Electrical rectification by Langmuir Blodgett Monolayers

Transcription

1 Electrical rectification by Langmuir Blodgett Monolayers R M Metzger Presented at the 8th International Conference on Electronic Materials (IUMRS-ICEM 22, Xi an, China, 1 14 June 22) 585

2 Advanced anomaterials and anodevices (IUMRS-ICEM 22, Xi an, China, 1 14 June 22) ELECTRICAL RECTIFICATIO BY LAGMUIR-BLODGETT MOOLAYERS Robert M. Metzger, Laboratory for Molecular Electronics, Chemistry Department, University of Alabama, Tuscaloosa, AL , USA Abstract. The electrical rectification by a Langmuir-Blodgett monolayer of γ-hexadecylquinolinium tricyanoquinodimethanide, C 16 H 33 Q-3CQ, 1 has been verified between Al electrodes, then Au electrodes: this is due to the asymmetry of the molecule, and the interplay between the zwitterionic ground state D + -π-a - and the less dissociated first electronic excited state D -π-a. Two new rectifers have been found recently. The first, 2,6-di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, (Bu 2 ϕv) 2 BuPy+I-, 2 is an interionic rectifier which uses the reverse charge transfer between the iodide ion and the pyridinium ring. The second is dimethylanilino-aza[c 6 ]fullerene, DMAn-C 6, 3, for which incredibly high currents (up to 1 Ampere /.283 mm 2 ) and high rectification ratios (up to 2,) are seen, probably because of the formation of Au stalagmites or other defects at grain boundaries within the monolayer. WHY WORRY ABOUT UIMOLECULAR DEVICES The interest in unimolecular electronic devices is driven by curiosity and also by the potential need for devices with small scale (1-3 nm) and fast response (< 1 ns). These devices may become a practical alternative to ever-shrinking and ever-faster Si-based integrated circuits [1]. It is possible that for design rules (i.e. distance between components) smaller than about 5 nm, insurmountable technical difficulties may arise in Si-based components, and that these difficulties may be solved by organic molecules. θ' =? C C C 1, C 16 H 33 Q-3CQ H 3 C CH 3 I (Bu 2 ϕ V) 2 BuPy + I -, 2 DMAn-C 6, 3 θ" θ" C 16 H 33 C C 16 H 33 C C C C C 1': zwitterionic ground stated + -π-a - 1": first excited stated -π-a FIGURE 1. Relevant molecular structures. 586

3 Advanced anomaterials and anodevices (IUMRS-ICEM 22, Xi an, China, 1 14 June 22) Direction of enhanced electron flow C 16 H 33 Q-3CQ, 1 D + π A - C C C Simplified molecular diagram I > I ~.6 mm Ga/In Au pad ~ 5 mm ~.6 mm 17 nm Au pad V Ga/In Au layer (5 nm thick) Cr adhesion layer enhanced electron flow under forward bias 2.3 nm glass substrate FIGURE 2. Geometry of IV measurements for LB monolayer of 1 between Au electrodes [8]..5.4 Current / µa Voltage / V FIGURE 3. I-V plot for a cell Al Al 2 O 3 monolayer of C 16 H 33 Q-3CQ, 1 Al 2 O 3 Al, with pads of area 4.5 mm 2, showing some hysteresis in the first full measurement cycle. At 1.5 V in the first cycle RR = 26.4 [4]. 587

