Low-Dimensional Materials and Devices 2024

Artificial intelligence (AI) techniques have boosted the capability of optical imaging, sensing, and communication. Concurrently, photonics facilitate the tangible realization of deep neural networks, offering potential benefits in terms of latency, throughput, and energy efficiency. In this talk, I will discuss our efforts in AI photonics with two examples. The first involves employing a convolutional neural network for achieving single-shot full-field measurement of optical signals. The second example pertains to implementing a deep neural network with a multiple-scattering system featuring structural nonlinearity, thereby enabling nonlinear computations using linear optics.

With the proliferation of networked sensors and artificial intelligence, there is an increasing need for edge computing where data is processed at the sensor level to reduce bandwidth and latency while still preserving energy efficiency. In this talk, I will discuss how meta-optics can be used to implement computation for optical edge sensors, serving to off-load computationally expensive convolutional operations from the digital platform, reducing both latency and power consumption. I will discuss how meta-optics can augment, or replace, conventional imaging optics in achieving parallel optical processing across multiple independent channels for identifying, and classifying, both spatial and spectral features of objects.

Aluminum thin films are often utilized for UV applications where other metals like gold and silver are ineffective, but the efficiency of Al in structures like mirror coatings and bandpass filters can be limited by the presence of native oxide. Previous work has investigated the development of thermal atomic layer etching (ALE) methods to remove this native oxide prior to subsequent encapsulation with fluoride dielectric materials deposited by atomic layer deposition (ALD). This ALE process has involved cyclic exposure to trimethylaluminum and HF in the presence of alkali materials which has been shown to enhance of the oxide removal efficacy at a given substrate temperature. In this current work we investigate ALE co-reaction with cesium fluoride which is shown to enable aluminum oxide etching at temperatures as low as 125 °C and reduce the amount of etch damage experienced by the Al layer. We report on the fabrication of broadband protected Al mirror coatings using this ALE/ALD approach and summarize ongoing and future NASA astrophysics missions that will benefit from this development.

Silver (Ag) and aluminum (Al) are common metal-bases for broadband telescope mirrors, but suffer from low durability. Ag, while highly reflective in the visible range, is readily corroded necessitating a protective coating like aluminum oxide (AlOx) that impairs the blue-UV response. Similarly, Al performs well from 90-2000 nm, but requires a protective coating like aluminum fluoride (AlFx), as the metal-base is readily oxidized. Optimizing the metal/dielectric interfaces of AlOx/Ag and AlFx/Al, then are necessary to create durable high-performing mirrors. Current manufacturing methods employ physical vapor deposition (PVD) and atomic layer deposition (ALD) independently; introducing failure points at the metal/dielectric interface. The study examines using sputtering atomic layer augmented deposition (SALAD) as a solution by seamlessly integrating the capabilities of PVD and ALD without breaking vacuum. Further, a focus is placed on multi-physics modeling, environmental testing, characterization, and assessment of degradation mechanisms.

High quality silver (Ag) thin films with low electrical resistivity and high reflectance were produced by vacuum evaporation and sputtering methods. Ag films with thermal stability and environmental resistance could be fabricated by depositing aluminum nanolayers on the surface and at the substrate interface. In the case of the sputtering method, Ag films with large crystallite size and low resistivity were obtained simply by changing the sputtering gas from Ar to Kr. This was due to the suppression of the uptake of noble gases into the film. Recently, a porous, black Ag film was obtained using the same equipments. By controlling the deposition parameters, films with various fill factors could be produced. In addition to optical applications, these porous films are expected to be used as highly sensitive gas sensors.

Traditional atomic layer deposition (ALD) systems, designed for small ~200 mm substrates, limit ALD's application in astronomical instrumentation. Our previous work introduced a meter-scale ALD (MSALD) system, accommodating larger ~900 mm substrates. Utilizing the MSALD system, we prepared aluminum oxide (AlOx) to protect silver-based telescope mirrors, demonstrating scalable ALD processes with optimized parameters. This opened avenues for diverse materials like aluminum in telescope mirror applications. Our current investigation explores the MSALD systems’ potential to create aluminum fluoride (AlFx) protection coatings for aluminum-based telescope mirrors operated in the far UV spectral range. Unlike traditional AlOx, ALD of AlFx is uncommon, posing new challenges for achieving a uniform 2 nm coating due to the oxidation of Al surfaces. Our study, along with MSALD system modifications, yields crucial insights into scaling ALD for AlFx coatings, offering unique solutions to enhance Al-mirrors' performance and durability.

