Hydrogen-Bonded and Methyl-Terminated (001) Cleavage Planes

the structure of the (001) plane as determined by previous single-crystal X-ray diffraction. ... For a more comprehensive list of citations to thi...
1 downloads 0 Views 152KB Size
Download PDF
Langmuir 2002, 18, 5551-5557

5551

Helium Diffraction Study of Organic Single-Crystal Surfaces: Hydrogen-Bonded and Methyl-Terminated (001) Cleavage Planes of a Guanidinium Methanesulfonate Crystal Gianangelo Bracco,*,† Jo¨rg Acker,‡,§ Michael D. Ward,| and Giacinto Scoles‡ INFM and Physics Department, University of Genoa, I-16146 Genoa, Italy, Department of Chemistry, Princeton University, Princeton, New Jersey 08540, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, and Department of Physical Chemistry, Freiberg University of Mining and Technology, D-09596 Freiberg, Germany Received February 14, 2002. In Final Form: May 3, 2002 The (001) surface of a single crystal of guanidinium methanesulfonate, which has a bilayer architecture, has been investigated and characterized by He diffraction in the temperature range between 40 and 100 K. The goal of these studies was to demonstrate that the surface structure of a single crystal could be determined by He diffraction and to identify the preferred cleavage plane in these crystals. The measured periodicity on different cleaved samples was consistent with the structure of the (001) plane as determined by previous single-crystal X-ray diffraction. The data, however, indicated that cleavage produced a surface terminated either by hydrogen-bonded guanidinium sulfonate sheets or by methyl groups projecting outward from these sheets. These different surfaces afforded slightly different He diffraction patterns along with different rates of decay in the thermal attenuation of the elastic scattered intensities. In particular, only one of the rates shows a dependence on the parallel momentum exchange, which is similar to that measured for self-assembled monolayers of alkanethiols on Au(111). These results suggest that He diffraction is sufficiently sensitive for distinguishing between the two possible (001) cleavage planes.

1. Introduction Organic materials, especially in thin film form, are increasingly attracting attention because of their potential in applications related to molecular electronics, optoelectronics, and bioactive interfaces. Much of the interest stems from the ability to modify their properties through molecular design. Central to the development of these materials is the characterization of their interface structure, which can be crucial with respect to both their intrinsic properties (e.g., electrical) and their use as substrates for the epitaxial growth of films with controlled orientation and structure. While the solid-state structure of organic crystals can be determined by single-crystal X-ray diffraction, the structures of organic crystal surfaces are much more difficult to characterize at the atomic level.1 Well-established diffraction techniques based on charged particles, such as low-energy electron diffraction and ion scattering, or direct imaging methods, such as scanning tunneling microscopy, can provide atomic-level characterization of crystal surfaces. These methods, however, are limited to materials with reasonable electrical conductivity, which is characteristic of only a limited number of organic crystals.2 This has prompted numerous studies of organic interfaces and crystal surfaces with atomic force microscopy (AFM),3 since this technique is not limited to * To whom correspondence should be addressed. E-mail: [email protected]. † University of Genoa. ‡ Princeton University. § Freiburg University of Mining and Technology. | University of Minnesota. (1) Matsumoto, A.; Odani, T.; Sada, K.; Miyata, M.; Tashiro, K. Nature 2000, 405, 328. (2) (a) Dvorak, M. A.; Ward, M. D. Chem. Mater. 1994, 6, 1206. (b) Li, S.; White, H. S.; Ward, M. D. J. Phys. Chem. 1992, 96, 9014. (3) Ward, M. D. Chem. Rev. 2001, 101, 1697.

