Quantum Size Confinement Research Articles

Enhanced photoluminescence from porous silicon by hydrogen-plasma etching

Porous silicon (PS) was etched by hydrogen plasma. On the surface a large number of silicon nanocone arrays and nanocrystallites were formed. It is found that the photoluminescence of the H-etched porous silicon is highly enhanced. Correspondingly, three emission centers including red, green, and blue emissions are shown to contribute to the enhanced photoluminescence of the H-etched PS, which originate from the recombination of trapped electrons with free holes due to SiO bonding at the surface of the silicon nanocrystallites, the quantum size confinement effect, and oxygen vacancy in the surface SiO2 layer, respectively. In particular, the increase of SiOx(x<2) formed on the surface of the H-etched porous silicon plays a very important role in enhancing the photoluminescence properties.

Size-Dependent Dissociation pH of Thiolate Ligands from Cadmium Chalcogenide Nanocrystals

A method, pseudo steady-state titration, is introduced for determining the precipitation pH of nanocrystals coated by electron-donating ligands. CdSe nanocrystals coated with hydrophilic deprotonated thiol (thiolate) ligands were studied systematically. For comparison, CdTe and CdS nanocrystals coated with the same types of ligands were also examined. The results show that the precipitation of the nanocrystals is caused by the dissociation of the nanocrystal-ligand coordinating bonds from the nanocrystal surface. The ligands are removed from the surface due to protonation in a relatively low pH range, between 2 and 7 depending on the size, approximately within the quantum confinement size regime, and chemical composition (band gap) of the nanocrystals. In contrast, the redispersion of the nanocrystals was found to be solely determined by the deprotonation of the ligands. The size-dependent dissociation pH of the ligands was tentatively used as a means for determining the size-dependent free energy associated with the formation of a nanocrystal-ligand coordinating bond.

Unaggregated silicon nanocrystals obtained by ball milling

We have investigated the formation of silicon nanocrystals (Si-nc) ex situ of the silicon dioxide host matrix as well as their structural and optical properties. Two different methods were used: Si-nc prepared by electrochemical etching as reference and Si-nc fabricated by ball milling. We show that both methods are able to prepare Si-nc with quantum confinement size (<10 nm). The Si-nc fabrication ex situ of host matrix allows investigating additional properties in colloidal ethanol dispersion by light scattering. The identification of Si-nc in the ethanol-based colloids has been performed by Malvern autosizer experiments. We show the clustering effect of Si-nc and as well as declustering step by adding ammonia to ethanol-based colloids. Reached Si-nc sizes from light scattering experiment corresponds to that evaluated by other independent techniques. Furthermore, we have prepared spin on glass-based self-supporting samples with high Si-nc concentration layers which allowed PL analysis. In all cases visible photoluminescence at room temperature was detected. However, for Si-nc prepared by ball milling a weak PL was detected, which likely originates rather from surface states then quantum confinement size effect.

Titania Nanoparticles Prepared with Pulsed Laser Ablation of Rutile Single Crystals in Water

The pulsed laser ablation (PLA) experiments of rutile single crystal surfaces were carried out in water solution to prepare nanosized titania particles. The solvated PLA species were transparent as produced and changed to a lighter blue solution with some precursors for filamentous species (in several days), then finally changed to white enlarged filamentous species (in 2−4 weeks). The solvated PLA species were measured as uniform nanoparticles with a size below 10 nm by TEM measurements and showed the absorption−photon energy relation for the direct transition with a band gap of 5.3−5.5 eV. The band-gap values were elucidated with quantum confinement size effects. It was assumed that the primary solvated species should have a size of about 1 nm and they were agglomerated to be the secondary species. The Mie scattering is responsible for the “blue” color, which proves that the size enlargement process exists on the PLA species in water. The filamentous species are composed of mainly the anatase form, which was analyzed with Raman spectroscopy. The XPS results indicate that the Ti species are tetravalent and bonded to oxygen atoms with O 1s binding energy at around 530 eV as TiO2. They were found to be composed to about 1 μm diameter with gathering nanosized particles on SEM pictures. The thin films composed of the PLA filamentous species showed no band gap increase. The process for PLA of rutile in water was studied with different temperatures (T in K) to elucidate the effect of viscosity (η). The yield of PLA species in water decreases nearly linearly with the ratio of T/η representing the diffusion coefficient of the solvated species. The result implies that the PLA species are confined in a media not diffused into water solution.

