Optical spectroscopy of lanthanides doped in wide band-gap semiconductor nanocrystals

Currently, tripositive lanthanide (Ln³⁺) ions doped wide band-gap semiconductor nanocrystals (NCs) have been the focus of research interest due to their distinct optical properties and potential applications in optical devices and luminescent biolabels. Because of the low absorptions of parity-forbidden 4f-4f transitions for Ln³⁺, it is highly anticipated that the luminescence of Ln³⁺ ions embedded in wide band-gap NC lattices can be sensitized efficiently via exciton recombination in the host. For this purpose, the successful incorporation of Ln³⁺ into the lattices of semiconductor NCs is of utmost importance, which still remains intractable via conventional wet chemical methods. Here, the most recent progress in the optical spectroscopy of Ln³⁺ ions doped wide band-gap semiconductor NCs is discussed. Much attention was focused on the optical properties including electronic structures, luminescence dynamics, energy transfer as well as the up-conversion emissions of Ln³⁺ ions in ZnO, TiO₂, SnO₂ and In₂O₃ NCs that were synthesized in our laboratory using wet chemical methods.     

Photoluminescence polarization of semiconductor nanocrystals

We review the polarization properties of photoluminescence (PL) in nanocrystals (NCs) from both theoretical and experimental points of view. We show that, under linearly polarized excitation, NCs emit partly polarized light owing to their uniaxial structure or their anisotropic shape. In elongated NCs, the anisotropy may have two origins, the electronic confinement or the effect of depolarizing field created by the light-induced charges on the interfaces. Results of polarization studies in porous silicon are presented. They are explained by the shape of the Si NCs. Experiments in CdSe NCs reveal the fine structure of the excitonic levels and show evidence of the enhancement of the electron-hole exchange energy with decreasing NC size. Spin orientation in wurtzite-type NCs is achieved by optical pumping with circularly polarized light. The effect of a magnetic field on the degree of circular polarization and the mechanisms of spin relaxation are discussed. Results in large-size NCs are presented.     

Enhancement of optical switching parameter and third-order optical nonlinearities in embedded Si nanocrystals: A theoretical assessment

Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs) embedded in a wide band-gap matrix representing silica are theoretically studied using an atomistic pseudopotential approach. Nonlinear refractive index, two-photon absorption and optical switching parameter are examined from small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs show higher third-order optical nonlinearities and much wider two-photon absorption-free energy gap which gives rise to enhancement in the optical switching parameter.     

Fundamental aspects of activated nanocrystal luminescence and possible applications

Presented results of complex study of relaxation processes and interionic interaction in Y₂SiO₅:Pr³⁺ and Lu₂SiO₅:Ce³⁺ nanocrystals clearly show two fundamental aspects: the phonon quantum confinement gives rise to the new fluorescence dynamics of doped ions; the developed surface of nanocrystals stimulates the irregular distribution of doped ions within the nanocrystal volume and could be the reason of new atomic arrangement of nanocrystal. Fluorescence spectrum of isolated Y₂SiO₅:Pr³⁺nanocrystal demonstrates the intense fluorescence from the high crystal field components of split ¹D₂ manifold of Pr³⁺ as the result of a suppression of phonon-assisted relaxation under the phonon quantum confinement. The direct comparison of the data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. This effect is caused by uphill diffusion of doped ions and preferred Pr segregation at the nanocrystal surface layer that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺. Lu₂SiO₅:Ce³⁺ nanocrystals which average size is 5 nm do not demonstrate the effect of energy storage as the result of atomic packing changing that does not permit the existence of electronic traps.     

