Quantum Confined Excitons in 2-Dimensional Materials by Carmen Palacios-Berraquero

Author:Carmen Palacios-Berraquero
Language: eng
Format: epub, pdf
ISBN: 9783030014827
Publisher: Springer International Publishing

We often observe multiple narrow emission lines from each nanopillar location. However, increasing the nanopillar height tends to reduce the number of peaks arising at each. We can observe this effect in the example spectra shown for each nanopillar height in Fig. 3.5a. The shaded bar highlights the position of the delocalised monolayer emission, for reference. In contrast to the 190 nm nanopillars, spectra taken at the 60 nm nanopillars have multiple peaks (instead of an isolated one). We verify this trend in Fig. 3.4e, a histogram of the probability that a given number of sub-nm emission peaks appear per nanopillar, for the different nanopillar heights (60, 130 and 190 nm in white, blue and purple, respectively). The likelihood of creating a single QE grows as nanopillar height is increased, and the spread in number of lines narrows as well. For the 190 nm nanopillars, 50 of all nanopillar sites host a single QE with one emission peak, as indicated by the purple bars.

Spectral wandering of the peaks as a function of time also displays a strong dependence on the nanopillar height: Fig. 3.5c shows an example measurement of spectral wandering for each nanopillar height. To quantify this dependence, we record the maximum range of emission wavelength wandering per QE over tens of seconds. The solid black circles in Fig. 3.4f correspond to the mean of these values for each group of QEs pertaining to each nanopillar height, for 17 different QEs in total, with the error bars displaying the standard deviation of these distributions. A total of 7 samples was used to collect the statistic for Fig. 3.4e and f. Here, we observe a reduction from up to a few meV for 60 nm height nanopillars to below 0.25 meV (average) for the tallest 190 nm nanopillars, reaching as low as 0.1 meV. To the best of our knowledge, this is the lowest spectral wandering seen to date in LM QEs on Si/SiO substrates [27–31]. The deterministic QEs are hence comparable, and even superior, in spectral stability to their randomly appearing counterparts. It is worth noting here that this is the first example, to the best of our knowledge, in which a single-photon emitting system which is deterministically positioned performs better than its naturally-appearing counterpart: a clear example of this are InGaAs-QDs versus their deterministically positioned counterparts [8]. Our system shows improvement at the very first stages, with little optimisation attempted, which highlights the potential of the technology. Very recently, a publication by Iff et al. [33] has shown that QDs on InGaP/GaAs substrates show even more reduced spectral wandering, which opens up exciting possibilities for QD optimisation.

Figure 3.5b shows a histogram of emission wavelength for all WSe QDs measured, for each nanopillar height. The QD emission wavelength ranges in total between 730 and 820 nm, which is equivalent to a redshift distributed between 50 and 280 meV from the delocalised exciton emission energy at 1.755 eV [34] (shown as a grey bar for reference). The emission wavelength range overlaps that observed for the naturally occurring QEs in WSe [27–31].

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