Nanometer-scale Area Research Articles

High Spatiotemporal Precision Mapping of Optical Nanosensor Array Using Machine Learning.

Optical nanosensors, including single-walled carbon nanotubes (SWCNTs), provide real-time spatiotemporal reporting at the single-molecule level within a nanometer-scale area. However, their superior sensitivity also makes them susceptible to slight environmental influences such as reference analytes in media, external fluid flow, and mechanical modulations. Consequently, they often fail to achieve the optimal limit of detection (LOD) and frequently convey misinformation spatiotemporally. To address this challenge, we developed a single-pixel mapping technique for optical nanosensor arrays that operates with high spatiotemporal precision using machine learning. We systematically measured the spatial sensing images of various analyte concentrations below the LOD by using a near-infrared (nIR) fluorescent SWCNT nanosensor array. For dopamine (DA) as an example analyte, we extracted single-pixel level sensing features such as entropy, the Laplacian operator, and neighboring values under noise levels. We then trained the artificial intelligence (AI) model to accurately identify specific reaction pixels of the nanosensor array, even below the LOD region. Additionally, our method can distinguish subtle noise caused by fluid in the media or mechanical modulation of the array substrate. As a result, our approach significantly improved the detection sensitivity of the nanosensor array, achieving a 13-fold increase over the original LOD and halving the detection time of the reporter pixels, with F1 scores exceeding 0.9. This method not only lowers the LOD of optical nanosensors but also isolates sensor responses specific to the analyte, providing accurate spatiotemporal information to the user, even in noisy conditions. It can be universally applied to various optical nanosensor materials and analytes, maximizing the sensitivity and accuracy of the nanosensors used in diagnostics and analysis.

Correlative study between the local atomic and electronic structures of amorphous carbon materials via 4D-STEM and STEM-EELS

Amorphous materials have been used in a range of electronic and photonic applications, and the need for quantitative analytical techniques on their local structural information is growing. We present a comprehensive analysis of the atomic and electronic structures of an amorphous material, amorphous carbon (a-C), with scanning transmission electron microscopy (STEM)-derived techniques, four-dimensional STEM (4D-STEM), and STEM-electron energy loss spectroscopy (STEM-EELS). Each diffraction pattern of an a-C layer stack acquired via 4D-STEM is transformed into a reduced density function (RDF) and a radial variance profile (RVP) to retrieve the information on the atomic structures. Importantly, a machine-learning approach (preferably cluster analysis) separates distinct features in the EELS and RDF datasets; it also describes the spatial distributions of these features in the scanned regions. Consequently, we showed that the differences in the sp2/sp3 ratio and the involvement of additional elements led to changes in the bond length. Furthermore, we identified the dominant types of medium-range ordering structures (diamond-like or graphite-like nano-crystals) by correlations among the EELS, RDF, and RVP data. The information obtained via STEM-EELS and 4D-STEM can be strongly correlated, leading to the comprehensive characterization of the a-C layer stack for a nanometer-scale area. This process can be used to investigate any amorphous material, thereby yielding comprehensive information regarding the origins of notable properties.

Polarized Raman Reveals Alignment of Few-Layer MoS2 Films

Ongoing interest in two-dimensional (2D) layered materials has motivated extensive studies of transitional metal dichalcogenides, especially its most pronounced representative, MoS2. The few-layer MoS2 exhibits distinct properties from those in bulk, which predetermine its potential usage in optoelectronics and flexible devices. Recently, it was found that the layer orientation in MoS2 thin films is a key parameter for their utilization in specific devices. Thus far, the alignment of MoS2 layers has been detected mostly by transmission electron microscopy (TEM). The drawback of this method is that it requires elaborate sample preparation and probes only a nanometer-scale area of the sample surface. Here we present polarized Raman spectroscopy which provides information about the MoS2 layer orientation on the area a few orders larger than in TEM. We show that the depolarization ratio of the significant Raman peaks A1g and E2g show specific values for the vertical and horizontal alignment of the MoS2 layers...

Two-probe STM experiments at the atomic level

Direct characterization of planar atomic or molecular scale devices and circuits on a supporting surface by multi-probe measurements requires unprecedented stability of single atom contacts and manipulation of scanning probes over large, nanometer scale area with atomic precision. In this work, we describe the full methodology behind atomically defined two-probe scanning tunneling microscopy (STM) experiments performed on a model system: dangling bond dimer wire supported on a hydrogenated germanium (0 0 1) surface. We show that 70 nm long atomic wire can be simultaneously approached by two independent STM scanners with exact probe to probe distance reaching down to 30 nm. This allows direct wire characterization by two-probe I–V characteristics at distances below 50 nm. Our technical results presented in this work open a new area for multi-probe research, which can be now performed with precision so far accessible only by single-probe scanning probe microscopy (SPM) experiments.

