Photoinduced Softening Research Articles

Extreme mechanical tunability in suspended MoS2 resonator controlled by Joule heating

Nanomechanical resonators are built into phones, as filters or accelerometers, but they lack a knob to effectively tune the frequency at the nanoscale when it’s easy to tune on an octave the tone of a classical musical instrument like a guitar string. Moreover, the control of deformation in nanomaterials, as two-dimensional (2D) materials, to tailor their electronic properties, i.e., straintronic, opens up avenues for applications in force detection, bolometry or quantum emitters. An accurate control of the deformation within these materials is thus necessary to fully exploit their potential. The precise study of deformations in 2D materials involves measurements of vibration modes and nanomechanics. By using a suspended MoS2 membrane heated by the Joule effect, we induce a strong softening of the mechanical resonance frequency as a function of the electrothermal heating, over one octave. A simple electrical tension is used to modulate the thermal mechanical tuning. Its amplitude is very large, greater than 100% modulation for one volt, compared to other approaches on 2D or 1D materials and, moreover, a very wide frequency range is accessible. Finally, we have related a photo-induced softening of the membrane over very long times with the current measurements and a photothermal effect.

Pump-Probe X-ray Photoemission Spectroscopy of Free-Standing Graphane

Free-standing nanoporous graphene was hydrogenated at about 60 at.% H uptake, as determined by the emerging of the sp3 bonding component in the C 1s core level investigated by high-resolution X-ray photoelectron spectroscopy (XPS). Fully unsupported graphane was investigated by XPS under optical excitation at 2.4 eV. At a laser fluence of 1.6 mJ/cm2, a partial irreversible dehydrogenation of the graphane was observed, which could be attributed either to the local temperature increase or to a photo-induced softening of the H-to-C stretching mode. The sub-ns dynamics of the energy shift and peak broadening of the C 1s core level revealed two different decay constants: 210 ps and 130 ps, respectively, the former associated with photovoltage dynamics and the latter with thermal heating on a time scale comparable with the synchrotron temporal resolution.

Optical Field-Induced Mass Transport in Soft Materials

The dependence of the surface relief formation in amorphous chalcogenide (As 2 S 3 and As-S-Se) and Disperse Red 1 dye grafted polyurethane polymer films on the polarization state of holographic recording light beams was studied. It is shown that the direction of lateral mass transport on the film surface is determined by the direction of light electric vector and photoinduced anisotropy in the film. We propose a photoinduced dielectropfhoretic model to explain the photoinduced mass transport in amorphous films. Model is based on the photoinduced softening of the matrix, formation of defects with enhanced or decreased polarizability, and their drift under the electrical field gradient of light.

Optical field-induced surface relief formation on chalcogenide and azo-benzene polymer films

The dependence of the surface relief formation in amorphous As2S3 and Disperse Red 1dye grafted polyurethane polymer films on the polarization state of recording light was studied. It is shown that the direction of mass transport on the film surface is determined by the direction of light electric vector and photoinduced anisotropy in the film. We propose a photoinduced dielectropfhoretic model to explain the photoinduced mass transport in amorphous films. Model is based on the photoinduced softening of the matrix, formation of defects with enhanced or decreased polarizability, and their drift under the electrical field gradient of light.

Light-responsive block copolymer vesicles based on a photo-softening effect

We report the synthesis and study of two series of amphiphilic block copolymers (BCPs) of which the hydrophilic block is either poly(acrylic acid) (PAA) or poly(N,N-dimethylacrylamide) (PDMA) and the hydrophobic block is a side-chain liquid crystalline polymer (SCLCP) bearing biphenyl mesogens in majority and azobenzene mesogens in minority, namely, a random copolymer of 6-[4-(4-cyanophenyl)phenoxyl]hexyl acrylate with 6-[4-(4-methoxyphenylazo)phenoxyl]hexyl acrylate (P(BiPA-co-Azo)). Samples of both BCPs, PDMA-b-P(BiPA-co-Azo) and PAA-b-P(BiPA-co-Azo), could form large vesicles in aqueous solution with a SCLCP membrane. The BCPs were designed for investigating photoinduced order-disorder transition of the mesogens confined inside the vesicle membrane as a result of the trans–cisphotoisomerization of azobenzene and the LC cooperative effect. From photo-optical measurements on a BCP vesicle solution, we found evidence that a photoinduced LC order–disorder transition could occur inside the vesicle membrane in aqueous solution. Moreover, using a pH-sensitive fluorescent probe, we studied the photo-softening effect on the proton transfer across the vesicle membrane. Similar to plasticization of the vesicle membrane upon addition of a good solvent in aqueous solution, photo-induced softening could increase the rate of proton diffusion from inside to outside of the vesicle through the SCLCP membrane.

Photostructural relaxation in As–Se–S glasses: Effect of network fragility

The effect of photoinduced structural relaxation in As–S–Se glass is investigated during sub-bandgap irradiation. It is shown that the glass undergoes rapid optically induced structural relaxation upon photoexcitation of bonding electrons. Fragile systems exhibit larger relaxation as expected from their enthalpy profile. This suggests that the process is thermodynamically driven and that the kinetic impediment to relaxation at low temperature is lifted through photoinduced softening of the glass matrix. Activation energy for enthalpy relaxation measurement and an annealing study near T g show that the photorelaxation effect is not a thermally activated process. The 〈 r〉 dependence of photostructural changes is addressed and explained using the energy landscape formalism.

Optical field-induced mass transport in As2S3 chalcogenide glasses.

We report the observation of a photorefractivelike nonlinearity responsible for the formation of giant relief modulations in amorphous semiconductor glasses. The photoinduced softening of the matrix, formation of defects with enhanced polarizability, and their drift under the optical field gradient force is believed to be the origin of the mass transport.

Photo-induced softening and hardening in Ge–As–S amorphous films

Hardness, X-ray diffraction and transmission as well as infrared reflection measurements on virgin and ultraviolet (UV)-irradiated amorphous films of the Ge x As 40− x S 60 system have been performed. The UV light induces film softening for x<19 and film hardening for x>19. The structure remains amorphous but the distance characteristic of medium range order decreases for x<19 and increases for x>19. Photo-contraction of the films (up to ∼12%) has been revealed from IR spectra and X-ray transmission experiments. A main effect of UV light is the release of sulphur.