CH3Br Structures on Pt(111): Kinetically Controlled Self-Assembly of

CH3Br Structures on Pt(111): Kinetically Controlled Self-Assembly of Weakly Adsorbed Dipolar Molecules. Todd C. Schwendemann, Indraneel Samanta, Tobia...
0 downloads 0 Views 1MB Size
Download PDF
J. Phys. Chem. C 2007, 111, 1347-1354

1347

CH3Br Structures on Pt(111): Kinetically Controlled Self-Assembly of Weakly Adsorbed Dipolar Molecules Todd C. Schwendemann, Indraneel Samanta, Tobias Kunstmann, and Ian Harrison* Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904-4319 ReceiVed: September 16, 2006; In Final Form: NoVember 13, 2006

Methyl bromide is a rodlike molecule with a large 1.8 D dipole moment that physisorbs on Pt(111) with its Br end preferentially bound to the surface. The kinetically controlled self-assembly of several ordered CH3Br structures following molecular adsorption at a surface temperature of 20 K and various annealing procedures is revealed by scanning tunneling microscopy (STM). Thermal programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) provide complementary evidence for kinetic control over the CH3Br monolayer structures at temperatures under ∼100 K. A well-ordered (6 × 3) CH3Br monolayer can be formed by dosing CH3Br multilayers at 20 K and slowly annealing the surface to 104 K over several minutes. The unit cell contains four CH3Br molecules assigned to adsorption at Pt(111) top sites and 3-fold hollow sites in equal measure. The saturation coverage is 0.22 ML with respect to the Pt(111) areal density. RAIRS shows that CH3Br within the (6 × 3) monolayer is aligned along the surface normal while at lower coverages some molecules are tilted. The RAIRS signature of the ordered (6 × 3) CH3Br monolayer is the disappearance of the ν5 asymmetric CH3 deformational mode near 1411 cm-1 and the adsorption-site-induced splitting of the ν2 symmetric CH3 deformational mode into a doublet at 1271 and 1277 cm-1 with full width at half maximums (fwhm’s) of just 3 cm-1. Other ordered structures and molecular aggregates were observed by STM at submonolayer coverages following particular annealing procedures even though TPD shows evidence for only repulsive molecular interactions. The range of structures observed for the relatively simple CH3Br/ Pt(111) system suggest it may be a useful proving ground for theoretical treatments of the kinetics of selfassembly of dipolar molecules at surfaces.

I. Introduction Although currently being phased out industrially because of its deleterious effects on the stratospheric ozone layer,1 methyl bromide has long been used as a methylating agent in organic chemistry2 and as a fumigant and biocide in agriculture. On surfaces, methyl halides are useful precursor species for delivering CH3 radicals and halogens to surfaces3 and also as model adsorbates for studies of surface reactivity,4 photochemistry,5 and photoinduced electron-transfer chemistry.6 Methyl bromide has been a particularly important polyatomic adsorbate for advancing our understanding of surface photochemistry on insulators,7,8 semiconductors,9,10 and metals.11-14 Unfortunately, detailed structural information about adsorbed CH3Br, that would be most helpful in rigorously interpreting and predicting its photochemical dynamics, has been largely lacking. The traditional structural probes of surface science such as lowenergy electron diffraction (LEED) or near-edge X-ray fine structure15 are difficult to apply to CH3Br because of the ease by which this molecule falls apart via dissociative electron attachment whenever low-energy secondary electrons are present. Consequently, structural information about adsorbed CH3Br has been derived primarily from helium beam scattering,16 photofragment angular distributions,9,12,13 and reflection absorption infrared spectroscopy (RAIRS).13,17 In this study, scanning tunneling microscopy (STM) with picoamp currents at low surface temperatures is employed to directly examine the ordering and structural behavior of CH3Br on Pt(111). Several * To whom correspondence should be addressed. Tel.: (434) 924-3639. Fax: (434) 924-3710. E-mail: [email protected].

ordered CH3Br structures are identified, and kinetic limitations on the molecular ordering behavior are observed. The nanoscale structure, orientation, and self-assembly of adsorbed dipolar molecules is a subject of considerable interest. Self-assembly of physisorbed CH3Br on Si(111)-(7×7) followed by adsorbate photochemistry has recently been shown to chemically imprint Br photofragments onto atomically precise locations of the underlying surface.10 The resulting nanofabricated Br/Si(111) pattern is stable to 600 K but owes its existence to the initial self-assembly of physisorbed CH3Br at 50 K. Although low-temperature STM was used to directly visualize these nanopatterns, the imprinting process was made to occur over the entire ultraviolet irradiated surface and so might be useful as a practical nanofabrication technique.18 The ability to flip the orientation of adsorbed CH3Br as a function of O coverage on O/Ru(001) surfaces has enabled fundamental studies of the steric effect in the dissociative electron attachment of electrons to an adsorbate (i.e., the electron/ molecule approach geometry is important).14,19 Helium beam scattering indicates that CH3Br antiferroelectrically orders on C(0001) graphite, NaCl(001), and LiF(001) surfaces.16 Theoretical analysis of the surface ordering of dipolar molecules, such as CH3Br on MgO,20 indicates a variety of ordered antiferroelectric, ferroelectric, and disordered phases are possible for different values of the coverage, temperature, adsorbate binding energy, and dipole moment. Understanding 2-D dielectric ordering behavior is also deemed important to the development of next generation nonvolatile memory and electronic devices based on ferroelectric thin films.21,22

