The material at the heart of the study belongs to a family called zero-dimensional metal halides. In the crystal Cs3GdCl6, negatively charged gadolinium-chlorine octahedra float in isolation, surrounded by cesium ions, with no continuous framework connecting them. This isolated architecture gives the compound exceptional chemical and thermal stability, a stark contrast to the moisture-fragile lead halide perovskites that have dominated optoelectronics research. Lead perovskites are brilliant light emitters, but their toxicity collides with regulations such as the Restriction of Hazardous Substances Directive, and their instability, while useful for sensing, ruins long-term device reliability. The cesium-gadolinium-chloride host sidesteps both problems while offering another crucial advantage: as a heavy halide lattice with low phonon energy, it suppresses the vibrational relaxation that normally kills upconversion, the nonlinear process by which materials absorb low-energy infrared photons and emit higher-energy visible light.

By replacing a fraction of the gadolinium ions with lanthanide emitters such as terbium, ytterbium, or erbium, the researchers created a versatile optical platform. X-ray diffraction with Rietveld refinement confirmed a pure monoclinic structure, and the unit cell contracted linearly with dopant concentration, exactly as Vegard’s law predicts for successful substitution. X-ray photoelectron spectroscopy verified that the dopants sat in their trivalent states within the halide coordination environment. The erbium-only composition Cs3Gd0.8Er0.2Cl6 emerged as the most interesting member of the family, because its emission color depended dramatically on which infrared laser was shining on it.

Under 1550-nanometer excitation, erbium ions absorb photons through the transition from their ground state to the first excited manifold, and a sequential three-photon process climbs the energy ladder to the green-emitting levels at 525 and 550 nanometers. Under 980-nanometer excitation, erbium’s absorption is inherently weak because the relevant transition has a low oscillator strength, and a two-photon pathway instead populates a higher level that relaxes nonradiatively, favoring red emission at 670 nanometers. Pump-power measurements confirmed the physics: intensity scaled with roughly the second power of laser power at 980 nanometers and between 2.5 and 3 at 1550 nanometers, matching the two- and three-photon mechanisms respectively.

Then came the surprise. When the erbium-only crystal was exposed to ambient air at 22 percent relative humidity, its bright green upconversion glow under 1550-nanometer light shifted to red within three minutes. Yet under 980-nanometer light, the same sample, in the same humid air, held a steady yellow hue with virtually unchanged spectra. Moisture alone, in other words, did not determine whether the material switched color. The deciding factor was whether the excitation wavelength could pump enough population into the moisture-sensitive excited states. This excitation selectivity is what the authors call hydrochromic upconversion, and it elevates the phenomenon from a simple wavelength-dependent color variation to a mechanistically defined optical response.

The mechanism hinges on water’s molecular vibrations. Adsorbed water molecules carry high-energy oxygen-hydrogen oscillations that act as exceptionally efficient quenching channels, draining energy nonradiatively from erbium’s green-emitting levels. But quenching can only occur if those levels are actually populated, which requires strong absorption at the excitation wavelength. Time-resolved photoluminescence sealed the argument. In a ytterbium-sensitized companion composition, where ytterbium absorbs 980-nanometer light strongly and transfers energy to erbium, the green emission decay curves showed a characteristic population buildup that vanished as moisture exposure progressed, alongside a shrinking lifetime, the classic signature of oscillation quenching. In the erbium-only crystal under 980 nanometers, no buildup existed and the decay curves stayed frozen throughout eight minutes of humidity exposure, because the excited-state population was simply too sparse for water to matter. Crucially, Raman spectra collected during exposure showed that the vibrational bands of the gadolinium-chloride octahedra never shifted or broadened, proving the color change involves no lattice disorder or defect formation, only surface-mediated energy quenching.

