Growth, Patterning, and One-Dimensional Electron -Transport

Growth, Patterning, and One-Dimensional Electron -Transport Properties of ... permitting the reproducible in situ growth of single Ag-TCNQF4 nanowire ...
0 downloads 0 Views 4MB Size
Download PDF
Chem. Mater. 2009, 21, 4275–4281 4275 DOI:10.1021/cm901431f

Growth, Patterning, and One-Dimensional Electron -Transport Properties of Self-Assembled Ag-TCNQF4 Organic Nanowires Kai Xiao,* Adam J. Rondinone, Alex A. Puretzky, Ilia N. Ivanov, Scott T. Retterer, and David B. Geohegan* Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6488 Received May 26, 2009. Revised Manuscript Received July 21, 2009

Controllable synthesis approaches for organic nanowires that permit the in situ fabrication of devices will enable future applications in nano-electronics and nano-optoelectronics. Here, the first synthesis of single-crystal silver-tetrafluorotetracyano-p-quinodimethane (Ag-TCNQF4) nanowires is reported. Ag-TCNQF4 is a good charge-transfer complex and nanowires of this organic semiconductor material were deterministically synthesized in a facile vapor-solid process on selected regions through the reaction of TCNQF4 vapor with patterned silver. Use of a growth barrier is shown to control the growth of Ag-TCNQF4 nanowires to horizontal alignment, permitting the reproducible in situ growth of single Ag-TCNQF4 nanowire devices and device arrays between prefabricated electrodes. The single-crystal nanowires are predominantly monoclinic in structure with efficient π-stacking of the TCNQF4 units, leading to a high conductivity along the nanowire. However, the electron-withdrawing fluorine groups on the π-delocalized ring in the TCNQF4 results in a distinctly different structure compared to that previously reported for Ag-TCNQ nanowires. The temperature- and bias-voltage-dependent electrical transport properties of in situ fabricated Ag-TCNQF4 organic nanowire devices were investigated and exhibit a power-law behavior characteristic of one-dimensional systems. Introduction The synthesis and electrical properties of organic semiconductor nanostructures has recently become the subject of intense interest1 because of their highly conjugated molecular structure, straightforward chemical modifiability, unique optical and electrical properties, and their promise for applications such as nanosensors,2 organic devices,3 integrated circuits,4 lasers,5 spin valves,6 and photodetectors.7 It is crucial that effective fabrication *Corresponding author. E-mail: [email protected] (K.X.); geohegandb@ ornl.gov (D.B.G.).

(1) Grimsdale, A. C.; Mullen, K. Angew. Chem., Int. Ed. 2005, 44, 5592. (2) (a) Naddo, T.; Che, Y.; Zhang, W.; Balakrishnan, K.; Yang, X.; Yen, M.; Zhao, J.; Moore, J. S.; Zang, L J. Am. Chem. Soc. 2007, 129, 6978. (b) Kemp, N. T.; McGrouther, D.; Cochrane, J. W.; Newbury, R. Adv. Mater. 2007, 19, 2634. (3) (a) Moran-Mirabal, J. M.; Slinker, J. D.; DeFranco, J. A.; Verbridge, S. S.; Ilic, R.; Flores-Torres, S.; Abruna, H.; Malliaras, G. G.; Craighead, H. G. Nano Lett. 2007, 7, 458. (b) Zhao, Y. S.; Di, C.; Yang, W.; Yu, G.; Liu, Y.; Yao, J. Adv. Funct. Mater. 2006, 16, 1985. (c) Xiao, S.; Tang, J.; Beetz, T.; Guo, X.; Tremblay, N.; Siegrist, T.; Zhu, Y.; Steigerwald, M.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 10700. (4) Briseno, A. L.; Mannsfeld, S. C. B.; Reese, C.; Hancock, J. M.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Nano Lett. 2007, 7, 2847. (5) (a) O’Carroll, D.; Lieberwirth, I.; Redmond, G. Nat. Nanotechnol. 2007, 2, 180. (b) Quochi, F.; Cordella, F.; Mura, A.; Bongiovanni, G.; Balzer, F.; Rubahn, H. G. Appl. Phys. Lett. 2006, 80, 041106. (6) Pramanik, S.; Stefanita, C. G.; Patibandla, S.; Nandyopadhyay, S.; Garre, K.; Harth, N.; Cahay, M. Nat. Nanotechnol. 2007, 2, 216. (7) (a) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 314, 1761. (b) O'Brien, G.; Quinn, A. J.; Tanner, D. A.; Redmond, G. Adv. Mater. 2006, 18, 2379. r 2009 American Chemical Society

