1. Introduction

CdTe semiconductor nanoparticles immersed in transparent matrices have currently many important applications in electrochemistry ¹, photoelectricity ², solar cells fabrication ³, chemical sensors ⁴, bio-labelling ⁵ etc. The achievement of desired particle sizes over the largest possible range, controlled particular sizes, and good crystallinity are some of the properties expected in nanocrystal growing methods. The transparent matrices can be solid or liquid and organic or inorganic, depending on their use and the growth method employed. Thus, hard matrices for CdTe such as silica ⁶ and soft ones like hydrogels ⁷ have been employed, which allow a large variety of applications. SnO₂ is a transparent oxide with the property of possessing high electric conductivity that places the oxide in a special position as matrix for electronic devices design, as instance. At present, CdS nanoparticles embedded in SnO₂ have been successfully utilized in photocurrent generation ⁸, and in solar cells based on CdTe nanoparticles immersed in SnO₂ matrix have been already studied with good results ⁹. Nowadays, a lot of growing techniques exist, physical or chemical, to prepare semiconducting nanoparticles, which have found a great acceptance among the researches of wide-world. In the last four years several works have been publishe about CdTe nanoparticles immersed in SiO₂, in which the good photoluminescent properties at energies larger than that of bulk CdTe have been reported ^10,11,12.

In this work the preparation of CdTe nanoparticles by means of r.f. sputtering with radius in the range 2.4 - 7.0 nm is reported. Actually, a CdTe-SnO₂ nanocomposite was grown on glass substrates. Within the framework of II-VI semiconductor quantum dots (QD) the sputtering growth technique offers controlled size distribution, also the preparation of CdTe crystallized in the metastable hexagonal wurtzite phase ¹³. Structural and optical studies include X-ray diffraction, transmission electron spectroscopy, optical absorption and Raman analysis to characterize the quantum confinement in the CdTe nanoparticles immersed in the also nanoparticulate SnO₂ matrix, both undoped. The variation at room temperature (RT) of the main vibrational modes of wurtzite CdTe nanoparticles, when their size goes to small values, is studied. The main contribution of this the analysis of the optical properties of hexagonal CdTe, since there are few reports in this crystalline phase of this important semiconductor.

2. Experimental Details

The CdTe + SnO₂ thin films were prepared on 7059 Corning glass substrates by means of the r.f. sputtering technique in an Ar ambient at 20 mTorr and at RT. The time for deposition was 180 min. The target was 5 cm² of diameter and was prepared with mixed 99.999 % purity SnO₂ and CdTe powders. The CdTe weight percentage in the target varied from 2% to 20% to get different nanoparticles size. After the growth the samples were annealed at 300°C in air during one hour. A variety of different crystallite size in the 4.8 - 14 nm range was obtained. Films have thickness of 200 ± 20 nm, as measured by using a Dektak II profilometer. For the crystalline structure analysis an X-ray Diffractometer Siemens D5000 (Cu- K α radiation) was utilized. The size of nanoparticles was determined by employing the Scherrer formula and transmission electron microscopy (TEM) images. TEM apparatus employed was a Jeol 2010 working at 200 KeV. The CdTe + SnO₂ layers were detached from the substrate and attached onto the TEM-grills. Indexation was computed by the relation d _t = λtL _t /r _t where λt=0.0027 nm is the electron wavelength at 200 keV, the steady camera for diffraction with L _t = 30 cm, and where r _t (mm) is the radius measured from the transmitted electron beam to the diffracted rings (Fig. 2b). Data were taken, from selected area of electron diffraction (SAED) and x-ray diffraction (XRD) patterns, to define the structure by Diamond 3.0 software. The band gap and higher electronic transitions were measured from optical absorption spectra (OA). OA spectra were measured by means of a UNICAM 8700 spectrophotometer. The Raman spectra of the samples were measured at room temperature with the LabRaman II Dilor spectrometer with excitation light of 514 nm wavelength. All the characterization was performed at RT.

Figure 1. (a) Typical X ray diffraction (XRD) pattern of the WCdTe embedded in SnO₂ matrix. Reflections labeled with H indicates CdTe in hexagonal wurtzite crystalline phase. XRD peaks corresponding to the tetragonal phase of the SnO₂ matrix can be observed. Insets illustrate the deconvolution fitting process followed to calculate the nanoparticle radius. (b) Deconvolution of hexagonal CdTe and SnO₂ broad X-ray diffraction bands. In this picture it can be appreciated that both types of nanoparticles have approximated the same average size. 

Figure 2. (a)TEM image of CdTe distribution of nanoparticles immersed in the SnO₂ matrix for an average diameter of ~ 12 nm. The inset illustrates the TEM image of a detail of CdTe QD prepared with average size of ~ 4 nm, the bar scale is 10 nm. (b) Electron diffraction pattern of nanoparticulate W-CdTe and SnO₂ matrix. 

