Amanda K. Petford-Long

Dynamic Atomic-Level Rearrangements in Small Gold Particles

Small metal particles (<5 nanometers), which are widely used in catalysis, have physical and chemical properties that are markedly different from those of the bulk metal. The differences are related to crystal structure, and it is therefore significant that structral rearrangements in small particles have been observed in real time by using high-resolution electron microscopy. A detailed investigation at the atomic level has been made of the factors affecting the dynamic activity of small gold crystals that are supported on thin films of amorphous carbon, silicon, and germanium. The rate of activity depends mainly on the current density of the incident electron beam and the degree of contact of the particle with the substrate, but this rate decreases rapidly as the particle size is increased. The activity of the particles is very similar on either carbon or silicon, but it is generally less marked on germanium because of increased contact between the particle and the substrate. The electron beam effectively heats the particles, and it appears that their dynamic behavior depends on their thermal contact with the substrate.

High‐resolution electron microscopy study of x‐ray multilayer structures

Multilayered structures containing thin (1–12 nm) layers of W or Mo, alternating with C or Si, have been prepared to produce thin cross‐sectional specimens, and direct structural information on the atomic scale has been obtained using an ultrahigh resolution electron microscope. Layer thickness and flatness have been analyzed—the average layer thickness varies by up to 0.6 nm from the average value, and the flatness of the layers depends on the quality of the substrate surface. The degree of crystallinity and crystal orientation within the layers has also been examined. This information should enable more accurate theoretical models to be proposed for the multilayer materials and their x‐ray optical properties. The results for the Mo/Si multilayers suggest a model for their growth when prepared by sputtering.

Atomic-resolution study of structural rearrangements in small platinum crystals

Abstract Structural rearrangements in small Pt crystals, which were initiated by the incident electron beam, have been studied in real time with a 400 kV high-resolution electron microscope equipped with TV image viewing and videorecording facilities. The phenomena of crystal growth, particle coalescence and atomic clouds above particular surfaces were recorded at the atomic level and have been analysed using frame-by-frame playback facilities.

In-situ electron-beam-induced reduction of CuO: A study of phase transformations in cupric oxide

Abstract The reduction of CuO to Cu has been studied using high resolution electron microscopy and electron energy loss spectroscopy. The reduction proceeds via at least two intermediate phases - Cu 4 O 3 and Cu 2 O. This is the first experimental observation and the first successful synthesis of the rare Cu 4 O 3 phase. The CuO particles were observed to reduce via these two phases with a continuous refinement of particle size and a redistribution of material over the support film (carbon). In the case of large CuO crystals, the entire particle transformed to Cu 4 O 3 before further reduction or decrease in particle size occurred. This is the first direct observation of the mechanism involved in the formation of the very small metallic Cu particles normally observed in reduced supported CuO. The reduction was effected by a high current density electron beam in the residual gas atmosphere of an electron microscope. The similarity between this technique and the normal hydrogen reduction at high temperatures is discussed. The presence of the Cu 2 O and Cu 4 O 3 was determined by application of the optical diffraction method to small regions of a high resolution electron micrograph. This enabled the measurement of both interplanar spacings and interplanar angles, which combine together to allow an unambiguous identification of the phases present.

On the growth of small crystals of Cd, Zn, Pt and Rh during electron microscope observations

Abstract The growth of small supported metal crystals under the influence of an electron beam has been studied in real-time using a 400 keV ultra-high-resolution electron microscope. Samples of Pt, Rh, Cd and Zn supported on amorphous C or Si films were prepared ex situ and crystal growth in situ was recorded directly using a TV imaging/video system attached to the microscope. The different types of observed crystal growth are reported: the fcc metals (Pt and Rh) grow by coalescence, or by the addition of atoms along the particle surface followed by structural rearrangements which result in approximately spherical particles. The hcp metals (Cd and Zn) grow in the form of long rafts along the surface of the substrate film.

