Showing papers on "Potential well published in 2007"
TL;DR: In this article, the authors showed that quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron-hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays.
Abstract: Semiconductor quantum dots may be used in so-called third-generation solar cells that have the potential to greatly increase the photon conversion efficiency via two effects: (1) the production of multiple excitons from a single photon of sufficient energy and (2) the formation of intermediate bands in the bandgap that use sub-bandgap photons to form separable electron–hole pairs. This is possible because quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays. For exciton multiplication, very high quantum yields of 300–700% for exciton formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap), respectively, indicating the formation of 3–7 excitons/photon, depending upon the photon energy. For intermediate-band solar cells, quantum dots are used to create the intermediate bands from the con fined electron states in the conduction band. By means of the intermediate band, it is possible to absorb below-bandgap energy photons. This is predicted to produce solar cells with enhanced photocurrent without voltage degradation.
222 citations
TL;DR: In this paper, the intrinsic defect states of ZnS nanoparticles were attributed to the intrinsic defects states of the nanoparticles and three peaks in the range of 390nm, 480nm and 525nm were observed.
Abstract: Thin films of ZnS: Cu nanoparticles were deposited in chemical bath by a pH controlled solution synthesis technique. The copper concentration was varied from 0 to 0.1M%. XRD and SEM indicated variations in diffracted intensity and morphology with Cu concentration. The PL spectrum of the undoped ZnS nanoparticles showed emission peaks at 393 and 432nm that could be attributed to the intrinsic defect states of ZnS nanoparticles. For ZnS: Cu samples three peaks in the range of 390nm, 480nm and 525nm were observed. With increase in Cu concentration from 0.001 to 0.1M%, the peak position of 480nm and 525nm did not change, whereas 390nm peak red shifted to longer wavelength region to 422nm. In addition, it was found that the overall photoluminescence intensity reached maximum at 0.01M% and quenched with further increase in Cu concentration. Enhancement of blue and green light emission by seven and twenty fivefold respectively compared to undoped ZnS was observed in ZnS: Cu with optimal dopant concentration. Time resolved decay of photoluminescence showed faster decay for 390 – 420nm purple/ blue emission compared to green (525nm) Cu related emission which is in the microsecond time scale. Optical absorption measurements indicate enhancement of band gap (3.89eV) for undoped ZnS suggesting the quantum confinement effect in the developed nanoparticles of size below the Bohr diameter. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
131 citations
TL;DR: In this paper, a chemical etching of microscale 3C-SiC grains and ultrasonic vibration was used to create a weakly interconnected nanostructure network and subsequent ultrasonic vibrations crumbled the interconnected network, forming small-size 3CSiC nanocrystals.
Abstract: Up to now, it is still a great challenge to obtain bulk quantities of luminescent 3C-SiC nanocrystals with sizes smaller than 10 nm, which have quantum confinement effect. We report in this paper on the fabrication of 3C-SiC nanocrystals via a chemical etching of microscale 3C-SiC grains and ultrasonic vibration. The sizes of the as-prepared 3C-SiC nanocrystals are smaller than 6.5 nm and have a centric distribution with the maximal probability of 3.6 nm. Due to the quantum confinement effect, the suspension of the 3C-SiC nanocrystals exhibits a tunable photoluminescence (PL), which is visible with the naked eye. As the excitation line increases from 260 to 480 nm, the PL peak position changes from 420 to 512 nm. Spectral analysis and microstructural observations show that the chemical etching leads to the formation of a weakly interconnected nanostructure network in the large 3C-SiC grains and subsequent ultrasonic vibration crumbles the interconnected network, forming small-size 3C-SiC nanocrystals.
92 citations
TL;DR: In this paper, the authors observed a quantum-confinement effect in individual Ge1−xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature.
Abstract: The authors observed a quantum-confinement effect in individual Ge1−xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ∼1.5eV with a decrease in QD diameter from 35to4nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two dimensionality in density of states of quasi zero-dimensional QDs with the QD flattening.
