Analog in-memory computing (AIMC) is an emerging paradigm that can enable energy efficient computing orders of magnitude beyond what is currently possible. Memory candidates for AIMC include SONOS (semiconductor oxide nitride oxide nitride), emerging resistive memory (ReRAM) and electrochemical memory (ECRAM). Electrical requirements for these memories are different than traditional digital memories in that the exact conductivity state of every device is used in every calculation. Effects including programming error and state drift are incorporated in the algorithm output. This new set of requirements has forced the development of a novel, holistic methodology for the electrical characterization and benchmarking of these devices. This talk will discuss these characterization and benchmarking methodology, and its application to SONOS, ReRAM, and ECRAM. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures and making outstanding progress on fundamental studies, practical applications of 2D materials will require a broader integration strategy. I will present our ongoing and recent work on integration of 2D materials with 3D electronic materials to realize logic switches and memory devices with novel functionality that can potentially augment the performance and functionality of Silicon technology. First, I will present our recent work on gate-tunable diode and tunnel junction devices based on integration of 2D chalcogenides with Si and GaN. Following this I will present our recent work on non-volatile memories based on Ferroelectric Field Effect Transistors (FE-FETs) made using a heterostructure of MoS2/AlScN, and also introduce our work on Ferroelectric Diode (FeD) devices also based on thin AlScN. In addition, I will also present how FeDs provide a unique advantage in compute-in-memory (CIM) architectures for efficient storage, search as well as hardware implementation of neural networks. I will conclude by providing a broad and optimistic outlook for integration of novel materials and devices in future classical computing chips.

The connection between device function and fundamental physical processes is a long-standing challenge. In the case of resistive and ferroelectric switching at the heart of emerging computing devices, the challenge is further exacerbated by non-equilibrium nature of the underlying dynamics. Scanning probe microscopy is a very promising technique, that can bridge the nanoscale gap between idealized atomic models and the functional scale of information devices. I will present our on-going work on methods of statistical analysis for multimodal switching microscopy, that enable robust inference of polarization dynamics in multiwell ferroelectrics enabling memcapacitive behavior and direct measurement of ionic processes underpinning resistive switching in amorphous oxides.

Resistive switching (RS) devices are key elements for future neuromorphic computing systems as they can emulate biological synaptic functions as well as short/long term memory effects. However, there are various challenges at the device level because of their operational stability, uniformity, and device-to-device variability, which are often originated by the switching mechanisms. Here, I will discuss on how fully interface controlled memristive devices can solve these issues. I will also discuss an example of Au/Nb:STO model system, which works based on interface charge trapping/detrapping and Schottky barrier modulation mechanism. Such device also shows analog RS switching characteristics with high stability, uniformity, and programmability, making it suitable for artificial synapse and neuromorphic computing.

Scattering-type scanning near-field optical microscopy (s-SNOM) is a useful tool for the non-destructive investigation of buried confined electron systems with nanoscale resolution, however, a clear separation of carrier concentration and mobility was often not possible. Here, we predict a characteristic (“fingerprint”) response of the LaAlO3/SrTiO3 2DEG in the mid-infrared spectral range, which was not experimentally accessible in the past, and verify this using a state-of-the-art tunable narrow-band laser in cryo-s-SNOM at 8 K. Our modelling allows us to separate the influence of carrier concentration and mobility on fingerprint near-field spectra, which we use to characterize 2DEG inhomogeneities on the nanoscale.

This talk will highlight recent efforts in my group that are aimed at characterizing the vibrational and electronic properties of low-dimensional material systems with joint femtosecond temporal and nanometer spatial resolution. I will first discuss commonly used techniques, such as TERS and TEPL, with emphasis on our own contributions to this vibrant field of study. For instance, I will describe recent efforts that enabled the visualization of phonons in a hetero-bilayer of transition metal dichalcogenides (TMDs) with few-nm spatial resolution under ambient laboratory conditions. I will then describe non-standard approaches to tracking excitons in TMDs, e.g., through excitation-tunable electronic 4-wave mixing. Finally, I will describe measurements that rely on nonlinear and interferometric photoemission electron microscopy to enable femtosecond-nanometer visualization of exciton generation and decay in TMDs. Overall, my talk will approach the problem from a measurement science perspective, with an ultimate goal of contributing to this ever-expanding field through the development and deployment of novel state-of-the-art characterization tools that provide detailed insights into the fundamental physics and chemistry of TMDs and related systems.

