PHENOMENOLOGICAL MODELING OF PHONON CONFINEMENT IN TRANSITION METAL DICHALCOGENIDE QUANTUM DOTS

Open Access
Author:
Fang, Lu
Area of Honors:
Engineering Science
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Kofi W Adu, Thesis Supervisor
  • S Ashok, Honors Advisor
Keywords:
  • PHENOMENOLOGICAL MODELING
  • PHONON CONFINEMENT
  • TRANSITION METAL DICHALCOGENIDE
  • QUANTUM DOTS
  • layered materials
  • TMD
  • TMDs
  • phonon dispersion
  • Raman
  • layer effect
  • phonon effect
Abstract:
In the past two decades, exfoliated layered materials have attracted much attention in the scientific community due to their potential applications in various emerging nanotechnologies. Graphene, which is a single layer of graphite, possesses intriguing properties that cannot be found in graphite. However, due to the absence of bandgap in graphene, it is very challenging in utilizing it in many electronic devices. Alternative methods such as such as quantum confinement in nanoribbons, deposition of a graphene monolayer on boron nitride etc., have been proposed to create a graphene based bandgap material. However, all of them exhibit a bandgap larger than 400meV, which still remains a challenge. On the other hand, they increase the mobility, which weakens the other property. [1] Therefore, fabrication of other layered materials with a small finite bandgap, such as MoS2, WS2 etc., have attracted tremendous interest. A detail understanding of their electronic and phononic properties are needed if they are to be integrated into the existing technologies. Whereas there are several reports on the electronic properties, there are very limited reports on the phononic properties, even though the phonons play a critical roll in the electronic behavior in these systems. Raman spectroscopy has become on of the useful technique to probe the phononic properties of structures and it is now widely used to elucidate on the phononic behavior in low-dimensional materials, such as 0D material: quantum dots, 2D material: nanowires, 2D: graphene and transition metal dichalcogenides (TMDs) and 3D material: bulk systems. It can provide the wealth of information, which can be used for sample identification and quantitation. Such as, chemical composition, structure and physical properties, just to mention a few. [2] The recent Raman results of layered transition metal dichalcogenides demonstrate the distinct vibrational properties that is dependent on the number of layers. These reports indicate that the optical E12g phonon and A1g modes shifts to higher and lower frequencies, respectively with decreasing number of layers. In Nano crystallites, however, the Raman selection rule (q = 0) of observing only zone center phononons breaks down allowing phonon away from the zone center to contributed to the Raman scattering, which leads to an asymmetry and broadening of Raman peaks. This phenomena pure depends on the phonon dispersion (wave vector) and should exhibit different behavior from that of the layered effect. However, there are very limited experimental and theoretical reports on the quantum confinement of the phonon states that also, clearly delineate the layered effects from the confinement effects. Thus, the purpose of this work is to develop a phenomenological model to elucidate on the origins of confined phonon states in TMDs, delineate the confinement effect from the layered effect and provide an experimental evidence that is consistent with quantum confinement effects in TMDs.