Supplementary Information

Cellulose Nanofiber Paper as an Ultra Flexible
Nonvolatile Memory

Kazuki Nagashima,1 Hirotaka Koga,1 Umberto Celano,2 Fuwei Zhuge,1 MasakiKanai,1 Sakon Rahong,1 Gang Meng,1 Yong He,1 Jo De Boeck,2 Malgorzata
Jurczak,2 Wilfried Vandervorst,2 Takuya Kitaoka,3 Masaya Nogi 1 and Takeshi
Yanagida 1

1The Institute of Scientific and Industrial Research, Osaka University, 8-1

Mihogaoka Ibaraki, Osaka, 567-0047, Japan
2K.U. Leuven at IMEC, Kapeldreef 75 B-3001 Leuven, Belgium

3Department of Agro-environmental Sciences, Graduate School of Bioresource

and Bioenvironmental Sciences, Kyushu University, Fukuoka, 812-8581, JapanS1

S1. Characterization of Ag-decorated cellulose nanofibers.

ab~4nm

200nm

20nm

dC Si

Intensity (a.u.)

cOPtAgCu

20nm
0Cu
2

468

Pt
10Figure S1 shows (a) low magnification and (b) high magnification transmission
electron microscopy (TEM) images of the Ag-decorated cellulose nanofibers
diluted by ethanol with 1:500 (Ag-decorated cellulose nanofibers solution :ethanol) ratio. The fiber-like structure with the minimum diameter of about 4 nmwas seen. Ag nanoparticles with the diameters of 1-30 nm were decorated on
the surface of cellulose nanofibers. (c) TEM image of cross-sectional Agdecorated cellulose nanofibers paper (CNP). TEM specimen was prepared bymaking a slice of Ag-decorated CNP device. The close packed structure of CNP
without any observable pore was seen. (d) Energy dispersive electron
spectroscopy (EDS) of Ag-decorated CNP taken near Pt electrode. Cu peaks
are signals from TEM grid.

S2

S2. Analysis of resistive switching in Ag-decorated CNP memory device.

Resistance (Ohm)bOFF state

108

R∝
1/A

106
104

ON state

102
100

250
Forming voltage (V)

1010

a150
100

103

104

105

106

Ag/Ag-CNP/Pt

20200

10
Forming notcompleted

0102

104

106

Pt/Ag-CNP/Pt

50

Readout 0.1V
10230

V limit

Ag/Ag-CNP/Pt

0102

107

103

Device size (µm2)

104

105

107

106

Device size (µm2)

S3. Top electrode material dependence on the nonvolatile resistive
switching properties of Ag-decorated CNP deviceab

Initial Resistance

100

Pt/Ag-CNP/Pt

109

Ag/Ag-CNP/Pt

Probability (%)

Resistance (Ohm)

1010

108
107
R

106
105

102

103

104

105

∝

1/106

Switching Probability

A

60
40
200

107

Pt/Ag-CNP/Pt
Ag/Ag-CNP/Pt

80

50

100

Device size (µm2)

200

300

500

Device size (µm)

Pt/Ag-CNP/Pt
500µm□

Compliance 10-3A

d 10

10-2

10-4

Retention

ON state

10-5
10-6

OFF state

10-7
10-8
-1.5

10-4
10-5
10-6~103
OFF statein air, RT
Readout 0.1V

10-7-1-0.5

00.5

Voltage (V)

1

1.5

10-8
100

101

102
103
Time (sec)

S3

104

ON state

10-3

10-3
ON state

Current (A)Current (A)10-3e 10-2-1

Current (A)

c

10-2

Endurance

10-4
10-5
10-6
OFF state
Readout 0.1V

10-7
10-8

020

406080
Cycles (times)100Current (A)

300µm

10-2
10-3 ON state
10-4
10-5
10-6 OFF state
10-7
10-8
10-9
10-10
Readout 0.1V
10-11
100
101
102
103
104
Time (sec)Current (A)

500µm

10-2
10-3
10-4 ON state
10-5
10-6
10-7 OFF state
10-8
10-9
10-10
Readout 0.1V
10-11
100
101
102
103
104
Time (sec)

100µm

10-2
10-3 ON state
10-4
10-5
10-6
10-7
10-8 OFF state
10-910-10
Readout 0.1V10-1101
1010102
103
104
Time (sec)

Ag/Ag-CNP/Pt
500µm

10-2
10-3
10-4 ON state
10-5
10-6
10-7
10-8 OFF state
10-9
10-10
Readout 0.1V
10-11100
101102
103
104
Time (sec)

