Control of piezoelectricity in amino acids by ... · Control of piezoelectricity in amino acids by supramolecular packing Sarah Guerin, Aimee Stapleton, Drahomir Chovan, Rabah Mouras,

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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT5045

NATURE MATERIALS | www.nature.com/naturematerials 1

S1

Supplementary Information

Control of piezoelectricity in amino acids by

supramolecular packing

http://dx.doi.org/10.1038/nmat5045




S2

Control of piezoelectricity in amino acids by supramolecular packing Sarah Guerin, Aimee Stapleton, Drahomir Chovan, Rabah Mouras, Matthew Gleeson, Cian McKeown, Mohamed Radzi Noor, Christophe Silien, Fernando M. F. Rhen, Andrei L. Kholkin, Ning Liu, Tewfik Soulimane, Syed A. M. Tofail, Damien Thompson Contents Page Figure S1: Dispersion-correction induced external pressure S3 Figure S2: XRD Data S4 Figure S3: Bulk Polymorphic Transformations S5 Figure S4: SEM Image, β-glycine S6 Figure S5: SEM Image, γ-glycine S7 Figure S6: Piezometer Measurement S8 Figure S7: Crystal Slicing S9 Figure S8: Crystal/Probe Alignment S10 Figure S9: Monoclinic Amino Acids S11 Figure S10: Dispersion Corrected DFT Graphs S12 Figure S11: Dispersion Corrected DFT Graphs S13 Figure S12: DFT Energy Convergence S14 Figure S13: DFT Parameter Convergence S15 Figure S14: Piezoelectric Constant Schematic S16 Figure S15: β-glycine grown on PMMA S17

Table S1: DFT Benchmarking S18 Table S2: DFT Benchmarking Percentage Errors S19 Table S3: Physical Constants, L-Asparagine S20 Table S4: Physical Constants, L-Aspartate S21 Table S5: Physical Constants, L-Methionine S22 Table S6: Physical Constants, L-Cysteine S23 Table S7: Physical Constants, L-Isoleucine S24 Table S8: Physical Constants, L-Histidine S25 Table S9: Physical Constants, L-Leucine S26 Table S10: Piezoelectric Matrices, γ-glycine S27 Table S11: Piezoelectric Matrices, β-glycine S28 Table S12: Packing Densities, Glycine Polymorphs S29 Table S13: Dispersion Corrections, Unit Cell Parameters S30 Table S14: Dispersion Corrections, Elastic Constants S31 Table S15: Young’s Moduli Approximations, Glycine Polymorphs S32 Table S16: Piezoelectric Constants, Finite Differences S33

Note: Computational Benchmarking S34 Note: Effectiveness of DFT Predictions S36 Note: Example d16 Calculation from Resonance Data S37 Note: Exploiting Piezoelectricity in β-glycine S40 Note: Experimental Precautions for β-glycine Characterisation S42 Note: Grimme DFT-D3 Dispersion Corrections S44 References S45





S3

Supplementary Figure 1: Plot of external pressure on the theoretical unit cell with

increasing van der Waals cut off radius. At the dispersion convergence point of 12Å, both β

and γ-glycine have begun to transform into δ and ϵ glycine respectively (Reference 24 in the

main text).





S4

Supplementary Figure 2: XRD spectrum for evaporated glycine droplets, showing a

mixture of alpha and beta polymorphs (as imaged in main text Fig. 3c). β-glycine needles

could be extracted from the centre of the droplet based on visual inspection. Relative

intensity is in arbitrary units.





