Article pubs.acs.org/Langmuir
Microstructure-Dependent Conformal Atomic Layer Deposition on 3D Nanotopography Qianqian Li,† Cezhou Dong,† Anmin Nie, Jiabin Liu, Wu Zhou, and Hongtao Wang* Institute of Applied Mechanics, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: The capability of atomic layer deposition (ALD) to coat conformally complex 3D nanotopography has been examined by depositing amorphous, polycrystalline, and single-crystal TiO2 films over SnO2 nanowires (NWs). Structural characterizations reveal a strong correlation between the surface morphology and the microstructures of ALD films. Conformal growth can only be rigorously achieved in amorphous phase with circular sectors developed at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e., sequential and self-limiting surface reactions result in smooth and conformal films. Orientation-dependent growth and surface reconstruction generally lead to nonconformal coating in polycrystalline and single-crystal films. Especially, an octagonal single-crystal TiO2 shell was derived from a rectangular SnO2 NW core, which was the consequence of both self-limited growth kinetics and surface reconstruction. Models were proposed to explain the conformality of ALD deposition over 3D nanostructures by taking account of the underlying microstructures. Besides the surface morphologies, the microstructures also have significant consequence to the surface electronic states, characterized by the broad band photoluminescence. The comparison study suggests that ALD process is determined by the interplay of both thermodynamic and kinetic factors.
1. INTRODUCTION Demanding scaling in ultralarge-scale integration-based technology has been exploiting the benefits of 3D architectures with increasing high aspect ratios, as demonstrated in DRAM trench capacitors1−4 and multigate transistors.5−7 Along with developments in electronic devices, the state-of-the-art technology in Li ion batteries as the next generation energy supply for electric vehicles requires ultrathin and conformal surface coating over the electrode materials, such as nanoflats,8 nanoparticles,9−13 or nanowires,14−18 in order to modify the solid electrolyte interphase. Thin film deposition over surfaces of these 3D structures in ultralarge scale demands high uniformity, to avoid malfunction or improve performance, which greatly challenges the capability of conventional technologies, such as physical vapor deposition and chemical vapor deposition. Atomic layer deposition (ALD) is an effective and scalable solution to the common demands because of its capability to coat complex 3D nanotopography with exceptional uniformity and conformality, as well as high quality.19,20 Great efforts have been focused on the mechanisms behind conformal ALD, by understanding the surface chemistry,21−24 developing kinetic models,25 and designing simulation schemes.26 Besides, numerous experimental studies have solidified the foundation to the theoretical analysis.25,27−30 All these observations have emphasized the built-in kinetic factors, as vividly pictured in the sequential process with strictly self-limiting surface reactions. Though thermodynamic factors may come into play for ALD metal thin films with sub-10 nm thickness, it has been well accepted that conformal ALD can be easily achieved in © 2012 American Chemical Society
deposition of simple binary nitrides and oxides, such as Al2O3 and TiO2, because of high bonding strength and low diffusional mobility.2 A recent study on surface evolution has demonstrated again the ability to apply conformal ALD of TiO2 over a highly scalloped surface with ultrasharp asperities.31 On the other hand, the microstructure of ALD thin films has not been explored for its dependence on 3D nanostructured substrates, which may affect the conformality in coating. It is expected that the downsizing trend will finally lead to the interplay among the film thickness and the characteristic lengths in microstructures and 3D nanotopography. In this paper, we explore these effects by systematically adjusting the TiO2 film microstructures and try to bring out the role of thermodynamic factors in conformal deposition.
