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Radiative/Nonradiative Recombination Affected by Defects and Electron−Phone Coupling in CdWO4 Nanorods Cuiling Zhang,†,§,∥ Donglin Guo,†,∥ Weina Xu,† Chenguo Hu,*,† and Yanxue Chen*,‡ †

Department of Applied Physics, Chongqing University, Chongqing 400044, P. R. China School of Physics, Shandong University, Jinan 250100, P. R. China § Chongqing Engineering Laboratory for Detection, Control and Integrated System, Chongqing Technology and Business University, Chongqing 400067, P. R. China ‡

S Supporting Information *

ABSTRACT: Tungstates are important photoluminescence (PL) materials owing to their unique luminescence center. However, radiative or nonradiative recombination affected by defects and electron-phone coupling have not been well understood. In this paper, we have synthesized CdWO4 nanorods and studied its temperature-dependent PL spectrum from 20 to 300 K. Theoretical calculations demonstrate that Cd vacancy (VCd) and O vacancies (VO0, VO1+, and VO2+) induce extra levels in the band gap, by which the VCd, VO0, and VO1+ defects mainly contribute to the absorption in 0−4 eV region, while VO2+ causes the emission bands peaked at 490 nm in PL spectrum. Because of the broken symmetry of octahedron two possible types of lowest unoccupied molecular orbital (LUMO) appear and the transitions from the each LUMO to the highest occupied molecular orbital (HOMO) contribute to the emission at about 410 and 436 nm. The emission intensity of the peaks decreases with an increase in temperature due to the thermal quenching by nonradiative recombination. This work insights into the understanding of physical nature of emission and presents a detailed analysis of the electron transition behaviors in the wolframite-type monoclinic CdWO4, which can be used not only to explain the photoluminescence mechanism of CdWO4, but also for the structure design to obtain better emission properties in tungstates.

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INTRODUCTION Since the luminescence properties of ZnWO4 were first reported in 1948, tungstates have attracted much attention for their excellent optical properties and have been widely used as phosphors.1−4 Tungstates have two main structures: scheelite-type tetragonal structure and wolframite-type monoclinic structure, and their radiative transitions are based on tetrahedral (WO4)2− and octahedral (WO6)2− group, respectively. The photoluminescence (PL) of scheelite tungstate crystals shows much similar excitonic emission spectra, which presents a narrower central peak surrounded by two broad shoulders. Meanwhile the excitonic emission of AWO4 (A = Pb, Ca, Ba, Sr) including extended temperature dependencies of emission properties have been studied by a unique set of parameters within a simple two-excited state level model.5 Usually, the wolframite tungstates have a dominant emission centered at 490 nm, which is often thought to be an intrinsic emission attributed to the charge transitions between the O2p orbital and the empty d orbital of the central W6+ in octahedral WO6.6,7 It is well-known that both the deformation of octahedron and the unavoidable intrinsic defect result in various PL. The hollow Mn-based heteropol tungstate microsphere showed an emission spectrum from the blue © XXXX American Chemical Society

(412 nm) to green (525 nm) region which is dominated by the WO6 group under the excitation of 260 nm, larger than the band gap (semiconductor with Eg = 2.93 eV). CoWO4 shows a strong blue-green peak around 495 nm with a shoulder at 530 nm and a wide emission band within 750−990 nm at temperature ≤250 K under excitation wavelength of 325 nm,3 while Lisitsyn found an emission at the region of 400 nm excited by wavelength of 337 nm and attributed the emission to the defects in ZnWO4 nanocrystals and considered that the luminescence centers at 490 nm come from the deep level.8 Therefore, the broader emission band and the multipeak emission indicates that more emission detail about the same emission center (WO6)6− should be discussed. The wolframitetype monoclinic tungstates CdWO4 possess excellent luminescent properties and used as prototypic scintillators for rareevent detections due to its relatively large X-ray absorption coefficient and scintillation output, but its long decay time is a disadvantage for scintillators. CdWO4 is constructed of edgesharing octahedral CdO6 with WO6 forming alternating zigzag Received: April 25, 2016 Revised: May 20, 2016

