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Micro- and Nanopyramids of Manganese-Doped Indium Oxide D. Maestre,† I. Martı´nez de Velasco,† A. Cremades,*,† M. Amati,‡ and J. Piqueras† Departamento de Fı´sica de Materiales, Facultad de Ciencias Fı´sicas, UniVersidad Complutense de Madrid, 28040 Madrid, Spain, and Sincrotrone Trieste, Area Science Park, 34012 BasoVizza-Trieste, Italy ReceiVed: March 1, 2010; ReVised Manuscript ReceiVed: June 11, 2010
Mn-doped In2O3 nanopyramids have been grown by a catalyst-free thermal process at 700 °C using InN and Mn2O3 powders as precursors. Energy dispersive spectroscopy, as well as X-ray photoelectron spectroscopy, demonstrate the presence of Mn in the pyramids in a content below 1 at. %. In addition to pyramids, nanowires with diameters of about 100 nm grow during treatments at 800 °C. Luminescence has been studied by cathodoluminescence in the scanning electron microscope, showing emissions at 1.9, 2.65, and 3.3 eV. Dopant incorporation into the nanostructures and their oxidation states, as well as the effect on the electronic structure, have been measured and discussed. Introduction Indium oxide is a wide band gap semiconductor with applications in fields such as gas sensing, optical transparency, optoelectronics, and others. As in the case of other oxides, there is an increasing interest in the fabrication of In2O3 nanostructures with potential applications in nanotechnology. In particular, elongated nanostructures have been grown by different thermal methods involving deposition on a substrate1-6 or on a compacted pellet from precursor powders.7,8 Also, small dimension In2O3 pyramids have been grown,9,10 and their efficient field emission has been demonstrated.9 Some other structures such as nano- and micropyramids grown at the tip of arrowlike In2O3 have also been reported.7 Doping of semiconductor nanostructures to modulate their physical properties is an active field of research, and In2O3 nanowires doped with Ga,11 Zn,12 or Sn13,14 have been recently synthesized. Incorporation of Mn into In2O3 has also attracted great interest due to its influence on the magnetic behavior of nanocrystalline and bulk material.15,16 Moreover, improvement of the In2O3 gas sensor selectivity to H2 and CH4 can be reached by Mn doping.17 In the present work, Mn-doped In2O3 micropyramids have been grown by a thermal evaporation-deposition method. Structural, compositional, and optical characterization of the doped structures was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) in SEM, cathodoluminescence (CL) in SEM, and XPS synchrotron microscopy. Experimental Methods The structures were grown by thermal treatment of compacted precursor powders under an argon flow. This method has been previously reported to lead to the growth of elongated nanoand microstructures of semiconductor oxides including In2O37,8 and SnO218 on the surface of the sample, so that neither a catalyst nor a foreign substrate is used. The starting materials used were InN powder with 99.9% purity and Mn2O3 powder with 99.999% purity. Mixtures of ball-milled precursor powders were compacted to form disks * To whom correspondence should be addressed: E-mail: cremades@ fis.ucm.es. † Universidad Complutense de Madrid. ‡ Sincrotrone Trieste.
