25 research outputs found
Exciton Dynamics of CdS Thin Films Produced by Chemical Bath Deposition and DC Pulse Sputtering
Exciton dynamics of CdS films have
been investigated using ultrafast
laser spectroscopy with an emphasis on understanding defect-related
recombination. Two types of CdS films were deposited on glass substrates
via direct current pulse sputtering (DCPS) and chemical bath deposition
(CBD) techniques. The films displayed distinct morphological, optical,
and structural properties. Their exciton and charge carrier dynamics
within the first 1 ns following photoexcitation were characterized
by femotosecond pump probe spectroscopy. A singular value decomposition
(SVD) global fitting technique was employed to extract the lifetime
and wavelength dependence of transient species. The excited electrons
of the DCPS sample decays through 1.8, 8, 65, and 450 ps time constants
which were attributed to donor level electron trapping, valence band
(VB) → conduction band (CB) recombination, shallow donor recombination,
and deep donor recombination, respectively. The CBD sample shows time
constants of 6, 65, and 450 ps which were attributed to CB →
VB recombination, sulfur vacancy (<i>V</i><sub>S</sub>)
recombination, and <i>V</i><sub>S</sub> → oxygen
interstitial (O<sub>i</sub>) donor–acceptor pair (DAP) recombination,
respectively. It was found that the DCPS deposition technique produces
films with lower defect density and improved carrier dynamics, which
are important for high performance solar cell applications
Experimental and TD-DFT Study of Optical Absorption of Six Explosive Molecules: RDX, HMX, PETN, TNT, TATP, and HMTD
Time dependent density function theory
(TD-DFT) has been utilized
to calculate the excitation energies and oscillator strengths of six
common explosives: RDX (1,3,5-trinitroperhydro-1,3,5-triazine), β-HMX
(octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), TATP (triacetone
triperoxide), HMTD (hexamethylene triperoxide diamine), TNT (2,4,6-trinitrotoluene),
and PETN (pentaerythritol tetranitrate). The results were compared
to experimental UV–vis absorption spectra collected in acetonitrile.
Four computational methods were tested including: B3LYP, CAM-B3LYP,
ωB97XD, and PBE0. PBE0 outperforms the other methods tested.
Basis set effects on the electronic energies and oscillator strengths
were evaluated with 6-31GÂ(d), 6-31+GÂ(d), 6-31+GÂ(d,p), and 6-311+GÂ(d,p).
The minimal basis set required was 6-31+GÂ(d); however, additional
calculations were performed with 6-311+GÂ(d,p). For each molecule studied,
the natural transition orbitals (NTOs) were reported for the most
prominent singlet excitations. The TD-DFT results have been combined
with the IP<sub>v</sub> calculated by CBS-QB3 to construct energy
level diagrams for the six compounds. The results suggest optimization
approaches for fluorescence based detection methods for these explosives
by guiding materials selections for optimal band alignment between
fluorescent probe and explosive analyte. Also, the role of the TNT
Meisenheimer complex formation and the resulting electronic structure
thereof on of the quenching mechanism of II–VI semiconductors
is discussed
Molecular Adsorption Mechanism of Elemental Carbon Particles on Leaf Surface
Plant leaves can
effectively capture and retain particulate matter
(PM), improving air quality and human health. However, little is known
about the adsorption mechanism of PM on leaf surface. Black carbon
(BC) has great adverse impact on climate and environment. Four types
of elemental carbon (EC) particles, carbon black as a simple model
for BC, graphite, reduced graphene oxide, and graphene oxide, and
C<sub>36</sub>H<sub>74</sub>/C<sub>44</sub>H<sub>88</sub>O<sub>2</sub> as model compounds for epicuticular wax were chosen to study their
interaction and its impact at the molecular level using powder X-ray
diffraction and vibrational spectroscopy (infrared and Raman). The
results indicate that EC particles and wax can form C–H···π
type hydrogen bonding with charge transfer from carbon to wax; therefore,
strong attraction is expected between them due to the cooperativity
of hydrogen bonding and London dispersion from instantaneous dipoles.
In reality, once settled on the leaf surface, especially without wax
ultrastructures, BC with extremely large surface-to-volume ratio will
likely stick and stay. On the other hand, BC particles can lead to
phase transition of epicuticular wax from crystalline to amorphous
structures by creating packing disorder and end-<i>gauche</i> defects of wax molecular chain, potentially causing water loss and
thereby damage of plants
Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr<sub>3</sub> Perovskite Quantum Dots
Exciton–phonon interactions elucidate structure–function
relationships that aid in the control of color purity and carrier
diffusion, which is necessary for the performance-driven design of
solid-state optical emitters. Temperature-dependent steady-state photoluminescence
(PL) and time-resolved PL (TRPL) reveal that thermally activated exciton–phonon
interactions originate from structural distortions related to vibrations
in cubic CsPbBr3 perovskite quantum dots (PQDs) at room
temperature. Exciton–phonon interactions cause performance-degrading
PL line width broadening and slower electron–hole recombination.
