High‐Performance Flexible Organic Nano‐Floating Gate Memory...

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High-Performance Flexible Organic Nano-Floating Gate Memory Devices Functionalized with Cobalt Ferrite Nanoparticles

Ji Hyung Jung , Sunghwan Kim , Hyeonjung Kim , Jongnam Park , * and Joon Hak Oh *

substrates using expensive fabrication processes that require

high temperatures and vacuum conditions. However, there is

currently increasing demand for fl exible memory elements

that can be manufactured using low-cost processes on fl ex-

ible or stretchable substrates. In this regard, organic memory

devices, which are inexpensive to fabricate, lightweight, and

amenable to fl exible plastic substrates, have attracted a great

deal of attention. [ 1–8 ] To realize fl exible fl ash memory devices,

extensive efforts have been made to optimize memory per-

formance using various fl oating gate materials, such as

polymer electrets, [ 9,10 ] organic semiconductors, [ 5,11 ] and nano-

structured materials. [ 12–14 ] Electrically bistable behaviors

can be observed in fl oating gate memory devices, with the

conductivity at a given gate–source bias voltage dependent

on the previous operation. [ 15 ] Charge carriers induced at

the semiconductor–dielectric interface by an external gate

voltage can be trapped into fl oating gates during a program

operation and released during an erase operation, resulting DOI: 10.1002/smll.201501382

Nano-fl oating gate memory (NFGM) devices are transistor-type memory devices that use nanostructured materials as charge trap sites. They have recently attracted a great deal of attention due to their excellent performance, capability for multilevel programming, and suitability as platforms for integrated circuits. Herein, novel NFGM devices have been fabricated using semiconducting cobalt ferrite (CoFe 2 O 4 ) nanoparticles (NPs) as charge trap sites and pentacene as a p -type semiconductor. Monodisperse CoFe 2 O 4 NPs with different diameters have been synthesized by thermal decomposition and embedded in NFGM devices. The particle size effects on the memory performance have been investigated in terms of energy levels and particle–particle interactions. CoFe 2 O 4 NP-based memory devices exhibit a large memory window (≈73.84 V), a high read current on/off ratio (read I on / I off ) of ≈2.98 × 10 3 , and excellent data retention. Fast switching behaviors are observed due to the exceptional charge trapping/release capability of CoFe 2 O 4 NPs surrounded by the oleate layer, which acts as an alternative tunneling dielectric layer and simplifi es the device fabrication process. Furthermore, the NFGM devices show excellent thermal stability, and fl exible memory devices fabricated on plastic substrates exhibit remarkable mechanical and electrical stability. This study demonstrates a viable means of fabricating highly fl exible, high-performance organic memory devices.

Organic Memory

J. H. Jung, Prof. J. H. Oh Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) Pohang , Gyeongbuk 790-784 , Korea E-mail: [email protected]

S. Kim, H. Kim, Prof. J. Park School of Energy and Chemical Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798 , Korea E-mail: [email protected]

1. Introduction

Conventional fl ash memory devices based on inorganic mate-

rials are typically produced on a limited number of rigid

small 2015, 11, No. 37, 4976–4984

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in a threshold voltage shift. [ 16 ] This shift is referred to as a

memory window or hysteresis.

Among the various fl ash memory devices, nano-fl oating

gate memory (NFGM) devices employing nanostructured

materials as the fl oating gates are particularly promising as

next-generation memory devices due to their high speed of

operation, outstanding reliability, capability of multilevel

programming, and suitability as platforms for integrated

circuits. [ 17–19 ] In particular, the electrical properties of an

NFGM device can be easily controlled by modulating the

energy levels of the embedded nanostructured materials, via

the manipulation of their size and shape. Most studies on

organic NFGM devices performed to date employed metallic

nanoparticles (NPs). [ 15,20–23 ] Au NPs have been used exten-

sively in NFGM devices because of their high work function

and chemical stability. [ 22 ] Thermal evaporation is commonly

used to deposit metallic NPs. This method, however, requires

high-vacuum conditions and does not allow for control-

ling the size, shape, and density of NPs because of Ostwald

ripening, in which small NPs tend to dissolve and redeposit

onto larger NPs during evaporation. [ 24 ] On the other hand,

semiconducting or metal oxide NPs are far less common in

memory devices despite their cost-effective production, due

to their relatively low electrical memory performance. [ 25–29 ]

