Enhancing the Thermal Conductance of Polymer and Sapphire Interface

Aug 8, 2016 - -H, were introduced into sapphire/PS interface, and their effects on ITC were ... by a factor of 7 through functionalizing the sapphire ...
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Enhancing the Thermal Conductance of Polymer and Sapphire Interface via SelfAssembled Monolayer Kun Zheng,†,∥ Fangyuan Sun,‡,∥ Jie Zhu,*,‡,⊥ Yongmei Ma,*,† Xiaobo Li,§ Dawei Tang,‡ Fosong Wang,† and Xiaojia Wang⊥ †

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China § School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China ⊥ Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡

S Supporting Information *

ABSTRACT: Interfacial thermal conductance (ITC) receives enormous consideration because of its significance in determining thermal performance of hybrid materials, such as polymer based nanocomposites. In this study, the ITC between sapphire and polystyrene (PS) was systematically investigated by time domain thermoreflectance (TDTR) method. Silane based self-assembled monolayers (SAMs) with varying end groups, -NH2, -Cl, -SH and -H, were introduced into sapphire/PS interface, and their effects on ITC were investigated. The ITC was found to be enhanced up by a factor of 7 through functionalizing the sapphire surface with SAM, which ends with a chloride group (-Cl). The results show that the enhancement of the thermal transport across the SAMfunctionalized interface comes from both strong covalent bonding between sapphire and silane-based SAM, and the high compatibility between the SAM and PS. Among the SAMs studied in this work, we found that the ITC almost linearly depends on solubility parameters, which could be the dominant factor influencing on the ITC compared with wettability and adhesion. The SAMs serve as an intermediate layer that bridges the sapphire and PS. Such a feature can be applied to ceramic-polymer immiscible interfaces by functionalizing the ceramic surface with molecules that are miscible with the polymer materials. This research provides guidance on the design of critical-heat transfer materials such as composites and nanofluids for thermal management. KEYWORDS: interfacial thermal conductance, solubility parameter, organic−inorganic interface, miscibility, time domain thermoreflectance layer.6−8 For these interfaces, transitioning from van der Waals to covalent bonding between gold (Au) and quartz (Qz) via a self-assembled monolayer (SAM) increases the thermal conductance by 80%.8 Similarly, incorporation of a strongly bonded SAM at the interface allows for a 4-fold increase of the ITC in the copper-silica system.7 The thermal conductance of the Al/monolayer graphene interface is also reported to be increased by oxygen functionalization.6 This effect demonstrates the powerful role of chemical bonding on thermal transport across graphene interfaces, and promises a way for tuning the ITC by surface modification with SAMs.6

P

olymer-matrix composite materials combine merits of their components: high thermal conductivity and strength from a metal or ceramic material, alongside low cost, lightweight, electrical insulation, easy processing, and corrosion resistance of the polymer matrix in which inorganic reinforcement is embedded. The ease of thermal transfer across their interfaces is the key to design and fabrication of thermally conductive organic−inorganic hybrids, and hence their use in heat-sinking applications.1−4 In general, the thermal conductance of an interface tends to scale with its bonding strength.5−7 Enhancement of either the mechanical or chemical adhesion of the interfaces has been carried out to improve and to understand the nature of thermal transport across the interface. The interfacial thermal conductance between metal and inorganic materials can be enhanced significantly by introducing a chemically bonded © 2016 American Chemical Society

Received: May 22, 2016 Accepted: August 8, 2016 Published: August 8, 2016 7792

DOI: 10.1021/acsnano.6b03381 ACS Nano 2016, 10, 7792−7798

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Figure 1. Sample preparation and characterization. (a) A schematic of formation of SAMs. (b) The contact angle of raw sapphire, hydoxylized sapphire and silanized sapphire. (c) Surface profile of the -Cl terminated SAM measured by AFM. Scan area is 3 μm by 3 μm, and the root square roughness is 0.3 nm. (d) The high-resolution core level XPS spectra in the Si2p region for SAMs. (e) The configuration of Al/PS/SAM/ Sapphire prepared for TDTR measurement.

