Modulation of the Structure and Electronic Density of Molecular

Jan 3, 2008 - Each structure corresponds to a respective electron density distribution along the chain direction. The latter two reflect the different...
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J. Phys. Chem. C 2008, 112, 1090-1093

Modulation of the Structure and Electronic Density of Molecular Chains on Organic Conductor Surfaces Feng Lin, Wei Zhou, Xiaoming Huang, Liang Ren, and Zhongfan Liu* Center for Nanoscale Science and Technology (CNST), Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: August 29, 2007; In Final Form: October 31, 2007

We used scanning tunneling microscopy (STM) to observe the bc-surface of the triethylammonium-7,7,8,8tetracyanoquinodimethane (TEA(TCNQ)2)charge-transfer complex crystal, which is terminated with the arrays of TCNQ chains. Three kinds of distinct structures of the quasi-one-dimensional (1D) TCNQ chains were found on different surface terraces, that is, the TCNQ molecules within a chain exhibit the monomerization, tetramerization, and octamerization, respectively. Each structure corresponds to a respective electron density distribution along the chain direction. The latter two reflect the different characteristics of the bulk and surface energy-band filling, respectively. The monomer structure indicates that the single molecular orbital was probed by STM due to the weakening of the overlapping between TCNQ molecules induced by the enlarging of the intermolecular distance. The two-dimensionality of the surface exerts an apparent influence on the quasi-1D TCNQ chains, with the specific manifestation of enhancing the interchain coupling even at the cost of the weakening of the intrachain interaction.

Introduction

Experimental Section

Organic conductors and superconductors based on chargetransfer (CT) complexes, such as TCNQ (7,7,8,8-tetracyanoquinodimethane)- and BEDT-TTF [(ethylenedithio)tetrathiafulvalene]-based species, have received considerable attention because of their intriguing electronic, optical, and magnetic properties.1-6 These novel properties, in essence, arise predominately from quasi one-dimensional (1D) molecular chain structures in solids formed by stacking π- acceptor or -donor molecules (like TCNQ and BEDT-TTF). So far, a great deal of works has been done for CT complexes on structural phase transitions as well as electrical and magnetic measurements in bulk materials, with main concerns in these properties related to the alteration of the molecular chains with temperatures or other stimuli.7-12 Since the molecular chains are embedded into the three-dimensional crystal materials and are the constituents of the CT salts, being subject to the severe constraint of crystal space group, the freedom of molecular chains is limited. On crystal surfaces, however, due to the symmetry broken, the structure of the molecular chains achieves larger freedom, which may be the cause of richer physical properties.

A single crystal of TEA(TCNQ)2 was synthesized according to the literature,13 usually exhibiting a needle shape with the size of several centimeters long, millimeters wide, and thick. The crystal was fixed onto a metallic plate with conductive silver epoxy, and then it was loaded into an ultrahigh vacuum chamber with a base pressure of about 1 × 10-10 mbar for STM measurements. All STM images were taken at room temperature (297 K), in constant-current mode using a tunneling current of 0.3 - 0.5 nA and a bias voltage of 40-80 mV (with respect to the sample). The STM tips used were mechanically cut from a wire of Pt(80%)Ir(20%) alloys. Lateral dimensions observed in the STM images were calibrated with a graphite standard.

In this paper, we present the bc-surface structure of TEA(TCNQ)2 (triethylammonium-7,7,8,8-tetracyanoquinodimethane) crystal as observed by a scanning tunneling microscope (STM). This surface is terminated with arrays of TCNQ molecular chains. Apart from the bulk tetramerization structure of the TCNQ chains, two new structures, the TCNQ monomerized and octamerized within a chain, were observed on the different surface terraces, and the corresponding electronic distribution along the chain was discussed. * Author to whom correspondence should be addressed. Tel. & Fax: 00-86-10-6275-7157. E-mail: [email protected].

