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
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 [
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:
The crystal structure was studied by the X-ray diffraction method (XRD) on the XD8-Advance Buker system with Cu-K
Figure
XRD patterns of the ZnS:Mn nanoparticles with different Mn contents.
Figure
TEM images of ZnS:Mn nanoparticles with Mn contents of (a) 0.1 mol% and (b) 9 mol%.
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
The PL spectra of the ZnS:Mn nanoparticles with different Mn contents.
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
The PL spectra of ZnS:Mn nanoparticles for Mn content of 9 mol% with different excitation power densities.
The dependence of
Figure
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
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
Using the photon energies of
To clarify the absorption transitions in Mn2+(3d5) configuration more, the absorption spectra of ZnS, ZnS:Mn samples were examined. Figure
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
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
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 [
The energy levels diagram and the absorption radiation transitions of Mn2+(3d5) ions in the ZnS:Mn crystal field.
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
The paper was completed with financial support from the National University Project (Code QG.11.07).