R Šmíd

A Granier

A Bousquet

G Cartry

L Zajíčková

Study of magnetic field influence on charged species in a low pressure helicon reactor

Keywords: 52.50.Qt helicon reactor, argon, oxygen, electron density, ion flux

The variation of electron density (n e ), ion flux and emission of argon lines were investigated in a rf helicon reactor for different static magnetic field amplitude and rf power using Langmuir probe, rf coupled planar probe and optical emission spectroscopy (OES). The static magnetic field was created by two Helmholtz coils positioned around the plasma source. Its amplitude B varied by changing the current I B =0÷ 2 A through the coils from 0 to 10 mT (0 ÷ 100 Gauss) in the helicon source, which corresponded to 0 ÷ 1.4m T (0 ÷ 14 Gauss) in the diffusion chamber. We studied Ar and O 2 discharges at 0.7 Pa with rf power 50 ÷ 600 W applied to the helicon antenna. The variations of n e , ion flux and intensity of Ar 750 nm with the rf power measured in Ar diffusion plasma were in a good agreement. It was found that n e increased monotonically with the rf power for I B < 1A whereas for higher current it exhibited a maximum, which position shifted from lower to higher rf power with increasing I B . The OES measurements in the plasma source for I B =1.5 A showed that the Ar emission line did not reach a maximum but continuously increased with the rf power. Hence, it is concluded that the maximum observed for n e in the diffusion chamber is likely to be due to some confinement of the plasma in the source. Measurements in oxygen discharge showed slightly different results.

Introduction

Helicon generation of plasmas was first employed by Boswell [1], following years of helicon propagation studies and applications in material processing (e. g. [2]). Helicons are propagating whistler wave modes in a finite diameter, axially magnetized plasma column, excited by an rf driven antenna that couples to the transverse mode structure across an insulating source wall [START_REF] Lieberman | Principles of Plasma Discharges and Materials Processing[END_REF]. Beside the wave coupling, two other coupling modes, capacitive and inductive, can be ignited in the source for certain input rf power and magnetic field [START_REF] Degeling | [END_REF].

Helicon applications to material processing utilize a process diffusion chamber downstream from the source. The present paper is focused on an investigation of the rf helicon reactor used for material processing, namely on an influence of dc magnetic field and rf power on the discharge parameters in the diffusion chamber. The reactor was previously used for the deposition of silicon oxides and organosilicon polymer films as well as plasma diagnostics [5]. It was, however, equipped with one magnetic coil around the source tube whereas here, two coaxial coils producing more homogeneous axial magnetic field were used. This study resumes on early results on the effect of the magnetic field on the discharge generated by the one-coil source [START_REF] Borvon | Dépôt PECVD de matériaux à faible constante diélectrique[END_REF] and deals with non-depositing gases only because of difficulties met in deposition conditions.

Experimental

The studied helicon reactor consisted of the helicon source made of a 30 cm long glass tube, 15 cm in diameter, and the stainless steel diffusion chamber with the length and diameter, both of 30 cm [5,[START_REF] Borvon | Dépôt PECVD de matériaux à faible constante diélectrique[END_REF]. The source tube was surrounded by two Helmholtz coils supplied by a dc current of 0 ÷ 2 A. The continuous wave rf power (13.56 MHz) in the range 50 ÷ 600 W was applied to the helicon antenna through an L-type matching box. Argon (Ar) or oxygen (O 2 ) were injected through a small tube above the source directly to the source tube. Pressure was in both cases 0.7 Pa.

The magnetic field lines were parallel to the reactor axis in the source and diverged in the diffusion chamber [5]. Compared to the one-coil source [5,[START_REF] Borvon | Dépôt PECVD de matériaux à faible constante diélectrique[END_REF] the amplitude of the axial magnetic field created by the two Helmholtz coils was more uniform. Calculations of this amplitude (B) showed that varying the current (I B ) in the coils from 0 to 2 A corresponded to B =0÷ 10 mT (0 ÷ 100 Gauss) in the source tube and 0 ÷ 1.4m T( 0÷ 14 Gauss) in the diffusion chamber (Fig. 1).

The discharge in the diffusion chamber was investigated by a commercial cylindrical rf compensated Langmuir probe (SmartProbe). It was assumed that the weak magnetic field did not influence the measurement. Since future experiments are planned to be performed in deposition mixtures (such as O 2 /hexamethyldisiloxane) the results from the Langmuir probe were compared with a rf driven planar probe that is not affected by the deposition of thin dielectric film [START_REF] St | [END_REF]. Both the probes had their active area in the center of the chamber, 15 cm under the source tube orifice and 4 cm above the substrate holder. The principle of the planar probe is as follows. The probe is rf biased through a capacitance and a dc self-bias appears on its surface. When the rf voltage is switched off, only ions can reach the probe. The variation of probe voltage with time is then linked to the ion flux [START_REF] St | [END_REF]. Our probe consisted of a concentric central disc (7 mm in diameter) and an outer guard ring (16 mm in outer diameter).

Additionally to the probe measurements, optical emission spectra were recorded using a fiber optics and a Jobin Yvon HR640 monochromator equipped with Hamamatsu R928S photomultiplier through quartz windows at two positions: at the top of the source tube in the reactor axis and in the mean height of the diffusion chamber. In our measurement we focused on Ar emission line at 750 nm that can be used as a measure of electron density under the assumptions of (a) direct electron excitation and (b) constant electron temperature. In O 2 discharge, 5 % of Ar was added to pure O 2 in order to observed Ar emission line too.

Results and disscussion

Langmuir probe characteristics revealed that the electron energy distribution could be approximated by Maxwellian in Ar as well as in O 2 . Therefore both, electron density and temperature, were evaluated from these measurements. The electron temperature in the diffusion chamber of Ar discharge was about 3.2 eV for all input powers of I B =1 .5 and 2 A. For weaker magnetic fields (I B ≤ 1 A), the electron temperature decreased strongly from about 3.2 eV for 25 W to 2.7 eV for 150 W and stayed constant for higher input powers. The electron temparature in the diffusion chamber of O 2 discharge decreased continuously with the input power from 3.8 eV to 2.8 eV for I B < 1A. For I B ≥ 1 A the electron temperature stayed constant around 4 eV. The dependence of electron density in the diffusion chamber of Ar discharge on the rf input power for different current to the coils (I B ) is shown in Fig. 2a. The electron density increased monotonically with the input power when a weak magnetic field was applied (I B < 1 A). For more intensive magnetic field (I B ≥ 1 A), the electron density exhibited a maximum which position and magnitude changed with I B . It reached 4 × 10 16 m -3 at 350 W for I B = 2 A and 2 × 10 16 m -3 at 200 W for I B =1.5A .F o rI B = 1 A, a very weak maximum of 4 × 10 15 m -3 appeared at 100 W.

Assuming a direct electron excitation and a weak dependence of the electron temperature on the rf power (see above), the intensity of Ar emission line at 750 nm is proportional to the electron density. The dependencies of Ar emission on rf power in the source tube are shown in Fig. 2b. For I B < 1 A, the intensity increased almost linearly and these results are comparable to the electron density measurements in the diffusion chamber. For I B =1.5 and 2 A, it increased strongly when increasing the rf power from 100 W to 250 W and from 100 W to 400 W, respectively, which corresponded to the same behavior of the electron density in the diffusion chamber. For higher input power the intensity increased weakly in the source whereas the electron density measured in the diffusion chamber strongly decreased.

