Crystal Growth, Structure, and Transport Properties of the Charge

Article pubs.acs.org/crystal

Crystal Growth, Structure, and Transport Properties of the ChargeTransfer Salt Picene/2,3,5,6-Tetrafluoro-7,7,8,8tetracyanoquinodimethane Benjamin Mahns,*,† Olga Kataeva,‡ Daut Islamov,§ Silke Hampel,† Frank Steckel,† Christian Hess,† Martin Knupfer,† Bernd Büchner,† Cameliu Himcinschi,∥ Torsten Hahn,∥ Roman Renger,∥ and Jens Kortus∥ †

IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences, A. E. Arbuzov Street 8, Kazan 420088, Russia § Kazan Federal University, Kremlevskaya Street 18, Kazan 420008, Russia ∥ Department of Theoretical Physics, TU Bergakademie Freiberg, Leipziger Strasse 23, D-09596 Freiberg, Germany ‡

S Supporting Information *

ABSTRACT: Single crystals of the charge-transfer salt picene/2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane have been grown using physical vapor transport. The crystal structure was determined using single-crystal X-ray diffraction. It was found that the crystals grow in a 1:1 molecular ratio and adopt a monoclinic structure with alternate stacking. Both Xray data and Raman measurements show that the grown crystals are of good quality. From structure and infrared data, the charge transfer between acceptor and donor molecules was estimated to be approximately 0.14−0.19 electron. Transport measurements indicate a nonmetallic ground state with an activation energy of 0.6 eV. The supporting density functional theory calculations on molecular model systems as well as on crystalline structures confirm the amount of charge transfer and provide first insights into the electronic structure of the new material. aromatic hydrocarbon (PAH) that consists of five benzene rings arranged in a zigzag-like manner and forms a herringbone, monoclinic crystal structure, similar to those of other aromatic molecules.7 In view of the superconducting properties of potassium-intercalated picene (K3picene), it is interesting to follow complementary routes for the formation of molecular crystals with positively charged picene molecules. This might lead to new properties, which could even be interesting in view of application aspects. To follow this route, we have grown molecular crystals of picene and 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ). The latter molecule is characterized by a rather high electron affinity, and it is known to form many charge-transfer crystals with appropriate partners. In this work, we demonstrate the successful growth of picene/F4-TCNQ single crystals. We provide the crystal structure as obtained by single-crystal X-ray diffraction. Furthermore, we have studied the vibrational properties of these newly synthesized crystals using infrared (IR) and Raman spectroscopy, and we report characteristic changes in the vibrational spectra caused by charge transfer between the two

1. INTRODUCTION The targeted modification of the physical properties of molecular materials by addition or removal of charge has a long and successful history. Often, the variations of the charge state are reached by the formation of mixed crystals, where the building blocks are chosen to allow a charge transfer between the two contributing components. Famous examples are the socalled charge-transfer salts demonstrating interesting and often unexpected physical properties ranging from metallicity and superconductivity over complex phase diagrams, including charge density and spin density wave phases, to highly correlated materials (Mott insulators).1,2 Intriguingly, even the formation of a two-dimensional metallic layer has been reported to be a consequence of charge transfer between the two insulating organic crystals (TTF and TCNQ).3,4 In addition, the discovery of a superconducting phase in the alkali metal-doped fullerides5,6 represented a breakthrough in the field of superconductivity and has attracted a lot of attention with respect to other molecular crystals built from π-conjugated molecules. Recently, superconductivity was reported for some alkali-intercalated molecular solids built from hydrocarbons such as picene, which renewed interest in the study of such materials and the variations of their physical properties upon charging and/or doping. Picene (C22H14) is a polycyclic © 2014 American Chemical Society