Flat and narrow band physics in moiré quantum matter has proven to be extremely rich with new emergent quantum phases. The topological properties of the eigenstates of the moiré Hamiltonian are critical for establishing the quantum phase of the system. While the emergence of non-trivial Chern numbers has been observed, it is important to characterize the quantum geometry in detail including Berry curvature and less known quantum metric effects throughout the bands. Using a local probe, we employ magnetic oscillations as a “ruler” for quantum geometry in small-angle twisted double bilayer graphene.

The unique sensitivity of local electronic properties in two-dimensional (2D) materials, such as their bandgap, to the underlying crystal arrangements provide exciting prospects for novel quantum nano-optoelectronic applications. However, the limitations of current characterization techniques in accessing this delicate relationship with nanoscale resolution have made it challenging to understand the impact of local structural phenomena such as strain, thickness, and layer rotation on electronic properties. Here, we present a comprehensive strategy for determining the relationship between bandgap energy and local thickness and strain fields in twisted van der Waals materials. Unveiling these structure/property correlations is crucial for gaining a complete understanding of the rich bandgap dynamics at the nanoscale in these materials. By combining advanced techniques such as electron energy-loss spectroscopy assisted with machine learning and 4D scanning transmission electron microscopy with an Electron Microscopy Pixel Array Detector, we evaluate the bandgap and local strain fields in twisted WS2 with nanoscale resolution. We demonstrate how strain can increase the bandgap by up to 30% in regions with significant twist angles between layers and hence pronounced local strain fields. This approach provides a flexible tool for uncovering the connection between strain and bandgap dynamics in 2D materials and can also be applied to more complex 2D material geometries and heterostructures, contributing to the development of novel technologies for quantum devices.

We report the first time nanoscale, tip enhanced Raman scattering (TERS) imaging of the SeMoS Janus monolayers crystals both as-grown on gold foil and transferred from the growth substrate to the gold-coated silicon wafers. Due to the preferential enhancement of the out-of-plane modes in the gap-mode of TERS, the TERS spectra of SeMoS differ from the conventional Raman spectra reported earlier [1 , 2]. The A11and A12 out-of-plane modes are shown to be the first and the second strongest Raman peaks in TERS, while in conventional Raman spectroscopy the A12 mode is extremely weak. Interestingly, the red shift of the spectral position of A12 mode correlates with a decrease of the contact potential difference in Kelvin probe force microscopy (KPFM) images. While the TERS maps mostly show the Raman spectra characteristic to the high quality SeMoS Janus monolayers, we observed in some cases narrow, below 20-30 nm, areas that featured a peak at 406 cm-1 which has been proved to be the A’ band of MoS2. The ability to detect the nanoscale imperfections in Janus monolayer crystals is a mandatory condition for optimizing their synthetic routes. TERS imaging cross-correlated with KPFM measurements demonstrate the applicability for the nanoscale assessment of the structural homogeneity of both the as-grown and transferred SeMoS Janus monolayer crystals. References 1. Z. Gan, I. Paradisanos, A. Estrada-Real et.al. ADVANCED MATERIALS 2022 34, 2205226 2. Marko M. Petrić, Malte Kremser , Matteo Barbone et.al. PHYSICAL REVIEW B 103, 035414 (2021)

Two-dimensional (2d) nano-electronics, plasmonics, and spintronics require clean and local charge control, calling for layered, crystalline acceptors or donors. Here I will describe how the Relativistic Mott Insulating state of RuCl3, a 2D antiferromagnet, provides a new opportunity to introduce modulation doping into 2D materials. Specifically, we demonstrate and optimize this charge transfer with extensive Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations. Also, we find the doping is exceptionally local, can occur through hBN, and works with various exfoliated, CVD, and MBE materials. Lastly, I will provide evidence that this doping is quite distinct from what is possible in typical MBE heterostructures and can provide doping levels compatible with ionic liquids without the disorder.

