Chemical Physics Letters 473 (2009) 321–325

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Chemical Physics Letters

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Layer growth and desorption kinetics of a discoid molecular acceptor on Au(1 1 1)

Paul Frank a, Norbert Koch b, Markus Koini a, Ralph Rieger c, Klaus Müllen c, Roland Resel a, Adolf Winkler a,*

a Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austriab Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germanyc Max Planck Institute of Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany

a r t i c l e i n f o

Article history:Received 4 February 2009In final form 8 April 2009Available online 11 April 2009

0009-2614/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cplett.2009.04.019

* Corresponding author. Fax: +43 316 873 8466.E-mail address: a.winkler@tugraz.at (A. Winkler).

a b s t r a c t

The layer growth and desorption kinetics of discoid organic acceptor molecules hexaazatriphenylene-hexacarbonitrile (HAT-CN) on Au(1 1 1) have been studied by thermal desorption spectroscopy (TDS),X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD).The molecules form two wetting layers of flat lying molecules when evaporated at or below room tem-perature. Increasing the coverage beyond two monolayers and upon heating of the sample a destabiliza-tion of the 2nd layer and subsequently of the 1st layer takes place and 3D islands are formed. A quiteunusual desorption behavior is observed for the individual layers, which is discussed in detail.

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1. Introduction

Thin films of conjugated organic molecules have attractedconsiderable interest due to the rapidly growing field of organicelectronics [1]. A key issue in this context is the preparation ofwell defined metal/organic and organic/organic interfaces [2,3],because the electrical and optical properties of devices dependcrucially on interface properties, such as morphology, energy levelalignment, or molecular orientation [4,5]. Therefore, a compre-hensive understanding of film growth phenomena that determinethe structure, morphology and stability of organic/metal inter-faces is indispensable.

Here we report on the complex coverage-dependent thin filmformation, thermal stability and desorption kinetics of hexaazatri-phenylene-hexacarbonitrile (HAT-CN) (chemical structure shownin Fig. 1) on a Au(1 1 1) surface. This discoid molecule is a strongelectron acceptor and may potentially be used to modify metal sur-face electronic properties, in analogy to existing examples [6,7].There is fundamental interest in the layer growth of disc shapedmolecules because they might be able to form columnar liquidcrystals [8]. We demonstrate that at and below room temperaturethe first two monolayers (ML) are wetting the Au(1 1 1) surface.Above a critical initial coverage (>2 ML) the 2nd monolayer isdestabilized during heating of the sample and forms islands onthe 1st monolayer. With further increase of the coverage the is-lands grow, and the molecules at the interface (1st layer) rear-range. The thermal stability (desorption temperature) of these re-structured molecules at the interface is smaller than that of the flatlying molecules on the uncovered 1st monolayer.

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2. Experimental

Thin films of HAT-CN were prepared by physical vapor deposi-tion in an ultra-high vacuum system from a Knudsen cell. The filmthickness was monitored by a calibrated quartz microbalance [9].The cleanliness and appropriate surface structure of the Au(1 1 1)substrate were confirmed with Auger electron spectroscopy(AES), X-ray photoelectron spectroscopy (XPS) and low energyelectron diffraction (LEED). For thermal desorption spectroscopy(TDS) an in-line positioned mass spectrometer was used. The mor-phology and crystallographic structure of the organic films wasinvestigated ex-situ with an atomic force microscope (AFM) andby the X-ray diffraction pole figure technique, respectively.

