A photoresponsive graphene oxide-C60 conjugate

[EN] An all-carbon donor–acceptor hybrid combining graphene oxide (GO) and C60 has been prepared. Laser flash photolysis measurements revealed the occurrence of photoinduced electron transfer from the GO electron donor to the C60 electron acceptor in the conjugate.This research was financially supported by the Spanish Ministry of Economy and Competitiveness of Spain (CTQ2010-17498, MAT2010-20843-C02-01 and PLE-2009-0038) and a Severo Ochoa operating grant from the Spanish Ministry of Economy and Competitiveness. We also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, Comunidad de Madrid (CAM 09-S2009_MAT-1467), Generalitat Valenciana (PROMETEO program), and VLC/Campus Microcluster "Nanomateriales Funcionales y Nanodispositivos".Barrejón, M.; Vizuete, M.; Gómez Escalonilla, M.; Fierro, J.; Berlanga, I.; Zamora, F.; Abellán, G.... (2014). A photoresponsive graphene oxide-C60 conjugate. Chemical Communications. 50(65):9053-9055. doi:10.1039/C3CC49589BS90539055506

Ultrafast charge transfer dynamics in photoexcited CdTe quantum dot decorated on graphene

We report synthesis and ultrafast charge transfer dynamics of photoexcited CdTe quantum dots (QDs) decorated on graphene. We have synthesized CdTe QD particles of 2.2 nm sizes with first exciton (1S3/2-1Se) band ∼450 nm and then decorated the QD particles on graphene which has been confirmed by HRTEM studies. The CdTe QD decorated graphene has been named as G-CdTe. Steady state emission studies revealed that on the graphene surface CdTe emission gets quenched drastically which indicates the charge transfer from photoexcited CdTe to graphene. To unravel the charge transfer dynamics in ultrafast time scale we have carried out femtosecond transient absorption studies by exciting the CdTe QD particles and monitoring the transients in the visible to near-IR region. Transient absorption studies indicate that exciton recombination time (as monitored the exciton bleach) of pure CdTe QD takes place within 50 ps; however, on graphene the surface exciton recombination time was found to be much longer (>1 ns). Our studies clearly indicate that charge separation of G-CdTe composite materials drastically improves as compared to that CdTe QD

Charge carrier cascade in type II CdSe–CdTe graded core–shell interface

We report the synthesis, structural characterisation and charge separation behaviour in an interface graded Type II CdSe–CdTe core–shell nanostructure. The gradation was accomplished by the use of two layers consisting of an alloyed composition with increasing Te composition between the CdSe core and finally a CdTe shell grown by the successive ionic layer adsorption and reaction (SILAR) method. The effect of gradation was analysed by steady state UV-Vis absorption, photoluminescence (PL) spectroscopy, time-resolved luminescence and femtosecond transient absorption spectroscopy and was found to be immensely beneficial in improving the quantum yield as compared to an ungraded core–shell. Improvement in charge separation was further ascertained by temperature dependent luminescence studies and time correlated single photon counting studies. Better charge separation behaviour accompanied by a more robust PL yield is indicative of better surface passivation and band alignment for charge carrier funnelling. The reduction in stress was further verified by Raman studies where the Raman peak position was used as an index for stress in the film

Charge separation by indirect bandgap transitions in CdS/ZnSe type-II core/shell quantum dots

Femtosecond time-resolved absorption and picosecond time-resolved emission studies have been carried out to study the indirect type exciton of CdS/ZnSe core/shell quantum dots (QDs). The CdS/ZnSe core/shell QD samples are synthesized with increasing thickness of ZnSe shell on CdS core QDs. In these CdS/ZnSe core/shell samples, a new energy band lower than the energy gap of both the CdS core and ZnSe shell has been observed and attributed to indirect bandgap transitions from the valence band of the ZnSe shell to the conduction band of the CdS core. The transient PL studies have revealed that the indirect type exciton, e(CdS)/h(ZnSe) due to photoexcitation of this low-energy band, endures less carrier trapping than selective excitation of the CdS core and charge transfer in the staggered photoexcited state. Femtosecond transient absorption studies have revealed that carrier trapping is as fast as 100 fs and interfacial charge recombination slows down with increasing ZnSe shell thickness on the CdS QD in CdS/ZnSe core/shell QDs

Newly designed resorcinolate binding for Ru(II)– and Re(I)–polypyridyl complexes on oleic acid capped TiO₂ in nonaqueous solvent: prolonged charge separation and substantial thermalized ³MLCT injection

