We study the origin of the spectral line shape in colloidal CdSe nanocrystal quantum dots. The three-pulse photon echo peak shift (3PEPS) data reveal a temperature-independent fast decay, obscuring the quantification of the homogeneous linewidth. The optical gap and Stokes shift are found to have an anomalous behavior with temperature, which is size, capping group, and surrounding polymer matrix independent. Using these results and combining them with simulations, we discuss the role of exciton-phonon coupling, static inhomogeneity, exciton fine structure, and exciton state disorder in the linewidth of the nanocrystal. In particular, our analysis shows that the disorder due to surface imperfections and finite temperature effects, as well as the relaxation within the fine structure, can have significant impact on the steady-state absorption spectrum, 3PEPS data, and dephasing processes.

Polydimethylaminoethyl methacrylate (PDMAEMA) was used as a multidentate ligand to modify the surface of CdSe/ZnS core-shell colloidal quantum dots in toluene with trioctylphosphine oxide (TOPO) as the surface ligand. Adsorption of PDMAEMA was accompanied by release of TOPO. The process is free of agglomeration, and the modified nanocrystals become soluble in methanol. The photoluminescence properties are well-preserved in either toluene or methanol.

The homogeneous line shape and inhomogeneous broadening of colloidal quantum dots are measured at ambient temperatures and in solution. Three-pulse echo peak shift (3PEPS) measurements of nanocrystalline CdSe semiconductor quantum dots in toluene solvent at 298 K are reported. Modeling of the 3PEPS data using a detailed model designed to account for the optical properties of semiconductor quantum dots, suggests that Debye-type relaxations attributed to reorientation of the trioctylphosphine and trioctyl phosphine oxide capping ligands are significant. It is shown that 3PEPS spectroscopy can quantify precisely the inhomogeneous broadening of the optical spectrum, brought about mainly by the distribution of nanocrystal sizes.

Quantum dots are a class of colloidal semiconductor nanocrystals (NCs) that show unique size-dependent optical properties.[1] These materials are of great interest for applications in light-emitting diodes, [2] photovoltaic devices, [3] and biological labels.[4] High-quality NCs of II-VI semiconductor materials such as CdSe and CdTe can be obtained with controlled sizes and shapes through an organometallic approach.[5] These as-prepared NCs, which are chemically passivated by a layer of organic ligands such as tri-noctylphosphine oxide (TOPO), are dispersible only in nonpolar organic solvents and are typically unstable in solution in the absence of excess free ligands. [1b, 6] These factors limit their use for biological and material applications. As a consequence, ligand-based functionalization of the NC surface is an essential step to control their physical properties as well as to provide the necessary chemical accessibility and biological functionality.For this purpose, various polymeric substances such as organic dendrons, [7] linear [8][9] or hyperbranched [10] polymers, chemically-modified proteins, [11] and amphiphilic polymers [12] have been employed to manipulate the surface properties of NCs through a ligand-exchange or micellar encapsulation process. Analytical methods such as dynamic light scattering, [9c, 10] gel electrophoresis, [7, 9b, 12c] fluorescence resonance energy transfer, [9e, 11] thermogravimetric analysis, [12b] and atomic force microscopy [13] have been used to characterize the polymer binding on NCs. However, the quantitative analysis of polymeric ligands on NCs still remains a considerable challenge. There are two distinct problems that need to be solved. First, one needs a methodology to separate the NC from free polymer. The polymer is normally added in excess to ensure that the NC surfaces are saturated or encapsulated, and this excess polymer is very difficult to remove from the sample. This problem can in principle be solved by centrifugation or dialysis, [12d] but then one encounters the second problem. One needs analytical methods to determine if any free polymer remains in the solution and to quantify the amount of polymer bound to the NC. This paper presents a quantitative analytical method based upon size-exclusion chromatography (SEC) that addresses this need.To facilitate the detection of polymeric ligands on NCs by spectroscopic methods, we synthesized a low-molecularweight sample of poly(dimethylaminoethyl methacrylate) labeled at one end with pyrene as a fluorescent tracer (Pyr-PDMAEMA, M n = 6700) by atom-transfer radical polymerization (ATRP; for details of the synthesis and characterization, see the Supporting Information). Pyr-PDMAEMA can bind to CdSe NCs as a multidentate ligand, as there is a dimethylamino function on each pendant group (Scheme 1). The CdSe NCs were prepared by an established organometallic synthesis with TOPO as ligands.[5] Two samples were used in this study. The first (CdSe 520 nm ) had a diameter of 3.4 AE 0.3 nm as determined by transm...

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