Physica B 334 (2003) 244–249

Study on the stages of N,N-dibutyl–3,4,9,10-perylenetetracarboxylic diimide growth on Ru (0 0 0 1) surface

Yang Fana, Yang xinguob, Wu Yuea, Zhang Jianhuaa, Sun Jingzhib,Wang Mangb, Li Haiyanga, He Pimoa, Bao Shininga,*

aPhysics Department, Zhejiang University, Hangzhou 310027, ChinabDepartment of Polymer Science and Engineering, Hangzhou 310027,China

Received 25 October 2002; received in revised form 21 January 2003

Abstract

The stages of N,N-dialkyl-3,4,9,10-perylene tetracarboxylic diimide derivatives growth on Ru (0 0 0 1) surface has

been studied with the ultraviolet photoemission spectroscopy measurements, three emission features of the organic

material are located at 2.9, 4.8 and 6.7 eV, respectively below the Fermi level. The angle-resolved ultraviolet

photoemission spectroscopy results suggest that the organic molecule is parallel to the substrate surface. Desorption of

the organic material in multilayer occurs with warming the substrate. The decomposition of the adsorbed organic

substance on Ru (0 0 0 1) surface is at roughly 125�C, the low decomposition temperature observed is attributed to the

Ru substrate.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 79.60.Jv; 71.20.Rv; 73.20.Dx

Keywords: Organic semiconductor material; Ruthenium surface; Electronic structure

1. Introduction

Organic semiconductor material thin films havereceived considerable attention over the past fewyears in view of their potential applications [1–4].N,N-dialkyl-3,4,9,10-perylene tetracarboxylic dii-mide derivatives, which contain special condensednuclear structures and p conjugated electronsystems, can be used as organic photoconductingmaterial due to its relatively high light and heat

durability and its excellent photoelectric pro-perties. Recently, application studies on N,N-

dialkyl-3,4,9,10-perylene tetracarboxylic diimidederivatives have been involved in researches intoorganic photoconductors (OPCs) [5], organic solarcells [6], etc. suggesting a favorable outlook forthese applications.

In most organic photoconducting devices thereare interfaces [7] between organic–organic, organ-ic–inorganic, inorganic–inorganic, etc. In somedouble-layered OPCs with the interface betweenthe electron-donor layer and the carrier-transport-ing layer, electronic properties of the interface maysignificantly influence the charge transfer and

*Corresponding author. Tel.: +86-0571-7951594; fax: +86-

0571-7951328.

E-mail address: phybao@dial.zju.edu.cn (B. Shining).

0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-4526(03)00072-3

charge separation [6]. Therefore, the impact ofinterfaces on the photosensitivity of OPCs is thekey to the operation of organic based devices. Thecontrolled growth of organic semiconductor ma-terials on inorganic substrates provides an oppor-tunity for producing hybrid organic–inorganicstructures with novel electronic properties [8–13].

The N,N-dibutyl-3,4,9,10-perylene tetracar-boxylic diimide molecule, as shown in Fig. 1, isan axisymmetric molecule in which there are twocarbonyls and one normal-butyl on either symme-trical part and there are condensed rings in thecenter. What we would like to mention in thispaper is to clarify, during the thin film growth onRu (0 0 0 1), which functional group in the N,N-

dibutyl-3,4,9,10-perylene tetracarboxylic diimidemolecule was involved in the interaction betweenthe adsorbate and the substrate, how did thefunctional groups contribute to the interfacialelectronic states and how did the charge trans-ferred between the two layers. We studied differentstages of the N,N-dibutyl-3,4,9,10-perylene tetra-carboxylic diimide film growth on Ru (0 0 0 1) andthen investigated its evolution with warming thesubstrate using ultraviolet photoelectron spectro-scopy (UPS). We also investigated the molecularorientation using angle-resolved ultraviolet photo-electron spectroscopy (ARUPS).

2. Experimental

The experiments were carried out in an ultra-high vacuum apparatus of VG ADES-400 electronenergy spectrometer with a base pressure betterthan 3� 10�10mbar. The apparatus was describedelsewhere [14]. In brief, it contained an argon-iongun, a low-energy electron diffraction (LEED)optic, an UV source, and a hemispherical electronanalyzer, etc. Angle-resolved photoemission mea-

surements can be performed by rotating thehemispherical analyzer. The cleaning of Ru(0 0 0 1) surface was obtained by several circles ofargon-ion sputtering and annealing. The organicthin films were deposited from a resistively heatedtantalum boat at an evaporation temperature ofbelow 500K. The thickness of each deposited layerwas monitored using a quartz crystal oscillator.The UPS measurements were performed with UVsource of He I (21.2 eV) and a sample bias of�5.0V to enable observation of the low energysecondary cut-off. The overall resolution is about0.05 eV. The temperature of the sample wasdetermined by a thermocouple attached to thesample.

