KIST 50 years, Global Leading Institute for Future www.kist.re.kr/materials
SEM image of the microcapsule
Materials Research Quarterly Magazine No.3
JULY
2011
Special Issue
Core-shell polymer microcapsules for dual growth factor delivery system
Focus inOrganic/Inorganic hybrid tandem photovoltaics with extended spectral responseNano-structured high temperature thermoelectric thin filmsGraphene-wrapped hybrid spheres with electrical conductivityA strategy for multifunctional nano/micro architectures using quantum dotsFlexible Organic Bistable Devices Based on Ultrathin Graphite SheetFirst Principles Calculation of Adsorption Energies of Mg, O, and MgO on GaAs
2 K ISToday MATERIALS
Carbon Fiber Research
-Carbon Fiber Process
Carbon fiber (CF) research in KIST Jeonbuk seeks
development of h igh st rength CFs with low
manufacturing costs. Several approaches to achieving
this goal are currently being advanced, including the
embedding of carbon nanotubes in CFs, control over
the microstructures of CF precursors, and development
of stabilization processes based on microwave-
assisted plasma. CFs are prepared by wet spinning
of polyacrylonitrile (PAN) precursors of the modified
comonomers. Plasma-based CF manufacturing
processes have shown that the tensile properties
of PAN-based carbon fibers can be significantly
stabilized by plasma treatment as a function of the
plasma exposure time. We are currently developing
new plasma processing equipment and process
protocols in collaboration with Dr. Sung-Moo Cho in
the nano-hybrid center at KIST, Seoul.
-Structural Composite Materials Research
The development of cost-effective processing
methods for preparing fiber composites is a priority
in the program, with the goal of extending the use of
fiber composites to various applications. Automobiles
and airplanes are key applications for which weight
reduction is critical to increasing fuel efficiency
and decreasing pollution. Weight reductions may
be achieved by replacing heavy components with
The Institute of Advanced Composite Materials, located in Jeonbuk, was established
in January 2008. The facilities are under construction in Wanju, Jeonbuk and will open
in 2013.
Composite materials have unique properties, including a high mechanical modulus and
strength, excellent abrasion resistance, and the ability to form lightweight structures,
which can enable eco-friendly cutting-edge 21st century technology development across
many industries. Most composite materials are currently imported due to a lack of core
technology in Korea.
The Institute of Advanced Composite Materials is a government supported research
institute designed to lead the development of the composite materials industry
by developing core technologies and fostering expertise and a knowledge base in
related areas. The vision is to become a focal point for the development of cutting-
edge technologies in Korea by building a world-class research institute specializing in
fundamental and applied science in the field of composite materials. Three main research
areas based on carbon materials are in active in progress in the Institute of Advanced
Composite Materials.
KIST Branch
Institute of Advanced Composite Materials
KIST Jeonbuk
>> Main functions of KIST Jeonbuk.
Development of core raw and engineered composite materials
• Research in response to national and industrial needs• Stabilization of domestic industries and pioneering emerging markets
• Science and technology graduate school• Support for venture businesses to boost regional economy
Support for businesses & cultivation of human resources
Institute of AdvancedComposite Materials
JULY 2011 3
carbon fiber composite components; however,
current processing methods are not suitable for
mass production. Fiber composite processing cycle
times must be shortened before industries can
incorporate fiber composites into their products. We
are simultaneously developing multi-scale composites
consisting of both micro- and nano-reinforcements
to achieve both structural integrity and multi-
functionality in the composite materials.
Nanocarbon Materials Research
-Synthesis of Functional nanocarbon materials
A current research focus of nanocarbon research
in KIST Jeonbuk is the development of nanocarbon
materials with novel functionalities, as well as
the use of these materials in various applications.
Graphenes and carbon nanotubes (CNTs) with
high-quality structures may be synthesized by arc
discharge, chemical vapor deposition (CVD), or wet
chemical processes that facilitate oxidation and
reduction. The synthesized nanocarbon materials may
be endowed with novel functionalities by introducing
dopant modifications or inorganic nanoparticles, and
the physical properties of the resultant materials
are currently being studied. Defect-free single-
walled nanotubes and graphenes exhibit unrivaled
electrical conduction and flexibility, suggesting
that they may replace ITO transparent electrodes in
displays and solar cells. We are also interested in
developing new nanomaterials made from boron, carbon, and nitrogen. The ability to
combine three atomic components in a nanotube or graphene structure can provide
band gap tunability, which is critical for nanoscale thin film transistor (TFT) fabrication.
