2705
Langmuir l a , 10, 2705-2709
Scanning Probe Microscopy Depth Measurements of
Self-AssembledMonolayer Structures on Gold Taejoon Han and Thomas P. Beebe, Jr.* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received March 28,1994@ Scanmng tunneling microscopy (STM) images of a self-assembled alkanethiol monolayer film on gold show a structure that contains flat areas with pits. An important characteristic of these pits, and one which has been a subject of interest in the recent literature, is their depth. For instances in which the measured depth is comparable to a single atomic step height of the gold substrate, the pits have been assigned as monolayer-deepdefects in the gold substrate, covered by alkanethiol molecules. For instances in which the measured depth is greater than a singleatomic step height ofgold, the pita have been assigned as defects in the alkanethiol monolayer itself. Since the diameters of those pits are small and the tips used for "hgprobe microscopy (SPM) imaging have a finite radius, it is possible that the structural interaction between the tip and surface prevents SPM from measuring the true depth of the pits in most cases. In this study, we construct a model to understand this phenomenon quantitatively, and analyze the measured depth of the pita. STM images of self-assembled octadecanethioVAu(ll1)from the same surface but obtained with tips of Merent sizes are presented. We show that in some STM images the apparent depth of the pits is coincidentlycomparablewith the singleatomicstepheight of the gold substrate surface. In other images we show that the true depth of the pit is measured, and that it is interpreted as the thickness of an odadecanethiol monolayer (-25.3 f 0.5A).
Introduction Self-assembled monolayer (SAM)filmshave attracted attention for their possible we in many fields such as nonlinear optics, chemical sensors,1 lithography,2 and other surfacerelated phenomena. In thiswork we studied the alkanethiollAu(ll1) system. Structural information about self-assembled alkanethiol monolayers on gold has been produced mainly by infrared ~pech.oscopy,3.~ ellip sometryp" el-mistry,? and ditii.action techniques?" From these studies, it is known that alkanethiol monolayers form a crystal-like, densely packed structure with a tilt angle of 20-40°.3.4 With minor discrepancies, the thickness of alkanethiol monolayers increases linearly with the number of methyIene groups in the chain3+ Electrochemicalanalysisof alkanethiolated gold s u r f a m predicts the possible presence of defects? and the crystalline structure of alkanethiol adsorbates on Au(ll1) is generally (J3xJ3)R3Oo as determined by difEaction
technique^.^-^ The techniques described above produced spatiallyaveraged, nonlocalized information. Naturally, with the
advent of scanning probe microscopy (SPM) techniques which produce highly localized information, scanning tunneling microscopy (STM)and atomic force microscopy (AFM)have been employed for structural characterizations of these monolayer films. Molecular-resolution images of alkanethiol monolayers on Au(ll1) have been obtained by STM and Al?M.'OJ1 Even though there are still questions about the imaging mechanism,12 those images show a (J3xJ3)R3Oo structure. Whether this periodicity is related to the adsorbate or a reconstructed substrate has been disputed.13 Typical large-scale STM images (-1000 A x 1000 A) show a structure which has atomically flat areas and naturally occurring pits with a diameter range of20-100 k14-16 These pits can migrate, coalesce, and even be etched by the STM tip during the scanning process.lhbJ"b AFM images of alkanethiolated gold surfaces have not been reported for this size range, except by written description."ja Despite intensestudy," there are still unresolved issues in alkanethiollgold systems. The imaging mechanism in
* To whom correspondence should be addressed.
* Abstract published inAduance ACS Abstracts, July 15,1994.