Received: December 10, 2013 Revised: January 28, 2014 Published: February 14, 2014 1338

dx.doi.org/10.1021/cg401841n | Cryst. Growth Des. 2014, 14, 1338−1346

Crystal Growth & Design

Article

Figure 1. Quartz ampule after the growth procedure (left). The insets show a detailed view of the obtained crystals of pure picene (right), pure F4TCNQ (left), and picene/F4-TCNQ charge-transfer crystals (middle). Sketch of the crystal growth setup (right). The temperature profile of the oven is shown in the bottom right panel. Temperatures T1 and T2 were directly measured at the ampule by two thermocouples. obtained via polarized optical microscopy using a ZEISS Axiovert 25 microscope. At the end of the tube, one can find flat crystals of F4TCNQ. The crystals of CT salts were found to be very stable in air at room temperature and can be kept under ambient conditions for a long time. The stability of the crystals was verified by IR spectroscopy, which was measured directly after the growth procedure and again after several months. 2.2. Determination of the Crystal Structure. The X-ray diffraction data of of the picene/F4-TCNQ charge-transfer crystals were collected on a Bruker AXS Kappa APEX Duo diffractometer, and the data for pure picene crystals were recorded on a Bruker AXS SMART APEX II diffractometer, both with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) using the complete sphere mode. APEX28 was used for data collection, SAINT9 for data reduction, and SADABS10 for absorption correction. The structure was determined by direct methods using SHELXS97.11 All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed into calculated positions and refined using a riding model. The structure full-matrix refinement against F2 was conducted using the SHELXL package.11 In the following, we summarize the results of the crystal structure analysis: formula, C22H14+C12F4N4; crystal size, 0.12 mm × 0.10 mm × 0.20 mm; Mr = 554.49; monoclinic; space group P21/n; a = 7.9197(1) Å; b = 6.9998(1) Å; c = 42.9408(8) Å; β = 91.429(1)°; V = 2379.75(7) Å3; Z = 4; ρc = 1.548 g/cm3; μ = 0.115 mm−1; T = 150(2) K; θ range from 0.95° to 30.6°; 63115 reflections collected; 7286 independent reflections (Rint = 0.0768); 4705 observed reflections [I ≥ 2σ(I)]; 379 refined parameters; R1 = 0.0699; wR2 = 0.1915 [I > 2σ(I)]; maximal residual electron density, 0.54 e Å−3. The orientation of the single crystals for further physical measurements was determined during the X-ray experiment, and the b direction was found to correspond to the longest dimension of the needlelike crystals. 2.3. Spectroscopic Analysis. Raman measurements were performed using a Horiba Jobin Yvon LabRam800 spectrometer equipped with a microscope, a charge-coupled device detector, and a grating of 1800 lines/mm. The 532 nm line of a frequency-doubled Nd:YAG laser was used for excitation. The measurements were performed on the crystals shown in Figure 1 in the backscattering geometry, the light being focused to a spot with a diameter of ∼2 μm and collected through a 50× magnification long distance objective. A laser power of 0.5 mW was used, which was low enough to avoid sample degradation. IR measurements were taken using a Bruker IFS 88 spectrometer with a spectral resolution of 2 cm−1 in a range of 400−4000 cm−1. The signal was counted by a DTGS-Detector, and light was emitted by a globar. The crystals were thoroughly ground in an agate mortar and mixed with KBr powder (Sigma-Aldrich Chemie GmbH). Afterward, the mixture was pressed to pellets and immediately transferred into the spectrometer, where the measurements were taken in absorption mode.

molecules involved. These changes and the analysis of the bond length of the molecules in comparison to the pristine materials prove that picene/F4-TCNQ is a charge-transfer crystal with a transferred charge of approximately 0.14−0.19 electron per molecule. Characterization of the picene/F4-TCNQ single crystals using electrical transport measurements yielded an activation energy of ∼0.6 eV. By theoretical calculations in the framework of density functional theory (DFT), we substantiate the experimental findings. To support the analysis of the IR and Raman measurements, we conducted calculations of the vibrational properties of individual molecules, a model dimer, and the bulk structure. The amount and mechanism of charge transfer were investigated, and a first picture of the electronic structure was derived.

