918
J. Phys. Chem. C 2008, 112, 918-925
A New Preparation Pathway to Well-Defined In2O3 Nanoparticles at Low Substrate Temperatures Harald Lorenz,† Michael Sto1 ger-Pollach,‡ Sabine Schwarz,‡ Kristian Pfaller,§ Bernhard Klo1 tzer,† Johannes Bernardi,‡ and Simon Penner*,† Institute of Physical Chemistry, UniVersity of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, UniVersity SerVice Centre for Transmission Electron Microscopy (USTEM), Vienna, UniVersity of Technology, Wiedner Hauptstrasse 8-10/052, A-1040, Vienna, Austria, and Section of Histology and Embryology, Medical UniVersity Innsbruck, Mu¨llerstrasse 59, A-6020 Innsbruck, Austria ReceiVed: September 13, 2007; In Final Form: NoVember 6, 2007
The structure and morphology of thin In2O3 films deposited on NaCl(001) single-crystal cleavage faces and thin sapphire sheets has been studied by transmission electron microscopy techniques and scanning electron microscopy. Both the substrate temperature and the nature of the substrate proved to be crucial for the formation of crystalline and well-defined In2O3 structures and morphologies. For In2O3 films grown on single-crystal NaCl(001) faces, a gradual transition from a quasi amorphous small-grain structure after deposition at 298 K to well-oriented In2O3 particles predominantly exhibiting pyramidal and octahedral shapes after deposition at 603 K has been observed. At the highest substrate temperatures, most particles were found to be oriented along a [001] zone axis and few particles in a [011] orientation. In contrast, depositing In2O3 on sapphire sheets under otherwise identical experimental conditions does not result in well-facetted In2O3 particles, but rather leads to rounded In2O3 agglomerates with a rough surface structure.
1. Introduction Because of its outstanding physicochemical properties as a transparent conductive oxide, In2O3 is renowned, for example, for its applications in electron devices or gas sensors.1-6 As these properties are strongly influenced by the preparation procedure of the In2O3 films,7 much effort has been placed into the development of methods allowing for easy tuning of the In2O3 structure and hence its material properties. These methods comprise chemical evaporation techniques,7,8 sputtering,9 laser deposition10 and spray pyrolysis,11 thermal oxidation of predeposited In,12 and electrochemical deposition.1 Recently, the preparation of low-dimensional In2O3 nanostructures (nanocrystals, nanowires, etc.) has gained increased attention as these structures are promising candidates for applications in chemical and biological sensors, too.13,14 As the detailed knowledge about the structure and morphology of the In2O3 films is a prerequisite for the understanding of their physicochemical properties, a number of studies have been performed on the structural characterization of differently prepared In2O3 films.7-22 Electron microscopy techniques have especially proven to be a powerful and versatile tool to examine structure, morphology, and chemical composition of In2O3 films.7,13,14,21-24 On the basis of previous studies on the influence of different preparation parameters (e.g., substrate temperature,16,21 oxygen background pressure,21 material of evaporation source in case of thermal evaporation)24 on the structure of the resulting In2O3 films, we aimed at a detailed investigation of the influence of the substrate temperature and substrate material on the structure and morphology of thermally deposited In2O3 films by transmis* To whom correspondence should be addressed. E-mail: simon.penner@ uibk.ac.at. Tel: 0043 512 507 5056. Fax: 0043 512 507 2925. † University of Innsbruck. ‡ University of Technology Vienna. § Medical University Innsbruck.
