J. Phys. Chem. 1995,99, 11770-11779
11770
Ordered Ultrathin Films of Perylenetetracarboxylic Dianhydride (PTCDA) and Dimethylperylenebis(dicarboximide) (Me-PTCDI) on Cu(100): Characterization of Structure and Surface Stoichiometry by LEED, TDMS, and XPS A. Schmidt? T. J. Schuerlein,' G. E. Collins,* and N. R. Armstrong" Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received: April 3, 1995@
Ordered monolayer and multilayer thin films of perylene-3,4:9,1O-tetracarboxylicdianhydride (PTCDA) have been grown on Cu(100) and MoS2(0001) single crystals. Low-energy electron diffraction (LEED) investigations have shown that PTCDA grows on Cu(100) in a commensurate rectangular lattice ( 4 d 2 x 5 d 2 ) R = 45", whose dimensions (bl = 14.5 A, b2 = 18.1 A) are significant different from those seen on the (0001) cleavage faces of single-crystal MoS2, where a coincident rectangular lattice with bl = 13.1 A and b2 = 21.2 A is formed. This latter structure is much closer to the packing in the (102) plane of the bulk crystal of PTCDA. X-ray photoelectron spectra (XF'S) and thermal desorption mass spectroscopy (TDMS) show that the first monolayer of PTCDA is strongly chemisorbed on the Cu surface. During the reaction with the copper surface each PTCDA molecule apparently loses the two bridging oxygen atoms in the anhydride groups, leading to a molecular system with close to the same dimensions and symmetry as the parent molecule. XPS data show that the adsorption on the surface of the weakly interacting metal dichalcogenide MoS2 is not accompanied by such a surface reaction. Support for this hypothesis was obtained by deposition of N,N-dimethylperylene3,4:9,1O-bis(dicarboximide) (Me-PTCDI) on the Cu(100) surface. Me-PTCDI forms a rectangular (6 x 8) structure on Cu(lOO), with bl = 15.3 8, and b2 = 20.4 A. XPS data obtained for the deposition of this molecule on clean copper show that Me-PTCDI apparently loses both nitrogen imide groups, for the first monolayer adsorbed. TDMS data confirm this reactivity of Me-FTCDI. Me-PTCDI thin films appear to grow as islands, starting with the second monolayer, in contrast to PTCDA, which appears to adopt a layerby-layer growth mode for the first six monolayers. Differences in the bulk crystal structures of these two molecules are believed to lead to these differences in growth mode; the layered bulk structure of PTCDA lends itself to a layered growth mode in multilayer ultrathin films.
Introduction There is presently widespread interest in the development of epitaxial thin films of large organic molecules on substrates ranging from insulators to semiconductors and metals.'-' A variety of interesting optical properties result when highly ordered films of molecules like the phthalocyanines, naphthalocyanines, perylenes, fullerenes, etc., are produced with welldefined molecular architectures, which optimizes the excitonic interactions in excited states of these molecules.2s6 Interfacing of two dissimilar molecular thin films, with sufficiently different electron affinities and ionization potentials, has also been shown to produce rectifying, diode-like, junction^.^^^.^ Electroluminescence with high efficiency has been recently observed from heterojunction organic diodes based upon strongly luminescent dyes and various organic hole and electron transport agents.I0 In addition, there is continued interest in the electronic properties which might be achieved in organic materials fabricated into heterojunction devices of molecular dimensions. Some of our recent studies have focused on the characterization of bilayer and multilayer thin films of various phthalocyanines (Pc) and the perylene dye perylene-3,4:9,lO-tetracarboxylic dianhydride (PTCDA) (Scheme 1).8b,c In addition to the
* To whom correspondence should be addressed.
' Present address:
Max Planck Institute fur Polymerforschung, Mainz, Germany. Present address: Department of Chemistry, Kansas State University, Manhatten, KS. 5 Present address: Naval Research Laboratories, Geo Centers, 10903 Indian Head Highway, Ft. Washington, MD 20744. Abstract published in Advance ACS Abstracfs, July 1, 1995.
*
@
SCHEME 1: Molecules Investigated in This Study and Metal-Phthalocyanine
3,49,1O-Tetracarboxylic-dianhydride (PTCDA)
H
rectification properties of the bilayers, it was observed that the transient photocurrent of such multilayers was directly proportional to the number of PcPTCDA interfaces, down to dimensions of each thin film of ca. 30 A. The origin of this enhancement in photoconductivity is presumed to lie in the facilitation of the dissociation of excitons, created in both the perylene and phthalocyanine layers, by the electric field gradients produced at the Pc/PTCDA interface.6.8-'0 Further exploration of this phenomenon is problematic without the use
0022-365419512099-11770$09.00/0 0 1995 American Chemical Society
Ordered Ultrathin Films of PTCDA and Me-PTCDI of highly ordered thin films, which permit the formation of moelcularly sharp interfaces, preferably on conductive substrates. Ordered monolayer films of large aromatic dyes on singlecrystal metal substrates have been achieved for molecules such as benzene, naphthalene, tetracene, tetracyano-p-quinodthane (TCNQ), phthalocyanines, and perylene dianhydrides."J2 Commensurate packing structures may be formed; however, chemisorption is presumed in many instances to involve some dissociation of these molecules or at least significant charge transfer to form products which may be different from the parent molecule. The boundaries between dissociative and molecular adsorption for such large molecules on a wide variety of metals have yet to be fully defined.'Ia Many of these same molecules have now been deposited epitaxially on substrates which interact weakly with organic overlayers, including the basal planes of layered semiconductors such as MoS2 and SnS2, the cleavage faces of single-crystal alkali halide salts, and highly ordered pyrolitic Coincident, rather than commensurate, surface lattices are mainly formed. The molecular architecture in such overlayers is controlled by a combination of intermolecular interactions and the summation of weak interactions between the substrate and molecular crystalline domains with diameters exceeding 5-10 molecular units.2f It has been demonstrated that ordered multilayers may also be occasionally formed, with structures which may be related to the bulk structures of these materials.2e-g It is believed that PTCDA can strongly chemisorb on different substrates via its anhydride oxygen groups, with the possibility of reaction on certain metal surface^.^'^-'^^^ The electrochemical reactivity (first reduction potential) and/or the electron affinity of PTCDA shows that this compound may act as a weak electron acceptor in the presence of low work function metal^.*^,'^ The potential reactivity of the anhydride groups in PTCDA also raises questions about its long-term stability in heterojunctions and superlattices, where a solid state reaction can take place between this dye and another material, which ultimately destroys both the geometric and electronic structure of the heterojunction.2J Reactive adsorption as monolayers and solid state reactions with other organic dyes could therefore strongly influence the result of investigations in thin organic superstructures. In this paper we present studies involving the formation of ordered PTCDA monolayers and multilayers on Cu(100) and MoS2(0001) substrates. Previously reported low-energy electron diffraction (LEED) data for PTCDA monolayers on both substrates are reviewed, and thermal desorption mass spectrometry (TDMS) data have been added to further characterize the structures of monolayers on the Cu(100) substrate. The interaction of PTCDA with the copper surface is sufficiently strong as to produce TDMS and X P S data consistent with reactive adsorption with the Cu surface. X-ray photoelectron spectroscopy studies ( X P S ) of polycrystalline ultrathin films of PTCDA on foils of atomically clean Cu, and on cleaved faces of MoS2, c o n f i i the reaction of PTCDA monolayers on Cu, while no decomposition of PTCDA was found on MoS2. To help determine what part of the perylene dianhydride molecule may be reacted during adsorption on the copper surface, we also investigated the adsorption of a related molecule, N~-dimethylperylene-3,4:9,1O-bis(dicarboximide) (Me-PTCDI) (see Scheme 1) on amorphous on Cu(100) surfaces. These two perylene derivatives differ in only one functionality. The oxygen in the anhydride is replaced by a N-CH3 group in the dicarboximide. Both molecules form similar bulk crystal structures (monoclinic, P21/c, Z = 2).5b9'5 Similar reactivity involving losses of the nitrogen heteroatoms
J. Phys. Chem., Vol. 99, No. 30, I995 11771 is indicated for adsorption of monolayer levels of this molecule on the Cu(100) surface, but multilayer Me-PTCDI formation appears to involve different growth mechanisms than for PTCDA thin films.
