Photoplastic Effect Research Articles

Photoplastic effect in MgO single crystals

Photoplastic effect in MgO single crystals

Bringing the photoplastic effect in ZnO to light: A photoindentation study on pyramidal slip

The external light environment has been reported to have a pronounced influence on the dislocation-based plasticity in various inorganic semiconductors. Herein, with a specific focus on the pyramidal slip in wurtzite structures, photoindentation tests were performed on the (0001) plane of ZnO crystal to activate the pyramidal slip and evaluate the effects of light on dislocation behaviors. Statistical analyses of the first pop-in events suggested that the corresponding maximum shear stresses in darkness were slightly smaller than those in 405 nm light, indicating a minor effect of light on dislocation nucleation event. Regarding nanoindentation creep tests, light illumination was found to suppress the indentation depth. Further transmission electron microscopy observations demonstrated that indentation-induced pyramidal dislocations were present deeper in darkness than in 405 nm light, revealing a suppressive effect of light illumination on the pyramidal slip in ZnO for the first time.

Repairable Macroscopic Monodomain Arrays from Block Copolymers Enabled by Photoplastic and Photodielectric Effects

Upon exposure to UV light (120 mW/cm2, λ = 365 nm), a trans-cis isomerization occurs in a cylinder-forming, azobenzene-containing block copolymer of polydimethylsiloxane-b-poly((4(phenyldiazenyl)phenoxy)hexyl acrylate) (PDMS-b-PPHA) that enables the generation of monodomains of healable, long-range ordered arrays of nanoscopic domains over macroscopic distances. The trans-cis isomerization gives rise to a significant increase in the dielectric constant (from 6.52 to 19.8 at 100 Hz, photodielectric behavior) and a dramatic decrease in the Tg (from 54 to 1 °C, photoplastic behavior) of the PPHA block. By combining these characteristics with an in-plane electric field, macroscopic monodomains of near-perfectly aligned cylindrical microdomains are achieved at low temperatures, and a damage repair is clearly uncovered, where the 300 nm wide scratches can be completely healed at 40 °C, leaving a smooth, uniformly thick film where the continuity and orientation of the aligned microdomains are restored. Subsequent exposure to visible light causes a cis-trans isomerization, increasing the matrix Tg to 54 °C, producing highly oriented and aligned PDMS cylindrical microdomains in a PPHA matrix.

Spectrally-dependent positive and negative photo-plastic effects in CdZnTe and CdZnTeSe

We studied the Vickers microhardness HV0.025 of CdZnTe and CdZnTeSe samples upon illumination with quasi-monochromatic light with wavelengths ranging between 750 and 1540 nm. We observed that light with a wavelength close to or above the bandgap of the samples results in a positive photo-plastic effect (hardening), and light in the spectral range well below the band gap can lead both to positive or negative effects depending on the parameters of deep levels in the bandgap. We performed measurements of spectrally dependent photoconductivity and compared them with HV0.025. Based on the analysis, we propose a complex qualitative model explaining the spectral dependence of HV0.025 by the charging and neutralization of deep levels by optical transitions between the valence and conduction bands and between the bands and deep levels in the band gap. Tuning of parameters of deep levels could be used to control the sign and magnitude of the photo-plastic effect.

Giant and Controllable Photoplasticity and Photoelasticity in Compound Semiconductors.

We show that the wide-band gap compound semiconductors ZnO, ZnS, and CdS feature large photoplastic and photoelastic effects that are mediated by point defects. We measure the mechanical properties of ceramics and single crystals using nanoindentation, and we find that elasticity and plasticity vary strongly with moderate illumination. For instance, the elastic stiffness of ZnO can increase by greater than 40% due to blue illumination of intensity 1.4 mW/cm^{2}. Above-band-gap illumination (e.g., uv light) has the strongest effect, and the relative effect of subband gap illumination varies between samples-a clear sign of defect-mediated processes. We show giant optomechanical effects can be tuned by materials processing, and that processing dependence can be understood within a framework of point defect equilibrium. The photoplastic effect can be understood by a long-established theory of charged dislocation motion. The photoelastic effect requires a new theoretical framework which we present using density functional theory to study the effect of point defect ionization on local lattice structure and elastic tensors. Our results update the longstanding but lesser-studied field of semiconductor optomechanics, and suggest interesting applications.