2. EXPERIMENTAL SECTION The model system was chosen to be ALD TiO2 thin films on single crystalline SnO2 and ZnO nanowires (NWs), which are direct band gap semiconductors (3.6 and 3.37 eV, respectively) and among the most studied functional materials with applications in gas sensors,32 electrodes,33 and photocatalystes.34 The NWs can be synthesized via a simple vapor transport (VLS) method. High-purity tin powder (Alfa Aldrich, 99.9%) was vaporized at 800 °C in a tube furnace under an ambient flow of mixing gas (Ar:O2 200 sccm:3 sccm). The base pressure was 2 Torr. The Si substrates coated with a thin layer of Au catalyst (nominal thickness ∼1 nm) were used to collect the Received: June 12, 2012 Revised: August 30, 2012 Published: October 18, 2012 15809
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
Langmuir
Article
Figure 1. (a) SEM image of as-synthesized SnO2 NWs on Si substrate. (b) Statistical distribution of NW diameters. (c) XRD spectrum using Cu Kα radiation. (d) Cross-sectional TEM image of a SnO2 NW. (e) High-resolution TEM image showing the orthogonal lattice planes. (f) The corresponding selected-area electron diffraction.
Figure 2. Schematic ALD process for α-TiO2 shell over SnO2 NW cores. The inset shows the overlayer geometry in the corner region, based on conformal deposition. Only the cross-section of the NW structure is considered. A conformal deposition requires the film thickness to be the same all around the NW. Therefore, the final morphology consists of parallel edges and round corners, which is substantially different from the c-TiO2/SnO2 NW structure.
Figure 3. (a) HAADF TEM image of a SnO2/α-TiO2 (900 ALD cycles) core−shell structure. High-resolution TEM image of (b) the interface and (c) the amorphous region. (d) The corresponding FFT of image c, which shows no speckle and confirms the amorphous structure. (e−g) The elemental mappings of Ti, O, and Sn.
as-synthesized NWs, as shown in Figure 1a. Particularly, the welldefined cross-sectional geometry makes these NWs as ideal 3D substrates with characteristic diameters in tens of nanometers. TiO2 thin films were deposited in a home-built tube ALD reactor, using titanium tetrakis(isopropoxide) (TTIP) and hydrogen peroxide (H2O2) as precursors. Three different deposition schemes were designed to obtain single-crystalline (c-TiO2), polycrystalline (p-TiO2), and amorphous (α-TiO2) phases. Crystalline TiO2 generally have two different phases, i.e., anatase phase (a = 3.785 Å and c = 9.514 Å, JCPDS 41-1445) and rutile phase (a = 4.593 Å and c = 2.959 Å, JCPDS 21-1276), while SnO2 has only one stable phase, i.e., rutile phase (a = 4.738 Å and c = 3.187 Å, JCPDS 41-1445). Direct deposition onto SnO2 NWs resulted in epitaxial growth of c-TiO2 shells for the small lattice mismatch of the rutile TiO2 phase to the SnO2 phase, i.e., (aSnO2 − aTiO2)/aTiO2 = 3.1% and (cSnO2 − cTiO2)/cTiO2 = 7.7%. The epitaxial growth can be suppressed if the initial NW surface is coated
with a thin layer of ALD Al2O3. Polycrystalline structures (p-TiO2) will be developed during TiO2 deposition. The local crystalline order in the TiO2 layer can be further frustrated by inserting five ALD cycles of Al2O3 per 100-cycle TiO2 deposition, which results in amorphous structures (α-TiO2). All other deposition conditions, such as the base pressure (320 mTorr), the substrate temperature (250 °C), the precursor exposure, and the purging time, remained the same for all the above depositions, which met the saturation condition. Detailed parameters are that both TMA and H2O2 were kept at room temperature and the TTIP was kept at 50 °C. The Al2O3 deposition cycle consisted of a 1 s pulse of TMA, followed by 40 s purge, then a 0.5 s pulse of H2O2, and another 60 s purge. 15810
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
Langmuir
Article
