Symposium F.EN01—Emerging Dielectric Materials—Applications in Energy Transmission, Storage and Conversion

Green and efficient energy technologies are a key point where nanoscience could make a difference in the paradigm shift from fossil fuels to renewable sources. One attractive possibility is the utilization of solar energy to obtain electricity or chemical fuel based on the ability of semiconductor nanomaterials to function as photocatalysts promoting various oxidation and reduction reactions under sunlight. We report on a novel class of Bi-based photocatalysts for hydrogen production via water splitting. A screening DFT investigation was performed on the Bi₂(MO₄)₃ (M= Cr, Mo, and W) systems. The Bi₂(CrO₄)₃ system exhibits the smallest bandgap energy, the highest dielectric constant, and the highest absorption in the visible region among the other counterpart materials. Consequently, Bi₂(CrO₄)₃ nanoparticles were synthesized via a simple one-pot method at room temperature and characterized by XRD, XPS, DRS, FE-SEM, HR-TEM, and Raman spectroscopy. The as-prepared yellow Bi₂(CrO₄)₃ nanoparticles exhibited direct bandgap energy of 2.45 eV. The photoactivity of the as-prepared Bi₂(CrO₄)₃ nanoparticles was tested toward the photocatalytic hydrogen production, where reasonable rates of 522.44, 174.15, and 88.24 μmol/g/h were achieved under UV, AM 1.5, and visible irradiations, respectively in the absence of any hole scavengers. Those rates are higher than those reported for Bi-based photocatalysts.

Hafnium oxide and its silicates are considered mainly as gate dielectrics owing to their high dielectric constant. Recently, their ferroelectric properties were discovered, whereas optical application of these materials was rarely addressed in spite of promised optical parameters and mechanical and thermal stability. Usually, CVD or ALD approaches are used for the fabrication of hafnia-based films. At the same time magnetron sputtering was considered only by some research groups. In this work, we report on the utility of reactive magnetron sputtering for the fabrication of composite thin films based on HfO₂ co-doped with Si, N, and Nd ions simultaneously.
The evolution of structural and optical properties of such films with annealing temperature was investigated by means of the scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman scattering, and photoluminescence techniques. It is observed that sputtering of HfO₂-Si-Nd composed target in argon-nitrogen plasma allowed producing homogeneous, dense and smooth films. The analysis of film properties with annealing temperature revealed the stability of film structure up to 950°C.
The light emission in the visible and IR spectral range was detected. It was found that bright Nd^3+-related emission can be achieved at lower temperatures (about 950°C) contrary to that reported for Nd-doped Si-SiO₂ or Si-Si₃N₄ materials. The Nd³⁺ PL intensity correlates with the evolution of HfO₂ monoclinic phase. It is shown that the excitation channel of Nd3₊ ions under non-resonant excitation involves host defects contained nitrogen ions. The desorption of nitrogen at high annealing temperature (1100°C) via the formation of volatile NO, as well as the formation of the Nd silicate phase results in the quenching of Nd³⁺ photoluminescence. This complex approach used for investigation of our films allowed demonstrating the utility of reactive magnetron sputtering for the fabrication of thin films with stable chemical composition and structure (up to 950°C). These results can be attractive for future applications of the hafnium silicate films (un-doped and doped with rare-earth ions) as emitting elements or waveguides.

TEG devices have potential to satisfy growing energy needs in portable electronics and sensors providing a clean alternative to conventional batteries.¹ TEG devices are produced from cheap, lightweight, flexible, widely used polymer materials and offer promise to capture neglected and unutilized forms of mechanical energy. TEG device consists from two conductive electrodes from which at least one is covered with polymer insulator film.¹ The two electrodes in TEG are connected by outer circuit and upon electrode oscillation surface charge on polymer layers and electric potential is created that drive electrons to flow between two electrodes in order to balance this electric potential difference. TEG devices can be operated in different modes – vertical separation, sliding, rotating, single electrode etc.² but the key feature for high TEG efficiency is surface charge from contact electrification.³
Different mechanisms are responsible for contact electrification and depend on material used. It is well demonstrated, that on metal-metal, metal-semiconductor or semiconductor-semiconductor contact the electron transfer occurs,^4,5 but this is not so obvious for polymer insulators.⁶ The three mechanisms for polymer insulator contact electrification are considered – electron transfer,⁷ ionic transfer,⁶ and covalent bond cleavage.⁸ Electron transfer between polymers is doubtful, because there are no available free electrons in insulators. Usage of the term “effective work function” in connection to the driving force for charge exchange between polymer insulators is also questionable even if the polymer is contacted with metal.⁶ Ion exchange between contacted polymer insulators has been considered, because water under ambient conditions is adsorbed even on hydrophobic polymers.⁹ The water layers on contacted surfaces fuse together upon contacting and as different polymer materials may have different affinity towards cations and anions in water, the imbalance between ions form during separation, thus creating surface charge.⁶ However, it has been proven, that contact electrification of polymer insulators also occurs in complete absence of water.¹⁰
In our work we show that polymer contact-electrification can be predicted using their cohesive energy density. This allows us to select the most suitable materials as the building blocks for efficient TEG devices.

References
[1] Z.L. Wang, ACS Nano, 2013, 7, 9533-9557.
[2] R. Hinchet, W. Seung, S.-W. Kim, ChemSusChem, 2015, 8, 2327-2344.
[3] J. Wang, C. Wu, Y. Dai, Z. Zhao, A. Wang, T. Zhang, Z.L. Wang, Nature Commun., 2017, 8, 88.
[4] J. Lowell, J. Phys. D: Appl. Phys., 1975, 8, 53-63.
[5] J. Liu, A. Goswami, K. Jiang, F. Khan, S. Kim, R. McGee, Z. Li, Z. Hu, J. Lee, T. Thundat, Nature Nanotechnol., 2018, 13, 112-116.
[6] L.S. McCarty, G.M. Whitesides, Angew. Chem. Int. Ed., 2008, 47, 2188-2207.
[7] M. Willatzen, Z.L. Wang, Nano Energy, 2018, 52, 517-523.
[8] H.T. Baytekin, A.Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, B.A. Grzybowski, Science, 2011, 333, 308-312.
[9] H.T. Baytekin, B. Baytekin, S.S., B.A. Grzybowski, Angew. Chem. Int. Ed., 2011, 50, 6766–6770.
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Traditional dielectric materials such as silicon dioxide (SiO₂) is the first choice and mostly used in many electronic devices including Metal Oxide Semiconductor Field Effect Transistors, Thin Film Transistors, and VLSI technology. However, SiO₂ has several disadvantages that researchers are currently looking for alternatives. Scaling down electronics are the most popular device approach where SiO₂ will not be able to fit in these ultralow dimensional adjustments. One approach to scaling down electronics, while sustaining adequate dielectric properties is incorporating high k dielectrics. High k dielectrics refer to materials with a high dielectric constant, which denotes the material ability to store an electrical charge. SiO₂ generally has a dielectric constant value of 4, while high k dielectrics have a dielectric constant greater than approximately 10. Noteworthy high k dielectrics include Hafnium Oxide (HfO₂), Zirconium Oxide (ZrO₂), and Aluminum Oxide (Al₂O₃). In this study, we perform important characterization techniques to analyze HfO₂, ZrO₂, and Al₂O₃ for the possible replacement of SiO₂. These high k dielectric thin films exhibited leakage currents lower than 9.45 x 10^-11 and with a thin film roughness of less than .5 nm. These high-quality high k dielectrics thin films were grown by Atomic Layer Deposition on n-type silicon at a different temperature and constant thickness for premise characterization. The effect of temperature on surface morphology, crystalline orientation, and dielectric properties were analyzed through Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), Keithley 4200 SCS, and Four Point Probe measurement system. AFM provided thin film roughness, grain size, and surface morphology for thin films. XRD provided the crystalline orientation of these high k dielectrics with the effect of temperature. Keithley 4200 was used to show and calculate the dielectric properties including; dielectric constant, I-V plots (leakage current), and C-V plots. Lastly, Four Point probe measurement system provided electrical properties for these high k dielectrics. The present work will provide valuable scientific input dielectric performance for the improvement of TFT devices.

Graphene, the two dimensional material with sp²-hybridized carbon atoms arranged in honeycomb-like structure has attracted greatest research interests because of their excellent mechanical and electrical properties. In particular, graphene have been considered as one of the most promising reinforcing nanofillers for developing cost-effective, high performance polymer composites. However, their main drawback is associated with their poor solution processability in organic solvents and agglomeration in polymer matrix due to their Van der Waals forces. Graphene oxide (GO), derived from acidic oxidation of natural graphite possesses abundant oxygen containing functionalities exhibit good solution processability, and therefore, receiving attention as precursor to the graphene-based nanofillers. On the other hand, poly(vinylidene fluoride) (PVDF) have been actively applied in piezoelectric sensors and nanogenerators because of their electroactive properties and flexibility. Generally, PVDF present five crystalline phases such as, α, δ, β, γ and ε, among them, β-phase PVDF with TTTT conformation shows the best piezoelectric properties, because all the dipoles are arranged parallel which resulted in highest dipole moment per unit cells. Incorporating GO into PVDF matrix can promote the formation of preferred β-phases owing to the interaction between the oxygen functionalities of GO and dipoles of the PVDF chains. In addition, the intrinsic high mechanical properties of GO can also enhance the mechanical properties of PVDF matrix. However, in most of the polymer composite systems, the strength and stiffness increased with the addition of nanofillers, meanwhile, the toughness decreased due to reduction in strain at break, which often restricts their applications in stretchable and flexible electronics. Herein, we report a method for fabricating PVDF/GO composite nanofibers with significantly enhanced mechanical strength and modulus without losing its toughness/stretchability. Isocyanate treated GO (i-GO) nanosheets were incorporated as nanofillers into PVDF nanofibers (PVDFNFs) using solution blending and electrospinning techniques. i-GO exhibit strong interfacial interaction with PVDF due to its reduced hydrophilicity and excellent dispersibilty in organic solvents. FTIR results revealed that the addition of i-GO induce the formation of PVDFNFs with high content of electroactive phases. Interestingly, the tensile strength, modulus and as well as strain-at-break increased with the addition i-GO. Furthermore, the composites displayed enhanced piezoelectric outputs than that of reference PVDFNFs during the static load analysis. The enhanced performance of the composites can be attributed to the strong interaction between i-GO and PVDF achieved through incorporation of chemically modified GO. The reported composites in this study hold a great potential in the field of flexible and stretchable piezoelectric sensors and nanogenerators.

Keywords: Graphene, PVDF Nanofibers, Piezoelectric Sensing, Nano Composites, Mechanical Properties

Acknowledgements

This work was supported by Grant NRF-2018R1A6A1A03025526 under the Priority Research Program through National Research Foundation of Korea(NRF) under Ministry of Education.

Triboelectric nanogenerators (TENG) are intriguing energy harvesting devices that convert mechanical energy into electricity and could power small portable devices or charge batteries [1]. Here we report contact electrification of glassy polymers at different temperatures and observing a dramatic three orders of magnitude increase in surface charge when passing the transition temperature to a rubbery state. Transition is accompanied by a larger force required for separation due to material softening and increased stickiness. This enhances material transfer during contact/separation cycles and indicates covalent bond breaking as the mechanism for increased polymer triboelectrification. Undeniably, surface roughness plays a great role in contact electrification, however in our study its impact is overrated when compared to the influence of physical properties.

[1] Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano 7 (2013) 9533-9557.

The field related to triboelectricnanogenerator (TENG) devices is emerging rapidly. Many original and creative TENG concepts have been presented in the literature for harvesting mechanical energy and converting it into electricity (Lee et al., 2019). The working principle of TENG devices is straightforward. Due to electrode oscillation or movement, a potential difference is created, which causes a current flow in the external electric circuit. TENG devices can be integrated into fabrics, wearables, interior objects, membranes (to harvest energy from sound), and even implantable devices.
To enhance the performance of TENG, different approaches have been used. The most obvious way is to increase the specific contact area via nanostructuring (Zhang et al.). Another possibility is the modification of surface or physicochemical properties of the triboelectric material (Sutka et al., 2019). The performance can be also enhanced by using ferroelectric polymer or composite films as the contacting surfaces (Bai et al.). State-of-the-art performance of ferroelectric material-based TENG devices can be expected when the ferroelectric material layers on contacting sides of the device are inversely polarized (Sutka et al). However, previous works related to TENG devices based on ferroelectric materials overlook the dipole that forms between the triboelectric surface charges on contacting surfaces. As soon as we consider this additional factor, it follows that the electric field direction from ferroelectric dipoles should match the direction of the electric field from triboelectric surface charge to achieve maximum electric field strength and electrostatic induction.
Polydimethylsiloxane (PDMS), ethylene-vinyl acetate copolymer (EVA), poly(vinyl acetate) (PVAc), and poly(methyl methacrylate) (PMMA) were used in our studies to prepare TENG devices. The polymer films were spin coated on indium tin oxide (ITO) conductive electrode and contacted against another ITO. The polymer films were given ferroelectric properties by adding 7.5 vol% BaTiO₃ nanoparticles. The sign of triboelectric surface charges formed on pure polymers after contacting against ITO was determined by measuring the current between the underlying electrode and the ground in Faraday cup mode. Polymers PDMS, PVAc, and EVA obtain a negative charge on their surface whereas for PMMA a positive charge is observed when contacted ITO. The sign of the net triboelectric charge did not change when BaTiO₃ nanoparticles were incorporated into the polymers. All poled BaTiO₃/polymer composites exhibit piezoelectric properties. The piezoelectric charges of 2.9 pC/cm² , 10.9 pC/cm² , 3.7 pC/cm² , and 2.4 pC/cm² were measured for BaTiO3/EVA, BaTiO3/PDMS, BaTiO3/PVAc, and BaTiO3/ PMMA composites, respectively. The higher piezoelectric response of BaTiO₃/PDMS could be attributed to its larger deformability under the constant loading force. Composite layer in each of these TENG devices was tested as non-poled and also positively and negatively poled, so that ferroelectric dipole is matched and mismatched with the previously determined surface charge.
As example BaTiO₃/PDMS TENG device performed excellently, and the output was superior to any other presented in this study. The peak open-circuit voltage (V_OC) of device with matched dipoles reached 460 V, instant energy and power densities of this TENG device reached 9.7 mJ/m² and 143.2 mW/m² , respectively. For comparison, a TENG device from the same polymers without BaTiO₃ NPs shows V_OC as small as 16 V and three orders of magnitude smaller energy and power densities of 0.012 mJ/m² and 0.104 mW/m². Also, the TENG device from the same inversely polarized composite films but with mismatched dipoles exhibited significantly lower output—V_OC of 300 V and instant energy and power densities of 4.0 mJ/m² and 91.6 mW/m² from the same contacting area.

