Volume 33 Issue 5
Nov.  2020
Turn off MathJax
Article Contents

Liang Zhang, Junjun Tan, Quanbing Pei, Shuji Ye. Film Thickness and Surface Plasmon Tune the Contribution of SFG Signals from Buried Interface and Air Surface†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 532-539. doi: 10.1063/1674-0068/cjcp2006113
Citation: Liang Zhang, Junjun Tan, Quanbing Pei, Shuji Ye. Film Thickness and Surface Plasmon Tune the Contribution of SFG Signals from Buried Interface and Air Surface[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 532-539. doi: 10.1063/1674-0068/cjcp2006113

Film Thickness and Surface Plasmon Tune the Contribution of SFG Signals from Buried Interface and Air Surface

doi: 10.1063/1674-0068/cjcp2006113
More Information
  • Corresponding author: Shuji Ye, E-mail:shujiye@ustc.edu.cn
  • Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
  • Received Date: 2020-06-28
  • Accepted Date: 2020-07-15
  • Publish Date: 2020-10-27
  • Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
  • 加载中
  • [1] P. Dhar, P. P. Khlyabich, B. Burkhart, S. T. Roberts, S. Malyk, B. C. Thompson, and A. V. Benderskii, J. Phys. Chem. C 117, 15213 (2013). doi:  10.1021/jp404846r
    [2] D. B. O'Brien and A. M. Massari, J. Chem. Phys. 142, 024704 (2015). doi:  10.1063/1.4904926
    [3] M. Y. Xiao, T. Y. Lu, T. Lin, J. S. Andre, and Z. Chen, Adv. Energy Mater. 10, 1903053 (2020). doi:  10.1002/aenm.201903053
    [4] J. G. C. Veinot, and T. J. Marks, Acc. Chem. Res. 38, 632 (2005). doi:  10.1021/ar030210r
    [5] T. C. Anglin, D. B. O'Brien, and A. M. Massari, J. Phys. Chem. C 114, 17629 (2010). doi:  10.1021/jp103636s
    [6] T. C. Anglin, J. C. Speros, and A. M. Massari, J. Phys. Chem. C 115, 16027 (2011). doi:  10.1021/jp2035339
    [7] X. L. Lu, G. Xue, X. P. Wang, J. L. Han, X. F. Han, J. Hankett, D. W. Li, and Z. Chen, Macromolecules 45, 6087 (2012). doi:  10.1021/ma301088g
    [8] Z. Chen, Prog. Polym. Sci. 35, 1376 (2010). doi:  10.1016/j.progpolymsci.2010.07.003
    [9] N. W. Ulrich, M. Y. Xiao, X. Q. Zou, J. Williamson, and Z. Chen, IEEE T. Comp. Pack. Man. 8, 1213 (2018).
    [10] C. Zhang, Appl. Spectrosc. 71, 1717 (2017). doi:  10.1177/0003702817708321
    [11] C. Y. Chen, J. Wang, M. A. Even, and Z. Chen, Macromolecules 35, 8093 (2002). doi:  10.1021/ma0205717
    [12] S. H. Wu, Polymer Interface and Adhesion, New York: Marcel Dekker, (1982).
    [13] Y. M. Li, J. X. Wang, and W. Xiong, J. Phys. Chem. C 119, 28083 (2015). doi:  10.1021/acs.jpcc.5b10725
    [14] Y. R. Shen, The Principles of Nonlinear Optics, New York: Wiley, (1984).
    [15] S. Hosseinpour, S. J. Roeters, M. Bonn, W. Peukert, S. Woutersen, and T. Weidner, Chem. Rev. 120, 3420 (2020). doi:  10.1021/acs.chemrev.9b00410
    [16] B. Ding, J. Jasensky, Y. X. Li, and Z. Chen, Acc. Chem. Res. 49, 1149 (2016). doi:  10.1021/acs.accounts.6b00091
    [17] E. T. Castellana and P. S. Cremer, Surf. Sci. Rep. 61, 429 (2006). doi:  10.1016/j.surfrep.2006.06.001
    [18] A. G. Lambert, P. B. Davies, and D. J. Neivandt, Appl. Spectrosc. Rev. 40, 103 (2005). doi:  10.1081/ASR-200038326
    [19] S. Roy, P. A. Covert, W. R. FitzGerald, and D. K. Hore, Chem. Rev. 114, 8388 (2014). doi:  10.1021/cr400418b
    [20] E. C. Yan, L. Fu, Z. G. Wang, and W. Liu, Chem. Rev. 114, 8471 (2014). doi:  10.1021/cr4006044
    [21] S. Nihonyanagi, S. Yamaguchi, and T. Tahara, Chem. Rev. 16, 10665 (2017).
    [22] X. L. Lu, N. Shephard, J. L. Han, G. Xue, and Z. Chen, Macromolecules 41, 8770 (2008). doi:  10.1021/ma801680f
    [23] X. L. Lu, D. W. Li, C. B. Kristalyn, J. L. Han, N. Shephard, S. Rhodes, G. Xue, and Z. Chen, Macromolecules 42, 9052 (2009). doi:  10.1021/ma901757w
    [24] X. L. Lu, M. L. Clarke, D. W. Li, X. P. Wang, G. Xue, and Z. Chen, J. Phys. Chem. C 115, 13759 (2011).
    [25] X. Li, B. L. Li, X. D. Zhang, C. C. Li, Z. R. Guo, D. S. Zhou, and X. L. Lu, Macromolecules 49, 3116 (2016). doi:  10.1021/acs.macromol.6b00389
    [26] C. Zhang, J. N. Myers, and Z. Chen, Langmuir 30, 12541 (2014). doi:  10.1021/la502239u
