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J. Phys. Chem. C 2008, 112, 4696-4703
Tetracene Monolayer and Multilayer Thin Films on Ag(111): Substrate-Adsorbate Charge-Transfer Bonding and Inter-Adsorbate Interaction† Grazia Gonella and Hai-Lung Dai* Department of Chemistry, Temple UniVersity, Philadelphia, PennsylVania 19122
Thomas J. Rockey‡ Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed: October 8, 2007; In Final Form: January 22, 2008
Temperature programmed desorption (TPD) is used for examining surface binding, intermolecular interaction, and morphology of mono- and multilayer films of tetracene on Ag(111). TPD of monolayer tetracene revealed strong inter-adsorbate repulsion caused by interaction among interface dipoles resulted from charge-transfer bonding. A modified Albano model, in which a point interface dipole is assigned to each of the aromatic rings of tetracene, is proposed to account for the interfacial dipole interaction at short range. It is found that desorption energy at the zero-coverage limit is 142 ( 7 kJ/mol. The interface dipole is determined as 8.2 ( 2.1 D, which corresponds to a partial charge transfer of 0.4 e per tetracene molecule to the Ag substrate. At full monolayer coverage, the strong inter-adsorbate repulsion reduces the desorption energy to 105 ( 14 kJ/mol. Annealing at elevated temperature (350-400 K) but below desorption temperature, on minute time scale followed by cooling, appears to produce a more stable structure. Multilayer TPD spectra show three separate half-order desorption peaks that merge into one bulk peak at higher coverage. The half-order kinetics agrees with the previously reported Stranski-Krastanov growth mode in which islands with high height-towidth ratio are formed. The desorption energies for these peaks are 100 ( 7, 110 ( 10, and 116 ( 4 kJ/mol respectively. Upon annealing, the lower energy structure transform into the higher energy ones.
I. Introduction Thin films of conjugated organic molecules have been of great interest recently because of their applications in organic thin film transistors (OTFT) and light emitting diodes.1 This type of thin film organic materials offers the possibility to combine electronic (charge-transport) and optoelectronic (luminescence) properties of the same material in a single device. Tetracene thin films have shown to be a potentially suitable material for this purpose. Both single-crystal and amorphous thin-films of this molecule can be used for the fabrication of field-effect transistors (FET).2,3 Also, a tetracene-film based light-emitting field-effect transistor has been fabricated.4 It is widely perceived that both the morphology of the film and that of the interfacial layer between the film and the substrate gravely affect the device performance. For example, nucleation induced grain boundaries substantially limit carrier mobility in vapor deposited films.5 The correlation between morphology and mobility in tetracene-film FET has been recently suggested.6 For this reason, many studies have been performed on the growth of films of aromatic molecules, with particular attention to acenes on solid substrates, for application in organic electronics.7 There have been many studies on tetracene thin films with thickness from a few to hundreds of layers. Most studies have centered on the characteristics of OTFT’s made with tetracene† This article was originally intended to be a contribution to the “Giacinto Scoles Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Present Address: Kulicke and Soffa Industries Inc., Willow Grove, PA 19090.
based semiconducting materials. In those studies, morphological and structural determination was performed utilizing typically atomic force microscopy,3,6,8-12 scanning tunneling microscopy (STM),13-15 and X-ray diffraction.8,10,16 Two separate studies of tetracene monolayers on Cu(110) show conflicting results. An earlier one using angle resolved photoemission spectroscopy and low-energy electron diffraction (LEED) has determined that the long axis of tetracene is perpendicular to the plane of the surface in a c(4 × 2) structure.17 The other study using highresolution electron energy loss spectroscopy (HREELS), LEED, and STM concluded that the tetracene molecular plane is parallel to the surface in a c(10 × 2) structure.18 The latter study suggested that a structural phase transition upon deposition of more than one monolayer is responsible for the different observations. Tetracene adsorbed on the Ag(111) surface has been studied using ultraviolet photoemission spectroscopy19 and near edge X-ray absorption.20 It was found that tetracene adsorbs with its molecular plane parallel to the surface. More recently, a high resolution, spot-profile analysis of LEED and temperature programmed desorption (TPD) study has found that monolayer adsorption takes place, depending on deposition conditions, in one of two different phases and that multilayer growth proceeds via a Stranski-Krastanov mode.21 In this paper, we report a detailed TPD study of tetracene adsorbed to Ag(111) in order to determine the binding energies of mono- and multilayer films. In addition the intermolecular interaction within the monolayer films is analyzed. The TPD spectra can be used to deduce desorption activation energy, which is related to the bonding of the molecule to the surface,
10.1021/jp709826q CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008
Tetracene on Silver
J. Phys. Chem. C, Vol. 112, No. 12, 2008 4697
Figure 1. Schematic diagram of the deposition source.
