Investigations on surface morphology and bandgap engineering of single crystal boron-doped silicon irradiated by a nanosecond laser.

We irradiate the single crystal boron-doped silicon (Si) at various laser fluences with 100 laser shots in ambient air at room temperature using an Nd:YAG laser and investigate its surface morphology and optical properties. The optical microscopy gives evidence of the formation of a crater and reveals that the heat-affected zone and melted area are increased with increase in laser fluence from 1.1 to 15.4 J/cm2. The micrographs obtained by scanning electron microscopy (SEM) show that the micro- and nano-structures such as microcracks, bubbles, nucleation sites, clusters, redeposited layered material, nanoparticles, and alike water droplet structures are formed on a laser-exposed Si surface. The optical profilometry of the irradiated Si further confirms the ablation and redeposition of the material and shows that the depth of the crater is increased from 12.1 to 15.2 µm with increase in fluence from 1.1 to 15.4 J/cm2. Raman spectroscopy of the samples shows that the irradiation generates anneal effects due to higher temperature, which increases the crystallinity of the Si. The ellipsometric analysis shows that the irradiation of Si with increasing laser fluence changes its optical constants (refractive index and extinction coefficient), which further influence its optical properties, e.g., reflectivity, absorptivity, and energy bandgap. The absorptivity of laser irradiated Si tends to increase with increasing laser fluence, and the energy bandgap is decreased accordingly due to increase in structural disorders. Our study shows that the controlled laser irradiation can tune the energy bandgap of exposed Si, and it makes the Si materials useful for the fabrication of optoelectronic devices such as solar cells, photovoltaic cells, and LEDs.

Slip-free processing of (001) silicon wafers under 1064 nm laser ablation.

Slip phenomenon on a (001) silicon surface under 1064 nm laser ablation was studied by experiments and simulations. The surface morphologies of the silicon wafers after laser irradiation were observed using an optical microscope. The slip patterns showed that slip that occurred before melting was responsible for a low-quality ablation surface. The slip damage was predicted by a three-dimensional finite element model based on heat transfer and thermoelasticity theory. The judging criterion of slip was explained in detail. The numerical results gave a better understanding of slip phenomenon in experiments. It is shown that low laser irradiances cause slip and high laser irradiances are helpful in preventing slip. The threshold irradiance is ∼1 MW/cm2. Lasers with higher irradiance are essential to obtain a slip-free ablation on a (001) silicon surface.

Surface damage induced by a combined millisecond and nanosecond laser.

The surface damage morphologies of single-crystal silicon induced by a combined pulse laser (CPL) and a single millisecond laser are investigated, respectively. The CPL includes a millisecond (ms) laser superposed by a nanosecond (ns) laser. Inspected by an optical microscope, it was found that the surface damage was more serious when the sample was irradiated by the CPL than by a single ms laser with the same incident laser energy. Besides surface cleavage, obvious ablation and fold areas were discovered by CPL irradiation. A two-dimensional spatial axisymmetric model was established to assess the difference between single ms laser and CPL irradiation and explain the generation mechanism of the different surface damage. This was attributed to the preheating effect by the ms laser and the surface damage caused by the ns laser.

Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays.

A millisecond laser combined with a nanosecond laser was applied to machining transparent materials. The influences of delay between the two laser pulses on processing efficiencies and modified sizes were studied. In addition, a laser-supported combustion wave (LSCW) was captured during laser irradiation. An optimal delay corresponding to the highest processing efficiency was found for cone-shaped cavities. The modified size as well as the lifetime and intensity of the LSCW increased with the delay decreasing. Thermal cooperation effects of defects, overlapping effects of small modified sites, and thermal radiation from LSCW result in all the phenomena.

An ROS-responsive and self-accelerating drug release nanoplatform for overcoming multidrug resistance.

An 'on-demand' drug release and ROS-responsive nanoparticle was prepared by chemically conjugating hydrophobic α-tocopheryl succinate to hydrophilic poly(ethylene glycol) via a thioketal linker. This nanoparticle encapsulated with doxorubicin and α-tocopheryl succinate exhibited remarkable efficiency in reversing multidrug resistance both in vitro and in vivo.

Mixed micelles based on a pH-sensitive prodrug and TPGS for enhancing drug efficacy against multidrug-resistant cancer cells.