(10) Widrig, C. A; Alves, C. A; Porter,M. D. J.Am. Chem. Soc. 1991,
(l)Ulman,kAnZnaDductiontoUltm-ThinFilmsFromLangmuir- 113(a), 2805. (11)Alves, C. A; Smith, E. L.; Porter, M.D. J.Am. Chem. Soc. 1992, Blodgett to SeylASsembly; Academic Press,Inc.: New Yo&, 1991. 114 (4), 1222. (2)Kumar, A; Whiteaides, G. M. Appl. Phys. Lett. 1993, 63 (141, 2002. (3)Porter, M.D.; Bright,T. B.; Allara, D. L.; Chidsey,C. E. D. J.Am. Chem. Soc. 1987,109,3559. (4) Nuzzo,RG.;Dubois,L.H.;AUara,D.L.J.Am.Chem.Soc. 1990, 112,558. (5)Bain, C. D.;Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuao, R G. J.Am. Chem.Soc. 1989,111,321. (6) For a CHs(CH2LSH monola r on gold, the measured thickness by ellipmetry is (1.5 x n) - 1.9 c a n dthe calculatedthickness when fully extended (all-tnurs configuration) with a 30"tilt angle is (1.27 x n)+4A(hmref5). F"ref3,themeasured . byellipsometry is (1.5 x n ) 3.8Akr n 2 9, and t h e c a l m E E E n e s s witha%" tilt angle is (1.1x n) 5.1 k The t h i h e a s obtained by capacitance measurement is in @ agreement with the calculated value of 1.12 A per methylene. h r d i n g to the above d t s the thickness of octadecanethiol on gold ranges from 23.6 to 29.3 A (measured), and from 23.8 to 25.9 A (calculated). The measured thickneas of hexaded o l ( n = 1 6 ) m g o l d f h m ref 4 is 22 k (7)Chidsey, C. E. D.;Loiacono, D. N. . 1990, 6 (3), 682. ( 8 ) strong,L;Whitesides, G. hi. ' 1988,4,546. (9) Chidsey, C. E. D.;Liu, G.; Rowntree, P.; Scoles, G. J.Phys. Chem. 1987,91 (7), 4421.
+
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(12) The molecular height information of alkanethiol monolayers ess ofthe chain produced by SITM in ref 10 is virtually the same re length. The total z-scale in the images is 6.2 for an ethanethiol adsorbate and 7.0 Afor an oetadecanethiolonAu(ll1). Therefore,the suggested imaging mechanism for STM is that the tip locates near the sulfurheadgroupandtunnelingoccursthroughthesulfuratom. There are speculations that the "ingpmcess actually breaks the S-C bond and the imaging take place over the remaining sulfur atoms. (13)(a) Kim,Y.-T.,McCarley, R.M., Bard, A J. J .Phys. Chem. l M , 96 (la), 7416. (b)McCarley,R.L.; Kim, Y.;Bard, A J. J . Phys. Chem. 1993,97,211. (14)Hiiussling, L.; Michel, B.; Rhgsdorf, H.; b h r e r , H. Angew. Chem., Znt.Ed. E&. lSSl,30 (5), 569 (15)(a) Kim, Y.;Bard,A. J. Langmuir 1 M , 8 (4), 1096. (b)Kim, Y.; McCarley, R L.;Bard, A J. LangrmLir 1993,9(8),1941. (c) Ross, C. B.; Sun,L.;Crooks, R. M.Lalrgnurir 1993,9 (3), 632. (16) (a)McCarley, R L.;Dunaway, D. J.; Wficut, R. J. Lungmuir 1993,9 ( l l ) , 2775. (b) Edinger, IC;GiiWuser, A; Demota, IC;Woll, Ch.; Grunze, M.Langmuir lSW, 9 (l), 4. (c) Dung, U.; Ziiger, 0.; Michel, B.; Houssltng, L.; Ringsdorf, H.Phys. Rev. B 1993,48 (3),1711. (17) Chailapakul, O., Sun, L.,Xu, C., Crooks, R. M.J. Am. Chem. Soc. 1993,115 (26),12459 and references therein.
