Absorption and Radiation Transitions in Mn²⁺(3d⁵) Configuration of Mn-Doped ZnS Nanoparticles Synthesized by a Hydrothermal Method

Abstract

The Mn-doped ZnS nanoparticles with Mn content of 0–15 mol% were synthesized by a hydrothermal method from the solutions Zn(CH₃COO)₂ 0.1 M, Mn(CH₃COO)₂ 0.01 M, and Na₂S₂O₃ 0.1 M at 220°C for 15 h. These nanoparticles presented the cubic structure with average particle size about 16 nm. The yellow-orange photoluminescence (PL) band at 586 nm was attributed to the radiation transition of the electrons in 3d⁵ unfilled shell of Mn²⁺ ions [⁴T₁(⁴G)-⁶A₁(⁶S)] in ZnS matrix. The photoluminescence excitation (PLE) spectra monitored at the yellow-orange band, the absorption spectra also showed the near band edge absorption of 336–349 nm and the characteristic absorption bands of Mn²⁺(3d⁵) ions at 392, 430, 463, 468, 492, and 530 nm. These bands should be attributed to the absorption transitions of 3d⁵ electrons from the ground state ⁶A₁(⁶S) to the excited states ⁴E(⁴D), ⁴T₂(⁴D), ⁴A₁(⁴G)-⁴E(⁴G), ⁴T₂(⁴G), and ⁴T₁(⁴G) of Mn²⁺ ions. The intensity of PL band and absorption bands of Mn²⁺(3d⁵) ions also increased with the Mn content from 0.1 to 9 mol%, but their peak positions were almost unchanged. The PLE spectra showed clearly the energy level splitting of Mn²⁺ ions in ZnS crystal field and allowed for the calculation of the splitting width between the excited states ⁴A₁(⁴G), ⁴E(⁴G) about of 229 cm⁻¹ (28.6 meV), and the Racah parameters 𝐵=559 cm⁻¹, 𝐶=3202 cm⁻¹(𝛾=𝐶/𝐵=5.7), and the crystal field strength 𝐷_𝑞=568 cm⁻¹. The PL spectra with different excitation wavelengths corresponding to absorption transition bands of the PLE spectra allow for the discussion of the indirect and direct excitation mechanisms of Mn²⁺(3d⁵) ions in the ZnS crystal.