Lithium Storage in Nanostructured TiO2 Made by Hydrothermal Growth.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Growth and characterization of single-crystal ZnSe nanorods via surfactant soft-template method

Stoichiometric ZnSe nanorods have been successfully synthesized through a simple and convenient surfactant-assisted reaction. The microstructures and the chemical compositions of the as-synthesized nanorods have been investigated by means of scanning electron microscope (SEM), transmission electron microscope (TEM), the energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results reveal that the as-synthesized materials selecting sodium laurylsulfonate (SDS) as surfactant consist of ZnSe nanorods with diameters and lengths ranging in 16–20 nm and 120–270 nm, respectively. Selected area electron diffraction (SAED) pattern shows the single-crystal nature of as-synthesized ZnSe nanorods. The room temperature photoluminescence (PL) measurements show a very broad peak centered at 453 nm (2.74 eV), which is attributed to the quantum size confinement effect of the nanorods.

Lithium Storage in Nanostructured TiO2 Made by Hydrothermal Growth

Nanostructured titania materials were prepared from TiCl4 via autoclaving in 10 M NaOH at 250 °C and subsequent treatment in aqueous and/or acidic media. XRD and Raman spectroscopy evidenced the presence of anatase as the main phase, but most materials contained also some XRD-silent component. Cyclic voltammograms of lithium insertion demonstrate two pairs of reversible pseudocapacitive peaks (S-peaks) in addition to the ordinary peaks of diffusion-controlled Li insertion into the anatase lattice. The occurrence of S-peaks is associated with the nanosheet- and/or nanotubular morphology of the materials. This structure developed at hydrothermal conditions via exfoliation of the layered Na+/H+ titanate precursors. The S-peaks were suggested to be the signatures of quantum-size confinement in titania nanosheets. The nanosheet-containing materials are reducible by n-butyllithium to cubic LiTiO2 (at conditions when the ordinary nanocrystalline anatase gives only the orthorhombic Li0.5TiO2). Consequently, the e...

Growth of group II–VI semiconductor quantum dots with strong quantum confinement and low size dispersion

AbstractCdTe quantum dots embedded in glass matrix are grown using two‐step annealing method. The results for the optical transmission characterization are analysed and compared with the results obtained from CdTe quantum dots grown using conventional single‐step annealing method. A theoretical model for the absorption spectra is used to quantitatively estimate the size dispersion in the two cases. In the present work, it is established that the quantum dots grown using two‐step annealing method have stronger quantum confinement, reduced size dispersion and higher volume ratio as compared to the single‐step annealed samples. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High‐Quality Ultra‐Fine GaN Nanowires Synthesized Via Chemical Vapor Deposition

GaN nanowires with diameters in the quantum‐confinement size regime (4–10 nm, ∼10 nm in Figure) are prepared on large‐area substrates through catalytic reaction of Ga and NH3 in a hot‐filament chemical vapor deposition system. The synthetic method is reproducible and could be applied to the growth of other semiconductor nanostructures.

