skip to main content

Title: Precursor reaction kinetics control compositional grading and size of CdSe 1−x S x nanocrystal heterostructures
We report a method to control the composition and microstructure of CdSe 1−x S x nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 °C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents ( k E ) spanning 1.3 × 10 −5 s −1 to 2.0 × 10 −1 s −1 . Depending on the relative reactivity ( k Se / k S ), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe 1−x S x alloys ( k Se / k S = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full more » conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed. « less
Authors:
; ; ; ; ; ; ; ; ; ; ; ; ;
Award ID(s):
1709464 1709287
Publication Date:
NSF-PAR ID:
10105710
Journal Name:
Chemical Science
Volume:
10
Issue:
26
Page Range or eLocation-ID:
6539 to 6552
ISSN:
2041-6520
Sponsoring Org:
National Science Foundation
More Like this
  1. The ternary manganese pnictide phases, MnAs 1− x Sb x , are of interest for magnetic refrigeration and waste heat recovery due to their magnetocaloric properties, maximized at the Curie temperature ( T C ), which varies from 580–240 K, depending on composition. Nanoparticles potentially enable application in microelectronics (cooling) or graded composites that can operate over a wide temperature range, but manganese pnictides are synthetically challenging to realize as discrete nanoparticles and their fundamental magnetic properties have not been extensively studied. Accordingly, colloidal synthesis methods were employed to target discrete MnAs x Sb 1− x nanoparticles ( x = 0.1–0.9) by arrested precipitation reactions of Mn 2 (CO) 10 with (C 6 H 5 ) 3 AsO and (C 6 H 5 ) 3 Sb in coordinating solvents. The MnAs x Sb 1− x particles are spherical in morphology with average diameters 10–13 nm (standard deviations <20% based on transmission electron microscopy analysis). X-Ray fluorescence spectroscopy measurements on ensembles showed that all phases had an excess of Sb relative to the targeted composition, whereas energy dispersive spectroscopic mapping data of single particles revealed that the nanoparticles are inhomogeneous, adopting a core–shell structure, with the amorphous shell rich in Mnmore »and O (and sometimes Sb) while the crystalline core is rich in Mn, As, and Sb. Magnetization measurements of the nanoparticle ensemble demonstrated the presence of both ferromagnetic and paramagnetic phases. By combining the magnetization measurements with precision chemical mapping and simple modeling, we were able to unambiguously attribute ferromagnetism to the MnAs x Sb 1− x crystalline core, whereas paramagnetism was attributed to the amorphous shell. Magnetization measurements at variable temperatures were used to determine the superparamagnetic transition of the nanoparticles, although for some compositions and particle sizes the blocking temperature exceeded room temperature. Preliminary magnetic studies also revealed a conventional dependence between core size and coercivity, in spite of variable compositions of the nanoparticles, an unexpected result.« less
  2. Liu, W. ; Wang, Y. ; Guo, B. ; Tang, X. ; Zeng, S. (Ed.)
    In Type-I X-ray bursts (XRBs), the rapid-proton capture (rp-) process passes through the NiCu and ZnGa cycles before reaching the region above Ge and Se isotopes that hydrogen burning actively powers the XRBs. The sensitivity study performed by Cyburt et al . [1] shows that the 57 Cu(p, γ ) 58 Zn reaction in the NiCu cycles is the fifth most important rp-reaction influencing the burst light curves. Langer et al . [2] precisely measured some low-lying energy levels of 58 Zn to deduce the 57 Cu(p, γ ) 58 Zn reaction rate. Nevertheless, the order of the 1 + 1 and 2 + 3 resonance states that dominate at 0:2 ≲ T (GK) ≲ 0:8 is not confirmed. The 1 + 2 resonance state, which dominates at the XRB sensitive temperature regime 0:8 ≲ T (GK) ≲ 2 was not detected. Using isobaric-multipletmass equation (IMME), we estimate the order of the 1 + 1 and 2 + 3 resonance states and estimate the lower limit of the 1 + 2 resonance energy. We then determine the 57 Cu(p, γ ) 58 Zn reaction rate using the full pf -model space shell model calculations. The new rate is up tomore »a factor of four lower than the Forstner et al . [3] rate recommended by JINA REACLIBv2.2. Using the present 57 Cu(p, γ ) 58 Zn, the latest 56 Ni(p, γ ) 57 Cu and 55 Ni(p, γ ) 56 Cu reaction rates, and 1D implicit hydrodynamic K epler code, we model the thermonuclear XRBs of the clocked burster GS 1826–24. We find that the new rates regulate the reaction flow in the NiCu cycles and strongly influence the burst-ash composition. The 59 Cu(p, γ ) 56 Ni and 59 Cu(p, α ) 60 Zn reactions suppress the influence of the 57 Cu(p, γ ) 58 Zn reaction. They strongly diminish the impact of the nuclear reaction flow that bypasses the 56 Ni waiting point induced by the 55 Ni(p, γ ) 56 Cu reaction on burst light curve.« less
  3. Abstract

