Martin R. Castell

Field-emission SEM imaging of compositional and doping layer semiconductor superlattices

Field-emission scanning electron microscopy (FE-SEM) has been used to study several semiconductor multilayer heterostructures. Compositional superlattices based on Ge x Si 1−x /Si and Al x Ga 1−x As/GaAs have been studied in both cross-sectional and oblique plan-views after indentation. Secondary and backscattered electron images reveal strong atomic number contrast which is primarily structural in origin. Secondly, for the first time, heterostructures containing n and p doping have been directly imaged at low voltages (0.5-1 kV) including: (i) Si- and Be-doped GaAs layers and (ii) B- and As-doped Si layers. Secondary electron images reveal strong contrast at doping concentrations as low as 10 17 cm −3 . The results have been interpreted in terms of energy band-bending effects between n- and p-doped layers

Scanning tunneling microscopy of reconstructions on the SrTiO3(001) surface

Scanning tunneling microscopy (STM) was used to investigate the (0 0 1) surface atomic structure of Nb-doped SrTiO3 single crystals which were annealed in ultra high vacuum. Atomic resolution images of the (2 � 1), c(4 � 4), and c(4 � 2) reconstructions are presented. Surface structure models are proposed based on qualitative interpretation of the STM images. Surface terminations with different Ti to O ratios are considered. 2002 Elsevier Science B.V. All rights reserved.

Smart Align—a new tool for robust non-rigid registration of scanning microscope data

Many microscopic investigations of materials may benefit from the recording of multiple successive images. This can include techniques common to several types of microscopy such as frame averaging to improve signal-to-noise ratios (SNR) or time series to study dynamic processes or more specific applications. In the scanning transmission electron microscope, this might include focal series for optical sectioning or aberration measurement, beam damage studies or camera-length series to study the effects of strain; whilst in the scanning tunnelling microscope, this might include bias-voltage series to probe local electronic structure. Whatever the application, such investigations must begin with the careful alignment of these data stacks, an operation that is not always trivial. In addition, the presence of low-frequency scanning distortions can introduce intra-image shifts to the data. Here, we describe an improved automated method of performing non-rigid registration customised for the challenges unique to scanned microscope data specifically addressing the issues of low-SNR data, images containing a large proportion of crystalline material and/or local features of interest such as dislocations or edges. Careful attention has been paid to artefact testing of the non-rigid registration method used, and the importance of this registration for the quantitative interpretation of feature intensities and positions is evaluated.

A homologous series of structures on the surface of SrTiO3(110)

Strontium titanate is seeing increasing interest in fields ranging from thin-film growth to water-splitting catalysis and electronic devices. Although the surface structure and chemistry are of vital importance to many of these applications, theories about the driving forces vary widely. We report here a solution to the 3 x 1 SrTiO(3)(110) surface structure obtained through transmission electron diffraction and direct methods, and confirmed through density functional theory calculations and scanning tunnelling microscopy images and simulations, consisting of rings of six or eight corner-sharing TiO(4) tetrahedra. Further, by changing the number of tetrahedra per ring, a homologous series of n x 1 (n > or = 2) surface reconstructions is formed. Calculations show that the lower members of the series (n < or = 6) are thermodynamically stable and the structures agree with scanning tunnelling microscopy images. Although the surface energy of a crystal is usually thought to determine the structure and stoichiometry, we demonstrate that the opposite can occur. The n x 1 reconstructions are sufficiently close in energy for the stoichiometry in the near-surface region to determine which reconstruction is formed. Our results indicate that the rules of inorganic coordination chemistry apply to oxide surfaces, with concepts such as homologous series and intergrowths as valid at the surface as they are in the bulk.

Nanostructures on the SrTiO3(001) surface studied by STM

Atomic resolution scanning tunnelling microscopy of the SrTiO3(0 0 1) surface reveals that certain treatments give rise to two types of self-assembled nanostructures. Surfaces prepared through Ar ion sputtering and ultrahigh vacuum (UHV) annealing up to 900 C form terraces with straight step edges that have a c(4 � 2) reconstruction. Following further UHV annealing above 900 C nanolines of 1 nm width, 0.2nm height, and variable length are created that run in h 100 i directions. On closest packing the nanolines are 2.4 nm apart and form a (6 � 2) overlayer. On further annealing the nanolines break up into nanodots of 0.2nm height that form regular arrays. It is proposed that both structure types are created through non-stoichiometry of the surface region that results in nanocrystalline growth of either SrO or TiOx. SrTiO3 surfaces patterned in this way have the potential to serve as a template for the growth of metallic or semiconducting quantum structures. 2002 Elsevier Science B.V. All rights reserved.