2. EXPERIMENTAL DETAILS 2.1. Crystal Growth. Co-crystals of picene (Sigma-Aldrich Chemie GmbH, 99.8%) and F4-TCNQ (Sigma-Aldrich Chemie GmbH, 97%) have been grown using physical vapor transport. The crystal growth was conducted in closed, evacuated quartz glass ampules (p ≈ 2 × 10−5 mbar) using a horizontal one-zone oven heated to 215 °C. The growth procedure lasted ∼7 days. The diameter of the quartz glass ampules was 10 mm, and the overall length was 100 mm. Twenty milligrams (0.072 mmol) of F4-TCNQ and 17.1 mg (0.061 mmol) of picene were ground together thoroughly in an agate mortar and transferred via a funnel into the glass tube that was afterward immediately evacuated and closed using an oxygen−hydrogen burner. Prior to growth, the ampule was cleaned using soap, demiwater, acetone, and alcohol, and then it was baked overnight in a compartment dryer at 110 °C to remove water and other solvents. Water can lead to the hydrolysis of the F4-TCNQ cyanonitrile groups, which then can form amines or carboxylic acids. Stretching vibrations of these carboxylic acids can be observed in respective IR spectra as distinct peaks at 1700 and 3300 cm−1, and our IR analysis demonstrated the absence of these contaminants. After being filled, the quartz glass ampule was placed into a glass tube (diameter 24 mm) of the preheated oven. It was placed in the oven in a way that there was a temperature gradient from 211 °C (ground material side) to 137 °C (end of tube side) (see Figure 1). The temperature was controlled using two thermocouples that were taped on both sides of the glass tube using alumina tape. After growth, the ampule was immediately removed from the hot oven. One can clearly distinguish four zones in the ampule (see Figure 1). In zone 1, parts of the mixed starting materials are still present. In zone 2, platelets of pure picene crystals are visible. In the center of the tube, the charge-transfer salt is formed as needle-shaped crystals with typical dimensions used for physical measurements of 2 mm × 0.05 mm × 0.1 mm. Parallel to this big needles, smaller crystals were also grown as seen in Figure 3, which shows a picture of different single crystals 1339



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Figure 2. Schematic drawings of picene (left) and F4-TCNQ (right). Bond lengths a, b, c, and d directly correlate with the charge transfer between the donor and acceptor (see the text).

Figure 3. Fragment of crystal packing of picene/F4-TCNQ (view along axis a) (left). Picture of grown crystals obtained using polarized optical microscopy (right). One can clearly observe several needles of different sizes that grow in the b direction.

Figure 4. Pairs of alternating picene/F4-TCNQ stacks viewed from the side (left) and top (right).

Figure 5. Projection showing a structural overlay (left) and the slipping arrangement of molecules in the stacks (right). 2.4. Electrical Transport Measurements. The voltage−current characteristics of the samples were measured using a Keithley 6527A high-resistance meter in a two-contact setup. The resistance of the contacts is negligible compared to the resistance of the sample; thus, a four-point measurement is not necessary. The contacts were made using either graphite or silver paste. Typically, the dimensions of the investigated samples have a diameter of 0.1 mm and a length along which the current flows of 1 mm. The measured current was at least 10 times higher than the background noise. With this setup, we were able to measure repeatedly currents down to 0.1 nA with voltages of up to

500 V. The same setup was used to measure the current at changing temperatures and at a constant set voltage to 100 V. This measurement was conducted in a cryostat, and the temperature change was achieved by varying the position of the sample in the temperature gradient of helium gas. 2.5. DFT Calculations. Starting from the experimentally determined X-ray structure of the picene/F4-TCNQ crystal, we generated molecular models of the free, isolated picene, and F4-TCNQ molecules (Figure 2). Additionally, we prepared the structure of a picene/F4-TCNQ dimer by extracting the relevant positions from the 1340



