TY - JOUR T1 - Chemistry and properties of nanocrystals of different shapes JF - Chemical Reviews Y1 - 2005 A1 - Burda, Clemens A1 - Chen, X. A1 - Narayanan, Radha A1 - El-Sayed, Mostafa A PB - ACS Publications VL - 105 SN - 0009-2665 UR - http://dx.doi.org/10.1021/cr030063a CP - 4 M3 - 10.1021/cr030063a ER - TY - JOUR T1 - Determination of the localization times of electrons and holes in the HgS well in a CdS/HgS/CdS quantum dot–quantum well nanoparticle JF - Physical Review BPhys. Rev. B Y1 - 2002 A1 - Braun, Markus A1 - Link, Stephan A1 - Burda, Clemens A1 - El-Sayed, Mostafa A AB - The femtosecond time-resolved electron-hole dynamics of the CdS/HgS/CdS quantum dot–quantum well system (QDQW) was investigated as a function of excitation energy. In the transient absorption spectra four bleach bands and a stimulated emission signal in the visible spectral range between 450 and 780 nm were resolved. By using an IR probe pulse at 4.7 μm a transient induced absorption due to intraband transitions was found. The decay and rise times of these signals were measured when the CdS core or the HgS well of the nanoparticles was excited by the pump pulse. After excitation within the HgS well the transient signals rise within the resolution of our pump pulse, while after core excitation slower rise times were measured. From the 1.5 ps rise time of the stimulated emission originating from the HgS well and the intraband hole IR absorption (150 fs) after excitation into the CdS core, the electron localization time (transfer time from the core to the well) is found to be 1.5 ps while that of the hole is ∼150 fs. This large difference in the observed dynamics of the electron and hole in crossing the CdS/HgS interface is discussed. PB - American Physical Society VL - 66 UR - http://link.aps.org/doi/10.1103/PhysRevB.66.205312 CP - 20 J1 - PRB M3 - 10.1103/PhysRevB.66.205312 ER - TY - JOUR T1 - The pump power dependence of the femtosecond relaxation of CdSe nanoparticles observed in the spectral range from visible to infrared JF - The Journal of chemical physics Y1 - 2002 A1 - Burda, Clemens A1 - Link, Stephan A1 - Mohamed, MB A1 - El-Sayed, Mostafa A AB - The pump power dependence of the relaxation dynamics of CdSe nanoparticles (NPs) was studied with femtosecond pump probe spectroscopy at observation wavelengths of the first exciton transition at 560 nm, the near infrared (NIR) absorption at 2 μm, and the transient mid-infrared (IR) absorption at 4.5 μm. Excitation with less than one photon per particle leads to bleaching of the excitonic transitions, and the bleach intensity is initially linear to the pump power. At higher pump power the bleach intensity levels off, when complete saturation of the excitonic transition is reached. At the same time, increasing pump power causes an acceleration of the bleach decay, which is due to additional Auger processes when multiple excitons are formed in the NPs. In addition, the pump power effect was investigated for the NIR and IR regions, at 2 and 4.5 μm wavelength, respectively. Whereas the IR transients are very similar to the ones observed for the bleach, the NIR transients behave completely different. No pump power dependence was found for the transients at 2 μm when pumped in a power range from 0.5 to 5 μJ per pulse. The results show that the fs transients in the visible (bleach) and IR (absorption) regions are due to electron relaxation in the conduction band and the NIR transients are due to the relaxation of the hole. Furthermore, it suggests that in the investigated CdSe NPs, Auger processes act much more efficiently on the electrons than for the holes. VL - 116 UR - http://link.aip.org/link/doi/10.1063/1.1446851 M3 - 10.1063/1.1446851 ER - TY - JOUR T1 - The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution JF - Photochemistry and Photobiology Y1 - 2002 A1 - Nikoobakht, Babak A1 - Burda, Clemens A1 - Braun, Markus A1 - Hun, M. A1 - El-Sayed, Mostafa A AB - The photoluminescence (PL) of CdSe quantum dots (QD) in aqueous media has been studied in the presence of gold nanoparticles (NP) with different shapes. The steady state PL intensity of CdSe QD (1.5-2 nm in size) is quenched in the presence of gold NP. Picosecond bleach recovery