50
High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Semiconducting Nanoparticles Ji Hyung Jung Department of Energy Engineering Graduate School of UNIST
High-Performance Flexible Organic Nano-Floating
Gate Memory Devices Functionalized with
Semiconducting Nanoparticles
Ji Hyung Jung
Department of Energy Engineering
Graduate School of UNIST
High-Performance Flexible Organic Nano-Floating
Gate Memory Devices Functionalized with
Semiconducting Nanoparticles
A thesis
Submitted to Department of Energy Engineering of UNIST
in partial fulfillment of the requirements
for the degree of Master of Science
Ji Hyung Jung
12. 19. 2014
Approved by
___________________________
Advisor
Byeong-Su Kim
High-Performance Flexible Organic Nano-Floating
Gate Memory Devices Functionalized with
Semiconducting Nanoparticles
Ji Hyung Jung
This certifies that the thesis of Ji Hyung Jung is approved.
12. 19. 2014
Signature
__________________________
Byeong-Su Kim
Signature
__________________________
Joon Hak Oh
Signature
__________________________
Hyunhyub Ko
Abstract
High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with
Semiconducting Nanoparticles, 2015, Ji Hyung Jung, Department of Energy Engineering, Ulsan
National Institute of Science and Technology (UNIST)
Transistor-type nano-floating gate memory (NFGM) devices have recently attracted great interest
because of their unique and compatible characteristics as suitable platforms for integrated circuits.
Their excellent memory properties with inexpensive fabrication processes make NFGM devices
highly promising for the next-generation data storage devices.
Herein, novel nonvolatile NFGM devices utilizing semiconducting cobalt ferrite (CoFe2O4)
nanoparticles (NPs) as the charge trap sites with p-type semiconductor, pentacene as active layer on
flexible and transparent polymer substrates as well as on conventional silicon wafers have been
prepared. Monodisperse CoFe2O4 NPs, which were synthesized in solution from cheap and nontoxic
metal-oleate complex precursor, provide facile and fast deposition on the target substrates via simple
spin-casting technique. The newly developed NFGM devices exhibit superb mechanical/electrical
stability against pure bending and repeated program/erase (P/E) operations without additional
tunneling dielectric layer which enhances data retention capacity and helps the charge carriers to be
trapped in the NPs. Furthermore, size effect of CoFe2O4 NPs (5, 8, and 11 nm) on electrical memory
performance in NFGM devices was investigated.
Keywords: Nano-floating gate memory, cobalt ferrite, nanoparticle, pentacene, organic memory
Blank page
Contents
I. Introduction ------------------------------------------------------------------------------------------------------ 1
1.1 Organic nano-floating gate memory (NFGM) devices --------------------------------------------- 1
1.2 Cobalt ferrite (CoFe2O4) nanoparticles (NPs) -------------------------------------------------------- 4
II. Experiments ----------------------------------------------------------------------------------------------------- 7
2.1 Synthesis of CoFe2O4 NPs ------------------------------------------------------------------------------ 7
2.2 Characterization of CoFe2O4 NPs ---------------------------------------------------------------------- 8
2.3 Fabrication of CoFe2O4 NFGM devices ------------------------------------------------------------- 15
III. Results & discussion ----------------------------------------------------------------------------------------- 19
3.1 Analysis of CoFe2O4 NFGM devices ---------------------------------------------------------------- 19
3.2 Electrical memory performance of CoFe2O4 NFGM devices ------------------------------------ 22
3.3 Electrical memory performance and mechanical stability test of flexible CoFe2O4 NFGM
devices -------------------------------------------------------------------------------------------------------- 33
IV. Conclusion ---------------------------------------------------------------------------------------------------- 36
V. Reference ------------------------------------------------------------------------------------------------------ 37
Acknowledgement ------------------------------------------------------------------------------------------------ 40
List of figures
Figure 1-1. Schematic illustrations and device operation schemes of transistor and floating gate flash
memory devices ---------------------------------------------------------------------------------------------------- 3
Figure 1-2. Various applications of magnetic NPs ----------------------------------------------------------- 5
Figure 1-3. Transmission electron microscopy (TEM) images and size distribution of size- and shape-
controllable CoFe2O4 NPs ---------------------------------------------------------------------------------------- 6
Figure 2-1. TEM images of synthesized CoFe2O4 NPs ------------------------------------------------------ 9
Figure 2-2. Size distribution of CoFe2O4 NPs --------------------------------------------------------------- 10
Figure 2-3. Cyclic voltammograms of CoFe2O4 NPs ------------------------------------------------------- 11
Figure 2-4. Plot of transformed Kubelka-Munk function versus the energy of the light absorption
from the UV-vis absorption spectra of CoFe2O4 NPs -------------------------------------------------------- 12
Figure 2-5. Energy band diagram of CoFe2O4 NPs --------------------------------------------------------- 13
Figure 2-6. Contact angle measurement of D. I. water on the Si wafer ---------------------------------- 16
Figure 2-7. Schematic configuration of the NFGM devices based on CoFe2O4 NPs ------------------- 17
Figure 2-8. Schematic illustration and photograph of the flexible CoFe2O4 NFGM devices ---------- 18
Figure 3-1. Atomic force microscopy (AFM) phase images of spin-coated CoFe2O4 NPs on the Si
wafer --------------------------------------------------------------------------------------------------------------- 20
Figure 3-2. Cross-sectional scanning transmission electron microscopy (STEM) images of 8 nm
CoFe2O4 NP-embedded NFGM devices ---------------------------------------------------------------------- 21
Figure 3-3. Transfer curves of the NFGM devices based on CoFe2O4 NPs ------------------------------ 25
Figure 3-4. Electrical memory characteristics of NFGM devices based on 8 nm CoFe2O4 NPs ------ 27
Figure 3-5. Memory characteristics of the NFGM devices based on 8 nm CoFe2O4 NPs depending on
the program/erase (P/E) operation bias voltage -------------------------------------------------------------- 28
Figure 3-6. Schematic energy band diagrams for the charge trap/release mechanism description --- 30
Figure 3-7. Electrical memory performance of flexible NFGM devices based on 8 nm CoFe2O4 NPs
----------------------------------------------------------------------------------------------------------------------- 34
Figure 3-8. Mechanical stability test of flexible NFGM devices based on 8 nm CoFe2O4 NPs ------- 35
List of tables
Table 2-1. Electrochemical and photochemical data of CoFe2O4 NPs ------------------------------------ 14
Table 3-1. Electrical memory characteristics of the NFGM devices with/without CoFe2O4 NPs ----- 26
Table 3-2. Memory window and read current on/off ratio (read Ion/Ioff) of the NFGM devices based on
8 nm CoFe2O4 NPs depending on the program/erase (P/E) operation bias voltage --------------------- 29
Table 3-3. Comparison of electrical memory performance between with/without additional tunneling
dielectric layer in NFGM devices ------------------------------------------------------------------------------ 31
Table 3-4. Comparison of data retention test between with/without additional tunneling dielectric
layer in NFGM devices ------------------------------------------------------------------------------------------ 32
1
I. Introduction
1.1 Organic nano-floating gate memory (NFGM) devices
As inorganic semiconductor-based electronic devices are processed on rigid and limited types of
substrates via expensive fabrication processes requiring high temperature and vacuum condition,
researches for organic semiconductor-based electronic devices have been intensively conducted over
the last decades for achieving not only low-cost, facile fabrication processes but also
commercialization for flexible and large-area electronics. To truly realize flexible electronics, all the
components of electronic devices including transistors,1 batteries,
2 and displays
3 should be
intrinsically flexible.
