Synthesis of colloidal silicon and germanium nanoparticles by laser ablation of solid Si and Ge targets in ethanol

Colloidal nanoparticles exhibit unique optical, electronic and magnetic properties which make them ideal candidates as novel tools for applications in several fields of science [1–3]. The interest in semiconductor nanoparticles is primarily because of the differences in electronic properties they exhibit from bulk material due to their small size. Nanoparticles are zero-dimensional materials, so their electronic structure may transform from indirect to direct band gap, which makes possible the emission of light by indirect gap semiconductors. Their band gap also increases due to the quantum confinement effect [4]. In recent years, Si and Ge nanoparticles (Si NPs and Ge NPs) have attracted much attention because of their possible integration with traditional Si transistors [5]. Synthesis of semiconductor nanoparticles in liquid environment has been achieved using techniques such as wet chemical methods, sol-gel and solvothermal [6–8]. Chemical synthesis may produce toxic residues from the precursors. Alternative green methods are being investigated in order to overcome the health and environmental issues regarding the production of nanomaterials [9, 10].

Laser ablation of solids in liquids has proven to be an excellent candidate process to synthesize nanoparticles [11–14]. By using this method, no toxic residues are produced; besides it allows the control of size and morphology of the particles and thus, the physical properties can be modified [15, 16]. Most of the literature that reports small nanoparticles refer the use of femtosecond pulses [13, 14, 17]; meanwhile, the use of nanosecond pulses is correlated with long ablation times and high fluences [12, 18]. In this work silicon and germanium nanoparticles were obtained by laser ablation in ethanol using nanosecond pulses, low fluence and short ablation time.

Silicon and germanium nanoparticles were synthesized by laser ablation of solids in liquids. For the synthesis a pulsed Nd:YAG laser with maximum output power of 800 mJ per pulse at 1064 nm, a pulse width and frequency of 5 ns and 10 Hz, respectively was used. Si or Ge pellets immersed in a glass vessel with 7 ml of ethanol, were irradiated during 1 min using a constant fluence of 0.86 J cm⁻². The samples were structurally characterized by Raman spectroscopy using a Labram Dilor micro Raman system employing a HeNe laser as excitation with the 632 nm line, and transmission electron microscopy using a Jeol JEM 2010 microscope. The accelerating voltage was 200 kV. Optical properties were characterized by UV–vis spectroscopy. These measurements were carried out in a Perkin Elmer Lambda 25 spectrophotometer.

Figure 1 shows the Raman spectra of the colloidal Si (black) and Ge (red) nanoparticles. As it can be seen, the Raman spectrum for the Si sample consists in a broad band centered at 478 cm⁻¹. This signal corresponds to amorphous Si [19]. A small peak can be observed at 519 cm⁻¹ which corresponds to Si–Si mode of nanocrystals, the shift and broadening are effect of size reduction [19, 20]. Semaltianos et al [13] reported a shift on the TO Raman mode of Si attributed to phonon confinement, they also found that a band centered at 490 cm⁻¹ can be associated to amorphous Si. The difference between the values reported in the present work and reference [13] can be produced by the pulse duration, they used femtosecond pulses which is known to produce different ablation processes than nanosecond lasers, in which thermal effects are predominant. In a different set of experiments, we have found that modification of the fluence, in either direction, helps to eliminate the amorphous silicon Raman signal. Thus, solutions with Si nanocrystals can be produced without the presence of amorphous Si. For the Raman spectrum of Ge sample, a peak centered at 287 cm⁻¹ can be noticed; this signal corresponds the Ge–Ge vibrations for nanocrystalline germanium, as indicated by the existence of a shift to lower frequencies with respect to the bulk signal (300 cm⁻¹). As it was mentioned above, a shift in the vibrational mode indicates the presence of nanocrystalline materials [21], thus, the ablation process for the case of germanium produced colloidal crystalline Ge nanoparticles without the presence of amorphous material. According to Raman results, the Si particles solution resulted in a mixture of nanocrystalline and amorphous Si, meanwhile the Ge solution consists only in colloidal germanium nanocrystals.

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The crystallization mechanism of the semiconductor nanocrystals involves the extreme thermodynamic conditions occurring during the ablation process [22]. Irradiation with high energy nanosecond pulses of a target produces the melting and evaporation of the surface, leading to a plasma expansion into the liquid, the pressure and temperature produced in the process are high enough to modify the structure of the particles; also, thermal quenching plays and important role on the crystalline structure of the obtained nanomaterial.

