纳米ZnO薄膜的激子光致发光特性

报道了纳米ZnO薄膜激子光致发光(PL)与温度的关系。首先利用低压金属有机化学气相沉积(LPMOCVD)技术生长ZnS薄膜,然后将ZnS薄膜在氧气中于800℃下热氧化2h获得纳米ZnO薄膜。X射线衍射(XRD)结果表明,纳米ZnO薄膜具有六角纤锌矿多晶结构且具有择优(002)取向。室温下观察到一束强的紫外(326eV)光致发光(PL)和很弱的深能级(DL)发射。根据激子峰的半高宽(FWHM)与温度的关系,确定了激子纵向光学声子(LO)的耦合强度(ГLO)。     

Charged Frenkel biexcitons in organic molecular crystals

It is known that the energy of the lowest electronic transition in the neutral molecules of anthracene, tetracene, and other polyacenes is blue-shifted in comparison with the corresponding transition energy in univalent molecular ions. This effect in a molecular crystal may be responsible for the attraction between a molecular (Frenkel) exciton and a charge carrier. Due to this attraction, a bound state of Frenkel exciton and free charge (charged Frenkel exciton) may be formed [5]. As we demonstrate below, the same mechanism can be responsible for the formation of a charged biexciton (bound state of two Frenkel excitons and a charge carrier). A one-dimensional lattice model is used which corresponds to J aggregates and is also a good approximation for quasi-one-dimensional crystals. Calculations are performed for molecular crystals like tetracene, where the exciton band at low temperature is much narrower than the band of the charge carrier.     

多组分纠缠在光子晶体中的存储

研究了激子的多组分纠缠相干态保真度在各向异性光子晶体中的演化行为.结果表明,当激子的跃迁频率处于光子晶体带隙时,保真度随时间变化作周期振荡,这与激子处于真空环境时,保真度振荡衰减的演化行为不同.此外,当激子跃迁频率离光子晶体带边较远时,其多组分纠缠相干态越容易被保存.     

ZnSe/ZnS抛物量子阱中激子的极化子效应(英文)

采用推广的LLP方法研究了ZnSe/ZnS抛物量子阱中激子的极化子效应。考虑电子和空穴与LO声子的相互作用,得到了激子基态能量和结合能随阱宽的变化关系。结果表明,阱宽较小时,能量随着阱宽的增大而急剧减小;阱宽较大时,能量减小的比较缓慢。和我们以前的工作对比,我们发现ZnSe/ZnS抛物量子阱对激子的束缚强于GaAs/Ga1-xAlxAs抛物量子阱。     

Exciton absorption of the GaAs semiconductor crystals under optical pumping to the conduction band

Modifications of the exciton structure of the fundamental absorption edge in the GaAs crystals is experimentally studied at T = 1.7 K using the optical pumping at a photon energy that is significantly greater than the band gap. An increase in the amplitude of the fundamental state of the exciton is observed at a stable maximum energy. The dependence of the integral absorption on the pump intensity is interpreted in the framework of the concept of the excitonic polariton using the dissipative scattering of the exciton by free electrons that are generated by the pumping radiation. The constant of the electron-exciton interaction can be estimated with the aid of the solution to the inverse problem for initial pump levels. The integral absorption of the fundamental exciton state at liquid-helium temperatures can be used to characterize the purity of an epitaxial layer. The reasons for the lower saturation level of the integral absorption that is significantly less than the calculated level determined by the exciton oscillator strength need to be further studied.     

Electromagnetically induced exciton mobility in a photonic band gap

It is suggested that an exciton in the engineered vacuum of a photonic-band-gap-quantum-well heterostructure exhibits electromagnetically induced anomalous quantum dynamics. The exciton is dressed by coherent emission and reabsorption of virtual photons near the photonic band edge and captured in momentum space, lowering its energy by 1-10 meV and lowering its effective mass by 4-5 orders of magnitude. The photonic band gap simultaneously enables strong coupling to confined optical modes and long exciton lifetime.     

Primary photoexcitations and the origin of the photocurrent in rubrene single crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.     

Purity of rubrene single crystals

Liquid chromatography connected with mass spectroscopy reveals that the oxidized form of rubrene is the major impurity in commercial powder of rubrene as well as in rubrene single crystals. One form of rubrene impurity can be transformed into the other. In solution, rubrene undergoes photo-oxidation completely until the red color of the rubrene solution disappears. Single crystals, due to compact packing of molecules and the required molecular shape change during oxidation, oxidize only on the surface.     

半导体量子点中弱耦合激子的性质

研究了抛物型半导体量子点中弱耦合激子的性质,在有效质量近似下,采用线性组合算符和幺正变换的方法,导出了抛物型半导体量子点中激子的基态能量。讨论了量子点半径和受限强度对半导体量子点中弱耦合激子的基态能量的影响。以GaAs半导体为例进行了数值计算,结果表明:在弱耦合情况下,重空穴激子和轻空穴激子的基态能量随量子点半径的减小而增大,随受限强度ω₀的增强而增大。     

Exciton polaritons in monoclinic zinc diphosphide

A classical theory based on excitons is insufficient to explain the reflectivity spectra of βZnP₂. Instead an exciton polariton picture is invoked and the polariton parameters are determined. Photoluminescence spectra are consistent with the polariton interpretation. Higher energy states of the exciton have been observed and from these the band gap and exciton binding energy in βZnP₂ have been deduced.     

