AVS 69 Session NS-TuP: Nanoscale Science and Technology Poster Session

There is growing interest in developing cost-effective and sustainable methods to synthesize photocatalysts that can be immobilized on substrates. In this study, we present a novel and ultra-scalable method for synthesizing ZnO nanowires that are immobilized on the surface of Zn plates using a hot water treatment (HWT) technique. This straightforward synthesis method involves immersing Zn coupons in deionized water at 75 °C without any chemical additives or precursors. SEM, TEM, EDS, XRD, XPS, and UV-Vis spectroscopy analysis on the surface of Zn after 5 hours of HWT showed the formation of well-developed, hexagonally-faceted, stoichiometric, and crystalline ZnO nanowires with a bandgap of 3.26 eV. To evaluate the photocatalytic activity of these ZnO nanowires, we performed methylene blue degradation tests under UV light. 93% of methylene blue was degraded in 120 minutes with a reaction rate constant of 0.021 min^-1. Overall, our findings demonstrate the potential of HWT as a scalable and environmentally-friendly method for synthesizing ZnO nanostructures with excellent photocatalytic properties.

Atomic force microscopy (AFM) is based upon a simple operational principle. However, the presentation and interpretation of AFM images can easily suffer from consequential artifacts that are easily overlooked. Here we discuss results from AFM and its companion variations PF-QNM (peak-force quantitative nano-mechanical mapping) and AFM-IR (AFM combined with infrared spectroscopy) by imaging “bee” structures in asphalt binder (bitumen) as examples. We show how common problems manifest themselves, with the intent that authors can present their results clearly and avoid interpreting artifacts as true physical properties, thereby raising the quality of AFM research.

Optical spectroscopies can be used to probe and characterize the composition as well as the physical and electronic structure of molecules and materials. By confining light-matter interactions into the near-field of a scanning probe microscope (SPM) the diffraction limit of light can be circumvented and the resulting measurements can consider composition and subtle local phenomena down to the nanoscale. In ultrahigh vacuum (UHV) and at cryogenic temperatures the spatial resolution of these methods can be pushed towards the atomic scale.

Scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the behavior and chemistry of molecular adsorbates with surfaces. Spectroscopic imaging provides the ability to visualize highly localized phenomena, while vibrational fingerprints can be used to understand their effects on a molecule’s or material’s structure. This yields insight into adsorbate-substrate interactions crucial to the growth of nanostructures through surface chemistry, as well as the functionalization or modification of 2D materials.

Moving beyond UHV-STM-TERS, ongoing work focuses on the development of a new versatile cryogenic UHV-SPM platform for other measurements of light-matter interactions, specifically STM-induced luminescence (STML) and coupling pulsed infrared light into the tip-sample junction. STML leverages the atomically sharp STM tip as a probe to locally excite light emission and captures spectra that can be used to characterize excitonic behavior and vibronic structure. On the other hand, many relevant quantum phenomena occur in the infrared regime, which remains relatively unexplored with atomic scale near-field microscopy. Together, these capabilities are expected to result in a platform capable of studying properties of materials that would otherwise be inaccessible.

In this study, a novel air spacer using plasma nitridation (PN) technique for dynamic random acess memory (DRAM) device is investigated to minimize parasitic bit-line capacitance (Cb). The ratio of Cb to cell capacitance (Cs) is increasing due to smaller cell size. Thus, DRAM development faced great difficulties due to the decrease in bit-line sensing margin [1]. The Cb/Cs is a key factor in determining the bit-line sensing margin. In addition, Cs is expected to continue to decrease due to the physical limitations of cell capacitor technology, so technology that can lower Cb has become a key technology [2]. 20nm DRAM developed bit-line air spacer technology and greatly improved Cb [1]. However, Cb still needs additional improvements due to the continued reduction in bit-line spacer dimension. We improved the Si3N4 transition layer by using the plasma nitridation techniques before the deposition of the outer Si3N4 film. The pre-plasma nitriding treatment process forms an inter-diffusion and additional protective layer and improves the surface properties of the deposited film [3]. We used this technology to the air spacer. As shown in figure 1, it was expected to maximize the air layer thickness formed in the subsequent process by reducing the thickness of the transition layer formed during outer Si3N4 deposition. The Cb was measured using the test element group, which can measure air spacer pattern and non-air spacer pattern. the Cb improved by 2%, as shown in figure 2. It is considered that the portion of air thickness is increased through surface transition layer improvement. This is a simple and innovative approach to improve the characteristics of DRAM. This study proposed a new direction to improve the Cb/Cs Characteristic for a sub 20 nm DRAM device.

