Most of the electronic devices in use today store, process, and transmit information using the charge of the electron. But electrons carry a second property that has long fascinated physicists: spin, an intrinsic form of angular momentum. Harnessing spin alongside charge is the goal of spintronics, a field that promises devices that are smaller, faster, and far more energy-efficient than their purely charge-based counterparts. The challenge lies in generating and controlling spin-polarized currents without bulky magnets or cryogenic conditions, which is where optical phenomena such as CPGE enter the picture.

The circular photogalvanic effect is a striking example of how light can do more than simply liberate charge in a material. When circularly polarized light—light whose electric field rotates in either a left- or right-handed spiral—falls on certain noncentrosymmetric materials, it can drive a net photocurrent whose direction flips depending on the handedness of the light. Because this effect is closely tied to spin-polarized electronic states, it offers a purely optical route to generating spin currents in materials that are not magnetized, making it especially attractive for studying and exploiting spin–orbit interactions, the quantum coupling between an electron’s motion and its spin.

Yet a fundamental question has clouded the field: when researchers measure a helicity-dependent photocurrent, is it truly coming from the bulk polarity of the crystal, or is it an artifact of surfaces and interfaces? In conventional CPGE measurements, signals from crystal surfaces often mix with bulk contributions, making it nearly impossible to determine the microscopic origin of the observed current. Professor Kouji Taniguchi of Science Tokyo, who led the new study, noted that CPGE is closely related to spin-polarized electronic states and has attracted attention as a route for generating spin-polarized photocurrents by light, but that this mixing of surface and bulk signals has made the microscopic origin difficult to pin down.

To resolve the ambiguity, the Science Tokyo team—including graduate student Ichi Naruse and Assistant Professor Po-Jung Huang of the Department of Chemistry—turned to two-dimensional organic–inorganic hybrid perovskites, abbreviated 2D-OIHPs. These layered crystals are built from alternating sheets of lead iodide and organic 1-(p-tolyl)ethylammonium cations. The design is deliberate: lead atoms endow the crystal with strong spin–orbit interaction, while the molecular cations carry permanent electric dipoles that confer a spontaneous macroscopic polarity on the whole crystal. In other words, the material combines the ingredients needed for strong CPGE with a tunable internal polarity set entirely by molecular arrangement.

The crucial experimental innovation was a symmetry-selective measurement geometry. The researchers illuminated the crystal with circularly polarized light at normal incidence—striking the surface at 90 degrees—and detected a photocurrent perpendicular to the crystal’s polarization axis that reversed direction when the light’s handedness was switched. When they rotated the electrodes by 90 degrees to measure along the polarization direction, the helicity-dependent current vanished entirely, exactly as the symmetry of the bulk crystal predicts. This directional asymmetry is the fingerprint of a bulk effect: surface contributions, which have different polarizations and spin orientations, are suppressed under normal incidence while the bulk response survives.

The importance of geometry became even clearer in control experiments. When the researchers tilted the light to a 45-degree incidence angle, helicity-dependent currents appeared in both the x- and y-axes, indicating that oblique illumination reintroduces contributions that obscure the intrinsic bulk signal. Normal incidence, the team concluded, is essential for cleanly isolating the bulk CPGE. The finding gives experimentalists a practical recipe: by simply choosing how light enters the crystal, they can separate bulk and surface photoresponses in atomically layered hybrid materials, a capability that should accelerate the characterization of spin-dependent properties across this growing material family.

To prove that bulk polarity truly dictates the direction of the photocurrent, the researchers went a step further and synthesized chiral-polar crystals containing either right-handed or left-handed versions of the organic molecule—mirror-image enantiomers. The chirality of the molecules determines the direction of the crystal’s overall polarization, so the two enantiomeric crystals exhibit opposite polarization directions. Remarkably, the sign of the CPGE photocurrent reversed between the two crystals, providing direct evidence that the molecular arrangement within the bulk sets the polarity of the crystal and, with it, the direction of the light-generated current. Flipping the handedness of the building blocks flips the response of the entire device.

The implications extend well beyond a single material system. By establishing that molecular design controls bulk polarity and thereby the direction of spin-polarized photocurrents, the study offers a rational strategy for engineering light-driven charge and spin responses from the bottom up. Taniguchi and his colleagues suggest that the findings could contribute to the development of helicity-sensitive photodetectors, spin-photonic devices, and next-generation opto-spintronic materials based on 2D hybrid perovskites. A photodetector that distinguishes left- from right-circularly polarized light, or a spin current source that requires nothing more than a laser and a cleverly designed crystal, are no longer distant concepts but plausible engineering targets.

The work, made available online on September 1, 2026, and published in Volume 26, Issue 36 of Nano Letters on September 16, 2026, also carries a broader lesson for materials science: symmetry is not merely a descriptive label but an operational tool. By aligning measurement geometry with the symmetry of the bulk crystal, researchers can silence unwanted surface signals and read out the intrinsic response of a material with unprecedented clarity. As hybrid perovskites continue to mature as a platform for optoelectronics, the ability to dial in polarity through molecular arrangement—and to verify it through symmetry-selective excitation—may prove to be one of the field’s most valuable design principles, bringing spin-based information technologies a step closer to practical reality.