Experimental Section PTCDA was obtained from Aldrich, and Me-PTCDI was obtained from Hoechst; both materials were purified by entrainer sublimation before use. Deposition and characterization of the ordered thin films for LEED and TDMS were carried out in a previously described vacuum chamber.2 A Cu(100) single crystal (ca. 2 cm2 total surface area) was obtained from Commercial Crystal Laboratories, Inc., aligned, and polished, with a mirror finish on one surface. Following routine cleaning of the surface the crystal was mounted on the sample stage of a Kurt Lesker VG-Omniax transfer rod, which allowed for heating of the crystal during argon ion sputtering. Several sputter/anneal cycles (sputtering: 3 keV argon ion beam, 10 pA/Cm2, Tsubsuate = 450 OC; annealing at 600 "C) were used to bring the crystal to an acceptable level of surface cleanliness (less than 5% of a monolayer carbon and oxygen and an absence of sulfur, as judged by Auger spectroscopy). Deposition of perylene dye layers was carried out through the use of previously described Knudsen cell sources, at a chamber base pressure of ca. Torr. Deposition rates were typically in the range of 10 equivalent monolayers (ML) per hour, in order to achieve the appropriately ordered layers. Coverages of the dyes in question were estimated from the response of a quartz crystal thickness monitor (QCM) placed in proximity to the Cu(100) crystal during deposition and from the deposition times necessary to degrade the LEED diffraction image from the Cu(100) substrate. The Cu( 100) crystal was maintained at a temperature of ca. 130 "C for the deposition of PTCDA and Me-PTCDI monolayers. After each organic depositiodcharacterizationcycle, the surfacecleaning procedure described above was repeated. For the growth of PTCDA on MoS2, a freshly cleaved crystal was first annealed overnight at ca. 200 OC. These surfaces were observed by Auger spectroscopy to be largely free of surface contamination (less than 5% of a monolayer of carbon and no oxygen present). Depositions on MoSz were carried out at a surface temperature of ca. 100 OC. LEED analysis was carried out using an Omicron SpectaLEED operated at ca. 80 eV to observe substrate diffraction spots (at these beam voltages overlayer diffraction features are very condensed) and ca. 15 eV for observation of the overlayer pattern (at these energies the substrate diffraction features were not observable). In all of the LEED experiments the beam current was held below 2 pA. At low overlayer coverages (1-3 ML) both overlayer and substrate diffraction spots were visible by adjusting the beam energy appropriately. The ability to observe both sets of diffraction features simplified the determination of the angle made by the overlayer unit cell with the substrate, once the overlayer symmetry was solved. The precision of these azimuthal angle and diffraction spot radius measurements for these overlayers was ca. &3%. LEED images were captured from the phosphor-coated back screen by means of a 5 12 x 512 pixel CCD camera, coupled to a Data Translation DT28.53 frame grabber.2a The LEED images were occasionally contrast enhanced by means of a Fourier filtering algorithm. Electron beam damage, as measured by changes in molecular ordering or changes in LEED diffraction spot intensity, was not detectable during these LEED investigations. The electron beam used for this experiment was sufficiently narrow (ca 0.1 mm diameter) that even if extensive electron beam damage did occur
Schmidt et al.
11772 J. Phys. Chem., Vol. 99, No. 30, I995 over the LEED analysis area, it would affect less than 5% of the overall sample area, as analyzed by the TDMS experiments (see below). TDMS experiments were carried out using a FISON S X P Elite quadrupole mass spectrometer, generally poised at a massto-charge (dz)ratio corresponding to a major fragment of the desorbing parent molecule. The major fragment d z ratios were identified by exposing the mass spectrometer directly to the Knudsen cell source operating to produce high fluxes of the perylene dye. The fragment used to identify the presence of intact PTCDA and Me-PTCDI was at a mass-to-charge ratio ( d z ) of 124, which corresponds to a naphthalene subunit of that molecule. The molecular ions of PTCDA and Me-PTCDI were not generally used because of instrument sensitivity limitations, although all of the general features of the TDMS data have been reproduced using mass spectrometer data from the molecular ion ( d z = 392), as reported previously.2i Since the fragmentation patterns are identical for the vapor-phase PTCDA and for the molecules desorbing from the Cu(100) surface, we assume that the majority of the desorbing dye leaves the surface as the intact fragment (except from the first monolayer) and that electron beam induced ionization in the mass spectrometer results in efficient fragmentation, to yield the most easily detected naphthalene ion. The concentration of this ion in the mass spectrometer is proportional to the coverage of the intact molecule on the copper surface. Heating of the Cu(100) crystal during TDMS experiments was achieved through a Eurotherm 815 temperature controller, coupled to a thermocouple feedback circuit which allowed for radiative heating of the Cu( 100) crystal by a 0.5 mm tungsten filament, producing changes of temperature at the surface of the crystal of 5 K/s for all desorption experiments described in this study. This configuration leads to very uniform heating across the crystal, as compared to experimental configurations where the crystal was directly heated by passing current through the sample. Temperature sensing occurred by means of a type K thermocouple, spot welded to the backside of the Cu crystal. The mass spectrometer RC time constant was set at 10 ms, which was low enough to ensure a lack of instrumental distortion of the desorption features. An aluminum mask with a ca. 7 mm hole was placed between the adsorbate-covered crystal and the ionization source of the mass spectrometer to prevent the introduction of any species that was not directly desorbed from the surface of the crystal into the spectrometer. XPS data were acquired at room temperature with a VG ESCALAB MKII spectrometer, using an Al Ka source (1486.6 eV), at a power of 280 W. The analyzer pass energy typically used was 50 eV and the energy increment was 0.1 eV for the measurements. Data analysis was carried out using standard inelastic photoelectron background correction schemes.15 Relative atomic ratios of the elemental constituents of the PTCDA film were determined as previously described,16 by correcting the areas of each peak for escape depth, photoemission probability, and the energy dependence of the analyzer transmission function. Overlapping peaks were determined by a nonlinear least-squares fitting routine, using Gaussianbrentzian peak shapes (typically 0.9/0.1) to approximate the real line shape. The binding energies were referred to the copper 2~312 peak for the clean surface, the position of which was set as 932.60 eV.17 For the XPS experiments clean polycrystalline copper surfaces were prepared by argon sputtering of high-purity metal foils inside the spectrometer sample preparation chamber. Naturally occurring MoS2 was freshly cleaved and put immediately into vacuum. Depositions of ultrathin perylene dye films for XPS
I
11-
e
e
e
e
e
y
.