To confirm the results of the Langmuir probe in the diffusion chamber we compared the electron density with ion flux measured by planar probe and intensity of Ar emission line. The comparison for Ar discharge without magnetic field (I B =0A) and for I B =1.5 A is given in Fig. 3a andb, respectively. All three diagnostics methods were in good agreement. In addition, the ion saturation flux taken from the Langmuir probe measurement was in good agreement with the ion flux measured by the planar probe. Similar agreement was achieved for O 2 discharges. Since the same trends were observed for all charged and excited particles in the diffusion chamber the results are quite reliable.

The trends in O 2 discharge are shown Figs. 4a and4b. The electron density increased strongly with the input power for all I B except 1 A where a plateau of about 7 × 10 15 m -3 was observed from 400 W (Fig. 4a). For low input power (< 100 W) the magnitude of the electron density was almost independent of I B . The maximum density of about 1.1 × 10 16 m -3 was obtained for I B =1 .5 A. The intensity of Ar emission line in the source, showed for I B = 1 A in Fig. 4b, exhibited similar dependence on rf power as the electron density in the diffusion chamber.

In order to propagate the helicon waves in the helicon reactor the power exceeding 1.5 kW is needed [START_REF] Boswell | A study of waves in gaseous plasmas[END_REF]. It was, however, impossible to reach it for our continuous wave mode due to extreme heating dangerous for the source tube. Such high power could be used only in a pulse mode. Therefore, it is assumed that the helicon mode was not coupled in the power range used in the present study. Only the capacitive or inductive coupling was possibly present [START_REF] Degeling | [END_REF].

The behavior of electron density observed especially in the diffusion chamber of Ar discharge is more likely due to some confinement of the plasma in the source, maybe due to magnetic field gradients at the source exit which prevents the diffusion. The diffusion coefficient D parallel to the magnetic field vector (and parallel to the reactor axis) is not influenced by magnetic field. The diffusion coefficient perpedicular to the magnetic field can be calculated as [START_REF] Chen | Introduction to Plasma Physics[END_REF]

D ⊥ = D 1 + ω c 2 τ 2 , (1)
where ω c is electron cyclotron frequency, τ -1 is mean collision frequency and D standard diffusion coefficient. The diffusion coefficient D ⊥ calculated from (1) using a reference data of electron to Ar collision cross-section [START_REF] Frost | [END_REF] vary in the range from 1t o0 .0005 × D for I B =0÷ 2 A, assuming an homogenous magnetic field and electron temperature around 3 eV. Similarly, the electron to O 2 collision crosssection taken from [11] gives the diffusion coefficient D ⊥ in the range from 1 to 0.0007 × D for I B =0÷ 2 A. The electron mean free paths calculated from the same data were about 12 cm and 8 cm for Ar and O 2 discharge, respectively. These values are comparable to the reactor dimensions.

Conclusion

The variation of the electron density and ion flux measured by the Langmuir probe, the ion flux measured by the rf coupled planar probe and emission of argon line in the diffusion chamber of helicon reactor were in very good agreement. Therefore, any of these three techniques can be used for a study of plasma parameters. This is an important information for future investigation of deposition processes in the same reactor.

In Ar and O 2 discharge, the intensity of Ar emission line (a small amount of Ar was added to O 2 ) in the helicon source did not present any maxima but continuously increased with the rf power. The electron density in the diffusion chamber of Ar discharge increased with the rf power for I B < 1 A, i. e. low magnetic field, whereas for higher current it exhibited a maximum, which position shifted from lower to higher rf power with increasing I B . These maxima were likely due to a plasma confinement in the source created by the magnetic field. The electron density measured in O 2 discharge did not show any maxima with the rf power. It increased strongly with the input power for all I B except 1 A where a plateau was observed.

Fig. 1 .

 1 Fig. 1. Calculations of the axial magnetic field in dependence on the current through the coils (I B ): (a) -whole reactor, (b) -detail of the diffusion chamber.

Fig. 2 .

 2 Fig. 2. Dependencies of (a) electron density in the diffusion chamber and (b) intensity of Ar emission line (750 nm) in the source tube at 0.7 Pa on rf input power for different current to the coils I B in argon discharge.

Fig. 3 .

 3 Fig. 3. Comparison of the variations of the electron density, ion flux and Ar 750 nm line intensity with the rf power at 0.7 Pa for I B = 0 A (a) and I B =1.5 A (b) in Ar discharge.

Fig. 4 .

 4 Fig. 4. Dependencies of (a) electron density in the diffusion chamber and (b) intensity of Ar emission line at 750 nm in the source on rf power for different current to the coils I B at 0.7 Pa in O 2 discharge.

This research has been supported by the Barrande project 2005-2006-023 and by the projects MSM 0021622411, 202/03/H162. The first author would like to thank the French government for granting his foreign student short stay in Nantes.


D Lupu

A R Biriş

I Baltoga

M Baibaraca

V Kuncsera

G Schinteiea

M Valeanua

S Lefrantb

A S Birisc

I Mişan

G Filotia
email: filoti@infim.ro

Carbon nanotubes from ethanol on Fe-Co/MgO catalysts and related interface phenomena

Keywords: Carbon nanotubes, Fe-Co/MgO catalyst, Mossbauer spectroscopy

Introduction

The synthesis of carbon nanotubes from alcohols on Fe-Co/zeolite catalysts, introduced by Maruyama et al. [1], proved to result in high quality Single Wall carbon nanotubes (SWNTs), almost free of amorphous carbon [1][2][3][4][5][6][7][8]. Various reaction conditions (temperature, catalytic metal loading on the support, low pressure or atmospheric, pre-activation of the catalyst, residence time) the formation of single (SWNTs), double (DWNTs), multiwall nanotubes (MWNTs) forms. In the last years efforts to improve the selectivity and yield of the nanotubes synthesized on solid state catalysts via Chemical Catalytic Vapour Deposition (CCVD) method were oriented in a search for new catalysts and adjusting the reaction conditions in close correlation with the factors affecting the size of the catalytic nanoparticles, critical for the growth of different nanotube types among which the DWNTs are of particular interest [9]. The reason for growing a mixture of DWNTs and SWNTs lies in their very similar catalytic growth conditions, including the high homogeneity distribution of metal particles [10].

MgO is a very attractive support, as it can be removed from the products by non-oxidizing acids like HCl without damaging the nanotubes [11]. Moreover, the reduced iron can be better dispersed on MgO than on Al 2 O 3 and SiO 2 [12] which should be an advantage for the formation of small catalytic nanoparticles. Ago et al. [13] proved the role of strong metal-support interactions in the formation of small size metal nanoparticles, suitable for SWNT(DWNT) synthesis on Fe/MgO catalysts and proposed a mechanism based on the observation that the dispersion of Fe 3+ ions into MgO bulk is favoured by the catalyst pre-treatment in argon at high temperature while the presence of hydrogen increases the particle size and suppresses the nanotube growth. A detailed investigation by Coquay et al. [14] of Fe/Co-MgO catalyst by Mössbauer spectroscopy revealed that Co ions readily substitute for Mg in MgO lattice, which hinders the dissolution of iron. Upon reduction with hydrogen, the well dispersed Fe 3+ ions in MgO tend to form Fe 2+ , resistant to some extent against further reduction to metallic iron. The well dispersed Fe 3+ ions and the existence of small MgFe 2 O 4 -like particles lead to efficient catalytic metal particles, while the iron rich Fe-Co system activated by Co, favours the growth of carbon nanotubes (mostly SWNTs and DWNTs) [14].

