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Langmuir 1993,9, 1556-1560
An Atomic Force Microscopy Study of a Self-Assembled Octadecyl Mercaptan Monolayer Adsorbed on Gold(11 1) under Potential Control J. Pan, N. Tao, and S. M. Lindsay' Department of Physics, Arizona State University, Tempe, Arizona 85287-1504 Received September 11, 1992. I n Final Form: March 22,1993 We have used an atomic force microscope (AFM)to study a self-awembled odadecyl mercaptan monolayer on Au(ll1) under 100mMNaC104solution. We controlledAu surfacepotential by electrochemical methods, so that the monolayer could be imaged in situ under different surface charge conditione. The monolayer is quite stable over a wide range of potentials but is destroyed when surface potential ie higher than 1590 mV or lower than a value between -370 and -755 mV vs AgIAgC1. The failure of the film at negative potentiale depends on how much 02 is diesolved in the solution. The disintegration of the film is found to be irreversible. We measured the film thickness, and, by comparing the imagea of film with that of Au after the monolayer was stripped off,the proposed (d3Xd3)R30° overlayer structure was confirmed. We have ab0 done electrochemical voltammetry measurements which agree with the AFM results.
Introduction Organosulfur compounds form well-ordered, self-assembled monoiayers at the solid-solution interface. This monolayer providesa controlled interface with well-defined composition, structure, and thickness, suitable for the study of catalysis, corroeion, Iubfication, adhesion, biblogical membranes, electron and ion transport, passivation, and wettability studied3 Recent experiments show that this monolayer is a promising candidate for microelectronic devices.3 Self-assembled organoeulfur compounds adsorbed on a Au(ll1) surface have been studied ertensively by a variety of methods including X-ray diffraction: low energy helium atom scattering," transmission electtion difYraction,2optical ellipsometay,'*6-8 contact angle measurementa,Sll X-ray photoelectFon spedroecopy,3fiJ2infrared spectroe~opy,1JJ*electrochemical measurements,lJ scapning tunneling micro~copy,,~3 and atomic force microscopy.14, These studies indicate ordered packing of organoeulfidea on a Au(ll1) surface. The usefulness of this film depends on the integrity of the 2-Dordered structure which could be disrupted by induced surface charge. More importantly, electrochemical stripping allows atomic resolution studiea sfboth the overlayer and the substrate a t the same locatiqp, permitting an unambiguous determination of the orientation of the overlayer. In this study, we have used AFM under electrolyte (1) Porter, M.D.;Bright, T. B.;Allma, D.L.; Chidney, C. E. D.J. Am. Chem. SOC.1987,109,3559-34568. (2) Strong, L.; Whiteaides, G. M.Langmuir 1988,4,646558. (3) Tarlov, M.J. Langmuir 1992, 8, 80-89. (4) Samant, M.G.; Brown, C. A.: Gordon, J. G..II Lonamuir 1991, 7, 437-439. (5) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scolee, G.J. Chem. Phyr. 1989,91,4421-4423. (6) Bain, C. D.; Troughton, E. B.;Tao, Y.-T.; Evall, J. J. Am. Chem. SOC.1989,111,321-335. (7) Chidney, C. E. D.;Loiacono, D.N. Longmuir 1390,6,682-691. (8) Folkere, J. P.; Laibinie, P. E.; Whiteaides, G. M.Langmuir 1992, 8, 1330-1341. (9) Bain, C. D.;Whitesides,G.M.J. Am. Chem. SOC.1988,110,58975898. (10) Bain, C. D.; Whiteaides, G. M.J.Am. Chem. SOC.1988,110,65606661. (11) Duboh, L. H.;Zegareki, B. R.; Nuzzo, R. G. J. Am. Chem. SOC. 1990,112,570-579. (12) Nuzzo, R. G.;Dubois, L. H.; Allara, D.L.J.Am. Chem. SOC. 1990, 112,558-569. (13) Kim, Y.-T.; Bard, A. J. Longmuir 1992, 8, 1096-1102. (14) Alvee, C. A,; Smith, E. L.; Porter, M.D.J. Am. Chem. SOC.1992, 114,1222-1227.
solution to control the surface potential while observing the change of the monolayer in situ. AFM is far less destructive to the film than STM because of the nonconducting nature of the monolayer. Operation of AFM under solution reduces the contamination problem and eliminates the capillary force between the AFM tip and monolayer, further minimizing destruction of the film.
