Surface Charging in CdSe Quantum Dots: Infrared and Transient Absorption Spectroscopy

Abstract

The surface states of selenium-rich CdSe particles are composed of occupied and unoccupied selenium p orbitals, which play a significant role in photophysical and photochemical properties of CdSe quantum dots. The empty p orbitals on the surface are involved in the recently reported phenomenon of thermal "surface charging". Surface charging occurs when electrons are thermally promoted from the valence band to empty surface orbitals at slightly higher energies. At room temperature, this equilibrium results in some fraction of the particles having a hole in the valence band; i.e., the surface acceptor states make the particle p-type. Photoexcitation of surface charged particles results in a positive trion which can undergo a rapid Auger process, making the nanocrystal nonluminescent. The extent of surface charging can be quantified by the amplitude of the trion decay component in the transient absorption kinetics. The presence of occupied p orbitals is quantitatively characterized by an 800 cm −1 feature in the infrared spectra, which is assigned to a Se−O stretch involving an oxidized surface selenium. We show that, because the extents of surface charging and surface selenium−oxygen bonds are both indicative of the density of unligated surface seleniums, the intensity of the 800 cm −1 IR feature correlates with the extent of surface charging.

Ultrafast Photoluminescence Kinetics from Hot Excitonic States in CdSe Nanocrystals

“…The X 1 (1S 3/2 1S) luminescence has been, by far, the most studied QD emission signal till date. The typical long decay of tens of nanoseconds in these nanocrystals has been attributed to dark-exciton and trapping effects, ,,− and the decay components which occur in the tens of picoseconds time scale have been assigned to transient trapping and to defects associated with dangling bonds. ,,− The samples studied here presented time constants for this trapping process of 70.0 ± 7.0 and 74.6 ± 9.4 ps for D = 4.4 and 5.4 nm, respectively (decays in the right section of the A and B graphs in Figure at 610 and 630 nm, respectively, for each QD size).…”

Photoinduced Surface Charging in Iron-Carbonyl-Functionalized Colloidal Semiconductor Nanocrystals

“…Redox processes at nanocrystal (NC) surfaces constitute a significant challenge that continues to adversely impact NC applications such as optoelectronics and photocatalysis. , NC surface charging and changes to trap state energies have been related to thermal photoluminescence quenching , and to photoactivated luminescence changes such as photobrightening and photodarkening. , Changes to NC surface dipoles have also been demonstrated to shift the band edge potentials by over 1 V. − Elucidating the local molecular structures and redox behavior of NC surface sites has therefore been the target of many experimental and computational studies. , In CdSe NCs, for example, chemical reduction experiments using strong reductants such as sodium biphenyl have demonstrated that the surfaces can accommodate the addition of tens of electrons prior to occupation of the conduction band (CB); , these redox changes have been assigned to reduction of oxidized selenium sites or to Cd 2+ reduction followed by loss of X-type ligands. , …”

“…R edox processes at nanocrystal (NC) surfaces constitute a significant challenge that continues to adversely impact NC applications such as optoelectronics and photocatalysis. 1,2 NC surface charging and changes to trap state energies have been related to thermal photoluminescence quenching 3,4 and to photoactivated luminescence changes such as photobrightening and photodarkening. 5,6 Changes to NC surface dipoles have also been demonstrated to shift the band edge potentials by over 1 V. 7−9 Elucidating the local molecular structures and redox behavior of NC surface sites has therefore been the target of many experimental and computational studies.…”

“…This correlation was used to study CdSe NC surface charging under photoexcitation as trapping of photoexcited carriers is a major loss pathway in NC applications. Figure 4a plots the IR spectra of a THF solution of CdSe-Fe(CO) 4 NCs irradiated with a 405 nm LED (3.6 mW/cm 2 ) over time, and Figure 4b shows the calculated number of additional surface electrons. During this experiment, the A 1 C−O stretch shifted from 2012.8 ± 0.2 to 2009.5 ± 0.4 cm −1 over a period of 140 min, corresponding to a calculated maximum charge of 8 ± 2 surface electrons/NC from the unexcited CdSe-Fe(CO) 4 NC sample.…”

Spectroscopy of Surface-State p-Doped CdSe/CdS Quantum Dots

“…16 The surface charging mechanism has also been proposed to understand the one-photon hot hole photochemistry observed for CdSe and CdSe/CdS QDs in polar solvents. 18,19 In these studies the hot hole produced by the trion Auger recombination oxidizes an alkylamine ligand. The charged ligand can then dissociate from the QD in polar solution.…”