Amir Sa'ar

Photoluminescence from silicon nanostructures: The mutual role of quantum confinement and surface chemistry

Recent developments in the field of silicon nanostructures, particularly those properties and phenomena that are related to the photoluminescence (PL) from silicon nanostructures, have attracted much attention lately. A major source of controversy and disagreement among researchers is the underlying mechanism behind the PL. Two classes of models, i.e., the quantum confinement model that assigns the PL to quantum size effects in the nanocrystalline silicon core of the nanostructures and the surface chemistry model that assign the PL to surface phenomena at the interface between the crystalline core and the host matrix that wrap the nanostructures, are the most notable ones. In recent years, alternative structures to porous silicon, which allow synthesizing high quality silicon nanostructures with better control of their dimensionality, shape and size distribution, have emerged. In particular, fabrication techniques of silicon nanocrystals embedded in silicon-dioxide (SiO2) matrices have reached a level where consistent investigation of surface and quantum size phenomena can be performed. Recent experimental results and theories suggest that none of the above models alone can explain the entire spectrum of optical phenomena in silicon nanostructures. Instead, a refined model that takes into account the mutual role of quantum confinement and surface chemistry in shaping the optical properties of these nanostructures should be considered.

Laser sintering of copper nanoparticles

Copper nanoparticle (NP) inks serve as an attractive potential replacement to silver NP inks in functional printing applications. However their tendency to rapidly oxidize has so far limited their wider use. In this work we have studied the conditions for laser sintering of Cu-NP inks in ambient conditions while avoiding oxidation. We have determined the regime for stable, low-resistivity copper (< × 3 bulk resistivity value) generation in terms of laser irradiance and exposure duration and have indicated the limits on fast processing. The role of pre-drying conditions on sintering outcome has also been studied. A method, based on spectral reflectivity measurements, was used for non-contact monitoring of the sintering process evolution. It also indicates preferred spectral regions for sintering. Finally, we illustrated how selective laser sintering can generate high-quality, fine line (<5 µm wide) and dense copper circuits.

Zinc oxide nanorods grown on two-dimensional macroporous periodic structures and plane Si as a pH sensor

pH determination is a strong prerequisite for many biochemical and biological processes. We used two methods, namely, the electrochemical potential method (experimental) and site binding method (theoretical), to study the sensitivity of zinc oxide (ZnO) nanorods grown on two-dimensional macroporous periodic structures (2DMPPS) (p-and n-type) and plane n-type Si substrates for use as an intracellular pH sensing device. The dimension of these nanorods varied in radius between 50 and 300 nm and lengths of 1–10 μm. We found that the sensitivity of ZnO nanorods increases with reductions in size, from 35 mV/pH for D=300 nm and L=10 μm, to 58 mV/pH for D=50 nm and L=1 μm, using the site binding model. The experimental electrochemical potential difference for the ZnO nanorods working electrode versus Ag/AgCl reference electrode showed a high sensitivity range for ZnO nanorods grown on 2DMPPS n-Si substrate as compared to plane n-Si at room temperature for pH ranging from 4 to 12 in buffer and NaCl solutions.

Optical biosensing of bacteria and cells using porous silicon based, photonic lamellar gratings

We report on a method to extend the optical sensing capabilities of conventional RIFTS (reflective interferometric Fourier transform spectroscopy) biosensors for real-time detection of large microorganisms, such as bacteria and cells. Using macro porous silicon based 2D arrays of phase (lamellar) grating, we demonstrate that the zero-order optical reflectivity exhibits a similar interference pattern to that obtained for ordinary RIFTS biosensors, which can be Fourier transformed into optical thickness and exploited for biosensing. The sensing capabilities are demonstrated for Escherichia coli bacteria that were captured inside the macro-pores. The entrapment process is monitored and verified by confocal laser scanning microscopy.

Tunability of the optical band edge in thin PbS films chemically deposited on GaAs(100)

The optical properties of chemical-solution-deposited thin films of lead sulfide (PbS) were investigated using infrared transmission and photoluminescence spectroscopies. The synthesized films are characterized by a wide range of microstructures, from 15 nm nanocrystals up to monocrystals. Energy bandgap values for bulk and nanostructured layers varied from 0.41 eV up to 0.48 eV, respectively. Blueshifts in both absorbance and emission peaks of the nanostructured layers were obtained due to quantum size effects. The optical properties of the films are shown to be size-dependent, with the band edge increasing with temperature.

