Electron Transfer through a Monolayer of Hexadecylquinolinium

P.O. Box 870336, Tuscaloosa, Alabama 35487-0336. Received February 1, 1999. In Final Form: March 19, 1999. Electron transfer through a Langmuir-Blodge...
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Langmuir 1999, 15, 4011-4017

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Electron Transfer through a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide Dominique Vuillaume* Institut d’E Ä lectronique et de Micro-e´ lectronique du Nord (IEMN), CNRS, and Department of Physics at the Institut Supe´ rieur d’Electronique du Nord (ISEN), Avenue Poincare´ , BP69, F-59652 Cedex, Villeneuve d’Ascq, France

Bo Chen and Robert M. Metzger Laboratory for Molecular Electronics, Department of Chemistry, University of Alabama, P.O. Box 870336, Tuscaloosa, Alabama 35487-0336 Received February 1, 1999. In Final Form: March 19, 1999 Electron transfer through a Langmuir-Blodgett monolayer of hexadecylquinolinium tricyanoquinodimethanide (C16H33Q-3CNQ) sandwiched between metal electrodes is studied carefully. Either current rectification (rectification ratio as high as ∼20) for positive bias applied on the metal overlayer or symmetric current voltage curves were observed for samples with the highest resistivity. More leaky devices (resistivity of about a decade lower) show rectification (with a smaller ratio e5) for a negative bias on the top electrode. These results are analyzed regarding various mechanisms: (1) Aviram and Ratner proposal for a molecular diode, (2) geometrical asymmetry of the metal/Langmuir-Blodgett monolayer/metal structure, and (3) a polarization charge density effect. This study leads us to confirm that the observed electron-transfer properties through the C16H33Q-3CNQ LB monolayers are not only due to possible geometrical asymmetry in the metal/LB monolayer/metal structure but are also related to the strong asymmetry of the donor-πbridge-acceptor C16H33Q-3CNQ molecules.

1. Introduction In 1974, Aviram and Ratner (AR) proposed the theoretical concept of a unimolecular diode, or rectifier of an electrical current.1 The molecular diode is based on an asymmetric organic molecule with structure D-σ-A, where D is a strong electron donor moiety, A is a strong electron acceptor moiety, and σ is a covalent (saturated) bridge. On the basis of the respective energy position of molecular orbitals of the donor and acceptor moieties, Aviram and Ratner postulated on an easier intramolecular electron transfer from A to D than from D to A. Therefore, when a single molecule, or an oriented monolayer of D-σ-A molecules, is sandwiched between two metallic electrodes M1 and M2 (M1|D-σ-A|M2), then a positive bias applied on M1 promotes electron transfer from the cathode (M2) to the anode (M1) along the pathway M2 f A f D f M1.1 The first step is an electron transfer from M2 to A and from D to M1, leading to the molecular excitedstate D+-σ-A-. Then an easy intramolecular electron transfer may occur from the high-lying LUMO of the A moiety to the lower-lying HOMO of the D moiety (Figure 1). Under reverse bias (M2 ) anode) the electron transfer from D to A is strongly improbable, because it would involve a large uphill inelastic tunneling. This proposal assumed that the bridge does not seriously affect the relative electron-donating/-accepting ability of D and of A; that is, in the molecular orbitals of D-σ-A the molecular orbitals of the fragments can be recognized as depicted in Figure 1 (although their ordering is affected by the formation of the single molecule). We remind ourselves of this for historical reasons;1 the more usual notation is that the whole molecule has one LUMO and one HOMO. Aviram and Ratner suggested a D-σ-A * To whom correspondence should be addressed. E-mail: [email protected]. (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277-283.

Figure 1. Schematic energy level (molecular orbitals) diagram showing the easy downhill inelastic tunneling electron transfer from the LUMO of the acceptor moiety to the HOMO of the donor moiety, according to the Aviram and Ratner model (ref 1).

prototype molecule based on D ) tetrathiafulvalene (TTF), the electron donor, and A ) tetracyanoquinodimethane (TCNQ), the electron acceptor, linked by a σ ) bicyclooctane bridge. However, this molecule was never synthesized. Metzger and co-workers synthesized many D-σ-A molecules and fabricated oriented molecular films by the Langmuir-Blodgett (LB) technique2 but no clear molecular rectification was seen. Geddes and co-workers3 observed that a LB monolayer of the dodecyloxyphenyl carbamate of 2-bromo-5 (2′-hydroxyethoxy) tetracyanoquinodimethane (DDOP-C-BHTCNQ) sandwiched between Pt and Mg electrodes behaves as a rectifying junction. However, it was suggested that this rectifying behavior may also be due to Schottky barrier formation between Mg and TCNQ (formation of Mg2+/TCNQ2- salt).4 (2) Metzger, R. M. Mater. Sci. Eng. C 1995, 3, 277-285. (3) Geddes, N. J.; Sambles, J. R.; Jarvis, D. J.; Parker, W. G.; Sandman, D. J. Appl. Phys. Lett. 1990, 56, 1916-1918.

