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Effect of Gd3+ doping on structural and optical properties of ZnO nanocrystals
Solid State Sciences 68 (2017) 47e54
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Effect of Gd3þ doping on structural and optical properties of ZnO nanocrystals Deepika Mithal*, Tapanendu Kundu Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2017 Received in revised form 23 April 2017 Accepted 26 April 2017 Available online 27 April 2017
To investigate the effect of rare earth ions on the excitation energy redistribution between the UV and visible photoluminescence, the Gd3þ doped ZnO nanocrystals prepared through Sol-Gel method were studied. Different structural characterizations and spectroscopic studies were carried out to establish a correlation between the structural effect and the photoluminescence. Our study indicates that the incorporation of Gd3þ in ZnO nanocrystals prepared through this synthesis takes place predominantly on the surface up to 0.18 mole fraction of Gd3þ. This is manifested by the observed lattice contraction due to the presence of Gd3þ on the surface. This structural defect introduces various defects that modifies the photoluminescence properties. For these modifications in crystal system, the A1 symmetry phonon modes of wurtzite structure are found to be gradually more Raman active than the other modes as the Gd3þ concentration is increased while the E modes are suppressed. These doping induced modification of phonon spectrum strongly influences the exciton phonon coupling and thereby the photophysical properties. Resulting crystal imperfections strongly enhance the defect band luminescence. It is shown that energy transfer from selectively excited Gd3þ centers to surface states can be effectively exploited for the sensitization of defect band luminescence in various fields such as bio imaging, light emitting devices etc. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Nanocrystals Semiconductor quantum dots Optical properties Photoluminescence Defects
1. Introduction In the past several years, research activities have been focused on Zinc Oxide (ZnO), a group II-VI semiconductor, for various optoelectronic applications due to its high quantum yield both in the UV and the visible wavelength range. While the UV emission often results from the near band edge transition, the origin of the broad visible band is mainly attributed to emissions from different defect states developed in the crystal system during synthesis process. At nanoscale, space confinement modifies the band gap of the semiconductor [1] and thus the fundamental optical transitions, absorption and UV emission, can be tailored favorably for the device applications. However, this advantage is offset by the emergence of defect states depending on the synthesis process of nanocrystals. A clever control over these defect states [2,3] thus offers
* Corresponding author. Department of Physics, Jamia Millia Islamia University, New Delhi 110025, India. E-mail addresses: [email protected], [email protected] (D. Mithal), [email protected] (T. Kundu). http://dx.doi.org/10.1016/j.solidstatesciences.2017.04.006 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.
opportunities to develop various tunable photonic devices for field applications [4e6]. Introduction of external dopants into the ZnO nanocrystals is one such way to remold these UV and visible emissions. Several rare earth ions such as Nd3þ, Dy3þ, Tb3þ, Gd3þ, Eu3þ etc. doped ZnO nanocrystals have been investigated previously in this regard [7e13]. In most of these cases, the intensity of the visible emission of the doped nanocrystals has been initially increased with the dopant concentration. This enhancement of the visible emission is generally attributed to the formation of various defects such as oxygen interstitials, oxygen vacancies and eOH groups as a result of doping [10,12]. For further increment of rare earth ions above a critical concentration, the intensity of visible emission has been found to quench monotonously. Not only the defect state emission of the doped ZnO nanocrytals, the enhancement of the intensity of characteristic rare earth transitions have also been observed due to the excitation energy redistribution. In Sm3þ, Eu3þ and Tb3þ doped ZnO films, the emission from rare earth (RE) ions has been found to be less prominent under the direct excitation of these ions [14e16]. However, excitation above the band edge absorption of ZnO, sharp
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features of these rare earth ions in the visible region have been clearly observed along with the broad yellow emission of ZnO. In all these cases, the energy transfer takes place from ZnO host matrix to the rare earth ions depending on the proximity of the lowest excited state of these rare earth ions with respect to the defect states of ZnO. This energy transfer is also efficient because of the long lifetime of the visible emission [17]. However, the reverse energy transfer from rare earth to ZnO host matrix has not been observed so far. From these observations, it can be inferred that rare earth ions can perturb the optical transitions of the ZnO host matrix in two ways; (i) by developing structural disorders in ZnO host matrix due to the ionic size mismatch with respect to zinc and, (ii) by invoking energy transfer processes that naturally depends upon the relative placement of the host and rare earth energy levels. These aspects play crucial role in determining the dynamics of the excitation energy distribution among the energy levels of the RE ions and host matrix. A systematic understanding of the relative contributions coming from structural disorders due to doping as well as energy transfer mechanisms is essential to tailor the optical properties of the rare earth doped ZnO nanocrystals. To decipher the role of these two aspects separately, we have investigated the Gd3þ doped ZnO nanocrystals. In the above said context, Gadolinium (Gd3þ) is interesting as its lowest excited state (6P) lies above the band edge absorption of ZnO nanocrystals reported here. Consequently, the direct interactions between the Gd3þ energy levels and the defect states are less probable. In this case, the characteristics of the visible emission would be governed by defect states arising from structural disorder only. Therefore, this is taken as a model system for understanding the correlation between the intensity of visible emission and the structural modification on incorporation of rare earth ions. Further, this system also provides possibility to excite both ZnO host nanocrystal and Gd3þ dopant simultaneously to understand the energy transfer mechanism between these two systems. In view of this, a systematic structural and optical studies have been carried out on Gd3þ doped ZnO (Gd3þ: ZnO) nanocrystals over a wide dopant concentration range. The extent of change in the crystal structure on Gd3þ incorporation has been investigated by different spectroscopic techniques such as UV-Vis absorption spectroscopy, X-ray diffraction (XRD), Raman spectroscopy and X- ray photoelectron spectroscopy (XPS). The photoluminescence (PL) spectra upon excitation above the band edge have been analyzed to correlate the defect mediated energy transfer mechanisms with the degree of structural disorder resulting from Gd3þ doping. The effect of Gd3þ doping on decay dynamics of the defect state emission has also been investigated using time resolved PL spectra. Photoluminescence excitation (PLE) spectroscopy have been exploited to get an insight into the role of different energy transfer mechanisms those are operative between the nanocrystal host matrix and Gd3þ dopant.
Gd3þ doping in ZnO nanocrystals, different concentrations (mol %) of Gd3þ were taken as precursors. The precursors Zinc acetate dehydrate and Gadolinium acetate hydrate were procured and used without any further purification. Compositions of the elements present in four prepared samples were derived from the energy dispersive X- ray spectrum. The observed ratio of Gd3þ at% and Zn at % (x ¼ Gd/Zn) for five prepared samples investigated here are 0.02, 0.059, 0.18 and 0.34. Crystal structure analysis of the synthesized nanocrystal was carried out by X-ray diffraction pattern from a PAN analytical X-ray powder diffractometer. To obtain the structural and the morphological characteristics of these prepared samples, Transmission electron microscopic (TEM) (Model JEOL, 2100F, accelerating voltage of 200 KV) images and the associated energy dispersive Xray (EDX) spectra were analyzed. Absorption spectra were measured by using JASCO V570 UV-Vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) was carried out by Thermo VG Scientific MultiLab with the concentric hemispherical analyzer. AlKa (1486.1 eV) radiation was used as an X-ray source. C1s at 284.6 eV was used as the reference for the binding energy calibration. Micro Raman spectroscopy was performed using Horiba Jobin Yvon HR640, using excitation source (514 nm) from Arþ ion laser. The continuous width (CW) excitation PL spectra were obtained from the drop cast samples on Si substrate by a CW He-Cd laser source (325 nm). An Acton monochromator attached with a cooled CCD camera (ANDOR) system was used for the photoluminescence measurement. For the time resolved spectroscopy, a nanosecond pulsed excitation PL measurement system was developed. A second harmonic (532 nm) of Q Switched LITRON Nd: YAG laser was used for pumping a tunable dye laser (Lamda Physik). The second harmonic output from the dye laser was used as an excitation source and the luminescence from the sample was collected through a double monochromator (SPEX 1680) attached with a photomultiplier tube (PMT). The signal output from the PMT was recorded using an oscilloscope (TDS 2024) and the oscilloscope was interfaced with the computer through the labview program. This program is also developed to control the monochromator and the tunable dye laser. The time resolved photoluminescence spectra and pulsed photoluminescence excitation (PPLE) spectra were obtained either by varying the wavelength of the monochromator or the excitation wavelength of the dye laser, respectively.
