Band Edge Excitons and Amplified Spontaneous Emission of Mercury Chalcogenide Nanoplatelets

Abstract

Colloidal nanoplatelets of HgSe and HgTe prepared indirectly through cation exchange reactions can transfer many of the advantageous properties of atomically precise, 2D cadmium chalcogenides to the near‐infrared (NIR) spectral window. In this work, HgSe and HgTe nanoplatelets are studied to understand their fundamental photophysical properties, particularly those areas of similarity and difference from cadmium‐based NPLs, and to examine their potential as optical gain media. Similar to cadmium chalcogenide NPLs, low‐temperature photoluminescence of HgTe NPLs displays two‐color emission that depends on temperature, sample, fluence, excitation frequency, and irradiation time. Both HgTe and HgSe show nanosecond emission dynamics at temperatures as low as 2.5 K, with no indication that bright‐dark excitonic splitting governs the low‐temperature photoluminescence. Collectively, experimental data is most consistent with emission from a negative trion state at low temperature. Although the mercury chalcogenide nanoplatelets are shown to have broadened optical resonances compared to the cadmium chalcogenides from which they are derived, they retain slow Auger recombination and can display low‐threshold amplified spontaneous emission in the NIR spectral window. Optical pumping thresholds for HgTe NPLs are observed as low as 4.4 µJ cm−2 and highlight the potential 2D nanoplatelets as gain medium in the near‐infrared.

Synthesis of Thick HgxCd1–xSe Nanoplatelets by Cation Exchange Catalyzed by Silver Ions, Showing Amplified Spontaneous Emission

“…These threshold fluences are higher than in HgTe NPLs (<5 μJ·cm –2 at 80 K) but substantially below the 300 μJ·cm –2 or higher reported in measurements of PbS at room temperature . An important point should be emphasized that due to the excitonic degeneracy, thresholds in undoped PbS should be higher on a per exciton basis as well. , It should nevertheless be stressed that the limited available data make a fair comparison (i.e., at the same temperature and for the same excitation).…”

“…The optical gain properties of the material were evaluated by using stripe illumination to observe amplified spontaneous emission (ASE). To overcome the thermal instability of the samples, thin films on sapphire were measured in a cryostat at 80 K. As also observed in HgTe NPLs, the photoluminescence spectrum at intermediate temperatures shows two features, one from the band edge emission and a second, substantially red-shifted band (see Figure a), previously attributed to a trion . As shown in Figure c, the emission spectrum narrows above a threshold fluence and shows a strong nonlinear increase of intensity with the excitation fluence, both of which are characteristic of ASE.…”