1. Introduction

In the Mn-doped A²B⁶ semiconductor crystals such as ZnS, ZnSe, and CdTe the energy levels ⁶S, ⁴G, ⁴P, and ⁴D of Mn²⁺ ions with a 3d⁵ unfilled electronic shell (called the Mn²⁺(3d⁵) configuration) are splitted into the multiple levels ⁶A₁(⁶S), ⁴T₁(⁴G), ⁴T₂(⁴G), ⁴E(⁴G), ⁴A₁(⁴G), ⁴T₂(⁴D), and ⁴E(⁴D) under the crystal field of the host matrix. The splitting of these energy levels has been studied both theoretically and experimentally [1–7]. Tanabe and Sugano have calculated the energy levels of Mn²⁺(3d⁵) configuration in the octahedral crystal field and obtained the energy levels diagram representing the dependence of the 𝐸/𝐵 ratio on Δ₀/𝐵 (which corresponds with 𝐶=4.77𝐵 [4]), where 𝐸 is the energy of the terms of the free Mn²⁺(3d⁵) ions, Δ₀ is the crystal field splitting energy, 𝐵 and 𝐶 are the Racah parameters characterizing the interaction between the 3d⁵ electrons of Mn²⁺ ions, and 𝐷_𝑞 is the crystal field strength. Fazzio and coworkers have calculated the energy structure of the Mn²⁺(3d⁵) ions in a tetrahedral crystal field of ZnS crystal particularly and determined the splitting width of ⁴G state of about 3242 cm⁻¹ (402 meV) and the energy splitting width of two very near energy levels ⁴E(⁴G) and ⁴A₁(⁴G) of about 390 cm⁻¹ (48.4 meV) [5]. Based on the PLE spectra monitored at the yellow-orange band, the groups of Chen et al. [6, 7] determined the Racah parameters 𝐵, 𝐶 and crystal field strength 𝐷_𝑞 by the following formulas: 10𝐵+5𝐶=𝐸₁,100𝐷²_𝑞−14𝐵+5𝐶−𝐸₂22𝐵+7𝐶−𝐸₂=12𝐵²𝐸₂−22𝐵−7𝐶13𝐵+5𝐶−𝐸₂,17𝐵+5𝐶=𝐸₃, (1) where 𝐸₁, 𝐸₂, and 𝐸₃ are the photon energies characterizing the absorption transition of electrons from the ground state ⁶A₁(⁶S) to the excited states ⁴A₁(⁴G)-⁴E(⁴G), ⁴T₂(⁴G), and ⁴E(⁴D) of Mn²⁺(3d⁵) ions in ZnS crystals, respectively. These results were calculated for bulk material: 𝐵=609 cm⁻¹, 𝐶=3111 cm⁻¹ (𝛾=𝐶/𝐵=5.1), and 𝐷_𝑞=667 cm⁻¹; for nanomaterials with particle size of 3.5, 4.5, and 10 nm, 𝐵=490, 551, 521 cm⁻¹; 𝐶=3357, 3246, 3306 cm⁻¹ (𝛾=𝐶/𝐵=6.8; 5.9; 6.3); 𝐷_𝑞=586, 600, and 637 cm⁻¹, respectively. However, the calculated results of these parameters for Mn-doped ZnS materials were only determined when the Mn²⁺(3d⁵) ions were well substituted into the positions of the Zn²⁺(3d¹⁰) ions. In this work, we systematically study the effect of Mn content on the absorption and radiation transitions of electrons in the Mn²⁺(3d⁵) configuration of Mn-doped ZnS nanoparticles synthesized by a hydrothermal method with a large range of Mn content from 0.1 mol% to 15 mol% using S²⁻ from Na₂S₂O₃. The Racah parameters 𝐵, 𝐶 and the crystal field strength 𝐷_𝑞 are calculated from the experimental results, and then the discussions are presented about the excitation mechanism of Mn²⁺(3d⁵) ions in the ZnS crystal at the increasing Mn content based on the dependence of PL spectra on excitation wavelengths.

2. Experimental

2.1. Synthesis of the Mn-Doped ZnS Nanoparticles

The Mn-doped ZnS nanoparticles (called ZnS:Mn nanoparticles) with different Mn contents were synthesized by the hydrothermal method according to the following process. The initial solutions Zn(CH₃COO)₂ 0.1 M (A), Mn (CH₃COO)₂ 0.01 M (B) were mixed together in the specified molar ratios to obtain 30 mL of solution (C) and stirred for 60 minutes. Then, Na₂S₂O₃ 0.1 M solution (D) with the volume of 30 mL was slowly dropped into solution (C) with continuous stirring for 60 minutes. This final mixture was put into the teflon-lined chamber steel vessel with a closed lid, after being annealed at 220°C for 15 h and cooled down to room temperature naturally. In the hydrothermal process, the ZnS nanocrystals were formed as follows: 4Na₂S₂O₃⟶Na₂S+3Na₂SO₄+4SZnCH₃COO₂+Na₂S⟶ZnS↓+2CH₃COONa (2) Besides ZnS, the small portion of MnS was also formed: MnCH₃COO₂+Na₂S⟶MnS↓+2CH₃COONa (3) As these two processes happened simultaneously, the Mn-doped ZnS nanoparticles were believed to be formed as a final product. In fact, the host material ZnS might exhibit defects associated with Zn or S vacancies or surface and interstitial defects. The obtained precipitation was filtered, washed by distilled water, and dried at 60°C for 10 h in ambient condition. By the above process, the Mn-doped ZnS nanoparticles were synthesized in the powder form with the different Mn contents of 0, 0.1, 0.4, 1, 2, 4, 6, 8, 9, 10, and 15 mol%.

2.2. Structural and Optical Measurements

The crystal structure was studied by the X-ray diffraction method (XRD) on the XD8-Advance Buker system with Cu-Kα radiation (𝜆=1.54056 Å). The surface morphology was examined using the transmission electron microscope (TEM) JEM-1010. The PL and PLE spectra were recorded at 300 K using the 325 nm excitation radiation of a He-Cd laser, radiation of XFOR-450 xenon lamp on the Oriel-Spec MS-257, FL3-22 spectrometers, respectively. The absorption spectra were examined by the reflection-scattering process and recorded at 300 K using radiation of detri, halogen lamp on Jasco-V670 spectrometer.

