Showing 1–18 of 18 results for author: Mannix, A J

Fermiology and the Candidate Chiral Superconductor in Rhombohedral Tetralayer Graphene

Authors: Sandesh S. Kalantre, Ben H. Alexander, Julian May-Mann, Jonah Herzog-Arbeitman, Marisa Hocking, Qingrui Cao, Kenji Watanabe, Takashi Taniguchi, David Goldhaber-Gordon, Andrew J. Mannix, Trithep Devakul, Yves H. Kwan, Daniel E. Parker, Aaron Sharpe

Abstract: Chiral superconductivity, in which the phase of the superconducting order parameter winds in momentum space, has long been sought for its close link to topological superconductivity. Recent work reported a superconductor in rhombohedral multilayer graphene emerging from a time-reversal symmetry broken normal state, suggesting that it could be a chiral superconductor. However, the possibility of ch… ▽ More Chiral superconductivity, in which the phase of the superconducting order parameter winds in momentum space, has long been sought for its close link to topological superconductivity. Recent work reported a superconductor in rhombohedral multilayer graphene emerging from a time-reversal symmetry broken normal state, suggesting that it could be a chiral superconductor. However, the possibility of chirality depends on the symmetry and structure of the normal-state Fermi surface, which have not been directly measured. Here we measure quantum oscillations in rhombohedral tetralayer graphene over a broad range of the phase diagram, including the superconducting region. At densities well above the onset of superconductivity, we reproduce previously-reported oscillations consistent with a spin- and valley-polarized quarter metal with a single simply-connected Fermi pocket. As the carrier density is reduced, we find a transition to a complex "multitone" state that persists through the superconducting region. This state's spectrum of quantum oscillations is incompatible with a simply-connected quarter metal. The next-simplest candidate normal states suggested by our microscopic modeling (fully-polarized annular, nematic, and three-pocket states) are inconsistent with our measurements, albeit difficult to rule out entirely. The normal state is thus seen to be richer than previously envisaged, reshaping the search for the superconducting mechanism and the possible chirality of the pairing channel. △ Less

Submitted 3 June, 2026; originally announced June 2026.

Comments: 12+60 pages, 4+52 figures

Interfacial control of hot-carrier extraction and photostability in two-dimensional materials

Authors: Claudia Gollner, Mohammad Taghinejad, Chenyi Xia, Zhepeng Zhang, Fang Liu, Francesco Laudani, Annette Foelske, Mark L. Brongersma, Andrew J. Mannix, Tony F. Heinz, Aaron Lindenberg

Abstract: Two-dimensional transition metal dichalcogenides (TMDCs) are promising materials for next-generation optoelectronic devices, yet their implementation is hindered by limited sample stability and challenges in forming reliable electrical contacts. Here, by utilizing time-domain THz emission spectroscopy we directly probe charge carrier dynamics in monolayer WS2 on gold (Au) and fused silica (SiO2) a… ▽ More Two-dimensional transition metal dichalcogenides (TMDCs) are promising materials for next-generation optoelectronic devices, yet their implementation is hindered by limited sample stability and challenges in forming reliable electrical contacts. Here, by utilizing time-domain THz emission spectroscopy we directly probe charge carrier dynamics in monolayer WS2 on gold (Au) and fused silica (SiO2) as a function of interface morphology. For laser excitation above the band gap of WS2, we independently extract effective transport times for both electrons and holes and find that discontinuous WS2 contacts on rough Au generate larger net photocurrents than uniform, strongly coupled interfaces - a counterintuitive observation attributed to imbalanced electron and hole transfer from WS2 to Au. Crucially, we demonstrate that ultrafast charge extraction and separation suppress recombination-driven energy release and thereby prevent photo-induced degradation under ambient conditions, eliminating the need for encapsulation. These findings redefine interfacial design as a central control parameter for both performance and stability in 2D optoelectronic devices. △ Less

Submitted 8 May, 2026; originally announced May 2026.

Scaling Nanoribbon Transistors with Monolayer Transition Metal Dichalcogenides

Authors: Tara Peña, Anton E. O. Persson, Andrey Krayev, Áshildur Friðriksdóttir, Haotian Su, Yuan-Mau Lee, Young Suh Song, Kathryn Neilson, Zhepeng Zhang, Anh Tuan Hoang, Jerry A. Yang, Lauren Hoang, Shan X. Wang, 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 and lengths down to 25-30 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 4 June, 2026; v1 submitted 12 September, 2025; originally announced September 2025.

