Reversible structure manipulation by tuning carrier concentration in metastable Cu ₂ S

Abstract

The optimal functionalities of materials often appear at phase transitions involving simultaneous changes in the electronic structure and the symmetry of the underlying lattice. It is experimentally challenging to disentangle which of the two effects--electronic or structural--is the driving force for the phase transition and to use the mechanism to control material properties. Here we report the concurrent pumping and probing of Cu 2 S nanoplates using an electron beam to directly manipulate the transition between two phases with distinctly different crystal symmetries and charge-carrier concentrations, and show that the transition is the result of charge generation for one phase and charge depletion for the other. We demonstrate that this manipulation is fully reversible and nonthermal in nature. Our observations reveal a phase-transition pathway in materials, where electron-induced changes in the electronic structure can lead to a macroscopic reconstruction of the crystal structure. functional and quantum materials--the key properties of functional materials are often born out of strong structural and electronic interactions that are quantum mechanical in nature. Notable examples include colossal magnetoresistance manganites (1), ferroelectrics (2), and valleytronic materials (3). The targeted functions are usually accompanied by symmetry breaking, induced through changes in temperature or under other external perturbations. A prominent question is whether the symmetry reduction has an origin in the lattice (e.g., in the form of displacement of atoms, and could be described reasonably well using first-principles calculations) or the electronic degrees of freedom (charge, spin, and orbital) (4-6). It is of great interest to distinguish the roles of these factors in phase transitions.Cu 2 S provides an intriguing example for addressing the above "chicken-and-egg" question (6), which is critical to the understanding of a wide range of functional and quantum materials. It is a fast ionic conductor (7) with highly mobile Cu ions. A phase transition in the bulk material occurs near 100°C from a semiconducting (8) monoclinic symmetry low-chalcocite phase (9) [hereafter called the "L-s phase" (i.e., low, semiconducting); space group P2 1 /c] to an electrically insulating (10) hexagonal symmetry high-chalcocite phase (7, 11) [hereafter called the "H-i phase" (i.e., high, insulating); space group P6 3 /mmc]. The coinciding changes in the bulk electrical conductivity and crystal structure present a possibility for exploring the relationship between electronic and structural phase transitions. Difficulties in the synthesis of stoichiometric Cu 2 S material and the lack of detailed theoretical treatments of both the crystal and the electronic structures of Cu 2 S have hindered the understanding of the phase transition (7, 11-13). Results and DiscussionWe have recently synthesized high-quality Cu 2 S nanoplates (Materials and Methods), which are on the order of 10-nm thick and 100 nm in lateral dimension ( Fig. 1 A and B)....

MOF-derived N,S Co-doped carbon matrix-encapsulated Cu2S nanoparticles as high-performance lithium-ion battery anodes: a joint theoretical and experimental study

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