3. Results and Discussion

3.1. The Structure and Morphology of ZnS:Mn Nanoparticles

Figure 1 shows the XRD patterns of ZnS, ZnS:Mn nanoparticles with different Mn contents. This patterns consists of the diffraction lines corresponding to the reflecting planes (111), (200), (220), and (311) in which the (111) line is the strongest. The obtained pattern demonstrate that the ZnS, ZnS:Mn nanoparticles possess a single-phase cubic structure with 𝑇²_𝑑−𝐹43𝑚 symmetry and calculated lattice constant of ZnS 𝑎=5.413 Å. This value is in good agreement with the lattice constant from JCPDS card number 05-0566 (𝑎=5.406 Å). For the ZnS:Mn nanoparticles with Mn contents of 0.1, 0.4 mol%, the lattice constant was unchanged 𝑎=5.404 Å. For the ZnS:Mn nanoparticles with Mn content from 1 to 15 mol%, the lattice constant increased slightly from 5.414 to 5.439 Å. This increase may be explained by that the ionic radius of Mn²⁺ was larger than that of Zn²⁺ 10% [8] and indicated that the Mn²⁺(3d⁵) ions were really substituted for Zn²⁺(3d¹⁰) sites. Based on the Debye-Scherrer’s formula𝐷=0.9𝜆,𝛽cos𝜃 (4) where 𝐷 (Å) is the particle size, 𝜆 (Å) is the X-ray wavelength of Cu-Kα, 𝛽 (rad) is the full width at half maximum (FWHM) of the diffraction line, and 𝜃 (rad) is the Bragg angle, the average sizes of the nanoparticles were calculated to be about 16 nm. This value did not vary with Mn content from 0.1 to 15 mol%.

(a)

(b)

Figure 1 

XRD patterns of the ZnS:Mn nanoparticles with different Mn contents.

Figure 2 shows the TEM images of ZnS:Mn nanoparticles with Mn contents of 0.1, 9 mol%. They showed that the nanoparticles are the quasi-spheres with average size of about 25–43 nm. Thus, this value is bigger than particle size calculated from Debye-Sherrer’s formula because of the aggregation of ZnS:Mn crystals.

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(b)

Figure 2 

TEM images of ZnS:Mn nanoparticles with Mn contents of (a) 0.1 mol% and (b) 9 mol%.

3.2. The Absorption and Radiation Transitions in the Mn²⁺(3d⁵) Configuration of ZnS:Mn Nanoparticles

The radiation transitions in the Mn²⁺(3d⁵) configuration were examined by the PL spectra according to Mn content, a power density, and a wavelength of the excitation radiation. Simultaneously, the absorption transitions in this configuration were studied by the PLE and absorption spectra. Figure 3 shows the PL spectra of ZnS, ZnS:Mn nanoparticles with different Mn contents. As seen, the PL spectrum of ZnS nanoparticles shows a wide blue band of about 450 nm (Figure 3(a)). This band is attributed to the origin of the Zn, S vacancies and their interstitial atoms in the host ZnS crystal [9–14]. For the samples with Mn content of 0.1 mol%, the blue band was almost quenching, but its PL spectra exhibited a broad yellow-orange band at 586 nm (Figure 3(b)) which should be characterized by the radiation transition of electrons in the Mn²⁺(3d⁵) configuration [⁴T₁(⁴G)→⁶A₁(⁶S)] [15]. Besides, for the samples with Mn content from 0.4 to 9 mol%, the substitution of Mn²⁺(3d⁵) ions into the Zn²⁺(3d¹⁰) sites increased, so the PL intensity of the yellow-orange band gradually increased and achieved the maximum with Mn content of 9 mol% while peak position was unchanged (Figures 3(c)–3(h)). By contrast, for the samples with Mn contents of 10, 15 mol%, the intersite interaction is possible between the Mn²⁺ ions themselves and between them and other ions in the lattice, so the intensity of the yellow-orange PL decreased (Figures 3(i) and 3(j)) [8, 16, 17]. The dependence of the yellow-orange PL intensity on Mn content is shown in Figure 4.

Figure 3 

The PL spectra of the ZnS:Mn nanoparticles with different Mn contents.

