Synthesis of blue luminescent nanoparticles based on sparked silicon

ScriptahIaterialia, Vol. 36, No. 5, pp. 503-508,1997 El.vevierScience Ltd Copyri&t 0 1997 Acta MetallurgicaInc. Printedin the USA. All rights reserved 1359-6462l97 $17.00 + .OO

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SYNTIKESIS OF BLUE LUMINESCENT NANOPARTICLES BASED ON SPARKED SILICON M. Hinojosa-Rivera’, U. Ortiz-MCndez’, C. Vhzquez-L6pe2, C. Falcony-Gmjardo2, 0. Alvarez-Fregoso3, J.G. Cab&as-Moreno and J. Ortiz-Lopez’ ‘Universidad Aut6noma de Nuevo Leon, Fat. de Ing. Me&mica y Electrica, DIMAT. A.P. 149-F, 66452 San Nicolas de 10s Garza, N.L. Mexico ‘Centro (deInvestigation y de Estudios Avanzados de1 I.P.N., Depto. de Fisica. A. P. 14-740, Mexico 07000, D.F. Mexico 3Universidad National Autonoma de Mexico, Instituto de Investigation de Materiales. 41nstituto Politecnico National. ESIQIE, Unidad Profesional Zacatenco Mexico 07738, D.F. Mexico ‘Institute Politecnico National. ESFM, Unidad Profesional Zacatenco Mexico 07738, D.F. Mexico (Received December 21, 1995) (Accepted September 9, 1996) Introduction Spark-processing is a new and useful alternative to prepare porous silicon. (1) In this process the affected area is formed by craters surrounded by microglobular luminescent regions. At present this is a very active area due to some potentially desirable features compared to other methods, such as electrochemical etching techniques, that have been extensively used (2,3), plasma CVD preparations (4), or focused ion beam implantation (5). Spark erosion has also been used as a low cost method to produce microparticles of TiC (6), Nd-Fe-B (7), ceramics (7), etc. The aim of this work is to report on the silicon-based luminescent particle formation in ambient air, using a modified spark process method (6): two pieces of silicon wafers were used as electrodes and a substrate situated 1 to 2 mm from the sparks. The particles are deposited with some adherence, which is convenient for the purposes of the present work. The phases, crystal structure, and composition of the particles made by spark erosion are reported. The light emission properties are correlated with the nanostructural nature of the silicon component of the particles. Experimental Procedures A tmipolar spark generator with a repetition rate of 20 Hz and voltages of 30,000 volts was used for the spark-processing of the samples studied. The high electric field at the edges of the electrodes results in the ionization of the adjacent gas molecules. The ions are then accelerated from one electrode

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to the other and transfer momentum to the material, resulting in localized vaporization. Then the condensation of spherical particles is observed during the off period of the sparks. Some of these particles fall on the (100) face of a crystalline silicon wafer located under the spark area which serves as substrate. This substrate is convenient for x-ray diffraction analysis and for infrared transmission spectroscopy in order to isolate the contribution of the deposited particles. Particle morphology was determined with a Nikon Epiphot TME metallographic microscope, a Jeol JSM-35 CF scanning electron microscope (SEM) and a Jeol JEM-2000 EX transmission electron microscope (TEM). For the TEM analysis a copper grid covered with a formvar thin film was used as substrate. In order to determine the types of reaction products after spark erosion, x-ray diffraction analysis and transmission infrared spectroscopy were performed. The x-ray diffraction was determined with a Siemens D-5000 diffractometer, using an operating voltage of 30 kV, and Cu Ka radiation (h = 1.541 A). The IR optical transmission was determined with a Nicolet Magna-IR 750 Spectrometer. The luminescence was obtained with a Perkin Elmer LSSOB Luminescence Spectrometer, using as excitation a wavelength of 2700 A . Results and Discussion The samples at simple sight have a cloudy aspect. The larger particles can be distinguished with an optical microscope. A 200x image is shown in Fig. 1. A typical scanning electron microscopy image of the spark processed particles is shown in Fig. 2. Particle dimensions range from several microns to sizes less than 100 nm distributed in two ranges: some are observed with dimensions in the range of 5 to 20 pm and others in the range of 5 to 50 mn. Transmission electron microscopy results indicate that the larger particles are cumulus of weakly bonded nanostructured particles. These large particles are probably ejected from the globular structures formed at the Si electrodes in the spark process and eventually reach the substrate. In Fig. 3 a TEM image of nanoparticles originated by the deagglomeration of a 1 pm particle as a result of the exposure

Figure 1. Optical micrograph showing the larger particles obtained in the spark process.

