arXiv subjects

Max C. Lemme

Publications and source records attributed to Max C. Lemme.

At least 19 recordsLinked to original sources

arXiv2026

Improved Heat Dissipation in CsPbBr${_3}$-hBN Heterostructures

Metal halide perovskite semiconductors are promising materials for optoelectronic and photonic devices, including solar cells and next-generation coherent light sources. However, their low thermal conductivity limits the practical operation of devices under high excitation levels. Integrating thermally conductive, large band-gap two-dimensional (2D) materials into perovskite devices could suppress heat accumulation, while preserving their optical properties. Here, we show that planar hot-pressed (PHP) cesium lead bromide (CsPbBr${_3}$) thin films capped with few-layer 2D hexagonal boron nitride (hBN) are less affected by laser-induced heating under high-power continuous-wave excitation than uncapped perovskite samples. A large-scale semidry transfer method was developed to integrate 2D hBN onto PHP CsPbBr${_3}$ thin films. The process is chemically and thermally compatible with perovskites. Microscopic and spectroscopic analyses show that the hBN capping layer does not alter the morphology and optical properties of the perovskite thin film. The PHP CsPbBr${_3}$-hBN heterostructure exhibits a thermal conductivity of 3 W/(m * K), approximately seven times higher than that of the bare perovskite films of 0.45 W/(m * K). Heat diffusion simulations confirm enhanced heat dissipation in the heterostructure relative to bare perovskite films. Our experiments demonstrate an effective approach to enhancing heat dissipation in perovskite devices using transparent, thermally conductive 2D materials.

cond-mat.mtrl-sci↗

arXiv2026

Benchmarking Dual-Polarization Silicon Nitride Photonic Integrated Circuits for Trapped-Ion Quantum Technologies

Trapped ions are one of the most advanced platforms for quantum technologies, with applications ranging from quantum computing to precision timekeeping. A crucial step towards more compact and scalable systems involves integrating photonic integrated circuits (PICs) into surface ion traps to enable on-chip light delivery and optical addressing of individual ions. Currently, most implementations rely solely on transverse-electric (TE) mode grating couplers, where the emitted light is polarized in the plane of the chip. In this work, we design, fabricate and characterize silicon nitride (Si$_3$N$_4$) PIC components, including incoupling structures, splitters, and grating couplers that support both TE and transverse-magnetic (TM) modes with comparable optical losses. We benchmark the PIC at 760\,nm, which is a typical wavelength for Yb$^{+}$-applications. The fabricated grating couplers enable the outcoupling of collimated free-space beams for both polarizations, exhibiting distinct emission angles. This dual-polarization capability gives more flexibility in polarization control and expands the accessible optical design space for trapped-ion quantum technologies.

physics.optics↗

arXiv2025

Reduced Variability in Threshold Switches Using Heterostructures of SiO${_x}$ and Vertically Aligned MoS${_2}$

Layered two-dimensional (2D) materials provide unique structural features, such as physical gaps between their layers that are only connected through van der Waals (vdW) forces. These vdW gaps can guide the migration of intercalated ions and thus regulate filament growth in resistive switching (RS) devices. Vertically aligned 2D materials and their heterostructures provide vdW gap-mediated ion transport in memristor crossbars, providing great potential for high-density integration and reliable RS performance. Nevertheless, the fundamental switching mechanisms and their contributions to the RS remain inadequately understood. In this work, we investigate silver (Ag) filament-based threshold switching (TS) in heterostructures comprising vertically aligned 2D molybdenum disulfide (VAMoS${_2}$) grown via sulfurization and silicon oxide (SiO${_x}$). Compared to SiO${_x}$-only devices, the SiO${_x}$/VAMoS${_2}$ devices exhibit TS with higher on-threshold and hold voltages, each approximately 0.4 V, faster switching times down to 356 ns under a 4 V pulse, and a lower cycle-to-cycle on-current variability of 3.0%. A physics-based, variability-aware model reveals that confined Ag ion migration within the vdW gaps in VAMoS${_2}$ forms ultrathin seed filaments, which guide filament growth in the SiO${_x}$ layer. These findings establish SiO${_x}$/VAMoS${_2}$ heterostructures as a promising concept for reliable TS in vertical device architectures for emerging memories and neuromorphic computing.

