Organic–organic interfaces and unoccupied electronic states of thin films of perylene and naphthalene derivatives

Journal of Molecular Structure 744–747 (2005) 145–149 www.elsevier.com/locate/molstruc

Organic–organic interfaces and unoccupied electronic states of thin films of perylene and naphthalene derivatives A.S. Komolova,b,*, P.J. Møllera,*, Y.G. Aliaevb, E.F. Laznevab, S. Akhremtchikb, F.S. Kamounahc, J. Mortensenc, K. Schaumburgc a Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark V. A. Fock Institute of Physics, St Petersburg State University, Uljanovskaja ul.1, St Petersburg 198504, Russia c Department of Chemistry and Biology, Roskilde University Center, Universitetsvej 1, DK-4000 Roskilde, Denmark b

Received 6 September 2004; accepted 11 January 2005 Available online 8 March 2005

Abstract Thin films of N,N 0 -Bis(benzyl)-3,4,9,10-perylenetetracarboxylic diimide (BPTCDI, Fig. 1b) and N,N 0 -Bis(benzyl)-1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI, Fig. 1d) were thermally deposited in UHV on 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, Fig. 1a) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA, Fig. 1c) film surfaces, respectively, in order to form organic–organic interfaces so that molecules constituting the interfacing layers differ by the substituent group. The surface potential and the density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level (EF) were measured during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Analysis of the TCS data allowed us to assign the p( band located 5–7.5 eV above EF for all the four films under study and the higher located s*1 and s*2 bands and the splitting within them. In order to perform the analysis the molecules were hypothetically divided into benzene-like, conjugated and non-conjugated fragments that may individually contribute to the peaks in the DOUS bands. It was shown that a non-conjugated fragment would serve for decreasing of the energy corresponding to the s*1 and s*2 bands and the sub-bands within them while an addition of a benzene-like fragment would do the opposite. The BPTCDI/PTCDA and BNTCDI/NTCDA interfaces were found non-reacted and a 4.1G0.1 eV work function value for both BPTCDI and BNTCDI films was determined, which is about 0.25 eV lower than the work functions of the PTCDA and the NTCDA films. q 2005 Elsevier B.V. All rights reserved. PACS: Condensed matter; Electronic properties Keywords: Surface electronic phenomena; Electron–solid interaction; Electronic band structure; Organic–organic semiconductor interfaces; Perylene and naphthalene derivatives

1. Introduction Thin films of perylene and naphthalene derivatives and other small conjugated molecules and their hetero-junctions have shown interesting electronic properties that can be used in photovoltaic applications, light-emitting diodes and gas sensing devices [1–3]. Electron spectroscopy techniques have been applied to study band alignment of such * Corresponding authors. Tel.: C45 35 32 02 64; fax: C45 35 32 0322. E-mail addresses: [email protected] (A.S. Komolov), [email protected] (P.J. Møller).

0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.047

molecular layers and to study the density of the occupied (valence) electronic states (DOS) and the density of the unoccupied electronic states (DOUS) [2,4,5]. Organic– organic interfaces have been shown mostly to be abrupt and unreactive [6], which can be considered as an advantage in respect to charge transport in the multilayer film structures. The energy structure of the electronic states is very sensitive to chemical modifications of a solid surface and DOUS are more sensitive than DOS because the unoccupied electron states have larger spatial delocalization. A number of inverse photo emission spectroscopy (IPES) studies of films made from 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and other perylene derivatives have revealed three p* bands at energies 2–5 eV above

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6

5

O

8

O

R = O,

(a)

R

R O

2

12

1

7

O

6

O 5

8 R

O

N

R = O,

(b)

(c)

R 1

4 O

11

3

2

O

N

(d)

Fig. 1. Chemical structure of the molecules under study (a–d): PTCDA, BPTCDI, NTCDA and BNTCDI.

