Langmuir 1993,9, 2775-2777
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Mobility of the Alkanethiol-Gold( 11 1) Interface Studied by Scanning Probe Microscopy Robin L. McCarley,' David J. Dunaway, and Robert J. Willicut Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803 Received August 24,1999 Scanning tunneling microscopy (STM) wae used to observe the motion of pita on the CH3(CH&SH/ Au(ll1) surface (n = 5, 15, and 17). These pita are shown to diffuse to step edges at rates independent of the alkanethiol chain length during STM imaging. Heating the CH3(CH2)nSH/Au(lll)surface (n = 15 and 17) at 100 "C causes removal of the pita from the surface without damage to the monolayer. The proposed mechanism for pit motion suggests that the Au-S bonds are labile and that only Au atoms diffuse. The pita are shown to be defects in the Au(ll1) surface formed during thiol adsorption. dissolution of the Auduring thiol adsorption.18 We present evidence here which indicates that the observed pits are We describe here the observation of mobile pita (0.22defectsin the Au substrate and that these pits are mobile 0.28 nm depth, 1-2 nm radius) on the CH~(CH~)~SH/AU-and can be removed by heating without removal of the (111)surface1P2( n = 5,15,and 17)usingscanningtunneling monolayer. microscopy (STMh3 During STM imaging, these pita migrate to crystal step edges at rates independent of the Results and Discussion alkanethiolchain length and then annihilate with the step Shown in Figure 1A is a constant-current STM hagel9 edge. Heating the CH3(CH2),SH/Au(lll) surface (n = of a hexadecanethiol/Au(lll) surfacemobtained in air.22 15 and 17) in air at 100 OC for 2 h causes the pita to be The pita in the surface have a depth of 0.25 f 0.03 nm, eliminated from the Au(ll1) plane. Pit motion is apparwhich is independent of tunneling parameters= and the ently due to labile Au-S bonds and mobile Au atoms. We chain length of the alkanethiol. We observe the same pit attribute the pita to defects in the Au(ll1) substrate that depth for octadecanethiol, hexadecanethiol, and hexform during adsorption of thiol and not to defects in the anethiol on Au(ll1). Areas where pita have annihilated thiol layer itself. with terrace edges show no difference in heights when Self-assembledmonolayers of thiols on Au and Ag have compared to the terrace below. SFM images of larger pita been widely investigateddue to their ease of preparation1p2 display the same depth.24 In addition, in situ STM and their use in studies of electron transfer eventat experiments in aqueous CN-702 do not indicate any chemical sensors,2wetting,5p6and molecular archite~ture.~?~ dissolution of Au from the pita,26726which would be Both STM and scanning force microscopy (SFMI9 have expected for defects in the monolayer."16 Although been used to obtain molecular-resolution images of several diffusion of the etchant may be slowed in the smallpores,n thiols on Au in air1g12and in s01ution.l~Large scale STM the pita do not change with increased exposure time in the images (50-1000 nm) of Au(ll1) modified with various CNIf the pita we observe were defects in thiols display depressionsor pita which have been argued the monolayer, the number would be too high (10l2cm-2) to be defects in the mon01ayer.l~~~ Recently, Edinger et al. observed a pit depth of 0.25 nm for CH3(CH2)nSH/ (18)Edinger, K.; Gdzhluzer, A.; Demota, K.; Wtill, Ch.; Grunze, M. Au(ll1) ( n = 17 and 21) by STM and reported spedroLangmuir 1993,90,4. (19) We used a Digital Instruments Nanoscope 111. scopic data which indicated that the pita were formed by Introduction
