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Effects of self-assembled multilayers on the evolution of surface physical properties of indium-tin-oxide
Applied Surface Science 219 (2003) 199–202
Short communication
Effects of self-assembled multilayers on the evolution of surface physical properties of indium-tin-oxide Lin Song Lia, Alexander D.Q. Lib, Q.X. Jiaa,* a
Materials Science and Technology Division, Los Alamos National Laboratory, Superconductivity Technology Center, MS K763, Los Alamos, NM 87545, USA b Department of Chemistry and Center for Materials Research, Washington State University, Pullman, WA 99164, USA Received 28 November 2001; accepted 28 May 2002
Abstract Using layer-by-layer self-assembly technique, we have deposited multilayers of poly(diallydimethylammonium) (PDDA) and nickel phthalocyanine (NiPc) onto indium-tin-oxide (ITO) coated glass substrate. To determine how self-assembled multilayers affect charge transfer properties of conductive ITO surface, Kelvin probe and surface photovoltage measurements have been carried out. The results showed an oscillation of the surface potential as the top layer was alternated between PDDA and NiPc. The changes in surface potential of ITO induced by the growth of PDDA or NiPc monolayers are attributed to the modulation in electron affinity. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Kelvin probe; Surface photovoltage spectroscopy; Layer-by-layer self-assembly
Layer-by-layer (LBL) assembly of oppositely charged polymeric and organic materials has recently been used to fabricate multilayers on various substrates. LBL assembly promises the construction of materials with well-controlled, nanometer-scaled architectures for applications in electronics, photonics and optoelectronics [1–3]. The interfaces between each layer become the key linkages between multilayers, and therefore the study of surface physical properties is critically important as each organic layer is assembled [4,5]. A Kelvin probe has been used to measure the work function qf defined as the difference between the vacuum level and Fermi level or surface potential f *
Corresponding author. Tel.: þ1-505-667-2716; fax: þ1-505-665-3164. E-mail address: [email protected] (Q.X. Jia).
of organic monolayers on semiconducting materials [6,7]. Previous studies have indicated that the physical properties near the surface of semiconducting materials such as CdTe, CdSe, CuInSe2, GaAs and InP are a function of molecular modifiers in systems composed of chemisorbed monolayers of dicarboxylic acids [8,9]. Surface photovoltage (SPV) spectroscopy, a very sensitive tool for measuring the charge transfer at the surface and interface, has also been used to study the change of charge distribution on semiconductor surfaces, buried interfaces, and heterojunctions [7,10,11]. In this paper, we systematically study the surface properties of multilayer thin films of positively charged poly(diallydimethylammonium) (PDDA) and negatively charged nickel phthalocyanine (NiPc) grown by LBL self-assembly technique on conductive indium-tin-oxide (ITO) coated glass substrates. We
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00540-3
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L.S. Li et al. / Applied Surface Science 219 (2003) 199–202
have also used SPV to study the photovoltaic behavior of multilayers (PDDA/NiPc)n (n ¼ 1; 2; 3; . . .) on ITO. This study reveals the effect of self-assembled multilayers on the work function of metal oxide. It also provides insightful information on materials design and leads to the control of physical properties near the surface of conducting oxides through molecular engineering. PDDA and NiPc were purchased from Aldrich and used without further purification. As positively or negatively charged materials, they were used to prepare self-assembled bilayers. The concentration of PDDA and NiPc aqueous solution are 10 and 1 mM, respectively. The ITO coated glass substrates (sheet resistance less than 40 O/2), purchased from Delta Technologies, were ultrasonically cleaned in an acetone solution. They were rinsed in deionized water and dried by N2. The ITO substrates were then immersed in the PDDA solution for 5 min for growth of PDDA layers. The attractive force between positively charged PDDA polymer and negatively charged (Sn–O terminated) [12] ITO surface leads to the growth of one PDDA monolayer through self-organization or association of charge pairs, a process that assembles solution-solvated oligomers into monolayer films. The PDDA-coated surface was washed with deionized water to remove