sion electron microscopy (TEM) techniques. Two substrate materials were studied in detail, namely vacuum-cleaved, crystallographically well-defined NaCl(001) cleavage faces and thin sapphire sheets. The substrate temperature will be varied between 298 and 600 K on either material. As it will be shown in detail below, this allowed us to induce a different growth of In2O3 on either material and opened a new pathway to the preparation of well-structured and well-defined In2O3 nanoparticles at surprisingly low substrate temperatures. 2. Experimental Methods All In2O3 films were prepared in a high-vacuum chamber operating at a base pressure of 10-4 Pa. Film thickness (usually 25 nm) was measured in situ by a quartz crystal microbalance. In2O3 powder (Indium (III) oxide, 99.99%, Alfa Aesar) was thermally evaporated from a tantalum crucible onto either vacuum-cleaved NaCl (001) surfaces or thin sapphire sheets at varying substrate temperatures (298-600 K) in 10-2 Pa O2 background pressure. A comment concerning the evaporation source should be added here: Although tungsten boats are often considered the best evaporator crucible for In2O3,25 they cause the serious drawback of being oxidized by the oxygen In2O3 loses upon heating. As the so-prepared tungsten oxides are highly volatile and exhibit a comparable vapor pressure as In2O3, serious contamination of the deposited In2O3 films takes place. This phenomenon has been documented by Nakao et al.24 A strong dependence of the film structure on the evaporation material (pure W boat versus Al-coated W crucible) has been observed. We experienced the same problems upon evaporating Ga2O3 from W boats. However, we found the method of choice to be evaporation from Ta crucibles (Ta oxides are much less volatile), which leads to clean and reproducible Ga2O3 and In2O3 films, as verified by regular XPS elemental analysis. In2O3 films
10.1021/jp077354y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
Well-Defined In2O3 Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 4, 2008 919
Figure 1. Overview TEM image of the In2O3 thin film after deposition in 10-2 Pa O2 at 298 K (a) and after irradiation by the electron beam in the electron microscope for 10 min (b). The corresponding SAED patterns are shown as insets. Important reflections have been marked.
Figure 2. High-resolution electron micrographs of the bright (a) and the dark (b) regions of an In2O3 thin film oxidized in 1 bar O2 at 473 K.
Figure 3. Overview TEM image of an In2O3 film deposited in 10-2 Pa O2 at 298 K subsequently oxidized in 1 bar O2 at 473 K (a) and the same area imaged by the HAADF method (b).
deposited onto NaCl(001) faces were floated and rinsed with distilled water, dried, and finally mounted on gold grids for electron microscopy. Annealing and oxidative treatments (1 bar He or O2 for 1 h at 473 K, respectively) were performed in a circulating batch reactor. The structure and morphology of these In2O3 thin films in the as-deposited state and upon oxidative and annealing treatments were monitored by TEM. Overview TEM imaging and selected area electron diffraction (SAED) were carried out with a ZEISS EM 10C microscope. The electron diffraction patterns were externally calibrated with
respect to the (111), (200), and (220) Pd reflections of an asdeposited, untreated Pd/Ga2O3 catalyst. High-resolution imaging (HRTEM), electron-energy loss spectroscopy (EELS), highangle annular dark-field imaging (HAADF), and weak-beam dark-field imaging were performed using a 200 kV FEI TECNAI F20 S-TWIN Analytical (scanning) transmission electron microscope (S)TEM equipped with a Gatan GIF 2001. The film composition was checked by energy-dispersive X-ray analysis. Basically, only peaks due to the evaporated elemental thin film constituents (In and O) and the gold grid (Au) were detected.
920 J. Phys. Chem. C, Vol. 112, No. 4, 2008
Lorenz et al.