A

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scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED), respectively. The bonding characteristics are determined by X-ray photoelectron spectroscopy (XPS). The diffuse reflectance spectra are measured by UV−vis−NIR Spectrophotometer (UV-3600, Shimadzu). The PL measurement is carried out by exciting the sample with a helium−cadmium laser operating at 325 nm. The emission spectra are measured by a monochromator in connection with a photomultiplier. And the temperature-dependent PL (from 20 K to 300 K) is carried out using a closed cycle cryo-refrigerator with a digital temperature controller. Calculation Details. The first-principles density functional theory (DFT) are performed using plane-wave pseudopotentials with generalized gradient approximation (GGA),14 which uses a gradient of the electron density to replace the heterogeneity of electron density, as implemented in the Vienna ab initio simulation package (VASP).15 The plane− wave basis sets an energy cutoff of 500 eV. The 5 × 5× 5 Monkhorst−Pack grid is used for the sampling of the Brillouin zone during geometrical optimization and a higher 9 × 9 × 9 Monkhorst−Pack grid for self-consistent calculations. The model of a 2 × 2 × 2 supercell with one Cd atom removed (Cd vacancy) or one O ion removed (O vacancy) is used to simulate the optical properties and, two and one electrons are added to the supercell for obtaining different charge states of VO0 and VO1+, respectively. The supercell is fully relaxed, and the convergence of force sets to 0.01 eV/Å.

chains. There are two energetically different oxygen positions in the CdWO4 lattice. Oxygen type 1 forms bonds with one W ion (with a distance of 0.1787 nm) and two Cd ions (0.2467 and 0.2205 nm) whereas oxygen type 2 forms bonds with two W ions (0.2139 and 0.1858 nm) and one Cd ion (0.224 nm).9−11 There are many defects during synthesis such as vacancies of cadmium and oxygen in the CdWO4 which distort the octahedral WO6 and the defects effect on the PL is unclear. Vedda considered the thermally stimulated luminescence emission spectrum features both the emission at 2.5 eV probably derived from the (WO6)6− group and a further band at 1.9 eV previously ascribed to a transition within a (WO6)6− group lacking of an oxygen ion.10 Thus, probing the radiative or nonradiative recombination affected by defects and electronphone coupling might be helpful to obtain the mechanism of PL. Meanwhile one emission at the maximum of 440 nm decays after 200 ns, and the other is the band peaked at 490 nm with characteristic decay time of 26 μs in pulsed PL spectra of the ZnWO4 single crystal.8 A similar phenomenon has found that fast (at 344 nm) and slow (at 496 nm) emissions occurred in ZnWO4, CdWO4, and CaWO4 crystals when excited by an electron beam. We usually attribute the fast and slow recombination emissions to the electrons released from shallow electron traps with self-trapped holes and to that from deeper electron traps. Moreover, the role of the chemical substitution on the luminescence properties under X-ray excitation of Ca(1−x)Cd(x)WO4(0 < x < 1) are investigated, and it shows that the emission (2.4−2.5 eV) is sensitive to the substitution of calcium by cadmium.12 The origin and the influence factor for decay time need to be discussed. All these results pose a challenge to the nature of the emission of tungstates and are help for improving their scintillating features. An investigation of the mechanism of PL of CdWO4 has been performed by kinetic study on peak position of thermally stimulated luminescence and by theoretical studies on its electronic band structure in comparison to that of the cadmium and oxygen related defects.10,11,13 While they did not give a satisfactory answer for the mechanism of PL including the effect of defect. For these reasons, we undertook an investigation of the radiative and nonradiative recombination affected by defects and electron-phone coupling in CdWO4 nanorods based on the absorption spectrum. First, the CdWO 4 nanorods are synthesized by the hydrothermal method. UV−visible reflection spectrum, photoluminescence (PL) spectra in the temperature of 20−300 K of CdWO4 nanorods are measured. To further understand the mechanism of these properties, the firstprinciples calculations of CdWO4 with oxygen vacancy and cadmium vacancy are performed, and then the photoluminescence mechanism affected by Cd vacancies and O vacancies is presented. This work offers a better understanding of physical nature of emission, which can be used for structure design to enhance the emission properties of tungstates.

