Elevated Temperature Scanning Tunneling Microscopy Research Articles

Real-time coarsening dynamics of Ge∕Si(100) nanostructures

The coarsening dynamics of Ge∕Si(100) nanostructures were monitored using real-time, elevated temperature scanning tunneling microscopy (STM). Gas-source molecular beam epitaxy from digermane onto Si(100) was used to produce mixed hut and pyramid cluster ensembles. The width of the most elongated rectangular-based hut clusters was always less than the side length of square-based pyramid clusters for the growth conditions employed. This suggests that pyramid elongation to form hut clusters occurred at early growth stages for some smaller clusters. A previously unidentified coarsening mechanism was characterized during growth temperature annealing and was interpreted using atomistic elastic modeling. Pyramid clusters were more stable than narrow hut clusters with larger volumes. These larger volume huts decayed by reducing their length at a constant width, finally becoming small pyramids. These small pyramids are less stable than those that never elongated to form huts and consequently dissolve. The decrease in the total island volume identified using STM was attributed to large (diameter>1μm), low chemical potential clusters. These clusters depleted the surface of Ge adatoms as evidenced by denuded zones devoid of smaller clusters observed for growth and annealing at higher temperatures. These results suggest that a preferred island size for pyramids and huts does not exist.

Planar vanadium oxide clusters: two-dimensional evaporation and diffusion on Rh(111).

The formation of novel vanadium oxide cluster molecules by oxidative two-dimensional evaporation from vanadium oxide nanostructures is reported on a Rh(111) metal surface. The structure and stability of the planar V6O12 clusters and the physical origin of their 2D evaporation process have been elucidated by high-resolution scanning tunneling microscopy (STM) and ab initio density functional theory calculations. The surface diffusion of the clusters has been followed in elevated-temperature STM experiments, and the diffusion parameters have been extracted, indicating diffusion by hopping of the entire surface stabilized cluster units.

Real-Time Coarsening Dynamics of Ge/Si(100) Nanostructures Using Elevated Temperature Scanning Tunneling Microscopy.

ABSTRACTCoarsening dynamics of Ge/Si(100) nanostructures were monitored using real-time, elevated temperature scanning tunneling microscopy. Gas-source molecular beam epitaxy from digermane at 0.2 ML / min onto Si(100) at temperatures near 500 °C produced mixed hut and pyramid cluster ensembles. The width of the most elongated rectangular-based hut clusters is always less than the side length of square-based pyramid clusters for island ensembles grown using these conditions. This suggests that pyramid elongation to form hut clusters occurred at early growth stages for some smaller clusters. Growth temperature annealing revealed that pyramid clusters are more stable than narrow hut clusters with larger volumes. These larger volume huts decay by reducing their length at constant width, finally becoming small pyramids. These pyramids, which are smaller than those which never elongated to form huts, are less stable and consequently dissolve. Atomistic elastic modeling confirms that hut clusters less efficiently store elastic energy than pyramid clusters which explains our observations. Large (> 1μm diameter), low chemical potential clusters deplete the surface of Ge adatoms as evidenced by the existence of denuded zones devoid of smaller clusters. These large clusters are responsible for the decrease in total island volume in the STM field of view during the anneal.

Microscopy of Metal Oxide Surfaces.

Elevated temperature scanning tunneling microscopy is used to study oxides that are room temperature insulators but become sufficiently electrically conducting at higher temperatures to allow imaging to be performed. Atomic resolution images of NiO, CoO, and UO(2) have been obtained in this fashion which allow surface structure and defect determination. To complement the experiments, modeling of the electronic surface structure reveals which atomic sites give rise to the contrast observed in the images. Low voltage scanning electron microscopy is used to image small equilibrium pores in UO(2) single crystals to evaluate the surface energy ratio of the (111) to (001) surfaces.

