(Nanowerk Spotlight) Electronic devices work through precise control over the flow of electricity. While typical materials act as either conductors or insulators, the ability to switch between these states within a single material could transform how we build and control electronic components. This capability becomes even more powerful when combined with other controllable electrical properties.
Creating such adaptive materials has challenged scientists since the discovery of materials that can switch between conducting and insulating states in the 1960s. The first examples, like chromium-doped vanadium oxide, demonstrated this switching behavior but proved extremely difficult to modify or improve. Any attempts to enhance their properties typically disrupted the very characteristics that made them useful.
Meanwhile, materials called ferroelectrics offered different but equally valuable properties. These materials maintain a switchable electrical polarization – essentially a separation of positive and negative charges that can be flipped by applying an electric field. This property makes them essential for various electronic components, from computer memory to sensors. However, ferroelectrics typically only work as insulators, limiting their potential applications.
A promising solution emerged through the development of hybrid materials that combine organic molecules with inorganic components. These materials offer more flexibility in design and the potential to merge multiple useful properties. Despite this advantage, creating a hybrid material that could both switch its conductivity and maintain ferroelectric properties remained out of reach – until now.
Reporting their findings in Advanced Materials (“The First Molecular Ferroelectric Mott Insulator”), team of researchers from several Chinese institutions has successfully created such a material. Their new substance, designated as (C7H14N)3V12O30, combines layers of vanadium oxide with organic molecules called quinuclidinium rings. This arrangement allows the material to switch between conducting and insulating states while preserving controllable ferroelectric properties.
Crystal structures of (C7H14N)3V12O30. Packing view in ac plane of a) LTP and b) HTP, respectively. Packing view in bc plane of c) LTP and d) HTP, respectively. (Image: Reprinted with permission by Wiley-VCH Verlag)
The material undergoes a remarkable transformation at -38.75 °C (234.4 K). Above this temperature, it conducts electricity relatively well. When cooled below this point, it becomes an insulator and develops ferroelectric properties, meaning its internal electrical polarization can be switched by applying an external electric field. This transition is accompanied by a sharp conductivity drop of three orders of magnitude within just 5 K, a hallmark of a Mott transition, where strong electron interactions prevent current flow.
The researchers found that this effect is closely linked to the formation of a charge density wave (CDW), a periodic arrangement of electron density that reinforces the insulating state. This change is driven by an order-disorder transformation of the quinuclidinium cations, which shifts from a freely rotating state at high temperatures to a fixed, asymmetric arrangement at lower temperatures. This structural shift breaks mirror symmetry and enables the emergence of ferroelectricity.
The material’s hybrid nature, combining organic quinuclidinium rings with inorganic vanadium oxide layers, allows independent tuning of its conductivity and ferroelectric properties, a key advantage over conventional ferroelectrics.
This behavior identifies the material as a Mott insulator – named after physicist Nevill Mott who first explained how electron interactions can cause materials to become insulators. In these materials, electrons become trapped in place at lower temperatures due to their mutual electrical repulsion, preventing current flow.
The researchers mapped out how this behavior emerges from the material’s structure. At room temperature, the organic quinuclidinium molecules can freely rotate. When cooled, they lock into specific positions, creating an ordered arrangement that enables both the insulating and ferroelectric properties. Advanced calculations revealed that this ordering causes electrons to arrange themselves in a specific pattern that supports both characteristics.
Unlike many similar materials, this new substance remains stable in humid conditions – a crucial practical advantage that could make it suitable for real-world applications. Previous molecular ferroelectric materials often degraded when exposed to moisture, limiting their usefulness.
The development of this material advances several areas of electronics research. Its ability to switch between conducting and insulating states while maintaining controllable ferroelectric properties could enable new types of electronic components that combine multiple functions. The material’s hybrid nature also means chemists might be able to modify its structure to adjust properties like its transition temperature.
This achievement shows that hybrid molecular materials can host sophisticated electronic behaviors previously found only in pure inorganic materials. The work establishes new possibilities for creating materials that combine multiple useful properties in ways that were previously impossible.
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