Direct measurement of biexcitons in monolayer WS 2

2022-02-04T15:16:44+00:00February 4th, 2022|Categories: Publications|Tags: |

The optical properties of atomically thin transition metal dichalcogenides are dominated by Coulomb bound quasi-particles, such as excitons, trions, and biexcitons. Due to the number and density of possible states, attributing different spectral peaks to the specific origin can be difficult. In particular, there has been much conjecture around the presence, binding energy and/or nature of biexcitons in these materials. In this work, we remove any ambiguity in identifying and separating the optically excited biexciton in monolayer WS 2 using two-quantum multidimensional coherent spectroscopy (2Q-MDCS), a technique that directly and selectively probes doubly-excited states, such as biexcitons. The energy difference between the unbound two-exciton state and the biexciton is the fundamental definition of biexciton binding energy and is measured to be 26 ± 2 meV. Furthermore, resolving the biexciton peaks in 2Q-MDCS allows us to identify that the biexciton observed here is …

Published in: "2DMaterials".

Surface and dynamical properties of GeI 2

2022-02-04T15:16:43+00:00February 4th, 2022|Categories: Publications|Tags: |

GeI 2 is an interesting two-dimensional wide-band gap semiconductor because of diminished edge scattering due to an absence of dangling bonds. Angle-resolved x-ray photoemission spectroscopy indicates a germanium rich surface, and a surface to bulk core-level shift of 1.8 eV in binding energy, between the surface and bulk components of the Ge 2p 3/2 core-level, making clear that the surface is different from the bulk. Temperature dependent studies indicate an effective Debye temperature ( ##IMG## [http://ej.iop.org/images/2053-1583/9/2/025001/tdmac4715ieqn1.gif] {${theta _D}$} ) of 186 ± 18 K for the germanium x-ray photoemission spectroscopy feature associated with the surface. These measurements also suggest an unusually high effective Debye temperature for iodine (587 ± 31 K), implying that iodine is present in the bulk of the material, and not the surface. From optical absorbance, GeI 2 is seen to have an indirect (direct) o…

Published in: "2DMaterials".

Shifts in valence states in bimetallic MXenes revealed by electron energy-loss spectroscopy (EELS)

2022-02-04T15:16:41+00:00February 4th, 2022|Categories: Publications|Tags: |

MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti 2− y Nb y CT x , Nb 2− y V y CT x , and Ti 2− y V y CT x ) were investigated with scanning transmission electron microscopy and electron energy-loss spectroscopy to determine the localized valence states of metals at the na…

Published in: "2DMaterials".

Molecular beam epitaxial growth of Sb 2 Te 3 –Bi 2 Te 3 lateral heterostructures

2022-02-04T15:16:38+00:00February 4th, 2022|Categories: Publications|Tags: , |

We report on heteroepitaxial growth of Sb 2 Te 3 –Bi 2 Te 3 lateral heterostructures using molecular beam epitaxy. The lateral heterostructures were fabricated by growing Bi 2 Te 3 islands of hexagonal or triangular nanostructures with a typical size of several 100 nm and thickness of ∼15 nm on graphene substrates and Sb 2 Te 3 laterally on the side facets of the nanostructures. Multiple-step processes with different growth temperatures were employed to grow the lateral heterostructures. Electron microscopy techniques indicate that the inner region is Bi 2 Te 3 and the outer Sb 2 Te 3 was formed laterally on the graphene in an epitaxial manner. The interface between Bi 2 Te 3 and Sb 2 Te 3 from planar and cross-sectional views was studied by the aberration-corrected ( C s -corrected) high-angle annular dark-field s…

Published in: "2DMaterials".

Raman spectra of twisted bilayer graphene close to the magic angle

2022-02-04T15:16:36+00:00February 4th, 2022|Categories: Publications|Tags: |

In this work, we study the Raman spectra of twisted bilayer graphene samples as a function of their twist-angles ( θ ), ranging from 0.03° to 3.40°, where local θ are determined by analysis of their associated moiré superlattices, as imaged by scanning microwave impedance microscopy. Three standard excitation laser lines are used (457, 532, and 633 nm wavelengths), and the main Raman active graphene bands (G and 2D) are considered. Our results reveal that electron–phonon interaction influences the G band’s linewidth close to the magic angle regardless of laser excitation wavelength. Also, the 2D band lineshape in the θ < 1° regime is dictated by crystal lattice and depends on both the Bernal (AB and BA) stacking bilayer graphene and strain soliton regions (SP) (Gadelha et al 2021 Nature 590 405–9). We propose a geometrical model to explain the 2D lineshape variations, and from it, we estimate the SP width when moving towards the magic a…

Published in: "2DMaterials".

