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 Biomaterialskpark@kist.re.kr

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

laye

r3

laye

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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 KimResearcherthkim1@kist.re.kr

Seung Hee HanPrincipal Researchershhan@kist.re.kr

Kyungkon KimPrincipal Researcherkimkk@kist.re.kr

EQE(

%)

Wavelength (nm)

Wavelength (nm)

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(c)

>> 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 Researchercykang@kist.re.kr

Min Gyu KangResearch Assistant mgkang@kist.re.kr

Jin Sang KimPrincipal Researcher jskim@kist.re.kr

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 s-slee@kist.re.kr

(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 kjwoo@kist.re.kr

Ho Seong Jangsenior research scientistmsekorea@kist.re.kr

Kipil Limresearch scientistkipil84@kist.re.kr

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 Scientistwkchoi@kist.re.kr

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 Centerleesc@kist.re.kr

Focus in

>> Computational Science

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

Editor-in-ChiefDr. Seok-Jin Yoon sjyoon@kist.re.kr

Editors Dr. Insuk Choi insukchoi@kist.re.kr Dr. Ho Won Jang hwjang@kist.re.kr Dr. Bong Soo Kim bongsoo@kist.re.kr Dr. Heesuk Kim heesukkim@kist.re.kr Dr. Sang Hoon Kim kim_sh@kist.re.kr Dr. Kwan Hyi Lee kwanhyi@kist.re.kr Dr. Jung Ah Lim jalim@kist.re.kr

Editorial OfficeMaterials Research Korea Instite of Science and TechnologyHwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, KoreaTel +82-2-958-5401 www.kist.re.kr

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