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Surface energy characterization of nanoscale metal using quantitative nanomechanical characterization of atomic force microscopy
Journal Pre-proofs Surface Energy Characterization of Nanoscale Metal using Quantitative Nanomechanical Characterization of Atomic Force Microscopy Woosu Park, Sebastian Müller, Roelf-Peter Baumann, Stefan Becker, Byungil Hwang PII: DOI: Reference:
S0169-4332(19)33858-9 https://doi.org/10.1016/j.apsusc.2019.145041 APSUSC 145041
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
19 July 2019 5 December 2019 10 December 2019
Please cite this article as: W. Park, S. Müller, R-P. Baumann, S. Becker, B. Hwang, Surface Energy Characterization of Nanoscale Metal using Quantitative Nanomechanical Characterization of Atomic Force Microscopy, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145041
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Surface Energy Characterization of Nanoscale Metal using Quantitative Nanomechanical Characterization of Atomic Force Microscopy
Woosu Parka,b, Sebastian Müllerb, Roelf-Peter Baumannb, Stefan Beckerb, and Byungil Hwanga,* a
School of Integrative Engineering, Chung-Ang University, Seoul 06974, Republic of
Korea b
Material Physics, Structures and Surfaces, RAA/OS, BASF SE, Ludwigshafen 67056,
Germany *
Corresponding author: E-mail:[email protected]
Abstract Characterization of surface energy is an important step in the development of nano/micro electromechanical systems. However, the conventional surface energy characterization technique is not ideal for nanoscale samples due to the limited volume of liquid droplets. In this study, a novel technique to evaluate the surface energy of nanoscale materials is investigated by using atomic force microscopy (AFM). The contact angle/surface energy of a nanoscale Au wire is determined by measuring the adhesion force by AFM PeakForce-QNM compared to the wetting behavior respectively adhesion force of various hydrophobic/hydrophilic thiol self-assembled monolayers (SAM).
Keywords: surface energy; quantitative nanomechanical characterization; Au; surface functionalization; atomic force microscopy; contact angle
1. Introduction With the emergence of low-dimensional electronic devices, there is an increasing need to formulate new methods to characterize the surface properties at the nanoscale [1]. Surface energy plays an important role in the fabrication of electronic devices such as nano/micro electromechanical systems (NEMS/MEMS), where different nanoscale materials are integrated sequentially using diverse liquid or non-liquid based fabrication processes [25]. Surface energy essentially determines the quality of the stacked layers and thus, its precise measurement in nanoscale materials is a prerequisite in the fabrication of reliable electronic devices. The most widely used method to determine the surface energy is by measuring the contact angle of a liquid droplet on a solid surface [6-11]. Once a liquid drop is placed on a solid substrate, the contact angle between them, and the liquid-gas (air) interface can be measured. According to the Young equation, γsg - γsl - γlg * cosθ = 0, where θ, γsg, γsl, and γlg are the contact angle, the interfacial energy at the substrate-gas, substrate-liquid, and liquid-gas interface, respectively. As the sample size decreases, however, the surface energy evaluation based on the contact angle is difficult. Therefore, a new method is required to characterize the surface energy of nanoscale materials. In this study, we demonstrate for the first time a method to characterize the surface energy of nanoscale metal by using an atomic force microscopy (AFM) with PF-QNM (Peak Force- Quantitative Nanomechanical Mapping) imaging mode. The AFM PF-QNM mode records and analyzes individual force-distance curves while scanning the sample surface under a constant, applied peak force to provide quantitative data on material properties such as adhesion, modulus, dissipation, or deformation with high resolution. In this
method, a calibration curve is obtained that is consisting of a plot of adhesion forces versus contact angle/surface energy for macro scale samples with different functional thiol self-assembled monolayers (SAMs) that are large enough to perform the liquid dropbased surface energy characterization. By correlating the measured AFM PF-QNM adhesion force on the nano scale with the contact angle/surface energy of different functional thiol self-assembled monolayers (SAMs) on the macro scale, the surface energy value of a nanoscale metal surface is evaluated.
