Probing Homogeneous Line Broadening in CdSe Nanocrystals Using Multidimensional Electronic Spectroscopy

Abstract

The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chemistry. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chemistry plays a critical role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Fröhlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales.

Broadband two-dimensional electronic spectroscopy in an actively phase stabilized pump-probe configuration

“…Intensity (a.u.) (a) The obtained 2D spectra of CdSe/ZnS NCs are consistent with literature results characterized by established approaches [58,59]. The two diagonal peaks correspond to the two excitonic transitions (i.e., 1S 1/2 (e) -1S 3/2 (h) and 1S 1/2 (e) -2S 3/2 (h)) sharing a common excited level in the conduction band [60].…”

“…The 2D spectra are recorded as a function of waiting time (T) (see Visualization 1). The signals exhibit oscillations along T due to strong vibrational couplings [58,59]. Phase retrieval is indispensable for 2DES.…”

Interference among Multiple Vibronic Modes in Two-Dimensional Electronic Spectroscopy

“…However, as seen in Figures 4 and 5 in Ref. [55] and in Figures 3 and S5 of Ref. [56], peaks do appear in beating maps at spacings other than the selected ω 2 value.…”

Measuring Ultrafast Spectral Diffusion and Correlation Dynamics by Two‐Dimensional Electronic Spectroscopy

“…Turner and co-workersm easured the 2DES spectra of CdSe QDs to characterize the homogeneous line broadening of the band-edge exciton peak (X 1 )a safunction of size from ar adius of 1.4 to 3.0 nm. [64] They measuredt hat the homogeneous linewidth narrows with increasings ize. They also observed that the 2DES spectra do not change with T from 75 fs to 900 ps.…”

“…Although it may be non‐trivial to quantitatively measure the FFCF of the excitonic transitions of CdSe QDs, it is still possible to qualitatively characterize the spectral diffusion dynamics. Turner and co‐workers measured the 2DES spectra of CdSe QDs to characterize the homogeneous line broadening of the band‐edge exciton peak (X 1 ) as a function of size from a radius of 1.4 to 3.0 nm . They measured that the homogeneous linewidth narrows with increasing size.…”