hydroxylated sapphire surface with a silane coupling reagent (see Experimental Section). The same length of carbon chain silane with different end groups, including Triethoxysilypropyl Chloride (-Cl), 3-Mercaptopropyltriethoxysilane (-SH), 3Aminopropyltriethoxysilane (-NH2) and Propyltriethoxysilane (-H), were used to fabricate SAMs with different end groups on sapphire.8,15 Prior to forming PS films on SAMs, all prepared SAMs were rinsed thoroughly with ethanol and then air-dried, as detailed in the Experimental Section. The surfaces of the SAMs had a root square roughness (Rq), ∼ 0.3 nm, as characterized by tappingmode atomic force microscopy (AFM, Bruker Mode 8). The surface morphologies are shown in Figure 1c. These smooth surfaces were favorable for forming more complete contact between the SAMs and PS films. A weak Si2p peak was observed in the spectrum of X-ray photoelectron spectroscopic (XPS) measurements of silanized sapphire (Figure 1d), confirming the presence of the silane layer. The Si2p 3/2 peak at binding energy of 104 eV was attributed to the Si−O bond formation.16 The sample configuration for TDTR measurement is shown schematically in Figure 1e, and further detailed in the Experimental Section. To further calibrate whether the SAMs are monolayer or not, we had characterized the thicknesses of the silane layers by ellipsometry. Unfortunately, due to the weak reflectance from sapphire substrate, it was difficult to get the ellipsometry signals with high enough signal-to-noise ratio, and thus the accurate thicknesses of the extremely thin SAMs had not been obtained. In order to fix this flaw, we had fabricated the same silane layers, under the same condition of the sapphire substrates samples, on silicon wafers whose reflectivity are high enough for acquiring strong ellipsometric signals. As shown in Figure S1, the thicknesses of the silane layers on silicon ranges from 0.7 to 1.0 nm, which is consist with the thickness of mono layer reported in literatures.17−19

To date, there have been few investigations of the use of SAMs to tune ITC between polymer and ceramic materials, although, there are some measurement results reported on the interfacial thermal conductance, such as polystyrene (PS)/ silicon,9 and poly(methyl methacrylate) (PMMA)/silicon,10 and high density polyethylene (HDPE)/sapphire.11 Understanding of the thermal transport mechanisms across these polymer-ceramic interfaces is the key for improving the thermal conductivity of polymer-ceramic composites, which is still lack of studies. This motivates our studies of the effects of SAMs on the ITC between polymers and ceramics. In this work, we had experimentally investigated how the ITC between PS and sapphire varies with different silane based self-assembled monolayers with short carbon chain (three carbon atoms), which are usually used in polymer-ceramic composites. A remarkable enhancement of the ITC between sapphire and PS is achieved by introducing SAMs with a chloride group. We attribute this significant enhancement of ITC to the similar solubility parameter between polystyrene and the SAM-modified sapphire surface. Materials with similar solubility parameters can mix at the molecular level, which is possibly the main factor affecting the ITC.

RESULTS AND DISCUSSION The ITC between PS and sapphire was measured using a twocolor time-domain thermoreflectance (TDTR) system with the pump laser centered at 400 nm and the probe laser at 800 nm (see Experimental Section).12−14 Figure 1a schematically shows that sapphire were first oxidized by piranha to grew hydroxyl groups, which could react with silane to form SAMs on sapphire.15 As shown in Figure 1b, the water contact angle of the hydroxylated sapphire surface is 22°, compared to the 60° angle expected for intact sapphire. This indicates that hydroxyl groups successfully formed on the sapphire surface. We then grew different type of SAMs on the 7793

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by fitting TDTR signals.9,11 The measurement error is between 5.1% to 26.4% (more detailed information about TDTR measurement please refer to Supporting Information (SI)). Figure 3 shows the representative thermoreflectance response of samples prepared with SAMs and the correspond-