10.1021/jp066062e CCC: $37.00 © 2007 American Chemical Society Published on Web 01/03/2007

1348 J. Phys. Chem. C, Vol. 111, No. 3, 2007 Methyl bromide has a substantial dipole moment of 1.8 Debye (D) and physisorbs, or very weakly chemisorbs, to Pt(111) through the more polarizable Br end of the molecule.17,23,24 The adsorption energy falls from 60 to 32 kJ/ mol as the coverage increases from 0 to the saturation coverage of 0.25 ( 0.02 ML13 (1 monolayer (ML) is the 1.5 × 1015 cm-2 areal density of the Pt(111) atoms). Only about one-third of this adsorption energy decrease can be attributed to dipolar repulsions, the remainder is attributed to the occupation of different adsorption sites or structures and electronic tempering of the surface. Temperaturedependent RAIRS experiments employing pre- or post-dosed CO, O2, and CD3Br to block and monitor different adsorption sites established that CH3Br preferentially adsorbs on Pt(111) top sites up until coverages of θ ∼ 0.1 ML and only at higher coverages do multifold adsorption sites begin to become occupied. At submonolayer coverages, both the CH3 photofragment angular distributions from the 193 nm photoinduced dissociative electron attachment (DEA) to physisorbed CH3Br and RAIRS indicate that the molecules tend to lie with the C-Br axis closer to the surface plane at low coverage (e.g., 42° by RAIRS at 0.1 ML) and tip toward the surface normal as the coverage increases (e.g., 30° by RAIRS at 0.18 ML). However, the results from these ensemble averaged techniques do not quantitatively agree with one another. Both RAIRS and photofragment angular distributions indicate that CH3Br diffusion is suppressed when adsorbed at a surface temperature of 20 K. In earlier work,13 attainment of equilibrium within the submonolayer was presumed to be achieved by annealing to 85 K, a temperature about 15 K below where a full monolayer would begin to desorb and a temperature that led to reproducible experimental results. As will be detailed below, even more aggressive annealing procedures are required to approximate thermal equilibrium conditions because the CH3Br ordering/ diffusion kinetics are surprisingly slow. The purpose of the current STM study is to provide a consistent microscopic picture of the CH3Br surface behavior and ordering phenomena that can aid in the interpretation of its photochemical dynamics and RAIRS spectra. II. Experimental Section STM experiments were performed in an ultrahigh vacuum (UHV) chamber with a working pressure of 4 × 10-11 Torr maintained by a 640 L/s ion pump and a titanium sublimation pump. The Pt(111) crystal (5 mm dia × 2 mm thick) was cooled by a flexible copper braid connection to an Oxford Instruments liquid helium cryostat and was heated by electron bombardment from a 2 mm coiled tungsten filament located directly behind the crystal. The sample temperature was monitored by a Eurotherm 9000 temperature controller connected to an alumel/ chromel thermocouple spotwelded to the sample. The temperature could be varied from 20 K to greater than 1250 K and was calibrated based on thermal programmed desorption (TPD) of CO multilayers as described by Schlichting and Menzel.25 Surface analytical techniques employed in the STM chamber were as follows: STM via a home-built, variable temperature, Besocke-beetle style STM driven by RHK electronics, Auger electron spectroscopy (AES) using a cylindrical mirror analyzer, and TPD to a twice differentially pumped quadrupole mass spectrometer. Further details of the STM chamber have been described elsewhere.26 STM images were typically obtained with currents in the 10-200 pA range and sample bias voltages, VB, were kept within a (2.5 V range to avoid dissociative electron attachment to CH3Br. Image acquisition times were typically 30 s, and the in-plane thermal drift at low temperatures was ∼4 Å/min.

Schwendemann et al.

Figure 1. TPD spectra (m/e ) 95 amu) of  ) 0.35 ML exposures of CH3Br dosed on to Pt(111) at 20 K, without and with slow annealing to 104 K over several minutes. The R, β, and γ TPD peaks of the monolayer remain following annealing, and the coverage within the R peak increases slightly.

