and redox properties), the bidentate bridging ligands, and the ... single bonds between the metals, and a Re2 complex [4] that ... coinage metal electrodes [8].
Conductivity Measurements of Paddlewheel Dimetal Complexes with Metal-Metal Multiple Bonds S.Rajagopal, N.Smith, Jan M. Yarrison-Rice
C.Urig, T.Scott, S.Zou, H.Zhou
Physics Department, Miami University, Miami University Center For Nanotechnology, Oxford, OH.
Chemistry Department, Miami University, Miami University Center For Nanotechnology, Oxford, OH.
Abstract—In this paper, we report conductivity measurements of dimetal paddlewheel inorganic complexes, using electrode pairs with nanometer separation. Compared to more extensively studied organic molecules, these inorganic complexes provide more versatile opportunities for tailoring electron transport properties. The synthesis of these complexes is briefly discussed and a detailed description of the fabrication of the nano-gap electrode with controlled gap size is provided. Results from the conductivity measurements using macro-pad electrodes for the ligands used to bind the dimetal complexes are discussed. Keywords: dimetal; conductivity lithography; Raman spectra
measurement;
e-beam
I. INTRODUCTION The miniaturization of silicon-based electronic devices is approaching a limit set by physical laws and economical constrains. In order to overcome this limit and continually increase computation speed, alternative technologies are required. One of the alternatives is molecular electronics, in which molecules are used as electronic devices. The most studied molecular electronics so far are made of organic molecules [1, 2]. Compared to organic molecules, dimetal inorganic complexes provide more versatile opportunities for tuning electron transport properties. In this paper, we report conductivity measurements of dimetal paddlewheel inorganic complexes, using electrode pairs with nanometer separation. Paddlewheel complex is a common structural motif in dimetal complexes where the metal-metal bond lies at the molecular axis and the bridging ligands form the paddles of the paddlewheel. In a paddlewheel complex there exist many possibilities to tune its electronic properties, such as by choice of the metal centers (with different metal-metal bond orders and redox properties), the bidentate bridging ligands, and the axial ligands (bound to the metal centers along the axis). In this paper, we first describe briefly the synthesis of a series of paddlewheel complexes that contain air-stable dimetal centers with metal-metal bond orders from single to quadruple. The fabrication of the nano-separation electrode pairs will then be described, followed by the preliminary results from the conductivity measurements of these complexes inserted in the gap of a nano-separation electrode pair. The relationship between metal-metal bond order and conductivity will also be discussed briefly.
II.
SYNTHESIS OF PADDLEWHEEL COMPLEXES
The synthetic methodology we used can be readily found in the literature. The initial compounds synthesized are a Co3 complex [3] that contains a chain of three metal centers with single bonds between the metals, and a Re2 complex [4] that contains a quadruple bond. These complexes were chosen for initial studies because they have a large difference in the metalmetal bond order and they are both robust and air stable. Following the initial synthesis of the paddlewheel complexes, the axial chlorides will be exchanged for labile ligands [5] and then replaced by a designed anchoring group [6], which contains a conjugated system [7] and can readily bind to coinage metal electrodes [8]. A schematic of the synthesis is shown in Fig. 1 and a drawing demonstrating the attachment of the complex to a Au nano-gap electrode pair is presented in Fig. 2. The fabrication of nano-separation electrode pairs is discussed in the following section. After initial studies confirm that a difference in conductance between single and quadruple metal-metal bonds can be observed, then Os2 (triple bond) [9] and Ir2 (double bond) [10] paddlewheels will be synthesized and measured in a similar manner. 4 LiCl (Bu4N)2[Re2 Cl8] 2 (Bu 4N)Cl 4 Li +
Cl S
N
S
N
-
Re N
S
N
S
Re Cl S
N
2 Ag(NO3) 2 AgCl THF
N
S
N
O3 N Re S
S
Re NO3
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S
O 2 Et3N + 2 H
S
N 2 (Et 3NH)(NO3 )
O
S
N S
Re S
N
N
S
Re S
O S
N
Figure 1. Synthetic scheme for the di-rhenium paddlewheel complex.
