Electrical characterization of rectifying and photoswitching molecular devices

Abstract

As one of the interesting research fields, the molecular electronics has been studied to be used as electronic device components such as rectifier, transistor, memories, and photoswitch. In particular, molecular self-assembled monolayer (SAM) can be used to a functional device structure due to several advantages such as low fabrication cost, high efficiency, less heat problem, and miniaturized junction size. However, the early day of molecular electronic devices had a problem of low device yield (typically less than 1 %) because of the electrical shorts that can occur as a result of the top electrodes penetration through the thin molecular layers. To solve this problem, several ways have been proposed. First, an intermediated protective layer composed of a conducting polymer, or graphene film have been introduced between the molecular layer and the top electrode. Or, top electrode has been directly transferred on the molecular layers. As a result, the yield of molecular electronic devices increased high (> 80 %) and the electrical properties of molecular devices were well maintained during a long period of operation. With the development of high-yield molecular devices, it became possible to fabricate molecular devices even on flexible substrates. Our research group previously reported that alkanethiol molecular devices could be fabricated on flexible substrates with showing operative electrical properties under bending conditions. However, alkanethiol molecules are insulating and not suitable for real application because of the absence of potential device functionality. Therefore, I started to fabricate functional molecular devices on flexible substrates. In this thesis study, I fabricated rectifying and photoswitching molecular devices on flexible substrates and studied the electrical properties under bent substrate conditions. First, ferrocene-alkanethiolate functional molecular devices showed asymmetric electrical characteristics on both rigid and flexible substrates. I observed asymmetric current-voltage behavior because of a redox process of ferrocene part of rectifying molecules although the current asymmetric ratio (~1.6) was rather low. The rectifying molecular devices were well maintained in the asymmetric electrical behaviors under bent conditions. Second, diarylethene molecules are known to have two electrical conductance states; a closed (high conductance) state or an open (low conductance) state can be created upon illumination with UV or visible light, respectively. These two electrical states were defined and fixed during the device fabrication with illuminations of either UV or visible light, and showed distinct current levels between the two states. However, the fabricated diarylethene molecular devices did not show reversible switching phenomena. Lastly, in order to demonstrate reversible photoswitching process, I fabricated and characterized diarylethene molecular devices by using reduced graphene oxide (rGO) as top electrode. The photoswitching molecular devices with rGO top electrode successfully exhibited two stabile electrical states with different current levels and with reversible photoswitching capability. This study has a promise towards functional molecular devices on flexible substrates.