PIEZOELECTRIC BIO-ORGANIC FILMS AND FABRICATION METHOD THEREOF

Abstract

The present invention provides piezoelectric bio-organic films resembling ceramic-based piezoelectric films, and also a fabrication method thereof. In particular, the bio-organic piezoelectric films are formed by compact nanocrystals resembling the inorganic ceramic structure, where nanocrystallization on biomaterials and in-situ electric field are applied to facilitate domain orientation alignment across the entire films. The present fabrication method provides flexibility to tune various parameters of the resulting bio-organic films according to the needs, and therefore is substantially applicable to a wide range of biomaterials to form piezoelectric bio-organic films comparable to those formed by conventional piezoceramics in terms of piezoelectricity, thermostability and durability.

Claims

1. A piezoelectric bio-organic film comprising compact nanocrystals of one or more biomaterials formed by homogenous nucleation and in-situ electric field, the nanocrystals having an average grain size between 100 and 800 nm, and the piezoelectric bio-organic film having a piezoelectric strain constant from 5 to 15 pm/V, piezoelectric voltage constant of at least 150 ×10.sup.-3 V m/N, and a relative permittivity of less than 10.

2. The piezoelectric bio-organic film of claim 1, wherein the biomaterials constituting the compact nanocrystals one or more materials of glycine, L-alanine, DL-alanine, DL-threonine, DL-leucine, and L-Phenylalanine-L Phenylalanine.

3. The piezoelectric bio-organic film of claim 1, wherein the biomaterials are glycine.

4. The piezoelectric bio-organic film of claim 3, wherein the compact nanocrystals of glycine are characterised by X-ray powder diffraction with major peaks at about 23.6 and 28.6 degrees two-theta.

5. The piezoelectric bio-organic film of claim 1, wherein the compact nanocrystals of the one or more biomaterials are deposited on a conductive substrate.

6. The piezoelectric bio-organic film of claim 5, wherein the conductive substrate comprises a substrate material and a conductive electrode, and wherein the substrate material comprises silicon, mica, glass, plastic and steel, or any combination thereof; the conductive electrode comprises gold, silver, magnesium, molybdenum and copper, or any combination thereof.

7. The piezoelectric bio-organic film of claim 5, wherein a polymer is coated on the piezoelectric bio-organic film before removal of the piezoelectric bio-organic film from the conductive substrate.

8. The piezoelectric bio-organic film of claim 7, wherein the polymer comprises polyvinylidene fluoride, polydimethylsiloxane and polylactic acid, and wherein the polymer is coated on the piezoelectric bio-organic film to facilitate removal of the piezoelectric bio-organic film by direct peeling off from the conductive substrate.

9. A method for fabricating the piezoelectric bio-organic film of claim 1, the method comprising: providing a homogenous solution of biomaterials; applying an electric field to the homogenous solution for overcoming surface tension of an aqueous portion of the homogenous solution to produce numerous nanodroplets containing the biomaterials; performing homogenous nucleation on the nanodroplets until nanocrystals are formed.

10. The method of claim 9, wherein said providing the homogenous solution of the biomaterials comprises dissolving the biomaterials into a solution followed by mixing under an elevated temperature until a homogenous solution is formed.

11. The method of claim 9, wherein the biomaterials are one or more of glycine, L-alanine, DL-alanine, DL-threonine, DL-leucine, and L-Phenylalanine-L Phenylalanine.

12. The method of claim 9, wherein said applying the electric field to the homogenous solution is through an electrohydrodynamic jet platform.

13. The method of claim 12, wherein the electrohydrodynamic jet platform comprises a needle with a syringe, a syringe pump, a power supply and an X-Y movement platform.

14. The method of claim 13, wherein the homogenous solution is introduced to the syringe of the electrohydrodynamic jet platform for subsequent atomization under an electric stimulation.

15. The method of claim 14, wherein the needle of the syringe is connected to the power supply, and the power supply provides an electric field to the needle of the syringe for generating a liquid jet.

16. The method of claim 13, wherein the X-Y movement platform is fully automated.

17. The method of claim 16, wherein a conductive substrate acting as a ground electrode is fixed on the fully automated X-Y movement platform and disposed at a distance from the needle tip of the syringe for deposition of nanocrystals of the biomaterials.

18. The method of claim 17, wherein the conductive substrate comprises a substrate material selected from silicon, mica, glass, plastic, steel, or any combination thereof, and a conductive material for forming a conductive electrode on said substrate selected from gold, silver, magnesium, molybdenum, copper, or any combination thereof, and wherein the conductive electrode is polished.

