Terms Of Quantum Confinement Research Articles

Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensionalMoS2

The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly in- vestigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the elec- tronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low energy valence electrons in the freestanding MoS$_2$ layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4$\pm$0.2 eV in monolayer MoS$_2$. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer MoS$_2$ systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation, for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement.

Analysis of EL Emitted by LEDs on Si Substrates Containing GeSn/Ge Multi Quantum Wells as Active Layers

We analyzed multi quantum well light emitting diodes, consisting of ten alternating GeSn/Ge-layers, were grown by molecular beam epitaxy on Si. The Ge barriers were 10 nm thick and the GeSn wells were grown with 7% Sn and thicknesses between 6 and 12 nm. Despite the high threading dislocation density of 109 to 1010 cm−2 the electroluminescence spectra measured at 300 and 80 K yield a broad and intensive luminescence band. Deconvolution revealed three major lines produced by the GeSn wells that can be interpreted in terms of quantum confinement. Biaxial compressive strain causes a splitting of light and heavy holes in the GeSn wells. We interpret the three lines to represent two direct lines, formed by transitions with the light and heavy hole band, respectively, andan indirect line.

Electroluminescence of GeSn/Ge MQW LEDs on Si substrate.

Multi-quantum well light-emitting diodes, consisting of ten alternating GeSn/Ge-layers, were grown by molecular beam epitaxy on Si. The Ge barriers were 10nm thick, and the GeSn wells were grown with 7% Sn and thicknesses between 6 and 12nm. The electroluminescence spectra measured at 300 and 80K yield a broad and intensive luminescence band. Deconvolution revealed three major lines produced by the GeSn wells that can be interpreted in terms of quantum confinement. We interpret that the three lines represent two direct lines, formed by transitions with the light and heavy hole band, respectively, and an indirect line. Biaxial compressive strain causes a splitting of light and heavy holes in the GeSn wells. This interpretation is supported by an effective mass band structure calculation.

Band-gap engineering of functional perovskites through quantum confinement and tunneling

An optimal band gap that allows for a high solar-to-fuel energy conversion efficiency is one of the key factors to achieve sustainability. We investigate computationally the band gaps and optical spectra of functional perovskites composed of layers of the two cubic perovskite semiconductors ${\mathrm{BaSnO}}_{3}$ and ${\mathrm{BaTaO}}_{2}\mathrm{N}$. Starting from an indirect gap of around $3.3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ for ${\mathrm{BaSnO}}_{3}$ and a direct gap of $1.8\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ for ${\mathrm{BaTaO}}_{2}\mathrm{N}$, different layerings can be used to design a direct gap of the functional perovskite between $2.3$ and $1.2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The variations of the band gap can be understood in terms of quantum confinement and tunneling. We also calculate the light absorption of the different heterostructures and demonstrate a large sensitivity to the detailed layering.

Tunneling in ZnO/ZnCdO quantum wells towards next generation photovoltaic cells

We report on the structural and photoluminescence (PL) properties of ZnO-based multiple quantum wells (MQWs) by metal organic vapor phase epitaxy. Two sets of ZnO/Zn0.75Cd0.25O MQW structures were grown on c- and r-plane oriented sapphire substrates to elucidate the carrier dynamics by varying the well (Zn0.75Cd0.25O) and barrier (ZnO) thicknesses. The high crystalline quality of the structures was confirmed by X-ray diffraction measurements. Importantly, no emission related to ZnO barriers could be resolved in PL spectra implying effective carrier confinement in the wells. This fact was further supported by a prominent blue-shift (∼0.5eV) of the dominant emission from the MQWs with respect to that in a single Zn0.75Cd0.25O layer. The structures with thicker wells are found to exhibit conventional quantum confinement, while samples with thinner wells demonstrate signs of excitonic wave functions penetrating into barrier layers according to results of carrier life-time measurements. It has been demonstrated that the well thickness also influences the tunneling in Zn1-xCdxO/ZnO MQWs.The estimated built-in electric field from optical transition is of the order of ∼1.75MV/cm. The observed spectral and carrier lifetime variations are discussed in terms of quantum confinement and internal electric field modulation.

