Two-dimensional gold quantum dots with tunable bandgaps

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Gold nanoparticles (Au NPs) are of interest for their intriguing properties. They are used in plasmonic and metamaterial devices(1) and field-effect transistors without semiconductors(2) and as catalysts for chemical reactions and biological sensing.(3−9) Recently interesting synthesis approaches of Au NPs have been demonstrated, including plasmonic driven photochemical reactions(10) and amino acid- and peptide-directed synthesis.(11) Apparently, Au nanostructures have continued to attract significant research interest, including NPs and Au clusters (Aun). For the case of Aun, theory suggests that planar two-dimensional (2D) Aun are stable in gas phase, but will transform into three-dimensional (3D) Aun when n is larger than about 8–12 Au atoms.(12−15) Experimentally, 2D Aun can be coated on oxide films such as TiO2(16) and MgO(17)under ultrahigh-vacuum conditions. It is reported that Aun coated on oxide films prefer to nucleate on the step edges of the oxide films with thicknesses as high as two to three Au atomic layers.(16) Since there are steps between and within the islands of the oxide films, it is challenging by using topographic imaging to determine if the Aun coated on top are constructed of mono- or multi-layered atoms.(18,19)

In this article, we describe the discovery of 2D Au quantum dots (Au QDs) with monolayered atoms and dot sizes as large as ∼4 nm (more than 100 atoms). This is consistent with our theoretical modeling, in which, 2D Aun with n as large as about 100 atoms can be stabilized on the hexagonal BN (h-BN) substrates. Our calculations further suggest that the bandgap of such a 2D Au QD is controllable by the number of Au atoms forming the QD. Experimentally, we successfully grew 2D Au QDs at room temperature on boron nitride nanotubes (BNNTs) with defect-free (no atomic steps) sp2-hybridized tubular surfaces. The atomically flat surfaces of BNNTs allow us to determine the Au monolayers with atomic resolution. In addition, the optically transparent and electrically insulating BNNTs enable the detection of sharp optical band gaps in the visible spectrum range. We further demonstrate that the size and shapes of 2D Au QDs could be atomically trimmed and restructured by electron beam irradiation. Our discovery will stimulate the search of fascinating properties in 2D QDs of Au and other metals for attractive electronic, photonic, plasmonic, chemical, and biological applications for which properties can be tuned atom by atom.

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© 2019 American Chemical Society. Publisher's version of record: https://dx.doi.org/10.1021/acsnano.8b09559

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ACS Nano