Atomic Layer Deposited Coatings on Nanowires for High Temperature

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Atomic Layer Deposited Coatings on Nanowires for High Temperature Water Corrosion Protection Alexander S. Yersak, Ryan J. Lewis, Li-Anne Liew, Rongfu Wen, Ronggui Yang, and Yung-Cheng Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11963 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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ACS Applied Materials & Interfaces

Atomic Layer Deposited Coatings on Nanowires for High Temperature Water Corrosion Protection Alexander S. Yersak#, *, Ryan J. Lewis*, Li-Anne Liew*, Rongfu Wen*, Ronggui Yang*, and YungCheng Lee* *

Department of Mechanical Engineering

University of Colorado, Boulder, Colorado 80309-0427, United States KEYWORDS: Atomic layer deposition, TiO2, Al2O3, copper, silicon, water corrosion, protective coating, and nanowires.

ABSTRACT

Two-phase liquid-cooling technologies incorporating micro/nanostructured copper or silicon surfaces have been established as a promising thermal management solution to keep up with the increasing power demands of high power electronics. However, the reliability of nanometerscale features of copper and silicon in these devices have not been well investigated. In this work, accelerated corrosion testing reveals that copper nanowires are not immune to corrosion in de-aerated pure hot water. To solve this problem, we investigate atomic layer deposition (ALD) TiO2 coatings grown at 150 oC and 175 oC. We measured no difference in coating thickness for a duration of 12 days. Using a core/shell approach we grow ALD TiO2/Al2O3 protective coatings

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on copper nanowires, and demonstrate a preservation of nano-engineered copper features. These studies have identified a critical reliability problem of nanoscale copper and silicon surfaces in de-aerated, pure, hot water, and have successfully demonstrated a reliable solution using ALD TiO2/Al2O3 protective coatings.

INTRODUCTION Micro/nanostructured copper-1–8 and silicon-5,9 enabled two-phase liquid-cooling technologies have been established as promising solutions for the increasing thermal management demands of high power electronics. Indeed, surface structuring for enhancing phase-change heat transfer is becoming one of the research frontiers in heat transfer.1,10,11 However, copper in de-aerated water has been reported to corrode at a rate of ~1 nm/day at room temperature12,13 and up to ~1 µm per year in the 73 oC water14 which raises significant reliability concerns for using nanoscale copper features in these novel thermal management devices. Likewise, the reliability of nanostructured silicon in these devices is also a concern as silicon is susceptible to dynamic process of oxidation15 and dissolution12,16,17 in water with a dissolution rate of the oxide, e.g., 2.5 nm/day at 101 oC for a wet thermally grown SiO2 on Si12. To solve this corrosion problem, a ceramic coating may be used in these nano-enabled devices as a corrosion barrier to protect nanoscale features of copper and silicon. Roll-to-roll (R2R) ALD is ideal for coating high aspect ratio micro/nano-structures18, and has already been demonstrated as a viable manufacturing method for conformal coatings of porous (i.e. high aspect ratio structures) battery materials.19,20 In this work, we study the stability of ALD TiO2 coatings grown at 150 oC and 175 oC and examine the capability of these coatings to uniformly protect copper nanowires using an accelerated corrosion test using 150 oC water.


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Along this route, previous studies have demonstrated protective ceramic coatings for silicon nanowires in 37 oC water using 10 nm thick ALD Al2O3 protective coatings15. However, ALD Al2O3 coatings have also been reported to suffer damaging effects when in contact with water21– 24

and were not suitable as protective coatings for water in a 90 oC environment24. To protect