    [27] J. N. Myers, C. Zhang, K. W. Lee, J. Williamson, and Z. Chen, Langmuir 30, 165 (2014). doi:  10.1021/la4037869
    [28] C. Zhang, J. Hankett, and Z. Chen, ACS Appl. Mater. Interface 4, 3730 (2012). doi:  10.1021/am300854g
    [29] N. W. Ulrich, J. Andre, J. Williamson, K. W. Lee, and Z. Chen, Phys. Chem. Chem. Phys. 19, 12144 (2017). doi:  10.1039/C7CP00567A
    [30] N. W. Ulrich, J. N. Myers, and Z. Chen, RSC Adv. 5, 105622 (2015). doi:  10.1039/C5RA24332G
    [31] C. Chen, J. Wang, C. L. Loch, D. Ahn, and Z. Chen, J. Am. Chem. Soc. 126, 1174 (2004). doi:  10.1021/ja0390911
    [32] C. Zhang and Z. Chen, J. Phys. Chem. C 117, 3903 (2013). doi:  10.1021/jp307472j
    [33] J. J. Tan, B. X. Zhang, Y. Luo, and S. J. Ye, Angew. Chem. Int. Ed. 56, 12977 (2017). doi:  10.1002/anie.201706996
    [34] J. H. Zhang, J. J. Tan, R. Q. Pei, and S. J. Ye, Langmuir 36, 1530 (2020). doi:  10.1021/acs.langmuir.9b03623
    [35] J. J. Tan, J. H. Zhang, Y. Luo, and S. J. Ye, J. Am. Chem. Soc. 141, 1941 (2019). doi:  10.1021/jacs.8b08537
    [36] J. H. Huang, K. Z. Tian, S. J. Ye, and Y. Luo, J. Phys. Chem. C 120, 15322 (2016). doi:  10.1021/acs.jpcc.6b05677
    [37] X. Hu, J. J. Tan, and S. J. Ye, J. Phys. Chem. C 121, 15181 (2017). doi:  10.1021/acs.jpcc.7b03092
    [38] B. X. Zhang, J. J. Tan, C. Z. Li, J. H. Zhang, and S. J. Ye, Langmuir 34, 7554 (2018). doi:  10.1021/acs.langmuir.8b00946
    [39] J. J. Tan, C. Z. Li, J. H. Zhang, and S. J. Ye, Chin. J. Chem. Phys. 31, 523 (2018). doi:  10.1063/1674-0068/31/cjcp1805128
    [40] J. J. Tan, S. J. Ye, and Y. Luo, J. Phys. Chem. C 119, 28523 (2015). doi:  10.1021/acs.jpcc.5b10632
    [41] S. J. Ye, H. C. Li, W. L. Yang, and Y. Luo, J. Am. Chem. Soc. 136, 1206 (2014). doi:  10.1021/ja411081t
    [42] W. T. Wang, J. J. Tan, and S. J. Ye, J. Phys. Chem. B 124, 5169 (2020). doi:  10.1021/acs.jpcb.0c02464
    [43] X. J. Zhang, D. G. Cahill, O. Coronell, and B. J. Marinas, J. Membr. Sci. 331, 143 (2009). doi:  10.1016/j.memsci.2009.01.027
    [44] R. H. Trivedi, L. Werner, D. J. Apple, S. K. Pandey, and A. M. Izak, Eye 16, 217 (2002). doi:  10.1038/sj.eye.6700066
    [45] H. H. Lin, Y. L. Liu, J. H. Liu, C. Y. Chou, Y. F. Yang, H. L. Kuo, and C. C. Huang, Artif. Organs 32, 468 (2008). doi:  10.1111/j.1525-1594.2008.00568.x
    [46] Y. Wang, B. Vaidya, H. D. Farquar, W. Stryjewski, R. P. Hammer, R. L. McCarley, S. A. Soper, Y. W. Cheng, and F. Barany, Anal. Chem. 75, 1130 (2003). doi:  10.1021/ac020683w
    [47] Q. F. Li, R. Hua, I. J. Cheah, and K. C. Chou, J. Phys. Chem. B 112, 694 (2008). doi:  10.1021/jp072147j
    [48] S. W. Kuan, C. W. Frank, Y. H. Lee, T. Eimori, D. R. Allee, R. F. W. Pease, and R. Browning, J. Vac. Sci. Technol. B 7, 1745 (1989).
    [49] J. B. Park and R. S. Lakes, Biomaterials: an Introducion, New York: Plenum Press, (1992).
    [50] J. Wang, C. Y. Chen, S. M. Buck, and Z. Chen, J. Phys. Chem. B 105, 12118 (2001). doi:  10.1021/jp013161d
    [51] S. G. Motti, L. S. Cardoso, D. J. C. Gomes, R. M. Faria, and P. B. Miranda, J. Phys. Chem. C 122, 10450 (2018). doi:  10.1021/acs.jpcc.8b01760
    [52] B. Zuo, Q. Y. Xu, T. C. Jin, H. M. Xing, J. H. Shi, Z. W. Hao, L. Zhang, K. Tanaka, and X. P. Wang, Langmuir 35, 14890 (2019). doi:  10.1021/acs.langmuir.9b02581
    [53] Y. M. Hong, H. Zhou, W. H. Qian, B. Zuo, and X. P. Wang, J. Phys. Chem. C 121, 19816 (2017). doi:  10.1021/acs.jpcc.7b06051
    [54] J. Q. Xu, Y. J. Liu, J. S. He, R. P. Zhang, B. Zuo, and X. P. Wang, Soft Matter. 10, 8992 (2014). doi:  10.1039/C4SM01743A
    [55] H. Zhu, N. Dhopatkar, and A. Dhinojwala, ACS Macro. Lett. 5, 45 (2016). doi:  10.1021/acsmacrolett.5b00834
    [56] M. Inutsuka, A. Horinouchi, and K. Tanaka, ACS Macro. Lett. 4, 1174 (2015). doi:  10.1021/acsmacrolett.5b00592
    [57] B. L. Li, J. Zhou, X. Xu, J. C. Yu, W. Shao, Y. Fang, and X. L. Lu, Polymer 54, 1853 (2013). doi:  10.1016/j.polymer.2013.02.002
    [58] A. Horinouchi, H. Atarashi, Y. Fujii, and K. Tanaka, Macromolecules 45, 4638 (2012). doi:  10.1021/ma3002559