techniques are developed for the controlled growth of organized, oriented organic nanostructures that can be assembled by design to build specific functional devices and integrated circuits. Metal-organic charge-transfer (CT) complex nanostructures based on TCNQ have long been regarded as attractive building blocks for molecular electronic devices because of their intriguing variety of novel electrical, optical, and magnetic properties.8 These properties make metal-TCNQ complexes good candidates for applications in organic electronic devices,9 optical and electrical media recording, energy and data storage, and sensors and catalysis, as well as electrochromic and magnetic devices.10 TCNQF4 is far less studied than TCNQ and is a much stronger electron acceptor because of the notable increase (about 0.4 eV) in the electron affinity consequent to fluorination. TCNQF4 has been widely (8) (a) Nafady, A.; Bond, A. M.; Bilyk, A.; Harris, A. R.; Bhatt, A. I.; O’Mullane, A. P.; De Marco, R. J. Am. Chem. Soc. 2007, 129, 2369. (b) Liu, Y.; Li, H.; Tu, D.; Ji, Z.; Wang, C.; Tang, Q.; Liu, M.; Hu, W.; Liu, Y.; Zhu, D. J. Am. Chem. Soc. 2006, 128, 12917. (9) (a) Bryee, M. R.; Murphy, L. C. Nature 1984, 309, 119. (b) Miller, J. S.; Calabrese, J. C.; Harlow, R. L.; Dixon, D. A.; Zhang, J.; Reiff, W. M.; Chittipeddi, S.; Selover, M. A.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 5496. (c) Mikuriya, M.; Yoshioka, D.; Handa, M. Coord. Chem. Rev. 2006, 250, 2194. (10) (a) Yamaguchi, S.; Potember, R. S. Synth. Met. 1996, 78, 117. (b) Fan, Z. Y.; Mo, X. L.; Chen, G. R.; Lu, J. G. Rev. Adv. Mater. Sci. 2003, 5, 72. (c) Wooster, T. J.; Bond, A. M. Analyst 2003, 128, 1386. (d) Perepichka, D. F.; Bryce, M. R.; Pearson, C.; Petty, M. C.; McInnes, E. J. L.; Zhao, J. P. Angew. Chem., Int. Ed. 2003, 42, 4636.

