Nanometer Spatial Resolution Research Articles

Electronic Structure and Band Gap of Fullerenes on Tungsten Surfaces: Transition from a Semiconductor to a Metal Triggered by Annealing.

The understanding and control of molecule-metal interfaces is critical to the performance of molecular electronics and photovoltaics devices. We present a study of the interface between C60 and W, which is a carbide-forming transition metal. The complex solid-state reaction at the interface can be exploited to adjust the electronic properties of the molecule layer. Scanning tunneling microscopy/spectroscopy measurements demonstrate the progression of this reaction from wide band gap (>2.5 eV) to metallic molecular surface during annealing from 300 to 800 K. Differential conduction maps with 104 scanning tunneling spectra are used to quantify the transition in the density of states and the reduction of the band gap during annealing with nanometer spatial resolution. The electronic transition is spatially homogeneous, and the surface band gap can therefore be adjusted by a targeted annealing step. The modified molecules, which we call nanospheres, are quite resistant to ripening and coalescence, unlike any other metallic nanoparticle of the same size. Densely packed C60 and isolated C60 molecules show the same transition in electronic structure, which confirms that the transformation is controlled by the reaction at the C60-W interface. Density functional theory calculations are used to develop possible reaction pathways in agreement with experimentally observed electronic structure modulation. Control of the band gap by the choice of annealing temperature is a unique route to tailoring molecular-layer electronic properties.

NanoSQUID magnetometry of individual cobalt nanoparticles grown by focused electron beam induced deposition

We demonstrate the operation of low-noise nano superconducting quantum interference devices (SQUIDs) based on the high critical field and high critical temperature superconductor YBa2Cu3O7 (YBCO) as ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have been patterned by focused ion beam milling. This allows us to precisely define the lateral dimensions of the SQUIDs so as to achieve large magnetic coupling between the nanoloop and individual MNPs. By means of focused electron beam induced deposition, cobalt MNPs with a typical size of several tens of nm have been grown directly on the surface of the sensors with nanometric spatial resolution. Remarkably, the nanoSQUIDs are operative over extremely broad ranges of applied magnetic field (–1 T 1 T) and temperature (0.3 K 80 K). All these features together have allowed us to perform magnetization measurements under different ambient conditions and to detect the magnetization reversal of individual Co MNPs with magnetic moments (1–30) . Depending on the dimensions and shape of the particles we have distinguished between two different magnetic states yielding different reversal mechanisms. The magnetization reversal is thermally activated over an energy barrier, which has been quantified for the (quasi) single-domain particles. Our measurements serve to show not only the high sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these sensors are exceptional magnetometers for the investigation of the properties of individual nanomagnets.

Enhanced Propagating Surface Plasmon Signal Detection

Overcoming the dissipative nature of propagating surface plasmons (PSPs) is a prerequisite to realizing functional plasmonic circuitry, in which large-bandwidth signals can be manipulated over length scales far below the diffraction limit of light. To this end, we report on a novel PSP-enhanced signal detection technique achieved in an all-metallic substrate. We take advantage of two strategically spatiotemporally separated phase-locked femtosecond laser pulses, incident onto lithographically patterned PSP coupling structures. We follow PSP propagation with joint femtosecond temporal and nanometer spatial resolution in a time-resolved nonlinear photoemission electron microscopy scheme. Initially, a PSP signal wave packet is launched from a hole etched into the silver surface from where it propagates through an open trench structure and is decoded through the use of a timed probe pulse. FDTD calculations demonstrate that PSP signal waves may traverse open trenches in excess of 10 μm in diameter, thereby al...

Retrieving plasmonic near-field information: A quantum-mechanical model for streaking photoelectron spectroscopy of gold nanospheres

Streaked photoemission from nanostructures is characterized by size- and material-dependent nanometer-scale variations of the induced nanoplasmonic response to the electronic field of the streaking pulse and thus holds promise of allowing photoelectron imaging with both subfemtosecond temporal and nanometer spatial resolution. In order to scrutinize the driven collective electronic dynamics in 10--200-nm-diameter gold nanospheres, we calculated the plasmonic field induced by streaking pulses in the infrared and visible spectral range and developed a quantum-mechanical model for streaked photoemission by extreme ultraviolet pulses. Our simulated photoelectron spectra reveal a significant amplitude enhancement and phase shift of the photoelectron streaking trace relative to calculations that exclude the induced plasmonic field. Both are most pronounced for streaking pulses tuned to the plasmon frequency and retrace the plasmonic electromagnetic field enhancement and phase shift near the nanosphere surface.