0743-7463/94/2410-2705$04.50/0 0 1994 American Chemical Society
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2706 Langmuir, Vol. 10, No. 8, 1994 Scheme 1 niicn 4u( g ) , v a (VIu:n
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STM a n d AFM,9-13t h e characteristics of pits, the mechanism of pit formation,15J6 a n d t h e kinetics of alkanethiol molecule adsorption5J8 a r e still controversial. Since the depth of pits in alkanethiolated gold surfaces measured by STM a n d AFM is not consistent with t h e depth predicted a n d measured by other techniques, the depth measurement of pits in alkanethiolated gold surfaces by these techniques generates considerable confusion. These pits have been ascribed to both defects in the alkanethiol monolayer15 a n d defects in the A u ( l l 1 ) substrate covered by the alkanethiol monolayer.16 Scheme 1indicates t h e various possibilities. The surface atoms a r e not to scale with the adsorbates, a n d t h e scheme is only meant to list t h e variouspossibilities. The pit depth in number of gold atoms is not meant to imply that only one-atom-deep pits occur. In this study, we describe and quantify t h e possible artifacts in depth measurement made by scanning probe microscopic techniques, which can be caused by t h e structural interaction between t h e finite size of t h e scanning tip a n d small surface depressions. Other papers have addressed t h e manifestation of this artifact with regard to t h e apparent width of protruding structures. This paper focuses on the interactions between depth a n d diameter measurements for features below the average surface plane.
Experimental Section Surfaces were prepared by vapor deposition of 2000-A-thick gold films onto freshly cleaved mica surfaces at P 5 5 x lo-' Torr. After the deposition, the goldmica films were heated to 550-600 "C for 1h under vacuum without being exposed to the ambient. The temperature was measured by a chromel-alumel thermocouple wedged between the back of the mica and the heating surface. A commercial vacuum evaporator (Edwards, Auto 306 Turbo) was used for these experiments. The thickness and deposition rate were monitored by a quartz crystal microbalance (Inficon) with a deposition controller. After annealing, the vacuum chamber was vented with nitrogen gas and the gold-coatedmica surfaces were immediately immersed into a large volume of octadecanethiol solution and kept in solution for 18 h prior to use. The concentration of octadecanethiol solution was -1 mM in ethanol. The alkanethiolated gold surfaces were washed thoroughly by ethanol and vacuum dried before image analysis. Successful adsorption was quantified by X-ray photoelectron spectroscopy (XPS)measurements in ultrahigh vacuum, and these results will be described in a forthcoming publication. (18)(a)Htihner, G.; Wo11, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9 (81,1955. (b) Milner, S.T. Science 1991,251,905.
-60
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Horizontal distance from apex of tip, x (A) Figure 1. (a, top) Schematic side view of the tip end radius of curvature calculation using atomic steps. r represents the tip end radius of curvature, h represents the atomic step height (2.36 A for Au), andx represents the width ofthe step measured in the SPM image. (b, bottom) Comparison of tip end shape models in scanning probe microscopy (side view) for and assumed tip radius of 100 A. The thick and the thin solid lines represent circular and parabolic tips, respectively. The dotted line represents the vertical height difference at the same lateral position. Since the circular tip is more sensitive in maximum depth measurement than the parabolic tip for a given radius of r , the circular tip was selected for modeling. STM imaging was performed with both custom-built and commercial (Topometrix) systems. The tip was a Pt/Rh (go%/ 10%)alloy, prepared by mechanical cutting; the tip end radius was measured by features in the image such as single or multiple atomic steps as discussed below. The measured tip end radius of curvature ranged from -50 to -1000 A. The tip end radius was usually between 150 and 300 A, and only rarely was it -50 A.19The method of measuring the tip end radius with steps is described schematically in Figure l a .
