Title: Structural, thermal and superconducting properties of Pb nanoparticles in Si, Al and Cu
Other Titles: Structurele, thermische en supergeleidende eigenschappen van Pb nanodeeltjes in Si, Al en Cu
Authors: Wang, Huan; S0169255
Issue Date: 8-Nov-2012
Abstract: In this thesis, we have investigated the structural, thermal and superconducting properties of Pb particles embedded in crystalline Si, Al and Cu matrices, which were synthesized by high fluence ion implan&shy;tation and subsequent annealing.In order to investigate the size dependent properties of embedded Pb particles, detailed knowledge on the formation and evolution of ion beam synthesized Pb nanoparticles during the implantation and sub&shy;sequent annealing processes is of paramount importance. Therefore, we initially studied the growth kinetics of the embedded Pb particles. The aim was to obtain a precise understanding of the basic depen&shy;dences of the average particle size on the main synthesis parameters such as im&shy;plantation fluence, annealing temperature and time, etc. It was de&shy;monstrated that Pb particles start to nucleate in a supersaturated solid solution due to the limited solubility of Pb in these three matrix materials (Al, Si and Cu). Subsequently, the size evolution of the Pb nanoparticles could be described by the classical particle nucleation and growth theory. Thus, by employing different para&shy;meters (such as implan&shy;tation fluence and temperature, annealing time and tempera&shy;ture), Pb nanoparticles with average diameters ranging from 5 to 20 nm are obtained, which can be accurately characterized by X-ray scattering techniques (including synchrotron based SAXS and conventional XRD) and TEM. In addition, the lattice structure, orientation and shape of the Pb nanoparticles are characterized.An epitaxial relationship between the fcc-Pb particles and the crystalline matrices is observed and their shape is a trun&shy;cated octa&shy;hedron bounded by {111} and {001} planes. Such a configuration can be attributed to the minimization of interface energy in the implanted systems, and more in general, the morphology of em&shy;bedded faceted metallic nanoparticles corresponds to a low-energy particle/matrix interface. In our study, special emphasis was put on the lattice parameter tuning of the embedded Pb parti&shy;cles, as this received little attention so far and is crucial for a better understanding of the melting behavior of embedded Pb particles. We have shown that the lattice constants of the embedded Pb precipitates can be tuned not only by their size but also by the matrix where they are embedded in. Such results can be qualita&shy;tive&shy;ly ex&shy;plained by two competing effects: (i) the influ&shy;ence of pseudomorphic growth on the lattice expansion/ contraction due to lattice mismatch; (ii) reduction of the lattice dilatation/su&shy;ppre&shy;&shy;ssion via dislocations formed at particle/matrix interfaces in order to de&shy;crease the corres&shy;ponding strain energy. Thus, with increasing particle size, due to (i) the decreasing surface-to- volume ratio; and (ii) the increa&shy;sing density of dislo&shy;cations at particle surfaces, the fraction of pseudomorphic Pb at particle/matrix interface is expected to decrease. As a result, the influence of pseudo&shy;morphic growth on the lattice constants becomes weaker with increasing particle size, which is observed in our experi&shy;ments. It is worth emphasizing that this structural information is crucial for a better understanding of the melting and superconducting properties for Pb nanoparticles em&shy;bedded in different matrices. Based on our experimental results, it can be generally predicted that uniform elastic strain, which is particle size and matrix dependent, exists in elemental particles having an epitaxial particle/matrix interface due to the lattice mismatch.Having examined in detail the structural properties, we subse&shy;quently elucidated the melting behavior of the embedded Pb particles. Since Pb particles experience different strain (pressure) conditions in different crystalline matrices, our implanted systems provide an opportunity to explore the elastic strain related pressure effect on the melting temperature of the Pb nanoparticles. In-situ high temperature XRD studies gave unam&shy;biguous evidence for a thermal hysteresis loop across the melting and solidification transition of Pb nanocrystals in the three matrices. Further on, analysis of their lattice constants during the heating pro&shy;cess shows that the elastic tensile/com&shy;pressive strain related pre&shy;ssure effect, which is size depen&shy;dent, can suppress/enhance the melting point of the embedded Pb particles in Si/Cu. This can be understood following the classical thermodynamic consideration, i.e. the Clausius-Clapeyron equation. Thus, the usual explanation advocated in order to explain super&shy;heating (for instance Pb nanoparticles in Al), namely, that low-energy epitaxial interfaces providing a barrier of melting nucleation, cannot fully account for the melting behavior of the embedded Pb particles in Si and Cu. Our data indicate that the elastic strain related pressure plays an important role in controlling their melting points. As a general conclusion, we can say that the melting behavior of embedded metallic particles is strongly dependent on their size and the interface configuration between particle and matrix. For embedded metallic particles without epitaxial particle/matrix interfaces, their melting point is usually smaller than the bulk value and decreases with decreasing particle size. For metallic particles with epitaxial particle/matrix interfaces, our experimental results imply that their melting