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The mechanical properties of crystalline materials are crucial knowledge for their screening, design, and exploitation. Density functional theory (DFT), remains one of the most effective computational tools for quantitatively predicting and rationalising the mechanical response of these materials. DFT predictions have been shown to quantitatively correlate to a number of experimental techniques, such as nanoindentation, high-pressure X-ray crystallography, impedance spectroscopy, and spectroscopic ellipsometry. Not only can bulk mechanical properties be derived from DFT calculations, this computational methodology allows for a full understanding of the elastic anisotropy in complex crystalline systems. Here we introduce the concepts behind DFT, and highlight a number of case studies and methodologies for predicting the elastic constants of materials that span ice, biomolecular crystals, polymer crystals, and metal–organic frameworks (MOFs). Key parameters that should be considered for theorists are discussed, including exchange–correlation functionals and dispersion corrections. The broad range of software packages and post-analysis tools are also brought to the attention of current and future DFT users. It is envisioned that the accuracy of DFT predictions of elastic constants will continue to improve with advances in high-performance computing power, as well as the incorporation of many-body interactions with quasi-harmonic approximations to overcome the negative effects of calculations carried out at absolute zero.

Here, we report the growth of bioelectret crystal films of L-leucine on conductive substrates and detail the first quantitative measurements of the direct piezoelectric effect in this proteinogenic amino acid. Through extensive electromechanical characterisation, we demonstrate that L-leucine is a promising candidate material for use in non-toxic, biocompatible and biodegradable energy harvesting and sensing devices. The data presented here is among, if not the, largest set of quasi-static, longitudinal piezoelectric measurements on crystalline films of a proteinogenic amino acid to date. We substantiate these measurements using a combination of density functional theory calculations and optical microscopy. Our data provides a further example of the enormous unexploited potential of amino acids and similar biological materials in flexible energy harvesting applications. Their combination of sizeable piezoelectric strain constants and extremely low elastic and dielectric constants makes them ideal as biocompatible replacements for toxic, expensive inorganic piezoelectric materials.

peer-reviewed Molecular stacking modes, generally classified as H-, J-, and X-aggregation, play a key role in determining the optoelectronic properties of organic crystals. However, the control of stacking transformation of a specific molecule is an unmet challenge, and a priori prediction of the performance in different stacking modes is extraordinarily difficult to achieve. In particular, the existence of hybrid stacking modes and their combined effect on physicochemical properties of molecular crystals are not fully understood. Herein, unexpected stacking transformation from H- to J- and X-aggregation is observed in the crystal structure of a small heterocyclic molecule, 4,4'-bipyridine (4,4'-Bpy), upon coassembly with N-acetyl-l-alanine (AcA), a nonaromatic amino acid derivative. This structural transformation into hybrid stacking mode improves physicochemical properties of the cocrystals, including a large red-shifted emission, enhanced supramolecular chirality, improved thermal stability, and higher mechanical properties. While a single crystal of 4,4'-Bpy shows good optical waveguiding and piezoelectric properties due to the uniform elongated needles and low symmetry of crystal packing, the significantly lower band gap and resistance of the cocrystal indicate improved conductivity. This study not only demonstrates cocrystallization-induced packing transformation between H-, J-, and X-aggregations in the solid state, leading to tunable mechanical and optoelectronic properties, but also will inspire future molecular design of organic functional materials by the coassembly strategy. ACCEPTED peer-reviewed