Results and Discussion The tip shape can be modeled in at least two ways: parabolic or circular. Comparison of a parabolic tip with a circular one at the same 100-A tip end radius is shown in Figure l b . The mathematical formulation for a parabolic tip is y = x2/2rwhere r represents the radius of
the tip.20a It can be seen that there is less than a 1-A height difference (for circular u s parabolic) within the
(19)STM tips prepared by chemical etching also show large variation in the tip end radius of curvature. G. Reiss et al. reported that only chemical etchin can generate tips with a tip end radius of curvature from 50 to 150 See ref 22. But most of the time chemical etching generates tips with tip end radii ofcurvatureranging from -500 A and upward. Tips which undergo an ion milling process after chemical etching show improvement in the tip end radius. These tips show less than a l0Od or even lo-A tip end radius of curvature. (a) Stemmer, A.; Hefti, A.; Aebi, U.; Engel, A. Ultramicroscopy 1989,30,263. (b) Morishita, S.;Okuyama, F. J.Vac. Sci. Technol.,A 1991,9(l), 167.(c) Biegelsen, D.K.; Ponce, F. A,; Tramontana, J. C.AppZ. Phys.Lett. 1989, 54 (131,1223. (20)(a)Keller, D.Su$. Sei. 1991,253,353.(b) Tersoff,J.;Hamann, D. R. Phys. Rev. B 1985,31(21,805.(c) Chemoff, D.A.; Chemoff, E. A. G. Atomic Force Microscopy of Collagen and other Extracellular Matrix Polymers. Presented at the 1992 Pittsburgh Conference, New
1.
Orleans.
Langmuir, Vol. 10,No. 8,1994 2707
Depth Measurements of SAM Structures on Gold lateral range of f50A from the apex of the tip. Since the pits in alkanethiol monolayers on Au(111)have average diameters of less than -100 the expected maximum depth will not be affected by more than -1 A by the selection of either model. Outside this limit, a circular tip has a steeper profile, and will produce deeper maximum depth information for a given diameter. It is known that the images obtained in scanningprobe microscopy are the result of "convolution" of surface topography with the shape of the tip. More correctly, the images represent a convolution of the outer envelope of the tip with comparably-sized surface features, since tip asperitiesbeyond the outer envelope of surface protrusions do not affect the resultant profile.20" The analysis and correction of this effect, and restoration of the real surface topography, has been discussed for both larger sized and atomic-scale tips.20a,bGenerally, in scanning probe microscopicimages, the surface structures that protrude from the flat areas appear with distorted lateral dimensions, always larger in apparent width, while in the absence of sample compressibility variations, the measured heights should be correct.21 In contrast, surface structures that are below the average surface plane (such as pits) appear correct in lateral dimension, while the depth information can be distorted.22 Since the pits which form naturally during the alkanethiol adsorption on gold have diameters from 20 to 100 and tip end radii of curvature prepared by mechanical cutting usually range from 150 to 300 the depth information measured by STM can be misleading due to the finite size of the tip. In order to understand this phenomenon quantitatively, a model was constructed. The geometry of this model is shown in Figure 2a. We assume that the surface and the tip are incompressible, and that the structure of the Fermi level and the total electron density of both the tip and surface are not significantly different from the topography. The tip end is assumed to be circular with a fixed radius of r, and the alkanethiol monolayer pits on gold have a diameter of d. From this geometry, the maximum depth information that can be measured, h,,, is calculated as h,, = r - (9- d2/4)1/2.The result of this calculation is shown in Figure 2b for a circular tip profile, and indicates that the measured depth depends strongly on the pit diameter. There are several important aspects of Figure 2b. First, the measured depth of small surface depressions can be limited by the diameter of their opening, causing an artifact in the measurement of the true depth, hheThe depth distribution measured for several pits is also dependent on the pit diameter distribution. The commercially prepared Si*, tips used in AFM imaging have approximately a 500-Atip end radius of curvature.23These tips will produce depth information from 70 to 100 diameter pits ranging from h,, = 1.3 A to h,, = 2.5 A (see right edge of Figure 2b). This information could be easily misinterpreted as the actual height of a single atomic step of gold.16" Second, even if a tip has a large radius of curvature, it will still measure the correct lateral dimension of a surface depression such as a pit, as long as the departure of the signal from the average surface plane outside the pit is above the detection limit of the microscope. Third, the profile of the pit structure measured by the scanning probe microscope will be v-shaped when the tip apex does not touch the bottom of the pit structure,16bindicating the regime in which h,, Ihhe. Fourth, in order to measurethe actual depth of pits formed by long alkanethiol monolayers on Au(11l), the tip should
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(21) Bustamante,C.;Vesenka,J.;Tang,C. L.;Rees, W.;Guthold,M.; Keller, R. Biochemistry 1992,31,22. (22) Reiss, G.; Vancea, H.; Witmann, H.; Zwwk, J.; Hoffmann, H. J . Appl. Phys. 1990,67(3), 1156. (23) Keller, D. J.; Chih-Chung, C. Suf. Sci. 1992,268,333.
