Ultrathin Films of Pentacene on Ag(111): Charge-Transfer Bonding and Inter-Adsorbate Interactions
Temperature programmed desorption (TPD) was used to examine the surface binding and intermolecular interactions of mono-and multi-layer thin films of the polycyclic aromatic acene, pentacene, deposited on a Ag(111) surface. The TPD spectra of sub-monolayer coverages revealed the presence of three distinct phases (denoted as α 1 , α 2 , and α 3 ). The α 1 phase was attributed to adsorption on step sites, while the α 2 and α 3 phases were assigned to adsorption on terrace sites under different local molecular densities. A physical model was constructed to describe the desorption kinetics from each of the three monolayer phases, including intermolecular repulsion from interfacial dipoles produced as a result of charge transfer bonding between pentacene and the Ag substrate. Fit analysis of the sub-monolayer spectra revealed desorption energies in the zero-coverage limit of 218±8, 166±8, and 162±9 kJ/mol for the α 1 , α 2 , and α 3 phases, respectively. The interface dipoles of the α 2 and α 3 terrace adsorption sites were found to be effectively invariant (within error) and deduced as 18±7 and 23±10 D, respectively. These values suggest a partial charge transfer of 0.6 to 0.7 electrons from each pentacene molecule to the Ag substrate and is equivalent to 0.13 electrons per aromatic ring. The TPD spectra from the multilayer films also exhibited three phases. Leading edge analysis of the lowest temperature multilayer peak yielded a desorption energy of 121±15 kJ/mole, while simulations predicted desorption energies ca. 10-15 kJ/mole higher for the higher temperature phases. The three multilayer phases were assigned, from lowest to highest temperature, as an amorphous bulk film, a thin film, and polycrystalline structures.
INTRODUCTION<!>MATERIALS AND METHODS<!>RESULTS<!>ANALYSIS<!>A. Sub-Monolayers of Pentacene: Structure Dependent Desorption Energies and Inter-Adsorbate Interactions<!>The α 2 Phase: Diffuse Terrace Adsorption<!>Modeling Sub-Monolayer Pentacene Desorption from Ag(111)<!>B. Multiple Overlayers<!>DISCUSSION<!>B. Charge-Transfer Bonding between Pentacene and Ag(111)<!>CONCLUSIONS
<p>Thin films of conjugated organic molecules, particularly fused polycyclic aromatic acenes, have been of great interest for study as organic optoelectronic materials. Ultrathin crystalline structures of these acenes, such as pentacene and hexacene, have shown spatially heterogeneous differences in optical exaction 1,2 and have been identified as media for singlet-triplet conversion. 3,4 These materials are equally relevant in organic thin film transistors (OTFT), consisting of semiconductors made of aromatic polyacenes, and offer the promise of inexpensive, lightweight, and flexible electronic devices. 5 It has been found that the structure of these films have a significant impact on device performance, 5 further highlighting the crucial importance of molecular interactions at surfaces. [6][7][8][9][10] In particular, the organic semiconductor is in direct contact with the metal electrodes, the source, and the drain. The interface of the metal conductor and the thin film semiconductor strongly influences the current-voltage characteristics of the OTFT. 11,12 Consequently, it is crucial to understand the various adsorbatesubstrate and adsorbate-adsorbate interactions which take place at the film-metal surface boundary, as these interactions critically influence the growth of the films as well as charge carrier transport through the interfacial layer.</p><p>Due to the intense interest in pentacene thin films as organic optoelectronic materials, a large body of work has been devoted to studying their deposition on various substrates. Over the last two decades, a large number of studies have been published on thin films and ultrathin films of pentacene on gold, [13][14][15][16][17][18] copper, [19][20][21] and silver surfaces. [22][23][24][25][26][27] For example, scanning tunneling microscopy (STM) studies of pentacene monolayers (ML) and sub-monolayers on the reconstructed Au(111) surface revealed that the molecules arrange themselves in rows with the molecules laying side by side and with the plane of the molecule perpendicular to the surface normal. [14][15][16] STM studies showed that within the coverage of 0.25 ML four different ordered structures 17 and for coverages close to one monolayer five different ordered structures were observed. 15 Further, a large interface dipole was measured (ca. 0.95-1.0 eV) and was attributed to charge transfer from pentacene to the gold surface. 11,15 For multilayers of pentacene on Au(111) a study 18 has found that the film grows in a Stranski-Krastanov mode where the first layer covers the surface and subsequent layers form 3D islands.</p><p>Likewise, pentacene on Ag(111) has been studied using a number of experimental and theoretical techniques, including: x-ray standing waves (XSW), STM, photoelectron spectroscopy, He diffraction, and density functional theory. [22][23][24][25][26][27][28][29][30] XSW revealed vertical bonding distances of 2.98 Å at 0.5 ML which increased to 3.12 Å at 0.75 ML. 22 The electronic structure of pentacene on Ag(111) was investigated with photoelectron spectroscopy and the hole barrier heights were determined to be 0.9 eV. 19 The He diffraction measurements showed that initial adsorption occurs at step edges. 28 A UPS study revealed that while pentacene initially lays flat on the (111) surface, it adopts a slightly tilted orientation at higher coverages. 