19. The method of claim 17, wherein the bio-organic films are further coated with a polymer to facilitate removal of the bio-organic films from the conductive substrate to obtain a freestanding bio-organic film, and wherein the polymer is selected from polyvinylidene fluoride, polydimethylsiloxane, or polylactic acid.

20. A piezoelectric device comprising the bio-organic film according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0058] The appended drawings, where like reference numerals refer to identical or functionally similar elements, contain figures of certain embodiments to further illustrate and clarify the above and other aspects, advantages and features of the present invention. It will be appreciated that these drawings depict embodiments of the invention and are not intended to limit its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0059] FIG. 1 shows a flowchart of a fabrication method of the bio-organic films according to certain embodiments of the present invention;

[0060] FIG. 2 shows an image of electrohydrodynamic jet platform used in fabrication of the bio-organic films according to certain embodiments of the present invention;

[0061] FIG. 3 schematically depicts an atomization process used in fabrication of the bio-organic films according to certain embodiments of the present invention;

[0062] FIG. 4 schematically depicts a piezoelectric bio-organic film formed on the conductive substrate according to certain embodiments of the present invention;

[0063] FIG. 5A shows a scanning electron microscope (SEM) image of the surface topography of β-glycine nanocrystal film;

[0064] FIG. 5B shows an SEM image of the cross-section of β-glycine nanocrystal film on the silicon substrate; thickness of the film: ~4 .Math.m;

[0065] FIG. 6A shows X-ray diffraction (XRD) spectrum obtained from the as-fabricated films of β-phase glycine according to certain embodiments of the present invention;

[0066] FIG. 6B shows Raman spectrum obtained from the as-fabricated films of β-phase glycine according to certain embodiments of the present invention;

[0067] FIG. 7A shows piezoresponse force microscopy (PFM) mapping of out-of-plane (OOP) amplitude of the β-glycine film fabricated according to certain embodiments which is overlaid on 3D topography;

[0068] FIG. 7B shows a correlation between the OOP amplitude and applied AC voltage on the β-glycine film;

[0069] FIG. 8A shows the piezoelectric voltage response of the β-glycine films deposited on a PMMA substrate coated with Au electrode fabricated according to certain embodiments of the present invention in a tapping test; film thickness: ~5 .Math.m;

[0070] FIG. 8B shows the results of a reversed connection tapping test with respect to that in FIG. 8A;

[0071] FIG. 9A shows the piezoelectric current response of the β-glycine films according to certain embodiments of the present invention with a thickness of approximately 5 .Math.m in a tapping test;

[0072] FIG. 9B shows the results of a reversed connection tapping test with respect to that in FIG. 9A;

[0073] FIG. 10A shows the in situ XRD test results of the β-glycine films fabricated according to certain embodiments of the present invention;

[0074] FIG. 10B shows the piezoelectric voltage response of β-glycine films fabricated according to certain embodiments of the present invention with a thickness of approximately 5 .Math.m upon 24,000 tapping cycles.

[0075] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0076] It will be apparent to those skilled in the art that modifications, including additions and/or substitutions, may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation.

Fabrication of Piezoelectric Bio-organic Thin Films on Conductive Substrate

[0077] Turning to FIG. 1, the fabrication of the present bio-organic films from biomaterials using in situ electric filed and nanocrystallization process to form the bio-organic films on a substrate is provided as a flowchart, including preparing a homogenous glycine solution, atomization of the homogenous glycine solution into nanodroplets, nucleation and nanocrystallization of nanodroplets to form glycine nanocrystals, coating a conductive solution on the substrate as an electrode, and depositing the glycine nanocrystals.

[0078] Initially, a 10% w/v of glycine powder is dissolved in deionized water, and mixed by stirring under magnetic rotator at 60° C. for 3 hours until a homogenous solution is obtained. The solution is covered in order to avoid the formation of a glassy solid layer at the air-solution interface during the preparation process. The as-prepared mixture solution will then be directly used for film growth.

[0079] The obtained glycine aqueous solution is then transferred to an electrohydrodynamic jet platform for subsequent film growth. An image of the electrohydrodynamic jet platform is provided in FIG. 2. In certain embodiments, the electrohydrodynamic jet platform consists of a needle (e.g., a stainless steel needle), a syringe pump, a power supply, and a X-Y movement platform controlled by computer. The obtained glycine aqueous solution is pumped by the syringe pump into the syringe needle of the electrohydrodynamic jet platform. An electric field is applied to the needle in order to overcome the surface tension of the aqueous solution to produce numerous nanodroplets, leading to the formation of nanocrystals. The nanocrystals are then formed by homogeneous nucleation, which makes it easier to control the crystallization process by the electric field. The in-situ electric field in the crystal growth process facilitates domain orientation alignment of nanocrystals across the entire film.

