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This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys.
Cite this: DOI: 10.1039/c3cp50993a
Reliable contact fabrication on nanostructuredBi2Te3-based thermoelectric materials†
Shien-Ping Feng,*a Ya-Huei Chang,a Jian Yang,b Bed Poudel,b Bo Yu,b
Zhifeng Ren*c and Gang Chen*d
A cost-effective and reliable Ni–Au contact on nanostructured
Bi2Te3-based alloys for a solar thermoelectric generator (STEG) is
reported. The use of MPS SAMs creates a strong covalent binding
and more nucleation sites with even distribution for electroplating
contact electrodes on nanostructured thermoelectric materials.
A reliable high-performance flat-panel STEG can be obtained by
using this new method.
Thermoelectric devices use thermoelectric materials to convertheat into electricity and vice versa. A thermoelectric coolercreates a temperature difference when a voltage is applied toit. Conversely, a thermoelectric generator creates a voltagewhen there is a different temperature on each side. Hence,thermoelectric devices can be used in solid-state cooling andpower generation, such as solar power conversion, waste-heatrecovery, CPU and high-power laser cooling. The performanceof thermoelectric materials depends on the dimensionlessfigure-of-merit ZT (S2sT/k), where S, s, k, and T are the Seebeckcoefficient, electrical conductivity, thermal conductivity, andabsolute temperature, respectively. The dimensionless ZT valuein Bi2Te3-based bulk alloys, which are the most widely usedthermoelectric materials near room temperature, has remainedat around 1 for more than 50 years. Over the last few years,significant improvements in the thermoelectric ZT value innanostructured thermoelectric alloys, particularly in nanostructuredBi2Te3-based alloys with a peak ZT value of 1.4, have been made.1
These nanocrystalline bulk materials were made by hot pressingnanopowders that were ball-milled from crystalline ingots underinert conditions. The improvement in ZT is the result of low thermal
conductivity caused by the increased phonon scattering bygrain boundaries and defects.1–3
The thermoelectric device performance depends not only onthe ZT value of the material, but also on the electrical andthermal contact performances between the metallic electrodesand the thermoelectric materials.4 In the thermoelectric coolerusing Bi2Te3-based alloys, a barrier layer of nickel (500 nm–1 mm)is coated on the Bi2Te3 surface to block inter-diffusion, followedby a protective layer of gold on nickel for soldering tin. Inthe cooling device, traditional methods of fabricating contactelectrodes on Bi2Te3 thermoelectric materials are sputtering,electroless plating or their combination. In the case of sputtering,compared with sputtering nickel on the metal substrate, therelatively poor wettability of the Bi2Te3-based alloys causes thenickel atoms to be more preferably bound to each other than toBi2Te3 and grow into three-dimensional islands so that it needs anadditional thin layer, such as sputtered chromium between nickeland Bi2Te3, to improve adhesion.5,6 A good contact performanceon Bi2Te3-based alloys can be achieved by sputtering chromium ofa few tens of nm and nickel of several hundred nm, whichrequires costly vacuum equipment and has low throughput.Electroless plated nickel on Bi2Te3-based alloys achieved by usingan initial coating of a palladium–tin catalyst can provide accep-table contact performance.7 Compared with the sputtered contact,the electroless plated contact has slightly high contact resistancebecause of the accumulation of unwanted impurities (such as apalladium–tin catalyst layer, reducing agents, and chelatingagents) and structural imperfections at the interface.8
Besides a thermoelectric cooler, a thermoelectric generator(TEG) uses thermoelectric materials to convert waste heator solar heat into electricity. Alongside a photovoltaic cellgenerating electricity by exciting electron–hole pairs, a TEG isanother way of converting solar energy into electricity, namedas a solar thermoelectric generator (STEG). Our previous workreported a 4.6% record efficiency for a flat-panel STEG usingnanostructured Bi2Te3-based alloys and a selective solar absorber(thermal concentration), which demonstrates that the solarthermoelectric generator is a promising approach to achieve
a Department of Mechanical Engineering, The University of Hong Kong, Pokfulam,
Hong Kong. E-mail: [email protected] GMZ Energy, 11 Wall Street, Waltham, Massachusetts 02458, USAc Department of Physics and Texas Center for Superconductivity, University of
Houston, Houston, TX 77204. E-mail: [email protected] Department of Mechanical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA. E-mail: [email protected]
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cp50993a
Received 6th March 2013,Accepted 14th March 2013
DOI: 10.1039/c3cp50993a
