Optical Switching Properties Research Articles

Shape Shifting and Locking in Mechanically Responsive Organic‐Inorganic Hybrid Materials for Thermoelastic Actuators

AbstractThe construction of mechanically responsive materials with reversible shape‐shifting, shape‐locking, and stretchability holds promise for a wide range of applications in fields such as soft robotics and flexible electronics. Here, we report novel thermoelastic one‐dimensional organic–inorganic hybrids (R/S‐Hmpy)PbI3 (Hmpy=2‐hydroxymethyl‐pyrrolidinium) to show mechanical responses. The single crystals undergo two phase transitions at 310 K and 380 K. When heated to 380 K, they show shape‐shifting and expansion along the b‐axis by about 13.4 %, corresponding to a larger deformation than that of thermally activated shape memory alloys (8.5 %), and exhibit a strong actuation force. During the cooling process, the stretched crystal shape maintains and a shape‐locking phenomenon occurs, which is lifted when the temperature decreases to 305 K. Meanwhile, due to the introduction of chiral ions, the thermal switching shows a 10‐fold second‐order nonlinear switching contrast (common values typically below 3‐fold). This study presents a thermoelastic actuator based on shape‐shifting and ‐locking of organic–inorganic hybrids for the first time. The dielectric and nonlinear optical switching properties of organic–inorganic hybrids broaden the range of applications of mechanically responsive crystals.

Shape Shifting and Locking in Mechanically Responsive Organic-Inorganic Hybrid Materials for Thermoelastic Actuators.

The construction of mechanically responsive materials with reversible shape-shifting, shape-locking, and stretchability holds promise for a wide range of applications in fields such as soft robotics and flexible electronics. Here, we report novel thermoelastic one-dimensional organic-inorganic hybrids (R/S-Hmpy)PbI3 (Hmpy=2-hydroxymethyl-pyrrolidinium) to show mechanical responses. The single crystals undergo two phase transitions at 310 K and 380 K. When heated to 380 K, they show shape-shifting and expansion along the b-axis by about 13.4 %, corresponding to a larger deformation than that of thermally activated shape memory alloys (8.5 %), and exhibit a strong actuation force. During the cooling process, the stretched crystal shape maintains and a shape-locking phenomenon occurs, which is lifted when the temperature decreases to 305 K. Meanwhile, due to the introduction of chiral ions, the thermal switching shows a 10-fold second-order nonlinear switching contrast (common values typically below 3-fold). This study presents a thermoelastic actuator based on shape-shifting and -locking of organic-inorganic hybrids for the first time. The dielectric and nonlinear optical switching properties of organic-inorganic hybrids broaden the range of applications of mechanically responsive crystals.

Enhanced visible transmittance with low phase transition temperature of VO2 enabled by W-Mg co-doping

Vanadium dioxide (VO2) is a promising thermochromic material for smart windows. However, the relatively high phase transition temperature (Tt) and low visible transmittance (Tlum) constrain its practical application. Cationic co-doping offers an effective strategy to optimize the optical switching properties near room temperature. In this line, herein, the W-Mg co-doped VO2 films were synthesized by a solution and annealing method from W-Mg co-doped ammonium citrato-oxovanadate (IV). The synergistic effect of W-Mg co-doping improves the performance of VO2 films with Tt of 42.6 °C, Tlum of 59.0 % and solar energy modulation of 12.8 %. By changing the amounts of Mg, the W-Mg co-doped VO2 film maintains the low Tt of W-VO2 film and meanwhile possesses higher Tlum, which is attributed to the band gap structure change induced by the W-Mg co-doping. The excellent optical, thermochromic and thermal insulation performances confirm that our research could provide guidance for advancing the application of VO2 in smart window fields.

Highly reproducible and CMOS-compatible VO2-based oscillators for brain-inspired computing.

With remarkable electrical and optical switching properties induced at low power and near room temperature (68°C), vanadium dioxide (VO2) has sparked rising interest in unconventional computing among the phase-change materials research community. The scalability and the potential to compute beyond the von Neumann model make VO2 especially appealing for implementation in oscillating neural networks for artificial intelligence applications, to solve constraint satisfaction problems, and for pattern recognition. Its integration into large networks of oscillators on a Silicon platform still poses challenges associated with the stabilization in the correct oxidation state and the ability to fabricate a structure with predictable electrical behavior showing very low variability. In this work, the role played by the different annealing parameters applied by three methods (slow thermal annealing, flash annealing, and rapid thermal annealing), following the vanadium oxide atomic layer deposition, on the formation of VO2 grains is studied and an optimal substrate stack configuration that minimizes variability between devices is proposed. Material and electrical characterizations are performed on the different films and a step-by-step recipe to build reproducible VO2-based oscillators is presented, which is argued to be made possible thanks to the introduction of a hafnium oxide (HfO2) layer between the silicon substrate and the vanadium oxide layer. Up to seven nearly identical VO2-based devices are contacted simultaneously to create a network of oscillators, paving the way for large-scale implementation of VO2 oscillating neural networks.

