Role of cohesive energy on the interparticle coalescence behavior of dispersed nanoparticles subjected to energetic ion
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The present work reports on the conditions of nanoparticle growth and splitting under energetic ion irradiation. Cohesive energy that determines the thermal stability of a given nanoparticle system was calculated by extending surface area difference (SAD) and liquid drop model (LDM). Based on the size-dependent cohesive energy calculations, the interparticle coalescence mechanism is discussed for a ZnS-based nanoparticle system with special reference to a variety of matrices. The interparticle separation is found to play key role in particle–particle coalescence leading to nanoparticle growth or partial evaporation that results in splitting. I. INTRODUCTION
Matrix-encapsulated metal/semiconductor nanostructures are considered technologically important assets for displaying size-dependent optoelectronic properties in the nanoscale regime. In recent years, size-selective quality nanostructure formation and modification by energetic ion beams using ion implantation,1 ion beam mixing,2 etc. have gained interest for obtaining better control over growth and recrystallization processes. It was demonstrated in previous studies that the energetic ion irradiation could play a major role in tailoring size, shape, and distribution of the nanostructures.3,4 This is because the energy deposited during irradiation leads to either particle growth or splitting of the nanostructures. Ion-induced particle melting and growth are generally attributed to the Ostwald ripening process.5 In contrast, nanoparticle fragmentation into still smaller particles was considered to be the result of the ion hammering effect.6 The cohesive energy of a material system is the amount of energy required to separate out its constituent neutral atoms. It is an important physical quantity as it is directly related to the melting temperature of the crystalline material.7 In earlier studies, the size-dependent cohesive energy of the nanoparticles was computed using different models such as surface area difference (SAD) model8,9 and liquid drop model (LDM).10,11 Though each of these models has its own merits and limitations, they adequately relate the cohesive energy, the melting temperature, and the onset temperature (temperature at which atoms escape from the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0119
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J. Mater. Res., Vol. 25, No. 5, May 2010
nanoparticle surface). The models can also be extended to explain the particle splitting or coalescence under ion-irradiation events. It is expected that ion irradiation will increase the internal energy of the nanoparticle system as a result of which cohesive energy will be suppressed. The reduction in interatomic cohesion results in competition with regard to particle evaporation, growth, and splitting, which in fact depends on the interparticle separation of the nanoparticles within the matrix host. To this end, we propose here a model that highlights the mechanism of nanoparticle growth/splitting under energetic ion irradiation. Using an expansion of t
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