[0080] At the same time, the substrate on which the bio-organic films will form is first coated with a conductive electrode, e.g., Au electrode. The electrode can be coated on the substrate by sputtering or any other coating method commonly used by skilled artisan in the same field.

[0081] Finally, piezoelectric bio-organic films are formed on the conductive electrode. To facilitate removal from the conductive electrode to obtain a free-standing piezoelectric bio-organic films, the piezoelectric bio-organic films can be coated with a polymer which allows direct peeling off the resulting bio-organic films from the conductive electrode.

[0082] Turning to FIG. 2, a customized electrohydrodynamic jet platform is provided for preparing glycine nanocrystals and deposition on the substrate with a conductive electrode according to certain embodiments of the present invention. In the setup shown in FIG. 2, an atomizing needle, with an outer/inner diameter of 0.31/0.16 mm, is inserted in epoxy resin for fixation and electrical insulation. The outlet of the needle is connected via metal wire to a high voltage power supply which is used to provide the electric field force for a liquid jet formation (4-4.5 KV). The inlet of the needle is connected to a microinjection pump via silicone rubber tube through which the solution is pumped (0.5-1.2 .Math.L/min). A polished aluminum plate with a dimension of 100 mm×100 mm×5 mm, acting as a ground electrode, is fixed on the computer-controlled X-Y movement platform and earthed reliably. The distance between the polished aluminum plate and needle tip is about 4-6 mm.

[0083] FIG. 3 depicts the atomization process used in the present invention, which the electric field applied to the needle overcomes the surface tension of the aqueous solution to produce numerous nanodroplets, leading to the nucleation and crystallization of β-glycine nanocrystals. The nanodroplets are affected by the force of viscous normal force, coulomb shearing force, electrical shearing force, coulomb normal force, gravitational force gravitational normal force and viscous shearing force.

[0084] FIG. 4 illustrates a basic structure of a piezoelectric bio-organic thin film formed on the conductive substrate including an n-type single crystal silicon, sputtered gold electrode and deposited glycine polycrystalline film. The electrode coating method includes sputtering using metal target and spin-coating using conductive paste. The glycine polycrystalline film is deposited by the customized electrohydrodynamic jet platform as shown in FIG. 2.

Characterization of Bio-organic Films

[0085] Turning to FIG. 5A, the scanning electron microscope (SEM) image of the surface topography of β-glycine nanocrystal film shows that the architecture of glycine nanocrystal film resembles the structure of inorganic ceramics. The microstructures of the surfaces of SIS are observed with scanning electron microscopy (SEM; FEI Quanta 450). Before the SEM measurements, all the samples are deposited with the silver electrode by magnetron sputtering (Q150TS). In FIG. 5A, an average grain size of the nanocrystals is measured to be with a range of 100 to 800 nm.

[0086] FIG. 5B shows an SEM image of the cross-section of β-glycine nanocrystal film deposited on a silicon substrate. The thickness of the film is about 4 .Math.m. From the SEM image, the deposited bio-organic film exhibit a uniform and compact distribution of nanocrystals, which may improve the piezoelectric performance.

[0087] Turning to FIG. 6A, an X-ray diffraction (XRD) spectrum obtained from as-fabricated films exhibit major characteristic peaks (at about 23.6° and 28.6° two-theta) of β-phase glycine, and no diffraction peaks from other phases could be observed, confirming that the as-fabricated film is dominated by the piezoelectric β-glycine crystals. The X-ray diffraction (XRD) pattern is obtained by a wide-angle X-ray diffractometer in the range of 10-50° (Ultima VI). The crystal structure of the β-phase glycine is further evidenced by Raman spectroscopy. FIG. 6B shows that four distinct Raman shifts of β-phase glycine crystals are observed at wavenumber of 1321, 1409, 2953 and 3009 cm.sup.-1, respectively.

[0088] Turning to FIG. 7A, piezoresponse force microscopy (PFM) is used in this study to quantify biological piezoelectricity in glycine film, by applying an AC voltage through the conductive atomic force microscopy (AFM) tip to excite the piezoelectric vibration of the sample. In FIG. 7A, the PFM mapping of out-of-plane (OOP) amplitude of the glycine film overlays on 3D topography in a 2×2 .Math.m.sup.2 area, exhibiting superb and uniform piezoelectric response of compact nanocrystals. PFM measurement in this study is conducted with an Asylum MFP 3D system. The used probe is an Asylum Research Arrow EFM with Pt/Ir coating on both cantilever and tip. The nominal resonance frequency of the probe was 75 kHz, and the tip radius is 33±10 nm according to the manufacturer. The inverse optical lever sensitivity and spring constant are calibrated before all measurements using Asylum’s software GetReal.

