Introduction

Laser ablation in liquids is an alternative to more common chemical methods for the fabrication of nanoparticles. This technique has been used to make a wide range of elemental and compound materials such as metals (e.g., Ag, NiFe) (Musaev et al. 2009; Tsuji et al. 2004), semiconductors (e.g., InP, ZnSe) (Anikin et al. 2002; Musaev et al. 2008b), and dielectrics (hydroxyapatite) (Musaev et al. 2008a). For laser pulse durations of the order of nanoseconds, sufficiently high fluences, and target materials with high absorption coefficients, explosive boiling is the accepted model of ablation (Miotello and Kelly 1995). In comparison with ablation in vacuum, ablation in liquids occurs in confinement and results in higher temperatures and pressures. The advantages of ablation in liquids for nanoparticle fabrication are its simplicity, the absence of precursors, and the use of a liquid to contain the nanoparticles.

Chemical methods have not been used to synthesize AuGe nanoparticles but have been used to make clusters. A structure composed of two Ge clusters connected to one another by a bridge of three Au atoms was synthesized recently, and other publications have reported synthesis of similar clusters (Li and Su 2009; Spiekermann et al. 2007a, b). Nanodrops of liquid Au72Ge28 were formed using a zepto-liter pipette (Sutter and Sutter 2007). The nanodrops crystallized to form homogeneous AuGe nanoparticles with Ge content close to the eutectic composition. This method produces individual nanoparticles but cannot be scaled to higher yields. Recently, AuGe nanowires with diameters of about 100 nm were fabricated (Chueh et al. 2010). Germanium nanowires were first formed using the vapor–liquid–solid process, an Au layer was then sputtered onto the wires which were capped with HfO2 and subsequently annealed. In order to determine if laser ablation in liquids will be effective for AuGe nanoparticle production, we have studied UV laser ablation of a AuGe target in water.

Gold nanoparticles have been fabricated by pulsed laser deposition in vacuum (Domingo et al. 2007; Donelly et al. 2007). There have also been several studies of laser ablation of gold in water (Kabashin et al. 2003; Kneipp et al. 2008; Mafune et al. 2001, 2003; Park et al. 2010; Sylvestre et al. 2005; Tarasenko et al. 2006). These studies used a variety of pulse durations (~10 ns and ~100 fs) and different wavelengths (1064, 780, 532, and 266 nm). In general, spherical particles with sizes in the range 1–100 nm were formed. Particle aggregation was observed and steps were taken to minimize aggregation. For example, the thickness of the water above the gold target affects the degree of particle aggregation (Kneipp et al. 2008).

Chainlike nanostructures with high aspect ratio were formed by laser irradiation of the surface of a solution containing gold nanoparticles (Mafune et al. 2003). The size of the irradiated area of the stirred solution was about 2 mm and nanoparticles formed chainlike structures (Mafune et al. 2003). Gold “nanopeanuts” and “nanowires” were obtained under similar conditions by laser irradiation of a solution of citrate-capped gold nanoparticles (Park et al. 2010).

To complement the previous work, and compare the results of ablation products from AuGe with those from Au, we also ablated Au in water.

For completeness, we have performed similar studies using pure Ge targets to fabricate Ge nanoparticles. Synthesis of Ge nanoparticles in solution was performed via chemical means (Ma et al. 2008) and recently Ge nanoparticle fabrication by laser ablation in liquids was reported (Jiang et al. 2011).

Experimental

UV radiation (337 nm) from a nitrogen pulsed laser (TEM10) with pulse duration of ~10 ns, was focused by a fused silica lens onto the surface of a AuGe eutectic alloy (26.9 at.% Ge), Au or Ge target immersed in distilled water. The resulting fluence at the surface of the target was ~50 J/cm2. The ablated materials were deposited onto carbon film supports for high-resolution transmission electron microscopy (HRTEM) investigations. The morphology of the deposited nanostructures was investigated in a JEOL 2100F and an FEI 80-300 Titan (with Cs-corrector) transmission electron microscopes, both equipped with energy dispersive spectroscopy (EDS) detectors. The microscopes were used to obtain the images and electron diffraction patterns for all of the samples. EDS measurements were carried out in scanning TEM mode with a beam size of 0.2 nm.

Results and discussion

HRTEM images of the nanostructures formed in the ablation process from the eutectic AuGe alloy are shown in Figs. 1 and 2. The images show the nanoparticles with chainlike morphology. The particles have polycrystalline structure and single crystalline segments can be as long as 10 nm. Sintering model is supported by experiments on formation of sintering of Au nanoparticles in a furnace in vacuum that result in chainlike structures (Magnusson et al. 1999) but with lower aspect ratio than for AuGe nanoparticles shown in Figs. 1 and 2. Change of the properties of a target by using low melting temperature eutectic alloy results in substantial change of morphology of resulting AuGe nanoparticles. Though the sintering of previously formed nanoparticles in the irradiated area in solution is possible, its contribution is insignificant since the laser spot area is about 100 μm, the solution is not stirred and the total time of irradiation is about 10 min.