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RESULTS AND DISCUSSION The crystal structure and chemical composition of the sample are confirmed by XRD. In Figure 1, the diffraction peaks can be

Figure 1. XRD of monoclinic CdWO4 nanorods.

readily interpreted as monoclinic phase CdWO4 (JCPDS: 140676) with lattice constants of a = 5.029 Å, b = 5.860 Å, and c= 5.071 Å. The sharp diffraction peaks demonstrate good crystallinity of the CdWO4 sample. The morphology of the CdWO4 sample is characterized by FESEM images at low (Figure 2a) and high magnification (Figure 2b), indicating that the sample possesses rod-like morphology with 80 nm in diameter and 200 nm in length. TEM image in Figure 2(c) further reveals a rod-like morphology with a length of 200 nm. The HRTEM image in Figure 2(d) suggests that the apparent boundary and the continuous lattice fringe indicate the crystalline nature. In Figure 2(d), the lattice spacing of 0.587 and 0.503 nm agrees with the (010) and (100) planes of CdWO4.

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EXPERIMENTAL SECTION Synthesis of CdWO4 Nanorods. CdWO4 nanorods are prepared by a simple hydrothermal reaction. Two mmol Cd(NO3)2 is dissolved into 40 mL of deionized water and then mixed with 2 mmol Na2WO4 to get a white solution. The white solution is then transferred to a 50 mL Teflon-lined stainless steel autoclave maintained at 473 K for 24 h. Characterization. The crystalline structure and morphology are investigated by X-ray diffraction (XRD), field-emission B

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used as a fingerprint to identify the presence of W6+, while the fitting peaks centered at 34.6 and 36.6 eV are related to the W 4f7/2 and W 4f5/2 core level, indicating W5+.18−20 As is shown in Figure 3(d), the peaks with binding energy of 530.2 and 531.1 eV are ascribed to O2− ions and oxygen deficit of the sample, respectively.21−23 The quantification of XPS peaks implies that the sample contains oxygen vacancy in Figure 3(d), which might act as donors for trapping charge carriers. In order to discuss the existence of W5+ in the prepared samples, we conduct XPS analysis and have calculated the atomic ratio of Cd/W/O from the XPS intensity by evaluating the peak area under the Cd 3d, W 4f and O 1s curves with using sensitivity factors. We find that the defects are oxygen vacancies (VO) with content of 2.9% and cadmium vacancy (VCd) with content of 0.7%, respectively. The oxygen vacancy is a hole-type defect that tends to attract electrons under O-rich conditions.24 The following equation presents W5+ forming process: 2+ 2[WO6 ]6 − → 2W 6 + + VO + 11O2 − + 0.5O2 ↑

Figure 2. SEM (a), FESEM (b), TEM (c), and HRTEM (d) of monoclinic CdWO4 nanorods.

0 → 2W 5 + + V O + 11O2 − + 0.5O2 ↑

(1)

2+ [WO6 ]6 − → W 6 + + VO + 5O2 − + 0.5O2 ↑

XPS is further performed to investigate the chemical composition and purity of the sample. In Figure 3, all the curves fitting processes use a mixed Gaussian−Lorentzian simulation method with the same full width at half-maximum. Figure 3(a) displays a typical XPS survey spectrum of the sample. No peaks of other elements except C, O, W, and Cd are observed in the spectrum, implying high purity. In Figure 3(b), two distinct peaks at 404.6 and 411.4 eV are assigned to Cd 3d5/2 and Cd 3d3/2 core level, which can be used as a fingerprint to identify the presence of Cd2+.16,17 Figure 3(c) shows a W 4f spectrum of the sample, which can be divided into four peaks by Gaussian fitting. The peaks at 35.5 and 37.6 eV are assigned to W 4f7/2 and W 4f5/2 core levels, which can be