Switching the fracture toughness of single-crystal ZnS using light irradiation

An enormous change in the dislocation-mediated plasticity has been found in a bulk semiconductor that exhibits the photoplastic effect. Herein, we report that UV (365 nm) light irradiation during mechanical testing dramatically decreases the fracture toughness of ZnS. The crack tip toughness on a (001) single-crystal ZnS, as measured by the near-tip crack opening displacement method, is increased by ∼45% in complete darkness compared to that in UV light. The increase in fracture toughness is attributed to a significant increase in the dislocation mobility in darkness, as explained by the crack tip dislocation shielding model. Our finding suggests a route toward controlling the fracture toughness of photoplastic semiconductors by tuning the light irradiation.

Spectral Dependence of the Photoplastic Effect in CdZnTe and CdZnTeSe.

We studied the spectral dependence of the Vickers microhardness HV0.025 of CdZnTe and CdZnTeSe samples upon illumination and found out that it increases over the entire applied spectral range of 1540–750 nm. We also found out that the photoconductivity and microhardness are correlated. We observed changes in the correlation diagram (change of slope and an abrupt change of HV0.025 at several wavelengths of the illuminating light). Based on measurements of the relative changes of the space charge upon illumination using the Pockels effect, we suggest that the observed spectral dependence of positive photoplastic effect in CdZnTe and CdZnTeSe can be explained by the trapping of photoinduced electrons and holes, which affects the motion of the partial dislocations. The underlying physical explanation relies on the assumption that reconstructed bonds break before dislocation glide.

Влияние освещения на пьезоэлектрические характеристики кварцевых резонаторов

The results of the effect of lighting on the piezoelectric characteristics of quartz resonators in the form of a tuning fork, which have been pre-trained by increased friction from the external environment, are presented. A significant change in the piezoelectric characteristics of such resonators was found depending on the intensity and spectrum of illumination. It is assumed that the detected change in the characteristics of the resonators is due to dislocations formed in the resonator during training with increased friction of the external environment, and is a manifestation of the well-known photoplastic effect and its quenching.

Structural and optical study of Zn-doped As2Se3 thin films: Evidence for photoinduced formation of ZnSe nanocrystallites

Amorphous Zn-doped As2Se3 films with a nominal zinc content x up to 10 at.% were prepared by thermal evaporation. Their structure is characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy. The AFM data show a considerable increase of the film surface roughness for films with x > 5 at.%. A strong gradient of the Zn content decreasing into the film depth is confirmed by the EDX and XPS data. Heavily Zn-doped (above 7 at.%) As2Se3 films reveal photostructural changes in the course of the Raman measurements. New Raman features are attributed to TO and LO vibrations of ZnSe nanocrystallites formed in the film under laser illumination. Depending on the laser wavelength and power density, the ZnSe nanocrystallites can experience tensile strain in the film due to a non-thermal photoplastic effect in the As2Se3 film resulting in a partial removal of the material from the laser spot. The tensile strain value, estimated from the TO and LO phonon frequency shift, is shown to reach up to 2.9 GPa.

Low-temperature mechanical properties of fullerites: structure, elasticity, plasticity, strength