and the inter-adsorbate interactions that affect the subsequent film growth. The coverage dependent TPD spectra are analyzed by a model in which the desorption energy at a specific coverage depends on the repulsive interaction between neighboring interface dipoles. Determination of the magnitude of the interface dipole gives information on the strength of the chargetransfer bonding between the aromatic rings of the tetracene molecule and the metal. Analysis of TPD spectra as a function of the film thickness reveals the structure and the growth mechanism of multilayer films II. Experimental Section All experiments were performed in a stainless steel ultrahigh vacuum chamber with a base pressure lower than 2 × 10-10 Torr. The chamber is equipped with an HREELS spectrometer (McAllister), a quadrupole mass spectrometer (QMS, UTI 100C), and a LEED spectrometer (OCI). The Ag(111) crystal (Monocrystals) was mounted on a button heater (Spectra-Mat) with tantalum clamps and attached to a liquid nitrogen reservoir for cooling. Temperature was measured with a chromel-alumel thermocouple spot welded to the tantalum clamps. The Ag(111) surface was routinely cleaned with several cycles of Ne+ sputtering. Surface order and cleanliness were verified by LEED and HREELS. The crystal was annealed to 720 K prior to each experiment. Tetracene (98% Aldrich) was purified by heating and pumping with a turbomolecular pump until no impurities were detected by residual gas analysis. The films deposition was performed with a homemade deposition system designed specifically for low vapor pressure molecules. The microcapillary array doser used in previous studies on benzene22 and naphthalene23 is more suitable for the deposition of high vapor pressure molecules but could not be used for tetracene. There are a few challenges for dosing a molecule like tetracene: the low vapor pressure requires an elevated temperature at the source to raise the vapor pressure, and condensation of the vapor on chamber walls has to be avoided. The deposition source is shown in Figure 1. The tetracene sample in solid powdery form was placed in a quartz crucible contained within a small vaporization chamber made of stainless steel. This vaporization chamber was attached to an all-metal valve which was connected to the doser in the main chamber. Tetracene vapor was produced by heating the all-metal valve which caused a temperature rise of the vaporization chamber. This arrangement ensured that the valve would always be warmer than the vaporization chamber, thus preventing condensation in the valve itself. The doser was a stainless steel
Figure 2. TPD spectra of tetracene films on Ag(111) with coverage 0.08, 0.18, 0.29, 0.36, 0.46, 0.54, 0.67, 0.82, 0.92, 1.08, 1.15, and 1.36 ML (from bottom up) respectively. The spectrum shown in bold is for a film annealed to 350 K for 10 min and is assigned as 1 ML.
tube, 1 cm in diameter and 20 cm long. It was heated separately by means of a ceramic covered tantanlum wire. Typically, the all-metal valve was heated to 410 K while the vaporization chamber was about 350 K (the temperature of the quartz crucible containing tetracene was not measured directly). The doser was pointed directly at the Ag(111) surface at a distance of 2.5 cm from the surface. The dosing pressure at the surface was not measured and the exposure is described in terms of dosing time, not Langmuirs. On the basis of the attribution of the monolayer coverage described later, the deposition rate was calibrated as approximately 0.2 monolayers per minute with the temperature of the crystal held at 90 K. All TPD experiments (m/z ) 228 amu) were performed with the Ag(111) surface normal pointed at the nose cone of the QMS at a distance of approximately 2.5 cm. The heating rate was 1 K/s for all the TPD spectra. III. Results Several TPD spectra of tetracene deposited on Ag(111) at low exposures are shown in Figure 2. At the lowest exposure, a broad peak centered at 525 K (R1) appears with a tail on the low-temperature side. Upon increasing the coverage, this peak broadens and shifts to lower temperatures until eventual saturation. Its tail portion, however, continues to grow into a second peak that is centered at 305 K (R2). Upon initial examination, the first peak appears to be the result of desorption of the first layer while the second lower temperature peak
4698 J. Phys. Chem. C, Vol. 112, No. 12, 2008
Figure 3. TPD spectra of tetracene films on Ag(111). (a) Coverage: 1.4, 1.8, 2.0, and 3.1 ML. (b) Coverage: 3.4, 3.9, 4.3, 5.1, 6.0, 11, 17, 22, 26, and 39 ML (from bottom up).
corresponds to desorption of the layer physisorbed on top of the first. This assignment is consistent with TPD of naphthalene multilayers on Ag(111)23 and also with observations made with a sample annealed at 350 K as shown below. Also shown in Figure 2, in bold, is a spectrum where the tail portion is not present. This spectrum was obtained by preparing a film with exposures about twice that of the highest exposure shown in this figure. The sample was then heated to 350 K for 10 min in order to desorb the physisorbed overlayers (the tail). The TPD spectrum (bold in Figure 2) of the film obtained following this treatment shows the highest integrated area for the R1 peak. We therefore assign the integrated area of this spectrum as one monolayer (1 ML). Figure 3a shows the TPD spectra of tetracene with coverage ranging from 1.4 to 3.1 ML. These spectra show that a total of four TPD peaks are observed. The peaks labeled R1 and R2 are the same as shown in Figure 2. At the coverage of ∼1.8 ML, a third peak, R3, appears around 320 K. A fourth peak, R4, appears around 330 K at the coverage of ∼3.1 ML. The TPD spectra with even higher coverage, from 3.4 to 39 ML, are shown in Figure 3b. At coverage above 6 ML, only R4 has been observed. The R4 peak continues to shift toward higher temperatures with increasing coverage. The effect of annealing on films with mono- and multilayer dosage was examined. Figure 4a shows the TPD spectrum of a film of 0.7 ML that was prepared by depositing several monolayers at 90 K followed by annealing at 400 K for 10 min. The sample was then cooled down to 90 K before running the TPD experiment. Also shown in Figure 4a is the TPD spectrum of an unannealed film with approximately the same coverage (∼0.7 ML) for comparison. As can be seen, annealing produces
Gonella et al.
