Article pubs.acs.org/Langmuir
Phase Formation Behavior in Ultrathin Iron Oxide Indrek Jõgi,†,∥ T. Jesper Jacobsson,† Mattis Fondell,† Timo Waẗ jen,‡ Jan-Otto Carlsson,† Mats Boman,† and Tomas Edvinsson*,†,§ †
Department of Chemistry - Ångström Laboratory, ‡Department of Engineering Sciences, Solid State Electronics, and §Department of Engineering Sciences, Solid State Physics, Uppsala University, Uppsala 75121, Sweden ∥ Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia ABSTRACT: Nanostructured iron oxides, and especially hematite, are interesting for a wide range of applications ranging from gas sensors to renewable solar hydrogen production. A promising method for deposition of low-dimensional films is atomic layer deposition (ALD). Although a potent technique, ALD of ultrathin films is critically sensitive to the substrate and temperature conditions where initial formation of islands and crystallites influences the properties of the films. In this work, deposition at the border of the ALD window forming a hybrid ALD/pulsed CVD (pCVD) deposition is utilized to obtain a deposition less sensitive to the substrate. A thorough analysis of iron oxide phases formation on two different substrates, Si(100) and SiO2, was performed. Films between 3 and 50 nm were deposited and analyzed with diffraction techniques, high-resolution Raman spectroscopy, and optical spectroscopy. Below 10 nm nominal film thickness, island formation and phase dependent particle crystallization impose constraints for deposition of phase pure iron oxides on non-lattice-matching substrates. Films between 10 and 20 nm thickness on SiO2 could effectively be recrystallized into hematite whereas for the corresponding films on Si(100), no recrystallization occurred. For films thicker than 20 nm, phase pure hematite can be formed directly with ALD/ pCVD with very low influence of the substrate on either Si or SiO2. For more lattice matched substrates such as SnO2:F, Raman spectroscopy indicated formation of the hematite phase already for films with 3 nm nominal thickness and clearly for 6 nm films. Analysis of the optical properties corroborated the analysis and showed a quantum confined blue-shift of the absorption edge for the thinnest films.
1. INTRODUCTION Nanostructured thin films of iron oxide are interesting in a wide range of applications including lithium ion batteries,1,2 water cleaning,3 magnetic materials,4 cancer treatment,5 spintronics,6,7 and gas sensors.8 Hematite, also known in rust, is the thermodynamically most stable phase of iron oxide. Hematite further demonstrates a set of close to ideal properties for photoassisted water splitting which have motivated a lot of research in this direction.9−13 With a band gap of around 2 eV, hematite absorbs in the visible, have a reasonable high absorption coefficient, and represent a stable, nontoxic, abundant, and low-cost material. Crucial for good efficiency in many of the aforementioned applications that depend on efficient charge transport is to have low-dimensional iron oxide, which is a consequence of poor transport properties. The diffusion length of the holes in hematite is in the order of a 2−4 nm,9 and together with a thin depletion layer, this is substantially shorter than the absorption depth. Utilizing a thin iron oxide, on the other hand, gives less optical absorption in the material and limits the light harvesting efficiency in optoelectronic and photocatalytic applications. This can be solved by making low-dimensional hematite via nanostructuration and to orthogonalize the process of absorption and charge transport.12 The charge transport is then effectively performed over the low dimensions in nanoparticles or ultrathin films while many such interfaces in the high surface area nanostructured device give enough optical © 2015 American Chemical Society
absorption for photocatalytic applications and also enough active surface area for gas sensing applications. One approach for obtaining high surface area is template based synthesis, where the iron oxide coating is prepared on a sacrificial template or by direct deposition on highly structured three-dimensional substrates. Atomic layer deposition (ALD) is a powerful method to obtain ultrathin coatings on complex 3D surfaces, and its usability has also previously been proved for the preparation of iron oxides.14−41 The resulting films are however very dependent on the nature of the surface and on the temperature used in the ALD process where a hybrid ALD/ pulsed CVD (pCVD) route is investigated in this work. The additional creation of iron oxide in the gas phase in the pulsed CVD can work in the direction of decreased substrate dependence while keeping the advantageous conformity of an ALD process.22−24,27,28 A more substrate independent deposition is of vital importance as the vast majority of practical applications are prepared on non-lattice-matching substrates. In addition to having low-dimensional materials with high coverage conformity on a large surface area, the phase content of the deposited films are important for the intended applications. Different studies concerning ALD of iron oxides demonstrate that hematite is the dominant phase in asReceived: January 30, 2015 Revised: October 26, 2015 Published: October 27, 2015 12372
DOI: 10.1021/acs.langmuir.5b03376 Langmuir 2015, 31, 12372−12381
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Figure 1. ALD reactor with four different temperature zones, three quartz tubes which separate the precursor pulses with a backflow in the inner quartz tube, which take the iron precursor in the opposite direction during the O2 pulse.