and nanosecond time-resolved luminescence measurements show a faster bleach recovery and decrease in the lifetime of the emitting states of CdSe QD in the presence of quenchers. Surfactant-capped gold nanorods (NR) with aspect ratio of 3 and surfactant-capped and citrate-capped nanospheres (NS) of 12 nm diameter were used as quenchers in order to study the effect of shape and surface charge on the quenching rates. The Stern-Volmer kinetics model is used to examine the observed quenching behavior as a function of the quencher concentration. It was found that the quenching rate of NR is more than 1000 times stronger than that of NS with the same capping material. We also found that the quenching rate decreases as the length of the NR decreases, although the overlap between the CdSe emission and the NR absorption increases. This suggests that the quenching is a result of electron transfer rather than long-range (Forster-type) energy transfer processes. The quenching was attributed to the transfer of electron with energies below the Fermi level of gold to the trap holes of CdSe QD. The observed large difference between NR and NS quenching efficiencies was attributed to the presence of the {110} facets only in the NR, which have higher surface energy. VL - 75 SN - 0031-8655 N1 - Nikoobakht, B Burda, C Braun, M Hun, M El-Sayed, MA M3 - 10.1562/0031-8655(2002)075<0591:tqocqd>2.0.co;2 ER - TY - JOUR T1 - Transfer times of electrons and holes across the interface in CdS/HgS/CdS quantum dot quantum well nanoparticles JF - Chemical physics letters Y1 - 2002 A1 - Braun, Markus A1 - Link, Stephan A1 - Burda, Clemens A1 - El-Sayed, Mostafa A AB - The electron and hole trapping times in the HgS well of a 6 nm CdS/HgS/CdS quantum dot quantum well nanoparticle were determined from the rise time of the ultrafast transient absorption in the visible and IR regions as well as the trap fluorescence when the CdS core is excited. From the 1.5 ps observed rise time of the well fluorescence (which is determined by the trapping time of the slowest carrier) and the intraband hole IR absorption (150 fs), the electron localization time is found to be 1.5 ps while that of the hole is ∼150 fs. This large difference in the observed dynamics of the electron and hole in crossing the CdS/HgS interface is discussed. PB - Elsevier VL - 361 SN - 0009-2614 UR - http://dx.doi.org/10.1016/S0009-2614(02)01001-1 CP - 5-6 M3 - 10.1016/S0009-2614(02)01001-1 ER - TY - JOUR T1 - Observation of large changes in the band gap absorption energy of small CdSe nanoparticles induced by the adsorption of a strong hole acceptor JF - Nano Letters Y1 - 2001 A1 - Landes, Christy F. A1 - Braun, Markus A1 - Burda, Clemens A1 - El-Sayed, Mostafa A AB - When butylamine In relatively high concentrations (0.1-0.25 M) is added to a colloidal solution of CdSe nanoparticles (NPs) that are approximately 1.6 nm in diameter, the band gap absorption changes from a broad, relatively weak absorption centered at 445 nm to a narrow, relatively strong absorption centered at 414 nm. The effect of concentration on the observed spectrum shows an isobestic point, suggesting an equilibrium between two species. The possible mechanisms for such a transformation are discussed. VL - 1 SN - 1530-6984 N1 - Landes, C Braun, M Burda, C El-Sayed, MA M3 - 10.1021/nl015619t ER - TY - JOUR T1 - Photoluminescence of CdSe nanoparticles in the presence of a hole acceptor: n-butylamine JF - Journal of Physical Chemistry B Y1 - 2001 A1 - Landes, Christy F. A1 - Burda, Clemens A1 - Braun, Markus A1 - El-Sayed, Mostafa A AB - Addition of butylamine to a solution of colloidal CdSe nanoparticles (NPs) caused a decrease in fluorescence intensity, with no effect on the picosecond bleach recovery of the exciton formation or on the luminescence dynamics. The relative fluorescence quantum yield was found to decrease with increasing butylamine concentration and to level off at high concentrations, but the fluorescence lifetimes were not influenced. The nonlinear concentration dependence of the fluorescence quantum yield did not follow the Stern-Volmer equation. This is in agreement with the observation that the CdSe luminescence lifetime was not affected by the addition