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Figure 2. Scanningelectronmicrographshowing differentparticlesizes.

to the focused electron beam of the TEM itself is shown. The larger particles are observed as black spots consisting in aggregated spherical nanostructured regions weakly bonded. In Fig. 4 a typical photoluminescence spectrum is shown. An intense high energy peak is centered at 328 mn, and a lower peak at 570 nm. In the following discussion, we consider four possible models for the blue emission from spark-processed Si: emission from the oxide, emission due to surface states, emission from contamination, and band to band recombination in silicon nanocrystals. Stoichiometric SiOZ has a band gap of 9 eV (8). This is the reason for the absence of luminescence in electronic-grade thermal oxides. However, in nonstoichiometric SiO, the band gap is smaller (Eg 2 3-4 eV for x = 1.4-l .6) (9) and then luminescence can be observed under near uv excitation. In SiO,, the quantity 12- x is reflected in the infrared spectra as low energy shifts corresponding to Si-0-Si vibration modes compared to those of SiOz. As we will discuss later, in our samples the features obtained are associated only with stoichiometric SiOZ;so the first model is not applicable to our samples. The second model implies that the emission has a surface state origin (10). It may involve states produced by Si-0 bonds at the surface of the nanocrystals or the interface between the silicon nanocrystals and the silicon dioxide (11). In this case the luminescence maximum would not depend appreciably on the parameters of sample preparation, in contradiction with the fact that the spectral position of the luminescence depends on the temperature substrate as has been observed by us and pointed out by R. E. Hummel and co-workers (12). The third model implies the participation of contaminants. This could be an effect similar to that of the blue luminescence that appears after the storage in air of oxidized light-emitting porous silicon (13). In order to verify this possibility, we have made samples in a controlled atmosphere of Ar. Without exposing the sample to the atmosphere we determined that optical emission is also present at a slight lower energy. So the contaminants do not contribute appreciably to the PL spectrum. The quantum confinement model was proposed for the case of electrochemically prepared porous silicon by Canham (2) and by Lehmann and Gosele (14). The fact that emission from spark-processed silicon is affected by the substrate temperature-among other parameters-suggests that the observed PL is originated by a quantum confinement effect (12). We can follow the arguments of M.H. Ludwig

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Figure 3. Transmission electron micrographshowing the deagglomerationof a large particle after focusing the TEM electron beam.

and co-workers (15). If this emission is originated by an opening of the band gap due to a quantum size effect, an estimate of the average diameter of the confinement region can be obtained using the effective mass approximation (16, 17). In this model the ground state for an excitonic transition can be described as a function of the confmement size diameter d, by:

1.4 -

1.2 -

si 1.0 -

1intund rtarldmdl

511111)

0.8 -

0.6 . 0.4

SiRItCt)

0.2 0.0 i 0.21

’

’ 4m

’ SC0

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* mo

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2

WAVELENGTH (nm)

Figure 4. Room temperaturephotoluminescence spectrmn of sparkprocessed obtmed nanoparticles.