Results and Discussion The crystal structure of TEA(TCNQ)2 determined by X-ray diffraction is triclinic, with the unit-cell parameters: a ) 14.328 Å; b ) 7.886 Å; c ) 13.22 Å; R ) 69.59°; β ) 82.62°; γ ) 72.75° at room temperature and the TCNQ chains along the c-axis.14 The details of the crystal structure are seen in the Supporting Information. Figure 1a shows that the bulk-truncated bc-surface is terminated with arrays of TCNQ chains along the c-axis, and the individual TCNQ molecule is in the stand-upright form along the molecular long axis of TCNQ, with two nitrogen atoms at the outermost surface. There are four TCNQ molecules along the c-axis in a unit cell, exhibiting a tetramer structure with the step-by-step increase in height for each molecule. There are two distinct face-to-face stacking modes between adjacent molecules in the TCNQ chains, the external-to-ring (two molecules slipped by about 2.1 Å along the long axis) and shifted external-to-ring motifs,14 as shown in Figure 1, parts b and c, respectively. The surface molecular corrugation originates from such stacking modes. The lowest unoccupied molecular orbital (LUMO) of a single TCNQ, calculated by density

10.1021/jp076925j CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008

Molecular Chains on Organic Conductor Surfaces

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Figure 1. The structures of the bulk-truncated bc-surface for TEA(TCNQ)2. (a) A space-filling model of the bulk-truncated bc-surface of TEA(TCNQ)2 terminated in the arrays of TCNQ chains along the c-axis. The unit b- and c-axes are labeled in the yellow arrows, which show the molecular topographical corrugation in the monotonous increase along the c-axis in a unit cell. The spacing between TCNQ molecules are indicated by four numbers whose unit is angstrom. (b) and (c) Two stacking modes in TCNQ chains, called “external-to-ring” and “shifted external-to-ring” motifs, respectively. Notably, the molecular corrugation arises from the two motifs due to the individual molecules standing upright on the surface. (d) The LUMO structure of a single TCNQ molecule calculated by density functional theory.

functional theory, is shown in Figure 1d, where it is noted that the electron density on the position of nitrogen atoms occupies the considerable weight. For a single crystal of TEA(TCNQ)2, the bc-surface, the largest among all surfaces due to the strong π-π overlapping interactions within TCNQ chains resulting in the lowest surface free energy,15-17 is readily identified. On this surface, molecularly flat terraces, appearing as rich shapes like long/short bands, squares, rectangles, mounds, grooves, and pits, scatter largely in size from tens to hundreds of nanometer, separated by holes or steps (Figure 2a). Figure 2b is a closeup that clearly exhibits surface steps on the bc-surface, and Figure 2c is the height profile measured along the double-headed arrow. As marked in the green labels, most of the molecularly flat terraces are terminated with TCNQ molecules, which will be verified in the latter text. The height difference of relatively larger neighboring terraces is about 1.45 nm (Figure 2c), which is consistent with the unit length of the a-axis, 1.43 nm. Occasionally, a small terrace patch (marked in TEA in Figure 2b) extends from the large terrace at the step edge, with step heights of 0.91 and 0.54 nm relative to the bottom and upper terraces, respectively, as shown in Figure 2c. On the basis of the unit cell of the TEA(TCNQ)2 crystal (see Supporting Information), which consists of alternately stacked TCNQ anion layers and TEA cation layers along the a-axis, it is reasonable that this small patch is identified as the TEA cation layer. Three distinct surface structures were found out on these terraces by high-resolution STM, which might result from the different balance states of all kinds of surface energy, such as surface free energy, terrace boundary free energy, and defect energy, if taking different shapes and sizes of the terraces into account.18 Figure 3a shows a high-resolution STM image obtained almost everywhere on the surface, with the unit cell (marked in green solid lines): b ) 8.7 Å, c ) 13.6 Å, γ ) 73.7°, which is consistent with the crystal structure aforementioned, 7.886 × 13.22 Å2, γ ) 72.75°, obtained by X-ray diffraction. This means that the surface herein is a bulk-truncated bc-surface; the (TCNQ)4 tetramer is, however, unresolved one-by-one, only

Figure 2. (a) A large-scale STM image of the bc-surface for TEA(TCNQ)2. On this surface, molecularly flat terraces appear as rich shape. Scalar bar ) 200 nm. (b) A closeup STM image showing the surface steps. Scalar bar ) 52 nm. (c) Height profile along the double-headed arrow in (b).

