Photo-induced Strain Research Articles

Photo-thermo-mechanically actuated liquid crystalline elastomer nanocomposite reinforced by polyurethane fiber-network

ABSTRACTIn this work, we develop a nanocomposite consisting of nematic liquid crystal elastomer (LCE) matrix incorporated with single-wall carbon nanotubes (SWCNTs) and the reinforcement phase of polyurethane fiber-network (PUFN). The photo-thermo-mechanical actuation of LCE matrix is realized by converting light into heat with assistant of SWCNTs. The PUFN enhances the mechanical properties of the material, thus the actuation capability. Under the irradiation from a wide-spectrum light source with an intensity on the order of 100 mW/cm2, the PUFNs/SWCNT/LCE nanocomposite contracts up to 27% of the original length in several seconds due to the photo-induced strain, and recovers to the original length in several seconds after the light source is switched off. The efficiency of actuation is not light-spectrum dependent. The maximum effective actuation force is about 260 kPa, much larger than that of the LCE material without the reinforcement of PUFNs. The tensile strength and the performance of anti-fatigue failure under multiple reversible actuations are also greatly improved. The mechanism of the actuation capability of LCE material involved in the fiber-network reinforcement phase is studied.

Implementation of ultrafast X-ray diffraction at the 1W2B wiggler beamline of Beijing Synchrotron Radiation Facility.

The implementation of a laser pump/X-ray probe scheme for performing picosecond-resolution X-ray diffraction at the 1W2B wiggler beamline at Beijing Synchrotron Radiation Facility is reported. With the hybrid fill pattern in top-up mode, a pixel array X-ray detector was optimized to gate out the signal from the singlet bunch with interval 85 ns from the bunch train. The singlet pulse intensity is ∼2.5 × 10(6) photons pulse(-1) at 10 keV. The laser pulse is synchronized to this singlet bunch at a 1 kHz repetition rate. A polycapillary X-ray lens was used for secondary focusing to obtain a 72 µm (FWHM) X-ray spot. Transient photo-induced strain in BiFeO3 film was observed at a ∼150 ps time resolution for demonstration.

Ultrafast transient dynamics in composite multiferroics

We investigate theoretically the dynamic multiferroic (MF) response of coupled ferroelectric (FE)/ferromagnetic (FM) composites upon excitation by a photo-induced acoustic strain pulse. Two magnetoelectric (ME) mechanisms are considered: interface strain- and charge-mediated ME couplings. The former results in demagnetization, depolarization and repolarization within tens of picoseconds via respectively magnetostriction and piezoelectricity. Charge ME interaction affects the FE/FM feedback response leading to magnetization recovery. Experimental realization based on time-resolved x-ray diffraction is suggested. The findings indicate the potential of composite MF for photo-steered, high-speed, multi-state electronic devices.

Light Actuation of Graphene-Oxide Incorporated Liquid Crystalline Elastomer Nanocomposites

The authors demonstrate high-performance photo-actuation of nematic liquid crystal elastomer (LCE) nanocomposites incorporating graphene oxide (GO). The nematic LCE serves as the matrix with reversible thermomechanical response. The incorporated GO absorbs photons and converts the photonic energy to heat, thus actuating the LCE nanocomposite. Both infrared and visible lights of wide spectrum (white light) or various wavelength ranges irradiations are able to effectively actuate the LCE nanocomposites, thus proves that they can fully utilize the photo energy of a light source for their mechanical actuation. Attributed to the well dispersity of GO in LCE matrix, sensitive (in seconds) and reversible photo-induced strain of LCE nanocomposites with consistent shape-changing ratio and significantly enhanced mechanical properties are observed. The contraction of the LCE nanocomposite films under light irradiation is about one third of the original length. The effective load-actuation capability is elevated about 50%.

