Nanotechnology

/Nanotechnology

Atomically thin van der Waals tunnel field-effect transistors and its potential for applications

2019-01-14T14:35:52+00:00January 14th, 2019|Categories: Publications|Tags: , |

Power dissipation is a crucial problem as the packing density of transistors increases in modern integrated circuits. Tunnel field-effect transistors (TFETs), which have high energy filtering provided by band-to-band tunneling (BTBT), have been proposed as an alternative electronics architecture to decrease the energy loss in bias operation and to achieve steep switching at room temperature. Very recently, the BTBT behavior has been demonstrated in van der Waals heterostructures by using unintentionally doped semiconductors. The reason of the BTBT formation is attributed to a significant band bending near the heterointerface, resulting in carrier accumulations. In this work, to investigate charge transport in type-III transistors, we adopted the same band-bending concept to fabricate van der Waals BP/MoS 2 heterostructures. Through analyzing the temperature dependence of their electrical properties, we carefully ruled out the contribution of metal-semiconductor contact…

Published in: "Nanotechnology".

Graphene and graphene nanomesh supported nickel clusters: electronic, magnetic, and hydrogen storage properties

2019-01-02T12:33:34+00:00January 2nd, 2019|Categories: Publications|Tags: , |

Small-sized nanoparticles are widely used in applications such as catalysis, nanoelectronics, and hydrogen storage. However, the small size causes a common problem: agglomeration on the support template. One solution is to use templates that limit the mobility of the nanoparticles. Graphene nanomeshes (GNMs) are two dimensional porous structures with controllably passivated pores. In this work, we employ first principles calculations to investigate the potential for using GNMs as support templates for Ni clusters and, at the same time, study their magnetic and hydrogen storage properties. We consider two Ni clusters (Ni 6 and Ni 13 ) and two GNMs (O-terminated and N-terminated), comparing our results to those of isolated Ni clusters and those of Ni clusters on graphene. High stability of the Ni clusters is found on the N-GNM in contrast to the O-GNM. We quantify the hydrogen storage capacity by calculating the adsorption energy for multiple H 2 molec…

Published in: "Nanotechnology".

Suppression of wrinkle formation in graphene on Ir(111) by high-temperature, low-energy ion irradiation

2018-12-31T10:33:46+00:00December 31st, 2018|Categories: Publications|Tags: , |

Graphene on Ir(111) is irradiated with small fluences of 500 eV He ions at temperatures close to its chemical vapor deposition growth temperature. The ion irradiation experiments explore whether it is possible to suppress the formation of wrinkles in Gr during growth. It is found that the release of thermal mismatch strain by wrinkle formation can be entirely suppressed for an irradiation temperature of 880 °C. A model for the ion beam induced suppression of wrinkle formation in supported Gr is presented, and underpinned by experiments varying the irradiation temperature or involving intercalation subsequent to irradiation.

Published in: "Nanotechnology".

Zero-energy-state-oriented tunability of spin polarization in zigzag-edged bowtie-shaped graphene nanoflakes under an electric field

2018-12-28T10:36:40+00:00December 28th, 2018|Categories: Publications|Tags: , |

A comprehensive first-principles study of the correlation between zero-energy states and the tunability of the spin-selective semiconducting properties of zigzag-edged bowtie-shaped graphene nanoflakes under an electric field is presented for the first time. We demonstrate that the spin degenerate semiconducting ground state can be lifted by the electric field. In particular, we find that the number of zero-energy states (’the nullity’) defined by the structural configuration determines the complexity and efficiency of the tunability of spin polarization. The fine-tuning of spin-dependent properties by the electric field originates from the manipulation of spin-polarized molecular orbital energies. We expect this study to aid the design of more effective and controllable low-dimensional molecular spintronics.

Published in: "Nanotechnology".

Phonon thermal conduction in a graphene–C 3 N heterobilayer using molecular dynamics simulations

2018-12-21T14:33:07+00:00December 21st, 2018|Categories: Publications|Tags: |

Two-dimensional (2D) graphene (GRA) and polyaniline (C 3 N) monolayers are attracting growing research interest due to their excellent electrical and thermal properties. In this work, in-plane and out-of-plane phonon thermal conduction of GRA–C 3 N heterobilayer are systematically investigated by using classical molecular dynamics simulations. Effects of system size, temperature and interlayer coupling strength on the in-plane thermal conductivity ( k ) and out-of-plane interfacial thermal resistance ( R ) are evaluated. Firstly, a monotonic increasing trend of k with increasing system size is observed, while a negative correlation between thermal conductivity and temperature is revealed. The interlayer coupling strength is found to have a weak effect on the in-plane thermal conductivity of the heterobilayer. Secondly, at T = 300 K and χ = 1, the predicted R of GRA → C 3 N and C 3 N → GRA are 1.29 × 10

Published in: "Nanotechnology".

