On the power-dependent spectral shift of photoluminescence from ensembles of silicon nanocrystals

ABSTRACT: Nanocrystals are widely studied for their tunable optical properties, most importantly increased luminescence efficiency and emission energy. Quantum confinement effects are found for many different types of nanocrystals and these introduce a relation between the emission wavelength and size of nanocrystals. When ensembles of nanocrystals with a distribution of sizes are studied, this can have profound effects on their luminescence spectra. Here we show how photoluminescence spectra of ensembles of silicon nanocrystals can shift under different excitation conditions, resulting from differences in absorption cross section of the individual nanocrystals sizes. This effect, together with the fact that after a pulsed excitation a silicon nanocrystal can only emit a single photon, determines how the distribution of excited nanocrystals changes and leads to the spectral shift for different excitation powers. Next to this effect, also the influence of different radiative rates in such ensembles are addressed. These notions are important for interpretation of photoluminescence data for silicon nanocrystals, but can be extended to any nanoparticle system comprising size-distributed ensembles.     

Opto-structural studies of well-dispersed silicon nano-crystals grown by atom beam sputtering

Synthesis and characterization of nano-crystalline silicon grown by atom beam sputtering technique are reported. Rapid thermal annealing of the deposited films is carried out in Ar + 5% H₂ atmosphere for 5 min at different temperatures for precipitation of silicon nano-crystals. The samples are characterized for their optical and structural properties using various techniques. Structural studies are carried out by micro-Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy, and selected area electron diffraction. The optical properties are studied by photoluminescence and UV-vis absorption spectroscopy, and bandgaps are evaluated. The bandgaps are found to decrease after rapid thermal treatment. The micro-Raman studies show the formation of nano-crystalline silicon in as-deposited as well as annealed films. The shifting and broadening in Raman peak suggest formation of nano-phase in the samples. Results of micro-Raman, photoluminescence, and TEM studies suggest the presence of a bimodal crystallite size distribution for the films annealed at higher temperatures. The results show that atom beam sputtering is a suitable technique to synthesize nearly mono-dispersed silicon nano-crystals. The size of the nano-crystals may be controlled by varying annealing parameters.     

Effect of the etching time on the morphological, optical and electronic properties of silicon nanowires

ABSTRACT: Owing to their interesting electronic, mechanical, optical and transport properties, silicon nanowires (SiNWs) have attracted much attention, giving opportunities to several potential applications in nanoscale electronic, optoelectronic devices and silicon solar cells. For photovoltaic (PV) application, a superficial film of SiNWs could be used as an efficient antireflection coating (ARC). In this work, we investigate the morphological, optical and electronic properties of SiNWs fabricated at different etching time. Characterizations of the formed SiNWs films were performed using a Scanning Electron Microscope (SEM), UV-Vis-NIR spectrophotometer and Light-Beam-Induced-Current (LBIC) technique. The later technique was used to determine the effective diffusion length in SiNWs films. From LBIC investigations, we deduce that the homogeneity of the SiNWs film play a key role on the electronic properties.     

Fabrication and characterization of magnetic porous silicon with curie temperature above room temperature

In this study, we demonstrate a completely novel synthesis route for producing magnetic porous silicon. The magnetic properties of this material are induced by manganese atoms. The Mn-doping in Si is achieved by ion implantation. A subsequent anodization of the substrate is done to turn it into porous silicon. Several characterization techniques, such as transmission electronic microscopy, atomic force microscopy and photoluminescence are combined to probe the structural and the optical properties of this material. Furthermore, temperature and magnetic field dependent magnetization is analyzed using superconducting quantum interference device. In addition to the well-reported structural and optical properties of the porous silicon, our Mn-doped porous silicon samples exhibit a magnetic behavior with a curie temperature (T_C) higher than room temperature. These results indicate that the magnetic porous silicon can be integrated with microelectronics and photonics technologies to produce new devices, such as magnetophotonic crystals and polarized emitting diodes.     

The modulation on luminescence of Er3+-doped silicon-rich oxide films by the structure evolution of silicon nanoclusters

A series of silicon-rich oxide (SRO) and erbium-doped SRO (SROEr) films imbedded with structural tunable silicon nanoclusters (Si NCs) have been fabricated using sputtering followed by post-annealing. The coalescence of Si NCs is found in the films with large Si excess. The energy transfer rate between Si NCs and Er³⁺ is enhanced, but the luminescence efficiencies of both Si NCs and Er³⁺ are reduced by the coalescent microstructures. Optimization of the microstructures of Si NCs is performed, and the preferential optical performance for both Si NCs and Er³⁺ could be achieved when Si NCs were separated in microstructures.     