Domoin 2 1
e
.&A
e
e
e
:b?&$g
e
e
e
e
e
e
e e
e e
e o e
e 0 e
e
e
e
e
0
.
e
e
e
e
e
e
:: : ::: e
e
e
I 1
I
e
e
e
e
e
e
e
e
e e
e e e
e
e e
e
e e
Figure 1. (a) LEED images from a 1.5 ML PTCDA/Cu( 100) assembly, (b) a schematic of this LEED data showing all the low index spots, some of which are not visible in (a) because of the glide plane symmetry in the unit cell of this overlayer (see text), and (c) proposed packing structure for a monolayer of PTCDA on the Cu( 100) surface. Intact molecules are shown in this packing structure merely to suggest the spacings determined from the LEED data.
studies were carried out at a rate of 0.1-0.3 ML/min done in an antechamber (base pressure Torr) coupled to the spectrometer. Deposition rates and film thicknesses were determined via a quartz crystal thickness monitor (QCM,fo = 10 MHz, Af = 12 Hz per equivalent monolayer) mounted within the deposition chamber. The repeatability and accuracy of the thickness measurements were ca. 10%.
Results and Discussion PTCDA on Cu(100) and MoS2(0001); Me-PTCDI on Cu(100): LEED. Figure la,b,c reproduces recently reported LEED data, a proposed packing structure for PTCDA on Cu( loo), and the expected LEED pattern from such an overlayer structure.2i Electron beam energies of 26.3 eV were used for the characterization of the PTCDA layers. The unit cell for this system may be characterized as ( 4 4 2 ~ 5 4 2 ) R= 45" with bl = 14.5 A, b2 = 18.1 A. The rotation of the unit cell for this overlayer, with respect to the Cu(100) substrate, was obtained from LEED data taken with electron beam energies of ca. 78 eV, at PTCDA, coverages of up to monolayer levels. This 2-fold symmetric overlayer has two nonequivalent domains on the surface, rotated with respect to each other by 90". The diffraction features shown in the simulated data indicate, by open and filled circles, which diffraction features originate from the same domain of the PTCDA overlayer. At the beam voltage used for this LEED pattern the substrate diffraction features are not visible. Observation of the substrate diffraction pattern, after the deposition of the first monolayer of PTCDA, showed that the substrate pattern was not altered in any way, as compared to the clean
J. Phys. Chem., Vol. 99, No. 30, 1995 11773
Ordered Ultrathin Films of PTCDA and Me-PTCDI surface. The result of PTCDA adsorption can be desribed as a commensurate rectangular lattice with two PTCDA molecules per unit cell on the Cu(100) surface. It should be noted that diffraction patterns such as these could only be obtained when the Cu(100) surface was held at temperatures greater than 100 "C during the deposition process. As with previous studies of PTCDA on various substrates, and for naphthalene on other metal single-crystal surfaces, diffraction patterns are seen which are controlled by the glide plane symmetry in the unit cell.11bJ8 Similar glide plane symmetry effects have also been recently observed after deposition of monolayers of pentacene mono- and diquinones, on the Cu( 100) These are molecular systems which produce nearly rectangular unit cells, through strong interaction between the quinone oxygens and the copper surface. Because of the glide symmetry in such molecular systems, some diffraction spots are extinguished for normal incidence LEED analysis (the (n,O) and (0,n)spots, where n is odd) in such systems. To produce the diffraction image in Figure la, electron beam energies were intentionally used which degraded the resolution in the (1,l) spots, in order to observe the higher order spots. These higher order spots are often critical in the interpretation of complex diffraction patterns. For precise measurement of the positions of the (1,l) diffraction spots, from which unit cell dimensions are confiied, slightly lower electron beam energies were used (data not shown here). In Figure IC the PTCDA molecules are schematically positioned on the Cu surface to place an overlayer unit cell coincident with an atop copper site (the copper atoms are represented by solid circles). Reactive adsorption occurs for PTCDA on copper (see below); therefore, this figure is only a representation of the orientation of the multilayer PTCDA unit cell with respect to the underlying substrate lattice. We hypothesize below that the first monolayer in contact with the surface would not consist of an intact molecule, but rather a perylene dianhydride parent species, missing the bridging oxygen atom from each end of the molecule. It has been shown that PTCDA forms a much more rectangular surface unit cell for adsorption on graphite (bl = 12.5 A,b2 = 19.1 and for adsorption on MoS2.1f In both cases coincident superlattices rather than commensurate lattices are observed, with much weaker interaction between the substrate and the overlayer PTCDA, than is apparently the case for PTCDNCu(l00). Figure 2 shows the LEED data of a thin film of PTCDA on MoS2, a proposed packing structure, and the LEED pattern expected from such a packing configuration. The adsorption of a molecule residing in a unit cell with 2-fold symmetry on a hexagonal substrate, such as MoS2, leads to three nonequivalent domains of material on the surface, if one of the axes of the overlayer unit cell is aligned with the axes of the substrate. In the case of PTCDA on MoS2 the unit cell axis does not align with a principal substrate axis, leading to the formation of six nonequivalent domains on the surface, which are indicated in Figure 2b. The rectangular superstructure has the dimensions bl = 13.1 8, and b2 = 21.2 8, and can be described by
with the dimensions a1 = a2 = 3.16 8,for the hexagonal MoS2 surface. The structure of PTCDA on MoS2 also possesses glide symmetry. The experimental LEED data and its interpretation for the deposition of Me-PTCDI on Cu(100) are shown in Figure 3a,b,c.
Domain2 Domain3
Domain 4
Damin Domain6 5
.
0
.
.
.
.
.
m
.
.
.
.
.
.
.
a''
0
.
. . .
.
e
0
.
"
Figure 2. (a) LEED image from 1 ML FTCDA on MoS2 and (b) a schematic of this LEED data obtained by simulating the diffraction data of the (c) proposed packing structure of a unit cell consisting of two PTCDA molecules.
This perylene derivative forms a (6 x 8) rectangular lattice, with bl = 15.3 8, and b2 = 20.4 8,. As for PTCDA, this rectangular unit cell has a lower aspect ratio than would be seen for a comparable plane of the bulk lattice. For this overlayer very weak {0,1} overlayer diffraction features were generally present, which are difficult to transmit to a printed LEED image like Figure 3a. These spots are much weaker than the secondorder peaks. There are two possible explanations for the observation of these { 1,O) reflections: (a) the glide symmetry of the overlayer cell may be weakly broken or (b) a separate domain of material is present on the surface which does not possess the same unit cell. Since the { 1,0} reflections evident for these films are of the correct dimension and orientation for their assignment to the previously described unit cell, it is believed that glide symmetry for this overlayer is not strictly adhered to. In PTCDA films, at coverages greater than 6 ML, weak {O,l} overlayer spots were also seen. In this case, however, as these diffraction features became evident, so did a series of other diffraction spots (see next section). This indicates that the appearance of these low-order spots is caused by the formation of a crystalline structure that does not possess the same symmetry as the major component of the overlayer. Further evidence for the difference in growth morphologies between PTCDA and Me-PTCDI on this surface will be given below. PTCDA and Me-PTCDI on Cu(lO0): TDMS. Figure 4 gives the TDMS data for a series of coverages of PTCDA on the Cu(100) surface. All of the desopriton peaks for this molecule have a fwhm of less than 25' K. These data should be viewed primarily as indicating the temperatures of sublimation of PTCDA from this unique ultrathin film environment, and the desorption energies are calculated recognizing that the system is not likely to be in local thermal equilibrium, as is often assumed for small molecule desorption from surfaces.l9
r
11774 J. Phys. Chem., Vol. 99, No. 30, I995
O Domain m i n2 1
..................... .. ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... . .. .. .. .. .. .... ..... ..... .. .. .. .. . .. .. .. .. .. . . . .. .. .. .. .. . . . . . . ..................... .................... 0
.
