Epitaxial Heterostructures of Lead Selenide Quantum Dots on Hematite Nanowires

We present a novel method for synthesizing epitaxial quantum dot-nanowire (QD-NW) heterostructures using the example of colloidal PbSe QDs decorated on furnace-grown hematite (α-Fe₂O₃) NWs. The direct heterogeneous nucleation of QDs on Fe₂O₃ NWs relies upon an aggressive surface dehydration of the as-synthesized Fe₂O₃ NWs at 350 °C under vacuum and subsequent introduction of colloidal reactants resulting in direct growth of PbSe QDs on Fe₂O₃. The synthesis is tunable: the QD diameter distribution and density of QDs on the NWs increase with increased dehydration time, and QD diameters and size distributions decrease with decreased injection temperature of the colloidal synthesis. Transmission electron microscopy (TEM) structural analysis reveals direct heteroepitaxial heterojunctions where the matching faces can be PbSe (002) and Fe₂O₃ (003) with their respective [11̅0] crystallographic directions aligned. This can be a general approach for integrating colloidal and furnace synthetic techniques, thus broadening possible material combinations for future high-quality, epitaxial nanoscale heterostructures for solar applications

Nitrogen-plasma treated hafnium oxyhydroxide as an efficient acid-stable electrocatalyst for hydrogen evolution and oxidation reactions

Development of earth-abundant electrocatalysts for hydrogen evolution and oxidation reactions in strong acids represents a great challenge for developing high efficiency, durable, and cost effective electrolyzers and fuel cells. We report herein that hafnium oxyhydroxide with incorporated nitrogen by treatment using an atmospheric nitrogen plasma demonstrates high catalytic activity and stability for both hydrogen evolution and oxidation reactions in strong acidic media using earth-abundant materials. The observed properties are especially important for unitized regenerative fuel cells using polymer electrolyte membranes. Our results indicate that nitrogen-modified hafnium oxyhydroxide could be a true alternative for platinum as an active and stable electrocatalyst, and furthermore that nitrogen plasma treatment may be useful in activating other non-conductive materials to form new active electrocatalysts

Spectral Isolation and Measurement of Surface-Trapped State Multidimensional Nonlinear Susceptibility in Colloidal Quantum Dots

Multiresonant coherent multidimensional spectroscopy (CMDS) is a powerful new method for probing the coupling between vibrational modes and their dynamics. The line narrowing that occurs because of the multidimensional nature of CMDS allows the separation of homogeneous and inhomogeneous broadening and enhances spectral resolution. Recent work has extended multiresonant CMDS to electronic resonances in quantum confined nanostructures. Vibrational modes of the solvent also appear in the CMDS spectra. The phase oscillations of the vibrational and electronic coherences interfere and change the line shapes. Since the form of the vibrational third-order susceptibility and hyperpolarizability are well-known and since they can be measured against known standards, it becomes possible to use the interference effects as a probe of the absolute magnitude and phase of the electronic resonances. This approach is demonstrated using PbSe quantum dots where incomplete capping causes ultrafast relaxation to a new electronic state that appears directly in the CMDS spectra. The new state is believed to be a mixed core/surface exciton. Closed-form expressions for the electronic nonlinearities are used to analyze the frequency dependence of the fully resonant complex hyperpolarizability of the 1S exciton and the surface-trapped state. The ability of mixed frequency/time domain multiresonant CMDS methods to spectrally resolve surface states promises to be an important new way to characterize the interface states in complex heterostructures and the surface states that define the stability of nanostructures resulting from different synthetic strategies

Element-specific magnetometry of EuS nanocrystals

A soft x-ray absorption and x-ray magnetic circular dichroism (XMCD) study of the ferromagnetism in solution-grown EuS nanocrystals (NCs) is reported. The absorption edges of Eu M5 and M4, S K, O K, and P K were probed to determine elementally specific contributions to the magnetism of EuS NCs. Differential spin absorption was observed by XMCD at the Eu M5,4 edges confirming the presence of a magnetic moment on the Eu2+ 4f shell. No dichroic signal was observed for S, O, or P. Vibrating sample magnetometry of ensembles of NCs shows ferromagnetic properties consistent with the XMCD studies

Spectral Isolation and Measurement of Surface-Trapped State Multidimensional Nonlinear Susceptibility in Colloidal Quantum Dots

Multiresonant coherent multidimensional spectroscopy (CMDS) is a powerful new method for probing the coupling between vibrational modes and their dynamics. The line narrowing that occurs because of the multidimensional nature of CMDS allows the separation of homogeneous and inhomogeneous broadening and enhances spectral resolution. Recent work has extended multiresonant CMDS to electronic resonances in quantum confined nanostructures. Vibrational modes of the solvent also appear in the CMDS spectra. The phase oscillations of the vibrational and electronic coherences interfere and change the line shapes. Since the form of the vibrational third-order susceptibility and hyperpolarizability are well-known and since they can be measured against known standards, it becomes possible to use the interference effects as a probe of the absolute magnitude and phase of the electronic resonances. This approach is demonstrated using PbSe quantum dots where incomplete capping causes ultrafast relaxation to a new electronic state that appears directly in the CMDS spectra. The new state is believed to be a mixed core/surface exciton. Closed-form expressions for the electronic nonlinearities are used to analyze the frequency dependence of the fully resonant complex hyperpolarizability of the 1S exciton and the surface-trapped state. The ability of mixed frequency/time domain multiresonant CMDS methods to spectrally resolve surface states promises to be an important new way to characterize the interface states in complex heterostructures and the surface states that define the stability of nanostructures resulting from different synthetic strategies

Multiresonant Coherent Multidimensional Electronic Spectroscopy of Colloidal PbSe Quantum Dots

We demonstrate the use of multiresonant coherent multidimensional spectroscopy (CMDS) for obtaining 2D spectra of the diagonal and cross-peaks of the 1S and 1P excitons and biexcitons in PbSe quantum dots and their coherent and incoherent dynamics. We show that multiresonant CMDS line narrows the inhomogeneous broadening and resolves the excitonic peaks from the background that often obscures peaks. We develop theoretical methods that extract details of the homogeneous and inhomogeneous broadening, the Coulombic coupling within the biexciton, and the relative exciton and biexciton transition moments. The population dynamics are measured by scanning the excitation pulse time delays over all time orderings. Phase modulations of individual coherences are observed because of heterodyning between scattered light and the four-wave mixing signal. The experiments demonstrate that CMDS can be used to obtain quantum state resolved dynamics of the electronic states in complex nanostructures and other important materials