Band gap tuning of ZnO nanoparticles via Mg doping by ... - Narod.ru

Accepted Manuscript Title: Band gap tuning of ZnO nanoparticles via Mg doping by femtosecond laser ablation in liquid environment Authors: E. Chelnokov, M. Rivoal, Y. Colignon, D. Gachet, L. Bekere, F. Thibaudau, S. Giorgio, V. Khodorkovsky, W. Marine PII: DOI: Reference:

S0169-4332(11)01381-X doi:10.1016/j.apsusc.2011.08.132 APSUSC 22384

To appear in:

APSUSC

Received date: Revised date: Accepted date:

13-5-2011 20-7-2011 31-8-2011

Please cite this article as: E. Chelnokov, M. Rivoal, Y. Colignon, D. Gachet, L. Bekere, F. Thibaudau, S. Giorgio, V. Khodorkovsky, W. Marine, Band gap tuning of ZnO nanoparticles via Mg doping by femtosecond laser ablation in liquid environment, Applied Surface Science (2010), doi:10.1016/j.apsusc.2011.08.132 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights

Highlights

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Femtosecond laser ablation synthesis of Mg doped ZnO nanoparticles > Electronic properties of ZnO are modified by Mg > Bang gap and exciton energy shifts to the blue> The exciton energy shift is saturated at Mg content of about 20%> Phase separation at Mg content more 25% > mechanism of exciton pinning – recombination via new surface states.

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*Manuscript

Band gap tuning of ZnO nanoparticles via Mg doping by femtosecond laser ablation in liquid environment. E. Chelnokov, M. Rivoal, Y. Colignon, D. Gachet, L. Bekere, F. Thibaudau,, S. Giorgio, V. Khodorkovsky, and W. Marine

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Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), UPR CNRS 3118, Case 913, 163, Avenue de Luminy, 13288, Marseille, France

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Abstract.

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We use multiphoton IR femtosecond laser ablation to induce non-thermal non-equilibrium conditions of the nanoparticle growth in liquids. Modifications of the electronic properties of ZnO NP were achieved by Mg ion doping of targets prepared from mixtures of Zn and Mg acetylacetonates. The nanoparticle sizes were 3-20 nm depending on the ablation conditions. X-ray fluorescence indicates that stoichiometric ablation and incorporation of Mg in nanocrystalline ZnO occurs. HRTEM observations show that nanoparticles retain their wurtzite structure, while at high Mg concentrations we detect the MgO rich domains. Exciton emissions exhibit relatively narrow bands with progressive and controlled blue shifts up to 184 meV. The exciton energy correlates to band edge absorption indicating strong modification of the NP band gaps. Stabilisation of the exciton blue shift is observed at high Mg concentration. It is accompanied by the formation of structure defects and ZnO/MgO phase separation within the nanoparticles.

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Keywords: laser ablation; zinc oxide; ZnO/MgO; exciton emission; HRTEM

Corresponding author: W. Marine, e-mail: [email protected], ph: 33 6 17 24 81 86 fax: 33 4 91 82 91 76

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Introduction. Nanohybrid materials composed of inorganic oxide nanoparticles (NP) and organic molecules are potential candidates to be employed as nano-markers for biomedical imaging. Zinc oxide is one of promising materials for these applications owing to its bio-compatibility. The wide band gap allows selection of a matching dye to achieve electron transfer from the ZnO NP onto the dye molecule. Large nonlinear absorption cross section of ZnO allows excitation of nanohybrids in the VIS and NIR spectral range. Although laser ablation in liquid provides ZnO nanoparticles of good quality [1,2], using the biocompatible solvents (mostly water and ethanol) severely limits the choice of the organic dyes. Thus, the organic chromophores should be soluble in water/ethanol and involve a functional group (usually the thiol group) capable to compete with the polar solvent molecules for the nanoparticle surface. The second selection criterion is the energies of the electronic levels (comparing to the ZnO valence/conduction band) of the dye molecules to be attached to ZnO. The lowest unoccupied molecular orbital (LUMO) energy of a grafted dye should be lower than the ZnO conduction band, whereas the highest occupied molecular orbital (HOMO) level of the dye should be also close to the valence band of the core material. Under these conditions, upon excitation of the hosting ZnO nanomaterials, the excited electron can be transfered onto the grafted dye molecule. In our previous works, we described the synthesis of ZnO nanoparticles possessing high optical quality using femtosecond laser ablation in absolute ethanol and preparation of the ZnO based nanohybrids employing commercially available organic fluorophores such as tetramethylrhodamine B isothiocyanate and rhodamine B. [3]. Band gap engineering based on the insertion of Mg into the ZnO crystalline structure is possible owing to the similarity of the ionic radii of Zn2+ (0.60Å) and Mg2+ (0.57Å). The thermodynamic limit of solubility of MgO in ZnO has been reported to be less than 4% [4]. In spite of the predicted low solubility, highly doped alloys, up to 35%, were prepared by the nonequilibrium pulsed laser deposition [5,6,7], by plasma assisted molecular beam epitaxy [8] and by thermal decomposition of zinc and magnesium acetates [9]. All these methods produced either supported films consisting of crystals within the micrometer range or the agglomerated collections of 40-100 nm nanocrystals on a support [9]. In this communication, we report the possibility of tuning ZnO optical band gap to expand the range of suitable organic fluorophores. The band gap modification of the isolated ZnO nanoparticles with small size (between 3-20 nm, depending on irradiation conditions) was achieved by the Mg ion doping during femtosecond laser ablation (f-PLA) in liquid environment.

