Confinement Energy Research Articles

Structural, optical, magnetic and dielectric studies of SnO2 nano particles in real time applications

Pure and Aluminium doped (x = 0.01, 0.02, 0.03, 0.04) SnO2 nano particles have been prepared by Sol Gel method. XRD studies confirm tetragonal structure of SnO2 and second phase peaks are absent. The obtained crystallite size of undoped SnO2 is 17.5 nm and with Al insertion Crystallite size reduced to 10 nm for Sn0.96Al0.04O2. FTIR data in the range of 550 cm−1–630 cm−1 confirms the vibration modes of SnOSn and SnO in SnO2 molecules. UV spectrum shows red shift due to the trapping of exitons by oxygen vacancies. In contrast to quantum confinement energy gap decreases from 3.7 eV for undoped SnO2 and to 2.91 eV for Sn0.97Al0.03O2 due to the band bending, which is the result of particle size reduction and then increases to 3.19 eV. This variation has been explained based on Burstein-Moss effect for Sn0.96Al0.04O2. Impedance studies of undoped and Al doped SnO2 samples have been investigated at room temperature and found that Sn0.97Al0.03O2 exhibits peculiar behaviour of having high dielectric constant, high A.C. conductivity, low dielectric loss and high theoretically calculated mobility among Al doped SnO2 samples. The data obtained from Vibration Sample Magnetometer shows typical conversion of magnetic nature of undoped SnO2 which is diamagnetic to paramagnetic in the case of Sn0.99Al0.01O2 and Sn0.98Al0.02O2 samples and to superparamagnetic nature for Sn0.97Al0.03O2 and Sn0.96Al0.04O2.

Acoustic phonon–exciton interaction by extremely strong exciton confinement and large phonon energy in CsPbBr3 perovskite

Recently, CsPbX3 (X: halogen) materials have been considered as promising light-emitting materials, because of their good environmental durability and outstanding optoelectronic properties. Understanding the exciton and phonon behaviors in CsPbX3 is crucial in developing advanced optoelectronic devices. In this work, distinct acoustic phonon–exciton interaction, which was first observed in perovskite, was discovered in our established highly luminescent and durable CsPbBr3 film. The acoustic phonon–exciton coupling was induced by strong exciton confinement (∼70 meV exciton binding energy) and accompanied by large phonon energy ascribed to local vibration modes. We believe that our results will serve to in improve performance of the corresponding optoelectronic devices.

Magnetoabsorption in HgCdTe/CdHgTe Quantum Wells in Tilted Magnetic Fields

Absorption spectra in a 30-nm Hg0.075Cd0.925Te/Cd0.74Hg0.26Te quantum well in tilted magnetic fields up to 13 T have been studied. Energy spectra in the conduction band (in the absence of the normal field component), as well as electron Landau levels spectra in the presence of the magnetic field component in the quantum well plane, have been calculated within the eight-band Kane model. In classical magnetic fields, the broadening of cyclotron resonance lines has been detected; this broadening is due both to the removal of the Kramers degeneracy of the lower quantum-confinement subband and to difference of cyclotron masses in split subbands. The longitudinal field component in quantizing magnetic fields is responsible for the hybridization of Landau levels, which is enhanced with an increase in their energy (on the scale of the quantum confinement energy), manifested in the appearance of “forbidden” transitions in magnetoabsorption spectra.

Towards unifying the concepts of catalysis in confined space

In this article, we reviewed the progress of theoretical and computational studies for confined catalysis in one-dimensional and two-dimensional systems. It was found that the chemical potential of reaction species was usually enhanced in confined space, validated in heterogeneous catalysis and electrocatalysis. Tunable chemical potential of reaction species in confined nanoscale reactors is recognized as a feasible avenue to establish exceptional activity and selectivity of catalytic reactions. A key concept, namely, “confinement energy”, has been defined based on our density functional theory calculations, which is able to unify a number of experimental and computational results. In addition, the concept of “confinement energy” has been able to successfully predict new experimental phenomena.

