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Two-Dimensional Gold Quantum Dots with Tunable Bandgaps Shiva Bhandari,† Boyi Hao,† Kevin Waters,† Chee Huei Lee,† Juan-Carlos Idrobo,‡ Dongyan Zhang,† Ravindra Pandey,† and Yoke Khin Yap*,† †
Department of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 3783, United States
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‡
S Supporting Information *
ABSTRACT: Metallic gold nanoparticles (Au NPs) with multilayer Au atoms are useful for plasmonic, chemical, medical, and metamaterial application. In this article, we report the opening of the bandgap in substrate-supported two-dimensional (2D) gold quantum dots (Au QDs) with monolayer Au atoms. Calculations based on density functional theory suggest that 2D Au QDs are energetically favorable over 3D Au clusters when coated on hexagonal boron nitride (BN) surfaces. Experimentally, we find that BN nanotubes (BNNTs) can be used to stabilize 2D Au QDs on their cylindrical surfaces as well as Au atoms, dimers, and trimers. The electrically insulating and optically transparent BNNTs enable the detection of the optical bandgaps of the Au QDs in the visible spectrum. We further demonstrate that the size and shapes of 2D Au QDs could be atomically trimmed and restructured by electron beam irradiation. Our results may stimulate further exploration of energetically stable, metal-based 2D semiconductors, with properties tunable atom by atom. KEYWORDS: two-dimensional materials, nanotubes, gold clusters, gold quantum dots, boron nitride nanotubes, pulsed-laser deposition
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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
old nanoparticles (Au NPs) are of interest for their intriguing properties. They are used in plasmonic and metamaterial devices1 and field-effect transistors without semiconductors2 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 reactions10 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 TiO216 and MgO17 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 © XXXX American Chemical Society
Received: December 18, 2018 Accepted: March 28, 2019
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DOI: 10.1021/acsnano.8b09559 ACS Nano XXXX, XXX, XXX−XXX
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Cite This: ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. SEM images of as-grown (a) BNNTs and (b) dispersed BNNTs coated with 6 nm-thick Au films. Raman (c) and FTIR (d) spectra of Au-coated BNNTs. TEM images (e) of BNNTs with Au NPs coated on one side of the nanotubes.
and other metals for attractive electronic, photonic, plasmonic, chemical, and biological applications for which properties can be tuned atom by atom.
resolution images (Figure 2a) obtained by aberration-corrected scanning transmission electron microscopy (STEM) show that Au NPs with diameters of 2−6 nm are coated on one side of the cylindrical BNNT surfaces. Thinner and smaller 2D Au QDs are seen on surfaces facing away from the Au vapor generated by PLD. The magnified view in Figure 2b allowed us to visualize the 2D Au QDs and some isolated Au atoms, dimers, trimers, etc. (as shown using red arrows). Moreover, Figure 2b suggests the formation of crystalline 2D Au clusters with single atom thickness and sizes ranging from a few to 100 Au atoms (see more examples in Figures S3 and S4). The lattice distance between two adjacent arrays of Au atoms was estimated to be about 0.266 nm, by calibrating the diffraction spots generated from fast Fourier transform (FFT) of the STEM images. This is consistent with the reported data in the Joint Committee on Powder Diffraction Standards (JCPDS: 4784). Growth Mechanism of 2D Au QDs. The formation mechanism of the 2D Au QDs is discussed in the following paragraph. Au vapor generated by laser ablation propagates from left to right toward a cylindrical BNNT (Figure 2c). The BNNT surface facing the Au vapor will be coated with Au adatoms, while the other surfaces will be blocked and shielded from the direct coating because of the directionality of the vapor generated by the PLD. As shown in Figure 2d, (1) most of the Au adatoms will be adsorbed on the left surface and it will continue to grow in thickness. (2) Some adatoms will be resputtered away. (3) Isolated adatoms near a larger Au island
RESULTS AND DISCUSSION 2D Au QDs were coated on BNNTs using pulsed-laser deposition (PLD)2,20 (see Supporting Information, Figures S1 and S2) at room temperature. These vertically aligned BNNTs, of diameters of ∼15−80 nm, are electrical insulators (band gap ∼6 eV) with minimal defects and surface impurities (Figure 1a).21−23 The BNNTs were then coated with Au films of 6 nm thickness to form large NPs that are detectable when dispersed on a fresh Si substrate (Figure 1b). The spectra of Raman and Fourier transformed infrared spectroscopy (Figure 1c,d) of the Au-coated BNNTs are similar to those of the as-grown samples, signifying almost no chemical and physical modification of the properties of the as-grown BNNTs. As shown in Figure 1e, Au NPs with diameters of