Memory devices are also one of most important elements for flexible electronic devices and have
been extensively developed so far. Among various kinds of memory devices, transistor-type flash
memory devices have been widely studied because of their suitability as platforms for integrated
circuits, and superb electrical performances. They can be fabricated by embedding floating gates
and/or tunneling dielectric layer between channel and gate dielectric layer in transistor structure. In
flash memory devices, electrically bistable behavior can be observed showing different conductivity
at the same gate-source bias voltage (VGS = 0 V) depending on the previous memory operation.4
Charge carriers in active layer can be induced and trapped in floating gates during program operation
and released during erase operation resulting in threshold voltage shift, also denoted as memory
window or hysteresis, via conductance change of the channel as shown in Figure 1-1.5
Among many kinds of flash memory devices, nanoparticle (NP)-embedded nano-floating gate
memory (NFGM) devices have attracted tremendous attention as promising next-generation memory
devices because they have the merit of facile control of electrical properties of NPs such as energy
level by modulating their sizes and shapes. To date, most of studies on organic NFGM devices have
been investigated utilizing metallic NPs.4, 6
Despite their high cost, Au NPs have been extensively
exploited because of their high work function and chemical stability, and thermal evaporation has
been widely used for the deposition of the NPs. This method, however, requires high-vacuum
condition and not a useful method for controlling the size, shape and the density of NPs because of
Ostwald ripening, where small Au NPs tend to dissolve and redeposit onto larger NPs during the
evaporation.7 On the other hand, semiconducting or metal oxide NPs have been far less utilized for
memory devices despite their lower cost compared with metallic NPs, presumably because of
comparatively poor electrical memory performance.8
In this study, the electrical memory characteristics of novel organic NFGM devices based on
2
semiconducting CoFe2O4 NPs were investigated showing comparable electrical memory
characteristics to the NFGM devices based on metallic NPs. In addition, the effect of size of CoFe2O4
on the NFGM devices has been studied thoroughly.
3
(a)
(b)
Figure 1-1. Schematic illustrations and device operation schemes of a) transistor and b) floating gate
flash memory device. (Han, S.-T. et al., Adv. Mater. 2013, 25, 5425-5449)5
4
1.2 Cobalt ferrite (CoFe2O4) nanoparticles (NPs)
Magnetic NPs have been investigated intensively in recent years because of their electrical, optical,
and magnetic property changes depending on their sizes, shapes, agglomerations.9 CoFe2O4 NPs are
one of the well-known hard magnetic materials having high coercivity, magnetocrystalline anisotropy,
and moderate saturation magnetization as well as outstanding physical/chemical stability.10
Owing to
their unique properties of the inverse spinel ferrites of ternary composition, CoFe2O4 NPs have
various application fields including sensors,11
data storage,12
and bio-applications such as drug-
delivery systems for biological labeling or magnetic hyperthermia.13
(Figure 1-2)
Accordingly, various size- and shape-controllable synthesis methods of monodisperse NPs have been
developed. CoFe2O4 NPs can be prepared via sol-gel,14
Langmuir-Blodgett,15 coprecipitation,
16
hydrothermal,17
bacterial18
and micellar synthesis,10
combustion,19
aerosol vapor,20
mechanical
grinding,21
or high-temperature decomposition of organic precursors.22
In non-hydrolytic reaction
method, size and shape of CoFe2O4 NPs can be controlled by adjusting the ratio between the quantity
of nano-crystalline seeds and precursors in the solution and regulating NP growth rate.23
(Figure 1-3)
Herein, CoFe2O4 NPs were synthesized via modified thermal decomposition method as previously
reported.24
Using this method, the size of CoFe2O4 NPs can be controlled simply by varying Ar
bubbling rate in the reacting solution. Furthermore, monodisperse NPs can be synthesized through
complete separation of nucleation and growth process using Ar bubbles, which absorb the heat
generated from exothermic multiple-bonds formation reactions in the nucleation step.25
5
Figure 1-2. Various applications of magnetic nanoparticles. (Jun, Y.-W. et al., Chem. Commun., 2007,
1203-1214)26
6
Figure 1-3. Size and shape control of CoFe2O4 NPs. TEM images of spherical CoFe2O4 nanoparticles
of (a) 5.2 ± 1.1, (b) 7.9 ± 0.5, and (e) 11.8 ± 1.3 nm and cubic nanoparticles of (c) 9.1 ± 0.5 and (d)
10.9 ± 0.6 nm with scale bar as 50 nm. (f) Size distribution of cubic nanoparticles in figure (d). The
inset of figure (f) shows the aspect ratio of cubic nanoparticles in figure (c) and (d). (Song, Q. et al., J.
Am. Chem. Soc. 2004, 126, 6164-6168)23
7
II. Experiments
2.1 Synthesis of CoFe2O4 NPs
In this study, CoFe2O4 NPs in three different sizes (5, 8, and 11 nm in diameter) have been
synthesized from low-cost and non-toxic metal-oleate complex precursor modifying the experimental
method as reported in previous study.24
(Co2+
Fe23+
)-oleate was used as precursor to synthesize CoFe2O4 NPs. To synthesize precursor,
iron(Ⅲ) chloride hexahydrate (FeCl3∙6H2O, 32 mmol), cobalt(Ⅱ) chloride hexahydrate (CoCl2∙6H2O,
16 mmol), and sodium oleate (128 mmol) were dissolved in a mixture of 80 ml of ethanol, 80 ml of D.
I. water, and 160 ml of n-hexane. Mixed solution was stirred until all reagents were dissolved
thoroughly. The reacted (Co2+
Fe23+
)-oleate was washed with 120 ml of D. I. water for 3 times and
residual solvent was evaporated in the rotary evaporator at 80 ˚C.
Then, (Co2+
Fe23+
)-oleate complex (2.5 g), oleic acid (OLA, 0.25 g), and octadecene (10 ml) were
mixed and evacuated at 80 ˚C for 1 h. Then, the mixture was heated to 310 ˚C at a heating rate of 1 ˚C
min-1
under Ar bubbling and stirred for 1 h. In this step, Ar bubbling rate into reacting solution was
adjusted to control the size of CoFe2O4 NPs because Ar bubbles in solution can absorb the local latent
heat generated during nucleation step, leading to continuous primary nucleation process for
monodisperse and smaller NPs in low temperature.
For the synthesis of 5 nm CoFe2O4 NPs, Ar was bubbled vigorously in the reacting solution. For 11
nm ones, on the other hand, the bubbling was stopped before the reacting solution was heated. After
growth of the NPs, the dispersion was cooled down to room temperature and rinsed with
acetone/ethanol mixture three times. As-prepared CoFe2O4 NPs were finally dispersed in n-hexane for
long-term storage.
8
2.2 Characterization of CoFe2O4 NPs
Physical and electrical properties of monodisperse CoFe2O4 NPs in three different sizes (5, 8, and 11
nm in diameter) have been chracterized using transmission electron microscopy (TEM), cyclic
voltammetry (CV), and UV-vis absorption spectroscopy.