Experiment Au(ll1) substrate were prepared by epitaxially evaporating Au on to a freshly cleaved mica surface." We ueed octadecyl mercaptan from Aldrich Chemical Co. (98% purity) with no further purification. Au substrata were immerwd into 1m M octadecyl mercaptan-pure ethanol eolutione for at least 24 h to ensure monolayer coverage. "be treated substrataewere rinaed thoroughly in pure ethanol to remove e x w molecules beyond the fmt layer. The quality of the monolayer was checked by applyinga drop of ethanol onto the surface. No wetting indicate a good monolayer. The treated subetrate were dried in air and then mounted in a Nanoecope II AFM (DigihI Instruments), using their standard liquid cell. The treated subetrate waa uaed as a working electrode, a 0.1 mm diameter silver wire was put into the liquid cell as quasi-referenceelectrode (AgQRE),and a 0.25 mm diameter Pt wire was used as counter electrode. We used 100mM NaClO, solution as supporting electrolyte because it doea not have any specific adsorption onto Au surfacea over a wide range of surface potentiale.u To check the effect of dieeolved 02,both unpurged and 99.999% pure Ns purged solutione were used in our experiment. We varied the potential on the Au surfacein both poeitive and negativedirectionerelative to AgQRE. The AgQRE was calibrated against a Ag/AgCl reference electrode and all potentiale are quoted ve Ag/AgCI. AFM imaging was carried out in situ with a net force of about 10nN wing 0.58 N/m triaagularcantilevere (DigitalInstrument& which had been calibrated optically. A similar electrochemical cell was used to measure the voltammograms separately. Results and Discussion Figure 1 shows the images we obtained under purged NaClO4 solution while substrate surface was scanned at a negative potential of 290 mV. A t 290 mV (Figure lA), we obtained a close-packed hexagonal structure. The lattice constant for the image, 5.2 f 0.6 A, is considerably (15) D e k , J. A.; Thundat, T.; Nagahnra, L. A,; Lindmy, S. M.Surf. Sei. 1991,266,102-108. (16) Hamelin, A; Sottomayor, M.J.; Silva, F.;Chang,S.-C.; Weaver, M.J. J. Ekctroanal. Chem. 1990,295,291-3 Do.
0743-7463/93/2409-1556%04.00/0 0 1993 American Chemical Society
AFM Study of Octadecyl Mercaptan on ALU
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Figure 1. 7 nm X 7 nm images of octadecyl mercaptan molecules adsorbed on Au(ll1) in purged 100 mM NaClO, solution under surface potentials lower than or equal to 290 mV; (A) 290 mV; (B)-110 mV; (C) -710 mV; (D)-725 mV; (E)-755 mV; (F)back to 290mV after disintegration of the monolayer. Images have been plane-fitted and Fourier filtered to remove noise.
larger than the Au(ll1) lattice constant, so we identify it as the monolayer octadecyl mercaptan tail group structure.14 As the surface potential is lowered, this structure remains more or less the same until the potential reaches -755 mV. In this potential range, we observed that as the surface potential goes more negative, images become more distorted and imaging is increasinglymore difficult (Figure 1B-D). For a more negative potentid, a larger negative charge collects on Au surface, disrupting the Au-S bond. At -755 mV, no periodic structure could be resolved (Figure 1E). We think that a t this potential, the monolayer film is destroyed, and the surface is completely changed. We then increased the surface potential back to 290 mV, obtaining the image shown in Figure lF, which has no sign of periodic structure, suggesting that the process is irreversible. We can follow the progress of disintegration by maintaining the surface potential at -795 mV and scanning a relatively large area. The first step of disintegration is the development of more pinholes around defects (Figure 2A). This process usually takes several minutes. These pinholes are then enlarged by repeated scanning of the AFM tip resulting in big holes (Figure 2B,C). Figure 2B is taken 2 min after Figure 2A. With a scan rate of 31 Hz, the creation of a hole takes about 19 scans. It takes one more minute or nine scans to further enlarge the hole as in Figure 2C. Nine minutes later, the surface finally gets very rough with mercaptan molecule clusters scattered all over the Au surface (Figure 2D). These clusters are very loose on the surface, making high resolution impossible. From this large area picture, we can see that the potential a t which the film gets destroyed is not a clear-cut value, because the AFM tip plays a role in destabilizing the film. From the depth of the hole in Figure 2C we find the thickness of the monolayer to be 18 f 3 A,which agrees with a previous resuk6
