Effects of Surface-Passivating Ligands and Ultrasmall CdSe Nanocrystal Size on the Delocalization of Exciton Confinement

Teunis, Meghan B.; Dolai, Sukanta; Sardar, Rajesh

doi:10.1021/la501533t

Cited by 40 publications

(66 citation statements)

References 64 publications

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“…37,38 Moreover, the surface ligand interaction has the potential to dramatically alter SCM electronic structure and dictate SCM function more than ligand interactions with larger nanocrystals because smaller SCMs have a greater surface-to-volume ratio that results in a large number of atoms becoming available to interact with the ligands. Our suggestion is supported by dramatic changes in the optical bandgap of (CdSe)34 SCMs upon functionalization of their surface with para-substituted Ph-NH-CS2 -NH4 + and Cd(carboxylate)2 ligands 10,39 as compared to larger CdSe nanocrystals under similar experimental conditions. 7,8,40 Moreover, DFT calculations demonstrate that the (CdSe)34 SCM possesses a stoichiometric (Cd28Se28) composition on its surface.…”

Section: Resultssupporting

confidence: 58%

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

et al. 2017

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This article describes the mechanisms underlying electronic interactions between surface passivating ligands and (CdSe) semiconductor cluster molecules (SCMs) that facilitate band-gap engineering through the delocalization of hole wave functions without altering their inorganic core. We show here both experimentally and through density functional theory calculations that the expansion of the hole wave function beyond the SCM boundary into the ligand monolayer depends not only on the pre-binding energetic alignment of interfacial orbitals between the SCM and surface passivating ligands but is also strongly influenced by definable ligand structural parameters such as the extent of their π-conjugation [π-delocalization energy; pyrene (Py), anthracene (Anth), naphthalene (Naph), and phenyl (Ph)], binding mode [dithiocarbamate (DTC, -NH-CS), carboxylate (-COO), and amine (-NH)], and binding head group [-SH, -SeH, and -TeH]. We observe an unprecedentedly large ∼650 meV red-shift in the lowest energy optical absorption band of (CdSe) SCMs upon passivating their surface with Py-DTC ligands and the trend is found to be Ph- < Naph- < Anth- < Py-DTC. This shift is reversible upon removal of Py-DTC by triethylphosphine gold(i) chloride treatment at room temperature. Furthermore, we performed temperature-dependent (80-300 K) photoluminescence lifetime measurements, which show longer lifetime at lower temperature, suggesting a strong influence of hole wave function delocalization rather than carrier trapping and/or phonon-mediated relaxation. Taken together, knowledge of how ligands electronically interact with the SCM surface is crucial to semiconductor nanomaterial research in general because it allows the tuning of electronic properties of nanomaterials for better charge separation and enhanced charge transfer, which in turn will increase optoelectronic device and photocatalytic efficiencies.

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Section: Resultssupporting

confidence: 58%

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

et al. 2017

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“…79 It was previously reported that surface passivating ligands can control exciton delocalization 80 and modulate photophysical properties and such effects are more profound for ultrasmall nanocrystals. 81 The trap state peak position of CdSe(BA/OLA) nanocrystal is red- Figure S9) demonstrated that no residual acetate ions or adsorbed water were present on the nanocrystal surface and it was coated almost exclusively with OLA. Our experimental data are in agreement with the literature that ultrasmall CdSe nanocrystals synthesized in the presence of alkylamines and Cd(acetate)2 produced a stiochiometric core, which is coated with only amines.…”

Section: Fluorescence Lifetime Measurement Of White Light-emitting CDmentioning

confidence: 99%

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

Dolai¹,

Dutta

Muhoberac³

et al. 2015

Chem. Mater.

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ABSTRACT:We have designed a new non-phosphinated reaction pathway, which includes synthesis of a new, highly reactive Se-bridged organic species (chalcogenide precursor), to produce bright white light-emitting ultrasmall CdSe nanocrystals of high quality under mild reaction conditions. The detailed characterization of structural properties of the selenium precursor through combined 77 Se NMR and laser desorption ionization-mass spectrometry (LDI-MS) provided valuable insights into Se release and delineated the nanocrystal formation mechanism at the molecular level. The 1 H NMR study showed that the rate of disappearance of Se-precursor maintained a single-exponential decay with a rate constant of 2.3 x 10 -4 s -1 at room temperature. Furthermore, the combination of LDI-MS and optical spectroscopy was used for the first time to deconvolute the formation mechanism of our bright white light-emitting nanocrystals, which demonstrated initial formation of a smaller key nanocrystal intermediate (CdSe)19. Application of thermal driving force for destabilization resulted in (CdSe)n nanocrystal generation with n = 29-36 through continuous dissolution and addition of monomer onto existing nanocrystals while maintaining a living-polymerization type growth mode. Importantly, our ultrasmall CdSe nanocrystals displayed an unprecedentedly large fluorescence quantum yield of ~27% for this size regime (<2.0 nm diameter). These mixed oleylamine and cadmium benzoate ligand-coated CdSe nanocrystals showed a fluorescence lifetime of ~90 ns, a significantly large value for such small nanocrystals, which was due to delocalization of the exciton wavefunction into the ligand monolayer. We believe our findings will be relevant to formation of other metal chalcogenide nanocrystals through expansion of the understanding and manipulation of surface ligand chemistry, which together will allow the preparation of "artificial solids" with high charge conductivity and mobility for advanced solid-state device applications.3

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“…25 Phenyl dithiocarbamate (PDTC, Scheme 1a) ligands have earned lots of attention quite recently, 26,27 because they may greatly influence the electronic and optical features of semiconductor QDs. [28][29][30][31] The absorption spectra of PDTC-capped CdSe QDs display a sizeable redshift with respect to their native counterparts, covered with carboxylate-terminated insulating ligands, e.g. oleic acid.…”

Section: Introductionmentioning

confidence: 99%

Ligand Induced Spectral Changes in CdSe Quantum Dots

Azpiroz

Angelis

2015

ACS Appl. Mater. Interfaces

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The rational design of ligand molecules has earned lots of attention as an elegant means to tailor the electronic and optical properties of semiconductor quantum dots (QDs). Aromatic dithiocarbamate molecules, in particular, are known to greatly influence the optoelectronic properties of CdSe QDs, red-shifting the absorption features and enhancing the photoluminescence. Here, we present an integrated computational study, which combines ab initio molecular dynamics and excited state calculations including thousands of excitations, aimed at understanding the impact of this kind of surface ligand on the optoelectronic properties of CdSe QDs. We demonstrate that the valence electronic states of the dithiocarbamate molecules, mostly localized in the anchoring moiety, are responsible for the red-shift of the absorption features of capped CdSe QDs. Ligands develop interfacial electronic states close to the band edges of the CdSe, which enhance the absorption features of the QD and might open new channels for the radiative decay from the excited state, improving optical emission. Hybridized QD/ligand states could also funnel interfacial charge transfer between the inorganic core and surface molecules, a process that lies at the heart of many photovoltaic and photocatalytic devices. This work may pave the way toward the design of new capping ligands that, adsorbed on the QD surface, could provide control over the optoelectronic properties of the semiconductor core.

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Effects of Surface-Passivating Ligands and Ultrasmall CdSe Nanocrystal Size on the Delocalization of Exciton Confinement

Cited by 40 publications

References 64 publications

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

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

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

Ligand Induced Spectral Changes in CdSe Quantum Dots

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