Effects of Dy on structural, morphology, optical, and resistive switching properties of HfO2 thin films

This paper studies the effect of Dy on the structure, morphology, and optical properties of HfO2 thin films deposited on fused silica substrates and the optical transparency and resistive switching (RS) behavior of the thin films as an active insulator layer in Ag/active insulator layer/ITO/glass memories. Thin films were deposited by spin-coating using a precursor solution obtained by the Pechini-type sol-gel method. The structural analysis revealed that the insertion of Dy ions into the HfO2 lattice caused the phase transformation from the monoclinic to the cubic phase. Morphological analysis revealed that the thin films had thicknesses between 98 and 101 nm, and their surfaces were composed of 3D islands formed by aggregates of spherical particles ranging from 8 to 5 nm. The active insulator layers had thicknesses ranging from 235 to 218 nm, with no cavities or cracks between them. The optical transparency of the active insulator layer/ITO/glass memory structures was between 70 and 87 %. Optical transmittance, optical bandgap, and optical constants were modified incorporating Dy ions into HfO2 thin films compared to the undoped HfO2 thin films. Incorporating Dy ions into HfO2 increased oxygen vacancies in the active insulator layers. The memory with 10 w% of Dy had the highest resistance off/on ratio of 9.04 compared with undoped HfO2 memory. Also, the conduction mechanism in the high resistive state (HRS) of the memory with 10 w% of Dy was related to ohmic conduction and space charge limited conduction (SCLC).

A High Working Temperature Multiferroic Induced by Inverse Temperature Symmetry Breaking.

Molecular-based multiferroic materials that possess ferroelectric and ferroelastic orders simultaneously have attracted tremendous attention for their potential applications in multiple-state memory devices, molecular switches, and information storage systems. However, it is still a great challenge to effectively construct novel molecular-based multiferroic materials with multifunctionalities. Generally, the structure of these materials possess high symmetry at high temperatures, while processing an obvious order-disorder or displacement-type ferroelastic or ferroelectric phase transition triggered by symmetry breaking during the cooling processes. Therefore, these materials can only function below the Curie temperature (Tc), the low of which is a severe impediment to their practical application. Despite great efforts to elevate Tc, designing single-phase crystalline materials that exhibit multiferroic orders above room temperature remains a challenge. Here, an inverse temperature symmetry-breaking phenomenon was achieved in [FPM][Fe3(μ3-O)(μ-O2CH)8] (FPM stands for 3-(3-formylamino-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium, which acts as the counterions and the rotor component in the network), enabling a ferroelastoelectric phase at a temperature higher than Tc (365 K). Upon heating from room temperature, two-step distinct symmetry breaking with the mm2Fm species leads to the coexistence of ferroelasticity and ferroelectricity in the temperature interval of 365-426 K. In the first step, the FPM cations undergo a conformational flip-induced inverse temperature symmetry breaking; in the second step, a typical ordered-disordered motion-induced symmetry breaking phase transition can be observed, and the abnormal inverse temperature symmetry breaking is unprecedented. Except for the multistep ferroelectric and ferroelastic switching, this complex also exhibits fascinating nonlinear optical switching properties. These discoveries not only signify an important step in designing novel molecular-based multiferroic materials with high working temperatures, but also inspire their multifunctional applications such as multistep switches.

Broadband nonlinear optical response and sub-picosecond carrier dynamics in graphene-SnSe2 van der Waals heterostructures.

The van der Waals (vdWs) heterostructures, with vertical layer stacking structure of various two-dimensional (2D) materials, maintain the reliable photonic characteristics while compensating the shortcomings of the participating individual components. In this work, we combine the less-studied multilayer tin selenide (SnSe2) thin film with one of the traditional 2D materials, graphene, to fabricate the graphene-based vdWs optical switching element (Gr-SnSe2) with superior broadband nonlinear optical response. The transient absorption spectroscopy (TAS) measurement results verify that graphene acts as the recombination channel for the photogenerated carrier in the Gr-SnSe2 sample, and the fast recovery time can be reduced to hundreds of femtoseconds which is beneficial for the optical modulation process. The optical switching properties are characterized by the I-scan measurements, exhibiting a saturable energy intensity of 2.82 mJ·cm-2 (0.425 µJ·cm-2) and a modulation depth of 15.6% (22.5%) at the wavelength of 1030 nm (1980nm). Through integrating Gr-SnSe2 with a cladding waveguide, high-performance picosecond Q-switched operation in the near-infrared (NIR) and mid-infrared (MIR) spectral regions are both achieved. This work experimentally demonstrates the great potential of graphene-based vdWs heterostructures for applications in broadband ultrafast photonics.