Subject of Research: Bulk circular photogalvanic effect in two-dimensional organic–inorganic hybrid perovskites

Article Title: Molecular arrangement controls crystal polarity and reverses photocurrent direction

Article References: Molecular arrangement controls crystal polarity and reverses photocurrent direction. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: circular photogalvanic effect, hybrid perovskites, spintronics, crystal polarity, circularly polarized light, spin–orbit interaction, chiral crystals, photocurrent, 2D materials, opto-spintronics, molecular design, Nano Letters

The new material belongs to a family of compounds known as two-dimensional hybrid organic-inorganic perovskites. These are layered semiconductors in which alternating sheets of organic and inorganic components stack into a highly ordered crystalline structure. The inorganic layers provide mechanical rigidity and structural definition, while the organic layers act as spacers whose chemical composition can be tuned almost at will. That tunability is the key to the new result. In their earlier work, the NC State team had already observed unusual combinations of stiffness and thermal behavior in this class of materials, but the new study represents a deliberate, targeted effort at molecular engineering rather than an incidental discovery.

The specific strategy involved modifying the carbon-carbon chains within the organic layers. By replacing a portion of those chains with a carefully tailored combination of benzene rings, the researchers gained precise control over how the material conducts heat and how rigid it is. Benzene rings are bulky, planar molecular units that disrupt the efficient pathways along which vibrational energy, and therefore heat, normally travels through a solid. Incorporating them in a controlled way scatters the lattice vibrations, known as phonons, that would otherwise carry thermal energy across the layers, while the overall layered architecture preserves the crystalline order that gives the film its stiffness.

The material the team produced is an azobenzene ethyl ammonium lead iodide thin film. When tested at room temperature, it exhibited a thermal conductivity of approximately 0.04 watts per meter-kelvin. To appreciate how remarkable that figure is, consider silicone, a soft polymer widely used to insulate against heat in everyday products such as oven mitts. Silicone has a thermal conductivity of about 0.2 watts per meter-kelvin, five times higher than the new perovskite film. Yet the engineered film is not a squishy insulating gel; it is a rigid semiconductor with mechanical stiffness exceeding that of silicone by a factor of roughly 700 to 10,000, depending on how the comparison is made. Combining those two figures underscores the scale of the achievement: a dense, load-bearing material that insulates better than the soft foams and elastomers engineers usually reach for when heat control matters.

Dali Sun, co-corresponding author of the study and a professor of physics at NC State, emphasizes why this combination is so valuable. Stiff materials that are good thermal insulators would have substantial utility across a wide range of applications, from cookware to electronic devices to space travel, he notes. The problem, he explains, is that in general stiff materials conduct heat well, and materials that insulate well are not stiff. The new compound is very stiff and extremely good at insulating against heat, outperforming any material one would find in nature. That framing is not marketing hyperbole but a statement about the material’s position relative to the entire known landscape of dense solids.

Jun Liu, co-corresponding author and an associate professor of mechanical and aerospace engineering at NC State, describes the work as a demonstration of what advanced molecular engineering can achieve when properties are designed intentionally rather than accepted as given. The team had previously demonstrated unusual behavior related to the combination of stiffness and thermal conductivity in a specific class of materials, she says, and for this study they engaged in more advanced molecular engineering to deliberately create an extreme combination of those properties. The result approaches the theoretical floor for thermal conductivity in a non-porous solid, meaning there is very little room left for any conventional material to do better without introducing porosity, which would sacrifice stiffness and structural integrity.

Equally important for practical adoption is the fact that the production method scales. Liu notes that the technique used to create the film can be scaled up fairly easily, allowing the material to be produced at fairly large scales, applied as a coating, and integrated into existing manufacturing workflows. This is a critical distinction from many laboratory-record thermal insulators, such as exotic aerogels or nanostructured ceramics, which achieve impressive numbers only through porous architectures that are difficult to produce uniformly over large areas and which lack the mechanical strength needed for demanding applications. A printable thin film that is both stiff and ultrainsulating opens the door to conformal thermal barriers on electronic components, protective layers in high-temperature environments, and insulation systems for aerospace structures where every gram and every millimeter counts.

The study, titled Extremely Low Thermal Conductivity in Rigid Layered Hybrid Perovskites, was published on September 18, 2026, in Science Advances. Co-lead authors are Ziqi Wang and Ankit Negi, Ph.D. graduates of NC State, along with Liang Yan of the University of North Carolina at Chapel Hill and Qingxuan Wang of Nanjing Normal University. The co-corresponding authors are Jun Liu and Dali Sun of NC State, Wei You of UNC Chapel Hill, and Jun Zhou of Nanjing Normal University. The broader author team includes contributors from NC State, UNC Chapel Hill, Texas A&M University, Yale University, Shanghai Polytechnic University, and Wenzhou University, reflecting the collaborative experimental and computational effort required to synthesize, characterize, and model the material’s behavior.

The research was supported by the National Science Foundation under multiple grants, by the U.S. Department of Energy, by the Office of Naval Research, and by the Goodnight Innovation Distinguished Professor Endowment. Beyond the specific compound, the authors point to the broader significance of the work: it highlights the potential of molecular engineering to fine-tune hybrid layered materials for applications that require novel combinations of stiffness and thermal insulation. If the same benzene-ring design principle can be extended across other members of the hybrid perovskite family and into related layered systems, the coming years could see a new generation of structural materials in which heat management is designed into the molecular architecture itself rather than bolted on afterwards. For engineers designing everything from smartphone processors to spacecraft heat shields, that prospect transforms thermal insulation from a compromise into a design variable.

Subject of Research: Development of a rigid two-dimensional hybrid organic-inorganic perovskite thin film with extremely low thermal conductivity achieved through molecular engineering of organic layers.

Article Title: New thermal insulator outperforms any material found in nature

Article References: New thermal insulator outperforms any material found in nature. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: thermal insulation, hybrid perovskites, molecular engineering, thin films, thermal conductivity, materials science, NC State University, Science Advances, stiffness, phonon scattering, benzene rings, thermal barriers