Mixed oxide thin films have found their way into numerous applications ranging from high-k dielectrics to gas sensors to optical coatings with tunable properties. The field is vast and most research conducted on mixed oxide coatings has been approached from the experimental side, leading to an arduous and time-consuming trial-and-error process. In the last decade, emerging computational methods have been conductive to accelerated materials discovery. In particular, the genomics approach behind open access initiatives such as the Materials Project aims to provide materials scientists with a new set of tools: large databases of computational calculations of known and predicted materials. This web-based database leads the way for in silico material design that can guide experimentation.

In this work, we carried out an extensive study of tantala (Ta₂O₅) amorphous thin films doped with Al, Si, Sc, Ti, Zn, Y, Zr, Nb and Hf grown by reactive sputtering. The influence of the dopant and post-deposition annealing on the film properties and structure was characterized by grazing-incidence x-ray diffraction, x-ray photoelectron spectroscopy and spectroscopic ellipsometry. Using the Materials Project database, compositional phase diagrams based on density functional theory calculations were constructed for all mixed oxide systems and the annealing process was evaluated via grand potential phase diagrams with varying oxygen chemical potential. The computational calculations agree well with experimental results, predicting formation of ternary compounds after annealing in some systems. When no ternary phase can be stabilized, the dopant acts as an amorphizer agent increasing the crystallization temperature of tantala. In the case where the dopant forms an oxide with similar formation enthalpy to tantala we experimentally observed effects of oxygen competition that lead to formation of phases that are predicted be unstable. Overall, the predictions based on the Materials Project database provide valuable information for tailoring dopant type and concentration in mixed oxide thin films towards specific applications that might require increased thermal stability or to adjust processing conditions for ternary oxide production.

Supercapacitors, with long cycle lives and long-term efficiency, have potential to become an appealing alternative to lithium-ion batteries; however, the specific energy of these devices lags well behind current state-of-the-art lithium-ion batteries. Nevertheless, numerous opportunities exist to optimize supercapacitors through the manipulation of the structure and morphology of novel materials. The implementation of hybrid supercapacitors, a device which combines a pseudo-capacitor with an electrical double layer capacitor, are of interest because material composites have greater charge storage capabilities than their components alone. The most effective hybrid capacitors for charge storage are ones possessing high specific capacitance (capacitance per unit mass). The optimization of a composite for high specific capacitance requires changes to intrinsic material properties, either by changing chemical or physical structures. The current study was aimed at the manipulation of the nanostructure of a manganese dioxide (MnO₂) – multi walled carbon nanotube (MWCNT) composite material via modulation of MWCNT composite concentration. The composite nanostructure was characterized using transmission electron microscopy (TEM), x-ray diffraction (XRD), charge-discharge tests, and cyclic voltammetry. Ten composites, each with different initial MWCNT concentrations, were synthesized via hydrothermal method. All ten concentrations were imaged via TEM. Of those ten, three were chosen to be characterized with electrochemical methods and XRD: 4.8%, 16.9%, and 33.7% mass MWCNT. TEM imaging reveals a nanostructure of polycrystalline MnO₂ adsorbed onto MWCNT. Results indicated that, on average, the thickness of MnO₂ coating decreased as percent mass MWCNT increased. At 33.7% MWCNT mass, MWCNT agglomeration became hard to mitigate and dominated MnO₂ adsorption structure. This is supported by XRD of 33.7% sample showing emergence of (002) plane of CNT, indicating widespread non-coating of MnO₂ in composite. Charge-discharge tests were conducted in with a 1 V window at 0.5, 1, and 2 A/g. Charge-discharge tests with 1 A/g indicate highest specific capacitance for 16.9% mass MWCNT at 150 F/g, compared to 70 and 45 F/g with 4.8% and 33.7% mass MWCNT, respectively. Preliminary results indicate that as MWCNT concentration increases, there is a decline in the average MnO₂ aggregation thickness, with this thickness potentially impacting the specific capacitance of the resulting device.

Oxide perovskite is an excellent platform for energy, sensing, and electronics applications, and their dielectric property has been extensively studied to date. Recently, superlattices of dielectric (e.g. SrTiO₃) and ferroelectric (e.g. PbTiO₃) materials have been shown to exhibit various emergent properties. An example is a newly discovered materials state of which could overcome fundamental limitations on the power consumption of field-effect transistors. Another emergent property is novel topological states of polarization such as vortex-antivortex arrays and with quantized skyrmion number. The formation of the polarization structures involves nanometric domains with opposite polarization orientation separated by domain walls (DWs). Highly complex DW characteristics as well as DW interactions dictates the design and control of functional ferroelectric devices. Furthermore, these topological phases can be switched on and off by ultrafast light pulses, opening up a fascinating opportunity for light-activated pathways to thermally-inaccessible and emergent metastable states. The scientific challenge here is to understand the intricate interplay between lattice polarization, strains in complex superlattice and polarization-vortex geometries, and excited electrons which triggers the light-induced structural transitions from first principles.
In this talk, I will discuss non-adiabatic quantum molecular dynamics (NAQMD) simulations of light switching behavior of topological states in a SrTiO₃/PbTiO₃ superlattice, as well as artificial neural network (ANN) MD simulations to study the structure and dynamics of polar vortex arrays encompassing sufficient length scales to represent realistic supercrystalline dimensions.
This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number The simulations were performed at the Argonne Leadership Computing Facility under the DOE INCITE and Aurora Early Science programs and at the Center for High Performance Computing of the University of Southern California.

BaZrO₃ is one of the end-members of remarkable perovskite systems: the binary Ba(Zr,Ti)O₃, studied for its relaxor-to-ferroelectric crossover, and the pseudo-ternary (Ba,Ca)(Ti,Zr)O₃, identified as promising for lead-free piezoelectrics. Pure BaZrO₃ exhibits the simple cubic structure down to 2 K, but theoretical DFT studies show that the cubic phase is slightly unstable, with an unstable phonon mode that should give rise to an antiferrodistortive transition driven by octahedral tilts. It has been also suggested that such distortions exist as distorted nano-domains. Experimentally, because of its very high melting point, investigations of BaZrO₃ have been mostly limited to polycrystalline samples. Here, we revisit its structure and properties using recently grown single crystals. In particular, we studied the unstable phonon mode by neutron scattering, and performed Raman measurements in a wide range of temperatures and pressures (up to 20 GPa). With the support of detailed calculations of the lattice dynamics, we show that the Raman spectrum does not confirm the hypothesis of distorted nano-domains, but instead can be explained by second-order scattering.

In recent years, environmentally friendly supercapacitors are gaining importance amongst researchers due to environmental concerns. Cellulose is one of the environmentally friendly biomaterials having properties such as renewability, degradability, biocompatibility, and costeffectiveness that has drawn a considerable amount of attention in the energy applications. In the present work, chemically synthesized cellulose nanoparticles were used in the energy storage applications to develop a supercapacitor. A mixture of carbon and cellulose nanoparticles was used as the electrode materials which were coated on a stainless-steel disk and assembled in a coin cell to develop a supercapacitor. Various electrolytes were used in the assembly to determine the best suitable one to obtain improved specific capacitance with large cyclic stability.

Europium titanate is known to crystalize in the pyrochlore crystal structure with the general formula A₂B₂O₇. Within the crystal structure, the A-site cations are occupied by Euⁿ⁺ ions whereas the B-sites are occupied by Tiⁿ⁺ ions in which the spins of the atoms are arranged on the vertices of tetrahedrons linked together at their corners. The high degree of freedom of rotation and vibration in the three-dimensional framework of tetrahedrons allows for a stronger spin interaction in the crystal lattice. The result of this interaction brings about a variety of magnetic properties that largely depends on the A-site cations, in this case, Euⁿ⁺ ions. In this work, novel piezoelectric europium titanate nanorods have been synthesized by using a simple solvothermal route in the temperature range 160-180 ^oC. TEM micrographs of the as-prepared samples show nanorods with lengths ranging from 0.3 to 1.2 µm and an average diameter of 40±5 nm. The nanorods become more elongated but rather thinner (length range = 0.5 – 1.6 µm, and diameter = 10±5 nm) when the as-synthesized powder was annealed at 800 ^oC. Powder X-ray diffraction pattern of the nanorods showed pure and highly crystalline pattern consistent with the diffraction patterns seen in Eu₂Ti₂O₇ nanocrystals. Dielectric spectroscopy of the pelletized Eu₂Ti₂O₇ revealed high-k dielectric constant up to e = 4000 at room temperature. In this study, scanning probe microscope was used to measure the magnetic interactions in the solution-processed Eu₂Ti₂O₇ nanorods thin-film. AFM topography images showed that the fabricated nanocrystals thin-film have an average thickness of 180 nm on a silicon substrate. The corresponding magnetic force microscope (MFM) images, were obtained my raising the magnetized MFM probe by 25 nm (delta height) above the sample surface in the tapping/lift mode. Since the probe tip uniformly scans at a height of 25 nm above the sample surface, the topography signal of the sample surface was drastically minimized allowing for the magnetic interactions to be mapped. This method allowed for the detection of the nanoscopic magnetic domains by measuring the interaction between the cantilever tip and sample surface. The widespread magnetic interactions can be visibly seen as a change in contrast (phase shift) in the amplitude and phase images.

The deposition of BiFeO₃ has been widely investigated in the recent years. Among multiferroic materials BiFeO₃ is of a special interest since its ferroelectric and magnetic transition temperatures are well above the room temperature (T_C ≈ 1100 K and T_N ≈ 650 K) [1]. Depositions of BiFeO₃ films by chemical routes: sol-gel, chemical solution deposition (CSD) and physical methods: sputtering, pulsed laser deposition (PLD) on single crystals such as SrTiO₃ or LaAlO₃ are well established. Despite its major role in industry MOCVD process is not as much investigated as other deposition techniques, but it has the major advantage to be easily scaled up. Recent works have been focused on A-site doping with rare-earth ions [2].

This work reports a simple MOCVD process for Dy-doped BiFeO₃ thin films on conductive Nb-doped SrTiO₃ (100) substrates. A multicomponent mixture composed of Bi(phenyl)₃, Dy(hfa)₃diglyme and Fe(tmhd)₃ (phenyl = –C₆H₅, H-hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, diglyme = 1-methoxy-2-(2-methoxyethoxy)ethane, H-tmhd = 2,2,6,6-tetramethyl-3,5-heptandione), has been used as precursor source. Nb:SrTiO₃ is acting simultaneously as a substrate and a bottom electrode, top gold electrodes have been sputtered after film deposition.

The piezoelectric, pyroelectric, and ferroelectric properties strongly depend on the structural and compositional properties of the deposited film with attention to the Dy doping. This correlation has been done using X-ray diffraction (XRD), for the structural characterization, field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPX) analyses for the morphologic and chemical characterizations, respectively. For the functional study, the permittivity, piezo-, pyro- and ferro-electric response of deposited films have been investigated by impedance spectroscopy, Piezoelectric Force Microscopy (PFM), pyroelectric measurement and P-V loop, respectively. Significant correlation between Dy doping and film properties have been observed and promising pyro-electric coefficients were measured.


Acknowledgment
This work is supported by the European Community under the Horizon 2020 Programme in the form of the MSCA-ITN-2016 ENHANCE project, Grant Agreement N.722496.