3. Results and Discussion

The crystalline structure of the CdTe nanoparticles immersed in SnO₂ is the hexagonal wurtzite (W-CdTe) phase as observed in Fig. 1(a), where a typical XRD pattern of the CdTe + SnO₂ nanocomposite, in the range 20 ≤ 2θ ≤ 50 degrees, is exhibited. XRD peaks which correspond to the tetragonal crystalline phase of SnO₂, the matrix, are also observed in Fig. 1(a). In the two insets of this figure are displayed in detail, in the interval 2θ=22-25 degrees, W-CdTe diffractograms with three reflections. These nearby three XRD signals can be considered as the finger print of the wurtzite phase of many materials which crystallize in hexagonal packing. Wurtzite is a metastable phase for CdTe, however, Farias et al. have demonstrated that, from the thermodynamic point of view, wurtzite phase should be more stable than the zinc blende for very small radius nanoparticles ¹⁴. The samples studied here were deposited at ∼1.1 nm/min, which can be considered as a very slow growing rate, in such a way that it enables the nanoparticles to grow, practically, in equilibrium conditions at RT. The XRD reflections of the SnO₂ do not always appear in the low 2θ values region, probably there the reflections are so weak because the existence of preferred orientation in other crystallographic directions. In Fig. 1(b), an XRD reflection of SnO₂ can be observed together with the triplet of W-CdTe. This diffractogram indicates that the size of SnO₂ nanoparticles is of the same order of the size of the CdTe ones. In order to calculate the average radius R of the nanoparticles, a deconvolution process with Gaussian profiles was carried out to determine the full width at half maximun (FWHM) of the XRD bands. This step allows obtain the center of each band also. The average radius of the particles, obtained from the Scherrer’s formula and from FWHM values of the bands in XRD patterns of a set of ten CdTe nanoparticles, lies in the 2.4 - 7.0 nm range. The TEM image of the CdTe-QD distribution grown with R=6.0±0.7 nm into the SnO₂ matrix is displayed in Fig. 2(a), the inset illustrates the TEM image nanoparticles with R=4.0± 0.5 nm. In Fig. 2(b) the electron diffraction pattern of the CdTe nanoparticles plus the SnO₂ nanoparticulate matrix is exhibited, where the pattern clearly evidences that CdTe nanoparticles have grown with the hexagonal symmetry.

From the UV-Vis optical absorption spectra of the CdTe + SnO₂ films and the relation (αhv) = A(Eg-nano-hν) the first electronic transition, i.e., the direct energy band gap (Eg-nano) was obtained for the samples. Here, α is the optical absorption coefficient, h the Plank’s constant, v the photon frequency, hv the photon energy, and A a constant. In Fig. 3(a) is shown the calculation of Eg-nano for the sample prepared with R=4.6±0.4 nm, where the extrapolation of the linear part of the curve until the hv axis indicates that Eg-nano=1.64 eV. In this same panel the first derivative with respect to hv of the absorbance [d(OA)/d(hv)] as a function of hv is outlined (thin line). The position of the maximum of d(OA)/d(hv) on the hv axis coincides with the value of Eg formerly calculated. Thus, the first derivative of OA will be used to estimate with good approximation all the electronic transition data, as has been used for other authors ¹⁵. Furthermore, the d(OA)/d(hv) lineshape allows observe other transitions with better resolution. In Fig. 3(b)d(OA)/d(hv) of two samples is displayed, the position of the first electronic transition is indicated with the first maximum.

Figure 3. (a)Calculation of the first exited energy of electrons in the nanoparticle with R = 4.5 nm employing both the first derivative of absorbance spectra and the (αhv)² = A(Eg-nano -hv) formula. (b) First derivative of absorbance of two samples, the first maximum indicates the position of the first quantized energy level. Arrows with asterisks as superindex indicate the energy of the second quantized energy level. 

The second electronic transition due to the next discrete level has been identified with the second band and was indicated with other small arrow accompanied by an asterisk. The third transition observed in Fig. 3(b) is a well pronounced maximum around 2.5 eV, which has been identified by us that is originated from CdO formed at the interface CdTe-SnO₂¹⁶, since the direct band gap of CdO is ~ 2.5 eV ¹⁷. Other possible phases formed in the CdTe surface ¹⁴ as product of oxidation are TeO₂ and CdTeO₃^16,18, however these oxides have band gap values of ~ 3.8 and ~ 3.9 eV, respectively ^19,20,21,22. Then, in our case, that a CdO layer has been formed in the CdTe/SnO₂ interface is highly probable. A small feature in the bottom spectrum in Fig. 3(b), which was indicated by the “hh” label, is due to first electronic transition from the heavy-holes in the valence band ²³. In this same spectrum a shoulder is observed at around 2.44 eV which is due to the spin- orbit plus the crystal field unfolding (Δso+Δcf). The values Δso=93 meV and Δcf=3 meV have been reported for W-CdTe ^24,25. The last unfolding arises because here CdTe has hexagonal symmetry ²⁶. For high energy values (Eg-nano≥1.64 eV) a deconvolution process was applied to have a better definition of the position of the second electronic transition.