Structural transition in II-VI nanofilms: Effect of molar ratio on structural, morphological, and optical properties

Cadmium sulphide nanofilms have been deposited by chemical bath deposition technique on glass substrates. The effect of S/Cd molar ratio on the structural, morphological, and optical properties of CdS nanofilms has been investigated. Structural parameters have been analyzed using x-ray diffractometer. Scanning electron microscope and energy dispersive x-ray analyzer have been used to study surface morphology and elemental composition of nanofilms, respectively. The optical properties of the nanofilms have been studied for varying S/Cd molar ratio. The variation in S/Cd molar ratio induces reorientation in lattice structure of the nanofilms and a phase transition has been observed with changing molar ratios. The modification in properties with varying S/Cd ratio has been discussed in terms of quantum confinement and compared with theoretical results based on effective mass approximation and tight binding model.

Phenomenological quantum confinement models for excitons and phonons applied to photoluminescence and Raman spectra of silicon nanocrystals

AbstractThe photoluminescence and vibrational properties of silicon nanocrystals are studied in a multilayered system elaborated by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The multilayer systems are deposited on a glass substrate (Herasil). The photoluminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. Contrary to the same study realized with multilayer systems deposited on silicon substrate, the two confinement models (phononic and excitonic) do not lead to the same size distribution. An amorphous silicon phase was also detected in Raman spectroscopy that prevented a good fitting accuracy by the model. Contribution of the substrate to the thermal crystallization process is thus discussed, as well as the origin of the photoluminescence and vibrational properties in terms of quantum confinement or interfacial defects.

ChemInform Abstract: Optical Properties of Amorphous ZnO, CdO, and PbO Nanoclusters in Solution.

The authors prepared ZnO, CdO, and PbO rock salt-coordinated and ZnO quartzite-coordinated nanoclusters with a microemulsion method in the AOT reverse micelle. X-ray and electron beam diffraction techniques indicate that all have amorphous structures. The similarity of the UV-visible absorption and photoluminescence spectra at room temperature for ZnO, CdO, and PbO rock salt-coordinated nanoclusters indicates that they are all similar in electronic structure. All prepared nanosystems show strong emission in the visible region, and the emission features depend on the wavelength of the excitation. This phenomenon is considered as a result of localization of the electronic state due to the amorphous structure of nanocluster and surface capping. Time-resolved photoluminescence studies on ZnO, CdO, and PbO nanoclusters were performed. The carrier lifetimes near the band-tail are in the sub-and nanosecond time ranges with different characteristics from that of bulk amorphous semiconductors and nanocrystals. The observed properties can be explained in terms of quantum confinement, structural disorder, and surface polarization.

Computational study of phonon modes in short-period AlN/GaN superlattices

The phonons and infrared absorption of short period [0001] superlattices ${(\text{AlN})}_{n}{(\text{GaN})}_{8\ensuremath{-}n}$, with $n=3,4,5$ were calculated by using the density-functional perturbation theory in the local-density approximation. The nature of the modes is discussed in terms of quantum confinement and interface localized modes. The spectrum for $e$-symmetry modes shows two TO peaks, of which the lowest corresponds to two closely spaced modes confined each near one half of the GaN part of the cell, and the higher one to two closely spaced AlN like modes with one mode localized at the interface showing the strongest oscillator strength and a bulklike LO-TO splitting. For ${a}_{1}$ symmetry, in contrast, a GaN vibration type mode localized near the interface shows the strongest LO-TO splitting and oscillator strength, while two higher modes are found to be only weakly localized in the AlN part of the cell. Lattice constant relaxation of free-standing superlattices leads to a decreasing lattice constant with increasing number of AlN layers and hence an overall increase in phonon frequencies. Qualitative agreement with experimental data is found for the $e$ symmetry but quantitative discrepancies exist and are interpreted as evidence of a different strain state in the experiment.

Near‐field investigation of porous silicon photoluminescence modification after oxidation in water

We report on local photo-induced oxidation of porous silicon in water at room temperature. Starting from a nonluminescent sample, the oxidation process induces luminescence which was found to first increase and then decrease as a function of the oxidation time. A clear blue shift is also observed. This effect is believed to be owing to size modification of silicon nanocrystallites and thus is explained in terms of quantum confinement. Optical near-field images and spectrum are used to monitor the photoluminescence modifications after oxidation. As the photoluminescence can be widely tuned in wavelength and intensity, this method offers a way to pattern the emission properties of the sample.