S5

Supplementary Figure 3: Bulk polymorphic transformations during β-glycine

crystallisation. a. α-glycine crystals cluster at the edges of glycine droplets and at the roots of

glycine needles within 24 hours of dropping an aqueous glycine solution. The red, green and

blue dots correspond to the red, green and blue spectra in the following panel. b. Raman

spectra showing the presence of α-glycine at three different points along the crystal face

shown in panel a. c. Over time, or if poorly grown, β-glycine needles become consumed by

their own transformation to the γ or α polymorph. The yellow, blue and pink dots correspond

to the yellow, blue and pink spectra in the following panel. d. Raman spectra showing the

presence of γ-glycine at three different points along the crystal face shown in panel b. The

crystal had been sliced from its substrate approximately 6 hours prior to measurement.

a b

c d

S5

Supplementary Figure 3: Bulk polymorphic transformations during β-glycine

crystallisation. a. α-glycine crystals cluster at the edges of glycine droplets and at the roots of

glycine needles within 24 hours of dropping an aqueous glycine solution. The red, green and

blue dots correspond to the red, green and blue spectra in the following panel. b. Raman

spectra showing the presence of α-glycine at three different points along the crystal face

shown in panel a. c. Over time, or if poorly grown, β-glycine needles become consumed by

their own transformation to the γ or α polymorph. The yellow, blue and pink dots correspond

to the yellow, blue and pink spectra in the following panel. d. Raman spectra showing the

presence of γ-glycine at three different points along the crystal face shown in panel b. The

crystal had been sliced from its substrate approximately 6 hours prior to measurement.

a b

c d





S6

Supplementary Figure 4: Morphologies of the glycine polymorphs as seen in this Scanning

Electron Micrograph (SEM) for a cleaved β-glycine needle, lying at ninety degrees to the

plane of growth. Needles were sliced to make samples for transverse shear resonance

measurements.





S7

Supplementary Figure 5: Morphologies of the glycine polymorphs as seen in this Scanning

Electron Micrograph (SEM) for a γ-glycine seed crystal used for energy harvesting

experiments (main text Figure 4).





S8

Supplementary Figure 6: Quasi-static direct measurement of longitudinal piezoelectric

coefficient using a commercial Berlincourt type piezometer. The figure shows an example

measurement of the d11 coefficient of a γ-glycine crystal that is clamped between two silver

electrodes for simultaneous straining and electrical measurement. The meter (left) reads a

value of -1.7 pC/N. Our DFT calculations predicted a value of-1.6 pC/N for d11 in γ-glycine

(Figure 2b in the main text). Note the instrument readout is labelled d33 because the d11

coefficient is measured by rotating the crystals so that their crystallographic a-axis is in line

with the 3 axis of the meter.

a b





S9

Supplementary Figure 7: Schematic (left) and corresponding optical images (right) of

monoclinic β glycine crystals illustrating the crystal cut required for piezoelectrically coupled

electromechanical resonance to take place in the thickness shear mode. The ratio of the

thickness, width and length in the samples is around 1:12:30 and a typical slice measures

~200 µm width, ~700 µm length and <20 µm thickness. Microscopic slices from a ~6000 µm

long needle provide samples for the resonance piezoelectricity measurements shown in

Supplementary Figure 9.





S10

Supplementary Figure 8: Photograph of four microprobes with 3.5 micron radius tips in

contact with a sliced β-glycine needle, as viewed on an ESP MON21 CCTV monitor

connected to a Costar SI-C400N camera.





S11

Supplementary Figure 9: The seven other smallest (non-glycine) monoclinic L-amino acid

crystals, showing their crystal packing, monoclinic angle, and highest predicted piezoelectric

strain constant. Carbon atoms are coloured grey, hydrogen atoms are white, oxygen atoms are

red, nitrogen atoms are blue and sulphur atoms are yellow.





S12

Supplementary Figure 10: Convergence in calculated energy, volume, and lattice parameter

values with respect to the vdW cut-off radius used for dispersion corrected DFT calculations,

for β-glycine, γ-glycine, and aluminium nitride (see Supplementary Note p S45). Literature

experimental crystal volumes and lattice constants are from refs 20 and 21 in the main text

and supplementary reference 28.





S13

Supplementary Figure 11: Computed changes in predicted elastic constant values with

increasing vdW cut-off radius for β-glycine, γ-glycine and aluminium nitride (see

Supplementary Note p. S45).





