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2021B1846

BL19B2Grazing Wide Angle X-ray Scattering of Ultrathin PVDF Thin Films
Pamarti Viswanath a, L.S.R Kumara b, Tomoyuki Koganezawa b, V. Kesava Rao a,
K. Kanishka H. De Silva a, Masamichi Yoshimura a.
aToyota Technological Institute, bJASRI

Abstract
In this study, ultrathin films of PVDF-TrFE (70:30 molar ratio) with thickness of 3~20 nm were fabricatedusing Langmuir Blodgett (LB) technique on various substrates like Si, Au(111) and highly oriented pyrolliticgraphite (HOPG). PVDF-TrFE is know to exhibit large ferroelectric character due to the presence of long
chains of C-F and C-H dipoles in its all trans (TTTT) configuration. In this study the polarization orientation
arising from the electric dipoles of PVDF-TrFE were investgated using grazing wide angle X-ray scattering(GIWAXS). From the GIWAXS measurement, it was found that the fabricated PVDF ultrathin films have aepitaxal crystal arrangement in the (200) direction on Au(111) and HOPG substrates. However, on Si wafers,
PVDF-TrFE exhibited a random crytal orientation. The GIWAXS measurements provide insights into crystal
arrangement of ferroelectric polymers like PVDF which will further help in improving the ferroelectric
properties.
Keywords: GIWAXS, PVDF, ferroelectricity, Langmuir Blodgett
Background and Objective
Ferroelectric (FE) materials are those which exhibit electrically switchable spontaneous polarization due tothe existence of non-centrosymmetric inversion symmetry1. Ultrathin FE materials are essential for thegrowing need for future miniaturized devices like FE random access memory. The direction of spontaneouselectric polarization (up or down) in these FE thin films can be switched by the application of an external
electric field leading to the formation of two electrically bi-stable states representing binary information “0”and “1”. The switched polarization is retained even after the removal of the external electric field giving rise
to nonvolatility which forms the basis in memory devices. In this context, we aim to develop highperformance polymer-based ultrathin organic FE materials as an alternative to the existing inorganic
materials. Currently, organic FE materials are promising alternatives for their counterparts mainly for theirflexibility, ease of fabrication, and performance. In the context of organic FE, polyvinylidene fluoride orPVDF has successfully emerged as a novel alternative FE polymer material for applications in piezoelectricenergy harvesting, wearable electronics, biomedical implants, wireless sensors, etc2.
These alternative materials can also be used for future FE memories and neuromorphic computation. FEbased memristor is one such two-terminal device technology wherein FE polarization controls the resistanceof the FE device thereby mimicking a human neuron3. For such applications, the purpose of our research isto explore the applicability of ultrathin organic FE polymers in FE memristors by fabricating FE tunnel
junctions for the first time. We employ ultrathin polyvinylidene fluoride (PVDF) with 3~20 nm thickness
fabricated by the Langmuir-Blodgett technique. We expect that electrons can tunnel through PVDF when
sandwiched between two metals with dissimilar work functions in a tunnel junction. Tunnel junctions at theatomic scale and nanoscale can also be fabricated using a scanning probe microscope (SPM) like scanningtunneling microscopy and atomic force microscopy4. These methods will not only provide the topography ofthe samples but also give us the current-voltage characteristics of the tunnel junctions. Combining all theSPM techniques we can demonstrate the tunneling effect in PVDF atomic layers along with crucial
information on the molecular arrangement and topography. In this context, material characterization of theatomically thin films is very crucial before fabricating a FE device. Since ferroelectricity is a directional
quantity, properties such as crystallinity, crystal plane orientation, and molecular configuration become
highly crucial for the overall FE properties. X-ray scattering experiments could provide information on thecrystal structure and orientations. Combining the results from and X-ray scattering and SPM, we will
demonstrate a high-performance ultrathin organic FEmemory technology in the future.