2. Experiment 2.1 Materials and sample preparation A template-stripped procedure, Figure 1, was used to fabricate micrometer-sized gold surfaces with atomically flat areas [12]. The deposition of a 50-100 nm thick gold film on an freshly cleaved mica sheet was performed using a thermal vacuum deposition method at a rate of 1 Å/s under a pressure of 10-5 mbar. The gold-deposited mica sheets were glued that gold face down onto a silicon wafer using superglue. Samples were stored for several hours under load until the superglue reached an adequate hardness. The mica layers on top of the atomically flat gold were successively stripped off mechanically using a razor blade or an adhesive tape. The quality of the gold surfaces was checked by their conductivity after each stripping.
Au deposition on mica substrate
Attaching Au thin film to the glued Si substrate
Au Mica Glue Si -CH3 -CF 3 -OH -COOH
SAM treatment
Removing mica substrate
Figure 1 Illustration of the preparation process of Au thin films with different surface functionalities. To cover the surface with a hydrophobic respective hydrophilic thiol self-assembled monolayer, the prepared gold surfaces were immersed in a 1 mM thiol-ethanol p.a. (≥ 99,8 %, Sigma Aldrich) solution for approximately 24 hours, and subsequently washed with pure ethanol to remove residues. For SAM preparation Alkanethiols with different functional end-groups, such as 1-Undecanethiol (CH3(CH2)9SH, molecular weight (MW) 188.37 g/mol), 11-mercapto-1-undecanol (OH(CH2)11SH, MW 204.37 g/mol), 11Mercaptoundecanoic acid (COOH(CH2)9CH2SH, MW 218.36 g/mol), and 1H,1H,2H,2HPerfluorodecanethiol (CF3(CF2)7CH2CH2SH, MW 480.18 g/mol) purchased from SIGMA-Aldrich were used to get a wide range of hydrophobicities. Table 1 shows the contact angle and surface energy of the used Alkanthiols. To fabricate the nanoscale Au wires, the Au films were deposited on the patterned dry film photoresist (DFR), which were prepared on the sapphire substrates using
photolithography. By stripping the DFR, the nanoscale Au wires were made to remain on the substrate. The detailed process of DFR patterning can be found in Ref. [13-16].
2.2 Characterization The adhesion force measurements were performed using a Bruker Dimension ICON AFM in PeakForce-QNM operation mode with a standard silicon cantilever RTESPA150 (Bruker, spring constant ~5 N/m, tip radius ~8 nm). For an exact adjustment of the applied tip force the deflection sensitivity and the spring constant must be calibrated. The adhesion force value is influenced by the contact radius of the AFM tip on the reference sample surface. During each scan, the applied force on the tip should be adjusted, that the deformation value is in a range between 0.5 nm to 1 nm. It is recommended to calculate the average adhesion force immediately after the image has been recorded, to have a real time feedback on changes in tip quality. The contact angle was evaluated by using a high-resolution drop shape analyzer (DSA 100, KRÜSS GmbH) with a sessile water drop. The surface energy was evaluated based on the Owens, Wendt, Rabel and Kaelble (OWRK) model, and water,diiodo-methane, and ethylene glycol drops were used for the surface energy characterization.
3. Results and discussion Figure 2 shows the schematic of the surface energy characterization process on nanoscale Au wires. The first step is to create a calibration plot of adhesion force versus contact angle/surface energy over a wide hydrophobicity range. The wetting behavior of the macro scale reference materials is tuned by utilizing different thiol SAMs to obtain different hydrophobicities. The macro scale contact angle/surface energy were
determined by the sessile drop method. Moreover, the force of adhesion between the AFM silicon tip and the modified surfaces were measured by AFM PF-QNM on nanoscale. The calibration curve is then used to determine the contact angle/surface energy values of the nanoscale material by translating the measured force of adhesion via the calibration curve into a contact angle.
