Sub-nanometer Level Research Articles

Preparation and characterization of B4C coatings for advanced research light sources.

X-ray optical elements are required for beam transport at the current and upcoming free-electron lasers and synchrotron sources. An X-ray mirror is a combination of a substrate and a coating. The demand for large mirrors with single layers consisting of light or heavy elements has increased during the last few decades; surface finishing technology is currently able to process mirror lengths up to 1 m with microroughness at the sub-nanometre level. Additionally, thin-film fabrication is able to deposit a suitable single-layer material, such as boron carbide (B4C), some tens of nanometres thick. After deposition, the mirror should provide excellent X-ray optical properties with respect to coating thickness errors, microroughness values and slope errors; thereby enabling the mirror to transport the X-ray beam with high reflectivity, high beam flux and an undistorted wavefront to an experimental station. At the European XFEL, the technical specifications of the future mirrors are extraordinarily challenging. The acceptable shape error of the mirrors is below 2 nm along the whole length of 1 m. At the Helmholtz-Zentrum Geesthacht (HZG), amorphous layers of boron carbide with thicknesses in the range 30-60 nm were fabricated using the HZG sputtering facility, which is able to cover areas up to 1500 mm long by 120 mm wide in one step using rectangular B4C sputtering targets. The available deposition area is suitable for the specified X-ray mirror dimensions of upcoming advanced research light sources such as the European XFEL. The coatings produced were investigated by means of X-ray reflectometry and interference microscopy. The experimental results for the B4C layers are discussed according to thickness uniformity, density, microroughness and thermal stability. The variation of layer thickness in the tangential and sagittal directions was investigated in order to estimate the achieved level of uniformity over the whole deposition area, which is considerably larger than the optical area of a mirror. A waisted mask was positioned during deposition between the sputtering source and substrate to improve the thickness uniformity; particularly to prevent the formation a convex film shape in the sagittal direction. Additionally the inclination of the substrate was varied to change the layer uniformity in order to optimize the position of the mirror quality deposited area during deposition. The level of mirror microroughness was investigated for different substrates before and after deposition of a single layer of B4C. The thermal stability of the B4C layers on the various substrate materials was investigated.

Revealing the semiconductor–catalyst interface in buried platinum black silicon photocathodes

Burying platinum black silicon photocathodes provides an atomically clean Pt catalyst/Si interface free of oxide or other phases.

Figuring large optics at the sub-nanometer level: compensation for coating and gravity distortions.

Large, precision optics can now be manufactured with surface figures specified at the sub-nanometer level. However, coatings and gravity deform large optics, and there are limits to what can be corrected by clever compensation. Instead, deformations caused by stress from optical mounts and deposited coatings must be incorporated into the optical design. We demonstrate compensation of coating stress on a 370mm substrate to λ/200 by a process of coating and annealing. We also model the same process and identify the leading effects that must be anticipated in fabrication of optics for future gravitational wave detectors and other applications of large, precisely figured optics, and identify the limitations inherent in using coatings to compensate for these deformations.

Plasmon coupling between silver nanoparticles: Transition from the classical to the quantum regime

We explore plasmon coupling between silver nanoparticles (AgNPs) as two AgNPs approach each other within a subnanometer distance. We prepare AgNP dimers with two 21-nm AgNPs separated by alkanedithiol linkers in high yield. Changing the length of the alkanedithiol linkers enables us to control the interparticle distance down to the subnanometer level on the molecular scale. We observe that the longitudinal plasmon coupling band, which is sensitive to the interaction between AgNPs, gradually redshifts as the interparticle distance decreases. This observation is fully consistent with the classical electromagnetic model. The redshift of the plasmon coupling, however, undergoes a drastic change when the interparticle distance reaches ∼1nm. The longitudinal plasmon coupling band vanishes and a new intense band appears at a shorter wavelength. This band redshifts as the nanogap further narrows, but crosses over to a blueshift at ∼0.7nm. A comparison of our observation with finite-difference time-domain simulations reveals that this band arises from quantum effects. Controlled assembly of AgNP dimers in combination with simulations allows us to observe the transition of the plasmon coupling from the classical to the quantum regime at the ensemble level.

