Coulomb scattering mechanism transition in 2D layered MoTe 2 : effect of high- κ passivation and Schottky barrier height

2018-11-16T12:35:23+00:00November 16th, 2018|Categories: Publications|Tags: |

Clean interface and low contact resistance are crucial requirements in two-dimensional (2D) materials to preserve their intrinsic carrier mobility. However, atomically thin 2D materials are sensitive to undesired Coulomb scatterers such as surface/interface adsorbates, metal-to-semiconductor Schottky barrier (SB), and ionic charges in the gate oxides, which often limits the understanding of the charge scattering mechanism in 2D electronic systems. Here, we present the effects of hafnium dioxide (HfO 2 ) high- κ passivation and SB height on the low-frequency (LF) noise characteristics of multilayer molybdenum ditelluride (MoTe 2 ) transistors. The passivated HfO 2 passivation layer significantly suppresses the surface reaction and enhances dielectric screening effect, resulting in an excess electron n -doping, zero hysteresis, and substantial improvement in carrier mobility. After the high- κ HfO 2 passivation, the obtained …

Published in: "Nanotechnology".

Growth of two-dimensional materials on hexagonal boron nitride ( h -BN)

2018-11-16T12:35:22+00:00November 16th, 2018|Categories: Publications|Tags: , |

With its atomically smooth surface yet no dangling bond, chemical inertness and high temperature sustainability, the insulating hexagonal boron nitride ( h -BN) can be an ideal substrate for two-dimensional (2D) material growth and device measurement. In this review, research progress on the chemical growth of 2D materials on h -BN has been summarized, such as chemical vapor deposition and molecular beam epitaxy of graphene and various transition metal dichalcogenides. Further, stacking of the as-grown 2D materials relative to h -BN, thermal expansion matching between the deposited materials and h -BN, electrical property of 2D materials on h -BN have been discussed in detail.

Published in: "Nanotechnology".

Dimensional reduction and ionic gating induced enhancement of superconductivity in atomically thin crystals of 2H-TaSe 2

2018-11-12T18:33:04+00:00November 12th, 2018|Categories: Publications|

The effects of dimensional reduction and ion intercalation on superconductivity (SC) in the presence of charge density waves (CDWs) in two-dimensional crystals of 2H-TaSe 2 were characterized. We prepared atomically thin crystals by mechanical exfoliation and performed electrical transport measurements on devices made by photolithography. The superconducting transition temperature ( T c SC ) was found to increase monotonically as the thickness decreased, changing from 0.14 K in the bulk to higher than 1.4 K for a 3-nm-thick crystal. The temperature dependence of upper critical field was found to be anomalous. The CDW transition temperature (T c CDW ) was found to decrease, but to a less extent than T c SC , from 120 K in the bulk to around 113 K for the 3-nm-thick crystal. In addition, ion intercalation was found to increase T …

Published in: "Nanotechnology".

Liquid-exfoliation of S-doped black phosphorus nanosheets for enhanced oxygen evolution catalysis

2018-11-12T18:33:03+00:00November 12th, 2018|Categories: Publications|Tags: |

Black phosphorus (BP) has recently drawn great attention in the field of electrocatalysis due to its distinct electrocatalytic activity for the oxygen evolution reaction (OER). However, the slow OER kinetics and the poor environmental stability of BP seriously limits its overall OER performance and prevents its electrocatalysis application. Here, sulfur (S)-doped BP nanosheets, which are prepared using high-pressure synthesis followed by liquid exfoliation, have been demonstrated to have much better OER electrocatalytic activity and environmental stability compared to their undoped counterparts. The S-doped BP nanosheets display a Tafel slope of 75 mV dec −1 , which is a favorable value refered to the kinetics of OER in electrochemical tests. Notably, there is no degradation of S-doped BP nanosheets after six days exposure to ambient, indicating an excellent environmental stability of the S-doped BP. The density functional theory calculations show that the OER activity …

Published in: "Nanotechnology".

