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Indium Tin Oxide Micro- and Nanostructures Grown by Thermal Treatment of InN/SnO2 David Maestre,† Ana Cremades,*,† Luca Gregoratti,‡ and Javier 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: NoVember 4, 2009; ReVised Manuscript ReceiVed: January 15, 2010
Mixtures of InN and SnO2 powders, with a weight ratio of 10:1, have been used as precursors for the thermal growth of arrow-shaped and other elongated micro- and nanostructures of indium-tin oxide (ITO) containing about 2.6 atom % of Sn. The temperatures used in the process, in the range 650-750 °C, favor the decomposition of InN and oxidation of In, with a limited incorporation of Sn in the resulting compound. Arrow-shaped indium-tin oxide structures are obtained and formation of stannates during the process is avoided. X-ray photoelectron spectroscopy indicates that tin incorporates into the In2O3 lattice mainly as Sn4+. Luminescence of the ITO microstructures has been studied by cathodoluminescence in the scanning electron microscope. Introduction Indium tin oxide is a semiconducting material of high technological interest. The applications of thin films of this oxide cover a wide field of optoelectronic devices, such as transparent contacts in solar cells,1 flat panel displays,2 heat reflecting mirrors,3 or radiation protection.4 Also, the growth and doping control of nanostructures reached in the last years enable potential applications of oxide semiconductors as active parts of nanoelectronics, nanophotonics, nanolasers, biosensors, and others. Tin-doped indium oxide (ITO) nanowires have been grown by a carbon-assisted technique,5 a coprecipitation-annealing process,6 and vapor-liquid-solid thermal processes.7,8 Also, a thermal evaporation-deposition process has been used to synthesize indium-doped tin oxide nano- and microstructures at temperatures between 800 and 1400 °C.9,10 In our previous work,9 ITO nano- and microstructures have been grown by thermal treatment of compacted mixtures of SnO2 and In2O3 powders, under argon flow at 1400 °C. This method has been previously reported to lead to the growth of elongated structures of different semiconductor oxides on the surface of the sample, so that neither a catalyst nor a foreign substrate is used.11-14 In the case of pure In2O3, wires, necklace-shaped, and arrow-shape structures were grown following a two-step thermal treatment with InN powder as precursor,11 with a significant reduction of the process temperature to around 600 °C. The same approach was followed to obtain TiO2 nanowires starting from TiN powders,12 and the temperature of 1500 °C needed to obtain TiO2 nanostructures from TiO2 powders15 was reduced to 800 °C when titanium nitride was used as precursor. InN has a low thermal stability at temperatures around 400 °C and a rapid dissociation into In and N2 above 550 °C by desorption of nitrogen has been reported.16,17 It was found11 that an initial step of the thermal treatment at 350 °C favors nitrogen desorption and the beginning of the oxidation of In by reaction with the atmosphere to form In2O3. A second step at 600 °C leads to the complete oxidation of the sample and the growth * To whom correspondence should be addressed. E-mail: cremades@ fis.ucm.es. † Universidad Complutense de Madrid. ‡ Sincrotrone Trieste.
of different In2O3 nanostructures on the surface of the sample. In this work, SnO2 powders have been added to the InN starting powder to obtain ITO nanostructures. The study of these structures is a step toward the functionalization of different nanoand microdevices. Recently, a layer of indium tin oxide nanowires18 has been used as fully transparent contact for lightemitting devices overcoming some of the limitations usually present when using ITO thin films. The investigation of the growth and properties of these nanoand microstructures has been carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), cathodoluminescence (CL) in SEM, X-ray energy dispersive spectroscopy (EDS) in SEM, and X-ray photoelectron spectroscopy (XPS). The luminescence properties and structural characterization are analyzed, and the formation of nanostructures of a multicomponent oxide ITO-indium stannate is discussed. A treatment to obtain ordered structures has been optimized. Experimental Methods A mixture of commercial InN (99.9% purity) and SnO2 (99.9%) powders with a InN/SnO2 weight ratio of 10/1 has been used as precursor. The powders have been mixed and milled for 5 h in a centrifugal ball mill, Retsch S100, with 20 mm agate balls, in order to make the mixture more homogeneous, as well as to reduce the initial grain size. Disk-shaped samples have been fabricated by compressing the milled mixture. The pellets were thermally treated in a furnace using a controlled argon flow. The furnace was not sealed for high vacuum conditions and worked at atmospheric pressure. All the treatments consist of an initial step at 350 °C during 3 h in order to favor the N desorption from InN, followed by a second step at 650 or 750 °C for 20 h, which controls the growth of the nanostructures. The crystalline structure of the samples was characterized by XRD with a Philips diffractometer working at 45 kV and 40 mA, using Cu KR radiation. Secondary electron (SE) observations were carried out in a Leica 440 SEM equipped with a Bruker EDX analysis system. The CL measurements were carried out at liquid nitrogen temperature with a beam energy of 15-20 keV. CL images in the visible range were recorded with a Hamamatsu R928
10.1021/jp911881s 2010 American Chemical Society Published on Web 02/05/2010
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Figure 1. SEM images showing beaded nanowires (a), arrows (b), beak-like (c), and some other ITO micro- and nanostructures (d) obtained at 650 °C.