In this paper, the various iron species appeared on MgO supported catalysts are studied by Mössbauer spectroscopy in order to correlate the type of nanotubes resulted from ethanol with the above mentioned mechanisms and roles [13,14]. Because the type of nanotubes strongly depends on the CCVD conditions, the ways to increase the DWNTs content have been also investigated by Raman Spectroscopy.

Experimental

The CCVD syntheses were performed in an induction heating set up described elsewhere in detail [15,16] using a quartz tube of 18 mm inner diameter. The pretreatments in argon (10% H 2 ) for Mössbauer spectroscopy were performed in the same tube. For CCVD in vacuum, 20-40 mg catalyst was disposed on a graphite cylinder of 12 mm outer diameter (10 mm inner diameter) was heated inductively at 400 o C for 40 min in argon (10% H 2 ), then pumped to vacuum. When the pressure decreased down to 10 -2 Torr, the ethanol was released in the system through suitable valves, keeping a constant ethanol pressure of 5 Torr while concomitantly increasing the temperature to 900 o C. After 15 min the ethanol flow was stopped and the high frequency inducing oven was turned off, cooling the catalyst-holder ensemble in pure argon flow.

The controlled ethanol flowing was settled to 0.027g/min. For syntheses in argon (without hydrogen), after the activation stage in the same conditions as above, the hydrogen from argon was cut off, the temperature was increased to 900 o C and the ethanol flow was ensured by bubbling a measured part of the argon through ethanol at room temperature, again for 15 min. It should be noted that the so called activation stage at 400 o C still does not lead to the complete reduction of iron species as shown below by Mössbauer spectroscopy.

The fitting procedure consists of a non-linear regression method using Lorentz line shapes and one common line-width per pattern. The temperature dependent Mössbauer measurements have been performed via a liquid nitrogen bath cryostat between 77 K and 300 K. The linear symmetrical wave-form mode of the velocity was utilized. The 57 Co source in Rhodium matrix had an average activity of around 1 GBq. A α-Fe foil was used for calibration, and the isomer shift is permanently referred to it. The absorber powder was stacked in a Cu holder, strongly pressed between two mica foils. The thickness of the absorber was of about 10 mg Fe/cm 2 .

The Raman spectra were obtained using the surfaceenhanced Raman scattering technique (SERS) that operates with 10 2 -10 4 enhanced Raman signals. Gold films of 60-150 nm thickness were used as SERS supports. The Raman spectra were recorded in a backscattering geometry using a FT Bruker RFS 100 Spectrometer working at λ exc = 1064 nm , a Jobin-Yvon Horiba T64000 or LabRAM HR 800 Micro Raman Spectrometer for λ exc = 676.4 , 632.8 and 514.5 nm, all equipped with a microprobe allowing to focus the laser beam on the sample within a one micron scale.

Results and discussion

Correlation with the nanotube growth mechanisms

The characteristics of nanotubes synthesized by CCVD strongly depend on the particle size of catalytic metal while the presence of hydrogen plays a dominant role in the development of metal nanoparticles on the catalyst surface. The reduction to metallic state may result either by an activation procedure in hydrogen or even during the CCVD via H 2 resulting from the decomposition of the carbon source.

Mössbauer spectroscopy on pre-treated catalysts

Even without a pre-activation treatment in hydrogen to reduce the iron species, H 2 produced during the CCVD process may increase the nucleation size of catalytic metal particles [13] and information on the effect of H 2 on the various Fe species in any particular catalyst is therefore useful for subsequent selective syntheses. Hydrogen was not introduced during the CCVD syntheses reported here from ethanol but it can arise from the ethanol decomposition (in the absence of water, at 800-900 o C up to 3moles H 2 /mole of ethanol [17] are released).

The Mössbauer studies on the catalysts after various pre-treatment in argon flow with 10% H 2 were performed in order to investigate the effect of hydrogen on the evolution of different oxidation states: such as the reduction of Fe 3+ to Fe 2+ and formation of Fe-Co alloy. The identification of the reduced states of iron (and their amount), induced via hydrogen presence, is important for their relationships with the CCVD mechanism on Fe:Co/MgO catalysts as reported in literature [13,14].

Two catalysts have been studied: 4.5%Fe:4.5%Co/MgO (abbreviated 78) and 2.5%Fe:1.25%Co/MgO (79), after the following pretreatments followed by cooling to room temperature: 78-N 1h at 400 o C in argon flow with 10% H 2 ; 78-A and 79-A 1h at 400 o C in argon flow with 10% H 2 then heated 2 min at 900 o C.

The sample 78-I is the CCVD product after dissolution of the catalyst by Soxhlet extraction in HCl 24 h, still contain encapsulated around 9.3% FeCo.

The Mössbauer spectra are presented in two figures with Fig. 1 showing the patterns for as prepared sample 78-N and the Fig. 2 showing the other four spectra for reason of easy visual comparison and understanding. The parameters describing various Fe species are grouped in table 1. Table 1 proves unambiguously the usefulness of measurements at two different temperatures. For example sample 78-N measured at room temperature (RT = 295 K) shows narrower lines and due to lower interactions a stronger superposition of patterns. The Mössbauer measurement at 80 K provides information about an increased distortion evidenced both by larger quadrupole splitting of the doublet as well as a noticeable (Fig. 1) larger line width for singlet. That could primarily be interpreted as a result of a low dimensionality of resulted grains (a super-paramagnetic state), with a rather broad range of dimension, but also as an evidence of more stronger interactions.

In this context the central line in spectra contains also some collapsing sextets (spins are not completely blocked at 80 K) appearing as magnetic ordered phase (see Fig. 2 and Table 1) in the other samples at the same temperature. The singlet represents an intimate Fe-Co compound , with high symmetry (HS in Table 1) of iron surroundings while the parameters of the doublet plead for Fe 3+ species as markedly mentioned in the last column of table containing the Mössbauer parameters. The evolution of pattern area at the mentioned two temperatures denotes a clear fingerprint of strong difference in the Debye temperatures of the involved species, standing for un-equivalent lattice vibrations. Compared with other samples, 78 -N clearly contains smaller particles and an homogeneous dispersion of Fe 3+ in MgO lattice as well as a result of only one hour treatment in argon (10%H 2 ) does not completely transform all Fe 3+ to the reduced Fe 2+ state. The sample 78-A which was treated in hydrogen at 900 o C for 2 min explicitly proved the reducing effect via presence of Fe 2+ species in Mössbauer spectra.