References
[1] T. Choi, S. Lee, Y. J. Choi, V. Kiryukhin, S.-W. Cheong, Science (2009), 324, 63
[2] M. R. Catalano, G. Spedalotto, G. G. Condorelli, G. Malandrino, Adv. Mater Interfaces (2017), 1601025

Corresponding author: Graziella Malandrino, +39-095-7385055, [email protected]

Lithium niobate (LiNbO₃) nanomaterials have recently attracted considerable attention due to their wide range of applications in the fields of ferroelectrics, piezoelectrics, and nonlinear optics. This material has been referred to as the “silicon of photonics” due to its exceptional optical properties. Applications for quantum photonics, surface acoustic wave devices, ferroelectric based devices, etc. require the availability of LiNbO₃ in thin-film form. To this aim, various techniques have been applied such as capillary liquid epitaxial technique [1], RF- magnetron sputtering [2], spin-coating [3], and metalorganic chemical vapor deposition (MOCVD) [4]. MOCVD represents one of the most appealing processes in view of its easy scalability, but its application requires the use of precursors with appropriate mass transport properties. Despite these promising advantages, industrial applications of an MOCVD process are still limited, likely because the commercial availability of starting molecular compounds is insufficient. Very few lithium precursors are known for vapor phase processes, either MOCVD or atomic layer deposition.
In this work, we report on the syntheses, through a single-step reaction, of novel adducts of lithium hexafluoroacetylacetonato {Li(hfa)} with polyethers (monoglyme = {CH₃OCH₂CH₂OCH₃}, diglyme = {CH₃O(CH₂CH₂O)₂CH₃}, triglyme = {CH₃O(CH₂CH₂O)₃CH₃} and tetraglyme {CH₃O(CH₂CH₂O)₄CH₃} [5]. They have been characterized by elemental analyses, FT-IR spectroscopy, ¹H, and ¹³C NMR. Single crystal X-ray diffraction studies provide evidence of interesting coordination moieties [7]. Lithium niobate films have been deposited on sapphire and Si using direct liquid injection (DLI-) MOCVD reactor. X-ray diffraction and scanning electron microscopy characterizations have allowed assessing the film quality.
The mass transport properties of these adducts have been investigated through thermogravimetric (TG) analysis. The TG curve of the “Li(hfa)monoglyme” shows a step mass loss with a residue of 6% at 338 ^oC. Thermal analysis data revealed a discrete behavior, when precursors are used in a suited temperature range, regarding conventional MOCVD applications, while its solubility and clean decomposition make these complexes interesting precursors for liquid assisted processes such as liquid-delivery and aerosol-assisted MOCVD or sol-gel applications.
The depositions have been carried in a pulsed liquid injection MOCVD reactor using the different lithium precursors and a correlation between the precursor nature and film properties have been derived.
Acknowledgments
This work is supported by the European Community under the Horizon 2020 Programme in the form of the MSCA-ITN-2016 ENHANCE project, Grant Agreement N.722496.

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[5] N. Peddagopu, P. Rossi, C. Bonaccorso, A. Bartasyte, P. Paoli & G. Malandrino, Dalton Trans., 49, (2020), 1002.

The need for high power density, flexible, pulsed power and lightweight energy storage devices requires the use of polymer film-based dielectric capacitors. The maximum energy storage density of a dielectric capacitor is directly proportional to the square of the maximum voltage that can be applied to the dielectric without causing the electrical breakdown. Theoretically, it has been shown that chain ends contribute adversely to the electrical breakdown of polymer dielectrics at high electric fields, resulting in low energy density in polymer capacitors. In this work, we enhanced the dielectric breakdown voltage and hence the energy density of the polymer capacitor by using well-ordered high molecular weight block copolymers (BCP), in which the chain ends are segregated to narrow zones. The well-ordered and easily processable amorphous BCP based capacitors exhibit an energy density more than 5 J/cm³, which is higher than that of industrially used crystalline biaxially oriented polypropylene. Cyclic homopolymers (no chain ends) and linear homopolymers having chemistry-controlled chain ends also show enhanced dielectric breakdown strength, resulting in higher energy density as compared to their linear counterparts. These novel insights into manipulating chain end distribution such as in BCPs and with molecular topology to increase the energy density of polymers will help fulfill next-generation energy demands.

Biochar carbon YP-50 exposed to gamma radiation at 50 kGy, 100 KGy, and 150 kGy was used as an electrode for an electric double-layer capacitor. The gamma radiation affected the pore structure and pore volume of the biochar electrodes. The optimized surface morphology, pore structure, and pore volume of the biochar with an irradiation dose of 100 kGy showed outstanding specific capacitance of 246.2 F g^–1 compared to the untreated biochar (115.3 F g^–1). The irradiation dose of 100 kGy exhibited higher specific power and specific energy of 0.1 kW kg⁻¹ and 34.2 W h kg⁻¹ respectively, with a capacity retention of above 96% after 10,000 cycles at a current density of 2 A g^-1. This improvement can be attributed to the decrease in average particle size, an increase in the porosity of biochar carbon. Besides, the charge transfer resistance of supercapacitor is significantly reduced from 21.7 Ω to 7.4 Ω after treating the biochar carbon with 100 kGy gamma radiation, which implies an increase in conductivity. This gamma radiation strategy to pretreat the carbon material for improving the properties of carbon materials can be promising for the development of high-performance supercapacitors for large-scale applications.

Current state-of-the-art biaxially oriented polypropylene (BOPP) film capacitors suffer from low temperature rating (<85 °C) and low energy density. At Case Western Reserve University, we have developed a novel multilayer film technology to meet the stringent requirements for next-generation dielectric film capacitors for electric vehicles. By multilayering a high breakdown strength polymer such as polycarbonate (PC) with a high dielectric constant polymer such as poly(vinylidene fluoride) (PVDF), we have achieved high breakdown strength (>600 MV/m), high energy density at breakdown (16 J/cm³), and relatively low dielectric dissipation factor (tanδ <0.005) and hysteresis loss (15% loop area). In this presentation, we will identify and discuss fundamental issues for these multilayer dielectric films, which include polarization of amorphous and crystalline dipoles, polarization of impurity ions, and interfacial polarization to achieve enhanced breakdown strength and lifetime at high temperatures.

We report on the successful synthesis of flower-like MoS₂/N-doped carbon dots (CDs) composite nanospheres via a facile solvothermal method making use of the layered structure of MoS₂ and the excellent conductivity of CDs. The structural, compositional, and morphological properties of the fabricated electrodes were investigated using XRD, XPS, EDS, Raman spectroscopy, FTIR, TEM, and FE-SEM techniques. The addition of carbon dots to MoS₂ improved the cyclic stability, wettability, and conductivity of the fabricated composites upon their use as supercapacitor electrodes. The supercapacitor electrodes showed a specific capacitance as high as 250.55 Fg^-1 at a scan rate of 10 mV/s and 149.21 Fg^-1 at 0.5 A/g. A symmetric supercapacitor device composed of MoS₂/NCDs composite and activated carbon as the positive and negative electrodes, respectively delivered an energy density and a power density of 20.01 Wh/kg and 350 W/kg, respectively with 98% Coulombic efficiency after 5000 cycles, indicating the high performance of the fabricated electrodes.

Achieving stable energy density at elevated temperature remains a great challenge for ferroelectric polymer-based capacitors. It seems that the emerging ferroelectric polymers (e.g. poly (vinylidene fluoride) (PVDF) and its copolymers), which possess spontaneous polarization and large permittivity, provide a promising solution to meet the ever-increasing requirement for high energy density. Nevertheless, the state-of-the-art PVDF-based ferroelectric polymer usually exhibits mediocre temperature stability in energy storage performance, which cannot fully satisfy the applications at elevated temperature caused by the energy consumption or nearby heat sources.
In this work, we report a strategy for stabilizing high energy density of ferroelectric polymer (PVDF) over a wide temperature range by blending with a miscible high-T_g polymer (polymethyl methacrylate (PMMA)). The blending material forms an alternating lamellar structure consisting of high-polarization crystalline regions and strengthened amorphous regions. It is found that this special morphology is able to confine the constructional change of stacked lamellar structure at elevated temperatures. Accordingly, the associated energy storage performances, including energy density, energy storage efficiency and breakdown strength, are remarkably enhanced in a wide temperature range (30 to 70^oC). At 70^oC, the energy density and efficiency of PVDF/PMMA(80/20) is 9.8J cm^-3 and 67.8%, which surpasses most neat ferroelectric polymers at such an elevated temperature. This work reveals the relationship between lamellar confinement of polymer blend and electric properties under temperature variation, and might provide a valid approach to attain high energy density and temperature stability in ferroelectric polymers.

As one of the most important renewable energy storage technologies, film capacitors have been used in modern electronics and power industries such as wearable electronics, hybrid vehicles, and other fields due to their high power density, long working hours, and environmental friendliness. With the development of electronic devices toward miniaturization and high performance, there is an urgent need for dielectric materials with high energy storage density. A high breakdown strength (E_b) is crucial for dielectric materials to ensure reliability of electric circuits and to improve electrostatic energy storage density.E_bis the result of space charge movement in the dielectrics under the applied electric field. Effectively reducing the density of charge carriers in the dielectrics is one of the keys to enhancing E_b. In this study, hybrid nanoparticles with special physical microstructure were designed and fabricated. When they were used as fillers,these particles possessing isoelectronic traps or heterojunction have proved efficient in confining space charge carriers generated under applied electric field and meanwhile suppressing their movement through the whole polymer system. As a result, electrical conductivity was reduced and E_bwas enhanced significantly, leading to improved energy storage density.

Carbon dioxide emissions and ever-growing environmental issues have stimulated the exploration of new energy sources that are required to be renewable and environmentally friendly. Most of the energy in nature is intermittent, such as solar, wind, and tidal energy, which puts forward critical requirements for energy conversion systems with ultrahigh power densities. Among all energy conversion technologies, the electrostatic capacitor has the fastest charge-discharge speed, which has been utilized as the invertors/convertor in hybrid electric vehicles. However, polymers, as the main functional part in electrostatic capacitors, are severely limited by unsatisfied energy densities due to their low dielectric constants, compared with ceramic counterparts.
Although much research work has been done to improve the dielectric polarization, e.g., introducing ceramic nanoparticles with high dielectric constant to construct nanocomposites, the increase in energy storage density is still restricted due to the compromise in electrical insulation capabilities. These ceramic nanoparticles generally have high electrical conductivity. When a large amount of the nanoparticles is introduced into polymers, conductive paths will be formed in the nanocomposites. As a result, charge migration will become easier, which will lead to not only a high energy loss but also a suppressed breakdown strength.
Here, to address this issue, layered nanocomposites reinforced by 2D interfaces are designed, which possess high dielectric polarization and insulation capabilities simultaneously. MMT nanosheets with anisotropic conductivities (surface conductivity > bulk conductivity) are adopted as the interface materials. It is demonstrated that this 2D interface plays different roles when it is interposed at different positions. In detail, it can block the charge injection when it locates between the electrodes and the surface of the polymers, while it tends to dissipate charges along the in-plane direction when they are inside of the polymers. The above two functions can simultaneously serve the purpose of preventing carriers from penetrating the polymer films. It causes a suppressed leakage current and energy loss, thus leading to an enhanced charge-discharge efficiency and energy density. This proposed method is applicable to a wide range of polymers. Favorable energy storage performance can be achieved in a harsh environment, e.g., high temperature (as high as 250°C), when high-temperature resistant polymers are selected as the matrix. For example, a high energy density of 4.3 J/cm³ is found in an interface-enhanced barium titanate/polyamideimide nanocomposite at 150°C, which can still be 2.2 J/cm³ even when the temperature is elevated to 250°C. This work systematically illustrates the impact of the interface reinforcement on the electrostatic energy storage performance and provides guidance for the exploration of high-performance dielectric nanocomposites for renewable energy storage and conversion.

Sustainable energy sources – such as wind, wave and solar – are attractive for reducing CO₂ emissions and tackling climate change. Often these energy sources are placed at large distances from where the energy is actually needed; off-shore wind farms and desert solar parks. To transfer the electrical power from such remote locations high voltage direct current (HVDC) is the preferred technology due to lower transmission losses. In the design of high voltage component the dielectric properties of the materials used are of significant importance. The conductivity of the leakage current in and the electrical breakdown field of the dielectric material impact and limits the design. To understand how these properties are affected by the materials chemical composition and morphology, material simulation is one approach to understand these phenomena in more detail.

Material simulation methods such as density functional theory (DFT) and molecular dynamics (MD) has an increased usage in literature to study dielectric properties of materials relevant for HV components. Basic properties such as band gaps and trap states, to more challenging properties to calculate such as mobility and electrical breakdown fields are reported. An additional important area is to include material simulation methods in multiscale simulation models to incorporate phenomena on longer time and length scales possible.

In this talk we will present result from several different studies of dielectric polymers using DFT, MD and kinetic Monte Carlo (KMC). Primarily the material of interest in these studies is polyethylene (PE) which is used as insulation in HVDC cables. PE is a semi crystalline material and have a complex structure at micrometer scale. Using linear scaling DFT (LSDFT) we have investigated the electronic structure of different parts of PE morphology. Band gaps and electron localization are calculated and used to determine electronic charge transport properties. A combination of DFT, MD and KMC are used to calculate mobility in PE. A coarse grained model for hopping transport in PE is also being developed. Recently we also applied LSDFT to investigate electronic structure of another dielectric polymer, poly(ethylene oxide), which may be used as polymer electrolyte in solid state batteries. Coupling to models at longer time and length scales are discussed.