all non-bound materials. To form a bilayer PDDA/NiPc, the PDDA-coated substrate was immersed in a NiPc aqueous solution for 2 min, followed by washing with deionized water. Repeating the above steps, additional bilayers of PDDA and NiPc were formed and a multilayer (PDDA/NiPc)n (n ¼ 1; 2; 3; . . .) was deposited on the ITO substrate. We utilized a Kelvin probe to study the evolution of electrical properties of ITO surfaces as a function of molecular layers. The Kelvin probe, a non-contact and non-destructive vibrating capacitor device, measures the surface potential difference (SPD) between a conducting specimen and a vibrating tip (reference plate) [6,7]. SPD is defined as the difference between the fr (potential of the reference plate connected to the preamplifier) and the fs (surface potential of the sample), i.e., fr fs . We integrated a commercial Kelvin probe (KP-6500 Digital Kelvin Probe System, McAllister Technical Services) with a light source (450 W xenon lamp through a double prism monochromator (SPEX FL3-21)) to measure changes of both surface potential and SPV. To evaluate the changes in surface potential before and after deposition of PDDA or NiPc onto ITO, the SPD measurement was done after each monolayer (PDDA or NiPc) growth. The photo-induced SPV spectra were
Fig. 1. Dependence of SPS on the terminating layer of PDDA and/or NiPc: (a) PDDA/ITO; (b) NiPc–PDDA/ITO; (c) PDDA–NiPc–PDDA/ ITO; (d) NiPc–PDDA–NiPc–PDDA/ITO; (e) PDDA–NiPc–PDDA–NiPc–PDDA/ITO; (f) NiPc–PDDA–NiPc–PDDA–NiPc–PDDA/ITO; (g) blank ITO.
L.S. Li et al. / Applied Surface Science 219 (2003) 199–202
obtained by scanning the wavelength of the incident light from red to the UV range (250–800 nm) at a typical rate of 30 nm/min. Fig. 1 shows the dependence of surface photovoltage spectra (SPS) of ITO on the terminating layer of either PDDA or NiPc. For reference purposes, blank ITO’s SPS was taken as the baseline (curve g in Fig. 1). The change in SPV was more than 500 mV when the photon energy was varied from about 2 to 4.5 eV. At photon energy of 3.75 eV, the spectra flatten out. This energy is related to the bandgap of ITO (Eg ¼ 3:74:3 eV) [13]. The negative photovoltaic response indicates that ITO is n-type and its surface is under depletion. After a single layer of PDDA was assembled on ITO surface, the change in photovoltaic response was 150 mV. When a NiPc layer was added on and form the first PDDA/NiPc bilayer on ITO, the change in photovoltaic response was about 80 mV. A similar response occurred when the second and/or third PDDA/NiPc bilayer was added. Fig. 2 shows the SPD measured after each deposition of PDDA or NiPc layer on ITO without (filled symbols) and with (open symbols) light illumination. After a single layer of PDDA was deposited onto an ITO, its SPD value increased by 500 mV (work
201
function decreased by 500 meV) in reference to blank ITO. After deposition of NiPc onto PDDA-coated ITO, the SPD value decreased to 200 mV (work function increased by 200 meV) in reference to blank ITO. Subsequent deposition of PDDA and NiPc layers continued the trend that results in a difference in SPD value of 300 mV. The oscillation of SPD with PDDA or NiPc deposition demonstrates that the relationship between surface potential and layer deposition (whether PDDA or NiPc) is periodic. In other words, the surface work function of the ITO is determined mainly by the terminating layer rather than the total number of layers. Surface work function change can be attributed to modification of the electron affinity and/or change in the Fermi level for a semiconductor. Since PDDA and ITO possess weak charge–charge bindings, we expect that changes in the surface work function from deposition of oppositely charged polymer or macrocycles on ITO result predominantly from the change of electron affinity. The presence of a dipole layer, interacting weakly with surfaces, affects the electron affinity of the substrate only. Both Fermi level and electron affinity can be changed simultaneously if a dipole layer interacts strongly with its adjacent material. Since each bilayer completed an
Fig. 2. Dependence of SPD on the terminating layer of PDDA (circle) or NiPc (triangle) versus the total number of individual layers (i.e., the number of PDDA layers plus NiPc layers). The filled and open symbols represent without and with light illumination (at 3.75 eV), respectively. The lines in the figure are added as guide for the eye.