the Fermi level (EF) and a s* band at 8–11 eV above the EF, which were in a good agreement with theoretical considerations [4,7]. Studies of DOUS by means of near-edge X-ray absorption spectroscopy (NEXAFS) have certain limitations with respect to smaller organic molecules [8]. However, it was possible to assign some unoccupied bands of PTCDA and related films as a result of the NEXAFS investigations [8,9]. Total current spectroscopy (TCS) [10] of unoccupied electronic states has been successfully used in studies of band alignment at interfaces [11] and a direct correspondence between TCS results and DOUS in the energy range 5–20 eV above the EF have been shown for metal oxide films [12] and for Cu-phthalocyanine films [13]. The peak assignment of the DOUS of the PTCDA films we reported earlier [14] turned out to be incomplete due to the lack of the reference data. In the present work, we study the BPTCDI/ PTCDA (Fig. 1b/a) and BNTCDI/NTCDA (Fig. 1d/c) interfaces in which the interfacing layers differ by the substitute group and we present our investigation of the DOUS and of the interface formation in these organic– organic structures by means of TCS.

2. Experimental The experiment was performed in a UHV system (base pressure 10K7 Pa), which had an Auger electron spectroscopy (AES) unit and a low energy electron diffraction (LEED) unit installed. The LEED unit was also used as a main instrumentation for the TCS measurements [10,12]. A probing electron beam was directed normally onto the surface under study and it had a 0.2–0.4 mm diameter in the surface plane. The total current J(E) in the sample circuit was monitored as a function of electron energy E in the range 0–30 eV. The low electron energies used and the low beam intensities (10K5–10K6 A/cm2) provided a non-destructive probing of the organic films under study. The derivative S(E) Z dJ(E)/dE was measured in our experiment by means of lock-in amplification technique and it will be further referred

to as a TC spectrum. A TC spectrum consists of a primary TCS peak and a TCS fine structure. The primary TC peak reflects the possibility for electrons to enter the surface under study from vacuum, and its maximum points out the Evac level of the surface. In order to be able to define the work function, we consider EF in an organic film deposited onto a metal or semiconductor substrate as continuation of the EF in the substrate. The absolute values of work functions EvacK EF could be obtained after the instrument was calibrated on a known sample surface, such as for instance an Au surface prepared by thermal deposition at 10K8 Pa that has 5.2 eV work function [15]. It has been shown theoretically and experimentally [16–21] that TCS fine structure describes changes in electron reflection coefficient of the surface. It is often assumed that the changes of the elastic component of the reflection with electron energy are predominant. In a forbidden energy region the electron reflection is high and the J(E) reaches its minimum and when the DOUS is high the J(E) reaches a maximum. DOUS analysis is usually carried out using the negative second derivative Kd2J (E)/ dE2ZKdS(E)/dE and its peaks are assumed to represent the DOUS peaks [10,18,20,21]. We note that a TCS fine structure is caused by the modulation of the J(E) after it had a step-wise increase due to the incident electron energy E approached Evac (primary peak). That is why the TCS fine structure can be observed starting from a few eV above Evac. Analysis of the TCS spectra that have contribution from both the substrate and the deposit allows us to describe the band alignment at the interface as we discussed in detail in [11]. Thin films of PTCDA and NTCDA (Aldrich) were thermally deposited in situ from the Knudsen cell onto the Cu(1 1 1) surface interspaced by approximately 10 cm distance. The thickness of the PTCDA and NTCDA films was from 10 to 15 nm and they will be also referred to as substrates in the scope of the present study. The thickness of the deposits was controlled by a quartz microbalance to which we assign a typical uncertainty of 0.1 nm. The N,N 0 Bis(benzyl)-3,4,9,10-perylenetetracarboxylic diimide (BPTCDI) and N,N 0 -Bis(benzyl)-1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI) molecules (Fig. 1) were synthesized and the thin films were thermally deposited onto the PTCDA and the NTCDA substrates in the way similar to the one described above. The thickness of the BPTCDI and BNTCDI overlayers in the structures under study was up to 12 nm. In addition to the quartz microbalance measurement, the deposit thickness was also controlled by means of monitoring the deposition time and the deposition rate (approximately 0.1 nm/min) and using attenuation rate of the TCS peaks from the substrate [11]. The AES analysis on all four films under study has shown a good correspondence between the atomic composition of the films measured and the chemical structure of the molecules. No LEED patterns from the films under study were observed manifesting formation of the disordered molecular films.