Breakdown strength (Ebd), the maximum electric field that can be applied on a dielectric polymer without destroying its insulating characteristics, sets an upper limit on the maximum energy that can be stored in this material. Despite its significance, the breakdown phenomena in polymers remain poorly understood owing to the complex nature of the phenomena. This is a major challenge in the development of new high-energy polymer dielectrics. In this work, we present a multi-step strategy to develop novel high breakdown polymers, sidestepping this challenge - by finding and improving properties that are correlated to Ebd (and easy to estimate) rather than directly attempting to improve Ebd. Specifically the steps involved include - (i) Identification of potential proxy properties correlated to Ebd utilizing available experimental data. (ii) Development and utilization of Density Functional Theory (DFT) based computational models to better understand the proxy properties and to create a dataset which can be used to build machine learning based prediction models of the proxy properties. (iii) Development of machine learning (ML) based prediction models capable of predicting the proxy properties of any given polymer. These models are used as screening tools to downselect potential high breakdown polymers from a candidate set which is much larger than what DFT can handle. (iv) Generation of novel potential high Ebd polymers utilizing optimization strategies (Genetic Algorithm, Graph to Graph translations etc.) and ML models. Finally, the obtained novel polymers are further down selected and suggested for synthesis.

Controlling thermal conductivities of materials is important for a wide range of applications, from thermoelectrics for clean energy generation to electronic devices and thermal barrier coatings. The thermal conductivity is commonly estimated using molecular dynamics simulations within the Green-Kubo formulation. This requires a force field that is both 1) an accurate estimate of the interatomic interactions and 2) fast enough to allow simulations with sufficiently large length and time scales. Traditionally, only empirical force fields have fulfilled both of these requirements, which severely limits the applicability of the method.

In this work, we employ the Gaussian Process-based FLARE force field, which automatically learns the interactions of more complex materials than empirical force fields. The resulting model can then be mapped to a low-dimensional, computationally efficient model. Through GPU-acceleration with LAMMPS and the Kokkos library, we achieve excellent performance and obtain well-converged estimates for the thermal conductivity. Furthermore, we investigate state-of-the-art sampling and spectral denoising methods for further acceleration of the simulations. We apply this machinery to dielectrics with high thermal conductivity and study the effect of impurities.

High fidelity prediction of dielectric permittivity is of critical importance for the design of better performing devices. However, conventional calculations that are based on quasi-harmonic approximation, are known to incur huge error in particular for ultrahigh-k material such as SrTiO₃. This is attributed to the missing of key physics i.e. lattice anharmonicity in the theory. To overcome this, we developed computational workflow that account for anharmonic contribution by performing self-consistent phonon (SCP) theory calculations using Alamode. Preceding preparation of higher order interatomic force constants are determined based on Hellmann-Feynman forces of standard DFT calculations. It turns out that efficient structure sampling is critical to reduce computational cost and maintaining errors in harmonic force constants as small as possible are very important for the accuracy of following SCP calculations. Resultant temperature dependency of transverse optical phonon frequency agrees well with experimentally measured ones. In addition, dielectric permittivity of SrTiO₃ calculated from phonon band structure is in good accord with values reported in literatures. We also confirmed that this workflow is able to predict para- to ferro-electric phase transition temperature of ferroelectric materials such as BaTiO₃ and PbTiO₃.

Metal oxide-based Resistive Random-Access Memory (RRAM) exhibits multiple resistance states, arising from the activation/deactivation of a conductive filament (CF) inside a switching layer. Understanding CF formation kinetics is critical to achieving optimal functionality of RRAM. Here a phase-field model is developed, based on materials properties determined by ab initio calculations, to investigate the role of electrical bias, heat transport and defect-induced Vegard strain in the resistive switching behavior, using MO_2-x systems such as HfO_2-x as a prototypical model system. It successfully captures the CF formation and resultant bipolar resistive switching characteristics. High-throughput simulations are performed for RRAMs with different material parameters to establish a dataset, based on which a compressed-sensing machine learning is conducted to derive interpretable analytical models for device performance (current on/off ratio and switching time) metrics in terms of key material parameters (electrical and thermal conductivities, Vegard strain coefficients). These analytical models reveal that optimal performance (i.e. high current on/off ratio and low switching time) can be achieved in materials with a low Lorenz number. This work provides a fundamental understanding to the resistive switching in RRAM and demonstrates a computational data-driven methodology of materials selection for improved RRAM performance, which can also be applied to other electro-thermo-mechanical systems.

The performance and reliability of organic dielectric devices depends critically on the chemistry and morphology of metal-organic semiconductor interfaces. Conjugated polymers represents a promising class of organic dielectrics due to their excellent electronic properties which can be tuned over 10 orders of magnitude through reversible doping processes. Aluminium (Al) is popular candidate for device electrodes on account of its low work function and capability to get thermally deposited on top these dielectric polymers to enable desired device architectures. The reliability and efficiency of these devices is dependent upon the stability of these Al-dielectric polymer interfaces since the charge injected from Al layer is a key contributor to polymer degradation and dielectric breakdown. Here, we perform a hybrid reactive molecular dynamics and density functional theory based study to understand how various interfacial features affect charge injection barriers. We utilize first-principles data on Al-polymer interfaces to design efficient ReaxFF force fields capable of simulating realistic interfaces. We demonstrate this approach on various dielectric polymers such as Polypropylene (PP), Polystyrene (PS), Polyethylene (PE) and Polyethylene terephthalate (PET) by carefully examining the effect of deposition rate and size of Al clusters, surface and bulk diffusion of Al clusters on polymer substrate and defects in polymer substrate.

Acknowledgments
This work was supported by the Office of Naval Research through a Multi-University Research Initiative (MURI) under grant number (N00014-17-1-2656). The simulations were performed at the Argonne Leadership Computing Facility under the DOE INCITE and Aurora Early Science programs and at the Centre for High Performance Computing of the University of Southern California.

A wide range of dielectric energy materials exhibit unusual properties that are due to soft, or highly polarisable constituent cations, e.g., Sn(II), Pb(II) or Bi(III). An accurate account of their behaviour may cause in modern first-principles calculations both methodological and technical difficulties, and, in general, requires high computational costs. Results of such calculations in turn are often difficult to analyse and interpret using chemical or physical intuition that is, however, essential in applied research. The oxides of these heavy main-group elements typically have a medium to wide band gap, and their materials show potential applications in many energy applications including solar/fuel cells and batteries. Examples include: SnO in both bulk and nanoparticle form have been studied as a photoelectric material and rechargeable batteries; whereas PbO and Bi₂O₃ have potential applications in thermoelectric materials and fuel cells, respectively; and SnTiO₃ and PbTiO₃ perovskites have ferro- and piezo-electric properties. Related compounds including transitions metals show exciting multiferroic properties.
Modelling the effect of the local environment on the Sn(II), Pb(II) and Bi(III) cations at the quantum-mechnical level of theory requires an accurate representation of, at the very least, their valence s and p orbitals; that can hybridise and occupied by the valence electrons form an sp-lone pair in the environment of closed-packed oxides, which avoids forming any direct chemical bonding and makes the cations relatively more polarisable than other cations with closed valence electron configuration. As a result, the formation of an sp-lone pair typically induces a local structural distortion about such a cation, i.e., a reduction of the symmetry of the crystal structure, that may have a significant effect on the desired material property and is, therefore, crucial to model correctly. Alkaline earth metal oxides, for example, adopt the NaCl rock salt structure with the space group symmetry of Fm3m where each cation is at the centre of a regular octahedron composed of six oxygen anions at the vertices, whereas SnO and PbO have the space group symmetry of P4/nmm with the six nearest neighbour oxygen anions showing a breakdown of the octahedral symmetry.
Atomistic simulations, based on semi-empirical interatomic potentials – a useful conventional tool for the study of simple ionic materials – are many orders of magnitude cheaper to conduct than a full electronic structure approach, but, alas, cannot successfully reproduce the atomic structure and dielectric properties of materials containing Sn(II), Pb(II) or Bi(III). To resolve this difficulty and model the effect of the sp-lone pair electron density we introduce a simple quantum mechanical treatment to the lone pair cations based on a one-centre Hamiltonian approach with an s and p atomic basis set. Polarisation effects of the other species are captured using a well-known Shell Model. The new approach has been implemented in our in-house software Sp-Lone pair involved Atomistic Model (SLAM) code and could easily be implemented in popular molecular-mechanical codes such as GULP and LAMMPS. During this presentation we will demonstrate the accuracy and efficiency of our model by comparison of our results with those from first principles calculations using Density Functional Theory (DFT) and previous interatomic-potential models for bulk materials doped with Sn(II) and nanosized molecular SnO clusters.

Flexible pressure sensors are the basic components for many different applications, such as electronic skins, robotics, health-monitoring devices, and human-machine interfaces. Among different working mechanisms, capacitive pressure sensors are widely-used owning to their low energy consumption, simple structure, and high reliability. Pressure sensors in the form of coatings have the advantages of facile applications on various complex surfaces because of the flowability before solidification, capability for large scale production, and low cost. But how to prepare pressure sensing coatings to meet the requirements for different applications remains to be addressed. This talk covers our recent work to enhance the performances of pressure sensing coatings through the rational design of dielectric materials.
Firstly, I will discuss a highly sensitive capacitive pressure sensing coating using thermally expandable microspheres/ polydimethylsiloxane (PDMS) composite as the dielectric layer. The microspheres can expand during the thermal curing process of PDMS, which greatly enhance the compressibility and thus increase the sensitivity of the pressure sensors. At the same time, it achieves a broad pressure sensing range from 10 Pa to 4.5 MPa and also features low hysteresis and good stability against repeated compression.
Secondly, I will discuss a strategy to achieve linear response of capacitive pressure sensor over a broad sensing range. Conventional capacitive pressure sensors usually exhibit a highly non-linear response as the sensitivity drops dramatically towards high pressure. Our strategy employs a percolative composite as the dielectric layer. The fast increase in dielectric constant during compression can compensate the sensitivity drop from the decreased compressibility, leading to a linear response. Owning to the nature of polymer composite, its dispersion not only can be conformally coated on surfaces with various complex shapes, but also can be molded into films with surface microstructures to achieve a unique combination of high sensitivity and linear response over a wide pressure sensing range.

Dielectric particles are often added to 3D printing inks to tailor the macro level permittivity of printed dielectric substrates and coatings. In these inks, the combined role of particle morphology, discrete spatial arrangement and material properties on variance is difficult to distinguish experimentally and hence poorly understood. This is primarily due to the large parameter space of processing variables as well as electrical sensitivity to local heterogeneities. We address this challenge by combining a finite element capacitor model with a neural network biased genetic algorithm (NBGA) to optimize the volume fraction, particle size and permittivity distributions of dielectric particles to identify systems with high capacitance variance. Analysis of the database generated from the optimization process provided insights on effect of polydisperse particles on the variance of the system. Design rules/strategies were also identified for achieving target variance. Unsupervised machine learning techniques were applied to the NBGA-created database to extract correlations between the spatial/material distributions of the dielectric particles and the capacitance variance. Collectively, this study provides a useful framework to correlate electrical performance with both macro- and microstructural variation sources, which is key to accelerating the development of 3D printing materials.

Highly thermally conductive and electrically insulating polymer composites are strongly demanded for thermal management applications including next-generation electronics and electric power systems. Anisotropic nanocarbons such as carbon nanotubes (CNTs) and graphenes are excellent nanofillers for improving thermal conductivity (TC) of polymers. However, nanocarbons exhibit high electrical conductivity and the addition of even small amounts of them into polymer matrices leads to decreased electrical insulation. Therefore, morphologically controlled CNT/polymer composites having highly electrical insulation were designed and fabricated [1,2]. However, these composites have not actually been applied towards highly electrically insulating materials requiring much higher TC (>10 W m^–1 K^–1). Meanwhile, boron nitride nanosheet (BNNS) is analogous to graphene and has an increasing interest for the abovementioned applications because of its extremely high TC and highly electrical insulation. BNNSs were easily prepared by direct exfoliation of h-BNs using carefully selected solvents such as ionic liquids (ILs) [3,4] and superacids [5]. ILs and superacids showed strong physical adsorption on h-BN surfaces, giving exfoliated, noncovalently functionalized BNNSs (NF-BNNSs) in high yields. The obtained NF-BNNSs showed high solubility in various solvents and maintained perfect crystallinity of h-BN structure. Therefore, NF-BNNSs acted as excellent nanofillers for dramatically improving TC of polymers. Highly thermally conductive and electrically insulating NF-BNNS/thermoplastic (TP) polymer composite films were produced through a simple wet-process using NF-BNNS solutions. NF-BNNSs were uniformly dispersed in the TP polymers, enhancing both the in- and through-plane TCs. Noncovalent functionalization of BNNSs with ILs or superacids gave high affinity between BNNSs and TP polymer matrices in the composites, leading to extremely high TC (≥15 W m^–1 K^–1) and improved volume resistivity compared with those of previously reported BNNS/TP polymer composites. Moreover, the composites exhibited high flexibility. The proposed approach is very promising for preparation of electrically insulating materials for various thermal management applications.

[1] T. Morishita, et al., J. Mater. Chem. 21, 5610 (2011).
[2] T. Morishita, et al., Compos. Sci. Technol. 142, 41 (2017).
[3] T. Morishita, et al., Chem. Commun. 51, 12068 (2015).
[4] T. Morishita, et al., RSC Adv. 7, 36450 (2017).
[5] T. Morishita, et al., ACS Appl. Mater. Interfaces 8, 27064 (2016).

It is important to determine the dielectric characteristics of semiconductor encapsulation materials based on epoxy resins. There are several such material systems employed to protect the active silicon circuitry. Some of the important properties of the materials are their glass transition temperature, filler content and the level of water absorption which leads to reliability challenges with respect to performance and lifetime. We study changes in the propeties by investigating dielectric relaxation using broadband impedance spectroscopy techniques. It is known that the dielectric relaxation in polymeric systems influenced by the presence of water, which demands special attention in sensitive applications. We will present measurement and analysis methods based on time and frequency domain techniques. It would be appreciated within the materials science community to improve material selection and performance evaluation efforts. We will also share information on the glass transition temperature estimates for a number of composites employed in semiconductor packaging.

