Piezoelectric peptides represent an emerging class of self-assembled biomimetic materials in which recently observed giant piezoelectric effect is a result of hierarchical structure of nanocrystalline blocks bonded together with non-covalent interactions (such as hydrogen bonds, van der Waals, electrostatic and π-stacking forces). The general aim of this project is to uncover the nature of these specific interactions and their effect on functional properties by studying a sequence of phase transitions and probable ferroelectricity and pyroelectricity. The study of these functional properties is extremely useful for further applications of self-assembled peptides in organic electronics, novel biosensors, microactuators, microresonators, etc.
(a) Schematic of the FF tube arrangement on the rigid substrate. AFM cantilever is used to excite piezoelectric resonances and to measure induced vibrations as a function of frequency. Red arrows are used to highlight inhomogeneous distribution of electric field that helps to induce bending resonances in the system,
(b) Piezoelectric resonances excited in the FF microtube of 885 μm long and 13 μm in diameter in a cantilever configuration. Solid line represents the resonance structure for the AFM tip located in the middle. Dotted line is the example of the resonance peaks for the AFM tip close to the support. Table in the set summarizes resonance frequencies and quality factors for both configurations. Source: Appl. Phys. Lett. 102, 073504 (2013)
(a) Optical image of the FF microtube bundle fixed between two electrodes by silver paste and (b) Representative waveform of the pyroelectric current of the FF microtubes bundle under irradiation with a CO2 laser with the duty cycle of 50% measured by fA-meter at room temperature. Source: Appl. Phys. Lett. 109, 142902 (2016)
The equivalent scheme of FF nanotubes. (a) Typical unit cell of an FF nanotube; (b) its mass-in-mass representation; and (c) nanotube formed by aligned interacting mass-in-mass units. Solid lines show interactions with spring constants α, β and γ. The arrow shows the tube’s axis direction. Source: Phys. Chem. Chem. Phys., 2016, 18, 29681-29685. Published by The Royal Society of Chemistry.
Values of ET measured directly by the nanoindentation method. The inset shows FF nanotubes with different degrees of filling by water: empty nanotubes, partially filled nanotubes, and completely filled nanotubes. Blue solid lines point out values obtained from the literature and calculated in this work. Source: Phys. Chem. Chem. Phys., 2016, 18, 29681-29685 [Published by The Royal Society of Chemistry]
FCT-Tubitak project # TUBITAK/0006/2014
Ferroelectricity in Amino acids
Optical images of β-glycine films at different magnifications (a-c), (d) Schematic of the film structure. Source: ACS Appl. Mater. Interfaces 2017, 9, 20029.
Supramolecular packing directs piezoelectric response in glycine amino acid crystals. (a) Computed molecular dipoles (green arrows) in the centrosymmetric α-glycine preclude piezoelectricity. Molecules are shown in the CPK representation, with carbon, hydrogen, oxygen, and and nitrogen atoms coloured cyan, white, red, and navy blue, respectively. Note: crystallographic a, b, and c axes of the crystals have been chosen to align with arbitrarily chosen 1-, 2- and 3-axes, respectively. The molecular dipoles in α-glycine sum to zero and produce no net polarization.
(b) Molecular dipoles in β-glycine sum to a spontaneous polarization (red arrow) along the 2-axis, which contributes to the longitudinal 22 piezoelectric coe‑cient.
(c) Molecular dipoles contributing to the experimentally observed high shear piezoelectricity in β-glycine. When strained in the shear plane normal to the 3-axis the distance between glycine molecules increases. The resulting strong net polarization along the 1-direction gives rise to a transverse (see Methods) shear piezoelectricity denoted by the 16 coe‑cient.
(d) Molecular dipoles in the unit cell of γ-glycine sum (red arrow) to a spontaneous polarization along the 3-axis, which corresponds to the longitudinal 33 piezoelectric coe‑cient.