Nanocarbon applications are not limited to flexible electronics and may be applied in
thermal management, NEMS, structural composites, and energy storage stemming due
to their excellent thermal and mechanical properties.
-Nanocarbon-Based Electrodes/Semiconductors and Electronic Applications
The emerging technologies of flexible printable electronics require development of
solution-processable transparent electrode and semiconductor materials that are low
in cost and easily applied over large areas. The ever-increasing costs of In and Si,
their limited mechanical flexibility, and their brittle mechanical properties under, for
example, substrate bending stress, prevent realization of flexible large-area electronics
based on these materials. It is clear that novel electrode and semiconductor materials
must be intensively developed to realize flexible low-cost electronics. We are currently
developing nanocarbon/polymer-based transparent electrode and semiconductor
materials to realize low-cost flexible electronics. Through the development of
graphene/conducting/conjugated polymers as well as solution-based mass production
and large-scale application of these polymers for use in novel transparent electrodes
and semiconductors, our convergent research program will support development of the
key materials, core technology, market, and industrialization of flexible electronics in
the near future.
KIST Branch
>> Carbon nanotubes and graphene.
>> Carbon fibers treated with plasma. >> Nanocarbon-based flexible electronics application.
>> Boeing 787 Dreamliner ZA003 and a 2012 McLaren MP4-12C are using CFRP lightweight technology. Picture from www.roadandtrack.com.
4 K ISToday MATERIALS
The development of appropriate carriers for biomolecule delivery
is critical in the area of drug delivery, pharmaceutics, tissue
engineering, and stem cell research. In the context of tissue
regeneration, growth factor delivery systems play a significant role
in regulating stem cell behavior because stem cell differentiation
into a specific lineage involves a cascade of multiple events during
which certain growth factors (GFs) or hormones are temporally and
spatially associated in a coordinated manner. Microencapsulation
of drugs, proteins, or other molecules by biodegradable polymers
has long been recognized as an effective way to deliver target
biomolecules. The primary interest of this work is to fabricate
microcapsules with a core-shell structure in which two different
bioactive molecules that are essential for stem cell differentiation
can be delivered together at different concentrations over
Special Issue
Core-shell polymer microcapsules for dual growth factor delivery system
Center for Biomaterials, Biomedical Research Institute, KIST>> Dong Hoon Choi, Dong Keun Han, Kwideok Park
Fig. 1 Formation of core-shell microcapsules using coaxial electrospraying, and the microcapsule (inset).
Fig. 2 Core-shell microcapsules: the PLGA core (black spot) was observed in the microcapsule after solidification (a), SEM image of the microcapsule (b), and the biomolecule release profile (BMP-2 and dexamethasone) (c).
Time (days)
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JULY 2011 5
time. To this end, we fabricated core-shell microcapsules composed of a
poly(lactide-co-glycolide) (PLGA) core and an alginate shell using a coaxial
electrospraying setup. PLGA and alginate are widely used as biomaterials
for drug delivery, as cell carriers, and as tissue engineering scaffolds. In this
work, under high-voltage conditions, PLGA emulsions and alginate solutions
were loaded into syringes and were slowly pumped through a nozzle. The
preparation conditions were optimized over several parameters to achieve
uniform, reproducible fabrication of the microcapsules. The size and shape of
microcapsules were greatly varying, depending on the variables: nozzle size,
applied voltage, volumetric feeding ratio (PLGA: alginate), feeding rate, and
polymer concentrations. Of particular interest was the unique release patterns
of the encapsulated individual GFs, and, more importantly, the crucial effects
of this release profile on the progress of stem cell differentiation. The release
profile of each GF may be manipulated through the technical design of the
microcapsule structure, for example, layer-by-layer (LBL) deposition, varying
the polymer viscosity, and using heparin. Such studies are underway. The
proposed dual GF delivery system can significantly impact the response of
stem cells to external signals, thereby providing a platform for improving our
understanding and regulation of stem cell differentiation.
Special Issue
References• H. M. Kronenberg, Nature 423, 332 (2003). • S. E. Bae, et al., J. Control. Rel. 143, 23 (2010). • R. R. Chen, et al., Pharm. Res. 20, 1103 (2003). • T. P. Richardson, et al., Nature Biotech. 19, 1029 (2001). • I. G. Loscertales, et al., Science 295, 1695 (2002). • Y. K. Hwang, et al., Langmuir 24, 2446 (2008). • D. H. R. Kempen, et al., Biomaterials 30, 2816 (2009).