Silver (Ag) excels as a metal-base for astronomical mirrors for its high reflectivity across the visible to infrared spectral range. However, Ag degrades quickly in an observatory environment, necessitating a protective coating like aluminum oxide (AlOx). Our study compares using water (H2O) and high-purity ozone (PO) as oxygen precursors for AlOx a protective coating on Ag using low-temperature atomic layer deposition (ALD). At ~80% purity, PO allows for higher quality films compared to that of H2O, while offering a reduced deposition time. After enduring high humidity high temperature (HTHH) testing, H2O samples showed a substantial reduction in reflectivity (~30%), while PO samples boasted a minimal reflectivity reduction (~12%). Ellipsometry revealed a 74 nm phase shift, compared to a 6 nm shift for H2O and PO respectively; indicating improved structural integrity. AFM and EDS analysis revealed H2O samples underwent erratic structural changes compromising integrity, while PO samples showed minimal structural change.

A phase field method is used to computationally study formation and morphology of multiple simultaneous conducting filaments, consistent with experimentally observed data of many oxide-based resistive switching thin films. In contrast to existing phase field methods that require an idealized pre-defined axisymmetric conducting filament model, producing only one conducting filament, our method produces multiple conducting filaments without a priori model constraints. Our computational results are consistent with observed irregular bulk current-voltage behavior associated with oxide-based thin films often attributed to existence of multiple conducting filaments, and usually understood to be a performance limiting factor.

Experimental data suggests that multiple conductive filament (CF) formation in oxide-based memristive thin films depends on the film's geometry. We used a computational phase-field model, without a priori model constraints, and studied the effects of the variation of geometry and dimensions in a three-dimensional sample on the formation of multiple conducting filaments and how they evolve within a sample. In this paper, we show the propensity of multiple CF to form within a variation of lateral and axial dimensions as well as different depths.

Analog neuromorphic computing hardware, despite their application-specific energy efficiency, are not easily reconfigured for generality, thus impeding them from competing with entrenched general purpose digital processors. Here we address this fundamental limitation by enabling a single neuromorphic component to be functionally reconfigured to express neuronal, synaptic, interconnect and switching behaviors. Via precise voltage-controlled on-chip injection of oxygen vacancy defects into VO2, a Mott insulator, we nucleate and stabilize phase coexistence across various oxides of vanadium, much like spinodal decomposition of a water-oil mixture, and tune the overall phase transition properties. Such phase coexistence is not possible in purely electronic components, and has not been achieved electrochemically under normal chip operating conditions. Using electrochemically controlled phase coexistence, we demonstrate both functionally tunable computing, such as reconfigurable logic gates, and also unusually stable information retention, with 1% loss over 14 years in ambient conditions. On-chip defect-tuned phase coexistence paves the path for functionally dense and dynamically reconfig

Transition metal dichalcogenides (TMD) flakes were obtained on SiO2/Si wafers using the metal-assisted-exfoliation technique. The flakes were transferred on Ag nanogratings (AgNGs), after being detached from SiO2/Si. Ag layers, evaporated and peeled off from patterned nano-templates, were attached to glass substrates to fabricate AgNGs. The optical reflectance spectra of AgNGs exhibited clear dips, which were dependent on the incident angle and polarization of light. These dips indicated surface plasmon (SP) excitation near the exciton resonance wavelengths of TMD. Thus, SP-exciton coupling phenomena significantly affected the optical spectra of TMD/AgNGs. Also, the surface potential distributions of TMD/AgNGs were studied using Kelvin probe force microscopy, while illuminating linearly polarized light with various incident angles. The light-induced potential changes can visualize how the SP-exciton coupling affects the charge carrier behaviors in TMD integrated with plasmonic nanostructures.

Monolayer WS2 flakes were exfoliated on Ag nanohole (AgNH) arrays with a period of 500 nm and a hole diameter of 250 nm. The AgNH arrays were fabricated using a template stripping (TS) method: evaporated Ag layers were peeled off from SiO2 nanopillar arrays and attached to quartz substrates with UV-curable epoxy. The surface of the TS-fabricated AgNH array was substantially smoother compared to the surface of the evaporated Ag thin film surface. Therefore, the very flat Ag surface enabled the direct exfoliation of a few layers of WS2 flakes with well-suspended and contamination-free surfaces on the periodic AgNH structures. The optical reflectance spectra, as well as PL and Raman spectra of WS2/AgNH, were obtained to investigate the plasmonic effects in the WS2-metal integrated nanostructures. Furthermore, Kelvin probe force microscopy was used to perform wavelength-dependent surface photovoltage characterizations of WS2/AgNH. This work can help us to improve the architectures of 2D material-based optoelectronic devices by harnessing the benefits of plasmonic effects.