deposited films at substrate temperatures above 250 °C while in some cases hematite was detected even in films deposited at temperatures below 200 °C.16,20,22,26,27,36−38,40,41 The composition of films deposited below 400 °C was often a mixture of different phases, i.e., hematite, maghemite, magnetite, and amorphous phases.16,22,27,28,36,41 However, these results have mainly been obtained for relatively thick films (20−60 nm), whereas in many applications dependent on diffusive charge transport, the film thicknesses should remain below 20 nm due to narrow depletion layer and the short hole diffusion length. One has to stress that conventional X-ray diffraction is not sufficient for unambiguous analysis for ultrathin films with a few dominating peaks and thus must be complemented with other methods in order to properly analyze the crystal phase of low portion phase contents as well as possible amorphous layers. In addition to the substrate temperature, the differences in phase content can be partially explained by the substrate material where for example SnO2 substrate has been observed to favor the growth of the hematite phase from the onset of deposition.26,40 The use of stronger oxidizer (O3 or O2 plasma) as oxygen precursor has shown to increase the amount of hematite27,38 while the presence of reducing agents (e.g., CO) in the purge gas favored the growth of maghemite.41 An additional parameter which may affect the crystalline properties is the film thickness, which is the case of several other metal oxides.42,43 It has also been reported that the preferred phase composition of iron oxide nanoparticles depends on the particle dimensions.4,44,45 The focus of this paper is to investigate how the film thickness, the deposition temperature, and the choose of substrate influence the structural and optical properties of ultrathin layers of iron oxide deposited by a ALD/pulsed CVD method using ferrocene, Fe(Cp)2, as an iron precursor. The temperature for deposition and postdeposition annealing was set to no more than 400 °C in order to comply with the low thermal budget necessary for cost-effective and temperaturesensitive material. At the same time, temperatures above 350 °C are required for the crystallization of hematite phase in the case of nanoparticles.44,45 The study is performed by deposition of a series of successively thicker iron oxide films at two different temperatures on two different substrates, silicon and quartz, representing examples of nonpolar and polar substrates with different lattice parameters. We especially focus on some potential problems for obtaining ultrathin hematite layers with ALD, involving nucleation and island formation as well as phase mixtures and crystallinity in the deposited films on nonlattice matched substrates compared to the situation on lattice matched F:SnO2 substrates. Finally, we analyze the indirect and direct band gaps and briefly discuss indications of quantum confinement with respect to the optical properties which would
be relevant from a technological perspective in photocatalytic applications.
2. METHODS 2.1. Synthesis. Atomic layer deposition of iron oxide was carried out in a lab-built hot-wall reactor of flow type.46 Several control experiments were also carried out with Picosun R150 reactor. The labbuilt reactor consisted of a 40 mm main quartz tube which was inserted in a four-zone furnace (see Figure 1). The first zone was used to evaporate ferrocene, Fe(Cp)2, at 50 °C from a quartz boat inserted in an inner quartz tube with 12 mm diameter. The inner tube was placed into a quartz tube, 20 mm diameter, and always purged by a N2 flow. Fe(Cp)2 was chosen for the suitable evaporation temperature, reasonable growth/cycle, and its high thermal stability17 and how the in house ALD-equipment was built. With different geometry of the ALD reactor and other target temperatures, FeCl3,18,36 Fe(thd)3,14,19,20,25 Fe(acac)3,15,33 Fe(tBuAMD)2, and Fe2(tBuO)631,32 can also be used. As the second precursor, O2 was chosen. The evaporation temperature of ferrocene was held constant at 50 °C. The pressure was kept constant at 3 Torr. During the ferrocene pulse, the N2 carrier gas was directed through the inner tube to the substrates. In the subsequent purging, no precursor was in the reactor, and the N2 flow direction in the inner tube was switched to backflow. Molecular oxygen, O2, was then introduced into the main tube after the first purge, and a second purge pulse followed. The last two heating zones of the reactor were used to heat the movable substrate holders to either 350 or 400 °C. Depositions were made on Si (100), SiO2 (quartz), and F:SnO2 (FTO) coated glass substrates, which were placed on the substrate holders. Pulse lengths for ferrocene and O2 were 4 and 8 s, respectively, whereas 6 s purge times were used between the precursor pulses. The number of applied cycles was varied to obtain films with different thicknesses (3−50 nm) and was typically below 200 cycles. Substrate temperatures of 350 and 400 °C were used in the present study because it has been shown that hematite becomes the dominant phase at around 400 °C.22 This temperature range should facilitate the transition to hematite also for iron oxide nanoparticles and thin films prepared by