Host–guest cooperative bridged bicyclopolyynic (BBP) open-molecular cages with optical-switching properties

A cyclo[18]carbon inspired open-polyynic cages promote host–guest interactions. The complex exhibits optical switching behaviour regulated by reversible transformation between endohedral and exohedral configurations.

Influences of Cu Doping on the Microstructure, Optical and Resistance Switching Properties of Zinc OxideThin Films.

Copper-doped zinc oxide films (Zn1-xCuxO) (x = 0, 2%, 4%, 6%) were fabricated on conductive substrates using the sol-gel process. The crystal structure, optical and resistive switching properties of Zn1-xCuxO films are studied and discussed. RRAM is made using Zn1-xCuxO as the resistive layer. The results show that the (002) peak intensity and grain size of Zn1-xCuxOfilms increase from 0 to 6%. In addition, PL spectroscopy shows that the oxygen vacancy defect density of Zn1-xCuxO films also increases with the increase in Cu. The improved resistive switching performance of the RRAM device can be attributed to the formation of conductive filaments and the destruction of more oxygen vacancies in the Zn1-xCuxO film. Consequently, the RRAM device exhibits a higher low resistance state to high resistance state ratio and an HRS state of higher resistance value.

Solvothermal synthesis of TiO2 nanospheres for non-volatile memory and synaptic learning applications

In this study, we used the one-pot solvothermal method to synthesize the TiO2 nanospheres (NSs) and used them for non-volatile memory and neuromorphic computing applications. Several analytical tools were used to understand the structural, optical, morphological, and compositional characteristics of synthesized TiO2 NSs. The tetragonal crystal structure of anatase TiO2 was formed, according to the Rietveld refined x-ray diffraction results. The NS morphology was confirmed by field emission scanning electron microscopy and transmission electron microscopy images. X-ray photoelectron spectroscopy was probed to understand the elemental composition and electronic states of the TiO2 NSs. We specifically looked at the impact of reaction time on the structural, optical, morphological, compositional, and resistive switching (RS) properties of TiO2 NSs. The fabricated devices (Ag/TiO2 NSs/FTO) exhibit bipolar RS behavior. The optimized RS device shows good endurance (5000 cycles) and memory retention (5000 s) properties. Moreover, fabricated devices showed double-valued charge-flux characteristics, whereas charge transport was caused by the Ohmic and space charge-limited current mechanisms. Additionally, the optimized device can mimic various synaptic characteristics including potentiation-depression, excitatory post-synaptic current, and paired-pulse facilitation.

Film thickness dependent electron transport and optical properties of thermochromic VO2

Vanadium dioxide (VO2) thin films are very well known for their efficient thermochromic properties, i.e. temperature dependent reversible switching of their optical and electrical properties, rooted in a first order phase change at the transition temperature of 68 °C. Despite a vast amount of basic and applied research on VO2 thin films, a simultaneous study of the optical and electrical switching properties, especially as functions of film thickness is scarcely reported. Such a study may provide interesting insight into the basics of thermochromic switching and lead to a better designing of more efficient thermochromically switching optical and electrical interactive devices. In this work we have studied the film thickness dependence of the optical and electrical properties of sputter deposited VO2 thin films. The size dependent electrical behavior of these films is examined in the light of the existing theories. Applying Drude model which combines the optical and electrical behaviour, all the relevant parameters (plasma frequency and free electron density) are calculated for the semi-conducting and metallic phases of the VO2 thin films. All the obtained parameters and the strong thermochromic switching behaviour indicate that the films are predominantly made up of VO2. Importance of distinction between optical hysteresis derived transition temperature and the one derived from resistance hysteresis is emphasized. This comprehensive and in-depth study is expected to lead to a better understanding and design of switching devices based on VO2 thin films.

