Raman Spectra and Bulk Modulus of Nanodiamond in a Size Interval of 2–5 nm
© The Author(s). 2017
Received: 23 August 2017
Accepted: 29 September 2017
Published: 10 October 2017
Nanodiamond in a 2–5-nm size interval (which is typical for an appearance of quantum confinement effect) show Raman spectra composed of 3 bands at 1325, 1600, and 1500 cm−1 (at the 458-nm laser excitation) which shifts to 1630 cm−1 at the 257-nm laser excitation. Contrary to sp2-bonded carbon, relative intensities of the bands do not depend on the 458- and 257-nm excitation wavelengths, and a halfwidth and the intensity of the 1600 cm−1 band does not change visibly under pressure at least up to 50 GPa. Bulk modulus of the 2–5-nm nanodiamond determined from the high-pressure study is around 560 GPa. Studied 2–5-nm nanodiamond was purified from contamination layers and dispersed in Si or NaCl.
Nanodiamond properties’ studies have attracted high interest of researchers during the past 30 years . Meanwhile, an important aspect of a quantum confinement effect influence on the mechanical properties and Raman spectra of diamond nanocrystals is practically omitted. A typical length scale for an appearance of quantum confinement effect is the Bohr radius of excitons ; the Bohr radius of exciton for diamond is 1.57 nm which is appropriate for the nanocrystal size of around 3 nm. Data of a parallel electron energy loss spectroscopy (PEELS)  provide more distinctive range for the size below 5 nm where new properties related to a modification of bonds in the nanodiamond appear. Nanodiamonds with non-modified surfaces of a size below 2 nm are not stable [1, 3] which restricts the size interval of nanodiamonds studied here by a 2–5-nm range.
According to both PEELS and nuclear magnetic resonance (NMR) spectroscopy data [3, 4], there is no sp2-bonded carbon in nanodiamond. As a result of quantum confinement effect, a bandgap of nanodiamond increases in the size interval of 2–5 nm, along with discrete energy levels arising at the band edges [1, 5]. In a case of covalently bonded solids, the bandgap growth means an increase in the chemical bond energy which means an increase in elastic moduli . Indeed, the increase of bulk modulus to 500 GPa was derived from a pressure-volume relationship of nanodiamond . Meanwhile, lattice parameters of nanodiamond including the 2–5-nm size interval correspond to those of natural diamond .
Raman spectra of nanodiamond are summarized in a review by Mochalin et al. . Due to phonon confinement effect, a triple-degenerated Raman band at 1333 cm−1 of bulk diamond crystal shifts to 1325 cm−1 in the 2–5-nm nanodiamond. In addition, a shoulder around 1250 cm−1 and bands at 1590, 1640, and 1740 cm−1 appear in Raman spectra of nanodiamond. A set of the 1590–1740 cm−1 bands is attributed to sp2-carbon (which, as mentioned above, is absent in nanodiamond), O-H and C=O groups . A relative intensity of the bands at 1325 cm−1 and around 1600 cm−1 depends on purification of nanodiamond. To avoid luminescence, spectra were usually recorded using the 325-nm laser excitation.
In the above assignment of Raman bands 1325 cm−1 to sp3- and 1600 cm−1 to sp2-bonded carbon, there is a self-contradiction related to a resonant Raman scattering effect. A scattering cross-section of sp2-bonded carbon exceeds the one of sp3-bonded carbon by a factor of 50–200 at the laser excitation in the visible range, and the cross-sections are mutually equal at the 257-nm laser excitation . We have revealed in our study that relative intensities of Raman bands at 1325 and 1600 cm−1 of the 2–5-nm nanodiamond purified from contamination layers do not depend on the excitation wavelength in the 257–532-nm range. We observed an addition Raman band which shifts from 1500 cm−1 at the 458-nm laser excitation to 1630 cm−1 at the 257-nm laser excitation. Bulk modulus of the 2–5-nm nanodiamond estimated in our study is around 560 GPa.
We used a detonation 2–5-nm diamond produced by the SINTA company (Republic of Belarus). For removing the rest of the contamination layers, the 2–5-nm nanodiamond was treated in a planetary mill with a mixture of 25 wt% of Si or NaCl. A Fritsch planetary mill with ceramic silicon nitride (Si3N4) bowls and balls of 10 mm in diameter was used. The treatment in the planetary mill provides preparation of homogeneous nanocomposites without contamination by material of the balls [10–12].
We also used a nanodiamond water suspension with a mean crystal size of 25 nm produced at the Microdiamant AG (MSY Liquid Diamond product; MSY diamond is a monocrystalline diamond powder produced by HPHT (high-pressure, high-temperature) synthesis) for high-pressure study. All structure studies of the 25-nm nanodiamond were done after drying the suspension.
The Raman spectra were recorded with a TRIAX 552 (Jobin Yvon Inc., Edison, NJ) spectrometer, equipped with a CCD Spec-10, 2KBUV Princeton Instruments 2048 × 512 detector and razor edge filters. Transmission electron microscope (TEM) and X-ray studies were done by a JEM 2010 high-resolution microscope (JEOL Ltd., Tokyo, Japan) and Empyrean (PANalytical) X-ray diffractometer. We used a diamond anvil cell (DAC) for a high-pressure study. The pressure was measured from the stress-induced shifts of the Raman spectra from the diamond anvil .