Ferroelectric materials are promising candidate for both conventional (e.g. low power, scalable and non-volatile memory) and emerging (e.g. in-memory computing, analog synapses, coupled oscillator networks, spiking neurons and quantum computing) nanoelectronics applications. Conventional ferroelectric materials suffer from lack of thickness scaling and compatibility with CMOS processing. Ultra-thin doped HfO₂ based ferroelectric thin-films have emerged as an attractive option for non-volatile memory and ultra-low power computing due to CMOS compatibility and precise thickness control through atomic layer deposition. In this work, we systematically investigate the scaling properties of ferroelectric Hf_0.5Zr_0.5O₂ in a metal/ferroelectric/insulator/semiconductor (MFIS) capacitors to identify the design space for logic and memory applications.
MFIS capacitors are fabricated on p-Si wafers using standard CMOS processing with different ferroelectric thickness. The ferroelectricity in the doped HfO₂ thin film is introduced by rapid thermal annealing. This high temperature annealing steps leads to structural phase change (monoclinic to orthorhombic) and non-centrosymmetric crystal structure in the doped HfO₂. The dielectric constant, k, of the orthorhombic Hf_0.5Zr_0.5O₂ film is higher than those of HfO₂ and ZrO₂(k = 25) for all thicknesses as observed from the small signal capacitance-voltage (C-V) characteristics. At low gate biases, the Hf_0.5Zr_0.5O₂ film is hysteresis-free and shows negligible frequency dispersion indicating a high-quality interface. This observation indicates that orthorhombic Hf_0.5Zr_0.5O₂ can be a promising way to achieve high- k gate dielectric. At high gate bias, the thinner Hf_0.5Zr_0.5O₂ films shows polarization switching when the effective electric field across the ferroelectric film exceeds the coercive field. However, in the thicker Hf_0.5Zr_0.5O₂ film required gate bias for polarization switching is higher the critical field for dielectric breakdown.
Our results indicate that orthorhombic Hf_0.5Zr_0.5O₂ can be an attractive option for gate dielectric for ubiquitous electronics. The high dielectric constant and relatively lower effective charge of the ferroelectric thin film can enable the polarization switching of the thinner dielectrics. However, to enable non-volatile memory with lower switching voltage additional materials and device engineering would be required as the switching voltage weakly scales with ferroelectric thickness.

Here we report the charge accumulation phenomena in Pt nanoparticle (NP) films deposited by the atomic layer deposition (ALD) on borosilicate glass (Pyrex®) substrates. The in-plane capacitance of Pt NP films was measured by AC impedance spectroscopy using Au electrodes patterned with an interval of 6.0 μm on Pyrex substrates. The experimental results show that annealed Pt NP films exhibit very high capacitance on Pyrex substrates. This phenomenon was observed in neither electron-beam-evaporation-deposited-Pt-NP films on Pyrex nor ALD-Pt-NP films on quartz. Although the mechanism of this charge accumulation phenomenon was not fully elucidated, it is considered that protons conduct in the in-plane direction of a Pt NP film via adsorbed water and behave like a supercapacitor to enhance the in-plane capacity. It is found that if the effective volume contributing to the charge accumulation is only the Pt NP film, quite large volume power and volume energy densities (6.3 × 10⁶ W L⁻¹, 7.0 × 10² Wh L⁻¹) are achieved.

Thin film oxide dielectrics boast high breakdown voltages, low leakage current densities, and high dielectric constants making them ideal for use as gate dielectrics in thin film electronics. However, to fully realize the benefit of thin film electronics, low-cost high-throughput production is necessary. This means reducing not only processing time, but also processing temperature to allow for production on plastic substrates. In this work, we discuss the effects of low temperature UV-assisted processing and high-speed photonic curing on the properties of solution-deposited oxide dielectric films. First, we show the results from exposing sol gel precursor films to UV-ozone (UVO) through a shadow mask. This process accomplishes both lowering the conversion temperature to 250 °C and direct patterning the precursor film with exposed areas converted to oxide after final annealing. We find drastically different capacitance-frequency behavior between Al₂O₃ and ZrO₂ dielectrics, which in turn affects the In₂O₃ transistors’ behavior. The origins of this finding are studied through Fourier transform infrared analysis and time dependent drain current measurements. Next, we adapt photonic curing, the use short but high intensity pulses of broadband light, to selectively heat and convert ZrO₂ precursor films on flexible polyethylene naphthalate (PEN) substrates. Photonic curing is typically used to sinter metal nanoparticle inks into conductive metal traces on plastic substrates, but it has not previously been used to fabricate oxide thin film dielectrics on plastic substrates. We demonstrate that the conversion time is reduced from 10s minutes to < 1s and patterning using a shadow mask during photonic curing does not need subsequent thermal annealing, unlike in the UVO processing. Furthermore, by adjusting the photonic curing conditions, the oxide film can be selectively converted on top of Al electrodes, which act as selective area heat absorbers, but not on the bare PEN substrate, resulting in a self-aligned oxide pattern. We achieve a ZrO₂ dielectric with breakdown field of 8 MV/cm and areal capacitance of 200 nF/cm² with little frequency dispersion. Finally, we show a pathway to converting ZrO₂ dielectric at a roll-to-roll web speed of up to 50 m/min.

The field of disordered materials is experiencing a meteoritic rise within materials engineering. In particular, several computational and experimental efforts are aimed at synthesizing novel alloy and highly doped materials, and predictive modeling is a powerful tool to guide the materials space exploration. However, the modeling of charge transport in disordered materials presents a number of challenges. In fact, charge transport in disordered materials is often not well described by common semiclassical methods, such as the Boltzmann transport equation, which have instead been developed for simple dielectric crystals such as silicon. In this work, we present some of our recent efforts aimed at bridging this gap in the modeling of charge transport in disordered materials. In an alloy, in the regime of weak electron localization, the electronic band structure consists of several overlapping flat bands. Here, the prevalent conduction mechanism is due to vertical electronic transitions, rather than diffusion due to electron group velocity. We introduce an ab-initio formalism based on the Wigner distribution that corrects predictions of electronic conductivity by including the effect of these vertical transitions. We exemplify the method with an application to a narrow gap topological semiconductor, Bi₂Se₃, showing that bulk transport is dominated by vertical electronic transitions between the valence and the conduction bands, a phenomenon also known as Zener tunneling effect. Our approach is general, applies to any system and provides a step forward for the modeling of charge transport in disordered materials.

Remarkable progress has occurred over the past 15 years through nanodielectric engineering involving inorganic nanofillers, coatings, and polymer matrices. The challenges of dielectric polymers remain primarily toward breakdown strength in thin polymer films for capacitor applications. There reported several major technical approaches to enhance the dielectric strength of polymers and nanocomposites, including nanoparticle incorporation in polymers, filler polymer interface engineering, and film surface coating. This work will direct the attention to interface contributions, including the rational design of core-shell structures, the use of low-dimensional fillers and thermally conducting fillers, and the inorganic surface coating of polymer films. These efforts demonstrated the enhancement in dielectric strength by 40–160% when controlling the fillers below 5 wt% in polyvinylidenedifluoride composites. This work will also discuss the possible dielectric mechanisms and the positive role of interfaces against charge transport traps for attaining higher breakdown strength. The investigation of low-dimensional filler/coating materials of high thermal conductivity is considered to be a key scientific subject for future research.

The process of charge injection at metal/polymer interfaces is crucial to many areas of research and technology, such as organic light emitting and harvesting devices, high-voltage capacitors and cables. In this work, we study charge injection at metal/polymer interfaces for two polymers commonly used in high-voltage applications, namely polyethylene (PE) and polypropylene (PP). Using first-principles electronic structure methods, we compute charge injection barriers at model aluminium/PE and aluminium/PP interfaces. We show that the introduction of polar chemical groups (e.g., -COOH, -CH₂Cl, and -CHO) in the polymer chains at the interface can tune the intrinsic charge injection barrier significantly. We take into account of thermal disorder by averaging over a large ensemble of interface structures obtained from first-principles molecular dynamics trajectories. Our results suggest the possibility of rational design of metal/polymer interfaces via localised chemical modification.

Molecular layer deposition (MLD) and atomic layer deposition (ALD) are sequential, self limiting thin film deposition techniques, used to fabricate organic, inorganic, and inorganic-organic hybrid thin films. The hybrid organic-inorganic films deposited using MLD are usually metal alkoxides that are also termed as metalcones. Metalcone films can be subjected to thermal processing to selectively remove the organic parts of the film and induce porosity to form porous dielectric materials. However the as deposited and thermally processed metalcone films are unstable in ambient atmosphere due to the moisture absorption and permeation, which alters the chemical properties of these films. Since the dielectric constant of the water is very high (70-80), the net dielectric constant of these films increases after moisture absorption thus affecting the electrical properties. To preserve the chemical stability and electrical properties of metalcones, this work examines the use of an ultra thin ALD film grown over the as deposited and thermally processed films.
We report the growth of alucone thin films using trimethylaluminum and ethylene glycol as precursors for MLD and water (H₂O) as an oxygen source for ALD reactions. The as deposited MLD films are subjected to thermal processing from 150-350 C in inert atmosphere and under UV light to remove the organic components and induce porosity to form in-organic dielectric films. We examined the thermally induced changes in film‘s chemistry, thickness, density, etch rate and dielectric constant.
Incorporation of the organic components, formation of M-O-C bonding and water absorption is confirmed using FTIR in the as deposited MLD films. By subjecting MLD films to thermal processing the decrease in intensity of hydrocarbon bonds and changes in thickness, density compared to pure MLD films can be observed by FTIR and x-ray reflectivity respectively. The dielectric constant of these films is measured using capacitance-voltage measurements. The dielectric constant of the thermally processed films with a thin ALD layer decreases compared to only thermally processed films indicating that the thin ALD layer thus preserves electrical properties of the film. The as deposited and thermally processed films with and without the ultrathin ALD layer are subjected to CF₄/O₂ plasma etch and the MLD films without an ALD layer shows more than double the etch rate compared to the MLD films with an ALD layer. Both the decreased dielectric contant and etch rates show that the ALD layer acts as a barrier to prevent moisture absorption and preserve the chemical stability and electrical properties of the film.

Taking advantage of phase transition and reconfigurable metamaterials, dynamic control of nanoscale thermal modulation can be achieved through the near-field radiative thermal rectification devices. An active-tuning near-field thermal rectifier using reconfigurable phase-transition metamaterials is explored. The rectifier has two terminals separated by vacuum, working under a controllable operational temperature around the critical temperature of the phase-transition material VO₂. One of the terminals is a stretchable structure made of PDMS thin film and grating consisting of various types of phase-transition material. The effects of various inclusion forms and all the related geometric parameters are well analyzed. The controllable nanoscale thermal modulation can be achieved and the ultrahigh rectification ratios of 23.7 and 19.8, the highest values ever predicted, can be obtained for two deformation scenarios, respectively. It will shed light on the dynamic tuning of small-scale thermal transport and light manipulation.

Use of dielectric materials such as lead zirconate titanate films for dielectric energy storage or for piezoelectric applications necessitates that they be able to sustain high DC electric fields. This paper will describe the coupled failure mechanisms that control these lifetimes for donor-doped, acceptor-doped, and films with graded doping. Curiously, the median failure time of PZT films can be increased on doping with either donors or Mn as an acceptor. It is found that lifetimes under DC fields are dominated by a combination of oxygen vacancy migration, coupled with changes in the interface barrier heights for conduction in thin films. This was assessed through a combination of highly accelerated lifetime testing, thermally stimulated depolarization currents, impedance spectroscopy, deep level transient spectroscopy, and electron energy loss spectroscopy. The relic defect chemistry of the film, including variations in this defect chemistry at interfaces strongly influences both the failure and the asymmetry of the failure as a function of the electrical polarity. As film thickness and/or the piezoelectric coefficients rise, the critical stress required for crack initiation decreases, and the failure modes can shift to electro-mechanically induced failure. Ball-on-three ball mechanical testing demonstrates that the critical flaw size in the failure of the stack becomes the thickness of the piezoelectric layer itself.

Thermoelectric materials directly convert waste heat to electricity and thus hold great potential for power generation with low environmental impact. To be able to compete with established technology and find broad applications, thermoelectric power generators must have high energy conversion efficiency, which is determined by their figure of merit ZT. This, in turn, is proportional to the material's power factor σS², where σ is the electrical conductivity and S is Seebeck coefficient. Optimization of PF is a challenging task, since σ and S have conflicting dependencies on different tuning parameters such as carrier concentration and their effective masses.

In this work we employ ab initio calculations to investigate the effects of the electron-phonon interaction on the transport properties of thermoelectric materials, focusing on the example of half-Heusler semiconductors. We calculate electron-phonon scattering rates using Wannier-Fourier interpolation of electron-phonon matrix elements and the recently developed electron-phonon averaged approximation. We find that the electron-phonon scattering can lead to anomalous behavior of the transport coefficients, namely the conductivity decreases with increasing number of charge carriers, and Seebeck coefficient changes sign, leading to an unusual peak in the PF at high carrier concentrations. We discuss the origin and magnitude of this enhancement effect starting from the electronic band structure and identify design rules to maximize the material's performance.

