Showing 1–17 of 17 results for author: Hoang, L

Robust Paramagnon and Acoustic Plasmon in a Photo-excited Electron-doped Cuprate Superconductor

Authors: Daniel Jost, Jiarui Li, Jordyn Hales, Jonathan Sobota, Giacomo Merzoni, Leonardo Martinelli, Shuhan Ding, Kejun Xu, Justine Schlappa, Andreas Scherz, Robert Carley, Benjamin E. Van Kuiken, Teguh C. Asmara, Le Phuong Hoang, Laurent Mercadier, Sergii Parchenko, Martin Teichmann, Patrick S. Kirchmann, Giacomo Ghiringhelli, Brian Moritz, Zhi-Xun Shen, Thomas P. Devereaux, Yao Wang, Wei-Sheng Lee

Abstract: Characterizing the spin and charge degrees of freedom in high-temperature superconducting cuprates under non-equilibrium conditions provides new insights into their electronic correlations. However, their collective dynamics have been largely unexplored due to experimental challenges. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to simultaneously tra… ▽ More Characterizing the spin and charge degrees of freedom in high-temperature superconducting cuprates under non-equilibrium conditions provides new insights into their electronic correlations. However, their collective dynamics have been largely unexplored due to experimental challenges. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to simultaneously track the collective spin (paramagnon) and charge (acoustic plasmon) dynamics in an optimally electron-doped cuprate driven out-of-equilibrium by a femtosecond pump laser pulse. Upon pumping, we observed an anti-Stokes signal associated with paramagnon generation, which modifies the paramagnon dispersion near the zone center, though the bandwidth remained unchanged, suggesting no significant alteration to spin exchange interactions. Simultaneously, in the charge sector, the acoustic plasmon's energy and spectral weight decreased, suggesting a light-induced redistribution of charge carriers. The variations of both the paramagnon and the plasmon were locked in time, demonstrating a robust intertwining between the spin and charge degrees of freedom on a femtosecond timescale, even in this non-equilibrium state. △ Less

Submitted 26 November, 2025; originally announced November 2025.

Scaling High-Performance Nanoribbon Transistors with Monolayer Transition Metal Dichalcogenides

Authors: Tara Peña, Anton E. O. Persson, Andrey Krayev, Áshildur Friðriksdóttir, Kathryn Neilson, Zhepeng Zhang, Anh Tuan Hoang, Jerry A. Yang, Lauren Hoang, Andrew J. Mannix, Paul C. McIntyre, Eric Pop

Abstract: Nanoscale transistors require aggressive reduction of all channel dimensions: length, width, and thickness. While monolayer two-dimensional semiconductors (2DS) offer ultimate thickness scaling, good performance has largely been achieved only in micrometer-wide channels. Here, we demonstrate both $\it{n}$- and $\it{p}$-type nanoribbon transistors based on monolayer 2DS, fabricated using a multi-pa… ▽ More Nanoscale transistors require aggressive reduction of all channel dimensions: length, width, and thickness. While monolayer two-dimensional semiconductors (2DS) offer ultimate thickness scaling, good performance has largely been achieved only in micrometer-wide channels. Here, we demonstrate both $\it{n}$- and $\it{p}$-type nanoribbon transistors based on monolayer 2DS, fabricated using a multi-patterning process, reaching channel widths down to 25 nm and lengths down to 50 nm. 'Anchored' contacts improve device yield, while nanoscale imaging, including tip-enhanced photoluminescence, reveals minimal edge degradation. The devices reach on-state currents up to 560, 420, and 130 $μ$A $μ$m$^{-1}$ at 1 V drain-to-source voltage for $\it{n}$-type MoS$_{2}$, WS$_{2}$, and $\it{p}$-type WSe$_{2}$, respectively, integrated with thin high-$κ$ dielectrics. These results surpass prior reports for single-gated nanoribbons, the WS$_{2}$ by over 100 times, even in normally-off (enhancement-mode) transistors. Taken together, these findings suggest that top down patterned 2DS nanoribbons are promising building blocks for future nanosheet transistors. △ Less

Submitted 12 September, 2025; originally announced September 2025.

