Structures,Oxidation, and Charge Transport of Phosphorus‐Doped Germanium Nanocrystals

The doping of semiconductor nanocrystals (NCs) is crucial for the optimization of the performance of devices based on them. In contrast to recent progress on the doping of compound semiconductor NCs and silicon NCs, the doping of germanium (Ge) NCs has lagged behind. Here it is shown that Ge NCs can be doped with phosphorus (P) during synthesis by a nonthermal plasma. It is found that there are more P atoms in the NC near‐surface region than in the NC core. P doping modifies the surface state of Ge NCs. Compressive strain can be incuced in Ge NCs by P which can explain the P‐doping‐enhanced oxidation resistance of Ge NCs. Stable dispersions of P‐doped Ge NCs in acetonitrile can be cast to produce films for field‐effect transistors (FETs). FET analysis shows that the electrical conductivity and electron mobility of a Ge‐NC film increase with the increase of the P doping level, although the electrical activation efficiency of P in the Ge‐NC film is low. Finally, atomic layer deposition of aluminum oxide at the surface of P‐doped Ge NCs is shown to improve the performance of the FETs.     

Boron‐ and Phosphorus‐Hyperdoped Silicon Nanocrystals

Hyperdoping silicon nanocrystals (Si NCs) to a concentration exceeding the solubility limit of a dopant may enable their novel applications. Here, the successful hyperdoping of Si NCs with boron (B) and phosphorus (P) is demonstrated, which are the most important dopants for Si. Despite the hyperdoping, the diamond structure of Si NCs is hardly modified. There are both electrically active B and P in hyperdoped Si NCs. It is proposed that the hyperdoping is made possible mainly by the kinetics in the nonthermal plasma synthesis of Si NCs. Collision between Si NCs and B or P atoms and the binding energy of B or P at the NC surface are critical to the understanding on the differences in the doping efficiency and dopant distribution between B and P. B‐hyperdoping‐induced tensile stress needs to be taken into account in the investigation on the doping and oxidation of Si NCs.     

Fluctuations of electroluminescence intensity of single CdSe nanocrystals excited by scanning tunneling microscope current

Electroluminescence from single CdSe nanocrystals (NCs) excited by tunneling current of scanning tunneling microscope (STM) has been measured. Two types of samples with low and high concentration of CdSe NCs deposited on the gold substrate have been prepared. Both types of samples had no plasmon emission. It enabled one to detect pure electroluminescence from single CdSe NCs. Samples with low concentration of NCs exhibit an intensive short-term luminescence of NCs for several seconds. Samples with high concentration of NCs exhibit a weak fluctuating long-term luminescence for thousand seconds. Fluctuations of NC electroluminescence differ considerably from those detected recently in photoluminescence of CdSe NCs embedded in polymer films. The difference in fluctuations results from the difference in physical conditions existing in electro- and photoluminescence. The distribution of photons w(N, T) emitted in time interval T has been found from statistical treating of fluctuating luminescence. Due to weakness of the pure signal, we paid a special attention to allowing for photomultiplier tube noise while treating these fluctuations. The photon distribution in pure signal is one of super-Poisson type, i.e. it is broader than Poisson distribution. A dynamical model for an absorber–emitter excited by tunneling current of STM has been offered. The model takes into account the thermal drift of STM tip.     

Exchange-coupled donor dimers in nanocrystal quantum dots

Doping of semiconductor nanocrystals (NCs) is expected to enable the control of key NC properties, yet its practical exploitation requires an understanding of exchange interactions when multiple dopants are incorporated in a single NC. Here, we experimentally probe the exchange of donor dimers in NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We show that the exchange coupling of the closely spaced donors can be well described by effective mass theory, which allows the consideration of statistical effects crucial in NC ensembles. While a dimer induces discrete states in a NC, their energy splitting differs by up to 3 orders of magnitude for randomly placed dimers in a NC ensemble, due to an enormous dependence of the exchange energy on the dimer configuration.     

Charge retention in quantized energy levels of nanocrystals

Understanding charging mechanisms and charge retention dynamics of nanocrystal (NC) memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium NCs embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of NCs and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different NC sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the NCs.     