(e), A ‘top-down’ view of the γ-glycine unit cell along the  crystallographic direction. A shear strain in the plane normal to the 1-axis increases the distance between the molecular dipoles (pointing into the page) that are orientated around the net dipole moment (marked by X). The shear strain will induce a polarization along the 1-axis, giving rise to a longitudinal shear piezoelectricity denoted by the 14 coe‑cient. Each glycine molecule has a computed molecular dipole of 13.98 D. Dipoles are visualized through the geometric centres of the molecules. Source: Nature Materials volume 17, pages 180–186 (2018)
Li-batteries at the nanoscale
Over the last decades, Li-ion batteries have become an integral part of portable devices on account of their light weight and compact size. Growth in popularity of electric vehicles, green and wireless technologies has driven a continuous development of cells. Conventional Li-ion batteries are only efficient for low current applications such as mobile devices, but do not satisfy needs of emerging high power automotive and renewable energy applications. Higher cycling rates in these applications cause faster degradation and lower specific capacity after prolonged use. Micromechanical effects associated with cycling significantly contribute to aging. Lithium intercalation and deintercalation results in volume expansion and contraction as well as in phase changes of active electrode particles. Resulting mechanical stress can cause microcracks, particle fracture, loss of contact among particles, leading to a reduction of the electrochemically active mass and capacity decrease. Therefore, a thorough understanding of functional properties and degradation mechanisms of electrode materials is indispensable. Scanning Probe Microscopy (SPM) based techniques such as Kelvin Probe Force Microscopy (KPFM) and recently implemented Electrochemical Strain Microscopy (ESM) are able to probe electronic and transport properties of ionically conducting materials at the nanoscale. This project aims at the development of electrochemical strain microscopy to investigate the microscopic origin of the degradation by measuring Li mobility in active particles at different charge and degradation states. The ESM method will be implemented in conventional multifrequency Band Excitation and/or dual AC resonance tracking regimes. The measurements will be performed on cathode materials such as LiFePO4, Li3V2(PO4)3 and LiMn2O4 cathodes of a commercial Li-ion battery. These measurements will be complemented with surface potential distribution study by KPFM associated with Li concentration in the cathodes and graphite anodes of the same battery.
Topography (a), resonance frequency (b), and ESM signal (c) of Li-battery cathode. Source: Appl. Phys. Lett. 108, 113106, 2016.
Capillary electrophoresis in PVDF and PVDF-CTFE. Typical PVDF and PVDF-CTFE contain the polar functional group −C–F. Schematic presentation of the possible mechanism of formation of negatively charged diffuse layer and positively charged capillary wall–dissociation fluorine ions into electrolyte solution. Source: Langmuir, 2016, 32 (21), pp 5267–5276.
FCT project PTDC/CTM-ENE/6341/2014
Piezoceramics serve as an enabling technology for many significant areas including microelectronics, medical diagnostics, sensors, actuators and energy harvesters. Our research group is extensively involved in lead-free materials synthesis and their characterization focussing primarily on their piezoelectric characteristics for energy harvesting applications. Our materials synthesis program includes: conventional bulk ceramic processing, sol-gel synthesis, chemical solution deposition for thin films and nano-structured materials. We have extended structural [XRD, Raman, SEM] and electrical characterizations [dielectric, ferroelectric, piezoelectric] to a number of different lead free compositions encompassing inorganic oxides [including BaTiO3-, (K,Na)NbO3– and BiFeO3-based compositions among others] and organic polar materials like dabcoHReO4, self-organized peptides etc. As an integral component of our lead-free research program, we have strong and active national, and international collaborations.
Few representative recent results on lead-free research topics are detailed below:
- Excellent piezoelectric properties in praseodymium-modified (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3
Optimal composition with high piezoelectric charge coefficient,d33 = 435 pC/N and transduction coefficient, d33•g33 = 11589×10-15 m2/N were identified and envisioned for vibration-based energy harvesters.
(a) Dielectric permittivity variation as a function of temperature for Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) + x wt% Pr6O11, and (b) Composition dependence of thermal hysteresis, peak dielectric constant and the longitudinal piezoelectric coefficient. Source: Reproduced from Phys.Chem.Chem.Phys., 2016, 18, 31184 with permission from The Royal Society of Chemistry.
- Energy harvesting characteristics in lead-free Fe2O3 modified KNN ceramics
A unimorph cantilever beam arrangement was utilized for studying energy harvesting characteristics using pristine K0.5Na0.5NbO3 (KNN) and Fe2O3 modified KNN (KNFN) as piezoelements. KNFN exhibited a superior piezoelectric performance: d33=100 pC/N and mechanical quality factor (Qm=135) as compared to KNN (d33=83 pC/N; Qm=76). The KNFN harvester generated an output power of 0.38 mW/cm3 at a load resistance of 470 kΩ for a transverse displacement amplitude of 1.2 mm.
A comparison of output power as a function of load resistance for the KNN and KNFN energy harvesters at a displacement of 2 mm. Source: J Electroceram (2015) 34:255–261.
- Energy harvesting from nanofibers of hybrid organic ferroelectric dabcoHReO4
Excellent piezoelectric properties of electrospun nanofibers based on the hybrid ferroelectric 1,4-diazabicyclo[2.2.2]octane perrhenate (dabcoHReO4) was observed. A flexible piezoelectric nanogenerator consisting of an aligned array of dabcoHReO4 fibers provides a voltage above 100 mV under a moderate strain level. Results show that the nanofibers based on dabcoHReO4 have a great potential for piezoelectric autonomous energy harvesting with natural advantages such as biocompatibility, flexibility, low cost, and easy fabrication.
(a) Illustration showing piezoelectric dabcoHReO4 – fibres device during bending and release, and (b) Voltage output dynamic bending tests at 2 and 5 Hz (output power is up to 90nW @100MΩ). Source: Appl. Phys. Lett. 104, 032907 (2014).