19 Pentacene multilayers deposited on Ag(111) using a hyperthermal molecular beam technique has also been studied using low energy atom diffraction (LEAD) 31 and a structure similar to the bulk crystalline was observed. Nevertheless, the binding energies and inter-adsorbate interactions of the sub-monolayer phases as well as of the multilayers have not yet been characterized.</p><p>In this paper, we report a temperature programmed desorption (TPD) study to quantitatively determine the binding energies of the mono-and multilayer thin films of pentacene adsorbed to Ag(111). Additionally, the various intermolecular interactions within the monolayer films are analyzed. Three separate phases are observed in the sub-monolayer coverage regime. Lateral repulsive interactions are analyzed using an interface dipole model that accounts for delocalization of the charge transfer within the pentacene molecule. The desorption energy and interface dipole are extracted with this model. Three separate phases are also observed for the multilayer TPD spectra in which two of these phases are observed to be metastable. The multilayer spectra are interpreted in terms of phase transitions and eventual 3D nucleation.</p><!><p>The details of the TPD experimental setup has been described previously. 32 Briefly, all experiments were performed in a stainless steel ultra-high vacuum chamber with a base pressure of less than 1x10 −10 Torr. The chamber is equipped with a high-resolution electron energy loss spectrometer (HREELS, McAllister), a quadrupole mass spectrometer (QMS, UTI 100C), and a LEED spectrometer (OCI). The Ag(111) crystal (Monocrystals, Inc) was mounted on a button heater (Spectra-Mat) using tantalum clamps and was attached to a liquid nitrogen reservoir for cooling. Temperature was measured with a chromel-alumel thermocouple that was spot welded to the tantalum clamps. Surface order and cleanliness was verified by HREELS and LEED. Prior to each experiment, the crystal was cleaned with Ne + sputtering and annealed to 720 K. The solid pentacene sample (98%, Acros) was purified by heating and pumping with a turbo molecular pump until no impurities were detected in a residual gas analysis.</p><p>Deposition of the film was performed with a homemade dosing system that was specifically designed for low vapor pressure materials. 32 The solid pentacene sample was placed in a quartz crucible contained within a small stainless steel vaporization chamber. Pentacene vapor was subsequently produced following a rise of temper-ature in the vaporization chamber, which was induced by direct heating of the all-metal valve with heat tape. As discussed previously, 32 this arrangement ensured that the valve would always be warmer than the vaporization chamber, thus preventing condensation of the sample within the valve. The dosing system consisted of a stainless steel tube (1 cm in diameter and approximately 20 cm long) and was heated separately by a ceramic covered tantalum wire. Under typical conditions, the metal valve was heated to an elevated working temperature of 430 K, which resulted in a chamber temperature of ca. 370 K. The dosing tube, located 2.5 cm from the surface, was aimed directly at the Ag(111) crystal. As the dosing pressure at the surface was not measured directly, the exposure is measured in terms of dosing time rather than Langmuirs.</p><p>Based upon attributes of the monolayer coverage (described in later sections, see Results), the pentacene deposition rate was calibrated as ca. 0.2 monolayers per minute when the Ag(111) crystal was held at a constant temperature of 90 K. All TPD experiments (measured with a m/z = 278 amu) were performed with an alignment such that the Ag(111) surface normal pointed directly at the nose cone of the QMS at a distance of 2.5 cm. All TPD spectra were performed at a heating rate of 1 K/s.</p><!><p>Fig. 1 depicts representative TPD spectra of pentacene adsorbed on Ag(111) for low exposures. The inset shows a 20x magnified view of the three lowest exposures measured (i.e., 0.02, 0.04, and 0.07 ML). The spectra for these exposures exhibit two well resolved peaks centered at approximately 680 K (α 1 ) and 620 K (α 2 ), respectively. The α 1 peak is comparatively broad and grows in slower than the α 2 peak. Following continued exposure, the α 1 peak gradually evolves into a shoulder on the high temperature side of the α 2 peak. The α 2 peak broadens and shows a peak maximum that shifts to lower temperatures with increasing coverage. A third peak, initially centered near 500 K (α 3 ), is observed to grow in following even longer exposure. Like α 2 , the α 3 peak also shifts to lower temperatures with increasing coverage. Prior to saturation of the α 2 and α 3 peaks, a fourth peak centered near 370 K (β 1 ) is observed to grow in with increasing exposure.