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Kelvin Probe Force Microscopy on Surfaces: Investigation of the Surface Potential of Self-Assembled Monolayers on Gold J. Lu¨,*,† E. Delamarche,‡ L. Eng,†,§ R. Bennewitz,† E. Meyer,† and H.-J. Gu¨ntherodt† Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland, and IBM Research, Zurich Research Laboratory, CH-8803 Ru¨ schlikon, Switzerland Received April 22, 1999 The local contact potential difference (CPD) between different linear alkanethiols self-assembled as monolayers on Au substrates was investigated with Kelvin probe force microscopy (KPFM). Our results demonstrate that KPFM simultaneously provides information on the sample topography and its contact potential down to a lateral resolution of 100 nm. The reported CPD measurements allow the distinction between regions in the monolayer comprising thiol molecules with chemically different terminal head groups down to a resolution of 3 meV. Furthermore, the CPD values measured in monolayers having one type of functionality vary with the chain length of the molecules in the film, which is consistent with a dipole-layer model. Variations in the length of the alkyl chain reveal a linear dependence as a function of the (-CH2-) units in the chain, with an increase in the CPD potential of 14.1 ( 3.1 mV per (-CH2-) unit.
1. Introduction The Kelvin probe method is frequently used to investigate electric properties of surfaces and to map the electric potential distribution on any material.1 Using an atomic force microscope (AFM), Martin et al.2 performed the first surface-potential measurements adapting the concept of Kelvin probing to the local scale accessible by scanning force microscopy. Further developments by Weaver and Abraham3 and Nonnenmacher et al.4 introduced the modulation method for measuring surface-potential differences between various metallic electrodes. Consecutive improvements by Lu¨ et al.5 and Kikukawa et al.6 proved to be advantageous for Kelvin probe force microscopy (KPFM) because they drastically increased the measurement sensitivity. Applications of KPFM to nonmetallic surfaces as semiconductor devices7 thus became feasible. Unlike scanning tunneling potentiometry (STP), which measures the local potential distribution on metallic surfaces,8-10 KPFM is not restricted to conductive samples because it is the local electrostatic forces that are investigated rather than measured tunneling currents. * To whom correspondence should be addressed. † University of Basel. ‡ Zurich Research Laboratory. § Present address: Institut fu ¨ r Angewandte Photophysik, Technische Universita¨t Dresden, Germany. (1) McClelland, G. M.; Erlandsson, R.; Chiang, S. Rev. Prog. Quant. Nondestr. Eval. 1987, 6B, 1307. (2) Martin Y.; Williams C. C.; Wickramasinghe, H. K. J. Appl. Phys. 1987, 61, 4723. (3) Weaver, J. M. R.; Abraham, D. W. J. Vac. Sci. Technol. B 1991, 9, 1561. (4) Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921. (5) Lu¨, J.; Guggisberg, M.; Lu¨thi, R.; Kubon, M.; Scandella, L.; Gerber, Ch.; Meyer, E.; Gu¨ntherodt, H. J. Appl. Phys. A 1998, 66, 273. (6) Kikukawa, A.; Hosaka, S.; Imura, R. Rev. Sci. Instrum. 1996, 67, 1463. (7) Jacobs, H. O.; Knapp, H. F.; Mu¨ller, S.; Stemmer, A. Ultramicroscopy 1997, 69, 39. (8) Muralt, P.; Pohl, D. W. Appl. Phys. Lett. 1986, 48, 514. (9) Pelz, J. P.; Koch, R. H. Rev. Sci. Instrum. 1989, 60, 301. (10) Kirtley, J. R.; Washburn, S.; Brady, M. J. Appl. Phys. Lett. 1988, 60, 1546.