3. Results and discussion

UPS spectra recorded from the clean Ru (0 0 0 1)surface and after deposition of different amountsof the organic material are shown in Fig. 2. Thespectra were collected in the normal direction fromthe Ru substrate and the subsequent depositedorganic thin layers with an incident angle of 30�

about the surface normal. The spectrum recordedfrom clean Ru surface clearly shows the character-istic valence structure of the Ru substrate with asharp Fermi edge. After deposition of the N,N-

dibutyl-3,4,9,10-perylene tetracarboxylic diimide,the features near the Fermi level decreased andthree main peaks, from the organic materials,appeared. The spectra b–g were recorded from thesurface with organic thin layers and the thicknessof the organic overlayer were 0.2, 0.4, 1.0, 4.0, 8.0and 12.0 monolayers, respectively. For the thick-ness of the organic overlayer larger than 8.0monolayers, the valence features of the Rusubstrate disappeared, indicating that the metalsurface is completely covered. The spectrum g,

NN

O

O

O

O

H3CH2CH2CH2C H2CH2CH2CH3C

Fig. 1. The chemical structure of the N,N-dibutyl-3,4,9,10-perylene tetracarboxylic diimide molecule.

Y. Fan et al. / Physica B 334 (2003) 244–249 245

corresponding to a 12.0 monolayers thickness ofthe overlayer is not different from that of 8.0monolayers and should represent typical emissionsfrom the surface of the organic material. Threeemission features of the organic are located at 2.9,4.8 and 6.7 eV, respectively below the Fermi level.

The variation in the secondary cut-off energywith thickness of organic material is shown in theright panel of Fig. 2. Since the Fermi level is fixed,the change in binding energy of the secondary cut-off corresponds directly to the change of the workfunction with the thickness. With the cut-offenergy of 17.0, 17.1, 17.2, 17.3, 17.35, 17.38 and17.4 eV for the thickness of 0, 0.2, 0.4, 1.0, 4.0, 8.0,12.0 monolayers, the work functions are deter-mined to be 4.2, 4.1, 4.0, 3.9, 3.85, 3.82 and 3.8 eV,respectively. Due to the sharp decrease in workfunction during deposition, it can be concludedthat the dipole formation at the beginning ofdeposition is related to polarization of the organicmolecule. The saturated value of 3.8 eV representsthe work function of the organic layer. The changein work function is very typical at the interfacebetween organic semiconductor and metal [15–17].

Compared to the molecular spectra, the peak at6.7 eV is assigned to the (CH2) orbits in the n-butylwith p character and the C–C orbits of theperylene in the core of the molecule with scharacter. The most intense peak, at 4.8 eV inspectra recorded from high coverage, is assigned tothe combination of the nN orbit in the n-butyl, theC–O p bond and nN orbit in the carbonyl and theC–C p bonds in the perylene core. The lowestbinding energy peaks at 2.9 eV cannot be assignedsolely to any functional group, it arises from a newmolecular orbit resulting from the interactionbetween functional groups in the molecule. Dueto the appearance of this peak the highest occupiedmolecular orbit (HOMO) of the organic materialon Ru is located at above 2.9 eV in BE.

Interfacial interactions occurred between layersduring the deposition of the organic adsorbate onthe substrate. Evidence of such interactions is theincrease in binding energy of the 4.8 eV peak withcoverage. The energy shifts of the other two peaks,however, have not been observed. Since that thepeak, at 4.8 eV, is assigned to the combination ofthe nN orbit in the n-butyl, the C–O p bond and nN

orbit in the carbonyl and the C–C p bonds in theperylene core, it shifts to higher binding energyduring the deposition indicates that Ru atoms inthe substrates donate electrons to one or moreorbits associated to the peak. During the perylene-3,4,9,10-tetracarboxylic dianhydride deposited onInAs (1 1 1) surface, XPS measurements show thatthe binding energies of the C1s and O1s core levelsare considerably lower in sub-monolayer filmscompared to those observed in thicker filmssuggesting significant charge transfer between theC–O bond and substrate at the interface [7]. Thestructure of C–O bond in the carbonyl in N,N-

dibutyl-3,4,9,10-perylene tetracarboxylic diimide isnot different from that in perylene-3,4,9,10-tetra-carboxylic dianhydride, the charge transfer in ourcase may be also between the C–O bond andsubstrate. The charge transfer between the C–Obond and substrate provides an opportunity forproducing hybrid organic–inorganic structures.