The results of long-term studies on the physical-mechanical properties of C60 and C70 molecular crystals (fullerites) were systematized and described. These materials are the new allotropic form of carbon in which fullerenes (stable molecules consisting of 60 and 70 atoms) represent the elementary base units. Molecules are combined into crystalline structures mainly by dispersive (van der Waals) forces with a small contribution from covalent bonds. The anomalies of the fullerite crystal structures and features of the phase transitions occurring in them, which are caused by dispersive forces, orientation ordering, and dynamics of the molecules’ rotational degrees of freedom, were discussed. The most interesting transformations of fullerite lattice structures and orientation states were observed in the temperature range of 77 K ≲ Т ≲ 350 K. The majority of the experimental investigations were carried out at these temperatures. The experiments were concentrated on the effects that lattice-orientation phase transitions had on the mechanical properties of single crystals, polycrystalline solids, and compacts. Acoustical spectroscopy at low and high oscillation frequencies, micro- and nanoindentation and macrodeformation methods were used in the experimental research. The crystallogeometric aspects and dislocation mechanisms of plastic slip in fullerites, as well as the methods of observing dislocations and studying their mobility, are described in detail. Also the influence of different external factors, namely, illumination (photoplastic effect), pulsed magnetic field (magnetoplastic effect), and sample compaction pressure (baropolymerization effect) are discussed. The effects of saturating samples of different morphology with hydrogen, oxygen, and inert gas impurities on the fullerites’ mechanical properties were considered. The discussion of the experimental results is accompanied by a brief description of their theoretical interpretation based on analyzing the interaction of elastic and plastic deformations of the fullerite lattice with the processes of orientational ordering, rotation, and librational vibrations of molecules.

Laser‐Induced Formation of CdS Crystallites in Cd‐Doped Amorphous Arsenic Sulfide Thin Films

As2S3:Cd films with nominal Cd content x up to 4 at.% were prepared by thermal evaporation. Their amorphous structure is confirmed by Raman spectroscopy. The film surface roughness estimated from atomic force microscopy does not exceed 1 nm. Energy‐dispersive X‐ray spectroscopy and X‐ray photoelectron spectroscopy show a strong decrease of Cd content with the film depth. Peaks of CdS longitudinal optical phonon and its overtones emerging in the Raman spectra of films with x ≥ 2 at.% Cd are the evidence for the laser irradiation‐induced formation of CdS crystallites in the As2S3:Cd films. The downward shift of the CdS Raman peak frequencies is mainly explained by tensile strain undergone by the CdS crystallites in the films caused by material transfer from the laser spot due to the photoplastic effect.

Formation of CdSe nanocrystals in Cd-doped thin arsenic selenide films under laser irradiation

Amorphous Cd-doped As2Se3 films with nominal Cd contents up to 12at.% were prepared by thermal evaporation. Atomic force microscopy studies confirm the uniform film structure with surface roughnesses below 1nm, independent of the Cd content. As shown by energy-dispersive X-ray spectroscopy, the Cd content in the film reveals a strong gradient and decreases with the film depth. For heavily Cd-doped (above 7at.%) As2Se3 films, Raman features attributed to CdSe longitudinal optical (LO) phonon and its overtone (2LO) are revealed in the Raman spectra as an evidence for the formation of CdSe nanocrystallites in the As2Se3:Cd film under above-bandgap or below-bandgap laser illumination. The CdSe nanocrystallites undergo tensile strain due to a photoplastic effect in the As2Se3 film, i. e. partial removal of the material from the laser spot. The tensile strain value, estimated from the LO phonon frequency shift, is shown to reach nearly 2GPa.

Detection of defects binding free polarons in colored alkali halide crystals

The assumption has been made that defects binding free polarons in colored alkali halide crystals are F'-center, i.e., defects that slow down the motion of dislocations (photoplastic effect). This assumption has been confirmed by the experiments performed in this study. Thus, the anion vacancy in alkali halide crystals at a low temperature can capture three electrons: two electrons at a deep level (F'-center) and one electron in a bound polaron state. This electron is retained due to the energy gain in the interaction of a local deformation of the polaron and a local deformation surrounding the F'-center, despite the presence of the Coulomb repulsion.

Enhancement of laser recording in gold/amorphous chalcogenide and gold/acrylate nanocomposite layers

Two types of amorphous materials with the same functional purpose – direct optical recording of optical amplitude–phase surface reliefs – were investigated with the aim to establish the influence of incorporated gold nanoparticles on the recording efficiency. It was shown that the laser recording of optical reliefs on the basis of photo-polymerization of the selected acrylates or on the basis of structural polymerization–destruction like processes in chalcogenides with photo-plastic effects can be enhanced in the presence of Au nanoparticles (AuNPs). The lateral mass-transport processes at the molecular and nanoparticle levels are initiated by the non-uniformly, periodically distributed recording optical fields and increase the resulting surface reliefs in these materials, which can be used as microstructured optical elements, holographic gratings.