conducting materials. Conventional AFM can be used to determine the lattice constants of crystalline interfaces; however, in almost all cases, these measurements rely on obtaining a Fourier contrast of a periodic surface rather than true atomic resolution. Furthermore, this method generally cannot provide unequivocal assignment of chemical composition. These features can limit the use of AFM for assignment of specific surface functionality on organic interfaces. Recent advances in grazing incidence X-ray diffraction have afforded substantial insight into the structure of organic thin films and crystal surfaces.4 These measurements, however, can be complicated by X-ray-induced damage and by the low Z number of the organic constituents, which results in low scattering intensity. The absence of a universal method for characterization of organic interfaces has stimulated the search for additional methods that address the current limitations. In this respect, the scattering of closed-shell helium atoms at thermal energies can provide structural information on both conductive and insulating systems. Moreover, the low energy of the probe eliminates any perturbation to, or damage of, the sample.5 Helium scattering is also very sensitive to small corrugation amplitude,6 surface disorder,7,8 and dynamic properties of a surface.9-12 In this paper, we describe the use of helium scattering for the (4) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Chem. Rev. 2001, 101, 1659. (5) Farias, D.; Rieder, K. H. Rep. Prog. Phys. 1998, 61, 1575. (6) Leung, T. Y. B.; Gerstenberg, M. C.; Lavrich, D. J.; Scoles, G.; Schreiber, F.; Poirier, G. E. Langmuir 2000, 16, 549. (7) Poelsema, B.; Comsa, G. In Springer Tracts in Modern Physics; Ho¨hler, G., Ed.; Springer-Verlag: Berlin, 1989; Vol. 115, p 1. (8) Wo¨ll, C.; Lahee, A. M. In Springer Series in Surface Science; Hulpke, E., Ed.; Springer-Verlag: Berlin, 1991; Vol. 27, p 73. (9) Bracco, G.; Bruschi, L.; Tatarek, R.; Franchini, A.; Bortolani, V.; Santoro, G. Europhys. Lett. 1996, 34, 687.

10.1021/la025629p CCC: $22.00 © 2002 American Chemical Society Published on Web 06/17/2002

5552

Langmuir, Vol. 18, No. 14, 2002

characterization of the (001) surfaces of single crystals of guanidinium methanesulfonate (GMS), an organic crystal with a well-defined layered architecture that results from the presence of two-dimensional hydrogen-bonded sheets of guanidinium ions and sulfonate moieties. The singlecrystal structure reveals that these sheets are assembled into bilayers through interdigitation of the methyl groups projecting from opposing sheets. Single crystals of this compound can be cleaved readily in a manner reminiscent of other layered materials such as mica and graphite. In the case of GMS, however, cleavage can occur, in principle, along two different crystal surfaces: one terminated with the guanidinium sulfonate (GS) hydrogen-bonded (HB) sheet and the other with a methyl-terminated (MT) surface. We have employed helium scattering to probe the surfaces of freshly cleaved GMS crystals and have detected the existence of both interface structures. Though the structure of organic thin film surfaces have been characterized by helium diffraction,13 to our knowledge, this is the first report of the application of this technique to the surface of an organic crystal. 2. Experimental Method The diffraction experiments were carried out using a He diffractometer described in ref 14 and recently modified in order to control the motion along all the rotational degrees of freedom by a personal computer to increase reliability and accuracy. Briefly, the beam is generated in the source chamber by expanding 100 psi of ultrahigh purity helium (99.9999%) through a nozzle of 0.020 mm diameter cooled by a closed-cycle refrigerator. The nozzle temperature, measured by a diode thermometer, can be varied by controlling the power fed to the source heater in order to change the beam incident energy and wave vector. For the present measurements, the chosen temperature was about 70 K, which corresponds to an incident energy of Ei ) 14.5 meV and an incident wave vector ki ) 5.26 Å-1. Under these conditions, the velocity distribution has a relative spread ∆v/v ) 1.7%. The beam passes through a skimmer and enters the scattering chamber where it is chopped at a 50% duty cycle and is collimated by a slit before striking the surface of a crystal mounted on a sample holder. The beam is well collimated in the horizontal (scattering) plane, while the angular resolution in the vertical plane is relaxed to optimize the intensity at the detector. The sample holder is connected to a manipulator with three rotational degrees of freedom in order to change the incident angle θi, the tilting χ of the surface normal with respect to the horizontal plane, and choice of the azimuthal direction φ. The sample holder can be cooled to 30 K and heated to 800 K, and its temperature is measured by a Pt thermometer mounted on the front side of the sample holder very close to the sample position. The scattered beams are detected by a commercial doped Si bolometer mounted on a 1.6 K liquid helium cryostat that can rotate over a wide range around the crystal in the horizontal plane to adjust the scattering angle θf. The bolometer signal is amplified by a lownoise preamplifier stage and is detected by a lock-in amplifier. Two concentric liquid nitrogen shields surround the crystal and the bolometer to protect them from the thermal radiation of the vacuum chamber walls, which are at room temperature. The pressure around the sample in this cryogenic environment is estimated to be lower than 10-11 mbar. All three rotations and the motion of the detector are driven by stepper motors and controlled by a personal computer, which also collects the lock-in signal during the angular scans. Since the GMS crystals are mechanically soft because of their molecular character, it is necessary to manipulate them without (10) Darling, S. B.; Rosenbaum, A. M.; Sibener, S. J. Surf. Sci. 2001, 478, L313. (11) Vollmer, S.; Fouquet, P.; Witte, G.; Boas, C.; Kunat, M.; Burghaus, U.; Woll, C. Surf. Sci. 2000, 462, 135. (12) Graham, A. P.; Toennies, J. P. Surf. Sci. 1999, 427, 1. (13) Schwartz, P.; Schreiber, F.; Eisenberger, P.; Scoles, G. Surf. Sci. 1999, 423, 208. (14) Aziz, R. A.; Buck, U.; Jo´nsson, H.; Ruiz-Sua´rez, J. C.; Schmidt, B.; Scoles, G.; Slaman, M. J.; Xu, J. J. Chem. Phys. 1989, 91, 6477.