Automatically Purification of Aqueous CdTe Nanocrystals in Water-ethanol Co-environment

Purification is a separated post-treatment step after the synthesis of nanocrystals (NCs) in order to exclude excess ligands and monomers in NC solution. The common purification process involves many manipulations, such as concentrating, addition of anti-solvents and centrifugation, which are troublesome and time consuming. In this work, we originally integrate NC synthesis and NC purification in one-pot via selecting water-ethanol co-environment for NC synthesis and NC purification. Our research shows that NCs can grow in water-ethanol mixture. When growing into critical size, NCs will automatically precipitate from the solution. Element analysis demonstrates that precipitates fraction fits well with stoichiometric of ligand-capped NCs. Excess monomers are left in supernatant, and thus achieving automatically purification of NCs in the water-ethanol co-environment. By adjusting the volume ratios of water and ethanol in bi-solvent system, different-sized purified NCs can be controlled. Besides, this water-ethanol co-environment can be used in both thermal-promoted and hydrazine-promoted growth.     

Highly efficient Raman conversion in O2 pumped by a seeded narrow band second-harmonic Nd:YAG laser

The first-Stokes conversion efficiency for a stimulated Raman scattering (SRS) is usually very low in gaseous oxygen media. In 3.0 Mpa O₂, a single longitudinal mode second harmonic Nd:YAG laser pump source gives a typical vibrational first-Stokes conversion efficiency of only 2.5%, In comparison, the accompanying stimulated Brillouin scattering (SBS) attains a reflectivity of 67%. However, by seeding an OPO beam into the Raman cavity, the first-Stokes photon conversion efficiency now attains a peak value of 54%, while the SBS reflectivity reduces to 5% in a 6.1 Mpa 41:59 O₂/ He mixture. This 54% efficiency was obtained for a seeder laser pulse-width less than one half that of pump laser (6.8 ns). A first-Stokes peak power conversion efficiency as high as 88% has been obtained when the pump and seeder pulse peaks coincide. So, we may expect a higher first-Stokes photon conversion efficiency if the seeder pulse-width can be made equal to or larger than that of the pump pulse. On the other hand, the beam quality of the first-Stokes in an O₂/ He mixture excels that of the pump laser for a seeder energy of 5 mJ and pump energy of 50 mJ. However, at pump energies higher than 105 mJ and a pump laser repetition rate of 10 Hz, the thermal defocusing effect worsens the first-Stokes beam quality. This thermal defocusing effect is a result of the Raman heat release and could be eliminated by fast circulating and cooling the Raman gas medium.     

两个独立全光纤多通道光子纠缠源的Hong-Ou-Mandel干涉

独立光子源的干涉是实现复杂量子体系应用(比如多光子纠缠态产生和量子隐形传态等)的核心技术.利用100 GHz密集波分复用技术,实现了1.55μm全光纤多通道独立纠缠光子源的Hong-Ou-Mandel干涉,在不去除暗符合(随机符合计数)的情况下,可见度为53.2%±8.4%,去除暗符合可见度可达到82.9%±5.3%.给出了关于色散位移光纤中基于自发四波混频过程产生的单光子光谱纯度严格的理论描述,模拟了抽运脉冲宽度和滤波器带宽对单光子光谱纯度的影响,并给出了理论上的最佳条件(最佳的抽运脉冲宽度为8 ps,高斯滤波器带宽为40 GHz及以下).在测量Hong-Ou-Mandel干涉之前,先测量了液氮冷却状态下的色散位移光纤关联光子源的符合和随机符合比率,在抽运功率为23μW的情况下,最大比率可以达到131.Hong-Ou-Mandel干涉在高精度光学测量、测量装置无关的量子密钥分配等应用中扮演着极为重要的角色.     

Generation of UV laser light by stimulated Raman scattering in D2, D2/Ar and D2/He using a pulsed Nd:YAG laser at 355nm

A pulsed Nd:YAG laser at 355nm is used to pump Raman cell filled with D_2, D_2/Ar and D_2/He. With adequately adjusted parameters, the maximum photon conversion efficiency of the first-order Stokes light (S_1, 396.796nm) reaches 33.33% in D_2/Ar and the stability of S_1 in pure D_2 is fairly high, the energy drift being less than 10% when the pump energy drifts in the range of 5%. The conversion efficiency and stability, which are functions of the composition and pressure of the Raman medium and the energy of pump laser, are investigated. The result has been used to optimize the laser transmitter system for a differential absorption lidar system to measure NO_2 concentration profiles.     

