Supplementary Information

All Nanocellulose Nonvolatile Resistive Memory

Umberto Celano, Kazuki Nagashima, Hirotaka Koga, Masaya Nogi, Fuwei
Zhuge, Gang Meng, Yong He, Jo De Boeck, Malgorzata Jurczak, Wilfried
Vandervorst and Takeshi Yanagida1Supplementary Information S1ab

500
Thickness (nm)0.02wt%cellulose nanofibers
solution (10µl)
ITO

40030020010000Si substrate

51015202530

Drop cycles (times)(a) Schematic illustration of drop casting method for fabricating theresistive-switching layer. 10 μl of cellulose nanofiber solution (0.02 wt%) wasdropped onto ITO coated Si substrate, followed by drying at 40 °C for 15 min.
The drop casting was repeated until the desired thickness was obtained. (b)Obtained thickness of the resistive-switching layer as a function of number
cycles of drop casting. 100 nm thickness was achieved by 3 cycles.

2

Supplementary Information S2aas-is (0.02wt%)

10 times diluted

50 times diluted

100 times diluted

1µm

1µm

1µm

1µm

500 times diluted

1000 times diluted

5000 times diluted

10000 times diluted

1µm

1µmb1µm

1µm

Density of CNFs (µm-2)200densely packed

150

100

50

10.50010-1102 103 104 105
100101102103

104105

Dilution ratio (1:x)

Density control of cellulose nanofibers. (a) Field emission scanning electron
microscopy (FESEM) images of cellulose nanofibers onto Si substrate withvarious dilution ratio. Cr with 40 nm was deposited prior to the observation foravoiding the charging effect. Accelerating voltage of 3 kV was utilized forobservation. (b) Number density of cellulose nanofibers as a function of dilutionratio. A single cellulose nanofiber was obtained over 1:1000 dilution ratio.

3Supplementary Information S3ab

20nm

0nm

c1nA0A(a) Topographic atomic force microscopy (AFM) image of single cellulose
nanofiber. (b) Magnified AFM image of projected area in (a). (c) conductive-AFM(C-AFM) image of single cellulose nanofiber. All the measurement wasperformed onto Ag coated Si substrate.

4

Supplementary Information S4a

Before metal contact

Single cellulose nanofiber device

2µm
SupportpadCellulose nanofiber

SiO2/Si
1µm

Pt

500nm

1Current (x10-13A)

bUnder lightDark

0.50-0.5-1-2-1012Voltage (V)(a) Left FESEM image of the nanogap test device consist of a single cellulose
nanofiber. Pt was utilized for contacting with cellulose nanofiber to avoid thepossible oxidization at electrode-cellulose nanofiber interface. Upper right image
shows the cellulose nanofiber prior to the electron beam (EB) lithography. Lowerright image shows the magnified nanogap device. The gap size is 100 nm. (b)Current-voltage (I-V) curves of single cellulose nanofiber device taken in 1x10-3
Pa at room temperature. The conductions under dark condition (blue) and lightirradiation (red) were almost identical, indicating the negligible photo carriers incellulose nanofiber. The resistance was over 100 TΩ, indicating the highly
insulative property of cellulose nanofiber.
5Supplementary Information S5
10-2

Current (A)

10-4

LRS
slope ~ 1.01

10-6
10-8

Measured data
Fitting curve

HRS
slope ~ 1.75

10-10slope ~ 1.02

10-12
10-2

10-1

100101

Voltage (V)Here we discuss the switching mechanism of the nanopaper memory. Among
various models, the one based on the formation and the rupture of Ag
conducting filaments [1,2] via the electrochemical redox process canappropriately explain our the resistive switching results in Ag/nanopaper/ITOstructure. In this model, Ag are ionized near the positively biased electrode, andthen the Ag+ ions migrate into the nanopaper layer toward negatively charged
counter electrode under the electric field. When the Ag+ ions reach to thecounter electrode, the Ag+ ions are reduced to be Ag by receiving the electrons
from the electrode. Such successive metallization process creates the Ag
conductive filaments between the electrodes. The rupture of Ag filaments occurs
through the ionization and the migration of Ag+ ions under the opposite electricfield polarity. To examine the conduction mechanism, we analyzed the I-Vcurves of HRS and LRS taken from 200 x 200 μm2 sample. The data fitting
indicated that the electrical conduction in LRS follows Ohmic law. This is6

consistent with the Ag filament based conduction model. On the other hand, theconduction in high resistance state (HRS) follows a space charge limited current(SCLC) law, which is consisting of three parts, (i) the Ohmic region (I∝V) atlower electric field, (ii) the Child’s law region (I∝V2) at middle electric field rangeand (iii) the steep current increase region at higher electric field [3]. Ohmic
conduction is derived from the thermally generated carrier in cellulose
nanopaper memory. The Child’s law and the steep current increase regions areassociated trap-unfilled and trap-filled SCLC transport, respectively. Therefore,
the main conduction mechanism in HRS is associated with the electric fieldand/or thermally driven electron hopping through the trap states in nanopaper
layer [4].

