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The discovery of the nanomesh for everyone
(read the publications for more detailed informations)

Introduction

In 2003 M. Corso et al. from Osterwalder's group at the University of Zurich, Switzerland, published in Science [M. Corso et al., Science 303, 217 (2004)] the discovery of a new inorganic nanostructured two dimensional material, called nanomesh, which so far has no analog in systems composed of carbon.

The discovered boron nitride nanomesh is composed of boron (B) and nitrogen (N) atoms, which form a highly regular mesh after high-temperature exposure of the clean rhodium (Rh (111)) single crystal to borazine . The nanomesh has a honeycomb-like superstructure (see figure) with apertures of 2nm and wires of 1nm. In 2003 a double-layer model was proposed, where each BN layer was offset in such a way as to expose a minimum metal surface area. In 2007 an alternative model emerged, which consists of a full single BN layer (no more holes), where "pores" are closer to the crystal surface than the wires.

The formation of the nanomesh is a self-assembly process, i.e. the organisation of the atoms is driven by the nature itself without any human intervention. The self-assembly process is likely driven due to close but different periodicities (lattice constants) of the BN nanomesh and the Rh substrate and a site dependent BN-bonding to the substrate.

The boron nitride nanomesh is stable towards air, vacuum and liquids, and it does not decompose up to temperatures of at least 796C (1070 K). In addition the BN nanomesh can serve as a template to organize molecules, as is exemplified by the decoration of the mesh with C60 molecules. These characteristics promise interesting applications of the nanomesh in areas like nanocatalysis, surface functionalisation, spintronics, quantum computing and data storage media like hard drives.


3D plot of an STM image



Preparation of the Nanomesh

Well-ordered nanomeshes are grown by thermal decomposition of borazine (HBNH)3, a colorless substance that is liquid at room temperature (see molecule). The Nanomesh results after exposing the atomically clean Rh (111) surface to borazine by chemical vapor deposition (CVD).

The Rh (111) substrate is kept at a temperature of 796°C (1070 K) when borazine is introduced in the vacuum chamber at a dose of about 40 L (1 Langmuir = 10-6 torr sec). A typical borazine vapor pressure inside the ultrahigh vacuum chamber during the exposure is 3x10-7 mbar. After cooling down to room temperature, the regular mesh structure is observed in STM images. A photograph of Zurich's experimental setup for borazine evaporation is presented on the right.
borazine

By courtesy of M. Corso

experimental setup



Experimental data from the boron nitride Nanomesh

Different complementary experimental techniques are used to study the boron nitride Nanomesh on Rh (111): Scanning tunneling microscopy (STM) allows to take a direct look on the local real space structure of the surface, while low energy electron diffraction (LEED) (more details) patterns are formed by surface structures ordered over a macroscopic sample area. Ultraviolet photoelectron spectroscopy (UPS) gives information about the electronic states in the outermost atomic layers of a sample, while X-Ray Photoelectron Spectroscopy (XPS) allows to get information about the electronic states of the innermost atomic layers.

Zurich's experimental setup is shown, where it is possible to perform photoemission, LEED and STM measurements on the very same sample surface under UHV.
experimental setup
By courtesy of M. Corso



Scanning tunneling microscopy (STM)

The left STM image below shows an 80 nm wide area of the Nanomesh. Two steps on the Rh (111) surface cross the image.

The middle figure shows a small area of perfect nanomesh and provides some further hints on the detailed structure of its unit cell. Inside each supercell, four distinct grey scale levels occur. This image was taken with a constant current of 1 n A. The sample bias was -2 eV and electrons tunnel from nanomesh occupied states into the tip. In a topographic interpretation (remember, no absolute height information can be obtained from STM!) we can identify the darker levels with regions near of the Rh surface, the brighter level with regions farer from the Rh surface.

The right image shows the nanomesh decorated with C60 molecules, where the periodicity of the mesh supercell is retained. The mesh wires are decorated by lines of individual molecules, whereas either six or seven molecules can be distinguished inside the pores.
STM small scale STM small scale STM large scale


Low energy electron diffraction (LEED)

The high degree of periodicity seen in the STM image is found throughout the macroscopic sample area, as is confirmed by the low-energy electron diffraction (LEED) pattern presented below. These data provide a first clue for the atomic structure within the nanomesh. The insets illustrate the assignment of the spots: The large brown circle corresponds to the Rh (111) substrate spots, the blue one to the h-BN principal spots and the olive ones stem from the nanomesh superlattice. The in-plane lattice constant of the hexagonal Rh (111) surface is 2.69 Å. From the principal spots of the nanomesh LEED pattern a hexagonal atomic lattice is deduced also for the boron nitride layer, with a lattice constant of 2.48±0.05 Å.
LEED image of clean Rh(111) LEED image of the Nanomesh
The superlattice spots around the principal spots indicate a periodicity of 32±1 Å, which corresponds to a supercell of (12x12) Rh unit cells, or (13x13) h-BN unit cells.


UV excited photoemission (UPS)

Upon exposure to borazine and nanomesh formation, the normal emission UPS spectrum shows an attenuation of the Rh signals and the appearance of two pairs of boron nitride related peaks. The binding energies of the individual pairs align well with the binding energies of the σ (5.3 eV) and the π (10.0 eV) states in normal emission spectra of h-BN monolayers formed on Ni (111). The spectra thus expose the presence of two species of h-BN that have their binding energies for the σ and π-band shifted by about 1 eV.
UPS spectrum
He Iα excited normal emission spectra:
bare Rh (111)
h-BN on Ni(111)
Nanomesh
This suggests that the lower binding energy component is associated with the loosely bound h-BN wires, while the higher binding energy component is from strong bonded pores of the nanomesh. The area ratio of pores:wires is expected to be 3:1.


 
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| Last update: 26.06.2013 by Ari P Seitsonen