Poly(Ethylene Oxide) Mediated Synthesis of Sub-100-nm Aluminum Nanocrystals for Deep Ultraviolet Plasmonic Nanomaterials

Abstract

Aluminum nanocrystals (Al NCs) are sustainable plasmonic nanomaterials with unique localized surface plasmonic resonance (LSPR) in the ultraviolet (UV) region. Chemical synthesis of sub-100-nm Al NCs remains a considerable challenge due to the lack of effective ligands to control their growth. Here, we describe a precise size-controlled synthesis of small colloidal Al NCs (25-100 nm) with strong and tunable LSPR peak from 250 to 372 nm in the UV spectral region by the use of poly(ethylene oxide) (PEO) as a polydentate surface ligand. The LSPR band matched well with the numerical simulation results. Additionally, the molecular weight of the PEO played an essential role in tuning the size of Al NCs. The PEO showed a strong affinity with Al {111} crystal facets, resulting in octahedral-and prism-shaped Al NCs. Owing to the passive oxide layer generated on the surface of Al NCs, their LSPR peak positions showed negligible changes after storage in tetrahydrofuran for 4 months. The passivation layer also impeded the metal Al core to react with deionized water. Further, the use of biocompatible PEO ligand and the subsequent generation of the sub-100-nm size of the Al NCs pave a path for bioapplications of such NCs.

Shining Light on Aluminum Nanoparticle Synthesis

“…Subsequently, there have been significant developments to better understand and control Al nanoparticle synthesis. [2][3][4]14,19,20 Electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopies were used to clarify the mechanism of the reduction of alane precursors using TIP, as well as the mechanism underlying size control using solvent ratios (Figure 1A and 1B). 2 EPR allows for direct observation of the catalyst/alane reaction in situ, as many of the catalysts and intermediates are paramagnetic and, therefore, NMR inactive.…”

“…Further developments of Al NC synthesis have focused on three main directions: alternate Al precursors, 3 ligand-based control, 14,19,20 and alternate catalysts. 4 In 2018, the first synthesis of anisotropic Al NCs was demonstrated.…”

“…This method produced quality crystalline particles of varying shape, but the underlying mechanism controlling size was unknown. Subsequently, there have been significant developments to better understand and control Al nanoparticle synthesis. − ,,, …”

“…11,12 Al has been shown to be a promising alternative for sustainability and for higher photon energy plasmonics, as it is far more abundant the most abundant metal at the earth's surfacewhile supporting resonances from the IR well into the UV region of the spectrum. 13,14 Similar to Au and Ag, Al is a FCC metal, therefore the same range of structures is theoretically available. Al NC precursors are highly reactive, however, resulting in a relative lack of development of Al NC synthetic methods compared to Au and Ag.…”

“…In addition to cost, the optical properties of Au and Ag in nanoparticle form are limited by their electronic structure. The LSPR of Au is restricted by the presence of its filled d-band to energies below the interband transition at nominally 520 nm, and Ag is similarly limited to wavelengths longer than 350 nm. , Al has been shown to be a promising alternative for sustainability and for higher photon energy plasmonics, as it is far more abundantthe most abundant metal at the earth’s surfacewhile supporting resonances from the IR well into the UV region of the spectrum. , Similar to Au and Ag, Al is a FCC metal, therefore the same range of structures is theoretically available. Al NC precursors are highly reactive, however, resulting in a relative lack of development of Al NC synthetic methods compared to Au and Ag.…”