Figure 4 

The dependence of the yellow-orange PL intensity of ZnS:Mn nanoparticles on Mn content.

The unchanged position and the increase of yellow-orange PL intensity were also observed with increasing power density of 325 nm excitation radiation from 0.10 to 0.27 W/cm² (Figure 5). Based on the relationship between the PL intensity 𝐼_PL and the excitation power density 𝐼_EX by equation: 𝐼_PL=𝑘𝐼^𝑛_EX, (5) where 𝑘 is a constant, 𝑛 is the coefficient dependent on radiation mechanism, the 𝑛 value was determined to be approximately 1 (Figure 6). This result agrees well with the radiation mechanism of impurity center in ZnS crystal [18]. This suggested that the yellow-orange PL was a characteristic radiation transition of electrons in the 3d⁵ unfilled shell of Mn²⁺(3d⁵) ions in ZnS crystals.

Figure 5 

The PL spectra of ZnS:Mn nanoparticles for Mn content of 9 mol% with different excitation power densities.

Figure 6 

The dependence of ln𝐼PL on ln𝐼EX of the yellow-orange PL intensity for ZnS:Mn nanoparticles with Mn content of 9 mol%.

Figure 7 shows the PLE spectra monitored at the yellow-orange PL band excited by the radiation from a xenon lamp. For the ZnS:Mn nanoparticles with Mn contents of 0.1, 0.4 mol%, there was a wide range with strong PL intensity at 336 nm (3.691 eV) which was characteristic for the near band edge absorption of the ZnS crystals because the photon energy corresponding to this transition is near the width of band gap [12, 19]. Besides, these PLE spectra also showed the bands with smaller PL intensity at 392, 430, 468, 492, and 530 nm (Figures 7(a) and 7(b)), which were attributed to the absorption transitions of the 3d⁵ electrons from the ground state ⁶A₁(⁶S) to the excited states ⁴E(⁴D), ⁴T₂(⁴D), ⁴A₁(⁴G)-⁴E(⁴G), ⁴T₂(⁴G), and ⁴T₁(⁴G) of Mn²⁺(3d⁵) ions, respectively [7].

Figure 7 

The PLE spectra monitored at the yellow-orange PL band of ZnS:Mn nanoparticles with different Mn contents.

For the ZnS:Mn nanoparticles with Mn content increased from 1 to 15 mol%, the PL band characterized to the near band edge absorption was shifted towards the longer wavelength from 342 to 349 nm (Figures 7(c)–7(h)). This shift might be caused by the s-d exchange interaction of the conduction electrons with the 3d⁵ electrons of Mn²⁺ ions. Due to this interaction, the band gap decreased slightly when Mn content increased [20, 21]. However, the PL intensity of Mn²⁺ absorption transition bands in the PLE spectra increased with increasing of Mn content from 0.1 to 8 mol% (Figures 7(a)–7(e)) and reached the maximum at 9 mol% (Figure 7(f)), then decreased again at 10 and 15 mol% (Figures 7(g) and 7(h)) but their peak positions were almost unchanged. This result agreed quite well with the observed PL spectra. Although the PL intensity of the Mn²⁺ absorption transition bands varied up and down with Mn content, in general the PL intensity ratio of Mn²⁺ absorption transition bands and the near band edge absorption transition band increased (Figure 8). This shows that the Mn²⁺(3d⁵) ions were doped into the crystal lattice at Zn²⁺(3d¹⁰) sites, including their vacancies sites, so the probability of the occurrence of near band edge absorption decreased while the probability of occurrence of absorption in the Mn²⁺(3d⁵) configuration increased.

Figure 8 

The PL intensity ratio dependence of Mn2+(3d5) absorption transition bands and near band edge absorption transition band on different Mn contents.

