# High-resolution thermal sensing using temperature-sensitive cathodoluminescence spectroscopy in nitrogen-doped nanodiamonds

Pablo Sáenz de Santa María Modroño, Hugues Girard, Jean-charles Arnault, Gwénolé Jacopin

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Pablo Sáenz de Santa María Modroño, Hugues Girard, Jean-charles Arnault, Gwénolé Jacopin. High-resolution thermal sensing using temperature-sensitive cathodoluminescence spectroscopy in nitrogen-doped nanodiamonds. Physica Status Solidi A (applications and materials science), 2025, 222 (5), pp.2400573. 10.1002/pssa.202400573. hal-04915770

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# Temperature dependent Cathodoluminescence spectroscopy of highly Nitrogen-doped nanodiamonds

Pablo Sáenz de Santa María Modroño¹ Hugues Girard² Jean-Charles Arnault² Gwénolé Jacopin¹

Pablo Sáenz de Santa María Modroño, Gwénolé Jacopin

¹ Université Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France

Email Address: gwenole.jacopin@neel.cnrs.fr

Hugues Girard, Jean-Charles Arnault

² Université Paris-Saclay, CEA, CNRS, NIMBE 91191 Gif sur Yvette, France

Keywords: Nanodiamond, NV Center, Cathodoluminescence, Nanothermometry

The evolution of the cathodoluminescence spectra from commercially available nanodiamonds with high density of nitrogen-vacancy defects has been studied in a large wavelength range, from 200 to 800 nm, and temperatures from 5 to 300 Kelvin. The full width at half maximum of the peak associated to the neutral charge state of the NV center has been found to evolve with temperature in a predictable way, although its value may vary from sample to sample. We have attributed this shift to the differences in the nanodiamonds surface chemistry.

## 1 Introduction

Diamond has a series of unique properties that make it interesting in the field of material physics. It boasts very high thermal conductivity [1], a ultra-wide indirect bandgap, chemical inertness and non-toxicity, and does not require rare elements for its synthesis. Furthermore, elemental defects can be inserted in the diamond lattice during growth, altering the material properties. Particularly, the Nitrogen-Vacancy (NV) defect is being widely studied for its large list of potential applications, with a special attention given to temperature sensing [2] and quantum computing [3].

Our study focuses on the cathodoluminescence (CL) properties of Nitrogen-doped nanodiamonds (ND). These types of ND have potential as a nano temperature probe, due to their high thermal conductivity allowing for quick thermalization. In fact, NV defects in ND are already being used as photoluminescence temperature probes [4]. However, no similar study of nanodiamond CL spectra exist to the best of our knowledge. CL main advantage over photoluminescence is the much larger spatial resolution provided by the shorter electron wavelengths [5]. In fact according to simulations we performed using Casino software [6], an electron beam under low acceleration voltage (1 keV) can reach a spatial resolution of 13 nanometers, corresponding to the width of the penetration area in nanodiamond.

For our study, we used a commercially available Sigma-Aldrich fluorescent ND powder, with a heavy nitrogen doping. Product specifications can be found on [7]. According to the information provided by the seller, the nanodiamonds contains 2.5ppm NV centers and have an average size of 70 nm.

In order to study the CL spectra of the ND, we deposited them on top of a silicon substrate. The silicon substrate was obtained by manually cutting a larger wafer into a square of approximately 1 cm size. The silicon was then cleaned by submerging it in ethanol and performing an ultrasonic bath for 2 minutes. The ND were on a colloidal suspension in deionized water. In order to perform the deposition, we place a droplet of the ND suspension on top of the silicon substrate, and waited for it to evaporate.

Our SEM observations reveal that the nanodiamonds are highly polydisperse in size and shape, with length oscillating from less than 10 nm for the smallest samples, to upwards of 300 nm. A SEM image of the samples can be seen in **figure 1**.

After evaporation, a thin layer of nanodiamond was formed. SEM imaging showed the ND density to be very heterogeneous, with some region where the ND were isolated from one another, and other regions where they were piled up forming large clusters. The estimated average height of the clusters was 90
<figure><img src="image_2.png" /><figcaption>Figure 1: SEM image of the Sigma-Aldrich nitrogen-doped nanodiamonds used in our study. The acceleration voltage has been set to 3 keV.</figcaption></figure>

nm. Both single ND and clusters were firmly attached to the surface after deposition, hinting at chemical bonds between sample and surface.