NO 2 gas sensor based on graphene decorated with Ge quantum dots

2018-12-19T14:33:44+00:00December 19th, 2018|Categories: Publications|Tags: , |

We report a NO 2 gas sensor based on germanium quantum dots (GeQDs)/graphene hybrids. Graphene was directly grown on germanium through chemical vapor deposition and the GeQDs were synthesized via molecular beam epitaxy. The samples were characterized by atomic force microscope, Raman spectra, scanning electron microscope, x-ray photoelectron spectroscope and transmission electron microscope with energy dispersive x-ray. By introducing GeQDs on graphene, the gas sensor sensitivity to NO 2 was improved substantially. With the optimization of the growth time of GeQDs (600 s), the response sensitivity to 10 ppm NO 2 can be as high as 3.88, which is 20 times higher than that of the graphene sensor without GeQDs decoration. In addition, the 600 s GeQDs/graphene hybrid sensor exhibits fast response and recovery rates as well as excellent stability. Our work may provide a new route to produce low-power consumption, portable, and room temperature gas sensor …

Published in: "Nanotechnology".

Recent progress in synthesis, properties, and applications of hexagonal boron nitride-based heterostructures

2018-12-17T12:33:59+00:00December 17th, 2018|Categories: Publications|Tags: , , |

Featuring an absence of dangling bonds, large band gap, low dielectric constant, and excellent chemical inertness, atomically thin hexagonal boron nitride (h-BN) is considered an ideal candidate for integration with graphene and other 2D materials. During the past years, great efforts have been devoted to the research of h-BN-based heterostructures, from fundamental study to practical applications. In this review we summarize the recent progress in the synthesis, novel properties, and potential applications of h-BN-based heterostructures, especially the synthesis technique. Firstly, various approaches to the preparation of both in-plane and vertically stacked h-BN-based heterostructures are introduced in detail, including top-down strategies associated with exfoliation transfer processes and bottom-up strategies such as chemical vapor deposition (CVD)-based growth. Secondly, we discuss some novel properties arising in these heterostructures. Several promising applications in ele…

Published in: "Nanotechnology".

SnO 2 -rGO nanocomposite as an efficient electron transport layer for stable perovskite solar cells on AZO substrate

2018-12-17T10:33:24+00:00December 17th, 2018|Categories: Publications|Tags: , |

Electron transport layer (ETL) plays an important role in realizing efficient and stable perovskite solar cells (PSCs). There are continuous efforts in developing new types of low cost ETLs with improved conductivity and compatibility with perovskite and the conducting electrode. Here, in order to obtain high efficient and stable PSCs on ZnO:Al (AZO) substrate, reduced graphene oxide (rGO) is incorporated into SnO 2 nanoparticles to form composite ETL. For planar PSC on AZO substrates, SnO 2 -rGO with a low incorporation ratio of 3 wt% rGO significantly enhances the device short circuit current density ( J sc ) and the fill factor when compared to the device with pristine SnO 2 ETL, leading to an overall power conversion efficiency of 16.8% with negligible hysteresis. The effectiveness of the excited charge transfer process of SnO 2 -rGO ETL is revealed by time-resolved photoluminescence decay, and by electrochemical impedanc…

Published in: "Nanotechnology".

MEH-PPV photophysics: insights from the influence of a nearby 2D quencher

2018-12-11T10:34:28+00:00December 11th, 2018|Categories: Publications|Tags: , |

The effect of 2D quenching on single chain photophysics was investigated by spin coating 13 nm thick films of polystyrene lightly doped with MEH-PPV onto CVD grown graphene and observing the changes in several photoluminescent (PL) observables. With 99% of the PL quenched, we found a 60% drop in the PL lifetime, along with a significant blue-shift of the PL emission due to the preferential quenching of emission at longer wavelengths. During photo-bleaching, the blue spectral shift observed for isolated polymers was eliminated in the presence of the quencher up until 70% of the polymer was photo-bleached. Results were interpreted using a static disorder induced conjugation length distribution model. The quencher, by opening up a new non-radiative decay channel, ensures that excitons do not have sufficient time to migrate to nearby lower energy chromophores. The reduction of energy transfer into the lowest-energy chromophores thus reduces their rate of photo-bleaching. Finally, th…

Published in: "Nanotechnology".

