Mn12 Molecular Redox Array Exhibiting One-Dimensional Coulomb

Apr 24, 2012 - dimensional (1D) Coulomb blockade (CB) model. The Vth value is 280 mV at 10 K and decreases linearly with increasing temperature...
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Mn12 Molecular Redox Array Exhibiting One-Dimensional Coulomb Blockade Behavior Yoshiaki Hirano,†,‡ Yuji Segawa,‡ Fumihiko Yamada,‡ Takayoshi Kuroda-Sowa,§ Tomoji Kawai,‡ and Takuya Matsumoto*,†,‡ †

Department of Chemistry, School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043 Japan Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan § Department of Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ‡

S Supporting Information *

ABSTRACT: We have found that nonlinear current−voltage characteristics (I−V curves) are observed in the Mn12 {[Mn12O12(O2CC6H5)12(O2CC6H4NH2)4(H2O)4]·2(CH2Cl2)} molecular redox array in the temperature range from 10 to 300 K. Among them, I−V characteristics with threshold voltages (Vth) are clearly observed from 10 to 80 K. The I−V curves with Vth can be well fitted by applying the onedimensional (1D) Coulomb blockade (CB) model. The Vth value is 280 mV at 10 K and decreases linearly with increasing temperature. These results indicate that each Mn12 acts as a CB element in the 1D array at 80 K or below. Thus, it is suggested that the Mn12 molecular redox system can be described by the CB behavior.

1. INTRODUCTION There has been a growing trend toward novel principle-based devices that will handle next-generation information processing and communication.1−5 One of the most attractive research subjects is the realization of devices that act according to the single-electron tunneling (SET) phenomena. Neural networks or cellular automaton can be constructed utilizing the SET phenomena. These devices are quite different from transistors that have been used in conventional integrated circuits. The advantages of realizing integrated circuits using devices based on the SET phenomena are low consumption of electricity, relaxation of action limitations, and reduction of fever.1−5 The basis of SET phenomena is to tune the tunneling of each electron in a desired manner by the fact that a Coulomb blockade (CB) is artificially produced or destroyed.1−3 While the CB behavior is well known to be observed in various conducting nanostructures with the size of a few nanometers, only two device prototypes with metal array structures have been reported and proposed to date. One is the fabrication of a two-dimensional (2D) metal−insulator−metal tunnel junction array using a lithographic technique.1,6 The other is based on a self-assembled thin-film array7 or a network8 composed of metal nanoparticles. In contrast, our group has focused on the emergence of electrical properties in the redox array system consisting of Mn12 {[Mn12O12(O2CC6H5)12(O2CC6H4NH2)4(H2O)4]·2(CH2Cl2), Figure 1a}, since redox systems of Mn12 are expected to behave as a CB (Figure 1b). A schematic illustration of the device consisting of the Mn12 molecular © 2012 American Chemical Society

redox array and two gold (Au) nanogap electrodes is shown in Figure 1c. The use of the Mn12 molecule yields the following advantages. First, Mn12 possesses a regular size (≑2 × 2 × 2 nm3) and a stable energy level, because the structure of a Mn12 molecule is determined in advance, and the redox system exists in the center portion. Second, since we utilize self-assembly, Mn12 array structures could be systematically controlled, and device fabrication with low-cost, high-speed, and clean processes can be achieved. Thus, a device prototype based on the Mn12 molecular redox array with these merits will create a next-generation architecture for future molecular electronics. In this paper, we report nonlinear current−voltage characteristics (I−V curves) with threshold voltages (Vth) in the Mn12 molecular redox array from 10 to 80 K, the interpretation of I−V curves with Vth by applying the one-dimensional (1D) CB model, and the description by the CB behavior of the Mn12 molecular redox system. As shown in Figure 1a, Mn12 possesses four Mn4+ (green color, spin quantum number S = 3/2 per Mn4+) in the center, and eight Mn3+ (brown color, S = 4/2 per Mn3+) surrounding the center.9−11 Four aniline groups reside on the exterior of the Mn12 molecule (yellow color), because 16 benzoic acid groups were substituted by 4 equiv of p-aminobenzoic acid (see the Experimental Details and the Supporting Information, Figure SI.1). While Mn12 is well known to be a single-molecule Received: February 22, 2012 Revised: April 18, 2012 Published: April 24, 2012 9895