Comments: 16 pages, 4 figures

Deep Learning to Automate Parameter Extraction and Model Fitting of Two-Dimensional Transistors

Authors: Robert K. A. Bennett, Jan-Lucas Uslu, Harmon F. Gault, Asir Intisar Khan, Lauren Hoang, Tara Peña, Kathryn Neilson, Young Suh Song, Zhepeng Zhang, Andrew J. Mannix, Eric Pop

Abstract: We present a deep learning approach to extract physical parameters (e.g., mobility, Schottky contact barrier height, defect profiles) of two-dimensional (2D) transistors from electrical measurements, enabling automated parameter extraction and technology computer-aided design (TCAD) fitting. To facilitate this task, we implement a simple data augmentation and pre-training approach by training a se… ▽ More We present a deep learning approach to extract physical parameters (e.g., mobility, Schottky contact barrier height, defect profiles) of two-dimensional (2D) transistors from electrical measurements, enabling automated parameter extraction and technology computer-aided design (TCAD) fitting. To facilitate this task, we implement a simple data augmentation and pre-training approach by training a secondary neural network to approximate a physics-based device simulator. This method enables high-quality fits after training the neural network on electrical data generated from physics-based simulations of ~500 devices, a factor >40$\times$ fewer than other recent efforts. Consequently, fitting can be achieved by training on physically rigorous TCAD models, including complex geometry, self-consistent transport, and electrostatic effects, and is not limited to computationally inexpensive compact models. We apply our approach to reverse-engineer key parameters from experimental monolayer WS$_2$ transistors, achieving a median coefficient of determination ($R^2$) = 0.99 when fitting measured electrical data. We also demonstrate that this approach generalizes and scales well by reverse-engineering electrical data on high-electron-mobility transistors while fitting 35 parameters simultaneously. To facilitate future research on deep learning approaches for inverse transistor design, we have published our code and sample data sets online. △ Less

Submitted 7 July, 2025; originally announced July 2025.

Comments: Main text + supplementary information

Low Resistance P-type Contacts to Monolayer WSe$_2$ through Chlorinated Solvent Doping

Authors: Lauren Hoang, Robert K. A. Bennett, Anh Tuan Hoang, Tara Pena, Zhepeng Zhang, Marisa Hocking, Ashley P. Saunders, Fang Liu, Eric Pop, Andrew J. Mannix

Abstract: Tungsten diselenide (WSe$_2$) is a promising p-type semiconductor limited by high contact resistance ($R_\textrm{C}$) and the lack of a reliable doping strategy. Here, we demonstrate that exposing WSe$_2$ to chloroform provides simple and stable p-type doping. In monolayer WSe$_2$ transistors with Pd contacts, chloroform increases the maximum hole current by over 100$\times$ (>200 $μ$A/$μ$m), redu… ▽ More Tungsten diselenide (WSe$_2$) is a promising p-type semiconductor limited by high contact resistance ($R_\textrm{C}$) and the lack of a reliable doping strategy. Here, we demonstrate that exposing WSe$_2$ to chloroform provides simple and stable p-type doping. In monolayer WSe$_2$ transistors with Pd contacts, chloroform increases the maximum hole current by over 100$\times$ (>200 $μ$A/$μ$m), reduces $R_\textrm{C}$ to ~2.5 k$Ω\cdotμ$m, and retains an on/off ratio of $10^{10}$ at room temperature. These improvements persist for over 8 months, survive annealing above 150 °C, and remain effective down to 10 K, enabling a cryogenic $R_\textrm{C}$ of ~1 k$Ω\cdotμ$m. Density functional theory indicates that chloroform strongly physisorbs to WSe$_2$, inducing hole doping with minimal impact on the electronic states between the valence band and conduction band edges. Auger electron spectroscopy and atomic force microscopy reveal that chloroform intercalates at the WSe$_2$ interface with the gate oxide, contributing to doping stability and mitigating interfacial dielectric disorder. This robust, scalable approach enables high-yield WSe$_2$ transistors with good p-type performance. △ Less

Submitted 29 April, 2025; originally announced April 2025.