Creating custom-designed patterns of nanoscale graphene quantum dots

2022-02-04T15:16:34+00:00February 4th, 2022|Categories: Publications|Tags: |

Graphene quantum dots (GQDs) have attracted extensive attention over the years because of their importance both in fundamental science and potential applications. However, fabricating patterns of the GQDs is still of great challenge in experiment. Here, we demonstrate a technique to create patterned nanometer-sized GQDs with nanoscale precision in their sites. By applying a voltage pulse from a scanning tunneling microscopy (STM) tip, we successfully create stationary nanoscale circular p–n junctions, i.e. GQDs, in a continuous graphene sheet on hydrogen terminated germanium (110) surface. With accurately tuning the coordinates of the STM tip, the designed patterns of the GQDs are successfully generated. Spatial-resolved measurements indicate that the patterns of the GQDs strongly affect the local electronic properties and two-dimensional distributions of local density of states in graphene.

Published in: "2DMaterials".

Designing spin-textured flat bands in twisted graphene multilayers via helimagnet encapsulation

2022-02-04T15:16:32+00:00February 4th, 2022|Categories: Publications|Tags: |

Twisted graphene multilayers provide tunable platforms to engineer flat bands and exploit the associated strongly correlated physics. The two-dimensional nature of these systems makes them suitable for encapsulation by materials that break specific symmetries. In this context, recently discovered two-dimensional helimagnets, such as the multiferroic monolayer NiI 2 , are specially appealing for breaking time-reversal and inversion symmetries due to their nontrivial spin textures. Here we show that this spin texture can be imprinted on the electronic structure of twisted bilayer graphene by proximity effect. We discuss the dependence of the imprinted spin texture on the wave-vector of the helical structure, and on the strength of the effective local exchange field. Based on these results we discuss the nature of the superconducting instabilities that can take place in helimagnet encapsulated twisted bilayer graphene. Our results put forward helimagnetic encapsulation as …

Published in: "2DMaterials".

Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene

2019-12-11T22:33:44+00:00December 11th, 2019|Categories: Publications|Tags: , |

Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr 2 Ge 2 Te 6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr 2 Ge 2 Te 6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic i…

Published in: "2DMaterials".

Tailoring the thermal transport properties of monolayer hexagonal boron nitride by grain size engineering

2019-12-11T22:33:41+00:00December 11th, 2019|Categories: Publications|Tags: |

The grain size effect on the thermal transport properties of hexagonal boron nitride (h-BN) thin films was experimentally investigated using the opto-thermal Raman technique. High-quality monolayer h-BN with mean grain sizes ranging from ~7 µ m to ~19 nm were successfully synthesized on Pt foil by chemical vapor deposition (CVD). The thermal conductivity ( κ ) of the single-crystalline h-BN was measured to be ~545 Wm −1 K −1 at 315K, well above the bulk value, and more than a factor of four higher than the value of poly-crystalline h-BN with mean grain size of ~19 nm. The very low thermal boundary conductance (deduced to be ~9.6 GW m −2 K −1 ) accounts for the significant reduction of κ for h-BN with small grain size. Molecular dynamics (MD) simulations reveal that due to the disordered vibrations of atoms along/near GB, the phonon scattering in poly-crystalline h-BN is greatly enhanced compared to large-grained or single-…

Published in: "2DMaterials".

Coupling of photonic crystal cavity and interlayer exciton in heterobilayer of transition metal dichalcogenides

2019-12-11T22:33:38+00:00December 11th, 2019|Categories: Publications|Tags: |

The advent of van der Waals heterostructures marks the emergence of a new class of synthetic materials with novel properties that are unattainable in their constituent materials. The 2D architecture of these layered materials makes them naturally suited for integration with a wide variety of planar nanophotonic cavities for next-generation low-power optoelectronic devices and explorations of fundamental physical effects in these new systems. Here, we report the coupling of the interlayer exciton in a transition metal dichalcogenide heterobilayer with a gallium phosphide photonic crystal defect cavity. The exciton-cavity coupling is found to be in the weak regime, resulting in ~15-fold increase in the photoluminescence intensity for interlayer exciton in resonance with the cavity. Simulation results suggest that the increased intensity stems from a Purcell enhancement of ~60. The order of magnitude enhancement of the photoluminescence yield offsets the low oscillator strength of …

Published in: "2DMaterials".

