True Atomic Resolution Research Articles

Electron holography for fields in solids: Problems and progress

Electron holography initially was invented by Dennis Gabor for solving the problems raised by the aberrations of electron lenses in Transmission Electron Microscopy. Nowadays, after hardware correction of aberrations allows true atomic resolution of the structure, for comprehensive understanding of solids, determination of electric and magnetic nanofields is the most challenging task. Since fields are phase objects in the TEM, electron holography is the unrivaled method of choice. After more than 40 years of experimental realization and steady improvement, holography is increasingly contributing to these highly sophisticated and essential questions in materials science, as well to the understanding of electron waves and their interaction with matter.

Aberration Corrected High-Resolution Transmission and Scanning Transmission Electron Microscopy of Thin Perovskite Layers

Abstract We have recently installed the first JEOL 3100Z R005 microscope in Europe, a 300 kV (scanning) transmission electron microscope with aberration correctors for both focused and parallel beam illumination, a cold field-emission electron gun and a high-resolution imaging energy filter. This instrument delivers 0.5 A resolution in both planar illumination and scanning convergent beam illumination at 300 kV [1] and [2] and can form an electron beam ∼2 A in diameter which is intense enough (∼1nA) to perform chemical mapping using either electron energy-loss or energy-dispersive X-ray spectroscopy. We show an application to perovskite thin films, demonstrating that the reduced delocalisation and improved signal-to-noise ratio due to aberration correction allow us to determine layer and interface widths from cross-sections at true atomic resolution. This type of equipment will also be ideally suited to investigate the atomic structure and chemistry of polycrystalline thin films such as typically produced from sintering of nano-particles deposited onto suitable substrates.

Research Progress in Atomic Resolution Microscopy

Tremendous progress has been made in non-contact atomic force microscopy (NC-AFM) recently. The spatial resolution of NC-AFM imaging and spectroscopy of individual molecules on surfaces has reached true atomic resolution and bond differentiation level. Combination of NC-AFM with other scanning probe techniques can open a new way for materials, physics, chemistry, and biochemistry studies. In this review, we first introduce the basic principle of NC-AFM and qPlus sensor. The interaction force at atomic scale and precise measurement of short-range force are discussed. We summarize the recent advances in structural determination of organic molecules, chemical identification, electronic structure, and atomic manipulation at the atomic scale. In addition, we also discuss the application of Kelvin probe force microscopy (KPFM) in measurement of local contact potential difference (LCPD). Finally, perspectives and challenges in NC-AFM techniques are presented.

Phase-Locked loops lock-in range in Frequency Modulated-Atomic Force Microscope nonlinear control system

Since the mid 1980s the Atomic Force Microscope is one the most powerful tools to perform surface investigation, and since 1995 Non-Contact AFM achieved true atomic resolution. The Frequency-Modulated Atomic Force Microscope (FM-AFM) operates in the dynamic mode, which means that the control system of the FM-AFM must force the microcantilever to oscillate with constant amplitude and frequency. However, tip-sample interaction forces cause modulations in the microcantilever motion. A Phase-Locked loop (PLL) is used to demodulate the tip-sample interaction forces from the microcantilever motion. The demodulated signal is used as the feedback signal to the control system, and to generate both topographic and dissipation images. As a consequence, a proper design of the PLL is vital to the FM-AFM performance. In this work, using bifurcation analysis, the lock-in range of the PLL is determined as a function of the frequency shift (Ω) of the microcantilever and of the other design parameters, providing a technique to properly design the PLL in the FM-AFM system.

Elucidation of the Complex Structure of Nanoparticles Composed of Bismuth, Antimony, and Tellurium Using Scanning Transmission Electron Microscopy

In this Article, we report on a study of the atomic structure of nanoparticles composed of bismuth, antimony, and tellurium with both wire and disk shape through use of the scanning transmission electron microscopy-high angle annular dark field detector technique. Recent advances in scanning transmission electron microscopy, such as aberration correction, have enabled a detailed structure analysis for materials with true atomic level resolution. Such ability is important for elucidating the underlying mechanisms, which give rise to the enhanced and novel properties displayed by nanoparticles. The atomic scale analysis of the particles revealed important insight into the nanoparticle structure. It was found that the nanowires are composed of both hexagonal tellurium and rhombohedral Bi2Te3 phases where the Bi2Te3 appears to have grown from the tips of the tellurium wires. The nanodiscs were found to be entirely rhombohedral in nature and were composed of both Bi2Te3 and Sb2Te3 phases with Bi2Te3 found at the particle center and Sb2Te3 appearing to subsequently grow at the periphery of the particles. The in-depth atomic structural characterization is expected to lead to a greater understanding of how to synthesize these thermoelectric type materials with more controllable properties such as size, shape, structure, and composition.

