Journal of Luminescence 174 (2016) 6–10 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin Full Length Article Effects of carbon on optical properties of ZnO powder Nguyen Tu, K.T Nguyen n, D.Q Trung, N.T Tuan, V Nam Do, P.T Huy n Nano Optoelectronic Laboratory (La Nopel), Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hanoi 10000, Vietnam art ic l e i nf o a b s t r a c t Article history: Received 18 March 2015 Received in revised form 21 January 2016 Accepted 25 January 2016 Available online 29 January 2016 We report on C-doped ZnO, with different weight percentages of dopant, prepared by a high-energy ball milling method The annealing conditions with temperature of 800 °C and in argon environment appear to be the optimal conditions for producing good quality crystals as well as pure UV emission XRD and FTIR analysis indicate the substitution of C for Zn In addition, Raman spectroscopy suggests a disordered graphitic layer covering the crystals Photoluminescence investigation reveals the continuous quenching of visible region upon increasing C concentration and the intensity ratio between defect-related and UV emission can be as negligible as 0.02 The passivation of surface defects and the creation of a nonradiative recombination pathway by carbon integration are proposed as possible origins of the suppression & 2016 Elsevier B.V All rights reserved Keywords: Pure UV emitter ZnO crystals Carbon doping High-energy ball milling Photoluminescence quenching Introduction Emission properties of ZnO have been at the center of ZnO research, besides synthesis methods and growth kinetics [1–3], due to its potential applications in optoelectronic devices, particularly ultraviolet-emitting devices Emission properties of ZnO have been at the center of ZnO research due to its potential applications in optoelectronic devices, particularly ultravioletemitting devices [4] However, besides UV emission, photoluminescence spectra of ZnO typically contain other bands in the visible region which are attributed to defect-related emission [5] even though the exact type of defect is not conclusively established The most commonly cited hypothesis for the green emission is the electronic transition from conduction band edge to oxygen vacancies (VO) or zinc vacancies (VZn) [6–9] The yellow luminescence band is attributed to excess oxygen [10], lithium impurities [11], or hydroxyl group [12] The orange/red emission is due to electronic transitions from zinc interstitials (Zni) to oxygen interstitials (Oi) [13–16] Nonetheless, surface defects [17] and defect complexes [18, 19] are also considered to explain the origin of visible emissions Particularly, carbon doping in ZnO has been extensively studied, apart from a large number of reports on p-type conduction or ferromagnetism properties, the luminescence properties of carbondoped ZnO has also aroused much scientific interest [18–21] n Corresponding authors Tel.: þ 84 436230435; fax: þ84 436230293 E-mail addresses: khoi.nguyenthi@hust.edu.vn (K.T Nguyen), huy.phamthanh@hust.edu.vn (P.T Huy) http://dx.doi.org/10.1016/j.jlumin.2016.01.031 0022-2313/& 2016 Elsevier B.V All rights reserved Despite of the fact that up to now not many studies were focused on the optical properties of carbon-doped ZnO, the reported data on luminescence of this material are rather complicated and far from unambiguous It was reported that blue emission was observed from the carbon modified ZnO particles [22] Carbon impurities were also found to enhance the blue/violet emission band from the ZnO thin film fabricated by pulse laser deposition [23] Green luminescence in carbon-doped ZnO films or in ZnO powder annealed with carbon black was also reported [19–20] Additionally, a broad emission band that covers the whole visible range from 400 to 800 nm was observed from a hollow microtube–nanowire structure made of carbon-doped ZnO [24] Moreover, the possibility to tune the optical bandgap of ZnO was also demonstrated for the C-implanted ZnO thin films [21] In this work, we report on the suppression of the defect-related emission by C doping in ZnO crystals XRD measurement shows a single phase hexagonal wurtzite structure of C-doped ZnO without any detectable secondary phase The introduction of C by a ballmilling method causes the blueshift of UV band and helps to suppress visible emission of ZnO crystals The intensity ratio between UV and visible emission gets its maximum value with