How does the substrate affect the Raman and excited state spectra of a carbon nanotube?

We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended across a trench and partially supported by a SiO₂-substrate. By tuning the laser excitation energy across the E ₃₃ excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial breathing mode (RBM), the D mode and the G mode show strong resonance enhancement of up to three orders of magnitude. In the supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E ₃₃ excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G band and D band is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect density in the supported CNT segments. By comparing the E ₃₃ resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements, however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser excitation power.     

Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass

Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z_1,2 excitonic emission of AgI exhibits fine structure with spacing of ∼0.20 eV (1610 cm⁻¹), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI-silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO₂ interface region before radiative recombination.     

Optical characterization of CdTe/ZnSe fractional monolayer structures grown by atomic layer epitaxy

Luminescence properties of CdTe/ZnSe fractional monolayer grown by atomic layer epitaxy have been investigated. To investigate the origin of the highly efficient luminescence, various optical spectroscopic methods such as, photoluminescence (PL), temporal/spatial resolved PL, temperature dependence PL, and excitation power dependence PL have been used. It is found that structural inhomogeneities affect dominant influence on the line width and line shape of luminescence. The luminescence intensity greatly enhanced by the localization of exciton at the disorder induced localized states.     

Variation of photoluminescence spectral line shape of monolayer WS2

The origin of the variation of photoluminescence (PL) spectra of monolayer tungsten disulfide (WS₂) is investigated systematically. Dependence of the PL spectrum on the excitation power show that the relatively sharp component corresponds to excitons whereas the broader component at slightly lower energy corresponds to negatively charged trions. PL imaging and second harmonic generation measurements show that the trion signals are suppressed more than the exciton signals near the edges, thereby relatively enhancing the excitonic feature in the PL spectrum and that such relative enhancement of the exciton signals is more pronounced near approximately armchair edges. This effect is interpreted in terms of depletion of free electrons near the edges caused by structural defects and adsorption of electron acceptors such as oxygen atoms.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Optical studies of localized excitons in symmetric coupled quantum wells

We present evidence for coupling between spatially separated excitonic states in a GaAs symmetric coupled quantum well. A split excitonic peak is observed in the photoluminescence spectrum of our sample and is identified as due to islands of monolayer fluctuation in the well width. We use the technique of far-infrared modulated photoluminescence to show that a resonant coupling exists between the excitonic states when the incident far-infrared photon energy is approximately equal to the splitting as measured in the photoluminescence spectrum. The most likely mechanism for this coupling is the dipole–dipole interaction.     

Förster Energy Transfer in Arrays of Epitaxial CdSe/ZnSe Quantum Dots Involving Bright and Dark Excitons

Using time-resolved photoluminescence (PL) spectroscopy, we establish the presence of the Förster energy transfer mechanism between two arrays of epitaxial CdSe/ZnSe quantum dots (QDs) of different sizes. The mechanism operates through dipole–dipole interaction between ground excitonic states of the smaller QDs and excited states of the larger QDs. The dependence of energy transfer efficiency on the width of barrier separating the QD insets is shown to be in line with the Förster mechanism. The temperature dependence of the PL decay times and PL intensity suggests the involvement of dark excitons in the energy transfer process.     

Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).     

Optical dynamics of energy-transfer from a CdZnO quantum well to a proximal Ag nanostructure

We studied photoluminescence (PL) and energy-transfer dynamics in a hybrid structure comprising a Cd(0.08)Zn(0.92)O quantum well (QW) and an Ag nanostructure. The observed PL quenching was dependent on the electronic states in the QW. Quenching occurred at low temperature where excited carriers recombined radiatively because of excitonic localization, which disappeared with increasing temperature due to delocalization of excitons. Furthermore, nanostructured Ag surfaces produced local surface plasmon (LSP) absorption that was resonant with the PL peak energy of the QW emission. These results indicate that the recombination energy of excitons transfers nonradiatively to induce LSP excitation, which was revealed using time-resolved PL measurements.     

紫外激发下锰掺杂纳米氧化锌

应用气相传输法, 以铜为催化剂, 在硅衬底上制备了锰掺杂氧化锌纳米四足晶须. 利用x射线与电子衍射谱、扫瞄电镜和高分辨率透射电镜, 对样品形貌、结构和成份进行表征与检测; 通过355nm与375nm紫外光激发下掺杂与未掺杂样品光致发光谱的对比, 结合拉曼散射谱与光致激发谱对掺杂样品中反常光致发光机理进行分析. 结果表明, 样品沿［0001］方向生长的前端和内部长有纤细刺状结构的中空六方管组成; 样品中掺入Mn2+的无辐射复合中心作用, 是掺杂样品紫外辐射强度和谱宽变小的主要原因, 氧化锌晶体中晶格周期对Mn2+中d-d电子跃迁过程的影响, 是375nm紫外光激发下掺杂样品中415nm辐射峰产生的主要原因.     

Optical orientation effects in the secondary radiation of excitonic molecules

The spectrum of secondary radiation emitted from excitonic molecules under resonant two-photon excitation is investigated theoretically. Taking into account the optical orientation for the final exciton states of the emission process, we derive polarization and angular features of the radiation, reflecting directly the relaxation stage of the molecules.     

