T3A.64.pdf
Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014
Influence of solvent on the optical and structural properties of Germanium nanoparticles synthesized by nanosecond laser ablation in liquids V. Saikiran, R. Kuladeep, Mudasir H. Dar and D. Narayana Rao* School of Physics, University of Hyderabad, Hyderabad 500046, India *e-mail:
[email protected] or
[email protected] Abstract Germanium nanoparticles are synthesized by using laser ablation of bulk Germanium in different liquids such as water, acetone, toluene and chloroform. Influence of solvent on the optical and structural properties of thus synthesized Germanium nanoparticles are discussed. Introduction Germanium nanoparticles (NPs) or quantum dots is a promising area of current research. A broad interest in this system is stimulated by its potential applications in light emitting devices [1], nonvolatile memory devices [2], and bioimaging [3]. The important difficulty on the way to a wide use of Ge NPs in different applications is the lack of simple and inexpensive method of their production. Besides, strict requirements are imposed for the NPs used in living systems, the chemical purity of the product is important, a requirement which is difficult to fulfill when the most commonly used chemical methods of synthesis are applied. Synthesis of germanium NPs in a simple, cheap and green procedure with sizes tuned to desired optical and electronic properties can be a multifaceted task. One relatively simple, inexpensive and green synthesis method for synthesizing nanoparticles of desired sizes at room temperatures is pulsed laser ablation in liquids (PLAL) from bulk targets. Laser ablation in liquids has the advantage that it can be done at room temperatures and control on the properties of the nanoparticles can be achieved by varying the liquid properties and laser ablation parameters. This technique is therefore considered effective for the synthesis of colloidal solutions of nanoparticles. Overall it is a fast, cheap and clean (green) method of preparation of a variety of nanoparticles. Here we report the structural and optical properties of Ge NPs synthesized in different liquids using nanosecond laser ablation. Experimental Details The NPs have been synthesized by nanosecond laser ablation and the detailed procedure followed was discussed in our earlier reports [4, 5]. In brief, the germanium target was placed at the bottom of the beaker filled with 10 ml of the liquid and then ablated with 10 Hz repetition rate nanosecond laser pulses of 6 ns pulse width at 1064 nm fundamental wavelength. During the laser ablation, the target was moved using a rotation system to achieve more and uniform irradiation of germanium surface. After 30 min irradiation, the solution became light brown in color which indicates the formation of Ge NPs. The ablated solution was then used to study the optical and structural properties by using micro Raman spectroscopy and TEM respectively. Results and Discussion Micro-Raman spectroscopy measurements were performed on all the samples synthesized in different liquids. We recorded the Raman spectra of Ge NPs by evaporating NP solution on a cover glass slide and the observed spectra for different liquids are shown in Fig 1. When we compare the spectra of Ge NPs in different liquids with that of the bulk crystalline Ge-Ge vibrational peak at 300 cm-1 peak (symmetrical at the position), we have observed that the Ge NPs peak shows increase in FWHM and a shift towards lower wavenumber side along with asymmetry. This is a consequence of formation of various size Ge NPs due to quantum confinement effects and also because of the changes in the structural characteristics of the Ge NPs which include strain, amorphous nature of Ge NPs. The broadening and shift towards the lower wavenumber side can be related to the nanoparticle size using the phonon confinement model [6].
T3A.64.pdf
Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014
Fig 1: Micro Raman spectra of Ge NPs formed in different liquids and the Raman spectra of NPs formed in toluene. No peak is observed around 440 cm-1 in all the samples which indicates that GeO/GeO2 phase has not been formed in the NPs. Only major peak corresponding to Ge-Ge optical phonon mode at 300 cm-1 with lower shift and broadening is observed in the Raman spectra of the NPs in acetone and water, whereas for the NPs in toluene and chloroform we have seen a broad peak at 260 cm-1 which indicates the amorphous nature of the NPs. Apart from the Ge-Ge vibrational peak we have also seen graphitic carbon related peaks (Fig 1) for the NPs in toluene. This is because of the formation of graphitic carbon layer due the laser irradiation of liquid (toluene). Thus pure crystalline Ge NPs have been observed in acetone and water and amorphous NPs with carbon network are observed in toluene and chloroform.
Fig 2: TEM images of Ge NPs obtained by laser ablation of Ge wafer in acetone, water, toluene and chloroform.
Fig 3: High resolution TEM image of Ge nanoparticles in acetone and the high resolution image of the graphitic layer formed in toluene.
T3A.64.pdf
Photonics 2014: 12th International Conference on Fiber Optics and Photonics © OSA 2014
Structural characterization of the synthesized Ge NPs was carried out using TEM imaging to understand the dispersion, average size of the Ge NPs in the solvent. Fig 2 show the TEM images of Ge NPs formed in different liquids. We have observed that the Ge nanoparticles are well dispersed in acetone and water without any agglomeration. These NPs are observed to be crystalline in nature. The NPs formed in solvents toluene and chloroform show different results. The NPs were observed to be formed over the edges of a carbon sheet/network. We have also observed the formation of amorphous graphitic sheets in the solvent. Fig 3 shows the high resolution images of the Ge NPs in acetone and the image of the graphitic carbon sheet formed in toluene. It was observed that the Ge NPs formed in the acetone and water are crystalline. The NPs formed in toluene and chloroform show amorphous nature. Conclusions We have synthesized Ge NPs in different liquids and studied the influence of solvent on the optical and structural properties of thus synthesized Ge NPs. The mean size of the Ge NPs is found to be less than the Bohrexciton diameter of the Ge which results into the quantum confinement effects in the NPs. We have explored the effect of solvent on the structural and optical properties of the Ge NPs in the colloidal solution. In acetone and water the Ge NPs are formed without agglomeration and exhibit good dispersion whereas in toluene and chloroform the NPs are either formed along with a graphitic carbon network or they are embedded inside graphite layer. These NPs are observed to be agglomerated. HRTEM measurements reveal that the NPs formed in water and acetone are highly crystalline whereas in toluene and chloroform they are partially crystalline and amorphous in nature. References 1. 2. 3. 4. 5. 6.
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