MoS 2 -enabled dual-mode optoelectronic biosensor using a water soluble variant of µ-opioid receptor for opioid peptide detection

2019-12-11T22:33:36+00:00December 11th, 2019|Categories: Publications|Tags: |

Owing to their unique electrical and optical properties, two-dimensional transition metal dichalcogenides have been extensively studied for their potential applications in biosensing. However, simultaneous utilization of both optical and electrical properties has been overlooked, yet it can offer enhanced accuracy and detection versitility. Here, we demonstrate a dual-mode optoelectronic biosensor based on monolayer molybdenum disulfide (MoS 2 ) capable of producing simultaneous electrical and optical readouts of biomolecular signals. On a single platform, the biosensor exhibits a tunable photonic Fano-type optical resonance while also functioning as a field-effect transistor (FET) based on a optically transparent gate electrode. Furthermore, chemical vapor deposition grown MoS 2 provides a clean surface for direct immobilization of a water-soluble variant of the μ -opioid receptor (wsMOR), via a nickel ion-mediated linker chemistry. We utilize a synthet…

Published in: "2DMaterials".

Scalable BEOL compatible 2D tungsten diselenide

2019-12-11T22:33:34+00:00December 11th, 2019|Categories: Publications|Tags: |

Low-temperature synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is a key challenge for their integration with complementary metal-oxide-semiconductor (CMOS) technology at ‘back-end-of-line (BEOL)’. Most low-temperature synthesis utilizes alkali salts, oxide-based metals, and methyl-group based chalcogen precursors which do not meet current BEOL requirements for contaminant-free manufacturing and process scalability. In this study, we benchmark a carbon and alkali salt-free synthesis of fully coalesced, stoichiometric 2D WSe 2 films on amorphous SiO 2 /Si substrates at BEOL- compatible temperatures (475 °C) via gas-source metal-organic chemical deposition. This work highlights the necessity of a Se-rich environment in a kinetically limited growth regime for successful integration of low-temperature 2D WSe 2 . Atomic-scale characterization reveals that BEOL WSe 2 is polycrystalline with domain size of ~200 nm and band…

Published in: "2DMaterials".

Potential dependent ionic sieving through functionalized laminar MoS 2 membranes

2019-12-11T22:33:33+00:00December 11th, 2019|Categories: Publications|Tags: |

Laminar MoS 2 membranes show outstanding potential for practical applications in energy conversion/storage, sensing, and as nanofluidic devices. The re-stacking of exfoliated MoS 2 creates nanocapillaries between the layers of MoS 2 nanosheets. These MoS 2 membranes have been shown to possess a unique combination of ionic rejection properties, high water permeation rates, and long-term solvent stability, with no significant swelling when exposed to aqueous or organic solvents. Chemical modification of MoS 2 membranes has been shown to improve their ionic rejection properties, however the mechanism behind this improvement is not well understood. In this work, we elucidate the ion-sieving mechanism by the study of potential-dependent ion transport through functionalized MoS 2 membranes. The ionic permeability of the MoS 2 membrane is transformed by chemical functionalization with a simple naphthalene sulfonate d…

Published in: "2DMaterials".

Graphene-based fiber sensors with high stretchability and sensitivity by direct ink extrusion

2019-12-11T22:33:32+00:00December 11th, 2019|Categories: Publications|Tags: |

Free-standing stretchable fibers with highly flexibility and sensitivity are the key components of the smart wearable electronic devices. In this work, one-dimensional graphene-based fibers with aligned morphology are fabricated by direct ink extrusion, in which graphene and poly(dimethyl siloxane) (PDMS) can be integrated into a conductive network. The graphene-based fibers can respond to multiple deformations such as bending, twisting, compressing, and stretching. The various response amplitude with a reversible electrical resistance change can be obtained from multiple strain cycles, which exhibits high sensitivity and broad range in strain sensing. The ultra-sensitive electromechanical property with a gauge factor of 65 under 6% strain is attributed to the interwoven graphene network and could be cycled 600 times of continuous stretching−releasing process with less than 6.2% attenuation in the response signal. The intrinsic dynamic fracture procedure and mechanism of graphen…

Published in: "2DMaterials".

Theory of coherent pump–probe spectroscopy in monolayer transition metal dichalcogenides

2019-12-05T12:33:26+00:00December 5th, 2019|Categories: Publications|

A valley-selective circular dichroism and a pronounced spin-valley locking in monolayer transition metal dichalcogenides (TMDCs) enable the investigation of new many-body physics revealed in the ultrafast nonlinear optical response of these atomically thin materials. While this topic is experimentally well studied in pump–probe spectroscopy, only a fragmented theoretical understanding is available due to the complexity and multitude of occurring effects. Here, a microscopic approach is presented to describe the ultrafast pump–probe response of monolayer TMDCs in the coherent limit. We focus on close to band edge excitations dominated by strongly correlated, bound electron–hole pairs, namely excitonic excitations. The approach includes Hartree–Fock and correlation effects up to two excitonic excitations known from conventional semiconductors as well as TMDC typical Coulomb mechanisms such as intra- and intervalley excitation and charge transfer. The investigated coherent limit is…

Published in: "2DMaterials".