TEM images of monodispersely synthesized CoFe2O4 NPs were shown in Figure 2-1. The size
estimation of CoFe2O4 NPs showed the values of 5.59 ± 0.65 (size variation ca.10 %), 8.05 ± 0.57
(size variation ca. 7 %), and 11.30 ± 0.76 nm (size variation ca. 5 %) for the 5, 8, and 11 nm NPs,
respectively. (Figure 2-2) The particle size distribution of NPs was estimated from 50 random
samples in TEM images.
The energy level of valence band of CoFe2O4 NPs can be estimated by cyclic voltammograms as
shown in Figure 2-3. The electrolyte was 0.1 M of tetabutylammonium hexafluorophosphate
(Bu4NPF6) in anhydrous acetonitrile and cyclic votammograms were measured at a scan rate of 100
mV s-1
at room temperature under N2 gas blowing. Indium tin oxide (ITO) glass deposited by each
CoFe2O4 NP dispersion was used as a working electrode. Platinum (Pt) wire and Ag/Ag+ electrode
containing 0.01 M of AgNO3 and 0.1 M of tetrabutyl ammonium perchlorate (TBAP) in acetonitrile
were served as a counter and a reference electrode, respectively. The Ag/Ag+ reference electrode was
internally calibrated by ferrocene/ferrocenium couple (Fc/Fc+) and the valence band energy level of
NPs can be estimated using equation,
𝐸𝑣𝑎𝑙𝑒𝑛𝑐𝑒 𝑏𝑎𝑛𝑑 (𝑒𝑉) = −[𝐸(𝑜𝑥)𝑜𝑛𝑠𝑒𝑡 − 𝐸(𝑓𝑒𝑟𝑟𝑜𝑐𝑒𝑛𝑒)
𝑜𝑛𝑠𝑒𝑡 + 4.8] (1)
Energy bandgap, the energy level difference between conduction band and valence band, can be
measured using a plot of the modified Kubelka-Munk function versus the energy of exciting light
derived from UV-vis spectra.27
(Figure 2-4) The spectra were measured for NP dispersion and spin-
coated NPs on the quartz plate.
The electromchemical/photochemical data of CoFe2O4 NPs (Figure 2-3, 2-4, and Table 2-1) showed
that the energy level of valence band increases gradually from -6.56 to -6.52 eV as the size of NPs
increases, and that of conduction band and energy bandgap decrease from -3.80 to -3.94 eV and 2.76
to 2.58 eV, respectively. (Figure 2-5)
9
(a) (b)
(c) (d)
(e) (f)
Figure 2-1. TEM images of (a, b) 5, (c, d) 8, and (e, f) 11 nm CoFe2O4 NPs.
10
(a) (b)
(c)
Figure 2-2. The size distribution of CoFe2O4 NPs in (a) 5.59 ± 0.65 (size variation ca. 10 %), (b) 8.05
± 0.57 (size variation ca. 7 %), and (c) 11.30 ± 0.76 nm (size variation ca. 5 %).
4.0 4.5 5.0 5.5 6.0 6.5 7.00
5
10
15
20
25
30
35
Perc
enta
ge (
%)
Diameter (nm)
7.0 7.5 8.0 8.5 9.0 9.50
5
10
15
20
25
30
Perc
en
tag
e (
%)
Diameter (nm)
9 10 11 12 13 140
10
20
30
40
50
Perc
enta
ge (
%)
Diameter (nm)
11
(a) (b)
(c)
Figure 2-3. Cyclic voltammograms of (a) 5, (b) 8, and (c) 11 nm CoFe2O4 NPs.
0 1 2 3
Curr
en
t (a
.u.)
Potential (V vs Fc/Fc+)
E onset
ox = 1.82 V
0 1 2 3
E onset
ox = 1.80 V
Curr
en
t (a
.u.)
Potential (V vs Fc/Fc+)
0 1 2 3
E onset
ox = 1.78 V
Curr
en
t (a
.u.)
Potential (V vs Fc/Fc+)
12
(a) (d)
(b) (e)
(c) (f)
Figure 2-4. Plots of modified Kubelka-Munk function versus the energy of the light absorption in the
UV-vis absorption spectra of (a ~ c) NP dispersion and (d ~ f) spin-coated NPs on the quartz plate: (a,
d) 5, (b, e) 8, and (c, f) 11 nm NPs.
2 3 4 5
Eg = 2.79 eV
[F(R
)*E
]1/2
Energy / eV
2 3 4 5
Eg = 2.76 eV
[F(R
)*E
]1/2
Energy / eV
2 3 4 5
Eg = 2.77 eV
[F(R
)*E
]1/2
Energy / eV2 3 4 5
Eg = 2.64 eV
[F(R
)*E
]1/2
Energy / eV
2 3 4 5
Eg = 2.76 eV
[F(R
)*E
]1/2
Energy / eV2 3 4 5
Eg = 2.58 eV
[F(R
)*E
]1/2
Energy / eV
13
Figure 2-5. Energy band diagrams of CoFe2O4 NPs in three different sizes.
14
Table 2-1. Electrochemical and photochemical data of CoFe2O4 NPs
Size of NPs Eox
onset
(V)[a]
Valence
band
(eV)[a]
Conduction
band
(eV)[b]
Eg, NPs
(eV)[c]
Eg, dispersion
(eV)[c]
5 nm 1.82 -6.56 -3.80 2.76 2.79
8 nm 1.80 -6.54 -3.90 2.64 2.77
11 nm 1.78 -6.52 -3.94 2.58 2.76
[a] Deduced from the onset oxidation potentials in the cyclic voltammograms.
[b] Calculated from Econduction (eV) = Evalence + Eg,NPs
[c] Extracted from UV-vis spectra using Kubelka-Munk function.
15
2.3 Fabrication of NFGM devices
n-octadecyltrimethoxysilane (OTS)-treated Si wafer was prepared for the fabrication of NFGM
devices based on CoFe2O4 NPs.28
The solution of 3 mM OTS dissolved in trichloroethylene (TCE)
was spin-coated on the highly n-doped (100) Si wafer (< 0.005 Ω cm) with thermally grown SiO2 100
nm (Ci = 32.8 nF cm-2
). The Si wafer was pre-cleaned in piranha solution (mixture of H2SO4 : H2O2 =
70 : 30 by volume ratio) before washing with D. I. water and UV/ozone plasma treatment. Spin-
coated Si wafer was exposed to ammonium hydroxide (NH4OH) vapor overnight in the desiccator for
the smooth formation of OTS self-assembled monolayer (SAM) on the surface of Si wafer.
Subsequently, the wafer was ultra-sonicated in toluene and rinsed with toluene, acetone, and isopropyl
alcohol (IPA) and dried with N2 gas. The contact angle of D. I. water on the surface of Si substrate
before and after the SAM treatment was ca. 28 and 107 ˚, respectively. (Figure 2-6)
The schematic configuration of the NFGM device based on CoFe2O4 NPs is shown in Figure 2-7.
The spin-casting technique was applied for the facile, fast deposition of CoFe2O4 NPs in the NFGM
devices. After spin-casting of 2 mg ml-1
of CoFe2O4 NP dispersion at 1000 rpm, the Si wafer was
annealed at 60 ˚C in the vacuum oven for the thorough solvent evaporation.
