Porous Silicon Quantum Research Articles

Broadband visible emission from photoelectrochemical etched porous silicon quantum dots containing zinc

The broadband visible photoluminescence (PL) from photoelectrochemical etched porous silicon quantum dots (PSi-QDs) containing zinc (Zn) powder is reported. The influence of the Zn content (0, 0.11, 0.17, and 0.25 g) on the structure, morphology, and room-temperature PL characteristics of the PSi-QDs were evaluated. The PSi-QDs were prepared in HF:ethanol solution by electrochemical etching of n-type Si at the optimum etching time and current density with varying masses of added Zn powder. The PL spectra of the as-prepared samples exhibited a broad emission peak in the region of 510–760 nm with the highest intensity at approximately 657 nm. The inclusion of Zn powder in the PSi-QDs was observed to enhance the PL peak intensity by a factor of seven. The improved visible light emitting traits were attributed to the quantum confinement (QC) effect, which widened the energy band gap (1.12–2.5 eV) of the Zn-included PSi-QDs. The present findings may contribute to the development of PSi-QD-based red phosphors and other optoelectronic applications. EDX and FESEM analysis showed that porous silicon etched with Zn mass is mainly composed of oxygen, fluoride, zinc, and silicon.

The effects of drying technique and surface pre-treatment on the cytotoxicity and dissolution rate of luminescent porous silicon quantum dots in model fluids and living cells.

Tailoring of the biodegradation of photoluminescent silicon quantum dots (Si QDs) is important for their future applications in diagnostics and therapy. Here, the effect of drying and surface pretreatment on the dissolution rate of Si QDs in model liquids and living cells was studied in vitro using a combination of photoluminescence and Raman micro-spectroscopy. Porous silicon particles were obtained by mechanical milling of electrochemically etched mesoporous silicon films, and consist of interlinked silicon nanocrystals (QDs) and pores. The samples were subjected to super-critical drying with CO2 solvent (SCD) or air drying (AD) and then annealed at 600 °C for 16 hours in 1% oxygen to obtain nano-sized Si QDs. The obtained samples were characterized by a core-shell structure with a crystalline silicon core and a SiO2 layer on the surface. The sizes of the crystalline silicon cores, calculated from Raman scattering spectra, were about 4.5 nm for the initial AD-SiQDs, and about 2 nm for the initial SCD-SiQDs. Both the AD-Si QDs and the SCD-Si QDs exhibited visible photoluminescence (PL) properties due to quantum confinement effects. The dissolution of the nanocrystals was evaluated through their PL quenching, as well as by the presence of a low-frequency shift, broadening, and a decrease in the intensity of the Raman signal. The stability of the AD-Si QDs and the complete dissolution of the SCD-Si QDs during 24 hours of incubation with cells have been demonstrated. This might explain the apparent lower cytotoxicity observed for SCD-Si QDs.

Two-Dimensional Fluorescent Strategy Based on Porous Silicon Quantum Dots for Metal-Ion Detection and Recognition

A two-dimensional photoluminescent (2D PL) detection strategy was established based on luminescent porous silicon (LuPSi) with wide-size-distributed silicon quantum dots and abundant surface chemis...

Synthesis of quantum dot porous silicon as extended gate field effect transistor (EGFET) for a pH sensor application

Porous silicon quantum dot (PSi-QD) was prepared from p-type silicon (100) using the electrochemical etching technique. The prepared PSi-QD was then applied as an extended gate field effect transistor (EGFET) in the synthesis of a PSi-QD based pH sensor. The morphology, structural properties, and pores of the prepared PSi-QD layer were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The average pore size of the PSi-QD was 5.8 nm with an average pore depth of 3.43 μm. The sensitivity, linearity, and hysteresis measurements of the sensor were determined for different pH buffer solutions. The sensor displayed a voltage sensitivity of 108.3 mV/pH with linearity of 98.97% for the linear regime, while the current sensitivity for the saturated regime was 2.65 μA1/2/pH with linearity of 99.81%. The net hysteresis value of the sensor was approximately 14 mV. The repeatability deviation at pH7 was 0.86%. The stability and reliability of the sensor in terms of coefficient of variation (C.V.) were 0.76%, 0.93%, and 0.98% for pH4, pH7, and pH10, respectively. Based on its high sensitivity, good linearity and low hysteresis as well as its good repeatability, stability and reliability, the PSi-QD EGFET based pH sensor is considered a viable device for the detection of hydrogen ions.