Ultrashort cantilever probes for high-speed atomic force microscopy

Since its invention in 1986, atomic force microscopy (AFM) has become the most widely used scanning-probe imaging technique.1 The microscope ‘maps’ the topography of the sample by scanning its surface with a small tip integrated near the free end of a flexible cantilever. In contrast to other microscope techniques, it is possible to image the surface of many materials—including electrically isolating ones, such as ceramics and glasses—with a resolution in both the vertical and horizontal direction of below 1nm. Figure 1 shows a typical example of an AFM measurement in vacuum, depicting a cerium oxide surface in true atomic resolution. The inherent mechanical characteristics of AFM requires serial acquisition of data, which limits the speed for obtaining high-resolution images. Using commercially available microscopes, a typical image of 256 256 pixels resolution can be obtained in 1–10min. This is significantly slower than the time resolution required, for example, to resolve biological processes such as the movement of motor proteins in cells.2 Enhancing imaging speed demands improved scanner technology and control electronics. Specifically, several limiting factors with respect to the cantilever probe must be addressed to reduce resolution time. First, the measurement bandwidth of the local interaction between the probe tip and sample, as well as the velocity at which the tip moves, must be increased. Additionally, the tip must be able to follow the sample’s topography at greater speed.3 Ideally, cantilever probes will function at resonance frequencies in the megahertz region with low force constants (approximately a few nano-Newtons per nanometer). Here, we report our progress in fabricating cantilever probes for highspeed imaging in AFM.4 Figure 1. Atomic force micrograph of a cerium oxide surface with true atomic resolution, imaged using a conventional atomic force microscope in noncontact mode.

Defects in oxide surfaces studied by atomic force and scanning tunneling microscopy.

Surfaces of thin oxide films were investigated by means of a dual mode NC-AFM/STM. Apart from imaging the surface termination by NC-AFM with atomic resolution, point defects in magnesium oxide on Ag(001) and line defects in aluminum oxide on NiAl(110), respectively, were thoroughly studied. The contact potential was determined by Kelvin probe force microscopy (KPFM) and the electronic structure by scanning tunneling spectroscopy (STS). On magnesium oxide, different color centers, i.e., F0, F+, F2+ and divacancies, have different effects on the contact potential. These differences enabled classification and unambiguous differentiation by KPFM. True atomic resolution shows the topography at line defects in aluminum oxide. At these domain boundaries, STS and KPFM verify F2+-like centers, which have been predicted by density functional theory calculations. Thus, by determining the contact potential and the electronic structure with a spatial resolution in the nanometer range, NC-AFM and STM can be successfully applied on thin oxide films beyond imaging the topography of the surface atoms.

Atomic structures of the defective SrTiO3 (001) surface

Surface structures of defective SrTiO(3) (001) have been studied by using scanning probe microscopy and density functional theory calculations. We observed several defective surface structures with true atomic resolution under reducing ultrahigh vacuum conditions. It is found that all the defects are terminated by (001), (100) and (010) microfacets of the TiO(2) plane. We propose microfaceting TiO(2) termination with Sr adatom models. The formation of various types of defects is driven by the changes of the surface stoichiometry depending on surface preparations.

True Atomic Level Imaging of Shaped Nanoparticles Composed of Bismuth, Antimony and Tellurium using Scanning Transmission Electron Microscopy.

