Controlling Densities of Manganese Ions and Cadmium Vacancies in

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Controlling Densities of Manganese Ions and Cadmium Vacancies in Cd1−xMnxTe Ultrasmall Quantum Dots in a Glass Matrix: x‑Concentration and Thermal Annealing Noelio O. Dantas,*,† Guilherme L. Fernandes,† Oswaldo Baffa,‡ Jorge A. Gómez,‡ and Anielle Christine A. Silva*,† †

Laboratório de Novos Materiais Isolantes e Semicondutores (LNMIS), Instituto de Física, Universidade Federal de Uberlândia, 38408-100 Uberlândia, MG Brazil ‡ Departamento de Física, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901 Ribeirão Preto, SP Brazil ABSTRACT: In this work, we used nominal concentration (x) and thermal annealing to control the density of manganese (Mn2+) ions and cadmium vacancies (VCds) in Cd1−xMnxTe ultrasmall quantum dots (USQDs) embedded in a glass matrix. The physical properties were investigated by photoluminescence (PL) and electron paramagnetic resonance (EPR). PL bands related to surface defects and Mn2+ ions intensified, whereas bands related to VCds decreased with increasing manganese concentration. Longer thermal annealing times caused decreases in the intensity of PL bands characteristic of Mn2+ ions and VCds. The EPR spectra confirmed that manganese exhibited +2 oxidation states and was incorporated at the core and surface of the Cd1−xMnxTe USQDs. Furthermore, the quantities of Mn2+ ions within the Cd1−xMnxTe USQDs confirmed that longer thermal annealing times were associated with constant diffusion of these ions from the core to the surface and then the glass. Therefore, we demonstrated that longer thermal annealing times and higher manganese concentrations make it possible to control both the emissions intensities related to VCds and Mn2+ ions and the diffusion of these ions to the surface and then the glass.

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INTRODUCTION Diluted magnetic semiconductor (DMS) nanocrystals (NCs) are materials in which some sites are substituted by impurities, typically manganese. DMSs are represented by A1−xMnxB, where x denotes the fraction of impurities. Manganese with +2 oxidation states (Mn2+ ions) is easily incorporated into the II− VI NCs by replacing group II cations. When Mn2+ ions occupy Cd sites in a cubic CdTe crystal, their free-ion terms split due to the cubic crystal field.1 Manganese-doped II−VI and III−V semiconductor NCs have been studied extensively because of their interesting properties and numerous applications.2−5 For example, group II cations (Zn, Cd, and Hg) and group VI anions (S, Se, and Te) doped with Mn2+ ions have been used in magneto-optical devices.6 Diverse applications are possible because of the exchange interaction between the electronic subsystem (sp-band electrons of the host semiconductor) and the electrons originating in the partially filled d or f levels of the magnetic ions constituting the DMS. This exchange interaction enables control over the electronic and optical properties of the material using external magnetic fields. The optical properties of semiconductor NCs can be altered by quantum confinement effects, which are tuned to the size and shape of nanocrystals called quantum dots (QDs). Regarding DMS, exchange interactions between magnetic ions and host nonmagnetic semiconductors strengthen as NC © 2015 American Chemical Society

sizes diminish. Furthermore, the electronic paramagnetic resonance (EPR) technique showed these ions may be incorporated at different crystallographic sites within the crystal, (e.g., in the nucleus or at the surface). Thus, identifying these magnetic ions makes it possible to trace their migration from the core to the surface regions of the NCs and to quantify their local densities.7−9 In relation to optical properties, materials doped with Mn2+ ions have an emission band in the orange region (580 nm) that results from 4T1 → 6A1 transition characteristic of Mn2+.13 Manganese-doped CdTe (Cd1−xMnxTe) NCs are promising materials in applications such as nuclear radiation detectors,10−12 terahertz electromagnetic radiation,13 photovoltaic energy conversion,14 and nonlinear Faraday processes.15 Given their potential, a new synthesis methodology for Cd1−xMnxTe NCs was developed.1,16−19 To understand Cd1−xMnxTe NCs, the magnetic and optical properties of CdTe and MnTe must be analyzed given that MnTe forms at manganese concentrations greater than x = 1 but forms CdTe in the absence of manganese.20 Cadmium telluride (CdTe) is classified as both a II−VI semiconductor with an exciton Bohr radius of 6.5 nm Received: April 16, 2015 Revised: July 8, 2015 Published: July 10, 2015 17416