Figure 3. Three distinct structures observed on the bc-surface. (a), (b), and (c) High-resolution STM images show the different structures of TCNQ chains on the different terraces of the bc-surface, exhibiting the tetramerization, monomerization, and octamerization in a unit cell, respectively. The scanning area is all 4.5 × 12.5 nm2. The unit cell is all marked in solid green lines in the images. The b- and c-axes are labeled at the bottom right of (a), which is also applicable to (b) and (c). The upper part of (b) shows the estimating positions, represented by solid green dots, of the unobserved nitrogen atoms. The bottom left of (b) indicates the distance 0.4 nm of two brighter spots in a pair.

with a relatively larger protrusion whose brightness gradually decays from one end to the other along the c-axis within a unit cell.19 For a bulk TEA-(TCNQ)2, the molecules are separated

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Lin et al.

Figure 4. The possible interactions between TCNQ molecules exclusively on the bc-surface. (a) The possible molecular arrangement of the monomer structure. Rotating the front TCNQ by some angles leads to the distance of the hydrogen atom in the back molecule and the nitrogen atom in the front, connected by a red dashed line, becoming shorter, about 2.3 Å, forming the CH‚‚‚N hydrogen bond. (b) The possible stacking mode for the monomer structure. (c) The interface configuration between the corner and center octamers in a unit cell.

by 3.24, 3.32, 3.24, and 3.30 Å spacing14 (Figure 1a) within a tetramer in the TCNQ chains, which leads to the strong π-electron overlapping among molecules in consideration of these short interplanar distances. With 0.5 electrons transferred to one TCNQ LUMO,14 such strong π-π overlapping interactions can form an extended linear 1/4-filled LUMO band, resulting in the electron not being localized in a single molecule. On the basis of the above analysis, the best interpretation of the feature in the observed STM image is that, besides the considerable height corrugation for each molecule, the energy levels near the Fermi level (kF) induced by the extended linear LUMO band, rather than a single molecular orbital, contribute greatly to the large protrusions in Figure 3a. Therefore, the individual TCNQ molecule within a tetramer is incapable of being discerned by STM. It is pointed out that the periodic structure in the tetrameric TCNQ form along a chain on the bc-surface is associated with the bulk state reflected on the surface since the TCNQ chains in bulk are tetramerization. In this case, the real “surface” structure has not been manifested yet. On some terraces, another surface structure was achieved in Figure 3b, with almost the same appearing frequency as the tetramer structure. Along the c-axis, a pair of brighter spots spaced by 4.0 Å (indicated by the arrows at the bottom left) and deviated from the c-axis about 44.7°, and a darker point accompanying a hole along the b-axis, appear alternately. The unit cell drawn in solid lines comprises two TCNQ molecules with the dimensions of b ) 9.0 Å, c ) 10.4 Å, and γ ) 74.5°. The distance of two brighter spots in a pair, 4.0 Å, is very close to the N-N distance (4.2 Å) on one end of a single TCNQ, which implies that the brighter spot arises from a single TCNQ LUMO on the position of a nitrogen atom (Figure 1d) at the outmost surface. The darker spot represents a nitrogen position in another TCNQ that is lower than the molecule at the position of the brighter pair from the surface. Since two nitrogen atoms are at one end in a TCNQ, the only one darker spot observed within a unit cell is indicative of another one unobserved by STM, which is interpreted in the latter part. At the upper part of Figure 3b, the green solid dots represent the unobserved nitrogen atoms whose positions are estimated approximately by the conformation of the TCNQ molecule. The spacing of two TCNQ molecules within a unit cell is c/2 ) 5.2 Å, which leads to a much weak π-electron overlapping between TCNQ molecules. Hence, a single TCNQ molecular orbital rather than an extended linear energy band is probed by STM, which accounts for the submolecular resolution of the image. The loose coupling between TCNQ molecules indicates the individual TCNQ along the chains exhibiting a “monomer” state. Quite rarely, but not never, we can acquire the third kind of surface structures (Figure 3c) on very small terraces (several tens of nanometers in size usually), for example, which prefers