of about 7 mm diameter and 2 mm thickness. The samples were prepared by using a mixture of InN powder and 2% wt Mn2O3 powder as precursors, and then the compacted disks were annealed at temperatures between 700 and 800 °C during 15 h under an argon flow. During the treatment, InN decomposes into In and N2 and indium oxide is formed. This treatment leads to the growth of different In2O3 nano- and microstructures on the surface of the sample. XRD measurements were performed in a Philips diffractometer. Secondary electron and CL measurements were carried out in a Leica 440 SEM and a Hitachi S2500 SEM. CL measurements were performed at liquid nitrogen temperature using a beam energy of 15-20 keV, with a Hamamatsu R928 photomultiplier and a PMA-11 charge coupled device camera. EDS was performed in a Leica 440 SEM with a Bruker AXS Quantax system working at 20 keV and 5 nA. Spatially resolved XPS measurements were carried out at the ESCA microscopy beamline of the Elettra synchrotron facility in Trieste (Italy). The scanning photoelectron microscope (SPEM) can work in both imaging and spectroscopy modes with a zone plate focusing optics, which produces a microprobe with a diameter of 150 nm. Photoemission spectra were measured by using a 640 eV photon energy with a 0.2 eV energy resolution. Results and Discussion XRD spectra show that thermally treated samples consist mainly of In2O3 and contain residual phases of MnO2, Mn2O3, and Mn3O4 (Figure 1). The average content of In2O3 in the treated disks is about 92 at. %, as estimated from EDS. Neither traces from the starting InN precursor nor ternary compounds or alloys have been detected in the XRD spectra. Previous studies reported on the presence of Mn-based secondary phases when Mn is used as a dopant of metallic oxides.19,20 In ref 19, Mn-doped ZnO thin films were found to present secondary phases of Mn2O3, causing the nominal doping of the films to be significantly higher than the measured doping level. The authors proposed a doping mechanism of diffusion through the interfaces between the indium oxide particles and the manganese oxide phases, which is only efficient near the surface of the particles and difficult to control. These secondary phases have been reported to be the origin of the extrinsic ferromagnetism observed in semiconducting oxides doped with magnetic elements.21,22
10.1021/jp103670b 2010 American Chemical Society Published on Web 06/22/2010
Pyramids of Manganese-Doped Indium Oxide
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Figure 1. XRD spectrum acquired on a sample treated at 700 °C during 15 h, showing the presence of In2O3, as well as MnO2, Mn2O3, and Mn3O4 phases.
The samples treated under 700 °C present a grainy structure, with grain sizes between 50 and 500 nm, and an inhomogeneous distribution of Mn as measured by EDS. The treatments carried out at and above 700 °C lead to the growth of micro- and nanopyramids as well as a small amount of nanowires on the sample surface. The pyramids, mainly formed during treatments at 700 °C for 15 h, present a square bottom face and four triangular faces, showing a wide range of sizes from few hundreds of nanometers to 6 µm, as shown in Figure 2. These pyramids, with smooth surfaces and sharp tips, appear randomly distributed and usually with a growth direction normal to the substrate (surface of the pellet). A structure formed by stacks of pyramids is sometimes observed when the treatment temperature is raised to 800 °C, as shown in Figure 2c. The content of Mn in these pyramids, as measured by EDS, is about 0.5 at. % or in some regions seemed to be in the detection limit of the technique. Undoped In2O3 nanopyramids with similar shape have been previously grown on silicon substrate and their efficient field emission related to their sharp tips has been assessed.9 The square and triangular faces of the pyramids were indexed as the (400) and (222) planes respectively of the body centered cubic structure. Undoped In2O3 pyramids have also been grown by the same method used in this work, using InN powder as precursor that decomposes during the thermal treatment.7,8 However, in refs 7 and 8, the pyramids appear as small crystallites connected by nanowires, in a necklace structure, or as the upper part of arrowlike structures. The present results show the influence of Mn dopant on the morphology of the obtained structures. Mn seems to favor the growth of the triangular faces of the pyramids as compared to the growth along the direction perpendicular to the square face. This prevents the formation of the elongated structures observed in ref 7 and leads to the formation of the stacks of pyramids shown in Figure 2c, as described in the inset diagram. The latter indicates that impurity incorporation influences the surface energy of certain facets, involving the preferential adsorption of impurities on specific surfaces and the inhibition of crystal growth. This agrees with the growth inhibition observed for Mn-doped colloidal In2O3 nanoparticles reported by Farvid et al.23 In ref 23 it is discussed that the incorporation of Mn into the indium oxide lattice depends on the heat of adsorption release upon trapping of the dopant ions on the nanocrystal surface. In this process, the dopant would undergo
Figure 2. (a,b) SEM images showing the pyramids obtained on a sample treated at 700 °C during 15 h. (c) Stacked structure of pyramids corresponding to a sample treated at 800 °C for 15 h.
