Atomic Structure of Single Dipole Emitters

Scanning Confocal Fluorescence Correlated HRTEM

  • Fig. 1: Process steps for Scanning confocal fluorescence correlated HRTEM. (a) 10 μl of CND solution dispersed on TEM grid. (b) Scanning of confocal laser beam on the CND coated TEM grids. (c) PL scan image with single excitation dipole patterns with spatial coordinates. (d) HRTEM at the SCFM obtained spatial coordinates. (e) Atomic resolution HRTEM images of single dipole emitters.

Structural information at atomic resolution is unavailable in any standalone fluorescence microscopy. For fundamental understanding of light-matter interaction [1,2] as well as to synthesize fluorescent probes for super-resolution microscopy [3], structural information of single dipole emitters are necessary to be known. To resolve this, here we present a single photon emitter sensitive correlative atomic resolution microscopy approach [4]. A scanning confocal fluorescence microscopy (SCFM) technique is correlated with high resolution transmission electron microscopy (HRTEM). Here, spatial correlation of photon count map and electron density map reveals underlying structures of single dipole emitters at atomic resolution. The motivation behind this technique was to investigate the atomic structure of hetero-dispersed carbon nanodots (CND) to understand their behavior of single dipole emission [4].

SCFM Correlated HRTEM

Here, position markers of standard TEM grids were utilized to locate CNDs of interest under SCFM and HRTEM. To avoid electron beam induced radiation damage, SCFM was performed prior to studying the CNDs under HRTEM. The photon emitting regions obtained from SCFM were overlapped and correlated with electron density map of HRTEM. The correlative investigation enables us to extract the atomistic parameters of CNDs.

Figure 2a demonstrates 10 μl CNDs were dispersed on standard TEM grids. The black lines represent copper support of the suspended amorphous carbon thin-film (gray regions). The droplet was dried overnight in a clean-room environment. The grids were placed on a silica coverslip and spatially scanned with a focused laser beam of SCFM [4]. The relative scanning was performed using an xy piezoelectric stage (fig. 2b). The photoluminescence map of the grid was recorded using single photon avalanche photodiodes. The spatial coordinates of the scanned grid was noted as marked in figure 2(b) with a red box. Figure 2c shows expected single excitation dipole patterns of the CNDs (the dipole pattern are simulated here, real excitation and emission dipole patterns are reported by Ghosh et al. [4]). The xy coordinates of the dipoles were recorded using a piezoelectric scanner.

The same grid was imaged under HRTEM as shown in figure 2d by coarse localization. The transmitted electron density map of the grid (fig. 2e - here simulated HRTEM image of CNDs) was spatially correlated with SCFM image (fig. 2c). The electron density map and photoluminescent map overlap with excellent agreement. The technique reveals the underlying atomistic structure the single dipole patterns. In other words, the technique enables us to identify the fluorescent CNDs. Furthermore, the extracted structures were used in a quantum mechanical modelling to investigate further the experimental light-matter interaction in CNDs. A complete details of the technique as well as the optical behavior of single CNDs can be found in Ghosh et al. reported investigation on ‘photoluminescence of carbon nanodots: dipole emission centers and electron–phonon coupling’.

Conclusion
In summary, we have presented an efficient correlative ex situ fluorescence-electron microscopy approach, which reveals underlying structural information of single dipole emitters at atomic resolution. The method was tested on CNDs. They had a size distribution from 2 nm to 20 nm [4]. In future, SCFM correlated HRTEM will answer scientific questions related to fundamental nanoscale photonics as well as photophysics of new fluorescent probes useful for biological as well as optoelectronic application. 

References
[1] Siddharth Ghosh, Moumita Ghosh, Michael Seibt, and G. Mohan Rao: Detection of quantum well induced single degenerate-transition-dipoles in ZnO nanorods, Nanoscale 8, 2632-2638 (2016) DOI 10.1039/C5NR06722G.
[2] Taubner R. T. Hillenbrand, and F. Keilmann: Phonon-enhanced light–matter interaction at the nanometre scale, Nature 418, 59-162 (2002) DOI 10.1038/nature00899.
[3] Bo Huang, Mark Bates, and Xiaowei Zhuang: Super resolution fluorescence microscopy, Annual review of biochemistry 78, 993 (2009) DOI 10.1146/annurev.biochem.77.061906.092014.
[4] Siddharth Ghosh et al. Photoluminescence of Carbon Nanodots: Dipole Emission Centers and Electron–Phonon Coupling, Nano Letters 14, 5656-5661 (2014) DOI 10.1021/nl502372x.

Author
Siddharth Ghosh

International Max Planck Research School for Physics of Biological and Complex Sytems
IIIrd Institute of Physics - Biophysics and Complex Systems
Georg-August-Universität Göttingen
Göttingen, Germany

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