Large Piezoelectric Strain in Sub-10 Nanometer Two-Dimensional

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Large Piezoelectric Strain in Sub-10 Nanometer TwoDimensional Polyvinylidene Fluoride Nanoflakes Naveed Hussain, Mao-Hua Zhang, Qingyun Zhang, Zhen Zhou, Xingyu Xu, Muhammad Murtaza, Ruoyu Zhang, HeHe Wei, Gang Ou, Dong Wang, Ke Wang, Jing-Feng Li, and Hui Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00104 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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Large Piezoelectric Strain in Sub-10 Nanometer Two-Dimensional Polyvinylidene Fluoride Nanoflakes Naveed Hussain,1† Mao-Hua Zhang,1† Qingyun Zhang,1 Zhen Zhou,1 Xingyu Xu, 1Muhammad Murtaza, 1 Ruoyu Zhang,1 Hehe Wei, 1 Gang Ou, 1Dong Wang,1 Ke Wang 1*, Jing-Feng Li1* and Hui Wu 1* Affiliations 1State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China † These authors contributed equally Email: [email protected]; [email protected]; [email protected] Abstract Functional polymers such as Polyvinylidene fluoride (PVDF) and its co-polymers, which exhibit room temperature piezoelectricity and ferroelectricity in two-dimensional (2D) limit, are promising candidates to substitute hazardous lead-based piezoceramics for flexible nanoelectronic and electromechanical energy-harvesting applications. However, realization of many polymers including PVDF in ultrathin 2D nanostructures with desired crystal phases and tunable properties remains challenging due to ineffective conventional synthesis methods. Consequently, it has remained elusive to obtain optimized piezoelectric performance of PVDF particularly in sub-10 nm regime. Taking advantage of its high flexibility and easy processing, we fabricate ultrathin PVDF nanoflakes down to 7 nm of thickness by using hot-pressing method. This thermo-mechanical strategy simultaneously induces robust thermodynamic α to electroactive β-phase transformation, with β fraction as high as 92.8 % in sub-10 nm flake. Subsequently, piezoelectric studies performed by using piezoresponse force microscopy (PFM) reveal an excellent piezoelectric strain of 0.7 % in 7 nm film and the highest piezoelectric coefficient (d33) achieved is -68 pm/V for 50 nm thick nanoflakes, which is 13 % higher than the piezoresponse from 50 nm thick PZT nanofilms. Our results further suggest thickness modulation as an effective strategy to tune the piezoelectric performance of PVDF and affirm its supremacy over conventional piezoceramics especially at nanoscale. This work aims not only to help understand fundamental piezoelectricity of pure PVDF ACS Paragon Plus Environment

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in sub-10 nm regime but also provides an opportunity to realize other polymer-based 2D nanocrystals. KEYWORDS: hot-pressing, 2D nanomaterials, polyvinylidene fluoride (PVDF), sub-10 nm nanoflakes, piezoelectricity Piezoelectricity defines a reversible interconversion between electrical charge and mechanical output and holds the key for energy harvesting 1 and sensing applications.2 Development of smart triboelectric nanogenerators,3 ultrasensitive nanoelectromechanical systems (NEMS)-based nanosensors 4 and ultra-high frequency oscillators 5 is tied immensely to the practical realization of miniaturized 2D piezoelectric materials that maintain large piezoelectric coefficient (d33) and high strain values in sub-10 nm regime. In addition, piezoelectric nanomaterials must qualify the prerequisites of being light weight, flexible, and bio-compatible for their employment in flexible mechanical sensors, nanorobotics and biomedical applications. However, despite of excellent bulk electromechanical response, conventional Pb(Zr,Ti)O3 (PZT) and BaTiO3 (BTO)-based sub-10 nm films exhibit poor d33 values of ~7-15 pm/V 6 and 10 pm/V,7 respectively, and the optimal d33 of 60 pm/V for 50 nm PZT nanofilms. Furthermore, these piezoceramics are incompatible with abovementioned applications because of their complex processing, stiffness and potential toxic issues. Group III-nitrides (AlN, GaN) and TMDCs (MoS2) on the other hand demonstrate even weaker values of d33 and mechanical strain in response to electrical signal, especially for films with thicknesses in sub-10 nm regime. 8-10 Thus, this non-suitability of current piezoelectric materials at nanoscale calls for alternatives with desired features and comparable or even superior d33 values.1113

Although, the exact competition between inorganic and organic material systems particularly in

sub-10 nm regime might not be appropriate especially under the intense electric field applied through the tip during the PFM measurements.14 However, the quantification of d33 in sub-10 nm regime is still essential for technological advancements in (nano)sensors and actuators. Owing to its advantages of high flexibility, wide frequency response, biocompatibility, low cost, high ACS Paragon Plus Environment

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negative piezoelectric coefficient and integration with traditional silicon based (micro)electronics,15 semi-crystalline polymer PVDF has emerged as a promising substitute for conventional piezoceramics. PVDF has already attracted phenomenal research efforts for energy scavenging and sensing applications involving the development of electromechanical actuators,16 pressure sensors2 and piezoelectric nanogenerators. 1, 17 However, an effective phase transformation from thermodynamically stable α- to technologically favorable ferroelectric β-phase (all trans TTTT planar zigzag structure)18 must be achieved in order to gain enhanced piezoresponse from PVDF nanostructures. Exhausting efforts are being poured in by the scientific community to achieve this phase transformation through various strategies such as nanoconfinement,18-20 nanocomposites,21 employing mechanical stretching,22 post-fabrication heat treatment.23 For instance, Hong et al. reported the average piezoelectricity of -72.7 pm/V from nanoimprinted PVDF-TrFE nanograss. 24 Whereas, piezoelectric co-efficient of -63.5 PC/N has been reported in bulk PVDF-TrFE films by exploiting stereochemistry that leads to highly order-disorder regions reminiscent of morphotropic phase boundary.25 Nevertheless, the enhanced values in the former are attributed to the pressure induced structural ordering by nanoimprinting also reported elsewhere