Here, we report the fabrication of high-performance

organic NFGM devices based on pentacene and cobalt ferrite

(CoFe 2 O 4 ) NPs as the semiconducting layer and charge trap-

ping sites, respectively. Pentacene was chosen as the active

layer as it is a model compound for p -channel organic semi-

conductors. CoFe 2 O 4 NPs were applied to organic NFGM

devices due to their relatively higher electrical conductivity,

excellent chemical stability, and mechanical strength com-

pared with other metal oxide NPs. [ 30–32 ] In addition, they

are highly suitable for the low-cost fabrication of NFGM

devices compared with conventional metallic NPs. These

unique properties have led to the use of CoFe 2 O 4 NPs in

various applications, including ionic fl uids, [ 33 ] sensors, [ 34 ] data

storage, [ 35 ] and bioapplications, such as drug delivery systems

for biological labeling or magnetic hyperthermia. [ 36 ] Accord-

ingly, many approaches to the synthesis of CoFe 2 O 4 NPs have

included alkalide reduction, [ 37 ] reverse micelles, [ 38,39 ] and

coprecipitation. [ 40 ] However, synthesized CoFe 2 O 4 NPs often

suffer from broad size distributions and particle aggregation.

In this study, monodisperse semiconducting CoFe 2 O 4 NPs

were synthesized and employed in organic NFGM devices for

the fi rst time. The CoFe 2 O 4 NPs were synthesized by thermal

decomposition of an inexpensive and nontoxic metal–oleate

complex precursor. [ 41–43 ] From the viewpoint of synthetic

methodologies for metal oxide NPs, although several methods

for synthesizing CoFe 2 O 4 NPs have been reported, [ 41,42 ] syn-

thesis of uniform NPs with several nanometer diameters in

high yield and good reproducibility has been challenging.

We successfully fabricated size-tunable and monodisperse

CoFe 2 O 4 NPs with the diameter of 5, 8, and 11 nm by con-

trolling the primary nucleation time via simply adjusting Ar

bubbling rate. [ 44 ] These solution-processed CoFe 2 O 4 NPs

enabled simple, cost-effective, fast, and density-controllable

deposition onto target substrates at room temperature via a

spin-casting technique. In addition, a self-assembled oleate

layer surrounding the NPs acted as a tunneling dielec-

tric layer. This reduced the number of fabrication steps and

enhanced the data retention properties of the memory device.

The resulting NFGM devices exhibited outstanding memory

performance, including a large memory window, fast and

reversible switching behavior, high read current on/off ratio

(read I on / I off ), and good data retention. These properties are

comparable to those of high-performance organic NFGM

devices based on expensive Au NPs. The excellent memory

performance of these CoFe 2 O 4 NP-embedded NFGM devices

resulted from the exceptional charge trapping and release

capability of CoFe 2 O 4 NPs.

2. Results and Discussion

2.1. Electrical Memory Characteristics of NFGM Devices

CoFe 2 O 4 NPs with diameters of 5, 8, and 11 nm were syn-

thesized from a low-cost and nontoxic (Co 2+ Fe 2 3+ )–oleate

precursor complex through a modifi cation of the previously

reported thermal decomposition method. [ 43 ] The details

about the synthetic procedures of CoFe 2 O 4 NPs are described

in Experimental Section. Transmission electron microscopy

(TEM) images of the resulting monodisperse CoFe 2 O 4 NPs

are shown in Figure 1 . The physical characteristics of the

CoFe 2 O 4 NPs were calculated from 50 randomly chosen

particles in the TEM images. The three different sized NPs

showed narrow size distributions of 5.59 ± 0.65 (size variation

≈10%), 8.05 ± 0.57 (size variation ≈7%), and 11.30 ± 0.76 nm

(size variation ≈5%) (Figure S1, Supporting Information).

Although various ferrite NPs have been synthesized, only

few experimental results on the synthesis of iron oxide NPs

smaller than 5 nm have been reported due to the large size

distribution of ultrasmall ferrite NPs. [ 39,44,45 ] The diameter

of the CoFe 2 O 4 NPs could be controlled by varying the Ar

bubbling rate in the reacting solution, which can strongly

affect the nucleation stage by absorbing the heat generated

from exothermic multiple-bonds formation reactions in the

nucleation step. [ 46 ] The resulting monodisperse NPs were

synthesized through the separation of nucleation and growth

step in the heating-up process. A solution of CoFe 2 O 4 NPs

(2 mg mL −1 ) was prepared in n -hexane and spin-coated onto

n -octadecyltrimethoxysilane (OTS)-modifi ed Si/SiO 2 wafers.

A pentacene layer (≈50 nm) was then thermally deposited

onto the CoFe 2 O 4 NP-coated substrates. Gold source–drain

electrodes (40 nm) were subsequently thermally deposited

onto the pentacene layer using shadow masks. Figure 2 shows

a schematic illustration of the CoFe 2 O 4 NP-based NFGM

device structure with cross-sectional bright-fi eld (BF), high-

angle annular dark-fi eld (HAADF) scanning transmission

electron microscopy (STEM, left inset), and TEM images

(right inset) of the 8 nm CoFe 2 O 4 NPs.

