MeV Si ions bombardment effects on the thermoelectric properties of nano-layers of nanoclusters of Ag in SiO2 host

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Abstract

We prepared 50 periodic nano-layers of SiO2/AgxSiO2(1  x) with Au layer deposited on both sides as metal contacts. The deposited multi-layer films have a periodic structure consisting of alternating layers where each layer is 10 nm thick. The purpose of this research is to generate nano-layers of nanocrystals of Ag with SiO2 as host and as buffer layer using a combination of co-deposition and MeV ion bombardment taking advantage of energy deposited in the MeV ion track to nucleate nanoclusters. Our previous work showed that these nanoclusters have crystallinity similar to the bulk material. Nanocrystals of Ag in silica produce an optical absorption band at about 420 nm. Due to the interaction of nanocrystals between sequential nanolayers there is widening of the absorption band. The electrical and thermal properties of the layered structures were studied before and after 5 MeV Si ions bombardment at various fluences to form nanocrystals in layers of SiO2 containing few percent of Ag. Rutherford Backscattering Spectrometry (RBS) was used to monitor the stoichiometry before and after MeV bombardments.

Introduction

Semiconductor and metal nanoclusters embedded in transparent matrices exhibit linear and nonlinear optical properties which are of interest to the field of opto-electronics. It is feasible to produce these clusters for converting heat into electrical power by using the same technique [1]. The efficiency of the thermoelectric devices and films are determined by the figure of merit ZT [2]. The figure of merit is ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity [3], [4], [5]. ZT can be increased by increasing S, by increasing σ, or by decreasing κ. Efficient thermoelectric devices have a high electrical conductivity and a low thermal conductivity as well as high thermopower coefficient [5]. The purpose of this research is to generate nano-layers of nanocrystals of Ag with SiO2 as host and as buffer layer using a combination of co-deposition and MeV ion bombardment taking advantage of energy deposited in the MeV ion track to nucleate nanoclusters.

Section snippets

Experimental

We have grown SiO2/AgxSiO2(1  x) nano-layers films on silica substrates using the Ion Beam Assisted Deposition (IBAD) system. The multilayer films were sequentially deposited to have a periodic structure consisting of alternating SiO2 and AgxSiO2(1  x) layers. The two electron-gun evaporators for evaporating the two solids were turned on and off alternately to grow the multilayers. The base pressure obtained in IBAD chamber was 6 × 10 6 Torr. The growth rate was monitored by an Inficon Quartz

Results and discussion

Fig. 3 shows the RBS spectrum and RUMP simulation [12] of 50 periodic nano-layers of SiO2/AgxSiO2(1  x) films on a Glassy Polymeric Carbon (GPC) substrate when the sample is at the normal angle. Each element which was used in the deposition is revealed in the RBS spectrum and the composition of the multilayer film during the RUMP simulation was shown in Fig. 3. RUMP simulation was used to approximate the layer thickness of about 10 nm. The total thickness is 486 nm with 50 layers. The initial

Conclusion

We have observed the effects of ion fluence on the thermoelectric properties of the SiO2/AgxSiO2(1  x) alternating layers. The data shows that the thermoelectric properties are positively impacted at the initial fluences. The properties quickly degrade or demonstrate limited response to increasing fluence. This behavior may be due to optimal relation between the Ag nanocrystal size and their limited spatial distribution along the ion track, which gets degraded at higher fluences. Future studies

Acknowledgement

Research sponsored by the Center for Irradiation of Materials, Alabama A&M University and by the AAMURI Center for Advanced Propulsion Materials under the contract number NNM06AA12A from NASA, and by National Science Foundation under Grant No. EPS-0447675.

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