(20) The Au(ll1) surfacea were prepared by evaporation of 120 nm of Au onto mica held at 310 OC in 2 X 10-8 Torr vacuum.zI No pita were ea Abstract published in Adoance ACS Abstracts, October 1,1993. e sof these freshly prepared surfacea nor was any observed in STM w mobility of the Au surface noted. Alkanethiol monolayers were formed (1)Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87, and on the Au(ll1) surfaces immediately after Au evaporation by immersion references therein. in 1 mM ethanolic solutions of the alkanethiol for 3-96 hours. There (2) Ulman,A. In AnZntroductionto Ultra-ThinFilmsFromLangmctirwere no large differencesin the number or size of pita observed for different Blodgett to Self-Assembly;Academic: San Diego, CA, 1991. incubation times or different alkanethiols. (3) Binnig, G.; Rohrer, H. ZBM J. Res. Deu. 1986,30,355. (21) Chidsey, C. E.D.;Loaicono, D. N.; Sleator, T.; Nakahara, S. Surf. (4) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. Sei. 1988, 200, 45. K.; Creager, S. E. Anal. Chem. 1993,65, 386, and references therein. (5)Laibinis,P.E.;Whitesides,G.M.;Allara,D.L.;Tao,Y.-T.;Parikh, (22) We have used tunneling parameters which do not cause disruption of the monolayer, as shown by Kim and Bard." Only at low tunneling A. N.; Nuzzo, R. G. J.Am. Chem. SOC. 1991,113, 7152. bias (10 mV) and high tunneling current (7-10 nA) do we observe (6) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; disruption of the surface by the tip. Chang, J. C. J. Am. Chem. SOC.1991,113, 1499. (23) When measuring the depth of the pita, we always used a nearby (7) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J . Am. Chem. monatomic Au step edge (0.25 nm) as an internal calibrati0n.l' We have SOC. 1988, 110, 618. measured hundredsof these pita on various alkanethioVAu(ll1) surfaces. (8)Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. STM currents are defined by the local electronic nature of the tip and Phys. Chem. 1988,92, 2597. may not yield the true topography of the sample. The tip is likely to be (9) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Reu. Lett. 1986,56,93. in the thiol layer and may or may not need to be closer to the surface than (10) Widrig. C. A.:. Alves.. C. A.:. Porter, M. D. J. Am. Chem. SOC.1991, for bare Au. The tip electronic environment should be homogeneous iij,2805. over the wholesample, and thus we feel the heighta we measure are indeed (11)Alves. C. A,: Smith. E. L.: Porter. M. D. J.Am. Chem. SOC.1992, true topographical heighta of the Au(ll1) surface. 114,1222. (24) The radius of curvature of the SFM tip only permita imaging of (12) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992,96, pita of sufficient size. Tip shape is variable due to manufacturing 7416. . limitations,andthusSFMimagesofalkanethiol-modifiedAu(lll) display (13) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. a few 7-10 nm pita only when terrace edges are well resolved. (14) Kim, Y.-T.; Bard, A. J. Langmuir 1992,8, 1096. (25) McCarley, R. L.; Bard, A. J. J . Phys. Chem. 1992, W,7410. (15) Roas, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993,9,632. (26) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. (16) Sun, L.; Crooks, R. M. J.Electrochem. SOC.1991,138, L23. Am. Chem. SOC.1992,119,4198. (17) Diirig, U.; Ztlger, 0.;Michel, B.; HHussling,L.; Ringsdorf, H. Phys. (27) Davison, M. G.; Deen, W. M. J. Membr. Sci. 1988,35, 167. Reu. 1993, B48, 1711. ~
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2776 Lcmngmuir, Vol. 9, No. 11, 1993 A.
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Figure 1. (A) 175 X 175 nm constantcurrent STM image of CH3(CH2)&H/Au(lll) immediately before imaging the 70 X 70 nm areas (approximate area contained in the box) in B-D. (BD) 70 X 70 nm constantcurrent images of right portion of A at times indicated. The asterisk indicates the same location on the surface. Thermal drift occurred during imaging. (E) Same area as A, but after imaging the 70 X 70 nm area of B-D. (F)175 X 175nm constantcurrent STM image of CH3(CH2)1&H/Au(lll) after heating for 2 h a t 100 "C. All images obtained with V = 0.5 V and it = 0.7 nA. Z-range is 5 nm full scale.