P-type semiconductor, pentacene was thermally deposited on the surface of NPs at 0.1 ~ 0.3 Å s-1
at 60 ˚C (substrate temperature) and 40 nm-thick gold source/drain electrodes were also formed via
thermal evaporation using shadow masks with 50 μm of channel length (L) and 1000 μm of channel
width (W). To confirm the effect of additional tunneling dielectric layer in NFGM devices, 10 nm-
thick Al2O3 thin-film was deposited between pentacene and NPs utilizing atomic layer deposition
(ALD).
For the flexible NFGM devices, transparent and flexible polyethylene terephthalate (PET) film was
prepared as a substrate. Cr (5 nm) and Au (100 nm) was thermally deposited on PET film and used as
adhesion layer and gate electrode, respectively. 100 nm-thick Al2O3 blocking dielectric layer (Ci = 4.0
nF cm-2
) was formed by radio frequency (RF) magnetron sputtering technique. The other processes
are the same as those in the fabrication on the Si wafer as described above. The schematic illustration
of the structure and photograph of flexible NFGM device based on CoFe2O4 NPs is shown in Figure
2-8.
16
(a) (b)
Figure 2-6. Contact angle of D. I. water (a) before and (b) after OTS treatment on the Si wafer.
17
Figure 2-7. Schematic configuration of the NFGM device based on CoFe2O4 NPs.
18
(a)
(b)
Figure 2-8. (a) Schematic image and (b) photograph of the flexible CoFe2O4 NFGM device.
19
III. Results & Discussion
3.1 Analysis of CoFe2O4 NFGM devices
The surface images of CoFe2O4 NPs deposited on Si wafer were obtained from atomic force
microscopy (AFM) phase images as shown in Figure 3-1. Uniformly coated NPs were observed in
AFM images of 5 and 8 nm CoFe2O4 NPs, while for 11 nm NPs, closely packed aggregated NPs were
observed. This agglomeration phenomena of 11 nm CoFe2O4 NPs may be attributed to their narrow
particle size distribution (standard deviation ca. 5 %), uniform shape, and van der Waals interaction
among NPs.29
Large NPs tend to be clustered together because large NPs usually have the stronger
attraction force among NPs than that of small NPs30
and it was experimentally verified that van der
Waals force increases as the size of CoFe2O4 NP increases because of their permanent dipole moment
in spinel structure.31
The agglomeration of 11 nm CoFe2O4 NPs was also confirmed in their TEM
images (Figures 2-1(e) and (f)).
Figure 3-2 shows cross-sectional bright-field (BF) and high angle annular dark field (HAADF)
scanning transmission electron microscopy (STEM) images of 8 nm CoFe2O4 NPs-embedded NFGM
devices showing that NPs can be deposited uniformly in monolayer on target substrates via facile and
simple spin-casting technique.
The sample for STEM analysis was prepared using dual-beam focused ion beam (FIB, Helios 450
HP, FEI, USA) on the copper grid and analyzed with high resolution-transmission electron
microscope (HR-TEM, Cs-corrected JEM-2100F, JEOL, Japan).
20
(a) (b)
(c)
Figure 3-1. AFM phase images of spin-coated (a) 5, (b) 8, and (c) 11 nm CoFe2O4 NPs on the OTS-
treated Si wafer.
21
(a) (b)
(c) (d)
Figure 3-2. Cross-sectional (a, b) bright-field (BF) and (c, d) high angle annular dark field (HAADF)
scanning transmission electron microscopy (STEM) images of NFGM devices based on 8 nm
CoFe2O4 NPs.
22
3.2 Electrical memory performances of the NFGM devices based on CoFe2O4 NPs
A Keithley 4200 semiconductor parametric analyzer was used to measure all the electrical
performance of NFGM devices based on CoFe2O4 NPs in an N2-filled glove box.
Figure 3-3 shows transfer curves of the NFGM devices based on 5, 8, and 11 nm CoFe2O4 NPs
during dual gate-source voltage sweep and their electrical memory characteristics were summarized in
Table 3-1. All the transfer curves showed typical counterclockwise hysteresis loop of p-type NFGM
devices resulting from increased memory window of NFGM devices by charge trapping/releasing of
NPs during dual gate-source voltage sweep as described in Chapter 1.1.
Memory window of 73.84 ± 6.34 V for NFGM devices based on 8 nm CoFe2O4 NPs was slightly
larger than that of 68.27 ± 2.77 V for 5 nm NP-based ones. Meanwhile, NFGM devices based on 11
nm CoFe2O4 NPs showed smaller memory window of 62.51 ± 7.16 V.
On the basis of energy level differences among CoFe2O4 NPs in 3 different sizes (Figure 2-5), it is
expected that charge trap capacity of NPs would be improved as their sizes increase because of their
lower energy level of conduction band. However, this experimental result showed that charge carrier
trap capacity of 11 nm CoFe2O4 NPs was decreased even though their energy levels are in equivalent
or even in better condition for the charge carrier transport than those of 5 or 8 nm CoFe2O4 NPs. This
result could be the consequence of the irregular aggregates of 11 nm CoFe2O4 NPs as previously
described in chapter 3.1. The agglomerated NPs may disturb the uniform deposition of pentacene
leading to interference in charge transfer between not only source and drain electrode but also
pentacene and NPs. This may bring about decreasing memory window as well as charge mobility.
Consequently, 8 nm CoFe2O4 NPs showed the best electrical memory performance, and therefore the
only 8 nm NPs will be embedded in NFGM devices for the efficient, in-depth study of memory effect
of CoFe2O4 NPs.
Figure 3-4 shows electrical memory performances of the NFGM devices based on 8 nm CoFe2O4
NPs. Typical output curves of p-type transistors were obtained in Figure 3-4(a). The drain current (ID)
response for repeating pulse bias of program/erase (P/E) operations was investigated to measure
memory switching speed and electrical stability (Figure 3-4(b)). 60 V, 0 V, -60 V, and 0 V were
sequentially and repeatedly applied to gate electrode for program, read on, erase, read off operation,
respectively. This switching cycle is usually called as write-read-erase-read (WRER) cycle. As shown
in Figure 3-4(b) and (c), electrically fast and stable switching ID response with the values of read
on/off current ratio (read Ion/Ioff) above 103 were maintained over 1,000 WRER cycles.
Good data storage capacity is one of the important factors in non-volatile memory devices and the
capacity was estimated from data retention test. (Figure 3-4(d)) The ID response at both programmed
23
(high conductance) and erased (low conductance) states was measured at VGS = 0 V after program
(VGS = 60 V) and erase (VGS = -60 V) operation, respectively. Both read Ion and Ioff values were
decreased slightly during first 100 s, but ca. 3 × 103 of read Ion/Ioff was maintained over 1,000 s.
To investigate the memory effect induced by P/E operation voltage level, applied bias was gradually
increased from ±10 to ±60 V at an interval of 10 V as shown in Figure 3-5 and the results were
summarized in Table 3-2. Both values, memory window and read Ion/Ioff remained almost constant in
the range from ±10 V to ±30 V but started to increase above ±30 V and reached ca. 76.79 V and 2.72
103 at ±60 V, respectively. It indicates that CoFe2O4 NPs start to function as charge trap sites from
±30 V.