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L.S. Li et al. / Applied Surface Science 219 (2003) 199–202
up-and-down oscillation cycle, we reasoned that the change in surface work function was due mainly to the modulation of electron affinity after each layer deposition because it would have ceased cycling if it were due to band bending. Interestingly, we also observed an oscillation pattern of SPD under light illumination. As can be seen in Fig. 2, it keeps the same oscillation pattern as compared with the case without light illumination. Since we expect that the electron affinity will remain constant with or without light illumination, we can deduce the photo-induced change of barrier height (dVb) where the barrier height is defined as the difference between the edge of the conduction band and the Fermi level at the surface. For bare ITO substrate, the dVb is 525 meV. This large change in barrier height indicates that excitation at the bandgap has a more pronounced photovoltaic effect. After the formation of a PDDA monolayer, the dVb value reduced to 158 meV. When a bilayer of PDDA/NiPc was formed, dVb was only 78 meV. Deposition of more PDDA or NiPc layers caused a repetition of the same trend as shown in Fig. 2. Again, the photo-induced change in barrier height depended mainly on the termination layer materials. In summary, self-assembled multilayers of PDDA/ NiPc have been grown on ITO coated glass substrates. We used a Kelvin probe and SPV spectroscopy to investigate the evolution of the surface physical properties of ITO due to the surface modification from the self-assembled multilayers. We found that the surface electronic potential is very sensitive to the presence of self-assembled molecular layers. The work function difference between PDDA-terminated and NiPc-terminated layers, is 300 meV. In other words, the surface potentials or surface work function is a periodic function of the terminating layer property, oscillating between 500 mV for PDDA and 200 mV for
NiPc on ITO. In addition, the photo-induced effects are also correlated to the terminating materials such as PDDA or NiPc.
Acknowledgements This work was supported as a Los Alamos National Laboratory directed research and development project under the auspices of the United States Department of Energy. ADQL acknowledges the support of the Center for Materials Science, Department of Chemistry, and College of Science at Washington State University.
References [1] G. Decher, Science 277 (1997) 1232. [2] J.W. Ostrander, A.A. Mamedov, N.A. Kotov, J. Am. Chem. Soc. 123 (2001) 1101. [3] Y.J. Liu, R.O. Claus, J. Appl. Phys. 85 (1999) 419. [4] D.Q. Li, A. Bishop, Y. Gim, X.B. Shi, M.R. Fitzsimmons, Q.X. Jia, Appl. Phys. Lett. 73 (1998) 2645. [5] J.B. Schlenoff, D. Laurent, H. Ly, J. Stepp, Adv. Mater. 10 (1998) 347. [6] H. Lu¨ tt, Surfaces and Interfaces of Solids, Springer, Berlin, 1993, p. 464. [7] L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1. [8] M. Bruening, E. Moons, D. Yaron-Marcovich, D. Cahen, J. Libman, A. Shanzer, J. Am. Chem. Soc. 116 (1994) 2972. [9] R. Cohen, L. Kronik, A. Shanzer, D. Cahen, A. Liu, Y. Rosenwakes, J.K. Lorenz, A.B. Ellis, J. Am. Chem. Soc. 121 (1999) 10545. [10] L.S. Li, J. Zhang, L.J. Wang, Y. Chen, Z. Hui, T.J. Li, L.F. Chi, H. Fuchs, J. Vac. Sci. Technol. B 15 (1997) 1618. [11] I. Shalish, Y. Shapria, L. Burstein, J. Salzman, J. Appl. Phys. 89 (2001) 390. [12] M. Lahav, V. HelegShabtai, J. Wasserman, E. Katz, I. Willner, H. Durr, Y.Z. Hu, S.H. Bossmann, J. Am. Chem. Soc. 122 (2000) 11480. [13] E. Moons, A. Goossens, T. Savenije, J. Phys. Chem. B 101 (1997) 8492.
