A mirage study of CdSe colloidal quantum dot films, Urbach tail, and surface states

Guyot‐Sionnest, Philippe; Lhuillier, Emmanuel; Liu, Heng

doi:10.1063/1.4758318

Cited by 42 publications

(53 citation statements)

References 29 publications

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“…Traditional approaches to fabricate conductive nanocrystal films displace native surfactant ligands with pyridine, 4,91,105-110 primary alkylamines, 15,73,111-116 hydrazine 73,84,106,108,116-126 and alkane diamines, 3,4,73,84,108,113,114,118,127-133 and are therefore likely to significantly reduce the metal-enrichment. Chelating alkane diamines have been used to displace native ligands and link nanocrystals, resulting in films where the inter-nanocrystal spacing depends on the diamine chain length.…”

Section: Resultsmentioning

confidence: 99%

Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

et al. 2013

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We demonstrate that metal carboxylate complexes (L–M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors like sterics and chelation, though also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L–M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd:Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se-2S3/2h absorption and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)2 at room temperature (~60 %) and fully reversed at elevated temperature. A model is proposed where electron accepting M(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.

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Section: Resultsmentioning

confidence: 99%

Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

et al. 2013

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“…The power law observed in the transient photocurrent is characteristic of a multiple trapping transport mechanism. 40,41 We recently detailed that this regime can be used to estimate the distribution of state within the gap, given by the Urbach energy 42…”

Section: Figure 1: Physical Properties Of Hgtementioning

confidence: 99%

Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals

Livache

Goubet

Martinez

et al. 2018

ACS Appl. Mater. Interfaces

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Mercury chalcogenide nanocrystals and especially HgTe appear as an interesting platform for the design of low cost mid-infrared (mid-IR) detectors. Nevertheless, their electronic structure and transport properties remain poorly understood, and some critical aspects such as the carrier relaxation dynamics at the band edge have been pushed under the rug. Some of the previous reports on dynamics are setup-limited, and all of them have been obtained using photon energy far above the band edge. These observations raise two main questions: (i) what are the carrier dynamics at the band edge and (ii) should we expect some additional effect (multiexciton generation (MEG)) as such narrow band gap materials are excited far above the band edge? To answer these questions, we developed a high-bandwidth setup that allows us to understand and compare the carrier dynamics resonantly pumped at the band edge in the mid-IR and far above the band edge. We demonstrate that fast (>50 MHz) photoresponse can be obtained even in the mid-IR and that MEG is occurring in HgTe nanocrystal arrays with a threshold around 3 times the band edge energy. Furthermore, the photoresponse can be effectively tuned in magnitude and sign using a phototransistor configuration.

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“…During this relaxation, surface trapping can compete with relaxation to the band-edge. A surface-trapped exciton has a weak oscillator strength, 72 reducing the stimulated emission pathway compared to the ESA pathway. ESA yields a negative signal at and below the band-edge due to the biexciton binding energy.…”

Section: A Features In 2d Spectra Of Qdsmentioning

confidence: 99%

Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy

Caram

Zheng

Dahlberg

et al. 2014

The Journal of Chemical Physics

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Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.

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A mirage study of CdSe colloidal quantum dot films, Urbach tail, and surface states

Cited by 42 publications

References 29 publications

Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals

Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy

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