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Home Search Collections Journals About Contact us My IOPscience Photoluminescence properties of Co-doped ZnO nanorods synthesized by hydrothermal method This article has been downloaded from IOPscience Please scroll down to see the full text article 2009 J Phys D: Appl Phys 42 065412 (http://iopscience.iop.org/0022-3727/42/6/065412) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 129.174.55.245 The article was downloaded on 27/07/2012 at 19:06 Please note that terms and conditions apply IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J Phys D: Appl Phys 42 (2009) 065412 (7pp) doi:10.1088/0022-3727/42/6/065412 Photoluminescence properties of Co-doped ZnO nanorods synthesized by hydrothermal method Trinh Thi Loan, Nguyen Ngoc Long1 and Le Hong Ha Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Vietnam E-mail: longnn@vnu.edu.vn Received November 2008, in final form January 2009 Published 26 February 2009 Online at stacks.iop.org/JPhysD/42/065412 Abstract Cobalt doped zinc oxide nanorods Zn1−x Cox O (x = 0.01, 0.10) have been synthesized by a hydrothermal process with Zn(NO3 )2 , Co(NO3 )2 , NH4 OH, CO(NH2 )2 and C2 H5 OH at 150 ◦ C for h X-ray diffraction and scanning electron microscopy were used to characterize the crystalline structure, size and morphology of the samples The photoluminescence (PL) and the PL excitation spectra of the nanorods were measured in the range of temperature from 15 K to room temperature The PL spectra at low temperatures exhibit a group of ultraviolet narrow lines in the near-band-edge region of 3.0–3.4 eV and a very broad band peaked at 3.20 eV The origin of the near-band-edge PL is interpreted as an emission from free excitons, neutral donor-bound excitons, radiative transitions from a donor to the valence band and donor–acceptor pairs In particular, a group of emission lines in the red region of 1.8–1.9 eV have been revealed These emission lines were assigned to the radiative transitions within the tetrahedral Co2+ ions in the ZnO host crystal (Some figures in this article are in colour only in the electronic version) chemical vapour deposition [6], metal-organic chemical vapour deposition [7, 8] and hydrothermal methods [9, 10] Recently, 3d transition-metal elements (Co, Ni, Mn and Cu) have been alloyed with ZnO and their properties have been investigated [11–15] Due to their spin-transport properties, transition-metal doped ZnO is a diluted magnetic semiconductor and has attracted much attention because of the possibility of its application in spintronic devices It is known that knowledge of the electronic structure of the dopants may enhance the understanding of the mechanisms inducing high-temperature ferromagnetism; therefore, optical properties such as absorption and photoluminescence (PL) have been of interest to many researchers However, in the existing literature, most of the papers deal with the absorption properties of Co-doped ZnO; meanwhile, there are few papers dealing with the PL properties of this material [11, 16] In this study, we have synthesized cobalt doped zinc oxide nanorods Zn1−x Cox O (x = 0.01, 0.10) by a hydrothermal process The x-ray diffraction (XRD) and the scanning electron microscopy (SEM) techniques were used Introduction One-dimensional (1D) nanostructures including nanowires, nanorods, nanobelts and nanotubes have attracted a great deal of interest not only because of their basic scientific richness but also for their potential application in electronic, optoelectronic, electrochemical and electromechanical nanodevices [1] In the past few years, considerable effort has been devoted to developing various 1D semiconductor nanostructures Many nanostructures based on various metal oxides, III–V and II–VI compound semiconductors were synthesized Zinc oxide (ZnO) is recognized as a promising material for photonics and optoelectronics because of its wide band gap Eg of 3.37 eV and a large exciton binding energy of 60 meV Furthermore, ZnO is bio-safe and biocompatible and may be used for biomedical applications without coating [2, 3] Many methods have been used to synthesize nanostructured ZnO: vapour-phase transport [4, 5], Author to whom any correspondence should be addressed 0022-3727/09/065412+07$30.00 © 2009 IOP Publishing Ltd Printed in the UK J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al to characterize the crystalline structure, the size and the morphology of the samples Our present studies were focused on the PL and the PLE spectra of the Zn1−x Cox O nanorods In the PL spectra at low temperatures apart