79 citations
TL;DR: In this article, X-ray diffraction and selected area electron diffraction studies confirmed the formation of nanocrystalline cubic phase of ZnS in the films, although the target material was hexagonal znS and the particle size, calculated from the XRD patterns of the thin films was found in the range 2.06-4.86-nm.
70 citations
TL;DR: The optical properties of silicon quantum dots (QDs) embedded in a SiO2 matrix are investigated at various temperatures using photoluminescence (PL) and time-resolved photolumininescence as discussed by the authors.
Abstract: The optical properties of silicon quantum dots (QDs) embedded in a SiO2 matrix are investigated at various temperatures using photoluminescence (PL) and time-resolved photoluminescence. Two broad luminescence bands, the S-band located at 600–850 nm and the F-band located at 450–600 nm, are observed. In the S-band a stretched exponential time evolution is observed and the short wavelengths have significantly shorter lifetimes than the long wavelengths. In the low temperature regime, the process of carrier delocalization from the defect states and capture into the QDs is dominant, which results in a decrease in PL intensity from the F-band and an increase from the S-band. In the high temperature regime, the carriers captured into the QDs decrease due to competition between the defect states, which results in a PL intensity decrease for both bands. The PL intensity on the high energy side of the S-band decreases more strongly than that on the low energy side due to the state filling effect, which results in a 30 nm red shift. The S-band is attributed mainly to zero-phonon electron–hole recombination due to enhancement of the quantum confinement effect. The F-band has a single exponential evolution with a much shorter lifetime of nanoseconds and is attributed to defect states of silicon oxide.
68 citations
TL;DR: It is suggested that the impact of the particle-size distribution accounts for the discrepancy between the energy-gap values derived from VEELS of single QDs and from optical methods of ensembles of QDs.
65 citations
TL;DR: In this paper, the main spectral feature of ultraviolet photoluminescence is attributed to the recombination of free exciton (FX) and the first surface phonon replica of FX emission.
Abstract: Crystalline ZnO nanoparticles with nearly uniform size were studied using photoluminescence and Raman spectroscopy. The main spectral feature of ultraviolet photoluminescence is attributed to the recombination of free exciton (FX) and the first surface phonon replica of FX emission. The energy of FX emission shows a clear blueshift as the size of nanoparticles decreases, indicating that the quantum confinement effect exists in the electronic structure of nano-ZnO, although no confinement effect on the vibrational modes has been found in the same series of samples.
50 citations
TL;DR: The evolution of the gap of a nanoscaled insulator material, namely, Gd(2)O(3), has been observed by means of vacuum ultraviolet excitation spectra of a dopant and shows that, in spite of the strong ionic character of the material, the amount of covalency is important enough for creating a significant delocalization of the electron with regard to its hole.
Abstract: The evolution of the gap of a nanoscaled insulator material, namely, Gd(2)O(3), has been observed by means of vacuum ultraviolet excitation spectra of a dopant (Eu(3+)). The nanoparticles have been synthesized by the low energy cluster beam deposition technique and grown afterward by different annealing steps. A gap shift towards the blue is observed, similar to what is observed in semiconductor nanoparticles. Despite the strong ionic character of the material, the evolution exhibits a behavior similar to covalent materials. The evolution of the gap for Gd(2)O(3) follows the same empiric rule that has been derived for semiconductors (ZnO, CuBr, Si, and CdS). It shows that, in spite of the strong ionic character of the material (0.9 on the scale of Phillips), the amount of covalency is important enough for creating a significant delocalization of the electron with regard to its hole.
47 citations
TL;DR: In this article, the excited state properties of allylamine-capped silicon quantum dots (SiQDs) from Si10 to Si59 are studied using a density-functional tight-binding method and compared with available experimental data.