photomultiplier, and the CL spectra were recorded with a Hamamatsu PMA-11 CCD camera. Spatially resolved XPS measurements were performed 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 that produces a microprobe with a diameter of 150 nm, which enables us to measure XPS on selected individual micro- and nanostructures.19,20 Photoemission spectra were measured by using a 640 eV photon energy with a 0.2 eV energy resolution. Results and Discussion Figure 1a shows a high density of different micro- and nanostructures, mainly arrows and wires, grown on the sample surface when the second step of annealing was carried out at 650 °C during 20 h. All the structures studied in this work grow on the surface of the corresponding compacted pellet, which acts as source and substrate during the thermal treatment, without the use of a foreign substrate. Some of the wires, with diameters ranging from 80 to 300 nm, show a beaded appearance formed by polyhedral beads distributed all along the wire (Figure 1a). Arrows formed by columns with a pyramidal or octahedral tip are also observed (Figure 1b). Some of these arrows show a high aspect ratio, with cross sections ranging from 150 to 600 nm and lengths up to dozens of micrometers. Many of the pyramidal tips at the top of these arrow-like nanostructures have submicrometer sizes. Some of these high aspect ratio nanostructures present a beak-like appearance (as observed in Figure 1c), with changes in the growth direction between 90° and 135°. Depending on the analyzed region of the surface, some other structures such as plates ending in a nanoneedle, as well as elbow-like structures, are also grown by this treatment, as observed in Figure 1d. In samples grown with the second step of the thermal treatment at 750 °C during 20 h, ordered nanostructures are observed in some cases as panels a and b of Figure 2 show. These well-oriented arrows present rather homogeneous tip sizes of about 1 µm. However, their columns, with uniform submicrometer cross sections, have lengths up to 20 µm. These structures represent different evolution stages in the arrow growth, starting by the tip formation followed by the progressive column growth21 (Figure 2c). The ceramic pellet acts as the source and simultaneously as the substrate for the growth of
Figure 2. Ordered arrow-like structures (a, b), as well as nanostructures showing the initial stages of the arrow’s growth (c), obtained at 750 °C.
these micro- and nanostructures at the pellet surface. Since no catalytic process or foreign substrate is involved, it is suggested that the formation of the structures takes place by a vapor-solid process. An initial step of the thermal treatment at 350 °C favors nitrogen desorption and the beginning of the oxidation of In by reaction with the atmosphere to form In2O3. A second step at 650 or 750 °C leads to the complete oxidation of the sample and the growth of different tin-doped In2O3 nanostructures on the surface of the sample. As a difference with pure In2O3 arrowlike structures reported in a previous work,11 in this case the Sn-doped In2O3 arrows show a longer and thinner appearance, induced by the incorporation of Sn during the growth process. The presence of Sn during the thermal process can modify the vapor pressures, supersaturation ratios, and surface energies, which are important factors governing the final morphology of the micro- and nanostructures. Surface energy is one of the main factors which can modulate the growth process. The presence of Sn would generate changes in the growth rates along specific crystallographic directions which results in variations in the final morphology. The influence of Sn doping on the morphology of elongated nanostructures has been reported for other oxides. In particular, Sn-doped GeO2 nanowires, grown by the method used in this work, are thinner and show fewer bends than undoped nanowires.14 Also, the presence of Sn in the precursor has been demonstrated to induce marked changes in the morphology of thermally grown ZnO elongated structures.22 Sn-doped ZnO hierarchical nanostructure arrays have been reported by other authors23 and attributed to the presence of Sn. Moreover, the growth of some new nanostructures, such as beak or elbow-like ones involving changes in the growth direction, also should be related to the incorporation of Sn into the In2O3 lattice. The XRD spectra (Figure 3) of the treated samples show no presence of residual indium nitride precursor and only In2O3 peaks are observed, which reveals a complete oxidation of the indium resulting from decomposition of InN into indium oxide
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Figure 4. Secondary electron (a) and EDX compositional mappings of Sn (b) and In (c) acquired on an individual ITO arrow from a sample grown at 650 °C.