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Tip Shank Angle Limit (45")
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200
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Tip end radius of curvature, r (A) Figure 2. (a, top) Schematic model of a scanning probe microscopy tip on a pit (side view). This geometry represents the case in which the tip end does not touch the bottom of the pit. If hme is deep enough, h , is what is measured, and is always dependent on the size and distribution of d. The straight tangent lines on both sides of the circular tip end represent the tip shank with an angle of 45". (b, bottom) Maximum depth measured, hm,, of hollow structures (such as pits) in SPM with a finite tip size. Each curve represents h , calculated with the indicated fixed pit diameter, as a function of the tip end radius of curvature. The straight line represents the h m , limit due to the tip shank. On the left side of this straight line, the tip shank determines h,, (see text).
have not only a small tip end radius of curvature but also a high aspect ratio. Beyond the apex, the tip shank has an angle, which a t a certain point starts to limit the measured maximum depth.24 This tip shank is indicated in Figure 2a, and the starting point of this limitation is indicated by the straight line on the left side of Figure 2b for a 45" tip shank angle. Fifth, although this model assumesthat the alkanethiol monolayer is incompressible (which it certainly must not be),25in the presence of some degree of monolayer compression, this error in depth measurement would be even greater, since one begins from an already-compressed monolayer. Figure 3 shows an STM image of a clean gold surface prepared according to the method described previously. It shows well-developed triangular facets, indicating flat terraces with the (111)structure. The measured atomic step height is 2.45 f 0.25 (number of measurements n = 10, la)while the expected step height based on the bulk structure is 2.36 There are also multiple atomic steps whose heights are usually measured up to -8 A. This measurement confirms the calibration of the z-piezo and the nominal accuracy of height information measured in our STM imaging (other artifacts, discussed below, notwithstanding). The tip end radius of curvature ob-
A
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(24)The mathematical formulation for a circular tip end is x2 + (y - r)2 = f ,and that of the tip shank isy = I&rl+ (1 - (1 + u2)ln)r,where a = tan 8 (8 represents the tip shank angle defined in Figure 2a). x and y represent the lateral and vertical dimensions from the apex of a tip. (25) P. Weiss, PennsylvaniaState University, private communication.
2708 Langmuir, Vol. 10,No.8,1994
Han and Beebe
Figure 3. A typical STM image of the bare gold substrate used. Vsample = -1.0 V, I t = 0.2 nA, scan speed 1.9 ,um*s-l, image size 2660 x 2660 x 22 Hi3.