The Influence of Matrix on Quantum Size Confinement of Semiconductor Microcrystals Doped in Glass Thin Films Prepared by RF-Sputtering

Various types of semiconductor microcrystal-doped glass thin films were successfully prepared by RF-sputtering. The blue shift of the absorption edge in the visible light region, caused by the quantum confinement effect, was observed in all the thin films. The amount of the blue shift was found to depend significantly not only on the types of semiconductor dopants and the microcrystal radius but also on the type of matrix. The blue shift of semiconductor microcrystals with a small exciton Bohr radius compared with the microcrystal radius, such as that of CuCl, showed an exciton confinement effect which was theoretically predicted. On the other hand, the blue shift of microcrystals with a large Bohr radius compared with the microcrystal radius, such as that of CdSe, has been expected to follow the electron–hole independent confinement model, but it was found to deviate significantly from the theoretical curve. In order to explain this deviation, we introduced factors, such as the Coulomb interaction between electron and hole, size-dependent dielectric constants of the dopants and the dielectric constant of matrix. Consequently, it was found that the matrices have a significant influence on the electron–hole independent confinement.

Variation of Raman feature on excitation wavelength in silicon nanowires

A variation of Raman feature on excitation wavelength in silicon nanowires was observed. Based on the quantum size confinement effect, the resonant Raman scattering phenomenon and the existence of different sizes of Si grains in the samples, a plausible mechanism to interpret the novel feature was proposed and supported by experimental facts.

IMPACT OF WIDE BANDGAP P-TYPE NC-SI ON THE PERFORMANCE OF A-SI SOLAR CELLS

This paper reports the impact of a wide bandgap p-type hydrogenated nanocrystalline silicon (nc-Si:H) on the performances of hydrogenated amorphous silicon (a-Si:H) based solar cells. The p-layer consists of nanometer-sized Si Crystallites and has a wide effective bandgap determined mainly by the quantum size-confinement effect (QSE). By incorporation of this p-layer into the devices we have obtained high performances of a-Si:H top solar cells with V oc = 1.045 V and FF = 70.3%, and much improved mid and bottom a-SiGe:H cells, deposited on stainless steel (SS) substrate. The effects of the band-edge mismatch at the p/i-interface on the I-V characteristics of the solar cells are discussed on the bases on the bases of the density-functional approach and the AMPS model.

Quantum-size confinement of excitons in CuBrnanocrystals embedded in a polymer

Quantum-size confinement of excitons in CuBr nanocrystals embeddedin polymethyl methacrylate has been studied. Both the absorptionband and the photoluminescence (PL) band of the Z1,2 freeexcitons in the nanocrystals show a blue-shift due to a quantum-sizeeffect, and the Stokes shift of the PL band becomes larger withdecrease in the nanocrystal size at 77 K. These blue-shifts areinterpreted by application of size quantization in the excitondispersion on the basis of Cho's k-linear theory.

Si-rich/SiO 2 nanostructured multilayers by reactive magnetron sputtering

Silicon-rich (SR)/SiO2 multilayered systems were produced by reactive magnetron sputtering, through an approach based on the ability of hydrogen, when alternatively mixed to the argon of the plasma, to reduce the oxygen originated from the SiO2 target. Optimum values of both hydrogen partial pressure (45 mTorr) and deposition temperature (500 °C) have led to the highest incorporation of Si in the SR layer which crystallizes after annealing. The SR/SiO2 superlattices grown with such conditions showed that the size of the Si nanocrystals is limited by the thickness of the SR layer. Considering the difference observed between the photoluminescence peak position and the predicted band gap for Si nanocrystals, the results suggest that we deal with a quantum-size confinement assisted by a Si–O vibration located at the interface.

Photoluminescence of Ge quantum dots prepared on porous silicon by ultrahigh vacuum chemical vapor deposition

This letter reports a way of preparing Ge quantum dots on anodized porous silicon layers by ultrahigh vacuum chemical vapor deposition at a low temperature of 720 °C. The porous silicon was formed by anodic conversion of p-type (100) oriented crystalline silicon in hydrofluoric acid diluted by alcohol. A clear phonon-resolved photoluminescence (PL), as a no-phonon (NP) and its transverse acoustic phonon replica, was observed from the Ge dots at the temperature of 10 K. The blueshift energy is as high as about 136 meV, but the full width at half maximum of the NP PL spectrum is only 1.23 meV. We attributed the very large blueshift in energy of the PL peak to quantum size confinement effect of the Ge quantum dots.

