2D proximate quantum spin liquid state in atomic-thin α -RuCl 3

2018-11-16T14:33:54+00:00November 16th, 2018|Categories: Publications|

Two-dimensional (2D) atomic crystals have made major inroads into condensed-matter physics and give rise to fascinating phenomena due to quantum confinement. Here we report the first Raman scattering study on phonon-magnetic scattering coupling, proximate quantum spin liquid ground state and collective fractionalized excitations in exfoliated α -RuCl 3 atomic layers. Our results uncover that 2D α -RuCl 3 could harbour the unusual magnetic continuum, serving as a hallmark of the 2D proximate quantum spin liquid state and frustrated magnetic interactions. More importantly, our work demonstrates that the unusual magnetic scattering, as compared with bulk, is more obvious in 2D α -RuCl 3 , indicating that the frustrated magnetic interactions are enhanced strongly. Such unusual enhancement of frustrated magnetic interactions may be responsible for the gigantic phonon-magnetic scattering coupling of 2D α -RuCl 3 and play a…

Published in: "2DMaterials".

Narrow photoluminescence and Raman peaks of epitaxial MoS 2 on graphene/Ir(1 1 1)

2018-11-14T18:33:53+00:00November 14th, 2018|Categories: Publications|Tags: |

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown ##IMG## [http://ej.iop.org/images/2053-1583/6/1/011006/tdmaaebd3ieqn001.gif] on graphene/Ir(1 1 1). This observation is explained in terms of a weak graphene-MoS 2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS 2 with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS 2 as well as a different thermal expansion of both materials. Molecular beam epitaxy grown MoS 2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties.

Published in: "2DMaterials".

Polytypism in ultrathin tellurium

2018-11-14T18:33:51+00:00November 14th, 2018|Categories: Publications|

We report the synthesis of ultrathin tellurium films, including atomically thin tellurium tri-layers, by physical vapor deposition (PVD) as well as larger area films by pulsed laser deposition (PLD). PVD leads to sub-nanometer, tri-layer tellurene flakes with distinct boundaries, whereas PLD yields uniform and contiguous sub-7 nm films over a centimeter square. The PLD films exhibit the characteristic hexagonal crystal structure of semiconducting tellurium, but high resolution transmission electron microscopy (HRTEM) reveals a unique stacking polytype in the thinner PVD-grown material. Density Functional Theory calculations predict the possible existence of three polytypes of ultrathin Te, including the α -type experimentally observed here. The two complementary growth methods afford a route to controllably synthesize ultrathin Te with thicknesses ranging from three atomic layers up to 6 nm with unique polytypism. Lastly, temperature dependent Raman studies suggest the pos…

Published in: "2DMaterials".

Ultrafast time-resolved investigations of excitons and biexcitons at room temperature in layered WS 2

2018-11-02T14:35:34+00:00November 2nd, 2018|Categories: Publications|

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel excitonic quasiparticle-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for substantial practical implications. Here, we explore the ultrafast excitonic phenomena in layered WS 2 (mono-to-quad) dispersion using broadband (350–750 nm) femtosecond pump-probe spectroscopy at room temperature (300 K) which are inaccessible to the steady-state absorption or emission spectroscopy. The transient absorption spectra (TAS) suggest that the mono-to-quad layered dispersion of WS 2 has similar spectral features as monolayer WS 2 in terms of saturation absorptions (SA) and excited s…

Published in: "2DMaterials".

Non equilibrium anisotropic excitons in atomically thin ReS 2

2018-11-02T14:35:32+00:00November 2nd, 2018|Categories: Publications|

We present a systematic investigation of the electronic properties of bulk and few layer ReS 2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by  ∼20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS 2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases.

Published in: "2DMaterials".

Thermal transport across graphene step junctions

2018-11-02T14:35:31+00:00November 2nd, 2018|Categories: Publications|Tags: |

Step junctions are often present in layered materials, i.e. where single-layer regions meet multilayer regions, yet their effect on thermal transport is not understood to date. Here, we measure heat flow across graphene junctions (GJs) from monolayer-to-bilayer graphene, as well as bilayer to four-layer graphene for the first time, in both heat flow directions. The thermal conductance of the monolayer-bilayer GJ ranges from ~0.5 to 9.1  ×  10 8 W m −2 K −1 between 50 K to 300 K. Atomistic simulations of such a GJ device reveal that graphene layers are relatively decoupled, and the low thermal conductance of the device is determined by the resistance between the two distinct graphene layers. In these conditions the junction plays a negligible effect. To prove that the decoupling between layers controls thermal transport in the junction, the heat flow in both directions was measured, showing no evidence of thermal asymmetry or rectification, within ex…

Published in: "2DMaterials".

