Substrate dependence of the surface structure and chain packing of docosylmercaptan selfassembled on the (111), (110), and (100) faces of single crystal goldNicholas Camillone III, Christopher E. D. Chidsey, Gangyu Liu, and Giacinto Scoles Citation: The Journal of Chemical Physics 98, 4234 (1993); doi: 10.1063/1.465030 View online: http://dx.doi.org/10.1063/1.465030 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/98/5?ver=pdfcov Published by the AIP Publishing

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Substrate dependence of the surface structure and chain packing of docosyl mercaptan self-assembled on the (111), (110), and (100) faces of single crystal gold

Nicholas Camillone III Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544

Christopher E. D. Chidsey AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974

Gang-yu Liu and Giacinta Scoles Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544

(Received 6 July 1992; accepted 19 November 1992)

Low-energy helium diffraction has been used to study the surface structure, chain packing, and thermal vibrations of docosyl mercaptan [CH3(CH2)21SH] self-assembled on single crys­tal Au(111), (110), and (100). The docosyl mercaptan molecules form monolayers with dif­ferent periodicity on the different surfaces of gold. On Au(111) at low temperatures ( < 100 K), the terminal methyl groups of the docosyl mercaptan molecules form domains of a hex­agonal lattice with a unit mesh constant of 5.01 ±0.02 A.. The sulfur head groups are ar­ranged in a commensurate (v:3xv:3)R30° structure and are believed to adsorb on the triple hollow sites of the Au(111) lattice. The unit mesh parameters for CH3(CH2h IS/Au(1lO) are a=b=4.99±0.08 A and a= 109S, suggesting that the chemisorbed sulfur atoms remove the "missing row" reconstruction of the Au (1 10) surface and form a commensurate c(2X2) lattice. The adsorption of docosyl mercaptan molecules on a Au(100) surface results in a complicated diffraction pattern. Analysis of the data reveals an oblique unit mesh with a=b =5.97 ±0.09 A and a=95°±5° with four kinds of equivalent domains present because of the fourfold symmetry of Au(100). The above results confirm that the sulfur-substrate interac­tion plays an important role in determining the periodicity and the packing density of the molecules within the monolayers. The estimated average domain size of the terminal methyl groups is 22.8,38.6, and 23.4 A for CH3(CH2)21SH self-assembled on Au(111), (110), and (100) faces, respectively. The chain packing and orientation within the-unit cell are also dis­cussed in this paper in conjunction with the latest results obtained via other techniques such as reflection IR spectroscopy and low-energy electron diffraction.

INTRODUCTION in spite of their relatively high complexity, their properties can still be calculated rather accurately by microscopic theories. I

,7 Consequently, these systems have been one of the main areas of activity in our laboratories for the past three years. 8-11

The study of the structure and molecular packing of monolayer and multilayer organic films is a vital and grow­ing area of technology and materials science I and is essen­tial to the understanding of the relationships between mi­croscopic structure and macroscopic physical and chemical properties. Knowledge of the structure of the films is the key to understanding the molecular processes that determine their wettability by liquids, their special optical and thermal properties, and their chemical reactiv­ity. As we come to a fuller understanding of these proper­ties we can move forward in the design and construction of new experiments, such as studies of protein-surface bind­ing,2 and new materials, such as those formed by the reac­tion of long chain organic molecules with the film to form thicker films. 3,4

Of the many organic thin films available, n-alkane thi­ols self-assembled on high quality gold surfaces have at­tracted considerable attention because they are easily pre­pared, chemically stable,5 and can be used as barriers in electron and ion transport studies.6 Furthermore, some of these systems are excellent models ofbiomembrances since,

If we limit ourselves to thermodynamic considerations, the surface structure, chain packing, and molecular orien­tation of an n-alkane thiol molecule in a self-assembled monolayer is the result of a balance struck among all the interactions contributing to the total energy. The most im­portant forces at work include the chemisorption of the sulfur to the gold, the intramolecular van der Waals inter­actions, the intermolecular van der Waals interactions, and electrostatic interactions. 12 In particular, in a monolayer of docosyl mercaptan [CH3(CH2h ISH, hereafter referred to as C22SH] on Au(111), the sulfur atoms form a commen­surate (vJxv:3)R30° latticeY This periodicity has also been confirmed via grazing incidence x-ray diffraction (GIXD) from C22S/Au(111)/mica.14 The nearest neigh­bor sulfur-sulfur distance is 4.99 A, more than 9% larger than the van der Waals diameter of the hydrocarbon chain, ~4.5 A. 15 One would expect that the thiol molecules

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TABLE I. Summary of the chain packing and orientation parameters for long chain thiols on Au ( 111 ) reported by different groups.

Techniques· IR MDS MMC GIXD GIXD

Systems XC1SS/ Au( 111 )/micab C16S/ Au( 111) C12SH C22S/ Au ( 111 ) /mica C22S/ Au (111 ) Tilt angles (a) 34· 35° 38° 12°± 1° 30o±lo Tilt direction" NNN NNN NNN NN NNN Rotation angle ([3) 55° 90· 46° Perpendicular chain-chain distance (A)d 4.37 4.33 4.23 4.88 4.34 Refs. 19-21 23 12 14 22

aIR: reflectance infrared spectroscopy, MDS: molecular dynamic simulations, MMC: molecular mechanics calculations, GIXD: glancing incidence x-ray diffraction.

bX=CH3, COOH, CONH2, and CH20H. cNNN: tilt is in the direction of the next nearest neighbor; NN: tilt is in the direction of the nearest neighbor. dThe perpendicular chain-chain distance is calculated for nearest neighbor chains using the value of a specified in the corresponding reference cited in the table.

