Showing papers on "Potential well published in 2011"

Origin of Blue Emission from Silicon Nanoparticles Direct Transition and Interface Recombination

Abstract: Blue light luminescent Si nanoparticles (NPs) have many potential applications in optoelectronics. However, blue light emission from Si is not stable in many cases, which severely hinders their real applications. Here, it is found that Si NPs prepared by laser ablation have strong and aging-enhancement blue light emissions. Subsequent annealing treatment of the aged Si NPs will weaken the blue emission dramatically. Through monitoring the photoluminescence (PL) evolutions (including the peak position and intensity) during different treatments of the Si NPs, we studied the origin of the blue light emission. In this case, such blue light emission cannot be simply ascribed to the quantum confinement effect of Si nanocrystals or surface states. The most plausible luminescent mechanism is that excitons first formed within Si nanocrystals by direct transitions at Γ or X point, then some of the formed excitons are trapped by nonradiative Pb centers, and the others transfer to and recombine at near-interface trap...

Matrix-embedded silicon quantum dots for photovoltaic applications: a theoretical study of critical factors

Abstract: Si Quantum dots (QD's) are offering the possibilities for improving the efficiency and lowering the cost of solar cells. In this paper we study the PV-related critical factors that may affect design of Si QDs solar cell by performing atomistic calculation including many-body interaction. First, we find that the weak absorption in bulk Si is significantly enhanced in Si QDs, specially in small dot size, due to quantum-confinement induced mixing of Γ-character into the X-like conduction band states. We demonstrate that the atomic symmetry of Si QD also plays an important role on its bandgap and absorption spectrum. Second, quantum confinement has a detrimental effect on another PV property – it significantly enhances the exciton binding energy in Si QDs, leading to difficulty in charge separation. We observe universal linear dependence of exciton binding energy versus excitonic gap for all Si QDs. Knowledge of this universal linear function will be helpful to obtain experimentally the exciton binding energy by just measuring the optical gap without requiring knowledge on dot shape, size, and surface treatment. Third, we evaluate the possibility of resonant charge transport in an array of Si QDs via miniband channels created by dot-dot coupling. We show that for such charge transport the Si QDs embedded into a matrix should have tight size tolerances and be very closely spaced. Fourth, we find that the loss of quantum confinement effect induced by dot-dot coupling is negligible – smaller than 70 meV even for two dots at intimate contact.

The role of the surfaces in the photon absorption in Ge nanoclusters embedded in silica.

Abstract: The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics. PACS: 81.07.Ta; 78.67.Hc; 68.65.-k

Freestanding silicon quantum dots: origin of red and blue luminescence

Abstract: In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface.

Synthesis and optical properties of Cr doped ZnS nanoparticles capped by 2-mercaptoethanol

Abstract: Nanoparticles of Zn1−xCrxS (x=0.00, 0.005, 0.01, 0.02 and 0.03) were prepared by a chemical co-precipitation reaction from homogenous solutions of zinc and chromium salts. These nanoparticles were sterically stabilized using 2-mercaptoethanol. Here a study of the effect of Cr doping on structural, morphological and optical properties of nanoparticles was undertaken. Elemental analysis, morphological, structural and optical properties have been investigated by energy dispersive analysis of X-rays (EDAX), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–visible spectroscopy .EDAX measurements confirmed the presence of Cr in the ZnS lattice. XRD showed that ZnS:Cr nanoparticles crystallized in zincblende structure with preferential orientation along (1 1 1) plane. The average sizes of the nanoparticles lay in the range of 3–6 nm and lattice parameters were in the range of 5.2–5.4 A. Lattice contraction was observed with an increase of Cr concentration. The particle size and lattice parameters obtained from TEM and SAED images were in agreement with the XRD results. The absorption edge shifted to lower wavelengths with an increase in Cr concentration as per UV–Vis spectroscopy. The band gap energy values were in the range of 3.85–4.05 eV. This blueshift is attributed to the quantum confinement effect.

