Many practical devices used in the industry and in various applications of technology continue to dramatically reduce in feature size. In the field of nanotechnology, device miniaturization has reached the boundary of the molecular level. Work is on-going to achieve practical devices, in areas such as microelectronics, chemical sensors, and bio-sensors, which are quantified in terms of numbers of molecules.
In the process of designing nanotechnology devices, there is a need for instrumentation and techniques of qualifying, measuring, and characterizing such devices. Analysis tools such as the Scanning Tunneling Microscope (STM) and Atomic Force Microscope (AFM), electromigration fabricated electrodes, break junctions, mercury drops, nanopores, nanorods, and cross-wire tunneling junctions, have been used to understand the characteristics and behavior of nanotechnology devices. The ability to measure and record physical phenomena at the molecular level is essential to understanding the attributes and behavior of molecules and advancing basic research into practical applications. For example, these tools have been used to gain knowledge about electron transport in molecules. It is important to fundamental nanotechnology research and development to be able to accurately and repeatable measure and characterize the conductivity or resistivity of one or more molecules. Understanding the conductivity of a molecule reveals significant insight into its physical and chemical makeup and interaction with other molecules.
In the work toward developing analysis tools aimed at quantifying molecular conductivity, a variety of techniques have been tried with varying degrees of success. In one approach, a molecule is anchored to a conducting substrate with covalent bonding. A STM tip or conducting AFM tip can be placed over the top of the molecule to measure the current through the molecule between the tip and the substrate. While the molecule can form a reproducible contact to the substrate via the covalent bond, the tip-molecule contact conductance remains undefined, which makes it difficult to determine the conductivity of the molecule itself.
Another related approach is to cap a metal particle onto the molecule. Again, the contact conductance between the metal particle and the tip is undefined. Moreover, the molecules prepared for the STM/AFM measurements are often imbedded in matrix of other molecules, which often prevents the molecule from binding to analyte molecules for sensor applications.
In break junctions, a pair of electrodes is formed separated with a molecular scale gap. The two electrodes originate by breaking a metal wire on the substrate to create the gap. By bridging the gap with molecules terminated with linkers that can bind to the electrodes, a molecular junction is formed which permits a measure of the electron transport properties of the molecular junction.
In a similar approach, the process of electromigration forms a molecular scale gap between two electrodes by passing an electrical current through a thin wire to break the wire into two electrodes via electromigration effect. When molecules are present during the electromigration process, a molecular junction is formed in which molecules bridge across two electrodes.
In a cross wire tunneling junction, a metal is first coated with a layer of molecules, and a second wire is placed over the first wire in perpendicular direction. A molecular junction can be formed by carefully controlling the separation between the two wires.
In an electrode-molecular film-electrode junction, the electrical properties of molecules can be measured by sandwiching a layer of the molecules between two electrodes. The layer of molecules is placed on a flat electrode by self-assembly or by using the Langmuir-Blodgett method. A metal film is evaporated on top of the molecular layer. For nanopore and nanorod molecular junctions, the nanopore and the nanorod junctions are formed in a similar manner with the electrode-molecular film-electrode sandwich structure formed in the nanoscale pores in a SiN membrane or alumina templates. In the case of nanorod method, molecular junctions are first formed in the pores of membrane templates electrochemically. The membranes are then dissolved to leave molecular junctions floating in solution. The molecular junctions are trapped electrically to the gaps between electrodes to allow conductivity measurement of the molecular junctions.
Unfortunately, for each of the above known approaches, it is difficult to accurately and repeatably determine how many molecules are involved in the molecular junctions. The conductivity test may involve one molecule or many molecules; there is no way to be certain as to specifically how many molecules are bridging the electrodes. Also, there is uncertainty as to how, or even if, the molecules are joined to the electrodes. In many tests, the electrodes are not properly coated or protected as required for electrical measurement in aqueous solutions. Finally, these processes rely on electron beam lithography or other expensive fabrication procedures, which may not be practical or available to users.
A need exists to accurately and repeatably measure the electrical conductivity of a determinable number of one or more molecules.
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