2. Experimental procedures ZnO nanocrystals were synthesized by sol-gel method following the process developed by Meulenkamp et al., [18]. In this method, 1.1 g of precursor zinc acetate dehydrate was refluxed at 60 C temperature with 50 ml of ethanol. In a separate flask, 0.29 g of LiOH was sonicated with 50 ml of ethanol at room temperature for 30 min. LiOH was then added drop wise to zinc acetate solution for 10 min, whole mixture was left for stirring and further sonicated for one hour. For removing the reaction by-products, the sample was mixed with hexane and left for hours. This supernatant was removed and again it was dispersed in ethanol with the addition of hexane. This procedure was repeated for a number of times. The product was dried in air and collected for measurements. For the
Fig. 1. Absorption spectra of Gd3þ: ZnO nanocrystals with x (ratio of Gd3þ at% and Zn at%)values (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. The inset shows the variation of average band gap values of Gd3þ: ZnO with the corresponding x values.
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3. Results and discussion The UV-Vis absorption spectra of prepared undoped ZnO (x ¼ 0.0) and doped Gd3þ: ZnO (x ¼ 0.02 to 0.34) nanoparticles dispersed in ethanol solvent are shown in Fig. 1. A monotonous blue shift of the absorption spectrum relative to that of undoped ZnO nanoparticles is observed with increasing concentration of Gd3þ ions. The average band gap values [18] obtained from these absorption spectra are shown in the inset of Fig. 1. It was observed that the average band gap of the undoped ZnO nanoparticles increased from 3.32 eV to 3.49 eV for x ¼ 0.34. Mean particle size was derived from TEM images of these samples. The reduction of the mean diameter was observed with respect to the undoped ZnO nanoparticles [11,12]. These values for x ¼ 0.00, 0.02, 0.059, 0.18, 0.34, are 6.7 nm, 6.3 nm, 5.9 nm, 5.5 nm and 4.4 nm, respectively. Thus the observed blue shift of the absorption spectra given in Fig. 1 and the corresponding average band gap values can very well be attributed to this reduction of particle size due to Gd3þ doping in the ZnO nanocrystals. The XRD pattern shown in Fig. 2 present the effect of the incorporation of Gd3þ ion on the crystal structural of these ZnO nanocrystals. All the observed reflection peaks of diffraction planes for the undoped nanocrystals (indicated in Fig. 2a) correspond to wurzite crystal structure of ZnO. No extra peaks related to the other crystalline phases were observed upon Gd3þ incorporation. However, it is observed that the intensities of the diffraction peaks
Fig. 2. (a) XRD Patterns for Gd3þ: ZnO nanocrystals and (b) Peak shift and broadening in high resolution diffraction pattern corresponding to plane (101) for x values of (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34.
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gradually diminish with the increment of the Gd3þ concentration. This is accompanied by the shift and the broadening of these peaks. Fig. 2b shows the extent of the shift and broadening obtained from the high resolution diffraction peak corresponding to plane (101). Both the peak shift and the full width at half maxima (FWHM) of the undoped ZnO nanocrystals increase gradually up to Gd3þ concentration x ¼ 0.059 and remain almost constant for further increment of the Gd3þ ion concentration. Our observations are consistent with the previously reported Gd doped ZnO nanocrystals [11,12] and these changes are attributed to the degradation of crystallinity due to Gd3þ doping. The increased diffuseness in the electron diffraction pattern as given in supplementary information [Fig. S1] for our samples also reflects the detoriation of the crystalinity due to the incorporation of Gd3þ. There are several mechanisms for the observed degradation of the crystal structure. The synthesis process of these doped nanocrystals through chemical route provides several possible doping sites for the Gd3þ ion such as substitutional and interstitial. Gd3þ ion might as well be accumulated on the highly reactive surface of these nanocrystals. The proper doping of Gd3þ ion creates oxygen vacancies in the ZnO crystalline structure and it is well known that more introduction of oxygen generates stronger tensile stress in the crystal structure [19]. Not only this, in case of substitutional doping, the mismatch of ionic radii between the Gd3þ also produces micro stress in the nanocrystals. Further, the accumulation of Gd3þ on the surface creates the compressive hydrostatic pressure on the nanocrystals. Fig. 3a shows variation of the lattice parameters a and c obtained from the XRD pattern shown in Fig. 2a with respect to x. It can be
Fig. 3. (a) Variation in lattice parameters a and c and (b) hydrostatic pressure for Gd3þ: ZnO nanocrystals with respect to different x values.
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seen that both a (¼3.231 Å) and c (¼5.179 Å) parameters decrease initially to the minimum values for x ¼ 0.18, and then tend to increase upon further increment of Gd3þ concentration. A similar trend has been observed by Archana et al. [9] for Tb doped ZnO nanoparticles. The plausible explanation of the observed trend is that for low concentration of Gd3þ, the lattice contraction takes place up to a critical concentration due to the hydrostatic pressure generated by the rare earth dopants situated on the surface of the ZnO nanocrystals. Upon further increment in dopant concentration, the crystal lattice expands due to the substitution of Zn atoms in the core due to relatively large ionic radii of rare earth dopant. The hydrostatic pressure was calculated from the derived0 a and c parameters using the relation P ¼ ðB0 =B0 0 Þ½ða20 c0 =a2 c0 ÞB0 1 where B0 and B0 0 are the bulk modulus and pressure derivative of bulk modulus having values 183 Gpa and 4 Gpa for ZnO, respectively. In this, a0 and c0 are the lattice constants for the bulk ZnO having values 3.2496 Å and 5.2042 Å respectively [10]. Fig. 3b shows the variation of the calculated hydrostatic pressure with respect to x. The pressure was found to vary in our case from 3.02 Gpa for x ¼ 0.0e4.8 Gpa for x ¼ 0.18 and decreased to 4.3 Gpa for x ¼ 0.34. The observed peak shift of plane (101) shown in Fig. 2b also provides direct measurement of the internal compressive microstress generated due to strain and can be obtained from sst ¼ 3*B*½ðDq101 Þ*cot q101 where Dq101 is the peak shift corresponding to the plane (101), B is the young modulus having value 143 Gpa for ZnO [19]. The maximum change in stress with respect to ZnO nanoparticle was found to be 1.6 Gpa that is in accordance with the pressure calculated using a and c parameters. Evidently, the stress originating from two distinctly different modes of Gd3þ incorporation, at the surface and inside the core, develops different defects in the ZnO nanocrystals. For deeper insight into the nature of these defects, Oxygen 1s XPS spectra for the ZnO and Gd3þ: ZnO nanocrystals shown in Fig. 4 were analyzed. The lower energy region corresponding to peak centered at 530.4 eV is attributed to the O2 ions in the wurtzite structure surrounded by the Zn atoms, i.e., oxygen atoms at the regular lattice sites [20,21]. The peak at 531.9 eV is due to the oxygen deficient regions and these oxygen vacancies can be in the core or on the surface in the form of eOH groups. Our observations point out that due to the increment of Gd3þ ion, the amount of oxygen in the lattice decreases whereas number of other types of oxygen increases. The XPS spectra of the energy region between 1180 eV and 1235 eV for Gd3þ: ZnO nanocrystals are given in supplementary information [Fig. S2]. The observed two features at 1189 eV and 1220 eV correspond to Gd3þ 3d5/2 and 3d3/2 states, respectively and the Zn 2s state is observed at 1198 eV. It is to be noted that the intensity of Zn 2s state is less affected than the Gd3þ states as concentration of Gd3þ increases. This implies that all Gd3þ ions may not be replacing the Zn atoms of the sub lattice, in accordance with our observation about contraction of lattice. Since the observed structural disorder affects the crystal symmetry and consequently the phonon vibrations, the Raman spectra of our samples shown in Fig. 5 were analyzed according to the reported experimental and abinitio calculated values [22,23]. The detailed analysis is given in the supplementary and the assigned peaks are marked in Fig. 5. Most of the peaks in the Raman spectrum of undoped ZnO (Fig. 5i) are in agreement with the other reported values [22]. However, E1 modes (410 cm1 and 581 cm1) [23,24] are not observed in our case, instead 2LO modes (1102 cm1, 1151 cm1) are found to be more Raman active. The comparison of the Raman spectra of Gd3þ doped ZnO samples brings out two major points. First, the gradual reduction in intensity of the high 437 cm1 band assigned as E2 symmetry mode corresponding to oxygen sublattice is generally observed in doped ZnO nanocrystals and is attributed to the increased disorder in the crystal structure
Fig. 4. Oxygen 1s XPS spectra for Gd3þ: ZnO nanocrystals with x values of (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. Spectra were decomposed into two regions: Region 1 centered at 530.4 eV as and Region 2 centered at 531.9 eV.