copper from corrosion in >90 oC water environment, amorphous ALD TiO2 coatings grown at 120 oC on a 8 nm seed layer of Al2O3 have been reported, but the coating dissolved at a rate of ~0.2 nm/day24. Dissolution of ALD TiO2 in 110 oC sulphuric acid has been reported to be a strong function of the growth temperature of the coatings on silicon with a dissolution rate of ~12 nm/min and below the measurement limit of 0.06 nm/min for growth temperatures of 100 oC and 175 oC, respectively.25 Enhanced stability of such ALD TiO2 films grown above 150 oC coincide with the reports of polycrystalline anatase regions in the coatings with reduced chemical defects.22,25–27 A reduction in the dissolution rate achieved for ALD TiO2 in hot sulphuric acid by increasing the deposition temperature from 100 oC to 150 oC is therefore a promising solution for achieving enhanced stability of ALD TiO2 coatings in other liquid media such as >90 oC deaerated pure water. Furthermore, from a thermodynamics perspective it has been reported that chemically pure tetravalent TiO2 is stable in pure water up to 150 oC with no hydroxide or oxyanion species with molar activity of 10-6 in solution with pH from -2 to 16.28 Above 150 oC, oxyanion species exist in water at a pH near 16.28 We suggest that if there exists an unknown or known thermodynamically stable species of aqueous TiO2 for ALD TiO2 coatings immersed in pure water at 90 oC, we can increase the kinetics of the reaction by immersing the coatings in superheated water at 150 oC, in a similar manner to that which has been done for silica.12,17 For example, the dissolution rate of ALD SiO2 immersed in water has been reported to increase by


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two orders-of-magnitude from 6.0 to 367 nm/day when the water temperature is increased from 101 oC to 159 oC.12 Herein, we investigate the capability of ALD TiO2 and Al2O3 coatings to protect copper nanowires under accelerated corrosion conditions. We show no thickness reduction of ALD TiO2 grown at 150 oC and 175 oC on Si samples under the accelerated superheated water conditions. ALD TiO2/Al2O3 films on copper nanowires under the identical superheated water conditions showed no measurable thickness reduction in ALD TiO2/Al2O3 coatings as measured by scanning electron microscopy (SEM). Conversely, unprotected Cu nanowires (NWs) were observed to corrode in less than two days. Hence, we demonstrate a stable ALD TiO2/Al2O3 coating to uniformly protect against high temperature water corrosion of copper and nanowires used in two-phase liquid-cooling technology for thermal management of high power electronics.

EXPERIMENTAL SECTION ALD Growth Conditions. ALD TiO2 coatings were grown in a Beneq TFS 200 reactor at a chamber temperature of 150 oC and 175 oC on a Si wafer. ALD cycle times were 125 ms TiCl4 pulse, 500 ms purge, 125 ms H2O pulse, and 750 ms purge. ALD Al2O3 and TiO2 coatings were grown in a Beneq TFS 200 reactor tool at a chamber temperature of 175 oC on copper nanowires. ALD cycle times for ALD Al2O3 were 1.25 s trimethylaluminum pulse, 10 s purge, 1.25 s H2O pulse, and 15 s purge. ALD cycle times for ALD TiO2 were 1.5 s TiCl4 pulse, 15 s purge, 1.5 s H2O pulse, and 15 s purge. Copper nanowires were coated with 145 cycles of ALD Al2O3 which serves as an adhesion layer to protect the native copper oxide from the byproducts of ALD TiO2 using TiCl4.24 Without an adhesion layer, ALD TiO2 suffers nucleation difficulties on the native oxide of copper, and its growth is dominated by an island growth mechanism which does not


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produce a uniform protective coating.24 Subsequently, 750 cycles of ALD TiO2 was coated on the ALD Al2O3 coated copper nanowires.

Hyperbaric Corrosion Chamber With in Situ Monitoring of Film Thickness. Both ALD coated and uncoated samples were immersed in 75 mL of degassed water (high performance liquid chromatography grade) in an aluminum hyperbaric chamber with a pressure-rated borosilicate glass window as shown in Figure 1.12

Figure 1. (a) Aluminum hyperbaric chamber with in situ reflectometry. (b) Cross-sectional drawing of (a). Reproduced with permission from ref 12.