Toward Fundamentals of Confined Electrocatalysis in Nanoscale Reactors.

A number of experiments have demonstrated that electrochemical reactions are feasible in confined nanoscale reactors, while what the fundamentals of confined electrochemistry are is not clear. Using first-principles calculations and electrochemical modeling, we find that the capacitance in the confined nanoscale reactors can be significantly enhanced, compared to an open electrode interface, essentially promoting the electrochemical reactions and charge transfer efficiency in nanoscale reactors. More importantly, this is a general character, as found in a variety of electrochemical and thermochemical reactions. At the end, we use the recently defined new concept of "confinement energy" for understanding the nature of confined electrochemistry from both thermochemical and electrochemical points of view.

Biexciton Auger recombination in mono-dispersed, quantum-confined CsPbBr3 perovskite nanocrystals obeys universal volume-scaling

Auger recombination has been a long-standing obstacle to many prospective applications of colloidal quantum dots (QDs) ranging from lasing, light-emitting diodes to bio-labeling. As such, understanding the physical underpinnings and scaling laws for Auger recombination is essential to these applications. Previous studies of biexciton Auger recombination in various QDs established a universal scaling of biexciton lifetime (τXX) with QD volume (V ): τXX = γV. However, recent measurements on perovskite nanocrystals (NCs), an emerging class of enablers for light harvesting and emitting applications, showed significant deviations from this universal scaling law, likely because the measured NCs are weakly-confined and also have relatively broad size-distributions. Here we study biexciton Auger recombination in mono-dispersed (size distributions within 1.7%–9.0%), quantum-confined CsPbBr3 NCs (with confinement energy up to 410 meV) synthesized using a latest approach based on thermodynamic equilibrium control. Our measurements clearly reproduce the volume-scaling of τXX in confined CsPbBr3 QDs. However, the scaling factor γ (0.085 ± 0.001 ps/nm3) is one order of magnitude lower than that reported for CdSe and PbSe QDs (1.00 ± 0.05 ps/nm3), suggesting unique mechanisms enhancing Auger recombination rate in perovskite NCs.

Efficient Nonthermal Particle Acceleration by the Kink Instability in Relativistic Jets.

Relativistic magnetized jets from active galaxies are among the most powerful cosmic accelerators, but their particle acceleration mechanisms remain a mystery. We present a new acceleration mechanism associated with the development of the helical kink instability in relativistic jets, which leads to the efficient conversion of the jet's magnetic energy into nonthermal particles. Large-scale three-dimensional abinitio simulations reveal that the formation of highly tangled magnetic fields and a large-scale inductive electric field throughout the kink-unstable region promotes rapid energization of the particles. The energy distribution of the accelerated particles develops a well-defined power-law tail extending to the radiation-reaction limited energy in the case of leptons, and to the confinement energy of the jet in the case of ions. When applied to the conditions of well-studied bright knots in jets from active galaxies, this mechanism can account for the spectrum of synchrotron and inverse Compton radiating particles, and offers a viable means of accelerating ultrahigh-energy cosmic rays to 10^{20} eV.

Hydrostatic and uniaxial effects in InGaN/GaN quantum wells

We calculate strains, polarizations, and electric fields in InGaN/GaN quantum wells (lattice matched to GaN) under the influence of hydrostatic and uniaxial (along the c-axis) pressure. We calculate the confinement energies for electrons and holes, and we derive simple expressions for the transition energies and their pressure derivatives. We include the changes of the dielectric constant with pressure. The results seem compatible with the experimental data.