→ W 5 + + V1O+ + 5O2 − + 0.5O2 ↑

(2)

In the process, the orbit of electrons captured by W5+ is a hydrogen-like atom that is easier to transfer. VCd is locally electrical negative divalent in the crystal and should trap positive center to maintain the local electrical neutrality. The existent form of the hole in CWO: VCd should be oxygen molecular ion (O3− 2 ). It can be written as follows: 0 0 [CdO6 ]10 − − V Cd → O32 − − V Cd − O32 − + 2O2 −

Therefore, a related color center − the VF center would form in the crystal.25 [O3− 2

V0Cd

−

O3− 2 ]

(3)

named

Figure 3. Typical XPS analysis of monoclinic CdWO4 nanorods. Survey spectrum (a), Cd 3d core level spectrum (b), W 4f core level spectrum (c), and O 2p core level spectrum (d). C

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construction of the surface, and the excitation wavelength all influence the emission peak. However, PL intensity depends strongly on the morphology of the nanocrystals rather than the crystallization.28 Therefore, our different emission bands imply the variation of band structure possibly caused by VCd, VO, or surface states, or the different electron transition behaviors under different excitation energy. Figure S1(a) in the Supporting Information (SI) shows the plots of emission intensity versus temperature. It is found that the emission intensity decreases with an increase in temperature for the blue peak at 490 nm (2.53 eV), the purple peak at 438 nm (2.83 eV), and at about 410 nm (3.02 eV). The strong temperature dependence of emission peaks could be explained by electron−phonon coupling. With increase in temperature, the intensity of the emission peak becomes weak due to the thermal quenching by nonradiative recombination. The emission intensity of CdWO4 versus temperature complies with the classical exponential form:

The reflection spectrum and temperature-dependent photoluminescence (PL) of the CdWO4 nanorods are shown in Figures 4 and 5. According to the Kubelka−Munk

Figure 4. UV−visible reflection spectrum (black-solid line) and Kubelka−Munk function (blue-dash line) of monoclinic CdWO4 nanorods.

I = I0/(1 + Ae−ΔE / kT )

function,17−22 the optical absorption band gap can be obtained from the reflection spectrum, and the related results are shown in Figure 4. From Figure 4, it is seen that the optical absorption band gap is 3.79 eV, and a weak absorption edge is found below 3.79 eV. From temperature-dependent photoluminescence (PL) in Figure 5(a), we can see a blue peak at 490 nm (2.53 eV) and a purple peak at 438 nm (2.83 eV), which can be resolved into three bands: a higher band at about 410 nm which is about 403 nm at 300 K, 417 nm at 20 K, and 436 and 490 nm in Figure 5b. Thus, the higher band shifts to blue with temperature increase, but the other two emissions do not shift. The emission spectra of the CdWO4 nanocrystalline show complex fine structures, which do not completely agree with the reported experimental data. In most research, CdWO4 shows an “intrinsic luminescence” centered at about 490 nm, which consists of a dominant emission band at 2.51 eV (495 nm) and two weak emission bands at 2.80 eV (444 nm) and 2.28 eV (545 nm). Yan et al. found that under the excitation of 350 nm, the CdWO4 nanorods prepared via a PEG-1000 polymer-assisted enhanced microwave synthesis route at 473 K for 1 min has a very strong photoluminescence peak at 475 nm with a decay time of 16.03 μs,26 while the CdWO4 nanorods prepared by the hydrothermal method present an emission peak at 460 nm.27 Ren et al. reported that both nanorods and microspheres of synthesized CdWO4 have the emission peaks at about 468 nm.28 It is known that the morphology,

