Thickness-Controlled Ferroelectric-to-Ionic Transport in CuCrP2S6 at Room Temperature

In the world of modern electronics, materials are often chosen for a single, specific talent. Some are excellent at conducting electricity, while others are masters at storing information by holding an electric charge in a fixed direction, a property known as ferroelectricity. A third group of materials allows ions, which are tiny charged atoms, to move freely through their structure, a behavior that mimics how neurons fire in the human brain. For decades, scientists have sought a material that can do all three things at once, hoping to build devices that are smaller, faster, and capable of learning like a biological system. The challenge has been that these properties often fight each other; the very movement of ions that enables brain-like behavior can scramble the stable electric charges needed for memory. Finding a material where these opposing forces coexist and cooperate, rather than cancel each other out, remains a significant hurdle in the quest for next-generation technology.

Researchers at Morgan State University have found a promising candidate in a crystal called copper chromium phosphorus sulfide. This material belongs to a family of substances known as van der Waals compounds, which are built like stacks of paper where the layers are held together by weak forces, allowing them to be peeled apart into incredibly thin sheets. The researchers discovered that the behavior of this material changes dramatically depending on how thick the sheet is. When the crystal is sliced into a very thin flake, measuring less than forty nanometers, it acts as a robust memory device, holding its electric charge firmly. However, as the crystal gets thicker, the ability to hold that charge fades away, and the material shifts into a different mode where electricity flows primarily through the movement of copper ions. This thickness-controlled switch between two different ways of conducting electricity offers a new way to tune materials for specific tasks, from memory storage to artificial synapses.

The team began by examining the crystal's ability to switch its electric polarization, which is the direction in which its internal charges point. Using a specialized microscope that can feel the tiny forces on a surface, they tested flakes of varying thicknesses. They found that in the thinnest samples, they could write patterns of electric charge that remained stable for hours, a hallmark of a good memory material. These thin flakes could be flipped back and forth reliably, much like a light switch. But as the flakes grew thicker, this ability to hold a stable electric pattern began to weaken. In the thickest samples, the electric charge would quickly fade, and the material stopped behaving like a memory device. Instead, the electrical current in these thick samples was driven by copper ions physically moving through the crystal to form conductive paths, a process that creates a different kind of electrical signature known as memristive behavior.

To understand what was happening inside the material, the researchers measured the electric potential on the surface of the flakes. They observed that the surface potential, which reflects the internal electric landscape, changed in a predictable way as the thickness varied. In the thin, memory-like flakes, the surface potential remained stable after being written, confirming that the electric charges were locked in place. In the thicker flakes, this stability vanished, and the surface potential relaxed quickly, indicating that the internal charges were not fixed but were instead shifting around. This shift suggested that in the thicker samples, the movement of copper ions was the dominant force, overpowering the stable electric order seen in the thinner versions. The researchers also noted that the crystal structure itself did not change physically during these tests, ruling out the possibility that the material was simply breaking down or deforming; the changes were purely electrical and ionic.

The study further explored how light affects these processes. When the researchers shined a blue laser on the crystals, they found that the light made it easier for electricity to flow in both thin and thick samples, but for different reasons. In the thick flakes, the light helped the copper ions move even faster, making the formation of conductive paths more efficient and lowering the voltage needed to turn the current on. This created a strong, light-sensitive switching effect. In the thin flakes, the light also increased the current, but the material maintained its stable memory-like behavior. The light simply enhanced the existing interaction between the fixed electric charges and the moving ions, rather than causing the ions to take over completely. This suggests that the thin flakes can operate as a combined system where light, memory, and ion movement work together, while the thick flakes operate more like a light-controlled switch driven by ion movement.

Finally, the team built actual electronic devices using the thicker crystals to see if these microscopic behaviors held up in a real-world setting. They created simple circuits with metal contacts on either side of a crystal flake and applied voltage to see how the current flowed. The results matched their microscopic observations: the current increased slowly over time as the copper ions moved, and the behavior changed depending on how fast the voltage was applied. When they turned a light on and off, the current responded instantly, rising when the light hit the material and falling when it was removed. This confirmed that the complex dance of ions and charges observed under the microscope translates directly to the function of a working device. The research demonstrates that by simply adjusting the thickness of the material, scientists can choose whether the crystal acts as a stable memory element or a dynamic, ion-driven switch, and that light can be used to control both modes. This discovery provides a clear path for designing future electronic components that can adapt their function based on their physical dimensions, opening the door to more versatile and efficient technologies.