DOI: 10.1021/acs.jpcc.5b06477 J. Phys. Chem. C 2015, 119, 17416−17420

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The Journal of Physical Chemistry C and a p-type semiconductor due to cadmium vacancies (VCd).21 In addition, CdTe has a band gap energy of 1.5 eV (826.5 nm) at room temperature and absorbs and emits in the infrared optical window.22,23 CdTe is used in several applications such as solar cells,24−26 photocatalytic activity,27 nuclear radiation detectors,28−30 chemical probes,24,26 and biomarkers.31,32 Manganese telluride (MnTe) is a semimagnetic semiconductor with a room-temperature energy gap (2.8 eV33) that is close to the optimum for photoconversion.34 It is also a p-type semiconductor with very high densities of impurity charge carriers. Because MnTe NCs are excellent materials for photovoltaic devices, they are mostly grown via solutions,35 thin films by eletrodeposition,36 and vacuum evaporation.37 The magnetic properties of Cd1−xMnxTe have been extensively investigated;38−42 however, the study of parameters that affect the diffusion of Mn2+ ions and VCd into ultrasmall quantum dots (USQDs) by optical properties have not yet been discussed. Therefore, we used photoluminescence (PL) and electron paramagnetic resonance (EPR) techniques to study the effect of manganese concentration and thermal annealing time on the optical properties of Cd1−xMnxTe USQDs.

Figure 1. (a) Room-temperature PL spectra of Cd1−xMnxTe USQDs with x nominal concentration (0%, 0.5%, 1%, 5%, and 10%) untreated normalized to excitonic emission (Eexn). (b) The energy diagram of all these transitions related to PL spectra of the Cd1−xMnxTe USQDs. The nonradiative processes associated with vacancies in the structures are also indicated. (c) Ratios of emission of Mn2+ to defect levels (EMn /E1, EMn/E2, and E Mn /ESDL) with increasing manganese concentration.

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MATERIALS AND METHODS USQDs were synthesized in a SNAB glass matrix with a nominal composition of 40SiO2·30Na2CO3·1Al2O3·29B2O3 (% mol) with 4 CdTe (wt % of SNAB) and x[metallic manganese] (wt % of cadmium (Cd)) (x = 0%, 0.5%, 1%, 5%, and 10% of the glass matrix). The powders were mixed in an alumina crucible, fused at 1250 °C for 15 min, and then cooled to 0 °C. Afterward, the samples were thermally annealed at 555 °C for 0, 2, 10, and 14 h. This temperature was chosen because it is approximately 30 °C higher than the glass transition temperature, the minimum temperature required for molecular mobility, diffusion of precursor ions (Cd2+ and Te2−), and subsequent formation/growth of nanocrystals (CdTe USQDs). This SNAB glass matrix was developed by NO Dantas and has a glass transition temperature of approximately 798.15 K (525.15 °C).43,44 The synthetic precursors were procured from Sigma−Aldrich and were nearly 99.9% pure. Photoluminescence spectra were recorded using a Jaz spectrometer (Ocean Optics Inc.) and a diode laser with an excitation wavelength of 409 nm. A JEOL FA-200 X-Band (∼9.5 GHz) spectrometer was used to record the electron paramagnetic resonance (EPR) spectra of the USQDs doped with Mn2+. The g factors of the EPR spectra were calibrated using the third and fourth lines of the Mn2+ marker. The quantity of Mn2+ ions embedded in the CdTe USQDs was determined by double integration of the EPR spectrum. Integration of the first derivative of the EPR spectrum yielded the absorption spectrum, while a second integration yielded the area under the absorption curve, which is proportional to the spin concentration of the sample.9,45,46

