Chemical Control of Electronic Coupling between a Ruthenium

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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

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Chemical Control of Electronic Coupling between a Ruthenium Complex and Gold Electrode for Resonant Tunneling Conduction Yoichi Otsuka, Satoshi Nishijima, Leo Sakamoto, Kentaro Kajimoto, Kento Araki, Tomoki Misaka, Hiroshi Ohoyama, and Takuya Matsumoto* Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka, Japan

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ABSTRACT: Current−voltage (I−V) nonlinearity is essential for information processing in molecular electronics. We used a nanoparticle bridge junction to investigate the effect of electronic coupling between a Ru complex and electrodes on nonlinear electrical properties. Two types of molecular layers, in which the Ru complex forms different chemical bondings to the electrode, were used for electrical measurements. The chemical bond and the surface potential of the Ru complex on Au electrodes were examined by X-ray photoelectron spectroscopy, infrared ray reflection absorbance spectroscopy and Kelvin probe force microscopy, respectively. The device, in which the Ru complex is directly bound to the Au electrode, indicated the nonlinear I−V characteristics with zero-bias conductance because of the direct tunneling conduction. Another device fabricated by inserting a spacer molecule between the Ru complex and the Au electrode realized nonlinear I−V characteristics with a clear threshold voltage and little zero-bias conductance. The I−V curves were well fitted by the resonant tunneling conduction model. The present results show the significance of controlling the electronic coupling for nonlinear I−V characteristics. KEYWORDS: ruthenium complex, resonant tunneling, nanoparticle, bridge junction, nonlinear I−V characteristics

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INTRODUCTION The unique nonlinear electrical properties of molecules are crucial for future molecular electronics. Nonlinear current− voltage (I−V) characteristics such as asymmetric I−V characteristic, current hysteresis, negative differential resistance, and current pulses can be used in applications such as as molecular diodes,1 memories,2 and switches.3 By utilizing the nonlinear I−V characteristics of a molecular network, neuromorphic spikes4 and stochastic resonance5 have been reported. Thus, nonlinearity is considered as the primitive function for mimicking biological information processing, such as that in the human brain6 and cells.7 One approach to obtaining nonlinear I−V characteristics is using resonant tunneling conduction.8 In resonant tunneling conduction, the electrical current is suppressed within the threshold voltage, whereas the current increases above the threshold voltage. This electrical property is promising for molecular devices because of the clear nonlinearity under a wide range of temperature conditions that can be obtained. Resonant tunneling conduction requires a stable electronic state of molecules against the injection of electrons/holes. In expanded π-conjugated molecules with narrow highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps, injection of conduction © 2019 American Chemical Society

carriers may produce unstable cationic/anionic radicals. In contrast, transition-metal complexes often have multiple nonbonding states with which the carrier injection/ejection is allowed to preserve the molecular structure. The ruthenium complex N-719 9 (di-tetra-butylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)) is one of the promising molecules to obtain a stable electrical conduction. N-719 has been widely used in dye-sensitized solar cells. The durable redox reaction would be beneficial for molecular electronics. Resonant tunneling conduction also needs a well-defined junction between the electrodes and molecules. Nanoscale electrical measurement techniques have been developed and used for molecular electronics. Crossbar junctions are used to create the electrode/molecule/electrode configuration to measure electrical properties. Problematic short circuits caused by Au atoms penetrating the molecular layer can be prevented by techniques such as stamp printing.10 The electrical measurements of a ruthenium complex containing an oligothiophene derivative indicated nonlinear I−V characterReceived: March 29, 2019 Accepted: June 6, 2019 Published: June 6, 2019 24331

DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

ACS Applied Materials & Interfaces

the effect of electronic coupling between molecules and electrodes on nonlinear I−V characteristics.