Mapping surface elastic properties of stiff and compliant materials on the nanoscale using ultrasonic force microscopy

Abstract The increasing production of nano-devices and nano-composite materials has prompted the development of new instruments to probe smaller and smaller volumes. Regarding mechanical properties in particular, modified atomic force microscopes using force modulation at frequencies below the cantilever resonance have been successfully employed to investigate relatively compliant materials such as bio-materials and polymers but have shown limitations to highly stiff materials. The alternative approach of ultrasonic force microscopy (UFM) uses sample vibration at frequencies far above the cantilever primary resonance, exploiting the inertial stiffness of an atomic force microscopy cantilever and detection of ultrasonic vibration via nonlinearity of the tip–surface force interaction. In this paper we demonstrate that UFM can discriminate elastic properties of materials ranging from quite stiff to relatively compliant with a lateral resolution of a few nanometres and with high sensitivity to the elastic modulus. Furthermore a phenomenon of ultrasonically induced friction reduction permits imaging of fragile samples otherwise swept away in conventional contact mode atomic force microscopes. The possible influence of adhesive properties also has been analysed and criteria for distinguishing elastic and adhesive contributions have been established. We also explore another promising application of UFM for detection of nanoscale subsurface delamination.

Understanding STM images and EELS spectra of oxides with strongly correlated electrons: a comparison of nickel and uranium oxides

Using a theoretical approach combining the local spin density approximation (LSDA) of density functional theory and the Hubbard U term (LSDA + U), we analyse the connection between the experimentally observed electron energy loss spectra and elevated temperature scanning tunnelling images of surfaces of semiconducting nickel monoxide NiO and uranium dioxide UO2. We show that a combination of electron energy loss spectroscopy, atomic-resolution tunnelling imaging and first-principles ab initio calculations provides a powerful tool for studying electronic and structural properties of surfaces of transition metal and actinide oxides.

High resolution scanning tunnelling microscopy of the rutile TiO2(110) surface

Abstract We present high resolution scanning tunnelling microscope (STM) images of the (1×1) and (1×2) reconstructions of the TiO 2 (110) surface. Under the conditions of low bias voltages and high tunnelling currents, we image two features per (1×1) surface unit cell. We observe that it is possible simultaneously to resolve the bridging oxygen atoms on the TiO 2 (110)-p(1×1) surface as well as the in-plane titanium atoms. When the surface is annealed in UHV, long straight rows form along the [001] direction. Evidence is provided which supports an “added row” model for these structures. After long annealing times they create a densely packed (1×2) reconstructed surface morphology with straight anti-phase boundaries running along the [001] direction and meandering anti-phase boundaries along [ 1 10] .

Pairs and heptamers of C70 molecules ordered via PTCDI-melamine supramolecular networks

In this paper, we report on the use of two PTCDI-melamine supramolecular networks on Au(111) to trap C70 molecules. The different supramolecular networks were formed by changing the postannealing temperature after molecular deposition. We observed, using scanning tunneling microscopy, that the deposition of C70 onto a PTCDI melamine network with parallelogram cavities results in the long-range ordering of paired C70, whereas the deposition of C70 molecules onto a PTCDI-melamine honeycomb network results in the trapping of C70 heptamers.

The evolution of Ni nanoislands on the rutile TiO2(110) surface with coverage, heating and oxygen treatment

Abstract Scanning tunnelling microscopy (STM) and reflection high-energy electron diffraction (RHEED) have been used to examine in situ the morphology and growth of nickel nanoislands on the rutile TiO 2 (110)-p(1×1) surface. Such analysis revealed growth at temperatures between 295 and 495 K via the Volmer–Weber (V–W) mode of three-dimensional islands, in agreement with thermodynamic expectations. The Ni islands grew first as small crystalline domes with no preferential alignment with the TiO 2 (110) surface, but with increasing Ni dose their orientation evolved, until almost perfect monocrystalline (110) Ni ∥(110) TiO 2 , [ 1 10] Ni ∥[001] TiO 2 orientation-relations were established. This arrangement has one of the lowest lattice mismatches if an average of the two orthogonal surface directions [001] TiO 2 and [110] TiO 2 is considered. The epitaxial relation persists upon annealing in ultra-high vacuum (UHV) to 1065 K and the heat treatment causes the particles to coarsen and grow in size. When the crystalline islands of nickel are oxidised, the lattice parameters are modified, consistent with the formation of NiO. A new epitaxial relation is established between the overlayer and the substrate: (001) layer ∥(110) TiO 2 , [1 1 0] layer ∥[001] TiO 2 . This produces a large strain parallel to the [1 1 0] TiO 2 direction and a very small strain along [001] TiO 2 (+0.13% mismatch). UHV annealing of the oxidised overlayer reduces the crystallites to islands with lattice parameters corresponding to nickel metal, but the original crystallography is not recovered. Instead, the nanoislands of reduced nickel maintain the same overlayer orientation as for the oxide. A model is presented in which the change in epitaxial relation is attributed to the formation of an interfacial NiO layer between the reduced Ni islands and TiO 2 substrate. The results demonstrate that the oxidation/reduction treatment of the substrate and overlayer can be used to modify the microstructure of the metal/oxide system, and have implications for the use of nickel as a supported metal catalyst.