Here we applied KPFM to the parallel investigation of the topography and local contact potential difference (CPD) of the examined surface. Microcontact printing (µCP) of alkanethiols on gold provided patterned and chemically controlled surfaces to test the resolution and the ability of this technique to differentiate the various chemical regions in the monolayers. Varying the chemistry of the terminal head group as well as changing the length of the alkyl chain attached to the thiol anchor yielded measurable changes of the surface potential, achieving a resolution of 3 meV. In the remainder of this paper, we will first discuss the origin of contrast in KPFM. A summary of how the samples were prepared will follow, and finally, the results obtained with KPFM for these alkanethiol monolayers will be discussed in the context of a dipole-layer model. 2. Theory of Material Contrast in KPFM When two metals are electrically connected, electrons flow immediately from one metal to the other until an equilibrium is established when both metals reach the same electrochemical potential. Subsequent to the charge transfer, both metals have small amounts of net surface charge, which causes a shift of the band structures in both metals so as to make the two Fermi levels coincide. The potentials outside the metals are no longer strictly constant due to these slight surface charges, creating a potential drop from one metal to the other, which is described as the CPD between these two metals. In other words, if an electron at the Fermi level is extracted through a face of the first metal (with a work function W) and reintroduced through a face of the second metal (with a work function W′) at the same Fermi level, conservation of the energy requires an external electric field to provide the work W - W′ to the electron. Thus, the potential difference between the two metallic interfaces is ∆U ) (W - W′)/e and represents the CPD between the two metals. Generally, CPDs are highly material-dependent and related to the work functions of pure material and to additional surface dipole moments. Here, we will discuss the dipole-layer model: The CPD between a sample and
10.1021/la9904861 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/08/1999
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and
φsample2 ) U02 - φtip
(6)
and the CPD between these two substrates is
φsample1 - φsample2 ) U01 - U02
(7)
which is independent of the properties of the tip. For two substrates having the same Volta potential, eqs 1 and 2 yield Figure 1. Experimental setup used for CPD measurements on regions comprising different alkanethiols chemisorbed on a Au substrate. The dc voltage U0 is adjusted until the electrostatic force between tip and sample is minimal. The surface potential varies for both variations in the dipole layer thickness ls and the effective surface charge σ.
a reference electrode is dependent not only on the material, i.e., the work function, but also on the state of the surface, such as its contamination and monolayer coating. Therefore, the electrical potential φ can be separated into two components, namely, ψ, which is due to the presence of an electrostatic charge on the surface and is called the Volta potential and χ, which is due to the presence of a dipole-charge distribution at the surface and is called the surface potential11
φ)ψ+χ
(1)
The different surface potentials of tip and sample cause an electrostatic force between the surfaces, which is given by
F)
1 ∂C (∆φ)2 2 ∂z
(2)
where C is the capacitance, ∆φ the contact potential between these two materials, and z the distance between the tip and the sample. As discussed above, the KPFM technique uses this electrostatic force as a control signal to adjust an external voltage to compensate for the contact potential so that the electrostatic force between the tip and the sample is minimized. This is achieved when the external voltage is equal to the difference of the two electrical potentials, i.e., ∆U ) ∆φ. Figure 1 illustrates this experimental setup. In this configuration, the dc voltage U0 is adjusted until a minimal electrostatic force acts between the tip and the sample. Under these conditions, the electrical potential φ of the tip and the sample must be equal across the gap between them
U0 ) φsample - φtip
(3)
Provided that the tip is well defined (i.e., φtip is known), the surface potential of any substrate is
φsample ) U0 - φtip
(4)
Therefore, if we study two substrates with the same tip, we have
φsample1 ) U01 - φtip
(5)
(11) Fujihira, M.; Kawate, H.; Yasutake, M. Chem. Lett. 1992, 2223.