300µm

10-2
10-3
10-4 ON state
10-5
10-6
10-7
10-8 OFF state
10-9
10-10
Readout 0.1V
10-11100
101
102
103
104
Time (sec)

100µm

Current (A)gCurrent (A)Pt/Ag-CNP/Pt

Current (A)Current (A)f

10-2
10-3 ON state
10-4
10-5
10-6
10-7
10-8
10-9 OFF state Readout 0.1V
10-10
10-11
100
101102
103
104
Time (sec)S4. Schematic illustration of resistive switching mechanism and band
diagram of Ag-decorated CNP device

S4bEVAC
1.6eVdEC
5.65eV

4.26eVcellulose

AgPt

Initial state, V=0Pt

SET process, V>0Pt

RESET process, V<05.6eV
AgPt
EVcelluloseAgfilament

Ag(αhν)2 (a.u.)

Absorption (%)

c100
80
60
40cellulose

220

3 4 5 6
Energy (eV)Ag02345
Energy (eV)

Agfilament

6cellulose

Here we discuss the mechanism for resistive switching in the Ag-decorated
CNP devices. Figure S2 (a) shows the device size dependence on the ON andOFF state resistances. The device size was varied from 50×50 μm2 to 500×500
μm2. The readout voltage is 0.1 V. The OFF state resistance inversely
correlates with the device size. On the other hand, the ON state resistance wasindependent of the device size, clearly indicating the presence of local
conduction

paths.

In

theframeworkofconventional

electrochemical

metallization mechanism, the resistive switching can be interpreted in terms of
the formation and rupture of Ag conductive filaments. [1-3] First, Ag in thepositively biased electrode is ionized to be Ag+ ions and migrate toward
negatively biased Pt counter electrode under the electric field. When the Ag+ions reach to the Pt electrode, the Ag+ ions are reduced by receiving theelectrons from the Pt electrode as following reaction; Ag+ + e- →Ag. SuchS5

successive metallization process creates the Ag conductive filaments from thePt electrode to the Ag electrode. CNP works as the solid electrolyte for Ag+ ionsmigration. The resistance state changes to ON state when the Ag filaments arecompletely bridged between the Ag electrode and the Pt electrode. The ruptureof Ag filaments occurs through the oxidization of Ag (Ag →Ag+ + e-) and themigration of Ag+ ions under the opposite electric field polarity. In such switchingmodel, the ionization and the migration of Ag might be critical for the resistive
switching phenomena. Figure S2 (b) shows the effect of top electrode on theforming voltage. In this experiment, we utilized Ag and Pt as the top electrodes
and the various device sizes ranging from 50×50 μm2 to 500×500 μm2 areexamined. Note that the initial resistance values for Ag and Pt top electrodes
were comparable (Figure S3 (a)), indicating that the migration of metal during
the electrode deposition process is negligible. For both Ag and Pt top electrodes,
the forming voltage increased as the device size decreased, which is consistent
with the typical electric breakdown phenomenon. [4,5] Remarkably, the forming
voltages of the Ag top electrode devices are lower than that of the Pt top
electrode devices for all device size. Also the resistive switching with Ag top
electrode was more stable than that with Pt top electrode. Figures S3 (b-e) arecomparison on (b) switching probability, (c) I-V curve, (d) retention and (e)endurance of Pt/Ag-decorated CNP/Pt device. Size dependent data retention isshown for (f) Pt top electrode and (g) Ag top electrode. These results indicate
the important role of Ag top electrode for the nonvolatile memory effect in CNP
device. Thus, the experimental results highlight that the resistive switching in
the Ag-decorated CNP device is based on the formation and rupture of AgS6

conductive filaments. The physical picture of discussed resistive switchingmechanism is given in figure S4 (a). In addition, the band diagram of present
CNP device is shown in figure S4 (b-d). To estimate the energy level ofcellulose, we referred the work function of 4.4 eV obtained by ultraviolet
photoelectron spectroscopy (UPS). [6] The band gap was estimated to be 5.6
eV by hypothesizing the direct transition of cellulose from UV absorption data as
shown in figure S4 (c). UV absorption data was obtained from the TEMPOoxidized cellulose nanofiber paper with the thickness of ~10 μm. α is absorption
coefficient and hν is photon energy, respectively.

S7

S5. Electric field simulation for metal nanoparticles embedded CNP.