3. Results and discussion

A series of thermal desorption spectra for HAT-CN films of dif-ferent initial coverages up to a mean film thickness of 7 Å is shownin Fig. 1. The adsorption temperature was 200 K and the heatingrate was 1 K/s. Quantitatively equivalent spectra were also ob-tained for HAT-CN adsorption at room temperature. The evolutionof the desorption spectra can be divided into four different re-gimes: (i) For coverages corresponding to less than 1.5 Å, (as basedon our coverage calibration with the quartz microbalance) HAT-CNdesorbs at �620 K. A small shift to lower temperature (by <10 K)with increasing coverage is observed. We designate this peak asb1 and it corresponds to a coverage of about 1/2 ML of flat lyingmolecules. (ii) For coverages above 1.5 Å the desorption spectra ex-hibit a shoulder at the leading edge of the b1 peak which shifts con-siderably (by more than 100 K) when increasing the coverage up toabout 3 Å (1 ML). This additional feature is designated as the b2

peak. (iii) With further increase of the coverage a third, sharpdesorption peak appears at�460 K. This peak increases in intensity

Fig. 3. XPS Au 4f substrate signal change during heating of the sample, for variousinitial coverages of HAT-CN: (a) 0 Å, (b) 2 Å, (c) 3 Å, (d) 5 Å, (e) 7 Å, (f) 8 Å, (g) 10 Å,(h) 15 Å. Substrate temperature during film deposition: 200 K, heating rate 1 K/s.The peak maxima of the corresponding desorption peaks are indicated.

Fig. 1. Series of thermal desorption spectra of HAT-CN on Au(1 1 1) as a function ofcoverage. Adsorption temperature: 200 K, heating rate: 1 K/s. The chemicalstructure of the HAT-CN molecule is depicted in the insert.

322 P. Frank et al. / Chemical Physics Letters 473 (2009) 321–325

up to a coverage of ca. 6 Å, with a slight shift of the peak to lowertemperature. We designated this peak as the c peak, which corre-sponds to a 2nd layer of flat lying molecules. (iv) Interestingly, fur-ther increase of the coverage leads to a disappearance of the c peakand a new peak at higher temperature (�480 K) emerges. This lat-ter peak, designated as the a peak, shows the typical feature of zeroorder desorption (common leading edge for various initial cover-ages), it corresponds to desorption from a multilayer.

A further remarkable observation was made for coveragesabove 7 Å thickness, as shown in Fig. 2. With increasing initial cov-erage the amount of molecules desorbing from the b1 and b2 peakdecreases continuously. At a coverage of 50 Å less than 20% of amonolayer (flat lying molecules) desorb from the b1 and b2 states.Such an incorporation of molecules, which are stable in the mono-layer at high temperature, into a 3D phase that desorbs at lowertemperature has, to the best of our knowledge, not been reportedbefore.

To further characterize the peculiar thermal stability and cover-age dependence of the HAT-CN films, additional experiments wereperformed. The attenuation of the substrate Au 4f core level signalintensity by the overlying HAT-CN film was measured with XPS as

Fig. 2. Series of thermal desorption spectra of HAT-CN on Au(1 1 1) for largercoverages. Adsorption temperature: 200 K, heating rate 1 K/s. (a–d): different initialcoverages.

a function of temperature. This is a suitable method to investigatemorphology changes in a film, e.g., a transition from a layer-likefilm to three-dimensional (3D) islands (i.e., dewetting) [10]. InFig. 3, the Au 4f intensity vs. temperature curves are compiledfor various initial HAT-CN coverages, after film formation at200 K. Trace (a) in Fig. 3 shows the XPS signal change vs. temper-ature without a film, indicating the signal attenuation due to theDebye–Waller effect and other experimental reasons. At theadsorption temperature of 200 K the attenuation of the substratesignal in XPS as a function of the initial coverage follows approxi-mately an exponential decay, indicating that at this temperaturethe multilayer grows in a layer-by-layer like fashion. From theexponential signal decay an elastic mean free path for the Au4fphotoelectrons (Ekin = 1170 eV) of k = 26 Å is determined. This va-lue is similar to the elastic electron mean free path in p-sexiphenyl(k = 25 Å) [11] and p-quaterphenyl (k = 24 Å) [9].