from a group of lines related to the near-band-edge emission, in particular, a group of emission lines related to radiative transitions within the tetrahedral Co2+ ions in the ZnO host crystal were observed and analysed Experimental 2.1 Materials All the chemicals used in our experiment, including zinc nitrate Zn(NO3 )2 ·6H2 O, CO(NH2 )2 , cobalt nitrate Co(NO3 )2 , sodium hydroxide NaOH and ethanol C2 H5 OH, are of analytic grade without further purification 2θ (º) Figure Typical XRD patterns for the samples of Zn1−x Cox O with x = 0.01, 0.10 2.2 Synthesis of Zn1−x Cox O nanorods Results and discussion The samples of Zn1−x Cox O (x = 0.01, 0.10) have been synthesized under hydrothermal conditions from Zn(NO3 )2 , Co(NO3 )2 with molar proportions (1 − x) : x The synthesis process of the samples is as follows: 3.00 g Zn(NO3 )2 · 6H2 O and 0.65 g CO(NH2 )2 were completely dissolved in 175 ml of double distilled water, forming a transparent solution Then, to this solution, 5.1 ml of 0.02 M Co(NO3 )2 solution for the sample with x = 0.01 or 5.1 ml of 0.2 M Co(NO3 )2 solution for the sample with x = 0.10 was added, followed by steady stirring for 30 An appropriate quantity of 10 M solution of NaOH and then an appropriate quantity of pure alcohol C2 H5 OH were added into the last solution, followed by continuous stirring for another 30 The above-mentioned solution mixture was placed in a sealed Teflon-lined autoclave, which was heated to 150 ◦ C and maintained at that temperature for 30 Then the temperature of the autoclave was raised to 200 ◦ C and kept constant for Finally, the temperature was reduced to 150 ◦ C and kept constant for 25 After that process, a precipitated product of navy blue colour was obtained The products with x = 0.10 are a darker blue than those with x = 0.01 3.1 Structure characterization and morphology Typical XRD patterns for the samples of Zn1−x Cox O with x = 0.01 and 0.10 are shown in figure 1, where the diffraction peaks corresponding to the (1 0), (0 2), (1 1), (1 2), (1 0), (1 3), (2 0), (1 2) and (2 1) diffraction planes can be seen All the peaks in the XRD patterns clearly indicate that the Zn1−x Cox O samples possess hexagonal wurtzite crystal structure No other diffraction peaks are detected except for the ZnO related peaks These results are in agreement with those of other authors [14, 15], who showed that no additional phase was observed in the samples with the doping level x below 0.10 A small amount of the CoO phase was detected in the samples when the doping level x reached 0.15 [15] Furthermore, from figure 1, it can be noted that the positions of all the diffraction peaks of the sample of Zn1−x Cox O with x = 0.10 are shifted towards those larger 2θ angle in comparison with that in the sample with x = 0.01 The lattice constants determined from the XRD patterns are a1 = 3.2476 Å, c1 = 5.2062 Å and a2 = 3.2247 Å, c2 = 5.1664 Å for the samples of Zn1−x Cox O with x = 0.01, 0.10, respectively The lattice constants in the ZnO bulk crystal are a = 3.2498 Å, c = 5.2066 Å [17] As compared with the lattice constants of the ZnO bulk crystal, we reveal that the lattice constants for the sample of Zn1−x Cox O with x = 0.10 are decreased remarkably (a − a2 = 0.0251 Å, b − b2 = 0.0402 Å) The reason for this is explained as follows: when ZnO crystals were doped with cobalt, the Co2+ ions substituted for Zn2+ ions [13–16] The effective ionic radius of Co2+ in the tetrahedral configuration (0.58 Å) is slightly smaller than that of Zn2+ (0.60 Å) [18]; the ionic radius of Co2+ in the 12-coordinated metal configuration (1.25 Å) is smaller than that of Zn2+ (1.39 Å) [19] Therefore, when Zn2+ ions are substituted by Co2+ ions, the lattice is shrunk so that the lattice constants are decreased Figure shows typical SEM images of the samples of Zn1−x Cox O with x = 0.01, 0.10 For the sample with 2.3 Characterization of the samples The crystal structure of the samples was analysed by using an x-ray diffractometer (SIEMENS D5005, Bruker, Germany) with Cu-Kα (λ = 0.154 056 nm) irradiation The morphology of the samples was characterized by using a scanning electron microscope (JSM 5410 LV, JEOL, Japan) Diffuse reflection spectroscopy measurements were carried out on a UV-VISNIR Cary 5G spectrophotometer Spectra were recorded at room temperature Transmission spectra of the samples were obtained from the diffuse reflectance values by using the spectrophotometer software The PL and the PLE spectra measured in the range of temperatures from 15 up to 300 K were carried out on a spectrofluorometer (Fluorolog FL 322 Jobin Yvon Spex, USA) with