compared to that determined by microelectrode array models (108-109 cm-2).30 There is evidence that Au may dissolvein the presence of thiols and oxygen,18$1and laserdesorption mass spectrometry studies have demonstrated oxidation of thiols on Au in air.32*33Based on the information presented here, we believe the pits are defects which form in the Au(ll1) during the alkanethiol adsorption process. With increased scanning (Figure 1B-D) at a given set of tunneling parameters, we observe coalescence of pits followed by diffusion of the pits to the step edges at a rate which is independent of the alkanethiol chain length. The pits diffuse at rates of 1X 10-15to 3 X 10-15cm2s-I under the conditions used here; these values were calcu(28) We do not believe that relaxed alkane chains laying on the Au surfaceexplaintheinaccerrsibdityofCN-tothe Au. XPSanalysisindicates no removal of alkanethiol under the conditions used here. (29)Crooks used underpotential deposition of Cu and STM to note the nature of the defects.16 In that study the defects were not filled by Cu, and the authors believed the Au was not acceasibleby electrochemical method8. (30) Chailapakul, 0.; Crooks, R. M. hngmuir 1993,9,884. (31)McCarley, R. L.;Bard, A. J. J. Phys. Chem. 1993,97, 211. (32) Tarlov, M. J.; Newman, J. G. Langmuir 1992,8, 1398. (33) Li, Y.;Huang, J.; McIver, R.T.; Hemminger, J. C. J. Am. Chem. Soc. 1992,114,2428.
Letters lated from observing the rate of pit motion toward an edge for various-sized pits.34 The majority of pit diffusion is restricted to the scan area and thus appears to be driven by tip/sample interactions.34 The tip is likely to penetrate the monolayer to some extent, even with the mild tunneling parameters used here. The frictional forces between tip and sample in STM experiments have been shown to be quite high:' and we believe that frictional energy diasipation causes pit diffusion. Chidsey has shown (with electrochemicalSTM)that diffusion of pita on unmodified Au(ll1) is an activated process, with the energy of activation decreasing with increasing Au/adsorbate interaction.36*36The mechanism of Au diffusion with an attached alkanethiol is not as straightforward astheatomic adsorbates used in Chidsey's studies. Although the alkanethiol should increase the Au mobility due to the strong Au-S bonding and a concomitant decrease in AuAu bonding, we believe the chain-chain interactions in the monolayer provide some barrier to motion. Until the necessary energy is provided to overcome this barrier, there should be little diffusion at room temperature. After heatinga C H ~ ( C H ~ ) I & H / A U samplesat ( ~ ~ ~ ) 100 OC in air for 2 h, we observe in Figure 1F almost complete removal of the 0.25 f 0.03 nm deep pits in the surface and an increase in crenulated step edges. We have noted similar decreases in pit number upon heating n = 17 ~~ heating times result alkanethiols on A ~ ( l l l ) .Shorter in pits accumulating at step edges or being trapped in thq middle of the terrace. The alkanethiols ( n = 15 and 17) are not removed during the heating, based on a voltammetric study of an adsorbed redox-activethiolmin a similar heating experiment:l Figure 2. The cyclic voltammetry of adsorbed (C5Hs)Fe(C5H4C02(CH2)16SH)on Au is virtually unchanged after heating for 1h.m If the monolayer were removed leaving a pit-free Au(ll1) surface behind, we would expect substantial removal of the monolayer and a large decrease in the voltammetric signal. Diffusion of the pits to the terrace edgescan be explained by mobile Au atoms under the alkanethiol and dynamic Au-S bonding. Assuming that the sulfur head group of the thiol sits in the 3-fold-hollowsite on the Au(ll1) surface and that there are a small number of unbound Au a t o m or Au vacancies at pit edges, it is possible for one of the bound Au atomsto leave the thiol binding site and diffuse to a vacancy at another location in the pit. The alkanethiol could then rebind to the exposed Au atoms directly below ~