26, 27

followed by the

additional post fabrication flip-stacking poling method. A fabrication mechanism such as hotpressing appears to be tailor made strategy that exploits pressure, thermal annealing and nanoconfinement, all in one step process to yield sub-10 nm 2D PVDF with potentially enhanced β-crystal phase content as compared to layer by layer assembly (LBL) 28 and Langmuir–Blodgett techniques.29, 30 Nanoimprinting lithography can also yield PVDF nanostructures by employing simultaneous pressure and heat-treatment, but this approach suffers from high degree of complexity and inaccessibility. Herein, we demonstrate the facile fabrication of ultrathin ferroelectric polymer PVDF NFs by thermal compressing of mixed α and β-phase PVDF nanoparticles (NPs) on highly polished ACS Paragon Plus Environment

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surfaces of Si, quartz and Pt (111)/Ti/SiO2/Si substrates by using facile one-step hot-pressing strategy. We further report the process-induced significant piezoresponse of -36 pm/V from an individual sub-10 nm NF (7 nm thick), accompanied by thickness–dependent piezoelectric studies using piezoresponse force microscopy (PFM). PVDF NFs demonstrate an optimal piezoresponse of -69 pm/V from 50 nm thick NF which is 15 %, 230 % and 1130 % higher than the reported piezoresponse of 50 nm PZT (60 pm/V),6 ferroelectric copolymer PVDF-TrFE thin films (-30 pm/V) 31 and pristine PVDF NPs (-6 pm/V) (This work), respectively. We believe that our work not only substantiate PVDF as an alternative for future NEMS to realize tactile sensors, soft robotics and biomedical nanodevices but also contribute in comprehending the behavior of other polymer systems at the nanoscale. RESULTS AND DISCUSSION Fabrication and morphology of PVDF NFs A diagram illustrating the schematic configuration of hot-pressing strategy for the fabrication of ultrathin 2D nanostructures has been presented in Figure 1.32, 33 PVDF NPs-Ethanol dispersion was dropped on highly polished and ultra-cleaned Pt (111)/Ti/SiO2/Si substrate in an Argon filled glove box in order to maintain the lowest possible exposure to oxygen. After 15 minutes, the dried Pt (111)/Ti/SiO2/Si substrate capped with small agglomerates of PVDF NPs was covered with another Pt (111)/Ti/SiO2/Si substrate of the same size, such that the PVDF NPs were sandwiched between the two substrates. Small agglomerates of PVDF NPs were subjected to optimized thermal compression under the ambient conditions. Under the constant applied pressure, the temperature is first raised to 150 oC and maintained for minutes before cooled down to room-temperature (tR). The temperature was intentionally raised to 150 oC in order to achieve melt and recrystallization of PVDF.34 The detailed fabrication procedure adopted to fabricate ultrathin PVDF NFs is presented in Text S1, while a typical hot-press machine with its core components and the XRD pattern of Pt (111)/Ti/SiO2/Si showing sharp single crystal (111) peak are presented in the Figure S1. The inset ACS Paragon Plus Environment

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circle on the left presents the highly disordered semi-crystalline configuration of molecular chains in the PVDF NPs that consists of mixed α and β-crystal phases. The crystal structure of thermodynamically stable α and electroactive β-phases is shown alongside. While, the inset circle on the right shows process-induced orientation and ordering of lamellae of intertwined chains in the β-crystal phase. Each molecular chain in PVDF comprises of a long zigzag monomers of carbon (C), fluoride (F) and hydrogen (H) atoms. Field emission scanning electron microscopy (FESEM) image of uniform sized PVDF nanoparticles is presented in Figure 2A. Ultrathin 2D morphology of PVDF nanoflakes (NFs) spanning several micrometers in lateral dimension prepared by using the hot-pressing method can be clearly observed in Figure 2B. Figure 2C is the magnified FESEM image, which revealed uniform surface of ultrathin NFs. Since our method involves melt and recrystallization mechanism which has been previously exploited to yield epitaxial grown PVDF-TrFE films on graphene and Rubrene single crystals.

34, 35

The reported films appear to be quite dense, uniformly aligned and

different in shape than those of grown on amorphous parts of substrates. However, the FESEM images of the hot-pressed NFs prepared on both the amorphous Si/SiO2 and single crystal Pt (111) substrates (Figure S2 A, B) exhibit no hint of alignment, density change or any considerable shape change and confirm non-epitaxial growth of the flakes. Atomic force microscopy (AFM) image of a large area and ultrathin NF with thickness of 7 nm, marked by the white line is shown in Figure 2D. It is evident that the hot-pressing strategy can easily yield sub-10 nm polymer NFs on the smooth surfaces of appropriate substrates. The area of the flakes prepared by hot-pressing measured by Image J software ranges from as small as 2 μm to as large as 30 μm and purely depends upon the size of the NPs agglomerate (Figure S3B). A transmission electron microscopy (TEM) image of pristine PVDF nanoparticles used to prepare nanoflakes is presented in Figure 2E. The average diameter of these NPs was measured to be 250 nm (Figure S4A). Selected area electron diffraction (SAED) pattern of pristine NPs revealing several diffraction rings is presented in Figure S4B, ACS Paragon Plus Environment