Generating strong room-temperature photoluminescence in black phosphorus using organic molecules

2018-10-30T12:39:51+00:00October 30th, 2018|Categories: Publications|Tags: |

Black phosphorus (BP) exhibits fascinating thickness dependent optical and electronic characteristics. However, photoluminescence (PL) emission in the visible spectrum does not exist for multilayer BP and requires the achievement of single layer, which are highly environmentally sensitive. This poses significant challenges in realizing the true potential of BP as multilayer BP exhibits exciting optical properties for a range of applications. Here, for the first time we reveal visible range room-temperature photoluminescence (PL) in multi-layered black phosphorus (BP) via chemical doping using organic molecules. We find the drastic enhancement of PL originates from the adsorption of p-type dopants and offer further insight using density functional theory (DFT) calculations. The reported non-destructive method creates a pathway to precisely control optical and electronic properties thereby expanding the application horizon for multilayer BP that is environmentally robust compared …

Published in: "2DMaterials".

Exfoliation of 2D materials by high shear mixing

2018-10-30T12:39:49+00:00October 30th, 2018|Categories: Publications|Tags: |

While it has been demonstrated that large scale liquid exfoliation of graphene is possible using high-shear exfoliation, it has not yet been shown to be applicable to a broader range of layered materials. In addition, it would be useful to determine whether the mechanisms reported for shear exfoliation of graphene also apply to other 2D materials. In this work we show that previous models describing high-shear exfoliation of graphene apply to MoS 2 and WS 2 . However, we find the minimum shear rate required to exfoliate MoS 2 and WS 2 to be ~3  ×  10 4 s −1 , somewhat higher than the value for graphene. We also demonstrate the scalability of shear exfoliation of WS 2 . By measuring and then optimising the scaling parameters, shear exfoliation of WS 2 is shown to be capable of reaching concentrations of 1.82 g l −1 in 6 h and demonstrating a maximum production rate of 0.95 g h −1 .

Published in: "2DMaterials".

Facile access to shape-controlled growth of WS 2 monolayer via environment-friendly method

2018-10-30T12:39:48+00:00October 30th, 2018|Categories: Publications|Tags: |

2D materials with tailored morphologies exhibit distinctive shape-dependent properties especially in electrocatalysis and gas sensing, extending their applications in nanoelectronics. Atomic-layer tungsten disulfide (WS 2 ) stands out as a promising candidate, but the controllable synthesis of WS 2 still faces unavoidable obstacles, such as strict parameter requirements, low efficiency and serious pollution during the preparation. Here we report an elegant technique for growing homogeneous-luminescence WS 2 monolayer with desired shapes and developed electrochemical properties, through the improved chemical vapor deposition method with a semi-closed airflow environment. It is an effective, economic and especially environment-friendly approach with atmospheric pressure and hydrogen-free condition, aiming at synthesizing high-quality and large-scale monolayer WS 2 crystals with only one heating zone. Compared with the existing methods, this o…

Published in: "2DMaterials".

Reduction of hysteresis in MoS 2 transistors using pulsed voltage measurements

2018-10-26T12:33:23+00:00October 26th, 2018|Categories: Publications|

Transistors based on two-dimensional (2D) materials often exhibit hysteresis in their electrical measurements, i.e. a dependence of measured current on voltage sweep direction due to charge trapping. Here we demonstrate a simple pulsed measurement technique which reduces this hysteretic behavior, enabling more accurate characterization of 2D transistors. We compare hysteresis and charge trapping in four types of devices fabricated from both exfoliated and synthetic MoS 2 , with SiO 2 and HfO 2 insulators, using DC and pulsed voltage measurements at different temperatures. Applying modest voltage pulses (~1 ms) on the gate significantly reduces charge trapping and results in the elimination of over 80% of hysteresis for all devices. At shorter pulse widths (~1 µ s), up to 99% of hysteresis is reduced for some devices. Our measurements enable the extraction of a unique value of field-effect mobility, regardless of voltage sweep direction, unlike …

Published in: "2DMaterials".

