Synthesis of colloidal solutions with silicon nanocrystals from porous silicon

In this work, we have obtained colloidal solutions of Si nanocrystals (Si-ncs), starting from free-standing porous silicon (PSi) layers. PSi layers were synthesized using a two-electrode Teflon electrochemical cell; the etching solution contained hydrogen peroxide 30%, hydrofluoric acid 40% (HF), and methanol. The anodizing current density was varied to 250 mA cm^-2, 1 A cm^-2, and 1.2 A cm^-2. Thus obtained, PSi was mechanically pulverized in a mortar agate; then, the PSi powders were poured into different solutions to get the final Si-ncs colloidal solutions. The different optical, morphological, and structural characteristics of the colloidal solutions with Si-ncs were measured and studied. These Si-ncs colloidal solutions, measured by photoluminescence (PL), revealed efficient blue-green or violet emission intensities. The results of X-ray diffraction (XRD) indicate that the colloidal solutions are mainly composed of silicon nanocrystallites. The result of UV–vis transmittance indicates that the optical bandgap energies of the colloidal solutions varied from 2.3 to 3.5 eV for colloids prepared in methanol, ethanol, and acetone. The transmission electron microscopy (TEM) images showed the size of the nanocrystals in the colloidal solutions. Fourier transform infrared spectroscopy (FTIR) spectra showed different types of chemical bonds such as Si-O-Si, Si-CH₂, and SiH _x , as well as some kind of defects.

PACS

61.46Df.-a; 61.43.Gt; 61.05.cp; 78.55.-m; 81.15.Gh     

Photoelectric Properties of Silicon Nanocrystals/P3HT Bulk-Heterojunction Ordered in Titanium Dioxide Nanotube Arrays

A silicon nanocrystals (Si-ncs) conjugated-polymer-based bulk-heterojunction represents a promising approach for low-cost hybrid solar cells. In this contribution, the bulk-heterojunction is based on Si-ncs prepared by electrochemical etching and poly(3-hexylthiophene) (P3HT) polymer. Photoelectric properties in parallel and vertical device-like configuration were investigated. Electronic interaction between the polymer and surfactant-free Si-ncs is achieved. Temperature-dependent photoluminescence and transport properties were studied and the ratio between the photo- and dark-conductivity of 1.7 was achieved at ambient conditions. Furthermore the porous titanium dioxide (TiO₂) nanotubes’ template was used for vertical order of photosensitive Si-ncs/P3HT-based blend. The anodization of titanium foil in ethylene glycol-based electrolyte containing fluoride ions and subsequent thermal annealing were used to prepare anatase TiO₂ nanotube arrays. The arrays with nanotube inner diameter of 90 and 50 nm were used for vertical ordering of the Si-ncs/P3HT bulk-heterojunction.     

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

Abstract

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.

PACS

78.55.Mb; 78.67.Rb; 78.47.jd     

Near-infrared optical absorption enhanced in black silicon via Ag nanoparticle-induced localized surface plasmon

Due to the localized surface plasmon (LSP) effect induced by Ag nanoparticles inside black silicon, the optical absorption of black silicon is enhanced dramatically in near-infrared range (1,100 to 2,500 nm). The black silicon with Ag nanoparticles shows much higher absorption than black silicon fabricated by chemical etching or reactive ion etching over ultraviolet to near-infrared (UV-VIS-NIR, 250 to 2,500 nm). The maximum absorption even increased up to 93.6% in the NIR range (820 to 2,500 nm). The high absorption in NIR range makes LSP-enhanced black silicon a potential material used for NIR-sensitive optoelectronic device.

PACS

78.67.Bf; 78.30.Fs; 78.40.-q; 42.70.Gi     

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al₂O₃ films (Si_x(Al₂O₃)_1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.     

