Epitaxially Grown Ferroelectric PVDF-TrFE Film on Shape-Tailored Semiconducting Rubrene Single Crystal

Accessing nanoscopic polarization reversal processes in an organic ferroelectric thin film

Towards eliminating toxic substances from electronic devices, Croconic Acid (CA) has great potential as a sublimable organic ferroelectric material. While studies on CA thin films are just beginning to emerge, its capability to be integrated in nanodevices remains unexplored. We demonstrate at the laterally nanoscopic scale robust ferroelectric switching of a stable enduring polarization at room temperature in CA thin films, without leakage. The challenging ferroelectric characterization at the nanoscale is performed using a unique combination of piezoresponse force microscopy, polarization switching current spectroscopy and concurrent strain response. This helps rationalize the otherwise asymmetric polarization-voltage hysteresis due to background noise limited undetectable switching currents, which are statistically averaged in macrojunctions but become prevalent at the nanoscale. Apart from successfully estimating the nanoscopic polarization in CA thin films, we show that CA is a promising lead-free organic ferroelectric towards nanoscale device integration. Our results, being valid irrespective of the ferroelectrics' nature; organic or inorganic, pave the way for fundamental understandings and technological applications of nanoscopic polarization reversal mechanisms.

Soft Ferroelectrics Enabling High-Performance Intelligent Photo Electronics

Soft ferroelectrics based on organic and organic-inorganic hybrid materials have gained much interest among researchers owing to their electrically programmable and remnant polarization. This allows for the development of numerous flexible, foldable, and stretchable nonvolatile memories, when combined with various crystal engineering approaches to optimize their performance. Soft ferroelectrics have been recently considered to have an important role in the emerging human-connected electronics that involve diverse photoelectronic elements, particularly those requiring precise programmable electric fields, such as tactile sensors, synaptic devices, displays, photodetectors, and solar cells for facile human-machine interaction, human safety, and sustainability. This paper provides a comprehensive review of the recent developments in soft ferroelectric materials with an emphasis on their ferroelectric switching principles and their potential application in human-connected intelligent electronics. Based on the origins of ferroelectric atomic and/or molecular switching, the soft ferroelectrics are categorized into seven subgroups. In this review, the efficiency of soft ferroelectrics with their distinct ferroelectric characteristics utilized in various human-connected electronic devices with programmable electric field is demonstrated. This review inspires further research to utilize the remarkable functionality of soft electronics.

Tailored Poly(vinylidene fluoride-co-trifluoroethylene) Crystal Orientation for a Triboelectric Nanogenerator through Epitaxial Growth on a Chitin Nanofiber Film

Tailoring the crystal orientation of poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) has attracted widespread interest because of its effects on the ferroelectric properties required for various electronic devices. In this study, we investigated the epitaxial growth of PVDF-TrFE on a chitin film for developing triboelectric nanogenerators (TENGs). The crystallographic match between the chitin and PVDF-TrFE enables the development of the intended crystal orientation, with the PVDF-TrFE polarization axis aligned perpendicular to the substrate. In addition, the epitaxially grown PVDF-TrFE on chitin not only enhances the performance of the TENG but also increases the stability of the hygroscopic chitin film against water. The corresponding TENG exhibits a significantly higher output current compared to that of a nonepitaxial PVDF-TrFE/chitin film. Furthermore, the triboelectric sensors based on epitaxial PVDF-TrFE/chitin films allow the monitoring of subtle pressures, suggesting that tailoring the crystal orientation of PVDF-TrFE is a promising approach for developing high-performance TENGs.

Morphology and transport characterization of solution-processed rubrene thin films on polymer-modified substrates

In this report, the morpho-structural peculiarities and the crystallization mechanisms in solution-processed, solvent vapor annealed (SVA) thin films of rubrene (5,6,11,12-tetraphenylnaphthacene) on different substrates were investigated. The high-quality rubrene crystal films with a triclinic crystal structure were successfully prepared on the FTO substrates (glass slide coated with fluorine-tin-oxide) modified by PLA (polylactic acid) for the first time. The area coverage of rubrene crystal and the sizes of rubrene dendritic crystals increased with increasing thickness of PLA film and concentration of rubrene solution. For rubrene molecules, FTO wafers with rough surface provided the possibility of heterogeneous nucleation. During the SVA process, there were two kinds of forces acting on the diffusion of rubrene molecules: one force was provided by the residual chloroform solvent, which was perpendicular to the substrate, and the other force was provided by gaseous dichloromethane, which was parallel to the substrate. The synergy of these two forces was proposed to explain the nucleation and the crystallization processes of rubrene films. The higher nucleus of PLA/rubrene dendrites and the layer-by-layer stacking of needle-shaped nanocrystalline PLA/rubrene were important for exploring their kinetic formation process.