We demonstrate the self-powered and flexible triboelectric sensors (TESs) using InN nanowires (NWs) as a response medium. For the device fabrication, 880-nm-long InN NWs formed on a Si(111) substrate were first filled with polydimethylsiloxane (PDMS). And then, the NW-PDMS structure with the thickness of 10 μm was separated from the substrate and transferred to a flexible and conductive polymer substrate. When touching, poking, and tapping the top surface of the TES with a finger, the output voltages were measured to be 6.6, 18.8, and 31.2 V, respectively, which are much higher than those of the previous reports. The output voltage of the TESs rarely changed under various device parameters such as touching frequency and operation time (up to 28 days). In addition, a sensor array composed of 16 TES chips was fabricated and evaluated under the motion of touching and sliding motion of a finger.

Large-sized MoS2 monolayer flakes were prepared on SiO2/Si wafers using the silver-assisted exfoliation method through the atomic spalling procedure. The flakes were picked up and transferred on ZnO-coated Ag-nanowire (AgNW) networks, using a viscoelastic transfer stamp consisting of polydimethylsiloxane (PDMS) layers coated with polypropylene carbonate (PPC). The PPC-PDMS stamp allows for the reliable and efficient transfer of the extremely thin MoS2 monolayers on the AgNW-containing non-flat surfaces. Optical reflectance spectra of the MoS2 flakes on AgNWs were obtained and then compared with numerical simulation results. The wavelength-dependent current-voltage and surface photovoltage characteristics of the MoS2 flakes were studied using current-sensing atomic force microscopy and Kelvin probe force microscopy, respectively. This work explores the possibility of using AgNW transparent electrodes in flexible optoelectronic devices based on atomically thin semiconductors.

Large-area metal-assisted exfoliated MoS2 monolayers (MLs) were integrated with plasmonic metal nanowires (NWs). Metal (Au and Ag) NWs were transferred on MoS2 MLs to form NW/MoS2 structures. In comparison, MoS2 MLs were transferred on NWs to fabricate MoS2/NW structures. Two kinds of NW-containing layers were prepared: NWs drop-casted on (NW-on-Sub) and NWs embedded in (NW-in-Sub) polymer layers. Comparative characterizations of MoS2/NW-on-Sub and MoS2/NW-in-Sub helped us to investigate the plasmon-exciton coupling effects in NW-integrated 2D van der Waals hybrid systems. To study the plasmon-enhanced photo-carrier generation behaviors, nanoscopic electrical characterizations employing Kelvin probe force microscopy and current-sensing atomic force microscopy were carried out in addition to optical spectroscopy analyses. This work can help us to propose high-performance 2D material-based optoelectronic devices and strategies to enhance the plasmonic contribution to the light-matter interaction.

Raman scattering is an optical spectroscopy technique that obtains information by exciting a sample and measuring the scattering spectrum. When the excitation beam interacts with molecules in the sample, some of the photon's energy is transferred and scattered out at different frequencies. This frequency difference is called Raman shift. This paper describes the design and integration of a in-situ Raman measurement of advanced semiconductor processes. Traditional ex-situ measurements can only be performed using various analytical tools after the process is completed. However, the sample contamination due to exposure to the atmosphere during transfer is a concern, especially for TMDs (Transition Metal Dichalcogenides). By implementing in-situ Raman measurement during the manufacturing process, we can prevent atmospheric contamination and this approach allows us to gain deeper insights into the growth mechanisms of 2D materials, benefiting semiconductor process development, ultimately enhancing process yield and reliability.

Silicon photonics is a promising platform for integrating various optical components on a single chip. However, one of the major challenges is to develop efficient and compact light sources due to the poor light emission efficiency of silicon. Semiconducting transition metal dichalcogenide (TMD) shows great potential to address this issue by efficient band engineering with stacking of different TMD monolayers. In this work, we observe the bright-light emission from TMD heterobilayers (MoS2/WSe2), where interlayer excitons dominate the optical properties of materials even at room temperature. Through integrating the heterobilayers with silicon topological cavities, we observe a dominant single emission mode around 1230 nm that is outcoupled to an on-chip waveguide. Our work demonstrates a new architecture for realizing silicon photonic chip-scale integrated light sources at room temperature.