Figure 2 Schematic of the process to evaluate the contact angle and surface energy of the nanoscale metal strip.
To characterize the surface energy of the Au nanowire, the adhesion force is measured by using the same AFM PF-QNM settings as were used to create the calibration curve. Subsequently, the contact angle/surface energy value of the nanoscale Au wire are obtained by comparing the measured adhesion forces with those from the previously prepared calibration curve. Table 1 shows the adhesion forces, contact angles and surface energy of the thiol modified references. The adhesion forces measured by PF-QNM varied from 1.27 nN to 2.92 nN, while the contact angles were found to be in the range of 100.18º to 53.12º. Accordingly, the surface energy was found to be between 23.35 mN/m and 48.39 mN/m. AFM PFQNM Adhesion Force images of each sample with various SAM layers are also shown in
Supporting information 01.
Table 1 The measured adhesion force, contact angle, and surface energy of the Au thin films with different SAM treatments. The samples were prepared as described in Figure 1. End group CF3
Thiol
Adhesion Force [nN]
Contact Angle [°]
Surface Energy [mN/m]
1.27
100.18
22.35
1.89
91.05
38.28
2.66
69.39
41.06
2.92
53.12
48.39
1H,1H,2H,2HPerfluorodecanethiol CF3(CF2)7CH2CH2SH
CH3
1-Undecanethiol CH3(CH2)9SH 11-Mercaptoundecanoicacid COOH(CH2)9CH2SH 11-mercapto-1-undecanol
COOH OH
OH(CH2)11SH
Based on these results, the calibration curve for the thiol-SAM references is plotted, as shown in Figure 3.
(a)
(b) 120 S u rfa c e e n e rg y (m N/m )
80
C o n ta c t A n g le ( o )
100 80 60
Estimated CA of Au nanowire
40
y = -27.53x + 138.58
20 0
Fa of Au nanowire
0
1
2
Adhes ion force (nN)
3
4
60 Estimated SE of Au nanowire
40
20
0
y = 16.26x + 1.01 Fa of Au nanowire
0
1
2
3
4
Adhes ion force (nN)
Figure 1 The calibration curves consisting of a plot of adhesion forces versus (a) contact angle and (b) surface energy for macro scale samples with different functional thiol SAMs.
The dark blue circles indicate the adhesion forces (Fa) of the nano scale Au wire measured by AFM PF-QNM, and corresponding macro contact angle (CA) and surface energy (SE) in the calibration curves.
The adhesion forces show a linear variation with both the contact angle and the surface energy as per slopes in Figure 3. To evaluate the surface energy of the Au wire (approximately 430 nm width), its adhesion force was measured by using PF-QNM, and found to be ~2.31 nN. The contact angle and surface energy corresponding to the measured adhesion forces of the nanoscale Au wire were extracted from the calibration curves and were determined to be 78.83º and 38.57 mN/m, respectively. The contact angle and surface energy of the macroscale Au sample measured by the conventional technique using a water droplet were ~74.38 º and ~38.59 mN/m, respectively. Thus, it was confirmed that the surface energy of the sub-micron thin Au wire was not significantly different from that of bulk Au. However, once the size of Au sample decreases to the scale of a few atoms or nanometers, even a small number of defects affect the surface properties significantly. In this case, the surface energy is expected to be significantly different from that of the bulk sample. Although the fabrication of an atomic scale metal strip is challenging, a detailed study on the size dependence of surface energy at the atomic scale will be of immense interest for the future.
