Different particle sizes of colloidal Mn doped ZnCuInS (Mn:ZCIS) nanocrystals were prepared with different reaction times in a noncoordinating solvent. Different test methods were used to reveal the successful incorporation of Mn2+ ion into ZCIS nanocrystals for obtaining the Mn d-state emission. We investigated the temperature-dependent PL spectra of Mn:ZCIS nanocrystals show a redshift with decreasing intensity at low temperature (20–300 K). The energy levels, FWHM of the PL peak and PL Intensity of nanocrystals with different diameters were investigated as a function of temperature in the range 20−300 K. We found that the variation of both energy level and PL peak broadening for Mn:ZCIS nanocrystals were most likely caused by the coupling of the carrier to acoustic phonon.
© 2015 Optical Society of America
Semiconductor nanocrystals (NCs) are receiving considerable attention due to their size-tunable spectroscopic properties and some important applications including light emitting diodes (LEDs) [1, 2 ], biomedical labeling [3, 4 ], lasers [5, 6 ], etc. Despite the traditional II-VI [7, 8 ] and IV-VI [9, 10 ] semiconductor NCs such as CdSe (or S) having appealing optical properties, the intrinsic toxicity of cadmium limits their applications, especially because of recent environmental regulations. Moreover, I-III-VI semiconductors have been arousing increased attention, such as CuInS2 , ZnCuInS [12, 14 ], CuInSe2  and ZnAgInS , AgInS , have been assigned as alternatives for low toxicity solar-harvesters and light-emitters [16–18 ]. But the current standing problem of internal electronic energy levels in NCs is critical for light generation and light harvesting . Nevertheless doping is a better method to possibly solve the relevant electronic energy levels for the photoelectron and photohole respectively so as to improve the whole optical properties such as photo- or electroluminescence, which is important for emerging photonics applications . Importantly, Mn doped nanocrystals with larger Stokes shift [19, 28 ] distinguish them from the band edge emitting nanocrystals. Until now, the high efficiency and temperature-dependent of the Mn emission has been mainly observed in Mn:ZnSe , Mn:ZnS , Mn:CdSe (or S) , and different binary alloyed group II−VI semiconductor nanocrystals. And the radiative and the nonradiative relaxation processes and the exciton−phonon coupling are discussed by temperature-dependent PL spectroscopy in colloidal NCs . The change in the PL intensity as a function of temperature can often reflect the evolution of the PL mechanism. The temperature-dependent of different sizes of ZCIS NCs and Mn doped II-VI and IV-VI semiconductor NCs have been reported [14, 23, 24 ]. However, there are seldom reports about different sizes of Mn:ZCIS NCs.
In this paper the preparation of nanocrystals ZCIS particles doped with Mn2+ is discussed. It will be shown that Mn2+ can be incorporated into ZCIS nanocrystals and made to luminesce by exciting in the ZCIS host lattice. To this end, photoluminescence emission and excitation spectra are presented and discussed. Furthermore, we report in this paper a study on the temperature dependence of the PL intensity, energy levels, FWHM in different sizes of Mn:ZCIS NCs. The energy levels were recorded as a function of temperature and were fitted with empirical expressions to derive the Huang−Rhys factor and the average phonon energy involved in the variation . Moreover, the size dependent parameters were calculated from the fitted data containing the average phonon energy, the Huang-Rhys factor and the carrier-acoustic phonon coupling coefficient . The aim of the work in this paper is to investigate Mn2+ ion emission.
2. Experiment section
All chemicals were used without further purification. Manganese acetate (Mn(OAc)2, 99.99%), zinc acetate (Zn(OAc)2, 99.99%), copperacetate (Cu(OAc)2, 99.99%), indium acetate (In(OAc)3, 99.99%), sulfur powder (99.5%), 1-octadecene (ODE, 90%), dodecanethiol (DDT, 98.0%), octadecylamine (OAm, 98.0%) and stearic acid (SA, 99.0%) were purchased from Aladdin in order to synthesize Mn doped ZCIS nanocrystals.
2.2 Precursor solution preparation
Mn(OAc)2 precursor solution: 0.0044 g (0.025 mmol) of Mn(OAc)2, 0.01348 g of OAm, and 2 mL of ODE were loaded in a three-neck flask clamped in a heating apparatus. The mixture was heated to 120 °C under an argon flow and remained at this temperature for about 10 min until forming a clear brown solution. The acquired Mn precursor solution was reserved at 50 °C for the following use.