behavior can be inter&shy;preted in terms of two effects: (i) the epitaxial particle/matrix interface and matrix confinement induced melting temperature increa&shy;se; and (ii) the strain induced depression/eleva&shy;tion of melting point. Although the melting transition of embedded Pb particles can be broaden due to their size distribution, clear trends of the average size dependent melting behavior are still observed.The third part of this thesis was dedicated to the systematic study of superconducting properties of the implanted systems. First, we investigated the proximity effect induced superconductivity in Pb nanoparticles embedded in an Al layer.Our observations show that the superconducting transition temperature (Tc) for the implanted Al layers is consistently larger than the Tc of virgin Al layer and con&shy;trolled by the volume ratio of the two components. The theory of the supercon&shy;ducting proximity effect adapted to granular nanocomposites can be applied to explain the Tc variations within the Cooper limit. In contrast with previous observations for granular nanocomposites [86, 87], we find that Pb nanoparticles behave as strong-coupling super&shy;conduc&shy;tors in Al. We further argue that the deviation of the measured Tc in granular nanocomposites with respect to the one calculated in the strong-coupling model might be attributed to the low interface trans&shy;parency related effective Tc reduc&shy;tion caused by e.g. surface rough&shy;ness or interface oxidization. For our implanted samples, after implanta&shy;tion and subsequent annealing, a thin passivation layer of alumina (~ 4 nm) is formed on the sample surface which helps to protect the samples from further internal oxidization. In addition, the epitaxial inter&shy;faces between the Pb particles and the Al matrix imply a high trans&shy;parency at the particle/matrix interfaces. Thus an excellent agreement between the experi&shy;mental results and the theoretical pre&shy;diction is obtained. More&shy;over, our results give strong support for the robustness and validity of SternfeldÂ’s approach for a correct ex&shy;planation of the superconducting proximity effect in such implan&shy;ted systems. In addi&shy;tion, other super&shy;conducting properties such as the supercon&shy;ducting phase boundary were studied to give a better description of this heterogeneous superconducting system. In general, it can be speculated that for other implanted systems such as In nanoparticles in an Al layer (with epitaxial particle/matrix interfaces [161]), in which both In and Al are weak-coupling supercon&shy;ductors, the Tc of the system is dependent on the volume fraction of In particles and the proximity induced superconductivity can be described by the weak-coupling model. For the Pb-implanted Cu system, although the Cooper limit is fulfilled, no superconducting transition was observed down to the lowest temperature (0.3 K) that our 3He cryostat can reach. This is due to the limited volume ratio of embedded Pb particles (< 5 %), which corresponds to a Tc less than 0.1 K according to the theoretical prediction. For Pb particles embedded in a semiconductor Si matrix, the proximity effect on the Tc of embedded Pb nanoparticles can be neglected. However, the Meissner effect was too weak to be observed in SQUID measurements for our implanted samples. This can be attributed to the significantly reduced dia&shy;magnetic response from a limited quantity of superconducting Pb nanoparticles in our implanted samples. Although a size and strain dependence of the Tc variation in the Pb particles is expected, it is impossible to observe it directly by SQUID measurements. From the above summary of our investigations, one can clearly see that the surface/&shy;interface effects, which are strongly dependent on particle size, matrix material and particle/matrix interface configura&shy;tion, play an important role in regulating the properties of embedded Pb particles. For instance, the influence of pseudomorphic growth on the elastic strain of embedded metallic par&shy;ticles via epitaxial particle/ matrix interfaces due to lattice mis&shy;match is also expected for larger particles, but only at the nano&shy;scale, the relative amount of the surface is large enough to influence the entire particle. Such particle size and matrix dependent elastic strain is directly connected to the lattice constants of the embedded Pb particles and can strongly affect their melting behavior. Moreover, since the size of embedded Pb parti&shy;cles is smaller than the corresponding superconducting coherence length, the Cooper limit of our implanted systems (Pb-in-Al and Pb-in-Cu) can be fulfilled. Also due to the epitaxial particle/matrix interfaces providing a high interface transparency between the Pb particles and the Al layer, a good agreement between the experi&shy;men&shy;tal data and theoretical prediction is obtained.In general, we can conclude that the Pb nanoparticles embedded in crystalline matrices exhibit novel structural, thermal and supercon&shy;ducting properties attributed to both the surface/&shy;interface effects and the quantum size effects by a proper choice of matrices.For the future work, investigations on the properties of other implanted systems with an epitaxial particle/matrix interface could be an interesting line, in which the structural, thermal, superconducting, optical and magnetic properties of embedded nanoparticles will be significantly changed with respect to their free-standing counterparts.
ISBN: 978-90-8649-571-9
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Solid State Physics and Magnetism Section
Nuclear and Radiation Physics Section

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