ABSTRACTNanotechnology is an area of research that is highly intriguing because of the novel properties often observed for materials whose sizes are reduced to the nanoscale. However, one of the biggest challenges is understanding the underlying principles that dictate the particles resulting properties. The atomic level structure for nanoparticles is suspected to vary from that for the corresponding bulk materials, however, direct observation of this phenomenon has proven difficult. Until recently only indirect information on the atomic level structure of such materials could be obtained with techniques such as XRD, HR-TEM, XPS, etc… However, recent advances in Transmission Electron Microscopy techniques now allow true atomic scale resolution, leading to definitive confirmation of the atomic structure. Namely, Scanning Transmission Electron Microscopy coupled with a High-angle Annular Dark Field detector (STEM-HAADF) has been demonstrated to be capable of achieving a nominal resolution of 0.8 nm (the JEOL JEM-ARM200F instrument). The ability is highly exciting because it will lead to an enhanced understanding of the relationship between atomic structure of nanoparticles and the resulting novel properties. In our own study, we focus on the analysis of the atomic level structure for nanoparticles composed of bismuth, antimony and tellurium for thermoelectric materials. This area has recently received much interest because of the realization that nanotechnology can be employed to greatly enhance the efficiency (dimensionless figure of merit ZT) of this class of materials. One of the most intriguing parameters leading to the enhanced TE activity is the relationship between composition and structure that exists within individual nanoparticles. We report our results on a study of the atomic level structure for both nanowires and nanodiscs composed of bismuth, antimony and tellurium. It was found that the nanoparticles have a complex structure that cannot be elucidated by conventional techniques such as XRD or HR-TEM. In addition, by employing Energy Dispersive Spectroscopy (EDS), a greater understanding of the composition-structure dependence was gained. The results are primarily discussed in terms of the atomic level resolution images obtained with the STEM-HAADF technique.

Phase-Locked Loop design applied to frequency-modulated atomic force microscope

The atomic force microscope (AFM) introduced the surface investigation with true atomic resolution. In the frequency modulation technique (FM-AFM) both the amplitude and the frequency of oscillation of the micro-cantilever must be kept constant even in the presence of tip–surface interaction forces. For that reason, the proper design of the Phase-Locked Loop (PLL) used in FM-AFM is vital to system performance. Here, the mathematical model of the FM-AFM control system is derived considering high order PLL. In addition a method to design stable third-order Phase-Locked Loops is presented.

Cysteine as a Modulator Residue in the Active Site of Xenobiotic Reductase A: A Structural, Thermodynamic and Kinetic Study

Xenobiotic reductase A (XenA) from Pseudomonas putida 86 catalyzes the NADH/NADPH-dependent reduction of various substrates, including 2-cyclohexenone and 8-hydroxycoumarin. XenA is a member of the old yellow enzyme (OYE) family of flavoproteins and is structurally and functionally similar to other bacterial members of this enzyme class. A characteristic feature of XenA is the presence of a cysteine residue (Cys25) in the active site, where in most members of the OYE family a threonine residue is found that modulates the reduction potential of the FMN/FMNH– couple. We investigated the role of Cys25 by studying two variants in which the residue has been exchanged for a serine and an alanine residue. While the exchange against alanine has a remarkably small effect on the reduction potential, the reactivity and the structure of XenA, the exchange against serine increases the reduction potential by +82 mV, increases the rate constant of the reductive half-reaction and decreases the rate constant in the oxidative half-reaction. We determined six crystal structures at high to true atomic resolution (dmin 1.03–1.80 Å) of the three XenA variants with and without the substrate coumarin bound in the active site. The atomic resolution structure of XenA in complex with coumarin reveals a compressed active site geometry in which the isoalloxazine ring is sandwiched between coumarin and the protein backbone. The structures further reveal that the conformation of the active site and substrate interactions are preserved in the two variants, indicating that the observed changes are due to local effects only. We propose that Cys25 and the residues in its place determine which of the two half-reactions is rate limiting, depending on the substrate couple. This might help to explain why the genome of Pseudomonas putida encodes multiple xenobiotic reductases containing either cysteine, threonine or alanine in the active site.

A low noise all-fiber interferometer for high resolution frequency modulated atomic force microscopy imaging in liquids

We have developed a low noise all-fiber interferometer for use as the deflection sensor in liquid environment frequency modulated atomic force microscopy (FM-AFM). A detailed description and rationale for the choice of the critical components are provided along with the design of a simple alignment assembly. The optimization of the deflection sensor toward achieving the highest possible sensitivity and lowest deflection noise density is discussed in the context of an ideal interference cavity. Based on the provided analysis we have achieved deflection noise densities of 2 fm/square root(Hz) on commercially available cantilevers in both ambient and liquid environments. The low noise interferometer works without the need for differential detection, special focusing lenses, or polarization sensitive optics, dramatically simplifying measurements. True atomic resolution imaging of muscovite mica by FM-AFM in water is demonstrated using the developed deflection sensor.