Structural distortions in cubic PQDs at room temperature exist as
the bending and stretching of the PbBr6 octahedra subunit.
The PbBr6 octahedral distortions cause symmetry breaking,
resulting in thermally activated longitudinal optical (LO) phonon
coupling to the photoexcited electron–hole pair that manifests
as inhomogeneous PL line width broadening. At cryogenic temperatures,
the line width broadening is minimized due to a decrease in phonon-assisted
recombination through shallow traps. A fundamental understanding of
these intrinsic exciton–phonon interactions gives insight into
the polymorphic nature of the cubic phase and the origins of performance
degradation in PQD optical emitters
Preparation and Photoelectrochemical Properties of CdSe/TiO<sub>2</sub> Hybrid Mesoporous Structures
We report on the design and synthesis of a novel CdSe/TiO<sub>2</sub> hybrid mesoporous structure and its implementation as a photoanode
for photoelectrochemical (PEC) application. The CdSe/TiO<sub>2</sub> hybrid mesoporous structure was produced by assembling CdSe quantum
dots (QDs) and TiO<sub>2</sub> nanocrystals into CdSe/TiO<sub>2</sub> hybrid colloidal spheres, followed by calcination to remove the
capping ligands between CdSe and TiO<sub>2</sub>. Compared to the
system involving CdSe QDs directly linked to TiO<sub>2</sub> through
molecular linkers, this CdSe/TiO<sub>2</sub> hybrid mesoporous structure
affords the advantage of better interfacial coupling between CdSe
and TiO<sub>2</sub> due to closer contact. As a result, the CdSe/TiO<sub>2</sub> hybrid mesoporous structure exhibits significantly improved
photoresponse as a photoanode, as demonstrated successfully in comparative
PEC studies. This study illustrates the importance of fundamental
structural control in influencing PEC properties of hybrid assembled
nanostructures
Tunable Photoluminescent Core/Shell Cu<sup>+</sup>‑Doped ZnSe/ZnS Quantum Dots Codoped with Al<sup>3+</sup>, Ga<sup>3+</sup>, or In<sup>3+</sup>
Semiconductor
quantum dots (QDs) with stable, oxidation resistant, and tunable photoluminescence
(PL) are highly desired for various applications including solid-state
lighting and biological labeling. However, many current systems for
visible light emission involve the use of toxic Cd. Here, we report
the synthesis and characterization of a series of codoped core/shell
ZnSe/ZnS QDs with tunable PL maxima spanning 430−570 nm (average
full width at half-maximum of 80 nm) and broad emission extending
to 700 nm, through the use of Cu<sup>+</sup> as the primary dopant
and trivalent cations (Al<sup>3+</sup>, Ga<sup>3+</sup>, and In<sup>3+</sup>) as codopants. Furthermore, we developed a unique thiol-based
bidentate ligand that significantly improved PL intensity, long-term
stability, and resilience to postsynthetic processing. Through comprehensive
experimental and computational studies based on steady-state and time-resolved
spectroscopy, electron microscopy, and density functional theory (DFT),
we show that the tunable PL of this system is the result of energy
level modification to donor and/or acceptor recombination pathways.
By incorporating these findings with local structure information obtained
from extended X-ray absorption fine structure (EXAFS) studies, we
generate a complete energetic model accounting for the photophysical
processes in these unique QDs. With the understanding of optical,
structural, and electronic properties we gain in this study, this
successful codoping strategy may be applied to other QD or related
systems to tune the optical properties of semiconductors while maintaining
low toxicity
Understanding and Mitigating the Effects of Stable Dodecahydro-<i>closo</i>-dodecaborate Intermediates on Hydrogen-Storage Reactions
Alkali
metal borohydrides can reversibly store hydrogen; however,
the materials display poor cyclability, oftentimes linked to the occurrence
of stable <i>closo</i>-polyborate intermediate species.
In an effort to understand the role of such intermediates on the hydrogen
storage properties of metal borohydrides, several alkali metal dodecahydro-<i>closo</i>-dodecaborate salts were isolated in anhydrous form
and characterized by diffraction and spectroscopic techniques. Mixtures
of Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, Na<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, and K<sub>2</sub>B<sub>12</sub>H<sub>12</sub> with the corresponding alkali metal hydrides were subjected to hydrogenation
conditions known to favor partial or full reversibility in metal borohydrides.