The size and shape of microcapsules were greatly varying, depending on the variables: nozzle size, applied voltage, volumetric feeding ratio (PLGA: alginate), feeding rate, and polymer concentrations.
Dong Hoon ChoiResearch AssistantCenter for Biomaterials
Dong Keun HanPrincipal Research ScientistCenter for Biomaterials
Kwideok ParkPrincipal Research ScientistCenter for [email protected]
Fig. 3 Optical images of a control microcapsule, an albumin–rhodamine (red) microcapsule, an LBL microcapsule containing alginate-FITC (blue)/chitosan, and a merged image.
No
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Optical image Alginate-FITC(blue) MergeAlbumin-rhodamine(red)
6 K ISToday MATERIALS
Thin-fi lm solar cells based on
hydrogenated amorphous silicon (a-Si:H)
have gained considerable attention for
their potential contributions toward
harnessing renewable solar energy
resources at low costs. a-Si:H solar cells
yield a relatively low power conversion
efficiency (PCE) compared to crystalline
silicon solar cells because the absorption
spectrum of a-Si:H is narrower than that of crystalline silicon.
The PCE of such cells may be improved by utilizing a wider solar
spectrum in tandem solar cells designed using the “micromorph”
concept, with an a-Si:H front cell and a microcrystalline silicon
(mc-Si:H) back cell. However, the low absorption coefficient
of mc-Si:H necessitates use of a thick absorber layer to obtain
sufficient light absorption and photocurrent generation. Because
thick mc-Si:H absorber layers are expensive to produce, new thin
photovoltaic (PV) systems that can potentially supersede the mc-
Si:H system are under development.
Recently, solution-processable bulk-heterojunction organic
photovoltaics (OPVs) have been extensively studied as a
promising future PV technology. OPV cell research has yielded
several OPV systems using low band gap
polymers with high PCEs (6–8%). Researchers
at KIST have demonstrated use of a series-
connected tandem multi-junction PV device that
combines a-Si:H with an OPV (Fig.1). [1]
In tandem devices, it is essential to engineer
the interface between the front and back cells
by developing compatible efficient interlayers.
Researchers at KIST found that the interfacial
series resistance generated at the interface
between the n-type a-Si:H and the hole-
transporting layer (HTL) is crucial to the PV
performance of hybrid tandem devices. A lower
interfacial series resistance in the tandem
device was achieved using thermally evaporated
Fig. 2 (a) Current density–voltage (J–V) characteristics of the hybrid tandem photovoltaic devices with and without an ITO interconnecting layer. (b) External quantum efficiency (EQE) of the hybrid tandem photovoltaic device with the ITO interconnecting layer.
Organic/Inorganic hybrid tandem photovoltaics with extended spectral response
Focus in
Fig. 1 (a) Schematic diagram showing the device architecture of the hybrid tandem photovoltaic cell. (b) Cross-sectional transmission electron microscopy image showing internal structures. (c) Normalized optical absorbance of the photoactive materials: an a-Si:H and a low band gap polymer.
MOO3 as the hole transporting layer, and the PCE of the hybrid
tandem photovoltaic cell was thereby increased from 0.83% to
1.84%. Conventional conducting polymers, which are usually
used as the HTL in OPVs, form poor interfacial contact due to
incompatibilities between the hydrophobic a-Si:H surface and
the hydrophilic polymer solution. On the other hand, thermally
evaporated MOO3 forms conformal contact at the interface. To
further enhance the interfacial connection between subcells,
KIST researchers inserted an optically transparent electrically
conductive indium tin oxide (ITO) thin layer at the interface.
The power conversion efficiency of the hybrid tandem solar
cell was thereby enhanced from 1.0% (VOC = 1.041V, JSC = 2.97
mA/cm2, FF = 32.3%) to 2.6% (VOC = 1.336V, JSC = 4.65 mA/
cm2, FF = 41.98%), as can be seen in Fig. 2(a).
Figure 2(b) shows the external quantum efficiency (EQE) of the
single-junction and tandem multi-junction solar cells as a function
of wavelength. The EQE measurements showed that the spectral
response extended into the near-infrared region with an EQE of
22.7% at 730 nm as a result of the low band gap polymer in the
OPV cell.