The evolution of parameters points to more emphasized effects at lower temperatures. The amounts of Fe 3+ and Fe 2+ remain comparable at both temperatures due to closed Debye temperatures of the involved species. There is a smaller amount of Fe-Co compound showing a value close to a quarter of total area of the spectrum. The magnetically ordered phase represents a Fe-Co solid solution conserving the body centered cubic structure of alpha iron phase. As expected the presence of Co in the neighborhood of Fe increased the effective field (B eff ) at nucleus on both iron coordination as derived from the parameters of ordered phase displayed in the Table 1. Lowering the temperature, the sample 78-A spectra are showing larger distribution of Fe 2+ ion sites (via increased line width) and local distortion (larger quadrupole splitting), similar to mentioned behaviour of the sample 78-N spectra.

In the sample 79-A, similar to 78-A, the iron ions are showing the same species but with a noticeable higher amount of Fe-Co and lower one for Fe 3+ and a less distorted surroundings for Fe 2+ , also in larger amount, which suggest a smaller penetration of iron into MgO support, this making it more accessible to reduction with hydrogen in 2 min la 900 o C.

Fig.2 The Mössbauer spectra of samples 78-A, 78-I and 79-A

The picture with the presence of non-reduced Fe 3+ bonded to oxygen is in agreement with literature [14] mentioning a high resistance at hydrogen reduction of such catalysts.

A careful scrutiny of data emphasizes the role played by the amount of metals (Fe+Co) dispersed on MgO substrate. The Fe-Co alloys appeared with higher percentage from total iron as compared to 78A sample.

Grains of order of one micron could be considered when magnetic phases are put in evidence in Mössbauer spectra close to room temperature and of a noticeable reduced dimensionality when appears only at 80 K.

Finally, the spectra for 78-I show mainly characteristic sextets of Fe-Co alloy, again with Fe accommodating in two different positions, as previously discussed, which pleads for stability of the Fe-Co intermetallic solution, probably encapsulated, located at the end in the nanotube as the nanotubes grow on these particles. There are small amount of Fe 3+ arising from catalyst particles covered by the nanotube bundles, not dissolved in acid during the first purification stage.

The values and temperature behaviour of dynamic Mössbauer parameters (line-widths and area mainly), suggests that the grains contain simultaneously large metallic particles and small size metal and oxide ones, showing both super-paramagnetic state, the dimension of the latter being influenced by their direct interaction with carbon nanotubes.

It has to be mentioned, based on above presentation that Fe-Co particles ≥7 nm grow during CCVD because no such particles appeared for a treatment of 1h in argon(10% H 2 ) as before shown .

Correlation between the nanotube yield and the previously accepted mechanisms quoted in the literature

The yield of carbonaceous products η (in weight %) obtained for the two samples are: η(78)=17% and η(79)=12% related to the overall catalyst weight. The values related to the content of catalytic metal Fe or (Fe+Co) are also given in Table 2 but they do not fit to the overall yield ratios. The question arises which could be the major species implied in the CCVD process. It seems interesting to calculate the yields / species amount normalized with their relative fraction obtained from the area of the specific patterns appearing in the Mössbauer spectra (e.g. for Fe in Fe-Co alloy, as Fe 2+ or their sum) as given below. It is interesting to note that only the yields normalized to Fe-Co, Fe 2+ or to their sum show a higher relative yield 78/79 as is the case. This is in agreement with the models proposed by Coquay et al. [14]. It is quite reasonable to refer the yield to the sum of Fe-Co alloy (directly implied in the nanotube growth) plus Fe 2+ ions (potential precursor of Fe-Co particles) according to the literature proposed models [13,14]. The iron appeared as Fe 2+ species following the samples treatment, as was evidenced from Mössbauer measurements, reveals that all Fe 3+ are easily available for reduction in sample 79, more or less close to the surface, showing also a poor dispersion.

As compared to sample 78, the diminishing of Co/Fe ratio in 79 proves to be less favourable for high quality SWNTs, which is indeed the case as evidenced below by the Raman spectra of the carbon nanotubes. The chemical analysis of the encapsulated metal revealed the overall composition Fe 0.53 Co 0.47 in the nanotubes 78 of better quality and Fe 0.73 Co 0.27 for 79, in agreement with the observations of Coquay et al. [14] that Co 2+ hinders the dissolution of iron in MgO and favours the formation of Fe-Co particles of size and composition close to Fe 0.50 Co 0.50 adequate for CNT formation.

The Mössbauer spectroscopy results proved to be a very useful tool providing parameters for calculating the yields of carbon nanotubes related to various iron species co-existing in the catalyst and also to evaluate their role in the growth process and the quality of the obtained carbon nanotubes. The chemical analysis of encapsulated metal and the characteristics of the nanotubes obtained on two different catalysts suggest that at low Co content, the rate of Co transfer from the bulk MgO to the catalyst surface, where the nanotubes growth takes place, has a detrimental effect on the quality of nanotubes.

Comparative Raman spectroscopic characterization of the products

The Raman spectrum of carbon nanotubes

Some information concerning the carbon nanotubes and their vibration properties are necessary to be reminded, before to develop the topic. A SWNT is a grapheme sheet rolled up into a cylinder with both ends capped with hemispheres made of hexagonal and pentagonal carbon rings. Regardless of the synthesis method, microscopic studies have revealed that samples consist of bundles of 20-100 individual nanotubes aligned in a two-dimensional crystal packing arrangement over essentially their entire length [START_REF] Dresselhaus | Phonons in carbon Nanotubes[END_REF]. The bundles, also known as nano-ropes, contain both metallic and semiconducting tubes. Raman spectroscopy is one of the most used technique in the characterization of CNs. An appropriate experimental method is the surface enhanced Raman scattering (SERS), which, owing to the resonant excitation of surface plasmons in a suitable metallic substrate operates with enhanced Raman signals (>1000 fold) [19]. According to the vibration calculus regarding the Raman active modes in SWNTs three spectral ranges are significant, i.e. 100-350, 1000-1700 and 1700-3000 cm -1 . In the first range one find the Raman bands associated with the radial breathing modes (RBM) , which do not exist in graphite. Their peaks position is related to the tube diameter according to the relation ν (cm -1 ) = 223.75/d (nm). Bands belonging to this group are very sensitive to the excitation wavelength. The intensity of each band is enhanced when the photon energy of the excitation radiation corresponds to a transition between the van Hove singularities (E ii ) in the valence and conduction bands of all possible nanotubes. The second group, consisting of G and D bands, covers the interval from 1000 to 1700 cm -1 . These bands are not only related to the nanotube structure: the former, peaking at about 1595 cm -1 , usually called as G, is a complex band attributed to the tangential vibration mode (TM). Recent data describe the G complex band as being formed from two bands noted as G + and G -, which in the case of the metallic SWNTs are attributed to the transversal optic (TO)-circumferential and longitudinal optic (LO) -axial vibration modes, respectively [20]. The D band, whose peak position varies with the excitation wavelength, is frequently associated with a disorder or defects state induced in the graphitic lattice or nanotubes. Another group, located in the high wave number range from 1700 to 3500 cm -1 , corresponds to the second order Raman spectrum. As a rule, the most intense bands are those detected at approximately twice the wavenumber of the D and G bands. Like their first-order counterparts, they behave resonantly when the excitation wavelength is changed.

