OPENSUBJECT AREAS:
INFORMATION STORAGE
ELECTRONIC PROPERTIES AND
MATERIALS

Received
3 April 2014
Accepted
13 June 2014
Published
2 July 2014

Correspondence andrequests for materials
should be addressed toT.Y. (yanagi32@
sanken.osaka-u.ac.jp)

Cellulose Nanofiber Paper as an Ultra
Flexible Nonvolatile Memory
Kazuki Nagashima1, Hirotaka Koga1, Umberto Celano2,3, Fuwei Zhuge1, Masaki Kanai1, Sakon Rahong1,
Gang Meng1, Yong He1, Jo De Boeck2, Malgorzata Jurczak2, Wilfried Vandervorst2,3, Takuya Kitaoka4,
Masaya Nogi1 & Takeshi Yanagida1
1

The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka, 567-0047, Japan, 2IMEC,
Kapeldreef 75, B-3001 Heverlee (Leuven), Belgium, 3KU Leuven, Department of Physics and Astronomy (IKS), Celestijnenlaan 200D,
3001 Leuven, Belgium, 4Department of Agro-environmental Sciences, Graduate School of Bioresource and Bioenvironmental
Sciences, Kyushu University, Fukuoka, 812-8581, Japan.

On the development of flexible electronics, a highly flexible nonvolatile memory, which is an importantcircuit component for the portability, is necessary. However, the flexibility of existing nonvolatile memory
has been limited, e.g. the smallest radius into which can be bent has been millimeters range, due to thedifficulty in maintaining memory properties while bending. Here we propose the ultra flexible resistive
nonvolatile memory using Ag-decorated cellulose nanofiber paper (CNP). The Ag-decorated CNP devices
showed the stable nonvolatile memory effects with 6 orders of ON/OFF resistance ratio and the small
standard deviation of switching voltage distribution. The memory performance of CNP devices can be
maintained without any degradation when being bent down to the radius of 350 mm, which is the smallest
value compared to those of existing any flexible nonvolatile memories. Thus the present device using
abundant and mechanically flexible CNP offers a highly flexible nonvolatile memory for portable flexible
electronics.

F

lexible electronics is an emerging research field in electronics, which has been intensively investigated in last
decade1. Novel devices such as rollable display, conformable sensor, biodegradable electronics and otherflexible electronic applications have been demonstrated and they have been achieved by utilizing mechanically flexible electronic circuit components1–12. For next-generation flexible electronics, it is strongly desired todevelop highly flexible circuit components, which can be tightly rolled and bent around sharp edges without
degradation of the electric performance. Therefore, recent studies on flexible electronics have focused on developing highly flexible circuit components8–12. For example, Sekitani et al. reported flexible organic thin filmtransistor (TFT) with the bending radius of 100 mm8. The performance of TFT was maintained as long as thegate insulator withstood the bending stress.
Nonvolatile memory, which is an essential component for portable and self-standing electronics, is an important flexible circuit component13–40. Previously, several types of flexible nonvolatile memory such as flexible flash
memory13–17, flexible ferroelectric memory18,19 and flexible resistive memory20–38 have been demonstrated. In theseprevious flexible nonvolatile memories, the smallest radius into which the memory devices can be bent has been
limited to several millimeters due to the strain-induced degradation of memory properties40. Needless to say,further flexibility of nonvolatile memory beyond the current limitation is desired toward highly flexible andportable electronics.
Here we demonstrate an Ag-decorated cellulose nanofiber paper (CNP) as ultra flexible nonvolatile resistive
memory41–44. The cellulose nanofibers, which are renewable and the most abundant biomass on earth, haveoutstanding properties including a high tensile strength (2–6 GPa) and a low thermal expansion coefficient(6 ppm/K)45,46. CNP has been applied to electronic device as a substrate and reinforcing agent for conductive
sheets of carbon nanotubes12,47–53, however none of previous investigations has utilized CNP as memory devices tostore electrical information. This study shows that the resistance of Ag-decorated CNP can be electricallyswitched with 6 orders of ON/OFF resistance ratio, and the resistance change can be utilized as nonvolatilememory effect. In addition, we found the excellent robustness of mechanical and electrical properties of thepresent CNP memory devices even for the bending radius of down to 350 mm, which is the smallest value so far
reported for flexible nonvolatile memories.
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Figure 1 | (a) Schematic illustration and (b) photograph of the Ag-decorated CNP device. (c) Cross-sectional TEM image and SAED pattern of the Agdecorated CNP.

