Elongation of metallic nanoparticles at the interface of silicon dioxide and silicon nitride

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Abstract

In the process of shape transformation of metal nanoparticles (MNPs) embedded in dielectrics resulting from swift heavy-ion irradiation, key parameters such as the energy deposition threshold, the necessity of a molten track and the NP size have been identified for amorphous silicon dioxide. The extension of such parameters to other dielectrics is yet unclear. We present experimental evidence of the shape transformation of nearly spherical NPs located at the interface of amorphous silicon nitride and silicon dioxide thin layers upon irradiation with 185 MeV Au ions at fluences of 0.3 and 1 × 1014 cm−2. After irradiation the ∼16–18 nm diameter Au and Ag NPs transformed into continuous nano-rods exhibiting a high aspect ratio with a clear preference of elongation into the silicon dioxide layer. The results are discussed in the context of Thermal Spike calculations, which indicate that the track formation timescales may have an important influence on the NP elongation process.

Introduction

Dielectric materials containing embedded metallic nanostructures with controlled anisotropy are of great interests due to their magnetic [1], and linear and nonlinear optical response arising from their plasmonics properties [2], [3], [4], [5], [6]. One method to fabricate such structures is by synthesis of nanoparticles in a dielectric thin film system followed by swift heavy-ion irradiation, which induces a shape transformation of the typically spherical nanoparticles to well-aligned nano-rods. This shape transformation process of MNPs has been studied predominantly with silicon dioxide as the host matrix [1], [7], [8], [9]; yet little work has been done in other materials with the exception of sapphire [6]. As the plasmonic response of these systems depends on the dielectric function of the host matrix [10], it is highly desired to extend the approach developed for silicon dioxide to other dielectric hosts.

It has previously been established that the energy deposited by swift heavy-ions can lead to the formation of so called “ion tracks”, which in silicon dioxide comprise of an underdense core surrounded by an overdense shell [11]. If the ion track and the NP dimensions are comparable, elongation can be achieved from single events [12] and appears to be related to the ion track dimensions [13], [14]. When the ion passes through the MNP, it increases its temperature starting from the metal-dielectric boundary inwards, as the thermal conductivity of silicon dioxide is two orders of magnitude lower than that of most metals and therefore, acts as a thermal insulator [15], resulting in complete NP melting. As the ion track formation time-scale is comparable to that of NP melting, the NP expands via longitudinal flow into the underdense core of the ion track and subsequent recrystallization [16].

In this work we present experimental results of the shape transformation of Au and Ag NPs located at the interface of amorphous silicon nitride and silicon dioxide thin films upon irradiation with 185 MeV Au ions. We selected silicon nitride due to the existing applications and the higher refractive index compared to silicon dioxide. Under these conditions, a shape transformation process with different modification rates is expected, as silicon nitride possesses a smaller track radius and higher thermal conductivity (still one order of magnitude below the value for metals) in comparison with amorphous silicon dioxide. We discuss our experimental results with respect to the thermal spike calculations of the time evolution of ion track formation in both materials.

Section snippets

Material and methods

An amorphous silicon nitride layer with a thickness of 600 nm was first deposited on top of a c-Si(100) wafer by plasma-enhanced chemical vapor deposition (PECVD). Afterwards, Au or Ag layers (5 nm in thickness) were deposited on top by thermal evaporation. Finally, a 500 nm thick top layer of amorphous silicon dioxide was deposited by PECVD. Subsequently, to promote the formation of nearly spherical Au and Ag NPs, rapid thermal annealing (RTA) was carried out at 1000 °C and 800 °C, respectively, in

Results and discussion

Figs. 1and 2 show the shape transformation of nearly spherical MNPs when located at the interface of the two dielectric materials after irradiation with two different ion fluences. At the selected fluences of 0.3 × 1014 cm−2 and 1 × 1014 cm−2 well-aligned and elongated single layer NPs can be observed (Figs. 1(d), (g) and 2(d), (g)).

Thermal deposition of 5 nm layers followed by an RTA process leads to the formation of single layers of nearly spherical NPs with an average diameter of 16.0 ± 4.5 nm in the

Conclusions

In this work we presented experimental observations regarding the shape transformation of nearly spherical NPs with average diameters 16–18 nm after irradiation with 185 MeV Au ions at the interface of silicon nitride and silicon dioxide layers. The resulting elongation process occurs preferentially towards the silicon dioxide layer while creating a high density of very small nanostructures with an average radius of 2 nm at the interface. Thermal Spike calculations suggest that the different

Acknowledgements

P. Mota-Santiago would like to thank the Consejo Nacional de Ciencia y Tencologia (CONACyT) for financial support. The authors acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the Centre of Advanced Microscopy, the Australian National University. We acknowledge access to NCRIS facilities (ANFF and the Heavy Ion Accelerator Capability) at the Australian National University. P. Kluth and M. C. Ridgway thank

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