## 2 Experimental Section

ND were first characterized in the solution by infrared spectroscopy, which revealed a partially oxidized surface. Indeed, C-H stretching bonds remain on the surface, which may originate from non-diamond phases. At the same time, C=O stretching mode appears at 1740 cm⁻¹, which is downshifted compared to those of equivalent air-treated particles (highly oxidized) which typically peak at 1780 cm⁻¹. The Zeta potential corroborates this, showing a value of only -20 mV, whereas highly oxidized particles often exhibit Zeta potentials as low as -50 mV [8]. However, surface oxidation is known to be detrimental to the stability of NV center [9], which may account for this partially oxidized surface.

IR spectra were measured on a Bruker Alpha II spectrometer equipped with a Platinum ATR system (diamond crystal), with 128 scans for the sample and 64 scans for the background. Zeta potentials were recorded by laser Doppler electrophoresis using a Horiba Nanopartica SZ100 system. Zeta measurements result from the average of 10 independent measurements.

The CL spectra was produced by studying the sample with a FEI Inspect F50 scanning electron microscope (SEM) [10]. The SEM was equipped with a Janis helium cryostage, which allows for a temperature range between 5 and 320 Kelvin. The spectra was collected using a parabolic mirror, whose position was controlled with a SmartAct micro-manipulator. The light was then sent into a spectrometer, where it was separated using a 600 g/mm diffraction grating. The spectra was measured with a series USB 2.0 Newton BU2 DU940P CCD camera from the Andor Oxford Instruments group [11].

The Silicon substrate was placed on top of a copper sample holder, connected to the helium cryostage. Silver paint was used to attach the substrate to the sample holder, allowing for faster thermalisation and to avoid charge accumulation from the electron beam.

To perform the CL measurements on the ND, a large cluster was selected, and a 5x5 µm section of said cluster was excited using an electron beam with 3 keV of acceleration voltage and a current of 511 pA. The spectra was measured between 200 and 800 nanometers, in sections of roughly 80 nm. The exposure time for each section was of 5 s and the opening slit was 0.2 mm. The procedure was repeated in a different cluster. No noticeable differences were found between the two spectra.

Another set of measurements was performed on a collection of four isolated nanodiamonds. The nanodiamonds selected were among the largest in region studied, with sizes between 250 and 300 nm. The
<figure><img src="image_3.png" /><figcaption>Figure 2: Full cathodoluminescence spectra of nanodiamonds deposited on Silicon for multiple temperatures. The intensity axis used a logarithmic scale. Measurements were done by irradiating a 5 µm x 5 µm region of a large nanodiamond cluster. The acceleration voltage was kept to keV, while the probe current was set to 511 pA, the exposure time was 5 s.</figcaption></figure>
measurement conditions were similar, except for the exposure time, which was increased to 10 seconds, due to the reduced brightness of single nanodiamond.

Temperature was regulated by modifying the Helium flow and the heater power at the cryostage. The temperature was measured at the base of the copper sample holder. A period of at least 5 minutes was waited between the stabilization of the desired temperature and the performance of measurements, to ensure the proper thermalisation of the nanodiamond.

The resulting spectra for the cluster, with intensity in logarithmic scale, can be seen in **figure 2**. The regions below 350 nm and above 700 nm did not show any noticeable features, so they have not been included in the image.

**3 Results and discussion**

The Cathodoluminescence spectra of the ND is comparable to that of bulk diamond with the main difference being the lack of phonon lines from the indirect bandgap, which normally appear between 225 and 250 nm [12].

The spectra displays two main features: The most dominant is the peak of the neutral charge state Nitrogen-vacancy center, commonly referred as NV0, with a Zero-Phonon-Line (ZPL) at 575 nm. It is followed in intensity by the 389 nm peak and its phonon replicas. The origins of the 389 nm peak are unclear, but is a common CL feature in CVD grown diamonds with Nitrogen implantation [13, 14, 15]. The behaviour of the NV0 peak was similar in clusters and isolated ND. However, the 389 nm was significantly weaker in isolated samples compared to clusters, and the relative intensity of the two peaks changed significantly from nanodiamond to nanodiamond.

