Programmable Colloidal Approach to Hierarchical Structures of Methylammonium Lead Bromide Perovskite Nanocrystals with Bright Photoluminescent Properties

Systematic tailoring of nanocrystal architecture could provide unprecedented control over their electronic, photophysical, and charge transport properties for a variety of applications. However, at present, manipulation of the shape of perovskite nanocrystals is done mostly by trial-and-error-based experimental approaches. Here, we report systematic colloidal synthetic strategies to prepare methylammonium lead bromide quantum platelets and quantum cubes. In order to control the nucleation and growth processes of these nanocrystals, we appropriately manipulate the solvent system, surface ligand chemistry, and reaction temperature causing syntheses into anisotropic shapes. We demonstrate that both the presence of chlorinated solvent and a long chain aliphatic amine in the reaction mixture are crucial for the formation of ultrathin quantum platelets (∼2.5 nm in thickness), which is driven by mesoscale-assisted growth of spherical seed nanocrystals (∼1.6 nm in diameter) through attachment of monomers onto selective crystal facets. A combined surface and structural characterization, along with small-angle X-ray scattering analysis, confirm that the long hydrocarbon of the aliphatic amine is responsible for the well ordered hierarchical stacking of the quantum platelets of 3.5 nm separation. In contrast, the formation of ∼12 nm edge-length quantum cubes is a kinetically driven process in which a high flux of monomers is achieved by supplying thermal energy. The photoluminescence quantum yield of our quantum platelets (∼52%) is nearly 2-fold higher than quantum cubes. Moreover, the quantum platelets display a lower nonradiative rate constant than that found with quantum cubes, which suggests less surface trap states. Together, our research has the potential both to improve the design of synthetic methods for programmable control of shape and assembly and to provide insight into optoelectronic properties of these materials for solid-state device fabrication, e.g., light-emitting diodes, solar cells, and lasing materials

Unraveling the Mechanism Underlying Surface Ligand Passivation of Colloidal Semiconductor Nanocrystals: A Route for Preparing Advanced Hybrid Nanomaterials

Optically bright colloidal semiconductor nanocrystals (CSNCs) are important nanomaterials because of their potential applications such as cellular imaging and solid-state lighting. The optoelectronic properties of CSNCs are strongly controlled by the chemical nature of the surface passivating ligands that are introduced during their synthesis. However, the existing LaMer growth model does not provide a clear understanding of the stage when ligands become attached onto the CSNC surface. Herein, apart from the three stage formation mechanism of CSNCs (supersaturation, nucleation, and growth), an entirely new stage—solely involving surface ligand attachment onto fully grown CSNCs—is now reported that controls their photoluminescence properties. Furthermore, we also demonstrate a fundamentally new surface modification approach using partially passivated CSNCs to introduce a variety of functional groups (azide, alkene, and siloxane), including photoisomerizable molecular machines (e.g., azobenzene), without the use of “state-of-the art” ligand exchange chemistry. Knowledge of the ligand adsorption phenomena and resulting adsorption time dependence expands our fundamental understanding of structure–property relationships while allowing us to engineer novel hybrid functional nanomaterials with both previously unknown optoelectronic properties and supermolecular assembly options for various applications

Dual Role of Electron-Accepting Metal-Carboxylate Ligands: Reversible Expansion of Exciton Delocalization and Passivation of Nonradiative Trap-States in Molecule-like CdSe Nanocrystals