physics.app-ph↗

arXiv2025

Intermediate Resistive State in Wafer-Scale MoS${_2}$ Memristors through Lateral Silver Filament Growth for Artificial Synapse Applications

Memristors based on two-dimensional materials (2DMs) have garnered significant attention due to their fast resistive switching (RS) behavior and atomic-level thickness, which enables low power consumption, making them promising candidates for neuromorphic computing. Among these, memristors based on molybdenum disulfide (MoS${_2}$) have been extensively studied. Their RS has been attributed to the formation and rupture of conductive filaments (CFs). However, the underlying mechanism of filament formation remains underexplored, and the inherently stochastic nature of RS leads to high variability and limited reproducibility. Additionally, the lack of scalable fabrication techniques for 2DM-based memristors restricts their integration into standard semiconductor technology. Here, we demonstrate memristors based on metal-organic chemical vapor-deposited MoS${_2}$ on the wafer-scale. Our devices exhibit volatile and nonvolatile RS behavior, tunable by modulating the current compliance. Notably, we observe stable RS characteristics in an intermediate resistive state (IRS), featuring set and reset voltages within $\pm$1 V, an endurance exceeding 2500 cycles in direct current mode, and a state retention over 10${^6}$ s. The experimental data, complemented with simulations, suggest that the IRS originates from the lateral growth of the CF within the MoS${_2}$ layer. Furthermore, the devices successfully emulate synaptic plasticity with current responses on the microsecond timescale, highlighting their potential for large-scale integration in neuromorphic computing architectures.

physics.app-ph↗

arXiv2025

Versatile Top-Down Patterning Technique for Perovskite On-Chip Integration

Metal-halide perovskites (MHPs) have exciting optoelectronic properties and are under investigation for various applications, such as photovoltaics, light-emitting diodes, and lasers. An essential step towards exploiting the full potential of this class of materials is their large-scale, on-chip integration with high-resolution, top-down patterning. The development of such patterning methods for perovskite films is challenging because of their intrinsic ionic nature and adverse reactions with the solvents used in standard lithography processes. Here, we introduce a versatile and precise method comprising photolithography and reactive ion etching (RIE) processes that can be tuned to accommodate different perovskite compositions and morphologies. Our method utilizes conventional photoresists at reduced temperatures to create micron-sized features down to 1 $μ$m, providing high reproducibility from chip to chip. The patterning technique is validated through atomic force microscopy (AFM), X-ray diffraction (XRD), optical spectroscopy, and scanning electron microscopy (SEM). It enables the scalable and high-throughput on-chip monolithic integration of MHPs.

cond-mat.mtrl-sci↗

arXiv2025

Wafer-scale Synthesis of Mithrene and its Application in 2D Heterostructure UV Photodetectors

Silver phenylselenide (AgSePh), known as mithrene, is a two-dimensional (2D) organic-inorganic chalcogenide (MOC) semiconductor with a wide direct band gap, narrow blue emission and in-plane anisotropy. However, its application in next-generation optoelectronics is limited by crystal size and orientation, as well as challenges in large-area growth. Here, we introduce a controlled tarnishing step on the silver surface prior to the solid-vapor-phase chemical transformation into AgSePh thin films. Mithrene thin films were prepared through thermally assisted conversion (TAC) at 100°C, incorporating a pre-tarnishing water (H${_2}$O) vapor pulse and propylamine (PrNH${_2}$) as a coordinating ligand to modulate Ag${^+}$ ion reactivity and facilitate the conversion of Ph${_2}$Se${_2}$ into an active intermediate. The AgSePh thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and grazing incidence wide-angle X-ray scattering (GIWAXS). The pre-tarnishing process, combined with organic ligands, resulted in large crystals exceeding 1 $μ$m and improved homogeneous in-plane orientation, while also enabling the selective, wafer-scale synthesis of mithrene on 100 mm wafers. Furthermore, the films were integrated on planar graphene field-effect phototransistors (GFETs) and demonstrated photoresponsivity beyond 100 A/W at 450 nm, highlighting mithrene's potential for blue light-detection applications.

