Communication Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
pubs.acs.org/crystal
Longitudinal Piezoelectricity in Orthorhombic Amino Acid Crystal Films Sarah Guerin,†,‡ Syed A. M. Tofail,*,†,‡ and Damien Thompson*,†,‡ †
Department of Physics and ‡Bernal Institute, University of Limerick, Limerick V94 T9PX, Ireland
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S Supporting Information *
ABSTRACT: The symmetry of orthorhombic amino acid single crystals precludes the existence of a longitudinal piezoelectric response. Here we report the growth of amino acid crystal films on conductive substrates that demonstrate measurable longitudinal piezoelectricity of the same order of magnitude as large piezoelectric biopolymers. Crystal films of the nonproteinogenic amino acid hydroxyproline show a response of the same magnitude as quartz single crystals. This response is deconstructed using density functional theory single crystal predictions and cross-sectional electron microscopy. These results verify that amino acid films can serve as simple piezoelectric transducers, which could be used for a variety of energy harvesting applications.
Statement of Urgency and Significant Findings. Recent publication of our findings on the piezoelectric properties of the glycine polymorphs, and the monoclinic amino acids, has opened up new avenues for practical biopiezoelectric sensing and energy harvesting.1 Rapid dissemination of the piezoelectric nature of the orthorhombic amino acids is of great importance to advance the understanding and exploitation of undoped amino acid crystals for device applications. The observation reported here of longitudinal piezoelectricity, which is otherwise forbidden in isolated single crystals, presents amino acid films as simple, easy to implement, high performance transducers. The significant magnitude of the piezoelectric response reported here places amino acids at the top of the hierarchy of electromechanical conversion in biological materials. Piezoelectricity is a property of all non-centrosymmetric crystals,2 whereby they strain linearly in the presence of an electric field and generate surface charge proportional to an applied stress.3 It has been demonstrated in a plethora of organic materials since the 1950s, including bone,4 wood,5 shells,6 peptides,7 proteins,8 and viruses.9 Biomolecular materials are of particular interest for energy harvesting applications,10,11 as their dielectric constants are 2 orders of magnitude lower than widely used perovskite piezoelectric materials,12 allowing for much higher voltage outputs per unit force and area. Amino acids are the building blocks of all biological materials.13 Their naturally occurring L-enantiomers, with the exception of L-tryptophan14 (triclinic, P1 symmetry), crystallize in either monoclinic or orthorhombic forms.15 This endows the majority of amino acid crystals with piezoelectric and nonlinear optical (NLO) properties.16−18 Here we present amino acid crystal films as simple piezoelectric transducers, which demonstrate a modest longitudinal piezoelectric response perpendicular to the film surface. We substantiate © XXXX American Chemical Society
this response using density functional theory (DFT) calculations, which allow us to predict the elastic, dielectric, and piezoelectric constants of crystals to a high accuracy.19 We present the predicted material properties of the seven orthorhombic L-amino acid crystals. Their single crystal P212121 symmetry (space group 19, class 222) allows for three shear piezoelectric strain constants: d14, d25, and d36 but no longitudinal piezoelectric coefficients, such as d11, d22, or d33. These data serve to guide experiments that explore and utilize the piezoelectric properties of amino acids and derivatives, as demonstrated herein for threonine and hydroxyproline films. This work also serves as a further advance in the use of DFT calculations to predict piezoelectricity in more complex crystalline aggregates. DFT is a quantum mechanical computational model that is used to investigate the electron structure of atoms and molecules. It is based on the assertion that the total ground state energy of a many-electron system is a functional of the electron density. To obtain the piezoelectric charge tensor, we use perturbation theory, which allows the calculation of the second derivative of the ground state energy (E) with respect to applied electric field (ϵ) and strain (η): eαj =
dPα ∂ 2E = ∂ϵαδηj dum
where P is polarization, um is atomic displacement, j is the direction of induced strain, and α is the direction of applied electric field. Received: May 31, 2018 Revised: July 8, 2018 Published: July 31, 2018 A