the annealing temperature of 800 °C Adding carbon might help to reduce the number of defect centers, passivate surface defects in ZnO crystals, or might create non-radiative recombination pathways that can compete with decay channels resulting in visible photoluminescence Above this optimal temperature, carbothermic reaction happens and creates more defects in the crystal, bringing the ratio back down N Tu et al / Journal of Luminescence 174 (2016) 6–10 Experimental Commercial ZnO (Merck, 99.99%) and Carbon (99.9%) powders were mixed together with different weight ratios (2, 3, and 4%) After coarse grinding for h, the mixture was grounded further by high-energy planetary ball milling (Restch PM 400) with the speed of 200 rpm for 60 h, and then annealed in Argon gas for h and at different temperatures from 200 to 1000 °C The crystal structure of the powders was investigated by X-ray diffraction (Bruker D8 Advance XRD) The surface morphology was characterized by ultra-high resolution scanning electron microscopy (Jeol JSM7600F) Chemical bonds and vibrational frequencies were analyzed by Fourier transform infrared (Nicole FTIR 6700) and Raman (Horiba Jobin-Yvon LabRAM HR Raman) techniques Emission spectrum of the samples was collected by using photoluminescence spectroscopy (Horiba Jobin-Yvon Nanolog) All measurements were carried out at room temperature The chemical bonding structure of obtained catalysts was demonstrated by X-ray photoelectron spectroscopy (XPS, VG Scientific, ESCALAB250 at 15 kV, 15 mA) Measurements were performed using a monochromatic Al Ka X-rays (1486 eV) source Results and discussion Fig shows FESEM images of the powders before and after milling with 4% C and post annealing at different temperatures The source ZnO material is in the form of rods and particles with diameters from 0.1 to around μm After grinding by high-energy planetary ball milling for 60 h and subsequent post annealing, small particles aggregate to form big particles Higher temperature leads to the formation of bigger particles, which are in order of micrometer size (i e after h annealing at 1000 °C in pure Ar gas environment) Fig shows X-ray diffraction patterns of ZnO and C-doped ZnO powders The patterns of the initial ZnO and ZnO doped with 4% C at various temperatures are shown in Fig 2a The diffraction peaks are broad at temperatures lower than 800 °C, indicating low crystallinity This is held true for other C concentrations as well The XRD patterns of ZnO with different levels of C doping and annealed at the same temperature of 800 °C are presented in Fig 2b All the diffraction peaks can be indexed as hexagonal wurtzite structure There is no measurable secondary phase or impurity peak, which indicates that C dopant is well integrated into the host The main peaks ((100), (002), (101)) are shown in the inset of Fig 2b The peaks shift to higher diffraction angles with increasing C content, except 4% C, in comparison with those of ZnO source The lattice constants are calculated and tabulated in Table They are smaller in C-doped ZnO compared to ZnO source Carbon doping in ZnO can happen by the substitution of C for either O or Zn or by taking the interstitial sites of C [25] The decreasing of lattice parameters when C is integrated suggests that the smaller C4 þ (0.15 Å) ions substitute for the larger Zn2 þ (0.74 Å) ions in the ZnO lattice At 4% C doping concentration, the diffraction peaks, however, not continue to shift to higher angles but the opposite side, suggesting that C dopant takes interstitial sites Fig FESEM images of original ZnO (a) and ZnO: 4% C after high-energy planetary ball milling for 60 h and post annealing at 600 °C (b), 800 °C (c), and 1000 °C (d) for h in pure Argon gas environment 8 N Tu et al / Journal of Luminescence 174 (2016) 6–10 besides Zn replacement, leading to a slight expansion of the host lattice compared to 3% C doping The substitution of C for Zn is also suggested by FTIR spectra shown in Fig Both the undoped and doped ZnO samples exhibit three absorption bands at 3446 cm À 1, 2361 cm-1, 1637 cm À which are attributed to O–H stretching vibration of H2O, the O¼ C¼ O bending vibration of CO2 in air, and the C ¼O bond vibrations [26–28], respectively The band at 430 cm À is assigned to Zn–O bond in undoped ZnO [29, 30] and it shifts to higher frequencies at 440 cm À 1, 454 cm À 1, and 480 cm À 1, respectively for 2, 3, and 4% C-doped ZnO Carbon atom is lighter