Growth of lateral graphene/h-BN heterostructure on copper foils by chemical vapor deposition

2018-11-12T16:34:33+00:00November 12th, 2018|Categories: Publications|Tags: , , |

The synthesis of lateral heterostructures assembled by atomically-thin materials with distinct intrinsic properties is important for future heterojunction-embedded two-dimensional (2D) devices. Here we report an etching-assisted chemical vapor deposition method to synthesize large-area continuous lateral graphene/hexagonal boron nitride (Gr/h-BN) heterostructures on carbon-containing copper foils. The h-BN film is first synthesized on the copper foil, followed by hydrogen etching, and then epitaxial graphene domains are grown to form continuous lateral heterostructures. Analyses, including Raman spectroscopy, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, and ultraviolet-visible absorption spectroscopy, are used to characterize the coexistence of both materials and the highly continuous nature of this lateral heterostructure. This facile and scalable synthesizing method enables the potential usage of Gr/h-BN heterostructure in both funda…

Published in: "Nanotechnology".

Terahertz time-domain spectroscopy as a novel metrology tool for liquid-phase exfoliated few-layer graphene

2018-11-06T12:46:15+00:00November 6th, 2018|Categories: Publications|Tags: |

Few-layer graphene (FLG) platelets exfoliated directly from graphite are finding a wide range of potential applications, including composites and printed electronics. However, characterisation of the FLG material following incorporation into polymers, including the quality of the dispersion, remains a challenge. Here, we present the use of terahertz time-domain spectroscopy as a potential solution to this challenge which could form the basis of a rapid characterisation tool. The THz refractive index was found to be highly sensitive to the loading of FLG, opening the route to mapping local FLG concentration within a polymer composite sample. By fitting the measured permittivity of the flakes to the Drude–Smith model of conductivity, we also show that the carrier concentrations of these materials are comparable to un-doped chemical vapour deposition produced materials. The ability to measure electronic properties of FLG following processing is important to ensure that defects have…

Published in: "Nanotechnology".

Simultaneous intercalated assembly of mesostructured hybrid carbon nanofiber/reduced graphene oxide and its use in electrochemical sensing

2018-11-02T10:34:01+00:00November 2nd, 2018|Categories: Publications|Tags: , |

Polyacrylonitrile nonwovens intercalated with graphene oxide (GO) sheets were prepared by a simultaneous electrospinning-spray deposition system. These hybrid nonwovens were carbonized in a two-stage process to obtain a mesostructured hybrid carbon containing carbon nanofibers (CNF) and reduced GO sheets (CNF/RGO). During the carbonization process, the CNF act as spacers between the RGO layers to prevent their compactation and restacking resulting in a three-dimensional structure. The presence of RGO increases the electrical conductivity in the CNF/RGO material. The resulting hybrid carbon is nitrogen-doped as indicated by x-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy. This N-doped porous carbon was used to prepare electrodes with improved sensitivity for the electrochemical detection of L-cysteine.

Published in: "Nanotechnology".

Direct nanomechanical measurements of boron nitride nanotube—ceramic interfaces

2018-11-02T10:33:59+00:00November 2nd, 2018|Categories: Publications|Tags: , |

Boron nitride nanotubes (BNNTs) are a unique class of light and strong tubular nanostructure and are highly promising as reinforcing additives in ceramic materials. However, the mechanical strength of BNNT-ceramic interfaces remains largely unexplored. Here we report the first direct measurement of the interfacial strength by pulling out individual BNNTs from silica (silicon dioxide) matrices using in situ electron microscopy techniques. Our nanomechanical measurements show that the average interfacial shear stress reaches about 34.7 MPa, while density functional theory calculations reveal strong bonded interactions between BN and silica lattices with a binding energy of –6.98 eV nm −2 . Despite this strong BNNT-silica binding, nanotube pull-out remains the dominant failure mode without noticeable silica matrix residues on the pulled-out tube surface. The fracture toughness of BNNT-silica ceramic matrix nanocomposite is evaluated based on the measured interfacial…

Published in: "Nanotechnology".