For initial HAT-CN coverages of 2 Å and 3 Å the Au 4f substratesignal intensity does not change during sample heating between200 K and 500 K, but then starts to increase corresponding to thedesorption from the b states (see Fig. 1). For the 5 Å film the sharpsignal increase around 450 K corresponds to the desorption fromthe c state, followed by the signal change between 500 K and600 K due to desorption from the b states. At higher initial cover-ages (>8 Å), the Au 4f signal intensity exhibits a first sharp increasealready in the range of 350–380 K, signified by Tr. The further sig-nal change up to 500 K correlates with the desorption of the mul-tilayer (a peak in Fig. 1). The low-temperature Au 4f intensityincrease is, however, not correlated with any desorption (cf. Fig. 2).This indicates that a morphological transition (Tr) from a layer-likefilm to 3D islands occurred in the range of 350–380 K. The Tr onsettemperature decreases with increasing coverage.

The layer like growth at and below room temperature is furthercorroborated by AFM investigations. In Fig. 4 AFM images areshown for a 20 Å thick HAT-CN film grown on the Au(1 1 1) sub-strate at 300 K (Fig. 4a) and at 400 K (Fig. 4b), respectively. TheAFM image of the film grown at 300 K indicates that the Au surfaceis nearly completely covered with the organic film, consisting ofgrains with an average diameter of ca. 200 nm. From cross sectionanalysis a mean grain height of 3–4 nm is obtained. A significantdifference was observed for the film prepared at 400 K substratetemperature, i.e., above the morphology transition temperaturederived from XPS (Fig. 3). The HAT-CN film shown in Fig. 4b con-sists of large islands of coalesced grains (typical size 5–10 lm)

Fig. 4. (a) AFM image in ‘shaded mode’ of HAT-CN films on Au(1 1 1) with a mean thickness of 20 Å, taken ex-situ at room temperature. Substrate temperature during HAT-CN adsorption: 300 K. The insert a1 shows an enlarged detail. The cross section is taken along the line depicted in the AFM image. (b) AFM image in ‘shaded mode’ of HAT-CNfilms on Au(1 1 1) with a mean thickness of 20 Å, taken ex-situ at room temperature. Substrate temperature during HAT-CN adsorption: 400 K. The cross section is takenalong the line depicted in the AFM image.

Fig. 5. X-ray pole figure of HAT-CN measured at 40� to detect the 143 diffractionpeaks. The strong intensity at the polar angle of 0� shows that the crystallites arealigned with their (143) plane parallel to the surface. The ring structure at 15� and30� stems from diffraction of equivalent planes (�153) and (413) of the hexagonal

P. Frank et al. / Chemical Physics Letters 473 (2009) 321–325 323

and the area between the islands appears not to be covered bymolecules, as concluded from the clear visibility of the steps ofthe gold surface. A cross section reveals a mean height of the grainsof about 10–20 nm.

X-ray diffraction pole figure measurements performed at a50 nm thick film (grown at 400 K) show that this film is indeedcomposed of small crystallites, which exhibit a structure corre-sponding to the bulk structure of HAT-CN. The molecules crystal-lize in a hexagonal symmetry with 18 molecules per unit cellresulting in a complex three-dimensional network formed by T-stacking of neighbouring molecules [21]. No significant line broad-ening of the diffraction peaks is observed which reveals the strongtendency of HAT-CN for crystallisation with crystallite sizes of atleast 100 nm. From the pole figure measurements (Fig. 5) we ob-tain a preferential orientation of the crystallites with the (143) ori-entation parallel to the substrate surface, but with randomazimuthal orientation.

Before we discuss the special coverage and temperature depen-dence of the film formation we will first analyze the thermaldesorption spectra (Fig. 1). An evaluation of the individual desorp-tion peaks with respect to desorption energy and pre-exponentialfactor can be performed based on the Polanyi–Wigner equation[12]:

rdes ¼ mnHn expð�Edes=RTsÞ ð1Þ

structure, indicating that no preferred azimuthal alignment of the crystallites exists.

Fig. 6. Schematic representation of the development of layer growth and thermalstability of HAT-CN on Au(1 1 1). b1,2: 1st monolayer, c: 2nd monolayer, a:multilayer islands.