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3. Results and discussion

147

(a) x5 BNTCDI

4

σ1∗

0

σ2∗

π∗

–4 (b)

8 x5

–dS(E) / dE,arb. u.

The evolution of the position of the primary TCS peak and of the TCS fine structure were monitored during the deposition of the BPTCDI and BNTCDI films onto the PTCDA and NTCDA substrates, respectively. The KdS(E)/dE spectra of the PTCDA (Fig. 2b) and NTCDA (Fig. 3b) films have undergone a gradual modification until the deposit thickness reached approximately 11 nm. The KdS(E)/dE spectra of the newly formed BPTCDI and BNTCDI overlayers are presented in Fig. 2a and Fig. 3a. The spectra of PTCDA and NTCDA (Figs. 2b and 3b) correspond well to the earlier TCS results reported for these films on a number of solid substrates [14,22]. As we discussed in the experimental section, the peak positions in the KdS(E)/dE spectra might directly reflect DOUS of certain materials as we have shown for films of the small conjugated molecules [13]. Let us assign the DOUS peaks for the films under present study. Highly ordered pyrolytic graphite (HOPG) basal plane might be a good starting point because it represents

8

σ2∗

4

NTCDA

0 π∗

σ1∗

–4

(c) 2

x5

1 0 σ1∗

–1

σ2∗

–2

(b)

–dS(E) / dE,arb. u.

(a)

x5

4 2 0 –2 –4 –6

0 σ1∗

σ2∗

BPTCDI

π∗

2

x5

1 0 –1

σ1∗

–2

1.0 –dS(E) / dE,arb. u. arb. u.

(d)

σ2∗

π∗

–3 (c)

π∗

–3

π∗

PTCDA

σ∗

0.5

PTCDA [4]

0.0 10 5 π∗

0

σ∗

–5

σ∗

π∗

π∗

σ∗ HOPG

–10 0

5

10

15 E-EF, eV

20

25

Fig. 2. DOUS of the 10 nm BPTCDI film (a) and of the 15 nm PTCDA film (b) obtained by TCS technique. DOUS of a PTCDA film (c) according to the IPES measurements [4]. DOUS of HOPG basal plane (d) obtained by TCS. p*, s*1 and s*2 bands in (a) and (b) contain main contributions from the CaC, C–C and CaC orbitals, respectively. See the text for further details. The peak assignment for HOPG (d) is made according to [13,16,19].

5

10

15 E-EF, eV

20

PTCDA 25

Fig. 3. DOUS of the 10 nm BNTCDI film (a) and of the 15 nm NTCDA film (b) obtained by TCS technique. p*, s*1 and s*2 bands in (a) and (b)—see Fig. 2. DOUS of the 15 nm PTCDA film (c)—the same as Fig. 2b.

a basic carbon-based surface and its DOUS has been well studied [21,23]. We have earlier found a good correspondence between the DOUS of HOPG and its KdS(E)/dE spectrum [13] and we present the peak assignment in the Fig. 2d. We should note that a feature-full DOUS structure of HOPG above 12 eV in the Fig. 2d and particularly the p* peaks at 13 and 19 eV are present due to the crystallinity of HOPG [21,23] and these peaks should not be observed in a study of a non-crystalline organic film. To a first approximation we assign the DOUS bands in PTCDA according to the Fig. 2b: 5–7.5 eV, 8–12 eV and a broader band 13–22 eV as p*, s*1(C–C) and s*2(CaC), respectively. The assignment of the first 2 bands corresponds well to the results of the combined theoretical and IPES studies of Hill et al. [4] and Hirose et al. [7]. A broader s*2(CaC) band in PTCDA and NTCDA was also reported by Taborski et al. [9]. The presence of p*1, p*2, s*1 and s*2 unoccupied bands in DOUS is typical for the DOUS of conjugated molecules according to Sto¨hr [24]. The p* band observed here by TCS corresponds well to the p*2 band, while the p*1 band is located below the typical energies at which we can monitor DOUS in a TCS experiment. The presence of the 2 peaks in the p* band (Fig. 2b) corresponds well to the results of the theoretical studies of perylene and a number of other conjugated molecules [8]. A contribution of the (C–H)