cond-mat.mtrl-sci↗

arXiv2025

Integrated, ultrafast all-optical polariton transistors with sub-wavelength grating microcavities

All-optical logic has the potential to overcome the operation speed barrier that has persisted in electronic circuits for two decades. However, the development of scalable architectures has been prevented so far by the lack of materials with sufficiently strong nonlinear interactions needed to realize compact and efficient ultrafast all-optical switches with optical gain. Microcavities with embedded organic material in the strong light-matter interaction regime have recently enabled all-optical transistors operating at room temperature with picosecond switching times. However, the vertical cavity geometry, which is predominantly used in polaritonics, is not suitable for complex circuits with on-chip coupled transistors. Here, by leveraging state-of-the-art silicon photonics technology, we have achieved exciton-polariton condensation at ambient conditions in fully integrated high-index contrast sub-wavelength grating microcavities filled with a pi-conjugated polymer as optically active material. We demonstrate ultrafast all-optical transistor action by coupling two resonators and utilizing seeded polariton condensation. With a device area as small as 2x2 um^2, we realize picosecond switching and amplification up to 60x, with extinction ratio up to 8:1. This compact ultrafast transistor device with in-plane integration is a key component for a scalable platform for all-optical logic circuits that could operate two orders of magnitude faster than electronic ones.

physics.optics↗

arXiv2025

Threshold Switching in Vertically Aligned MoS${_2}$/SiO${_x}$ Heterostructures based on Silver Ion Migration

Threshold switching (TS) is a phenomenon where non-permanent changes in electrical resistance of a two-terminal device can be controlled by modulating the voltage bias. TS based on silver (Ag) conductive filaments has been observed in many materials, including layered two-dimensional (2D) transition metal dichalcogenides (TMDs). 2D TMDs are particularly promising for metal ion movement due to their van der Waals (vdW) gaps between their sheets, facilitating ion migration and filament formation without disturbing covalent chemical bonds. In this work, we demonstrate the heterostructure growth of vertically aligned molybdenum disulfide (VAMoS${_2}$) with an amorphous silicon oxide (SiO${_x}$) layer on top after sulfurization. We show that Ag ions migrate through this material stack, enabling TS. Our Ag/SiO${_x}$/VAMoS${_2}$/gold (Au) devices exhibit TS at low voltages of ~0.63 V, with high on-state currents over 200 $μ$A and stable switching exceeding 10${^4}$ cycles. Moreover, we identify two rate-limiting steps for filament formation through a physics-based dynamical model and simulate the switching kinetics. Our devices show a fast on-switching time of 311 ns and spontaneous relaxation in 233 ns. These findings deepen the understanding of SiOx/MoS${_2}$-based RS devices and demonstrate the promise for applications in emerging memories and neuromorphic computing systems.

cond-mat.mtrl-sci↗

arXiv2025

Volatile and Nonvolatile Resistive Switching in Lateral 2D Molybdenum Disulfide-Based Memristive Devices