The transfer characteristics of NFGM devices based on 5,

8, and 11 nm CoFe 2 O 4 NPs are presented in Figure 3 . The

electrical memory characteristics are summarized in Table

1 . Compared with a small memory window (≈7.60 V) of typ-

ical organic thin-fi lm transistors (OTFTs), all of the transfer

curves of the NFGM devices based on CoFe 2 O 4 NPs exhibited

small 2015, 11, No. 37, 4976–4984



much larger memory window (>62 V). The NFGM devices

also exhibited a counterclockwise hysteresis loop, indicating

p -type behavior, due to the trapping and release of charge

carriers by the NPs. After functionalization with CoFe 2 O 4

NPs, the average read I on / I off increased from ≈0.621 to >10 3

with a sweep of the gate voltage between −60 and +60 V.

Although it is diffi cult to compare the electrical character-

istics of memory devices with those of other devices com-

posed of different materials, the memory performance of

8 nm CoFe 2 O 4 NP-embedded NFGM devices (a large

memory window of ≈73.84 V and high read I on / I off of

≈2.98 × 10 3 ) is comparable to those of the best-performing

organic NFGM devices based on Au NPs. [ 20 ] However, the

charge carrier mobility of the memory devices was decreased

after the integration of NPs, most likely due to the more

disturbed molecular packing by underlying NPs (Figure S2,

Supporting Information). It is also common to observe

such mobility degradation in organic NFGM devices as

small 2015, 11, No. 37, 4976–4984

2 0 n m

aa) b)

2 0 n m

c) d)

2 0 n m

e) f)

Figure 1. TEM images of CoFe 2 O 4 NPs with diameters of a,b) 5, c,d) 8, and e,f) 11 nm.



considerable charge carriers forming drain current in the

active layer can be trapped in nano-fl oating gates. [ 15 ]

The energy levels of the NPs were estimated from cyclic

voltammograms (CVs) and UV–vis absorption spectra. These

data provided a means of determining the charge trapping

and release mechanism of the NFGM devices and effects of

NP size on the electrical memory characteristics. The energy

level of the valence band of CoFe 2 O 4 NP was obtained from

the cyclic voltammetric data shown in Figure S3 (Supporting

Information). The energy bandgap was determined from a

plot of the modifi ed Kubelka–Munk function as a function of

the energy of exciting light derived from the UV–vis absorp-

tion spectra (Figure S4, Supporting Information). [ 47 ] The

energy band diagrams of pentacene [ 48,49 ] and the various sizes

of CoFe 2 O 4 NPs are shown in Figure 4 a. The energy levels

of CoFe 2 O 4 NPs derived from electrochemical/photochem-

ical measurements are summarized in Table S1 (Supporting

Information). As the diameter of the NPs increased from 5 to

11 nm, the energy level of the valence band increased gradu-

ally from −6.56 to −6.52 eV, while that of the conduction band

decreased from −3.80 to −3.94 eV. Concurrently, the energy

bandgap decreased from 2.76 to 2.58 eV. Schematic energy

band diagrams for program/erase (P/E) operations and charge

trapping/release mechanisms are shown in Figure 4 b. During a

program operation (at V GS = 60 V), electrons at the semicon-

ductor–dielectric interface can be transferred by the strong

electric fi eld from the lowest unoccupied

molecular orbital (LUMO) of pentacene

to the conduction band of CoFe 2 O 4 NPs

by passing through the thin oleate layer

(≈2 nm). As a result, negatively charged

NPs generated by the accumulation of

electrons can induce a negative internal

electric fi eld. This leads to accumulation of

holes in the channel region and a positive

threshold voltage shift, which maintains a

high conductance state during the subse-

quent read on operation (at V GS = 0 V).

Conversely, during an erase operation (at

V GS = −60 V), a negative threshold voltage

shift is induced by the release of electrons

from the NPs or by recombination with

holes transported by the strong negative

external electric fi eld, which results in a

low conductance state during the read off

operation (at V GS = 0 V).

Although the charge carriers that contribute directly to

the drain current of p -type pentacene active layer are holes,

the fl oating gate architecture allows trapped negative charge

carriers (electrons) to also affect output current. In addi-

tion, the energy barrier for electron transport between the

LUMO of pentacene and the conduction band of CoFe 2 O 4

NP is lower than that for hole transport between the highest

occupied molecular orbital (HOMO) of pentacene and the

valence band of CoFe 2 O 4 NPs.

