Dual Role of Electron-Accepting Metal-Carboxylate Ligands: Reversible Expansion of Exciton Delocalization and Passivation of Nonradiative Trap-States in Molecule-like CdSe Nanocrystals

Abstract

This paper reports large bathochromic shifts of up to 260 meV in both the excitonic absorption and emission peaks of oleylamine (OLA)-passivated molecule-like (CdSe) nanocrystals caused by postsynthetic treatment with the electron accepting Cd(OCPh) complex at room temperature. These shifts are found to be reversible upon removal of Cd(OCPh) by N,N,N',N'-tetramethylethylene-1,2-diamine. H NMR and FTIR characterizations of the nanocrystals demonstrate that the OLA remained attached to the surface of the nanocrystals during the reversible removal of Cd(OCPh). On the basis of surface ligand characterization, X-ray powder diffraction measurements, and additional control experiments, we propose that these peak red shifts are a consequence of the delocalization of confined exciton wave functions into the interfacial electronic states that are formed from interaction of the LUMO of the nanocrystals and the LUMO of Cd(OCPh), as opposed to originating from a change in size or reorganization of the inorganic core. Furthermore, attachment of Cd(OCPh) to the OLA-passivated (CdSe) nanocrystal surface increases the photoluminescence quantum yield from 5% to an unprecedentedly high 70% and causes a 3-fold increase of the photoluminescence lifetime, which are attributed to a combination of passivation of nonradiative surface trap states and relaxation of exciton confinement. Taken together, our work demonstrates the unique aspects of surface ligand chemistry in controlling the excitonic absorption and emission properties of ultrasmall (CdSe) nanocrystals, which could expedite their potential applications in solid-state device fabrication.

Unraveling the Mechanism Underlying Surface Ligand Passivation of Colloidal Semiconductor Nanocrystals: A Route for Preparing Advanced Hybrid Nanomaterials

“…B2). We believe that this shift is because of delocalization of the excitonic wavefunction 49,67 from the CSNC to the highly conjugated trans azobenzene moiety, from alteration in the local dielectric environment around the CSNC, or a combination of both, but it is not due to changes of the CSNC core diameter as confirmed by TEM analysis ( Figure S9C). As mentioned above, the selection of AAB for hybrid nanomaterial synthesis was to allow tuning of the band-gap through photoisomerization of azobenzene.…”

“…It is known that the delocalization of exciton wave functions modulates the band-gap and optoelectronic properties of CSNCs. 49 Figure S9A &B). In the PL analysis, the broad spectrum of partially OLA-passivated CSNCs turned into a band-edge peak for mixed OLA-/AHA-passivated CSNCs (Figure 4.A1), which is in agreement with our previous observation for ex-situ ligand treatment (Figure 3).…”

Plasmoelectronic-Based Ultrasensitive Assay of Tumor Suppressor microRNAs Directly in Patient Plasma: Design of Highly Specific Early Cancer Diagnostic Technology

“…Most important, this plasmoelectronic effect should be reversible by disrupting the delocalization process. Recently, we 33,34 and others 35 have demonstrated reversible electron wavefunction delocalization of CdSe quantum dots and manipulated their optoelectronic properties.…”

Elucidating the role of surface passivating ligand structural parameters in hole wave function delocalization in semiconductor cluster molecules

“…Furthermore, we were able to restore the original band-gap of (CdSe)34 SCMs by removing Py-DTC from their surface through treatment with triethylphosphine gold(I) chloride (Et3PAuCl). Although very recently Buhro and coworkers, 9 and our group 10 reported reversible shifts of optical band-gaps of semiconductor nanocrystals upon treatment with Cd(carboxylate)2 ligands, to the best of our knowledge here is the first example where reversible band-gap modulation is demonstrated for semiconductor nanocrystals functionalized with a series of DTC-type ligands. 7,8,11 Exciton delocalization is a ground-state electronic phenomenon at the nanocrystal surfaceligand interface in which electron and/or hole wave functions expand beyond the nanocrystal core boundary into either covalently-attached 7,11,12 or electrostatically-interacting ligand monolayers, 9,10 or into adjacent nanocrystals, [13][14][15][16][17][18] thus decreasing the band-gap of nanocrystals by "increasing the confinement box size".…”