Low Laser Fluence Research Articles

Effects of Thermal Energy on the Formation of Lattice Strain in VO2 Thin Films Grown on TiO2(001).

Film thickness is a well-known experimental parameter for controlling lattice strain in oxide films. However, due to environmental and resource conservation considerations, films need to be as thin as possible, increasing the need to find alternative factors for strain management. Herein, we present the importance of thermal energy as a factor for the formation of lattice strain in the oxide films, specifically focusing on the effects of laser fluence during pulsed laser deposition (PLD) on the in-plane lattice strain of vanadium dioxide (VO2) thin films grown on titanium dioxide (TiO2) (001). VO2 thin films were deposited using a KrF excimer laser (λ = 248 nm) at laser fluences ranging from 0.88 to 1.70 J/cm2. The film thickness ranged from 10-15 nm, below the critical thickness. Films grown at higher laser fluences exhibited smooth surfaces and completely strained in-plane lattices. In contrast, films grown at lower laser fluences displayed numerous small islands and relaxed in-plane lattice strain. The metal-insulator transition (MIT) temperature was lower for films grown at higher laser fluencies compared to those grown at lower laser fluences. It was also revealed that Ti-V interdiffusion occurs, forming a solid solution (V1-xTixO2) near the interface. These observations suggest that the thermal energy of the particles, influenced by laser fluence, is a critical factor in the formation of lattice strain in metal oxide films and also that laser fluence in PLD is an effective experimental parameter for strain management in oxide films. Our findings enhance the understanding of lattice strain formation in metal oxides and offer insights for establishing effective methods for controlling lattice strain in metal oxide films.

Melasma removal using Q-Switched Neodymium-doped Yttrium Aluminium Garnet laser toning and high-intensity focussed ultrasound.

To evaluate the effectiveness of combining high-intensity focussed ultrasound and a Q-switched neodymium-doped yttrium aluminium garnet 1064nm low-fluence toning laser in the treatment of melasma. The experimental study was conducted from November 2020 to August 2021 at the medical physics laboratory, College of Medicine, Mustansiriyah University, Baghdad, Iraq, and comprised individuals of either gender aged >25 years having mixed form melasma in malar and forehead areas. Each participant received high-intensity focussed ultrasound treatment on the right side. After 2 weeks, Q-switched neodymium-doped yttrium aluminium garnet 1064nm low-fluence toning laser was applied to the melasma patches 4 times at one-week intervals. Melasma area and severity index scores were noted at baseline and at each visit for each patient. Visual analogue scale was used for objective evaluation. After the 4 laser sessions, all the patients completed a 3-month follow-up. Data was analysed using SPSS 24. Of the 25 subjects, 15(60%) were females and 10(40%) were males. The overall mean age was 41.6±8.46 years (range: 22-60 years). There was a significant improvement with high-intensity focussed ultrasound (p<0.05). In the 3rd laser session, there was a significant difference in melasma area and severity index score of 2.48 for the right side and 3.12 for the left side (p<0.05). In the 4th session, there was a significant difference in melasma area and severity index score of 1.68 for the right side and 2 for the left side (p<0.05). No post-inflammatory hyperpigmentation or rebound was noted. High-intensity focussed ultrasound combined with toning laser was more effective than Q-switched neodymium-doped yttrium aluminium garnet 1064nm low-fluence laser alone in removing difficult melasma from the malar area in the 2nd, 3rd and 4th sessions, and from the forehead in the 3d and 4th sessions.

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge-Transfer Phase Transition.