TABLE 1: Interplanar Distances dhkl [Å] Measured on e-Beam Irradiated Amorphous In2O3 Films and on In2O3 Films Deposited at 373 and 600 K, Respectivelya In2O3 e-beam irradiated Assignment
In2O3 deposited at 373 K Assignment
In2O3 deposited at 600 K Assignment
d(hkl)exp
d(hkl)theor
lattice plane
d(hkl)exp
d(hkl)theor
lattice plane
d(hkl)exp
d(hkl)theor
lattice plane
7.199 5.039 4.156 3.568 3.225 2.921 2.706 2.536 2.386 2.265 2.156 2.067 1.996 1.849 1.784 1.738 1.680 1.646 1.594 1.557 1.527 1.493
7.155 5.059 4.131 3.577 3.200 2.921 2.704 2.530 2.384 2.262 2.157 2.065 1.984 1.847 1.788 1.735 1.686 1.641 1.599 1.561 1.525 1.491
110 200 211 220 310 222 321 400 411 420 332 422 431 521 440 433 600 611 620 541 622 631
7.124 5.039 4.114 3.536 3.200 2.921 2.706 2.536 2.386 2.265 2.156 2.067 1.986 1.849 1.784 1.730 1.680 1.646 1.606 1.563 1.527 1.488
7.155 5.059 4.131 3.576 3.200 2.921 2.704 2.530 2.384 2.262 2.157 2.065 1.984 1.847 1.788 1.735 1.686 1.641 1.599 1.561 1.525 1.491
110 200 211 220 310 222 321 400 411 420 332 422 431 521 440 433 600 611 620 541 622 631
7.093 5.053 4.168 3.578 3.234 2.930 2.713 2.543 2.392 2.284 2.163 2.063 1.967 1.833 1.784 1.716 1.673 1.632 1.594 1.557 1.513 1.482
7.155 5.059 4.131 3.576 3.200 2.921 2.704 2.530 2.384 2.262 2.157 2.065 1.984 1.847 1.788 1.735 1.686 1.641 1.599 1.561 1.525 1.491
110 200 211 220 310 222 321 400 411 420 332 422 431 521 440 433 600 611 620 541 622 631
a
A correlation of the experimentally determined lattice spacings to the body-centered cubic In2O3 structure is also shown.
Different amounts of Na (but always less than about 3%) were sometimes detected, too. However, Cl was never observed. Surface carbon impurities present on the films were removed by plasma cleaning using Ar+ ions prior to TEM imaging. The purity of the substrate was ensured by freshly cleaving the NaCl(001) crystals immediately before deposition of the oxide. Scanning electron microscopy (SEM) was used to characterize the surface structure of In2O3 films grown on both NaCl(001) faces and thin sapphire sheets. All SEM experiments were conducted using an SM 982 GEMINI ZEISS field emission scanning electron microscope. Prior to SEM imaging, the samples were coated with 5 nm Au/Pd to improve its conductance and were fixed with conducting carbon paste.
changed very much, although some darker areas can easily be detected in the TEM images. Moire´ fringes in the darker domains already indicate a transformation to a more crystalline structure, which is further corroborated by the SAED patterns (inset). For the darker areas, a Debye-Scherrer type ring pattern of poorly oriented bcc In2O3 is usually observed. Two reflections have been indexed, and a detailed attribution of the measured distances to lattice spacings of the cubic In2O3 structure is given in Table 1. From the SAED analysis, we conclude that the darker domains are associated with a crystalline, single bcc In2O3 phase. The brighter areas are still attributed to amorphous In2O3, as confirmed by the SAED patterns. It is worth noting that a comparable structure of brighter and darker grains can also be prepared by post-oxidizing the amorphous In2O3 samples in 1
3. Results 3.1. Structure and Morphology of In2O3 Thin Films Deposited at 298 K. Figure 1a shows an overview TEM image of the as-deposited, untreated In2O3 film grown on NaCl(001) cleavage faces at 298 K. The structure consists of an array of interconnected, round-shaped grains exhibiting average diameters between 15 and 20 nm. However, the corresponding SAED pattern (inset) reveals an almost amorphous structure with one broad Debye-Scherrer-type ring reflection and one faint and also broadened diffraction ring at ∼2.7 and 1.7 Å. These are attributed to the (321) and (600) reflections of the body-centered cubic (bcc) In2O3 structure. These observations are in agreement with previous X-ray diffraction (XRD) studies on the influence of the substrate temperature on the crystallinity of In2O3 and Sn-doped In2O3 films prepared by radio frequency-sputtering, flash evaporation, and spray pyrolysis.16,22,26 The film structure also closely resembles atomic force microscopic and Z-contrast images of Sn-doped In2O3 films deposited by radio frequency sputtering at 300 K.26 The In2O3 film was observed to be prone to crystallization if illuminated by the electron beam in the electron microscope for a longer time, typically after 10-15 min. Figure 1b depicts the film structure of Figure 1a after irradiation by the electron beam for about 10 min. The overall grain structure has not been
Figure 4. In M4,5 and O K EELS spectra of the In2O3 thin film after various treatments: (a) an amorphous region of an untreated In2O3 film deposited in 10-2 Pa O2 at 298 K, (b) a crystalline region of an In2O3 film deposited in 10-2 Pa O2 at 298 K subsequently oxidized in 1 bar O2 at 473 K, (c) a bright grain of the film shown in Figure 3a, (d) a dark grain of the film shown in Figure 3a, (e) a bright area in the HAADF image shown in Figure 3b, (f) a dark area in the HAADF image shown in Figure 3b, and (g) an In2O3 thin film deposited in 10-2 Pa O2 at 600 K.