of butylamine. A mechanism is proposed in which the emission observed in CdSe is assumed to result from the combination of surface-trapped electrons and holes. n-Butylamine occupies hole sites, thus blocking the recombination process, which results in decreasing the density of luminescent centers. These results will be discussed in terms of the nature of the binding sites of the amine on the nanoparticle surface. VL - 105 SN - 1089-5647 N1 - Landes, C Burda, C Braun, M El-Sayed, MA M3 - 10.1021/jp0041050 ER - TY - JOUR T1 - Variation of the thickness and number of wells in the CdS/HgS/CdS quantum dot quantum well system JF - Journal of Physical Chemistry A Y1 - 2001 A1 - Braun, Markus A1 - Burda, Clemens A1 - El-Sayed, Mostafa A AB - We report on the first chemically prepared multilayer quantum well structure in a semiconductor quantum dot. By subsequent precipitation of HgS, CdS, HgS, and again CdS from aqueous solution, we obtained nanoparticles which contain two HBS quantum wells separated by a double layer of CdS. The:core:and the capping material is also CdS. The two-well system was characterized by absorption and emission spectroscopy, which clearly reveal the formation of a two-well and not a single double-layered quantum well system. This system allows to study the interaction of quantum wells that are separated by different thicknesses of the CdS barriers. The radiative and relaxation dynamics of the new two-well system are compared with the dynamics of systems having a single-layer well and a double-layer well system. VL - 105 SN - 1089-5639 N1 - Braun, M Burda, C El-Sayed, MA M3 - 10.1021/jp010002l ER - TY - JOUR T1 - Femtosecond dynamics of a simple merocyanine dye: Does deprotonation compete with isomerization? JF - Journal of the American Chemical Society Y1 - 2000 A1 - Burda, Clemens A1 - Abdel-Kader, M. H. A1 - Link, Stephan A1 - El-Sayed, Mostafa A AB - The primary photochemistry of the trans isomer of a simple merocyanine dye of the stilbazolium betaine type 1-methyl-4-(4'-hydroxytyryl)pyridinium betaine (M-trans) and its conjugate acid MHtrans+ in aqueous solution is studied by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primary photodynamics is determined to be k = 1.1 x 10(12) s(-1) for M-trans at pH 10 and 0.8 x 10(12) s(-1) for MHtrans+ at pH 6. This was assigned to either conformational changes or a simple vibrational relaxation before the actual isomerization takes place. Wavelength excitation studies give support for the former assignment. These results an discussed in terms of the recent results found for the primary processes of retinal in bacteriorhodopsin. Time-resolved transient measurements show that no excited-state deprotonation of MHtrans+ occurs in aqueous solutions at pH 6 or pH 0, suggesting that the deprotonation occurs on a longer time scale than the picosecond time domain. This is in agreement with present theories of intermolecular proton-transfer reactions, which require solvent reorganization as well as the time of deprotonation estimated from the pK(a) value of this molecule in the excited state. The results of our MO calculations on the electronic structure of these two compounds could account for the fact that, while MHtrans+ photoisomerizes, its deprotonated form does not. VL - 122 SN - 0002-7863 N1 - Burda, C Abdel-Kader, MH Link, S El-Sayed, MA M3 - 10.1021/ja993940w ER - TY - JOUR T1 - Femtosecond transient-absorption dynamics of colloidal gold nanorods: Shape independence of the electron-phonon relaxation time JF - Physical Review B Y1 - 2000 A1 - Link, Stephan A1 - Burda, Clemens A1 - Mohamed, MB A1 - Nikoobakht, Babak A1 - El-Sayed, Mostafa A AB - We studied the femtosecond dynamics of colloidal gold nanorods encapsulated in micelles after excitation with 400 nm pulses of 100 fs duration. It is found that the laser heating of the electron gas of gold nanorods with an average aspect ratio of 3.8 leads to the bleaching of both the transverse and longitudinal mode of the surface plasmon oscillation at 520 and 750 nm. The bleach recovers with the same time constant for both the transverse and longitudinal oscillation, for gold nanodots prepared by photothermal reshaping of the rods as well as for nanodots