A single layer of hexagonal boron nitride (h-BN) is an isoelectronic structure ana-logue to graphene, consisting of equal numbers of boron and nitrogen atoms. In contrast to freestanding graphene which behaves as a zero bandgap semiconduc-tor, BN polymorphs includingh-BN are wide-bandgap electric insulators. In recent years, an increasing interest in single layerh-BN on metal surfaces, in particular on Rh(111), emerged because of its ability to act as a nano-template for the growth of atoms [157, 158], clusters [159] and molecules [157, 160–163].
Preparation
Preparation ofh-BN/Rh(111) was performed as follows: The Rh(111) substrate was cleaned by several cycles of Ar+ sputtering (2 kV), heating in oxygen atmosphere (5×10−7mbar, 650–900◦C) and annealing in UHV (1100◦C). For the formation of theh-BN layer, the substrate was kept at 800◦C while it was exposed to a borazine (B3N3H6) vapor pressure of 1×10−7mbar for 26 min. The borazine precursor was purified prior to preparation by several freeze-pump-thaw cycles.
Properties
Hexagonal boron nitride on Rh(111) forms a highly regular hexagonal superstructure with a periodicity of around 3.2 nm, which is often referred to asnanomesh. The term goes back to an early work which suggested a two layer model of h-BN/Rh(111), in which the formation of each BN layer is incomplete [160]. Considering the necessarily large number of broken B-N bonds, this model turned out to be not realistic and consensus is reached today that h-BN forms a single, complete layer on Rh(111) with a highly corrugated moiré pattern [164, 165].
LEED measurements confirm that h-BN/Rh(111) preferably grows in R0 ori-entation (see Figure 7.5a). The moiré superstructure is reflected by a hexagonal pattern of satellite spots accompanying the principal substrate reflexes. Based on
(a) (b) (c)
10 nm 2 nm
Rh [1
10] pore
BN wire Rh
Figure 7.5 |Hexagonal boron nitride on Rh(111). (a)LEED pattern recorded with an electron energy of 63 eV. (b) Large scale STM image of the h-BN moiré superstructure.(c)Atomically resolved close up image. The moiré unit cell as well as pore and wire sites are indicated. Scanning parameters: (b)V =−1.1 V,I = 500 pA, T = 6.1 K, (c)V = 70 mV,I = 2 nA,T = 7.8 K.
wire
pore Figure 7.6 | Site resolved STS spec-tra, recorded on pore and wire regions of theh-BN superstructure. A wide conduc-tion gap around EF reflects the insulat-ing nature of theh-BN layer. Parameters:
Vs = 3 V, Is = 100 pA, Vmod = 40 mV, T = 1.9 K.
the distances between the substrate and h-BN primary spots, a real space lattice ratio between h-BN and the metal substrate of 12:13 is deduced, corresponding to a moiré unit cell which contains (13×13)h-BN units on (12×12) Rh atoms [166].
A peculiarity of the h-BN/Rh(111) moiré pattern is the relatively abrupt tran-sition between regions of low and high corrugation. In STM images, strongly bound areas (‘pores’) appear as relatively flat holes of ca. 2 nm diameter, which are sep-arated from elevated regions (‘wires’) by an almost stepwise change in height (see Figure 7.5b). The unique structure is well reproduced by DFT calculations, which predict a corrugation of around 0.55 Å, the h-BN layer being closest to the metal surface in regions where B and N atoms are close to fcc and atop sites, respec-tively [164]. While atomic resolution on the wire sites is routinely achieved in STM, no structure is usually visible inside the pore regions (see Figure 7.5c). Adjacent atomic protrusions on the wire sites are measured to be separated by about 2.50 Å.
According to DFT, only N atoms are probed by STM in the vicinity ofEF [165].
An intriguing question is whether the boron nitride layer remains an electric insulator on the Rh surface. Insight into the local electronic structure can be obtained based on STS measurements. Spectra taken at different sites of the h-BN moiré pattern are shown in Figure 7.6. A band gap of around 6 eV is observed on the pore
regions, while a smaller gap of ca. 4 eV is visible in curves taken on wire sites. STS data reported in literature [159] show slightly larger band gap values, but are in overall good agreement with the measurement performed here.
Results and Discussion
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Application Examples
This chapter provides several examples of molecular compounds that were deposited by means of ESD in the course of this thesis. The aim is to briefly demonstrate the capabilities as well as the limitations of the ESD setup, without going into details concerning the physical properties of the systems.
8.1 Fe-Phthalocyanine
Metal phthalocyanines (MPcs) are planar coordination complexes with a single metal ion M, located at the center of an aromatic macrocycle that is formed by alternating carbon and nitrogen atoms. The structure of the molecule shows a remarkable ther-mal and chemical stability, which enables deposition of MPcs onto surfaces by means of thermal sublimation in UHV. Since MPcs are utilized in a wide field of industrial applications, most importantly in dyeing, they are commercially available in huge amounts and in high purity. All these aspects – combined with a great interest in the electronic properties of MPcs in fundamental research [167–170] – have led to a vast amount of literature that is available on MPcs on surfaces today [171]. Therefore, MPcs an ideal test bed for verifying the ESD setup functionality. Here, M = Fe was chosen as central metal ion. The structure of the FePc complex is shown in Figure 8.1a.
FePc was purchased from Sigma-Aldrich as FePcCl powder (95 % purity). The chloride salt rather than the neutral complex was used due to the generally bad solubility of phthalocyanines in polar organic solvents. Electrospray deposition of the compound was performed on Au(111), using a 2×10−5M solution in pure methanol.
Typical STM images of the surface after deposition are shown in Figure 8.1b,c.
Most of the molecules occur as individual objects that preferably occupy the elbow sites of the Au(111) herringbone reconstruction. A clear intramolecular structure is resolved in the STM topography, reflecting the fourfold symmetry of the molecule with the four benzene rings appearing as well-separated lobes and the Fe center appearing as a small protrusion. It is therefore be concluded that the molecules lay
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Fe
(a) (b) (c)
10 nm 1 nm
Figure 8.1 |(a)Molecular structure of FePc.(b)STM image showing the Au(111) surface after electrospray deposition of FePc molecules. Dashed circles indicate for-mation of multimers.(c) Magnification of the molecule marked by an arrow in (b).
Scanning parameters:V = 1 V, I = 400 pA, T = 9.8 K.
on the substrate in a flat geometry. Besides individual molecules, also formation of dimers and trimers is observed (see Figure 8.1b).
By comparing our results to other STM studies of FePc on Au(111) [172,173], we find that the sample quality provided by the ESD process is equal to that of thermal sublimation. In particular, no residuals of the solvent and only small amounts of im-purities are found in large scale STM images. The intramolecular structure resolved in STM topography provides no evidence for molecular fragmentation. Furthermore, the adsorption behavior of the molecules is the same as reported in [172, 173]. These findings demonstrate the high potential for an application of ESD in the field of ultra-high vacuum scanning tunneling microscopy.