In this study, we prepared mixed micelles composed of a pH-sensitive poly(ethylene glycol)-doxorubicin conjugate prodrug and d-alpha-tocopheryl polyethylene glycol succinate (TPGS). The average hydrodynamic size of the mixed micelles was approximately 144nm, measured by dynamic light scattering. In an MTT assay the pH-sensitive prodrug was non-cytotoxic at low concentration but inhibited drug-resistant cancer cell (MCF-7/ADR) growth at high dose. The mixed micelles showed concentration-dependent cytotoxicity and significantly increased the cytotoxicity of the prodrug in MCF-7/ADR cells. Confocal laser scanning images revealed that higher concentrations of doxorubicin were successfully delivered into cell nuclei, enabling effective drug-induced cell death. Fluorescence microscopy indicated that there was less escape of the internalized doxorubicin from cells. Therefore, the enhanced drug efficacy in MCF-7/ADR cells is most likely attributed to a synergistic effect of drug-release from the pH-sensitive prodrug inside cells and suppression of P-glycoprotein efflux activity by TPGS.

Enhancement of all-trans retinoic acid-induced differentiation by pH-sensitive nanoparticles for solid tumor cells.

Cancer differentiation therapy is an attractive concept and has been clinically used to treat leukemia. However, it is limited to date for solid tumors. In this study, the pH-sensitive nanoparticles based on poly(amidoamine) (PAMAM) dendrimers are synthesized by coupling 3,4,5,6-tetrahydrophthalic anhydride with the first generation PAMAM. The modified dendrimers can self-assemble in aqueous solution to form nanoparticles with a diameter of 125-435 nm. The nanoparticles are relatively stable at physiological pH (pH 7.4) but dissociated in acidic environments (pH 5.0 or 6.0). The present studies show that the proliferation inhibition and albumin secretion of hepatoma carcinoma cells are enhanced with all-trans retinoic acid (ATRA) encapsulated in the nanoparticles. The enhancement of induced differentiation is due to the high internalization of ATRA in the cells by the nanoparticles. These experimental results demonstrate that pH-sensitive nanoparticles may be efficient for improving the differentiation therapy for solid tumor.

Intraoperative diffusion tensor imaging predicts the recovery of motor dysfunction after insular lesions.

Insular lesions remain surgically challenging because of the need to balance aggressive resection and functional protection. Motor function deficits due to corticospinal tract injury are a common complication of surgery for lesions adjacent to the internal capsule and it is therefore essential to evaluate the corticospinal tract adjacent to the lesion. We used diffusion tensor imaging to evaluate the corticospinal tract in 89 patients with insular lobe lesions who underwent surgery in Chinese PLA General Hospital from February 2009 to May 2011. Postoperative motor function evaluation revealed that 57 patients had no changes in motor function, and 32 patients suffered motor dysfunction or aggravated motor dysfunction. Of the affected patients, 20 recovered motor function during the 6-12-month follow-up, and an additional 12 patients did not recover over more than 12 months of follow-up. Following reconstruction of the corticospinal tract, fractional anisotropy comparison demonstrated that preoperative, intraoperative and follow-up normalized fractional anisotropy in the stable group was higher than in the transient deficits group or the long-term deficits group. Compared with the transient deficits group, intraoperative normalized fractional anisotropy significantly decreased in the long-term deficits group. We conclude that intraoperative fractional anisotropy values of the corticospinal tracts can be used as a prognostic indicator of motor function outcome.

Plasticity of language pathways in patients with low-grade glioma: A diffusion tensor imaging study.

Knowledge of the plasticity of language pathways in patients with low-grade glioma is important for neurosurgeons to achieve maximum resection while preserving neurological function. The current study sought to investigate changes in the ventral language pathways in patients with low-grade glioma located in regions likely to affect the dorsal language pathways. The results revealed no significant difference in fractional anisotropy values in the arcuate fasciculus between groups or between hemispheres. However, fractional anisotropy and lateralization index values in the left inferior longitudinal fasciculus and lateralization index values in the left inferior fronto-occpital fasciculus were higher in patients than in healthy subjects. These results indicate plasticity of language pathways in patients with low-grade glioma. The ventral language pathways may perform more functions in patients than in healthy subjects. As such, it is important to protect the ventral language pathways intraoperatively.

Magnetic resonance diffusion tensor imaging-based evaluation of optic-radiation shape and position in meningioma.

Employing magnetic resonance diffusion tensor imaging, three-dimensional white-matter imaging and conventional magnetic resonance imaging can demonstrate the tumor parenchyma, peritumoral edema and compression on surrounding brain tissue. A color-coded tensor map and three-dimensional tracer diagram were applied to clearly display the optic-radiation location, course and damage. Results showed that the altered anisotropy values of meningioma patients corresponded with optic-radiation shape, size and position on both sides. Experimental findings indicate that the magnetic resonance diffusion tensor imaging technique is a means of tracing and clearly visualizing the optic radiation.