Figure 4. TPD spectra of annealed (dotted line) and unannealed (continuous line) tetracene films. (a) Both films have a coverage of 0.7 ML. Film annealing was performed at 400 K for 10 min. (b) Both films have a coverage of 1.8 ML. Film annealing was performed at 275 K for 10 min.
a more symmetric peak with less tail on the low-temperature side. Figure 4b shows the comparison of TPD spectra between an annealed and an unannealed multilayer film. The annealed film was prepared by depositing several monolayers at 90 K and then heating up the surface to 275 K for 10 min. The surface was then cooled down to 90 K before running the TPD experiment. Both films have approximately the same coverage of 1.8 ML. As can be seen in Figure 4b, after annealing, the intensity of the R2 peak decreases while that of the R3 peak increases. In addition, a small shoulder appears at 338 K which can be assigned to the R4 peak. The effect of the annealing (275 K for 10 min) on thicker films was also examined. For films with a final coverage greater than 2 ML, no significant change was observed in their TPD spectra when compared with the unannealed spectra. IV. Analysis A. Inter-Adsorbate Repulsion within the Monolayer: TPD Analysis using the Albano Model Modified for Large Molecules. In general, TPD curves give information on the desorption energy of the molecules in the film which depends on their binding energy with the substrate and on the intermolecular interaction strength within the film. The TPD signal, QMS intensity measured as a function of temperature T, is proportional to the desorption rate, the change of coverage θ with time, which can be modeled by the Polanyi-Wigner equation assuming Arrhenius behavior for the desorption rate constant:24
(
)
k0 -Ed(θ) dθ ) θn exp dt β RT
-
(1)
Tetracene on Silver Here, n is the desorption order, k0 is the pre-exponential factor, β is the heating rate, Ed(θ) is the desorption energy which may be a function of the coverage, and R is the gas constant. For monolayer desorption, in the absence of surface reactions, n is typically equal to one. The pre-exponential factor, k0, is assumed to be 1013 s-1 (see discussion below). The heating rate, β was set to 1 K/s in all experiments. Therefore, the only unknown in eq 1 is the desorption energy, Ed(θ). The broadness of the R1 peak together with the fact that the peak shifts to lower temperatures at higher coverage suggests repulsive interaction within the monolayer.21,25-30 Ed(θ), therefore, depends on coverage, and the contribution of this repulsive interaction must be included in its description. The repulsion most likely originates from interactions between dipole moments generated at the adsorption sites due to charge-transfer bonding between the aromatic molecule and the metal surface. Previous measurements of the surface work function during adsorption of acenes have shown a negative work function change upon adsorption, suggesting a donation of a negative charge from tetracene to the Ag(111) surface.19 This partial charge transfer results in a positive charge at the molecule and its negative image charge in the metal, forming an interface dipole. Interactions between these interface dipoles, parallel to each other, result in a repulsive inter-adsorbate interaction. A model has been proposed by Albano to account for the contribution of lateral repulsion due to these adsorption-induced dipoles on the desorption energy.31 This model was originally devised for the calculation of the coverage dependence of the desorption energy for alkali metal atoms on transition metal surfaces and has been shown effective in describing the coverage-dependent desorption energy of aniline,32,33 benzene22 and napthalene23 on Ag(111). Before applying the Albano model to acenes with multiple aromatic rings we need to consider if a single point dipole is appropriate to represent the charge distribution of a molecule with possible multiple charge-transfer sites. Tetracene has the ability to delocalize charge over its entire molecule19 but the Albano model in its original form only considers one point dipole per molecule. When the inter-adsorbate distance becomes comparable to the size of the molecule itself, it is doubtful that one dipole per molecule, as in the Albano model, would be adequate to describe the repulsion energy as a function of coverage (or interadsorbate distance). We therefore propose a modified Albano model that accounts for the repulsion energy by assuming that at each ring in the molecule there is an individual interface dipole. Langner et al.21 have proposed an adsorption pattern of the tetracene monolayer on Ag(111), designated as the R phase which has been shown to be unstable at the desorption temperature. Because of the strong intermolecular repulsion that would repel the nearest-neighbor molecules in order to maximize the inter-adsorbate distance and the mobility of tetracene molecules at desorption temperatures, it is reasonable to assume a hexagonal arrangement similar to the one illustrated in Figure 5 for the adsorption pattern. In this pattern, each molecule has two different kinds of nearest neighbors, hereto labeled A and B as in Figure 5b. Each molecule has four A-type neighbors that are side-by-side but offset by 30° and two B-type neighbors that are end-on-end. The distance between the centers of mass of the molecules is labeled R. This distance can be related to the coverage. In our treatment of the inter-adsorbate interaction at different coverage, we have assumed that the adsorption pattern remains
J. Phys. Chem. C, Vol. 112, No. 12, 2008 4699
Figure 5. (a) Structure of the submonolayer tetracene films used for the modified Albano model calculation. (b) Definition of type A and type B nearest neighbor molecules. R is the distance between the centers-of-mass of the molecules. The distance between neighboring dipoles within a tetracene molecule is d. rij labels the distance between the point dipoles of two different molecules.