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Supporting information. The TEM image of a hot-pressed PVDF NF clearly suggests the sheet like 2D morphology and ultrathin nature of as-prepared PVDF NFs (Figure 2F). Two-dimensional elemental mapping results acquired from the same NF shown in TEM image are presented in Figure 2G and clearly reveal that the elemental composition comprises of distinct carbon and fluoride content in PVDF NFs. The AFM topography image of three NFs with thicknesses of 10 nm, 12 nm and 15 nm lying on Pt (111)/Ti/SiO2/Si substrate is presented in Figure 2H. The corresponding height profiles of these NFs marked with I, II and III are shown in Figure 2I. The energy dispersive spectra (EDS) captured from the zone 1 in Figure 2K is presented in Figure 2J and indicates the fabrication of large area pure PVDF NFs on highly polished and ultraclean surface of Si substrate. The percentile elemental composition by weight and by atom acquired from EDS analysis is presented in the inset of Figure 2J. The EDS shown in Figure 2L is collected from the zone 2 in Figure 2K and reveals 100 % Si content depicting the ultra-cleaned surfaces of the substrates used. Thickness distribution histogram of as-prepared PVDF NFs, acquired with the help of AFM topography in tapping mode conducted over 100 thinnest NFs, revealed that the thickness distribution spans primarily from 7 nm to 30 nm (Figure S5). However, the thickness density of NFs was considerably higher from 7 nm to 12 nm flake thickness, which indicates high efficiency of hot-pressing method to fabricate large area sub-10 nm 2D nanostructures. Structural Transformation in PVDF NFs Comparative XRD patterns in Figure 3A suggest that the pristine PVDF NPs consist of mixed α and β-phases (illustrated in Figure 1) by the peaks (020) at 18.53o (α-phase) and (110) at 20.2o (βphase), respectively. 1 However, the patterns for PVDF NFs after hot-pressing reveals preferential orientation of the (110) or (200) planes assigned to ferroelectric β-phase.36 The absence of diffraction peaks at 35.2° and 40.8° in XRD patterns of NFs further suggests significant phase transformation from mixed α and β phases to electroactive β-phase with fairly high crystallinity which could be attributed to the simultaneous nanoconfinement effect,19, 37 heat treatment 23, 38 and ACS Paragon Plus Environment

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compressive stretching.22 The magnified XRD scans reveal very weak signal from α-phase peak at 18.53° (Figure S6A). Thus, it is rather possible to realize a high degree of β-phase crystalline content by a robust phase transformation as a synergistic effect of nanoconfinement, thermal annealing and compressive stretching applied simultaneously during the hot-pressing of PVDF to fabricate 2D PVDF nanostructures. The blue line in Figure 3A is the background XRD scan obtained from bare quartz substrate. The overall degree of crystallinity (φTotal) of PVDF NFs, calculated by exploiting XRD analysis came out to be 52 % and is in agreement with the reported values. 39 Detailed calculations are presented in Text S2, Supporting information. To further assess the structural/phase transformation, transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM) was carried out (Figure S6 B, C). HRTEM analysis show highly crystalline nature of PVDF NFs. Whereas, the selected area electron diffraction (SAED) pattern obtained from NFs (Figure 3B) revealed sharp and distinct diffraction points signifying highly oriented single crystal-like reflections. Additional pattern analysis discloses that the reflections can be indexed to (200)/(110) reciprocal lattice conformation as reported elsewhere. 26, 36

In contrast, the SAED pattern obtained from PVDF NPs consist of several diffraction rings

indicating their randomly oriented polycrystalline nature (Figure S4B). Since, the planes (200)/(110) of β-phase lie parallel with molecular chains, which indicates that the molecular chains are oriented in plan with the substrate.

40

Therefore, our XRD and SAED analyses both confirm

highly preferential orientation corresponding to β-phase of as-prepared PVDF NFs on Pt (111) substrates. To further support our conclusions, we performed Fourier-transform infrared spectroscopy both in absorption and reflection absorption spectroscopy (RAS) mode to determine the molecular orientation of PVDF NFs. The Fourier-transform infrared spectroscopy (FTIR) in absorption mode in Figure 4A presents the absorption spectra of both the pristine NPs and as-prepared NFs. The absorption peaks at 763, 795, and 974 cm-1 are assigned to α-phase, while the absorption peaks ACS Paragon Plus Environment

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centered at 841, 1280, and 1402 cm-1 are allocated to β-phase of pure PVDF.41 Figure 4B is the magnified FTIR spectra (Highlighted in gray, Figure 4A) showing noticeable suppression in absorption peaks of α-phase at 763 cm-1, 795 cm-1, and considerable enhancement in absorption peaks of β-phase at 841 cm-1 , which is in agreement with our XRD results and propose considerable improvement in β-phase content in as-prepared NFs. Moreover, the broadening of the characteristic peak at 1403 cm-1 associated with β-phase for 2D PVDF in Figure 4A provided further evidence of enhancement in crystallinity.38 The relative polar β-phase fraction (Fβ) was calculated by assuming that FTIR absorption follows the Lambert–Beer law 39 and using the following equation:

F  

A

K      A A  K  

100

(1)

Where, Aα and Aβ are the absorption intensities at 763 cm-1 and 840 cm-1, respectively. Kα (=6.1  104 cm2mol-1) and Kβ (=7.7  104 cm2mol-1) are the absorption coefficients at the respective wavenumbers, respectively. On the basis of these calculations, the average β fraction (Fβ) came out to be 87.04 %. The detailed calculations are presented in Text S3, Supporting information. The crystal conformation of PVDF NFs lying on Pt (111) substrate is critical for its piezoelectric performance. This information can be extracted by comparing the bands A1, B1 and B2 in FTIR spectra acquired in RAS mode (Figure 4C). It has been proposed that an IR band usually appears when a component of its transition dipole is perpendicular to flake surface or parallel to the electric field. In this case, the electric field is parallel to the plan of the substrate. The bands at 1288 cm-1 (A1, µ || b) corresponding to symmetric stretching vibration of CF2 groups, 1187 cm-1 (B1, μ || a), antisymmetric stretching and rocking vibrations of CF2 groups and at 1400 cm-1 (B2, μ || c), wagging vibration of CH2 groups are assigned to the transition dipole moments parallel to polar b-axis, chain c-axis and a-axis, respectively.42 It must be acknowledged that the peaks were normalized to absorption band B2 at 1187 cm-1. The prominent appearance of bands B1 and B2 and the reduction in the intensity of dipole sensitive A1 band at 1288 cm-1 in FTIR spectra of hot-pressed PVDF NFs ACS Paragon Plus Environment

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suggests that the orientation of electroactive dipoles (C-F) is strongly tilted away from the longitudinal axis or the plane of Pt (111) substrate. Whereas, a and c axes are aligned in-plane with the substrate.