to appear on the right bottom of Figure 2a where the density of small terraces is relatively higher. This structure is made up of long strips about 28.2 Å long along the c-axis, which do not display any fine structures but the different brightness along the strip, forming arrays of bands side by side. Similar to the tetramer structure, the strip structure reflects the surface electronic structure in the form of an extended linear TCNQ LUMO band. The strip, whose length is approximately two times that of the tetramer, should contain eight TCNQ molecules, and it is named an octamer here. Band filling of the charge-transfer molecular crystal depends on the amount of molecular charge from cations to anions. In the case of TEA(TCNQ)2, if the charge transfer is completed, the TCNQ molecules which organize the conductive band retain an average negative charge of 0.5 electrons per molecule. For the surface layer, however, the molecular charge is reduced to 0.25 electrons per molecule due to the missing TEA cation layer on one side. Therefore, the TCNQ LUMO band filling in the surface layer changes from 1/4 to 1/8. The tight-binding approximated band calculation17 demonstrated that the Fermi surface (FS) nesting vector becomes smaller on surfaces than in bulk when taking into account this change in molecular charge. The formation of CDW depends strongly on the degree of FS nesting, the smaller FS nesting vector, and the larger wavelength of 2kF CDW. In a more explicit view, since 0.5 electron is transferred to a TCNQ in the bulk, leading to the tetramerization, the octamerization originates from that the charge per TCNQ on the surface being reduced to 0.25. Hence, the octamerization structure is a real surface CDW that reflects the characteristic of surface energy filling. Along the c-axis, the quasi-1D TCNQ chains are not continuous, and the neighboring strips are shifted by about half a unit along the b-axis, resulting in a two-dimensional superstructure, with a unit cell drawn in green solid lines in Figure 3c, which consists of two strips, one is in the corner, and the other in the center, with the unit dimensions b ) 7.5 Å, c ) 56.4 Å, and γ ) 75.6°. Here, the distance of interchains is 7.5 Å, apparently smaller than the equilibrium distance 7.886 Å in the bulk, which leads to the interchain interaction being significantly enhanced. Hence, from the energy point of view, if the chains are broken, the structure should become relatively more stable. On the basis of the STM data above, the possible arrangement of TCNQ molecules within a unit cell for the “monomer” structure is depicted in Figure 4a, where the front molecules, that are lower than the back ones (the requirement of the external-to-ring motif), rotate some angles around the molecular long axis, which results in one nitrogen atom in TCNQ locating more deeply under the surface. Such a configuration can account for the only one darker spot observed by STM in Figure 3b. Meanwhile, the distance between the hydrogen atom in the back molecule and the deeper nitrogen atom in the front molecule,

Molecular Chains on Organic Conductor Surfaces

J. Phys. Chem. C, Vol. 112, No. 4, 2008 1093 terraces with different sizes. These terraces are mainly terminated with TCNQ molecules in a chain form. Three kinds of distinct structures, monomerization, tetramerization, and octamerization, of the TCNQ chains were found on different surface terraces, with the different distribution of electron density. For CT complexes possessing linear molecular chains, the characteristic 1D molecular stacking arrangement plays a crucial role in their electronic properties, such as spin/Peierls transitions, and Mott-Hubbard transitions, superconductivity. On a surface arena, as exemplified in this paper, occur abundant sorts of 1D structures that are anticipated to have novel properties.

