ABSTRACT
In this chapter, we examine the various approaches to making electrical contact to large area molecular junctions. We will highlight the experimental concerns that one must consider and the various approaches to make reliable structures and characterize the junctions electrically, structurally, and chemically. 2.1 IntroductionMolecular electronics was proposed in the early 1970s with molecules performing electronic functions and the notion that
molecules could be incorporated into advanced electronic architectures as conventional silicon-based components continue to scale to smaller and smaller dimensions. However, understanding of the electronic properties of molecules and single molecular layers were slow until advances in nanotechnology enabled researchers to reliably fabricate and characterize properties on the nanoscale. Investigating the electrical properties of molecules remains an active area of research because molecules are among the smallest sized objects that can be mass-produced through synthetic chemistry with millions to choose from in high yield and high purity. The properties of molecules can also be easily changed through synthetic chemistry enabling researchers to probe the link between molecular structure and the electronic function. In addition, self-assembly can be used on many surfaces and surface reactions can be tailored for more reactive surfaces to make high-quality monolayers for electronic applications at lower cost and utilizing different materials than conventional electronics. Many molecules exhibit selectivity that can be utilized for sensors, catalysts, or other applications and can be incorporated with nanoparticles, microfluidics, flexible electrodes, or other specialized surfaces, for nanoscale engineering of optimal properties.Research in molecular electronics has largely been conducted through two focus areas: investigation of single molecules and investigation of thousands of molecules. In the case of single molecules, these investigations typically involve a break junction or scanned probe technique where a small number of molecules (ranging from one to several hundreds) are trapped between two electrodes and the resulting properties are measured. These studies have the advantage of investigating the properties of molecules in a predefined environment, which provides a fundamental understanding of the electronic properties of molecules with proper data collection and analysis and have been the subject of several recent reviews [1, 2]. However, the transient nature of this measurement approach limits the ultimate utility of these structures, and it is not well understood how the properties of isolated molecules compare with the properties of many molecules in a thin film where many-body effects are present. The second experimental approach involves investigating thin molecular films where the active device area could range from 103 to 1010molecules. Often these molecular electronic junctions are fabricated
in a more permanent substrate-molecular layer-substrate approach that have the advantage that they are stable and can be measured multiple times and in multiple locations under differing conditions of temperature, pressure, and magnetic field. As such, they more closely align with conventional device structures and fabrication techniques with promise to be more easily integrated into existing technologies. However, making reliable structures on the nanoscale is non-trivial as the larger area structures are more likely to contain defects or other artifacts that alter the electronic properties of the structure. We will focus further on approaches to fabricating reliable electrical contacts on large area ensembles of molecules and characterization approaches to interrogate these structures. 2.2 Challenges Facing Ensemble Molecular
A molecule is typically 0.5 nm wide and 0.5 nm to several nm long depending on the specific molecule. In order to make a molecular junction, the bottom substrate, molecular layer, and top substrate all need to be controlled within these nm length scales over the entire range of the junction, which could extend laterally several micrometers to millimeters. Let us first consider the bottom electrode where evaporated metals can have a root-mean square roughness of several nanometers, making the grains of the substrate nearly equivalent to the molecular length. Thus, alternative approaches are needed to prepare substrates that are atomically smooth on large area length scales. For metals, this is often accomplished by using a template stripping approach where metal is initially evaporated onto a very smooth surface and the incident kinetic and thermal energy enable the impinging metal to form an intimate, smooth interface between the deposited metal and the smooth substrate. Often, another layer or polymer is attached to the top of this metal surface for ease of handling. Next, the metal is peeled off in a template stripping fashion to expose the smooth underside of the evaporated metal. Alternatively, there are carbon-based and polymer-based substrates that are typically quite smooth on these relevant length scales. A third approach is to use a crystal substrate, such as silicon, which can be processed to remove the oxide and form an atomically smooth surface both in solution and under ultrahigh vacuum (UHV) conditions.
