Article pubs.acs.org/cm
Transparent Conductive Oxide Nanocrystals Coated with Insulators by Atomic Layer Deposition John Ephraim,† Deanna Lanigan,† Corey Staller,‡ Delia J. Milliron,‡ and Elijah Thimsen*,† †
Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Thin films comprised of transparent conductive oxide (TCO) nanocrystals are attractive for a number of optoelectronic applications. However, it is often observed that the conductivity of such films is very low when they are in contact with air. It has recently been demonstrated, somewhat surprisingly, that filling in initially insulating films comprised of TCO nanocrystals with another insulator by atomic layer deposition (ALD) dramatically increases the conductivity by many orders of magnitude. This work aims to elucidate the mechanism by which the ALD coating increases conductivity. We examined the effect of removing two adsorbed oxygen species (physisorbed molecular water and chemisorbed hydroxide) on sheet resistance and compared this result to the results with thin films comprised of ZnO nanocrystals coated with Al2O3 and also HfO2 by ALD. Although both insulating infills decrease the sheet resistance and increase the stability of the films, there is a stark discrepancy between the two. From the in situ measurements, it was found that coating with Al2O3 removes both physisorbed water and chemisorbed hydroxide, resulting in a net reduction of the ZnO nanocrystals. Coating with HfO2 removes only physisorbed water, which was confirmed by Fourier transform infrared spectroscopy. A similar phenomenon was observed when thin films comprised of Sn-doped In2O3 nanocrystals were coated, suggesting Al2O3 can be used to reduce and stabilize metal oxide nanocrystals in general. hin films comprised of nanocrystals are attractive for optoelectronic applications.1−3 Part of the attraction is related to processing: films comprised of nanocrystals can be deposited at very high rates and low temperatures over large areas, for example, by spray coating. For many applications, such as transparent conductive oxide (TCO) thin films, it is essential to maintain optimal charge carrier concentrations and high mobility.4−6 Nanocrystals have a large surface area to volume ratio, and as a result, adsorbed chemical species have a dramatic effect on electronic conductivity.7 It is desirable for TCO films to maintain high electrical conductivity in ambient air. However, this goal has proven to be difficult to achieve, and it has often been reported that films comprised of metal oxide nanocrystals are highly resistive when in contact with air.8 The high resistance of metal oxide nanocrystal thin films in contact with air is believed to be caused by adsorbed oxygen species, which essentially act as acceptor defects.9,10 Oxygen species, such as water and dioxygen, are known to adsorb to the nanocrystal surfaces.7,11,12 The adsorbed oxygen species cause electrons to become trapped at the surface, which decreases the free electron concentration in the n-type semiconductor, thereby increasing the sheet resistance. Some of these oxygen species are physisorbed, and some are chemisorbed. Physisorbed species, for example, multilayers of water piled on the ZnO surface,11,12 can be removed by heating the sample to moderate temperatures of approximately 200 °C in an
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© XXXX American Chemical Society
atmosphere with a low partial pressure of water and oxygen, for example, ultrapure argon.11−13 However, under such conditions, the chemisorbed species remain unaffected.11 To remove the chemisorbed species, specifically chemisorbed hydroxyl,11,12 one must use more aggressive treatments. The chemisorbed hydroxyl can be removed by heating to high temperatures of approximately 500 °C under vacuum.11 Alternatively, a reducing atmosphere containing hydrogen at a partial pressure of approximately 0.01 bar can be used to remove the chemisorbed hydroxyl at lower temperatures14,15 (i.e., 200 °C), resulting in a net reduction, because it removes oxygen associated with the ZnO. However, at 200 °C with a partial pressure of H2 of ∼0.15 bar, the reduction is superficial, because H2 has insufficient reduction potential to reduce bulk ZnO under these conditions, given typical water contents of 5 ppm in ultrapure cylinder gas. Unfortunately, such treatments are in vain if the pores become filled with air afterward, because all of the adsorbed oxygen species return to the surface, causing the material to be insulating. Thus, if the goal is to have a highly conductive film comprised of TCO nanocrystals, then both types of adsorbed oxygen species must be removed, and the Received: June 16, 2016 Revised: July 19, 2016