10.1021/la990099r CCC: $18.00 © 1999 American Chemical Society Published on Web 04/30/1999

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In a previous report7 a detailed study of the electrontransfer process through the “metal/LB monolayer of C16H33Q-3CNQ/metal” sandwich was not presented. This is given in this paper on the basis of the large number of samples and the role of the oxide Al2O3 between the Al electrodes and the LB monolayer of C16H33Q-3CNQ is also discussed. The results are discussed considering the AR proposal and other possible mechanisms, including geometric asymmetry of the electrode/LB monolayer/ electrode structure and polarization charge density associated with such a zwitterionic molecule. 2. Experimental Section

Figure 2. Schematic representation of the chemical structures of the ground state (zwitterion) and excited state (neutral) of the C16H33Q-3CNQ molecule.

In 1990, Ashwell et al.5 reported a similar rectification for a “Ag/Mg/LB layer/Pt” sandwich structure using an LB monolayer and LB multilayers of Z-β-(1-hexadecyl-4quinolinium)-R-cyano-4-styryldicyano-methanide, or Nhexadecyl-γ-quinolinium tricyanoquinodimethanide (C16H33Q-3CNQ, Figure 2). When an insulating layer (ωtricosenoic acid LB film) was incorporated between the C16H33Q-3CNQ film and the Mg electrode, asymmetric J-V behaviors were still observed, and this was claimed as proof of molecular rectification against Schottky barrier formation (Mg-3CNQ salt).6 Additional proof is the fact that no rectification was observed for LB films of “bleached” molecules (i.e., molecules which had been protonated and were no longer zwitterionic). More recently, we observed that macroscopic and nanoscopic current-voltage measurements exhibit rectification in the electron transfer through LB multilayers and monolayers of C16H33Q3CNQ sandwiched between similar metal electrodes (Al).7 There are two differences between the Aviram-Ratner proposed molecule D-σ-A and the molecule C16H33Q3CNQ. First, the three-carbon σ-bridge in D-σ-A is replaced by an ethylenic two-carbon π-bridge in C16H33Q3CNQ; however, the D+ and A- moieties are not coplanar, but twisted by some unknown twist angle, thus destroying the conjugation. If this twist angle were zero, then the zwitterion D+-π-A- and the neutral state D0-π-A0 would be degenerate resonance structures. The nonzero twist angle makes the π-bridge behave as if it were a σ-bridge. The second difference is that the zwitterion is now the ground state, and the neutral state is the excited one, inverting the order of states of the Aviram-Ratner molecule. This is no difficulty. One just uses the AviramRatner intramolecular through-bond tunneling mechanism “in reverse”, as discussed in detail elsewhere:7 for D+-π-A- one first has intramolecular transfer (IMT) within the molecule (from A to D) and then electron transfer (ET) across the two metal-molecule interfaces, for D-σ-A molecules IMT follows ET. (4) Geddes, N. J.; Sambles, J. R.; Jarvis, D. J.; Parker, W. G.; Sandman, D. J. J. Appl. Phys. 1992, 71, 756-768. (5) Ashwell, G. J.; Sambles, J. R.; Martin, A. S.; Parker, W. G.; Szablewski, M. J. Chem. Soc., Chem. Commun. 1990, 1374-1376. (6) Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Phys. Rev. Lett. 1993, 70, 218-221. (7) Metzger, R. M.; Chen, B.; Ho¨pfner, U.; Lakshmikantham, M. V.; Vuillaume, D.; Kawai, T.; Wu, X.; Tachibana, H.; Hughes, T. V.; Sakurai, H.; Baldwin, J. W.; Hosch, C.; Cava, M. P.; Brehmer, L.; Ashwell, G. J. J. Am. Chem. Soc. 1997, 119, 10455-10466.

