高增益复合泵浦O^＋7X射线激光的理论研究

用一维流体力学程序MEDUSA耦合原子物理程序NIMP,对超短脉冲激光辐照Mylar纤维靶和平面靶产生复合泵浦O~(17)X射线激光的物理过程进行了理论研究,探讨了利用VULCAN激光装置的CPA皮秒脉冲光束进行饱和增益实验的可行性.结果表明,超短脉冲辐照激光靶可能是产生饱和增益的复合泵浦X射线激光的一条有效途径.     

Unknots and Molecular Biology

This article shows that an unknot is obtained as the numerator closure of the sum of two rational tangles, whose respective fractions are P/Q and R/S, if and only if PS − QR = ± 1. This result is used to construct many examples of unknotted diagrams, including “hard” unknot diagrams that require non-simplifying Reidemeister moves in order to be unknotted. The paper then discusses minimal hard unknots and the applications of these results to DNA recombination. Lecture held in the Seminario Matematico e Fisico on July 7, 2005 Received: July 2006     

Temperature dependence of ultra-exothermic charge recombinations.

We measured the temperature dependence (from +32 to -50 degrees C) of charge-recombination rates between contact radical ion pairs in isopropyl ether. In the systems selected for this study, aromatic hydrocarbon cations are the electron acceptors and the fumaronitrile anion is the electron donor. Nearly quantitative electron transfers occur at all temperatures. The charge recombinations have excess exothermicities of -60 kcal mol(-1) and exhibit a very weak temperature dependence. Our observations emphasize the absence of solvent effects and the relevance of nuclear tunneling in charge recombinations.     

Chemical and electronic properties of sulfur-passivated InAs surfaces

Treatment with ammonium sulfide ((NH₄)₂S_x) solutions is used to produce model passivated InAs(0 0 1) surfaces with well-defined chemical and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aqueous solutions, as characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the associated downward band bending.     

双层有机发光二极管中复合效率和复合区域宽度的研究

本文建立了双层有机发光二极管中载流子的注入、输运和复合的理论模型.模型中采用了较合理的无序跳跃模型来处理界面问题.计算和讨论了空穴传输层厚度和内界面处的空穴势垒对器件复合效率和复合区域宽度的影响.结果表明:器件结构的变化导致电场强度在器件中的重新分布,空穴传输层厚度和内界面处空穴势垒的变化对器件的复合效率和复合区域宽度有重要影响.     

III‐nitride quantum dots for ultra‐efficient solid‐state lighting

III‐nitride light‐emitting diodes (LEDs) and laser diodes (LDs) are ultimately limited in performance due to parasitic Auger recombination. For LEDs, the consequences are poor efficiencies at high current densities; for LDs, the consequences are high thresholds and limited efficiencies. Here, we present arguments for III‐nitride quantum dots (QDs) as active regions for both LEDs and LDs, to circumvent Auger recombination and achieve efficiencies at higher current densities that are not possible with quantum wells. QD‐based LDs achieve gain and thresholds at lower carrier densities before Auger recombination becomes appreciable. QD‐based LEDs achieve higher efficiencies at higher currents because of higher spontaneous emission rates and reduced Auger recombination. The technical challenge is to control the size distribution and volume of the QDs to realize these benefits. If constructed properly, III‐nitride light‐emitting devices with QD active regions have the potential to outperform quantum well light‐emitting devices, and enable an era of ultra‐efficient solid‐state lighting.

     

Site-Specific Fluorescent Labeling of RNA Interior Positions

The introduction of fluorophores into RNA for both in vitro and in cellulo studies of RNA function and cellular distribution is a subject of great current interest. Here I briefly review methods, some well-established and others newly developed, which have been successfully exploited to site-specifically fluorescently label interior positions of RNAs, as a guide to investigators seeking to apply this approach to their studies. Most of these methods can be applied directly to intact RNAs, including (1) the exploitation of natural posttranslational modifications, (2) the repurposing of enzymatic transferase reactions, and (3) the nucleic acid-assisted labeling of intact RNAs. In addition, several methods are described in which specifically labeled RNAs are prepared de novo.     