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The fiberglass insulated chamber was pressurized to ~11 bar to allow liquid water at 150 oC. The chamber temperature was monitored with an external thermocouple at the vertical height of the samples. The in situ and ex situ ALD TiO2 coating thickness on the Si samples were monitored using reflectometry (FILMETRICS F20-UV), following a method developed previously in conjunction with the hyperbaric chamber.12 The reflectometer was baselined using Si at 150 oC. The optical model was air (medium), roughness (layer), TiO2 (layer), and Si (substrate). The geometrical thickness was calculated using FILMeasure 7.0 software by analyzing the pure reflectance spectra (400 to 900 nm). Both the surface roughness and film thickness were allowed to vary when calculating the best fit to the raw reflectance spectra. FILMeasure 7.0 software accounts for expected dispersion optical properties of the medium, layers, and substrate in the optical model.29

Ex Situ Characterization. All samples were heated in a rough vacuum at 90 oC for 15 minutes to remove excess water after immersion in the hyperbaric corrosion chamber. Samples were imaged at 20 kV using a JEOL JSM6480LV SEM. Cross-sections were imaged at 5 kV with a FEI Nova 600 NanoLab with a 52o tilt. Atomic force microscopy (AFM) scans were performed using a NanoSurf EasyScan. Energy dispersive x-ray spectroscopy (EDS) measurements were performed in a JEOL JSM7610F SEM. X-ray diffraction (XRD) measurements were performed with a D2 PHASER.

Copper Nanowire Growth. Porous anodic alumina (PAA) templates (nominal pore size 0.2 µm, GE Healthcare) were used to fabricate copper nanowires by the template-assisted electroplating


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method.30–32 As shown in Figure 2, we employed a two-step electroplating process for copper nanowire growth. The PAA template was first placed on the clean copper sample and the electrolyte solution composed of cupric pyrophosphate (Cu2P2O7·xH2O, Sigma-Aldrich 344699), potassium pyrophosphate (K4P2O7, Sigma-Aldrich 322431), ammonium citrate tribasic (C6H17N3O7, Sigma-Aldrich A1332), and DI water (6:25:2:100wt%) was pipetted on top of the template. After sufficiently wetting the template, a filter pater wetted with electrolyte solution was placed on the template and create a conductive channel. Another copper plate was then used as counter electrode on the filter paper. The uniform pressure was exerted to make the PAA template adhere on the sample surface. A constant voltage of -0.8 V (Electrochemical Workstation, CH Instrument, CHI760C) was applied between the counter electrode and the sample for 15 min. During this first step, the nanowires were grown on the surface to serve as the screws and to bond the PAA template on copper substrate. Then, the copper sample with PAA template on the top was released from the sandwich structure and placed in a 3-electrode electroplating cell. During this second step, a constant voltage of -0.8 V versus reference electrode (Ag/AgCl) was applied for 20 min to deposit the nanowires. The obtained copper nanowires were released from the PAA template by immersing the sample into 2.0 M sodium hydroxide solution, and then the sample was washed with deionized water to remove the residual solution and dried with clean nitrogen gas. The length of the nanowires are around 30 µm to 35 µm. The high surface tension of water caused agglomeration of nanowires when the nanowires were released and dried.


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Figure 2. Schematics of the two-step porous anodized alumina (PAA)-templated nanowire fabrication. (a) Bonding process. (b) Electro-deposition process.

RESULTS AND DISCUSSION In Situ Dissolution Measurements of ALD TiO2 on Si. To assess the long-term reliability and stability of ALD TiO2 coatings we monitored the thickness of ALD coatings grown on Si substrates in a closed chamber using an in situ reflectometer. Such witness tests, under identical accelerated superheated water conditions of 150 oC as the ALD coated copper nanowire tests, allowed for quantitative measurements of a dissolution rate that are difficult for coatings on copper nanowires. ALD TiO2 films with different thicknesses were measured ex situ and in situ to validate the in situ reflectometry technique to measure nm-scale thickness reductions of nmthick coatings in this hyperbaric chamber. Ex situ and in situ film thickness measurements for ALD TiO2 films grown on Si for 250, 500, 750, and 1000 ALD cycles are shown in Figure 3. A growth rate at 0.057 and 0.058 nm/cycle was obtained from in situ and ex situ reflectometry, respectively, which shows great agreement between in situ and ex situ reflectometry. This measured growth rate is also in great agreement with the reported values of ALD TiO2 coatings using TiCl4 and water precursors.27 In situ TiO2 measurements were taken from samples immersed in water at room temperature in the hyperbaric chamber. To confirm the reflectometry


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measurements, a step height at the ALD TiO2 and Si edges were measured using AFM, as shown in Figure 3b, with values of ~17, 27, 44, and 59 nm for 250, 500, 750, and 1000 ALD cycles, respectively. The measurement using in situ reflectometry showed that ALD TiO2 thicknesses were 15, 29, 43, and 58 nm for 250, 500, 750, and 1000 ALD cycles, respectively. AFM measurements were comparable to the thickness values of the reflectometer validating the reflectometry methods to machine accuracy.