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(34)We have observed a small dependence of pit diffueion rate on tunneling parameters, but it has been difficult to quantitate this dependence due to the relatively rapid coalescence rate and the time required to acquire each image. Pit diffusivityis approximatelyinversely proportional%* to (rpi33; we also observe qualitatively that smaller pits diffuse more rapidly than larger pits. (35) Trevor, D. J.;Chidsey, C. E. D. J. Vac.Sci. Technol.199l,B9,964. (36) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D.N. Phys. Reu. Lett. 1989,62,929. (37) Salmeron, M.; Folch, A.; Neubauer, G.;Tomitmi, M.; Ogletree,D. F. hngmuir 1992,8,2832. (38)Heating untreated Au(ll1) samplesa t 100OC does not caw any pit formation or grom topographical changes to occur. (39) We observe no changes in the surface after reimmersingthe heattreated samples in the alkanethiol solution. (40) We adsorbed (C~H~)F~(C~HJCO~(CH~)~~SH) from millimolar solutions in absolute ethanol onto electrochemicallypolished polycrystalline Au electrodes and observed the voltammetry in 0.1 M HClOb These electrodes were heated in the oven a t 100 O C for various length of time, allowed to cool, rinsed with ethanol, dried under N2, and then placed in the electrochemical cell again. These electrodes showed substantial monolayer removal only when they had been heated for more than 24 h. (41)Bain et al. have also reportedJ2 that long-chain alkanethiob adsorbed on Au are stable upon heating in &.for several hours, but quickly desorb in hot hexadecane. (42) Bain, C. D.; Troughton, E.B.; Tao, Y.-T.; ball, J.; whitesidea, G. M.; Nuzzo, R. G. J . Am. Chem. Soc. 1989,112,321.
Langmuir, Vol. 9, No. 11,1993 2777
Letters Voltammetry of: CpFeCpC02 (CH2 ) 16 SWAu
V
n
100 mV s-l 0.1 M HC104
-
1.o
V
0.5 vs. SSCE
0.0
Figure2. Cyclicvoltammetryof (CaHa)Fe(C~COz(CH2)lgH)/ Au in 0.1 M HClOd at a scan rate of 0.1 V s-l; upper trace, immediatelybefore heating; lower trace, after heating at 100 O C for 1 h. The reference and counter electrodes were a sodium calomel electrode (SSCE) and a Pt gauze, respectively.
the original binding site. Repeating this process would came net motion of the pit. In order for the pits in the Au(111)to move, the Au-S bonds must be There is evidence that alkanethiols on Au become bonded to Ag
that is vapor deposited onto the alkanethiol ~urface.4~ For such an exchange of metals to occur, the alkanethioUAu bond must be dynamic. In addition,exchangeexperiments of various thiols in solution have shown that the thiol can be displaced from the Au surface, indicating a reversible RS/Au interaction." The mechanism we propose is likely to be a lower energy process than one involving simultaneous diffusion of alkanethiolattached to three Au atoms. In addition, this mechanism, based on rate-limiting diffusion of Au atoms, would explain the independence of pit diffusivity on alkanethiol chain length. In conclusion, we have shown that mobile pits on the CH3(CH&SH/Au(lll) surface ( n = 5, 15, and 17) are expelled from the terraces during STM scanning or by gentle heating without damage to the monolayer. It appears that the pita are not defects in the monolayer, as noted by their 0.25 f 0.03 nm depth23and their passive nature with respect to aqueous CN- etchant. We are currently investigating the electron transfer rates of electroactive monolayers before and after heating in order toprobe subtle differences in the environment of the redox POUP.
Acknowledgment. This work was supported by the National Science Foundation (CHE-9221646), a LSU Council on Research Grant, and the Louisiana Education Quality Support Fund (LEQSF(1993-96)-RD-A-09). We thank Mr. Z. Ling for performing photoelectron spectroscopy measurements. Helpful discussions with A. W. Maverick are acknowledged. (43)Tarlov, M.J. Langmuir 1992, 8, 80. (44) Collard, D.M.;Fox, M.A. Langmuir 1991, 7,1193.