Influence of the catalyst

Two catalysts of different catalytic metal loading 4.5%Fe:4.5%Co/MgO (78) and 2.5%Fe:1.25%Co/MgO (79) were studied in order to reduce the particle size of catalytic metal by lowering their concentrations on the support. However, as the Mössbauer spectra revealed, the Fe dispersion is much higher for 78 despite the lower loading of 79, pointing to the importance of Co and of a Co/Fe ratio close to 1 as reported [14].

The Raman spectra in Fig. 3-4 of the products synthesized at 900 o C on each of the two catalysts show poor quality nanotubes appeared for 79: intense D bands (defect rich) and / or amorphous carbon and no radial breathing modes (RBMs) modes at excitation with 514 nm. The better quality, RBMs at all excitations, high ratios of G/D intensities, intense 2D(G ' ) band, of 78-I (Fe to Co ratio 1:1) is also in agreement with the observation [14] that ratio close to 1 is favourable for SWNT(DWNT) formation. The dominant diameters for 78-I are found in the ranges 0.7-0.9; 1.2-1.5 and 2-2.3 nm.

A method for evaluation the DWNTs content in a SWNTs/DWNTs mixture, based on the fact that 2D band splits in two bands for DWNTs, was proposed [24]. The FWHM at 514 nm excitation and 633 nm of the 2D band in the table above are significantly higher than 50-60 cm -1 for pure SWNTs [24,25] which suggest a significant amount of DWNTs in 78.

The spectra of 78-I, 78-II (78-I subjected to a second purification by heating in air to 500 o C with 5 o C/min followed by Soxhlet extraction) or 78-III (78-I heated in air saturated with water vapour for 24 h at 280 o C) closely resemble the Aldrich DWNTs as shown in Fig. 5. However, the treatments in acid show that for 78-II all the RBMs corresponding to nanotube diameters between 0.7 and 2.3 nm strongly decrease in intensity while for Aldrich DWNTs, the bands at 264, 315, 331 cm -1 remain almost unchanged, as expected for inner nanotubes. This suggests that the amount of DWNTs in 78 is lower as compared to Aldrich DWNTs

Synthesis conditions -effects at the interface with carbon source

Aiming to an increased content of DWNTs, syntheses in other conditions were performed on the same catalyst, as given in Table 4. The residence time τ=ρV/G of the ethanol on the catalyst was calculated as in [23] but for a volume V=1 cm 3 of the reaction tube, taking into account the ethanol density ρ in the total feed mass flow rate G (ethanol and argon) . The I G /I D ratios of 80, 80b show an increased quality ( high I G /I D ratios) of the nanotubes and a significant increase of all the RBMs at 1064 nm in Fig. 6, the spectra resembling closely to those of Aldrich DWNTs from Fig. 5 and also of other high quality DWNTs reported in literature [24,25].

Tentatively, the histogram given in Fig. 7, based on the Raman spectra may be used for a comparative analysis of the products. As compared with the low pressure (5 Torr ethanol) conditions, where the sum of the all nanotube diameters in the range 0.7-1nm correspond to a fraction ~0.23, for 80 and 80b the fraction (resulting from the histogram in Fig. 7) of diameters in this range increases to 0.52-0.54. Thus in argon flow and low residence time about half of the nanotubes are found within this range. The increase of the low diameter nanotubes with the decreasing partial pressure of hydrogen is in perfect agreement with the effect of increasing the catalytic Fe particle size in the presence of hydrogen reported by Ago et al. [13]. The partial pressure of H 2 evaluated from ethanol equilibrium thermodynamics [17] The close resemblance of the Raman spectra for 80 and 80b with Aldrich DWNTs, suggests that the DWNT content increases for CCVD performed in argon flow at the atmospheric pressure. Two possible pairs of tubes satisfying the condition of diameter difference 0.67-0.82nm may be identified as corresponding to DWNTs: with 0.7 and, respectively 1.4nm as inner tubes and outer tubes of 1.4 and, respectively, 2-2.3nm diameters.

As concerning the role of hydrogen, it was revealed that reactive H species, unavoidably arising in CCVD with carbon sources containing hydrogen [START_REF] Zhang | Ultra-high-uield growth of vertical single-walled carbon nanotubes: Hidden roles of hydrogen and oxygen[END_REF], the small diameter tubes are preferentially attacked by reactive H species during CCVD, finally resulting in larger diameter SWNTs. This is also in agreement with the results in the series 78-80-80b: as the partial pressure of hydrogen is lower, a significant increase of low diameter tubes (d<1 nm) can be observed in the histogram for 80, 80b as compared to 78. Selective syntheses from ethanol on mixed Fe/Co catalysts reported that "SWNTs synthesized on the iron-rich catalyst have narrow diameters" [27].

For Fe-Co alloys the hyperfine field increases with their Co content and the B eff values from Table 1 supply experimental evidence on the presence of different magnetic phases encapsulated in 78-I nanotubes. From the histogram, the fraction of tubes of three dominant diameter ranges 0.7 -1 , 1.1 -1.5 and 2 -2,3 nm are 0.24, 0.61 and 0.15 respectively and they may not be co-related with only two types of Fe-Co observed. It is interesting to note that a ratio of the two Fe-Co alloys with different B eff encapsulated in nanotubes 78-I is close to 1:2 (32.1: 62.3), the theoretical ratio of metallic to semiconducting nanotubes [START_REF] Dresselhaus | Phonons in carbon Nanotubes[END_REF]. A detailed discussion [START_REF] Little | Mechanistic Aspects of Carbon Nanotube Nucleation and Growth[END_REF] of the intermediate stages involved in CCVD process emphasizes that the reconstructive nucleation at the catalytic metal nanoparticles/carbon nanotubes interface causes structural and magnetic phase changes during the stage of melting and reconstruction of metal nanoparticles, which modulate the carbon nanotubes growth. Further Mössbauer studies on the encapsulated metal resulting from different catalysts and synthesis conditions might give additional information in this direction. In Fig. 8 the TEM image shows indeed that CCVD in vacuum results in a rather low content of DWNTs, a single double-walled tube can be clearly identified.

Conclusions

1. Mössbauer spectroscopy studies of Fe-Co/MgO catalysts proved that a high dispersion degree of Fe may be achieved for catalysts with Fe/Co ratio close to 1. They also showed that the presence of hydrogen at high temperatures increases the particle size of catalytic Fe.

2. The reduced iron species (Fe 2+ and metallic Fe-Co alloy) resulting from highly oxidized species of Fe 3+ inside the catalysts can be quantitatively evaluated from their areas in the Mössbauer spectra of catalyst reduced for short time with hydrogen. This may be a measure of the readiness to reduction of a given catalyst, depending on the initial degree of iron dispersion.

3. The nanotube yields for two catalysts with rather different (Fe,Co) concentrations are proportional to the amount of (Fe-Co alloy + Fe 2+ ) which occur in the catalyst after a short time reduction with hydrogen. This relationship is in pretty good agreement with the mechanisms proposed in literature [13,14] pointing out to the importance of these two species in balancing the growth process of SWNTs(or DWNTs).

