Optical investigation on cadmium-doped zinc oxide nanoparticles synthesized by using a sonochemical method

Abstract

Cadmium-doped zinc oxide Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles were synthesized by using a sonochemical method. The X-ray diffraction (XRD) patterns show that Cd_xZn_1−xO nanoparticles keep a wurtzite structure up to x = 0.40. However, the lattice parameters a, c, and internal parameter u gradually increase with increasing x. The Raman scattering investigation indicates that the Raman mode shows a significant softening in Cd-doped ZnO nanoparticles with increasing x, which suggests an expansion of the Zn–O bond. We also observe a significant red-shift of the band gap for Cd_xZn_1−xO nanoparticles as x increases, i.e., the gap energy E_g ∼ 3.21 eV for x = 0 shifts to 2.92 eV for x = 0.40. In addition, the Urbach band tail gradually increases with x-value, which indicates an enhancement of the defect states in samples with Cd incorporation. In Cd_xZn_1−xO nanoparticles, we further find that the near band-edge (NBE) emission band shows a red-shift with increase in x-value. A broad photoluminescence (PL) band is also observed at around 590 nm, which enhances from x = 0 to 0.15. With further increases in the x-value to 0.40, a weak PL band appears around 450 nm while the broad band around 590 nm is suppressed. The variation of PL intensity is ascribed to the change of the Cd-doping induced defect states and the nonradiative energy transfers.

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1. Introduction

II–VI semiconductor materials have already been used for a variety of applications, e.g., in gas sensors, surface acoustic wave (SAW) devices and transparent contacts. Among them, zinc oxide (ZnO) has attracted extensive interest because of its novel properties such as direct wide band gap, large exciton binding energy, high chemical stability and environment-friendly applications.¹ These properties make ZnO a highlight in the field of optoelectronic semiconductors.^2,3 The main focus for the application of ZnO materials is in optoelectronic devices, which means that it is important to modulate the band gap. Doping by other elements is an effective way to adjust the band gap of ZnO.^3–7 Among these doping elements, cadmium is a suitable material for reducing the band gap of ZnO,^5,6,8–16 and a decrease in the band gap down to 2.99 eV has been achieved in Cd_xZn_1−xO by incorporating Cd²⁺ with x = 0.07.¹⁷

ZnO has a wurtzite structure, which is incompatible with the rock-salt structure of CdO, suggesting that Cd_xZn_1−xO with heavy Cd-doping cannot be stable. Actually, the thermodynamic solubility limit of CdO in the CdO–ZnO system is 2 mol% under thermal equilibrium conditions.¹⁸ Thus, phase segregation often occurs in heavy Cd-doped ZnO as they are subjected to conventional thermal processing. In order to overcome the limit, some techniques such as laser molecular beam epitaxy (MBE),¹⁹ pulse-laser deposition,²⁰ and metal organic vapor phase epitaxy (MOVPE),^5,9 have been used to prepare Cd_xZn_1−xO films under non-thermal equilibrium conditions. By using these techniques, the maximum content of cadmium in the wurtzite Cd_xZn_1−xO film is 0.32.¹³ In addition, there are quite a few reports on the synthesis of cadmium doped ZnO nanoparticles.^21–23 However, the maximum content of cadmium is below 0.1 in the wurtzite Cd-doped ZnO nanoparticles. On the other hand, it is well known that ultrasonic irradiation causes the occurrence and collapse of cavitation bubbles in solution, which can give rise to extreme conditions such as high temperature and high pressure. These extreme conditions may induce non-thermal equilibrium conditions, which are suitable for the synthesis of stable Cd_xZn_1−xO nanoparticles with heavy cadmium doping.

In this paper, a sonochemical method has been used for the first time to synthesize Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles. By XRD investigation, we have found that Cd_xZn_1−xO nanoparticles can keep the wurtzite structure with increasing x up to 0.40, which indicates the stability of the Cd_xZn_1−xO nanoparticles synthesized by the sonochemical method. In the wurtzite structure, the lattice parameters gradually increase with increasing x. The findings of the Raman investigation are consistent with the XRD results. We also observed Cd-doping induced red-shifts of the band gap and a near band-edge (NBE) emission band for Cd_xZn_1−xO nanoparticles. In addition, we observed broad photoluminescence (PL) around 590 nm, which increases with increasing x up to 0.15 and decreases when the x-value further increases to 0.40. Simultaneously, a weak PL band seems to appear around 450 nm. The variation of the PL bands has been ascribed to the change of the defect states induced by Cd-doping and the nonradiative energy transfers.

