Recent advances in electron phase microscopy

Electron microscopy utilizing the phase information of an electron beam has recently seen remarkable progress due to the development of a “coherent” field-emission electron beam and the corresponding image-processing techniques. This paper reviews these developments and their applications.     

Transmission electron microscopy investigation of an ordered metastable phase in Zr-N alloys

Supersaturated hcp f-Zr(N) alloys containing 22-28 at.% N were prepared by nitriding sheets of Zr in an atmosphere of high-purity N 2 , followed by homogenization under high-purity Ar gas. Quenching and isothermal ageing of the alloys for various times between 500 and 650C resulted in precipitation of a metastable phase, rather than the equilibrium phase ZrN. This investigation focused on determining the structure, orientation relationship, habit plane, morphology, growth kinetics and atomic growth mechanism of this non-equilibrium precipitate using transmission electron microscopy imaging and diffraction techniques, high-resolution transmission electron microscopy, electron-energy-loss spectroscopy and various simulation programs. The precipitate, which was arbitrarily designated the ξ phase, has a monoclinic Bravais lattice. Its lattice parameters are a = 0.32 nm, b = 0.60 nm, c = 0.56 nm, f= g= 90 and n= 121.5. Its orientation relationship with the fmatrix is $$ (0001)_\alpha /\mskip-2/ (020)_\xi\quad \a\n\d\quad [01\bar 10] _\alpha /\mskip-2/ [001]_\xi, $$ and the average habit plane of lenticularly shaped precipitates is $(01\bar 12)_\alpha$ . Determination of the structure and other aspects of the phase transformation are discussed.     

Transmission electron microscopy observation of a deformation twin in TWIP steel by an ex situ tensile test

The formation of deformation twins in twinning-induced plasticity steels was observed in transmission electron microscope by an ex situ tensile test. The twinning process initially includes formation of extended dislocations at primary slip plane, then cross-slip to a conjugate slip plane with dissociation of the leading partial into a stair-rod dislocation and an emitted partial, and finally un-faulting of the original faults and formation of Frank partials. Repetition of the operation of the process on successive conjugate planes results in the formation of deformation twins. The formation mechanism of deformation twins can thus be explained by improving the stair-rod cross-slip model.     

Direct observation of biological molecules in liquid by environmental phase-plate transmission electron microscopy

We have been developing a combination method for environmental TEM (E-TEM) and phase-plate TEM (P-TEM) that enables direct observations of the structure of biological molecules in aqueous solution. It is clearly demonstrated that the biological molecules in a water layer can be imaged by the combined method without any stain. The spatial resolution obtained in the present study was about 10nm. This should be improved by using energy filtering. The image contrast of the specimen in water was reduced in comparison with that in vacuum. A model calculation that includes the effects of beam broadening, intensity decrease, and background increase caused by scattering from the water layer around the specimen shows that an increase in the thickness of the water layer reduces the contrast, intensity, and resolution of the image.     

Direct observation of geometrical phase transitions in mesoscopic superconductors by scanning tunneling microscopy

Using scanning tunneling microscopy, we mapped the distribution of the local density of states in a single crystal superconductor heterostructure with an array of submicron normal metal islands. We observe the coexistence of strongly interacting multiquanta vortex lattice with interstitial Abrikosov vortices. The newly formed composite magnetic flux structure undergoes a series of phase transitions between different topological configuration states. The vortex configuration states are strongly dependent on the number of flux quanta and the nanoscale confinement architecture of the mesoscopic superconductor. Here, we present images of vortex phase transitions due to confinement effects when the number of magnetic flux quanta in the system changes. The vortex dynamics in these systems could serve as a model for behavior of confined many-body systems when the number of particles changes.     

Video-frequency scanning transmission electron microscopy of moving gold nanoparticles in liquid

Immobilized gold nanoparticles were imaged in a liquid containing water and 50% glycerol with scanning transmission electron microscopy (STEM). The specimen was enclosed in a liquid compartment formed by two silicon microchips with electron transparent windows. A series of images was recorded at video frequency with a spatial resolution of 1.5nm. The nanoparticles detached from their support after imaging them for several seconds at a magnification of 250,000. Their movement was found to be much different than the movement of nanoparticles moving freely in liquid as described by Brownian Motion. The direction of motion was not random-the nanoparticles moved either in a preferred direction, or radially outwards from the center of the image. The displacement of the gold nanoparticles over time was three orders of magnitude smaller than expected on the basis of Brownian Motion. This finding implies that nanoscale objects of flexible structure or freely floating, including nanoparticles and biological objects, can be imaged with nanoscale resolution, as long as they are in close proximity to a solid support structure.     

Annealing effects on the microstructure of sputtered gold layers on oxidized silicon investigated by scanning electron microscopy and scanning probe microscopy

The structure of Au layers deposited by sputtering on oxidized p-type Si(100) substrates is investigated by a combination of scanning electron microscopy and scanning probe microscopy. The effect of the temperature on the grain structure of the layers has been determined, revealing that an annealing temperature of 300° C results in a larger grain size and smoother surfaces but generates some cracks in the film surface. At an annealing temperature of 500° C, further grain growth is observed, but a high density of cracks and voids also results while there is little enhancement regarding the smoothness of the grain surfaces.     