2 Experimental

For the preparation of targets, we used Zn(C5H7O2)2 and Mg(C5H7O2)2 (Zn and Mg acetylacetonates) in the weight proportion corresponding to the number of Mg/Zn atoms ratio: 0; 0.02; 0.05; 0.10; 0.20; 0.30. After manual mixing and heating during 1 hour at 130°C to evaporate water, the mixture was calcined during 3 hours at 350°C to destroy organics. The target had been prepared by compressing at 12.5 tons with follow-up sintering at 700◦C for 2 days. Femtosecond laser ablation was carried out with an amplified Ti:Sapphire laser system (TSA, Spectra Physics) generating 90 fs pulses with a central wavelength of 800 nm. The laser beam was focused onto a sintered target, placed on the bottom of a 20 ml glass vessel filled with absolute ethanol with the incident angle of 90◦ and fluence about 5 J/cm2. The choice of these conditions was guided by our previous results [3]. A typical ablation run was set to 9000 shots with a repetition rate of 10 Hz. During the ablation procedure, the target was continuously rotated to

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avoid cratering. All ablation experiments were carried out at room temperature and atmospheric pressure. The optical properties of MgxZn1-xO nanoparticles were determined by the photoluminescence (PL) spectroscopy. Single photon induced PL was studied using a Horiba (Jobin-Yvon) fluorometer under Xe lamp or He–Cd laser excitation at 325 nm. The optical transmission spectra of the samples were obtained by a two-path UV–Vis optical spectrometer (Cary 1E VARIAN). The morphology of the pure and Mg-doped ZnO nanoparticles was investigated by high resolution transmission electron microscopy (HRTEM) with a JEOL 3010 microscope. After ablation, the solutions were added drop wise on a copper mesh covered by an amorphous carbon film and dried at room temperature, for further studies by a HRTEM and Scanning Electron Microscope (FEG-SEM 6320S, JEOL) with attached energy-dispersive x-ray spectrometer (EDS, Bruker).

3.1. MgO/ZnO femtosecond pulsed laser ablation.

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3 Results and discussion.

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In this work we used femtosecond laser irradiation with λ= 800 nm, where the photon energy 1.55 eV is well below the band gap energy of ZnO at room temperature (~3.37eV). Under these conditions, we consider two main absorption processes: the two photon absorption and the two-step sequential absorption through the defects and impurities levels located in the gap of ZnO and ZnO/MgO. In the case of nanocrystalline ZnO thin films the effective two-photon absorption coefficient is β800 = 110 cm/GW [10]. In our experiments, the ablation threshold was ~1.5 J/cm2, however, it depends somewhat on the Mg content in the target. Most of the ablation runs were carried out at fluence 5J/cm2. Immediately after the beginning of ablation, the formation of pure and stable colloidal solution was observed. To increase the quantity of the synthesized nanoparticles, the number of laser pulses was fixed at 9000, leading inevitably to the interaction of the laser pulses with already formed NPs. At this moment we can not evaluate the influence of this interaction on the properties of NP (size, size distribution). Nevertheless, the fragmentation of a number of NPs near the target surface is possible. To reduce this effect we have used relatively large volume of the liquid to disperse the NPs. Therefore, we consider the effect of the NP/laser interaction to be of minor influence on the composition changes of NPs.

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3.2 Structural properties.