Published on Web 08/27/2009

pubs.acs.org/cm

4276

Chem. Mater., Vol. 21, No. 18, 2009

used in organic light-emitting diodes to reduce the hole injection barrier by forming a narrow space-charge region near the metal contact.11 TCNQF4 also allows precise tuning of the hole concentration (p-type doping) of singlewall carbon nanotubes in field effect transistors, as well as for graphene and organic semiconductors such as zincphthalocyanine.12 Ag-TCNQF4 is a good charge-transfer organic semiconductor compound and is regarded as an ideal candidate for the reversible bistable electrical and optical switches together with memory effects.13 Compared to Ag-TCNQ, Ag-TCNQF4 should have significantly enhanced charge transfer because TCNQF4 is a notably stronger electron acceptor. In addition, AgTCNQF4 has a distinct crystal structure compared with either phase of Ag-TCNQ, which should result in different electrical properties. Although metal-TCNQ nanostructures have been studied recently,14 to the best of our knowledge, no Ag-TCNQF4 nanostructure has been reported so far. It is necessary to introduce a facile, selective growth method to produce organized, oriented Ag-TCNQF4 nanowires with controllable structure and location to meet the requirements for potential uses in nanodevices. A variety of methods have been utilized for the growth of metal-TCNQ nanowires, including vacuum vapor deposition,14b solution processing in organic solvents,15 as well as chemical and electrochemical reduction of TCNQ in the presence of metallic Ag or Agþ ions.16 A vapor-solid chemical reaction method has also been reported for the growth of high-quality, single-crystalline metal-TCNQ nanowires, highlighting this synthesis route as a promising one for the facile synthesis of novel semiconductor nanowires.17 In this work, we report the direct synthesis, structural characterization, patterning, and electrical transport properties investigation of single-crystal organic nanowires of Ag-TCNQF4 using such a vapor-solid chemical reaction method. Vertical or horizontal patterns of dense AgTCNQF4 nanowire assemblies are achieved through the application of growth barriers. Moreover, the growth of Ag-TCNQF4 nanowires between prefabricated electrodes into device structures is demonstrated using a method that we previously used to grow organic nanowires of (11) Hanson, E. L.; Guo, J.; Koch, N.; Schwartz, J.; Bemasek, S. L. J. Am. Chem. Soc. 2005, 127, 10058. (12) (a) Takenobu, T.; Kanbara, T.; Akima, N.; Takahashi, T.; Shiraishi, M.; Tsukagoshi, K.; Kataura, H.; Aoyagi, Y.; Iwasa, Y. Adv. Mater. 2005, 17, 2430. (b) Chen, W.; Chen, S.; Qi, D.; Gao, X.; Wee, A. T. S. J. Am. Chem. Soc. 2007, 129, 10418. (c) Harada, K.; Werner, A. G.; Pfeiffer, M.; Bloom, C. J.; Elliott, C. M.; Leo, K. Phys. Rev. Lett. 2005, 94, 036601. (13) Potember, R. S.; Poehler, T. O. Synth. Met. 1982, 4, 371. (14) (a) Ye, C.; Cao, G.; Fang, F.; Xu, H.; Xing, X.; Sun, D.; Chen, G. Micro 2005, 36, 461. (b) Fan, Z.; Mo, X.; Lou, C.; Yao, Y.; Wang, D.; Chen, G.; Lu, J. G. IEEE Trans. Nanotechnol. 2005, 4, 238. (15) Liu, H.; Zhao, Q.; Li, Y.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D.; Yu, D.; Chi, L. J. Am. Chem. Soc. 2005, 127, 1120. (16) Harris, A. R.; Nafady, A.; O’Mullane, A. P.; Bond, A. M. Chem. Mater. 2007, 19, 5499. (17) (a) Xiao, K.; Tao, J.; Pan, Z.; Puretzky, A. A.; Ivanov, I. N.; Pennycook, S. J.; Geohegan, D. B. Angew. Chem., Int. Ed. 2007, 46, 2650. (b) Xiao, K.; Ivanov, I. N.; Puretzky, A. A.; Liu, Z.; Geohegan, D. B. Adv. Mater. 2006, 18, 2184.

Xiao et al.