This research demonstrates the possibility of fabricating tandem
PVs with hybrid active materials and straightforward interfacial
engineering. The development of new efficient low band gap
polymers can potentially improve the PV performance of hybrid
tandem devices.
References• T. Kim et al., “Organic-inorganic hybrid tandem multi-junction photovoltaics with extended spectral response” Appl. Phys. Lett., 98, 183503 (2011).
Taehee [email protected]
Seung Hee HanPrincipal [email protected]
Kyungkon KimPrincipal [email protected]
EQE(
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>> Solar Cell Research
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JULY 2011 7
Nano-structured high temperature thermoelectric thin films >> Electronic Materials
Fig. 2 XRD patterns of Ca3Co4O9 thin films deposited at (a) 500°C, (b) 600°C and (c) 700°C. The inset shows the XRD patterns of Ca3Co4O9 thin films for 40°~44°.
Fig. 1 Surface morphology of a nanostructured Ca3Co4O9 thin film.
Thermoelectric (TE) materials have been of particular interest
to the solid-state physics and materials research communities
for several years because of their practical applications in
power generators, thermal sensors, and coolers. The quality of
thermoelectric materials can be quantified in term of the figure
of merit (ZT), which is calculated with via Eq. (1):
(1)
where, S , T, ρand, κare the Seebeck coefficient, temperature, electrical resistivity, and thermal conductivity,
respectively. Recent studies showed that ZT could be enhanced
in nanostructured TE materials by increasing phonon boundary
scattering at a number of nanoscale interfaces, which reduced
the thermal conductivity of the material.
At KIST, we prepared nanostructured Ca3Co4O9 (CCO) thin films
as high-temperature thermoelectric materials by RF-sputtering,
as shown in Figure 1. The CCO thin films were easily grown
with a c-axis-oriented alignment similar to that found in
epitaxial films, although these films were grown on amorphous
substrates. Recently, some reports have described possible
approaches to the control of epitaxial growth orientation in
CCO thin films using sapphire substrates. Because the lattice
parameters of Al2O3 and CCO are closely matched on the (00l)
plane of each material, the c-axis orientation of the CCO film
is expected to be controllable via the (00l) plane of the Al2O3
substrate. Poly-crystalline Al2O3 substrates were used to obtain
nanostructured CCO films with a c-axis oriented alignment.
The grains of the Al2O3 substrate contained various crystal
orientations, and the (00l) planes were present in random
orientations among the grains. The deposited atoms were
more strongly bound to the (00l) plane than the other planes,
and stable clusters easily nucleated on the (00l) plane with
application of the same activation energy during deposition.
This process resulted in island growth in three dimensions. The
grains in the (00l) plane of poly-crystalline Al2O3 were expected
to act as seeds for localized epitaxial growth, resulting in
nanostructure growth. We confirmed that localized epitaxial
growth occurred on the seed grains that dominated the (006)
plane (See Figure 2). The nanostructured CCO thin film had
a large specific surface area with a long phonon conduction
path and many interfaces. Phonons of long wavelength were
scattered at the grain boundary, the film surface, and the film/
substrate interface. Such scattering blocked propagation of
long wavelength phonons, which reduced heat conduction
(See Figure 3). We expect that ZT may be further improved by
reducing the thermal conductivity of the film, which relies on
the effects of the nanostructure.
References• M. G. Kang, et al., “High-temperature thermoelectric properties of nanostructured Ca3Co4O9 thin films”, Appl. Phys. Lett., 98, 142102 (2011)
Focus in
Chong-Yun KangPrincipal [email protected]
Min Gyu KangResearch Assistant [email protected]
Jin Sang KimPrincipal Researcher [email protected]
Fig. 3 Schematic diagram illustrating various phonon scattering mechanisms within a thermoelectric material.
Inte
nsity
(a.u
.)
ZT= ρκS2T
10 20 30 40 50 6020 (Degree)
Short wavelength phononMid/long wavelength phonon
Cold electronHot electron
8 K ISToday MATERIALS
Graphene, which is a two-dimensional graphitic nanosheet,
exhibits remarkable electronic properties that could be
advantageous to the development of novel cutting-edge devices.