Abstract: Excited-state properties of allylamine-capped silicon quantum dots (SiQDs) from Si10 to Si59 are studied using a density-functional tight-binding method and compared with available experimental data. Signatures in vibrational and optical absorption spectra are revealed, which show the detailed effect of modification of the SiQDs with allylamine. It is verified that the modification could be expected to not only reduce the surface oxidation rate but also maintain an efficient electronic transition feature that facilitates blue emission. The optical properties show significant size dependence due to the quantum confinement effect. The increase in the number of allylamine molecules could only result in a slight red shift of emission spectra.
41 citations
TL;DR: In this article, the role of quantum confinement effect and surface passivation of nanograins in optical properties is examined in detail, and the coupling between surface vibrations and fundamental gap Eg as well as the increase of interaction between them at the strong confinement regime (⩽2nm) are proposed for the observable pinning of Eg in luminescence measurements.
Abstract: We report on a spectroscopic study of very thin nanocrystalline silicon films varying between 5 and 30nm. The role of quantum confinement effect and surface passivation of nanograins in optical properties are examined in detail. The coupling between surface vibrations and fundamental gap Eg as well as the increase of interaction between them at the strong confinement regime (⩽2nm) are proposed for the observable pinning of Eg in luminescence measurements.
TL;DR: In this paper, the exciton-longitudinal-optical-phonon (LO phonon) interaction was observed to decrease with reducing ZnO particle size to its exciton Bohr radius (aB).
Abstract: The exciton-longitudinal-optical-phonon (LO-phonon) interaction was observed to decrease with reducing ZnO particle size to its exciton Bohr radius (aB). The unapparent LO-phonon replicas of free exciton (FX) emission and the smaller FX energy difference between 13 and 300K reveal decreasing weighting of exciton-LO phonon coupling strength. The diminished Frohlich interaction mainly results from the reducing aB with size due to the quantum confinement effect that makes the exciton less polar.
TL;DR: In this paper, the structure and composition of the as-prepared nanocrystals were confirmed by scanning electron microscopy, X-ray diffraction, transmission electron microscope and energy dispersive x-ray spectroscopy.
TL;DR: In this paper, the interaction of two-dimensional quasiparticles characterized by a linear dispersion E = ±u|p| (graphene) with impurity potentials is studied.
Abstract: The interaction of two-dimensional quasiparticles characterized by a linear dispersion E = ±u|p| (graphene) with impurity potentials is studied. It is shown that discrete levels corresponding to localized states are present in a one-dimensional potential well (quantum wire), whereas such states are absent in a two-dimensional well (quantum dot). The cross section for the scattering of electrons (holes) of graphene by an axially symmetric potential well is determined. It is shown that the cross section tends to a constant value in the limit of infinite particle energy. The effective Hamiltonian is derived for a curved quantum wire of graphene.
TL;DR: In this paper, photoluminescence in visible range of spectra from nanohills formed on a surface of Ge single crystal by JAG:Nd laser radiation was found.
Abstract: Photoluminescence in visible range of spectra from nanohills formed on a surface of Ge single crystal by JAG:Nd laser radiation was found. This photoluminescence is explained by quantum confinement effect in quantum dots on a top of nanohills. Red shift of 300 cm–1 line micro-Raman back scattering spectrum is an evidence of this suggestion. The calculation of QDs diameter using band gap shift photoluminescence spectra and peak frequency in micro-Raman spectra as a function of diameter for spherical Ge particles gives diameters of nano-balls on the top of nanohills is 4 nm and 6 nm. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, a blueshift of the photocurrent features has been detected by reducing the Ge dot size, interpreted as due to quantum confinement effect, which suggests that Ge dots could be applied in photovoltaic nanodevices and quantum dot based lasers.
Abstract: Photocurrent generation of nanometric Ge dots has been investigated by using electrochemical measurements. Photocurrent features have been ascribed, for large Ge dots, to Ge bulk direct electronic transitions at L and X points as evidenced by their close correspondence with the optical absorption coefficient. A blueshift of the photocurrent features has been detected by reducing the Ge dot size. These changes have been interpreted as due to quantum confinement effect. This result suggests that Ge dots could be applied in photovoltaic nanodevices and quantum dot based lasers.