Developing electronic devices capable of emulating biological functions is essential for advancing brain-inspired computation paradigms such as neuromorphic computing. In recent years, two-dimensional materials have emerged as promising candidates for neuromorphic electronic devices. This work addresses the coexistence of volatile and nonvolatile resistive switching in lateral memristors based on molybdenum disulfide with silver as the active electrode. The fabricated devices exhibited switching voltages of ~0.16 V and ~0.52 V for volatile and nonvolatile operation, respectively, under direct-current measurements. They also displayed the essential synaptic functions of paired-pulse facilitation and short- and long-term plasticity under pulse stimulation. The operation mechanism was investigated by in-situ transmission electron microscopy, which showed lateral migration of silver ions along the molybdenum disulfide between electrodes. Based on the experimental data, a macroscopic semi-classical electron transport model was used to reproduce the current-voltage characteristics and support the proposed underlying switching mechanisms.

cond-mat.mtrl-sci↗

arXiv2025

Unraveling the dynamics of conductive filaments in MoS${_2}$ based memristors by operando transmission electron microscopy

Advanced operando transmission electron microscopy (TEM) techniques enable the observation of nanoscale phenomena in electrical devices during operation. They can be used to study the switching mechanisms in two-dimensional (2D) materials-based memristive devices, which is crucial to tailor their operating regimes and improve reliability and variability. Here, we investigate lateral memristive devices composed of 2D layered molybdenum disulfide (MoS${_2}$) with palladium (Pd) and silver (Ag) electrodes. We visualized the formation and migration of Ag conductive filaments (CFs) between the two electrodes under external bias voltage and their complete dissolution upon reversing the bias voltage polarity. The CFs exhibited a wide range of sizes, ranging from several Ångströms to tens of nanometers, and followed diverse pathways: along the MoS${_2}$ surfaces, within the van der Waals gap between MoS${_2}$ layers, and through the spacing between MoS${_2}$ bundles. Notably, the Ag electrode functioned as a reservoir for the CFs, as evidenced by the shrinking and growing of the Ag electrode upon switching. Our method enabled correlating the current-voltage responses with real-time TEM imaging, offering insights into failed and anomalous switching behavior, and providing clarity on the cycle-to-cycle variabilities. Our findings provide solid evidence for the electrochemical metallization mechanism, elucidate the formation dynamics of CFs, and reveal key parameters influencing the switching performance. Our approach can be extended to investigate similar memristive devices.

cond-mat.mtrl-sci↗

arXiv2025

Influence of Humidity on the Resistive Switching of Hexagonal Boron Nitride-Based Memristors

Two-dimensional material-based memristors have recently gained attention as components of future neuromorphic computing concepts. However, their surrounding atmosphere can influence their behavior. In this work, we investigate the resistive switching behavior of hexagonal boron nitride-based memristors with active nickel electrodes under vacuum conditions. Our cells exhibit repeatable, bipolar, nonvolatile switching under voltage stress after initial forming, with a switching window > 10${^3}$ under ambient conditions. However, in a vacuum, the forming is suppressed, and hence, no switching is observed. Compact model simulations can reproduce the set kinetics of our cells under ambient conditions and predict highly suppressed resistive switching in a water-deficient environment, supporting the experimental results. Our findings have important implications for the application of h-BN-based memristors with electrochemically active electrodes since semiconductor chips are typically processed under high vacuum conditions and encapsulated to protect them from atmospheric influences.

cond-mat.mtrl-sci↗

arXiv2024

Room-temperature Distributed Feedback CsPbBr$_3$ Perovskite Laser Integrated on a Silicon Nitride Waveguide Platform