Experimental:
Grazing incidence X-ray scattering is technique to analyze the crystal structure of thin films mainly fororganic and semiconductor materials which can reveal the crystal structure within 10 nm from the surface
distinctly. Ultrathin Organic materials and polymers have weakly scattering crystals unlike in-organic
materials with high orderliness and crystallinity, hence synchrotron radiation of high brilliance and photon
flux is necessary. Refractive indices of many materials have a refractive index less than one for X-rays
resulting in total reflection.
The ultrathin PVDF samples were fabricated by the Langmuir-Blodgett technique. In this method, 0.01 wt%
of PVDF-TrFE in dimethylsulfoxide was dropped on the Langmuir trough containing ultrapure water. The
surface pressure of the water was monitered using a Wilhemy plate. About 10 microliters of PVDF-TrFEsolution was dropped using a syringe 30 times resulting in a total volume of 300 microliters. The amount ofthe dropping solution depends on the Langmuir trough size. After the solution was dropped, the PVDF layers
were compressed by the barriers at a speed of 5 mm/s. A waiting time of 20 mins was needed after dropping
the PVDF solution and compression. The PVDF layers were lifted by horizotal dipping of the substrate (Si,Au and HOPG) after the surface pressure of 5 N/m2 was reached. To achieve multiple layers, the substratedipping process was repeated (drying the substrate with N2 gas between cycles) while manintaing the surfacepressure. After PVDF was deposited, the samples were annealed at 140 ºC for 2 hours and cooled naturally
to room temperature.
Grazing incidence angles produce a large beam footprint which increases the X-ray sample interaction. Asimple experimental schematic of a typical Grazing incidence wide angle scattering (GIWAXS) as shown inFig. 1(a), where X-rays are incident on the sample at the grazing angle (0.5° ~ 2°). A beam stopper wasplaced on the direct beam path to avoid damage to the detector. For collecteing the the scattered X-rays atwide angles, a 2D detector (Pilatus 300 K) was placed at 10~30 cm from the samples. Beam energies of~12.3 keV along with a are used in the current experiment. Figure 1(b) shows schematics of ultratin PVDF
crystallized epitaxially with edge-on arrangement of nano lamellae on Au (111) and HOPG substrates. Figure1(c) shows schematics of randomly oriented PVDF nanolamellae crsystallized on Si wafer.

Fig. 1: (a) Schematics of the GIWAXS experiment where an X-ray beam grazes the substrate. The scattered
X-rays are collected on a 2D detector placed behind the sample. (b) Schematics of 2~3 nm ultrathin PVDF
nano lamellae arranged as edge-on lamellae on Au (111) substrate. (c) Schematics of 2~3 nm ultrathin PVDF
lamellae arranged randomly on Si wafer.
Resuls and discussion:
Figure 2(a) shows the GIWAXS pattern of 20 nm ultrathin PVDF on Au (111) and Si substrates. In ourexperiment, we did not observe X-ray scattering signals from the samples with 3 nm and 10 nm thickness ofPVDF. Probably, different measurements and beam with more photon flux must be implemented in the future.
For the 20 nm thick samples, distinct (200) and (100) reflections are observed as bright spots for PVDF
crystallized on Au (111). Similar result was observed for ultrathin PVDF crystallized on HOPG surfaces. TheGIWAXS pattern thus confirm the epitxial growth of PVDF in (200) direction. This also confirmed that
PVDF posess in-plane ferroelectricity in these cases. When ultrathin PVDF was crystallized on bare Si wafers,continuous reflections of PVDF (200/110) are observed. This indicated that the scattering vectors of(200/110) are randomly oriented. This also confirms that the ferroelect ric polarization direction of PVDF on
Si is randomly oriented.

Fig. 2: (a) GIWAXS pattern of 2~3 nm ultrathin PVDF on Au (111) and Si substrates. Distinct (200) and(100) reflections are observed as bright spots for PVDF crystallized on Au (111). Continuous reflections ofPVDF (200/110) are observed for the case of crystallization on a Si wafer. (b&c) AFM height images ofPVDF crystallized on Au (111) surface. (d) Average azimuthal profiles of 2D GIWAXS pattern for epitaxial
PVDF on Au (111). (e) AFM height profile for 1L and 0L showing layer thickness of 0.5 nm corresponding
to ‘b’ axis of PVDF orthorhomibic crystal.
The samples were also observed from atomic force microscopy (AFM). Figure 2(b) shows the AFM height
images of 2~3 nm PVDF crystallized on Au(111). From Fig. 2(c), which shows a mangnified AFM image, itis seen that PVDF crystallizes layer by layer. The number of layers are indicated on the AFM image as 0L,
1L and 2L. Figure 2(d) shows the average azimuthal profiles of 2D GIWAXS pattern for epitaxial PVDF on
Au (111) which reveals the presence of highly oriented 110 and 200 planes of PVDF orthorhombic crystal assharp peaks in the azimuthal direction. Figure 2(e) shows the cross-sectional profile of 0L to 1L showing alayer thickness of 0.5 nm corresponding to ‘b’ axis of PVDF orthorhomibic crystal. This also agrees with theepitaxial arrangement of PVDF on Au (111) as observed from GIWAXS. The current measurements showthat the arrangement of PVDF molecules depend on the subtsrate on which it is crystallized. It is currently
assumed that there is crystallographic interaction between Au (111) and HOPG on PVDF enabling an
epitaxial edge-on arrangement as compared to that of Si requiring further detailed investigation.
References:
1. Sachio, H. et al. Organic ferroelectrics. Nat. Mater. 7, 357–366 (2008).
2. Chen, X. et al. PVDF-Based Ferroelectric Polymers in Modern Flexible Electronics. Adv. Electron.
Mater. 3, (2017).
3. Tian, B. B. et al. Tunnel electroresistance through organic ferroelectrics. Nat. Commun. 7, 1–6 (2016).
4. Wang, Z. et al. Optically controlled electroresistance and electrically controlled photovoltage inferroelectric tunnel junctions. Nat. Commun. 7, 1–9 (2016).