Therefore, we can conclude qualitatively that the monolayer SAMs of our samples had been formed. PS films were then prepared on SAMs by spin-coating PStoluene solution at a rotation speed of 2000 rpm.20 The thicknesses of the prepared PS films were measured by ellipsometry and fall in a narrow range from 64 to 66 nm, as listed in Table 1. As shown in Figure 2 and Table 1, the Table 1. Thickness and Roughness of PS Films type of silane layer end groups

thickness of PS film (nm)

roughness of PS film (nm)

-Cl -NH2 -SH -H

64.8 63.6 66.7 64.5

0.46 0.57 0.36 0.37

Figure 3. TDTR signals (symbols) and corresponding best-fit curves (solid lines). The values of effective thermal conductance are 58.0 MW m−2 K−1 for SAMs with a -Cl group, 22.0 MW m−2 K−1 for SAMs with a -SH group, and 16.5 MW m−2 K−1 for SAMs with a -NH2 group.

ing best-fit curves from the thermal conduction model. Each individual sample was measured at three different locations. Figure 4 shows the thermal conductance of the PS/Sapphire interfaces modified by silane agents with different terminal

Figure 2. Surface profile of PS films on SAMs terminated with different groups measured by AFM. Scan area is 3 μm by 3 μm, and the root square roughness is ∼0.5 nm.

morphology of prepared films appeared similar and the Rq, was ∼0.5 nm, which is consist with literatures,21,22 suggesting that all of the films were sufficiently smooth for the TDTR measurements. As shown in Figure 2, the morphology of PS on the silane layer with varying end groups appear similar and the layers are continuous, which means that the structure of PS film is not affected by varying silane layers. Furthermore, the roughness of PS films vary from 0.4 to 0.6 nm, as listed in Table 1, much thinner than thickness of PS film (>60 nm). Therefore, no patchiness should exist in the PS film allowing the aluminum penetration to form “shorter” between the Al layer and substrate. Aluminum layers of approximately 100 nm, which were deposited on the top of PS films by e-Beam evaporation, were used as a transducer for TDTR measurements. The TDTR measurement data were fitted using a multilayer thermal conduction model.12 Heat capacities of Al,23 sapphire and PS were taken from literatures.24,25 The thermal conductivities of sapphire and PS were measured by TDTR method separately. All the electrical conductivities of Al films were measured by a four point type probe. The thermal conductivities of Al were calculated by the Wiedemann−Franz Law and used as known parameter in TDTR data analysis. In the following ITC measurement process, only the ITCs of Al/ PS and PS/sapphire are the unknown parameters and extracted

Figure 4. Interfacial thermal conductance of the PS/SAM/Sapphire junction compared with that of PS/Sapphire.

groups. The ITC of the PS/sapphire without SAMs was 7.6 ± 0.4 MW m−2 K−1, the same order of magnitude as the thermal conductance of interfaces bonded with a weak van der Waals force (∼10 MW m−2 K−1), such as PS/Sapphire, PS/Si and HDPE/Sapphire interfaces.9,11,26 Functionalization of sapphire surfaces with silane SAMs significantly enhanced the ITC even with the extra sapphire-SAM interface being added. For the PS/ Sapphire interface, the ITC increased from 7.6 ± 0.4 MW m−2 K−1 to 58.0 ± 15.3 MW m−2 K−1 after the interfaces were modified with chloride-group SAMs. Unusual high ITC was measured in Losego’s10 study and they doubted that hole was formed, but without any proofs. But the ITC obtained in this paper is one magnitude smaller than Losego’s, and the enhancement of ITC with different end group is remarkable. In literature, the possible explanations on this enhancement of the interfacial thermal conductance are usually based on either surface wettability27,28 or interfacial adhesion.6−8 There7794