χsample1 - χsample2 ) U01 - U02
(8)
which attributes the CPD to a net difference in dipolecharge distribution. The monolayers adsorbed onto the Au substrate may be considered a two-dimensional ensemble of dipoles with a layer of negative charges residing close to the Au/ monolayer interface and a layer of positive charges closer to the monolayer/air interface. The orientation of the dipoles is inferred from the positive sign of the change in the surface potential with respect to the bare Au reference electrode. In molecular terms, this implies that the effective R+-S- dipole (where R ) CnH2n+1) must be larger than the Au+-S- dipole, which seems reasonable because the Au+ can be screened within a very short distance by the electrons within the metal, whereas the charges in the monolayer have a net dipole perpendicular to the surface and oriented as shown in Figure 1. If +σ and -σ are the positive and negative surface charges per unit area, respectively, and ls is the separation between them (see Figure 1), then the surface potential χ will be directly proportional to σls, where σ is the absolute value of +σ and -σ. If we assume that ls varies linearly with increasing chain length (n) of the thiol, then the CPD values are expected to show the corresponding linear behavior. Please note that in this simple model, the distance between charged layers ls may be considerably different from the monolayer thickness d of the alkanethiol molecules. Nevertheless, both ls and d may depend linearly on the alkyl chain length n. 3. Sample Preparation The set of experiments presented here examines surfacepotential properties in self-assembled monolayers formed by µCP, a widely used technique for transferring patterns from an elastomeric “stamp” to a solid substrate.12,13 With µCP, the stamp is first formed by curing poly(dimethylsiloxane) (PDMS) on a master with the negative of the desired surface, resulting in an elastomeric solid with a pattern of reliefs typically a few micrometers deep on its surface.14 In a subsequent step, the stamp inked with alkanethiols localizes the formation of self-assembled monolayers of alkanethiols on gold in the printed region (see Figure 2). Here, the value of µCP is to localize the self-assembly of alkanethiols from the ink onto particular regions of a gold substrate. Unprinted regions can therefore be derivatized with a different molecule, providing two types of layers on the substrate for simultaneous exploration and comparison of the surface-potential properties of these films at the submicrometer scale. (12) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (13) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (14) Delamarche, E.; Schmid, H.; Michel, B.; Biebuyck, H. A. Adv. Mater. 1997, 9, 741.
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Figure 3. Experimental setup to perform KPFM. Feedback I maintains a constant tip-sample distance, whereas feedback II balances the electrostatic potential differences between the tip and the sample.
Figure 2. Combination of microcontact printing alkanethiols ((a) inking of the stamp and (b) printing) and (c) self-assembly of thiols from solution on gold enables (d) the patterning of a gold substrate with two types of functionalized thiols.
In practice, 15 nm of gold (>99.99%, Goodfellow, U.K.) was evaporated onto a clean silicon surface using a BAE250 evaporation chamber (Balzers, Liechtenstein) at a base pressure of 10-7 mbar. During evaporation, Au was deposited at a rate of 0.5 nm/s at room temperature. The elastomeric stamp was formed by pouring a mixture of PDMS prepolymers and its curing agent (10:1 by weight) onto a photolithographically patterned silicon wafer and left at 60 °C for at least 12 h to ensure a complete cure of the polymer mixture. Stamps ≈5 × 5 mm2 in size were cut from the cured PDMS, and the side of the stamp, which contacted the surface of the silicon wafer, was used for printing. Stamps were rinsed several times with ethanol and dried under a flow of Ar for 10 s. A drop of a freshly prepared solution of alkanethiols in ethanol was placed on the stamp to cover its surface. The ink remained there for 30 s before the stamp was blown dry. A monolayer resulted from a 3-s print between the stamp and the gold substrate, for which no pressure was applied. The complementary surface of the gold not initially printed by µCP was functionalized by immersing the printed sample in an ethanolic solution with different thiols for 10 s. If the second thiol had protic groups, the samples were rinsed with HCl (pH 2) to leave all groups at the monolayer/air interface in the same protonated state. Following this approach, samples with patterned alkanethiols (purified from Fluka15) and ω-functionized mercaptoalkanoic acid (synthesized following ref 16) were prepared for experiment in the dry N2-vented chamber of KPFM. 4. KPFM Experiments The home-built Kelvin force microscope based on optical beam-deflection AFM5 used for this work is schematically (15) Libioulle, L.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 1999, 15, 300.