The integration of any new material into device architectures necessarily requires interfaces with dissimilar materials. In the case of semiconducting transition metal dichalcogenide (TMDCs) WSe2 the interface with a gate dielectric is extremely important. Presented will be our work on two approaches to WSe2 integration. The first considers the direct growth of WSe2 on and insulating substrate. Here we consider the impact of the WSe2 on the dielectric itself. In the second approach we investigate the deposition of dielectrics by atomic layer epitaxy onto WSe2 with a focus on enhancing nucleation and the considering the impact of surface functionalization on device performance

We discuss a new enhancement effect in the external quantum efficiency (EQE) of AlGaN fin light emitting diodes (LEDs) as the aspect ratio of the fin increases. We show electroluminescence results on single n-AlGaN fin/p-GaN heterojunctions that are arranged in an array format with their aspect ratio increasing from 0.2 to 3, while fin width reduces from 3000 nm to 200 nm. The UV excitonic emission of the AlGaN fins are studied as the aspect ratio increases at a fixed current density. We observe an average 7x increase in the EQE in transitioning from 3000 nm wide fins to 200 nm wide fins. This geometrical advantage allows a 200 nm wide fin to operate at 1/3rd the current density compared to a 3000 nm wide fin while generating a UV emission with a comparable power of 1 microWatt. These results show new parameters that can be used for developing brighter light sources free of efficiency droop at the micro- or nano-scale. Efficiency droop is the decline in internal quantum efficiency with increasing current density, which is one of the significant challenges facing wide bandgap LEDs.

This work investigates the novel electrical and magnetic properties exhibited by nanoscale materials. Here we outline some unique nanomaterials synthesis approaches such as nanoparticles catalyzed vapor-liquid-solid synthesis, doped oxide nanowires synthesis, and high temperature synthesis of nanomaterial. We examine magnetic and electrical properties enabled by the nanodimensions. For example, electrical conductivity is significantly enhanced in Te nanostructures compared to its bulk phase when its thickness is reduced to a few nanometers. We investigate the magnetic properties in oxide nanostructures (Ga2O3, ZnO, ITO). These results provide key new insights to understand the electrical and magnetic properties originated in the nanoscale dimensions. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

Indirect excitons offer a platform to gain an understanding of the realization of the excitonic-based device and the physics of excitonic condensation. Exploiting transport properties of the indirect excitonic requires a large interface where electrons and holes are spatially separated like polytype type-II interface consisting of the same compound, such as zinc blende and wurtzite InP. The indirect excitons are dipoles and aligned at the type-II interface. Indirect excitons transport a long distance before recombination from the excitation spot by the presence of high-density excitons due to the strong repulsive interaction between dipolar indirect excitons.

The two degenerate valleys, K and K’, in 2D transition metal dichalcogenides (TMD) can be used as information carriers for quantum information science and technologies. Obtaining valley polarization in TMDs, which is the equivalent of encoding or writing data, is the first step towards these applications. Here, we report valley polarization in monolayer TMD/chiral perovskite heterostructures at room temperature via spin selective charge transfer. We obtained 7% degree of polarization from the heterostructures after photoexcitation with a linearly polarized laser. We further demonstrate that charge transfer occurs within picoseconds using ultrafast transient absorption spectroscopy. Our results pave the way for practical valleytronics devices based on TMD/perovskite heterostructures.

Photodetection plays a key role in basic science and technology, with exquisite performance having been achieved down to the single photon level. Further improvements in photodetectors would open new possibilities across a broad range of scientific disciplines, and enable new types of applications. However, it is still unclear what is possible in terms of ultimate performance, and what properties are needed for a photodetector to achieve such performance. In this presentation, I will discuss recent theoretical and experimental work to address this question. On the theoretical front, we present a new general framework to establish the fundamental properties of photodetectors from a fully quantum perspective, and show what basic features are needed to achieve high performance. Novel photodetector designs emerge from these considerations, and we present experiments with carbon nanotube devices to test these new designs.