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ACS Nano fore, we had characterized both of them in this study to comprehensively investigate the mechanism of the thermal transfer on the interfaces. SAMs with different end groups could change the surface chemistry, and thus change the wettability, which is proportional to the interface adhesion energy.28,29 Therefore, the functionalization of a surface with hydrophilic molecules is the main cause of the increase in solid−water ITC.27−29 In this study, we characterized the wettability of all sapphires surfaces modified by SAMs by measuring the water contact angles (Figure 1b). The contact angle changed from 60° on bare sapphire to 22° on hydroxylized sapphire, and then increased significantly when the sapphire surfaces were functionalized with SAMs, suggesting that the sapphire/water interface adhesion energy is decreased when the SAMs are formed. However, the ITC did not appear linearly depending on water contact angle, as shown in Figure 5. The contact angle could be

Figure 6. (a) Scratch of different SAM modified Sapphire/PS interface. (b) Interfacial thermal conductance varies with interfacial adhesion.

well with literatures.8,37,38 And also suggests that the improved interfacial adhesion enhances the ITC, which is consistent with SAMs modified metal/inorganic interface.7,8 Silane with -NH2 forms a stronger bond to the styrene than -Cl. However, among the interfaces modified by different SAMs, the nonmonotonic trend between the ITC and the interfacial adhesion hints that the interface adhesion should not be the dominant influencing factor for the thermal transport when SAMs had been introduced into the interface. As we did not find the directly relationship between interface thermal conductance and wettability or adhesion, we calculated the solubility parameters (δ) of SAMs and PS, which is also a candidate for the explanation on how well the molecular bonding on the interfaces.40−42 The parameters used to calculate the solubility parameters are listed in Table 2.41

Figure 5. Relationship between interfacial thermal conductance and water contact angle.

affected by many factors, such as the chemical composition and the micro structure of surfaces,30 but not all of them related to the interfacial thermal transport. Because of this, the wettability cannot be alone used to correlate with the ITC, as the similar conclusion from the studies on gold/PE interfaces.31 Enhancing interfacial adhesion has also been considered as an effective way to improve ITC.32 Thermal transport across Gold/Quartz, Cu/SiO2 and sapphire/PS interfaces grew significantly with greater interfacial adhesion.7,8,26 In this study, interfacial adhesion strengths between PS film and sapphire were measured using a CSM nano scratch tester in ambient air (see Experimental Section).33,34 In this test, as shown in Figure 6a the diamond indenter was drawn across the PS film under an increasing continuous load until at certain load, termed as the critical load, Lc, a well-defined failure event occurred. If this failure event represented coating detachment then the critical load could be used as a measure of coatingsubstrate interfacial adhesion.35−38 The interfacial adhesion can be influenced by many factors, such as the thickness and the roughness of films.39 In this study, the Rq (root square roughness) and thicknesses of PS films used for interfacial adhesion measurements were 0.5 ± 0.1 nm and 70 ± 2 nm respectively, whose uncertainty are small enough for neglecting their influence on adhesion. As shown in Figure 6b, both the interfacial adhesion and interface thermal conductance of PS/SAMs/Sapphire are higher than those of bare PS/Sapphire sample, indicating that SAMs can improve inorganic−organic interfacial adhesion and interfacial thermal conductance at the same time, which agree

Table 2. Molar Attraction Constants (G) of Different Functional Groups groups

Gdi (J1/2 cm3/2 mol−1)

Gpi (J1/2 cm3/2 mol−1)

Ehi (J1/2 cm3/2 mol−1)

-SH40 -CH2-NH2 -Cl -CH3

645.1 270 280 450 420

0 0 0 550 0

0 0 8400 400 0

Cohesion parameters are derived from the energy required to convert a liquid to a gas (vaporization), which is used to predict molecular interaction force.40,42 Two materials with similar solubility parameters can be compatible, which results in favorable contact or soluble at the molecular level.41 As shown in Figure 7, the ITC decease almost linearly with the increasing discrepancy in solubility parameters, which clearly shows us the strong relevance between those two parameters. 7795