Materials. Hexadecylquinolinium tricyanoquinodimethanide (C16H33Q-3CNQ, Figure 2) was synthesized as reported elsewhere7 from N-cetyllepidinium tosylate and the lithium salt of the TCNQ radical anion, in dry DMSO solution in the presence of pyridine. The final product was characterized by 1H NMR, UV-vis spectroscopy.7 Sample Preparation. Langmuir-Blodgett (LB) monolayers were made onto aluminum electrodes. We used quartz slides covered by a 140 nm thick evaporated Al film (∼5 Å s-1, ∼2 × 10-6 Torr, Edwards EL306 coater, oil diffusion pump, base pressure about 10-6 Torr). C16H33Q-3CNQ molecules dissolved (∼0.1 mg mL-1) in methylene chloride (CH2Cl2) were spread onto the water subphase (deionized water, temperature controlled at 14 °C) on a Nima model 622D2 Langmuir trough, in an airfiltered room. Since the Al surface, unavoidably covered by a thin (∼2 nm) oxide, is hydrophilic, the Al-coated quartz slides were immersed in the water subphase before spreading the molecules, and the LB monolayers were transferred on the upstroke at a speed of 10 mm min-1 using a film pressure of 25 mN m-1 under a green safelight (C16H33Q-3CNQ molecules are very sensitive to photooxidation, with a strong absorption peak at 570 nm7). Details on the pressure isotherms of C16H33Q-3CNQ are given elsewhere.7 The existence of aluminum oxide on the base electrode makes the surface hydrophilic and allows for excellent LB transfer ratios, close to 100%. Thus, the LB monolayers are Z-type, the acceptor moiety close to the base Al electrode, and the alkyl tail oriented toward the monolayer surface. The monolayer thickness (ellipsometry) is 22 ( 2 Å.7 After the LB monolayer transfer, the samples were dried for 2 days in a vacuum desiccator containing P2O5. Counter electrodes (Al) were evaporated upward through a shadow mask atop the LB monolayers. The shadow mask was maintained a few hundred microns far-off the surface of LB monolayers to avoid mechanical destruction of the organic films. The samples were mounted on a coldfinger (77 K) at a large distance from the Al source (21 cm), and Al was evaporated very slowly (∼1 Å s-1) to minimize the generation of evaporation-induced defects in the LB monolayers. In previous work, Mg was deposited onto a LB film to minimize thermal damage of the film by the hot metal vapor impinging onto it.3-6,8 Then the Mg had to be protected by a noble metal, to prevent hydrolysis of the Mg pad in air. We found, instead,7 that cryocooling the substrate to 77 K allows Al to be used, even though the Al vapor is hotter than Mg vapor. Counter electrode areas were 3.1, 4.9, and 7.1 mm2, with thicknesses in the range 100-350 nm, depending on the batch. Then, the samples were placed for 1 day in a rotary pump evacuated desiccator containing P2O5. Contact Angles. We measured with a contact angle goniometer (GBX, France) the static contact angle of a sessile droplet (a few 10 µL) of deionized water deposited on the LB film. Measurements were made at room temperature and pressure. Electrical Measurements. The electrical measurements (dc and ac current-voltage) were performed using a computercontrolled Hewlett-Packard 3245A universal source, a HewlettPackard 3457A multimeter, and a Hewlett-Packard HP4263B LCR meter (100 Hz to 100 kHz). All measurements were made at room temperature, in the dark. The samples were connected electrically by Au wire (0.05 mm diameter) wetted by Ga/In (8) Geddes, N. J.; Sambles, J. R.; Martin, A. S. Adv. Mater. Opt. Electron. 1995, 5, 305-320.

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eutectic or Ag pastes. Voltages were applied on the counter electrode, and the base electrode was grounded. Unless specified, a low voltage scan rate (10 mV s-1) was selected, to avoid a large displacement current (I ) C (∂V/∂t)) due to the capacitive behavior of the samples. To minimize the large hysteresis effects often observed in such organic monolayers, when voltages are swept back and forth between positive and negative voltages,9,10 all measurements were started from zero.

3. Results In a first control experiment, it was checked that no asymmetric current was seen in a simple “Al/Al2O3/Al” sandwich. In a second control experiment, only symmetric J-V currents were found in a sandwich “Al/Al2O3/LB multilayer of arachidic acid C19H39COOH/Al2O3/Al”, provided that the multilayer was dried for 2 days in a vacuum desiccator containing P2O5, as described above. Typical values for the static water contact angles on our C16H33Q3CNQ LB monolayers were 100 ( 2°. For a densely packed monolayer (not a LB but a self-assembled monolayer) of methyl-terminated alkyl chains, the typical water contact angle is 110-115°;11-13 thus our measured value of 100° indicates that the C16H33Q-3CNQ molecules have predominantly their alkyl chains pointing toward the free surface (Z-type LB film). Compared to a compact methylterminated surface, some methylene groups are probably present at the surface, because of some molecular disorder, that explains the lowered water contact angle. The Al metal is thus deposited onto an organic layer that is not perfectly ordered in the plane of the film. Accordingly, many junctions, particularly when depositing atop a single monolayer, are electrically shorted. The Q-3CNQ “head group” is more bulky than the alkyl chains, which probably leave some space between one chain and the next, allowing some Al atoms (and/or oxide) to crowd into the interstices. Cooling the LB monolayer might create defects (in view of its likely thermal contraction) that lead to electrically shorted junctions. All these electrically shorted junctions were removed from consideration. Of 72 measured samples (corresponding to 6 LB transfer batches of C16H33Q-3CNQ), 37 were electrically shortcircuited before any measurement, 8 were leaky (generally, they became short-circuited during measurements), 27 were highly resistive (>1 MΩ, up to few tens of MΩ). Ten out of these 27 samples experienced breakdown (a sudden large increase in the current density when the voltage was increased too much) during current density-voltage (J-V) measurements. The 17 remaining samples survived repeatedly to under-applied voltages up to (1.8 V, and therefore, were deemed suitable for detailed study and analysis. From the J-V measurements, we identified three different behaviors carefully discussed below. Positively Rectifying Junctions. Figure 3a shows a typical rectification curve, with a much higher current at positive bias (on the counter electrode) than at a negative one. The positive rectification ratio (PRR) is defined by PRR ) J(V+)/J(V-) where J(V+) is the current density at a given positive bias and J(V-) is the current density at the corresponding negative bias. A high PRR is observed for the highest applied voltages (∼18 at 1.5 V). A positive threshold voltage of ∼1 V is clearly mandatory to observe the rectification behavior. Of our 17 “high-resistance” samples, 5 showed this behavior, with PRR in the range (9) Boulas, C. Ph.D. Thesis, University of Lille, 1996. (10) Tanguy, J. Thin Solid Films 1972, 13, 33-39. (11) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-496. (12) Wasserman, S. R.; Tao, Y.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (13) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-assembly; Academic Press: Boston, 1991.