Electron Microscopy Reveals Structural and Chemical Changes at the Nanometer Scale in the Osteogenesis Imperfecta Murine Pathology.

Alternations of collagen and mineral at the molecular level may have a significant impact on the strength and toughness of bone. In this study, scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) were employed to study structural and compositional changes in bone pathology at nanometer spatial resolution. Tail tendon and femoral bone of osteogenesis imperfecta murine (oim, brittle bone disease) and wild type (WT) mice were compared to reveal defects in the architecture and chemistry of the collagen and collagen-mineral composite in the oim tissue at the molecular level. There were marked differences in the substructure and organization of the collagen fibrils in the oim tail tendon; some regions have clear fibril banding and organization, while in other regions fibrils are disorganized. Malformed collagen fibrils were loosely packed, often bent and devoid of banding pattern. In bone, differences were detected in the chemical composition of mineral in oim and WT. While mineral present in WT and oim bone exhibited the major characteristics of apatite, examination in EELS of the fine structure of the carbon K ionization edge revealed a significant variation in the presence of carbonate in different regions of bone. Variations have been also observed in the fine structure and peak intensities of the nitrogen K-edge. These alterations are suggestive of differences in the maturation of collagen nucleation sites or cross-links. Future studies will aim to establish the scale and impact of the modifications observed in oim tissues. The compositional and structural alterations at the molecular level cause deficiencies at larger length scales. Understanding the effect of molecular alterations to pathologic bone is critical to the design of effective therapeutics.

Spatially resolved spectroscopic differentiation of hydrophilic and hydrophobic domains on individual insulin amyloid fibrils.

The formation of insoluble β-sheet-rich protein structures known as amyloid fibrils is associated with numerous neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease. A detailed understanding of the molecular structure of the fibril surface is of interest as the first contact with the physiological environment in vivo and plays a decisive role in biological activity and associated toxicity. Recent studies reveal that the inherent sensitivity and specificity of tip-enhanced Raman scattering (TERS) renders this technique a compelling method for fibril surface analysis at the single-particle level. Here, the reproducibility of TERS is demonstrated, indicating its relevance for detecting molecular variations. Consequently, individual fibrils are systematically investigated at nanometer spatial resolution. Spectral parameters were obtained by band-fitting, particularly focusing on the identification of the secondary structure via the amide III band and the differentiation of hydrophobic and hydrophilic domains on the surface. In addition multivariate data analysis, specifically the N-FINDR procedure, was employed to generate structure-specific maps. The ability of TERS to localize specific structural domains on fibril surfaces shows promise to the development of new fibril dissection strategies and can be generally applied to any (bio)chemical surface when structural variations at the nanometer level are of interest.

Quantification of surface displacements and electromechanical phenomena via dynamic atomic force microscopy

Detection of dynamic surface displacements associated with local changes in material strain provides access to a number of phenomena and material properties. Contact resonance-enhanced methods of atomic force microscopy (AFM) have been shown capable of detecting ∼1–3 pm-level surface displacements, an approach used in techniques such as piezoresponse force microscopy, atomic force acoustic microscopy, and ultrasonic force microscopy. Here, based on an analytical model of AFM cantilever vibrations, we demonstrate a guideline to quantify surface displacements with high accuracy by taking into account the cantilever shape at the first resonant contact mode, depending on the tip–sample contact stiffness. The approach has been experimentally verified and further developed for piezoresponse force microscopy (PFM) using well-defined ferroelectric materials. These results open up a way to accurate and precise measurements of surface displacement as well as piezoelectric constants at the pm-scale with nanometer spatial resolution and will allow avoiding erroneous data interpretations and measurement artifacts. This analysis is directly applicable to all cantilever-resonance-based scanning probe microscopy (SPM) techniques.