Short communication
Effects of self-assembled multilayers on the evolution of surface physical properties of indium-tin-oxide Lin Song Lia, Alexander D.Q. Lib, Q.X. Jiaa,* a
Materials Science and Technology Division, Los Alamos National Laboratory, Superconductivity Technology Center, MS K763, Los Alamos, NM 87545, USA b Department of Chemistry and Center for Materials Research, Washington State University, Pullman, WA 99164, USA Received 28 November 2001; accepted 28 May 2002
Abstract Using layer-by-layer self-assembly technique, we have deposited multilayers of poly(diallydimethylammonium) (PDDA) and nickel phthalocyanine (NiPc) onto indium-tin-oxide (ITO) coated glass substrate. To determine how self-assembled multilayers affect charge transfer properties of conductive ITO surface, Kelvin probe and surface photovoltage measurements have been carried out. The results showed an oscillation of the surface potential as the top layer was alternated between PDDA and NiPc. The changes in surface potential of ITO induced by the growth of PDDA or NiPc monolayers are attributed to the modulation in electron affinity. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Kelvin probe; Surface photovoltage spectroscopy; Layer-by-layer self-assembly
Layer-by-layer (LBL) assembly of oppositely charged polymeric and organic materials has recently been used to fabricate multilayers on various substrates. LBL assembly promises the construction of materials with well-controlled, nanometer-scaled architectures for applications in electronics, photonics and optoelectronics [1–3]. The interfaces between each layer become the key linkages between multilayers, and therefore the study of surface physical properties is critically important as each organic layer is assembled [4,5]. A Kelvin probe has been used to measure the work function qf defined as the difference between the vacuum level and Fermi level or surface potential f *
Corresponding author. Tel.: þ1-505-667-2716; fax: þ1-505-665-3164. E-mail address: [email protected] (Q.X. Jia).
of organic monolayers on semiconducting materials [6,7]. Previous studies have indicated that the physical properties near the surface of semiconducting materials such as CdTe, CdSe, CuInSe2, GaAs and InP are a function of molecular modifiers in systems composed of chemisorbed monolayers of dicarboxylic acids [8,9]. Surface photovoltage (SPV) spectroscopy, a very sensitive tool for measuring the charge transfer at the surface and interface, has also been used to study the change of charge distribution on semiconductor surfaces, buried interfaces, and heterojunctions [7,10,11]. In this paper, we systematically study the surface properties of multilayer thin films of positively charged poly(diallydimethylammonium) (PDDA) and negatively charged nickel phthalocyanine (NiPc) grown by LBL self-assembly technique on conductive indium-tin-oxide (ITO) coated glass substrates. We
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00540-3
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L.S. Li et al. / Applied Surface Science 219 (2003) 199–202
have also used SPV to study the photovoltaic behavior of multilayers (PDDA/NiPc)n (n ¼ 1; 2; 3; . . .) on ITO. This study reveals the effect of self-assembled multilayers on the work function of metal oxide. It also provides insightful information on materials design and leads to the control of physical properties near the surface of conducting oxides through molecular engineering. PDDA and NiPc were purchased from Aldrich and used without further purification. As positively or negatively charged materials, they were used to prepare self-assembled bilayers. The concentration of PDDA and NiPc aqueous solution are 10 and 1 mM, respectively. The ITO coated glass substrates (sheet resistance less than 40 O/2), purchased from Delta Technologies, were ultrasonically cleaned in an acetone solution. They were rinsed in deionized water and dried by N2. The ITO substrates were then immersed in the PDDA solution for 5 min for growth of PDDA layers. The attractive force between positively charged PDDA polymer and negatively charged (Sn–O terminated) [12] ITO surface leads to the growth of one PDDA monolayer through self-organization or association of charge pairs, a process that assembles solution-solvated oligomers into monolayer films. The PDDA-coated surface was washed with deionized water to remove