istics with a clear threshold voltage. The temperature dependence of the conductivity also suggested that intermolecular interactions suppress the single-molecule electrical properties above 145 K.11 To obtain an electrode/molecule junction for resonant tunneling conduction, the number of molecules between electrodes should be minimized. The single-molecule electrical properties have been measured using mechanically controllable break junctions,12 scanning tunneling microscopy,2 and conductive probe atomic force microscopy.13 Although these methods achieve precise control between two electrodes, the dynamically controlled nanogap disturbs the formation of a stable electrode/ molecule/electrode junction, resulting in the temperature dependence of the electrical conductivity. Nanoparticle (NP) bridge junctions, in which AuNPs bridge nanogap electrodes that are modified with molecules, have been used to study the stable nanometer-scale electrical contact between electrodes and molecules.14−16 With these junctions, the electrical properties of the two molecular layers can be measured by treating the AuNPs as the middle electrode. The advantage of this type of junction is the reduced intermolecular interactions because of the small contact area between NPs and molecules. Furthermore, the stable electrical contact in the device configuration allows us to measure the temperature dependence of the electrical conductivity. Considering the appropriate interface between electrodes and molecules to realize the resonant tunneling conduction, a weak electronic coupling between the Fermi energy of the electrodes and the electronic state of the molecules is essential. Using alkanethiols as a tunneling barrier is a promising way to reduce the electronic coupling between molecules and electrodes because the thickness of the tunneling barrier can be controlled by the molecular length of the alkanethiol selfassembled monolayers (SAMs). The molecular-length-dependent decay parameter, β, is around 1.1 per carbon atom.14 Attempts to reduce the electrode/molecule interaction from this viewpoint were recently reported. The electronic properties of a heterolayer composed of an alkanethiol and N-719 have been measured by a spectroscopic method. The weak electronic coupling was achieved by inserting an alkanethiol between the Au electrode and N-719 isolated the N-719 molecular electronic state.17 Furthermore, we reported the nonlinear I−V characteristics of a heterolayer of 2-aminoethanethiol and N-719 by using NP bridge junctions. The I−V curves showed a rapid current increase above a threshold voltage of ±1.2 V, without obvious temperature dependence. The conduction mechanism was assumed as the resonant tunneling via the HOMO of N-719, which was weakly coupled to the Au electrode and AuNPs.18 In this study, the chemical control of electronic coupling between a ruthenium complex and Au electrodes for nonlinear electrical properties is examined by using an NP bridge junction. In addition to the electrical measurement, the chemical bonding and the surface potential of N-719 on the Au electrodes were evaluated by X-ray photoelectron spectroscopy (XPS), infrared ray reflection absorbance spectroscopy (IRRAS), and Kelvin probe force microscopy (KFM), respectively. It was found that the weak electronic coupling between N-719 and Au electrodes is essential to obtain nonlinear I−V characteristics which can be well fitted by the resonant tunneling conduction model. These methods presented here also propose the general approach to study

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EXPERIMENTAL SECTION

Nanogap electrodes were fabricated on a 300 nm thick SiO2 layer with a Si substrate using angle-controlled thermal deposition, as reported previously.19 The substrate was subjected to ultrasonic cleaning in acetone and ozone/UV treatment (NL-UV253, Filgen, Japan). Half of the substrate was covered with a cleaved silicon mask, and the Au/Cr electrode was fabricated on the uncovered area. The mask was removed, and the substrate was covered with a micrometer-scale patterned metal mask. The Au/Cr counter electrodes were fabricated using oblique angle thermal deposition to form a nanogap between the first and second electrodes. The nanogap electrodes were cleaned with ozone/UV treatment and pure ethanol before chemical modification. We prepared three types of NP bridge junctions such as the weak coupling device, the strong coupling device, and the control device (Figure 1) to study the effect of an electronic coupling between a Ru