Synthesis-related nanoscale defects in Mo-based Janus monolayers revealed by cross-correlated AFM and TERS imaging

Authors: Tianyi Zhang, Andrey Krayev, Tilo H. Yang, Nannan Mao, Lauren Hoang, Zhien Wang, Hongwei Liu, Yu-Ren Peng, Yunyue Zhu, Eleonora Isotta, Maria E. Kira, Ariete Righi, Marcos A. Pimenta, Yu-Lun Chueh, Eric Pop, Andrew J. Mannix, Jing Kong

Abstract: Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are promising candidates for various applications in non-linear optics, energy harvesting, and catalysis. These materials are usually synthesized via chemical conversion of pristine TMDs. Nanometer-scale characterization of the obtained Janus materials' morphology and local composition is crucial for both the synthesis optimization… ▽ More Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are promising candidates for various applications in non-linear optics, energy harvesting, and catalysis. These materials are usually synthesized via chemical conversion of pristine TMDs. Nanometer-scale characterization of the obtained Janus materials' morphology and local composition is crucial for both the synthesis optimization and the future device applications. In this work, we present a cross-correlated atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) study of Janus $\mathrm{Mo}_{\mathrm{Se}}^{\mathrm{S}}$ and Janus $\mathrm{Mo}_{\mathrm{S}}^{\mathrm{Se}}$ monolayers synthesized by the hydrogen plasma-assisted chemical conversion of $\mathrm{MoSe}_2$ and $\mathrm{MoS}_2$, respectively. We demonstrate how the choice of the growth substrate and the starting TMD affects the morphology of the resulting Janus material. Furthermore, by employing TERS imaging, we demonstrate the presence of nanoscale islands (~20 nm across) of $\mathrm{MoSe}_2$-$\mathrm{Mo}_{\mathrm{Se}}^{\mathrm{S}}$ ($\mathrm{MoS}_2$-$\mathrm{Mo}_{\mathrm{S}}^{\mathrm{Se}}$) vertical heterostructures originating from the bilayer nanoislands in the precursor monolayer crystals. The understanding of the origins of nanoscale defects in Janus TMDs revealed in our study can help with further optimization of the Janus conversion process towards uniform and wrinkle-/crack-free Janus materials. Moreover, our work shows that cross-correlated AFM and TERS imaging is a powerful and accessible method for studying nanoscale composition and defects in Janus TMD monolayers. △ Less

Submitted 28 March, 2025; originally announced March 2025.

Ultrafast decoupling of polarization and strain in ferroelectric BaTiO$_3$

Authors: Le Phuong Hoang, David Pesquera, Gerard N. Hinsley, Robert Carley, Laurent Mercadier, Martin Teichmann, Saptam Ganguly, Teguh Citra Asmara, Giacomo Merzoni, Sergii Parchenko, Justine Schlappa, Zhong Yin, José Manuel Caicedo Roque, José Santiso, Irena Spasojevic, Cammille Carinan, Tien-Lin Lee, Kai Rossnage, Jörg Zegenhagen, Gustau Catalan, Ivan A. Vartanyants, Andreas Scherz, Giuseppe Mercurio

Abstract: A fundamental understanding of the interplay between lattice structure, polarization and electrons is pivotal to the optical control of ferroelectrics. The interaction between light and matter enables the remote and wireless control of the ferroelectric polarization on the picosecond timescale, while inducing strain, i.e., lattice deformation. At equilibrium, the ferroelectric polarization is prop… ▽ More A fundamental understanding of the interplay between lattice structure, polarization and electrons is pivotal to the optical control of ferroelectrics. The interaction between light and matter enables the remote and wireless control of the ferroelectric polarization on the picosecond timescale, while inducing strain, i.e., lattice deformation. At equilibrium, the ferroelectric polarization is proportional to the strain, and is typically assumed to be so also out of equilibrium. Decoupling the polarization from the strain would remove the constraint of sample design and provide an effective knob to manipulate the polarization by light. Here, upon an above-bandgap laser excitation of the prototypical ferroelectric BaTiO$_3$, we induce and measure an ultrafast decoupling between polarization and strain that begins within 350 fs, by softening Ti-O bonds via charge transfer, and lasts for several tens of picoseconds. We show that the ferroelectric polarization out of equilibrium is mainly determined by photoexcited electrons, instead of the strain. This excited state could serve as a starting point to achieve stable and reversible polarization switching via THz light. Our results demonstrate a light-induced transient and reversible control of the ferroelectric polarization and offer a pathway to control by light both electric and magnetic degrees of freedom in multiferroics. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Journal ref: Nat. Commun. 16 (2025) 7966