Molecular dynamics simulations of the thermal conductivity of graphene for application in wearable devices

2018-11-02T10:33:57+00:00November 2nd, 2018|Categories: Publications|Tags: |

Graphene has been highlighted as a great potential material in wearable devices, owing to its extraordinary properties such as mechanical softness, high electrical conductivity and ultra-thin thickness. However, there are remaining challenges in thermal dissipation of graphene under such complicated conditions, which significantly affect the performance of portable electronics. Using molecular dynamics simulations, we have performed systematic analysis of thermal performance for the application in wearable devices in terms of graphene with defects, under uniaxial tensile strain, and vertical stress. Three kinds morphology of defects (horizontal line defect, circular defect, and vertical line defect) are constructed by deleting atoms on the pristine graphene plane. The thermal conductivity is related to the projected defected area perpendicular to the direction of the heat current. The relative thermal conductivity displays a cubic decreasing trend with the increase of uniaxial t…

Published in: "Nanotechnology".

Electron effective attenuation length in epitaxial graphene on SiC

2018-11-01T10:33:29+00:00November 1st, 2018|Categories: Publications|Tags: , |

The inelastic mean free path (IMFP) for carbon-based materials is notoriously challenging to model, and moving from bulk materials to 2D materials may exacerbate this problem, making the accurate measurements of IMFP in 2D carbon materials critical. The overlayer-film method is a common experimental method to estimate IMFP by measuring electron effective attenuation length (EAL). This estimation relies on an assumption that elastic scattering effects are negligible. We report here an experimental measurement of electron EAL in epitaxial graphene on SiC using photoelectron spectroscopy over an electron kinetic energy range of 50–1150 eV. We find a significant effect of the interface between the 2D carbon material and the substrate, indicating that the attenuation length in the so-called ‘buffer layer’ is smaller than for free-standing graphene. Our results also suggest that the existing models for estimating IMFPs may not adequately capture the physics of electron interactions in…

Published in: "Nanotechnology".

Structural, chemical, and magnetic properties of cobalt intercalated graphene on silicon carbide

2018-11-01T10:33:28+00:00November 1st, 2018|Categories: Publications|Tags: , |

We report on a study of the Co intercalation process underneath the ##IMG## [] {$(6sqrt{3}times 6sqrt{3})$} R30° reconstructed 6H-SiC(0001) surface for Co film-thicknesses in a range of 0.4–12 nm using a combination of surface sensitive imaging, diffractive, and spectroscopic methods. In situ photoemission electron microscopy reveals a dependence of the intercalation temperature on the Co film-thickness. Using low energy electron diffraction and photoemission spectroscopy (XPS), we find that the SiC surface reconstruction is partially lifted and transformed. We show that the ##IMG## [] {$(6sqrt{3}times 6sqrt{3})$} R30° reconstruction does not prevent silicide formation for Cofilm-thicknesses ≥0.4 nm according to XPS and x-ray absorption spectra. Our results indicate that the silicide formati…

Published in: "Nanotechnology".

Shot noise in electrically-gated silicene nanostructures

2018-10-30T14:33:28+00:00October 30th, 2018|Categories: Publications|Tags: , |

We have theoretically studied fundamental shot noise properties in single- and dual-gated silicene nanostructures. It is demonstrated here that due to the intrinsic spin–orbit gap, the Fano factor ( F ) in the biased structures does not coincide with the characteristic value F = 1/3, a value frequently reported for a graphene system. Under gate-field modulations, the F in the gated structure can be efficiently engineered and the specific evolution of the F versus the field strength is symmetric with the center of spectra oppositely shifting away from the zero field condition for the valley or spin-coupled spinor states. This field-dependent hysteretic loop thus offers some flexible methods to distinguish one spinor state from its valley or spin-coupled state via their numerical difference in the F once the incident beam is spin or valley-polarized.

Published in: "Nanotechnology".