RAIRS and TPD experiments were performed in a separate UHV chamber with a working pressure of 6 × 10-11 Torr maintained by a titanium sublimation pump and a 240 L/s turbopump backed by a diffusion pump. The Pt(111) sample (15 mm diameter × 2 mm) was cooled by a Cu braid connection to a closed cycle helium refrigerator and heated by electron bombardment to achieve a temperature range from 18 K to greater than 1250 K. The RAIRS chamber13 has RAIRS, AES, TPD, LEED, and X-ray photoelectron spectroscopy (XPS) capabilities. The Pt(111) samples were cleaned by Ar+ ion sputtering at 800 K followed by a high temperature annealing at 1100 K for 5 min. After Ar+ sputtering, the cleanliness of the surface was monitored by AES. Once the surface was initially free of carbon and other contaminates, the subsequent day-to-day cleaning of the surface could typically be done by oxidation of the surface at 700 K in ∼5 × 10-8 Torr O2 for 5 min. Following oxidative cleaning, a final oxygen TPD was taken and the ratio of the oxygen recombinative desorption peak (∼800 K) to the molecular oxygen desorption peak (∼150 K) was used to confirm the surface cleanliness before dosing the molecules of interest. Methyl bromide, UHP grade from Matheson Gases, was purified by several freeze/pump/thaw cycles using an acetone/ dry ice bath and stored in a stainless steel bottle on a gas manifold that led through a leak valve to a capillary array (or cosine effusive doser) pointed directly at the sample surface. A trapped volume was filled to a pressure measured by a Baratron capacitive manometer and then evacuated though a leak valve to dose the sample in a reproducible manner (ca. 2-5%). Gas exposures were calibrated using TPD of CO dosed at 20 K where the CO sticking coefficient is assumed to be unity and the chemisorbed CO coverage saturates at 0.50 ML.27 In this report, the Pt(111) sample was typically dosed with CH3Br at 20 K and slowly annealed to 104 K over several minutes prior to STM and RAIRS experiments at low temperature. III. Results and Discussion A. Thermal Programmed Desorption (TPD). The TPD spectra of Figure 1 illustrate the thermal behavior of CH3Br on Pt(111). Briefly summarizing earlier work,13 small coverages of CH3Br yield a γ TPD peak at 230 K which broadens smoothly to lower temperature with increasing coverage to yield the β TPD peak at 155 K. Additional submonolayer coverage

CH3Br Structures on Pt(111)

Figure 2. STM image of a CH3Br monolayer formed by dosing multilayers and annealing to 98 K. Within the ordered monolayer there are defect structures labeled A and B. Circle A contains a single missing molecule defect, while circle B contains a “dog bone” defect corresponding to two molecules missing from the lattice (245 Å × 245 Å image, I ) 100 pA, VB ) +300 mV).

populates the R TPD peak at 120 K which is kinetically slow to complete when CH3Br is dosed at 20 K and the surface temperature is ramped at 2 K s-1 in TPD. Further CH3Br exposure at 20 K populates a transitional feature, δ, that appears at 103 K in the TPD spectrum before multilayers, M, are populated that desorb at 100 K. The 20 K sticking coefficient of CH3Br is constant and assumed to be unity up until exposures greater than 0.23 ML that lead to the appearance of the δ peak in TPD spectra and a concurrently reduced sticking coefficient. The postannealing TPD spectrum of Figure 1 shows that by dosing CH3Br multilayers at 20 K and slowly annealing the CH3Br to 104 K over several minutes it is possible to further populate and complete the R TPD peak of the CH3Br monolayer and avoid populating the surface with any residual overlayer molecules. B. Molecular Orientation within the CH3Br Monolayer. The orientation of CH3Br within the annealed monolayer could be established by examination of the monolayer’s defect structures by STM and by the monolayer’s RAIRS spectrum. B.1. Scanning Tunneling Microscopy (STM). Initial STM images of an ordered CH3Br monolayer were observed by dosing multilayers and annealing slowly to 95 K over several minutes to anneal away the overlayer. The resulting images obtained at 30 K showed patches of ordered structures, but there was substantial nonuniform noise consistent with the presence of loosely bound molecules diffusing and aggregating randomly on the surface. Accordingly, TPD following STM images of samples prepared in this manner showed the presence of a small multilayer/δ peak at 103 K. After some experimentation with RAIRS, STM, and TPD it was found that annealing multilayers slowly to 104 K over several minutes gave the most perfectly ordered CH3Br monolayer with a minimum of the loosely bound overlayer molecules that can degrade STM imaging. Figure 2 shows a constant current STM topographic image of a fairly well-ordered CH3Br monolayer that contains several kinds of defect structures. The molecules appear as bright dots, and the underlying Pt(111) lattice cannot be simultaneously resolved. The van der Waals size of the molecules (∼3.8 Å

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1349

Figure 3. RAIRS spectra of a well-annealed CH3Br submonolayer and monolayer. The absence of the ν5 mode near 1410 cm-1 in the ordered monolayer spectrum requires that all molecules within the monolayer are oriented along the surface normal, unlike their behavior in submonolayers. The splitting of the monolayer ν2 band into two sharp symmetric peaks at 1270 and 1277 cm-1 (3 cm-1 fwhms), and a small satellite peak at 1288 cm-1, provides evidence for CH3Br adsorption at primarily two different sites or local environments, in equal measure.

