Surface defects in carbon-doped hexagonal boron nitride for negative-contrast direct laser writing

Abstract

Radiative defects in hexagonal boron nitride (hBN) are active in a broad spectral range from deep ultraviolet to near-infrared wavelengths. Representatives of these defects act as bright single photon sources, spin-1 systems, and multiproperty atomic-scale sensors. They are predominantly investigated in bulk hBN films, where defects are decoupled from surface and interfacial effects. Here, we demonstrate a novel class of surface defects optically active in the green/yellow visible spectral range, which exhibit photophysical properties distinct from their bulk counterparts. High-power resonant laser illumination quenched the emission from the ensemble of such defects, which was attributed to a light-driven structural reconfiguration. The quenched defects were found to recover their emissive capabilities via a thermal cycling process, revealing an activation energy of 24.5 meV for the structural transition. Alternatively, permanent quenching of the defects was triggered by surface chemistry, involving lithiation-enabled attachment of functional groups. These mechanisms were utilized to realize negative-contrast direct laser writing, designing arbitrary geometric emissive patterns on demand in a microscopic configuration. The surface-active radiative centers in hBN appear particularly attractive for exploring environmental sensitivity, surface science, and coupling to photonic structures or electronic devices by taking unique advantage of the two-dimensional characteristics of the host lattice.

Figures (9)

• Figure 1: Optical response and structural characterization of carbon-doped hBN films. (a) The low temperature (4 K) photoluminescence (PL) spectrum of a mechanically exfoliated carbon-doped hBN film. The spectrum is spatially averaged over the entire area of the hBN flake based on raster mapping of the PL spectra in a microscopic configuration. The red star indicates the Raman scattering resonance related to the optical phonon from the silicon substrate. A, B, and C mark the positions of zero-phonon lines for different radiative defect centers. (b) An atomic force microscopy image of a carbon-doped hBN flake. (c) A cross-sectional height profile along line 1 from panel (b). (d-f) - $\upmu$PL maps of the carbon-doped hBN flake obtained by monitoring the integrated PL intensity of defects A, B, and C, respectively. The intensity color scale is logarithmic.
• Figure 2: The coupling of the surface defect to phonons and the characterization of the reversible photoluminescence (PL) quenching mechanism. (a) PL spectrum of the surface defect A presented in an energy scale relative to the position of the zero-phonon lines. (b) PL (black curve) and PL excitation (PLE, red curve) spectra for the surface defect A. (c) A spatially averaged low temperature (4 K) PL spectrum of defect A measured via a read-out protocol described in the main text in three states of the sample: 1) initial state (black curve), 2) quenched state (blue curve), and 3) recovered state (red curve). (d) The low temperature (4 K) PL spectra measured after 5 thermal cycling procedures with different upper temperature limits. A thermal cycle involved heating the sample to the upper temperature limit indicated in the legend, maintaining the sample at that temperature for 30 minutes, and cooling it back to 4 K. (e) The dependence of the recovering efficiency (defined in the main text) on the temperature presented in a logarithmic scale as a function of $1/T$. Black points represent the experimental data, and the red curve demonstrates the least-squares fit of a biexponential function. (f) A schematic diagram showing the minimal electronic structure of the defect, which accounts for the reversible quenching mechanism.
• Figure 3: The permanent quenching of the photoluminescence (PL) via surface classical chemistry. (a) The schematic diagram presenting the chemical reactions performed for the carbon-doped hBN flakes and the plausible product of the reactions. (b) Spatially averaged low temperature (4 K) PL spectra of the defect A before (red curve) and after (blue curve) the chemical reactions. (c) Atomic force microscopy image of the carbon-doped hBN flake. (d) The cross-sectional height profiles along the line 1 indicated in panel (c), before (red curve) and after (black curve) the chemical reactions. The two curves are shifted horizontally by 2 $\upmu$m for clarity.
• Figure 4: Direct laser writing of negative-contrast emissive patterns. (a-c) Low temperature (4 K) photoluminescence (PL) maps demonstrating the integrated intensity of defect A averaged within and outside of a geometrically defined pattern indicated by a dashed line. The maps were obtained via a read-out protocol described in the main text for three conditions of the sample: 1) initial state (panel a), 2) quenched state achieved via selective irradiation of the patterned area (panel b), and 3) recovered state after a thermal cycling procedure up to room temperature (panel c). (d-f) PL spectra spatially averaged within the pattern area (red curves) and outside of the pattern area (blue curves) for the (d) initial, (e) quenched, and (f) recovered states of the sample.
• Figure S1: Quenching efficiency as a function of excitation wavelength. Blue and red solid curves are photoluminescence excitation and photoluminescence spectra, respectively.