The detailed study about 468 nm (2.650 eV) band in the PLE spectra showed that the peak of 463 nm (2.678 eV) appeared unclearly in the left shoulder of this band for ZnS:Mn samples with Mn contents of 0.1, 0.4, 1 mol% (Figures 7(a)–7(c)) and more clear for the Mn content from 6 to 15 mol% (Figures 7(d)–7(h)). The bands at 463 and 468 nm were attributed to the absorption transitions of the electrons from the ground state ⁶A₁(⁶S) to the excited states ⁴A₁(⁴G) and ⁴E(⁴G) of Mn²⁺(3d⁵) ions, respectively. The energy difference between these excited states is about 229 cm⁻¹ (28.6 meV) and the splitting energy gap for the ⁴G state of Mn²⁺(3d⁵) ions is around 2734 cm⁻¹ (339 meV). This value agrees well with the result of the calculation provided by Fazzio and coworkers and by the experiment of MnS material (about 200 cm⁻¹ (24.8 meV)) [5]. Similar results were obtained for the Mn²⁺(3d⁵) absorption band at the range from 480 to 515 nm. For the ZnS:Mn samples with Mn contents of 0.1, 0.4, 1 mol%, a wide band appeared with a maximum at about 492 nm (Figures 7(a)–7(c)). For the ZnS:Mn samples with Mn content from 6 to 15 mol%, near the band at 492 nm, also the new bands at 482 and 501 nm appeared (Figures 7(d)–7(h)). Among them, the band at 492 nm is the characteristic absorption transition of the electrons from the ground state ⁶A₁(⁶S) to the excited state ⁴T₂(⁴G) of Mn²⁺(3d⁵) ions. The calculated energy gaps between ⁴T₂(⁴G) state and states corresponding to the bands at 482 and 501 nm are 53.3 and 50.2 meV, respectively.

Using the photon energies of 𝐸₁, 𝐸₂, and 𝐸₃ corresponding to the wavelengths of 392, 468, and 492 nm in the PLE spectra monitored at the yellow-orange PL band of ZnS:Mn nanoparticles and (1), the Racah parameters and the crystal field strength were determined: 𝐵=559 cm⁻¹, 𝐶=3202 cm⁻¹, (𝛾=𝐶/𝐵=5.7), and 𝐷_𝑞=568 cm⁻¹. These values ​​are in good agreement with parameters 𝐵, 𝐶, and 𝐷_𝑞 obtained for ZnS:Mn nanomaterials [7].

To clarify the absorption transitions in Mn²⁺(3d⁵) configuration more, the absorption spectra of ZnS, ZnS:Mn samples were examined. Figure 9 shows the absorption spectra of these samples with different Mn contents. As seen, the absorption spectrum of ZnS nanoparticles appears as a wide band of about 334 nm with strong absorption (Figure 9(a)), which is attributed to the near band edge absorption of ZnS crystal [10]. For the ZnS:Mn samples with the Mn content from 0.1 to 15 mol%, the near band edge shifted to the longer wavelength at 337 nm (Figures 9(b)–9(i)). For the ZnS:Mn sample with Mn content of 8 mol%, besides that band, there appeared bands at 392, 429, 467, and 500 nm, in which the band at 467 nm was the strongest absorption (Figure 7(f)).

Figure 9 

The absorption spectra of the ZnS:Mn nanoparticles with different Mn contents.

The positions of these bands were identical to the ones seen in the PLE spectra monitored at the yellow-orange band, but their absorptions were smaller because the PL bands in the PLE spectra were characteristic by the resonance absorption transitions of Mn²⁺(3d⁵) ions. With the increase of Mn content from 9 to 15 mol%, their absorptions increased, but their peak positions remained unchanged (Figures 9(g)–9(i)). This suggested that these bands must also be characteristic by the absorption of Mn²⁺(3d⁵) ions. Thus, the presence of Mn²⁺(3d⁵) ions in ZnS host material has created a yellow-orange band at 586 nm in the PL spectra and bands at 392, 430, 463, 468, 492, and 530 nm in the PLE spectra and the absorption spectra. These peak positions were consistent with the positions of the emission bands of ZnS:Mn nanoparticles calculated from the Kohl-Sham Density Functional Theory equation [22].