Photo-isomerization effect of the azobenzene chain on the opto-mechanical behavior of nematic polymer: A molecular dynamics study

The opto-mechanical properties of a photo-responsive nematic polymer network (PRPN) are investigated using molecular dynamics simulation. For the implementation of the trans-to-cis isomerization of azo compounds, a switchable potential formalism for the N = N bond is applied to the crosslinked PRPN unit cell model. During the light switch-on and heating-up simulations at a wide range of temperatures, the scalar orientational order parameter for the mesogenic side group molecules, the effective photo-induced strain of the bulk polymer network, and the opto-mechanical properties are characterized. The correlation between the microstate which belongs to the molecular location and the macroscopically observed photostrain is identified according to the isomerization ratio of the diazene groups.

Giant ultrafast photo-induced shear strain in ferroelectric BiFeO3.

Generation of strain using light is a key issue for future development of ultrasonic devices. Up to now, photo-induced GHz-THz acoustic phonons have been mainly explored in metals and semiconductors, and in artificial nanostructures to enhance their phononic emission. However, despite their inherent strong polarization (providing natural asymmetry) and superior piezoelectric properties, ferroelectric oxides have been only poorly regarded. Here, by using ultrafast optical pump-probe measurements, we show that photogeneration/photodetection of coherent phonons in BiFeO3 ferroelectric leads, at room temperature, to the largest intensity ratio ever reported of GHz transverse acoustic wave versus the longitudinal one. It is found that the major mechanism involved corresponds to screening of the internal electric fields by light-induced charges, which in turn induces stress by inverse piezoelectric effect. This giant opto-acoustic response opens new perspectives for the use of ferroelectric oxides in ultrahigh frequency acoustic devices and the development of new GHz-THz acoustic sources.

Measurement of transient atomic displacements in thin films with picosecond and femtometer resolution.

We report measurements of the transient structural response of weakly photo-excited thin films of BiFeO3, Pb(Zr,Ti)O3, and Bi and time-scales for interfacial thermal transport. Utilizing picosecond x-ray diffraction at a 1.28 MHz repetition rate with time resolution extending down to 15 ps, transient changes in the diffraction angle are recorded. These changes are associated with photo-induced lattice strains within nanolayer thin films, resolved at the part-per-million level, corresponding to a shift in the scattering angle three orders of magnitude smaller than the rocking curve width and changes in the interlayer lattice spacing of fractions of a femtometer. The combination of high brightness, repetition rate, and stability of the synchrotron, in conjunction with high time resolution, represents a novel means to probe atomic-scale, near-equilibrium dynamics.

Photo-induced doping and strain in exfoliated graphene

The modification of single layer graphene due to intense, picoseconds near-infrared laser pulses is investigated. We monitor the stable changes introduced to graphene upon photoexcitation using Raman spectroscopy. We find that photoexcitation leads to both a local increase in hole doping and a reduction in compressive strain. Possible explanations for these effects, due to photo-induced oxygenation and photo-induced buckling of the graphene, are discussed.

Colorimetric UV sensors with tunable sensitivity from diacetylenes

A series of diacetylene compounds containing β-alanine ethyl ester and di-β-alanine ethyl ester groups were synthesized and dip-coated on paper for colorimetric UV sensing study. The white-to-blue-to-red colorimetric responses of the sensor strips were recorded by a photo scanner. The color images are analyzed based on the RGB color system to determine the color transition points and sensitivity. Coarse and fine tuning of the UV sensitivity was achieved by varying the head group and aliphatic chain length of the diacetylene lipids. Comparison of the UV sensitivity trends with the thermochromic sensitivity trends revealed that not only the photo-induced thermochromism but also photo-induced mechanical strain of the polydiacetylene backbone contributed to the color transition upon the UV irradiation.