Engineering Lipid Bilayer Membranes for Protein Studies

Lipid membranes regulate the flow of nutrients and communication signaling between cells and protect the sub-cellular structures. Recent attempts to fabricate artificial systems using nanostructures that mimic the physiological properties of natural lipid bilayer membranes (LBM) fused with transmembrane proteins have helped demonstrate the importance of temperature, pH, ionic strength, adsorption behavior, conformational reorientation and surface density in cellular membranes which all affect the incorporation of proteins on solid surfaces. Much of this work is performed on artificial templates made of polymer sponges or porous materials based on alumina, mica, and porous silicon (PSi) surfaces. For example, porous silicon materials have high biocompatibility, biodegradability, and photoluminescence, which allow them to be used both as a support structure for lipid bilayers or a template to measure the electrochemical functionality of living cells grown over the surface as in vivo. The variety of these media, coupled with the complex physiological conditions present in living systems, warrant a summary and prospectus detailing which artificial systems provide the most promise for different biological conditions. This study summarizes the use of electrochemical impedance spectroscopy (EIS) data on artificial biological membranes that are closely matched with previously published biological systems using both black lipid membrane and patch clamp techniques.     

Photoluminescence properties of porous silicon layers prepared by electrochemical etching in extremely dilute HF solutions

Dilute HF solutions with concentrations down to 0.03% have been used to obtain luminescent porous silicon (PSi) layers on p-type Si wafers. The experimental results show that with a constant etching time of 30 min, PSi layers with sufficient luminescence efficiencies can be formed for HF concentrations as low as 0.1%. Because of a significantly lowered critical current density, only very low etching current densities of  ≤0.1 mA cm⁻² can result in the formation of luminescent PSi samples in 0.1% HF solutions. A notable result is that these low etching current densities cannot be used to form luminescent PSi layers in concentrated ( ≥1%) HF solutions. The behavior of PL intensity as a function of etching current density has been analyzed over a wide range of HF concentration. The PL intensity is determined by the ratio of the etching current density to the critical current density, suggesting that the presence of silicon oxides plays an important role in the formation of luminescent Si nanostructures in PSi layers.     

Silicon quantum dot superlattice solar cell structure including silicon nanocrystals in a photogeneration layer

The solar cell structure of n-type poly-silicon/5-nm-diameter silicon nanocrystals embedded in an amorphous silicon oxycarbide matrix (30 layers)/p-type hydrogenated amorphous silicon/Al electrode was fabricated on a quartz substrate. An open-circuit voltage and a fill factor of 518 mV and 0.51 in the solar cell were obtained, respectively. The absorption edge of the solar cell was 1.49 eV, which corresponds to the optical bandgap of the silicon nanocrystal materials, suggesting that it is possible to fabricate the solar cells with silicon nanocrystal materials, whose bandgaps are wider than that of crystalline silicon.

PACS

85.35.Be; 84.60.Jt; 78.67.Bf     

The Electrical Band-Gap Energy of Porous Silicon Measured Versus Sample Temperature

Photocurrent measurements have been carried out on a series of samples consisting of porous silicon on top of crystalline silicon, in the temperature range 10–300 K. From the experimental data set, the electrical band-gap energy of porous silicon is deduced to be (1.80 ± 0.01) eV, independent of sample temperature. The results are discussed with the conclusion that for the samples studied here, the electrical bandgap in porous silicon is of molecular nature and cannot be related to quantum-confinement properties of nanocrystals of elemental silicon.     

Al掺杂对甲醇醇解法合成ZnO纳米粉体的影响

采用醇解法,在130℃的甲醇溶液中分别合成纯的和Al掺杂纳米氧化锌(ZnO)晶体.使用X射线衍射仪,透射电子显微镜,Fourier红外光谱和偏振稳态荧光光谱对其晶体结构和光学性能进行了表征.结果表明:在甲醇溶液中,在较低的温度(130℃)下,成功制备出纳米ZnO晶体.Fourier红外吸收光谱表明醇解法合成的ZnO纳米晶体含有较少的有机物杂质.荧光光谱结果可以看出,纯ZnO和Al掺杂的ZnO纳米晶体在可见光范围(400nm～700nm)内有一个高的蓝光发光带(峰位为440nm)和一个绿光发光带(纯的和Al掺杂的峰位分别为520nm和530nm).通过对比发现掺杂Al可以有效的改变ZnO纳米粉体的可见光发光特性.     

