Charge Transfer and Molecular Orientation of Tetrafluoro

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Charge Transfer and Molecular Orientation of Tetrafluorotetracyanoquinodimethane on a Hydrogen-Terminated Si(111) Surface Prepared by a Wet Chemical Method Masayuki Furuhashi 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 chemical state and molecular orientation of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) adsorbed on a hydrogen-terminated Si(111) (1 1) surface using transmission infrared (IR) spectroscopy. We deposited F4-TCNQ molecules on H-Si(111) by a wet chemical method. Similar to evaporated F4-TCNQ molecules on various substrates in vacuum, we observed anionized F4-TCNQ on the H-Si(111) substrate. The incident angle dependence of the IR spectra reveals that this F4-TCNQ anion lies flatly on the surface. On the other hand, minority neutral F4-TCNQ species assume random orientation, judging from the comparison between s- and p-polarized IR spectra. We conclude that the first layer on the H-Si surface is a flat-lying anion species and the upper layers consist of randomly oriented neutral molecules. SECTION Surfaces, Interfaces, Catalysis

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rganic molecule-inorganic substrate interfaces have been widely investigated from the viewpoint of modification of surface electronic structures and novel hybrid device applications.1,2 The adsorption of the molecules often causes charge transfer, polarization, and chemical bond formation between the molecules and substrates.3 These electronic modifications are the key points in controlling device performance; comprehensive understanding of the surface modification has been sought because of the diversity of organic molecules. 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a strong acceptor with an extremely large electron affinity (5.24 eV),4 and is expected to be a useful material for organic devices. Recently it has been reported that deposited F4-TCNQ species can induce hole-doping into semiconducting and insulating substrate surfaces, such as diamond and graphene.5,6 The massive carrier doping into the surface on the nonmetal substrates causes the variation of not only their work functions, but also band bending.5,6 We recently investigated surface transfer doping with F4TCNQ on a 2-methylpropene-terminated Si(100) surface.7 Studies on silicon substrates would bring the benefit of connection with the current integrated circuit technology. Furthermore, investigation for a more simply terminated substrate is required for a fundamental understanding of surface transfer doping. The hydrogen-terminated Si(111) (1 1) surface is one of the simplest structures and is easily prepared with low defects by the wet chemical method.8-10 Usually we must pay attention to prevent the surface from contamination by air, because the surface condition sometimes leads to unexpected device performance.11 The H-Si(111) surface has the advantage that it is relatively stable to handle under atmospheric conditions.

r 2010 American Chemical Society

In addition, the adoption of wet chemical processes can help the manufacturing of industrial applications. In this study, we deposited F4-TCNQ on H-Si(111)(1 1) by a wet chemical method. The vibrational states were observed by incident angle dependent transmission infrared (IR) spectroscopy. By comparison with the spectra for neutral F4-TCNQ, we elucidated the chemical states of first-layer and multilayer F4-TCNQ on the H-Si(111) surface. In addition, we discuss the molecular orientations on the surface using the IR spectra as a function of incident angle. Figure 1a shows the transmission IR spectrum of the F4TCNQ adsorbed on H-Si(111) in CtN and CdC stretching regions, and Figure 1b gives the active vibrational modes of neutral bulk F4-TCNQ on an oxidized Si(111) substrate. We assigned the observed peaks in reference to the reports by Meneghetti et al.12 Four peaks at 2227, 2215, 1601, and 1550 cm-1 in the spectra of the neutral F4-TCNQ are assigned to b1uν18, b2uν32, b2uν33, and b1uν19 modes, respectively.12 The peaks at the same frequencies with the neutral species appear in Figure 1a, and additional peaks are observed at 2213, 2193, 1540, and 1501 cm-1. Therefore we consider that neutral and other species coexist in the F4-TCNQ/ H-Si system by the present wet chemical method. The CtN stretching modes at 2213 and 2193 cm-1 are in good agreement with the result of Rb(F4-TCNQ) powder.12 Hence these peaks can be assigned to b1uν18 and b2uν32 modes of the F4TCNQ anion, respectively. The peaks at 1540 and 1501 cm-1 in the CdC stretching region are b2uν33 and b1uν19 modes, Received Date: April 9, 2010 Accepted Date: May 4, 2010 Published on Web Date: May 06, 2010