other methods.44,45 At the substrate temperature of 400 °C, one can expect decomposition of FeCp27,28 and hence obtain a pulsed CVD like growth but with repeatability and conformal growth on high-aspect structures similarly to earlier work.22,23 For some samples, the postdeposition annealing was carried out in an ambient atmosphere at 400 °C for 2 h. The same precursor set, carrier gas, and substrate temperatures were used for the deposition of iron oxide on the Si substrate in the Picosun reactor. Other deposition parameters were altered to adjust for different reactor types. The base pressure was 2.3 Torr in a nitrogen atmosphere whereas the pulse and purge times were 1−12−16−12 s. 2.2. Material Characterization. The phase composition was analyzed by grazing incident X-ray diffraction (GI-XRD) at an incident angle 0.5° using a Siemens D5000 diffractometer. Film thickness was measured by X-ray reflectivity (XRR) using a Siemens D5000 X-ray diffractometer with Cu Kα radiation. The thicknesses were additionally determined from iron content, obtained by X-ray florescence spectroscopy (XRFS), using a Spectrolab X2000 spectrometer. This mass thickness was obtained by calibrating the iron content with thicknesses determined from XRR data for iron oxide films of uniform 12373
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Langmuir thickness. Information about phase compositions was obtained from Raman shifts, determined by a high-resolution confocal Renishaw micro 2000 spectrometer using an argon-ion laser operating at 514 nm. The power of the laser was set to between 0.7 and 5 mW and concentrated to a laser spot of approximately 3 μm on the iron oxide samples. Lower laser intensities were routinely applied to certify that no heat-induced phase transitions occurred. The data were averaged over 20 scans with 30 s duration, and no detectable changes at the film surface or in the Raman signal were observed during the scans. The surface morphology of the films was determined by highresolution scanning electron microscope (SEM), Leo 1550. Highresolution transmission electron microscopy (TEM) was used for the visualization of cross sections of iron oxide films by using a FEI Tecnai F30ST (300 kV) instrument. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a Phi Quantum 2000 device where monochromatic Al Kα (1486.6 eV) was used for sample excitation. For depth profiling, argon ion sputtering with energy 0.5 keV was used. The UV−vis absorption measurements were performed on an Ocean Optics spectrophotometer HR-2000+ with deuterium and halogen lamps. In all measurements, a full spectrum from 190 to 1100 nm with 2048 evenly distributed points was sampled.
XRR or XRFS on quartz substrate were approximately the same for thicknesses above 20 nm. In the case of thinner films, the XRR measurements yielded thicker films as compared to XRFS (17 nm vs 14 nm, for example). The deviation of up to 20% found between the methods can likely be ascribed to the fact that the roughness (3−5 nm) of the films perturbs the constructive interference when the films are thinner than 20 nm. The use of thicknesses measured by XRR showed good correlation with the optical absorption curves, showing that optical cross section in X-ray and in the visible range are linearly related for the different iron oxide polymorphs. 3.1.2. Morphology. SEM micrographs of iron oxide films deposited on Si (100) with native oxide are shown in Figure 2. The micrographs taken from the top surface of the samples reveal separate islands at the sample with 7 nm nominal film thickness. These islands coalesce after applying larger number of ALD cycles. The iron oxide thus appears to start to grow as separate islands and at a nominal film thickness about 10 nm, as determined by XRFS, the islands start to coalesce whereupon compact films are obtained. With increasing film thickness the smaller features with dimensions less than 20 nm also merged into larger ones, 40−50 nm in size, resulting in a decreased film roughness. The results were similar for both the deposition temperatures, 350 and 400 °C, and for the films deposited in the Picosun reactor. Cross-section TEM was performed on one of the samples with a nominal thickness of 18 nm, as determined by XRFS and showed in Figure 3. TEM diffraction at the interface reveals
3. RESULTS AND DISCUSSION 3.1. Iron Oxide Deposition on Si(100). 3.1.1. Thickness Determination. Only film thicknesses above 10 nm were reliably determined by XRR due to the relatively high roughness of the films compared to their thicknesses. The surface roughness was determined by fitting the XRR curves, and the root-mean-square variation of the surface roughness was between 3 and 5 nm for the thickest films measured by XRR. The nominal film thickness was determined from XRFS spectra, a method not sensitive to the roughness of the samples. The integrated intensity of the iron peak in the XRFS spectra was correlated with the thickness of films determined by XRR. The derived correlation coefficient was then used to determine the mass thickness of the iron oxide samples via XRFS as summarized in Table 1. The film thicknesses determined by Table 1. Summary of the ALD Cycles and Thicknesses of the Deposited Films sample
substrate
cycle nos.