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Table 1. Comparison of Bond Lengths and Estimated Degrees of Charge Transfer According to refs 27 and 28in F4-TCNQ Charge-Transfer Complexesa picene/F4-TCNQ F4-TCNQ (BTT)2/F4TCNQ DBTTF/F4-TCNQ M2P/F4-TCNQ NBP/F4-TCNQ

a (Å)

b (Å)

c (Å)

d (Å)

c/(b + d)

predicted CT

ref

1.343(3) 1.334(2) 1.338(18) 1.354(4) 1.356(4) 1.350(4)

1.441(3) 1.437(4) 1.430(10) 1.418(4) 1.414(4) 1.410(4)

1.383(3) 1.372(2) 1.369(18) 1.416(4) 1.415(4) 1.413(4)

1.429(3) 1.437(4) 1.411(12) 1.440(7) 1.430(6) 1.419(5)

0.481 0.477 0.482 0.495 0.498 0.499

0.19 − 0.24 0.85 1 1.047

this work 31 28 32 30 33

a Abbreviations: BTT, benzo[1,2-c:3,4-c′:5,6-c″]trithiopene; DBTTF, dibenzotetrathiafulvalene; M2P, 5,10-Dihydro-5,10-dimethylphenazine; NBP, N-butylphenazinium.

where αn = c/(b + d), where b−d represent the bond lengths (Figure 2), and subscripts ct, 0, and −1 denote the chargetransfer complex, the neutral molecule, and the anion, respectively. Because of the similarity of the bond ratios in TCNQ and F4-TCNQ, we adopt this model for our F4-TCNQ charge-transfer complex as it has also been used successfully in previous studies.28,29 The neutral F4-TCNQ acceptor adopts a quinoid structure. Additional charge in the lowest unoccupied molecular orbital (LUMO) of the acceptor molecule leads to a variation of different bond lengths and results in a more aromatic structure in which the length of the bonds becomes more equal. The bond length most sensitive to the charge transfer appears to be the quinoid double bond c as depicted in Figure 2, which is elongated the most compared to a slight increase in b and a decrease in d. Table 1 lists the bond lengths, used for the determination of charge transfer, and also an overview of estimated charge transfers for other CT compounds containing F4-TCNQ . According to ref 28, the bond lengths for α−1 were taken from the M2P/F4-TCNQ complex that was found to be nearly purely ionic.30 From a comparison of our structural data and those of pure F4-TCNQ crystals as well as F4TCNQ−, we arrive at a charge transfer of 0.19 e in our picene/F4-TCNQ charge-transfer crystal. This value is relatively small in comparison to those of other charge-transfer crystals containing non-PAHs listed in Table 1 but significantly different from zero. It is also consistent with the IR and Raman observations for the vibrational CC and CN modes (see below). 3.2. Vibrational Spectra and Calculations. The degree of charge transfer can also be analyzed using infrared absorption spectroscopy; this empirical method has been widely used in the case of TCNQ as well as in some F4TCNQ compounds.34,35 In particular, the F4-TCNQ IR bands of the CC and CN stretching modes are sensitive to the amount of charge in the molecules.35 To estimate the charge transfer, we will focus on the CN stretching modes, because pristine picene also absorbs in the same CC region as F4TCNQ, and therefore, it is difficult to clearly differentiate between peaks of both components in picene/F4-TCNQ crystals. As seen in Figure 6, which shows the absorption Fourier transform infrared (FT-IR) spectra of picene/F4TCNQ bulk crystal powder in a KBr pellet, picene has no observable features in the region of the F4-TCNQ CN stretching modes. For neutral F4-TCNQ, we observe a strong CN stretching mode at 2227 cm−1 and a weaker one at 2214 cm−1. According to ref 35, these bands can be assigned to vibrational modes with b1u and b2u symmetry. In picene/F4-TCNQ crystals, one can