Multilayer Quantum Well–Dot InGaAs Heterostructures in GaAs-based Photovoltaic Converters

GaAs photovoltaic converters containing quantum well-dot (QWD) heterostructures are studied. The QWD properties are intermediate between those of quantum wells (QWs) and quantum dots. The QWDs are obtained by the epitaxial deposition of In0.4Ga0.6As with a nominal thickness of 8 single layers by metal-organic vapor phase epitaxy. QWDs are a dense array of elastically strained islands that localize carriers in three directions and are formed by a local increase in the indium concentration and/ or InGaAs-layer thickness. There are two quantum-well levels of varied nature in structures with QWDs. These levels are manifested in the spectral characteristics of GaAs photovoltaic converters. A short-wavelength peak with a maximum at around 935 nm is associated with absorption in the residual QW, and the long-wavelength peak (1015–1030 nm) is due to absorption in the QWDs. Investigation by transmission electron microscopy demonstrates that an increase in the number of InGaAs layers leads to stronger elastic stresses, which, in turn, increases the carrier confinement energy in the QWDs and lead to a corresponding long-wavelength shift of the internal quantum efficiency spectrum.

Band structure and absorption properties of (Ga, In)/(P, As, N) symmetric and asymmetric quantum wells and super-lattice structures: Towards lattice-matched III-V/Si tandem

Quaternary dilute nitride compound semiconductors like GaAsyP1−x−yNx and Ga1−zInzP1−xNx are lattice matched with silicon when y = 4.7 * x − 0.1 and z = 2.2 * x − 0.044 and also have direct bandgaps (with N > 0.6%), thus allowing for monolithic integration of III-V optoelectronics with silicon technology as well as III-V/Si tandem junction solar cells. By applying an eight-band k.p strained (tensile or compressive) Hamiltonian and a Band Anti-crossing model (to account for small amounts of nitrogen impurities) to the conduction band, the electronic band structure and the dispersion relation of these alloys can be determined near the center of Brillouin Zone. In this work, by minimizing the total mechanical energy of the stack of alternating layers of GaP1−xNx and GaAs1−xNx, we have evaluated the ratio of thickness of the respective layers for a strain-balanced superlattice GaAs1−xNx/GaP1−xNx structure on silicon. We calculated the confinement energies and the corresponding states of the respective carriers inside a quantum well (with and without resonantly coupled) or in the miniband of a superlattice structure as a function of the nitrogen composition using a transfer matrix approach under the envelope function approximation. Incorporating only a small amount of nitrogen (<5%), the bandgap of these lattice matched structures fulfils the optimum bandgap requirement of (1.65–1.8) eV for III-V/Si tandem solar cells and optoelectronic devices. The optical-absorption coefficient, in both symmetric and asymmetric quantum wells, is then evaluated with respect to nitrogen composition and temperature by using the Fermi-golden rule for both TE and TM polarization of incident light, including the effect of excitons and thermal broadening.

Suppression of the quantum-confined Stark effect in polar nitride heterostructures

Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.

Energy confinement of hydrogen and deuterium electron-root plasmas in the Large Helical Device

The dependence of the energy confinement and energy transport on the isotope mass is a long-standing open question in the stellarator community. With the recent upgrade of the Large Helical Device to allow for deuterium plasma operation, systematic isotope experiments could be carried out for the first time in a major non-axisymmetric device.Within this framework, electron-cyclotron-resonance heated (ECRH) hydrogen and deuterium plasmas were investigated varying both density and heating power to establish a broad data set. Even at low power the central ECRH heating is sufficient to lead to stellarator-specific core-electron-root-confinement which features a peaked electron temperature profile and a positive radial electric field. For this data set, the energy confinement time and energy transport is investigated in detail and compared to the neoclassical theory.Over the whole data set, the energy confinement time of deuterium is statistically 10%–20% larger than in hydrogen indicating that the ‘isotope effect’ also exists in non-axisymmetric devices. Both the electron and ion temperature are elevated in deuterium compared to hydrogen at the same effective absorbed power and density. From a neoclassical point-of-view, the electron-root and the positive electric field extend over nearly the entire plasma radius. Good agreement is found between the measured and theoretical neoclassical ambipolar electric field. The neoclassical energy-flux can account for up to half the experimental flux implying that turbulence is responsible for a significant fraction of the entire energy-flux.