26, 43

Additionally, it has been well documented that the dipole moments owing to

their constrained freedom in 2D-restricted ferroelectric films, are only allowed to rotate about the long chain axis (i.e. c-axis in our case) and are prohibited to rotate about other axes.30 This is in good agreement with our SAED analysis and confirms the conclusions drawn about the crystal conformation of PVDF NFs which is shown by the corresponding model in Figure 4D. Micro-Raman spectra of pristine NPs (black line) and as-prepared NFs (red line) recorded from 100 cm-1 to 1600 cm-1 shows vibrational features at 286, 412, 487, 537, 612, 797, 838, 847 and 1427 cm-1 (Figure 4E). The intense vibrational modes at 797 cm-1 and weak 839 cm-1 (indicated by the pointer) in pristine PVDF NPs spectrum are assigned to α and β-phase, respectively.44 Significant suppression of the peak associated with α-phase and rigorous enhancement of the peak assigned to β-phase is observed and is consistent with all the previous results. Nevertheless, in the context of investigating thickness-dependent structural evolution of β-phase by calculating β fraction F(β), we performed thickness-dependent micro-Raman experiments to probe the structural evolution from NPs (250 nm) to NFs (7 nm) by carefully monitoring the vibrational features from 600 to 1400 cm-1 (Figure 4F). A considerable suppression in the intensity of vibrational mode associated α-phase (797 cm-1) along with a little blue-shift of about 3 cm-1 in the spectrum of PVDF NFs was observed with decreasing flake thickness. In contrast, there was a significant enhancement in the peak intensity of out-of-plane vibrational mode assigned to β-phase (839 cm-1), accompanied by the same amount of blue-shift as observed in case of peak at 797 cm-1. We speculate that this systematic but comprehensive weakening and enhancement in intensities of vibrational modes of α and β-phase, respectively, are attributed to the robust process-induced alignment/ordering of molecular chains with decreasing thickness of PVDF from NPs to ultrathin NFs. This results in significant transition from α to β-phase and refers to an overall improvement of crystallinity in as-prepared PVDF NFs, ACS Paragon Plus Environment

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which is in line with our HRTEM, XRD and FTIR results. The evidence of systematic blue-shift in as-prepared PVDF NFs is presented in magnified micro-Raman spectra in Figure 4G and might be associated with the process-induced stretching in crystal structure and reduced dimensionality of PVDF. 32 Raman modes around 840 cm-1 and 794 cm-1 are characteristic modes that are assigned to β and α-phases of PVDF respectively. Therefore, the relative polar β fraction F(β) and the corresponding β to α ratio (β/α) for each corresponding thicknesses were precisely calculated by comparing the intensities at these wavenumbers 39 and presented in Figure 4H (details in Text S4, supporting information). The F (β) (red curve) was estimated to boost from 42 % to as high as 92.8 % for decreasing flake thickness from 300 nm to 7 nm, respectively. Whereas, (β/α) increased from 0.9 to 6.4 (blue curve). AFM images of PVDF NFs and the corresponding height profiles at particular points where the thickness-dependent Raman studies were performed are presented in Figure S8. X-ray photoelectron spectroscopy (XPS) was performed to investigate any mechanical deformation under the applied pressure during hot-press (See Text S6, Figure S7, Supporting Information), which suggests no evidence of mechanical or thermal breakdown in as-prepared NFs. Piezoelectric response of PVDF NFs For the characterization of piezoelectricity of PVDF NFs, switching spectroscopy PFM (SS-PFM) technique

45-48

which has been successfully applied to probe the electromechanical properties of

molecular 12, 49 and perovskite 50-52 ferroelectric films was employed. We used Pt (111)/Ti/SiO2/Si substrate for PFM measurements, where Pt acted as a bottom electrode to minimize the electrostatic effect. During SS-PFM operation, the electrode was well-connected to a ground wire, precluding the accumulation of bound charges. The piezoresponse obtained by using dual amplitude resonance tracking (DART) mode was then corrected via simple harmonic oscillator (SHO) model.53 PFM tip calibration and spring constant measurements were performed prior to piezoresponse experiments and PHLs characterized at OFF-state were used in order to ensure the authenticity of obtained results (Figure S9). The first and second harmonic PFM measurements were carried out under an ACS Paragon Plus Environment