</p><p>The bold black trace depicted in Fig. 1 is a TPD spectrum corresponding to a saturated monolayer of pentacene. This spectrum was obtained by depositing multiple pentacene layers (later deduced as roughly 4 monolayers) on the Ag(111) surface. This film was then annealed at 350 K for 10 minutes to desorb any physisorbed overlayers (the β peaks, see assignment below), leaving behind only a monolayer film of pentacene. This spectrum noticeably lacks the sharp low temperature β 1 peak observed in the unannealed spectra. Further, the resulting spectrum has peak intensities which closely match the Fig. 1 Representative low coverage TPD spectra of pentacene adsorbed on Ag(111). The spectrum shown in black is for a film annealed to 350 K for 10 min and is assigned as 1 ML. The legend labels the number of equivalent ML's in each of the unannealed spectra (based upon integrated area), which contain contributions from both monolayer and multilayer phases. The inset shows a 20× magnified view of the three lowest coverages and highlights the location of the α 1 phase. saturation levels for the three α peaks observed in the low coverage cases. Consequently, the three α peaks are assigned as three phases within the sub-monolayer coverage, while the β peak corresponds to the multilayer. The integrated area of the annealed spectrum is assigned as one monolayer (ML) and can be used as a reference for calibrating the coverage at a specific exposure. Note that a small amount (< 5%) of the β 1 peak appears to be present in the low temperature shoulder of this reference spectrum. Going forward, all coverages are assigned with respect to the integrated area of the annealed spectrum, in which contribution from the minor presence of the β 1 multilayer peak has been removed.</p><p>With the ML thus defined, it is now possible to describe all TPD spectra of pentacene films grown on Ag(111) at 90 K in terms of MLs. However, it is important to note that the unannealed spectra (colored curves in Fig. 1) contain simultaneous contributions from both monolayer and multilayer phases, even before saturation of the monolayer. Consequently, the color legend in Fig. 1 assigns equivalent monolayers, which are based upon comparison of the integrated areas of the unannealed spectra relative to the reference 1 ML annealed spectrum. For instance, while the onset of the α 3 phase occurs at a coverage near 0.26 ML (aqua trace), the β 1 multilayer peak occurs at an equivalent sub-monolayer coverage of only 0.40 ML (light green trace). Consequently, the α 2 (light orange trace) and α 3 (orange trace) peaks are observed to saturate at coverages of 1.4 and 1.8 equivalent MLs, respectively. The fact that saturation of these monolayer phases occurs at equivalent coverages well above 1 ML simply reflects the concurrent presence of multilayer phases before saturation of the monolayer in the unannealed spectra. However, when contributions of the β 1 multilayer phase are removed, saturation of the α 2 and α 3 phases are found to occur for 0.9 and 1.0 ML, respectively.</p><p>The TPD spectra of pentacene adsorbed at higher coverages (0.8 -25 ML) on Ag(111) are shown in Fig. 2. For coverages of 2.4 ML (yellow trace) and above, the β 1 peak maximum shifts down to ca. 340 K (Fig. 2a). At 3.7 ML (orange trace), a distinct high temperature shoulder appears and at 4.4 ML (red trace) a discernable peak is observed. This new multilayer peak centered near 370 K is labeled β 2 . Interestingly, the shift to lower temperatures stops at the same coverage as the appearance of β 2 . The broad feature near 400 K (Fig. 2a) in the 3.7 and 4.4 ML spectra does not grow in intensity and is likely an artifact of the experiment (e.g., desorption from the crystal hardware). Coverages from 2.4 to 25 ML are shown in Fig. 2b. The peak maximum of β 1 shifts back to higher temperatures with increasing coverage between 8.3 (light blue trace) and 25 ML (purple trace). The peak maximum of β 2 also shifts to higher temperatures with increasing coverage. A small shoulder is observed on the high temperature side of β 2 at a coverage of 4.4 ML (yellow trace). It continues to grow and is observed as a separate multilayer peak near 400 K for a coverage of 25 ML (purple trace) and is labeled β 3 . Higher coverages were not attempted due to the required dosing time (>1 hour), the slow pumping speed of pentacene from the chamber, and the possibility of overall vacuum system contamination from the sticky pentacene molecule.</p><p>Fig. 3 shows a comparison of TPD spectra for multilayers of pentacene adsorbed to Ag(111) at 300 K (blue trace) and 90 K (red trace). The pentacene films in both cases have the same coverage of 8 ML. As depicted in Fig. 3, for adsorption at 300K, both the β 1 and β 2 peaks are significantly reduced in intensity, though both are clearly still present. The β 1 peak can be seen at approximately 335 K and the β 2 peak is now observed as a low temperature shoulder centered near 360 K. Conversely, adsorption at an elevated temperature caused the β 3 peak to grow in significantly, to the point that it now dominates the resulting spectrum with a peak maximum at 393 K.</p><!><p>TPD spectra contain information regarding the desorption energy of surface adsorbed molecules, which is dependent upon the binding energy to the substrate as well as the strength of intermolecular interactions within the film. The analysis of the TPD curves for obtaining the energetic information here is performed in two regimes: the sub-monolayers and the multi-overlayers.</p><!><p>The TPD signal corresponds to the mass spectrometer intensity measured (i.e., m/z = 278 amu for pentacene) as a function of temperature, which is directly proportional to the change of the surface coverage as the surface temperature increases. The Arrhenius description of the temperature-dependence of the surface coverage during the TPD process is the Polanyi-Wigner equation: 33</p><p>where θ is the surface coverage, n the order of the desorption, k 0 the pre-exponential factor, β the heating rate, E d [θ] the desorption energy (which may be a function of coverage), R the ideal gas constant, and T the temperature.