The bottom up approach, where assembling basic units (atoms or molecules) into larger nanostructures on surfaces, is of great recent interest both on fundamental researches and technological applications.
In particular, molecules have clearly demonstrated their potential to be building units for practical applications, such as, organic light emitting diodes (OLEDs), organic photovoltaic solar cells, and organic field- effect transistors (OFETs). At the nanoscale, the controlled assembly of highly organized architectures on surfaces using functional molecules is highly demanded. The central challenging is how to well perceive and manipulate the interactions between molecules and supporting substrates.
Due to their wild applications both in Chemistry and life science, polyenic and aromatic compounds have been studied as the molecular layers adsorbed on various surfaces. Within this class, porphyrins and metalloporphyrins are preferentially investigated; as these compounds
are very robust, polyfunctional, volatile and readily available with a range of different electronic and steric properties. For sub-monolayer studies, moreover, they are perfectly suitable because they can be prepared on surfaces in vacuum, which can be explored by a high-spatial resolution local probe technique, scanning tunneling microscopy (STM).
Many STM studies of tetraphenylporphyrin (TPP) molecules adsorbed on various metallic surfaces have been carried out recently.
The interaction of the phenyl legs with the surface is found to play a key role in their physical properties, from molecular adsorption conformations to growth of larger molecular complexes [1 - 3]. By replacing phenyl substituents, the adsorption property of TPP molecules varied from forming highly ordered stacking molecular structures to staying as an isolated molecule [4, 5]. On the contrary, efforts have been also made by replacing the H2 center with metallic ions, no influence on the adsorption but only on electronic properties were reported [6, 7]. One can therefore consider an alternative way: to modify the surface property in order to obtain desired molecular structures. We therefore propose to place an atomic layer of “soft”
material between molecules and surfaces as a tuning layer. By properly controlling the structure of the tuning layer, the properties of
molecules are expected to be modulated accordingly.
In this study, cobalt (II) - tetraphenylporphyrin (Co-TPP) molecules were first time prepared on one monolayer (ML) of Pb on Si (111) surface. We chose Pb/Si (111) system because it has been well studied for decades and most importantly, various surface reconstructions can be prepared with Pb coverage’s, annealing history and temperatures [8 - 12]. This is then a perfect tuning layer for us to learn how Co-TPP molecules interacting with substrates. At a Pb coverage of about 1 ~ 1.2 ML, Co-TPP molecules formed short-ranged ordered domains where three adsorption conformations were observed.
With a Pb coverage of about 1.3 ML, surprisingly, the orientation of molecular domains changed and arranged along with one of the Pb surface directions. On this surface, only two adsorption conformations were observed and a preferred adsorption site was found.
3.1.1 Molecule H2-TPP and Co-TPP
Tetraphenylporphyrin, abbreviated H2-TPP, is a synthetic heterocyclic compound that resembles naturally occurring porphyrins, which form the base structure for both heme and chlorophyll and vitamin B12, as well as a variety of other hemoproteins consisting of porphyrin combined with metals and proteins. The name of porphyrin
comes from the Greek word porphyra, which means purple.
Porphyrins absorb light in the visible spectrum and have been used as dyes. The study of naturally occurring porphyrins is complicated by their low symmetry and the presence of polar substituents.
Tetraphenylporphyrin is hydrophobic, symmetrically substituted, and easily synthesized.
The porphyrin can coordinate with many metal ions (such as Fe, Co, Cu, and Zn) to form stable metal–porphyrin coordination complexes, which are widely existed in nature with special physiological activity and excellent properties in electron transfer, energy conversion, and nonlinear optical-limiting [13], [14] and [15].
There has a H2-TPP model show in Fig. 3.1.1, it like a macrocyclic conjugated organic molecules with four phenyl rings bonding to single bond. There two hydrogen atom in the center of the macrocycle, it can be easily replaced with a metal atom becomes metal- tetraphenylporphyrin.
Fig. 3.1.1 Illustrate the molecule models of the tetraphenylporphyrin.
3.1.2 Si (111)
The Si (111) is chosen as the sample in the experiments. The silicon atom has four bonding possibilities, and the crystal structure of silicon is a diamond cubic lattice, as shown in Fig. 3.1.2 (a). The sample is cut from a 4” Si (111) wafer. The miscut angle is 0.1∘and the dimension of the sample is 16 mm × 2 mm × 0.3 mm. Before transferring to the UHV chamber, the Si sample is cleaned by ultrasonic vibration in acetone, methylalcohol and deionized water for 10min, respectively.