Figure 3. XRD spectrum acquired on a sample treated at 650 °C.
for all the samples. From the analysis of the XRD spectra, a lattice parameter of 10.135 Å has been estimated, which means an increase with respect to the undoped In2O3 one (10.117 Å). It is well accepted that Sn4+ substitutes In3+ in the ITO compound, and according to their ionic radii (0.71 and 0.81 Å for Sn4+ and In3+, respectively), a lattice contraction should be expected. However, in our case tin doping generates a lattice expansion. In the bixbyite crystal structure of indium oxide there are two nonequivalent cation sites denoted as b and d cations.24 The tin substitution for indium on b and d sites depends on tin content. The preference for tin to occupy the d sites is observed for doping levels lower than 8 atom %, above this doping level the b site is preferred.25 Tin doping creates a charge imbalance due to the different valence of tin and can be compensated by the incorporation of oxygen interstitials Oi. Franck and Ko¨stlin26 proposed charge neutral clusters containing tin atoms and oxygen interstitials formed during the synthesis. Gonzalez et al.24 calculated the local geometry and energetics of these point defects and concluded that energetics are most favorable when substitutional Sn and interstitial oxygens are in closer proximity than in the clusters proposed by Franck and Ko¨stlin. In particular, the Sn(d) is preferred over the Sn(b) as a first nearest neighbor of oxygen interstitials, while the prevalence of Sn(b) increases as a second nearest neighbor. These calculations are confirmed by experimental observations based on Mo¨ssbauer spectroscopy and high-resolution synchrotron X-ray diffraction experiments.19 Therefore the changes in lattice parameter observed for our ITO samples, also reported by other authors,18,26 are influenced by the cation distribution and the repulsive forces between Sn ions incorporated in In positions, the incorporation of interstitial oxygens, and the formation of neutral clusters partially shielding the neighboring charge. For low doping levels the first two effects are dominant leading to an unexpected expansion of the lattice, while for Sn levels above 8 atom %, the third effect dominates, explaining the contraction of the lattice observed in highly doped samples. As a difference with some other works, neither traces of rhombohedral structure nor In4Sn3O12 compounds have been observed in the XRD spectra of our samples. The formation of stannates such as In4Sn3O12 requires high temperatures (more than 1300 °C) during the thermal process. The low temperatures used in our case, due to the presence of InN in the precursor mixture, inhibit the growth of this stannate. The detailed study by EDS in the SEM of the samples grown at 650 °C with the higher amount of nanostructures enabled regions presenting a high concentration of nanostructures as well as bulk regions to be analyzed. Moreover in order to avoid the contribution of the bulk to the measured signal, some individual
structures removed from the surface also have been probed. EDS mapping images for Sn and In presented in Figure 4 for an individual arrow show that the averaged compositional signals coming out from the entire volume of the arrow are constant along the growth direction within the limitations imposed by the image resolution. The cation percent of Sn (calculated as Sn/(In + Sn) % from EDS measurements) is around 6 cation % for individual nanostructures, corresponding nominally to a 2.6 atom % Sn doping level. To conserve neutrality, one oxygen interstitial per two tin atoms should incorporate into the lattice following the stoichiometry of In2-xSnxO3+x/2, which makes nominally 1.3 atom % of interstitial oxygens. To investigate the effect of the incorporation of tin to the nanostructures, photoemission spectra were measured by XPS in samples grown at 650 °C. Spectromicroscopy methods using X-ray sources and detection of photoelectrons are surface sensitive and provide information on the chemical state and electronic structure of the surface, which is an important factor that determines the properties of the nanostructures. The results are summarized in Figure 5. The C (1s) peak (284.6 eV) from residual carbon has been used to calibrate the peak positions. Significant differences have been observed between the spectra of the Sn (3d), In (3d), and O (1s) core levels of the pellet surface and the nanostructures (Figure 5a). XPS spectra show that the nanostructures exhibit a higher In/Sn ratio as compared to the pellet (5.2 and 1.5, respectively). These tendencies are in agreement with the EDS results, although the ratios measured by both techniques are quantitatively different. This fact is related to the volume of the sample probed with each technique: for EDS the entire volume of individual structures is probed, whereas for XPS the signal generation is limited to several atomic layers of the surface. Therefore the quantitative differences measured by EDS and XPS should be attributed to an inhomogeneous composition along the radial direction of the structures. As the temperature of the growth treatment is only of 650 °C, the tin oxide of the precursor mixture does not evaporate in high concentrations as compared to the In obtained from the indium nitride, whose decomposition is very effective at this temperature. This effect is associated with a low Sn content on the nanostructures, which is compensated by the growth of an Sn-rich compound at the pellet surface. The reverse situation has been reported in a previous work9 on ITO structures grown at high temperatures (1350 °C). In that case, the pellet surface consists of an In-rich compound whereas the elongated structures formed on the surface mainly consist of tin oxide doped with indium, due to the high temperatures used during the treatment. The XPS spectra show binding energies of 444.6 eV for In (3d5/2), 487.0 eV for Sn (3d5/2), and two values, 530.2 and 532.0 eV, for O (1s), in agreement with previously reported results in different indium-tin oxide systems.9,27 According to the model
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Figure 5. (a) XPS spectra acquired on the pellet surface (grown at 650 °C) and the nanostructures showing In (3d), Sn (3d), and O (1s) core levels. (b) XPS spectra of the O (1s) core level resolved into two components.