[0089] Turning to FIG. 7B, to further quantify the piezoelectric strength of the glycine film, the PFM amplitudes averaged over the mapping under applied AC voltage of 0.5, 1, 2, 3, 4, 5 V, respectively, are measured in Dual AC Resonance Tracking (DART) mode. DART mode is used to reduce noise and topography crosstalk when determining the phase responses of the array. The probe is excited at 1 V amplitude and contact resonance frequency is typically around 300 kHz. To determine the effective piezoelectric coefficient d.sub.33, an area of 300×300 nm.sup.2 is scanned under AC voltages between 0.5 and 5 V. The average vibration amplitudes in each scanned area are recorded to calculate the effective d.sub.33. As shown in FIG. 7B, the OOP amplitude yielded increases linearly as a function of the applied AC voltage, and the slope yields the effective shear piezoelectric coefficient around 11.2 pm/V. In all PFM scans, the probe is typically pressed on the sample surface with a force of about 100 nN.

[0090] FIG. 8A shows the piezoelectric voltage response of glycine films deposited on the PMMA substrate coated with Au electrode. The film thickness of the sample in this study is about 5 .Math.m, and a PMMA plate coated with Au is served as the top electrode. In the tapping mode, when a compressive force is loaded under a tapping process with a frequency of 30 Hz, high open-circuit voltage output occurs. Under 10 N applied tapping force, the output voltage reaches 15 V, which is superior to any piezoelectric bio-organic materials previously reported. FIG. 8B shows a reversed connection test of the tapping test as in FIG. 8A, and the results demonstrate that all outputs are reversed. The switching-connection test excludes the errors from the variation of contact resistance or parasitic capacitance and confirms that the detected electrical signal is truly from the piezoelectric glycine films. The piezoelectric output voltage is measured by a digital oscilloscope (Rohde & Schwarz RTE1024).

[0091] Turning to FIG. 9A, the piezoelectric current response of as-fabricated glycine films with a thickness of approximately 5 .Math.m is tested. The piezoelectric output current is measured by a low-noise current preamplifier (Stanford Research SR570). Under 10 N applied tapping force, the output voltage reaches 4 .Math.A, which is an order of magnitude larger than any piezoelectric bio-organic materials previously reported. Similar to the reverse connection setup used in FIG. 8B, the reversed connection test results in FIG. 9B show that all outputs are reversed with respect to those in FIG. 9A.

[0092] Turning to FIG. 10A, besides excellent piezoelectric responses, the bio-organic β-glycine films as-fabricated also exhibit excellent thermostability and superb durability in longtime deformation. The in situ XRD test result in FIG. 10A shows that the bio-organic β-glycine films exhibits anomalous stability without phase transition until melting (at a melting temperature of about 180° C.) resulting from nanoconfinement effects. FIG. 10B shows durability of β-glycine films by measuring the piezoelectric voltage response in longtime deformation cycles. Upon 24,000 tapping cycles, the output voltage of the piezoelectric β-glycine films is kept unchanged. In this study, the bio-organic β-glycine films have a uniform thickness of approximately 5 .Math.m.

[0093] In summary, the present bio-organic films and the related fabrication method have the following characteristics and advantages: [0094] 1) The bio-organic films exhibit an outstanding piezoelectric property, as well as anomalously excellent thermodynamic stability resulting from the nanoconfinement effect; [0095] 2) The fabrication method of bio-organic films is extricated from the interface dependency of traditional self-assembly methods due to the homogeneous nucleation of β-glycine nanocrystals; [0096] 3) The flexibility of the present invention to produce films with variable sizes, programmable structures, and diverse materials forms increases the potential of biomaterials as materials of piezoelectric thin film in a wide variety of products like piezoelectric ceramics.

[0097] Although the invention has been described in terms of certain embodiments, other embodiments apparent to those of ordinary skill in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims which follow.

INDUSTRIAL APPLICABILITY

[0098] Due to superb piezoelectricity, excellent thermostability, biocompatibility, accessibility, and environmental sustainability, the present invention can be used as high-performance implantable sensors, actuators, energy harvesters.

[0099] The present fabrication method for the piezoelectric bio-organic films based on the electric field-driven nanoconfinement technique can also be applied to other biomaterials, achieving excellent piezoelectric output performance.

[0100] Various tunable parameters of the bio-organic films provide a flexibility of the present invention to fit into different applications such as flexible and wearable electronics with irregular shape or size.

REFERENCES

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