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AC voltage of 4 V, reported elsewhere.54, 55 The 1st harmonic response was much larger than the second one (See Figure S10A), which suggests that the PFM response of PVDF nanoflakes mainly comes from its piezoelectricity. The schematic illustration of the piezoresponse studies of PVDF NFs on Pt (111)/Ti/SiO2/Si substrates and the atomic force microscopy (AFM) image of a 7 nm thick PVDF NF used as a model for our sub-10 nm piezoelectric measurements is presented in Figure 5A and 5B, respectively. The diameter of PFM Cantilever tip used to perform piezoelectric experiments was measured to be around 50 nm (Figure 5C). The local piezoresponse of pristine PVDF NPs (250 nm) and as-prepared sub-10 nm (7 nm) NF as a function of DC bias voltage are compared in Figure 5D. The piezoresponse amplitude divided by the thickness defines local effective strain. A fine-looking butterfly loop obtained for 7 nm thick NF provides a clear indication of ferroelectricity on the nanoscale, leading to a characteristic polarization switching behavior and a fairly high amplitude in sub-10 nm PVDF NFs when compared with their pristine NPs. It must be noted that all the amplitude PHLs were acquired after applying SHO filter as presented in Figure S11A. The combined strain-voltage butterfly piezoresponse hysteresis loop (PHLs) for each various flake thicknesses are presented in Figure S12, Supporting information. Characteristic piezoresponse amplitude and piezoresponse phase loops as a function of DC bias voltage for the thinnest and thickest i.e. 7 nm and 50 nm NFs are shown in Figure 5E and 5F, respectively. The piezoresponse characterization was performed within a 1 μm × 1 μm-sized mesh of 100 points to ensure the consistency of the results. Both piezoresponse amplitude loops for 7 nm and 50 nm NFs demonstrate well-defined butterfly shapes and a clear contrast of 180o is observed in piezoresponse phase loops for both the NFs. This indicates the existence of ferroelectricity for the NFs with different thicknesses on the nanoscale. A comparative PHL of local piezoelectric constant (d33) as a function of applied voltage for pristine 250 nm NPs and 7 nm NFs is presented in Figure 6A. The d33 value obtained from 7 nm thick NF (-36 pm/V) is 6-fold higher than that of 250 nm NPs (-6 pm/V). Piezoelectric constant obtained as ACS Paragon Plus Environment

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a function of flake thickness is plotted in Figure 6B, which shows an almost linear increase in d33 values except for sub-10 nm NFs. The PHLs for individual NFs are shown in Figure S13 and S14, Supporting information. The local piezoelectric strain as a function of DC bias voltage for NPs and various NFs, i.e. 7 nm, 10 nm, 12 nm, 15 nm, 20 nm, 30 nm and 50 nm are displayed individually in Figure 6C. An ultra-large strain of 0.7 %, which is approximately 117-fold larger than the effective strain of pristine PVDF NPs (0.006 %) was observed for sub-10 nm NF. Furthermore, asymmetric piezoresponse strain loops are observed for NFs with thickness lower than 12 nm. This is due to an overall improvement of relative polar β fraction F(β) in NFs with decreasing thickness and might also be attributable to the manifestation of non-uniformly scattered charged defects.56 The calculations used to determine the % piezoelectric strain for each flake thicknesses are presented in Text S5 Supporting information. The piezoresponse strain as a function of flake thickness is shown in Figure 6D. A decrease in strain from 0.7 % for 7 nm to only 0.15 % for 50 nm thick NFs can be clearly observed. To confirm the uniformity of piezoelectric response, multiple tests were performed on different valleys of the same flake to determine the piezoelectric response for each thickness presented in the study. The values of d33 and piezoelectric strain are the mean values of at least three measurements for each flake thickness presented by corresponding error bars in Figure 6B and 6D, respectively. Figure 6E shows a 180o phase switching in PVDF NFs which advocates a robust room-temperature ferroelectricity. Detailed discussions are addressed below. The piezoresponse amplitude divided by the driving voltage of the tip defines local effective piezoelectric coefficient d33. A d33 value of -36 pm/V for sub-10 nm PVDF NFs is the highest value reported for sub-10 nm PVDF nanostructures to the best of our knowledge. This value of d33 is ~ 6-fold and 3-fold and ~ 4-fold higher than that of pristine PVDF NPs (-6 pm/V), sub-10 nm PZT 6 and BTO nanofilms,7 respectively. The piezoresponse coefficient d33 as a function of flake thickness is shown in Figure 6B. An almost linear increase in d33 with increasing thickness, except for sub10 nm NF, is observed in our results. The highest value obtained was -68 pm/V for 50 nm thick ACS Paragon Plus Environment

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PVDF NF which is ~ 13 % higher than the optimal d33 of PZT for the same thickness. This trend of increasing d33 with increasing flake thickness is consistent with previous report on PZT

6

but in

striking contrast to that of diameter-dependent d33 of PVDF nanofibers, where the d33 decreases with increasing thickness.20 It is believed that the enhanced values of overall piezoresponse are the results of synergistic effects of mechanical confinement and thermal treatment, the effect of mechanical confinement which is in direct proportion to that of the applied pressure plays vital role in ordering the molecular conformation of as-prepared NFs, as compared with the thermal annealing. Notably, it is highly anticipated that flakes thinner than 7 nm could exhibit even higher piezoelectric strain. But unfortunately, 7 nm seems to be the critical thickness achieved through current strategy. We speculate that this trend of almost linear increase in piezoresponse (d33) with increasing flake thicknesses originate from the contributions from strong electromechanical couplings between intermixed crystalline lamellae and amorphous valleys.57 This increasing trend could further be assigned to the aggregated sum of piezoelectric contributions from each PVDF layer in out-of-plane direction, resulting in higher d33 values. However, both the piezoelectric strain and piezoelectric coefficient d33 exhibit vastly different behavior in response to flake thickness. For the NFs with a thickness of 7 nm, the intense increase in β fraction F(β) gets the better of the decrease in volumes of crystalline PVDF chains, leading to a singular value of d33. The relationship between field-induced piezoresponse S and piezoelectric coefficient d33 satisfies Equation: S=d33E

(2)

where E is the local electric field under which SS-PFM is conducted. For NFs with decreasing thicknesses, the reduction of d33 is compensated by the increase in E, which is boosted by decreasing the thickness while maintaining the same electric voltages. As, the hot-pressing mechanism involves simultaneous employment of pressure and annealing, we believe that our values are the results of synergistic effect of simultaneous pressure and thermal annealing of mechanically ACS Paragon Plus Environment

