Semiclassical Boltzmann transport theory of few-layer black phosphorus in various phases

2019-02-13T14:35:37+00:00February 13th, 2019|Categories: Publications|Tags: , |

Black phosphorus (BP), a two-dimensional (2D) van der Waals layered material composed of phosphorus atoms, has been one of the most actively studied 2D materials in recent years due to its tunable energy band gap (tunable even to a negative value) and its highly anisotropic electronic structure. Depending on the sign of the band gap tuning parameter, few-layer BP can be in a gapped insulator phase, gapless Dirac semimetal phase, or gapless semi-Dirac transition point between the two phases. Using the fully anisotropic multiband Boltzmann transport theory, we systematically study the dc conductivity of few-layer BP as a function of the carrier density and temperature by varying the band gap tuning parameter, and determine the characteristic density and temperature dependence corresponding to each phase.

Published in: "2DMaterials".

Complementary inverters based on low-dimensional semiconductors prepared by facile and fully scalable methods

2019-02-12T16:42:25+00:00February 12th, 2019|Categories: Publications|

Two-dimensional (2D) semiconductors offer great potential in nanoelectronics due to their unique electrical and optical properties mainly attributed to their atomically thin nature. To make use of 2D semiconductors for practical applications, it is highly desired to develop scalable methods for integration of complementary circuits rather than a discrete device. We report a simple, scalable method for fabricating complementary inverters based on n-type monolayer MoS 2 , grown by a chemical vapor deposition (CVD), and inkjet-printed p-type SWCNTs. The CVD and inkjet printing methods are effectively combined to show the feasibility in terms of the integration of low-dimensional materials for complementary circuits. In the complementary circuits, cost-effectively printed Ag is utilized as the source and drain electrodes for both MoS 2 and SWCNT transistors. Both the n- and p-type transistors show decent device characteristics at low operating voltages under ambie…

Published in: "2DMaterials".

Highly resonant graphene plasmon hotspots in complex nanoresonator geometries

2019-02-12T16:42:22+00:00February 12th, 2019|Categories: Publications|Tags: |

Van der Waals surface polariton nanostructures are promising candidates for miniaturisation of electromagnetic devices through the nanoscale confinement of infrared light. To fully exploit these nanoresonators, a computationally efficient model is necessary to predict polariton behaviour in complex geometries. Here, we develop a general wave model of surface polaritons in 2D geometries smaller than the polariton wavelength. Using geometric approximation widely tuneable infrared nanoimaging and local work function microscopy, we test this model against complex mono-/bi-layer graphene plasmon nanoresonators. Direct imaging of highly resonant graphene plasmon hotspots confirms that the model provides quantitatively accurate, analytical predictions of nanoresonator behaviour. The insights built with such models are crucial to the development of practical plasmonic nanodevices.

Published in: "2DMaterials".

Freestanding carbon nanomembranes and graphene monolayers nanopatterned via EUV interference lithography

2019-02-08T12:39:35+00:00February 8th, 2019|Categories: Publications|Tags: , |

Two-dimensional materials, such as graphene, molecular nanosheets, hexagonal boron nitride or transition metal dichalcogenides have recently attracted substantial interest due to their potential use in electronics, chemical and biological sensors, nanooptics, and catalysis. For most of these applications, the functional nanostructures have to be prepared lithographically. In this respect, extreme ultraviolet interference lithography provides both high-resolution patterning with an ultimate limit in the sub-10 nm range and high throughput capability. Here we present the preparation of nanopatterned 1 nm thick freestanding molecular nanosheets—carbon nanomembranes (CNMs)—and single layer graphene employing this method and their characterization with a helium ion microscope. We demonstrate periodic arrays of suspended nanostructures of CNMs and graphene including nanoribbons with width between 20 nm and 500 nm and nanomeshes with openings between 140 nm and 300 nm and mesh lines do…

Published in: "2DMaterials".