Other notable features include the A band, a broad but weak emission band that ranges from 380 to 480 nm. The band has been attributed to $sp^2$ defects in the network of $sp^3$ structures of diamond at incoherent grain boundaries and/or dislocations.[16]. The intensity of the A band was weaker in single ND compared to the large cluster.

A couple peaks at 503 and 496 nm are attributed, respectively, to the H3 and H4 bands. The bands lose intensity as the temperature raises and disappear completely at 300 K. These bands are attributed to radiation damage [17], and its formation has been observed to be enhanced by the presence of both Nitrogen defects [18] and vacancies [19]. The bands have a series of phonon replicas, stretching all the way to 530 nm.

Notably, despite the extremely high intensity of NV0, there is no signal associated to the negatively charged Nitrogen-Vacancy center, known as NV-. This is a common occurrence in diamond CL, as NV- have been observed to switch to NV0 state under electron beam radiation [20]. This represents a significant hurdle for nano thermometry applications, since nanodiamond photoluminescence thermometry is based on studying the NV- peak [21].

As the dominant feature of the CL spectra, the NV0 peak at 575 nm is a good candidate for a temperature probe. The peak's shape and intensity can be seen to evolve with temperature in a significant manner, and the high intensity means the exposure time required to infer the temperature can be made very short.

In order to properly study the evolution of the NV0 peak with temperature, we fitted it to a function with the goal to extract its key parameters. After some experimentation, we settled on a Voigt profile as the model for the peak, with an added lineal background noise, also to be fitted. Examples of the fit for the spectra at various temperatures can be seen in **figure 3**.

The Voigt profile is a convolution of a Gaussian and a Lorentzian profiles [22], and it has two parameters: $\sigma$, which corresponds to the standard deviation of the underlying Gaussian peak; and $\gamma$, which corresponds to the median absolute deviation of the Lorentzian peak. If one of those parameters is very
<figure><img src="image_4.png" /><figcaption>Figure 3: Voigt fit for the NV0 peak of the nanodiamond cluster at different temperatures. The model included a lineal term for the background, to model what appears to be an underlying broad band between 550 and 800 nm.</figcaption></figure>
close to zero, the associated function will be a Dirac Delta, and therefore the Voigt profile will be equal to the other function in the convolution. Interestingly, outside very low temperatures (50 Kelvin or below), the value of $\sigma$ for the spectral fits in the cluster region is of the order of $10^{-8}$. Hence, computational cost could be reduced by fitting the peak to a simpler Lorentzian profile above these temperatures. In the case of the isolated ND, the Gaussian $\sigma$ term was present at potentially any temperature.

From the profile fits, a series of peak parameters can be obtained: peak position, peak intensity and full width at half maximum. In order to obtain a good temperature probe, the parameter needs to follow predictable evolution with temperature that is consistent between samples.

Peak intensity is the simplest parameter to obtain, potentially not even requiring model fitting. However, there is a large intrinsic limitation to intensity as a temperature sensor, as this parameter will depend on a large number of factors: The concentration of NV0 in the irradiated region, the electron beam conditions, the aperture of the slit and the spectrometer characteristics, to name a few. Due to the infeasibility of separating those different contributions to the value of the peak intensity, the parameter can not be used as absolute temperature sensor.

Due to these reasons, peak intensity between the different samples has been normalised in order to visualize all of them simultaneously. These normalised intensities can be found in **figure 4 b**).

The parameter seems to decrease linearly for the ND clusters, but its decrease is much less linear for the isolated samples. Due to all these reasons, we do not believe that NV0 peak intensity can be used as a temperature sensor.

Peak position also shifts slightly with temperature, in a way that appears to be linear above 125 Kelvin, as seen in **figure 4 c**). However, the shift is very small (less than 1 nm for the cluster) so precision is limited. The peak position is also not consistent between samples, changing from ND to ND, and between ND clusters. For all these reasons, we don't believe it is a good candidate for an absolute temperature sensor, although it may have some applications as a relative temperature sensor.

The last parameter studied was the full width at half maximum (FWHM) of the NV0 peak, as seen in **figure 4 a**). The FWHM of the Voigt profile does not have a known analytical function. In order to calculate its value, we used the expression obtained by J.J. Olivero and R.L. Longbothum [23], which has an error rate of 0.02%.