As summarized in Table 1 , the memory window

(73.84 ± 6.34 V) of NFGM devices based on 8 nm CoFe 2 O 4

NPs was slightly larger than that of 5 nm NP-based ones

(68.27 ± 2.77 V). NFGM devices based on 11 nm CoFe 2 O 4

NPs had the smallest memory window of 62.51 ± 7.16 V,

although they were expected to exhibit the largest memory

window based on their relative energy levels compared to

those of the 5 or 8 nm CoFe 2 O 4 NPs. This is most likely due to

the formation of irregular aggregates of 11 nm CoFe 2 O 4 NPs.

Atomic force microscopy (AFM) phase images indicated that

5 and 8 nm CoFe 2 O 4 NPs on Si/SiO 2 wafers (Figure S5, Sup-

porting Information) were relatively uniformly dispersed.

In contrast, the 11 nm CoFe 2 O 4 NPs formed close-packed

aggregates. This aggregation was confi rmed on TEM images

(Figure 1 e,f). This aggregation likely prevents the uniform

deposition of pentacene during thermal evaporation, thereby

hindering charge trapping and release, resulting in a smaller

small 2015, 11, No. 37, 4976–4984

-60 -40 -20 0 20 40 6010-11

10-10

10-9

10-8

10-7

10-6

10-5

-ID (

A)

VGS

(V)

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)

erase program

a) b)

-60 -40 -20 0 20 40 6010-11

10-10

10-9

10-8

10-7

10-6

10-5

-ID (

A)

VGS

(V)

erase program

c)

Figure 3. Transfer curves of the NFGM devices based on a) 5, b) 8, and c) 11 nm CoFe 2 O 4 NPs at V DS = −30 V.

Figure 2. A schematic confi guration of the NFGM devices based on CoFe 2 O 4 NPs with cross-sectional bright-fi eld (BF) and high-angle annular dark fi eld (HAADF) scanning transmission electron microscopy (STEM) images (left inset), and a TEM image of 8 nm CoFe 2 O 4 NPs (right inset).



memory window and decreased charge mobility. The aggrega-

tion of 11 nm CoFe 2 O 4 NPs may be attributed to the stronger

coercivity and van der Waals interactions among the larger

NPs. [ 50,51 ] It has been experimentally verifi ed that van der

Waals forces increase with the size of CoFe 2 O 4 NPs because

of permanent dipole moments in their spinel structures. [ 52 ]

The inverse spinel structure of CoFe 2 O 4 NPs was confi rmed

in X-ray patterns (Figure S6, Supporting Information, XRD).

Among the devices tested, the best electrical memory

performance was observed from NFGM devices based on

8 nm CoFe 2 O 4 NPs. Therefore, these devices were selected

for further characterization. The typical output curves of

8 nm CoFe 2 O 4 NP-embedded NFGM devices are shown in

Figure 5 a. The drain current ( I D ) responses for the repeated

pulse bias of P/E operations were investigated to measure

memory switching speed and electrical stability (Figure 5 b).

Gate voltages of 60, 0, −60, and 0 V were sequentially and

repeatedly applied for program, read on, erase, read off oper-

ation, respectively. This switching cycle is usually called as

write-read-erase-read (WRER) cycle. As shown in Figure 5 b,c,

fast and stable switching I D responses with the value of read

I on / I off above 10 3 was maintained over 1000 WRER cycles.

In addition, data storage capability estimated from data

retention test is one of the important factors in nonvolatile

memory devices. A read I on / I off (at V GS = 0 V) of ≈1.62 × 10 3

was maintained over 1000 s as well (Figure 5 d).

To investigate the effects of P/E operation bias volt-

ages on memory performance, the gate–source voltage was

gradually increased from ±10 to ±60 V at intervals of 10 V, as

shown in Figure S7 (Supporting Information). The results are

summarized in Table S2 (Supporting Information). Both the

memory window and read I on / I off remained low and constant

from ±10 to ±30 V, but started to increase above ±30 V. These

observations indicated that the CoFe 2 O 4 NPs start to behave

as charge trap sites above ±30 V. The memory window and

read I on / I off of these devices increased to ≈76.79 V and

≈2.72 × 10 3 , respectively, at ±60 V.

The following equation can be used to estimate the

charge carrier storage capacity of 8 nm CoFe 2 O 4 NPs: [ 53 ]

n V Cet iΔ = Δ

(1)

where Δ n is the amount of transferred charge carriers, C i is the capacitance of the blocking dielectric layer, and e is

the elementary charge. Equation ( 1) indicates that ca. 1.57

× 10 13 charge carriers are expected to be transferred. The

deposition density of the 8 nm CoFe 2 O 4 NPs in the NFGM

devices is ≈8.29 × 10 11 cm −2 based on AFM phase images

of NPs on OTS-treated Si/SiO 2 wafers. These data indicate

that approximately 18 charge carriers were trapped in each

CoFe 2 O 4 NP. It is noteworthy that the charge trapping capa-

bility of the CoFe 2 O 4 NPs is comparable to that of Au NPs

without the aid of additional tunneling dielectric or self-

assembled monolayer (SAM) treatments on the dielectric

layer. [ 6,20,24 ]

The thermal stability of the memory devices has also been

investigated for the practical application in electronic appli-

ances. The NFGM devices showed thermally and electrically

stable behaviors in repeating P/E cycles and data retention

test at 100 °C (Figure S8, Supporting Information). In addi-

tion, it is notable that not only charge carrier mobility but

also I D increases as the temperature of the memory device

small 2015, 11, No. 37, 4976–4984

Table 1. Electrical memory characteristics of the NFGM devices based on CoFe 2 O 4 NPs in various sizes and pentacene-based OTFTs.