Reference

[1] Park, J. M., et al. IEEE International Electron Devices Meeting (IEDM) (2015)

[2] Hwang, Yoosang, et al. Solid State Devices and Materials (2012)

[3] Bashir, M. I., et al. Surface and Coatings Technology Vol.327, pg 59-65 (2017)

In recent years, nanoporous thin films are widely studied as an effective way to manipulate the thermal transport within thin-film-based devices. In practice, nanoporous patterns can effectively cut off the heat flow and thus guide the thermal transport along the desired direction. However, a better design of these thermal devices is not addressed, such as thermal cloaking as the thermal counterpart for optical invisibility cloaks. In existing designs based on the Fourier's law, composite materials with varied structures are often introduced to achieve the required location-dependent thermal conductivities to distort the heat flux. At the micro-to nano-scale, such designs are difficult to be implemented and factors such as the interfacial thermal resistance must be further considered. In this work, inverse thermal designs of a nanoporous material are used to achieve the thermal cloaking effect for 2D materials or 3D thin films, without introducing any other variation of the composition or material to tune the local thermal conductivity. This simple approach can be widely used for thin-film-based devices to protect heat-sensitive regions or function as thermal camouflaging devices. The proposed nanoporous structures can also be used to tune the local properties of a thin film for general applications, such as graded thermoelectric materials.

When a magnetic field larger than a lower critical field is applied to a type-II superconductor, magnetic fluxes will pass through the volume of the superconducting material, creating a vortex state. Vortices exhibit a rich structural diagram, with the possibility of vortex liquid, glass, and lattice states determined by the material properties and external parameters. On a fundamental level, single and few vortex manipulation opens a pathway to understand vortex properties and to tailor their dynamics to prospective applications in quantum computing.

We will discuss two experiments using scanning tunneling microscopy (STM) which demonstrate direct control over vortex positions in FeSe superconductor. First, we observed that the twin boundary in the FeSe superconductor traps a relatively high density of vortices and acts as a barrier that aligns the vortices on the terrace parallel to the twin boundary. The alignment effect causes various phases of vortex lattice structures such as rectangular and one-dimensional vortex lattices – both with ordering qualitatively different from the commonly observed vortex glass. The analysis of these interactions also reveals further clues regarding vortex dynamics, inter vortex interaction, and vortex pinning, and directly points to strain fields as possible effective approach for accurate control over vortex structures. Second, we found that the vortex shape can be controllably varied with the imaging conditions in STM, particularly at the extreme limit of very large local current density. We attribute the observations as direct evidence of vortex manipulation, by contrasting the behaviors of various types of vortices in proximity to twin boundaries on FeSe. Altogether we suggest that precise control over the high tunneling current, combined with specific structural topological defects, can translate into an effective strategy for vortex manipulation approach without destruction of the superconducting state, enabling STM to become a quantitative nanoscale probe of vortex dynamics and a platform to explore vortex manipulation in the context of topological quantum computing.

Work supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Experiments were carried out as part of the user project at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, which is a US Department of Energy Office of Science User Facility.

S. Y. Song, C. Hua, L. Bell, W. Ko, H. Fangohr, J. Yan, G. B. Halász, E. F. Dumitrescu, B. J. Lawrie, P. Maksymovych, Nano Lett.23(2023)2822.