4. The Raman spectra show that the carbon nanotubes synthesized from ethanol in argon flow on 4.5%Fe:4.5%Co/MgO are abundant in low diameter (0.7-1.0nm) tubes than those obtained at low ethanol pressure. This is in agreement with the effects generated by hydrogen, reported in literature, both on the catalyst [13,14] and on nanotubes [START_REF] Zhang | Ultra-high-uield growth of vertical single-walled carbon nanotubes: Hidden roles of hydrogen and oxygen[END_REF]: i) as the partial pressure of hydrogen (resulting from ethanol decomposition at 900 o C) decreases, there are less favourable conditions for an increase of catalytic metal particle size by additional reduction of iron species to metal [13] and ii) preferential availability of small diameter tubes to attract reactive H species. The close similarity with the Raman spectra of Aldrich and literature DWNTs suggest that syntheses in argon flow at short residence time (low ethanol flux) are more favourable for the formation of double-walled nanotubes on this catalyst type.

5. Two different non equivalent sites were found in Fe-Co alloys encapsulated in carbon nanotubes. Their ratio is very close to the theoretical ratio of metallic to semiconducting nanotubes 1:2. It is premature to conclude that the two types of nanotubes grow on Fe-Co particles with different magnetic characteristics. However, this supplies experimental evidence for the presence of different magnetic phases and their implication in the stages of CCVD, in agreement with the importance of these phase changes emphasized in literature [START_REF] Little | Mechanistic Aspects of Carbon Nanotube Nucleation and Growth[END_REF].

Fig. 1 .

 1 Fig. 1. Mössbauer spectra for catalysts 78-N at two different temperatures 80 and 295 K

Fig. 3 .Fig. 4

 34 Fig. 3. Raman spectra (SERS) for nanotubes synthesized from ethanol (5 Torr) at 900 o C on 4.5%Fe:4.5%Co/MgO catalyst sample 78-I (solid curves) and DWNT-Aldrich (inner tube diameter d i =1.3 nm) for comparison (dashed curves)

Fig. 5

 5 Fig. 5 Transformation of 78-II and DWNT-ALDRICH carbon nanotubes by a chemical treatment with an acid and base solution. The thick solid lines illustrate the initial states, the dashed spectra were obtained plugging the samples for 15 min. in 0.5M H2SO4 solution and the thin solid lines show the recovered states after the sample immersion for 10 min. in 0.5M KOH solution.

Fig. 6 .

 6 Fig.6. Raman spectra (SERS) for nanotubes synthesized from ethanol at 900 o C on 4.5%Fe:4.5%Co/MgO catalyst in argon flow 170 sccm argon for the sample 80 (solid curve) and 300 sccm for 80b (dashed curve)

Fig. 7 .

 7 Fig. 7. A histogram based on (I RBM /I G )/(sum I RBM /I G ) vs. the tube diameter for the samples 78, 80 and 80b and Aldrich DWNTs (d i =1.3nm) . The sum refers to all the RBM peaks observed in the Raman spectra at 1064, 676.4 and 514.5 nm. Open bars refer to semiconductor and filled bars to metallic nanotubes according the Kataura diagrams.

Fig. 8 .

 8 Fig. 8. TEM image for nanotubes synthesized at 900 o C, 5 Torr ethanol, 15 min on Fe4.5%:Co4.5%:MgO in graphite boat. Mostly SWNTs of 1-1.3 nm diameter are present and a DWNT in the bottom right corner marking with arrows the outer and inner tubes.

Table 1 .

 1 Mössbauer parameters at 80 K or room temperature for samples 78 (N and A type) as well as for 79 and 78

	Sample	Area (%)	IS or ε	QS (mm/s)	B eff	Species
			(mm/s)		(T)
	78-N	30.3	0.13	0.00	0.0	Fe-Co
	RT	69.7	0.14	0.64	0.0	Fe 3+
	78-N 80K	34.0	0.20	0.00	0.0	Fe-Co
		66.0	0.25	0.98	0.0	Fe 3+
		12.2	0.05	-0.02	36.5	Fe-Co
	78-A	9.8	0.02	0.14	34.4	Fe-Co
	RT	48.0	0.20	0.82	0.0	Fe 3+
		26.4	1.01	0.78	0.0	Fe 2+
		3.6	1.10	1.68	0.0	Fe 2+
		18.2	0.14	0.06	37.0	Fe-Co
	78-A	6.2	0.13	-0.11	34.5	Fe-Co
	80 K	40.6	0.29	0.59	0.0	Fe 3+
		20.5	1.27	1.16	0.0	Fe 2+
		14.5	1.23	1.98	0.0	Fe 2+
		13.9	0.10	0.11	38.9	Fe-Co
	79-A	21.9	0.11	0.14	35.0	Fe-Co
	80 K	19.7	0.31	0.92	0.0	Fe 3+
		38.1	0.85	0.97	0.0	Fe 2+
		6.4	1.10	1.36	0.0	Fe 2+
		32.1	0.14	0.00	36.7	Fe-Co
	78-I	62.3	0.15	0.00	34.5	Fe-Co
	80 K	2.6	0.25	0.38	0.0	Fe 3+
		3.0	-0.05	0.00	0.0	Fe-Co s.p.

Table 2 .

 2 The yield of carbonaceous products normalized to various iron species for the samples 78 and 79

	sample	η/ total	η/ Fe in	η/ (Fe+Co) in	η/ Fe-Co alloy	η/ Fe 2+	η/ (Fe-Co)+ Fe 2+
		catalyst	catalyst	catalyst	area	area	area
	78	17	378	189	1718	1260	727
	79	12	480	320	1340	1078	598

Table 3 .

 3 RBM bands observed for the sample 78-I, the diameters calculated by ω RBM =224/d +14 (left ) or ω RBM =223.75/d and the nanotube type according to Kataura diagram.

	λ laser	[cm -1 ] RBM bands	[nm] Diameter	Type
	1064 nm	100-120	2-2.4	1.9-2.2	metallic
	SERS	166 i	1.47	1.35	semiconductor
		266 i	0.95	0.84	semiconductor
		312;330 m	0.71-0.75	0.68-0.72	semiconductor
	676,4 nm	125 w	2.0	1.79	semiconductor
	SERS	173 vi	1.51	1.37	metallic
		198 vi	1.22	1.13	metallic
		285 w	0.83	0.79	semiconductor
	633 nm	130 (145.8) w	1.82 -1.93	1.53-1.72
	micro Raman	189 vi	1.28	1.18	semiconductor
	2D FWHM =80 cm -1	213 vi	1.12	1.05	metallic
					metallic
	514 nm	183 i	1.33	1.22	semiconductor
	SERS	204 m	1.18	1.1	metallic
		265 w	0.95	0.84	metallic
	514 nm	140-156w	1.58-1.78	1.43-1.6	semiconductor
	micro Raman	170m	1.43	1.32	semiconductor
	2D FWHM =88 cm -1	185vi	1.31	1.21	semiconductor
		248-267m	0.88-0.96	0.85-0.9	metallic

vi=very intense; i=intense; m=moderate; w=weak; vw=very weak

Table 4 .

 4 Residence time for various CCVD conditions for nanotube synthesis from ethanol at 900 o C on 4.5%Fe:4.5%Co/MgO in quartz tube of 18 mm inner diameter.