Typical HRTEM images of crystallized Mg-doped ZnO NP are shown in Fig. 1. As can be seen in the image (Fig.1a), at low Mg concentrations the shapes of the NP are mostly spherical. The mean size is about 2-4 nm. The size dispersion is about 20%, however, we observed the formation of a number of larger nanoparticles with the size of about 15 - 20 nm. Typical NPs size distribution is shown in the inset of Fig.1a. The concentration of Mg in NPs after ablation was verified by EDS measurements on samples prepared by dispersion of a large number of NPs on a ~1 cm2 solid substrate. The deviation in Mg concentration for the particles in different areas of the substrate is about ± 5% as compared to the initial Mg concentration in the corresponding target. This feature demonstrates that the transfer of atoms from the macro sample irradiated by the IR femtosecond laser pulse to the nanosized particles occurs stoichiometrically. By increasing the Mg ion concentration, we noticed the appearance of the faceted structure of ZnO nanoparticles, meaning that these nanocrystals retain the wurtzite structure. At the Mg ion concentration larger than 10% (Fig.1b), the difference in morphology between the low and high doped ZnO nanoparticles consists

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3.3. Optical Properties.

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in the formation of microfacets. Lattice constants of Mg-doped ZnO NPs changed slightly (~1,4%), resulting in an increase of a axis and a decrease of c-axis with increasing Mg concentration. Similar observations (variation of about 1%) were reported for ZnO/MgO thin epitaxial films on ZnO [5]. At high Mg ion content (> 20%) we observed the appearance of Mg rich zones characterized by the difference in HREM contrast. For example, Fig.1c shows the HRTEM image of 20 nm nanoparticles with well-developed Mg rich zones of about 4-5 nm. This observation clearly indicates the saturation in ZnO nanocrystal by Mg atoms and the beginning of the phase separation between MgO and ZnO. It is interesting to note that Mg rich zones are formed generally at the periphery of the large nanoparticles or at the interface of the well-developed structural imperfections like twins. We assume that the NP growth involves atom by atom addition within the ablation plume forming a hot homogenized mixed region (MR), confined by the surrounding liquid. The initial temperature of MR and NP should be relatively high, near the melting temperature of the materials. The NPs cooling time from high to ambient temperature for the case of NP synthesis by laser ablation in gases was considered by Luk’yanchuk et al [11]. Unfortunately, owing to the lack of information concerning the thermodynamic parameters of the ZnO/MgO system it is difficult to perform similar calculations. However, we estimated that cooling time depends on the NP size and it is proportional to R2 [11], where R is the radius of the NP. Thus, NP of small size will retain higher Mg concentration without formation of Mg rich zones. Fig.2 shows the collection of HRTEM images obtained from the same colloid after ablation of target with 30% Mg concentration. The sizes of the NPs in Fig. 2.a and b are 18 and 22 nm, respectively. It can be seen that 22% variation in the NP size induces more pronounced phase separation on the final NPs. We note that the Mg rich zones are not yet detectable in 3 nm particles shown in Fig. 2c. Indirectly, it shows that the origin of the Mg rich zone formation is the diffusion of the Mg atoms during NP cooling, which results in the segregation of MgO at the NPs’ surface [12].

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In this part we present studies of the optical properties of Zn1-xMgxO nanocrystalline particles below the phase separation range. Large nanoparticles presented in the colloidal solution after laser synthesis and agglomerated NPs were separated by centrifugation. We note that for the Mg composition range used here 0 – 30%, the beginning of phase separation was observed only for NP size > 17 nm and for concentrations higher than 25%. All optical spectra in this study were obtained at room temperature under He-Cd laser excitation (325 nm, 10 mW maximal power) and Xe lamp excitation. The both light sources, with the same wavelength, gave rise to the PL spectra with the identical shapes. Observed difference concerns only the PL intensity variation of the excitonic and defects bands versus laser excitation power. Multiple acquisitions didn’t show any significant photoluminescence (PL) spectra fluctuations vs. time that confirm a high stability and homogeneity of the NPs. Fig.3 shows a general view of PL spectra of pure ZnO and ZnO/MgO nanoparticles with 10% content of Mg under He-Cd laser excitation. For the undoped sample (black curve) we can see a narrow excitonic peak at 376 nm (3.29 eV) and a wide band between 430 and 650 nm related to deep defect levels. The PL spectrum of the doped ZnO NPs (green curve) features three main differences. First of all, we note that both bands exhibit an increased PL intensity. Secondly, the exciton peak is blue-shifted. Finally, the defect band is blue-shifted and its shape is modified. To analyze the exciton peak shift as a function of the Mg concentration, the PL spectra of Zn1-xMgxO with different Mg concentrations are shown in Fig. 4. The PL band undergoes an apparent blue shift upon increased Mg concentration as a direct outcome of the band gap widening. Here, we used low intensity excitation at 275 nm to reduce the overlap of the Raman peak from the