DOI: 10.1021/acs.cgd.8b00835 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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(XRD) was used to confirm the film compositions (Supporting Information), and scanning electron microscopy (SEM) was used to characterize the cross section of the films, showing the different orientations of single crystals within the aggregate structures (Figure 2). We note the broad diffraction peaks for threonine crystal films, which is consistent with the high degree of polycrystallinity observed in these samples. SEM images verify the random in-plane orientation of hydroxyproline crystals. By contrast, threonine crystals selfassemble into more controlled dispersive needle structures in a number of “islands” across the film. Both films had a thick, dense crystalline outer ring due to the “coffee-stain effect” associated with drop casting of solution.28,29 The effective longitudinal (d33) piezoelectric constants of the polycrystalline amino acid films were measured using a commercial piezometer with an accuracy of 0.01 pC/N. Hydroxyproline crystal films give consistent piezoelectric constants of approximately ±1 pC/N (Table 1), with a maximum recorded value of 2.48 pC/N. Threonine films showed a piezoelectric response an order of magnitude lower than hydroxyproline films, of approximately ±0.1 pC/N. The nonzero value of this modest response was confirmed by a polarity switch on inversion of the sample. Though small compared to other measurements of piezoelectric response in undoped amino acid single crystals,1,18,23 it is of similar magnitude to the typical shear response of biopolymers.31 This result is exciting, as a d33 response is forbidden by the orthorhombic symmetry of the single crystals that make up these films. This suggests that amino acid networks obey classical piezoelectric rules, whereby each crystal can be thought of as a polarization vector, or electrical dipole. The growth of the polycrystalline aggregates is such that the effective piezoelectric response is a vector sum of a multitude of randomly orientated dipoles, which could indeed result in an overall longitudinal polarization. This is substantiated by recent d33 measurements of 18 pC/N in aligned diphenylalanine crystal structures.7 Table 2 shows the predicted piezoelectric strain constants for the seven orthorhombic L-amino acid single crystals, calculated using DFT (i.e., the piezoelectric properties of an individual crystal if extracted from the film, or grown in isolation). The values range in absolute magnitude from 0.5 pC/N (glutamate) to 28 pC/N (hydroxyproline). We very recently experimentally verified the high predicted d25 value for hydroxyproline single crystals23 and discussed its contribution to the piezoelectric response of collagena well-known biopolymer, and the main protein in connective tissue. The second highest predicted piezoelectric strain constant is a d25 value of −9.73 pC/N in tyrosine single crystals, which matches the longitudinal piezoelectricity calculated and measured in rhombohedral γ-glycine crystals32,33 and is higher than the piezoelectric constants of widely used inorganic materials aluminum nitride34−36 and quartz.37−39 The films chosen for experimental investigation, threonine and hydroxyproline, have predicted shear piezoelectric constants ranging from 3.4 pC/N to 28 pC/N in single crystal form, an order of magnitude higher than their respective measured polycrystalline d33 values. If we convert these single crystal piezoelectric strain constants, (dij) to piezoelectric voltage constants (gij), we get values (Table 3) of the same order as barium titanate (BaTiO3) and PZT ceramics,12,40 due to their predicted dielectric constant values of 2.3−2.6.
We recently showed that calcite (Icelandic spar) can generate small but nonzero piezoelectricity of 0.03 pC/N,20 despite being presumed to belong to a centrosymmetric space group. The work presented here also substantiates the longitudinal piezoelectricity recently observed in polycrystalline films of the globular protein, lysozyme wherein a longitudinal piezoelectricity is also forbidden in isolated single crystals.21,22 The piezoelectricity measured here in polycrystalline films is far larger than calcite and on the order of 1 pC/N. While weaker than the order of 10 and 100 pC/N1,23 measured in single crystals, the presence of longitudinal piezoelectricity suggests that amino acid films are also pyroelectric.24 Amino acid films were grown from saturated aqueous solutions following literature procedures.25−27 Bright field optical microscopy showed remarkable film growth dynamics, particularly when compared to the relatively slow growth of single crystals. Threonine crystals grew as dense, high aspect ratio needle clusters that radiated from a number of nucleation sites over a 2 cm × 2 cm area (Figure 1). Hydroxyproline crystals were very similar in size and aspect ratio in film form to single crystals,27 but grew in dense, warped clusters, with few isolated fully formed orthorhombic crystals (Figure 1). Single crystals of each amino acid formed within the respective drop cast solutions in less than 1 h, and films formed after complete droplet evaporation over 24 h. Transmission X-ray diffraction
Figure 1. (a) Threonine and (b) hydroxyproline crystal films grown from aqueous solution. B
DOI: 10.1021/acs.cgd.8b00835 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. SEM characterization of orthorhombic amino acid crystal films. SEM images of (a) hydroxyproline single crystal with longitudinal 3 axis perpendicular to the substrate, (b) threonine crystal clusters orientated at various acute angles to the substrate, (c) internal structure of an individual hydroxyproline single crystal, showing inner pores known to increase the piezoelectric response of inorganic materials,30 (d) smaller, hollow threonine crystal clusters growing perpendicularly to the substrate.
