Vibrational and Optical Studies of Organic Conductor Nanoparticles

To create free electrons in organic solids and thus generate an organic material exhibiting electrical conductivity, a simple way is to build an organic complex, in which there is a charge transfer from the atoms or molecules of an electron donor (D) to those of an electron acceptor (A). In 1973, the charge transfer salt TTF·TCNQ (donor: tetrathiafulvalene, TTF; acceptor: tetracyanoquinodimethane, TCNQ) was synthesized (figure 1) (Ferraris et al., 1973). In TTF·TCNQ single crystals, TTF and TCNQ form segregated columnar stacks along the b-axis of the crystal structure. The interplanar spacings in the TTF and TCNQ molecular stacks at room temperature are 3.47 A and 3.17 A, respectively (Kistenmacher et al., 1974). The TTF and TCNQ molecular planes tilt at an angle of 24.5 ° and 34.0 °, respectively with respect to the b-axis forming a herringbone arrangement. As single crystals, this compound behaves like a metal (the dc conductivity increases with decreasing temperature) down to 54 K, temperature at which it undergoes a metal-to-semiconductor transition. The maximum of conductivity in TTF·TCNQ is along the b-axis (about 600 1 cm1). Conductivity values range from 10−2 to 1 1 cm1 in a direction perpendicular to the direction of maximum conductivity. The amount of charge transfer from the TTF donor molecule to the TCNQ acceptor molecule has been investigated by various techniques. Using X-ray photoelectron spectroscopy, a value of 0.56  0.05 has been extracted from the shape of the S2p signal (Ikemoto et al., 1977). Values in the range 0.500.60 have been obtained by a numerical integration of X-ray diffraction amplitudes (Coppens, 1975). However, the most convenient technique is based on infrared spectroscopy. Using the linear correlation of the nitrile stretching mode for TCNQ as a function of the degree of charge transfer, a value of 0.59  0.01 has been obtained (Chappell et al., 1981). Although synthesized for many years, TTF·TCNQ currently attracts much interest because of its interesting physical properties. Moreover, it is the one-dimensional conductor which is the most intensively processed in forms others than single crystals. For instance, TTF·TCNQ was prepared as thin films on (100)-oriented alkali halide substrates (Fraxedas et al., 2002), as self-organized monolayers on Au(111) (Yan et al., 2009), or as nanowires on stainless steel conversion coatings (Savy et al., 2007). We report in this chapter the preparation and spectral studies of TTF·TCNQ prepared as nanoparticles.


Introduction
To create free electrons in organic solids and thus generate an organic material exhibiting electrical conductivity, a simple way is to build an organic complex, in which there is a charge transfer from the atoms or molecules of an electron donor (D) to those of an electron acceptor (A).In 1973, the charge transfer salt TTF•TCNQ (donor: tetrathiafulvalene, TTF; acceptor: tetracyanoquinodimethane, TCNQ) was synthesized (figure 1) (Ferraris et al., 1973).In TTF•TCNQ single crystals, TTF and TCNQ form segregated columnar stacks along the b-axis of the crystal structure.The interplanar spacings in the TTF and TCNQ molecular stacks at room temperature are 3.47 Å and 3.17 Å, respectively (Kistenmacher et al., 1974).The TTF and TCNQ molecular planes tilt at an angle of 24.5 ° and 34.0 °, respectively with respect to the b-axis forming a herringbone arrangement.As single crystals, this compound behaves like a metal (the dc conductivity increases with decreasing temperature) down to 54 K, temperature at which it undergoes a metal-to-semiconductor transition.The maximum of conductivity in TTF•TCNQ is along the b-axis (about 600   1 cm  1 ).Conductivity values range from 10 −2 to 1   1 cm  1 in a direction perpendicular to the direction of maximum conductivity.The amount of charge transfer from the TTF donor molecule to the TCNQ acceptor molecule has been investigated by various techniques.Using X-ray photoelectron spectroscopy, a value of 0.56  0.05 has been extracted from the shape of the S2p signal (Ikemoto et al., 1977).Values in the range 0.500.60 have been obtained by a numerical integration of X-ray diffraction amplitudes (Coppens, 1975).However, the most convenient technique is based on infrared spectroscopy.Using the linear correlation of the nitrile stretching mode for TCNQ as a function of the degree of charge transfer, a value of 0.59  0.01 has been obtained (Chappell et al., 1981).Although synthesized for many years, TTF•TCNQ currently attracts much interest because of its interesting physical properties.Moreover, it is the one-dimensional conductor which is the most intensively processed in forms others than single crystals.For instance, TTF•TCNQ was prepared as thin films on (100)-oriented alkali halide substrates (Fraxedas et al., 2002), as self-organized monolayers on Au(111) (Yan et al., 2009), or as nanowires on stainless steel conversion coatings (Savy et al., 2007).We report in this chapter the preparation and spectral studies of TTF•TCNQ prepared as nanoparticles.