displayed in Figure 3. KPFM measurements were performed in a modified noncontact AFM (nc-AFM) mode, where the cantilever is vibrated slightly above its fundamental resonance frequency f0. As all measurements were performed in ambient air, the cantilever quality factor typically measures around 100. Therefore slope detection, also referred to as the amplitude modulation (AM) technique, is suitable to monitor the motion of the tip attached to the cantilever. Using lock-in techniques, both the sample topography and the surface CPD are monitored and regulated simultaneously. First, the cantilever vibration amplitude is kept constant by using a feedback loop, which then reveals the topographic information of the system under investigation. Constant interaction between tip and sample, however, results only when the electrostatic interaction is kept constant. This can be achieved by applying an additional dc voltage to the conductive AFM tip with the sample being on ground potential. The electrostatic force between the tip and the sample can be written
∂C 1 F ) (U + Uc)2 2 ∂z
(9)
where U, Uc, C, and z are the potential differences caused by the applied voltage, the CPD, the capacitance, and the distance between the tip and the sample, respectively. There are two ways to determine the CPD: (i) by using a second feedback loop or (ii) by performing spectroscopic measurements. The second method has been reported elsewhere.5 Here, the first method was applied to enhance statistics by recording CPD images (see section 5). When a second feedback loop is used, a sinusoidal voltage signal U ) U0 + Us sin(ωt) is applied to the tip. In the absence of surface charge, the electrostatic force may be approximated as1,2,17
∂C 1 F ) (Uc + U0 + Us sin ωt)2 2 ∂z
(10)
The cantilever is driven by a sinusoidal driving force with constant frequency f ) f0 + ∆f (slightly off the resonance frequency at f0 ) 160 kHz, ∆f being typically 1-2 kHz) and an oscillation amplitude of ∼10-20 nm. (16) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (17) Hao, H. W.; Baro, A. M.; Saenz, J. J. Vac. Sci. Technol., B 1991, 9, 1323.
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Figure 4. KPFM investigation on a gold substrate patterned with HS-(CH2)15-COOH (region I) self-assembled from solution and microcontact-printed HS-(CH2)15-CH3 (region II): (a) CPD on the sample; (b) simultaneously recorded topography of the sample. Gray-scale variations correspond to 0.4 eV and 0.4 nm for CPD and topographic images, respectively.
The cantilever resonance shift is detected indirectly from its vibration amplitude variation and used in the nc-AFM mode to control the distance between the tip and the sample. We then used a second feedback loop to measure the CPD by minimizing the electrostatic field between tip and sample (U0 ) -Uc), while maintaining a constant tip-sample separation. The minimal detectable potential difference is given as18
∆U )
x
2kBTkB 3
π Qf0
z 0UsR
(11)
Figure 5. Histograms of CPD measurements for two types of thiols patterning gold substrates and measured with KPFM: (a) HS-(CH2)21-CH3 and HS-(CH2)15-COOH; (b) HS-(CH2)19CH3 and HS-(CH2)15-COOH; (c) HS-(CH2)15-CH3 and HS(CH2)15-COOH; (d) HS-(CH2)11-CH3 and HS-(CH2)15COOH. Table 1 (a) (b) (c) (d)
-COOH-
-CH3
0.210 V 0.222 V 0.181 V 0.134 V
0.538 V 0.479 V 0.392 V 0.310 V
Figure 4 presents the CPD and topographic results of two alkanethiol monolayers patterned on gold. As the stamp used for µCP has an asymmetric structure, areas I and II are clearly attributable to hexadecanethiol (HS(CH2)15-CH3 (HDT)) and mercaptohexadecanoic acid (HS-(CH2)15-COOH) molecules, respectively, occupying stripes 1.2 and 0.8 µm in width (see Figure 4). Si (n-doped) cantilevers covered with Au derivatized with HDT were used for these experiments and provided chemically stable tips. The CPD images reveal a clear contrast of ≈0.4 eV between the two regions of different terminal groups. The corresponding topographic variations are very small, measuring only ≈0.4 nm. For the experiments reported in Figure 4 we matched the chain length of the two thiol
molecules used.19 The observed topographic contrast therefore arises from the different conformation of the two terminal groups, i.e., the -COOH and -CH3 units, because both the number of -CH2 units and of the thiol headgroups are the same. According to the dipole-layer model, the CPD consists of two contributions, the Volta potential and the surface potential caused by the surface dipole charge distribution; see eq 1. For our experiments reported in Figure 4, both areas show the same Volta potential ψ because the thiol terminal headgroup is attached to the same polycrystalline gold substrate. Using eq 3, we therefore conclude that the observed variations of the CPD values must stem from the net difference in dipole-charge distribution at the two surfaces. In a further step, alkanethiol molecules having the same terminal group but different chain length are inspected with KPFM. In these investigations a series of gold substrates stamped with thiol monolayers of various chain length (HS-(CH2)n-CH3, with n ) 11, 15, 19, and 21) but filled with HS-(CH2)15-COOH are examined using a goldcoated, n-doped Si tip. Figure 5 shows histograms of the CPD measurements on these samples. The peak positions are listed in Table 1. The two peaks of each curve represent the CPD value measured between the tip and the two different regions. Left peaks always represent the CPD between the tip and -COOH-terminated molecules, which again are structurally identified by the stamp asymmetry. These peaks are quite stable in CPD position except for those in curve d, which we suppose is due to a tip change. Peaks on the right side show the CPD between the gold-coated Si tip and the methyl-terminated alkanethiols of various length. These peaks are seen to shift in CPD position, indicating a chainlength dependence of the CPD.