0
0
.
.
0
.
0
.
0
.
.
0
.
Figure 3. (a) LEED image from 1 ML Me-PTCDI on Cu( 100) and (b) a schematic of this LEED data obtained by simulating the diffraction data of the (c) proposed packing structure of a unit cell consisting of two Me-PTCDI molecules.
These TDMS data are consistent with the presence of a chemically reacted first monolayer of PTCDA, overlayed by second and subsequent ordered monolayers, which desorb at unique temperatures, depending upon the crystallographic environment developed in these ultrathin films.
I00
200
300
Schmidt et al. Intact parent molecular fragments were desorbed at only trace trace levels when the coverage of PTCDA was 1 ML or less, even though these were the same coverages which gave the welldefined LEED data shown above. After taking a 1 ML PTCDN Cu(100) surface to ca. 600 “C (the normal protocol for our TDMS experiments) the surface was examined via Auger spectroscopy, and a graphitic carbon signal was detected, with an intensity suggesting nearly monolayer coverage. The’ reactivity of the first monolayer of PTCDA on the copper surface, and the relative stability of the subsequent monolayers deposited, is further confirmed by the X P S studies discussed below. Increases in PTCDA coverage to ca. 1.5, 2.5, and 3 ML showed the desorption of intact parent molecular species at unique temperatures, depending upon coverage. The main desorption features seen for coverages of 1-2 mL occur at ca. 303 f 2 “C, indicating a desorption energy of ca. 35.5 f 0.2 kcal/mol (determined after the method described in ref 19). This TDMS peak is the result of the desorption of PTCDA molecules from a unique substrate: the monolayer of strongly adsorbed (reacted) PTCDA on the Cu( 100) surface. The intensity of this desorption peak is constant after coverages of ca. 2 ML are reached, indicating that this desorption process results from a single monolayer of PTCDA in direct contact with the first reacted (and tightly held) PTCDA monolayer on the Cu( 100) surface. Upon completion of 2 ML coverages an additional desorption feature grows in at ca. 282 f 2 “C, corresponding to a desorption energy of ca. 34.1 kcaYmo1, and continues to increase in intensity up to PTCDA coverages of 6 ML. This new desorption feature suggests the formation of a less stable (strained) crystallographic form of PTCDA, in monolayers 3-6. Once coverages of greater than ca. 6 ML were achieved (seen in the expanded left inset in Figure 4), an additional very small desorption peak was observed at ca. 330 “C (37.2 kcaYmol), which was accompanied by a new set of diffraction spots, marked “a” and “b”, in the LEED data as seen in the right inset of Figure 4. This diffraction and desorption data suggest the formation of an additional phase of PTCDA at these higher coverages, possibly through rearrangement of the already
400
500
600
Temperature [“C] Figure 4. TDMS data for ordered PTCDA layers on Cu(100) using the m/z = 124 fragment to indicate desorption of the intact molecule. PTCDA coverages of 1, 1.5, 2.5, and 3 ML. The left inset shows the growth of a new peak at 330 “C at 8 ML coverage at a reduced scale compared to the main figure. The right inset is a LEED pattern observed for a 8 ML film of PTCDA on Cu( 100). New diffraction features are labeled “a” and “b”. The spots labeled “b” replace a pair of spots in the lower coverage LEED pattern of this molecule.
Ordered Ultrathin Films of PTCDA and Me-PTCDI
J. Phys. Chem., Vol. 99,No. 30, 1995 11775 -3ML
-.-..-35ML
. 96ML
a) Carbon I s
1
b) Oxygen IS
538
536 534 532 530 520 electron binding energy [ev]
526
Figure 6. Photoelectron spectra (normalized intensity versus binding energy) of the (a) carbon and (b) oxygen 1s region determined by XPS experiments for different coverages (3, 35, 96 ML) of PTCDA on polycrystalline copper. The peak positions and relative peak areas change with film thickness.
0.5ML
0.0
.
200
300
400
500
Temperature ("C) Figure 5. TDMS data for ordered Me-FTCDI layers on Cu(100) using the d z = 124 fragment (inset) for 0.5, 1.0, 1.5, and 2.5 ML.
deposited material, to form a more stable molecular crystal thin film.5a-c The rapid appearance of such new features in both the TPD and LEED experiment indicates an abrupt change in packing structure at this critical coverage, which we hypothesize is due to the onset of island formation. Island formation has been seen for PTCDA thin films on A u , and ~ ~ NaCl and KC1 single crystals,5eat coverages thick enough to be investigated by electron microscopy. It is likely that on Cu(100) after a critical coverage is achieved (ca. 6 ML) that island growth commences for these PTCDA thin films. FTCDA thin films have been observed to form strained overlayers on the relatively inert substrates such as HOPG,Sbwhere the molecule appeared to relax into a more stable structure over a period of several mgnolayers, without the occurrence of an abrupt change in packing structure, as we observed for PTCDNCu(100) thin films. Ultrathin films of Me-FTCDI on Cu(100) show TDMS behavior which is consistent with the formation of ordered islands composed of reacted Me-PTCDI and intact parent molecules capable of desorbing as such (Figure 5). Unlike PTCDA/Cu( loo), desorption of the intact molecule fragment is seen at Me-PTCDI coverages of ca. 0.5 ML. At a Me-FTCDI coverage above monolayer a fully developed desorption peak at 281 & 2 "C is seen, with a fwhm of ca. 33 K (versus the 22 K fwhm seen for comparable peaks for PTCDA desorption). If it were possible to desorb intact molecular fragments at all surface coverages and thin film thicknesses, it is expected that the TDMS peak areas would increase linearly with coverage. Instead, the TDMS peak areas increase sharply in intensity as the equivalent coverage passes 1 ML (e.g., the TDMS peak area ratio for Me-PTCDI coverages of 1.5 ML versus 1.0 ML is ca. 3.2). This desorption peak temperature is in the same range as those seen for PTCDA desorption, but the larger distribution of desorption energies suggests considerably less order in the Me-FTCDI thin films than was observed for PTCDA. Peak widths in TDMS are a function of many parameters including thermal scan rate, uniformity in surface temperature (the Boltzman pre-exponential factor), instrumental response time, the number of desorption sites, and their energy.Ig In a system
as homogeneous as the one investigated, employing one substrate crystal, one heating rate, and similar molecules, with similar functionalities and heats of sublimation, the difference in desorption peak widths can be attributed to a difference in the energetic homogeneity of the ultrathin film crystalline environment, including the adsorption sites for the first monolayer. The TDMS data for these Me-PTCDI thin films can be explained by the occurrence of island formation (VolmerWeber of Me-PTCDI at low coverages, where the dimensions of these islands must be greater than the coherence length requirements of LEED, to explain the good electron diffraction data obtained (this coherence length is estimated to be not greater than 100-500 A).21bThis growth mode leads to molecular desoprtion from a variety of energetically dissimilar sites, such as those from large Me-PTCDI islands and also from intact molecular fragments at the Cu/Me-PTCDI interface. The occurrence of island formation indicates there are regions on the surface where coverages of the molecule may reach many molecular layers, adjacent to regions on the surface which are bare. These regions of multilayer coverage allow for the desorption of intact Me-PTCDI molecules. Me-PTCDI coverages beyond 5 ML were not investigated by TDMS; however, no additional diffraction spots were detected in LEED experiments which indicated the growth of a new phase, as was seen at the higher coverages of PTCDA. XPS Characterization of PTCDA and Me-PTCDI on PolycrystallineCopper and PTCDA on MoS2. The chemical reactions undergone by ultrathin films of PTCDA and MePTCDI on atomically clean copper foils at near room temperature were investigated in a separate deposition chamber, directly coupled to the X-ray photoelectron spectrometer. These studies c o n f i i e d the reaction of both PTCDA and Me-PTCDI monolayers with the copper surface, relative to the MoS2 surface, where no reaction was observed. Figure 6a,b shows examples of O(1s) and C( 1s) peaks for three different film thicknesses of PTCDA on a clean copper foil surface (normalized to the same absolute intensity to clarify peak shapes and positions). The assignment of these peaks to different functionalities on the perylene core is aided by comparison to photoelectron spectroscopic studies of a wide variety of oxygen-functionalized organic materials.21 C( 1s) peaks 1 and 1' are attributed to carbon in the aromatic portion of the molecule and carbon directly bonded to oxygen in the anhydride groups, respectively. With increasing coverage the area ratio between peak 1 and peak 1' in the C( 1s) spectra decreases and peak 1 is shifted to a higher (corrected) binding
11776 J. Phys. Chem., Vol. 99, No. 30, 1995
Schmidt et al.
