Probing the electronic properties and charge state of gold nanoparticles on ultrathin MgO versus thick doped CaO films

Electron transfer into metal nanoparticles on oxide supports is associated with unusual morphological, electronic, and chemical properties of the charged system. Two fundamental charging routes have been identified so far, which are electron tunneling through ultrathin oxide films supported by a bulk metal and charge donation from single-ion impurities embedded in the oxide matrix. In this study, we have investigated whether both routes lead to the formation of metal deposits with identical properties. For this purpose, Au islands have been prepared on $1-2\mathrm{ML}$ thin $\mathrm{MgO}/\mathrm{Ag}\left(001\right)$ layers and on 25-ML-thick CaO films doped with Mo impurities. The morphological and electronic properties of the islands were analyzed with low-temperature scanning tunneling microscopy (STM) and spectroscopy. In both systems, pronounced electron confinement effects are observed in the nanostructures, arising from the quantization of one and the same Au electronic band. Moreover, clear experimental signatures for a charge transfer into the islands are found, such as a layer-by-layer growth of the ad-metal and a negative contrast of the oxide region around the deposits in STM images. Our data provide evidence that the charged nanostructures exhibit comparable properties independent of the origin of the extra electrons. This agreement suggests that ultrathin oxide films may be used as model systems for doped bulk oxides, as used in heterogeneous catalysis.

Figure 1

STM images in pseudo-3D representation of (a) several $(40\times 40\mathrm{n}{\mathrm{m}}^2)$ and (b) a single Au monolayer island $(11\times 11\mathrm{n}{\mathrm{m}}^2)$ grown on a 25-ML-thick, Mo-doped CaO film $(V_s=4.0\mathrm{V},15\mathrm{pA})$. The stripes in (b) arise from a moiré pattern formed between the hexagonal Au(111) and the square $\mathrm{CaO}\left(001\right)$ lattices, as depicted in (c).

Figure 2

STM images of (a) several $(70\times 70\mathrm{n}{\mathrm{m}}^2)$ and (b) a single Au monolayer island $(11\times 11\mathrm{n}{\mathrm{m}}^2)$ on 2 ML $\mathrm{MgO}/\mathrm{Ag}\left(001\right) \left(V_s=1.7\mathrm{V},15\mathrm{pA}\right)$. (c) Filling of island (b) with a hexagonal lattice of $2.89\AA$ periodicity, as in bulk gold, gives an approximate atom number of 319. The inset displays the hexagonal symmetry of Au atoms in the top facet $(2\times 2\mathrm{n}{\mathrm{m}}^2)$.

Figure 3

(a) Topographic and (b) conductance maps of an Au island on $\mathrm{MgO}/\mathrm{Ag}\left(001\right)$ taken at different bias voltages $(11\times 11\mathrm{n}{\mathrm{m}}^2)$. Note the bias-dependent evolution of a standing-wave pattern inside the island.

Figure 4

(a)–(c) Experimental dispersion relations of QWSs in three differently sized Au islands on MgO thin films. The number of atoms per island amounts to approximately (a) 300, (b) 200, and (c) 44. The respective islands are shown in the insets [(a) $3.4\mathrm{V},11\times 11\mathrm{n}{\mathrm{m}}^2$; (b) $0.5\mathrm{V},11\times 8\mathrm{n}{\mathrm{m}}^2$; (c) $-1.3\mathrm{V},6.5\times 5.5\mathrm{n}{\mathrm{m}}^2]$. Results for island (a) are based on the $dI/dV$ maps shown in Fig. 3. (d) Calculated band structure for bulk gold, taken from Ref. [36]. The electronic band that experiences in-plane quantization in the Au islands is marked by an ellipse. Despite the limited size of the nanostructures, the QWSs already reproduce main properties of the associated bulk band.

Figure 5

(a) Topographic and (b) conductance maps of an Au island on 25 ML $\mathrm{CaO}\left(001\right)$ taken at different bias voltages $(12\times 12\mathrm{n}{\mathrm{m}}^2)$. The evolution from standing waves to a bias-independent moiré pattern is clearly discernible.

Figure 6

(a) Experimental dispersion relation of QWSs in the CaO-supported Au island shown in Fig. 5 (200 atoms). After correction for tip-induced band bending, the eigenstates shift down in energy and approach the values obtained for MgO-grown nanostructures. Above a critical $k$ of $3.8\mathrm{n}{\mathrm{m}}^{-1}$, the wave pattern locks into the periodicity of the Moiré pattern and becomes constant. (b) Potential diagrams of STM junctions containing a thick CaO as well as a thin MgO film. Band-bending effects are relevant only for the polarizable CaO layer.

Figure 7

Series of $dI/dV$ spectra taken along a path from (a) bare CaO and (b) bare MgO to an Au island, as shown in the insets [(a) $3.5\mathrm{V},11\times 25\mathrm{n}{\mathrm{m}}^2$; (b) $1.7\mathrm{V},7\times 15\mathrm{n}{\mathrm{m}}^2]$. The spectra have been acquired with enabled feedback loop and 25 pA set-point current. The high-bias $dI/dV$ oscillations are FERs that shift up in energy when approaching the negatively charged ad-gold. The spectral region marked with a dashed box in (a) displays the Au QWSs, uncorrected for the effect of tip-induced band bending.

Figure 8

(a) STM image (3.5 V, $12\times 12\mathrm{n}{\mathrm{m}}^2$) and (b) corresponding height profiles across an Au monolayer island on $\mathrm{CaO}\left(001\right)$. (c) 1D model and (d) assumed charge distribution within the island. The exclusive appearance of edge electrons and a mixture of edge and inner charges $\left(0.1e\right)$ are depicted by solid and dashed lines, respectively. The resulting shift of the oxide conduction band is shown in turquoise for both scenarios. (e) Simulated height of the STM tip above a charged Au island; the pronounced negative contrast at the metal-oxide boundary is clearly discernible.