Simulation-based systematic error compensation for nanoprofiler using normal vector tracing method

We have developed a nanoprofiler that relies on the use of the normal vector. Our aim was to enable the measurement of the profile of free-form surfaces with high precision. Since the nanoprofiler does not use a reference surface, it should be capable of measuring free-form surfaces with a high degree of accuracy. Repeatability at the sub-nanometer level has been achieved. In this study, with a goal of reducing and evaluating the related uncertainty, we set out to estimate the degree of systematic error. We investigated the effect of the systematic error on the measurement results by computer simulation. Then, by comparing the measured results with those of the simulation, we estimated the systematic error.

High-resolution AFM in liquid: what about the tip?

Atomic Force Microscopy relies on a nanoscale tip to image and probe samples, often down to the sub-nanometre level. The measurement process depends on the precise geometry and chemical nature of the tip apex, and is therefore difficult to control. In the current issue of Nanotechnology, Akrami and co-workers show that, for measurements in aqueous solutions and on flat samples, the presence of stable hydration sites at the tip apex is key to achieving high-resolution images. These sites can be created on commercial tips using a simple preparation strategy that prevents build-up of interfering contaminants. The findings by Akrami et al also suggest a possible way forward to control the influence of the tip on high-resolution measurements.

Ultrafast Tracking of a Single Live Virion During the Invagination of a Cell Membrane.

The first step in most viral infections is the penetration of the cell membrane via endocytosis. However, the underlying mechanism of this important process has not been quantitatively characterized; for example, the velocity and force of a single virion during invagination remain unknown. Here, the endocytosis of a single live virion (Singapore grouper iridovirus, SGIV) through the apical membranes of a host cell is monitored by developing and using a novel ultrafast (at the microsecond level) tracking technique: force tracing. For the first time, these results unambiguously reveal that the maximum velocity during the cell entry of a single SGIV by membrane invagination is approximately 200 nm s(-1), the endocytic force is approximately 60.8 ± 18.5 pN, and the binding energy density increases with the engulfment depth. This report utilizing high temporospatial resolution (subnanometer and microsecond levels) approaches provides new insight into the dynamic process of viral infection via endocytosis and the mechanism of membrane invagination at the single-particle level.

Sub-nanometer dimensions control of core/shell nanoparticles prepared by atomic layer deposition

Bimetallic core/shell nanoparticles (NPs) are the subject of intense research due to their unique electronic, optical and catalytic properties. Accurate and independent control over the dimensions of both core and shell would allow for unprecedented catalytic performance. Here, we demonstrate that both core and shell dimensions of Pd/Pt core/shell nanoparticles (NPs) supported on Al2O3 substrates can be controlled at the sub-nanometer level by using a novel strategy based on atomic layer deposition (ALD). From the results it is derived that the main conditions for accurate dimension control of these core/shell NPs are: (i) a difference in surface energy between the deposited core metal and the substrate to obtain island growth; (ii) a process yielding linear growth of the NP cores with ALD cycles to obtain monodispersed NPs with a narrow size distribution; (iii) a selective ALD process for the shell metal yielding a linearly increasing thickness to obtain controllable shell growth exclusively on the cores. For Pd/Pt core/shell NPs it is found that a minimum core diameter of 1 nm exists above which the NP cores are able to catalytically dissociate the precursor molecules for shell growth. In addition, initial studies on the stability of these core/shell NPs have been carried out, and it has been demonstrated that core/shell NPs can be deposited by ALD on high aspect ratio substrates such as nanowire arrays. These achievements show therefore that ALD has significant potential for the preparation of tuneable heterogeneous catalyst systems.