Bracco et al. resorting to glues or other bonding agents typically used for sample mounting. Consequently, the crystals were mounted by means of a square of thin copper foil on a circular 0.5 mm thick copper disk. The disk diameter was 11 mm larger than the average crystal size (∼7 mm). At the center of the foil, a circular hole slightly smaller than the maximum linear size of the crystal provides access to the crystal surface when the foil is carefully wrapped around the crystal and the disk. The foil is then gently pressed against the disk to firmly block the crystal and to avoid shadowing effects on the beam due to the hole edge. This assembly was mounted on the sample holder by means of metallic clips screwed on the holder that was pressed on the wrapped copper disk. The crystal was cleaved in air by using adhesive tape until a flat surface with just a few macroscopic steps was exposed. The holder was then mounted in the scattering chamber, which was immediately pumped down. At the beginning of any experimental run, the crystal was brought to 300 K to desorb contaminants while most of the measurements were performed at 40 K to minimize inelastic scattering. The temperature was not lowered below this value to reduce the possibility of adsorbing contaminants. In fact, the specular intensity was observed to decrease over time for temperatures below 35 K while the initial intensity was recovered by heating to 40 K where the signal is stable during a typical run of 10 h. Heating and cooling rates were kept below 5 K/min to avoid thermal stress on the crystal.

3. Results and Discussion The solid-state structure of GMS, as determined by single-crystal X-ray diffraction,15 reveals a monoclinic three-dimensional unit cell and the existence of quasihexagonal (001) HB sheets of guanidinium cation and sulfonate moieties (Figure 1).16 These planes can be viewed as consisting of a sulfonate ion linked by two S-O‚‚‚H-N hydrogen bonds to each guanidinium ion, this unit repeating to form a “ribbon” (Figure 1A). These ribbons fuse along their edges through S-O‚‚‚H-N hydrogen bonds to form the quasihexagonal GS sheet, with the methyl groups all projecting out from the same side. For a given sheet, the sulfonate groups all lie in a plane that is elevated slightly above a plane that contains the guanidinium ions. The (001) surface unit cell, based on a single-crystal X-ray structure determination, is described by a ) 12.778 Å, b ) 7.342 Å, and γ ) 90°. The a axis is perpendicular to the ribbons, and because of a registry shift of adjacent ribbons along the b direction, the periodicity is equivalent to two ribbons (this can be visualized readily in Figure 1A), whereas along the b axis the periodicity is determined by the S‚‚‚S distance.17 The (001) GS sheet can also be described by an alternative unit cell, with a′ ) 14.737 Å, b ) 7.342 Å, and γ′ ) 120.42° (as indicated by the dashed outline in Figure 1A). The solid-state structure of GMS suggests that the exposed surface resulting from the cleavage can be either a MT surface, with methyl groups protruding from the GS sheet, or a HB surface, with the methyl groups buried in the bilayer below so that only the HB sheet is exposed (Figure 1B). Symmetry dictates that the two types of (001) surfaces must have the same periodicity. The structure of each surface can be described by a real-space net and its corresponding reciprocal net (Figure 2). The real-space net illustrated in Figure 2A is characterized by the lattice constants a′ ) 14.737 Å, b ) 7.342 Å, and γ′ ) 120.42° using the aforementioned alternative description of the 2D (001) unit cell assigned from the single-crystal (15) Russell, V. A.; Etter, M. C.; Ward, M. D. J. Am. Chem. Soc. 1994, 116, 1941. (16) Summary of the lattice parameters for GMS: space group C2/ m, a ) 12.778 Å, b ) 7.342 Å, c ) 9.998 Å, β ) 126.96°. (17) According to a convention described previously,15 where a puckering angle is defined by the dihedral angle, θIR, between adjacent GS ribbons, for GMS θIR ) 180°.

