Dimer rows

STM has not as yet proved to be easily applicable to the area of ultrafast surface phenomena. Nevertheless, some success has been achieved in the direct observation of dynamic processes with a larger timescale. Kitamura et al [23], using a high-temperature STM to scan single lines repeatedly and to display the results as a time-ver.sn.s-position pseudoimage, were able to follow the difflision of atomic-scale vacancies on a heated Si(OOl) surface in real time. They were able to show that vacancy diffusion proceeds exclusively in one dimension, along the dimer row. 

Reactions of alkenes with H-Si(l 0 0)-2 x 1 surfaces have been shown to yield films with one-dimensional (ID) molecular lines through Si-C linkages, contrary to formation of the islands observed on H-Si(l 11). The reaction can be initiated from isolated surface silyl radicals created using the tip of the STM. The STM images showed molecular lines running along and across the dimer rows depending on the chemical constituent of R in the CH2 = CH-R molecules. 

Scheme 22 Mechanism of radical chain reactions of the growth of styrene line along the edge of a dimer (left side) and of the growth of allyl mercaptan line across the dimer rows (right side) of a H-Si(l 0 0)-2 x 1 surface.

The rapid fabrication of covalently bonded ID functional molecular lines with predefined location, direction, and length provides a means to make a predesigned interconnection of molecular lines running along and across the dimer rows. Indeed, the perpendicularly connected allyl mercaptan and styrene lines or allyl mercaptan and acetone lines have been fabricated on the H-Si(l 00)-2 X 1 surface. °° 2 ... 

Figure 4.14 Top and side view of Si(001) in its dimer row reconstruction. Coordinates used for this illustration come from a fully relaxed DFT calculation.

We begin our discussion with the diffusion of a Si adatom over a flat terrace. This problem has previously been addressed with ab initio calculations for the case of symmetric dimers. The main result is that diffusion is highly anisotropic on the surface, with fast diffusion taking place over the top of the dimers with a saddle point energy of about 0.60 eV. Slow adatom diffusion is predicted to take place across the dimer rows with a barrier of 1.0 eV. Experiments based on a number counting of the island density are in agreement with these results. ... 

Diffusion over the surface is still highly anisotropic. For motion over the top of the dimers, we obtain intervening barriers of 0.70 and 0.55 eV, respectively. To move across the dimer rows, a barrier of 0.95 eV must be surmounted. The barriers for diffusion in the channels separating the dimer rows are quite sensitive to the tilt of the dimer, with 0.75 eV being the lowest barrier encountered when the adatom moves past a down dimer. 

Most of the adatoms will reach this step edge from the upper terrace. Adatoms coming in over the top of the upper terrace dimers encounter a barrier of 0.85 eV, while the barrier is only 0.70 eV if the adatoms come in from between the dimer rows. A relatively large activation barrier of more than 1.0 eV is required to come in from the lower terrace, so that this direction of approach is unlikely. To escape onto the upper terrace requires hops of 1.1 eV for C and 0.70 eV for hops B D. Clearly, in contast to the step, because the minima are deeper and the barriers for escape... 

Due to the relative ease of carrying out the reaction and the versatility of the process, the hydrosilylation reaction has been used in a number of interesting extensions and applications. Here several of them are highlighted. In one report, Lop-inski and coworkers used the same concept of the radical-initiated hydrosilylation reaction on the Si(100)-2 x 1 surface to induce self-directed growth of molecular wires on the surface [141]. On the Si(100)-2 x 1 surface, the radical chain reaction propagates primarily along the direction of the dimer row, leading to lines of... 

Figure 5.8. STM images of the Si(100)—(2 x 1)—H surface after hydrosilylation reaction using both styrene and allyl mercaptan, from Hossain, Kato and Kawai. The image shown in (a) contains two allyl mercaptan lines. Image (b) shows the system after a new dangling bond is generated using the STM tip. After the surface is exposed to styrene molecules, a styrene line forms which joins the two allyl mercaptan lines (c). The allyl mercaptan lines form across the Si dimer rows, while the styrene line forms along the dimer row. Figure reproduced with permission from Ref. [143]. Copyright 2005 American Chemical Society.

Alkynes have also been shown to form the [2 + 2] cycloaddition product. Acetylene (H—C=C—H), the simplest alkyne, forms an interesting adsorption case, because the specific adsorption geometries of acetylene on Si(100)-2 x 1 have been debated [11,201,207,210,224-236]. Acetylene was first found experimentally to form a [2 + 2] C=C cycloaddition product that exhibits a cyclobutene-like surface structure on Si(100)-2 x 1 [210,227]. Later STM measurements revealed that at least two different surface products were present [228,231,233], and identified a product that is oriented perpendicularly to the dimer row. From these images, it was argued that in addition to an intradimer [2 + 2] C=C cycloaddition geometry, acetylene also forms a surface adduct that bridges two dimers along a row. Several theoretical... 

From the ethylene results, and similar results on acetylene [306], it is evident that interdimer reactions play an important role in the chemistry of organic molecules on Ge(100)-2 x 1. The simple picture of reaction across a single Ge-Ge dimer, while capturing a number of important reaction pathways, is incomplete. Even small C2 molecules such as ethylene and acetylene can bridge across dimers along a dimer row. Other molecules are found to bridge across the wider trench. Furthermore, these studies indicate that multiple reaction products can form even for simple systems. 

Hossain, M. Z., Kato, H. S. and Kawai, M. Fabrication of interconnected ID molecular lines along and across the dimer rows on the Si(100)-(2 x 1)-H surface through the radical chain reaction. Journal of Physical Chemistry 109, 23129 (2005). 

Therefore, the Si(001) surface covered with ZrCT groups between dimer rows was taken as the initial structure for the simulation of zirconia film growth. The following set of chemical reactions was used as kinetic mech-... 

Fig. 8. STS of Si(100)-2 X 1 (a) Spatially average current-voltage (I/V) curve (b) the calculated dljdVjil/V) versus V curve shows the peaks related to the surface states, (c) Cross-sectional dl/dV/(I/V) versus V, perpendicular to dimer rows. Figure courtesy of T. C. G. Reusch.

Organic Molecules Styrene [79], Vinylferrocene [95] Initial adsorption at dangling bond and chain reaction assembly of molecular wires along dimer row direction that are predicted to permit charge transport [96]. 

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