The surface representing the object of investigation of my research is silicon. In particular, Si(111)-7x7 and Si(100)-2x1 reconstructed surfaces.
Scanning probe devices provide the quantum mechanical tool needed to characterize solid materials surfaces and investigate their properties on a nanometre scale. Two of the most common techniques, the ones I use at the University of Nottingham laboratory, are scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Silicon surface is also considered an ideal substrate for molecule deposition. This is another area of my research, devoted especially to the deposition of C60 molecules.
When investigating the structure of a solid surface, the first step is to consider the arrangement of atoms on the plane of the crystalline solid chosen as the surface to be exposed. Different spacing and symmetries of the atoms on different planes will lead to significant differences in the atomic pattern. The surface of a solid crystal is created by breaking interatomic bonds within the 3-D periodicity, which leaves these bonds dangling into the vacuum. In order to minimize the number of dangling bonds, a wide range of surfaces undergo a reconstruction, which consists in a rearrangement of the surface atoms with respect to the bulk structure and can affect one or more layers at the surface.
Fig. 1 shows the atomic arrangement of the Si(111)-7x7 reconstructed surface described by the DAS (dimer-adatom-stacking fault) model . The reconstruction of this sample is one of the most complicated as it not only affects the surface atom layer, but the topmost four layers, resulting in a new unit cell which is 7x7 as large as the bulk terminated structure (v=3.84 Ǻ is the length of the unit vector of the bulk terminated Si(111), whereas w= 26.9 Ǻ is the length of the unit vector of the reconstructed surface). According to this model, twelve adatoms and six rest atoms are evenly distributed in the unit cell, where two different sides can be distinguished, the faulted half unit cell (FHUC) and unfaulted half unit cell (UHUC). Each 7x7 unit cell contains 19 dangling bonds against the 49 originally contained in the unreconstructed unit cell: 12 for the adatoms, six for the rest atoms (ReF, ReU) and one for the atom in the center of the corner hole (CoH). These dangling bonds originate the tunneling current in STM when scanning the Si(111)-7x7 surface.
Fig. 1 Schematic diagram (top and side view) for the Si(111)-7x7 "DAS" model. (Taken from Ref.).
If compared with the Si(111)-7x7 surface, the Si(100)-2x1 surface experiences a simpler reconstruction. The bulk terminated surface will be a (1x1) structure as shown in Fig. 2. It can be seen that, based on the fact that the surface silicon atoms have a tendency to reach a coordination number of 4, each Si atom at the first layer on Si(100)-1x1 surface is connected to the bulk by two bonds; in addition it has two bonds unoccupied, namely two dangling bonds.
Fig. 2 Top view (left) and side view (right) schematic model of Si(100)-1x1 ideal surface. (Taken from Ref. ).
To decrease the number of dangling bonds, adjacent Si atoms form rows of Si atom pairs called dimers. By lateral translation of the surface atoms, two dangling bonds in each Si dimer (one per each silicon atom) form a very strong σ bond, leaving one dangling bond per atom. These remaining dangling bonds interact by forming a π bond, causing the Si(100) surface to constitute a self-organized 2x1 dimer row structure as shown in Fig. 3.
Fig. 3 Top view (left) and side view (right) schematic models of Si(100)-2x1 reconstructed surface. (Taken from Ref. ).
When a charge transfer from the lower to the upper dimer atom occurs, dimers configuration at equilibrium turns asymmetric due to their buckling and a small amount of energy is gained. Depending on the relative orientation of the buckling in adjacent dimers, two other surface reconstructions become possible, in addition to the (2x1) phase: p(2x2) in which all dimers in each row buckle in reverse directions, but the overall structure is such that neighboring rows are buckled in phase; and c(4x2) in which dimers not only buckle along each row, but also perpendicularly to the row direction forming qusi-hexagonal rings, both illustrated in Fig. 4.
Fig. 4 Buckling structure: p(2x2) (left) and c(4x2) (right) reconstruction. (Taken from Ref. ).
Here is shown an example of Si surfaces scanned with the STM technique. All the experiments were performed using a LT UHV system, with a base pressure of better than 5x10-11 mbar at a temperature of 77 K. All silicon samples investigated, of 3x10 mm2 size, are B-doped wafers, with a resistivity of 0.01÷0.02 Ωcm for Si(111), and 0.004 Ωcm for Si(100).
Figs. 5(a)-(b) show high contrast topographic images of Si(111)-7x7 clean surface.
They were taken on an area of 15x15 nm2 in dual mode, with sample bias of -1.5 V and +1.5 V respectively, and tunnelling current of 1.0 nA. Fig. 5(a) illustrates the atomic arrangement typical of the filled-state reconstruction. The (7x7) unit cells are distributed in a perfect symmetry, each one clearly showing the FHUC and UHUC sides, corner holes, as well as the simultaneous appearance of both adatoms and rest atoms. Generally, it can be seen that the adatoms on the faulted half look brighter than those on the unfaulted half. As regards the rest atoms, they can be recognized on both halves as darker spots between the adatoms. Fig. 5(b) is a representative example of the empty-state reconstruction. Unlike the corresponding occupied state image, only adatoms are visible in this case, which present all the same brightness.
Fig. 5 STM topographic images of filled and empty states of Si(111)-7x7 surface taken in dual mode on an area of 15x15nm2 with a current setpoin I=1.0 nA. Sample bias (a) V=-1.5V, (b) V=+1.5V.
Figs. 6(a)-(b) show STM images of filled and empty states of Si(100) surface. They were acquired with a sample bias of -1.5 V and +2.5 V, over a scan area of 30x25 nm2 and 50x50 nm2 respectively. They illustrate on a large scale the characteristic reconstruction of Si(100) surface, as typical features like steps and terraces can be observed. In addition to the step edges, parallel dimer rows are visible in both images as well as intrinsic defects, appearing like dark regions on each of the terraces, known as missing dimers.
Fig. 6 Filled and empty states topographic images of Si(100) surface. Sample bias voltage: (a) -1.5V, (b) 2.5 V. Current setpoint: (a) I=30 pA, (b) 200 pA. Scan area: (a) 30x25nm2, (b) 50x50nm2.
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