* Applied Quantum and Statistical Mechanics
* New Multiferroic Materials
* Functional Nanostructures and Devices
* Quantum-Dot-Sensitized and Dye-Sensitized Solar Cells
* Applied Quantum and Statistical Mechanics
We are able to predict electronic structure and spin configuration of various functional oxides by exploiting first-principles quantum mechanical method, called density-functional theory (DFT). We compute the three-dimensional (3-D) electronic density contour and the corresponding energy accurately by solving the Kohn-Sham independent-electron equation. In particular, we are quite successful in accurately predicting the orbital-interaction mechanisms that are responsible for the manifestation of ferroelectricity in multiferroic materials. These first-principles calculations greatly help us to obtain a useful insight into design and fabrication of advanced functional materials.
The research topics that we are currently investigating include: (i) novel hexagonal ferroelectricity and multiferroism by structurally tailoring rare-earth ferrites, ReFeO3 with nonpolar orthorhombic symmetry [J. Am. Chem. Soc., 134, 1450 (2012)], (ii) improper ferroelectricity in antiferromagnetic SmFeO3 by a Si•Sj-type exchange-striction mechanism [A Highlight Article in Phys. Rev. Lett, 107, 117201 (2011)], (iii) the origin of covalent-bond-driven ferroelectricity and 4d-5p orbital self-mixing in InMnO3 hexagonal antiferro-magnet [Phys. Rev. Lett, 106, 047601 (2011)], (iv) variations of ferroelectric off-centering distortion and 3d-4p orbital mixing in La-doped BiFeO3 multiferroics [Phys. Rev. B, 82, 045113 (2010)], (v) first-principles prediction of the morphotropic phase boundary in doped BiFeO3 multiferroics [J. Mater. Chem., 22, 1667 (2012)], (vi) asymmetric Ho 5d-O 2p hybridization as the origin of hexagonal ferroelectricity in multiferroic HoMnO3 [Phys. Rev. B, 84, 153106 (2011)], and most recently (vii) first-principles prediction of the evolution of a long-range spin-density wave and the associated polarization flipping in rare-earth manganites that are characterized by an incommensurate spiral spin ordering.
Prof. Hyun M. Jang is also actively doing research in statistical mechanics of displacive ferroelectricity and phonon softening, in addition to Landau-Lifshitz-Ginzburg (LLG) theory of ferroelectricity and multiferroism. His research topics in these areas include: (i) statistical mechanical theory of displacive ferroelectricity, phonon softening, and mode-mode coupling, (ii) relaxation mode versus oscillation mode in relaxor ferroelectrics, (iii) statistical mechanical theory of multiferroism in BiFeO3, (iv) Landau-Lifshitz-Ginzburg theory of multiferroism in BiFeO3, and (v) Statistical mechanical theory of improper ferroelectricity.
A schematic representation of the cycloidal spin-density wave (SDW) characterized
by the wavevector k of 0.25 along the orthorhombic b axis of TbMnO3. This cycloidal SDW on the a-b plane was formed by the flipping of
the incommensurate SDW (k=0.28) on the b-c plane upon applying a bias magnetic field along the wave-propagation
direction, b. The red-colored arrow
which is parallel to the a-axis
designates an improperly developed small polarization according to the reverse
Dzyaloshinskii-Moriya interaction of a SixSj-type.
(3-D) electron-density contour of the paraelectric P63/mmc phase of
YbFeO3 is compared with those of the two ferroelectric phases, P63mc
and P63cm [from J. Am. Chem. Soc., 134, 1450-1453
The 3-D contour was calculated using the
density-functional theory. Upon the
transition to the ferroelectric P63cm ground state, there occurs a
strong asymmetric covalent bonding interaction between the Yb ion and one of the
two axial oxygen ions (OA) along the c-axis. This clearly shows that the ferroelectricity
in the P63cm
phase is covalent in nature.
Schematic diagrams of the four distinct
possibilities of the Ho-OA orbital interactions in HoMnO3:
(a) 6pz(Ho)-2pz(OA), (b)
6px(Ho) or 6py(Ho)-2pz(OA), (c)
5dz2(Ho)-2pz(OA), and (d) 6s(Ho)-2pz(OA)
orbital interactions. Examination of the orbital-resolved partial density of
states (PDOS) indicates that the asymmetric 5dz2(Ho)-2pz(OA)
hybridization is primarily responsible for the manifestation of the hexagonal
ferroelectricity along the c-axis of
HoMnO3 with the non-centrosymmetric P63cm symmetry.