While ascorbic acid (AA) has been the ubiquitous choice as a mild reductant in the aqueous synthesis of noble metal nanoparticles (NPs), its role beyond the reducing powers has been mostly overlooked. Despite the fact that reduction kinetics of metal salt precursors are most susceptible to modifications by reducing agents, it has frequently been relegated to concentration and pH variations for AA, without delving into the more complex chemistry associated with the latter. The mechanism behind AA-mediated reduction is often stymied by an oversimplified presentation in literature wherein the diprotic acid is assumed to undergo consecutive oxidations via a radical intermediate to form dehydroascorbic acid (DHA). In reality, these assumptions may not accurately describe the aqueous behaviour of AA in a wide array of synthetically relevant conditions.

To overcome the limitation of conventional planar flash memory in scaling down, gate-all-around (GAA) charge trapping flash (CTF) memory gradually become the most promising alternative due to remarkably larger storage and less disturbance. However, as stacking more layers vertically and getting smaller in feature size, it is inevitable that device failures attributed to interference or leakage such as band-to-band tunneling (BTBT) and hot carrier injection (HCI) increase rapidly. Furthermore, a suitable framework to analyze failure mechanisms and optimize design by using circuit simulation is insufficient. In this paper, we proposed circuit-level device modeling as a framework focused on HCI failure analysis in GAA CTF, which is achieved by establishing new method in measuring HCI effect and optimizing model parameters by derived formula. As shown in Fig. 1, threshold voltages in non-programmed memory cell can be increased unexpectedly by HCI effect, which results in critical failure and degradation of performance. In conventional junction-less GAA CTF memory devices, electrons generated by BTBT in a large gap in boosted channel potential and high electric field in vertical direction contribute to HCI effect. As a result, circuit-level modeling for accurate calculation of boosted channel potential and determination of gate voltages in a cell string is essential in analyzing HCI effect. Fig. 2(a) and (b) illustrate single modeling structure and equivalent circuit attaching voltage-controlled-current sources which execute essential current flow in GAA CTF such as Fowler-Nordheim (FN) tunneling, BTBT and HCI leakage by using derived formula in our work. By connecting single modeling units serially as shown in Fig. 2(c), circuit-level modeling structure with a cell string unit is proposed which makes prediction of boosted channel potential and BTBT/HCI current more accurate. Fig. 3 presents mechanisms of HCI in the proposed experimental method. Since the amount of HCI leakage is determined by 2 dominant conditions which are potential difference in lateral direction generating BTBT and electric field in vertical direction, we separated 2 measurement conditions, as shown in Fig. 3(a) and (b), to extract model parameters compactly by breaking down 2 terms. Furthermore, since HCI failure commonly occurs at the end of channel, we enhanced experimental progress by measuring near gate select line (GSL) transistor, as shown in Fig. 3(c). Finally, the fitting results show good agreement with experimental data, as shown in Fig. 4, which means the proposed model would equip circuit designer of GAA CTF by analyzing failure mechanism and optimizing the performance.

Due to the importance of oxide surfaces in heterogeneously catalyzed oxidation reactions, it is necessary to gain a fundamental understanding and behavior of oxygen on transition metal surfaces. Additionally, the atomic arrangement of the metal surface plays an important role in the behavior of the oxygen on the surface, which provides a need to study high defect density surfaces that are more akin to industrial catalysts. The research presented herein utilizes a curved rhodium crystal c-Rh(111) with two different well-defined defects on either side to conduct a systematic study of the influence of defect geometry on the kinetics and dynamics of different oxygen species present on the surface. Scanning tunneling microscopy (STM), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and Meitner-Auger electron spectroscopy (MAES) will be used to look at the surface on an atomic scale and to observe what chemical species are on the surface after introducing oxygen into the vacuum environment.