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nm (1.39 eV). These bands are characteristic of excitonic emissions (Eexn), surface defects (ESDL), Mn2+ ions (EMn), and two types (E 1 and E2) of cadmium vacancies (VCd), respectively. ESDL and EMn emissions intensified with increasing manganese concentrations. The intensification of E SDL emissions is related to a broadening and red shift of the PL band, which is induced by increasing manganese concentration. Furthermore, ESDL emission intensification is also related to an increase in the density of shallow level defects caused by the accumulation of dopant ions on and near the surface of the USQDs.47 More specifically, the broadening and red shift in the PL spectra result from sp−d exchange interactions as manganese concentration increases.47 EMn emissions occur between the 4T1 → 6A1 levels,48 which are characteristic of the d-orbital of Mn2+ ions when incorporated substitutionally in II−VI semiconductors.49−55 Mn2+ ions can be incorporated at the core (SI signal)56−58 and/or at or near the surface (SII signal).59−61 The EMn emission is suppressed when Mn2+ ions are incorporated at or near the surface (SII signal).62,63 Therefore, EMn emissions at higher manganese concentrations indicate that Mn2+ ions are also incorporated in the core of the CdTe USQDs.54 E1 and E2 emission intensities decreased with increasing manganese concentration, suggesting that Mn2+ ions are replacing VCd vacancies in the Cd1−xMnxTe USQDs.51,64 The energy diagram in Figure 1(b) shows all of these transitions. To understand the changes in the optical properties of the Mn-doped CdTe USQDs, we determined ratios of Mn2+ ions to VCd (EMn/E1 and EMn/E2) and to emissions from surface level defects (EMn/ESDL) (Figure 1(c)). The EMn/E1 ratio increased more sharply than the EMn/E2 ratio indicating that the Mn2+ ions occupied more VCds corresponding to E1 than to E2. This provides strong evidence that the vacancies were located in a different crystallographic position.51,65 The Figure 1(c) inset shows that the ratio of the EMn/ESDL emission

RESULTS AND DISCUSSION

The optical absorption spectra of CdTe and Cd1−xMnxTe were investigated in a recent paper.1 Room-temperature photoluminescence spectra (PL) from untreated samples containing Cd1−xMnxTe USQDs with x nominal concentration (0%, 0.5%, 1%, 5%, and 10%) were normalized to excitonic emission intensity (Eexn) (Figure 1). The PL spectra show five luminescence bands at approximately 528 nm (2.34 eV), 564 nm (2.19 eV), 617 nm (2.01 eV), 753 nm (1.65 eV), and 892 17417

DOI: 10.1021/acs.jpcc.5b06477 J. Phys. Chem. C 2015, 119, 17416−17420

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The Journal of Physical Chemistry C intensities increases as manganese concentration rises, confirming that Mn2+ ions are located near the surface. These results show that we were able to control luminescent intensities from surface defects and VCd by varying manganese concentration. Figure 2 shows the room-temperature PL spectra of Cd1−xMnxTe USQDs with x nominal concentration of (a)

Figure 2. Room-temperature PL spectra of Cd1−xMnxTe USQDs in a SNAB matrix glass with x nominal concentration (a) 1% and (b) 5% post thermal annealing at 555 °C for 0, 2, 10, and 14 h.

Figure 3. (a) EPR spectra of Cd1−xMnxTe USQDs grown in a SNAB glass matrix with x nominal concentration 1% and 5% and post thermal annealing at 555 °C for 0 and 10 h, (b) expanded view, and (c) the quantity of Mn2+ ions within the Cd1−xMnxTe USQDs as a function of thermal annealing time and Mn concentration.