Phototransistor using 2D semiconductor material as a channel material has shown promising potential for high sensitivity photo detection. The atomically thin layer structure allows photo excited charge carriers to be highly sensitive to external electric field modulation. Gate dielectric material and its deposition processing conditions can have great effect on the interface states. Here, we report a high photoresponsivity MoS2 phototransistor using properly annealed HfO2 gate dielectric. When the deposited HfO2 is annealed in H2 atmosphere, the photoresponsivity is enhanced by an order of magnitude as compared with HfO2 without annealing or annealed in Ar atmosphere. The enhancement is attributed to the hole trapping states introduced at HfO2 interface, which greatly enhances photogating effect. The phototransistor exhibits a very large photoresponsivity of 1.1 × 107 A W-1 and photogain of 3.3 × 107 under low light illumination intensity. This study demonstrates a processing technique to fabricate highly sensitive 2D material phototransistor.

Silver (Ag) based mirrors boast high reflectivity in the visible spectral range and low emissivity in the thermal infrared, but suffers from low durability; requiring regular recoating. Conventional coatings extend the operational life of Ag-mirrors, but significantly reduce the mirror’s efficacy in the blue-UV spectral range. The solution our study proposes is a protective coating of diamond-like carbon (DLC); known for its hydrophobicity and abrasion resistance. DLC coating were produced at room temperature using filtered cathodic arc (FCA) deposition, which resulted in a high sp3 to sp2 bond ratio. This ratio and the abrasion resistance can be further improved by tuning the substrate bias. High humidity high temperature testing demonstrated DLC’s ability to resist hydrolyzation while optical characterization shows only minor impairment to the Ag-mirror’s overall spectral response. FCA-deposited DLC shows promise as an alternative protective coating providing an increase in durability over conventional coatings without impairing the UV response.

GAA-NWFETs are gaining attraction due to their superior electrostatic control and low leakage currentsin low-power electronic circuits. Therefore, it is crucial to consider the impact of temperature on the efficiency and reliability of these devices, especially in low-power circuit applications that requires minimizing of power loss. This paper presents a comprehensive thermal variation analysis of these devices to better understand the effect of temperature on GAA-NWFETs and explored ways to reduce thermal impacts. Using TCAD simulations, we investigate the temperature-dependent metrics that include drain current, subthreshold swing, threshold voltage, energy band, electric potential, and acceptor concentration taking into account changes in ambient temperature and device design.The superior electrostatic control and low leakage currents of GAA-NWFETs make them desirable choices for low-power electronic circuits. Comparing the ION/IOFF ratios with various variants, there is a 21.59% improvement.

We present the fabrication and operation of GaN vacuum electron nanodiodes operating by field emission and in air. The devices exhibit low turn-on voltage, high field emission current, and excellent radiation hardness. Experimental and modeling results on the characteristics of these devices at various nanogap sizes, operating pressures, temperature, and radiation environments are discussed. Preliminary results on the fabrication and characteristics of lateral GaN nano vacuum transistors will also be shown. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

We present results on III-V semiconductor nanowires and metal halide perovskite nanowires. Focus is on optical and electrical design and characterization, especially of nanowire arrays for optoelectronic applications. Emphasis is given to the strong impact of material and geometry choice on device performance. We give examples of how electromagnetic optics modelling and drift-diffusion electrical modelling can be used to aid in both device design and analysis of characterization results.

Wide bandgap lateral fin light-emitting diodes show a droop free behavior. This property allows their brightness to increase at higher current density leading to the possibility of lasing. In this presentation, we will investigate the optical loss due to the presence of threading dislocation and surface defects in wide bandgap fins with different aspect ratios. Furthermore, the light extraction efficiency and external quantum efficiency will be discussed as the height to width ratio of a fin changes. Further understanding and optimization of these parameters are expected to lead to identification of robust, bright and wavelength tunable nanolasers.

Photovoltaic solar cells (SCs) based on dense arrays of III-V compound nanowires (NWs) possess potentials for enormous improvement in their solar power conversion efficiency. A strategy to achieve this goal is through improved light management of the incident sun light and multi-band absorption. The former is demonstrated by GaAs near band-edge absorption in GaAs−AlGaAs core−shell NWs grown by MOVPE in our Lab: much higher values of absorption enhancement factor have been experimentally estimated for the first time in these structures (up to 200 times that of homogeneous - only core - NWs), an effect ascribed to wave-guiding of incident light into the GaAs cores by the surrounding shell. Secondly, adoption of an intermediate-band gap semiconductor (IBGS) as the SC active material allows to combine the multiband functionality with advantages associated to nanowire-based SCs, avoiding complicated multi-junction architectures. The use of dilute nitrides (III-N-V) as IBGS within core-multishell NW-based SCs is a very promising solution, never reported so far. The challenges of self-assembling such SC structures by MOVPE will be discussed.