High-resolution chemical depth profiling of solid material using a miniature laser ablation/ionization mass spectrometer.

High-resolution chemical depth profiling measurements of copper films are presented. The 10 μm thick copper test samples were electrodeposited on a Si-supported Cu seed under galvanostatic conditions in the presence of particular plating additives (SPS, Imep, PEI, and PAG) used in the semiconductor industry for the on-chip metallization of interconnects. To probe the trend of these plating additives toward inclusion into the deposit upon growth, quantitative elemental mass spectrometric measurements at trace level concentration were conducted by using a sensitive miniature laser ablation ionization mass spectrometer (LIMS), originally designed and developed for in situ space exploration. An ultrashort pulsed laser system (τ ∼ 190 fs, λ = 775 nm) was used for ablation and ionization of sample material. We show that with our LIMS system, quantitative chemical mass spectrometric analysis with an ablation rate at the subnanometer level per single laser shot can be conducted. The measurement capabilities of our instrument, including the high vertical depth resolution coupled with high detection sensitivity of ∼10 ppb, high dynamic range ≥10(8), measurement accuracy and precision, is of considerable interest in various fields of application, where investigations with high lateral and vertical resolution of the chemical composition of solid materials are required, these include, e.g., wafers from semiconductor industry or studies on space weathered samples in space research.

Assessment of the high precision of global least squares stitching over large measuring areas

A 3D stitching algorithm was developed to measure wide-range data without reducing lateral resolution. This algorithm can correct X- and Y-axis rotational errors and Z-axis translational errors, as well as minimize the propagation of stitching error using an overall squared stitching method. Furthermore, the coordinate relation of adjacent areas is clearly defined to facilitate proper implementation of the algorithm. The developed algorithm has been applied with simulated and actual measured data to verify that it can stitch various types of data. The stitching accuracy on the common stitching area was evaluated using the Monte-Carlo method as the noise level increased to determine the criteria on measuring conditions. The stitching error was validated to be very small and to occur at the sub-nanometer level.

Enhancing ionic conductivity in lithium amide for improved energy storage materials**Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2–6 November 2014, Ha Long, Vietnam.

Non-stoichiometry and bulk cation transport have been identified as key factors in the release and uptake of hydrogen in the Li–N–H system. Amide halide phases have been synthesized that have ionic conductivities several orders of magnitude greater than lithium amide, a faster rate of hydrogen release and elimination of the by-product, ammonia. Here we report the effect of both anion- and cation-doping on the hydrogen desorption properties of lithium amide, focusing in particular on how the presence of chloride anions and magnesium cations affects and controls the structure of the amide and imide compounds at the sub-nanometre level. Reducing the chloride content resulted in new low-chloride rhombohedral phases that contain around half of the chloride present in earlier amide chlorides, but maintained the enhancements seen in hydrogen desorption properties when compared to the halide-free system. These materials may also have potential in a range of other energy applications such as all solid state lithium ion batteries, supercapacitors, and CO2 capture and storage membranes.

Local Plasmonic Studies on Individual Core–Shell Gold–Silver and Pure Gold Nano-Bipyramids

Bimetallic systems present new opportunities to tailor the optical properties of nanoparticles. Morphology, structural, and optical properties of gold and of gold–silver nanoparticles have been studied by three-dimensional (3D) scanning transmission electron microscopy (STEM), STEM imaging, and monochromated subnanometer electron beams. The 3D morphology of these nanoparticles consists of bipyramidal prisms with well-defined facets. Furthermore, in the case of the core–shell nanoparticles, from these tomographical studies, we have determined the silver (shell) distribution and their atomic arrangement. Multipolar localized surface plasmon resonances (LSPR) have been studied, at subnanometer level and at high-energy resolution, as a function of their shape, their size (aspect ratio), and the surrounding environment. These results have been interpreted in the framework of the discrete dipole approximation (DDA) simulations. The effect of the silver outer-shell has been elucidated. We observed a significant damping of plasmon excitations due to the difference of dielectric function of these two metals. In addition, we have shown that the combination of the tomographical and plasmonic (experiments and simulations) studies with such high spatial resolution constitutes a very powerful and fundamental tool for understanding and optimizing the photonics properties of nanomaterials.