synthesized chemically by citrate reduction (and known to have twin boundaries and surface defects). Since the bleach recovery on the 3 ps time scale is assigned to electron-phonon relaxation processes, these results suggest that phonon dependent relaxation processes in gold nanoparticles are independent of the shape, size, type of the surfaces, or the mode of the surface plasmon, oscillation excited. The fact that the mean free path of the electron in metallic gold is in the nanometer length scale (similar to 50 nm) raised the question of the importance of surface scattering to the electron-phonon relaxation process in gold nanoparticles. Our previous studies showed little dependence of the relaxation rate of the size of gold nanodots (from 9 to 48 nm). In the present study, the electron-phonon relaxation is measured in gold nanorods, which have different facets from those of gold nanodots. VL - 61 SN - 1098-0121 N1 - Link, S Burda, C Mohamed, MB Nikoobakht, B El-Sayed, MA M3 - 10.1103/PhysRevB.61.6086 ER - TY - JOUR T1 - High-density femtosecond transient absorption spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality JF - Pure and Applied Chemistry Y1 - 2000 A1 - Burda, Clemens A1 - El-Sayed, Mostafa A AB - At the high laser excitation intensities used in our experiments, more than 50 electron-hole pairs are formed in colloidal semiconductor nanoparticles used in our studies. At this density of charge carriers, new transient absorptions are observed in the femtosecond transient spectra in the 450 to 700 nm region with unresolved fast rise (<100 fs) and two decay components of 660 fs and >150 ps. The absorption at 510 nm could be quenched with the adsorption of electron accepters (e.g., benzoquinone, 1,2-naphthoquinone), whereas the low-energy transient absorption was not affected. For CdS NPs, we found that passivation eliminated most of the transient absorption. The transient absorptions are thus proposed to result from either trap-state absorption, trapped dimers (or complexes) and/or Stark-shifted exciton absorption resulting from surface electric field of the uncompensated trapped electron-hole pairs. All these possibilities require effective surface trapping at these high levels of excitation. VL - 72 SN - 0033-4545 N1 - Burda, C El-Sayed, MA1st IUPAC Workshop on Advanced Material (WAM1)Jul 14-18, 1999Hong kong, peoples r chinaIupac M3 - 10.1351/pac200072010165 ER - TY - JOUR T1 - Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses JF - Journal of Physical Chemistry B Y1 - 2000 A1 - Link, Stephan A1 - Burda, Clemens A1 - Nikoobakht, Babak A1 - El-Sayed, Mostafa A AB - Gold nanorods have been found to change their shape after excitation with intense pulsed laser irradiation. The final irradiation products strongly depend on the energy of the laser pulse as well as on its width. We performed a series of measurements in which the excitation power was varied over the range of the output power of an amplified femtosecond laser system producing pulses of 100 fs duration and a nanosecond optical parametric oscillator (OPO) laser system having a pulse width of 7 ns. The shape transformations of the gold nanorods are followed by two techniques: (1) visible absorption spectroscopy by monitoring the changes in the plasmon absorption bands characteristic for gold nanoparticles; (2) transmission electron microscopy (TEM) in order to analyze the final shape and size distribution. While at high laser fluences (similar to 1 J cm(-2)) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observed when the laser energy is lowered. Upon decreasing the energy of the excitation pulse, only partial melting of the nanorods takes place. Shorter but wider nanorods are observed in the final distribution as well as a higher abundance of particles having odd shapes (bent, twisted, phi-shaped, etc.). The threshold for complete melting of the nanorods with femtosecond laser pulses is about 0.01 J cm(-2). Comparing the results obtained using the two different types of excitation sources (femtosecond vs nanosecond laser), it is found that the energy threshold for a complete melting of the nanorods into nanodots is about 2 orders of magnitude higher when using nanosecond laser pulses than