To calculate the size of the Si nanocrystals, two theoretical models that describe the size dependence on the shift of the phonon peak were used; the bond polarizability model (BPM) [19], and the modified one-phonon confinement model (PCM) [20]. BPM describes the polarizability of a nanocrystal system calculated by the sum of the contributions of each bond for a particular size. PCM describes the Raman spectrum of a nanocrystal, as a function of its crystal momentum, phonon frequency, phonon dispersion, and the degree of confinement. The degree of the phonon confinement (the smaller the size, the stronger the confinement) influences the width and position of the phonon peak. Figure 2 shows the Normalized deconvoluted Raman spectrum for the Si nanocrystals from 510 to 530 cm⁻¹. A cumulative fit was used to calculate the size of the Si NCs. With the BPM, it was found that for the Raman shift Δω of −1.2 cm⁻¹, the size of the nanocrystal to be about 5.88 nm. And for the PCM, a mean size of 5.2 nm was calculated. The inset of figure 2 shows the plot for the function used to represent the size distribution of the nanocrystals for the PCM. The size of Si nanoparticles was estimated to confirm that the small signal centered at 519 cm⁻¹ indeed was associated to phonon vibrations of silicon nanocrystals.

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In order to confirm the results regarding the crystal size, transmission electron micrographs were recorded. Figures 3(a) and (b) show the TEM images of Si and Ge nanoparticles solution, respectively. The insets show selected area electron diffraction patterns were the crystalline structure of the particles can be observed. For the case of Si (figure 3(a)) a distribution of particles ranging from 5 to 20 nm approximately can be observed. The size, shape and distribution of colloidal nanoparticles synthesized by laser ablation depend on the confining media and the laser energy, wavelength, etc. Liu et al [12] reported the synthesis of 40–60 nm Si nanoparticles produced by a 10 ns laser pulse with a wavelength of 532 nm, 2 h ablation time and a 120 V applied potential, with a wide distribution of sizes and the presence of microparticles. For the present work, no microparticles were observed. The inset in figure 3(a) shows the presence of well-defined diffraction rings together with a diffuse background, which confirms the presence of a mixture of nanocrystalline together with amorphous silicon, as observed by Raman spectroscopy.

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In figure 3(b) it can be observed that the size distribution is narrower than for the case of Si. Ge Nanoparticles with diameter sizes ranging approximately from 10 to 15 nm can be observed. It could be possible that the lower melting point of Ge (938 °C) favors the production of near monodisperse crystalline nanoparticles. The inset of figure 3(b)) showing the selected area electron diffraction pattern, is composed of diffraction spots with rings, which indicates crystalline material with some preferred orientations. The better crystalline quality of Ge nanoparticles confirms the observed behavior of Raman spectrum for Ge.

Figure 4 shows the UV–vis transmittance spectra for Si (black continuous line) and Ge (red dashed line) nanoparticles. As it can be seen for Ge, absorption starts to become important from 900 nm, allowing a transmission of 50% for wavelengths above this value. For the Si case, optical transmission reach 45% for wavelengths above 1000 nm. The inset on figure 4 shows a Tauc plot of (O.D.* hν)^1/2 versus hν, an extrapolation of the linear part of the plots gives the band gap energy values for Si and Ge nanoparticles. As it can be seen, for both cases the band gap is above the bulk value [5], which means that quantum confinement effect is occurring due to size reduction. No absorption edge for amorphous nanoparticles was observable, since they absorb on shorter wavelengths [23]. The band gap widening confirms the results observed by Raman spectroscopy and transmission electron micrographs, where existence of Si and Ge nanoparticles was revealed.

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Colloidal Si and Ge nanocrystals were synthesized by low fluence laser ablation of solid Si and Ge in ethanol. Structural analysis revealed that for the case of Si, a mixture of amorphous and crystalline material was obtained while for Ge pure crystalline material was observed. The Si nanocrystals resulted smaller than Ge due to the difference in thermodynamic properties. The band gap of the nanocrystals was found to be larger than the band gap for the bulk material as an effect of quantum confinement.

The authors acknowledge the technical assistance of Alejandra García and Marcela Guerrero.