Schematic energy band diagrams of the elements of the NFGM devices were illustrated to propose
the possible charge trapping/releasing mechanism of the NFGM devices based on CoFe2O4 NPs.
(Figure 3-6)
Under high positive gate bias (at VGS = 60 V, program operation), electrons in the lowest unoccupied
molecular orbital (LUMO) of pentacene can be attracted and trapped in the conduction band of
CoFe2O4 NPs through oleates by strong positive external electric field. As a result, negatively charged
NPs induced by significant amount of electrons can cause the negative internal electric field and start
to attract the holes in pentacene to the interface between pentacene and oleates to form p-channel
resulting in positive threshold voltage shift and maintenance of high conductance state in read on
operation (at VGS = 0 V) after program operation. In erase operation (at VGS= -60 V), on the other hand,
negative threshold voltage shift can be given via released electrons from NPs or recombination with
transferred holes by strong negative external electric field resulting in low conductance state in read
off operation (at VGS = 0 V) after erase operation.
Tunneling dielectric layer, which has been usually deposited in most NFGM devices for good data
storage capacity, was not embedded in this study. It has been reported that SAM alkyl chains
surrounding NPs can play a role as alternative tunneling dielectric layer. To verify the effect of
additional tunneling dielectric layer, 10 nm-thick Al2O3 thin-film was deposited between CoFe2O4
NPs and pentacene by ALD. Al2O3 has been used widely as an insulator in various electronic devices
because of its high electrical breakdown field, high dielectric constant, and large bandgap32
and ALD
has been considered as a useful method for thin-film deposition due to the merits of facile and
accurate thickness control, making dense and pinhole-free thin-films with excellent thickness
uniformity in large area.33
As summarized in Table 3-3 and 3-4, both memory window and data retention capacity were
lowered by embedding Al2O3 thin-film in NFGM devices indicating that additional Al2O3 tunneling
dielectric layer rather disturbs charge carriers to transfer and oleates capping CoFe2O4 NPs are thick
24
enough to be a tunneling layer in NFGM devices.
25
(a) (b)
(c)
Figure 3-3. Transfer curves of the NFGM devices based on (a) 5, (b) 8, and (c) 11 nm CoFe2O4 NPs.
-60 -40 -20 0 20 40 6010
-11
10-10
10-9
10-8
10-7
10-6
10-5
-ID (
A)
VGS
(V)
Initial
Erase
Program
-60 -40 -20 0 20 40 6010
-11
10-10
10-9
10-8
10-7
10-6
10-5
-ID (
A)
VGS
(V)
Initial
Erase
Program
-60 -40 -20 0 20 40 6010
-10
10-9
10-8
10-7
10-6
10-5
-ID (
A)
VGS
(V)
Initial
Erase
Program
26
Table 3-1. Electrical memory characteristics of the NFGM devices based on CoFe2O4 NPs in different sizes and pentacene-based organic thin-film
transistors (OTFTs) without NPs
Size of
CoFe2O4
NPs
μavg, initial
(cm2
V-1
s-1
)
μmax, initial
(cm2
V-1
s-1
)
Vt, initial
(V)
Vt, program
(V)
Vt, erase
(V)
ΔVt
(V)
Ion, read[a]
(-A)
Ioff, read[a]
(-A)
Read
Ion/Ioff
5 nm
2.3310-3
(±1.95
10-3
)
2.6110-3
(±1.92
10-3)
4.53
(±8.13)
32.46
(±1.78)
-35.82
(±2.09)
68.27
(±2.77)
8.6010-7
(±7.52
10-7
)
1.6410-10
(±3.06
10-11
)
5.13103
(±3.86
103)
8 nm
1.9610-3
(±6.81
10-4
)
2.2410-3
(8.44±
10-4)
-3.38
(±5.39)
31.63
(±4.25)
-42.21
(±3.48)
73.84
(±6.34)
5.9710-7
(±4.08
10-7
)
2.2810-10
(±1.07
10-10
)
2.98103
(±5.29
102)
11 nm
1.0410-3
(±3.05
10-4
)
1.1810-3
(±2.56
10-4)
-6.18
(±3.65)
30.52
(±3.16)
-31.99
(±4.05)
62.51
(±7.16)
4.4910-7
(±1.61
10-7
)
3.2710-10
(±8.87
10-11
)
1.35103
(±1.99
102)
No NPs
5.1710-1
(±1.12
10-1
)
5.2110-1
(±1.11
10-1)
-20.75
(±4.67)
-24.91
(±2.05)
-32.51
(±2.26)
7.60
(±1.13)
1.3210-10
(±1.12
10-11
)
2.2510-10
(±7.48
10-11
)
6.2110-1
(±1.24
10-1)
[a]I(on/off, read) was obtained at VGS = 0 V after the program operation (VGS = 60 V), and erase operation (VGS = -60 V).
27
(a) (b)
(c) (d)
Figure 3-4. Electrical memory characteristics of programmable 8 nm CoFe2O4 NP-based NFGM
devices. (a) Output curves, (b) drain current response for P/E cycles, (c) electrical endurance for 1,000
repeating P/E cycles, and (d) data retention time test.
-60 -50 -40 -30 -20 -10 00.0
0.1
0.2
0.3
0.4
0.5
0.6
-ID (A
)
VDS
(V)
VGS
60 V
50 V
40 V
30 V
20 V
10 V
0 V
100 150 200 250 30010
-11
10-9
10-7
10-5
10-3
10-1
Time (s)
-ID (
A)
Program
Erase
Read ON
Read OFFION
IOFF -250
-200
-150
-100
-50
0
50
100
VG
S (V)
0 200 400 600 800 100010
-11
10-10
10-9
10-8
10-7
10-6
-ID (
A)
Time (s)
Erased state
Programmed state
0 200 400 600 800 100010
-10
10-9
10-8
10-7
10-6
Programmed state
Erased state
-ID (
A)
P/E cycles
28
(a) (b)
Figure 3-5. Memory effect depending on P/E bias voltage for NFGM devices based on 8 nm CoFe2O4
NPs. (a) Transfer curves and (b) summarized values of memory window and read Ion/Ioff.
-60 -40 -20 0 20 40 6010
-11
10-10
10-9
10-8
10-7
10-6
10-5
-ID (
A)
P/E voltage (V)
10 V
20 V
30 V
40 V
50 V
60 V
10 20 30 40 50 60
0
20
40
60
80
Memory window
ON/OFF ratio
P/E voltage (V)
Me
mo
ry w
ind
ow
(V
)
10-1
100
101
102
103
104
ON
/OF
F ra
tio
29
Table 3-2. Memory window and read Ion/Ioff depending on the program/erase operation bias voltage
for NFGM devices based on 8 nm CoFe2O4 NPs
P/E voltage
(V)
Memory window
(V)
Read Ion/Ioff
(read at 0 V)
±10 0.30 9.59 10-1
±20 1.19 9.71 10-1
±30 4.91 7.74 10-1
±40 25.93 8.15 102
±50 52.06 2.69 103
±60 76.79 2.72 103
30
(a) (b)
(c)
Figure 3-6. (a) Schematic energy band diagrams of program and (b) erase operation for the charge
trap/release mechanism description and (c) energy band diagram of pentacene and CoFe2O4 NPs in
different sizes.