Femtosecond pump–probe spectroscopic studies on a series of newly synthesized resorcinol-based Ru(II) and Re(I) complexes on oleic acid capped TiO2 nanoparticles have been carried out in chloroform, and the results are compared with those of the catechol analogues. The ruthenium complex shows biexponential injection; the second component arises due to injection from the thermally equilibrated 3MLCT states as a result of a weaker strength of the resorcinolate binding. Also, in comparison with catechol binding, as a result of a greater diffusion of the injected electrons into TiO2 , the back electron transfer (BET) is slowed down significantly for the ruthenium complex. These are distinctive observations for any mononuclear ruthenium–polypyridyl–enediol complex reported thus far. However, the rhenium complex educes single exponential ultrafast injection (<120 fs) because of apparent injection in a high density of states and shows the most prominent results with ∼50% slowdown in the charge recombination rate as compared to the analogous catechol bound system. These results exemplify the probable development of highly capable sensitizer dyes with resorcinol as the anchoring group for the development of efficient dye-sensitized solar cells

Interfacial electron transfer dynamics in quinizarin sensitized ZnS nanoparticles: monitoring charge transfer emission

Water soluble cubic ZnS nanoparticles (NPs) have been synthesized at room temperature by using 3-mercaptopropionic acid (MPA) as a modifier molecule and characterized by X-ray diffraction (XRD), steady-state absorption, and emission spectroscopy. Electron transfer (ET) dynamics have been carried out in ZnS semiconductor nanoparticles and quinizarin (Qz) molecules as studied by picosecond time-resolved fluorescence spectroscopy. We have proposed that electron injection takes place from photoexcited Qz molecules into the surface states of wide band gap ZnS NPs. We have revealed that the formation of a charge transfer complex between the Qz molecule and ZnS nanoparticles facilitates electron injection into the surface states of nanoparticles. In the present investigation, we have detected charge transfer (CT) emission in the Qz−ZnS system as the injected electrons from surface states return back to the parent Qz cation radical. We have determined back ET rates by monitoring the CT emission

Ultrafast charge carrier relaxation and charge transfer dynamics of CdTe/CdS core−shell quantum dots as studied by femtosecond transient absorption spectroscopy

We are reporting ultrafast charge carrier and charge transfer dynamics of the CdTe quantum dot (QD) and type II CdTe/CdS core−shell QD materials with different shell (CdS) thicknesses. Herein, we have synthesized CdTe and CdTe/CdS core−shell quantum dots by using 3-mercaptopropionic acid as a capping agent. Steady state absorption and emission studies confirmed successful synthesis of CdTe QD and CdTe/CdS core−shell QD materials. Time-resolved emission studies indicate a longer emission lifetime of the CdTe/CdS core−shell as compared to CdTe QD materials, where in both cases only CdTe gets excited. We have carried out femtosecond transient absorption studies of these QD and core−shell materials by exciting them with 400 nm laser light and monitoring the transients in the visible to near-IR region to study charge carrier and charge transfer dynamics in the ultrafast time scale. On laser excitation, electron−hole pairs are generated which are confirmed by induced absorption signal for the charge carriers in the visible and near-IR region and an immediate bleach at excitonic position for both QD and QD core−shell. The carrier relaxation was found to be slower and the carrier lifetime was found to be longer in the QD core−shell as compared to the QD indicating charge transfer from core to shell. Carrier quenching studies have been carried out for both CdTe and CdTe/CdS by using benzoquinone (BQ, electron quencher) and Pyridine (Py, hole quencher) to assign the different relaxation processes. Details about the relaxation of hot carriers and the quenching effect on the relaxation dynamic of the charge carriers have been discussed for both QD and core−shell nanostructures

Ultrafast hole transfer in CdSe/ZnTe type II core−shell nanostructure

We have synthesized thiol-capped CdSe/ZnTe quantum dot core−shell nanostructures by colloidal methods, have characterized them by steady-state absorption and photoluminescence (PL) spectroscopy and further confirmed by high resolution transmission electron microscopy and X-ray diffraction measurements. Clear red shift on shell formation was observed in optical absorption and photoluminescence studies. Time-resolved emission studies indicate longer emission lifetime of CdSe/ZnTe core−shell as compared to CdSe QD material where in both cases only CdSe gets excited, which indicates spatial charge separation in type-II core−shell. Ultrafast photoinduced charge transfer dynamics in type-II CdSe/ZnTe donor−acceptor core−shell were studied in real-time using femtosecond broadband pump−probe spectroscopy. Our transient absorption studies suggests that on photoexcitation core−shell hole transfer from CdSe core to ZnTe shell takes place in pulse-width limited time scale as evidenced by an increase in cooling dynamics of the charge carriers from 150 fs for CdSe to 300 fs for thickest CdSe/ZnTe core−shell. Increase in cooling dynamics in core−shell has been explained due to decoupling of electron and hole in photoexcited core−shell. Trapping dynamics play a major role in the excited dynamics of the photoexcited charge carriers of quantum dot materials. Bleach recovery kinetics of the photoexcited QD materials fitted multi-exponentially where 2.5 ps (first component) has been attributed to the electron trapping dynamics and the longer components (30−50 ps and >400 ps) attributed to the charge recombination dynamics