Results
The memory effect in Ag-decorated CNP device was firstly examinedby utilizing devices fabricated onto the rigid Si substrate before examining its flexibility. Figure 1 (a) and (b) show the schematic illustration and photograph of the Ag-decorated CNP memory device on Si
substrate. In this study, we utilized 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO)-oxidized cellulose nanofibers of 3–4 nm width52–55 as
the base matrix of CNP. The CNP layer was fabricated by drop
casting of the cellulose nanofibers slurry onto Pt/Ti/SiO2/Si substrateand drying at 40uC for 24 hrs. The Ag nanoparticles were in situ
synthesized onto the cellulose nanofibers prior to the fabrication of
CNP (see the details in Method section and Supplementary
Information S1). The thickness of Ag-decorated CNP can be readily
controlled by varying the amount of cellulose nanofibers slurry. Then
Ag electrodes of sizes ranged from 50 3 50 mm2 to 500 3 500 mm2
were deposited onto the Ag-decorated CNP device. Figure 1 (c)shows a cross-sectional TEM image of the device. Although theindividual cellulose nanofibers were not visible due to the denselypacked CNP structure, the nanoparticles were observable and thesize was ranged from 1 nm to 30 nm. Selected area electron diffraction (SAED) pattern identified that these nanoparticles were composed of Ag. Figure 2 (a) shows the typical current-voltage (I-V) dataof Ag-decorated CNP device. The measurements were performed atroom temperature in atmospheric condition. The thickness of the
Ag-decorated CNP was ca. 2.9 mm. The Ag-decorated CNP was
sandwiched by Ag top electrode and Pt bottom electrode. As shownin the inset of figure 2 (a), initially we performed the forming processwith the current compliance of 1023 A, which is a preparative process
for resistive switching memory56. The applied voltage was controlled
by the Ag top electrode, whereas the Pt bottom electrode was
grounded. When we increased the voltage, the current abruptly
increased at 4.7 V. After the forming process, the resistance statechanged from insulative pristine state to conductive ON state.
Then we performed so-called RESET process by applying negative
voltage, which changed the resistance state back to insulative OFF
state. Subsequently, ON state was recreated by applying positive
voltage, which is so-called SET process. The stability of resistive
switching in Ag-decorated CNP devices was confirmed by retention
and endurance measurement. Figure 2 (b) and (c) show the data
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retention and the switching endurance of Ag-decorated CNP device.
The readout voltages were 0.01 V for retention and 0.1 V for endurance, respectively. Both of the ON and OFF states were maintainedfor 105 s with 6 orders of ON/OFF resistance ratio, indicating the
nonvolatility of the present device. The endurance was confirmed atleast up to 100 switches. The distribution of resistances in endurance
data was caused by the small SET voltages of Ag-decorated CNP
devices and should be improved by optimizing the thickness andconcentration of Ag nanoparticles in CNP devices. Figure 2 (d)shows the distribution of the applied voltages required for SET and
RESET processes. The data were taken from 100 continuous
switches. The SET voltage is defined as a voltage when the current
reaches to the current compliance value-1023 A during SET process,while the RESET voltage is defined as the voltage when the current
starts to drop during RESET process. The average voltage values
required for SET and RESET processes were 10.28 V and
20.22 V, respectively. The standard deviations for SET and
RESET voltages were 0.26 V and 0.56 V, respectively, indicating
the small variation of switching voltages for memory operation.
Thus, these results highlight that the present Ag-decorated CNP
devices exhibit reliable nonvolatile memory effects with unambiguous memory properties including retention, endurance, ON/OFF
resistance ratio and small statistical variations of operation voltage.
Here we discuss what governs the resistive switching in the present
Ag-decorated CNP devices. First, we examined the area dependenceof ON and OFF state resistances to clarify the spatial inhomogeneityof electrical conduction within matrix57. As shown in Figure 3 (a), theOFF state resistance increased with decreasing the device size,
whereas the ON state resistance did not show the size dependence.
Considering previous implications, the present data infers the presence of local conduction paths within an insulating matrix56,57.
Within the framework of conventional electrochemical metallization
mechanism, the resistive switching can be interpreted in terms of theformation and rupture of Ag conductive filaments42–44. Ag are
ionized near the positively biased electrode, and then the Ag1 ionsmigrate and diffuse into CNP layer toward negatively charged Pt
counter electrode. When the Ag1 ions reach to the Pt electrode,
the Ag1 ions become Ag by receiving the electrons from the Pt
electrode. Such successive metallization process creates the Ag con2