Guanidinium Methanesulfonate Crystal

Langmuir, Vol. 18, No. 14, 2002 5553

Figure 1. The (001) plane of guanidinium methanesulfonate. (A) Structure of the plane with the basic unit of sulfonate and guanidinium ions that form a ribbon (shaded region). Me is a methyl group protruding perpendicularly from only one side of the plane. The rectangular unit cell with edges a and b (solid lines) and the alternative unit cell with edges a′ and b (dashed lines) are also shown. (B) Schematic view of the 3D crystal with its bilayer structure. The ribbons are depicted as rectangles. The height of the unit cell is 9.998 Å. The shortest distance between the hydrogen-bonded planes of a bilayer is 4.04 Å. On the right, after cleavage, the crystal is terminated with a hydrogen-bonded (001) plane (lower part) or with a methyl-terminated (001) surface (upper part).

direction, new crystallites, generally with different orientation, can be illuminated and the diffraction pattern can change abruptly in intensity. In this case, the optimum χ changes abruptly from the value of the previous azimuth. Nevertheless, for any given sample, it was possible to find an azimuthal region of about 60° 18 in which χ optimization produced smooth and rather small changes on χ and where the diffraction peaks are well resolved, since they were related mainly to a few (one to three) crystallites. The specular peak width is near the value calculated taking into account the parameters of the scattering instrument alone.19 In the following discussion, unless stated otherwise, we will refer to measurements performed in this ordered region where the analysis is simpler. Polar scans were transformed to momentum space by the equation Figure 2. Real-space net of (001) surface (A). The unit vectors with length a′ ) 14.737 Å and b ) 7.342 Å are shown. (B) Corresponding reciprocal surface net is shown (all points). Larger points refer to the integer net generated by the reciprocal unit vectors Ga′ and Gb (see text for details).

structure. For comparison with experimental results, the reciprocal net (Figure 2B) consists of points that have been separated into an integer set, related to the realspace unit cell defined by (1/2)a′, b, and γ′, and a halfinteger set formed by the remaining points of the GMS full lattice periodicity (real-space unit cell described by a′, b, and γ′). The reciprocal unit vectors of the (half) integer net have lengths Ga′ ) 0.988 Å-1 ((1/2)Ga′ ) 0.494 Å-1) and Gb ) 0.992 Å-1, forming an angle of 59.58°. While the He diffraction measurements aimed at the determination of the surface periodicity were carried out at 40 K, measurements were also performed up to 100 K in order to evaluate the thermal attenuation of the scattered intensity, i.e., the Debye-Waller factor. Polar scans in which θf is varied were collected at a fixed azimuthal direction every 5° in a range of 110° to obtain the reciprocal surface net. For all azimuthal directions, the optimization of the tilt angle χ to maximize the specular peak intensity preceded the polar scan. It is worth noting that the beam spot on the surface covers an area of about 0.5 mm2 and the diffraction pattern is the incoherent sum of contributions coming from the different crystallites in the spot. When the crystal is rotated to a new azimuthal

∆K ) ki(sin(θf) - sin(θi))

(1)

where ∆K is the change in momentum parallel to the surface in the scattering plane. Figure 3 shows diffraction scans measured for one of the samples, hereafter referred to as sample A, as a function of the azimuthal direction. The specular peak (∆K ) 0 Å-1) is always the most intense peak, and the scattered intensity decreases rapidly at high values of momentum exchange. The other peaks are well resolved and the diffuse background is small, which means that the surface presents a good long-range order. The direction φ ) 0° was set along the direction where many diffraction peaks (up to the sixth-order) were detected. In parts A and B of Figure 4, the scans carried out along φ ) 0° for two different incident angles are shown. All peaks are clearly split in two, a fingerprint of the presence of two crystallites that give the largest contribution to the detected intensity. The tilt between them (estimated by the angular separation of the two specular components) is 0.7°. The first-, second-, and third-order peaks are detectable for both angles of incidence, and the fifth one is always small. The fourth- and sixth-order peaks increase (18) The value of this azimuthal angular range is most probably related to the triangular shape of the crystal, since the crystallite shape should be similar. (19) Schwartz, P. V. Ph.D. Thesis, Princeton University, Princeton, NJ, 1998.

