ABSTRACT

Non-technical abstract:
In the information age, affordable and efficient optical and electrical memory is foundational to the preservation and dissemination of knowledge and ideas. Materials which undergo a phase transition, such as chalcogenides that are commonly used in DVDs, are especially promising for emerging applications which combine memory with high-speed computing but require relatively large programming energies which is proportional to the volume of material switched. Encoding data in two-dimensional (2D) materials such as molybdenum tellurides (MoTe2) provides a direct route to overcome this fundamental limitation. Among available 2D materials which can undergo a phase transition, MoTe2 is predicted to be the most energy efficient, but there is a distinct lack of experimental evidence to support conflicting theoretical models governing the mechanisms, dynamics, and limitations of optically-induced phase transformations in MoTe2. The team proposes to address this knowledge gap using dynamic optical measurement techniques in combination with ultrahigh-resolution transmission electron microscopy. The project overcomes the experimental limitations of prior works to shed new light on related 2D materials for applications requiring high-speed, reliable, and efficient optoelectronic memory. The team seeks to educate middle- and high-school students on topics related to nanomaterials in daily life from districts with historically under-represented minorities in STEM fields using a combination of interactive workshops and virtual reality tools. This project also provides training for two graduate students in nanofabrication and characterization techniques and hosts undergraduates from underrepresented groups during the summer months to broaden participation in STEM-related fields.

Technical abstract:
Phase-change materials that enable optoelectronic memory have the potential to transform low-energy, non-von Neumann computing architectures by processing information in memory at the speed of light. A phase-change material that is atomically flat (e.g. MoTe2 and its alloy Mo1-xWxTe2) would further reduce the energy required to configure its state by drastically reducing the active volume undergoing a phase transition. While optically induced phase transformations have been observed in MoTe¬2 and related materials, these transformations have been irreversible unlike similar observations employing electrochemical doping and mechanical strain. Limited empirical evidence and theoretical modeling indicates Te vacancies play a central role in the phase transition process, but a clear understanding of the dynamics and physical mechanism of optical switching between the 2H and 1T? phases in MoTe2 remains elusive to date. The team proposes that optically induced structural transformations can be controlled in MoTe2 through material synthesis, encapsulation, and W-alloying, resulting in higher operating speeds, improved reliability, and lower switching energies. To test this hypothesis, the project contains the following three aims: (1) determine the influence of Te vacancies on the optical switching power by engineering the concentration of Te vacancies during the MoTe2 growth process; (2) encapsulate MoTe2 to reduce Te loss during optical excitation?the expected mechanism preventing reversible optical switching; and (3) alloy MoTe2 with W to engineer an optimal 2D material for efficient and rewriteable optoelectronic phase-change memory. The proposed approach overcomes the temporal limitations of prior experimental techniques by probing the phase-transition process in the optical domain. The proposed research is expected to enable the development of high-speed, non-volatile, and efficient data storage by exploiting structural transformations in MoTe2 to encode information. This study is the first to use a combination of optical and electro-optical techniques to resolve conflicting theoretical models regarding the phase transformation mechanisms, dynamics, and optimal stoichiometry of MoTe2 and its alloy Mo1-xWxTe2. New insights into phase-transformation process of MoTe2 are expected to have broad application to fields beyond data storage, such as neuromorphic computing, electro-optic conversion, flexible electronics, and reconfigurable topological and quantum devices.

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