Resonance exciton-phonon spectra in open ZnCdSe/ZnSe nanowires: Raman scattering and hot luminescence,extended and localized exciton states

Low-temperature (T ～ 8 K) secondary emission spectra of open Zn_0.87Cd_0.13Se/ZnSe nanowires were studied comprehensively in the region of the ground excitonic state under tunable laser excitation. The spectra revealed a fine structure produced in the interaction of excitons with optical phonons. The observed resonance exciton-phonon (REP) lines are shown to be formed through two different mechanisms. The strongest component is due to Raman scattering via free excitonic states. The other REP lines are generated in the hot luminescence of localized excitons. The spectrum of the optical phonons involved in the formation of the REP lines in the biaxially strained Zn_0.87Cd_0.13Se/ZnSe structures was analyzed.     

电场诱导聚合物分子取向对单线态和三线态激子形成截面的影响

通过对聚乙烯咔唑(PVK) 掺杂三(2-苯基吡啶)铱(Ir(ppy)₃)和4-二氰亚甲基-2-叔丁基-6-(1,1,7,7-四甲基久咯呢定基-9-烯基)-4H-吡喃(DCJTB)，PVK 掺杂DCJTB和PVK掺杂Ir(ppy)₃聚合物在成膜时高压电场作用下分子取向变化对单线态和三线态激子形成截面的研究，发现，随着成膜时电场的增强，单线态激子的形成截面在增加，而三线态激子的形成截面却减小. 关键词： 分子取向 激子形成截面 三线态 单线态     

Excitonic state in quantum wells formed from “above-barrier” electronic states

Lines corresponding to localized excitonic states formed from “above-barrier” electron and/or hole states (specifically, excitation lines of excitons formed by an electron localized in a QW and a free heavy hole) have been observed in the photoluminescence excitation spectra of GaAs/Al_0.05Ga_0.95As structures with quantum wells (QWs), each containing one single-particle size-quantization level for charge carriers of each type. A computational method is proposed that permits finding the binding energy and wave functions of excitons in QWs taking the Coulomb potential into account self-consistently. The computed values of the excitonic transition energies agree quite well with the experimental results. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 613–619 (10 November 1999)     

Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al_0.35Ga_0.65As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e₁-hh₁ transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed.     

Exciton luminescence and many-body effect of monolayer WS2 at room temperature

Monolayer transition metal dichalcogenides favor the formation of a variety of excitonic quasiparticles, and can serve as an ideal material for exploring room-temperature many-body effects in two-dimensional systems. Here, using mechanically exfoliated monolayer WS₂ and photoluminescence (PL) spectroscopy, exciton emission peaks are confirmed through temperature-dependent and electric-field-tuned PL spectroscopy. The dependence of exciton concentration on the excitation power density at room temperature is quantitatively analyzed. Exciton concentrations covering four orders of magnitude are divided into three stages. Within the low carrier concentration stage, the system is dominated by excitons, with a small fraction of trions and localized excitons. At the high carrier concentration stage, the localized exciton emission from defects coincides with the emission peak position of trions, resulting in broad spectral characteristics at room temperature.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Optical transitions and carrier dynamics in self-organized InAs quantum dots grown on In0.52Al0.48As/InP(0 0 1)

Optical transitions in self-organized InAs quantum dots (QDs) grown on In_0.52Al_0.48As layer lattice matched to InP(0 0 1) substrate, have been studied by continuous wave (cw) photoluminescence (PL) and time-resolved PL. The dependence of the PL transition on excitation power and photoluminescence excitation measurements clearly shows that the multi-component cw-PL spectrum is related to emission coming from ground and related excited states of QDs with heights varying by monolayer fluctuations. While decay times measured by time-resolved PL are in the nanosecond range for the ground states, shorter decay times related to relaxation of carriers down directly to the ground state are determined for the excited states.     

Observation of long-lived excitons in InAs quantum dots under thermal redistribution temperature

We study the temperature-dependent time-resolved photoluminescence (TRPL) of self-assembled InAs quantum dots (QDs). Under low excitation power, a surprisingly long PL decay time is observed at about 60 K, under the thermal redistribution temperature. The long decay time decreases with increasing excitation power but is nearly independent of the detection energy of TRPL measurements. A model considering the spin relaxation through the excited excitonic state is proposed to quantitatively explain the unusual phenomena. The rate equation analysis indicates that the observation of long-lived excitons is caused by the shortened spin-flip time.     

Resonant Raman scattering spectra in a ZnCdSe/ZnSe structure with a quantum well and open nanowires

Resonance Raman scattering (RS) spectra of a ZnCdSe/ZnSe sample containing a single quantum well and quantum well-based open nanowires were studied at T=300 K. The longitudinal optical (LO) phonons involved in the formation of the observed spectra of the quantum-well and nanowire regions differ noticeably in energy. The LO phonon energies in the structures under study were calculated taking into account the compositional effect (doping of Cd into ZnSe) and biaxial strain. When excited in the exciton resonance region, RS is shown to occur via free (extended) excitonic states with the involvement of LO phonons of the ZnCdSe strained layer with final wave vectors near the Brillouin zone center. When excited below the excitonic resonance in the ZnCdSe layer, resonance scattering via localized exciton states provides a noticeable contribution to the observed RS lines. Because of the finite size of a localized state, phonons with large wave vectors are involved in these scattering processes. The RS lines produced under excitation in the excitonic region of the thick barrier layers are due to scattering from the ZnSe barrier phonons.