Effect of local atomic disorder on the half-metallicity of full-Heusler Co2FeSi alloy: a first-principles study

This paper investigates the effect of atomic disorder on the electronic structure, magnetism, and half-metallicity of full-Heusler Co2FeSi alloy by using the full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA) and GGA+U schemes. It considers three types of atomic disorders in Co2FeSi alloy: the Co-Fe, Co-Si, and Fe-Si disorders. Total energy calculations show that of the three types of disorders, the Fe-Si disorder is more likely to occur. It finds that for the Co-Si disorder, additional states appear in the minority band-gap at the EF and the half-metallcity is substantially destroyed, regardless of the disorder level. On the other hand, the Co-Fe and Fe-Si disorders have little effect on the half-metallicity at a low disorder level. When increasing the disorder levels, the half-metallcity is destroyed at about 9 % of the Co-Fe disorder level, while that stays at 25 % of the Fe-Si disorder level.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

非晶化与量子限域效应

当材料尺度减小到几个纳米时，材料内部电子结构会表现为分立能级，这就是所谓的量子限域效应。通过晶态和非晶Pd纳米颗粒的单电子隧穿实验发现，在晶态Pd颗粒中能观察到量子限域效应，而在同样大小的非晶Pd颗粒中却没有观察到。考虑到有序／无序结构的静态效应并结合电子散射等动态效应，解释了非晶Pd颗粒实验中没有观察到量子限域效应的原因。这一结果表明，尺寸减小并不足以使纳米体系表现量子行为，原子结构有序度对于决定纳米体系表现经典行为或量子行为具有同等重要作用。     

Quantum confinement effect in β-SiC nanowires

The quantum confinement effect is important in nanoelectronics and optoelectronics applications; however, there is a discrepancy between the theory of quantum confinement, which indicates that band-gap widening occurs only at small sizes, and experimental observations of band-gap widening in large-diameter nanowires (NWs). This paper reports an obvious blue shift of the absorption edge in the UV-visible absorption spectra of SiC NWs with diameters of 50–300 nm. On the basis of quantum confinement theory and high-resolution transmission electron microscopy images of SiC NWs, band-gap widening in SiC NWs with diameters of up to hundreds of nanometers is fully explained; the results could help to explain similar band-gap widening in other NWs with large diameters.     

Theory of radiative and nonradiative transitions for semiconductor nanocrystals

Existing calculations on the radiative and nonradiative transitions in semiconductor crystallites are reviewed with particular emphasis on indirect band-gap materials like silicon for which the quantum confinement effects are more spectacular. It is shown that the crystallite gaps and radiative recombination rates can be predicted with fair accuracy. Effects related to atomic relaxation in the excited state (Stokes shift) are calculated and it is shown that small enough crystallites lead to self-trapped excitons which provide another source of luminescence, much less dependent on size effects. Nonradiative processes are then examined: intrinsic, due to Auger recombination, and extrinsic, due to dangling bond surface states. Both are found to play an essential role in the interpretation of experimental data. Finally, dielectric screening is studied, justifying the use of a reduced internal dielectric constant and providing an estimate of the Coulomb shift due to charging effects.     

Quantum confinement effect in <Emphasis Type="Italic">β</Emphasis>-SiC nanowires

The quantum confinement effect is important in nanoelectronics and optoelectronics applications; however, there is a discrepancy between the theory of quantum confinement, which indicates that band-gap widening occurs only at small sizes, and experimental observations of band-gap widening in large-diameter nanowires (NWs). This paper reports an obvious blue shift of the absorption edge in the UV-visible absorption spectra of SiC NWs with diameters of 50–300 nm. On the basis of quantum confinement theory and high-resolution transmission electron microscopy images of SiC NWs, band-gap widening in SiC NWs with diameters of up to hundreds of nanometers is fully explained; the results could help to explain similar band-gap widening in other NWs with large diameters.     