Research on the piezoelectric response of biomolecules has intensified following demonstration of open circuit voltages of over 20 V in biopiezoelectric generators. Organic piezoelectric nanotubes, fibers, and micro-islands have been grown and studied; however, the lack of fundamental understanding of the piezoelectric effect in nature hinders the rational design of biomaterials to provide a tailor-made piezoelectric response. Advances in high performance computing have facilitated the use of quantum mechanical calculations to predict the full piezoelectric tensor of biomolecular crystals, including amino acids and small peptides. By identifying directions of high piezoelectric response, the simulations can guide experimental crystal growth, device fabrication and electrical testing, which have led to the demonstration of unprecedented piezoelectric responses in organic crystals on the order of 200 pC/N. These large responses arise from strong supramolecular dipoles, which can be tuned by molecular chemistry and packing, opening new opportunities for the realization of technologically useful piezoelectric devices from renewable materials. The amino acids predicted to exhibit the highest piezoelectric response, such as glycine, hydroxyproline and lysine, are anticipated to be used to engineer highly piezoelectric peptides in the future. With improved scaling of advanced computational methods, such as density functional perturbation theory, the research community can begin to efficiently screen peptide structures for enhanced electromechanical properties. This capability will accelerate the experimental development of devices and provide much-needed insight into the evolution of a hierarchical relation in biological materials starting from strongly piezoelectric building blocks. Biological structures such as amino acids, peptides, and proteins are emerging as promising candidates for piezoelectric energy harvesting and sensing. Here we highlight the position of biological materials in the diverse world of piezoelectric structures, and emphasise how a nanoscale insight into these assemblies, particularly in crystalline form, can pave the way for development of a diverse new array of biocompatible sensors for a greener future. By harnessing advances in high performance computing, we can begin to screen the vast library of biomolecules for optimum candidates, with the ultimate goal of re-engineering biological piezoelectricity by first principles design.

Cocrystallization of two or more molecular compounds can dramatically change the physicochemical properties of a functional molecule without the need for chemical modification. For example, coformers can enhance the mechanical stability, processability, and solubility of pharmaceutical compounds to enable better medicines. Here, we demonstrate that amino acid cocrystals can enhance functional electromechanical properties in simple, sustainable materials as exemplified by glycine and sulfamic acid. These coformers crystallize independently in centrosymmetric space groups when they are grown as single-component crystals but form a noncentrosymmetric, electromechanically active ionic cocrystal when they are crystallized together. The piezoelectricity of the cocrystal is characterized using techniques tailored to overcome the challenges associated with measuring the electromechanical properties of soft (organic) crystals. The piezoelectric tensor of the cocrystal is mapped using density functional theory (DFT) computer models, and the predicted single-crystal longitudinal response of 2 pC/N is verified using second-harmonic generation (SHG) and piezoresponse force microscopy (PFM). The experimental measurements are facilitated by polycrystalline film growth that allows for macroscopic and nanoscale quantification of the longitudinal out-of-plane response, which is in the range exploited in piezoelectric technologies made from quartz, aluminum nitride, and zinc oxide. The large-area polycrystalline film retains a damped response of ≥0.2 pC/N, indicating the potential for application of such inexpensive and eco-friendly amino acid–based cocrystal coatings in, for example, autonomous ambient-powered devices in edge computing., Glycine and sulfamic acid form an ionic cocrystal that demonstrates a measurable longitudinal piezoelectric response in single-crystal and polycrystalline form. The crystals are characterized using piezoresponse force microscopy and second harmonic generation, and the material properties are rationalized and quantified using density functional theory calculations.

Realization of a self-assembled, nontoxic and eco-friendly piezoelectric device with high-performance, sensitivity and reliability is highly desirable to complement conventional inorganic and polymer based materials. Hierarchically organized natural materials such as collagen have long been posited to exhibit electromechanical properties that could potentially be amplified via molecular engineering to produce technologically relevant piezoelectricity. Here, by using a simple, minimalistic, building block of collagen, we fabricate a peptide-based piezoelectric generator utilising a radically different helical arrangement of Phe-Phe-derived peptide, Pro-Phe-Phe and Hyp-Phe-Phe, based only on proteinogenic amino acids. The simple addition of a hydroxyl group increases the expected piezoelectric response by an order of magnitude (d35 = 27 pm V−1). The value is highest predicted to date in short natural peptides. We demonstrate tripeptide-based power generator that produces stable max current >50 nA and potential >1.2 V. Our results provide a promising device demonstration of computationally-guided molecular engineering of piezoelectricity in peptide nanotechnology. Piezoelectric materials which are non-toxic and eco-friendly are of interest. Here, the authors report on the creation of collagen-mimetic peptides which can be self-assembled into piezoelectric materials and study the design characteristics required for optimized power generation.

Large reversible shear strain has been achieved by electric-field-driven bipolar switching in a hybrid ferroelectric, facilitating development of shape-memory-type actuators with outstanding figures of merit.

