Insights into the Structural Complexity of Colloidal CdSe Nanocrystal Surfaces: Correlating the Efficiency of Nonradiative Excited-State Processes to Specific Defects.

II-VI colloidal semiconductor nanocrystals (NCs), such as CdSe NCs, are often plagued by efficient nonradiative recombination processes that severely limit their use in energy-conversion schemes. While these processes are now well-known to occur at the surface, a full understanding of the exact nature of surface defects and of their role in deactivating the excited states of NCs has yet to be established, which is partly due to challenges associated with the direct probing of the complex and dynamic surface of colloidal NCs. Here, we report a detailed study of the surface of cadmium-rich zinc-blende CdSe NCs. The surfaces of these cadmium-rich species are characterized by the presence of cadmium carboxylate complexes (CdX2) that act as Lewis acid (Z-type) ligands that passivate undercoordinated selenide surface species. The systematic displacement of CdX2 from the surface by N,N,N',N'-tetramethylethylene-1,2-diamine (TMEDA) has been studied using a combination of 1H NMR and photoluminescence spectroscopies. We demonstrate the existence of two independent surface sites that differ strikingly in the binding affinity for CdX2 and that are under dynamic equilibrium with each other. A model involving coupled dual equilibria allows a full characterization of the thermodynamics of surface binding (free energy, as well as enthalpic and entropic terms), showing that entropic contributions are responsible for the difference between the two surface sites. Importantly, we demonstrate that cadmium vacancies only lead to important photoluminescence quenching when created on one of the two sites, allowing a complete picture of the surface composition to be drawn where each site is assigned to specific NC facet locale, with CdX2 binding affinity and nonradiative recombination efficiencies that differ by up to two orders of magnitude.

Harvesting 88Zr from heavy-ion beam irradiated tungsten at the National Superconducting Cyclotron Laboratory.

Tungsten is a commonly used material at many heavy-ion beam facilities, and it often becomes activated due to interactions with a beam. Many of the activation products are useful in basic and applied sciences if they can be recovered efficiently. In order to develop the radiochemistry for harvesting group (IV) elements from irradiated tungsten, a heavy-ion beam containing 88Zr was embedded into a stack of tungsten foils at the National Superconducting Cyclotron Laboratory and a separation methodology was devised to recover the 88Zr. The foils were dissolved in 30% hydrogen peroxide, and the 88Zr was chemically purified from the tungsten matrix and from other co-implanted radionuclides (such as 85Sr and 88Y) using strong cation-exchange (AG MP-50) chromatographic resin in sulfuric acid media. The procedure provided 88Zr in approximately 60 mL 0.5 M sulfuric acid with no detectable radio-impurities. The overall recovery yield for 88Zr was (92.3 ± 1.2)%. This proof-of-concept experiment has facilitated the development of methodologies to harvest from tungsten and tungsten-alloy parts that are regularly irradiated at heavy-ion beam facilities.

Branching ratios for the three most intense gamma rays in the decay of 47Ca.

A sample of 47Ca produced through isotope harvesting at the National Superconducting Cyclotron Laboratory was used to measure branching ratios of 7.17(5)%, 7.11(5)%, and 75.0(5)% for the 489.2, 807.9, and 1297.1 keV characteristic gamma rays, respectively. Based on these updated branching ratios, the ground state to ground state 47Ca to 47Sc beta decay branching ratio has been indirectly measured as 17.7(5)% and the ground state to 1297.1 keV excited state as 82.2(5)%. These values represent a greatly increased precision for all five branching ratios compared to the currently accepted values (Burrows, 2007). The measurements presented here were made relative to the ingrown 47Sc daughter in a47Ca sample and the well-established 159.4 keV gamma-ray branching ratio and the half-life for the decay of 47Sc (Reher et al., 1986; Meadows and Mode, 1968; Mommsen et al., 1980). These measurements were supported by verifying that the half-lives measured from characteristic gamma-ray peaks over multiple spectra for both 47Ca and 47Sc were consistent with previously reported values. Additionally, the half-lives of both 47Ca and 47Sc were independently measured with Liquid Scintillation Counting to reverify the previously reported values used in this study to find updated gamma-ray branching ratio values.

Harvesting krypton isotopes from the off-gas of an irradiated water target to generate 76Br and 77Br.

