Technology and Integration Roadmap for Optoelectronic Memristor

Optoelectronic memristors (OMs) have emerged as a promising optoelectronic Neuromorphic computing paradigm, opening up new opportunities for neurosynaptic devices and optoelectronic systems. These OMs possess a range of desirable features including minimal crosstalk, high bandwidth, low power consumption, zero latency, and the ability to replicate crucial neurological functions such as vision and optical memory. By incorporating large-scale parallel synaptic structures, OMs are anticipated to greatly enhance high-performance and low-power in-memory computing, effectively overcoming the limitations of the von Neumann bottleneck. However, progress in this field necessitates a comprehensive understanding of suitable structures and techniques for integrating low-dimensional materials into optoelectronic integrated circuit platforms. This review aims to offer a comprehensive overview of the fundamental performance, mechanisms, design of structures, applications, and integration roadmap of optoelectronic synaptic memristors. By establishing connections between materials, multilayer optoelectronic memristor units, and monolithic optoelectronic integrated circuits, this review seeks to provide insights into emerging technologies and future prospects that are expected to drive innovation and widespread adoption in the near future.

Triple Thorpe-Ingold Effect in the Synthesis of 18-Membered C3 Symmetric Lactams Stacking as Endless Supramolecular Tubes

Three C₃ symmetric macrolactams were very efficiently cyclized from their linear precursors. Adequately located substituents are responsible for the enhancement of reactivity that is not observed in the unsubstituted parent. DFT calculations show that the properly folded cyclization precursor, the reactive conformer, is more populated than other conformers, leading to a decrease of free energy of activation. The crystal structure of the ring substituted with three very bulky esters indicates that tubular stacking is preserved.

Large-Area Flexible Memory Arrays of Oriented Molecular Ferroelectric Single Crystals with Nearly Saturated Polarization

Molecular ferroelectrics (MFs) have been proven to demonstrate excellent properties even comparable to those of inorganic counterparts usually with heavy metals. However, the validation of their device applications is still at the infant stage. The polycrystalline feature of conventionally obtained MF films, the patterning challenges for microelectronics and the brittleness of crystalline films significantly hinder their development for organic integrated circuits, as well as emerging flexible electronics. Here, a large-area flexible memory array is demonstrated of oriented molecular ferroelectric single crystals (MFSCs) with nearly saturated polarization. Highly-uniform MFSC arrays are  prepared on large-scale substrates including Si wafers and flexible substrates using an asymmetric-wetting and microgroove-assisted coating (AWMAC) strategy. Resultant flexible memory arrays exhibit excellent nonvolatile memory properties with a low-operating voltage of <5 V, i.e., nearly saturated ferroelectric polarization (6.5 µC cm^-2 ), and long bending endurance (>10³ ) under various bending radii. These results may open an avenue for scalable flexible MF electronics with high performance.

A study of the dynamics and structure of the dielectric anomaly within the molecular solid TEA(TCNQ)2

With rising interest in organic-based functional materials, it is important to understand the nature of magnetic and electrical transitions within these types of systems. One intriguing material is triethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)₂) where there is an order-disorder transition at ∼220 K. This work focuses on novel neutron scattering techniques to understand the motion of the TEA cations at this transition and explain why we see the dielectric behaviour and possible ferroelectricity within this type of system. We show that the motion of the methyl groups of the TEA cation is spatially restricted below 220 K, whereas above the dielectric anomaly at 220 K, they are free to re-orientate, which ultimately leads to some rich behaviour that could be further exploited. Lastly, we also study the dynamics at this transition using a variety of additional techniques, helping to provide a consistent picture of the motions of the cations.

Hybridized Nanogenerators for Multifunctional Self-Powered Sensing: Principles, Prototypes, and Perspectives

Sensors are a key component of the Internet of Things (IoTs) to collect information of environments or objects. Considering the tremendous number and complex working conditions of sensors, multifunction and self-powered feathers are two basic requirements. Nanogenerators are a kind of devices based on the triboelectric, piezoelectric, or pyroelectric effects to harvest ambient energy and then converting to electricity. The hybridized nanogenerators that combined multiple effects in one device have great potential in multifunctional self-powered sensors because of the unique superiority such as generating electrical signals directly, responding to diverse stimuli, etc. This review aims at introducing the latest advancements of hybridized nanogenerators for multifunctional self-powered sensing. Firstly, the principles and sensor prototypes based on TENG are summarized. To avoid signal interference and energy insufficiently, the multifunctional self-powered sensors based on hybridized nanogenerators are reviewed. At last, the challenges and future development of multifunctional self-powered sensors have prospected.

