Sergei Rouvimov

Automated nanocrystal orientation and phase mapping in the transmission electron microscope on the basis of precession electron diffraction

Abstract An automated technique for the mapping of nanocrystal phases and orientations in a transmission electron microscope is described. It is primarily based on the projected reciprocal lattice geometry that is extracted from electron diffraction spot patterns. Precession electron diffraction patterns are especially useful for this purpose. The required hardware allows for a scanning-precession movement of the primary electron beam on the crystalline sample and can be interfaced to any older or newer mid-voltage transmission electron microscope (TEM). Experimentally obtained crystal phase and orientation maps are shown for a variety of samples. Comprehensive commercial and open-access crystallographic databases may be used in support of the nanocrystal phase identification process and are briefly mentioned.

MBE-grown 232–270 nm deep-UV LEDs using monolayer thin binary GaN/AlN quantum heterostructures

Electrically injected deep ultra-violet emission is obtained using monolayer thin GaN/AlN quantum structures as active regions. The emission wavelength is tuned by controlling the thickness of ultrathin GaN layers with monolayer precision using plasma assisted molecular beam epitaxy. Single peaked emission spectra are achieved with narrow full width at half maximum for three different light emitting diodes operating at 232 nm, 246 nm, and 270 nm. 232 nm (5.34 eV) is the shortest electroluminescence (EL) emission wavelength reported so far using GaN as the light emitting material and employing polarization-induced doping.

Precession electron diffraction and its advantages for structural fingerprinting in the transmission electron microscope

Abstract The foundations of precession electron diffraction in a transmission electron microscope are outlined. A brief illustration of the fact that laboratory-based powder X-ray diffraction fingerprinting is not feasible for nanocrystals is given. A procedure for structural fingerprinting of nanocrystals on the basis of structural data that can be extracted from precession electron diffraction spot patterns is proposed.

Precession electron diffraction & automated crystallite orientation/phase mapping in a transmission electron microscope

The basics of precession electron diffraction (PED) in a transmission electron microscope (TEM) are briefly discussed. An automated system for the mapping of nanocrystal phases and orientations in a TEM is briefly described. This system is primarily based on the projected reciprocal lattice geometry as extracted from experimental precession electron diffraction spot patterns. Comprehensive open-access crystallographic databases may be used in support of the automated crystallite phase identification process and are, therefore, also briefly mentioned.

Automated crystal orientation and phase mapping of iron-oxide nanocrystals in a transmission electron microscope

The development of novel materials for micro- and nano-electronics requires reliable characterizations of structures, sizes, and orientations of thin polycrystalline films and ensembles of nanocrystals. The electron backscatter diffraction (EBSD) technique (also known as orientation imaging microscopy or back-scatter Kikuchi diffraction method) in a scanning electron microscope (SEM) is often employed for structural characterization of this kind of materials. EBSD in SEM is, however, sensitive to the plastic deformation state of the crystals as well as to structural damage or contamination of the crystal surfaces.

Thermoelectric and Structural Characterization of In x Rh 4 Sb 12 (0 <x< 0.2) Skutterudites

Thermoelectric materials play a unique and import role in the global effort toward energy diversification. Skutterudite-based materials have been extensively studied due to their highly tunable transport properties and show promise as a viable substitute for current thermoelectric (TE) materials. The skutterudite crystal (a naturally occurring mineral) exhibits a unique open crystal structure with two icosahedral void-sites per unit cell. Interstitial filling of the icosahedral void-sites has been a heavily exploited approach that reliably reduces the lattice thermal conductivity and produces a concomitant enhancement of the thermoelectric figure of merit (ZT). Partial indium-void-site filling of cobalt antimonide skutterudites has proven an effective strategy in achieving a ZT namely through a reduction in lattice thermal conductivity. As little work has been published on void-filled rhodium antimonide (RhSb3) skutterudite compounds, a series of In-filled rhodium antimonides skutterudites (InxRh4Sb12) were synthesized and their thermoelectric properties and microstructure characterized. Polycrystalline samples of InxRh4Sb12 (0 <x< 0.3) were prepared by solid-state reaction [1]. The crystal structure was characterized by powder X-Ray diffraction. The principle thermoelectric properties; Seebeck coefficient, electrical resistivity and thermal conductivity, were measured from 300–600K. Temperature dependence of the Seebeck coefficients and electrical resistivity of InxRh4Sb12 are shown in Figs. 1 and 2, respectively. The unfilled RhSb3 structure exhibits typical semiconducting behavior over the temperature range; however the In-filled compounds become increasingly degenerate with greater indium filling at high temperatures. Unexpectedly no reduction of lattice thermal conductivity was observed with indium filling (Fig. 3). Microstructural studies were performed using SEM (Fig. 4) and TEM/HREM (Fig. 5) analysis in order to further understand the unexpected trend in thermal transport with indium filling as well as to verify the presence and distribution of indium within the sample. All samples exhibit a high degree of porosity with an increasing crystallite size with indium content. The bright contrast in STEM image may indicate a variation in composition, e.g on possible segregation of In. EDS spectra of local areas of In0.2Rh4Sb12 sample (not shown) indicate that there are In-rich areas in the sample. The microstructural analysis reveals that the unusual increase in lattice thermal conductivity with indium filling can be correlated with an increasing grain size which results in fewer acoustic-phonon grain boundary scattering events.

