Hexagonal Boron Nitride Nanomesh Research Articles

Enhancing the dipole ring of hexagonal boron nitride nanomesh by surface alloying

Surface templating by electrostatic surface potentials is the least invasive way to design large-scale artificial nanostructures. However, generating sufficiently large potential gradients remains challenging. Here, we lay the groundwork for significantly enhancing local electrostatic fields by chemical modification of the surface. We consider the hexagonal boron nitride (h-BN) nanomesh on Rh(111), which already exhibits small surface potential gradients between its pore and wire regions. Using photoemission spectroscopy, we show that adding Au atoms to the Rh(111) surface layer leads to a local migration of Au atoms below the wire regions of the nanomesh. This significantly increases the local work function difference between the pore and wire regions that can be quantified experimentally by the changes in the h-BN valence band structure. Using density functional theory, we identify an electron transfer from Rh to Au as the microscopic origin for the local enhancement of potential gradients within the h-BN nanomesh.

Adsorption of Azobenzene on Hexagonal Boron Nitride Nanomesh Supported by Rh(111).

Adsorption properties of azobenzene, the prototypical molecular switch, were investigated on a hexagonal boron nitride (h-BN) monolayer (“nanomesh”) prepared on Rh(111). The h-BN layer was produced by decomposing borazine (B3N3H6) at 1000–1050 K. Temperature-programmed desorption (TPD) studies revealed that azobenzene molecules adsorbed on the “wire” and “pore” regions desorb at slightly different temperatures. Angle-resolved high-resolution electron energy loss spectroscopy (HREELS) measurements demonstrated that the first molecular layer is characterized predominantly by an adsorption geometry with the molecular plane parallel to the surface. Scanning tunneling microscopy (STM) indicated a clear preference for adsorption in the pores, manifesting a templating effect, but in some cases one-dimensional molecular stripes also form, implying attractive molecule–molecule interaction. Density functional theory (DFT) calculations provided further details regarding the adsorption energetics and bonding and confirmed the experimental findings that the molecules adsorb with the phenyl rings parallel to the surface, preferentially in the pores, and indicated also the presence of an attractive molecule–molecule interaction.

Correction to “Self-Assembly of a Hexagonal Boron Nitride Nanomesh on Ru(0001)”

In our paper(1) the N 1s core level energy as obtained by Al Kα excited X-ray photoelectron spectroscopy (XPS) of boron nitride on Ru(0001) is not correctly reproduced, it is 398.63 ± 0.15 eV and not 389.63 ± 0.15 eV. Table 1 reproduces the B 1s and N 1s binding energies and work functions for h-BN on Rh(111), Ru(0001), Ni(111), and Pd(111).(2)Figure 1 reproduces the corresponding core level data with the correct energy scale. We are grateful to W. Auwarter for spotting these inconsistencies.

A Microscopy Approach to Investigating the Energetics of Small Supported Metal Clusters

Metal clusters are partway between molecular and bulk systems and thus exhibit special physical and chemical properties. Atoms can rearrange within a cluster to form different structural isomers. Internal degrees of freedom and the interaction with the support - which both are dependent on cluster size - can promote diffusion across a support. Here, we show how fast scanning tunneling microscopy (FastSTM) can be used to investigate such dynamical behavior of individual clusters on the example of Pdn (1<=n<=19) on a hexagonal boron nitride nanomesh on Rh(111), in particular pertaining to minority species and rare events. By tuning the cluster size and varying the temperature to match the dynamics to the FastSTM frame rate, we followed steady state diffusion of clusters and atoms inside the nanomesh pore and reversible cluster isomerization in situ. While atoms diffuse along the rim of a pore, a small cluster experiences a corrugation in the potential energy landscape and jumps between six sites around the center of the pore. The atom and cluster diffusion between pores is strongly influenced by defects.

An electron acceptor molecule in a nanomesh: F4TCNQ on h-BN/Rh(111)

The adsorption of molecules on surfaces affects the surface dipole and thus changes in the work function may be expected. The effect in change of work function is particularly strong if charge between substrate and adsorbate is involved. Here we report the deposition of a strong electron acceptor molecule, tetrafluorotetracyanoquinodimethane C12F4N4 (F4TCNQ) on a monolayer of hexagonal boron nitride nanomesh (h-BN on Rh(111)). The work function of the F4TCNQ/h-BN/Rh system increases upon increasing molecular coverage. The magnitude of the effect indicates electron transfer from the substrate to the F4TCNQ molecules. Density functional theory calculations confirm the work function shift and predict doubly-charged F4TCNQ2− in the nanomesh pores, where the h-BN is closest to the Rh substrate, and to have the largest binding energy there. The preferred adsorption in the pores is conjectured from a series of ultraviolet photoelectron spectroscopy data, where the σ bands in the pores are first attenuated. Scanning tunneling microscopy measurements indicate that F4TCNQ molecules on the nanomesh are mobile at room temperature, as “hopping” between neighboring pores is observed.

Tailoring the hexagonal boron nitride nanomesh on Rh(111) with gold.