[12,25e27]. However, there is a small variation of the FWHM of this band (6e8 cm1) observed due to doping. Second, the monotonous increment in intensity of all the A1 symmetry modes with the Gd3þ doping (1102 cm1, 1151 cm1, 650 cm1-750 cm1) is observed in our samples. This implies that the weakening/degradation of the crystallinity results in mixing of different phonon modes and this modification increases the Raman activity of the A1 modes. It is expected that this modifications profoundly influence the excitonphonon coupling in this system and consequently all its physical properties, especially the photoemission and its dynamics (radiative and nonradiative processes). The PL spectra of undoped ZnO and Gd3þ: ZnO obtained under CW excitation (325 nm) are presented in Fig. 6. The observed PL spectra consist of one very less intense peak in UV wavelength range (350e380 nm) and a broad feature in the visible wavelength range (450e675 nm). The spectra presented here are normalized with respect to peak intensity of UV emission as shown in the inset of Fig. 6. The observed blue shift of the peak of this band edge emission can be correlated with the observed shift in the absorption spectra. As seen, the defect state emission increases up to x ¼ 0.18 (curve (iv)) and decreases beyond that. Not only the change in intensity, but also the peak position of this emission shifts towards blue side of the spectrum till the intensity is maximum. Red shift of the peak occurs for further increment of Gd3þ
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Fig. 5. Raman spectra of Gd3þ: ZnO nanocrystals with x values of (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. The peak at * represents the S-M branch phonon in ZnO crystal structure (Ref. [28]).
Fig. 7. Decomposition of normalized visible spectra for x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. The sharp peak at 650 nm corresponds to the second order of excitation wavelength.
Fig. 6. Photoluminescence (PL) spectra of ZnO and Gd3þ: ZnO nanocrystals for various x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. Spectra are normalized with respect to UV peak intensity.
concentration. Generally, it has been shown that different parts of this broad emission contribute to the overall spectrum, originate from different defects states, mainly different kinds of oxygen vacancies, surface defect states, Zn vacancies and interstitial both oxygen and zinc [3]. To understand the contributions from these defect states in determining the overall shape of the broad emission, the observed spectra 450e675 nm shown in Fig. 6 were deconvoluted into three basic regions. These deconvoluted spectra are shown in Fig. 7. All the spectra were normalized to the peak intensity to understand the percentage contribution arising from these three regions. The deconvoluted PL spectra of ZnO nanocrystals (Fig. 7 (i)) consists of a band centered at 565 nm having
maximum intensity corresponds to the contribution from different kinds of oxygen vacancies. The peak centered at 653 nm results from the transitions from (Zni) to (Oi) [3,29]. The small peak centered at 595 nm is from the surface defects such as eOH groups and oxygen antisites [29]. The overall small blue shift of the PL spectrum for x value of 0.02 in Fig. 7(ii) compared to Fig. 7(i) is observed due to the blue shift of the three regions upon Gd3þ incorporation. It is observed that the contributions of both regions from oxygen vacancies and transition from Zni to Oi are almost same. A small contribution of surface defects centered at 595 nm is observed and this emission increases compared to the other two regions as the Gd3þ concentration increases. At optimum concentration where the PL intensity is maximum (x ¼ 0.18), it is seen that the contribution of surface defect dominates over the other defects. For x ¼ 0.34, it is observed that the contribution of region centered at 626 nm originating due to the increment of transition from Zni to Oi defects relatively dominates and thus the peak of the overall spectrum gets shifted towards the red side. In case of Sn-doped ZnO nanobelts and nanorings, Ramin Yousefi [30] has shown that there is a definite trend between the relative percentage change of green emission and the oxygen vacancies in the nonlattice observed in oxygen XPS spectrum. From detailed investigation on photocurrent in doped ZnO nanoparticles/nanocomposites, it is concluded that the enhancement of photocurrent in these doped systems is due to the observed oxygen vacancies in XPS and these composite systems
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are the promising materials for photodetectors [31e33]. In light of this, we have investigated the variation of PL corresponding to normalized contribution from oxygen defects such as surface oxygen and antisites as shown in Fig. 7 (area of the peak at 595 nm) with the contribution of the oxygen vacancies in XPS spectrum as shown in Fig. 4 (area of the peak centered at 531.9 eV) and plotted in Fig. 8. It is observed that there is a linear correlation between these two observed values upto the Gd concentration of 0.18. The departure from this linearity in higher concentration is obvious due to the dominant contribution in normalized PL from other than oxygen defects such as Zni to Oi as mentioned above. The overall increment of defect states emission with Gd3þ doping can be rationalized in a way that due to the particle size reduction, the surface to volume ratio increases and this increases the defect states concentration. As Gd3þ concentration increases, the crystallinity of the nanocrystals degrades as observed in XRD patterns and Raman spectra and this weakening in crystallinity alters the radiative transition probability. The reduction of PL intensity of visible emission thus results from the relative dominance of the radiative and nonradiative pathways. The above observation made for Gd3þ in ZnO is related to the structural modification and this insight can be extrapolated to other rare earth ions also. Thus by controlling the concentration, relative contributions of these defect states can be modified and hence the color of emission can be tuned. To understand the energy distribution in different time domain, the PL spectra of Gd3þ: ZnO colloidal solutions as shown in Fig. 9a were recorded immediately after excitation by the 5e7 ns 295 nm laser pulse. It is seen that the UV emission is more pronounced here as compared to cw excitation (Fig. 6). In all other respects, the CW and pulsed excited spectral profiles behave almost the same way. This difference in relative intensity arises due to the different population distribution in equilibrium (cw excitation) and nonequilibrium conditions (pulsed excitation). It also indicates longer relaxation time to populate defect states. Fig. 9b shows the inverted output of the PMT when the UV and visible emissions were monitored upon pulsed excitation at wavelength 295 nm. The decay profiles of the UV emission monitored at 370 nm (Curve (ii)) almost resembles the instrument response of the 5e7 ns excitation pulse (Curve (i)). Since the UV band edge emission is very fast
Fig. 9. (a) PL spectra of Gd3þ: ZnO upon excitation with 295 nm (5e7ns) light pulse for corresponding x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. (b) The decay profile upon excitation with 295 nm 5e7 ns pulse for (i) Instrument response function (IRF) (ii) UV emission at 370 nm ZnO. Decay profiles with x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34 with monitoring wavelength at peak of visible emission.