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related orbitals and of the (CaO) orbitals to this band should be also assumed according to NEXAFS results on a large number of organic molecules [24,25]. In order to further analyze the structure of the unoccupied bands (Fig. 2b) we hypothetically divide the PTCDA molecules into fragments to which we would assign the peaks within the bands. We expect this approach to be analogous to building-block method used in NEXAFS investigations [24]. We base our selection of the fragments on the carbon–carbon bond length which is 1.39, 1.42 and ˚ in benzene, HOPG and some non-conjugated 1.5 A molecules, respectively [25]. A linear dependence of the energy of the s*1(C–C) unoccupied band on the bond length was obtained and the approximate 1.5 eV difference of the s*1(C–C) band energy for benzene and HOPG was reported [24,25]. In PTCDA we distinguish fragments that have benzene character (sites 1, 2, 5, 6, 7, 8, 11 and 12 in Fig. 1.), the central ring that has HOPG character and the weakly conjugated fragment, which is dicarboxylic acid anhydride terminated by the in-ring carbon atoms. According to the approach above we assign the 11 eV peak in the PTCDA s*1(C–C) band to the benzene-type fragments and the 9 eV peak in this band to the weakly conjugated fragments. Although one may expect the contribution of the central ring of the PTCDA to the s*1(C–C) band at 10.5 eV according to the TCS results on the HOPG surface (Fig. 2d), that can hardly be distinguished in Fig. 2b due to the low partial weight of this fragment in the PTCDA molecule. We assume that a similar approach to the peak assignment of the 13–22 eV s*2(CaC) band (Fig. 2b) could be applied as the energy positions of s*2(CaC) bands are typically 5–7 eV higher than the ones of s*1(C–C) bands [24,25]. We also suppose that the sub-band at 19–22 eV in Fig. 2b contains a contribution from the from s*(CaO) orbitals as their position typically 4–6 eV above s*(CaC) related orbitals has been reported [24,25]. The KdS(E)/dE spectrum of the BPTCDI film (Fig. 2a) demonstrates also the p*, s*1(C–C) and s*2(CaC) bands at 5–7.5 eV, 8–12 eV and a broader band 12.5–19 eV, respectively. The profile of the DOUS within the bands and the position of the s*2(CaC) band are different from the case of the PTCDA film. One can observe two separate peaks at 9 and 12 eV (Fig. 2a) that point to a higher splitting the s*1(C–C) band in the BPTCDI film compared to the PTCDA film (Fig. 2b). According to the approach we used above to analyze the DOUS bands of PTCDA, the difference of the BPTCDI molecular structure from the PTCDA molecular structure may be described as a presence of the additional fragments that would contribute to the different sub-bands within the s* bands. We distinguish the additional benzene-like fragments and the non-conjugated fragments, which are butyl-imide terminated by the in-ring carbon (Fig. 1) that both serve for a higher splitting of the s*(C–C) band in the BPTCDI film. The KdS(E)/dE spectrum of the NTCDA film (Fig. 3b) has the p* band at the 5–7.5 eV energy similar to