a 450 W xenon lamp as an excitation source J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al Figure Room temperature optical transmittance spectra of the samples of Zn1−x Cox O: (a) x = 0.001 and (b) x = 0.01 Figure Typical SEM images of the samples of Zn1−x Cox O: (a) flower-like nanostructures in the samples with x = 0.01 and (b) nanorods in the samples with x = 0.10 x = 0.01, two kinds of flower-like Zn1−x Cox O are formed (figure 2(a)) One is large scale, which is composed of needlelike rods with average diameters of ∼800 nm and lengths of ∼4 µm, and another is small scale, which is composed of thin rods with average diameters and lengths of ∼170 nm and ∼2 µm, respectively Meanwhile, for the sample with x = 0.10, rods with various diameters in the range from 100 to 250 nm are arranged chaotically (figure 2(b)) Figure PL spectrum of the Zn1−x Cox O nanorods with x = 0.10 at a temperature of 50 K, excited with the wavelength of 325 nm of a xenon lamp The open circles represent the experimental PL spectrum, while the solid line shows the calculated spectrum Individual contributions are represented by the dashed lines 3.2 Absorbance and PL properties (UV) region (3.4–3.2 eV) and the other in the red region (1.9–1.8 eV) The group of UV lines in both the samples of Zn1−x Cox O with x = 0.01 and x = 0.10 shows the same characteristics Figure represents the PL spectrum of the Zn1−x Cox O nanorods with x = 0.10 at a temperature of 50 K, excited with the wavelength of 325 nm (3.815 eV) In figure the fitted result of the PL spectrum is shown, where the open circles represent the experimental PL spectrum, while the solid line shows the calculated result The experimental PL spectrum can be analysed into several lines, whose individual contributions are represented by the dashed lines As seen from figure 4, the UV emission can be assigned to the free excitons (denoted by XA ) at 3.376 eV, the excitons bound to neutral donor (D◦ X) at 3.367 eV, the recombination of electrons bound on a donor with free holes in the valence band (BF) at 3.314 eV, its longitudinal optical (LO) phonon Figure shows the optical transmission spectra of Zn1−x Cox O with x = 0.001 and x = 0.01 measured at room temperature Unlike the samples with x = 0.001, for the samples with x = 0.01, apart from an absorption edge (∼3.22 eV), which corresponds to the band gap Eg for the host ZnO material, a group of absorption bands at around eV are observed The absorption bands at about 1.907 eV, 2.035 eV and 2.189 eV are assigned to A2 (4 F) → E(2 G), A2 (4 F) → T1 (4 P) and A2 (4 F) → A1 (2 G) transitions, respectively, in the highspin state of the tetrahedrally coordinated Co2+ (3d7 ) ions substituting Zn2+ ions [20] The PL spectra of the Zn1−x Cox O rods were measured in the temperature range from 15 K to room temperature under various excitation wavelengths The PL spectra for the Zn1−x Cox O rods with x = 0.01, 0.10, basically exhibited the same behaviour The emission spectra at low temperatures were composed of two groups of lines, one in the ultraviolet J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al Figure The temperature dependence of the peak energies for the XA emission (solid squares), the D◦ X emission (reverse solid triangles), BF emission (solid circles) and the BF-1LO emission (solid triangles) Solid lines are calculated using Varshni’s formula The dotted line is calculated using the formula hν = Eg − ED + kB T peaked at 3.370 eV and had a full width at half-maximum of 18 meV As can be seen from figures and 6, the intensity of the D◦ X line is decreased rapidly and its position is slightly shifted to the low-energy side with increasing measuring temperature At 100 K, this line becomes much weaker than the BF line It is also evident from figures and that beginning from 50 K a shoulder with an energy value of 3.376 eV appears at the high-energy side of the D◦ X line The position of the shoulder is shifted to the low-energy side with increasing measuring temperature This shoulder is still maintained up to 250 K and is mixed with the BF line at higher temperatures This shoulder is interpreted to originate from radiative recombination of free excitons (XA ) Indeed, as the temperature is increased, the thermal activation energy is enough for the release of excitons from the neutral donor (D◦ X → D◦ + X), then radiative transitions take place via states of the free excitons Assuming that the peak position of the free exciton and the donor-bound exciton emission varies with temperature as the energy band gap, we tried to fit the observed temperature dependence to