Luminescence enhancement of ZnS:Mn nanoclusters in zeolite

The luminescence intensity of Mn2+ in the ZnS:Mn nanoclusters formed in an ultrastable zeolite-Y is seven orders of magnitude stronger than that of other nanoparticles deposited out of solutions. This remarkable effect is attributed to a strong quantum size confinement, the location of Mn2+ ions at the near-surface sites, and good surface passivation. The lowering of the energy loss in the E4(D4 or/and T24(D4) states during the energy transfer from the ZnS host to the emitting state T14(G4) of Mn2+ may also contribute to the observed fluorescence enhancement.

Light-emission properties in nanocrystalline BaTiO3

This letter reports photoluminescence properties of nanocrystalline BaTiO3. Two intense emission bands centered at 543 and 694 nm and a weak one at 574 nm have been observed at room temperature. The change of luminescence peak with different heat treatments correlates to the evolution of Ti4+ defects in the nanocrystalline BaTiO3 matrice. The light-emission mechanism is explained within the framework of self-trapped excitons by combining quantum-size confinement and dielectric confinement effects.

Luminescence enhancement of EuS nanoclusters in zeolite

Large luminescence enhancement was observed in EuS clusters formed in ultrastable zeolite-Y by solid-state diffusion in vacuum at 600 °C. The emission of EuS clusters shifts to higher energies relative to that of bulk EuS powder. The blueshift and the emission enhancement are attributed to quantum-size confinement and are discussed based on the magnetic exciton and the magnetic polaron models, respectively.

Quantum-size shift in PbTe/BaF2MQW structure photoluminescence spectra

PbTe/BaF2multi-quantum-well (MQW) structures were grown on cleaved (111) BaF2substrates by the MBE method. The substrate temperature was 330°C. The MQWs consist of 10 pairs of 10 nm thickness BaF2barriers and PbTe wells with widths ranging from 5 to 20 nm. For this highly mismatched system we have measured photoluminescence spectra at 77K and have demonstrated the quantum-size confinement consistent with the model of wells with infinite barriers. A remarkable deformation shift due to the thermal expansion coefficient difference is observed at cryogenic temperatures.

Some interesting behaviors observed in nanostructured magnetic multilayers with alloy spacers

This paper summarizes our recent experimental results on investigating magnetic multilayers with alloy spacers. The main results are: the doping of Cu, Al, and Ag into a Pt layer and Cu into a Pd layer result in the polar Kerr rotation enhancement and oscillation with the increasing doping density in PtCu(Al, Ag)/Co and PdCu/Co multilayers. In the dilute PtNi alloy/Co multilayers, the Curie temperature of the multilayers increases monotonically with increasing of the Ni concentration in the Pt spacer layer and the saturation magnetization shows an oscillation-like behavior with the increase of the Ni concentration in the Pt spacer at room temperature. The temperature dependence of the saturation magnetization and effective perpendicular anisotropy in the dilute PtNi alloy/Co multilayers also shows some interesting results, i.e. the broad peaks of the magnetization and anisotropy appear nearly at the same temperature and both peak values show a simultaneous oscillation-like behavior as the Ni concentration increases. Within the framework of quantum confinement size, the oscillation of the polar Kerr rotation has been discussed. On the basis of the RKKY interaction we have calculated the exchange coupling between Co-Ni and Ni-Ni atoms and found that the total coupling energy is responsible for the elevation of the Curie temperature for the samples with Ni doping. The calculation also indicates that the oscillations of the magnetization may arise from Fermi vector reduction. Using the mean field theory we have discussed the temperature dependence of the magnetization and anisotropy in the dilute PtNi alloy/Co multilayers.