29th International Scientific Conference of Young Scientists and Specialists (AYSS-2025)

Absorption properties of boron nitride quantum dots: Effects of solvents

27 Oct 2025, 18:30

2h

JINR International Conference Centre, 2 Stroiteley st.

Poster Condensed Matter Physics Poster session & Welcome drinks

Speaker

Mr Ilia Simonenko (BLTP)

Description

Abstract

Boron nitride quantum dots (BNQDs) are promising for Boron Neutron Capture Therapy (BNCT) due to their unique properties. This work analyzes their electronic structure using DFT and TD-DFT to understand their absorption characteristics. We demonstrate both theoretically and experimentally that solvents used in synthesis do not act merely as a medium but chemically attach to the BNQD surface, altering their optical properties. A fundamental absorption edge is identified at 215-220 nm for BNQDs with diameters of 10-15 nm, resulting from a size-saturation effect.

Introduction

Boron Neutron Capture Therapy (BNCT) requires effective agents for delivering $^{10}$B, a role for which boron nitride quantum dots (BNQDs) are well-suited due to their high boron content and stability.$^{1}$ Their excellent biocompatibility and tunable optical properties make them promising for theranostics, though shifting their absorption into the near-infrared (NIR) window remains a challenge.$^{2}$ This work investigates two key factors governing their absorption: the quantum confinement effect and surface functionalization by synthesis solvents.

Results and Discussion

BNQDs were synthesized via a "top-down" solvothermal method. Bulk h-BN was first exfoliated into nanosheets (BNNSs, avg. size 0.56 µm) using ultrasonication, which were then converted into quantum dots (avg. diameter 4.9 nm) by solvothermal treatment at 200 °C (Fig. 1).

To understand their intrinsic optical properties, we performed TD-DFT calculations (ORCA$^{4}$, B3LYP/6-31++G**) on BNQD models ranging from 7 to 61 rings. As dot size increases, the band gap ($E_g$) systematically decreases from 6.62 eV to 5.96 eV due to quantum confinement (Fig. 2). This narrowing of the HOMO-LUMO gap is clearly visible in the Density of States (DOS) plots (Fig. 3).

However, this electronic modulation results in only a minor bathochromic shift of the absorption maximum ($\lambda_{abs}$) from ~199 nm to ~210 nm (Fig. 4). The reason is a strong size-saturation effect, which limits the fundamental absorption edge of pristine BNQDs to ~215–220 nm for diameters above 10 nm (Fig. 5). This highlights the limitations of size-tuning alone and confirms the necessity of using hybrid functionals for accurate predictions.

Given the limits of quantum confinement, we investigated surface functionalization by solvent molecules, a phenomenon supported by our experimental FTIR and XPS data.$^{3}$ Our theoretical approach explicitly models covalently bonded solvent molecules, diverging from common continuum models.$^{5,6}$ Natural Transition Orbital (NTO) analysis revealed that the frontier orbitals become localized on attached nitrogen-containing solvents (DMF, NMP) but remain delocalized for ethanol (Fig. 6).

This localization is caused by the formation of a new occupied mid-gap state at -6.2 eV, arising from orbital overlap between nitrogen atoms in the solvent and the BNQD (Fig. 7).

This modification of the electronic structure induces a more significant bathochromic shift than size-tuning, and our calculations show good agreement with experimental spectra (Fig. 8). Nevertheless, this chemical effect also saturates, capping the absorption maximum in the UV region (~230-235 nm for DMF).

Summary

In summary, we have shown that the UV absorption of BNQDs is determined by both quantum confinement and surface chemical functionalization. Crucially, both mechanisms exhibit saturation effects that limit the absorption edge to the UV region, hindering a significant shift into the NIR therapeutic window. This understanding highlights the need for alternative strategies, such as defect engineering or atomic doping, to achieve the desired optical properties for advanced BNCT applications. A full account of this research is detailed in our recent publication in PCCP.$^{3}$

Acknowledgements

The research was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (theme No. 124110600041-0).

References

1. I. V. J. Feiner et al., J. Mater. Chem. B, 2021, 9, 410.
2. A. P. Shmidberskaya et al., JETP Letters, 2025, 121, 611.
3. E. A. Sidorov et al., Phys. Chem. Chem. Phys., 2025.
4. F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2012, 2, 73.
5. A. Haridas et al., J. Phys. Chem. Solids, 2025, 198, 112447.
6. Y.-J. Gao et al., Phys. Chem. Chem. Phys., 2023, 25, 3912.