4. Conclusion In this study, a novel technique to characterize the surface energy of nanoscale metals by using an AFM PF-QNM was explored. Au thin films as references were treated with
different thiol- SAMs that provided hydrophobic to hydrophilic surface functionalities. By measuring the adhesion force, contact angle, and surface energy of the references, the calibration curves were plotted. The adhesion forces of the Au wire having a width of ~430 nm were measured by using PF-QNM. By extracting the corresponding values of contact angle and surface energy from the prepared calibration curves, the surface energy and contact angle for the nanoscale Au wires were obtained. The surface energy and contact angle of an Au wire of sub-micron width showed similar values to those of the bulk samples, which confirmed that the surface energy of the Au was not significantly altered at the sub-micron scale.
Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea, which is funded by the Ministry of Science, Information and Communications Technology (Grant No. NRF-2018R1C1B5043900).
References [1] S. Trostová, I. Stibor, J. Karpíšková, Z. Kolská, V. Švorčík, Characterization of surface chemical modified carbon nano-particles, Materials Letters 102-103 (2013) 83-86. [2] H. Kim, G. Lee, S. Becker, J.-S. Kim, H. Kim, B. Hwang, Novel patterning of flexible and transparent Ag nanowire electrodes using oxygen plasma treatment, Journal of Materials Chemistry C 6(35) (2018) 9394-9398. [3] S. Kim, T.G. Yun, C. Kang, M.-J. Son, J.-G. Kang, I.-H. Kim, H.-J. Lee, C.-H. An,
B. Hwang, Facile fabrication of paper-based silver nanostructure electrodes for flexible printed energy storage system, Materials & Design 151 (2018) 1-7. [4] A. Gopalan, U.K. Kommuri, Design and development of miniaturized low voltage triangular RF MEMS switch for phased array application, Applied Surface Science 449 (2018) 340-345. [5] M. Kolahdoozan, A. Kiani, P. Heidari, S. Oveissi, Investigating the effect of varying coating thickness on the surface roughness and adhesion forces of MEMS surfaces utilizing a theoretical and experimental approach, Applied Surface Science 481 (2019) 531-539. [6] R.Z. Moghadam, M.H. Ehsani, H.R. Dizaji, P. Kameli, M. Jannesari, Modification of hydrophobicity properties of diamond like carbon films using glancing angle deposition method, Materials Letters 220 (2018) 301-304. [7] B. Hwang, T.G. Yun, Stretchable and patchable composite electrode with trimethylolpropane formal acrylate-based polymer, Composites Part B: Engineering 163 (2019) 185-192. [8] S.Y. Misyura, Contact angle and droplet heat transfer during evaporation on structured and smooth surfaces of heated wall, Applied Surface Science 414 (2017) 188-196. [9] K. Zhang, J. Zhao, S. Chen, Y. Wang, Y. Liu, Effects of a chemically heterogeneous island on the dynamic contact angles of droplets, Applied Surface Science 486 (2019) 337-343. [10] J. Qu, Y. Yang, S. Yang, D. Hu, H. Qiu, Droplet impingement on nano-textured superhydrophobic surface: Experimental and numerical study, Applied Surface Science 491 (2019) 160-170.
[11] Y. Wang, X. Luo, W. Qin, F. Jiao, New insights into the contact angle and formation process of nanobubbles based on line tension and pinning, Applied Surface Science 481 (2019) 1585-1594. [12] M. Hegner, P. Wagner, G. Semenza, Ultralarge atomically flat template-stripped Au surfaces for scanning probe microscopy, Surface Science 291(1) (1993) 39-46. [13] C.-H. An, S. Kim, H.-J. Lee, B. Hwang, Facile patterning using dry film photoresists for flexible electronics: Ag nanowire networks and carbon nanotube networks, Journal of Materials Chemistry C 5(19) (2017) 4804-4809. [14] S. Kim, B. Hwang, Ag nanowire electrode with patterned dry film photoresist insulator for flexible organic light-emitting diode with various designs, Materials & Design 160 (2018) 572-577. [15] S. Kim, B. Kim, S.M. Cho, H.-J. Lee, B. Hwang, Etchant-free patterning of silver nanowire transparent electrode using dry-film photoresists for organic light-emitting diodes, Materials Letters 209 (2017) 433-436. [16] S. Kim, T.-Y. Eom, M. Cho, K.-Y. Song, C.-H. An, H.-J. Lee, B. Hwang, Simple transfer of Ag nanowires by dry film photoresist for paper-based flexible electronics, Materials Letters 199 (2017) 196-199.