The FWHM of the NV0 peak has a consistent evolution with time. Particularly, the parameter is stable for different ND clusters. Its value spreads for 1 to 6 nanometers, allowing for good precision. Its evolution is very slow at lower temperatures, but the slope becomes significant enough above 150 Kelvin.

However, the FWHM appears to behave differently in single ND and large clusters. In isolated samples, the trend with temperature appears to be the same as for clusters, but it is displaced by a fixed value, which is different for each ND.

To understand the source of the shift in FWHM on the NV0 peak, we performed a series of measurements on NDs spread on top of an InGaN LED. The measurements were done also in a 5 µm square, on two different positions in two different ND clusters. Each measurement was done for multiple acceleration voltages. The obtained FWHM obtained from these measurements can be seen in figure 5.

These measurements show no effect from the acceleration voltage on the value of the FWHM for the acceleration voltage used. Since modifying the acceleration voltage of the electron beam also changes the electron current, the FWHM is not affected by neither of these parameters.

Figure 5 also shows a clear shift on the FWHM between the two substrates. This is similar to what was observed between isolated ND in figure 4 a), with the notable difference that in this case the shift is the same for both clusters. Hence, the both the shift observed in single samples and the shift between substrates may have a similar origin.

Our current explanation for these shifts if that they are caused by changes in the surface chemistry. As
<figure><img src="image_5.png" /><figcaption>Figure 4: NV0 parameters from fit of all samples at different temperatures. a) Full width at half maximum, with a trend-line for the cluster. b) intensity normalised for each sample c) peak position.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5: FWHM for clusters on top of two different substrates: pure Silicon and a InGaN LED with a GaN uppermost layer. The trendline is the same for both substrates, except for a constant shift.</figcaption></figure>

the chemical analysis shows, the surface of the ND is below the oxidation expected for air-exposed particles, which indicates the samples have been subjected to a chemical surface treatment. Figure 1 shows the samples are inhomogeneous in shape and size, causing differences in surface-to-volume ratio. Hence, between single samples the effect of the surface on the NV could be equally inhomogeneous, while averaging out when studying multiple ones.

The two measurements on ND clusters shown in figure 4 a) were done a month apart, so the difference in air exposure does not affect the FWHM. However, the exposure to different surface chemistries appears to have an impact, as per figure 5. Si wafers are expected to have a negative surface potential [24], while the surface of the LED was positively doped. Hence, the interaction between the surface of the ND and the substrate may differ in the two cases, explaining the difference in the aggregated case.

Surface chemistry has been observed to affect the properties of the NV center before [9]. Hence, we propose that the surface chemistry of the nanodiamond is affecting the NV0 peak, causing a shift in its FWHM. The effect is homogeneous when the ND are concentrated in clusters, but unpredictable when the samples are isolated. The reason for this unpredictability could be heterogeneity in the original surface composition and surface-to-volume ratio of each ND, or even local differences in surface chemistry of the substrate.

## 4 Conclusion

We have measured the Cathodoluminescence spectra of Nitrogen-doped commercially available nanodiamonds, for both isolated and clustered samples at temperatures ranging from 5 to 300 Kelvin. We have analysed the different features of their spectra, in order to find a good temperature sensor. We have found that the main peak of the spectra, associated to the NV0 defect, changes drastically with temperature. We have extracted the peak parameters for each temperature by using a Voigt fit, found that the FWHM was a potential good candidate for a temperature sensor, and that it was not affected by beam conditions. However, we have seen that the value of the FWHM shifts between isolated samples. We have found that the shift does not occur when the ND are concentrated in clusters. However, the FWHM of the cluster will shift when they are placed on different surfaces. We believe both shift are caused by changes in the ND surface chemistry, which in short is due to the low stability of the ND surface treat-
ment.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

This work is supported by the French National Research Agency in the framework of the INDIANA Project No. ANR-21-CE42-0020-01 and with the help of the “Plateforme Technologique Amont” de Grenoble, with the financial support of CNRS Renatech network. The authors would like to thank Fabrice Donatini for his help in the development of the SRTC-CL.