diameter × 6.4 Å long), the substantial surface work function drop as the CH3Br monolayer is formed,24 and an earlier RAIRS study13 argue that the molecules are oriented close to the surface normal and bind with the Br end against the surface. The STM image is consistent with these expectations. Nevertheless, it is conceivable that CH3Br could image either as a dot-like projection if the C-Br axis is oriented along the surface normal or possibly as two distinct projections if the molecule lies in the surface plane and tunneling is enhanced through both the CH3 and Br ends of the molecule. In the latter case, “dog bone” defects involving the loss of two bright dots as illustrated in circle B of Figure 2 should be the minimal building block of any more complicated defects. However, as can be seen in circle A of Figure 2, a defect involving the loss of a single bright dot is also possible and so we assign the dots to individual standingup molecules. Furthermore, if the molecules were lying down, then one might expect to see a differential change in the recorded topography as a function of the biasing voltage for tunneling into either the CH3 or Br ends of the molecule. In practice, the topographic images were almost identical over bias voltages from +1.8 to -1.8 V. The highest resolution molecular images were obtained at very low bias voltages ((50 mV), and biases greater than ( 2.5 V were found to lead to fragmentation of the molecules, presumably by dissociative electron attachment. B.2. Reflection Absorption Infrared Spectroscopy (RAIRS). Figure 3 compares 2 cm-1 resolution RAIRS spectra for submonolayer and multilayer exposures of CH3Br on Pt(111) dosed at 20 K and annealed to 104 K over several minutes. The multilayer annealing procedure yields the annealed TPD trace of Figure 1 and best prepares the well-ordered monolayer observed in Figure 2. Important for making assignments of the CH3Br orientation are the ν2 and ν5 “CH3” deformational modes which have dipole derivatives parallel and perpendicular to the molecular C-Br axis, respectively.28 The presence of both the ν2 band at 1278 cm-1 and the ν5 band at 1412 cm-1 in the submonolayer RAIRS spectrum is evidence that not all molecules are lying either parallel or perpendicular to the surface normal. This follows because the IR electric field in RAIRS on metals is directed only along the surface normal, and hence,

1350 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Schwendemann et al.

TABLE 1: RAIRS Active Modes of Methyl Bromide (cm-1) ν2 deformation (| to C-Br axis); (fwhm) gas phase multilayer 3-fold hollow 1305.1

1300

top site

satellite

1270 (3 cm-1) 1277 (3 cm-1) 1288 (4 cm-1) 1268 -trace- 1278 (8 cm-1)

only modes with a dipole derivative that have a nonzero projection on the surface normal (electric field) will be RAIRS active. Consequently, the submonolayer RAIRS spectrum indicates that at this coverage some molecules may lie down while some may stand up, or the molecular angular distribution may be broad or peaked toward an angle tilted away from the surface normal. The disappearance of the ν5 band in the monolayer RAIRS spectrum requires that all the molecules must be standing up and oriented along the surface normal within the monolayer. The annealed monolayer RAIRS shows no evidence for overlayer molecules that would have given rise to additional absorption peaks (e.g., another ν2 peak at 1300 cm-1).13 The splitting of the ν2 band of the monolayer into two sharp symmetric peaks at 1270 and 1277 cm-1, and a small satellite peak at 1288 cm-1, provides evidence for CH3Br adsorption at primarily two different sites or local environments. The ν2 mode of gas-phase CH3Br is nondegenerate, and so the symmetric RAIRS splitting on the surface indicates that two different local environments are populated in equal measure. The integrated absorbance of the smaller 1288 cm-1 satellite peak is only 6% of the total for the ν2 band. Since the defect density of the Pt(111) surface has been titrated by CO RAIRS as less than 1%, it seems likely that the 6% satellite RAIRS peak does not derive from molecules adsorbed at Pt(111) defects but rather from molecules at the boundaries of ordered domains or other defects in the CH3Br monolayer lattice. The sharpness of the ν2 band peaks with 3 cm-1 full width at half-maximums (fwhm’s) argues for the structural homogeneity of the monolayer structure. An earlier RAIRS study13 performed with CH3Br annealing to 85 K and a lower 4 cm-1 resolution never observed the ν5 band to disappear as a function of coverage, nor could the ν2 band splitting be completely resolved as seen in Figure 3. However, the ν2 band was clearly observed to be split at high submonolayer coverage, and line shape analysis in conjunction with pre- and post-dosing of CD3Br, CO, and O2 made possible an assignment of the CH3Br adsorption to different Pt(111) sites on the basis of the ν2 band peak positions. It was found that CH3Br preferentially occupies top sites at low coverage up to ∼0.1 ML, and multifold, presumably 3-fold hollow, sites become increasingly occupied at higher coverage. Table 1 summarizes the RAIRS observations of Figure 3 and provides consistent site assignments. The weak satellite peak of the ν2 band is relatively little perturbed in frequency away from CH3Br in multilayers or in the gas-phase, suggesting that the associated molecules may be relatively loosely bound (e.g., at domain walls). The top and 3-fold hollow site ν2 peaks are increasingly perturbed in frequency consistent with increasing electron back-donation from the surface into the molecule’s antibonding lowest unoccupied molecular orbital. The ν1 C-H symmetric stretch frequency of the tilted molecules in the submonolayer is substantially lower than that for the standing-up molecules of the monolayer, and again it may be that more efficient electron back-donation into the tilted molecule lowers the C-H vibrational frequency. An alternative explanation for the ν2 splitting may be possible on the basis of dipolar or other coupling of the vibrations between molecules occupying different sites

