Nanogap structures for molecular nanoelectronics
© Motto et al; licensee Springer. 2012
Received: 28 November 2011
Accepted: 9 February 2012
Published: 9 February 2012
This study is focused on the realization of nanodevices for nano and molecular electronics, based on molecular interactions in a metal-molecule-metal (M-M-M) structure. In an M-M-M system, the electronic function is a property of the structure and can be characterized through I/V measurements. The contact between the metals and the molecule was obtained by gold nanogaps (with a dimension of less than 10 nm), produced with the electromigration technique. The nanogap fabrication was controlled by a custom hardware and the related software system. The studies were carried out through experiments and simulations of organic molecules, in particular oligothiophenes.
Electrical nanogap devices are emerging because of their possibility to be the building blocks for connecting , analyzing , and using molecules, and so for implementing nano-metric electronic devices . The main advantage of these systems is, in general, the ability to measure and to transduce events of specific molecules into useful electrical signals . As a consequence, nanogaps have nowadays a high level of interest in research. There are a lot of techniques for obtaining nanogaps, but a process to totally control the gap size has not been found yet. Electromigration effect is the simplest technique useful for obtaining the break of the two terminals structures where the nanogap is built [5, 6]. Electro-induced break junction (EIBJ) can generate an instantaneous and random break, but to obtain reproducible and stable devices it is very important to control the width of the nanogap [7, 8]. For this reason, the quantity of current used to stimulate the electromigration effect must be controlled with a custom feedback circuit that manages all the fabrication steps. The authors defined a method for producing nanogaps inside gold structures, and the controlled use of the electromigration enabled to build gaps under ten nanometers.
2 Experimental section
2.1 Realization of the chip
The electromigration is mainly dominated by the current density  and by the temperature of the wire . Both these quantities can be controlled by a proper geometry of the probe  and by the applied voltage waveform . Using electromigration as technique for creating nanogaps, the wire has to have optimized geometries to facilitate the phenomenon and make it more controllable. From this point, to avoid a too high input current, it is necessary to have a small section of the wire ; moreover, if the section of the wire is too small, the thermal conductance decreases and the temperature of the wire tends to become excessive, leading to the melting of the wire.
The realization of the chip starts from a silicon wafer capped by 200 nm of SiO2; the wafer is, then, inserted in a plasma oxygen machine for increasing the oxygen atoms concentration on the surface. Hydroxylation process is developed with a piranha solution, and so, after rinsing and drying, the surface of the wafer exposes -OH groups, fundamental for the anchoring of the organic compound that is evaporated on. In fact, to promote the adhesion of gold on the SiO2 surface, an insulating layer of MPTMS
The custom hardware
The custom system consists of a driver board that hosts a digital-to-analog converter that realizes the input voltage waveform; a measurement board composed by a transimpedance amplifier with variable feedback, in order to measure a wide range of currents, and an analog-to-digital converter; a switch board to allow the connection of external instrumentation; a digital board that provides electrical power supplies and the bus connection between all boards. The embedded Linux system is built with a real-time custom kernel, so that the electronic components of the other boards are driven in a deterministic way. We have developed a wireless connection between this board and a host computer for sending the experimental data. This solution allows also the use of the system in chambers where a wire interconnection can create difficulties.
The custom algorithm
The higher resistance causes a current reduction, but increasing the voltage in this case creates a mechanism by which the current flow tends to be constant. In fact the first increase in resistance (linear growth) is only due to this heat effect, but, when the temperature reaches high values and the current density is near to 108 A/cm2, the electromigration starts (exponential growth) and the structure begins to change.
The molecular length plays a key role in the electrical conduction.
A system composed by a software interface and an electronic control circuit for the nanogap realization has been implemented, and all the technological steps for arriving at the final nanogap production has been presented in this study. The probe geometries were optimized through electrothermal simulations performed with the COMSOL Multi-physics software. The method applied demonstrated the possibility to build nanogaps under 3 nm with controlled feedback, having a good statistical yield with about the 80% of the nanogaps below 10 nm (Figure 9). In the experimental phase an oligothiophene molecule was successfully inserted in the nanogap, producing a first Metal-Molecule-Metal system, and it was characterized by current-voltage (Figure 10) measurements, taking into account that the coupling between metal and molecule plays a key role. Future study will be focused on the optimization of the system for the realization of integrated molecular devices.
The authors would like to acknowledge Dr. Valentina Cauda for the precious advices about chemistry, Dr. Salvatore Guastella for the FESEM microscope characterizations, Dr. Dario Trimarchi and Dr. Davide Daprà for the electronic system design. They want to acknowledge too the group of Prof. Mucci at University of Modena for the synthesis of the oligothiophene molecules.
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