Investigation of Auger recombination in Ge and Si nanocrystals embedded in SiO2 matrix

We study theoretically the optical properties of embedded Ge and Si nanocrystals (NCs) in wide band-gap matrix and compared the obtained results for both NCs embedded in SiO₂ matrix. We calculate the ground and excited electron and hole levels in both Ge and Si nanocrystals (quantum dots) in a multiband effective mass approximation. We use the envelope function approximation taking into account the elliptic symmetry of the bottom of the conduction band and the complex structure of the top of the valence band in both Si and Ge (NCs). The Auger recombination (AR) in both nanocrystals is thoroughly investigated. The excited electron (EE), excited hole (EH) and biexciton AR types are considered. The Auger recombination (AR) lifetime in both NCs has been estimated and compared.     

Optical band-edge absorption of oxide compound SnO2

Tin oxide (SnO₂) is an important oxide for efficient dielectrics, catalysis, sensor devices, electrodes and transparent conducting coating oxide technologies. SnO₂ thin film is widely used in glass applications due to its low infra-red heat emissivity. In this work, the SnO₂ electronic band-edge structure and optical properties are studied employing a first-principle and fully relativistic full-potential linearized augmented plane wave (FPLAPW) method within the local density approximation (LDA). The optical band-edge absorption α(ω) of intrinsic SnO₂ is investigated experimentally by transmission spectroscopy measurements and their roughness in the light of the atomic force microscopy (AFM) measurements. The sample films were prepared by spray pyrolysis deposition method onto glass substrate considering different thickness layers. We found for SnO₂ qualitatively good agreement of the calculated optical band-gap energy as well as the optical absorption with the experimental results.     

An open-shell method for neutral vacancy in silicon and diamond

The restricted Hartree-Fock-Roothaan method with closed and open electronic shells projected by electron density matrices and the quasi-molecular large-unit-cell (LUC) model have been applied to calculate the electronic structure of monovacancy and semivacancy in the neutral charge state in the totally symmetric atomic configuration with relaxation and symmetry-lowering distortions. The difference in the energies of states with total spins of 1 and 3/2 for a neutral monovacancy is determined within the ΔSCF approximation.     

First-Principles Calculations of Electronic Structures of New Ⅲ-Ⅴ Semiconductors： BxGa1-xAs and TlxGa1-xAs alloys

We investigate the electronic structures of new semiconductor alloys BxGa1-xAs and TlxGa1-xAs, employing first-principles calculations within the density-functional theory and the generalized gradient approximation. The calculation results indicate that alloying a small TI content with GaAs will produce larger modifications of the band structures compared to B. A careful investigation of the internal lattice structure relaxation shows that significant bond-length relaxations takes place in both the alloys, and it turns out that difference between the band-gap bowing behaviours for B and TI stems from the different impact of atomic relaxation on the electronic structure. The relaxed structure yields electronic-structure results, which are in good agreement with the experimental data. Finally, a comparison of formation enthalpies indicates that the production Tlx Ga1-xAs with TI concentration of at least 8% is possible.     

Investigation of magnetic active core sizes and hydrodynamic diameters of a magnetically fractionated ferrofluid

In this work we address the question which relates between the size of the magnetically active core of magnetic nanoparticles (MNPs) and the size of the overall particle in the solution (the so-called hydrodynamic diameter d _hyd) exists. For this purpose we use two methods of examination that can deliver conclusions about the properties of MNP which are not accessible with normal microscopy. On the one hand, we use temperature dependent magnetorelaxation (TMRX) method, which enables direct access to the energy barrier distribution and by using additional hysteresis loop measurements can provide details about the size of the magnetically active cores. On the other hand, to determine the size of the overall particle in the solution, we use the magnetooptical relaxation of ferrofluids (MORFF) method, where the stimulation is done magnetically while the reading of the relaxation signal, however, is done optically. As a basis for the examinations in this work we use a ferrofluid that was developed for medicinal purposes and which has been fractioned magnetically to obtain differently sized fractions of MNPs. The two values obtained through these methods for each fraction shows the success in fractioning the original solution. Therefore, one can conclude a direct correlation between the size of the magnetically active core and the size of the complete particle in the solution from the experimental results. To calculate the size of the magnetically active core we found a temperature dependent anisotropy constant which was taken into account for the calculations. Furthermore, we found relaxation signals at 18 K for all fractions in these TMRX measurements, which have their origin in other magnetic effects than the Néel relaxation.     

