27 research outputs found
Quantitative optical mapping of two-dimensional materials
The pace of two-dimensional materials (2DM) research has been greatly accelerated by the ability to identify exfoliated thicknesses down to a monolayer from their optical contrast. Since this process requires time-consuming and error-prone manual assignment to avoid false-positives from image features with similar contrast, efforts towards fast and reliable automated assignments schemes is essential. We show that by modelling the expected 2DM contrast in digitally captured images, we can automatically identify candidate regions of 2DM. More importantly, we show a computationally-light machine vision strategy for eliminating false-positives from this set of 2DM candidates through the combined use of binary thresholding, opening and closing filters, and shape-analysis from edge detection. Calculation of data pyramids for arbitrarily high-resolution optical coverage maps of two-dimensional materials produced in this way allows the real-time presentation and processing of this image data in a zoomable interface, enabling large datasets to be explored and analysed with ease. The result is that a standard optical microscope with CCD camera can be used as an analysis tool able to accurately determine the coverage, residue/contamination concentration, and layer number for a wide range of presented 2DMs
Charge-transfer Contact to a High-Mobility Monolayer Semiconductor
Two-dimensional (2D) semiconductors, such as the transition metal
dichalcogenides, have demonstrated tremendous promise for the development of
highly tunable quantum devices. Realizing this potential requires
low-resistance electrical contacts that perform well at low temperatures and
low densities where quantum properties are relevant. Here we present a new
device architecture for 2D semiconductors that utilizes a charge-transfer layer
to achieve large hole doping in the contact region, and implement this
technique to measure magneto-transport properties of high-purity monolayer
WSe. We measure a record-high hole mobility of 80,000 cm/Vs and access
channel carrier densities as low as cm, an order of
magnitude lower than previously achievable. Our ability to realize transparent
contact to high-mobility devices at low density enables transport measurement
of correlation-driven quantum phases including observation of a low temperature
metal-insulator transition in a density and temperature regime where Wigner
crystal formation is expected, and observation of the fractional quantum Hall
effect under large magnetic fields. The charge transfer contact scheme paves
the way for discovery and manipulation of new quantum phenomena in 2D
semiconductors and their heterostructures
Single-crystalline gold nanodisks on WS mono- and multilayers: Strong coupling at room temperature
Engineering light-matter interactions up to the strong-coupling regime at
room temperature is one of the cornerstones of modern nanophotonics. Achieving
this goal will enable new platforms for potential applications such as quantum
information processing, quantum light sources and even quantum metrology.
Materials like transition metal dichalcogenides (TMDC) and in particular
tungsten disulfide (WS) possess large transition dipole moments comparable
to semiconductor-based quantum dots, and strong exciton binding energies
allowing the detailed exploration of light-matter interactions at room
temperature. Additionally, recent works have shown that coupling TMDCs to
plasmonic nanocavities with light tightly focused on the nanometer scale can
reach the strong-coupling regime at ambient conditions. Here, we use ultra-thin
single-crystalline gold nanodisks featuring large in-plane electromagnetic
dipole moments aligned with the exciton transition-dipole moments located in
monolayer WS. Through scattering and reflection spectroscopy we demonstrate
strong coupling at room temperature with a Rabi splitting of 108 meV. In
order to go further into the strong-coupling regime and inspired by recent
experimental work by St\"uhrenberg et al., we couple these nanodisks to
multilayer WS. Due to an increase in the number of excitons coupled to our
nanodisks, we achieve a Rabi splitting of 175 meV, a major increase of
62%. To our knowledge, this is the highest Rabi splitting reported for TMDCs
coupled to open plasmonic cavities. Our results suggest that ultra-thin
single-crystalline gold nanodisks coupled to WS represent an exquisite
platform to explore light-matter interactions
High-quality graphene flakes exfoliated on a flat hydrophobic polymer
We show that graphene supported on a hydrophobic and flat polymer surface results in flakes with extremely low doping and strain as assessed by their Raman spectroscopic characteristics. We exemplify this technique by micromechanical exfoliation of graphene on flat poly(methylmethacrylate) layers and demonstrate Raman peak intensity ratios I(2D)/I(G) approaching 10, similar to pristine freestanding graphene. We verify that these features are not an artifact of optical interference effects occurring at the substrate: they are similarly observed when varying the substrate thickness and are maintained when the environment of the graphene flake is completely changed, by encapsulating preselected flakes between hexagonal boron nitride layers. The exfoliation of clean, pristine graphene layers directly on flat polymer substrates enables high performance, supported, and non-encapsulated graphene devices for flexible and transparent optoelectronic studies. We additionally show that the access to a clean and supported graphene source leads to high-quality van der Waals heterostructures and devices with reproducible carrier mobilities exceeding 50 000 cm2V-1s-1at room temperature