Vertical GaN nanocolumns grown on graphene intermediated with a thin AlN buffer layer

2018-10-30T12:38:13+00:00October 30th, 2018|Categories: Publications|Tags: |

We report on the self-assembled growth of high-density and vertically-oriented n -doped GaN nanocolumns on graphene by radio-frequency plasma-assisted molecular beam epitaxy. Graphene was transferred to silica glass, which was used as the substrate carrier. Using a migration enhanced epitaxy grown AlN buffer layer for the nucleation is found to enable a high density of vertical GaN nanocolumns with c -axis growth orientation on graphene. Furthermore, micro-Raman spectroscopy indicates that the AlN buffer reduces damage on the graphene caused by impinging active N species generated by the radio-frequency plasma source during the initial growth stage and nucleation of GaN. In addition, the grown GaN nanocolumns on graphene are found to be virtually stress-free. Micro-photoluminescence measurements show near band-edge emission from wurtzite GaN, exhibiting higher GaN bandgap related photoluminescence intensity relative to a reference GaN bulk substrate and the absence o…

Published in: "Nanotechnology".

Toward barrier free contact to MoSe 2 /WSe 2 heterojunctions using two-dimensional metal electrodes

2018-10-30T12:38:11+00:00October 30th, 2018|Categories: Publications|

In the design of electronic devices based on two-dimensional heterojunctions, the contact between electrodes and different surfaces of two-dimensional heterojunctions may produce different effects. Furthermore, metal–semiconductor contact plays an important role in modern devices. However, due to the Fermi level pinning effect (FLPE), it is difficult to tune the Schottky barrier height between common metals (e.g. Au, Ag, and Cu) and semiconductors. Fortunately, the FLPE becomes weak at the contact between the 2D metal and 2D semiconductor, due to the suppression of metal-induced gap states. Here, we choose monolayer NbS 2 as the electrode to be in contact with the MoSe 2 /WSe 2 bilayer. The interfacial properties as well as the stacking dependence are discussed based on the density functional theory, combined with the nonequilibrium Green’s functions. Two configurations are considered, i.e. the WSe 2 /MoSe 2 /NbS 2 and MoS…

Published in: "Nanotechnology".

Robust visibility of graphene monolayer on patterned plasmonic substrates

2018-10-29T10:33:07+00:00October 29th, 2018|Categories: Publications|Tags: |

Single and few-layer graphene flakes, while visible on a dielectric surface with customized thickness, cannot be optically imaged when exfoliated directly on semiconductors or metal substrates with arbitrary thickness. In this paper, we show that such graphene flakes become visible through a conventional microscope on a substrate patterned with a submicron sized, hexagonally packed array of gold disks. The interaction of the metal pattern with the incident light generates surface plasmon polaritons (SPPs) and results in enhanced reflectivity for certain angles and wavelengths. In the areas where graphene flakes are present, the interaction of the SPP with incident radiation is altered and consequently decreases the reflectivity in this region and increases the contrast, which accounts for the visibility of the graphene flakes on such substrates. We validate the observed contrast in visibility utilizing an in-house developed modified form of rigorous coupled wave analysis algorit…

Published in: "Nanotechnology".

Enhancing the adhesion of graphene to polymer substrates by controlled defect formation

2018-10-26T10:33:40+00:00October 26th, 2018|Categories: Publications|Tags: |

The mechanical integrity of composite materials depends primarily on the interface strength and the defect density of the reinforcement which is the provider of enhanced strength and stiffness. In the case of graphene/polymer nanocomposites which are characterized by an extremely large interface region, any defects in the inclusion (such as folds, cracks, holes, etc) will have a detrimental effect to the internal strain distribution and the resulting mechanical performance. This conventional wisdom, however, can be challenged if the defect size is reduced beyond the critical size for crack formation to the level of atomic vacancies. In that case, there should be no practical effect on crack propagation and depending on the nature of the vacancies the interface strength may in fact increase. In this work we employed argon ion (Ar + ) bombardment and subsequent exposure to hydrogen (H 2 ) to induce (as revealed by x-ray and ultraviolet photoelectron spectroscopy …

Published in: "Nanotechnology".

