skip to main content

This content will become publicly available on September 21, 2024

Title: Facet-Specific Electron Transfer in Pseudo-Two-Dimensional Wurtzite Cadmium Selenide Nanocrystals
Ligand-exchange reactions of wurtzite CdSe quantum platelets (QPs) and quantum belts (QBs) with methyl viologen (MV2+) and the derivative ligands MV2+(CH2)nNH2 (n = 2, 4, or 6) are investigated. The QP and QB photoluminescence is quenched after partial ligand exchange. Spectroscopic and compositional data establish that this initial ligand substitution occurs on the thin QP and QB edges. The MV2+(CH2)nNH2 ligands are shown to be more-efficient photoluminescence quenchers than the parent MV2+ ion. The ligands on the thin, nonpolar, long-edge facets quench the photoluminescence via the trapping of excitons. Transient absorption experiments indicate the excitons dissociate, and electron transfer to the MV2+(CH2)nNH2 ligands only occurs at the polar, short-edge facets of the wurtzite CdSe QPs and QBs. Electron transfer to the MV2+(CH2)nNH2 ligands occurs within 100 fs when exciting at the band edge and on longer time scales, due to intraband relaxation, when exciting at higher energies.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
American Chemical Society
Date Published:
Journal Name:
The Journal of Physical Chemistry C
Page Range / eLocation ID:
18506 to 18517
Subject(s) / Keyword(s):
Cadmium selenide, Charge transfer, Ligands, Ligation, Quenching
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Wurtzite CdSe quantum belts with L-type n-octylamine, L-type ammonia, or Z-type Cd(oleate)2 ligands are exchanged for several metal-dithiocarbamate ligands [M(S2CNR1R2)2]: Cd(S2CNPhMe)2, Cd(S2CNEt2)2, Zn(S2CNPhMe)2, and Zn(S2CNEt2)2. Successful ligand exchange with all M(S2CNR1R2)2 compounds occurs from {CdSe[Cd(oleate)2]0.19} quantum belts (QBs), which induce similar spectral shifts in the absorption spectra of the ligand-exchanged QBs. Spectroscopic data, experimentally determined lattice strains, and ligand exchanges with [Na][Et2NCS2] and [NH4][MePhNCS2] establish that the [M(S2CNR1R2)2] ligands bind as bound-ion-paired X-type ligands with (S2CNR1R2)− groups ligated directly to the QB surfaces and [M(S2CNR1R2)]+ groups serving as the charge-balancing ion-paired countercations. The X-type dithiocarbamate ligands do not impart any special electronic effects to the CdSe QBs. 
    more » « less
  2. Spatial confinement of charge carriers in nanosize semiconductor quantum dots (QDs) results in highly tunable, size-dependent optoelectronic properties that can be utilized in various commercial applications. Although in such nanostructures, non-stoichiometry is frequently encountered using conventional synthesis techniques, it is not often addressed or considered. Here, we perform ab initio molecular dynamics simulations on non-stoichiometric CdSe clusters to study the phonon-mediated charge carrier relaxation dynamics. We model cation-rich and anion-rich QDs passivated with monocharged neutralizing ligands of different sizes. Our studies confirm the presence of localized trap states at the valence band edge in only anion-rich QDs due to the presence of undercoordinated exposed surface Se atoms. Noteworthily, these localized states disappear when using bulkier ligands. Calculations reveal that the size of the ligands controls the crystal vibrations and electron–phonon coupling, while ligand coordination number affects the electronic structure. For a particular non-stoichiometric CdSe QD, a change of a ligand can either increase or decrease the total electron relaxation time compared to that of stoichiometric QDs. Our results emphasize the importance of ligand engineering in non-stoichiometric QDs for photoinduced dynamics and guide future work for the implementation of improved materials for optoelectronic devices. 
    more » « less
  3. Morkoç, Hadis ; Fujioka, Hiroshi ; Schwarz, Ulrich T. (Ed.)
    Although AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been studied extensively, their quantum efficiency and optical output power still remain extremely low compared to the InGaN-based visible color LEDs. Electron leakage has been identified as one of the most possible reasons for the low internal quantum efficiency (IQE) in AlGaN based UV LEDs. The integration of a p-doped AlGaN electron blocking layer (EBL) or/and increasing the conduction band barrier heights with prompt utilization of higher Al composition quantum barriers (QBs) in the LED could mitigate the electron leakage problem to an extent, but not completely. In this context, we introduce a promising approach to alleviate the electron overflow without using EBL by utilizing graded concave QBs instead of conventional QBs in AlGaN UV LEDs. Overall, the carrier transportation, confinement capability and radiative recombination are significantly improved. As a result, the IQE, and output power of the proposed concave QB LED were enhanced by ~25.4% and ~25.6% compared to the conventional LED for emission at ~254 nm, under 60 mA injection current. 
    more » « less
  4. null (Ed.)
    To prevent electron leakage in deep ultraviolet (UV) AlGaN light-emitting diodes (LEDs), Al-rich p-type AlxGa(1−x)N electron blocking layer (EBL) has been utilized. However, the conventional EBL can mitigate the electron overflow only up to some extent and adversely, holes are depleted in the EBL due to the formation of positive sheet polarization charges at the heterointerface of the last quantum barrier (QB)/EBL. Subsequently, the hole injection efficiency of the LED is severely limited. In this regard, we propose an EBL-free AlGaN deep UV LED structure using graded staircase quantum barriers (GSQBs) instead of conventional QBs without affecting the hole injection efficiency. The reported structure exhibits significantly reduced thermal velocity and mean free path of electrons in the active region, thus greatly confines the electrons over there and tremendously decreases the electron leakage into the p-region. Moreover, such specially designed QBs reduce the quantum-confined Stark effect in the active region, thereby improves the electron and hole wavefunctions overlap. As a result, both the internal quantum efficiency and output power of the GSQB structure are ~2.13 times higher than the conventional structure at 60 mA. Importantly, our proposed structure exhibits only ~20.68% efficiency droop during 0–60 mA injection current, which is significantly lower compared to the regular structure. 
    more » « less
  5. Abstract

    The relationship between emission and ligand restriction of a series of ZnSe/ZnS quantum dots (QDs) encapsulated in nanoparticles is investigated systematically via experiments and quantum theory. The QDs have a ZnSe core and a ZnS shell, capped with hydrophobic ligands (triotylphosphine oxide/hexadecylamine), allowing them to be entrapped in a model biomembrane, bicelle, made of zwitterionic dipalmitoyl and dihexanoyl phosphatidylcholines and charged dipalmitoyl phosphatidylglycerol. Enhanced photoluminescence is observed upon encapsulation, depending on the QD‐to‐lipid ratio. Transmission electron microscopy and small‐angle X‐ray scattering confirm that QDs are preferably situated at the rim of bicellar discs. A simplified quantum dissipation heat‐bath theory is proposed to correlate the enhancement with slower nonradiative processes caused by the restriction‐in‐motion (RIM) of the surface ligands. However, Förster resonance energy transfer due to QD aggregation counteracts the effect.

    more » « less