6.2 Setup Performance and Ion Beam Characterization
6.2.3 Deposition Spot Profile
The diameter of the deposition spot was measured by driving the edge of the aperture plate of the UHV sample holder across the profile of the ion beam and simultaneously
measuring the current on the plate IH as a function of the position z. In order compensate for fluctuations of the ion beam intensity, the signal is normalized to the current on the second apertureIA2. The result of the measurement is shown in Figure 6.7a.
The spatial dependence of the ion beam intensity can be extracted from the measurement by calculating the derivative dzd(IH/IA2). The corresponding curve exhibits a surprisingly flat profile with a FWHM of around 3.6 mm (Figure 6.7b).
Taking into account the distance between UHV sample holder and second aperture, the opening angle of the beam is calculated to be 0.4◦.
Substrates for deposition were prepared by means of standard UHV techniques.
Unless otherwise stated, the preparation was performed at one of the UHV set-ups described in chapter 5.1. Cut and polished single crystals were purchased from Mateck GmbH (Jülich, Germany) or SPL (Zaandam, Netherlands). All crystals were mounted on molybdenum or tungsten sample holders.
7.1 Single Crystal Noble Metal Surfaces: Au(111), Ag(111) and Cu(111)
The (111) surfaces of Au, Ag and Cu can be considered as standard substrates for the investigation of atomic and molecular assemblies. They were widely used within this thesis as supports for ESD and wet chemical preparation procedures.
Preparation
Cleaning of the crystal surfaces was performed by repeated cycles of Ar+ sputtering (Ag: 800 eV, Au & Cu: 1000–1500 eV) and subsequent annealing to high temperature (Ag: 550◦C, Au & Cu: 600◦C). The surface quality was occasionally checked by means of STM and XPS measurements.
The Au(111) Surface Reconstruction
Gold is the only fcc metal that exhibits a reconstruction of the close-packed (111) surface [138–140]. As the topmost atomic layer is contracted along the [110] di-rections, periodic transitions between regions of fcc and hcp stacking occur with a periodicity of 22 bulk atomic distances. The corresponding height variation is visi-ble in STM topographic images as characteristic lines propagating along the [112]
directions, which are often referred to as herringbone structure (see Figure 7.1a,b).
Typical STM corrugations are in the range of 10–20 pm. At the boundary between domains of different (crystallographically equivalent) contraction orientations, so called elbow sites are formed, which provide nucleation centers for the growth of atomic and molecular adsorbates.
71
50 nm
(a)
3 nm
(b) (c)
[112]
[110]
fcc hcp
hcp fcc
difference hcp
elbows
Figure 7.1 | STM and STS measurements on Au(111). (a) Large scale image showing the herringbone pattern of the surface reconstruction.(b) Close up image with atomic resolution. Regions with fcc and hcp stacking and crystallographic orientations are indicated. The white rectangle marks the (22×√
3) reconstruction unit cell.(c)STS spectra recorded on fcc and hcp sites of the surface reconstruction.
Scanning parameters: (a) V = 0.1 V, I = 950 pA, T = 15.8 K, (b) V = 0.1 V, I = 3.2 nA, T = 10.0 K. Parameters for spectroscopy: Vs = 200 mV, Is = 3 nA, Vmod= 10 mV.
Surface States
The electronic density of states of all three substrates comprises a surface state in the vicinity of EF, which is reflected in STS measurements as a step-like increase in differential conductivity. Since the onset energies of the surface states are well known [141–143], they can be used as a spectroscopic reference when performing STS measurements on not yet characterized adsorbates. Figure 7.1c shows the appearance of the surface state in an STS measurement on Au(111). In accordance with the literature, the surface state band edge is located at around−0.5 eV, coinciding with the minimum energy of the parabolic surface state dispersion. As first reported by Crommieet al. [141], a slight difference in spectroscopic shape is observed between measurements performed on fcc and hcp sites of the Au(111) surface reconstruction.
7.2 NaCl(100) on Au(111)
Ultrathin insulating layers on metallic substrates facilitate the investigation of asor-bates on insulator surfaces with scanning tunneling microscopy. A possible approach is to use multi-layered NaCl islands, which have been successfully grown on a variety of different crystalline metal surfaces [144–151].
Preparation
Preparation of NaCl on Au(111) was performed by thermal sublimation of a NaCl single crystal at around 500◦C. The substrate was kept at room temperature during the deposition process. Before each preparation, the temperature of the evaporator
2
Figure 7.2 | STM measurement of NaCl(100) on Au(111). (a)Large scale image of several NaCl islands. Numbers indicate the amount of layers.(b)Close up image showing the atomically resolved NaCl lattice superimposed to the Au(111) herringbone reconstruction. Green and red circles indicate the positions of Cl– and Na+ions, respectively. A point defect of the NaCl lattice is visible in the upper part (arrow). (c) Height profile along the dashed line in (a). Scanning parameters: (a) V =−2.5 V,I = 10 pA, room temperature, (b)V =−2 V, I = 150 pA,T = 7.5 K.
was fine adjusted using a quartz crystal microbalance to achieve deposition rates in the range of 1–2 Å/min. Typical coverages are between 2 and 4 Å.
Properties
NaCl on Au(111) grows epitaxially as islands with (100) orientation [144, 145]. The first plateau is formed by an atomic double layer – single layered islands are not observed. Further layers start growing before the first double layer is completed.
An STM image of NaCl islands comprising up to 5 layers is shown in Figure 7.2a. The first NaCl double layer forms straight edges in 90◦ angles to each other.
Edges of higher layers exhibit an increased roughness.
The apparent height of NaCl in STM is generally smaller than the physical height, owing to the insulating nature of the material. Bulk NaCl crystallizes in fcc structure with a lattice constant of 564 pm [144], corresponding to the physical height of two atomic layers. As it can be seen in Figure 7.2c, the apparent height of the first double layer of around 350 pm is already well below this value. Furthermore, the apparent height of each additional layer is successively reduced, making STM measurements on multi-layered islands increasingly difficult. For the comparable system NaCl(100)/Cu(111), it was reported that tunneling is possible through up to five layers of NaCl, provided that tunneling currents of only a few pA are used [152].
It should be noted that the apparent height of the first double layer, while being smaller than the physical height of two layers, is still larger than the physical height of a single layer. This verifies that the first plateau of NaCl on Au(111) is indeed formed by more than one atomic layer.
An atomically resolved STM image of NaCl is shown in Figure 7.2b. The atomic corrugation exhibits a square lattice with a lattice constant of around 400 pm,
super-imposed to the Au(111) surface reconstruction. Since the periodicity coincides with the closest separation between ions of the same species, it is concluded that only one of the two element sublattices is imaged in STM. According to [144], protrusions in the STM topography are attributed to Cl– anions.