Thermally Activated Transformation from a Charge-Transfer State to a

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Thermally Activated Transformation from a Charge-Transfer State to a Rehybridized State of Tetrafluoro-tetracyanoquinodimethane on Cu(100) Tetsuo Katayama, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu* The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8581, Japan

ABSTRACT We investigated the adsorbed states of 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ) on Cu(100) using high-resolution electron energy loss spectroscopy (HREELS) and ultraviolet photoelectron spectroscopy (UPS). In the case of deposition at 90 K, the workfunction change increases up to ∼1.5 eV. In the case of deposition at 300 K, it saturates at ∼0.9 eV. In both cases, an occupied lowest unoccupied molecular orbital (LUMO) state of F4-TCNQ has been observed at 1.2 eV below the Fermi level. Both UPS and vibrational EELS results show that F4-TCNQ becomes an anion at 90 K. In addition, the extra weakening of CtN bonds occurs at 300 K, involving the local interaction between the cyano groups in F4-TCNQ and surface Cu atoms, where the transformation into the rehybridized state is thermally activated. The bidirectional interaction including the donation from F4-TCNQ and the back-donation from Cu(100) can explain the difference in workfunction change. SECTION Surfaces, Interfaces, Catalysis

geometry.14 In this system, the combination of adsorptioninduced geometric distortion of the molecules and charge redistribution between the molecule and the surface leads to a net workfunction change. Very recently, Tseng et al. reported that the adsorption of TCNQ on Cu(100) involves the charge transfer and the substrate buckling.15 In this study, we investigated the adsorbed states of F4-TCNQ on Cu(100) using high-resolution electron energy loss spectroscopy (HREELS) and ultraviolet photoelectron spectroscopy (UPS). We found that the workfunction change strongly depends on deposition temperature. Two different states, that is, “charge-transfer state” and “rehybridized state” of F4-TCNQ on Cu(100), are observed as a function of substrate temperature. Figure 1 shows the workfunction change of F4-TCNQ on Cu(100) as a function of coverage. Filled circles and open circles correspond to the deposition temperature at 300 and 90 K, respectively. In both cases, the work function increases with increasing coverage. These results indicate that the charge transfer from the Cu surface to adsorbed F4-TCNQ occurs. The coverage of F4-TCNQ (molecule/cm2) was estimated on the basis of in situ X-ray photoelectron spectroscopy (XPS) results. We assumed that the monolayer completion corresponds to 1.1 1014 molecule/cm2 from the saturation of the workfunction change. In the case of deposition at 300 K,

O

rganic molecules having a strong electron affinity character, such as tetracyanoquinodimethane (TCNQ) and its derivatives, have received considerable attention because of the electronic modification of interfaces when these molecules adsorb on various substrates.1 In particular, a fluorinated TCNQ derivative, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), which has a high electron affinity (EA = 5.24 eV), has been studied on various substrates from the viewpoint of the p-type doping of organic films,2-6 the energy level alignment on metal surfaces,7-9 and the surface transfer doping of semiconductor surfaces.10,11 In particular, at organic-organic or organic-metal interfaces, adsorbed F4-TCNQ species result in the lowering of a hole injection barrier, and it is possible to improve organic molecular device performance such as organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs).4-7 Therefore, understanding of structural and electronic properties of metal-organic interfaces is crucial in improving such organic electronic devices. Recently, the structural distortion of strong acceptor molecules and the adsorbate-induced reconstruction of the substrate have been reported on Cu surfaces.12-15 The scanning tunneling microscopy (STM) and density functional theory (DFT) studies show that tetracyanoethylene (TCNE) molecules form ordered straight chains along the [110] and [110] directions on Cu(100) and induce the buckling of surface Cu atoms.12,13 The X-ray standing wave (XSW) and DFT study reported that the adsorption of F4-TCNQ on Cu(111) induces a strong molecular distortion from the quinonoid to benzenoid

r 2010 American Chemical Society

Received Date: June 5, 2010 Accepted Date: September 14, 2010 Published on Web Date: September 20, 2010

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DOI: 10.1021/jz100766k |J. Phys. Chem. Lett. 2010, 1, 2917–2921

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Figure 1. Workfunction change of F4-TCNQ on Cu(100) as a function of coverage. Filled and open circles correspond to the deposition at 300 and 90 K, respectively.