Comments: 22 pages, 9 figures

Journal ref: Nature Nanotechnology 21 (2026) 803-809

High-current p-type transistors from precursor-engineered synthetic monolayer WSe$_2$

Authors: Anh Tuan Hoang, Kathryn Neilson, Kaikui Xu, Yucheng Yang, Stephanie M. Ribet, Tara Peña, Giulio D'Acunto, Young Suh Song, Anton E. O. Persson, William Millsaps, Colin Ophus, Matthew R. Rosenberger, Eric Pop, Andrew J. Mannix

Abstract: Monolayer tungsten diselenide (WSe$_2$) is a leading candidate for nanoscale complementary logic. However, high defect densities introduced during thin-film growth and device fabrication have limited p-type transistor performance. Here, we report a combined strategy of precursor-engineered chemical vapor deposition and damage-minimizing fabrication to overcome this limitation. By converting tungst… ▽ More Monolayer tungsten diselenide (WSe$_2$) is a leading candidate for nanoscale complementary logic. However, high defect densities introduced during thin-film growth and device fabrication have limited p-type transistor performance. Here, we report a combined strategy of precursor-engineered chemical vapor deposition and damage-minimizing fabrication to overcome this limitation. By converting tungsten trioxide and residual oxyselenides into reactive suboxides before growth, and precisely regulating selenium delivery during deposition, we synthesize uniform, centimeter-scale monolayer WSe$_2$ films with charged defect densities as low as $5 \times 10^{9}$ cm$^{-2}$. Transistors fabricated from these films achieve record p-type on-state current up to $888 μ$A$\cdotμ$m$^{-1}$ at $V_{\mathrm{DS}}=-1$ V, matching leading n-type devices. This leap in material quality closes the p-type performance gap without exotic doping or contact materials, marking a critical step towards complementary two-dimensional semiconductor circuits. △ Less

Submitted 8 September, 2025; originally announced September 2025.

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

Wide-field Hyperspectral Optical Microscopy for Rapid Characterization of Two-Dimensional Semiconductors and Heterostructures

Authors: Zhenghan Peng, Adeyemi Uthman, Zhepeng Zhang, Anh Tuan Hoang, Xiang Zhu, Eric Pop, Andrew J. Mannix

Abstract: Electronic and optoelectronic applications of two-dimensional (2D) semiconductors demand precise control over material quality, including thickness, composition, doping, and defect density. Conventional benchmarking methods (e.g., charge transport, confocal mapping, electron or scanning probe microscopy) are slow, perturb sample quality, or involve trade-offs between speed, resolution, and scan ar… ▽ More Electronic and optoelectronic applications of two-dimensional (2D) semiconductors demand precise control over material quality, including thickness, composition, doping, and defect density. Conventional benchmarking methods (e.g., charge transport, confocal mapping, electron or scanning probe microscopy) are slow, perturb sample quality, or involve trade-offs between speed, resolution, and scan area. To accelerate assessment of 2D semiconductors, we demonstrate a broadband, wide-field hyperspectral optical microscope for 2D materials (2D-HOM) that rapidly captures a spatial-spectral data cube within seconds. The data cube includes x-y spatial coordinate (a 300 * 300 $μ\mathrm{m}^2$ field, with ~ 1 $μ\mathrm{m}$ resolution) and a selectable wavelength range between 1100 to 200 nm at each pixel. Using synthesized films and heterostructures of transition metal dichalcogenides ($\mathrm{MoS}_{2}$, $\mathrm{WS}_{2}$, $\mathrm{V}_{x}\mathrm{W}_{1-x}\mathrm{S}_{2}$, and $\mathrm{WSe}_{2}$), we show that this cost-effective technique detects spectral fingerprints of material identity, doping, grain boundaries, and alloy composition, and enables advanced analysis, including unsupervised machine learning for spatial segmentation. △ Less

Submitted 23 June, 2025; originally announced June 2025.

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.

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.

Quantitative determination of twist angle and strain in Van der Waals moiré superlattices

Authors: Steven J. Tran, Jan-Lucas Uslu, Mihir Pendharkar, Joe Finney, Aaron L. Sharpe, Marisa Hocking, Nathan J. Bittner, Kenji Watanabe, Takashi Taniguchi, Marc A. Kastner, Andrew J. Mannix, David Goldhaber-Gordon