Project SGH – SMART GREEN HOMES (POCI-01-0247-FEDER-007678)
Graphene and 2D Materials
- Graphene and its band gap engineering by in-situ doping by heteroatoms
The isolation of single layer of carbon atoms in the form of graphene emerged as one of the greatest discoveries in the 21st century. It has attracted a lot of attention in recent years owing to the amazing new physics, high electronic mobility, and ultra-thin dimensionality.
Our group’s research is focused on the development of technologies for deposition of monolayer/few layers graphene on different substrate (copper/nickel) by using hydrocarbon as precursor by home-built Chemical Vapor Deposition (CVD) system that can be scaled up for industrial scale production.
- Pure graphene on copper substrate
Full Raman spectrum of pure graphene on copper substrate. The spectrum shows major peaks associated with graphene, namely, D, G and 2D. The inset shows the optical image of graphene flakes.
- Band-gap Engineering: Defect creations by in-situ ammonia doping
Our group is exploring graphene’s unique materials properties in the context of tunable band-gap engineering by in-situ ammonia doping. By changing the substrate thickness, we could create defects in graphene. Tuning the band-gap of graphene is a current need for real device applications. Copper (Cu) as a substrate plays a crucial role in graphene deposition. The results suggest that Cu substrate of 20 μm in thickness exhibits higher defect density (1.86 x 1012 cm-2) as compared to both 10 and 25 μm thick substrates (1.23 x 1012 cm-2 and 3.09 x 1011 cm-2).
Defect concentration in SL graphene grown on 10, 20 and 25μm Cu substrates vs. deposition time. Source: Physica B 513 (2017) 62–68.
HR-XPS comparison of nitrogen (N) – doped graphene grown on copper substrate of 10 (brown), 20 (black spectra) and 25 (red) μm in thickness. (a) Shows the overview spectra of n-doped graphene. (b) C 1s and (c) N 1s core levels. The best fits are also included in green. In the case of the N 1s spectra of graphene grown on 25 μm Cu substrate sample (bottom spectra) the blue line is a guide for the eyes. Source: Physica B 513 (2017) 62–68.
- Graphene on nickel: charge injection
The group also carries out extensive atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) studies. We performed charge injection using AFM and KPFM for the quantitative characterization of nanoscale electrostatic properties of as-grown multilayer-graphene (MLG) sheets.
Charge injection experiments on MLG/nickel surface based on AFM. (a) Schematic illustration of KPFM and contact mode for charge injection process. (b) 3D example of charge injection schematic presentation. The topography is mapped onto the third dimension (z-axis) and the recorded surface potential is color coded. Source: Applied Materials Today 8 (2017) 18–25.
Topography with KPFM image superimposed (colored coded) before (a), immediately after (b) poling, +5 V, 10 s, and (c) after prolonged aging 3 h (c). Source: Applied Materials Today 8 (2017) 18–25.
Charge injection experiments on MLG/nickel. AFM topography image before (a) and after (b) charge injection, performed with the conducting tip (Vinj = -5 V, Vinj = +5 V; and injection time tinj = 10 s). Surface potential image before (c) and after (d) injection. (e) Represents the difference between images (c) and (d) for more clarity. (e and f) Profile of the surface potential signal across red dotted lines. Source: Applied Materials Today 8 (2017) 18–25.
2. Transition metal dichalcogenides: MoS2
The group also aims at fabrication of other atomically thin 2D materials especially molybdenum disulfide (MoS2). MoS2 is one of the most studied layered TMDCs with a direct bandgap of 1.8 eV, providing a possibility for 2D materials to be used in the next generation of switching and optoelectronic devices. So far, MoS2 has achieved primary progress in several application areas, such as energy conversion and storage devices. Additionally, MoS2 with odd number of layers could produce notable piezoelectric voltage and current outputs, indicating its potential for powering nanodevices and stretchable electronics.
We successfully synthesized large-area MoS2 via sulfurization of MoO3 using CVD at different argon (Ar) base pressures and found the optimal pressure for the growth (50 mbar).
Combined Raman imaging using the distinct Raman spectra (left) and respective Raman spectra used for the combined Raman image (right) of MoS2 grown on SiO2/Si by CVD. The blue spectra correspond to the blue region and the red spectra correspond to the red region in Raman imaging respectively. The differences (Δ) between (E12g – A1g) were found to be 25.7 and 28.55 cm-1. Source: Materials Research Bulletin 97 (2018) 265–271.
X-Ray photoelectron spectroscopy spectra of (a) Mo and (b) S respectively. The black dots are experimental data; red line is resultant fitting; green, blue, cyan and yellow are different components used for fitting. Source: Materials Research Bulletin 97 (2018) 265–271.