Yutaka Noguchi

Effect of the metal/organic interface phenomena on the current–voltage characteristics of organic single electron tunneling device

The current-voltage (I-V) characteristic of Au/PI/(Rh-G2 + PI)/PI/Au (or Al) junctions was examined. An asymmetrical I-V characteristic with several steps was observed for Au/PI/PI:Rh-G2/PI/Al junctions, whereas a symmetric I-V characteristic with steps was obtained for Au/PI/PI:Rh-G2/PI/Au junctions. It was concluded that the presence of metal/organic interfacial states made a significant contribution to the I-V characteristics of these junctions.

Space charge effect and the step voltages in metal/polyimide/rhodamine–dendorimer/polyimide/metal junctions

An electrostatic space charge layer of nanometer thickness is formed at organic film/metal interfaces due to the displacement of electrons from the metal to the film. This space charge layer directly affects the current–voltage (I–V) characteristics of artificially layered rhoadamine–dendorimer thin films, which show a step structure similar to a Coulomb staircase. The capacitance between one rhodamine–dendorimer molecule and a metal electrode is calculated using a sphere-plane model, assuming that excess electronic charges are displaced from the plane metal electrode into the film. This capacitance is found to increase as the excess charge increases which leads to a change in the voltage steps in the I–V characteristics as observed in metal/polyimide/ rhodamine–dendorimer/polyimide/metal junctions. Such a voltage step is derived theoretically, and will explain the difference between Au/polyimide/rhodamine–dendorimer/ polyimide/Al and Al/polyimide/rhodamine–dendorimer/polyimide/Al junctions.

Single-electron transport in metal/polyimide:C60/metal junction

Abstract Metal/organic film/metal (MIM) junctions were fabricated using C 60 dispersed in spin-coated polyimide (PI) ultra-thin film. The current–voltage ( I – V ) characteristics of these new types of junctions exhibit a Coulomb staircase behavior at a temperature of 15.5 K, a behavior not observed with C 60 -free MIM junctions. It is obvious that the Coulomb staircase behavior observed was induced by C 60 acting as a central electrode of the single-electron tunneling junction. We then analyzed the I – V characteristics of this MIM junction, taking into account both the Coulomb charging energy and the discrete energy states of the molecular central electrode. Comparing the experimental ( V / I )d I /d V – V characteristics with the calculated ones, a possible energy structure for C 60 in the junction was estimated.

Analysis of Potential Distribution and Current-Voltage Characteristic in Polyimide Langmuir-Blodgett Films

The current–voltage (I–V) characteristics of Au electrode (M)–polyimide (PI) Langmuir–Blodgett (LB) film (I)–Al electrode (M)-MIM element are analyzed, taking into account the interfacial electrostatic phenomena and the presence of the interfacial electronic states. On the basis of the Richardson–Schottky model, the attractive force and potential energy created by a series of mirror image charges against both electrodes are calculated. A positive bias (Vex>0) applied to an Au–PI LB film–Al element gives rise to the injection of electrons from the Au electrode into the lowest unoccupied molecular orbitals (LUMO) of PI LB film, whereas a negative bias (Vex<0) causes the injection from Al electrode. The potential distributions across the PI LB film and the potential barrier height under the condition of various externally applied voltages (Vex) are calculated, assuming the presence of surface states at the metal/film interface. It is determined that the calculation results of the I–V characteristics can predict the experimental results.

Analysis of Step Voltages in Single Electron Tunneling Devices Using Organic Thin Films

During the past ten years, electron tunneling devices such as Josephson junctions and single electron tunneling devices have been fabricated using organic thin films, where the successful preparation of polyimide Langmuir-Blodgett (PI LB) films has made a significant contribution. The electrical properties of PI LB films were also examined carefully, and electrostatic phenomena, interfacial phenomena, electrical breakdown behaviors and so on have been elucidated. Based on these studies, the effects of space charge and coupling capacitance upon the operation of single electron tunneling (SET) devices using organic molecules have been investigated. The equation of step voltage is derived taking into account space charge and coupling capacitance, using an equivalent circuit model. It was concluded that knowledge of interfacial phenomena is very important for a better understanding of the operation of such devices.