S14

Supplementary Figure 12: Convergence of the calculated DFT energies of the α, β, and γ

glycine crystals respectively with respect to k-point sampling. The energies were obtained

from ionic relaxations performed using 1x1x1 to 8x8x8 k-point grids.





S15

Supplementary Figure 13: Graphs showing the convergence in predicted (top) piezoelectric

and (bottom) elastic constant values for (left) beta and (right) gamma glycine with respect to

k-point sampling.





S16

Supplementary Figure 14: Relationship between longitudinal (σ) and shear (τ) stress and

corresponding piezoelectric polarisation (P) acting along arbitrarily chosen mutually

orthogonal 1-2-3 axes for four piezoelectric strain constants, d22, d33, d16 and d14. Grey

shading indicates the corresponding surfaces on which the electrodes are contacted to

measure the polarisation.





S17

Supplementary Figure 15: A selection of images showing β-glycine needles grown on a

layer of PMMA. The PMMA was spincoated onto Kapton film and sprayed with ethanol1,2 to

encourage the growth of the β-polymorph.





S18

Supplementary Table 1: Calculated piezoelectric and elastic constants for aluminium

nitride, zinc oxide and α-quartz. Other theoretical and experimental results are shown for

comparison3-9. In cases where many studies exist, the values shown are averages across all

studies.





S19

Supplementary Table 2: Percentage errors in physical constants calculated directly by

VASP, when compared to experimental studies. Percentage errors for other DFT studies are

shown for comparison.





S20

Supplementary Table 3: Calculated piezoelectric charge tensor components eij (in units of

C/m2), strain tensor components dik (pm/V), voltage tensor components gij (mV m/N), elastic

constants (GPa), and dielectric constants (unitless) of L-Asparagine.

Charge Tensor (C/m2)

(0 0 0 0.13 0 −0.09

0.11 0.05 0.09 0 −0.05 00 0 0 −0.01 0 0.04

)

Strain Tensor (pm/V)

(0 0 0 12.0 0 −13.02.1 2.0 1.1 0 −4.6 00 0 0 −0.9 0 5.7

)

Voltage Tensor (mV m /N)

(0 0 0 490 0 530

110 100 60 0 240 00 0 0 40 0 240

)

Elastic Stiffness Constant (GPa)

c11

52

c22

25

c33

83

c44

11

c55

11

c66

7

Dielectric Constant (unitless)

ε1 2.76

ε2 2.19

ε3 2.70

εr 2.55





S21



constants (GPa), and dielectric constants (unitless) of L-Aspartic acid.


(0 0 0 0.03 0 −0.05

−0.08 −0.05 0.06 0 0 00 0 0 −0.13 0 0.15

)


(0 0 0 3 0 3

−0.73 −1.85 −1.82 0 0 00 0 0 13 0 10

)


(0 0 0 126 0 12637 94 93 0 0 00 0 0 575 0 442

)


ε1 2.69

ε2 2.21

ε3 2.55

εr 2.48


c11

109

c22

27

c33

33

c44

10

c55

11

c66

15





S22



constants (GPa), and dielectric constants (unitless) of L-Methionine.


(0 0 0 −0.01 0 0.09

0.08 0.18 0.07 0 0.02 00 0 0 0.03 0 0

)


(0 0 0 5 0 13

2.42 6.9 4.1 0 5 00 0 0 15 0 0

)


(0 0 0 228 0 594

106 304 181 0 220 00 0 0 694 0 0

)


ε1 2.48

ε2 2.57

ε3 2.44

εr 2.50


c11

33

c22

26

c33

17

c44

2

c55

4

c66

7





S23



constants (GPa), and dielectric constants (unitless) of L-Cysteine.