The stoichiometric mixtures of MH and M<sub>2</sub>B<sub>12</sub>H<sub>12</sub> salts form the corresponding metal borohydrides MBH<sub>4</sub> (M = Li, Na, K) in almost quantitative yield at 100 MPa H<sub>2</sub> and 500 °C. In addition, stoichiometric mixtures of
Li<sub>2</sub>B<sub>12</sub>H<sub>12</sub> and MgH<sub>2</sub> were
found to form MgB<sub>2</sub> at 500 °C and above upon desorption
in vacuum. The two destabilization strategies outlined above suggest
that metal polyhydro-<i>closo</i>-polyborate species can
be converted into the corresponding metal borohydrides or borides,
albeit under rather harsh conditions of hydrogen pressure and temperature
Dependence of Interfacial Charge Transfer on Bifunctional Aromatic Molecular Linkers in CdSe Quantum Dot Sensitized TiO<sub>2</sub> Photoelectrodes
Quantum dot (QD) sensitization of
TiO<sub>2</sub> is a powerful
method to improve its performance as a photoanode material in solar
energy conversion. The efficiency of sensitization depends strongly
on the rate of interfacial electron transfer (ET) from the QDs to
TiO<sub>2</sub>. To understand the key factors affecting the ET, arene-substituted
(ortho, meta, and para) bifunctional linkers with single or double
aromatic rings were employed to link CdSe QDs to TiO<sub>2</sub> and
control the strength of their interaction as well as the rate of interfacial
ET. Interestingly, the para-substituted aromatic linker, 4-mercaptobenzoic
acid (4MBA) with the longest distance between the carboxyl and thiol
groups, shows the best photoelectrochemical (PEC) performance, when
compared to those of ortho-subtituted (2-mercaptobenzoic acid,
2MBA) and meta-substituted (3-mercaptobenzoic acid, 3MBA) aromatic
linkers. Two other bifunctional linkers with double aromatic rings,
4′-mercapto-[1,1′-biphenyl]-4-carboxylic acid (4M1B4A)
and 6-mercapto-2-naphthioc acid (6M2NA), were also studied for comparison.
Ultrafast transient absorption (TA) spectroscopy was used to study
the exciton dynamics in CdSe QDs and determine the interfacial ET
rate constant (<i>k</i><sub>ET</sub>). The <i>k</i><sub>ET</sub> results are consistent with the trend of PEC measurements
in that 4MBA shows the highest <i>k</i><sub>ET</sub>. To
gain further insight into the ET mechanism, we performed density functional
theory (DFT) calculations to examine the intrinsic properties of the
linkers. The results revealed that the favorable wave function distribution
of the molecular orbitals of 4MBA and 4M1B4A are responsible for the
higher interfacial ET rate and PEC performance due to better interfacial
coupling, a factor that dominates over distance. The present study
provides important new insight into the mechanism of interfacial ET
using aromatic bifunctional linkers, which is useful in designing
QD sensitized semiconductor metal oxide nanostructures for applications
including photovoltaics and solar fuel generation
Coherent Vibrational Oscillations of Hollow Gold Nanospheres
Ultrafast pump−probe spectroscopy was used to characterize coherent vibrational oscillations of hollow gold nanospheres (HGNs) composed of a polycrystalline Au shell and a hollow, solvent-filled interior. Different HGN samples show heavily damped radial breathing mode oscillations with a period ranging from 28 ± 2 to 33 ± 3 ps. We theoretically modeled the oscillation period of HGNs while varying both the shell thicknesses and particle radii. Creation of a hollow cavity was predicted to increase the oscillation period relative to solid gold nanoparticles, and this result was verified experimentally. Our theoretical predictions of oscillation period are significantly lower than the experimental measurements. We propose that this difference is due to the polycrystalline nature of HGNs that softens the vibration of the lattice compared with a single-crystalline shell. We compare our system to solid Au nanoparticles and Au nanoparticle aggregates and find a general trend of longer oscillation period with increasing particle polycrystallinity
Ultrafast Exciton Dynamics in Silicon Nanowires
Ultrafast exciton dynamics in one-dimensional (1D) silicon
nanowires
(SiNWs) have been investigated using femtosecond transient absorption
techniques. A strong transient bleach feature was observed from 500
to 770 nm following excitation at 470 nm. The bleach recovery was
dominated by an extremely fast feature that can be fit to a triple
exponential with time constants of 0.3, 5.4, and ∼75 ps, which
are independent of probe wavelength. The amplitude and lifetime of
the fast component were excitation intensity-dependent, with the amplitude
increasing more than linearly and the lifetime decreasing with increasing
excitation intensity. The fast decay is attributed to exciton–exciton
annihilation upon trap state saturation. The threshold for observing
this nonlinear process is sensitive to the porosity and surface properties
of the sample. To help gain insight into the relaxation pathways,
a four-state kinetic model was developed to explain the main features
of the experimental dynamics data. The model suggests that after initial
photoexcitation, conduction band (CB) electrons become trapped in
the shallow trap (ST) states within 0.5 ps and are further trapped
into deep trap (DT) states within 4 ps. The DT electrons finally recombine
with the hole with a time constant of ∼500 ps, confirming the
photophysical processes to which we assigned the decays