(4)

where I0, k, and ΔE is the intensity at 0 K, Boltzmann constant, and activation energy in thermal quenching, respectively, and A is a constant. The extra vibration energy ΔE corresponding to nonradiation transition is 8.8, 9.0, and 11.0 meV for the blue peak at 490 nm, the purple peak at 438 nm, and at about 410 nm, respectively, as shown in Figure S1(b−d). Thus, the PL emissions probably originate from the excitons. In conclusion, there are three noticeable effects of temperature on the PL of CdWO4: (1) there are three emission centers at about 410, 438, and 490 nm; (2) the emission intensity decreases with temperature; and (3) with the increase in temperature the peak position at 490 and 436 nm are invariant and the emission at about 410 nm is blue-shift. In order to know the role of VO and VCd in absorption spectrum and PL spectra for CdWO4, a 2 × 2 × 2 supercell is constructed in Figure 6. On the basis of the results of XPS, we mainly take into account two defects, VCd and VO (Figure 6). However, as the O atom has two different positions, we consider VO1 and VO2 in the supercell, separately. Tran-Blaha modified Becke-Johnson GGA functional (mBJ-GGA) is used to compensate for the weakness of density functional theory (DFT),29 which usually underestimates band gap width. On the 2+ basis of V0O, V1+ O , VO , and VCd, we calculate the optical properties of these defects, as shown in Figure 7. From Figure 7, it is seen that the absorption of VO1 and VO2 are similar. V0O

Figure 5. Temperature-variable PL (a) and Gaussian fitting of emission (b) at 20 and 300 K of monoclinic CdWO4 nanorods. D

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Figure 6. Calculated structure of monoclinic CdWO4 with possible defect.

and V1+ O in VO1 and VO2 generate absorption at about 1.5−2.8 eV, 2.2−3.7 eV, and V2+ O in VO1 and VO2 only has a strong absorption higher than 3.6 eV. The absorption spectrum of the crystal with VCd is broader, stronger and more asymmetric than that of VO at 0−4 eV region, which can be divided into three peaks: 1.1 eV, 1.5 and 2.0 eV, displaying more effect on optical properties than VO. Roughly, the results are similar to that of the influence of VZn and VO on the absorption spectrum at 0−4 eV region.30 The experimental absorption of CdWO4 showed in Figure S2 have a weak absorption from 800 to 400 nm which should come from the absorption of VCd and VO. To get a better understanding of the defect, the total density of states (TDOS) with and without defect are studied (Figure S3). Both VO1 and VO2 produce similar impurity levels in the band gap and have similar absorption spectra, which is probably due to the fact that the distortion caused by oxygen vacancy is larger than that by different oxygen position. Thus, discuss of VO on optical properties is no need to distinguish the oxygen position. The main effects of oxygen vacancies V0O and V1+ O in CdWO4 are the introduction of W character states into the band gap with the change of the W (nearest O vacancy) valence into W5+ according to the eq 1 and 2. The energies of these defect states are very sensitive to their occupancy (Figure 8). An isolated O vacancy (V0O) with capturing two electrons forming F color center causes a defect state of W5d Cd5s below the conduction band. Since the V0O state is filled with 2 electrons and its energy approximately appears near the CB, it can be an effective electron donor. V1+ O with capturing one electron, namely the F+ color center, results in the additional states of W 6s5d and Cd5s which widens the impurity band. Meanwhile, as the V1+ O energy is farther away from the conduction band minimum than V0O energy, it can be an electron donor. Furthermore, V2+ O is a deep electron acceptor and also induces the defect states of W5d O2p Cd5s in the band gap, and the valence of W5d ion maintains W6+ for capturing

Figure 8. Partial density of states of monoclinic CdWO4 with possible defect: (a) VO0 defect; (b) VO1+ defect; (c) VO2+ defect; and (d) VCd defect.