than zinc atom, so the replacement could result in the stretching frequency upshift In order to confirm that the incorporation of C is the culprit of the shift, we have repeated the same milling and annealing processes but on original ZnO only and its XRD patterns at different states are shown in Fig 2c The crystallinity is drastically improved at 800 °C which is indicated by sharp diffraction peaks However, no significant peak shift has been observed, differentiating Cdoped ZnO from original ZnO The integration of C results in lattice contraction The ZnO:C system is further investigated by Raman spectroscopy and the results are shown in Fig The spectra show two new peaks at 1324 and 1594 cm À in comparison to source ZnO The Raman mode at 1594 cm À is a characteristic of graphitic Fig FTIR spectra of ZnO source material and ZnO:C (2, 3, and 4%C) annealed at 800 °C The peak corresponding to ZnO bond shifts to higher wavenumber with increasing C concentration Fig XRD patterns of ZnO source material and ZnO: 4% C at different annealing temperatures (a), ZnO:C (2, 3, 4%) at the same annealing temperature of 800 °C The inset shows XRD patterns of the three peaks ((100), (002), and (101)), which shift to the higher angles with C doping Table Lattice constant a and c of ZnO and C-doped ZnO powders with different C doping concentrations The lattice constant a and c a (Å) c (Å) ZnO 2%C 3%C 4%C 3,279 3,268 3,262 3,265 5,249 5,232 5,224 5,230 Fig Raman spectra of C-doped ZnO exhibit characteristic peaks of graphitic material N Tu et al / Journal of Luminescence 174 (2016) 6–10 Fig Photoluminescence spectra of ZnO source material and ZnO: 4%C annealed at various temperatures (a) The inset shows the dependence of the intensity ratio between UV and visible regions on annealing temperature The evolution of PL spectra upon C integration at the annealing temperature of 800 °C (b) The inset presents the shift of the UV peak toward lower wavelength with increasing C doping Fig XPS spectra of 4% C-doped ZnO powders annealed at 800 °C in argon: Zn 2p core level spectrum (a) C 1s core level spectrum (b) O 1s core level (c) materials and the mode at 1324 cm À is attributed to defects in graphitic structure Since intensities of the two bands are comparable, ZnO crystals appear to be covered by a disordered graphitic layer This layer may play a role in changing the emission spectrum of ZnO crystals as presented in Fig The formation of the graphitic shell around the ZnO core indicates that the actual dopant concentration is less than the initial doping concentration, i.e smaller than 2%, 3% or 4% for corresponding experimental conditions In order examine the stoichiometry and the chemical bonding in C-doped ZnO powders, XPS measurements were carried out for the powder doped with 4% C annealed at 800 °C XPS data for the C 1s, O 1s and Zn 2p binding energy regions is shown in Fig The Zn 2p spectrum in Fig 5a shows a doublet with the binding energies at 1020.4 eV and 1043.5 eV which correspond to the reported binding energies of the Zn 2p3/2 and Zn 2p1/2 states, respectively The difference between the binding energy of Zn 2p3/ and Zn 2p1/2 states is found to be 23.1 eV and is in accordance with the standard reference value [21] Both the binding energies and their difference value infer that Zn ions are in þ2 oxidation state The XPS spectrum for the C 1s is shown in Fig 5(b) Fitting the spectrum with Gaussian-Lorentzian functions, it can be deconvoluted into three peaks centered at binding energies of 284.7, 285.9 and 288.9 eV The intense peak at binding energy of 284.7 eV could be attributed to C–C bonds of the “free carbon” (graphite or carbon contamination) This is in good agreement with Raman spectra and confirms the presence of graphitic shell The peak at 288.9 eV is assigned to carbonate species [31] It has been reported that if C substitutes for Zn then the C 1s binding energy shifts to a slightly higher value [32–33] Thus, the appearance of the peak centered at 285.9 eV indicates the successful doping of carbon into ZnO crystal by substituting for zinc, in good agreement with the XRD and FTIR results as shown Fig and Fig.3 One of the possibilities of the substitution is the formation of O–C–O bond The O 1s XPS spectrum as shown in Fig (c) can be de-convoluted into two peaks at 529.4 eV and 530.8 eV The peak at 529.4 eV is assigned to O2 À ions of Zn–O bonds in 10 N Tu et al / Journal of Luminescence 174 (2016) 