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Figure 2 | (a) I-V characteristics of the Ag-decorated CNP device. Current compliance of 1023 A was applied. Inset shows the initial forming process. (b)Data retention and (c) switching endurance of the Ag-decorated CNP device. Readout voltages were 0.01 V for retention and 0.1 V for endurance,
respectively. (d) Statistical distribution data of operation voltages for SET and RESET processes. The data is analyzed by continuous 100 switches.

ductive filaments between the Pt electrode and the Ag electrode. The
rupture of Ag filaments occurs through the ionization and the migration of Ag1 ions under the opposite electric field polarity. In suchswitching model, the ionization and migration of Ag top electrodemight be critical for the resistive switching phenomena.
Experimentally, the forming voltages for devices with the Ag top
electrodes was found to be much lower than that for devices with
Pt top electrodes, as shown in Figure 3(b). In addition, the resistive
switching of Ag top electrode device was confirmed to be much morestable than that of Pt top electrode devices (Supplementary
Information S2–3). These results indicate the critical role of Ag top
electrode for the nonvolatile memory effect in CNP device. Moredetails can be seen in Supplementary Information. Thus, the experimental results highlight that the model based on the formation andrupture of Ag conductive filaments is plausible for the resistive
switching in the Ag-decorated CNP device (Supplementary
Information S4).

Next we discuss the switching voltage value required for the initial
forming process of the Ag-decorated CNP devices (Vforming 5 4.7 V
in Figure 2(a)). The calculated electric field intensity for the Agdecorated CNP device is ca. 1.6 3 1022 MV/cm, which is much lower
than that for conventional resistive memory devices (0.6–1.2 MV/cm)58. Since the presence of metal nanoparticles is well known toenhance the electric field intensity around the nanoparticles59–61,
such effect might explain the lower forming electric field intensity
for the Ag-decorated CNP device. We performed the electric field
simulation in the presence of metal nanoparticles, as shown in
Supplementary Information S5. For qualitative understanding andsimplicity for simulation, we assumed the simple series alignment of
the nanoparticles to calculate the spatial distribution of electric field.
The electric field intensity increases several times by introducing themetal nanoparticles, because of 1) the enhancement of electric fieldaround the nanoparticles’ edge and 2) the reduction of working
distance. Since the actual forming electric field intensity for the