Figure 3. Measurement techniques are compared. (a) Growth rate of 0.057 (solid line) and 0.058 nm/cycle (dashed line) were measured using in situ and ex situ reflectometry, respectively, for


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ALD TiO2 films grown at 150 oC. Error bars are for machine error of ±2 nm. (b) AFM step height profiles for 250, 500, 750, and 1000 cycles of ALD TiO2 on Si.

Stability of ALD TiO2 in 150 oC Water. The in situ thickness reflectometry was used as a monitoring technique to quantify the dissolution rate in the 150 °C water, of the two ALD TiO2 coatings, grown at 150 oC and 175 oC, as shown in Figure 4. The ALD TiO2 films grown at both 150 oC and 175 oC were observed by reflectometry with no measurable dissolution during the measuring period of ~12 days as measured by the reflectometer and focused ion beam-scanning electron microscopy (FIB-SEM). After ~12 days, the root-mean-square (RMS) micro-roughness was ~6 nm and was measured by AFM over a 4x4 µm area for the ALD TiO2 grown at 150 oC, as shown in Figure 4a. Before immersion the RMS micro-roughness of the ALD TiO2 coatings was ~1 nm as measured by AFM.


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Figure 4. Topography (a) of ALD TiO2 grown at 150 oC after immersed in 150 oC water for ~12 days, and (b) Dissolution rates of ALD TiO2 grown at 150 oC and 175 oC in 150 oC water for 12 days. To investigate the micro-roughness and TiO2 film thickness after 12 days, we used FIB-SEM cross-sectional imaging to measure the thickness of the ALD TiO2 coating grown at 150 oC before and after 12 days of immersion in 150 oC water as shown in Figure 5. The initial ALD TiO2 film thickness of ~45 nm was unchanged after 12 days as observed by a distinct film thickness on the Si substrate. Corrosion products from the glass window of this hyperbaric chamber were observed as scattered islands on the ALD TiO2 coating as shown in Figure 5b. Although during this test the TiO2 film RMS micro-roughness increased from ~1 nm to ~6 nm as


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measured by AFM, a longer test would be needed to further investigate the effects of microsurface roughening on the thickness reduction rate of ALD TiO2 films.

Figure 5. FIB-SEM cross section measurements of the thickness of ALD TiO2 coating grown at 150 oC: (a) before immersion and (b) after 12 days of immersion in 150 oC water showing no reduction in coating thickness, but does show deposition of islands of glass corrosion products (arrow labeled island). The step edge in (a) was created by masking the Si sample with the Kapton tape prior to ALD TiO2 growth process. Step edges were used to measure the ALD TiO2 coating thickness with AFM.


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To assess the survival and global capability of the ALD TiO2 coatings to uniformly protect Si substrates from corrosion, we selectively etched the samples in XeF2 for 1 hour as shown in Figure 6. ALD TiO2 will not be attacked by Si etchant XeF2.33

Figure 6. Optical microscope images of ALD TiO2 coatings after 12 days of immersion in 150 oC water, shown are ALD TiO2 grown at 150 oC (a) and 175 oC (b) on Si are shown. Note the discoloration of the film, which results from the deposited corrosion products of glass. Subsequent decoration of pinholes in the ALD TiO2 coatings by using XeF2 Si etching for 1 hour, shown are ALD TiO2 grown at 150 oC (c) and 175 oC (d). Note the XeF2 dissolved the deposited glass corrosion products, and the underlying TiO2 suffers less discoloration.