Although they are good candidates for structural, electrical,
and thermal applications, the use of graphene nanosheets is
hindered by the tedious exfoliation process of bulk graphite and
difficulties associated with their manipulation due to insolubility
and poor dispersion in common organic solvents or polymeric
matrices. Recently, our group described the fabrication of three-
dimensional functional materials from two-dimensional graphene
nanosheets, including graphene–polymer spheres prepared by
wrapping nanosheets via ionic interaction-based self-assembly,
which is a highly efficient method for manipulating graphene
while retaining the intrinsic structural nature.
Figure 1 shows the procedure for preparing graphene-wrapped
polymer nanospheres. The chemical oxidation of graphite easily
generates hydrophilic graphite oxide, which can be readily
exfoliated as individual graphene oxide (GO) sheets. GO can
be converted back to graphene by chemical reduction. The
formation of stable GO colloids relies largely on electrostatic
repulsion as a result of ionization of the carboxylic acid and
phenolic hydroxyl groups present on the GO sheets. Under the
controlled reduction of GO, copious amount of carboxylic acid
groups can be alive, generating negatively charged graphene
sheets. The negatively charged graphene sheets are mixed with
cationic polymer spheres, and attractive forces between anions
and cations should result in graphene-wrapped hybrid spheres.
Graphene-wrapped hybrid spheres with electrical conductivity
Fig. 1 Schematic illustration of the preparation of graphene-wrapped conductive hybrid nanospheres.
Figure 2 shows SEM images of the polymer spheres successfully
decorated by graphene nanosheet patches. Whereas the pristine
colloidal polymer spheres in Figure 2(a) did not exhibit notable
features, except for a smooth surface, the surfaces of the
polymer spheres after assembly with graphene sheets showed
a rough texture, as shown in Figure 2(b), indicating the presence
of an adsorbed layer of texturing molecules on the polymer
spheres. The deposition of graphene sheets on the polymer
spheres was supported by TEM images, shown in Figure 2(c),
which reveals the presence of a thin layer of graphene with a
thickness of 5 to 16 nm.
The electrical conductivities of thin layers of the graphene-
wrapped hybrid spheres were measured using the four-probe
measurement technique. The thin layers were prepared by
spray-coating suspensions containing pristine graphite, reduced
graphene, or graphene-wrapped hybrid spheres of various
diameters in alcohol media onto glass slides heated at 70°C.
Whereas the pristine graphite showed an electrical conductivity
of 15.5 S/m, the conductivity of the reduced graphene was
lowered to 10.7 S/m, indicating imperfect reduction of GO
to graphene. Hybrid spheres 5 μm in diameter exhibited an
electrical conductivity of 1.33 S/m, and decreasing the diameter
of the spheres to 400 nm increased the conductivity to 4.21 S/
m. The lower electrical conductivity of the hybrid spheres may
have arisen from the partial lack of ohmic contact in the cells as
well as the electronic interfacial change. Regardless, electron
transfer along the graphene sheets was preserved, even after
formation of the hybrid polymer spheres. Optimization of the
preparation conditions can potentially increase the conductivity
of the hybrid spheres, for example, by using graphene sheets
with higher electrical conductivity or more sophisticated
assembly geometries. It should be noted that the concept
of using graphene-wrapped hybrid spheres as a conductive
medium worked well, even under the unoptimized preparation
conditions.
References• Sang Ah Ju, Kyunghee Kim, Jung-Hyun Kim , Sang-Soo Lee, ACS Appl. Mater. Interfaces. 2011, in press.
Fig. 2 Scanning electron microphotographs of polymer sphere of 5 μm diameter (a) before mixing with graphene, (b) after mixing with graphene and (c) transmission electron microphotographs showing highly thin graphene layer adsorbed on polymer sphere.
Sang-Soo LeePrincipal Researcher [email protected]
(a)
(b)
(c)
Focus in
Self-assembly through ionic interactions
Deposition of graphene
Graphene oxide Graphite Graphene with anions
Cationic polymer nanospheres
Graphene-wrapped conductive spheres assemblies
Controlled reduction of graphene oxide to form graphene nanosheets with limited surface anions
>> Nanohybrids Materials
JULY 2011 9
Nanoscale materials show properties that are distinct from
their bulk counterparts. Quantum dots (QDs), a representative
nanomaterial, have held particular interest to many scientists
and engineers due to their unique properties, including quantum
size effects, their potential for applications in the biomedical,
electronic, and energy-related fields. Since Bawendi’s group
described nearly monodisperse Cd-based nanocrystals,
research into semiconductor nanocrystals, called QDs, has
exploded across the world. Once the size of a CdSe particle
reaches a sufficiently small size comparable to its exciton
Bohr radius (5.7 nm), the CdSe band gap exceeds that of bulk
CdSe. Under such circumstances, the exciton is confined in the
nanoparticle and the band gap energy of CdSe depends on the
particle size. CdSe QDs can emit visible light from blue to red
by varying the particle size, as shown in Figure 1(a). Realizing
emission across the full color spectrum from CdSe QDs with
high quantum efficiency broadens their applicability relative to
other semiconductor QDs. At KIST, we synthesize high-quality
CdSe QDs with good crystallinity and high monodispersity,
which causes regular ordering as shown in Figure 1(b).