4 Advanced anomaterials and anodevices (IUMRS-ICEM 22, Xi an, China, 1 14 June 22) Current I / milliamperes (1) (2) -1 (4) (3) Bias V / Volts FIGURE 4. I-V plot for a cell Au monolayer of C 16 H 33 Q-3CQ, 1 Au, showing the hysteresis in the first full measurement cycle. At 2.2 V in the first cycle, R = 268 Ω, I = 8.2 ma = electrons molecule -1 s -1, and RR = 6.62 [8]. RECTIFICATIO BY C 16 H 33 Q-3CQ, 1 Unimolecular rectification by an ordered Langmuir-Blodgett monolayer of γ- hexadecylquinolinium tricyanoquinodimethanide, C 16 H 33 Q-3CQ (Fig. 1, 1) has been studied for some time [2], [3], [4], [5], [6], [7], [8], first between Pt and Mg electrodes [2], [3], then between Al electrodes [4], [5], as a function of temperature [6], and finally between oxide-free Au electrodes (Fig. 2) [7], [8]. The monolayer thickness is about 2.3 nm [4].. The rectification ratio RR is defined as the ratio of the current at a large voltage V divided by the negative of the current at the corresponding negative potential V. Between Al electrodes, RR was as high as 26.4 (Fig. 3) [4]. Between Au electrodes, large currents are encountered (Fig. 4), with RR as large as 27, and currents as large as 9,4 electrons per molecule per second [8]. It seems clear that rectification in this molecule is due to the interplay between the electronic ground state, which is zwitterionic, D + -π-a -, with a large dipole moment (43 ± 8 Debyes) (Fig. 1, 1 ) [4] and the first excited electronic state, D -π-a, with a lower polarity (3 to 9 Debyes) [9] (Fig. 1, 1 ). The mechanism can be explained by a trivial modification [4] of the mechanism proposed for a molecular rectifier by Aviram and Ratner [1]. When the measurement cycle is repeated, the currents decrease, as do the RR. These changes are probably because under the large electric fields ([2.3 V/ 2. 3 nm] = 1 GV m -1 ) the molecules flip end-over-end to lower the total energy. 588

5 Advanced anomaterials and anodevices (IUMRS-ICEM 22, Xi an, China, 1 14 June 22) FIGURE 5. Current-voltage plots for (Bu 2 ϕv) 2 BuPy + I -, 2 measured in a Au LB monolayer of 2 Au cell, for six successive cycles of measurement. the rectification ratios are RR = 12, 7, 5, 4, 3, 3, for cycles 1 through 6, respectively [11]. RECTIFICATIO BY (Bu 2 ϕv) 2 BuPy + I -, 2 2,6-Di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, (Bu 2 ϕv) 2 BuPy + I - (Fig. 1, 2) can be transferred as a LB monolayer onto an Au substrate, and, using the same cold gold evaporation technique previously developed for 1 [7], [8], a second Au electrode was evaporated on top by the cold gold technique [11]. An LB multilayer of 2 exhibits an absorption maximum at 49 nm (which is slightly hypsochromic in solution) attributable to iodide to pyridinium back charge transfer, and a second harmonic signal χ (2) = 5 pm V -1 at normal incidence (λ =164 nm). The rectification is shown in Fig. 5: once again, there is a decrease of rectification upon successive cycles (RR = 12, 7, 5, 4, 3, 3 in cycles 1 through 6 respectively.). Some cells have initial RR as high as 6. The direction of enhanced electron flow is from the gegenion to the pyridinium ion, i.e. in the direction of back charge transfer, and the rectification is therefore attributed to an interionic electron transfer, rather than to an intramolecular electron transfer [11]. RECTIFICATIO BY DMAn-C 6, 3 Dimethylanilino-aza[C 6 ]fullerene, DMAn-C 6, 3 is a blue fullerene, thanks to its intramolecular charge-transfer band (or intervalence band, IVT), with λ max = 72 nm [12]. The pressure-area isotherm reveals a very rigid Langmuir, or Pockels- Langmuir film at the air-water interface, with collapse areas of about 4 Å 2 (compared to a calculated 98 Å 2 for C 6 ); this suggests that the molecules are staggered, as shown in Fig. 6, at the air-water interface, and also when transferred onto an Au electrode (and covered by a second Au pad by the cold gold technique [8]), as shown in Fig. 6 [12]. The current-voltage plots (Fig. 7) are asymmetrical, but reveal a large ohmic component (which decreases upon successive cycles), that exists between +1.5 V and + - A D e - FIGURE 6. Schematic of monolayer of DMAn-C 6 on gold substrate, including Au stalagmites. The direction of preferred electron flow under forward bias is also shown [12]. 589