Phase Switching as the Origin of Large Piezoelectric Response in Organic-Inorganic Perovskites: A First-Principles Study

Piezoelectrics are critical functional components of many practical applications such as sensors, ultrasonic transducers, actuators, medical imaging, and telecommunications. So far, the best performing piezoelectrics are ferroelectric ceramics, many of which are toxic, heavy, hard, and cost-ineffective. Recently, a groundbreaking discovery of extraordinarily large piezoelectric coefficients in the family of organic-inorganic perovskites gave a hope for a cheaper, environmentally friendly, inexpensive, lightweight, and flexible alternative. However, the origin of such a response in organic-inorganic ferroelectrics whose spontaneous polarization is an order of magnitude smaller than for inorganic counterparts remains unclear. In our study, we employ first-principles simulations to predict that the mechanism associated with large piezoelectric constants is of extrinsic origin and associated with switching between the stable phase and a previously overlooked energetically competitive metastable phase that can be stabilized by the external stress. The phase switching changes the polarization direction and therefore produces a large piezoelectric response similar to PbZr_{1-x}Ti_{x}O_{3} near the morphotropic phase boundary. The existence of such metastable phases is likely to manifest as the dynamical molecular disorder above the Curie temperature and therefore could be intrinsic to the entire family of organic-inorganic ferroelectrics with such disorder.

Pressure-Induced Amorphization of Diisopropylammonium Perchlorate Studied by Raman Spectroscopy and X-ray Diffraction

Diisopropylammonium salts have drawn attention in recent years due to their room-temperature ferroelectric properties. Triclinic diisopropylammonium perchlorate (DIPAP) exhibits ferroelectricity at room temperature. We have carried out density functional theory calculations to assign the phonon modes in DIPAP. High-pressure Raman spectra of DIPAP are recorded up to ∼3 GPa. Discontinuity in the NH₂ bending and stretching mode frequencies and the appearance of new bands at 0.7 GPa suggest a phase transition by a rearrangement in the hydrogen network. Broadening of lattice modes at 1.3-1.7 GPa indicates a loss of crystalline nature above 1.7 GPa. High-pressure synchrotron X-ray diffraction of DIPAP shows an isostructural phase transition at 0.6 GPa and confirms amorphization at 1.5 GPa that may lead to a loss of ferroelectricity above this pressure. The ambient phase becomes reversible after releasing the pressure. The bulk modulus of DIPAP is determined to be 16.5 GPa.

High-Temperature Molecular Ferroelectric Tris(2-hydroxyethyl) Ammonium Bromide with Dielectric Relaxation

We obtained one new molecular ferroelectric material tris(2-hydroxyethyl) ammonium bromide (TAB) that crystallizes in aqueous solution at room temperature with a space group of R3m which belongs to ten polar space groups. There is a paraelectric-to-ferroelectric phase transition at 424 K (from hexagonal R3̅m to hexagonal R3m phase). Such a high transition temperature is close to that of diisopropylamine bromide (426 K) and higher than that of many other molecular ferroelectrics, such as triethylmethylammonium tetrabromoferrate(III) (360 K); some of the organic-inorganic perovskite ferroelectrics, such as (cyclohexylammonium)₂PbBr₄ (363 K); and some inorganic ferroelectrics, including BaTiO₃ (393 K). The saturated polarization and the coercive field of TAB measured from the ferroelectric hysteresis loop are about 0.54 μC·cm^-2 and 0.62 kV/cm, respectively. Given its superior performance, including high phase transition temperature, room-temperature ferroelectricity, small coercive electric field, and adjustable ladder-shaped dielectric constant, TAB will have many potential applications.

Self-Assembly of C3 Symmetric Rigid Macrolactams into Very Polar and Porous Trigonal Crystals

Cyclohexane and cyclotri-β-alanyl have been used as scaffolds for the design of new C₃ -symmetric rings incorporating conjugated alkenes and dienes. All three C₃ -symmetric lactams share the same triangular shape and their crystal system is trigonal. They all belong to the R3 space group, R3m, R3 and R3c, for the increasingly large 12-, 18- and 24-membered rigid rings, respectively. All lactams stack on top of each other, through H-bonds and van der Waals noncovalent interactions, leading to endless supramolecular cylinders and tubes. The largest member of the family leads to tubes, the central pores of which is wide enough to let water in. A common feature of all the lactams is their very large dipole, of around 9 D, according to DFT calculations. Surprisingly, all the resulting cylinders and tubes pack side by side in the crystals, with all the dipoles pointing to the same direction. As a result, all three crystals are anisotropic and appear to be the first members of a new kind of highly polar crystals.