Well-Defined In2O3 Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 4, 2008 921
Figure 5. Overview TEM images of an In2O3 thin film deposited in 10-2 Pa O2 at 373 (a) and 473 K (c). The corresponding SAED patterns are shown in (b) and (d), respectively.
Figure 6. Overview TEM image of an In2O3 thin film deposited in 10-2 Pa O2 at 600 K (a) with the corresponding SAED pattern shown in (b).
bar O2 at 373-473 K. The question now arises how to interpret these darker domains. A similar structure with darker grains has been observed by Nakao et al. after vacuum evaporation of In2O3 onto KBr pellets without oxygen background pressure. SAED patterns revealed the simultaneous presence of both In2O3 and metallic In.24 We, however, can exclude the presence of metallic In after the analysis of the diffraction patterns, and it appears highly unlikely that metallic In (if any has been present during the preparation procedure) persists after the abovementioned oxidation treatment. Figure 2a,b shows two HRTEM images of a brighter (a) and a darker grain (b) of a film subjected to an oxidative treatment at 473 K. The most common lattice
fringe distance that has been observed on both areas is ∼2.9 Å, corresponding to the most intense reflection in the electron diffraction patterns, and is attributed to the (222) lattice spacing of the bcc In2O3 structure. This strongly points to the fact that after the oxidative treatment both the brighter and the darker areas can be associated with crystalline cubic In2O3. The chemical composition of the films was further checked by using the energy loss near edge structure (ELNES) fingerprint method (Figure 3). The In M4,5 and O K ELNES have routinely been collected for In2O3 films on different areas and after various treatments. Spectra a-d of Figure 3 correspond to the amorphous In2O3 film, an amorphous film subsequently oxidized in
922 J. Phys. Chem. C, Vol. 112, No. 4, 2008
Figure 7. High-resolution TEM image of a coalesced In2O3 grain oriented along a [001] zone axis. The respective FFT is shown as an inset.
Figure 8. High-resolution TEM image of an octahedral In2O3 particle oriented along a [011] zone axis. The respective FFT is shown as an inset.
1 bar O2 at 473 K and a bright and a dark area of an oxidized In2O3 film, respectively. Both the In M4,5 and the O K ELNES resemble typical In2O3 spectra,14,27 and one does not differ substantially from another. Hence, we conclude that (i) the stoichiometry after deposition is already close to In2O3 and (ii) that both the bright and the dark areas also exhibit stoichiometries close to In2O3. Comparing spectra c and d, we also rule out a significant contribution from thickness variations between the brighter and the darker areas, because the overall intensity of the spectra is more or less the same. Thus, the contrast difference between the darker and the brighter areas is not primarily due to mass contrast. To further clarify the nature of the darker areas, high-angle annular dark-field (Z-contrast) imaging has been performed on the film subjected to an oxidative treatment in 1 bar O2 at 473 K. Figure 4a shows a
Lorenz et al. bright-field TEM overview image, and Figure 4b shows the corresponding Z-contrast image recorded by using the HAADF detector in scanning mode of the (S)TEM. As this method would allow the detection of In-depleted or In-richer areas in this case, we would expect the darker areas of the bright-field image to be different in contrast also in the Z-contrast images, if the darker areas in the bright-field images are enriched or depleted in In. However, exactly the opposite is observed. The Z-contrast image does not show any contrast difference between the brighter and the darker area in the corresponding bright-field images, but rather an even contrast distribution is observed. EELS spectra taken on the brighter and darker areas in the Z-contrast image (Figure 3e,f, respectively) do also not exhibit substantial differences in intensity and the energy loss near edge structure. By comparing the ELNES a-f of Figure 3, we conclude that the contrast variations in the bright-field TEM images are also not due to differences in the chemical composition but are caused only by diffraction contrast. 