Size of NPs [nm]

μ avg, initial [cm 2 V −1 s −1 ]

μ max, initial [cm 2 V −1 s −1 ]

V t, initial [V]

V t, program [V]

V t, erase [V]

Δ V t [V]

Read I on [−A]

Read I off [−A]

Read I on / I off

5 2.33 × 10 −3

(±1.95 × 10 −3 )

2.61 × 10 −3

(±1.92 × 10 −3 )

4.53

(±8.13)

32.46

(±1.78)

−35.82

(±2.09)

68.27

(±2.77)

8.60 × 10 −7

(±7.52 × 10 −7 )

1.64 × 10 −10

(±3.06 × 10 −11 )

5.13 × 10 3

(±3.86 × 10 3 )

8 1.96 × 10 −3

(±6.81 × 10 −4 )

2.24 × 10 −3

(±8.44 × 10 −4 )

−3.38

(±5.39)

31.63

(±4.25)

−42.21

(±3.48)

73.84

(±6.34)

5.97 × 10 −7

(±4.08 × 10 −7 )

2.28 × 10 −10

(±1.07 × 10 −10 )

2.98 × 10 3

(±5.29 × 10 2 )

11 1.04 × 10 −3

(±3.05 × 10 −4 )

1.18 × 10 −3

(±2.56 × 10 −4 )

−6.18

(±3.65)

30.52

(±3.16)

−31.99

(±4.05)

62.51

(±7.16)

4.49 × 10 −7

(±1.61 × 10 −7 )

3.27 × 10 −10

(±8.87 × 10 −11 )

1.35 × 10 3

(±1.99 × 10 2 )

No NPs 5.17 × 10 −1

(±1.12 × 10 −1 )

5.21 × 10 −1

(±1.11 × 10 −1 )

−20.75

(±4.67)

−24.91

(±2.05)

−32.51

(±2.26)

7.60

(±1.13)

1.32 × 10 −10

(±1.12 × 10 −11 )

2.25 × 10 −10

(±7.48 × 10 −11 )

6.21 × 10 −1

(±1.24 × 10 −1 )

Figure 4. a) Energy band diagrams of pentacene and CoFe 2 O 4 NPs in different sizes and b) a schematic energy band diagrams during program (left) and erase operations (right) for the charge trapping/release mechanism description.



rises, which can be attributable to the facilitated charge hop-

ping. [ 54 ] The mechanism can be expressed as: [ 55–58 ]

exp2

I Sqa n vqa V

dkTqkTD m e f

m t= − ∅⎛⎝⎜

⎞⎠⎟

⎡⎣⎢

⎤⎦⎥

(2)

where S is the contact area, d is assumed to be the fi lm thick-

ness, q is the electronic charge, V is the applied bias, a m is the

mean hopping distance, n e is the density of space charge, v f is

the intrinsic vibration frequency, and k is Boltzmann’s con-

stant. Thermally excited electrons in pentacene layer can hop

from one trap state to another over barrier height between

the localized sites. In addition to hopping mechanism, carrier

mobility may be increased as the interconnectivity between

pentacene grains is improved as the substrate temperature

increases, leading to the reduced number of traps at the grain

boundaries under a given channel length. [ 59 ] Furthermore,

hysteresis can be enhanced as the temperature increases due

to the fl uctuation in the number of trapped charges resulting

from the change in the values of q∅ t and a m , which are

affected by the gate voltage. [ 60 ]

2.2. Effects of an Additional Tunneling Dielectric Layer on Memory Characteristics

The outstanding electrical memory characteristics of

CoFe 2 O 4 NP-based NFGM devices in the absence of a tun-

neling dielectric layer may be attributed to the exceptional

charge trapping and release capability of oleate-capped

(≈2 nm) CoFe 2 O 4 NPs. A coating consisting of a SAM of

alkyl chains can act as an alternative tunneling dielectric layer

due to the insulating nature of hydro-

carbon chains. [ 15 ] In conventional NFGM

devices, a tunneling dielectric layer is usu-

ally prepared at high temperatures and

under high vacuum over a long period.

