Memory Effect and Negative Differential Resistance by Electrode‐ Induced Two‐Dimensional Single‐ Electron Tunneling in Molecular and Organic Electronic Devices

Abstract

Beyond the demonstration of single-molecule (or singleatom) and organic-thin-film transistors, [1±4] a reliable electrode contact remains one of the most critical challenges in the development of molecular and organic electronics. Except for the tip of a scanning tunneling microscope, [5±7] more practical connections often involve a metal/ insulator(semiconductor)/metal thin-film structure, either through a gate thin-film dielectric assembly below the source±drain, [1±4] or a insulating thin-film separator which runs parallel to the molecular path. [8,9] To achieve practical applications, not only does one need ohmic contacts so that any nonlinearity can be correctly attributed, but also a much more insulating medium to minimize the influence from the surroundings.[10] However, one often encounters unexpected current±voltage characteristics which do not arise from the given atoms or molecules. For example, negative differential resistance (NDR) [11±15] and the memory effect (hysteresis) [16±21] were found not only in singlemolecule devices, but also in organic thin-film transistors, organic light-emitting devices, and metal/inorganic insulator/ metal systems.[22±26] Despite many years of effort, the precise origin of the phenomena remains unknown, which has become a major obstacle in the research and development of new electronic devices. In this report, we try to reveal the origin of such phenomena, based on new experiment results from various metals and both organic and polymeric thin films. The answer is surprisingly found to be two-dimensional (2D) single-electron tunneling due to nanometer-sized metal islands, which were manifested unexpectedly through a wellknown process where some nuclei were blocked from further growth by the sealing of electrode outside crevices. The results not only provide a satisfactory explanation of the anomalous phenomena, but also open a new door to the construction and fabrication of molecular electronic and memory devices.The widespread nature of the phenomena is shown by its easy duplication, using any metal electrode and any organic or inorganic thin-film insulator, or even semiconductors, since the anomalous current is usually several orders of magnitude higher than that of the nominal conductance. The typical current±voltage (I±V) curve showing NDR and a memory effect has two distinctive features. One is a local current maximum (when V = V max ), followed by a region of NDR. The other feature is the dual states for V < V max , where the ªonº state is obtained by reaching the V max first, and the ªoffº state is recovered by visiting a voltage beyond NDR before rapidly setting back to zero.[21] In addition, there are intermediate states available, [21,22] and the phenomena could be removed by exposure to oxygen gas. [13] Many models have been proposed, but so far none offers a satisfactory explanation. The arbitrary choice of metals and insulators of various energies immediately rejects any speculation related to, for example, a particular type of insulating or semiconduc...

Electrical Switching and Bistability in Organic/Polymeric Thin Films and Memory Devices

“…[59,65] On the other hand, Tang et al argued that the SV model may not be a good one for the conductance switching, since it disregards potential energy changes resulting from the charging process. [64] They believe that the electrical behavior in their devices was caused by 2D single-electron tunneling by nanometer-sized metal islands, formed unexpectedly by the nucleation and growth of a metal thin-film electrode. Yet another model was proposed by Rozenberg et al [66] They proposed three kinds of metal domains in the film between the top and bottom electrodes: the top domain, the middle domain, and the bottom domain (Fig.…”

Multi-layer stackable polymer memory devices

“…From the damages shown on the failed devices, these spots were likely to be concentrated along the edges of the device. If the conduction paths were formed as a result of dust particles (Tang et al 2005), the on state current would be likely to be proportional to the device area. The off current showed a higher dependence on device area indicating a more uniform conduction mode.…”

Advancements in organic nonvolatile memory devices

“…This charge-trapping mechanism was also adopted by various other groups [37,46]. Tang et al [47], however, argued that the SV model may not be a good one for conductance switching as it disregards potential energy changes resulting from the charging process. They believed that the electrical behavior in their devices was caused by 2D single-electron tunneling by nanometer-sized metal islands, formed unexpectedly by the nucleation and growth of a metal thin-film electrode.…”