3.2. Structure and Morphology of In2O3 Thin Films Deposited at Temperatures 298 < T e 600 K. To obtain more regular and well-defined In2O3 particles, the substrate temperature was increased in steps to 600 K. Figure 5a and c show the evolution of the structure of the In2O3 film deposited at 373 and 473 K, respectively. As it is immediately obvious by comparing Figure 5a and Figure 1a, there has been a rather drastic transformation of the In2O3 film morphology. Some rounded grains can still be detected, although they have considerably increased in size with average diameters of about 30 nm. Moire´ fringes are sometimes observed, too, indicating a crystalline In2O3 film. Apart from the rounded aggregates, some elongated patches are observed, obviously formed by coalescence of neighboring particles. This points to an enhanced mobility of In2O3 and material transport already at these rather low temperatures. The corresponding SAED pattern (Figure 5b) in turn can be interpreted as a crystalline, single bcc In2O3 phase with still no pronounced ordering yet. A detailed addressing of the experimentally determined lattice distances to the theoretical lattice spacings of the bcc In2O3 structure is outlined in Table 1. A similar trend has been observed by Kaleemulla et al. for flash-evaporated In2O3 films. On the basis of XRD studies, they showed that in contrast to films deposited at 300 K films prepared at 373 K and above are crystalline in nature.16 Raising the substrate temperature to 473 K again causes a change in the In2O3 film structure and morphology (Figure 5c). The rounded In2O3 particles have almost completely vanished and have been replaced by rather sharp-edged particles exhibiting square, triangular, and sometimes rectangular outlines. The SAED patterns (Figure 5d), however, have not changed as compared to the 373 K substrate temperature and still consist of a set of ring reflections arising from the bcc In2O3 structure. This trend of structural and morphological transformation to particles with well-defined outlines is continued and is further pronounced if the substrate temperature is raised to about 600 K (Figure 6a). The TEM overview image reveals well-defined In2O3 particles with square, triangular, and sometimes quasirhombohedral outlines. Particle sizes, measured along the longest edge-to edge distance, range between 25 and 40 nm for particles with square and about 45-50 nm for particles with rhombohedral outlines. The morphological alterations are accompanied by corresponding changes of the SAED patterns (Figure 6b). Although some particles remain poorly oriented, a preferred orientation for many particles is now observed in contrast to the diffraction patterns obtained at lower substrate temperatures.
Well-Defined In2O3 Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 4, 2008 923
Figure 9. Bright-field (a) and weak-beam dark-field image (b) of a single pyramidal-shaped In2O3 particle obtained after deposition of In2O3 in 10-2 Pa O2 at 600 K.
Figure 10. Scanning electron microscopic images of a In2O3 film deposited in 10-2 Pa O2 at 600 K onto a vacuum-cleaved NaCl(001) face (a) and a thin sapphire sheet (b).