Figure 2. The di-rhenium paddlewheel bound to the electrode surface.
III. ELECTRODE FABRICATION The fabrication of nano-separation gold electrode pairs consists of two steps. The first entails fabrication of electrode pairs with a larger gap (> 30 nm) by e-beam lithography and the gap is subsequently closed to smaller than 5 nm by electrodeposition. These procedures are discussed below. The overall design of the microscopically larger gap electrode system consists of single gold electrodes of 500 nm wide and 500 µm long attached to 300 × 300 µm2 contact pads fabricated on an insulated silicon substrate. About 10 pairs of electrodes were created on a single chip. The electrodes are addressed to the external package terminals with gold wires through wire bonding. The electrodes are designed using Raith GDS2 software. The actual gap between the electrode fingers after e-beam exposure is always less than the designed gap because of the proximity effect (electron scattering effect in the resist and the substrate). Designed gap sizes from 50 to 150 nm were tested. The designed gap size that resulted in actual inter-electrode gap (after e-beam exposure) of 75 nm (Fig. 3) was selected. Careful consideration was given to avoid write-field errors at critical junctions of different parts of the design. The chip (sample) is prepared by spinning 300 nm thick PMMA (Microchem 495 Anisole 6%) on a degenerately doped silicon substrate covered with a 300 nm thick thermal oxide layer. The pattern of the GDS2 design is then transferred onto PMMA using a Raith 150 e-beam lithography system with a resolution of 5 nm. The e-beam dose parameters are calculated and fine tuned after performing several trial exposures with dose matrices. To streamline the process further, different exposure parameters were used for the microscopically large contact pads and for the microscopically small leads with contacts. Immediately after exposure, the sample is developed in the MIBK-IPA (ratio 1:3) solution and N2 dried. Chromium/Gold (thickness 40/240nm) are used as electrode materials. The electrodes were fabricated by thermal evaporation in a vacuum of 5 × 10–7 torr. A 40 nm layer of Cr was evaporated onto the sample to provide a good adhesion between silicon dioxide and Au. A 240 nm Au layer was then evaporated on top of Cr layer. The Au layer has to be sufficiently thick for wire bonding of contact pads with the package [11]. The chip is adhered to suitable packaging to make it robust for
Figure 3. SEM image of an electrode pair with a microscopically large gap before electrodeposition.
measurement. The contacts in the sample are then wire bonded to the package terminals. The wire bonds were then protected from physical stress by securing it with appropriate nonconducting resin to the package. This resin also forms a cavity, which serves as the bath for subsequent electrochemical deposition, which is used to close and control the electrode gap to below 5 nm. To close the electrode gap, electrodeposition is used [12, 13]. A schematic of the electrodeposition system is shown in Fig. 4. A conventional three-electrode configuration was used for the deposition [12, 13]. One of the gold electrodes in the nano-separation electrode pair serves as the working electrode (WE) and the other as the reference electrode (RE). A gold wire immersed in the bath acts as the counter electrode (CE). A constant DC current was applied across the working electrode and the counter electrode with a galvanostat and the gold plating occurs at the working electrode. The deposition was conducted at room temperature. The voltage between the working electrode and reference electrode was monitored for controlling the gap size [12, 13]. The electroplating solution follows the work by Green et al. [14]. The composition of the solution is 0.42 M Na2SO3, 0.42 M Na2S2O3 and 0.05 M NaAuCl4. Compared to the plating bath used in [12, 13], which contains cyanide, this deposition solution is not toxic and without strongly adsorbed ions. This latter property is important for molecular electronics since the molecules to be tested must bind covalently to the electrode pairs. With strongly adsorbed species present on the electrode surface, such bonding will be inhibited. To control the electrode gap size, the potential difference across the working electrode and the reference electrode was monitored. As the Au deposition on the working electrode proceeds, the distance between the WE and the RE decreases. When the RE is in the double layer region of the WE, the potential difference across the two electrodes decreases with the electrode distance. By recording the potential (E) vs time (t) response, control of the gap size can be obtained. Typically it takes 300-500 seconds to completely close the gap, which is indicated by a zero potential response in the E-t plot and subsequently shown by SEM images. For the completely closed electrode pairs, a gap with a controlled size can still be obtained by electromigration.
