https://doi.org/10.3390/nano13040696
“We observed resonance effects in the Raman scattering of nanodiamonds with an average size of 2–5 nm excited at a wavelength of 1064 nm (1.16 eV). The resonant Raman spectrum of the 2–5 nm nanodiamonds consists of bands at wavelengths of 1325 and 1600 cm−1, a band at 1100–1250 cm−1, and a plateau in the range from 1420 to 1630 cm−1. When excited away from the resonance (at a wavelength of 405 nm, 3.1 eV), the Raman spectrum consists of only three bands at 1325, 1500, and 1600 cm−1. It is important to note that the additional lines (1500 and 1600 cm−1) belong to the sp3-hybridized carbon bonds. The phonon density of states for the nanodiamonds (~1 nm) was calculated using moment tensor potentials (MTP), a class of machine-learning interatomic potentials. The presence of these modes in agreement with the lattice dynamics indicates the existence of bonds with force constants higher than in single-crystal diamonds. The observed resonant phenomena of the Raman scattering and the increase in the bulk modulus are explained by the presence of Tamm states with an energy of electronic transitions of approximately 1 eV, previously observed on the surface of single-crystal diamonds.”
“The Raman spectra were recorded with a Renishaw inVia Raman microscope (excitation wavelength 405 nm), TRIAX 552 (Jobin Yvon Inc., Edison, NJ, USA) spectrometer equipped with a CCD Spec-10, 2KBUV Princeton Instruments 2048 × 512 detector and razor edge filters (excitation wavelength 257 and 458 nm), and Raman–Fourier spectrometer (RFS) 100/s (Bruker company, Germany), excitation wavelength 1064 nm. The spectra were acquired at a laser radiation power density of 108 W/m2. Shifts of the line at 1325 cm−1 into the low-frequency region by more than 1 cm−1 due to the sample heating were observed at the laser radiation power density of more than 3 × 108 W/m2.”
”
The Raman spectra of the 2–5 nm nanodiamonds encapsulated in NaCl excited at wavelengths of 257, 405, 458, and 1064 nm are presented in a. The spectra are marked by the corresponding excitation wavelengths. As we discussed earlier [
1,
2], the Raman spectra of a nanodiamond consists of three bands located at 1325, 1600, and 1500 cm
−1 (with laser excitation of 458 nm). The band at 1500 cm
−1 shifts to 1630 cm
−1 when excited by a laser with a wavelength of 257 nm [
1]. Also, a wide band from 1100 to 1250 cm
−1 is observed in all the measured spectra.Usually, this band is referred to as disordered sp
3 carbon [
34]. The narrow lines observed at 1064 nm excitation (upper spectrum in a) at a frequency slightly higher than 1960 cm
−1 are due to the absorption of residual water vapor in the atmosphere (a band with the rotational structure centered at 1.38 μm).
Figure 1. Raman spectra of the studied 2–5 nm nanodiamonds encapsulated in NaCl. (a) Raman spectra measured with four different excited wavelengths in the region of 257–1064 nm; red arrows mark the dispersive band position. (b) Lorentz multi-peak fits of Raman spectrum excited with 1064 nm.
The observed dispersion of the band between 1500 and ~1630 cm
−1 is typical for various sp
3 carbon clusters (for example, 3D C
60, ultrahard fullerite, or nanodiamond) [
1,
2,
35,
36,
37]. For diamond-like carbon, the peak dispersion linearly depends on the exciting wavelength in the range of 200–800 nm [
38]. The resonant Raman spectra of tetrahedral amorphous carbon was calculated, and a model of resonant Raman spectra of carbon films was presented [
38]. According to the model, the dispersed band is attributed to the G peak, which arises from chains of sp
2-bonded atoms. Significantly, the model contains 28% of the sp
2-bonded carbon atoms.
In practice, there is no sp
2-bonded carbon in pure 2–5 nm nanodiamonds, according to both parallel electron energy loss spectroscopy and nuclear magnetic resonance spectroscopy data (the sensitivity of the methods is better than 1%) [
10,
11].No such chains were observed in the sp
3 carbon clusters described in Refs. [
1,
2,
35,
36,
37], although in all these works the effect of dispersion was observed, including 3D C
60 with 92% of the sp
3 bonds [
35]. Moreover, the effect of the dispersion at a pressure up to 75 GPa was described in Ref. [
35], when the presence of sp
2 bonds in the sample at a pressure above 40 GPa is impossible [
39]. Thus, in the case of sp
3-bonded 2–5 nm nanodiamonds, the nature of the dispersion is not clear.
When excited with a wavelength of 1064 nm, the resonant Raman spectrum is observed (a,b). Let us look at the Raman spectrum of 2–5 nm nanodiamonds when excited at a wavelength of 1064 nm in more detail. Lorentz multi-peak fits of the Raman spectrum excited at 1064 nm are plotted in b. The bands at approximately 1325 and 1600 cm−1 are distinctly present in the spectra. A plateau (marked by the red color) that is composedof multi-bands in the range from 1423 to 1713 cm−1 is observed. The appearance of this plateau is a new feature of the Raman spectrum of 2–5 nm nanodiamonds, which manifests itself when excited at 1064 nm. The plateau covers all possible positions of 1500 to ~1630 cm−1 of the dispersed line, which it occupies when excited at wavelengths of 458–257 nm.
The relative intensities of the Raman lines also changed significantly when the wavelength of the exciting radiation changed from 458–257 nm to 1064 nm. In particular, the relative intensities of the bands at approximately 1325 and 1600 cm−1 and the plateau at the wavelength range 1423–1713 cm−1 as well as the band at 1100–1250 cm−1 are changed. When excited at 458 to 257 nm, the intensity of the band at approximately 1325 cm−1 is approximately 2-times higher than the intensities of the bands at 1100–1250 cm−1, the band at 1600 cm−1, and the bands at 1500–1630 cm−1. However, when excited at 1064 nm, the intensity ratio changes to the opposite (see a). The intensity of the plateau and the band at 1600 cm−1 exceeds the intensity of the bands at 1325 cm−1 by a factor of 2 (see b). Thus, with the resonant excitation, the relative intensities of the bands at 1100–1250 cm−1, 1600, and 1500–1630 cm−1 increased 4-fold, and additional lines appeared, which expanded the previously observed dispersion interval of 1500–1630 cm−1 to 1423–1713 cm−1. Thus, the full spectrum of the Raman-active excitations of the nanodiamonds under consideration consist of bands at 1100–1250 cm−1 and the plateau in the range from 1423 to 1713 cm−1.
Let us take a closer look at the 4-fold increase in the intensity of the Raman spectrum of bands in the range from 1423 to 1713 cm
−1. Such an increase in the intensity of the spectrum with the change in the exciting wavelength has been described in detail within the framework of the concept of resonant Raman scattering [
16]. In particular, resonance amplification of the spectrum is observed when the energy of the exciting radiation is near the absorption edge. In our case, the resonance is observed when excited at a wavelength of 1064 nm (1.16 eV). In terms of energy, this corresponds to the surface Tamm states with a band gap of approximately 1 eV [
13,
14,
15], as we discussed above in the Introduction section.
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