Photoinduced perturbations of the magnetic superexchange in core@shell Prussian blue analogues

Cubic heterostructured core@shell particles of Prussian blue analogues, composed of a shell (∼80nm thick) of ferromagnetic K0.3Ni[Cr(CN)6]0.8·1.3H2O (A), Tc∼70K, surrounding a bulk (∼350 nm) core of photoactive ferrimagnetic Rb0.4Co[Fe(CN)6]0.8·1.2H2O (B), Tc∼20K, have been studied. Below nominally 70K, these CoFe@NiCr samples exhibit a persistent photoinduced decrease in low-field magnetization, and these results resemble data from other BA core@shell particles and analogous ABA heterostructured films. This net decrease suggests that the photoinduced lattice expansion in the B layer generates a strain-induced decrease in the magnetization of the NiCr layer, similar to a pressure-induced decrease observed by others in a similar, pure NiCr material and by us in our CoFe@NiCr cubes. Upon further examination, the data also reveal a significant portion of the NiCr shell whose magnetic superexchange, J, is perturbed by the photoinduced strain from the CoFe constituent.

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s.

Observation of anomalous acoustic phonon dispersion in SrTiO3 by broadband stimulated Brillouin scattering

Femtosecond laser pulses have been used to generate and detect longitudinal acoustic phonons along the [001] direction of SrTiO3 single crystals coated with a thin absorbing layer. Two-dimensional maps of the reflectivity as a function of probe wavelength and time delay display persistent oscillations, from which the frequency-dependent group velocity of the photoinduced strain pulse can be directly measured. By applying the theory of stimulated Brillouin scattering we found that the longitudinal sound speed varies from 7900 to 7520 m/s in the 60–74 rad μm−1 wave vector range, thus indicating a clear deviation from a linear phonon dispersion relationship.

Reversible white-light actuation of carbon nanotube incorporated liquid crystalline elastomer nanocomposites

We present single-wall carbon nanotube (SWCNT) incorporated liquid crystal elastomer (LCE) nanocomposites that demonstrate strong, reversible photoactuation. The matrix nematic LCE material possesses reversible thermal deformation, while SWCNTs perform photothermal energy conversion and local heat dissipation upon irradiation. The resultant SWCNT–LCE nanocomposites exhibit effective photoactuation not only by infrared (IR) irradiation, but also white light with an intensity on the order of 100 mW cm−2. Rapid and reversible photo-induced strain was observed. The nanocomposite films contracted up to one third of the original length in several seconds under the irradiation of white light, and recovered to the original length in several seconds after the light source was switched off. Moreover, the nematic–isotropic transition temperatures of the SWCNT–LCE nanocomposites were evidently lower than that of the blank LCE by up to 19 °C.

Constitutive equations for 0-3 polarized PLZT actuators

This article presents constitutive equations of transparent lead lanthanum zirconate titanate (PLZT) materials poling along thickness direction or with 0-3 polarization. Discussed first are the Beer’s law of light transmission in transparent solid and the photovoltaic current density in PLZT materials. Formulas for field strength and electrical potential between two electrodes of a PLZT wafer are then derived from the basic photovoltaic equations. By superposing the photovoltaic and the converse piezoelectric effects, we formulate a novel expression for photo-induced strain that is nonlinearly dependent of incident light intensity and varies with light penetration depth in through-thickness direction. On the basis of the derived photo-induced strain, linear and nonlinear constitutive equations of 0-3 polarized PLZT actuators are formulated. The present model is validated by using the available experimental data in the literature. By using the present model with an exponential variation of strain through the thickness, closed-form solutions for the equivalent loads acting on a PLZT unimorph, bimorph and intelligent beam are obtained and then compared with numerical results available in the literature.

Constitutive Modeling of Photostrictive Materials and Design Optimization of Microcantilevers

This article presents constitutive models for photo-induced strains in photostrictive materials and an optimal design of microcantilevers driven by light illumination. Behaviors of photo-induced strains in semiconductors and polymeric materials are reviewed first, and then new phenomenological constitutive models are developed that depict two distinctive features of photo-induced strain profile: (a) dependence on variation of light intensity which decreases with the depth measured from the illuminated surface due to light attenuation and energy absorption; and (b) anisotropy or dependence on direction of light polarization. On the basis of the constitutive modeling of photo-induced strain in wafers, design optimization of microcantilevers is then investigated for both geometrically linear and non-linear deformations.