Figure 5. The I-V relationship for the three surface structures. The black, blue, and red curves show the I-V relationship of the tetramer, octamer, and monomer structure, respectively.

connected by a red dashed line in Figure 4a, is about 2.3 Å, which falls into the interacting range of a weak hydrogen bond of CH‚‚‚N.20 The formation of the hydrogen bond, which does not exist in the bulk, enhances the interchain interaction on the bc-surface. As a consequence, the π-π overlapping interaction in the TCNQ chains becomes much weaker, as shown in Figure 4b, in which shows a distorted external-to-ring motif is shown. Figure 4c shows the possible interface configuration between the corner and center octamers proposed from Figure 3c. Such one-to-two external-to-ring motif allows the π-π overlapping interaction to enlarge from intrachains to interchains, despite not being very strong. These two additional structures of TCNQ chains, monomer and octamer, reflect that the structures on the surface have a tendency to enhance the interchain coupling in comparison to that in the bulk, even at the expense of the weakening of the intrachain interaction. In order to study the electronic property of the three structures on the bc-surface of TEA(TCNQ)2, I-V (tunneling currentbias voltage) curves were carried out. Figure 5 shows a series of I-V curves obtained under the same initial conditions of tip bias voltage Vb ) 70 mV and set current Is ) 0.4 nA. Obviously, the conductive property of the tetramer and octamer structures is almost similar, while significantly better than the monomer structure. The former two show a little metallic behavior, and the latter one is more inclined to a semiconductor I-V relationship. Being lesser conductive of the monomer structure is reasonably assigned to the weaker overlapping between TCNQ molecules in chains due to the larger molecular spacing. In summary, we have used STM to observe the bc-surface of the CT complex TEA(TCNQ)2 crystal at room temperature. It was found that the surface is molecularly flat and full of

Acknowledgment. This work was supported by NSFC (90301006,50521201)andMOST(2007CB936203,2006CB932403, 2006CB932602). Supporting Information Available: A description of the unit cell (Figure S1) and the ac-surface (Figure S2) for the TEA(TCNQ)2 crystal. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shibaeva, R. P.; Yagubskii, E. B. Chem. ReV. 2004, 104, 5347. (2) Chollet, M.; Guerin, L.; Uchida, N.; Fukaya, S.; Shimoda, H.; Ishikawa, T.; Matsuda, K.; Hasegawa, T.; Ota, A.; Yamochi, H.; Saito, G.; Tazaki, R.; Adachi, S.; Koshihara, S. Science 2005, 307, 86. (3) Horiuchi, S.; Okimoto, Y.; Kumai, R.; Tokura, Y. Science 2003, 299, 229. (4) Coronado, E.; Day, P. Chem. ReV. 2004, 104, 5419. (5) Xiao, K.; Ivanov, I. N.; Puretzky, A. A.; Liu, Z.; Geohegan, D. B. AdV. Mater. 2006, 18, 2184. (6) Peng, H. L.; Ran, C. B.; Yu, X. C.; Zhang, R.; Liu, Z. F. AdV. Mater. 2005, 17, 459. (7) Ota, A.; Yamochi, H.; Saito, G. J. Mater. Chem. 2002, 12, 2600. (8) Parkin, S. S. P.; Miljak, M.; Cooper, J. R. Phys. ReV. B 1986, 34, 1485. (9) Sakano, T. I.; Kawamoto, T.; Shimoi, Y.; Abe, S. Phys. ReV. B 2004, 70, 085111. (10) Clay, R. T.; Mazumdar, S.; Campbell, D. K. Phys. ReV. B 2003, 67, 115121. (11) Mazumdar, S.; Clay, R. T.; Campbell, D. K. Phys. ReV. B 2000, 62, 13400. (12) Monceau, P.; Ya, F.; Brazovskii, S. Phys. ReV. Lett. 2001, 86, 4080. (13) Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson, R. E.; Mochel, W. E. J. Am. Chem. Soc. 1962, 84, 3374. (14) Filhol, A.; Thomas, M. Acta Crystallogr., Sect. B 1984, 40, 44. (15) Li, Sh. L.; White, H. S.; Ward, M. D. Chem. Mater. 1992, 4, 1082. (16) Dvorak, M. A.; Li, Sh. L.; Ward, M. D. Mater. 1994, 6, 1386. (17) Ishida, M.; Mori, T.; Shigekawa, H. Phys. ReV. Lett. 1999, 83, 596. (18) Zhang, Z. Y.; Lagally, M. G. Science 1997, 276, 377. (19) Magonov, S. N.; Schuchhardt, J.; Kempf, S.; Keller, E.; Cantow, H. J. Synth. Met. 1991, 40, 59. (20) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997.