(18) Nonnenmacher, M.; Wickramasinghe, H. K. Ultramicroscopy 1992, 42, 351.
(19) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, Ch. Adv. Mater. 1996, 8, 719.
where kB is the Boltzmann constant, T is the temperature, k is the cantilever spring constant, B is the instrumental bandwidth, Q is the cantilever quality factor, f0 is the cantilever resonance frequency, z is the tip-to-sample distance, 0 is the dielectric permittivity in vacuum, and R is the tip radius. In our experimental setup, the values are k ) 25 N/m, B ) 3 Hz, Q ) 100, f0 ) 160 kHz, Us ) 0.5 V, z ) 10 nm, and R ) 20 nm. Under these conditions, the minimal detectable potential difference at room temperature is about 1 mV. In order to minimize the influence of humidity and contamination on high free-energy surface monolayers, experiments were performed in a chamber filled with dry N2, inside which samples were transferred immediately after their preparation. 5. Results and Discussion
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however, were recorded on macroscopic functionalized areas. The difference between those results and ours may be due to the fact that local inhomogeneities of the sample surface play a minor role. Therefore, our value of 14.1 mV per (-CH2-) unit appears reasonable in light of the macroscopically reported findings. 6. Conclusions
Figure 6. CPD values as a function of the chain length of the methyl-terminated alkanethiols chemisorbed on gold. The linear fit shows an increase of 14.1 ( 3.1 mV per (-CH2-) unit.
As discussed above, the magnitude of the CPD depends on the tip material and properties in use. Nevertheless, when different surface regions are investigated with one and the same tip, the tip potential may be regarded as the CPD reference electrode, which allows the relative comparison of CPD values recorded on two differently functionalized regions on our sample (see Figure 5). As now the tip material and the Volta potential of the underlying gold substrate are identical for both measurements, CPD contrast arises due to the difference in the molecular layers deposited onto the gold substrate, which is simply the number of (-CH2-) units. Figure 6 shows the linear CPD change as a function of chain length. The slope indicates that the CPD change is 14.1 ( 3.1 mV per (-CH2-) unit. Previous experiments with self-assembled monolayers reported by Evan et al. using the Kelvin probe method yield a CPD change of about 9.3 mV per (-CH2-) unit.20 Those experiments, (20) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462.
We demonstrated that KPFM is suitable for investigating the local electrical properties of self-assembled monolayer films deposited onto gold substrates in a controlled atmosphere. Topographic changes as well as variations of the CPD values are recorded simultaneously with 0.05 nm and 3 meV vertical resolution, respectively, and down to a 100-nm lateral scale. We propose that the relative difference of CPD values between self-assembled monolayers either of different terminal groups or of different alkyl-chain lengths may be investigated further with high accuracy. We find that the surface potential varies linearly with the number n of carbon within an alkyl chain, at least for the regime 12 e n e 22. We attribute the observed effect to the changing dielectric behavior of the hydrocarbon tail region. KPFM appears thus to be an interesting technique to investigate interfaces and is not constrained to ultrahigh vacuum conditions but is operable under ambient pressure provided that the lower Q value of KPFM under ambient pressure does not adversely affect its resolution. Acknowledgment. J. L. Lu¨ thanks all her colleagues for their helpful and stimulating discussions, and all authors express their gratitude to H. Breitenstein, P. Fornaro, H.-R. Hidber, R. Maffiolini, S. Messmer, and A. Tonin for technical help. This work was supported in part by the Swiss National Science Foundation, the “Kommission zur Fo¨rderung der wissenschaftlichen Forschung”, and a joint study agreement with the IBM Zurich Research Laboratory and the Swiss priority program MINAST. LA9904861
