AVS 72 Session NS-ThP: Nanoscale Science and Technology Poster Session

Nanostructured carbon used as solid contact enables stable, calibration-free all-solid-state ion-selective electrodes (SC-ISEs) in miniaturized formats. It is typically coated on top of a metallic electrode, such as a microneedle, followed by an ion-selective membrane (ISM) coating. When this microneedle is inserted into artificial skin, the coatings do not always stay intact. Here we investigate an alternate approach by preparing self-supporting mesoporous carbon-based microneedles, which can be infiltrated and covered with the ISM. We hypothesize that this helps to maintain the ISM coating during insertion of the microneedle into skin. Silicone molds were prepared from steel microneedle arrays and then infiltrated with a resol containing colloidal silica as sacrificial template for the mesopores. After polymerization and carbonization, the resulting microneedle arrays were etched to remove the silica, introducing mesopores. Structural features were verified by scanning-electron microscopy. Future work will incorporate reference membranes and assess ion selectivity under physiological conditions, advancing SC-ISE microneedles for real-time on-skin or in vivo ion monitoring.

Understanding structure-reactivity relationships in heterogeneous catalysis requires well-defined model systems that enable atomic-scale characterization. In this work, Rhodium (Rh) nanoclusters were deposited on Highly Oriented Pyrolytic Graphite (HOPG), an inert support, to investigate their nucleation, growth and changes in morphology under ultra-high vacuum (UHV) conditions. By systematically varying Rh coverage (0.09 ML to 0.6 ML), Scanning Tunneling Microscopy (STM) was used to resolve cluster size, morphology, and spatial distribution. Our experiments reveal the formation of highly dispersed Rh nanoclusters with a notably narrow size distribution; for instance, a 0.09 ML deposition produced particles with an average width of 1.74 ± 0.5 nm and height of 0.85 ± 0.1 nm. Crucially, these nanostructures exhibit exceptional thermal stability, maintaining their size and distribution even after annealing to elevated temperatures up to 850 K. Subsequent CO exposure at both 300 K and 500 K allowed for the visualization of surface interactions. While CO induced streaking was observed in STM images due to adsorbate mobility, the Rh clusters remained structurally stable without significant redispersion or morphological changes. This study establishes the robustness of Rh nanoclusters in response to various stimuli that would be present in an industrial reaction, such as high temperatures and gas exposure, further deepening our knowledge of surface catalysis.

This study investigates the characterization and analysis of rhodium oxide (RhOₓ) nanoparticles formed on highly oriented pyrolytic graphite (HOPG), with the aim of understanding how oxygen influences nanoparticle stabilization and behavior on a weakly interacting support. HOPG serves as an ideal model surface for probing intrinsic nanoparticle properties in the absence of strong substrate interactions. Rh nanoparticles are vapor-deposited onto freshly cleaved HOPG under ultra-high vacuum conditions and subsequently oxidized at controlled temperatures (300–850 K) to generate RhOₓ nanostructures. Scanning Tunneling Microscopy (STM) is employed to examine nanoparticle morphology, size distribution, and structural evolution as a function of temperature and Rh coverage. Oxidation at room temperature results in the formation of RhOₓ nanoparticles with an apparent reduction in particle density and surface coverage relative to metallic Rh. At elevated temperatures, larger RhOₓ nanoparticles are observed, with average widths of 9.14 nm. High temperatures like 850 K, result in the formation of triangular channels on HOPG surface, originating from step edges. These features are not due to direct structural failure of HOPG but rather arise from oxygen mediated anisotropic etching of the graphite surface. Oxygen can dissociate on RhO_x which then migrate onto the HOPG surface where the atomic oxygen react with uncoordinated carbon atoms at defect sites and step edges which lead to the removal of carbon atoms as CO/CO₂ and leads to anisotropic etching on surface. Post oxidation exposure to carbon monoxide (CO) is performed to evaluate redox reversibility and stability under reducing conditions, with the evolution of gaseous species such as CO₂, O₂, and CO observed, while the crack and particle size remains unchanged. In parallel, Density Functional Theory (DFT) calculations using (PBE-GGA) model RhOₓ adsorption on graphite, identifying favorable configurations, and electronic properties with nanoparticle stability, reactivity, and catalytic performance on carbon supports.

