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1.
Sci Rep ; 10(1): 4237, 2020 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-32144312

RESUMO

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments.

2.
ACS Appl Mater Interfaces ; 11(50): 47289-47298, 2019 Dec 18.
Artigo em Inglês | MEDLINE | ID: mdl-31746197

RESUMO

Development of n-/p-type semiconducting graphenes is a critical route to implement in graphene-based nanoelectronics and optronics. Compared to the p-type graphene, the n-type graphene is more difficult to be prepared. Recently, phosphorous doping was reported to achieve air-stable and high mobility of n-typed graphene. The phosphorous-doped graphene (P-Gra) by ion implantation is considered as an ideal method for tailoring graphene due to its IC compatible process; however, for a conventional ion implanter, the acceleration energy is in the order of kiloelectron volts (keV), thus severely destroys the sp2 bonding of graphene owing to its high energy of accelerated ions. The introduced defects, therefore, degrade the electrical performance of graphene. Here, for the first time, we report a low-damage n-typed chemical vapor deposition (CVD) graphene by an industrial-compatible ion implanter with an energy of 20 keV where the designed protection layer (thin Au film) covered on as-grown CVD graphene is employed to efficiently reduce defect formation. The additional post-annealing is found to heal the crystal defects of graphene. Moreover, this method allows transferring ultraclean and residue-free P-Gra onto versatile target substrates directly. The doping configuration, crystallinity, and electrical properties on P-Gra were comprehensively studied. The results indicate that the low-damaged P-Gra with a controllable doping concentration of up to 4.22 at % was achieved, which is the highest concentration ever recorded. The doped graphenes with tunable work functions (4.85-4.15 eV) and stable n-type doping while keeping high-carrier mobility are realized. This work contributes to the proof-of-concept for tailoring graphene or 2D materials through doping with an exceptional low defect density by the low energy ion implantation, suggesting a great potential for unconventional doping technologies for next-generation 2D-based nanoelectronics.

3.
ACS Omega ; 4(17): 17536-17541, 2019 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-31656926

RESUMO

The multilayer 1T-TaSe2 is successfully synthesized by annealing a Se-implanted Ta thin film on the SiO2/Si substrate. Material analyses confirm the 1T (octahedral) structure and the quasi-2D nature of the prepared TaSe2. Temperature-dependent resistivity reveals that the multilayer 1T-TaSe2 obtained by our method undergoes a commensurate charge-density wave (CCDW) transition at around 500 K. This synthesis process has been applied to synthesize MoSe2 and HfSe2 and expanded for synthesis of one more transition-metal dichalcogenide (TMD) material. In addition, the main issue of the process, that is, the excess metal capping on the TMD layers, is solved by the reduction of thickness of the as-deposited metal thin film in this work.

4.
Nanotechnology ; 30(24): 245709, 2019 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-30731440

RESUMO

In this work, we study surface functionalization effects of artificially stacked graphene bilayers (ASGBs) to control its wetting properties via low-damage plasma. The ASGBs were prepared on a SiO2/Si substrate by stacking two monolayer graphene, which was grown by chemical vapor deposition. As a result, the low-damage plasma functionalization of ASGBs could hold both the key characteristics of surface functionalization and electrical transport properties of graphene sheets. To characterize ASGBs, Raman and x-ray photoelectron spectroscopy (XPS) were used to determine the degree of defect formation and functionalization. Meanwhile, the degree of the wettability of the ASGBs surface was determined by optical contact angle (CA) measurements. Based on experimental results, the compositional ratio of C-OH + COOH was found to increase 67% based on the analysis of XPS spectra after low-damage plasma treatment. This treatment effect can also be found with 75.3% decrease in the CA of water droplet on graphene. In addition, we found that the ratio of 2D/(D + G') in Raman spectra shows strong correlation to the measured CA; it can be a reliable indicator of ASGBs surface wettability modification. This work showed that we obtained a higher degree functionalization of ASGBs without degrading the under-layer structure of ASGBs due to the moderate low-damage plasma treatment. The presented process technique of controllable wettability through low-damage plasma treatment can be employed for potential application in graphene-based sensors/devices.

5.
Nanotechnology ; 30(4): 045706, 2019 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-30479310

RESUMO

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 outside the polygonal regions due to different surface wettability. Hence, it is possible to manipulate the frictional properties of SLG-coated silica by the amount of hydration layer confined underneath SLG. Our results may find applications in friction control for future nano-devices.

6.
Nanotechnology ; 30(6): 065702, 2019 Feb 08.
Artigo em Inglês | MEDLINE | ID: mdl-30524048

RESUMO

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, the difference between the quenched and non-quenched spectra allows the rate of energy transfer along the polymer backbone to be estimated at ∼2 ns-1.