Si(111)衬底上生长的ZnO薄膜的光学特性研究

利用等离子体分子束外延(P-MBE)设备在Si(111)衬底上制备了高质量的ZnO薄膜.通过扫描电镜观察了ZnO薄膜的表面形貌为的六角结构.X射线衍射谱显示ZnO薄膜为C轴择优取向的,ZnO(002)取向X射线衍射峰的最大半宽度仅0.18°.并通过室温和变温发光谱对ZnO薄膜的发光特性进行了研究.在低温下ZnO的发光以...     

Spatial organization of the sp-hybridized carbon atoms and electronic density of states of hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films prepared by plasma decomposition of hydrocarbons exhibit a wide variety of electronic and mechanical properties depending on their deposition conditions, which makes them very interesting for applications in several domains. This versatility is essentially due to the presence of both sp²- and sp³-hybridized carbon atoms in variable proportions, and to the tendency of the sp² C atoms to gather into π-bonded clusters with different bonding configurations. The relationships between the film microstructure and their electronic density of states, as deduced from a detailed analysis of their optical properties over a large spectral range, are described and discussed, taking as reference materials the purely sp² (graphite) and purely sp³ (diamond) carbon crystalline phases, as well as the prototype hydrogenated amorphous tetra-coordinated semiconductor, hydrogenated amorphous silicon. It is shown that the type of clustering of the sp² C atoms is certainly more determinant for the electronic density of states, and especially for the optical gap value, than the proportion of these atoms in the material.     

Controllable size and photoluminescence of ZnO nanorod arrays on Si substrate prepared by microwave-assisted hydrothermal method

High-quality ZnO nanorod arrays were grown on silicon substrates by microwave-assisted hydrothermal method. A ZnO seed layer deposited by magnetron sputtering was used for promoting nanorod growth. Process optimization indicates that the size and surface density of nanorods can be controlled individually by varying process parameters including precursor concentration, heating temperature, and heating time. The photoluminescence performance of the nanorods is closely dependent on the mean size of the rods. Reducing rod diameter leads to decreased UV emission and visible emission intensity ratio, which has been attributed to the increased impurities or defects on the rod surface. The present results provide a feasible approach to modify the optical properties of transparent ZnO nanorod arrays.     

First-principles density-functional investigation on the electronic properties and field emission of a hydrogenated nanodiamond

First-principles calculations using quantum-mechanical density functional theory (DFT) are carried out to investigate the geometrical structure and electronic properties for hydrogen terminated nanometer-sized diamonds. The results reveal that the size dependent feature in the electronic structures for nanodiamonds is different from that of Si clusters. The field emission properties for nanodiamonds are also explored, and it is found that under applied electric field Mulliken charges redistribute and accumulate on the emission side. Furthermore, the emission currents from the occupied orbitals for nanodiamond are calculated and it is revealed that the largest emission current comes from the third highest occupied molecular orbital.     

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.     

On the origin of emission and thermal quenching of SRSO:Er3+ films grown by ECR-PECVD

Silicon nanocrystals embedded in a silicon-rich silicon oxide matrix doped with Er³⁺ ions have been fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition. Indirect excitation of erbium photoluminescence via silicon nanocrystals has been investigated. Temperature quenching of the photoluminescence originating from the silicon nanocrystals and the erbium ions has been observed. Activation energies of the thermally activated quenching process were estimated for different excitation wavelengths. The temperature quenching mechanism of the emission is discussed. Also, the origin of visible emission and kinetic properties of Er-related emission have been discussed in details.     

Spectroscopic characterization of thin SiC films

The electrical, structural and optical properties of thin SiC films were investigated. A new approach based on high temperature annealing of layered carbon–silicon structures was used for the formation of the films. The SiC films were prepared by deposition of 30 nm thick carbon films on crystalline silicon (c-Si) and on porous silicon layers grown on c-Si. The layers were annealed to temperatures between 800 and 1400°C for different annealing times ranging between 15 and 180 s. The structure of the resulting SiC films was analyzed by Raman spectroscopy. The Raman spectra of as-deposited films consist of two broad bands at 1350 and 1580 cm⁻¹ characteristic of the presence of amorphous carbon. These bands were shifted to lower frequencies in the spectra of annealed layers and were assigned to the hexagonal and cubic SiC phases. The photoluminescence spectra of the studied layers show a broad band at 550 nm. The most intense photoluminescence was observed from non-annealed porous silicon layers covered with thin carbon films. A degradation of the luminescence and a simultaneous increase of the conductivity of the layers with increasing annealing temperature and/or duration of annealing was observed. This behavior strongly suggests the creation of defect states which determine the conductivity of the layers and at the same time act as non-radiative centers. The increase of defect states was explained as originating from the dehydrogenation of the silicon carbide layers by annealing.     

