Soft lattices of metal halide perovskite (MHP) nanocrystals (NCs) are considered responsible for many of their optical properties associated with excitons, which are often distinct from other ...semiconductor NCs. Earlier studies of MHP NCs upon compression revealed how structural changes and the resulting changes in the optical properties such as the bandgap can be induced at relatively low pressures. However, the pressure response of the exciton transition itself in MHP NCs remains relatively poorly understood due to limitations inherent to studying weakly or nonconfined NCs in which exciton absorption peaks are not well-separated from the continuum interband transition. Here, we investigated the pressure response of the absorbing and emitting transitions of excitons using strongly quantum-confined CsPbBr
quantum dots (QDs) and nanoplatelets (NPLs), which both exhibit well-defined exciton absorption peaks. Notably, the reversible vanishing and recovery of the exciton absorption accompanied by reversible quenching and recovery of the emission were observed in both QDs and NPLs, resulting from the reversible pressure modulation of the exciton oscillator strength. Furthermore, CsPbBr
NPLs exhibited irreversible pressure-induced creation of trap states at low pressures (∼0.1 GPa) responsible for trapped exciton emission that developed on the time scale of ∼10 min, while the reversible pressure response of the absorbing exciton transition was maintained. These findings shed light on the diverse effects the application of force has on the absorbing and emitting exciton transitions in MHP NCs, which are important for their application as excitonic light emitters in high-pressure environments.
Background
The COVID-19 pandemic intersected with a housing crisis for unsheltered Veterans experiencing homelessness (VEHs); congregate settings became high risk for viral spread. The VA Greater Los ...Angeles responded by creating the Care, Treatment, and Rehabilitation Service (CTRS), an outdoor, low-barrier-to-entry transitional housing program on VA grounds. This novel emergency initiative offered a protected outdoor environment (“sanctioned encampment”) where VEHs lived in tents and had access to three meals a day, hygiene resources, and health and social services.
Objective
To identify contextual factors that supported and impeded CTRS participants’ access to healthcare and housing services.
Design
Multi-method, ethnographic data collection.
Participants
VEHs residing at CTRS, CTRS staff.
Approach
Over 150 hours of participant observation were conducted at CTRS and at eight town hall meetings; semi-structured interviews were conducted with 21 VEHs and 11 staff. Rapid turn-around qualitative analysis was used to synthesize data, engaging stakeholders in iterative participant validation. Content analysis techniques were used to identify key factors that impacted access to housing and health services among VEHs residing in CTRS.
Key Results
Staff varied in their interpretation of CTRS’ mission. Some conceptualized access to health services as a central tenet, while others viewed CTRS as an emergency shelter only. Regardless, staff burnout was prevalent, which lead to low morale, high turnover, and worsened access to and quality of care. VEHs endorsed trusting, long-term relationships with CTRS staff as paramount for facilitating access to services. Though CTRS addressed basic priorities (food, shelter, etc.) that traditionally compete with access to healthcare, some VEHs needed on-site healthcare services, at their tents, to access care.
Conclusions
CTRS provided VEHs access to basic needs and health and housing services. To improve access to healthcare services within encampments, our data suggest the value of longitudinal trusting relationships, adequate staff support, and on-site health services.
We report a pressure-induced phase transition in the frustrated kagomé material jarosite at ∼ 45 GPa , which leads to the disappearance of magnetic order. Using a suite of experimental techniques, we ...characterize the structural, electronic, and magnetic changes in jarosite through this phase transition. Synchrotron powder x-ray diffraction and Fourier transform infrared spectroscopy experiments, analyzed in aggregate with the results from density functional theory calculations, indicate that the material changes from a R 3 m structure to a structure with a R3 c space group. The resulting phase features a rare twisted kagomé lattice in which the integrity of the equilateral Fe3+ triangles persists. Based on symmetry arguments we hypothesize that the resulting structural changes alter the magnetic interactions to favor a possible quantum paramagnetic phase at high pressure.