7.
Phys Rev Lett ; 121(1): 018101, 2018 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-30028147

RESUMO

We report the experimental observation of speeded-up collective motion of the monolayer endothelia-cancer mixture on a collagen-coated substrate, after the invasion of a small fraction of motile cancer cells into the confluent endothelial monolayer, through disrupting cell-cell junctions. It is found that, with an increasing waiting time, the cancer-free confluent endothelial monolayer exhibits a dynamical slowing-down of liquidlike micromotion with a gradually decreasing degree of superdiffusion. After invasion, cancer cells aggregate and exhibit turbulentlike cooperative motion, which is enhanced with the increasing size of gradually aggregated cancer clusters, confined by the fluctuating boundaries of surrounding endothelial cells. It, in turn, enhances the surrounding endothelial cell motion and speeds up the originally slowed-down motion.


Assuntos
Células Endoteliais da Veia Umbilical Humana/patologia , Modelos Biológicos , Neoplasias Nasofaríngeas/patologia , Neoplasias da Medula Óssea/secundário , Agregação Celular/fisiologia , Comunicação Celular/fisiologia , Técnicas de Cocultura , Humanos , Invasividade Neoplásica
8.
ACS Appl Mater Interfaces ; 9(39): 34204-34212, 2017 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-28891290

RESUMO

Lateral transition-metal dichalcogenide and their heterostructures have attracted substantial attention, but there lacks a simple approach to produce large-scaled optoelectronic devices with graded composition. In particular, the incorporation of substitution and doping into heterostructure formation is rarely reported. Here, we demonstrate growth of a composition graded doped lateral WSe2/WS2 heterostructure by ambient pressure chemical vapor deposition in a single heat cycle. Through Raman and photoluminescence spectroscopy, we demonstrate that the monolayer heterostructure exhibits a clear interface between two domains and a graded composition distribution in each domain. The coexistence of two distinct doping modes, i.e., interstitial and substitutional doping, was verified experimentally. A distinct three-stage growth mechanism consisting of nucleation, epitaxial growth, and substitution was proposed. Electrical transport measurements reveal that this lateral heterostructure has representative characteristics of a photodiodes. The optoelectronic device based on the lateral WSe2/WS2 heterostructure shows improved photodetection performance in terms of a reasonable responsivity and a large photoactive area.

9.
Nano Lett ; 17(10): 6469-6474, 2017 10 11.
Artigo em Inglês | MEDLINE | ID: mdl-28926715

RESUMO

Atomically thin materials, such as graphene, are the ultimate building blocks for nanoscale devices. But although their synthesis and handling today are routine, all efforts thus far have been restricted to flat natural geometries, since the means to control their three-dimensional (3D) morphology has remained elusive. Here we show that, just as a blacksmith uses a hammer to forge a metal sheet into 3D shapes, a pulsed laser beam can forge a graphene sheet into controlled 3D shapes in the nanoscale. The forging mechanism is based on laser-induced local expansion of graphene, as confirmed by computer simulations using thin sheet elasticity theory.

10.
Nanotechnology ; 28(39): 395704, 2017 Sep 27.
Artigo em Inglês | MEDLINE | ID: mdl-28715345

RESUMO

Micrometer sized oxidation patterns were created in chemical vapor deposition grown graphene through scanning probe lithography (SPL) and then subsequently reduced by irradiation using a focused x-ray beam. Throughout the process, the films were characterized by lateral force microscopy, micro-Raman and micro-x-ray photoelectron spectroscopy. Firstly, the density of grain boundaries was found to be crucial in determining the maximum possible oxygen coverage with SPL. Secondly, the dominant factor in SPL oxidation was found to be the bias voltage. At low voltages, only structural defects are formed on grain boundaries. Above a distinct threshold voltage, oxygen coverage increased rapidly, with the duration of applied voltage affecting the final oxygen coverage. Finally, we found that, independent of initial conditions, types of defects or the amount of SPL oxidation, the same set of coupled rate equations describes the reduction dynamics with the limiting reduction step being C-C â†’ C=C.

11.
Nanotechnology ; 28(3): 035603, 2017 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-27934779

RESUMO

We present a study on the formation of p-type ZnO thin film through ion implantation. Group V dopants (N, P) with different ionic radii are implanted into chemical vapor deposition grown ZnO thin film on GaN/sapphire substrates prior to thermal activation. It is found that mono-doped ZnO by N+ implantation results in n-type conductivity under thermal activation. Dual-doped ZnO film with a N:P ion implantation dose ratio of 4:1 is found to be p-type under certain thermal activation conditions. Higher p-type activation levels (1019 cm-3) under a wider thermal activation range are found for the N/P dual-doped ZnO film co-implanted by additional oxygen ions. From high resolution x-ray diffraction and x-ray photoelectron spectroscopy it is concluded that the observed p-type conductivities are a result of the promoted formation of PZn-4NO complex defects via the concurrent substitution of nitrogen at oxygen sites and phosphorus at zinc sites. The enhanced solubility and stability of acceptor defects in oxygen co-implanted dual-doped ZnO film are related to the reduction of oxygen vacancy defects at the surface. Our study demonstrates the prospect of the formation of stable p-type ZnO film through co-implantation.