The destabilizing effect of the AFM tip on the monolayer is important only when the bonding between the monolayer and Au is loosened by the mechanisms discussed below. With the surface potential close to the potential of zero charge (PZC) of Au (300 mV for pure A d 7 ) , we varied the operating force between the AFM tip and the monolayer from 1to 50 nN without having difficulty in imaging the hexagonal monolayer structure. We first considered the possibility that the Au-S bond is disrupted by driving the surface potential negative of the PZC of Au substrate. Another mechanism could be the H2 evolution reaction which happens around -710 mV on bare Au. This reaction takes place through defects and could displace the film. We also noticed that the negative potential at which the film fails is dependent upon how well the NaC104 solution is purged. This leads us to believe that the dissolved 0 2 is playing an important role. In one extreme we used an unpurged solution in our experiment. The image a t 290 mV is shown in Figure 3A, with a hexagonal periodic mercaptan monolayer structure displayed. A t -370 mV the film in the scan region is completelystripped off, revealing the underlyingAu lattice as shown in Figure 3B. The presence of dissolved 02shifts the failure potential closer to PZC by 385 mV. Our explanation is that 02 reacts with Au a t a much higher potential through defects, thus destroying the film. This view is c o n f i i e d by cyclicvoltammetry as will be discussed later. In most of our experiments, the Au substrate became very rough after the monolayer was destroyed, and no atomic resolution of Au could be obtained. Desorption of the monolayer occurred very violently; we were able to obtain atomic resolution of the underlying Au lattice. One (17)Hamelin,A.; Vitanov, T.;Sevaetyanov,E.;Popov,A. J.Electmml. Chem. 1983,145,225.
1558 Langmuir, Vot. 9, No.6, 1993
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Figure 2. 506 nm X 506 nm images showing monolayer disintegration process in purged 100 mM NaClO, solution when the surface potential is -795 mV. (A) More pinholes are formed in the monolayer. (B)Two minutes after (A) was taken, AFM tip sweeps off the loosely bond mercaptan molecules and creates a big hole on the surface. (C) One minute after (B), the hole is further enlarged by repeated scanning of the AFM tip. (D)Nine minutes later, the surface is completely modified with mercaptan molecule clusters scattered around rough Au substrate.
of these luckily-obtainedimages is shown as Figure 3. When the surface potential was lowered to -370 mV, the image of the surface changed from a large hexagonal lattice like in Figure 3A to a smaller one as in Figure 3B after just one scan. A t a scan rate of 31 Hz,this quick appearance of the Au lattice indicatesa violet desorption of the mercaptan monolayer. The images shown in parts A and B of Figure 3 were taken of the same area on a single Au terrace, so we can compare the two images to find the registration of the two lattices (Figure 3C). We measured the ratio of lattice constants to be 1.8 f 0.2 (-d3) with a rotation angle of 28 f 3" (-30'). This confirms directly the proposed (d3Xd3)R30° structure of the octadecyl mercaptan monolayer on the Au(111)~urface.2.~~~ Octadecyl mercaptan monolayer film also fails at certain positive surface potential vs Ag/AgCl. Figure 4 shows a series of images when the surface potential is higher than or equal to 290 mV. In the range of surface potentials from 290 to 1540 mV, the images (Figure 4A-C) show (d3Xd3)R3Oo monolayer structure, although with increasing distortions. A t a potential of 1565 mV, the monolayer structure is barely resolvable (Figure 4D). A t 1590 mV, no periodic structure is observable, indicating
a totalfailure of the monolayer (Figure 4E). Irreversibility of the desorption is confirmed by decreasing the surface potential to 290 mV without recovery of periodic structure (Figure 4F). Around 1290 mV, 02 evolution takes place on bare Au, so this reaction probably drives desorption at defects. Other Au oxidation reactions will not contribute significantly because they occur a t a lower potential. There is no obvious dependence of the positive failure potential on the degree of purging of the NaClO4 solution, so this is probably the only mechanism for disintegration. We have also measured cyclic voltammograms. The current-voltage (CV) curves we obtained in the unpurged NaC104 solution are shown in Figure 5. The working electrode potential is scanned from 290 mV to as negative as -210 mV with no observable current on the first cycle. When we increased the scan range from 290 to -310 mV, a small current flows out from the substrate, but the magnitude of the current is independent of time (Figure 5A, curve 1). A t this potential range, the film is still resolvable by AFM as a periodic 2-D lattice, so that this small time-independent current does not lead to failure of the film. This current could be attributed to reactions taking place through defects in the monolayer. If, however,
