Trapping High-Pressure Nanophase of Ge upon Laser Ablation in Liquid P Liu, Y. L. Cao, X. Y. Chen, and G. W. Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, School of Physics Science & Engineering, Zhongshan UniVersity, Guangzhou 510275, P. R. China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1390–1393
ReceiVed June 17, 2008; ReVised Manuscript ReceiVed NoVember 25, 2008
ABSTRACT: The high-pressure nanophase, that is, the metastable tetragonal structure, of germanium is trapped by a facile technique named electrical-field assisted pulsed laser ablation in liquid at ambient pressure and temperature. On the basis of X-ray diffraction, transmission electron microscopy, and Raman scattering analyses, the trapped Ge nanophase is identified to be the tetragonal structure rather than the diamond structure of bulk germanium. First-principles calculations are used to clarify the physical and chemical mechanisms of the tetragonal Ge formation upon laser ablation in liquid.
1. Introduction Nanocrystals of Ge, as well as Si, have attracted significant attention for many years due to the discovery of their various quantum phenomena, especially the emitting of visible photoluminescence (PL) in nanostructures,1 and their possible integration with traditional Si transistors.2 Moreover, compared with Si nanocrystals,3,4 Ge nanocrystals may be more interesting for device applications. For instance, Ge nanostructures had been trusted to be a material with a smaller band gap and have exhibited the stronger quantum confinement effect, which makes it more suitable than Si for photovoltaic applications.5,6 Note that an unusual metastable phase of the tetragonal structure, the so-called ST-12 structure, was discovered by compressing the cubic Ge at the pressures exceeding 12 GPa in 1965.7 Interestingly, some theoretical studies have predicted the ST-12 structure of Ge to be a semiconductor with a direct gap of 1.47 eV,8 which implied that this metastable phase of Ge seems most attractive for device applications. It is, thus, of great interest to trap this metastable phase at ambient conditions. Although the ST-12 structure of Ge is a high-pressure phase, some theoretical and experimental investigations have indicated that the nanophase of the metastable structure may be reserved at ambient conditions.8-10 In this contribution, we report that the highpressure nanophase, that is, the metastable tetragonal structure, of germanium is trapped by a facile technique named electricalfield assisted pulsed-laser ablation in liquid at ambient pressure and temperature.
2. Experimental Procedures Similar to our previous works,11-14 in this case a single crystalline cubic Ge target with 99.99% purity, which can be used as the starting material, is first fixed on a stainless steel fitting that be hung upsidedown in a quartz chamber, and a volt d.c. of - 47 V is applied on it. A piece of single crystalline Si substrate is placed on the bottom of the chamber under the target, and a volt d.c. of + 47V is applied on it. The whole set is immersed in a pure toluene (purity >99.5%) environment, and the liquid toluene bath is maintained at roomtemperature. The distance between the Ge target and the substrate surface is about 5 mm. Then a second harmonic produced by a Q-switched Nd:YAG laser device, with a wavelength of 532 nm, pulse width of 10 ns, power density of 1010 W/cm2, and repetition frequency of 1 Hz, is induced and focused onto the target surface. Note that an optical limiting flake is employed between the original laser beam and * Corresponding author. E-mail:
[email protected].
quartz lens, which makes the laser finely focalized, and the diameter of the focus is estimated to be less than 0.5 mm. This restricted laser ablation leads to a quite slow but fine-particle deposition, and during this process the whole system is maintained at room temperature. After the interaction of laser and target for about 60 h, the Si substrates are taken out from the chamber, carefully washed with deionized water, and then desiccated at 50 °C in a vacuum oven, which should cause hardly any changes in the finally crystalline structure or states. Note that the thickness of deposition on the Si substrate is about several hundreds nanometers after 60 h of operation. Scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman scattering spectroscopy are employed to characterize the structure of the products.