In Situ TEM Observation on Redox Cycling of SOFC Anode

Solid Oxide Fuel Cell (SOFC) technology is a promising energy conversion option which generates electricity with high efficiency. SOFCs have additional advantages including fuel flexibility (e. g. CO, H2 and CH4), distributed power generation, and reduction of pollutants such as NOx and SOx. The standard SOFC design is based on the use of a porous cermet composite anode made of yttria (Y2O3)-stabilized zirconia (YSZ) and Ni. NiO particles are sintered with YSZ and then reduced to metallic Ni before normal operation of the cell. Ni-based anode-supported cells display sufficient performance and steady state durability, however, the anode exhibits structural instability in successive redox cycles: although Ni is kept in its reduced state under normal operating conditions, the Ni may reoxidize due to factors such as seal leakage, shutdown in the absence of protecting gas, or high fuel utilization. It has been suggested that coalescence and sintering of Ni during reduction and the formation of porosity on reoxidation may account for the instability of Ni on redox cycling. However, there have been only a few reports on the reduction and oxidation processes of Ni-zirconia cermets observed directly with nanometer spatial resolution in real time. Environmental transmission electron microscopy (ETEM) can be used to provide dynamic in situ observations of chemical reactions at the nano- to micrometric scale. It allows gases at pressures above 10 mbar to be introduced into the electron microscope column with the sample at elevated temperature and without compromising other aspects of the performance of the microscope. In this study, in situ reduction and reoxidation for Ni and Sc2O3-stabilized zirconia (ScSZ) cermet anode were performed in a differentially pumped FEI TitanTM ETEM with the specimen mounted in a NanoExTM heating holder. This configuration allows in situ studies in the microscope in gaseous atmospheres up to 20 mbar and at temperatures up to 1000℃. The SOFC anode samples for the ETEM examination were prepared by a focused ion beam (FIB) lamella lift-out method on MEMS Chips for the NanoEX holder using a FEI Versa 3D. Prior to heating the sample, H2 or O2was introduced into the column, corresponding to measured pressures of 3-5 mbar at the sample. After that, scanning TEM (STEM) high-angle annular dark-field and bright-field (BF) images of the SOFC lamella were continuously taken by the BB FlashBack software. The temperature was kept at 300 ℃, 400 ℃, 500 ℃, 600 ℃, and 700 ℃ for 5 minutes during heating up to 800 ℃. In the range between 400 ℃ and 800 ℃, the specimen was heated at a ramp of 1 ℃/sec, although it was possible to heat and to cool it immediately in the holder. In the case of heating as-sintered NiO-ScSZ cermet in a H2 atmosphere of 3 mbar, nanovoids were observed at around 400 ℃ at the grain boundaries of NiO/ScSZ and NiO/NiO. Nano-pores were formed in the NiO grains to accommodate the volume shrinkage that resulted from the reduction of NiO to Ni. When the temperature was over 500 ℃, the NiO-free surface was reduced directly, and porosity formed uniformly across the NiO grains. At higher temperatures, coalescence of the porosity was observed to result from Ni reorganization. This sample was reoxidized with heating again up to 800 ℃ under an O2atmosphere of 3 mbar in the ETEM. When the Ni-ScSZ cermet samples were heated in the O2 atmosphere of 3-5 mbar, both the grain boundary of Ni/ScSZ and the Ni free surface were reduced at around 500 ℃. Electron energy loss spectroscopy revealed that Ni particles were completely oxidized to NiO after heating up to 800 ℃. From these ETEM studies, it was found that the formation of porous NiO and the coarsening of Ni during the redox cycling result in the reduction of triple-phase boundaries in the cells. This work is partly supported by the Japan Science and Technology Agency (JST) through its Center of Innovation (COI) Program. International Institute for Carbon-Neutral Energy Research (I2CNER) is supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. Figure 1

Live endothelial cells imaged by Scanning Near‐field Optical Microscopy (SNOM): capabilities and challenges

The scanning near-field optical microscopy (SNOM) shows a potential to study details of biological samples, since it provides the optical images of objects with nanometric spatial resolution (50-200 nm) and the topographic information at the same time. The goal of this work is to demonstrate the capabilities of SNOM in transmission configuration to study human endothelial cells and their morphological changes, sometimes very subtle, upon inflammation. Various sample preparations were tested for SNOM measurements and promising results are collected to show: 1) the influence of α tumor necrosis factor (TNF-α) on EA.hy 926 cells (measurements of the fixed cells); 2) high resolution images of various endothelial cell lines, i.e. EA.hy 926 and HLMVEC (investigations of the fixed cells in buffer environment); 3) imaging of live endothelial cells in physiological buffers. The study demonstrate complementarity of the SNOM measurements performed in air and in liquid environments, on fixed as well as on living cells. Furthermore, it is proved that the SNOM is a very useful method for analysis of cellular morphology and topography. Changes in the cell shape and nucleus size, which are the symptoms of inflammatory reaction, were noticed in TNF-α activated EA.hy 926 cells. The cellular structures of submicron size were observed in high resolution optical images of cells from EA.hy 926 and HLMVEC lines.