TL;DR: In this article, the authors report clear crystallite size dependencies of the transport and phototransport properties in solid-state ensembles of semiconductor quantum dots and show that the above dependencies are associated with the quantum confined induced variation of the band gap in the individual dots.
Abstract: We report clear crystallite size dependencies of the transport and phototransport properties in solid-state ensembles of semiconductor quantum dots. By finding a Meyer-Neldel-like behavior for the former and by comparing the experimental results with computer simulations for the latter, we show that the above dependencies are associated with the quantum confined induced variation of the band gap in the individual dots. These findings go beyond the available knowledge of interparticle conduction mechanisms by providing a basis for the corresponding physical statistics of such quantum dot ensembles.
TL;DR: In this paper, a single exciton eigenstate in a spherical quantum shell and its linear optical absorption was analyzed using the parabolic band model and a matrix diagonalization method.
Abstract: We present the analysis of a single exciton eigenstate in a spherical quantum shell and its linear optical absorption by using the parabolic band model and a matrix diagonalization method. The result shows that the confinement effect for an exciton in a spherical quantum shell structure is stronger than that in a spherical quantum dot structure, in general. This attributes mainly to the exclusion of $2s$ single particle state in forming an exciton state in a quantum shell. The emission spectra from an exciton in a quantum shell would show blueshifts compared to those in a quantum dot. The eigenlevels of an exciton state in a quantum shell are much denser than those in a quantum dot. We found that the total absorption is proportional to the volume of the structure, as well.
TL;DR: In this paper, it was shown that as the diameter and length of Bi nanorods decrease below ∼10 and ∼35 nm, respectively, the quantum confinement effect drastically increases.
Abstract: From the electron energy loss spectroscopy (EELS) study of plasmon excitation of Bi nanorods, we find that as the diameter and length of Bi nanorods decrease below ∼10 and ∼35 nm, respectively, the quantum confinement effect drastically increases. Thus, such Bi nanorods would be extremely useful for thermoelectric devices and applications.
TL;DR: In this paper, the authors used cross-sectional scanning tunneling microscopy to examine strain relaxation profiles of InAs quantum dots with In0.33Ga0.67As layers overgrown by three distinct capping methods.
Abstract: The authors have used cross-sectional scanning tunneling microscopy to examine strain relaxation profiles of InAs quantum dots with In0.33Ga0.67As layers overgrown by three distinct capping methods. A statistical analysis of strain relaxation profile allowed them to infer that the long wavelength emission (>1.3μm) of InAs quantum dots capped with sequential GaAs∕InAs binary growth is mainly due to a weaker quantum confinement effect. This particular capping method is better than the traditional molecular beam epitaxy with simultaneous In∕Ga∕As deposition, and much better than a capping method with separated Ga deposition followed by As and InAs growth.
Journal Article•
TL;DR: The influence of varying etching time in the anodizing solution on structural and optical properties of porous silicon has been investigated in this article, where it is observed that pore size increases with the etch time and reaches maximum for 20 minutes and then decreases.
Abstract: Porous silicon layers have been prepared from n-type silicon wafers of (100) orientation. SEM, FTIR and PL have been used to characterize the morphological and optical properties of porous silicon. The influence of varying etching time in the anodizing solution, on structural and optical properties of porous silicon has been investigated. It is observed that pore size increases with etching time and attain maximum for 20 minutes and then decreases. The PL spectrum peak shifts towards the higher energy side, which supports the quantum confinement effect in porous silicon. The FTIR shows that the Si-Hn peaks are observed at the surface of the PS layer and these chemical species also give raise the PL in PS.
TL;DR: In this article, the energy levels of the electron and hole states of the Mn-doped ZnO quantum wires (x=0.0018) were investigated in the presence of the external magnetic field.