3. Results and Discussion The spherical nanocrystals deposited on Si substrate are shown by the images of SEM in Figure 1a. One can see the diameters of most nanocrystals are in the range of 10-40 nm. The inset gives a size-distribution of these synthesized nanocrystals based on the SEM statistics, which shows the maximal probability of the nanocrystal diameters is about 20 nm. A corresponding XRD pattern is shown in Figure 1b. Compared with the XRD pattern of the Ge target (pattern ii), we can definitely see that there are seven new peaks, 19.6°, 24.8°, 29.62°, 32.86°, 36.24°, 45.28°, and 53.7°, in the XRD pattern i of sample, which are identified to the (101), (111), (102), (201) and (211) crystalline planes of the tetragonal Ge (T-Ge) (JCPDS card, No. 180549),7 while the two peaks located at 45.28° and 53.7° come from the (220) and (311) crystalline planes of the cubic Ge (JCPDS card, No. 894895). Therefore, the XRD analyses clearly exhibit that the T-Ge nanophase is prepared in our studies. Note that the XRD peaks of the cubic Ge in the XRD pattern of the as-prepared sample should be originated from the residual of the target. Thus, the T-Ge preponderates over the cubic phase in our as-prepared products. Since there are numbers of agglomeration of the nanoparticles intermixed in final products, they could be diffracted more strongly and easily hide the small nanocrystals signature. Moreover, our data processing of those weak signals may also induce the slim peaks present in final spectra construction, which thereby allows the broadening of our XRD spectrum to seem not obviously in the pattern. Figure 2a shows a low magnification TEM image of the prepared nanocrystals, with the corresponding select area electronic diffraction (SAED) pattern shown in the top left
10.1021/cg800633j CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
Trapping High-Pressure Nanophase of Ge
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Figure 1. (a) SEM image of the synthesized Ge nanocrystals; the inset is a size-distribution of the as-prepared sample. (b) XRD patterns of (i) the synthesized products, and (ii) the Ge target.
corner of Figure 2a, which clearly displays that the synthesized samples are single crystals with the tetragonal structure. On the basis of TEM detection, a more appropriate analysis of sizedistribution is obtained and also indicated in the inset of Figure 2a. This analysis considers the influence of the oxide layer that surrounds nanoparticles, and finally shows that the maximal probability of the nanocrystal core diameters should be about 17-18 nm. Meanwhile, an EDS spectrum within the measurement error of 2% is exhibited in Figure 2b, which clearly indicates that these nanocrystals are mainly composed of Ge (93%). Additionally, the O (4%) peak is supposed to come from the exterior oxide layer, which further implies that there is undoubtedly a thin oxide layer on the outer surface of the Ge nanoparicles, and the Cu, Cr, and C peaks are originated from the copper grid and amorphous carbon film support, respectively. Furthermore, the high-resolution TEM (HRTEM) analysis
Figure 2. (a) Bright-field TEM image of the synthesized products with a corresponding SAED pattern and a size-distribution analysis based on TEM is shown in the inset. (b) The corresponding EDS spectrum. (c) HRTEM image of a nanocrystal, and the inset shows a corresponding fast Fourier transform (FFT) analysis, which can be indexed to the (201) and (111) directions of the T-Ge structure.
(Figure 2c) shows that the two interplanar spacing of 0.357 and 0.272 nm, respectively, correspond to the crystallographic planes of (111) and (201) of the T-Ge structure quite well. In addition, an angle of Φ ) 69.2° between the two interplanar spacings is
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Figure 4. Total energy of T-Ge and C-Ge calculated by the firstprinciple calculations, in which an accurate full-potential, linearized, and augmented plane wave method is used, and the exchange and correlation effects are treated within the generalized gradient approximation (GGA).