Figure 1. (a−c) Schematic illustration of the energy diagrams of the three devices. complex and electrodes on nonlinear I−V characteristics. In the weak coupling device, 6-amino-1-hexanethiol (6-AHT, Dojindo, Japan) SAM was formed on the nanogap electrodes by immersing them in a solution of 6-AHT [1 mM hydrochloric acid (20 mL) in ethanol (120 mL)] for 24 h. The N-719 (Sigma-Aldrich, Japan) layer was formed on the 6-AHT SAM by immersing the SAM-modified electrodes in a 1 mM N-719 solution in ethanol for 24 h. The electrodes were rinsed with ethanol after removing them from each solution. Finally, AuNPs capped with citric acid (Tanaka Kikinzoku Kogyo, Japan; 150 nm average diameter; solution pH 3) were deposited on the electrodes and allowed to air dry. The strong coupling device was fabricated in the same manner except that the 6-AHT SAM was omitted. The control device was also fabricated in the same way, except that a 10carboxy-1-decanethiol (10-CDT, Dojindo, Japan) SAM was formed on the nanogap electrodes by immersing them in 1 mM 10-CDT solution in ethanol for 24 h. The nanogap and the bridging AuNPs were examined by a scanning electron microscope (JSM-7600F, JEOL, Japan). Two-terminal electrical measurements were conducted using a current source meter (2636B, Keithley, USA) and a cryostat (ST-500, Janis, USA) under a vacuum of 5 × 10−4 Pa and dark conditions. The sample temperature was controlled by the temperature controller (model 330, Lake Shore, USA) using liquid helium flow and the heater system in the cryostat. XPS measurements were conducted for the molecular layers on the Au/Cr/SiO2/Si substrate. The layers were fabricated using the same method as for the nanogap electrodes. The samples were kept in a nitrogen-purged container until the measurements. The XPS measurements were performed using an X-ray photoelectron spectrometer (AXIS 165x, Kratos, UK) in hybrid mode with a monochromatic Al Kα X-ray source. The photoelectron takeoff angle with respect to the surface was 45°. All spectra were collected with a pass energy of 80 eV, and the binding energy was referenced to the Au 4f peak at 84 eV. Background correction was conducted by the Shirley method and the spectra were fitted with a Gaussian function (OriginPro, OriginLab, USA). IRRAS measurements were conducted for the evaluation of the orientation of N-719 in the weak and the strong coupling devices. A Fourier transform infrared spectrometer (FT/IR6100, JASCO, Japan) 24332

DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Schematic illustration of the NP bridge junction. Magnified images of the molecular layer at the interface between the gold electrodes and NPs for the (b) strong coupling, (c) weak coupling, and (d) control devices. (e) SEM image of the surface structure of the strong coupling device. (f) Magnified SEM image between the gold electrodes. and a grazing angle reflectance accessory (RAS-PRO410-H) were used. KFM measurements were performed using a scanning probe microscope (JSPM-4200, JEOL, Japan) and an oscillation controller (OC4, Nanonis, Switzerland). The frequency shift of the conductive probe (Pt−Ir-coated, PPP-NCHPt-50, NanoWorld, Switzerland) was measured while scanning the sample surface. Feedback control was executed with the amplitude constant mode. The groups of Au and N719/Au (strong coupling), Au and 6-AHT/Au, and 6-AHT/Au and N-719/6-AHT/Au (weak coupling) were fixed on the same sample folder and their surface potentials were measured with the same probe. The measurement area was 1 × 1 μm. The contact potential difference images were analyzed by WinSPM (JEOL, Japan). The potential values of all pixels were converted into a histogram.20 Histogram peaks for the same type of sample were aligned and the potential values were subtracted from the peak value of the Au surface. Density functional theory (DFT) calculations were performed using the Gaussian 09 program package for N-3 (cis-RuL2(NCS)2, cisbis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II), where L = 2,2′-bipyridyl-4,4′-dicarboxylic acid), because the two tetra-n-butylammonium (TBA) ions in N-719 ([RuL2(NCS)2]· 2TBA) were expected to be substituted with protons during device fabrication. The DFT calculations were performed for N-3 in the ground state with an optimized structure using the B3LYP exchange correlation functional and the LANL2DZ basis set to gain insights into the electronic structure (i.e., HOMO and LUMO), molecular structure, and orbital energies. The output file generated from Gaussian 09 was used to visualize the HOMO and LUMO in GaussView 5. The fitting of the I−V characteristics with the resonant tunneling conduction model was performed with OriginPro.