(0 0 0 −0.03 0 0.09

0.15 0.36 −0.04 0 0.01 00 0 0 0 0 −0.01

)


(0 0 0 10.7 0 10.64.1 11.4 2.9 0 2.6 00 0 0 0 0 1.2

)


(0 0 0 487 0 487

175 487 124 0 111 00 0 0 0 0 55

)


c11

37

c22

32

c33

14

c44

3

c55

4

c66

9


ε1 2.55

ε2 2.64

ε3 2.49

εr 2.56





S24



constants (GPa), and dielectric constants (unitless) of L-Isoleucine.


(0 0 0 0.0049 0 −0.11

−0.05 0.24 −0.02 0 0.03 00 0 0 0.01 0 0.02

)


(0 0 0 12.25 0 18.32.6 10 1.1 0 10 00 0 0 25 0 3.33

)


(0 0 0 587 0 876

122 469 52 0 469 00 0 0 1233 0 164

)


ε1 2.36

ε2 2.41

ε3 2.29

εr 2.35


c11

19

c22

24

c33

19

c44

0.4

c55

3

c66

6





S25



constants (GPa), and dielectric constants (unitless) of L-Histidine.


(0 0 0 0.04 0 −0.09

0.04 0.02 −0.04 0 −0.02 00 0 0 0.01 0 −0.01

)


(0 0 0 4 0 181.6 1.25 0.43 0 6.67 00 0 0 2.5 0 2

)


(0 0 0 183 0 82681 63 21.8 0 340 00 0 0 103 0 80

)


ε1 2.46

ε2 2.23

ε3 2.75

εr 2.48


c11

25

c22

16

c33

92

c44

4

c55

3

c66

5





S26



constants (GPa), and dielectric constants (unitless) of L-Leucine.


(0 0 0 0 0 0.01

0.06 −0.18 0.03 0 0.05 00 0 0 −0.02 0 0.01

)


(0 0 0 0 0 12.52.7 8.6 1.5 0 12.5 00 0 0 20 0 12.5

)

Voltage Tensor (mV m/N)

(0 0 0 0 0 593

126 400 70 0 580 00 0 0 976 0 611

)


ε1 2.38

ε2 2.43

ε3 2.31

εr 2.37


c11

22

c22

21

c33

20

c44

1

c55

4

c66

8





S27

Supplementary Table 10: Piezoelectric charge tensor components eij (in units of C/m2),

strain tensor components dik (pm/V, equivalent to pC/N – we use the two units

interchangeably), and voltage tensor components gij (mV m/N) of γ-glycine. The format of

the tensor table is described further in main text Methods. For comparison, the shear

piezoelectric strain constants of other biomaterials10 include cellulose (0.15 pC/N), amylose

(2.0 pC/N), collagen (2.0 pC/N), and keratin (1.8 pC/N).

γ-glycine Charge Tensor (C/m2)

(0.04 -0.04 0 -0.07 0.10 0.030.03 -0.03 0 0.09 0.07 -0.04-0.01 -0.01 0.81 0 0 0

)

γ-glycine Strain Tensor (pm/V)

(1.6 -1.1 0 -5.8 7.2 5.61.2 -1.1 0 7.5 5.6 -5.6-0.8 -0.7 10.4 0 0 0

)

γ-glycine Voltage Tensor (mV m /N)

(73 51 0 -266 331 25755 51 0 344 257 25735 31 455 0 0 0

)





S28

Supplementary Table 11. Piezoelectric charge tensor components eij (in units of C/m2),

strain tensor components dik (pm/V), and voltage tensor components gij (mV m/N) of β-

glycine. These matrices emphasise how precise sample preparation and orientation can

maximise output voltage along shear planes for device applications.

β-glycine Charge Tensor (C/m2)

(0 0 0 -0.12 0 0.26

0.10 -0.13 0.13 0 -0.08 00 0 0 0.01 0 -0.05

)

β-glycine Strain Tensor (pm/V)

(0 0 0 15.8 0 195

1.8 -5.7 1.9 0 5.1 00 0 0 1.3 0 7.5

)

β-glycine Voltage Tensor (mV m /N)

(0 0 0 658 0 813194 -296 98 0 265 00 0 0 57 0 328

)





S29

Supplementary Table 12: Computed Packing Densities and Monoclinic Angles of the three

glycine polymorphs. Numbers of atoms include hydrogen atoms.