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.     

Deciphering the Structure and Chemical Composition of Drug Nanocarriers: From Bulk Approaches to Individual Nanoparticle Characterization

Drug nanocarriers (NCs) with sizes usually below 200 nm are gaining increasing interest in the treatment of severe diseases such as cancer and infections. Characterization methods to investigate the morphology and physicochemical properties of multifunctional NCs are key in their optimization and in the study of their in vitro and in vivo fate. Whereas a variety of methods has been developed to characterize “bulk” NCs in suspension, the scope of this review is to describe the different approaches for the NC characterization on an individual basis, for which fewer techniques are available. The accent is put on methods devoid of labelling, which could lead to artefacts. For each characterization method, the principles and approaches to analyze the data are presented in an accessible manner. Aspects related to sample preparation to avoid artefacts are indicated, and emphasis is put on examples of applications. NC characterization on an individual basis allows gaining invaluable information in terms of quality control, on: i) NC localization and fate in biological samples; ii) NC morphology and crystallinity; iii) distribution of the NC components (drugs, shells), and iv) quantification of NCs’ chemical composition. The individual characterization approaches are expected to gain increasing interest in the near future.     

Size-dependent intrinsic radiative decay rates of silicon nanocrystals at large confinement energies

We study ultrafast photoluminescence (PL) dynamics of Si nanocrystals (NCs). The early-time PL spectra (<1 ns), which show strong dependence on NC size, are attributed to emission involving NC quantized states. The PL spectra recorded for long delays (>10 ns) are almost independent of NC size and are likely due to surface-related recombination. Based on instantaneous PL intensities measured 2 ps after excitation, we determine intrinsic radiative rate constants for NCs of different sizes. These constants sharply increase for confinement energies greater than approximately 1 eV indicating a fast, exponential growth of the oscillator strength of zero-phonon, pseudodirect transitions.     

Energy dispersion of localized states in light-sensitive nanocrystals

The kinetics of the formation and thermal destruction of color centers in CuCl and AgCl nanocrystals (NCs) distributed in a glass matrix is described on the basis of the band model of an NC with colloidal color centers and with hole traps of one species. The possibility of experimentally determining the relative depth distribution of hole states in light-sensitive NCs in glass is demonstrated. The observed energy dispersion of localized hole states and its variation in NCs are associated, in accordance with Dexter’s idea, with large-scale thermal fluctuations of the crystal field. The presence of an excess charge on a colloidal particle and its influence on localized hole states are presumed.     

Electroluminescence of single CdSe nanocrystals induced by scanning tunneling microscope

Electroluminescence intensity of single CdSe nanocrystals (NCs) excited by the tunnel current of a scanning tunneling microscope (STM) was measured. Samples with NCs do not exhibit plasmon emission. This enables one to measure pure emission from a single NC. Electroluminescence, measured in time, differs considerably from that of photoluminescence in similar NCs. This difference in temporal behavior results from the difference in physical conditions of experiments on photo- and electroluminescence.     

极化子效应对ZnS/CdSe核壳量子点光吸收系数的影响

在有效质量近似下,利用量子力学密度矩阵理论,从理论上研究了考虑极化子效应后核壳量子点中线性、三阶非线性以及总的光吸收系数在不同条件下随入射光能量变化的关系。通过数值计算,分析了电子-LO声子和电子-IO声子相互作用对ZnS/CdSe柱型核壳结构量子点光吸收系数的影响。结果表明,极化子效应对光吸收系数有很大影响,不同声子模式对光吸收系数影响大小不同。考虑电子-LO声子后,光吸收系数被大大提高。另外,入射光强和弛豫时间对系统的吸收系数也有很大影响。     

Mechanism and enhancement of photoluminescence from silicon nanocrystals implanted in SiO2 matrix

Photoluminescence (PL) spectra of Si nanocrystals (NCs) prepared by 130 keV Si ions implantation onto SiO2 matrix were investigated as a function of annealing temperature and implanted ion dose. PL spectra consist of two PL peaks, originated from smaller Si NCs due to quantum confinement effect (QCE) and the interface states located at the surface of larger Si NCs. The evolution of number of dangling bonds (DBs) on Si NCs was also investigated. For hydrogen-passivated samples, a monotonic increase in PL peak intensity with the dose of implanted Si ions up to 3×1017 ions /cm2 is observed. The number of DBs on individual Si NC, the interaction between DBs at the surface of neighbouring Si NCs and their effects on the efficiency of PL are discussed.     