1% and (b) 5% and subsequent thermal annealing at 555 °C for 0, 2, 10, and 14 h. The PL spectra were normalized to the excitonic emission intensity (Eexn). The EMn emission intensity decreases with increasing thermal annealing time, indicating Mn2+ ion diffusion from the core to the surface of the CdTe USQDs. This result is more apparent in the PL spectra where x nominal is 5% (Figure 2(b)). Figure 2 also shows that increasing thermal annealing time reduces E1 and E2 emission intensities. This process is called “self-purification” and occurs for different types of impurities such as defects and ions.65,66 Thus, impure atoms must be able to diffuse readily through NCs to be purified. The PL spectra in Figure 2(b) show the suppression of E1 luminescence intensity with longer thermal annealing. This indicates that vacancies related to E1 emissions diffuse more easily through the CdTe USQDs than those associated with the E2 emission. Thus, this result provides reinforcement that the VCds are located at different crystallographic orientations.51,66 Longer thermal annealing also intensifies ESDL emission intensity and increases the density of surface defects due to the diffusion of Mn2+ ions and VCds to the surface of the CdTe USQDs. Figure 3a and an expanded view in Figure 3b show the EPR spectra of Cd1−xMnxTe USQDs grown in the SNAB glass matrix with x nominal concentration of 1% and 5% and subsequent thermal annealing at 555 °C for 0 and 10 h. The six strongest resonance lines in these EPR spectra are associated with dipole-allowed transitions (ΔMS = ±1 with ΔMl = 0, where S = 1/2), which confirms that the manganese exhibits a +2 oxidation state and is incorporated within the CdTe USQDs.67 Thus, the main aspects of the experimental EPR lines provided evidence of Mn2+ incorporation at the core and surface of the Cd1−xMnxTe USQDs.1,7,8 The ratios between the SI and SII EPR intensities were constant for all manganese

concentrations and thermal annealing times.51 These results provide strong evidence of constant Mn2+ diffusion from core to surface and from surface to glass systems (Figure 3(b)). This evidence is confirmed by quantifying Mn2+ concentration within the Cd1−xMnxTe USQDs as a function of thermal annealing time on samples with 1% and 5% x nominal concentration (Figure 3(c)). An increase in Mn2+ ions within the CdTe USQDs with increasing manganese concentrations is in excellent agreement with the PL spectra (Figure 1). However, longer annealing times are linked to lower Mn2+ concentrations within the CdTe USQDs, which is in turn associated with the diffusion of Mn2+ ions to the surface of the USQDs, as also observed in the PL spectra (Figure 2).

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CONCLUSION We controlled the diffusion of manganese ions and cadmium vacancies of Cd1−xMnxTe ultrasmall quantum dots in a glass matrix as a function of x nominal concentration and thermal annealing time. The increase in manganese concentration intensified emission bands associated with surface defects originated by the accumulation of ions on the surface of the QDs and emission of Mn2+ ions that appeared due to the substitutional incorporation into the QDs.The intensity of emissions related to E1 and E2 vacancies decreased, providing evidence that Mn2+ ions replaced vacancies in the Cd1−xMnxTe USQDs. Longer thermal annealing times decreased EMn, E1, and E2 emission intensities due to the diffusion of both Mn2+ ions from the core to the surface and VCds on the surface, respectively. This diffusion is also evidenced by increasing ESDL 17418

DOI: 10.1021/acs.jpcc.5b06477 J. Phys. Chem. C 2015, 119, 17416−17420

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emission intensities that are related to surface defect densities. EPR spectra confirmed that Mn2+ ions are incorporated at the core and surface of the Cd1−xMnxTe USQDs. The quantity of Mn2+ ions within the Cd1−xMnxTe USQDs as a function of thermal annealing time and manganese concentration confirmed that higher manganese concentrations increased the quantity of Mn2+ ions within the CdTe USQDs. Mn2+ quantities also showed that thermal annealing time significantly decreased the number of Mn2+ ions within the CdTe USQDs, which occurs in conjunction with the diffusion of Mn2+ ions to the surface of USQDs. Therefore, we demonstrated that it is possible to control E1 and E2 emission intensities by incorporating Mn2+ ions into VCds of CdTe USQDs and that the diffusion of these ions and VCds can be controlled by varying thermal annealing time.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the following agencies: CAPES, FAPEMIG, MCT/CNPq. REFERENCES

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DOI: 10.1021/acs.jpcc.5b06477 J. Phys. Chem. C 2015, 119, 17416−17420