with femtosecond laser pulses. This is explained in terms of the successful competitive cooling process of the nanorods when the nanosecond laser pulses are used. For nanosecond pulse excitation, the absorption of the nanorods decreases during the laser pulse because of the bleaching of the longitudinal plasmon band. In addition, the cooling of the lattice occurring on the 100 ps time scale can effectively compete with the rate of absorption in the case of the nanosecond pulse excitation but not for the femtosecond pulse excitation. When the excitation source is a femtosecond laser pulse, the involved precesses (absorption of the photons by the electrons (100 fs), heat transfer between the hot electrons and the lattice (<10 ps), melting (30 ps), and heat loss to the surrounding solvent (>100 ps) are clearly separated in time. VL - 104 SN - 1089-5647 N1 - Link, S Burda, C Nikoobakht, B El-Sayed, MA M3 - 10.1021/jp000679t ER - TY - JOUR T1 - Electron dynamics in gold and gold-silver alloy nanoparticles: The influence of a nonequilibrium electron distribution and the size dependence of the electron-phonon relaxation JF - Journal of Chemical Physics Y1 - 1999 A1 - Link, Stephan A1 - Burda, Clemens A1 - Wang, Z.L. A1 - El-Sayed, Mostafa A AB - Electron dynamics in gold nanoparticles with an average diameter between 9 and 48 nm have been studied by femtosecond transient absorption spectroscopy. Following the plasmon bleach recovery after low power excitation indicates that a non-Fermi electron distribution thermalizes by electron-electron relaxation on a time scale of 500 fs to a Fermi distribution. This effect is only observed at low excitation power and when the electron distribution is perturbed by mixing with the intraband transitions within the conduction band (i.e., when the excitation wavelength is 630 or 800 nm). However, exciting the interband transitions at 400 nm does not allow following the early electron thermalization process. Electron thermalization with the lattice of the nanoparticle by electron-phonon interactions occurs within 1.7 ps under these conditions, independent of the excitation wavelength. In agreement with the experiments, simulations of the optical response arising from thermalized and nonthermalized electron distributions show that a non-Fermi electron distribution leads to a less intense bleach of the plasmon absorption. Furthermore, the difference between the response from the two electron distributions is greater for small temperature changes of the electron gas (low excitation powers). No size dependence of the electron thermalization dynamics is observed for gold nanoparticles with diameters between 9 and 48 nm. High-resolution transmission electron microscopy (HRTEM) reveals that these gold nanoparticles possess defect structures. The effect of this on the electron-phonon relaxation processes is discussed. 18 nm gold-silver alloy nanoparticles with a gold mole fraction of 0.8 are compared to 15 nm gold nanoparticles. While mixing silver leads to a blue-shift of the plasmon absorption in the ground-state absorption spectrum, no difference is observed in the femtosecond dynamics of the system. (C) 1999 American Institute of Physics. [S0021-9606(99)71427-3]. VL - 111 SN - 0021-9606 N1 - Link, S Burda, C Wang, ZL El-Sayed, MA M3 - 10.1063/1.479310 ER - TY - JOUR T1 - Electron Shuttling Across the Interface of CdSe Nanoparticles Monitored by Femtosecond Laser Spectroscopy JF - The Journal of Physical Chemistry B Y1 - 1999 A1 - Burda, Clemens A1 - Green, T.C. A1 - Link, Stephan A1 - El-Sayed, Mostafa A AB - The formation and decay of the optical hole (bleach) for 4 nm CdSe nanoparticles (NPs) with adsorbed electron acceptors (1,4-benzoquinone and 1,2-naphthoquinone) and the rise and decay of the reduced electron acceptors formed after interfacial electron transfer from the CdSe NPs were investigated by femtosecond laser spectroscopy. The ultrashort (200?400 fs) rise times of the bleach at the band-gap energy of the CdSe NP as well as of the acceptor radical anion are found to increase with increasing the excitation energy. This suggests that the electron transfer from the CdSe NP to the quinone electron acceptor occurs after thermalization