Catalyst Interface Engineering for Improved 2D Film Lift-Off and Transfer
The mechanisms by which chemical vapor deposited (CVD) graphene and hexagonal boron nitride (h-BN) films can be released from a growth catalyst, such as widely used copper (Cu) foil, are systematically explored as a basis for an improved lift-off transfer. We show how intercalation processes allow the local Cu oxidation at the interface followed by selective oxide dissolution, which gently releases the 2D material (2DM) film. Interfacial composition change and selective dissolution can thereby be achieved in a single step or split into two individual process steps. We demonstrate that this method is not only highly versatile but also yields graphene and h-BN films of high quality regarding surface contamination, layer coherence, defects, and electronic properties, without requiring additional post-transfer annealing. We highlight how such transfers rely on targeted corrosion at the catalyst interface and discuss this in context of the wider CVD growth and 2DM transfer literature, thereby fostering an improved general understanding of widely used transfer processes, which is essential to numerous other applications.We acknowledge funding from the ERC (InsituNANO, grant 279342). R.W. acknowledges an EPSRC Doctoral Training Award (EP/M506485/1). During this work, S.T. was supported in parts by a DFG research fellowship under grant TA 1122/1-1:1. J.A.A.-W. acknowledges a Research Fellowship from Churchill College, Cambridge. Z.A.V.V. acknowledges funding from ESPRC grant EP/L016087/1. P.B. and B.S.J. thank the Danish National Research Foundation Centre for Nanostructured graphene, DNRF103, and EU Horizon 2020 âGraphene Flagshipâ 696656. T.J.B. and P.R.W. acknowledge financial support from EU FP7-6040007 âGLADIATORâ and Innovation Fund Denmark Da-Gate 0603-005668B. P.R.K. acknowledges a Lindemann Trust Fellowship
Nanometer-Scale Lateral pân Junctions in Graphene/Îą-RuCl3 Heterostructures
[EN] The ability to create nanometer-scale lateral p-n junctions is essential for the next generation of two-dimensional (2D) devices. Using the charge-transfer heterostructure graphene/alpha-RuCl3, we realize nanoscale lateral p-n junctions in the vicinity of graphene nanobubbles. Our multipronged experimental approach incorporates scanning tunneling microscopy (STM) and spectroscopy (STS) and scattering-type scanning near-field optical microscopy (s-SNOM) to simultaneously probe the electronic and optical responses of nanobubble p-n junctions. Our STM/STS results reveal that p-n junctions with a band offset of 0.6 eV can be achieved with widths of 3 nm, giving rise to electric fields of order 108 V/m. Concurrent s-SNOM measurements validate a point-scatterer formalism for modeling the interaction of surface plasmon polaritons (SPPs) with nanobubbles. Ab initio density functional theory (DFT) calculations corroborate our experimental data and reveal the dependence of charge transfer on layer separation. Our study provides experimental and conceptual foundations for generating p-n nanojunctions in 2D materials.Research at Columbia University was supported as part of the Energy Frontier Research Center on Programmable Quantum Materials funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No DE-SC0019443. Plasmonic nano-imaging at Columbia University was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No DE-SC0018426. J.Z. and A.R. were supported by the European Research Council (ERC-2015-AdG694097), the Cluster of Excellence âAdvanced Imaging of Matterâ (AIM) EXC 2056-390715994, funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under RTG 2247, Grupos Consolidados (IT1249-19), and SFB925 âLight induced dynamics and control of correlated quantum systemsâ. J.Z. and A.R. would like to acknowledge Nicolas Tancogne-Dejean and Lede Xian for fruitful discussions and also acknowledge support by the Max Planck Institute-New York City Center for Non-Equilibrium Quantum Phenomena. The Flatiron Institute is a division of the Simons Foundation. J.Z. acknowledges funding received from the European Union Horizon 2020 research and innovation programme under Marie SkĹodowska-Curie Grant Agreement 886291 (PeSD-NeSL). STM support was provided by the National Science Foundation via Grant DMR-2004691. C.R.-V. acknowledges funding from the European Union Horizon 2020 research and innovation programme under the Marie SkĹodowska-Curie Grant Agreement 844271. D.G.M. acknowledges support from the Gordon and Betty Moore Foundationâs EPiQS Initiative, Grant GBMF9069. J.Q.Y. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. S.E.N. acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Scientific User Facilities. Work at University of Tennessee was supported by NSF Grant 180896
Graphene/-RuCl: An Emergent 2D Plasmonic Interface
Work function-mediated charge transfer in graphene/-RuCl
heterostructures has been proposed as a strategy for generating highly-doped 2D
interfaces. In this geometry, graphene should become sufficiently doped to host
surface and edge plasmon-polaritons (SPPs and EPPs, respectively).