In the weak coupling device, 6-AHT SAM was inserted between N-719 and the Au electrode (Figure 1b). Because the electronic state of N-719 is separated from the Au electrode, the energy shift of the molecular electronic state would be determined by the thickness of the 6-AHT SAM and the electric field between the electrode and NPs. Once the alignment between the molecular electronic state and the Fermi energy of the electrode was achieved, a resonant tunneling conduction via the molecular electronic state can occur. This device was expected to indicate nonlinear I−V characteristics with a threshold voltage. The large distance between the electrode and NPs was also expected to suppress the direct tunneling, minimizing the current below the threshold voltage. Ru complex was absent in the control device. The 10-CDT SAM was formed on the Au electrode (Figure 1c). Because of the short distance between the AuNP and electrodes and the absence of a molecular electronic state close to the Fermi energy of the electrode, a direct tunneling conduction was expected. In the present study, we used 6-AHT to form reproducible SAMs with which the appropriate tunneling conduction is expected. The thickness of 6-AHT SAMs was estimated around 0.9 nm according to the previous report.21 The thickness of N-719 was estimated at around 1.0 nm assuming that the carboxy groups are attached on the amino groups of 6AHT.22 The length of 10-CDT on Au was calculated as 1.6 nm by ChemOffice (PerkinElmer, USA). We assumed that the effective tunneling distance of the weak coupling device (N719 layer on 6-AHT) would be the summation of the 6-AHT SAMs and the tunneling distance in N-719. The tunneling distance in N-719 was also assumed as 0.6 nm from the distance between the carboxy group and Ru because the HOMO of N-719 is localized around Ru and isothiocyanate groups18 and the carboxy group faced the 6-AHT SAMs. The effective tunneling distance was estimated as 1.5 nm, and thus we selected 10-CDT for the control experiment. On the basis of the considerations above, we prepared the NP bridge junction. AuNPs covered with citric acid bridged the nanogap electrode and functioned as a middle floating electrode (Figure 2a). The I−V measurements of the three types of devices were executed by sweeping the voltage difference between two Au electrodes. Figure 2b−d show the schematics of the interfaces between the Au electrodes and NPs. In the strong coupling device, N-719 was immobilized on the Au nanogap electrodes with the isothiocyanate group (Figure 2b). In the weak coupling device, N-719 was

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RESULTS AND DISCUSSION Design of Nanoparticle Bridge Junction Incorporating N-719. To examine the effect of the electronic coupling between molecules and electrodes on nonlinear I−V characteristics, we designed three kinds of molecular devices (Figure 1a−c) in which different types of molecular layers were formed between the Au electrode and AuNPs to control the electronic coupling between them. In the strong coupling device, N-719 is immobilized on the Au electrode via the covalent bond. The strong electronic coupling between N-719 and the Au electrode was expected to pin the electronic state of N-719 on the Au electrode (Figure 1a). The coupling between N-719 and AuNPs was weak because of the absence of a covalent bond between them. As the distance between the electrode and NPs is short, a direct tunneling between the Au electrode and NPs was expected in this device. 24333

DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

ACS Applied Materials & Interfaces

immobilization of 6-AHT on Au via the thiol group. In the weak coupling device (N719-modified 6-AHT on Au), the peak was fitted by the S 2p3/2 and S 2p1/2 peaks at 162 and 163.2 eV for an Au−S−CH2 bond, respectively, and the S 2p3/2 and S 2p1/2 peaks for the −CS bonds at 162.8 and 164 eV, respectively. 25 These results would indicate that the isothiocyanate groups of N-719 were not bound to the Au electrodes. In the control device (10-CDT on Au), the peak was fitted by the S 2p3/2 and S 2p1/2 peaks for an Au−S−CH2 bond at 162 and 163.2 eV, respectively, suggesting the immobilization of 10-CDT on Au via the sulfur group (Figure S1). Evaluation of N-719 by IRRAS. N-719 used in the strong and weak coupling devices was evaluated by IRRAS. Because N-719 contains isothiocyanate and 2,2′-bipyridyl-4,4′-dicarboxylate ligands, the signals of isothiocyanate and carboxylate were examined (Figure 4). The absorbance peak of

immobilized directly on the 6-AHT SAM which was formed on Au nanogap electrodes (Figure 2c). In the control device, N719 is not present, and 10-CDT SAM was formed on the Au nanogap electrode (Figure 2d). The deposition of each layer was confirmed by the elemental analysis using XPS. Figure 2e,f shows the representative scanning electron microscopy (SEM) images of the strong coupling device. Multiple AuNPs bridged the nanogap electrodes; thus, the electrical properties of multiple molecular junctions were obtained. In the present study, we designed the gap size of electrodes to immobilize a single AuNP between electrodes. I− V characteristics are considered to indicate the electrical property of parallel NP bridge junctions. Therefore, we do not discuss the absolute current value of I−V characteristics but discuss the nonlinearity of I−V characteristics in terms of electronic coupling between electrodes and N-719. Evaluation of Molecular Layers by XPS. The molecular layers used in each device were evaluated by XPS. Because N719 contains Ru ions, isothiocyanate, and 2,2′-bipyridyl-4,4′dicarboxylate ligands, the C 1s and Ru 3d peaks were examined (Figure 3a). The C 1s peak at 284 eV for the Au surface was