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DOI: 10.1021/jz100463q |J. Phys. Chem. Lett. 2010, 1, 1655–1659

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Figure 1. Transmission IR spectra of F4-TCNQ (a) on an H-Si(111) substrate and (b) on an oxidized Si(111) substrate in CtN and CdC stretching regions with an incident angle of 15. The peak positions are indicated in the figures.

respectively.12 The frequencies of CdC stretching modes are good diagnostic parameters for the degree of charge transfer. (Note that the frequencies of CtN stretching modes are sensitive to local environment.)12 Since the present frequencies agree well with those of completely anionized F4-TCNQ,12 we conclude that the F4-TCNQ on the H-Si(111) surface accepts nearly one electron per molecule from the substrate. Figure 2a shows the incident angle dependence of transmission IR spectra in CtN and CdC stretching regions. The peaks assigned to neutral F4-TCNQ molecules (2227, 1601, and 1550 cm-1) increase in intensity as a function of the incident angle. On the other hand, the peaks assigned to the F4-TCNQ anion (2213, 2193, 1540, and 1501 cm-1) decrease with increasing the angle. The origin of the incident angle dependences may be due to the anisotropy of refractive indexes, because the anion species did not obey LambertBeer's law; absorbance should increase with increasing the effective film thickness, which is increased at a larger incident angle. However, in realty, the peaks of the anion species decrease in intensity as the angle becomes larger. Hence we must consider the direction of dynamic dipole moments, i.e., the molecular orientation on the surface. The orientation can be quantitatively estimated from the incident angle dependence of IR spectra.13-16 However, we qualitatively analyze the orientations, because the distribution of the adsorbed F4TCNQ in the present study could be microscopically inhomogeneous on the surface. In order to discuss the molecular orientation, we presuppose several simplifications. First, there is no optical rotation and multiple reflections (when more accurate analyses are required, these optical phenomena should not be neglected).14 Second, neutral F4-TCNQ and its anion have a planar structure when adsorbed on the H-Si surfaces. In this situation, the directions of dynamic dipole moments for b1u and b2u modes (in D2h symmetry) are perpendicular to each other and parallel to the molecular plane (See the calculated atomic motions in Figure 2b). Third, we presume that the F4-TCNQ


Figure 2. (a) Incident angle-dependent transmission IR spectra of F4-TCNQ on H-Si(111) in CtN and CdC stretching regions. (b) Theoretically calculated atomic motions in the ν18 and ν32 normal modes associated with CtN stretching modes. The x axis comes from the rear to the front of the paper. (c) Comparison of transmission IR spectra with s- and p-polarized lights. These spectra were measured at an incident angle of 44.

has a uniform orientation angle to the surface normal, but randomly orients in the surface plane. The peak intensity in the transmittance of IR spectra is proportional to the square of transition dipole moment Æφf|μ 3 E|φiæ2, where the product μ 3 E strongly depends on the relation between the molecular orientations and the incident angle of p-polarized light.14 On the other hand, the film thickness to be passed through by the IR light monotonically increases with the incident angle. The measurable absorbance is proportional to the product of these two effects.14,17 Qualitatively speaking, the increase of absorbance with incident angle means that the transition dipole moments (or the symmetries of normal vibrational modes) are perpendicular to the substrate surface, and the decrease indicates a parallel orientation.14 As mentioned above, the b1u and b2u modes of F4-TCNQ anion species show monotonic decrease with the incident angle, which is typical behavior of the parallel orientation in

