Position-controlled quantum emitters with reproducible emission wavelength in hexagonal boron nitride

Single photon emitters (SPEs) in low-dimensional layered materials have recently gained a large interest owing to the auspicious perspectives of integration and extreme miniaturization offered by this class of materials. However, accurate control of both the spatial location and the emission wavelength of the quantum emitters is essentially lacking to date, thus hindering further technological steps towards scalable quantum photonic devices. Here, we evidence SPEs in high purity synthetic hexagonal boron nitride (hBN) that can be activated by an electron beam at chosen locations. SPE ensembles are generated with a spatial accuracy better than the cubed emission wavelength, thus opening the way to integration in optical microstructures. Stable and bright single photon emission is subsequently observed in the visible range up to room temperature upon non-resonant laser excitation. Moreover, the low-temperature emission wavelength is reproducible, with an ensemble distribution of width 3 meV, a statistical dispersion that is more than one order of magnitude lower than the narrowest wavelength spreads obtained in epitaxial hBN samples. Our findings constitute an essential step towards the realization of top-down integrated devices based on identical quantum emitters in 2D materials.


Supplementary note 1: Characterization of the spot diameter
In the first round of irradiation (sample 1, corresponding to the figure 1 of the main text), the electron beam diameter has been calibrated by irradiating the substrate and imaging the modification it yielded to the secondary electron imaging signal in the SEM (supplementary figure 1a). The irradiation leads to a reduction of the signal from the substrate within a slightly elliptic Gaussian spot. We summed the spot image along two orthogonal direction and fitted the result to obtain a mean FWHM of 315 nm (supplementary figure 1b).
The spatial distribution of the PL signal from SPE ensembles has been characterized with a similar procedure: the room-temperature PL signal (supplementary figure 1c) was summed along two orthogonal direction and the resulting Gaussian shape was fitted, yielding the FWHM of the intensity distribution. Statistics from 10 irradiation spots lead to an average FWHM of 850 nm. This value is slightly greater than the spot size convolved by the emission wavelength (which yields 550 nm). The mismatch could be attributed to the relatively long duration of the irradiations leading to drifts in the beam position and therefore to an effective broadening of the spot, as well as to secondary electrons creating colour centres in the close vicinity of the spot.

Supplementary note 2: Spectral shape and phonon replica
In order to better appreciate the spectral lineshape of the SPEs, we show supplementary figure 2 an ensemble spectrum -calculated as the sum of the spectra shown supplementary figure 1c of the main text -in logarithmic scale. We can distinguish an acoustic phonon sideband [1], as well as two phonon replica, labelled LO1 and LO2, respectively redshifted by 155 meV and 185 meV relatively to the ZPL. We note that these values are slightly lower than usually observed for other SPEs [2], with however a similar splitting of about 30 meV between LO1 and LO2.

Supplementary note 3: Ensemble linewidth
To estimate the probability distribution of the ZPL wavelength, we have summed the spectra taken over the 26 irradiations we performed on sample 1. This corresponds to several hundreds of emitters (see next section). The ensemble spectrum we obtain (supplementary figure

Supplementary note 5: Isolation of individual SPEs
As mentioned in the main text, a 30 nm flake has been irradiated with a lower dose, spread over larger areas. Supplementary figure 5 shows a confocal map of two irradiated spots with 10 min irradiation time. The orange circles denote the nominal beam diameter (FWHM ∼ 1 µm) used for this flake. With these irradiation parameters, individual SPEs can be isolated at the vicinity of the spots as indicated by the red circles. The rest of the luminescence originates from small ensembles of two SPEs or more, as confirmed by both g (2) measurements and spectroscopy, with the presence of several ZPLs.

Supplementary note 8: SPEs in 2D materials
In this section, we compare relevant properties of the main SPE families in 2D materials. For a fair comparison, count rates of SPEs coupled to optical microstructures have been discarded.
Part of the table content has been extracted from reference [6]. MoS2