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cost-effective conversion of solar energy into electricity.4 Theoperating temperature of 200–250 1C in a TEG is higher than theoperating temperature of 100–150 1C in a thermoelectric cooler.Therefore, the nickel barrier layer with 500 nm to 1 mm for thethermoelectric cooler cannot effectively block inter-diffusion inthe TEG case. Recent studies indicated that the thickness of theNi barrier layer for TEG needs to be at least several mm.9,10
However, sputtering is a very time-consuming process for achievinga thick nickel layer and the increased film stress with increasing Nithickness (>1 mm) causes the film to peel off. Similarly, electrolessplating also cannot coat a several-mm nickel layer having strongadhesion on Bi2Te3-based alloys. To date, thermal spraying hasbeen the most common method to fabricate thick Ni contactelectrodes on bulk Bi2Te3-based alloys for TEG. In the thermalspraying process, Ni is melted and sprayed over the cold samplesurface via a jet stream. The bulk Bi2Te3-based alloys can bear thethermal shock during thermal spraying while the nanostructuredBi2Te3-based alloys will have cracks propagating randomly alongthe grain boundaries because of low thermal conductivity anddisordered grain textures. Hence, the contact fabrication on thenanostructured Bi2Te3-based alloys limits the impact of theseadvances in material properties. Most fundamental research hasbeen conducted on nanostructured thermoelectric materials, butvery little has been focused on the contact interfacial region. Thispaper therefore aims to fabricate reliable contact electrodes at lowtemperature, low cost and high throughput on nanostructuredBi2Te3-based thermoelectric materials for STEG. The findings areexpected to stimulate research interest for industrial applica-tions of thermoelectric devices using nanostructured thermo-electric materials for energy generation and energy savings.
As mentioned, thermal spraying causes sample cracking.Sputtering was chosen as the initial method of fabricatingcontact electrodes on nanostructured Bi2Te3-based alloys forSTEG.4 The open circles in Fig. 1(a)–(c) show the efficiency,open circuit voltage (Voc) and power output of the flat-panelSTEG, which comprises a p-type and an n-type leg of a nano-structured Bi2Te3-based alloy with sputtered electrodes (50 nmCr/500 nm Ni/500 nm Au) sandwiched between a solar absorberplate (hot side 220 1C) and a copper plate (cold side 20 1C),which decrease over time during consecutive illumination. Thetotal device resistance at the operating temperature (DT =200 1C) would increase by 1.5 times compared with the initialdevice resistance at room temperature due to the increasedresistance of the Bi2Te3-based thermoelectric alloy itself. There-fore, the ratio of the total resistance at the operating tempera-ture to the initial resistance at room temperature shouldremain at around 1.5. However, this ratio increases over timefrom 1.5 to 1.63, as shown using open circles in Fig. 1(d). Allmaterial properties of nanostructured Bi2Te3-based alloys arethe same before and after the operation. It indicates that themain problem arises from the increase in the interfacial contactresistances between metallic electrodes and thermoelectricmaterials. Based on the data given in Fig. 1(d), we can estimatethe specific contact resistance that increases initially from1 � 10�6 O cm2 to 1.4 � 10�5 O cm2 after illumination for2500 minutes, as shown using open circles in Fig. 2 (see ESI†).
It was reported that a reasonably low ohmic contact at each end ofthe Bi2Te3-based alloys should be within 5% of the resistance ofthe thermoelectric leg, representing the specific contact resistanceof about 9 � 10�6 O cm2 in our STEG system (see ESI†).4,11,12
Electrochemical deposition is a low temperature, low costand high throughput process, and has been widely used insteadof sputtering in a wide variety of applications. In the case ofelectroplating of Bi2Te3-based thermoelectric semiconductors,the lower surface energy leads to higher nucleation energy and thusthe scattered growth of nuclei on a small number of nucleationsites causes weak interface contact and poor adhesion.14,15
Nanostructured surfaces having broad distribution of surfaceenergy would cause electroplating nucleation to be moreuneven.16,17 One approach to improve electroplating nucleationis to yield a wettable surface beneficial to subsequent metalli-zation, and the other is to use a bridging ligand, such as
Fig. 1 Unicouple STEG performance characteristics as a function of time underillumination at incident solar radiation fluxes of 1 kW m�2. (a) STEG efficiency;(b) open circuit voltage; (c) power output; and (d) ratio of device resistance at theoperating temperature to the initial resistance at room temperature. Open circlesare for STEG with sputtered contacts, open triangles are for STEG with electro-plated contacts. Open circle symbols (J) indicate sputtered 500 nm Ni–500 nmAu and open triangle symbols (n) indicate MPS/electroplated 5 mm Ni–3 mm Au.