the same in which R is equivalent for type-A and type-B nearest neighbors. We have also assumed that each aromatic ring within the tetracene molecule has an individual point dipole µ that is equivalent to the others no matter which ring is considered. On the basis of the dimensions of a free tetracene molecule, these dipoles are separated by a distance d ) 2.4 Å. The interaction energy between two dipole moments, each perpendicular to the surface and separated by distance r is given as E ) µ2/r3. In our model, µ is an unknown to be determined from data analysis, while r can be calculated as a function of R using simple geometric relations. As an example, r34 shown in
Figure 5b, is r34 ) x(R2 - Rd + d2). The total dipole of each molecule is assigned as µ0 and the dipole moment for each ring is µ0/4. 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 interactions between point dipoles i and j of the two molecules respectively as:
EA(B) )
µ02
4
∑ 16
1
i,j ) 1r
3
(2)
ij
with i representing the position of the ring in the molecule under consideration and j the position of the ring in an A- or B-type nearest neighbor molecule. Equation 2 can be equivalently expressed as: EA(B) )
µ02 S (r ) 16 A(B) i,j
(3)
where SA(B)(rij) indicates the sum for A-type (or B-type) molecules. The interaction energy experienced by any molecule on the surface has, in addition to the nearest neighbor interactions, contributions from molecules further away. The long-range interaction at distances larger than the molecular size can be calculated by treating the interface dipoles of one molecule as a one-point dipole. The interaction energy of an hexagonal array of interface dipoles each with magnitude µ0 has been calculated by Topping34 as 11.034µ02/R3. The long-range interaction can then be obtained using the Topping energy formula and subtracting the nearest neighbor contribution which is 6µ02/R3 according to Figure 5a. The long-range interaction, which
4700 J. Phys. Chem. C, Vol. 112, No. 12, 2008
Gonella et al.
Figure 6. Experimental TPD spectra (circles) and fits (lines) of 0.7 and 1.0 ML annealed tetracene films on Ag(111). The 0.7 ML film was annealed at 400 K while the 1.0 ML film at 350 K, both for 10 min. The dashed line represents the fit to the ideal model using the predetermined ai parameters while the solid line represents the best fit allowing a2 and a3 to vary.
accounts for all interactions outside the nearest neighbor shell, can then be calculated as: Elong ) ET - Eshort ) 5.034
µ02
(4)
R3
With all contributions included, the total energy experienced by a tetracene molecule with four point (interface) dipoles at each of the four rings can be expressed as: ET ) µ02
(
4EA µ0
2
+
2EB 2
µ0
) (
1 + 5.034 3 ) R SA(rij) SB(rij) 1 µ02 + + 5.034 3 (5) 4 8 R
)
ET is only a function of µ0 and R. The latter, for a hexagonal array, can be related to the coverage:34 R ) {(4/3)1/4}/xθ. To reduce the complexity of eq 5 resulting from the large number of terms in SA and SB, ET was fitted to an empirical polynomial expression as a function of coverage as ET(θ) ) µ02(a1θ1.5 + a2θ2 + a3θ2.5)
(6)
where the parameters ai can be determined as a1 ) 14.8 ( 0.4, a2 ) (-18.8 ( 0.9) 10-9 m and a3 ) (16.4 ( 0.6) 10-18 m2 for the hexagonal pattern of adsorbates in Figure 5. Details of the calculation are reported in Appendix. The desorption energy in eq 1 is related to the inter-adsorbate interaction as: Ed(θ) ) E0 - ET(θ)
(7)
where E0 is the zero-coverage limit desorption energy. The combination of eqs 1 and 7 gives:
(
)
E0 - ET(θ) dθ ) θk0 exp dt RT
-
(8)
for n ) 1 and β ) 1 K/s. Equation 8, with ET expressed by eq 6, has been used to fit two TPD spectra obtained with annealed (at 359 and 400 K respectively) samples of 1.0 and 0.7 ML. The experimental spectra and fitting results are shown in Figure 6. The experimental data was first fitted according to the values of ai (i ) 1, 2, 3) determined above with only E0 and µ0 as variable parameters. The fit yielded E0 ) 141.7 ( 0.2 kJ/mol and µ0 )
6.7 ( 0.6 D. The fit indeed reproduced the general shape of the TPD spectra including the low temperature shoulder at higher coverage and clearly shows the importance of intermolecular interactions in affecting the adsorption and desorption behavior. In an effort to improve the description of the TPD spectra within the framework of the inter-adsorbate repulsion-dominated model, we have conducted another fit that retains the a1 parameter fixed at the value reported above, but allows the a2 and a3 parameters to vary together with E0 and µ0. The a2(R-4) and a3(R-5) terms have primarily the function of accounting for the contributions to the adsorption energy in addition to nearest neighbor point dipole-point dipole (per molecule) interaction that is described by the a1(R-3) term. In reality, we have to consider the following facts: the crystalline surface is not perfect for defects and terraces as was observed in the pentacene case by Scoles and co-workers;35 the adsorption pattern may not maintain the 6-fold symmetry at all coverage; at high coverage the inter-adsorbate adsorption is not all pointdipole interactions; and the molecules may have different orientations. Accordingly, these two R-4 and R-5 correctional terms are allowed to vary to account for these effects. Indeed the fit using four variables produced better results statistically. The best fit yielded a2 ) (-27.5 ( 1.5) 10-9 m, a3 ) (20.5 ( 1.5) 10-18 m2, E0 )141.8 ( 0.2 kJ/mol and µ0 ) 8.2 ( 0.2 D. Even though it is reasonable to speculate that the tetracene molecules at desorption temperatures assume an ordering mimicking the hexagonal symmetry