University of Groningen
Revisiting the delta-phase of poly(vinylidene fluoride) for solution-processed ferroelectric thin
films
Li, Mengyuan; Wondergem, Harry J.; Spijkman, Mark-Jan; Asadi, Kamal; Katsouras, Ilias;
Blom, Paul W. M.; de Leeuw, Dago M.
Published in:
Nature Materials
DOI:
10.1038/NMAT3577
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Publication date:
2013
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Citation for published version (APA):
Li, M., Wondergem, H. J., Spijkman, M-J., Asadi, K., Katsouras, I., Blom, P. W. M., & de Leeuw, D. M.
(2013). Revisiting the delta-phase of poly(vinylidene fluoride) for solution-processed ferroelectric thin films.
Nature Materials
,
12
(5), 433-438. https://doi.org/10.1038/NMAT3577
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Revisiting the δ-phase of poly(vinylidene fluoride)
for solution-processed ferroelectric thin films
1
The forgotten δ-phase of poly(vinylidene-fluoride) for
solution-processed ferroelectric thin films
Supplemental Information
By Mengyuan Li
1
*, Harry J. Wondergem
2
, Mark-Jan Spijkman
1
, Kamal Asadi
2
, Ilias
Katsouras
1
, Paul W. M. Blom
1,3
and Dago M. de Leeuw
1,3
1
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,
9747 AG, Groningen, The Netherlands
2
Philips Research Laboratories, High Tech Campus 4, 5656 AE, Eindhoven, The
Netherlands
3
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany
[*] E-mail: mengyuan.li@rug.nl
SUPPLEMENTARY INFORMATION
DOI: 10.1038/NMAT3577
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2
Table of Contents
1. Supplemental Introduction
2. Supplemental Methods
3. Microstructure
4. Thin film capacitors
5. Alternative chain packing model for α
-PVDF
6. Molecular mobility of δ
-PVDF
7. Field-effect transistors
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1. Supplemental Introduction
The supplemental information contains the experimental details about film
preparation, microstructure characterization, dielectric measurements and electrical
characterization such as polarization data retention and programming cycle endurance.
AFM images of as deposited films are given. Capacitors made with smooth, thinner
d-PVDF films are presented. The molecular mobility of δ-PVDF is investigated by
dielectric spectroscopy at temperatures between -80 °C and 200 °C. The dielectric
constants are given as a function of frequency and temperature. A preliminary
interpretation is discussed. Finally, the first field-effect transistors using a ferroelectric
PVDF gate dielectric are presented. Both a ferroelectric single channel transistor and
a ferroelectric dual-gate transistor are presented.
2. Supplemental Methods
PVDF with molecular weights, M
w
, of 180, 275, and 534 kg/Mol was purchased from
Sigma-Aldrich. PVDF has a limited solubility in common organic solvents such as
alcohols, chlorinated solvents and acids. Good solvents are e.g.
N,N-dimethylformamide (DMF), dimethylsulfoxide and N-methyl-2-pyrrolidone.
Here we used solutions of PVDF in DMF, typically 10% by weight for wire-bar
coating and 5% for spin-coating. Thin PVDF films were made in a standard class
10,000 clean room with a temperature of 20
o
C and a relative humidity of 45% by
both wire-bar (Meyer rod) coating and spin-coating. The substrate temperature for the
wire-bar coater was controlled using a K202 control coater (RK Print) between room
temperature and 140
o
C. The substrate used for spin-coating was heated in situ with a
heat gun and the temperature was measured with a contactless IR thermometer
(RS-components 1327K). After coating, the films were either a) dried, annealed at
150 °C and slowly cooled down to room temperature or b) melted at 200 °C,
quenched in ice water and annealed at 150 °C.
The film thickness was measured with a Dektak profilometer. The surface
morphology of the films was characterized by atomic force microscopy (AFM)
(Nanoscope Dimension 3100 Bruker). To ascertain the crystal phase of the films, both
grazing incidence X-ray diffraction (GI-XRD) and Fourier-transform infrared
spectroscopy (FTIR) were used. GI-XRD scans were obtained with a Philips X’pert
MRD diffractometer, using the line focus of a Cu-anode X-ray tube, a Göbel mirror in
the primary beam and a parallel plate collimator in front of the detector. The
incidence angle was fixed during the measurement at an angle of 0.23° just above the
critical angle of total diffraction. Infrared spectra were recorded using a Bruker
Vertex spectrometer attached to a Hyperion FT-IR microscope. The scans were
performed with a resolution of 4 cm
−1
.
Capacitors were fabricated on thermally oxidized silicon monitor wafers on which 50
nm thick Au bottom electrodes on a 2 nm Ti adhesion layer were photo-
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lithographically defined. After film deposition, and/or annealing, the capacitors were
finished by evaporating an Au top electrode through a shadow mask. The device area
varied from 0.059 to 1.38 mm
2
.
To allow phase identification with GI-XRD, large device areas of 1 cm
2
were
fabricated without the strongly diffracting Au top electrodes. A thin film of the
amorphous conducting polymer PEDOT:PSS, a water-based suspension of
poly(3,4-ethylenedioxythiophene) stabilized with poly(4-styrenesulphonic acid), was
used instead. As bottom electrodes we used Pd. A PVDF film was applied and the
PEDOT:PSS, AGFA ICP 1020 (Agfa-Gevaert) was spin-coated from a formulation
containing a few drops of the nonionic Zonyl FSO-100 (DuPont) fluoro-surfactant.
The PEDOT:PSS thickness amounted to 100 nm and the conductivity amounted to
300 S/cm. Finally, the redundant PEDOT:PSS was removed by reactive ion etching
using a shadow mask.
Electric displacement loops versus electric field for the capacitors were measured
using a Sawyer-Tower circuit, consisting of a Tektronix AFG3102 function generator,
a Tektronix TDS3032B oscilloscope and a Krohn-Hite 7600 wide-band amplifier. The
capacitors were measured with a continuous triangular wave signal, to reduce the time
at maximum bias, at a frequency of 100 Hz and using a reference capacitor of 216 nF.
Data retention was measured in a Janis probe station in a dynamic vacuum of 10
-4
mbar, using a Precision Workstation (Radiant Technologies). A write pulse was
applied, followed by two read pulses of the same amplitude but opposite polarity, at
fixed intervals ranging between 1 s and 300.000 s. All pulse widths were fixed at 10
ms. The programming cycle endurance was measured using the same setup. A
triangular pulse of 160 V
pp
with a frequency of 200 Hz was applied for spans of time
ranging between 1 s and 3000 s. The resulting number of cumulative cycles exceeded
10
6
. The remanent polarization after each span was measured with a
Positive-UP-Negative-Down (PUND) sequence of 10 ms wide pulses. The molecular
mobility of the films was investigated by dielectric spectroscopy at temperatures
between –80 °C and 200 °C, using a Schlumberger SI1260 Impedance Gain Phase
Analyzer.
3. Microstructure
Thin films were made by wire-bar coating and spin-coating. They behaved
qualitatively the same. The crucial parameter is the substrate temperature, as it
determines the film roughness. Films deposited at low substrate temperatures exhibit a
high roughness. Especially spin-coated films are extremely rough; the rms roughness
is comparable to the layer thickness.
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