Fig. 2 The ratio of contact resistance to the resistance of the thermoelectric legand its corresponding specific contact resistance. Open circle symbols (J)indicate sputtered 500 nm Ni–500 nm Au and open triangle symbols (n) indicateMPS/electroplated 5 mm Ni–3 mm Au.
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sulfide, to link metal ions as a promotion agent during electro-plating.18,19 The former method usually involves different stepsof chemical treatments, which is likely to damage the nano-structured surface.13 Therefore, a promotion layer on nano-structured Bi2Te3-based alloys that can provide more nucleationsites for electroplating metal would be ideal in order to achieve agood contact. Functionalized self-assembled silane monolayers(SAMs) have been used for many years in corrosion preventionfor a wide range of substrates and adhesion promoters forjoining two substrates (metal sheets, polymer films) by chemicalor electrostatic binding.20,21 Recently, it was discovered thatsome SAMs in the range of 1 to 3 nm are able to transportelectrons by a hopping process.22 To integrate the conceptsmentioned above, a new method can be used to promotenucleation and adhesion for the electroplating process by usingthe hydrolysable methoxy end-group and sulfur functional groupon 3-mercaptopropyl-trimethoxysilane (MPS), as shown in Fig. 3.The MPS with the methoxy group would adhere to the surface ofthe nanostructured Bi2Te3-based alloy and its sulfur functionalgroup can form a bridging link to nickel ions in the electrolyte.When electroplating, electrons hop onto the sample surface toreduce nickel ions to atoms, forming strong covalent bonds atthe interface. Another advantage of using MPS is that it forms a
stable neutral solution in ethanol, thus avoiding oxidation andcorrosion on nanostructured surfaces.
Fig. 4(a) shows a cathodic wave from cyclic voltammetry fornickel on nanostructured Bi2Te3-based alloys with and withoutMPS treatment. A Gamry PCI4/300 potentiostat/galvanostat wasused in the electrochemical measurement in a standard three-electrode system with a platinum mesh as the counter electrodeand a saturated calomel electrode (SCE) as the referenceelectrode. Compared with the onset voltage of 0.6 V for thebare surface, the sample surface pretreated with 1% MPS showsa small peak ranging from �0.15 to �0.5 V in the current–potential curve, indicating another redox reaction occurring ata more positive potential. It is believed that this additional peakresults from the reduced formation energy by an MPS bridginglink to nickel ions in the electrolyte so that a seed layer of nickelis deposited prior to bulk electroplating. The peak in the rangeof �0.15 to �0.5 V is unpronounced for the sample pretreatedwith 5% MPS, meaning that an excess of MPS would retardcharge transfer at the interface.
The chronoamperograms in Fig. 4(b) show the currentresponse at an operating voltage of 0.85 V for electroplatingnickel on nanostructured Bi2Te3-based alloys without and with1% MPS treatment. The electroplating rate in the initial fewseconds increases by 1.5 times after MPS treatment, whichindicates that more nucleation sites are being generated at the
Fig. 3 The sequence of steps for electroplating of a nickel layer on a nanos-tructured Bi2Te3-based substrate pretreated with functionalized self-assembledMPS. (a) After surface cleaning, the nanostructured Bi2Te3-based alloy wasimmersed into 1% MPS in ethanol for 50 min to form a MPS monolayer;(b) immersion into the electrolyte for 30 s to form a bridging link with nickel ionsbefore electroplating; and (c) electroplated nickel onto the pretreated surface.
Fig. 4 (a) Cathodic wave from cyclic voltammetry for nanostructured Bi2Te3-basedalloys in a nickel electrolyte without and with 1% and 5% MPS treatment; and(b) chronoamperometry of nickel during electrodeposition on nanostructuredBi2Te3-based alloys under an operating voltage of 0.85 V without and with 1%MPS treatment.
