Oluwaseyi Balogun

The frequency domain thermoreflectance (FDTR) technique is a nondestructive method for thermal characterization and imaging with microscale spatial resolution. The technique relies on a pump laser to generate a modulated temperature change in a sample and a probe laser to monitor the sample's local thermal response. The sample's thermal properties are determined by fitting a thermal model to the sample's thermoreflectance response. This article presents detailed protocols for implementing the technique and conducting local measurements of a substrate's local thermal conductivity. Special attention is devoted to discussing how the measurement is influenced by the laser source parameters, error sources in an FDTR measurement, and quantifying the measurement uncertainty. Finally, we discuss a thermal conductivity imaging experiment conducted near a single vertical interface between two surface-activated bonded single-crystal silicon substrates. The thermal conductivity at the interface is suppressed by 3% compared to the bulk value in the adjoining single crystals. The ability to probe interface thermal properties with the FDTR technique may facilitate local studies of heat flow around individual grain boundaries, particularly in thermoelectric materials, where they can be tuned to optimize the performance of thermoelectric devices.

Abstract The ability to control material heat transport properties over space and time can drive advanced functionalities in thermal management for electronics and system‐on‐chip, and enable thermal circuits. Despite the technological relevance, there are limited demonstrations of local thermal property control. Rotating lattice single (RLS) crystals—formed via laser‐induced crystallization of an amorphous substrate—offer a novel avenue for local crystal engineering, unlocking opportunities for microscale property patterning. Here, thermal conductivity (𝜅) imaging is applied to RLS crystals of Sb 2 S 3 to resolve microscale 𝜅 variations across patterned regions. Amorphous areas exhibit 𝜅 as low as 0.6 Wm −1 K −1 , while crystalline regions display periodic 𝜅 variations from 0.7 to over 2.5 Wm −1 K −1 . These variations correspond to changes in crystal orientation, revealing marked 𝜅 anisotropy. The crystal out‐of‐plane direction (c axis)—featuring van der Waals bonds—shows amorphous‐like transport, whereas in‐plane directions (a, b axes) exhibit 3.5x and 1.7x larger 𝜅, respectively. First‐principles calculations, in excellent agreement with experiments, suggest that the in‐plane anisotropy originates from expressed Sb lone pairs, which impart a corrugation along the b axis affecting bond stiffness and 𝜅. These findings demonstrate microscale control of thermal properties via laser‐processed metastructures, with significant implications for next‐generation thermal management.

Recently, concerns have arisen over the use of most organic and inorganic substances as corrosion inhibitors due to their expensive and toxic nature. This has necessitated the search for alternative cheap and eco-friendly inhibitors. Nanoparticles, especially trimetallic nanoparticles has of recent found unique applications in various field especially in corrosion studies owing to its superior synergistic properties compared to its monometallic and bimetallic counterparts. This study addresses the need for eco-friendly inhibitors by biosynthesizing novel Ag-Cu-Al trimetallic nanoparticles using aqueous leaves extract of Hierochloe odorata. UV-Vis analysis revealed a peak maximum of 405 nm, FTIR characterization identified the possible biomolecules responsible for biosynthesis of the nanoparticles, XRD revealed an average crystalline particle size of 36 nm while SEM-EDS analysis revealed the cylindrical nature of the nanoparticles. The effect of Ag-Cu-Al nanoparticles on corrosion of mild steel in 1 M HCl was investigated by weight loss method. The corrosion rate of the steel sample was observed to decrease with increase in concentration of the nanoparticles and increased with increasing temperature, while the inhibition efficiency increased with increasing inhibitor concentration and decreased with increasing immersion time and temperature. The highest inhibition efficiency under optimum conditions was found to be 95 %. Kinetic data revealed a first order kinetic reaction while thermodynamic investigations showed that Ag-Cu-Al NPs were spontaneously adsorbed onto the mild steel surface by mixed adsorption (physical and chemical adsorption); the Langmuir adsorption isotherm model was found to have the best linear relationship. The process was endothermic as confirmed by the positive values of ΔH. SEM analysis of mild steel samples revealed the presence of adsorbed deposits of Ag-Cu-Al NPs which indicates the adsorption of the inhibitor onto the mild steel surface confirming its effectiveness in protecting the mild steel surface used in petroleum industries.

thermal conductivity between grain boundaries?