Hydrostatic pressure effect on photoinduced insulator-to-metal transition in the layered organic saltα−(BEDT−TTF)2I3

The effect of high pressure on the photoinduced insulator-to-metal transition in charge-ordered [bis(ethylenedithio)]tetrathiafulvalene (BEDT-TTF) salt $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{I}}_{3}$ was investigated using femtosecond spectroscopy under hydrostatic pressure. By applying pressure $(P)$, the lifetime of the initially produced microscopic metallic domain becomes shorter, reflecting that condensation to the semimacroscopic metallic state is disturbed for $P<0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. However, condensation is promoted for $P=0.5--1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, thereby enhancing the nonlinearity of the photoinduced insulator-to-metal transition. Such nonmonotonous pressure dependence suggests competition between the increase of electronic bandwidth and photoinduced anisotropic lattice strain under pressure.

Mechanism of the Initial Conformational Transition of a Photomodulable Peptide

A light switch: The mechanism of transmission of the photoinduced torsional strain along a cyclic peptide, incorporating a biomimetic photochemical switch, is resolved by quantum mechanics/molecular mechanics and molecular dynamics simulations. The excited-state cis→trans motion of the switch does not affect the conformation of the peptide which only begins to change after decay to the ground state at a conical intersection (CI; see scheme).

Ferroelectric and Photostrictive Properties of Fine‐Grained PLZT Ceramics Derived from Mechanical Alloying

Single‐phase lead lanthanum zirconate titanate (PLZT) solid solution powder was synthesized from the constituent oxides at ambient temperature through a mechanical alloying (MA) process and was then densified to fine‐grained ceramics by sintering and hot‐pressing. The anomalous photovoltaic effect (APV) and photoinduced strain of the resultant PLZT ceramics were investigated and analyzed in association with the influence of grain size. It was found that a photoinduced voltage up to 6000 V·cm−1 can be obtained as the grain size is reduced to 0.42 μm. This is extremely high and about three times that achievable in normal micrometer‐grained PLZT ceramics. The maximum photoinduced strain of the PLZT ceramics with an average grain diameter of 0.54 μm reached 0.01%, which is equivalent to electric‐field‐induced strain of common piezoelectric materials.

Photostrictive Characteristics of Fine-Grained PLZT Ceramics Derived from Mechanically Alloyed Powder

Lanthanum-modified lead zirconate titanate (PLZT) ceramics possess photostrictive effect that is applicable to photo-driven and wireless actuators. The photo-induced strain is expressed in a product of piezoelectric constant and anomalous photovoltage, and it is theoretically explained that grain size reduction enhances photovoltage. Finegrained PLZT ceramics that have mean grain sizes of 2.0-0.2μm were prepared through the mechanical alloying (MA) synthesis and normal or hot-press sintering process. Their piezoelectric constant, dielectric constant and electric resistivity up to a grain size of 0.5μm were not dependent on grain size and agreed with the conventional one. The photovoltage significantly increased with a decrease in grain size. These results suggested that a lowering of grain size by MA leads to a remarkable improvement of the photo-induced strain value without a decline in strain response. However, the photo-induced strain value within grain size less than 0.5μm extremely declined due to a little remnant polarization. The photo-induced strain with illumination of 15mWcm-2 intense was maximally estimated to be 1×10-4.

Effect of ceramic processing methods on photostrictive ceramics

Lanthanum-modified lead zirconate titanate (PLZT) ceramics (3/52/48) doped with 0.5 at.% WO3 were prepared by three different processing methods, coprecipitation, sol-gel and conventional oxide mixing techniques, to investigate the effect of processing route on photovoltaic and photostrictive properties. The piezoelectric constant (d33) was observed to be independent of processing technique. Photovoltaic and photo-induced strain were found to get enhanced with decreasing grain size and increasing relative density. PLZT ceramics prepared by the coprecipitation route possess unique combination of favorable properties namely: high purity, higher degree of homogeneity, uniform distribution of doping, stoichiometric compositions, finer grain size and highest density. This resulted into enhanced photostrictive properties in coprecipitated PLZT ceramics.