Micro- and Nanopatterning of Halide Perovskites Where Crystal Engineering for Emerging Photoelectronics Meets Integrated Device Array Technology

Tremendous efforts have been devoted to developing thin film halide perovskites (HPs) for use in high-performance photoelectronic devices, including solar cells, displays, and photodetectors. Furthermore, structured HPs with periodic micro- or nanopatterns have recently attracted significant interest due to their potential to not only improve the efficiency of an individual device via the controlled arrangement of HP crystals into a confined geometry, but also to technologically pixelate the device into arrays suitable for future commercialization. However, micro- or nanopatterning of HPs is not usually compatible with conventional photolithography, which is detrimental to ionic HPs and requires special techniques. Herein, a comprehensive overview of the state-of-the-art technologies used to develop micro- and nanometer-scale HP patterns, with an emphasis on their controlled microstructures based on top-down and bottom-up approaches, and their potential for future applications, is provided. Top-down approaches include modified conventional lithographic techniques and soft-lithographic methods, while bottom-up approaches include template-assisted patterning of HPs based on lithographically defined prepatterns and self-assembly. HP patterning is shown here to not only improve device performance, but also to reveal the unprecedented functionality of HPs, leading to new research areas that utilize their novel photophysical properties.

High-resolution structural mapping and single-domain switching kinetics in 2D-confined ferroelectric nanodots for low-power FeRAM

Ferroelectric nanostructures have received much attention because they can be used for the next generation of ferroelectric random-access memory (FeRAM) in flexible electronic devices. Manipulation of domain reversal in ferroelectric nanostructures is extremely important, but rarely studied. Herein, we present generic and reusable fabrication of 2D-confined P(VDF-TrFE) nanodots with an integration density of up to 4 Gbit per inch2, and then investigate the structural maps and the corresponding domain switching kinetics of P(VDF-TrFE) nanodots by atomic force microscope-based (AFM-based) technology. Meanwhile, their storage features, such as precise programmability and data stability, are well characterized by piezoresponse force microscopy (PFM). Remarkably, the ferroelectric crystals in single-confined P(VDF-TrFE) nanodots simultaneously aligned in a plane over the whole patterned region. 2D-confined P(VDF-TrFE) 50 : 50 nanodots has high-temperature ferroelectric (HT FE) phase with all-trans conformations, which endows them with excellent memory characteristics, such as a low operating voltage of 3 V, a short domain nucleation of 100 ms (by V = 10 V), a fast domain growth, an excellent writing-erasing repeatability, and a long retention time. Compared with normal ferroelectric materials, like P(VDF-TrFE) 70 : 30, approximately 150% ratio of energy loss and a 5-fold duration for domain nucleation can be saved. Especially, written domains were well confined in the P(VDF-TrFE) 50 : 50 nanodots, which attains precise programmability on a single nanodot. Our systematic study provides an alternative route for the fabrication of ferroelectric nanostructures that are worth considering for the next generation of flexible FeRAM in all-organic nanoelectronic devices.

Controlled polymer crystal/two-dimensional material heterostructures for high-performance photoelectronic applications

The control of atomically thin two-dimensional (2D) crystal-based heterostructures wherein the interfaces of 2D nanomaterials are vertically stacked with other thin functional materials via van der Waals interactions is highly important for not only optimizing the excellent properties of 2D nanomaterials, but also for utilizing the functionality of the contact materials. In particular, when 2D nanomaterials are combined with soft polymeric components, the resulting photoelectronic devices are potentially scalable and mechanically flexible, allowing the development of a variety of prototype soft-electronic devices, such as solar cells, displays, photodetectors, and non-volatile memory devices. Diverse polymer/2D heterostructures are frequently employed, but the performance of the devices with heterostructures is limited, mainly because of the difficulty in controlling the molecular structures of the polymers on the 2D surface. Thus, understanding the crystal interactions of polymers on atomically flat and dangling-bond-free surfaces of 2D materials is essential for ensuring high performance. In this study, the recent progress made in the development of thin polymer films fabricated on the surfaces of various 2D nanomaterials for high-performance photoelectronic devices is comprehensively reviewed, with an emphasis on the control of the molecular and crystalline structures of the polymers on the 2D surface.