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initial stage. Fig. 5 shows that an MPS pretreated surfaceincreases the contact angle between the water drop and thesample surface from 381 to 861, implying that the increase inthe initial electroplating rate is promoted by the bridging linkeffect, not by the surface wettability.
Fig. 6(a) shows an AFM image of the bare surface ofnanostructured Bi2Te3-based alloys and Fig. 6(b) and (c) showAFM images of electroplated nickel in the initial 10 secondsat an operating voltage of 0.85 V on a bare nanostructuredBi2Te3-based alloy, aiming to compare the nucleation formationwith and without MPS treatment at the very initial stage. ANanoscope IIIa atomic force microscope (AFM) was used to scan
the surface and calculate the root mean square (RMS) roughness.The rather high nucleation energy for electroplating causesuneven island growth on a small number of nucleation sites,leading to irregularities in the grain size. Stacking faults andmismatch in the lattice spacing would also generate the largestrains and stresses in the electroplated film. Hence, the islandgrowth mode is usually not desirable for technological applicationsdue to its poor adhesion and non-uniform deposition. In contrast,Fig. 6(b) shows that more nucleation sites were evenly generated onthe MPS pretreated surface. In addition, the pull-off adhesion testperformed using 3M Flatback Masking Tape 250 (ASTM D3359)shows that no coating was stripped off for a 5 mm layer ofelectroplated nickel on nanostructured Bi2Te3-based alloys withthe MPS treatment, but almost all of the coating on the barenanostructured Bi2Te3-based alloy came off.
Fabricating reliable ohmic contacts by this new method is ofprime importance. Table 1 lists the electrical contact resistancemeasured by a 4-wire AC method for small cubes cut from adisk of n-type or p-type nanostructured Bi2Te3, both sides ofwhich were treated with MPS followed by electroplating a 5 mmlayer of nickel and a 3 mm layer of gold.7,23 As observed, thespecific contact resistance at each end of thermoelectric legs isapproximately 1 � 10�6 O cm2 and contributes around 1% ofthe total resistance, showing the low ohmic contacts betweenthe electroplated electrodes and nanostructured Bi2Te3 legsbefore the STEG operation. The performances of a STEGcomprising a p-type and an n-type leg of nanostructuredBi2Te3-based alloys with electroplated electrodes of 5 mm nickeland 3 mm gold are shown by the open triangles in Fig. 1(a)–(d).As seen, a good improvement of the device reliability of the flat-panel STEG in terms of the efficiency, open circuit voltage (Voc)and power output can be achieved due to a reliable contactresistance between nanostructured Bi2Te3 alloys and metallicelectrodes. As shown by the open triangles in Fig. 1(d), the ratioof the total resistance at the operating temperature to the initialresistance at room temperature increases slightly over time from1.5 to 1.52. This ratio does not improve much even when thenickel barrier is thickened. A possible reason is the formation ofnickel telluride at the interface, which strengthens the mechanicaljoint but degrades the electrical contact.7,8,10 Further studies onthe interfacial reactions for nanostructured Bi2Te3-based alloysand their improvement are underway. The increase in specific
Fig. 5 Contact angles of nanostructured Bi2Te3-based alloys on (a) bare surfaceand (b) the surface pretreated with 1% MPS for 50 min.
Fig. 6 Atomic force microscopy images of (a) the bare surface of nanostruc-tured Bi2Te3-based alloys, and electroplating nickel at an operating voltage of0.85 V at the initial 10 seconds on the surface of nanostructured Bi2Te3-basedalloys (b) without MPS treatment and (c) with MPS treatment. AFM scan size is5 mm, scan rate is 1 Hz. The Z axis for the height is 3 mm per division and the X axisis 1 mm per division.