On the characterization of ultra-precise X-ray optical components: advances and challenges in ex situ metrology.

To fully exploit the ultimate source properties of the next-generation light sources, such as free-electron lasers (FELs) and diffraction-limited storage rings (DLSRs), the quality requirements for gratings and reflective synchrotron optics, especially mirrors, have significantly increased. These coherence-preserving optical components for high-brightness sources will feature nanoscopic shape accuracies over macroscopic length scales up to 1000 mm. To enable high efficiency in terms of photon flux, such optics will be coated with application-tailored single or multilayer coatings. Advanced thin-film fabrication of today enables the synthesis of layers on the sub-nanometre precision level over a deposition length of up to 1500 mm. Specifically dedicated metrology instrumentation of comparable accuracy has been developed to characterize such optical elements. Second-generation slope-measuring profilers like the nanometre optical component measuring machine (NOM) at the BESSY-II Optics laboratory allow the inspection of up to 1500 mm-long reflective optical components with an accuracy better than 50 nrad r.m.s. Besides measuring the shape on top of the coated mirror, it is of particular interest to characterize the internal material properties of the mirror coating, which is the domain of X-rays. Layer thickness, density and interface roughness of single and multilayer coatings are investigated by means of X-ray reflectometry. In this publication recent achievements in the field of slope measuring metrology are shown and the characterization of different types of mirror coating demonstrated. Furthermore, upcoming challenges to the inspection of ultra-precise optical components designed to be used in future FEL and DLSR beamlines are discussed.

Nanoscale dynamics of MEMS TFC active slider

With enabling of a thermal actuator, an air bearing slider can fly at sub-nanometer level spacing on a magnetic disk while the recording elements are functioning. At such spacing, the slider stability and head-disk interface reliability remains to be understood. In this study, a novel understanding on dynamics of the MEMS thermal flying-height control (TFC) slider in touchdown process is developed. By using average and variational-iteration methods, closed-form spectrum estimations of slider vibration are derived. The derived formulation offers an insight of the relationship between spectrum and interface parameters. Physics-based simulation is also conducted to quantify the spectrum of slider vibrations as a function of varied interfacial parameters. To further extend the analytical and numerical analysis, the experimental study of a TFC slider while flying on a rotating disk at sub-nanometer spacing are performed. The analysis reveals the dominance of the air bearing force among other interfacial forces at sub-nanometer spacing.

Identification of contact bouncing vibrations using TFC active slider and adaptive fuzzy control for advanced active slider design

The recent advance of magnetic storage technology includes the active thermal fly-height control (TFC) technique that is able to reduce the clearance between read/write heads and disk surface to be of few nanometers. However, it has been estimated that TFC technique alone cannot provide the even smaller clearances necessary to achieve Tbit/in2 recording densities due to the presence of bouncing vibrations and instability. In this work we investigate hybrid active slider technique with both thermal and piezoelectric actuators to achieve extremely high-density recording. The nonlinear system dynamics of thermal actuated slider is established by test-based identification. We propose an adaptive fuzzy control hybrid active slider design in which piezoelectric actuation is added to the existing thermal actuated slider design, so as to fully eliminate the high frequency bouncing vibrations. It can yield more than Tbit/in2 recording density with sub-nanometer level clearance, while using the state-of-the-art and matured manufacturing techniques for active piezoelectric slider and TFC slider.