be [101]̅ and further confirmed by the corresponding electron diffraction in Figure 1f. Figure 2 schematically sketches the ALD process for α-TiO2 films onto SnO2 NWs. The crystallization tendency is inhibited by inserting a thin layer (∼0.5 nm) of Al2O3 after every 100cycle TiO2 deposition (∼3 nm). Due to the atomic mass difference between Al and Ti, the contrast from different constituents can be identified in the cross-sectional transmission electron microscopy. Therefore, the morphology evolution in the ALD process can be tracked by the alternative mass contrast. For conformal coating, it is expected that the film has uniform thickness and a corner shape of circular sector enclosed by normals of neighboring edges (inset to Figure 2). The cross-sectional high-angle annular dark field (HAADF) image in Figure 3a clearly reveals a core−shell structure with TiO2 overlayer (gray) conformally wrapping a SnO2 NW (white). The abrupt change of contrast at the interface indicates no intermixing of the two phases (Figure 3b). Neither the highresolution TEM image nor the fast Fourier transformation (FFT) (Figure 3c,d) show features of crystallinity, confirming the amorphous nature of TiO2. Elemental mappings show the distribution of Ti, O, and Sn (Figure 3e−g). Titanium and tin are located in the outer layer and the core part, respectively, while oxygen distributes throughout in the whole structure. The geometry of the TiO2 layer is characterized by the uniform thickness along the four sidewalls of the core region. Four quadrants are enclosed by two radii normal to the neighboring edges and an arc centered at the corners. The radius is exactly the same as the film thickness, and the outer surface is atomically smooth. The HAADF contrast comes from the incoherently scattered electrons and is highly sensitive to variations in the atomic number (Z) with the intensity approximately proportional to Z2.35 Consequently, the blurred darker lines in the outer shell have higher concentration of aluminum due to the periodical insertion of Al2O3. Every layer has exactly the same thickness with boundaries perfectly parallel to each other. The layered structure convincingly demonstrates the long-held
The structural characterizations were done with a scanning electron microscopy (SEM, Hitachi S-4800) operated at 8 kV and highresolution transmission electron microscopy (TEM, FEI F20) operated at 200 kV. X-ray diffraction patterns were obtained on an X’Pert PRO instrument with Cu Kα radiation. Laser Raman spectroscopy was obtained in a range of 200−800 cm−1 at room temperature. Photoluminescence (PL) was measured at room temperature with a Hitachi luminescence spectrometer using a xenon discharge lamp as excitation light source. The excited wavelength was 325 nm.
3. RESULTS AND DISCUSSION Figure 1a is a typical SEM image of the as-synthesized SnO2 NWs with a length of several tens of micrometers. An examination of ∼500 NWs shows that all of them are straight and uniform along the length with diameters mainly ranging from 70 to 100 nm (Figure 1b). The X-ray diffraction (XRD) peaks closely match those appearing in powder diffraction of the tetragonal phase of SnO2 (JCPDS 41-1445) with lattice constants of a = 4.738 Å and c = 3.187 Å. Cross-sectional TEM images (Figure 1d) reveal the rectangular geometry. The single crystallinity is confirmed by high-resolution TEM images. The orthogonal lattice fringes have spacings of 0.45 and 0.26 nm (Figure 1e) and can be indexed to be (010) and (101), respectively. Thus, the growth direction can be determined to
Figure 4. (a) TEM image of a ZnO/α-TiO2 core−shell structure. (b) Schematic representation of the film morphology at a corner.
Figure 5. (a) The HAADF image of a SnO2/p-TiO2 NW. (b) Line profiles of elemental distribution. The intensity of the collected signal represents the relative concentration of elements. (c) Raman spectra of NWs with c-TiO2 and p-TiO2 shells. 15811
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
Langmuir
Article
Figure 6. (a) TEM image of a SnO2/p-TiO2 NW (900 ALD cycles). (b) High-resolution TEM image of the interface region. The inset is the FFT. (c) Cross-sectional TEM image (600 ALD cycles). (d and e) Magnified TEM images show the crystalline structure and a grain boundary.