In this work, we present a systematic study of the dependence of the gap-mode tip-enhanced Raman scattering (TERS) response of the mono- and bilayer WS 2 and MoS 2 on silver as a function of the excitation laser energy in a broad spectral range from 473 to 830 nm. For this purpose, we collected consecutive TERS maps of the same area in the sample containing mono- and bilayer regions with the same TERS probe with 6 different excitation lasers. To decrease the number of collected TERS maps, we used for the first time, to the best of our knowledge, concurrent excitation and collection with two lasers simultaneously. We found that the E 2g /A 1g peak intensity ratio for the bilayer WS 2 @Ag and the ratio of the A′/A 1g peak intensity of the out-of-plane mode for the mono- and the bilayer change in a significantly nonmonotonous way as the excitation laser energy is swept from 1.58 to 2.62 eV. The former ratio increases at energies corresponding to A and B excitons (∼2.0 and 2.4 eV, respectively) in bilayer WS 2 . The absolute intensity of the A′ peak in the monolayer, and correspondingly the A′/A 1g ratio, is surprisingly high at lower excitation energies but dips dramatically at the energy corresponding to the A exciton, being restored partially in between A and B excitons, but still showing the descending trend as the excitation laser energy increases. A somewhat similar picture was observed in mono- and bilayers of MoS 2 @Ag, though the existing set of excitation lasers did not match the excitonic profile of this material as nicely as for the case of WS 2 . We attribute the observed behavior to the presence of intermediate (Fano resonance) or strong (Rabi splitting) coupling between the excitons in transition metal dichalcogenides (TMDs) and the plasmons in the tip–substrate nanocavity. This is akin to the so-called “Fano” (Rabi) transparency experimentally observed in far-field scattering from TMDs between two plasmonic metals. The possibility of the formation of intermediate/strong coupling between the excitonic resonances in TMDs and the nanocavity reevaluates the role of various resonances in gap-mode TERS, and should become an important factor to be considered by TERS practitioners during experiment planning. Finally, based on the observed phenomena and their explanation, we propose the “ideal” substrate for efficient TERS and tip-enhanced photoluminescence (TEPL) measurements.

This research explores the rheological behavior of cassava starch, focusing on its dilatant and pseudoplastic responses under varying shear stresses and shear rates, with and without the influence of a magnetic field. The study aims to empirically validate theoretical projections by determining the power law exponent, which characterizes the starch’s flow behavior, and to examine its viability for functional applications, particularly in protective materials such as military gear. Two cassava starch varieties were locally obtained and manually processed. A base sample of 20g was prepared and subjected to rheological evaluation. Subsequently, iron filings were incorporated at 2g and 4g concentrations to investigate the influence of magnetic particulates on viscosity. Flow behavior analysis was conducted by applying the linearized form of the power law model, and data visualization was executed using Microsoft Excel. Findings demonstrate that cassava starch at a 20g concentration displays shear-thinning (pseudoplastic) behavior. Upon the addition of iron fillings, an increase in the power law exponent was observed, signifying heightened apparent viscosity and greater resistance to flow under applied stress. These results suggest that magnetic additives significantly influence the rheological profile of cassava starch, enhancing its potential as a tunable non-Newtonian fluid. The study concludes that cassava starch, inherently pseudoplastic, can be effectively modified with magnetic components to exhibit tailored flow characteristics, offering promising implications for smart materials designed to absorb and dissipate impact. Future investigations are recommended to refine the formulation and assess its integration into real-world protective systems.

ABSTRACT OBJECTIVES Imaging phantoms for training and validation are vital to improving the performance and adoption of ultrasound imaging modalities in clinical and pre-clinical applications, and the goal of this study was to assess the viability of 3D printed muscle ultrasound phantoms to meet this need. METHODS We used a soft stereolithography resin to 3D print phantoms that mimicked the fascicle- and perimysium-scale structure of skeletal muscle and compared the long axis B-mode imaging quality and pattern of the phantom to that of healthy, adult Biceps brachii. We used a pulse-echo, time-of-flight method to measure the acoustic impedance of the resin for comparison to skeletal muscle and common soft tissue mimicking materials. We analyzed the echo intensity (EI) of muscle images to establish a physiological range and compared the EI of different phantom designs to assess the ability to control imaging brightness through structural modification. RESULTS A linear, striated hyper-/hypo-echoic B-mode imaging pattern mimicking long axis Biceps brachii muscle images was achieved with two 3D structure paradigms, rod and honeycomb. Acoustic impedance of Elastic 50A resin is higher than skeletal muscle in bulk, but appears suitable for use in a 3D structured phantom. EI measured in the Biceps images were found to vary both within and across images with an overall mean ± SD of 87 ±13 AU. EI measured in honeycomb phantoms (55 ±15 AU) was higher than in rod phantoms (42 ±13 AU), and a latticed honeycomb further increased EI (90 ±11 AU). CONCLUSIONS This study serves as proof-of-concept for soft, 3D printed phantoms that replicate the characteristic muscle ultrasound imaging pattern with the ability to tune clinically relevant EI values via structural design.