2. Experimental

Cd-doped ZnO nanoparticles were synthesized by the sonochemical reaction. Zn(CH₃COO)₂·2H₂O and Cd(CH₃COO)₂·2H₂O were chosen as the zinc precursor and cadmium source, respectively. The molar ratio of Cd and Zn was kept at about 1 : 20, 3 : 20, 2 : 5 and 4 : 5 for preparing 5%, 15%, 40% and 80% Cd-doped ZnO, respectively. Two separate solutions of Zn(CH₃COO)₂·2H₂O and Cd(CH₃COO)₂·2H₂O were prepared in diethanolamine. Then the two solutions were mixed and stirred for 3 min under ultrasonic irradiation. Water was added into the solution for precipitation. The precipitate was annealed in open air at 70 °C for one hour and the Cd-doped ZnO nanoparticles were thus obtained. The ultrasonic irradiation was generated by an ultrasonic generator (VCX750) made by Sonics & Materials Inc. with a frequency of 20 kHz and a power of 500 W.

The crystal structures of the samples were analyzed by a Rigaku D/max 2500 diffractometer with Cu-Kα radiation and a graphite monochromator. A step scan mode was employed with a step width of 2θ = 0.02° and a sampling time of 1s. The average size for the undoped sample was determined from the full-width at half-maximum (FWHM) of the most intense peak such as (101) peak by using Scherrer's equation, i.e., D = 0.9λ/Bcos(θ),²⁴ where D is the average grain size, λ is the X-ray wavelength, B is the FWHM of (101) peak and θ is the diffraction angle, and thus the average size of the particles was determined to be 12 nm. Fig. 1 shows the TEM images of (a) pure ZnO and (b) 40% Cd-doped ZnO nanoparticles. It is found that the size of the Cd-doped ZnO nanoparticles lies in the same range as that of the pure sample. The lattice parameters are calculated by using the equation: 1/d² = (H² + K²)/a² + L²/c², where H, K and L stand for the crystal face index, and d is the interplanar spacing.

Fig. 1 TEM images of (a) pure ZnO and (b) 40% Cd-doped ZnO nanoparticles.

In Raman experiments, Raman scattering light was detected with a Renishaw spectrometer excited at 632.8 nm. Measurements were performed from 0.5 to 2.4 mW for incident laser power, which can avoid sample damage or laser induced heating. No significant spectral change was observed in the range of 0.5 to 2.4 mW for incident laser power. The UV-Visible absorption spectra of the samples were recorded on a Shimadzu UV-Visible Spectrophotometer (UV-2550) using barium sulfate as the standard. A deuterium lamp D2 and a halogen lamp WI served as the excitation sources. The spectrometer was directly linked to a computer by communication cables. The obtained results can be analyzed by the software named UVPROBE in PC client. Photoluminescence (PL) experiments were conducted at room temperature with an excitation source of 325 nm (3.81 eV) He–Cd laser. PL spectra for the samples were detected with a grating monochromator and a standard CCD detection system.

3. Results and discussion

Fig. 2 shows powder X-ray diffraction (XRD) patterns for Cd_xZn_1−xO (x = 0, 0.05, 0.15, 0.40 and 0.80) nanoparticles. For the pure sample, XRD peaks are observed at 2θ = 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8° and 68.0°, which correspond to (100), (002), (101), (102), (110), (103) and (112) lattice reflection planes of a typical wurtzite structure [JCPDS Card 36-1451]. The intense sharp XRD peaks suggest that the undoped sample has a good crystalline nature. The wurtzite structure is kept up to x = 0.40. It is noted that the XRD bands become broad and blurred with an increase in Cd-doping, indicating a deterioration in crystal quality. When the x-value reaches 0.80, some new peaks are observed at 2θ = 23.5°, 30.4°, 44.0° and 49.9°, as shown by the star marks in Fig. 2, which suggests the appearance of a new phase or phase segregation in the 80% Cd-doped sample. In order to investigate the Cd-doping effect on the lattice parameters of the wurtzite structure for Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40), the (100), (002) and (101) diffraction peaks are fitted with the Gaussian functions. The solid curves in Fig. 3 are the best-fitted results. For the (100) peak, the position shows a slight shift towards a low diffraction angle from 2θ = 31.7° at x = 0 to 31.5° at x = 0.40, and (002) peak shifts from 2θ = 34.4° at x = 0 to 34.0° at x = 0.40, while that for the (110) peak are from 36.2° at x = 0 to 35.7° at x = 0.40. The shifts for the diffraction peaks towards a low diffraction angle indicate lattice expansion in Cd_xZn_1−xO nanoparticles with Cd-doping.

Fig. 2 Powder X-ray diffraction (XRD) patterns for Cd_xZn_1−xO (x = 0, 0.05, 0.15, 0.40 and 0.80) nanoparticles.