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Without a protective coating, the Si substrate would have been attacked by the XeF2 and would roughen, with dark features appearing on optical microscopy. Thus, if the ALD TiO2 coating were dissolved and/or compromised by some unknown mechanism in the 150 oC water after 12 days, the samples would be subject to systematic roughening across the surface and would appear dark to optical microscopy. Any nanoscale pinholes, cracks, and significant roughening (i.e. substrate exposure) in the ALD film would result in an undercutting process for the silicon by which an unobservable feature would be enlarged and visible as a black artifact (i.e. roughening Si substrate). No cracking was observed in Figure 6c,d. A pinhole density was measured as ~54 pinholes/cm2 and 0 pinholes/cm2 for silicon samples with ALD TiO2 grown at 150 oC and 175 oC growth temperatures, respectively. Such pinhole density is common for asgrown ALD films, due to extrinsic defects such as particle contaminants in the reactor.34,35 A low pinhole density, no cracking, and no significant roughening (i.e. substrate exposure) in the ALD films revealed a global capability of the ALD TiO2 coatings to uniformly protect Si substrates from corrosion when immersed in water at 150 oC for 12 days.

Stability of Copper Nanowires in 150 oC Water. Without a protective barrier, copper nanowires grown using porous alumina templates were observed to corrode in less than two days after immersing in deareated 150 oC water as shown in Figure 7. We propose that copper nanowires act as ultramicroelelectrodes36 and corrode into Cu(I) species37 via local anodes, and redeposit on local cathodes as HxCuOy.12,13,36,37 Areas on the copper nanowires serving as local cathodes continue to grow via a diffusion-controlled process, and consume Cu(I) species37 dissolved in the water from the corrosion of copper sources serving as local anodes. We suggest a formation of HxCuOy nanowires based on reports of HxCuOy as the majority corrosion species


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for corrosion of copper foil in deareated, pure, water.13,14 Further, the cone-shaped outward growth of the HxCuOy nanowires in Figure 7d suggests that copper nanowire cathodes act as the seed for the growth, and this shape is also observed in free suspended copper particle growth of HxCuOy nanowires.36 Copper nanowires may also corrode directly into Cu2O wires.38

Figure 7. (a) In situ optical images of uncoated copper nanowires immersed for 0, 2, 16, and 48 hours, showing the onset of corrosion by discoloration after 2 hours. False-colored SEM images of uncoated copper nanowires before (b, c) and after two days immersed in 150 oC water (d, e). To further investigate the corrosion mechanisms of the copper nanowires that were immersed in 150 oC water for 2 days from Figure 7, the crystal structure and elemental signals were probed


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with XRD and EDS, as shown in Figure 8a respectively. Elemental analysis for the copper nanowires immersed in 150 oC water for 2 days in Figure 8a indicate increased oxygen content in comparison with the sample that was not exposed to water, which supports the evidence that both HxCuOy and Cu2O may form on the nanowire. The crystal structure of the nanowires before and after immersion in 150 oC water for 2 days in Figure 8b shows the presence of a crystal structure of Cu2O. However, the crystal structure of Cu2O and HxCuOy and hydrated HxCuOy are identical38, and cannot be distinguished with XRD patterns. As a result, we show evidence that copper nanowires are either consumed and reformed under our test conditions as HxCuOy wires and/or Cu2O wires.

Figure 8. (a) EDS and (b) XRD of copper nanowires before and after immersion in 150 oC water for 2 days.


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Stability of ALD TiO2/Al2O3 Coated Copper Nanowires.

With ALD TiO2 and Al2O3

protective coatings the copper nanowires were intact under SEM inspection after 14 days of immersion in 150 oC water as shown in Figure 9. These samples were immersed nonstop for 12 days, and then for an additional two days with fresh water to remove any the corrosion products of the glass window, such as shown in Figure 5b, from the copper NW surfaces. In situ optical images in Figure 9a showed no discoloration of the ALD coated copper nanowires as compared to the discolorations observed in the uncoated nanowires presented in Figure 7a. Further, no discoloration was observed up to the duration of 14 days which indicates no corrosion of the copper.


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Figure 9. (a) In situ optical images of ALD TiO2/Al2O3 coatings grown at 175 oC on copper nanowires immersed for 0, 2, 16, and 48 hours. False-colored SEM images of copper nanowires with ALD TiO2/Al2O3 coatings grown at 175 oC before (b, c) and after 14 days immersed in 150 o

C water (d, e).