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Si−O−Al. It has been previously found that junctions with covalent bond, such as the gold/SAM interface, have large thermal conductance of ∼200−400 MW m−2 K−1,43 suggesting that the thermal transport across interface with covalent bond is enhanced. Second, unlike the sapphire/SAM interface which is connected by strong covalent bonds (Al−O−Si), the SAM/ PS interface is connected by weak van der Waals force, which is usually unfavorable for thermal transport. However, the chemical compositions of the SAMs and those of the PS molecules are almost the same. The SAMs and PS could have similar solubility parameters suggesting that SAMs and PS could contact better at molecular level,40−42 which hence produces less void in the interface. Between PS and sapphire, SAMs act as a strongly bonded thermal bridges that enlarge the contacting area and thus form more channels for thermal transportation and improve the ITC.44−46

Figure 7. Relationship between interfacial thermal conductance and difference in solubility parameters.

The Hildebrand solubility parameter is defined as the square root of the cohesive energy density: δ = (E /V )1/2

CONCLUSION In summary, the ITC between sapphire and PS can be enhanced by a factor of 7 through functionalizing the sapphire surface with SAMs with a chloride end group. The results show that the enhancement comes from strong covalent bonding, which facilitates thermal transfer from sapphire to the silanebased SAM, and then from SAM to PS due to the high compatibility between the SAM and PS. Among the SAMs used in this study, we found that the ITC almost linearly depends on solubility parameters, which could be the dominant factor influencing on the ITC compared with wettability and adhesion. The SAMs serve as an intermediate layer that bridges the sapphire and PS. Such a feature can be applied to other ceramic-polymer immiscible interfaces by functionalizing the ceramic surface with molecules that are miscible with the polymer materials. This research opens up a strategy to manipulate interfacial thermal transfer and provides guidance on the design of critical-thermal transfer materials such as composites and nanofluids for thermal management.

(1)

where V is the molar volume of the pure solvent, and E is its (measurable) energy of vaporization. The basic equation governing the assignment of Hansen parameters is that the total cohesion energy, E, must be the sum of the individual energies Ed, Ep and Eh E = Ed + Ep + Eh

(2)

Dividing this by the molar volume (V) gives the square of the total (or Hildebrand) solubility parameter as the sum of the squares of the Hansen D, P, and H components. E /V = Ed /V + Ep /V + Eh /V

(3)

δ 2 = δd 2 + δp 2 + δh 2

(4)

or where, δd, dispersion cohesion parameter; δh, hydrogen bonding cohesion parameter; δp, polar cohesion parameter. (δ, total (Hildebrand) cohesion (solubility) parameter).

δd = (Ed /V)1/2 1/2

(5)

δp = (Ep /V )

(6)

δh = (Eh /V )1/2

(7)

EXPERIMENTAL SECTION Materials. Sapphire (0001) substrates were supplied by Hefei Kejing Materials Technology CO., Ltd. The substrates were 1 cm × 1 cm × 400 μm, with a root square roughness (Rq) is less than 0.4 nm. The polystyrene (Mw: 65 000 g mol−1 and Mw/Mn = 1.06) was purchased from Alfa-Aesar. Self-Assembled Monolayer (SAM) preparation. The sapphire substrates were cleaned in piranha solution (3 H2SO4: 1 H2O2 by volume) at 110 °C for 60 min in order to remove organic contamination on the surface. Then the cleaned substrates were soaked in a mixture of H2O, H2O2 and NH4OH and heated at 70 °C.15 The substrates were rinsed with ethanol, dried by nitrogen flow, and further dried in an oven at 120 °C for 30 min. This procedure was done to leave only a monolayer of water on the surface as the activator for silane attachment. General procedures for SAM preparation involve soaking the substrates in ethanol solution of 10 mM silane in a sealed vials. The vials containing sapphire substrates and this solution were sonicated for 5 h at 25 °C, and then kept overnight at room temperature. These substrates were rinsed thoroughly by methanol and dried again by nitrogen flow. Polystyrene Film Preparation. The polystyrene was dissolved in toluene (>99.9%) with concentration of 1.4 wt % and was spin-coated on sapphire substrates (treated and untreated) with a typical rotation speed of 2000 rpm.