    The millimeter-wave spectrum of the SiP radical (X2Πi) has been measured in the laboratory for the first time using direct-absorption methods. SiP was created by the reaction of phosphorus vapor and SiH4in argon in an AC discharge. Fifteen rotational transitions (J+ 1 ←J) were measured for SiP in the Ω = 3/2 ladder in the frequency range 151–533 GHz, and rotational, lambda doubling, and phosphorus hyperfine constants determined. Based on the laboratory measurements, SiP was detected in the circumstellar shell of IRC+10216, using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1 mm and 2 mm, respectively. Eight transitions of SiP were searched: four were completely obscured by stronger features, two were uncontaminated (J= 13.5 → 12.5 and 16.5 → 15.5), and two were partially blended with other lines (J= 8.5 → 7.5 and 17.5 → 16.5). The SiP line profiles were broader than expected for IRC+10216, consistent with the hyperfine splitting. From non-LTE radiative transfer modeling, SiP was found to have a shell distribution with a radius ∼300R*, and an abundance, relative to H2, off∼ 2 × 10−9. From additional modeling, abundances of 7 × 10−9and 9 × 10−10were determined for CPmore »and PN, respectively, both located in shells at 550–650R*. SiP may be formed from grain destruction, which liberates both phosphorus and silicon into the gas phase, and then is channeled into other P-bearing molecules such as PN and CP.

    « less
  4. Copper-antimony-sulfide compounds have desirable earth-abundant compositions for application in renewable energy technologies, such as solar energy and waste heat recycling. These compounds can be synthesized by bottom-up, solution-phase techniques that are more energy and time efficient than conventional solid-state methods. Solution-phase methods typically produce nanostructured materials, which adds another dimension to control optical, electrical, and thermal material properties. This study focuses on a modified-polyol, solution-phase synthesis for tetrahedrite (Cu 12 Sb 4 S 13 ), a promising thermoelectric material with potential also for photovoltaic applications. To dope the tetrahedrite and tune material properties, the utility of the modified polyol synthetic approach has been demonstrated as a strategy to produce phase-pure tetrahedrite that incorporates transition metal (Fe, Co, Ni, Zn, Ag) dopants for Cu, Te dopant for Sb, and Se for S. Six of these reported tetrahedrite compounds have not previously been made by solution-phase methods. For the bottom-up formation of the tetrahedrite nanomaterials, the evolution of the chemical phases has been determined by an investigation of the reaction progress as a function of temperature and time. Digenite (Cu 1.8 S), covellite (CuS), and famatinite (Cu 3 SbS 4 ) are identified as key intermediates and are consistently observed for bothmore »undoped and doped tetrahedrites. The effect of nanostructuring and doping tetrahedrite on thermal properties has been investigated. It was found that nanostructured undoped tetrahedrite has reduced thermal stability relative to samples made by solid-state methods, while the addition of dopants for Cu increased the thermal stability of the material. Crystallinity, composition, and nanostructure of products and intermediates were characterized by powder X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and transmission electron microscopy. Thermal properties were investigated by differential scanning calorimetry and thermal gravimetric analysis. This synthetic study with thermal property analysis demonstrates the potential of the modified polyol method to produce tetrahedrite and other copper-antimony-sulfide compounds for thermoelectric and photovoltaic applications.« less
  5. ZrS2, ZrSe2 and mixed alloy ZrSxSe2−x materials were achieved through chemical vapor transport. The incongruent melting system of Zr-S-Se formed crystalline layered flakes as a transport product that grew up to 2 cm in lateral size with cm-scale flakes consistently obtained for the entire compositional range exhibiting visible hexagonal features. Bulk flakes of the series ZrSxSe2−x (x=0, 0.15, 0.3, 0.6, 1.05, 1.14, 1.51, 1.8 and 2) were analyzed through Raman spectroscopy revealing significant convolution of primary bonding modes and shifting of Raman features as a function of increasing sulfur composition. Additionally, activation of new modes not present in the pure compounds are observed as effects which result from disorder introduced into the crystal due to the random mixing of S-Se in the alloying process. Further structural characterization was performed via x-ray diffraction (XRD) on the layered flakes to evaluate the progression of layer spacing function of alloy composition which was found to range between 6.24 Å for ZrSe2 and 5.85 Å for ZrS2. Estimation of the compositional ratios of the alloy flakes through energy dispersive spectroscopy (EDS) large-area mapping verified the relation of the targeted source stoichiometry represented in the layered flakes. Atomic-resolution high angle annular dark field (HAADF)-scanning transmissionmore »electron microscopy (STEM) imaging was performed on the representative Zr (S0.5Se0.5)2 alloy to validate the 1T atomic structure and observe the arrangement of the chalcogenide columns stacks. Additionally, selected area diffraction pattern generated from the [0 0 0 1] zone axis revealed the in-plane lattice parameter to be approximately 3.715 Å.« less