Resolving the Electron Plume within a Scanning Electron Microscope

Authors: Francis M. Alcorn, Christopher Perez, Eric J. Smoll, Lauren Hoang, Frederick Nitta, Andrew J. Mannix, A. Alec Talin, Craig Y. Nakakura, David W. Chandler, Suhas Kumar

Abstract: Scanning electron microscopy (SEM), a century-old technique, is today a ubiquitous method of imaging the surface of nanostructures. However, most SEM detectors simply count the number of secondary electrons from a material of interest, and thereby overlook the rich material information contained within them. Here, by simple modifications to a standard SEM tool, we resolve the momentum and energy i… ▽ More Scanning electron microscopy (SEM), a century-old technique, is today a ubiquitous method of imaging the surface of nanostructures. However, most SEM detectors simply count the number of secondary electrons from a material of interest, and thereby overlook the rich material information contained within them. Here, by simple modifications to a standard SEM tool, we resolve the momentum and energy information of secondary electrons by directly imaging the electron plume generated by the electron beam of the SEM. Leveraging these spectroscopic imaging capabilities, our technique is able to image lateral electric fields across a prototypical silicon p-n junctions and to distinguish differently doped regions, even when buried beyond depths typically accessible by SEM. Intriguingly, the sub-surface sensitivity of this technique reveals unexpectedly strong surface band bending within nominally passivated semiconductor structures, providing useful insights for complex layered component designs, in which interfacial dynamics dictate device operation. These capabilities for non-invasive, multi-modal probing of complicated electronic components are crucial in today's electronic manufacturing but is largely inaccessible even with sophisticated techniques. These results show that seemingly simple SEM can be extended to probe complex and useful material properties. △ Less

Submitted 10 January, 2025; originally announced January 2025.

Comments: Main text: 16 pages, 5 figure. Supporting information: 18 pages, 14 figures

Journal ref: ACS Nano 2024, 18, 49, 33479-33490

Enabling P-type Conduction in Bilayer WS2 with NbP Topological Semimetal Contacts

Authors: Lauren Hoang, Asir Intisar Khan, Robert K. A. Bennett, Hyun-mi Kim, Zhepeng Zhang, Marisa Hocking, Ae Rim Choi, Il-Kwon Oh, Andrew J. Mannix, Eric Pop

Abstract: Two-dimensional (2D) semiconductors are promising for low-power complementary metal oxide semiconductor (CMOS) electronics, which require ultrathin n- and p-type transistor channels. Among 2D semiconductors, WS2 is expected to have good conduction for both electrons and holes, but p-type WS2 transistors have been difficult to realize due to the relatively deep valence band and the presence of mid-… ▽ More Two-dimensional (2D) semiconductors are promising for low-power complementary metal oxide semiconductor (CMOS) electronics, which require ultrathin n- and p-type transistor channels. Among 2D semiconductors, WS2 is expected to have good conduction for both electrons and holes, but p-type WS2 transistors have been difficult to realize due to the relatively deep valence band and the presence of mid-gap states with conventional metal contacts. Here, we report topological semimetal NbP as p-type electrical contacts to bilayer WS2 with up to 5.8 microamperes per micron hole current at room temperature; this is the highest to date for sub 2 nm thin WS2 and more than 50 times larger than with metals like Ni or Pd. The p-type conduction is enabled by the simultaneously high work function and low density of states of the NbP, which reduce Fermi level pinning. These contacts are sputter-deposited at room temperature, an approach compatible with CMOS fabrication, a step towards enabling ultrathin WS2 semiconductors in future nanoelectronics. △ Less

Submitted 27 September, 2024; originally announced September 2024.

