Applied Surface Science
Volume 219, Issues 3-4 , 15 December 2003, Pages 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 and Q. X. Jia
,
,a
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. ; Available online 7 May 2003.
Abstract
Using layer-by-layer self-assembly technique, we have deposited multilayers of
poly(diallydimethylammonium) (PDDA) and nickel phthalocyanine (NiPc) onto indium-tinoxide (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.
Author Keywords: Kelvin probe; Surface photovoltage spectroscopy; Layer-by-layer selfassembly
Article Outline
Acknowledgements
References
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, 2 and 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 and 5].
A Kelvin probe has been used to measure the work function q defined as the difference
between the vacuum level and Fermi level or surface potential of organic monolayers on
semiconducting materials [6 and 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 and 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 and 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-tinoxide (ITO) coated glass substrates. We have also used SPV to study the photovoltaic
behavior of multilayers (PDDA/NiPc)n (n=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 /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=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 and 7]. SPD is defined as the difference
between the r (potential of the reference plate connected to the preamplifier) and the s
(surface potential of the sample), i.e., r- s. We integrated a commercial Kelvin probe (KP6500 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 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=3.7–4.3 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. 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.
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 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 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.
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.
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 ( Vb) 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 Vb 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 Vb value reduced to 158 meV. When a
bilayer of PDDA/NiPc was formed, Vb 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), p. 1232. Abstract-EMBASE | Full Text via CrossRef
2. J.W. Ostrander, A.A. Mamedov and N.A. Kotov. J. Am. Chem. Soc. 123 (2001), p. 1101.
Abstract-MEDLINE | Abstract-Beilstein Abstracts | Abstract-Elsevier BIOBASE | AbstractEMBASE | Full Text via CrossRef
3. Y.J. Liu and R.O. Claus. J. Appl. Phys. 85 (1999), p. 419. Abstract-MEDLINE | OJPS full
text | Full Text via CrossRef
4. D.Q. Li, A. Bishop, Y. Gim, X.B. Shi, M.R. Fitzsimmons and Q.X. Jia. Appl. Phys. Lett. 73
(1998), p. 2645. OJPS full text | Full Text via CrossRef
5. J.B. Schlenoff, D. Laurent, H. Ly and J. Stepp. Adv. Mater. 10 (1998), p. 347. Full Text via
CrossRef
6. H. Lütt, Surfaces and Interfaces of Solids, Springer, Berlin, 1993, p. 464.
7. L. Kronik and Y. Shapira. Surf. Sci. Rep. 37 (1999), p. 1. Abstract
8. M. Bruening, E. Moons, D. Yaron-Marcovich, D. Cahen, J. Libman and A. Shanzer. J. Am.
Chem. Soc. 116 (1994), p. 2972. Abstract-Beilstein Abstracts | Abstract-Compendex
9. R. Cohen, L. Kronik, A. Shanzer, D. Cahen, A. Liu, Y. Rosenwakes, J.K. Lorenz and A.B.
Ellis. J. Am. Chem. Soc. 121 (1999), p. 10545. Abstract-Beilstein Abstracts | AbstractElsevier BIOBASE | Abstract-EMBASE | Full Text via CrossRef
10. L.S. Li, J. Zhang, L.J. Wang, Y. Chen, Z. Hui, T.J. Li, L.F. Chi and H. Fuchs. J. Vac. Sci.
Technol. B 15 (1997), p. 1618. OJPS full text | Full Text via CrossRef
11. I. Shalish, Y. Shapria, L. Burstein and J. Salzman. J. Appl. Phys. 89 (2001), p. 390.
Abstract-INSPEC | Full Text via CrossRef
12. M. Lahav, V. HelegShabtai, J. Wasserman, E. Katz, I. Willner, H. Durr, Y.Z. Hu and S.H.
Bossmann. J. Am. Chem. Soc. 122 (2000), p. 11480. Abstract-Beilstein Abstracts | AbstractEMBASE | Abstract-Elsevier BIOBASE | Full Text via CrossRef
13. E. Moons, A. Goossens and T. Savenije. J. Phys. Chem. B 101 (1997), p. 8492. AbstractBeilstein Abstracts | Abstract-Compendex | Abstract-INSPEC | Full Text via CrossRef
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