Graphical Abstract Sample preparation
AF, CA/SE measurement
AF vs. CA/SE curves
SE at nanoscale
AFM- PQNM
Extracting matching CA/SE from calibration curve
AF characterization by AFM PF-QNM
AFM PF-QNM Adhesion force by PQNM Calibration curve
Functional group by SAMs R RR R R RR R R RR RR R
Metal
Metal
Source: Bruker Adhesion force (AF)
Substrate
Substrate Adhesion force of nanoscale metal
R=CH3, CF3, OH, COOH
Expected contact angle / surface energy of nanoscale metal sample Source: KRÜSS GmbH Contact angle (CA) / Surface Energy (SE)
Highlights
l Adhesion forces measured by AFM PF-QNM were used to evaluate surface energy l Au thin films were modified with thiol-SAMs to have varied surface functionalities l Calibration curves (adhesion forces vs contact angle/surface energy) plotted for Au l Adhesion force of Au strip with ~430 nm width was ~2.31 nN as measured by PF-QNM l Surface energy of Au wire was 38.57 mN/m by correlating with calibration curve
Author contribution Woosu Park: Investigation, Data Curation, Validation, Writing-Original Draft, Visualization, Sebastian Müller: Conceptualization, Methodology, Validation, Data Curation, Writing-Original Draft, Writing-Review&Editing, Roelf-Peter Baumann: Conceptualization, Methodology, Writing-Original Draft, Stefan Becker: Conceptualization, Project administration, Supervision, Funding acquisition, Byungil Hwang: Conceptualization, Methodology, Validation, Writing-Original Draft, WritingReview&Editing, Visualization, Supervision, Project administration, Funding acquisition.
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S0169-4332(19)33858-9 https://doi.org/10.1016/j.apsusc.2019.145041 APSUSC 145041
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
19 July 2019 5 December 2019 10 December 2019
Please cite this article as: W. Park, S. Müller, R-P. Baumann, S. Becker, B. Hwang, Surface Energy Characterization of Nanoscale Metal using Quantitative Nanomechanical Characterization of Atomic Force Microscopy, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145041
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Surface Energy Characterization of Nanoscale Metal using Quantitative Nanomechanical Characterization of Atomic Force Microscopy
Woosu Parka,b, Sebastian Müllerb, Roelf-Peter Baumannb, Stefan Beckerb, and Byungil Hwanga,* a
School of Integrative Engineering, Chung-Ang University, Seoul 06974, Republic of
Korea b
Material Physics, Structures and Surfaces, RAA/OS, BASF SE, Ludwigshafen 67056,
Germany *
Corresponding author: E-mail:[email protected]
Abstract Characterization of surface energy is an important step in the development of nano/micro electromechanical systems. However, the conventional surface energy characterization technique is not ideal for nanoscale samples due to the limited volume of liquid droplets. In this study, a novel technique to evaluate the surface energy of nanoscale materials is investigated by using atomic force microscopy (AFM). The contact angle/surface energy of a nanoscale Au wire is determined by measuring the adhesion force by AFM PeakForce-QNM compared to the wetting behavior respectively adhesion force of various hydrophobic/hydrophilic thiol self-assembled monolayers (SAM).
Keywords: surface energy; quantitative nanomechanical characterization; Au; surface functionalization; atomic force microscopy; contact angle
1. Introduction With the emergence of low-dimensional electronic devices, there is an increasing need to formulate new methods to characterize the surface properties at the nanoscale [1]. Surface energy plays an important role in the fabrication of electronic devices such as nano/micro electromechanical systems (NEMS/MEMS), where different nanoscale materials are integrated sequentially using diverse liquid or non-liquid based fabrication processes [25]. Surface energy essentially determines the quality of the stacked layers and thus, its precise measurement in nanoscale materials is a prerequisite in the fabrication of reliable electronic devices. The most widely used method to determine the surface energy is by measuring the contact angle of a liquid droplet on a solid surface [6-11]. Once a liquid drop is placed on a solid substrate, the contact angle between them, and the liquid-gas (air) interface can be measured. According to the Young equation, γsg - γsl - γlg * cosθ = 0, where θ, γsg, γsl, and γlg are the contact angle, the interfacial energy at the substrate-gas, substrate-liquid, and liquid-gas interface, respectively. As the sample size decreases, however, the surface energy evaluation based on the contact angle is difficult. Therefore, a new method is required to characterize the surface energy of nanoscale materials. In this study, we demonstrate for the first time a method to characterize the surface energy of nanoscale metal by using an atomic force microscopy (AFM) with PF-QNM (Peak Force- Quantitative Nanomechanical Mapping) imaging mode. The AFM PF-QNM mode records and analyzes individual force-distance curves while scanning the sample surface under a constant, applied peak force to provide quantitative data on material properties such as adhesion, modulus, dissipation, or deformation with high resolution. In this
method, a calibration curve is obtained that is consisting of a plot of adhesion forces versus contact angle/surface energy for macro scale samples with different functional thiol self-assembled monolayers (SAMs) that are large enough to perform the liquid dropbased surface energy characterization. By correlating the measured AFM PF-QNM adhesion force on the nano scale with the contact angle/surface energy of different functional thiol self-assembled monolayers (SAMs) on the macro scale, the surface energy value of a nanoscale metal surface is evaluated.
2. Experiment 2.1 Materials and sample preparation A template-stripped procedure, Figure 1, was used to fabricate micrometer-sized gold surfaces with atomically flat areas [12]. The deposition of a 50-100 nm thick gold film on an freshly cleaved mica sheet was performed using a thermal vacuum deposition method at a rate of 1 Å/s under a pressure of 10-5 mbar. The gold-deposited mica sheets were glued that gold face down onto a silicon wafer using superglue. Samples were stored for several hours under load until the superglue reached an adequate hardness. The mica layers on top of the atomically flat gold were successively stripped off mechanically using a razor blade or an adhesive tape. The quality of the gold surfaces was checked by their conductivity after each stripping.
Au deposition on mica substrate
Attaching Au thin film to the glued Si substrate
Au Mica Glue Si -CH3 -CF 3 -OH -COOH
SAM treatment
Removing mica substrate
Figure 1 Illustration of the preparation process of Au thin films with different surface functionalities. To cover the surface with a hydrophobic respective hydrophilic thiol self-assembled monolayer, the prepared gold surfaces were immersed in a 1 mM thiol-ethanol p.a. (≥ 99,8 %, Sigma Aldrich) solution for approximately 24 hours, and subsequently washed with pure ethanol to remove residues. For SAM preparation Alkanethiols with different functional end-groups, such as 1-Undecanethiol (CH3(CH2)9SH, molecular weight (MW) 188.37 g/mol), 11-mercapto-1-undecanol (OH(CH2)11SH, MW 204.37 g/mol), 11Mercaptoundecanoic acid (COOH(CH2)9CH2SH, MW 218.36 g/mol), and 1H,1H,2H,2HPerfluorodecanethiol (CF3(CF2)7CH2CH2SH, MW 480.18 g/mol) purchased from SIGMA-Aldrich were used to get a wide range of hydrophobicities. Table 1 shows the contact angle and surface energy of the used Alkanthiols. To fabricate the nanoscale Au wires, the Au films were deposited on the patterned dry film photoresist (DFR), which were prepared on the sapphire substrates using
photolithography. By stripping the DFR, the nanoscale Au wires were made to remain on the substrate. The detailed process of DFR patterning can be found in Ref. [13-16].
2.2 Characterization The adhesion force measurements were performed using a Bruker Dimension ICON AFM in PeakForce-QNM operation mode with a standard silicon cantilever RTESPA150 (Bruker, spring constant ~5 N/m, tip radius ~8 nm). For an exact adjustment of the applied tip force the deflection sensitivity and the spring constant must be calibrated. The adhesion force value is influenced by the contact radius of the AFM tip on the reference sample surface. During each scan, the applied force on the tip should be adjusted, that the deformation value is in a range between 0.5 nm to 1 nm. It is recommended to calculate the average adhesion force immediately after the image has been recorded, to have a real time feedback on changes in tip quality. The contact angle was evaluated by using a high-resolution drop shape analyzer (DSA 100, KRÜSS GmbH) with a sessile water drop. The surface energy was evaluated based on the Owens, Wendt, Rabel and Kaelble (OWRK) model, and water,diiodo-methane, and ethylene glycol drops were used for the surface energy characterization.
3. Results and discussion Figure 2 shows the schematic of the surface energy characterization process on nanoscale Au wires. The first step is to create a calibration plot of adhesion force versus contact angle/surface energy over a wide hydrophobicity range. The wetting behavior of the macro scale reference materials is tuned by utilizing different thiol SAMs to obtain different hydrophobicities. The macro scale contact angle/surface energy were
determined by the sessile drop method. Moreover, the force of adhesion between the AFM silicon tip and the modified surfaces were measured by AFM PF-QNM on nanoscale. The calibration curve is then used to determine the contact angle/surface energy values of the nanoscale material by translating the measured force of adhesion via the calibration curve into a contact angle.
Figure 2 Schematic of the process to evaluate the contact angle and surface energy of the nanoscale metal strip.
To characterize the surface energy of the Au nanowire, the adhesion force is measured by using the same AFM PF-QNM settings as were used to create the calibration curve. Subsequently, the contact angle/surface energy value of the nanoscale Au wire are obtained by comparing the measured adhesion forces with those from the previously prepared calibration curve. Table 1 shows the adhesion forces, contact angles and surface energy of the thiol modified references. The adhesion forces measured by PF-QNM varied from 1.27 nN to 2.92 nN, while the contact angles were found to be in the range of 100.18º to 53.12º. Accordingly, the surface energy was found to be between 23.35 mN/m and 48.39 mN/m. AFM PFQNM Adhesion Force images of each sample with various SAM layers are also shown in
Supporting information 01.
Table 1 The measured adhesion force, contact angle, and surface energy of the Au thin films with different SAM treatments. The samples were prepared as described in Figure 1. End group CF3
Thiol
Adhesion Force [nN]
Contact Angle [°]
Surface Energy [mN/m]
1.27
100.18
22.35
1.89
91.05
38.28
2.66
69.39
41.06
2.92
53.12
48.39
1H,1H,2H,2HPerfluorodecanethiol CF3(CF2)7CH2CH2SH
CH3
1-Undecanethiol CH3(CH2)9SH 11-Mercaptoundecanoicacid COOH(CH2)9CH2SH 11-mercapto-1-undecanol
COOH OH
OH(CH2)11SH
Based on these results, the calibration curve for the thiol-SAM references is plotted, as shown in Figure 3.
(a)
(b) 120 S u rfa c e e n e rg y (m N/m )
80
C o n ta c t A n g le ( o )
100 80 60
Estimated CA of Au nanowire
40
y = -27.53x + 138.58
20 0
Fa of Au nanowire
0
1
2
Adhes ion force (nN)
3
4
60 Estimated SE of Au nanowire
40
20
0
y = 16.26x + 1.01 Fa of Au nanowire
0
1
2
3
4
Adhes ion force (nN)
Figure 1 The calibration curves consisting of a plot of adhesion forces versus (a) contact angle and (b) surface energy for macro scale samples with different functional thiol SAMs.