31
Table 3-3. Electrical memory performances of the NFGM devices based on 8 nm CoFe2O4 NPs with/without additional Al2O3 tunneling dielectric layer
Thickness
of Al2O3
μavg, initial
(cm2
V-1
s-1
)
μmax, initial
(cm2
V-1
s-1
)
Vt, initial
(V)
Vt, program
(V)
Vt, erase
(V)
ΔVt
(V)
Ion
(-A)
Ioff
(-A)
Read
Ion/Ioff
0 nm
1.9610-3
(±6.81
10-4
)
2.2410-3
(±8.44
10-4)
-3.38
(±5.39)
31.63
(±4.25)
-42.21
(±3.48)
73.84
(±6.34)
5.9710-7
(±4.08
10-7)
2.2810-10
(±1.07
10-10
)
2.98103
(±5.29
102)
10 nm
5.7810-2
(±1.30
10-2
)
5.8410-2
(±1.25
10-2)
-13.38
(±2.81)
20.98
(±2.61)
-38.74
(±1.05)
59.72
(±2.10)
1.7110-5
(±4.92
10-6)
2.3410-10
(±7.57
10-11
)
7.47104
(±1.42
104)
32
Table 3-4. Data retention time test of the NFGM devices based on 8 nm CoFe2O4 NPs with/without additional Al2O3 tunneling dielectric layer
Thickness of
Al2O3
Ion
(at 0 s, -A)
Ion
(at 103 s, -A)
Ion, retention[a]
(%)
Ioff
(at 0 s, -A)
Ioff
(at 103 s, -A)
Ioff, retention[a]
(%)
0 nm 8.2910-7
1.04 10-7
12.65 3.53 10-10
6.42 10-11
18.21
10 nm 1.42 10-5 5.53 10
-7 3.87 7.17 10
-10 6.49 10
-11 9.06
[a] Ion(off), retention = [Ion(off) (at 10
3 s)/Ion(off) (at 0 s)] 100 (%)
.
33
3.3 Electrical memory performance and mechanical test of the flexible NFGM devices based on
8 nm CoFe2O4 NPs
8 nm CoFe2O4 NPs were embedded on the bendable and transparent PET film with 100 nm-thick
Al2O3 blocking dielectric layer to investigate their charge trap capacity in the flexible NFGM devices.
Electrical memory performance of flexible NFGM devices based on 8 nm CoFe2O4 NPs was shown
in Figure 3-7. The memory window was enhanced to ca. 31 V by embedding 8 nm CoFe2O4 NPs in
flexible NFGM devices (Figure 3-7(a)), compared with flexible organic thin-film transistor (OTFT)
without CoFe2O4 NP showing less than 2 V of memory window (Figure 3-7(b)). The value below 2 V
of the memory window of OTFTs indicates that there were few charge trap sites in the active layer,
blocking dielectric layer, and interface between them.
In Figure 3-7(c) and (d), electrical stability of the memory devices was examined by 700 P/E cycles
of dual gate voltage sweep from 20 V to -20 V. The memory window remained stable ca. 30 V
without considerable electrical degradation.
The mechanical reliability of the NFGM devices based on CoFe2O4 NPs has been also tested by
measuring electrical memory performance after repeating pure bending.
The tensile strain at the surface (εtop) of the flexible memory devices in pure bending can be
estimated from the following equation:
𝜀𝑡𝑜𝑝 =(𝐷𝐹+𝐷𝑆)(1+2𝜂+𝜒𝜂
2)
2𝑅(1+𝜂)(1+𝜒𝜂) ≒
𝐷𝑆
2𝑅 (2)
where η = DF/DS and χ = YF/YS. R is bending radius, D is the thickness and YF and YS are the Young’s
modulus of the thin-film (F) and substrate (S), respectively. εtop can be simply calculated as DS/2R.5
As shown in Figure 3-8, the memory window above 26 V was retained with little electrical
degradation during dual gate voltage sweep from 20 V to -20 V against ca. 0.53 % of tensile strain
over 500 bending cycles, showing their potential possibilities of application in flexible memory
devices.
34
(a) (b)
(c) (d)
Figure 3-7. Transfer curves of the 1st dual gate voltage sweep (a) with and (b) without 8 nm CoFe2O4
NPs. (c) Transfer curves for electrical stability test and (d) summarized values of threshold voltage in
programmed/erased state for 700 P/E cycles of the flexible NFGM devices based on 8 nm CoFe2O4
NPs.
-20 -10 0 10 2010
-10
10-9
10-8
10-7
-ID (
A)
VGS
(V)
Program
Erase
0 200 400 600-20
-10
0
10
20
Programmed state
Vt
P/E Cycles
Erased state
-20 -10 0 10 20 30
10-9
10-8
- I D
(A
)
VGS
(V)
100
200
300
400
500
600
700
-20 -15 -10 -5 0 510
-10
10-9
10-8
10-7
10-6
10-5
-ID (
A)
VGS
(V)
Erase
Program
35
(a) (b)
Figure 3-8. (a) Transfer curves and (b) summarized values of threshold voltage in mechanical
stability tests against pure bending.
-20 -10 0 10 2010
-10
10-9
10-8
10-7
- I D
(A
)
VGS
(V)
Initial
After 100
After 200
After 300
After 400
After 500
0 100 200 300 400 500-20
-10
0
10
20
Programmed state
Vt
Bending Cycles
Erased state
36
IV. Conclusion
This study is to fabricate high-performance flexible NFGM devices based on semiconducting
CoFe2O4 NPs. Electrical memory performance depended on the size (5, 8, and 11 nm) of the NPs and
the dependence was explained in terms of the energy level difference. Monodisperse NPs could be
synthesized by facile thermal decomposition using (Co2+
Fe23+
)-oleate complex as a precursor and the
size of NPs can be controlled by regulating Ar bubbling rate in the reacting solution through complete
separation of nucleation and growth process.
The NFGM devices based on 8 nm CoFe2O4 NPs showed the best electrical memory performance
among 3 different sizes of NPs. The devices showed the excellent memory performance: large
memory window of ca. 73.84 V, fast and reversible switching behavior, ca. 3 × 103 of high read Ion/Ioff
at VGS = 0 V, and outstanding data retention capability with an aid of hydrocarbon chains capping NPs
as alternative tunneling dielectric layer. Furthermore, electrical memory operations of the NFGM
devices on the flexible PET substrates have been also investigated and they showed superb, stable
electrical characteristics in repeating P/E cycles and mechanical stability against pure bending. These
results are expected to open-up wide possibilities for the flexible integrated circuits in data storage
technologies.
37
V. References
1. Yi, H. T.; Payne, M. M.; Anthony, J. E.; Podzorov, V., Ultra-flexible solution-processed
organic field-effect transistors. Nat. Commun. 2012, 3, 1259.
2. Lee, Y. H.; Kim, J. S.; Noh, J.; Lee, I.; Kim, H. J.; Choi, S.; Seo, J.; Jeon, S.; Kim, T. S.; Lee, J. Y.;
Choi, J. W., Wearable textile battery rechargeable by solar energy. Nano Lett. 2013, 13 (11), 5753-
61.