Light-induced mass transport in amorphous chalcogenides: Toward surface plasmon-assisted nanolithography and near-field nanoimaging

Two types of amorphous functional materials, based on lightsensitive inorganic compounds like Se and As20Se80 chalcogenide glass (ChG) were investigated with the aim to establish the influence of plasmonic fields, excited by the band-gap light in nanocomposite layers made of these compounds and gold nanoparticles on their photomechanical response. Both these basic materials are characterized by pronounced photoplastic effect and used for real-time optical recording of optoelectronic elements (based mainly on surface relief gratings) due to high photofluidity and polarization-dependent masstransport. We established that mass-transport processes in these ChG can be enhanced in the presence of localized plasmonic fields generated by light if the condition of surface plasmon resonance (SPR) is fulfilled. The subjects of special interest are the mass-transport processes at the nano-scale stimulated in the nano-copmosite layers of either by the uniform or periodically distributed optical fields. It was found that irradiation by light with SPR really enhance the efficiency of masstransport and produce surface nanostructurizations. The variation in the topography follows closely and permanently the underlying near field intensity pattern.

In situ Raman observation of laser-induced formation of TlInSe2 crystallites in Tl–In–As–Se glass

Tl–In–As–Se glasses are obtained by co-melting of As2Se3 and TlInSe2. Laser-induced crystallization of TlInSe2 in the glass due to local laser beam heating is observed in the course of micro-Raman measurements. Dependence of the crystallization process on the laser power is studied. Simultaneously a photoplastic effect in the glass is observed leading to the mass transfer from the laser spot.

Width-to-thickness ratio dependence on photoplastic effect of ZnS nanobelt

Photoplastic effect (PPE) of individual ZnS nanobelt with different width-to-thickness (w/t) ratios was investigated by using a nanoindenter in conjunction with an incident ultraviolet light source system. The results show that the positive and negative PPEs are observed for the w/t ratios 4.9–9.6 and 3.2–4.1. At the peak indentation loads 20–50 μN, the positive and negative PPEs occur for the w/t ratios 4.9 and 3.2, however, they disappear at 60 μN. PPE is determined by w/t ratio not peak indentation load, and it is interpreted by the competing mechanisms of increase of Peierls barrier and radiation enhanced dislocation glide.

Photoinduced Changes in the Structure of As2S3-Based SbSI Nanocrystal-Containing Composites Studied by Raman Spectroscopy

Raman studies of (As2S3)1- x (SbSI) x glasses (0.2 ≤ x ≤ 0.7) are performed. Laser-induced crystallization of SbSI crystallites in the As2S3 glass composite in the course of the measurement is observed as a result of local laser beam heating. Simultaneously a photoplastic effect in the As2S3/SbSI composite leading to a mass transfer from the laser spot is observed.

Photoinduced stiffening and photoplastic effect of ZnS individual nanobelt in nanoindentation

The photoinduced stiffening (PIS) and photoplastic effect (PPE) of ZnS individual nanobelt (NB) were observed by using a nanoindenter in conjunction with an incident ultraviolet (UV) light source system. The results show that the elastic modulus and hardness of ZnS individual NB under UV illumination are at least 32% and 20% larger than those in darkness. The mechanisms of PIS and PPE are interpreted by the increase in electronic strain and Peierls barrier due to the photogeneration of free carriers in ZnS individual NB. The research may offer useful guidelines to the application of optoelectronic devices based on individual nanostructures.

Photoplastic effects in chalcogenide glasses: A review

AbstractA synopsis of various photoinduced changes of rheological, mechanical and elastic properties is presented in the first part of the article. After a critical appraisal of a large body of experimental data, it is suggested that the photoviscous effect, that is, the athermal decrease of viscosity of a non‐crystalline chalcogenide upon illumination, is the key for a plethora of photoinduced effects reported so far in the literature under different names. Morphic effects (shape or surface morphology) may appear either in the presence or absence of external mechanical stimuli leading to the fabrication of a variety of technologically important photoprocessed structures. A few representative examples of photoplastic effects are described in some detail in the second part of the paper, based on information provided by in situ Raman scattering and nanoindentation experiments.