Great blue-shift of luminescence of ZnO nanoparticle array constructed from ZnO quantum dots

Abstract: ZnO nanoparticle array has been fabricated on the Si substrate by a simple thermal chemical vapor transport and condensation without any metal catalysts. This ZnO nanoparticles array is constructed from ZnO quantum dots (QDs), and half-embedded in the amorphous silicon oxide layer on the surface of the Si substrate. The cathodoluminescence measurements showed that there is a pronounced blue-shift of luminescence comparable to those of the bulk counterpart, which is suggested to originate from ZnO QDs with small size where the quantum confinement effect can work well. The fabrication mechanism of the ZnO nanoparticle array constructed from ZnO QDs was proposed, in which the immiscible-like interaction between ZnO nuclei and Si surface play a key role in the ZnO QDs cluster formation. These investigations showed the fabricated nanostructure has potential applications in ultraviolet emitters.

Photoluminescence Blue-Shift of CdSe Nanoparticles Caused by Exchange of Surface Capping Layer

Abstract: TOPO- and pyridine-capped cubic CdSe quantum dots were prepared, and the effect of surface ligand exchange on the photoluminescence (PL) peak shift was investigated. From the analysis of PL spectra and TEM images, it was found that the decrease in diameter in the CdSe quantum dot and redistribution of its surface electronic density play different roles in the PL peak shift. The reduction in size of CdSe by ligand exchange caused a blue shift of the PL peak due to the quantum confinement effect, whereas the redistribution of surface electronic density of CdSe by the exchange of TOPO with pyridine resulted in a red shift of 23 meV on the average for the samples investigated. The result was also supported by the XPS characterizations.

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Abstract: Si quantum dots (QDs) were formed by thermal annealing the hydrogenated amorphous silicon carbide films (a-SiCx:H) with different C/Si ratio x, which were controlled by using a different gas ratio R of methane to silane during the deposition process. By adjusting x and post annealing temperature, the QD size can be changed from 1.4 to 4.2 nm accordingly, which was verified by the Raman spectra and transmission electron microscopy images. Size-dependent electroluminescence (EL) was observed, and the EL intensity was higher for the sample containing small-sized Si QDs due to the quantum confinement effect (QCE). The EL peak energy as a function of the Si QDs size was in good agreement with a modified effective mass approximation (EMA) model. The calculated finite barrier potential of the Si QDs embedded in SiC matrix is 0.4 and 0.8 eV for conduction and valence band, respectively. Moreover, the current-voltage properties and the linear relationship between the integrated EL intensity and injection current i...

Structural and optical characterization of Ni-doped CdS quantum dots

Abstract: Ni-doped CdS quantum dots have been prepared by chemical precipitation technique. The X-diffraction results indicated that the particle size of Ni-doped CdS nanoparticles is smaller than that of undoped CdS and no secondary phase was observed. The average grain size of the nanoparticles is found to lie in the range of 2.7–4 nm. The compositional analysis results show that Cd, Ni, and S are present in the samples. HRTEM studies reveal that the average particle size of undoped and Ni-doped CdS quantum dots is 2 and 3 nm, respectively. Raman spectra shows that 1LO, 2LO, and 3LO peaks of the Ni-doped CdS samples are slightly red shifted when compared to that of undoped CdS. The absorption edge of Ni-doped CdS nanoparticles is found to shift towards the higher-wavelength (red shift) side when compared to that of undoped CdS and the band gap is observed to lie in the range of 3.79–3.95 eV. This band gap is higher than that of the bulk CdS and is due to quantum confinement effect present in CdS nanoparticles.

