A programmable semiconductor changes its electronic properties under UV light and reverts under a different wavelength, enabling optical writing and reading for potential computing applications.
(Nanowerk Spotlight) Computing hardware faces an impasse. Two fundamental constraints loom: as transistors approach atomic dimensions, quantum effects like electron tunneling cause current to leak through barriers that should be impermeable; and the von Neumann bottleneck limits how quickly data can shuttle between processor and memory. These problems have pushed researchers to explore radically different approaches.
One promising avenue involves semiconductors whose optical and electronic properties can be altered after device construction, enabling materials that combine computation and memory storage in ways conventional semiconductors cannot. Such adaptive materials could support neuromorphic computing, which mimics biological neural networks, or optical computing systems that process information using photons rather than electrons.
Creating these materials presents formidable challenges. A semiconductor must maintain structural integrity while undergoing reversible chemical changes, and those changes must translate into measurable differences in electronic behavior. Previous attempts using layered hybrid perovskites with azobenzene photoswitches showed initial promise, but subsequent studies revealed that switching occurred only at particle surfaces rather than throughout the material bulk, severely limiting practical utility.
A research team from Leibniz Universität Hannover and the Paul-Drude-Institut für Festkörperelektronik in Berlin has now overcome this limitation. Their work, published in Advanced Functional Materials (“A Programmable Semiconductor Containing Active Molecular Photoswitches Located in the Crystal’s Volume Phase”), describes a layered hybrid perovskite incorporating molecular photoswitches that toggle using ultraviolet light, altering the material’s bandgap, the energy barrier electrons must overcome to participate in conduction, throughout its entire volume. The research represents a materials-level proof of concept rather than a finished device, but establishes the fundamental physics for future development.
Layered hybrid perovskites consist of alternating organic and inorganic layers. The inorganic component, typically built from lead and halide atoms arranged in corner-sharing octahedra (geometric units with eight triangular faces), determines the material’s electronic properties. The organic layer provides structural spacing and can host functional molecules.
Characterization of the Coumarin-based layered hybrid perovskite (LHP). a) Crystal structure of the novel perovskite containing the Coumarin-derivative as organic spacer. The two crystallographically different Coumarin species are presented in bright and dark blue, respectively. b) Relative orientation and distance analysis of the carbon atoms to participate in the [2+2]-cycloaddition. c) pXRD pattern of the novel perovskite (red line) with reflection positions (blue bars) calculated from the unit cell and space group, fit after Rietveld refinement (black line) and difference plot (grey line). d) SEM-micrograph of the LHP particles. Scale bar = 1 μm. e) Normalized diffuse reflectance (F(R), solid lines) and fluorescence spectrum (I, dashed lines, excited @350 nm) of the perovskite (blue) and the bromide salt (black) of the Coumarin derivative. (Image: Reproduced from DOI:10.1002/adfm.202524426, CC BY) (click on image to enlarge)
The researchers built their material using a derivative of Coumarin, a ring-shaped organic molecule that undergoes a well-studied photochemical reaction called [2+2] cycloaddition. When exposed to UV light at 365 nm wavelength, two adjacent Coumarin molecules link together, forming a four-membered carbon ring. This dimerization is reversible: illumination with higher-energy UV light at 254 nm breaks the ring apart, regenerating the original molecules.
For this reaction to occur in a solid material, participating molecules must be positioned correctly. The Schmidt rule states that [2+2] cycloadditions require the reactive double bonds to be roughly parallel and within about 4 Å of each other. Crystal structure analysis confirmed that these geometric requirements were satisfied in the new perovskite.
The team synthesized their material by dissolving lead bromide and the Coumarin derivative in dimethylformamide, then adding this solution to dichloromethane, which causes the perovskite to precipitate out. The resulting slightly yellow powder formed microscopic, rod-like crystals less than 10 μm in size. X-ray diffraction revealed two crystallographically distinct Coumarin molecules positioned to form exactly one dimerization pair per molecule.
Illumination experiments demonstrated that UV light at 365 nm could convert up to 70% of the Coumarin monomers to dimers. This high conversion rate confirms that photoswitching occurs throughout the crystal volume, not merely at particle surfaces, representing a significant advance over previous azobenzene-based systems.
The chemical transformation produced measurable changes in the semiconductor’s properties. Both the excitonic absorption peak and the absorption edge shifted toward shorter wavelengths as dimerization progressed. Excitons are bound pairs of electrons and the positively charged holes they leave behind; their behavior serves as a signature of a material’s electronic structure.
Photoluminescence spectra revealed even more dramatic changes. The narrow emission characteristic of free excitons progressively weakened with increasing dimerization. Simultaneously, a broad emission peak emerged around 550 nm, attributed to defect states where excitons become trapped.
X-ray diffraction patterns explained the structural origin of these electronic changes. The layer-to-layer spacing increased, while the in-plane area per cation decreased. These geometric changes indicate increased tilting of the lead bromide octahedra, which reduces orbital overlap between neighboring units and widens the bandgap.
The researchers demonstrated spatial control by illuminating a film through an optical grating with 100 μm openings. Fluorescence microscopy revealed a clear pattern: illuminated regions showed green defect fluorescence, while protected regions retained blue emission. Raman spectroscopy, which detects molecular vibrations, confirmed the spatial distribution of monomers and dimers across the patterned surface.
Reversibility testing showed that irradiating dimerized samples with 254 nm light reduced dimerization by 14% after four hours, starting from 67.5%. The incomplete reversal reflects competing processes: higher-energy UV light triggers both cycloreversion and, more slowly, new dimerization events. Quantum efficiency calculations yielded 0.16% for dimerization and 0.23% for cycloreversion, indicating the reversion faces no structural hindrance. Three complete switching cycles confirmed consistent alternation between higher and lower dimerization states, though full reversal to the original condition was not achieved.
The material’s ability to be optically written and read through multiple mechanisms suggests potential applications in all-optical data storage. The continuous tunability of electronic properties, rather than simple binary switching, could support neuromorphic or synaptic devices that mimic biological neural networks. The demonstrated resolution of at least 100 μm indicates that electronic patterns can be inscribed within homogeneous films, though smaller feature sizes remain untested. As a proof of concept, this work offers a potential pathway toward light-controlled semiconductor materials.
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