The performance of photocatalysts, sensors, and oxide-based devices is governed by how charge carriers migrate, recombine, and interact with defects at surfaces and interfaces. Titanium dioxide (TiO₂), a model photocatalyst, provides a versatile platform for exploring these dynamics. Recent advances intime-resolved atomic force microscopy (TR-AFM) now allow direct nanoscale mapping of carrier motion under realistic conditions, uncovering defect-controlled pathways that were previously inaccessible.

We establish findings from our recent studies to build a coherent picture of light-induced defect dynamics in TiO₂. In "thin" TiO₂films, it was shown that ultraviolet irradiation generates photoinduced surface oxygen vacancies (PI-SOVs) that shorten carrier migration time constants but raise activation barriers, indicating a trade-off between faster trapping and reduced mobility. Extending this framework,werevealed that PI-SOVs impact both fast and slow timescales, with spatially heterogeneous distributions leading to locally distinct transport regimes.Moreover, we explored surface chemistry by introducing methanol as a prototypical hole scavenger: methanol adsorption further reduced time constants and modified activation energies, demonstrating how molecular adsorbates tune vacancy-carrier interactions.

By bridging molecular adsorption, defect generation, and irradiation penetration, this body of work highlights spatiotemporal defect engineering as a promising strategy for controlling charge transport in oxides. TR-AFM emerges as a uniquely powerful platform for resolving these processes at the nanoscale, offering pathways to rationally design oxide-based systems with tunable photocatalytic efficiency, sensor responsiveness, and energy-conversion robustness.

Ce-doped TiO₂ nanoparticles were synthesized by modifying commercial Degussa P25 and flame-made TiO₂ produced by flame spray pyrolysis (FSP), followed by thermal treatment at 500 °C. The effects of Ce doping on the structural, morphological, optical, and photocatalytic properties of TiO₂ were investigated for undoped and Ce-doped samples containing 5 and 40 wt% Ce.

X-ray diffraction (XRD) analysis showed that both P25 and FSP TiO₂ consisted primarily of anatase and rutile phases, while Ce doping suppressed the anatase-to-rutile transformation and promoted CeO₂ phase formation at higher Ce loading. Ce doping reduced the anatase crystallite size and altered the phase composition, with brookite contributions observed in CE5-P25 and CE5-TiO₂ samples. SEM and TEM analyses revealed morphological differences between P25 and FSP TiO₂ particles, where P25 exhibited irregular elongated and polygonal particles, while FSP-derived TiO₂ showed predominantly spherical particles. Ce doping decreased particle size to approximately 14–34 nm, whereas high Ce loading (40 wt%) resulted in particle agglomeration and cube-like Ce-rich structures. SAED and HRTEM analyses confirmed the coexistence of anatase and rutile phases and verified lattice fringes corresponding to anatase (101) and rutile (110) planes.

UV–Vis spectroscopy demonstrated enhanced visible-light absorption and a red shift in the absorption edge with increasing Ce content. The bandgap decreased from approximately 3.06 eV for pure TiO₂ to 2.48 eV for Ce-doped samples, indicating improved visible-light response. Photocatalytic performance was evaluated through methylene blue degradation under visible-light irradiation using a Xe lamp with AM 1.5G and a 400 nm cut-on filter. Among all samples, CE5-TiO₂ exhibited the highest photocatalytic activity, achieving approximately 90% methylene blue degradation under visible light.

The enhanced photocatalytic performance is attributed to reduced crystallite size, suppression of rutile phase growth, modified phase composition, and improved visible-light absorption induced by Ce doping. These results demonstrate that Ce modification effectively enhances the photocatalytic activity of both commercial P25 and flame-made TiO₂ for environmental remediation applications.

Nanoimprint Lithography (NIL) fabrication technique consists of transferring a pattern from a template by pressing it onto a resin coated substrate, yielding structures with nanometer resolution. The resin can be cured thermally or by exposure to UV radiation. This is a simple, low-cost and high-throughput technique, which has been used as an alternative to photolithography in semiconductor industry, optics and photonics, and biomedical field. NIL roll-to-plate method was used in this project to replicate microstructures of Cicada wings which are of interest due to their antibacterial properties. We report the results on the fabrication of these biomimetic structures and on their characterization by scanning electron microscopy.