all non-bound materials. To form a bilayer PDDA/NiPc, the PDDA-coated substrate was immersed in a NiPc aqueous solution for 2 min, followed by washing with deionized water. Repeating the above steps, additional bilayers of PDDA and NiPc were formed and a multilayer (PDDA/NiPc)n (n ¼ 1; 2; 3; . . .) was deposited on the ITO substrate. We utilized a Kelvin probe to study the evolution of electrical properties of ITO surfaces as a function of molecular layers. The Kelvin probe, a non-contact and non-destructive vibrating capacitor device, measures the surface potential difference (SPD) between a conducting specimen and a vibrating tip (reference plate) [6,7]. SPD is defined as the difference between the fr (potential of the reference plate connected to the preamplifier) and the fs (surface potential of the sample), i.e., fr fs . We integrated a commercial Kelvin probe (KP-6500 Digital Kelvin Probe System, McAllister Technical Services) with a light source (450 W xenon lamp through a double prism monochromator (SPEX FL3-21)) to measure changes of both surface potential and SPV. To evaluate the changes in surface potential before and after deposition of PDDA or NiPc onto ITO, the SPD measurement was done after each monolayer (PDDA or NiPc) growth. The photo-induced SPV spectra were
Fig. 1. Dependence of SPS on the terminating layer of PDDA and/or NiPc: (a) PDDA/ITO; (b) NiPc–PDDA/ITO; (c) PDDA–NiPc–PDDA/ ITO; (d) NiPc–PDDA–NiPc–PDDA/ITO; (e) PDDA–NiPc–PDDA–NiPc–PDDA/ITO; (f) NiPc–PDDA–NiPc–PDDA–NiPc–PDDA/ITO; (g) blank ITO.
L.S. Li et al. / Applied Surface Science 219 (2003) 199–202
obtained by scanning the wavelength of the incident light from red to the UV range (250–800 nm) at a typical rate of 30 nm/min. Fig. 1 shows the dependence of surface photovoltage spectra (SPS) of ITO on the terminating layer of either PDDA or NiPc. For reference purposes, blank ITO’s SPS was taken as the baseline (curve g in Fig. 1). The change in SPV was more than 500 mV when the photon energy was varied from about 2 to 4.5 eV. At photon energy of 3.75 eV, the spectra flatten out. This energy is related to the bandgap of ITO (Eg ¼ 3:74:3 eV) [13]. The negative photovoltaic response indicates that ITO is n-type and its surface is under depletion. After a single layer of PDDA was assembled on ITO surface, the change in photovoltaic response was 150 mV. When a NiPc layer was added on and form the first PDDA/NiPc bilayer on ITO, the change in photovoltaic response was about 80 mV. A similar response occurred when the second and/or third PDDA/NiPc bilayer was added. Fig. 2 shows the SPD measured after each deposition of PDDA or NiPc layer on ITO without (filled symbols) and with (open symbols) light illumination. After a single layer of PDDA was deposited onto an ITO, its SPD value increased by 500 mV (work
201
function decreased by 500 meV) in reference to blank ITO. After deposition of NiPc onto PDDA-coated ITO, the SPD value decreased to 200 mV (work function increased by 200 meV) in reference to blank ITO. Subsequent deposition of PDDA and NiPc layers continued the trend that results in a difference in SPD value of 300 mV. The oscillation of SPD with PDDA or NiPc deposition demonstrates that the relationship between surface potential and layer deposition (whether PDDA or NiPc) is periodic. In other words, the surface work function of the ITO is determined mainly by the terminating layer rather than the total number of layers. Surface work function change can be attributed to modification of the electron affinity and/or change in the Fermi level for a semiconductor. Since PDDA and ITO possess weak charge–charge bindings, we expect that changes in the surface work function from deposition of oppositely charged polymer or macrocycles on ITO result predominantly from the change of electron affinity. The presence of a dipole layer, interacting weakly with surfaces, affects the electron affinity of the substrate only. Both Fermi level and electron affinity can be changed simultaneously if a dipole layer interacts strongly with its adjacent material. Since each bilayer completed an
Fig. 2. Dependence of SPD on the terminating layer of PDDA (circle) or NiPc (triangle) versus the total number of individual layers (i.e., the number of PDDA layers plus NiPc layers). The filled and open symbols represent without and with light illumination (at 3.75 eV), respectively. The lines in the figure are added as guide for the eye.