The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon

The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced.     

Comparative study of initial stages of copper immersion deposition on bulk and porous silicon

Initial stages of Cu immersion deposition in the presence of hydrofluoric acid on bulk and porous silicon were studied. Cu was found to deposit both on bulk and porous silicon as a layer of nanoparticles which grew according to the Volmer-Weber mechanism. It was revealed that at the initial stages of immersion deposition, Cu nanoparticles consisted of crystals with a maximum size of 10 nm and inherited the orientation of the original silicon substrate. Deposited Cu nanoparticles were found to be partially oxidized to Cu₂O while CuO was not detected for all samples. In contrast to porous silicon, the crystal orientation of the original silicon substrate significantly affected the sizes, density, and oxidation level of Cu nanoparticles deposited on bulk silicon.     

Nanoparticles prepared from porous silicon nanowires for bio-imaging and sonodynamic therapy

Evaluation of cytotoxicity, photoluminescence, bio-imaging, and sonosensitizing properties of silicon nanoparticles (SiNPs) prepared by ultrasound grinding of porous silicon nanowires (SiNWs) have been investigated. SiNWs were formed by metal (silver)-assisted wet chemical etching of heavily boron-doped (100)-oriented single crystalline silicon wafers. The prepared SiNWs and aqueous suspensions of SiNPs exhibit efficient room temperature photoluminescence (PL) in the spectral region of 600 to 1,000 nm that is explained by the radiative recombination of excitons confined in small silicon nanocrystals, from which SiNWs and SiNPs consist of. On the one hand, in vitro studies have demonstrated low cytotoxicity of SiNPs and possibilities of their bio-imaging applications. On the other hand, it has been found that SiNPs can act as efficient sensitizers of ultrasound-induced suppression of the viability of Hep-2 cancer cells.     

Effect of thermal treatment on the growth,structure and luminescence of nitride-passivated silicon nanoclusters

Silicon nanoclusters (Si-ncs) embedded in silicon nitride films have been studied to determine the effects that deposition and processing parameters have on their growth, luminescent properties, and electronic structure. Luminescence was observed from Si-ncs formed in silicon-rich silicon nitride films with a broad range of compositions and grown using three different types of chemical vapour deposition systems. Photoluminescence (PL) experiments revealed broad, tunable emissions with peaks ranging from the near-infrared across the full visible spectrum. The emission energy was highly dependent on the film composition and changed only slightly with annealing temperature and time, which primarily affected the emission intensity. The PL spectra from films annealed for duration of times ranging from 2 s to 2 h at 600 and 800°C indicated a fast initial formation and growth of nanoclusters in the first few seconds of annealing followed by a slow, but steady growth as annealing time was further increased. X-ray absorption near edge structure at the Si K- and L_3,2-edges exhibited composition-dependent phase separation and structural re-ordering of the Si-ncs and silicon nitride host matrix under different post-deposition annealing conditions and generally supported the trends observed in the PL spectra.     

多孔碳化硅表面改性及其生物相容性的研究

采用浸渍提拉的方法将聚乙烯醇、硅溶胶、海藻酸钙等涂敷于多孔碳化硅板上对其进行改性,并通过SEM观察改性后多孔碳化硅的生物相容性,并对其处理COD的能力进行测定。实验表明：聚乙烯醇和硅溶胶可以对多孔碳化硅进行改性,改性后多孔碳化硅板的生物相容性提高,微生物负载量增大,处理COD的能力明显提高。反应5d后,PVA改性的多孔碳化硅板COD去除率由改性前的15％提高到80％;硅溶胶改性的多孔碳化硅板COD去除率由改性前的15％提高到78％。     