The equivalence of protonic water to an electronic semiconductor is undeniable: each undergoes detailed-balance generation, recombination, and transport of mobile charge carriers driven by gradients in their electrochemical potentials. During my presentation, I will further explain how water is a protonic semiconductor and show that it can be doped through addition of salts of H+ and/or OH– that when fixed in place using polymeric bipolar membrane scaffolds, or by freezing, enables protonic diodes. Their electrochemical evaluation requires fabrication of membrane–electrode–assemblies that drive reversible H2 redox chemistry in order to transduce electronic electrochemical potentials into protonic electrochemical potentials, and vice versa. Analysis of impedance spectroscopy data afforded quantification of the so-called “flatband” potential (i.e. when the electric potential difference across the pn-junction is zero), “electroactive” dopant density, minority carrier collection length, and distribution of quasi-electrochemical potential splitting. Collectively, these efforts form the foundational framework for new devices and functions that benefit from purely protonic transport and reactivity. We are hopeful that it motivates participants to help us expand our platform to protonic versions of other condensed matter phenomena, such as 2D gases, spintronic devices, and topological insulators.

By expressing more than two logic states within given operating voltages without device scaling, multi-valued logic (MVL) circuits perform the same functions much more efficiently with less number of devices compared to binary logic circuits. Such MVL circuits can be often implemented by employing negative transconductance (NTC) devices whose channels are composed of van der Waals (vdW) pn heterojunctions of dissimilar semiconductors. In this work, we will demonstrate NTC devices and ternary inverter circuits using single semiconducting material. The pn homojunction-based devices show anti-ambipolar behavior with NTC characteristics. The resultant ternary inverters with three distinct logic states are successfully demonstrated.

The discovery of complex phenomena in solids has relied upon the creation of stable low-dimensional crystals approaching the atomic scale. While 2D van der Waals (vdW) solids have gained tremendous interest, very little is known about the chemistry and physics of their more confined 1D counterparts. To this end, I will describe our efforts towards elucidating the chemical bonding interactions which define the structure and properties of optically-active crystals comprised of vdW-bound 1D chains. I will conclude by sharing insights on how these phases can be used as building blocks in next-generation devices for quantum, optoelectronic, sensing, and energy technologies.

Research into electrochemical energy storage sits at the intersection between competing electronic, thermal, chemical, and mechanical forces. Motivated by the pursuit of 3D thin film battery architectures which deliver high power density without sacrificing energy, we have been developing new methodologies and fabrication strategies to study the impact of these forces on actual devices. Here, we will first present predictions of device operation based on continuum level modeling, followed by several experiments that address specific questions of electrochemo-mechanical coupling, interfacial impedances, and other nanoscale phenomena. Finally, we will discuss the consequences for real world operation.

The performance of energy storage and conversion devices, including batteries, fuel cells, and photovoltaics, is defined by the delicate interplay of electrical response and charge carrier migration at the nanoscale. Although physical behavior and macroscopic functional response is well established, intrinsic chemical phenomena associated with ionic motion or localized electrochemical reactions can dramatically alter behavior and restrict utility of these materials. Over the last decade, advancements in development of novel characterization tools such as atomic force microscopy (AFM) have revolutionized our understanding of the electrical and mechanical response of materials; however, dynamic electrochemical behavior and ion migration remain poorly understood. Recently time-of-flight secondary ion mass spectrometry (ToF-SIMS) has proven to be effective tool for characterization of static chemical states in energy materials. Here we introduce approach based on combined AFM/ToF-SIMS platform for correlated studies of the dynamic chemical phenomena on the nanoscale in operando conditions. Being used for characterization of the perovskite photovoltaic and ferroelectric materials it allowed direct observation of the ionic migration within the device in externally applied electric fields and under different temperatures. This is important for fundamental understanding of the material functionality. Altogether, developed approach enables direct characterization of interplay between chemical and functional response in energy materials and aids in the development and optimization of novel devices. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility and using instrumentation within ORNL's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy.

Solar-powered water splitting using nanoparticle photocatalyst suspensions is a promising route to a clean hydrogen economy. A key step in the water-splitting process is the transport of photo-excited electrons and holes to the photocatalyst surface, where they undergo redox reactions. Here we characterize charge transport in individual SrTiO3:Rh and BiVO4 nanoparticles using a nanoprobe within a scanning electron microscope, and directly map photocarrier diffusion lengths with electron-beam induced current. We find that performance in this system is limited by poor e-h transport within the hydrogen-evolving SrTiO3:Rh nanoparticles.

