1H NMR Characterization of the CdSe Nanocrystallite Surface

Majetich, Sara A.; Carter, Adriaan C.; Belot, John A.; McCullough, Richard D.

doi:10.1021/j100102a042

Cited by 65 publications

(68 citation statements)

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“…[13,27,28]. As compared to these microscopic investigations, the approach presented here provides a macroscopic characterization of the capping, based on T 2 relaxation dynamics, homogeneous versus heterogeneous broadening and a set of thermodynamic quantities characterizing the ligands and their interaction.…”

Section: Conclusion and Prospectsmentioning

confidence: 99%

“…A capping of colloidal metal and semiconductor nanocrystals, providing chemical and structural information on the ligands and on ligand exchange [13][14][15][16][17][18]. Here, we explore the possibilities of solution 1 H NMR to yield qualitative and quantitative information on the ligands of colloidal nanocrystals using InP nanocrystals with trioctylphosphine oxide ligands as an example.…”

Section: Introductionmentioning

confidence: 99%

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Solution NMR techniques for investigating colloidal nanocrystal ligands: A case study on trioctylphosphine oxide at InP quantum dots

Moreels

Martins

Hens

2007

Sensors and Actuators B: Chemical

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Section: Conclusion and Prospectsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Solution NMR techniques for investigating colloidal nanocrystal ligands: A case study on trioctylphosphine oxide at InP quantum dots

Moreels

Martins

Hens

2007

Sensors and Actuators B: Chemical

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“…Magnetic resonance studies (18)(19)(20)(21)(22)(23) have probed the nature of NC-ligand-binding interactions and it is typically found (24,25) that surface coverage is incomplete. Unbound surface atoms constitute a distribution of localized sites that carry slight positive or negative charge after local distortions minimize surface free energy (26).…”

mentioning

confidence: 99%

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

Jones

Scholes

2009

Proc. Natl. Acad. Sci. U.S.A.

330

383

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Charge carrier trapping is an important phenomenon in nanocrystal (NC) decay dynamics because it reduces photoluminescence (PL) quantum efficiencies and obscures efforts to understand the interaction of NC excitons with their surroundings. Particularly crucial to our understanding of excitation dynamics in, e.g., multiNC assemblies, would be a way of differentiating between processes involving trap states and those that do not. Direct optical measurement of NC trap state processes is not usually possible because they have negligible transition dipole moments; however, they are known to indirectly affect exciton photoluminescence. Here, we develop a framework, based on Marcus electron transfer theory, to determine NC trap state dynamics from time-resolved NC exciton PL measurements. Our results demonstrate the sensitivity of PL to interfacial dynamics, indicating that the technique can be used as an indirect but effective probe of trap distribution changes. We anticipate that this study represents a step toward understanding how excitons in nanocrystals interact with their surroundings: a quality that must be optimized for their efficient application in photovoltaics, photodetectors, or chemical sensors.quantum dot ͉ states ͉ electron transfer ͉ time-correlated single-photon counting ͉ fluorescence intermittency C olloidal semiconductor nanocrystals (NCs) are potentially useful in a variety of photoactive applications because of their widely tunable electronic band gaps and their ease of processability. Different from well-studied molecular fluorophores, the excited electronic states of NCs with diameters in the range 1 to 10 nm are examples of nanoscale excitons (1). To be used in solar photovoltaics, for example, photo-generated NC excitons must be able to dissociate and transfer charge carriers to their surroundings in a controlled fashion; however, this process is impeded in NCs by the considerable influence of surface-localized states that trap carriers (2). Perturbations due to traps are important because of the significant surface-to-volume ratios characteristic of small colloids (3, 4). Certain surface sites act as charge acceptors that dissociate excitons and therefore reduce the photoluminescence (PL) quantum yield. Changes in solvent or surface-bound coordinating ligands have been found to affect these surface traps and thereby influence steady-state (5-9) and time-resolved (10-12) NC PL. The ability of surface traps to act as charge acceptors makes them excellent model systems for elucidating exciton dissociation processes occurring on the NC surface. Properties intrinsic to NC excitons have been examined in detail (13-17) but comparatively little is understood about the interplay among intrinsic excitons and surface states. Here, we report investigations of CdSe/CdS/ZnS core/shell/shell NC ensemble PL and establish a quantitative model of interfacial trapping and its effect on NC PL dynamics.Colloidal CdSe NCs are coated with passivating ligands that sustain their dispersion in solution and minimize ...