dx.doi.org/10.1021/jp301778r | J. Phys. Chem. C 2012, 116, 9895−9899

The Journal of Physical Chemistry C

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Figure 1. (a) Structure of the Mn12 molecule. (b) Energy diagrams of the Coulomb blockade and the redox system of Mn12. (c) Schematic representation of the device composed of the Mn12 molecular redox array and two gold nanogap electrodes. The red line corresponds to the onedimensional (1D) current path produced by the Mn12 molecular array redox system at 80 K or below. (d) AFM image of the Mn12/poly(dA)poly(dT)DNA complex fixed on the SiO2 substrate.

magnet9−11 or single-molecule transistor,12 our focus is on the charge transport redox system in the Mn12 molecular redox array. For the formation of the Mn12 molecular redox array, we utilized DNA for the scaffolding. Since phosphate groups exist on the exterior of DNA, it was predicted that the phosphate group would bind to the aniline groups of Mn12 by Coulombic interactions, resulting in DNA covered by Mn12 (see the Supporting Information, Figures SI.1−SI.4). Among the various DNA species, we selected the artificial poly(dA)-poly(dT)DNA (bp = 50 mer), which consists of only adenine (A) and thymine (T). The length of poly(dA)-poly(dT)DNA is approximately 17 nm, which is much shorter than that (about 16 μm) of natural λDNA. Thus, it was anticipated that 1D structures of the Mn12/poly(dA)-poly(dT)DNA complex would be formed by blowing dry nitrogen gas from one direction against the mixed solution of Mn12 and poly(dA)-poly(dT)DNA complex dropped on a SiO2 substrate.13

C114H96Cl4Mn12N4O48 (as a bisdichloromethane solvate): C, 44.30; H, 3.13; N, 1.81%. Found: C, 43.93; H, 3.16; N, 1.91%. 2.2. Preparation of Mn12 and DNA Solutions. A 1.1 mg portion of Mn12 powder was added to 1 mL of ultrapure water, and then the Mn12 powder was well dispersed by ultrasonic waves for 5 min. Next, the solution was centrifuged at 10 000 rpm for 15 min at room temperature to remove the insoluble Mn12 powder from the ultrapure water. The concentration of the clear layer at the top of the solution was measured to be 0.128 mM by UV−visible near-infrared absorption spectroscopy and inductively coupled plasma-atomic emission spectrometry. Poly(dA)-poly(dT)DNA (bp = 50 mer) composed of adenine and thymine was purchased from Amersham Pharmacia Biotech Inc. (Uppsala, Sweden). The length of the DNA was approximately 17 nm. The poly(dA)-poly(dT)DNA powder was dissolved in sterilized water at a concentration of 250 μg/mL. 2.3. Preparation of Mn12/DNA Complex and Its Fixing Method on Substrates. The Mn12 solution was added to the DNA solution at a concentration ratio of [Mn12]/[phosphate group in poly(dA)-poly(dT)DNA] = 1:6. The mixed solution of Mn12/poly(dA)-poly(dT)DNA was allowed to sit for a day. The fixing method of the Mn12/DNA complex on the substrate is as follows (see the Supporting Information, Figure SI.5). A 15 μL portion of the mixed solution was dropped onto a SiO2 substrate (100) that had been pretreated by ozone-UV cleaner for 60 min, and the solution was maintained under 100% humidity for 30 min. Any excess solution was then eliminated from one direction using dry nitrogen gas (0.1 MPa). 2.4. AFM Measurements. AFM measurements were carried out using a JEOL SPM 4200. AFM images of the Mn12/poly(dA)-poly(dT)DNA complex on the SiO2 substrate, and of Mn12, poly(dA)-poly(dT)DNA, and the Mn12/poly(dA)-poly(dT)DNA complex on the cleaved mica substrate