Decoration of Cu 2 O photocathode with protective TiO 2 and active WS 2 layers for enhanced photoelectrochemical hydrogen evolution

2018-10-18T10:34:51+00:00October 18th, 2018|Categories: Publications|Tags: |

A Cu 2 O based multi-layered photocathode was fabricated with a layer-by-layer assembly method for enhanced photoelectrochemical (PEC) hydrogen evolution. Au was first electrodeposited on the fluorine-doped tin oxide glass to decrease the electrochemical impedance of the Cu 2 O photocathode. A layer of TiO 2 was then coated to increase the light-to-electricity energy conversion efficiency and the chemical stability by forming a p – n junction with Cu 2 O. Exfoliated WS 2 nanosheets obtained from lithium insertion were then coated as the electron acceptor to facilitate the hydrogen evolution. This photocathode is effective for PEC hydrogen evolution, and a photocurrent of −10 mA cm −2 can be obtained at −0.33 V versus RHE in a phosphorus buffer (pH = 6.0) under visible light (λ ≥ 420 nm, 100 mW cm −2 ) on the optimized photocathode.

Published in: "Nanotechnology".

2D vertical field-effect transistor

2018-10-17T12:36:21+00:00October 17th, 2018|Categories: Publications|Tags: |

Within the framework of 2D materials, we present four theoretical models of a vertical field-effect transistor (FET) composed of simple alternate graphene and MoS 2 layers. The electronic transport properties at a specific graphene/MoS 2 interface in each configuration are investigated by focusing in particular on the current as a function of the gate voltage. The gate voltage, simulated with a shift of the bands of a specific layer, allows us to tune the current at the interface and the charge transfer between the planes. This analysis of the charge transfer as a function of the gate voltage reveals a strong connection with the transport characteristics as the slope of the current curve. The analysis of physical phenomena at the graphene/MoS 2 interface can further improve the 2D vertical FET performance and contribute to the development of new 2D nanotechnology.

Published in: "Nanotechnology".

Synergistic antibacterial effect of tetracycline hydrochloride loaded functionalized graphene oxide nanostructures

2018-10-17T12:36:19+00:00October 17th, 2018|Categories: Publications|Tags: , |

With the high demand for developing novel composites with integrated performance, graphene-based nanostructures have been drawing great attention in environmental and biomedical applications because of their extraordinary physicochemical properties and biocompatibility. Although graphene oxide (GO) nanosheets exhibit some antibacterial activities, novel GO based nanostructures with enhanced antibacterial activities are highly desired. To realize this aim, polyethyleneimine (PEI) modified GO as a tetracycline hydrochloride (TCH) carrier and release platform was constructed (pGO-TCH). The nanostructures were fully characterized by TEM, AFM, FTIR and Raman spectra, which demonstrated that TCH were uniformly and compactly deposited on PEI modified GO nanosheets. The antibacterial performances of the prepared nanostructures were investigated by disk diffusion method and bacterial growth kinetics method towards Gram-positive S. aureus and Gram-negative E. coli . Results s…

Published in: "Nanotechnology".

Graphene/transition metal dichalcogenides hybrid supercapacitor electrode: status, challenges, and perspectives

2018-10-16T10:38:32+00:00October 16th, 2018|Categories: Publications|Tags: , |

Supercapacitors, based on fast ion transportation, are among the most promising energy storage solutions that can deliver fast charging–discharging within seconds and exhibit excellent cycling stability. The development of a good electrode material is one of the key factors in enhancing supercapacitor performance. Graphene (G), an allotrope of carbon that consists of a single layer of carbon atoms arranged in a hexagonal lattice, elicits research attention among scientists in the field of energy storage due to its remarkable properties, such as outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. Furthermore, numerous studies focus on 2D materials that are analogous to graphene as electrode supercapacitors, including transition metal dichalcogenides (TMDs). Recently, scientists and researchers are exploring TMDs because of the distinct features that make 2D TMDs highly attractive for capacitive energy storage. This study provides an ov…

Published in: "Nanotechnology".

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