Figure 3 | (a) The device size dependence on the ON and OFF state resistances. The Ag/Ag-decorated CNP/Pt device structure was employed. The
device size was varied from 50 3 50 mm2 to 500 3 500 mm2. The readout voltage is 0.1 V. (b) The material dependece of top electrode on the forming
voltage. In this experiment, we utilized Ag and Pt as the top electrodes and the device sizes ranged from 50 3 50 mm2 to 500 3 500 mm2 were examined.
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Ag-decorated CNP device was 2 orders lower than that for the conventional resistive memory devices, the electric field enhancement
effect alone cannot explain the present experimental results in thisstudy. Although we do not have the direct experimental evidence toexplain the discrepancy, we speculate that the bonding weakness
between cellulose nanofibers and the water adsorbed on Ag-decorated CNP might lower the forming voltage via increasing the ion
mobility of Ag1 ions within CNP layer. Since the cellulose nanofibers
are weakly connected by hydrogen bonds and/or van der Waals
forces, the cellulose nanofibers can be easily disconnected by external
stimuli. In addition, the ions can stably exist and easily move inwater62,63. If the forming process of Ag-decorated CNP device isrelated to the migration of Ag1 ions, the presence of water cruciallyaffects the forming voltage. In fact, the water is known to adsorb on
the surface of cellulose nanofibers due to the presence of OH bonds64.
Although the further investigations should be undertaken to directly
identify the exact origin of lowering the forming voltage, our experimental results highlight that the present Ag-decorated CNP devices
have the advantages for memory operations working at low voltage
range even for micrometer-scale thickness.
Here we examine the flexibility of Ag-decorated CNP devices. We
utilized an aluminium foil as flexible substrate. Figure 4 (a) and (b)show the schematic illustration and the photograph of Ag-decorated
CNP device on aluminium foil substrate. The Ag-decorated CNP wasselectively deposited onto the aluminium foil substrate by chemicallymodifying the substrate surface (see details in Method section and
Supplementary Information S6). The bending test was performed bywrapping the Ag-decorated CNP devices around various columnshaped staffs. Figure 4 (c) shows the typical photograph of the Agdecorated CNP device wrapped around the glass rod (w 5 5 mm).
The thicknesses of Ag-decorated CNP and aluminium foil were
560 nm and 12 mm, respectively. The device can be flexibly bent,
and any cracks or exfoliations were not observed even the device
was bent down to the radius of 350 mm (see Supplementary
Information S7). Figure 4 (d) shows the typical I-V curve of the
Ag-decorated CNP device on aluminium foil. The aluminium foil
substrate was used as the bottom electrode since the aluminium foil
can maintain the electrical conductivity even bending. The clear
resistive switching events and good retention were confirmed forthe present aluminium foil based memory devices. Next we examine
the flexibility of the present Ag-decorated CNP memory devices.
Figure 5 (a) shows the ON and OFF state currents as a function ofcurvature radius r. The data was collected from the I-V curves performed at each curvature radius. The readout voltage was 0.1 V.
Remarkably, both of the ON and OFF state currents were stably kept
to be almost constant without any degradations even for r 5 350 mm.
In addition, we performed the bending cycle measurement of the Agdecorated CNP device. Figure 5 (b) shows the ON and OFF state
currents as a function of bending cycles. The curvature radius r andthe readout voltage were 2.5 mm and 0.1 V, respectively. The data
were collected from the I-V curves performed after the bending
cycles. No significant changes for both ON and OFF state currents
were observed up to 1000 cycles, indicating the excellent stability of
the Ag-decorated CNP devices for the bending cycles. Now we willcompare the flexible memory performance of Ag-decorated CNP
device with the recent studies on flexible nonvolatile memories asfunctions of the ON/OFF resistance ratio and the curvature radius in
Figure 6. The data contains several types of nonvolatile memory
devices including flash memory13–16, FeRAM18,19 and resistive memory20–38 and several types of material including graphene, otherorganic materials and oxides. Our Ag-decorated CNP device shows
superior flexible memory performance to any existing flexible nonvolatile memories in terms of the flexibility and the ON/OFF resistance ratio.
We discuss what produces the flexibility of the Ag-decorated CNP
devices. It is noted that individual cellulose nanofiber is not flexible,
e.g. the Young’s modulus is about 145 GPa65. In addition, the Agdecorated CNP utilized in this study was close packed and the space
between nanofibers was not observable at nanometer scale in the
TEM image of figure 1 (c). Therefore, the degree of spatial freedom
for the deformation of cellulose nanofibers must be much lower thanthat for the conventional paper46,66. In respect to the mechanics ofmaterials, the following two factors seem to be responsible for theflexibility of the Ag-decorated CNP devices. They are 1) the small
width of cellulose nanofiber component and 2) the network structurein CNP. Considering the width of individual cellulose nanofiber4 nm, the maximum strain for each cellulose nanofiber can be estimated to be 0.0006% for the curvature radius of 350 mm, which is too