2. EXPERIMENTAL DETAILS 2.1. Synthesis and Characterization of Manganese Complex {Mn12, [Mn12O12(O2CC6H5)12(O2CC6H4NH2)4(H2O)4]·2(CH2Cl2) = 3091.1 (g/mol)}. Four equivalents of p-aminobenzoic acid (20 mg, 0.14 mM) was added to a solution of [Mn 12 O 12 (O 2 CPh) 16 (H 2 O) 4 ] (100 mg, 0.035 mM) in CH2Cl2 (50 mL). The solution was stirred overnight in a closed flask, then evaporated to dryness. A minimum amount of CH2Cl2 was added to the residue, and the solution was stirred for 4 h and filtered to remove any undissolved solid. Hexane was added to the filtrate until precipitation of a dark brown solid was observed. The resulting solid was collected by filtration, and the above treatment was repeated twice to ensure the substitution reaction. The yield was 85%. Anal. Calcd for 9896

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temperature-dependent I−V curves of the Mn12/poly(dA)poly(dT)DNA complex from 10 to 300 K. The I−V curves with Vth at 10, 50, and 80 K added in the inset will be discussed later. Since the electric property of poly(dA)-poly(dT)DNA under vacuum is that of an insulator,15 the observed I−V curves must originate from the Mn12 molecular array. At first glance, the I−V curve with a weak nonlinear property can be seen even at 300 K. All the I−V curves were analyzed by the Frankel− Poole conduction model based on the hopping conduction mechanism. In this conduction model, the I−V relationship can be written as

were measured with the tapping mode. A silicon cantilever was used. 2.5. Fabrication of Nanogap Electrodes. The gold electrodes on the Mn12/poly(dA)-poly(dT)DNA complex fixed on the SiO2 substrate were formed by the angled deposition method under vacuum, as reported by Otsuka et al.14 (see the Supporting Information, Figures SI.6 and SI.7). 2.6. Current−Voltage (I−V) Measurements. The current−voltage (I−V) measurements at room temperature were performed using the prober system of a Keithley 4200SCS under a vacuum of 5 × 10−2 Pa or less. Temperaturedependent I−V characteristics were measured at intervals of 10 K from 10 K up to 300 K with a Quantum Design, Ever Cool physical properties measurement system under 5 × 10−2 Pa or less.

⎛I⎞ ln⎜ ⎟ ∼ V (1/2) ⎝V ⎠

(1)

where I and V are the current and bias voltage, respectively.16 Not only barriers between Mn12 molecules but also trap levels are involved in the concept of the hopping model, and both the barriers and the trap levels are changed by the externally applied potential V (see the Supporting Information, Figure SI.8). Figure 2b shows ln(I/V) at 200, 250, and 300 K as a function of V(1/2), assuming the Frankel−Poole conduction model. In the inset, the results from 10 to 300 K are added. At 250 and 300 K, a linear dependence is observed, indicating Frankel− Poole conduction based on the hopping process. At 200 K, the deviation from a straight line is recognized around smaller V(1/2) values, becoming greater upon decreasing the temperature from 200 to 10 K. These results suggest the other electric conduction mechanism but the Frankel−Poole conduction gradually dominates at 200 K or below. Moreover, we have estimated the activation energy from an Arrhenius plot using values of I−V characteristics at 0.1 V. The equation can be expressed as

3. RESULTS AND DISCUSSION The AFM image of the Mn12/poly(dA)-poly(dT)DNA complex fixed on the SiO2 substrate is shown in Figure 1d. The 1D structure along the top and bottom sides of the image is achieved by a simple solution process. The average height of the complex is approximately 2 nm, suggesting the formation of the Mn12/poly(dA)-poly(dT)DNA complex (see the Supporting Information, Figures SI.1−SI.4). The I−V measurements of Mn12/poly(dA)-poly(dT)DNA were then carried out by fixing two Au nanogap electrodes on the upper and lower sides of the 1D structure, as shown in Figure 1d, with the top contact configuration by angled deposition under vacuum.14 Figure 2a shows the results of

⎛ E ⎞ I ∼ V exp⎜ − a ⎟ ⎝ kBT ⎠

(2)

where I, V, Ea, kB, and T are the current, bias voltage, activation energy, Boltzmann constant, and temperature, respectively.17 The graph of ln(I/V) as a function of 1/T by applying eq 2 is shown in Figure 3. The estimated Ea values are 32.4 and 20.3 meV from 150 to 300 K and from 90 to 150 K, respectively. These results suggest a decrease in the path of the hopping conduction by thermal activation upon decreasing temperature.