nothing. When excited, the electrons transfer from VC to F and F+, causes the absorption at about 1.5−2.8 eV and 2.2−3.7 eV. To maintain the local electrical neutrality, VCd shares one hole with the nearest two oxygen ions, which induces O2pCd5s in band gap, as is shown in Figure 8d. The double-hole color 0 3− center [O3− 2 − VCd − O2 ] is denoted as VF. Oxygen ions with trapping the holes in different patterns cause different absorption bands. For example, the strong absorption at 330, 370, 420 nm and the wide band region in 500−700 nm is 31,32 0 3− attributed to [O3− The extra 2 − VPb − O2 ] in PbWO4. electronic state of CdWO4 caused by O 2p forming the VF color center within the band gap would result in the broad 0 3− absorption peak. Meanwhile [O3− 2 − VCd − O2 ] in CdWO4 also destroys the symmetry of octahedron of CdO6 group, and then destroys the molecular orbital. Therefore, the nonbonding can be obtained by Cd 5s and O 2p near Fermi surface, as is shown in Figure 8d. These characters are beneficial to the mobility of the photogenerated carriers in Cd 5s states. Under excitation, electrons in VB transfer to the impure level and then to CB by strong absorption and some of them may be captured by WO5 or WO6 group and then go through radiative recombination or nonradiative recombination. The PDOS of CdWO4 in Figure 8 presents that W5d6s, O2p, and Cd5s occupy the bottom of CB. Compared CdWO4:VO with the perfect CdWO4 (Figure S4 in the SI) and CdWO4:VCd, the W5d states in CdWO4:VO have more obvious split and broad, which means that VO especially V2+ O has more effect on PL of CdWO4. According to calculation that the perfect CdWO4 almost does not have optical absorption in 0−4

Figure 7. Optical property of monoclinic CdWO4 calculated by DFT with possible defect: (a) VO at Oxygen type 1, (b) VO at Oxygen type 2, and (c) VCd. E

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The Journal of Physical Chemistry C eV range, electron in CdWO4:VO might be excited from VB to CB or from the defect level to CB under radiation with wavelength 325 nm (3.82 eV, larger than the band gap 3.79 eV). W5d6sCd5s in CB would capture more excited electrons. The itinerant electrons in Cd 5s states are likely to be captured by localized electronic states W5d due to their similar level. Thus, a pair of holes (O−) and the trapped electron (W5+) emerge, and they may cause to the radiative recombination during the transition from metal to ligand. Undoubtedly, the existence of an oxygen and cadmium vacancies may lead to a change in the interatomic distance via correlated deformation and rotation of neighboring ions.24 The shortened TiTi and MnMn distances by nearest oxygen-vacancy are observed in the octahedron SrTiO3 and LaMnO3 because of the electrostatic and mechanical potential, respectively.24,33 Therefore, especially VO and VCd, destroys the symmetry of octahedron and relaxes the octahedron, which localizes the W6s electrons and splits the W5d degenerate states of t2g and eg into additional levels of W 5d state, O 2p state and Cd 5s state in band gap.8 For the octahedron WO6 in CdWO4 near defect, the OO distance slightly increases along two axes and decrease along one axe as compared with the ideal system. The perfect octahedron [WO6]6− near oxygen or cadmium vacancies is compressed and two possible types of the LUMO appear, which are antibonding molecular orbitals of W5d (dz2 and dx2−y2) with ligand O2p character (px, py, pz). The transition from the each LUMO to the HOMO [an nobonding molecular orbital with W5d character (dxy, dyz, dzx)] results in the emission in the blue-green region at about 410 and 436 nm. That is, the intrinsic luminescence is the result of recombination of these free self-trapped excitons. When temperature increases, the thermal activation of free self-trapped excitons leads to the thermal quenching and the decrease in the emission at about 410 and 436 nm. Furthermore, carriers are rapidly captured by the emission center and then produce an emission at about 410 nm in PL, which makes the emission drop more slowly than other two emission bands (Figure 5b). The blue-shift of the emission at about 400 nm may attribute to the following reasons: (1) ligands may change their HOMO and LUMO energy levels after their coordination to metal centers and (2) the broadening of emission at about 410 nm. Therefore, with temperature increasing, thermalization of the free self-trapped excitons forms the continuous electron−hole band and the excitons absorb phonon energy to enter into the exciton band. The exciton band makes the emission band broaden and the enhanced electron−phonon interaction makes the emission blue shift. The light absorption and luminous mechanism can be described by molecular orbital theory showed in Figure 9. The impure levels are sensitive to the electrons occupancy 2+ and the W5dO2pCd5s states arising from V0O, V2+ O , and VO , respectively, have different positions in band gap shown in Figure 9. Under excitation, some electrons in VB band directly transfer to CB and some electrons in the impurity levels (V0O 1+ and V1+ O , especially VO ) also transfer to CB. And then the impurity levels of V0O and V1+ O lose their most electrons and turn 2+ level. Thus, V makes the majority contribution to into V2+ O O W5d in band gap under excitation. It is well-known that as an excellent photocatalytic material CdWO4 has a large photoelectric current because of the itinerant electron in Cd6s.34 It means that under excitation the excited electrons in CdWO4 are captured by W5dCd6s first and then some electrons captured by Cd6s may be captured by W6+ in CB or by the impurity levels W5d in V2+ O . The radiative recombination from