Fig. 1
figure 1

HRTEM image of a AuGe nanowire formed by laser ablation in water

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Fig. 2
figure 2

a HRTEM image of AuGe nanoparticles formed by laser ablation in water. The ‘+’ denotes the location that EDS was performed. b The EDS spectrum from figure a, which shows that the nanoparticle has 6.4 at.% Ge. The unlabeled peaks are from the support. c HRTEM image of AuGe nanoparticles formed by laser ablation in water. The ‘+’ denotes the location that EDS was performed. d The EDS spectrum from c, which shows that the nanoparticle has 4.5 at.% Ge. The unlabeled peaks are from the support

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The nanowires and elongated rods are polycrystalline and appear to form by attachment of nanoparticles. In some cases, the individual single crystalline segments can be as long as 10 nm. EDS spectra collected at different positions along the nanowires invariably show small but well-distinguishable Ge peaks (Fig. 2) indicating that the nanowires contain Ge. The Ge content is evaluated to be 4–6 at.%. This amount is substantially lower than the Ge content (26.9 at.%) of the target material. However, the partial pressure of germanium is around three orders of magnitude higher than the partial pressure of gold, and thus more Ge than Au evaporates during ablation. This is the probable cause for the resulting higher Au content in the nanowires in comparison with the target material. The AuGe nanowires formed by laser ablation have a higher Ge content than the solid solubility of Ge in Au, which is below 0.1 at.% for the thermodynamically stable α-AuGe phase (Okamoto and Massalski 1984). However, in Au–Ge rapidly cooled alloys, the α-AuGe phase was found to contain up to 8 at.% Ge. Laser ablation is a non-equilibrium process similar to the method in which high Ge content α-AuGe was produced.

Ablation in water causes oxidation of the resulting nanoparticles that depends on ablated material. Even oxidation of ablated gold was observed on the level of 3.3–6.6% of surface atoms (Muto et al. 2007). The EDS spectra in Fig. 2b, d show strong oxygen peaks at 0.5 keV, but they should be attributed to the copper grid and the contribution of AuGe is negligible. Electron diffraction of the AuGe nanowires shows that they crystallize in the Au fcc structure (α-AuGe phase) with a unit cell parameter of 4.1 Å (Fig. 3), close to 4.08 Å for pure gold as well as for bulk α-AuGe with a similar Ge content (Okamoto and Massalski 1984). It should be noted that for similar nanostructures formed from pure gold in solution by laser irradiation of nanoparticle solution values for lattice constant 4.08 Å as well as 4.10 Å in the different locations of the same nanostructure were observed (Park et al. 2010). The evaporated Ge may form clusters, however, they were not observed in this study.

Fig. 3
figure 3

Electron diffraction pattern from AuGe nanoparticles, taken with an accelerating voltage of 20 kV and a camera length of 2155.0 mm. The rings identify the d-spacings. From the inside outwards, they are (111), (200), (220), (311), and (222)

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To compare the results from ablation of the AuGe alloy with its constituent elements, ablation of Ge and Au was performed under identical conditions. In Fig. 4, an HRTEM image of typical nanostructures formed during the ablation of Au is shown. Au ablation results in spherical and oblong particles, with the aspect ratio for pure Au particles substantially lower than for AuGe chains. Electron diffraction was performed and a unit cell parameter of 4.0 Å was observed, comparable to what has been previously reported (Mays et al. 1968). Ablation of Ge results in a round-shaped particles that form aggregates in solution. The size of particles is in 20–40 nm range (Fig. 5).

Fig. 4
figure 4

HRTEM image of Au nanoparticles formed by laser ablation in water

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Fig. 5
figure 5

HRTEM image of Ge nanoparticles formed by laser ablation in water

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Chainlike nanoparticles of AuGe alloy shown in Figs. 1 and 2 are of interest for the following reasons. Doping of nanoparticles is a challenging problem (Erwin et al. 2005). Using a doped or alloyed target for laser ablation can be a way to bypass this problem to form doped or alloy nanoparticles. Other reason is that control of shapes of nanoparticle agglomerates by using alloy targets for laser ablation can be useful for many applications such as catalysis (Koenigsman et al. 2010) and improvement of properties of composite materials (Rong et al. 2004).

Chainlike morphology of nanoparticle structures has also been observed for particles formed in a furnace in vacuum (Magnusson et al. 1999; Kim et al. 2009). To gain an understanding of the mechanisms influencing the shapes of the ablated particles, aerosol formation models can be applied to ablation (Aricidiacono et al. 2004; Ehrman et al. 1998; Lehtinen and Zachariah 2001). The formation of chains can be explained by random collisions of primary particles that were formed in explosive boiling, which adhere to each other during plume expansion. These can form different types of particles ranging from big round particles to long chains of primary particles, which may be of interest because shape control of noble metal nanoparticles has relevance for catalysis (Koenigsman et al. 2010).

For aerosol particle formation, the exact morphology of the nanoparticle aggregates is determined by the interplay of the following parameters: coalescence time, inter-collision time, quench time, surface tension and viscosity temperature dependence, and melting temperature (Ehrman et al. 1998). For the case of laser ablation of AuGe, additional parameters such as dependence on concentration of Ge in gold, time and space dependence of temperature during the super-boiling process and subsequent expansion of the plume should be considered.

Summary

The ablation in water of eutectic AuGe and pure Au results in fabrication of nanostructures, i.e., a mixture of nanowires and nanoparticles, while the ablation of Ge results in highly aggregated nanoparticles. For the case of AuGe, nanowires with a high aspect ratio and Ge content of ~4–6 at.% form. Because of the high partial pressure of Ge, the particles formed through ablation of AuGe, have a composition that is depleted in Ge, as compared to the target material.