Surface,interface and electronic properties of F8:F8BT polymeric thin films used for organic light‐emitting diode applications

Ultrathin polymeric films consisting of poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (F8) blended with poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) grown onto PEDOT:PSS/ITO/PET were investigated by X‐ray photoelectron spectroscopy (XPS), depth‐profiling XPS, reflection electron energy loss spectroscopy (REELS) and angle‐dependent X‐ray absorption spectroscopy (XAS) to gain information on the films' electronic, order and interface properties. AFM studies provide valuable information on the films' nanotopographical properties and homogeneity. Spectroscopic ellipsometry and photoluminescence spectroscopy were used also to obtain information on the optoelectronic properties. Well‐ordered films were observed from the XAS analysis, measured at the sulfur K absorption edge. XPS measurements demonstrated that the surface composition of the polymer thin films prepared by a spin‐coating wet‐chemical deposition method matches the expected F8:F8BT blend stoichiometry. The interfacial properties were studied through an argon ion sputtering process coupled to the XPS acquisition, showing an enhancement of oxygen components at the interface. The films' inhomogeneity was verified by AFM images and analysis. We obtained a value of 3.1 eV as the electronic bandgap of the F8:F8BT film from REELS data, whereas analysis of the spectroscopic ellipsometry spectra revealed that the optical bandgap of F8:F8BT has a value of 2.4 eV. A strong green emission was obtained for the produced films, which is in agreement with the expected emission due to the 1:19 ratio of the F8 and F8BT blended polymers. © 2018 Society of Chemical Industry     

Lateral electrical transport,optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO<Subscript>2</Subscript>

In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.     

Optical Characterization of Free-Standing Porous Silicon Films

We report here the optical characterization of two free-standing porous silicon (PS) films of porosity 70% and 85%, grown on a p- and a n-type Si. We determine the optical band gap in these films from the excitation wavelength dependence of the photoluminescence band. As the excitation wavelength is changed from red (800 nm) to UV (355 nm), a blue shift of the photoluminescence (PL) band is observed. We attribute the observed blue shift of the PL band to the emission due to the distribution of bandgap in PS. Both samples are oxidized in air and we believe that the observed bandgap in these films arises from the inhomogeneous distribution of Si particle sizes. However, we find that intrinsic defects play a dominant role in the process of luminescence. Electron spin resonance measurement indicates the presence of defects leading to the saturation of the optical absorption spectra and a decrease in intensity of the PL band. The lineshape of the PL band is modeled using a weighted asymmetric Gaussian that selects a gap distribution function at each excitation wavelength. From Raman measurements in these two films, the quantum confinement effect in Si nanostructures is clearly observed in both films.     

Morphological,compositional, structural,and optical properties of Si-nc embedded in SiOx films

Structural, compositional, morphological, and optical properties of silicon nanocrystal (Si-nc) embedded in a matrix of non-stoichiometric silicon oxide (SiO_x) films were studied. SiO_x films were prepared by hot filament chemical vapor deposition technique in the 900 to 1,400°C range. Different microscopic and spectroscopic characterization techniques were used. The film composition changes with the growth temperature as Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy reveal. High-resolution transmission electron microscopy supports the existence of Si-ncs with a diameter from 1 to 6.5 nm in the matrix of SiO_x films. The films emit in a wide photoluminescent spectrum, and the maximum peak emission shows a blueshift as the growth temperature decreases. On the other hand, transmittance spectra showed a wavelength shift of the absorption border, indicating an increase in the energy optical bandgap, when the growth temperature decreases. A relationship between composition, Si-nc size, energy bandgap, PL, and surface morphology was obtained. According to these results, we have analyzed the dependence of PL on the composition, structure, and morphology of the Si-ncs embedded in a matrix of non-stoichiometric SiO_x films.     

