University of Arkansas

About: University of Arkansas is a education organization based out in Fayetteville, Arkansas, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 17225 authors who have published 33329 publications receiving 941102 citations. The organization is also known as: Arkansas & UA.

Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.

Abstract: High-quality colloidal semiconductor nanocrystals are nanometer-sized, single crystalline fragments of the corresponding bulk crystals, which have well-controlled size and size distribution and are dispersible in desired solvents/media. Recently, semiconductor nanocrystals are of great interest for both fundamental research and technical applications, 1-8 due to their strong size dependent properties and excellent chemical processibility. Synthesis of highquality semiconductor nanocrystals has been playing a critical role in this very active field. 1,9-15 As the most developed system in terms of synthesis, 1,9,10,15high-quality CdSe nanocrystals with nearly monodisperse size and shape are in active industrial development for biological labeling reagents. 5,6 Since Murray et al. 15 reported the synthesis of high quality cadmium chalcogenides nanocrystals using dimethyl cadmium (Cd(CH 3)2) as the cadmium precursor, the synthesis of CdSe nanocrystals using this precursor has been well developed. 1,9,10In comparison, the synthesis of CdTe and CdS15,16are not as advanced. For instance, there is no method to controllably vary the shape of CdTe and CdS nanocrystals. Cd(CH3)2 is extremely toxic, pyrophoric, expensive, unstable at room temperature, and explosive at elevated temperatures by releasing large amount of gas. Due to these reasons, the Cd(CH 3)2related schemes require very restricted equipments and conditions and are not suited for large-scale synthesis. In this paper, we will prove that Cd(CH3)2 can be replaced by CdO. Surprisingly, this new synthetic scheme works significantly better than the Cd(CH3)2-related ones. Without any size-sorting, the quality of quantum-confined dots and rods (quantum dots and quantum rods) of all cadmium chalcognides formed by the new method is comparable to that of the best CdSe nanocrystals reported in the literature. The new scheme is reproducible and simple and thus can be readily scaled up for industrial production. Recently, we identified that Cd(CH 3)2 decomposes in hot trioctylphosphine oxide (TOPO) and generates insoluble metallic precipitate. 9 With a strong ligand, either hexylphosphonic acid (HPA) or tetradecylphosphonic acid (TDPA), Cd(CH 3)2 is immediately converted into cadmium HPA/TDPA complex (Cd HPA/Cd-TDPA) if the cadmium to HPA/TDPA ratio is lower than 1. After the formation of the complex, an injection of Se dissolved in tributylphosphine (TBP) generates high-quality CdSe nanocrystals. This result implies that Cd(CH 3)2 may not be necessary, if we can generate the complex by other means. We first synthesized and purified Cd -HPA from CdCl 2 or Cd(CH3)2. High-quality CdSe nanocrystals were indeed yielded from this complex. This success encouraged us to develop a one-pot synthesis which does not require separated preparation of cadmium complex. We failed to make high-quality CdSe nanocrystals using CdCl2 by the one-pot approach although CdCl 2 can be dissolved in the reaction mixture at elevated temperatures. In contrast, CdO works very well for the one-pot approach. We think this is due to the low stability of CdO relative to phosphonic acids, compared to that of CdCl 2. Experimentally, CdO, TOPO, and HPA/TDPA were loaded in a three-neck flask. At about 300 °C, reddish CdO powder was dissolved and generated a colorless homogeneous solution. Introducing tellurium, selenium, and sulfur stock solutions yields high quality nanocrystals. 17 The samples for all of the measurements shown in this paper are directly from synthesis without any size separation. The growth kinetics of nanocrystals grown by the new approach possesses a pattern similar to that of the best CdSe nanocrystals formed by the Cd(CH3)2 approach (Figure 1). 10 Figure 1 and Figure 2 further reveal that the size of all three kinds of nanocrystals can be close to monodisperse, represented by the sharp absorption peaks if the growth stops in the “focusing of size distribution” regime. 10 Transmission electron microscopy (TEM) measurements indicate that these nanocrystals have very narrow distribution. The relative standard deviation of the size of the nanocrystals shown in Figure 3 (top) is about 10%. The high crystallinity of these wurtzite nanocrystals was confirmed by X-ray powder diffraction. For this CdO approach, the size of relatively monodisperse CdSe nanocrystals can be continuously tuned down to the sizes with the first absorption peak at 440 nm (see the first absorption spectrum in Figure 1). Relatively monodisperse CdSe nanocrystals with the first exciton absorption peak below 480 nm are difficult to synthesize directly with the existing Cd(CH 3)2-related approach. 10,18 (1) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature2000, 404, 59-61. (2) Heath, J. R. (editor). Acc.f Chem. Res. 1999. (3) Alivisatos, A. P.Science1996, 271, 933-937. (4) Huynh, W.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923927. (5) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (6) Chan, W. C. W.; Nie, S. M. Science1998, 281, 2016-2018. (7) Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys.1997, 82, 5837-5842. (8) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys.1998, 83, 7965-7974. (9) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. , in revision. (10) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (11) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J Am. Chem. Soc. 1993, 115, 8706-8715. (12) Nozik, A. J.; Micic, O. I.MRS Bull.1998, 23, 24-30. (13) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029. (14) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (15) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 76657673. (16) Mikulee, F.; Ph.D. Thesis, MIT, Boston, 1998. (17) A typical synthesis for CdTe nanocrystals: 0.0514 g of CdO, 0.2232 g of TDPA and 3.7768 g of TOPO were loaded into a 25 mL flask. The mixture was heated to 300 -320 °C under Ar flow, and CdO was dissolved in TDPA and TOPO. The temperature of the solution was cooled to 270 °C, tellurium stock solution (0.0664 g of tellurium powder dissolved i n 2 g of TOP) was injected. After injection, nanocrystals grew at 250 °C to reach desired size. (18) Bawendi, M. G. Private communication. Figure 1. Temporal evolution of size and size distribution of CdTe, CdSe, and CdS nanocrystals studied by UV -vis. 183 J. Am. Chem. Soc. 2001,123,183-184