No reduction in the film thickness of the ALD TiO2/Al2O3 protective shell was observed after 14 days of immersion in 150 oC water as shown in Figure 10. That no reduction in the total ALD TiO2/Al2O3 coating thickness of ~55 nm was observed further substantiates film thickness measurements of the ALD TiO2 coatings on Si as shown in Figure 4. The ALD TiO2/Al2O3 coated copper nanowires in the SEM images in Figure 10(b-d) is shown as the darker region between edge charging of the coating and the observed brighter copper nanowires in the center. We grew ALD TiO2/Al2O3 coatings of ~110 nm on another sample of copper nanowires which substantiated that the dark shell with edge charging around the copper nanowire coating in fact corresponds to actual the thickness of the ALD coating as shown in the Supporting Information (Figure S1).


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Figure 10. False-colored SEM images of ALD TiO2/Al2O3 coatings grown at 175 oC on copper nanowires before (a, b) and copper nanowires with ALD TiO2/Al2O3 after two days (c) and 14 days (d) immersed in 150 oC water.

Titanium and aluminum signals were both detected by EDS in Figure 11 for the copper nanowires with ALD TiO2/Al2O3 coatings from Figure 10, and substantiated the survival of the


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ALD coatings. These samples were immersed nonstop for 12 days, and then for an additional two days with fresh water to remove any trace corrosion products of the glass window (i.e. island surface contaminates as shown in Figure 5b) from the copper NW surfaces. As a result, no Si signals were detected after 14 days immersed in 150 oC water. Before the samples were immersed in fresh water for a further two days at 150 oC, Si signals were detected by EDS as shown the Supporting Information (Figure S2).

Figure 11. EDS (100 s) of ALD TiO2/Al2O3 coatings grown at 175 oC on copper nanowires after 14 days immersed in 150 oC water.

CONCLUSIONS In summary, we have demonstrated robust ALD TiO2/Al2O3 protective coatings on copper nanowires in 150 oC water. ALD TiO2 coatings grown at 150 oC and 175 oC were measured by in situ reflectometry with no difference in coating thickness for a duration of 12 days immersed in 150 oC water. We show that copper nanowires without a protective ALD coating will corrode in less than two days. This work establishes a reliable core/shell nanowire approach to protect nmscale engineered copper and silicon features in two-phase liquid-cooling technologies with water


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> 90 oC for thermal management of high power electronics. Finally, we note that ALD coatings of Al2O3 and TiO2 generally reduce the effective thermal conductivity of copper or silicon nanowires due to the lower thermal conductivity of ALD coatings, usually at 1-3 W/mK, in comparison with that of copper and silicon. If the diameters of the NWs are 200-500 nm and the ALD shell is 10-20nm, the effective thermal conductivity along the wire axis is reduced by 1020%. However, the thermal conductivity reduction might not be as important as the gains in corrosion resistance and the potential in altering the wettability using ALD coatings for phasechange heat transfer enhancement. More research is needed for systematic optimization.

ASSOCIATED CONTENT Supporting Information Available: SEM of ~110 nm ALD TiO2/Al2O3 copper nanowires and EDS of ALD TiO2/Al2O3 coatings on copper NWs after 12 days of immersion in water. AUTHOR INFORMATION Corresponding Author #

E-mail: [email protected]

ACKOWEDGEMENTS This work was supported by the NSF through SNM: Roll-to-Roll Atomic/Molecular Layer Deposition Award No. CBET 1246854 awarded to the University of Colorado. This work was also supported by a grant from the Intelligence Community Postdoctoral Research Fellowship Program through funding from the Office of the Director of National Intelligence. All statements of fact, opinion, or analysis expressed are those of the author and do not reflect the official positions or views of the Intelligence Community or any other U.S. Government agency.


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Nothing in the contents should be construed as asserting or implying U.S. Government authentication of information or Intelligence Community endorsement of the author’s views. The authors would like to thank C. Holub for fabricating the hyperbaric test chamber and Tomoko Borsa from the NCF at Boulder for FIB-SEM and EDS.