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confined PVDF NFs. A comparison of piezoelectricity for the PVDF nanoflakes with various benchmarked 2D piezoelectric materials is provided in Table1. Our NFs exhibit superior d33 in the sub-10 nm and 10-300 nm regimes than previously reported 2D PVDF. CONCLUSIONS In conclusion, we employ a facile hot-pressing strategy to fabricate sub-10 nm PVDF NFs which demonstrate an overall excellent piezoresponses, owing to the process-induced structural transition from α to β-phase. The average β fraction (Fβ) in as-prepared PVDF NFs was estimated to be around 87.04 %. Our results indorse the superiority of PVDF over lead-based piezoceramics and other inorganic materials at the nanoscale. Further, the thickness modulation is employed to tune the piezoelectric strain in PVDF NFs. We believe that the proposed strategy can be extended directly for the practical realization of previously inaccessible polymer-based 2D nanomaterials. METHODS Materials Semi-crystalline Poly (vinylidene fluoride) (PVDF) powder was obtained from Sigma-Aldrich (St. Louis, MO, USA), having an average molecular weight of 534 000 g mol−1. The purchased powder had a melting temperature of 171 °C with density of 1.74 g/mL at 25 °C. Poly (vinylidene fluoride) (PVDF) nanopowder (250 nm, 99.99 % pure) in the form of uniform-sized nanoparticles purchased from Sigma Aldrich was used as received without any further processing. A homogenous dispersion was prepared by dispersing 1 mg of PVDF nanopowder in 30 mL of ethanol. Highly polished Pt (111)/Ti/SiO2/Si substrates with the dimensions of 1×1 cm2 were purchased commercially from Beijing Top Vendor Technology Co., Beijing, China for piezoelectric measurements. Whereas, highly polished silicon (Si) and quartz (SiO2) substrates (1×1 cm2) used for scanning electron microscopy (SEM), X-ray diffraction (XRD) and FTIR measurements were purchased from KYKY Technology Co. LTD. Intensive washing of all the substrates was performed by acetone, ethyl alcohol/ethanol and distilled water in an ultrasonicator to attain ultraclean surfaces. All the substrates were placed in an electric heating furnace maintained at 70°C for 20 min to obtain ACS Paragon Plus Environment

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moisture-free surfaces before use. Hot-press system (AH-4015, 200V-20A, Japan) used to fabricate ultrathin PVDF nanoflakes mainly consists of a hydraulic cylinder with two large-size plates for generating a large compressing force and controlled heating unit (See Figure S1A, Supporting Information). Fabrication of sub-10 nm PVDF nanoflakes 10 µL of PVDF-Ethanol dispersion was dropped on highly polished and ultra-cleaned Pt (111)/Ti/SiO2/Si substrate by a pipette gun in an Argon filled glove box (MB200MOD) in order to retain the lowest possible oxygen exposure. After 15 minutes, the dried Pt (111)/Ti/SiO2/Si substrate capped with small agglomerates of PVDF NPs was taken out of glove box and covered with another Pt (111)/Ti/SiO2/Si substrate of the same size, such that the PVDF NPs are sandwiched between the two substrates (Figure 1). The pair of substrates with the configuration of substratePVDF NPs-substrate were then positioned at the middle of the metal plates of the hot-press machine. We raised the temperature of plates from room-temperature to 150 °C and the applied pressure from atmospheric pressure to 0.542 GPa, simultaneously. Pt (111)/Ti/SiO2/Si substrates were pressed for 20 min at 150 °C under the constant applied pressure before permitting it to naturally cool down to room temperature. The applied pressure was kept constant throughout (once attained) until the system reached back to room temperature. No post fabrication heat treatment was done in our experiment. Fabrication Mechanism Polymers such as PVDF are known for their light-weight, flexibility and easy processing. Pristine PVDF NPs consist of mixed crystalline and amorphous phase. The crystalline phase content in PVDF being equal to or less than 50 % cannot endure much resistance to any external stimuli such as pressure. Therefore, it’s reasonable to expect PVDF to undergo considerable plastic deformation very easily when reasonably high temperature and pressure are applied simultaneously. During the deformation process, the microstructure changes mainly due to the modification in the shape and ACS Paragon Plus Environment

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size of the domains, which facilitates self-assembly during the fabrication process. Additionally, the deformation process under higher temperatures causes thermal expansion in microstructure, which makes them soft enough to mold effectively to a large area sheet-like morphology under persistent high pressure. Material Characterizations Ultrathin PVDF NFs lying on quartz substrate were characterized by using X-ray diffractometer (Rigaku D/Max 2500), equipped with a graphite monochromator and Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 0.04o/min in 2θ range of 20–80°. Electric current and voltage were fixed at 30 mA and 40 kV for the measurement. Field Emission scanning electron microscopy (FESEM, MERLIN VP compact, Carl Zeiss, Germany) was employed for microstructure and morphology analysis. TEM/HRTEM measurements were performed using a JEOL-2100 operated at 200kV or 80kV. Chemical states and composition of as-prepared PVDF NFs were performed by X-ray Photoelectron spectroscopy (XPS, Escalab, 250 Xi, Thermo Fisher Scientific, MA, USA) equipped with A1Kα radiation source (1487.6/eV). Binding energy calibration was done by using the C1s peak (284.8eV) of the substrates. FTIR spectroscopic measurements in Reflection-absorption spectroscopy (RAS) mode were performed by using Bruker-V70 equipped with vacuum tight chamber and has the range from 80 to 6000 cm-1 and a spectral range of 0.4 cm-1. Thicknessdependent micro-Raman spectra of PVDF NFs were obtained by using a Horiba LabRAM HR spectrometer with a 532 nm Nd: YAG (Nd: Y3Al5O12) laser and equipped with CCD detector. The laser spot size is roughly 1μm, which is focused by an objective lens (Nikon Plan Fluor 50 X, N.A. = 0.4). Asylum Research Cypher AFM (SPM, SHIMADZU Corporation, spm-9600) in tapping mode was used to perform Atomic Force Microscopy imaging. TEM/HERTEM images were measured by a JEOL-2100F microscope. The operating voltage was maintained at 200 kV. The machine we used for our PFM measurements was MFP-3D, from Asylum Research, while the custom code is available at https://github.com/JunxiYu/point-wise. ACS Paragon Plus Environment