AFM Study of Octadecyl Mercaptan on Au
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Langmuir, Vol. 9, No. 6,1993 1559 c
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A
B
C
Figure 5. Current-voltage characteristicsof octadecyl mercaptan monolayer on Au(ll1) in unpurged 100 mM NaClO, solution. (A) Voltage is scanned lower than or equal to 290 mV curve 1correaponde to a scan range of 290 to -310 mV, showing a small timeindependent current; curves 2,3, and 4 correspond to a scan range of 290 t~-610 mV, showing rapidly increasing currents with each repeated scan. (B) After scanning the voltage in the range 290 to -610 mV for 6 min,the CV curve becomes characteristic of bare Au surface. (C) Voltage is scanned higher than or equal to 290 mV curve 1correaponde to a scan range of 290 to 1290 mV, which show small time-independent current; curves 2 and 3 correspond to a scan range of 290 to 1690 mV, which show a rapidly-increasing current with each subsequent scan. Curve 3 is almost identicalto CV curve on bare Au surface. The scan rate is 26 mV/s for all curves.
the scan range is extended to 290 to -510 mV, an abrupt current increase is observed. We noticed that the current at a given potential increases very rapidly with each subsequent scan even without increasing the scan range (Figure 5A, curves 2,3, and 4). Thus, when the substrate potential is more negative than some value between -510 and -310 mV, mercaptan molecules are leaving the substrate and exposing more and more bare Au on each cycle, leadirigto an increasing current. If we keep scanning between 290 and -510 mV, after about 5 min, the CV curve between -10 and 1490 mV (Figure 5B) is essentially identical to that obtained on bare Au surface1*(the scan rate is 25 mV/s). This shows that almost all the molecules were stripped off the substrate and also that this disintegration is irreversible, CV curves for scanning at potentials higher than 290 mV are shown in Figure 5C. There is no noticeable current in either direction until the maximum potential reaches 1290 mV (Figure 5C, curve 1). When the scan range extends to 290 to 1590 mV, current increams with each scan and finally the CV curve becomes characteristic of bare Au substrate (Figure 5C, curves 2 and 3), indicating that when the substrate potential is higher than a value between 1290 and 1690 mV, the film is destroyed. This process is irreversible. Purging with 99.999% pure N2 causes little change in the CV curve in the positive direction. The negative direction is altered considerably as shown in Figure 6, indicating the role of dissolved 0 2 .
Conclusions We have studied a self-"bled octadecyl mercaptan monolayer film on Au(l1l) under potential controlusing electrochemical AFM with a 100 mM NaClOr supporting electrolyts. Thisf iis stable under a wide range of surface
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Figure 6. CV curve comparison of octadecyl mercaptan monolayer on Au(ll1) when the voltage is scanned lower than or equal to 290 mV with a scan rate of 25 mVls. Curve 1is in unpurged 100mM NaClO4 solution and curve 2 is under purged 100 mM NaCIO4 aohtion. There ia a big shift in the voltage at which the current becomea observable due to the presence of On.
potentials although the film distortsunder scanning when the variance of the surface potential away from 300 mV (the PZC on bare Au) gets larger. The f h is destroyed when the surface potential is more positive than 1590mV. Failure at negative potential is more complicated because it depends on dissolved 02,varying from -370 mV for unpurged solution to -755 mV for solution purged by 99.999% pure Nz for 3 h. The disintegration of the monolayer is found to be irreversible. We also confirmed the (d3Xd3)R30° monolayer structure on the Au(ll1) surface by direct comparison of the substrate image (after stripping) with the adlayer image before stripping. Electrochemical CV c w e measurements agree quite well with all the AFM results.
Acknowledgment. We thank Y. Li, P. Oden, J. DeRoee, and H. Song for help in the lab. We also acknowledgeM.Weaver, X.Gao, and W. Lo for interesting discussions. We also thank Jack Lamen and Chrissie Manion for assistance in reviewing this manuscript.