Figure 3. HREELS spectra of F4-TCNQ on Cu(100) deposited at (a) 90 and (b) 300 K, respectively. (c) HREELS spectra of F4-TCNQ on Cu(100) deposited at 90 K and after heating at 180 K. The coverage is 1.2 ML. The coverages were estimated assuming that the appearance of ν(CdC) of neutral F4-TCNQ at 1460 cm-1 corresponds to 1 ML with the constant deposition rate. Each spectrum has an offset.

d bands.16 In the submonolayer coverages, the adsorbatederived peaks were observed at 1.2, 6.3, 7.4, and 8.3 eV under both temperature conditions. According to the previous studies, occupied lowest unoccupied molecular orbital (LUMO) states of F4-TCNQ were observed in the region from 1.2 to 0.45 eV below the Fermi level on various surfaces.7,14,17,18 Therefore, the observed peak at 1.2 eV is attributed to the occupied LUMO of F4-TCNQ on Cu(100). In both cases, the charge transfer occurs from the Cu(100) surface to LUMO of F4-TCNQ (back-donation). The UPS spectrum at 3.1 ML at 90 K is similar to that of a pristine F4-TCNQ film.4 The UPS spectra at high coverages at 300 K are different from those. These results indicate that the interface morphology, orientation, or both of the multilayer changes because of the mobility difference as a function of deposition temperature. To investigate vibrational states of F4-TCNQ on Cu(100), HREELS measurements were carried out. Figure 3a,b shows HREELS spectra of F4-TCNQ on Cu(100) as a function of coverage. In the case of 90 K deposition (Figure 3a), peaks were observed at 205, 276, 506, 876, 960, 1230, 1413, 1635, 2124, and 2200 cm-1 in the low coverage region. With increasing coverage, new peaks appear at 1460 and 2044 cm-1. In the deposition at 300 K (Figure 3b), peaks were

Figure 2. UPS spectra of F4-TCNQ on Cu(100) as a function of coverage. (a) 90 K deposition and (b) 300 K deposition. 1 ML = 1.1 1014 molecule/cm2.

the workfunction change saturates at ∼0.9 eV. In the case of 90 K deposition, it saturates at ∼1.5 eV. Therefore, the surface dipole induced by adsorbed F4-TCNQ species is larger in the 90 K deposition than that in the 300 K deposition. To investigate valence electronic states of adsorbed F4-TCNQ, UPS measurements were carried out. Figure 2a,b shows UPS spectra of F4-TCNQ on Cu(100) at 90 and 300 K as a function of coverage, respectively. On the clean Cu(100) surface, the peaks were observed at 2.4, 2.8, and 3.3 eV in binding energy (BE); the peak at 2.4 eV is derived from the s-p band, and the peaks at 2.8 and 3.3 eV are derived from


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Table 1. Assignments of Observed HREELS peaks (inverse centimeters: cm-1) in Figure 3 Based on the Vibrational Energies of Neutral F4-TCNQ and its Charge Transfer Complex (CTC)19 vibration mode ν(CtN)

symmetry

F4-TCNQ/Cu(100)

F4-TCNQ

Rb-F4-TCNQ

modes except for b2u b2u

2200 2124 (90 K) 2044 (300 K) 1635

2228, 2227, 2219 2214

2221, 2210, 2194 2190

ν(CdC), inside of the ring

ag

1665

1626

ν(CdC), outside of the ring

ag

1460a 1413

1456

1404

ν(C-F)

ag

1230

1273

1266

ν(C-F) ν(C-C), C-CtN bonds

b2u ag

960 876

976 878

977 877

δ(C-CtN), out of plane

b3u

506

564

552

276 (90 K) 205 (90 K) 180 (300 K) 120 (300 K)

hindered translation/hindered rotation

a

Peak attributed to F4-TCNQ in multilayer.