ν5 deformation (⊥ to C-Br axis) ν1 C-H sym. stretch exposure annealed 1412 (15 cm-1)

2958 (3 cm-1) 2922 (7 cm-1)

0.35 ML 0.19 ML

within the unit cell.29 In this way, the satellite peak might derive from an asymmetrically phased combination of ν2 vibrational modes on different molecules within the unit cell. C. Structure of the (6 × 3) CH3Br Monolayer. A high resolution, constant current STM image of the ordered CH3Br monolayer formed by dosing multilayers and slowly annealing to 104 K over several minutes is shown in Figure 4. The bright, gold-colored spots are assigned as standing-up molecules on the basis of their defect pattern and RAIRS spectrum as described above. The molecules are arranged in a modulated hexagonal pattern around either round or elliptical depressions. The lattice’s most easily seen distinguishing feature is the alternating diagonal rows of round and elliptical depressions that run from top left to bottom right across the STM image. A few of these characteristic round and elliptical depressions are highlighted in green and blue, respectively, in the upper right corner of Figure 4a. The A defects of Figure 2 can be seen to derive from removing any single molecule from the lattice, and the dog bone B defects derive from removing any two molecules facing one another across a depression. The minimal repetitive unit cell for the CH3Br monolayer can be seen to be fairly large. Although it was not possible to simultaneously resolve the CH3Br and Pt(111) lattices, we tentatively assign the CH3Br monolayer as a (6 × 3) pattern. The STM length scale was calibrated by separately imaging an O p(2 × 2) layer on Pt(111) at 30 K. Figure 5 provides a comparison of a theoretical model of a (6 × 3) CH3Br overlayer on Pt(111) with a corresponding highresolution STM image. Important in assigning this structural model were constraints introduced by RAIRS. We assume that the symmetric splitting of the ν2 band in the monolayer RAIRS of Figure 3 indicates that there are two kinds of CH3Br local environments populated in equal measure. These environments are assigned to CH3Br adsorption at Pt(111) top and 3-fold hollow sites on the basis of earlier RAIRS work as indicated in Table 1. The proposed unit cell can be seen to contain four molecules, with the molecules labeled 1 and 2 being on top sites and those labeled 3 and 4 being on 3-fold hollow sites. The STM image of Figure 5b shows that molecules at top sites image slightly higher than those on 3-fold hollow sites. The saturation coverage of the (6 × 3) monolayer is 0.22 ML which compares with the 0.25 ( 0.02 ML value calculated by French and Harrison13 on the basis of TPD and the assumption of an initial sticking coefficient of unity for both CO and CH3Br on Pt(111) at 20 K. Livneh and Asscher determined the saturation coverage of CH3Br on Ru(0001) to be 0.22 ML (i.e., the CH3Br/Ru ratio) using TPD in conjugation with surface passivation by pre-adsorbed D atoms.30 Because the face-centered cubic (fcc) Pt(111) and hcp Ru(001) surfaces are both hexagonal and their atom spacings differ by just 4% it may be that similar CH3Br adsorption structures are initially formed on both surfaces. Unlike on Pt(111), CH3Br begins to dissociate on Ru(001) at Ts g 125 K. Figure 6 illustrates how the (6 × 3) lattice of upright CH3Br molecules leads to the distinctive rows of round and elliptical depressions within the monolayer STM images. D. Submonolayer CH3Br Structures. When multilayers of CH3Br were dosed at 20 K and then quickly flashed to ∼115

CH3Br Structures on Pt(111)

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1351

Figure 5. (a) Model of the ordered CH3Br monolayer (red) on top of a Pt(111) atom lattice (green). (b) High-resolution STM image of the ordered CH3Br monolayer with a proposed grid of Pt(111) atoms overlaid. The proposed (6 × 3) unit cell is outlined and a few of its constituent molecules are labeled. Figure 4. High-resolution STM image of the ordered CH3Br monolayer displayed (a) as a 2-D contour map and (b) in 3-D. The 40 Å × 40 Å image was taken at taken at Ts ) 30 K, I ) 10 pA, and VB ) -10 mV. In the upper right corner of (a), several of the characteristic “circular” depressions are highlighted in green and several “elliptical” depressions are highlighted in blue. Outlined in white is a proposed (6 × 3) unit cell for the molecular lattice whose unit vectors of lengths 16.8 Å and 8.5 Å have an angle of 61° between them.