Prediction of an excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.     

氧分压对PLD制备ZnO薄膜结构和发光性质的影响

采用脉冲激光沉积(PLD)技术,在Si(100)衬底上制备出高度c轴取向的ZnO薄膜。通过X射线衍射(XRD)谱,扫描电镜(SEM)和室温光致发光(PL)光谱的测量,研究了生长气氛压强的改变对薄膜结构和光致发光的影响。实验结果表明,当氧压从10Pa升高到100Pa时ZnO(002)衍射峰的半峰全宽(FWHM)增大。可以认为这是由于较高的氧压下,到达衬底表面的离子动能减小。这样部分离子没有足够的能量迁移到生长较快的(002)面,c轴取向变差,导致(002)衍射峰的强度降低,半峰全宽增大。随着氧压增大,紫外发光强度增强。这可能是氧压变大,薄膜的化学配比升高,说明化学配比对UV发光的影响要大于薄膜微结构的影响。改变氧气压强对薄膜的表面形貌也有较大的影响。     

Exciton dispersion and electronic excitations in hcp 4He

We present first measurements of the dispersion of excitons in solid helium, taken on a single hcp 4He crystal along the c axis. In agreement with studies on helium clusters, the major energy-loss peak can be interpreted as an intermediate molecular-type exciton, as we do not observe Wannier-like excitations. The measurements are in the (0 0 2) periodic zone, with the exciton energy dispersing along the c axis with a minimum at the gamma point. A calculated conduction band minimum at 31.0 eV above the valence band at gamma is supported by our data at energies above the exciton energy, leading to an exciton binding energy of 8.4 eV.     

Excitons in optical spectra of boron-nitride (BN) nanotubes

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the on-site interaction U=2t with the hopping integral t=1.1 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes, which show metallic and semiconducting properties depending on the chiralities.     

Coherent anti-stokes Raman spectroscopy of highly compressed solid deuterium at 300 K: evidence for a new phase and implications for the band gap

Coherent anti-Stokes Raman spectroscopy has been used to study deuterium at ambient temperature to 187 GPa, the highest pressure this technique has ever been applied. The pressure dependence of the nu1 vibron line shape indicates that deuterium has a rho direct=0.501 and rho exciton=0.434 mol/cm3 for a band gap of 2omega P=4.66 eV. The extrapolation from the ambient pressure band gap yields a metallization pressure of 460 GPa, confirming earlier measurements. Above 143 GPa, the Raman shift data provide clear evidence for the presence of the ab initio predicted I' phase of deuterium.     

Direct imaging of anisotropic exciton diffusion and triplet diffusion length in rubrene single crystals

We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.     

Temperature dependence of excitonic emission in cubic CdSe thin film

A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic CdSe film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton-phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16±1.5 meV was determined from the Arrhenius analysis.     

Photoluminescence study of polycrystalline CsSnI3 thin films: Determination of exciton binding energy

We report on the determination of exciton binding energy in perovskite semiconductor CsSnI₃ through a series of steady state and time-resolved photoluminescence measurements in a temperature range of 10–300 K. A large binding energy of 18 meV was deduced for this compound having a direct band gap of 1.32 eV at room temperature. We argue that the observed large binding energy is attributable to the exciton motion in the natural two-dimensional layers of SnI₄ tetragons in this material.     

二维共价有机骨架中的直接激子和间接激子

本文利用基于GW方法和Bethe-Salpeter方程的第一性原理计算，研究了两种二维共价有机骨架材料(COF)的激发态性质. 单层COF是直接带隙材料，而体相COF呈现间接带隙. 根据直接激子计算的体相COF的光学带隙和吸收光谱与实验一致，而由位于导带底的光生电子和位于价带顶的空穴形成的间接激子能量的理论计算值远低于实验荧光光谱的测量值. 研究表明，可以排除间接带隙COF材料的发光由声子主导的可能性. 研究认为体相COF的发光可能源于缺陷处直接激子的复合. 体相COF的AA堆叠结构导致其带隙是间接的. 如果将堆叠方式由AA变成AB，体相COF将转变成直接带隙材料，它的发光效率可能会增强.     

Superliquid state of an exciton mixture with small exciton radii in molecular crystals

The energy spectrum of excitons with a small radius in a superliquid state in molecular crystals has been investigated. A more complicated structure of the exciton energy spectrum with a number of equivalent minima for k 0 has been considered. The problem was solved as that of determining the energy spectrum of a mixture of various types of non-ideal exciton gases in a superliquid state, corresponding to the individual exciton energy minima. The appropriate branches of the energy spectrum of elementary excitations (hydrons) have been determined and the conditions for their stability have been investigated.