Table 1 Electrical contact resistances for small cubes cut from a disk of n-type or p-type nanostructured Bi2Te3, both sides of which were treated with MPS followedby electroplating of a 5 mm layer of nickel and a 3 mm layer of gold (the initial contact resistance of the sputtered Ni samples has been reported and is around1–5 � 10�6 O cm2 (ref. 4))
Type No.Length(mm)
Width(mm)
Height(mm)
Resistivity ofTE legs (O cm)
Total resistance(TE leg + contact) (O)
Specific contactresistance (O cm2)
Contact/TEleg (%)
n 1 1.07 1.06 1.54 1.29 � 10�3 1.76 � 10�2 8.79 � 10�7 0.892 1.11 0.96 1.52 1.04 � 10�3 1.48 � 10�2 4.21 � 10�7 0.543 1.21 1.21 1.48 1.08 � 10�3 1.09 � 10�2 2.92 � 10�7 0.374 1.07 1.26 1.56 1.23 � 10�3 1.44 � 10�2 1.07 � 10�6 1.11
p 1 0.89 1.01 1.48 1.17 � 10�3 1.95 � 10�2 1.02 � 10�6 1.182 1.13 1.19 1.54 1.10 � 10�3 1.27 � 10�2 6.88 � 10�7 0.813 1.20 1.20 1.69 1.00 � 10�3 1.19 � 10�2 9.90 � 10�7 1.174 0.80 1.23 1.50 1.18 � 10�3 1.82 � 10�2 1.17 � 10�6 1.32
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contact resistance for this electroplated electrode can be modeledas shown using the open triangles in Fig. 2, showing a stable valueof around 2.5% of the resistance of the thermoelectric leg afterconsecutive illumination for 2500 min (see ESI†).
In summary, we have presented a new technique for deposi-tion of an adhesive, uniform and thick metallic layer onto ananostructured Bi2Te3-based material pretreated with functio-nalized self-assembled MPS. The use of an MPS thin layer notonly provides a strong covalent binding but also creates morenucleation sites with even distribution for nickel electroplating.The deposition of MPS SAMs followed by the electroplatingprocess on the nanostructured thermoelectric materials can beachieved by low temperature and solution-based processing,which is significantly more cost-effective. Measurements ofcontact resistance and device characteristics demonstrate thata reliable high-performance flat-panel STEG can be achievedby using this new electroplated contact on nanostructuredBi2Te3-based alloys.
ExperimentalFabrication of nanostructured thermoelectric materials
Nanopowders are made by ball milling bulk Bi2Te3-based alloyingots (p-type alloyed with Sb as BixSb2�xTe3 and n-type alloyedwith Se as Bi2SeyTe3�y). The disk samples of 25 mm in diameterand 2 mm in thickness are made by hot pressing the nano-powders loaded in graphite dies. After hot pressing, both sidesof the disk sample are polished using sand paper. To achieve agood ZT value, the size and quality of the initial nanoparticlesmust be controlled. For good electrical conductivity, it isespecially important to prevent oxidation.1
Contact fabrication
The disk samples of the n-type and p-type nanostructuredBi2Te3 alloys were cleaned by immersion into 0.5% bromine–ethanol at room temperature for 1 min and then 5% CLEAN-100 (Wako Pure Chemical Industries Ltd) at 45 1C for 5 minwith sonication.16 For sputtered contacts, the samples weredeposited by 500 nm nickel followed by 500 nm gold withoutvacuum break. For electroplated contacts, the samples wereimmersed into a solution of 1% MPS (3-mercaptopropyl-trimethoxysilane) for 50 min. A 5 mm layer of nickel waselectroplated onto the pretreated surface at an operatingvoltage of 0.85 V from a commercial electrolyte (Caswell Inc.)for 50 min at 53 1C. Before triggering electroplating, the MPSpretreated surface should be immersed into the electrolyte for30 s to form a bridging link to nickel ions. The process flowdiagram is shown in Fig. 3. A 3 mm layer of gold was thenelectroplated on nickel at an operating voltage of 1.0 V from acommercial electrolyte (Samtec Inc.) for 30 min at roomtemperature.