Perseverance of direct bandgap in multilayer 2D PbI 2 under an experimental strain up to 7.69%

2019-02-08T12:39:34+00:00February 8th, 2019|Categories: Publications|Tags: |

The two-dimensional (2D) materials are naturally suitable for various flexible 2D optoelectronic devices, in which the direct band gap perseverance is crucial because the flexibility deformations often cause a bandgap transition and thus break performance of the devices. Most of 2D transition metal dichalcogenides (TMDs) materials such as monolayer MoS 2 , WS 2 and MoSe 2 have been thought to be not suitable for flexible optoelectronic devices due to their direct-to-indirect bandgap transition even under a small strain (~1%–2%) for any flexibility deformations. So far, only 2D phosphorene has been theoretically predicted to be able to keep direct bandgap property under a large strain. Here we report a 2D material lead iodide (PbI 2 ) mutilayer with a direct band gap and find by photoluminescence (PL) measurements that it maintains a direct bandgap nature under a large experimental strain up to 7.69%. Theoretical simulations support and expla…

Published in: "2DMaterials".

Giant spin Hall effect in two-dimensional monochalcogenides

2019-02-08T12:39:32+00:00February 8th, 2019|Categories: Publications|Tags: |

One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M  =  Ge, Sn and X  =  S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelectric properties. Here, using calculations from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transition that makes a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensio…

Published in: "2DMaterials".

Boron nitride monolayer growth on vicinal Ni(1 1 1) surfaces systematically studied with a curved crystal

2019-02-08T12:39:31+00:00February 8th, 2019|Categories: Publications|Tags: |

The structural and electronic properties of hexagonal boron nitride (hBN) grown on stepped Ni surfaces are systematically investigated using a cylindrical Ni crystal as a tunable substrate. Our experiments reveal homogeneous hBN monolayer coating of the entire Ni curved surface, which in turn undergoes an overall faceting. The faceted system is defined by step-free hBN/Ni(1 1 1) terraces alternating with strongly tilted hBN/Ni(1 1 5) or hBN/Ni(1 1 0) nanostripes, depending on whether we have A-type or B-type vicinal surfaces, respectively. Such deep substrate self-organization is explained by both the rigidity of the hBN lattice and the lack of registry with Ni crystal planes in the vicinity of the (1 1 1) surface. The analysis of the electronic properties by photoemission and absorption spectroscopies reveal a weaker hBN/Ni interaction in (1 1 0)- and (1 1 5)-oriented facets, as well as an upward shift of the valence band with respect to the band position at the hBN/Ni(1 1 1) t…

Published in: "2DMaterials".

The reactivity of reduced graphene depends on solvation

2019-02-01T12:55:39+00:00February 1st, 2019|Categories: Publications|Tags: |

The reactivity of reduced graphene sheets (graphenide), obtained by dissolution of graphite intercalation compounds, in particular KC 8 , was systemically studied by covalent functionalization with 4-fluorobenzenediazonium tetrafluoroborate and 4-iodoaniline in different solvents (5 pure and 7 solvents with addition of K + chelating agents). Interestingly, proper solvation of the potassium cations was beneficial to the covalent functionalization, while excessive solvation led to destabilization and flocculation, with decreased reactivity of graphenide. The results were confirmed by scanning Raman spectroscopy (SRS) and x-ray photoelectron spectroscopy (XPS).

Published in: "2DMaterials".

Probing the origin of lateral heterogeneities in synthetic monolayer molybdenum disulfide

2019-02-01T12:55:33+00:00February 1st, 2019|Categories: Publications|Tags: |

Synthetic two-dimensional (2D) materials provide an opportunity to realize large-scale applications in next generation electronic and optoelectronic devices. One of the biggest challenges of synthetic 2D materials is the lateral heterogeneity such as non-uniform strain, composition and defect density. The electronic and optical properties are found to be not uniform in many cases, even within a single crystalline domain, potentially limiting synthetic 2D materials in advanced devices. In this work, we probe the origin of the widely observed lateral heterogeneities in synthetic monolayer MoS 2 . Epitaxial single crystalline domains (~10 µ m) are optically homogeneous and uniform with 0.3%–0.4% tensile strain, while misoriented domains (>20 µ m) exhibit distinct photoluminescence (PL) emissions from the center to the edge, along with released strain at the center. Temperature-dependent Raman and PL mapping reveals that the center of non-epitaxial domains ex…

Published in: "2DMaterials".

