The research team led by Professor Jeong Kwang-seob of the Department of Chemistry, Korea University and Professor Kim Ju-yeong of the Department of Chemistry, Gyeongsang National University, observed intraband transition and localized surface plasmon resonance of self-doped quantum dot.

The results from the research team were published in Nano Letters, an international journal in nanoscience.

▲(From the left) Professor Jeong Kwang-seob of the Department of Chemistry, Korea University; Professor Kim Ju-yeong of the Department of Chemistry, Gyeongsang National University; and Doctor Choi Dong-sun and Doctor Son Ju-hee of the Department of Chemistry, Korea University.

The research team led by Professor Jeong Kwang-seob of the Department of Chemistry, Korea University and Professor Kim Ju-yeong of the Department of Chemistry, Gyeongsang National University reported that the team induced a structural change and an increase of electron density of quantum dot, and observed quantum plasmon where both the characteristics of electron transition and localized surface plasmon resonance are found together.

Stained glass, often found in the churches and cathedrals of the Middle Ages, is the result of a phenomenon called plasmon resonance, which is caused by collective resonance of the electrons on a metal surface. A material having a high electron density such as metal shows different colors depending on the size, when the particle size is decreased to several nanometers.

On the other hand, semiconductor nanoparticles (colloid quantum dots) have an increased level of kinetic energy, as the electrons and holes are contained in a limited space. As a result, the semiconductor nanoparticles obtain higher energy than other materials. In contrast to plasmon resonance, when light is irradiated, quantum dots show a variety of colors depending on the types and sizes of the materials. This means that the wavelength of light may be controlled. The colloidal quantum dots are the main optical materials of the next-generation quantum dot light-emitting diodes (QLED) TV.

The electron density of the conventional quantum dots is hardly increased because of oxidation where the internal electrons are discharged by an external factor when the electron density is increased. Professor Jeong’s team has successfully demonstrated self-doping of silver selenide through the fine tuning of the material composition. Going one step further, his team observed that the absorption wavelength peak of the colloidal quantum dots of silver selenide was slightly divided into two wavelengths in the infrared spectra, showing that the phenomenon is the result of a structural change in the material. Specifically, the peaks are overlapped with each other because the transition* energy levels in the band are the same. As the material structure is turned asymmetrical, the energy levels are split. This phenomenon, a maximization of the quantum properties, is applicable to multi-numeration quantum memory.

*Intraband transition refers to electron transition between discontinuous energy levels in a conduction band or balance band. It was found in 2014 that steady-state intraband transition is observable in colloidal quantum dots.

In addition, an increase of electron density was found with the change in the structure. With the increase of the density, both quantum properties and localized surface plasmon resonance occurred simultaneously. This suggests the possibility of using more energy levels of the colloidal quantum dots, which are a nanoscale quantum material, and thus selectively utilizing the properties of semiconductor and metallic nanomaterials.

▲A schematic diagram of quantum plasmon occurring in colloidal silver selenide quantum dots

Professor Jeong stated, “We expect that quantum plasmon, which is observed simultaneously with broken degeneracy depending on the colloidal quantum dot* structure of low-toxicity silver chalcogenide, can widen the scope of the quantum nanomaterial applications in physical chemistry, material chemistry and optics.” He also added, “I appreciate the graduate students, Son Ju-hee, Choi Dong-sun, and Park Mi-hyeon, and Professor Kim Ju-yeong of the Gyeongsang National University for their continuous discussion and efforts for this work.”

*Colloidal quantum dot refers to a zero-dimensional spherical semiconductor of several nanometers formed by wet chemical synthesis. The material is used as an energy conversion material for TV, displays and solar cells.

The study was supported by the Individual Basic Science and Engineering Research Program (Korean SGER) funded by the Ministry of Education and the National Research Foundation of Korea and the Creative Materials Discovery Program (led by Jeong So-hee) funded by the Ministry of Science and ICT and the National Research Foundation of Korea, and the relevant article was published in the June 9 online issue of Nano Letters, an international journal in nanoscience.

*Article title: Transformation of Colloidal Quantum Dot: From Intraband Transition to Localized Surface Plasmon Resonance.

*Authors: Son Ju-hee (Department of Chemistry, Korea University, first co-author), Choi Dong-sun (Department of Chemistry, Korea University, first co-author), Park Mi-hyeon (Department of Chemistry, Korea University, co-author), Kim Ju-yeong (Department of Chemistry, Gyeongsang National University, co-author) and Jeong Kwang-seob (Department of Chemistry, Korea University, corresponding author).