due to the underlying substrate, we test the accuracy of the analysis by assuming a square (cross-shaped) adsorption pattern instead of hexagonal. In this square pattern, each tetracene molecule has a total of four neighbors: two head-to-tail and two side-by-side. In this framework, eqs 2 and 3 still hold true. The interaction energy of a square array of dipoles as calculated by Topping34 is 9.033µ02/R3. The long-range interaction in eq 4, after subtracting the nearest neighbor contribution, is 5.033 µ02/R3. ET in eq 5 is subsequently equal to µ02({SA(rij)}/8 + {SB(rij)}/8 + 5.033 1/R3). Noting that for a square array R ) 1/xθ, we use eq 6 to fit ET as a function of the coverage. For the square dipole array, we obtain a1 ) 24.5 ( 1.0, a2 ) (-48.3 ( 2.5) 10-9 m and a3 ) (39.2 ( 1.5) 10-18 m2. Holding the ai (i ) 1, 2, 3) values fixed, the best fit for the TPD spectra of the annealed 0.7 and 1 ML samples resulted in E0 ) 141.4 ( 0.3 kJ/mol and µ0 ) 6.5 ( 1.2 D. Alternatively, allowing the parameters a2 and a3 free to vary during the fit, we obtain E0 ) 141.6 ( 0.2 kJ/mol and µ0 ) 6.4 ( 0.2 D for a2 ) (-46.8 ( 0.2) 10-9 m and a3 ) (36.5 ( 0.2) 10-18 m2. As can be seen in comparing the results for the hexagonal and square arrays, the values obtained for E0 and µ0 are less sensitive to the molecular arrangement since the intermolecular distance is more of a determining factor for intermolecular interactions. Noticeable changes in the value of the dipole moment can be obtained if a different surface molecular density is assumed for the monolayer due to the fact that the dipole strength has to compensate for the decrease in surface molecular density. In our study, we have used a value of 100 Å2 for the area of the adsorbed tetracene molecule implying that our annealed monolayer is as densely packed as the R phase.21 This assumption resulted reasonable a posteriori when we realized that our attribution of the coverage to the TPD spectra reported in Figure 2 well compared with those reported in Figure 7a in the work of Langner et al. in the same coverage range.21 In this analysis, we have assumed that the pre-exponential factor k0 ) 1013 s-1. To test this assumption and its effect on
Tetracene on Silver
J. Phys. Chem. C, Vol. 112, No. 12, 2008 4701 The four-parameter fitting model is more physically sensible and also generates a better description of the TPD spectra, and therefore, its results are reported as the values for E0 and µ0 but with larger uncertainties to reflect the values generated from different models and the uncertainty of the pre-exponential factor: E0 ) 142 ( 7 kJ/mol and µ0 ) 8.2 ( 2.1 D. Finally, we note that analyses using the original Albano model with one interface dipole localized at the center of the tetracene molecule cannot adequately describe the TPD curve shape. Upon applying the original Albano model to the TPD spectra of tetracene, fits to the data resulted in unphysical polarizability with negative values. B. Multilayer TPD. The analysis of multilayer TPD spectra is complicated by the very close desorption temperatures of the R2, R3 and R4 peaks. The PJK analysis can give the desorption energies if the peaks are well separated, which is not the case here. Nonetheless, the PJK analysis can still be applied by carefully selecting regions of the spectrum where the overlap is minimal. Our study of the multilayer regime included 33 different spectra for films whose coverage ranges from 1.3 to 39 ML. In all cases where an order determination was possible from the PJK analysis, the most linear behavior was obtained for n ) 1/2 in the case of R2 and R4 and n ) 1 in the case of R3, though for the R3 peak, because of the small temperature range, the fitting of the data cannot rule out a fractional order desorption. The desorption energies obtained from the fit for the R2, R3, and R4 peaks, in Figure 7, are Ed2 ) 100 ( 7, Ed3 ) 110 ( 10 and Ed4 ) 116 ( 4 kJ/mol, respectively. These values are reasonable considering the peak temperature positions. V. Discussion
Figure 7. PJK analysis results obtained for three films with different thickness: 1.4 (for R2), 3.4 (for R3) and 39 ML(for R4). Calculations for n ) 0 (circles), n ) 1/2 (diamonds) and n ) 1 (squares) are shown. The fit to a linear line (continuous line) was obtained for desorption order of n ) 1/2 in the case of R2 and R4 and n ) 1 in the case of R3.
the value of E0, a leading edge analysis was performed. This analysis, when only a small portion of the leading edge is used, yields a desorption energy that is independent of k0. The desorption energy obtained in the leading edge analysis compares well with the value of E0 obtained from the fit and validates the use of k0 ) 1013 s-1 for calculating the TPD curve and extracting the interface dipole magnitude: the values of E0 obtained by the two methods fell within the uncertainty range of ( 7 kJ/mol. This observation is further supported by the PJK analysis. Parker et al.36 have shown that, when the intrinsically correct desorption order is used, a plot of [ln(-dθ/dt) - n lnθ] versus 1/T should yield a straight line the slope of which can be used to determine the desorption energy and the intercept the pre-exponential factor. We have performed this analysis for the low coverage films where no repulsive interaction among the molecules is present. For example, in the case of the 0.08 ML film, we have obtained the most linear behavior for n ) 1 and from the linear fit E0 ) 141.5 ( 1.5 kJ/mol and k0 ) (6.8 ( 0.7) 1012 s-1. These results support our assumption on the order of desorption as well as the choice of the pre-exponential factor.