A versatile route to synthesize MgO nanocrystals by combustion technique

Abstract: II – VI semiconductor nanocrystals are recently developed class of nanomaterials whose unique photophysical properties are helping to create a new generation in the field of photonics and microelectronics. In this review, we examine the progress in adapting these nanomaterials for several optoelectronics application followed by characterization studies. Magnesium oxide (MgO) nanoparticles were characterized by X-ray powder diffraction and the peaks are quite agreeable with the pure phase cubic structure. The XRD pattern confirms the crystallinity and phase purity of the sample. Photoluminescence measurement reveals the systematic shift of the emission band towards the lower wavelength thereby ascertaining the quantum confinement effect. Photoluminescence spectra of pure MgO were investigated, showing emission peaks around 475 nm relating to new energy levels induced by defects or defect levels generation. The SEM results reveal that the resultant nanopowders are porous and agglomerated with polycrystalline nano entities. Field emission scanning electron microscopic studies showed that the average size of the nanoparticles were 20 nm and 33 nm respectively. The dielectric loss of the MgO samples decreases with increase in frequency. Similar trend is observed for the dielectric constant also.

Ballistic electron emission from quantum-sized nanosilicon diode and its applications

Abstract: A quantum confinement effect renders silicon a functional wide-gap material with useful functions. For instance, a diode based on nanocrystalline silicon (nc-Si) exhibits characteristic quasi-ballistic emission effects in various media. As means for physical excitation and probing, the applicability to parallel electron beam lithography and high-sensitivity image-pickup has been demonstrated in vacuum. The energetic electron incidence into air and Xe ambient induces negative ion generation by electron attachment into oxygen molecules and vacuum ultraviolet light emission by internal excitation of Xe molecules, respectively. Another effect is that the nc-Si ballistic emitter can supply highly reducing electrons into aqueous and metal-salt solutions without the use of counter electrodes. This is an attractive process that will be applicable to hydrogen generation and thin metal film deposition.

Properties of nanocones formed on a surface of semiconductors by laser radiation: quantum confinement effect of electrons, phonons, and excitons

Abstract: On the basis of the analysis of experimental results, a two-stage mechanism of nanocones formation on the irradiated surface of semiconductors by Nd:YAG laser is proposed for elementary semiconductors and solid solutions, such as Si, Ge, SiGe, and CdZnTe. Properties observed are explained in the frame of quantum confinement effect. The first stage of the mechanism is characterized by the formation of a thin strained top layer, due to redistribution of point defects in temperature-gradient field induced by laser radiation. The second stage is characterized by mechanical plastic deformation of the stained top layer leading to arising of nanocones, due to selective laser absorption of the top layer. The nanocones formed on the irradiated surface of semiconductors by Nd:YAG laser possessing the properties of 1D graded bandgap have been found for Si, Ge, and SiGe as well, however QD structure in CdTe was observed. The model is confirmed by "blue shift" of bands in photoluminescence spectrum, "red shift" of longitudinal optical line in Raman back scattering spectrum of Ge crystal, appearance of Ge phase in SiGe solid solution after irradiation by the laser at intensity 20 MW/cm2, and non-monotonous dependence of Si crystal micro-hardness as function of the laser intensity.

Controllable growth and optical properties of ZnO nanostructures on Si nanowire arrays

Abstract: In this paper, two types of hybrid semiconductors made up of silicon nanowires and ZnO nanostructures, namely Si/ZnO core–shell nanowire arrays and ZnO quantum dots (QDs)-decorated Si nanowire arrays, have been prepared by combining metal-assisted wet-chemical etching and metal–organic chemical vapor deposition (MOCVD). We demonstrate that ZnO QDs and thin ZnO layers can be grown on Si nanowires in a controlled manner by varying growth parameters including working pressure and growth time. Meanwhile, porous silicon and porous Si/ZnO nanowire arrays have also been fabricated. The morphology and optical properties of both hybrid nanostructures have been carefully investigated for their potential applications in nanowire optoelectronics. A quantum confinement effect in ZnO QDs was confirmed by the blue-shifted photoluminescence. Porous Si/ZnO core–shell nanowires display a very broad emission band throughout the entire visible light range.

