Using a diamond anvil cell and x-ray diffraction (XRD), researchers at Washington State University (WSU) and the University of North Carolina at Charlotte investigated the structural response of the II–IV organic-inorganic hybrid semiconductor β-ZnTe(en)0.5 to applied hydrostatic pressure. Their findings suggest that β-ZnTe(en)0.5, which consists of alternating layers of two-monolayer-thick zinc telluride (ZnTe) and the organic molecule ethylenediamine (en), could be used for phase change memory, an ultrafast, long-lasting form of data storage that does not need a constant power source. The material could also be valuable in photonic applications, where light instead of electricity is used to move and store information. Organic-inorganic hybrid semiconductors combine the flexibility of organic materials with the electronic properties of inorganic materials. The layered, hybrid semiconductor β-ZnTe(en)0.5 is known to exhibit high crystallinity, stability, and tunable optical properties. When the researchers conducted XRD experiments on β-ZnTe(en)0.5, they found that the material underwent significant pressure-induced phase transitions at relatively low pressures of 2.1 and 3.3 gigapascals (GPa). In both cases, the structure of the material changed dramatically, shrinking by up to 8%. The organic layer of the material was especially responsive to changes in pressure. Researcher Julie Miller (left) and professor Matt McCluskey conduct research into the hybrid semiconductor β-ZnTe(en)0.5 using the X-ray beamline at WSU’s Dodgen Research Facility. Courtesy of Robert Hubner, WSU Photo Services. The researchers verified the phase transitions observed in XRD with Fourier transform infrared (FTIR) spectroscopy. XRD measurements showed splittings in the (020) and (130) peaks, and FTIR spectroscopy showed changes in the vibrational modes at both phase transition pressures. The transitions occurred at pressures significantly lower than the lowest reported phase change for pure ZnTe. “Most materials like this need huge amounts of pressure to change structure, but this one started transforming at a tenth of the pressure we usually see in pure zinc telluride,” researcher Julie Miller said. “That’s what makes this material so interesting — it’s showing big effects at much lower pressures.” The researchers also found that β-ZnTe(en)0.5 behaved differently depending on the direction in which it was squeezed. The directional sensitivity of the material, combined with its layered structure, makes it more tunable and versatile. The phase transitions observed in β-ZnTe(en)0.5 occurred between two solid states, with the same atoms rearranging into a denser configuration. This type of phase transition can cause radical changes in a material’s physical properties, including how the material conducts electricity or emits light. “We discovered that the material didn’t just compress — it actually changed its internal structure in a big way,” professor Matt McCluskey said. Different structural phases often have different electrical and optical characteristics. This observation has led researchers to hypothesize that materials like β-ZnTe(en)0.5, which exhibit structural phase transitions, could be used to encode digital information for phase change memory. Because β-ZnTe(en)0.5 emits ultraviolet (UV) light, the researchers suspect its glow might shift depending on its phase, potentially making β-ZnTe(en)0.5 a useful material for fiber optics and optical computing. Next, the team plans to investigate how the material responds to temperature changes and how it reacts when both pressure and heat are applied to it. These future experiments will help the researchers build a more complete map of how the material behaves under various conditions and its future potential. The team conducted its research into β-ZnTe(en)0.5 on an XRD system acquired by WSU in 2022 with support from the Murdock Charitable Trust. The system enabled the researchers to observe minute structural changes in the material as the changes transpired. Typically, experiments like these require equipment available only at national facilities like the Advanced Light Source at Berkeley National Laboratory. “Being able to do these high-pressure experiments on campus gave us the flexibility to really dig into what was happening,” McCluskey said. While the study of β-ZnTe(en)0.5 as a potential commercial memory material is still in its early stages, the work of the WSU team is a step forward in this direction. “We’re just beginning to understand what these hybrid materials can do,” Miller said. “The fact that we could observe these changes with equipment right here on campus makes it that much more exciting.” The research was published in AIP Advances (www.doi.org/10.1063/5.0266352).