Fig. 3 The fitting curves for (100), (002) and (101) diffraction peaks for Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles by using Gaussian functions. The solid curves are the best-fitted results.

We further calculated the lattice parameters for Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles. The x dependence of the lattice parameters a and c for Cd_xZn_1−xO nanoparticles are shown in Fig. 4 (a) and (b), respectively. With increasing x, the a-value shows a continuous increase from 3.2453 Å at x = 0 to 3.2623 Å at x = 0.40, while the c-value increases gradually from 5.2036 Å at x = 0 to 5.2173 Å at x = 0.40. The lattice expansion is reasonable because the ionic radius (0.97 Å) of the Cd²⁺ ion is larger than that (0.74 Å) of the Zn²⁺ ion. We further calculated the internal parameter u, which shows the deformation ratio for a regular tetrahedron²⁵

u = (a/c)²/3 + 1/4. (1)

Fig. 4 Lattice parameters (a) a and (b) c for Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles. (c) Relation between structure parameters u and (a/c)².

The correlation between the internal parameter u and (a/c)² is shown in Fig. 4(c). The u value for undoped ZnO nanoparticles is 0.3796, which is larger than that (0.3750) for the ZnO single crystal. With an increase in Cd-doping, the u value gradually increases and reaches 0.3803 for x = 0.40. The increase in the u value indicates an enhancement of lattice distortion with Cd-doping, which can be ascribed to the size difference between Cd²⁺ (0.97 Å) and Zn²⁺ ions (0.74 Å).

In order to confirm the XRD results, we further investigated the Raman spectra for Cd_xZn_1−xO nanoparticles. ZnO has a wurtzite structure and belongs to the space groupC_6v with two formula units in the primitive cell.²⁶ The group theoretical calculation predicts six optical modes: A₁ + 2B₂ + E₁ + 2E₂.²⁷ Among them, A₁ and E₁ modes are active in both Raman and IR while E₂ modes are only Raman active, and B₂ modes are silent modes. Fig. 5 illustrates the Raman spectra of Cd_xZn_1−xO (x = 0, 0.05, 0.15, 0.40 and 0.80) nanoparticles. For x = 0, a broad peak is found at 438 cm⁻¹, which corresponds to the E₂-high mode for ZnO with wurtzite structure.²⁸ With increasing x, the E₂ mode shows a significant softening from 438 cm⁻¹ at x = 0 to 424 cm⁻¹ at x = 0.40, suggesting expansion of the Zn–O bond in Cd_xZn_1−xO nanoparticles with Cd-doping.³ In order to compare these results with the XRD results, we also measured the Raman spectra for 80% Cd-doped ZnO nanoparticles and CdO nanoparticles, as shown in Fig. 5 as well. A new peak appears at 260 cm⁻¹ (shown by an upward arrow in Fig. 5) for the 80% Cd-doped sample, which is close to the TO mode of CdO with a rock salt structure. Thus, a new phase or phase segregation occurs in the 80% Cd-doped sample. Therefore, the Raman investigation is consistent with the XRD results.

Fig. 5 Raman spectra of Cd_xZn_1−xO (x = 0, 0.05, 0.15, 0.40 and 0.80) and CdO nanoparticles. The Raman mode at 438 cm⁻¹ for x = 0 corresponds to the E₂-high mode for ZnO with a wurtzite structure.

We then continued to investigate the optical absorption for Cd_xZn_1−xO nanoparticles. Fig. 6 shows the absorption spectra of Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles. The energy gap E_g of Cd_xZn_1−xO nanoparticles can be obtained with the equation (αhν)² = B(hν − E_g), where α is the absorption coefficient, hν is the photon energy and B is a function of index of refraction and hole/electron effective masses.²⁹ The broken curves show the best fitted results. The extrapolation of the linear portion of the plot onto the energy axis gives the band gap energy E_g of Cd_xZn_1−xO nanoparticles. The obtained E_g-value decreases as 3.21, 3.18, 3.09 and 2.92 eV for x = 0, 0.05, 0.15 and 0.40, respectively. In doped semiconductors, there is a possibility of widening of the optical band gap due to the Burstein Moss (BM) effect. An enhancement of the band gap was observed in Co-doped ZnO.³⁰ In the present case, the decrease in E_g-value with Cd-doping is not in accordance with the BM effect, which can be attributed to band gap renormalization because the E_g-value (∼2.3 eV) for CdO is smaller than that (∼3.2 eV) for ZnO. A similar effect has been reported for Cd-doped ZnO films.¹⁷

Fig. 6 Absorption spectra of Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles. The gap energy E_g of Cd_xZn_1−xO nanoparticles can be obtained with the equation (αhν)² = B(hν − E_g).