A flowing-water target was irradiated with a 150 MeV/nucleon beam of 78Kr at the National Superconducting Cyclotron Laboratory to produce 77Kr and 76Kr. Real-time gamma-imaging measurements revealed the mass transport of the krypton radioisotopes through the target-water processing, or "isotope harvesting", system. The production rates were determined to be 2.7(1) × 10-4 nuclei of 76Kr and 1.18(6) × 10-2 nuclei of 77Kr formed per incident 78Kr ion. Utilizing an off-gas processing line as part of the isotope harvesting system, a total of 7.2(1) MBq of 76Kr and 19.1(6) MBq of 77Kr were collected in cold traps. Through the decay, the daughter radionuclides 76Br and 77Br were generated and removed from the traps with an average efficiency of 77 ± 12%. Due to the differences in half-lives of 76Kr and 77Kr, it was possible to isolate a pure sample of 76Br with 99.9% radionuclidic purity. The successful collection of krypton radioisotopes to generate 76Br and 77Br demonstrates the feasibility of gas-phase isotope harvesting from irradiated accelerator cooling-water. Larger-scale collections are planned for collecting by-product radionuclides from the Facility for Rare Isotope Beams.

Solid-phase isotope harvesting of 88Zr from a radioactive ion beam facility.

During routine operation of the Facility for Rare Isotope Beams (FRIB), radionuclides will accumulate in both the aqueous beam dump and along the beamline in the process of beam purification. These byproduct radionuclides, many of which are far from stability, can be collected and purified for use in other scientific applications in a process called isotope harvesting. In this work, the viability of 88Zr harvesting from solid components was investigated at the National Superconducting Cyclotron Laboratory. A secondary 88Zr beam was stopped in a series of collectors comprised of Al, Cu, W, and Au foils. This work details irradiation of the collector foils and the subsequent radiochemical processing to isolate the deposited 88Zr (and its daughter 88Y) from them. Total average recovery from the Al, Cu, and Au collector foils was (91.3 ± 8.9) % for 88Zr and (95.0 ± 5.8) % for 88Y, respectively, which is over three times higher recovery than in a previous aqueous-phase harvesting experiment. The utility of solid-phase isotope harvesting to access elements such as Zr that readily hydrolyze in near-neutral pH aqueous conditions has been demonstrated for application to harvesting from solid components at FRIB.

Radiolysis and radionuclide production in a flowing-water target during fast 40Ca20+ irradiation.

A flowing-water target was irradiated with a 140 MeV/u, 8 nA 40Ca20+ beam to test the feasibility of isotope harvesting at the upcoming Facility for Rare Isotope Beams. Among other radionuclides, 2.6(2)E-6 48Cr and 5.6(5)E-6 28 Mg nuclei were formed for every impingent 40Ca and were collected through ion exchange. Radiolysis-induced molecular hydrogen evolved from the target at an initial rate of 0.91(9) H2 molecules per 100 eV of beam energy deposited. No radiation-accelerated corrosion of the target material was observed.

Production, Collection, and Purification of 47Ca for the Generation of 47Sc through Isotope Harvesting at the National Superconducting Cyclotron Laboratory.

An experiment was performed at the National Superconducting Cyclotron Laboratory using a 140 MeV/nucleon 48Ca beam and a flowing-water target to produce 47Ca for the first time with this production route. A production rate of 0.020 ± 0.004 47Ca nuclei per incoming beam particle was measured. An isotope harvesting system attached to the target was used to collect radioactive cationic products, including 47Ca, from the water on a cation-exchange resin. The 47Ca collected was purified using three separation methods optimized for this work: (1) DGA extraction chromatography resin with HNO3 and HCl, (2) AG MP-50 cation-exchange resin with an increasing concentration gradient of HCl, and (3) AG MP-50 cation-exchange resin with a methanolic HCl gradient. These methods resulted in ≥99 ± 2% separation yield of 47Ca with 100% radionuclidic purity within the limits of detection for HPGe measurements. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to identify low levels of stable ions in the water of the isotope harvesting system during the irradiation and in the final purified solution of 47Ca. For the first time, this experiment demonstrated the feasibility of the production, collection, and purification of 47Ca through isotope harvesting for the generation of 47Sc for nuclear medicine applications.