thickness (nm)
A B C D E F G H I
Si(100) Si(100) Si(100) Si(100) Si(100) quartz quartz quartz quartz
50 100 150 200 250 25 50 100 200
7 10 18 22 28 6 13 22 50
annealing 400 400 400 400 400 400 400 400 400
°C, °C, °C, °C, °C, °C, °C, °C, °C,
2 2 2 2 2 2 2 2 2
Figure 3. (left) TEM image of polycrystalline iron oxide film deposited at 400 °C and (right) the same TEM image with ellipsoids illustrating the approximate grain boundaries and parallel lines to emphasize the orientation of the grains as seen in close-ups of the TEM image.
h h h h h h h h h
both Si diffraction from the substrate and diffraction from polycrystalline iron oxide particles. Looking closely at the TEM micrographs, one can observe crystalline nanoparticles of iron oxide starting at the substrate−film interface and in many
Figure 2. SEM images of iron oxide films of different thicknesses deposited on silicon at 400 °C: (a) 7, (b) 10, and (c) 22 nm. 12374
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Figure 4. (a) XRD patterns for iron oxide films of different thicknesses deposited on Si (100) substrates at 400 °C. (b) Raman spectra for the corresponding films. Different phases of iron oxides are labeled as H = hematite, M = maghemite, and m = magnetite.
preferential ordering of the crystal planes in the thinnest films. Here, the multiplicity of the [104] diffraction in hematite is expected to be 9 times the intensity of the [113] peak for randomly oriented crystals, indicating random ordering of the hematite grain orientation in the films with 22 and 28 nm thickness, whereas for thinner films, no conclusive intensity analysis can be performed here due to too low intensity of the [113] peak. The intensity for the maghemite and magnetite peaks did not increase once hematite has started to form, indicating that hematite then became the dominant deposition product. Analysis of the broadening of the diffraction via the Scherrer equation and neglecting strain give a grain size estimate of 16 nm in the film nominally determined to 22 nm thickness. Although a coarse estimation, it is in good agreement with the TEM analysis showing grains with dimensions both corresponding to the film thickness as well as smaller grains. Raman spectroscopy was performed for all the samples and supplements the information given from XRD, and data are given in Figure 4b. No crystalline hematite peaks are seen for the thinnest films conforming with the XRD data. In the case of films deposited on F:SnO2, the hematite peaks were visible already for 3 nm thick iron oxide films (see Figure 9). This confirms that the initial iron oxide deposition was not hematite but either amorphous, maghemite, or magnetite. For film thicknesses of 18 nm the peaks corresponding to hematite start to emerge. The hematite peaks gets more pronounced with increasing film thickness, while no increase is seen in peaks attributable to other phases. This correlates well with the XRD data, indicating that hematite starts to be the dominant phase for films thicker than approximately 20 nm, and combined with the information from TEM, the hematite crystallites seem to be formed also from crystallization of amorphous material in earlier cycles. Maghemite and magnetite are hard to separate with XRD due to almost identical lattice distances, but due to difference in the local bonding they are distinguishable by Raman spectroscopy. Unfortunately, many of the magnetite and maghemite peaks in the Raman spectra50,51 coincide with peaks from Si or SiO2. The monotonic growth of the Raman signal between 640 and 690 cm−1 does, however, indicate the presence of magnetite phase in the films, but it is too vague to be used without large uncertainty. Films deposited at 350 °C had essentially the same characteristic features in terms of microstructure, crystallinity, and phase composition as given by XRD and Raman