TABLE 1: PTCDA on Polycrystalline Cu, XPS Data for Selected Coverages O(1s) peak 3 O(1s) peak 2 O(1s) peak difference 2 - 3 O(1s) peak area coverage (eV) 1 0 . 1 (eV) 1 0 . 1 (eV) +0.2 3/2 h15% 1 2 7 16 35 53 141
530.94 531.15 531.40 531.52 531.46 531.58 531.57
533.19 533.22 533.34 533.30 533.32 533.5 1 533.39
2.25 2.07 1.94 1.87 1.86 1.84 1.82
O(1s) fwhm3 (eV) 1 1 5 %
C(1s) peak 1 (eV) hO.1
C(1s) fwhm'
1.94 1.96 1.70 1.51 1.41 1.41 1.40
284.04 284.10 284.25 284.35 248.26 284.45 284.40
1.87 1.77 1.69 1.62 1.56 1.61 1.62
2.33 1.62 1.11 0.91 0.85 0.80 0.80
(eV) 1 1 5 %
oxygen 1s
8 7
#.-
5.01,.
, ,
,
,
, , ,
,
,
,
,
,
,
,
,
;, ,
,
,
,,,
,
, ,.,
,I 0 '
0
10
20
30
40
50
60 0
film thickness [ML]
20
Figure 7. Relative atomic ratios of carbon to oxygen of PTCDA on
polycrystalline copper as a function of coverage. The ratios were determined from the 1s peak areas after correction for the escape depth, photoemission probability, and analyzer transmission function. The two symbols refer to two different films. The inset shows the ratio of the two oxygen peak areas as a function of coverage. These experiments prove that the anhydride oxygen 2 is lost during adsorption of PTCDA on copper. energy, consistent with a small change in the chemical environment of the anhydride carbons in the first deposited material (see below). The O(1s) peaks 2 and 3, arise from the two different forms of oxygen in the anhydride groups. Peak 3 on the low binding energy side of the O(1s) spectra can therefore be identified as the oxygen in the C=O group, while peak 2 arises from the singly bound oxygen in the anhydride moiety (C-0-C). CEO groups bonded to an aromatic system typically have a binding energy for the oxygen atoms ranging from 531.6 to 531.7 eV. For thick (bulk) films of PTCDA the O(1s) peak ( 2 ) attributed to the C-0-C group is in between the binding energies typically observed for (a) a singly bonded oxygen in a system such as O=C-0-C=O, where both carbons are attached to a doubly bound oxygen (binding energies for such groups range from 533.9 to 534.0 eV) and (b) the binding energies observed for oxygen in a 0 4 - 0 groups, where the carbon is attached to an aromatic group (533.0-533.2 eV).2' At the lowest PTCDA coverages on copper, O( 1s) peak 2 is significantly diminished in intensity relative to peak 3, consistent with the hypothesis that this form of oxygen in involved in a chemical reaction with the copper surface. Each set of C( 1s) and O( 1s) peaks were curve fit assuming only two peaks in each spectrum, and the binding energies and peak area ratios for PTCDA coverages from 1- 141 equivalent monolayers are shown in Table 1. The total peak areas of both C( 1s) peaks, and the two O( 1s) peaks, corrected for photoemission and instrumental sensitivity factor~,'~ were used to calculate the relative atomic ratio of carbon to oxygen as a function of PTCDA film thickness (Figure 7). The C/O ratio is ca. 6.0 at the lowest PTCDA coverages and decreases to ca. 4.75 at equivalent coverages of ca. 20 ML. The C/O ratio for an intact PTCDA molecule should be 4.0. Stoichiometries of standard aromatic hydrocarbons determined from X P S have typical uncertainties of ca. 10-20%; therefore the carbon-to-oxygen ratio of 4.75 is probably representative of the stoichiometric PTCDA thin film. These results are consistent with a reaction
60 80 100 I20 140 film thickness [ML]
40
Figure 8. Binding energy of the oxygen 1s peaks of PTCDA on copper as a function of surface coverage. This shows that the interaction between the copper surface and PTCDA is mainly mediated by the
C=O groups.
TABLE 2: PTCDA on MoS2, XPS Data for Selected Coverages O(1s) peak O(1s) C(1s) difference O(1s) area fwhm 3 fwhm 1 stoichiometric 2 - 3 (eV) ratio 3/2 (eV) carbon-to-oxygen (eV) coverage f 0 . 2 eV f 1 5 % fO.l eV 10.1 eV ratio 115% 1 ML 5 ML 37 ML
1.83 1.89 1.95
0.57 0.74 0.65
1.80 1.60 1.56
1.67 1.62 1.63
3.94 4.36 4.04
of the first PTCDA monolayer, in direct contact with copper, to cause loss of oxygen (mainly the oxygen species responsible for peak 2 in Figure 7b) from the surface region. The interaction between PTCDA and copper is also reflected in the charge-shift-corrected variation in binding energies of the oxygen and carbon photoemission peaks. Figure 8 shows how the binding energies for the O(1s) peaks 2 and 3 increase with increasing coverage. Peak 2 has a slightly lower binding energy for thin films and reaches a constant value of 533.4 eV at a coverage of ca. 10 ML. At low PTCDA coverages the binding energy for O( 1s) peak 3 is 529.9 eV and reaches a value of 53 1.6 eV at a PTCDA coverage of 10 ML. Since the binding energies of these peaks were corrected versus the binding energies of the CuQp) transitions, we can conclude that there is an additional surface work function final state effect, at low PTCDA coverages, which lowers the binding energies in the initial adsorbate layer. The strong change with coverage for peak 3 leads to the conclusion that the electrons in the n-system of the C=O group are strongly influenced by the copper surface. This observation is in agreement with UV-photoemission spectroscopy studies of this molecule bound on several different metallic substrate^'^^^^ and with the investigation of the bonding nature of carbonyl-copper organometallic complexes.'3c Similar changes in binding energies were observed for other O( 1s) and C( 1s) peaks, as given in Table 1. A strong change is observed until about 10 ML, followed by a constant value; the rate of change of these values characterizes the PTCDA thickness dependence of such binding energy shifts. Exponential decay fits of the changes with PTCDA coverage gave a decay constant of ca. 5 ML for all of the C( 1s) and O(1s) peaks.