A graphical representation of the eigenvectors
of the two ‘soft’ phonon modes that are responsible for the manifestation of
displacive ferroelectricity: (i) the non - degenerated A1(1TO) mode
and (ii) the doubly degenerated E (1TO) mode of a ABO3-type
perovskite (e.g., PbTiO3)
having tetragonal 4mm symmetry.
* New Multiferroic Materials
exhibit simultaneous ferroic properties with coupled electric, magnetic, and
structural orders in a single phase. Multiferroic materials have received a
great deal of attention because of their potential for enabling entirely new
device paradigms. We are currently
developing several new multiferroics that are characterized by exotic ferroic
properties which include a strong magnetoelectric (ME) coupling.
Several typical ferroic
systems that we are currently investigating include: (i) artificially tailored epitaxial
thin films of hexagonal ferrites that show a stepwise ferroelectric transition
and a spontaneous magnetization reversal, (ii) an orthorhombic ferrite that
exhibits an interesting phenomenon of spin-canting-induced improper
ferroelectricity, (iii) a hexagonal manganite (RMnO3) that involves
an anomalous covalent-bonding mechanism of R10-ness-driven
ferroelectricity, and (iv) a new class of Pb-hexaferrites that shows local spin
reversal and collinear magnetostriction-induced ferroelectricity.
* Functional Nanostructures and Devices
are currently designing and practically implementing a variety of novel
functional nanostructures and devices, in collaboration with Prof. Jong Yeog
Son currently at Dept. of Applied Physics, Kyung Hee University (former research professor of
our group). Recently, we have developed
a graphene nano-ribbon (GNR) based field-effect transistor that shows bipolar
FET behavior with a high electronic mobility and a low operation voltage at
room temperature. This work was recently selected as a ‘Featured Highlight’
by Nature Publishing Group (NPG) Asia Materials:
research topics that we are currently investigating in functional nano-scale
devices include: (i) a new nano-intaglio process to form a nano - template and a
series of nano-scale grooves using atomic force microscopy [J. of Phys. Chem. C, 115, 14077-14080 (2011)], (ii) development of a graphene nano-ribbon (GNR)
based field-effect transistor [J. Am. Chem. Soc., 133, 5623-5625 (2011)], (iii) a nonvolatile memory device made of a
ferroelectric polymer gate nanodot and a single - walled carbon nanotube [ACS Nano, 4,
(iv) development of NiO resistive random access memory (R-RAM) capacitors by
exploiting self-formed exchange bias [ACS
Nano, 4, 3288-3292 (2010)], (v) development of NiO resistive random access
memory (R-RAM) nano-capacitor array on graphene [ACS Nano, 4,
and (vi) four - states nonvolatile multiferroic memory devices based on
vertically aligned nano-rods.
* Dye-Sensitized and Quantum-Dot-Sensitized Solar Cells
◇ Dye-sensitized solar cells
increasing demand of sustainable renewable energies, dye-sensitized solar cells
(DSCs) have been considered to be a
promising alternative to relatively expensive conventional silicon-based solar
cells. DSC is made of n-type wide-band semiconductor (mostly TiO2
and ZnO), which is sensitized with dye that excites an electron by absorbing
photons to produce photocurrent. It is also called a ‘kinetic cell’ because
there is no charge separation caused by built-in potential, but difference of
rate constants between electron excitation, diffusion and recombination gives
rise to power generation. Since the announcement of a sensitized
electrochemical photovoltaic device by O’Regan and Grätzel in 1990 with 7 % of
overall efficiency, the development of DSC has continued to achieve certified
efficiency up to 11% which is half level of silicon single crystal solar cells.
If its efficiency is reached close to theoretically predicted value, its cost
competitiveness will be strikingly improved, compared to silicon-based photovoltaics.
Typical structure of DSC is as follows. It consists
of conducting glass substrate (FTO), wide - band gap semiconductor (TiO2),
sensitizer, electrolyte including redox mediator, and Pt counter electrode.