Figure 3. Raman spectrum of (a) the prepared samples, and (b) the Ge target.
carefully detected. Using the crystallogram expression of the tetragonal structure as
(h1h2 + k1k2)/a2 + l1l2/c2 cos φ ) r*1r*2
(1)
we obtain the theoretic value Φcalc between the (111) and (201) crystallographic planes of the T-Ge structure to be 69.14°, which is in good agreement with the Φexp value above. Since the angle between the crystallographic planes is one and only in different crystal structures, the measured Φexp value further confirms that the prepared nanocrystals are indeed the tetragonal structure. Note that the nanocrystals with less than 30 nm in diameter are the tetragonal phase, and the others with more than 200 nm in diameter are the cubic phase in the TEM analysis. Figure 3 shows the two Raman scattering spectra from the prepared sample and the Ge target, respectively. Clearly, there are four peaks in the Raman spectrum of sample. The peak at 520.2 cm-1 undoubtedly originates from the silicon substrate, and a small peak located at 299.5 cm-1 is suggested to be the Raman peak of the residual cubic Ge, which is consistent with the XRD and TEM analyses. Kobliska et al.15 reported that the ST-12 structure of Ge has two Raman peaks at 246 ( 3 (Γ5) and 273 ( 3 (Γ3) cm-1, respectively. Accordingly, the two unusual peaks at 248 and 279 cm-1 in our case are suggested to be from the ST-12 structure of Ge. Considering the oxide component contained in the samples, they may just be a factor to cause the little red shift of the Raman peaks. Therefore, the Raman scattering analyses confirm that the ST-12 phase of Ge is trapped by laser ablation in liquid. It is well-known that laser ablation in liquid is a very fast and far from equilibrium process, in which many metastable phases forming in the synthesis process can be reserved in the final products.11 The initial process of laser ablation at liquid-solid interface is an interaction between laser and Ge target. Ge species having a large initial kinetic energy would form a dense region in the vicinity of the solid-liquid interface, called the laser-induced plasma plume. A shock wave will be created at supersonic velocity in front, which will induce extra
pressure in the plasma plume, called the laser-induced pressure, because the plasma is strongly confined in the liquid. Then, the laser-induced pressure induces a temperature increase in the plasma plume. Therefore, the plasma plume from laser ablation in the liquid is in the high-temperature, high-density, and highpressure state. For the utilization of organic intermedia,16,17 the pure toluene environment, which is used in our case, is appreciated to be a good transparence for the pulsed laser and stable under an electric-field. Moreover, the low dielectric constant (about 2.38) of toluene makes it easy to form a fine construction of an inner-electric-field, which should be the direct factor that acts on the reactive clusters. On the other hand, the applied electric-field on the target helps the plasma plume keep a high energy state; the electrical charge that is insulated over the nanoparticles could become a transitory energy protection of a metastable stage. Importantly, the high-temperature, highdensity, and high-pressuresstate in the plasma plume provides an advantageous thermodynamic environment for the nucleation of the high-pressure phase of Ge. Therefore, the nucleation of the ST-12 phase would take place when the plasma plume starts condensing. As a result of the liquid confinement, the growth time (the plasma quenching time) of the nuclei of the ST-12 phase is very short. Thus, the size of the grown crystals upon laser ablation in liquid is usually on the nanometer scale.18 Interestingly, the proposed growth mechanisms are consistent with the experimental evidence of the trapped high-pressure nanophase of Ge in our case. First-principles calculations are used to further clarify the physical mechanisms of the tetragonal Ge formation upon laser ablation in liquid. Figure 4 shows the total energies of the cubic and ST-12 phases of Ge through the first-principle calculations, which indicate that the difference between the equilibrium total energies of these two phases is a little bit large. Then, the ST12 phase forming in the plasma plume is possibly frozen in the final products in the far from equilibrium process of laser ablation in liquid due to the very short quenching time of the plasma plume.11,19 Therefore, our studies indicate that laser ablation in liquid can be expected to be a general strategy for trapping the metastable nanophase.
4. Conclusion In summary, the metastable tetragonal nanophase of Ge has been trapped at ambient conditions by laser ablation in liquid, and the physical and chemical mechanisms of the high-pressure
Trapping High-Pressure Nanophase of Ge
nanophase formation upon laser ablation in liquid were pursued experimentally and theoretically. Acknowledgment. The National Natural Science Foundation of China (50525206 and U0734004) and the Ministry of Education (106126) supported this work.
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