The continued scaling of ferroelectric tunnel junctions (FTJs) has placed unprecedented importance on understanding and controlling nanoscale surface and interface interactions within ultrathin HfO₂–based ferroelectrics. At thicknesses below 6 nm, the electronic structure, bonding environment, and screening behavior at metal/ferroelectric interfaces dominate the tunneling landscape, often more strongly than the bulk ferroelectric properties themselves. Conventional symmetric Metal-Hf_0.5Zr_0.5O₂-Metal (MFM) stacks such as TiN/HZO/TiN exhibit negligible tunneling electroresistance (TER) because their interfaces screen polarization charges in an identical manner, suppressing any barrier asymmetry. While inserting dielectric interlayers can restore asymmetry, these additional interfaces introduce variability, trap formation, and integration challenges.

This contribution presents a materials–driven strategy that leverages intrinsic nanoscale surface and interface asymmetry between dissimilar metal electrodes to enhance FTJ performance without modifying the ferroelectric layer. We focus on five CMOS compatible metals such as TiN, W, Ru, TaN, and Mo about possessing distinct electronic screening behavior, interfacial bonding characteristics, and surface chemistries with HZO. These differences affect how polarization-bound charges are compensated at the interface, reshaping the tunneling barrier.

To assess these nanoscale interfacial effects, multiple devices including Ta/HZO/TaN were developed and their nanoscale interfacial structures and chemistry were compared in agreement with theoretical research for MFM devices. This study incorporates realistic ferroelectric switching, nonlocal tunneling transport, and metal–specific screening behavior, enabling direct correlation between nanoscale interface physics and device–level performance. Surface and interface characteristics with multiple HZO thicknesses were quantified to confirm how interfacial screening, depolarization stability, and surface-induced barrier shaping collectively determine TER, read current, and write voltage.

Our results show that electrode pairs with different interfacial screening length create strong asymmetry in the tunneling barrier, enabling enhanced TER without the need for additional interlayers. Energy band analysis reveals how nanoscale surface interactions such as metal-oxygen bonding, interfacial dipole formation, and screening charge distribution directly affect the tunneling mechanism in opposite polarization states. Finally, we mapped optimal metal combinations that support low–voltage switching and stable ferroelectric behavior <6nm HZO for interface–engineered MFM devices.

In neuromorphic and memory devices, accessible multistability is enabled by the dynamics of the constitutive materials rather than by their equilibrium structures alone. Hysteresis loops, oscillatory responses, and other time-dependent signatures reflect the multiplicity of accessible material states. Yet deciphering the underlying dynamics remains persistently elusive — in no small part because the dynamic information itself is treated qualitatively rather than as a quantitative observable.

I will present a systematic approach based on time-delayed embedding and dynamic mode decomposition that treats dynamic data as a structured object, extracting its nonlinearity, topology, and a predictive numerical model of the underlying process. Applied to resistive switching in memory oxide materials [1,2], nonlinearity alone can discriminate between specific microscopic mechanisms. In ferroelectrics, the same framework separates genuine switching from spurious hysteresis and detects the signatures of a multiwell potential [3,4]. Effective denoising strategies extend the approach to noisy nanoscale and device measurements without loss of physical interpretability.

Time-delayed methodologies provide a transparent and interpretable connection between hysteresis loops and physical mechanisms, complementing machine learning methods and enabling parameter-free comparison between modeling and experiments of driven dynamics. R

esearch sponsored in part by Division of Materials Science and Engineering, Basic Energy Sciences, Office of Science, US DOE at the Oak Ridge National Laboratory, and, at Clemson University by the State of South Carolina through funding for the Battelle Savannah River Alliance Workforce Development Program.

1. O. Popova, et al., “Nanoscale imaging of He-ion irradiation effects on amorphous tantalum oxide toward electroforming-free neuromorphic functions“,Appl. Phys. Lett. 123, 153503 (2023)

2. S. M. Neumayer et al., “Nanoscale Probing of Electrical Memory Effects in van der Waals Layered PdSe, ACS Applied Materials & Interfaces. 16 (2024) 3665-3673.

3. S. M. Neumayer, et al., "Dynamic Stabilization of Metastable States in Triple-Well Ferroelectric Sn₂P₂S₆", Advanced Materials 20 (2023) 2211194.

4. S. M. Neumayer, et al, “Polarization-controlled volatile ferroelectric and capacitive switching in Sn₂P₂S₆”, Neuromorph. Comput. Eng.3 (2023) 014005.