A pronounced 4-fold splitting of the bcc In2O3 (110), (200), (220), (400), and (440) reflections is observed with the splitting of the (110) reflection being the least pronounced. As expected, the (200) and the (400) spots are in parallel aligned and rotated by 45° against the (110), (220), and (440) reflections. Hence, most particles are oriented along a [001] zone axis and expose a (001) basal plane perpendicular to the electron beam. HRTEM images confirm this preferred orientation of the In2O3 particles. Figure 7 shows a part of a three-particle agglomerate with lattice fringes of ∼2.5 Å, corresponding to the (400) lattice distance of the bcc In2O3 structure, extending along two perpendicular directions. The fast fourier transform (FFT) of the particle (inset) reveals a diffraction pattern of a bcc crystal oriented along the [001] beam direction. To the right, a coalesced second [001] oriented In2O3 particle is clearly visible. In contrast, Figure 8 depicts a HRTEM image of a more well-shaped, rhombohedral In2O3 particle. Lattice fringes of ∼5.1and ∼7.2 Å are observed in two perpendicular directions, corresponding to the (200) and (01h1) lattice planes of the bcc In2O3 structure. Its FFT reveals the [011] orientation of the particle. The In2O3 stoichiometry is further confirmed by the corresponding EELS spectrum (Figure 3g). So far only the orientation and the chemical composition of the particle have been considered. However, for a detailed analysis of the In2O3 film morphology, the three-dimensional
habitus of the individual particles has to be determined. As it has been shown in a range of previous publications, the weakbeam dark-field technique of Yacaman et al.28 is a powerful method to reveal three-dimensional (3D) particle shapes. It has especially proven to be an excellent method for determining the 3D structures of small, well-oriented metal particles.29 We have taken advantage of this method to study the shapes of individual In2O3 particles. Figure 9a,b shows a representative bright-field and weak-beam dark-field image of a single particle with square outline, respectively. Figure 9b unambiguously reveals the pyramidal or octahedral habitus of this individual In2O3 particle. In addition, two particles with a less pronounced and defined shape are visible to the top and to the right of the central pyramidal particle. 4. Discussion It is useful to compare our results to previous studies on the preparation and characterization of nanoparticulate In2O3. As already mentioned above, different forms like In2O3 nanowires, nanobelts, nanosheets, and nanocrystals have been prepared by the so-called vapor-liquid-solid mechanism on crystalline Si substrates using gold catalysts, by a catalyst-free vapor-solid mechanism on crystalline silicon substrates or also by a catalystfree physical vapor deposition of In metal in a wet oxidizing
924 J. Phys. Chem. C, Vol. 112, No. 4, 2008 atmosphere.14,30-34 The common feature of all the mentioned methods is the rather high substrate temperature (usually between 873 and 1573 K). Most interesting for the results discussed in the present contribution is the preparation and characterization of well-defined indium oxide octahedrons and pyramids. Jia et al.32 focused on the formation of well-aligned, tetragonal, single crystalline In2O3 pyramids prepared on Nicoated Si(100) by chemical vapor deposition. The formation of In2O3 octahedrons on a Si single crystal through deposition of In metal under wet oxidizing conditions was reported by Hao et al.33 and was tentatively explained on the basis of surface energy relationships in combination with a high supersaturation ratio of oxidized In in case that metallic In was used as a source material. Octahedrons oriented along the [110], [100], and [111] beam directions were observed, owing to rhombohedral, square, and hexagonal particle outlines in the bright-field TEM images. It is worth noting that for our preparation at 600 K