Conductivity studies of the size-induced metal-insulator transition (SIMIT) in silver nanoparticles

Thesis

In his tenth decade, Professor Sir Nevill Mott, in a letter to Professor Peter P. Edwards, reminiscenced about the fundamental difference between metals and non-metals; thus he wrote of the situation at the absolute zero of temperature (T = 0 K) '...there a metal conducts, and a non-metal doesn't'. This simple but amazingly powerful commentary stands as a powerful abiding description of the challenge of the fascinating question as to what makes a metal conduct. Many years of experimental studies of metals and nonmetals, targeted at the experimental verification in divided-metal systems ranging from transition mesoscopic metal compounds to microscopic colloidal metals, the occurrence of the electronic transition between these two canonical states of matter have always an enduring fascination where the possibility of a Size-Induced Metal-Insulator Transition (SIMIT) within a small particle of a bulk metal is accessed. In contrast to the situation of a discontinuous Metal-to-Insulator Transition (MIT) in macroscopic systems containing huge numbers of interacting particles and electrons (i.e. a doped semiconductor), the onset of the SIMIT in a single, isolated microscopic particle of a metallic element of the Periodic Table is expected to be a continuous metal-insulator transition.

Here, we report an experimental study on the microwave electrical conductivity across the possible SIMIT in particles of the most conductive of all metals in Periodic Table - silver (Ag). Particular emphasis has been placed on the development and application of a non-intrusive microwave measurement technique, taken across a broad range microwave frequencies. The aim is to look for, and interrogate the nature of the SIMIT in such ultra-fine Ag particles. We have attempted to develop a coherent description of the different dielectric responses that yield electrical conductivity over a wide range of mesoscopic and microscopic Ag particles, from which the SIMIT is identified and rationalised. We find that, at 294 K, the mesoscopic Ag particles approximately 5 nm exhibit an 'effective' microwave conductivities which is reduced by some two orders-of-magnitude below that of bulk metallic Ag.

Authors: Lah, N

• Alternative title: Size-induced metal-insulator transition
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Microwave Cavity Resonator; Silver Nanoparticles; Electrical Properties; Size-Induced Metal-Insulator Transition
• Subjects: Metal Nanoparticles; Nanotechnology; Silver Nanoparticles