3. White, M. S.; Kaltenbrunner, M.; Głowacki, E. D.; Gutnichenko, K.; Kettlgruber, G.; Graz, I.;
Aazou, S.; Ulbricht, C.; Egbe, D. A. M.; Miron, M. C.; Major, Z.; Scharber, M. C.; Sekitani, T.; Someya,
T.; Bauer, S.; Sariciftci, N. S., Ultrathin, highly flexible and stretchable PLEDs. Nat. Photonics 2013, 7
(10), 811-816.
4. Chiao-Wei Tseng, Y.-T. T., Electric bistability in pentacene film-based transistor embedding
gold nanoparticles. J. Am. Chem. Soc. 2009, 131, 12441-12450.
5. Han, S. T.; Zhou, Y.; Roy, V. A., Towards the development of flexible non-volatile memories.
Adv. Mater. 2013, 25 (38), 5425-49.
6. (a) Chang, H. C.; Lee, W. Y.; Tai, Y.; Wu, K. W.; Chen, W. C., Improving the characteristics of
an organic nano floating gate memory by a self-assembled monolayer. Nanoscale 2012, 4 (20),
6629-36; (b) Lee, K.; Weis, M.; Taguchi, D.; Manaka, T.; Iwamoto, M., Memory effect in organic
transistor: Controllable shifts in threshold voltage. Chem. Phys. Lett. 2012, 551, 105-110; (c) Han, S.
T.; Zhou, Y.; Xu, Z. X.; Huang, L. B.; Yang, X. B.; Roy, V. A., Microcontact printing of ultrahigh density
gold nanoparticle monolayer for flexible flash memories. Adv. Mater. 2012, 24 (26), 3556-61; (d)
Kim, S.-J.; Park, Y.-S.; Lyu, S.-H.; Lee, J.-S., Nonvolatile nano-floating gate memory devices based on
pentacene semiconductors and organic tunneling insulator layers. Appl. Phys. Lett. 2010, 96 (3),
033302.
7. Baeg, K.-J.; Noh, Y.-Y.; Sirringhaus, H.; Kim, D.-Y., Controllable shifts in threshold voltage of
top-gate polymer field-effect transistors for applications in organic nano floating gate memory.
Adv. Funct. Mater. 2010, 20 (2), 224-230.
8. (a) Choi, S.; Cha, Y.-K.; Seo, B.-S.; Park, S.; Park, J.-H.; Shin, S.; Seol, K. S.; Park, J.-B.; Jung,
Y.-S.; Park, Y.; Park, Y.; Yoo, I.-K.; Choi, S.-H., Atomic-layer deposited IrO2 nanodots for charge-trap
flash-memory devices. J. Phys. D: Appl. Phys. 2007, 40 (5), 1426-1429; (b) Maikap, S.; Wang, T. Y.;
Tzeng, P. J.; Lin, C. H.; Lee, L. S.; Yang, J. R.; Tsai, M. J., Charge storage characteristics of atomic layer
deposited RuOx nanocrystals. Appl. Phys. Lett. 2007, 90 (25), 253108; (c) Verrelli, E.; Tsoukalas, D.;
Normand, P.; Kean, A. H.; Boukos, N., Forming-free resistive switching memories based on
titanium-oxide nanoparticles fabricated at room temperature. Appl. Phys. Lett. 2013, 102 (2),
022909; (d) Yu-Hsien Lin, C.-H. C., Ching-Tzung Lin, Chun-Yen Chang, Tan-Fu Lei, Novel two-bit
HfO2 nanocrystal nonvolatile flash memory. IEEE Trans. Electron Devices 2006, 53 (4), 782-789; (e)
Kajimoto, K.; Matsui, D.; Uno, K.; Tanaka, I., Organic memory transistors using monolayer of
38
semiconductor colloidal nano-dots as a floating gate. Jpn. J. Appl. Phys. 2013, 52 (5S1), 05DC04.
9. Souad Ammar, A. H., Noureddine Jouini, Fernand FieÂvet,a Izio Rosenman,; FrancËoise
Villain, P. M. a. M. D., Magnetic properties of ultrafine cobalt ferrite particles synthesized by
hydrolysis in a polyol medium. Journal of Materials Chemistry 2001, 11, 186-192.
10. Rana, S.; Philip, J.; Raj, B., Micelle based synthesis of cobalt ferrite nanoparticles and its
characterization using fourier transform infrared transmission spectrometry and thermogravimetry.
Mater. Chem. Phys. 2010, 124 (1), 264-269.
11. Paulsen, J. A.; Ring, A. P.; Lo, C. C. H.; Snyder, J. E.; Jiles, D. C., Manganese-substituted
cobalt ferrite magnetostrictive materials for magnetic stress sensor applications. J. Appl. Phys.
2005, 97 (4), 044502.
12. Giri, A. K.; Kirkpatrick, E. M.; Moongkhamklang, P.; Majetich, S. A.; Harris, V. G.,
Photomagnetism and structure in cobalt ferrite nanoparticles. Appl. Phys. Lett. 2002, 80 (13), 2341.
13. Q A Pankhurst, J. C., S K Jones, J Dobson, Applications of magnetic nanoparticles in
biomedicine. J. Phys. D: Appl. Phys. 2006, 36, R167-R181.
14. Fuxiang Cheng; Zuoyan Peng; Chunsheng Liao; Zhigang Xua Song Gao; Chunhua Yan;
Dongjun Wang; Wang, J., Chemical synthesis and magnetic study of nanocrystalline thin films of
cobalt spinel ferrites. Solid State Commun. 1998, 107 (9), 471-496.
15. Meron, T.; Rosenberg, Y.; Lereah, Y.; Markovich, G., Synthesis and assembly of high-quality
cobalt ferrite nanocrystals prepared by a modified sol–gel technique. J. Magn. Magn. Mater. 2005,
292, 11-16.
16. Kim, Y. I.; Kim, D.; Lee, C. S., Synthesis and characterization of CoFe2O4 magnetic
nanoparticles prepared by temperature-controlled coprecipitation method. Physica B: Condensed
Matter 2003, 337 (1-4), 42-51.
17. Zhao, L.; Zhang, H.; Xing, Y.; Song, S.; Yu, S.; Shi, W.; Guo, X.; Yang, J.; Lei, Y.; Cao, F.,
Studies on the magnetism of cobalt ferrite nanocrystals synthesized by hydrothermal method. J.
Solid State Chem. 2008, 181 (2), 245-252.
18. Victoria S. Coker; Neil D. Telling; Gerrit van der Laan; Richard A. D. Pattrick; Carolyn I.
Pearce; Elke Arenholz; Floriana Tuna; Richard E. P. Winpenny; Lloyd, J. R., Harnessing the
extracellular bacterial production of nanoscale cobalt ferrite with exploitable magnetic properties.
ACS nano 2009, 3 (7), 1922-1928.
19. C.-H. Yan; Z.-G. Xu; F.-X. Cheng; Z.-M. Wang; L.-D. Sun; C.-S. Liao; Jia, J.-T., Nanophased
CoFe2O4 prepared by combustion method. Solid State Commun. 1999, 111, 287-291.
20. Sophie Laurent; Delphine Forge; Marc Port; Alain Roch; Caroline Robic; Luce Vander Elst;
Muller, R. N., Magnetic iron oxide nanoparticles: synthesis, stabilization, vectorization,
physicochemical characterizations, and biological applications. Chem. Rev. 2008, 108, 2064-2110.