would tilt from the surface normal and rotate about their molecular axes in order to minimize the total free energy of the system, which is similar to what n-paraffin chains do in a variety of crystalline phases. 15- 18 Indeed, quantitative re­flection IR spectroscopy has previously shown that the alkyl chains maintain an all-trans, zigzag conformation, that the chains are tilted ~ 34° from the surface normal, and that the C-C-C plane is rotated ~ 55° with respect to the plane defined by the chain axis and the surface normal vector. 19-21 Very recently, the tilting angle of the chain has also been measured by two groups using grazing incidence x-ray diffraction with a synchrotron x-ray source. 14,22 Though there are discrepancies between the two GIXD measurements (see Table I), the tilting angle for C22S/ Au( 111) is likely to be ~ 30° because single crystal Au was used in the second G IXD experiment and because this value is also consistent with the results obtained by other experimental techniques. 19-21

Because of its extremely high surface sensitivity and specificity and its completely nondestructive nature, 24-27 low-energy helium atom diffraction has been used in our laboratory during the past few years to successfully mea­sure the surface structure of n-alkane thiols chemisorbed on Au(111) and Ag(111) surfaces. 8-1 I In the present study we report the use of helium diffraction to probe the surface of C22SH self-assembled on the (111), (110), and ( 100) faces of single crystal Au. The purpose of this study is to examine the effect of varying the substrate surface structure, e.g., its density and symmetr~ on the surface structure, chain packing, and molecular orientation of the thiols. The results can also provide sensitive tests for sim­ulations of such layers.

EXPERIMENT

The C22SH was synthesized according to literature methods.5,28 The single crystals were purchased from Ae­sar polished to within 1° of the designated orientation. The final surface orientation was verified via Laue x-ray diffrac­tion, and surface contamination and polish damage were removed via Ar+ ion bombardment. High quality surfaces were obtained by annealing at ~ 700 cc, and the surface

purity and structure of the substrates were checked by Au­ger electron spectroscopy and low-energy electron diffrac­tion (LEED), respectively. A quick LEED experiment showed (lX2), approximately (5X20), and (23Xl) re­construction structures at the Au (110), (100), and (111) surfaces, respectively, consistent with common observa­tions made for these surfaces under ultrahigh vacuum con­ditions at room temperature.29-32 Each substrate was im­mersed in an ~ 1 mM solution of C22SH in ethanol for at least 36 h prior to helium atom diffraction measurements.

The low-energy helium diffraction apparatus has been described in detail previously.33,34 The samples are mounted on a three-axis manipulator with a silver paste to ensure good thermal contact between the crystal and the manipulator which is cooled via a closed-cycle helium re­frigerator. The helium beam is generated by expanding ~40 psi (gauge) of ultrahigh purity helium (Spectra Gases, 99.9999%) through a 20 f.Lm nozzle into the vac­uum chamber. The incident wave vector (k) can be ad­justed by changing the nozzle temperature. Velocity dis­persion in the beam is typically ~2%. In our experiments, carried out in the so-called in-plane scattering configura­tion, only the diffracted flux in the plane defined by the incident beam and the surface normal is detected. The beam fluxes are detected by a commercial silicon bolometer mounted on a 1.6 K liquid helium cryostat.

One of the most important pieces of structural infor­mation that can be extracted from helium diffraction spec­tra. is the value o[ the surface unit mesh parameters.-A diffraction spectrum is collected by scanning the detector within the diffraction plane with the incident beam and surface orientation fixed. After one scan is completed, the surface is rotated to a new azimuthal angle, ¢J, and a second scan is collected. A considerable number of scans must be gathered before we can construct a meaningful reciprocal space pattern, a map of diffraction peak positions in coor­dinates ilKIl and ¢J. For in-plane diffraction, the momen­tum transfer parallel to the surface, Mil ' for a given dif­fraction peak is given by

(1)

where ei and e f represent the angles between the surface

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4236 Camillone et al.: Structure of self-assembled docosyl mercaptan

normal and the incident beam and the surface normal and the scattered flux, respectively. k i is the value of the wave vector for the incident helium atoms. aKll can also be calculated via the two-dimensional, in-plane version of Bragg's law:

where m and n are the indices of the two-dimensional re­ciprocal space vector, a and b are the magnitudes of the real space unit mesh parameters, and a is the angle be­tween the unit mesh vectors a and h. In principle, direct comparison of the reciprocal space maps constructed from the diffraction spectra [Eq. (1)] with the reciprocal space maps constructed from a given set of a, b, and a values and the kinematic condition for helium diffraction [Eq. (2)] can be used to validate a proposed structural model. 35

In addition to the unit mesh structure, helium diffrac­tion can also provide information about the overall quality of the surface. In particular, it is possible to estimate the average size of ordered domains at the surface. The for­malism for this estimation is summarized here and a more detailed discussion can be found in Refs. 36 and 37. The experimentally observed peak width !l.e f can be expressed as36,37

(3)

where !l.eIfJ f represents the broadening contribution due to both the finite size and the velocity dispersion of the beam,37 !l.be f represents the contribution due to the finite size of the bolometer window, and !l.fi f represents the influence of the imperfections of the surface, namely the existence of domains of finite size. The average size of the domains can be estimated by the following formula:36

RESULTS AND DISCUSSION

CH3(CH2h1S1 Au(111)

(4)

The surfaces of n-alkane thiols [CH 3 ( CH2 ) nSH] self­assembled on mica supported Au ( 111) thin films and on a Au(111) single crystal substrate have been studied exten­sively in this laboratory using helium diffraction.8- 10 The main results for Cz2S/ Au( 111) are summarized here to facilitate comparison with the C22S/ Au( 110) and C22S/ Au(100) systems. Figure 1 (a) shows helium diffraction patterns obtained from C22S/ Au( 111) at 12 azimuthal an­gles (<,6=0°-60°). The corresponding reciprocal space map is plotted in Fig. 1 (b), more clearly displaying the overall azimuthal symmetry of the diffraction peaks. The recipro­cal space map suggests that the surface unit mesh must be hexagonal with lattice parameter 5.01 ±0.02 A [see Fig. 2(a)]. To demonstrate the consistency between the exper­imental data and our unit mesh model (a=b=5.01 A, a= 120°), the reciprocal space map constructed from the model is plotted in Fig. 1 (b) together with the reciprocal

.:.e ---.. ~

(J) -

>­:::: (J) s:: (l,) -s::

8

6

4

2

0

-2

[a] (0,-3)

(0,0)

(0,-4) (0,_2)(0,-1)

(O''-5i~·.~ $~60'~·"""· .".,.. -".,..