Zn(OAc)2 precursor solution: 0.02195 g (0.1mmol) of Zn(OAc)2, 0.0645 g of OAm, and 2.0 mL of ODE were loaded in a three-neck flask clamped in a heating apparatus. The mixture was heated to 160 °C under an argon flow and remained at this temperature for about 10 min until forming a clear colorless solution. The acquired Zn precursor solution was reserved at 50 °C for the following use.
S precursor solution: sulfur precursor solution was prepared by dissolving 0.32 mmol S (0.01026 g) in 2.0 mL ODE at 120 °C, the acquired S precursor solution was reserved at 50 °C for the following use.
2.3 Synthesis of Mn doped ZCIS nanocrystals
0.2 mmol Cu(OAc)2 (0.03993 g), 0.2 mmol In(OAc)3 (0.0584 g), 1 ml DDT, 1.2 mmol SA and 7 ml ODE were all taken into a three-neck flask. The reaction mixture was then degassed by purging Argon for 15 minutes. Then the temperature was increased to 130 °C. Once a clear solution was obtained, 2 ml S precursor solution was injected into the flask and reacted five minutes kept the temperature at 130 °C. Then the above Zn and Mn precursor solutions were reinjected into the reaction system and increased the temperature to 220 °C to allow the growth of Mn:ZCIS nanocrystals. Aliquots of the sample were taken at different time intervals and injected into cold toluene to terminate the growth of nanocrystals immediately for recording their optical spectra. The obtained Mn:ZCIS nanocrystals were purified with centrifugation by adding methanol and acetone respectively into the toluene solution and stored in chloroform.
Jobin Yvon FluoroLog-3 fluorescence spectrometer was used to measure the photoluminescence spectra (excitation and emission spectra) and temperature dependence of the PL spectra, and the excitation light sources were a 450-W xenon lamp. JEM-2100F transmission electron microscopes equipped with a field-emission gun were used to characterize the morphology and the sizes of the different samples. The phase structure of the sample was investigated by using a Rigaku D/max 2500v/pc X-ray diffractometer with a Cu Kα source. The PL lifetime measurement was performed on a Jobin Yvon FluoroLog-3-221-TCSPC fluorescence spectrometer equipped with an EPL456 laser diode. X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the surface composition of the Mn:ZCIS NCs by multi-function services of Kratos Axis Ultra DLD and an Al Kα-raysource 1486.6 eV. The nanocrystals were dissolved into chloroform.
4. Results and discussion
Figures 1(a)-1(c) show the transmission electron microscope (TEM) photos and HRTEM photos (inset) of the different reaction times of the Mn:ZCIS nanocrystals. The corresponding average sizes of the samples were closed to (a) 3.5 nm, (b) 4.2 nm and (c) 4.8 nm. The lattice fringes were clearly observed in the high resolution TEM (HRTEM) images (inset of Figs. 1(a)-1(c)). It reveals the highly crystalline nature of the as-synthesized Mn:ZCIS QDs. The energy dispersive X-ray spectroscopy (EDS) spectrum of the Mn:ZCIS nanocrystals prepared in 45 min (Fig. 1(d)) shows that the as-prepared nanocrystals have a quinary composition, containing Mn, Zn, Cu, In, and S. Figure 1(e) shows the XRD patterns of the ZCIS NCs with and without Mn dopants, indicating no apparent difference between them. As seen from the figure, the three characteristic peaks are located between the distinct diffraction peaks of CuInS2 and ZnS, which indicates the formation of ZCIS quaternary NCs and clarifies that the NCs should be zinc blended phase of CIS-ZnS.
To further characterize the as-synthesized samples, XPS measurement was carryed out to investigate the surface compositions and chemical state of Mn doped ZCIS nanocrystals. The X-ray photoelectron spectroscopy (XPS) spectrum was also used to support the presence of Mn in the doped nanocrystals, and the result was shown in Fig. 2(a) . The higher carbon peak must be mainly due to the surface adsorption of organic solvents. To reveal the type of chemical bonds of manganese in these nanocrystals, a high resolution Mn (2p) spectrum was recorded for the Mn:ZCIS nanocrystals, shown in Fig. 2(b). In contrast to the previous report  we can affirm that manganese element is mainly + 2 valence in Mn:ZCIS nanocrystals.