of the excited hot electrons. The decay times of the transient absorption for the electron acceptor radical anions are found to be comparable to that of the CdSe NP bleach recovery time (3 ps). This suggests that the surface quinones shuttle the electron from the conduction band to the valence band of the excited NP. We contrast this behavior with the excited-state dynamics of the recently investigated CdS?MV2+ system in which the electron acceptor does not shuttle the accepted electron back to the hole in CdS.The formation and decay of the optical hole (bleach) for 4 nm CdSe nanoparticles (NPs) with adsorbed electron acceptors (1,4-benzoquinone and 1,2-naphthoquinone) and the rise and decay of the reduced electron acceptors formed after interfacial electron transfer from the CdSe NPs were investigated by femtosecond laser spectroscopy. The ultrashort (200?400 fs) rise times of the bleach at the band-gap energy of the CdSe NP as well as of the acceptor radical anion are found to increase with increasing the excitation energy. This suggests that the electron transfer from the CdSe NP to the quinone electron acceptor occurs after thermalization of the excited hot electrons. The decay times of the transient absorption for the electron acceptor radical anions are found to be comparable to that of the CdSe NP bleach recovery time (3 ps). This suggests that the surface quinones shuttle the electron from the conduction band to the valence band of the excited NP. We contrast this behavior with the excited-state dynamics of the recently investigated CdS?MV2+ system in which the electron acceptor does not shuttle the accepted electron back to the hole in CdS. PB - American Chemical Society VL - 103 SN - 1520-6106 UR - http://dx.doi.org/10.1021/jp9843050 CP - 11 N1 - doi: 10.1021/jp9843050 J1 - J. Phys. Chem. B M3 - doi: 10.1021/jp9843050 ER - TY - JOUR T1 - How long does it take to melt a gold nanorod? A femtosecond pump-probe absorption spectroscopic study JF - Chemical Physics Letters Y1 - 1999 A1 - Link, Stephan A1 - Burda, Clemens A1 - Nikoobakht, Babak A1 - El-Sayed, Mostafa A AB - Using pump-probe femtosecond transient absorption spectroscopy, we determined the rate of the bleach of absorption around 700-800 nm due to the longitudinal surface plasmon band of gold nanorods. Using TEM of the spotted, completely irradiated solutions suggest that the dominant products of the photothermal conformation of the rods are spheres of comparable volume. This lead to the conclusion that the melting of the rods is at least 30-35 ps, independent of the power used (5-20 mu J) or the nanorod aspect ratio (1.9-3.7). (C) 1999 Elsevier Science B.V. All rights reserved. VL - 315 SN - 0009-2614 N1 - Link, S Burda, C Nikoobakht, B El-Sayed, MA M3 - 10.1016/s0009-2614(99)01214-2 ER - TY - JOUR T1 - Laser photothermal melting and fragmentation of gold nanorods: Energy and laser pulse-width dependence JF - Journal of Physical Chemistry A Y1 - 1999 A1 - Link, Stephan A1 - Burda, Clemens A1 - Mohamed, MB A1 - Nikoobakht, Babak A1 - El-Sayed, Mostafa A AB - We studied the shape transformation (by use of TEM and optical absorption spectroscopy) of gold nanorods in micellar solution by exposure to laser pulses of different pulse width (100 fs and 7 ns) and different energies (mu J to mJ) at 800 nm, where the longitudinal surface plasmon oscillation of the nanorods absorb. At moderate energies, the femtosecond irradiation melts the nanorods to near spherical particles of comparable volumes while the nanosecond pulses fragment them to smaller near-spherical particles. At high energies, fragmentation is also observed for the femtosecond irradiation. A mechanism involving the rate of energy deposition as compared to the rate of electron-phonon and phonon-phonon relaxation processes is proposed to determine the final fate of the laser-exposed nanorods, i.e., melting or fragmentation. VL - 103 SN - 1089-5639 N1 - Link, S Burda, C Mohamed, MB Nikoobakht, B El-Sayed, MA M3 - 10.1021/jp983141k ER - TY - JOUR T1 - New transient absorption observed in the spectrum of colloidal CdSe nanoparticles pumped with high-power femtosecond pulses JF - Journal of Physical Chemistry B Y1 - 1999 A1 - Burda, Clemens A1 - Link, Stephan A1 - Green, T.C. A1 - El-Sayed, Mostafa A AB - The power dependence of the transient absorption spectrum of CdSe nanoparticle colloids with size distribution of 4.0 +/- 0.4 nm diameter is studied with femtosecond pump-probe techniques. At the lowest pump laser power, the absorption bleaching (negative spectrum) characteristic of the exciton spectrum is observed with maxima at 560 and 480 nm, As the pump laser power increases, two new transient absorptions at 510 and 590 nm with unresolved fast rise (<100 fs) and long decay times (much greater than 150 ps) are observed. The energy of each of the positive absorption is red shifted from that of the bleach bands by similar to 120 meV. The origin of this shift is discussed in terms of the effect of the internal electric field of the many electron-hole pairs formed within the quantum dot at the high pump intensity, absorption from a metastable excited state or the formation of biexcitons. VL - 103 SN - 1089-5647 N1 - Burda, C Link, S Green, TC El-Sayed, MA M3 - 10.1021/jp991503y ER - TY - JOUR T1 - Charge separation effects on the rate of nonradiative relaxation processes in quantum dots quantum well heteronanostructures JF - Journal of Physical Chemistry A Y1 - 1998 A1 - Little, Reginald A1 - Burda, Clemens A1 - Link, Stephan A1 - Logunov, Stephan L. A1 - El-Sayed, Mostafa A AB - Using time-resolved optical hole (oh)-burning techniques with femtosecond lasers, the time dependence of the spectral diffusion of the oh is examined for both the CdS quantum dot (QD) and the CdS/HgS/CdS quantum dot quantum well (QDQW) nanoparticles. It is found that the nonradiative relaxation of the optical hole is at least 3 orders of magnitude slower in the QDQW than in the QD system. Analysis of the second derivative of the broad transient bleach spectrum of the QDQW system in the 1.6-2.5 eV energy region at 50 fs delay time is found to have a minimum at 2.1 eV, corresponding to a minimum in the radiative probability. Around this energy, the rise and decay times of the transient bleach in the spectrum an found to change greatly. These results suggest that spectral diffusion in the QDQW is a result of relaxation from high- to low-energy exciton states, involving an intervening dark state at an energy of similar to 2.0 eV. The energies of the maxima and minimum of the second-derivative curve are found to be in good agreement with recent theoretical calculations by Jaskolski and Bryant(1) of the energies of the radiative and dark charge-separated state, respectively. In the latter, the hole is in the CdS clad and the electron is in the HgS well. The slow nonradiative relaxation processes involving this state are expected to be slow owing to the large change in the charge carrier effective masses as they cross from the CdS clad to the HgS well. VL - 102 SN - 1089-5639 N1 - Little, RB Burda, C Link, S Logunov, S El-Sayed, MA M3 - 10.1021/jp9822687 ER - TY - JOUR T1 - Femtosecond Interfacial Electron Transfer Dynamics of CdSe Semiconductor Nanoparticles JF - MRS Proceedings Y1 - 1998 A1 - Burda, Clemens A1 - Green, T.C. A1 - Link, Stephan A1 - El-Sayed, Mostafa A AB - The effect of the adsorption of an electron donor (thiophenol, TP) on the surface of CdSe nanoparticles (NPs) on the emission and electron-hole dynamics is studied. It is found that while the emission is completely quenched, the effect on the transient bleach recovery of the band gap absorption is only slight. This is explained by a mechanism in which the hole in the valence band of the NP is rapidly neutralized by electron transfer from the TP. However, the excited electron in the conduction band is not transferred to the TP cation, i. e. the electron does not shuttle via the organic moiety as it does when naphthoquinone is adsorbed [1]. The excited electron is rather trapped by surface states. Thus the rate of bleach recovery in the CdSe NP system is determined by the rate of electron trapping and not by hole trapping. Comparable conclusions resulted previously [2] for the CdS NP when the CdS-MV2+ system is studied. A comparative discussion of the electron-hole dynamics in these systems (CdSe-NQ, CdS-MV2+ and CdSe-TP) is given. VL - 43 CP - 4 M3 - 10.1557/PROC-536-419 ER -