Abstract: Based on the effective-mass model and the mean-field approximation, we investigate the energy levels of the electron and hole states of the Mn-doped ZnO quantum wires (x=0.0018) in the presence of the external magnetic field. It is found that either twofold degenerated electron or fourfold degenerated hole states split in the field. The splitting energy is about 100 times larger than those of undoped cases. There is a dark exciton effect when the radius R is smaller than 16.6 nm, and it is independent of the effective doped Mn concentration. The lowest state transitions split into six Zeeman components in the magnetic field, four sigma(+/-) and two pi polarized Zeeman components, their splittings depend on the Mn-doped concentration, and the order of pi and sigma(+/-) polarized Zeeman components is reversed for thin quantum wires (R < 2.3 nm) due to the quantum confinement effect.
TL;DR: In this paper, double-step annealing has been used to grow quantum dots of CdSxSe1−x embedded in a borosilicate glass matrix.
Abstract: Quantum Dots of CdSxSe1−x embedded in borosilicate glass matrix have been grown using Double-Step annealing method. Optical characterization of the quantum dots has been done through the combinative analysis of optical absorption and photoluminescence spectroscopy at room temperature. Decreasing trend of photoluminescence intensity with aging has been observed and is attributed to trap elimination. The changes in particle size, size distribution, number of quantum dots, volume fraction, trap related phenomenon and Gibbs free energy of quantum dots, has been explained on the basis of the diffusion-controlled growth process, which continues with passage of time. For a typical case, it was found that after 24 months of aging, the average radii increased from 3.05 to 3.12 nm with the increase in number of quantum dots by 190% and the size-dispersion decreased from 10.8% to 9.9%. For this sample, the initial size range of the quantum dots was 2.85 to 3.18 nm. After that no significant change was found in these parameters for the next 12 months. This shows that the system attains almost a stable nature after 24 months of aging. It was also observed that the size-dispersion in quantum dots reduces with the increase in annealing duration, but at the cost of quantum confinement effect. Therefore, a trade off optimization has to be done between the size-dispersion and the quantum confinement.
TL;DR: In this paper, the authors investigated the properties of the conveyance of quantum particles by a moving potential well and found that the tunneling effect reduces the number of particles carried up.
Abstract: The properties of the conveyance of quantum particles by a moving potential well are studied. We sweep a bell-shape potential well with constant velocity, and study how the potential well traps the particle. In particular, we study the case where we suddenly change the velocity up to a constant velocity. We investigate the number of particles conveyed as a function of the sweep velocity. The effect of the discretization of space in the numerical method is also investigated. We also study how the potential well carries the particle up from a region where the potential energy is low, to another region of high potential energy. Here, we find that the tunneling effect reduces the number of particles carried up. This process is well described by the Landau–Zener mechanism. The dynamics of the density of particles under the sweeping potential well is also studied in noninteracting Fermi particle systems, where the position of the Fermi level plays an important role.
TL;DR: In this paper, the donor/acceptor doping in ZnO quantum dots (QDs) grown by a metal-organic chemical vapor deposition method was reported, with a combination of valence band XPS and scanning tunneling microscopy.
Abstract: The authors report on donor/acceptor doping in ZnO quantum dots (QDs) grown by a metal-organic chemical vapor deposition method. The Ga donor and N acceptor, as identified by x-ray photoelectron spectroscopy (XPS), are introduced into ZnO QDs. They demonstrate, with a combination of valence band XPS and scanning tunneling microscopy, that the electrical properties as well as Fermi level of the ZnO QDs can be well tuned by the donor/acceptor doping. In addition, photoluminescence from the ZnO QDs with quantum confinement effect is observed.
TL;DR: In this paper, the size of InGaN quantum dots can be well controlled by SiNx nanomasks, enabling the manipulation of quantum confinement effect, which is very useful for the application of optoelectronic devices.