K to remove the overlayers and some of the R submonolayer TPD peak at 120 K, it was possible to see some interesting additional structures. Figure 7a shows a “square” looking lattice with a nominal local coverage of 0.12 ML. The average spacing between rows of molecules is 6.9 Å in one direction and 8.0 Å in the other. The angle between the rows is 82°. Figure 7b shows that the square lattice can coexist with the (6 × 3) hexagonal or “hex” looking lattice of the ordered monolayer with a saturation coverage of 0.22 ML. TPD analysis suggests that interactions between the dipolar CH3Br molecules are purely repulsive and the adsorption energy (Ead) falls monotonically from 42 kJ/mol at 0.12 ML to 29 kJ/mol at 0.22 ML.13 Only about one-third of this reduction in Ead can be attributed to electrostatic repulsion, and it is over this range of coverage that steric interactions between neighboring CH3Br molecules are increasingly constraining and the less favorable 3-fold hollow sites begin to become occupied. The reduction in Ead with increasing coverage could be attributed to several causes other than simply increasing the fractional representation of the hex rather than the square CH3-

Br lattice phase. The entropy of the more loosely packed square lattice is doubtlessly greater than the more closely packed (6 × 3) hex lattice whose molecules are strictly constrained to orient along the surface normal. On both enthalpic and entropic grounds, the Gibbs free energy of the square lattice should be less than that for the hex lattice. Consequently, in areas where space permits, the square lattice should be more thermodynamically stable. On the other hand, the square lattice was not observed by STM when CH3Br submonolayers, rather than multilayers, were dosed and annealed. It is not known whether having an overlayer of dipolar molecules above the monolayer affects the structure of the underlying monolayer. It could be that the square lattice can coexist only metastably with the hex lattice, and it is the kinetics of the multilayer desorption and rapidly changing local electrostatics that allow the square lattice to form. Figure 8 provides an STM image showing more localized coexistence of square- and hex-like CH3Br lattices at slightly less than saturation coverage. Several structures and repetitive molecular groupings are seen in these images. What is not seen is evidence for sufficient repulsion between the molecules that the spacings between the molecules are maximized as is typically assumed in models of adsorbed dipolar molecules.31,32 Instead, the molecules are often found in relatively close-packed linear chains and 2D groupings. The 3-fold symmetry and pseudo-6-fold symmetry of the underlying fcc Pt(111) surface can be identified through the site occupation of CH3Br molecules in the lower coverage regions on the left side of the image (e.g.,

1352 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Schwendemann et al.

Figure 6. (a) Model of the ordered CH3Br monolayer (red) on top of a Pt(111) atom lattice (green) in which the repetitive molecular patterns framing the round and elliptical depressions of the high-resolution STM image of (b) are highlighted.

linear chains of CH3Br molecules coming to a vertex with angles of ∼120° or 60° between the chains). This behavior is consistent with the idea that at low coverage a single type of adsorption site is preferred (i.e., the top site). Given the requirement of prolonged annealing for several minutes at 104 K to form the (6 × 3) hex monolayer, we assume that diffusion under the dipolar electrostatic fields of the monolayer is relatively slow and so Figure 8 may be providing a snapshot of the surface prior to establishment of a thermal equilibrium site occupancy. If this is the case, and desorption occurs preferentially from the more weakly bound CH3Br at 3-fold hollow sites, then this might lead to a greater occupancy of top sites in Figure 8. However, it is not our purpose here to analyze the slow diffusional behavior of CH3Br but rather to simply point out that its existence may complicate the analysis and interpretation of CH3Br experiments involving RAIRS, TPD, photochemistry, and so forth. Low-energy helium beam scattering would likely be ideal for further and faster kinetic studies of the diffusion and phase behavior of the CH3Br/Pt(111) system. E. Implications for Other Methyl Halide Adsorption Systems. Slow diffusion-limited ordering of methyl halides monolayers on metal surfaces may be relatively common, particularly at high coverage where repulsive electrostatic interactions will be most pronounced. In this study, we find that with sufficient annealing at a surface temperature of 104 K it is possible to obtain a relatively homogeneous (6 × 3) monolayer of CH3Br on Pt(111) with a coverage of 0.22 ML in which all the molecules are aligned along the surface normal, both top and 3-fold hollow sites are occupied in equal measure,

Figure 7. (a) STM image of a “square” ordered pattern of CH3Br formed after annealing away some of the monolayer by flashing the surface temperature to ∼115 K (142 Å × 142 Å Ts ) 30 K, I ) 100 pA, and VB ) 250 mV). (b) STM image showing coexistence of “square” and “hex” (6 × 3) ordered structures after a brief annealing with oscillations at 106 ( 3 K (335 Å × 500 Å; Ts ) 30 K, I ) 1 nA, and VB ) -2.01 V).

and the RAIRS spectral peaks are very narrow. The RAIRS spectral signature of the (6 × 3) monolayer is the splitting of the ν2 band and the disappearance of the ν5 band. In related RAIRS studies in which a saturation coverage of CH3Br13 or CH3I33,34 on Pt(111) was dosed or annealed at 90 K or less, the ν2 band was split but there remained sufficient ν5 band intensity to indicate that the average molecular tilt angle to the surface normal was 23-30°. Similar behavior was seen by RAIRS for CH3Cl on Pd(100),35 a system that also exhibits a structured TPD spectrum that displays M, R, β, and γ TPD peaks similar to those in Figure 1. Just as for CH3Br, it might be possible to further orient these methyl halide molecules toward the surface normal with higher temperature annealing/dosing. Indeed, the packing densities of methyl halides in their crystalline solids36 are much higher than the saturation values reported in two dimensions on metal surfaces. So, there is reason to believe that the van der Waals size of the molecules (e.g., a roughly 3.8 Å diameter × 6.4 Å long rodlike shape for CH3Br) does not limit the molecular packing on the surface but rather it is the electrostatics and surface bonding. An upright orientation for ferroelectrically oriented molecules affords the highest packing density, and so it seems likely that with careful slow annealing/dosing at relatively high temperatures it may be

