Effect of Lateral Size and Surface Passivation on the Near-Band-Edge Excitonic Emission from Quasi-Two-Dimensional CdSe Nanoplatelets

Abstract

As a new type of quasi-two-dimensional nanomaterial, CdSe nanoplatelets (NPLs) possess excellent properties such as narrow emission peak, large absorption cross section, and a low threshold of amplified spontaneous emission. However, the origin of emission especially at low temperatures has not been studied clearly up till now. Here, we study the temperature-dependent photoluminescence of CdSe NPLs which show two emission peaks at low temperatures. It is interesting to note that the intensity of the low-energy peak shows a correlation with laser irradiation time. Moreover, the low-temperature PL spectra of four CdSe NPLs with different lateral sizes demonstrate the relationship of low-energy peaks with the surface. It has been confirmed that CdSe NPLs with larger surface areas to volume ratio have stronger low-energy emissions, which is ascribed to the surface-state-related emission. Finally, surface passivation of CdSe NPLs attenuates the intensity of the low-energy peak, which further verifies our model. Our results demonstrate the critical significance of surface in CdSe NPLs for their optical properties, which is crucial for the application of optoelectronic devices.

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

“…These unpassivated surface states act as nonradiative centers that accelerate nonradiative recombination processes and impede the performance of CdSe NPL–based photonic devices. [ 9,10 ] For this reason, passivation strategies, such as CdSe‐based NPL heterostructures [ 11,12 ] with inorganic semiconductor layers on the surface and optimization of the surface‐ligand chemistry, [ 13 ] are proposed in the subsequent research. CdSe NPLs with several monolayer (ML) thickness show great sensitivity on the dielectric environment.…”

Nature and Control of Shakeup Processes in Colloidal Nanoplatelets

“…In core-only CdSe NPLs, the low temperature photoluminescence is thought to arise from subpopulations of excitons and negative trions. 24,33,46 Very recently, Antolinez and co-workers have reported that the X − emission shows a distinct peak or a shoulder (depending on the film thickness) redshifted from the trion band edge transition. The redshift is ∼ 19 meV and the relative height 15 − 25% that of the main peak.…”

“…22,38,47 The coexistence of X − and trapped surface charges in CdSe NPLs is backed up by studies reporting correlation between surface-to-volume ratio, laser irradiation time and trion emission intensity. 46 A plausible location for surface charges are the lateral sidewalls of the NPL (as in Fig. 1b).…”

Nature and Control of Shakeup Processes in Colloidal Nanoplatelets

“…In core-only CdSe NPLs, the low temperature photoluminescence is thought to arise from subpopulations of excitons and negative trions. 21,41,42 Very recently, Antolinez and co-workers have reported that the X − emission shows a distinct peak or a shoulder (depending on the film thickness) redshifted from the trion band edge transition. The redshift is ∼ 19 meV and the relative height 15 − 25% that of the main peak.…”