$~SS>~

-20 40 60

Polar Angle / Degrees

[b]

0t> "0

o· o ()o '"0 ()

0 . 0 @o O~ V

o~ 0 Q @CO ~ ®

0

~O, 0 0 0

"'0 <;)

~ 0 ® ~ ® ()

-2 o 2 4 6 8

FIG. 1. (a) Helium diffraction from CH3(CH2h ISH chemisorbed on single crystal Au( Ill) at different azimuthal angles. The surface temper­ature was -35 K. The incident wave vector (kj ) was 5.36 A -I and the incident angle (8;) was 59. 3D

• The base line for 8[<45° has been shifted downward for visual clarity. The definitions of 8j and (J [are given in the experimental section and the azimuthal zero is chosen as the next nearest neighbor direction of the terminal groups. (b) A reciprocal space map (0) of the diffraction patterns [some of them are shown in (a)] and our unit mesh model (e), a=b=5.01 A and a=120°.

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Camillone et al.: Structure of self~assembled docosyl mercaptan 4237

space pattern calculated from Fig. l(a). Note that, within our experimental uncertainty, every observed data point can be accounted for by this model.

These results are in good agreement with those found by means of other surface probes. Previously, Strong and Whitesides obtained an intermolecular (or mainly sulfur­sulfur) separation of 4.97 ± O. OS A using electron diffrac­tion. 13 More recently, scanning tunneling microscopy (STM) has been carried out by Porter and co-workers on a shorter chain thiol monolayer [CH3(CH2) 17S1 Au( 111)1 mica].38 It was proposed that the tunneling occurred be­tween the microscope tip and the gold-bonded sulfur at­oms, and the atomically resolved images indicated a hexagonal array of sulfur atoms with a=b=S.0±0.2 A, also consistent with our observations.

Our calculated lattice constant for the CH3 groups (S.Ol A) as well as the sulfur-sulfur distance measured by electron diffraction on C22S1 Au( 111 )/mica13 and STM on C18S1 Au(111 )/mica38 (4.97 and S.O A, respectively) cor­respond to the next nearest neighbor separation or y'j times the nearest neighbor separation of gold atoms at the Au( 111) surface. A reinterpretation39 of the transmission electron diffraction results13 indicated that the thiol mole­cules are commensurate with the underlying Au( 111) and form a simple (v'5Xv'5)R30° overlayer. Ulman and co­workers have recently performed ab initio calculations for HS and CH3S adsorbed on the (111) face of gold (simu­lated by a cluster of gold atoms) to find that the optimum adsorption geometry involves the epitaxial chemisorption of sulfur in triple hollow sites on the Au ( 111) surface.4O

Ignoring chain tilt and orientation for the sake of simplic­ity, we can picture C22S/Au(111) as shown in Fig. 2.

In principle, we can construct a helium-surface poten­tial by assuming a certain orientation for the chains. Using such a potential in atom-surface scattering calculations would provide information about the distribution of scat­tered intensity into the various diffraction channels under a given set of experimental conditions.34,41 By varying the assumed chain orientation iteratively, comparison of the calculations with the observed peak intensities would even­tually result in the determination of the chain orientation and packing by finding a best fit of the calculation to the experimental data. However, while such a calculation for this complicated system is feasible, it is still very cumber­some and has not been attempted by us.

Fortunately, in the case of n-alkane thiols on Au(111), IR, molecular-dynamics (MD) simulations, molecular mechanics (MM) calculations, and GIXD have been car­ried out to probe the molecular orientation. 12,19-23 Since the sulfur atoms are separated by S.OO A, corresponding to the next nearest neighbor spacing of gold atoms on the (111) face, the hydrocarbon chains must tilt from the surface normal and rotate about their axes to form a close-packed structure. The corresponding tilt (a) and twist angles (f3) reported by different groups are summarized in Table I. With respect to the orientation of the chains for C22S/ Au( 111), two possible models come to mind. The first one is shown in Fig. 2(c), where the molecules are all parallel within each domain. Another is illustrated in Fig. 2(d),

[a]

[b]

[c]

tilt

[d]

tilt

FIG. 2. Ca) A schematic diagram of the terminal methyl groups of CH3CCH2hlS/Au(111). This unit mesh structure is obtained directly from the helium diffraction scans shown in Fig. 1 Cal. Cb) A schematic picture of the top view of CH3 C CH2htS/ AuC 111) illustrating the rela­tionship between the thiol chains and the substrate Au(111) lattice. Please note that the actual position of the methyl groups is different from that of the sulfur head groups. For clarity in this figure, the chain tilt and rotation are ignored. Sulfur atoms are bonded to the triple hollow sites and form a commensurate (v:3xv'3)R30° structure on the Au(lll) sub­strate lattice. C c) A schematic of the surface structure indicating the chain tilting direction. The orientation of the all-trans carbon plane with respect to the tilting plane is indicated by the projection of the terminal CH2-CH3 bond ( .... ) onto the surface. Cd) An illustration of the static disorder model where the molecules are randomly distributed between two orientations.

where the molecules are randomly oriented in one of two equivalent orientations. Since the second model assumes a static disorder of the CH3 groups, an intensity calculation should be performed to check if the diffraction peaks aris­ing from the main (or approximate) hexagonal Fourier component of the potential are still strong enoUgh -to be observed.