Soft lattices of metal halide perovskite (MHP) nanocrystals (NCs) are considered responsible for many of their optical properties associated with excitons, which are often distinct from other ...semiconductor NCs. Earlier studies of MHP NCs upon compression revealed how structural changes and the resulting changes in the optical properties such as the bandgap can be induced at relatively low pressures. However, the pressure response of the exciton transition itself in MHP NCs remains relatively poorly understood due to limitations inherent to studying weakly or nonconfined NCs in which exciton absorption peaks are not well-separated from the continuum interband transition. Here, we investigated the pressure response of the absorbing and emitting transitions of excitons using strongly quantum-confined CsPbBr3 quantum dots (QDs) and nanoplatelets (NPLs), which both exhibit well-defined exciton absorption peaks. Notably, the reversible vanishing and recovery of the exciton absorption accompanied by reversible quenching and recovery of the emission were observed in both QDs and NPLs, resulting from the reversible pressure modulation of the exciton oscillator strength. Furthermore, CsPbBr3 NPLs exhibited irreversible pressure-induced creation of trap states at low pressures (∼0.1 GPa) responsible for trapped exciton emission that developed on the time scale of ∼10 min, while the reversible pressure response of the absorbing exciton transition was maintained. These findings shed light on the diverse effects the application of force has on the absorbing and emitting exciton transitions in MHP NCs, which are important for their application as excitonic light emitters in high-pressure environments.
Covalency in Ln-Cl bonds of Oh-LnCl6(x-) (x = 3 for Ln = Ce(III), Nd(III), Sm(III), Eu(III), Gd(III); x = 2 for Ln = Ce(IV)) anions has been investigated, primarily using Cl K-edge X-ray absorption ...spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6(x-) (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce(III) and Ce(IV)). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce(III) to Ce(IV) had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl6(3-) (formally Ln(III)), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce(IV) 4f-orbital mixing (t1u* + t2u*) in CeCl6(2-). This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl6(2-) (M = Ti, Zr, Hf, U).
Soft X‐ray spectromicroscopy at the O K‐edge, U N4,5‐edges and Ce M4,5‐edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on ...the sub‐micrometer scale. To analyze these data, a modification to non‐negative matrix factorization (NMF) was developed, in which the data are no longer required to be non‐negative, but the non‐negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K‐edge to distinguish between two components, one present in the bulk of the sample similar to UO2 and one present at the interface of the sample which is a hyperstoichiometric UO2+x species. The species maps are consistent with a model of a thin layer of UO2+x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in the spent fuel.
Soft X‐ray spectromicroscopy at the O K‐edge, U N4,5‐edges and Ce M4,5‐edges has been performed on focused ion beam sections of spent nuclear fuel for the first time. Analysis of oxygen spectra using a modified non‐negative matrix factorization method is consistent with a thin layer of hyperstoichiometric uranium oxide having been formed at the interface of a sample consisting of primarily UO2, probably formed after sample preparation. The cerium oxidation state is shown to be predominantly trivalent, with a possible small contribution from tetravalent Ce.
The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3 (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy ...(XAS) with a scanning transmission X-ray microscope (STXM). The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.
Computationally Directed Discovery of MoBi2 Altman, Alison B; Tamerius, Alexandra D; Koocher, Nathan Z ...
Journal of the American Chemical Society,
01/2021, Letnik:
143, Številka:
1
Journal Article
Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of ...diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)–aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide–metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a Cptt 2ThH3AlCTMS3 TMS = Si(CH3)3; Cptt = 1,3-di-tert-butylcyclopentadienyl complex with bridging hydrides and (ii) an actinide-free Cp2TiH3AlCTMS3 (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, 1H and 27Al, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (a iso = −9.7 and −10.7 MHz, respectively), the spin density on the Al ion in the Th3+ complex is ∼5-fold larger than that in the titanium(3+) (Ti3+) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.
The spin state in heterobimetallic complexes heavily influences both reactivity and magnetism. Exerting control over spin states in main group-based heterobimetallics requires a different approach as ...the orbital interactions can differ substantially from that of classic coordination complexes. By deliberately engendering an energetic mismatch within the two metals in a bimetallic complex we can mimic the electronic structure of lanthanides. Towards this end, we report a new family of complexes,
Ph,Me
TpMSnPh
3
where M = Mn (
3
), Fe (
4
), Co (
5
), Ni (
6
), Zn (
7
), featuring unsupported bonding between a transition metal and Sn which represent an unusual high spin electronic structure. Analysis of the frontier orbitals reveal the desired orbital mismatch with Sn 5s/5p primarily interacting with 4s/4p M orbitals yielding localized, non-bonding d orbitals. This approach offers a mechanism to design and control spin states in bimetallic complexes.
We report a series of high spin bimetallic transition metal-tin complexes. The unusual high spin configuration in a bimetallic complex is enabled by an energetic mismatch in the orbital energies, leading to lanthanide-like nonbonding interactions.
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