UHV TEM Research Articles

Lattice-resolved frictional pattern probed by tailored carbon nanotubes

In this study, we demonstrate a high-resolution friction profiling technique usingsynchronous atomic/lateral force microscopy (AFM/LFM). The atomic resolution isachieved by our special carbon nanotube (CNT) probes made via in situ tailoring andmanipulation inside an ultra-high vacuum transmission electron microscope (UHV TEM).The frictional pattern mapped on graphite displays a periodic distribution similar to theatomic (0001)-oriented graphite lattice structure. Furthermore, the electrothermalprocess in the UHV TEM renders a graphite-capped CNT tip, which delivers thenanotribology study within two graphite layers by the LFM measurement ongraphite. The synchronous AFM and LFM images can discern a spatial shiftbetween the atomic points and local friction maxima. We further interpret thisshift as caused by the lattice distortion, which in turn induces irreversible energydissipation. We believe this is the origin of atomic friction on the sub-nanonewton scale.

The fabrication of carbon nanotube probes utilizing ultra-high vacuum transmissionelectron microscopy

An application of ultra-high vacuum transmission electron microscopy (UHV TEM) isdemonstrated for the fabrication of carbon nanotube (CNT) probes. In this study, all thefabrication processes—such as CNT attachment, CNT orientation manipulation, and apextrimming—are integrated into a single UHV TEM system. The in situ work under UHV conditions(<5 × 10−10 mbar) allows us to clean the tip surface at the start of the fabrication process to ensurea good contact between the tip and CNT. Furthermore, the CNT size can beuser-selected to meet the various needs for scanning probe microscopy (SPM). Mostimportantly, the in situ trimming enables a multi-walled CNT to have the sharpness of asingle-walled CNT. The three advantages mentioned above are designed to improveconventional methods and will be shown in detail as the procedures of CNT probefabrication by a series of high-resolution TEM images. Finally, we compare the scannedimage via our CNT probes and conventional probes, where the typical artefactscoming from the conventional ones are addressed. We believe the technique wehave developed here will further enhance the resolution of SPM measurements.

Development of a miniature STM holder for study of electronic conductance of metal nanowires in UHV–TEM

A new miniature scanning tunneling microscope (STM) holder was developed in order to simultaneously investigate electronic conductance and structure of nanowires in an ultra high-vacuum electron microscope (UHV–TEM). A thin gold wire held between the STM tip and substrate stage of the specimen holder was stretched to form a suspended gold nanowire. The new TEM–STM system allowed us to measure electronic conductance at intervals of 20 ms, and to record high-resolution TEM images on videotape at 30 fps. Suspended gold nanowires formed from [1 1 0] oriented electrodes were well-elongated. In contrast, [1 0 0] and [1 1 1] electrodes produced nanowires with short necks. Electronic conductance was found to change as nanowire structure changed, with conductance quantization in units of 2 e 2/ h, where e is the electron charge and h is Planck’s constant, only being exhibited for well-elongated nanowires.

Temperature effect on the Cu 2O oxide morphology created by oxidation of Cu(0 0 1) as investigated by in situ UHV TEM

The temperature effect on the Cu 2O oxide morphology was investigated by oxidizing Cu(1 0 0) thin films at the temperature ranging from 150 to 1000 °C and constant oxygen partial pressure of 5×10 −4 Torr. The evolution of the oxide island size and shape was followed inside an in situ ultrahigh vacuum transmission electron microscope (UHV TEM). Of particular interest, we find that the oxide morphology can be triangular, hut, rod or pyramid shaped depending only on the oxidation temperature.

In-situ observation of Pd 2Si islands on Si by UHV–TEM/STM

Pd deposited clean Si(1 1 1) and (1 1 0) substrates were examined with UHV–TEM and STM that are part of an UHV–TEM/STM integrated characterization system. Annealing of Pd deposited substrates at 673 K showed the formation of Pd 2Si islands on the Pd 2Si layer. Superstructure of (3×3) was observed on the Pd 2Si surface. Deposition of Pd to TEM substrates at 673 K also resulted in the formation of Pd silicide islands. Structural analysis identified these islands to be Pd 2Si. On the surface of the islands, superstructure with 3 times periodicity was found. STM height distribution and EELS spectrum suggest that this structure is rather Si-rich and has a sub-surface layer. Small Pd 2Si islands whose height is less than 3 ML did not show surface structure.