Towards structural studies of the old yellow enzyme homologue SYE4 fromShewanella oneidensisand its complexes at atomic resolution

Shewanella oneidensis is an environmentally versatile Gram-negative gamma-proteobacterium that is endowed with an unusually large proteome of redox proteins. Of the four old yellow enzyme (OYE) homologues found in S. oneidensis, SYE4 is the homologue most implicated in resistance to oxidative stress. SYE4 was recombinantly expressed in Escherichia coli, purified and crystallized using the hanging-drop vapour-diffusion method. The crystals belonged to the orthorhombic space group P2(1)2(1)2(1) and were moderately pseudo-merohedrally twinned, emulating a P422 metric symmetry. The native crystals of SYE4 were of exceptional diffraction quality and provided complete data to 1.10 A resolution using synchrotron radiation, while crystals of the reduced enzyme and of the enzyme in complex with a wide range of ligands typically led to high-quality complete data sets to 1.30-1.60 A resolution, thus providing a rare opportunity to dissect the structure-function relationships of a good-sized enzyme (40 kDa) at true atomic resolution. Here, the attainment of a number of experimental milestones in the crystallographic studies of SYE4 and its complexes are reported, including isolation of the elusive hydride-Meisenheimer complex.

‘All-inclusive’ imaging of the rutileTiO2(110) surface using NC-AFM

Non-contact atomic force microscopy (NC-AFM) at true atomicresolution is used to investigate the (110) surface of rutileTiO2. We are able to simultaneously resolve both bridging oxygen and titanium atoms of thisprototypical oxide surface. Furthermore, the characteristic defect species, i.e. bridgingoxygen vacancies, single and double hydroxyls as well as subsurface defects, are identified inthe very same frame. We employ density functional theory (DFT) calculationsto obtain a comprehensive understanding of the relation between the tip apexstructure and the observed image contrast. Our results provide insight into thephysical mechanisms behind atomic-scale contrast, indicating that electrostaticinteraction can lead to a far more complex contrast than commonly assumed.

Imaging Beyond Topography

As a relatively new scanning probe technique, Dynamic Force Microscopy [1] has proved to be a powerful tool, allowing for imaging the topography of a sample surface with true atomic resolution. Besides topographic imaging, the simultaneously recorded damping signal is related to non-conservative interaction between tip and sample. For a molecular system, we show the tip-induced switching of the functionalized groups of the organic molecule leading to an enhanced damping signal.

Compositional analysis of mixed–cation‐anion III–V semiconductor interfaces using phase retrieval high‐resolution transmission electron microscopy

Employing exit-plane wave function (EPWF) reconstruction in high-resolution transmission electron microscopy (HRTEM), we have developed an approach to atomic scale compositional analysis of III-V semiconductor interfaces, especially suitable for analyzing quaternary heterostructures with intermixing in both cation and anion sub-lattices. Specifically, we use the focal-series reconstruction technique, which retrieves the complex-valued EPWF from a thru-focus series of HRTEM images. A study of interfaces in Al(0.4)Ga(0.6)As-GaAs and In(0.25)Ga(0.75)Sb-InAs heterostructures using focal-series reconstruction shows that change in chemical composition along individual atomic columns across an interface is discernible in the phase image of the reconstructed EPWF. To extract the interface composition profiles along the cation and anion sub-lattices, quantitative analysis of the phase image is performed using factorial analysis of correspondence. This enabled independent quantification of changes in the In-Ga and As-Sb contents across ultra-thin interfacial regions (approximately 0.6 nm wide) with true atomic resolution, in the In(0.25)Ga(0.75)Sb-InAs heterostructure. The validity of the method is demonstrated by analyzing simulated HRTEM images of an InAs-GaSb-InAs model structure with abrupt and graded interfaces. Our approach is general, permitting atomic-level compositional analysis of heterostructures with two species per sub-lattice, hitherto unfeasible with existing HRTEM methods.

Low Amplitude High Frequency Atomic Force Microscopy

Heterodyne laser Doppler interferometry was used to implement a high frequency atomic force microscope (AFM) that can accommodate a cantilever with a natural frequency of up to 200 MHz. Since Doppler interferometry measures velocity, the signal level increases proportionally with frequency for a given amplitude of oscillation, or the noise floor in terms of displacement decreases with 1/f. A noise level of 0.5 fm/√Hz was attained around 2 MHz, giving a large signal to noise margin for high frequency cantilevers and higher vibration mode detection. We have so far confirmed the following: (i) atomic resolution of Si(111) 7×7 with an amplitude of drive as low as 0.03 nm at 1.6 MHz, (ii) true atomic resolution dynamic lateral force microscopy, (iii) lateral force gradient detection at the atomic level, (iv) manipulation of Si atoms at room temperature, (v) vibration measurement of Si nanowires, tungsten oxide whiskers and graphene cantilevers above 100 MHz, (vi) atomic resolution imaging with the 2nd and 3rd mode of deflection and (v) true atomic resolution imaging in water using deflection or torsion with a few 10 pm amplitude of drive.

