Date of Award

Summer 8-18-2023

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Yingchao Yang

Second Committee Member

Liping Yu

Third Committee Member

Sheila Edalatpour

Abstract

Single or few atomic layered two-dimensional (2D) materials have gained significant research interest due to their unique physical properties and outstanding applications as high-performance functional nanomaterials. The intrinsic electrical, mechanical, and optical properties of 2D materials (e.g., graphene, transition metal dichalcogenides (TMDs)) were found to be remarkably distinct compared to their bulk counterparts. In this research, two unique categories of atomically thin 2D materials: (i) 2D high entropy materials (HEMs) and (ii) 2D silica (SiO2) have been fabricated for their potential applications in energy storage devices like batteries or supercapacitors, electrocatalysis and energy capture.

In this thesis, Chapter 2 introduces the concept of entropy stabilized 2D HEMs which consist of four or more elements in nearly equal concentrations to achieve outstanding features typically absent in conventional alloys. The design of novel compositions of HEMs is very challenging due to the limitations in existing methodologies and competing factors for enthalpy-entropy relationship. To address this, a novel descriptor called Mixed Enthalpy-Entropy Descriptor (MEED) is proposed. MEED successfully predicts the most thermodynamically stable phases for high-entropy transition metal ditellurides (HETs) and disulfides. Experimental synthesis for the top five candidates

of five-element tellurides (MoNbTaVW)Te2, (MoNbTaTiV)Te2, (HfNbTiVZr)Te2, (HfNbTaTiV)Te2 and (HfMoNbTiV)Te2 were conducted using the conventional solid-state reaction (SSR) method to validate the theoretical prediction. Optimized growth conditions (1000°C for 7-14 days) followed by ultrasonication of 4.5 hours resulted in the successful synthesis of atomically thin layered 2D HET nanoflakes (NFs). The 1st, 2nd and the 4th composition resulted in uniform atomic distribution of all five metal elements, which was confirmed by advanced characterization techniques (TEM combined with EDS).

In Chapter 3, the effectiveness of chemical vapor deposition (CVD) technique was demonstrated in synthesizing MEED predicted HE disulfides with a stable composition of VTaTiNbMoS10 using a mixture of transition metal chlorides as precursors. Optimized growth conditions (750 °C) under an Ar atmosphere resulted in the formation of contamination-free high quality 2D triangular crystals composed primarily of Mo and Ta, with minor elements V and Ti, along with a sulfur content of 69.81 at. %. This research highlights the optimistic use of CVD for achieving high quality 2D HEMs directly with potential catalytic applications.

Furthermore, the exploration of CVD synthesis of 2D HEMs involved a mixture of multiple transition metal powders (Hf, Nb, Ta, Ti, Zr) and NaCl salt. At a growth temperature of 950°C, a unique hexagonal 2D silica structure was unexpectedly obtained instead of 2D HEMs. In Chapter 4, experiments with individual transition metals from groups IVB and VB were conducted on SiO2/Si substrate to investigate this interesting growth phenomenon. The study reveals that group VB transition metals have a stronger influence on the formation of hexagonal structured 2D silica compared to group IVB metals. This detailed understanding of the growth mechanism of 2D silica expands its scope for potential applications in the energy capture sector.

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Available for download on Thursday, October 09, 2025

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