Large Coulomb-Blockade Oscillations and Negative Differential Conductance in Silicon Single-Electron Transistors with [100]- and [110]-Directed Channels at Room Temperature

Single-electron transistors (SETs) in the form of ultranarrow wire channel metal–oxide–semiconductor field-effect-transistors (MOSFETs) in both [100] and [110] directions were fabricated and large Coulomb-blockade oscillations with a peak-to-valley current ratio (PVCR) of as high as 140, which is the highest ever reported in a single-dot-system SET, were observed at room temperature. Clear negative differential conductance (NDC), whose PCVR is also the highest ever reported, has been observed in both directions at room temperature for the first time. The difference in Coulomb-blockade oscillations and NDC characteristics between [100]- and [110]-directed SET is discussed in terms of quantum confinement.

First-principles optical properties of silicon and germanium nanowires

In this work we study the optical properties of hydrogen-passivated, free-standing silicon and germanium nanowires, oriented along the [1 0 0], [1 1 0], [1 1 1] directions with diameters up to about 1.5 nm, using ab-initio techniques. In particular, we show how the electronic gap depends on wire’s size and orientation; such behaviour has been described in terms of quantum confinement and anisotropy effects, related to the quasi one-dimensionality of nanowires. The optical properties are analyzed taking into account different approximations: in particular, we show how the many-body effects, namely self-energy, local field and excitonic effects, strongly modify the single particle spectra. Further, we describe the differences in the optical spectra of silicon and germanium nanowires along the [1 0 0] direction, as due to the different band structures of the corresponding bulk compounds.

Recrystallization behavior in chiral sculptured thin films from silicon

Chiral sculptured thin films, which contain amorphous silicon screws with a fiberlike fine structure, were grown by ion-beam-assisted glancing-angle deposition at room temperature. The thin films were postgrowth annealed in the temperature range from 400to1000°C. Raman spectroscopy and transmission electron microscopy investigations performed before and after sample annealing reveal a recrystallization of silicon at temperatures above 800°C, with a persistence of the chiral structure geometry and fine structure. The Raman results are further discussed in terms of quantum confinement and coexisting phase effects.

Spectra of photoluminescence from silicon nanocrystals

The evolution of time-resolved photoluminescence (PL) spectra in Au-doped nanocrystalline silicon films produced by laser ablation has been studied. The PL spectra with a relaxation time of nanoseconds are broad; they lie in the energy range 1.4–3.2 eV with a peak at 2.4–2.8 eV. At the longest times of tens of microseconds, the spectra become narrower, with a peak at 1.6 eV. At intermediate times, two bands are observed: low-energy (1.6 eV) and high-energy, with the peak shifting from 2.7 to 2.1 eV with time increasing. The data are discussed in terms of quantum confinement, dielectric amplification, and manifestation of kinetically coupled electron-hole and exciton subsystems. Ions and atoms of gold passivate dangling bonds on the Si surface and serve as catalysts for the oxidation of nanocrystals. The similarity between recombination processes responsible for the visible PL in oxidized por-Si layers and in the studied Au-doped films is discussed.

Effect of passivation and aging on the photoluminescence of silicon nanocrystals

Silicon nanocrystals were prepared by laser pyrolysis of silane in a gas flow reactor and deposited on substrates by cluster beam deposition. The evolution of the photoluminescence was measured as a function of the time the samples were exposed to air. With gradual air exposure and oxidation, the photoluminescence increases in intensity and its peak wavelength shifts to the blue. At the same time, the photoluminescence band becomes wider. To remove the oxide layer, the samples were exposed to hydrofluoric acid (HF) vapor. The HF attack has no influence on the band position but leads to a smaller width. The observations can be explained in terms of quantum confinement as the origin of the photoluminescence. Only for the smallest Si nanocrystals, an interface state seems to limit the maximum photoluminescence energy to 2.1 eV.

Optical Properties of Amorphous ZnO, CdO, and PbO Nanoclusters in Solution

The authors prepared ZnO, CdO, and PbO rock salt-coordinated and ZnO quartzite-coordinated nanoclusters with a microemulsion method in the AOT reverse micelle. X-ray and electron beam diffraction techniques indicate that all have amorphous structures. The similarity of the UV-visible absorption and photoluminescence spectra at room temperature for ZnO, CdO, and PbO rock salt-coordinated nanoclusters indicates that they are all similar in electronic structure. All prepared nanosystems show strong emission in the visible region, and the emission features depend on the wavelength of the excitation. This phenomenon is considered as a result of localization of the electronic state due to the amorphous structure of nanocluster and surface capping. Time-resolved photoluminescence studies on ZnO, CdO, and PbO nanoclusters were performed. The carrier lifetimes near the band-tail are in the sub-and nanosecond time ranges with different characteristics from that of bulk amorphous semiconductors and nanocrystals. The observed properties can be explained in terms of quantum confinement, structural disorder, and surface polarization.