Cu-TCNQ17 and Ag-TCNQ,18 whereby nanowires are grown within the device and electrical connections are made in situ during the growth process. This technique avoids the time-consuming and challenging task of manipulating nanowires into position and making electrical contacts post-synthesis. Finally, we present electrical transport measurements on self-assembled Ag-TCNQF4 nanowire devices for temperatures 100 K < T < 330 K to provide insight on the mechanisms and the applicability of theories of electrical transport in such 1D organic nanowires. Single-crystal AgTCNQF4 nanowires that are made of conjugated molecules and contain arrays of infinite, parallel, periodic 1D stacking of planar anions are of particular interest as model systems. Experimental Section Nanowire Synthesis. In general, a 50 nm thick film of Ag was deposited by e-beam evaporation onto substrates of Si, glass, and PET foil. TCNQF4 powders were loaded into a ceramic boat and the substrate was placed on the top of the ceramic boat with the Ag film coating face down. The boat with substrates was placed into a 2 in. quartz tube that was inserted into a tube furnace. The reaction temperature and time were precisely controlled. The temperature was increased to 170 or 180 °C and then kept at that temperature for 2 h. Finally, the temperature was decreased to room temperature. In the process, the argon gas flow rate was kept at 90 standard cubic centimeters per minute (sccm). Following evaporation, the TCNQF4 vapor reacts with the Ag on the substrate surface to form Ag-TCNQF4 nanowires. Nanowire Characterization. SEM observations were performed with a Hitachi S4700. TEM measurements were performed with a Hitachi HF2000 (operated at 200 kV). Samples were prepared by dropping a methanol solution of suspended Ag-TCNQF4 nanowires onto a lacey carbon film-coated Cu TEM grid. Diffraction patterns were recorded by controlling the electron dose with low illumination intensity to reduce radiation damage of the sample. Raman spectra of the Ag-TCNQF4 nanowires were recorded by a Renishaw 1000 confocal microRaman spectrometer utilizing an excitation wavelength of 633 nm. XRD analysis was performed on a PANalytical X’pert pro powder diffractometer with Cu K radiation. Rietveld analysis was performed using X’pert Highscore Plus using single crystal data supplied by O’Kane et al. Device Fabrication and Testing. Highly doped (001) Si with a 500-nm thickness thermal oxide SiO2 was used as a substrate. Patterns designed for large electrical contact pads on the substrate were made on spin-coated photoresist by photolithography and used as masks to deposit Ti/Au metal electrodes. Smaller three-layer electrodes of Ti (10 nm), Ag (50 nm), Ni (100 nm) were patterned with e-beam lithography methods to connect these large pads. Prefabricated electrodes with different gaps were thereby formed after lift off. The I-V curves of the device were tested in vacuum with a Keithley 4200 semiconductor parameter analyzer using a low-temperature probe station.

Results and Discussion All nanowires were prepared in a controllable manner from reactions between the Ag film and TCNQF4 vapor (18) Xiao, K.; Tao, J.; Puretzky, A. A.; Ivanov, I. N.; Retterer, S. T.; Pennycook, S. J.; Geohegan, D. B. Adv. Funct. Mater. 2008, 18, 3043.

Article

Figure 1. (a) SEM image of Ag-TCNQF4 nanowires prepared in an argon flow on the Ag foil at 170 °C; (b) STEM image of nanowires grown from predeposited Ag (nominally 100 nm) on a Mo TEM grid.

at low temperatures (150-170 °C). A boat containing TCNQF4 powder was heated within a tube furnace to sublime TCNQF4 in an Ar atmosphere. Silver was provided either as a Ag foil or as an e-beam evaporated Ag film on various substrates (such as Si wafer, glass, quartz, or flexible plastic). After a 2 h reaction, the Ag surface became black and fluffy, indicating the formation of dense Ag-TCNQF4 nanowire arrays. Figure 1a shows typical SEM images of the as-prepared Ag-TCNQF4 nanowire arrays on the Ag foil. The nanowires are 1-200 μm long and 30-200 nm in diameter as measured by scanning transmission electron microscopy (STEM) observations (Figure 1b). The nanowires tended to grow quasi-aligned to the normal to the substrate surface. Chemical composition imaging of individual AgTCNQF4 nanowires using energy-dispersive X-ray spectroscopy (EDS) within the STEM showed C, N, F, and Ag elements uniformly distributed throughout the nanowires. As result of the vapor-solid reaction, the Ag-TCNQF4 nanowires grew only in areas coated with Ag, readily yielding reliable patterns of nanowires. Images a and b in Figure 2 present SEM images of Ag-TCNQF4 nanowires grown from the Ag dots and lines that were patterned on a silicon substrate. The nanowire growth was found to conform to the shape of the Ag film with high fidelity. The nanowire growth direction was influenced by carefully exposing only selected regions of the silver. For example, a dense array of quasi-vertically organic nanowires could be converted to nanowire growth with horizontal alignment through the use of a growth barrier. Image c and panel e in Figure 2 show the quasi-aligned nanowire array achieved from the reaction between TCNQF4 vapor and a Ag film (50 nm thick) deposited on a Si film at 170 °C. Using Ni metal as a growth barrier on the Ag resulted in horizontal alignment of the organic nanowires. Image d and panel f in Figure 2 show the SEM image and schematic of the nanowires grown horizontally from the edge of the Ni/Ag film where the Ag metal had been exposed. The top layer of the Ag metal encapsulated by the Ni layer did not show any signs of nanowire growth from the top surface. The results show the effectiveness of the Ni layer not only as a growth barrier to confine the growth of nanowires horizontally on a planar substrate, but also as a good electrical and mechanical contact with the nanowires. The thickness of the Ag film and the carrier gas flow direction were critical to obtaining precise control on the horizontal nanowire length, density, and