Silicon photonic integrated circuits (PICs) require cost-effective laser sources that can be monolithically integrated. The low cost and low-temperature solution processability of metal halide perovskites (MHPs) make them attractive alternatives to established III-V compound semiconductors for on-chip laser sources in PICs. Cesium lead bromide (CsPbBr$_3$) perovskites are emerging materials for green light-emitting diodes and lasers. To date, amplified spontaneous emission (ASE) at room temperature has been frequently achieved in CsPbBr$_3$ thin films, while reports on lasing are more limited. Here, we demonstrate a first-order grating distributed feedback (DFB) CsPbBr$_3$ thin-film laser operating at room temperature. Planar hot-pressed (PHP)-CsPbBr$_}$, with a low ASE threshold of 14.5 $μ$Jcm$^{-2}$ under 0.3 nanosecond (ns) pump pulses, was monolithically integrated into a silicon nitride (Si$_3$N$_4$) waveguide platform via a compatible top-down patterning process. The first-order grating DFB PHP-CsPbBr$_3$ thin-film laser operated at 540 nm in the green spectral region, where III-V lasers have limitations, and exhibited a lasing threshold of 0.755 mJcm$^{-2}$ at room temperature. This work marks a significant step toward utilizing MHPs for on-chip green lasers in PICs for commercial applications.

physics.app-ph↗

arXiv2024

Contact Resistance Optimization in MoS${_2}$ Field-Effect Transistors through Reverse Sputtering-Induced Structural Modifications

Two-dimensional material (2DM)-based field-effect transistors (FETs), such as molybdenum disulfide (MoS${_2}$)-FETs, have gained significant attention for their potential for ultra-short channels, thereby extending Moore's law. However, MoS${_2}$-FETs are prone to the formation of Schottky barriers at the metal-MoS${_2}$ interface, resulting in high contact resistance (R${_c}$) and, consequently, reduced transistor currents in the ON-state. Our study explores the modification of MoS${_2}$ to induce the formation of conductive 1T-MoS${_2}$ at the metal-MoS${_2}$ interface via reverse sputtering. MoS${_2}$-FETs exposed to optimized reverse sputtering conditions in the contact area show R${_c}$ values reduced to less than 50% of their untreated counterparts. This reduction translates into improvements in other electrical characteristics, such as higher ON-state currents. Since reverse sputtering is a standard semiconductor process that enhances the electrical performance of MoS${_2}$-FETs, it has great potential for broader application scenarios in 2DM-based microelectronic devices and circuits.

cond-mat.mtrl-sci↗

arXiv2024

Spectroscopic ellipsometry of CsPbCl${_3}$ perovskite thin films

Designing optoelectronic devices based on cesium lead chloride (CsPbCl${_3}$) perovskites requires accurate values of their optical constants. Unfortunately, experimental data for this material is very limited thus far. Therefore, here, we applied spectroscopic ellipsometry (SE) to measure the complex optical constants of thermally evaporated CsPbCl${_3}$ thin films with different thicknesses on Si/SiO${_2}$ substrates. The data were corroborated with scanning electron microscopy (SEM) images and absorption spectroscopy. An optical dispersion model was developed to derive the complex optical constants and film thicknesses. The Tauc-Lorentz model, in conjunction with two harmonic oscillators, was used to extract the required parameters. The extinction coefficient spectrum exhibited a sharp absorption edge at 411 nm, consistent with the absorption spectrum. In addition, the optical bandgap of the film was calculated from the absorption spectra and SE data. The experimental values agree well with the simulation results, with values of $\sim$ 2.99 eV for different film thicknesses. This work provides fundamental information for designing and modeling CsPbCl3-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Dry Transfer Based on PMMA and Thermal Release Tape for Heterogeneous Integration of 2D-TMDC Layers

A reliable and scalable transfer of 2D-TMDCs (two-dimensional transition metal dichalcogenides) from the growth substrate to a target substrate with high reproducibility and yield is a crucial step for device integration. In this work, we have introduced a scalable dry-transfer approach for 2D-TMDCs grown by MOCVD (metal-organic chemical vapor deposition) on sapphire. Transfer to a silicon/silicon dioxide (Si/SiO$_2$) substrate is performed using PMMA (poly(methyl methacrylate)) and TRT (thermal release tape) as sacrificial layer and carrier, respectively. Our proposed method ensures a reproducible peel-off from the growth substrate and better preservation of the 2D-TMDC during PMMA removal in solvent, without compromising its adhesion to the target substrate. A comprehensive comparison between the dry method introduced in this work and a standard wet transfer based on potassium hydroxide (KOH) solution shows improvement in terms of cleanliness and structural integrity for dry-transferred layer, as evidenced by X-ray photoemission and Raman spectroscopy, respectively. Moreover, fabricated field-effect transistors (FETs) demonstrate improvements in subthreshold slope, maximum drain current and device-to-device variability. The dry-transfer method developed in this work enables large-area integration of 2D-TMDC layers into (opto)electronic components with high reproducibility, while better preserving the as-grown properties of the layers.