Benchmarking of graphene-based materials: real commercial products versus ideal graphene

2019-01-24T14:35:33+00:00January 24th, 2019|Categories: Publications|Tags: |

There are tens of industrial producers claiming to sell graphene and related materials (GRM), mostly as solid powders. Recently the quality of commercial GRM has been questioned, and procedures for GRM quality control were suggested using Raman Spectroscopy or Atomic Force Microscopy. Such techniques require dissolving the sample in solvents, possibly introducing artefacts. A more pragmatic approach is needed, based on fast measurements and not requiring any assumption on GRM solubility. To this aim, we report here an overview of the properties of commercial GRM produced by selected companies in Europe, USA and Asia. We benchmark: (A) size, (B) exfoliation grade and (C) oxidation grade of each GRM versus the ones of ‘ideal’ graphene and, most importantly, versus what reported by the producer. In contrast to previous works, we report explicitly the names of the GRM producers and we do not re-dissolve the GRM in solvents, but only use techniques compatible with industrial p…

Published in: "2DMaterials".

Tunable Schottky barriers in ultrathin black phosphorus field effect transistors via polymer capping

2019-01-24T14:35:31+00:00January 24th, 2019|Categories: Publications|Tags: |

It is still a great challenge to avoid the degradation of ultrathin black phosphorus (BP) since its discovery in 2014. Various methods have been explored to stabilize the properties of ultrathin BP through capping technology or chemical passivation. Besides, the large metal–semiconductor contact resistance is also one of the critical issues. The two problems hinder the further development of ultrathin BP devices. Herein, we demonstrate that polymethyl methacrylate (PMMA) capping can not only enhance the durability of the ultrathin BP effectively and nondestructively, but also tune the effective Schottky barriers (SBs) formed at the interfaces between the metal and semiconductor dramatically. Particularly, the Schottky barrier (SB) for electron injection from metal to semiconductor is decreased by ~13 meV and the performance of the BP field effect transistor (FET) is strongly enhanced with the current on/off ratio increased by 6.8 times for the hole conduction after the PMMA capp…

Published in: "2DMaterials".

Mapping the mechanical properties of a graphene drum at the nanoscale

2019-01-23T14:41:23+00:00January 23rd, 2019|Categories: Publications|Tags: , |

The operation of graphene-based nanoelectromechanical systems (NEMS) crucially depends on the local mechanical characteristics of the graphene drum resonator. In particular, inhomogeneity in the residual strain (pre-strain) of the graphene membrane may affect the vibration dynamics as well as the energy dissipation. Despite its importance, achieving a precise local mapping of the pre-strain of a graphene membrane remains challenging. Here, we correlate scanning-probe force microscopy and Raman spectroscopy to map the local mechanical properties of circular monolayer-graphene drums. At odds with other techniques, we obtain maps of the membrane pre-strain with nanometric resolution and measure the effective Young’s modulus in a non-invasive way. Moreover, we show that the common topographic artefacts stemming from tip-induced deformations can be precisely corrected using the information derived from force-spectroscopy data. As a result, the local map of the pre-strain can be corre…

Published in: "2DMaterials".