Tuning the electronic structure of single-walled carbon nanotube by high-pressure H 2 exposure

2018-12-07T10:34:45+00:00December 7th, 2018|Categories: Publications|Tags: |

We report on an electronic structure change of single-walled carbon nanotube (SWNT) on hexagonal boron nitride due to electron doping via high-pressure H 2 exposure. The fractional coverage of hydrogenated carbon atom is estimated to be at least θ = 0.163 from the in situ I ds – V g measurements of the release process. Raman spectroscopy and x-ray photoelectron spectroscopy were carried out to support the in situ electrical measurements. In particular, we used the dissociative Langmuir-type model to yield the desorption coefficient k des by fitting it to the in situ electrical data. Finally, we applied this hydrogenation method to the SWNT network on the commercial Si/SiO 2 substrate to open the possibility of the scalable n -type semiconducting SWNT FETs.

Published in: "Nanotechnology".

Carbon-based polymer nanocomposites as dielectric energy storage materials

2018-12-07T10:34:43+00:00December 7th, 2018|Categories: Publications|Tags: , |

Nanostructured polymeric materials based on conductive nanofillers have promising applications in the energy storage field owing to the extraordinary characteristics of the nanofillers. Conductive nanofillers, such as graphene nanoplatelets, are characterized by small size, extraordinary surface area to volume ratio, high aspect-ratio and extremely low electrical resistivity. In this work, the dielectric behaviors and the corresponding energy storage capabilities of high aspect-ratio carbon nanofiller/polymer composites were reviewed. At the electrical percolation point, a conductive composite exhibits a sudden and remarkable enhancement in dielectric constant and dielectric loss. The challenge is to maintain the increase in dielectric constant while preventing the increase in dielectric loss. Various physical and chemical methodologies have been followed to overcome this challenge including surface chemistry modifications, physical alignment of nanofillers and utilizing of hybr…

Published in: "Nanotechnology".

Fast water desalination by carbon-doped boron nitride monolayer: transport assisted by water clustering at pores

2018-12-05T12:33:35+00:00December 5th, 2018|Categories: Publications|Tags: |

The well-being of the ever-escalating world population hinges largely upon the adequacy of clean, fresh water. Desalination is one of the most promising approaches in such an endeavor. Using molecular dynamics simulations, we take a close look at nanoporous hexagonal boron nitride nanosheets as desalination membranes, and study how C dopants affect their performance. The calculations predict that the desalination performance of C-doped BN membranes compares favorably to that of MoS 2 membranes: the water flux through the 0% (0C B –0C N ), 25% (3C B –0C N ), 75% (3C B –6C N ), and 100% C terminated BN membrane (6C B –6C N ) is 29.9, 47.5, 95.3, and 81.5 molecules ns −1 per pore, respectively, and there is a strong correlation between the water flux and the axial diffusion coefficient. Through our study of the effect of C content on the desalination performance, it is found that more clu…

Published in: "Nanotechnology".

Structural reconfiguration and stress relaxation in twisted epitaxial graphene by annealing

2018-11-28T12:33:34+00:00November 28th, 2018|Categories: Publications|Tags: |

The possibility to engineer the van der Waals interactions between graphene layers is crucial for controlling the electronic properties. Using epitaxial graphene with preferential orientations grown on the C-face 6H-SiC as a prototype, we have addressed the annealed structural reconfiguration of graphene layer in view of the evolutions of surface ripples and relative rotation angle (RRA) between lattices. It was found that the heat treatment of graphene layers under vacuum deformed the arcuate ripple surface and subsequently split one ripple into parallel twin pleats, which drastically increased the strains in the films. The originally oriented graphene layers, with small RRA between adjacent layers, were rearranged by the annealing resulting in disordered orientations and larger RRA. After a sufficient annealing, the compressive stress stored in the films was well released to give undistorted graphene lattices. The vacuum annealing is an effective treatment for irreversibly rel…

Published in: "Nanotechnology".

Electronic textiles based on aligned electrospun belt-like cellulose acetate nanofibers and graphene sheets: portable, scalable and eco-friendly strain sensor

2018-11-27T12:34:39+00:00November 27th, 2018|Categories: Publications|Tags: , |

Recently, there has been strong interest in flexible and wearable electronics to meet the technological demands of modern society. Environmentally-friendly and scalable electronic textiles is a key area that is still significantly underdeveloped. Here, we describe a novel strain sensor composed of aligned cellulose acetate (CA) nanofibers with belt-like morphology and a reduced graphene oxide (RGO) layer. The unique spatial alignment, microstructure and wettability of CA nanofibrous membranes facilitate their close contact with deposited GO colloids. After a portable and fast hot-press process within 700 s at 150 °C, the GO on CA membrane can be facilely reduced to a conductive RGO layer. Moreover, the connection among contiguous CA nanofibers and the interaction between the GO and CA substrate were both highly enhanced, resulting in superior mechanical strength with Young’s modulus of 1.3 GPa and small sheet resistance lower than 10 kΩ. Therefore, the conductive RGO/CA membrane…

Published in: "Nanotechnology".