Quantifying and enforcing two-dimensional symmetries in scanning probe microscopy images of periodic objects

The defining features of a scanning probe microscope (SPM) are a very fine “probe” that is scanned in two dimensions (2D), in very fine steps, very close to the surface of a sample, and a “probesample interactions signal” that is recorded at each scanning increment. This signal may then be digitized and displayed as a function of the magnified scanning steps. A 2D-image of the probe-sample interactions may, thus, be obtained.

Precession electron diffraction and its utility for structural fingerprinting in the transmission electron microscope

Precession electron diffraction (PED) in a transmission electron microscope (TEM) is discussed in order to illustrate its utility for structural fingerprinting of nanocrystals. While individual nanocrystals may be fingerprinted structurally from PED spot patterns, ensembles of nanocrystals may be fingerprinted from powder PED ring patterns.

Effect of annealing on ZnO nanowires grown at low temperature

Our research demonstrates that thermal and pulsed laser annealing improve the structural and optical properties of ZnO nanowire films grown electrochemically from aqueous electrolytes at a temperature below 90° C. The structural and optical properties of the grown ZnO nanowires were characterized by transmission electron microscopy and by their photo- and electroluminescence. Our results indicate that the as-grown nanowire structures have considerable internal lattice strain. However, moderate thermal annealing and laser annealing induce strain relaxation, considerably improving optical and electrical properties, and thereby allowing the fabrication of devices, such as solar cells and light emitting diodes.

Crystallographic characterization of polycrystalline materials: High resolution automated crystallite orientation & phase mapping and Precession electron diffraction ring patterns

The electron backscatter diffraction (EBSD) technique (also known as orientation imaging microscopy or the back-scatter Kikuchi diffraction method) in a scanning electron microscope (SEM) is often employed for structural characterization of thin polycrystalline films and ensembles of nanocrystals in the powder form. This technique is, however, sensitive to the plastic deformation state of the crystals as well as to structural damage or contamination of the crystal surfaces. In addition, its spatial resolution is limited to somewhere between 20 to 80 nm. Parallel illumination electron diffraction in the nano-probe mode in a transmission electron microscope (TEM) that is equipped with a field emission gun, on the other hand, delivers a significantly higher spatial resolution as compared to EBSD in a SEM and is also less sensitive to the plastic deformation state and the surface of nanocrystals. An automated technique for the mapping of crystallite phases and orientations of polycrystalline materials in a TEM has, therefore, been developed recently [1]. This technique is based on template matching of experimental electron diffraction spot patterns to their pre-calculated theoretical counterparts. Very promising results have so far been obtained with this technique for polycrystalline metal films, microelectronic composite structures (Fig. 1), and inorganic nanocrystalline powders [15]. The procedure of crystallite orientation and phase mapping comprises the automated collection of single crystal precession electron diffraction patterns on an external digital camera while scanning the area of interest with a nanometer-sized primary electron beam, followed by off-line data processing. Spatial resolutions of a few nanometers can be obtained on a fieldemission TEM. Figure 1 shows a typical crystallite orientation map (courtesy of JEOL, Tokyo). The software that goes with this hardware is flexible in its intake of experimental data so that it can also create crystallite orientation and phase maps of nanocrystal from the amplitude part of Fourier transforms of high resolution TEM images [4,5]. For inorganic nanocrystals with small to medium sized unit cells, an objective-lens aberration corrected TEM needs to be utilized [5]. It has also been demonstrated that the single crystal precession electron diffraction (PED) mode [6,7] improves the reliability of this technique significantly as the 180o ambiguity in the indexing of spot patterns from the zero order Laue zone can be reliably overcome [2,4]. This is because more reflections are excited in PED patterns and there are frequently also reflections from higher order Laue zones. For inorganic nanocrystal without heavy elements and sizes of below some 10 to 50 nm, the intensities of PED reflections are quasi-kinematical (i.e. roughly proportional to the square of the modulus of the structure factors) [6,7]. Such PED patterns are, therefore, very useful for advanced structural fingerprinting of nanocrystals in a TEM [5,8,9]. 768 doi:10.1017/S1431927610059052 Microsc. Microanal. 16 (Suppl 2), 2010