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DOI: 10.1021/acs.chemmater.6b02414 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials pores in the film must be sealed to prevent their return upon exposure to air. Recently, it has been demonstrated that if the pores in a film comprised of ZnO nanocrystals are filled with insulating Al2O3 by atomic layer deposition (ALD), the electrical conductivity increases by 7 orders of magnitude.16 In other words, combining an insulator with another insulator makes a conductor! The increase in conductivity after Al2O3 infilling was correlated to a decrease in the density of OH groups measured on the surface by Fourier transform infrared (FTIR) spectroscopy. In addition, it was found that the conductivity of a film with no Al2O3 coating increased by 4 orders of magnitude when physisorbed species originating from air were removed using ultrahigh vacuum.16 However, the remaining 3 order of magnitude increase in the conductivity caused by the Al2O3 coating process was not explained. Thus, the mechanism by which the Al2O3 infill increases the conductivity requires further elucidation. The result also raises questions. Would other insulators deposited by ALD, such as HfO2, have the same effect on conductivity? Is a similar effect observed if the TCO nanocrystals have a different composition, for example, Sn-doped In2O3? In this work, the effect of filling in films comprised of ZnO nanocrystals with insulators by ALD on conductivity is elucidated. Specifically, evidence will be presented that supports the hypothesis that coating films comprised of ZnO nanocrystals with HfO2 by ALD removes physisorbed oxygen species. Evidence will also be presented that supports the hypothesis that coating of ZnO nanocrystals with Al2O3 by ALD results in the removal of both physisorbed oxygen species and chemisorbed hydroxyl, which amounts to a surface reduction. Finally, it will be demonstrated that a similar effect is observed when films comprised of Sn-doped In2O3 nanocrystals are coated with Al2O3 by ALD: the conductivity increases and is stable in air. ZnO nanocrystals were synthesized in the gas phase using a nonthermal Ar−O2 plasma, as described previously.16 As deposited, all films comprised of ZnO nanocrystals in this study had a nominal thickness of 205 ± 11 nm and a ZnO volume faction of 34 ± 1% (i.e., 76% porosity), as measured by spectroscopic ellipsometry. From analysis of transmission electron microscopy images, the average ZnO nanocrystal diameter was determined to be 7.5 ± 1.5 nm (Figure 1a). For details, see the Supporting Information. The sheet resistance of films comprised of uncoated ZnO nanocrystals was monitored in situ in controlled gas mixtures at a total pressure of 1.1 bar in a tube furnace. The sheet resistance of uncoated nanocrystals, measured by these in situ experiments under controlled environmental conditions, was compared to the sheet resistance of nanocrystals that had been coated with a sufficient number of ALD cycles to fill in the pores with HfO2 or Al2O3, to a >90% solid volume fraction. Representative cross-sectional SEM images of an uncoated film with no ALD infill (Figure 1b), as well as films after coating with HfO2 (Figure 1c) and Al2O3 (Figure 1d), are presented in Figure 1. The idea is to determine a relationship between the ZnO surface chemistry and sheet resistance using in situ measurements and then determine a relationship between sheet resistance and ALD infill (HfO2 or Al2O3) in comparison, to establish the connection between ZnO surface chemistry and ALD infill. In situ measurements were used to establish the relationship between sheet resistance and the species present on the nanocrystal surfaces. The experiment was conducted as follows.
Figure 1. TEM and cross-sectional SEM images of thin films comprised of ZnO nanocrystals: (a and b) as-deposited samples and (c and d) samples coated with HfO2 and Al2O3 by ALD, respectively. The scale bars are 50 nm for panel a and 200 nm for panels b−d.