ligand substitution and eventually a change in coordination. The dopant incorporation depends on how similar are the crystalline structures of the host (In2O3) and of the stable compound of the dopant with the host lattice anion. In our case, the decomposition of the precursor oxide Mn2O3 by the thermal treatment should contribute with Mn3+ ions to the gas mixture, although Mn3+ easily transforms into Mn2+ and Mn4+. The stable compound of Mn3+ with oxygen is Mn2O3, which presents the same bixbyite crystalline structure as the In2O3 obtained in our samples. In the case of Mn2+, the MnO is the stable compound with a rock salt structure and octahedral coordination of both cations and anions, while for Mn4+, MnO2 should be considered, with up to five crystalline structures, depending on how the MnO6 octahedra are interlinked.24 Therefore, a high incorporation of Mn into In2O3 would imply the 3+ charge state. The limited availability of Mn3+ could also be due to the strong stabilization of Mn cations in the Mn2O3 precursor or its gasphase intermediates, as observed previously for other Mn-doped nanowires from different manganese oxides. For example, Djurisˇic´ et al.25 observed that an increase in the mass ratio of the MnO2 in the precursor mixture did not result in the increase
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Figure 3. Nanowires obtained on terraced structures grown under treatments at 800 °C for 15 h.
of Mn content in the GaN nanowires. As the incorporation of Mn in the samples is low, we conclude that the limitation on the dopant incorporation is due to the lack of enough Mn3+ available in the gas mixture, which causes the growth inhibition. In the samples treated at 800 °C, some regions with a terraced structure present nanowires with cross sections below 100 nm (Figure 3). The wires are mainly associated to steps and show often preferential directions nearly parallel to the surface. The low density of nanowires and their reduced size do not enable the measurement of the Mn content by EDS, as the contribution from the bulk is always included in the results. XPS measurements with spatial resolution in the submicrometer range were performed in the ESCA microscopy beamline at the Elettra Synchrotron in Trieste. Different regions of the samples, as for example grains or pyramids, were probed and XPS spectra were locally recorded. XPS peak positions have been calibrated with respect to the C (1s) peak of carbon (284.6 eV). XPS measurements show that Mn is not uniformly distributed on the surface of the samples. Figure 4 shows the photoelectron peaks In (3d), O (1s), and Mn (3p) corresponding to the nano- and micropyramids on a sample grown at 700 °C during 15 h with a 2% wt Mn2O3 in the precursor powders. The O peak recorded on points for which no manganese could be detected is shown for comparison in Figure 4d. In (3d) and Mn (3p) core levels appear centered at 444.6 and 47.7 eV, respectively, as shown in Figure 4a,b. These values agree with those reported by different authors.26-28 However, in this case, the Mn (3p) spectrum does not show any shoulder or satellite peak at higher binding energies, as those reported in ref 27 and
Maestre et al. 28. The Mn content in the pyramids, estimated from the XPS spectra, is 0.8 at. %. This value is close, but somewhat higher, than that measured by EDS, which can be explained by the different volume of the sample probed with each technique (some atomic layers of the surface by XPS). The O (1s) core level presents two components at 530.4 and 532.0 eV. A similar feature has been previously observed for Sn-doped indium oxide nanostructures26,29 and other compounds. In this case, the higher binding energy peak (532.0 eV) is significantely reduced for regions containing Mn, as observed in Figure 4c. The nanopyramids present a cleaner surface and a less defective surface than the bulk material related to the reduced incorporation of manganese. The lower binding energy has been attributed