Amino Acid

Number of Atoms

(and Molecules) in

Unit Cell11-13

Crystal Unit

Cell Volume

(Å3)

Atomic (and

Molecular) Packing

Densities

Atoms (molecules)/Å3

Monoclinic

Angle (°)

α-glycine 40

(4) 321

0.125

(0.0125)

Not

applicable

β-Glycine 20

(2) 164

0.122

(0.0122) 112

γ-glycine 30

(3) 245

0.122

(0.0122)

Not

applicable





S30

Supplementary Table 13: Analysis of the unit cell parameters of beta and gamma glycine

crystals calculated with and without dispersion corrections. Values in parentheses are

literature experimental values, as referenced in Supplementary Figure 10.

No Correction DFT-D3 a (Å) 5.13 5.06

(5.08) b(Å) 6.39 6.20

(6.27) β- c(Å) 4.99 5.39

(5.38) glycine Cell Volume

(Å3) 164.03 155.2 (157)

E0 (eV) -119.147 -120.392 a (Å) 7.16 6.95

(7.04) γ- b(Å) 7.16 6.95

(7.04) glycine c(Å) 5.52 5.48

(5.48) Cell Volume

(Å3) 244.82 229.27 (235)

E0 (eV) -178.712 -180.546





S31

Supplementary Table 14: Elastic constant values of AlN, calculated with and without

dispersion corrections. All values are in GPa. Values in parentheses are literature

experimental values, as referenced in Supplementary Table 1.

No Correction DFT-D3 c11 377 430 (378)

c12 130 122 (137)

Aluminium c33 358 363 (392)

Nitride c44 112 113 (122)

c66 123 154 (121)





S32

Supplementary Table 15: Calculation of the average Young’s Moduli from the DFT data

for the three ambient polymorphs of glycine, using three methods of approximation. All three

methods give reasonable match with known experimental values14 for α and γ-glycine, so an

arithmetic mean is taken over the three analysis methods to estimate the previously unknown

Young’s Modulus of β-glycine.All units are in GPa.

Analysis Method Alpha Beta Gamma

Voigt-Reuss15,16 28.32 19.37 24.58

Orthorhombic Approximation17 (Nye) 31.21 13.84 31.51

Triclinic Approximation17 (Nye) 30.00 13.22 31.27

Experiment14 33 - 28

Predicted Average Young’s Modulus 29.84 15.48 27.78





S33

Supplementary Table 16: Comparison of piezoelectric charge coefficients calculated with

Finite Differences and Density Functional Perturbation Theory (DFPT) methods. All units

are C/m2. Deviations between Finite Differences and DFTP range from 0-15%, and the

maximum deviation is used as our theoretical error in this work.

e21 e22 e23 e33 e14 e16 e25 e34 e36

Beta Finite Differences 0.09 -0.11 0.11 - -0.12 0.25 -0.11 0.01 -0.06

Glycine DFPT 0.10 -0.13 0.13 - -0.12 0.26 -0.08 0.01 -0.05

Gamma Finite Differences 0.03 -0.03 - 0.83 -0.06 0.03 0.06 - -

Glycine DFPT 0.03 -0.03 - 0.81 -0.07 0.03 0.07 - -





S34

Supplementary Note: Computational benchmarking against known inorganic piezoelectric

crystals

The accuracy of the computational methods used in the simulation of polarisation, elastic

stiffness and piezoelectricity has been benchmarked with respect to three well-known

inorganic piezoelectric materials, namely aluminium nitride (AlN), zinc oxide (ZnO) and α-

quartz (SiO2). We have compared elastic and piezoelectric coefficients of these materials

computed by us with those obtained in previous experimental and computational studies3-9.

Table S1 shows the calculated piezoelectric and elastic constants of α-quartz, zinc oxide and

aluminium nitride. Table S2 shows the percentage error in our values, and those of other DFT

studies, when compared to the experimental works referenced in Table S1.