We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge-transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+-N-C-Fe2+ tetragonal phase to the high temperature Mn2+-N-C-Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out-of-equilibrium dynamics leading to this phase is multi-scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N-C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+-N-C-Fe2+ phase creates small charge-transfer polarons [Mn2+-N-C-Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge-transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+-N-C-Fe2+ species. Above a threshold fluence, when a critical fraction of small CT-polarons is reached, the macroscopic phase transition to the persistent Mn2+-N-C-Fe3+ cubic phase occurs within ≃ 100 ps. This non-linear photo-response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge‐Transfer Phase Transition

AbstractWe study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge‐transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+−N−C−Fe2+ tetragonal phase to the high temperature Mn2+−N−C−Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out‐of‐equilibrium dynamics leading to this phase is multi‐scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N−C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+−N−C−Fe2+ phase creates small charge‐transfer polarons [Mn2+−N−C−Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge‐transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+−N−C−Fe2+ species. Above a threshold fluence, when a critical fraction of small CT‐polarons is reached, the macroscopic phase transition to the persistent Mn2+−N−C−Fe3+ cubic phase occurs within ≃ 100 ps. This non‐linear photo‐response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.

Investigation of the laser fluence and wavelength dependence in surface-assisted laser desorption/ionization mass spectrometry using gold nanoparticles.

Surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) builds on the use of nanostructured surfaces (e.g., coatings of colloidal nanoparticles) to promote analyte desorption and ionization. The SALDI process is believed to occur mainly through thermal processes, resulting from heating of the nanosubstrate upon absorption of the photon energy, and by assisting ionization steps. Mostly due to the accessibility of the respective hardware, the majority of SALDI-MS studies use standard laser wavelengths for MALDI (i.e., 337 or 355 nm), even though peak absorption of the SALDI nanosubstrate might completely differ from these values. Here, we investigated the wavelength dependence in SALDI-MS to determine if wavelength adjustment would be beneficial, and to provide new experimental data for a better understanding of the SALDI mechanism. To this end, gold nanoparticles (AuNPs) sprayed onto microscope glass slides were employed as SALDI nanosubstrates and L-arginine as a model analyte. In addition, we used 2,5-dihydroxyacetophenone (2,5-DHAP) for classical MALDI-MS using the same experimental setup. Arginine ion signals were recorded as a function of laser wavelength and laser fluence. Mass spectra were acquired in the wavelength range between 310 and 630 nm, including the absorption maximum of the sprayed AuNPs around 550 nm and that of 2,5-DHAP around 380 nm. Laser fluence thresholds for the generation of arginine ions were found to be dependent on the laser wavelength and to inversely correlate with the absorbance profiles of the deposited AuNPs and 2,5-DHAP, respectively. Very differently to MALDI, in SALDI ionization efficiency was found to strictly linearly decrease with increasing laser wavelength. Our results, therefore, corroborate the general assumption that material ejection in SALDI-MS is mainly driven by thermal processes in the low laser fluence range and add new evidence that the ionization process is directly influenced by photon energy when AuNPs are employed as nanosubstrates.

Ultrasound-assisted laser therapy for selective removal of melanoma cells.

The current study explores the potential of ultrasound-assisted laser therapy (USaLT) to selectively destroy melanoma cells. The technology was tested on an ex vivo melanoma model, which was established by growing melanoma cells in chicken breast tissue. Ultrasound-only and laser-only treatments were used as control groups. USaLT was able to effectively destroy melanoma cells and selectively remove 66.41% of melanoma cells in the ex vivo tumor model when an ultrasound peak negative pressure of 2MPa was concurrently applied with a laser fluence of 28mJ/cm2 at 532nm optical wavelength for 5min. The therapeutic efficiency was further improved with the use of a higher laser fluence, and the treatment depth was improved to 3.5mm with the use of 1,064nm laser light at a fluence of 150mJ/cm2. None of the laser-only and ultrasound-only treatments were able to remove any melanoma cells. The treatment outcome was validated with histological analyses and photoacoustic imaging. This study opens the possibility of USaLT for melanoma that is currently treated by laser therapy, but at a much lower laser fluence level, hence improving the safety potential of laser therapy.