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 substra...

Here we report a preliminary quantitative investigation into calcite piezoelectricity using a combination of quantum mechanical modelling and electromechanical characterisation of naturally occurring calcite crystals and structures. The low symmetry of crystallised biomolecules — from amino acids and peptides to viruses and fibrous proteins — can result in useful electrical properties, including ferroelectricity, pyroelectricity, and piezoelectricity. Biominerals such as hydroxyapatite and calcite are generally not considered piezoelectric. This data represents the first quasi-static, longitudinal piezoelectric measurements on macroscopic calcite based materials; namely Icelandic spar and a variety of beach specimens. We substantiate these measurements using Density Functional Theory (DFT) calculations, which attribute the measured weak, residual piezoelectric response in all directions to centrosymmetric calcite crystals. Our data is consistent with the observation of second harmonics generation by calcite microcrystals occurring in the human pineal gland.

Collagen piezoelectricity has been the focus of a large number of experimental and theoretical studies for over fifty years. Less is known about the piezoelectric properties of its building blocks, in particular but not limited to, proline and hydroxyproline. Spurred by the recent upsurge of interest in piezoelectricity in organic crystals including our own report of unprecedentedly high piezoelectricity in amino acid glycine, we predict and measure the piezoelectric properties of collagen subcomponents in single crystalline forms and the collagen-like alanine-hydroxyproline-glycine trimer peptide. We map the modulation of piezoelectric charge constants in collagen building blocks as the crystal symmetry is lowered and the molecule size increases, finding strong evidence for amino acid-level barcoding of collagen piezoelectricity that can in turn be tuned using very simple chemistry. The simple addition of an -OH group can increase piezoelectric constants by up to two orders of magnitude (d25 = 25 ± 5 pC N-1) as measured in Y-cut hydroxyproline crystals. The value is similar to that obtained from thermoelectrically poled commercial polyvinylidene di fluoride (PVDF) films. Overall, our findings support a simple block by block approach in which first principles calculations can guide the understanding and re-engineering of piezoelectric biomolecules.

Bioinspired assemblies bear massive potential for energy generation and storage. Yet, biological molecules have severe limitations for charge transfer. Here, we report l-tryptophan-d-tryptophan assembling architectures comprising alternating water and peptide layers. The extensive connection of water molecules results in significant dipole–dipole interactions and piezoelectric response that can be further engineered by doping via iodine adsorption or isotope replacement with no change in the chemical composition. This simple system and the new doping strategies supply alternative solutions for enhancing charge transfer in bioinspired supramolecular architectures.

Amino acid chirality plays an important role in conveying directionality and specificity to their supramolecular organization. However, the impact of enantiopure and racemic amino acids on the favorable packing and macroscopic properties of organic cocrystals with nonchiral coformers is poorly understood. Herein, we performed a systematic study of the effect of chirality on the macroscopic properties of acetylated alanine (AcA) single crystals and cocrystals with a nonchiral photo-sensitive bipyridine derivative (BPE). Cocrystallization with BPE produced a marked morphology transition that improved the supramolecular chirality, thermal stability and mechanical strength of AcA crystals. The distinct supramolecular packing modes were analyzed by X-ray crystallography. The highest rigidity was observed for BPE/ dl -AcA, while BPE/ d -AcA and BPE/ l -AcA crystals exhibited higher efficiency of photo-induced emission for fluorescent imprinting, as well as significantly higher piezoelectricity. This work provides a striking illustration that subtle differences in amino acid stereochemistry translate into tunable macroscopic properties of organic cocrystals for future applications in rigid solids, fluorescent imprinting, and energy harvesting.