Ordered Ultrathin Films of PTCDA and Me-PTCDI
J. Phys. Chem., Vol. 99,No. 30, 1995 11777
TABLE 3: Me-PTCDI on Polycrystalline Cu, XPS Data for Selected Coverages coverage [MI-I
O(1s) peak 3 (eV) f 1 5 %
O(1s) fwhm 3 (eV) fO.l eV
N(1s) peak (eV) f O . l eV
N( 1s) fwhm (eV) f O . l eV
C(1s) peak 1 (eV) f O . l eV
C(1s) fwhm 1 (eV) *0.1 eV
2 5 9 16 24 45
530.9 531.2 53 1.2 531.3 531.2 531.2
2.17 2.11 2.07 2.04 1.95 1.97
400.26 400.56 400.58 400.65 400.90 400.82
2.18 2.10 1.97 1.97 1.89 1.94
284.3 284.4 284.4 284.5 284.4 284.5
1.91 1.91 1.90 1.87 1.89 1.90
a) copper 2p I
-
I
960 955 950 945 940 935 930 925 electron binding energy [ev]
1 1 0
3
6
9
12
15
18
b) Cu 2p,,:
h
= 9 f 3 ML
I
21
film thickness [ML] Figure 9. Relative atomic ratios of (a) oxygen to nitrogen and (b)
carbon to nitrogen in films of Me-PTCDI on polycrystalline copper as a function of Me-PTCDI coverage. The two symbols refer to different films. These experiments show that the imide groups are lost during adsorption of Me-PTCDI on copper. PTCDA thin films at any coverage on MoS2 qualitatively give the same photoemission peaks as on copper (Table 2). The relative atomic ratio C/O is, however (within experimental error), independent of coverage and equal to 4.0, the expected value for an intact PTCDA molecule. In addition, no significant change of O(1s) and C(1s) peak energies with coverage were observed for any PTCDA coverage on MoS2. The adsorption on MoS2 does not appear to involve reaction with the substrate. The binding energy values for thick films of PTCDA on copper agree with the results on MoS2, indicating that thick PTCDA films on copper are chemically identical to ultrathin films of PTCDA on MoS2. LEED studies show that the reacted PTCDA structure, at monolayer coverage on the Cu( 100) surface, has glide symmetry and a lattice size which is reasonably close to that expected for a form of close-packed PTCDA. This is only possible if the reacted molecular fragment has the same symmetry as PTCDA itself. We hypothesize that the reaction of the fiist monolayer of PTCDA causes the loss of both anhydride oxygens, which would increase the ratio C/O to 6 (24/4), near that seen for thin PTCDA films, instead of the ratio near 4 (24/6), observed for thick PTCDA films. The loss of other functional groups, such as CO, C02, or C2O3, can be excluded, since their loss would result in C/O ratios vastly different than those observed and/or would lead to adsorbate structures with different symmetries and sizes than those observed. As pointed out above, there is also evidence that Me-PTCDI monolayers undergo irreversible reaction with the clean copper surface. The results from XPS measurements of Me-PTCDI films deposited on similar copper foils are given in Table 3 and suggest a strong similarity between the reaction of PTCDA and the reaction of Me-PTCDI on copper. Loss of the central portion of the anhydride functionality is indicated for both molecules, resulting in line-shape and peak area changes for photoemission from O(ls), N(ls), and C(ds) levels. Figure 9 shows the relative atomic ratio of total oxygen to nitrogen (9a) and total carbon to nitrogen (9b) for coverages of Me-PTCDI from ca. 3-21 equivalent monolayers. These data indicate a loss of nitrogen from the Me-PTCDYCu interface. After
0
40 60 80 film thickness [ML]
20
100
Figure 10. Copper 2p peaks: (a) intensity of the peaks as a function of binding energy for 3, 35, and 96 ML coverage with PTCDA; (b) peak area as a function of PTCDA coverage. From the fit (solid line) the escape depth for the 0.55 keV electrons was determined.
coverages of 9 ML are surpassed, the expected O/N and C/N atomic ratios for a stoichiometric thin film of Me-PTCDI are achieved. The O(ls), C(ls), and N(1s) photoemission peak features of Me-PTCDI on copper (Table 3) also show binding energy changes with increasing Me-PTCDI coverage similar to those seen for PTCDA on copper. Charge transfer interactions with the copper surface which might accompany these reactions are difficult to detect when examining the major photoemission bands of copper. Similar problems have been detailed before, for the adsorption of a strong electron acceptor such as TCNQ on Cu, Au, and PtZ3 Stoichiometric mixtures of TCNQ and copper produce XPS Cu(2p), C(ls), and N(1s) line-shape changes which clearly indicate the transfer of charge from copper to TCNQ.24 X P S studies of TCNQ monolayers on the copper surface, however, show the expected chemical shifts in the C(1s) and N(1s) levels of the adsorbate, but little or no change in the binding energy of the Cu(2p) levels. Electronic changes which produce these changes in orbital energies for bulk charge transfer salts are not easily seen in the near surface region of a clean metal, where chargecompensating effects can dominate. In addition, for molecular adsorbates the size of TCNQ, PTCDA, etc., the number of copper atoms actually participating in the charge transfer event may be only 10% of the total number sampled by the XPS experiment, further decreasing the likelihood of detecting binding energy shifts for the core levels of the metal. Three examples of Cu(2p) spectra are shown in Figure 10 as a function of PTCDA coverage. The separation between 2~112 and 2~312peaks stays constant at 19.9 eV, a value expected for both Cu(0) and Cu(1) states. The presence of a Cu(II) species can be c o n f i i e d in X P S studies by the observance of shakeup losses associated with the Cu 2~112and 2~312photoelectron peaks. Their absence in this case indicates that the Cu(I1) species is not present on the surface at a detectable concentration.I7 The Auger parameter (kinetic energy of the Cu Auger peak L3VV plus binding energy of Cu 2~312)for PTCDA films of different thickness on copper was found to be 1851.6 f 0.1
11778 J. Phys. Chem., Vol. 99, No. 30, 1995
Schmidt et al. monolayers give LEED data consistent with flat-lying molecules, thus contributing to considerable lattice strain in the second layer of Me-PTCDI deposited. Such lattice strain can be alleviated by the production of islands at low coverage of this molecule. The PTCDA monolayer structure closely mimics the bulk structure with regard to the orientation of adjacent layers. Molecules in the second layer can be therefore orientated parallel to the first layer with some lattice strain, indicated by the shift in TDMS peaks. This parallel orientation may facilitate a layerby-layer growth mode, for several monolayers.