Chem. Mater., Vol. 21, No. 18, 2009

4277

growth direction (see the Supporting Information Figure 2). Coated Ag films with thicknesses less than 20 nm resulted in the growth of low density horizontal nanowires. Taken together, our results demonstrate the ability to form a two-dimensional dense array of nanowires assembled with either quasi-vertical or horizontal patterns, which makes it possible to pattern arrays of Ag-TCNQF4 nanowires into designs for their integration into devices. To understand the optical properties, chemical composition, and structure of the Ag-TCNQF4 nanowires, we used UV-vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), Raman scattering, and X-ray diffraction (XRD). An absorption spectrum of the nanowires is shown in Figure 3a. There are two peaks centered around 335 and 659 nm in the spectral ranges of 300-440 nm and 540-760 nm, respectively. The peak at ∼335 nm can be attributed to neutral Ag-TCNQF40, and the peak at ∼659 nm can be assigned to the TCNQF4 anion radical of Ag-TCNQF4.19 The FTIR spectrum of the nanowires is shown in Figure 3b. The bands at 2213, 2197 cm-1 are typical of CN stretching modes, and the absorption at 1502 cm-1 can be assigned to π(CdC) ring stretching. The appearance of an absorption peak at 1502 cm-1 indicates that the TCNQF4 has changed to TCNQF4 anion radicals.20 Figure 3c shows Raman spectrum of as-grown Ag-TCNQF4 nanowires, confirming that the nanowires are Ag-TCNQF4 via the characteristic principal vibrational modes at 1453 cm-1 (C-CN wing stretch), 1644 cm-1 (CdC ring stretching) and 2224 cm-1 (C-N stretch). However, the principal vibrational modes of the Ag-TCNQF4 nanowires expected on the basis of TCNQF4 powder at 1644 cm-1 (CdC ring stretching) is, in fact, red-shifted by 16 cm-1. This is quite different from that observed for the C-CN wing stretch peak shift for the TCNQ ionization due to the much higher electron affinity of TCNQF4.17 According to previous studies, this decrease in vibrational energy can be attributed to charge transfer between atomic Ag and free TCNQF4.21 Figure 3d shows a representative XRD pattern of assynthesized nanowires on the Si substrate and demonstrates clearly that the nanowire products are highly crystalline. Rietveld analysis of the XRD pattern confirms that these nanowires were crystallized in same monoclinic structure as bulk Ag-TCNQF4, with lattice constants a=13.35 A˚, b=7.02 A˚, and c=25.78 A˚.20 The refinement also indicates strong preferential orientation favoring the [010] direction, consistent with SEM imaging observations that the nanowires tend to align perpendicular to the silver substrate. The detailed morphology, growth direction, and crystal structure of the Ag-TCNQF4 nanowires were determined by TEM and electron diffraction. Figure 4 shows the TEM images and the selected area electron diffraction (19) Kotsiliou, A. M.; Risen, W. M. Solid State Commun. 1988, 68, 503. (20) O’Kane, S. A.; Clerac, R.; Zhao, H.; Ouyang, X.; Glan-Mascaros, J. R.; Heintz, R.; Dunbar, K. R. J. Solid State Chem. 2000, 152, 159. (21) Li, Z.; Matsuzaki, S.; Onomichi, M.; Sano, M. Synth. Met. 1986, 16, 71.

4278

Chem. Mater., Vol. 21, No. 18, 2009

Xiao et al.

Figure 2. SEM images of patterned Ag-TCNQF4 nanowires on Si substrate. (a) Line-patterned nanowires. Ag thickness, 50 nm (50° tilted view); (b) nanowires grown from patterned 200 nm dots of Ag with 30 nm thickness; (c) vertically aligned Ag-TCNQF4 nanowires grown on the Si substrate without the use of growth barrier (50° tilted view); (d) horizontally aligned Ag-TCNQF4 nanowires grown on the Si substrate with the Ni layer as growth barrier. (e, f) Schematics of the vertical and horizontal growth of nanowires, respectively.