DSC is fabricated on conducting glass,
fluorine-doped SnO2 (FTO) so that irradiated light can pass through
the substrate. Anatase TiO2 nanoparticles are generally deposited on
the FTO substrate as form of colloidal suspensions, then annealed at 500 °C to ensure inter-particle connectivity. TiO2
nanoparticles are 10~20 nm in diameter, and prepared as a mesoporous film,
giving sufficient surface area to adsorb enough amount of sensitizer. Typical
nano-structured TiO2 film provides 1000 times larger surface area
than flat TiO2 film.
The electrolyte of the DSC primarily uses triiodide/iodide (I3-/I-) as a redox couple. Therefore, it is essential to understand the regeneration and recombination kinetics of the I3-/I- redox couples in the device. In this context, controlling the total and local concentrations of the I3-/I- redox couples is an important parameter that can influence the DSC performance. Here, we propose that the introduction of a sodium bis (2-ethylhexyl) sulfosuccinate (AOT)/water system to the I3-/I- electrolyte enables the control of the concentration of the redox couples, which consequently achieves a high power conversion efficiency of ~11% for ~1000 h (under one sun illumination) owing to the enhanced dye-regeneration efficiency and the reduced recombination rate. This novel concept assists in the comprehension of the regeneration and recombination kinetics and develops highly efficient DSCs. [Adv. Energy Mater., 3, 1344-1350 (2013)]
Ion exchange is a versatile method for efficient dye-adsorption. Herein, we show that the ion exchange using aerosol OT (AOT)offers twice as fast as known methods in the dye-loading process. Moreover, it suppresses the dye-agglomeration that may cause insufficient dye-coverage on the photoelectrode surface. Consequently, this bi-function of fast dye-loading and higher dye-coverage significantly improves the power conversion efficiency of dye-sensitized solar cells. [Chem. Commun., 49, 6671-6673 (2013)]
structural viewpoint, multi-layered DSCs have been reported to show superior
efficiencies to single- and bi-layered devices, because of the optimized light
trapping within the photoelectrodes. However, their structural complexity and
restricted dye-loading still remains a challenge, which may limit wider
applications to the DSCs. To deal with this problem, we have implemented optically-tunable
TiO2 hierarchical nanomaterials with three distinct configurations.
Since these nanostructures are formed by aggregation of mesoporous TiO2
particles, they turn out to have sufficient surface area of > 70 m2∙g-1.
Then, the hierarchically-structured multi-layer was demonstrated by simple layering
of three TiO2 hierarchical nanomaterials. As a result, effective
light confinement was achieved over a wide wavelength range without
compromising the dye-loading issue. [J. Mater. Chem. A, DOI: 10.1002/aenm.201300275 (2013)]
Instead of using a dye, the sensitization of a photoelectrodes
can be achieved through modification of the oxide surface with a narrow
band-gap semiconductor quantum-dot (QD). Quantum-dot-sensitized solar cells
(QDSCs) recently have attracted a great deal of attention owing to their
advantages over DSCs. The advantages include (i) higher molar extinction
coefficient of QDs than ruthenium complexes, (ii) tunable energy gaps, and
(iii) multiple exciton generation which may potentially lead to a theoretical maximum
efficiency with cheap manufacturing cost over that of DSCs.
The sea urchin TiO2 (SU TiO2) particles composed of radially aligned rutile TiO2 nanowires are successfully synthesized through the simple solvothermal process. SU TiO2 was incorporated into the TiO2 nanoparticle (NP) network to construct the SU-NP composite film, and applied to the CdS/CdSe/ZnS quantum-dot-sensitized solar cells (QDSSCs). A conversion efficiency of 4.2% was achieved with a short-circuit photocurrent density of 18.2 mA cm2 and an open-circuit voltage of 531 mV, which corresponds to ~20% improvement as compared with the values obtained from the reference cell made of the NP film. We attribute this extraordinary result to the light scattering effect and efficient charge collection. [Phys. Chem. Chem. Phys., 14, 4620-4625 (2012)]
As a modification version
of chemical bath deposition, SILAR (successive ionic layer adsorption and
reaction) technique was used to deposit CdS QDs on tertiary-structured
mesoporous spherical TiO2 films (mean diameter of 1190 ± 60-nm,
which consists of ~100-nm-sized secondary particles formed by clustering of
15-nm-sized primary nanocrystallites). A conversion efficiency of 1.9 % was
achieved by the MS TiO2 device, noticeably higher than 1.2 % for
conventional QDSCs made of nanocrystalline TiO2. [Electrochim. Acta, 56, 7371-7376 (2011)]