High-strength and impermeable sapphire/aluminum joints fabricated by ultrafast laser microwelding: Microstructures and joining mechanism

In this work, robust joints of sapphire and aluminum were successfully achieved by direct ultrafast laser transmission microwelding. Effects of laser parameters on the microstructures and mechanical properties were comprehensively studied. The welding process highly depended on the laser ablated metal nanoparticles within the heterojunction, as well as the thermal accumulation under multiple laser pulses irradiation. It showed that lack of fusion at the sapphire/aluminum interface can occur due to the incomplete melting of ablated nanoparticles at low laser fluence (<0.48 J/cm2). While serious cavities and cracks inside the brittle sapphire were formed with the intensified stress concentration after melting-solidification process at excessive laser fluence (>0.64 J/cm2). A nearly defect-free joint was obtained with max. shear strength of 128.1 ± 8.2 MPa at room temperature, when the laser fluence was 0.56 J/cm2. The high shear strength can be attributed to the tortuous interface with wide elemental transition layer. Meanwhile, the welded sapphire/aluminum joints exhibited high reliability under extreme service temperatures, which the shear strength can reach 159.8 ± 8.96 MPa at −50 °C and 80.6 ± 11.1 MPa at 200 °C. The sealing tests also showed the excellent impermeability of laser welded sapphire/aluminum structure, confirming the micro-crack free joint formation. This work demonstrates the feasibility of using ultrafast laser to achieve high-quality sapphire/aluminum joint, which is promising in heat-dissipation substrate manufacturing in power semiconductor devices.

A comparative study of laser fluence effect on surface modification and hardness profile of austempered ductile iron

The impact of laser transformation hardening (LSH) and Laser surface melting (LSM) on an austempered ductile iron (ADI) alloy was investigated. Various laser fluences (J mm−2) were irradiated on the ADI specimens, obtained by different beam powers and scanning speeds to reach the optimum parameters to attain a surface with a good combination of high hardness and deep hardened depth, together with obtaining minimal distortion and crack-free surface. ADI surfaces were treated by means of high-power Nd:YAG laser in continuous wave mode. A detailed investigation of the obtained microstructure was characterized via optical and scanning electron microscopy attached to an EDX analyzer as well as X-ray diffraction analysis. Such a wide laser fluence range has a considerable effect on the achieved microstructure. A process parameter map was established which indicated that three different laser treatments were induced under such values of laser fluences: a laser transformation hardening, a partial laser melting, and a complete laser melting. The results revealed that to produce a hardened microstructure, specimens have to be operated within a very small range of laser fluence from 9.6 up to 29 J mm−2, achieving surface hardness values around 900 HV0.1 and depths of approximately 184–700 μm, respectively. Meanwhile, partial melting of the treated layer took place at a medium energy density range from 37 to 112 J mm−2, by a further increase in laser fluence (120–360 J mm−2), complete melting occurred which resulted in a deeper treated layer that reached 3.5 mm with an increase in microhardness value to almost 1100 HV0.1 at 360 J mm−2. The obtained microstructure in case of low laser fluence consisted of an austenite dendritic structure with undissolved graphite nodules, however, at medium and high laser fluence, the microstructure contains large cementite plates, iron-silicon carbides, a small area of ledeburite structure, and some retained austenite. The quantitative amounts of such phases directly depend on the applied laser fluence. Such phases were identified by means of EDX analysis.

Development of a New Binary Matrix for the Comprehensive Analysis of Lipids and Pigments in Micro- and Macroalgae Using MALDI-ToF/ToF Mass Spectrometry.