	Sample	Synthesis	Residence	I G /I D	Partial
		conditions	time τ [s]	1064nm 676.4mm	pressure of H 2 resulting
				514.5nm	from
						ethanol
						[Torr]
	78	5 Torr	0.067	14		11
		ethanol		3.4
		0.027 g
		ethanol/min
	80	Argon flow	0.0045	20	11	8
		170		9
		sccm/min
		(10.5
		sccm/min
		through
		ethanol)
	80b	Argon flow	0.0014	16	5	3.3
		300		13
		sccm/min
		(10.5
		sccm/min
		through
		ethanol)


O V Sedelnikova

C P Ewels

L G Bulusheva

A V Okotrub

OPTICAL PROPERTIES OF CdS QUANTUM DOTS ON GRAPHENE

Keywords: graphene, cadmium sulfide, quantum dot, permittivity, density functional theory

Hybrid systems based on graphene and semiconductor quantum dots are prospective materials for optoelectronics and photonics. In this work, electronic structure and dielectric properties of small particles of cadmium sulfide on the surface of graphene were studied using the density functional theory. The optical spectrum of this hybrid structure depends on the orientation of the nanoparticle relative to graphene due to the interaction between electrons of sulfur atoms on the surface of the CdS particle and π-orbitals of carbon atoms.

INTRODUCTION

Hybrid materials based on carbon materials and semiconductor nanoparticles (CdS [1][2][3][4][5][6][7][8][9][10][11], CdSe [12], CdTe [13], TiO 2 [14], nanodiamonds [15], etc.) are currently subject to extensive research, mainly with regard to their application in optoelectronics and power engineering. Such systems combine the properties of quantum dots, which provide effective interaction between nanoparticles and the incident electromagnetic radiation of the visible spectrum, and the ability to transfer excitation energy to conductive substrates [4,10,11].

Cadmium sulfide, a direct gap semiconductor with a band gap of 2.4 eV, is widely used as an active medium in photocells [16]. Quantum dots are semiconductor nanocrystals whose size is comparable to the Bohr radius of the exciton in bulk material. The effects of quantum confinement of electron energy spectra, holes, and excitons can be used to control the relative positions of energy levels in the quantum dot and "tune" the system to interact with photons of a certain energy. Thus, CdS nanoparticles can be used to extend the spectral response to the infrared region relative to the bulk material.

Combining CdS nanoparticles with a semiconductor or a metal substrate enhances transmission of the excitation localized at the quantum dot. Graphene stands out among other allotropic forms of carbon due to its extraordinary conductive properties [17,18]. Since graphene exhibits outstandingly high mobility of charge carriers, it can serve as a current collector for the electrons of nanoparticles excited by external photons into the conduction band [4,10]. Note that one substantial disadvantage of graphene photodetectors is associated with low quantum yield due to almost absolute transparency of graphene in the visible frequency range [19]. Introduction of structural defects (graphene bending [20][21][START_REF] Sedelnikova | Fullerenes, Nanotubes[END_REF][23][24], disorientation of layers in bilayer graphene [25,26], synthesis of graphene nanodisks [27]) creates new long-wave absorption bands in the spectrum of graphene-based materials.

In this work, optical properties of small CdS particles adsorbed on graphene are studied using quantum chemical methods. We will discuss how graphene's electronic structure and optical spectrum are affected by the change in the distance between graphene and CdS particles and by their relative orientation, and consider contributions of surface atoms and band hybridization.

CALCULATION DETAILS

Density functional calculations were performed within the local density approximation [28] using the AIMPRO software package [29][30][31]. This software contains basis sets of atomic orbitals in the form of localized Gaussian based functions (38, 28, and 40 independent functions for carbon, sulfur, and cadmium atoms, respectively) with angular momenta up to l = 0, 1, 2. For better convergence, the effective temperature of the electron system was taken equal to 0.04 eV. The charge density plane wave energy cut-off was 150 Hartrees. Self-consistent calculations were carried out with an accuracy of 10 -5 eV for the total energy gradient. The total and partial densities of electronic states were searched using a 20×20×10 grid of k-points. The binding energy between the CdS particle and graphene was calculated as:

E b = E tot (CdS/graphene) -E tot (graphene) -E tot (CdS)
, where E tot (CdS/graphene) is the total energy of the hybrid structure; E tot (graphene) and E tot (CdS) are total energies of isolated graphene and CdS particle, respectively.

The frequency dependence of the imaginary component of permittivity ε 2 was calculated within the random phase approximation.

Fig. 1 shows the CdS/graphene models. The periodic graphene supercell consists of 128 carbon atoms arranged at a distance of 1.42 Å from each other. The clusters of 22 atoms (11 sulfur atoms and 11 cadmium atoms, Fig. 1a,b) and 72 atoms (37 sulfur atoms and 35 cadmium atoms, Fig. 1c,d) denoted below CdS1 and CdS2, respectively, were cut out from the CdS crystal hexagonal structure (wurtzite structure, lattice parameters a = 4.136 Å and c/a = 1.62 [32]). The CdS particle was placed above the center of the graphene supercell to make Cd-S hexagons projected onto the carbon hexagons. We further varied the distance d between the components of the hybrid structure and the orientation of the CdS particle on graphene (angle α = 0°, 30°, 60°, and 90°). The atomic structures of the components were kept unchanged. The planar hexagonal CdS monolayer was calculated using the equilibrium unit cell parameter a = 4.24 Å from work [33].

RESULTS AND DISCUSSION

Fig. 2 shows binding energy E b as a function of the distance between CdS1 and graphene for α = 0°. The energy is negative for d = 2.6-3.5 Å, i.e. formation of the hybrid structure is energetically advantageous at such distances. The energy becomes positive below 2.5 Å to indicate repulsion between the particle and graphene. There is a minimum in the E b energy at about d = 3.1 Å. The energy of the system slightly changes when the particle rotates in the graphene plane at this optimal distance (up to 15 meV). The minimal E b corresponds to the CdS1/graphene models with angles α = 30° and 90°, while the models with α = 0° and 60° are metastable. The small value of the difference between these binding energies indicates that small CdS particles can rotate freely on the graphene surface.

Fig. 3 shows total and partial densities of electronic states for the CdS monolayer from -5.0 eV to 5.0 eV with respect to the Fermi level. The valence band of the CdS monolayer is due to 3p-electrons of sulfur and 5s-and 4p-electrons of cadmium (with small participation of 4d-electrons). The geometry of overlapping atomic orbitals indicates formation of σ-orbitals due to s-and p xy -electrons of cadmium and p xy -electrons of sulfur, and formation of π-orbitals mainly due to p z -electrons of sulfur (a peak at about -1.5 eV in the density of states). The bottom of the conduction band is formed by antibonding σ*-orbitals. The peak in the density of states near 3.3 eV is associated with a π*-orbital of CdS monolayer formed mainly by p z -electrons of cadmium. Besides, 4d-orbitals of cadmium and 3d-orbitals of sulfur provide noticeable contributions to the conduction band and the valence band, respectively. The obtained partial densities of the CdS monolayer agree with previous data [33]. The value of the band gap is 1.6 eV. Underestimation of band gaps obtained for semiconductors and insulators within the approximation of the local density exchange-correlation functional is associated with an incorrect description of the exchange component and is characteristic of the method.