This paper reports large bathochromic shifts of up to 260 meV in both the excitonic absorption and emission peaks of oleylamine (OLA)-passivated molecule-like (CdSe)34 nanocrystals caused by postsynthetic treatment with the electron accepting Cd(O2CPh)2 complex at room temperature. These shifts are found to be reversible upon removal of Cd(O2CPh)2 by N,N,N′,N′-tetramethylethylene-1,2-diamine. 1H NMR and FTIR characterizations of the nanocrystals demonstrate that the OLA remained attached to the surface of the nanocrystals during the reversible removal of Cd(O2CPh)2. On the basis of surface ligand characterization, X-ray powder diffraction measurements, and additional control experiments, we propose that these peak red shifts are a consequence of the delocalization of confined exciton wave functions into the interfacial electronic states that are formed from interaction of the LUMO of the nanocrystals and the LUMO of Cd(O2CPh)2, as opposed to originating from a change in size or reorganization of the inorganic core. Furthermore, attachment of Cd(O2CPh)2 to the OLA-passivated (CdSe)34 nanocrystal surface increases the photoluminescence quantum yield from 5% to an unprecedentedly high 70% and causes a 3-fold increase of the photoluminescence lifetime, which are attributed to a combination of passivation of nonradiative surface trap states and relaxation of exciton confinement. Taken together, our work demonstrates the unique aspects of surface ligand chemistry in controlling the excitonic absorption and emission properties of ultrasmall (CdSe)34 nanocrystals, which could expedite their potential applications in solid-state device fabrication

Elucidating the role of surface passivating ligand structural parameters in hole wave function delocalization in semiconductor cluster molecules

This article describes the mechanisms underlying electronic interactions between surface passivating ligands and (CdSe)34 semiconductor cluster molecules (SCMs) that facilitate band-gap engineering through the delocalization of hole wave functions without altering their inorganic core. We show here both experimentally and through density functional theory calculations that the expansion of the hole wave function beyond the SCM boundary into the ligand monolayer depends not only on the pre-binding energetic alignment of interfacial orbitals between the SCM and surface passivating ligands but is also strongly influenced by definable ligand structural parameters such as the extent of their π-conjugation [π-delocalization energy; pyrene (Py), anthracene (Anth), naphthalene (Naph), and phenyl (Ph)], binding mode [dithiocarbamate (DTC, –NH–CS2−), carboxylate (–COO−), and amine (–NH2)], and binding head group [–SH, –SeH, and –TeH]. We observe an unprecedentedly large ∼650 meV red-shift in the lowest energy optical absorption band of (CdSe)34 SCMs upon passivating their surface with Py-DTC ligands and the trend is found to be Ph- < Naph- < Anth- < Py-DTC. This shift is reversible upon removal of Py-DTC by triethylphosphine gold(I) chloride treatment at room temperature. Furthermore, we performed temperature-dependent (80–300 K) photoluminescence lifetime measurements, which show longer lifetime at lower temperature, suggesting a strong influence of hole wave function delocalization rather than carrier trapping and/or phonon-mediated relaxation. Taken together, knowledge of how ligands electronically interact with the SCM surface is crucial to semiconductor nanomaterial research in general because it allows the tuning of electronic properties of nanomaterials for better charge separation and enhanced charge transfer, which in turn will increase optoelectronic device and photocatalytic efficiencies

Seroprevalence of Zika virus in wild African green monkeys and baboons

ABSTRACT Zika virus (ZIKV) has recently spread through the Americas and has been associated with a range of health effects, including birth defects in children born to women infected during pregnancy. Although the natural reservoir of ZIKV remains poorly defined, the virus was first identified in a captive “sentinel” macaque monkey in Africa in 1947. However, the virus has not been reported in humans or nonhuman primates (NHPs) in Africa outside Gabon in over a decade. Here, we examine ZIKV infection in 239 wild baboons and African green monkeys from South Africa, the Gambia, Tanzania, and Zambia using combinations of unbiased deep sequencing, quantitative reverse transcription-PCR (qRT-PCR), and an antibody capture assay that we optimized using serum collected from captive macaque monkeys exposed to ZIKV, dengue virus, and yellow fever virus. While we did not find evidence of active ZIKV infection in wild NHPs in Africa, we found variable ZIKV seropositivity of up to 16% in some of the NHP populations sampled. We anticipate that these results and the methodology described within will help in continued efforts to determine the prevalence, natural reservoir, and transmission dynamics of ZIKV in Africa and elsewhere. IMPORTANCE Zika virus (ZIKV) is a mosquito-borne virus originally discovered in a captive monkey living in the Zika Forest of Uganda, Africa, in 1947. Recently, an outbreak in South America has shown that ZIKV infection can cause myriad health effects, including birth defects in the children of women infected during pregnancy. Here, we sought to investigate ZIKV infection in wild African primates to better understand its emergence and spread, looking for evidence of active or prior infection. Our results suggest that up to 16% of some populations of nonhuman primate were, at some point, exposed to ZIKV. We anticipate that this study will be useful for future studies that examine the spread of infections from wild animals to humans in general and those studying ZIKV in primates in particular. Podcast: A podcast concerning this article is available

