Evidence for Exciton Crystals in a 2D Semiconductor Heterotrilayer

Two-dimensional (2D) transition metal dichalcogenides (TMDC) and their moire interfaces have been demonstrated for correlated electron states, including Mott insulators and electron/hole crystals commensurate with moire superlattices. Here we present spectroscopic evidences for ordered bosons - interlayer exciton crystals in a WSe2/MoSe2/WSe2 trilayer, where the enhanced Coulomb interactions over those in heterobilayers have been predicted to result in exciton ordering. While the dipolar interlayer excitons in the heterobilayer may be ordered by the periodic moire traps, their mutual repulsion results in de-trapping at exciton density larger than 10^11 cm^-2 to form mobile exciton gases and further to electron-hole plasmas, both accompanied by broadening in photoluminescence (PL) peaks and large increases in mobility. In contrast, ordered interlayer excitons in the trilayer are characterized by negligible mobility and by sharper PL peaks persisting to nex larger than 10^12 cm^-2. We find that an optically dark state attributed to the predicted quadrupolar exciton crystal transitions to the bright dipolar excitons either with increasing nex or by an applied electric field. These ordered interlayer excitons may serve as models for the exploration of quantum phase transitions and quantum coherent phenomena.Comment: 16 pages, 4 figures, S

Realization of multiple charge density waves in NbTe2 at the monolayer limit

Abstract: Layered transition-metal dichalcogenides (TMDCs) down to the monolayer (ML) limit provide a fertile platform for exploring charge-density waves (CDWs). Though bulk NbTe2 is known to harbor a single axis 3*1 CDW coexisting with non-trivial quantum properties, the scenario in the ML limit is still experimentally unknown. In this study, we unveil the richness of the CDW phases in ML NbTe2, where not only the theoretically predicted 4*4 and 4*1 phases, but also two unexpected sqrt(28)*sqrt(28) and sqrt(19)*sqrt(19) phases, can be realized. For such a complex CDW system, we establish an exhaustive growth phase diagram via systematic efforts in the material synthesis and scanning tunneling microscope characterization. Moreover, we report that the energetically stable phase is the larger scale order (sqrt(19)*sqrt(19)), which is surprisingly in contradiction to the prior prediction (4*4). These findings are confirmed using two different kinetic pathways, i.e., direct growth at proper growth temperatures (T), and low-T growth followed by high-T annealing. Our results provide a comprehensive diagram of the "zoo" of CDW orders in ML 1T-NbTe2 for the first time and offer a new material platform for studying novel quantum phases in the 2D limit

PTCDA molecular monolayer on Pb thin films: An unusual π-electron Kondo system and its interplay with a quantum-confined superconductor

The hybridization of magnetism and superconductivity has been an intriguing playground for correlated electron systems, hosting various novel physical phenomena. Usually, localized d- or f-electrons are central to magnetism. In this study, by placing a PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) molecular monolayer on ultra-thin Pb films, we built a hybrid magnetism/superconductivity (M/SC) system consisting of only sp electronic levels. The magnetic moments reside in the unpaired molecular orbital originating from interfacial charge-transfers. We reported distinctive tunneling spectroscopic features of such a Kondo screened pi-electron impurity lattice on a superconductor in the regime of TK>>delta suggesting the formation of a two-dimensional bound states band. Moreover, moiré superlattices with tunable twist angle and the quantum confinement in the ultra-thin Pb films provide easy and flexible implementations to tune the interplay between the Kondo physics and the superconductivity, which are rarely present in M/SC hybrid systems.Center for Dynamics and Control of Material

(Invited) Excited-State Dynamical Studies of Positively and Negatively Charged Excitons in Polymer-Wrapped Single-Walled Carbon Nanotube Superstructures