CdS particles exhibit a discrete energy spectrum. The band gap in CdS1 does not exceed 0.3 eV, which is due to the contributions of p-and s-electrons of surface sulfur and cadmium atoms to the valence and conduction bands (see Fig. 3b). Passivation of unsaturated surface bonds opens the band gap of the nanoparticle [34]. The states of the particle in the valence band near the Fermi level are determined mainly by p-electrons, while the contribution of s-electrons of sulfur atoms becomes substantial in the interval from -5.0 eV to-3.0 eV. In the conduction band, s-and p-electrons have equal contributions to the density of states in the energy interval of 0-2.7 eV, while p-electrons of cadmium with small participation of s-electrons of cadmium and d-electrons of sulfur are prevailing in the region of higher energies.

Fig. 4 shows the dispersions of electronic levels of CdS1/graphene structures for different d values. As can be seen, hybridization between particle bands and graphene bands substantially decreases the dispersion for small d, and Dirac cones disappear in graphene. The interaction between the components of the hybrid system decreases when the distance grows to 2.5 Å. The density of electronic states confirms overlapping of graphene and CdS orbitals to indicate strong interaction between the components of the system.

To study the influence of the interaction between the CdS particle and graphene on the optical properties of the hybrid system, we calculated frequency dependences of the imaginary component of the permittivity ε 2 for the CdS monolayer, CdS1 particle, graphene, CdS1/graphene model for various orientations of the particle relative to graphene absorption edge is near 1.6 eV, which correlates with the band gap width. The analysis of the contribution of interband transitions to the absorption spectrum indicates that singularities at 1.6-4.5 eV are caused by transitions between σ-orbitals near the band gap of the CdS monolayer, with predominant contribution from p xy -electrons of sulfur and s-and p xy -electrons of cadmium. The peak at 6.8 eV corresponds to the transitions between p z -orbital of sulfur and p z -orbital of cadmium. The peak at 8.7 eV is due to electron excitations from the σ-orbital (s-symmetry) to the antibonding σ*-orbital. A number of resonances at 10.0-13.5 eV are due to interband transitions involving 4d-orbitals of cadmium and 3d-orbitals of sulfur.

Comparison of 2 ⊥ ε spectra of the CdS1 nanoparticle and the spectra of the monolayer suggests common nature of their absorption mechanisms. This assumption is supported by the fact that most atoms of the CdS particle are surface atoms with monolayer-like surrounding.

The optical spectra of CdS1/graphene and CdS2/graphene are similar. The dielectric function in the interval from 1.0 eV to 4.5 eV exhibits absorption bands associated with graphene (peak A) and surface states of the CdS nanoparticle. The most energetically preferable CdS1/graphene model (α = 30°) possesses peaks denoted B and C with an energy of about 6.7 eV and 8.0 eV, respectively. We suppose, that the first resonance arises from the transition between p z -orbitals of sulfur and cadmium atoms, the second one can be associated with the transitions between the particle's bonding and antibonding orbitals formed mainly by s-and p xy -electrons. Changing orientation of the CdS1 particle increases the contribution of surface interband transitions in the interval of 1.0-4.0 eV and substantially decreases the intensity of the peak B. Suppression of interband transitions due to excitation of p z -electrons of the sulfur atoms can be attributed to the interaction between the CdS nanoparticle and graphene. Indeed, for the CdS1/graphene model with α = 0°, sulfur atoms are located above the centers of С-С bonds . In this configuration, p z -orbitals of sulfur will overlap significantly with p z -orbitals of carbon to form a hybrid orbital which is symmetrical with respect to the reflection operation (see Fig. 5b). According to the selection rules, the interband transition between this joint state and the antibonding p z -orbital of cadmium is forbidden. For the model with the angle α = 30° (sulfur atoms appear above the centers of carbon hexagons, Fig. 5c), the peak B occurs due to weaker overlapping of electron densities of graphene and nanoparticles.

CONCLUSIONS

Quantum chemical methods have been used to study electronic structure and optical properties of hexagonal CdS quantum dots on graphene. The sulfur atoms of the nanoparticles were oriented towards graphene. Calculation of bonding energy indicates that formation of hybrid structure is energetically advantageous at a distance of 3.1 Å between graphene and sulfur atoms. The intensity of the absorption peak associated with S(p z ) → Cd(p z ) transitions changes as a result of nanoparticle rotation relative to the surface of graphene. This effect is weaker in the model where projections of Cd-S hexagons coincide with carbon hexagons. This can be explained by suppression of interband transitions due to formation of the S(p z ) → C(p π ) bonding orbital.

The work was supported by the Russian Foundation for Basic Research (grant No. 16-33-00515). O. V. Sedelnikova is grateful for being awarded the Scholarship of the President of the Russian Federation (SP-3530.2016.1). Some of these results were obtained using the CCIPL, "Centre de Calculs Intensif Pays de la Loire".

Fig. 1 .

 1 Fig. 1. Top (a) and side (b) views of the unit cell of the CdS1/graphene model and top (c) and side (d) views of the CdS2/graphene model. Darker atoms of the particle represent sulfur atoms.

Fig. 2 .

 2 Fig. 2. Binding energy E b in the CdS1/graphene model as a function of the distance d between the CdS1 particle and graphene for α = 0°.

Fig. 3 .

 3 Fig. 3. Density of electronic states of the CdS monolayer and partial densities of the electronic states of cadmium and sulfur atoms (a). Density of electronic states of the CdS1/graphene model with the parameters d = 3.1 Å and α = 30° as compared to the individual CdS1 particle and graphene (b). Partial densities for the CdS1 particle include total densities of s-, p-, and d-orbitals.

Fig. 5 shows 2 ⊥ε 2 εFig. 4 .

 224 Fig. 5 shows 2 ⊥ ε spectra for the case of electromagnetic wave normally incident to graphene surface (E⊥c, where с is the normal to graphene surface). If the field is polarized along the axis с (E||c), the permittivity values || 2 ε of graphene and the particle belong to different regions of the spectrum, therefore, the change in the geometry of the CdS/graphene model does substantially change the optical properties of the hybrid structure. The graphene's optical spectrum agrees well with the literature data [21, 24]. The component 2 ⊥ ε demonstrates peaks at about 4.0 eV and 14.0 eV, corresponding to π → π* and σ → σ* transitions, respectively. The absorption spectrum of the CdS monolayer appears mainly in the ultraviolet region, the

Fig. 5 .

 5 Fig. 5. Imaginary component of the permittivity 2 ⊥ ε of CdS1/graphene models (d = 3.1 Å, α = 0° and 30°) and CdS2/graphene model (d = 3.1 Å, α = 30°) as compared to graphene and CdS monolayer (a). Overlapped p z -orbitals of sulfur and carbon for hybrid structures with α = 0° ( b) and α = 30° (c).

Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia; o.sedelnikova@gmail.com.

Novosibirsk State University, Novosibirsk, Russia.

Institut des Matériaux Jean Rouxel, CNRS-Université de Nantes, UMR6502, Nantes, France. Translated from Zhurnal Strukturnoi Khimii, Vol. 59, No.

4, pp. 907-913, May-June, 2018. Original article submitted December 1, 2017.