Figure 3. Typical rectifying current-voltage characteristics (J-V) of the “Al/Al2O3/LB monolayer of C16H33Q-3CNQ/Al2O3/ Al” junction. The junction has the structure “Al (base substrate)|A-π-D-T|Al (top counter electrode)” and the bias is applied to the top counter electrode, the base substrate grounded. (a) lin-lin scale; (b) same as (a) in lin-log scale. Straight line in (b) is the fitted slope of the Ln(J) ∝ V law (see text). In (a) the back and forth traces (at a voltage scan rate of 10 mV s-1) are displayed.

3-18 at the highest applied voltage ((1.5 to (1.8 V) and a positive threshold voltage (Vth) in the range 0.8-1.3 V. A detailed analysis of the shape of the J-V curves reveals that the conduction is ohmic between approximately -0.3 and 0.3 V. In the reverse regime (negative voltage), the current follows a linear relationship Ln(J) ∝ V from -0.5 V to the highest negative bias applied (Figure 3b). In the forward regime, we also observed a Ln(J) ∝ V law for V > 0.5 V up to the threshold voltage, above which the current rises severalfold. However, above this threshold, the law remains Ln(J) ∝ V, but with a higher slope. Time relaxation phenomena were found in these samples. When a constant positive or negative bias is applied, the current decreases with time according to a J ∝ (Ln(t) law (Figure 4). The relaxation rate, ∂J/∂ Ln(t), is weak, always e10-4 mA‚cm-2 decade-1. After a long time under an electric field, the currents at both positive and negative voltages decrease and PRR tends to unity. As discussed below (see discussion) the Ln(t) relaxation may be due to electron trapping and detrapping in the oxide, but the decrease of PRR is, in our opinion, evidence that some molecular rearrangements occur in the monolayer (Al2O3 has no role to play in the rectification behavior). The J-V curves show a small hysteresis (Figure 3a). All the current measurements were performed in two steps, one by cycling the voltage from 0 to +1.5 V and back to 0 V, the second step by cycling from 0 to -1.5 V and back to 0 V. Between the two steps, the samples were biased to 0 V during a “rest” time of about 1 min. This procedure reduced the hysteresis, which may be due to mobile ionic contamination in the LB films or in the oxide.

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Figure 4. Typical time relaxation behavior of the “Al/Al2O3/ LB monolayer of C16H33Q-3CNQ/Al2O3/Al” rectifying junctions (owning typical J-V as those reported in Figure 3). The current decreases as Ln(t) for both positive and negative biases.

Figure 5. Typical current density-voltage characteristics (JV) of the “Al/Al2O3/LB monolayer of C16H33Q-3CNQ/Al2O3/Al” reverse rectifying junctions. The current at a negative bias is higher than the one at a positive bias. (a) Characteristics are plotted in a lin-lin scale at two voltage scan rates: line ) 1 mV s-1; square ) 50 mV s-1. (b). (c) Same data as (a) at 50 mV s-1 plotted as log(J) versus V1/4. Circle ) negative bias; square ) positive bias.

Negatively Rectifying Junctions. Four junctions showed a completely different behavior. The current was higher for a negative bias than for a positive one (Figure 5a). The negatively rectification ratios, NRR ) J(V-)/J(V+) were small, 1.5-4, compared to the PRR values of the positively rectifying junctions. However, their J-V shape and time relaxation were completely different from those of the former case. First, it is clear that the J-V curves do not follow a Ln(J) ∝ V behavior. They more or less obey a Ln(J) ∝ V1/n behavior with n ∼ 4 for both polarities (Figure 5b). Moreover, at low voltages (-0.5-0.5 V) the current

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Figure 6. Typical time-dependent behaviors of the “Al/Al2O3/ LB monolayer of C16H33Q-3CNQ/Al2O3/Al” reverse rectifying junction (owning typical J-V as those reported in Figure 5).