δd, δh, δp can be calculated by following formulations: δd =

∑ Gdi /V

(8)

δp =

∑ Gpi /V

(9)

δh =

∑ Ehi /V

(10)

where Gdi, Gpi Ehi are dispersion, polarization and hydrogen molar attraction constant, respectively. The ITC of the interfacial structure consisting of the sapphire/SAM and SAM/PS interfaces and the SAM layer is 7 times higher than that of the bare PS/sapphire interface. We attribute the remarkable improvement of thermal conductance to the following reasons: First, through the sapphire/SAM interface the thermal transport is facilitated by strong covalent bonds formed as 7796

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ACS Nano Characterization. Thickness Measurement. Thickness measurements for SAM and PS films were done by using a spectroscopic ellipsometer (M-2000 V, J. A. Woollam) were conducted at an incidence angle of 70° and a wavelength scan from 370.1 to 999.1 nm. By ellipsometry, the complex reflection coefficient is measured as a function of wavelength expressed by the following equation:

tan(ψ )eiΔ =

Rp Rs

contact angle. At least three readings from each sample were averaged to give the water contact angle data. XPS Measurements. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. The hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for energy referencing.

, where tan(ψ) denotes the amplitude ratio of the

reflection coefficient of p-polarized light (Rp) to that of s-polarized light (Rs), and Δ is the phase difference.47 The quantities ψ and Δ were measured directly in experiments and the physical parameters, such as thickness and refractive index, were obtained by numerical fitting using the appropriate model. In this study, the well-established Cauchy dispersion model was used to fit the ellipsometry data.47,48 Interfacial Thermal Conductance Measurement. Thermal properties of the prepared samples were measured using the ultrafast laserbased time-domain thermoreflectance (TDTR) method, which is a well-accepted measurement technique for characterizing thermal properties of bulk and thin film materials.11−14,49 The sample configuration used for ITC measurements is schematically shown in Figure 1e. An aluminum thin film (100 nm) was evaporated onto the spin-coated PS thin films by electron beam evaporation.7,31,50 The thickness of Al was measured using the picosecond acoustic reflections during TDTR measurement.51This aluminum film serves as both a heater that absorbs pump laser energy, and as a thermometer, due to the temperature-induced reflectivity change that can be measured with the probe laser beam. The recorded reflectivity response of the Al thin film was then fitted with a thermal transfer model to extract the unknown thermal properties of the materials, such as the ITC between the PS film and the sapphire substrate. The pump beam (400 nm wavelength) was used to heat sample at a modulation frequency of 1 MHz. The probe beam (800 nm wavelength) was time delayed relative to the pump beam by passing through a mechanical stage. The diameters of the pump and probe beam are approximately 30 and 15 μm, respectively. The TDTR system was calibrated and checked by measuring many standard materials, such as silicon, silicon dioxide, and sapphire, prior to measuring the new samples. TDTR measurement were performed at three different locations for a single sample and three scans were taken and averaged for each location to reduce the measurement uncertainty. The temperature rise on the sample was controlled to be less than 20 K by carefully selecting the power of the pump beam. Scratch Test. The interfacial adhesion between the PS film and sapphire was measured using a CSM nano scratch tester in ambient air, at a temperature of 23 °C and a relative humidity of 20%.33,34 The following controlled test parameters were used to produce at least three identical scratches on each specimen: • • • • • •

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03381. Supporting figures and tables (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