Vibration detection schemes based on absorbance tuning in monolayer molybdenum disulfide mechanical resonators

2018-10-24T12:35:08+00:00October 24th, 2018|Categories: Publications|Tags: |

Mechanical resonators based on monolayer molybdenum disulfide, a two-dimensional semiconductor, offer the possibility of harnessing light-matter interaction to develop sensitive detection schemes. These schemes consist in measuring fluctuations in the mechanical response of the resonator, such as fluctuations of its displacement spectrum, induced by minute changes in its environment. The accuracy of detecting such changes depends on how accurately the temperature of vibrations is known. However, using light to measure the mechanical response modifies vibrational temperature, making this detection challenging. We propose two complementary detection schemes that may enable us to measure the response of the resonator optically with optimal accuracy. The first scheme is based on minimizing the absorbance of the monolayer while ensuring that the reflectance of the resonator strongly varies with displacement. This scheme minimizes absorptive heating of the monolayer and optomechanical…

Published in: "2DMaterials".

Atomic layer deposition of stable 2D materials

2018-10-22T18:33:47+00:00October 22nd, 2018|Categories: Publications|Tags: |

Following the graphene isolation, strong interest in two dimensional (2D) materials has been driven by their outstanding properties. Their typical intrinsic structure, including strong in-plane covalent bonding and weak out-of-plane Van der Waals interaction, makes them highly promising in diverse areas such as electronics, catalysis, and environment. Growth of 2D materials requires a synthesis approach able to control the deposition onto a support at the atomic scale. Thanks to their simplicity, versatility and ability to control thickness at the angstrom level, atomic layer deposition (ALD) and its variant atomic layer etching (ALET) appear as ones of the most suited techniques to synthesize 2D materials. The development of ALD technique for fabricating 2D materials in the last ten years justifies reviewing its most recent groundbreaking discoveries and progresses. Particular attention will be paid to stable 2D materials especially graphene, h-BN, Mo and W dichalcogenides and …

Published in: "2DMaterials".

2D material printer: a deterministic cross contamination-free transfer method for atomically layered materials

2018-10-22T16:34:16+00:00October 22nd, 2018|Categories: Publications|

Precision and chip contamination-free placement of two-dimensional (2D) materials is expected to accelerate both the study of fundamental properties and novel device functionality. Current deterministic transfer methods of 2D materials onto an arbitrary substrate deploy viscoelastic stamping. However, these methods produce (a) significant cross-contamination of the substrate inherent from typical dense sources of 2D material flakes and (b) are challenged with respect to spatial alignment, and (c) multi-transfer at a single step. Here, we demonstrate a novel method of transferring 2D materials resembling the functionality known from printing; utilizing a combination of a sharp micro-stamper and viscoelastic polymer, we show precise placement of individual 2D materials resulting in vanishing cross-contamination to the substrate. Our 2D printer-method results in an aerial cross-contamination improvement of two to three orders of magnitude relative to state-of-the-art transfer metho…

Published in: "2DMaterials".

STM study of exfoliated few layer black phosphorus annealed in ultrahigh vacuum

2018-10-22T16:34:15+00:00October 22nd, 2018|Categories: Publications|Tags: |

Black phosphorus (bP) has emerged as an interesting addition to the category of two-dimensional materials. Surface-science studies on this material are of great interest, but they are hampered by bP’s high reactivity to oxygen and water, a major challenge to scanning tunneling microscopy (STM) experiments. As a consequence, the large majority of these studies were performed by cleaving a bulk crystal in situ . Here we present a study of surface modifications on exfoliated bP flakes upon consecutive annealing steps, up to 550 °C, well above the sublimation temperature of bP. In particular, our attention is focused on the temperature range 375 °C–400 °C, when sublimation starts, and a controlled desorption from the surface occurs alongside with the formation of characteristic well-aligned craters. There is an open debate in the literature about the crystallographic orientation of these craters, whether they align along the zigzag or the armchair direction. Thanks to the atom…

Published in: "2DMaterials".

