Imaging the Dimers in Si(111)-(7 x 7)

Deciphering the Atomistic Mechanism of Si(111)-7 × 7 Surface Reconstruction Using a Machine-Learning Force Field

While the complex 7 × 7 structure that arises upon annealing the Si(111) surface is well-known, the mechanism underlying this unusual surface reconstruction has remained a mystery. Here, we report molecular dynamics simulations using a machine-learning force field for Si to investigate the Si(111)-7 × 7 surface reconstruction from the melt. We find that there are two possible pathways for the formation of the 7 × 7 structure. The first path arises from the growth of a faulted half domain from the metastable 5 × 5 phase to the final 7 × 7 structure, while the second path involves the direct formation of the 7 × 7 reconstruction. Both pathways involve the creation of dimers and bridged five-membered rings, followed by the formation of additional dimers and the stabilization of the triangular halves of the unit cell. The corner hole is formed from the joining of several five-member rings. The insertion of atoms below the adatoms to form a dumbbell configuration involves extra atom diffusion or rearrangement during the evolution of triangular halves and dimer formation along the unit cell boundary. Our findings may provide insights for manipulating the surface structure by introducing other atomic species.

A re-evaluation of diffraction from Si(111) 7 × 7: decoding the encoded phase information in the 7 × 7 diffraction pattern

The diffraction features of Si(111) 7 × 7 are analyzed and related to various structural models of the Si(111) 7 × 7 surface as one part of a multivariate analysis of this system. The limitations in early sample preparation and measurements produce some uncertainty in previously proposed structures. More recent data is considered here. In addition, models used early on to evaluate the structure of 7 × 7 have been over simplified, idealized models. More complex models are considered within the projection rod method as used for surface crystallography. The origin of numerous diffraction features can be determined via their Fourier components for a wide range of 2-D layers, which provides new insight into the structure as well as the limitations of prior projection analyses. Structures which produce the key elements of the 7 × 7 diffraction are presented and various distortions are considered consistent with other experimental results. In general it is found that the presence of a strong set of 3/7th order beams and near extinction of neighbouring fractional order beams are features which are found experimentally and distinguish an important class of structures. This class has a particular type of 3-fold mirror symmetry, which is not apparent in the widely accepted dimer-adatom-stacking fault, DAS, model. Higher order diffraction features, of which many are weak, are also considered and provide important new structural information. Several new polymorphs of the 7 × 7 are identified which may also satisfy the diffraction derived features and possess some degree of pi-bonding so as to enable magnetic surface states not possible in a pure covalently bonded system such as DAS. The Patterson map of the 7 × 7 surface provide insight into the lost phase information encoded in diffraction and reveal why the DAS structure was experimentally favored. An unusual non-primitive 7 × 7 unit cell is also derived from the Patterson map that possesses unusual symmetry properties, a non-standard surface Brillouin zone with potentially unusual electronic properties.

When does atomic resolution plan view imaging of surfaces work?

Surface structures that are different from the corresponding bulk, reconstructions, are exceedingly difficult to characterize with most experimental methods. Scanning tunneling microscopy, the workhorse for imaging complex surface structures of metals and semiconductors, is not as effective for oxides and other insulating materials. This paper details the use of transmission electron microscopy plan view imaging in conjunction with image processing for solving complex surface structures. We address the issue of extracting the surface structure from a weak signal with a large bulk contribution. This method requires the sample to be thin enough for kinematical assumptions to be valid. The analysis was performed on two sets of data, c(6×2) on the (100) surface and (3×3) on the (111) surface of SrTiO₃, and was unsuccessful in the latter due to the thickness of the sample and a lack of inversion symmetry. The limits and the functionality of this method are discussed.

Surface determination through atomically resolved secondary-electron imaging

Unique determination of the atomic structure of technologically relevant surfaces is often limited by both a need for homogeneous crystals and ambiguity of registration between the surface and bulk. Atomically resolved secondary-electron imaging is extremely sensitive to this registration and is compatible with faceted nanomaterials, but has not been previously utilized for surface structure determination. Here we report a detailed experimental atomic-resolution secondary-electron microscopy analysis of the c(6 × 2) reconstruction on strontium titanate (001) coupled with careful simulation of secondary-electron images, density functional theory calculations and surface monolayer-sensitive aberration-corrected plan-view high-resolution transmission electron microscopy. Our work reveals several unexpected findings, including an amended registry of the surface on the bulk and strontium atoms with unusual seven-fold coordination within a typically high surface coverage of square pyramidal TiO5 units. Dielectric screening is found to play a critical role in attenuating secondary-electron generation processes from valence orbitals.

Reversible wurtzite-tetragonal reconstruction in ZnO(1010) surfaces

Bistable surface: The reversible phase transition between wurtzite (WZ) and body-centered-tetragonal (BCT) lattice was activated in ZnO(1010) surfaces and directly imaged at atomic scale by using aberration-corrected electron microscopy. A nucleation-growth mechanism for the surface reconstruction is further proposed based on observations and calculations of the WZ-BCT domain boundary.

Surface crystallography via electron microscopy

The study of atomic structure of surfaces is fundamental to the understanding of electronic, chemical and mechanical properties of surfaces and numerous techniques have been developed to this end. Transmission Electron Microscopy techniques, namely transmission electron imaging (TEM) and diffraction (TED), due to their ability to provide structural information at very high resolutions, have emerged as powerful tools for the study of surface structure. In this article we review the experimental method alongside the various post-processing routines that are necessary to extract vital structural information from experimental data.

The structure and chemistry of the TiO(2)-rich surface of SrTiO(3) (001)

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of 'dangling bonds' by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO(3)--a standard model for oxides with a perovskite structure--have been studied extensively. Here we report a solution of the 2 x 1 SrTiO(3) (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO(6-x) units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO(3). We suggest that this structural concept can be extended to perovskite surfaces in general.

Maximum entropy methods in electron crystallography

The maximum entropy (ME) method of solving crystal structures in two or three dimensions from electron diffraction data is described. Applications to organic and inorganic molecules, membrane proteins and surface structures are outlined, and the power of the ME formalism to deal with incomplete and error prone data is demonstrated.

In situ growth and characterization of ultrahard thin films

Results concerning the operation of a new ultrahigh vacuum (UHV) ion-beam assisted deposition system for in situ investigation of ultrahard thin films are reported. A molecular beam epitaxy (MBE) chamber attached to a surface science system (SPEAR) has been redesigned for deposition of cubic-boron nitride thin films. In situ thin film processing capability of the overall system is demonstrated in preliminary studies on deposition of boron nitride films on clean Si (001) substrates, combining thin film growth with electron microscopy and surface characterization, all in situ.