Vibrational spectroscopic and computational studies on diisopropylammonium bromide

Diisopropylammonium bromide (DIPAB) can be crystallized either in an orthorhombic (P2₁2₁2₁) or in a monoclinic (P2₁) structure at room temperature depending on synthesis conditions. The non-polar orthorhombic structure exhibits a subtle, irreversible transformation into the ferroelectric monoclinic-II (m-II) phase above ~421K. At a slightly higher temperature of 426K this m-II (P2₁) phase reversibly transforms into a disordered, paraelectric monoclinic-I (P2₁/m) structure. We synthesized DIPAB in the orthorhombic structure, heated it to obtain the m-II phase and carried out a systematic study of their Raman and IR spectra. We obtained the phonon irreducible representations from factor group analysis of the orthorhombic and m-II structures based on the reported structural information. DIPAB is an organic molecular crystal, and the vibrational spectra in the intramolecular region (200-3500cm^-1) of the two different phases are identical to each other, indicating weak inter-molecular interactions in both crystalline structures. In the low wavenumber region (10-150cm^-1) the Raman spectra of the two phases are different due to their sensitivity to molecular environment. We also carried out first principles calculations using Gaussian 09 and CASTEP codes to analyze the vibrational frequencies. Mode assignments were facilitated by isolated molecule calculations that are also in good agreement with intramolecular vibrations, whereas CASTEP (solid state) results could explain the external modes.

Novel photoswitchable dielectric properties on nanomaterials of electronic core-shell γ-FeOx@Au@fullerosomes for GHz frequency applications

We unexpectedly observed a large amplification of the dielectric properties associated with the photoswitching effect and the new unusual phenomenon of delayed photoinduced capacitor-like (i.e. electric polarization) behavior at the interface on samples of three-layered core-shell (γ-FeOx@AuNP)@[C60(>DPAF-C9)](n)2 nanoparticles (NPs) in frequencies of 0.5-4.0 GHz. The detected relative dielectric constant amplification was initiated upon switching off the light followed by relaxation to give an excellent recyclability. These NPs having e(-)-polarizable fullerosomic structures located at the outer layer were fabricated from highly magnetic core-shell γ-FeOx@AuNPs. Surface-stabilized 2 in a core-shell structure was found to be capable of photoinducing the surface plasmonic resonance (SPR) effect by white LED light. The accumulated SPR energy was subsequently transferred to the partially bilayered C60(>DPAF-C9) fullerosomic membrane layer in a near-field (∼1.5 nm) region without producing radiation heat. Since the monostatic SAR signal is dielectric property-dependent, we used these measurements to provide evidence of derived reflectivity changes on a surface coated with 2 at 0.5-4.0 GHz upon illumination of LED white light. We found that a high, >99%, efficiency of response amplification in image amplitude can be achieved.

Temperature-triggered order–disorder phase transition in molecular-ionic material N-butyldiethanolammonium picrate monohydrate

We report an organic–ionic material that undergoes a first-order structural phase transition, induced by order–disorder of oxygen atoms in picrate anion. This strategy offers a potential pathway to explore new switchable dielectric materials.

Introducing DDEC6 atomic population analysis: part 2. Computed results for a wide range of periodic and nonperiodic materials

DDEC6 atomic population analysis gives excellent performance for small and large molecules, porous solids, dense solids, solid surfaces, organometallic complexes, nanoclusters, and magnetic materials.

Polarization-driven catalysis via ferroelectric oxide surfaces

Ferroelectric polarization can tune the surface chemistry: enhancing technologically important catalytic reactions such as NO_x direct decomposition and SO₂ oxidation.

Two reversible ferroelectric phase transitions in diisopropylammonium perchlorate

Two reversible first-order phase transitions were found in an improper ferroelectric, diisopropylammonium perchlorate, as the crystal symmetry transforms from P2₁/c to P1 at 296 K, and subsequently to P2₁/c at 338 K in the heating process.

4-Methoxyanilinium perrhenate 18-crown-6: a new ferroelectric with order originating in swinglike motion slowing down

A supramolecular adduct 4-methoxyanilinium perrhenate 18-crown-6 was synthesized, which undergoes a disorder-order structural phase transition at about 153 K (T(c)) due to slowing down of a pendulumlike motion of the 4-methoxyanilinium group upon cooling. Ferroelectric hysteresis loop measurements give a spontaneous polarization of 1.2  μC/cm2. Temperature-dependent solid-state nuclear magnetic resonance measurements reveal three kinds of molecular motions existing in the compound: pendulumlike swing of 4-methoxyanilinium cation, rotation of 18-crown-6 ring, and rotation of the methoxyl group. When the temperature decreases, the first two motions are frozen at about 153 K and the methoxyl group becomes rigid at around 126 K. The slowing down or freezing of pendulumlike motion of the cation triggered by temperature decreasing corresponds to the centrosymmetric-to-noncentrosymmetric arrangement of the compound, resulting in the formation of ferroelectricity.