The (111) - orientation for samples results in an extremely flat surface that is suited well for the purpose of this project. Figures 3.1.2. (b) and (c) show the structure of the (111) layers [16]. If this is the configuration of the atoms at the surface, it is denoted as a 1 × 1 surface.
Fig. 3.1.2 The basic Si unit cell and (111) planes. (a) Si unit cell.
Diamond cubic lattice. (b) Si (111) surface, side view. (c) Si (111) surface, top view.
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b c
The clean Si (111) - 7 × 7 surface is prepared by high temperature annealing from 900 K. The Si (111) - 7 × 7 is the thermal stable equilibrium structure at the end of the reaction pathways. The surface structure is observed with 12 adatoms and large holes at the corners of the Si (111) - 7 × 7 unit cells from STM image. Further, the unit cell is divided into two equilateral triangles called the faulted half unit cells and unfaulted half unit cells resulting in unbalanced density of electron states at the special tunneling conditions. The Si (111) - 7 × 7 reconstruction is described by the model of the Dimer- Adatom-Stacking-Fault (DAS) family of reconstructions referred to as the 7 × 7 DAS model as shown in the model of Fig. 3.1.3 [17, 18].
The unfaulted domain means that the atomic stacking is following the sequence of A - B - C and so on in the direction normal to the surface as indicated by the half cell on the right side of the model, but the faulted domain is not allowing the stacking arrangement in the left half cell of the model. The unbalanced contrast between the faulted and unfaulted domains in the same unit cell is due to the DOS feature that the faulted stacking domain is observed resulting in an only chlorine-terminated rest-adatom layer without adatoms.
Fig. 3.1.3 Atomically clean surface of Si (111) - 7 × 7. STM images of (а) filled and unfilled (b) electron states of surface; (c) schematic representation of surface (plan and side views) in accordance with Takayanagi DAS model (dimer- adatom- stacking fault) [18]. Yellow circles represent Si atoms, red circles - dimerizated Si atoms, and blue - second layer Si rest-atoms. Elementary cell 7 × 7 is highlighted with a diamond. Half of the elementary cell with package defect is marked as FH (faulted half); the half with no package defect is marked as UH (unfaulted half). You may see that the half of the cell with package defect depicted in the STM image of filled states (a) appears brighter than the other half. Maximums in the STM image correspond to adatoms.
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3.1.3 Pb phase on Si (111) surface
The Pb phase on Si (111) surface is a complicated system. There have various phase structures can be formed on the Si (111) substrate depending on the coverage and different thermal treatment [19 - 22].
In this thesis, the different phase of Pb interlayer face are occupies an important impact of the molecules self-assembly. We will briefly illustrate the different structural phase for 1 ~ 1.3 ML coverage Pb on Si (111) system.
We first prepared Si (111) - 7 × 7 surface with deposition 1 ML Pb atoms (1ML = 7.84 × 1014 atoms/cm2) at room temperature, followed by annealing at 600 K for a few seconds (about 3 ~ 4 sec).
After that, we often could see the Pb/Si (111) - 1 × 1 phase. At low temperature, the Pb/Si (111) - 1 × 1 phase is transformed to row- like
√7 × √3 phase. Next, we added more Pb atoms onto the above first-stage deposition sample at room temperature. It could be called second-stage deposition. Then we can be found Pb/Si (111) - HIC or SIC phase when the Pb coverage increase to 1 ~ 1.3 ML.
The detailed phase structure of Pb on Si (111) substrate will be described as follows.
Fig. 3.1.4 (a) Room-temperature STM topograph showing regions of the 1 × 1 phase surrounded by Si (111) - 7 × 7. (Info: 2.0 V, 0.15 nA).
Red scale bar: 5 nm. (b) High-resolution image of the 1×1 phase.
Blue scale bar: 1 nm. (c) STM image of √7 × √3 domains at 190 K.
(Info: 2 V, 0.15 nA) (d) High-resolution image of the √7 × √3 phase [10]. (e) Illustrate the atomic models of the 1 × 1 and √7 × √3
e
A. 1 ML coverage of the Pb/Si (111) - 1 × 1 phase, RT
After the above procedures of sample preparation, the Pb atoms destroy Si (111) - 7 × 7 and rearrange to form Pb/Si (111) - 1 × 1 phase [23] surrounded by Si (111) - 7 × 7 at room temperature, as shown in Fig. 3.1.4 (a). The Pb- covered regions is mainly a 2D Pb monolayer structure on Si (111) substrate. Fig. 3.1.4 (b) shows the atomic resolution image which can be taken at low bias for both polarities. The image exhibits a hexagonal 1 × 1 structure with a small corrugation of ~ 0.2 Å . The Pb coverage of Pb/Si (111) - 1 × 1 phase has been determined to be about one monolayer by Rutherford backscattering (RBS) experiments [23 - 25]. The atomic model for 1
× 1 phase is drawing in left side of Fig. 3.1.4 (e); the Pb atom is adsorbed on the Si adatom (T1 site).