Results and Discussion
The resonance behavior of the band at 1600 cm−1 under the 458 nm and 257 nm laser excitations was not observed: the intensity of the band is the same at both excitations. Raman spectra at the 458-nm excitation include the peaks at 1325 cm−1 (with the shoulder around 1250 cm−1), 1500 cm−1, and 1600 cm−1. In addition, sp2-bonded pollution at 1360 and 1620 cm−1 (D and G bands) are present in the spectra.
Lorentz multi-peaks fits are plotted in Fig. 3. Raman spectra at the 257-nm excitation consist of the same peaks 1325 cm−1 (with shoulder around 1250 cm−1) and 1600 cm−1. The D and G bands of the pollution disappeared from the spectra, because the Raman scattering cross-section of sp2-bonded carbon decreases by the factor of 50–200 upon changing the excitation wavelength from 458 to 257 nm as mentioned above. The band around 1500 cm−1 shifts to 1630 cm−1. The observed resonance shift (dispersion) of the band around 1500 to 1630 cm−1 is typical for different carbon clusters with conjugated bonds where carbon atoms have 3 and 4 neighbors (for example, 3D C60, ultrahard fullerite or diamond-like carbon) [15–17]. In Ref. , the resonant Raman spectra of tetrahedral amorphous carbon were calculated and the dispersion of the band around 1500 cm−1 was attributed to a presence of sp2 chains. Nevertheless, no chains are expected in nanodiamond; there are no place for sp2 chains in a structure of 3D C60, and no chains were observed in ultrahard fullerite. Thus, the reason of the dispersion in the last group of carbon clusters is not clear.
The absence of the resonance effect for the 1600 cm−1 band indicates an attribution of the band to phonon features of the 2–5-nm nanodiamond instead of an sp2-bonded fraction. Consequently, force constants appropriate to Raman bands 1333 cm−1 (this one is shifted to 1325 cm−1 because of a phonon confinement effect ), 1500–1630 cm−1, and 1600 cm−1 determine the elastic module of the 2–5-nm nanodiamond according to the dynamical theory of crystal lattices . Typically, the Raman frequency ω scales upon the force constant k as ω~(k/m)2 where m is an atom mass, and the presence of the additional higher frequency bands in the Raman spectra means an increasing elastic module.
The mixture of 2–5-nm nanodiamond and NaCl (as mentioned in the “Methods” section, the 2–5-nm nanodiamond was treated in a planetary mill with a mixture of 25 wt% of NaCl) was loaded in a DAC. NaCl acts as a pressure-transmitting medium: under pressure below 50 GPa, a yield strength of NaCl varies from 0.08 to 0.65 GPa depending on pressure  (the strength increases upon pressure growth to 28 GPa and decreases about 50% at higher pressures). Consequently, a value of non-hydrostaticity  (σ 1 − σ 2)/σ 1 (σ 1 and σ 2 are major stresses in the sample) is below 5%.
Taking into account that ω 0 = 1325 cm−1 in the relation (1) and after the simplest calculations from Eqs. (1–3), we could conclude that the absence of the band with ω 0 = 1325 cm−1 under pressure of 50 GPa is possible only for a case when the bulk modulus of the 2–5-nm nanodiamond exceeds 524 GPa.
From a least-squares fit of the dependence in Fig. 6 for the 2–5-nm nanodiamond and Eq. (1), we obtain the bulk modulus of the 2–5-nm nanodiamond B 2-5nm = 564 GPa for γ ≈ 1, as mentioned above. For comparison, the dependence for DLC gives the 392 GPa bulk modulus for γ ≈ 1.
Raman spectra of a 2–5-nm nanodiamond consist of 3 bands at 1325 cm−1, 1500–1630 cm−1 (depending on the excitation wavelength of 458–257 nm, accordingly), and 1600 cm−1. The 1600 cm−1 band cannot be attributed to a fraction of sp2-bonded carbon, because the intensity of this band does not depend on the excitation wavelengths of 458 and 257 nm (while the intensity of sp2-bonded carbon depends essentially on these wavelengths), and one halfwidth and the intensity do not change visibly under pressure at least up to 50 GPa (contrary to pressure-induced transformations of sp2-bonded carbon). The presence of the additional high-frequency (comparing to diamond) bands in the Raman spectra means an increase (comparing to diamond) in the elastic module according to dynamical theory of crystal lattices. The dependence of Raman spectra of the 2–5-nm nanodiamond upon pressure provides information on bulk modulus which we estimate as 564 GPa.
The authors thank P. Belobrov, P. Sorokin, and B. Mavrin for fruitful discussions.
This work was supported by the Ministry of Education and Science of the Russian Federation (project ID RFMEFI59317X0007; the agreement No. 14.593.21.0007); the work was done using the Shared-Use Equipment Center “Research of Nanostructured, Carbon and Superhard Materials” FSBI TISNCM.
MP, VC, DO, and AK prepared the samples and performed high pressure and Raman studies. VD performed UV Raman study. BK, IP, and VB carried out TEM studies. VA carried out X-ray measurements. All the authors took part in the discussions and in the interpretation of the results and have read and approved the final manuscript.
The authors declare that they have no competing interests.
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