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Sample Preparation for TEM/HRTEM measurements TEM/HRTEM measurements were performed by transferring PVDF NFs on to a Cu grid after isolating the flakes from the substrate using ample ultrasonication in ethanol. Soon after the fabrication, Si/SiO2 substrate capped with PVDF NFs was dropped in small amount of ethanol and ultrasonicated subsequently for two hours. Majority of PVDF NFs that were separated from the substrate to form a PVDF NF/ethanol dispersion. The dispersion was then dropped on Cu grid for TEM/HRTEM measurements. Piezoresponse Measurements PFM cantilever tip was a silicon probe with tip reflex coated by conductive Platinum-Iridium (PtIr). The cantilever had a spring constant of 2.8 N/m with the frequency of 75 kHz. For the quantification of switching and piezoelectric coefficient of PVDF NFs, we employed a dual ac resonance tracking piezoresponse force microscopy (DART-PFM) also known as SS-PFM technique which permits us to probe the piezoresponse originated within a single domain with a spatial resolution up to submicrometers.58 Furthermore, (DART-PFM) is comparatively a reliable technology to probe the piezoresponse from atomically thin and rough NFs, because it uses dual ac resonance tracking to quantify the shift of resonance to avoid the noise effects of the surface height topography and exterminate the contributions from electrostatic effects.45 DART-PFM has been successfully applied to measure the electromechanical elastic properties of inorganic ceramics and biological systems. 46 Piezoresponse hysteresis loops (amplitude-voltage loops) were acquired by applying a periodic DC biasing with a superimposed AC signal of 1 V (Vtip = Vdc + Vcos(ωt)) to nanoflakes via conductive tip while scanning the NFs. The PHLs characterized at OFF-state were used to reduce the effect from static electric. Various tip DC voltages with an amplitude ranging from 10 V to 40 V over different thicknesses of NFs from 7 nm to 50 nm were applied to obtain complete domain switching behavior. The instrument and tip calibration was performed to ensure the accurate values of piezoresponse (Figure S9). ACS Paragon Plus Environment

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Pressure Calculations Pressure applied by the hot-press machine onto silicon wafers alongside a gradual rise in heat was calculated by the following simple formula,

P

( P0  A0 ) (40 106    (24 103 m) 2 )   0.724GPa a (1104 m 2 )

(3)

Where, P0  Applied Pressure, A0  Area of Cylinder, a  Area of substrate ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ----------/ acsnano nn-2019-001042. Supporting Notes include supplementary texts and figures describing fabrication mechanism, degree of crystallinity/phase content calculations and PFM characterizations data (PDF). AUTHOR INFORMATION Corresponding authors *Email: [email protected] *Email: [email protected] *Email: [email protected] Author contributions: N.H. and Q.Z. initiated the project. N.H. synthesized and performed structural characterizations of PVDF nanoflakes. N.H. fabricated the samples for piezoresponse measurements. M.H. and N.H. performed PFM experiments. W.K. and H.W. performed the data analysis. N.H. and M.H. wrote the manuscript with contributions from all other authors. H.W., J.F.L. and W.K. supervised the project. ACKNOWLEDGMENTS We acknowledge the financial support from the National Basic Research of China (Grants 2015CB932500) and National Natural Science Foundations of China (Grant 51661135025 and

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51522207). We also acknowledge the contributions from Prof. Li and Junxi who helped us with second harmonic response. REFERENCES (1) Pi, Z.; Zhang, J.; Wen, C.; Zhang, Z.; Wu, D. Flexible Piezoelectric Nanogenerator Made of Poly(Vinylidenefluoride-Co-Trifluoroethylene) (PVDF-TrFE) Thin Film. Nano Ener. 2014, 7, 3341. (2) Sharma, T.; Je, S. S.; Gill, B.; Zhang, J. X. J. Patterning Piezoelectric Thin Film PVDF–TrFE Based Pressure Sensor for Catheter Application. Sensor. Actuat. A: Phys. 2012, 177, 87-92. (3) Chang, C.; Tran, V. H.; Wang, J.; Fuh, Y. K.; Lin, L. Direct-Write Piezoelectric Polymeric Nanogenerator with High Energy Conversion Efficiency. Nano Lett. 2010, 10, 726-731. (4) Y. T. Yang.; C. Calligari.; X. L. Feng.; K. L. Ekinci.; M. L. Roukes. Zeptogram-Scale Nanomechanical Mass Sensing. Nano Lett. 2007, 6, 583-586. (5) Feng, X. L.; White, C. J.; Hajimiri, A.; Roukes, M. L. A Self-Sustaining Ultrahigh-Frequency Nanoelectromechanical Oscillator. Nat. Nanotechnol. 2008, 3, 342-346. (6) Nagarajan, V.; Junquera, J.; He, J. Q.; Jia, C. L.; Waser, R.; Lee, K.; Kim, Y. K.; Baik, S.; Zhao, T.; Ramesh, R.; Ghosez, P.; Rabe, K. M. Scaling of Structure and Electrical Properties in Ultrathin Epitaxial Ferroelectric Heterostructures. J. Appl. Phys. 2006, 100, 051609. (7) Baturin, A. S.; Bulakh, K. V.; Zenkevich, A. V.; Minnekaev, M. N.; Chuprik, A. A. Study of the Ferroelectric Properties of BaTiO3 Films Grown on an Iron Sublayer Using Atomic Force Microscopy. J. Surf. Investigation. X-ray, Synch. Neutron. Techn. 2012, 6, 733-737. (8) Brennan, C. J.; Ghosh, R.; Koul, K.; Banerjee, S. K.; Lu, N.; Yu, E. T. Out-of-Plane Electromechanical Response of Monolayer Molybdenum Disulfide Measured by Piezoresponse Force Microscopy. Nano Lett. 2017, 17, 5464-5471. (9) Lueng, C. M.; Chan, H. L. W.; Surya, C.; Choy, C. L. Piezoelectric Coefficient of Aluminum Nitride and Gallium Nitride. J.Appl. Phys. 2000, 88, 5360-5363. (10) Tonisch, K.; Cimalla, V.; Foerster, C.; Romanus, H.; Ambacher, O.; Dontsov, D. Piezoelectric Properties of Polycrystalline Aln Thin Films for Mems Application. Sensor. Actuat. A: Phys. 2006, 132, 658-663. (11) Liao, W. Q.; Tang, Y. Y.; Li, P. F.; You, Y. M.; Xiong, R. G. Large Piezoelectric Effect in a Lead-Free Molecular Ferroelectric Thin Film. J. Am. Chem. Soc. 2017, 139, 18071-18077. ACS Paragon Plus Environment