Relationship Between Spectroscopic and Structural Properties of Poly(meta/para Phenylene)

ABSTRACTWe have used electron paramagnetic resonance (EPR) spectroscopy for investigating the properties of spins, such as those carried by polarons which carry both spin and charge in an electrosynthesized poly (meta/para phenylene) PMPP.Indeed, the study of their mobility provides an insight into the charge transport properties of the conjugated polymer in a microscopic scale.Moreover, we report a correlation between thin film morphology, chainlength and optical gap.Particularly, we show a blue shift upon decreasing chainlength. Furthermore, we have observed a blue shift of the energy band gap with the decrease of the surface grain size deduced from atomic force microscopy (AFM) analyses. This result has been explained in terms of quantum confinement.

Size Effect in Germanium Nanostructures Fabricated by Pulsed Laser Deposition

ABSTRACTWe have fabricated Ge nanostructures buried in AlN and Al2O3 matrices grown on Si(111) and sapphire substrates by pulsed laser deposition. Our approach involved three-dimensional island growth of low band-gap material followed by a layer of wide band-gap material. The nanodots were uniformly distributed in between alternating layers of AlN or Al2O3. It was observed that these nanodots exhibit crystalline structure when grown at 300-500 °C. The average size of Ge islands was determined to be ∼5-15 nm, which could be varied by controlling laser deposition and substrate parameters. The Raman spectrum showed a peak of the Ge-Ge vibrational mode downward shifted upto 295 cm− which is caused by quantum confinement of phonons in the Ge-dots. The photoluminescence of the Ge dots (size ∼15nm) was blue shifted by ∼0.266 eV from the bulk Ge value of 0.73 eV at 77 K, resulting in a distinct peak at ∼1.0 eV. The spectral positions of both E1 and E2 transitions in the absorption spectra at room temperature and 77K shift toward higher energy as the Ge dot size decreases. The interpretation of these behaviors in terms of quantum confinement is discussed in this work, and the importance of pulsed laser deposition in fabricating novel nanostructures is emphasized

Quantum Confinement of Above-Band-Gap Transitions in Ge Quantum Dots

AbstractA number of research efforts have been focused on self-assembled germanium quantum dots in which indirect optical transitions take place across the band gap. However, many questions regarding the confined electronic state transitions of Ge quantum dots still remain unanswered. In the present report, we have deposited ten alternating layers of crystalline Ge quantum dots embedded in an Al2O3 or an AIN matrix on sapphire substrates by pulsed laser deposition. The average dot sizes (73 Å to 260 Å) were controlled by the laser energy density, deposition time and substrate temperature. The spectral positions of both the E1 and the E2 transitions in the absorption spectra at room temperature and 77 K shift toward higher energy (ΔE1=1.19 eV, ΔE2 =0.57 eV) as the Ge dot size decreases (73 Å). Structural analysis using transmission electron microscopy and atomic force microscopy and the interpretation of optical absorption measurements in terms of quantum confinement of carriers in both transitions are presented.

Optical Analysis of Plasma Enhanced Crystallization of Amorphous Silicon Films

AbstractLow-temperature crystallization of a-Si is important for display and Silicon-On- Insulator (SOT) technologies. We present optical characterization (Raman scattering and photoluminescence) of H2 and O2 plasma enhanced crystallization of a-Si:H films. H2 plasma treatment is shown to be the most efficient, leading to larger grain sizes, and both H2 and O2 plasma lead to visible photoluminescence (PL). Recently, the PL of re-crystallized a-Si films has been explained in terms of quantum confinement [1]. The mean size of the crystallites in our re-crystallized films is determined by Raman scattering for different treatments parameters. No correlation between size and the photon energy of the visible emission is found. However, we can clearly distinguish between the PL from purely amorphous and re-crystallized a-Si:H films: Their PL temperature dependence and spectra are very different. The origin of the visible PL in re-crystallized thin Si films is discussed.