Abstract: InGaN quantum dots (QDs) deposited on SiNx nanomasks have been investigated by atomic force microscopy, photoluminescence (PL), and photoluminescence excitation (PLE) measurements It was found that the size of QDs can be well controlled by SiNx nanomasks, enabling the manipulation of quantum confinement effect The PL spectra of InGaN QDs contain several fine structures, and the main peaks can be attributed to families of QDs with different sizes The emission arising from InGaN QDs and GaN buffer layer can be clearly distinguished based on PLE measurement, which can be used to improve the interpretation in the previous reports Our study indicates that the quantum confined Stark effect due to piezoelectric field plays a very important role in the optical properties of InGaN QDs, which is very useful for the application of optoelectronic devices
TL;DR: In this article, photoluminescence was measured in one single sample with an excitation wavelength selectable within 360-420 nm, and the typical decay time was found to be below 3.0 ns.
Abstract: Silicon particles approaching the size of 1 nm were grown along with the confining SiC films by employing a low-temperature chemical vapor deposition procedure. The resulting amorphous composite structure enables an experimental study of the quantum confinement effect in extremely narrow potential wells, as exemplified here by photoluminescence measurement. Owing to the enhanced energy fluctuation for such small particles, strong photoluminescence centered at 450–540 nm, and of comparable profiles, was measured in one single sample with an excitation wavelength selectable within 360–420 nm. Moreover, the typical decay time was found to be below 3.0 ns. These properties hold promise for the fabrication of wide-spectrum photoreceptors, ultraviolet-light detectors, and other optoelectronic devices.
TL;DR: In this paper, the authors studied the optical properties of Si nanohills formed on the SiO2/Si interface by the pulsed Nd:YAG laser radiation and found a new method for graded band gap semiconductor formation using an elementary semiconductor.
Abstract: The aim of this work is to study optical properties of Si nanohills formed on the SiO2/Si interface by the pulsed Nd:YAG laser radiation. Nanohills which are self-organized on the surface of Si, are characterized by strong photoluminescence in the visible range of spectra with long wing in the red part of spectra. This peculiarity is explained by Quantum confinement effect in nanohillsnanowires with graded diameter. We have found a new method for graded band gap semiconductor formation using an elementary semiconductor. Graded change of band gap arises due to Quantum confinement effect.
01 Jun 2007
TL;DR: In this paper, a single crystalline nature and microstructure of a single cupric oxide (CuO) nanowire was investigated using confocal Raman spectroscopy.
Abstract: Spatially well separated cupric oxide (CuO) nanowires grown using thermal oxidation method were examined to directly observe the scale structure evolution of lattice vibration modes. Scanning electron microscopy and high-resolution transmission electron microscopy studies revealed the single crystalline nature and microstructure of a single CuO nanowire. Phonon spectral evolution along the wire axis was investigated using confocal Raman spectroscopy by scanning a single nanowire. The Ag, Bg(1), and Bg(2) phonon modes of CuO that are quantum-confined in radial directions of the thin nanowire evidenced from the observed systematic red-shift, broadening of the peak profile. These results can be well explained based on the phonon confinement model.
TL;DR: In this article, it is proposed that nucleation and growth of bubbles and particles in the chemical reaction, and their kinetics and interactions are responsible for the formation of nanotubes.
Abstract: CdS nanotubes with wall thickness comparable to excitonic diameter of the bulk material are synthesized by a chemical route. A change in experimental conditions result in formation of nanowires, and well-separated nanoparticles. The diameter and wall thickness of nanotubes measured to be 14.4 $\pm$ 6.1 and 4.7 $\pm$ 2.2 nm, respectively. A large number of CdS nanocrystallites having wurzite structure constitute these nanotubes. These nanotubes show high energy shifting of optical absorption and photoluminescence peak positions, compared to its bulk value, due to quantum confinement effect. It is proposed that nucleation and growth of bubbles and particles in the chemical reaction, and their kinetics and interactions are responsible for the formation of nanotubes.