A fluorescent color center in meteoritic Lonsdaleite

TL;DR

This work demonstrates, for the first time, that lonsdaleite can host optically active color centers by identifying RU1 in meteoritic ureilite NWA 7983. RU1 exhibits broad visible-to-NIR emission with a peak near $700\ \mathrm{nm}$, excitation around $455\ \mathrm{nm}$, and a long excited-state lifetime of $14\ \mathrm{ns}$, with a fast component of $2\ \mathrm{ns}$ and no detectable photobleaching. Correlative electron microscopy links RU1 to the lonsdaleite lattice, while surrounding Ni, Si, or N impurities are implicated as possible defect constituents; RU1 is not reconstructible as a simple diamond NV center. The findings position hexagonal diamond as a promising platform for quantum emitters and suggest a new family of hexagonal-diamond color centers, motivating further synthesis and defect engineering of lonsdaleite for quantum technologies.

Abstract

Lonsdaleite -- hexagonal diamond -- has only recently been proposed as a wide-bandgap host capable of supporting optically active point defects, but no such centres have yet been observed. Here we provide the first experimental evidence that lonsdaleite does in fact host photoluminescent color centres. In meteoritic lonsdaleite grains from the ureilite NWA7983, we identify a new defect, RU1, which exhibits bright and stable emission across 550-800 nm, with optimal blue excitation (~455 nm) and a peak at ~700 nm. Time-resolved photoluminescence reveals an excited-state lifetime of 14 ns with no detectable blinking, bleaching, or charge conversion. From the excitation-emission energetics we infer an unresolved zero-phonon line near 550 nm. Correlative electron microscopy confirms the lonsdaleite host lattice, and compositional analysis suggests N, Si, or Ni as plausible defect constituents. These results suggest lonsdaleite could become a new quantum-grade crystalline platform and indicate that hexagonal-diamond color centres may form a new and unexplored family of solid-state quantum emitters.

Figures (3)

• Figure 1: Electron microscopy analysis of a region of interest in ureilite NWA 7983. A: Scanning electron microscope image of a hard carbon-rich region. B: Elemental intensity map of carbon, showing that the smooth elongated grain and speckled crystals surrounding it, seen in (A), are pure carbon. C: Elemental map of the region of interest showing the surrounding minerals are comprised of Fe, O and Si. D: Elemental map highlighting the carbon-rich region, as well as surrounding Si and Ni. E: Cathodoluminescence spectra from lonsdaleite (black line) and diamond (red line) which have unique features. The peak at 890 nm in the diamond spectrum likely arises from Ni-related color centers. F: De-convoluted CL response for diamond and lonsdaleite, with lonsdaleite projected in blue and diamond in green.
• Figure 2: A: Photoluminescence spectra of RU1 under excitation wavelengths from 450520 nm (right axis) with accompanying excitation spectrum for the same wavelength range (left axis). The shaded region between 550600 nm corresponds to the spectral window of the excitation-wavelength dependent signal, which was isolated into (B). B: Isolated $\lambda_{\text{ex}}$-dependent signals extracted from the total emission spectrum in (A) by approximating the PL as the sum of to skewed Gaussians. The inset shows the calculated Stokes shifts ($\Delta\nu$) based on the peak positions of two Gaussian fits. C: Time resolved PL of RU1 for excitation wavelengths ranging 440500 nm using a 20 MHz pulsed laser.
• Figure 3: A: Cathodoluminescence map of a second region containing diamond (green), lonsdaleite (blue), and nickel (red). The CL spectra (\ref{['fig:1as']}E) was de-convoluted to extract each component. The boxed region marked along the bottom half of the map corresponds to the region shown in (B). B: PL confocal map of the sub-region marked in (A), using 480 nm excitation with collection above 550 nm. We note that the bright feature in the center-left of the map is an anomaly. The inset shows a zoomed in map of the RU1 color center. C: PL spectra of lonsdaleite RU1 compared to PL from nearby diamond which can be attributed to NV centers. Lonsdaleite and diamond were excited with 480 and 520 nm, respectively, marked by the shaded regions. Solid vertical lines indicate the locations of the diamond NV zero-phonon lines. D: PL intensity of lonsdaleite RU1 under continuous pulsed excitation with average power of 500 , averaged (dark blue) over 10 sequential measurements (gray). A linear fit (light blue) suggests that RU1 is highly photostable.