Fig. 4. Doping-mediated piezoelectricity enhancement of Ww crystals.

(a) Comparison of the d₃₃ coefficients of pristine and doped Ww crystals measured using PFM. (b) Comparison of d₃₃ coefficients among different classes of organic and inorganic materials. The magenta, blue, cyan and grey regions show the range of d₃₃ values for peptide/amino acid self-assemblies, non-peptide organic materials, organic–inorganic hybrids and inorganic constituents, respectively. The Ww series (pristine and doped with iodine or neutrons) are denoted in red for comparison. From left to right: M13 bacteriophage nanopillars (phage-p),²² γ-glycine (γ-G),²⁴ dl-alanine (dl-A),²⁵ FF microrod arrays prepared using electric field (FF array-E),²⁶ FF microrod arrays (FF array),²⁷ M13 bacteriophage thin films (phage-f) and type I collagen films (collagen);²³ nylon, Rochelle salt (E337) and triglycine sulfate (TGS),³¹ diisopropylammonium bromide (DIPAB),³² croconic acid (CRCA),³³ poly(vinylidene fluoride) (PVDF),³⁴ poly(vinylidene fluoride/trifluoroerhylene) (P(VDF-TrFE)),²⁸ imidazolium perchlorate (Im-ClO₃);³⁵ trimethylchloromethyl ammonium trichloromanganese(ii) (TMCM-MnCl₃);³¹ ZnO and CdS,³⁶ periodically poled lithium niobate (LiNbO₃),²³ potassium titanyl phosphate (KTP), non-poled and poled BaTiO₃ [001],³¹ lead zirconate titanate (PZT).³⁷ (c) Schematic cross-section diagram of the proof-of-concept generator used as a direct power source comprising the peptide crystals as the active components. The inset shows a photographic picture of the device. (d) Comparison of the output signals (V_oc, I_sc) from the generators utilized as a direct power source using undoped or doped Ww crystals as the active components, as designed in (c).