Our previous study on CH3(CH2)nS/Au(111)/mica with n=5, 9, 13, 16, 17, and 21 suggests that there are at least two sUrface phases observed among these self-

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4238 Camillone et al.: Structure of self-assembled docosyl mercaptan

assembled monolayers. lO One phase is the above mentioned structure found for freshly prepared C22S1 Au (111 ). An­other phase, evidenced by a different diffraction pattern, has been found for monolayers of shorter chain thiols (n<17). A pattern very similar to that for the shorter chains has also been observed for a C22S1 Au ( 111) Imica sample which was "aged" for eight months at room tem­perature. Though diffraction from this second phase gives the same basic periodicity as the first, there is an obvious difference in the distribution of scattered intensity into the various diffraction channels. lO In particular, at </1=0° for freshly prepared C22S1 Au ( 111 ), up to 5 orders of diffrac­tion are clearly visible. For the other samples, only the first-order peak is easily observed while the higher orders are almost completely washed out (see Figs. 3 and 5 in Ref. 10). A recent study of CH3 (CH2 ) 17SH self-assembled on single crystal Au(111) (Ref. 42) has enabled us to determine that the intensity distribution observed for this and other shorter chains is the consequence of a c ( 4 X 2 ) superlattice [with respect to the commensurate ( V3" xV3")R30° lattice], which is due to patterned orientations of the hydrocarbon zigzag planes.42 This structure is ap­parently more difficult to establish in the spontaneous for­mation of the longer chain (e.g., C22SH) monolayer.

With respect to the presence of defects, one should note that the diffraction peaks shown in Fig. 1 (a) are much broader than those observed for helium diffraction from high-quality single crystal surfaces, e.g., NaCl(001).lO This indicates that the domain size of the terminal groups is much smaller than that of NaCl(001). Quantitative analysis of the peak width results in an aver­age domain size of the CH3 groups for C22S1 Au( 111) of -23 A.lO Finally, we should mention our previous study of the thermal motion behavior of the C22S1 Au( 111) mono­layer. We have found the temperature coefficients for the mean square displacements of the terminal methyl groups along the surface normal and the next nearest neighbor directions to be d(U;)ldT=(2.50±0.70)XlO-4 and d(U;)ldT= (7.2±2.4) X 10-4 A2/K, respectively. This implies that the diffraction intensities fall below our sensi­tivity at any temperature higher than 110 K.

CH3{CH2hlS1 Au{11 0)

Two CH3(CH2h 1S/Au(11O) [Cn S/Au(1lO)] sam­ples have been studied in this laboratory. The helium dif­fraction patterns have been taken in - 5° intervals between </1= _110° and 100°. Most of the scans at important azi­muthal angles are shown in Fig. 3(a). The corresponding reciprocal space map for all the scans is plotted in Fig. 3 (b). Three simple and important observations that can be made from Fig. 3 are (1) that the pattern taken at </1=0° is not the same as that taken at </1-60°, (2) that the pattern at </1=30° does not reappear at </1- _30° or ±90°, and (3) that the diffraction pattern at </1=0° is nearly identical to those at </1= 109.5° and 70S. Those observations indicate that, unlike helium diffraction from C22S1 Au( 111) in which each diffraction pattern repeats itself every 60° in </1, the reciprocal space map of C22S1 Au ( 110) does not have hexagonal symmetry. The highest symmetry of the data

points in Fig. 3(b) is only twofold symmetry. The unit mesh extracted from this data has a distorted hexagonal symmetry with a=b=4.99±0.08 A and a=109S [see Fig.4(a)].

The unreconstructed Au(1lO) surface has a rectangu­lar unit mesh with a=2.88 A and b=4.07 A. The next nearest neighbor distance of the gold atoms is (a2 + b2

) 1/2

=4.99 A, which is identical to the C22SH monolayer sur­face unit mesh constant. In addition, the C22SH monolayer unit mesh constants exactly correspond to a rhombic unit mesh constructed from the diagonals of four neighboring substrate unit meshes [see Fig. 4(b)]. Comparison of the orientation of the Au( 110) lattice determined via x-ray diffraction and that of the C22SH determined by helium diffraction has shown that the relative orientation between the two lattices shown in Fig. 4(b) is correct to within 5°. Therefore, the thiol molecules are commensurate with the substrate lattice within our experimental uncertainty. By analogy with C22S1 Au ( 111 ), the sulfur head groups in this case may favor the rectangular hollow site on the Au( 110) surface which provides coordination of the sulfur atoms with four nearest neighbor gold atoms, as shown in Fig. 4(b). However, it is a well known phenomena that the bare surface of Au ( 11 0) undergoes a 1 X 2 (or missing row) reconstruction with a=2.88 A and b=8.15 A.29 As a mat­ter of fact, we did observe the LEED pattern of Au(110) corresponding to this 1 X 2 reconstruction under UHV conditions prior to the deposition of the organic mono­layer. It is also known that chemisorbed molecules on metal surfaces may modify the surface to make it more similar to the bulk. In other words, chemisorbed molecules may remove the surface reconstruction of the metal sub­strate. In the case of C22S/Au(11O), the unit mesh mea­sured by helium diffraction is indirect evidence of removal of the Au ( 110) 1 X 2 reconstruction via chemisorption of thiols. If the Au( 110) surface maintained its reconstruc­tion, we would expect to see a different periodicity.

To the best of our knowledge, there have been no pre­vious reports on the structure ofC22S/Au(11O). However, we may be able to get some hints about the chain packing and orientation within the unit cell by analogy with similar systems such as C22S1 Au(111). Since the area per chain for CnSI Au( 110) is 23.47 A2 [larger than the 21.74 A2 per chain found for C22S1 Au (111 )], we expect the chains in C22S1 Au( 110) to tilt more from the surface normal than in the case of C22S1 Au (111). If we treat the chains as hard cylinders39 of diameter 4.6 A, a simple geometric calcula­tion indicates that the chains must tilt - 27° for C22S1 Au(111) and _37° for C22S/Au(11O) along the close­packed rows of gold atoms to achieve the highest packing density.