and brookite,36 among which the brookite phase is rarely observed in ALD. Raman scattering was employed to identify the phase (Figure 5c). Anatase phase has six active Raman modes, i.e. 144 cm−1 (Eg), 197 cm−1 (E2g), 399 cm−1 (B1g), 513 cm−1 (A1g), 519 cm−1 (B2g), and 639 cm−1 (E3g).37 For rutile phase, the Raman peaks appear at 396, 517, and 629 cm−1, corresponding to B1g, A1g (or B2g), and Eg, respectively. A simple comparison shows TiO2 and p-TiO2 are of rutile (a = 4.593 Å and c = 2.959 Å, JCPDS 21-1276) and anatase phase (a = 3.785 Å and c = 9.514 Å, JCPDS 41-1445), respectively. The morphology was visualized by the nonuniform thickness along the SnO2 NW, as shown in Figure 6a. The surface of a single TiO2 grain is perfectly flat, paralleling the substrate. The grain boundary (GB) can be identified at the location with a
dogma that sequential and self-limiting surface reactions result in smooth and conformal ALD. Amorphous TiO2 films were also deposited onto the ZnO NWs with hexagonal cross sections (Figure 4) and other irregular tetragonal cross sections. Similar layered structure with smooth surface further confirmed the proposed model in Figure 2. Direct deposition on SnO2 NWs forms c-TiO2 or p-TiO2, depending on whether the NW surface is coated with amorphous layer of Al2O3. The HAADF image of a SnO2/p-TiO2 NW is shown in Figure 5a. The elemental distribution was characterized by energy-dispersive spectroscopy (EDS). The valley in the Ti profile coincides with the peak of the Sn concentration (Figure 5b), which confirms the core−shell-like heterostructure. Crystalline TiO2 has three different phases, i.e. anatase, rutile, 15812
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
Langmuir
Article
step due to the difference in thickness. This observation clearly shows the strong orientation-dependence of the growth rate. The roughness is higher than that of the amorphous oxide films with the same thickness deposited by ALD, which is usually similar to the bare substrate roughness.38 By considering the relative large grain size, the roughness is quite rare and reveals growth mechanisms different from most polycrystalline films, i.e., nucleation and coalescence. Our experiments show that the TiO2 film is amorphous below a critical thickness of 3−5 nm. Thicker film is always fully crystallized without amorphous underlayer, as shown in Figure 6b. This observation suggests a sudden phase transformation during the ALD process. The coating morphology has been further investigated by crosssectional TEM (Figure 6c). The overlayer thickness is 18, 14, 15, and 12 nm, respectively, along the sidewalls of the rectangular SnO2 core. Similar observation was found in other two cross-section geometries. Different grain orientations were identified in different regions of the cross-section, as shown in Figure 6d,e. In Figure 6e, two neighboring grains can be distinguished by the different orientated lattice fringes. A curved GB is located in the blurred region, which confirms the polycrystalline structure of the TiO2 shell. It is noted that the p-TiO2 shell has both round and faceted corners and does not retain a conformal geometry to the core region. No specific crystal planes can be assigned to the round corners. The resemblance to the circular sector in amorphous coatings suggests the dominance of the kinetic factors. On the other hand, small facets are clear evidence of surface reconstruction during growth, which is driven by the tendency of lowering surface energy. Similar growth behaviors were also observed in coating hexagonal ZnO NWs with p-TiO2 (Figure 7).
Figure 8. (a) Schematic process in ALD p-TiO2 shell on SnO2 NW cores. (b) 3D model of a SnO2/p-TiO2 NW. Visualization is aided by coloring different grains.