While edible algae might seem low in fat, the lipids they contain are crucial for good health and preventing chronic diseases. This study introduces a binary matrix to analyze all the polar lipids in both macroalgae (Wakame-Undaria pinnatifida, Dulse-Palmaria palmata, and Nori-Porphyra spp.) and microalgae (Spirulina-Arthrospira platensis, and Chlorella-Chlorella vulgaris) using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). The key lies in a new dual matrix made by combining equimolar amounts of 1,5-diaminonaphthalene (DAN) and 9-aminoacridine (9AA). This combination solves the limitations of single matrices: 9AA is suitable for sulfur-containing lipids and acidic phospholipids, while DAN excels as an electron-transfer secondary reaction matrix for intact chlorophylls and their derivatives. By employing the equimolar binary matrix, a wider range of algal lipids, including free fatty acids, phospholipids, glycolipids, pigments, and even rare arsenosugarphospholipids were successfully detected, overcoming drawbacks related to ion suppression from readily ionizable lipids. The resulting mass spectra exhibited a good signal-to-noise ratio at a lower laser fluence and minimized background noise. This improvement stems from the binary matrix's ability to mitigate in-source decay effects, a phenomenon often encountered for certain matrices. Consequently, the data obtained are more reliable, facilitating a faster and more comprehensive exploration of algal lipidomes using high-throughput MALDI-MS/MS analysis.

Subpicosecond laser ablation behavior of a magnesium target and crater evolution: Molecular dynamics study and experimental validation

Abstract The micro-ablation processes and morphological evolution of ablative craters on single-crystal magnesium under subpicosecond laser irradiation are investigated using molecular dynamics (MD) simulations and experiments. The simulation results exhibit that the main failure mode of single-crystal Mg film irradiated by a low fluence and long pulse width laser is the ejection of surface atoms, which has laser-induced high stress. However, under high fluence and short pulse width laser irradiation, the main damage mechanism is nucleation fracture caused by stress wave reflection and superposition at the bottom of the film. In addition, Mg[0001] has higher pressure sensitivity and is more prone to ablation than Mg [ 10 1 ¯ 0 ] . The evolution equation of crater depth is established using multi-pulse laser ablation simulation and verified by experiments. The results show that, under multiple pulsed laser irradiation, not only does the crater depth increase linearly with the pulse number, but also the quadratic term and constant term of the fitted crater profile curve increase linearly.

Physical Regimes and Mechanisms of Picosecond Laser Fragmentation of Gold Nanoparticles in Water from X-ray Probing and Atomistic Simulations.

Laser fragmentation in liquids has emerged as a promising green chemistry technique for changing the size, shape, structure, and phase composition of colloidal nanoparticles, thus tuning their properties to the needs of practical applications. The advancement of this technique requires a solid understanding of the mechanisms of laser-nanoparticle interactions that lead to the fragmentation. While theoretical studies have made impressive practical and mechanistic predictions, their experimental validation is required. Hence, using the picosecond laser fragmentation of Au nanoparticles in water as a model system, the transient melting and fragmentation processes are investigated with a combination of time-resolved X-ray probing and atomistic simulations. The direct comparison of the diffraction profiles predicted in the simulations and measured in experiments has revealed a sequence of several nonequilibrium processes triggered by the laser irradiation. At low laser fluences, in the regime of nanoparticle melting and resolidification, the results provide evidence of a transient superheating of crystalline nanoparticles above the melting temperature. At fluences about three times the melting threshold, the fragmentation starts with evaporation of Au atoms and their condensation into small satellite nanoparticles. As fluence increases above five times the melting threshold, a transition to a rapid (explosive) phase decomposition of superheated nanoparticles into small liquid droplets and vapor phase atoms is observed. The transition to the phase explosion fragmentation regime is signified by prominent changes in the small-angle X-ray scattering profiles measured in experiments and calculated in simulations. The good match between the experimental and computational diffraction profiles gives credence to the physical picture of the cascade of thermal fragmentation regimes revealed in the simulations and demonstrates the high promise of the joint tightly integrated computational and experimental efforts.

