title: Solid-solid phase transformations induced through cation exchange and strain in 2D heterostructured copper sulfide nanocrystals.
abstract: We demonstrate dual interface formation in nanocrystals (NCs) through cation exchange, creating epitaxial heterostructures within spherical NCs. The thickness of the inner-disk layer can be tuned to form two-dimensional (2D), single atomic layers (<1 nm). During the cation exchange reaction from copper sulfide to zinc sulfide (ZnS), we observe a solid-solid phase transformation of the copper sulfide phase in heterostructured NCs. As the cation exchange reaction is initiated, Cu ions replaced by Zn ions at the interfaces are accommodated in intrinsic Cu vacancy sites present in the initial roxbyite (Cu1.81S) phase of copper sulfide, inducing a full phase transition to djurleite (Cu1.94S)/low chalcocite (Cu2S), a more thermodynamically stable phase than roxbyite. As the reaction proceeds and reduces the size of the copper sulfide layer, the epitaxial strain at the interfaces between copper sulfide and ZnS increases and is maximized for a copper sulfide disk ∼ 5 nm thick. To minimize this strain energy, a second phase transformation occurs back to the roxbyite phase, which shares a similar sulfur sublattice to wurtzite ZnS. The observation of a solid-solid phase transformation in our unique heterostructured NCs provides a new pathway to control desired phases and an insight into the influence of cation exchange on nanoscale phase transitions in heterostructured materials.
authors.affiliation: Department of Materials Science and Engineering, ‡Cornell High Energy Synchrotron Source (CHESS), §School of Applied and Engineering Physics, ∥Kavli Institute at Cornell for Nanoscale Science, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Don-Hyung
authors.initials: DH
authors.lastname: Ha
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Caldwell
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Ward
authors.forename: Shreyas
authors.initials: S
authors.lastname: Honrao
authors.forename: Kiran
authors.initials: K
authors.lastname: Mathew
authors.forename: Robert
authors.initials: R
authors.lastname: Hovden
authors.forename: Margaret K A
authors.initials: MK
authors.lastname: Koker
authors.forename: David A
authors.initials: DA
authors.lastname: Muller
authors.forename: Richard G
authors.initials: RG
authors.lastname: Hennig
authors.forename: Richard D
authors.initials: RD
authors.lastname: Robinson
mesh_terms: D002412:Cations; D003300:Copper / Q000737:chemistry*; D055119:Elastic Modulus; D008422:Materials Testing; D053768:Metal Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure*; D054118:Nanospheres / Q000737:chemistry* / Q000648:ultrastructure*; D010316:Particle Size; D044367:Phase Transition; D013314:Stress, Mechanical; D013440:Sulfides / Q000737:chemistry*; D013718:Tensile Strength; D017967:Zinc Compounds / Q000737:chemistry
chemical_list: D002412:Cations; D013440:Sulfides; D017967:Zinc Compounds; D003300:Copper; C031238:zinc sulfide
keywords: 2D heterostructure; Cation exchange; copper sulfide; diffusion; phase transformation; plasmonic