Effect of aluminum doping on the structural and luminescent properties of ZnO nanoparticles synthesized by wet chemical method

Abstract: The critical role that dopants play in semiconductor devices has stimulated research on the properties and the potential applications of semiconductor nanocrystals. Hence the investigation of the role of dopant concentration on the properties of semiconductor nanoparticles is very important from the viewpoints of basic physics as well as applications. In this context, in the present work Al-doped ZnO (AZO) nanoparticles were synthesized by simple wet chemical route. The structure and morphology of the nanoparticles analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed hexagonal wurtzite structure with flower-like clusters consisting of multi-nanorods. Energy Dispersive Spectrum (EDS) confirms the substitution of Al into ZnO lattice. Defect analysis and excitonic effect of the nanoparticles were investigated by photoluminescence (PL) and UV–Vis absorption measurements, respectively. Optical absorption showed band gap broadening due to quantum confinement effect. PL measurements exhibited both near band edge (NBE) and deep level (DL) emissions. The effect of doping concentration on the growth, crystallization and defect distribution of AZO nanoparticles was studied.

Polymer wires with quantum dots grown by molecular layer deposition of three source molecules for sensitized photovoltaics

Abstract: Molecular layer deposition (MLD) can be used to grow organic tailored materials, where different molecules are sequentially connected in designated arrangements. Using MLD, polymer wires containing quantum dots (QDs) of lengths ∼0.8, ∼2, and ∼3 nm were grown by connecting three source molecules in monomolecular steps. The peak energy of the QD absorption shifted to higher energy with decreasing QD length, due to the quantum confinement effect. Consequently, the polymer wires exhibited a widening of their absorption spectra from ∼480 to ∼300 nm. This was attributed to the superposition of individual absorption bands of different QDs. A sensitization model for ZnO in photovoltaic devices is proposed, in which polymer wires with QDs as the sensitizing layer reduce the heat energy loss during absorption.

Synthesis, characterizations, and optical properties of copper selenide quantum dots

Abstract: We demonstrate the synthesis of copper selenide quantum dots (QDs) by element directed, inexpensive, straight forward wet chemical method which is free from any surfactant or template. Copper selenide QDs have been synthesized by elemental copper and selenium in the presence of ethylene glycol, hydrazine hydrate, and a defined amount of water at 70 °C within 8 h. The product is in strong quantum confinement regime, phase analysis, purity and morphology of the product has been well studied by X-ray diffraction (XRD), UV–Visible spectroscopy (UV–Vis), Photo-luminescent spectroscopy (PL), Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM), High resolution transmission electron microscopy (HRTEM), and by Atomic force microscopy (AFM) techniques. The absorption and photoluminescence studies display large “blue shift”. TEM and HRTEM analyses revealed that the QDs diameters are in the range 2–5 nm. Due to the quantum confinement effect copper selenide QDs could be potential building blocks to construct functional devices and solar cell. The possible mechanism is also discussed.

CONTROLLED SYNTHESIS AND CHARACTERIZATION OF CERIUM-DOPED ZnS NANOPARTICLES IN HMTA MATRIX

Abstract: Cerium-doped ZnS nanoparticles have been synthesized through hydrothermal method. The nanoparticles were stabilized using hexamethylenetetramine (HMTA) as surfactant in aqueous solution. The average particle size of the prepared samples is about 2 nm. The structure of the as-prepared ZnS nanoparticles is cubic (zinc blende) as demonstrated by X-ray powder diffraction (XRD) and selected area electron diffraction (SAED) analysis. TEM results showed that the synthesized nanoparticles were uniformly dispersed in the HMTA matrix without aggregation. The UV–Vis absorption spectra of the prepared ZnS nanoparticles show a considerable blueshift in the absorption band edge compared to bulk ZnS indicating a strong quantum confinement effect. Formation of HMTA-capped ZnS nanoparticles was confirmed by FTIR studies. Photoluminescence studies showed that the relative emission intensity of Ce3+-doped ZnS nanoparticles is higher than that of undoped ZnS nanoparticles, which is due to the enhancement of radiative recombination in the luminescence process. The PL spectra showed two emission peaks at around 420 nm and 442 nm, which may be attributed to deep-trap emission or defect-related emission of ZnS and presence of various surface states.