Continuous GHz femtosecond laser interacting with aluminum film: Simulation and experiment

GHz femtosecond laser has attracted more research interest due to the so-called high-efficiency cold ablation effect. In this study, behavior of continuous GHz (cGHz), i.e., GHz repetition frequency femtosecond laser with no burst mode, interacting with aluminum (Al) film has been investigated numerically and experimentally. For this purpose, an axisymmetric two-temperature model was used, accounting for the thermodynamical evolution of Al film. Laser irradiating Al film experiments were also carried out simultaneously. The calculated temperature field evolution was in good agreement with the observed experimental results. Effects of laser fluence and scanning speed on morphology, area, height, and volume of the residual Al film were then sequentially studied. Interestingly to see, for this cGHz femtosecond laser source, melting and re-solidification lead to the material removal under low laser fluence, say, ≤0.0337 J/cm2 in this study. For the whole Al film, with laser fluence increasing, the outmost lattice evaporation and beneath lattice melting occur together, both responsible for material removal and morphology variation. Finally, a potentially beneficial GHz femtosecond laser mode for high-quality microfabrication is proposed.

Laser-Induced Intracellular Delivery: Exploiting Gold-Coated Spiky Polymeric Nanoparticles and Gold Nanorods under Near-Infrared Pulses for Single-Cell Nano-Photon-Poration.

This study explores the potential of laser-induced nano-photon-poration as a non-invasive technique for the intracellular delivery of micro/macromolecules at the single-cell level. This research proposes the utilization of gold-coated spiky polymeric nanoparticles (Au-PNPs) and gold nanorods (GNRs) to achieve efficient intracellular micro/macromolecule delivery at the single-cell level. By shifting the operating wavelength towards the near-infrared (NIR) range, the intracellular delivery efficiency and viability of Au-PNP-mediated photon-poration are compared to those using GNR-mediated intracellular delivery. Employing Au-PNPs as mediators in conjunction with nanosecond-pulsed lasers, a highly efficient intracellular delivery, while preserving high cell viability, is demonstrated. Laser pulses directed at Au-PNPs generate over a hundred hot spots per particle through plasmon resonance, facilitating the formation of photothermal vapor nanobubbles (PVNBs). These PVNBs create transient pores, enabling the gentle transfer of cargo from the extracellular to the intracellular milieu, without inducing deleterious effects in the cells. The optimization of wavelengths in the NIR region, coupled with low laser fluence (27 mJ/cm2) and nanoparticle concentrations (34 µg/mL), achieves outstanding delivery efficiencies (96%) and maintains high cell viability (up to 99%) across the various cell types, including cancer and neuronal cells. Importantly, sustained high cell viability (90-95%) is observed even 48 h post laser exposure. This innovative development holds considerable promise for diverse applications, encompassing drug delivery, gene therapy, and regenerative medicine. This study underscores the efficiency and versatility of the proposed technique, positioning it as a valuable tool for advancing intracellular delivery strategies in biomedical applications.

Unraveling the formation dynamics of metallic femtosecond laser induced periodic surface structures

Femtosecond laser surface processing (FLSP) is an emerging fabrication technique to efficiently control the surface morphology of many types of materials including metals. However, the theoretical understanding of the FLSP formation dynamics is not a trivial task, since it involves the interaction of various physical processes (electromagnetic, thermal, fluid dynamics) and remains relatively unexplored. In this work, we tackle this problem and present rigorous theoretical results relevant to low-fluence FLSP that accurately match the outcomes of an experimental campaign focused on the formation dynamics of laser induced periodic surface structures (LIPSS) on stainless steel. More specifically, the topography and maximum depth of LIPSS trenches are theoretically and experimentally investigated as a function of the number of laser pulses. Moreover, precise LIPSS morphology measurements are performed using atomic force microscopy (AFM). The proposed comprehensive simulation study is based on two-temperature model (TTM) non-equilibrium thermal simulations coupled with fluid dynamic computations to capture the melting metal phase occurring during FLSP. Our rigorous simulation results are found to be in excellent agreement with the AFM measurements. The presented theoretical framework to model FLSP under low-fluence femtosecond laser pulses will be beneficial to various emerging applications of LIPSS on metallic surfaces, such as cooling high-powered laser diodes and controlling the thermal emission or absorption of metals.