Nanoparticle synthesis
The TTF•TCNQ charge transfer salt is prepared by slow diffusion of an organic solution of TTF into an organic solution of TCNQ.Due to its quasi-one-dimensional character, TTF•TCNQ is commonly grown as long needles or wires.To control the growth of this material as nanospheres or nanoplatelets, a stabilizing agent is added.The stabilizing agent forms a very thin protective layer around each particle thereby preventing their aggregation.
We have used ionic liquids (IL), ionic liquid/oleic acid (OA) mixtures or various other as protecting agents or stabilizing media.
Pure ionic liquids such as imidazolium salts are known to stabilize metal nanoparticles (Ru or Pd for example) exhibiting sizes lower than 10 nm (Gutel et al., 2007).The imidazolium salt alone is not suitable for the growth of TTF The TTF precursor is solubilized in a mixture of BMIMBF 4 (or DMIMBF 4 ) and S whereas the TCNQ precursor is solubilized in S. The TTF solution is slowly added to the TCNQ solution at room temperature.A fine black precipitate appears throughout the addition.The airstable black solid, filtered off, washed and finally dried under vacuum, consists in TTF•TCNQ nanoparticles (yield ~ 5080 %).Mean size, morphology and state of dispersion of the nanoparticles depend on the volume ratio V r = BMIMBF 4 /S (or DMIMBF 4 /S), see table 1 and figures 3 and 4. The use of a tertiary mixture (BMIMBF 4 /OA/S) in a 1:1 ratio (vol./vol.)BMIMBF 4 /OA leads to clusters of nanoparticles (cluster size: 150400 nm, size of individual particles in a cluster: 825 nm).For a 1:3 ratio (vol./vol.)BMIMBF 4 /OA, well dispersed nanoparticles exhibiting a mean diameter of 40 nm are obtained.

Optical properties of TTF•TCNQ nanoparticles
The electronic properties of one-dimensional systems are governed by three types of interactions among the unpaired electrons occupying the highest molecular orbital in the solid.These interactions are (i) the overlap of the wave functions of these electrons between adjacent sites in the crystal, (ii) the interactions of the electrons with their surroundings (e.g.phonons), and (iii) the Coulomb interaction between electrons.Theoretical models taking into account interactions (i) are usually based on a tight-binding method for the band structure.The energy bands are of the form: where k is the wave vector, a the lattice constant, and t is the transfer matrix element between sites, given by: where H is the total Hamiltonian of the systems, and  i is the appropriate molecular orbital wave function.
The width of the band, w = 4t, depends upon the overlap of electronic wave functions, and a one-dimensional character for the bands is obtained by allowing this overlap to occur only in one direction.For partially filled bands, the system will be a metal, with a complex dielectric function: where ε ∞ is the dielectric constant at high frequency arising from core polarisability, τ is the electron relaxation time and ω p is the plasma frequency.For typical one-dimensional chargetransfer based organic conductors, it has been found that ε ∞ ~ 3, τ ~ 10 −15 s, and ω p ~ 10000 cm  1 .Several peaks (undoubtedly characterizing a charge-transfer-based organic conductor) can be seen on electronic spectra recorded either in solution or in the solid state.In addition to the plasma frequency, four absorption bands are usually observed: the first one is at (24)×10 3 cm  1 , the second at about (1012)×10 3 cm  1 , the third one at about (1618)×10 3 cm  1 , and the fourth one at about (2531)×10 3 cm  1 .The first one is assigned to the charge transfer of the type A  A 0 → A 0 A  , where A  and A 0 denote the anion and the neutral molecule of the electron acceptor, respectively (CT1 band).The second one is attributed to the charge transition of the type A  A  → A 0 A 2  (CT2 band).The third one is the local excitation associated with the lowest intramolecular transition of A  (LE1 band), whereas the fourth one is due to the local excitation associated with the lowest intramolecular transition of A 0 (LE2 band).The room temperature reflectance spectrum of a nanoparticle film of TTF•TCNQ has been recorded in the 900025000 cm  1 range (figure 5).The general shape of this spectrum is in good agreement with that of TTF•TCNQ single crystals, recorded for an electric field polarized parallel to the b crystallographic axis (Grant et al., 1973).The plasma reflection is clearly seen at 10700 cm  1 (about 1.32 eV), in excellent agreement with that obtained for TTF•TCNQ single crystals (1.38 eV) (Grant et al., 1973).The conduction-band width (w = 4t) can be estimated from the measured plasma frequency, using the expression: where N denotes the electron density and b the lattice parameter (3.819 Å).Taking the value of 4.7×10 21 cm  3 for N (Bright et al., 1974), we find w = 0.59 eV, in good agreement with the w value calculated for TTF•TCNQ single crystals (0.62 eV) (Graja, 1997).It is to be noticed that the X-ray diffraction pattern of the nanoparticle film is dominated by the (002), (004), and (008) lines.This indicates a preferential orientation of the film, i.e. the ab plane being parallel to the substrate surface.In our case, the light is unpolarized and interacts with the substrate surface according to all possible angles (integrating sphere).However, our reflectance spectrum is as good as that recorded for a light polarized parallel to the b axis.Furthermore, four broads signals are observed at 12600 (CT2), 18300 (LE1), 21700, and 24000 (LE2) cm  1 (figure 5).These four signals are also observed in the reflectance spectrum of TTF•TCNQ single crystals (12000, 17000, 22000, and 25800 cm  1 ) (Grant et al., 1973).The absorption spectrum of TTF•TCNQ nanoparticles dispersed in acetonitrile (figure 6) also clearly evidences CT2, LE1, and LE2 bands at 11900, 1640019000, and 26000 cm  1 , respectively