Pressure-induced successive phase transitions and Fano resonance engineering in lead-free piezoceramics KNbO3

We carried out detailed high-pressure Raman scattering studies on lead-free perovskite piezoceramics KNbO3 (KNO) with a smaller pressure interval so that the successive phase transition pressures are precisely determined (PO→T: 8.2 GPa; PT→C: 10.9 GPa). The pressure-quenched phase does not return to the ambient orthorhombic phase completely. In this work, the pressure-induced phase transition sequence is discussed in detail. Also, based on the pressure-dependent Raman spectra of KNO, we have discovered clear evidence of a pressure-induced Fano resonance. It reveals the role played by the Raman-active continuum and discrete energy states in the condensed matter system under high pressure and enables the high-pressure Fano resonances engineering in arbitrary geometries [q∈(−∞, 0)]. These findings highlight the opportunity for designing and tuning the wave transmission and optical switch properties of lead-free piezoceramics KNO via high pressure.

A Flexible Circularly Polarized Luminescence Switching Device Based on Proton‐Coupled Electron Transfer

Flexible circularly polarized luminescence (CPL) switching devices have been long‐awaited due to their promising potential application in wearable optoelectronic devices. However, on account of the few materials and complicated design of manufacturing systems, how to fabricate a flexible electric‐field‐driven CPL‐switching device is still a serious challenge. Herein, a flexible device with multiple optical switching properties (CPL, circular dichroism (CD), fluorescence, color) is designed and prepared efficiently based on proton‐coupled electron transfer (PCET) mechanism by optimizing the chiral structure of switching molecule. More importantly, this device can maintain the switching performance even after 300 bending‐unbending cycles. It has a remarkable comprehensive performance containing bistable property, low open voltage, and good cycling stability. Then, prototype devices with designed patterns have been fabricated, which opens a new application pattern of CPL‐switching materials.

Sol-Gel Processed ZrO2 Based Forming-Free Resistive Switching Memory Devices

In this work, we fabricated ZrO2 based resistive random access memory by sol-gel spin coating technique and investigated its structural, optical and resistive switching properties. The X-ray diffraction pattern revealed that 400 °C annealed ZrO2 thin film has tetragonal structure. The optical band gap value of ZrO2 thin film obtained was 5.51 eV. The resistive switching behaviour of W/ZrO2/ITO capacitor like structure was studied. It was found that no initial electroforming process required for the device. The fabricated devices show a self-compliance bipolar resistive switching behaviour and have high on off ratio (>102). Our result suggests that solution processed ZrO2 has great potential to develop transparent and flexible resistive random access memory devices.

High Temperature CsPbBrxI3-x Memristors Based on Hybrid Electrical and Optical Resistive Switching Effects.

The emergence of perovskite-based memristors associated with the migration of ions has attracted attention for use in overcoming the limitations of the von Neumann computing architecture and removing the bottleneck of storage density. However, systematic research on the temperature dependence of halide perovskite-based memristors is still required due to the unavoidable thermal stability limits. In this work, mixed halide CsPbBrxI3-x-based (X = 0, 1, 2) memristors with unique electrical and optical resistive switching properties in an ambient atmosphere from room temperature to a 240 °C maximum have been successfully achieved. At room temperature, the CsPbBrxI3-x-based memristors exhibit outstanding resistive switching behaviors such as ultralow operating voltage (∼0.81, ∼0.64, and ∼0.54 V for different devices, respectively), moderate ON/OFF ratio (∼102), stable endurance (103 cycles), and long retention time (104 s). The CsPbBrxI3-x-based memristors maintain excellent repeatability and stability at high temperature. Endurance failures of CsPbI3, CsPbBrI2, and CsPbBr2I memristors occur at 90, 150, and 270 °C, respectively. Finally, nonvolatile imaging employing CsPbBr2I-based memristor arrays based on the electrical-write and optical-erase operation at 100 °C has been demonstrated. This study provides utilization potentiality in the high temperature scenarios for perovskite wearable and large-scale information devices.

Influence of the charge compensation effect on the metal–insulator transition of Mg-W co-doped VO2

Cation co-doping was proved an effective approach to reduce the metal–insulator transition (MIT) temperature (θc) of vanadium dioxide (VO2) for realizing excellent optical switching properties near room temperature, but its mechanism is remaining a great challenge. Herein, the Mg-W co-doped VO2 films are fabricated through hydrothermal growth. By changing the Mg content with maintaining 1 at% W doping level, it is observed that Mg dopants raise the θc of co-doped VO2 at a rate of ∼ 22 ℃ per at% Mg as the Mg content is less than ∼ 1 at%, but loses efficacy more than 1 at% Mg. The resistance of the co-doped VO2 films show the similar variation trend. The valence-band XPS spectra indicate that the Mg dopants counteract the VBE uplifting of VO2 caused by W dopants, consequently enlarging the optical band gap of W-doped VO2, which would partially explain the improvement of the visible light transmittance of W-doped VO2 induced by Mg dopants. The results clearly evidence that the charge compensation takes effect in the co-doped VO2, demonstrating the co-doping of high- and low-valent cations is unfavorable for effectively decreasing θc of VO2, and reveal the electron doping plays a key role in reducing θc.