For aluminium nitride, our calculation generally predicts better piezoelectric and elastic

coefficients than those reported in the literature. There is one exception, the e15 coefficient,

which deviates from the experimental value by ~37%. This however compares well with the

deviation (35%) of the values predicted by current literature. For zinc oxide, the agreement of

our predictions and those predicted and measured in the literature is also good (Table S1 and

S2). This lends further confidence to our predicted values for the glycine biomolecules (Table

1).

We note that the results for both zinc oxide and aluminium nitride are both highly dependent

on the number of k points used in the calculations, with an 8x8x8 mesh giving a better match

with experimental results. Both crystals have a wurtzite structure, and belong to the space

group P 63/mc (number 186). This emphasises the value of more extensive k point sampling

(which requires more computational expense) when calculating second order derivatives of

hexagonal crystals, as has been observed in other computational studies 18-20.





S35

In the third and final set of benchmarking, we again find our predicted coefficients in α-

quartz (P3121) matching experimental values better than those predicted by previous DFT

studies, except for the coefficients c44 and c66, for which Ren et al8 predicts slightly better.

Yet, our computed values deviate from experimental values of c44 and c66 only by 3% and 9%

respectively. Interestingly, we note that both the elastic and piezoelectric constants for α-

quartz exhibit negligible dependence on the number of k points used. As both α-quartz and γ-

glycine form trigonal crystals, the accuracy obtained by us in predicting coefficients of α-

quartz build further confidence in the predicted physical constants for glycine polymorphs

(Table 1 main text).





S36

Supplementary Note: The effectiveness of density functional theory predictions of

piezoelectric glycine polymorphs

The excellent quantitative correspondence of DFT predicted stiffness, permittivity and

piezoelectricity tensors with those obtained from experiment conducted in the present study

and in the literature highlights the potential that DFT calculations now demonstrate to

identify materials with significant piezoelectric response, and to estimate the expected

magnitudes of individual piezoelectric constants. The recent upsurge in interest in biological

energy harvesting (and very promising results)21 highlights the need for efficient candidate

screening with DFT. This can pave the way for an exciting and more diverse range of

bioelectronic components.

We use DFT to quantify all eighteen components in the piezoelectric matrices of two non-

centrosymmetric polymorphs of glycine. Such quantitative predictions are desirable to guide

experiments and in the discussion of technological applications. These matrices can be

further separated into their ionic and electronic contributions. Induced dipole moments are

made up of contributions from both distortions of the symmetrical electronic charge

distributions relative to the nucleus of each ion (electronic), and from displacements of the

entire ion from its normal lattice site (ionic).

The DFT values are calculated at 0 K, so it should be noted that systems could exhibit a

deviation between predicted DFT values and properties measured at room temperature,

especially if there is a phase transition between 0 and 300 K. No such deviation has been

observed in this work, neither for the well-characterised inorganic crystals (in benchmarking

the periodic DFT methods used for the organic crystals), nor in the experimental confirmation

of the glycine longitudinal strain coefficients.





S37

Supplementary Note: Example calculation in the determination of piezoelectric constants

e16 and d16 of β-Glycine following methodologies outlined in supplementary references 22,23

for resonance piezoelectricity measurements in a monoclinic crystal using thickness mode

vibration measurements.

For all calculations:

ρ is defined as the average density of β-glycine, taken as 1610 kg/m3

Fa, is the anti-resonance frequency at the maximum impedance

Fr is the resonance frequency at minimum impedance

For capacitance measurements, crystal slice dimensions were

Thickness, t = 5.21 μm

Width, w = 212 μm

Length, l = 692 μm

Area, A = Area of the XY plane (w*l)= 1.467 x 10-7 m2

A. Calculation of Relative Permittivity, εr

ε11= C11tA

= C11tw x l

From direct measurement, capacitance in the 11 direction, C11 = 0.8 pF, so that

ε11=0.8 pF x 5.21μm212μm x 692μm

= 2.84 x 10-11 F/m

Therefore,

εr= ε11

ε0=

2.84 x 10-11

8.85 x 10-12





S38

Where ε0= permittivity of the free space = 8.85x 10-12 F/m and

εr= 3.21

B. Calculation of Elastic Stiffness Constant, c66

For measurements of the 16 coefficient, the thickness: width: length ratio should not be more

than 1:12:30 in accordance with CENELEC standards. The following dimensions were used

in cutting the samples from the needles

Thickness, t = 8.43 μm = 8.43x10-6 m

Width, w = 117 μm =117x10-6 m

Length = 265 μm = 265x10-6 m

Shear compliance s66 is given by:

s66= 1

4ρFa2l2

= 1

4 x 1610 x (1626000)2x (265×10−6)2

=1.20 GPa

c66=0.84 GPa

C. Calculation of Electromechanical Coupling Constant, k16

k162= π

2

FrFa

tan (π2

∆FFa

)

= π2

409

1626 tan (π

2 12171626

)





S39

= 1.57 x 0.25 x 2.40

= 0.94

k16 = √0.94 = 0.97

D. Calculation of Piezoelectric Charge Constant, e16

e16 = k16 √ε11c66

= 0.97 √2.84 x 10−11 x 0.84 x 109

= 0.15 C/m2

E. Calculation of Piezoelectric Strain Constant, d16

d16= e16

c66

= 0.15 C 𝑚𝑚−2

0.84 GPa

= 178 pm/V





S40

Supplementary Note: Exploiting piezoelectricity in β-glycine for technological applications

At first glance, it is natural to assume that such a hidden piezoelectric response, in a

metastable crystal, would be extremely difficult to utilise for technological applications.

However in recent years much progress has been made in the stabilisation of the

microcrystals, in the hope of exploiting the effective longitudinal and shear piezoelectric

constants in β-glycine. Values ranging from 6-13 pm/V were measured using Piezoresponse

Force Microscopy (PFM) 24, and are discussed in the main text. So far, these needles have

been successfully stabilised on (111) platinum thin films25, and also made into β-glycine

fibres via electrospinning26. Both of these processes, as discussed in the respective papers,

have huge technological potential, but do not exploit our measured value of 178 pm/V.

Hence the challenge for us was the technological exploitation of our previously unknown d16

and g16 values of 178 pm/V and 8131 V m/N respectively. For this, the crystal would have to

be electroded on its x-axis, which has an average length of 200 micron. The easiest way to

induce a shear stress along the z-axis would be to bend the crystal. For a single β-glycine

crystal, or even a poly-crystalline needle aggregate, the crystal would not be expected to

withstand any significant manual force. Supplementary Figure 15 shows our attempts to grow

-glycine layers on a flexible substrate,27 but we expect that extensive further research will be

required to fully realise exploitation of our predicted g16 value in this way.

It is worth noting that the γ-glycine device we report (to the best of our knowledge) is the first

energy harvesting device reported using undoped organic crystals, and is most certainly the

first energy harvesting device that uses undoped amino acid crystals. This proof of concept

device for a (relatively) modest predicted voltage constants (51 mV m/N), should stimulate

more research into the challenges surrounding organic energy harvesting, particularly for





S41

microneedle strucures such as β-glycine and L-isoleucine (predicted g values of 8131

and 1233 mV m/N respectively).





S42

Supplementary Note: Experimental precautions for characterisation of metastable β-glycine

As discussed above, there are many ways to stabilise β-glycine needles for technological

exploitation; however for our experimental verification the needles needed to be removed

from their growth substrate, sliced, and tested in ambient conditions. We found during our

work that coating any glass substrate in ethanol before coating with a 0.133 M solution of

glycine, yielded needles that were stable for 4-6 weeks. Outer regions transforming into α-

glycine or γ-glycine could be observed visually on a micron scale (main text Figure 3c), and

was confirmed at the molecular level using Raman Spectroscopy and X-Ray Diffraction.