We discuss the fabrication and characterization of flexible piezoelectric motion sensors (PMSs) with single-electrode (SE) configuration. As a response medium of the devices, InN nanowires (NWs), formed on Si(111) substrates by a plasma-assisted molecular beam epitaxy, were used. For comparison, the PMSs with two electrodes (TEs) were prepared. The performances of the PMSs were analyzed depending on device parameters such as bending frequency, operation time, relative humidity, and bending cycle. For example, the output voltage of the SE-PMS with the InN NWs aligned along the bending direction was measured to be 10.65 V, which is higher than that (7.1 V) of the TE device. To evaluate the possibility for real-life applications, the flexible PMSs were attached to a human finger and their output signals were measured depending on the movement angle.

The continuous research on electronics, biocompatible materials and nanomaterials has led to the design of a new generation of wearable devices that can be employed in direct contact with the body of the user, which is attractive for real-time, non-invasive health monitoring. For the satisfaction of such requirements, hydrogel-based conductive devices are often proposed as promising candidates for these applications, thanks to their softness, flexibility, and biocompatibility. Here we report the synthesis of conductive hybrid hydrogels containing two-dimensional (2D) MoS2. The nanoflakes are integrated in the polymeric matrix creating an anisotropic structure, which helps to generate mismatch stress for a strain sensing under a certain stimulus, thus allowing the gel to give an electrical response to pressure.

Colloidal quantum dots (QDs) integrated Micro-LEDs are key technologies for next-generation displays such as AR/VR. However, fundamental scientific issues such as the low internal quantum efficiency (IQE) of Micro-LEDs and low quantum dots light conversion efficiency (LCE) remain to be resolved for the commercialization of this technology. In this talk, the impact of Micro-LED Epi and chip designs as well as GaN free-standing substrates on the improvement of IQE of Micro-LEDs will be reported. Mechanisms of proposed 0D–2D hybrid optical scatterers for improving the LCE of QDs will also be discussed.

Micro light-emitting diodes (μ-LEDs) coupled with color conversion phosphors are among the most promising technologies for future display and artificial light sources. Here, we demonstrate down-converting μ-LED phosphors based on CsPbBr3 perovskite nanocrystals grown directly within perfectly sealed mesoporous silica nanospheres (NSs). Through a selective sintering technology, we can synthesize CsPbBr3/SiO2 NSs with emission efficiency >87% at high temperatures (>500oC) without causing interparticle cross-linking or aggregation. The resulting CsPbBr3-SiO2 NSs with uniform size, good solution dispersion, ultra-stable, and high brightness meet the technical requirements of photolithographic inks and can achieve highly uniform μ-LED color conversion patterns with pixels smaller than 20 μm.

In this study, we utilized various Zn precursors (ZnBr2, ZnCl2,Zn(ud)) and adjusted the Zn to In ratio (Zn/In=0~3) to synthesize ZnInP quantum dot (QDs) cores. After optimizing the nucleation conditions, InP QDs were coated with heteroalloy shell, ZnSeS,to form core-shell QDs. After that, DDT was injected and reacted for 5 hr to form ZnInP/ZnSeS/ZnS. Among them, the prepared ZnInP/ZnSeS/ZnS core/shell/shell QDs exhibited an emission wavelength of 603 nm with a full width at half maximum (FWHM) of 52 nm. The narrower the FWHM, the more effectively beneficial for the application of InP QDs in the display industry.

We report a reversible three-state and dual color photoluminescence (PL) intensity modulation of quantum dots (QDs) by electrochemically applying voltages on the Prussian blue (PB) substrate. PB acts as the electro-switchable materials because the applied voltage controls the oxidation state of iron ions in PB. Depending on the oxidation states of iron ions and their redox potential, the charge transfer from QDs to PB can be allowed or blocked, acting as a main mechanism of PL intensity modulation of QDs in multistate. Engineering heterostructures of QDs give rise to additional controllability of PL intensity modulation. The CdS shell on top of CdSe core QDs acts as a hole blocking layer, whereas the ZnSe shell acts as an electron blocking layer. With the combination of the applied voltage and its core/shell heterostructure, we could selectively quench or recover PL intensity of two different QDs which gives dual color tunability.