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Figure 1. A typical hot-pressing method adopted to fabricate ultrathin 2D PVDF NFs. PVDF NPs on the left show intermixed lamellar structure with mixed α and β-phases. The NFs on the right show highly aligned lamellar crystal with major β-phase.

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Figure 2. (A) FESEM image of uniformly sized pristine PVDF NPs. (B) FESEM images of ultrathin PVDF NF lying on surface of Si substrate. (C) Highly magnified FESEM images of PVDF NF. (D) AFM image of a sub-10 nm thick PVDF NF. (E) TEM image of pristine NPs used for fabricating NFs. (F) TEM images of a NF lying in Cu grid. (G) Two-dimensional elemental mapping results captured from Figure 2. (F) showing the elemental composition of as-prepared NFs. (H) AFM topography image of three NFs of different thicknesses used to conduct PFM studies. (I) Corresponding height profiles of three NFs shown in Figure 2(H) for thicknesses estimation. (J) EDS mapping of a PVDF NF from the area highlighted by box 1 in Figure 2(K). (K) FESEM image used for detailed EDS analysis. (L) EDS mapping acquired from the bare area in Figure 2(K) highlighted by box 2.

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Figure 3. (A) X-ray diffraction patterns of PVDF NPs, NFs and quartz substrate. (B) Selected area electron diffraction (SAED) pattern of as-prepared PVDF NF showing high degree of crystallinity and preferential orientation assigned to (110) facet of β-crystal phase.

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Figure 4. (A) FTIR spectra of PVDF NPs and NFs in absorption mode. (B) Magnified FTIR spectra from gray highlighted region in (A). (C) FTIR spectra of PVDF NPs and NFs in RAS mode. (D) Proposed crystal conformation of PVDF NFs lying on Pt (111) substrate favoring the easy rotation of the dipole moment P about the c (chain) axis. (E) Comparative micro-Raman spectra of pristine NPs and as-prepared NFs. The insets show corresponding morphology and major crystal phases. (F) Thickness-dependent micro-Raman spectra for structural evolution of PVDF NFs (G) Magnified Micro-Raman spectra showing the evidence of systematic blue-shift in Raman bands with decreasing flake thickness. (H) Variations in the relative polar β-phase fraction (Fβ) (red curve) and β to α ratio (blue curve) with decreasing flake thickness of PVDF NFs.

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Figure 5. (A) Schematic illustration showing measurement of local vertical piezoelectricity and the lattice contraction of PVDF flake for both, +ve and –ve voltages. PFM measurements are conducted on NFs lying on Pt (111)/Ti/SiO2/Si substrate. (B) AFM topography image of a typical sub-10 nm PVDF flake lying on Pt (111)/Ti/SiO2/Si substrate. The yellow box shows PFM tip position on the flake. (C) The FESEM image of PFM Cantilever tip with the diameter of 50 nm, measured by Image J software. (D) A typical butterfly shaped piezoresponse hysteresis loop for strain-applied voltage for PVDF NPs and 7 nm NF (E&F) Combined amplitude-voltage and phase-voltage hysteresis loops for 7 nm and 50 nm thick flakes, respectively.

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Figure 6. (A) Piezoresponse hysteresis loops of piezoelectric constant-voltage for pristine NPs and as-prepared NFs. (B) Thick-driven piezoresponse from 7 nm to 50 nm PVDF NFs. (C) Butterfly shaped strain-voltage hysteresis loops for individual NFs of different thicknesses. (D) Thicknessdriven piezoelectric strain from 7 nm to 50 nm PVDF NFs. (E) Phase Vs time graph showing phase reversal in response to time. The error bars in B and D are standard deviations from the mean value acquired from at least three measurements for each thickness of the flakes.

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Table1. Various 2D piezoelectric materials and their d33 for sub-10 nm and 10-300 nm Piezoelectric

Type of 2D

Sub-10 nm

10-300 nm

Material

nanostructure

d33 (pm/V)

d33 (pm/V)

PVDF

nanoflakes

-36 (7 nm)

-68 (50 nm)

This work

PZT

nanofilm

7-15 (4-10 nm)

60 (50 nm)

6

BTO

nanofilm

8 (8 nm)

------*

7

CdS

nanoplates

32.8 (~ 4nm)

------*

13

MoS2

Triangular nanoflakes

1.35 (monolayer)

------*

8

ZnO

nanoflakes

27.3 (2nm)

------*

12

GaN

nanofilms

------*

3.1 (140 nm)

9

AlN

nanofilms

------*

5.1 (200 nm)

10

*No reports found Table 1. d33 values for our work and preciously reported 2D piezoelectric materials.

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Reference

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TOC Graphic

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