Kun Zheng and Fangyuan Sun contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51373184 and 51336009), the National Plan for Science & Technology Support, China (Grant No. 2014BAC03B05), the MoST (Ministry of Science and Technology) 973 Research Programme (Grant Nos. 2014CB931803 and 2012CB933801), and Ministry of Science and Technology of the People’s Republic of China (Grant No. 2013YQ120355). X. Wang would like to acknowledge the support from UMN CSE startup. The authors also would like to thank the help from Prof. Hang Zhang and Mr. Zhe Chen. REFERENCES (1) Pop, E. Energy Dissipation and Transport in Nanoscale Devices. Nano Res. 2010, 3, 147−169. (2) Luo, T.; Lloyd, J. R. Enhancement of Thermal Energy Transport across Graphene/Graphite and Polymer Interfaces: a Molecular Dynamics Study. Adv. Funct. Mater. 2012, 22, 2495−2502. (3) Hung, M. T.; Choi, O.; Ju, Y. S.; Hahn, H. Heat Conduction in Graphite-Nanoplatelet-Reinforced Polymer Nanocomposites. Appl. Phys. Lett. 2006, 89, 023117. (4) Huxtable, S. T.; Cahill, D. G.; Shenogin, S.; Xue, L.; Ozisik, R.; Barone, P.; Usrey, M.; Strano, M. S.; Siddons, G.; Shim, M.; Keblinski, P. Interfacial Heat Flow in Carbon Nanotube Suspensions. Nat. Mater. 2003, 2, 731−734. (5) Collins, K. C.; Chen, S.; Chen, G. Effects of Surface Chemistry on Thermal Conductance at Aluminum-Diamond Interfaces. Appl. Phys. Lett. 2010, 97, 083102. (6) Hopkins, P. E.; Baraket, M.; Barnat, E. V.; Beechem, T. E.; Kearney, S. P.; Duda, J. C.; Robinson, J. T.; Walton, S. G. Manipulating Thermal Conductance at Metal-Graphene Contacts via Chemical Functionalization. Nano Lett. 2012, 12, 590−595. (7) O’Brien, P. J.; Shenogin, S.; Liu, J.; Chow, P. K.; Laurencin, D.; Mutin, P. H.; Yamaguchi, M.; Keblinski, P.; Ramanath, G. BondingInduced Thermal Conductance Enhancement at Inorganic Heterointerfaces Using Nanomolecular Monolayers. Nat. Mater. 2013, 12, 118−122. (8) Losego, M. D.; Grady, M. E.; Sottos, N. R.; Cahill, D. G.; Braun, P. V. Effects of Chemical Bonding on Heat Transport Across Interfaces. Nat. Mater. 2012, 11, 502−506.

indenter (stylus) type: Rockwell diamond stylus radius: 10 μm initial load: 0.3 mN end load: 20 mN scratch length: 200 μm stylus velocity: 400 μm/min.

The diamond indenter was drawn across the PS film under an increasing continuous load until at a certain load, termed as the critical load, Lc, a well-defined failure event occurred. If this failure event represents coating detachment then the critical load can be used as a qualitative measure of coating-substrate interfacial adhesion.35,36 Surface morphology. The root square roughness (Rq) of SAMs and prepared PS thin films was characterized by tapping-mode atomic force microscopy (AFM), using Bruker Nanoscope V Multimode 8. Water Contact Angle Measurement. The contact angles were recorded on a VCA 1000 video contact angle system (AST Products, Inc., Billerica, MA). A droplet of ∼3 μL of high purity Milli-Q water (resistivity of >18 MΩ-cm) generated by a NANO pure system (Barnstead, Dubuque, IA) was injected onto the sample surface from a syringe, and then the needle was retracted from the droplet. An image of the static water droplet was recorded by a digital camera and analyzed using the software provided by AST to provide a sessile 7797

DOI: 10.1021/acsnano.6b03381 ACS Nano 2016, 10, 7792−7798

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DOI: 10.1021/acsnano.6b03381 ACS Nano 2016, 10, 7792−7798