Diphenylalanine (FF) represents the simplest peptide building block that self-assembles into ordered nanostructures with interesting physical properties. Among self-assembled peptide structures, FF nanotubes display notable stiffness and piezoelectric parameters (Young’s modulus = 19–27 GPa, strain coefficient d33 = 18 pC/N). Yet, inorganic alternatives remain the major materials of choice for many applications due to higher stiffness and piezoelectricity. Here, aiming to broaden the applications of the FF motif in materials chemistry, we designed three phenyl-rich dipeptides based on the β,β-diphenyl-Ala-OH (Dip) unit: Dip-Dip, cyclo-Dip-Dip, and tert-butyloxycarbonyl (Boc)-Dip-Dip. The doubled number of aromatic groups per unit, compared to FF, produced a dense aromatic zipper network with a dramatically improved Young’s modulus of ∼70 GPa, which is comparable to aluminum. The piezoelectric strain coefficient d33 of ∼73 pC/N of such assembly exceeds that of poled polyvinylidene-fluoride (PVDF) polymers and compares well to that of lead zirconium titanate (PZT) thin films and ribbons. The rationally designed π–π assemblies show a voltage coefficient of 2–3 Vm/N, an order of magnitude higher than PVDF, improved thermal stability up to 360 °C (∼60 °C higher than FF), and useful photoluminescence with wide-range excitation-dependent emission in the visible region. Our data demonstrate that aromatic groups improve the rigidity and piezoelectricity of organic self-assembled materials for numerous applications.

Discovery of novel cocrystal systems and improvement of their physicochemical properties dominates the current literature on cocrystals yet the required end-product formulation is rarely addressed. Drug product manufacturing includes complex API solid state processing steps such as milling, granulation, and tableting. These all require high mechanical stress which can lead to solid-state phase transformations into polymorphs and solvates, or lead to dissociation of cocrystals into their individual components. Here we measured the effect of tablet excipients on solid-state processing of a range of pharmaceutical cocrystal formulations. Our findings were rationalised using Density Functional Theory (DFT) calculations of intermolecular binding energies of cocrystal constituents and co-milling excipients. A 1:1 stoichiometric ratio of API Theophylline (THP) and co-former 4-Aminobenzoic acid (4ABA) was co-milled with five different excipients: hydroxypropylmethylcellulose (HPMC), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), lactose, and microcrystalline cellulose (MCC). The experiments were carried out in 10 and 25 mL milling jars at 30 Hz for different milling times. Co-milled samples were characterised for formation of cocrystals and phase transformation using powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC). Our data shows that co-milling in the presence of PEG, HMPC or lactose yields purer cocrystals, supported by the calculated stronger excipient interactions for PVP and MCC. We identify a suitably-prepared THP–4ABA pharmaceutical cocrystal formulation that is stable under extended milling conditions.

Molecular electronics aims to control charge transport at the molecular level, but it is not always clear which factors are important to control because it is difficult to distinguish interface effects from molecular effects. Investigating so-called “odd–even” effects in molecular tunnel junctions provides an opportunity to study molecular effects while keeping the nature of the molecule–electrode interactions unchanged. Odd–even effects in charge tunneling rates have been observed in self-assembled monolayer (SAM)-based tunnel junctions with strong covalent molecule-bottom electrode interactions, but it is not clear whether these odd–even effects originate from the intrinsic properties of the SAM or strong molecule-electrode interactions. Herein, we report tunnel junctions based on SAMs that form on graphene through weak noncovalent interactions (i.e., van der Waals interactions) and also form a van der Waals contact with the top electrode. We found that odd–even effects in charge tunneling rates persist...

Biomolecular crystals are an emerging class of piezoelectric materials that are both biocompatible and biodegradable, which enables their use in biomedical applications and smart devices while ensuring eco-friendly production and disposal. However, accurate quantification of the piezoelectric response of soft sub-micron crystals remains a significant challenge, as conventional piezoelectric measurement techniques are suited to ceramics, thin films, and polymers. Here, we demonstrate the use of a novel piezoresponse force microscopy (PFM) methodology for robust, reliable quantification of the electromechanical response of biomolecular crystals. As a strong test of high accuracy and precision, we show that PFM, integrated with quantum mechanical (QM) density functional theory (DFT) calculations, can distinguish the piezoelectric responses of near-isopiezoelectric amino acid crystals. We show that a statistical approach, combined with experimental best practices, provides effective piezoelectric coefficients of biomolecular single crystals accurately and unambiguously. This work opens the door to high-throughput screening and characterisation of natural and engineered soft piezoelectric crystals for eco-friendly energy harvesters and biodegradable medical implants, reducing dependence on lead-based and rare-earth-containing piezoelectric materials.

