R. Singhal

Au–ZnO: A tunable localized surface plasmonic nanocomposite

In this letter, we report the thermal processing controlled tunability of localized surface plasmon resonance (LSPR) of Au nanoparticles embedded in ZnO matrix. Au–ZnO nanocomposite films were prepared by atom beam cosputtering and were annealed from 200to600°C in Ar. A regular redshift ∼110nm (from 505to615nm) in LSPR peak with increase in annealing temperature up to 600°C is observed. Transmission electron microscopy results confirm the formation of Au nanoparticles supported by ZnO nanorods at annealing temperature of 600°C. The Au–ZnO nanocomposite exhibits significant enhancement in the Raman signal for C70 molecules.

Formation of Self-organized Silver Nanocup-Type Structures and Their Plasmonic Absorption

The present work reports on the formation of extremely low volume, silver nanocup-type structures on the surface by annealing of ultra-thin silver film on quartz in inert environment. Atomic force microscopy studies together with scanning electron microscopy confirmed the formation of Ag nanocup-type structures at the surface. A basic physical model for the formation of nanocups in terms of buckling and Oswald ripening due to surface-induced morphological instability and diffusional mass transport under thermal treatment is demonstrated. Surface plasmon resonance absorptions of nanocup structures are studied and preliminary experiment for observing the surface-enhanced Raman scattering of fullerene C70 molecules has been shown.

Electronic excitation induced tuning of surface plasmon resonance of Ag nanoparticles in fullerene C70 matrix

We report the electronic excitation induced controlled tuning of the surface plasmon resonance (SPR) wavelength of Ag nanoparticles (NPs) in fullerene C70 matrix. The transformation of fullerene C70 into amorphous carbon (a:C) under ion irradiation is used to tune the SPR wavelength of C70–Ag nanocomposite thin films. A 100 nm blue shift was recorded for irradiation at a fluence of 3 × 1013 ions cm−2 by 120 MeV Ag ions. A growth of Ag NPs from 7.0 ± 0.8 to 11.0 ± 0.4 nm with increasing fluence was observed by transmission electron microscopy and it is explained in the framework of thermal spike model. The transformation of fullerene C70 into amorphous carbon with ion irradiation was confirmed by Raman spectroscopy. This work demonstrates the possibility to locally excite the SPR at a desired wavelength and therefore, acquiring multiple SPR bands at a single substrate which could be useful in developing more efficient optical sensors.

Synthesis and characterizations of silver-fullerene C70 nanocomposite

Films of C70 fullerene containing silver nanoparticles were synthesized by thermal codeposition. Optical absorption studies revealed that surface plasmon resonance of Ag nanoparticles occurs at unusually large wavelength, which showed a regular redshift from 521 to 581 nm with increase in metal content from 4.5% to 28%. It is explained by the Maxwell–Garnett theory considering the absorbing nature of fullerene matrix. Rutherford backscattering and transmission electron microscopy were performed to quantify metal content and the particle size, respectively. A better detection of low intensity vibrational modes of C70 in Raman scattering is observed due to surface enhanced Raman scattering.

Synthesis, characterizations, and thermal induced structural transformation of silver-fullerene C60 nanocomposite thin films for applications in optical devices

Nanocomposite thin films of fullerene C60 containing Ag nanoparticles (NPs) were synthesized by thermal codeposition. The surface plasmon resonance (SPR) band of the nanocomposite film was observed in the region 450–550 nm, showing a large redshift with increasing metal concentration. This is explained by the Maxwell–Garnett effective medium theory considering the absorbing nature of fullerene C60. The C60–Ag nanocomposite thin film with lowest Ag concentration was annealed at increasing temperatures in neutral atmosphere. The SPR band showed first a small redshift after annealing at small temperature then progressive blueshift at higher temperature. This behavior of SPR is explained by the increased particle–particle interaction due to the compaction of the fullerene C60 film upon annealing at low temperature and the transformation of fullerene C60 matrix into amorphous carbon at higher temperature. Rutherford backscattering spectrometry and transmission electron microscopy were used to quantify Ag metal...

A comparative study of ion-induced damages in C60 and C70 fullerenes

The stability of fullerenes (C60 and C70) under swift heavy ion irradiation is investigated. C60 and C70 thin films were irradiated with 120 MeV Ag ions at fluences from 1×1012 to 3×1013 ions/cm2. The damage cross-section and radius of damaged cylindrical zone were found to be higher for C60 than C70 as evaluated by Raman spectroscopy, which shows that the C70 molecule is more stable under energetic ion impact. The higher damage cross-section of the C60 molecule compared with that of the C70 molecule is explained on the basis of thermal conductivity in the framework of the thermal spike model. The surface morphology of pristine C60 and C70 films is studied by atomic force microscopy. UV-visible absorption studies revealed that band gap for C60 and C70 fullerenes thin films decreases with increasing ion fluence. Resistivity of C60 and C70 thin films decreases with increasing ion fluence but the decrease is faster for C60 than C70, indicating higher damage in C60. Irradiation at a fluence of 3×1013 ions/cm2 results in complete damage of fullerenes (C60 and C70) into amorphous carbon.