In-Situ Study of the Oxide Mediated Epitaxy of CoSi2 on Si

AbstractOxide Mediated Epitaxy (OME) shows promise as a method to form good quality, thin epitaxial CoSi2 films on most Si surfaces. We have performed an in-situ study of the OME of CoSi2, on the Si (001) surface. Our work was carried out with our specially modified ultra-high vacuum transmission electron microscope (UHV TEM) SHEBA (Surface High Energy Electron Beam Apparatus). With SHEBA we were able to monitor the diffraction pattern and therefore the phase formation throughout the anneal. Our results confirm the suppression of intermediate phases during CoSi2 formation in the OME process. We also see a difference in the as deposited Co film when the oxide coated silicon surface is used rather than a clean substrate. From combined imaging and diffraction studies we will shed some light on the mechanism behind the success of OME.

In situ transmission electron microscopy observations of the formation of self-assembled Ge islands on Si

The in situ transmission electron microscope allows us to visualise processes occurring at surfaces and interfaces in real time and is therefore capable of providing detailed, quantitative information about reaction mechanisms. We have used a UHV TEM equipped with in situ growth capabilities to study the process of chemical vapour deposition of Ge on Si(100), with particular emphasis on the formation of self-assembled, nanosize Ge islands. Video-rate image acquisition enables us to track the development of individual islands from nucleation onwards and to observe the introduction of dislocations as the strained islands relax. For islands less than 80 nm in diameter, which are coherently strained, we observe an interesting coarsening process during growth. This coarsening results in a bimodal distribution of island sizes at certain times and a narrow size distribution at later times. We explain the phenomenon by a model in which coarsening occurs among a population of islands for which the equilibrium island shape depends on the size. Numerical simulations of coarsening in the presence of a shape transition are in good agreement with experiment. As the islands grow larger, dislocations form and we observe rapid shape changes associated with dislocation introduction. These changes can also be understood by considering a strain-dependent island shape. The insight that these results provide into the understanding of island growth and evolution can be used to develop arrays of uniformly sized islands ("quantum dots") for a variety of potential applications.

In situ transmission electron microscopy observations of the formation of self‐assembled Ge islands on Si

The in situ transmission electron microscope allows us to visualise processes occurring at surfaces and interfaces in real time and is therefore capable of providing detailed, quantitative information about reaction mechanisms. We have used a UHV TEM equipped with in situ growth capabilities to study the process of chemical vapour deposition of Ge on Si(100), with particular emphasis on the formation of self-assembled, nanosize Ge islands. Video-rate image acquisition enables us to track the development of individual islands from nucleation onwards and to observe the introduction of dislocations as the strained islands relax. For islands less than 80 nm in diameter, which are coherently strained, we observe an interesting coarsening process during growth. This coarsening results in a bimodal distribution of island sizes at certain times and a narrow size distribution at later times. We explain the phenomenon by a model in which coarsening occurs among a population of islands for which the equilibrium island shape depends on the size. Numerical simulations of coarsening in the presence of a shape transition are in good agreement with experiment. As the islands grow larger, dislocations form and we observe rapid shape changes associated with dislocation introduction. These changes can also be understood by considering a strain-dependent island shape. The insight that these results provide into the understanding of island growth and evolution can be used to develop arrays of uniformly sized islands (“quantum dots”) for a variety of potential applications. Microsc. Res. Tech. 42:281–294, 1998. © 1998 Wiley-Liss, Inc.

Sintering of silver and copper nanoparticles on (001) copper observed by in-situ ultrahigh vacuum transmission electron microscopy

The sintering of copper and silver nanoparticles with single crystal copper substrates has been studied using a novel in-situ ultrahigh vacuum transmission electron microscope (UHV TEM). The system is equipped with a UHV DC sputtering attachment enabling metal nanoparticles to be generated in-situ and transferred directly into the microscope in the gas phase. In both cases, we find the particles to be of initially random orientation on the substrate. Upon annealing, however, the particles reorient and assume the same orientation as the substrate. The process apparently occurs by a mechanism involving sintering and grain growth. In the case of silver on copper, grain growth cannot occur since the metals are immiscible. Our observations show that, upon annealing, the particles wet the substrate surface and form epitaxially oriented islands by surface diffusion and grain boundary migration. The post-anneal islands exhibit the orientation relationship (111) Ag∥001) Cu, [110] Ag∥[110] Cu.