is no longer ohmic. Contrary to the case reported above, the time relaxation depends on the bias polarity (Figure 6). Under a negative bias, we again observed a decrease in the current according to J ∝ Ln(t), but the relaxation rates were now always larger than 2 × 10-4 mA‚cm-2/ decade. For the positive bias, the current stayed almost constant, or for some junctions weakly increased or decreased linearly with time. Related to this is the observation that NRR increases (Figure 6a) when the voltage scan speed is increased (typically from 1.5 at 1 mV s-1 to 4 at 50 mV s-1). This is because the current under a positive bias is not strongly affected by a dynamic process, while the current at a negative bias tends to decrease with the time elapsed during its measurement (Figure 6). Symmetric Junctions. Eight junctions out of the 17 “highly resistive” ones showed a rather symmetric J-V over the whole voltage range, or a very small rectification ratio, PRR or NRR < 2. Such a small value is not clear proof of the occurrence of rectification. Because of time relaxation behavior, the comparison between negative and positive traces (and thus RR values) may vary, depending on the measurement history. A clear relationship between small RR, time relaxation phenomena, and measurement history is the fact that junctions displaying a small RR, when measured at a low voltage scan rate, are strictly symmetric at a higher scan rate, since time relaxation phenomena have larger time constants than the J-V measurement time. Such samples, with a RR value smaller than 2, are referred to as “symmetric”. For both negative and positive bias, the current follows Ln(J) ∝ V for |V| > 0.3-0.5 V and it is ohmic below 0.3-0.5 V. Time relaxation phenomena (i.e., current decreasing as time elapses) were observed just as for the positively rectifying junctions, with the same Ln(t) law and relaxation rate lower than 2 × 10-5 mA‚cm-2‚decade-1. So, except for the current increase for positive bias larger than Vth, the electrical characteristics for these junctions are very similar to those of the positively rectifying ones. Note that the samples in all three cases exhibited a similar reproducibility. Figure 7 is a plot of the resistivities of the three classes of samples; the currents, and the resistivities are within about a decade in each case. 4. Discussion In the AR model, the rectification mechanism depends on a match (resonant tunneling) between the Fermi levels of the metal electrodes and the HOMO and LUMO levels of the molecule sandwiched between the metal layers,

Electron Transfer through C16H33-Q-3CNQ

Figure 7. Comparison of the resistivities of the three classes of samples. R+ and R- stand for a positively and negatively rectifying junction, respectively. Resistivity was calculated from the first derivative of the J-V curves around a dc bias of +1.5 V for the positively rectifying junction and symmetric junction and -1.5 V for the negatively rectifying junction.

followed or preceded by inelastic tunneling within the molecule.1 Fermi levels of inorganic metals and molecular orbitals of organic molecules are not easily matched. Fermi levels of noble metals (Au, Pt) are not close to LUMOs of organic acceptors. Fermi levels of more active metals (Mg, Al) are closer to the LUMOs of interest, but oxides will form on the metal surface, except when one uses deposition in an ultrahigh vacuum. In particular, Al will oxidize in air to form a layer of Al2O3 that is between 2 and 4 nm thick. However, this oxide (in opposition to anodized Al films) is not impervious to electron migration through filamentary defects in the oxide. But the number of current-carrying filaments across the oxide limits the current (compared to oxide-free metals). Much effort was devoted to the conductivity of “Al/Al2O3/LB multilayer/ Al2O3/Al” sandwiches.14-16 Generally speaking, the use of metal electrodes with different work functions may also hinder the observation of molecular rectification, or at least complicate the interpretation of the current voltage (J-V) curves of the metal/LB film/metal junctions. It is well-known that higher currents are always observed in metal-insulator-metal junctions when a positive bias is applied on the electrode with the lower work function. This consideration leads us to use Al for both electrodes. The existence of an aluminum oxide will increase the resistance of the metal/LB monolayer/metal junction (compared to oxide free metals), but in any case, it cannot explain a rectification behavior. “Al/Al2O3/Al” junctions have symmetrical J-V curves. Positively Rectifying Junctions. The “Al/Al2O3/ C16H33Q-3CNQ/Al2O3/Al” junctions belonging to the first family (rectification ratio observed for the positive bias) seem to behave according to the modified Aviram and Ratner model of rectification in an A-π-D molecule (vide supra, Introduction).7 Since the Al bottom electrode is hydrophilic, the junctions have the following structure: “Albase/Al2O3/A--π-D+-T/Al2O3/Altop” where A- is the tricyanoquinodimethanide acceptor moiety, π is the π-electron bridge, D+ is the quinolinium donor, and T is the alkyl tail. Thus, upon the application of a positive bias on the top electrode, the electric field promotes electron transfer from the acceptor to the donor. The interesting (14) Roberts, G. G.; Vincett, P. S.; Barlow, W. A. J. Phys. C: Solid State Phys. 1978, 11, 2077-2085. (15) Tredgold, R. H.; Vickers, A. J.; Allen, R. A. J. Phys. D 1984, 17, L5-L8. (16) Geddes, N. J.; Sambles, J. R.; Parket, W. G.; Couch, N. R.; Jarvis, D. J. J. Phys. D 1990, 23, 95-102.