    [59] K. C. Jena, P. A. Covert, S. A. Hall, and D. K. Hore. J. Phys. Chem. C 115, 15570 (2011). doi:  10.1021/jp205712c
    [60] Y. Tateishi, N. Kai, H. Noguchi, K. Uosaki, T. Nagamura, and K. Tanaka, Polym. Chem. 1, 303 (2010). doi:  10.1039/B9PY00227H
    [61] A. Rao, H. Rangwalla, V. Varshney, and A. Dhinojwala, Langmuir 20, 7183 (2004). doi:  10.1021/la049413u
    [62] J. H. Zhang, W. L. Yang, J. J. Tan, and S. J. Ye, Phys. Chem. Chem. Phys. 20, 5657 (2018). doi:  10.1039/C7CP07389E
    [63] J. J. Tan, Y. Luo, and S. J. Ye, Chin. J. Chem. Phys. 30, 671 (2017). doi:  10.1063/1674-0068/30/cjcp1706114
    [64] C. Humbert, T. Noblet, L. Dalstein, B. Busson, and G. Barbillon, Materials 12, 836 (2019). doi:  10.3390/ma12050836
    [65] K. P. Cheung, R. Grover, Y. Wang, C. Gurkovich, and G. Wang, Appl. Phys. Lett. 87, 214103 (2005). doi:  10.1063/1.2133926
    [66] O. Selig, R. Siffels, and Y. L. A. Rezus, Phys. Rev. Lett. 114, 233004 (2015). doi:  10.1103/PhysRevLett.114.233004
    [67] M. M. Coleman and P. C. Painter, Prog. Polym. Sci. 20, 1 (1995). doi:  10.1016/0079-6700(94)00038-4
    [68] X. H. Liu, Y. Zhao, Z. Liu, D. J. Wang, J. G. Wu, and D. F. Xu, J. Mol. Struct. 892, 200 (2008). doi:  10.1016/j.molstruc.2008.05.025
    [69] S. J. Ye, J. J. Tan, K. Z. Tian, C. Z. Li, J. H. Zhang, and Y. Luo, Chem. Commun. 55, 541 (2019). doi:  10.1039/C8CC08452A
    [70] E. Tyrode, C. M. Johnson, S. Baldelli, C. Leygraf, and M. W. Rutland, J. Phys. Chem. B 109, 329 (2005). doi:  10.1021/jp047337y
    [71] S. L. Ma, H. C. Li, K. Z. Tian, S. J. Ye, and Y. Luo, J. Phys. Chem. Lett. 5, 419 (2014). doi:  10.1021/jz402537w
    [72] K. Kneipp, M. Moskovits, and H. Kneipp, Surface-Enhanced Raman Scattering: Physics and Applications, Berlin: Springer Science & Business Media, (2006).
    [73] B. Sharma, R. R. Frontiera, A. I. Henry, E. Ringe, and R. P. Van Duyne, Mater. Today 15, 16 (2012). doi:  10.1016/S1369-7021(12)70017-2
    [74] E. C. Le Ru and P. G. Etchegoin, Annu. Rev. Phys. Chem. 63, 65 (2012). doi:  10.1146/annurev-physchem-032511-143757
    [75] T. Schmid, L. Opilik, C. Blum, and R. Zenobi, Angew. Chem. Int. Ed. 52, 5940 (2013). doi:  10.1002/anie.201203849
    [76] C. Zong, M. X. Xu, L. J. Xu, T. Wei, X. Ma, X. S. Zheng, R. Hu, and B. Ren, Chem. Rev. 118, 4946 (2018). doi:  10.1021/acs.chemrev.7b00668
    [77] A. B. Zrimsek, N. H. Chiang, M. Mattei, S. Zaleski, M. O. McAnally, C. T. Chapman, A. I. Henry, G. C. Schatz, and P. R. Van Duyne, Chem. Rev. 117, 7583 (2017). doi:  10.1021/acs.chemrev.6b00552
    [78] O. Pluchery, C. Humbert, M. Valamanesh, E. Lacaze, and B. Busson, Phys. Chem. Chem. Phys. 11, 7729 (2009). doi:  10.1039/b902142f
    [79] C. Humbert, O. Pluchery, E. Lacaze, A. Tadjeddine, and B. Busson, Gold Bull. 46, 299 (2013). doi:  10.1007/s13404-013-0126-5
    [80] D. Lis, Y. Caudano, M. Henry, S. D. Champagne, E. Ferain, and F. Cecchet, Adv. Optical Mater. 1, 244 (2013). doi:  10.1002/adom.201200034
    [81] E. V. Alieva, L. A. Kuzik, V. A. Yakovlev, G. Knippels, A. F. G. van der Meer, and G. Mattei, Chem. Phys. Lett. 302, 528 (1999). doi:  10.1016/S0009-2614(99)00155-4
    [82] E. V. Alieva, L. A. Kuzik, and V. A. Yakovlev, Chem. Phys. Lett. 292, 542 (1998). doi:  10.1016/S0009-2614(98)00738-6
    [83] S. Baldelli, A. S. Eppler, E. Anderson, Y. R. Shen, and G. A. Somorjai, J. Chem. Phys. 113, 5432 (2000). doi:  10.1063/1.1290024
    [84] Q. F. Li, C. W. Kuo, Z. Yang, P. L. Chen, and K. C. Chou, Phys. Chem. Chem. Phys. 11, 3436 (2009). doi:  10.1039/b821045d
    [85] W. T. Liu and Y. R. Shen, Proc. Natl. Acad. Sci. USA 111, 1293 (2014). doi:  10.1073/pnas.1317290111
    [86] M. Gao, Y. H. He, Y. Chen, T. M. Shih, W. M. Yang, H. Y. Chen, Z. L. Yang, and Z. H. Wang, Nanophotonics 9, 815 (2020). doi:  10.1515/nanoph-2019-0447
    [87] N. L. Gruenke, M. F. Cardinal, M. O. McAnally, R. R. Frontiera, G. C. Schatz, and R. P. Van Duyne, Chem. Soc. Rev. 45, 2263 (2016). doi:  10.1039/C5CS00763A
  • 加载中
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