Excitation laser energy dependence of the gap-mode TERS spectra of WS$_2$ and MoS$_2$ on silver

Authors: Andrey Krayev, Eleonora Isotta, Lauren Hoang, Jerry A. Yang, Kathryn Neilson, Minyuan Wang, Noah Haughn, Eric Pop, Andrew Mannix, Oluwaseyi Balogun, Chih-Feng Wang

Abstract: We present a systematic study of the dependence of gap mode tip-enhanced Raman scattering (TERS) of mono- and bi-layer WS$_2$ and MoS$_2$ as a function of excitation laser energy. We collected consecutive TERS maps of mono-and bi-layer regions with 6 different excitation lasers. To decrease the acquisition time, we used for the first time concurrent excitation and collection with two lasers simult… ▽ More We present a systematic study of the dependence of gap mode tip-enhanced Raman scattering (TERS) of mono- and bi-layer WS$_2$ and MoS$_2$ as a function of excitation laser energy. We collected consecutive TERS maps of mono-and bi-layer regions with 6 different excitation lasers. To decrease the acquisition time, we used for the first time concurrent excitation and collection with two lasers simultaneously. We found that the E$_{2g}$/A$_{1g}$ peak intensity ratio for bilayer WS$_2$@Ag and the A'/A$_{1g}$ peak intensity ratio of the out-of-plane modes for mono- and bilayer change in a significantly non-monotonous way with excitation laser energies from 1.58 to 2.62 eV. The former ratio increases at energies corresponding to A and B excitons in bilayer WS$_2$. The intensity of the A peak in the monolayer, and hence the A/A$_{1g}$ ratio, is surprisingly high at low excitation energies, dips dramatically at energy corresponding to the A exciton, and is restored partially in between A and B excitons, though still showing a descending trend with increasing energy. A similar picture was observed in mono- and bi-layer MoS$_2$, though the existing set of lasers did not match its excitonic profile as nicely as for WS$_2$. We attribute the observed behavior to intermediate (Fano resonance) or strong (Rabi splitting) coupling between the excitons in transition metal dichalcogenides (TMDs) and the plasmons in the tip-substrate nanocavity. This is akin to the so-called Fano (Rabi) transparency experimentally observed in far field scattering from TMDs between two plasmonic metals. The possibility of intermediate/strong coupling between excitonic resonances in TMDs and the nanocavity re-evaluates the role of resonances in gap-mode TERS and should become an important factor to be considered by TERS practitioners when planning experiments. Finally, we propose the ideal substrate for efficient TERS and tip enhanced photoluminescence measurements. △ Less

Submitted 18 July, 2024; originally announced July 2024.

Comments: 21 pages, 10 figures

Mobility and Threshold Voltage Extraction in Transistors with Gate-Voltage-Dependent Contact Resistance

Authors: Robert K. A. Bennett, Lauren Hoang, Connor Cremers, Andrew J. Mannix, Eric Pop

Abstract: The mobility of emerging (e.g., two-dimensional, oxide, organic) semiconductors is commonly estimated from transistor current-voltage measurements. However, such devices often experience contact gating, i.e., electric fields from the gate modulate the contact resistance during measurements, which can lead conventional extraction techniques to estimate mobility incorrectly even by a factor >2. This… ▽ More The mobility of emerging (e.g., two-dimensional, oxide, organic) semiconductors is commonly estimated from transistor current-voltage measurements. However, such devices often experience contact gating, i.e., electric fields from the gate modulate the contact resistance during measurements, which can lead conventional extraction techniques to estimate mobility incorrectly even by a factor >2. This error can be minimized by measuring transistors at high gate-source bias, |$V_\mathrm{gs}$|, but this regime is often inaccessible in emerging devices that suffer from high contact resistance or early gate dielectric breakdown. Here, we propose a method of extracting mobility in transistors with gate-dependent contact resistance that does not require operation at high |$V_\mathrm{gs}$|, enabling accurate mobility extraction even in emerging transistors with strong contact gating. Our approach relies on updating the transfer length method (TLM) and can achieve <10% error even in regimes where conventional techniques overestimate mobility by >2$\times$. △ Less

Submitted 20 February, 2025; v1 submitted 29 April, 2024; originally announced April 2024.