Once individual needles were removed and sliced, they had to be placed on a slide and

positioned in the probe station. Crystals would be in ambient conditions for an average of five

minutes before microprobe alignment, which could take another ten minutes. Extraction of

correctly compensated resonance data using LabVIEW takes approximately three minutes.

Only Raman Spectroscopy, calibrated and ready for analysis, could be used to confirm the

presence of β-glycine post-measurement. The logistics for X-Ray Diffraction characterisation

require more time than β-glycine has been known to remain stable for in ambient conditions.

X-Ray Diffraction was used only to confirm the presence of pre-measurement β-glycine

which had been grown on glass substrates and stored below room temperature and humidity.

The fast transformation reported by Isakov et al. (reference 46 in the main text) was induced

by rapidly increasing the relative humidity, and the crystals were grown on a different

substrate. We have not observed such a rapid transformation during our work. When growing

crystals by droplet evaporation, the edges of the droplet do transform into α-glycine crystals

in a matter of hours (see Figure 3c) but the macroscopic needles at the centre of the droplet,

and those grown in petri dishes with ethanol, have remained stable for much longer than is

reported in the literature. When grown on platinum25 substrates β-glycine crystals have been

stabilised for one month. For our work we have microcrystals grown in petri dishes that have





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remained stable after three months. In terms of crystals that are suitable for resonance

measurements (neither too thin nor too strongly adhered to the substrate) we see no signs of

transformation for 2-3 weeks when stored slightly below room temperature and humidity.

However, given that the crystals do spend finite periods of time in environments with a

higher relative humidity than that in which they are stored, it is definitely possible that

surface transformations could take place on the crystal slices. A complete transformation to

the γ or α form can be observed using optical microscopy or Raman spectroscopy, as shown

in Supplementary Figure 3. This can happen to sporadically to needles that are removed from

the substrate, or do not adhere strongly to the substrate during crystallisation. We feel it is

more realistic to acknowledge the potential development of stable polymorphs within β-

needles during measurement, which can lead to a reduction in our measured piezoelectric

constant (d16 = 0 pm/V and 5.6 pm/V) for α and γ glycine respectively).





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Supplementary Note: Grimme DFT-D3 Dispersion Corrections

For β and γ glycine, convergence of unit cell volumes (and individual lattice parameters) with

respect to cut-off radius is achieved at 12Å and 8Å respectively. At this convergence point

we find a reduction in energy of ~1.6 eV and a corresponding reduction of ~4% in the cell

volume (Supplementary Figure 10) to just below the experimental volume (Supplementary

Table 15) for both polymorphs. Van der Waals binding between the molecules results in a

proportional increase in elastic tensor values (Supplementary Figure 11) and a corresponding

decrease in piezoelectric constants (discussed in the main text), which moves the predicted

values away from the measured experimental values (main text Fig. 2).

As a control, we tested also the sensitivity of AlN dispersion corrections to cut-off values,

AlN28 being a far less malleable, inorganic structure that requires at least 14 GPa29 to undergo

a pressure induced transformation. The addition of vdW forces reduces the cell volume by

just 1%, but the calculated elastic constants change significantly. We find a percentage

deviation from experimental values of 14%, 7% and 27% for c11, c33 and c66 respectively,

larger than the corresponding deviations in the purely electronic DFT calculations of 0%, 9%,

and 2% (see Supplementary Figure 11 and Supplementary Table 1).

We suspect that this discrepancy between experimental measurements and predictions could

stem from increased external pressures acting on the crystals in the dispersion-corrected

calculations, which could possibly be used to simulate transformations between glycine

polymorphs in future studies.





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Supplementary References

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2 Ferrari, E. S., Davey, R. J., Cross, W. I., Gillon, A. L. & Towler, C. S. Crystallization in polymorphic systems: the solution-mediated transformation of β to α glycine. Crystal growth & design 3, 53-60 (2003).

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