Spatially selective reversible charge carrier density tuning in WS 2 monolayers via photochlorination

2018-10-17T12:37:49+00:00October 17th, 2018|Categories: Publications|Tags: , |

Chlorine-doped tungsten disulfide monolayer (1L-WS 2 ) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal’s electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron s…

Published in: "2DMaterials".

Edge currents driven by terahertz radiation in graphene in quantum Hall regime

2018-10-10T12:33:59+00:00October 10th, 2018|Categories: Publications|Tags: |

We observe that the illumination of unbiased graphene in the quantum Hall regime with polarized terahertz laser radiation results in a direct edge current. This photocurrent is caused by an imbalance of persistent edge currents, which are driven out of thermal equilibrium by indirect transitions within the chiral edge channel. The direction of the edge photocurrent is determined by the polarity of the external magnetic field, while its magnitude depends on the radiation polarization. The microscopic theory developed in this paper describes well the experimental data.

Published in: "2DMaterials".

Effect of nucleation density on the crystallinity of graphene grown from mobile hot-wire-assisted CVD

2018-10-05T12:33:41+00:00October 5th, 2018|Categories: Publications|Tags: |

Chemical vapor deposition (CVD) is widely used for producing high-quality graphene in large area. However, grain boundaries (GBs) are inevitably generated when isolated graphene domains, grown from numerous graphitic nuclei, unify, resulting in severe degradation of intrinsic material properties of graphene. Using mobile hot-wire-assisted CVD (MHW-CVD), here, we successfully demonstrated the progress toward single-crystal graphene; graphene grown from MHW-CVD on pure copper substrate generally has large density of high angle tilt boundary. The main strategy of the improvement is the composition control of catalytic substrate to copper-nickel alloy (4.25 at% of Ni), via both increasing the initial density of graphitic nuclei and facilitating the rotations of isolated graphene domains during the recrystallization process of 2D graphene. Compared to graphene grown from the pure copper substrate, the areal density of the single-crystal graphene from the alloy increases from 22% to 8…

Published in: "2DMaterials".

Revealing the 3D structure of graphene defects

2018-09-25T12:33:46+00:00September 25th, 2018|Categories: Publications|Tags: |

We demonstrate insights into the three-dimensional (3D) structure of defects in graphene, in particular grain boundaries, obtained via a new approach using two transmission electron microscopy images recorded at different angles. The structure is revealed through an optimization process where both the atomic positions as well as the simulated imaging parameters are iteratively changed until the best possible match to the experimental images is found. We first demonstrate that this method works using an embedded defect in graphene that allows direct comparison to the computationally predicted 3D shape. We then apply the method to a set of grain boundary structures with misorientation angles spanning nearly the whole available range (2.6°–29.8°). The measured height variations at the boundaries reveal a strong correlation with the misorientation angle with lower angles resulting in stronger corrugation and larger kink angles. Our results allow for the first time a direct compariso…

Published in: "2DMaterials".

Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures

2018-09-25T12:33:45+00:00September 25th, 2018|Categories: Publications|Tags: , , |

Two-dimensional (2D) semiconductors—atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers—are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found (Bandurin et al 2017 Nat. Nanotechnol . 12 223–7) that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided…

Published in: "2DMaterials".

Controlling local deformation in graphene using micrometric polymeric actuators

2018-09-25T12:33:43+00:00September 25th, 2018|Categories: Publications|Tags: |

Lattice deformation and electronic properties are closely linked in two-dimensional materials such as graphene. However, a fine control of the spatial strain distribution is crucial to correctly engineer the electrical properties of atomic-thick materials. Although several solutions have been proposed so far, the flexibility required to fully master and investigate arbitrary strain profiles remains challenging. Here, we locally deform graphene using the poly-methyl-methacrylate (PMMA) shrinkage induced by electron-beam irradiation. Arbitrary design of pulling geometries and different actuation magnitudes can be both defined in the PMMA by electron-beam patterning. Specific graphene strain fields can be obtained using reverse engineering of the PMMA micro-actuators geometry. As proof of principle of operation, we target and we successfully demonstrate a strongly localized and virtually-pure uniaxial strain profile. This configuration is promising for the implementation of the pse…

Published in: "2DMaterials".

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