THE EFFECTS OF MUSCLE CROSS-SECTIONAL AREA ON THE PHYSICAL WORKING CAPACITY AT THE FATIGUE THRESHOLD

Purpose: The purpose of this study was to examine the effects of quadriceps cross-sectional area (CSA) of the dominant quadriceps muscle in the assessment of the physical working capacity at the fatigue threshold (PWCFT) during incremental cycle ergometry. Methods: Eighteen adults (9 men and 9 women; mean age ± SD = 20.5 ± 1.04 yr; mean body weight ± SD = 73.9 ± 18.2 kg; mean height ± SD = 172.3 ± 11.5 cm; mean dominant quadriceps CSA ± SD = 68.7 ± 14.5 cm2) performed an incremental cycle ergometry test to exhaustion while the electromyographic (EMG) signals were recorded from the vastus lateralis (VL) muscles. Fatiguing and non-fatiguing power outputs were differentiated by examining the slope coefficients for the EMG amplitude versus time relationship at each power output throughout the incremental cycle ergometry test. Quadriceps CSA was estimated from an equation. Subjects were divided into groups of small quadriceps CSA (57.3 ± 10.0 cm2) and large quadriceps CSA (80.0 ± 7.6 cm2). Results: Independent t-test results indicated no significant mean differences between the PWCFT for the large and small quadriceps CSA groups (p=0.456). Conclusion: The findings of the study suggest that muscle CSA may not have a significant effect on the assessment of the PWCFT, and therefore that PWCFT may be a determinant of neuromuscular fatigue independent of muscle CSA. Future research could explore the contributions of muscle fibertype predominance to CSA and PWCFT and provide more conclusive evidence relating these variables

Handoffs and Transitions in Critical Care (HATRICC): Protocol for a Mixed Methods Study of Operating Room to Intensive Care Unit Handoffs

Background: Operating room to intensive care unit handoffs are high-risk events for critically ill patients. Studies in selected patient populations show that standardizing operating room to intensive care unit handoffs improves information exchange and decreases errors. To adapt these findings to mixed surgical populations, we propose to study the implementation of a standardized operating room to intensive care unit handoff process in two intensive care units currently without an existing standard process. Methods/Design: The Handoffs and Transitions in Critical Care (HATRICC) study is a hybrid effectiveness- implementation trial of operating room to intensive care unit handoffs. We will use mixed methods to conduct a needs assessment of the current handoff process, adapt published handoff processes, and implement a new standardized handoff process in two academic intensive care units. Needs assessment: We will use non-participant observation to observe the current handoff process. Focus groups, interviews, and surveys of clinicians will elicit participants’ impressions about the current process. Adaptation and implementation: We will adapt published standardized handoff processes using the needs assessment findings. We will use small group simulation to test the new process’ feasibility. After simulation, we will incorporate the new handoff process into the clinical work of all providers in the study units. Evaluation: Using the same methods employed in the needs assessment phase, we will evaluate use of the new handoff process. Data analysis: The primary effectiveness outcome is the number of information omissions per handoff episode as compared to the pre-intervention period. Additional intervention outcomes include patient intensive care unit length of stay and intensive care unit mortality. The primary implementation outcome is acceptability of the new process. Additional implementation outcomes include feasibility, fidelity and sustainability. Discussion: The HATRICC study will examine the effectiveness and implementation of a standardized operating room to intensive care unit handoff process. Findings from this study have the potential to improve healthcare communication and outcomes for critically ill patients. Trial registration: ClinicalTrials.gov identifier: NCT02267174. Date of registration October 16, 2014