places penetrating the whole thickness of the film with diffraction lines showing the crystalline particle orientation. One can also see a uniform amorphous layer between the Si(100) substrate and the deposited iron oxide with a thickness of about 1.5 nm, which most likely is a silicon oxide layer existing already before deposition. TEM images for a large part of the cross section show an average film thickness of 20 nm with a standard deviation of the roughness of about 4 nm, with some grains smaller than the film thickness and some grains with the same size as the film thickness. The lateral feature size of approximately 10 nm conforms to the SEM pictures. The relatively uneven film as it appears in the TEM figure thus seems to be a result from the crystallization of particles. The penetration of crystalline grains from the substrate through the films suggests a growth of the film via a Volmer−Weber (VW) mechanism47 where separate particles form and then coalesce. This can be contrasted to the more common Stranski− Krastanov (SK) mechanism48 where a few monolayers of films would form where after particles grow on top of the film. The lattice distances of many of the iron oxides, FeO(111), Fe3O4 (111), α-Fe2O3 (0001), and γ-Fe2O3, are not ideally matching when compared to the 5.4 Å of Si(100) which would give nonideal conditions for either epitaxial growth or initial film formation as in the SK mechanism.49 Here the thin native SiO2 seem to relax the lattice mismatch condition to a small extent, resulting in a VW growth mechanism. 3.1.3. Phase Composition. XRD data for films deposited on Si (100) at 400 °C are given in Figure 4a. For the samples below 10 nm no diffraction peaks are seen, indicating that the initial deposited layer consist of amorphous iron oxide islands. For films thicker than approximately 10 nm, where the initial islands start to coalesce, peaks that could be attributed to either crystalline maghemite or magnetite are seen in XRD. Maghemite could be described as magnetite with vacancies of Fe(II) in the octahedrical sites of the structure and thus have almost the same lattice constants as magnetite, making it hard to distinguish between the two phases with XRD. The relative intensity between the different diffraction peaks vary as a function of film thickness. At around 18 nm hematite starts to emerge and seems to be the dominant phase for thicker films. Films with thickness of 10−18 nm reveal diffraction at 2Θ = 35.61 that can be assigned to either the [110] in hematite or [311] in maghemite whereas the lack of 2Θ = 33.1 and thus a [104] diffraction from hematite indicates the presence of maghemite but also lack the 2Θ = 30.3 peak of maghemite. The lack of peaks and change in relative intensity can also be from 12375
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Figure 5. SEM images of iron oxide films of different thicknesses deposited on quartz at 400 °C: (a) 6 nm; (b) 13 nm; (c) 22 nm; (d) 50 nm; (e) 13 nm annealed at 400 °C for 2 h; (f) 22 nm annealed at 400 °C for 2 h.