1. Yang, Y., Gao, P., Gaba, S., Chang, T., Pan, X. & Lu, W. ‘Observation ofconducting filament growth in nanoscale resistive memories’ Nat. Commun. 3,732 (2012). DOI: 10.1038/ncomms1737
2.

Valov, I. et al. ‘Nanobatteries in redox-based resistive switches require
extension of memristor theory’ Nat. Commun. 4, 1771 (2013). DOI:
10.1038/ncomms2784

3. Lampert, M. A. & Mark, P. ‘Current Injection in Solids’ Academic Press: NewYork, NY, USA, (1970). ISBN: 978-0124353503
4. Lim, E. W. & Ismail, R. ‘Conduction Mechanism of Valence Change Resistive
SwitchingMemory:ASurvey’

DOI:10.3390/electronics4030586
7Electronics

4,586-613(2015).

Supplementary Information S6ca nanopaper memory b

e1cm

1cm1cm1cm

DI waterdAg
electrode

After sonication 28kHz 5min

Fresh sample

propanolacetoneethanol

DMF

waterfElectrode residual ratio (%)

Metal electrode
Nanopaper
120g

acetone

10080

elec
tr

propanol60

ethanol
DMF

40

nanowater

20

odepolar solvent

paper

045678910cellulose

Polar index of solvent(a-d) Removal of Ag electrodes from nanopaper memory device by applying
the sonication into DI water. (a) Prior to immerse the nanopaper memory device
into DI water, (b) just after immersion, (c) 1 min after immersing without
sonication, and (d) after 5 min sonication and dried up. (e) Examination ofelectrodesorting

invarioussolution(propanol,acetone,ethanol,N,N-dimethylformamide (DMF) and water). The sonication was applied for 5 min.
In water and DMF, the electrode was fully removed while it partially or fully
remained in the other solvents. (f) Residual ratio of electrode (%) as a function ofpolar index of solvent. The electrode tended to be removed as the polarity ofsolvent increases. This trend can be understood by taking into account the8surface hydroxyl groups of cellulose nanofibers. Since the cellulose nanofibers
are constructed by compiling the pyranose ring of cellulose through thehydrophobic interaction, many hydroxyl groups are exposed to the surface ofcellulose nanofibers. The polar surface of cellulose nanofiber is familiar with thepolar solvent, which allows the penetration of solvent underneath the electrodeand/or into the nanopaper. (g) Schematic illustration for the mechanism ofelectrode removal in polar solution.

9Supplementary Information S7

Natural soil 4L
sampling in Septemberat34:49:30 north latitude
135:31:28 east longitude
Temperature 22±2ºC
Humidity 92±3%

Plastic bag
For the biodegradation experiment, we utilized the natural soil. The 4L of soil
was collected in September at 34:49:30 north latitude and 135:31:28 east
longitude and kept in a plastic bag. The temperature and the humidity of soil
were controlled to be 22 ±2 °C and 92 ±3 %, respectively, during the experiment.

10Supplementary Information S8acDay 0Day 36
Room air
1cmb5ml

Humid-airImmersion
into water25ºC

10ml
90ºC

Not
examined
1cm

Since the biodegradation test in natural soil has several factors for thedegradation of nanopaper such as humidity, temperature and bacterial (enzyme)
effect, we conducted the biodegradation test without soil (bacterial effect). Figure
S8(a) shows the nanopaper memory prior to the biodegradation test. In thisstudy, we kept the samples (1) in room-air-condition (humidity 50-60%), (2) inhumid-air-condition (humidity 100%) and (3) in water. Figure S8(b) shows theexperimental set-up for the humid-air and water-immersion system. For thesample in humid-air-condition, 5 ml DI water was prepared in the 20 ml glass
bottle and the nanopaper memory was set out of the water. For the sample inwater, the nanopaper memory was immersed in 10 ml DI water.
Figure S8(c) shows the results of biodegradation test obtained after 35 days. Inwater, the resistive-switching layer was fully removed with the electrode inseveral minutes, while the nanopaper substrate was fully maintained after 35days. On the other hand, in humid-air-condition and in room-air-condition, both11

of the resistive-switching layer and the nanopaper substrate were partially(humid-air-condition) and fully (room-air-condition) remained after 35 days,respectively. We further examined temperature effect in both humid-air andwater condition. At 90ºC, there were no significant differences in biodegradation
effect compared with the same test performed at room temperature. These
clearly indicate that the biodegradation of our nanopaper memory was caused
by a bacterial effect in natural soil.

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