Photoinduced gate modulation and temperature dependence in the Coulomb staircase of organic single electron tunneling junctions

Threshold voltage shift in the Coulomb staircase induced by photoirradiation, that is, photoinduced gate modulation, was observed at a temperature of 15 K. The samples denoted by Au/PI/CuttbPc/PI/Al consisted of Kapton-type polyimide (PI) spin-coated films with copper tetra-t-butyl phthalocyanine (CuttbPc), sandwiched between Au and Al electrodes. The temperature dependence of the Coulomb staircase was also examined within the range of 15 K to 300 K. The single electron tunneling process was dominant below 150 K, whereas a thermally assisted electron conduction mechanism such as the hopping mechanism was dominant above 150 K. It was found that the photoinduced gate modulation and the temperature dependence observed in the Coulomb staircase of our junctions could be well explained by taking into account the space charge behavior at the metal/organic ultra-thin film interface.

Addendum: “Space charge effect and the step voltages in metal/polyimide/rhodamine–dendorimer/polyimide/metal junctions” [J. Appl. Phys. 90, 1368 (2001)]

Recently, we reported the observation of step structure, similar to Coulomb staircase, in the current–voltage characteristic of metal/polyimide(PI)/rhodamine-dendrimer(Rh-G2)/PI/metal junctions prepared by the Langmuir–Blodgett(LB) technique [J. Appl. Phys. 90, 1368 (2001)]. Calculating the additional apparent capacitance formed by electrostatic charge at the interface, the equation of threshold voltage of the step structure was derived and the interfacial space charge effect was pointed out. However, this analysis is not sufficient. In this Addendum, the equation of step voltage is derived taking into account both electrostatic charges and residual charges by electron tunneling. It was found that the derived equation is more satisfactory to explain our previous experimental result.

STM observation of Coulomb staircase behavior through C60 clusters

Abstract Two samples of C60 clusters were prepared on highly oriented pyrolytic graphite (HOPG) substrate by thermal evaporation. One is ∼1 nm and the other is ∼15 nm in radius. The current flowing across C60 clusters was measured using scanning tunneling microscope in atmosphere at room temperature. Coulomb staircase behavior was successively observed in the current–voltage ( I – V ) characteristics. Interestingly, the capacitances between HOPG and C60 clusters, estimated from the I – V characteristics, were almost the same between these two samples. We discussed this result taking into account the relaxation and localization time of electrons injected into C60 clusters.

Step voltage in metal/polyimide/organic molecule/polyimide/metal junction

Abstract A step structure, very similar to a Coulomb staircase, has been successfully observed in the current–voltage ( I – V ) characteristic of metal/polyimide (PI)/rhodamine-dendorimer (Rh-G2)/PI/metal junctions prepared by the Langmuir–Blodgett technique. Assuming Rh-G2 molecule functions like a metallic sphere and PI works as tunneling barriers, the I – V characteristic of the junctions was analyzed based on Coulomb blockade theory. Taking into account the presence of space charge at the PI-metal interface, which has been revealed by the surface potential measurement, the equation of threshold voltage of the step structure is derived. It was found that the derived equation is satisfactory to explain the I – V characteristic.

Single electron tunneling organic devices

Publisher Summary Until now, much effort has been done on the fabrication of novel devices based on physics of quantum mechanics, principally using the nano-fabrication technique developed in semiconductor device technology. As a result, single electron tunneling (SET) devices using small particles in their systems have been successfully prepared. Nano-fabrication technology may lead to a revolution in the field of electronics, and many novel electronic devices such as high-density memory devices, high-speed low-power switching devices, and highly sensitive electrometer devices will be produced in near future. The study of observing specific functions of organic molecules and applying these functions to electronic and optical molecular devices is of crucial help because one can realize novel functional devices only by using organic molecules without using the maturing nano-fabrication technology in the field of semiconductor device technology. This chapter introduces single electron tunneling (SET) devices using organic molecules prepared by the Langmuir–Blodgett (LB) technique. For the realization of SET devices, it is necessary to design the device system so that the one-electron charging energy of e 2 /2C is greater than the thermal energy. In this sense, the so-called small particles, whose size is less than several nanometers, must be introduced into the molecular systems to be used. However, it is difficult to do this. Overcoming the difficulties, an attempt to use an organic mono-molecule as a so-called “Coulomb island” has been made. The chapter also describes single electron tunneling devices prepared using organic molecules.