Charge transfer across monolayer/bilayer MoS 2 lateral interface and its influence on exciton and trion characteristics

2019-01-17T16:36:39+00:00January 17th, 2019|Categories: Publications|

The charge transfer phenomenon is identified to be a major factor determining exciton and trion characteristics of atomically thin MoS 2 layers in various stacking configurations. We report photoluminescence (PL) from CVD-grown layered MoS 2 in the presence of a skewed or a deformed triangular-shaped monolayer/bilayer (1L/2L) lateral interface. Integrated PL mapping images over the 1L and 2L MoS 2 regions revealed that the neutral exciton emission was significantly enhanced and exhibited an oscillatory behavior in its intensity in the 1L region near the 1L/2L boundary, whereas the negative trion emission remained unchanged. The interplays among the number of MoS 2 layers, a substrate, and a geometric boundary structure of the 1L/2L lateral interface turned out to be important in charge transfer due to a modulation in work functions. Density functional theory predicted that the work functions of 1L and 2L MoS 2 were strongly influ…

Published in: "2DMaterials".

A roadmap for electronic grade 2D materials

2019-01-17T16:36:36+00:00January 17th, 2019|Categories: Publications|Tags: |

Since their modern debut in 2004, 2-dimensional (2D) materials continue to exhibit scientific and industrial promise, providing a broad materials platform for scientific investigation, and development of nano- and atomic-scale devices. A significant focus of the last decade’s research in this field has been 2D semiconductors, whose electronic properties can be tuned through manipulation of dimensionality, substrate engineering, strain, and doping (Mak et al 2010 Phys. Rev. Lett . 105 136805; Zhang et al 2017 Sci. Rep . 7 16938; Conley et al 2013 Nano Lett . 13 3626–30; Li et al 2016 Adv. Mater . 28 8240–7; Rhodes et al 2017 Nano Lett . 17 1616–22; Gong et al 2014 Nano Lett . 14 442–9; Suh et al 2014 Nano Lett . 14 6976–82; Yoshida et al 2015 Sci. Rep . 5 14808). Molybdenum disulfide (MoS 2 ) and tungsten diseleni…

Published in: "2DMaterials".

WSe 2 homojunctions and quantum dots created by patterned hydrogenation of epitaxial graphene substrates

2019-01-17T16:36:34+00:00January 17th, 2019|Categories: Publications|Tags: |

Scanning tunneling microscopy (STM) at 5 K is used to study WSe 2 layers grown on epitaxial graphene which is formed on Si-terminated SiC(0 0 0 1). Specifically, a partial hydrogenation process is applied to intercalate hydrogen at the SiC–graphene interface, yielding areas of quasi-free-standing bilayer graphene coexisting with bare monolayer graphene. We find that an abrupt and structurally perfect homojunction (band-edge offset ~0.25 eV) is formed when WSe 2 overgrows a lateral junction between adjacent monolayer and quasi-free-standing bilayer areas in the graphene. The band structure modulation in the WSe 2 overlayer arises from the varying work function (electrostatic potential) of the graphene beneath. Scanning tunneling spectroscopy measurements reveal that this effect can be also utilized to create WSe 2 quantum dots that confine either valence or conduction band states, in agreement with first-principles band structure calculatio…

Published in: "2DMaterials".

Reversible electron doping in monolayer WS 2 via a chemical strategy

2019-01-09T16:34:49+00:00January 9th, 2019|Categories: Publications|Tags: |

Controlled doping, as a cornerstone of the semiconductor industry, becomes one of the most important topics for two-dimensional (2D) semiconductors such as tungsten disulfide (WS 2 ), with intriguing physical properties. Here, we present a facile, controllable and reversible strategy for surface charge transfer doping in the monolayer WS 2 crystals. After triethylamine treatment, the field effect transistors (FETs) based on monolayer WS 2 exhibit enhanced mobility up to 28.6 cm 2 V −1 s −1 , which is much larger than that of the pristine one (9.8 cm 2 V −1 s −1 ). In addition, the n-doping via triethylamine treatment can also improve the photoresponsivity of the device, i.e. from pristine 6.4  ×  10 −3 AW −1 to the doped 21 AW −1 . Interestingly, the electron transport properties of the doped WS 2 can be recovered to be close to that of the intrinsic WS

Published in: "2DMaterials".