**References**

[1] A. Ward, D. A. Broido, D. A. Stewart, G. Deinzer, *Phys. Rev. B* **2009**, *80* 125203.

[2] T. Plakhotnik, M. W. Doherty, J. H. Cole, R. Chapman, N. B. Manson, *Nano letters* **2014**, *14*, 94989.

[3] L. Childress, R. Hanson, *MRS Bulletin* **2013**, *38*, 2 134–138.

[4] D. M. Toyli, C. F. de Las Casas, D. J. Christle, V. V. Dobrovitski, D. D. Awschalom, *Proceedings of the National Academy of Sciences* **2013**, *110*, 21 8417.

[5] W. Zhou, Z. L. Wang, *Scanning microscopy for nanotechnology: techniques and applications*, Springer science & business media, **2007**.

[6] D. Drouin, A. R. Couture, D. Joly, X. Tastet, V. Aimez, R. Gauvin, *Scanning* **2007**, *29*, 3 92.

[7] Sigma-Aldrich, URL https://www sigmaaldrich com/FR/fr/product/aldrich/798169.

[8] L. Saoudi, H. A. Girard, E. Larquet, M. Mermoux, J. Leroy, J.-C. Arnault, *Carbon* **2023**, *202* 438.

[9] R. Kumar, D. K. Singh, P. Kumar, R. Kumar, S. Dhakate, *Diamond and Related Materials* **2019**, *97* 107431.

[10] FEI company - inspect™ community, manuals and specifications | LabWrench, URL https://www.labwrench.com/equipment/7220/fei-company-inspect-trade.

[11] Scientific cameras, microscopy systems & spectrographs - andor, URL https://andor.oxinst.com/.

[12] T. Ruf, M. Cardona, H. Sternschulte, S. Wahl, K. Thonke, R. Sauer, P. Pavone, T. Anthony, *Solid State Communications* **1998**, *105*, 5 311.

[13] Y. Mori, N. Eimori, H. Kozuka, Y. Yokota, J. Moon, J. Ma, T. Ito, A. Hiraki, *Applied physics letters* **1992**, *60*, 1 47.

[14] S. Charles, J. Butler, B. Feygelson, M. Newton, D. Carroll, J. Steeds, H. Darwish, C.-S. Yan, H. Mao, R. Hemley, *physica status solidi* (a) **2004**, *201*, 11 2473.

[15] H. Yagyu, M. Deguchi, J. H. Won, Y. Mori, A. Hatta, M. Kitabatake, T. Ito, T. Hirao, A. Hiraki, *Diamond and related materials* **1995**, *4*, 5-6 575.

[16] D. Takeuchi, H. Watanabe, S. Yamanaka, H. Okushi, H. Sawada, H. Ichinose, T. Sekiguchi, K. Kajimura, *Physical review B* **2001**, *63*, 24 245328.

[17] A. M. Zaitsev, *Optical properties of diamond: a data handbook*, Springer Science & Business Media, **2013**.

[18] J. Walker, *Reports on progress in physics* **1979**, *42*, 10 1605.
[19] G. Davies, S. C. Lawson, A. T. Collins, A. Mainwood, S. J. Sharp, *Phys. Rev. B* **1992**, *46* 13157.

[20] M. Solà-Garcia, S. Meuret, T. Coenen, A. Polman, *ACS photonics* **2019**, *7*, 1232.

[21] P. Neumann, I. Jakobi, F. Dolde, C. Burk, R. Reuter, G. Waldherr, J. Honert, T. Wolf, A. Brunner, J. H. Shim, D. Suter, H. Sumiya, J. Isoya, J. Wrachtrup, *Nano Letters* **2013**, *13*, 62738, pMID: 23721106.

[22] F. Olver, D. Lozier, R. Boisvert, C. Clark, *The NIST Handbook of Mathematical Functions*, Cambridge University Press, New York, NY, **2010**.

[23] J. Olivero, R. Longbothum, *Journal of Quantitative Spectroscopy and Radiative Transfer* **1977**, *17*, 2233.

[24] D. S. L. Mui, E. H. Lenz, C. Cyterski, K. Venkataraman, M. Kawaguchi, *IEEE Transactions on Semiconductor Manufacturing* **2011**, *24*, 4552.