Titanium content influence on the optical response characteristics of TiO2/ormosils composite films doped with azobenzene.

TiO2-based organic-inorganic composite films doped with azobenzene and photosensitive groups were prepared by combining a low-temperature solgel technique and a spin-coating method. The influence of TiO2 content on the optical and structural properties of the composite films including the film thickness, the refractive index, the transmission loss, the thermal gravity analysis, and Fourier transform infrared spectroscopy spectra was studied. Photoisomerization and optical switching characteristics of the composite film were investigated under the irradiation of 365 nm ultraviolet light and 450 nm visible light. Results indicate that several micrometer thick films can be easily obtained at room temperature and there is a proportional relationship between the refractive index value and the TiO2 content. In addition, the composite films have a low optical propagation loss of about 0.1 dB/cm. The composite films with 0.2 M TiO2 content have an obvious photoisomerization and good optical switching properties. Finally, the hexagonal microlens array was fabricated in the composite films by using an ultraviolet nanoimprint technology. All these results above indicate that the as-prepared TiO2-based organic-inorganic composite film has potential applications in optical switching devices and photonic elements.

Effects of strain on the electronic, optical, and ferroelectric transition properties of HfO2: ab initio simulation study

The amazing ferroelectricity observed in Hafnium oxide (HfO2) thin films is due to the existence of non-centrosymmetric orthorhombic Pca21 phase. In this study, the structural, electronic, optical, and polarization switching properties of the orthorhombic Pca21 phase with various strains are investigated by employing density functional theory (DFT) calculations based on the DFT. Out-of-plane and in-plane strains are taken into consideration. The results indicate that the band-gap values of orthorhombic HfO2 increase at first and then decrease from compressive strain to tensile strain due to the variation of the valence band maximum and the conduction band minimum. Moreover, orthorhombic HfO2 transforms into a direct bandgap material when compressive strain along the z direction is larger than or equal to 5%. A red-shift occurs under both tensile and compressive strains in the absorption edge compared with instinct HfO2 in the absorption spectra. The absorption intensity of the strained HfO2 is enhanced with the energy below 5.5 eV. The energy barriers of path A and path B of the polarization switching mediated by non-polar tetragonal phase are 116.8 and 91.2 meV per formula unit (meV/f.u.), respectively. In polarization switching without mediated by non-polar phase, the energy barriers decrease with the increasing of out-of-plane compressive strain along the x direction and tensile strain along the y direction compared with strain free structures.

Unusual reversible fast-response dielectric and nonlinear optical switches in homochiral compounds

Two new materials with a fast dielectric response and a quadratic nonlinear optical switching property, (R)- and (S)-3-hydroxypiperidinium perchlorate ((R)- and (S)-3-HP-ClO4), have been synthesized. The homochirality of the cation causes asymmetry breaking, making the compounds crystallize in a chiral space group with special optical properties. By using differential scanning calorimetry (DSC) measurements, (R)- and (S)-3-HP-ClO4 undergo a typical first-order phase transition at 360 and 352 K, respectively. Similarly, the dielectric constants of the two enantiomers exhibit remarkable step-like anomalies around the phase transition temperatures. Moreover, the two enantiomers both show distinct switchable nonlinear optical (NLO) behaviors, which are caused by order–disorder transitions of the (R)- and (S)-3-(hydroxypiperidinium) cations and perchlorate anions. These findings have great potential in next-generation sensors, switches and actuator devices.

Self-assembled single crystal VO2 (A) microbelts by the reduction of V2O5 thin films: synthesis, structure and optical properties

In this work, single crystal VO2 (A) microbelts (MBs) were synthesized for the first time via the reduction of V2O5 thin films without a catalyst and without a hydrothermal process. The morphology and microstructure of the as-prepared VO2 (A) MRs was characterized by SEM, fluorescence mapping, Raman, XRD, XPS and FTIR. Results show that the as-prepared VO2 (A) MBs exhibit belt-like morphology with a rectangular cross section. The formation mechanism was proposed as melting of V2O5 thin film, VO2 nucleation in V2O5 droplets, VO2 growth. Light scattering measurements were carried out at varying sample temperatures. The results found that the as-prepared VO2 (A) MBs exhibit optical switching properties at a high temperature around 170 ℃, which is promising for designing thermally stable optical switching devices.