Light-biased IV characteristics of a GaAsBi/GaAs multiple quantum well pin diode at low temperature

While recent work developing GaAsBi for opto-electronic applications has shown promise, it has also indicated that the large valence band offset of GaAsBi/GaAs may cause undesirable hole-trapping in GaAsBi quantum wells. In this work, hole-trapping is demonstrated to be the cause of the reduced depletion width of GaAsBi/GaAs multi-quantum well solar cell devices under illumination. Modelling of the quantum confinement energies in these devices shows how the carrier escape times vary as functions of temperature, providing a tool for the design for future GaAsBi based photovoltaic devices.

Field Confinement Energy at a Nonlinear Interface between Nonlinear Defocusing Media

A model of two media with Kerr defocusing nonlinearity separated by a planar interface interacting nonlinearly with excitations has been considered. Two new types of stationary states describing the transformation of a soliton lattice to a damping field at the passage through such an interface have been found. These two types of states differ in the energy range of existence and in the character of field damping with the distance from the interface. The energies of long-wavelength states have been obtained. The conditions of the existence of such states have been determined depending on the characteristics of media and the interface between them.

H2 storage by confinement inside germanium nanotubes

This work enabled us to compare the behaviour of different types of CNTs and BNNTs with that of various germanium nanotubes with a hydrogen molecule introduced perpendicularly or parallel to their axis. The confinement of the H2 molecule in GeNTs in both cases showed that the confinement energy is large for the smallest diameter, but it becomes close to zero for larger diameters. The main difference with respect to carbon nanotubes is the disappearance of H-H bond activation due to the absence of interactions between the π electrons and the hydrogen atoms. Compared to the BNNTs, we noted elongation of the H-H bond in both orientations for small diameters instead of breaking or shortening. This elongation is due to the strong interaction between the van der Waals radius of hydrogen and the atomic radius of germanium. This leads to a strong attraction between germanium and hydrogen for both orientations. This situation engenders a significant polarization. The most important consequence of the existence of this induced dipole moment is the IR activation of the H-H stretching vibration, which becomes very intense and whose value depends on the diameter of the nanotube. As an application, this vibration frequency can be used to detect and assay H2 contained in nanotubes or deduce the diameter of the host tube. This result shows that it is possible to detect H2 molecules inside GeNTs as well as in CNTs and BNNTs. However, in the germanium nanotubes, the most remarkable result that emerges from this study is that it is possible to detect and clearly differentiate the orientation of the H2 molecule inside the nanotubes, in contrast to other types of nanotubes.

Determination of strain relaxation in InGaN/GaN nanowalls from quantum confinement and exciton binding energy dependent photoluminescence peak

GaN based nanostructures are being increasingly used to improve the performance of various devices including light emitting diodes and lasers. It is important to determine the strain relaxation in these structures for device design and better prediction of device characteristics and performance. We have determined the strain relaxation in InGaN/GaN nanowalls from quantum confinement and exciton binding energy dependent photoluminescence peak. We have further determined the strain relaxation as a function of nanowall dimension. With a decrease in nanowall dimension, the lateral quantum confinement and exciton binding energy increase and the InGaN layer becomes partially strain relaxed which decreases the piezoelectric polarization field. The reduced polarization field decreases quantum confined Stark effect along the c-axis and increases electron-hole wave-function overlap which further increases the exciton binding energy. The strong dependency of the exciton binding energy on strain is used to determine the strain relaxation in these nanostructures. An analytical model based on fractional dimension for GaN/InGaN/GaN heterostructures along with self-consistent simulation of Schrodinger and Poisson equations are used to theoretically correlate them. The larger effective mass of GaN along with smaller perturbation allows the fractional dimensional model to accurately describe our system without requiring first principle calculations.

Enhanced transient photovoltaic characteristics of core–shell ZnSe/ZnS/L-Cys quantum-dot-sensitized TiO2 thin-film**Project supported by the Natural Science Foundation of Hebei Province, China (Grant Nos. E2013203296 and E2017203029).