to lattice O2-, while the higher energy one has been related to adsorbed oxygen species. A contribution in this energy region (531.5 eV) has also been related to oxygen in oxygen-deficient regions (i.e., oxygen ions whose neighboring In atoms do not have their full complement of six nearest-neighbor O2- ions).30,31 Mn ion acts as a magnetic impurity and also as an acceptor in semiconductor oxides, such as indium oxide. The Mn3+ cation has a smaller ionic radius (0.64 Å) than that of In3+ (0.80 Å), whereas for Mn2+ (0.83 Å), the ionic radius is comparable to the one of In3+. According to our XRD spectra, no changes in the lattice parameter have been detected in the Mn-doped samples, which could indicate that Mn2+ replaces preferentially In3+ in the indium sublattice, although this involves charge imbalance. This replacement enhances the hybridization of d-state of Mn2+ with s- and p-states of indium oxide involving the free electrons of oxygen deficient centers.32 On the other hand, Kantcheva et al.33 reported that Mn2+ and Mn3+ can both be present in Al2O3, and Farvid et al.23 reports on the incorporation of Mn3+ to colloidal In2O3, so that the presence of Mn3+ cannot be excluded in our samples. Experimental measurements of the effective magnetic moment of Mn atoms in indium oxide thin films suggest that the coordination of Mn should be octahedral or tetrahedral (or highly distorted octahedral), as the coordination of the different indium sites in the indium oxide lattice.34 Figure 5 shows the XPS spectra corresponding to the valence bands acquired in regions from the sample surface with (e.g.,
Figure 4. (a-c) XPS spectra of In (3d), Mn (3p), and O (1s) core levels spectra obtained on pyramids. (d) O (1s) spectrum recorded on a region where Mn has not been detected.
Pyramids of Manganese-Doped Indium Oxide
Figure 5. Valence band XPS spectra acquired in regions with and without Mn, showing a decrease of 0.47 eV in the relative position of the Fermi level.
Figure 6. CL spectra acquired on different regions on the surface of a pellet as well as in the pyramids of a sample treated at 700 °C for 15 h, showing peaks at 1.9, 2.65, and 3.3 eV.
pyramids) and without Mn. The relative position of the Fermi level with respect to the valence band minimum decreases around 0.47 eV, which is consistent with a Mn induced p-doping. CL spectra acquired on the surface of the samples after the treatment show emission bands centered at 1.90, 2.65, and 3.30 eV in areas without pyramids, as observed in Figure 6. The relative intensity of these bands depends on the region probed, but in general, the 1.90 eV band is dominant in all the spectra. CL spectra recorded on the pyramids show only the 1.9 eV band, as observed in Figure 6. This result agrees with previous CL measurements on the above-mentioned arrowlike structures of undoped In2O3 with well-formed pyramids at the top7,8 that show a single broad band at 1.9 eV. It appears that this emission is dominant in the case of micro- and nanopyramids obtained by the method used here and that the Mn incorporation does not modify the CL emission. The 1.9 eV transition has also been observed in In2O3 thin films35 and tentatively attributed to oxygen deficiency or defects. Seo et al.36 associate this emission with transitions between the shallow donor level related with oxygen vacancies or with In interstitial deep acceptors related to In vacancies. The 2.65 eV emission has been previously reported in In2O3 and attributed to the presence of single ionized oxygen vacancies.7,10,37 Higher energy emissions centered around 3.3 eV (Figure 6) are indeed formed by different sharp peaks (0.03 eV width) at 3.25, 3.32, and 3.35 eV, which could be tentatively related to excitonic emissions due to an exciton bound to a donor level, as is also observed in other doped In2O3 systems.36 The presence of a Mn dopant would favor the formation of exciton coupled with lattice phonons, as stated by Kundu et al.38 in Mn-doped ITO compounds.