Our research activities on the energy-related dielectric polymer nanocomposites will be reviewed. Micro-energy storage devices are highly demanded for diverse miniaturized electronic devices. [1,2] However, making high-energy microcapacitors with currently available printing technologies remains challenging. Based on a composite material of latex polyvinylidene fluoride and polyvinyl alcohol, we created high-energy microcapacitors with inkjet printing technique. A discharged energy density of 12 Jcm^-3 is achieved at 550 MVm^-1.[3] The printed microcapacitors demonstrate mechanical robustness and dielectric stability over time. on the other hand, it is well known that all dielectric materials have electrostriction. They have capability of changing their dielectric properties upon an external mechanical deformation. Based on this effect, we have developed giant electrostriction soft nanocomposites which can serve as an active dielectric layers in a variable capacitor to convert the vibrational mechanical energy into electricity. To make such energy harvester, we prepare an electrostrictive nanocomposite composed of reduced graphene oxide in soft polydimethylsiloxane matrix.[4] The near percolated composite exhibits a large permittivity variation in response to small strain deformation, giving rise to a giant electrostrictive coefficient -4.3×10^-14 m²/V²,[5] indicating that our materials are particularly promising for harvesting mechanical energy applications. Their implementation in actual MEMS generator devices has been also investigated. [6]

References
[1] Z. S. Wu, X. Feng, H. M. Cheng, Natl. Sci. Rev. 2014, 1, 277.
[2] M. Beidaghi, Y. Gogotsi, Energy Environ. Sci. 2014, 7, 867.
[3] F. Torres-Canas, et al, Adv. Funct. Mater. 2019, 1901884.
[4] J. Yuan, et al, Nat. Commun. 2015, 6, 8700.
[5] J. Yuan, et al. ACS Nano 2018, 12, 1688-1695
[6]. Nessera, H. et al, Nano Energy, 2018, 44, 1–6.

To overcome the energy crisis and its efficient utilization, there is a great need to harvest clean and green energy from waste-heat, which is abundant, ubiquitous, and free. Energy harvesting, using waste-heat, is one of the most encouraging methods to capture freely accessible heat and converting it to electrical energy. Ferroelectric materials are the most suitable candidates to harvest energy for low power electronic devices, as they exhibit switchable polarization, excellent piezoelectric and pyroelectric coefficients. The most important characteristic of ferroelectric materials is their ability to generate electric power via the pyroelectric effect. In this work, we have grown superlattice of BaZr_0.2Ti_0.8O₃ (barium zirconium titanate, BZT)/ Ba_0.7Ca_0.3TiO₃ (barium calcium titanate, BCT) on SrRuO₃ (strontium ruthenate, SRO) deposited on SrTiO₃ (strontium titanate, STO) single crystalline substrate via optimized pulsed laser deposition. We observed only the substrate peaks without any secondary peaks in the x-ray diffraction patterns, which confirms the epitaxial nature and phase purity of the grown material. Interface and domain engineering are integrated on these BZT / BCT structures for slim and well-saturated polarization-electric field (P-E) loops with improved saturation polarization.
Solid-state thin-film devices are demonstrated by the conversion of waste-heat into electrical energy by measuring P-E loops at various temperatures using pyroelectric Ericsson cycles and measured an energy conversion density ~ 10.97 MJ/m³ per cycle. These devices exhibit a large pyroelectric current of ~ 600 nA with a temperature fluctuation of ~ 11°C. Our findings suggest that these lead-free relaxor-ferroelectric superlattices might be superior to the well-known thermoelectric materials for thermal energy harvesting.

Electrocaloric cooling is a solid-state cooling technology, offering several advantages over traditional cooling systems, but few EC devices have been reported with high specific cooling power and high coefficient of performance (COP). We have developed solution processes to fabricate poly(vinylidene fluoride-trifluoroethylene-chlororfluoroethylene) terpolymer thin films. Various material treatment and characterizations were conducted to study and optimize the performance of the terpolymer thin films. We present several EC cooling device designs with compact and desired form factors to transfer heat between a heat source and heat sink, with the ultimate objective to pave the way for new applications that cannot be obtained using existing technologies.

Mechanical energy harvesters are important for powering wearable devices and microelectronics. Most mechanical energy harvesters supply high voltages (10⁰-10² V), low currents (10^-4-10^-2 mA/cm²), and operate at high frequencies (~20-100 Hz). The high voltage and low current outputs, however, do not interface well with many microelectronics (which use low voltage and high current supplied from a battery). Additionally, the high operation frequency of mechanical energy harvesters does not match the low frequency of most mechanical energy sources in daily life.

Here, we report lithium cobalt oxide (LCO) based mechanical energy harvesters that deliver ultra-high current from low-frequency mechanical energy input. LCO is widely used as a cathode material for lithium ion batteries, where its crystal volume changes as lithium ions intercalate/deintercalate during battery cycling. We utilize the reverse process, where a change in volume in the crystal due to external stress causes a change in potential that drives a large current. LCO is conductive (9.25×10^-4 S/cm) and exhibits a piezoelectrochemical effect, like many dielectrics, that converts mechanical to chemical or electrical energy. Our energy harvester consists of a pair of LCO electrodes with identical composition and different thicknesses. When an external load is applied to the device, the corresponding stress is concentrated on the thick electrode. As the thick electrode volume changes, the free energy of LCO bonds is modified and results in a shift in the electrochemical voltage. When the unloaded LCO is electrically connected to the loaded LCO, the potential difference drives a current through an external circuit while lithium ions migrate between electrodes through an electrolyte. The current continues until there is 0 V between the electrodes. When the pressure is removed, an opposite potential difference is generated between electrodes. The device can be cycled between low displacements (<100 μm) loads at a wide range of frequencies (10^-4-10^-1 Hz) to convert mechanical energy in into ultra-high currents (0.87 mA/cm²) and low voltage (~10 mV) power.

Molten salt electrodeposition allowed the synthesis of up to 98% dense and hundreds of micrometer thick monolithic LCO electrodes with no additives, which improved the performance over slurry cast electrodes with binders found in battery electrodes. By reducing mechanical loses that occur in the polymer binders, the high density and single component electrodes enabled a high voltage output close to the theoretical value. Furthermore, the laser-based fabrication process enabled small electrode gaps which contribute to low internal resistance and high output current. The design can be scaled to form array of harvesters connected in series or parallel to generate higher current or voltage to meet the demands of different applications. The usage of aqueous electrolyte in our energy harvester decreases the potential toxic and combustion hazards of organic solvent based electrolytes and avoids the hermetic requirements for battery device packaging. Our work demonstrated an energy harvester with high current capabilities that will create new opportunities for powering wearables and microelectronics with increased functionality.

Relaxor ferroelectric exhibits striking physical properties including ultrahigh piezoelectric coefficients, large dielectric permittivity over a broad temperature range, and strong frequency dependence in dielectric response, which enables a wide range of applications in acoustic sensors, transducers and actuators. Understanding of the origin of unusual properties in relaxor ferroelectrics, however, remains one of the central challenges in ferroelectric materials despite more than 50 years of investigations. Here we reveal that the relaxor behavior of ferroelectric polymers originates from conformational disorder (Y. Liu et al, Nature Materials, in press), which is completely different from classic perovskite relaxors typically characterized by the existence of chemical disorder. We show that chain chirality is essential to the formation of disordered helix conformation arising from local distortions of gauche torsional angles, which consequently give rise to relaxor ferroelectric properties in polymers. Our insights into the origin of relaxor ferroelectric properties in polymers shed new light on fundamental theory of relaxor ferroelectric materials and enable the investigation of the mechanisms of ferroelectric relaxor from a new perspective. The structural model established in this work provides unprecedented guidance for the design of new organic relaxor materials for flexible electronics and energy applications.

For the purposes of powering always-on, self-powered sensors and portable/wearable electronics, new reliable and sustainable energy harvesting technologies are required. Piezoelectric generators show exceptional promise to fill this gap, owing to their ability to convert mechanical energy from external sources to electricity, with energy conversion efficiencies >35%.¹ Recent advances in piezoelectric fluoropolymers (e.g., poly(vinylidene floride), PVDF) further enable the use of flexible polymer-based piezoelectric generators with tailored optical transmittance for wearable or implantable energy harvesting from the movements of the human body.

Despite the promising nature of piezoelectric polymers, the extraordinary energy input required during the manufacture steps limits the commercial applicability for energy harvesting. Namely, the electrical energy (>200 MV m^-1) and thermal energy (>100 °C), applied to the fluoropolymer in order to align the dipole moments and induce polarization, negates the energy harvested over the device lifetime.² Hence, the elimination of this so-called electrical poling step will enable the commercial and environmental viability of flexible piezoelectric generators.

Here, we demonstrate for the first time the dipole templating and polarization locking of a copolymer of PVDF, poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), in the presence of 1D (carbon nanotube) and 2D (MXene) nanomaterials. The alignment of polarization at the interface, predicted by molecular dynamics models, improves energy harvesting performance and decreases the energy input for the manufacture of flexible piezoelectric energy harvesters. The presence of single-walled carbon nanotubes (0.02 wt%) exhibits a generated power density of 70.6 μW cm^-3, whereas the incorporation of Ti₃C₂T_x MXenes (0.50 wt%) results in a piezoelectric charge coefficient of 52.0 pC N^-1, without the use of electrical poling. The composites produced through this method are fully recyclable in acetone and can be reprinted into various form factors without loss of performance, enabling future use for designer energy harvesting systems.

1. Shepelin, N. A., et al., Energy Environ. Sci. 2019, 12, 1143-1176.
2. Lovinger, A. J., Science 1983, 220, 1115-1121.

Poly(vinylidene fluoride) (PVDF) have received applications in piezoelectric sensors, actuators and nanogenerators due to their flexibility and electroactive properties. In general, PVDF exhibit five crystalline polymorphs namely, α, δ, β, γ and ε, among them, β (TTTT conformation) and γ (TTTGTTTG' conformation) phases are the electrically active phases. Moreover, γ-phase offers higher breakdown strength and discharge energy density as compared to β-phase, therefore, receiving greater attention as a ferroelectric material. Until now, various approaches such as, high electric field poling, mechanical stretching, electrospinning and incorporation of nanofillers have been established for fabricating PVDF with high content of electroactive phases. In comparison, incorporation of nanofillers can simultaneously enhance the mechanical properties of polymer matrix and can also promote the formation of preferred electroactive phases owing to the interaction between the surface of the fillers and dipoles of the PVDF chains. Recently, mono or few-layered two-dimensional (2D) boron nitride nanosheets (BNNS) have been actively applied as nanofillers for polymer composites because of their extraordinary properties such as, high thermal conductivity, bandgap, thermal stability, Young’s modulus and piezoelectric properties. However, their main drawback is related with their poor dispersion in polymer matrix due to their Van der Waals forces. Herein, we report the fabrication of high performance PVDF nanofibers (PVDFNFs) based composites using oleylamine functionalized boron nitride nanosheets (OLABN) as nanofillers. First, PVDFNFs-OLABN composite nanofiber membranes were prepared by liquid-phase exfoliation (LPE) and electrospinning process, and then, the effect of OLABN on the crystalline phases, mechanical and piezoelectric properties of PVDFNFs were systematically investigated. FTIR results revealed that the OLABN promote the formation of polar γ-phase in PVDFNFs. Impressive 343% and 823% enhancements in tensile strength and Young’s modulus can be achieved even with addition of only 0.1 wt% OLABN, respectively. The composites exhibited enhanced piezoelectric outputs as compared to reference PVDFNFs. Furthermore, rapid and sharp piezoelectric outputs can be achieved upon force applied by finger tapping/releasing on the PVDFNFs-OLABN (4 wt%) composite based piezo sensors. The output power density of the composites with 4 wt% OLABN was 133% higher than that of reference PVDFNFs. The findings of this study hold a great potential in high performance polymer based piezoelectric sensors/actuators and nanogenerators applications.

Keywords: Boron Nitride, Piezoelectric Sensing, Polymer Nanofibers, Nano Composites, Mechanical Properties

Acknowledgements

This work was supported by Grant NRF-2018R1A6A1A03025526 under the Priority Research Program through National Research Foundation of Korea(NRF) under Ministry of Education.

The interest for antiferroelectric materials has been increasing over the last years, largely because of their potential for energy storage and electrocaloric applications, but also because they still are insufficiently known compared to ferroelectrics. Their key distinctive signature is the antiferroelectric switching revealed by a double hysteresis loop. However, this switching is not often demonstrated, largely due to the large electric fields involved. In particular, the details of the switching behavior of the model antiferroelectric perovskite PbZrO3 are still unclear.

Here, we synthesize and characterize polycrystalline films of PbZrO3, with thicknesses ranging from 85 nm to 850 nm. The samples have been processed via chemical solution deposition on Pt/TiOx/SiO2/Si and fused silica substrates. For the films processed on platinized silicon, platinum top electrodes have been sputtered to apply the field out-of-plane in a metal/insulator/metal (MIM) geometry. For the samples deposited on fused silica substrates, interdigitated platinum electrodes (IDE) have been deposited to apply the field in-plane.

