Supplemental Information
High-performance nonvolatile write-once-read-many-times memory devices with
ZnO nanoparticles embedded in polymethylmethacrylate
Dao Thanh Toan, Tran Viet Thu, Koichi Higashimine, Hiromasa Okada, Derrick Mott,
Shinya Maenosono and Hideyuki Murata
1. ZnO NP synthesis
Monodispersed ZnO NPs of mean size 9.2  1.4 nm were chemically synthesized using
our own method (J. Appl. Phys. 107, 014308 (2010)) with some modifications (Fig. S1).
Briefly, 20 mmol of oleylamine (OLA) and 20 mmol of 1-octadecene (ODE) were
loaded into a three-necked flask. Separately, 1 mmol of zinc acetylacetonate was
dissolved in 3 mL of toluene to form a stock solution. The OLA/ODE mixture was
heated at 100°C for 15 min under an argon atmosphere, and then, the temperature was
raised to 280°C. Subsequently, the stock solution was rapidly injected into the flask.
After 1 hour of reaction at 280°C, the temperature was cooled down to room
temperature. After that, ZnO NPs were separated from the matrix by poor solvent
precipitation and centrifugation followed by washing with ethanol. The ZnO NPs were
redispersed in a chloroform/n-butylamine mixture. The resulting dispersions were
totally transparent and stable for several days.
Fig. S1. Size distribution and TEM image of as-synthesized ZnO NPs.
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2. Emission image microscopy
The emission image scope is equipped with a cooled high-sensitive charge-coupled
device sensor (back-illumination type) as a detector, which is able to detect the emission
with high quantum efficiency at the wavelength ranging from 300 nm to 1000 nm. The
emission image from a WORM device was observed through a glass substrate during
the voltage sweep from 0 V to 4 V.
Figure S2 shows the emission images of memory devices observed during changing the
applied voltage. For the first scan, the device was in the OFF state and no emission was
observed at 3 V (Fig. S2(a)). By increasing the voltage to 4 V, the current abruptly
increased at 3.2 V and the device entered the ON state. Many bright spots appreared at 4
V as shown in Fig. S2(b). The visible spots remained even after the voltage was
decreased to 3 V (Fig. S2(c)), indicating that conductive filament formation in the
resistive memory device occured. Indeed, the current at 3 V in the ON state was higher
than that for the first scan.
Fig. S2. Emission images of memory devices were obtained under (a) a voltage of 3
V (OFF state) (b) a voltage of 4 V (ON state) (c) a voltage of 3 V again after
applying a voltage of 4 V (ON state).The color does not represent the emission
color and is just used for enhancing the visibility.
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3. The measurements of cross-sectional TEM, STEM-HAADF images and EDS
The specimen for cross-sectional TEM observation was prepared using a SMI 3050
focused ion beam (FIB) bombardment (SII Nanotechnology) instrument. Four steps in
the FIB analysis were performed. First, carbon and Pt-Pd layers were deposited onto the
surface of the specimen. Second, the target location (bright spot) was marked to indicate
the precise cross section position. Third, FIB milling with a Ga ion source (30 keV) was
performed to prepare a 2-μm-thick specimen for cross-sectional TEM measurements.
Finally, the specimen was transferred to a TEM grid using a microprobe, and then, the
thickness of the specimen was reduced to 0.1 μm by crude processing and low-energy
Ga ion milling. The cross-sectional TEM analysis was performed on an Hitachi
H-9000NAR transmission electron microscope operated at 300 kV.
Scanning TEM - high angle annular dark field (STEM-HAADF) imaging and energy
dispersive X-ray spectroscopy (EDS) elemental mapping mesurements at emission point
were carried out using a JEOL JEM-ARM200F instrument operated at 200 kV with a
spherical aberration corrector (nominal resolution 0.8 Å).
Fig. S3. STEM-HAADF images, EDS elemental mapping images and EDS spectra
of cross section of emission point.
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4. Reproducibility of threshold voltage (Vth)
To find the reproducibility of the Vth in WORM memory devices, we characterized a
total number of 136 devices, where 132 devices switched to the ON state at positive
bias (97%) and 54 % of the devices turned on at the narrow voltage range of +3.2 V to
+3.4 V. In contrast, only 4 devices switched to the ON state at negative bias.
Fig. S4. Histogram of threshold voltage of WORM memory devices.
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