(~picosecond) [34], this could not be resolved in our experiment. As seen in Fig. 9b, decay profile of the visible emission of Gd3þ: ZnO consists of initially a fast component, followed by a slow and a very long decay component. It is seen that due to incorporation of Gd3þ, the shape of the decay profile changes. To understand the variation of these shapes, these decay profiles were deconvoluted into three exponential decay function using the instrument response function of the excitation pulse (curve (i)). The fitted values of the relative contribution and the corresponding lifetimes for these samples are given in Table 1. For undoped ZnO, the lifetime of the fast component (t1) was calculated as 0.6 ns whereas that of the slow component (t2) was 23 ns. The very long decay was having a lifetime (t3) around 0.6 ms. The contribution of the very fast
Table 1 Lifetimes and their relative contributions determined from the decay profiles.
Fig. 8. Variation of Pl corresponding to normalized contribution from oxygen defects such as surface oxygen and antisites (area of the peak at 595 nm) with the contribution of the oxygen vacancies in XPS spectrum (area of the peak centered at 531.9 eV).
x¼ (Gd3þ/Zn)
A1
t1 (ns)
A
0.0 0.02 0.059 0.18 0.34
(1.1%) (0.8%) (0.03%) (0.02%) (0.09%)
0.60 0.50 0.55 0.51 0.57
(76.3%) (80.1%) (59.4%) (44.4%) (62.1%)
2
t2 (ns)
A
23.4 22.6 42.7 50.8 26.4
(22.2%) (18.6%) (39.6%) (54.6%) (36.2%)
3
t3 (ms) ~0.60 ~0.68 ~0.70 ~0.84 ~0.89
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component is relatively much less as compared to others two components. As seen, both t2 and t3 increase gradually with Gd3þ concentration up to x ¼ 0.18. on the other hand, the contribution of t2 decreases while that of t3 increases as Gd3þ concentration increases. Since Gd3þ has an effect on these long components, these two lifetimes throughout the visible emission under the excitation in the vicinity of the first excited state of Gd3þ were studied. The fnfn transitions of Gd3þ ions between the ground state 8S and first excited state 6P lies around at 311 nm [35]. Fig. 10 shows the variation of t2 and t3 of the visible emission wavelength region (450e650 nm) under excitation of wavelength 311 nm where both Gd3þ as well as ZnO host matrix are excited. For comparison, the variation of these parameters due to the excitation of only ZnO at wavelength 309 nm is also presented in Fig. 10. As seen in both the cases, the lifetime of the defect state emission increases as the emission wavelength is changed towards the red side of the spectrum. However, for the excitation wavelength 311 nm, a clear increase in longer lifetime component is observed in the wavelength region 530e570 nm. As analyzed previously, this region is corresponding to the emission from surface absorbed spices like eOH groups. Since excitation at 311 nm affects the surface defect emission lifetime in presence of Gd3þ, the photoluminescence excitation spectrums were obtained by monitoring this peak of the visible emission at ~550 nm.
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Fig. 11. Photoluminescence excitation spectra for monitoring the peak of defect state emission for Gd3þ concentrations with x value of (i) x ¼ 0.0, (ii).
Fig. 11 shows the PPLE spectra for undoped and Gd3þ doped ZnO nanocrystals in the wavelength region 308.5 nme313.5 nm. The appearance of the peak at 311 nm in the PPLE spectra of doped ZnO samples signifies the existence of 8S to 6P transitions of Gd3þ. Although one photon absorptions within fn states are electric dipole forbidden due to parity selection rules, but may occur due to the admixture of odd parity states such as fn1d states. The comparison between the PPLE spectrum for undoped ZnO and Gd3þ: ZnO reveals that when the Gd3þ is excited not only the lifetime, but also the intensity of emission from the surface defect states increases. This observation suggests that the intensity increment upon doping results from an energy transfer from Gd3þ to ZnO surface defect states. There are several mechanisms for this energy transfer from Gd3þ to the surface states. One possible mechanism could be the reabsorbtion of emission from Gd3þ centers by ZnO nanocrystals host. Since the transition of Gd3þ is at 311 nm and this energy is greater than the band edge of the absorption of ZnO, resonant energy transfer can take place from Gd3þ to ZnO. The photogenerated charge carriers from this reabsorbtion may get trapped in surface states as most of Gd3þ are residing on the surface. Hence this emission as well as the lifetime of these surface states gets affected when the Gd3þ and ZnO both are excited simultaneously. For excitation other than 311 nm, visible emission results only from structural disorders due to the incorporation of Gd3þ ions.
4. Conclusion
Fig. 10. Variation of the long components of the visible emission lifetime (a) t2 and (b) t3 for Gd3þ: ZnO nanocrystals (x ¼ 0.18) at excitation wavelength of 309 nm and 311 nm.
To decipher the role of rare earth ions in the enhancement of the photoluminescence of the ZnO system, Gd3þ doped ZnO nanocrystals have been studied. Our systematic observations raise several important issues those need to be considered for developing the rare earth doped ZnO based optoelectronic devices. When the nanocrystals are synthesized through chemical route, depending on the mode of incorporation of Gd3þ, different kind of defect states such as oxygen vacancy, surface groups, oxygen interstitial, zinc interstitial and oxygen antisites are generated. Simultaneously, the crystallinity of the ZnO structure is gradually reduced due to the microstress developed from the incorporation of Gd3þ ion. As a result, both the radiative and nonradiative transition probabilities get modified due to the mixing of the wavefunctions of different symmetries and hence the re distribution of
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excitation energy governs the intensities of UV and visible emissions in the doped ZnO system. In our study, it was found that upon excitation above band edge absorption of Gd3þ doped ZnO nanocrystals, the contribution in the visible emission from the surface defect states or surface groups dominates when the Gd3þ concentration was increased. However, beyond a critical concentration, decrement of the intensity as well as red shift of the spectrum was observed. This accounts for the major contribution from the Zni to Oi transitions in the visible emission. Our study suggests that by controlling the Gd3þ concentration, the relative contributions of these defect states can be modified and hence the color of visible emission can be tuned. Role of Gd3þ ions is explored further more in effecting the defect luminescence observed from selective excitation of rare earth Gd3þ ions in ZnO nanocrystals. It was observed that Gd3þ doping significantly influences the long lifetime of the visible emission. When the Gd3þ is excited to its first excited state, the absorption by Gd3þ generates more charge carriers, which are trapped in the surface defect states. Consequently, the lifetime as well as the intensity of surface states emission in the wavelength region 530 nme570 nm is increased. This enhancement in intensity as well as lifetime is useful in various fields of research and applications such as LEDs, lasers and bioimaging etc. Our observation provides a correlation between the structural modifications and photoluminescence in wide concentration range of Gd3þ doping in ZnO nanocrystals. This study thus offers a way to control the optical properties of ZnO nanocrystals by merely adjusting the concentration of Gd3þ ions and the excitation wavelengths. This inference can also be extended to other rare earth ion. Acknowledgements Authors acknowledge the Sophisticated Analytical Instrument Facility (SAIF) of IIT Bombay for providing the Raman, FEG-TEM measurement facilities and Central facilities for XPS measurements. One of the authors acknowledges the CSIR (SRF award no, 20-6/2008(ii) EU-IV), India and IRCC, IIT Bombay for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2017.04.006. References [1] S.V. Gaponenko, Optical Properties of Semiconductor Nanocrystals, Cambridge
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Effect of Gd3þ doping on structural and optical properties of ZnO nanocrystals Deepika Mithal*, Tapanendu Kundu Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2017 Received in revised form 23 April 2017 Accepted 26 April 2017 Available online 27 April 2017