almost no octahedrons oriented along the [111] beam direction are encountered (i.e., “hexagonal” particles), but the structure is mostly made up by octahedrons oriented along the [001] and [011] beam directions. This can be tentatively explained in terms of the use of single-crystalline NaCl(001) cleavage faces mediating the preferential growth of bcc In2O3. This behavior bears some similarities to the growth of small well-defined and well-oriented face-centered cubic (fcc) noble metal particles (Pt, Rh, Pd, Re, Ir) on NaCl(001) cleavage faces at elevated substrate temperatures (573-623 K).29 For these metals, the close structural and crystallographic relationship between the single cubic NaCl structure and the fcc noble metal structure caused the formation of mostly half-octahedral and half-tetrahedral particles typically oriented along the [001] beam direction and sharing a (001) plane with the underlying NaCl substrate. However, this behavior is not limited to noble metal particles but can be extended to oxides like CeO2, VO, or V2O3.35,36 The former two also crystallize in the fcc structure, the latter in a rhombohedral lattice. Nevertheless, although highly oriented oxide structures could be observed, the preparation did never lead to well-shaped particles like the ones obtained for In2O3. The necessity to use well-defined NaCl(001) cleavage faces to grow the octahedral nanoparticles is further corroborated by the comparison of the SEM images of In2O3 particles grown at 600 K on NaCl(001) faces (Figure 10a) and on thin sapphire sheets (Figure 10b). The SEM images of the particles grown on NaCl(001) nicely show the smooth surface structure of the differently oriented octahedra. In contrast, more rounded particles with a rough and porous surface structure are observed on sapphire. Similar structures have been obtained for electrostatically spraypyrolyzed In2O3 deposited onto Pt-coated alumina at 673 K, where rounded particles fragmented into tiny aggregates have been observed. This behavior was explained by exceeding the Raleigh-limit for the maximum attainable charge density for a liquid droplet and the subsequent disrupture into smaller droplets.18 By comparing panels a and b in Figure 10, we conclude that well-shaped In2O3 crystallites are only obtained in the case of well-ordered nuclei for crystallization being present on and mediated by NaCl(001) during In2O3 growth. These are apparently missing if In2O3 is grown on sapphire. Hence, a less regular growth is observed. It seems, however, that the mobility of the growth species is comparable on both substrates because the size of the resulting In2O3 entities is very similar on NaCl(001) and sapphire. It is also worth noting that epitaxial In2O3 thin films have been successfully grown on MgO(001) single-crystal substrates by Sieber et al. at temperatures between 873 and 1123 K21 and
Lorenz et al. by Tarsa et al. at substrate temperatures between 673 and 723 K.37 Cube-to-cube MgO[100] // In2O3 [100] and MgO // In2O3 [11h0] orientations were found to be dominating. Reduced misfit relations between MgO and In2O3 lattice planes have been found to account for the preferential growth. A similar argument can be stressed for the growth of In2O3 on NaCl(001). The lattice mismatch between the NaCl(001) plane (aNaCl ) 5.64 Å) and the (200) lattice plane of bcc In2O3 (dtheor(200) ) 5.06 Å) is about 11%. Hence, In2O3 grows on NaCl(001) preferentially in the relationship In2O3 [001] // NaCl [001]. 