Abstract: Scanning probe techniques are popular, non-destructive ways to visualize the real space structure of Van der Waals moirés. The high lateral spatial resolution provided by these techniques enables extracting the moiré lattice vectors from a scanning probe image. We have found that the extracted values, while precise, are not necessarily accurate. Scan-to-scan variations in the behavior of the piezo… ▽ More Scanning probe techniques are popular, non-destructive ways to visualize the real space structure of Van der Waals moirés. The high lateral spatial resolution provided by these techniques enables extracting the moiré lattice vectors from a scanning probe image. We have found that the extracted values, while precise, are not necessarily accurate. Scan-to-scan variations in the behavior of the piezos which drive the scanning probe, and thermally-driven slow relative drift between probe and sample, produce systematic errors in the extraction of lattice vectors. In this Letter, we identify the errors and provide a protocol to correct for them. Applying this protocol to an ensemble of ten successive scans of near-magic-angle twisted bilayer graphene, we are able to reduce our errors in extracting lattice vectors to less than 1%. This translates to extracting twist angles with a statistical uncertainty less than 0.001° and uniaxial heterostrain with uncertainty on the order of 0.002%. △ Less

Submitted 12 June, 2024; originally announced June 2024.

Comments: 20 pages including supplementary material and 3 main figures

Deterministic fabrication of graphene hexagonal boron nitride moiré superlattices

Authors: Rupini V. Kamat, Aaron L. Sharpe, Mihir Pendharkar, Jenny Hu, Steven J. Tran, Gregory Zaborski Jr., Marisa Hocking, Joe Finney, Kenji Watanabe, Takashi Taniguchi, Marc A. Kastner, Andrew J. Mannix, Tony Heinz, David Goldhaber-Gordon

Abstract: The electronic properties of moiré heterostructures depend sensitively on the relative orientation between layers of the stack. For example, near-magic-angle twisted bilayer graphene (TBG) commonly shows superconductivity, yet a TBG sample with one of the graphene layers rotationally aligned to a hexagonal Boron Nitride (hBN) cladding layer provided the first experimental observation of orbital fe… ▽ More The electronic properties of moiré heterostructures depend sensitively on the relative orientation between layers of the stack. For example, near-magic-angle twisted bilayer graphene (TBG) commonly shows superconductivity, yet a TBG sample with one of the graphene layers rotationally aligned to a hexagonal Boron Nitride (hBN) cladding layer provided the first experimental observation of orbital ferromagnetism. To create samples with aligned graphene/hBN, researchers often align edges of exfoliated flakes that appear straight in optical micrographs. However, graphene or hBN can cleave along either zig-zag or armchair lattice directions, introducing a 30 degree ambiguity in the relative orientation of two flakes. By characterizing the crystal lattice orientation of exfoliated flakes prior to stacking using Raman and second-harmonic generation for graphene and hBN, respectively, we unambiguously align monolayer graphene to hBN at a near-0 degree, not 30 degree, relative twist angle. We confirm this alignment by torsional force microscopy (TFM) of the graphene/hBN moiré on an open-face stack, and then by cryogenic transport measurements, after full encapsulation with a second, non-aligned hBN layer. This work demonstrates a key step toward systematically exploring the effects of the relative twist angle between dissimilar materials within moiré heterostructures. △ Less

Submitted 28 May, 2024; originally announced May 2024.

Comments: 40 pages, 15 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

Thermal relaxation of strain and twist in ferroelectric hexagonal boron nitride moiré interfaces

Authors: Marisa Hocking, Christina E. Henzinger, Steven Tran, Mihir Pendharkar, Nathan J. Bittner, Kenji Watanabe, Takashi Taniguchi, David Goldhaber-Gordon, Andrew J. Mannix

Abstract: New properties can arise at van der Waals (vdW) interfaces hosting a moiré pattern generated by interlayer twist and strain. However, achieving precise control of interlayer twist/strain remains an ongoing challenge in vdW heterostructure assembly, and even subtle variation in these structural parameters can create significant changes in the moiré period and emergent properties. Characterizing the… ▽ More New properties can arise at van der Waals (vdW) interfaces hosting a moiré pattern generated by interlayer twist and strain. However, achieving precise control of interlayer twist/strain remains an ongoing challenge in vdW heterostructure assembly, and even subtle variation in these structural parameters can create significant changes in the moiré period and emergent properties. Characterizing the rate of interlayer twist/strain relaxation during thermal annealing is critical to establish a thermal budget for vdW heterostructure construction and may provide a route to improve the homogeneity of the interface or to control its final state. Here, we characterize the spatial and temporal dependence of interfacial twist and strain relaxation in marginally-twisted hBN/hBN interfaces heated under conditions relevant to vdW heterostructure assembly and typical sample annealing. We find that the ferroelectric hBN/hBN moiré relaxes minimally during annealing in air at typical assembly temperatures of 170°C. However, at 400°C, twist angle relaxes significantly, accompanied by a decrease in spatial uniformity. Uniaxial heterostrain initially increases and then decreases over time, becoming increasingly non-uniform in direction. Structural irregularities such as step edges, contamination bubbles, or contact with the underlying substrate result in local inhomogeneity in the rate of relaxation. △ Less

Submitted 14 March, 2024; originally announced March 2024.

