1. Atomic Layer Deposition (ALD); Ph.D. research (NCSU)
ALD is a thin film deposition technique using the sequential dose and purge of gas-phase precursors. Basically, the film growth relies on self-limiting reactions on a substrate, such that no more reaction- and no film growth occurs once the surface of substrate is saturated with an atomic layer of precursor vapor. Those key features of ALD facilitate to control the film thickness as well as the composition in molecular and even atomic levels precisely. In the Ph.D. study, my research interest preliminary focused on ALD metal oxides (TiO2, ZnO2, Al2O3, and Fe2O3) with several nanometer thicknesses integrating them with dye-sensitized solar cells (DSSCs). Because the photoelectrodes of DSSCs are composed of mesoporous structures in nano-scale, there are a lot of opportunities for ALD films to engineer the surfaces. Nanoscale ALD metal oxide films were exploited to enhance the performance and long-term stability of DSSCs. Other than DSSCs project, my ALD metal oxide films functionalized fibers, synthesized bi-component nanosheets, and developed novel structures for photoelectrochemical cells through collaborative research.
My personal view on the ALD technology for DSSCs is summarized in a progress report published recently;
D. H. Kim, M. D. Losego, Q. Peng, and G. N. Parsons, “Atomic layer deposition for sensitized solar cells: Recent Progress and Prospects”, Advanced Materials Interfaces, (2016) DOI:10.1002/admi.201600354
- Passivation films: ALD films, thick of < 1 nm on dye molecules of mesoporous TiO2 made dye-attachment durable in low pH or aqueous electrolytes. This is the first multilayer composed of ALD metal oxides to stabilize the dye molecules and study the effect on the long-term stability of DSSCs.
D. H. Kim, M. D. Losego, K. Hanson, L. Alibabaei, K. Lee, T. J. Meyer, and G. N. Parsons, “Stabilizing chromophores for dye-sensitized solar cells using multicomponent, sub-nanometer atomic layer deposition”, Physical Chemistry Chemical Physics, 16(18), 8615 (2014). DOI:10.1039/c4cp01130a
- Electrocatalytic films: Developed a platinum-free cathode with the comparable performance of conventional platinum cathode using oxidative molecular layer deposition (oMLD) of conductive polymers (PEDOT) on a mesoporous ITO structure.
D. H. Kim, S. E. Atanasov, P. Lemaire, K. Lee, and G. N. Parsons, “Platimum-free cathode for dye-sensitized solar cells using poly(3,4-ethylenedioxythiophene) (PEDOT) formed via oxidative molecular layer deposition” (2015) ACS Applied Materials & Interfaces, 17, 3866 (2015) DOI: 10.1021/am5084418
- Blocking layer: The thinnest (5-10 nm) and the most efficient blocking layer was synthesized by ALD for dye-sensitized solar cells (DSSCs) and systematically examined.
D. H. Kim, M. Woodroof, K. Lee, G. N. Parsons, “Atomic layer deposition of high performance ultra-thin (< 10 nm) TiO2 blocking layers for dye-sensitized solar cells”, ChemSusChem, 6 (6), 1014, (2013). [featured on front cover] [highligted in Chemistryviews] DOI: 10.1002/cssc.201300067
- Light-scattering layer: Developed a highly light scattering layer; coated quartz fibers with TiO2 ALD. TiO2 ALD on quartz fibers thermally stabilized anatase TiO2 even over 1050 °C and outperformed conventional light scattering layer for DSSCs.
D. H. Kim, H. Koo, J. S. Jur, M. Woodroof, B. Kalanyan, K. Lee, C. K. Devine, and G. N. Parsons, “Stable anatase TiO2 coating on quartz fibers by atomic layer deposition for photoactive light-scattering in dye-sensitized solar cells”, Nanoscale, 4, 4731-4738 (2012). DOI: 10.1039/c2nr30939d
- Non-DSSCs projects:
1) Investigated hydrophilic/hydrophobic property transition of cellulose cotton with a few ALD cycles: assisted with in-situ FT-IR analysis.