Photoanodic properties greatly determine the overall performance of quantum-dot-sensitized solar cells (QDSCs). In the present report, the microdynamic behaviors of carriers in the nanocomposite thin-film, a ZnSe QD-sensitized mesoporous La-doped nano-TiO2 thin-film, as a potential candidate for photoanode, are probed via nanosecond transient photovoltaic (TPV) spectroscopy. The results confirm that the L-Cys ligand has a dual function serving as a stabilizer and molecular linker. Large quantities of interface states are located at the energy level with a photoelectric threshold of 1.58 eV and a quantum well (QW) depth of 0.67 eV. This QW depth is approximately 0.14 eV deeper than the depth of QW buried in the ZnSe QDs, and a deeper QW results in a higher quantum confinement energy. A strong quantum confinement effect of the interface state may be responsible for the excellent TPV characteristics of the photoanode. For example, the peak intensity of the TPV response of the QD-sensitized thin-film lasts a long time, from 9.40 × 10−7 s to 2.96 × 10−4 s, and the end time of the PTV response of the QD-sensitized thin-film is extended by approximately an order of magnitude compared with those of the TiO2 substrate and the QDs. The TPV characteristics of the QD-sensitized thin-film change from p-type to n-type for the QDs before and after sensitizing. These properties strongly depend on the extended diffusion length of the photogenerated carries and the reduced recombination rate of photogenerated electron-hole pairs, resulting in prolonged carrier lifetime and an increased level of electron injection into the TiO2 thin-film substrate.

Correlating Photoluminescence and Structural Properties of Uncapped and GaAs-Capped Epitaxial InGaAs Quantum Dots

The understanding of the correlation between structural and photoluminescence (PL) properties of self-assembled semiconductor quantum dots (QDs), particularly InGaAs QDs grown on (001) GaAs substrates, is crucial for both fundamental research and optoelectronic device applications. So far structural and PL properties have been probed from two different epitaxial layers, namely top-capped and buried layers respectively. Here, we report for the first time both structural and PL measurements from an uncapped layer of InGaAs QDs to correlate directly composition, strain and shape of QDs with the optical properties. Synchrotron X-ray scattering measurements show migration of In atom from the apex of QDs giving systematic reduction of height and enlargement of QDs base in the capping process. The optical transitions show systematic reduction in the energy of ground state and the first excited state transition lines with increase in capping but the energy of the second excited state line remain unchanged. We also found that the excitons are confined at the base region of these elliptically shaped QDs showing an interesting volume-dependent confinement energy scaling of 0.3 instead of 0.67 expected for spherical dots. The presented method will help us tuning the growth of QDs to achieve desired optical properties.

Bright Colloidal PbS Nanoribbons

Colloidal lead sulfide (PbS) nanoribbons are synthesized using organometallic precursors with chloroalkane cosolvents. The few-atom-thick nanoribbons have a typical width 20 nm and a length more than 50 nm. Different from a nanosheet where the quantum confinement energy is mainly determined by the thickness, the narrow width of the nanoribbon has an additional contribution to the increase of energy gap. In contrast to nanosheets, the nanoribbons are much brighter. At room temperatures, well-passivated nanoribbons have achieved more than 30% photoluminescence quantum yield in the infrared spectrum, competing with the well-developed colloidal lead chalcogenide quantum dots of the similar energy gap.

HgTe, the Most Tunable Colloidal Material: from the Strong Confinement Regime to THz Material

ABSTRACTHgTe nanocrystals are extremely interesting materials to obtain a highly tunable absorption spectrum in the infrared range. Here, we discuss the two extreme cases of strongly confined and barely confined HgTe nanocrystals. We discuss the synthesis and optoelectronic properties of HgTe 2D nanoplatelets where the confinement energy can be as large as 1.5 eV. This material presents enhanced (mostly narrower) light emitting properties compared to spherical nanocrystals emitting at the same wavelength. Moreover, absorption spectra, majority carriers and time response can be tuned by carefully choosing the surface chemistry and applying a well-chosen gate bias. HgTe can also be used to explore the effect of vanishing confinement and to obtain quasi bulk properties with tunable absorption in the THz, up to 150 µm.