Unlike helium diffraction from C22S1 Au(111) whose diffraction peaks have a Gaussian distribution, the diffrac­tion peaks for C22S1 Au(1lO) are not well fitted by Gauss­ian functions, but instead show a Lorentzian line shape. The physical implication of the peak shape is still unclear to us. Peak width analysis of the strongest peak, (1,-2), indicates that the average size of the domains of the or­dered terminal methyl groups is -39 A, larger than that

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Camillone et at.: Structure of self-assembled docosyl mercaptan 4239

(0,0) [a2] (-1,0)

(-4,0) (-2,0)

(0,0)

~~/.\l$~ (_2~~3\) ~ 1(-3,0)

(-5,0)

(-6,0) $_-68'

+=-73' ~ $=-5' (-1,2)

C/) -'2 $ =·78' ::J >-... (II

-= :c +.-84' ... c:I;

~ 'iii c: Q) -.E

$.-94'

Ii .-99'

-75 ·55 -35 -15 5 25 45 65 ·55 ·35 -15 25 45 65 -55 -35 -15 25 45 65

Polar Angle / Degrees Polar Angle / Degrees Polar Angle / Degrees

8

6

4

2

o

-2

-4

-6

-8

[b] ~

o

• 0 <l!lo Of)oocP~ •

'"' -3 -1 1 3 5

o ( ~

7

for C22S/Au(111) and C22S/Au(100) (see Table II). Following the example of our previous study,10 the

temperature dependence of the diffraction patterns have been studied for Cz2S1 Au( 110) for the purpose of probing the thermal motion of the terminal methyl groups. Figure 5 shows helium diffraction from freshly prepared e22s/

FIG. 3. Ca) Helium diffraction scans from CH3 CCH2lzIS/Au(11O) at different azimuthal angles. The incident angles C 0i) and incident beam wave vector Ck;l are 62.1· and 5.17 A -I for 1/>= -109.5" to _55.4'; 62.8' and 5.26 A -I for 1/>= _30' to 23'; 63.9' and 4.85 A -I for 1/>=33° to 100·, respectively. The surface temperature for all scans is -38 K. Cb) The reciprocal space map CO) of the data in C a) clearly shows the azimuthal symmetry of the diffraction pattern. The reciprocal space map of our model Ca=b=4.99 A and a= 109.5") is also plotted here for comparison ce).

Au ( 110) at surface temperatures in the range of 30-130 K. The intensity of the diffraction peak decreases as the sur­face temperature is increased, and vanishes at ~ 130 K. No phase transitions are observed: the diffraction peak posi­tions and the relative intensities remain approximately con­stant. Using the standard formulas, we calculated the tem-

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4240 Camillone et at.: Structure of self-assembled docosyl mercaptan

[a]

[b]

FIG. 4. (a) A schematic diagram of the surface structure for the terminal methyl groups of CH3 (CH2}zIS/Au ( 110). This .unit mesh is based solely on the helium diffraction data shown in Fig. 3(a). (b) An illustration of the relationship between the thiol chains and the substrate Au( I 10) lat­tice. The chain tilt and rotation are ignored in this drawing for clarity. Sulfur atoms are bonded to the rectangular hollow sites and form a com­mensurate c(2X2) structure with respect to the substrate Au(1lO) lat­tice.

perature coefficient of the mean squared displacement along the surface normal direction for the terminal methyl groups to be d(U;)/dT=(2.32±0.70)XlO-4 A.2/K, which is similar to that found for C22S/Au(111), d(U;)/ dT=(2.50±0.70) X 10-4 A?/K.

CHa(CH2h1S1 Au(1 00)

Three CH3(CH2h 1S/Au(1oo) samples have been studied in this laboratory, and the helium diffraction pat­terns have been taken in ~ 5° intervals between cp = - 12° and 136° [see Fig. 6(a)]. Two observations suggest that the diffraction patterns for C22S/ Au( 100) have characteristics that are very different from those of C22S/ Au(111) and C22S/Au(110). Both of these factors complicate the deter­mination of the surface structure of this system.

First, if we chose any diffraction peak from C22S/ Au(l11) or C22S/Au(11O), we are able to determine at which azimuth such a peak reaches its maximum intensity. In the case ofC22S/Au(111), the (-1,0) and (-1,-1) peaks have their maximum intensities at cp=Oo (or 60°, 120°, ... due to hexagonal symmetry) and 30° (or 90°, ... ), respectively [see Fig. l(a)]. Their intensities decrease rap-

TABLE II. Estimated average domain size of the terminal methyl groups.

CI)

::= c:

:::::>

>-.... C1l .... ::= .0 .... « ->-::= CI)

c: CI> -c:

T s

t 33.8 K

39.1 K

42.9 K

52.2 K

62.7 K

78.2K

86.4 K

-45 -25 -5 15 35 55 75

Polar Angle I Degrees

FIG. 5. Helium diffraction from CH3(CH2}zIS/Au(11O) as a function of surface temperature. The incident angle and beam wave vector are 63.0' and 4.90 A -I, respectively. The scans were taken at an azimuthal angle of 100'.

idly away from the best azimuths. In the case of C22S/ Au(1lO), the (-1,0) and (1,-2) peaks reach their max­ima at cp=O° and 100°, respectively [see Fig. 3(a)]. This c1earcut azimuthal dependence of the diffraction peaks makes the determination of the symmetry of the reciprocal space maps, and therefore the unit mesh parameters, rela­tively simple for C22S/ Au( 111) and C22S/ Au( 110). How­ever, for C22S/ Au (1 00), it is almost impossible to deter­mine the azimuthal dependence of any peak's intensity. This is due to shifts of the peak positions with cp. The peaks seem to trace out straight lines instead of arcs in reciprocal space. For exampte, ilKll values monotonically increase as

Domain size cp is moved from 0° (or 90°) to 45° [see Fig. 6(a)]. Systems (A) The second factor that complicates the data analysis is ---------------------- - the variety of peak width and shapes. Assuming that every ~::~~~:g~~~ ;~:: peak in Fig. 6(a) is a single diffraction peak (as opposed to C

22S/Au(1oo) 23.4 llnresolved multiple peaks), we can measure the full width

at half maximum (FWHM) of each diffraction peak. The

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Camillone et al.: Structure of self-assembled docosyl mercaptan 4241

diffraction peaks with the narrowest peak widths appear at cp=O· and 90· in Fig. 6(a), while the broadest peaks are observed at cp=45°, The measured peak widths at these two azimuths are listed in Table III together with the peak widths for C22S1 Au (111) and C22S1 Au (110) measured at

corresponding azimuths. In contrast to the patterns for C22S/Au(111) and C22S/Au(11O), whose peaks are nar­rower than those of C22S1 Au( 100) and whose peak widths increase only slightly with the increasing of Mil ' the FWHM for diffraction from C22S/Au(100) at cp=45· is

9

6

c'< 3 ;:.::>.