The thin film sample was situated on a glass sample holder with electrical contacts in the Van der Pauw electrode configuration. The sample holder was then placed in a tube furnace, and the temperature was increased from room temperature to 200 °C and allowed to stabilize over a period of 0.8 h. During this time, the atmosphere was air. At 0.8 h, continuously flowing Ar was introduced into the sample compartment. The Ar was allowed to flow through the sample compartment for 24 h. After 24 h, the H2 flow was turned on and the rates were adjusted to maintain the atmosphere as a 15% volume fraction of H2, with the balance being Ar, at a total pressure of 1.1 bar. This hydrogen/argon mixture was allowed to flow for 24 h. After the Ar/H2 mixture had been allowed to flow for 24 h, the hydrogen was turned off. Upon a return to a pure Ar atmosphere, the furnace was turned off, and the entire system was allowed to cool under flowing Ar back to room temperature. Finally, with the system at room temperature, ambient air was reintroduced into the chamber. The experiment is illustrated in Figure 2. Each colored box corresponds to a period with a different gas atmosphere. During the entire course of the experiment, the sheet resistance was measured. As produced, the uncoated films of ZnO nanocrystals were insulating. The measured sheet resistance was at the upper limit of our measurement apparatus, which is approximately 1 × 109 Ω sq−1 (Figure 2). The sheet resistance remained at the upper limit of measurement until Ar was introduced at 0.8 h, at which time the sheet resistance started to fall (Figure 2). Eventually, the sheet resistance reached a plateau in pure Ar at 3.0 × 106 Ω sq−1, at least 3 orders of magnitude smaller than it was initially in air. At 24.8 h, the atmosphere was switched to the Ar/H2 mixture, and the sheet resistance again began to fall, ultimately reaching a plateau at 1.1 × 105 Ω sq−1. When the atmosphere was switched back to pure Ar at 48.8 h, there was a slight blip in the sheet resistance. As the sample cooled, there was a slight increase in the sheet resistance, which is expected for a variable range hopping transport mechanism.16 The sheet resistance at room temperature in pure Ar was 1.3 × 105 Ω sq−1 (t = 49.6 h). After air had been introduced into the system at room temperature, the sheet resistance very quickly returned to the upper limit of the measurement apparatus (109 Ω sq−1) within B
DOI: 10.1021/acs.chemmater.6b02414 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 2. In situ sheet resistance of uncoated ZnO nanocrystals (black curve) as a function of time, temperature, and gas composition. The total pressure was 1.1 bar. The measured sample temperature is the blue curve. The colored boxes correspond to different gas atmospheres. From left to right, the gas atmospheres were air, Ar, the Ar/H2 mixture (15% H2 by volume), Ar, and air, respectively. The horizontal lines are the sheet resistance values measured from films comprised of nanocrystals coated with HfO2 (top) and Al2O3 (bottom).
a matter of minutes, meaning the film returned to its insulating state. Thus, we may conclude the following. If the physisorbed oxygen species are completely removed (Ar at 200 °C), then the sheet resistance is approximately 3.0 × 106 Ω sq−1, and if the surface is reduced by removal of the chemisorbed hydroxyl (Ar/H2 at 200 °C), then the sheet resistance is 1.1 × 105 Ω sq−1. These numbers can be compared to the sheet resistance of samples that have been filled in with HfO2 or Al2O3, to draw conclusions about how the ALD coating process is modifying the ZnO surface chemistry. The sheet resistances of nanocrystal films filled in with HfO2 or Al2O3 are in good agreement with the plateaus observed in an Ar or Ar/H2 atmosphere, respectively (Figure 2). For the HfO2-infilled films, the sheet resistance was 7.2 × 106 Ω sq−1, in good agreement with the sheet resistance obtained after heating uncoated ZnO nanocrystals in Ar (3.0 × 106 Ω sq−1). This suggests that HfO2 removes only physisorbed oxygen species from the ZnO nanocrystal surfaces. The slight discrepancy could be a result of the HfO2 coating process not completely removing all of the physisorbed water.17 To verify this assertion, we measured the FTIR absorption spectrum of uncoated nanocrystals deposited on KBr substrates, as well as nanocrystals on KBr substrates infilled with HfO2 (Figure 3). A variety of features can be observed in the FTIR absorption spectra that correspond to different adsorbed water species. The features around 1500 cm−1 arise from the presence of physisorbed water. The peak at 3400 cm−1 is also from the presence of physisorbed water but is broadened by hydrogen bonding. Chemisorbed water on specific crystal faces results in sharp peaks at different wavenumbers around 3600 cm−1. For example, chemisorbed water on the welldefined (101̅0) and (0001̅) faces appears at 3670 and 3620 cm−1, respectively.11 Our samples have many different exposed crystal faces; therefore, the chemisorbed water peak is broadened in this range, and we observe this feature as a shoulder at approximately 3600 cm−1 (Figure 3). When the
Figure 3. FTIR spectra of ALD-coated and uncoated thin films comprised of ZnO nanocrystals with nominally the same thickness on polished KBr substrates. Films comprised of as-deposited ZnO nanocrystals (blue), HfO2-coated nanocrystals (red), and Al2O3coated nanocrystals (black).
