Reactivity and Epitaxy of Metal Thin-Films on Few-Layer Molybdenum Disulfide

Restricted (Penn State Only)
Author:
Domask, Anna Catherine
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 22, 2017
Committee Members:
  • Suzanne E Mohney, Dissertation Advisor
  • Suzanne E Mohney, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Nasim Alem, Committee Member
  • Jun Zhu, Outside Member
Keywords:
  • Contacts
  • Thermodynamics
  • van der Waals Solid
  • Transition Metal Dichalcogenide
  • Raman spectroscopy
  • Transition electron microscopy
Abstract:
Transmission electron microscopy, Raman spectroscopy, Auger electron spectroscopy, and time-of-flight secondary ion mass spectrometry were used to better understand the interactions between molybdenum disulfide and a large number of metal thin films. Thermodynamic modeling of the ternary metal--Mo--S systems is presented. The results allow one to predict the equilibrium reaction products resulting from the interaction of a metal and molybdenum disulfide. Overall, many early transition metals were predicted to react to form either a metal sulfide or ternary compound, while most late transition metals were predicted to be in equilibrium with molybdenum disulfide. Experimental investigation of the interaction between various metals and few-layer molybdenum disulfide found that Ti, Nb, Re, and Al were not reactive on molybdenum disulfide after annealing to 673 K for 4 h. On the other hand, Ni and Ag both diffused into the molybdenum disulfide (potentially into the van der Waals gaps between layers), with signs of diffusion of Ni appearing at temperatures as low as 473 K. The change of the Raman spectrum for molybdenum disulfide after indiffusion with Ni and Ag was investigated. This study is the first of Raman spectra changes due to transition metal indiffusion. Indiffusion of the contact metal has been shown to decrease contact resistance for Ag contacts to molybdenum disulfide system due to doping and improved carrier transport between the molybdenum disulfide layers beneath the contact. Lastly, the crystallographic orientation between various metals deposited at room temperature and molybdenum disulfide was explored. Epitaxy of Pd, Ag, Al, and Zn on molybdenum disulfide was seen but Mn, Mo, Ru, Re, and Ni were not epitaxial. Metals with face-centered cubic, body-centered cubic, and hexagonal close packed structures were tested, and a number (although not all) FCC and HCP metals were found to be epitaxial. The metals that were epitaxial on molybdenum disulfide all had a close packed plane with a hexagonal structure, a high homologous temperature, and a low barrier to surface diffusion on molybdenum disulfide---criteria that could apply to other metal/transition metal dichalcogenide systems. Based on this work, epitaxial metals on molybdenum disulfide could be deployed as contact metals, heterostructure layers, and TMD growth substrates.