The absorption-coefficient explanation passed a decisive test. Quantified from diffuse-reflectance spectra, the ratio of absorbance at 1550 nanometers to that at 980 nanometers was 3.11 for the erbium-only crystal but inverted to 0.56 for the ytterbium-codoped one. When both compositions were excited at 1550 nanometers, a wavelength both absorb strongly, both showed the rapid green-to-red hydrochromic transition. The rule held at every level of the analysis: steady-state color, decay kinetics, and absorption strength all pointed to the same criterion. A secondary contribution from moisture-altered erbium-erbium cross-relaxation cannot be fully excluded, the authors note, but vibrational quenching by adsorbed water is clearly the dominant driver.

To showcase the practical power of this selectivity, the team arranged three compositions, the terbium emitter, the ytterbium-erbium codoped crystal, and the erbium-only crystal, into a single pattern plate that is completely invisible under daylight. Under 254-nanometer ultraviolet light, the terbium regions glow green in a circular pattern; under x-rays, the same circle appears on a CMOS sensor array, since the terbium emission at 548 nanometers matches the sensitivity of commercial imaging chips and scales linearly with x-ray dose. Under 1550-nanometer infrared light, both erbium compositions reveal a green comet pattern that turns entirely red when moisture arrives. Under 980 nanometers, however, a yellow heart shape appears, and it stubbornly refuses to change even in humid air. Because the heart is moisture-insensitive while the comet is not, the same plate carries an internal reference that separates genuine moisture signals from any general drift in emission, a built-in control that most single-stimulus sensors lack.

The researchers then pushed beyond qualitative color tricks into quantitative dosimetry. They built an automated analysis pipeline in Python that extracts video frames, detects the crystal region of interest, and converts emission colors into the CIE Lab* color space, tracking the a* coordinate that captures the green-to-red transition. The a* value climbs from minus 33.21 in the dry state to 20.37 at full saturation, defining fixed boundaries that normalize the response to a 0-to-100 percent saturation scale. Experiments across relative humidities from 11 to 93 percent, controlled with saturated salt solutions, showed that the color evolution follows the long-time solution of Fick’s second law of diffusion, with fitting quality above 0.97 under high humidity. The extracted diffusion coefficients rose exponentially with relative humidity, correlating above 0.96 across the entire range. Because the measured color reflects moisture that has already diffused into the lattice rather than the instantaneous external humidity, the sticker records cumulative exposure, so the readout remains meaningful even when humidity fluctuates unpredictably over time.

The end result is a smartphone-based dosimeter that requires no external power. A 1550-nanometer laser illuminates the sticker, the phone camera captures the emission, an inverse calculation based on Fick’s law converts the color coordinates into a saturation level, and the user receives a concrete number, say 68 percent, instead of a vague color warning. Unlike conventional binary indicators such as cobalt chloride, this system quantifies accumulated moisture uptake nondestructively through sealed packaging, making it attractive for precision chemicals, pharmaceutical storage, and heritage conservation. The luminescent material itself can even be regenerated by heating, although the assembled sticker is designed as a single-use record. Beyond the device, the deeper contribution is conceptual: the demonstration that hydrochromic switching in upconversion materials obeys an excitation-selective criterion, governed by whether the pump wavelength can populate moisture-sensitive excited states, gives materials designers a general principle for building optical sensors whose sensitivity can be switched on and off simply by changing the color of invisible light.

Subject of Research: Excitation-selective hydrochromic upconversion in lanthanide-doped zero-dimensional lead-free metal halides for optical moisture sensing

Article Title: Excitation‐Selective Hydrochromic Upconversion in Zero‐Dimensional Metal Halides

Article References: Han, J. H., Seo, J. M., Jang, S. W., Park, Y. M., Choi, S. H., Cha, J. H., & Im, W. B. (2026). Excitation‐Selective Hydrochromic Upconversion in Zero‐Dimensional Metal Halides. Advanced Science, Article e78059. https://doi.org/10.1002/advs.78059

Image Credits: AI Generated

DOI: 10.1002/advs.78059

Keywords: upconversion, metal halides, hydrochromic, lanthanide doping, erbium, ytterbium, moisture sensing, humidity dosimetry, Fick's second law, lead-free perovskite alternative, anti-counterfeiting, optical readout