cary 100 Bio‐UV‐Visible Spectrophotometer. ICP mass for elemental analysis was performed by a NuPlasma Multi‐Collector ICP Mass Spectrometer (MC–ICPMS). ImageJ software is an image processing program developed at the National Institutes of Health (NIH) that was used to measure the size distribution of synthesized nanoparticles based on the image obtained either from SEM or TEM microscope instrument.

113Cd NMR Measurements

The reported 113Cd NMR measurements were carried out using a Tecmag Apollo 200 MHz spectrometer (44.44 MHz) with a broadband Bruker probe. Spectra were recorded using a Hahn spin‐echo pulse sequence (π/2‐τ‐π‐τ‐acquisition). Spin lattice relaxation measurements were undertaken by the saturation recovery technique using typical experimental parameters that included: π/2 pulse length of 4 μs; recycle delay of 30–50 s; and number of scans of 2000–4000. To obtain one spectrum requires approximately 16 hours. An aqueous solution of 0.1 M Cd(ClO4)2 served as reference at zero ppm.63 Resolution was ∼0.3 ppm, and the spectra were recorded at room temperature. We realize that there is a new technique,67 DNP‐enhanced 113Cd spin diffusion MAS NMR, that can obtain solid state 113Cd NMR spectra much more quickly,41, 68 which may render practical more detailed studies.

Supporting Information Summary

Supporting information contains the detail data of XRD, additional of 113Cd NMR spectra, spin‐lattice relaxation T1 measurement data and process and some correlation plots as well as the measured nanoparticle diameter distributions (Table S1–S4 and Figure S1–S14).

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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