To conclude this section, we can control the molecular self-assembly structure is something special. Due to the Pb/Si (111) have different phases such as √7 × √3, HIC and SIC with different coverage of deposition from 1 to 1.3 ML of Pb. We discovered two macroscopic views of self-assembly on different phases. Therefore, it is expected that the control of self -assembly can be achieved through the different deposition of Pb.
The detail finishing in the Table 3.3, with 1 ML deposition of Pb prepared as substrate, Co-TPP molecules form three different conformations absorbed on √7 × √3 phase. Saddle, planar, and iso-planar conformation are distinguished by various distortions. It is very common to see saddle conformation absorbed on noble metals.
Besides, planar and iso-planar conformations distinguished by phenyl rings angle difference switch through the change of temperature.
Therefore, only saddle and planar conformations appear under the room temperature. Furthermore, parts of planar conformations will change into iso-planar conformations under low temperature. This phenomenon is probably caused by the transition of the substrate.
Each kind of conformation forms a domain individually and each domain possesses a same pattern.
With 1.3ML deposition of Pb/Si (111), SIC phase is formed. On this phase, a periodic structure is built by saddle and planar conformations alternatively. One of most seen period is 14 × 2√3, which is composed by one saddle stripe and three planar stripes aligning along [-101] direction. This structure is denser than the structure on √7 × √3 phase. This fact is a possible reason for lack of iso-planar conformation. In this assembling structure, we realize all the centers of the saddle type are always on T4 sites. For this reason, we consider that the specific interaction between Cobalt atom of the molecule and the substrate results in an orderly appearance.
Table 3.3. The table is easy summarizing our experiment results.
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Chapter Four
Fabrication of Mo pyramidal-shape single atom tips covered by a
noble metal
4.1 Introduction
A super-sharp tip is one of the most important elements in nano-science and nano-technology. The sharpest tip is the single atom tip (SAT), which has only one atom, with a diameter of about 10-10 m.
The SAT has attracted much research effort due to its unique properties and potentially wide range of applications. First, it is an optimal point electron and ion source. Its advantages include high brightness, stability and long time maintainability. Second, it is an ion beam with good focus [1 - 4] of the smaller half angle from a local enhanced electric field, which can resolve the problem of chromatic aberration. Third, its electron beam has high coherence, so the wave properties of the spatially well-defined electron source can be used in many applications, including high-resolution transmission electron microscope and shadowing electron microscope with holographic
microscopes to achieve the best spatial resolution.
In the 1990s, Madey et al. found that ultrathin noble metal films grown on a tungsten W (111) surface can undergo massive reconstruction upon annealing to form three-sided pyramids with {211} facets [6 - 9]. The driving force of the facet formation was attributed to an increase in surface-free-energy anisotropy, as later confirmed by theoretical calculations [10, 11]. Inspired by this adsorbate-induced faceting process, the SAT can simply be completed via thermodynamic equilibrium in an ultra-high vacuum (UHV). The W (111) faceting to a single atom tip is induced by heating 1 ~ 2ML noble metals, including Pd, Pt, Rh, and Ir to 1000K after adsorption [2, 12]. Further, Madey presented the contribute to an SAT application. Molybdenum (Mo) and W have the same body-centered cubic structure as well as similar lattice constant,
physical and chemical properties [16, 17]. However, the oxygen desorption temperature of Mo is about ~1600 K [18, 19], lower than that of W, which is about ~1800 K [20]. Importantly, removing oxides and cleaning the Mo surface is easier than for W. In addition, according to the Zhang et al’s simulation , which was calculated by using the second nearest–neighbor modified embedded atom method, the surface free energy E(211) = 3277 ergs/cm2 is lower than the E(111)
= 3429 ergs/cm2 for Mo surfaces [21]. Therefore, the area of {211}
facets will increase relative to (111). SAT might be formed if {211}
facets replaced (111) completely after reconstruction.
Using first principle calculations, Cle et al. that found faceting agents such as Pd and Pt induce significant anisotropy in the surface energy when ultra-thin layers of noble metals are grown on a Mo (111) substrate[10, 22 - 24]. For these elements, to lower the surface formation energy is a substantial task. The surface-free-energy anisotropy of the noble metal covered surfaces is much larger than that of the clean surfaces. Therefore, the three {211} facets will increase in area to form a triangular pyramidal structure on (111) facets. On the other hand, both Madey and Dańko et al used field emission microscopy (FEM) [25], low-energy electron diffraction (LEED) [16, 26], X-ray photoelectron spectroscopy (XPS) [27] and
scanning tunneling microscope [28] to observe and found that the adsorption of more than one physical layer of Pd will induce Mo (111) to rearrange to form three symmetric pyramidal structure by {211}
facets after heating to 1100K. Therefore, we believe it is possible to produce Mo ultra-sharp tips and further SATs.
In the present paper, we report the FIM (field ion microscopy) research on Mo SAT growth. The possible growth conditions including annealing temperature and various noble metals are discussed.