The Urbach band tails were also investigated for Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles, as shown in Fig. 7. Several factors such as the carrier-impurity interaction, carrier-phonon interaction and structure disorder are responsible for the Urbach band tail in semiconductors.³¹ We evaluated the Urbach band tails by fitting the absorption coefficient (α) as a function of photon energy (E) in the vicinity of the fundamental absorption edge³²

where α₀ is the pre-exponential factor and ΔE is the Urbach band tail. The ΔE values for Cd_xZn_1−xO nanoparticles were found to be 0.043 eV at x = 0, 0.072 eV at x = 0.05, 0.125 eV at x = 0.15 and 0.130 eV at x = 0.40. A similar band tail structure was found for N-doped ZnO layers,³³ where the Urbach band tail is related to N-doping induced defects. Therefore, we consider that the increase in ΔE with Cd-doping is due to an increase in the defects in the samples with Cd incorporation, which results in an extension of the parabolic density of states into the band edge.

Fig. 7 Urbach band tails evaluated by fitting the absorption coefficient α as a function of photon energy in Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles.

We further investigated the photoluminescence (PL) spectra of Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles. Fig. 8 displays the PL spectra of the Cd_xZn_1−xO nanoparticles with different Cd content measured at room temperature in the range of 340–750 nm. For the undoped ZnO nanoparticles, two obvious PL bands are observed around 383 nm and 590 nm, respectively. The one around 383 nm is a strong emission band, which originates from a near band-edge (NBE) transition of the wide band gap of ZnO,³⁴ while the other broad band around 590 nm is currently attributed to native defects such as singly ionized oxygen vacancies.³⁵ Compared to the NBE emission band, the intensity of the defect related band is very weak, which indicates that the undoped ZnO nanoparticles have a few defects. With an increase in the x-value up to 0.15, the intensity of the NBE band gradually decreases while the position shows a slight red-shift due to the Cd-doping induced narrowing of the band gap. Simultaneously, the intensity of the broad band increases significantly while the position shows a negligible change. The quenching of the NBE emission and the enhancement of the defect related band indicate that the Cd-doping induced an increase in defects in the samples. When the x-value further increases to 0.40, the intensities for both the PL bands shown suddenly decrease. We have ascribed the suppression of the PL to the non-radiative energy transfers between defect states to lattice. The non-radiative recombination centers should be significantly enhanced in heavy Cd doped ZnO nanoparticles, leading to suppressed PL emission. Similar non-radiative energy transfers have been reported for Eu-doped ZnO nanowires.³⁶ In addition, a weak blue band seems to appear around 450 nm in addition to the red-shifted NBE band when x = 0.40, as shown by a downward arrow in Fig. 8. The new blue PL band may be caused by the defect levels created between the conduction and valence bands such as the shallow donor levels arising from Zn defects.^37,38

Fig. 8 Photoluminescence (PL) spectra of Cd_xZn_1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles measured at 300 K in the range of 340–750 nm.

Recently, we have successfully synthesized heavy Mg-doped ZnO nanoparticles by using the sonochemical method. It is found that Mg-doping can significantly increase the gap energy E_g of ZnO nanoparticles, i.e., ∼3.74 eV for Mg_0.4Zn_0.6O nanoparticles (not shown). Therefore, the energy difference of 0.82 eV between the highest and lowest E_g in doped ZnO nanoparticles is substantially higher than that typically achieved in ZnO nanoparticles by size tuning. We believe that successful doping in the present case is due to the extreme conditions such as high temperature and high pressure under ultrasound irradiation. Thus, the controlling of the ultrasound power and frequency as well as the irradiation time may be important for the synthesis of doped samples with good crystalline quality.

4. Conclusion

We have successfully synthesized heavy Cd-doped ZnO nanoparticles by a sonochemical reaction. X-ray diffraction (XRD) and Raman scattering spectroscopy indicate that the wurtzite structure can be kept in Cd_xZn_1−xO nanoparticles up to x = 0.40. UV-Visible and photoluminescence spectroscopy indicate that both the band gap and the NBE emission band for Cd_xZn_1−xO nanoparticles show red-shift with an increase in Cd-doping. The calculation results on the Urbach band tails reveal an increase in the defects in Cd_xZn_1−xO nanoparticles with Cd incorporation. We further found that the Cd-doping can modify the photoluminescence bands of Cd_xZn_1−xO nanoparticles. Therefore, the sonochemical method is a useful tool for the synthesis of doped nanoparticles.

Acknowledgements

This work was supported by NSFC under Grant Nos. 11074124, 11104139 and 10874087, and the National Basic Research Program of China under grant No. 2012CB921504.

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