Melting of 2D van der Waals material systems

The thermodynamic properties of two-dimensional (2D) van der Waals (vdW) materials are modulated by the layer-to-layer vdW coupling. Such structure-property interplay is of fundamental interest and promises even more tunability in the emergent 2D vdW heterostructures/superlattices. This research studies the melting in 2D vdW chalcogenides (silver alkanethiolate, AgSCn) and superlattices (Ge2Sb2Te5/Sb2Te3, mix-chain AgSCn). Calorimetry is coupled with structure characterization techniques (NMR, XRD, and TEM) as well as first-principles calculation to uncover unique structural signatures responsible for the modulation of melting, providing a fundamental understanding of the thermodynamics in 2D vdW chalcogenides. A model 2D vdW system, AgSCn lamellae, is chosen due to the strong parity effect in its alkyl chain melting (Tm). To investigate the structural origin of such effects, we synthesize extremely large (3 mm) single crystals, and the atomic-resolution crystallography of AgSCn (n=3-8) is determined by coupling X-ray/electron diffraction and first-principles calculation. Results reveal orthogonal H-H packing configurations at the vdW interface for odd and even chain AgSCn, providing insights into its thermodynamic parity effect. Besides, a strong in-plane anisotropy is also uncovered, which triggers an ~8× distinction of in-plane electrical conductivity as well as a strong optical birefringence with dielectric frame rotations. The findings suggest potential applications of AgSCn crystals as birefringent color filters. The first-principles calculation also reveals different in-plane lattices for monolayer AgSCn with different chain lengths, suggesting a possible heterostructure, if those monolayers were stacked up. Inspired by this idea, AgSCn crystals with different chain lengths are dissolved and mixed in solutions. The precipitates are confirmed as single-phase superlattice stacks consisting of alternating crystalline and glassy sublayers, based on the combined calorimetry, TEM, and XRD studies. The melting can be continuously tuned and is described by a modified Gibbs-Thomson model, with variable excess free energy developed for the glassy sublayers. The methodology developed for AgSCn can also be applied to the melting of its inorganic counterparts (Ge2Sb2Te5, Ge2Sb2Te5/ Sb2Te3 superlattice) used in fast-switching phase-change memory (PCM). To validate that ultrafast NanoDSC can probe the thermal metrics accurately, the crystallization of 10-40 nm Ge2Sb2Te5 (GST) is studied. The findings confirm the negligible thermal lag, repeatability of Cp (T), and stability of temperature calibration even at extremely high heating rates (1,000,000 K/s) in NanoDSC. The crystallization growth rates of GST are derived from kinetic analysis and agree with those measured in PCM cells. Compared to Ge2Sb2Te5, PCM devices made from 2D vdW superlattice (Ge2Sb2Te/ Sb2Te3) require 8 times lower Joule heating power during the melting cycle. The NanoDSC study on PCM superlattices reveals a novel low-T melting which is ~240 °C lower than that of bulk GST (627 °C) and Sb2Te3 (620 °C), along with an 8 times smaller transition enthalpy which is consistent with the low switching current in devices. Such melting also shows up in pristine Sb2Te3 with inherent vdW interfaces but is absent in GST (without vdW interfaces). It is thus proposed that the high density of vdW interfaces may facilitate the formation of Te-rich metastable phases and initiate the pre-melting of the lattice stacks. The findings provide critical guidance in material search and device operation for interfacial engineering of future 2D vdW PCM devices.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Zhao, Jie

Contributors dc:contributor

• Allen, Leslie H
• Zuo, Jian-Min
• Schleife, Andre
• Braun, Paul V
• Lyding, Joseph W

• melting transition
• 2D vdW material
• Nanocalorimetry
• phase-change memory

Statement dc:rights

• Copyright 2025 Jie Zhao

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129158

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129158