0

-3

~ '00 c .$ c

[b]

0

-6

(0,0) [a2] (0,0) [a3] (0,0)

~ ~ ~( $31350~~ I\.

.~.~r .~.~ .,,~~ ... ,/ .. fi/v,.bv

.-~~ '_'~~~Il '-~'A' '-'~\/~ $=150~' ~ H150~ i~

.~ ~fl.-~!l._".~'/I~

.-,.~~ .~~ H~~~(

._,.~A .u.,.,.I\/V,.-v,J\ ,-~, ( ·75 -55 -35 ·15 5 25 45 65 ·55 -35 ·15 5 25 45 65

Polar Angle I Degrees Polar Angle I Degrees

0000000000 o 0

o 00000000000000000 o

o 0000000 0 0 00 ~0C@ 0

00 ooooooooooo~ 0

o 0 !Ch

rO~ e,'J:: 0"' 0'B 0o

I \~ ~, J@9:J8 0

8 ~ 8

-3 o 3 6 Kx / A:I

9

6

3

o

-3 9 -6

, , , ·55 ·35 ·15 5 25 45 65

Polar Angle I Degrees

-3 o 3

Kx / A:I 6

FIG. 6. (a) Helium diffraction from CH3(CH2h1SH chemisorbed on single crystal Au(1W) at different azimuthal angles with a surface temperature of -35 K, k;=5.18 A-I, and 8;=58°. The base line for 8/<45° has been shifted downward for visual clarity. (b) A reciprocal space map (0) of the diffraction peaks in (a). (c) The reciprocal space map of the proposed model plotted together with the data from (b) for comparison, where e, +, .. , and. represent four equivalent domains with the same unit mesh parameter: a=b=5.97 A and a=95°. (d) A blowup of the proposed model, with the relationship of the four unit meshes of four kinds of equivalent domains more clearly shown.

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4242 Camillone et al.: Structure of self-assembled docosyl mercaptan

3 [d]

2

1

o

-1 -

-2

-3 -3

• : J,. ...

". :";:;CE:!:~::: ":. ... ITJ . . . -2

(0,-1), (-1,0),

... : . -1 0 1

K, / A-I

FIG. 6. (Continued.)

. . . 2 3

almost twice as broad as that at cp=O° (or 90°). In addition, each diffraction peak at cp=45° seems to have its own in­dividualized shape. The above observation strongly sug­gests that peaks at certain azimuthal angles, e.g., 45°, are unresolved multiple peaks. Alternatively, one might pro­pose some type of defect structure which gives rise to a different pattern where the peak width is some function of both cp and AKII . To explain our data, one would need this function to be such that peaks at some azimuths are broader than peaks at others, and the widths of peaks along a particular azimuth (e.g., 45°) do not follow a simple monotonic increase with increasing Mil . However, we see no physically reasonable mechanism for this second pros­pect.

Figure 6(b) represents the corresponding reciprocal space map for Fig. 6(a) where we assume that every peak in Fig. 6(a) is a single peak in order to begin our data analysis. It is obvious that there are peaks closer to the

TABLE III. Examples of some observed peak widths for helium diffrac­tion from C22SH on Au(11!), (110), and (100) surfaces.

~kll ~ FWHM Systems (m,n) (A-I) (deg) (deg)

~2S/ Au( 111) (-1,0) 1.448 0 4.0 C22S/ Au( 111) (-2,0) 2.896 0 3.6 C22S/ Au(111) ( -3,0) 4.344 0 4.1 C22S! Au( 111) (-1,-1) 2.508 30 4.2

C22S/Au(1lO) C-l,O) 1.340 0 4.5 C22S/Au(1lO) ( -2,0) 2.680 0 5.2 C22S/Au(1lO) (-3,0) 4.020 0 5.3 C22S/Au(1lO) (1,-2) 2.536 100 5.3

~2S/Au(100) (-1,0) 0.977 0 4.2 ~2S/Au(100) ( -2,0) 2.115 0 4.0 C22S/Au(100) ( -5,0) 5.349 0 4,3 C22S/Au(100) 1st peak 1.351 45 6.5 ~2S/Au(100) 2nd peak 2.868 45 8.1 C22S/ Au (100) 3rd peak 4.338 45 6.9 ~2S/Au(I00) 4th peak 5.792 45 6.9 Cz2S/Au (1 00) 5th peak 7.531 45 10.5

9

6

3

()

--3 -6 -3 6 9

FIG. 7. The reciprocal space plot of our helium diffraction results (0) from C22S/ Au (111 ), together with a square unit mesh model (.) with a=5.95 A.

(0,0) peak and many more reciprocal space points in the case of C22S/ Au( 100) than for C22S/ Au( 111) and C22S/ Au(1W) [Fig. 6(b) vs Fig. l(b) and Fig. 3(b)], indicating a larger unit mesh. In addition, the reciprocal space points are distributed approximately in a concentric square pat­tern [see Fig. 6(b)]. Therefore the reciprocal space map of C22S/Au(100) has fourfold symmetry, which is quite rea­sonable since the substrate has fourfold symmetry.