K. Lee, Jesse S. Jur, D. H. Kim, and G. N. Parsons, “Mechanisms for hydrophilic/hydrophobic wetting transitions on cellulose cotton fibers coated using Al2O3 atomic layer deposition”, Journal of Vacuum Science and Technology. A 30, 01A163 (2012). DOI: 10.1116/1.3671942
2) Developed metal oxide nanosheets using ALD which showed outperforming (×4) photocatalytic properties comparing with those of commercial catalytic powders: established a kinetic model of photodegradation.
K. Lee, D. H. Kim, and G. N. Parsons, “Free-floating synthetic nanosheets by atomic layer deposition”, ACS Applied Materials & Interfaces, 16 (14), 10981, (2014) DOI: 10.1021/am502850p
3) Developed ALD TiO2 coated conductive meoporous structures for photoelectrochemical cells: investigated systematically the roughness factors of the coated conductive mesoporous structure.
Q. Peng, B. Kalanyan, P. G. Hoertz, A. Miller, D. H. Kim, K. Hanson, L. Alibabaei, J. Liu, T. J. Meyer, G. N. Parsons, J. T. Glass, “Solution-processed, antimony doped tin oxide colloids film enabled high-performance TiO2 photoanodes for water splitting”, Nano Letters, 13 (4) 1481, (2013). DOI:10.1021/nl3045525
2. initiated Chemical Vapor Deposition (iCVD); Postdoctoral research (MIT)
In iCVD invented by Prof. K. Gleason at MIT, vapors of monomer and initiator flow into the vacuum chamber, in which initiator vapor contacts to resistively hot filaments (250-300 C) breaking into free-radicals while monomer is liable at the temperature. Then a free-radical polymerization of monomer begins at a substrate cooled down. iCVD is a vapor version of conventional solution-phase free-radical polymerization. My postdoctoral research is on utilizing and understanding the unique properties of iCVD films in directed self-assembly (DSA) process.
- Nanoscalable nanomufacturing of sub-10 nm patterns: Invents a sub-10 nm patterning technology using iCVD films and directed self-assembly (DSA) process (Chemoexpitaxy). The project is seeking a nano-scalable manufacturing technology which can be readily employed to the semiconductor industry, understanding fundamentals of polymer physics observed in exploiting of two processes.
H. S. Suh*, D. H. Kim*, P. Moni, S. Xiong, L. C. Ocola, N. J. Zaluzec, K. K. Gleason, and P. F. Nealey, “Sub-10 nm patterning via directed self-assembly of block copolymer films with a vapour-phase deposited topcoat”, Nature Nanotechnology (2017) (In press) *equally contributed. DOI:10.1038/nnano.2017.34
3. Non-convetional lithography; M.S. research (SNU)
The semiconductor industry has made a great effort to produce fine patterns in a cost-effective way as the feature size is getting close to the limit of the wavelength of UV light source in current photolithography. Thus, the thermodynamic nature of polymer films has been explored as an enabling technology, which is broadly called as soft lithography. My MS research contributed to control the dewetting phenomena of polymer films and the adhesion strength between polymer films and substrates. They were successfully employed in fabricating fine patterns below ~1 um with no light source and masks.
- Selective dewetting of polymer films: Invented a new method to make metal nano-patterns without photolithographic process, in which the selective dewetting of polymer films on heterogeneous structures was introduced to fabricate nano-sized Cr/Metal metal patterns.
D. H. Kim, M. J. Kim, J. Y. Park, H. H. Lee "Low-duty-ratio patterning on a heterogeneous surface", Advanced Functional Materials 15, 1445-1450 (2005). DOI: 10.1002/adfm.200500137
- Detachment-patterning: Developed a simple technique, as like stamping, to fabricate patterned light emitting films for organic light emitting display (OLED), in which the difference of adhesion strength between soft-molds (stamps), flims, and substrate at the elevated temperature was used and validated with the work of adhesion.
J. H. Choi, D. H. Kim, H. H. Lee “Simple detachment patterning of organic layer and its application to organic light emitting diodes”, Advanced Materials 17,166-171 (2005). DOI: 10.1002/adma.200400223