</p><p>The TPD signal is directly proportional to dθ/dT and the integrated area under the curve is θ. For submonolayer desorption from systems not exhibiting surface reactions, n equals one. As discussed below, the pre-exponential factor (k 0 ) can reasonably be assumed to be ca. 10 13 s −1 . For all experiments, the heating rate, β, was manually set as 1 K/s while temperature was directly measured during the experiment. Therefore, the only unknown in Equation ( 1) is the desorption energy, E d [θ]. As highlighted in Fig. 1, the sub-monolayer TPD spectra of pentacene on Ag(111) consist of three closely overlapping phases. For a quantitative analysis of the TPD spectra (Fig. 1), we first construct the most appropriate model for the desorption energy for each of the observed sub-monolayer phases (i.e., α 1 , α 2 , and α 3 ) based on their physical structures that we can best determine.</p><p>In general, aromatic molecules, like pentacene, are known to accumulate at step edges under conditions of low coverage 34 and this has previously been observed for pentacene on Ag(111). 28,31 Adsorption at step edges is expected to have high binding energies compared to terrace sites. Therefore, the α 1 peak is assigned to adsorption on step edges.</p><p>On the Au(111) surface, pentacene was shown to form a number of different ordered structures on terraces. 15,17 For example, at low coverages (ca. 0.25 ML), four different ordered structures have been identified. 17 All of these structures appear simultaneously within separate domains of the same film. As shown schematically in Fig. 4a, three of these structures have a pseudohexagonal structure (structures 2-4), while the remaining one (structure 1) has a non-centrosymmetric regular structure. 17 Each of these structures has a relatively low packing density ranging from 3.6x10 13 to 4x10 13 molecules/cm 2 . The α 2 peak, which desorbs from lowto-intermediate coverages, is therefore assigned to desorption from these diffuse structures on terraces. Likewise, for comparatively higher coverages (e.g., between 0.75 and 1.0 ML), five separate ordered structures have been observed. Similar to the α 2 terrace structures, each of the high coverage structures have been simultaneously observed in separate domains of the same film. 15,17 As depicted in Fig. 4b, each of these structures can be described as a row-like structure where the pentacene molecules are aligned side-by-side within the row. The primary difference between these structures is the length of the long axis of the unit cell, which is the distance separating the rows. The α 3 peak (which grows in as the coverage approaches a full ML) is assigned to this row structure. Though the structures described here were determined for pentacene on a gold surface, we use these structures as the (111) surface is the flattest and we assume similarities between the pentacene structures on the least corrugated surfaces of the two inert metals. therefore do not specifically express the influence of interadsorbate interactions within the α 1 phase. Instead we empirically model the coverage dependence of the desorption energy, E d [θ], for the α 1 phase as:</p><p>where E dα1 [θ α1 ] is the coverage dependent desorption energy for the α 1 phase, E dα1(0) the desorption energy at the zero coverage limit, θ α1 the coverage of the α 1 phase, and δ an empirical fitting parameter. Equation ( 2) can be inserted into Equation (1) to describe the desorption kinetics of the α 1 phase.</p><!><p>The α 2 phase also shows the influence of repulsive interactions, as is evident from the width of the peak and the shift of the peak maximum to lower desorption temperatures. [35][36][37][38][39] Pentacene on Au has a large interface dipole due to charge transfer from pentacene to the metal surface. 11,15 These interface dipoles are responsible for the lateral repulsive intermolecular interactions. Albano previously proposed a model that accounts for such interactions based on dipole-dipole repulsion. 40 In this model, the molecules are assumed to form either a hexagonal or square array. Though it is not known whether this phase is an ordered structure, use of the Albano model here is justified because the interactions in the α 2 phase are primarily long-range and the average distance between molecules can be approximated as a square or hexagonal array. Based on the Albano model, the desorption energy of the α 2 phase is expressed as:</p><p>where E dα2 [θ α2 ] is the coverage dependent desorption energy for the α 2 phase, E dα2(0) the zero coverage desorption energy, µ α2(0) the magnitude of the zero coverage interface dipole moment, α p the adsorbate-substrate complex polarizability, θ α2 the coverage for the α 2 phase, and the coefficient 9 is a geometric factor for a hexagonal or square lattice based on the Topping formula. 41 Equation (3) can be inserted into Equation (1) to describe the desorption kinetics of the α 2 phase.</p><p>For the α 3 phase, a number of densely packed rowlike structures, as discovered on the Au surface, are possible. 