In the past years, we have been working with "self-assembly" --- Self-assembly of atoms, molecules and nanomaterials on two-dimensional (2D) surface. For 2D self-assembly, asymmetric interface is required such as air-liquid, gas-solid and liquid-solid interfaces, even for the case of nanomaterials. Self-assembly of metal nanoparticles revealed great potential for both fundamental and application via coupling of localized plasmons. Regularly aligned small sized metal nanoparticles (< 10 nm) with nanogap can excite collective mode on their surface. It results in extremely high refractive index and extinction coefficient, regarded as metamaterials (metasurfaces), and exhibits quite unique optical properties. One example is "Electromagnetically induced transparency (EIT)" induced by multilayered metal nanoparticles on mirror and brought up vivid metallic full-colors as a result of LSPR band splitting. In this talk, 2D self-assembly of luminescent QDs such as CdSe/ZnS and perovskite nanocrystals are also introduced with their unique optical properties.

Metal halide perovskite solar cells have achieved efficiencies exceeding 26%, at par with crystalline Silicon. However, concerns of long-term stability and open questions about upscaled manufacturing persist. I will show how atomic layer deposition (ALD) can unlock further progress towards increased efficiency and long-term stability. Permeation barriers prepared by ALD as integral part of the device architecture suppress thermally driven decomposition of the perovskite and inhibit detrimental diffusion of halide species [1]. At the same time, ALD enables novel processing options for the preparation of semitransparent cells [2] and ultra-thin loss-less interconnects for tandem architectures [3] with the prospects to reach efficiency levels beyond 30% [4]. As ALD is originally a vacuum-based batch-processing technique, I will address the prospects of upscaling ALD for high-throughput manufacturing by the introduction of spatial ALD (S-ALD). [1] K. O. Brinkmann et al., Nat. Comms. 2017, 8, 13938. [2] T. Gahlmann et al., Adv. Energy Mater. 10, 1903. [3] K. O. Brinkmann et al., Nature 604, 280 (2022). [4] K.O. Brinkmann et al. Nat. Rev. Mater. DOI: 10.1038/s41578-023-00642-1.

Laser, as a widely used light source, has had a significant impact on our scientific advancements and engineering applications for over 60 years. However, due to the diffraction limit, lasers have been unable to shrink to the nanoscale, thereby limiting their applications at the micro/nanoscale. In recent years, researchers have begun exploring new applications of the surface plasmon effect, allowing electromagnetic fields to be confined within regions smaller than the diffraction limit. In this study, we employ a semiconductor-insulator-metal structure to fabricate substrate-free surface plasmon polariton lasers, reducing the device footprint while also lowering the laser threshold. Importantly, the substrate-free design holds promise for facilitating integration into micro/nanoscale biological applications.

We present a novel photodetector that involves the metasurface-induced scattering of a vertically oriented photon beam into a circularly oriented directional and guided light propagation, resulting in enhanced detection efficiency in an ultra-thin photoabsorption layer. The higher absorption efficiency is enabled by an enhanced photon density of states while substantially reducing the optical group velocity of light and extending the photon material interaction time compared to traditional semiconductor photodetectors without an integrated metasurface. Such detectors with a very thin absorption layer can enable high-efficiency and high-speed photodetectors needed in ultrafast computer networks, data communication, imaging and quantum systems.

CVD growth of graphene from CH4 requires temperatures >900°C. Aromatic hydrocarbons, like benzene and toluene, have been proposed to lower the growth temperatures <600°C, while preserving graphene quality. We investigated the decomposition steps of toluene adsorbed onto Cu(111) and c(4x2)-reconstructed Si(100) surfaces through DFT calculations. The geometry and energy of aromatic adsorbates were analysed in various structural configurations, identifying most stable adsorption sites. Early decomposition reactions were studied through investigation of minimum energy pathway and transition states. Low activation energies were found for H removal from the methyl group of toluene adsorbed on Cu(111) (1.20 eV) or chemisorbed on Si(100) (1.39 eV), leading to benzyl radicals. Anthracene formation from two close-by C7H5 has been studied through meta-dynamics and umbrella sampling applied to molecular dynamics simulations. Two anthracene configurations were obtained onto Cu(111): the zig-zag and armchair ones. The energy cost to produce anthracene does not exceed 1 eV in both cases. A prohibitive energy was obtained for Si, hindering anthracene formation under practical CVD conditions.