Ⅳ族FCC晶体基态结构和电子性质第一性原理研究

采用基于密度泛函理论的第一性原理方法,在广义梯度近似下,计算了Ⅳ族元素晶体的面心立方结构和电子性质.结果表明:Ⅳ族元素晶体的面心立方结构均可存在,面心立方结构Ge晶体的结合能最大,结构最稳定;面心立方结构C、Si、Ge和Sn的晶格常数分别为0.3509nm、0.4322nm、0.4225nm、0.4903nm,不随原子序数的增加而单调增加,是由面心立方锗晶体比面心立方硅晶体中电子云交叠小,产生的排斥较弱所导致的;面心立方结构C晶体是间接能隙为6.5e V的宽禁带半导体,面心立方结构Si晶体的导带和价带存在较小的交叠而呈现出半金属性,面心立方结构Ge和Sn的电子结构相似均表现为金属性,Ⅳ族元素面心立方结构晶体的电学性质由宽禁带半导体向金属转变.     

Microscopic phonon theory of Si/Ge nanocrystals

Microscopic phonon theory of semiconductor nanocrystals (NCs) is reviewed in this paper. Phonon modes of Si and Ge NCs with various sizes of up to 7 nm are investigated by valence force field theory. Phonon modes in spherical SiGe alloy NCs approximately 3.6 nm (containing 1147 atoms) in size have been investigated as a function of the Si concentration. Phonon density-of-states, quantum confinement effects, as well as Raman intensities are discussed.      

Theoretical study of influencing factors on the dispersion of bulk band-gap edges and the surface states in topological insulators Bi2Te3 and Bi2Se3

The dispersion of the band-gap edge states in bulk topological insulators Bi₂Te₃ and Bi₂Se₃ is considered within density functional theory. The dependences of this dispersion both on the approximation used for an exchange-correlation functional at fixed unit cell parameters and atomic positions and on these parameters and positions that are obtained upon structural relaxation performed using a certain approximated functional are analyzed. The relative position of the Dirac point of topologically protected surface states and the valence band maximum in the surface electronic structure of the topological insulators is discussed.     

Al-doped ZnO: Electronic, electrical and structural properties

Changes in structural, electrical and electronic properties of zinc oxide (ZnO) due to Al doping are studied using a quantum-chemical approach based on the Hartree-Fock theory. A periodic supercell of 128 atoms has been exploited throughout the study. The atomic parameters for Zn atom were obtained by reproducing the main properties of ZnO crystal as well as the first three ionization potentials of Zn atom. The perturbation imposed by Al atom incorporation leads to the atomic relaxation, which is computed and discussed in detail. A novel effect of electron density redistribution between different atomic orbitals within the same atom has been found. This phenomenon influences atomic rearrangement near Al impurity. The Al doping generates a free electron in the conduction band, which can be considered as a large radius electron polaron increasing the n-type electrical conductivity in the crystal in agreement with the known experimental data. The obtained small increase in the band-gap width due to the impurity incorporation resolves existing experimental debates on this point.     

Hydrogen adsorption on graphene: a first principles study

We present a systematic ab initio study of atomic hydrogen adsorption on graphene. The characteristics of the adsorption process are discussed in relation with the hydrogenation coverage. For systems with high coverage, the resultant strain due to substrate relaxation strongly affects H atom chemisorption. This leads to local structural changes that have not been pointed out to date, namely localized surface curvature. We demonstrate that the hydrogen chemisorption energy barrier is independent of the optimization technique and system size, being associated with the relaxation and rehybridization of the sole adsorbent carbon atom. On the other hand, the H desorption barrier is very sensitive to a correct structural relaxation and is also dependent on the degree of system hydrogenation.