In order to determine the excitation mechanism of Mn²⁺(3d⁵) ions, the PL spectra of ZnS:Mn samples with Mn contents of 0.1, 1, 9, 15 mol% were investigated by different excitation wavelengths. Using in turn radiations of the xenon lamp corresponding to the wavelengths of 325, 336, 349, 392, 430, 468, and 492 nm in the PLE spectra to excite these ZnS:Mn samples, their PL spectra present only a yellow-orange band at 586 nm. This proved that even with small Mn content of 0.1 mol% the Mn²⁺(3d⁵) ions were really substituted with the Zn²⁺(3d¹⁰) sites. The peak position of the yellow-orange band was almost unchanged but its intensity depended on the excitation wavelength. For the ZnS:Mn samples with Mn content of 0.1, 1 mol%, the PL intensity of yellow-orange band was strongest as excited by the wavelengths of 336, 349 nm, and then the intensity decreased gradually as excited by 325, 392, 492, 468, and 430 nm (Figures 10(a) and 10(b)). On the contrary, for ZnS:Mn samples with Mn contents of 9, 15 mol%, the PL intensity was the strongest as excited by the wavelength of 468 nm, and then it decreased gradually as excited by the wavelengths of 392, 492, 430, 349, 325 nm (Figures 10(c) and 10(d)). The obtained results showed that there might be two dominant excitation mechanisms for the 3d⁵ electrons of Mn²⁺ ions: (i) the indirect and (ii) the direct excitations [9]. The absorption and radiation transitions caused by the excitation radiation of 325, 336, and 349 nm with photon energy near the band gap energy of ZnS mainly belong to (i) the indirect excitation [19, 23].

(a)

(b)

(c)

(d)

Figure 10 

The PL spectra of ZnS:Mn nanoparticles with Mn contents of (a) 0.1 mol%, (b) 1 mol%, (c) 9 mol%, and (d) 15 mol% excited by different excitation radiations.

Under radiation, the imbalance electron-hole pairs can be bound with the Mn²⁺ ions; these carrier pairs can recombine nonradiatively and transfer the energy to the 3d⁵ electrons of Mn²⁺ ions. However, the absorption and radiation transitions caused by 392, 492, 468, and 430 nm radiations with the photon energy smaller than the band gap of ZnS belong to the (ii) direct excitation [19, 23]. When the electrons in the 3d⁵ unfilled shell of the ion Mn²⁺(3d⁵) absorb the photon, they may transfer from the ground state ⁶A₁(⁶S) to the excited states ⁴E(⁴D), ⁴T₂(⁴D), ⁴A₁(⁴G)-⁴E(⁴G), ⁴T₂(⁴G), and ⁴T₁(⁴G), from where they are transited nonradiatively down to the state ⁴T₁(⁴G) and eventually transit down the ground state ⁶A₁(⁶S) to emit the yellow-orange band at 586 nm. The absorption and radiation transitions of electrons in the unfilled 3d⁵ shell of Mn²⁺ ions in the ZnS crystals and excitation energy transfer mechanism are shown in Figure 11.

Figure 11 

The energy levels diagram and the absorption radiation transitions of Mn2+(3d5) ions in the ZnS:Mn crystal field.

4. Conclusion

By the hydrothermal method we have successfully synthesized ZnS:Mn nanoparticles with Mn content variying in a wide range from 0.1 to 15 mol%, in which the Mn²⁺(3d⁵) ions substituted well into ZnS matrix even at the Mn content of 0.1 mol%. The substitution of the Mn²⁺(3d⁵) ions in the positions of Zn²⁺(3d¹⁰) sites and their vacancies caused a small change in the lattice structure of the ZnS host material but affected significantly its optical behavior. The Mn²⁺ ions quenched the blue band at 450 nm and induced the appearance of a yellow-orange band at 586 nm in the PL spectra and the new bands at 392, 430, 463, 468, 492, and 530 nm in the PLE spectra and in the absorption spectra. When increasing the Mn content, the probability of absorption, radiation transitions in Mn²⁺(3d⁵) ions increases, with the maximum at 9 mol% of Mn content after being reduced. Then, the energy level splitting of Mn²⁺ ions in ZnS crystal got more clear. The clear appearance of the absorption bands of Mn²⁺(3d⁵) ions in the PLE spectra allows to determine the energy gap of two states ⁴A₁(⁴G), ⁴E(⁴G) about of 28.6 meV, the Racah parameters 𝐵, 𝐶, and the crystal field strength 𝐷_𝑞. The unchanged yellow-orange PL peak position at 586 nm of the samples excited by different excitation wavelengths showed the presence of the indirect and direct excitation mechanisms for the 3d⁵ electrons of Mn²⁺ ions. When the Mn content increased from 0.1 to 15 mol%, the probability of the indirect excitation decreased, while the one of direct excitation increased.

Acknowledgment

The paper was completed with financial support from the National University Project (Code QG.11.07).