ZnO nanocrystals were coated with HfO2, the intensity of the maximum of the broad feature at 3400 cm−1 decreased and the feature shifted to approximately 3500 cm−1 in the HfO2-coated sample (Figure 3), which is consistent with the removal of physisorbed water from the ZnO nanocrystal surfaces.11 The HfO2 ALD coating process also significantly reduced the intensity of the features near 1500 cm−1 compared to that seen in the uncoated case (Figure 3), which is also consistent with the removal of physisorbed water. However, after HfO2 infill, there was still a measurable signal from a small amount of adsorbed water, which is consistent with previous characterization of HfO2 deposited on Si substrates.18 The sheet resistance of the Al2O3 infilled film (0.94 × 105 Ω −1 sq ) was in excellent agreement with the value obtained after heating the uncoated nanocrystals in an Ar/H2 atmosphere (1.1 × 106 Ω sq−1), which suggests that coating ZnO nanocrystals C
DOI: 10.1021/acs.chemmater.6b02414 Chem. Mater. XXXX, XXX, XXX−XXX
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maintained its stability over several weeks, whereas the sheet resistance of uncoated ITO nanocrystals steadily increased over this period of time (Figure 4). This renders the Al3+ doping hypothesis unlikely. We believe that the reduction of the ZnO and the ITO nanocrystals is related to the exposure to TMA under the ALD conditions. There is extensive evidence of TMA generating oxygen vacancies in ZnO by removing associated oxygen resulting in a net reduction of the ZnO.18,23 It is plausible that a similar reaction occurs on ITO nanocrystal surfaces24 because of the presence of surface hydroxide. While this hypothesis for how the Al2O3 coating process reduces the ZnO and ITO is consistent with our results and the published literature, more work is clearly required to prove the mechanism. In conclusion, from in situ measurements of sheet resistance under a controlled atmosphere, we have shown that the removal of physisorbed oxygen species in the presence of Ar and the removal of chemisorbed water in the presence of H2 are both reversible processes characteristic of a surface effect. Via comparison of sheet resistances measured in situ under these different gas atmospheres to sheet resistances after ALD coating, our results indicate that the ALD coatings modify the interfacial layer by removing surface species. We have demonstrated that physisorbed water species on thin films comprised of ZnO nanocrystals can be removed by coating with either insulating Al2O3 or HfO2 by ALD, resulting in decreased sheet resistance that is stable over long periods of time. Furthermore, evidence from in situ electrical measurements indicates that coating with Al2O3 by ALD reduces the surfaces of ZnO nanocrystals. The reducing effect was also observed when ITO nanocrystals were coated with Al2O3 by ALD, suggesting that the effect may be general.
with Al2O3 by ALD removes physisorbed oxygen species and also reduces the ZnO. Presumably, the reduction is related to the removal of chemisorbed hydroxyl from the nanocrystal surfaces. The FTIR spectra show the disappearance of both the physisorbed and chemisorbed water features after coating with Al2O3. However, a broad peak at approximately 2250 cm−1 appears that can be attributed to surface plasmon resonance resulting from free carrier absorption (Figure 3).16,19−21 Thus, the evidence presented is consistent with the hypothesis that the Al2O3 infill, or more specifically the Al2O3 ALD process, reduces the ZnO surface, in addition to removing physisorbed water. Besides the increased carrier concentration, it is possible that the removal of adsorbed water may also improve coupling between adjacent nanocrystals. The evidence presented so far does not completely rule out the possibility of Al3+ doping of the ZnO, which would also result in a decrease in sheet resistance. We tend to discount the Al3+ doping hypothesis because it would require a coincidence to explain the result in Figure 2. Nevertheless, to test this alternative hypothesis, we performed the following experiment. To examine the possibility of aluminum doping of ZnO, a similar experiment was performed in which a 200 nm thick film comprised of Sn-doped In2O3 (ITO) nanocrystals was filled in with Al2O3 by ALD using the same procedure and number of cycles as for the ZnO nanocrystals. Colloidal ITO nanocrystals with a diameter of 9.5 ± 1.75 nm were synthesized by a previously published procedure22 and deposited by spin-coating (see the Supporting Information for details). Following infill with Al2O3, the sheet resistance was monitored as a function of time in ambient air (Figure 4). If Al3+ doping were the cause of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02414. Plasma and ALD synthesis techniques; characterization methods, including TEM, SEM, ellipsometry, and FTIR; and the in situ electrical measurement configuration (PDF)
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AUTHOR INFORMATION
Corresponding Author
Figure 4. Time-resolved sheet resistance measurements of uncoated (blue triangles) and ALD Al2O3-coated (black squares) films comprised of colloidal ITO nanocrystals. The measurements were performed in ambient air.