This step is considered vital for charge car-

rier storage in NPs, which is required for

data retention. In addition, optimization of

the thickness of the tunneling layer, which

separates charge trap sites from the semi-

conducting layer, is important to achieve

a balance between data retention and

P/E speed. [ 22 ] A thick tunneling dielectric

layer can degrade P/E speed and increase

power consumption. [ 18 ] Conversely, a thin

layer can enhance the P/E operation speed

but an excessively thin layer can limit

the number of P/E cycles and/or result

in charge loss or leakage current through

defects in the layer. [ 61 ]

To verify the effects of an additional

tunneling dielectric layer in our memory

devices, 10 nm thick Al 2 O 3 thin fi lm was

deposited by atomic layer deposition (ALD)

between the CoFe 2 O 4 NPs and the penta-

cene layer. Al 2 O 3 is widely used as an insu-

lator in various electronic devices because of

its high electrical breakdown fi eld, high dielectric constant, and

large bandgap. [ 62 ] In addition, ALD methods have simple and

accurate thickness control, resulting in dense and pinhole-free

thin fi lms with excellent thickness uniformity over large areas. [ 63 ]

The electrical memory characteristics of 8 nm CoFe 2 O 4

NP-based NFGM devices with and without the Al 2 O 3 tun-

neling dielectric layer are summarized in Table S3 (Supporting

Information). Although the integration of the Al 2 O 3 layer

enhanced several parameters, including charge mobility and

read I on / I off by isolating the NPs from pentacene (Table S4,

Supporting Information), the memory window was reduced

from ≈73.84 to ≈59.72 V. These results indicate that the addi-

tional Al 2 O 3 tunneling dielectric layer disrupts the transfer of

charge carriers into the CoFe 2 O 4 NPs and the oleate layer is a

suffi cient tunneling layer in our NFGM devices.

2.3. Electrical Memory Characteristics of Flexible NFGM Devices

The CoFe 2 O 4 NP-embedded NFGM devices were prepared

on polyethylene terephthalate (PET) fi lms to demonstrate

the low voltage operation and the feasibility of fabricating

fl exible organic memory devices since they are essential

prerequisites for the practical application in fl exible devices.

Most of the organic NFGM devices reported so far required

gate voltages of at least ±30 V for effi cient fi eld-effect mod-

ulation and maximization of the charge trap in NPs [ 12,53,64 ]

and a small memory window less than 20 V was often

obtained at higher P/E operation voltage in fl exible NFGM

devices. [ 6,22 ] Al 2 O 3 dielectric layer (≈100 nm) was subse-

quently deposited using radio-frequency (RF) sputtering.

small 2015, 11, No. 37, 4976–4984

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0.1

0.2

0.3

0.4

0.5

0.6-I

D( µ

A)

VDS

(V)

VGS

(V)

60 50 40 30 20 10 0

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 off Ion

Ioff -250

-200

-150

-100

-50

0

50

100

VG

S (V)

0 200 400 600 800 100010-10

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10-8

10-7

10-6

programmed state

erased state

-ID (

A)

P/E cycles0 200 400 600 800 1000

10-11

10-10

10-9

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10-7

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-ID (

A)

Time (s)

erased state

programmed state

a) b)

c) d)

Figure 5. Electrical memory characteristics of programmable 8 nm CoFe 2 O 4 NP-based NFGM devices: a) Output curves, b) drain current response for P/E cycles, c) electrical endurance for 1000 P/E repeating cycles, and d) data retention test.



The CoFe 2 O 4 NPs and pentacene layer were deposited con-

secutively, followed by the formation of gold source–drain

electrodes. Figure 6 shows a schematic, photograph, and

electrical memory characteristics of the resulting fl exible

NFGM devices. Despite the risks of current leakage and

performance degradation due to the high surface rough-

ness and temperature susceptibility of the PET substrate,

the fl exible NFGM devices exhibited excellent electrical

memory performance with a large memory window of over

32 V, even after 700 P/E cycles in low gate–source voltage

sweep between −20 and +20 V (Figure 6 c,d, and Figure S9a,

Supporting Information). The mechanical stability of the

CoFe 2 O 4 NP-based NFGM devices was tested by measuring

electrical memory performance after repeated bending

cycles. The tensile strain at the top surface ( ε top ) of the fl ex-

ible memory devices can be calculated from the following

equation for a simple bending: [ 65 ]

1 2

2 1 1 2top

F S2

sD D

RD

Rε

η χηη χη( )

( )( )( )

=+ + +

+ +≈ (3)

where η = D F / D S and χ = Y F / Y S . R is the bending radius, D is

the thickness and Y F and Y S are the Young’s modulus of the

thin-fi lm (F) and substrate (S), respectively. ε top can be simply

calculated as D S /2 R .