To investigate the effect of rare earth ions on the excitation energy redistribution between the UV and visible photoluminescence, the Gd3þ doped ZnO nanocrystals prepared through Sol-Gel method were studied. Different structural characterizations and spectroscopic studies were carried out to establish a correlation between the structural effect and the photoluminescence. Our study indicates that the incorporation of Gd3þ in ZnO nanocrystals prepared through this synthesis takes place predominantly on the surface up to 0.18 mole fraction of Gd3þ. This is manifested by the observed lattice contraction due to the presence of Gd3þ on the surface. This structural defect introduces various defects that modifies the photoluminescence properties. For these modifications in crystal system, the A1 symmetry phonon modes of wurtzite structure are found to be gradually more Raman active than the other modes as the Gd3þ concentration is increased while the E modes are suppressed. These doping induced modification of phonon spectrum strongly influences the exciton phonon coupling and thereby the photophysical properties. Resulting crystal imperfections strongly enhance the defect band luminescence. It is shown that energy transfer from selectively excited Gd3þ centers to surface states can be effectively exploited for the sensitization of defect band luminescence in various fields such as bio imaging, light emitting devices etc. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Nanocrystals Semiconductor quantum dots Optical properties Photoluminescence Defects
1. Introduction In the past several years, research activities have been focused on Zinc Oxide (ZnO), a group II-VI semiconductor, for various optoelectronic applications due to its high quantum yield both in the UV and the visible wavelength range. While the UV emission often results from the near band edge transition, the origin of the broad visible band is mainly attributed to emissions from different defect states developed in the crystal system during synthesis process. At nanoscale, space confinement modifies the band gap of the semiconductor [1] and thus the fundamental optical transitions, absorption and UV emission, can be tailored favorably for the device applications. However, this advantage is offset by the emergence of defect states depending on the synthesis process of nanocrystals. A clever control over these defect states [2,3] thus offers
* Corresponding author. Department of Physics, Jamia Millia Islamia University, New Delhi 110025, India. E-mail addresses: [email protected], [email protected] (D. Mithal), [email protected] (T. Kundu). http://dx.doi.org/10.1016/j.solidstatesciences.2017.04.006 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.
opportunities to develop various tunable photonic devices for field applications [4e6]. Introduction of external dopants into the ZnO nanocrystals is one such way to remold these UV and visible emissions. Several rare earth ions such as Nd3þ, Dy3þ, Tb3þ, Gd3þ, Eu3þ etc. doped ZnO nanocrystals have been investigated previously in this regard [7e13]. In most of these cases, the intensity of the visible emission of the doped nanocrystals has been initially increased with the dopant concentration. This enhancement of the visible emission is generally attributed to the formation of various defects such as oxygen interstitials, oxygen vacancies and eOH groups as a result of doping [10,12]. For further increment of rare earth ions above a critical concentration, the intensity of visible emission has been found to quench monotonously. Not only the defect state emission of the doped ZnO nanocrytals, the enhancement of the intensity of characteristic rare earth transitions have also been observed due to the excitation energy redistribution. In Sm3þ, Eu3þ and Tb3þ doped ZnO films, the emission from rare earth (RE) ions has been found to be less prominent under the direct excitation of these ions [14e16]. However, excitation above the band edge absorption of ZnO, sharp
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features of these rare earth ions in the visible region have been clearly observed along with the broad yellow emission of ZnO. In all these cases, the energy transfer takes place from ZnO host matrix to the rare earth ions depending on the proximity of the lowest excited state of these rare earth ions with respect to the defect states of ZnO. This energy transfer is also efficient because of the long lifetime of the visible emission [17]. However, the reverse energy transfer from rare earth to ZnO host matrix has not been observed so far. From these observations, it can be inferred that rare earth ions can perturb the optical transitions of the ZnO host matrix in two ways; (i) by developing structural disorders in ZnO host matrix due to the ionic size mismatch with respect to zinc and, (ii) by invoking energy transfer processes that naturally depends upon the relative placement of the host and rare earth energy levels. These aspects play crucial role in determining the dynamics of the excitation energy distribution among the energy levels of the RE ions and host matrix. A systematic understanding of the relative contributions coming from structural disorders due to doping as well as energy transfer mechanisms is essential to tailor the optical properties of the rare earth doped ZnO nanocrystals. To decipher the role of these two aspects separately, we have investigated the Gd3þ doped ZnO nanocrystals. In the above said context, Gadolinium (Gd3þ) is interesting as its lowest excited state (6P) lies above the band edge absorption of ZnO nanocrystals reported here. Consequently, the direct interactions between the Gd3þ energy levels and the defect states are less probable. In this case, the characteristics of the visible emission would be governed by defect states arising from structural disorder only. Therefore, this is taken as a model system for understanding the correlation between the intensity of visible emission and the structural modification on incorporation of rare earth ions. Further, this system also provides possibility to excite both ZnO host nanocrystal and Gd3þ dopant simultaneously to understand the energy transfer mechanism between these two systems. In view of this, a systematic structural and optical studies have been carried out on Gd3þ doped ZnO (Gd3þ: ZnO) nanocrystals over a wide dopant concentration range. The extent of change in the crystal structure on Gd3þ incorporation has been investigated by different spectroscopic techniques such as UV-Vis absorption spectroscopy, X-ray diffraction (XRD), Raman spectroscopy and X- ray photoelectron spectroscopy (XPS). The photoluminescence (PL) spectra upon excitation above the band edge have been analyzed to correlate the defect mediated energy transfer mechanisms with the degree of structural disorder resulting from Gd3þ doping. The effect of Gd3þ doping on decay dynamics of the defect state emission has also been investigated using time resolved PL spectra. Photoluminescence excitation (PLE) spectroscopy have been exploited to get an insight into the role of different energy transfer mechanisms those are operative between the nanocrystal host matrix and Gd3þ dopant.
Gd3þ doping in ZnO nanocrystals, different concentrations (mol %) of Gd3þ were taken as precursors. The precursors Zinc acetate dehydrate and Gadolinium acetate hydrate were procured and used without any further purification. Compositions of the elements present in four prepared samples were derived from the energy dispersive X- ray spectrum. The observed ratio of Gd3þ at% and Zn at % (x ¼ Gd/Zn) for five prepared samples investigated here are 0.02, 0.059, 0.18 and 0.34. Crystal structure analysis of the synthesized nanocrystal was carried out by X-ray diffraction pattern from a PAN analytical X-ray powder diffractometer. To obtain the structural and the morphological characteristics of these prepared samples, Transmission electron microscopic (TEM) (Model JEOL, 2100F, accelerating voltage of 200 KV) images and the associated energy dispersive Xray (EDX) spectra were analyzed. Absorption spectra were measured by using JASCO V570 UV-Vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) was carried out by Thermo VG Scientific MultiLab with the concentric hemispherical analyzer. AlKa (1486.1 eV) radiation was used as an X-ray source. C1s at 284.6 eV was used as the reference for the binding energy calibration. Micro Raman spectroscopy was performed using Horiba Jobin Yvon HR640, using excitation source (514 nm) from Arþ ion laser. The continuous width (CW) excitation PL spectra were obtained from the drop cast samples on Si substrate by a CW He-Cd laser source (325 nm). An Acton monochromator attached with a cooled CCD camera (ANDOR) system was used for the photoluminescence measurement. For the time resolved spectroscopy, a nanosecond pulsed excitation PL measurement system was developed. A second harmonic (532 nm) of Q Switched LITRON Nd: YAG laser was used for pumping a tunable dye laser (Lamda Physik). The second harmonic output from the dye laser was used as an excitation source and the luminescence from the sample was collected through a double monochromator (SPEX 1680) attached with a photomultiplier tube (PMT). The signal output from the PMT was recorded using an oscilloscope (TDS 2024) and the oscilloscope was interfaced with the computer through the labview program. This program is also developed to control the monochromator and the tunable dye laser. The time resolved photoluminescence spectra and pulsed photoluminescence excitation (PPLE) spectra were obtained either by varying the wavelength of the monochromator or the excitation wavelength of the dye laser, respectively.