17 Just as for α 1 and α 2 , the width of the α 3 peak and the observed shift to lower temperatures at higher coverages highlights the influence of repulsive intermolecular interactions [35][36][37][38][39]42 due to interface dipoles formed by charge-transfer adsorbate-substrate bonds. Despite the repulsive interactions, the existence of the more tightly packed row structures (as was found on the gold surface) suggests the influence of attractive intermolecular interactions, which pulls nearby pentacene molecules into a parallel alignment in close proximity. Molecules within the α 3 phase are present at higher densities than in the α 2 phase. As a result, for the α 3 phase, short-range interactions must also be taken into consideration for the desorption energy. The formation of stable row structures implies a balance between attractive and repulsive forces for nearest neighbor molecules within the row and suggests a simplified way to express the nearest neighbor interactions as a net zero interaction. This simplification is supported by the fact that attractive interactions (due to dispersion forces) scale as r −6 , whereas repulsive dipole-dipole interactions scale as r −2 . As a result, beyond nearest neighbor molecules, only effects of repulsion need to be considered. Consequently, in our model we only include repulsive interactions between molecules across different rows. Fig. 5a illustrates our treatment of the inter-adsorbate interactions. From the point of view of the center molecule (highlighted in black), the attractive and repulsive interactions with the nearest neighbor molecules (light grey) within the same row are nulled as a simplifying assumption. Rather, the nearest repulsive interactions considered are those between the center molecule and molecules in adjacent rows, including type A (yellow highlight) and type B (green highlight) neighboring molecules. Additionally, more distant long-range repulsive interactions were also considered for molecules in rows extending beyond those depicted in Fig. 5a.</p><p>As the distance between molecules becomes shorter such that the intermolecular distance is comparable to the length of the molecule (as in the α 3 phase), we need to examine whether the standard Albano model is suitable to account for the intermolecular interactions. In its original formulation, the Albano model was constructed to describe the coverage dependence of alkali metal atoms on metal surfaces. 40 Each alkali atom is assigned a single point dipole. This model has since been successfully applied to smaller aromatic molecules (i.e., containing only one or two aromatic rings, such as benzene, 43 aniline, 44,45  and naphthalene 46 ) on Ag(111), but its application to larger aromatic molecules like pentacene may not be appropriate. The primary concern here is the delocalization of charge over the length of the molecule, which makes the single point dipole assumption employed in the Albano formulation insufficient.</p><p>This effect of delocalized charge has previously been treated in our analysis of the TPD spectra of tetracene on Ag(111). 32 In that study, we developed a modified Albano model to account for the repulsion energy by evenly partitioning the total interface dipole across the body of the molecule. Here, we accomplish this by explicitly localizing a fractional point dipole centered in each of the five aromatic rings of pentacene. The interaction of neighboring molecules is then calculated by summing together all individual point dipole interactions.</p><p>The interaction energy between a pair of point dipoles i and j can be expressed as E = µ 2 /r 3 i,j , where µ is the dipole moment and r i,j the distance between the dipoles. In this model, µ is an unknown and must be deduced from the data through fit analysis. However, r i,j can be calculated as right angles (B-type) or straight lines (A-type) defined by combinations of the distances: R N , R r , and d using the geometries shown in Fig. 5b. Specifically, the distance between the center of mass of molecules within a row is defined as R N and is treated as a constant of 5.8 Å (based on STM images of the row structure). 15 This value was previously deduced for pentacene on Au(111). Again, given that (111) is the flattest surface, we assume that this value should likewise hold for silver. The distance between the rows is defined as R r and varies as a function of coverage. The coverage and distances are related by θ = 1/(R r × R N ). R N is set as a constant, whereas R r varies with coverage. Specifically, as coverage and hence density increases (decreases), the value of R r becomes smaller (larger). The individual dipoles within a molecule are localized on each aromatic ring and are separated by a calculated distance of d=2.4 Å (based on the dimensions of a free pentacene molecule). As a representative example, the dipole-dipole interaction between the 4 th point dipole of a reference pentacene and the 1st point dipole of a B-type nearest neighbor (i.e., r 41 ) is depicted in Fig. 5b where r 41 = (R r + 3d)</p><p>2 + R 2 N . All other dipole interactions can be deduced in a similar fashion. The repulsive interactions of a molecule with either type A or type B nearest neighbors can be calculated by taking the sum of all pair dipole interactions between point dipole i and j of the molecules, respectively:</p><p>where µ α3 is the total dipole of the molecule for the α 3 phase and the subscripts i and j are used to identify the individual point dipoles, as shown in Fig. 5b. The 1/25 coefficient is included to account for the fact that each of the five aromatic rings of pentacene contributes only a 5 th of the total dipole, µ α3 . Equation ( 4) above can also be expressed in simpler terms as:</p><p>where the S A(B) r ij term represents the sum for A-type (or B-type) molecules. All possible interactions for type A and B molecules were calculated for coverages ranging from 1.0 to 0.0 ML using 150 data points to obtain a coverage dependence of the short-range interactions.