2.3

a
100nm

700nm

1.0B

ACD
0.0

Electric field (105 V/cm)+7Vb

Electric field intensity (105 V/cm)

GND
2.5
2.0

DC

1.5

B

1.0

A

0.5
00

100

200

300

400

500

600

700

Distance from bottom electrode (nm)Figure S5 (a) shows the electric field distribution for metal nanoparticles
embedded CNP. In this simulation, 2-dimensional electric field simulator, μExcel (μ-TEC Co. Ltd.) was utilized. The CNP thickness is 700 nm and the sizeof metal nanoparticles is 100 nm. For the calculation, the dielectric constant of
7.5 was utilized for CNP. +7 V was applied to the top electrode while the bottom
electrode was grounded. We simulate the electric field intensity with varying thenumber of metal nanoparticles in CNP (A) 0, (B) 1, (C) 2 and (D) 3, respectively.
Note that for the case of 3 metal nanoparticles, working distance reduces to57 % of total CNP thickness, which is less than the substantial working distanceof Ag-decorated CNP used in this study. It is clearly seen that the electric fieldnear the metal nanoparticles is enhanced. In addition, the reduction of working

S8

distance enhances the apparent electric field. Figure S5 (b) shows the analyzed
data of the electric field intensity along the thickness direction. As discussed
above, the electric field is enhanced via 1) the effect of metal nanoparticle edge
and 2) the decrease of working distance. However, the maximum electric field
intensity of metal nanoparticles embedded CNP is just 2.3 times higher thanthat of CNP without metal nanoparticles. Therefore, only the enhancement ofelectric field intensity by the metal nanoparticles cannot explain the extremely
low electric field intensity for forming process of Ag-decorated CNP device(1.6×10-2 MV/cm).

S9S6. Selective deposition of Ag-decorated CNP on chemically modifiedsurface.

Ag-CNP (on hydrophilic area)d10

Hydrophobic area

Siin air
100µm□

cc.10-3A

10-3
Current (A)

Pt-1

10-5
10-7

SETAgRESETc

10-9
10-11-1

e 100Voltage (V)-2

ON state

10-4
Current (A)-0.5

10-6
10-810-10OFF state

10-12100

101

RT in airReadout 0.1V

102
Time (s)103

104

S100.51

Printing the functional materials on desired surface at low temperature isinteresting on the viewpoint of large area, low cost, low energy consumption
processing. Here we demonstrate the printability of Ag-decorated CNP devices
ondesiredsurface.

Tocreatethehydrophilic/hydrophobicpattern,

photolithography and 30 kV electron beam (EB) lithography were utilized. Priorto the lithography, 5 nm thick Ti adhesion layer and 100 nm thick Pt bottom
electrodes were deposited onto Si. For the photolithography, AZ5206E photoresist (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 (ZEONChemicals) was used as EB resist. After the lithography process, SiO2 wasdeposited on the exposed surface area, which was used as a reactive layer forhydrophobic self-assembly monolayer (SAM). Then the substrate wasimmersed into trichloro-(1H, 1H, 2H, 2H perfluorooctyl)-silane (so-called FDTS)
1 vol% solution in perfluorooctane for 2 hrs, followed by rinsing withperfluorooctane. Subsequently, the resist was lifted-off from the substrate by

S11

sonicatingin

N,N-dimethylformamide(DMF).

Finally

weobtained

thehydrophilic/hydrophobic pattern where the surface of Pt is hydrophilic and FDTS
is hydrophobic. Since the Ag-decorated cellulose nanofibers were suspended in
the water, the solution didn’t spread to the hydrophobic surface when thesolution was dropped onto the hydrophilic surface but not onto the hydrophobic
surface, resulting in the selective deposition of the Ag-decorated CNP. FigureS6 (a-b) shows the deposition process of Ag-decorated CNP on lithographicallypatterned substrate surface. Figure S6 (a) shows photographs before (upper)
and after (lower) the deposition of the Ag-decorated CNP on patterned
substrate by photolithography. The deposition of Ag-decorated CNP wasperformed at room temperature. The colored area in the photograph indicatesthe presence of Ag-decorated CNP. Clearly, Ag-decorated CNP was selectively
deposited on the hydrophilic Pt surface. Furthermore, we examine themicrometer-scale patterning of Ag-decorated CNP as shown in the dark field
optical microscopy images of figure S6 (b). The surface pattern was designed
by EB lithography. The presence of Ag-decorated CNP could be confirmed by
the Ag nanoparticles as bright spots in the figures. Clearly, selective depositionof Ag-decorated CNPs could be seen outside (upper) and inside (lower) of theletters ‘OSAKA UNIVERSITY & imec’. The width of letter is ~5 μm. Surprisingly,
the Ag-decorated CNP was selectively deposited onto hydrophobic surface
when the solution was dropped over the whole area. It might be due to thechemical interaction between substrate surface and TEMPO-oxidized cellulose
nanofibers. Although the mechanism of this abnormal phenomenon isambiguous, this methodology offers the micro pattering of the Ag-decorated

