The challenge of realizing electronic devices based on organic materials started in the early 70‟s. The discovery and advent of conducting polymers in the 80‟s paved the way to new solution-based process and realization of cheap and...
moreThe challenge of realizing electronic devices based on organic materials started in the early 70‟s. The discovery and advent of conducting polymers in the 80‟s paved the way to new solution-based process and realization of cheap and flexible electronic devices. Mastering thin films fabrication by evaporation techniques also made possible devices realization with optimal properties and control. Organic electronics is nowadays becoming a mainstream innovative field, with perspectives in solar cells, lightning and cheap electronics. The first industrial products are emerging in our every day‟s life, for example in screens for portable electronics. From another perspective, electrical transport through molecules, or entities of molecular size or thickness, created a growing interest in the research community. The development has been slower, mostly hindered by the technical issue of matching the nm molecular size with the typical 100 nm electrical interconnects sizes. This is illustrated by the scientific literature on the topic, with large fractions of the publications related to theoretical studies and nanofabrication methodologies, leaving aside only a marginal numbers of experimental results reports. The field of molecular electronics has nevertheless matured, and initial high expectations are nowadays moderated by the reality of difficulties in reuniting the molecular and macroscopic worlds. In particular, the scanning probe studies on molecules deposited on surfaces illustrated that stability usually require cryogenic temperatures, and the intrinsic conductivity of the molecules under study can be remarkably modified by the environment, the molecules conformations, and details of its interactions with the substrate. These points illustrate how difficult it is to create molecular devices of reproducible and robust properties. Tackling molecular electronics problems through size reduction of „standard‟ organic electronics systems also have stringent limits. The progress in this field cannot be compared to progress in miniaturization of inorganic silicon-based electronics. The primary technical bottleneck to miniaturization is metal-organic interfaces, which have a large importance for organic devices, and easily become predominant for sub-micrometer sizes. A huge research effort has been dedicated to this problem in the last 15 years, aiming at better for better characterization and understanding of dielectric-organic and metal-organic interfaces joining two materials of very different electronic properties. In the midway between bulk organic electronics and single molecule devices, are molecular electronic devices in the size range of 10-100 nm. This thesis is mostly dedicated to investigate these intermediate size devices. We envision several key advantages: (1) direct top-down nanofabrication tools can be used to fabricate reliable and reproducible interconnects in the 50 – 100 nm size range, (2) we can use bottom up fabrication methodologies to create molecular-based materials of size exceeding a few tens of nanometers, (3) by targeting devices where transport occurs though a significant number of molecules, we get access to average properties, with the advantage of studying more robust and reproducible samples, with expected environmental stability making ambient conditions measurements possible. Our ambition is to convince the reader that this mid-sized devices approach is promising, with high potentials. Our ambition is to provide a solid ground for molecular electronics devices realization, where reliability and results confidence are priorities. This explains why, all along this thesis, a large number of control experiments have been performed. This thesis is essentially articulated in three sections. In the first section (Chapter II), we present our methodology for creating the electrodes circuit, the interconnects, the excitation and measurement environments. Light is used as a trigger or excitation source, illustrating how molecular devices can have controlled properties, making them potential candidates for devices properties beyond those of standard inorganic electronics. For this aim, the setup for electrical measurements is placed in a home built-interconnect setup over an inverted optical microscope where we can apply simultaneous optical-electrical measurements followed by temperature and magnetic field complementary studies. We report the fabrication of lateral electrodes with high aspect ratio (104) separated by a 20-100 nm distance, and using simple optical lithography techniques. These „nanotrenches‟ are our basic top-down tool for interfacing organic materials. The next section (Chapter III) is a proof-of-principle experiment, showing that we can create robust molecular electronics devices. The best experimental confirmation for the occurrence of molecular-type transport relates to the study of „switching‟ molecules. Such systems exhibit a reversible, and possibly hysteretic, modification of their properties under external stimulus. For experimental convenience reasons, aiming at minimizing the risks of experimental artifacts, we use a benchmark molecular system, choosing a photochromic molecular film. These molecules are known to reversibly exhibit a change of conformation (cis↔trans) under light excitations in the UV and blue ranges. We use a microsphere coated with a film of these molecules, and trapped over the nanotrench. The sphere is the intermediate size connector, closing the junction between metallic electrodes through a double molecular layer. While a light induced change of conduction has been shown in a vertical geometry in these molecules, the lateral geometry is unexplored. On-switching molecular films as check system, we provide quantitative success rate in observing electrical transport unambiguously related to molecules properties, reaching a success rate better than 90%. The third section (chapters IV and V) presents the use of our methodology to investigate original new molecular materials. The spincrossover phenomenon, occurring as a collective transition with hysteretic behavior triggered by a change in temperature, pressure or irradiation in transition metal complexes, from paramagnetic high spin state (HS, S=2) to diamagnetic low spin state (LS, S=0) has been studied thoroughly in the literature, and well suites as switching molecular system. The transition is often accompanied by a change in color, dielectric constant and volume of the bulk material. Studies have shown that due to the collective nature of the transition, the hysteresis occurs in an ensemble of molecules and not in single molecules, thus spincrossover nanostructures are ideal candidates for observing the physical change (volume) due to the transition and we aimed to detect this change in their transport properties in chapter IV. We therefore used original spin crossover nanoparticles, of well-characterized spin transition properties, positioned over 100 nm-size gaps. The obtained results do not relate to spin transition, but to other intrinsic properties of the particles (high conductivity and photoconductivity) and confirmed that this geometry and size scale opens a view to novel properties investigation. The other molecular system is designed for supramolecular electronics studies, where the bottom-up fabrication involves the construction of a large molecular architecture, of size matching the nanotrenches widths. We investigate a self-assembled molecular system based on a triarylamine derivative as building block. Light triggers a polymerization of these molecules, through radicals creation in the solution. When this growth procedure is probed between metallic electrodes, a surprising self-fabrication of highly conductive molecular wires parallel to the linesof electric field in the gap is observed. The wires have an ohmic character with conductivity values of 104 S.m-1 combined with an extremely low interface resistance, typically six orders of magnitude lower than conventional polymers, samples exhibit high environmental stability persisting for a long time. This "discovery‟, detailed in chapter V, represents a milestone discovery for organic electronics since it describes the first molecular self-assembly having intrinsic metallic behavior in both bulk and at the metal/organic interface when lowering the temperature down to 1.5 K. Once again we emphasize the importance of the intermediate sized devices in making progress in the field of molecular electronics, by discovering materials with novel intrinsic properties and taking a great step towards lowering the parasitic interface resistance of organic materials with metals using self-fabrication procedures.