Figure 3. Characterization of the Ag-TCNQF4 nanowires. (a) UV-vis absorption spectrum, (b) FTIR spectrum, (c) Raman spectrum, (d) XRD pattern. The UV-vis absorption spectrum and FTIR spectrum were recorded at room temperature for the Ag-TCNQF4 grown on a glass substrate; XRD and Raman spectra were measured using a nanowire array grown on a Si substrate.

(SAED) pattern of single Ag-TCNQF4 nanowire. Images b and f in Figure 4 show the TEM images of the straight

Ag-TCNQF4 nanowires with a uniform diameter about 100 nm. Two electron diffraction patterns (patterns a and e

Article

Chem. Mater., Vol. 21, No. 18, 2009

4279

Figure 4. TEM images and the SAED pattern of single Ag-TCNQF4 nanowire. (a, e) Corresponding SAED patterns along the [001] and [110] zone axis taken from single Ag-TCNQF4 nanowires in panels b and f, respectively. (c, g) Derived structural model of the stacking of the a-c planes (d) along the b-axis direction of the Ag-TCNQF4 organic nanowires based upon the [001] and [110] zone axis SAED data, respectively.

in Figure 4) recorded perpendicular to the nanowires’ long axis can be indexed to the [001] and [110] zone axes of the bulk crystal structure of the Ag-TCNQF4.20 It can be concluded that the Ag-TCNQF4 nanowires are single crystal and grow along the [010] direction, which coincides with the π-π stacking direction of the TCNQF4 molecules. A schematic representation of the crystal structure of the Ag-TCNQF4 nanowire indicating the growth direction is presented in panels c and g in Figure 4. The Ag atoms in the structure are coordinated to four nitrogen atoms in a highly distorted tetrahedral environment and the quinoid rings of the TCNQF4 units are engaged in face-to-face stacking (Figure 4d). The stacking interactions are exceedingly short and are related to the electron withdrawing fluorine groups on the π-delocalized ring. Adjacent TCNQF4 stacks have relative orientation of 90° with respect to each other and the two independent networks bring the TCNQF4 molecules together to give a column stack parallel to the a-axis with the closest distance (Figure 4g). This structure is different from either phase of Ag-TCNQ. The strong π-π stacking of the Ag-TCNQF4 molecules along the b-axis direction favors the one-dimensional growth of the crystalline nanowire. This π-π stacking may enhance the chargecarrier mobility, which is believed to be favored along this direction. The vapor-solid chemical reaction responsible for the growth of the Ag-TCNQF4 nanowires, coupled with the simple growth barrier methods demonstrated above, generally offer versatile opportunities to pattern nanowires at well-defined sites on surfaces to produce preplanned oriented growth in different directions for

Figure 5. (a) Schematic of nanowires grown directly between the prefabricated electrodes. SEM images of (b) a single Ag-TCNQF4 nanowire (10 nm Ti/ 20 nm Ag/ 100 nm Ni) and (c) multiple nanowires (10 nm Ti/ 40 nm Ag/ 100 nm Ni) grown between prefabricated multilayered metal electrodes. (d) SEM image of device arrays of Ag-TCNQF4 nanowires.

integration into electrode structures for characterization or device architectures. Here, Ag-TCNQF4 nanowires were directly and selectively formed across prefabricated electrodes on Si substrates. Figure 5 shows the AgTCNQF4 nanowires grown by the reaction of TCNQF4 vapor with the Ag layer in the electrodes via the vaporsolid reaction. The electrodes were defined using electron beam lithography followed by electron beam assisted deposition of a Ti adhesion layer, a Ag reaction layer, and a Ni barrier layer. Ag-TCNQF4 nanowires were formed almost exclusively across the two prefabricated

4280

Chem. Mater., Vol. 21, No. 18, 2009

Xiao et al.