</p><p>As shown in Fig. 5a, each pentacene molecule exhibits repulsive interactions with a maximum of six neighbor molecules, two of type A (yellow) and four of type B (green). As discussed above, intermolecular repulsion within the rows is assumed to be balanced by the attractive interactions which give rise to the row structures and are therefore not included in our analysis. As a result, the total short-range repulsive interaction can be written as the sum,</p><p>The contribution from molecules beyond the six neighboring molecules is treated as single point dipole-dipole interactions and calculated also based upon the geometry proposed in Fig. 5. Here, the sum of the longer range repulsions was calculated using the expression E = µ 2 /r 3 . All dipole interactions within a closed area of 56 R N units × 28 R r units were calculated for 150 coverages ranging from 1.0 to 0.0 ML to obtain a coverage dependence of the longer range repulsion. Recall that R N is held constant and R r varies inversely with coverage (and hence density). To ensure the accuracy of this exercise, we also calculated (for one monolayer) this contribution from all molecules within an extended area defined by 200 R N units and 100 R r units. The result from the larger area was found to be within 0.2% of the result for the original smaller area. Thus, the finite sampled area (i.e., 56 R N units × 28 R r units) was deemed sufficient for evaluating the interactions in the limit of the infinite sum. It should be noted that all interactions within this area of 56 R N × 28 R r included a total of 6266 individual interactions. These interactions were summed together to deduce the coverage dependence of the overall repulsive interactions of the</p><p>The total repulsive energy (taking into account both short-and long-range interactions) of the α 3 phase can now be expressed as:</p><p>where µ α3 is the magnitude of the interface dipole moment for the α 3 phase, and E long [θ α3 ] and E short [θ α3 ] are defined above. The expressions for E short [θ α3 ] and E long [θ α3 ] were numerically evaluated for a variety of coverages (described above), then plugged into Equation ( 7) and plotted as a function of coverage. This plot was then fit empirically to a much simpler exponential form:</p><p>where θ α3 is the coverage for the α 3 phase, and ρ and λ are empirical fitting parameters with no physical meaning. Evaluation of Equation ( 8) in the limit of zero coverage (i.e., θ α3 = 0) yields a value of the zero coverage desorption energy, E dα 3(0) , from which an expression of the desorption energy for the α 3 phase (E dα3 ) can be deduced:</p><p>and thereafter used in Equation ( 1) to model desorption from the α 3 phase.</p><!><p>Now that individual expressions for the desorption energies of all three phases have been defined (i.e., Equations 2, 3, and 9), a set of rate equations can be constructed for each of the sub-monolayer phases (α 1 , α 2 , and α 3 ) using Equation ( 1). Here we note that the α 1 phase, assigned to step edge adsorption, is treated as being independent from the other two phases which involve adsorption on the terraces. The populations of the α 2 and α 3 phases are not distinct and should be coupled as interconversion between the two phases occurs during desorption from α 3 to α 2 . As the repulsive energies of α 2 and α 3 depend on the total population on the terraces, the coverage dependence of the desorption energies should depend on the sum of the coverages of α 2 and α 3 . Consequently, setting n=1 and β= 1 K/s, the set of rate equations for the three sub-monolayer phases can be written as:</p><p>The sum of these rate equations is proportional to the total TPD signal. The series of rate equations were solved simultaneously and used to fit the measured TPD spectra using a non-linear least squares fit routine written in Igor Pro (Wavemetrics, Inc). The series of parameters allowed to vary during the fit were: E dα1(0) , E dα2(0) , E dα3(0) , µ α2(0) , µ α3(0) , α p , and δ. Initial values of θ α1 , θ α2 , θ α3 were chosen in order to give the lowest chi-square value.</p><p>Representative fit results are shown in Fig. 6 for coverages of 1.0, 0.9, 0.8, 0.7, and 0.6 ML. The actual coverages may be higher due to adsorption in the second layer before the first layer is complete. The 1.0 ML spectrum shown (bold black trace) was annealed at 350 K for 10 minutes. The monolayer is assigned a coverage of 1x10 14 molecules based on the dimensions of the unit cell of a monolayer given in Ref. 15. The averaged zero coverage desorption energies and interface dipole moments from all five fittings are shown in Table 1. Dissimilar from our prior study of tetracene on Ag(111), 32 the validity of our assumption that k 0 = 1×10 13 s −1 could not be directly tested on this data set. This stems primarily from the substantial overlap of the three sub-monolayer phases (Fig. 1). Nevertheless, given that this assumption was previously shown to be valid for tetracene suggests that it may likewise be valid for pentacene. 32 Afterall, pentacene is only 20% longer (and heavier) than tetracene, hence it is reasonable to expect only a small variation in k 0 . Furthermore, the reported desorption energies for the α 2 and α 3 phases are reasonable compared with the desorption energies of similar polycyclic acenes and the observed desorption temperatures, suggesting that our assumption regarding the pre-exponential factor is reasonable.</p><p>Fig. 6 Representative TPD spectra of sub-monolayer films of pentacene adsorbed to Ag(111), along with fit spectra (blue dashed lines). The spectrum shown in black is for a film annealed to 350 K for 10 min and is assigned as 1 ML. All other films were deposited at a surface temperature of 90 K. The β 1 peak at 350 K was not included in the fit analysis.