21. Elina Manova; Boris Kunev; Daniela Paneva; Ivan Mitov; Petrov, L., Mechano-synthesis,
characterization, and magnetic properties of nanoparticles of cobalt ferrite, CoFe2O4. Chem. Mater.
39
2004, 16, 5689-5696.
22. Taeghwan Hyeon; Yunhee Chung; Jongnam Park; Su Seong Lee; Young-Woon Kim; Park, B.
H., Synthesis of highly crystalline and monodisperse cobalt ferrite nanocrystals. J. Phys. Chem. B
2002, 106, 6831-6833.
23. Qing Song; Zhang, Z. J., Shape control and associated magnetic properties of spinel
cobalt ferrite nanocrystals. J. Am. Chem. Soc. 2004, 126, 6164-6168.
24. Lynch, J.; Zhuang, J.; Wang, T.; LaMontagne, D.; Wu, H.; Cao, Y. C., Gas-bubble effects on
the formation of colloidal iron oxide nanocrystals. J. Am. Chem. Soc. 2011, 133 (32), 12664-74.
25. Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon,
T., Ultra-large-scale syntheses of monodisperse nanocrystals. Nat. Mater. 2004, 3 (12), 891-5.
26. Jun, Y. W.; Choi, J. S.; Cheon, J., Heterostructured magnetic nanoparticles: their versatility
and high performance capabilities. Chem. Commun. 2007, (12), 1203-14.
27. Sakthivel, S.; Kisch, H., Daylight photocatalysis by carbon-modified titanium dioxide.
Angew. Chem. 2003, 42 (40), 4908-11.
28. Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin, J.; Bao, Z., Crystalline
ultrasmooth self-assembled monolayers of alkylsilanes for organic field-effect transistors. J. Am.
Chem. Soc. 2009, 131, 9396-9404.
29. Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D., Assembly and self-organization of
silver nanocrystal superlattices. J. Phys. Chem. B 1998, 102, 8379-8388.
30. Ohara, P.; Leff, D.; Heath, J.; Gelbart, W., Crystallization of opals from polydisperse
nanoparticles. Phys. Rev. Lett. 1995, 75 (19), 3466-3469.
31. Shim, M.; Guyot-Sionnest, P., Permanent dipole moment and charges in colloidal
semiconductor quantum dots. J. Chem. Phys. 1999, 111 (15), 6955.
32. Voigt, M.; Sokolowski, M., Electrical properties of thin rf sputtered aluminum oxide films.
Mater. Sci. Eng., B 2004, 109 (1-3), 99-103.
33. L. Niinistö, J. P., J. Niinistö, M. Putkonen, M. Nieminen, Advanced electronic and
optoelectronic materials by Atomic Layer Deposition: An overview with special emphasis on recent
progress in processing of high-k dielectrics and other oxide materials. Phys. Status Solidi A 2004,
201 (7), 1443-1452.
40
Acknowledgement (감사의 글)
어느덧, 울산에서의 짧고도 긴 2년간의 석사연구를 마무리 하는 시기가 다가왔습니다.
석사 연구를 마무리하면서, 우선 항상 촐랑대며 사고만 치고 다니는 막내 아들을 꿋꿋
이 믿고 응원해 주신 가족들에게 감사의 말을 전하고 싶습니다. 가장으로서 항상 멋진
모습 보여주시는 든든한 아버지, 직장 다니랴 집안일 하랴 몸이 10개라도 모자라지만 힘
든 내색 안하며 항상 웃는 모습 보여주시는 엄마, 그리고 서울에서 열심히 꿋꿋하게 미
래를 향해 달려가고 있는 형 모두 사랑합니다.
2년동안 학문적으로, 그리고 인간적으로 큰 가르침을 주신 오준학 교수님께 깊은 감사
드립니다. 불철주야 연구에 힘쓰시는 교수님의 모습을 본받아 포항공과대학교에서의 박
사과정에서도 유니스트 입학 당시의 열정과 초심을 잃지 않고 항상 열심히 하는, number
one이 아닌 only one이 될 수 있도록 노력 하겠습니다. 앞으로도 호된 꾸짖음과 질책, 많
은 지도 편달 부탁 드리겠습니다.
그리고 2년 동안 SNDL 연구실에서 동고동락을 함께하며 많은 도움을 준 호정이, 아름
이, 은광이, 문정이와 사랑하는 동기들, 무열이, 윤호, 자연이, 은엽이, 그리고 무럭무럭
자라고 있는 인호와 철희, 그리고 해랑이 모두 고맙습니다. 2년간의 연구실 생활을 뒤돌
아보며 여러분들의 소중함을 다시 한번 더 느낄 수 있었습니다. 앞으로도 부끄럽지 않은
SNDL 연구원으로서 최선을 다할 것을 약속 드립니다.
바쁘신 와중에 흔쾌히 석사학위 논문심사를 허락해 주시고 아낌없는 조언을 해주신 김
병수 교수님, 고현협 교수님 감사 드립니다. 또한 나노입자 합성과 논문작성에 도움을 주
신 박종남 교수님과 김성환 학생께도 감사의 말씀을 드립니다. 위에서 언급 드린 분들뿐
만 아니라, 제 주변 모든 분들의 응원과 격려, 도움 덕분에 제가 이 자리까지 올 수 있지
않았나 생각해 봅니다.
41
SNDL 연구실에서 2년 동안 배운 지식, 그리고 행복한 추억 모두 영원히 잊지 않겠습
니다. SNDL 출신 연구원으로서, 어디를 가더라도 부끄럼 없는 사람이 되도록 노력하겠습
니다. 항상 곁에서 따뜻한 격려와 응원을 해주시는 모든 분들의 사랑과 은혜 잊지 않고
살아가겠습니다.
감사합니다.
2014 년 12 월
정 지 형
I. Introduction 1.1 Organic nano-floating gate memory (NFGM) devices 1.2 Cobalt ferrite (CoFe2O4) nanoparticles (NPs)
II. Experiments 2.1 Synthesis of CoFe2O4 NPs 2.2 Characterization of CoFe2O4 NPs 2.3 Fabrication of CoFe2O4 NFGM devices
III. Results & discussion 3.1 Analysis of CoFe2O4 NFGM devices 3.2 Electrical memory performance of CoFe2O4 NFGM devices 3.3 Electrical memory performance and mechanical stability test of flexible CoFe2O4 NFGM devices
IV. Conclusion V. Reference Acknowledgement
10I. Introduction 1 1.1 Organic nano-floating gate memory (NFGM) devices 1 1.2 Cobalt ferrite (CoFe2O4) nanoparticles (NPs) 4II. Experiments 7 2.1 Synthesis of CoFe2O4 NPs 7 2.2 Characterization of CoFe2O4 NPs 8 2.3 Fabrication of CoFe2O4 NFGM devices 15III. Results & discussion 19 3.1 Analysis of CoFe2O4 NFGM devices 19 3.2 Electrical memory performance of CoFe2O4 NFGM devices 22 3.3 Electrical memory performance and mechanical stability test of flexible CoFe2O4 NFGM devices 33IV. Conclusion 36V. Reference 37Acknowledgement 40