5554

Langmuir, Vol. 18, No. 14, 2002

Bracco et al.

Figure 3. Diffraction scans measured along different azimuthal direction φ on sample A. The curves are shifted by a constant value for the sake of clarity.

strongly in intensity as the angle of incidence changes from 40.5° to 65.7°. To estimate the size of the unit cell in reciprocal space, a fit by means of Lorentzian functions plus a sloping background was carried out on the scans of Figure 3. The peak positions are reported in Figure 5 as open circles.20 It is worth noting that the broad vertical (out-of-plane) resolution of the apparatus allows the detection of peaks off their azimuthal direction. As a consequence, the angular resolution close to the specular direction is low, while it improves toward larger values of the momentum exchange.21 This explains why points at small momentum exchange or corresponding to structures with high-intensity group themselves along a curved line. In the same figure, the reciprocal surface net of Figure 2 is shown as solid squares. The measured periodicity along φ ) 0° is about 1.01 Å-1, which is close to the length of Gb (the reciprocal unit vector of the bulk lattice which equals 0.992 Å-1). Therefore, this direction was tentatively assigned to be parallel to Gb. In the explored region, the agreement between our measurements and the integer net points is good with an overall discrepancy smaller than 2%. This minor discrepancy can be probably explained by taking into account that the X-ray data were collected at room temperature while He measurements were performed at 40 K. In fact, the reciprocal net (20) The peak positions -∆K are reported in Figures 5 and 7. (21) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 98, 3503.

Figure 4. Diffraction scans measured on sample A along φ ) 0° at different incident angles θi ) 40.5° (A) and θi ) 65.7° (B) and along φ ) -31° at θi ) 67.1° (C). The intensity is normalized to the incident beam intensity.

Figure 5. Diffraction peak positions of Figure 3 are reported in reciprocal space as open circles. Solid (small) squares represent the reciprocal (half) integer surface net of Figure 1.

measured by He diffraction seems larger with respect to that determined by X-ray diffraction, suggesting a thermal contraction of the unit cell in real space.22 However, there is no clear evidence for half-integer peaks.

Guanidinium Methanesulfonate Crystal

Langmuir, Vol. 18, No. 14, 2002 5555

Figure 7. Diffraction peak positions of Figure 6 are reported in reciprocal space as open circles. Solid (small) squares represent the reciprocal (half) integer surface net of Figure 1.

Figure 8. Diffraction scans measured on sample A along φ ) 0° at different sample temperatures. Figure 6. Diffraction scans as a function of the azimuthal direction φ for sample B. The intensity is normalized to the incident beam intensity, and the incidence angle is θi ) 55.0°.

On the other hand, Figure 4C shows the scan measured along φ ) -31°, i.e., in the disordered region, where a peak is clearly discernible around ∆K ) -0.5 Å-1, which can be associated with a half-integer peak. To investigate the presence of half-integer peaks, sample A was replaced by a second crystal whose surface was prepared with the same procedure. Two diffraction scans measured on this sample, hereafter referred to as sample B, are plotted in Figure 6. The peaks are very sharp, suggesting that a single crystallite contributes to the scattered intensity. Moreover, small peaks that can be associated with half-integer peaks are clearly detected around the specular peak. In contrast with sample A, along φ ) 0°, the integer diffraction peaks were only detected up to the fourth order. The same fitting procedure as for sample A was performed, and in Figure 7 the peak positions are reported in reciprocal space with the reciprocal surface net determined by X-ray diffraction. The picture is similar to that of sample A, but because of the overall smaller intensity, the number of detected peaks is less than for sample A. Instead, structures that can be (22) We also tested the possibility of a structural phase transition at low temperature by performing X-ray measurements below 100 K. The data analysis shows that the 3D structure of the guanidinium methanesulfonate remains the same at this low temperature.