The smallest parallel momentum transfer calculated from Fig. 6(b) is 1.056±0.025 J... -1 along both the x and y directions. There are many real space models which can satisfy this basic periodicity. The simplest one is a square unit mesh with a=b=5.95 J.... It can been seen from Fig. 7 that such a model reflects the basic periodicity and can account for most of the peak positions. However, this model has trouble in explaining the much broader peak width and individualized peak shape for the scans taken at certain azimuths (e.g., cp=45°). In addition, this model fails to account for some diffraction peaks (see Fig. 7).

One can adjust a, b, and a to satisfy the basic period­icity and to fit the overall data better. The best model is

-u=b=5.97 ±0.09 J... and a=95°±5°, corresponding to an oblique unit mesh. If the monolayer lattice is oriented in a certain way with respect to the substrate Au(100) lattice, four types of equivalent domains may exist as a result of the fourfold symmetry of the Au(100) surface. The real and reciprocal space maps for such a model are shown in Fig. 8(a) and Figs. 6(c) and 6(d), respectively. It can be seen from Fig. 6(c) that this model can account for all of the observed peak positions within our experimental un­certainty. It can also be seen by comparing Fig. 6(c) with Fig. 7, or by looking at Fig. 9, that this model can fit the experimental data better than the square model. In addi­tion, this model also suggests that the unusually broad peaks, such as the peaks at cp=45°, are in fact multiple peaks. For example, Fig. 10 demonstrates how the fifth diffraction peak (the broadest one) at cp=45° can be sep-

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Camillone et al.: Structure of self-assembled docosyl mercaptan 4243

[a]

I

II

[b]

IV

III

IV

III

FIG. 8. (a) A schematic diagram of the proposed surface structure for the terminal methyl groups of the CH3(CH2}zIS/ Au(lOO), with a=b =5.97 A and a=95°. (b) An illustration the relationship between the thiol chains and the substrate Au(lOO) lattice. The chain tilting and rotation are ignored in this drawing for clarity. Sulfur atoms are incom­mensurate with the substrate Au( 100) lattice.

arated into three peaks, corresponding to three pairs of peaks: (-5,5h and (5,-5)4' (-5,-5)1 and (5,5h, and (-6,5h and (5,-6)4'

In addition to the square lattice shown in Fig. 7, we have also compared our data with other possible structural models. These models include a c( lOX 10) (or a=b=6.42 A and a=900) unit mesh proposed on the basis of electron diffraction experiments,13 a c(2X2) unit mesh observed for CH3S on the Au(100) surface by LEED,43 and a one-

[aJ

lllilinllli~" '" nm'n t r-----------------~ 'c

::J

.Q ...

...:

~ 'in = .,

r-------------.-~]

.55 -35 -15 5 25 45 65 85 ·55 ·35 ·15 5 25 45 65 85

Polar Angle I Degrees Polar Angle I Degrees

FIG. 9. Three scans from Fig. 6(a) are chosen to check the validity of the square and oblique unit mesh model. These scans represent helium dif­fraction from CH3(CH2lzIS/Au(100) at ~35 K, with k i =5.l8 A -1 and Oi=SSo. The straight lines indicate the peak positions calculated from (a) a square unit mesh model with a=5.95 A and (b) the four-domain ob­lique unit meSh model with a=h=5.97 A a;;d a=95°. Clearly, not all the observed peaks can be accounted for by the model with a square unit mesh.

dimensional disorder model with a=a'=5.95 A. The agreement between the first two models and our data is poor. Although the one-dimensional disorder model can account for all the observed diffraction peaks, it fails to explain the fact that peaks at certain azimuthal angles, e.g., 0/=45°, have individualized line shapes.

-50 -45

--..... --40

\ .-\

\ \

\ \

-35

\ \.

-30

Polar Anglc I Degrces

-25 -20

FIG. 10. Decomposition of the fifth diffraction peak at azimuthal angle, </>=45°, into three diffraction peaks. The first peak ( ... ) corresponds to the (-5,S}z and (5,-5)4 peaks in the model. Their maximum intensity should appear at </>=42.5" and </>=47.5", respectively, and they both have the same ilKn at </>=45°. Similarly, the second peak (---) represents (-5,-5)1 and (5,S)3' and the third peak (_. -) corresponds to (-6,S)2 and (5,6)4' where the subscripts represent the domains. The background is subtracted for clarity.

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4244 Camillone et al.: Structure of self-assembled docosyl mercaptan

TABLE IV. Summary of the surface structure measurements of the terminal methyl groups for C22SH self-assembled on Au(1ll), (110), and (100) surfaces.

Systems C22S/ Au( 111) C22S/ Au (110)

Unit mesh a=b=5.01±0.02 A a=b=4.99±0.OS A a=120' a=109S

Relationship with respect commensurate commensurate to the substrate (v'3 X Y3) R30' c(2X2)

Average domain size (A) 22.S 3S.6

Area per unit mesh (A2) 21.74±0.20 23.47±0.SO

Number of molecules/mesh

Area per molecule (A2) 21.74±0.20 23.5S±0.SO

Real space map Fig.2(a) Fig.4(a)

d(U;)/dT (10- 4 A2/K) 2.50±0.70 2.32±0.70

The unreconstructed Au(100) surface has a square unit mesh with a=2.88 A, while the CH3 groups of the chemisorbed C22SH layer show an oblique unit mesh with a=b=S.97±0.09 A and a=9So±So. Therefore the thiol molecules are incommensurate with the substrate lattice. It has been observed that the bare Au(100) surface under­goes a (26 X 68) reconstruction.30-32 However, there is no simple commensurate relationship between the oblique lat­tice we observe for the thiol monolayer and this recon­structed Au( 100) surface lattice either, assuming naturally that the chain and the methyl groups have the same unit mesh. Although the unit mesh for both the Au( 100) sub­strate and the C22SH surface are known at this point, we are not yet able to determine the relationship between them. According to electron diffraction experiments, the sulfur head groups form a square lattice that is parallel to that of the metal substrate.43,44 By analogy with the results of LEED experiments,43 the relationship between the chemisorbed ~2SH and the Au( 100) can be pictured as in Fig. 8 (b) if we ignore the chain tilt and rotation as well as the substrate reconstruction for the time being. Synchro­tron x-ray diffraction experiments are currently in progress to check if the thiols remove the reconstruction of the Au(100) and to reveal the orientation of the thiol unit mesh with respect to the Au(100) lattice.