*E-mail:
[email protected]. Phone: +1 314 935 6103. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. A. B. F. Martinson and Prof. W. E. Buhro for providing useful feedback in the early stages of this work. We thank the School of Engineering and Applied Sciences at Washington University in Saint Louis (WUStL) for financial support. This work was performed in part using the nanoscale research facility (NRF) at WUStL. D.J.M. and C.S. acknowledge support from the Welch Foundation (F-1848) and the National Science Foundation (CBET-1511826).
the ZnO result, then the Al2O3 coating would have an insignificant effect on the sheet resistance of the film comprised of ITO nanocrystals, because Al3+ is isoelectronic with In3+. However, if the Al2O3 ALD coating process reduces TCO nanocrystals, then we expect a qualitatively similar result (i.e., a decrease in sheet resistance compared to that of uncoated nanocrystals). For ITO nanocrystals, a qualitatively similar decrease in sheet resistance was observed after Al2O3 coating compared to the sheet resistance of the uncoated nanocrystals (Figure 4). This is consistent with the explanation of the ZnO results, suggesting that the Al2O3 coating process reduced the ITO nanocrystals by a similar mechanism. Furthermore, the Al2 O3 coating
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ABBREVIATIONS TCO, transparent conductive oxide; ALD, atomic layer deposition; TEM, transmission electron microscopy; SEM, D
DOI: 10.1021/acs.chemmater.6b02414 Chem. Mater. XXXX, XXX, XXX−XXX
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Tunable via Al Doping and Quantum Confinement. Nano Lett. 2015, 15, 8162−8169. (20) Buonsanti, R.; Llordes, A.; Aloni, S.; Helms, B. A.; Milliron, D. J. Tunable Infrared Absorption and Visible Transparency of Colloidal Aluminum-Doped Zinc Oxide Nanocrystals. Nano Lett. 2011, 11, 4706−4710. (21) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361−366. (22) Choi, S.-I.; Nam, K. M.; Park, B. K.; Seo, W. S.; Park, J. T. Preparation and Optical Properties of Colloidal, Monodisperse, and Highly Crystalline ITO Nanoparticles. Chem. Mater. 2008, 20, 2609− 2611. (23) Elam, J. W.; George, S. M. Growth of ZnO/Al2O3 Alloy Films Using Atomic Layer Deposition Techniques. Chem. Mater. 2003, 15, 1020−1028. (24) Personal communication with A. Mane and J. Elam at Argonne National Laboratory (Argonne, IL).