</p><p>Depolarization effects were not included in our model of the α 3 phase for the same reasons pointed out for the tetracene model previously reported. 32 To begin with, inclusion of a polarizability term (similar to that of the Albano model used to treat the α 2 phase) would render the model mathematically intractable. However, to test the influence of polarizability on the deduced dipoles, we consider our prior investigations of benzene and aniline, both of which are sufficiently small as to be accurately treated by the Albano model. Specifically, omission of the polarizability term resulted in a ca. 15% reduction in the deduced interface dipoles for each of these adsorbates. 45 Consequently, given that the uncertainty in the fit-deduced dipole moment for the α 3 phase of pentacene is over 25%, no adjustments to the uncertainty of µ α3 will be made.</p><!><p>Numerical analysis of the multilayer peaks, β 1 , β 2 , and β 3 was limited due to the significant overlap of each of these peaks. A leading edge analysis was performed on the β 1 peak for coverages ranging from 1.1 ML to 25 ML, from which a desorption energy of 121±15 kJ/mole was deduced. Desorption energies for the β 2 and β 3 phases could not be analyzed, but it was estimated that the desorption energies are roughly 10-15 kJ/mole higher than the β 1 phase, based on simulated TPD spectra and the observed peak positions.</p><!><p>A. The Three Binding Sites in the Pentacene Monolayer</p><p>The TPD spectra of pentacene on Ag(111) shows complex desorption kinetics within the monolayer. Even for very low coverages (i.e., 0.02 ML), two adsorption phases are clearly identifiable in the TPD spectra (Fig. 1 inset). The comparably high desorption energy of 218 kJ/mole deduced for the α 1 phase is consistent with adsorption on step edges. 31 Given the much higher adsorption energy at the step sites compared to terrace sites (166 kJ/mole), how can we account for the co-presence of the α 2 phase, which is observed to grow in prior to saturation of the α 1 phase? One likely possibility is diffusion limited sampling of the substrate surface. At 90 K, surface diffusion (though significantly hindered) should still allow pentacene to sample sub-domains of the total substrate surface. In the limit of sparsely available step sites, the amply available nearby terrace sites could be thermodynamically favorable for binding in a limited available diffusion range. Such a mechanism could reasonably explain the simultaneous coexistence of the energetically distinct α 1 and α 2 phases at very low coverages.</p><p>Distinct from the α 1 phase, the α 2 and α 3 phases exhibit similar desorption energies (i.e., 166±8 kJ/mole and 162±9 kJ/mole, respectively) and interface dipole moments (i.e., 18±7 D and 23±10 D, respectively) within the limit of the measured uncertainty. This comparison highlights the similarity of the pentacene-Ag interaction in these phases. The α 3 phase has pentacene molecules more densely packed than in α 2 . Closer proximity of the molecules results in both stronger repulsion (due to interface dipoles) as well as stronger attraction (from dispersion forces). The variety of different structures which have been observed within the pentacene monolayer on inert metal surfaces 15,17 must therefore result from an intricate balance between these attractive and repulsive forces.</p><p>The attractive interactions which give rise to the observed row structures of the α 3 phase may involve tilting of the molecule on the metal surface. It has been speculated that, since the distance between molecules within a row (ca. 5.8 Å) is slightly smaller than the van der Waals width (ca. 6.0 Å), pentacene likely does not sit flat and is instead somewhat tilted with respect to the metal surface. 15 Indeed, photoelectron spectroscopy has revealed that pentacene initially lays flat on Ag(111), but adopts a tilted structure at higher coverages. 19 This allows for a partial π − π interaction between neighboring molecules in a row and gives the monolayer a structure similar to the (100) plane of bulk crystalline pentacene.</p><p>The fact that the coverages for the α 2 and α 3 phases were approximately equal for the range of fitted spectra obscures the relative presence of a coverage dependent phase transition between the high-density row structures (α 3 ) and the more diffuse low coverage structures (α 2 ). It is likely that both phases α 2 and α 3 exist simultaneously on the surface in different domains. Consequently, variations in local coverage likely have more of an influence on the resulting structure compared to the overall coverage.</p><!><p>The interaction of an organic molecule with the surface of a metal substrate gives rise to an interface dipole moment. 47 This stems from the partial charge donation from the adsorbed molecule and the change in the electron density of the metal surface in response to the presence of the adsorbate. 47 As each adsorbed molecule gives rise to an individual interface dipole moment, a film of such molecules produces a layer of dipoles at the metal surface. Consequently, the cumulative electric field of this film of interface dipoles results in a measurable reduction in the work function of the metal substrate. Similar to tetracene, it has previously been shown that adsorption of pentacene results in a negative work function change in silver. 