The growth mode of the organic thin film is notclear. Since that the thickness of each depositedlayer was monitored using a quartz crystaloscillator, the measurement of the thickness is

Fig. 2. UPS spectra recorded from the clean Ru(0 0 0 1) surface

and after deposition of different amounts of the organic

materials. The spectra were collected in the surface normal

with an incidence angle of 30� about the surface normal. The

spectra a–g were recorded from the surface with thin organic

layers, the thickness of the organic overlayer was 0, 0.2, 0.4, 1.0,

4.0, 8.0 and 12.0 monolayers, respectively.

Y. Fan et al. / Physica B 334 (2003) 244–249246

dependent on its weight, the given thicknessshould be an average value. The spectrum (shownin Fig. 2) of the organic overlayer of 1.0 mono-layers thickness shows the characteristic valencestructure near the interface. In the case, thefeatures from both organic layer and the substratecan be observed. Compared to the spectrum ofthick organic layer, the second peak arising fromthe organic is not at 4.8, but at 4.5 eV. ARUPSspectra from an organic of 1.0 monolayersthickness recorded with fixed incidence angle at30� and fixed emission angle at 30� are shown inFigs. 3a and b, respectively. In both figures themost intense peak at 4.5 eV is normalized forcomparison purpose. In spite that the peakintensity did not change in the case of fixedincidence angle, shown in Fig. 3a, the change inintensity of the peaks can be observed with varyingincidence angle, shown in Fig. 3b. In the case of

fixed emission angle, the peaks at 2.9 and the6.7 eV decrease in intensity with increasing inci-dence angle. The peak at 6.7 eV mainly comesfrom the C–C orbits of the perylene with scharacter, the ARUPS results suggest that theorganic molecule is parallel to the substratesurface. Since the peak at 2.9 eV arises from theinteraction between functional groups in themolecule, decreasing its intensity with increasingincidence angle is reasonable when the moleculelies on the substrate.

The UPS spectra evolution as a function oftemperature has been plotted in Fig. 4. The spectrawere collected in the surface normal direction withan incident angle of 30� about the surface normal.The spectrum g in Fig. 4 was taken from 12.0monolayers thickness of the overlayer identical tothe spectrum g in Fig. 2, three emission features ofthe organic are located at 2.9, 4.8 and 6.7 eV,

Fig. 3. ARUPS spectra from an organic thickness of 1.0 monolayers (a) recorded with fixed incidence angle at 30� (b) recorded with

fixed emission angle at 30�.

Y. Fan et al. / Physica B 334 (2003) 244–249 247

respectively below the Fermi level. The spectra h–lwere recorded with warming the substrate at100�C, 125�C, 150�C, 200�C and 300�C, respec-tively. All the three emission features of theorganic decrease in intensity with warming thesubstrate due to the desorption of the organicmaterial in multilayer. After the desorption ofmultilayer the characteristic valence structures inUPS spectrum is not the same as that from sub-monolayer films. Disappearance of the peak at2.9 eV and a relatively significant shift in bindingenergy of both two peaks at 4.8 and 6.7 eV isobserved between 125�C and 150�C. This suggeststhat decomposition of the adsorbed organicsubstance occurs at roughly 125�C rather thaninterfacial interaction, since the gradual and tinyshift in binding energy resulting from interfacialinteraction is not comparable with the discontin-uous and considerable shift in this case. Thedecomposition temperature of 125�C is muchlower than that of the bulk organic material, it isclear that the low decomposition temperatureobserved is attributed to the Ru substrate.

The further decomposition occurs with furtherwarming the substrate. The change of the second-

ary cut-off shows the work function increases, seethe right panel of Fig. 4. The variation in thesecondary cut-off energy with warming the sub-strate is different from that with decreasing thethickness of organic layers, since both desorptionand decomposition occurred.

In summary, according to the performed ultra-violet photoemission spectroscopy measurements,three emission features of the N,N-dibutyl-3,4,9,10-perylene tetracarboxylic diimide are lo-cated at 2.9, 4.8 and 6.7 eV, respectively below theFermi level, the HOMO of the organic material onRu is located at above 2.9 eV in BE. The ARUPSresults suggest that the organic molecule is parallelto the substrate surface. Desorption of the organicmaterial in multilayer occurs with warming thesubstrate. The decomposition of the adsorbedorganic substance at the interface is at roughly125�C, the low decomposition temperature ob-served is attributed to the Ru substrate.

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