Figures(8)

Article Metrics

Article views(15) PDF downloads(3) Cited by()

Proportional views
Related

Film Thickness and Surface Plasmon Tune the Contribution of SFG Signals from Buried Interface and Air Surface

doi: 10.1063/1674-0068/cjcp2006113
Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
Liang Zhang, Junjun Tan, Quanbing Pei, Shuji Ye. Film Thickness and Surface Plasmon Tune the Contribution of SFG Signals from Buried Interface and Air Surface†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 532-539. doi: 10.1063/1674-0068/cjcp2006113
Citation: Liang Zhang, Junjun Tan, Quanbing Pei, Shuji Ye. Film Thickness and Surface Plasmon Tune the Contribution of SFG Signals from Buried Interface and Air Surface[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 532-539. doi: 10.1063/1674-0068/cjcp2006113
  • Buried interfaces are ubiquitous in devices such as photovoltaics [1-3], light emitting diodes [4], and field effect transistors [5, 6]. Buried interfaces are also associated with many applications including microelectronics, anticorrosion coatings or painting, and adhesion [7-9]. In situ probing the molecular structures of the molecules at buried interfaces can not only provide important information for optimizing desired properties of these interfaces, but also help to understand many important interface-related mechanisms that are beneficial to modern polymer science and chemical engineering [10]. For example, understanding polymer/polymer buried interfacial structures is critical to assess miscibility and bulk properties of polymer blends and adhesion of polymer/polymer interfaces [11, 12]. However, determination of molecular structures at buried interfaces is a very challenging task because buried interfaces are always in contact with the thicker part of the solid materials. Few nondestructive-characterization techniques can provide useful information in situ [7, 13].