CH3Br Structures on Pt(111)

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1353 molecules were alternately oriented CH3 up, CH3 down, and CH3 up over the first three monolayers. In contrast, here we have shown by TPD, RAIRS, and STM that R, β, and γ TPD peaks can be observed from a single, well-annealed (6 × 3) CH3Br monolayer on Pt(111) in which all molecules are oriented CH3 up and the saturation coverage is 0.22 ML. It would be interesting to determine if d∆Φ/dT < 0 as some of the β TPD peak desorbs for the CH3Br/Pt(111) system in analogy to the earlier observations for CH3Cl/Ru(001) to more firmly establish a commonality of methyl halide/metal surface interactions and to possibly more firmly correlate ∆Φ-TPD spectral features with adsorbate structural changes. IV. Summary

Figure 8. STM image of CH3Br submonolayer prepared by dosing multilayers, flashing to 107 K, and annealing at 98 K for several minutes. The average coverage is 0.20 ML. Three interesting structures are noted: box A shows an ensemble of molecules in rows containing three molecules each that begin to form a “square” lattice structure; box B shows some three-molecule rows alternately angling in and out forming a zigzag backbone; the arrow identifies a ring structure that has a small opening on one side, suggesting that the “hex” rings seen in the (6 × 3) monolayer are a stressed form that does not allow the molecules to occupy more favorable adsorption sites available at lower coverage (140 Å × 140 Å, Ts ) 30 K, I ) 200 pA, and VB ) +327 mV).

possible to generally prepare saturated methyl halide monolayers uniformly oriented along the surface normal. At less than saturation coverage, the repulsive interactions between molecules clearly observed in TPD should tend to keep molecules away from one another and the electrostatics of halogen-anchored molecules should further conspire to tip any two molecules away from each other. The observation of closely associated aggregates of CH3Br molecules in our STM images at less than saturation coverage therefore suggests that the molecular distributions were defined by the kinetics of their formation rather than equilibrium thermodynamics or that unanticipated short-range attractions between the molecules are present. Asscher and co-workers have examined the adsorption behavior of CH3Br and CH3Cl on Ru(001) using pressure monitored (∆p) and work function monitored (∆Φ) TPD.30,37 Methyl bromide dissociates on Ru(001) at 125 K whereas CH3Cl desorbs intact. The CH3Cl/Ru(001) system exhibits a structured ∆p-TPD spectrum similar to that in Figure 1 for CH3Br/Pt(111) although the multilayer M and R peaks coalesce. Most interesting is that the ∆Φ-TPD spectra shows d∆Φ/dT < 0 as molecules desorb over a substantial portion of the β TPD peak. Asscher assumed that the γ TPD peak correlates with a 0.22 ML saturation coverage of CH3Cl, the β peak correlates to the second monolayer, and the R peak with the third monolayer and subsequent multilayers. The reduction in the work function as molecules leave the surface during the β TPD peak was interpreted to indicate that (i) the molecules desorbing in the β peak (second monolayer) had been oriented with CH3 end pointing toward the surface or (ii) that there was substantial orientational disorder in the second monolayer that was being relieved. It was also speculated that the alternating sign of d∆Φ/ dT as the R, β, and γ TPD peaks desorbed might signal that the