2. Experimental procedures ZnO nanocrystals were synthesized by sol-gel method following the process developed by Meulenkamp et al., [18]. In this method, 1.1 g of precursor zinc acetate dehydrate was refluxed at 60 C temperature with 50 ml of ethanol. In a separate flask, 0.29 g of LiOH was sonicated with 50 ml of ethanol at room temperature for 30 min. LiOH was then added drop wise to zinc acetate solution for 10 min, whole mixture was left for stirring and further sonicated for one hour. For removing the reaction by-products, the sample was mixed with hexane and left for hours. This supernatant was removed and again it was dispersed in ethanol with the addition of hexane. This procedure was repeated for a number of times. The product was dried in air and collected for measurements. For the
Fig. 1. Absorption spectra of Gd3þ: ZnO nanocrystals with x (ratio of Gd3þ at% and Zn at%)values (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. The inset shows the variation of average band gap values of Gd3þ: ZnO with the corresponding x values.
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3. Results and discussion The UV-Vis absorption spectra of prepared undoped ZnO (x ¼ 0.0) and doped Gd3þ: ZnO (x ¼ 0.02 to 0.34) nanoparticles dispersed in ethanol solvent are shown in Fig. 1. A monotonous blue shift of the absorption spectrum relative to that of undoped ZnO nanoparticles is observed with increasing concentration of Gd3þ ions. The average band gap values [18] obtained from these absorption spectra are shown in the inset of Fig. 1. It was observed that the average band gap of the undoped ZnO nanoparticles increased from 3.32 eV to 3.49 eV for x ¼ 0.34. Mean particle size was derived from TEM images of these samples. The reduction of the mean diameter was observed with respect to the undoped ZnO nanoparticles [11,12]. These values for x ¼ 0.00, 0.02, 0.059, 0.18, 0.34, are 6.7 nm, 6.3 nm, 5.9 nm, 5.5 nm and 4.4 nm, respectively. Thus the observed blue shift of the absorption spectra given in Fig. 1 and the corresponding average band gap values can very well be attributed to this reduction of particle size due to Gd3þ doping in the ZnO nanocrystals. The XRD pattern shown in Fig. 2 present the effect of the incorporation of Gd3þ ion on the crystal structural of these ZnO nanocrystals. All the observed reflection peaks of diffraction planes for the undoped nanocrystals (indicated in Fig. 2a) correspond to wurzite crystal structure of ZnO. No extra peaks related to the other crystalline phases were observed upon Gd3þ incorporation. However, it is observed that the intensities of the diffraction peaks
Fig. 2. (a) XRD Patterns for Gd3þ: ZnO nanocrystals and (b) Peak shift and broadening in high resolution diffraction pattern corresponding to plane (101) for x values of (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34.
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gradually diminish with the increment of the Gd3þ concentration. This is accompanied by the shift and the broadening of these peaks. Fig. 2b shows the extent of the shift and broadening obtained from the high resolution diffraction peak corresponding to plane (101). Both the peak shift and the full width at half maxima (FWHM) of the undoped ZnO nanocrystals increase gradually up to Gd3þ concentration x ¼ 0.059 and remain almost constant for further increment of the Gd3þ ion concentration. Our observations are consistent with the previously reported Gd doped ZnO nanocrystals [11,12] and these changes are attributed to the degradation of crystallinity due to Gd3þ doping. The increased diffuseness in the electron diffraction pattern as given in supplementary information [Fig. S1] for our samples also reflects the detoriation of the crystalinity due to the incorporation of Gd3þ. There are several mechanisms for the observed degradation of the crystal structure. The synthesis process of these doped nanocrystals through chemical route provides several possible doping sites for the Gd3þ ion such as substitutional and interstitial. Gd3þ ion might as well be accumulated on the highly reactive surface of these nanocrystals. The proper doping of Gd3þ ion creates oxygen vacancies in the ZnO crystalline structure and it is well known that more introduction of oxygen generates stronger tensile stress in the crystal structure [19]. Not only this, in case of substitutional doping, the mismatch of ionic radii between the Gd3þ also produces micro stress in the nanocrystals. Further, the accumulation of Gd3þ on the surface creates the compressive hydrostatic pressure on the nanocrystals. Fig. 3a shows variation of the lattice parameters a and c obtained from the XRD pattern shown in Fig. 2a with respect to x. It can be
Fig. 3. (a) Variation in lattice parameters a and c and (b) hydrostatic pressure for Gd3þ: ZnO nanocrystals with respect to different x values.
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seen that both a (¼3.231 Å) and c (¼5.179 Å) parameters decrease initially to the minimum values for x ¼ 0.18, and then tend to increase upon further increment of Gd3þ concentration. A similar trend has been observed by Archana et al. [9] for Tb doped ZnO nanoparticles. The plausible explanation of the observed trend is that for low concentration of Gd3þ, the lattice contraction takes place up to a critical concentration due to the hydrostatic pressure generated by the rare earth dopants situated on the surface of the ZnO nanocrystals. Upon further increment in dopant concentration, the crystal lattice expands due to the substitution of Zn atoms in the core due to relatively large ionic radii of rare earth dopant. The hydrostatic pressure was calculated from the derived0 a and c parameters using the relation P ¼ ðB0 =B0 0 Þ½ða20 c0 =a2 c0 ÞB0 1 where B0 and B0 0 are the bulk modulus and pressure derivative of bulk modulus having values 183 Gpa and 4 Gpa for ZnO, respectively. In this, a0 and c0 are the lattice constants for the bulk ZnO having values 3.2496 Å and 5.2042 Å respectively [10]. Fig. 3b shows the variation of the calculated hydrostatic pressure with respect to x. The pressure was found to vary in our case from 3.02 Gpa for x ¼ 0.0e4.8 Gpa for x ¼ 0.18 and decreased to 4.3 Gpa for x ¼ 0.34. The observed peak shift of plane (101) shown in Fig. 2b also provides direct measurement of the internal compressive microstress generated due to strain and can be obtained from sst ¼ 3*B*½ðDq101 Þ*cot q101 where Dq101 is the peak shift corresponding to the plane (101), B is the young modulus having value 143 Gpa for ZnO [19]. The maximum change in stress with respect to ZnO nanoparticle was found to be 1.6 Gpa that is in accordance with the pressure calculated using a and c parameters. Evidently, the stress originating from two distinctly different modes of Gd3þ incorporation, at the surface and inside the core, develops different defects in the ZnO nanocrystals. For deeper insight into the nature of these defects, Oxygen 1s XPS spectra for the ZnO and Gd3þ: ZnO nanocrystals shown in Fig. 4 were analyzed. The lower energy region corresponding to peak centered at 530.4 eV is attributed to the O2 ions in the wurtzite structure surrounded by the Zn atoms, i.e., oxygen atoms at the regular lattice sites [20,21]. The peak at 531.9 eV is due to the oxygen deficient regions and these oxygen vacancies can be in the core or on the surface in the form of eOH groups. Our observations point out that due to the increment of Gd3þ ion, the amount of oxygen in the lattice decreases whereas number of other types of oxygen increases. The XPS spectra of the energy region between 1180 eV and 1235 eV for Gd3þ: ZnO nanocrystals are given in supplementary information [Fig. S2]. The observed two features at 1189 eV and 1220 eV correspond to Gd3þ 3d5/2 and 3d3/2 states, respectively and the Zn 2s state is observed at 1198 eV. It is to be noted that the intensity of Zn 2s state is less affected than the Gd3þ states as concentration of Gd3þ increases. This implies that all Gd3þ ions may not be replacing the Zn atoms of the sub lattice, in accordance with our observation about contraction of lattice. Since the observed structural disorder affects the crystal symmetry and consequently the phonon vibrations, the Raman spectra of our samples shown in Fig. 5 were analyzed according to the reported experimental and abinitio calculated values [22,23]. The detailed analysis is given in the supplementary and the assigned peaks are marked in Fig. 5. Most of the peaks in the Raman spectrum of undoped ZnO (Fig. 5i) are in agreement with the other reported values [22]. However, E1 modes (410 cm1 and 581 cm1) [23,24] are not observed in our case, instead 2LO modes (1102 cm1, 1151 cm1) are found to be more Raman active. The comparison of the Raman spectra of Gd3þ doped ZnO samples brings out two major points. First, the gradual reduction in intensity of the high 437 cm1 band assigned as E2 symmetry mode corresponding to oxygen sublattice is generally observed in doped ZnO nanocrystals and is attributed to the increased disorder in the crystal structure
Fig. 4. Oxygen 1s XPS spectra for Gd3þ: ZnO nanocrystals with x values of (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. Spectra were decomposed into two regions: Region 1 centered at 530.4 eV as and Region 2 centered at 531.9 eV.