Preparation and characteristics of ZnO films on freestanding diamond substrates

In this paper, preferred and fine grained polycrystalline ZnO films were deposited on smooth nucleation surfaces of freestanding thick diamond films (FTDF) by plasma-assisted metal organic chemical vapor deposition. The properties of the ZnO films were characterized by scanning electron microscopy, X-ray diffraction, glancing angle X-ray diffraction, room temperature photoluminescence spectra, and electron probe microanalysis. The results indicate that the morphological, structural, and optical properties of ZnO films are strongly dependent on the deposition procedures, especially the deposition temperature. The Zn/O atomic ratio plays an important role in the optical properties of ZnO films. The experimental results can help us improve our understanding of how to obtain ZnO films with excellent properties deposited on FTDF. The most significant improvements in morphological, structural, and optical properties of ZnO films are obtained by using the proper deposition temperature of 500 °C.     

Role of defects in tailoring optical,electronic, and luminescent properties of Cu-doped ZnO films

We present a detailed characterization study on copper-doped ZnO films to correlate the films' electronic and optical properties with the existing native defects in the lattice. In addition, we describe the variation in the concentration of these defects with Cu dopant and temperature. The results of XRD confirmed the single-phase würtzite-structure of the synthesized films. The SEM images showed a homogeneous and dense grain morphology with a granular form and a signature for a hexagonal-like shape. The EDX, XPS, and UV–Vis spectra showed the proper doping of Cu ions into the lattice. The XPS analysis indicated mixed electronic states of both Cu²⁺ and Cu¹⁺ and showed a clear increase in the Cu²⁺ intensity relative to Cu¹⁺, with Cu dopant. The transmittance spectra exhibited an average value above 80% in all doped films in the visible and infrared regions. The overall results indicated a clear link between the films’ optical and electronic responses and the level of the intrinsic defects in the lattice. By increasing the Cu dopant, we find a slight reduction in the energy bandgap (E_g). This is correlated with a clear reduction in the blue emission luminescence band associated with the V_Zn and in the yellow emission band associated with the O_i. On the other hand, we observed a clear enhancement in the green emission band originating from the V_O, and in the emission band related to possible transitions from Zn_i levels to O_i levels. The slight reduction in the E_g signals a weak sp-d hybridization between the ZnO conduction band electrons and the Cu²⁺ ions, which is mediated by the intrinsic defects. With reducing the temperature, the photoluminescence temperature profiles indicated a slight increase in the E_g values and a negligible effect on the distribution of the native defects.     

Investigation into the effect of substrate material on microstructure and optical properties of thin films deposited via magnetron sputtering technique

This study aims at investigating the effect of the substrate material on growth mechanism and also microstructure of Ta₂O₅ thin films. For this purpose, atomic force microscopy, scanning electron microscopy, and interferometry analyses were implemented to reveal the influence of silicon wafer and amorphous BK7 glass substrates on the nucleation and growth mechanisms of Ta₂O₅ thin films deposited via the radio frequency magnetron sputtering technique. Results indicated that those films with finer morphologies had relatively higher nucleation densities. Compared with BK7 glass substrate, crystals formed on the silicon wafer were shown to be finer and had lower mean areas in more nucleation sites. Moreover, optical properties and morphological characteristics of the films on the silicon substrates had much more endurance after the annealing treatment. It was observed that shift in the transmission spectra of the deposited films after the treatment was insignificant, implying high packing density of the films. However, a 6-nm shift in the transmission spectra indicated low density and high porosity of the films. Finally, atomic force microscopy analysis along with the light scattering measurements confirmed the formation of a low-roughness film on the silicon wafer substrates.     

控电位法制备CdSe/PANI复合薄膜及其光致发光性

采用控电位法,以PANI薄膜作为载体,成功制备出CdSe/PANI复合薄膜。采用扫描电子显微镜(SEM)、X射线衍射(XRD)、荧光光谱仪等仪器对所制备的薄膜进行比较与表征。CdSe/PANI复合薄膜中CdSe微粒粒径约为150~200 nm,且随沉积时间的延长,粒径逐渐增大。复合薄膜中含有聚苯胺和CdSe 2种成分的晶体,并且复合薄膜的晶型结构与沉积CdSe薄膜的厚度无关。与PANI薄膜和CdSe薄膜比较,CdSe/PANI复合薄膜的荧光强度明显增强,PANI增强了CdSe的光致发光性能,光致发光发射峰位置发生红移。