Two-dimensional heterostructures based on ZnO

Abstract: The multiple quantum wells (MQW) Mg0.27Zn0.73O/ZnO have been grown by the pulsed laser deposition method with different well widths L w . The interface roughness of quantum wells was inherited from the bottom one and did not exceed 1 nm. We observed the quantum confinement effect showing up in the blue shift of the exciton peak in the low temperature (8 K) photoluminescence and absorption spectra at well width reduction. The exciton binding energy of the two-dimensional structures Mg0.27Zn0.73O/ZnO was two times higher in comparison with the bulk ZnO. It has been established that Einstein’s characteristic temperature Θ E sharp increase with reduction of well width L w up to L w =2.6 nm. It has been revealed that the discontinuity ratio of conduction and valence bands in the heterostructure Mg0.27Zn0.73O/ZnO is 0.65/0.35. We demonstrated the abrupt increase in quantum efficiency at a reduction of the well width that allowed us to observe the optically excited stimulated emission in ZnO quantum wells with the excitation threshold of ∼210 kW/cm2.

Syntheses of CdTe Quantum Dots and Nanoparticles through Simple Sonochemical Method under Multibubble Sonoluminescence Conditions

Abstract: Colloidal cadmium telluride (CdTe) quantum dots (QDs) and their nanoparticles have been synthesized by one pot sonochemical reactions under multibubble sonoluminescence (MBSL) conditions, which are quite mild and facile compared to other typical high temperature solution-based methods. For a typical reaction, and tellurium powder with hexadecylamine and trioctylphosphine/trioctylphosphineoxide (TOP/TOPO) as a dispersant were sonicated in toluene solvent at 20 KHz and a power of 220W for 5-40 min at 60 . The sizes of CdTe particles, in a very wide size range from 2 nm-30 , were controllable by varying the sonicating and thermal heating conditions. The prepared CdTe QDs show different colors from pale yellow to dark brown and corresponding photoluminescence properties due mainly to the quantum confinement effect. The CdTe nanoparticles of about 20 nm in average were found to have band gap of 1.53 eV, which is the most optimally matched band gap to solar spectrum.

Bandgap expansion and dielectric suppression of self-assembled Ge nanocrystals

Abstract: The bandgap and optical properties (dielectric functions and optical constants) of dome-shaped Ge nanocrystals (nc-Ge) with average sizes of ∼6 nm in height and ∼13 nm in diameter have been investigated using spectroscopic ellipsometry based on the Forouhi-Bloomer optical dispersion model. As compared to bulk crystalline Ge, the nc-Ge exhibited a bandgap expansion of ∼0.2 eV and a significant reduction in the dielectric function. The bandgap expansion and dielectric suppression are discussed in terms of the quantum confinement effect as well as the bond contraction model.

Temperature dependent transport studies in InN quantum dots grown by droplet epitaxy on silicon nitride/Si substrate

Abstract: InN quantum dots (QDs) were fabricated on silicon nitride/Si (111) substrate by droplet epitaxy. Single-crystalline structure of InN QDs was verified by transmission electron microscopy, and the chemical bonding configurations of InN QDs were examined by x-ray photoelectron spectroscopy. Photoluminescence measurement shows a slight blue shift compared to the bulk InN, arising from size dependent quantum confinement effect. The interdigitated electrode pattern was created and current-voltage (I-V) characteristics of InN QDs were studied in a metal-semiconductor-metal configuration in the temperature range of 80-300K. The I-V characteristics of lateral grown InN QDs were explained by using the trap model. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651762]

