The research focus of our group centers on (1) synthesis and characterization of quantum dot/grapheme hybrid nanomaterials, (2) photocatalysis of metal/semiconductor heterostructure nanocrystals, (3) spintronic devices, and (4) design and fabrication of ultracapacitors based on colloidal nanocrystals and nanowires.
- Synthesis and Characterization of Quantum Dot/Graphene Hybrid Nanomaterials
Novel size-dependent properties of nanomaterials provide
many exciting opportunities to solve important problems in
optoelectronic applications. Successful integration of such
devices requires reliable and reproducible colloidal synthesis,
in which nanomaterials are produced with controlled size,
shape and composition with a reasonably narrow size distribution.
In a technique called arrested precipitation, molecular reactants are decomposed to a crystalline solid in a solvent.
The presence of organic ligands controls the size of the crystals and prevents surfaces from undergoing unwanted reactions. This approach has provided a unique solution to precise control over nanocrystal size and shapes in a colloidal phase. The versatility of the synthesis sparked research interest in the study of the size-dependent properties of colloidal inorganic nanocrystals. For instance, the energy gap (or optical gap) of semiconductor nanocrystals is size-tunable as a result of quantum confinement.
Despite the outstanding control of their structural and optical properties, the use of the nanocrystals in electronic devices has remained elusive, as the organic ligands generally make the nanocrystal film insulating. One way to address the problem is to replace the organic ligands with graphitic carbon materials. In this study, we synthesize nanometer-scale graphenes colloidally and use them as a passivation layer for the nanocrystals. The novel hybrid structures with improved conductivity ensure the use of the materials in a range of device applications, such as photodetectors, photodiodes, photovoltaics, and spintronics.
- Photocatalysis of Metal/Semiconductor Heterostructure Nanocrystals
A relatively new trend in the colloidal nanocrystals field is the synthesis of heterostructure nanomaterials comprising various functional components. The heterostructure architecture enables the fine-tuning of optical and electrical properties. This motif has been utilized for producing new emergent properties arising from strong interactions between different components located in the nanoscale proximity from each other. Control of the dimension of each component permits the comprehensive engineering of electronic energy state configuration within the nanoscale architecture.
Our group studies the metal/semiconductor heterostructure nanocrystals and their use in photocatalytic reactions. To have a better understanding of how we can improve catalytic activity and selectivity of the metal components, we investigate the charge transfer, charge carrier dynamics, and electron-hole separation within the heterostructure nanocrystals.
- Spin Coherence Control in Magnet/Semiconductor Hybrid Nanocrystals
The spin of the electron is a binary information carrier as an electron is a spin-1/2 fermion. In a technology called spin transport electronics, or spintronics, the spin is polarized, injected, and transported through a channel without an unwanted spin relaxation. This use of spin offers opportunities for a new generation of devices that utilize the interaction between spin of the carriers and the magnetic moments in the material. Spin-based electronics based on semiconductor architecture is expected to find applications in a greater variety of areas. However, the emerging semiconductor-based spintronics encounters a number of challenges, including the optimization of spin lifetime and the detection of spin coherence. For example, spin relaxation in a bulk semiconductor shows a characteristic time scale of <100 ps, faster than radiative recombination decay, and this fast relaxation impairs the reliable deployment of the spin polarization detection through light emitting diodes.
The discrete energy levels in semiconductor nanocrystals (quantum dots) lead to a predicted quenching of the dominant spin relaxation channels and account for spin relaxation time considerably longer than the exciton lifetimes. Therefore, a light-emitting diode using quantum dots as an optical marker is an effective means of detecting spin-polarized carriers. The degree of polarization of photoluminescence from quantum dots is a direct measure of the spin polarization of the carriers injected into the quantum dots. In this regard, the search for nanomaterials combining the properties of quantum dots and magnets has been a long-standing goal.
Schematic illustration of spin-polarized emission mechanism (A), spin flip during intrabad relaxation (B), and spin exchange at a radiative energy level (C). Semiconductor-spacer-magnet heterostructure nanocrystals (D) will be explored in this study.
Our group explores hybrid nanocrystals comprising a magnetic metal and a semiconductor in core/shell architecture. The hybrid nanocrystals permit the study of the effect of the magnetic component on the optical properties of the semiconductor. The understanding of magnetic interactions is important in ultimate realization of long spin-coherence time detection scheme. We study the correlation between semiconductor degeneracy and the magnetic properties of a proximal nanomagnet. Atomic precision in the synthesis of nanocrystals enables the design of structures for better control of spin coherence, which would be a significant milestone in spin coherence detection technology. In this project, by using magnetic metal as a component, we control the magnetic interactions separately.
- Ultracapacitors based on Colloidal Nanocrystals and Nanowires
Energy harvested from various energy sources, especially from solar radiation, needs to be stored effectively without significant loss to energies of less useful forms. Aside from the cost and efficiency issues addressed above, broad dissemination of solar energy use is limited by the inherent intermittency of the source. Ultracapacitors, also known as electrochemical double-layer capacitors, are among the fastest charging/discharging energy storage media, and the pursuit of appropriate materials is a quintessential part in the development of high-power density capacitors.
One significant question to be asked prior to the use of the nanomaterials as a capacitor component is ‘can we synthesize the nanomaterials in technologically
meaningful quantities?’ Nanomaterial-based capacitors have lacked scalability in synthesis of the nanomaterials.
In the pursuit of high-quantity synthetic routes, our group employs the solution-based growth of nanocrystals and nanowires.
Doh Chang Lee (이도창)