Anisotropic infrared light emission from quasi-1D layered TiS 3

2019-12-05T12:33:20+00:00December 5th, 2019|Categories: Publications|Tags: |

Atomically thin semiconductors hold great potential for nanoscale photonic and optoelectronic devices because of their strong light absorption and emission. Despite progress, their application in integrated photonics is hindered particularly by a lack of stable layered semiconductors emitting in the infrared part of the electromagnetic spectrum. Here we show that titanium trisulfide (TiS 3 ), a layered van der Waals material consisting of quasi-1D chains, emits near infrared light centered around 0.91 eV (1360 nm). Its photoluminescence exhibits linear polarization anisotropy and an emission lifetime of 210 ps. At low temperature, we distinguish two spectral contributions with opposite linear polarizations attributed to excitons and defects. Moreover, the dependence on excitation power and temperature suggests that free and bound excitons dominate the excitonic emission at high and low temperatures, respectively. Our results demonstrate the promising properties of TiS

Published in: "2DMaterials".

The emergence of one-dimensional channels in marginal-angle twisted bilayer graphene

2019-12-05T12:33:16+00:00December 5th, 2019|Categories: Publications|Tags: |

We generalize the continuum model for Moiré structures made from twisted graphene layers, in order to include lattice relaxation and the formation of channels at very small (marginal) twist angles. We show that a precise description of the electronic structure at such small angles can be achieved by (i) calculating first the relaxed atomic structure, (ii) projecting the interlayer electronic hopping parameters using a suitable basis of Bloch states, and (iii) increasing the number of harmonics in the continuum approximation to interlayer hopping. The results show a complex structure of quasi one dimensional states when a finite bias is applied.

Published in: "2DMaterials".

Unveiling multiferroic proximity effect in graphene

2019-12-05T12:33:12+00:00December 5th, 2019|Categories: Publications|Tags: |

We demonstrate that electronic and magnetic properties of graphene can be tuned via proximity of multiferroic substrate. Our first-principles calculations performed both with and without spin–orbit coupling clearly show that by contacting graphene with bismuth ferrite BiFeO 3 (BFO) film, the spin-dependent electronic structure of graphene is strongly impacted both by the magnetic order and by electric polarization in the underlying BFO. Based on extracted Hamiltonian parameters obtained from the graphene band structure, we propose a concept of six-resistance device based on exploring multiferroic proximity effect giving rise to significant proximity electro- (PER), magneto- (PMR), and multiferroic (PMER) resistance effects. This finding paves a way towards multiferroic control of magnetic properties in two dimensional materials.

Published in: "2DMaterials".

Magnetic field mixing and splitting of bright and dark excitons in monolayer MoSe 2

2019-11-22T12:33:08+00:00November 22nd, 2019|Categories: Publications|Tags: , |

Monolayers of semiconducting transition metal dichalcogenides (TMDCs) with unique spin-valley contrasting properties and remarkably strong excitonic effects continue to be a subject of intense research interests. These model 2D semiconductors feature two fundamental intravalley excitons species–optically accessible ‘bright’ excitons with anti-parallel spins and optically inactive ‘dark’ excitons with parallel spins. For applications exploiting radiative recombination of bright excitons or long lifetime dark excitons, it is essential to understand the radiative character of the exciton ground state and establish the energy separation between the lowest energy bright and dark excitons. Here, we report a direct spectroscopic measure of dark excitons in monolayer MoSe 2 encapsulated in hexagonal boron nitride. By applying strong in-plane magnetic field, we induce mixing and splitting of bright and dark exciton branches, which enables an accurate spectroscopic determination…

Published in: "2DMaterials".

Screen-printed and spray coated graphene-based RFID transponders

2019-11-22T12:33:05+00:00November 22nd, 2019|Categories: Publications|Tags: |

We report ultra-high-frequency (UHF, 800 MHz–1 GHz) radio frequency identification (RFID) transponders consisting of printed dipole antennas combined with RFID microchips. These are fabricated on Kapton via screen printing and on paper via spray coating, using inks obtained via microfluidization of graphite. We introduce a hybrid antenna structure, where an Al loop (small compared to the overall size of the antenna) is connected to a microchip with the double function of matching the impedances of antenna and microchip and avoiding bonding between exfoliated graphite and chip. The transponders have reading distance  ∼11 m at UHF RFID frequencies, larger than previously reported for graphene-based RFID and comparable with commercial transponders based on metallic antennas.

Published in: "2DMaterials".

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