Homogeneous molecular catalysts provide precise control over the steric and electronic properties of active sites, enabling tunable catalytic activity and selectivity. However, these systems often suffer from limited stability, poor recyclability and reduced long-term durability under homogeneous operating conditions. Consequently, strategies that combine the tunability of molecular catalysts with the robustness of heterogeneous platforms have attracted considerable attention. Despite advances in catalyst heterogenization, developing structurally well-defined systems that bridge homogeneous and heterogeneous catalysis remains a challenge.

Porphyrin-based systems have emerged as attractive molecular platforms owing to their structural versatility, electronic tunability and catalytic relevance. Among them, iron porphyrins are particularly promising for electrocatalytic CO₂ reduction through successive electron-transfer processes that have demonstrated efficient, CO-selective and durable catalytic performance.

Understanding molecule-surface interactions at the nanoscale is essential for rational catalyst heterogenization. Scanning tunneling microscopy (STM), with submolecular and atomic-scale spatial resolution, provides a powerful platform for directly visualizing molecular arrangements, conformations, electronic structures and intermolecular interactions at surfaces. STM has proven especially effective for investigating the adsorption behavior and self-assembly of porphyrin systems on conductive substrates.

The present study investigates the adsorption behavior of iron tetraphenylporphyrin (FeTPP) on highly oriented pyrolytic graphite (HOPG) using STM as the primary investigative technique. By correlating molecular structure with surface organization, intermolecular interactions, and local electronic properties, this work aims to provide fundamental insights into molecule-surface interactions relevant to catalyst heterogenization and to establish structure–property relationships for the development of heterogenized iron porphyrin systems for efficient electrocatalytic CO₂ reduction.

References:

S.Rana, N.Arisnabarreta, A.Salamé, et al. “Metalloporphyrin Monolayers as Tunable Platforms for CO2 Electroreduction.” Chemistry – A European Journal32, no. 9 (2026): e03187. https://doi.org/10.1002/chem.202503187

The confinement of charge carriers within an anisotropic two-dimensional (2D) geometry leads to reduced, variable dielectric screening, which gives rise to strongly bound excitons, correlated electron phases, and band gap renormalization. We employ electronic structure probes (photoelectron and electron energy loss spectroscopies) to assess the impact of the supporting substrate and the sulfur vacancy defects on the screening characteristics of the 2D semiconductor WS₂. Exploiting the contrasting external dielectric screening environments achieved in gold-supported and free-standing (i.e. suspended) configurations, we show how the electronic states of one-layer WS₂ align at a built-in junction across the effective and ineffective screening environments. Photoelectron spectroscopy points to the close alignment of the charge neutrality levels of WS₂ between each screening environment. We further reveal through ion irradiation that the introduction of a modest defect density (n_V) of 1×10¹³ cm^-2 reduces the electronic band gap from 2.4 eV to 2.2 eV in poorly-screened suspended WS₂ because of the exciton binding energy reduction. The band gap renormalization accompanies the diminishing photoelectron core-hole relaxation and the expansion of the screened radius beyond the inter-defect distance around 4 nm when n_V reaches ~8×10¹² cm^-2. Collectively, these findings provide key insights into the interrelationships of the polarizability of WS₂ with its electronic behavior and photoemission process.

We thank P. Mantos for measurement support. A.B., C.M.S., D.B., T.L., C.D.S., and T.O. acknowledge support from the Laboratory Directed Research and Development program at Sandia National Laboratories. A.R.K. acknowledges support from the U.S. Department of Energy, Office of Science, Division of Materials Sciences and Engineering (grant BES 20-017574). J.J.F. and J.T.R. were supported through Base Programs and the Nanoscience Institute at the Naval Research Laboratory via the Office of Naval Research. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly-owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This abstract describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the abstract do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Despite long-standing efforts to understand the interface between epitaxial graphene and the Si-terminated silicon carbide (0001) substrate, the exact atomic-scale structure, its complex bonding configurations, and the resulting electronic properties of the first epitaxial graphene carbon layer (C_buffer) remain open problems. We will present how to control the local structure of the epitaxial graphene-SiC interface with a low-temperature scanning tunneling microscope (STM) [1]. We will show that the covalent bonds between C_buffer and the SiC substrate may be reversibly broken and restored using the polarity of the electric field from the STM tip, generating protrusions at the interface of slightly higher amplitude. This buried interface manipulation enabled us to pattern epitaxial graphene with lateral precision, reaching the scale of a single 1.8 nm unit cell of the graphene-SiC interface (6×6)_SiC moiré lattice. However, the STM manipulation experiments also provide direct evidence of silicon vacancies at the topmost reconstructed SiC(0001) layers [2]. Bias-voltage- and epitaxial graphene thickness-dependent characterization of the collective C_buffer-SiC interface showed that 'Si' vacancy sites exhibit stable, non-switching behavior under STM electric fields. Moreover, vacancies introduce localized electronic states below the Fermi level. Our experimental results directly elucidate the atomic-scale interface and the influence of atomic and bonding configurations on the local density of states for graphene of different thicknesses on SiC(0001). Finally, we will present the consequences of the local epitaxial graphene structure on electronic transport properties.