related to half-integer peaks were clearly detected around the specular peak in a broad angular region because of the above-mentioned out-of-plane resolution, which is low near the specular peak. These results clearly show that two different surfaces are observed by He diffraction. On the other hand, the assignment of one set of diffraction patterns to the MT or HB surface requires a calculation of the elastic intensities. This quantitative calculation is not feasible, since a detailed knowledge of the He-surface interaction potential is needed and a very high corrugation, especially in the case of MT surfaces, is expected, which gives convergency problems in the solutions of the close-coupled multichannel scattering problem. To overcome these points, we used a different approach, investigating the vibrational dynamics of both surfaces through the thermal attenuation of the diffracted peaks. In fact, diffraction peak intensities decrease as a function of the sample temperature because of the thermal vibrations that reduce the coherence of the lattice, and the surface vibrational spectrum of the HB surface should be different from that of the MT surface. In Figures 8 and 9, polar scans measured along φ ) 0° in the temperature range between 40 and 100 K are shown for samples A and B, respectively. The peak intensity decreases, and the inelastic background increases. To perform a quantitative analysis, the integrated intensities of the peaks were estimated by a fitting procedure. The reliability of the fit parameters depends on the ratio of the peak intensity to the subtracted background, and in

5556

Langmuir, Vol. 18, No. 14, 2002

Bracco et al.

Figure 9. Diffraction scans measured on sample B along φ ) 0° at different sample temperatures.

Figure 11. Integrated intensity of the peaks shown in Figure 9: specular (squares), first-order (up triangles), second-order (down triangles), and third-order (circles) peaks. The solid lines are linear fits for each peak.

the incident beam intensity. The Debye-Waller exponent 2W is

2W ) ∆kz2〈uz2〉 + ∆K2〈U2〉

(3)

where ∆kz2 is the perpendicular momentum transfer to the surface and 〈uz2〉 and 〈U2〉 are the perpendicular and parallel mean square displacements in the scattering plane. In the case of He scattering, eq 2 is not valid, and corrections have been proposed to account for the presence of an attractive well that accelerates the impinging atoms toward the surface,23 the size of the incident atom,24 long interaction time with respect to vibrational periods,25 and correlations among surface atomic displacements.26 Generally the first correction is the most important and will be the only one we will take into account. The correction modifies the perpendicular momentum exchange in the following way: Figure 10. Integrated intensity of the peaks shown in Figure 8: specular (squares), first-order (up triangles), second-order (down triangles), and third-order (circles) peaks. The solid lines are linear fits for each peak.

what follows, the analysis was limited to peaks whose integrated intensities have relative error bars lower than 50%. In particular, up to 100 K, the specular peak and first-order peaks for both samples gave relative error bars within 10% and 20%, respectively, whereas only the second- and third-order peaks fulfilled the above requirement at least up to 80 K. Half-integer peaks measured on sample B were too small to get reliable results above 40 K. In Figures 10 and 11, the integrated intensities for the analyzed peaks are shown on a logarithmic scale. In the case of X-ray or neutron scattering, for which the interaction is localized or fast, the decreasing of the diffracted intensities can be well described by a Debye-Waller factor

I ) I0P exp(-2W)

(2)

where I and P are the intensity and the diffraction probability of a diffraction peak, respectively, while I0 is

∆kz ) ki(xcos2(θf) + /Ei + xcos2(θi) + /Ei)

(4)

where  is the well depth of the He-surface potential V and Ei is the incident beam energy. In harmonic approximation, i.e., using a Debye model, the mean square displacements are proportional to the crystal temperature T, and the linear trend of Figures 10 and 11 is in agreement with this prediction. The lines shown in both figures are linear fits to the points and the slopes

σ)-

(

)

d〈uz2〉 d〈U2〉 d(2W) ) - ∆kz2 + ∆K2 dT dT dT

(5)

are displayed in Figure 12. Data relative to sample A (sample B) show a weak (strong) dependence on ∆K2; therefore, d〈U2〉/dT is greater for sample B with respect to A, since for the specular peak (∆K ) 0 Å-1) the same value is found in both cases. The vibrational modes of the (23) Beeby, J. L. J. Phys. 1971, C4, L359. (24) Hoinkes, H.; Nahr, H.; Wilsch, H. Surf. Sci. 1972, 33 516; 1973, 40, 457. (25) Levi, A. C.; Suhl, H. Surf. Sci. 1979, 88, 221. (26) Armand, G.; Manson, J. R. Surf. Sci. 1979, 80, 532.