STM, combined with TPD and RAIRS, was used to study the adsorption behavior of CH3Br on Pt(111). By dosing multilayers of CH3Br at 20 K and annealing to 104 K for several minutes, it was possible to prepare a (6 × 3) self-assembled monolayer of CH3Br in which all the molecules were oriented along the surface normal and bound through their Br against the surface. The (6 × 3) self-monolayer has a coverage of 0.22 ML, and half the molecules are assigned to top sites and the other half to more weakly binding 3-fold hollow sites. Earlier RAIRS studies of methyl halide adsorption on metals involving dosing or annealing at temperatures less than 90 K observed molecules somewhat tilted away from the surface normal (ca. 25°), presumably in metastable nonequilibrium configurations because the kinetics of self-assembly are slow at these low temperatures. Although TPD showed evidence for only repulsive interactions between molecules, STM images following a brief flash annealing of the (6 × 3) CH3Br/Pt(111) monolayer to 115 K that removed some molecules showed additional ordered structures and some aggregation of molecules that were likely formed under kinetic, rather than thermodynamic, control. The slow ordering kinetics observed for CH3Br/Pt(111) suggests that metastable structural traps are likely observable for other methyl halide/metal systems and also for water which, though bent, has the same 1.8 D dipole moment as CH3Br. We anticipate that methyl halides on metals may prove to be useful model systems for theoretical studies of the kinetics of self-assembly of weakly adsorbing dipolar molecules because of the diverse range of experimental observations that are possible. Acknowledgment. Financial support from the Department of Energy, Basic Energy Sciences Grant No. DE-FG0201ER15149 and the University of Virginia is gratefully acknowledged. References and Notes (1) McCauley, S. E.; Goldstein, A. H.; DePaolo, D. J. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 10006. (2) Schmatz, S. ChemPhysChem 2004, 5, 600. (3) Zaera, F. Prog. Surf. Sci. 2001, 69, 1. (4) Lin, J. L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 2849. (5) Zhou, X. L.; Zhu, X. Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73. (6) Marsh, E. P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. ReV. Lett. 1988, 61, 2725. (7) Bourdon, E. B. D.; Cowin, J. P.; Harrison, I.; Polanyi, J. C.; Segner, J.; Stanners, C. D.; Young, P. A. J. Phys. Chem. 1984, 88, 6100. (8) Lee, T. G.; Liu, W.; Polanyi, J. C. Surf. Sci. 1999, 426, 173. (9) Yang, Q. Y.; Schwarz, W. N.; Lasky, P. J.; Hood, S. C.; Loo, N. L.; Osgood, R. M. Phys. ReV. Lett. 1994, 72, 3068. (10) Dobrin, S.; Lu, X. K.; Naumkin, F. Y.; Polanyi, J. C.; Yang, J. S. Y. Surf. Sci. 2004, 573, L363. (11) Costello, S. A.; Roop, B.; Liu, Z. M.; White, J. M. J. Phys. Chem. 1988, 92, 1019. (12) Ukraintsev, V. A.; Long, T. J.; Harrison, I. J. Chem. Phys. 1992, 96, 3957.

1354 J. Phys. Chem. C, Vol. 111, No. 3, 2007 (13) French, C.; Harrison, I. Surf. Sci. 1997, 387, 11. (14) Lilach, Y.; Asscher, M. J. Phys. Chem. B 2004, 108, 4358. (15) Lasky, P. J.; Lu, P. H.; Yang, M. X.; Osgood, R. M.; Bent, B. E.; Stevens, P. A. Surf. Sci. 1995, 336, 140. (16) Robinson, G. N.; Camillone, N.; Rowntree, P. A.; Liu, G. Y.; Wang, J.; Scoles, G. J. Chem. Phys. 1992, 96, 9212. (17) Zaera, F.; Hoffmann, H.; Griffiths, P. R. J. Electron Spectrosc. Relat. Phenom. 1990, 54, 705. (18) Osgood, R. Surf. Sci. 2004, 573, 147. (19) Jorgensen, S.; Dubnikova, F.; Kosloff, R.; Zeiri, Y.; Lilach, Y.; Asscher, M. J. Phys. Chem. B 2004, 108, 14056. (20) Burns, T. E.; Dennison, J. R.; Kite, J. Surf. Sci. 2004, 554, 211. (21) Dearaujo, C. A. P.; Cuchiaro, J. D.; McMillan, L. D.; Scott, M. C.; Scott, J. F. Nature 1995, 374, 627. (22) Fong, D. D.; Stephenson, G. B.; Streiffer, S. K.; Eastman, J. A.; Auciello, O.; Fuoss, P. H.; Thompson, C. Science 2004, 304, 1650. (23) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184, L325. (24) White, J. M. In Chemistry and Physics of Solid Surfaces VIII; Vanselow, R., Howe, R., Eds.; Springer: New York, 1990; p 29.

Schwendemann et al. (25) Schlichting, H.; Menzel, D. ReV. Sci. Instrum. 1993, 64, 2013. (26) Xu, H.; Yuro, R.; Harrison, I. Surf. Sci. 1998, 411, 303. (27) Nekrylova, J. V.; French, C.; Artsyukhovich, A. N.; Ukraintsev, V. A.; Harrison, I. Surf. Sci. 1993, 295, L987. (28) Herzberg, G. Molecular Spectra and Molecular Structure II. Infrared and Raman Spectroscopy of Polyatomic Molecules; van Nostrand Reinhold Company: New York, 1945. (29) Davydov, A. S. Theory of Molecular Excitons; McGraw-Hill Book Co.: New York, 1962. (30) Livneh, T.; Asscher, M. J. Phys. Chem. B 1997, 101, 7505. (31) Albano, E. V. J. Chem. Phys. 1986, 85, 1044. (32) Maschhoff, B. L.; Cowin, J. P. J. Chem. Phys. 1994, 101, 8138. (33) French, C.; Harrison, I. Surf. Sci. 1995, 342, 85. (34) Fan, J. F.; Trenary, M. Langmuir 1994, 10, 3649. (35) Berko, A.; Erley, W.; Sander, D. J. Chem. Phys. 1990, 93, 8300. (36) Kawaguchi, T.; Hijikigawa, M.; Hayafuji, Y.; Ikeda, M.; Fukushima, R.; Tomiie, Y. Bull. Chem. Soc. Jpn. 1973, 46, 53. (37) Livneh, T.; Lilach, Y.; Asscher, M. J. Chem. Phys. 1999, 111, 11138.