tained from this image using the method illustrated in Figure l a is 571 f 89 The observation of pits on bare Au(ll1) surfaces is quite rare (see the upper right part of Scheme 1). Figure 4a shows an STM image of an octadecanethiol monolayer film on gold. The measured apparent depth of pits in this image ranges from 0.7 to 3.8 A, with an average value of 2.43 f1.07A(n = 14, la). This measured depth is close to that of an atomic step height of gold and comparable to the reported apparent depth of alkanethiol monolayer pits.16 In order to determine whether the pits are true atomicsteps of gold or one of the other possibilities shown in Scheme 1,the tip end radius of curvature for the tip used to acquire this image was calculated from the linear steps. The average value is r = 151.4 f 27.8 A (n = 15, la). There is naturally some variability in the measurement of T as a result of actual variations in r depending on the side of the tip producing the profile, since tips are not necessarily axially symmetric. The diameter of the pits, d, us the apparent depth, happarent, of the pits in Figure 4a is plotted in Figure 4b. This plot shows the strong dependence of the apparent depth on the diameter of the pits, which was predicted from the model discussed above. The lines in the plot represent the calculated maximum depth that could be measured, hm,, based on the actual tip end radius of curvature. The solid line represents this calculated h m , with a 151.4-A tip end radius of curvature. The upper dotted line represents this calculated h m , with a 123.6-A tip end radius of curvature which can be obtained by the lower statistical limit, rav - la, or 151.4 -27.8 The lower dotted line represents this calculated h m , with a 179.2-A tip end radius of curvature which was obtained in a similar manner. In this case due to the structural interaction between the finite size of the tip and relatively small diameters of the pits, the tip did not touch the bottoms of any of the pits, and thereby produced misleading depth information in all cases. Figure 5a shows an STM image of an octadecanethiol monolayer film on gold obtained from the same surface as Figure 4a, but with a much sharper tip. The tip radius of curvature used to obtain this image was also calculated using the linear atomic steps, and found to be rav = 39.4 f 8.2 (n = 17, la). The measured depth of pits in this
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Figure 4. (a, top) STM image of an octadecanethiol monolayer on gold. The happarent obtained by measuring the depth of the pits from this image is 0.7-3.8 A. Vsample = -0.8 V, I t = 0.2 nA, scan speed 0.95 pm-s-l, image size 760 x 760 x 12 Hi3. (b, bottom) Pit diameter us measured depth from (a). The solid squares represent happarent for each pit, and the solid line represents h m , calculated with an T of 151.4 f 27.8 A. The upper and lower dotted lines represent hm, calculated with T values of 123.6 and 179.2 A,respectively (ravf la).
image ranged from approximately4 to 25 A. The diameter of the pits, d, US the apparent depth, happarent, of the pits in this image is plotted in Figure 5b. This plot also shows the strong dependence of the apparent depth on the diameter of the pits. The solid line represents the calculated h m , with a tip end radius of r = 39.4 and the upper and lower dotted lines represent h m , calculated with tip end radii of 31.2 and 47.6 A, respectively (rav f l a limits). The effect of a tip shank on h m , is only important for sharper tips. h m , calculated with tip shank angles of 45" and 55" for a tip with a 47.6-A end radius is shown in the figure with dotted-dashed lines. Depth measurements of pits less than 80 in diameter remain incorrect due to the structural interaction arising from finite tip size see Figure 2b). The largest pit has a diameter of 94 and an apparent depth of 25.3 A,which is -15 A less than h m , calculated for a tip end radius of curvature of 47.6 and well below the limiting depth. Even including the effect of the tip shank, the apparent depth of this pit is still less than h m a . In this case the scanning tip touches the bottom of the pit and this depth can be correctly assigned to the thickness of the octade-
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Depth Measurements of SAM Structures on Gold
Langmuir, Vol. 10, No. 8,I994 2709
