Observation of a 1.979-eV spectral line of a germanium-related color center in microdiamonds and nanodiamonds

Color centers in diamond are considered as a platform for quantum computing and communications, biomedical markers, and nanosensors. Negatively charged split-vacancy centers have outstanding properties due to bright and almost monochromatic luminescence, but they have poor spin coherence and relaxation times. This drawback is believed to be absent in the neutral charge state of the defects. So far only the neutral silicon-vacancy center has been observed in luminescence and absorption spectra. Here we report the observation of its germanium-based analog in luminescence spectra with the zero-phonon line (ZPL) at 1.979 eV in samples containing negatively charged ${\mathrm{GeV}}^{-}$ centers. The relationship between the center and Ge dopant is unambiguously confirmed by studies of ${}^{12}\mathrm{C}$ diamonds containing ${}^{70}\mathrm{Ge}$, ${}^{73}\mathrm{Ge}$, or ${}^{76}\mathrm{Ge}$ isotopes. The intensity ratio of the ZPL of these centers varies with the crystal size, and the intensity of the ${\mathrm{GeV}}^0$ centers reaches its maximum in samples 150 nm in size. In the vibronic sideband of this center the local vibrational mode with an energy of 23 meV was identified. The density functional theory calculations yield a ZPL energy value of the ${\mathrm{GeV}}^0$ center which is 150 meV higher than the experimentally observed one and matches the values of its relaxational energy as well as the local phonon mode frequency.

Figure 1

Morphology of samples with (a) the 50-nm and (b) 150-nm nanodiamonds and (c) microcrystalline diamonds.

Figure 2

XRD patterns of nano- and microcrystalline diamond samples. Broadening of the diffraction lines of nanodiamonds is clearly detected (inset).

Figure 3

Spectra of low-temperature (5 K) photoluminescence of micro- and nanocrystals of diamond in the region of radiation of the ZPL (right) of the ${\mathrm{GeV}}^{-}$ center and its phonon replicas (left). The average size of the crystals is shown. In all spectra, an additional line $I_X$ is registered, the small width of which is not typical for the vibronic ${\mathrm{GeV}}^{-}$ peaks.

Figure 4

Spectra of low-temperature (5 K) photoluminescence of diamond nanocrystals with an average size of 150 nm in the region of GeV-center emission under conditions of resonant excitation of the zero-phonon line (upper curve) and nonresonant excitation by radiation with a wavelength of 472 nm. Under conditions of resonant excitation, the line $I_X$ is not registered.

Figure 5

Excitation spectra of luminescence at wavelengths corresponding to the $I_X$ line (green and red curves), ZPL ${\mathrm{GeV}}^{-}$ (blue curve), and the vibronic peak corresponding to the local phonon mode of the ${\mathrm{GeV}}^{-}$ center (gray curve) in diamond nanocrystals with an average size of 100 nm. Since $I_X$ is superimposed on the sideband of the ${\mathrm{GeV}}^{-}$ center, to extract the signal $I_X$, we used two different subtraction procedures described in the text. All spectra are normalized to maximum intensity.

Figure 6

The ratio of the intensity of the line $I_X$ and ZPL ${\mathrm{GeV}}^{-}$ depending on the integral intensity of the ZPL ${\mathrm{GeV}}^{-}$ normalized by the value of the Raman signal corresponding to the $\mathrm{\Gamma }$ phonon scattering of the diamond matrix. The blue dots correspond to microcrystals with an average size of 1 $\mu \mathrm{m}$. The dashed line represents the approximation of data for a microcrystal with a linear dependence. The green and red dots are data for nanocrystals with average sizes of 50 and 100 nm, the spectra of which are shown in Fig. 1.

Figure 7

The change in the spectral position of line $I_X$ in ${}^{12}\mathrm{C}$ diamond microcrystals doped with germanium isotopes ${}^{70}\mathrm{Ge}$ (green curve), ${}^{73}\mathrm{Ge}$ (purple curve), and ${}^{76}\mathrm{Ge}$(red curve) and in ${}^{13}\mathrm{C}$ diamond microcrystals doped with ${}^{73}\mathrm{Ge}$ (black curve) at a temperature of 5 K.

Figure 8

Photoluminescence spectra in the emission region of the ${\mathrm{GeV}}^{-}$ center at a temperature of 5 K obtained when excited by radiation with a quantum energy of 2.709 eV (gray curves) and 2.753 eV (blue curve). The spectra in the region of the selected peaks are normalized to the maximum intensity of the LM line. The red curve is the result of subtracting the gray and blue curves corresponding to the spectra in the region of the ${\mathrm{GeV}}^{-}$ peak sideband.

Figure 9

Emission spectra in region $I_X$ (top) and the ZPL ${\mathrm{GeV}}^{-}$ (bottom) recorded at temperatures of 5 K (blue curves) and 40 K (red curves) for ${}^{12}\mathrm{C}{}^{73}\mathrm{Ge}$ microcrystals.

Figure 10

Pressure dependence of the $I_X$ line.

Figure 11

Dependence of the formation energy of the substitutional germanium atom and three impurity vacancy complexes in neutral and negative charge states on the Fermi level $E_F$ (relative to valence band maximum $E_{\mathrm{VBM}}$). The kinks in the dependencies mark the $\left(0\right|-)$ charge transition level. Vertical dashed lines mark positions of the Fermi level in pure (2.7 eV) diamond and typical HPHT diamond doped with substitutional nitrogen (3.7 eV).

Figure 12

The Kohn-Sham orbitals of GeV and related centers. Up and down triangles mark the highest occupied orbitals in spin-majority and spin-minority channels, respectively. Arrows designate the optically active electronic transitions considered in the text.