Acknowledgements: This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division at the Ames National Laboratory, which is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358. M.K. and U.T. acknowledge support through a DOE Early Career Project.

References:

[1]M. Kolmer, J. Hall, S. Chen, S. Roberts, Z. Fei, Y. Han, and M. C. Tringides, Atomic-scale manipulation of buried graphene–silicon carbide interface by local electric field, Communications Physics, 7, 16 (2024).

[2]U. Thupakula, S. Chen, Y. Han, M. C. Tringides, and M. Kolmer, Subsurface Interface Structure Controlling Local Electronic Properties of Epitaxial Graphene on SiC(0001), The Journal of Physical Chemistry Letters, 16, 10487−10498 (2025).

A unique photoinduced modulation doping is achieved via integration of common 2D materials and inexpensive light sources. Under applied electric field and incident light, the doping level of 2D materials heterostructures consisting of multiple dielectric layers can be precisely controlled and tuned in situ. This photodoping also offers local control of charge with submicron spatial resolution with lifetimes on the order of months to years. For 2D materials systems there is no current agreement on the exact mechanism underlying this effect, with studies pointing to charge generation and trapping in various layers/interfaces. In this work, we present photoinduced doping of van der Waals heterostructures with various compositions in order to pin-point the exact spatial location of the photoactive defects that responsible for the doping phenomena. Regardless of the mechanistic details, this photodoping effect yields high-quality reconfigurable devices in a variety of 2D material systems including graphene, TMDs, and 2D superconductors.

Reference

Son T. Le et al., 2D Mater. 12 015006 (2025)

Exciton-polaritons (EPs), hybrid light–matter quasiparticles formed by strong coupling between excitons and confined photons, offer a route to engineer semiconductor optical response without chemical modification. Here, I present strong and ultrastrong coupling in metal-organic chalcogenolates (MOCs), layered van der Waals hybrid semiconductors that act as natural multi-quantum-well structures with strong excitons, large oscillator strengths, and high refractive indices. These properties make MOCs promising self-hybridized polaritonic materials from the ultraviolet (UV) to visible range.

I will first discuss silver phenylselenolate (AgSePh, mithrene), which has a direct optical bandgap of ~2.65 eV, exciton binding energy of ~400 meV, and refractive index of ~2.5. Mueller matrix ellipsometry reveals giant in-plane birefringence (~1.01) and three anisotropic exciton resonances, enabling polarization-dependent linear dichroism up to ~77% when coupled to an optical self-cavity. Thickness-controlled mithrene flakes on reflective substrates support self-hybridized EPs in the ultrastrong coupling regime, with normalized coupling g/Ex = 0.13–0.14 and Rabi splitting >650 meV, the largest reported for a non-organic semiconductor in the visible range.

I will then present self-hybridized EP photodetectors in mithrene, where thickness-tunable multimode polariton states enable sub-bandgap photodetection extending >130 nm (~0.55 eV) below the optical bandgap. Trap-assisted two-photon absorption sustains strong coupling under sub-bandgap excitation, addressing a key limitation for polaritonic light harvesting. The polariton dispersion yields ultrafast group velocities (~65 μm ps−1), extending effective exciton diffusion from hundreds of nanometers to several micrometers and producing a 2.38-fold enhancement in photo-to-dark current ratio.

Finally, I will extend these concepts to the UV using silver phenylthiolate (AgSPh, thiorene), a wider-bandgap MOC with a sharp excitonic resonance at 3.46 eV (~60 meV linewidth) and refractive index of ~2.1. In open and closed cavities, thiorene shows clear anticrossing with Rabi splittings of 488 and 512 meV, respectively, among the largest reported in the UV and 6–10× larger than conventional ZnO and GaN platforms. Together, these results establish MOCs as a versatile platform for polariton-enhanced optoelectronics.