Two-dimensional materials, including transition metal dichalcogenides (TMDs), have attracted attention for potential use in electronic, photonic, and optoelectronic applications. Molybdenum disulfide (MoS2) is a widely studied TMD that offers potential for improving speed and efficiency in scaled electronic devices. However, advancing MoS2 and other 2D materials into high volume device manufacturing requires scalable deposition and etching processes that are compatible with manufacturing constraints, such as back-end-of-line (BEOL) compatibility. Atomic layer deposition (ALD) and atomic layer etching (ALE) are scalable deposition processes that deposit and etch films at relatively low temperatures. Together, atomic layer deposition and atomic layer etching constitute complementary facets of atomic layer processing. Here, we describe our recent progress in thermal ALD and thermal ALE of MoS2 films. Combining the two processes offers greater control over MoS2 films. Using ALD followed by ALE and post-deposition annealing, we achieved few-layer MoS2 films. These combined thermal processes represent a pathway for integration of MoS2 films into device manufacturing.

In nanoelectronics, Transition Metal Dichalcogenides (TMDs) like MoS2 are prized for their semiconducting properties. Successful device integration requires a high-quality interface between the TMD and a deposited dielectric. Sulfur-terminated surfaces are hydrophobic and typical atomic layer deposited films are non-continuous. We have studied MoS2 flakes obtained by mechanical exfoliation and Chemical Vapor Deposition (CVD). Mechanical exfoliation can be used to quickly obtain few-layer MoS2 surfaces, but these surfaces exhibit significant variability in quality and cleanliness, leading to unpredictable film growth. CVD-grown MoS2 provides a more reliable starting but still TiO2 and Al2O3 ALD films up to 6 nm thick exhibit a large concentration of pin-hole type features. To enhance ALD film growth we treat the MoS2 surface with mercaptoethanol (ME). Experimental and computational findings reveal that employing ME, coupled with creating surface sulfur defects

This study proposes a novel approach to streamline the manual identification and classification of 2D materials on-chip devices, crucial for rapid prototyping. Leveraging high-resolution imaging and smart stitching techniques, our method achieves a comprehensive representation of the material landscape. Advanced image processing algorithms, including mask-RCNN segmentation, extract key material attributes such as surface area and morphology. A tailored U-Net model is trained for precise material identification, encompassing parameters like composition and thickness. Performance evaluation involves state-of-the-art model architectures and hyperparameter optimization. By automating the material identification process and integrating with a sophisticated transfer system, manual intervention is minimized, expediting prototyping workflows. This framework not only enhances efficiency but also aligns with contemporary trends in materials science and machine learning research, fostering advancements in rapid prototyping capabilities.

2D transition metal dichalcogenides (TMD) are direct-gap semiconductor in the monolayer limit and exhibit strong room-temperature photoluminescence (PL) due to the formation and recombination at room temperature of stable exciton states. Spatial nonuniformities in the intensity and emission wavelength are commonly observed and can be correlated to local deviations from ideal crystalline structure such as inhomogeneities, defects, strain or oxidation. Here we combine high resolution confocal optical microscopy with several complementary spectroscopies such as PL, Reflectance and Raman. We compare these imaging techniques by correlating basic parameters such as resonance wavelengths, width and intensity. This method allows us to identify the effect of the different local variations from ideal crystalline structure by means of a well-defined set of values of the spectroscopic parameters.

In the course of our study, we characterized MoSSe and MoSeS Janus materials derived from MoS2 and MoSe2 correspondingly. Preliminary Raman characterization of the as-synthesized crystal performed in a single point with the single excitation wavelength (532nm) showed weak Raman peaks of the precursors, what was misinterpreted as incomplete conversion. SPM and TERS imaging revealed that the precursor monolayers featured multiple nanoscale multi-layer islands. These islands have been identified in the Janus crystals transferred to Au or Ag via the SPM imaging and their composition was confirmed by TERS imaging. The morphology of the Janus crystals derived from MoS2 and MoSe2 was also fundamentally different. In the course of conversion of MoSe2 to MoSeS accumulated tensile strain led to physical breakage of the crystals. TERS showed that the gaps between the domains in MoSeS monolayers seen in SPM images were physical cracks. Conversely, compressive strain appearing in MoSSe results in the formation of wrinkles that after the transfer to Au or Ag look like cracks in MoSeS, but in reality there is no physical breakage in these crystals.