Figure 4. IRRAS spectrum of N719 of the strong coupling and weak coupling devices.

isothiocyanate26 at 2100 cm−1 was obtained with the strong and weak coupling devices. This result indicates the presence of N-719 on Au or 6-AHT SAMs. On the other hand, the peak of the symmetric carboxylate stretch mode27 at 1370 cm−1 was obtained with the weak coupling device. This result suggested that N-719 molecules were oriented on 6-AHT by the electrostatic adsorption via ion pair formation between the carboxylate and the ammonium group of 6-AHT. Surface Potential of Molecular Layer on Au by KFM. The chemical modification of the Au electrode shifts the surface potential because of the charge transfer between the Au electrode and molecules. Because the energy alignment between the Fermi energy of the electrode and the electronic state of the molecules determines the electrical properties of devices, understanding the shift of the surface potential before/ after the chemical modification of the electrode is important. We used KFM to evaluate the change in surface potential caused by the molecular layer in both the strong and weak coupling devices. Figure 5 shows the surface potential difference between Au and the molecular layer on Au. The N-719 layer, which was directly immobilized on the Au surface, caused a small potential shift of 0.05 eV (Figure 5a). The change in surface potential arose from the charge transfer between N-719 and Au by tunneling through the covalent bond identified by XPS. The immobilization of 6-AHT on the Au surface resulted in a larger potential shift of 0.44 eV (Figure 5b), owing to the amino group of 6-AHT.28 The indirect immobilization of N-719 on 6-

Figure 3. XPS spectrum of (a) C 1s and Ru 3d peaks and (b) S 2p peaks of the strong coupling, weak coupling, control, and of bare gold. The deconvoluted S 2p peaks are also shown in (b).

the hydrocarbon layer. In the strong coupling device, the peaks at 281, 285, and 288 eV corresponded to Ru 3d5/2, C 1s for multiple chemical bonds (−CH2−CH2−, −CH2−NH2, −CH2−S, −CHCH−, NCS), and Ru 3d3/2 or C 1s for (−CO), respectively. The weak coupling device had a similar pattern to the strong coupling device. The XPS measurement of the reference sample (6-AHT on Au) showed only a single peak at 285 eV corresponded to C 1s.21 These results indicate that N-719 molecules were immobilized on the electrode in both the strong and weak coupling devices. In the control device, the C 1s peaks at 284.8 and 289 showed the immobilization of 10-CDT22 on the Au surface (Figure S1). The S 2p peaks were examined to evaluate the chemical bonding between the molecules and Au (Figure 3b). In the strong coupling device (N-719 on Au), the peak was fitted by the S 2p3/2 and S 2p1/2 peaks for an Au−S−C bond and S2 species on Au at 162.2 and 163.4 eV, respectively.23 This result indicated that N-719 would be bound to the Au surface via the isothiocyanate group. Considering the control XPS measurement of the weak coupling device (6-AHT on Au), the peak was fitted by the S 2p3/2 and S 2p1/2 peaks for an Au−S−CH2 bond at 162 and 163.2 eV, respectively,24 suggesting the 24334

DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

ACS Applied Materials & Interfaces

little variation with temperature from 10 to 310 K, and the zero-bias conductance was about 1.2 × 10−10 (A/V) at 130 K. The current increased beyond the ±1 V bias voltage threshold. The weak coupling device showed different nonlinear I−V characteristics from the strong coupling device. The clear threshold voltage around ±1 V and the suppressed current within the threshold voltage was measured (Figure 6b). The zero-bias conductance was about 8.8 × 10−12 (A/V) at 130 K, which was the smallest among all the devices. The control device also showed nonlinear I−V characteristics (Figure 6c). The temperature dependence was the smallest among the devices, whereas the zero-bias conductance of about 1.2 × 10−9 (A/V) at 130 K was the largest. Overall, the small temperature dependence of I−V characteristics supports the predominant tunneling conduction and the suppressed thermally activated conduction. In general, the tunneling current decreases exponentially with an increasing distance between electrodes. The difference in the zero-bias conductance between the three devices can be considered in terms of the device structure. The zero-bias conductance in the strong coupling device and the control device indicates the direct tunneling conduction. The difference in the zero-bias conductance would be because of the thickness of the molecular layer (N-719 > 10-CDT). The little zero-bias conductance in the weak coupling device also corresponds to the suppression of the direct tunneling conduction. This is because of the thick tunneling barrier composed of the 6-AHT SAM and N-719 under the condition that the resonant tunneling has not occurred. The I−V characteristics with a clear threshold are considered in the next section. Figure 6d shows the temperature dependence of the current under different bias voltages for the weak coupling device. The current beyond the threshold voltage (1.5, 2.0 V) increased with increasing temperature. The current increase above the threshold voltage was small compared with previous results,11 probably because of the reduction of the intermolecular coupling in the present device. This is plausible because the NP bridge junction enables us to reduce the contact area between the molecular layer and the electrode compared to the crossbar sandwich junction. The current within the threshold voltage (0.5 V) was low and it was temperature-independent. Conduction Mechanism. To consider the conduction mechanism of the strong coupling device, the HOMO and LUMO energy levels for the model complex cis-RuL2(NCS)2 (N-3) and N-3 immobilized on Au were calculated by DFT. The molecular structure is common in both N-3 and N-719 except the counter ions. The two TBA ions in N-719 were expected to be substituted with protons to form N-179 during device fabrication. The HOMO and LUMO of N-3 were −5.34 and −3.87 eV, respectively, whereas those of N-3 on Au were −5.97 and −4.07 eV, respectively. For N-3 on Au, the N3 thiocyanate groups faced the Au(111) surface (Figure S2). This result indicates that the stabilization of the N-3 HOMO on Au (−0.63 eV) was larger than that of the LUMO (−0.20 eV). This was because the HOMO and LUMO of N-719 were localized around the spatially separated thiocyanate and biisonicotinic acid ligands, respectively, and thus each energy level could be changed independently. This result shows that the direct bonding of N-719 to the Au surface via thiocyanate groups increased the electronic coupling between the Fermi energy of Au and the energy level of the N-719 HOMO. Figure S3 shows the I−V characteristics fitted with the direct

Figure 5. Surface potentials measured by KFM of (a) Au and N-719/ Au, (b) Au and 6-AHT/Au, and (c) N-719/6-AHT/Au and 6-AHT/ Au.

AHT on Au showed a potential shift of 0.17 eV (Figure 5c). This result would indicate that the positive charges of the 6AHT SAM were partially neutralized by the carboxyl group of N-719 because of the electrostatic interaction. The larger positive surface potential of the weak coupling device (0.17 eV) than that of the strong coupling device (0.05 eV) showed that the HOMO of N-719 in the weakly coupled device was closer to the Fermi energy of Au than that in the strongly coupled device. This finding supports the resonant tunneling conduction via the HOMO of N-719 in the weak coupling device as noted later. Comparison of I−V Characteristics. The effects of the electronic coupling between molecules and electrodes on electrical properties were studied by I−V measurements at different temperatures. The strong coupling device showed nonlinear I−V characteristics (Figure 6a). The results showed

Figure 6. Temperature dependence of the I−V characteristics of the (a) strong coupling, (b) weak coupling, and (c) control devices. (d) Temperature dependence of the current under the specific bias voltage of the weak coupling device. 24335

DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

ACS Applied Materials & Interfaces

tunneling conduction model (eq S1). The good fitting indicates that the direct tunneling conduction was dominant in the strong coupling device. The I−V characteristics of the weak coupling device were fitted with the resonant tunneling model. We assumed the asymmetric resonant tunneling model with two tunneling barriers between Au electrodes and AuNPs. The tunneling barrier is composed of the HOMO and LUMO of N-719 and the two barriers between N-719 and AuNP or the Au electrode. The fitting equation is based on the Simmons model29 (eq S3). Figure 7 shows the energy diagrams for the weak coupling device at (a) zero-bias voltages and (b) bias condition in which the Fermi energy of the Au electrode, AuNPs, and the HOMO of N-719 were aligned on one side of the device. In the consideration of the conduction model, we assumed the following conditions; (a) The resonant tunneling conduction occurred when the energy of an electron from the electrode and that of a bound electron in N-719 is aligned under the condition that the molecular orbital of N-719 is positioned between the occupied and unoccupied state of electrodes. (b) The energy alignment between the molecular orbital of N-719 and the Fermi energy of electrodes does not occur around the zero-bias voltage. In addition, the direct tunneling conduction is suppressed because of the long distance between the electrodes. (c) The electronic potentials, both of the molecule and AuNP, are floated and determined by the external electric field between Au electrodes. (d) The surface potential of N-719 on the Au electrode is higher than that of Au electrodes (KFM measurement). Thus, the energy difference between the Fermi energy of the electrode and the HOMO of N-719 is smaller than that between the Fermi energy of the electrode and the LUMO of N-719. (e) The work function of AuNP is larger than that of the bulk Au generically. Hence, the Fermi energy of AuNP is deeper than that of the Au electrode. (f) The threshold voltage means the appropriate bias voltage between two Au electrodes to realize the energy alignment between the molecular orbital and the Fermi energy of both the Au electrodes. (g) The threshold voltage is determined by the balance of the energy difference for ΔE1 (between the Fermi energy of the gold electrode and the HOMO of N-719) and ΔE2 (between the Fermi energy of the gold electrode and that of AuNP). (h) The bias voltage between two Au electrodes is equal to the sum of voltage differences in four tunneling barriers (V1, V2, V3, V4). The four tunneling barrier thicknesses originated from 6-AHT (d2, d3) and citric acid (d1, d4), respectively.

Figure 7. Energy diagram of the weak coupling device at a bias voltage of (a) zero and (b) condition in which the Fermi energy of the Au electrode, the HOMO of N-719, and the Fermi energy of AuNP are aligned on one side because of the external electric field between the electrodes.

The experimental results under different temperature conditions fitted well for all temperature regions (Figures 8a−e). The fitting parameters are also shown in Table S1. The tunneling barrier thicknesses, d1 and d4, were similar to the thickness of the 6-AHT monolayer on Au,21 and d2 and d3 were similar to the size of citric acid on Au30 (Figure 8f). These results indicated that the resonant tunneling between one side of the Au electrode and AuNPs via the N-719 HOMO

Figure 8. Fitting of I−V characteristics at (a) 10, (b) 70, (c) 130, (d) 190, and (e) 290 K with the resonant tunneling conduction model. (f) Thickness of the tunneling barriers used as the fitting parameters of the resonant tunneling conduction model for the weak coupling device.

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DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

ACS Applied Materials & Interfaces

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was the main mechanism of nonlinear I−V characteristics with a clear threshold voltage over a wide temperature range. In the future perspective, the nonlinear I−V characteristics of a smaller amount of molecules would be measured by using the smaller nanogap electrode31,32 and the single AuNP bridge junction.

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CONCLUSIONS We have studied the effect of electronic coupling between a Ru complex (N-719) and Au electrodes on nonlinear I−V characteristics. NP bridge junctions, in which N-719 was electronically coupled to Au electrodes strongly or weakly, exhibited the prerequisite for sharp nonlinear properties. Our results show that designing the electronic coupling between the molecular electronic states and the Fermi energy of chemically modified electrodes is significant for achieving resonant tunneling conduction and for obtaining nonlinear I−V characteristics with a clear threshold voltage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05569. XPS results for the control sample (10-CDT/Au); results of DFT calculation of N-3; I−V characteristics of the strong coupling device and its fitted results by the direct tunneling conduction model; and fitting parameters for the resonant tunneling conduction model for the weak coupling device (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yoichi Otsuka: 0000-0003-2304-6637 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant numbers JP25110014, JP24360011, JP16K21143, and JP16K13667. We also thank Mr. Kawamura of analytical instrument facility, graduate school of science, Osaka university for the supporting of IRRAS.

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REFERENCES

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DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338

Research Article

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DOI: 10.1021/acsami.9b05569 ACS Appl. Mater. Interfaces 2019, 11, 24331−24338