Comments: Updated to include citation information for published manuscript on page 1; corrected direction of increasing channel length in Fig. 2b; minor changes to wording/phrasing throughout

The Heisenberg-RIXS instrument at the European XFEL

Authors: Justine Schlappa, Giacomo Ghiringhelli, Benjamin E. Van Kuiken, Martin Teichmann, Piter S. Miedema, Jan Torben Delitz, Natalia Gerasimova, Serguei Molodtsov, Luigi Adriano, Bernard Baranasic, Carsten Broers, Robert Carley, Patrick Gessler, Nahid Ghodrati, David Hickin, Le Phuong Hoang, Manuel Izquierdo, Laurent Mercadier, Giuseppe Mercurio, Sergii Parchenko, Marijan Stupar, Zhong Yin, Leonardo Martinelli, Giacomo Merzoni, Ying Ying Peng , et al. (22 additional authors not shown)

Abstract: Resonant Inelastic X-ray Scattering (RIXS) is an ideal X-ray spectroscopy method to push the combination of energy and time resolutions to the Fourier transform ultimate limit, because it is unaffected by the core-hole lifetime energy broadening. And in pump-probe experiments the interaction time is made very short by the same core-hole lifetime. RIXS is very photon hungry so it takes great advant… ▽ More Resonant Inelastic X-ray Scattering (RIXS) is an ideal X-ray spectroscopy method to push the combination of energy and time resolutions to the Fourier transform ultimate limit, because it is unaffected by the core-hole lifetime energy broadening. And in pump-probe experiments the interaction time is made very short by the same core-hole lifetime. RIXS is very photon hungry so it takes great advantage from high repetition rate pulsed X-ray sources like the European XFEL. The hRIXS instrument is designed for RIXS experiments in the soft X-ray range with energy resolution approaching the Fourier and the Heisenberg limits. It is based on a spherical grating with variable line spacing (VLS) and a position-sensitive 2D detector. Initially, two gratings are installed to adequately cover the whole photon energy range. With optimized spot size on the sample and small pixel detector the energy resolution can be better than 40 meV at any photon energy below 1000 eV. At the SCS instrument of the European XFEL the spectrometer can be easily positioned thanks to air-pads on a high-quality floor, allowing the scattering angle to be continuously adjusted over the 65-145 deg range. It can be coupled to two different sample interaction chamber, one for liquid jets and one for solids, each equipped at the state-of-the-art and compatible for optical laser pumping in collinear geometry. The measured performances, in terms of energy resolution and count rate on the detector, closely match design expectations. hRIXS is open to public users since the summer of 2022. △ Less

Submitted 13 March, 2024; originally announced March 2024.

Comments: 43 pages, 12 figures, Supplemental Material

Chemically Tailored Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition

Authors: Zhepeng Zhang, Lauren Hoang, Marisa Hocking, Jenny Hu, Gregory Zaborski Jr., Pooja Reddy, Johnny Dollard, David Goldhaber-Gordon, Tony F. Heinz, Eric Pop, Andrew J. Mannix

Abstract: Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are an exciting platform for new excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, and heterostructures. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly cry… ▽ More Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are an exciting platform for new excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, and heterostructures. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly crystalline 2D TMDCs with controlled doping. Here, we report a hybrid MOCVD growth method that combines liquid-phase metal precursor deposition and vapor-phase organo-chalcogen delivery to leverage the advantages of both MOCVD and SS-CVD. Using our hybrid approach, we demonstrate WS$_2$ growth with tunable morphologies - from separated single-crystal domains to continuous monolayer films - on a variety of substrates, including sapphire, SiO$_2$, and Au. These WS$_2$ films exhibit narrow neutral exciton photoluminescence linewidths down to 33 meV and room-temperature mobility up to 34 - 36 cm$^2$V$^-$$^1$s$^-$$^1$). Through simple modifications to the liquid precursor composition, we demonstrate the growth of V-doped WS$_2$, MoxW$_1$$_-$$_x$S$_2$ alloys, and in-plane WS$_2$-MoS$_2$ heterostructures. This work presents an efficient approach for addressing a variety of TMDC synthesis needs on a laboratory scale. △ Less

Submitted 6 March, 2024; originally announced March 2024.