324 P. Frank et al. / Chemical Physics Letters 473 (2009) 321–325

where mn is the pre-exponential factor of the desorption processwith the order n, H is the coverage, Edes the desorption energy, Rthe gas constant, and Ts the sample temperature.

According to classical transition state theory (TST) the desorp-tion rate for a first order process, i.e., for non-associative moleculardesorption, can be described in the following way [13]:

rdes ¼ jHkBTh

qgas

qadsexp

�Edes

RTs

� �

¼ jHkBTh

expDSR

� �exp

�Edes

RTs

� �ð2Þ

where j is a transmission coefficient, which is usually taken asunity, qgas and qads are the partition functions of the molecules inthe free gas phase and the adsorbed phase, respectively, DS the en-tropy change during desorption, kB Boltzmann’s constant and hPlanck’s constant. Note that the transition state for a system withnon-activated adsorption is equivalent to the final state, i.e., thegas phase [16]. Therefore, the desorption rate is a function of bothEdes and DS, i.e., the ratio of the molecular partition functions inthe gaseous and adsorbed state. For a simple (mono-atomic)desorption system the entropy change can thus be neglected, andthe pre-exponential factor reduces to kBT/h, which is 1 � 1013 s�1

for T = 500 K. However, for desorption of large organic moleculessignificant changes of the partition functions of the adsorbed andthe free gaseous state are expected. Actually, there is substantialexperimental [14–16] and theoretical [17,18] evidence that gener-ally very large pre-exponential factors determine the desorptionrate of large organic molecules. In particular in the Letter by Taitet al. [16] the correlation between the entropy of large organic mol-ecules in the adsorbed state and the pre-exponential factors fordesorption is discussed in great detail.

The evaluation of the a-peak can easily be performed because itis of zero order desorption. In this case the desorption rate is only afunction of the temperature and independent of the coverage [12].The desorption energy can be deduced from the slope of the plotln(rdes) vs. 1/T and yields Edes = 1.84 ± 0.10 eV. From the interceptwith the y-axis of this quantitative plot a pre-exponential factorof m = 1 � 1019±1 s�1 is determined.

For the b and the c peaks, where the desorption rate is coveragedependent, such an evaluation is not applicable. However, by vary-ing the heating rates d and from the corresponding shifts of thedesorption peak maxima, the desorption energies and the pre-exponential factors can be obtained without further assumptions.For a first-order desorption process the following relation holds[12]:

Edes

RT2m

¼ md

exp�Edes

RTm

� �ð3Þ

here d is the heating rate and Tm the peak maximum. Therefore, aplot of lnðd=T2

mÞ vs. 1/Tm results in a straight line with a slope of�Edes/R. From the intercept of the slope with the y-axis the pre-exponential factor can be deduced.

Various sets of desorption spectra were measured with differentheating rates between 0.3 K/s and 3 K/s. The quantitative evalua-tion of the spectra yield the following values: b1-state: Edes =1.7 ± 0.2 eV, m = 1 � 1012±2 s�1; c-state: Edes = 2.0 ± 0.2 eV, m = 1 �1021±2 s�1. For the b2 state the peak maxima are not defined wellenough to perform a quantitative evaluation, but from the qualita-tive similarity to the b1 peak behavior we propose a similarpre-exponential factor as for the b1 state. The error for this typeof evaluation is by nature larger than for a leading edge evaluation,due to the limited range of the heating rate variation. Nevertheless,the result of this evaluation is very intriguing. The desorptionenergies for all three adsorption states (a, b1, c) are nearly thesame (within the error). Actually, there is even an opposite trendbetween the desorption peak maxima and the desorption energies.

Hence, the difference of the desorption temperatures for the indi-vidual adsorption states is mainly determined by the pre-exponen-tial factors. Both, the multilayer peak (a) as well as the secondlayer peak (c) exhibit the large pre-exponential factor expectedfor large molecules. Surprisingly, the monolayer peaks (b) show avery small pre-exponential factor, usually only observed for smallparticle (atom, diatomic molecule) desorption.