Controlling the surface roughness of epitaxial SiC on silicon

The surface of cubic silicon carbide (3C-SiC) hetero-epitaxial films grown on the (111) surface of silicon is a promising template for the subsequent epitaxial growth of III-V semiconductor layers and graphene. We investigate growth and post-growth approaches for controlling the surface roughness of epitaxial SiC to produce an optimal template. We first explore 3C-SiC growth on various degrees of offcut Si(111) substrates, although we observe that the SiC roughness tends to worsen as the degree of offcut increases. Hence we focus on post-growth approaches available on full wafers, comparing chemical mechanical polishing (CMP) and a novel plasma smoothening process. The CMP leads to a dramatic improvement, bringing the SiC surface roughness down to sub-nanometer level, though removing about 200 nm of the SiC layer. On the other hand, our proposed HCl plasma process appears very effective in smoothening selectively the sharpest surface topography, leading up to 30% improvement in SiC roughness with only about 50 nm thickness loss. We propose a simple physical model explaining the action of the plasma smoothening.

Effect of interfaces on the magnetoelectric properties of Co/PZT/Co heterostructures

We have studied Co (film)/PZT (substrate)/Co (film) heterostructures with plane-parallel interfaces, produced by direct growth of cobalt films 0.5 to 3.5 μm thick on PZT surfaces smoothed to a subnanometer level (where PZT stands for a ceramic PbZr0.45Ti0.55O3 lead zirconate titanate ferroelectric substrate). The results demonstrate that they possess magnetoelectric properties comparable in magnitude to those characteristic of known structures but, in contrast to these latter, allow one to dispense with the condition that the volume fractions of the ferromagnetic and ferroelectric components be roughly equal. The interface is shown to play a key role in determining the magnetoelectric response of the heterostructures: above 9 mV/(cm Oe) (11.7 mV/A) in a magnetic field of 50 Oe (3980 A/m) at a frequency of 100 Hz and room temperature. The heterostructures are potentially attractive for use as nonvolatile sensors in household devices.

Discovery of nanoscale reduced surfaces and interfaces in VO2 thin films as a unique case of prewetting

VO2 thin films grown on SiOx/Si substrates have been characterized at the sub-nanometer level by Cs-corrected scanning transmission electron microscopy along with electron energy loss spectroscopy. Reduced transitional regions of 2–3 nm thick were found at both the surface and the interface, where the vanadium valence progressively changes from +4 to +2. The formation of these nanometer-thick surficial and interfacial layers can be interpreted as a unique case of prewetting, and it explains the degradation of metal-to-insulator transition properties in VO2 thin films.

Optical nanolithography with λ/15 resolution using bowtie aperture array

We report optical parallel nanolithography using bowtie apertures with the help of the interferometric-spatial-phase-imaging (ISPI) technique. The ISPI system can detect and control the distance between the bowtie aperture, and photoresist with a resolution of sub-nanometer level. It overcomes the difficulties brought by the light divergence of bowtie apertures. Parallel nanolithography with feature size of 22 ± 5 nm is achieved. This technique combines high resolution, parallel throughput, and low cost, which is promising for practical applications.

Extreme ultraviolet lithography resist-based aberration metrology

Extreme ultraviolet lithography (EUVL) at 13.5 nm is currently the most promising technology for advanced integrated circuit manufacturing nodes. Since the wavelength for EUVL is an order of magnitude smaller than current optical lithography systems (193 nm), aberration tolerances on lens manufacturing must be tightened to avoid image distortion and contrast loss as they scale with wavelength. Therefore, understanding the aberrations of an EUVL system both in idle and production conditions is paramount. This study aims to assess a photoresist-based aberration metrology technique for capturing pupil information of EUVL systems that can be implemented during full system use. Several datasets have been collected on a full-field EUVL system. Various one-dimensional and two-dimensional binary structures were imaged and used for pupil wave front extraction in conjunction with computational modeling and simulations. Results show a successful extraction of a stable aberration signature over several measurements, showing the method to be sensitive to subnanometer levels of intentional aberration change through lens manipulation.