The Aharonov-Bohm effect and its applications to electron phase microscopy

The Aharonov-Bohm effect was conclusively established by a series of our electron interference experiments, with the help of some advanced techniques, such as coherent field-emission electron beams and microlithography. Using this fundamental principle behind the interaction of an electron wave with electromagnetic fields, new observation techniques were developed to directly observe microscopic objects and quantum phenomena previously unobservable.     

The use of electron microscopy for the study of phase transitions

Two examples of the study of phase transitions by means of electron diffraction and electron microscopy are discussed.     

The ferroelectric phase transition in tridymite type BaAL2O4 studied by electron microscopy

The paraelectric-ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl₂O₄ was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accompanied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400–670 K.) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6₃22, a≈ 5.22 Å, c ≈ 8.8 Å) and of the FE phase (space group P6₃, a= 10.4469(1)Å, c = 8.7927(1)Å) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.     

Atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) of nanoscale plate-shaped second phase particles

The feasibility of accurately measuring the size and the volume fraction of nanoscale plate-shaped precipitates by atomic force microscopy (AFM) has been explored. For quantitative evaluations their unhandy geometry is conveniently described by superellipsoids. The experimental alloy Ni₆₉Co₉Al₁₈Ti₄ served as a model system: plate-shaped disordered γ-precipitates form in the L1₂ long-range ordered γ′-matrix. The results obtained by AFM are compared with those derived from transmission (TEM) and from high-resolution scanning electron microscopy (SEM). The agreement between the AFM and the TEM results is good. In spite of the low number of SEM images taken, the same holds for the SEM results. In addition, magnetic force microscopy was applied; its results are acceptable. The main advantages of AFM are (i) the numerical output for all three dimensions, (ii) the simplicity of its operation and (iii) the lower cost of the microscope itself. The first point allows the numerical AFM output data to be directly subjected to automated computer-based evaluations. All present experimental and evaluation procedures are also applicable to cube-shaped particles with rounded edges and corners as found, for example, in γ′-strengthened nickel-based superalloys.     

Inelastic electron scattering observation using energy filtered transmission electron microscopy for silicon -- germanium nanostructures imaging

This paper presents a new technique using energy filtered TEM (EFTEM) for inelastic electron scattering contrast imaging of Germanium distribution in Si-SiGe nanostructures. Comparing electron energy loss spectra (EELS) obtained in both SiGe and Si single crystals, we found a spectrum area strongly sensitive to the presence of Ge in the range [50-100 eV]. In this energy loss window, EELS spectrum shows a smooth steeply shaped background strongly depending on Ge concentration. Germanium mapping inside SiGe can thus be performed through imaging of the EELS background slope variation, obtained by processing the ratio of two energy filtered TEM images, respectively, acquired at 90 and 60 eV. This technique gives contrasted images strongly similar to those obtained using STEM Z-contrast, but presenting some advantages: elastic interaction (diffraction) is eliminated, and contrast is insensitive to polycrystalline grains orientation or specimen thickness. Moreover, since the extracted signal is a spectral signature (inelastic energy loss) we demonstrate that it can be used for observation and quantification of Ge concentration depth profile of SiGe buried layers.     

High resolution transmission electron microscopy observation of thermally fluctuating phasons in decagonal Al-Cu-Co

In situ high-temperature, high resolution transmission electron microscopy (HRTEM) was performed on an Al-Cu-Co decagonal quasicrystal, to investigate thermal fluctuation of phasons. A tiling pattern constructed from the HRTEM image was analyzed in the framework of the strip-projection method. Transitions between two local tile arrangements were observed at high temperature for the first time, and were shown to correspond to a thermal phason fluctuation.     

Trapping of an electron in the transmission through two quantum dots coupled by a wire

We consider single-channel transmission through a double quantum dot that consists of two identical single dots coupled by a wire. The numerical solution for the scattering wave function shows that the resonance width of a few of the states may vanish when the width (or length) of the wire and the energy of the incident particle each take a certain value. In such a case, a particle is trapped inside the wire as the numerical visualization of the scattering wave function shows. To understand these numerical results, we explore a simple model with a small number of states, which allows us to consider the problem analytically. If the eigenenergies of the closed system cross the energies of the transmission zeroes, the wire effectively decouples from the rest of the system and traps the particle.     

High-resolution transmission electron microscopy of phase formation and growth in metal-Si-Ge systems

High-resolution transmission electron microscopy (HRTEM) in conjunction with nano-beam (NB) analysis as well as energy dispersive spectrometry analysis have been fruitfully utilized to study the interfacial reactions in the metal-Si-Ge systems. In this paper we report the results of TEM study of the phase formation and growth in Ti-Si-Ge, Cu-Si-Ge and Ni-Si-Ge systems.     

Measurement of gold surface self-diffusion by scanning tunneling microscopy

Measurement of gold surface self-diffusion by the method of surface profile decay, using a scanning tunneling microscope (STM) have been done on a polycrystalline gold film deposited on a glass substrate. The peak-to-peak surface roughness was measured as a function of annealing time after annealing at 170 °C with a special pan-cake furnace in the STM. The gold surface diffusion coefficient at 170 °C can then be extracted from these measurements.