In search of nanoperfection: Experiment and Monte Carlo simulation of nucleation-controlled step doubling

In order to make effective use of the extreme density of nanoscale elements that form spontaneously in self-assembling architectures, one must address the associated issue of minimizing defect creation during the formation of such structures. In this article we examine the competing roles that nucleation kinetics and two-dimensional growth processes play in nanostructure formation and defect minimization. We employ oxygen-induced step doubling of vicinal Ni(977) surfaces as our physical system, using elevated temperature scanning tunneling microscopy and Monte Carlo simulations to extract the desired details of interface evolution. Two interesting topological defect features are observed on the surface after doubling reaches its asymptotic limit: (i) “frustrated ends,” which form when two counter-propagating step-doubling events having a single step in common intersect, leaving a stable topological defect, and (ii) residual “isolated single steps,” which form when a single step is unable to partner with an adjacent step. This latter defect occurs when a single step is surrounded on both sides by previously doubled structures. In an attempt to understand and control these results, Monte Carlo simulations indicate that experimental control of the delicate and competing interplay of nucleation kinetics and two-dimensional growth kinetics is the key to the formation of more perfect interfaces. In this instance this corresponds to using a small initial oxygen exposure and reduced substrate temperature to achieve a doubled surface of higher perfection. Such optimized interfaces can act as templates for guiding the hierarchical assembly of nanowires and other nanoscale molecular assemblies.

Distribution of dangling bond pairs on partially hydrogen-terminated Si(100) surface observed by scanning tunneling microscopy

Hydrogen recombinative desorption from a Si 2H 2 monohydride dimer on the Si(100) surface regenerates a dangling bond (DB) pair on the dimer. In this paper, we investigated the spatial distribution of the regenerated DB pairs on the Si(100)-2×1:H and disilane-passivated Si(100) surface using elevated temperature scanning tunneling microscopy (HT-STM) in the temperature range between 590 and 622 K. Experimental results indicate that the ends of one-dimensional (1D) monohydride dimer islands are preferred sites for DB pairs and a repulsive interaction occurs between two neighboring DB pairs in the same dimer row.

Proximity heater for elevated temperature in situ vacuum scanning tunneling microscopy of metal surfaces

The design and application of a radiant heater assembly for elevated temperature scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) is presented. The proximity heater is a noninvasive modification to an existing commercial room-temperature microscope and is capable of radiatively heating samples up to 650 K in situ. Imaging at higher temperatures should be readily accessible with other microscope construction designs. It is demonstrated that this heater is well suited for enabling an STM to capture surface morphological transformations such as the motion of atomic steps on metal surfaces at elevated temperature. Various design issues and solutions related to variable temperature UHV-STM are also discussed. We believe the approach described to be general in nature, offering a direct route to adapting UHV-STM designs for elevated temperature imaging.

Two (1×2) Reconstructions ofTiO2(110): Surface Rearrangement and Reactivity Studied Using Elevated Temperature Scanning Tunneling Microscopy

We have employed elevated temperature scanning tunneling microscopy to elucidate the reactivity and surface structure of the $(1\ifmmode\times\else\texttimes\fi{}2)$ reconstructed $\mathrm{TiO}{}_{2}(110)$ surface. We find two distinctly different ordered surface reconstructions depending upon the level of bulk reduction of the crystal (degree of nonstoichiometry). On the near stoichiometric surface reactivity to oxygen is low and attributed to the formation of the stable $\mathrm{Ti}{}_{2}\mathrm{O}{}_{3}$ type termination. For heavily reduced crystals a cross-linked $(1\ifmmode\times\else\texttimes\fi{}2)$ reconstruction forms with high activity to oxygen resulting in a well-defined cyclic reaction ( $\mathrm{TiO}{}_{2}$ growth).