X-ray data. For the calculations on the free molecules and the dimer, NRLMOL (Naval Research Laboratory Molecular Orbital Library),12−17 which is an all-electron implementation of DFT, was used. For the calculations on the bulk structure, PWSCF18 was used together with a set of norm-conserving pseudopotentials for all atoms. All calculations were conducted using the generalized gradient approximation (GGA), and the PBE functional19 was used to approximate exchange and correlation.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure of Picene/F4-TCNQ and Determination of the Charge from Bond Lengths. The X-ray single-crystal diffraction study revealed the formation of 1:1 molecular crystals of F4-TCNQ and picene. Two major factors determine the packing in this molecular complex: chargetransfer interactions that favor the arrangement with alternating face-to-face picene/F4-TCNQ stacks (Figure 3) and the dimensions of the co-crystallizing molecules. Both picene and F4-TCNQ are planar and have similar widths, while the longest dimensions are quite different and are roughly equal to 13 and 9 Å, respectively. To compensate for this size discrepancy, the alternating stacks are arranged in pairs (Figure 4) to provide close packing. The projection orthogonal to the mean plane of picene (Figure 5) shows the slipped orientation of molecules in the stacks, with the average distance between the F4-TCNQ molecule and the picene plane being equal to 3.25 Å, which is the first evidence of the comparatively strong interaction between the donor and acceptor molecules. In general, the interplanar mean plane distances after charge transfer between F4-TCNQ acceptor and donor molecules containing different aromatic fragments vary from 3.25 to 3.29 Å in compounds with cyclophane derivatives containing a benzene fragment,20−22 from 3.31 to 3.35 Å in compounds with 5,10dithia- and -diselenanthrene derivatives,23 and to 3.36 Å in the compound with trans-stilbene.24 One of the fundamental properties of charge-transfer crystals, which is also relevant to the understanding of many physical properties, is the amount of charge that is transferred during crystal formation. Intramolecular geometric changes, especially those of bond lengths, are directly connected to this amount of charge transfer. They are therefore a useful tool for estimating the amount of transferred charge between donor and acceptor molecules. This has been conducted for TCNQ over a wide range of compounds.25,26 As proposed for TCNQ by Kistenmacher et al.,27 we estimate charge transfer ρ by using the equation ρ=

αct − α0 α −1 − α 0

(1) 1341



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transfer salt, respectively. When ν0 = 2227 cm−1, ν1 = 2190 cm−1 (according to ref 35), and νct = 2222 cm−1, we end up at a charge transfer ρ of 0.14 e, in good agreement with our bond length analysis described above. These empirical findings were verified by means of DFT calculations. We performed calculations of the vibrational modes of the F4-TCNQ molecule with stepwise variation of the molecule charge between 0.0 and 1.0 electron. For every charge state, we fully relaxed the geometry and obtained the vibrational spectrum. In Figure 7 (right), the shift of the CN stretching mode obtained from the DFT results is shown. Additionally, we show the shift of the more intense C−C streching mode at 1590 cm−1. Assuming a simple linear model, the slope of the theoretical curves was used to obtain the amount of transferred charge from the experimental data. In both cases (C−C and CN mode), we end up with a charge transfer of ≈0.13 electron, which verifies the empirical findings. The determination of charge transfer using a complementary approach is possible via the calculation of the Mulliken electronegativity χm = (Ea + Ip)/2 (where Ea is the electron affinity and Ip is the ionization potential). In general, charge transfer takes places until the χm values of donor and acceptor are equal. Furthermore, the value of χm depends quadratically on the amount of a partial charge. In Figure 7 (left), we show the calculated values. From the intersection of χm for picene and F4-TCNQ, one can directly obtain the amount of transferred charge. We obtained a ρ value of 0.13 electron, which again agrees perfectly with the findings from the vibrational analysis. To investigate the electronic structure of the material and to detail the origin of the charge transfer, we performed DFT calculations on molecular model systems. In Figure 8 (left), we show the electronic structure of the molecular components compared to the results for a picene/F4-TCNQ model dimer. For the molecular orbitals relevant for the charge transfer, the isosurfaces of the orbital layouts are also shown. The comparison of the dimer orbitals to the molecular orbitals of the components reveals that the LUMO+1 orbital of the dimer is mainly formed by the LUMO of picene. The LUMO of the dimer has mainly the character of the F4-TCNQ LUMO. The dimer HOMO is nearly identical to the picene HOMO. The HOMO-1 of the dimer is a hybrid orbital constructed from the HOMO-1 orbital of picene and the F4-TCNQ LUMO. The