Adhesion and frictional force measurements employing scanning probe microscopy in a pentafluoropropane gas atmosphere

Ultraviolet (UV) nanoimprinting is a useful technique to fabricate nanostructure devices with high throughput, high resolution, and low cost. To eliminate bubble defects, one of the main problems in UV nanoimprinting, UV nanoimprinting in an atmosphere of pentafluoropropane (PFP) gas, which is a condensable gas, is proposed. This process has been reported to reduce the demolding force. To examine the PFP gas effect in a nanometer-scale area, adhesion and frictional forces were measured by scanning probe microscopy in a PFP gas atmosphere. The results show that the adhesion and frictional forces measured by scanning probe microscopy in a PFP gas atmosphere were lower than those measured in air and a N2 gas atmosphere. Furthermore, the adhesion and frictional forces were found to depend on the PFP gas pressure.

Engineering space for light via transformation optics

Conceptual studies and numerical simulations are performed for imaging devices that transform a near-field pattern into magnified far-zone images and are based on high-order spatial transformation in cylindrical domains. A lens translating a near-field pattern from an almost circular input boundary onto a magnified far-field image at a flat output boundary is considered. The lens is made of a metamaterial with anisotropic permittivity and permeability both depending on a single "scaling" parameter of the transformation. Open designs of the lens with a truncated body (3/4-body and 1/4-body lenses) are suggested and analyzed. It is shown that the ideal full lens and the 3/4-body lens produce identical images. Numerical simulations of 1/4-body designs indicate that further truncation of the lens could limit its performance. A light concentrator "focusing" far-zone fields into a nanometer-scale area is also considered.

Light emission induced by tunneling electrons from surface nanostructures observed by novel conductive and transparent probes

We have developed an ultrahigh-vacuum low-temperature scanning tunneling microscope (STM) equipped with a near-field optical detection system using novel conductive and optically transparent probes. Tunneling-electron induced photons generated in a nanometer-scale area just under the STM probe can be collected directly into the core of the optical fiber probe within the optical near-field region. Firstly, optical fiber probes coated with indium-tin-oxide thin film are applied to quantitative analysis of p-type GaAs(110) surface, where a decrease of light emission in photon mapping clearly extracts the existence of Zn accepter atoms located at the sub-surface layers. Secondly, in order to enhance the efficiency for inelastic tunneling excitation of a tip-induced plasmon mode, a STM probe coated with an Ag/ITO dual-layer film has been developed and applied to an Ag(111) surface, where photon mapping with a step resolution has been achieved by near-field detection.

Photon emission induced by tunneling electrons from a Cu([formula omitted]) surface covered with porphyrin molecules

An ultrahigh-vacuum scanning tunneling microscope with an optically transparent tip was used to excite photon emission from a clean Cu(100) surface and a sub-monolayer of Cu-tetra-[3,5-di-t-butylphenyl]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(100) surface. Photons generated in a nanometer-scale area just under the tip were collected within the near-field region through the apex of the optical fiber tip. Two kinds of mechanisms for the photon emission from a Cu-TBPP/Cu(100) surface are proposed. The quantum efficiency for molecular fluorescence induced by inelastic tunneling was estimated to be ∼3×10−6 photons/electron.

Heterostructures of single-walled carbon nanotubes and carbide nanorods

A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes (SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces. The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube, represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important role in the future fabrication of hybrid nanodevices.

Surface processes of selective growth by atomic layer epitaxy

Characteristics and mechanisms, of the crystallographic selective growth by atomic layer epitaxy (ALE) and the localized ALE growth over the nanometer scale area, are discussed focusing on the surface process. The experimental results indicate that higher growth temperature, and longer hydrogen purge time after AsH 3 supply, promote the crystallographic selectivity of GaAs ALE growth. The characteristics of the ALE selective growth include high crystallographic selectivity, and high uniformity in the nanometer scale. The mechanisms of ALE selective growth seem to be caused by the separate enhancement of As desorption process and the self-limiting mechanism of Ga precursor adsorption process on the GaAs surface.