spectroscopy. In the case of films deposited by the Picosun reactor, the first signs of hematite appeared at a film thickness of about 20 nm, which is in the limits of uncertainty for the thickness measurement. Postannealing at 400 °C for 2 h did not affect the phase content in the films deposited on Si(100) for either deposition temperature. To obtain ultrathin layers ( epsilon -> beta -> alpha-Phase). J. Am. Chem. Soc. 2009, 131 (51), 18299−18303. (5) Islam, M. S.; Kusumoto, Y.; Abdulla-Al-Mamun, M.; Horie, Y. Synergistic Cell Killing by Magnetic and Photoirradiation Effects of Neck-structured alpha-Fe2O3 against Cancer (HeLa) Cells. Chem. Lett. 2011, 40 (7), 773−775. (6) Eerenstein, W.; Palstra, T. T. M.; Saxena, S. S.; Hibma, T. Spinpolarized Transport Across Sharp Antiferromagnetic Boundaries. Phys. Rev. Lett. 2002, 88, 247204. (7) Nagai, S.; Hata, K.; Okada, M.; Mimura, H. Verwey Transition in Spin Polarization of Field-emitted Electrons from < 1 1 0 > -oriented Single Crystal Magnetite Whisker. Appl. Surf. Sci. 2009, 256 (4), 1058−1060. (8) Gou, X. L.; Wang, G. X.; Kong, X. Y.; Wexler, D.; Horvat, J.; Yang, J.; Park, J. Flutelike Porous Hematite Nanorods and Branched 12380
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O-2 Precursor Combination. Chem. Vap. Deposition 2003, 9 (1), 21− 25. (47) Volmer, M.; Weber, A. Keimbildung in Ü bersättigten Gebilden (Nucleation of Supersaturated Structures). Z. Phys. Chem. 1926, 119, 277−301. (48) Stranski, I. N.; Krastanow, L. Abhandlungen der MathematischNaturwissenschaftlichen Klasse IIb. Akademie der Wissenschaften Wien 1938, 146, 797−810. (49) Ketteler, G.; Weiss, W.; Ranke, W.; Schlogl, R. Bulk and Surface Phases of Iron Oxides in an Oxygen and Water Atmosphere at Low Pressure. Phys. Chem. Chem. Phys. 2001, 3 (6), 1114−1122. (50) de Faria, D. L. A.; Silva, S. V.; de Oliveira, M. T. Raman Microspectroscopy of Some Iron Oxides and Oxyhydroxides. J. Raman Spectrosc. 1997, 28 (11), 873−878. (51) Lubbe, M.; Gigler, A. M.; Stark, R. W.; Moritz, W. Identification of Iron Oxide Phases in Thin Films Grown on Al2O3(0001) by Raman Spectroscopy and X-Ray Diffraction. Surf. Sci. 2010, 604 (7− 8), 679−685. (52) Massey, M. J.; Baier, U.; Merlin, R.; Weber, W. H. Effects of Pressure and Isotopic-Substitution on the Raman-Spectrum of AlphaFe2O3 - Identification of 2-Magnon Scattering. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41 (11), 7822−7827. (53) Lewin, E.; Gorgoi, M.; Schafers, F.; Svensson, S.; Jansson, U. Influence of Sputter Damage on the Xps Analysis of Metastable Nanocomposite Coatings. Surf. Coat. Technol. 2009, 204 (4), 455− 462. (54) He, Y. P.; Miao, Y. M.; Li, C. R.; Wang, S. Q.; Cao, L.; Xie, S. S.; Yang, G. Z.; Zou, B. S.; Burda, C. Size and Structure Effect on Optical Transitions of Iron Oxide Nanocrystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71 (12), 125411. (55) Sherman, D. M.; Waite, T. D. Electronic-Spectra of Fe-3+ Oxides and Oxide Hydroxides in the Near IR to Near UV. Am. Mineral. 1985, 70 (11−12), 1262−1269. (56) Vayssieres, L.; Sathe, C.; Butorin, S. M.; Shuh, D. K.; Nordgren, J.; Guo, J. H. One-dimensional Quantum-Confinement Effect in Alpha-Fe2O3 Ultrafine Nanorod Arrays. Adv. Mater. 2005, 17 (19), 2320−2323. (57) Dirac, P. A. M. The Quantum Theory of the Emission and Absorption of Radiation. Proc. R. Soc. London, Ser. A 1927, 114 (767), 243−265. (58) Elliott, R. J. Intensity of Optical Absorption by Excitons. Phys. Rev. 1957, 108 (6), 1384−1389. (59) Sahana, M. B.; Sudakar, C.; Setzler, G.; Dixit, A.; Thakur, J. S.; Lawes, G.; Naik, R.; Naik, V. M.; Vaishnava, P. P. Bandgap engineering by tuning particle size and crystallinity of SnO2-Fe2O3 nanocrystalline composite thin films. Appl. Phys. Lett. 2008, 93 (23), 231909. (60) Mitchell, D. R. G.; Attard, D. J.; Triani, G. Transmission Electron Microscopy Studies of Atomic Layer Deposition TiO2 Films Grown on Silicon. Thin Solid Films 2003, 441 (1−2), 85−95.