Varshni’s semiempirical formula [21]: Figure PL spectra of the samples of Zn1−x Cox O with x = 0.10 at different temperatures in the range 15–300 K, excited with the 325 nm light of a xenon lamp E(T ) = E(0) − αT , T +β where E(0), α and β are fitting parameters As can be seen in figure 7, the experimental values for the XA line and the D◦ X line fit rather well to Varshni’s curve with fitting parameters: α = −4.5 × 10−4 eV K−1 , β = −650 K and E(0) = 3.380 eV and 3.370 eV for XA and D◦ X lines, respectively It can be noted that Varshni’s formula is an empirical one The coefficients α and β are fitting parameters In some cases, β is supposed to be related to the Debye temperature However, in a number of cases (for example, diamond, 6H–SiC [21], ZnO [22, 23]) the coefficients α and β turn out to be negative, which in general have not found a physical interpretation While the D◦ X line decreases rapidly in intensity and almost disappears at 135 K, the BF line is still maintained up to room temperature This line is dominant and located Figure PL spectra of the samples of Zn1−x Cox O with x = 0.10 at 50, 75, 100 and 135 K The spectra exhibit the appearance of the shoulders due to emission of free excitons A(XA ) replica (BF-1LO) at 3.243 eV and donor–acceptor pair (DAP) emission at 3.139 eV (DAP1) and at 3.010 eV (DAP2) In order to investigate the origins of emission lines, we have measured the PL spectra of the sample of Zn1−x Cox O in the temperature range from 15 to 300 K (figures and 6) In the spectrum at 15 K, the D◦ X bound exciton line dominated, J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al at 3.243 eV at 300 K This line cannot be attributed to a LO phonon replica of the free exciton line, on the one hand, because while the line at 3.314 eV appeared at very low temperatures (15 K), the free exciton line appeared only at temperatures higher than 50 K On the other hand, the energy distance between the free exciton line and the BF line was decreased from 62 meV at 50 K to 49 meV at 200 K These energy distances are smaller than the energy of an LO phonon (72 meV) The BF line cannot be attributed to an exciton bound to a neutral acceptor either, because of the low binding energy for this complex In the case of the exciton bound to a neutral acceptor, the excitons are thermally detached from these acceptors even at low temperatures, which is contrary to our experimental results For the origin of the BF emission line, we believe that this line probably corresponds to the recombination of carriers bound on a defect with free carriers in some band In our case, the sample is an n-type semiconductor, in which the donor concentration is larger than the acceptor concentration, so it is more likely that electrons bound on donors recombine with free holes in the valence band, causing the BF line The same BF transition was observed in n-type GaP [24] and n-type GaN [25] In that case, the peak position of the BF emission should vary with the temperature more slowly than the energy band gap varies according to Varshni’s model, as may be seen in figure In addition, the experimental values of the BF peak position at various temperatures fit rather well to the formula describing the peak position of emission due to transitions from a donor to the valence band [24–26]: hν = Eg − ED + kB T , where Eg is the band-gap energy, ED , as mentioned above, is the binding energy of the donor, T is the temperature and kB is the Boltzmann constant The free-to-bound radiative transitions have been observed in ZnO both at low temperatures and at room temperature by other authors [27, 28] The BF-1LO line is assigned to a LO phonon replica of the BF line, because the energy distance between them at different temperatures is 72.4 meV on average, which is equal to the energy of an LO phonon The very broad DAP1 band centred at 3.139 eV at 50 K is considerably shifted to the low-energy side with increasing temperature (see figure 5) This emission band is interpreted as a DAP emission It is known that the DAP emission energy is described as [29] hν = Eg − EA − ED + Figure PL spectra of the rods of Zn1−x Cox O: (a) with x = 0.01 and (b) x = 0.10 at different temperatures, excited with the 565 nm light of a xenon lamp peaked at 3.272, 3.205 and 3.166 eV (figure 4) were observed at temperatures lower than 75 K The origins of these lines are not yet clear at the present time, but they, perhaps, can be related to the neutral donor-bound exciton because of their simultaneous appearance at low temperatures Currently the nature of defects related to near-band-edge emission is not