Vibrational properties of TTF•TCNQ nanoparticles
Vibrational spectroscopy (infrared and Raman) is a powerful tool to investigate onedimensional organic charge-transfer complexes.At large distances between the donor (D) and the acceptor (A) molecules, the vibrational spectrum of D + A is just the sum of those of the free molecules.As the distance between them becomes shorter (formation of the D−A complex), the electrostatic field of one molecule begins to influence the second one, and viceversa.This causes changes in the frequency and intensity from the spectra of isolated molecules.Moreover, due to the fact that the D (or A) molecule may have a lower symmetry in D−A, additional frequencies appear in the complex because forbidden modes may become active due to mixing with other internal modes of D (or A).The vibrational spectrum consists of a primary electronic charge transfer band (CT1) and a of oscillations driven by totally symmetric internal molecular vibrations (a g modes) of one or both parts of the complex.
In the infrared spectrum of ionic liquid-stabilized TTF•TCNQ nanoparticles, vibration bands for BMIMBF 4 or DMIMBF 4 are not observed.However, its presence and thus its stabilizing role have been evidenced by X-ray photoelectron spectroscopy.Indeed, the X-ray photoelectron spectrum shows boron and fluorine lines which can only be due to the ionic liquid.Moreover, the S2p 3/2 signal (163.6 eV) is in excellent agreement with that previously reported for TTF•TCNQ single crystals, i.e. 163.8 eV (Butler et al., 1974).In the spectrum of ionic liquid/oleic acid (OA)-stabilized TTF•TCNQ nanoparticles, vibration bands for OA are present, thus confirming that oleic acid forms a protecting layer around the nanoparticles.However, this stabilizing role is effective when oleic acid is at least introduced in a volume three times higher than that of the ionic liquid.Whatever the stabilizing agent, the infrared of nanoparticles recorded at room temperature in a KBr matrix (figure 7) is quite similar to that previously described for TTFTCNQ processed as thin films (Wozniak et al., 1975;Benoit et al., 1976).Moreover, the infrared spectrum is weakly particle size-dependent.Peak positions and assignments are given in table 2.   , 1975;3: Benoit et al., 1976 Our spectrum clearly evidences C(sp 2 )−H at 3093 and 3073 cm −1 .These modes are surprisingly not observed for TTF•TCNQ thin films deposited on NaCl or KBr crystals (Wozniak et al., 1975;Benoit et al., 1976).In our case, the characteristic nitrile doublet is located at 2204 and 2182 cm −1 .The more intense signal at 2204 cm −1 allows us to determine the amount of charge transfer from the TTF donor molecule to the TCNQ acceptor molecule.
Using the linear correlation of the nitrile stretching mode for TCNQ as a function of the degree of charge transfer, we obtain a value of 0.56, in relatively good agreement with that for single crystals, i.e., 0.59 (Chappell et al., 1981).Thus, the charge transfer in an assembly of nanoparticles is rather similar to that on a macroscopic single crystal.The position, the intensity and the width at half maximum for carbon-carbon double bond modes (in the 1500−1600 cm −1 range) are usually sample preparation and temperature dependent.For TTF•TCNQ nanoparticles, they are located at 1571 and 1518 cm −1 .Finally, the characteristic doublet for TTF (C−S stretch) is located at 832/817 cm −1 for us and at 797/786 cm −1 for TTF•TCNQ thin films.This large difference (about 30 cm −1 ) can be an effect of the temperature (spectra recorded at room temperature in our case and at 10 K for TTF•TCNQ thin films, see Wozniak et al., 1975).Raman spectroscopy is widely used to study the vibrational and structural properties of molecule-based conductors as single crystals.This form is the best suited for obtaining the best signal to noise ratio.However, this technique is also well suited for studying thin molecular layers (oriented, Langmuir-Blodgett, nanoparticle, or polymer films).The TTF•TCNQ nanoparticles are dispersed in diethyl ether and deposited on a glass slide.The solvent is then evaporated slowly.Raman spectra are obtained at 77 K using the 647 nm line of a Kr laser (power ~ 1.7×10 6 W cm −2 ).The incident beam is focused onto the film through the ×100 microscope objective, giving a spot size of ~ 1 m 2 .The back-scattered light is collected through the same objective, dispersed and then imaged onto a CCD detector.
Whatever the ionic liquid used as stabilizing agent, the Raman spectrum of the nanoparticle film (figure 8) is quite similar to that previously described for TTFTCNQ single crystals  Graja, 1997or Kuzmany & Stolz, 1977(Kuzmany & Stolz, 1977).An assignment of nearly all of the peaks can be found by comparing our results with the latter.In table 3, we compile the most important lines, their assignment, and their symmetry.
Except the nitrile stretching mode, all peak positions for TTF•TCNQ nanoparticles are very similar to those on single crystals (see values in parentheses on table 3).The C≡N stretching mode for TTF•TCNQ nanoparticles is located at 2210 cm −1 in the Raman spectrum whereas it is located at 2204 cm −1 in the infrared spectrum (figure 7 and table 2).This difference is still relatively low given the uncertainties on the positions of signals (± 4 cm −1 ) for both spectral techniques.The following discussion will be mainly based on the totally symmetric a g modes.These modes (10 for the TCNQ molecule and 7 for the TTF molecule) have been intensively studied for neutral (TTF 0 , TCNQ 0 ), anionic (TCNQ − ), or cationic (TTF + ) species (Graja, 1997).The conduction electrons in TTF•TCNQ are significantly coupled to almost all intramolecular vibration (a g ) modes.The more significant ones (from  2 to  6 ) have been used to investigate the charge transfer in TTF•TCNQ single crystals (Kuzmany & Stolz, 1977).
where  is the vibrational frequency for TTF•TCNQ,  0 the vibrational frequency for neutral TCNQ, and  I the vibrational frequency for TCNQ  .Values of  obtained from equation ( 5) are given in table 4. Kuzmany  Stolz admit that the negative charge borne by the TCNQ molecule in TTF•TCNQ is the average of all charges as compiled in table 4 (column five).In our case, the average charge is found to be 0.58, in excellent agreement with that for single crystals, i.e., 0.59 (Chappell et al., 1981).