The dark blue circles indicate the adhesion forces (Fa) of the nano scale Au wire measured by AFM PF-QNM, and corresponding macro contact angle (CA) and surface energy (SE) in the calibration curves.
The adhesion forces show a linear variation with both the contact angle and the surface energy as per slopes in Figure 3. To evaluate the surface energy of the Au wire (approximately 430 nm width), its adhesion force was measured by using PF-QNM, and found to be ~2.31 nN. The contact angle and surface energy corresponding to the measured adhesion forces of the nanoscale Au wire were extracted from the calibration curves and were determined to be 78.83º and 38.57 mN/m, respectively. The contact angle and surface energy of the macroscale Au sample measured by the conventional technique using a water droplet were ~74.38 º and ~38.59 mN/m, respectively. Thus, it was confirmed that the surface energy of the sub-micron thin Au wire was not significantly different from that of bulk Au. However, once the size of Au sample decreases to the scale of a few atoms or nanometers, even a small number of defects affect the surface properties significantly. In this case, the surface energy is expected to be significantly different from that of the bulk sample. Although the fabrication of an atomic scale metal strip is challenging, a detailed study on the size dependence of surface energy at the atomic scale will be of immense interest for the future.
4. Conclusion In this study, a novel technique to characterize the surface energy of nanoscale metals by using an AFM PF-QNM was explored. Au thin films as references were treated with
different thiol- SAMs that provided hydrophobic to hydrophilic surface functionalities. By measuring the adhesion force, contact angle, and surface energy of the references, the calibration curves were plotted. The adhesion forces of the Au wire having a width of ~430 nm were measured by using PF-QNM. By extracting the corresponding values of contact angle and surface energy from the prepared calibration curves, the surface energy and contact angle for the nanoscale Au wires were obtained. The surface energy and contact angle of an Au wire of sub-micron width showed similar values to those of the bulk samples, which confirmed that the surface energy of the Au was not significantly altered at the sub-micron scale.
Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea, which is funded by the Ministry of Science, Information and Communications Technology (Grant No. NRF-2018R1C1B5043900).
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Graphical Abstract Sample preparation
AF, CA/SE measurement
AF vs. CA/SE curves
SE at nanoscale
AFM- PQNM
Extracting matching CA/SE from calibration curve
AF characterization by AFM PF-QNM
AFM PF-QNM Adhesion force by PQNM Calibration curve
Functional group by SAMs R RR R R RR R R RR RR R
Metal
Metal
Source: Bruker Adhesion force (AF)
Substrate
Substrate Adhesion force of nanoscale metal
R=CH3, CF3, OH, COOH
Expected contact angle / surface energy of nanoscale metal sample Source: KRÜSS GmbH Contact angle (CA) / Surface Energy (SE)
Highlights
l Adhesion forces measured by AFM PF-QNM were used to evaluate surface energy l Au thin films were modified with thiol-SAMs to have varied surface functionalities l Calibration curves (adhesion forces vs contact angle/surface energy) plotted for Au l Adhesion force of Au strip with ~430 nm width was ~2.31 nN as measured by PF-QNM l Surface energy of Au wire was 38.57 mN/m by correlating with calibration curve
Author contribution Woosu Park: Investigation, Data Curation, Validation, Writing-Original Draft, Visualization, Sebastian Müller: Conceptualization, Methodology, Validation, Data Curation, Writing-Original Draft, Writing-Review&Editing, Roelf-Peter Baumann: Conceptualization, Methodology, Writing-Original Draft, Stefan Becker: Conceptualization, Project administration, Supervision, Funding acquisition, Byungil Hwang: Conceptualization, Methodology, Validation, Writing-Original Draft, WritingReview&Editing, Visualization, Supervision, Project administration, Funding acquisition.
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