ABBREVIATIONS ALD, atomic layer deposition; roll-to-roll, R2R; SEM, scanning electron microscopy; nanowires, NWs; atomic force microscopy, AFM; energy dispersive x-ray spectroscopy, EDS; X-ray diffraction, XRD; focused ion beam-scanning electron microscopy, FIB-SEM; porous anodized alumina, PAA; and root-mean-square, RMS.


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For Table of Contents Only.


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Figure 1. (a) Aluminum hyperbaric chamber with in situ reflectometry. (b) Cross-sectional drawing of (a). Reproduced with permission from ref 12. 107x135mm (300 x 300 DPI)


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Figure 2. Schematics of the two-step porous anodized alumina (PAA)-templated nanowire fabrication. (a) Bonding process. (b) Electro-deposition process. 51x31mm (300 x 300 DPI)



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Figure 3. Measurement techniques are compared. (a) Growth rate of 0.057 (solid line) and 0.058 nm/cycle (dashed line) were measured using in situ and ex situ reflectometry, respectively, for ALD TiO2 films grown at 150 oC. Error bars are for machine error of ±2 nm. (b) AFM step height profiles for 250, 500, 750, and 1000 cycles of ALD TiO2 on Si. 132x207mm (300 x 300 DPI)


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Figure 4. Topography (a) of ALD TiO2 grown at 150 oC after immersed in 150 oC water for ~12 days, and (b) Dissolution rates of ALD TiO2 grown at 150 oC and 175 oC in 150 oC water for 12 days. 124x183mm (300 x 300 DPI)



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Figure 5. FIB-SEM cross section measurements of the thickness of ALD TiO2 coating grown at 150 oC: (a) before immersion and (b) after 12 days of immersion in 150 oC water showing no reduction in coating thickness, but does show deposition of islands of glass corrosion products (arrow labeled island). The step edge in (a) was created by masking the Si sample with the Kapton tape prior to ALD TiO2 growth process. Step edges were used to measure the ALD TiO2 coating thickness with AFM. 117x161mm (300 x 300 DPI)


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Figure 6. Optical microscope images of ALD TiO2 coatings after 12 days of immersion in 150 oC water, shown are ALD TiO2 grown at 150 oC (a) and 175 oC (b) on Si are shown. Note the discoloration of the film, which results from the deposited corrosion products of glass. Subsequent decoration of pinholes in the ALD TiO2 coatings by using XeF2 Si etching for 1 hour, shown are ALD TiO2 grown at 150 oC (c) and 175 oC (d). Note the XeF2 dissolved the deposited glass corrosion products, and the underlying TiO2 suffers less discoloration. 129x98mm (300 x 300 DPI)



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Figure 7. (a) In situ optical images of uncoated copper nanowires immersed for 0, 2, 16, and 48 hours, showing the onset of corrosion by discoloration after 2 hours. False-colored SEM images of uncoated copper nanowires before (b, c) and after two days immersed in 150 oC water (d, e). 138x112mm (300 x 300 DPI)


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Figure 8. (a) EDS and (b) XRD of copper nanowires before and after immersion in 150 oC water for 2 days. 120x170mm (300 x 300 DPI)



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Figure 9. (a) In situ optical images of ALD TiO2/Al2O3 coatings grown at 175 oC on copper nanowires immersed for 0, 2, 16, and 48 hours. False-colored SEM images of copper nanowires with ALD TiO2/Al2O3 coatings grown at 175 oC before (b, c) and after 14 days immersed in 150 oC water (d, e). 135x107mm (300 x 300 DPI)


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Figure 10. False-colored SEM images of ALD TiO2/Al2O3 coatings grown at 175 oC on copper nanowires before (a, b) and copper nanowires with ALD TiO2/Al2O3 after two days (c) and 14 days (d) immersed in 150 o C water. 166x162mm (300 x 300 DPI)



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Figure 11. EDS (100 s) of ALD TiO2/Al2O3 coatings grown at 175 oC on copper nanowires after 14 days immersed in 150 oC water. 69x57mm (300 x 300 DPI)


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Atomic Layer Deposited Coatings on Nanowires for High Temperature

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