Carbon Nanolights in Piezopolymers are Self-Organizing Toward Color Tunable Luminous Hybrids for Kinetic Energy Harvesting

Herein, an all-solid-state sequential self-organization and self-assembly process is reported for the in situ construction of a color tunable luminous inorganic/polymer hybrid with high direct piezoresponse. The primary inorganic self-organization in solid polymer and the subsequent polymer self-assembly are achieved at high pressure with the first utilization of piezo-copolymer (PVDF-TrFE) as the host matrix of guest carbon quantum dots (CQDs). This process induces the spontaneous formation of a highly ordered, microscale, polygonal, and hierarchically structured CQDs/PVDF-TrFE hybrid with multicolor photoluminescence, consisting of very thermodynamic stable polar crystalline nanowire arrays. The electrical polarization-free CQDs/PVDF-TrFE hybrids can efficiently harvest the environmental available kinetic mechanical energy with a new large-scale group-cooperation mechanism. The open-circuit voltage and short-circuit current outputs reach up to 29.6 V cm^-2 and 550 nA cm^-2 , respectively. The CQDs/PVDF-TrFE-based hybrid nanogenerator demonstrates drastically improved durable and reliable features during the real-time demonstration of powering commercial light emitting diodes. No attenuation/fluctuation of the electrical signals is observed for ≈10 000 continuous working cycles. This study may offer a new design concept for progressively but spontaneously constructing novel multiple self-adaptive complex inorganic/polymer hybrids that promise applications in the next generation of self-powered autonomous optoelectronic devices.

Large Piezoelectric Strain in Sub-10 Nanometer Two-Dimensional Polyvinylidene Fluoride Nanoflakes

Functional polymers such as polyvinylidene fluoride (PVDF) and its copolymers, which exhibit room-temperature piezoelectricity and ferroelectricity in two-dimensional (2D) limit, are promising candidates to substitute hazardous lead-based piezoceramics for flexible nanoelectronic and electromechanical energy-harvesting applications. However, realization of many polymers including PVDF in ultrathin 2D nanostructures with desired crystal phases and tunable properties remains challenging due to ineffective conventional synthesis methods. Consequently, it has remained elusive to obtain optimized piezoelectric performance of PVDF particularly in sub-10 nm regimes. Taking advantage of its high flexibility and easy processing, we fabricate ultrathin PVDF nanoflakes with thicknesses down to 7 nm by using a hot-pressing method. This thermo-mechanical strategy simultaneously induces robust thermodynamic α to electroactive β-phase transformation, with β fraction as high as 92.8% in sub-10 nm flakes. Subsequently, piezoelectric studies performed by using piezoresponse force microscopy reveal an excellent piezoelectric strain of 0.7% in 7 nm film and the highest piezoelectric coefficient ( d₃₃) achieved is -68 pm/V for 50 nm-thick nanoflakes, which is 13% higher than the piezoresponse from 50 nm-thick PZT nanofilms. Our results further suggest thickness modulation as an effective strategy to tune the piezoelectric performance of PVDF and affirm its supremacy over conventional piezoceramics especially at nanoscale. This work aims not only to help understand fundamental piezoelectricity of pure PVDF in sub-10 nm regimes but also provides an opportunity to realize other polymer-based 2D nanocrystals.

Flexible Robust and High-Density FeRAM from Array of Organic Ferroelectric Nano-Lamellae by Self-Assembly

Ferroelectric memories are endowed with high data storage density by nanostructure designing, while the robustness is also impaired. For organic ferroelectrics favored by flexible memories, low Curie transition temperature limits their thermal stability. Herein, a ferroelectric random access memory (FeRAM) is demonstrated based on an array of P(VDF-TrFE) lamellae by self-assembly. Written data shows enhanced thermal endurance up to 90 °C and undergoes 12 thermal cycles between 30 and 80 °C with little volatilization. The promoted thermal stability is attributed to pinning effect at interfaces between grain boundaries and lamellae, where charged domain walls and charged defects are coupled. These results provide a strategy for improving robustness of organic flexible FeRAMs, and reveal an attracting coupling effect between different phases of ferroelectric polymer.