Table 2. Frequency of Four Different Symmetric ν(CtN) Modes in TCNQ, F4-TCNQ, and these CTCs19,22-28

observed at 120, 180, 370, 506, 876, 960, 1230, 1413, 1635, 2044, and 2200 cm-1 in the low coverage region. With increasing coverage above 1 ML, a new peak appears at 1460 cm-1. The assignments of observed peaks in Figure 3 are summarized in Table 1, based on the vibrational energies of neutral F4-TCNQ and a charge transfer complex (CTC).19 Because the ν(CdC) (outside of the ring) of neutral F4-TCNQ was observed at 1456 cm-1,19 the observed peak at 1460 cm-1 at higher coverages is attributed to ν(CdC) of neutral F4-TCNQ in the multilayer. In the submonolayer region in Figure 3a,b, the ν(CdC) was observed at 1413 cm-1. This red shift indicates that the charge transfer from the surface occurs (back-donation). It is well known that the frequency of ν(CdC) is a good diagnostic parameter for the degree of charge transfer (γ).19-21 The empirical relation between the frequency of ν(CdC) and γ about TCNQ complexes was reported;20,21 the 1413 cm-1 peak observed under both deposition conditions corresponds to γ = 0.7. Note that the integer charge transfer model has been proposed for adsorbed F4-TCNQ species on various surfaces.8 The frequency of the b2u mode of ν(CtN) in 300 K deposition (2044 cm-1) is significantly lower than that in 90 K deposition (2124 cm-1) and those of previously reported CTCs. This indicates that the CtN bonds in 300 K deposition become weaker than those in 90 K deposition. In addition, the low-energy hindered modes are significantly different. Therefore, an additional weakening mechanism of CN bonds should operate, which will be discussed later. F4-TCNQ has four modes for ν(CtN), and these are affected by not only charge transfer but also molecular distortion and local interaction.19 In neutral F4-TCNQ and TCNQ, these four modes appear at similar vibrational energies above 2200 cm-1. Charge transfer and molecular distortion induce the red shift and the splitting of four ν(CtN) modes in CTCs.19,22-28 The observed energies of ν(CtN) modes in CTCs are summarized in Table 2. The vibrational energy of b2u mode is observed lower than those of other symmetric modes in CTCs.


b1u TCNQ, F4-TCNQ, and these CTCs TCNQ

γa 0

F4-TCNQ

ag

b3g

b2u

cm-1 2225

2229

2223

2225

2228

2227

2219

2214

TTF-TCNQ

0.59

2204

2209

Na-TCNQ

1

2206

2214

2214

2175

2167 2210

2168

K-TCNQ

1

2198

2210

M2P-TCNQ

0.5

2205

2200

2182

M2P-F4-TCNQ

1

2188

2200

2169

F4-TCNQ/Cu(100)@90 K F4-TCNQ/Cu(100)@300 K

0.7

2200

a

2124 2044

Degree of charge transfer estimated from ν(CdC).19,22-28

In the deposition at 90 K (Figure 3a), two peaks are observed at 2124 and 2200 cm-1 at 0.6 ML. Because other peaks do not split in the submonolayer region, we interpret that these two peaks are due to the split of four ν(CtN) modes of low-temperate molecular species. Two ν(CtN) peaks are observed at 2044 and 2200 cm-1 in the deposition at 300 K (Figure 3b). Because other peaks do not split in the submonolayer region, we interpret that these two peaks are due to the split of four ν(CtN) modes of high-temperature molecular species. The frequency of b2u mode of ν(CtN) in the 300 K deposition (2044 cm-1) is significantly lower than that in the 90 K deposition (2124 cm-1) and those of previously reported CTCs. (See Table 2.) This indicates that the CtN bonds in the 300 K deposition become weaker than those in the 90 K deposition. In addition, the low-energy hindered modes are different. Because the ν(CtN) and the low-energy hindered modes are quite sensitive to the local environment of cyano groups and the substrate,19 the direct interaction between the cyano groups and the Cu substrate must be involved. Note that however other intramolecular modes such as ν(CdC), ν(C-F), and so on do not differ significantly.

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spectrometer (LK technologies, ELS5000), a low-energy electron diffraction (LEED) apparatus, and a quadrupole mass spectrometer (QMS). The other UHV system contained an electron spectrometer (SCIENTA, R3000), a LEED apparatus, and a QMS. The Cu(100) surface was cleaned by cycles of Ne ion sputtering and annealing to 700 K. We observed vibrational spectra in the HREELS chamber and XPS and UPS spectra in the photoelectron spectroscopy (PES) chamber, respectively. The orderliness of the Cu(100) surface was confirmed by LEED. We checked the cleanliness of the Cu(100) surface by XPS spectra in the PES chamber and vibrational spectra in the HREELS chamber. F4-TCNQ molecules were evaporated at 370 K from a homemade miniature effusion cell and deposited on the Cu(100) surface kept at 90 or 300 K. The details of the cell were described elsewhere.18,29 In the HREELS measurements, the incident electron energy was 5.0 eV, and the HREELS spectra were recorded in specular reflection at an incident angle of 60°. Intensity of 105 order counts was obtained in the reflected elastic beam at 4 meV resolution. The measurements of UPS using He I (21.22 eV)and XPS using Al KR (1486.6 eV) were carried out under a normal emission condition. The work function was measured from the cutoff of secondary electrons in UPS spectra. From XPS measurements, we estimated the coverage (molecules/cm2) of F4-TCNQ by comparing the C 1s peak intensity of F4-TCNQ on Cu(100) with that of the saturation coverage of CO on the clean Cu(100) surface (0.57 ML = 8.8 1014 molecule/cm2).30-32

In the previous study of F4-TCNQ on Cu(111), the local interaction between highest occupied molecular orbitals (HOMO) 9-12 and the surface causes the lowering of the occupation in these molecular states (donation).14 This donation from F4-TCNQ is accompanied by the strong molecular distortion from the quinonoid to the benzenoid conformation. On the basis of this report, the observed frequency difference of the b2u mode of ν(CtN) as a function of deposition temperature is interpreted as follows. In the case of deposition at 90 K, the charge transfer from Cu(100) (= back-donation) is predominantly responsible for the weakening of the CtN bonds, and thus the red shift of the b2u mode of ν(CtN) is observed at 2124 cm-1. In the 300 K deposition, both the donation from F4-TCNQ and the back-donation from Cu(100) cause the weakening of the CtN bonds and the large red shift of the b2u mode of ν(CtN) at 2044 cm-1. Therefore, the extra weakening of the CtN bonds at 300 K is caused by the rehybridization between the CN bonds and surface Cu atoms. The transformation from the charge-transfer state to the rehybridized state is thermally activated, as evidenced in Figure 3c. After 180 K heating, the 2124 cm-1 peak becomes weak in intensity, and the 2044 cm-1 peak develops dominantly. At the same time, the hindered modes are drastically changed. Note that other intramolecular modes do not change significantly. Although thermally induced morphology and orientation change of adsorbed species might be involved, the present vibrational results indicate that the local bidirectional interaction between F4-TCNQ and surface Cu atoms (= rehybridization) is thermally activated. This rehybridized state may include the bond orientation change in F4-TCNQ and the buckling of surface Cu atoms. These bidirectional interactions have been reported in the cases of F4-TCNQ on Cu(111)14 and TCNQ on Cu(100).15 On the whole, we can explain the workfunction change in Figure 1. In the case of 90 K deposition, the charge transfer from the surface to F4-TCNQ (back-donation) dominates, resulting in the workfunction change up to 1.5 eV. When the donation from F4-TCNQ becomes significant because of the local interaction between CN groups and Cu atoms by thermal activation, the net surface dipole becomes smaller, and the workfunction would be less increased (∼0.9 eV). In summary, we investigated the adsorption states of F4-TCNQ on Cu(100) as a function of deposition temperature. At 90 K, F4-TCNQ adsorbs on Cu(100) mainly via the charge transfer from Cu(100) (back-donation), and this state classified into “charge transfer state”. At 300 K, F4-TCNQ adsorbs on Cu(100) via both the back-donation from the surface and the donation from F4-TCNQ; this state is classified into “rehybridized state”. On the basis of vibrational EELS results, we conclude that the local interaction between the cyano groups and surface Cu atoms occurs at the rehybridized state. The net effect in the rehybridized state causes a weaker surface dipole than that of the charge-transfer state; it appears as the difference of the workfunction change.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: yoshinobu@ issp.u-tokyo.ac.jp.

ACKNOWLEDGMENT T.K. is supported by the program “The 21st Century Global-COE Program for Applied Physics on Strong Correlation” at the University of Tokyo. This research was supported by the Giant-in-Aid for Scientific Research on “Electron Transport through a linked molecule in nanoscale” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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Thermally Activated Transformation from a Charge-Transfer State to a

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