Measuring the Ultrafast Spectral Diffusion Dynamics of Colloidal CdSe Nanomaterials
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.38
Measuring the Ultrafast Spectral Diffusion Dynamics of Colloidal CdSe Nanomaterials Thanh Nhut Do1, Cheng Zhang1, Xuanwei Ong2, Jie Lian2, Yinthai Chan2, and Howe-Siang Tan1 1
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
2
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
Abstract
We use ultrafast coherent two-dimensional electronic spectroscopy (2DES) to study the ultrafast spectral diffusion dynamics of colloidal CdSe quantum dots (QDs) and CdSe nanoplatelets (NPLs). The Center Line Slope (CLS) and Nodal Line Slope (NLS) techniques were employed to analyse the 2DES spectra. We show that no spectral diffusion dynamics occurs for the CdSe QDs. On the other hand, spectral diffusion was observed in the CdSe 5 mono-layers NPLs heavy-hole transition. The normalized Frequency Fluctuation Correlation Function (FFCF) of the CdSe NPLs heavy-hole transition was measured to have a major fast decay component at 140 fs.
INTRODUCTION One of the many challenges to the performances of nanomaterial-based applications such as biological imaging, solid state lightings and lasers, is the control of the ensemble emission linewidth [1]. This linewidth is usually assumed to compose of homogeneous and inhomogeneous contributions. The inhomogeneous linewidth is typically associated with the size distribution while the homogeneous condition is dictated by the spectral linewidths due to processes affecting individual particles such as phonon broadening [1-2]. In the context of the Kubo lineshape theory, we can consider the total lineshape of a material as the ensemble average of the time dependent transition frequency fluctuation that arises from the environment interactions [3]. In the Kubo lineshape
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theory, the timescale associated to the homogeneous linewidth is infinitely short as it can be thought of the individual transition instantaneously fluctuating over a frequency bandwidth. On the other hand, the inhomogeneous linewidth arises from static inhomogeneity and hence is infinitely long in timescale. This naturally introduces a regime in between these two timescale extremes, which is due to fluctuation on the order of the timescale of the measurement. This is typically known as spectral diffusion. For nanomaterials, the processes happening in the femtosecond to picosecond timescale are very important, as it includes processes like hot excitons relaxation [4] and fast interplane exciton transport in nanoplatelets [5]. These processes affect the ultrafast spectral diffusion dynamics of the materials. Conversely, measuring the femtosecond to picosecond timescale sp
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