Conjugated, highly charged aryleneethynylene polymers exfoliate, individualize, and disperse single-walled carbon nanotubes (SWNTs) via a single-chain helical wrapping mechanism. These semiconducting polymer-SWNT superstructures are robust in a wide range of aqueous and organic solvents, maintaining a fixed polymer helical pitch length on the SWNT surface; importantly, this single chain polymer wrapping of the SWNT surface solubilizes the nanotube at a minimal polymer:SWNT molar ratio, and provides a facile means to organize functional organic moieties at predefined intervals along the SWNT surface. We have investigated the transient absorptive and dynamical properties of positively and negatively charged excitons (i.e., hole- and electron-trions) in these semiconducting polymer-wrapped SWNT superstructures. This work exploits redox titration experiments that fix SWNT electron or hole polaron concentrations, and ultrafast pump-probe transient spectroscopic measurements that determine trion transient absorptive signatures and dynamics as functions of absolute electron- or hole-doping levels. Global analysis of these data over the entire vis-NIR spectral domain, where these systems display characteristic transient absorptive spectral signatures, provide fundamental new understanding of trion formation and decay mechanisms in semiconducting SWNTs essential for developing electro-optic and photonic materials. Figure. (A) Structural schematic of a chiral aryleneethynylene polymer-wrapped SWNT. (B) Molecular structure of the binaphthalene-based conjugated, polyanionic semiconducting polymer, S -PBN(b)-Ph 5 . (C) Schematic illustration highlighting semiconducting SWNT hole polaron, exciton, positively charged exciton (hole-trion) states. Figure

Dissecting Interlayer Hole and Electron Transfer in Transition Metal Dichalcogenide Heterostructures via Two-Dimensional Electronic Spectroscopy

[Image: see text] Monolayer transition metal dichalcogenides (ML-TMDs) are two-dimensional semiconductors that stack to form heterostructures (HSs) with tailored electronic and optical properties. TMD/TMD-HSs like WS(2)/MoS(2) have type II band alignment and form long-lived (nanosecond) interlayer excitons following sub-100 fs interlayer charge transfer (ICT) from the photoexcited intralayer exciton. While many studies have demonstrated the ultrafast nature of ICT processes, we still lack a clear physical understanding of ICT due to the trade-off between temporal and frequency resolution in conventional transient absorption spectroscopy. Here, we perform two-dimensional electronic spectroscopy (2DES), a method with both high frequency and temporal resolution, on a large-area WS(2)/MoS(2) HS where we unambiguously time resolve both interlayer hole and electron transfer with 34 ± 14 and 69 ± 9 fs time constants, respectively. We simultaneously resolve additional optoelectronic processes including band gap renormalization and intralayer exciton coupling. This study demonstrates the advantages of 2DES in comprehensively resolving ultrafast processes in TMD-HS, including ICT

SAR Time Series Despeckling Based on Additive Signal Component Decomposition in Logarithm Domain

With the substantial improvement of Synthetic Aperture Radar (SAR) regarding swath width and spatial and temporal resolutions, a time series obtained by registering SAR images acquired at different times can provide more accurate information on the dynamic changes in the observed areas. However, inherent speckle noise and outliers along the temporal dimension in the time series pose serious challenges for subsequent interpretation tasks. Although existing state-of-the-art methods can effectively suppress the speckle noise in a SAR time series, outliers along the temporal dimension will interfere with the denoising results. To better solve this problem, this paper proposes an additive signal decomposition method in the logarithm domain that can suppress the speckle noise and separate stable data and outliers along the temporal dimension in a time series, thus eliminating the impact of outliers on the denoising results. When the simulated data are disturbed by outliers, the proposed method can achieve an approximately 3 dB improvement in the Peak Signal-to-Noise Ratio (PSNR) compared to the other state-of-the-art methods. On Sentinel-1 data, the proposed method robustly suppresses the speckle noise in a time series, and the obtained outliers along the temporal dimension provide reference data for subsequent interpretation tasks

(Invited) Photoinduced Charge Transfer Reactions in Chiral, Semiconducting Polymer-Wrapped Single-Walled Carbon Nanotube Superstructures