Figure 5. X-ray diion ited on Si (100).

patternof nanoparticles depos-

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E(d) = Eg + (h*/2&)(l/nq* + Urni,*) - 3.572 e2/Ed,

(1)

where Eg is the optical bandgap energy of the bulk (which in this case it is assumed to correspond to Si), me* and rnh* are the effective masses of the electrons and holes, respectively, and E is the relative dielectric constant of the material. Eq. (1) is written in c.g.s. units. The second term of the right hand side represents the confinement energy of electrons and holes, while the third term is the coulombic interaction. The result of the calculation for the grain size is: d = 14 A. For this calculation the position of the high energy peak of the photoluminescence (E(d) = 3.8 eV) was used. The material parameters for bulk Si used are: m* = 0.43m,; mh* = 0.537 m,; E = 11.9; Eg = 1.13 eV. In Fig. 5 an x-ray diffraction pattern corresponding to the spark-processed particles is shown. This spectrum is analyzed using the Si (100) as internal standard, since the particles are adhering to a Si (100) wafer substrate. The finite size of the particles causes a broadening of the diffraction lines according to the Debye-Scherrer formula. From Fig. 5, some particles have a clear composition of crystalline Si, with (100) preferential orientation following that of the substrate. The calculation of the crystalline grain size in these particles results in d = 14 + 2 A. Some particles have also cristalline Si with (111) preferential orientation with a grain size of 82 f 2 A. The corresponding lattice parameter of these nanostructured regions is 3.108 A, which means a contraction of 0.0275 A compared to that of bulk Si single crystals.(l8) This is an effect induced by surface strain, as has been observed in nanostructures of CdS (19). Also in this spectrum a peak corresponding to SiO2 crystalline clusters is observed. These clusters are oriented in the (101) direction (20) and have a grain size of 90 + 2 A. The IR transmission spectrum in Fig. 6 presents absorption peaks that indicate the presence of SiO2 and C - H bonds with Si as well as Si - H bonds. In particular, the 446,800, and 1075 cm-’ peaks are associated with the bending, rocking and stretching vibration modes of the Si - 0 - Si bonds in SiO2 (21). Si - CH2 vibration modes are also observed at 937 cm-’ and 1416 cm-’ associated with rocking or wagging mod.es and with scissor or bending modes, respectively (22). Also peaks associated with vibration modes of CH2 by itself are present at 1346 (wagging), and at 2890 (asymmetric stretching). Also a small peak at 2007 cm-’ usually associated to Si - H stretching vibration mode in an amorphous Si matrix is present. There is also a broad peak at 3223 cm“ that is likely to be associated with adsorbed water or OH bonds. From these results it is clear that an oxidation of the Si particles generated during the spark is occurring as well as some incorporation of C - H probably from the carbon present as CO2 in the atmosphere. However, the small peak associated with the stretching vibration mode of the S - H bond at

Figure 6. Infiared transmission spectrum for nanoparticles deposited on Si. This spectrum was obtained after removing the backgroundcornzspondingto a non-exposed region of the substrate.

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2007 cm-’ indicates that small particles of Si remain without reacting. This peak tends to shift towards 2270 cm-’ if it is embedded in an SiOz matrix. It is possible that the 0 - H absorption peak is related to water adsorption after the formation of the particles.

Conclusions We have produced nanoparticles of Si and SiOz composition by spark-processing single crystalline Si in ambient air. The silicon nanostructures show luminescence in the blue region of the visible spectrum, Considering this emission as a quantum confinement effect and using the effective mass approximation we fmd that the average diameter of silicon nanocrystals is of 14 A. Consistent with these observations, x-ray diffiction determines the presence of a bimodal distribution of nanocrystals of Si: particles of grain size of 14 A oriented in the (100) direction and particles of crystalline regions with grain size of 82 A oriented in the (111) direction. The presence of nanostructured SiOa was also determined. By IR transmission spectroscopy the chemical composition of the samples was complemented, resulting in the presence of stoichiometric SiOl , non-reactive Si and some contaminants. Acknowledgements This work was partially supported by Consejo National de Ciencia y Tecnologia (CONACyT, Mexico). Two of the authors (J.G.C.M. and J.O.L.) acknowledge COFAA-IPN fellowships. The technical assistance of M. Guerrero, Blanca Zendejas and Rogelio Fragoso is also acknowledged. The assistance of Unidad de Microscopia Electronica de1 CMVESTAV is also acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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