A. Charge-Transfer Bonding and Interface Dipole Moment. As explained in the work by Crispin et al.37 the interface dipole formed at an organic/metal interface arises from two contributions: one induced by a partial charge transfer (as opposed to a complete charge transfer as in the case of alkali atoms on transition-metal surfaces38) between the layers of organic molecules and the metal substrate upon chemisorption, and the other related to the change in the metal surface electron density in response to the presence of the adsorbed organic molecules on the surface. The presence of an interface dipole causes a change in the surface work function of the Ag(111) substrate (4.46 ( 0.02 eV39) due to the electric field of the dipole layer.37 For tetracene, a negative work function change upon adsorption has been observed,19 suggesting donation of negative charge from the molecule to the Ag substrate. Without taking into account any depolarization effect, the work function change, ∆Φ, can be related heuristically to the dipole moment through the Helmholtz equation:40,41∆Φ ) 2πµθ. Using the coverage of 1014 molecules/cm2 21 and the dipole moment obtained for the tetracene monolayer µ0 ) 8.2 ( 2.1 D, we estimate a work function change of -1.5 ( 0.3 eV, which is yet to be confirmed experimentally. Frank and co-workers found a decrease in work function of 0.5 eV for a film of 2-3 monolayers of tetracene on Ag(111).19 The bigger decrease in work function we estimate in our work can be explained by the fact that we are studying tetracene films in the submonolayer regime, and the work function change is known to decrease drastically after the first monolayer is completed because of depolarization effects related to increase in coverage.41-45 The value of -1.5 ( 0.3 eV is also comparable to the value of -0.8 eV found for a monolayer of tetracene on Ag(110)44 and of -0.7 eV found in the case of Au(111).46 There are, however, several factors to take into account in explaining
4702 J. Phys. Chem. C, Vol. 112, No. 12, 2008 the different values in the work function change such as the adsorption site and substrate roughness.47 For example, Gland and Somorjai compared the systems of benzene, naphthalene, and pyridine on Pt(111) or Pt(100).45 They showed that the deposition flux as well as thermal treatments may influence the ordering and structure of the film which in turn effects the work function change. Two very recent works by White and coworkers on phenylacetylene42 and phenyl isocyanide43 on Cu(111) enhance this point. The two molecules appear to weakly chemisorb on Cu(111) with desorption activation energy of 108 and 105 kJ/mol, respectively. Surprisingly, the change in work function induced by adsorption is very different: -0.75 eV for phenylacetylene and -2.55 eV for phenyl isocyanide. This striking difference is attributed to factors such as the adsorption geometry as well as the molecular dipole moment (phenyl cyanide has a large permanent dipole ∼4 D compared with phenylacetilene’s 0.66 D). The amount of partial charge transfer in the interface bonding can be calculated from the interface dipole moment assuming that there is only the contribution due to charge transfer. Assuming that the distance between the tetracene molecular plane and the Ag surface electron density boundary is the same as the one estimated for benzene/Ag, 2.3 Å,48 we estimate the partial charge transfer to be approximately 0.4 ( 0.1 e. Obviously this value is strongly dependent upon the value of the interfacial bonding distance. This estimate suggests that each ring in the tetracene molecule on average donates a charge of about 0.1 e to the metal surface. This is consistent with the value we obtained for naphthalene adsorbed to Ag(111) which is also 0.1 e per ring.23 Adsorption-induced charge transfer influences also the magnitude of the adsorption energy. Usually molecules with larger electron affinities or lower ionization potentials are likely to transfer more charge47 as we have found comparing tetracene with benzene22 and naphthalene.23 B. Effect of Annealing on the Monolayer Structure. The information concerning the adsorption structure and energy deduced from our TPD experiments should be considered in light of the LEED study by Langner et al.21 This study revealed a number of temperature-dependent phases within the monolayer coverage. Briefly, they reported that deposition with saturation coverage at 300 or 230 K with subsequent cooling results in two phases labeled R and β respectively. In the R phase, tetracene is presumed to have a flat orientation parallel to the surface with one molecule per unit cell, whereas in the metastable β phase, tetracene tilts with respect to the surface with two molecules per unit cell. Long-range order of either phase is lost at room temperature. Deposition below 100 K produces an amorphous film due to reduced surface mobility of tetracene at low temperatures. Nonetheless, it seems that, no matter which is the initial phase, monolayer desorption proceeds from a disordered film since neither of the two phases is stable above 300 K. Considering what Langner et al.21 have found, we expect the TPD spectra of submonolayer tetracene films to be identical regardless of any prior annealing treatment. This is, however, not what we observed. In fact, we found that annealing tetracene films has resulted in different TPD spectra as shown in Figure 4a. Likely when the amorphous film deposited at 90 K is heated above 300 K (350 K or 400 K) for tens of minutes and then cooled to 90 K, the more stable long-range R phase forms. Then during the TPD process, desorption occurs from a structure with maximum inter-adsorbate distance because of the strong intermolecular repulsion. This structure may have somewhat ordered geometry with hexagonal symmetry on the (111) surface. The