We present a systematic study of the dependence of gap mode tip-enhanced Raman scattering (TERS) of mono- and bi-layer WS$_2$ and MoS$_2$ as a function of excitation laser energy. We collected consecutive TERS maps of mono-and bi-layer regions with 6 different excitation lasers. To decrease the acquisition time, we used for the first time concurrent excitation and collection with two lasers simultaneously. We found that the E$_{2g}$/A$_{1g}$ peak intensity ratio for bilayer WS$_2$@Ag and the A'/A$_{1g}$ peak intensity ratio of the out-of-plane modes for mono- and bilayer change in a significantly non-monotonous way with excitation laser energies from 1.58 to 2.62 eV. The former ratio increases at energies corresponding to A and B excitons in bilayer WS$_2$. The intensity of the A peak in the monolayer, and hence the A/A$_{1g}$ ratio, is surprisingly high at low excitation energies, dips dramatically at energy corresponding to the A exciton, and is restored partially in between A and B excitons, though still showing a descending trend with increasing energy. A similar picture was observed in mono- and bi-layer MoS$_2$, though the existing set of lasers did not match its excitonic profile as nicely as for WS$_2$. We attribute the observed behavior to intermediate (Fano resonance) or strong (Rabi splitting) coupling between the excitons in transition metal dichalcogenides (TMDs) and the plasmons in the tip-substrate nanocavity. This is akin to the so-called Fano (Rabi) transparency experimentally observed in far field scattering from TMDs between two plasmonic metals. The possibility of intermediate/strong coupling between excitonic resonances in TMDs and the nanocavity re-evaluates the role of resonances in gap-mode TERS and should become an important factor to be considered by TERS practitioners when planning experiments. Finally, we propose the ideal substrate for efficient TERS and tip enhanced photoluminescence measurements.

Abstract Engineering microstructural defects, like grain boundaries, offers superior control over transport properties in energy materials. However, technological advancement requires establishing microstructure‐property relations at the micron or finer scales, where most of these defects operate. Here, the first experimental evidence of thermal resistance for individual silicon grain boundaries, estimated with a Gibbs excess approach, is provided. Coincident site lattice boundaries exhibit uniform excess thermal resistance along the same boundary, but notable variations from one boundary to another. Boundaries associated with low interface energy generally exhibit lower resistances, aligning with theoretical expectations and previous simulations, but several exceptions are observed. Transmission electron microscopy reveals that factors like interface roughness and presence of nanotwinning can significantly alter the observed resistance, which ranges from ∼0 to up to ∼2.3 m 2 K/GW. In stark contrast, significantly larger and less uniform values ‐ from 5 to 30 m 2 K/GW ‐ are found for high‐angle boundaries in spark‐plasma‐sintered polycrystalline silicon. Further, finite element analysis suggests that boundary planes that strongly deviate from the sample vertical (beyond ∼45°) can show up to 3‐times larger excess resistance. Direct correlations of properties with individual defects enable the design of materials with superior thermal performance for applications in energy harvesting and heat management.

Biofilms are complex biomaterials comprising a well-organized network of microbial cells encased in self-produced extracellular polymeric substances (EPS). This paper presents a detailed account of the implementation of optical coherence elastography (OCE) measurements tailored for the elastic characterization of biofilms. OCE is a non-destructive optical technique that enables the local mapping of the microstructure, morphology, and viscoelastic properties of partially transparent soft materials with high spatial and temporal resolution. We provide a comprehensive guide detailing the essential procedures for the correct implementation of this technique, along with a methodology to estimate the bulk Young's modulus of granular biofilms from the collected measurements. These consist of the system setup, data acquisition, and postprocessing. In the discussion, we delve into the underlying physics of the sensors used in OCE and explore the fundamental limitations regarding the spatial and temporal scales of OCE measurements. We conclude with potential future directions for advancing the OCE technique to facilitate elastic measurements of environmental biofilms.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings, and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted by employing a Gibbs excess approach - is found to be correlated with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices.

Two-dimensional (2D) semiconductors exhibit unique physical properties at the limit of a few atomic layers that are desirable for optoelectronic, spintronic, and electronic applications. Some of these materials require ambient encapsulation to preserve their properties from environmental degradation. While encapsulating 2D semiconductors is essential to device functionality, they also impact heat management due to the reduced thermal conductivity of the 2D material. There are limited experimental reports on in-plane thermal conductivity measurements in encapsulated 2D semiconductors. These measurements are particularly challenging in ultrathin films with a lower thermal conductivity than graphene since it may be difficult to separate the thermal effects of the sample from the encapsulating layers. To address this challenge, we integrated the frequency domain thermoreflectance (FDTR) and optothermal Raman spectroscopy (OTRS) techniques in the same experimental platform. First, we use the FDTR technique to characterize the cross-plane thermal conductivity and thermal boundary conductance. Next, we measure the in-plane thermal conductivity by model-based analysis of the OTRS measurements, using the cross-plane properties obtained from the FDTR measurements as input parameters. We provide experimental data for the first time on the thickness-dependent in-plane thermal conductivity of ultrathin MoS 2 nanofilms encapsulated by alumina (Al 2 O 3 ) and silica (SiO 2 ) thin films. The measured thermal conductivity increased from 26.0 ± 10.0 W m –1 K –1 for monolayer MoS 2 to 39.8 ± 10.8 W m –1 K –1 for the six-layer films. We also show that the thickness-dependent cross-plane thermal boundary conductance of the Al 2 O 3 /MoS 2 /SiO 2 interface is limited by the low thermal conductance (18.5 MW m –2 K –1 ) of the MoS 2 /SiO 2 interface, which has important implications on heat management in SiO 2 -supported and encased MoS 2 devices. The measurement methods can be generalized to other 2D materials to study their anisotropic thermal properties.