CE
WE
Electrochemical Bath
RE
Galvanostat
Chart Recorder
Figure 4. Schematic of the electrodeposition setup using a three-electrode system where WE is the working electrode, CE is the counter electrode, and RE is the reference electrode.
A scanning electron micrograph of an electrode pair after electrodeposition with a current of 25 µA is shown in Fig. 5. Optimization of the bath concentration for chemical stability as well as gap control sensitivity, and the deposition current for a smoother surface is underway. The electrode pairs with small gap will be used to measure the conductivity of the dimetal paddlewheel complexes. The results will be included in this paper in the final form. IV.
CONDUCTIVITY MEASUREMENTS FOR THE ANCHORING LIGANDS To measure the conductivity of the paddlewheel complexes, it is necessary to attach these complexes to the electrodes. One of the anchoring ligands we plan to use is isocyanides, which has been shown to be a better anchoring group in molecular electronics as compared to the much more extensively studied thiols, because the barrier for electron transport is lower.[15-17] To understand the results from the conductivity measurements for the paddlewheel complexes attached with isocyanides to the nano-gap electrodes, it is important to know the electron transport characteristics of the ligand itself. As a first step, we measured the current-voltage (I-V) curve of 1, 4-phenylene diisocyanide sandwiched between two gold electrodes in a macro-pad device.[18] The macro-pad device consists of a Au film electrode covered with a monolayer of 1, 4-phenylene diisocyanide. A second Au
Figure 5. SEM image of an electrode pair after electrodeposition.
electrode was thermally evaporated onto the molecular layer through a shadow mask containing millimeter size holes. To monitor the integrity of the molecular structure in the device under applied voltages, surface-enhanced Raman spectra were recorded simultaneously with the I-V measurements.[18] The in-situ spectroscopic approach could also provide valuable information for understanding the molecular negative differential resistance effect and other interesting I-V characteristics. The I-V curve of 1, 4-phenylene diisocyanide suggests the ligand behaves as a molecular wire and the molecular structure is intact within the potential region where the device is stable. A schematic of the coupled Raman and IV measurement is shown in Fig. 6. The I-V curve and the Raman spectra in Fig. 6 are results from real simultaneous measurements. Conductivity measurements for the ligands using the nano-gap electrode pairs are underway. V.
CONCLUSIONS
Electrode pairs with a controlled separation have been fabricated through a combination of e-beam lithography and electrodeposition. The gap size can be controlled to the scale suitable for characterizing electron transport properties for a single molecule. Conductivity of the potential ligands for attaching the inorganic complexes has been examined using a macro-pad device. The integrity of the molecule under applied potential was confirmed using simultaneous I-V and Raman spectroscopic measurements. The conductivity measurements of the dimetal paddlewheel complexes with varying metal-metal bond order are the first of its kind and will provide a promising route to find suitable molecules with interesting electron transport properties.
Figure 6. Schematic of coupled surface-enhanced Raman spectroscopic and IV measurement of 1, 4-phenylene diisocyanide in a macro-pad device. the Raman spectra and the I-V curve are results from real measurements.
VI. ACKNOWLEDGMENT We wish to thank NSF-MRI for their support of the Raith 150 E-beam lithography system through Grant #0216374. We also would like to acknowledge NSF-NER-ECS grant #0403669. VII. REFERENCES [1] [2] [3]
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