Interest in low-dimensional magnetic systems surged with the discovery of magnetism in some quasi-2D materials. While ferro- and ferrimagnetic compounds gained attention, antiferromagnetic semiconductors like Metal-transition phospho-trichalcogenides (MPX3) remained less studied. MPX3s are quasi-2D van der Waals semiconductors with diverse antiferromagnetic spin configurations. In this talk, I will describe the properties of acoustic phonons in such materials probed by Brillouin-Mandelstam inelastic light scattering. Acoustic phonons carry heat and contribute to electron–phonon, and magnon–phonon scattering processes. We observed significant variations in acoustic phonon velocities among materials with similar structures. Correlations with available thermal transport data underscore the importance of our findings for understanding layered vdW semiconductors. Authors acknowledge the NSF DMR project No. 2205973 “Controlling Electron, Magnon, and Phonon States in Quasi‐2D Antiferromagnetic Semiconductors for Enabling Novel Device Functionalities” and NSF MRI project No. 2019056 “Development of a Cryogenic Integrated Micro-Raman-Brillouin-Mandelstam Spectrometer.”

Recent advancements have positioned van der Waals heterostructures as a platform for developing optical and optoelectronic devices with unprecedented properties. This talk focuses on our recent research investigating interaction among excitons in two-dimensional materials, exploring possible realization of macroscopic quantum coherent states and applications to nonlinear optics. I will describe the observation of the giant excitonic optical nonlinearity facilitated by exciton-hole interactions. I will also introduce our recent efforts to achieve tightly bound interlayer excitons with low disorder, which lays the groundwork for forming exciton condensates. Finally, we will discuss how these findings have substantial implications for advancing classical and quantum optical information processing and communication.

The family of 2D layered III-VI transition-metal chalcogenides, including GaS, GaSe, GaTe, InS, InSe, InTe, and also Janus structures like GaSSe, exhibit exceptional nonlinear optical properties. The most energetically favorable crystal ordering for this family is AB layer stacking, which breaks central inversion symmetry for an arbitrary number of layers, resulting in non-zero off-diagonal elements of the chi(2) tensor for arbitrary thickness of the materials. Experimentally, the nonresonant second harmonic response of GaSe is the strongest among all the 2D layered crystals. In this talk we will discuss methods to increase the nonlinear response in III-VI transition-metal chalcogenides by material design, using first-principles method based on many-body GW theory, Bethe-Salpeter equation, and Kadanoff-Baym equations, thereby taking band gap renormalization and excitonic effects into account. M. N. L. acknowledges support by the Air Force Office of Scientific Research (AFOSR) under award no. FA9550-23-1-0455. M. N. L. and D. R. E. acknowledge support by the AFOSR under award no. FA9550-23-1-0472 and DARPA under grant no. HR00112220011.

The bright emission from thick flakes makes gallium selenide a fantastic material for understanding the relationship between local strain and optical response. Here, we investigate complex strain distributions by transferring gallium selenide flakes onto nanostructures patterned in close proximity, enabling the study of a variety of strain distributions, such as uniaxial, biaxial, and triaxial strain within a single flake. Our findings reveal that finite strain distributions and resulting bandgap shifts occur in regions of gallium selenide suspended between closely-spaced nanostructures, in good agreement with strain distributions simulated using finite element analysis. This research paves the way for designer strain distributions and tailorable nanophotonic behavior in two-dimensional materials.

Beyond-silicon technology demands ultrahigh-performance field-effect transistors (FETs). Transition metal dichalcogenides (TMDs) provide an ideal material platform, but the device performances such as contact resistance, on/off ratio, and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate (PPC) with a negligible residue coverage of ~0.08% for monolayer MoS2 in the centimeter scale. By incorporating bismuth semimetal contact with atomically-clean monolayer MoS2-FET on h-BN substrate, we obtain an ultralow Ohmic contact resistance RC of ~78 Ω-µm, approaching the quantum limit, and a record-high on/off ratio of ~10^11 at 15 K. Such an ultraclean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting TMDs.

A key signature of topological boundary modes, namely a non-local response, has remained elusive in topological superconductors(TSC). Here we focus on 1D higher-order TSC (HOTSC) chiral modes in FeTe0.55Se0.45, demonstrating they mediate electron co-tunneling (EC) over macroscopic distances, owing to the topological-protection long-term coherence. We found the hinge-mediated EC emerges in an anomalous large range, and produces a robust conductance plateau. Meanwhile, such plateau is robust against increasing temperature and magnetic field, and disappear without hinge contact or topological nontrivial phase of materials. Thus, our experiments reveal the first proof of robust, long-range, and non-local response from the 1D chiral hinge modes in \ch{FeTe_{0.55}Se_{0.45}}, providing a new methodology to explore TSC.

When an intense laser interacts with a gas of atoms, high-order harmonics are generated. In the time domain, this radiation forms a train of extremely short light pulses, of the order of 100 attoseconds. Attosecond pulses allow the study of the dynamics of electrons in atoms and molecules, using pump-probe techniques. This presentation will highlight some of the key steps of the field of attosecond science.