substrate temperature of 250 °C. The thermal activation energy is overwhelmed by the enthalpy from the surface reaction, which determines the rate of chemisorptions. The Al2O3 underlayer has an amorphous surface, which provides randomly distributed active sites. Therefore, chemisorbed TTIP molecules retain the nonperiodical feature from the amorphous substrate and pass it the next layer of ALD deposition. A transition to the crystalline phase has been observed at a critical thickness tcr ∼ 3−5 nm and can be explained by a simple thermodynamic argument. Taking the α-TiO2/α-Al2O3 stack as reference, the specific free energy per volume (Δg) of the p-TiO2/α-Al2O3 stack with the same thickness (t) can be expressed by Δg = Δγ/t + Δu, where Δγ and Δu are the difference in the interfacial energy and the internal energy, respectively. The crystalline structure leads to a smaller internal energy and hence a negative Δu, while its incompatibility to the amorphous substrate results in a larger interfacial energy and thus a positive Δγ. The critical film thickness is given by tcr = −Δγ/Δu. To lower the total free energy, the local crystal orientation will be predominately determined by the lowenergy surfaces. Since the in-plane grain size (a few hundred nanometers) is much larger than the film thickness (Supporting Information, Figure S2), a column grain structure will be developed during ALD. A schematic 3D model is sketched in Figure 8b. The orientation-dependent growth rate and the possible surface reconstruction will generally lead to nonconformal coating by ALD. Direct deposition of TiO2 forms single crystal shells on SnO2 NW cores (Figure 9a). The marked feature is the derived octagonal cross-section from a rectangular core (Figure 9b). The corresponding orientations are labeled in the reconstructed 3D model (Figure 9c). The newly developed surfaces are the consequence of the staircase-like growth mode at the sharp corners, which is a manifestation of the inherent layer-by-layer growth mode in ALD, as reported in our early study.39 The final morphology is also subjected to the surface reconstruction in order to lower the system energy. The device application of NWs strongly relies on the large surface-to-volume ratio, which renders surface modification an important tool to tune the physical properties. For example, the transport properties of SnO2 depend strongly on surface states induced by gaseous adsorption.40 The photoluminescence is
Figure 7. (a) Cross-sectional TEM image (600 ALD cycles) of ZnO/ p-TiO2 nanowire. (b and c) Magnified TEM images show the crystalline structure and a grain boundary.
On the basis of the above observations, a schematic ALD process is proposed in Figure 8a. For the strong ionic bonding nature in TiO2, the diffusional mobility is rather low at the 15813
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
Langmuir
Article
thermodynamic and kinetic factors. The derivation of an octagonal TiO2 shell from a rectangular SnO2 NW core was caused by both self-limited growth kinetics and surface reconstruction. The morphologies are also manifested by the passivation effect on the surface electronic states. A comparison study shows that the broad band luminescence was substantially suppressed by the amorphous coating due to the atomically smooth surface. Models were proposed to explain the ALD deposition onto 3D nanostructures. A relatively conformal deposition can be achieved only when the microstructural length scale is much less than the characteristic length of the 3D nanotopography.
■
Figure 9. (a) Planar and (b) cross-sectional TEM images of a SnO2/ c-TiO2 NW (250 ALD cycles). (c) The constructed 3D model with the crystal planes labeled, which are all low-energy surfaces.
ASSOCIATED CONTENT
S Supporting Information *
Additional information and Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
also sensitive to the surface states of NWs and is employed here to characterize the effect of passivation by ALD. Broad luminescence bands between 500 and 700 nm have been reported from SnO2 NWs at room temperature, which is mainly attributed to the surface oxygen vacancies.40−42 Our early research shows that the lattice mismatch between SnO2 and TiO2 induces a dramatic increase of peak intensity in the band at 500−600 nm. A comparison study reveals that α-TiO2 and p-TiO2 can effectively passivate the SnO2 surface states and introduce few new states (Figure 10). This difference is also
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
Both authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work has been supported by the National Science Foundation of China (Grant No. 11002124 and No. 11090333), Science Foundation of Chinese University (Grant No. 2011QNA4038), Scientific Research Fund of Zhejiang Provincial Education Department (Grant No. Z200906194), and Science and Technology Innovative Research Team of Zhejiang Province (No. 2009R50010).
