Ultrasmall Nanoplatelets: The Ultimate Tuning Of Optoelectronic Properties

 Semiconductor nanocrystals with unique optoelectronic properties have emerged as promising materials for applications in solar technologies, including solar cells, solar-driven hydrogen production, and luminescent solar concentrators.

Among them, ultrathin two-dimensional (2D) semiconducting nanoplatelets (NPLs) are sheet-like structures with single- or few-layer thickness (typically less than 5 nm), with lateral size ranging from 100 nm to tens of micrometers. They exhibit ultrahigh specific surface area and strong one dimensional (1D) quantum confinement, leading to novel physical, optical, chemical, and electronic properties. These features include relatively large absorption coefficient, high carrier mobility, and unique thickness-dependent optical transitions, and make it favorable for fabricating high-performance optoelectronic devices for solar technologies.

Recent advances are focused on the synthesis of ultrathin visible-active Cd-based NPLs and near-infrared (NIR) NPLs with large lateral sizes. In order to enhance solar harvesting, colloidal NIR lead (or tin) chalcogenides based NPLs enable a thickness-tunable wide absorption spectrum, ranging from ultraviolet (UV) to NIR which matches well with the spectrum of the sun. However, the bandgaps of as-obtained NIR NPLs with large lateral sizes are small (< 0.9 eV) and the corresponding energy levels are not suitable for solar-driven hydrogen generation. Typically, the decrease in the lateral size of NPLs may increase the bandgap of NPLs with similar thickness. Up to now, there is still no report for producing ultrasmall NPLs (less than 10 nm in lateral size) due to a lack of reliable synthetic methodologies.

To address the above-mentioned issues, the research group led by Professor Federico Rosei from INRS-EMT center reported a cation-exchange route to synthesize ultrasmall ternary NPLs optically active in the NIR range with engineered bandgaps (Figure 1). The ultrasmall PbSe_1-xS_x NPLs with a lateral size of 4-10 nm, a controlled thickness of ~ 2 nm, and different compositions are obtained via a cation exchange process using large lateral size CdSe or alloyed CdSe_1-xS_x NPLs as templates. The bandgaps of as-synthesized ultrasmall NIR NPLs are larger than 1.0 eV, which will favor their applications in solar cells and solar-driven H₂ generation.

The photoluminescence (PL) emission is tunable in the range of 1180-1380 nm thanks to the variation of the S/Se ratio. A high PL quantum yield (defined as the ratio of numbers of emitted photons and numbers of absorbed photons) of up to 60% is achieved, suggesting the potential applications in NIR photodetectors and photosensors. In addition, theoretical simulations of the bandgap as a function of thickness, geometry, and size reveal that ultrasmall NPLs exhibit strong quasi-3D quantum confinement, compared with 1D confinement in larger lateral sized NPLs with similar thickness. Their results indicate that ultrasmall NIR NPLs is suitable for solar energy-related applications, through efficient bandgap engineering.

As a proof-of-concept, the ultrasmall NIR NPLs are firstly post-treated with Cd ion to enhance the stability and used as photosensitizers for solar-driven photoelectrochemical (PEC) H₂ generation. The energy levels in ultrasmall NPLs/TiO₂ heterostructure is appropriate for efficient electron injection from NIR NPLs to TiO₂. The photogenerated electrons transfer to Pt electrode for the reduction of water, leading to high H₂ generation rate in PEC system. As a result, a photocurrent density of ~5 mA cm^-2 is obtained in this system under one sun illumination, equals to the H₂ generation rate of 44 mL cm^-2 d^-1.

In summary, we demonstrated a simple template-assisted cation exchange approach to synthesize ultrasmall NIR NPLs. The ultrasmall NIR NPLs are good candidates for solar energy applications, through efficient bandgap engineering and surface passivation. This approach can be extended to synthesize other small-sized semiconductors NPLs, such as HgS(Se), BiS(Se), CuSnS(Se), CuZnS(Se), and CuInS(Se).

This study, Ultrasmall Nanoplatelets: The Ultimate Tuning of Optoelectronic Properties was recently published in the journal Advanced Energy Materials.