Long-lived Single Excitons,Trions, and Biexcitons in CdSe/CdTe Type-Ⅱ Colloidal Quantum Wells

Light-harvesters with long-lived excited states are desired for efficient solar energy conversion systems. Many solar-to-fuel conversion reactions, such as H₂ evolution and CO₂ reduction, require multiple sequential electron transfer processes, which leads to a complicated situation that excited states involves not only excitons (electron-hole pairs) but also multi-excitons and charged excitons. While long-lived excitons can be obtained in various systems (e.g., semiconductor nanocrystals), multi-excitons and charged excitons are typically shorted-lived due to nonradiative Auger recombination pathways whereby the recombination energy of an exciton is quickly transferred to the third carrier on a few to hundreds of picoseconds timescale. In this work, we report a study of excitons, trions (an exciton plus an additional charge), and biexcitons in CdSe/CdTe colloidal quantum wells or nanoplatelets. The typeⅡ band alignment effectively separates electrons and holes in space, leading to a single exciton lifetime of 340 ns which is ~2 order of magnitudes longer than that in plane CdSe nanoplatelets. More importantly, the electron-hole separation also dramatically slows down Auger decay, giving rise to a trion lifetime of 70 ns and a biexciton lifetime of 11 ns, among the longest values ever reported for colloidal nanocrystals. The long-lived exciton, trion, and biexciton states, combined with the intrinsically strong light-absorption capability of two-dimensional systems, enable the CdSe/CdTe type-Ⅱ nanoplatelets as promising light harvesters for efficient solar-to-fuel conversion reactions.     

Bis(phenothiazyl‐ethynylene)‐Based Organic Dyes Containing Di‐Anchoring Groups with Efficiency Comparable to N719 for Dye‐Sensitized Solar Cells

A new series of acetylene‐bridged phenothiazine‐based di‐anchoring dyes have been synthesized, fully characterized, and used as the photoactive layer for the fabrication of conventional dye‐sensitized solar cells (DSSCs). Tuning of their photophysical and electrochemical properties using different π‐conjugated aromatic rings as the central bridges has been demonstrated. This molecular design strategy successfully inhibits the undesirable charge recombination and prolongs the electron lifetime significantly to improve the power conversion efficiency (η ), which was proven by the detailed studies of electrochemical impedance spectroscopy (EIS) and open‐circuit voltage decay (OCVD). Under a standard air mass (AM) 1.5 irradiation (100 mW cm⁻²), the DSSC based on the dye with phenyl bridging unit exhibits the highest η of 7.44 % with open‐circuit photovoltage (V _oc) of 0.796 V, short‐circuit photocurrent density (J _sc) of 12.49 mA cm⁻² and fill factor (ff) of 0.748. This η value is comparable to that of the benchmark N719 under the same conditions.     

Power‐ and polarization dependence of two photon luminescence of single CdSe nanowires with tightly focused cylindrical vector beams of ultrashort laser pulses

We investigate two‐photon luminescence (TPL) of single CdSe nanowires (NWs) excited by tightly focused cylindrical vector beams of 150 fs pulses, achieving an optical resolution better than λ/4. Comparing the TPL images recorded by scanning the nanowires through the focal fields created either by a radially or an azimuthally polarized vector beam we observe a distinct anisotropic excitation efficiency which depends on the orientation of the electric field component with respect to the nanowire. The excitation anisotropy of the linear photoluminescence (PL) and TPL can directly be derived from the respective image patterns. Our results show that the highest TPL signal from a single NW is detected when the electric excitation field is parallel to the long axis of the NW, i.e. about one order of magnitude stronger than that excited along the short axis. We attribute the TPL to an emission mechanism based on the recombination of free carriers, which exhibits a fourth order excitation power dependence in low power regime and a second order power dependence in high power regime.