Frictional characteristics of nano-confined water mediated hole-doped single-layer graphene on silica surface

2018-11-27T12:34:37+00:00November 27th, 2018|Categories: Publications|Tags: |

We have investigated the frictional properties of single-layer graphene (SLG) coated rough silica substrate under the influence of nano-confined hydration layer underneath SLG. Through the friction and surface potential measurements by atomic force microscopy (AFM), we found polygonal features in AFM images of SLG-protected silica surface that exhibit simultaneously larger friction and higher surface potential as compared to their surrounding areas due to water layers confined under SLG. Nano-confined water layers at the SLG-silica interface can induce the hole-doping effect in SLG, resulting in a more positively-charged and hydrophilic surface that favors adsorption of ambient water molecules. Therefore, during friction measurements, nanoscale capillary bridges can form within the interstices of AFM probe-SLG contact, leading to larger adhesion and friction. The friction forces were found to respectively have negative and positive dependence on the sliding velocity inside and o…

Published in: "Nanotechnology".

Low-temperature one-pot synthesis of WS 2 nanoflakes as electrocatalyst for hydrogen evolution reaction

2018-11-27T12:34:35+00:00November 27th, 2018|Categories: Publications|Tags: |

Transition metal dichalcogenides have unique physicochemical properties. Herein, a low-temperature facile method is demonstrated to synthesize ultrathin tungsten disulfide nanoflakes. They are loosely stacked between layers with highly exposed edges, which provide lots of active sites for electrochemical applications. The by-product of crystalline carbon improves their conductivity, which also enhances their performance in hydrogen evolution reaction.

Published in: "Nanotechnology".

Grinding of nano-graphite inkjet inks for application in organic solar cells

2018-11-22T12:33:26+00:00November 22nd, 2018|Categories: Publications|Tags: |

The production of printable graphene flakes is not easy to scale up when produced by ultrasonication and purified by centrifugation. In this work, natural graphite flakes were exfoliated by wet ball milling in water supported by the addition of sodium deoxycholate as a surfactant and the dispersion was formulated for inkjet printing. By subsequent dilution and filtration of the milling paste, more than 45 l of a stable dispersion of nano-graphite particles in one batch process was obtained. The dispersion was characterized by thermogravimetric analysis and UV–vis spectroscopy to determine concentration, and experiments to measure long-term stability were conducted. The nano-graphite particles were analyzed by optical microscopy, scanning electron microscopy and Raman spectroscopy, revealing 300–400 nm sized particles. The dispersion was formulated into an inkjet ink and tested as interfacial hole transport layer between the anode and the photo-active bulk-heterojunction layer of…

Published in: "Nanotechnology".

Pseudocapacitive Li-ion storage boosts high-capacity and long-life performance in multi-layer CoFe 2 O 4 /rGO/C composite

2018-11-22T12:33:24+00:00November 22nd, 2018|Categories: Publications|Tags: , , |

Due to the intrinsic low electrical conductivity and large volume expansion of the CoFe 2 O 4 based active materials, designing more novel structures is still one of the most important challenges for its lithium ion battery application. In this work, the CoFe 2 O 4 /reduced graphene oxide/carbon (CFO/rGO/C) composite with integrated multi-layer structure has been synthesized through a facial two-step hydrothermal method. Benefiting from the introduction of the graphene network and amorphous carbon coating layer, as well as the accompanying synergistic effect, this composite can exhibit fast and reversible lithium intercalation/deintercalation reactions. With the aid of a surface-induced capacitive process, the CFO/rGO/C composite delivers a superior specific capacity (945 mA h g −1 at 0.1 A g −1 ) and excellent long-term cyclic stability (421 mA h g −1 at 4 A g −1 with closely 100% Coulombic efficiency a…

Published in: "Nanotechnology".

Adiabatic and nonadiabatic charge separation dynamics in graphene oxide quantum dots for overall water splitting

2018-11-20T14:34:10+00:00November 20th, 2018|Categories: Publications|Tags: , |

Chemical functionalization and heteroatom doping are two effective strategies for improving the conductivity of a graphene lattice. Nitrogen-doped graphene oxide quantum dot (GOQD) has been reported to possess both p- and n-conductivity that is induced by an oxygen functional group and nitrogen doping, respectively, and is suitable for catalyzing hydrogen and oxygen evolution reactions for complete water splitting. The experimental study shows that the hydrogen evolution reaction occurs considerably faster than the oxygen evolution reaction. However, the mechanism of this phenomenon remains unknown, which poses a challenge to the chemical modification of such classes of materials. In the present work, we perform nonadiabatic ab initio molecular dynamics to explore the charge separation dynamics in N-doped GOQD with oxygen functional groups. Our results show that there exists multiple charge decay channels governed by different mechanisms, which complicates the overall cha…

Published in: "Nanotechnology".

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