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reference to the substrate surface. Here we consider the direction of the transition dipole moments for the b1u and b2u modes in the molecule. These modes are in-plane modes and parallel to the z and y axes, respectively (see Figure 2b). We can uniquely determine the molecular orientation in which both axes are parallel to the substrate surface. Because the z and y axes are parallel to the molecular plane, the F4TCNQ anion must lie down on the silicon surface flatly. If the anion stands on the surface, either b1u or b2u would show different incident angle dependence. The parallel (or flat) orientation agrees with the result of F4-TCNQ adsorption on a clean Au surface.18 By contrast, the neutral F4-TCNQ shows opposite dependence with the incident angle from that of the anion. From the above discussion, the z and y axes of the neutral species should tilt in reference to the surface plane. However, we consider that the neutral F4-TCNQ molecules do not have a uniform tilted orientation, because the variations of both b1u and b2u modes are similar. If the molecules have a unique tilted orientation, b1u and b2u should show different incident angle dependence in absorbance from each other. Thus, the neutral molecules in the present system are adsorbed with random orientation. In this case, the neutral F4-TCNQ is expected to have a macroscopically isotropic refractive index. If the refractive index is anisotropic, we can see a prominent difference between the spectra with s- and p-polarized lights at an oblique incident angle. Figure 2c shows the compared IR spectra with s- and p-polarized lights in the CdC stretching region at an incident angle of 44. The modes belonging to anion species (1540 and 1501 cm-1) show a prominent absorbance difference between the polarizations; however, the neutral species (1601 and 1550 cm-1) give similar intensities in spite of the polarizations. These results strongly indicate that the neutral F4-TCNQ has an isotropic refractive index. Therefore, we consider that the random adsorption model is reasonable for explaining the incident angle dependence. Note that the peaks for the F4-TCNQ anion slightly shift with the incident angle. The b1uν18 (2213 cm-1), b2uν32 (2193 cm-1), and b1uν19 (1501 cm-1) modes at an incidence of 15 prominently shift to higher frequencies by 2 cm-1 in the case of a grazing angle of 71. However, the origin of the peak shifts is not clear. From the previous studies, the first-layer F4-TCNQ becomes an anion, but the multilayer F4-TCNQ species are neutral on various surfaces.5-7,19,20 N 1s X-ray photoelectron spectra show that the initially adsorbed F4-TCNQ species become an anion and that the molecules in the multilayer are neutral.5,6,19 The valence photoelectron spectra of the F4TCNQ on the 2-methylpropene-terminated Si(100) surface show that anion species are observed at the initial stage of the deposition.7 A similar situation occurs in the present F4-TCNQ/ H-Si(111) system. We estimated the number of adsorbed F4TCNQ molecules as ca. 6.5 1014 molecules per one droplet from the solution density (3.6 10-5 M) and the amount of a droplet (0.03 mL). Since the number of surface Si atoms on the Si(111) surface is 3.9 1015 atoms per single side, the density of F4-TCNQ is 0.17 molecules per Si atom. On the other hand, the saturation coverage on the modified Si(100)


Figure 3. Schematic diagram of the geometry of the F4-TCNQs adsorbed on H-Si(111). The red and black backbones indicate anions and neutral molecules, respectively. The designation of N and F atoms is omitted for ease of viewing.