As shown in Figure 6 e and Figure S9b (Supporting Infor-

mation), the memory window was retained above 26 V over

500 bending cycles with minimal electrical degradation during

gate–source voltage sweeps between −20 V and +20 V against

a tensile strain of ≈0.54%. This strain was induced by a bending

radius of 7 mm, indicating that these devices have high poten-

tial for use in fl exible or stretchable electronic devices. Con-

versely, a memory window smaller than 2 V was observed with

fl exible devices prepared without CoFe 2 O 4 NPs (Figure S9c,

Supporting Information). This confi rmed that the CoFe 2 O 4

NPs acted as charge trap sites for memory performance.

3. Conclusion

High-performance fl exible organic NFGM devices were

fabricated using pentacene and CoFe 2 O 4 NPs as the semi-

conductor and charge trapping agent, respectively. Monodis-

perse CoFe 2 O 4 NPs with three different diameters (5, 8, and

11 nm) were synthesized by a simple thermal decomposition

method starting from a low-cost and non-toxic metal–oleate

complex precursor. NFGM

devices based on 8 nm CoFe 2 O 4

NPs exhibited the best electrical

memory performance due to

favorable energy levels and uni-

form dispersion on the substrate.

These factors resulted in a large

memory window of ≈73.84 V,

fast and reversible switching

behavior, a high read I on / I off of

≈2.98 × 10 3 , and outstanding

data retention, facilitated by an

oleate capping layer that acted

as an alternative tunneling die-

lectric layer. It is noteworthy

that the electrical memory per-

formance of the NFGM devices

based on CoFe 2 O 4 NPs was

comparable to that of Au NP-

embedded ones. Furthermore,

the NFGM devices were electri-

cally and mechanically stable on

fl exible PET substrates at low

operation voltage and retained

a memory window over 32 and

26 V with little electrical degra-

dation after 700 P/E cycles and

500 bending cycles, respectively.

Solution-processed, monodis-

perse CoFe 2 O 4 NPs allowed

simple and cost-effective depo-

sition of nano-fl oating gates

with controllable size and den-

sity, which has been challenging

in the fabrication processes of

NFGM devices.

small 2015, 11, No. 37, 4976–4984

a) b)

c) d)

e)

-20 -10 0 10 2010-10

10-9

10-8

10-7

-ID (

A)

VGS

(V)

program erase

0 100 200 300 400 500-20

-10

0

10

20

programmed state

Vt

bending cycles

erased state

0 200 400 600 800-20

-10

0

10

20

programmed state

Vt

P/E cycles

erased state

Figure 6. a) A schematic image, b) photograph, c) transfer curves (at V DS = −20 V), d) electrical, and e) mechanical stability test of the fl exible NFGM devices based on 8 nm CoFe 2 O 4 NPs on PET substrate.



In addition, the oleate capping layer signifi cantly simpli-

fi ed the fabrication process by eliminating the need for an

additional tunneling dielectric layer. Our fi ndings demon-

strate that sophisticated control of methodologies when syn-

thesizing semiconducting or metal oxide NPs can yield novel

charge trapping agents for memory devices and open up new

possibilities for the simple and low-cost fabrication of high-

performance data storage devices.

4. Experimental Section

Synthesis of (Co 2+ Fe 2 3+ )–Oleate Precursor : FeCl 3 ·6H 2 O (8.64 g, 32 mmol), CoCl 2 ·6H 2 O (3.808 g, 16 mmol), and sodium oleate (38.96 g, 128 mmol) were dissolved in a mixture of 80 mL of ethanol, 80 mL of D. I. water, and 160 mL of n -hexane. The solution was stirred until all the reagents were dissolved com-pletely. Then, the solution was refl uxed at 60 °C for 4 h. The reacted (Co 2+ Fe 2 3+ )–oleate was washed three times with 120 mL of D. I. water and residual solvent was evaporated in the rotary evaporator at 80 °C.

Synthesis of 8 nm CoFe 2 O 4 NPs : (Co 2+ Fe 2 3+ )–oleate complex (2.5 g), OLA (0.25 g), and octadecene (10 mL) were mixed and evacuated at 80 °C for 1 h. The mixture was stirred with Ar bub-bling at rising temperature. After 30 min, the mixture was heated to 310 °C at a heating rate of 1 °C min −1 and maintained at this temperature for 1 h. The reacted solution was cooled to room temperature and washed three times with acetone/ethanol mix-ture. Finally, the purifi ed CoFe 2 O 4 NPs were dispersed in 10 mL of n -hexane for long-term storage.

CoFe 2 O 4 NPs in different sizes can be synthesized by the same procedures, except for the Ar bubbling fl ow rate, which optimizes the NP size. 5 nm CoFe 2 O 4 NPs were synthesized under vigorous Ar bubbling. Reversely, 11 nm ones were synthesized when Ar bub-bling was stopped before the solution was heated.