of Han et al.,28 tin incorporates into two different configurations in the In2O3 lattice, as electrically active (Sn donors associated to Sn4+) and electrically inactive defects (related to the presence of Sn2+). Only the first ones should be present in our samples, as the XPS Sn (3d) spectrum (not shown here) consists of one single peak centered in 487.0 eV corresponding to the Sn4+ state. The peak form and position indicates that no significant chemical reduction of the tin cation to Sn2+ has occurred. The peak position related to the In (3d5/2) corresponds also to the cation In3+, with no presence of reduced species In+ or metallic indium at the surface. The 530.2 eV band is attributed to oxygen (1s) in indium oxide. The relative intensities of the two O (1s) components (Figure 5b) depend on the region probed, being the 532.0 eV component more intense in regions from the surface without (or with a lower) the presence of nanostructures. The higher energy contributions (532.0-533.0 eV) have been related to the presence of O-H, or amorphous In2O3. However, recently Kim et al.29 associated the increase of this peak with the adsorption and/or incorporation of peroxo species (O2)2near the ITO surface. A contribution in this energy region (531.5 eV) has also been related to oxygen in oxygen-deficient regions (i.e., they do not have neighboring In atoms with their full complement of six nearest-neighbor O2- ions).30,31 In our case the nanostructures present a cleaner surface or/and a less defective surface than the bulk material related to the reduced incorporation of tin. CL spectra and images have been recorded at 80 K in order to analyze the luminescence from the ITO nanostructures and the surface of the samples. The spectra acquired in the nanostructures obtained from samples grown at 650 °C show a characteristic CL band centered at 1.9 eV, while the surface without nanostructures shows in addition a band at 2.58 eV (Figure 6). This luminescence centered at 1.9-2.0 eV has been previously reported and attributed to oxygen deficiency.32,9 Kundu et al.33 attribute emissions at 2.56 and 2.34 eV to excitons trapped at the defect centers in ITO; however, these luminescence emissions have been previously observed in ITO microstructures and related to the presence of both SnO2 and In2O3 oxides.9 CL images also have been recorded in some individual arrowlike structures removed from the surface (Figure 7) in order to analyze their luminescent behavior, avoiding any possible contribution from the surface. As the CL spectra characteristic of these nanostructures present only one emission centered at 1.9 eV, the CL image can be considered as monochromatic. This CL image shows a contrast variation along the arrow, which
Figure 6. Normalized CL spectra acquired on the nanostructures and the pellet surface of a sample grown at 650 °C, showing two main components centered at 1.9 and 2.58 eV.
Figure 7. SEM (a) and CL (b) images obtained from one individual arrow removed from the pellet grown at 650 °C.
could indicate a different distribution of defects incorporated during the growth process. Conclusions Thermal treatment of compacted mixtures of InN and SnO2 powders at temperatures in the range 650-750 °C under argon flow causes decomposition of the nitride and formation of arrowshaped ITO micro- and nanostructures with Sn content of about 2.6 atom %. Oxygen interstitials are also formed in the lattice to maintain charge neutrality. XPS shows the 487 eV peak corresponding to the Sn4+ state. Cathodoluminescence of the ITO structures is dominated by a 1.9 eV band. Acknowledgment. This work was supported by MEC Project No. MAT2006-01259. References and Notes (1) Ginely, D. S.; Bright, C. MRS Bull. 2000, 25, 15.
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