Electron holography—basics and applications

Despite the huge progress achieved recently by means of the corrector for aberrations, allowing now a true atomic resolution of 0.1 nm, hence making it an unrivalled tool for nanoscience, transmission electron microscopy (TEM) suffers from a severe drawback: in a conventional electron micrograph only a poor phase contrast can be achieved, i.e. phase structures are virtually invisible. Therefore, conventional TEM is nearly blind for electric and magnetic fields, which are pure phase objects. Since such fields provoked by the atomic structure, e.g. of semiconductors and ferroelectrics, largely determine the solid state properties, hence the importance for high technology applications, substantial object information is missing.Electron holography in TEM offers the solution: by superposition with a coherent reference wave, a hologram is recorded, from which the image wave can be completely reconstructed by amplitude and phase. Now the object is displayed quantitatively in two separate images: one representing the amplitude, the other the phase. From the phase image, electric and magnetic fields can be determined quantitatively in the range from micrometre down to atomic dimensions by all wave optical methods that one can think of, both in real space and in Fourier space.Electron holography is pure wave optics. Therefore, we discuss the basics of coherence and interference, the implementation into a TEM, the path of rays for recording holograms as well as the limits in lateral and signal resolution. We outline the methods of reconstructing the wave by numerical image processing and procedures for extracting the object properties of interest. Furthermore, we present a broad spectrum of applications both at mesoscopic and atomic dimensions.This paper gives an overview of the state of the art pointing at the needs for further development. It is also meant as encouragement for those who refrain from holography, thinking that it can only be performed by specialists in highly specialized laboratories. In fact, a modern TEM built for atomic resolution and equipped with a field emitter or a Schottky emitter, well aligned by a skilled operator, can deliver good holograms. Running commercially available image processing software and mathematics programs on a laptop-computer is sufficient for reconstruction of the amplitude and phase images and extracting desirable object information.

Dynamic force microscopy study of the Ga-rich c(8×2) and As-rich c(4×4) reconstructions of the GaAs(001) surface

As-rich and Ga-rich GaAs(001) surfaces were studied with frequency-modulation dynamic force microscopy. By simply changing the parameters of argon sputtering and annealing during sample preparation, surfaces reconstructed with the As-rich c(4×4) phase or the Ga-rich c(8×2) phase could be obtained. True atomic resolution of the c(8×2) reconstruction is achieved by using constant frequency shift imaging. We show that tip functionalization allows selective species imaging. The presence at the tip apex of empty Ga dangling bonds or of fully filled As dangling bonds leads to selective atomic resolution of the As or Ga sublattices of the c(8×2) reconstructed surface, respectively. Our observations support the ζ model for the c(8×2) reconstruction (but no dimers were found) and the α model for the c(8×2) reconstruction (individual As–As dimers were resolved by dynamic force microscopy).

Chemical identification of individual surface atoms by atomic force microscopy

Scanning probe microscopy is a versatile and powerful method that uses sharp tips to image, measure and manipulate matter at surfaces with atomic resolution. At cryogenic temperatures, scanning probe microscopy can even provide electron tunnelling spectra that serve as fingerprints of the vibrational properties of adsorbed molecules and of the electronic properties of magnetic impurity atoms, thereby allowing chemical identification. But in many instances, and particularly for insulating systems, determining the exact chemical composition of surfaces or nanostructures remains a considerable challenge. In principle, dynamic force microscopy should make it possible to overcome this problem: it can image insulator, semiconductor and metal surfaces with true atomic resolution, by detecting and precisely measuring the short-range forces that arise with the onset of chemical bonding between the tip and surface atoms and that depend sensitively on the chemical identity of the atoms involved. Here we report precise measurements of such short-range chemical forces, and show that their dependence on the force microscope tip used can be overcome through a normalization procedure. This allows us to use the chemical force measurements as the basis for atomic recognition, even at room temperature. We illustrate the performance of this approach by imaging the surface of a particularly challenging alloy system and successfully identifying the three constituent atomic species silicon, tin and lead, even though these exhibit very similar chemical properties and identical surface position preferences that render any discrimination attempt based on topographic measurements impossible.