physics.app-ph↗

arXiv2024

Graphene MEMS and NEMS

Graphene is being increasingly used as an interesting transducer membrane in micro- and nanoelectromechanical systems (MEMS and NEMS, respectively) due to its atomical thickness, extremely high carrier mobility, high mechanical strength and piezoresistive electromechanical transductions. NEMS devices based on graphene feature increased sensitivity, reduced size, and new functionalities. In this review, we discuss the merits of graphene as a functional material for MEMS and NEMS, the related properties of graphene, the transduction mechanisms of graphene MEMS and NEMS, typical transfer methods for integrating graphene with MEMS substrates, methods for fabricating suspended graphene, and graphene patterning and electrical contact. Consequently, we provide an overview of devices based on suspended and nonsuspended graphene structures. Finally, we discuss the potential and challenges of applications of graphene in MEMS and NEMS. Owing to its unique features, graphene is a promising material for emerging MEMS, NEMS and sensor applications.

cond-mat.mes-hall↗

arXiv2024

Piezoresistive PtSe$_2$ pressure sensors with reliable high sensitivity and their integration into CMOS ASIC substrates

Membrane-based sensors are an important market for microelectromechanical systems (MEMS). Two-dimensional (2D) materials, with their low mass, are excellent candidates for suspended membranes to provide high sensitivity, small footprint sensors. The present work demonstrates pressure sensors employing large-scale-synthesized 2D platinum diselenide (PtSe${_2}$) films as piezoresistive membranes supported only by a thin polymer layer. We investigate three different synthesis methods with contrasting growth parameters and establish a reliable high yield fabrication process for suspended PtSe${_2}$/PMMA membranes across sealed cavities. The pressure sensors reproducibly display sensitivities above 6 x 10${^4}$ kPa${^{-1}}$. We show that the sensitivity clearly depends on the membrane diameter and the piezoresistive gauge factor of the PtSe${_2}$ film. Reducing the total device size by decreasing the number of membranes within a device leads to a significant increase in the area-normalized sensitivity. This allows the manufacturing of pressure sensors with high sensitivity but a much smaller device footprint than the current state-of-the-art MEMS technology. We further integrate PtSe${_2}$ pressure sensors with CMOS technology, improving the technological readiness of PtSe${_2}$-based MEMS and NEMS devices.

physics.app-ph↗

arXiv2024

High-yield large-scale suspended graphene membranes over closed cavities for sensor applications

Suspended membranes of monoatomic graphene exhibit great potential for applications in electronic and nanoelectromechanical devices. In this work, a "hot and dry" transfer process is demonstrated to address the fabrication and patterning challenges of large-area graphene membranes on top of closed, sealed cavities. Here, "hot" refers to the use of high temperature during transfer, promoting the adhesion. Additionally, "dry" refers to the absence of liquids when graphene and target substrate are brought into contact. The method leads to higher yields of intact suspended monolayer CVD graphene and artificially stacked double-layer CVD graphene membranes than previously reported. The yield evaluation is performed using neural-network-based object detection in SEM images, ascertaining high yields of intact membranes with large statistical accuracy. The suspended membranes are examined by Raman tomography and AFM. The method is verified by applying the suspended graphene devices as piezoresistive pressure sensors. Our technology advances the application of suspended graphene membranes and can be extended to other two-dimensional (2D) materials.

physics.app-ph↗