clear Maybe they are some lattice defects or uncontrollable impurities On the other hand, because the surface-to-volume ratio of the nanoscale materials is larger than that of the bulk materials, the surface states become important in the emission process Thus, the above-mentioned group of UV lines in both the samples of Zn1−x Cox O with x = 0.01 and x = 0.10 is mainly related to the near-band-edge emission of the host ZnO materials, while the group of lines in the red region (1.9– 1.8 eV) is related to the emission transitions within Co2+ ions Figure illustrates the PL spectra of the rods of Zn1−x Cox O with x = 0.01, 0.10 in the temperature range from 15 K to room temperature excited with the 565 nm (2.194 eV) light of the xenon lamp From figure one can note that the fine structure of the spectra is influenced by changing the Co2+ content Indeed, the PL spectrum of the q2 , εr where Eg is the band gap, EA and ED are the acceptor and the donor binding energy, respectively, q is the electrical charge of the acceptor and the donor ions, ε is the dielectric constant and r is the distance between the donor and the acceptor With increasing temperature, carriers on DAP with a small distance r are released into the bands, which results in extinguishing the high-energy side of the DAP emission band, and the band is shifted to the low-energy side as observed in our experiment At 135 K, when the DAP1 band completely extinguishes, a DAP2 band can be observed (figure 5) Some weak lines J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al sample of the Zn1−x Cox O with x = 0.01 at 15 K shows two lines overlaying each other: 669.1 nm (1.853 eV) and 681.2 nm (1.820 eV) (figure 8(a)), while the PL spectrum of the sample of Zn1−x Cox O with x = 0.10 at 15 K shows three separate lines: 662.0 nm (1.873 eV), 667.3 nm (1.858 eV) and 681.2 nm (1.820 eV) and a shoulder at 672.4 nm (1.844 eV) (figure 8(b)) In addition, as the temperature is increased, the above-mentioned lines broaden and overlap each other, and the fine structure of the spectra disappears The line at 1.873 eV can be interpreted as a E(2 G) → A2 (4 F) transition in the Co2+ ions with 3d7 high-spin configuration under the tetrahedral (Td ) crystal field formed by neighbouring O2− ions [20] The energy distance between the lines at 1.873 and 1.820 eV is found to be 0.053 eV which is in good agreement with the energy for the E2 (high) mode (0.054 eV) in the Raman spectrum of a polycrystalline ZnO sample [15] The energy distances between the lines at 1.873 eV and 1.858 eV and the lines at 1.858 eV and 1.844 eV are found to be 0.015 eV and 0.014 eV, respectively, which are in agreement with the energy for the E2 (low) mode (0.013 eV) in the Raman spectrum of a ZnO crystal [30] The participation of these phonons in the Co2+ emission transitions clearly demonstrates that the Co2+ ions are indeed incorporated in the ZnO host crystal lattice The PLE spectra monitored at the emission lines 662.0 nm (1.873 eV), 667.3 nm (1.858 eV), 672.4 nm (1.844 eV) and 681.2 nm were measured The results showed that the PLE spectra for all these emission lines were the same This fact indicates that these emission lines were generated from the same luminescence centre Here we show only the PLE spectra of the emission line at 1.820 eV, because in the PLE spectra of the emission lines at the higher energy side we could not get the excitation peak at 1.914 eV The reason for this is that the emission photon and the excitation photon are too close to each other in the energy position The PLE spectra monitored at the emission line 681.2 nm (1.820 eV) for both the samples of Zn1−x Cox O with x = 0.01 and x = 0.10 were measured at a temperature of 15 K (figure 9) The PLE spectra exhibit fine structure As seen from figure 9, the Co2+ ion-related PL can be excited both at energies near the band edge of the ZnO host (the UV region) and at energies below the band edge (the visible region) The UV group consists of a peak at 3.341 eV for the sample with x = 0.01 and a peak at 3.357 eV for the sample with x = 0.10 (figure 9(a)) These peaks are related to the near-band-edge absorption of the ZnO host materials The visible group consists of four peaks at 1.914, 2.023, 2.195 and 2.506 eV (figure 9(b)), and the first three peaks among them are very close to the absorption peaks in the above-mentioned transmission spectrum These four peaks are attributed to the transitions from the basic state A2 (4 F) to the E(2 G), T1 (4 P), A1 (2 G) and T1 (2 P) excited states of tetrahedrally coordinated Co2+ ions, respectively Figure 10 schematically