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1.
Nano Lett ; 2020 Mar 24.
Artigo em Inglês | MEDLINE | ID: mdl-32191480

RESUMO

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the efficient charge transport of photogenerated carriers across the interface is a critical factor to determine the device performances. Here, we report an unexplored approach to boost the optoelectronic device performances of the WSe2-MoS2 p-n heterojunctions via the monolithic-oxidation-induced doping and resultant modulation of the interface band alignment. In the proposed device, the atomically thin WOx layer, which is directly formed by layer-by-layer oxidation of WSe2, is used as a charge transport layer for promoting hole extraction. The use of the ultrathin oxide layer significantly enhanced the photoresponsivity of the WSe2-MoS2 p-n junction devices, and the power conversion efficiency increased from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment, as confirmed by band alignment analyses and first-principle calculations. Our work suggests a new route to achieve interface contact engineering in the heterostructures toward realizing high-performance 2D optoelectronics.

2.
Adv Mater ; 31(39): e1903424, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31389640

RESUMO

A new compound material of 2D hydrofluorinated graphene (HFG) is demonstrated whose relative hydrogen/fluorine concentrations can be tailored between the extremes of either hydrogenated graphene (HG) and fluorinated graphene (FG). The material is fabricated through subsequent exposures to indirect hydrogen plasma and xenon difluoride (XeF2 ). Controlling the relative concentration in the HFG compound enables tailoring of material properties between the extremes offered by the constituent materials and in-plane patterning produces micrometer-scale regions with different surface properties. The utility of the technique to tailor the surface wettability, surface friction, and electrical conductivity is demonstrated. HFG compounds display wettability between the extremes of pure FG with contact angle of 95° ± 5° and pure HG with contact angle of 42° ± 2°. Similarly, the HFG surface friction may be tailored between the two extremes. Finally, the HFG electrical conductivity tunes through five orders of magnitude when transitioning from FG to HG. When combined with simulation, the electrical measurements reveal the mechanism producing the compound to be a dynamic process of adatom desorption and replacement. This study opens a new class of 2D compound materials and innovative chemical patterning with applications for atomically thin 2D circuits consisting of chemically/electrically modulated regions.

3.
Nanotechnology ; 30(40): 404002, 2019 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-31234153

RESUMO

We have performed density functional theory calculations to study the effects caused by the interfacial structure between 2D-MoS2 and 3D-GaN. Two different surface terminations of GaN are considered: Ga-terminated (0001) (Ga-GaN) and N-terminated ([Formula: see text]) (N-GaN) configurations. We confirm that Rashba spin splitting occurs in band structure of MoS2 on GaN. We also find that the surface states of GaN move to the deep position in band structure in the MoS2/Ga-GaN case, while the surface states of GaN are hybridized with MoS2 near the Fermi level for the MoS2/N-GaN case. Furthermore, we investigate the variation in electronic structure of MoS2/GaN heterostructures depending on the number of MoS2 layers. Especially, the top layer MoS2 of the 2L-MoS2/GaN structures shows both n-type and p-type properties depending on the GaN surface termination. As a result, we suggest that the electrical characteristics of the 2D/3D heterostructures could be controlled by the surface terminations of substrate materials.

4.
Sci Rep ; 9(1): 5811, 2019 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-30967593

RESUMO

The valid strong THz absorption at 1.58 THz was probed in the organic-inorganic hybrid perovskite thin film, CH3NH3PbI3, fabricated by sequential vacuum evaporation method. In usual solution-based methods such as 2-step solution and antisolvent, we observed the relatively weak two main absorption peaks at 0.95 and 1.87 THz. The measured absorption spectrum is analyzed by density-functional theory calculations. The modes at 0.95 and 1.87 THz are assigned to the Pb-I vibrations of the inorganic components in the tetragonal phase. By contrast, the origin of the 1.58 THz absorption is due to the structural deformation of Pb-I bonding at the grain boundary incorporated with a CH3NH2 molecular defect.