According to our model, the unit mesh area of C22S1 Au( 100) is 3S.S A2, about SO% larger than that of C22S1 Au(111) (21.7 A2) or C22S/Au(11O) (23.S A2). The first question that arises from this comparison is whether there can be more than one molecule per unit mesh.

Assuming that there is only one molecule per unit mesh, the nearest neighbor molecular separation is S.97 A, much larger than that of C22SH on Au ( 111) and Au ( 110). To achieve the strongest interchain interaction, the mole­cules would need to tilt as much as ~ 60° from the surface normal. A much smaller tilt angle ( < ISO) is observed by reflection IR spectroscopy carried out for CH 3 ( CH2) nSI Au(100) (n=O to n=17).43 Therefore, it is unlikely that

~2S/Au(100)

a=b=5.97 ±0.09 A a=95'±5' four kinds of domains

incommensurate

23.4

35.50±0.95

2

17.75±0.48

Fig. Sea)

there is only one molecule per unit mesh for ~2S1 Au(100).

Assuming that there are two molecules per unit mesh, the area per chain is then 17.7S±0.48 A2, much smaller than 21.74±0.20 A2 for C22S/Au(11l) and 23.S8±0.80 A2 for C22S/Au(11O). In other words, thiol chains are packed ~20% and 28% more densely on Au(100) than those on Au( 111) and Au( 110) surfaces, respectively. The nearest neighbor distances in this case are 4.03 and 4.40 A, respectively, along the two diagonals of the unit mesh. At such close distances, the chains must tilt less to minimize the repulsive interactions with their nearest neighbors. This is consistent with the observation made by reflection IR spectroscopy that thiols on Au( 100) are tilted much less «ISO) than thiols on Au(111) (~34o).19-21,43 The closer packing is also consistent with the observation made above that the incommensurability between the monolayer and the support unit meshes indicates a decreased relative strength of the thiol-substrate interaction as compared to the other substrates.

The thermal motion measurements have not been car­ried out for C22S1 Au( 100) because of the relatively poor resolution of the diffraction peaks. At 0/=0° where the peaks are better resolved than at other azimuths, the dif­fraction peak intensity is weak, which would result in larger uncertainty in the mean squared displacement mea­surements.

The average size of the ordered domains has been es­timated using the relatively well resolved diffraction peaks at 0/=0°. The value of the average domain size is ~23.4 A (see Table II), about the same as C22S1 Au ( 111) (~23 A) and smaller than that ofC22S/Au(11O) (39 A).

SUMMARY

In summary, helium diffraction has been used in our laboratory to probe the surface structure and the thermal vibrations of CH3(CH2)zlSH self-assembled on Au(lll),

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Camillone et al.: Structure of self-assembled docosyl mercaptan 4245

(110), and (100) surfaces. Furthermore, our data have helped us to understand the chain packing and orientation, and have given us some hints as to the influence of the adsorbate on the substrate reconstruction. The main results we have obtained via helium diffraction are summarized in Table IV. The docosyl mercaptan (C22SH) molecules self­assembled on different gold single crystal surfaces [Au(l1l), (110), and (100)] form stable and ordered monolayers. The terminal methyl groups have a triangular unit mesh for C22S/Au(111) with a=b=5.0l±0.02 A. This result is consistent with the (v'JXv3)R30° structure of sulfur atoms on the Au(111) surface. The surface of C22S1 Au( 110) has only twofold symmetry. The unit mesh constants of C22S/Au(1lO) are a=b=4.99±0.08 A and a= 109.5". This result indicates that in all probability the thiols remove the 1X2 reconstruction of Au(110) surface. The diffraction pattern of C22S1 Au ( 100) is complicated in comparison to the other two systems. An oblique unit mesh with the a=b=5.97±0.09 A and a=95°±5° best explains the observed diffraction pattern. Four kinds of equivalent domains must be present because of the square symmetry of the Au( 100) surface. The estimated domain size of the terminal methyl groups is -23,39, and 23 A for C22SH on Au(111), (110), and (100), respectively.

In addition to the surface structure, we have discussed the sulfur-substrate relationship as well as chain packing and orientation in conjunction with information from other researchers in the field. It is relatively certain that the sulfur atoms of C22SH bond to the triple hollow sites of the Au (111) substrate. The thiol chains in C22S1 Au ( 111) tilt - 34° from the surface normal along the next nearest neighbor direction with the carbon planes rotated _ 55° from the tilting plane. After the sulfur head groups chemi­sorb onto the substrate, the C22SH molecules remove the so-called missing row reconstruction of the Au( 110) sur­face and form a commensurate c(2X2) lattice. It is very likely, by analogy with C22S/ Au( 111), that the sulfurs bond to the rectangular hollow sites of the Au ( 11 0) sur­face. The all-trans carbon chains tilt more from the surface normal than the chains in C22S1 Au( 111) because the sur­face density for C22S1 Au (110) is smaller than that for C2zS/Au(111). Cz2SH molecules on Au(lOO) have the highest density among all three systems and, therefore, the chains are almost perpendicular to the surface of the sub­strate. Synchrotron x-ray diffraction experiments currently in progress will elucidate the exact chain packing and mo­lecular orientations within the unit cell of CH3(CHzh 1SI Au(1lO) and CH3(CH2)21S/Au(100).

ACKNOWLEDGMENTS

This work has been supported by Princeton University and AT&T Bell Laboratories. We thank Ralph Nuzzo, Lawrence Dubois, Jun Li, Peter Eisenberger, Paul Fenter, and Ken Liang for helpful discussions.

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