Characterization of the SPP and EPP behavior as a function of frequency and
temperature can be used to simultaneously probe the magnitude of interlayer
charge transfer while extracting the optical response of the interfacial doped
-RuCl. We accomplish this using scanning near-field optical
microscopy (SNOM) in conjunction with first-principles DFT calculations. This
reveals massive interlayer charge transfer (2.7 10 cm)
and enhanced optical conductivity in -RuCl as a result of
significant electron doping. Our results provide a general strategy for
generating highly-doped plasmonic interfaces in the 2D limit in a scanning
probe-accessible geometry without need of an electrostatic gate.Comment: 22 pages, 5 figure
Programming moir\'e patterns in 2D materials by bending
Moir\'e superlattices in twisted two-dimensional materials have generated
tremendous excitement as a platform for achieving quantum properties on demand.
However, the moir\'e pattern is highly sensitive to the interlayer atomic
registry, and current assembly techniques suffer from imprecise control of the
average twist angle, spatial inhomogeneity in the local twist angle, and
distortions due to random strain. Here, we demonstrate a new way to manipulate
the moir\'e patterns in hetero- and homo-bilayers through in-plane bending of
monolayer ribbons, using the tip of an atomic force microscope. This technique
achieves continuous variation of twist angles with improved twist-angle
homogeneity and reduced random strain, resulting in moir\'e patterns with
highly tunable wavelength and ultra-low disorder. Our results pave the way for
detailed studies of ultra-low disorder moir\'e systems and the realization of
precise strain-engineered devices
Nanometer-scale lateral p-n junctions in graphene/Îą-RuCl3 heterostructures
The ability to create nanometer-scale lateral pân junctions is essential for the next generation of two-dimensional (2D) devices. Using the charge-transfer heterostructure graphene/Îą-RuCl3, we realize nanoscale lateral pân junctions in the vicinity of graphene nanobubbles. Our multipronged experimental approach incorporates scanning tunneling microscopy (STM) and spectroscopy (STS) and scattering-type scanning near-field optical microscopy (s-SNOM) to simultaneously probe the electronic and optical responses of nanobubble pân junctions. Our STM/STS results reveal that pân junctions with a band offset of âź0.6 eV can be achieved with widths of âź3 nm, giving rise to electric fields of order 108 V/m. Concurrent s-SNOM measurements validate a point-scatterer formalism for modeling the interaction of surface plasmon polaritons (SPPs) with nanobubbles. Ab initio density functional theory (DFT) calculations corroborate our experimental data and reveal the dependence of charge transfer on layer separation. Our study provides experimental and conceptual foundations for generating pân nanojunctions in 2D materials.Research at Columbia University was supported as part of the Energy Frontier Research Center on Programmable Quantum Materials funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No DE-SC0019443. Plasmonic nano-imaging at Columbia University was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No DE-SC0018426. J.Z. and A.R. were supported by the European Research Council (ERC-2015-AdG694097), the Cluster of Excellence âAdvanced Imaging of Matterâ (AIM) EXC 2056-390715994, funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under RTG 2247, Grupos Consolidados (IT1249-19), and SFB925 âLight induced dynamics and control of correlated quantum systemsâ. J.Z. and A.R. would like to acknowledge Nicolas Tancogne-Dejean and Lede Xian for fruitful discussions and also acknowledge support by the Max Planck Institute-New York City Center for Non-Equilibrium Quantum Phenomena. The Flatiron Institute is a division of the Simons Foundation. J.Z. acknowledges funding received from the European Union Horizon 2020 research and innovation programme under Marie SkĹodowska-Curie Grant Agreement 886291 (PeSD-NeSL). STM support was provided by the National Science Foundation via Grant DMR-2004691. C.R.-V. acknowledges funding from the European Union Horizon 2020 research and innovation programme under the Marie SkĹodowska-Curie Grant Agreement 844271. D.G.M. acknowledges support from the Gordon and Betty Moore Foundationâs EPiQS Initiative, Grant GBMF9069. J.Q.Y. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. S.E.N. acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Scientific User Facilities. Work at University of Tennessee was supported by NSF Grant 180896.Peer reviewe