In both geometries, we could observe the double hysteresis loop, signature of antiferroelectric switching. We compare the switching characteristics in the two types of sample, with adequate corrections in order to take into account the differences in geometry in the calculation of the applied field. A saturation polarization of up to 36 µC/cm2 was reached in the IDE geometry. The switching peaks in the current vs. voltage characteristic are generally broader in the IDE geometry as compared to the MIM geometry. Orientation effects alone decrease the switching fields in MIM geometry from 495 kV/cm to 286 kV/cm in the IDE geometry. We discuss these differences with a simple switching model. Additionally, in both geometries, the films display a “ferroelectric-like” contribution at lower fields. This works paves the way for possible applications based on antiferroelectric switching of polycrystalline PbZrO3 thin films.

Since the discovery of ferroelectricity in doped-HfO₂ by Böscke et. al in 2011, doped-HfO₂ has been regarded as an eventual replacement of lead zirconate titanate (PZT), and as a highly promising functional oxide for information storage devices, electromechanical systems, energy storage, and piezoelectric-power generation. Highly ordered, highly piezoelectric orthorhombic and rhombohedral doped-HfO₂ film structures have been obtained by physical deposition techniques such as ALD, PLD and sputtering deposition. Such methods have produced groundbreaking results and astonishing conceptual devices. Unfortunately, they require high-vacuum and expensive high-precision equipment. Chemical solution deposition methods have also yielded decent results, albeit less surprising. Yet, they are more suitable for large scale production, particularly for applications where large areas are needed. Nevertheless, current chemical approaches make use of highly unstable or toxic compounds, which require restrictive inert-atmosphere processing conditions. In our work, we have focused on the sustainable synthesis of piezoelectric doped-HfO₂ good-quality films by employing much less or non-toxic chemical precursors, and short/lower thermal treatments. Besides, no special synthesis conditions are required, which makes the processing cheaper.

We have employed Zr and Ca as dopants of HfO₂ because such elements are relatively abundant and, like Hf, they do not present risks for human health or for the environment. Chemical precursor solutions of each corresponding metal oxide are prepared in normal air atmosphere from simple and low toxicity compounds dissolved in alcoholic solvents. Solvation-enhancing agents are used to suspend the precursor metal salts. Combustion-boosting compounds are also added to produce an increased annealing temperature, which promotes crystallization. The solutions are mixed in calculated ratios. Thin films of doped-HfO₂ are deposited in normal conditions via spin-coating on platinized silicon substrates. Each layer is dried at 200 to 400 °C for 5 minutes. The process is repeated several times for increased thickness. The films are later annealed in Ar or air ambience at temperatures between 500 and 800 °C. Top Pt electrodes are deposited by e-beam evaporation.

Uniform and dense doped-HfO₂ layers are deposited from the evaporation and Metal-Organic Decomposition (MOD) of the precursor solution components. The solution can be prepared and safely managed in normal ambient because it does not contain sol-gel-type precursors. The most stable crystalline phase of HfO₂, monoclinic, is obtained for drying temperatures of 200-300 °C after sintering at 500-800 °C under conventional annealing in Ar and air. However, cubic and/or tetragonal phases mixed with monoclinic phase can be obtained for drying temperature of 400 °C and annealing at 500 °C in Ar and air.

Interestingly, pure cubic or tetragonal phase is obtained at a drying temperature of 400 °C and Rapid Thermal Annealing (RTA) at 500 – 700 °C for 5 minutes. Cubic and/or tetragonal phase is required because it has been demonstrated each can undergo phase-transition to ferroelectric orthorhombic phase during wake-up polarization cycling. The cubic, tetragonal and orthorhombic phases are stabilized by a small crystallite size. On the contrary, bigger crystals lead to the irreversible formation of the monoclinic phase. The use of a relatively high drying temperature of 400 °C promotes the nucleation of small HfO₂ crystals rather than growth. Besides, annealing in RTA does not allow enough time for such crystals to increase in size, thus stabilizing the higher symmetry phases of HfO₂.

The fabrication method developed can be used to make films with piezoelectric response, of easy scalability, and with low-toxicity precursor materials.

Two dimensional dielectric fillers have been considered to serve as high performance fillers to enhance both the dielectric constant and breakdown strenght of various dielectric polymer composites. Nevertheless, the fabrication of these 2D fillers is quite often complicated, and usually rely on the nature availability of these materials. Herein, we demonstrate systematic study on the fabrication of 2D TiO2 nanofillers through tunning various processing condition of electrospinning. This research work was carried out to investigate the effects of different solution formulas and electrospinning conditions on the morphology of the spinning fibers. The influence of calcination temperature on the preparation of dielectric fillers was studied, and it was applied to the preparation of dielectric composite materials based on PVDF and PI. The energy density of two-dimensional nano-sheets dielectric fillers was investigated, and shows high dielectric constant, high Effects, high breakdown strength and low dielectric loss. The extended application of 2D lamellar fillers in polyimade system under high temperature conditions is further discussed achieving an energy density of 3 J/cm3 at 150 degree C; 3.5 J/cm3 at 100 degree C; a charge-dischange efficiency of around 90%. This could provides a theoretical and technical guidelines for the practical application.

The critical size limit of ferroelectric order has extensive implications for the future scaling of energy-efficient electronics, particularly due to the intrinsic ability to control electric polarization with an applied voltage. As ferroelectric materials are made thinner however, polarization is typically suppressed, as size effects in ferroelectrics have been thoroughly investigated in the prototypical perovskite oxides. Furthermore, perovskites suffer from various chemical, thermal, electrical, and interfacial incompatibilities with silicon and modern semiconductor processes. Recently, fluorite-structure binary oxides have attracted considerable interest as they overcome many of the silicon compatibility issues afflicting its perovskite ferroelectric counterparts. Here we report ferroelectricity in ultrathin Zr-doped HfO₂ (HZO) [1], a fluorite-structure oxide grown by atomic layer deposition (ALD) on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to a thickness of one nanometer, equivalent to just two fluorite-structure unit cells. Second harmonic generation (SHG) and advanced scanning probe techniques – microwave-frequency scanning capacitance microscopy (SCM) and piezoresponse force microscopy (PFM) coupled with an interferometric displacement sensor (IDS) – establish the presence of inversion symmetry breaking and switchable electric polarization in one nanometer HZO films, respectively. Synchrotron X-ray characterization indicate that polar distortions increase in the ultrathin limit; diffraction markers (structural aspect ratio, interplanar lattice spacing) and spectroscopic signatures (orbital polarization, crystal field splitting) of polarization all demonstrate ultrathin enhancement.

These results indicate not only the absence of a ferroelectric critical thickness, but also ultrathin-enhanced ferroelectricity in HZO thin films on silicon. Notably, such ‘reverse’ size effects oppose conventional perovskite ferroelectric trends, which can be understood from the crystal symmetries involved in the competing fluorite-structure polymorphs. Therefore, this work shifts the search for the fundamental limits of ferroelectricity to simpler transition-metal oxide systems – that is, from perovskite-derived complex oxides to fluorite-structure binary oxides – in which ‘reverse’ size effects can counterintuitively stabilize polar symmetry in the ultrathin regime. From a synthesis perspective, these results suggest that harnessing confinement strain – required to crystallize ultrathin HZO into the metastable ferroelectric orthorhombic phase – offers a new mechanism to control electric polarization at the atomic-scale beyond epitaxial strain. Furthermore, this approach to integrate nanoscale ferroelectricity in doped-HfO₂ thin films directly on silicon via ALD provides a route towards realizing emerging hyper-scaled energy-efficient nanoelectronics [2]. In particular, we have recently demonstrated ferroelectric tunnel junctions [1] and negative capacitance ferroelectric field-effect transistors [3,4] exploiting ultrathin ferroelectric HZO films on silicon, critical steps towards realistic polarization-driven resistive switching memories and ultralow-power advanced transistors, respectively.

[1] S Cheema^*, D Kwon^* […] S Salahuddin. “Enhanced ferroelectricity in ultrathin films grown directly on silicon.” Nature 580, 478–482 (2020).
[2] S Salahuddin, K Ni & S Datta. “The era of hyper-scaling in electronics.” Nat. Electron. 1, 442–450 (2018).
[3] D Kwon^*, S Cheema^* […] S Salahuddin. “Negative Capacitance FET with 1.8-nm-Thick Zr-Doped HfO₂ Oxide.” IEEE Electron Device Lett. 40, 993–996 (2019).
[4] D Kwon^*, S Cheema^* […] S Salahuddin. “Near Threshold Capacitance Matching in a Negative Capacitance FET with 1 nm Effective Oxide Thickness Gate Stack.” IEEE Electron Device Lett. 41, 179–182 (2020).

We report on the high temperature thin film growth of BaTiO₃ on Ti₃C₂T_x (where T_x represents surface terminations, such as O and F) MXene flakes using van der Waals epitaxy on a degradable template layer. MXene was deposited on amorphous and crystalline substrates by spray- and dip-coating techniques, while the growth of BaTiO₃ at 700 °C was accomplished using pulsed laser deposition in an oxygen rich environment. We demonstrate that the MXene flakes act as a temporary seed layer, which promotes highly oriented BaTiO₃ growth along <111> independent of the underlying substrate. The lattice parameters of the BaTiO₃ films are close to the bulk value suggesting that the BaTiO₃ films remains unstrained, as expected for van der Waals epitaxy. The initial size of the MXene flakes has an impact on the orientation of the BaTiO₃ films with larger flake sizes promoting a higher fraction of the polycrystalline film to grow along the <111>. The deposited BaTiO₃ film adopts the same morphology as the original flakes and piezoresponse force microscopy shows a robust ferroelectric behavior for individual grains. Transmission electron microscopy results indicate that the Ti₃C₂T_x MXene fully decomposes during the BaTiO₃ deposition and the surplus Ti atoms are readily incorporated into the BaTiO₃ film. Electrical measurements show a similar dielectric constant as a BaTiO₃ film grown without the MXene seed layer. The demonstrated process has the potential to overcome the longstanding issue of integrating highly oriented complex oxide thin films directly on any desired substrate. Work supported by the Hearst Foundation, the U.S. Dept. of Energy under DE-SC0018618, the ONR under N00014-15-11-2170, the ARO under W911NF-14-1-0500, the AFOSR under FA9550-13-1-0124, NSF under DMR 1608887, and the NSF under NNCI-1542153.

Current state-of-the-art biaxially oriented polypropylene (BOPP) film capacitors suffer from low temperature rating (<85 °C) and low energy density. At Case Western Reserve University, we have developed a novel multilayer film technology to meet the stringent requirements for next-generation dielectric film capacitors for electric vehicles. By multilayering a high breakdown strength polymer such as polycarbonate (PC) with a high dielectric constant polymer such as poly(vinylidene fluoride) (PVDF), we have achieved high breakdown strength (>600 MV/m), high energy density at breakdown (16 J/cm³), and relatively low dielectric dissipation factor (tanδ <0.005) and hysteresis loss (15% loop area). In this presentation, we will identify and discuss fundamental issues for these multilayer dielectric films, which include polarization of amorphous and crystalline dipoles, polarization of impurity ions, and interfacial polarization to achieve enhanced breakdown strength and lifetime at high temperatures.

Achieving stable energy density at elevated temperature remains a great challenge for ferroelectric polymer-based capacitors. It seems that the emerging ferroelectric polymers (e.g. poly (vinylidene fluoride) (PVDF) and its copolymers), which possess spontaneous polarization and large permittivity, provide a promising solution to meet the ever-increasing requirement for high energy density. Nevertheless, the state-of-the-art PVDF-based ferroelectric polymer usually exhibits mediocre temperature stability in energy storage performance, which cannot fully satisfy the applications at elevated temperature caused by the energy consumption or nearby heat sources.
In this work, we report a strategy for stabilizing high energy density of ferroelectric polymer (PVDF) over a wide temperature range by blending with a miscible high-T_g polymer (polymethyl methacrylate (PMMA)). The blending material forms an alternating lamellar structure consisting of high-polarization crystalline regions and strengthened amorphous regions. It is found that this special morphology is able to confine the constructional change of stacked lamellar structure at elevated temperatures. Accordingly, the associated energy storage performances, including energy density, energy storage efficiency and breakdown strength, are remarkably enhanced in a wide temperature range (30 to 70^oC). At 70^oC, the energy density and efficiency of PVDF/PMMA(80/20) is 9.8J cm^-3 and 67.8%, which surpasses most neat ferroelectric polymers at such an elevated temperature. This work reveals the relationship between lamellar confinement of polymer blend and electric properties under temperature variation, and might provide a valid approach to attain high energy density and temperature stability in ferroelectric polymers.

As one of the most important renewable energy storage technologies, film capacitors have been used in modern electronics and power industries such as wearable electronics, hybrid vehicles, and other fields due to their high power density, long working hours, and environmental friendliness. With the development of electronic devices toward miniaturization and high performance, there is an urgent need for dielectric materials with high energy storage density. A high breakdown strength (E_b) is crucial for dielectric materials to ensure reliability of electric circuits and to improve electrostatic energy storage density.E_bis the result of space charge movement in the dielectrics under the applied electric field. Effectively reducing the density of charge carriers in the dielectrics is one of the keys to enhancing E_b. In this study, hybrid nanoparticles with special physical microstructure were designed and fabricated. When they were used as fillers,these particles possessing isoelectronic traps or heterojunction have proved efficient in confining space charge carriers generated under applied electric field and meanwhile suppressing their movement through the whole polymer system. As a result, electrical conductivity was reduced and E_bwas enhanced significantly, leading to improved energy storage density.