6–10 Wurtzite structure with Zn2 þ in hexagonal coordination and the peak at 530.8 eV can be attributed to Zn–Vo and Zn–O–C bonds because C-dopant and Vo usually induce blue shift of Zn–O bond [31,33] The above results and the disappearance of the oxygen vacancies peak at higher binding energy suggest that C atoms incorporated in ZnO crystals may work to passivate defects as such oxygen vacancies in the host, leading to the reduction of defectrelated emission Fig shows photoluminescence spectra of undoped and Cdoped ZnO samples annealed in argon at different temperatures from 200 to 1000 °C The PL spectrum of the starting material (Fig 6a, black curve) consists of two emission features: a sharp peak at around 383 nm and a wide band in visible region that are attributed to near band edge (NBE) and defect-related emissions, respectively [5] Upon C integration, the evolution of PL spectra at different annealing temperatures is shown in Fig 6a The intensity ratio between the UV and visible regions is an important quantity to characterize the emission spectrum purity For all C concentrations, the intensity ratio keeps increasing until 800 °C (Fig 6a inset) where the visible region is remarkably reduced In addition, the ratio at 800 °C is proportional with carbon concentration The quenching upon C doping at 800 °C is shown in Fig 6b This indicates that the defect level is suppressed with C addition By both diffusing in and forming a graphitic cover of ZnO crystals, C integration might help to reduce the number of defect centers, passivate surface defects [34], or provide a non-radiative recombination channel that is competitive to decay channels resulting in visible emission, leading to the defect-related PL quenching When the annealing temperature increased to 1000 °C, the ratio is reduced and the visible emission reappears again The enhancement is probably due to carbothermic reaction in ZnO–C system [35], that creates more defects (Zn and O vacancies) in the host crystals The temperature of 800 °C appears to be the optimal annealing temperature at which the system shows good crystallinity (from XRD and SEM measurements) as well as the suppression of the defect-related emission In addition to the quenching of visible emission, the NBE peak shifts to lower wavelength upon the increasing of C doping level and annealing at 800 °C as shown in Fig 6b inset The peak position is observed at 383, 382, 380, and 379 nm for ZnO source and ZnO doped with 2, 3, and 4% C, respectively Since the particle size of the samples is much larger than the exciton Bohr radius of ZnO, which is around 10 nm [36,37], the quantum confinement cannot explain the blue shift When ZnO is doped with carbon, the dopant and their complexes can act as donors, providing excess carriers which occupy impurity energy levels close to the conduction band edge When the concentration of excess electrons is high enough, they start to occupy states above the conduction band edge, shifting up the lowest available energy level for electrons in the valence band to be excited to Therefore, C dopant can cause degenerate doping and make the bandgap appear to be larger, known as Burstein–Moss effect [38], resulting in the blue shift of UV peak as indicated in the inset of Fig 6b The higher the dopant level is, the lower wavelength the UV peak position can be Conclusions In conclusion, C-doped ZnO powder with good quality single crystals has been prepared successfully by high-energy ball milling technique and with appropriate annealing conditions, i e at 800 °C and in argon environment XRD and FTIR measurements indicate the substitution of C for Zn Nonetheless, Raman analysis suggests a graphitic layer covering ZnO crystals The incorporation of C can happen in both ways, diffusing in and forming a graphitic structure on the surface The integration of C shows strong effects on PL spectrum of ZnO powder Besides the slight blueshift of UV band, the defect-related emission is drastically quenched and the quenching is proportional with dopant level, making C doping an effective method to produce almost pure UV emission from ZnO crystals The C incorporation might help to decrease defect concentration, passivate the surface, and/or provide a non-radiative recombination pathway, resulting in the suppression of visible band Acknowledgments This work was supported by the National Application-Oriented 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