The following conclusion can be drawn from this result: At thedesorption temperature the partition function of the molecules inthe first monolayer is similar to that in the gaseous phase. In otherwords, the translational and rotational degrees of freedom are al-ready highly excited in the adsorbed phase when desorption starts.Apparently, the activation energy for surface diffusion of the largeHAT-CN molecules on the inert gold surface is very small and themolecules behave like a two-dimensional gas. The high pre-expo-nential factors for desorption from the 2nd layer and the bulkphase indicate that only little excitation of the various degrees offreedom takes place prior to desorption. This is the case if the mol-ecules are rather immobile.

Considering all the data obtained from TDS, XPS, AFM and XRDwe propose the following film formation and desorption kinetics ofHAT-CN on a Au(1 1 1) surface, as schematically depicted in Fig. 6.At low substrate temperature (200 K) HAT-CN molecules first ad-sorb flat lying on the surface without considerable lateral interac-tions between the molecules up to 1/2 ML. Molecules from thislayer desorb with a peak maximum at ca. 610 K from the surface(b1-peak), due to a desorption energy of 1.7 ± 0.2 eV and a pre-exponential factor m = 1 � 1012±2 s�1. A viable explanation for thesmall pre-exponential factor is a rather weak corrugation of thediffusion potential for the large HAT-CN molecules on Au(1 1 1),with the result that the molecules behave like a two-dimensionalgas prior to desorption.

With increasing coverage (between 1/2 ML and 1 ML) repulsivedipole interactions between the flat lying molecules lead to a con-tinuous decrease of the desorption temperature (b2-peak). The pre-exponential factor for this state is similar to that of the b1 state,indicating similar excitations of the degrees of freedom in thisstate. After completion of the monolayer a second layer of flat lyingmolecules forms. Molecules from this layer desorb with a peakmaximum of ca. 460 K in form of a single peak (c-peak). For thisstate a desorption energy of 2.0 ± 0.2 eV and a pre-exponential fac-tor m = 1 � 1021±2 s�1 was derived. The high value for m is expectedfor large organic molecules when the partition function of theadsorbed state is much smaller than that of the gaseous state.

P. Frank et al. / Chemical Physics Letters 473 (2009) 321–325 325

Apparently, the interaction of molecules in the second layer withthose in the monolayer leads to an immobilization of the mole-cules. This corresponds to large lateral diffusion barriers, which re-sults in a comparably low desorption temperature, as the degreesof freedom for lateral motion are hardly excited.

Just a small further increase of the coverage by about 0.3 MLleads to a destabilization of the second layer. It transforms at ca.410 K into an island shaped film which then desorbs with a peakmaximum at around 480 K. The first monolayer is not influencedby this phase transition. A similar transition of a low-temperature(c-peak) to a higher temperature desorption state (a-peak) hasalso been reported for benzene adsorbed on Ru(0 0 1) [19,20].The reason for this phase transition is most probably the increasedinterface energy between the 2nd and the 3rd layer. According toSzalay et al. [21] the HAT-CN molecules avoid p-stacking. The crys-tal packing of HAT-CN is instead dominated by perpendicular CN-pinteractions (T-stacking). Therefore, small amounts of molecules inthe 3rd layer act as nucleation centers for the formation of three-dimensional islands. Interestingly, the dewetting involves onlymolecules of the 2nd layer, but not of the 1st layer. Although themolecules in the 1st monolayer are laterally very mobile a totaldewetting is apparently energetically not favorable.

Continued HAT-CN adsorption at 200 K leads to a multilayerfilm which is meta-stable up to 360–410 K, depending on the cov-erage. Above this temperature the molecules of the multi-layersand the second layer transform into 3D islands involving dewett-ing. These islands desorb at ca. 490 K with zero-order kinetics(a-peak). This desorption is characterized by Edes = 1.84 ± 0.10 eVand m = 1 � 1019±1 s�1. Again, the large pre-exponential factor indi-cates that the molecules are largely immobile in this bulk phaseand that the internal degrees of freedom are only weakly excitedprior to desorption.