However, high-quality CdSe QDs are usually synthesized using
organometallic synthesis method and, thus, as-synthesized QDs
have hydrophobic surfaces. The hydrophobicity of a CdSe QD
surface complicates their use in the biomedical field, although
they may potentially present an alternative to
conventional organic fluorophores. Although
ligand exchange on the CdSe QD surfaces can
alter the surface properties from hydrophobic
to hydrophilic, such surface modifications
weaken the luminescence. To overcome this
problem and improve the multifunctionality
of nanoparticles, we suggested silica nano-
or micro-spheres encapsulating a QD layer
(Figure 2). Silica has many advantages,
including photonic properties, biocompatibility,
diverse surface functionalization chemistries,
and stability. To realize functional silica-QD
composites, the surface ligands of originally
synthesized CdSe/CdS core/shell QDs were exchanged
from octadecylamine to mercaptopropionic acid (MPA), and
hydroxyl group-terminated silica was converted into amine
group-terminated silica by refluxing with 3-(aminopropyl)
trimethoxysilane (APTMS). In basic media, CdSe/CdS-MPA
QDs are negatively charged, and silica-APTMS spheres are
positively charged in acidic media. Simply by mixing the QD
and silica solutions, the QDs electrostatically self-assembled
onto the surfaces of silica spheres. These QDs were then
arranged equidistance from the center of a silica sphere and
separated from one another by a small gap stabilized by
electrostatic repulsion. This structure minimized self-quenching
among QDs. Indeed, QD-decorated silica spheres showed
higher photoluminescence (PL) intensities than the pure CdSe/
CdS-MPA QDs under identical measurement conditions. It is
believed that the enhanced PL properties were attributed to
coupling between the silica photonic dots and the QDs, and
self-quenching was minimized. When thin silica shell was
additionally overcoated on the QD-attached silica sphere to
increase the robustness of the silica-QD composites, further
PL enhancement was observed. The silica spheres containing
an encapsulated QD layer can be functionalized to present
additional luminescent, magnetic, and plasmonic properties by
encapsulating a variety of nanoparticles. This research program
is ongoing in the Nano-Materials Research Center.
A strategy for preparing multifunctional nano/micro architectures using quantum dots
References• K. Woo et al. J. Phys. Chem. C 113, 7114 (2009)• K. Woo et al. Chem. Commun. 46, 5584 (2010)
Fig. 1 (a) Photograph showing the quantum confinement effects observed in CdSe QDs illuminated by ultraviolet light. (b) Transmission electron microscopy images of a CdSe QD monolayer (inset scale bar = 5 nm).
Fig. 2 Schematic diagram and TEM images showing the synthesis of silica spheres that encapsulate a QD layer (scale bar = 50 nm).