Guanidinium Methanesulfonate Crystal

Langmuir, Vol. 18, No. 14, 2002 5557

find, therefore, that both the structural and vibrational properties can be made consistent with the present findings if we assume that the HB surface terminates sample A and the MT surface terminates sample B. The experimental evidence that fewer peaks were detected on sample B than on sample A does not conflict with the expected larger corrugation of the MT surface. In fact, the smaller relative intensity can be ascribed to a larger parallel mean square displacement, which reduces exponentially the intensity with increasing ∆K values (eqs 2 and 3) because of thermal motion. 4. Conclusions

Figure 12. Slope σ calculated by the fits of Figure 10 for sample A (squares) and Figure 11 for sample B (circles).

MT surface, which mainly involve bending modes of the methyl groups that are more than 7 Å apart from each other, should be softer with respect to the correlated motions between nearest-neighbor atoms in the more rigid net of the hydrogen-bonded plane. Moreover, measurements of the thermal decay of He diffracted intensities carried out for a self-assembled monolayer (SAM) of CH3(CH2)21SH chemisorbed on Au(111)27 yield an estimated value d〈U2〉/dT ) (7.2 ( 2.4) × 10-4 Å2/K. Like the MT (001) plane of guanidinium methanesulfonate, the topmost layer of these SAMs is made of methyl groups. The well depth of V for the SAM was estimated to be  ) 6.8 meV. Assuming the same value for the MT (001) guanidinium methanesulfonate surface, the data of sample B, which show a dependence on ∆K2, yield d〈U2〉/dT ) (5.0 ( 1.1) × 10-4 Å2/K.28 This value is very near the value obtained for the SAM of CH3(CH2)21SH. On the other hand, the corrugation of the MT surface is larger than that of the HB surface because of the methyl groups that protrude perpendicularly from the hydrogen-bonded surface. Moreover, the methyl hydrogens in GMS have two different equilibrium positions,15 and at this low temperature, nearest-neighbor methyl groups can be locked in two different positions. This alignment can double the periodicity, resulting in detection of half-integer peaks. We (27) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Putvinski, T. M.; Scoles, G. J. Chem Phys. 1991, 94, 8493. (28) In the case of CH3(CH2)16SH, a well depth of 8.07 meV was estimated.27 Assuming this value for  of the MT surface, the calculation of the parallel mean square displacement derivative yields d〈U2〉/dT ) (6.0 ( 1.3) × 10-4 Å2/K.

The (001) hydrogen-bonded and methyl-terminated surfaces of guanidinium methanesulfonate were studied by means of He diffraction. The data show that both surfaces can be exposed during the cleavage of the crystal. This was confirmed by atomic force microscope measurements, which revealed that the step height between different crystallites is not simply a multiple of the 3D unit cell height of about 10 Å;29 therefore, both hydrogenbonded and methyl-terminated regions can coexist on the same surface. The diffraction pattern and in particular the thermal decay of the diffracted intensities allowed discrimination between the two surfaces. Both surfaces have the same measured periodicity, which is in good agreement with that determined by X-ray measurements for the three-dimensional unit cell. The differences between the two surfaces are related to the presence of the half-integer peaks (in particular around the specular peak), which are detected only for the MT surface because of a probable alignment of the methyl groups. Moreover, the parallel mean square displacement increases more quickly with temperature for the MT surface. This can be related to the soft in-plane modes of the methyl groups. In fact, the measured derivative d〈U2〉/dT is equal within error bars to the value previously obtained for the SAM of alkanethiols on Au(111). The sensitivity of He diffraction to the two types of (001) surfaces suggests that each surface can be unambiguously identified to the extent that the adsorption of molecular species on the different surfaces can be examined by He diffraction. Acknowledgment. We gratefully acknowledge Ying Hu for the atomic force microscopy measurements and Loredana Casalis for fruitful discussions. G.B. acknowledges the financial support from the INFM unit of Genova (Italy). This work was supported by the Department of Energy under Grant DE-FG02-93ER45503 from BES Material Science Division. LA025629P (29) In fact, step heights of 10, 14, 16, and 19 Å were observed. Within the error bars, only the first value and the last one can correspond to multiples of the unit cell height.