With further increase of the initial coverage, the desorptionfrom the monolayer (b-peaks) successively decreases and the mol-ecules are incorporated into the 3D islands (a peak). It is a quiteunusual behavior that ‘more strongly bound’ molecules are trans-formed into ‘less strongly bound’ molecules prior to desorption.The amount of molecules transformed into the less stable phasecorrelates roughly with the area covered by the 3D islands, as ver-ified by AFM measurements on films with varying film thickness.Thus we assume that the molecules in the first layer, which areoriginally flat lying, are incorporated into the bulk structure, whencovered by the multilayer islands. As a result of desorption thethree-dimensional islands shrink, both in height and in diameter,thus ‘cleaning’ part of the surface. On the rest of the surface the flatlying molecules remain on the surface up to higher temperature.The reason for this unusual behavior is that, in addition to the min-imization of the surface free energy, the entropy term plays a keyrole for this system. The degrees of freedom of the molecules in themonolayer (b-state) can be excited to a great extent prior todesorption, due to the high lateral mobility. Therefore, the entropychange during desorption, and hence the pre-exponential factor, ismuch smaller then for those molecules which desorb from verylocalized adsorption sites (c-state, a-state).

The reason that a phase transition of this type is generally notobserved for organic films is that the interaction between the first(wetting) layer and a clean metal substrate is generally muchstronger than that between additional layers, e.g., for p-quater-phenyl [9] or p-sexiphenyl [11] on Au(1 1 1). However, for PTCDA(3,4,9,10-perylene tetracarboxylicacid dianhydride) physisorbedon inert NaCl [22] and KCl [23] surfaces (weak bonding to the sub-strate) a similar dewetting behavior was recently observed. It wasargued that the molecule arrangement in the monolayer is incom-patible with the multilayer structure. Thus, as the coverage is in-creased beyond the first monolayer the interface structure isadapted to accommodate the growth of additional layers triggering

a dewetting transition [22]. This idea is also appropriate in ourcase. On the other hand, PTCDA on Ag(1 1 1) (strong bonding tothe substrate) exhibits classical Stranski–Krastanov growth behav-ior, where the first two layers grow in a layer-by-layer mode, fol-lowed by 3D island formation and no dewetting occurs duringsample heating [24].

4. Conclusions

In conclusion, we have shown by TDS, XPS, AFM, and XRD thatHAT-CN thin films on Au(1 1 1) exhibit a quite unusual tempera-ture stability, depending on the amount of initially adsorbed mate-rial. The first two layers grow in a layer like fashion (wetting layersof flat lying molecules) at and below room temperature, which aremeta-stable (desorption peak maxima at 450 K and 620 K, respec-tively). Increasing the coverage beyond two monolayers leads to adestabilization of the 2nd layer and a transition to a Stranski–Kras-tanov film takes place. The molecular arrangement within the is-lands corresponds to that of the HAT-CN bulk structure and themolecules desorb from these islands at 490 K. At the substrate-is-lands interface, the primarily flat lying molecules of the 1st mono-layer adapt to the bulk structure. With increasing initial coveragethe average island size also increases and thus the amount of mol-ecules of the 1st layer incorporated into 3D islands increases too.The molecules in the first layer, which are not covered by islandsremain stable up to a higher temperature than those which havebeen incorporated into the islands. The reason for this is that boththe binding energy and the entropy of the adsorbed phase are deter-mining the thermal stability of the molecular layer.

Acknowledgements

Many helpful discussions with E. Zojer, B. Bröker, R.-P. Blum andP. Hadley are sincerely acknowledged. This work was supported bythe Austrian Science Funds, Project No. P19197, and by the Euro-pean Commission under Contract No. NMP-3-CT-2006-033197(ICONTROL). N.K. acknowledges financial support by the EmmyNoether-Program (DFG).

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