Chemically deposited PbS thin film photo-conducting layers for optically addressed spatial light modulators

Lead sulfide semiconducting thin films were chemically deposited on indium tin oxide coated glass plates for use as photoreceptor layers in conjugation with optically addressed spatial light modulators (OASLMs). Deposition conditions such as temperature, reagent concentration, pH and deposition time were optimized in order to achieve homogeneous, continuous and adherent films. Mirror-like films with tunable particle size and film thickness were obtained. The microstructure and morphology evolution of the films were investigated using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Electrical and optical properties were studied using four-point probe measurements, FTIR spectroscopy, photoluminescence spectroscopy, photo-current and photo-voltage measurements. Blue shift of the band gap to the short wavelength infra-red (SWIR) range was obtained as a function of particle size, and significant photovoltaic effect was measured. The resistivity of the films, as well as their photo-voltage response, were enhanced after thermal annealing. These results indicate that PbS films can serve as effective photoreceptors in OASLMs for applications including SWIR detection for night vision purposes.

Supersonic laser-induced jetting of aluminum micro-droplets

The droplet velocity and the incubation time of pure aluminum micro-droplets, printed using the method of sub-nanosecond laser induced forward transfer, have been measured indicating the formation of supersonic laser-induced jetting. The incubation time and the droplet velocity were extracted by measuring a transient electrical signal associated with droplet landing on the surface of the acceptor substrate. This technique has been exploited for studying small volume droplets, in the range of 10–100 femto-litters for which supersonic velocities were measured. The results suggest elastic propagation of the droplets across the donor-to-acceptor gap, a nonlinear deposition dynamics on the surface of the acceptor and overall efficient energy transfer from the laser beam to the droplets.

Radiative and nonradiative relaxation phenomena in hydrogen- and oxygen-terminated porous silicon

Using time-resolved photoluminescence spectroscopy over a wide range of temperatures, we were able to probe both radiative and nonradiative relaxation processes in luminescent porous silicon. By comparing the photoluminescence decay times from freshly prepared and oxidized porous silicon, we show that radiative processes should be linked with quantum confinement in small Si nanocrystallites and are not affected by oxidation. In contrast, nonradiative relaxation processes are associated with the state of oxidation where slower relaxation times characterize hydrogen-terminated porous silicon. These results are in a good agreement with the extended vibron model for small Si nanocrystallites.PACS78.55.Mb; 78.67.Rb; 78.47.jd

Dielectric relaxation and porosity determination of porous silicon

Abstract Results of the dielectric spectroscopy study of porous silicon (PS) samples in the frequency range, 20 Hz–1 MHz, and in the temperature range, 173–493 K, are presented. Three relaxation processes that dominate at low, moderate and high temperatures respectively were observed. The low temperature dispersion follows the Cole–Cole type with two activation energies of 0.28–0.4 and 0.2–0.3 eV for samples with thickness 20 and 30 μm, respectively. At moderate temperatures the time domain dielectric response function demonstrates stretch exponential behaviour associated with the percolation of charge carries near threshold. At temperatures above 400 K, we found a strong contribution of dc-conductivity, with approximately the same activation energy of 0.47±0.01 eV for both samples. An additional Havriliak–Negami relaxation process with typical relaxation times of the order of 10−3 s is observed in this temperature interval. The dielectric response is found to be very sensitive to the geometrical micro- and meso-structural features of the PS. The dynamical aspects of the processes are discussed.

Hydrogels synthesized in electrochemically machined porous Si hosts: effect of nano-scale confinement on polymer properties

Imprisonment of polymers and hydrogels in inorganic nanostructures is of growing interest in many areas of materials research and various biomedical devices. In this study, we synthesize the hydrogels polyacrylamide and poly(N-isopropylacrylamide) in situ within nanostructured porous Si (PSi) hosts and characterize their thermal properties by differential scanning calorimetry (DSC) and thermal gravimetry. The PSi hosts are electrochemically machined as thin optical Fabry–Perot films with a well-characterized interconnected nanostructure. Confining hydrogels in these hosts is found to significantly decrease the transition temperatures of the polymers i.e., glass transition temperature (Tg) and temperature-dependent volume phase transition (VPT), in comparison to those of the neat hydrogels. We show that incorporation of a responsive hydrogel, poly(N-isopropylacrylamide), into nanochannels of the PSi thin film permits optical transduction of the hydrogel behavior during its volume phase transition. The “real-time” optical response is correlated with DSC data and provides an alternate means for studying nano-scale confinements of responsive hydrogels. In addition, we found significant differences between the thermal degradation behavior of the confined hydrogels compared with those of neat hydrogels and thin hydrogel films on planar Si surfaces. The confined hydrogels have an inferior thermal stability than that of the neat hydrogels. These findings suggest that the in situ polymerization and the hydrogel confinement conditions have a profound effect on the nanostructure and resulting behavior of the hydrogel phase.