Size dependent thermal properties of embedded crystalline germanium nanowires

Abstract

Here we report the size-dependent melting points of crystalline germanium nanowires confined within the pores of hexagonal mesoporous silica templates. A supercritical fluid deposition technique was used to form the nanowire–template composite materials and differential thermal analysis, coupled to thermal gravimetric analysis, was used to determine the melting points of the embedded Ge nanowires with mean diameters ranging from 22 to 85 Å. The melting points of the Ge nanowires within the templates were found to be higher than the melting point of bulk germanium (937 °C), typically by 60 °C, and with a broad melting range (∼80 °C). Extended X-ray absorption fine structure (EXAFS) analysis of the Ge K-edge from the nanocomposite materials revealed a linear increase in the Ge–Ge nearest neighbour distance with decreasing nanowire diameter over the size range investigated. In all cases the Ge–Ge first shell distance in the nanowires was greater than in the bulk. This observed lattice expansion can be attributed to crystallographic deformation resulting from the strain imposed on the nanowires by the template. EXAFS studies also revealed an increase in the average number of oxygen atoms at the Ge/silica interface with decreasing diameter due to the increasing surface area. Interfacial effects and interactions at the nanowires/matrix interface are believed to delay the melting point of these systems.