    As a second-order nonlinear optical laser technique, sum-frequency generation vibrational spectroscopy (SFG-VS) has been demonstrated to be a powerful tool for providing the molecular structures of various surfaces and interfaces under different chemical conditions. SFG-VS has been applied to probe many interfaces that are accessible to laser light, including buried interfaces [8, 10, 14-21]. For instance, Chen group has extensively investigated solid/solid or solid/liquid buried interfacial structures, including low-dielectric-constant materials relevant to microelectronics and polymer packaging materials [8, 9, 11, 26-32]. Lu et al. studied the polymer/solution and polymer/metal interfaces [22-25]. Our group has also applied SFG to probe molecular structures and ultrafast vibrational dynamics of proteins and peptides at the lipid membranes/water buried interfaces [33-42]. It is noticed that many of the above mentioned studies do not involve air surface. In fact, many studies often use different substrates to support the films of the studied materials. At this case, there exist two symmetry-broken environments: substrates/materials buried interface and materials/air surface. In terms of SFG principle, SFG signals may come from buried interface and air surface. Distinguishing the contribution of SFG signals from these two environments is crucial to reveal the behavior of buried interfacial molecules.

    In this study, we use polymer poly(methyl methacrylate) (PMMA) as the model, and show how to distinguish the contribution of SFG signals from buried interface and surface. PMMA is used because it has been widely used as materials for barriers, membranes for water filtration and dialysis, artificial lenses in ophthalmology, biomedical implants, photo lithography, nano-imprinting, and bio-tips in micro total analysis systems due to its unique mechanical, thermal, and optical properties [43-50]. PMMA has been used as the dielectric layer in organic field-effect transistors [51]. In these applications, PMMA is generally in contact with other materials to form buried interface. Although the molecular structures of PMMA surface have been investigated by several groups [52-61], it is highly desirable to investigate buried interfacial molecular structures to reveal how PMMA functions in devices. Here we employ a near-total-internal-reflection geometry (FIG. 1) to produce CaF2/PMMA buried interface and PMMA/air surface. It is found that SFG signals from buried interface become much more prominent when increasing the thickness of PMMA films or using surface plasmon-induced enhancement.

    Figure 1.  A schematic of the near-total-internal-reflection geometry.

  • PMMA ($ M_{\rm{W}} $ = 35000) polymer molecules were purchased from Huite Optoelectronic Technology Co., Ltd. Gold nanorod with an aspect ratio of 2.5 was purchased from Zhongkeleiming (Beijing) Technology Co., Ltd. PMMA was dissolved in chloroform to achieve target concentrations of 0.1 wt%, 0.25 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt%. PMMA solution with different concentrations was spin-coated on the surface of CaF2 prisms with a spin-coating speed of 3000 r/min to prepare PMMA films. For the plasmon-enhanced SFG experiments, gold-nanorod in chloroform solution was initially deposited on the surface of CaF2 prisms. After that, PMMA solution with different concentrations was spin-coated on the Au-deposited CaF2 surface. We labeled the samples with gold nanorod as Au-PMMA. The prism-cleaning was performed using a standard procedure given in a previous publication [62]. All of the solvents were used without further purification.

  • All the SFG experiments were carried out by a femtosecond time-resolved SFG-VS system. Detailed information about instrumental parameters can be found in our published articles [33, 39, 63]. The SFG spectra were normalized by the energy profiles of the IR pulses determined through measuring the SFG signals from the gold surface coated at the prism. All SFG experiments were carried out at room temperature (24 ℃).

  • The SFG signals were fitted using a standard procedure described by Eq.(1) [64]:

    where $ A_\nu $, $ \omega_\nu $, and $ \Gamma _\nu $ are the strength, resonant frequency, and damping coefficient of the vibrational mode ($ \nu $), respectively. $ \Phi $ and $ \varphi_\nu $ are the phases of non-resonance and the vibrational mode, respectively. $ A_\nu $, $ \omega_\nu $, and $ \Gamma _\nu $ can be extracted by fitting the spectrum. We define the effective peak strength ($ \chi _\nu^{(2)} $) as the fitting strength $ A_\nu $ divided by $ \Gamma _\nu $.