Highly charged, chiral anionic semiconducting polymers helically wrap single-walled carbon nanontubes (SWNTs) at periodic and constant morphology. These polymers can be used as tools to modulate SWNT electronic properties, provide expansive solubility, or engineer electron acceptor units at rigorously defined intervals along the nanotube backbone. We detail (SWNT)-based nanohybrid compositions based on (6,5) chirality-enriched SWNTs ([(6,5) SWNTs]) and chiral n-type polymers such as ( S -PBN(b)-Ph 4 PDI), that feature a perylenediimide (PDI)-containing repeat unit; S -PBN(b)-Ph 4 PDI-[(6,5) SWNT] superstructures, for example, feature a PDI electron acceptor unit positioned at 3 nm intervals along the nanotube surface, thus controlling rigorously SWNT–electron acceptor stoichiometry and organization. Potentiometric studies determine driving forces for photoinduced charge separation (CS) and thermal charge recombination (CR) reactions, as well as spectroscopic signatures of SWNT hole polaron and PDI radical anion states. Femtosecond pump-probe transient absorption spectroscopic experiments show that excitation into the SWNT E 11 transition of a S -PBN(b)-Ph 4 PDI-[(6,5) SWNT] superstructure generates SWNT hole polaron [(6,5) SWNT (•+)n ] and PDI radical anion (PDI −• ) states. Here we describe related compositions that detail reaction driving force- and solvent-dependent photoinduced CS and thermal CR reactions, and provide insights into the factors that govern photoinduced charge transfer reactions at soft matter-hard matter interfaces defined by polymers and SWNTs. Figure

Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and marten-site exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as "non-inclusion induced crack initiation, NIICI", is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identi-fies the knowledge gap through an examination of subsurface crack initiation and interaction with the lo -cal microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indi-cated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, includ-ing two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. An-other important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack ini-tiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure. (c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology

(Invited) Chiral, Semiconducting Polymer-Wrapped Single-Walled Carbon Nanotube Superstructures for Opto-Electronic Applications

We demonstrate that highly charged, chiral anionic semiconducting polymers helically wrap single-walled carbon nanontubes (SWNTs) at periodic and constant morphology.  These polymers can be used as tools to modulate SWNT electronic properties, provide expansive solubility, or engineer electron acceptor units (e.g., perylene diimide, PDI) at rigorously defined intervals along the nanotube backbone. Femtosecond pump-probe transient absorption spectroscopic experiments show that excitation into the SWNT E 11 transition of a S -PBN(b)-Ph 4 PDI-[(6,5) SWNT] superstructure generates SWNT hole polaron [(6,5) SWNT (•+)n ] and PDI radical anion (PDI −• ) states.  These studies demonstrate for the first time a photoinduced electron transfer process involving a SWNT and a semiconducting polymer in which: (i) the charge-separated products, and (ii) photoinduced charge separation and thermal charge recombination dynamics, are fully characterized.  To fully exploit these and related polymer-SWNT hybrids in opto-electronic applications, we have developed solution-based processes to structure dense, highly aligned arrays of these nano-objects on solid substrates.  Polyanionic semiconducting polymers designed to wrap SWNTs with a fixed helical screw axis, used in combination with ionic self assembly approaches, enable for the first time the production of functionalized SWNTs that are fully soluble in organic solvents and capable of assembly into complex hierarchical structures that feature aligned nanotubes at high areal density (2.5 x10 10 SWNTs cm -2 ) that maintain the opto-electronic properties characteristic of individualized SWNTs.  These porphyrin-polymer/SWNT superstructures can be engineered to undergo both photoinduced electron- and hole-transfer reactions, thereby defining unique energy conversion assemblies, compositions with which to interrogate mechanistic issues regarding photoinduced charge transfer reactions involving nanoscale objects, and utilized as building blocks for highly organized mesoscopic materials that make possible investigation of electron and hole polaron transport phenomena in the solid state, as well as new types of electro-optic materials. Figure