The atomic structure of the UO 2+ x(110) surface and the effects of interstitial oxygen: an elevated-temperature STM study

The (110) surface of uranium dioxide (UO 2+ x ) single crystals has been studied by elevated-temperature scanning tunneling microscopy (STM). STM images of stoichiometric UO 2(110) surfaces display a row structure which corresponds to a (1×1) bulk termination with missing rows along the [1 1 ̄ 0] direction. Uranium sites are visible with atomic resolution in empty-states images above ∼1.6 V sample bias. Interstitial oxygen in hyperstoichiometric UO 2+ x leads to a local redistribution of charge which appears in the STM image as dimer pairs. After dosing a UO 2(110) surface with oxygen, another type of superstructure was observed which results from oxygen adsorption at surface interstitial sites.

Elevated temperature scanning tunneling microscopy study of formic acid adsorption and reaction on oxygen (2×1) covered Cu(110)

The adsorption and reaction of formic acid on a fully oxygen (2×1) covered Cu(110) surface maintained at 353 K has been studied by scanning tunneling microscopy (STM). Formic acid adsorption is followed in a sequence of images that show the removal of oxygen (2×1) rows preferentially in the 〈001〉 direction. The added copper atoms which were associated with the oxygen reconstruction are liberated and diffuse to create significant areas of the high oxygen coverage O-c(6×2) structure. The adsorbed formate produces a (4×1) structure. Analysis of the STM images allows limits to be placed upon the surface structure of the final formate (4×1) surface revealing a lower local formate density than produced for room temperature adsorption. These two observations lead to an understanding of the significant drop in formic acid uptake which has been reported for a narrow temperature window between 350–420 K on this surface.

Bias-dependent STM investigations of trimethylgallium adsorption on Si(001) at elevated temperatures

We have studied the change in surface reconstruction of Si(001) after adsorption of 1 L trimethylagllium, Ga(CH3)3 (TMGa) on Si(001). The investigations were carried out by elevated temperature scanning tunneling microscopy (STM) and electron diffraction (LEED, RHEED). Dosing TMGa at a substrate temperature of 500°C leads to a c(4 × 4) surface reconstruction which develops preferentially at trench defects and step edges. These isolated patches grow over time and eventually cover the entire surface. Bias dependent imaging shows that electronic effects dominate the contrast in the STM empty states images. Electron diffraction (LEED, RHEED) confirmed the c(4 × 4) surface reconstruction. Based on our STM images and the results of the electron diffraction experiments, we propose a structural model for the Si(001) c(4 × 4) reconstructed surface after adsorption of TMGa.

Defect Structure of NonstoichiometricCeO2(111)Surfaces Studied by Scanning Tunneling Microscopy

The surface structure of nonstoichiometric ${\mathrm{CeO}}_{2}(111)$ has been studied by scanning tunneling microscopy (STM). By using extremely small tunneling currents it was possible to obtain the first ever reported atomically resolved images of this oxide. STM imaging was possible at a sample bias voltage between $\ensuremath{-}2$ and $\ensuremath{-}3.5\mathrm{V}$. Comparing this with the band structure of ceria we claim that the main contribution to the image contrast results from oxygen in the topmost layer. The dominant defect type on the surface at RT is of triangular shape and contains three oxygen vacancies. Elevated temperature STM at a substrate temperature of 500 \ifmmode^\circ\else\textdegree\fi{}C revealed an alignment of oxygen vacancies on the surface. The defect shapes are in qualitative agreement with previous energy calculations.

Atomic-resolution STM of a system with strongly correlated electrons:NiO(001) surface structure and defect sites

Elevated-temperature scanning tunneling microscopy has been used to obtain atomic resolution images of (001) surfaces of NiO single crystals. Empty-state images of the (1\ifmmode\times\else\texttimes\fi{}1) surface show the nickel-atom sites and filled-state images show the oxygen ions in agreement with recent theoretical studies of the electronic structure of NiO. Empty-state and filled-state images of point defects and patterning effects observed on the upper terraces of 〈100〉 step edges are explained by directional 2p-3d covalent bonding between the oxygen and nickel sublattices.