In this work, we perform spectroscopic ellipsometry measurements at room temperature on mechanically exfoliated α-RuCl3 nanoflakes of different thickness for photon wavelengths ranging between 400 and 1000 nm. Our measurements allow us to estimate the wavelength-dependent complex refractive index along the crystal directions parallel and perpendicular to the layers, which reveal an anisotropy between the in-plane and out-of-plane optical properties of the material. Our results provide a valuable information about the optical properties of 2D α-RuCl3 flakes in the visible and near infrared, which are crucial to exploit this material in nanodevices with enhanced light-matter interactions.

The fundamental problem that limits the solar energy conversion efficiency of conventional semiconductors such as Si is that all absorbed photon energy above the band gap is lost as heat. The critical question that our research addresses is: Can we avoid energy losses in semiconductors? Hot-carrier systems that avoid such losses have tremendous potential in photovoltaics and solar fuels production, with theoretical efficiencies of 66% (well above the detailed-balance limit of 33%). Ultrathin 2D semiconductors such as monolayer (ML) MoS2 and WSe2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The specific knowledge gap in the field is how the energy levels of 2D semiconductors move with applied potential and/or illumination, making the driving force for charge transfer (G0´) unclear. Since G0´ governs the hot-carrier extraction rate (kET), understanding how and why G0´ changes under solar fuel generation conditions is critical to controlling kET relative to the cooling rate. Absence of this critical information is limiting our ability to perform hot-carrier photochemistry. Our research team has employed photocurrent spectroscopy, steady-state absorption spectroscopy, and in situ femtosecond transient absorption spectroscopy as a function of applied potential to characterize underlying steps in a ML MoS2 photoelectrochemical cell. The rich data set informs us on the timescales for hot-carrier generation/cooling and exciton formation/recombination, as well as the magnitudes of changes in exciton energy levels, exciton binding energies, and the electronic band gap. These findings open the possibility of tuning the hot-carrier extraction rate relative to the cooling rate to ultimately utilize hot-carriers for solar energy conversion applications.

WS2 multilayer flakes were prepared on template-stripped ultra-flat Ag layers using a metal-assisted exfoliation technique. Using a shadow mask consisting of holey carbon films, 2-micron-sized Au top electrodes were evaporated on the WS2 flakes to fabricate vertical Au/WS2/Ag devices. The photovoltaic characterization of the devices indicated the formation of Schottky diodes, and the estimated power conversion efficiency at 625-nm visible light was as high as 5.0%. Moreover, our Au/WS2/Ag devices exhibited broadband and incident-angle-insensitive absorption capability. The lithography-free processes suggested in this work enabled us to fabricate high-yield and high-performance devices.

WS2 flakes were prepared on template-stripped ultra-flat Au and Ag layers using a metal-assisted exfoliation technique, and their physical characteristics were investigated. The identification of the thickness for each flake is confirmed by the agreement between the measured and calculated optical reflectance spectra. Despite the extremely small flake thickness, the resonant cavity modes can appear in WS2/Au and WS2/Ag, according to the anticipated phase shifts of light. The contact potential difference of the flake was studied using Kelvin probe force microscopy to propose the interfacial band alignment. This work can provide valuable insights into the use of the 2D-semiconductor/metal structures for optoelectronic device applications.

Despite the encouraging inherent properties of and research progress on two-dimensional (2D) MoS2, an ongoing issue associated with oxidative instability remains unsolved for practical optoelectronic applications. Thus, in-depth understanding of the oxidation behavior of large-scale and homogeneous 2D MoS2 is imperative. In this study, the structural and chemical transformations of large-area MoS2 multilayers were investigated by air-annealing at various temperatures and times by performing Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The results explicitly indicate the temperature- and time-dependent oxidation effects: heat-driven elimination of redundant residues, internal strain stimulated by the formation of Mo–O bonds, deterioration of the MoS2 crystallinity, layer thinning, and morphological transformation from 2D MoS2 layers into particles. Photoelectrical characterization of the air-annealed MoS2 was implemented to capture the link between the oxidation behavior of large-scale MoS2 multilayers and their photoelectrical properties.