Figure 6. (a) Low bias conductivity vs T for the Ag-TCNQF4 nanowire devices; solid lines are fits to the power-law, G-T R. (b) I-V data taken at different temperatures; all curves show the power-law dependence at a temperature-dependent high bias voltage. (c) 1/T Rþ1 determined from the I-V data plotted against eV/kBT; solid line is the fitting curve based on eq 1.

electrodes. Ag-TCNQF4 nanowires with diameters between 20 and 100 nm were routinely found to bridge the opposing electrodes through the sidewalls of the Ag layer. The distances between the two electrodes are from 300 nm to 1 μm. However, for large-sized gaps (>1 μm), this was less reliable, as long nanowires do not always grow in the desired direction because of bending during the growth. Single-crystal Ag-TCNQF4 nanowires are of particular interest as model systems for transport studies because they have a high degree of crystallinity and a relatively simple structure consisting of arrays of infinite, parallel, periodic 1D stacking of planar anions, which are for some extent coupled and disordered as a result of defects. The 1D transport properties of Ag-TCNQF4 nanowires were studied by measuring the I-V curves of the devices as a function of temperature. Dozens of Ag-TCNQF4 nanowire devices were characterized, however, and most of the nanowires did not display a memory effect previously observed from Ag-TCNQ nanowires, perhaps because of the structural differences between Ag-TCNQ and AgTCNQF4. In this study, we focus on the devices mainly containing those nanowires displaying a linear currentvoltage dependence at low V and with an absence of memory effect. Figure 6a shows the temperature dependence of the conductance G as determined by measuring the dc I-V characteristics and taking the slope at zero bias for the representative devices. The conductance increases by up to over 3 orders of magnitude with increasing temperature from 100 to 300 K, displaying a typical semiconductor

behavior. A power-law (I ≈ VTR at low V and I ≈ Vβ at high V) dependence G ≈ TR is indeed observed in all the samples with a single R in the range of 9.0-2.0. Figure 6b shows the log-log plot of I-V curves at different temperatures for one of the samples. The exponent β for the power-law behavior (I - Vβ) obtained from the data in the voltage range between 7 and 14 V is slightly temperaturedependent and increases from 2.65 to 3.93 as the temperature decreases from 300 to 160 K. Power-law behavior, I - Vβ with β = 2, is seen for a space-charge limited current (SCLC) transport mechanism in semiconductors.22 The values β > 2 observed for Ag-TCNQF4 nanowires thus render the SCLC model inapplicable here. According to the Luttinger liquid (LL) theory, I-V curves taken at different temperatures should be fitted by the general equation23 I ¼ I0 T

Rþ1

    2 eV β eV Γ 1þ þiγ sinh γ kB T 2 πkB T

ð1Þ

Where Γ is the gamma function, V is the voltage bias, and γ, I0 are constants independent of T and V. The parameters R and β correspond to exponents estimated from the G(T) and I(V) plots. As shown in Figure 6C, the series of I-V curves measured at different temperatures for the same sample collapse remarkably well onto a single curve described by (22) Kao, K. C.; Hwang, W. Electrical Transport in Solids; Pergamon: Oxford, U.K., 1981. (23) Slot, E.; Holst, M. A.; van der Zant, H. S. J.; Zaitsev-Zotov, S. V. Phys. Rev. Lett. 2004, 93, 176602.