Figure 2. (a) Temperature-dependent current−voltage characteristics (I−V curves) of the Mn12 molecular redox array in the Mn12/ poly(dA)-poly(dT)DNA complex from 10 to 300 K. In the inset, I−V curves at 10, 50, and 80 K are displayed. (b) ln(I/V) at 200, 250, and 300 K as a function of V(1/2), assuming the Frankel−Poole conduction model. The results from 10 to 300 K are presented in the inset.

Figure 3. ln(I/V) as a function of 1/T by applying the Arrhenius equation. 9897

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At 80 K or below, the Ea value is undefined because zero conductance is involved within Vth, as shown in the inset of Figure 2a. Although we examined the interpretation of I−V curves with Vth using tunneling-based models, including cotunneling,14 direct tunneling,17 and Fowler−Nordheim tunneling18 mechanisms, no good agreement was obtained. We have found that the sole model that can explain the I−V curves with Vth is the low-dimensional CB one. This model is represented as

I ∝ {(V /Vth) − 1}ζ

pathways. Thus, our result suggesting a 1D CB pathway is in good agreement with the morphology of a Mn12 molecular redox array in the Mn12/poly(dA)-poly(dT)DNA complex, as shown in the AFM image of Figure 1d. As shown in Figure 5a, the I−V curves at 10, 50, and 80 K from Figure 2a are well fitted by the 1D CB model, postulating

(3)

where I, V, Vth, and ζ are the current, bias voltage, threshold voltage, and index indicating the dimension of the electrical conduction path, respectively.7,8,19−21 Vth is equal to ∑Ecn, where Ecn corresponds to the charging energy of each CB element in the current path.7,8,19−21 The schematic illustration of the 1D and two-dimensional (2D) CB arrays is represented in Figure 4a. Arrows in the 2D CB array correspond to current

Figure 5. (a) I−V curves at 10, 50, and 80 K fitted by the 1D CB model. (b) Vth as a function of temperature.

ζ = 1.9. According to ref 7, the temperature dependence of Vth in the low-dimensional CB model is described as Vth(T ) = Vth(0) × (1 − αT )

(4)

where Vth(T), Vth(0), α, and T are the temperature-dependent threshold voltage, threshold voltage at 0 K, coefficient, and temperature, respectively. Equation 4 suggests that the Vth(T) value disappears due to the increase in thermal activation in higher-temperature regions. Figure 5b shows the results of Vth(T) as a function of T. The Vth value is estimated as 280 mV at 10 K. In addition, the Vth(T) values at each T show a linear dependence, and Vth is expected to disappear in the region between 80 and 90 K. Vth(0) and α values are estimated to be 309 mV and 0.0117, respectively. Consequently, the results in Figures 4b and 5a,b indicate that individual Mn12 molecules act as CB elements in the 1D array at 80 K or below. If we assume that the Ecn value of Mn12 is approximately equal to the Ea value (20.3 meV) estimated from 90 to 150 K (see Figures 1b and 3, and Supporting Information, Figure SI.8), then the number of Mn12 molecules between the Au nanogap electrodes is calculated to be 14. In addition, the distance between electrodes is 28 nm, postulating that the size per Mn12 molecule is 2 nm. The effective gap distance estimated is shorter than the gap length (50−100 nm),14 which is obtained by the angled deposition under vacuum. This distance is possibly caused by

Figure 4. (a) Schematic illustration of 1D and two-dimensional (2D) Coulomb blockade (CB) arrays. Arrows in the 2D CB array correspond to current paths. The trend of I−V curves characteristic of 1D and 2D CB arrays is shown, where Vth is the threshold voltage. (b) ln(I) plotted against ln[(V/Vth) − 1] at 10 K, postulating eq 3 and Vth = 250 mV.

paths. The trend of the I−V curves characteristic of the 1D and 2D CB arrays is also added. It is noted that the I and Vth values in the 2D CB array become larger than those in the 1D array due to the presence of many current paths. According to the theoretical estimation, ζ values for 1D and 2D CB systems are known to be 1 and 5/3, respectively.7 Figure 4b shows ln(I) as a function of ln[(V/Vth) − 1] at 10 K, assuming eq 3 and Vth = 250 mV. We obtained ζ = 1.9 from the slope of the fitted straight line. The ζ value is consistent with that obtained for multilayer films of benzenedimethanethiol-linked 5.3 nm Au nanoparticles constructed on 20 nm spaced electrodes (ζ = 1.7),7,22 where the current in the films was interpreted to be mainly passing through quasi-1D 9898