多孔碳化硅表面改性及其生物相容性的研究

采用浸渍提拉的方法将聚乙烯醇、硅溶胶、海藻酸钙等涂敷于多孔碳化硅板上对其进行改性,并通过SEM观察改性后多孔碳化硅的生物相容性,并对其处理COD的能力进行测定。实验表明:聚乙烯醇和硅溶胶可以对多孔碳化硅进行改性,改性后多孔碳化硅板的生物相容性提高,微生物负载量增大,处理COD的能力明显提高。反应5d后,PVA改性的多孔碳化硅板COD去除率由改性前的15%提高到80%;硅溶胶改性的多孔碳化硅板COD去除率由改性前的15%提高到78%。     

Transient surface photovoltage studies of bare and Ni-filled porous silicon performed in different ambients

Mesoporous silicon and porous silicon/Ni nanocomposites have been investigated in this work employing light-dark surface photovoltage (SPV) transients to monitor the response of surface charge dynamics to illumination changes. The samples were prepared by anodization of a highly n-doped silicon wafer and a subsequent electrodepositing of Ni into the pores. The resulting pores were oriented towards the surface with an average pore diameter of 60 nm and the thickness of the porous layer of approximately 40 μm. SPV was performed on a bare porous silicon as well as on a Ni-filled porous silicon in vacuum and in different gaseous environments (O₂, N₂, Ar). A significant difference was observed between the ‘light-on’ and ‘light-off’ SPV transients obtained in vacuum and those observed in gaseous ambiences. Such behavior could be explained by the contribution to the charge exchange in gas environments from chemisorbed and physisorbed species at the semiconductor surface.

PACS

81.05.Rm; 73.20.-r; 75.50.-y; 82.45.Yz     

Nanostructures formed by displacement of porous silicon with copper: from nanoparticles to porous membranes

ABSTRACT: The application of porous silicon as a template for the fabrication of nanosized copper objects is reported. Three different types of nanostructures were formed by displacement deposition of copper on porous silicon from hydrofluoric acid-based solutions of copper sulphate: (1) copper nanoparticles, (2) quasi-continuous copper films, and (3) free porous copper membranes. Managing the parameters of porous silicon (pore sizes, porosity), deposition time, and wettability of the copper sulphate solution has allowed to achieve such variety of the copper structures. Elemental and structural analyses of the obtained structures are presented. Young modulus measurements of the porous copper membrane have been carried out and its modest activity in surface enhanced Raman spectroscopy is declared.     

High resolution TEM and triple-axis XRD investigation into porous silicon formed on highly conducting substrates

The microstructure of thin porous silicon films (∼1 μm) formed potentiostatically on p⁺-type silicon in dilute HF solutions is investigated via high resolution TEM and triple-axis XRD. Average pore diameters were found to increase with increasing etching potential, changing from a mixture of micro- and mesopores to predominately square macropores once oxide growth commences. It is postulated that these square pores result from crystallographic etching. High resolution TEM images revealed that stresses within porous silicon are sufficient to cause lattice distortions of the order of a few percent and that once oxide formation occurs areal defects are generated. Although the high resolution TEM analysis suggests that the lattices of the majority of the nanocrystals are compressed relative to bulk silicon and explanations for this are proposed, the combination of the porous silicon film's high porosity and thinness meant that this could not be confirmed by the more reliable triple-axis XRD measurements. Conversely, the triple-axis XRD revealed that thicker films grown under the same conditions had expanded lattices. The results also highlight the drawbacks of relying solely on high resolution TEM or double-axis XRD data when investigating the nature of lattice distortions.     

Laser Structuring of Luminescent Porous Silicon During Etching

A new approach to the problem of preparation of laterally structured luminescent porous silicon is proposed. The effect is based on the photosensitivity of chemical etching of silicon. Contrary to the other technique recently reported where the porous layer was modified with light assisted chemical dissolution, a one stage anodisation-free process is used. Any desired lateral structure can be produced, depending on the illumination pattern, which is defined by optical imaging. A 2D micro-array was prepared as an example of this fast mask-free technology with an accuracy of 5 m. Samples have a broadband visible photoluminescence centred at 680 nm when illuminated with UV or blue light. The results are analysed to determine the possible achievable accuracy of the technique and to improve our understanding of the mechanism of light assisted etching.