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feature that a certain threshold voltage has to be overcome to observe the rectification effect may be related to the energy barriers at the electron-injecting electrode (the “Albase/Al2O3/C16H33Q-3CNQ” interface). Under a forward bias, we can assume that the positive electric field has to move the LUMO down to allow the electron to elastically tunnel from Al to the LUMO of the molecule (here we consider that the molecule has one LUMO and one HOMO, and we do not discuss on the relative position of the molecular orbitals of its acceptor and donor fragments). Using the Gauss law, we can calculate the electric field distribution in the oxide and in the LB monolayer and the average potential Vmol in the C16H33Q-3CNQ molecules (we assume that Vmol is the potential at the middle of the LB monolayer). The total applied potential V is V ) Foxdox + FLBdLB where F and d are the electric field and the thickness (subscript “ox” and “LB” refer to the oxide and LB monolayer, respectively), and with Fox ) (LB/ox)FLB. If we consider a symmetric structure with the same oxide thickness at both interfaces, Vmol is simply V/2. If we consider an asymmetric one, let us say with no oxide at the interface between the LB monolayer and the top electrode to set a limit case, we have

Vmol )

( (

)

dLB V LB dox + + LBdox + oxdLB 2

)

dLB (1) 2

(ox - LB)

We need an estimate of LB from the high-frequency capacitance of our samples. Capacitance values for all samples are rather frequency-independent (within a factor of 1.5) in the range from 100 Hz to 100 kHz and are in the range 0.6-1 µF/cm2. If we assume a thickness of 2-4 nm for the aluminum oxide, with a dielectric constant of 8,17 and using dLB ) 2.2 nm, these values lead to a dielectric constant of 3.7 ( 1.9 for the C16H33Q-3CNQ monolayer. A value larger than that of monolayers of alkyl chains (2-2.5 13) is expected, because of the presence of the dipolar Q-3CNQ moiety, and the large dipole moment of the ground state of C16H33Q-3CNQ. We see that Fox is only about half of FLB, thus two-thirds of the total electric field is applied on the LB monolayer. All these data allow us to estimate that Vmol is comprised between V/2 and V/1.4, and thus at the onset of rectification (Vth from 0.8 to 1.4 V), the energy difference between the aluminum Fermi energy and the LUMO of C16H33Q-3CNQ is 0.4-1 eV. The work function of aluminum is - 4.2 eV (relative to vacuum level); thus, the LUMO level is ELUMO ) -3.5 ( 0.3 eV. This result clarifies previous similar estimates (-3.0 ( 0.3 eV) made from measurements on LB multilayers and monolayers of C16H33Q-3CNQ with various metal electrodes (Al, Mg, HOPG),7 but for which the aluminum oxide was not taken into account. Both these two values remain deeper than the calculated electron affinity of C16H33Q-3CNQ that are -2.4 eV (PM3) and -2.7 eV (LDA18). Below this threshold voltage (or in the reverse bias regime), the current flows via tunneling (Figure 3b), as revealed by the classical tunneling law Ln(J) ∝ V when V is lower than ∆/e (with ∆ the effective tunneling energy barrier and e the electron charge).19-22 The slope of the (17) Fisher, J. C.; Giaever, I. J. Appl. Phys. 1961, 32, 172. (18) Krzeminski, C.; Delerue, C. Private communication. (19) Simmons, J. G. J. Appl. Phys. 1963, 34, 2581-2590. (20) Simmons, J. G. J. Appl. Phys. 1963, 34, 238-239. (21) Simmons, J. G. J. Appl. Phys. 1963, 34, 1793-1803. (22) Stratton, R. J. Phys. Chem. Solids 1962, 23, 1177-1190.

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Ln(J)-V plot is simply

a)

Rd 4x∆

(2)

