Combining in situ micro-photoluminescence and cathodoluminescence to understand defects photophysics in nanodiamonds

Session

PST.4 - Spectroscopies in Electron, X-ray and Ion Microscopy

Authors

Dr Noémie Bonnet (1), Dr Luiz Tizei (1), Dr Mathieu Kociak (1)

Affiliations

1. Laboratoire de Physique des Solides

Keywords

cathodoluminescence, diamonds, electron spectroscopy, in-situ, photoluminescence

Abstract text

In this contribution, the measurements of confocal photoluminescence microscopy in a scanning transmission electron microscope (STEM) will be demonstrated. These measurements allow the combined detection of photoluminescence (PL), cathodoluminescence (CL), and energy electron loss spectroscopy (EELS) with atomic resolution imaging of the sample.

 

Confocal photoluminescence microscopy is a widely used technique allowing the optical characterization of a wide range of challenging samples, such as single defects, monolayers, quantum dots etc. [1]. However, even super-spatially-resolved techniques like stimulated emission depletion (STED) or photoactivated localization microscopy (PALM) [2], cannot precisely measure some local variations in the structure of samples. A useful tool for more precise measurements is the aberration corrected scanning transmission electron microscope. With a scanning probe smaller than an Ångström, atomic resolution can be reached and direct structural information can be gathered, on top of different types of spectroscopies such as EELS and cathodoluminescence.

The combination of both confocal photoluminescence and STEM microscopy enables a deeper characterization of samples, with a diffraction-limited resolution in photoluminescence, and an atomic resolution in imaging.

 

We have added the photoluminescence setup to a VGHB501 STEM electron microscope already equipped with an Attolight Mönch cathodoluminescence detection setup, made of an adjustable parabolic mirror (NA=0.6) and an optical spectrometer.

Lasers are brought to the sample in free space, directed at the parabolic mirror, and focused on the sample inside the STEM microscope. The emitted light is then collected and dispersed in a spectrometer. The spatial resolution reached with this setup is limited by diffraction.

Hyperspectral images can be measured with the laser and/or the electrons: the electron and/or laser beam have a fixed position on the sample, and the sample is being translated in the x and y direction of the plane. For each position of the sample, a luminescence spectrum is recorded. In the measured hyperspectral image, the shade of each pixel corresponds to the overall intensity of the spectrum contained in that pixel.

 

Preliminary results are shown in Fig. 1 for a sample containing small aggregates of fluorescent nanodiamonds (FNDs). Those FNDs have a high number of nitrogen-vacancy (N-V) centres that are extremely bright and stable photon emitters. Another interest of those centres is that their luminescence differs when they are excited with photons or with electrons [3]. They can be in two charge-state: neutral (NV⁰) and negatively charged (NV^-). The zero phonon line is at 575 nm for NV⁰, which emission is mostly caused by electrons, and at 637 nm for NV^-, which is only caused by photons (in our case the excitation is at 543nm). The curves in (b) show the spectra for respectively: electron beam on (CL), 543 nm laser beam on (PL), and both beams on (CL+PL).

The displayed hyperspectral maps (c) and (d) were measured both at once, with both the electron beam and the laser beam focused at the same place. The map (c) corresponds to the intensity integrated between 700 and 800nm (NV^- PL emission) and (d) between 575 and 590nm, (NV⁰ CL emission). The displacement of the sample is limited to 1 µm, which is the size of the pixel of the map. A reference image of the FNDs aggregate laying on an amorphous carbon net is shown. The spectra displayed are measured in the STEM microscope with liquid nitrogen cooled stage. 

 

A setup combining confocal photoluminescence in a STEM electron microscope has been successfully built, which allows combined PL, CL and EELS measurements, with high resolution imaging. The system is, at the time of writing, adapted to a highly monochromated NION Hermes 200, and we hope to present related results on few different emitters at the time of the conference [4].

Figure 1. CL and µPL measurements of an aggregate of nanodiamonds. (a) Dark-field STEM image of the aggregate, (b) scheme of the setup: the optical system is on a table next to the electron microscope, (c) reference spectra obtained when exciting with electrons (CL), photons at 543nm (PL), and electrons and photons simultaneously (PL+CL), (d) PL filtered luminescence map (700-800nm), (e) CL filtered map (570-590nm)

References

[1] Z. Liu, L. D. Lavis and E. Betzig, Molecular Cell 58 (2015), p.644.

[2] E. Rittwege et al. Nature Photonics 3, 144-147 (2009).

[3] M. Solà-Garci et al. ACS Photonics 1, 232-240 (2020)

[4] The authors gratefully acknowledge funding from SIRTEQ and Région Ile-de-France.