Table 1. Longitudinal Piezoelectric Measurements on Orthorhombic Amino Acid Crystal Films Grown on Copper Substrates
Table 3. Predicted Piezoelectric Voltage Constants (gij) and Electromechanical Coupling Constants (kij) for the Seven Orthorhombic L-Amino Acidsa
crystal film
position
1
2
3
crystal
g14
g25
g36
k14 (%)
hydroxyproline
upright inverted upright inverted
1.28 −1.46 0.16 −0.09
0.68 −0.97 0.09 −0.07
1.06 −1.12 0.09 −0.08
threonine tyrosine glutamine glutamate serine hydroxyproline alanine
0.18 0.24 0.09 0.03 0.22 0.18 0.30
0.16 0.46 0.18 0.08 0.20 1.31 0.18
0.23 0.27 0.54 0.15 0.14 0.21 0.30
7 9 5 1 12 7 10
6 13 8 3 8 27 7
PZT12 AlN42 ZnO43 BaTiO311
0.025 0.037 0.16 0.013
(g33) (g33) (g33) (g33)
75 23 48 50
(k33) (k33) (k33) (k33)
threonine
Table 2. Predicted Piezoelectric Strain Constants for the Seven Orthorhombic L-Amino Acidsa crystal
d14
d25
d36
threonine tyrosine glutamine glutamate serine hydroxyproline alanine
3.78 −5.00 −1.85 0.54 4.69 3.72 −6.26
−3.40 −9.73 −3.78 −1.60 4.33 −27.75 −3.78
−4.90 −5.81 −11.40 −3.18 −3.00 4.55 −6.30
a
k25 (%)
k36 (%) 12 8 30 8 3 7 11
All gij values are in V m/N.
in needle-shaped crystals, which also grow out of plane (Figure 2), reducing structural distortion. Epitaxial or intercrystalline strain could slightly alter the symmetry of the single crystals within the film to allow a net longitudinal polarization. Another possibility lies in the porous structure of the single crystals, as seen in Figure 2c,d. The internal structure of the hydroxyproline crystals appears to contain a number of voids, and the threonine crystals appear almost hollow in polycrystalline aggregate form. This could be an indicator of ferroelectret behavior, most commonly observed in cellular polymers. A longitudinal piezoelectric effect manifests due to individually charged voids, as opposed to surface charge on the film. The weak response observed in these films is consistent with their high Young’s moduli,41 and the lack of poling in this work. Verifying such ferroelectret behavior and potentially increasing the piezoelectric response reported here with the application of a high external electric field is an exciting area of future work.
a
All values are in pC/N.
For the polycrystalline threonine films, the modest d33 values measured using the piezometer are much less than their DFT predicted single crystal shear piezoelectric constants of 4−6 pC/N (Table 2). Threonine crystal films in particular are quite thin, and the quality of the response could potentially be improved by using different growth techniques or electrical contacting methods. By contrast, the longitudinal response of hydroxyproline films is relatively high. Looking at the bright field microscopy images in Figure 1, it is clear that hydroxyproline crystals deform more during film formation than threonine. While the plate-like structure of the hydroxyproline crystals results in stronger in-plane growth, which induces stress on neighboring crystals, threonine grows C
DOI: 10.1021/acs.cgd.8b00835 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Table 4. Predicted Elastic Stiffness Constants for the Seven Orthorhombic L-Amino Acid Crystalsa coefficient
threonine
tyrosine
glutamine
glutamate
serine
hydroxy-proline
alanine
c11 c22 c33 c44 c55 c66
67.4 −161.3 13.9 7.3 5.1 11.5
16.2 34.2 44.1 5.2 3.7 5.5
25.6 34.6 36.7 8.1 11.1 16.8
37.9 41.1 22.5 7.4 9.4 13.2
27.7 23.1 58.9 12.6 6.3 2.3
14.8 16.8 31.1 8.1 −2.3 7.7
67.4 16.5 25.8 6.9 8.7 7.0
a
All values are in GPa.