Substrates with Tin Oxide Nanoparticles. J. Cryst. Growth 2012, 343 (1), 21−27. (29) Kukli, K.; Dimri, M. C.; Tamm, A.; Kemell, M.; Kaambre, T.; Vehkamaki, M.; Puttaswamy, M.; Stern, R.; Kuusik, I.; Kikas, A.; Tallarida, M.; Schmeisser, D.; Ritala, M.; Leskela, M. Structural and Magnetic Studies on Iron Oxide and Iron-Magnesium Oxide Thin Films Deposited Using Ferrocene and (Dimethylaminomethyl)ferrocene Precursors. ECS J. Solid State Sci. Technol. 2013, 2 (3), N45−N54. (30) Escrig, J.; Bachmann, J.; Jing, J.; Daub, M.; Altbir, D.; Nielsch, K. Crossover Between Two Different Magnetization Reversal Modes in Arrays of Iron Oxide Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77 (21), 214421. (31) Bachmann, J.; Escrig, J.; Pitzschel, K.; Moreno, J. M. M.; Jing, J.; Gorlitz, D.; Altbir, D.; Nielsch, K. Size Effects in Ordered Arrays of Magnetic Nanotubes: Pick Your Reversal Mode. J. Appl. Phys. 2009, 105 (7), 07B521. (32) Bachmann, J.; Jing, J.; Knez, M.; Barth, S.; Shen, H.; Mathur, S.; Gosele, U.; Nielsch, K. Ordered Iron Oxide Nanotube Arrays of Controlled Geometry and Tunable Magnetism by Atomic Layer Deposition. J. Am. Chem. Soc. 2007, 129 (31), 9554. (33) Zhou, K.; Huang, J. Q.; Zhang, Q.; Wei, F. Multi-Directional Growth of Aligned Carbon Nanotubes Over Catalyst Film Prepared by Atomic Layer Deposition. Nanoscale Res. Lett. 2010, 5 (10), 1555− 1560. (34) Hamann, T. W. Splitting Water with Rust: Hematite Photoelectrochemistry. Dalton Transactions 2012, 41 (26), 7830− 7834. (35) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co-Pi″-Coated Hematite Electrodes. J. Am. Chem. Soc. 2012, 134 (40), 16693−16700. (36) Klug, J. A.; Becker, N. G.; Riha, S. C.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J.; Proslier, T. Low temperature atomic layer deposition of highly photoactive hematite using iron(III) chloride and water. J. Mater. Chem. A 2013, 1, 11607−11613. (37) Riha, S. C.; Racowski, J. M.; Lanci, M. P.; Klug, J. A.; Hock, A. S.; Martinson, A. B. F. Phase Discrimination through Oxidant Selection in Low-Temperature Atomic Layer Deposition of Crystalline Iron Oxides. Langmuir 2013, 29, 3439−3445. (38) Ramachandran, R. K.; Dendooven, J.; Detavernier, C. Plasma enhanced atomic layer deposition of Fe2O3 thin films. J. Mater. Chem. A 2014, 2, 10662−10667. (39) Zandi, O.; Beardslee, J. A.; Hamann, T. Substrate dependent Water Splitting with Ultrathin α-Fe2O3. J. Phys. Chem. C 2014, 118, 16494−16503. (40) Fondell, M.; Jacobsson, J. T.; Boman, M.; Edvinsson, T. Optical quantum confinement in low dimensional hematite. J. Mater. Chem. A 2014, 2, 3352−3363. (41) Fondell, M.; Johansson, F.; Goroi, M.; Fieandt, L.; Boman, M.; Lindblad, A. Phase control of iron oxides grown in nano-scale structures on FTO and Si(100): Hematite, maghemite and magnetite. Vacuum 2015, 117, 85−90. (42) Aarik, J.; Aidla, A.; Mändar, H.; Uustare, T. Atomic layer deposition of titanium dioxide from TiCl4 and H2O: investigation of growth mechanism. Appl. Surf. Sci. 2001, 172 (1−2), 148−158. (43) Aarik, J.; Aidla, A.; Mändar, H.; Uustare, T.; Sammelselg, V. Growth kinetics and structure formation of ZrO2 thin films in chloride-based atomic layer deposition process. Thin Solid Films 2002, 408 (1−2), 97−103. (44) Navrotsky, A.; Mazeina, L.; Majzlan, J. Size-Driven Structural and Thermodynamic Complexity in Iron Oxides. Science 2008, 319, 1635−1638. (45) Machala, L.; Tuček, J.; Zbořil, R. Polymorhpous Transformations of Nanometric Iron(III) Oxide: A Review. Chem. Mater. 2011, 23, 3255−3272. (46) Sundqvist, J.; Tarre, A.; Rosental, A.; Harsta, A. Atomic Layer Deposition of Epitaxial and Polycrystalline SnO2 Films from the SnI4/ 12381
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