■
REFERENCES
(1) Lutzen, J.; Birner, A.; Goldbach, M.; Gutsche, M.; Hecht, T.; Jakschik, S.; Orth, A.; Sanger, A.; Schroder, U.; Seidl, H. Integration of capacitor for sub-100-nm DRAM trench technology. VLSI Technol. 2002, 178−179. (2) Gerritsen, E.; Emonet, N.; Caillat, C.; Jourdan, N.; Piazza, M.; Fraboulet, D.; Boeck, B.; Berthelot, A.; Smith, S.; Mazoyer, P. Evolution of materials technology for stacked-capacitors in 65nm embedded-DRAM. Solid-State Electron. 2005, 49 (11), 1767−1775. (3) Seidl, H.; Gutsche, M.; Schroeder, U.; Birner, A.; Hecht, T.; Jakschik, S.; Luetzen, J.; Kerber, M.; Kudelka, S.; Popp, T. A fully integrated Al2O3 trench capacitor DRAM for sub-100 nm technology. IEDM 2002, 839−842. (4) Golz, J.; Safran, J.; He, B.; Leu, D.; Yin, M.; Weaver, T.; Vehabovic, A.; Sun, Y.; Cestero, A.; Himmel, B. 3D stackable 32nm high-K/metal gate SOI embedded DRAM prototype. VLSI Circuits 2011, 228−229. (5) Elam, J.; Routkevitch, D.; Mardilovich, P.; George, S. Conformal coating on ultrahigh-aspect-ratio nanopores of anodic alumina by atomic layer deposition. Chem. Mater. 2003, 15, 3507−3517. (6) Sinha,S.; Yeric, G.; Chandra, V.; Cline, B.; Cao, Y. Exploring sub20nm FinFET design with predictive technology models.Proceedings of the 49th Annual Design Automation Conference ; ACM: New York, 2012; pp 283288 (7) Colinge, J. P.; Lee, C. W.; Afzalian, A.; Akhavan, N. D.; Yan, R.; Ferain, I.; Razavi, P.; O’Neill, B.; Blake, A.; White, M. Nanowire transistors without junctions. Nat. Nanotechnol. 2010, 5, 225−229. (8) Snyder, M. Q.; Trebukhova, S. A.; Ravdel, B.; Wheeler, M. C.; DiCarlo, J.; Tripp, C. P.; DeSisto, W. J. Synthesis and characterization of atomic layer deposited titanium nitride thin films on lithium titanate
Figure 10. The photoluminescence (PL) spectra excited at 325 nm at room temperature for (a) SnO2, (b) SnO2/α-TiO2, (c) SnO2/p-TiO2 NWs, and (d) TiO2 film on Si substrate.
manifested from the surface morphology. Less residue stress, as implied from the uniform contrast in the TEM images (Figures 4a, 6a, 7a), results in a nearly atomically smooth surface of both amorphous and polycrystalline TiO2 coatings. On the contrary, the lattice mismatch at the interface of SnO2/TiO2 NWs results in a rough surface, which is the source of surface states.
4. CONCLUSION α-TiO2, p-TiO2, and c-TiO2 films were deposited via ALD onto SnO2 NWs. Structural characterizations reveal the strong correlation between morphology and the microstructures of the ALD films. Conformal growth can be achieved in the amorphous phase, where circular sectors develop at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e., sequential and self-limiting surface reactions result in smooth and conformal films. Orientation-dependent growth in crystalline films generally leads to nonconformal coating. Corner morphology is determined by the interplay of both 15814
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
Langmuir
Article