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.

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

Torsional Force Microscopy of Van der Waals Moirés and Atomic Lattices

Authors: Mihir Pendharkar, Steven J. Tran, Gregory Zaborski Jr., Joe Finney, Aaron L. Sharpe, Rupini V. Kamat, Sandesh S. Kalantre, Marisa Hocking, Nathan J. Bittner, Kenji Watanabe, Takashi Taniguchi, Bede Pittenger, Christina J. Newcomb, Marc A. Kastner, Andrew J. Mannix, David Goldhaber-Gordon

Abstract: In a stack of atomically-thin Van der Waals layers, introducing interlayer twist creates a moiré superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system's electronic properties. Setting a precise and uniform twist angle for a stack remains difficult, hence determining that twist angle and mapping its s… ▽ More In a stack of atomically-thin Van der Waals layers, introducing interlayer twist creates a moiré superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system's electronic properties. Setting a precise and uniform twist angle for a stack remains difficult, hence determining that twist angle and mapping its spatial variation is very important. Techniques have emerged to do this by imaging the moiré, but most of these require sophisticated infrastructure, time-consuming sample preparation beyond stack synthesis, or both. In this work, we show that Torsional Force Microscopy (TFM), a scanning probe technique sensitive to dynamic friction, can reveal surface and shallow subsurface structure of Van der Waals stacks on multiple length scales: the moirés formed between bi-layers of graphene and between graphene and hexagonal boron nitride (hBN), and also the atomic crystal lattices of graphene and hBN. In TFM, torsional motion of an AFM cantilever is monitored as it is actively driven at a torsional resonance while a feedback loop maintains contact at a set force with the sample surface. TFM works at room temperature in air, with no need for an electrical bias between the tip and the sample, making it applicable to a wide array of samples. It should enable determination of precise structural information including twist angles and strain in moiré superlattices and crystallographic orientation of VdW flakes to support predictable moiré heterostructure fabrication. △ Less

Submitted 20 December, 2023; v1 submitted 17 August, 2023; originally announced August 2023.

Comments: 31 pages, 14 figures and 1 table including supplementary materials

Report number: 121 (10) e2314083121

Journal ref: PNAS (2024)

Torsional Periodic Lattice Distortions and Diffraction of Twisted 2D Materials

Authors: Suk Hyun Sung, Yin Min Goh, Hyobin Yoo, Rebecca Engelke, Hongchao Xie, Kuan Zhang, Zidong Li, Andrew Ye, Parag B. Deotare, Ellad B. Tadmor, Andrew J. Mannix, Jiwoong Park, Liuyan Zhao, Philip Kim, Robert Hovden

Abstract: Twisted 2D materials form complex moiré structures that spontaneously reduce symmetry through picoscale deformation within a mesoscale lattice. We show twisted 2D materials contain a torsional displacement field comprised of three transverse periodic lattice distortions (PLD). The torsional PLD amplitude provides a single order parameter that concisely describes the structural complexity of twiste… ▽ More Twisted 2D materials form complex moiré structures that spontaneously reduce symmetry through picoscale deformation within a mesoscale lattice. We show twisted 2D materials contain a torsional displacement field comprised of three transverse periodic lattice distortions (PLD). The torsional PLD amplitude provides a single order parameter that concisely describes the structural complexity of twisted bilayer moirés. Moreover, the structure and amplitude of a torsional periodic lattice distortion is quantifiable using rudimentary electron diffraction methods sensitive to reciprocal space. In twisted bilayer graphene, the torsional PLD begins to form at angles below 3.89° and the amplitude reaches 8 pm around the magic angle of 1.1°. At extremely low twist angles (e.g. below 0.25°) the amplitude increases and additional PLD harmonics arise to expand Bernal stacked domains separated by well defined solitonic boundaries. The torsional distortion field in twisted bilayer graphene is analytically described and has an upper bound of 22.6 pm. Similar torsional distortions are observed in twisted WS$_2$, CrI$_3$, and WSe$_2$ / MoSe$_2$. △ Less

Submitted 28 December, 2022; v1 submitted 12 March, 2022; originally announced March 2022.

Journal ref: Nature Communications 13, 7826 (2022)