Observation of PT-Symmetry Breaking in Complex Optical Potentials

Abstract: In 1998, Bender and Boettcher found that a wide class of Hamiltonians, even though non-Hermitian, can still exhibit entirely real spectra provided that they obey parity-time requirements or PT symmetry. Here we demonstrate experimentally passive PT-symmetry breaking within the realm of optics. This phase transition leads to a loss induced optical transparency in specially designed pseudo-Hermitian guiding potentials.

Meta-analytic reviews of board composition, leadership structure, and financial performance

Abstract: Careful review of extant research addressing the relationships between board composition, board leadership structure, and firm financial performance demonstrates little consistency in results. In general, neither board composition nor board leadership structure has been consistently linked to firm financial performance. In response to these findings, we provide metaanalyses of 54 empirical studies of board composition (159 samples, n = 40,160) and 31 empirical studies of board leadership structure (69 samples, n= 12,915) and their relationships to firm financial performance. These—and moderator analyses relying on firm size, the nature of the financial performance indicator, and various operationalizations of board composition— provide little evidence of systematic governance structure/financial performance relationships. © 1998 John Wiley & Sons, Ltd.

Cognitive and behavioral outcomes of school-aged children who were born preterm: a meta-analysis.

Abstract: ContextThe cognitive and behavioral outcomes of school-aged children who were born preterm have been reported extensively. Many of these studies have methodological flaws that preclude an accurate estimate of the long-term outcomes of prematurity.ObjectiveTo estimate the effect of preterm birth on cognition and behavior in school-aged children.Data SourcesMEDLINE search (1980 to November 2001) for English-language articles, supplemented by a manual search of personal files maintained by 2 of the authors.Study SelectionWe included case-control studies reporting cognitive and/or behavioral data of children who were born preterm and who were evaluated after their fifth birthday if the attrition rate was less than 30%. From the 227 reviewed studies, cognitive data from 15 studies and behavioral data from 16 studies were selected.Data ExtractionData on population demographics, study characteristics, and cognitive and behavioral outcomes were extracted from each study, entered in a customized database, and reviewed twice to minimize error. Differences between the mean cognitive scores of cases and controls were pooled. Homogeneity across studies was formally tested using a general variance-based method and graphically using Galbraith plots. Linear meta-analysis regression models were fitted to explore the impact of birth weight and gestational age on cognitive outcomes. Study-specific relative risks (RRs) were calculated for the incidence of attention-deficit/hyperactivity disorder (ADHD) and pooled. Quality assessment of the studies was performed based on a 10-point scale. Publication bias was examined using Begg modified funnel plots and formally tested using the Egger weighted-linear regression method.Data SynthesisAmong 1556 cases and 1720 controls, controls had significantly higher cognitive scores compared with children who were born preterm (weighted mean difference, 10.9; 95% confidence interval [CI], 9.2-12.5). The mean cognitive scores of preterm-born cases and term-born controls were directly proportional to their birth weight (R2 = 0.51; P<.001) and gestational age (R2 = 0.49; P<.001). Age at evaluation had no significant correlation with mean difference in cognitive scores (R2 = 0.12; P = .20). Preterm-born children showed increases in externalizing and internalizing behaviors in 81% of studies and had more than twice the RR for developing ADHD (pooled RR, 2.64; 95% CI, 1.85-3.78). No differences were noted in cognition and behaviors based on the quality of the study.ConclusionsChildren who were born preterm are at risk for reduced cognitive test scores and their immaturity at birth is directly proportional to the mean cognitive scores at school age. Preterm-born children also show an increased incidence of ADHD and other behaviors.

Multicultural Counseling Competencies and Standards: A Call to the Profession

Abstract: In April 1991, the Association for Multicultural Counseling and Development (AMCD) approved a document outlining the need and rationale for a multicultural perspective in counseling. The work of the Professional Standards committee went much further in proposing 31 multicultural counseling competencies and strongly encouraged the American Association for Counseling and Development (AACD) and the counseling profession to adopt these competencies in accreditation criteria. The hope was to have the competencies eventually become a standard for curriculum reform and training of helping professionals.