Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Photocharging ZnO Nanocrystals: Picosecond Hole Capture, Electron Accumulation, and Auger Recombination

Photochemical charging of colloidal ZnO nanocrystals has been studied using continuous-wave and time-resolved photoluminescence spectroscopies in conjunction with electron paramagnetic resonance spectroscopy. Experiments have been performed with and without addition of alcohols as hole quenchers, focusing on ethanol. Both aerobic and anaerobic conditions have been examined. We find that ethanol quenches valence-band holes within ∼15 ps of photoexcitation, but does not quench the trapped holes responsible for the characteristic visible photoluminescence of ZnO nanocrystals. Hole quenching yields “charged” nanocrystals containing excess conduction-band electrons. The extra conduction-band electrons quench visible trap-centered luminescence via a highly effective electron/trap-state Auger-type cross-relaxation process. This Auger process is prominent even under aerobic photoexcitation conditions, particularly when samples are not stirred. Charging also reduces exciton nonradiative decay rates, resulting in increased UV luminescence. The dependence of charging on ethanol concentration and the reduced exciton nonradiative decay rates of charged ZnO nanocrystals are discussed. Finally, the results here provide a kinetic basis for understanding photochemical electron accumulation in colloidal ZnO nanocrystals

Photochemical Electronic Doping of Colloidal CdSe Nanocrystals

A method for electronic doping of colloidal CdSe nanocrystals (NCs) is reported. Anaerobic photoexcitation of CdSe NCs in the presence of a borohydride hole quencher, Li­[Et₃BH], yields colloidal <i>n</i>-type CdSe NCs possessing extra conduction-band electrons compensated by cations deposited by the hydride hole quencher. The photodoped NCs possess excellent optical quality and display the key spectroscopic signatures associated with NC <i>n</i>-doping, including a bleach at the absorption edge, appearance of a new IR absorption band, and Auger quenching of the excitonic photoluminescence. Although stable under anaerobic conditions, these spectroscopic changes are all reversed completely upon exposure of the <i>n</i>-doped NCs to air. Chemical titration of the added electrons confirms previous correlations between absorption bleach and electron accumulation and provides a means of quantifying the extent of electron trapping in some NCs. The generality of this photodoping method is demonstrated by initial results on colloidal CdE (E = S, Te) NCs as well as on CdSe quantum dot films

Size Dependence of Negative Trion Auger Recombination in Photodoped CdSe Nanocrystals

We report a systematic investigation of the size dependence of negative trion (T^–) Auger recombination rates in free-standing colloidal CdSe nanocrystals. Colloidal n-type CdSe nanocrystals of various radii have been prepared photochemically, and their trion decay dynamics have been measured using time-resolved photoluminescence spectroscopy. Trion Auger time constants spanning 3 orders of magnitude are observed, ranging from 57 ps (radius <i>R</i> = 1.4 nm) to 2.2 ns (<i>R</i> = 3.2 nm). The data reveal a substantially stronger size dependence than found for bi- or multiexciton Auger recombination in CdSe or other semiconductor nanocrystals, scaling in proportion to <i>R</i>^4.3

Optical absorption and spectral photoconductivity in α-(Fe \u3csub\u3e1-x\u3c/sub\u3eCr\u3csub\u3ex\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e solid-solution thin films

Hematite, α-Fe2O3, is an attractive narrow gap oxide for consideration as an efficient visible light photocatalyst, with significant potential for band gap engineering via doping. We examine optical absorption in α-(Fe1-xCrx)2O3 epitaxial films and explain the observed excitations, and the nature of the band gap dependence on x, through first-principles calculations. The calculated and measured optical band gap becomes smaller than that of bulk α-Fe 2O3 and reaches a minimum as the Cr cation fraction increases to 50%. The lowest energy transitions in the mixed-metal alloys involve electron excitation from occupied Cr 3d orbitals to unoccupied Fe 3d orbitals, and they result in a measurable photocurrent. The onset of α-Fe2O3 photoconductivity can be reduced by nearly 0.5 eV (to 1.60 eV) through addition of Cr. © 2013 IOP Publishing Ltd