[12,25e27]. However, there is a small variation of the FWHM of this band (6e8 cm1) observed due to doping. Second, the monotonous increment in intensity of all the A1 symmetry modes with the Gd3þ doping (1102 cm1, 1151 cm1, 650 cm1-750 cm1) is observed in our samples. This implies that the weakening/degradation of the crystallinity results in mixing of different phonon modes and this modification increases the Raman activity of the A1 modes. It is expected that this modifications profoundly influence the excitonphonon coupling in this system and consequently all its physical properties, especially the photoemission and its dynamics (radiative and nonradiative processes). The PL spectra of undoped ZnO and Gd3þ: ZnO obtained under CW excitation (325 nm) are presented in Fig. 6. The observed PL spectra consist of one very less intense peak in UV wavelength range (350e380 nm) and a broad feature in the visible wavelength range (450e675 nm). The spectra presented here are normalized with respect to peak intensity of UV emission as shown in the inset of Fig. 6. The observed blue shift of the peak of this band edge emission can be correlated with the observed shift in the absorption spectra. As seen, the defect state emission increases up to x ¼ 0.18 (curve (iv)) and decreases beyond that. Not only the change in intensity, but also the peak position of this emission shifts towards blue side of the spectrum till the intensity is maximum. Red shift of the peak occurs for further increment of Gd3þ
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Fig. 5. Raman spectra of Gd3þ: ZnO nanocrystals with x values of (i) 0.00, (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. The peak at * represents the S-M branch phonon in ZnO crystal structure (Ref. [28]).
Fig. 7. Decomposition of normalized visible spectra for x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. The sharp peak at 650 nm corresponds to the second order of excitation wavelength.
Fig. 6. Photoluminescence (PL) spectra of ZnO and Gd3þ: ZnO nanocrystals for various x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. Spectra are normalized with respect to UV peak intensity.
concentration. Generally, it has been shown that different parts of this broad emission contribute to the overall spectrum, originate from different defects states, mainly different kinds of oxygen vacancies, surface defect states, Zn vacancies and interstitial both oxygen and zinc [3]. To understand the contributions from these defect states in determining the overall shape of the broad emission, the observed spectra 450e675 nm shown in Fig. 6 were deconvoluted into three basic regions. These deconvoluted spectra are shown in Fig. 7. All the spectra were normalized to the peak intensity to understand the percentage contribution arising from these three regions. The deconvoluted PL spectra of ZnO nanocrystals (Fig. 7 (i)) consists of a band centered at 565 nm having
maximum intensity corresponds to the contribution from different kinds of oxygen vacancies. The peak centered at 653 nm results from the transitions from (Zni) to (Oi) [3,29]. The small peak centered at 595 nm is from the surface defects such as eOH groups and oxygen antisites [29]. The overall small blue shift of the PL spectrum for x value of 0.02 in Fig. 7(ii) compared to Fig. 7(i) is observed due to the blue shift of the three regions upon Gd3þ incorporation. It is observed that the contributions of both regions from oxygen vacancies and transition from Zni to Oi are almost same. A small contribution of surface defects centered at 595 nm is observed and this emission increases compared to the other two regions as the Gd3þ concentration increases. At optimum concentration where the PL intensity is maximum (x ¼ 0.18), it is seen that the contribution of surface defect dominates over the other defects. For x ¼ 0.34, it is observed that the contribution of region centered at 626 nm originating due to the increment of transition from Zni to Oi defects relatively dominates and thus the peak of the overall spectrum gets shifted towards the red side. In case of Sn-doped ZnO nanobelts and nanorings, Ramin Yousefi [30] has shown that there is a definite trend between the relative percentage change of green emission and the oxygen vacancies in the nonlattice observed in oxygen XPS spectrum. From detailed investigation on photocurrent in doped ZnO nanoparticles/nanocomposites, it is concluded that the enhancement of photocurrent in these doped systems is due to the observed oxygen vacancies in XPS and these composite systems
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are the promising materials for photodetectors [31e33]. In light of this, we have investigated the variation of PL corresponding to normalized contribution from oxygen defects such as surface oxygen and antisites as shown in Fig. 7 (area of the peak at 595 nm) with the contribution of the oxygen vacancies in XPS spectrum as shown in Fig. 4 (area of the peak centered at 531.9 eV) and plotted in Fig. 8. It is observed that there is a linear correlation between these two observed values upto the Gd concentration of 0.18. The departure from this linearity in higher concentration is obvious due to the dominant contribution in normalized PL from other than oxygen defects such as Zni to Oi as mentioned above. The overall increment of defect states emission with Gd3þ doping can be rationalized in a way that due to the particle size reduction, the surface to volume ratio increases and this increases the defect states concentration. As Gd3þ concentration increases, the crystallinity of the nanocrystals degrades as observed in XRD patterns and Raman spectra and this weakening in crystallinity alters the radiative transition probability. The reduction of PL intensity of visible emission thus results from the relative dominance of the radiative and nonradiative pathways. The above observation made for Gd3þ in ZnO is related to the structural modification and this insight can be extrapolated to other rare earth ions also. Thus by controlling the concentration, relative contributions of these defect states can be modified and hence the color of emission can be tuned. To understand the energy distribution in different time domain, the PL spectra of Gd3þ: ZnO colloidal solutions as shown in Fig. 9a were recorded immediately after excitation by the 5e7 ns 295 nm laser pulse. It is seen that the UV emission is more pronounced here as compared to cw excitation (Fig. 6). In all other respects, the CW and pulsed excited spectral profiles behave almost the same way. This difference in relative intensity arises due to the different population distribution in equilibrium (cw excitation) and nonequilibrium conditions (pulsed excitation). It also indicates longer relaxation time to populate defect states. Fig. 9b shows the inverted output of the PMT when the UV and visible emissions were monitored upon pulsed excitation at wavelength 295 nm. The decay profiles of the UV emission monitored at 370 nm (Curve (ii)) almost resembles the instrument response of the 5e7 ns excitation pulse (Curve (i)). Since the UV band edge emission is very fast
Fig. 9. (a) PL spectra of Gd3þ: ZnO upon excitation with 295 nm (5e7ns) light pulse for corresponding x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34. (b) The decay profile upon excitation with 295 nm 5e7 ns pulse for (i) Instrument response function (IRF) (ii) UV emission at 370 nm ZnO. Decay profiles with x values of (i) 0.0 (ii) 0.02, (iii) 0.059, (iv) 0.18 and (v) 0.34 with monitoring wavelength at peak of visible emission.