  • We first investigate the influence of PMMA film thickness on the molecular structures at buried interface and air surface. We control the thickness by spin-coating different-concentration PMMA solution on CaF2 prisms with the same spin speed. It has been evident that there is a positive correlation between the concentration of PMMA solutions and the thickness of PMMA films. Namely, higher concentration solutions result in thicker films [22, 54, 65]. Because of lack of the instrument to measure the film thickness value, we did not measure the actual thickness value of the films. It is highly desirable to correlate the experimental film thickness and SFG intensity in a future study. FIG. 2 shows the ssp and ppp SFG spectra of carbonyl vibrations of PMMA. The IR spectrum of PMMA films indicates that the carbonyl vibration locates at 1730-1735 cm-1 [66]. However, in SFG spectra (FIG. 2 (a) and (b)), besides the peak at $ \sim $1730 cm-1 (labeled as Peak 2), another peak at 1720 cm-1 (labeled as Peak 1) is observed in both ssp and ppp spectra. For the samples prepared with 0.1 wt% and 0.25 wt% PMMA solution, the intensity of the 1730 cm-1 peak is stronger than the one of the 1720 cm-1 peak. In contrast, for the samples prepared with 1 wt% and 2 wt% PMMA solution, the intensity of the 1720 cm-1 peak is much stronger than the one of the 1730 cm-1 peak. Recently, Dhinojwala et al. observed that the carbonyl vibrations at C=O/substrate and C=O/solvent interfaces have different frequencies [55]. The carbonyl groups at C=O/substrate shift to low frequency (1710 cm-1) because of acid-base interactions with the surface silanol groups. Therefore, these two peaks in FIG. 2 most likely originate from carbonyl groups at PMMA/air surface (1730 cm-1) and CaF2/PMMA buried interface (1720 cm-1), respectively. The surface mode is dominated in the thin films prepared by 0.1 wt% and 0.25 wt% PMMA solution while the buried interface mode is dominated in the thick films prepared by 1 wt% and 2 wt% PMMA solution. It is because the local energy of IR beam at the PMMA surface is much smaller than the one at CaF2/PMMA interface due to the absorption of the thick PMMA film. Earlier studies indicated that formation of hydrogen bond between carbonyl group and the hydrogen bond donor leads to a red shift in the C=O frequency [60,67]. In addition, interaction between Ca$^{2+}$ cation and the carbonyl group can also shift the C=O group to low frequency [68,69].

    Figure 2.  The SFG spectra of the carbonyl groups of CaF2 substrate-supported PMMA films prepared with different concentration (0.1 wt%, 0.25 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt%). (a) ssp and (b) ppp.

    Such contribution of SFG signals from buried interface and air surface can be further confirmed by the SFG spectra in the C-H region (FIG. 3). The C-H stretching of PMMA has been well studied using SFG-VS [50, 52-61]. Previous studies indicated that the spectra show the peaks at 2908 cm-1 (symmetric stretching of methylene groups), 2930 cm-1 (antisymmetric stretching of methylene groups), and 2955 cm-1 (symmetric stretching of ester methyl groups) at the PMMA/air surface, while only 2955 cm-1 peak appears at the PMMA/H2O interface [58, 60]. The 2908 cm-1 and 2930 cm-1 peaks appear at the PMMA/air surface because segregation of hydrophobic methylene groups to air surface can minimize the surface free energy. In our study, the ppp and ssp spectra are both composed of the peaks at 2847 cm-1 (symmetric stretching of -CH2 groups), 2875 cm-1 (symmetric stretching of -CH3 groups), 2910 cm-1, 2930 cm-1 and 2955 cm-1 for the thin films prepared by 0.1 wt% and 0.25 wt% PMMA solution; but for the thick films prepared by 1 wt% and 2 wt% PMMA solution, the spectra are dominated by two peaks at 2955 cm-1 and 2990 cm-1 (antisymmetric stretching of ester methyl groups). The C-H results, taking into account the C=O results in FIG. 2, indicate that surface is covered by the -CH2, -CH3, -OCH3 and C=O groups, while only -OCH3 and C=O groups are present at the buried interface.

    Figure 3.  The SFG spectra in the C-H stretching region of CaF2 substrate-supported PMMA films prepared with different concentrations (0.1 wt%, 0.25 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt%). (a) ssp and (b) ppp.