Gonella et al. TPD spectra of the unannealed monolayer are, on the other hand, from the amorphous phase since the reduced mobility at 90 K does not allow the formation of the R phase with long-range order. Heating of the amorphous film during the TPD process anneals the structure to a more ordered phase at the higher temperature regime and consequently TPD curves at higher temperatures appear similar. Using our results and eq 8, we calculate the desorption energy at the onset of desorption for one full monolayer to be 105 ( 14 kJ/mol. This low value is apparently due to strong interadsorbate repulsion at the saturated coverage. C. Multilayer Structure. The multilayer TPD spectrum of tetracene on Ag(111) shows three separate peaks very close in desorption temperature and energy. This result is not unexpected due to polymorphism that is commonly associated with films of organic molecules.1,7 Line shape analyses of the peaks revealed an order of n ) 1/2 or n ) 1 for the desorption kinetics. This determination is mainly significant for the R4 peak that is the TPD feature clearly associated with the bulk multilayer formation. The other two features, R2 and R3, are transitional layers in between the first monolayer and the multilayer. Fractional order desorption kinetics are typically associated with desorption from islands or clusters whose desorption area decreases as desorption progresses. This result confirms the Stranski-Krastanov growth mode revealed by Langner et al.21 for tetracene on Ag(111): In that study, the monolayer LEED pattern and the LEED spots of the Ag(111) substrate could still be detected even up to 50 ML, suggesting the presence of small crystallites with a large height-to-width ratio covering only a minor fraction of the monolayer surface. It is shown that R2 can be transformed into R3 and possibly a small amount into R4 by annealing the multilayer films. This observation indicates that R2 is metastable and is consistent with a common feature in the deposition of aromatic multilayers: As the bulk crystal forms, it incorporates the metastable, first physisorbed layer (R2 peak) into its structure.32,49,50 Since no evidence was found for transformation of R3 into R4 upon annealing, we speculate that the R3 and R4 peaks are due to crystallites having slightly different morphologies with R4 being slightly more stable with a higher minimum nucleation size. Moreover, the desorption energy of R4, 116 ( 4 kJ/mol, is closer to the value reported by Langner et al. for the multilayer (120130 kJ/mol)21 as well as to the bulk sublimation energy of 126 ( 9 kJ/mol.51 VI. Conclusions TPD of monolayer tetracene film adsorbed on Ag(111) revealed strong inter-adsorbate repulsion caused by interaction among interface dipoles. This is a result of charge-transfer bonding between the aromatic molecule and the metal surface. A modified Albano model, in which the molecule-substrate interfacial dipole is treated as consisting of four point dipoles, one for each aromatic ring, is proposed to account for the interfacial dipole interaction. The desorption energy at the zerocoverage limit is determined to be E0 ) 142 ( 7 kJ/mol while the interface dipole is µ0 ) 8.2 ( 2.1 D. The dipole roughly corresponds to a partial charge transfer of 0.4 e from the tetracene molecule to the Ag substrate. At full monolayer coverage, the strong inter-adsorbate repulsion reduces the desorption energy to 105 ( 14 kJ/mol. Annealing at elevated temperature (350-400 K), but below the desorption temperature, on minute time scale, followed by cooling to 90 K, appears to produce a more stable structure, likely the R phase reported in ref 21.
Tetracene on Silver Multilayer TPD spectra show three separate half-order or firstorder desorption peaks that merge into one peak at higher coverages. The half-order kinetics agrees with the previously reported Stranski-Krastanov growth mode in which islands with high height-to-width ratio are formed. The desorption energies for these peaks are 100 ( 7, 110 ( 10, and 116 ( 4 kJ/mol, that well compares with the value reported in literature for the multilayer and the tetracene bulk sublimation energy of 126 ( 9 kJ/mol. Upon annealing, the lower energy structure transforms into the higher energy ones. Acknowledgment. This work is supported in part by a grant from the Air Force Office of Scientific Research. Appendix For a hexagonal array as the one reported in Figure 5a, SA(rij) and SB(rij) can be expressed in eq 5 as: SA(rij) )
4 3 2 + + + R3 (R2 - Rd + d2)1.5 (R2 - 2Rd + 4d2)1.5 1 1 + + (R2 - 3Rd + 9d2)1.5 (R2 + 3Rd + 9d2)1.5 2 3 + (R2 + 2Rd + 4d2)1.5 (R2 + Rd + d2)1.5
SB(rij) )
4 3 2 1 + + + + R3 (R - d)3 (R - 2d)3 (R - 3d)3 1 2 3 + + (9) (R + 3d)3 (R + 2d)3 (R + d)3
For convenience of description, we designate the quantity in parentheses on the right-hand side of eq 5 as ST. The relation R ) {(4/3)1/4}/xθ for an hexagonal array34 and the value of 100.0 Å2 for the geometric area of the planar adsorbed tetracene molecule as obtained for the R phase21 are used. (No change is observed if the value from the hard sphere model for tetracene,21 95.9 Å,2 is used.) After substituting the quantities in eq 9, expressed as a function of coverage, in eq 5, we have in SI units ET/µ02 ) 6.025 × 10-29(a1θ1.5 + a2θ2 + a3θ2.5) where ET is in kJ/mol, µ0 is in D, and θ is in m-2. ST is first calculated for the 0-1 ML coverage range using eq 9 and then fitted to the polynomial form of θ as reported in eq 6. From the fit, we obtain a1 ) 14.8 ( 0.4, a2 ) (-18.8 ( 0.9) 10-9 m and a3 ) (16.4 ( 0.6) 10-18 m2. References and Notes (1) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. DeV. 2001, 45, 11. (2) de Boer, R. W. I.; Klapwijk, T. M.; Morpurgo, A. F. Appl. Phys. Lett. 2003, 83, 4345. (3) Gundlach, D. J.; Nichols, J. A.; Zhou, L.; Jackson, T. N. App. Phys. Lett. 2002, 80, 2925. (4) Hepp, A.; Heil, H.; Weise, W.; Ahles, M.; Schmechel, R.; von Seggern, H. Phys. ReV. Lett. 2003, 91. (5) Verlaak, S.; Steudel, S.; Heremans, P.; Janssen, D.; Deleuze, M. S. Phys. ReV. B 2003, 68.