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(9) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141−143. (10) Gatteschi, D.; Caneschi, A.; Pardi, L.; Sessoli, R. Science 1994, 265, 1054−1058. (11) Bian, G.-Q.; Kuroda-Sowa, T.; Konaka, H.; Hatano, M.; Maekawa, M.; Munakata, M.; Miyasaka, H.; Yamashita, M. Inorg. Chem. 2004, 43, 4790−4792. (12) Jo, M.-H.; Grose, J. E.; Baheti, K.; Deshmukh, M. M.; Sokol, J. J.; Rumberger, E. M.; Hendrickson, D. N.; Long, J. R.; Park, H.; Ralph, D. C. Nano Lett. 2006, 6, 2014−2020. (13) Ma, Y.; Zhang, J.; Zhang, G.; He, H. J. Am. Chem. Soc. 2004, 126, 7097−7101. (14) Otsuka, Y.; Naitoh, Y.; Matsumoto, T.; Mizutani, W.; Tabata, H.; Kawai, T. Nanotechnology 2004, 15, 1639−1644. (15) Taniguchi, M.; Kawai, T. Physica E 2006, 33, 1−12. (16) Harrell, W. R.; Frey, J. Microelectron. Eng. 1993, 22, 281−284. (17) McCreery, R. L. Chem. Mater. 2004, 16, 4477−4496. (18) Lenzlinger, M.; Snow, E. H. J. Appl. Phys. 1969, 40, 278−283. (19) Middleton, A. A.; Wingreen, N. S. Phys. Rev. Lett. 1993, 71, 3198−3201. (20) Narumi, T.; Suzuki, M.; Hidaka, Y.; Kai, S. J. Phys. Soc. Jpn. 2011, 80, 114704−114711. (21) Narumi, T.; Suzuki, M.; Hidaka, Y.; Asai, T.; Kai, S. Phys. Rev. E 2011, 84, 051137−051141. (22) Xu, C.; van Zalinge, H.; Pearson, J. L.; Glidle, A.; Cooper, J. M.; Cumming, D. R. S.; Haiss, W.; Yao, J.; Schiffrin, D. J.; Proupín-Pérez, M.; Cosstick, R.; Nichols, R. J. Nanotechnology 2006, 17, 3333−3339.

the migration of Au nanoparticles as the electrodes were formed.

4. CONCLUSIONS Nonlinear I−V curves have been observed in the Mn12 molecular redox array from 10 to 300 K. From 250 to 300 K, the conduction mechanism of Mn12 is ascribed to the Frankel− Poole conduction on the basis of the hopping process. At 200 K or below, the deviation from hopping conduction by thermal activation has been suggested. Furthermore, I−V curves with Vth are clearly observed from 80 to 10 K, which has been interpreted by application of the 1D CB model. The Vth(T) values increase linearly with decreasing temperature. These results indicate that each Mn12 molecule acts as a CB element in the 1D array at 80 K or below. It has been, therefore, suggested that the Mn12 molecular redox array can be described by the CB behavior. The similarity between CB systems based on metallic nanoparticles (NPs) and redox molecules is that current−voltage characteristics (I−V curve) with threshold voltage (Vth) can be well fitted by applying the low-dimensional (LD) CB model, suggesting equivalent CB systems. For the difference, the single-electron charging energy (Ec) of metallic NPs can be tuned by the electrostatic capacitance, whereas that of Mn12 molecules can be controlled by the redox potentials.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the structural characterization of Mn12, poly(dA)poly(dT)DNA, the and Mn12/poly(dA)-poly(dT)DNA complex on the cleaved mica substrate by AFM, the schematic illustration in the Experimental Details, and the schematic representation of Frankel−Poole conduction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], matsumoto@ sanken.osaka-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research in Innovative Areas (No. 20111016), 2008-2012, and a Grant-in-Aid for Young Scientists (B) (No. 22760007) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), 2010-2011.

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REFERENCES

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