Noncollinear electric dipoles in a polar, chiral phase of CsSnBr$_3$ perovskite

Authors: Douglas H. Fabini, Kedar Honasoge, Adi Cohen, Sebastian Bette, Kyle M. McCall, Constantinos C. Stoumpos, Steffen Klenner, Mirjam Zipkat, Le Phuong Hoang, Jürgen Nuss, Reinhard K. Kremer, Mercouri G. Kanatzidis, Omer Yaffe, Stefan Kaiser, Bettina V. Lotsch

Abstract: Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two unstudied phases of the 3D perovskite, CsSnBr$_3$, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II… ▽ More Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two unstudied phases of the 3D perovskite, CsSnBr$_3$, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K<$T$<85 K, space group $P2_1/m$) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I ($T$<77 K, space group $P2_1$) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and a large, intrinsic polarization$-$electrostriction coefficient along the polar axis ($Q_{22}\approx$1.1 m$^4$ C$^{-2}$), resulting in acute negative thermal expansion ($α_V=-9\times10^{-5}$ K$^{-1}$) through the onset of spontaneous polarization. The unprecedented structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and $^{81}$Br nuclear quadrupole resonance spectroscopies. Relativistic electronic structure scenarios compatible with reported photoluminescence measurements are discussed. Together, the polar symmetry, small bandgap, large spin-orbit splitting of Sn 5$p$ orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr$_3$ or its neighboring solid solutions as strong candidates for bulk Rashba effects. △ Less

Submitted 25 April, 2024; v1 submitted 15 January, 2024; originally announced January 2024.

Biaxial Tensile Strain Enhances Electron Mobility of Monolayer Transition Metal Dichalcogenides

Authors: Jerry A. Yang, Robert K. A. Bennett, Lauren Hoang, Zhepeng Zhang, Kamila J. Thompson, Antonios Michail, John Parthenios, Konstantinos Papagelis, Andrew J. Mannix, Eric Pop

Abstract: Strain engineering can modulate the material properties of two-dimensional (2D) semiconductors for electronic and optoelectronic applications. Recent theory and experiments have found that uniaxial tensile strain can improve the electron mobility of monolayer MoS$_2$, a 2D semiconductor, but the effects of biaxial strain on charge transport are not well-understood in 2D semiconductors. Here, we us… ▽ More Strain engineering can modulate the material properties of two-dimensional (2D) semiconductors for electronic and optoelectronic applications. Recent theory and experiments have found that uniaxial tensile strain can improve the electron mobility of monolayer MoS$_2$, a 2D semiconductor, but the effects of biaxial strain on charge transport are not well-understood in 2D semiconductors. Here, we use biaxial tensile strain on flexible substrates to probe the electron mobility in monolayer WS$_2$ and MoS$_2$ transistors. This approach experimentally achieves ~2x higher on-state current and mobility with ~0.3% applied biaxial strain in WS$_2$, the highest mobility improvement at the lowest strain reported to date. We also examine the mechanisms behind this improvement through density functional theory simulations, concluding that the enhancement is primarily due to reduced intervalley electron-phonon scattering. These results underscore the role of strain engineering 2D semiconductors for flexible electronics, sensors, integrated circuits, and other optoelectronic applications. △ Less

Submitted 28 November, 2023; v1 submitted 19 September, 2023; originally announced September 2023.