spinel powder as a lithium-ion battery anode. J. Power Sources 2007, 165, 379−385. (9) Tsai, S. S. H.; Wexler, J. S.; Wan, J. D.; Stone, H. A. Conformal coating of particles in microchannels by magnetic forcing. Appl. Phys. Lett. 2011, 99, 153509. (10) Myung, S. T.; Izumi, K.; Komaba, S.; Sun, Y. K.; Yashiro, H.; Kumagai, N. Role of alumina coating on Li−Ni−Co−Mn−O particles as positive electrode material for lithium-ion batteries. Chem. Mater. 2005, 17, 3695−3704. (11) Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S. H.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S. H. Ultrathin direct atomic layer deposition on composite electrodes for highly durable and safe Li-ion batteries. J. Adv. Mater. 2010, 22, 2172−2176. (12) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Jung, Y. S.; Yan, Y.; Lee, S. H.; Dillon, A. C. Conformal surface coatings to enable high volume expansion Li-ion anode materials. ChemPhysChem 2010, 11, 2124−2130. (13) Scott, I. D.; Jung, Y. S.; Cavanagh, A. S.; Yan, Y.; Dillon, A. C.; George, S. M.; Lee, S. H. Ultrathin coatings on nano-LiCoO2 for Liion vehicular applications. Nano Lett. 2011, 11, 414−418. (14) Mendoza, F.; Kumar, A.; Martinez, R.; Scott, J. F.; Weiner, B.; Katiyar, R. S.; Morell, G. Conformal coating of ferroelectric oxides on carbon nanotubes. EPL 2012, 97, p1−p5. (15) Gowda, S. R.; Reddy, A. L. M.; Shaijumon, M. M.; Zhan, X. B.; Ci, L. J.; Ajayan, P. M. Conformal coating of thin polymer electrolyte layer on nanostructured electrode materials for three-dimensional battery applications. Nano Lett. 2011, 11, 101−106. (16) Jiang, J.; Li, Y.; Liu, J.; Huang, X. Building one-dimensional oxide nanostructure arrays on conductive metal substrates for lithiumion battery anodes. Nanoscale 2011, 3 (1), 45−58. (17) Kim, D. W.; Hwang, I. S.; Kwon, S. J.; Kang, H. Y.; Park, K. S.; Choi, Y. J.; Choi, K. J.; Park, J. G. Highly conductive coaxial SnO2− In2O3 heterostructured nanowires for Li ion battery electrodes. Nano Lett. 2007, 7, 3041−3045. (18) Cui, L. F.; Yang, Y.; Hsu, C. M.; Cui, Y. Carbon−silicon core− shell nanowires as high capacity electrode for lithium ion batteries. Nano Lett. 2009, 9, 3370−3374. (19) George, S. M. Atomic layer deposition: An overview. Chem. Rev. 2009, 110, 111−131. (20) Leskela, M.; Ritala, M. Atomic layer deposition (ALD): From precursors to thin film structures. Thin Solid Films 2002, 409 (1), 138−146. (21) George, S.; Ott, A.; Klaus, J. Surface chemistry for atomic layer growth. J. Phys. Chem. 1996, 100, 13121−13131. (22) Rai, V. R.; Vandalon, V.; Agarwal, S. Influence of surface temperature on the mechanism of atomic layer deposition of aluminum oxide using an oxygen plasma and ozone. Langmuir 2012, 28, 350−357. (23) Goldstein, D. N.; McCormick, J. A.; George, S. M. Al2O3 atomic layer deposition with trimethylaluminum and ozone studied by in situ transmission FTIR spectroscopy and quadrupole mass spectrometry. J. Phys. Chem. C 2008, 112, 19530−19539. (24) Tanskanen, J. T.; Bakke, J. R.; Bent, S. F.; Pakkanen, T. A. ALD growth characteristics of ZnS films deposited from organozinc and hydrogen sulfide precursors. Langmuir 2010, 26, 11899−11906. (25) Gordon, R. G.; Hausmann, D.; Kim, E.; Shepard, J. A kinetic model for step coverage by atomic layer deposition in narrow holes or trenches. Chem. Vap. Deposition 2003, 9, 73−78. (26) Knizhnik, A.; Bagaturyants, A.; Belov, I.; Potapkin, B.; Korkin, A. An integrated kinetic Monte Carlo molecular dynamics approach for film growth modeling and simulation: ZrO2 deposition on Si (100) surface. Comput. Mater. Sci. 2002, 24, 128−132. (27) Kucheyev, S. O.; Biener, J.; Baumann, T. F.; Wang, Y. M.; Hamza, A. V.; Li, Z.; Lee, D. K.; Gordon, R. G. Mechanisms of atomic layer deposition on substrates with ultrahigh aspect ratios. Langmuir 2008, 24, 943−948. (28) Elam, J. W.; Routkevitch, D.; Mardilovich, P. P.; George, S. M. Conformal coating on ultrahigh-aspect-ratio nanopores of anodic