J. Phys. Chem. C, Vol. 112, No. 12, 2008 4703 (6) Cicoira, F.; Santato, C.; Dinelli, F.; Murgia, M.; Loi, M. A.; Biscarini, T.; Zamboni, R.; Heremans, P.; Muccini, M. AdV. Funct. Mater. 2005, 15, 375. (7) Witte, G.; Woll, C. J. Mater. Res. 2004, 19, 1889. (8) Brinkmann, M.; Graff, S.; Straupe, C.; Wittmann, J. C.; Chaumont, C.; Nuesch, F.; Aziz, A.; Schaer, M.; Zuppiroli, L. J. Phys. Chem. B 2003, 107, 10531. (9) Abthagir, P. S.; Ha, Y. G.; You, E. A.; Jeong, S. H.; Seo, H. S.; Choi, J. H. J. Phys. Chem. B 2005, 109, 23918. (10) You, E.-A.; Ha, Y.-G.; Choi, Y.-S.; Choi, J.-H. Synth. Met. 2005, 153, 209. (11) Santato, C.; Manunza, I.; Bonfiglio, A.; Cicoira, F.; Cosseddu, P.; Zamboni, R.; Muccini, M. App. Phys. Lett. 2005, 86. (12) Shi, J.; Qin, X. R. Phys. ReV. B 2006, 73, 121303. (13) Rada, T.; Chen, Q.; Richardson, N. V. J. Phys.: Condens. Mater. 2003, 15, S2749. (14) Rada, T.; Chen, Q.; Richardson, N. Phys. Status Solidi B 2004, 241, 2353. (15) Lu, B.; Zhang, H. J.; Huang, H.; Mao, H. Y.; Chen, Q.; Li, H. Y.; He, P.; Bao, S. N. Appl. Surf. Sci. 2005, 245, 208. (16) Milita, S.; Servidori, M.; Cicoira, F.; Santato, C.; Pifferi, A. Nucl. Instrum. Meth. B 2006, 246, 101. (17) Yannoulis, P.; Frank, K. H.; Koch, E. E. Surf. Sci. 1991, 243, 58. (18) Chen, Q.; McDowall, A. J.; Richardson, N. V. Langmuir 2003, 19, 10164. (19) Frank, K. H.; Yannoulis, P.; Dudde, R.; Koch, E. E. J. Chem. Phys. 1988, 89, 7569. (20) Yannoulis, P.; Dudde, R.; Frank, K. H.; Koch, E. E. Surf. Sci. 1987, 189, 519. (21) Langner, A.; Hauschild, A.; Fahrenhoz, S.; Sokolowski, M. Surf. Sci. 2005, 574, 153. (22) Rockey, T. J.; Yang, M.; Dai, H.-L. J. Phys. Chem. B 2006, 110, 19973. (23) Rockey, T.; Dai, H.-L. Surf. Sci. 2007, 601, 2307. (24) King, D. A. Surf. Sci. 1975, 47, 384. (25) Berko, A.; Erley, W.; Sander, D. J. Chem. Phys. 1990, 93, 8300. (26) Lu, P. H.; Lasky, P. J.; Yang, Q. Y.; Wang, Y. B.; Osgood, R. M. J. Chem. Phys. 1994, 101, 10145. (27) Garrett, S. J.; Holbert, V. P.; Stair, P. C.; Weitz, E. J. Chem. Phys. 1994, 100, 4615. (28) Maschhoff, B. L.; Ledema, M. J.; Kwini, M.; Cowin, J. P. Surf. Sci. 1996, 359, 253. (29) Livneh, T.; Asscher, M. J. Phys. Chem. B 1997, 101, 7505. (30) Livneh, T.; Lilach, Y.; Asscher, M. J. Chem. Phys. 1999, 111, 11138. (31) Albano, E. V. J. Chem. Phys. 1986, 85, 1044. (32) Yang, M. C.; Rockey, T. J.; Pursell, D.; Dai, H. L. J. Phys. Chem. B 2001, 105, 11945. (33) Rockey, T. J.; Yang, M. C.; Dai, H. L. Surf. Sci. 2005, 589, 42. (34) Topping, J. Proc. R. Soc. London A 1927, 114, 67. (35) Danisman, M. F.; Casalis, L.; Scoles, G. Phys. ReV. B 2005, 72. (36) Parker, D. H.; Jones, M. E.; Koel, B. E. Surf. Sci. 1990, 233, 65. (37) Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Bredas, J. L. J. Am. Chem. Soc. 2002, 124, 8131. (38) Lang, N. D. Phys. ReV. B 1971, 4, 4234. (39) Chelvayohan, M.; Mee, C. H. B. J. Phys. C: Solid State Phys. 1982, 15, 2305. (40) Macdonald, J. R.; Barlow, C. A. J. Chem. Phys. 1963, 39, 412. (41) Schmidt, L.; Gomer, R. J. Chem. Phys. 1965, 42, 3573. (42) Sohn, Y.; Wei, W.; White, J. M. J. Phys. Chem. C 2007, 111, 5101. (43) Sohn, Y.; White, J. M. J. Phys. Chem. C 2007, 111, 7816. (44) Tao, Y. Z.; Hanjie, Lu, Bin, Huang, Han, Li, Haiyang, Bao, Shining, He, Pimo. Zhenkong Kexue Yu Jishu Xuebao 2005, 25, 1. (45) Gland, J. L.; Somorjai, G. A. Surf. Sci. 1973, 38, 186. (46) Lindstrom, C. D.; Muntwiler, M.; Zhu, X. Y. J. Phys. Chem. B 2007, 111, 6913. (47) Somorjai, G. A. Introduction to surface chemistry and catalysis; Wiley: New York, 1994. (48) Anderson, A. B.; McDevitt, M. R.; Urbach, F. L. Surf. Sci. 1984, 146, 80. (49) Jakob, P.; Menzel, D. J. Chem. Phys. 1996, 105, 3838. (50) Huang, W. X.; White, J. M. J. Phys. Chem. B 2004, 108, 5060. (51) Oja, V.; Suuberg, E. M. J. Chem. Eng. Data 1998, 43, 486.