Kyoungja Wooprincipal research scientistcenter head [email protected]
Ho Seong Jangsenior research [email protected]
Kipil Limresearch [email protected]
Focus in
>> Nano-Materials
10 K ISToday MATERIALS
Hybrid inorganic/organic composite structures are interesting
in terms of their fundamental physical properties, but also
because composites containing inorganic nanoparticles,
for example, bistable organic memory (BOM) cells [1], have
recently emerged as excellent candidates for potential
applications in next-generation nonvolatile memory
devices. Among BOM cells, hybrid BOM devices comprising
inorganic nanoparticles blended into an organic host have
been extensively investigated for their device performance,
abundance of nanoparticle species, and cost-effective
manufacturing processes. As reported previously [2], flexible
BOM devices based on PMMA/Ultrathin Graphites (about
30 layers)/PMMA composite structures were fabricated (see
Figure 1). I–V curves at 300 K for Al/PMMA/UGS/PMMA/
ITO/PET devices exhibited electrical bistability before and
after bending. Current–time and current–cycle measurements
under flat or bent conditions demonstrated the memory
stability of the BOM devices. Figure 2 shows the I–V curves
for Al/PMMA/UGS/ PMMA/ITO/PET devices after bending
(bending to a radius of 10 mm). The voltage
applied to the device was scanned cyclically
from −5 to 0 to 5 to 0 to −5 V in all the cases. The
I–V curves for all devices displayed electrical
hysteresis, which in an essential feature of
memory devices. States ‘1’ and ‘0’ correspond
to a relatively high-current state (ON state)
and a relatively low-current state (OFF state),
respectively. The state transition from the OFF to
the ON state is equivalent to the ‘writing’ process
in a digital memory cell. The re-writability of the
nonvolatile memories was investigated using
a write-read-erase-read sequence executed in
air. The write, read, and erase voltage pulses for
the I–t characteristics were set to be +5, +1.8,
and −5 V, respectively. The retention of the BOM
devices was also assessed by keeping the device
in the OFF state at +1.8 V or in the ON state at
Flexible Organic Bistable Devices Based on Ultrathin Graphite Sheet
Fig. 1 (a) Schematic representation of the multilayer structured Al/PMMA/UGS/PMMA/ITO/PET OBDs fabricated in this work. (b) Cross-sectional TEM image of the PMMA/UGS/PMMA/ITO layers obtained from the cross-section of a sample cut using the FIB technique. The inset shows an enlarged HRTEM image of the UGS (about 30 layers).
+1.8 V under ambient conditions. The performance of the
flexible organic memory device was similar to that of a BOM
containing embedded graphene sheets, including a maximum
Ion/Ioff ratio for the I–V curves of 5x106, a retention stability of
4.8x104 s, and an endurance to electrical stress beyond 1x105
cycles.[3] Figure 3 shows a log–log plot of the I–V results
shown in Figure 2(d) for BOM devices after bending. In the
BOM devices after bending, low currents, below 2 V, arose
from thermally generated carriers at the interface between
the UGS and PMMA layers. On the other hand, the slope of
the fitted line above 2 V was as large as 11.3. Such a large
increase in the slope suggested that the trap density was
exponentially distributed over the energy in the PMMA band
gap, largely due to mechanical bending.
References• W.K. Choi et. al., “Bistable Organic memory device with Gold Nanoparticles Embedded in a Conducting Poly(N-vinylcarbazole) Colloids Hybrid,” J. Phys. Chem. C, 115, 2342 (2011).• W.K. Choi et. al., “Flexible Organic Bistable Devices Based on Graphene Embedded in an Insulating Poly(methyl methacrylate) Polymer Layer ,” Nano Lett., 10, 2442 (2010).• W.K. Choi et al., “Polymer-Ultrathin Graphite-Polymer composite structured Flexible Nonvolatile Bistable Organic Memory Devices,” Nanotechnology, 22, 295203 (2011).
Fig. 2 Characterization of the memory properties of BOM devices after bending (to a bending radius of 10 mm). (a) Current–voltage curves for the Al/PMMA/UGS/PMMA/ITO/PET sheet devices, and (b) operation of a write–read–erase–read (+5/1.8/ − 5/1.8 V) sequence.
Fig. 3 A log–log plot of the current as a function of the applied voltage for Al/PMMA/UGS/PMMA/ITO/PET devices. The curves were fit according to an SCLC mechanism.
Won-Kook ChoiPrincipal Research [email protected]
Focus in
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>> Opotoelectric Materials
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JULY 2011 11
The spin injection efficiency may be increased by inserting a
MgO spacer layer between a ferromagnetic metal (Fe or FeCo)
and a semiconductor (GaAs). The crystallinity and directionality
of a MgO film on GaAs plays a critical role in achieving a high
spin injection ratio. Many experimental methods have been
proposed for growing MgO thin films on GaAs, including
magnetron sputtering methods, diode sputtering methods,
e-beam evaporation. Among these methods, sputter-based
methods have been widely used due for their high productivity
and controllability.
The quality of a MgO film depends on the preparation
conditions. For example, a crystalline MgO film was obtained
at high temperatures by sputtering from a MgO target in the
presence of an oxygen gas stream. In contrast, a Mg-rich film
was obtained at room temperature from a MgO target alone.