Conclusions
Figure 11. (a) View of the (102) plane of Me-PTCDI, (b) view parallel to the (102) plane, and (c) view along one of the Me-PTCDI molecules.
eV, independent of coverage. Literature values of 1851.6 eV for the Auger parameter are reported for both Cu(0) and Cu(1) states.I7 The plot of the normalized intensity of Cu 2~312as a function of PTCDA coverage is shown in Figure lob. The fit obtained from two samples with an exponential decay gives an escape depth for the Cu(2p) photoelectrons of 9 f 3 ML through the organic overlayer. Assuming a thickness of about 0.35 mm for a single, flat-lying PTCDA monolayer, we obtain an escape depth for the Cu(2p) photoelectrons of ca. 3.1 nm. This value is close to the escape depth of the same kinetic energy electrons through multiple layers of barium stearate layers (3.5 which are believed to grow in a layer-by-layer mode, and selfassembled monolayers of variable chain length alkanethiolates (2.3 nm),24bwhere the increase in the hydrocarbon chain length simulates layered growth. These data suggest that PTCDA films grown on the copper surface are of morphology (layered growth) similar to these previously described systems. Observations of intensities and binding energies of the Cu(2p) bands for multiple coverages of Me-PTCDI agree with the PTCDA experiments, with one exception. The escape depth of the electrons determined from the Cu 2~312peak is ca. 18 equivalent monolayers (6.2 nm), twice that in PTCDA. The same differences in escape depths are also found in the escape depth of the oxygen and carbon photoelectrons in PTCDA and Me-PTCDI on copper. PTCDA and Me-PTCDI are very similar molecules; therefore, this finding can only be understood as arising from different thin film morphologies. The TDMS experiments indicate island growth at the lowest coverages for Me-PTCDI thin films and layered growth at low coverages for PTCDA thin films, which is consistent with the finding that the apparent escape depth for Cu(2p) photoelectrons through Me-PTCDI thin films is twice that for PTCDA thin films. The bulk crystallographic structures of these two molecules suggest a reason for the differences in growth mode for these two molecular systems. Figure l l a shows the (102) plane of Me-PTCDI, which is similar to that of PTCDA.15 This plane is similar in appearance to the packing geometry of these molecules on many surfaces, including Cu(100). The solid line in this figure shows the symmetry of the overlayer unit cell for this molecule. It is known that the plane of a PTCDA molecule is parallel to the (102) crystal plane in its bulk structure. If one observes along the (102) crystal plane of Me-PTCDI, it is seen that the molecule is not aligned along this axis (Figure 1lb). By observing along one of the molecular planes of this structure, it is seen that the molecules lie f6.5" out of this crystallographic plane (Figure 1IC). PTCDA and Me-PTCDI
We have shown that the deposition of PTCDA and MePTCDI on copper surfaces (deposition temperatures of 130 "C to achieve a good electron diffraction data, lower temperatures in the XPS studies) is accompanied by reaction, causing the loss of the two bridging oxygens in the anhydride groups in PTCDA and the methyl imido group in Me-PTCDI. The perylene/Cu interface appears to consist of molecular fragments which retain approximately the size and symmetry of the parent molecule, but form a rectangular unit cell with a smaller aspect ratio than for comparable planes in the bulk crystals. PTCDA on Cu(100) grows in a commensurate rectangular lattice (4J2xSJ2)R = 45", and Me-PTCDI on Cu(100) forms a commensurate (6 x 8) rectangular lattice. On the unreactive hexagonal MoS2 surface, F'TCDA builds a coincident superlattice with a much larger and more rectangular unit cell, much closer to the unit cell dimensions of a single layer in the (102) plane of the bulk structure. The fate of the oxygen or imide portions of these molecules is not clear at present. Desorption of volatile products is indicated by the XPS results, which show substantially higher C/O ratios (considering total carbon and oxygen) after deposition of the first monolayer, relative to thicker films. 0 2 may be the volatile product following PTCDA reaction, although H2O cannot be dismissed as a possibility, since hydrogen coadsorption on the copper surface is possible and undetectable with the current experimental capabilities. In building multilayer thin films the growth modes for these two perylene dyes are quite different. The origin of this difference may lie in the difference in the bulk structures of these two materials. For PTCDA, flat-lying nearest neighbors are energetically favorable in all layers of the bulk structure, which is not the case for Me-PTCDI. Extrapolation of these studies to a wide variety of perylenes, phthalocyanines, naphthalocyanines, pentacenes, coronene, and fullerenes, and to different adsorption strength substrates, is underway.2k It is clear that molecules with high electron affinity heteroatoms will tend to adsorb strongly to substrates such as copper and gold; therefore, the electronic properties of heterojunctions involving these materials must be viewed as consisting of interfaces of unique composition relative to the bulk of either the metal or the organic semiconductor. It is also clear that it will be possible to produce several exotic organic thin film surfaces, on single-crystal metal or semiconductor supports, with which to study the onset of organic heterojunction formation and small molecule chemisorption.
Acknowledgment. A.S. is grateful to the Alexander vanHumboldt Foundation for its financial support (Feodor Lynen Fellowship VB2-FLF). We acknowledge helpful discussions with H. Yanagi, L.-K., Chau, K. Nebesny, and P. Lee. This research was supported in part by the National Science Foundation (Chemistry), Air Force Office of Scientific Research, and the Materials Characterization Program-State of Arizona.