12.
Nanoscale ; 7(37): 15362-73, 2015 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-26332120

RESUMO

Large-scale production of uniform and high-quality graphene is required for practical applications of graphene. The electrochemical exfoliation method is considered as a promising approach for the practical production of graphene. However, the relatively low production rate of graphene currently hinders its usage. Here, we demonstrate, for the first time, a rapid and high-yield approach to exfoliate graphite into graphene sheets via an electrochemical method with small molecular additives; where in this approach, the use of melamine additives is able to efficiently exfoliate graphite into high-quality graphene sheets. The exfoliation yield can be increased up to 25 wt% with melamine additives compared to electrochemical exfoliation without such additives in the electrolyte. The proposed mechanism for this improvement in the yield is the melamine-induced hydrophilic force from the basal plane; this force facilitates exfoliation and provides in situ protection of the graphene flake surface against further oxidation, leading to high-yield production of graphene of larger crystallite size. The residual melamine can be easily washed away by water after collection of the graphene. The exfoliation with molecular additives exhibits higher uniformity (over 80% is graphene of less than 3 layers), lower oxidation density (C/O ratio of 26.17), and low defect level (D/G < 0.45), which are characteristics superior to those of reduced graphene oxide (rGO) or of a previously reported approach of electrochemical exfoliated graphene (EC-graphene). The continuous films obtained by the purified graphene suspension exhibit a sheet resistance of 13.5 kΩ â–¡(-1) at ∼95% transmittance. A graphene-based nanocomposite with polyvinyl butyral (PVB) exhibits an electrical conductivity of 3.3 × 10(-3) S m(-1) for the graphene loading fraction of 0.46 vol%. Moreover, the melamine functionalized graphene sheets are readily dispersed in the aqueous solution during the exfoliation process, allowing for the production of graphene in a continuous process. The continuous process for producing graphene was demonstrated, with a yield rate of 1.5 g h(-1). The proposed method can produce high-crystallinity graphene in a fast and high-yield manner, which paves the path towards mass production of high-quality graphene for a variety of applications.

13.
Nanotechnology ; 19(32): 325703, 2008 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-21828825

RESUMO

A two-dimensional (2D) dopant profiling technique is demonstrated in this work. We apply a unique cantilever probe in electrostatic force microscopy (EFM) modified by the attachment of a multiwalled carbon nanotube (MWNT). Furthermore, the tip apex of the MWNT was trimmed to the sharpness of a single-walled carbon nanotube (SWNT). This ultra-sharp MWNT tip helps us to resolve dopant features to within 10 nm in air, which approaches the resolution achieved by ultra-high vacuum scanning tunnelling microscopy (UHV STM). In this study, the CNT-probed EFM is used to profile 2D buried dopant distribution under a nano-scale device structure and shows the feasibility of device characterization for sub-45 nm complementary metal-oxide-semiconductor (CMOS) field-effect transistors.

14.
Phys Rev Lett ; 93(22): 220602, 2004 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-15601077

RESUMO

We experimentally demonstrate shear banding and construct a microscopic dynamic picture of a sheared 2D mesoscopic dust Coulomb liquid at the kinetic level. Under the topological constraints from the discreteness and finite boundary, the nonlinear threshold-type response of motion to the local stress induced by thermal and external drives leads to shear thinning and the enhanced avalanche-type local topological transitions with stress relaxation in the form of clusters. It causes the formation of the outer shear bands in which the mean shear rate, the velocity fluctuations, and the structural rearrangement rate are all enhanced, and leaves a weakly perturbed center band. The typical size of the cooperative hopping vortex (about three interparticle distance) sets up a common length scale for the widths of the confinement induced layering and the shear band.

15.
Phys Rev Lett ; 92(6): 065003, 2004 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-14995246

RESUMO

We report the intermittent and heterogeneous plastic structural rearrangement at the kinetic level in a quasi-2D creeping dusty-plasma Coulomb liquid through directly tracking each particle trajectory and measuring the evolution of each associated local bond-orientational order. The thermal agitation and external shear have comparable strengths. Their interplay with the Coulomb coupling enhances particle hopping, and induces the loss of local structural memory and avalanche-type excitation of topological defect clusters following the universal scaling behaviors of cluster size in xyt space, akin to the generic behavior of self-organized criticality. Increasing shear tends to reduce the aspect ratio of the temporal to the spatial spans of the defect cluster in xyt space.

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