The first (Eg-nano) and the second electronic transitions as a function of the inverse square radius of QD are displayed in Fig. 4. Despite the scattering of the experimental points, it can be concluded that both electronic transitions satisfy the well known Eg versus R ^-2 dependence. The solid lines on the experimental data have been drawn according with the relation E _n,l (R) = Eg+(∇2/2R2)[(xn _e,le )² /m _e + (xn _hlh )²/m _h ], where n, l are the electron or hole eigenstates as 1s, 1p, 1d, etc. Eg=1.43 eV is the band gap energy of the bulk CdTe at RT, m _e and m _h are the effective masses of electron and holes, respectively. xne,le, xnh,lh are the roots of the spherical Bessel functions ^27,28. The Bohr Radius of CdTe is 7.3 nm, which means that our analysis has been totally developed in the strong quantum confinement regime. The optical properties of absorbance and photoluminescence of the CdTe nanoparticles do not overlap with those of SnO₂ because the difference in the band gap values (Eg=3.8 eV for bulk SnO₂).

Figure 4. First two electronic transitions of W-CdTe versus the square inverse of nanoparticle radius. 

High resolution of CdTe Raman spectra was carried out in the set of samples. The Raman spectra of two CdTe representative samples (samples with R=5.4±0.4 and 2.4±0.5 nm) in the range 100-400 cm^-1 are depicted in Figs. 5(a) and 5(b), respectively. In both cases the deconvolution process with Lorentzian curves of the spectrum is also illustrated. Five modes are observed in the Raman spectra of CdTe in the 100 - 170 cm^-1 range. The modes which practically coincide are those of E ₂-Te and TO-CdTe, both at ~ 140 cm^-1. However, in past we have found that the TO-CdTe mode lies slightly at low energies of E ₂-Te ²⁹. A lower vibrational band observed at slightly low energy than ~ 140 cm in CdTe nanocrystals, has been associated to DALA mode (disorder activated longitudinal acoustic) by other authors ³⁰. In Figs. 5(a) and 5(b) the mode E _2-Te has been fitted at lower energy than that of TO-CdTe. The mode at The Raman spectrum of CdTe has been The modes centered at ~ 140 and ~ 160 cm^-1 correspond to the transversal optic (TO) and to the longitudinal optic (LO) vibrational modes of cubic and hexagonal CdTe, respectively ^31,32,33, and the mode located at about 120 cm^-1 has been associated to with the A₁ (TO) of hexagonal Te ^33,34. The positions of the centers of both the TO and the LO modes of CdTe as a function of R are shown in Fig. 6(a). The most important modes in nanometric SnO₂ are observed at wavenumbers above the 450 cm^-135. Even with the large scattering of experimental points, a tendency of the LO mode to soften and the TO mode to harden can be ensured. In Fig. 6(b) the dispersion curves of cubic zincblende CdTe in the 160-180 cm^-1 interval are shown ^34,35. No specific information exists in the literature about the dispersion curves of the hexagonal wurtzite CdTe, nevertheless, given the similarities existent between hexagonal-Wurtzite (HW) and cubic-Zinc blende (CZB) crystalline phases, the unit cell length of HW along the (0001) axis is double with respect to CZB along the (111) direction (Γ→ L) . The dispersion curves of CZB along the Γ-X and Γ-L ³⁶, high symmetry directions, of the 1^st Brillouin zone can be used to analyze the variation of the TO and LO modes around the Γ point when R decreases. The very low dimensional character of nanoparticles breaks the translational periodicity, then, the selection rules of crystal momentum conservation are relaxed when R→0, the momentum conservation is less and less preserved. Then, transitions out the Γ point are allowed, as outlined by the dashed arrow in Fig. 6(b). In this way, as R goes to lower values, the dashed arrow can move away the Γ point more and more. This fact indicates that when R decreases LO decreases, and on the contrary, TO increases. The shift to lower frequency values of the LO mode of CdTe when R decreases has been observed by Rolo et al²⁷. This behavior is consequence of quantum confinement effects. Theoretically, Trallero-Giner et al. have developed a model for the LO mode of CdS for nanocrystals, in which the mode is softens when nanoparticles size decreases ³⁷.

Figure 5. Raman spectra of two CdTe nanoparticles. (a) Deconvolution to separate the positions of the modes of Te and of the TO and LO modes of the CdTe nanoparticle with R = 5.4 0.4 nm. (b) Of sample with R = 2.4 0.5 nm. 

Figure 6. (a) Section of the dispersion curves of cubiczincblende CdTe found in the literature. (b) Positions of the LO and TO modes of the W-CdTe nanoparticles as a function of the particle radius. 

4. Conclusions

Wurtzite CdTe nanoparticles embedded in SnO₂ matrix in the radius range 2.4-7.0 nm were succesfully prepared by means of the r.f. sputtering technique. X-ray diffraction shows that W-CdTe and Tetragonal SnO₂ nanocrystals have sizes of the same order. W-CdTe is the metastable phase of bulk CdTe, however for nanoparticles is a stable phase. The quantum confinement occurs in the strong regime as the Bohr radius for CdTe is 7.3 nm. From optical absorbance the first and second electronic transition were determined. In Raman spectra the LO and TO modes of W-CdTe were measured where, as expected from dispersion curves, the LO mode hardens and the TO softens when the radius of nanoparticles tends to lower values.