the PTCDA and BPTCDI films. The s*1(C–C) and s*2(CaC) bands of the NTCDA film (Fig. 3b) have a lower energy position compared to the PTCDA bands (Fig. 2b). We assign the position of the s*1(C–C) band in NTCDA and the domination peak at 8.5 eV (Fig. 3b) to the larger partial contribution of the dicarboxylic acid anhydride in the NTCDA molecule. A similar difference in the s*1(C–C) bands of PTCDA and naphthalene dicarboxylic acid anhydride (NDCA, one half of PTCDA) could be seen [9]. This difference may have a lower extent in the case of PTCDA and NDCA because NDCA has only 1 of the dicarboxylic acid anhydride fragments while NTCDA has 2 of them. The substitution of the anhydride groups in NTCDA by the benzyl-imide groups resulted in the higher splitting in the s*1(C–C) band and the shift of the s*2(CaC) band towards higher electron energies in BNTCDI (Fig. 3a and b). As we have discussed above, an addition of a non-conjugated fragment would serve for decreasing of the energy corresponding to the s* bands while an addition of a benzene-like fragment would do the opposite. We assume that the effect of the addition of the benzene-like fragments within the benzyl-imide groups was dominating in the case of BNTCDI because the number of the benzene-like sites in NTCDA (Fig. 1, sites 2, 3, 6 and 7) was increased by 10 in BNTCDI. On the other hand, the substitution of the anhydride groups in PTCDA by the benzyl-imide groups resulted in the shift of the s*2(CaC) band towards lower electron energies in BPTCDI (Fig. 2a and b). Apparently, the partial effect of the non-conjugated fragments on the DOUS structure upon the benzyl-imide substitution of PTCDA was larger than the effect of the conjugated fragments because PTCDA had already a larger aromatic core. We have conducted the analysis of the subsequent changes in the dS(E)/dE spectra during the formation of the BPTCDI/PTCDA and BNTCDI/NTCDA interfaces according to the method we discussed in refs. [11,13] and we have observed no evidence of chemical interaction or polarization at these interfaces. The low reactivity at the BPTCDI/ PTCDA and BNTCDI/NTCDA interfaces corresponds well to the results of Hill et al. [6] who studied a number of organic–organic interfaces that contained a PTCDA layer. The surface potential was measured by means of monitoring the position of the primary TCS peak during the deposition of BPTCDI on PTCDA and of BNTCDI on NTCDA, according to the TCS method [10]. Since no interface interaction was detected to affect the surface potential, the changes of the latter upon the film deposition reflect the changes of the work function [11]. The work function changes during the formation of the BPTCDI/PTCDA and BNTCDI/NTCDA interfaces are shown in Fig. 4a and b, respectively. The 4.4G0.1 eV work function value of the PTCDA film and the 4.3G0.1 eV work function value of the NTCDA film correspond well to the results of our earlier measurements [14] and to the results of the photo-electron spectroscopy studies of PTCDA films that report EF located

A.S. Komolov et al. / Journal of Molecular Structure 744–747 (2005) 145–149

(a)

149

was determined, which is about 0.25 eV lower than work functions of PTCDA and NTCDA films.

4.4

(b)

Work function, eV

4.2

Acknowledgements

4.0 0

2 4 6 8 BPTCDI film thickness, nm

10

2 4 6 8 BPTCDI film thickness, nm

10

The work was supported by the Danish Research Agency, the Russian Foundation for Basic Research (0203-32751),the Russian state program ‘Surface atomic structures’, and the STRP ‘Nanochemical sensors’ and COST D15 Chemistry ‘Interfacial Chemistry and Catalysis’ programs of the European Commission.

4.4 4.2 4.0 0

Fig. 4. Work function changes during the deposition of the BPTCDI film on the PTCDA film (a) and during the deposition of the BNTCDI film on the NTCDA film (b).

about 2 eV above the top of the valence band [7]. The work function values decreased monotonically (Fig. 4) until they reached a 4.1 eV value for the 8–10 nm thick BPTCDI and BNTCDI films. The values of the work function for these films are very similar to each other while they differ about 0.25 eV from the work functions of PTCDA and NTCDA. We assume that this difference may be due to the difference of the electron affinity of the interfacing films although a direct explanation of the difference observed could be a subject of a separate investigation.

4. Conclusions Our TCS results obtained on the BPTCDI/PTCDA and on the BNTCDI/NTCDA organic-organic interfaces allowed us to determine and to assign the DOUS features of the interfacing organic films in the 5–25 eV range above the EF and to study the band alignment at the interfaces. The p* band was found at 7–7.5 eV above EF for all the four films under study. The bands located at approximately 8–13 eV and at 13–22 eV above EF were assigned as s*1(C–C) and s*2(CaC) bands, respectively. It was shown that a weakly-or a non-conjugated fragment within the molecules under study would serve for decreasing of the energy corresponding to the s*1 and s*2 bands and for distinguishing of the lower lying sub-bands within them while an addition of a benzene-like fragment would do the opposite. The BPTCDI/PTCDA and BNTCDI/NTCDA interfaces were found non-reacted and the 4.1G0.1 eV work function value for both BPTCDI and BNTCDI films

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