Minimally destructive laser-induced breakdown spectroscopy of brass assisted by a low-power atmospheric pressure plasma jet

Minimizing sample damage is crucial in laser-induced breakdown spectroscopy (LIBS) for applications involving valuable samples and elemental mapping. In this study, we introduced a low-power atmospheric pressure plasma jet (APPJ) to reduce sample damage by obtaining LIBS signals at significantly lower laser fluences. The proposed technique, APPJ-assisted LIBS (APPJ-LIBS), utilized an argon APPJ to provide seed electrons and enhance the excitation. The APPJ was generated by a 10 kHz alternating current power supply and made contact with the surface of a brass sample at a 30° angle. An infrared nanosecond Nd:YAG laser was focused onto the contacting zone, allowing the resulting laser-induced plasma to evolve within the surrounding APPJ and produce optical emission. The optimized APPJ-LIBS system reduced the laser fluence threshold for spectral detection of the brass sample by 97 %, from 1.43 J/cm2 to 0.05 J/cm2, which represented the lowest laser fluence threshold reported in LIBS studies on copper-based materials. Micrographs of the sample surface showed no visible damage after the APPJ-LIBS measurement at a near-threshold laser fluence and an APPJ input power as low as 6.0 W. Furthermore, gated images showed the plasma evolution in APPJ-LIBS and confirmed the excitation capability of the APPJ for the laser-ablated materials.

Numerical modeling to predict threshold fluence for material ejection in Laser-Induced forward transfer of metals

Laser-induced forward transfer (LIFT) uses a pulsed laser beam to propel material from a donor (containing a glass coated with a thin material film) onto a receiving substrate, resulting in pixellated material deposition. The deposition characteristics depend on the material ejection modes that vary with film thickness and laser parameters. This study develops a computational model based on the finite volume method for LIFT of gold films using nanosecond pulses, which captures two different ejection modes for droplet depositioncap ejection and jet ejection. The model computes the temperature distribution and predicts potential ejection regimes for different film thicknesses, providing an understanding of the material removal process. Cap ejection occurs at lower laser fluences when the entire film thickness in the irradiated zone is in the molten phase. In comparison, jet ejection occurs at higher laser fluences caused by vapor pocket formation at the glass-film interface. The model predicts threshold fluences with greater than 90% accuracy for film thickness less than 583 nm. However, for the film thickness of more than 583 nm, the simulations underpredict the threshold fluence, suggesting that laser absorption by the film decreases due to vapor formation at the glass-film interface. Besides, it is observed that higher fluences can cause melting and vaporization of the glass at the interface leading to possible contamination of the deposited material. The proposed model can be used to choose the operating window for laser parameters for droplet deposition in LIFT, which otherwise is challenging using experiments.

Grain visualization of FeCoCrMnNi and AlCoCrFeNi2.1 high-entropy alloys by nanosecond pulse laser irradiation

Characterizing grains is the most common method in materials science research, but the existing techniques for grain characterization are generally expensive and time-consuming. In this study, nanosecond pulse laser irradiation with low laser fluence is attempted to visualize the grains of two typical high-entropy alloys (HEAs) with different crystal structures and elemental compositions. The effects of laser parameters and gas atmospheres on the laser visualization of grains are comparatively investigated. Under specific laser irradiation conditions, the grains of these two HEAs could be well visualized by producing different surface nanostructures. By experiments and analysis, the underlying mechanisms of laser visualization of grains under different conditions are elucidated, and the role of O element on the promotion of the visualization of grain boundaries is confirmed. This study provides a simple and efficient method to visualize the grains of HEAs, and on the other hand, enhances the understanding of nanosecond laser-HEA interaction.

