Photodoping and Transient Spectroscopies of Copper-Doped CdSe/CdS Nanocrystals

Abstract

Colloidal Cu-doped CdSe/CdS core/shell semiconductor nanocrystals (NCs) are investigated in their as-prepared and degenerately n-doped forms using time-resolved photoluminescence and transient-absorption spectroscopies. Photoluminescence from Cu:CdSe/CdS NCs is dominated by recombination of delocalized conduction-band (CB) electrons with copper-localized holes. In addition to prominent bleaching of the first excitonic absorption feature, transient-absorption measurements show bleaching of the sub-bandgap copper-to-CB charge-transfer (MLCT) absorption band and also reveal a photoinduced midgap valence-band (VB)-to-copper charge-transfer (LMCT) absorption band that extends into the near-infrared, as predicted by recent computations. The photoluminescence of these NCs is substantially diminished upon introduction of excess CB electrons via photodoping. Time-resolved photoluminescence measurements reveal that the MLCT excited state is still formed upon photoexcitation of the n-doped Cu:CdSe/CdS NCs, but its luminescence is quenched by a fast (picosecond) three-carrier trap-assisted Auger recombination process involving two CB electrons and one copper-bound hole.

n-Type PbSe Quantum Dots via Post-Synthetic Indium Doping

“…A ligand-assisted dopant outdiffusion has been previously observed in Cu + -doped CdSe QDs. 41,42 Therefore, the surface chemistry of doped QDs plays a critical role in stabilizing electron impurity dopants, and extra 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 caution must be taken when characterizing the n-type nature of In:PbSe films. In fact, when ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) were performed on both PbSe and In:PbSe QDs to probe the shift of the QD Fermi-level, we were not able to gain any insightful results on the valence band maximum due to the high sensitivity of surface treatments on doped QDs (Fig.…”

Heterogeneity in Local Chemical Bonding Explains Spectral Broadening in Quantum Dots with Cu Impurities

“…Radiative recombination occurs through a hole localized at a Cu impurity and a delocalized conduction band electron. 4,[7][8][9][10][11][12]15 This is distinct from Mn doped QDs where Mn transitions localize both carriers. [16][17][18] Consequently, Mn emission can only shift within a small range of the visible spectral window due to changes in local strain environment altering the crystal field splitting energy.…”

“…[2][3] While environmental concerns have spurred research interest in heavymetal-free multinary and transition metal doped structures, their optical transitions are not well understood, and significantly differ from Cd and Pb chalcogenides. [4][5][6][7][8][9][10][11][12][13] For example, QDs with Cu, or Mn cations have large Stokes shifts (∆ " , defined as the energy difference between absorption and emission energies), broad ensemble spectral linewidths, long radiative lifetimes, and tunable magnetic exchange interactions. 6,14 One possible reason for these peculiar properties is the smaller ionic radii of 3d transition metals, which have greater polarizing power than Cd or Pb, and should form more covalent bonds with early chalcogens.…”

Cu‐doped quantum dots: a new class of near‐infrared emitting fluorophores for bioanalysis and bioimaging

“…The luminescence mechanism of Cu doping has been reported in several previous publications, has a great effect on tuning and modifies the photoluminescence properties of Cu‐doped QDs. It is usually accepted that the fluorescence of the Cu dopant is closely related to band gap energy and originates from exciton relaxation via recombination of the electron in the CB of the host material and the hole in the Cu T 2 state .…”