Article

eq 1 by plotting 1/T Rþ1 vs eV/kBT, where R ≈ 11.0 is the power-law exponent estimated from the G(T) ≈ T R. The exponents of the power law depend on the number of onedimensional channels;24 the LL state survives for a few onedimensional chains coupled by Coulomb interactions and the LL state can be stabilized in the presence of impurities for more than two coupled one-dimensional chains.25 Power-law behavior has been observed in various 1D systems, including molybdenum selenide nanowires, single and multiwall carbon nanotubes, and conducting polymers.26 The power-law dependences in these systems were attributed to the tunneling of electrons into a LL. Our data agree with the 1D quantum dot chain model,27 which gives a power-law with large R and R/β ratio, indicating that the Coulomb interactions in our Ag-TCNQF4 nanowires can not be neglected. Moreover, similar to conductive polymer nanofibers, the Ag-TCNQF4 nanowires are quasi-1D systems with a large numbers of nearly independent 1D conducting channels in parallel, which also contributes to the large exponents of the power law in nanowire devices. Mesoscopic fluctuations that would normally obscure the power-law behavior in strictly 1D wires are averaged out in our systems. Similar with quasi-one-dimensional systems such as polymer nanofibers, this theory implies that both tunneling along LLs through impurity barriers and tunneling between the chains with LLs provides the conduction of a set of coupled LLs. The system is characterized by measuring the LL interaction parameters from the tunneling density of states.26b LL theory predicts conduction exponents R, as (1/g - 1)/2 for impurity barriers, and (g þ 1/g - 2)/4 for interchain tunneling.28 The latter equation for the interchain tunneling allows estimating g = 0.02 for most of the Ag-TCNQF4 nanowires, which is similar to the g = 0.08 reported for conductive polymer (24) (a) Matveev, K. A.; Glazman, L. I. Phys. Rev. Lett. 1993, 70, 990. (b) Aleshin, A. N.; Lee, H, J.; Akagi, K.; Park, Y. W. Microelectron. Eng. 2005, 81, 420. (25) Mukhopadhyay, R.; Kane, C. L.; Lubensky, T. C. Phys. Rev. B 2001, 64, 045120. (26) (a) Venkataraman, L.; Hong, Y. S.; Kim, P. Phys. Rev. Lett. 2006, 96, 076601. (b) Bockrath, M.; Cobden, D. H.; Lu, J.; Rinzler, A. G.; Smalley, R. E.; Balents, L.; McEuen, P. L. Nature 1999, 397, 598. (c) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Nature 1999, 402, 273. (d) Bachtold, A.; de Jonge, M.; Grove-Rasmussen, K.; McEuen, P. :: L.; Buitelaar, M.; Schonenberger, C. Phys. Rev. Lett. 2001, 87, 166801. (e) Aleshin, A. N.; Lee, H. J.; Park, Y. W.; Akagi, K. Phys. Rev. Lett. 2004, 93, 196601. (27) Fogler, M. M.; Malinin, S. V.; Nattermann, T. Phys. Rev. Lett. 2006, 97, 096601. (28) Hausler, W.; Kecke, L.; MacDonald, A. H. Phys. Rev. B 2002, 65, 085104.

Chem. Mater., Vol. 21, No. 18, 2009

4281

nanofibers. The low g value indicates that the strong repulsive electron-electron interactions (EEI), which are characterized by g , 1, affect transport in organic nanowires, whereas for noninteracting electrons, g = 1. We believe that further experimental and theoretical studies are necessary to clarify the origin of the power law in G(T) observed in quasi 1D organic nanowires. Conclusion In summary, our study provides a general and rational approach for the patterned growth and assembly of single-crystal organic nanowires through vapor-solid chemical reactions at low temperatures. Growth barriers were employed to control the directional growth of twodimensional dense arrays of Ag-TCNQF4 nanowires into vertical or horizontal patterns. Single devices and device arrays of nanowires were then fabricated by in a singlestep in situ between prefabricated gap electrodes. The electron transport studies of the single-crystal, semiconductor Ag-TCNQF4 nanowires demonstrates that the conductance and the current-voltage characteristics follow the power law: G(T) ≈ T R with R = 9.0-12.0 and I(V) ≈ Vβ with β = 2.65-3.93. Both G(T) and I(V) show some characteristic features of similar one-dimensional systems with the existence of Coulomb interactions as Luttinger liquids. The controlled, in situ growth of these highly aligned self-assembled organic nanowires between prefabricated electrodes provides a promising tool to study electronic transport in nanostructures and may enable their integration into multiarray sensitive sensors, field-emission settings, and high-density memories. Acknowledgment. The authors gratefully acknowledge technical assistance by Pamela Fleming. We also thank Dr. Lu J. for writing the program for the LL equation fitting. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy, managed by UTBattelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725. Supporting Information Available: The molecular structure of TCNQF4; the schematic reaction of TCNQF4 and Ag; the vertically and horizontally patterned growth of Ag-TCNQF4 nanowires with or without the use of growth barrier (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.