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

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

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

Effect of surface states on charge-transfer dynamics in type II CdTe/ZnTe core–shell quantum dots: a femtosecond transient absorption study

We have synthesized CdTe/ZnTe type II core–shell quantum dots at lower temperature by using 3-mercaptopropionic acid (SHCH2CH2COOH, thiol) as a capping agent, and they are characterized by steady-state absorption, emission studies, and high-resolution transmission electron microscopy. We observed a red shift in optical absorption and emission spectra with the increase in ZnTe shell thickness. Time-resolved emission studies indicate a longer average emission lifetime for CdTe/ZnTe core–shell QDs as compared with that for CdTe QDs. Femtosecond transient absorption spectroscopy was employed to study charge carrier relaxation in CdTe QDs and charge carrier relaxation and charge-transfer dynamics in CdTe/ZnTe core–shell QDs by monitoring the transients in the visible-to-near-IR region. After laser excitation of QDs, electron–hole pairs are generated, which are confirmed by bleach at the excitonic position and induced absorption signal in the visible and near-IR regions for both QDs and core–shell QDs. In the transient absorption spectrum of CdTe/ZnTe QDs, exciton bleach (negative absorption) has been superimposed by positive absorption caused by trapped carriers, which indicates the presence of trap states. The carrier cooling time was found to be slower (250 fs) in CdTe/ZnTe core–shell QDs as compared with that in CdTe QDs (185 fs), indicating charge transfer from CdTe to the ZnTe shell. Ultrafast transient absorption studies suggest an influence of trap states in relaxation dynamics of photoexcited charge carriers in core–shell QDs

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

Synthesis and optical properties of Type I CdSe/ZnSe core–shell quantum dot

We have synthesized thiol-capped CdSe/ZnSe core–shell quantum dot nanostructures at low temperature; have characterized them by steady-state absorption and photoluminescence (PL) studies. Band-gap energy alignment for CdSe and ZnSe semiconductor materials suggested that they can form Type-I core–shell quantum dot. Our steady state absorption and emission studies confirmed the formation of Type-I core–shell band alignment between CdSe and ZnSe quantum dots. On excitation of CdSe quantum dot only surface states emission was observed, however on ZnSe shell formation we could also observed band-edge emission of CdSe. Time-resolved emission studies have also been carried out in CdSe core and CdSe/ZnSe core–shell QD materials. Band-edge emission which appeared due to shell formation for CdSe found to decay faster as compared to surface state emission of CdSe QD and CdSe-ZnSe core–shell

Interfacial electron transfer dynamics of two newly synthesized catecholate bound Ru<SUP>II</SUP> polypyridyl-based sensitizers on TiO<SUB>2</SUB> nanoparticle surface - a femtosecond pump probe spectroscopic study

Two new catecholate-bound RuII-polypyridine based sensitizers, (2,2'-bipyridine){ethyl 3-(4-hydroxyphenyl)-2-[(4'-methyl-2,2'-bipyridinyl-4-carbonyl)amino]propionate}{4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)}ruthenium(II) hexafluorophosphate (5) and [(2,2'-bipyridine)-(4-2,2'-bipyridinyl-4-yl-phenol)-(4-{2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexafluorophosphate (6) with secondary electron-donating groups (tyrosine and phenol, respectively) were synthesized and characterized. Steady-state optical absorption and emission studies confirm strong coupling between the sensitizers and TiO2 nanoparticles. Femtosecond visible transient absorption spectroscopy has been employed to study interfacial electron transfer (IET) dynamics in the dye-nanoparticle systems to explore the influence of the secondary electron-donating groups on IET dynamics. Electron injection into the conduction band of nanoparticulate TiO2 has been confirmed by detection of the conduction band electrons in TiO2 ([e-]TiO2CB) and radical cation of the adsorbed dye (D&#183;+) in real time monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (&lt; 100 fs) electron injection has been observed. Back electron transfer (BET) dynamics have been studied by monitoring the decay kinetics of the injected electron in the conduction band of TiO2 and by the recovery of the ground state bleach. BET dynamics in dye-TiO2 systems for complexes 5 and 6 have been compared with those of [bis(2,2'-bpy)-(4-{2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexafluorophosphate (7), which does not have a secondary electron-donating group

Spectroscopy and femtosecond dynamics of water soluble type I CdSe/ZnS core–shell quantum dot

Thiol-capped type I CdSe/ZnS core–shell quantum dot nanostructures have been synthesized at low temperature in water; and then characterized by steady-state absorption and photoluminescence (PL) studies and high resolution TEM (HRTEM) measurements. On excitation of CdSe quantum dot predominantly surface state emission was detected, however on ZnS shell formation prominent exciton emission of CdSe with much higher overall quantum yield was observed. Femtosecond up-conversion measurements reveal that exciton emission found to decay much faster as compared to that of surface state emission. Lifetime of CdSe exciton emission found to increases with ZnS shell thickness. Femtosecond transient absorption studies have been carried out on these QD and core–shell material at 400 nm laser light and monitored the transients in the visible region to study charge carrier dynamics in ultrafast time scale. On laser excitation electron–hole pairs are generated which are detected by induced absorption signal for the charge carriers in visible region and immediate bleach at excitonic position for both QD and QD core–shell. Carrier quenching studies has been carried out for both CdSe and CdSe/ZnS by using benzoquinone (BQ, electron quencher) and pyridine (Py, hole quencher) suggest that although CdSe/ZnS form type I core–shell, still both electron and hole found to be leaked through ZnS shell from CdSe core

Does bridging geometry influence interfacial electron transfer dynamics? Case of the enediol-TiO₂ system

We have employed femtosecond transient absorption spectroscopy in enediol-TiO₂ systems (catechol, resorcinol, and quinol) to understand localized vs delocalized interfacial electron transfer dynamics in dye-nanoparticle systems. Optical absorption studies confirmed the formation of a charge transfer (CT) complex between the enediols and TiO₂ nanoparticles. CT interaction between enediols and TiO₂ was found to be decreased from catechol to resorcinol to quinol. The decrease in interaction strength from catechol to quinol was explained on the basis of a reduced overlap between the HOMO localized on the enediol and the conduction band of TiO₂. Femtosecond transient absorption studies confirmed an ultrafast electron injection (&#60;50 fs) into the conduction band of TiO₂ in all enediol-TiO₂ systems. Interestingly back electron transfer (BET), which follows multiexponential dynamics, is faster in the catechol-TiO₂ system as compared to the other two enediol-TiO₂ systems. As we increase the distance between the bridging ligand from catechol to quinol, we find that decay time increases proving the influence of bridging distance between enediols and TiO₂. Our theoretical studies indicate an increase in the delocalization of the injected electron over several Ti atoms as the distance between the bridge linkers increases. Analysis of BET results in the framework of the Marcus theory indicated a significant influence of electronic coupling on BET in the enediol-TiO₂ systems

Surface-state-mediated charge-transfer dynamics in CdTe/CdSe core-shell quantum dots

Herein, we report the synthesis of aqueous CdTe/CdSe type-II core–shell quantum dots (QDs) in which 3-mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core–shell QDs are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), steady-state absorption, and emission spectroscopy. A red shift in the steady-state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR-TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time-resolved emission studies, we observe that the average emission lifetime (τav) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τav=12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type-II CdTe/CdSe core–shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge-transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core–shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core–shell QDs