6 Advanced anomaterials and anodevices (IUMRS-ICEM 22, Xi an, China, 1 14 June 22) 1.8 Current I / Amperes Bias V / Volts FIGURE 7. Current-voltage plots for DMAn-C 6, 3 measured in a Au LB monolayer of 3 Au cell, with numbers indicating the measuring sequence. The current is dominated by extrinsic effects (stalagmites, or oxidation/reduction couples at grain boundaries) [12]..6 Current I / milliamperes Bias V / Volts FIGURE 8. Current-voltage plots for DMAn-C 6, 3 measured in a Au LB monolayer of 3 Au cell, with numbers indicating the measuring sequence (RR=2). The current is dominated by intramolecular electron transfer [12]. 59

7 Advanced anomaterials and anodevices (IUMRS-ICEM 22, Xi an, China, 1 14 June 22).5 V. This ohmic component contributes a large current (up to 1 Ampere across a pad area of.285 mm 2!), but may be due to extrinsic phenomena, such as either Au stalagmites that form under bias, as shown schematically in Fig. 6, or to some oxidation/reduction couple, or some other effect [12]. Some pads show much lower currents (very likely due to the molecules themselves in the absence of stalagmites) with a rectification ratio of about 2 (Fig. 8) [12]. ACKOWLEDGMETS This research was made possible by the United States ational Science Foundation (grant SF-DMR-FRG Oligomolecular anodevices ), and with the hard work, the help, the ideas, and the friendship of many colleagues, students, and postdoctoral fellows. REFERECES [1] G. E. Moore, Electronics (19 April 1965) pp [2] G. J. Ashwell, J. R. Sambles, A. S. Martin, W. G. Parker, and M. Szablewski, J. Chem. Soc. Chem.Commun (199). [3] A. S. Martin, J. R. Sambles, and G. J. Ashwell, Phys. Rev. Lett 7, (1993). [4] R. M. Metzger, B. Chen, U. Höpfner, M. V. Lakshmikantham, D. Vuillaume, T. Kawai, X. Wu, H. Tachibana, T. V. Hughes, H. Sakurai, J. W. Baldwin, C. Hosch, M. P. Cava, L. Brehmer, and G. J. Ashwell,, J. Am. Chem. Soc. 119, (1997). [5] D. Vuillaume, B. Chen, and R. M. Metzger, Langmuir 15, (1999). [6] B. Chen and R. M. Metzger, J. Phys. Chem. B13, (1999). [7] T. Xu, I. R. Peterson, M. V. Lakshmikantham, and R. M. Metzger, Angew. Chem. Intl. Ed.. 4, (21). [8] R. M. Metzger, T. Xu, and I. R. Peterson, J. Phys. Chem. B15, (21). [9] J. W. Baldwin, B. Chen, S. C. Street, V. V. Konovalov, H. Sakurai, H., T. V. Hughes, C. S. Simpson, M. V. Lakshmikantham, M. P. Cava, L. D. Kispert, and R. M. Metzger, J. Phys. Chem B13, (1999). [1] A. Aviram and M. A. Ratner, Chem. Phys. Lett. 29, (1974). [11] J. W. Baldwin, R. R. Amaresh, I. R. Peterson, W. J. Shumate, M. P. Cava, M. A. Amiri, R. Hamilton, G. J. Ashwell, and R. M. Metzger, submitted to J. Phys. Chem. B. [12] R. M. Metzger, J. W. Baldwin, W. J. Shumate, I. R. Peterson, P. Mani, G. J. Mankey, T. A. Morris, G. J., Szulczewski, S. Bosi, M. Prato, A. Comito, and Y. Rubin, submitted to J. Am. Chem. Soc. 591