(~picosecond) [34], this could not be resolved in our experiment. As seen in Fig. 9b, decay profile of the visible emission of Gd3þ: ZnO consists of initially a fast component, followed by a slow and a very long decay component. It is seen that due to incorporation of Gd3þ, the shape of the decay profile changes. To understand the variation of these shapes, these decay profiles were deconvoluted into three exponential decay function using the instrument response function of the excitation pulse (curve (i)). The fitted values of the relative contribution and the corresponding lifetimes for these samples are given in Table 1. For undoped ZnO, the lifetime of the fast component (t1) was calculated as 0.6 ns whereas that of the slow component (t2) was 23 ns. The very long decay was having a lifetime (t3) around 0.6 ms. The contribution of the very fast
Table 1 Lifetimes and their relative contributions determined from the decay profiles.
Fig. 8. Variation of Pl corresponding to normalized contribution from oxygen defects such as surface oxygen and antisites (area of the peak at 595 nm) with the contribution of the oxygen vacancies in XPS spectrum (area of the peak centered at 531.9 eV).
x¼ (Gd3þ/Zn)
A1
t1 (ns)
A
0.0 0.02 0.059 0.18 0.34
(1.1%) (0.8%) (0.03%) (0.02%) (0.09%)
0.60 0.50 0.55 0.51 0.57
(76.3%) (80.1%) (59.4%) (44.4%) (62.1%)
2
t2 (ns)
A
23.4 22.6 42.7 50.8 26.4
(22.2%) (18.6%) (39.6%) (54.6%) (36.2%)
3
t3 (ms) ~0.60 ~0.68 ~0.70 ~0.84 ~0.89
D. Mithal, T. Kundu / Solid State Sciences 68 (2017) 47e54
component is relatively much less as compared to others two components. As seen, both t2 and t3 increase gradually with Gd3þ concentration up to x ¼ 0.18. on the other hand, the contribution of t2 decreases while that of t3 increases as Gd3þ concentration increases. Since Gd3þ has an effect on these long components, these two lifetimes throughout the visible emission under the excitation in the vicinity of the first excited state of Gd3þ were studied. The fnfn transitions of Gd3þ ions between the ground state 8S and first excited state 6P lies around at 311 nm [35]. Fig. 10 shows the variation of t2 and t3 of the visible emission wavelength region (450e650 nm) under excitation of wavelength 311 nm where both Gd3þ as well as ZnO host matrix are excited. For comparison, the variation of these parameters due to the excitation of only ZnO at wavelength 309 nm is also presented in Fig. 10. As seen in both the cases, the lifetime of the defect state emission increases as the emission wavelength is changed towards the red side of the spectrum. However, for the excitation wavelength 311 nm, a clear increase in longer lifetime component is observed in the wavelength region 530e570 nm. As analyzed previously, this region is corresponding to the emission from surface absorbed spices like eOH groups. Since excitation at 311 nm affects the surface defect emission lifetime in presence of Gd3þ, the photoluminescence excitation spectrums were obtained by monitoring this peak of the visible emission at ~550 nm.
53
Fig. 11. Photoluminescence excitation spectra for monitoring the peak of defect state emission for Gd3þ concentrations with x value of (i) x ¼ 0.0, (ii).
Fig. 11 shows the PPLE spectra for undoped and Gd3þ doped ZnO nanocrystals in the wavelength region 308.5 nme313.5 nm. The appearance of the peak at 311 nm in the PPLE spectra of doped ZnO samples signifies the existence of 8S to 6P transitions of Gd3þ. Although one photon absorptions within fn states are electric dipole forbidden due to parity selection rules, but may occur due to the admixture of odd parity states such as fn1d states. The comparison between the PPLE spectrum for undoped ZnO and Gd3þ: ZnO reveals that when the Gd3þ is excited not only the lifetime, but also the intensity of emission from the surface defect states increases. This observation suggests that the intensity increment upon doping results from an energy transfer from Gd3þ to ZnO surface defect states. There are several mechanisms for this energy transfer from Gd3þ to the surface states. One possible mechanism could be the reabsorbtion of emission from Gd3þ centers by ZnO nanocrystals host. Since the transition of Gd3þ is at 311 nm and this energy is greater than the band edge of the absorption of ZnO, resonant energy transfer can take place from Gd3þ to ZnO. The photogenerated charge carriers from this reabsorbtion may get trapped in surface states as most of Gd3þ are residing on the surface. Hence this emission as well as the lifetime of these surface states gets affected when the Gd3þ and ZnO both are excited simultaneously. For excitation other than 311 nm, visible emission results only from structural disorders due to the incorporation of Gd3þ ions.
4. Conclusion
Fig. 10. Variation of the long components of the visible emission lifetime (a) t2 and (b) t3 for Gd3þ: ZnO nanocrystals (x ¼ 0.18) at excitation wavelength of 309 nm and 311 nm.
To decipher the role of rare earth ions in the enhancement of the photoluminescence of the ZnO system, Gd3þ doped ZnO nanocrystals have been studied. Our systematic observations raise several important issues those need to be considered for developing the rare earth doped ZnO based optoelectronic devices. When the nanocrystals are synthesized through chemical route, depending on the mode of incorporation of Gd3þ, different kind of defect states such as oxygen vacancy, surface groups, oxygen interstitial, zinc interstitial and oxygen antisites are generated. Simultaneously, the crystallinity of the ZnO structure is gradually reduced due to the microstress developed from the incorporation of Gd3þ ion. As a result, both the radiative and nonradiative transition probabilities get modified due to the mixing of the wavefunctions of different symmetries and hence the re distribution of
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excitation energy governs the intensities of UV and visible emissions in the doped ZnO system. In our study, it was found that upon excitation above band edge absorption of Gd3þ doped ZnO nanocrystals, the contribution in the visible emission from the surface defect states or surface groups dominates when the Gd3þ concentration was increased. However, beyond a critical concentration, decrement of the intensity as well as red shift of the spectrum was observed. This accounts for the major contribution from the Zni to Oi transitions in the visible emission. Our study suggests that by controlling the Gd3þ concentration, the relative contributions of these defect states can be modified and hence the color of visible emission can be tuned. Role of Gd3þ ions is explored further more in effecting the defect luminescence observed from selective excitation of rare earth Gd3þ ions in ZnO nanocrystals. It was observed that Gd3þ doping significantly influences the long lifetime of the visible emission. When the Gd3þ is excited to its first excited state, the absorption by Gd3þ generates more charge carriers, which are trapped in the surface defect states. Consequently, the lifetime as well as the intensity of surface states emission in the wavelength region 530 nme570 nm is increased. This enhancement in intensity as well as lifetime is useful in various fields of research and applications such as LEDs, lasers and bioimaging etc. Our observation provides a correlation between the structural modifications and photoluminescence in wide concentration range of Gd3þ doping in ZnO nanocrystals. This study thus offers a way to control the optical properties of ZnO nanocrystals by merely adjusting the concentration of Gd3þ ions and the excitation wavelengths. This inference can also be extended to other rare earth ion. Acknowledgements Authors acknowledge the Sophisticated Analytical Instrument Facility (SAIF) of IIT Bombay for providing the Raman, FEG-TEM measurement facilities and Central facilities for XPS measurements. One of the authors acknowledges the CSIR (SRF award no, 20-6/2008(ii) EU-IV), India and IRCC, IIT Bombay for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2017.04.006. References [1] S.V. Gaponenko, Optical Properties of Semiconductor Nanocrystals, Cambridge
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