Comparative study on the photovoltaic characteristics of A–D–A and D–A–D molecules based on Zn-porphyrin; a D–A–D molecule with over 8.0% efficiency

The synthesis of two symmetrical molecules with A–D–A (1) and D–A–D (2) structures is reported. The compounds have Zn-porphyrin as the donor component and 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DT-DPP) as the acceptor unit, linked through ethynyl π-bridges. The effect of scaffold modification on the electrochemical, optical and photovoltaic characteristics has been investigated and favourable properties are found for the D–A–D system compared to the A–D–A. These two Zn-porphyrin-based small molecules are used as donors along with PC71BM as the acceptor for the fabrication of solution processed bulk heterojunction organic solar cells in order to obtain information about the effect of donor and acceptor positions on the photovoltaic response of the devices. A strategy for optimizing the morphology of the active layer using a binary solvent additive has been used. After the optimization of the weight ratios between 1 or 2 and PC71BM and the solvent additives, the devices based on 1 : PC71BM and 2 : PC71BM active layers show remarkable PCE values of 7.04% and 8.03%, respectively. The higher PCE obtained for 2 : PC71BM compared to 1 : PC71BM can be related to the increase in Jsc and FF and attributed to lower energy loss, strong π–π intermolecular stacking and more balanced charge transport.

Unprecedented low energy losses in organic solar cells with high external quantum efficiencies by employing non-fullerene electron acceptors

In order to realize high photocurrent generation in the low-energy region of the solar spectrum, two conjugated A–D–A oligomers, PY-1 and DCI-2 comprising a central dithieno[3,2-b:2′,3′-d]pyrrole donor (D) unit and end-capping acceptors (A) 1-butyl-4-methyl-2,6-dioxopyridine-3-carbonitrile (PY) or 3-dicyanomethyleneindan-1-one (DCI) have been synthesized and characterized. The oligomers showed strong absorptions in the red and near-IR region in solution and in the solid state. As a consequence of the strong electron-accepting character, the absorption band of DCI-2 in thin films is significantly red-shifted compared to that of PY-1 resulting in a low optical energy gap of 1.23 eV. In optimized solution-processed bulk-heterojunction solar cells using a polymeric donor P1, the new non-fullerene acceptor DCI-2 provides an excellent power conversion efficiency of 6.94% which is noticeably higher than that of PY-1-based devices (PCE = 4.89%). Most importantly, a high open-circuit voltage (VOC) of ∼0.8 V with unprecedented energy losses, between the optical energy gap and the open-circuit voltage, between 0.39 and 0.43 eV concomitant with excellent external quantum efficiencies of 69%@880 nm in the NIR-regime have been achieved for DCI-2-based devices.

Ferrocene-diketopyrrolopyrrole based non-fullerene acceptors for bulk heterojunction polymer solar cells

Herein we have investigated the photovoltaic properties of ferrocenyl tetracyanobutadiene derivatives of diketopyrrolopyrroles SM1 and SM2 as efficient non-fullerene acceptors along with a donor–acceptor (D–A) conjugated polymer P as a donor for polymer solar cells. The solar cell devices showed a maximum power conversion efficiency of 6.44% and 6.89% for the vacuum dried P:SM1 and P:SM2 active layers respectively. The solar cells based on SM2 showed higher PCE compared to SM1 which may be due to the larger values of both Jsc and FF. The results presented here demonstrate that employing ferrocene as a donor unit can provide great scope for the molecular design of highly efficient non-fullerene acceptors and provides important progress in the development of non-fullerene organic solar cells.

Efficient Polymer Solar Cells with High Open-Circuit Voltage Containing Diketopyrrolopyrrole-Based Non-Fullerene Acceptor Core End-Capped with Rhodanine Units

Herein we report the synthesis of a novel A-D-A-D-A non-fullerene small-molecule acceptor (NFSMA) bearing a diketopyrrolopyrrole (DPP) acceptor central core coupled to terminal rhodanine acceptors via a thiophene donor linker (denoted as MPU1) for use in non-fullerene polymer solar cells (PSCs). This NFSMA exhibits a narrow optical band gap (1.48 eV), strong absorption in the 600-800 nm wavelength region of the solar spectrum, and a lowest unoccupied energy level of -3.99 eV. When the mixture of a medium band gap D-A copolymer P (1.75 eV) was used as donor and MPU1 as acceptor, the blend film showed a broad absorption profile from 400 to 850 nm, beneficial for light harvesting efficiency of the resulted polymer solar cell. After optimization of the donor-to-acceptor weight ratios and concentration of solvent additive, the P-MPU1-based PSC exhibited a power conversion efficiency of 7.52% (Jsc= 12.37 mA/cm2, Voc = 0.98 V, and fill factor = 0.62), which is much higher than that for a P3HT-MPU1-based device (2.16%) prepared under identical conditions. The higher value for the P-MPU1-based device relative to the P3HT-MPU1-based one is related to the low energy loss and more balanced charge transport in the device based on the P donor. These results indicate that alteration of the absorption spectra and electrochemical energy levels of non-fullerene acceptors, and appropriate selection of the polymer donor with complementary absorption profile, is a promising means to further boost the performance of PSCs.