alumina by atomic layer deposition. Chem. Mater. 2003, 15 (18), 3507−3517. (29) Becker, J. S.; Suh, S.; Wang, S.; Gordon, R. G. Highly conformal thin films of tungsten nitride prepared by atomic layer deposition from a novel precursor. Chem. Mater. 2003, 15, 2969−2976. (30) Hausmann, D.; Becker, J.; Wang, S.; Gordon, R. G. Rapid vapor deposition of highly conformal silica nanolaminates. Science 2002, 298 (5592), 402−406. (31) Cleveland, E. R.; Banerjee, P.; Perez, I.; Lee, S. B.; Rubloff, G. W. Profile evolution for conformal atomic layer deposition over nanotopography. ACS Nano 2010, 4, 4637−4644. (32) Reddy, C. V. G.; Cao, W.; Tan, O.; Zhu, W.; Akbar, S. Preparation of Fe2O3 (0.9)−SnO2 (0.1) by hydrazine method: Application as an alcohol sensor. Sens. Actuators B 2002, 81, 170−175. (33) Hasobe, T.; Imahori, H.; Kamat, P. V.; Fukuzumi, S. Quaternary self-organization of porphyrin and fullerene units by clusterization with gold nanoparticles on SnO2 electrodes for organic solar cells. J. Am. Chem. Soc. 2003, 125 (49), 14962−14963. (34) Li, L.; Liu, J.; Su, Y.; Li, G.; Chen, X.; Qiu, X.; Yan, T. Surface doping for photocatalytic purposes: Relations between particle size, surface modifications, and photoactivity of SnO2:Zn2+ nanocrystals. Nanotechnology 2009, 20, 1−9. (35) Hartel, P.; Rose, H.; Dinges, C. Conditions and reasons for incoherent imaging in STEM. Ultramicroscopy 1996, 63, 93−114. (36) Balachandran, U.; Eror, N. G. Raman-spectra of titanumdioxide. J. Solid State Chem. 1982, 42, 276−282. (37) Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman-spectrum of anatase, TiO2. J. Raman. Spectrosc. 1978, 7, 321−324. (38) Wang, H. T.; Wang, J. J.; Gordon, R.; Lehn, J. S. M.; Li, H. Z.; Hong, D.; Shenai, D. V. Atomic layer deposition of lanthanum-based ternary oxides. Electrochem. Solid-State Lett. 2009, 12, G13−G15. (39) Nie, A.; Liu, J.; Li, Q.; Cheng, Y.; Dong, C. Z.; Zhou, W.; Wang, P.; Qingxiao, W.; Yang, Y.; Zhu, Y. Epitaxial TiO2/SnO2 core-shell heterostructure by atomic layer deposition. J. Mater. Chem. 2012, 22, 10665−10671. (40) Faglia, G.; Baratto, C.; Sberveglieri, G.; Zha, M.; Zappettini, A. Adsorption effects of NO2 at ppm level on visible photoluminescence response of SnO2 nanobelts. Appl. Phys. Lett. 2005, 86, 011923. (41) Mendes, P. G.; Moreira, M. L.; Tebcherani, S. M.; Orlandi, M. O.; Andrés, J.; Li, M. S.; Diaz-Mora, N.; Varela, J. A.; Longo, E. SnO2 nanocrystals synthesized by microwave-assisted hydrothermal method: Towards a relationship between structural and optical properties. J. Nanopart. Res. 2012, 14 (3), 1−13. (42) Cai, D.; Su, Y.; Chen, Y.; Jiang, J.; He, Z.; Chen, L. Synthesis and photoluminescence properties of novel SnO2 asterisk-like nanostructures. Mater. Lett. 2005, 59 (16), 1984−1988.
15815
dx.doi.org/10.1021/la302391u | Langmuir 2012, 28, 15809−15815