Hot Electron Generation and Cathodoluminescence Nanoscopy of Chiral Split Ring Resonators.

Three-dimensional chiral plasmonic nanostructures have been shown to be able to dramatically boost photon-spin selective light-matter interactions, potentially leading to novel photonics, molecular spectroscopy, and light-harvesting applications based on circularly polarized light. Here, we show that chiral split-ring gold nanoresonators interfaced to a wide band gap semiconductor exhibit a contrast in hot-electron transfer rate between left-handed and right-handed visible light that essentially mimics the far-field circular dichroism of the structures. We trace down the origin of this effect to the differential excitation of the thinnest part of the split-ring structures using dichroic-sensitive cathodoluminescence imaging with nanometer spatial resolution. The results highlight the intricate interplay between the near-field and far-field chiral response of a nanostructure and establishes a clear link to the emerging field of hot carrier plasmonics with numerous potential applications in photocatalysis and solar light harvesting.

Two Mechanisms of Tip Enhancement of Raman Scattering by Protein Aggregates

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for probing the surface of biological species with nanometer spatial resolution. Here, we report the TER spectra of an individual insulin fibril, the protein cast film and a short peptide (LVEALYL) microcrystal mimicking the fibril core. Two different types of TER spectra were acquired depending on the "roughness" of the probed surface at the molecular level. A fully reproducible, low-intensity, normal Raman-type spectrum was characteristic of the top flat surface of the microcrystal while highly variable, higher intensity TER spectra were obtained for the edges of the microcrystal, cast film, and fibril. As a result, two tip enhancement mechanisms of Raman scattering, long- and short-range, were proposed by analogy with the physical and chemical enhancement mechanisms, respectively, known for surface-enhanced Raman spectroscopy.

Nanoscale Probing of Bandgap States on Oxide Particles Using Electron Energy-Loss Spectroscopy

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2 showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2 nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.

Nanoscale probing of bandgap states on oxide particles using electron energy-loss spectroscopy

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2 showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2 nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.

Integrated magnetic tweezers and single-molecule FRET for investigating the mechanical properties of nucleic acid

Many enzymes promote structural changes in their nucleic acid substrates via application of piconewton forces over nanometer length scales. Magnetic tweezers (MT) is a single molecule force spectroscopy method widely used for studying the energetics of such mechanical processes. MT permits stable application of a wide range of forces and torques over long time scales with nanometer spatial resolution. However, in any force spectroscopy experiment, the ability to monitor structural changes in nucleic acids with nanometer sensitivity requires the system of interest to be held under high degrees of tension to improve signal to noise. This limitation prohibits measurement of structural changes within nucleic acids under physiologically relevant conditions of low stretching forces. To overcome this challenge, researchers have integrated a spatially sensitive fluorescence spectroscopy method, single molecule-FRET, with MT to allow simultaneous observation and manipulation of nanoscale structural transitions over a wide range of forces. Here, we describe a method for using this hybrid instrument to analyze the mechanical properties of nucleic acids. We expect that this method for analysis of nucleic acid structure will be easily adapted for experiments aiming to interrogate the mechanical responses of other biological macromolecules.

NanoARPES of twisted bilayer graphene on SiC: absence of velocity renormalization for small angles.

The structural and electronic properties of twisted bilayer graphene (TBG) on SiC(000) grown by Si flux-assisted molecular beam epitaxy were investigated using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy with nanometric spatial resolution. STM images revealed a wide distribution of twist angles between the two graphene layers. The electronic structure recorded in single TBG grains showed two closely-spaced Dirac π bands associated to the two stacked layers with respective twist angles in the range 1–3°. The renormalization of velocity predicted in previous theoretical calculations for small twist angles was not observed.

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy.

Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.