the electronic work function or the tunneling probability between the octadecanethiol monolayer and the exposed gold causethe 8-A depth measurement ofthe pits. Rather, a structural interaction between the finite-sized tip and the smaller-diameter pits is sufficient to explain in quantitative detail the observed effects, including how it would be possibleto measure an 8-A depth. Acombination of the two explanations may also be operating. It is also possible that the scanning tip resides close to the terminal methyl group in some cases, and compresses the alkanethiol molecules during the scanning process. Even though the alkanethiol monolayer can be compressed,26 it is unlikely that the tip can compress the long odadecanethiol molecule up to one-third of its length under relatively mild STM imaging ~ o n d i t i o n s .It ~ ~would be instructive to perform simultaneous force measurements during STM imaging on these systems to determine the amount of applied force as a function of tunnel gap resistance. Conclusion In the plots of the apparent depth us diameter of the pits, the data show a trend which indicates a strong functional relationship between these two parameters, amrdingto a simplemodel. This model works well within a statistical limit and shows a tip-pit structural interaction in the depth measurement of alkanethiol monolayer pits. The model assumes an incompressible monolayer, in the more realistic situation with a compressible monolayer, the underestimation of the depth will be even worse. Therefore, care must be taken in the depth or structure measurement of small hollow structures such as pits in SAMs or defeds in Langmuir-Blodgett films, artificially generated holes in surfaces by SPM tips, etc. The information about the tip end shape should be considered togethered with the scale of the features being measured. 30 40 50 60 70 80 90 100 From the above results, the expected thickness of an Diameter of pit, d (A) octadecanethiol monolayer was measured only when the tip was sharp enough relative to the opening diameter of the pit. Within the scope of Scheme 1presented earlier, Figure 5. (a,top) STM imageof an octadecanethiolmonolayer on gold obtained on the same surface as that of Figure 4a,but we can conclude that the pits are as deep as what is with a much sharper tip (rav= 39.4 f 8.2 A). The h-t expected for molecular defects in the alkanethiol monoobtained by measuring the depth of the pits in this image is layer in the cases observed here. This is valid under the -4-25k V-I,= -1.OV,I~=O.2nA,s~anspeedO.55~ms~~,assumption that alkanethiol molecules form a monolayer image size 1160 x 1160 x 67Hi3. (b) Pit diameter us measured on gold, and that no surface reconstruction occurs on the depth from (a). The solid squares represent h a v t for each gold substrate which would result in a topography change pit, and the solid line represents h , calculated with an r of causing unknown effects. It is likely that previous depth 39.4 f8.2 k The upper and lower dotted lines representh , calculated with r values of 31.2 and 47.6 respectively. (rav measurements of the pit structures in question were f la). The dotted-dashed lines representh , caolculatedwith underestimated for the reasons presented here. Assigntip shank angles of 45" and 55" at an r of 47.6 A. ments must therefore be based on information obtained from acombinationofseveral techniques,notjust scanning canethiol monolayer on gold.6 It was mentioned above probes. that the curvature of the profile was, in principle, Acknowledgment. The authors wish to thank Dr. indicative of whether or not the tip was able to touch the Richard M.Crooks of Texas A&M University for helpfirl bottom of the pit. Unfortunately, the profiles of the pits discussions. This work is supported by grants from the have neither clearly discernible v- or u-shapes, nor do Center for Biopolymers at Interfaces at the University of they clearly exhibit a crossover betwwn those two shapes. Utah, the National Science Foundation National Young The measurement of the pit's depth is important because Investigator Award (CHE-9357188), and the National it can tell the position of the tip during scanning. Since Institutes of Health (R21-HG00613-02). the actual thickness ofodadecanethiol has been measured by STM,and since this depth is very close to that expected (26) Houston, J. E.; Michalske, T. A. Nature 1992,356,266. for this molecule, we conclude that in these experiments (27)In an AFM study of an alkanethiol monolayer on gold, the alkanethiol layer is compressed by about 2Wo of its thickness by a 1-pN the tip located near the terminal methyl group rather force (seeref 26). In an STM study of alkanethiol on gold, the contacting than at the s u l k head group. The STM imaging area and applied force to the alkanethiol monolayer by the scanningtip mechanism suggested by Kim and Bard1& is still valid. have been measured. The suggested force during imaging is on the It is not necessary to invoke a model in which changes in order of 0.5 nN per alkanethiol molecule (see ref 1 6 ~ ) .