Agglomeration in Porous Silicon Prepared from Si-Nanowires Structures

Silicon nanowires (SiNW’s) arrays prepared by electroless etching method are attractive due to their significant light absorption properties across the visible spectrum [1]. Typical silicon nanowires grown by electroless etching is shown in Figure 1a. Following this preparation of silicon nanowires, these samples are etched in an electrolytic HF and ethanol solution to produce porous silicon quantum confined structures. Mechanical behavior of materials at nanoscale is different due to their quantum confinement effect. When porous silicon nano-structures are prepared from SiNWs obtained through electroless process, they may have a tendency to become thin. Ultimately these nano-structures may collapse and form bunches on the surface [2], as shown in Figure 1b. However, bunched silicon nanowires structures may be desirable in some applications but may not be for some others. In order to avoid bunching of SiNWs, it may be necessary to reduce the surface tension during the growth of nanowires [3]. (a) (b) Fig 1. Cross-section images of silicon nanowires (a) grown by electroless etching, (b) Porous silicon structures prepared from SiNWs. SiNW’s were grown by eletroless etching in a solution of hydrofluoric acid (HF) and silver nitrate (AgNO3) at room temperature [4]. The prepared nanowires were then etched in an electrolytic solution containing HF and ethanol to make porous Si quantum confined structures. Reflectance and photoluminescence studies are performed on this structures Figure 1

Linear Polarization of Photoluminescence Excitation and Emission Spectra of Porous Silicon

We have found a separation of the photoluminescence band caused by a strong spin-orbit splitting of holes in a porous silicon quantum well. The porous silicon has a red-orange photoluminescence (480 ∼ 800 nm; λmax. about 600 nm) due to the quantum-confinement effect. In this research, the existence of sub-bands was newly confirmed by using the polarized excitation and photoluminescence spectroscopy. These sub-bands depended on the harmony of the rotation angle of the excitation and/or emission polarizers, respectively, and the degree of polarization was negative or positive values as a rotation angle of the excitation and emission polarizers. From these results, the quantumconfined band structure of the porous silicon is more concretely explained by using the spin-orbit splitting model. The photoluminescence band of the porous silicon was composed of an electronheavy hole and electron-light hole recombination, and its relaxation process is in good agreement with the electron transition selection rule. Namely, the polarization of the nanoporous material is caused by a strong spin-orbit splitting of the quantum-confined valence band.

Localized electron state on porous silicon quantum dots

Porous silicon can be fabricated by laser irradiation and annealing. The center wavelength of photoluminescence （PL） band is pinned in the region of 700—780 nm and its intensity increases obviously after oxidation of the sample. It was found that the PL intensity changes with time of laser irradiation and annealing. Calculation shows that some localized states appear in the band gap of the smaller nanocrystal when SiO bonds or Si—O—Si bonds are passivated on the surface. It was discovered that the number of SiO bonds or Si—O—Si bonds, which are related to irradiation and annealing time, obviously affects the generation of localized gap states.

Enhancement of Raman scattering intensity in porous silicon

An enhancement in inelastic light scattering intensity from porous-silicon quantum wires has been discovered. It is shown that this effect is caused by a decrease in the absorption coefficient of the optical medium formed by quasi-one-dimensional structures, with the crystal structure of the wires themselves remaining unchanged.

Electronic States and Luminescence in Porous Silicon Quantum Dots: The Role of Oxygen

Depending on the size, the photoluminescence (PL) of silicon quantum dots present in porous silicon can be tuned from the near infrared to the ultraviolet when the surface is passivated with Si-H bonds. After exposure to oxygen, the PL shifts to the red by as much as 1 eV. This shift and the changes in PL intensity and decay time, show that both quantum confinement and surface passivation determine the electronic states of silicon quantum dots. A theoretical model in which new electronic states appear in the band gap of the smaller quantum dots when a $\mathrm{Si}=\mathrm{O}$ bond is formed, is in good agreement with experiments. This result clarifies the controversy regarding the PL mechanisms in porous silicon.