Creating zinc-hyperdoped silicon with modulated conduction type by femtosecond laser irradiation

Silicon, as the earth-abundant semiconductor, has low cost and good compatibility with mature complementary metal-oxide-semiconductor technology. Therefore, instead of choosing the proper semiconductor with a specific bandgap energy, hyperdoping technique, which can introduce deep-level dopants and intermediate band into silicon, enable silicon to operate in infrared wavebands. Particularly, the hyperdoped silicon with transition metals, having low ionized impurity scatting, makes silicon a promising infrared detection material. In present work, zinc-hyperdoped silicon is fabricated by femtosecond pulsed laser irradiation. The doping concentration of zinc has exceeded 1019 cm−3, and even reached 1022 cm−3, which is about five orders of magnitude greater than the solid solubility of zinc in crystalline Si. The zinc-hyperdoped silicon shows sub-bandgap absorptance of 87.86 % at 1310 nm. Temperature-dependent Hall effect measurements prove that zinc doping results in acceptor energy level located at 0.520 eV above the valence band maximum. It is worth nothing that, zinc-oxygen related defect state results in donor energy level located at 0.252 eV below the conduction band minimum when lower laser fluence is used, which means the energy level of impurity is altered by the presence of undesignedly doped oxygen atoms, suggesting compound formation between the zinc atoms and oxygen atoms.

Quinoxalineimide-Based Semiconducting Polymer Nanoparticles as an Effective Phototheranostic for the Second Near-Infrared Fluorescence Imaging and Photothermal Therapy.

Multifunctional theranostics play a critical role in improving the efficacy of photothermal therapy and tumor fluorescence imaging; however, they require the integration of complex components into a single theranostic system, and their response in the second near-infrared (NIR-II) region is constrained by wavelengths of a photosensitizer. To address this issue, we herein developed a novel multifunctional thiazole-fused quinoxalineimide semiconducting polymer (named PQIA-BDTT), which exhibits NIR-II fluorescence and photothermal properties. PQIA-BDTT nanoparticles achieved an impressively high photothermal conversion efficiency (72.6%) in laser (1064 nm)-induced photothermal therapy at a safe maximum permissible exposure, demonstrating their capability as an effective photothermal agent. Moreover, PQIA-BDTT nanoparticles can be used as a reference for NIR-II fluorescence imaging under a low laser fluence. The tumor size and location in 4T1 mice intravenously injected with the PQIA-BDTT nanoparticles could be precisely identified through NIR-II fluorescence imaging, which also exhibited remarkable photothermal antitumor efficacy by in vitro and in vivo therapy. Overall, this study demonstrates that introducing a thiazole-fused quinoxalineimide acceptor unit into a donor-acceptor conjugated polymer is an effective strategy for the synthesis of novel multifunctional theranostic systems, which provides a novel platform for designing theranostic agents for biomedical applications.

Control of resistive switching type in BaTiO3 thin films grown by high and low laser fluence

A ferroelectric memristor has attracted much attention due to convenient controlling by polarization switching, but the resistive switching has been attributed to the drift or charge trapping of defects. To distinguish the resistive switching mechanism between ferroelectric polarization switching and the normal resistive switching mechanism such as the drift or charge trapping of defects, BaTiO3 (BTO) thin films were grown on a (001) Nb:SrTiO3 single crystal substrate by pulsed laser deposition with high and low laser energy density. Based on a piezoelectric force microscope, ferroelectricity is found in BTO thin films grown at high laser energy density. X-ray photoelectron spectroscopy further confirms the existence of defects in the BTO films grown at low laser energy density. The high energy sample with low density of defects exhibits a resistance hysteresis loop but little current hysteresis loop, while the low energy sample with high density of defects shows a significant resistance and current hysteresis loop simultaneously. These results provide a deep understanding about the resistive switching from ferroelectric polarization switching and the drift or charge trapping of defects.