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L.S. Li et al. / Applied Surface Science 219 (2003) 199–202
up-and-down oscillation cycle, we reasoned that the change in surface work function was due mainly to the modulation of electron affinity after each layer deposition because it would have ceased cycling if it were due to band bending. Interestingly, we also observed an oscillation pattern of SPD under light illumination. As can be seen in Fig. 2, it keeps the same oscillation pattern as compared with the case without light illumination. Since we expect that the electron affinity will remain constant with or without light illumination, we can deduce the photo-induced change of barrier height (dVb) where the barrier height is defined as the difference between the edge of the conduction band and the Fermi level at the surface. For bare ITO substrate, the dVb is 525 meV. This large change in barrier height indicates that excitation at the bandgap has a more pronounced photovoltaic effect. After the formation of a PDDA monolayer, the dVb value reduced to 158 meV. When a bilayer of PDDA/NiPc was formed, dVb was only 78 meV. Deposition of more PDDA or NiPc layers caused a repetition of the same trend as shown in Fig. 2. Again, the photo-induced change in barrier height depended mainly on the termination layer materials. In summary, self-assembled multilayers of PDDA/ NiPc have been grown on ITO coated glass substrates. We used a Kelvin probe and SPV spectroscopy to investigate the evolution of the surface physical properties of ITO due to the surface modification from the self-assembled multilayers. We found that the surface electronic potential is very sensitive to the presence of self-assembled molecular layers. The work function difference between PDDA-terminated and NiPc-terminated layers, is 300 meV. In other words, the surface potentials or surface work function is a periodic function of the terminating layer property, oscillating between 500 mV for PDDA and 200 mV for
NiPc on ITO. In addition, the photo-induced effects are also correlated to the terminating materials such as PDDA or NiPc.
Acknowledgements This work was supported as a Los Alamos National Laboratory directed research and development project under the auspices of the United States Department of Energy. ADQL acknowledges the support of the Center for Materials Science, Department of Chemistry, and College of Science at Washington State University.
References [1] G. Decher, Science 277 (1997) 1232. [2] J.W. Ostrander, A.A. Mamedov, N.A. Kotov, J. Am. Chem. Soc. 123 (2001) 1101. [3] Y.J. Liu, R.O. Claus, J. Appl. Phys. 85 (1999) 419. [4] D.Q. Li, A. Bishop, Y. Gim, X.B. Shi, M.R. Fitzsimmons, Q.X. Jia, Appl. Phys. Lett. 73 (1998) 2645. [5] J.B. Schlenoff, D. Laurent, H. Ly, J. Stepp, Adv. Mater. 10 (1998) 347. [6] H. Lu¨ tt, Surfaces and Interfaces of Solids, Springer, Berlin, 1993, p. 464. [7] L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1. [8] M. Bruening, E. Moons, D. Yaron-Marcovich, D. Cahen, J. Libman, A. Shanzer, J. Am. Chem. Soc. 116 (1994) 2972. [9] R. Cohen, L. Kronik, A. Shanzer, D. Cahen, A. Liu, Y. Rosenwakes, J.K. Lorenz, A.B. Ellis, J. Am. Chem. Soc. 121 (1999) 10545. [10] L.S. Li, J. Zhang, L.J. Wang, Y. Chen, Z. Hui, T.J. Li, L.F. Chi, H. Fuchs, J. Vac. Sci. Technol. B 15 (1997) 1618. [11] I. Shalish, Y. Shapria, L. Burstein, J. Salzman, J. Appl. Phys. 89 (2001) 390. [12] M. Lahav, V. HelegShabtai, J. Wasserman, E. Katz, I. Willner, H. Durr, Y.Z. Hu, S.H. Bossmann, J. Am. Chem. Soc. 122 (2000) 11480. [13] E. Moons, A. Goossens, T. Savenije, J. Phys. Chem. B 101 (1997) 8492.