Ordered Ultrathin Films of PTCDA and Me-PTCDI References and Notes (1) (a) Hara, M.; Sasabe, H.; Yamada, A.; Garito, A. F. Jpn. J . Appl. Phys. 1989, 1306, 28. (b) Tada, H.; Saiki, K.; Koma, A. Jpn. J. Appl. Phys. 1991, L306, 30. (c) Dann, A. J.; Hoshi, H.; Maruyama, Y. J . Appl. Phys. 1990,67, 1371, 1845. (d) Yanagi, H.; Ashida, M.; Elbe, J.; Worhle, D. J . Phys. Chem. 1990, 94, 7056. (e) Yanagi, H.; Kouzeki, T.; Ashida, M. J . Appl. Phys. 1993, 73, 3812. (0 Yanagi, H.; Dauke, S.; Ueda, Y.; Ashida, M.; Worhle, D. J. Phys. Chem. 1992, 96, 1366. (g) Terasaki, A.; Hosoda, A.; Wada, T.; Tada, H.; Koma, A.; Yamada, S.; Sasabe, H.; Garito, A. F.; Kobayashi, T. J . Phys. Chem. 1992, 96, 10534. (2) (a) Nebesny, K. W.; Collins, G. E.; Lee, P. A.; Chau, L.-K.; Danziger, J. L.; Osbum, E.; Armstrong, N. R. Chem. Mater. 1991, 3, 829. (b) Armstrong, N. R.; Collins, G.E.; Nebesny, K. W.; England, C. D.; Chau, L.-K.; Lee, P. A.; Parkinson, B. A. J . Vac. Sci. Technol. A 1992, 10, 2902. (c) Armstrong, N. R.; Arbour, C.; Chau, L.-K.; Collins, G.E.; Nebesny, K. W.; Lee, P. A.; England, C. D.; Parkinson, B. A. J . Phys. Chem. 1993, 97, 2690. (d) Armstrong, N. R.; Chau, L.-K.; England, C. D.; Chen, S.-Y. J . Phys. Chem. 1993, 97, 2699. (e) Anderson, M. L.; Collins, G. E.; Williams, V. S.; England, C. D.; Chau, L.-K.; Schuerlein, T. J.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R. Surf. Sci. 1994,307309, 551. (0 England, C. D.; Collins, G.E.; Schuerlein, T. J.; Anderson, M. L.; Armstrong, N. R. In Interfacial Design and Chemical Sensing; T.E. Mallouk, T. E., Harrison, J. E., Eds.; ACS Symposium Series 561; Amercian Chemical Society: Washington, DC, 1994; p 202. (g) Chau, L.-K.; Chen, S.-Y.; Armstrong, N. R.; Collins, G.E.; England, C. D.; Williams, V. S.; Anderson, M. L.; Schuerlein, T. J.; Lee, P. A.; Nebesny, K. W. Mol. Cryst. Liq. Cryst., in press. (h) Collins, G.E. Ph.D. Dissertation, University of Arizona, 1992. (i) Schuerlein, T. J.; Armstrong, N. R. J. Vac. Sci. Technol. 1994, A12 (4), 1992. (i) Schmidt, A.; Chau, L.-K.; Valencia, V.; Armstrong, N. Chem. Mater. 1995, 7, 657. (k) Schuerlein, T. J.; Schmidt, A. J.; Armstrong, N. R. Jpn. J. Appl. Phys., in press. (1) Schuerlein, T. J. Ph.D. Dissertation, University of Arizona, 1995. (3) (a) Buchholz, J. C.; Somojai, G.A. J . Chem. Phys. 1977,66,573. (b) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; Wo11, Ch.; Chiang, S. Phys. Rev. Lett. 1989, 62, 171. (4) (a) Fryer, J. R.; Kenney, M. E. Macromolecules 1988, 21, 259. (b) Fryer, J. R. Mol. Cryst. Liq. Cryst. 1986, 137, 49. (c) Ashida, M. In Electron Crystallography of Organic Molecules; Freyer, J. R., Dorset, D. L., Eds.; Fluwer Academic Publishers: Netherlands, 1990; pp 227-240. (5) (a) Zimmerman, U.; Karl, N. SUI$ Sci. 1992, 2168, 296. (b) Ludwig, C.; Gompf, B.; Glatz, W.; Petersen, J.; Eisenmenger, W.; Mobus, M.; Zimmerman, U.; Karl, N. Z. Phys. B 1992, 86, 397. (c) Jung, M.; Gaston, U.; Schnitzler, G.;Kaiser, M.; Papst, J.; Ponvol, T.; Freund, H. J.; Umbach, E. J . Mol. Struct. 1993, 293, 239. (d) Danziger, J.; Dodelet, J.P.; Lee, P.; Nebesny, K. W.; Armstrong, N. R. Chem. Mater. 1991,3, 821. (e) Mobus, M.; Karl, N. J . Crystal Growth 1992, 116, 495. (0 Hoshino, A.; Isod, S.; Kurata, J.; Kobayashi, T. J . Appl. Phys. 1994, 76, 4113. (6) (a) Haskal, E. I.; So, F. F.; Burrows, P. E.; Forrest, S. R. Appl. Phys. Lett. 1992, 60, 3223. (b) So, F. F.; Forrest, S. R. Phys. Rev. Lett. 1991, 66, 2649. (c) So, F. F.; Forrest, S. R.; Shi, Y. Q.;Steier, W. Appl. Phys. Lett. 1992, 56, 674.
J. Phys. Chem., Vol. 99, No. 30, 1995 11779 (7) Weaver, J. H. Acct. Chem. Res. 1992,143,25 and references therein. (8) (a) Forrest, S. R.; Leu, L.-Y.; So, F. F.; Yoon, W. Y. J . Appl. Phys. 1989, 66, 5908. (b) Danziger, J.; Dodelet, J. P.; Lee, P.; Nebesny, K. W.; Amstrong, N. R. Chem. Mater. 1991,3, 812. (c) Danziger, J.; Dodelet, J. P.; Armstrong, N. R. Chem. Mater. 1991,3, 82 1. (d) Popovic, Z. D.; Hor, A. M.; Loutfy, R. 0. Chem. Phys. 1988, 127, 4. (9) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (IO) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (1 1) (a) Albert, M. A.; Yates, J. T., Jr. The Surface Scientist's Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987; pp 91-104 and references therein. (b) Dahlgren, D.; Hemminger, J. C. SUI$ Sci. 1982, 114, 459. (c) Dahlgren, D.; Hemminger, J. C. Surf. Sci. 1983, 134, 836. (12) Erley, W.; Ibach, H. Surf. Sci. 1986, 178, 565. (13) (a) Zimmermann, U.; Schnitzler, G.; Karl, N.; Umbach, E.; Dudde, E. Thin Solid Films 1989, 175, 85. (b) Jung, M.; Baston, U.; Schnitzler, G.;Kaiser, M.; Papst, J.; Ponvol, T.; Freund, H. J.; Umbach, E. J . Mol. Struct. 1993,293, 239. (c) Jeon, N. J.; Nuzzo, R. G. Langmuir 1995, 11, 341. (14) (a) Berkowitz, J. J . Chem. Phys. 1979, 70, 2819. (b) Viehbeck, A.; Goldberg, M. J.; Kovac, C. A. J . Electrochem. SOC.1990, 137, 1460. (c) Chen, E. C. M.; Wentworth, W. E. J . Chem. Phys. 1975, 63, 3183. (d) Berger, S. Spectrochim. Acta 1967, 23A, 2213. (e) DeLuca, C.; Giomini, C.; Rampazzo, L. J . Electroanal. Chem. 1987, 238, 215. (15) Haedicke, E.; Graser, F. Acta Crystallogr. 1986, C42, 189. (16) Nebesny, K. W.; Maschhoff, B. L.; Armstrong, N. R. Anal. Chem. 1989, 61, 469A. (17) Wagner, C. D. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons, Inc.: New York, 1987; 477 ff. (18) Holland, B. W.; Woodruff, D. P. Surf. Sci. 1973, '36, 488. (19) (a) Redhead, P. A. Vacuum 1962, 12, 203. (b) Adamson, A. W. Physical Chemistry of Sutfaces, 5th ed.; Wiley: New York, 1990; pp 690692. (c) Wu, M.-C.; Truong, C. M.; Goodman, D. W. J . Phys. Chem. 1993, 97,4182. (d) Heitzinger, J. M.; Gebhard, S. C.; Koel, B. E. J . Phys. Chem. 1993, 97, 5321. (20) Lovinger, A. J.; Forrest, S. R.; Kaplan, M. L.; Schmidt, P. H.; Venkatesan, T. Bull. Am. Phys. Soc. 1983, 28, 476. (21) (a) Tsao, J. Y. Materials Fundamentals of Molecular Beam Epitaxy; Academic Press: San Diego, 1993. (b) Liith, H. Surfaces and Interfaces of Solids; Springer Series in Surface Science; Springer-Verlag: New York, 1993; Vol. 15, pp 201-209. (22) (a) Briggs, D.; Beamson, G.Anal. Chem. 1993, 65, 1517. (b) Laibinis, P.; Bain, C.; Whitesides, G. J . Phys. Chem. 1991, 95, 7017. (23) Patterson, T.; Pankow, J.; Armstrong, N. R. Langmuir 1991, 7, 3160. (24) Lindquist, J. M.; Hemminger, J. C. Chem. Mater. 1998, 1, 72. (25) Hall, S.; Andrade, J.; Ma, S.; King, R. J . Electron Spectrosc. Relat. Phenom. 1979, 17, 181.
JP9509525
