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 sp³ lattice is examined by investigating the bond length and tetrahedral character of the carbons comprising nanodiamonds for the appearance of aromatic sp² 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-C₆₀-butyric acid methyl ester (PC₆₀BM), 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 [C₆₀]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC₆₀BM 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/cm²) 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:PC₆₀BM > 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