Figure 6. FT-IR absorption spectra of a KBr pellet containing the grown crystals in the CN (left) and CC (right) stretching mode regions.

clearly identify a new peak at 2193 cm−1 that indicates the charge transfer. A similar peak at 2190 cm−1 was also observed for the anion of Rb/F4-TCNQ crystals by Meneghetti et al.,35 and it was found to have b2u symmetry. There are also two further bands at 2222 and 2212 cm−1. Compared to what was observed in pristine F4-TCNQ, the relative intensity of these bands has changed dramatically. Also, the asymmetry of the peaks with respect to their center and their broadening compared to the pristine acceptor is different. It is obvious that the excitations of the CN stretching modes are red-shifted in the case of picene/F4-TCNQ crystals within a range of 2−5 cm−1. In general, this is an indication of charge transfer in which in most cases the red shift grows linear with increased charge.34,36−38 Having now a closer look at the CC bands, one can clearly identify two signals at 1599 cm−1 (b1u) and 1549 cm−1 (b2u) in pristine F4-TCNQ.35 Picene/F4-TCNQ also shows these peaks that appear to be red-shifted at 1593 and 1546 cm−1. The degree of charge transfer can be calculated from infrared absorption spectroscopy using the empiric relation ρ = 2Δν /ν0(1 − ν12/ν0 2)−1

(2)

where Δν = ν0 − νct as proposed by ref 39 and used for TCNQ as well as F4-TCNQ compounds.37−39 The values ν0, ν1, and νct denote the CN stretching modes of pristine F4-TCNQ, the F4-TCNQ anion (ρ = 1), and the picene/F4-TCNQ charge-

Figure 7. DFT-calculated Mulliken electronegativity of picene and F4-TCNQ as a function of partial charge (left) and calculated shifts of selected vibrational modes of the F4-TCNQ molecule as a function of its charge (right). 1342



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Figure 8. Energy level diagram obtained by DFT calculations of the F4-TCNQ and picene molecules as well as for the model dimer (left). Calculated band structure of picene/F4-TCNQ (right).

Figure 9. Raman spectra of picene, picene/F4-TCNQ mixed crystals, and F4-TCNQ in the spectral regions of 160−900 cm−1 (left) and 1000−2300 cm−1 (right).

formation of this hybrid orbital is the origin of the charge transfer, which can be seen as the (partial) filling of the F4TCNQ LUMO resulting in the HOMO-1 of the dimer as a new bonding state. The band structure of the material was obtained by using the measured crystallographic data as input parameters for a DFT calculation using periodic boundary conditions together with the PWSCF code. The result is shown in Figure 8 (right). The band gap was determined to be 0.55 eV, and the bandwidths of the bands are very narrow and between 0.15 eV for the valence band and 0.25 eV for the conduction band. In Figure 9, the Raman spectra of picene, picene/F4-TCNQ mixed crystals, and F4-TCNQ are shown in the spectral regions of 160−900 and 1000−2300 cm−1. The frequency positions of the modes measured in the spectra of F4-TCNQ are similar to those measured for neutral molecules by Meneghetti et al.35 The spectrum of picene is very similar with the spectra of monoclinic pristine picene reported by Girlando et al.40 The Raman spectra of picene are dominated by the peak at 1379 cm−1, which corresponds to the C−C stretching mode.41,42 This vibrational mode was predicted to be very sensitive to electron−phonon coupling in picene41,42 and was found to be downshifted up to 70 cm−1 by K doping.42 Moreover, a downshift of up to 120 cm−1 of this Raman mode was theoretically predicted when picene was doped with K or Rb in K5picene or Rb5picene crystals.42