surface is about 0.25 F4-TCNQs per Si atom.7 If only the nearest Si atoms to the F4-TCNQ first layer participate in the charge transfer, the saturation coverage for the present Si(111) systems may have a similar value to that for the modified Si(100) surface. The estimated F4-TCNQ/Si ratio of 0.17 is close to 0.25; therefore the present system prepared by the wet chemical method corresponds to the initial stage of deposition, and the multilayer would be partly formed. Figure 3 is a proposed adsorption model of F4-TCNQ on H-Si(111); the first layer on the H-Si(111) consists of flat-lying F4-TCNQ anion species, which obtain nearly one electron per molecule from the substrate. The upper layer consists of randomly oriented neutral molecules. Here we consider why the multilayer does not strongly interact with the first layer. Since the electron affinity of neutral F4-TCNQ (5.24 eV) is larger than the work function of n-type H-Si(111) (4.42 eV),4,21 charge transfer spontaneously occurs between the first layer and the H-Si surface. In the case of a metal substrate, the Fermi level of the F4-TCNQ deposited surface is pinned at 5.55 eV.20 If the integer chargetransfer model is applied to the H-Si(111) substrate,22 the F4TCNQ-deposited H-Si(111) surface would have a Fermi level of around 5.55 eV. Therefore, hole-injection from neutral F4TCNQ in the multilayer to the anion species in the first layer would not occur spontaneously. In summary, we prepared an F4-TCNQ layer on H-Si(111) (1 1) using the wet chemical method, and observed their vibrational states by transmission IR spectroscopy. The observed frequency shifts of CtN and CdC stretching modes clearly indicate the existence of F4-TCNQ anion species. The variation of band intensities as a function of the incidence angle elucidates that the F4-TCNQ anion species have flat orientation to the surface. On the other hand, the neutral molecules assume random orientation based on the incident angle dependence and the comparison between s- and p-polarized IR spectra.

EXPERIMENTAL AND COMPUTATIONAL METHODS Pieces of double-sided polished n-type Si(111) wafer (25 mm 10 mm) were cleaned by a hot sulfuric acid/hydrogen peroxide mixture (SPM) and 5% hydrofluoric acid (HF) aqueous solution. The cleaned wafers were etched with NH4 aqueous solution, and then we obtained H-Si(111)(1 1) surfaces. We used a wet process for the adsorption of F4-TCNQ

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molecules on the H-Si(111) surface as follows: F4-TCNQ and acetonitrile were purchased from Tokyo Kasei Industry and Wako Pure Chemicals, respectively. After 0.1 mg of F4-TCNQ was dissolved into 10 mL of acetonitrile in a Teflon container, the solution was preserved in a refrigerator until just before use. One droplet of the solution (ca. 0.03 mL) was dripped on each side of the freshly prepared H-Si surfaces as soon as possible. After spontaneously drying the solvent, we entered the substrates into the vacuum chamber of our FT-IR spectrometer. A bulky F4-TCNQ layer, which mostly contains neutral species, was obtained by the 10-fold repetition of the dripping and drying processes on a single side of the oxidized Si(111) substrate that was prepared by dipping a H-Si(111) substrate into the SPM solution; here the SiO2 layer prevents charge transfer from the Si substrate to adsorbed F4-TCNQ. Transmission IR spectra were obtained using an FT/IR6100 (JASCO Co., Japan), which was evacuated by a scroll pump and operated at room temperature. IR light was p-polarized with a KRS-5 wire grid polarizer and detected with a mercury-cadmium-telluride (MCT) detector cooled by liquid nitrogen. The incident angle was controlled by a home-built angle adjustable sample holder. The spectra were obtained by an average of 50 repeated alternate measurements of the F4-TCNQ-deposited wafer and a reference (fresh H-Si), where each measurement consisted of an average of 20 scans with a resolution of 4 cm-1, using a commercial shuttle device (SSH-4000 by JASCO Co.). We also calculated the normal vibrational modes of an isolated neutral F4-TCNQ based on the density functional theory method, where the 6-31G split valence basis set and B3LYP exchange-correlation functional were used.23,24 The results of the calculation were used for the estimation of the atomic motion in each vibrational mode; thus we derived the direction of dynamic dipole moments. The molecular geometry was optimized by the same calculation level maintaining the symmetry D2h. All calculations were performed on a personal computer by the GAMESS program package.25

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected].

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ACKNOWLEDGMENT This research was supported by a Grant-inAid for Scientific Research on Priority Areas “Electron transport through a linked molecule in nano-scale” from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Charge Transfer and Molecular Orientation of Tetrafluoro

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