TY - JOUR T1 - Why is the thermalization of excited electrons in semiconductor nanoparticles so rapid? Studies on CdSe nanoparticles JF - Chemical Physics Letters Y1 - 2003 A1 - Darugar, Q. A1 - Landes, Christy F. A1 - Link, Stephan A1 - Schill, A. W. A1 - El-Sayed, Mostafa A AB - Quantum confinement of electronic motion in semiconductor nanoparticles leads to quantization of its band continua of the bulk. The relaxation between the resulting quantized levels by electron phonon coupling was expected, but not found, to be slow due to the small phonon frequencies (phonon bottleneck). Studying the electronic relaxation from the band gap and a higher excited state in CdSe dots and rods under different perturbations suggest the importance of coupling the excited electron to the surface. The surface species act as an efficient heat bath or as electron trapping sites in the linear or nonlinear (Auger) relaxation processes. (C) 2003 Elsevier Science B.V. All rights reserved. VL - 373 SN - 0009-2614 N1 - Darugar, Q Landes, C Link, S Schill, A El-Sayed, MA M3 - 10.1016/s0009-2614(03)00213-6 ER - TY - JOUR T1 - The effect of surface adsorption on the hyper-Rayleigh scattering of large and small CdSe nanoparticles JF - Chemical Physics Letters Y1 - 2002 A1 - Landes, Christy F. A1 - Braun, Markus A1 - El-Sayed, Mostafa A AB - Hyper-Rayleigh scattering is used to compare the effect of the addition of n-butylamine to large (3.2 nm) and small (1.6 nm) Use nanoparticles (NPs). It is found that although the adsorption of n-butylamine on large NPs enhances their nonlinear optical response, it has the opposite effect when adsorbed on the small nanoparticles for which the amine adsorption was found previously to induce structural changes. This observation is consistent with an increase in the symmetry of the nanocrystal structure such as would be observed in a phase change from a 4-coordinate wurtzite crystal structure to that of a 6-coordinate zinc-blend form. (C) 2002 Published by Elsevier Science B.V. VL - 363 SN - 0009-2614 N1 - Landes, C Braun, M El-Sayed, MA M3 - 10.1016/s0009-2614(02)01169-7 ER - TY - JOUR T1 - Thermodynamic and kinetic characterization of the interaction between N-butylamine and similar to 1 nm CdSe nanoparticles JF - Journal of Physical Chemistry A Y1 - 2002 A1 - Landes, Christy F. A1 - El-Sayed, Mostafa A AB - When butylamine (0.1-0.25 M) is added to a colloidal solution of CdSe nanoparticles (NPs) that are 1-2 nm in diameter, the band gap absorption changes from a broad, relatively weak band 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. Thermodynamic and kinetic studies of this process were carried out. The transition was found to be exothermic, with a size-dependent heat of reaction,that suggests that there is a direct proportionality between the NP size and the exothermicity of the interaction. The decay kinetics of the original peak were studied and compared to the rise kinetics of the 414 nm peak. The decay of the original NPs was biexponential, with a lifetime that depends on the NP size, concentration of reactants,. and the temperature. The rise in the 414 nm peak was found to be multiexponential, reflecting 2 transformations from the ensemble having different NP sizes. A mechanism is proposed for this interaction that depends on the binding of butylamine to the NP surface, followed by a release in energy. The nature of the new species absorbing at 414 nm is discussed. VL - 106 SN - 1089-5639 N1 - Landes, C El-Sayed, MA M3 - 10.1021/jp0201130 ER - TY - JOUR T1 - On the Nanoparticle to Molecular Size Transition:  Fluorescence Quenching Studies JF - The Journal of Physical Chemistry B Y1 - 2001 A1 - Landes, Christy F. A1 - Braun, Markus A1 - El-Sayed, Mostafa A AB - Semiconductor nanoparticles, NPs, exhibit fluorescence properties that are closely related to the nature of their surface. CdSe NPs that range in size from 2 to 15 nm in diameter fluoresce with both near-band-edge emission and deep-trap emission, depending on the quality of the surface. When butylamine is added to colloidal solutions of NPs in this size range, the amine is thought to bind to the NP surface and eliminate radiant recombination pathways that lead to luminescence. Since the amine binds to the NP surface, the decrease in fluorescence intensity does not follow standard molecular models of collisional fluorescence quenching. NPs that are smaller than ?2 nm are composed entirely of a discontinuous arrangement of atoms that are all in constant chemical contact with capping material, solvent, and contaminants. When butylamine is added to solutions of these smaller NPs, the fluorescence quenching follows more standard collisional quenching models. Thus, by monitoring the interaction between NPs and a hole acceptor such as butylamine, one can observe the transition from NP, with atoms both in the core and on the surface, to molecular cluster, with no core atoms to dominate the electron density, but with only surface atoms that can participate in molecular processes such as electron transfer.Semiconductor nanoparticles, NPs, exhibit fluorescence properties that are closely related to the nature of their surface. CdSe NPs that range in size from 2 to 15 nm in diameter fluoresce with both near-band-edge emission and deep-trap emission, depending on the quality of the surface. When butylamine is added to colloidal solutions of NPs in this size range, the amine is thought to bind to the NP surface and eliminate radiant recombination pathways that lead to luminescence. Since the amine binds to the NP surface, the decrease in fluorescence intensity does not follow standard molecular models of collisional fluorescence quenching. NPs that are smaller than ?2 nm are composed entirely of a discontinuous arrangement of atoms that are all in constant chemical contact with capping material, solvent, and contaminants. When butylamine is added to solutions of these smaller NPs, the fluorescence quenching follows more standard collisional quenching models. Thus, by monitoring the interaction between NPs and a hole acceptor such as butylamine, one can observe the transition from NP, with atoms both in the core and on the surface, to molecular cluster, with no core atoms to dominate the electron density, but with only surface atoms that can participate in molecular processes such as electron transfer. PB - American Chemical Society VL - 105 SN - 1520-6106 UR - http://dx.doi.org/10.1021/jp0118726 CP - 43 N1 - doi: 10.1021/jp0118726 J1 - J. Phys. Chem. B 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 -