scanning electron microscopy; FTIR, Fourier transform infrared spectroscopy; ITO, indium:tin oxide; TMA, trimethylaluminum
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REFERENCES
(1) Luo, L.; Bozyigit, D.; Wood, V.; Niederberger, M. High-Quality Transparent Electrodes Spin-Cast from Preformed Antimony-Doped Tin Oxide Nanocrystals for Thin Film Optoelectronics. Chem. Mater. 2013, 25, 4901−4907. (2) Lee, J.; Zhang, S.; Sun, S. High-Temperature Solution-Phase Syntheses of Metal-Oxide Nanocrystals. Chem. Mater. 2013, 25, 1293− 1304. (3) Hoel, C. A.; Mason, T. O.; Gaillard, J.-F.; Poeppelmeier, K. R. Transparent Conducting Oxides in the ZnO-In2O3-SnO2 System. Chem. Mater. 2010, 22, 3569−3579. (4) Diroll, B. T.; Gordon, T. R.; Gaulding, E. A.; Klein, D. R.; Paik, T.; Yun, H. J.; Goodwin, E. D.; Damodhar, D.; Kagan, C. R.; Murray, C. B. Synthesis of N-Type Plasmonic Oxide Nanocrystals and the Optical and Electrical Characterization of Their Transparent Conducting Films. Chem. Mater. 2014, 26, 4579−4588. (5) Liang, X.; Ren, Y.; Bai, S.; Zhang, N.; Dai, X.; Wang, X.; He, H.; Jin, C.; Ye, Z.; Chen, Q.; Chen, L.; Wang, J.; Jin, Y. Colloidal IndiumDoped Zinc Oxide Nanocrystals with Tunable Work Function: Rational Synthesis and Optoelectronic Applications. Chem. Mater. 2014, 26, 5169−5178. (6) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 1776−1779. (7) Li, Y.; Della Valle, F.; Simonnet, M.; Yamada, I.; Delaunay, J.-J. Competitive Surface Effects of Oxygen and Water on UV Photoresponse of ZnO Nanowires. Appl. Phys. Lett. 2009, 94, 023110. (8) Gu, H.; Wang, Z.; Hu, Y. Hydrogen Gas Sensors Based on Semiconductor Oxide Nanostructures. Sensors 2012, 12, 5517−5550. (9) Ahn, S. E.; Lee, J. S.; Kim, H.; Kim, S.; Kang, B. H.; Kim, K. H.; Kim, G. T. Photoresponse of Sol-Gel-Synthesized ZnO Nanorods. Appl. Phys. Lett. 2004, 84, 5022−5024. (10) Takahashi, Y.; Kanamori, M.; Kondoh, A.; Minoura, H.; Ohya, Y. Photoconductivity of Ultrathin Zinc Oxide Films. Jpn. J. Appl. Phys. 1994, 33, 6611−6615. (11) Noei, H.; Qiu, H.; Wang, Y.; Löffler, E.; Wöll, C.; Muhler, M. The Identification of Hydroxyl Groups on ZnO Nanoparticles by Infrared Spectroscopy. Phys. Chem. Chem. Phys. 2008, 10, 7092−7097. (12) Yasumoto, I. Adsorption of Water, Ammonia, and Carbon Dioxide on Zinc Oxide at Elevated Temperatures. J. Phys. Chem. 1984, 88, 4041−4044. (13) Li, C.; Li, L.; Du, Z.; Yu, H.; Xiang, Y.; Li, Y.; Cai, Y.; Wang, T. Rapid and Ultrahigh Ethanol Sensing Based on Au-Coated ZnO Nanorods. Nanotechnology 2008, 19, 035501. (14) Thomas, D. G.; Lander, J. J. Hydrogen as a Donor in Zinc Oxide. J. Chem. Phys. 1956, 25, 1136−1142. (15) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Fabrication and Ethanol Sensing Characteristics of ZnO Nanowire Gas Sensors. Appl. Phys. Lett. 2004, 84, 3654−3656. (16) Thimsen, E.; Johnson, M.; Zhang, X.; Wagner, A. J.; Mkhoyan, K. A.; Kortshagen, U. R.; Aydil, E. S. High Electron Mobility in Thin Films Formed via Supersonic Impact Deposition of Nanocrystals Synthesized in Nonthermal Plasmas. Nat. Commun. 2014, 5, 5822. (17) Ho, M.-T.; Wang, Y.; Brewer, R. T.; Wielunski, L. S.; Chabal, Y. J.; Moumen, N.; Boleslawski, M. In Situ Infrared Spectroscopy of Hafnium Oxide Growth on Hydrogen-Terminated Silicon Surfaces by Atomic Layer Deposition. Appl. Phys. Lett. 2005, 87, 133103. (18) Lin, Y.-Y.; Hsu, C.-C.; Tseng, M.-H.; Shyue, J.-J.; Tsai, F.-Y. Stable and High-Performance Flexible ZnO Thin-Film Transistors by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2015, 7, 22610− 22617. (19) Greenberg, B. L.; Ganguly, S.; Held, J. T.; Kramer, N. J.; Mkhoyan, K. A.; Aydil, E. S.; Kortshagen, U. R. NonequilibriumPlasma-Synthesized ZnO Nanocrystals with Plasmon Resonance E
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