Second-harmonic generation (SHG) is a nonlinear optical process that can provide disease diagnosis through characterization of biological building blocks such as amino acids, peptides, and proteins. The second-order nonlinear susceptibility tensor χ(2) of a material characterizes its tendency to cause SHG. Here, a method for finding the χ(2) elements from polarization-resolved SHG microscopy in transmission mode is presented. The quantitative framework and analytical approach that corrects for micrometer-scale morphology and birefringence enable the determination and comparison of the SHG susceptibility tensors of β- and γ-phase glycine microneedles. The maximum nonlinear susceptibility coefficients are d33 = 15 pm V-1 for the β and d33 = 5.9 pm V-1 for the γ phase. The results demonstrate glycine as a useful biocompatible nonlinear material. This combination of the analytical model and polarization-resolved SHG transmission microscopy is broadly applicable for quantitative SHG material characterization and diagnostic imaging.

Wrinkles are a unique class of surface corrugations present over diverse length scales from Kinneyia-type wrinkles in Archean-era sedimentary fossils to nanoscopic crinkling in two-dimensional crystals. Lately, the role of wrinkles on graphene has been subject to debate as devices based on graphene progress towards commercialization. While the topology and electronic structure of graphene wrinkles is known, data on wrinkle geometrical effects on molecular adsorption patterns and resonance states is lacking. Here, we report molecular superstructures and enhancement of free-molecular electronic states of pentacene on graphene wrinkles. A new trend is observed where the pentacene energy gap scales with wrinkle height, as wrinkles taller than 2 nm significantly screen metal induced hybridization. Combined with density functional theory calculations, the impact of wrinkles in tuning molecular growth modes and electronic structure is clarified at room-temperature. These results suggest the need to rethink wrinkle engineering in modular devices based on graphene and related 2D materials interfacing with electronically active molecules. The morphology of out-of-plane wrinkles in graphene determines the molecular growth and electronic structure of adsorbed pentacene molecules. A team led by Peter Nirmalraj at IBM Research (Zurich) performed a combined experimental and theoretical study on the interplay between the ubiquitous corrugations naturally present in graphene and the structure and electronic properties of pentacene molecular superstructures. The atomic scale wrinkle landscape was found to trigger substantial variations in molecular growth, along with pronounced electronic decoupling of pentacene adsorbed on wrinkles. This is in contrast to the behavior of pentacene on flat graphene, which featured broadened molecular energy levels owing to electronic interaction with the underlying substrate. These results may facilitate the understanding of interfacial effects between graphene and molecules and pave the way to improved design of nanoscale molecular electronic devices.

In molecular electronics, it is important to control the strength of the molecule-electrode interaction to balance the trade-off between electronic coupling strength and broadening of the molecular frontier orbitals: too strong coupling results in severe broadening of the molecular orbitals while the molecular orbitals cannot follow the changes in the Fermi levels under applied bias when the coupling is too weak. Here, a platform based on graphene bottom electrodes to which molecules can bind via π-π interactions is reported. These interactions are strong enough to induce electronic function (rectification) while minimizing broadening of the molecular frontier orbitals. Molecular tunnel junctions are fabricated based on self-assembled monolayers (SAMs) of Fc(CH2 )11 X (Fc = ferrocenyl, X = NH2 , Br, or H) on graphene bottom electrodes contacted to eutectic alloy of gallium and indium top electrodes. The Fc units interact more strongly with graphene than the X units resulting in SAMs with the Fc at the bottom of the SAM. The molecular diodes perform well with rectification ratios of 30-40, and they are stable against bias stressing under ambient conditions. Thus, tunnel junctions based on graphene with π-π molecule-electrode coupling are promising platforms to fabricate stable and well-performing molecular diodes.

Piezoelectricity, the linear relationship between stress and induced electrical charge, has attracted recent interest due to its manifestation in biological molecules such as synthetic polypeptides or amino acid crystals, including gamma (γ) glycine. It has also been demonstrated in bone, collagen, elastin and the synthetic bone mineral hydroxyapatite. Piezoelectric coefficients exhibited by these biological materials are generally low, typically in the range of 0.1-10 pm V