  • To quantitatively analyze the influence of PMMA film thickness on the intensity of C = O peaks, we fit the spectra using Eq.(1) by assuming that the 1720 cm-1 and 1730 cm-1 modes adopt the same phase sign. FIG. 4(a) shows the dependence of the fitting strength ratios of $ \chi _{ \rm{Peak1}}^{(2)}/\chi _{ \rm{Peak2}}^{(2)} $ on the solution concentrations used in film preparation. It can be seen that the ratio of $ \chi _{ \rm{Peak1}}^{(2)}/\chi _{ \rm{Peak2}}^{(2)} $ increases as the solution concentration ($ C $) increases. $ \chi _{ \rm{Peak1}}^{(2)}/\chi _{ \rm{Peak2}}^{(2)} $ is smaller than 1 at $ C $$ \leq $0.25 wt%, while it is larger than 1 at C≥0.5 wt%. Such transition from surface mode to buried interfacial mode occurs at C=0.5 wt%, as illustrated by the C−H stretching spectra in FIG. 3.

    Figure 4.  The fitting strength ratios of (a) $\chi _{\textrm{Peak1}}^{(2)}/\chi _{\textrm{Peak2}}^{(2)}$ and (b) $\chi _{{\rm{ppp}}}^{(2)}/\chi _{{\rm{ssp}}}^{(2)}$ are plotted against solution concentrations used in film preparation.

    The molecular orientation angle ($ \theta $) of a functional group can be deduced by measuring the ssp and ppp spectral intensity ratio through relating SFG susceptibility $ {\chi _{ijk}} $($ i, j, k $ = $ x, y, z $) to the molecular hyperpolarizability $ {\beta _{lmn}} $($ l, m, n $ = $ a, b, c $) [14, 17, 18]. Tyrode et al. have developed a method to determine the orientation of C = O bond [70]. The components of $ \chi _{{\rm{eff}}, {\rm{ssp}}}^{(2)} $ and $ \chi _{{\rm{eff}}, {\rm{ppp}}}^{(2)} $ are given in Eq.(2) and Eq.(3) in the lab coordinate system in which the $ z $-axis is defined along the surface normal and the $ x $-axis in the incident plane [14, 17, 18, 70, 71].

    where $ \beta_{\rm{SF}} $, $ \beta_{\rm{Vis}} $ and $ \beta_{\rm{IR}} $ are the angles between the surface normal and the sum frequency beam, the input visible beam, and the input IR beam, respectively. $ L_{ii} $ ($ i $ = $ x $, $ y $, or $ z $) denotes the Fresnel coefficients. After considering the Fresnel coefficient constants under this experimental geometry, Eq.(2) and Eq.(3) are then given by Eq.(4, 6) and Eq.(5, 7), respectively.

    For the CaF2/PMMA buried interface:

    For the PMMA/air surface:

    For $ C_{\propto \rm{v}} $ symmetry of C = O group, $ \chi _{xxz}^{(2)} $ equals to $ \chi _{yyz}^{(2)} $. Therefore, the $ \chi _{yyz}^{(2)} $ and $ \chi _{zzz}^{(2)} $ susceptibility components are the main contributors to the ssp and ppp signals, respectively. The dependence of $ \chi _{yyz}^{(2)} $ and $ \chi _{zzz}^{(2)} $ susceptibility components on the molecular hyperpolarizability averaged over the azimuthal angles can be described by the following expressions.

    For the CaF2/PMMA buried interface:

    For the PMMA/air surface:

    where $ r $ = $ {\beta _{aac}}/{\beta _{ccc}} $. The value of $ r $ = 0.3 is given by Tyrode et al. [70]. According to Eq.(10) and Eq.(11), the orientation angle ($ \theta $) of C = O bond can be obtained by measuring the ppp and ssp spectral intensity ratio. Resulting relation between the $ \chi _{{\rm{ppp}}}^{(2)}/\chi _{{\rm{ssp}}}^{(2)} $ ratio and $ \theta_{ \rm{C} = \rm{O}} $ for C = O bond at buried interface and air surface is present in FIG. 5(a). According to the experimentally measured $ \chi _{{\rm{ppp}}}^{(2)}/\chi _{{\rm{ssp}}}^{(2)} $ ratio of the peaks at 1720 cm-1 and 1730 cm-1 in FIG. 4(b), the tilt angle of $ \theta_{ \rm{C} = \rm{O}} $ versus the surface normal is deduced with the assumption of a $ \delta $-distribution (FIG. 5(b)). At the buried interface, the tilt angle $ \theta_{\rm{C = O}} $ decreases from 65° to 43° as the film preparation concentration increases. The tilt angle $ \theta_{\rm{C = O}} $ of C = O at the buried interface equals to 45°$ \pm $2° at $ C $$ \geq $0.5 wt%. At the air surface, the tilt angles of C=O with different film preparation concentrations all fall in the range of 38◦ to 21◦.

    Figure 5.  (a) The relation between the $ \chi _{{\rm{ppp}}}^{(2)}/\chi _{{\rm{ssp}}}^{(2)} $ ratio and tilt angle $ \theta_{ \rm{C} = \rm{O}} $ for C = O bond at buried interface (top panel) and air surface (bottom panel). (b) The deduced tilt angle of C = O bond at buried interface (top panel) and air surface (bottom panel).