casting method. XRD, SEM, and bright field microscopy were used to characterize the films, revealing the orientation of crystalline aggregates on the surface. The observed longitudinal response matches the response of larger biopolymers, such as cellulose5 and DNA,46 supporting the hypothesis that amino acids have the highest inherent piezoelectric response in the biological hierarchy.1,15,21,47 The presence of a longitudinal piezoelectricity in amino acid films, substantiated by DFT calculations, indicates a spontaneous polarization or pyroelectricity in such aggregates. This opens up the use of amino acid films as a convenient source of both piezoelectric and pyroelectric materials.
The effective longitudinal piezoelectricity in hydroxyproline crystal films is of similar magnitude to X-cut quartz and could be exploited for sensing and energy harvesting applications. These films could theoretically generate an open circuit piezoelectric voltage as high as γ-glycine1 or phage viruses.9 The predicted single crystal piezoelectric voltage constants are shown in Table 3. The orientation of the hydroxyproline crystals (shown in the SEM images, Figure 2) is with the longitudinal 3 axis perpendicular to the substrate, whereas the threonine crystals are in randomly orientated clusters. The alignment of the hydroxyproline crystals is more likely to favor a measurable piezoelectric response, due to alignment of the induced single crystal dipole moments when stressed. Table 3 also shows the single crystal electromechanical coupling constants for the amino acids, which indicates the conversion efficiency between electrical and mechanical energy. The k-values range from 1% to 30%. The maximum k-values for commercial piezoelectrics is shown for comparison, all of which are longitudinal k33 values. As the amino acid values correspond to shear electromechanical coupling, they are expectedly lower than the commercial k33 values, which range from 23% for AlN, to 75% for PZT. However, both hydroxyproline and glutamine have k-values that exceed that of AlN, indicating that they would be suited to resonator technologies. Generally speaking, the relatively low k-values, and very high g-values highlight that amino acid crystals would be best suited as microgenerators for self-powered electronics and medical devices. Prediction of elastic constants is strongly desirable for piezoelectric and pyroelectric device applications.40,44 Table 4 shows the predicted elastic stiffness constants for seven orthorhombic L-amino acid crystals. The piezoelectric strain constants dij are inversely proportional to the elastic stiffness constants. To calculate the predicted piezoelectric constants for orthorhombic amino acids, the most important elastic stiffness constants are c44, c55, and c66. These range from 2 to 17 GPa across all of the amino acids presented. The c66 value predicted for glutamine of 17 GPa is the highest predicted shear elastic constant in this work. We note the very high negative c22 value of 161 GPa in threonine single crystals, which is the highest predicted amino acid crystal elastic constant to date,1,18,41 of similar magnitude to the c11 elastic constant of calcite.20 We attribute this mechanical strength to the second chiral center of the threonine molecules, which appears to provide extra stability along the 2 axis during growth. The only other amino acid with a predicted elastic constant greater than 100 GPa is aspartate,1 from which threonine is synthesized.45 The negative sign could indicate mechanical instability, but experimental investigation is needed to understand this further. In conclusion, crystalline films of orthorhombic L-amino acids were prepared for the first time using a simple drop
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00835. DFT calculation details, characterization specifications (methodology) and transmission XRD spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(D.T.) E-mail:
[email protected]. *(S.A.M.T.) E-mail:
[email protected]. ORCID
Sarah Guerin: 0000-0002-2442-4022 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This publication has emanated from research conducted with the financial support of Science Foundation Ireland (SFI) and is cofunded under the European Regional Development Fund under Grant Number 13/RC/2073. D.T. acknowledges support from SFI under Grant Number 15/CDA/3491 and for provision of computing resources at the SFI/Higher Education Authority Irish Center for High-End Computing (ICHEC).
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