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“…[12][13][14][15][16][17] Herein, we present the results of a quantitative one-dimensional 1 H NMR study on suspensions of InP nanocrystals with trioctylphosphine oxide (TOPO) ligands. It is shown that the ligands are involved in an adsorption/desorption equilibrium, and that the adsorption isotherm can be obtained from the NMR data.…”

mentioning

confidence: 99%

“…Previous NMR studies on the ligands of colloidal semiconductor nanocrystals often studied the adsorbate-adsorbent bond on a molecular scale (adsorption-site selectivity, adsorbate-adsorbent bond, ligand orientation, etc.). [12,21,22] Compared to these microscopic investigations, the thermodynamic approach presented here provides a macroscopic characterization of the capping. It is based on a quantification of 1 H NMR data and provides a limited set of quantities characterizing the ligands, such as the free energy of adsorption and the adsorbate-adsorbate interactions.…”

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confidence: 99%

Ligand Adsorption/Desorption on Sterically Stabilized InP Colloidal Nanocrystals: Observation and Thermodynamic Analysis

2006

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Colloidal semiconductor nanocrystals are widely studied in nanoscience and nanotechnology, since they implement almost perfectly the concept of a quantum dot. Even at room temperature, they have truly discrete energy levels, providing them with unique electrical and optical properties.[1] Typically, colloidal quantum dots come with an organic layer of ligands. These ligands play a key role during nanocrystal synthesis, as they form an adsorbed monolayer (capping) that slows down nanocrystal growth and stabilizes the nanocrystals in suspension.[2] In addition, the capping determines the properties and the processing possibilities of colloidal nanocrystals. For example, the improvement of photoluminescence efficiency, [3][4][5] chemical reactivity, [6] the formation of two-dimensional and three-dimensional aggregates, [7,8] and the attachment of quantum dots to surfaces [6,[9][10][11] all involve the appropriate engineering of the ligands.A number of literature reports demonstrate that solution 1 H NMR spectroscopy is a suitable technique for studying the capping of colloidal metal and semiconductor nanocrystals in situ, providing chemical and structural information on the ligands and on ligand exchange. [12][13][14][15][16][17] Herein, we present the results of a quantitative one-dimensional 1 H NMR study on suspensions of InP nanocrystals with trioctylphosphine oxide (TOPO) ligands. It is shown that the ligands are involved in an adsorption/desorption equilibrium, and that the adsorption isotherm can be obtained from the NMR data. In the case of TOPO adsorption at InP nanocrystals, the adsorption isotherm fits a Fowler isotherm, an extension of the Langmuir isotherm incorporating adsorbate-adsorbate interactions. [18,19] This fitting leads to a set of parameters characterizing the adsorption equilibrium, most notably, the equilibrium constant, the adsorbate density at full coverage, and the free energy of the adsorbate-adsorbate interaction. This work indicates that solution NMR spectroscopy not only provides an identification of the ligands, but may also be used to obtain quantitative, thermodynamic data on ligand adsorption. Figure 1 shows the aliphatic part of the 1 H NMR spectrum of a suspension of trioctylphosphine (TOP)-TOPO capped InP nanocrystals in [D 8 ] toluene (see Methods). As TOP and TOPO cannot be distinguished by 1 H NMR spectroscopy, we will only mention TOPO when discussing the capping molecules. The distinct resonances were attributed previously to free (N) and adsorbed TOPO (*).[17] For free TOPO, the end methyl group is found at 0.92 ppm; for adsorbed TOPO, it yields a broad resonance centered at 1.05 ppm. Both resonances are well resolved, and are integrated reliably by decomposing the spectrum into separate contributions (see Figure 1, inset). In this way, the concentrations of free and adsorbed TOPO can be determined quantitatively (see Methods). Figure 2 shows the con-

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1H NMR Characterization of the CdSe Nanocrystallite Surface

Cited by 65 publications

References 4 publications

Solution NMR techniques for investigating colloidal nanocrystal ligands: A case study on trioctylphosphine oxide at InP quantum dots

Solution NMR techniques for investigating colloidal nanocrystal ligands: A case study on trioctylphosphine oxide at InP quantum dots

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

Ligand Adsorption/Desorption on Sterically Stabilized InP Colloidal Nanocrystals: Observation and Thermodynamic Analysis

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