  • Surface-enhanced Raman scattering (SERS) has been demonstrated to be able to enhance Raman signals of molecules by several orders of magnitude through the amplification of electromagnetic fields generated by the excitation of localized surface plasmons [72-77]. Since SFG signals rely on a tensor product of the IR transition dipole moment and the Raman polarizability tensor, coupling of SFG-VS with surface plasmon is expected to largely enhance the signals from the metal/material buried interface. In fact, several surface plasmon-enhanced SFG (PE-SFG) phenomena have been explored. For example, Humbert et al. found that the SFG signals of thiophenols grafted on gold nanospheres can be detected even if the surface coverage of thiophenol molecules is as low as 1% [78, 79]. The enhancement is reported ranging from several tens to ten-thousand times [64, 78, 80-87]. Because PMMA is widely used as dielectric layer in organic field-effect transistors, it is very important to obtain the molecular structure at the metal/PMMA buried interface. Here we examine the influence of gold nanorod on the molecule structures at buried interface and air surface. FIG. 6 shows the ssp and ppp SFG spectra of the PMMA carbonyl groups at the CaF2/gold nanorod/PMMA/air interfaces. We collect the spectra from different positions and find that the interface is totally homogeneous. The SFG intensity does not change much in different positions. It is evident that the SFG signals of these samples all get more than several-time enhancements. Although the spectral features are similar to FIG. 2, the low-frequency peak (Peak 1, green peak in FIG. 6) that arises from the Au/PMMA buried interface is largely enhanced by the local electric field induced by gold nanorod plasmon. It is because the strength of the local electric field depends on the distance between the molecule and the metal surface ($r$) by $E(r)$$\propto$(1+$r/a$)$^{-3}$ [76]. Here, $a$ is the radius of nanorod. This relationship indicates that the SFG intensity will decrease significantly with the increase of the distance. Therefore, the intensity of Peak 2 (blue peak in FIG. 6) which arises from the PMMA/air surface is much smaller than the intensity of Peak 1. It is noticed that the intensity of Peak 1 is nearly as strong as the one of Peak 2 at the preparation concentration of 0.1 wt%. The ratio of $\chi _{\textrm{Peak1}}^{(2)}/\chi _{\textrm{Peak2}}^{(2)}$ is already larger than 1 at $C$$\geq$0.25 wt%. The contribution from the PMMA/air surface is even ignored at $C$$\geq$1 wt%, which is further illustrated by FIG. 7. FIG. 7 plots the fitting strength ratio of $\chi _{\textrm{Peak1}}^{(2)}/\chi _{\textrm{Peak2}}^{(2)}$ against solution concentrations used in film preparation. The ratio of the films without gold nanorods (FIG. 4(b)) is also given for comparison (empty circle). The ratio of $\chi _{\textrm{Peak1}}^{(2)}/\chi _{\textrm{Peak2}}^{(2)}$ in the presence of gold nanorod is always larger than the films without gold nanorod. These results indicate that surface-plasmon enhancement provides an effective method to obtain structural information at buried interface.

    Figure 6.  The SFG spectra of the carbonyl groups of CaF2-Au nanorod-supported PMMA films prepared with different concentration (0.1 wt%, 0.25 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt%). (a) ssp and (b) ppp.

    Figure 7.  The fitting strength ratio of $\chi _{\textrm{Peak1}}^{(2)}/\chi _{\textrm{Peak2}}^{(2)}$ is plotted against solution concentrations used in film preparation. The solid circle represents Au-PMMA films and the empty circle represents the pure PMMA films.

  • We have visualized the variations in the contribution of SFG signals of substrate-supported PMMA films from buried interface and air surface. Investigations on film thickness-dependent spectral changes of the carbonyl and CH stretching groups indicate that SFG signals are dominated by the chemical moieties segregated at the PMMA/air surface for the thin films (preparation concentration$ \leq $0.25 wt%) while they are mainly contributed by the groups appearing at the substrate/PMMA buried interface for the thick films (preparation concentration$\geq$0.5 wt%). The surface is covered by the -CH2, -CH3, -OCH3 and C=O groups, while the buried interface is only composed of -OCH3 and C=O groups. In the presence of gold nanorod, the surface plasmon largely enhances the SFG signals, particularly the signals from the buried interface. Our study depicts that SFG signals from the chemical components at buried interface and air surface can be distinguished by tuning the film thickness and using surface-plasmon enhancement, which is important to determine the buried interfacial molecular structures of the devices applied in the fields such as electronics, photovoltaics, coating, and adhesion.

  • This work was supported by the National Key Research and Development Program of China (No.2018YFA0208700 and No.2017YFA0303500), the National Natural Science Foundation of China (No.21925302, No.21633007, and No.21873090), and Anhui Initiative in Quantum Information Technologies (AHY090000).

Reference (87)

Catalog

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return