Knowing the polymer triboelectrification mechanisms is the key to provide high-performance triboelectric nanogenerators for mechanical energy harvesting from movement or, vice versa, to eliminate charge accumulation on the surface for safety. Moreover, the triboelectric surface charge has attracted significant interest among scientists recently for particle separation and liquid droplet transport, as well as nanoparticle deposition and surface patterning.
The three main mechanisms considered for polymer electrification are (i) electron transfer [1], (ii) heterolytic covalent bond break and material transfer [2], and (iii) ion transfer [3]. The ion transfer, where ions are separated between water adsorbate layers on contacted polymers, seems not the dominating mechanism as the polymer triboelectrification has been observed in the complete absence of water and between identical materials which should exhibit the same affinity between cationic and ionic species in water. In the past few years, the debate has concentrated on electron transfer and covalent bond breaking as possible dominating polymer triboelectrification mechanisms.
Recently a vast number of triboelectrification experiments has been performed and reported for various polymers with the different structural state, surface adhesion, and morphology. The experimental results indicate that the triboelectrification mechanism for polymer insulators is heterolytic covalent bond cleavage (heterolysis or heterolytic fission) alongside material transfer [4-6]. The surface charge increases with decreasing cohesion energy [4] or increasing surface adhesion [5]. A dramatic three orders of magnitude increase in surface charge when passing the transition temperature from a glassy to a rubbery state also has been observed due to the adhesion, softness, and deformation increases, thus enhance the covalent bond cleavage, material transfer, and surface charge formation [6].
Herein we are highlighting these aspects and illuminating the pathways to enhance the triboelectrification avoiding expensive and complicated nanostructuring approaches.

References:
[1] J. Chen and Z. L. Wang, Reviving vibration energy harvesting and self-powered sensing by a triboelectric nanogenerator, Joule, 1, 480-521, (2017).
[2] H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh and B. A. Grzybowski, The mosaic of surface charge in contact electrification, Science, 333, 308, (2011).
[3] L. S. McCarty and G. M. Whitesides, Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets, Angew. Chem., Int. Ed., 47, 2188-2207, (2008).
[4] A. Šutka, K. Mālnieks, L. Lapčinskis, P. Kaufelde, A. Linarts, A. Berzina, R. Zābels, V. Jurkāns, I. Gornevs, J. Blums and M. Knite, The role of intermolecular forces in contact electrification on polymer surfaces and triboelectric nanogenerators. Energy Environ. Sci., 12, 2417-2421, (2019).
[5] L. Lapčinskis, K. Mālnieks, J. Blums, M. Knite, S. Oras, T. Käämbre, S. Vlassov, M. Antsov, M. Timusk and A. Šutka, The Adhesion-Enhanced Contact Electrification and Efficiency of Triboelectric Nanogenerators, Macromol. Mater. Eng., 305, 1900638, (2020).
[6] A. Šutka, A. Linarts, K. Mālnieks, K. Stiprais and L. Lapčinskis, Dramatic increase in polymer triboelectrification by transition from a glassy to rubbery state, Mater. Horiz., 7, 520-523, (2020).

Two dimensional dielectric fillers have been considered to serve as high performance fillers to enhance both the dielectric constant and breakdown strenght of various dielectric polymer composites. Nevertheless, the fabrication of these 2D fillers is quite often complicated, and usually rely on the nature availability of these materials. Herein, we demonstrate systematic study on the fabrication of 2D TiO2 nanofillers through tunning various processing condition of electrospinning. This research work was carried out to investigate the effects of different solution formulas and electrospinning conditions on the morphology of the spinning fibers. The influence of calcination temperature on the preparation of dielectric fillers was studied, and it was applied to the preparation of dielectric composite materials based on PVDF and PI. The energy density of two-dimensional nano-sheets dielectric fillers was investigated, and shows high dielectric constant, high Effects, high breakdown strength and low dielectric loss. The extended application of 2D lamellar fillers in polyimade system under high temperature conditions is further discussed achieving an energy density of 3 J/cm3 at 150 degree C; 3.5 J/cm3 at 100 degree C; a charge-dischange efficiency of around 90%. This could provides a theoretical and technical guidelines for the practical application.

Flexible pressure sensors are the basic components for many different applications, such as electronic skins, robotics, health-monitoring devices, and human-machine interfaces. Among different working mechanisms, capacitive pressure sensors are widely-used owning to their low energy consumption, simple structure, and high reliability. Pressure sensors in the form of coatings have the advantages of facile applications on various complex surfaces because of the flowability before solidification, capability for large scale production, and low cost. But how to prepare pressure sensing coatings to meet the requirements for different applications remains to be addressed. This talk covers our recent work to enhance the performances of pressure sensing coatings through the rational design of dielectric materials.
Firstly, I will discuss a highly sensitive capacitive pressure sensing coating using thermally expandable microspheres/ polydimethylsiloxane (PDMS) composite as the dielectric layer. The microspheres can expand during the thermal curing process of PDMS, which greatly enhance the compressibility and thus increase the sensitivity of the pressure sensors. At the same time, it achieves a broad pressure sensing range from 10 Pa to 4.5 MPa and also features low hysteresis and good stability against repeated compression.
Secondly, I will discuss a strategy to achieve linear response of capacitive pressure sensor over a broad sensing range. Conventional capacitive pressure sensors usually exhibit a highly non-linear response as the sensitivity drops dramatically towards high pressure. Our strategy employs a percolative composite as the dielectric layer. The fast increase in dielectric constant during compression can compensate the sensitivity drop from the decreased compressibility, leading to a linear response. Owning to the nature of polymer composite, its dispersion not only can be conformally coated on surfaces with various complex shapes, but also can be molded into films with surface microstructures to achieve a unique combination of high sensitivity and linear response over a wide pressure sensing range.

Use of dielectric materials such as lead zirconate titanate films for dielectric energy storage or for piezoelectric applications necessitates that they be able to sustain high DC electric fields. This paper will describe the coupled failure mechanisms that control these lifetimes for donor-doped, acceptor-doped, and films with graded doping. Curiously, the median failure time of PZT films can be increased on doping with either donors or Mn as an acceptor. It is found that lifetimes under DC fields are dominated by a combination of oxygen vacancy migration, coupled with changes in the interface barrier heights for conduction in thin films. This was assessed through a combination of highly accelerated lifetime testing, thermally stimulated depolarization currents, impedance spectroscopy, deep level transient spectroscopy, and electron energy loss spectroscopy. The relic defect chemistry of the film, including variations in this defect chemistry at interfaces strongly influences both the failure and the asymmetry of the failure as a function of the electrical polarity. As film thickness and/or the piezoelectric coefficients rise, the critical stress required for crack initiation decreases, and the failure modes can shift to electro-mechanically induced failure. Ball-on-three ball mechanical testing demonstrates that the critical flaw size in the failure of the stack becomes the thickness of the piezoelectric layer itself.

Thin film oxide dielectrics boast high breakdown voltages, low leakage current densities, and high dielectric constants making them ideal for use as gate dielectrics in thin film electronics. However, to fully realize the benefit of thin film electronics, low-cost high-throughput production is necessary. This means reducing not only processing time, but also processing temperature to allow for production on plastic substrates. In this work, we discuss the effects of low temperature UV-assisted processing and high-speed photonic curing on the properties of solution-deposited oxide dielectric films. First, we show the results from exposing sol gel precursor films to UV-ozone (UVO) through a shadow mask. This process accomplishes both lowering the conversion temperature to 250 °C and direct patterning the precursor film with exposed areas converted to oxide after final annealing. We find drastically different capacitance-frequency behavior between Al₂O₃ and ZrO₂ dielectrics, which in turn affects the In₂O₃ transistors’ behavior. The origins of this finding are studied through Fourier transform infrared analysis and time dependent drain current measurements. Next, we adapt photonic curing, the use short but high intensity pulses of broadband light, to selectively heat and convert ZrO₂ precursor films on flexible polyethylene naphthalate (PEN) substrates. Photonic curing is typically used to sinter metal nanoparticle inks into conductive metal traces on plastic substrates, but it has not previously been used to fabricate oxide thin film dielectrics on plastic substrates. We demonstrate that the conversion time is reduced from 10s minutes to < 1s and patterning using a shadow mask during photonic curing does not need subsequent thermal annealing, unlike in the UVO processing. Furthermore, by adjusting the photonic curing conditions, the oxide film can be selectively converted on top of Al electrodes, which act as selective area heat absorbers, but not on the bare PEN substrate, resulting in a self-aligned oxide pattern. We achieve a ZrO₂ dielectric with breakdown field of 8 MV/cm and areal capacitance of 200 nF/cm² with little frequency dispersion. Finally, we show a pathway to converting ZrO₂ dielectric at a roll-to-roll web speed of up to 50 m/min.

Electrocaloric cooling is a solid-state cooling technology, offering several advantages over traditional cooling systems, but few EC devices have been reported with high specific cooling power and high coefficient of performance (COP). We have developed solution processes to fabricate poly(vinylidene fluoride-trifluoroethylene-chlororfluoroethylene) terpolymer thin films. Various material treatment and characterizations were conducted to study and optimize the performance of the terpolymer thin films. We present several EC cooling device designs with compact and desired form factors to transfer heat between a heat source and heat sink, with the ultimate objective to pave the way for new applications that cannot be obtained using existing technologies.

Our research activities on the energy-related dielectric polymer nanocomposites will be reviewed. Micro-energy storage devices are highly demanded for diverse miniaturized electronic devices. [1,2] However, making high-energy microcapacitors with currently available printing technologies remains challenging. Based on a composite material of latex polyvinylidene fluoride and polyvinyl alcohol, we created high-energy microcapacitors with inkjet printing technique. A discharged energy density of 12 Jcm^-3 is achieved at 550 MVm^-1.[3] The printed microcapacitors demonstrate mechanical robustness and dielectric stability over time. on the other hand, it is well known that all dielectric materials have electrostriction. They have capability of changing their dielectric properties upon an external mechanical deformation. Based on this effect, we have developed giant electrostriction soft nanocomposites which can serve as an active dielectric layers in a variable capacitor to convert the vibrational mechanical energy into electricity. To make such energy harvester, we prepare an electrostrictive nanocomposite composed of reduced graphene oxide in soft polydimethylsiloxane matrix.[4] The near percolated composite exhibits a large permittivity variation in response to small strain deformation, giving rise to a giant electrostrictive coefficient -4.3×10^-14 m²/V²,[5] indicating that our materials are particularly promising for harvesting mechanical energy applications. Their implementation in actual MEMS generator devices has been also investigated. [6]

References
[1] Z. S. Wu, X. Feng, H. M. Cheng, Natl. Sci. Rev. 2014, 1, 277.
[2] M. Beidaghi, Y. Gogotsi, Energy Environ. Sci. 2014, 7, 867.
[3] F. Torres-Canas, et al, Adv. Funct. Mater. 2019, 1901884.
[4] J. Yuan, et al, Nat. Commun. 2015, 6, 8700.
[5] J. Yuan, et al. ACS Nano 2018, 12, 1688-1695
[6]. Nessera, H. et al, Nano Energy, 2018, 44, 1–6.

It is important to determine the dielectric characteristics of semiconductor encapsulation materials based on epoxy resins. There are several such material systems employed to protect the active silicon circuitry. Some of the important properties of the materials are their glass transition temperature, filler content and the level of water absorption which leads to reliability challenges with respect to performance and lifetime. We study changes in the propeties by investigating dielectric relaxation using broadband impedance spectroscopy techniques. It is known that the dielectric relaxation in polymeric systems influenced by the presence of water, which demands special attention in sensitive applications. We will present measurement and analysis methods based on time and frequency domain techniques. It would be appreciated within the materials science community to improve material selection and performance evaluation efforts. We will also share information on the glass transition temperature estimates for a number of composites employed in semiconductor packaging.

Sustainable energy sources – such as wind, wave and solar – are attractive for reducing CO₂ emissions and tackling climate change. Often these energy sources are placed at large distances from where the energy is actually needed; off-shore wind farms and desert solar parks. To transfer the electrical power from such remote locations high voltage direct current (HVDC) is the preferred technology due to lower transmission losses. In the design of high voltage component the dielectric properties of the materials used are of significant importance. The conductivity of the leakage current in and the electrical breakdown field of the dielectric material impact and limits the design. To understand how these properties are affected by the materials chemical composition and morphology, material simulation is one approach to understand these phenomena in more detail.

Material simulation methods such as density functional theory (DFT) and molecular dynamics (MD) has an increased usage in literature to study dielectric properties of materials relevant for HV components. Basic properties such as band gaps and trap states, to more challenging properties to calculate such as mobility and electrical breakdown fields are reported. An additional important area is to include material simulation methods in multiscale simulation models to incorporate phenomena on longer time and length scales possible.

In this talk we will present result from several different studies of dielectric polymers using DFT, MD and kinetic Monte Carlo (KMC). Primarily the material of interest in these studies is polyethylene (PE) which is used as insulation in HVDC cables. PE is a semi crystalline material and have a complex structure at micrometer scale. Using linear scaling DFT (LSDFT) we have investigated the electronic structure of different parts of PE morphology. Band gaps and electron localization are calculated and used to determine electronic charge transport properties. A combination of DFT, MD and KMC are used to calculate mobility in PE. A coarse grained model for hopping transport in PE is also being developed. Recently we also applied LSDFT to investigate electronic structure of another dielectric polymer, poly(ethylene oxide), which may be used as polymer electrolyte in solid state batteries. Coupling to models at longer time and length scales are discussed.