Conclusion
In this chapter, we have shown that the famous organic charge transfer-based conductor TTF•TCNQ can be processed as roughly spherical nanoparticles, while this compound has a natural tendency to grow as needles.The growth as nanospheres is controlled by the use of an ionic liquid introduced together with a conventional solvent.Under certain conditions, well dispersed nanoparticles exhibiting sizes lower than 50 nm can be obtained.Optical and vibrational studies have been performed on either TTF•TCNQ dispersed in an infrared transparent matrix, or on a nanoparticle film, or dispersed in solution.Optical and vibrational signatures for TTF•TCNQ as nanoparticles are in good agreement with those on single crystals, on powders exhibiting micrometer-sized grains, or on thin films.We do expect transport properties similar as the two latter.Preliminary conductivity measurements on a nanoparticle film (mean diameter of individual nanoparticles: 4 nm or 35 nm) have been performed using the four probe technique.TTF•TCNQ nanoparticle films exhibit a semiconducting behavior, which is not surprising for nanopowdered materials (roomtemperature conductivity: 10−20 S cm −1 , activation energy: 15−40 meV).We are currently working on the use of dielectric spectroscopy to characterize an assembly of TTF•TCNQ nanoparticles.We are also developing the use of Raman spectroscopy (at low temperatures) to investigate the charge transfer in nanoparicles of organic or metallo-organic superconducting phases.

Table 4 .
Table4gathers a g  2 to  6 modes (column two) for TTF•TCNQ nanoparticles, for neutral TCNQ (column three) and for the TCNQ − anion (column four).Comparing column two with columns three and four in table 4, evidences that the Raman lines of TTF•TCNQ are generally within the limits of neutral and completely charged TCNQ.A g modes (in cm −1 ) for TTF•TCNQ nanoparticles, neutral TCNQ and radical anion, and evaluation of the charge transfer in TTF•TCNQ as nanoparticles According to Graja, the charge () borne by the TCNQ molecule can be calculated as follows: