Optical properties of negatively charged germanium-vacancy centers in detonation nanodiamonds with an average single-digit nanometer particle size

Nanodiamonds that contain germanium-vacancy centers (GeV-NDs) exhibit significant potential for biomedical and quantum science applications. GeV-NDs with an average particle size of 9 nm were recently fabricated through a detonation process that enables the practical-scale production of detonation NDs (DNDs). However, the optical properties of the GeV centers in the DNDs have not been studied thoroughly. In particular, the luminescence spectrum of these GeV-DNDs had an unassigned peak at 1.98 eV. Here, we investigate the optical properties of GeV-DNDs under various conditions. Although the GeV-DNDs exhibit a zero-phonon line (ZPL) with similar excitation energy dependence and photostability to their bulk counterparts, the ZPL linewidth is broader. The 1.98 eV-peak is attributed to a composite phonon sideband peak. The unique properties of the GeV centers in these small DNDs are explained by enhanced electron–phonon coupling.


Introduction
][15] However, the progress of such applications is limited by the weak ZPL accompanied by the broad phonon sideband of the NV − center, which exhibits a Debye-Waller factor (DWF) of ca.][18] Negatively charged group IV-vacancy (G4V − ) centers have recently attracted attention as alternatives to the NV − center for fluorescent marker and SPS applications. 1,14,17,18)][21][22] This inversion-symmetric and phonon-insulated configuration means the G4V − centers have a weak electron-phonon coupling and thus exhibit narrow and strong ZPLs. 23)Negatively charged silicon-and germanium-vacancy (SiV − and GeV − , respectively) centers are representative G4V − centers, of which the ZPLs in bulksized diamonds are located at 1.68 and 2.06 eV with linewidths of ca. 12 and 15 meV, and with DWFs of ca.][26] Therefore, SiV − or GeV − center-containing NDs (SiV-or GeV-NDs, respectively) have emerged as promising candidates for fluorescent markers and SPSs. 1,18,26,27)][30] Some high crystalline SiV-and GeV-NDs synthesized by highpressure-high-temperature (HPHT) techniques have been reported to exhibit optical properties comparable to their bulk counterparts. 26,31)In addition to these fundamental studies at the laboratory scale, researchers have also conducted practical studies on the application of SiV-and GeV-NDs.One of the fabrication methods that has been examined is the mass production of SiV-and GeV-NDs using a detonation process. 32)The detonation process produces NDs with an average particle size of <10 nm through the detonation of explosives such as a mixture of 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX). 33)This simple process enables the production of detonation NDs (DNDs) in large quantities (tens or hundreds of tons per year) at low cost. 34)In 2021 and 2022, we demonstrated the direct syntheses of SiV-or GeV-NDs (SiV-or GeV-DNDs, respectively) via the detonation process using a mixedexplosive containing aromatic compounds as group IV dopants. 35,36)According to optical property evaluation of the SiV − centers in DNDs, the linewidth was slightly broader (32 meV), the DWF was smaller (0.47), and the luminescence lifetime was shorter (0.56 ns) than those of typical SiV − centers in bulk-sized diamonds. 37)The reason for these changes was explained by the average 10 nm particle size of the DNDs.The extremely small particle size distorted the SiV − structure and enhanced its electron-phonon interactions.Therefore, while the direct synthesis of the SiV-DNDs via the detonation process allows for their mass production, the optical properties are changed from those typical SiV − centers in bulk-sized diamonds.The optical properties of GeV − centers in DNDs have not been investigated in detail, with only a touch on their photoluminescence (PL) spectra under room and cryogenic temperatures in an existing report. 38)Furthermore, a previous paper on GeV-DND synthesis reported the simultaneous generation of not only GeV-DNDs that exhibited similar luminescence to the bulk counterpart but also another type of GeV-DNDs with an unassigned peak at 1.98 eV adjacent to the ZPL. 36)Therefore, the optical properties of GeV-DNDs also require systematic evaluation before they can be applied practically.
In this article, the detailed optical properties of the GeV − centers in DNDs were investigated using temperature-and excitation energy-dependent PL, and time-resolved PL spectroscopy.The origin of the unassigned 1.98 eV-peak is also discussed on the basis of these optical studies.

Production and structural characterization of detonation nanodiamonds
A cylindrical explosive with a total mass of 1000 g was prepared by mixing and then compressing 60 wt% TNT and 40 wt% RDX powder.The explosive serves as a raw material for synthesizing standard DNDs (Std-DNDs), and is referred to as the Std-explosive.An explosive for production of GeV-DNDs was prepared by adding 10 g of tetraphenylgermane (TPG; Tokyo Chemical Industry Co., Ltd.) powder during the process of preparing the Std-explosive, and is referred to as the Ge-explosive.The bulk densities of the Std-and Geexplosives were 1.59 and 1.55 g cm −3 , respectively.The Stdand Ge-explosives were individually detonated under a CO 2 atmosphere.The detonation products underwent purification using an acid mixture (H 2 SO 4 +HNO 3 ) at 150 °C for 5 h.After cooling at 70 °C, the reaction mixtures were added to deionized (DI) water and heated again at 150 °C for 5 h.The purpose of the acid treatment was to eliminate sp 2 carbons and metal impurities.The resulting precipitates were washed with DI water and subjected to drying.To remove GeO 2 as a by-product derived from TPG, the dry crude products were treated with aqueous 8 M NaOH at 70 °C for 8 h.Although the product from the Std-explosive did not contain GeO 2 , the Std-explosive product was subjected to the alkali treatment to compare the optical properties of the Std-and GeV-DNDs under the same purification conditions.The alkali-treated precipitates were combined with DI water at room temperature, and the pH of the resultant mixtures was adjusted to 3-4 by the addition of aqueous 1 M HCl.The crude suspensions were centrifuged (CR22G, Hitachi Koki) at 8000 × g for 10 min, and the precipitates were separated and added to DI water.The procedures involving aqueous HCl addition and centrifugation were repeated.The collected precipitates were subsequently rinsed with DI water and dried.Finally, the purified samples underwent air-oxidation in O 2 /N 2 (4/96 vol%) at 570 °C for 2 h to remove trace amounts of sp 2 carbon.Through these procedures, the Std-and GeV-DNDs were obtained from the Std-and Ge-explosives, respectively.Both DNDs were characterized using powder X-ray diffraction (XRD; SmartLab, Rigaku) analysis with Cu-K α1 radiation (λ = 1.54 Å) and transmission electron microscopy (TEM; JEM-1400 Plus, JEOL; acceleration voltage: 120 kV).

Measurements of the optical properties
Spectroscopic measurements of the Std-and GeV-DNDs were performed on their drop-cast samples prepared by dropping 100 μl of 10 wt% aqueous suspension of each DND onto a glass substrate and drying.All PL measurements were conducted using a custom-built optical system illustrated in Fig. 1.A Yb:KGW laser (Pharos, Light Conversion) was operated at 1 kHz with a 200 fs pulse width and 0.2 mJ pulse energy at an output energy of 1.204 eV to pump an optical parametric amplifier (OPA; Orpheus-HP, Light Conversion).The system generates excitation pulses with energies in the range of 2.30-3.54eV.The excitation laser beam was guided through circular pinholes to align the optical path for all measurements.Luminescence underwent spectral dispersion via a spectrometer (SpectraPro HRS-300, Acton Research) equipped with a 150 grooves mm −1 grating and was detected using a thermoelectrically-cooled (CCD; PIXIS 256, Princeton Instruments).For low-temperature measurements, the drop-cast samples were cooled by a He cryostat and excited by the laser with an energy of 2.34 eV.In the case of time-resolved PL measurements for the GeV-DNDs, a streak camera was used in place of the CCD camera in the optical system.The drop-cast sample of GeV-DNDs was excited by a 2.38 eV laser.Luminescence was detected by the streak camera (C14831, Hamamatsu Photonics) interfaced with a spectrometer (SpectraPro 300i, Acton Research) equipped with a 50 grooves mm −1 grating.This optical system recorded time-resolved spectra at 1000 and 100 ns timescales with 1.4 and 0.14 ns time resolutions, respectively.
To evaluate the photostability of the GeV − centers in the DNDs, a 1 wt% GeV-DNDs-water colloidal solution sealed in a glass capillary was employed, following the methodology reported by Reineck et al. 39) Our previous report on the photostability evaluation of SiV − centers in DNDs describes the process for preparing the colloidal solution. 37)he PL intensity of the colloidal sample was monitored over time using a similar optical system shown in Fig. 1, except that the excitation source was substituted with a continuouswave (CW) laser of 2.33 eV (LCX-532S, Oxxius).The sample was irradiated with the CW laser at 7.1 kW cm −2 , and their PL was monitored every 15 s.The Japan Society of Applied Physics by IOP Publishing Ltd average crystallite sizes of the Std-and GeV-DNDs, which roughly correspond to the average particle sizes of the DNDs, were determined using the Scherrer formula on the basis of the (111) diffraction peak with the highest intensity.The calculated average sizes were 9.2 nm for Std-DNDs and 9.0 nm for GeV-DNDs.TEM observations of each DND were performed to investigate the particle size distributions.Figures 3(a) and 3(b) show TEM images of the Std-and GeV-DNDs, respectively.The sizes of particles in the TEM images were calculated by assuming a spherical shape because the particle sizes of non-spherical particles are difficult to determine.In addition, the calculation did not include particles that had unclear boundaries with adjacent particles.Figure 3(c) shows the particle size distributions calculated from the TEM images.The particle sizes of both DNDs were from a few nanometers to over 15 nm.Assuming Gauss distributions for these size distributions, the average particle sizes of the Std-and GeV-DNDs were determined as 7 and 9 nm, respectively.The average particle sizes of GeV-DNDs calculated from the XRD pattern and the TEM image were generally consistent.However, the average particle size estimated from the TEM image for the Std-DNDs was smaller than that calculated from the XRD pattern.The TEM image most likely represents a local area that contained a small-sized Std-DND aggregate.The spectra show the GeV − -ZPL at 2.06 eV, the unassigned 1.98 eV peak, and a broadband peak as background.These spectra are the average for different GeV − centers in DNDs because the excitation laser has a beam size of ca. 30 μm and thus excites many GeV-DNDs.This broad-area excitation could be the reason for the observation of the unassigned peak at 1.98 eV, which was only recorded in some GeV-DNDs in the previous report.The broadband peak has also been observed in other types of diamond containing GeV − centers, 40,41) which can be attributed to surface defects for 5-50 nm-sized NDs. 42,43)To remove the broadband peak from the DND structures, PL spectra (red dots) of the Std-DNDs shown in Figs.4(a) and 4(b) were measured at corresponding temperatures, where the PL intensities were normalized with respect to the PL intensities of the GeV-DNDs at 2.20 eV.The blue dots in Figs.4(a) and 4(b) represent difference spectra between the GeV-and Std-DNDs spectra, excluding the luminescence from their DND structure.Figures 4(c) and 4(d) show that the GeV − -ZPL in the difference spectra can be well-fitted with Gaussian curves.The ZPL linewidths (FWHM: full width at half maximum) at 300 and 4 K were 59 and 41 meV, respectively.According to Wang et al., the linewidths of the GeV − centers in bulk-sized diamond were 17 and 10 meV at 280 and 80 K, respectively. 44)Therefore, the ZPL of GeV − centers in DNDs has greater homogeneous and inhomogeneous broadening than that of typical GeV − centers.Such broadening has also been observed in SiV-DNDs synthesized directly by the detonation process. 37)The wider homogeneous broadening could originate from an enhancement of electron-phonon coupling in the internal GeV − centers in DNDs due to the extremely small DND particle size effect.The inhomogeneous broadening most likely originates from the detonation process.The detonation process does not allow for precise control of the particle sizes and positions of the GeV − centers within the DNDs; therefore, GeV-DNDs with different particle size effects are generated.The presence of these diverse GeV − centers in the ca.30 μm observations spot leads to the inhomogeneous broadening.

Structural characterization
Spectral variations at temperatures between in the range of 4-300 K are shown in Fig. 4(e) as difference spectra with the background removed according to the described procedure.Peak intensities of the ZPL and 1.98 eV-peak were calculated by Gaussian fittings and plotted at measured temperatures, as shown in Fig. 4(f).The peak intensities I follow temperature dependence expressed by the Arrhenius equation: 45) ( ) where I , 0 A, E , a k , B and T are the PL intensity at 4 K, a preexponential factor, activation energy, the Boltzmann constant, and the measurement temperature, respectively.All the fitting parameters are summarized in Table I.The ZPL and 1.98 eV-peak indicate the same thermal behavior with approximately identical E .

Excitation energy dependence photoluminescence spectra
The excitation energy dependence of the PL spectra from the GeV-DNDs was investigated in the range of 2.38-3.54eV. Figure 5(a) shows difference PL spectra of the GeV-and Std-DNDs following the procedure described in Sect.3.2.Figure 5(b) shows PL excitation (PLE) spectra of the ZPL of the GeV − centers at 2.06 eV and the 1.98 eV-peak, derived from Fig. 5(a).The excitation energy dependence of the PL intensity of the 1.98 eV-peak shows the same behavior as that of the GeV − -ZPL.Both PLE spectra have a peak at 2.8 eV

035003-3
© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd and a second peak on the lower energy side that is presumed to correspond to the ZPL.The shape of the second peaks is consistent with the PLE spectra of GeV − centers in bulksized diamonds reported by Häußler et al. 24)

Time-resolved photoluminescence spectra
The PL dynamics of the ZPL of GeV − centers at 2.06 eV and the unassigned peak at 1.98 eV were investigated using timeresolved spectroscopy.Firstly, a time-resolved PL spectrum was recorded over a time range of approximately 1000 ns with 1.4 ns time resolution and the time-integrated spectrum was calculated, as shown in Figs.6(a) and 6(b), respectively.These results indicate the GeV − -ZPL and the 1.98 eV-peak.The relative intensity of the 1.98 eV-peak to the GeV − -ZPL in Fig. 6(b) is low compared to the other PL spectra shown in Figs.4(c) and 5(a).It has already been reported that there is a mixture of GeV-DNDs with and without the peak at 1.98 eV.Therefore, the measured spot has a lower proportion of GeV-DNDs with the 1.98 eV-peak than the spots observed in the other PL measurements.The black and red dots in Fig. 6(c) show decay curves extracted by integration over the ranges of 2.01-2.07eV (including the GeV − -ZPL) and 1.94-1.99eV (including the 1.98 eV-peak) in Fig. 6(a), respectively.The decay curves for the GeV − -ZPL and the 1.98 eV-peak are identical within the experimental error bounds.They reach the baseline by ca.250 ns [150-400 ns in Fig. 6(c)]; therefore, the luminescence from the GeV-DNDs should have components with several dozen to roughly a hundred nanosecond lifetimes.Such lifetimes are too long compared with the well-known lifetimes of the GeV − centers in bulksized diamond, i.e. 1.4-5.5 ns. 46)To determine the long-time components, the decay curve of 2.01-2.07eV in the time domain after 220 ns was fitted using where F 0 and t are the baseline and time, and F Long and t Long denote the pre-exponential factor and the timeconstant, respectively.As shown in Fig. 6(d), the decay curve is well-expressed as Eq. ( 2) using the fitting parameters summarized in   © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd with a lifetime of 35 ns. 47)The presence of the NV centers in DNDs derived from nitro groups in explosive molecules has also been reported. 43,48)Therefore, we consider the long-lived component in the GeV-DNDs luminescence to be the NV centers.Subsequently, the lifetime of the GeV − centers in DNDs was investigated by luminescence observations from the GeV-DNDs in an earlier time region with higher temporal resolution.Decay curves in the ranges of 2.01-2.07eV and 1.94-1.99eV are plotted in Fig. 7(c) as black and red dots, respectively.These decay curves are also in complete agreement, even under the 0.14 ns time resolution, which leads to the conclusion that the PL decay of the GeV − -ZPL and 1.98 eV-peak are the same.Here, we examine the time constant of the GeV − -ZPL using its decay curve during 100 ns [black dots in Fig. 7(c)].This decay curve can be represented by where ¢ F 0 and t are the baseline and time, and F i and t i denote the pre-exponential factor and the time-constant, respectively.As shown in Fig. 7(d), the luminescence decay curve including the GeV − -ZPL is well-fitted with the triexponential model [N = 3 in Eq. ( 3)] using the values in Table III.The longest time constant of t 3 = 25 ns may include the effect of the long-lived component, a time constant of 35 ns, which is unrelated to the luminescence from GeV − centers.Therefore, the luminescence lifetime of the GeV − centers in DNDs could range from 1.3 to 7.1 ns, which is consistent with the typical lifetimes (1.4-5.5 ns) of GeV − centers in bulk-sized diamonds. 46)5.Attribution of the peak at 1.98 eV Here, we discuss the origin of the unassigned peak at 1.98 eV in the GeV-DND luminescence.Essential to this inquiry is the work by Krivobok et al. 49) They fabricated an ND containing GeV centers using the HPHT technique and  035003-6 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd measured its PL spectrum.The PL spectrum has the ZPL of the GeV − centers at 2.058 eV accompanied by the following phonon-related peaks: (i) local vibration mode (LVM) related peaks of the GeV − centers at 2.013 eV; (ii) longitudinal acoustic (LA) or transverse acoustic (TA) phonon related peaks of the diamond lattice coupled with the GeV − centers at 1.933 eV; (iii) longitudinal optical (LO) or transverse optical (TO) phonon related peaks of the diamond lattice coupled with the GeV − centers at 1.903 eV.In addition to these known peaks, a new unidentified peak emerged in the PL spectrum at 1.979 eV.After their comprehensive study, including an isotopic experiment and a theoretical calculation, they assigned the 1.979 eV-peak to the ZPL of a neutral GeV 0 center.The peak position of the GeV 0 -ZPL is in good agreement with the unassigned peak at 1.98 eV from the GeV-DNDs in the present work.However, the optical studies of the GeV-DNDs reveal that the GeV − -ZPL and 1.98 eVpeak exhibited identical temperature and excitation energy dependence as represented in Figs.4(f) and 5(b), respectively.Furthermore, the luminescence decay curves from those peaks were perfectly consistent, as shown in Figs.6(c) and 7(c).The concordance of these optical responses for both emissions indicates that the 1.98 eV-peak is a luminescence related to the ZPL of GeV − centers.Even if it remains possible that GeV 0 -ZPL is also one of the components of the 1.98 eV-peak, its contribution, which should show different responses, is limited.Thus, in the case of the GeV − centers in DNDs, it is most plausible that the LVM, LA/TA, and LO/ TO phonon-related peaks were not individually discerned and were observed at 1.98 eV as a phonon sideband peak compared to those of bulk-sized GeV-diamond, due to the enhanced electron-phonon coupling and inhomogeneity distribution of the GeV − centers in DNDs explained in Sect.3.2.

Photostability
Photostability is essential for the use of GeV-DND as fluorescent markers and SPSs.The photostability of the GeV − centers in DNDs was evaluated according to the method of Reineck et al., 39) where the photostabilities of various fluorescent materials, such as an organic dye, nanorubies, and NV-NDs, were investigated.An evaluation result of this study is shown in Fig. 8.The GeV − centers in DNDs exhibit stable luminescence with no bleaching, similar to other fluorescent diamonds.

Conclusions
The optical properties of the GeV − centers in DNDs were systematically investigated.The ZPL exhibited the same excitation energy dependence as that of the GeV − centers in bulk-sized diamonds and showed non-bleaching similar to the NV-NDs.However, the ZPL linewidth was broader than   © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd its bulk counterparts.This broadening is considered to be caused by the inhomogeneity inherent to the detonation process and the enhanced electron-phonon coupling due to the small average DND particle size of 9 nm.In addition, the unassigned peak at 1.98 eV not only showed the same temperature and excitation energy dependence as the GeV − -ZPL, but also the corresponding luminescence decay curves were virtually the same.Consequently, the 1.98 eVpeak was attributed as an emission related to the GeV − -ZPL, possibly interpreted as a broadened phonon sideband due to the strong electron-phonon coupling.035003-8 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

3. 2 .
Temperature dependent photoluminescence spectra Figures 4(a) and 4(b) show PL spectra (black dots) of the GeV-DNDs excited by 2.34 eV at 300 and 4 K, respectively. a

Fig. 2 .
Fig. 2. XRD patterns of detonation products obtained from the (a) Std-and (b) GeV-DNDs.Asterisks denote a cubic diamond structure.

Fig. 3 .
Fig. 3. TEM images of aggregated (a) Std-and (b) GeV-DNDs.The yellow squares in (a) and (b) indicate the areas that were excluded from the particle size estimation due to unclear boundaries between adjacent particles.(c) Particle size distributions were obtained from the TEM images.The gray and blue bars indicate the number of particles of Std-and GeV-DNDs, respectively.
Figure 7(a) shows a timeresolved PL spectrum of the GeV-DND luminescence during approximately 100 ns with 0.14 ns time resolution, and the time-integrated spectrum is shown in Fig. 7(b).

Fig. 4 .
Fig. 4. PL spectra of GeV-DNDs recorded at (a) 300 and (b) 4 K, indicated by black dots.Red dots in (a), (b) show PL spectra of Std-DNDs with the intensity normalized with respect to the PL intensity of GeV-DNDs at 2.20 eV.Blue dots in (a), (b) show difference spectra between GeV-DNDs and Std-DNDs.(c) and (d) are Gaussian fitting curves for difference spectra in (a) and (b), respectively (Reprinted with permission from Ref. 38).Black dots and red lines in (a), (b) indicate original data and fitted curves, respectively.(e) Difference spectra between PL spectra of GeV-and Std-DNDs at each temperature.(f) Temperature dependence of PL intensities of ZPL of GeV − centers in DNDs (black dots) and unassigned peak at 1.98 eV (circles).Red and blue lines in (f) are curve-fitting using the Arrhenius equation.

Fig. 5 .
Fig. 5. (a) PL spectra versus excitation energy for GeV − centers in DNDs prepared from the difference PL spectra of GeV-and Std-DNDs.(b) PLE spectra were constructed by plotting the PL intensity of the GeV-ZPL at 2.06 eV and the unassigned peak at 1.98 eV against excitation energy in (a) as black dots and circles, respectively.The PLE spectrum of typical GeV − centers from Häußler et al. is plotted as red circles. 24)This PLE spectrum is normalized for comparison with the GeV − centers in DNDs.(a)(b)

Fig. 6 .
Fig. 6.(a) Time-resolved PL spectrum of luminescence from GeV-DNDs during 1000 ns.(b) Time-integrated spectrum at all measurement times in (a).(c) Decay curves of the luminescence in the energy ranges of 2.01-2.07 and 1.94-1.99eV plotted with black and red dots, respectively.(d) Decay curves of 2.01-2.07eV in the time region from 220 to 1000 ns.Black dots and the red line in (d) indicate the experimental data and fitted curve based on Eq. (2), respectively.

Fig. 7 .
Fig. 7. (a) Time-resolved PL spectrum of luminescence from GeV-DNDs during 100 ns.(b) Time-integrated spectrum at all measurement times in (a).(c) Decay curves of luminescence in the energy ranges of 2.01-2.07 and 1.94-1.99eV plotted by black and red dots, respectively.(d) Decay curves of 2.01-2.07eV [same as red dots in Fig. 7(c)] and fitting curve based on Eq. (3), plotted as black dots and a red line, respectively.

Fig. 8 .
Fig. 8. Luminescence intensity for the ZPL of GeV − centers in DNDs, recorded at 15 s intervals.The ZPL intensity was plotted after baseline removal and was normalized with respect to the initial intensity at 0 min.
Table II with a time-constant of 35 ns.Such long-lived components have been recognized as NV centers, particularly the neutral NV 0 center

Table I .
Fitting parameters of Eq. (1) by Arrhenius plot for PL intensities of GeV − -ZPL and unassigned peak.

Table III .
Fitting parameters of tri-exponential for a decay curve of luminescence of 2.01-2.07eV from GeV-DNDs.