In situ oxidation processes for In III-V compound semiconductors studied by high-resolution electron microscopy

Abstract Oxidation of the amorphous and crystalline material at, and near, the surfaces of InP, InAs and InSb crystals takes place in situ during observation with a 400 kV atomic-resolution electron microscope. The phases present in the surface layers can be identified by comparison of lattice spacings with those of the bulk material, using the optical Mractogram technique. In general, the most prevalent material formed is found to be In2O3 and, for InAs, there is often an epitaxial relationship between the oxide and the substrate. However, in some regions of the original amorphous layer, small crystah (< 50 A) of the corresponding In compound semiconductors are recrystallized, and small amounts of metallic As are observed for Ids, and metailic In for InP. It is concluded that electron-beam-stimulated desorption of the anion species takes place, followed by oxidation of the residual In metal.

Tem and x-ray analysis of multilayer mirrors and beamsplitters

Recent advances in the development of lasers at soft x-ray wavelengths has spurred increasing interest in the production of cavity components using multilayer technology. We have established a comprehensive capability to design, fabricate, and characterize multilayer x-ray optics directed towards the goal of building the first x-ray laser cavity. High quality multilayer structures have been fabricated using magnetron sputtering. In addition, we have applied microfabrication technology to create freestanding beamsplitters and three-dimensional diffracting structures, as is discussed in another paper at this conference. The x-ray reflectivity and transmission of the multilayer components have been measured using synchrotron radiation. We have also characterized the microstructure of these devices using high-resolution transmission electron microscopy (TEM). This information provides structural parameters that are incorporated into computer codes to calculate the theoretical performance of the multilayer components. Comparison of the calculated reflectivity and transmission with the measured performance of the multilayer optics provides insight into the physics of these devices. In addition, a successful modeling capability allows us to iterate the fabrication cycle, modifying the design of the multilayer components to optimize their performance.

The real-time growth of atom planes on Ru, Rh and Sn microcrystals observed at atomic resolution

Abstract The growth of small Ru, Rh and Sn crystals, atomic column by atomic column, has been recorded in real time using a 400 kV high-resolution electron microscope, operated in the surface profile imaging mode. A common observation preceding the addition of a new atomic plane on the surface of the small (2–4 nm) metal crystal was the occurence of a diffuse “cloud”. Moreover, the sequential growth of an atomic plane was often preceded by the presence of such a cloud at the growth front (especially for Ru and Rh). Real-time video sequences showed that the addition of a new atomic layer typically took about 1 min.

Transmission-electron microscope studies of Au-Ni-Ge based ohmic contacts to GaAs-AlGaAs MODFET device

A technique is developed to obtain cross‐sectional transmission‐electron microscope specimen of a specifically desired region of semiconducting devices. A Au‐Ge‐Ni based metallization scheme is used to obtain low resistivity ohmic contact to GaAs‐AlGaAs based modulation‐doped field‐effect transistors device. Cross‐sectional electron microscopy and energy dispersive x‐ray analysis revealed that a uniformly alloyed region (Ni‐Ge‐As) can be obtained through the proposed metallization scheme. Transmission‐electron diffraction, high‐resolution electron microscopy, and optical‐diffraction analyses are employed to determine the various phases in the contact region. Movement of contact materials both parallel and perpendicular to the device surface is observed at the metallization edges after alloying. Contacts stressed at elevated temperatures to the point of incipient instability are found to exhibit a rougher metal/semiconductor interface.

Atomic Resolution Study of Structural Rearrangements in Metal Clusters

Dynamic events in small particles of Au, Pt, Rh and Ru have been observed inside the electron microscope. Depending on the particle size and the beam current density, the small clusters undergo structural rearrangements, with rapid changes in orientation and shape with respect to the beam direction. Various forms of surface activity have been recorded including atom hopping, twinning and atomic clouds above some surfaces. In a fraction of a second, a 2.5nm ruthenium crystal changed its internal structure from cubic-close-packing to hexagonal-close-packing. Twinning was comparatively common, even in particles with diameters of 2nm or less. From hot stage observations the speed of structural rearrangements was found to be temperature dependent.