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solvent and the PL spectra. For low Mg doping (2%), the exciton energy is blue shifted by ~47 meV relatively to the undoped ZnO. The shift is linear with the Mg concentration up to doping levels about 10%. At higher Mg concentration, we observe a saturation of the shift, with a maximal shift of 184 meV. From absorption spectra we observed an absorption edge shift between undoped and strongly doped samples of about 226 meV. The difference between absorption edge and exciton energy shifts confirms a complex nature of exciton emission in ZnO/MgO nanoalloys and indicates that relatively shallow in-gap defect states exits, limiting the exciton energy increase upon increasing Mg doping. The maximum of the exciton emission energy of pure nanocrystalline ZnO synthesized by fPLA is 3.29 eV, that is lower by few tens of meV compared to the exciton of bulk ZnO. It is widely accepted that at room temperature, this 3.29 eV transition is related to a transition of conduction band electrons to the acceptor-like states. The acceptor states stem either from the surface defects or from the stacking faults (see for example [13, 14]). These assumptions are in agreement with our observations. The NPs exhibit the large surface to volume atomic ratio and the existence of the surface defects can be clearly seen in the HRTEM images. The appearance of a new kind of defects during Mg doping is one of the possibilities to explain the saturation of the exciton energy with doping. The formation of interstitials Mgi has been proposed in [15] after analysis of deep levels of ZnMgO emission from samples with similar Mg content. It was concluded that the Mgi levels are situated well below the conduction band with typical emission at 3.04 eV. This type of interstitial Mg can also contribute to the shift of the defects band demonstrated in Fig.3, but its contribution seems to be very small. The formation of Mg vacancies or antisites can also be ruled out because this kind of defects should correspond to the deep level emission. In this case only modification of the native shallow surface defect states induced by band gap widening can be the reason for the exciton pinning. Further information about this kind of defects can be obtained from experiments involving nonlinear or IR spectroscopy.

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Conclusion. Zn1-xMgxO crystalline nanoparticles with tunable optical properties were synthesized by femtosecond laser ablation in ethanol. To our knowledge, it is the first demonstration of doped NPs synthesis in colloidal state using physical approaches. The observed bang gap increase is limited by 15-20% Mg content. At higher Mg concentration NPs exhibit the beginning of ZnO/MgO phase separation. At room temperature, the exciton energy transition is shifted by 184 meV toward the higher energy compared to the exciton transition of pure ZnO.

Acknowledgements. . This work was supported by ANR (French Agency for National Research), project NEM, grant: #09-0107-01

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[11] B. Luk’yanchuk and W. Marine, Applied Surface Science, 154–155 (2000) 314–319 [12] D. Gouveaa, G. J. Pereira, L. Gengembre, M. C. Steil, . Roussel, A. Rubbens, Pilar Hidalgo, R.R. Castro, Applied Surface Science, 7 (2011) 4219 [13] M. Schirra, R. Schneider, A. Reiser, G. M. Prinz, M. Feneberg, J. Biskupek, U. Kaiser, C. E. Krill, K. Thonke, and R. Sauer, Phys. Rev. B 77, (20081) 25215 [14] J. Fallert, R. Hauschild, F. Stelzl, A. Urban, M. Wissinger, H. Zhou, C. Klingshirn, and H. Kalt, J. of Appl. Phys., 101 (2007) 073506 [15] M. Trunk, V. Venkatachalapathy, A. Galeckas, and A. Yu. Kusnetsov, Appl. Phys. Lett. 97 (2010) 211901

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Figure captions:

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Fig. 1. a. HRTEM image of the Mg-doped ZnO nanoparticles prepared in pure ethanol. Inset shows NP size distribution corresponding to this ablation run. b. Image of a single dimly Mg-doped ZnO NP prepared from a target with 5% Mg concentration. c. Image of a single highly Mg-doped ZnO NP prepared from a target with 30% Mg concentration. White areas in this image reflect partial charging of the Mg rich zones under electron irradiation.

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Fig. 2. HRTEM images of NPs from the same batch. Mg concentration was 30%. a. Isolated NP (~18 nm) with a twinned structure. Mg rich regions exhibit bright contrast. b. Isolated NP (~22 nm) with larger and more visible Mg rich regions. Their brighter contrast indicates higher diffusion of Mg. c. Small NPs without formation of Mg rich zones.

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Fig. 3 (color online). PL spectra of ZnO/MgO under He-Cd laser excitation. The two first narrow peaks on the black spectra (pure ZnO) correspond to Raman scattering of ethanol and water. The intensity of the PL spectra corresponding to ZnO/MgO with 10% Mg concentration is shown in green. Fig. 4 (color online). PL spectra of the exciton emission for NPs with different Mg concentrations.

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Figure 1.

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Figure 2.

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Figure 3

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Figure 4.

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Fig.1

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Fig.2

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Fig.3

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Fig.4

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