B. 1 ML coverage of the Pb/Si (111) - √7 × √3 phase, LT
When the sample is cooled to low temperature, the Pb/Si (111) - 1 × 1 structure is transformed reversibly into a low- symmetry row- like √7 × √3 structure [26 - 28]. Figure 3.1.4 (c) shows the typical image of Pb/Si (111) - √7 × √3 surface structure. The image exhibits that the Pb- covered regions have the stable row-like structure with {-211} orientations, as expected from the substrate symmetry. The
atomic resolution image, as shown in Fig. 3.1.4 (d), demonstrates the trimer row-like structure. Figure 3.1.4 (e) illustrated the phase transition atomic model of Pb/Si (111) - √7 × √3 phase. In this model, some atoms are displaced from their T1 sites slightly to H3 and some still on the T1 sites. Therefore, the √7 × √3 structure can be seen as a distorted 1 × 1 structure and the coverage of Pb of the √7×√3 structure is also 1 ML.
C. 1~1.2 ML coverage of the Pb/Si (111) - HIC phase, LT
In particular after the second-stage deposition, two different phases have been discussed extensively: the HIC (hexagonal incommensurate phase) and the SIC (striped incommensurate phase) [29]. When the increase the Pb coverage to 1 ~ 1.2 ML, the surface will change to HIC phase in the monolayer. Based on STM images of Fig. 3.1.5 (a), it was suggested that these phases are six-fold degenerate, which implies that Pb can occupy two different binding sites (i.e., H3 and T4 sites) since the symmetry of the √3 × √3 unit cell is only three-fold. The HIC structure attributed to the added Pb atoms squeeze inner to monolayer. Therefore, this stress caused by like-trimer pattern of Pb clusters on H3 and T4 sites in a triangle domain and the domain walls formed by dimer on B2 site. The HIC
model is show in Fig. 3.1.5 (b). Ex: according to the coverage are 1 ~ 1.2 ML, so the trimer pattern should form by four atoms (tetramer) [30]. Our model simply presents the STM image. We do not participate in the discussion of this result of trimer or tetramer.
Fig. 3.1.5. (a) STM image is the Pb/Si (111) - HIC phase. The two different triangle domains mean the Pb atoms trimer (tetramer) close on H3 and T4 sites [31]. (b) Illustrate the atomic models of the HIC model. There marks by H3 and T4 site domains and a √3 × √3 unit
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D. 1.3 ML coverage of the Pb/Si (111) - SIC phase, LT
When the Pb coverage increase to 1.3 ML [30, 32], the monolayer surface will be rearranged to Pb/Si (111) - SIC phase.
Based on STM images of Fig. 3.1.6 (a), it phase pattern look like striped along to {-211} orientations, each striped line is a domain of H3 or T4. Similarly, like the HIC phase, that Pb can occupy H3 and T4 two different binding sites since the symmetry of the √3 × √3 unit cell. High-resolution image shows in Fig. 3.1.6 (b), the different direction trimer pattern of Pb clusters are adsorbed on H3 and T4 sites and the domain walls formed by dimer on B2 site. The SIC model is show in Fig. 3.1.6 (c). In this model, we can simply distinguish Pb clusters adsorbed on which site. Due to the Pb clusters are arrangement along the direction [-211], like clusters line.
However, we model show four lines on H3 site and five lines in T4 site of the Pb trimer with dimer cluster between there. Interestingly, the phase call incommensurate is mean not very orderly phase. So we often can to found random distribution of the different number of Pb cluster lines on H3 or T4 domains.
Fig. 3.1.6. (a) Over view the STM image of Pb/Si (111) - SIC phase, the striped line parallel the [1-21] direction (Info: 2.0 V, 1.0 nA). (b) High-resolution image of SIC phase (Info: -0.05 V, 0.2 nA) . It show the different direction trimer on H3 and T4 sites marks by arrows. (c) Illustrate the atomic models of the SIC model. There marks by H3 and T4 site domains and a √3 × √3 unit cell.
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