Figure 9. Possible mechanism of absorption (left) and PL spectrum (right) with the split orbits and defect level under asymmetric crystalline field. The emissions at 410, 436, and 490 nm are indicated by the arrows of a, b, and c, respectively, on the right.

electrons and holes captured by impure level V2+ O is responsible for the strong emission at 490 nm. The intensity of the broad emission band at 490 nm at low temperature is larger than the other two bands and drops faster at high temperature than the other two emissions because of the stronger interaction between impure center V2+ O and phonon (shown in Figure 5b), which results in the larger recombination cross section and higher quantum yield. The recombination cross-section affects carrier transport in the model of the PL enhancement which is confirmed as a reduction of the surface recombination velocity.35 With increasing temperature, the interaction between the impure center V2+ O and the phonon becomes stronger, and the effect of temperature is more explicit. Therefore, thermal quenching with the peak at 490 nm is more remarkable than the other two emission bands.

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CONCLUSIONS

In summary, the radiative/nonradiative recombination caused by defects and electron-phone coupling in monoclinic CdWO4 nanorods is investigated. The temperature-dependent photoluminescence from 20 to 300 K is studied, showing two peaks at 436 nm (2.83 eV) and 490 nm (2.53 eV), which can be resolved to three emission peaks: 410, 436, and 490 nm. The emission intensity decreases with an increase in temperature, due to the thermal quenching by nonradiative recombinations. Strong temperature dependence of emission peaks could be explained by electron−phonon coupling. The theoretical 2+ calculations demonstrate that VCd, V0O, V1+ O , and VO induce extra level in band gap. VCd and V0O are dominant in the absorption at 0−4 eV region, and VCd contributes more than V0O. The deep level of V2+ O causes the emission band peaked at 490 nm in PL spectrum. Because the broken symmetries of octahedron results from the different WO bond and defect of VCd and V0O, two possible types of the LUMO appear and the transition from each LUMO to the HOMO results in the emission at about 410 and 436 nm. This work insights into the understanding of physical nature of emission and presents a detailed analysis of electron transition behaviors in the wolframite-type monoclinic CdWO4. F

DOI: 10.1021/acs.jpcc.6b04145 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

These materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04145. (Figure S1) The emission peaks vs temperature (a) and fit of emission peaks vs temperature (b−d) of monoclinic CdWO4 nanodods; (Figure S2) the absorption spectrum of monoclinic CdWO4 nanorods; (Figure S3) the total DOS of pure and defect CdWO4 with possible defects at different O positions; and (Figure S4) the total and partial density of states of pure monoclinic CdWO4 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 23 65678362. Fax: +86 23 65678362. E-mail: hucg@ cqu.edu.cn (C.H.). *Tel.: +86 23 65678362. Fax: +86 23 65678362. E-mail: cyx@ sdu.edu.cn (Y.C.). Author Contributions ∥

C.Z. and D.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFCQ (cstc2015jcyjA20020, cstc2012jjA50024), The Science and Technology Research Project of Chongqing Municipal Education Commission of China (KJ1400607, KJ130603), and the National High Technology Research and Development Program (863 program) of China (2015AA034801).

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DOI: 10.1021/acs.jpcc.6b04145 J. Phys. Chem. C XXXX, XXX, XXX−XXX