represents the energy levels split in the crystal-field of Co2+ ions [20, 31] This figure also shows the excitation transitions and the emission transitions within the tetrahedral Co2+ ions Indeed, based on the PL and the PLE spectral analysis, we reveal that visible light can immediately excite electrons from the basic state A2 (4 F) to the E(2 G), T1 (4 P), A1 (2 G) and T1 (2 P) excited states of Figure PLE spectra monitored at the emission line 681.2 nm (1.820 eV) for the samples of Zn1−x Cox O with x = 0.01 and x = 0.10 measured at a temperature of 15 K: (a) for the UV region and (b) for the visible region Figure 10 The energy levels split in the crystal field of Co2+ ions and the excitation and emission transitions within the tetrahedral Co2+ ions J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al tetrahedrally coordinated Co2+ ions The electrons relax from the higher excited states (4 T1 (4 P), A1 (2 G) and T1 (2 P)) to the 2 E( G) lowest excited state and then return to the A2 (4 F) basic state, emitting the photon of 1.873 eV In order to explain the existence of excitation peaks observed near the band edge of the ZnO host, an energy transfer mechanism is proposed as follows: when the Zn1−x Cox O nanocrystals are excited by UV light, electrons in the valence band of the ZnO host absorb the photon energy and transfer to the conduction band, generating free electrons in the conduction band and free holes in the valence band These photogenerated electrons and holes then recombine via the near-band-edge states, exhibiting band-edge emission In this process, some of the photogenerated charge carriers transfer their energy to the Co2+ ions, exciting the Co2+ ions, resulting in red luminescence thank Dr Nguyen Hoang Nam (Graduate School of Science, Osaka University) for transmission measurements References [1] Xia Y N, Yang P D, Sun Y G, Wu Y Y, Mayers B, Gates B, Yin Y D, Kim F and Yan H Q 2003 Adv Mater 15 353 [2] Yi G-C, Wang C and Park W I 2005 Semicond Sci Technol 20 S22 [3] Wang Z L 2004 J Phys.: Condens Matter 16 R829 [4] Huang M, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R and Yang P 2001 Science 292 1897 [5] Pan Z W, Dai Z R and Wang Z L 2001 Science 291 1947 [6] Najafov H, Fukada Y, Ohshio S, Iida S and Saitoh H 2003 Japan J Appl Phys 42 3490 [7] Park W I, Kim D H, Jung S W and Yi G C 2002 Appl Phys Lett 80 4232 [8] Zhang B P, Binh N T, Segawa Y, Wakatsuki K and Usami N 2003 Appl Phys Lett 83 1635 [9] Liu B and Zeng H C 2003 J Am Chem Soc 125 4430 [10] Guo M, Diao P and Cai S 2005 Appl Surf Sci 249 71 [11] Schulz H-J and Thiede M 1987 Phys Rev B 35 18 [12] Polyakov A Y, Govorkov A V, Smirnov N B, Pashkova N V, Pearton S J, Overberg M E, Abernathy C R, Norton D P, Zavada J 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spectra of the Zn1−x Cox O nanorods were composed of two groups of emission lines, one in the UV region (3.4–3.2 eV) and another in the red region (1.9–1.8 eV) The group of UV lines is mainly related to the near-band-edge emission of the host ZnO materials The free excitons (XA ), the neutral donor-bound excitons (D◦ X), the bound-to-free (BF) transitions and the DAPs were observed in the low temperature PL spectra Cobalt doping leads to a group of emission lines in the red region The origins of these lines can be interpreted as a E(G) → A2 (F) transition and its phonon replicas in the Co2+ ion with 3d7 high-spin configuration under the tetrahedral (Td ) crystal field formed by neighbouring O2− ions The Co2+ related PL can be excited at energies below the band edge The visible peaks in the PLE spectra are attributed to the transitions from the basic state A2 (4 F) to the E (2 G), T1 (4 P), A1 (2 G) and the T1 (2 P) excited states of the tetrahedrally coordinated Co2+ ion Acknowledgments The authors thank the European Commission Project Selectnano-TTC (Contract No 516922) and the Natural Science Council, Ministry of Science and Technology of Vietnam (Project 055 06), for financial assistance They also ... JOURNAL OF PHYSICS D: APPLIED PHYSICS J Phys D: Appl Phys 42 (2009) 065412 (7pp) doi:10.1088/0022-3727/42/6/065412 Photoluminescence properties of Co-doped ZnO nanorods synthesized by hydrothermal method. .. (PL) and the PL excitation spectra of the nanorods were measured in the range of temperature from 15 K to room temperature The PL spectra at low temperatures exhibit a group of ultraviolet narrow... energy of the donor, T is the temperature and kB is the Boltzmann constant The free-to-bound radiative transitions have been observed in ZnO both at low temperatures and at room temperature by other