Figure 4 | (a) Schematic illustration and (b) photograph of the Ag-decorated CNP devices on aluminium foil. (c) Photograph of the Ag-decorated CNP
device wrapped around the glass rod (w 5 5 mm). Inset shows the magnified image near the device stack. (d) I-V characteristics of the Ag-decorated CNP
device on aluminium foil (curvature radius r 5 ‘). Inset shows data retention taken at 0.1 V at room temperature in atmospheric condition.
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Figure 5 | (a) ON and OFF state currents as a function of curvature radius. The data is collected from I-V curves performed at each curvature radius. (b)ON and OFF state currents as a function of bending cycles. Curvature radius of 2.5 mm is utilized. The data is collected from I-V curves performedat each bending cycles.

small value to shear the cellulose nanofiber. Although the strain of
560 nm thick sheet on 12 mm thick aluminium foil reaches to 1.79%
at the curvature radius of 350 mm, the substantial strain for the CNP
with the fiber network structure is supposed to be smaller. Based on
the Cox model, the load applied to the fiber network structure is
mitigated by the random orientation of fibers, resulting in the reduction of Young’s modulus of the network structure67. Such effect isexperimentally confirmed by Sehaqui et al.; the Young’s modulus of
CNP decreased and the critical strain for breaking increased as thedegree of orientation of cellulose nanofibers decreases, indicating thehigh flexibility of CNP with randomly oriented cellulose nanofibers
network68. Remarkably, the critical strain for breaking of randomly
oriented CNP reached to 5.26%68, which is much higher value than
the strain applied in this study. Thus these two factors enhance theflexibility of Ag-decorated CNP devices although enough space is notsecured between nanofibers.

Discussion
In summary, we demonstrated the Ag-decorated CNP as ultra flexible electronic information storage. The Ag-decorated CNP memory
device showed the nonvolatile resistive switching with 6 orders of
ON/OFF resistance ratio and small variation of operation voltage. In
addition, the Ag-decorated CNP devices showed the great mechanical flexibility without degradation of memory performance(bending radius of 350 mm), which was the smallest value so far
reported for existing any flexible nonvolatile memory devices.
Although cellulose nanofibers, which are main component of CNP
and a raw material of paper, have been utilized as an ink-written

Figure 6 | Flexible properties on nonvolatile memory devices as functionsof ON/OFF resistance ratio and curvature radius. Several types of flexible
nonvolatile memories including flexible flash memory13–16, flexible
FeRAM18,19 and flexible resistive memory20–38 are shown.
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information-transfer medium, these results highlight the potential
use of cellulose nanofibers as functional electric information storage.

