Education & Certifications
Postdoc, Stanford University, Materials Science & Engineering (2015)
Ph.D, Seoul National University, Chemical and Biological Engineering (2010)
B.S., Seoul National University, Chemical Engineering (2003)
Towards clinically translatable in vivo nanodiagnostics
Nature Reviews Materials
View details for DOI 10.1038/natrevmats.2017.14
Two-dimensional limit of crystalline order in perovskite membrane films.
2017; 3 (11): eaao5173
Long-range order and phase transitions in two-dimensional (2D) systems-such as magnetism, superconductivity, and crystallinity-have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO3 membrane lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. The transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.
View details for DOI 10.1126/sciadv.aao5173
View details for PubMedID 29167822
View details for PubMedCentralID PMC5696264
Vertical Heterostructure of Two-Dimensional MoS2 and WSe2 with Vertically Aligned Layers.
2015; 15 (2): 1031-1035
Two-dimensional (2D) layered materials consist of covalently bonded 2D atomic layers stacked by van der Waals interactions. Such anisotropic bonding nature gives rise to the orientation-dependent functionalities of the 2D layered materials. Different from most studies of 2D materials with their atomic layers parallel to substrate, we have recently developed layer vertically aligned 2D material nanofilms. Built on these developments, here, we demonstrate the synthesis of vertical heterostructure of n-type MoS2 and p-type WSe2 with vertically aligned atomic layers. Thin film of MoS2/WSe2 vertical structure was successfully synthesized without significant alloy formation. The heterostructure synthesis is scalable to a large area over 1 cm(2). We demonstrated the pn junction diode behavior of the heterostructure device. This novel device geometry opens up exciting opportunities for a variety of electronic and optoelectronic devices, complementary to the recent interesting vertical heterostructures with horizontal atomic layers.
View details for DOI 10.1021/nl503897h
View details for PubMedID 25590995
In situ observation of divergent phase transformations in individual sulfide nanocrystals.
2015; 15 (2): 1264-1271
Inorganic nanocrystals have attracted widespread attention both for their size-dependent properties and for their potential use as building blocks in an array of applications. A complete understanding of chemical transformations in nanocrystals is important for controlling structure, composition, and electronic properties. Here, we utilize in situ high-resolution transmission electron microscopy to study structural and morphological transformations in individual sulfide nanocrystals (copper sulfide, iron sulfide, and cobalt sulfide) as they react with lithium. The experiments reveal the influence of structure and composition on the transformation pathway (conversion versus displacement reactions), and they provide a high-resolution view of the unique displacement reaction mechanism in copper sulfide in which copper metal is extruded from the crystal. The structural similarity between the initial and final phases, as well as the mobility of ions within the crystal, are seen to exert a controlling influence on the reaction pathway.
View details for DOI 10.1021/nl504436m
View details for PubMedID 25602713
- Two-dimensional layered transition metal disulphides for effective encapsulation of high-capacity lithium sulphide cathodes NATURE COMMUNICATIONS 2014; 5
- High-resolution three-photon biomedical imaging using doped ZnS nanocrystals NATURE MATERIALS 2013; 12 (4): 359-366
Giant Zeeman splitting in nucleation-controlled doped CdSe:Mn2+ quantum nanoribbons
2010; 9 (1): 47-53
Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.
View details for DOI 10.1038/NMAT2572
View details for Web of Science ID 000272854800018
View details for PubMedID 19915554
Magnetic fluorescent delivery vehicle using uniform mesoporous silica spheres embedded with monodisperse magnetic and semiconductor nanocrystals
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2006; 128 (3): 688-689
We synthesized uniform pore-sized mesoporous silica spheres embedded with magnetite nanocrystal and quantum dots. The magnetic separation, luminescent detection, and controlled release of drugs were demonstrated using the uniform mesoporous silica spheres embedded with monodisperse nanocrystals.
View details for Web of Science ID 000234815000010
View details for PubMedID 16417336