S12

CNP with large area, low cost, low energy consumption processing. Thus we
successfully demonstrate the printability of Ag-decorated CNP devices at
micrometer-scale resolution.
Then the resistive switching effect was confirmed in the selectively deposited
Ag-decorated CNP. Figure S6 (c) shows the schematic illustration of Agdecorated CNP device selectively deposited on hydrophilic surface. Ag/Agdecorated CNP/Pt structure was fabricated by deposition of Ag top electrode.
Figure S6 (d) and (e) show the I-V characteristics and data retention of thedevice. The measurement was performed at room temperature in air condition.
The thickness of Ag-decorated CNP was 750 nm. The compliance current of 103

A was applied. The clear resistive switching and the good retention properties

were confirmed.
Next we examined the feasibility of hand-drawing process for the selective
deposition of Ag-decorated CNP on the arbitrarily designed surface. Figure S6(f) shows the fabrication procedure of the arbitrarily designed Ag-decoratedCNP. We utilize an aluminium foil as a substrate. The surface pattern wasdrawn by using a plastic pencil (Coupy pencil; SAKURA Color Product Corp.).
Since the plastic pencil is made of hydrophobic resin, the line drawn by theplastic pencil works as the guide for flowing the Ag-decorated CNP solution. As
shown in figure S6 (f), we successfully deposited the Ag-decorated CNP on thearbitrarily designed characters ‘CNF’. Thus these results offer the feasibility ofAg-decorated CNP memory by low cost, low energy consumption printingprocessing.

S13

S7. Micro structure of Ag-decorated CNP on aluminium foil.

S14

Figure S7 shows the micro structure of Ag-decorated CNP on aluminium foil
observed by optical microscopy. (a) The aluminium foil before Ag-decorated
CNP deposition. The maximum height difference between top and valley was
about 1 μm. (b) The micro structure of Ag-decorated CNP near the edge, which
was taken after wrapped around a toothpick (ϕ=2.2 mm) as shown in the inset.
The thickness of Ag-decorated CNP was 560 nm. (c) The micro structure of Agdecorated CNP near the edge, which was taken after wrapped around a metal
wire (ϕ=0.7 mm) as shown in the inset. None of any cracks and exfoliation wasseen for all over the sheet, indicating the mechanical flexibility of Ag-decoratedCNP.
To further investigate the flexible performance of the Ag-decorated CNP, weexamined the folding tests as shown in figure S7 (d). Firstly, we prepare the 1μm thick Ag-decorated CNP on aluminium foil. Then we fold up the aluminium
foil as shown in inset of figure S7 (d). After expanding, we perform the optical
microscopy observations. Lower figure S7 (d) shows the optical microscopy
images taken at the mountain-fold area and the valley-fold area. The surface
cannot be back to totally flat after folding process due to the shape memory
effect of aluminium foil. Therefore the image near the valley looks dark since theoptical light on the surface reflect to the inclined angle. There were noobservable cracks and exfoliations at the folded area. Here we estimate thecurvature radius for the folding test. In the optical microscopy images, thewidths of folded area are 78.5 μm for mountain-fold and 52.6 μm for valley-fold,respectively. Note that the larger width for mountain-fold area is reasonable bytaking into account the thickness of Ag-decorated CNP and aluminium foil.

S15

From the widths, the curvature radii can be roughly calculated to be ~25.0 μm(mountain-fold) and ~16.8 μm (valley-fold). As such, the structure of Agdecorated CNP is mechanically sustainable for both tensile and compressive
strain at least down to several tens micrometers of curvature radius.
Taking the advantage of foldability, here we demonstrate an arbitrarily formed
Ag-decorated CNP. Figure S7 (e) shows the photograph of ‘origami crane’
fabricated by folding the Ag-decorated CNP on aluminium foil. Although thesheet was folded many times during the origami process, the cracks and theexfoliation were not observed. Thus we successfully demonstrated the excellent
mechanical flexibility of the Ag-decorated CNP for the flexible resistive switchingmemory devices.

S16

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