where d is the tunneling distance and R is a constant that depends on the effective mass of the electron. In principle, the value of ∆ can be deduced from the slope. This is not very accurate in practice because of the large uncertainty in the effective mass in such a complicated heterostructure, and in the real thickness of the oxide. Nevertheless, note that the slope in both regimes (reverse and forward below Vth) are quite similar, |a| ) 3.4 ( 0.1 and 4.4 ( 0.05, respectively (Figure 3b). These values are reproducible from junction to junction. This symmetry is due to the use of the same metal for both electrodes. In the forward regime above Vth, the current behavior always follows a Ln(J) ∝ V (Figure 3b), but with a higher slope (a ) 7.1 ( 0.05). This suggests that the effective energy barrier ∆ has been lowered (by ∼60%) by the onset of the rectification. Below Vth, if we consider the difference between the calculated ionization energy of C16H33Q-3CNQ and the aluminum Fermi energy, we get ∆ ) 1.5-1.8 eV, and thus a reduction by 60% gives an effective barrier of 0.6-0.72 eV, a value in very close agreement with that determined above from the Vth values. Note that the Ln(J) ∝ V law was also observed for a negative bias on “Mg/ C16H33-Q-3CNQ monolayer/Ag”.3,4,8 Contrary to these authors, we have not observed the unexplained Ln(J) ∝ V3 law for the forward bias. Symmetric Junctions. For the family of “Al/Al2O3/ C16H33Q-3CNQ/Al2O3/Al” junctions displaying a symmetric J-V characteristic, a Ln(J) ∝ V law is observed with a rather similar energy barrier (|a| ∼ 3.7-3.9). The fact that no rectification was seen is ascribed to a possible “bleaching” of the molecules due to ionic contamination in the subphase during LB fabrication or during light exposure to the rest of the process and measurements. Note that, except for the increased current above a certain threshold voltage, these samples exhibited characteristics similar to those of the positively rectifying junctions; in particular, the resistivity of these samples is the same as that for positively rectifying junctions: 1012-1013 Ω‚cm (Figure 7). Negatively Rectifying Junctions. For the family of “Al/Al2O3/C16H33Q-3CNQ/Al2O3/Al” junctions exhibiting a negatively rectifying ratio (NRR), a completely different J-V law is observed. The Ln(J) ∝ V1/n with n ∼ 4 has been previously observed in LB mono- and multilayers sandwiched between metal electrodes.3,4,14-16 Even though the physical meaning of this law is not clear, such a behavior has been attributed to conduction dominated by defects in the LB films.15 This hypothesis is consistent with our present observation that the samples exhibiting the negative rectification behavior also have the lowest resistivity (1010-(3 × 1011) Ω‚cm against 1012-1013 Ω‚cm for the symmetric and AR junctions), Figure 7. Also, note that the negatively rectifying junctions have been found in two distinct batches out of the six we made. They were mixed with symmetric junctions, but never with positively rectifying junctions. Positively rectifying junctions, again mixed with symmetric ones, were observed in three other batches, while one batch exhibited only symmetric junctions. The fact that positively rectifying junctions and negatively rectifying junctions were found in distinct batches (except one sample that is discussed below) leads us to the conclusion that small irreproducibilities in the substrate preparation and LB transfer are responsible for a more disordered molecular architecture in the

Figure 8. Evolution with time of the behavior of one sample from a positively rectifying junction (fresh sample, square) to a negatively rectifying junction (aged sample, triangles).

negatively rectifying junctions than in the positively rectifying junctions. For instance, we can surmise that more than 50% of the molecules are “upside down” in these LB monolayers (i.e., the acceptor moiety is close to the top electrode). Molecular rearrangements in the monolayer may arise after the LB deposition, during storage periods, during metal deposition, or even during electrical measurements, because of possible interaction between dipole moments and the applied electric field, if the molecular density in the monolayer is low enough to allow molecules to have enough free volume to bend and “flip-flop”. Supporting these hypotheses is the observation of a chance from a positively rectifying behavior to a negatively rectifying one after several measurement cycles (Figure 8). This sample was a positively rectifying junction with a high resistivity (1012 Ω.cm) when freshly prepared, and it turned to be a negatively rectifying junction with a lower resistivity (2 × 1011 Ω‚cm) after several measurement cycles. More evidence of the peculiar feature of the negatively rectifying junctions is their time relaxation behaviors, which markedly differ from those of symmetric and positively rectifying junctions (compare Figures 4 and 7). For these two latter cases, it is difficult to say whether the Ln(t) behavior is due to molecular dynamics in the LB monolayers or due to trapping-detrapping of electrical charges in the oxide between the LB monolayers and electrodes. It is well-known that a Ln(t) relaxation is due to electron trapping-detrapping on oxide defects via tunneling from the metal electrodes, with a tunneling time constant exponentially dependent on defect depth distribution in the oxide.23,24 In addition to time relaxation due to oxide traps, the different behavior observed for the negatively rectifying junctions may be due to possible molecular rearrangements in the LB film. This may explain the larger relaxation rate observed for the negatively rectifying junctions under large applied negative electric fields. Under positive bias, the reason for the quasi time-invariant behavior of the current is not understood. In more densely packed films (as those exhibiting positively rectifying and symmetrical junctions) these molecular rearrangements are inhibited, and only oxide trap contributions are observed. More experiments are needed to analyze this time relaxation data in detail. Note that our Ln(t) current decrease is similar to that reported for the DDOP-C-BHTCNQ monolayer under a negative bias;3,4 however, we have never observed a huge (23) van Staa, P.; Rombach, H.; Kassing, R. J. Appl. Phys. 1983, 54, 4014. (24) Heiman, F. P.; Warfield, G. IEEE Trans. Electron Devices 1965, ED-12, 167.