5. Conclusions We have demonstrated a novel catalyst-free, epitaxy-promoted preparation pathway to well-defined and well-oriented pyramidal and octahedral In2O3 particles at a surprisingly low-substrate temperature of about 600 K. This is in striking contrast to previous studies on the preparation of nanoparticulate In2O3, which usually requires much higher substrate temperatures. Tentatively, we explain this behavior by the crystallographic matching of the structures of the cubic NaCl substrate and the bcc In2O3 structure. The unusually low preparation temperature might also have implications for the use of In2O3 nanostructures as nanoscale devices, because it is economically desirable to prepare In2O3 nanostructures for these purposes at the lowest temperatures possible. References and Notes (1) Ho, W. H.; Yen, S. K. Thin Solid Films 2006, 498, 80. (2) Sauter, D.; Weimar, U.; Noetzel, G.; Mitrovics, J.; Go¨pel, W. Sens. Actuators, B 2000, 69, 1. (3) Hana, A. K. J. Photochem. Photobiol. A 2000, 132, 1. (4) Kominami, H.; Nakamura, T.; Sowa, K.; Nakanishi, Y.; Hatanaka, T.; Shimaoka, G. Appl. Surf. Sci. 1997, 113-114, 519. (5) Chung, W. Y.; Sakai, G.; Shimanoe, K.; Miura, N.; Lee, D. D.; Yamazoe, N. Jpn. J. Appl. Phys. 1998, 37, 4994. (6) Ivanovskaya, M.; Gurlo, A.; Bogdanov, P. Sens. Actuators, B 2001, 77, 264. (7) Wang, Ch.; Cimalla, V.; Romanus, H.; Kups, Th.; Niebelchu¨tz, M.; Ambacher, O. Thin Solid Films 2007, 515, 6611. (8) Kane, J.; Schweitzer, H. P. Thin Solid Films 1975, 29, 155. (9) Gagoudakis, E.; Bender, M.; Douloufakis, E.; Katsirakis, N.; Natasakou, E.; Cimalla, V.; Kiriakidis, G. Sens. Actuators, B 2001, 80, 155. (10) Marotta, V.; Orlando, S.; Parisi, G. P.; Giardini, A.; Perna, G.; Santoro, A. M.; Capozzi, V. Appl. Surf. Sci. 2000, 168, 141. (11) Prince, J. J.; Ramamurthy, S.; Subramanian, B.; Sanjeeviraja, C.; Jayachandran, M. J. Cryst. Growth 2002, 240, 142. (12) Das, V. D.; Kirupavathy, S.; Damodare, L. J. Appl. Phys. 1996, 79, 8521. (13) Cheng, Z.; Dong, X.; Pan, Q.; Zhang, J.; Dong, X.-W. Mater. Lett. 2006, 60, 3137. (14) Cheng, G.; Stern, E.; Guthrie, S.; Reed, M. A.; Klie, R.; Hao, Y. Meng, G.; Zhang, L. Appl. Phys. A 2006, 85, 233. (15) Malar, P.; Vijayan, V.; Tyagi, A. K.; Kasiviswanathan, S. Nucl. Instrum. Methods Phys. Res., Sect. B 2005, 229, 406. (16) Kaleemulla, S.; Sivasankar Reddy, A.; Uthanna, S.; Sreedhara Reddy, S. Mater. Lett. 2007, 21, 4309. (17) Cho, J.; Yoon, K.; Koh, S. Thin Solid Films 2000, 368, 111. (18) Ghimbeu, C.; Schoonamn, J.; Lumbreras, M. Ceram. Int. 2008, 34, 95. (19) Korotcenkov, G.; Brinzari, V.; Cerneavschi, A.; Cornet, A.; Morante, J.; Cabot, A.; Arbiol, J. Sens. Actuators, B 2002, 84, 37. (20) Blyth, R. I. R.; Netzer, F. P.; Ressel, R.; Ramsey, M. G. Surf. Sci. 2000, 446, 137. (21) Sieber, H.; Senz, St.; Hesse, D. Thin Solid Films 1997, 303, 216. (22) Golovanov, V.; Ma¨ki-Jaskari, M.; Rantala, T.; Korotcencov, G.; Brinzari, V.; Cornet, A.; Morante, J. Sens. Actuators, B 2005, 106, 563. (23) Ratajczak, J.; Malag, A.; Sobkowicz, W.; Katcki, J. Inst. Phys. Conf. Ser. 1999, 164, 251. (24) Nakao, T.; Nakada, T.; Nakayama, Y.; Miyatani, K.; Kimura, Y.; Saito, Y.; Kaito, C. Thin Solid Films 2000, 370, 155.
Well-Defined In2O3 Nanoparticles (25) Handbook of Thin Film Process Technology; Glocker, D. A., Shah, S. I., Eds.; Institute of Physics Publishing: Phildalphia, PA, 1995. (26) Yan, Y.; Zhou, J.; Wu, X. Z.; Moutinho, H. R.; Al-Jassim, M. M. Thin Solid Films 2007, 515, 6686. (27) Ahn, C. C.; Krivanek, O. L. EELS Atlas, Gatan, Warrendale, USA 1983. (28) Yacaman, M. J.; Ocan˜a, T. Phys. Status Solidi A 1977, 42, 571. (29) Rupprechter, G.; Hayek, K.; Rendon, L.; Yacaman, J. M. Thin Solid Films 1995, 260, 148. (30) Chun, H. J.; Choi, Y. S.; Bae, S. Y.; Park, J. Appl. Phys. A 2005, 81, 539. (31) Pan, Z.; Dai, Z.; Wang, Z. Science 2001, 291, 1947.
J. Phys. Chem. C, Vol. 112, No. 4, 2008 925 (32) Jia, H.; Zhang, Y.; Chen, X.; Shu, J.; Luo, X.; Zhang, Z.; Yu, D. Appl. Phys. Lett. 2003, 82, 4146. (33) Hao, Y.; Meng, G.; Ye, C.; Zhang, L. Cryst. Growth Des. 2005, 5, 1617. (34) Jeong, J. S.; Lee, J. Y.; Lee, C. J.; An, S. J.; Yi, G.-C. Chem. Phys. Lett. 2004, 384, 246. (35) Penner, S.; Wang, D.; Schlo¨gl, R.; Hayek, K. Thin Solid Films 2005, 484, 10. (36) Penner, S.; Wang, D.; Podloucky, R.; Schlo¨gl, R.; Hayek, K. Phys. Chem. Chem. Phys. 2004, 6, 5244. (37) Tarsa, E. J.; English, J. H.; Speck, J. S. Appl. Phys. Lett. 1993, 62, 2332.