WEAK QUANTUM CONFINEMENT IN ZnO NANORODS: A ONE DIMENSIONAL POTENTIAL WELL APPROACH

Abstract: We report here the weak quantum confinement effect in zinc oxide nanorods fabricated by a simple wet chemical method at room temperature. The formation of nanorods was confirmed through X-ray diffraction measurements. The particle size was also determined from the X-ray diffraction pattern and was found to be 20 nm. The band gap was calculated from the UV-Visible spectrum and found to be 3.72 eV, which is higher as compared to the bulk ZnO. It owes its value to the quantum confinement effect. However, the large particle size indicates that the confinement is weak in nature. The photoluminescence spectrum shows a strong emission peak at 421 nm accompanied by several much weaker defect related emissions in the visible region. Using the weak confinement model, we identified the transition levels for those emissions.

Distribution Pattern of Length, Length Uniformity, and Density of TiO32− Quantum Wires in an ETS‐10 Crystal Revealed by Laser‐Scanning Confocal Polarized Micro‐Raman Spectroscopy

Abstract: ETS-10 is a highly intriguing microporous titanosilicate that has shown an excellent propensity for the selective removal of harmful heavy-metal ions, the potential to work as an effective catalyst for various reactions, and that can be used as a material for solar cells. Such important features arise from the TiO3 2 quantum wires with the diameter (d) of approximately 0.67 nm running along the [110] and [110] directions in the crystal (Figure 1). The TiO3 2 quantum wire is a one-dimensional (1D) extreme of three-dimensional (3D) bulk titanates, which are widely used in industry as, for example, capacitors. It also exhibits an interesting 1D quantum confinement effect. The TiO3 2 quantum wires are not expected to be connected all the way from one face to the opposite face of a crystal owing to the large number of randomly distributed defects. Now the questions are what is the average length of the wires, to what degree do the lengths vary (how does the length homogeneity vary), how does the local density of the quantum wire vary from one region to another within a crystal, do they vary randomly or in accordance with a certain pattern? Answers to the above questions will be highly useful for understanding the mechanism of ETS-10 formation and growth, the refinement of its structure, improvements of its catalytic activities, and its future applications. However, there have been no methods to gain such information. The TiO3 2 quantum wire in ETS-10 gives a strong Raman shift band between 724 and 840 cm , arising from a longitudinal vibrational mode of the -Ti-O-Ti-Ochain. Its frequency at the band maximum (nmax), its bandwidth (full width at half maximum, fwhm), and intensity (I) reflect the relative average length, length homogeneity, and density of the quantum wire, respectively. The Raman band frequency decreases as the length increases, owing to the increase in the reduced mass of the quantum wire. The smallest frequency ever observed is 724 cm . Bandwidths between 23 and 120 cm 1 have been observed, and the bandwidth decreases as the length uniformity increases. The intensity increases as the number of the TiO3 2 quantum wire increases. Accordingly, the frequency, bandwidth, and intensity have served as the three important criteria for comparison of the relative average lengths, relative average length uniformities, and relative average densities of the TiO3 2 quantum wires in the ETS-10 crystals. This information indicates that we can also apply the same principle to obtain their distribution pattern within an ETS-10 crystal if we can obtain a matrix of Raman spectra measured from a large number of artificially divided very small sections of a crystal. Furthermore, the obtained data would be more informative if we can obtain a map of these three data sets for the TiO3 2 quantum wires running along the [110] and [110] directions, respectively. We now report that laser scanning confocal polarized micro-Raman (LSC-PMR) spectroscopy is a highly useful tool for the above purpose and the novel fact that the TiO3 2 quantum wires are not evenly distributed within ETS-10 crystals but distributed in a symmetrical manner according to an interesting pattern. Figure 1. a) Illustrations of a typical morphology (truncated bipyramid) of an ETS-10 crystal and three-dimensional networks of SiO2 channels (cyan) and TiO3 2 quantum wires (red) in the case of polymorph B and b) a single TiO3 2 quantum wire.