The spectrum measured for the mixed picene/F4-TCNQ crystal shows, besides the vibrational peaks that are present in the spectra of picene and F4-TCNQ, additional new modes. The larger number of peaks indicates a lowering of the symmetry of the molecules in the CT crystal. On the other hand, some of the peaks seen in pure picene and F4-TCNQ are clearly shifted in the CT crystal, as compared with those of their pure counterparts. For instance, the CN stretching vibration at 2225 cm−1 in the Raman spectrum of F4-TCNQ is red-shifted by 5 cm−1 compared to that in the spectrum of picene/F4-TCNQ. A similar shift was also found in the IR spectra measured for our crystals, being attributed to the charge-transfer complex29 formed in the CT crystal. Another very clear shift that can be attributed to charge transfer is observed via the mode at 1620 cm−1 in pristine picene, which is red-shifted to 1615 cm−1 in the CT crystal. In addition, as shown in Figure 10, the Raman spectrum of the picene/F4-TCNQ crystal exhibits a very strong polarization dependence. The spectra were recorded in backscattering geometry having the polarization of the incident light (y) parallel with the length of a crystal needle (as shown in the inset of Figure 10), while the polarization of scattered light was analyzed depolarized (y,−), parallel (y,y), and perpendicular (y,x) to the incident polarization. This very strong polarization dependence demonstrates the good quality of the picene/F4TCNQ crystals investigated in this work. 1343



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4. CONCLUSIONS We have demonstrated the successful growth of picene/F4TCNQ single crystals using physical vapor transport. The crystal structure was obtained by single-crystal X-ray diffraction. It was found that the crystals grow in a 1:1 molecular ratio and adopt a monoclinic structure with alternate stacking. Raman measurements show a very strong polarization dependence, which confirms the high quality of the picene/F4-TCNQ crystals investigated in this work. By using IR and structural data, we were able to estimate a charge transfer of 0.14−0.19 electron, which is relatively small in comparison to those of other charge transer salts with F4-TCNQ containing nonPAHs. The value of the charge transfer was verified by our DFT calculations. The charge transfer takes place mainly between the HOMO-1 orbital of picene and the LUMO of F4-TCNQ. The transport gap derived from temperature-dependent conductivity measurements was found to be 0.60 eV. The gap derived from the DFT calculations on the bulk structure was found to be 0.55 eV.

Figure 10. Polarization dependence of the Raman spectra for a picene/F4-TCNQ needle crystal.

3.3. Electrical Transport. The measured needlelike crystals grow in the b direction, and therefore, all transport measurements were also measured along this direction. The voltage− current (I−V) characteristics were repeatedly checked in different samples [cf. Figure 11 (left)]. The measurement was taken at room temperature. Both curves resemble the same linear slope and yield a resistance of ≈1.5 GΩ of the sample. The difference in the absolute value of the two curves is given by the differing sample geometry. The linear characteristic is typical of an ohmic resistance with a very high resistivity; thus, the electrical conductivity is very low. No excitation or different than linear behavior has been found for any forced voltage through the sample. The dependence of the current on temperature at a constant voltage of 100 V is plotted in the inset of Figure 11 (right). One sees that the current and thus the conductivity decreases with a decrease in temperature. In a temperature range from room temperature to 230 K, the conductivity follows an exponential law, visualized with the Arrhenius plot in Figure 11 (right). By fitting the high-temperature exponential behavior, we found an energy gap Δ of 0.59 eV. For another sample, we found an energy gap Δ of 0.65 eV. Thus, it is a reproducible value of the compound. Thus, we can state that above 230 K and up to room temperature the compound shows thermally activated behavior with an open gap of ∼0.6 eV, which is in good correspondence with the DFT calculations.