The inset of Fig. 3(a) displays the optical excitation and PL spectra recorded at room temperature for the Mn:ZCIS nanocrystals with different reaction times. It is clear from Fig. 3(a) that in different emission peaks having the same PLE position. The PL spectra exhibited a strong peak around 620 nm from an internal electronic transition of the Mn (4T1-6A1). When Mn:ZCIS host nanocrystals was excited by photons with energy higher than its band gap, the energy of a photogenerated electron–hole pair could be transferred into the electronic d–d levels of the Mn2+ ions . Compared to traditional Mn2+ emission (580-600nm), we got a new emission peak which own to the various lattices. The internal electronic transition of the Mn (4T1-6A1) led to the characteristic dopant emission around 620 nm. The similar result has been reported by Cao et al., which emission peak position was around 610 nm . The emission peak position of Mn:ZCIS nanocrystals redshift from 626 to 639 nm compared to ZCIS nanocrystals (528-672 nm) which redshift was not obvious shown in Figs. 3(b) and 3(c). We can see from the photos all emitting light color is red and much stronger than ZCIS nanocrystals under the same conditions. However, the weak redshift of Mn:ZCIS nanocrystals is weak dependent with quantum confined effect which owns to the variation of ZCIS crystalline field.
To explore the possible luminescence mechanisms involved, the PL decay time of the Mn:ZCIS NCs excited by a 456 nm wavelength laser was measured. The PL decays of the Mn:ZCIS NCs were detected at the wavelengths of 626 nm, 629nm, and 639nm, respectively, as shown in Fig. 4 . The PL decay curves of the ZCIS NCs can be fitted well by at biexponential function I(t) = A1exp(−t/τ1) + A2exp(−t/τ2), where τ1, τ2, represent the decay time of the PL emission and A1, A2, represent the relative weights of the decay component. The average lifetime τav were calculated according to τav = ∑Aiτi 2/∑Aiτi . The corresponding PL Lifetime were 2.86 ms, 2.89 ms, and 2.97 ms for Mn:ZCIS NCs with different reaction time. The excited-state lifetime of the emission also shows significant difference in comparison to the ZCIS nanocrystals with a short PL lifetime often scaled in nanosecond . The millisecond lifetime in Mn emission distinguishes it from the trap state emission of the undoped nanocrystals .
Figure 5 Shows Mn d-states emission mechanism in Mn:ZCIS nanocrystals where Mn ground state (6A1) above both the trap state (ts) and the host valence band. Once excitated, the trap state emission was quenched by the Mn2+ emission. In our example, Mn:ZCIS nanocrystals did not show the band edge excitonic emission, it is the exciton confined within the conduction band that transfers the energy to the Mn state for obtaining the Mn d-state emission .
We show the samples of different reaction times of photoluminescence (PL) test at low temperature which between 20 k and 300 k shows in Figs. 6(a)-6(c) . With increasing the temperature, the emission peak was observed with a red-shift [31, 32 ] as well as a broadening [23, 25 ].
To investigate the nonradiative relaxation processes in the nanocrystals, we analyzed the temperature dependence of the integrated PL intensity. The PL intensities of Mn:ZCIS nanocrystals with different reaction times as a function of temperature are shown in Fig. 7 . The PL intensity of the nanocrystals decreases rapidly when temperature increases. The solid lines are the fitted curves using equation Eq. (1) Table 1 . The ∆E of Mn:ZCIS nanocrystals is much smaller than ZCIS (115meV)  nanocrystals. The nonradiative relaxation process with a small activation energy results in a decrease in the PL intensity.
Figure 8 shows the energy levels as a function of temperature for Mn:ZCIS nanocrystals with different reaction time. The energy levels values in this work were calculated from the peak position acquired from the PL spectra. The data in Fig. 8 could be fitted to the expression (2) [23, 26 ], since the parameters used in the equation are related to an intrinsic interaction with in semiconductors, namely, the electron-phonon couplingTable 2 . The fitting value of Eg(0) at low temperatures is 1.97, 1.93 and 1.91 eV respectively with different reaction time. While the value of Eg(0) differs significantly from that of ZCIS (2.669 eV)  owing to the Mn2+ (4T1-6A1) emission. A more specific explanation for the red shift of the Mn2+ related emission band with increasing temperature is the thermal expansion of the host lattice with increasing temperature. The Tanabe-Sugano diagram for the Mn2+ ion shows that the 4T1→6A1 transition in the Mn2+ ion strongly depends on the crystal field induced by the host lattice. Therefore, for increasing temperature, the thermal expansion of the ZCIS host lattice results in a decrease of the crystal field and a concomitant increase in the Mn2+ related emission energy. The lattice deformation potential and the exciton–phonon coupling cause a band gap shrinkage, which induces the PL spectral peak red shift with increasing temperature. The fitting values of S signify that the electron−phonon coupling increases as the diameter of the nanocrystal increases and are consistent with the data as shown in Fig. 1. The tendency of the S increases with the sizes of nanocrystal was also reported in previous publication . Compared to defect emission , the fitting values of S is much smaller shows the weak coupling regime which resulting from Mn2+ (4T1→6A1) emission.
Figure 9 shows the full width at half-maximum (FWHM) of the emission spectra of Mn:ZCIS nanocrystals with different particle sizes as a function of temperature. Due to the electron-phonon coupling this bandwidth changes with temperature, so to gain a deeper insight into the carrier-phonon scattering processes involved in the line broadening, we fitted the experimental data to the expression given in Eq. (3) . This equation describes the temperature dependence of the excitonic peak broadening in bulk semiconductors and has been used for nanocrystals.Table 3 . The full width at half-maximum (FWHM) is observed to decrease with the increasing reaction time, indicating that the sizes of the nanocrystals increase, which is in accord with the analyses of TEM. In our case, we have obtained similar values for the phonon energy in FWHM and energy levels, showing that the thermal expansion of the lattice can explain the shift of the Mn2+ related emission.
In summary, Mn:ZCIS nanocrystals have been prepared and their optical properties are tuned by simply adjusting the reaction time. The PL spectra were recorded for several Mn:ZCIS nanocrystals with different particle sizes. Their weak redshift were weak dependent with quantum confined effect which owns to the variation of ZCIS crystalline field. The X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and the energy dispersive X-ray spectroscopy (EDS) spectrum were also used to support the presence of Mn in the doped nanocrystals. In our examples, Mn:ZCIS nanocrystals did not show the band edge excitonic emission, it is the exciton confined within the conduction band that transfers the energy to the Mn state for obtaining the Mn d-state emission. Furthermore, we report in this paper a study on the temperature dependence of the PL intensity and energy levels in different sizes of Mn:ZCIS NCs. The ∆E of Mn:ZCIS nanocrystals is much smaller than ZCIS (115meV)  nanocrystals which owns to Mn2+ emission. The nonradiative relaxation process with a small activation energy results in a decrease in the PL intensity. The energy level of nanocrystals with different diameters were investigated as a function of temperature in the range 20−300 K and fitted with an empirical expressions, from which the Huang−Rhys factor and the average phonon energy were achieved. The tendency of the S increases with the sizes of nanocrystal was also reported in previous publication. The fitting values of S is much smaller shows the weak coupling regime which resulting from Mn2+ (4T1→6A1) emission. The FWHM of the PL peak and PL Intensity were also investigated as a function of temperature. The full width at half-maximum (FWHM) is observed to decrease with the increasing reaction time, indicating that the sizes of the nanocrystals increase, which is in accord with the analyses of TEM. In our case, we found that the thermal expansion of the lattice can explain the shift of the Mn2+ related emission. In the temperature range 20−300 K, we found that the variation of both the energy level and the PL peak broadening for Mn:ZCIS nanocrystals were most likely caused by the coupling of the carrier to acoustic phonon.
This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant No. 2013AA014201), the Natural Science Foundation of Tianjin (Grant Nos. 11JCYBJC00300, 14JCZDJC31200, 15JCYBJC16700 and 15JCYBJC16800), the National Key Foundation for Exploring Scientific Instrument of China (Grant No. 2014YQ120351) and International Cooperation Program from Science and Technology of Tianjin (Grant No. 14RCGHGX00872).
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