Measurement of contact resistance
The electrical contact resistance was measured by a 4-wireAC method (Keithley 2300) for small cubes (around 1 mm �1 mm� 1.5 mm) cut from a disk of n-type or p-type nanostructured
Bi2Te3, both sides of which were treated with MPS followed byelectroplating of a 5 mm layer of nickel and a 3 mm layer of gold. Asmall current (Joule heating can be neglected) with alternatingpolarity at a high frequency of 1000 kHz (the Peltier heat cancelsdue to periodic heating and cooling at the junction) from 0.03 mAto 0.1 mA was input into the sample from top to bottom. The totalresistance can be obtained using the slope of the voltage drop andthe current. The intrinsic resistance of nanostructured Bi2Te3 alloyscan also be obtained by measuring the voltage drop across thesample body itself using a known distance. Therefore, the electricalcontact resistance at both sides is the difference between the totalresistance and intrinsic resistance.23
Acknowledgements
This work was supported partially as part of the Solid StateSolar-Thermal Energy Conversion Center (S3TEC), an EnergyFrontier Research Center funded by the U.S. Department ofEnergy, Office of Science, Office of Basic Energy Sciences underAward Number: DE-SC0001299 (G.C. and Z.F.R.). This work wasalso supported by the General Research Fund from ResearchGrants Council of Hong Kong Special Administrative Region,China, under Award Number: HKU 719512E.
Notes and references
1 B. Poudel, Q. Hao, Y. Ma, Y. C. Lan, A. Minnich, B. Yu, X. Yan,D. Wang, A. Muto, D. Vashaee, X. Y. Chen, J. Liu,M. S. Dresselhaus, G. Chen and Z. F. Ren, Science, 2008, 320, 634.
2 Y. C. Lan, B. Poudel, Y. Ma, D. Z. Wang, M. S. Dresselhaus,G. Chen and Z. F. Ren, Nano Lett., 2009, 9, 1419.
3 M. Zebarjadi, K. Esfarjani, M. S. Dresselhaus, Z. F. Ren andG. Chen, Energy Environ. Sci., 2012, 5, 5147.
4 D. Kraemer, B. Poudel, H. P. Feng, J. C. Caylor, B. Yu, X. Yan,Y. Ma, X. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa,Z. F. Ren and G. Chen, Nat. Mater., 2011, 55, 532.
5 S. S. Coffee and J. G. Ekerdt, J. Appl. Phys., 2007, 102, 114912.6 T. Kacsich, E. Kolawa, J. P. Fleurial, T. Caillat and
M. A. Nicolet, J. Phys. D: Appl. Phys., 1998, 31, 2406.7 C. N. Liao, C. H. Lee and W. J. Chen, Electrochem. Solid-State
Lett., 2007, 10, 23.8 W. J. Dressick, L. M. Kondracki, M. S. Chen, S. L. Brandow,
E. Matijevi and J. M. Calvert, Colloids Surf., 1996, 108, 101.9 W. P. Lin, D. E. Wesolowski and C. C. Lee, J. Mater. Sci.,
2011, 22, 1313.10 T. Y. Lin, C. N. Liao and A. T. Wu, J. Electron. Mater., 2011,
41, 153.11 A. M. Pettes, R. Melamud, S. Higuchi and K. E. Goodson, Int.
Conf. Thermoelectr., 2007, 283.12 L. W. Silva and M. Kaviany, Int. J. Heat Mass Transfer, 2004,
47, 2417.13 A. J. Bard, R. Parsons and J. Jordan, Standard Potentials in
Aqueous Solutions, Marcel Dekker, New York, 1985.14 E. Budevski, G. Staikov and W. J. Lorenz, Electrochim. Acta,
2000, 45, 2559.15 V. M. Kaganer, B. Jenichen, R. Shayduk, W. Braun and
H. Riechert, Phys. Rev. Lett., 2009, 102, 016103.
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16 H. P. Feng, B. Yu, S. Chen, K. C. Collins, C. He, Z. F. Ren andG. Chen, Electrochim. Acta, 2011, 56, 3079.
17 H. P. Feng, T. Paudel, B. Yu, S. Chen, Z. F. Ren and G. Chen,Adv. Mater., 2011, 23, 2454.
18 R. L. Cohen and R. L. Meek, J. Colloid Interface Sci., 1976,55, 156.
19 C. H. Yang, Y. Y. Wang and C. C. Wan, J. Electrochem. Soc.,1999, 146, 4473.
20 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo andG. M. Whitesides, Chem. Rev., 2005, 1103.
21 K. L. Mittal, Silanes and Other Coupling Agents, Utrecht, TheNetherlands, 2003.
22 T. Morita and S. Kimura, J. Am. Chem. Soc., 2003, 125,8732.
23 A. Muto, D. Kraemer, Q. Hao, Z. F. Ren and G. Chen, Rev.Sci. Instrum., 2009, 80, 093901.
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