Controlled growth of atomically thin MoSe 2 films and nanoribbons by chemical vapor deposition

2019-01-09T16:34:48+00:00January 9th, 2019|Categories: Publications|

Atomically thin transition metal dichalcogenides (TMDCs) have drawn much interest for their promising applications in electronic, optoelectronic, valleytronic and sensing fields. Controlled growth of large-scale and high-quality TMDC nanostructures is highly desirable but remains challenging. In the present work, large-scale monolayer, bilayer and few-layer MoSe 2 films have been controllably synthesized by ambient pressure chemical vapor deposition (APCVD). Hydrogen flow rate, growth temperature as well as selenium–metal flux ratio have been systematically investigated, which were demonstrated to play a key role in the synthesis of MoSe 2 nanostructures. We have also reported the successful growth of MoSe 2 nanoribbons with controlled width and length on diverse substrates by APCVD with the assistance of sodium chloride and corresponding growth mechanism was proposed. Our findings highlight the prospects for the controlled growth of novel 1D and 2D…

Published in: "2DMaterials".

Unveiling the competitive role of etching in graphene growth during chemical vapor deposition

2018-12-21T14:34:35+00:00December 21st, 2018|Categories: Publications|Tags: |

During the growth of graphene via chemical vapor deposition, its recessive process, that is, etching, is often neglected. However, recent experimental studies showed that etching is not only able to give rise to complex morphologies that cannot be achieved by pure growth, but it also can be used to create designed patterns. In this work, we develop a kinetic Monte Carlo model based on the underlying mechanisms and growth kinetics of graphene to predict the formation of various morphologies during growth and etching. The simulation results reproduce a variety of experimentally observed morphologies of graphene domains with six-fold, four-fold and three-fold symmetries. In addition, we propose analytical relations between the gas flow rate in the experiments and the growth and etching parameters used in our simulations. We also present a phase diagram for the domain morphology from the attachment-limited regime to the diffusion-limited regime to guide the control of domain morphol…

Published in: "2DMaterials".

Electric-field modulation of linear dichroism and Faraday rotation in few-layer phosphorene

2018-12-21T14:34:33+00:00December 21st, 2018|Categories: Publications|Tags: |

Electro-optical modulators, which use an electric voltage (or an electric field) to modulate a beam of light, are essential elements in present-day telecommunication devices. Using a self-consistent tight-binding approach combined with the standard Kubo formula, we show that the optical conductivity and the linear dichroism of few-layer phosphorene can be modulated by a perpendicular electric field. We find that the field-induced charge screening plays a significant role in modulating the optical conductivity and the linear dichroism. Distinct absorption peaks are induced in the conductivity spectrum due to the strong quantum confinement along the out-of-plane direction and to the field-induced forbidden-to-allowed transitions. The field modulation of the linear dichroism becomes more pronounced with increasing number of phosphorene layers. We also show that the Faraday rotation is present in few-layer phosphorene even in the absence of an external magnetic field. This optical H…

Published in: "2DMaterials".

Chiral phonons in two-dimensional materials

2018-12-17T14:34:01+00:00December 17th, 2018|Categories: Publications|

There has been growing interest in investigating chiral phonons since they are theoretically found and experimentally verified recently. In a magnetic system with time inversion symmetry breaking, phonon can have nonzero angular momentum, which makes a correction to the gyromagnetic ratio measured in the Einstein–de Haas effect. Though total phonon angular momentum is zero in a nonmagnetic two-dimensional (2D) hexagonal system with space inversion symmetry breaking, phonons at high symmetry points of the Brillouin zone can have nonzero phonon angular momentum, which means they are chiral phonons. Chiral phonons decide selection rules in the electronic intervalley scattering, which has been experimentally verified in tungsten-diselenide monolayers very recently (Zhu et al 2018 Science 359 579). In this review, after a brief introduction of related background and some basic concepts, we mainly report recent progress of phonon angular momentum in magnetic syste…

Published in: "2DMaterials".

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