Optoelectronic properties of octahedral molybdenum cluster-based materials at a single crystal level

E. Segura-Sanchis; A. Moreno; F. Ramiro-Manzano; R. Fenollosa; M. Feliz; P. Atienzar

doi:10.1039/D3DT02501B

Research · Publication & note

Optoelectronic properties of octahedral molybdenum cluster-based materials at a single crystal level

Research article

E. Segura-Sanchis, A. Moreno, F. Ramiro-Manzano, R. Fenollosa, M. Feliz and P. Atienzar

Dalton Transactions 52, 17818–17825 (2023) · Published 9 November 2023

Abstract

Octahedral molybdenum (Mo₆) clusters constitute suitable building blocks for the design of promising single crystal materials in the field of optoelectronics. Here, we prepared single crystals composed of hydroxo Mo₆X₈ (X = Br, Cl) cluster complexes interconnected by H-bonding interactions with water molecules and protons. The optoelectronic responses and the absorption and emission spectra of these cluster-based single crystals were acquired upon light irradiation, and they show dependency on the nature of the halogens, with the brominated cluster being the most conductive. A fast photoelectrical response was recorded and it showed remarkable stability after multiple illumination on/off cycles. The results obtained provide relevant information for the development of photonic and optoelectronic devices, sensors and photocatalysts.

Reference

Initials first

E. Segura-Sanchis, A. Moreno, F. Ramiro-Manzano, R. Fenollosa, M. Feliz and P. Atienzar. “Optoelectronic properties of octahedral molybdenum cluster-based materials at a single crystal level.” Dalton Transactions 52, 17818–17825 (2023). DOI: 10.1039/D3DT02501B.

Family name first

Segura-Sanchis, E., Moreno, A., Ramiro-Manzano, F., Fenollosa, R., Feliz, M. and Atienzar, P. (2023). “Optoelectronic properties of octahedral molybdenum cluster-based materials at a single crystal level.” Dalton Transactions, 52, 17818–17825. https://doi.org/10.1039/D3DT02501B

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Figures

Schematic representation of hydrogen bonding in layers of the MoBr structure — **Figure 1.** Schematic representation of H-bonding (dotted lines) in the layers of the MoBr structure (atom color code: Mo in blue, Br in green and O in red). H atoms are omitted for clarity.

Optical absorption and emission — **Figure 2.** (a) Diffuse reflectance and (b) steady state PL of a collection of MoBr (black line) and MoCl (red line) single crystals, registered at λ_exc = 405 nm under room conditions. The inset shows the time-resolved PL monitored at 720 nm.

Photoluminescence and lifetime — **Figure 3.** (a) Experimental scheme of the single crystal contact; (b) photocurrent spectra of a single crystal of MoBr embedded into BMIMBF₄ (black line) and a drop of BMIMBF₄ as a blank (red line), under excitation with a supercontinuum laser at different wavelengths; (c) MoBr and (d) MoCl current–time (I–t) curves under 405 nm LED light with 0.1 Hz light on/off cycles.

Figure 4. (a) Photocurrent mapping of a MoBr single crystal embedded in BMIMBF₄; (b) PL map of the MoBr single crystal. Both measurements were performed at room temperature with excitation using a 405 nm diode laser; (c) photocurrent profile along the dashed line depicted in (a). The maximum value corresponds to the electronic tip contact; (d) optical microscopy image of a single crystal of MoBr cluster.

Source: E. Segura-Sanchis et al., Dalton Transactions 52, 17818–17825 (2023), CC BY-NC 4.0.

RESEARCH TOPICS

EmissionBoth cluster crystals emit in the red/near-infrared from the triplet state of the {Mo₆X₈}⁴⁺ core after intersystem crossing. Figure 2 places the steady-state maximum at 740 nm for MoBr and 752 nm for MoCl, but time-resolved luminescence reveals a much larger difference: 80.9 μs for the bromide and 13.7 μs for the chloride. The comparison shows why spectral colour alone is insufficient to characterise how halogen substitution changes excited-state dynamics.

MaterialsMeasurements are performed at single-crystal scale to exclude grain-boundary and amorphous-domain contributions present in pressed powders or polycrystalline films. Figure 4 maps photocurrent and photoluminescence over the same MoBr crystal; the photocurrent maximum lies near the electronic-tip contact, while PL follows the optically excited area. Current–time traces under 405 nm illumination remain repeatable over multiple on/off cycles, showing that the photoresponse is a property of the crystal rather than an irreversible charging artefact.

ChemistryMoCl and MoBr retain comparable octahedral Mo₆ cluster frameworks while changing the inner halogen. Figure 1 shows clusters connected into layers by hydrogen bonding to water molecules and protons. MoCl crystals are grown by slow hydrolysis of the chloride precursor at pH 10.0–11.5 and are more prone to dehydration and cracking than MoBr; XRD and Raman checks verify that the cluster framework survives ageing. This chemical control lets conductivity and lifetime differences be related to halogen identity while keeping crystal architecture comparable.