Contact resonance atomic force microscopy (CR-AFM) in applied mineralogy: the case of natural and thermally treated diaspore

Contact resonance atomic force microscopy (CR-AFM) is a nondestructive technique based on atomic force microscopy (AFM) that allows one to perform single-point measurements as well as surface mapping of the indentation modulus of a material. In this work, we demonstrate the potential of CR-AFM in applied mineralogy research. As a case study, we report the characterization of natural and thermally treated diaspore. Natural diaspore samples from the Ilbirdagi diasporic metabauxite (diasporite) deposit in the Milas (Mugla) region in Turkey were heat treated in a muffle furnace causing them to transform from diaspore to corundum. After the treatment, the samples had a polycrystalline structure with ordered micrometer-size rectangular grains of uniform crystallographic orientation. Nanomechanical characterization by CR-AFM allowed us to visualize inclusions with different mechanical properties and to determine the average indentation modulus of the surface of the thermally treated diaspore. Quantitative maps of the indentation modulus reveal the variation of the indentation modulus on the surface of single micro-grains with nanometer spatial resolution.

Depth profiles of the interfacial strains of Si0.7Ge0.3/Si using three-beam Bragg-surface diffraction.

Interfacial strains are important factors affecting the structural and physical properties of crystalline multilayers and heterojunctions, and the performance of the devices made of multilayers used, for example, in nanowires, optoelectronic components, and many other applications. Currently existing strain measurement methods, such as grazing incidence X-ray diffraction (GIXD), cross-section transmission electron microscope, TEM, and coherent diffractive imaging, CDI, are limited by either the nanometer spatial resolution, penetration depth, or a destructive nature. Here we report a new non-destructive method of direct mapping the interfacial strain of [001] Si0.7Ge0.3/Si along the depth up to ~287 nm below the interface using three-beam Bragg-surface X-ray diffraction (BSD), where one wide-angle symmetric Bragg reflection and a surface reflection are simultaneously involved. Our method combining with the dynamical diffraction theory simulation can uniquely provide unit cell dimensions layer by layer, and is applicable to thicker samples.

Direct observation of intermediate states in model membrane fusion.

We introduce a novel assay for membrane fusion of solid supported membranes on silica beads and on coverslips. Fusion of the lipid bilayers is induced by bringing an optically trapped bead in contact with the coverslip surface while observing the bead’s thermal motion with microsecond temporal and nanometer spatial resolution using a three-dimensional position detector. The probability of fusion is controlled by the membrane tension on the particle. We show that the progression of fusion can be monitored by changes in the three-dimensional position histograms of the bead and in its rate of diffusion. We were able to observe all fusion intermediates including transient fusion, formation of a stalk, hemifusion and the completion of a fusion pore. Fusion intermediates are characterized by axial but not lateral confinement of the motion of the bead and independently by the change of its rate of diffusion due to the additional drag from the stalk-like connection between the two membranes. The detailed information provided by this assay makes it ideally suited for studies of early events in pure lipid bilayer fusion or fusion assisted by fusogenic molecules.

High speed direct imaging of thin metal film ablation by movie-mode dynamic transmission electron microscopy.

Obliteration of matter by pulsed laser beams is not only prevalent in science fiction movies, but finds numerous technological applications ranging from additive manufacturing over machining of micro- and nanostructured features to health care. Pulse lengths ranging from femtoseconds to nanoseconds are utilized at varying laser beam energies and pulse lengths, and enable the removal of nanometric volumes of material. While the mechanisms for removal of material by laser irradiation, i.e., laser ablation, are well understood on the micrometer length scale, it was previously impossible to directly observe obliteration processes on smaller scales due to experimental limitations for the combination of nanometer spatial and nanosecond temporal resolution. Here, we report the direct observation of metal thin film ablation from a solid substrate through dynamic transmission electron microscopy. Quantitative analysis reveals liquid-phase dewetting of the thin-film, followed by hydrodynamic sputtering of nano- to submicron sized metal droplets. We discovered unexpected fracturing of the substrate due to evolving thermal stresses. This study confirms that hydrodynamic sputtering remains a valid mechanism for droplet expulsion on the nanoscale, while irradiation induced stress fields represent limit laser processing of nanostructured materials. Our results allow for improved safety during laser ablation in manufacturing and medical applications.