3 research outputs found
Effect of Surface Passivation on Nanodiamond Crystallinity
Diamonds
approaching the nanoscale have the potential for use as
probe materials as their optical properties can be sensitive to optical/electric
fields, mechanical stress/pressure, and the configuration of nuclear
spins. The surface of nanodiamonds impacts their optical properties
and sensing capabilities, and examining the nanodiamond surface through
X-ray absorption can give insights into molecular surface structures.
Here, quantum dot models with varying amounts of surface carbon passivation
are prepared, optimized, and compared. The loss of the diamond sp<sup>3</sup> lattice is examined by investigating the bond length and
tetrahedral character of the carbons comprising nanodiamonds for the
appearance of aromatic sp<sup>2</sup> surface domains. Electronic
transitions in the carbon K-edge region, using the energy-specific
time-dependent density functional theory method, as well as vibrational
spectra are computed from the optimized models. The surface reorganization
is shown to affect the electronic characteristics of the nanodiamond.
As a result, there is a distinct absorption peak in the carbon K-edge
region, along with stretching modes in the vibrational spectra, that
can be correlated to the nature of the surface hybridization of the
nanodiamond
Optomechanical Thermometry of Nanoribbon Cantilevers
Cadmium sulfide (CdS) nanostructures
have attracted a significant
amount of attention for a variety of optoelectronic applications including
photovoltaic cells, semiconductor lasers, and solid-state laser refrigeration
due to their direct bandgap around 2.42 eV and high radiative quantum
efficiency. Nanoribbons (NRs) of CdS have been claimed to laser cool
following excitation at 514 and 532 nm wavelengths by the annihilation
of optical phonons during anti-Stokes photoluminescence. To explore
this claim, we demonstrate a novel optomechanical experimental technique
for microthermometry of a CdSNR cantilever using Young’s modulus
as the primary temperature-dependent observable. Measurements of the
cantilever’s fundamental acoustic eigenfrequency at low laser
powers showed a red-shift in the eigenfrequency with increasing power,
suggesting net heating. At high laser powers, a decrease in the rate
of red-shift of the eigenfrequency is explained using Euler–Bernoulli
elastic beam theory, considering Hookean optical-trapping force. A
predicted imaginary refractive index for CdSNR based on experimental
temperature measurement agrees well with a heat transfer analysis
that predicts the temperature distribution within the cantilever and
the time required to reach steady state (<100 μs). This approach
is useful for investigating solid-state laser refrigeration of a wide
variety of material systems without the need for complex pump/probe
spectroscopy
Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics
New
electron-acceptor materials are long sought to overcome the
small photovoltage, high-cost, poor photochemical stability, and other
limitations of fullerene-based organic photovoltaics. However, all
known nonfullerene acceptors have so far shown inferior photovoltaic
properties compared to fullerene benchmark [6,6]-phenyl-C<sub>60</sub>-butyric acid methyl ester (PC<sub>60</sub>BM), and there are as
yet no established design principles for realizing improved materials.
Herein we report a design strategy that has produced a novel multichromophoric,
large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T,
whose π-conjugated framework occupies space comparable to an
aggregate of 9 [C<sub>60</sub>]-fullerene molecules. Comparative studies
of DBFI-T with its planar monomeric analogue (BFI-P2) and PC<sub>60</sub>BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration
and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a
high short-circuit current (10.14 mA/cm<sup>2</sup>) with a high open-circuit
voltage (0.86 V) to give a power conversion efficiency of 5.0%. The
external quantum efficiency spectrum of PSEHTT:DBFI-T devices had
peaks of 60–65% in the 380–620 nm range, demonstrating
that both hole transfer from photoexcited DBFI-T to PSEHTT and electron
transfer from photoexcited PSEHTT to DBFI-T contribute substantially
to charge photogeneration. The superior charge photogeneration and
electron-accepting properties of DBFI-T were further confirmed by
independent Xenon-flash time-resolved microwave conductivity measurements,
which correctly predict the relative magnitudes of the conversion
efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC<sub>60</sub>BM > PSEHTT:BFI-P2. The results demonstrate that the large
size, multichromophoric, nonplanar 3D molecular design is a promising
approach to more efficient organic photovoltaic materials