Gazi N. Aliev

Hydrothermal conversion of one-photon-fluorescent poly-(4-vinylpyridine) into two-photon-fluorescent carbon nanodots

A novel two-photon-fluorescent N,O-heteroatom-rich carbon nanomaterial has been synthesized and characterized. The new carbon nanoparticles were produced by hydrothermal conversion from a one-photon-fluorescent poly(4-vinylpyridine) precursor (P4VP). The carbonized particles (cP4VP dots) with nonuniform particle diameter (ranging from sub-6 to 20 nm with some aggregates up to 200 nm) exhibit strong fluorescence properties in different solvents and have also been investigated for applications in cell culture media. The cP4VP dots retain their intrinsic fluorescence in a cellular environment and exhibit an average excited-state lifetime of 2.0 ± 0.9 ns in the cell. The cP4VP dots enter HeLa cells and do not cause significant damage to outer cell membranes. They provide one-photon or two-photon fluorescent synthetic scaffolds for imaging applications and/or drug delivery.

Quasi-periodic Fibonacci and periodic one-dimensional hypersonic phononic crystals of porous silicon: Experiment and simulation

A one-dimensional Fibonacci phononic crystal and a distributed Bragg reflector were constructed from porous silicon. The structures had the same number of layers and similar acoustic impedance mismatch, and were electrochemically etched in highly boron doped silicon wafers. The thickness of the individual layers in the stacks was approximately 2 μm. Both types of hypersonic band gap structure were studied by direct measurement of the transmittance of longitudinal acoustic waves in the 0.1–2.6 GHz range. Acoustic band gaps deeper than 50 dB were detected in both structures. The experimental results were compared with model calculations employing the transfer matrix method. The acoustic properties of periodic and quasi-periodic structures in which half-wave retarding bi-layers do not consist of two quarter-wave retarding layers are discussed. The strong correlation between width and depth of gaps in the transmission spectra is demonstrated. The dominant mechanisms of acoustic losses in porous multilayer structures are discussed. The elastic constants remain proportional over our range of porosity, and hence, the Gruneisen parameter is constant. This simplifies the expression for the porosity dependence of the Akhiezer damping.

Porous silicon bulk acoustic wave resonator with integrated transducer.

AbstractWe report that porous silicon acoustic Bragg reflectors and AlN-based transducers can be successfully combined and processed in a commercial solidly mounted resonator production line. The resulting device takes advantage of the unique acoustic properties of porous silicon in order to form a monolithically integrated bulk acoustic wave resonator.

Hypersonic phononic stopbands at small angles of wave incidence in porous silicon multilayers

We report theoretical simulation and experimental observation of the mode conversion effect in a hypersonic distributed Bragg reflector of porous silicon. Acoustic transmission of longitudinal waves through the multilayered structure has been measured in the frequency range 0–3 GHz. It is found that the measured transmittance at the gap frequencies is always higher than that theoretically predicted for normal incidence. We attribute this to non-perpendicular wave propagation that was not deliberately sought, which subsequently increases the center gap transmittance due to the mode conversion effect. Oblique incidence with angles of about 1° results in truncated gap depth in acoustic transmission spectra from about −80 dB, and deeper, to about −40 dB and shallower. The spectra were simulated by employing the stiffness matrix method. Porosity-dependent acoustic viscous damping was included in the calculations. A way to optimize reflectors in the frequency range, where the forbidden gaps for longitudinal and shear waves overlap, is discussed.

Magnetic field dependence of singlet oxygen generation by nanoporous silicon

Energy transfer from photoexcited excitons localized in silicon nanoparticles to adsorbed oxygen molecules excites them to the reactive singlet spin state. This process has been studied experimentally as a function of nanoparticle size and applied external magnetic field as a test of the accepted understanding of this process in terms of the exchange coupling between the nano-Si exciton and the adsorbed O2 molecules.

Porous silicon as an acoustic material for BAW

Layers of porous silicon with a varying degree of porosity between 30% and 75% were prepared by electrochemical etching from bulk-Si. Thickness of the layers was in a range of 2 to 4 μm. Acoustic impedance and velocity for longitudinal waves in a frequency range of 2 GHz were evaluated utilizing a BAW resonator built on top of the porous Si layers. Material parameters were extracted from the electrical impedance characteristics of the BAW resonators. Special preprocessing of the frequency dependent impedance data was applied to simplify an initial fit to a Mason-Model. A subsequent fit to full data was performed to refine acoustic material parameters and estimate the propagation loss occurring in the porous Si. TEM cross-sections were prepared to verify the thickness of the porous Si layers. The longitudinal acoustic impedance was found to be in the range between 4.6 and 11.1 Mrayl while velocity was 4900 to 6950 m/s. Propagation loss was found to be lower than one would expect from a porous film.

Hypersonic spectroscopy of porous silicon for acoustic devices

I will review our work on porous silicon (pSi) presenting achievements while highlighting underlying physical questions that remain to be answered. pSi is produced by the electrochemical etching of crystalline silicon. It is typically mesoporous, having pores of 10–30nm diameter. The etching current density determines the final porosity, the volume fraction of air, with a wide range of porosities, 25%–95%, achievable. For wavelengths much greater than the pore size, pSi gives a tunable effective medium for light and sound waves. We have characterized pSi acoustic properties using transmission spectroscopy with matched transducer pairs working at 0.5–2.5 GHz. The results for velocity, v, have fitted to a general law of v=v0(1−φ)k, where v0 is the velocity in bulk silicon, φ is porosity, and k is the fitting parameter. We have investigated the variation of k with the direction of propagation and the etching conditions used to extract the dependence of the elastic constants on porosity. The measurement of ve...

The morphology of nanoporous silicon investigated by time-resolved measurement of changes to the porosity during dissolution

The dissolution rate of porous silicon layers was measured in-situ during etching by HF and NaOH solutions. Cellular models were applied to the structure of the pSi from which the expected rates of change of porosity with time were calculated. Experimental results indicate that the nanostructure of porous silicon may be best modeled as an open-cell foam. This is in contrast to the honeycomb model often used which is not supported by the experimental data.

Nanoscale multilayers in porous silicon for THz phonon engineering

A model predicting the frequency of folded longitudinal acoustic phonon modes in porous silicon based superlattices has been developed. The model includes the measured dependence of the acoustic velocity on porosity and the expected effect of acoustic attenuation on the dispersion relation and on line broadening. We predict that folded phonons with low folding index can be observed with the use of Raman spectroscopy in appropriately designed samples.