■

APPENDIX With respect to picene, the crystal structure of only one molecular complex with picric acid has been studied;43 however, no atomic coordinates are available in the Cambridge Structural Database. The structure of pure picene was previously studied44 at room temperature using Cu radiation. We have re-determined the crystal structure of picene at 150 K using Mo radiation. The picene molecules form a herringbone packing motif in the crystal with no stacking arrangement (Figure 12) but with multiple weak C−H···π interactions, and the angle between the interacting picene molecules is 58.5°. The results of the picene crystal structure analysis are as follows. Crystal data for picene: formula, C22H14; crystal size, 0.13 mm × 0.18 mm × 0.40 mm; Mr = 278.33; monoclinic; space group P21; a = 8.048(3) Å; b = 6.082(2) Å; c = 13.429(5) Å; β = 90.023(5)°; V = 686.7(4) Å3; Z = 2; ρc = 1.346 g/cm3; μ = 0.076 mm−1; T = 150(2) K; θ range from 1.52° to 26.24°; 5511 reflections collected; 2716 independent reflections (Rint = 0.0216); 1930 observed reflections [I ≥ 2σ(I)]; 199 refined parameters; R1 = 0.0427; wR2 = 0.1000 [I > 2σ(I)]; maximal residual electron density, 0.19 e Å−3. The absolute structure of the picene crystal was not determined, the Flack parameter being meaningless, because

Figure 11. Current−voltage plot of two picene/F4-TCNQ crystals at room temperature. Both curves resemble the same linear slope and yield a resistance of ≈1.5 GΩ (left). Conductivity dependence vs the inverse temperature at 100 V. The linear fit yields an energy gap Δ of 0.59 eV. The inset shows the dependence of the current on temperature at a constant voltage of 100 V (right). 1344



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Figure 12. Fragment of crystal packing of pure picene.

picene is a weak anomalous scatterer. CCDC-972624-picene and 972625-charge-transfer complex contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.).

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ASSOCIATED CONTENT

S Supporting Information *

CheckCif/Platon report for the studied crystals and tables of observed and calculated structure factors. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*IFW Dresden, Institute for Solid State Research, P.O. Box 270116, D-01171 Dresden, Germany. Phone: +49-351-4659735. Fax: +49-351-4659-313. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Naumann, A. Schubert, R. Hübel, and S. Leger for technical assistance. This work was performed within the Cluster of Excellence Structure Design of Novel HighPerformance Materials via Atomic Design and Defect Engineering (ADDE), which is financially supported by the European Union (European regional development fund) and by the Ministry of Science and Art of Saxony (SMWK). This work was also partially supported by the Deutsche Forschungsgemeinschaft DFG (KN393/20 and KN393/14). We further thank the ZIH TU-Dresden for providing computer resources, technical expertise, and assistance.

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REFERENCES

(1) Toyota, N.; Lang, M.; Müller, J. Low-Dimensional Molecular Metals; Springer: Berlin, 2007. (2) Ouahab, L. Organic Conductors, Superconductors and Magnets; Kluwer Academic: Dordrecht, The Netherlands, 2004. (3) Alves, H.; Molinari, A. S.; Xie, H.; Morpurgo, A. F. Nat. Mater. 2008, 7, 574−580. (4) Wen, S.; Deng, W.-Q.; Han, K.-L. Chem. Commun. 2010, 46, 5133−5135. (5) Hebard, A.; Rosseinsky, M.; Haddon, R.; Murphy, D.; Glarum, S.; Palastra, T.; Ramirez, A.; Kortan, A. Nature 1991, 350, 600−6012. (6) Gunnarson, O. Alkali Doped Fullerides; World Scientific: Singapore, 2004. 1345



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Crystal Growth, Structure, and Transport Properties of the Charge

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