J. Phys. Chem. C, Vol. 114, No. 27, 2010 11751 There are many reports on Mn intraionic luminescence in different hosts. For instance, Mn2+ luminescence in ZnS nanoparticles has been observed at 2.13 eV39 and attributed to the 4T1 f 6A1 transition of 3d states of the ion in a tetrahedral coordination symmetry. A similar result has been reported for CdS,40 while a luminescence band at 2.76 eV observed in Mndoped ZnO was attributed to the presence of the dopant.41 Characteristic intraionic transitions of Mn2+ in octahedral symmetry have characteristic energies in the visible range between 300 and 600 nm, depending sligthly on the host material. The most commonly reported transition of Mn3+ in octahedral symmetry is 5Eg f 5T2 g, with an energy around 2.48 eV. In our samples, the possible contribution of Mn in octahedral symmetry sites, either Mn2+ or Mn3+, to the luminescence could overlap with the broad CL band and remain unresolved. Conclusions Manganese-doped In2O3 micro- and nanopyramids have been grown by a catalyst-free thermal method, with mixtures of compacted InN and Mn2O3 powders as precursors. Several physicochemical aspects involved in the incorporation of Mn to the In2O3 structure concerning the growth mechanism, the charge state of the Mn, and the effect of the incorporated Mn on the electronic structure and optical properties have been analyzed. Manganese doping of In2O3 inhibits the growth of elongated structures at the surface and reduces the grain size of the resulting nanostructured In2O3 ceramic when the treatment temperatures are below 700 °C. Mn is not homogeneously distributed on the sample surface, and residual phases of manganese oxides aggregate, forming micrometer grains at temperatures under 700 °C. Micro- and nanopyramids grow at temperatures in the range 700-800 °C. EDS and XPS results demonstrate the presence of Mn in these structures with a content below 1 at. %. The results of the XPS measurement of the relative position of the Fermi level with respect to the valence band minimum is consistent with a Mn-induced p doping due to the incorporation of Mn2+; however, the incorporation of Mn3+ cations cannot be excluded. The cathodoluminescence signal from these pyramids consists of a broad band centered at 1.9 eV, while emissions at 2.65 and 3.3 eV are also observed in regions without pyramids. Acknowledgment. This work was supported by MEC (MAT2006-01259 and MAT2009-07782) and BSCH-UCM (Group 910146). References and Notes (1) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (2) Peng, X. S.; Meng, G. W.; Zhang, J.; Wang, X. F.; Wang, Y. W.; Wang, C. Z.; Zhang, L. D. J. Mater. Chem. 2002, 12, 1602. (3) Liang, C.; Meng, G.; Lei, Y.; Phillipp, F.; Zhang, L. AdV. Mater. 2001, 13, 1330. (4) Vomiero, A.; Ferroni, M.; Comini, E.; Faglia, G.; Sberveglieri, G. Cryst. Growth Des. 2010, 10, 140. (5) Yin, W.; Cao, M.; Luo, S.; Hu, C.; Wei, B. Cryst. Growth Des. 2009, 9, 2173. (6) Zeng, F.; Zhang, X.; Wang, J.; Wang, L.; Zhang, L. Nanotechnology 2004, 15, 596. (7) Magdas, D. A.; Cremades, A.; Piqueras, J. Appl. Phys. Lett. 2006, 88, 113107. (8) Magdas, D. A.; Cremades, A.; Piqueras, J. J. Appl. Phys. 2006, 100, 094320. (9) Jia, H.; Zhang, Y.; Chen, X.; Shu, J.; Luo, X.; Zhang, Z.; Yu, D. Appl. Phys. Lett. 2003, 82, 4146. (10) Guha, P.; Kar, S.; Chaudhuri, S. Appl. Phys. Lett. 2004, 85, 3851. (11) Chun, H. J.; Choi, Y. S.; Bae, S. Y.; Choi, H. C.; Park, J. H. Appl. Phys. Lett. 2004, 85, 461. (12) Hsin, C. L.; He, J. H.; Chen, L. J. Appl. Phys. Lett. 2006, 88, 063111.
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