19,32 This suggests that pentacene donates negative charge to the silver surface. By combining the extrapolated dipole moment at one full monolayer (ca. 10 14 molecules / cm 2 ) for the α 3 phase (23±10 D) and the deduced polarizability from the α 2 phase (200±130 Å3 ), the change in the work function of Ag(111) following adsorption of pentacene can be deduced as -1.5±0.9 eV. 48 Within the experimental uncertainty, this value is consistent with previously reported work function change for pentacene on a silver surface (i.e., -0.7 eV 11 and -0.58 eV 19 ). The apparent overestimate of the deduced change in the work function is fully consistent with prior studies of aromatic molecules adsorbed on metal surfaces. 32,43,45,46 Changes in the substrate work function deduced from interface dipole moments tend to be roughly two times larger compared to values deduced from photoelectron spectroscopy results (e.g., -1.2 vs. -0.7 eV for benzene, -1.5 vs. -0.7 eV for tetracene). 19,43,46,49 Overestimation of the dipolededuced ∆Φ values has previously been attributed to contributions from additional repulsive interactions (e.g., inter-adsorbate repulsion), rather than simply the dipoledipole repulsion, which results in an overestimation of the actual interface dipole moment. 40 The amount of charge transfer (q) from the adsorbate to the substrate can be estimated using the dipole moment expression: µ = 2d 0 q based upon the distance from the molecule to the surface, d 0 = 0.3 nm. 14,22,50 Charge transfers of q = 0.6±0.3 e and q = 0.7±0.3 e were estimated for the α 2 and α 3 phases, respectively. It should be noted that this charge is delocalized over the entire length of the molecule. This estimate therefore suggests that each ring in the pentacene molecule donates a charge of roughly 0.13 e to the silver surface. This charge transfer value is in agreement with our prior studies of naphthalene, 46 tetracene, 32 benzene, 43 and aniline, 45 in which each phenyl ring of the adsorbate was found to contribute roughly 0.1 e (naphthalene and tetracene) to 0.2 e (aniline and benzene) to the Ag(111) surface. Similar to the sub-monolayer spectra (Fig. 1), the multilayer spectra (Fig. 2) also show complex desorption kinetics. A previous study 51 showed that pentacene deposited on SiO 2 at 77 K forms an amorphous film which develops sub-micron sized crystallites within the amorphous matrix when heated to room temperature. Two other phases were observed for deposition of pentacene at higher temperatures on SiO 2 . 51 Deposition at 300 K produced a 'thin-film phase' which was not characterized, while deposition at 328 K produced a film of mixed phases consisting of a thin-film phase and a single crystal phase (similar to bulk crystalline pentacene). 51 In the present case, deposition at 90 K is expected to form an amorphous layer. The multiple peaks observed in Fig. 2 are likely due to phase transitions occurring during the heating of the TPD experiment. The fact that the β 1 and β 2 peaks do not saturate also supports our assertion that the observed multiple peaks are from thermally induced phase transitions. The β 2 peak is likely due to desorption from the thin-film phase, while the β 3 peak is likely due to desorption from the single crystal phase. The thin-film phase was previously described as being a metastable phase, 51 which is consistent with our observation that β 2 is metastable as evidenced by the deposition at 300 K shown in Fig. 3. Interestingly, the formation of the single crystal phase occurs at a lower deposition temperature on Ag(111) compared to SiO 2 . 51 The term 'single crystal phase' here indicates that the film is not a single crystal but is instead composed of numerous 3D clusters having a crystalline structure similar to the bulk crystal structure. Pentacene films on Au(111) at room temperature grow in a Stranski-Krastanov mode. After the first layer of the film is formed, the film proceeds to grow through formation of 3D crystallites. 18 Pentacene films on Ag(111) should likewise show a similar growth mode. The strong β 3 peak in the TPD spectrum of the film grown at 300 K in Fig. 3 is consistent with this expectation.</p><!><p>TPD spectra of pentacene adsorbed on Ag(111) show complex behavior due to substantial charge transfer to the substrate and strong intermolecular interactions among the pentacene molecules. Three separate phases (α 1 , α 2 , and α 3 ) are observed in the sub-monolayer TPD spectra. The α 1 phase is assigned to adsorption at step sites, while the α 2 and α 3 phases are assigned to adsorption on terrace sites with unique structures at varying densities. A model is proposed to describe the desorption kinetics including repulsion from the interfacial dipoles which result from charge transfer bonding between pentacene and Ag. The repulsive energy can be calculated based upon the geometries of previously determined unit cell structures 17 and row structures 15 of pentacene molecules on a Au surface and charge delo-calization within the pentacene molecule. Fit analysis of the exposure dependent TPD spectra using this model allowed the deduction of the desorption energies and the interfacial dipoles of each of the three phases (Table 1). The deduced desorption energies are consistent with the proposed assignments of the three TPD peaks. The interfacial dipoles correspond to a 0.6-0.7 electron transfer from an individual pentacene molecule to Ag, which is equivalent to 0.13 electron transfer per aromatic ring, consistent with previous studies of similar aromatic molecules. Desorption from multilayer films also shows numerous phases, likely due to phase transitions that occur during the heating of the TPD experiment. The three desorption peaks in increasing temperature can be assigned to desorption from amorphous bulk films, thin films, and polycrystalline structures.</p>