Single PbS Colloidal Quantum Dot Transistors

Colloidal quantum dots (CQDs) are small semiconductor crystals with a diameter of several nanometers [1]. CQDs exhibit excellent light emission/absorption characteristics, and their optical bandgaps are widely tunable by adjusting their size. CQDs can be treated by liquid processes, and their functionality can be controlled by selecting suitable ligands [2]. These properties make CQDs attractive candidates for use as transport channels in single-electron transistors (SETs) operating at high temperatures. However, there have been only a few evaluations of carrier transport through single CQDs in transistor geometries because of the technical difficulties in electrically accessing single CQDs prepared by bottom-up methods.<br/><br/>In this work, we report the first demonstration of single-CQD transistors based on commercially available high-quality PbS CQDs with oleic acid as a ligand. We made electrical contact to a single CQD using nano-gap metal electrodes and measured single-electron tunneling through the CQDs. The transport characteristics measured at 4 K strongly depend on the quantum dot size; a few-electron regime is observed in small PbS CQDs, while a many-electron regime is observed in large CQDs. From the orbital-dependent electron charging energy and conductance, we demonstrate that the tunneling barrier in this system is formed not only by the capping material but also by the intrinsic gap between the electron wavefunction in the CQDs and electrodes. Analysis of the excited states observed in the Coulomb stability diagram indicates that the confinement potential of electrons in CQDs is strongly affected by the external electric field induced by gate and source-drain voltages, suggesting the tunability of the effective confinement size of electrons and the bandgap in CQDs by the external electric field. Moreover, spin-correlated coherent carrier transport (the Kondo effect) has been observed for the first time in the CQD system; this indicates strong coupling between the electrodes and the CQDs despite the use of long-chain insulating oleic acid ligand. These results provide nanoscopic insight into carrier transport through CQDs at the single quantum dot level, which is essential for developing CQD applications in optoelectronic devices, such as solar cells and photodetectors.<br/><br/>Furthermore, the large charging energy in small CQDs enables SET operation at high temperature. While there is a tendency for the noise to increase at higher temperatures, we obtained diamond-like Coulomb stability diagrams even at room temperature, confirming that the devices operate as SETs at room temperature [3]. This is the first demonstration of room-temperature SET operation in semiconductor nanocrystal systems. Considering that PbS CQDs are excellent emitters and absorbers of light, our device is a platform for room-temperature SETs with good optical properties, increasing the functionality and versatility of single-electron devices. This will bring about innovation in quantum information technology.<br/><br/>[1] Y. Yin and A. Alivisatos, Nature 437, 664-670 (2005).<br/>[2] F. de Arquer, D. Talapin, V. Klimov, Y. Arakawa, M. Bayer, and E. Sargent, Science 373, eaaz8541 (2021).<br/>[3] K. Shibata, M. Yoshida, K. Hirakawa, T. Otsuka, S. Z. Bisri, and Y. Iwasa, Nature Communications, in press (2023).<br/><br/>This work was partly carried out at the Fundamental Technology Center and the Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University. This work was partly supported by the Cooperative Research Projects of RIEC, JKA and its promotion funds from KEIRIN RACE, and Grants-in-Aid from the JSPS (JP20H05660, JP21K04815, and JP21K04820).