According to theoretical calculations, the oxygen binding energy
on GaAs is much higher than on Mg. Because of this, a Mg-rich
phase cannot be formed on a GaAs surface. These contradictory
results were investigated using theoretical approaches based on
first principle calculations.
As a first step toward understanding the thin film growth, the
adsorption energies of Mg and O on GaAs(001)-β (2x4) were
calculated. The atomic structure of the GaAs surface is shown
in Fig. 1. The left hand side of Fig. 1 shows the atomic structure
of GaAs(001)- β (2x4), which is the most stable
structure for a GaAs(001) surface. The gray and
black spheres represent Ga and As, respectively.
The right hand side shows the adsorption sites that
Mg or O can occupy. The orange and red spheres
represent Mg and O, respectively. A calculation
of the binding energies of Mg and O on the GaAs
surface reveals that oxygen binds more strongly
than Mg. The chemical binding characteristics
of Mg and O to the surface were investigated by
modeling the charge density differences, as shown
in Fig. 2. Figure 2 shows the charge density of the oxygen bridge
site. Yellow and magenta represent the charge density increase
and decrease relative to the reference state, respectively. Charge
transfer from As to O was observed, which was interpreted as
ionic bonding characteristics. In contrast, the charge density
increased during Mg adsorption over the whole area, including
As, indicating formation of a metallic bond, as shown in Fig. 1.
The theoretical calculations showed that O bound to the GaAs
surface more strongly than Mg. Further insight into the growth
of the thin films requires additional calculations of the chemical
potentials of Mg and O.
First Principles Calculation of Adsorption Energies of Mg, O, and MgO on GaAs
References• K.H. Kim et al. “Microstructural Changes of Epitaxial Fe/MgO Layers Grown on InAs(001) Substrates”, Cryst. Growth Des. 2011, 11, 2889.• R. Tamarany et al., “Energetics of Mg and O atoms on As-Terminated GaAs(001) Surface”, Psi-k 2011, Berlin, 2011
Fig. 1 Atomic structure of GaAs(001)-(2x4) surface. (Left hand side). Gray and black spheres represent Ga and As atoms respectively. The right-hand side figure indicate the possible adsorption sites for Mg and O atoms on GaAs surface. Orange and red sphere represent Mg and O atom respectively.
Fig. 2 Charge density difference of O and Mg with respect to their reference state. Green, purple, and red spheres represent Ga, As, and O(Mg) atoms, respectively. Yellow and magenta represent charge density increased and decreased area, respectively.
Seung-Cheol LeePrincipal Research ScientistComputational Materials Design GroupComputational Science [email protected]
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The International Materials &
Components Industry Show 2011
( IMAC’11 ) wh ich i s the mos t
prominent, large-scale show for the
materials and components industries
in Korea, was held at the Korea
International Exhibition Center (KINTEX) in Goyang, northwest of
Seoul over 4 days starting on May 25, 2011. KIST exhibited key nine
recently developed technologies for advanced materials, including
solid-state fuel cells, piezoelectric energy harvesting system, a RAN
system based on plastic optical fibers, dye-sensitized photovoltaic
cells, multifunctional nanostructures composed of metal oxide thin
films, biochemical sensors based on plasmonics, photoelectrodes
based on hierarchical nanostructures, polymer electrolytes for
fuel cells, and plasma spraying technologies for multifunctional
nanostructures. One hundred fifty companies with 400 booths
participated, and nearly 11,000 visitors attended the show.
The “KBS Open Concert” which has been a leading music program
on Korean TV since 1993, was held on a grassy field at the KIST
campus on May 3, 2011, to celebrate the launch of a new science and
technology council, the National Science and Technology Commission
(NSTC) and to encourage scientists. NSTC aims to improve the
efficiency and accountability of R&D projects that in the past have
been controlled by several separate ministries. KIST’s president, Dr.
Kil-Choo Moon, delivered a welcoming speech, and commissioner
of NSTC Do-Yeon Kim offered his congratulations in advance of the
performance. The concert included performances by several of Korea’s
popular stars, Shin Hyung-won, Byun Jin-sup, Kim Jong-suh, Kim Yun-
ja, Lee Hyun-woo, Hong Kwang-ho, and Jung Sun-ah, and more than
6,000 people, including KIST employees, family members, alumni, and local residents attended the concert.
KBS Open Concert in KIST
KIST exhibited key technologies for advanced materials in IMAC’11
KISToday MaterialsMaterials Research Quarterly Magazine
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