It is known that the hexagonal boron nitride (h-BN) monolayer has a periodically corrugated structure on Rh(111), termed "nanomesh", while the h-BN layer is planar on the close packed surfaces of coinage metals (Cu, Ag, Au) due the weak interactions. Our studies are aimed at understanding the metal-h-BN interaction, when both Rh and Au are present. On the one hand, the growth and thermal properties of gold deposited on h-BN nanomesh prepared on Rh(111) were studied. On the other hand, the formation of h-BN was examined on Au/Rh surface alloys prepared by the deposition of Au on Rh(111) and subsequent annealing at 1000 K. In each case, the h-BN was prepared by the decomposition of borazine at about 1000 K. Low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) measurements revealed that the growth of Au on h-BN/Rh(111) at room temperature leads to the formation of mainly three dimensional (3D) gold nanoparticles, although at low coverages (<0.2 ML) 2D particles formed as well. Stepwise annealing to higher temperatures induces the intercalation of Au below the nanomesh, which was complete at around 1050 K. Some agglomeration and desorption of Au also took place. Interestingly, the nanomesh structure was observable after intercalation up to relatively large Au coverages. Measurements performed in the reverse order, namely exposing a Au/Rh(111) surface alloy to borazine, revealed that Rh atoms get covered by h-BN (or by its precursors) at significantly smaller borazine exposures than Au atoms. The nanomesh structure was essentially present up to a gold coverage of 0.9 ML, but with a smaller pore diameter, while it gradually disappeared at higher gold amounts. In this way the application of surface alloy supports provides a key for gradual tuning of the mesh morphology. Density functional theory calculations confirmed the decreased pore diameter of the BN layer upon the formation of a surface Rh-Au alloy layer.

Chiral Distortion of Confined Ice Oligomers (n = 5,6)

Ice nuclei have been studied on the hexagonal boron nitride nanomesh (h-BN/Rh(111)), a template with 2 nm wide molecule traps. Scanning tunneling microscopy shows confined clusters, where oligomers with three protrusions are particularly abundant. Together with local barrier height dI/dz maps, it is found that the dipoles of the water molecules arrange in a homodrome, which is consistent with density functional theory calculations. Hydrogen bonds toward the substrate identify h-BN/Rh(111) to be hydrophilic. The substrate distorts the hexamers (n = 6) and possibly pentamers (n = 5), where the experimentally observed footprints of the three protrusions appear more chiral than expected.

Effect of substrate nanopatterning on the growth and structure of pentacene films

The effect of modulating the structure of thin pentacene $({\text{C}}_{22}{\text{H}}_{14})$ films by a nanopatterned inert substrate, known as hexagonal boron nitride nanomesh, is reported. Films of different thickness are grown and characterized by x-ray absorption, core-level photoemission, low-energy electron microscopy, microbeam low-energy electron diffraction, and scanning tunneling microscopy. Initially the pentacene molecules adsorb with the molecular plane lying flat on the substrate but they tend to flip up with increasing coverage, forming well-ordered monolayer-thick islands of upright molecules with low nucleation density. The herringbone packing of the upright molecules is observed with scanning tunneling microscopy. The electronic structure of the adsorbed molecules is very similar to that of the gas-phase pentacene, implying weak interaction with the substrate and between the molecules. The periodic corrugation of the substrate surface causes the monolayer of upright pentacene molecules to form two different coincidence superstructures. The lattice parameters of the pentacene unit cell for each of these two substrate-induced domains are determined from the microdiffraction patterns. Both domains can occur in several equivalent configurations, thus resulting in a number of twins with a typical size of a few micrometers. The first monolayer grows in a layer-by-layer mode until it is completed while the second monolayer forms diffusion-limited fractal islands. Upon annealing, the pentacene films are thermally stable up to approximately $80\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and thereafter the onset of desorption is observed.

Comparison of electronic structure and template function of single-layer graphene and a hexagonal boron nitride nanomesh on Ru(0001)

The structure of a single-layer graphene on Ru(0001) is compared with that of a single-layer hexagonal boron nitride nanomesh on Ru(0001). Both are corrugated $s{p}^{2}$ hybridized networks and display a $\ensuremath{\pi}$ band gap at the $\overline{\text{K}}$ point of their $1\ifmmode\times\else\texttimes\fi{}1$ Brillouin zone. In contrast to $h\text{-BN}/\text{Ru}(0001)$, $g/\text{Ru}(0001)$ has a distinct Fermi surface. Together with the band structure measurements this indicates that $0.1e$ per $1\ifmmode\times\else\texttimes\fi{}1$ unit cell are transferred from the Ru substrate to the graphene. Photoemission from adsorbed xenon on $g/\text{Ru}(0001)$ identifies two $\text{Xe}\text{ }5{p}_{1/2}$ lines, separated by 240 meV, which reveals a corrugated electrostatic potential energy surface like on $h\text{-BN}/\text{Rh}(111)$. These two Xe species are related to the topography of the template and have different desorption energies.

Self-Assembly of a Hexagonal Boron Nitride Nanomesh on Ru(0001)

Hexagonal boron nitride (h-BN) nanostructures were grown on Ru(0001), and are very similar to those previously reported on Rh(111). They show a highly regular 12 x 12 superstructure, comprising 2 nm wide apertures with a depth of about 0.1 nm. Valence band photoemission reveals two distinctly bonded h-BN species, and X-ray photoelectron spectroscopy indicates an h-BN monolayer film. The functionality of the h-BN/Ru(0001) nanomesh is demonstrated by using this structure for the assembly of gold nanoclusters.