Methods
Synthesis and characterization of Ag-decorated cellulose nanofibers. Cellulose
nanofibers with width 3–4 nm (COONa content: 1.2 mmol g21) were prepared frombleached softwood kraft pulp, using the 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)-mediated oxidation system50. Then, the surfaces of cellulose nanofibers
were oxidized by sodium periodate (NaIO4) to introduce aldehyde groups, as follows.
The aqueous dispersion of cellulose nanofibers (1 wt%, 50 mL) was mixed with theaqueous solution of NaIO4 (12 mM, 50 mL) and then stirred at room temperature for
48 hrs. The as-treated cellulose nanofibers were washed with a mixture of 70/30 (v/v)
ethanol/water (100 mL) five times, followed by removal of ethanol at room
temperature under reduced pressure and dilution with deionized water. For thedecoration of cellulose nanofibers with Ag nanoparticles, silver ammonia aqueous
solution was prepared by adding silver nitrate (0.59 M, 2 mL) aqueous solution toammonia aqueous solution (1 M, 2 mL), followed by dilution with deionized water(96 mL). Subsequently, the aqueous suspension of the NaIO4-treated cellulose
nanofibers (0.1 wt%, 16 mL) was mixed with polyvinylpyrrolidone aqueous solution(1 wt%, 28 mL) and the silver ammonia aqueous solution (56 mL), in that order. The
resulting mixture was stirred at room temperature for 3 hrs to form the Ag-decorated
cellulose nanofibers.
The width and the microstructure of fabricated Ag-decorated cellulose nanofibers
were evaluated by transmission electron microscopy (TEM; JEOL JEM 3000F) at anaccelerating voltage of 300 kV. A TEM specimen was prepared by diluting the Agdecorated cellulose nanofibers solution (cellulose nanofibers content: 0.016 wt%) byethanol with 15500 volume ratio and then dropping the suspension onto a microgrid.
Fabrication and characterization of Ag-decorated CNP devices. To fabricate theAg-decorated CNP, the Ag-decorated cellulose nanofibers solution was dropped on
Pt/Ti/SiO2/Si(100) substrate or aluminium foil and then dried up at 40uC for 24 hrs.
The thickness of Ag-decorated CNP can be controlled via the amount of Agdecorated cellulose nanofibers solution and the spreading area. Prior to the Agdecorated CNP deposition, Ti adhesion layer and Pt bottom electrode were sputteredon SiO2/Si(100) substrate with the thickness of 5 nm and 100 nm, respectively. After
the Ag-decorated CNP deposition, Ag or Pt top electrode was fabricated with varying
the size ranged from 50 3 50 mm2 to 500 3 500 mm2 by means of the metal mask. The
microstructure, composition and crystal structure of the Ag-decorated CNP were
evaluated by TEM equipped with energy dispersive electron spectroscopy (EDS).
Cross-sectional TEM specimen was prepared by milling the Ag-decorated CNP
device. The structural change for bending experiments was evaluated by optical
microscopy equipped with digital CCD camera (OLYMPUS; BXFM-DP21).
Transport properties of the devices were characterized using a semiconductor
parameter analyser (Keithley 4200SCS). I-V curve, retention and bending
experiments were performed at room temperature and in atmospheric condition.
Hydrophilic/hydrophobic pattern preparation for selective Ag-decorated CNP
deposition. To create the hydrophilic/hydrophobic pattern, several techniques
including photolithography and 30 kV electron beam (EB) lithography were utilized.
Prior to the lithography, 5 nm thick Ti adhesion layer and 100 nm thick Pt bottom
electrode were deposited onto glass substrate. For the photolithography, AZ5206E
photo resist (AZ electronic materials) was spin coated at 5500 rpm for 60 s and UVlight was irradiated using Xe lamp. For the EB lithography, ZEP520A-7 (ZEON
Chemicals) was used as EB resist. After the lithography process, SiO2 was depositedon the exposed surface area, which is used as a reactive layer for hydrophobic selfassembly monolayer (SAM). Then the substrate was immersed into trichloro-(1H,
1H, 2H, 2H perfluorooctyl)-silane (so-called FDTS) 1 vol% solution in
perfluorooctane for 2 hrs, followed by rinsing with perfluorooctane. Subsequently,
the resist film was lifted-off from the substrate by dipping in N,N-dimethylformamide(DMF). After the lift-off process, FDTS is absorbed only onto the SiO2 surface. Since5www.nature.com/scientificreports
the surface of Pt was hydrophilic and FDTS was hydrophobic, finally we obtained thehydrophilic/hydrophobic surface pattern. For arbitrary shaped hydrophobic patternby hand-drawing process, we utilized a plastic pencil (coupy pencil; SAKURA Color
Product Corp.). Since the plastic pencil is made of hydrophobic resin, the line drawn
by the plastic pencil works as a guide for the water-based solution flow.
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Acknowledgments
This study was supported by NEXT and Grant-in-Aid for Scientific Research (S) (No.
26220908). K.N. was supported by Grant-in-Aid for Challenging Exploratory Research(No.24651138). U.C. acknowledges the Agency for Innovation by Science and Technology(IWT).

manuscript during the preparation. All authors discussed the results and implications andcommented on the manuscript at all stages.

Additional information
Supplementary information accompanies this paper at http://www.nature.com/
scientificreports
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Nagashima, K. et al. Cellulose Nanofiber Paper as an Ultra Flexible
Nonvolatile Memory. Sci. Rep. 4, 5532; DOI:10.1038/srep05532 (2014).
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Author contributions
K.N., H.K., M.N. and T.Y. designed this work and K.N. prepared the manuscript. The
experiments were carried out by K.N., H.K., T.K., C.U., F.Z., S.R., G.M., Y.H. and K.N., H.K.,
M.N., T.Y., M.K., K.S., J.D.B., M.J., W.V. have analyzed the results and discussed the

SCIENTIFIC REPORTS | 4 : 5532 | DOI: 10.1038/srep05532

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