NFGM Device Fabrication : CoFe 2 O 4 NP solution (2 mg mL −1 ) was spin-coated at 1000 rpm for 60 s on the OTS-treated Si/SiO 2 wafer (D. I. water contact angle: ≈107° (Figure S10, Supporting Informa-tion)) [ 66,67 ] and annealed at 60 °C for 3 h in the vacuum oven to evaporate the solvent thoroughly. 50 nm thick pentacene was ther-mally deposited on the surface of NPs at a rate of 0.01–0.03 nm s −1 at 60 °C (substrate temperature) under a pressure of 5.0 × 10 −6 Torr. Gold source–drain electrodes (40 nm) were also thermally depos-ited in the evaporation chamber at room temperature using shadow masks with 50 µm of channel length (L) and 1000 µm of channel width (W). For the fl exible NFGM device fabrication, 100 nm thick Al 2 O 3 gate dielectric layer was deposited on the PET substrate by RF sputtering technique after the gate electrode (Cr (5 nm)/Au (100 nm)) was formed by thermal evaporation. The other processes are the same as those in the fabrication on the Si wafer as described above. Additional tunneling dielectric layer, 10 nm Al 2 O 3 thin fi lm was deposited by ALD at a substrate temperature of 150 °C.

Energy Level Measurement : All the cyclic voltammetry (CV) measurements were performed in 0.1 m of tetabutylammonium hexafl uorophosphate (Bu 4 NPF 6 ) in anhydrous acetonitrile elec-trolyte at room temperature at a scan rate of 100 mV s −1 under N 2 gas blowing for the estimation of the valence band energy level of the CoFe 2 O 4 NPs. CoFe 2 O 4 NP solution was deposited on indium tin oxide (ITO) glass and used as a working electrode.

The counter electrode and the reference electrode were platinum (Pt) wire and Ag/Ag + electrode containing 0.01 m of AgNO 3 with 0.1 m of tetrabutyl ammonium perchlorate (TBAP) in acetonitrile, respectively. The Ag/Ag + reference electrode was internally cali-brated by ferrocene/ferrocenium couple (Fc/Fc + ) and the valence band energy level of NPs can be estimated using:

E E EeV 4.8valenceband ox

onsetferrocene

onset( ) = − − +⎡⎣

⎤⎦( ) ( )

(4)

For the estimation of the energy bandgap of CoFe 2 O 4 NPs, both NP solution and spin- coated samples were measured using double-beam mode. The sample for the solution-based UV–vis absorption spectrum was prepared by diluting 100 µL of extracted 5, 8, and 11 nm CoFe 2 O 4 NP solution in 3 mL of n -hexane. For the spin-coated samples, UV–vis absorption spectra can be obtained by similar procedures except the quartz cuvette was replaced with quartz plate which was spin-coated at 1000 rpm using of CoFe 2 O 4 NP solution (10 mg mL −1 ) before loaded on the sample holder.

Instruments for Characterization : All the electrical perfor-mances of the NFGM devices were measured in an N 2 -fi lled glove box using a Keithley 4200 semiconductor parametric analyzer.

The size and shape of NPs were characterized using transmis-sion electron microscopy (TEM; JEM-2100, JEOL) operated at 200 kV. Specimens for TEM were prepared by conventional methods, drop-ping the diluted CoFe 2 O 4 NP solution onto the copper grid. Inverse spinel structure of NPs was characterized using X-ray diffraction (Figure S6, Supporting Information, XRD; Bruker, Germany). Rotating anode was used as X-ray source, and the measurement was oper-ated at 220 V. The sample was prepared in the form of solid powder.

For the surface analysis of the NFGM devices, tapping-mode atomic force microscope (AFM; Veeco (USA)) was used and the sample for cross-sectional 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 elec-tron microscope (HR-TEM, Cs-corrected JEM-2100F, JEOL, Japan). The bandgaps of CoFe 2 O 4 NPs were measured using UV-1800 (Shi-madzu Corp.) and VARIAN CARY 100 (Agilent Tech).

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program (2013M3A6A5073175), the National Research Foundation of Korea (2014R1A2A2A01007467) of the Ministry of Science, ICT & Future Planning, Korea, and the New & Renewable Energy Core Technology Program (20133030000180) of the Korea Institute of Energy Tech-nology Evaluation and Planning (KETEP), granted fi nancial resource from the Ministry of Trade, Industry & Energy, Korea. J.H.J. acknowl-edges Moo Yeol Lee for his help in drawing 3D illustrations.

small 2015, 11, No. 37, 4976–4984


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Received: May 15, 2015 Published online: July 7, 2015

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