Collective spontaneous emission of dye molecules and lasing

The threshold pump power density for lasing in dye solutions is found to depend on the photon energy of pumping radiation. An increase in the pumping photon energy can significantly lower the threshold pump power of dye lasers. For an ethanol solution of rhodamine 6G with a concentration of 4×10¹⁸ cm^?3, the threshold power density for pumping radiation with a wavelength of 532 nm is 20-fold higher than for pumping radiation with a wavelength of 347 nm. This phenomenon is associated with the competition of collective spontaneous emission, which can lead to the efficient deactivation of excited molecules in femtosecond times, and the dephasing of excited molecules due to the intramolecular nonradiative processes of absorbed-energy conversion. An increase in the dephasing rate with the increasing energy of exciting photons lowers the efficiency of collective spontaneous emission and increases the concentration of dephased excited molecules responsible for lasing.     

Theoretical analysis on the efficiency of optical-optical double-color double-resonance multiphoton ionization

Analytic formula of the efficiency of optical-optical double-color double-resonance multi-photon ionization （OODR-MPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the influence of characteristic of the pump and probe laser on the ionization efficiency of （1＋2＋1） OODR-MPI process is simulated theoretically. It is shown that the pump laser will affect the ionization efficiency by the number control of the molecules excited to the first resonance state. The ionization efficiency is decided by the probe laser directly. Both of the excited molecules and ionization efficiency increase with the intensity and pulse duration of the laser until saturation. It is also found that the longer the delay time of the probe laser to the pump one is, the lower the ionization efficiency would be. The delay time ought to be smaller than the lifetime of the excited molecule in the practical use of the OODR-MPI technique.     

Ultrasound-assisted synthesis of nanocomposites based on aromatic polyamide and modified ZnO nanoparticle for removal of toxic Cr(VI) from water

In this study, novel nanocomposites (NCs) of aromatic polyamide (PA) and surface modified ZnO nanoparticle with s-triazine heterocyclic ring was introduced for efficient removal of toxic hexavalent chromium (VI) from aqueous solution. The surface of ZnO nanoparticle was modified by s-triazine core silane coupling agent (ZnO-TSC) and PA/ZnO-TSC NCs with different amount of ZnO-TSC nanoparticles (0, 5, 10 and 15 wt%) were prepared by ultrasonic irradiation. The synthesized PA/ZnO-TSC NCs were characterized by FT-IR, XRD, FE-SEM, TEM and TGA methods. TEM images showed that ZnO nanoparticles were dispersed homogeneously in the polymer matrix. The adsorption experiments were carried out in batch mode to optimize various parameters like contact time, pH and concentration of metal ion that influence the adsorption rate. The maximum uptakes of Cr(VI) at pH 4.0 was 72%, 81%, 89% and 91% for pure PA, NC5%, NC10% and NC15%, respectively. The kinetic of adsorption was investigated and the pseudo second-order model is an appropriate model for interpretation of adsorption mechanism of Cr(VI) ions.     

Upconversion luminescent characteristics and peak shift of CdSeS nanocrystals under different wavelength laser excitation

Upconversion luminescence was obtained from CdSeS nanocrystals (NCs) under 800 nm femtosecond laser excitation. The structural and optical characteristics of the CdSeS NCs were investigated experimentally by use of UV–visible absorption spectroscopy, transmission electron microscopy, X-ray diffractometry, and time-resolved luminescence dynamics. Peak shift of luminescence in CdSeS NCs can be readily observed under different wavelength femtosecond excitation. The pump power dependence of the luminescence intensity and time-resolved decay revealed that one, two, and three-photon absorption occur. It was found that upconversion luminescence is composed of photoinduced trapping and a band-edge excitonic state, and two types of species are involved in the biexponential luminescence decay kinetics. With increasing Se-doped composition, luminescence lifetimes of CdSeS NCs with similar sizes become shorter. This is not consistent with the changes of undoped CdS NCs and is ascribed to impurity level increased doping in the energy gap, which is favorable for trapping luminescence. A simple energy level of doping NCs is used to interpret upconversion luminescence and the peak shift of steady-state emission.