Electron Transfer through C16H33-Q-3CNQ

Figure 9. Schematic representation of current asymmetries coming from geometric asymmetries of the metal/molecule/ metal structure.

current increase for a positive bias as also reported by these authors. Alternative Explanations of the Asymmetry. (i) Current-voltage asymmetries may result from geometrical asymmetries in the “electrode/molecules/electrode” structure.25-27 In our case, a positively rectifying junction may arise if the C16H33Q-3CNQ molecules are closer to the top electrode than to the bottom one (Figure 9). In that case we assume that Vmol is at the same potential as the top electrode, and a small positively bias is required to align the LUMO with the Fermi energy of the bottom electrode. While the HOMO of C16H33Q-3CNQ is deeper in energy (-8 eV from PM3; - 6.9 eV from LDA 18), a larger negative bias is required to align the HOMO with the Fermi energy. A negatively rectifying junction arises from the reverse case, while a symmetric junction arises if the molecule is equidistant from both electrodes. In our case, these different structures may exist because of different oxide thicknesses at both electrodes that are not well-controlled. However, with this model it is difficult to explain the observed conversion from a positively to a negatively rectifying junction. Although such geometrical asymmetry may be present in our samples, molecular asymmetry is also required to explain all our experimental observations. (ii) The zwitterionic C16H33Q-3CNQ molecules have a high dipole moment7 leading to the existence of a built-in potential in the “Al/Al2O3/C16H33Q-3CNQ/ Al2O3/Al” device. For our Z-type LB monolayers, a positive bias on the top electrode adds an external potential to the built-in potential, while a negative bias subtracts it. Thus, a higher current is expected at a positive bias as observed in our positively rectifying junctions. This effect is (25) Pomerantz, M.; Aviram, A.; McCorkle, R. A.; Li, L.; Schrott, A. G. Science 1992, 255, 1115-1118. (26) Ottaviano, L.; Santucci, S.; Nardo, S. D.; Lozzi, L.; Passacantando, M.; Picozzi, P. J. Vac. Sci. Technol. B 1997, 15, 1014-1019. (27) Kergueris, C.; Bourgoin, J.-P.; Palacin, S.; Esteve, D.; Urbina, C.; Magoga, M.; Joachim, C. Phys. Rev. B, in press.

Langmuir, Vol. 15, No. 11, 1999 4017

molecular by nature, even if it is not explained in terms of energy levels as in the AR model. The above discussions for symmetric and negatively rectifying junctions also apply in this case. The polarization charge density28 Qp ) µ/Ω with µ the dipole moment and Ω the volume of a molecule is related to the current voltage asymmetry. If we define ∆V ) V+ - V- as the difference between the positive bias V+ and the negative one V- given the same level of current, we have Qp ) C∆V. In our positively rectifying junction, we have ∆V ∼ 0.5-1 V (see Figure 3b) and Ω for a densely packed C16H33Q-3CNQ monolayer is about 33 × 7 × 4.5 Å3;7 thus, the corresponding dipole moment should be 1.5-3 D. Such a value is much lower than the measured 43 ( 8 D.7 However, this latter value comes from measurements in dilute solutions of molecules, while we consider a dense array of molecules sandwiched between metallic electrodes, a situation for which screening effects have to be accounted for. This will be further discussed elsewhere.29 Conclusion The electrical behaviors of C16H33Q-3CNQ LangmuirBlodgett monolayers sandwiched between aluminum electrodes has been extensively studied on a large number of samples. Three classes of behaviors have been observed. Current rectification (rectification ratio as high as ∼20) for a positive bias applied on the metal overlayer and symmetric current voltage curves has been observed for samples with the highest resistivity. More leaky devices (resistivity of about a decade lower) have shown rectification (with a smaller ratio e5) for a negative bias on the top electrode. The positively rectifying junctions seem in qualitative agreement with the Aviram and Ratner proposal of the molecular diode, while others mechanisms were also discussed. The positive rectification mechanism is triggered by the application of a forward bias above a threshold voltage that corresponds to an energy offset between the Fermi energy of the metal (aluminum) and the LUMO of the molecule of 0.4-1 eV. Time relaxation behaviors of the current are similar for the positively rectifying and symmetric junctions, while they markedly differ for the negatively rectifying junctions. Also an “aging” effect turning a positively rectifying junction to a negative one has been reported. Both these results lead us to confirm that the observed electron-transfer properties through the C16H33Q-3CNQ LB monolayers are not only due to possible geometrical asymmetry in the metal/LB monolayer/metal structure but are also related to the strong asymmetry of the zwitterionic donor-π-bridgeacceptor C16H33Q-3CNQ molecules. Acknowledgment. One of us (D.V.) wishes to thank NSF and DoE for financial support during his sabbatical leave at the University of Alabama. LA990099R (28) Larkins, G. L.; Fung, C. D. Thin Solid Films 1989, 179, 319325. (29) Krzeminski, C.; Delerue, C.; Vuillaume, D.; Metzger, R. M. Manuscript in preparation.