In-Situ Observation Of Aln Formation During Nitridation Of Sapphire By Ultrahigh Vacuum Transmission Electron Microscopy

AbstractUsing a novel ultrahigh vacuum transmission electron microscope (UHV TEM) with insitu molecular beam epitaxy capability we have studied the nitridation of (0001) sapphire upon exposure to ammonia. Atomically flat sapphire surfaces for the experiments were obtained by high temperature annealing. Subsequent exposure to ammonia flow at 950°C led to the successful synthesis of epitaxial AIN; the films were characterized in-situ using TEM. Complimentary ex-situ atomic force microscopy (AFM) was also performed in order to characterize the surface morphology before and after nitridation.The experiments indicate that AIN grows by a 3D island growth mechanism. Electron diffraction patterns suggest an abrupt AIN/sapphire interface with no evidence of the formation of Al–O–N compounds. The rate limiting step in the nitridation reaction appears to be the diffusion of nitrogen and oxygen species between the free surface of the growing AIN film and the reaction interface. It is inferred from kinetic measurements that diffusion of these species occurs along the boundaries between coalescing AIN islands.

In-Situ UHV Tem Investigations of the Initial Oxidation Stage of Copper Thin Films

ABSTRACTThe nucleation and growth of Cu2O due to oxidation of Cu(001) films were monitored at various temperatures and oxygen partial pressures. For all examined temperatures and pressures, Cu2O islands were observed to form epitaxially with respect to the copper film. The nucleation of these oxide islands was homogeneous –no clear evidence was observed for either steps or dislocations being preferential nucleation sites. Based on this data, we have developed a semiquantitative model of the initial oxidation stage where the dominant mechanism for transport, nucleation and growth of oxide islands is oxygen diffusion on the Cu surface. We are presently comparing our experimental data with nucleation rate theory, where the predictions qualitatively describe our observations, but not quantitatively.

Sintering of Sputtered Copper Nanoparticles on (001) Copper Substrates

ABSTRACTThe sintering of sputtered copper nanoparticles with a thin film copper substrate has been studied in real time using a novel in-situ UHV TEM. Particles were generated by dc sputtering in argon and transferred directly into the microscope along a connecting tube. The particles were deposited onto a clean (001) copper substrate of thickness 40nm. As-deposited particles assume a random orientation on the copper substrate as evidenced by electron diffraction patterns. Upon annealing, however, the particles were observed to reorient and assume the orientation of the substrate. Reorientation of individual, isolated particles occurred at ∼200°C, primarily by grain-boundary motion and not by surface diffusion.

TEM study of Si surfaces

UHV TEM studies of surfaces have been successfully applied in surface science. Low resolution TEM can characterize surface atomic steps, monolayer adsorbate, reconstruction of surfaces and surface structure domains. TED from surfaces of thin films can be used to analyze surface atomic structure. On the other hand high resolution TEM can seen atomic structure of surfaces either in profile or in plan view modes. The profile mode is effective for surfaces with short periods along the beam direction and is sensitive to displacements of surface atoms normal to the surface and along a direction parallel to the surface and perpendicular to the beam direction. Image contrast of high resolution plan view images is very weak except cases of heavy adsorbed atoms on a substrate of light atoms. The present paper shortly reviews recent studies of Si surfaces done with use of a low resolution UHV TEM and high resolution TEM.

Applications of In-Situ UHV and High Resolution Tem to the Study of Small Particles

In-situ electron microscopy is a powerful tool for the study of small particles. Since most of the interesting phenomena take place in particles smaller than ˜5 nm, high resolution is highly desirable. In-situ and high resolution conditions are difficult to achieve in a single instrument. We have combined the in-situ UHV capabilities of a modified microscope at Stanford University with the high resolution capabilities of a 200 kV and a UHV-400 kV microscopes at Xerox PARC. Examples are presented, pointing out the advantages of in-situ deposition and treatment, and post deposition ex-situ observation at atomic resolution. Advantages and limitations of this approach are discussed.