Evaluation of semiconductor colloidal quantum dots using environmentally compatible materials

Professor Kenji Shibata and Undergraduate Students Tomoki Takiguchi, Akira Sato, and Yuto Sasaki from the Faculty of Engineering at Tohoku Institute of Technology, together with Associate Professor Tomohiro Otsuka from the Advanced Institute for Materials Research at Tohoku University, have succeeded in evaluating and magnetically controlling single spin states in individual semiconductor colloidal quantum dots using RoHS-compliant environmentally compatible materials. For the first time, they achieved room-temperature operation of a single-electron transistor. Their results were published in ACS Nano.

Semiconductor colloidal quantum dots are extremely small semiconductor particles, also called artificial atoms. They have attracted attention as active layers in optoelectronic devices such as solar cells, and their optical properties have been relatively well-studied to date. On the other hand, research on the electrical properties of colloidal quantum dots has been limited, and evaluation of the electrical conduction of individual colloidal quantum dots in particular has rarely been conducted due to technical difficulties, leaving many problems to be clarified.

Two years ago, the research group fabricated a single-electron transistor (SET) using a single PbS semiconductor colloidal quantum dot, successfully conducted detailed evaluation of the electrical conduction of a single quantum dot, which had previously been difficult, and realized SET operation at room temperature for the first time. This achievement marked an important first step toward the application of colloidal quantum dots to optoelectronic devices and quantum information devices.

In this study, SET structures were fabricated using commercially available InAs colloidal quantum dot solutions. InAs quantum dots are compliant with RoHS regulations and have superior environmental compatibility compared with PbS quantum dots. A conductive silicon substrate was used as the gate electrode to control the number of electrons in the quantum dots. Dispersed placement of quantum dots is possible through solution processing.

Through measurements at low temperatures, the researchers observed diamond-shaped structures (Coulomb diamonds), clearly demonstrating that electrons flow one by one through the quantum dots and confirming that the device functions as a SET. Notably, it was demonstrated that SETs using quantum dots of approximately 6 nanometers operate as SETs even at room temperature because the electron-electron interaction greatly exceeds the thermal energy at room temperature. This is the second example of a room-temperature-operating SET using colloidal quantum dots, following a SET using PbS, and represents a new development due to the difference in materials.

Furthermore, analysis of characteristic changes in the magnetic field revealed that two conductance peaks corresponding to two electron levels with different spin orientations formed in the same orbital within the quantum dot shifted significantly with the magnetic field. The interval between the two peaks increased in proportion to the magnetic field, clearly capturing the splitting of electron levels due to the Zeeman effect. The g-factor representing the magnetic field sensitivity of electrons derived from this behavior showed a very large value of 15. Behind this remarkable spin response lies the fact that InAs is a material with inherently strong spin-orbit interaction, and this is considered to be the main factor that enabled the detection and control of single spin states.

These results not only contribute to the application of colloidal quantum dots to optoelectronic devices but also open up new possibilities for development in quantum information devices and spintronics technologies that utilize extremely small quantum dots with prominent quantum effects.

Journal Information
Publication: ACS Nano
Title: Magnetotransport and Coulomb Blockade in Single InAs Colloidal Quantum Dot Transistors
DOI: 10.1021/acsnano.5c14784