Comments: Corrected author list

Probing the Surface Polarization of Ferroelectric Thin Films by X-ray Standing Waves

Authors: Le Phuong Hoang, Irena Spasojevic, Tien-Lin Lee, David Pesquera, Kai Rossnagel, Jörg Zegenhagen, Gustau Catalan, Ivan A. Vartanyants, Andreas Scherz, Giuseppe Mercurio

Abstract: Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, it is demonstrated that the X-ray standing wave technique allows the polarization near the surface of a ferroelectric thin film to be probed directly. The… ▽ More Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, it is demonstrated that the X-ray standing wave technique allows the polarization near the surface of a ferroelectric thin film to be probed directly. The X-ray standing wave technique is employed to determine, with picometer accuracy, Ti and Ba atomic positions near the surface of three differently strained $\mathrm{BaTiO_3}$ thin films grown on scandate substrates, with a $\mathrm{SrRuO_3}$ film as bottom electrode. This technique gives direct access to atomic positions, and thus to the local ferroelectric polarization, within the first 3 unit cells below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface, upon exposure to ambient conditions, is obtained. The combination of structural and spectroscopic information allows us to conclude on the most plausible mechanisms that stabilize the surface polarization in the three samples under study. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to the type, amount and spatial distribution of the oxygen-containing adsorbates. △ Less

Submitted 4 September, 2023; originally announced September 2023.

Correlation between Macroscopic and Microscopic Relaxation Dynamics of Water: Evidence for Two Liquid Forms

Authors: Nguyen Q. Vinh, Luan C. Doan, Ngoc L. H. Hoang, Jiarong R. Cui, Ben Sindle

Abstract: Water is vital for life, and without it biomolecules and cells cannot maintain their structures and functions. The remarkable properties of water originate from its ability to form hydrogen-bonding networks and dynamics, which the connectivity constantly alters because of the orientation rotation of individual water molecules. Experimental investigation of the dynamics of water, however, has prove… ▽ More Water is vital for life, and without it biomolecules and cells cannot maintain their structures and functions. The remarkable properties of water originate from its ability to form hydrogen-bonding networks and dynamics, which the connectivity constantly alters because of the orientation rotation of individual water molecules. Experimental investigation of the dynamics of water, however, has proven challenging due to the strong absorption of water at terahertz frequencies. In response, by employing a high-precision terahertz spectrometer, we have measured and characterized the terahertz dielectric response of water from supercooled liquid to near the boiling point to explore the motions. The response reveals dynamic relaxation processes corresponding to the collective orientation, single-molecule rotation, and structural rearrangements resulting from breaking and reforming hydrogen bonds in water. We have observed the direct relationship between the macroscopic and microscopic relaxation dynamics of water, and the results have provided evidence of two liquid forms in water with different transition temperatures and thermal activation energies. The results reported here thus provide an unprecedented opportunity to directly test microscopic computational models of water dynamics. △ Less

Submitted 31 May, 2023; originally announced May 2023.

Journal ref: Journal of Chemical Physics 158, 204507 (2023)

Exact Solution of Two-Dimensional Screened Donor State in a Magnetic Field

Authors: Le Van Hoang, Le Tran The Duy, Hoang Do Ngoc Tram, Ngo Dinh Nguyen Thach, Le Thi Ngoc Anh

Abstract: The use of Levi-Civita transformation allows us to formulate the problem of two-dimensional screened donor states in a magnetic field as that of two-dimensional anharmonic oscillator. Therefore, the operator method can be directly used for the first problem and the exact solutions of Schrodinger equation are obtained correspondently. In our approach, wave-functions are constructed in the represe… ▽ More The use of Levi-Civita transformation allows us to formulate the problem of two-dimensional screened donor states in a magnetic field as that of two-dimensional anharmonic oscillator. Therefore, the operator method can be directly used for the first problem and the exact solutions of Schrodinger equation are obtained correspondently. In our approach, wave-functions are constructed in the representation of annihilation and creation operators, which permits one to use purely algebraic method in further calculations of other characteristics. The considered problem is related to the motion of 2D electron gas in GaAs/AlGaAs multiple-quantum well structures with the presence of a magnetic field, which continues to provide new and fascinating phenomena. △ Less

Submitted 16 October, 2004; v1 submitted 15 October, 2004; originally announced October 2004.

Comments: 9 pages, 3 figures