11 research outputs found
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-Fe<sub>2</sub>O<sub>3</sub>) NWs.
The direct heterogeneous nucleation of QDs on Fe<sub>2</sub>O<sub>3</sub> NWs relies upon an aggressive surface dehydration of the
as-synthesized Fe<sub>2</sub>O<sub>3</sub> NWs at 350 °C under
vacuum and subsequent introduction of colloidal reactants resulting
in direct growth of PbSe QDs on Fe<sub>2</sub>O<sub>3</sub>. 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 Fe<sub>2</sub>O<sub>3</sub> (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
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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