5.
Sci Rep ; 9(1): 3623, 2019 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-30842541

RESUMO

Using first-principles calculations, we investigate an atomic impurity at the interface of a van der Waals heterostructure (vdW heterostructure) consisting of a zigzag graphene nanoribbon (ZGNR) and a hexagonal boron nitride (h-BN) sheet. To find effects of atomic intercalation on geometrical and electronic properties of the ZGNR on the h-BN sheet, various types of impurity atoms are considered. The embedded atoms are initially placed at the edge or the middle of the ZGNR located on the h-BN sheet. Our results demonstrate that most of the impurity atoms are more stable at the edge than at the middle in all cases we consider. Especially, a nickel atom has the smallest energy difference (~0.15 eV) between the two embedding positions, which means that the Ni atom is relatively easy to intercalate in the structure. Finally, we discuss magnetic properties for the vdW heterostructure with an intercalated atom.

6.
ACS Appl Mater Interfaces ; 11(11): 10959-10966, 2019 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-30807091

RESUMO

There have been a few studies of heterojunctions composed of two-dimensional transition-metal dichalcogenides (TMDs) and an oxide layer, but such studies of high-performance electric and optoelectronic devices are essential. Such heterojunctions with low-resistivity metal contacts are needed by the electronics industry to fabricate efficient diodes and photovoltaic devices. Here, a van der Waals heterojunction composed of p-type black phosphorus (p-BP) and n-type indium-gallium-zinc oxide (n-IGZO) films with low-resistivity metal contacts is reported, and it demonstrates high rectification. The low off-state leakage current in the thick IGZO film accounts for the high rectification ratio in a sharp interface of p-BP/n-IGZO devices. For electrostatic gate control, an ionic liquid is introduced to achieve a high rectification ratio of 9.1 × 104. The photovoltaic measurements of p-BP/n-IGZO show fast rise and decay times, significant open-circuit voltage and short-circuit current, and a high photoresponsivity of 418 mA/W with a substantial external quantum efficiency of 12.1%. The electric and optoelectronic characteristics of TMDs/oxide layer van der Waals heterojunctions are attractive for industrial applications in the near future.

7.
Nanoscale ; 10(48): 22970-22980, 2018 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-30500036

RESUMO

Two-dimensional atomic layered materials (2d-ALMs) are emerging candidates for use as epitaxial seed substrates for transferrable epilayers. However, the micrometer-sized domains of 2d-ALMs preclude their practical use in epitaxy because they cause crystallographically in-plane disordering of the overlayer. Ultrathin graphene can penetrate the electric dipole momentum from an underlying crystal layer to the graphene surface, which then drives it to crystallize the overlayer during the initial growth stage, thus resulting in substantial energy saving. This study demonstrates the remote homoepitaxy of ZnO microrods (MRs) on ZnO substrates across graphene layers via a hydrothermal method. Despite the presence of poly-domain graphene in between the ZnO substrate and ZnO MRs, the MRs were epitaxially grown on a- and c-plane ZnO substrates, whose in-plane alignments were homogeneous within the wafer's size. Transmission electron microscopy revealed a homoepitaxial relationship between the overlayer MRs and the substrate. Density-functional theory calculations suggested that the charge redistribution occurring near graphene induces the electric dipole formation, so the attracted adatoms led to the formation of the remote homoepitaxial overlayer. Due to a strong potential field caused by long-range charge transfer given from the substrate, even the use of bi-layer and tri-layer graphene resulted in remote homoepitaxial ZnO MRs. The effects of substrate crystal planes were also theoretically and empirically investigated. The ability of graphene, which can be released from the mother substrate without covalent bonds, was utilized to transfer the overlayer MR arrays. This method opens a way for producing well aligned, transferrable epitaxial nano/microstructure arrays while regenerating the substrate for cost-saving device manufacturing.

8.
Phys Chem Chem Phys ; 20(39): 25240-25245, 2018 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-30270382

RESUMO

For utilization of two-dimensional (2D) materials as electronic devices, their mixed-dimensional heterostructures with three-dimensional (3D) materials are receiving much attention. In this study, we have investigated the atomic and electronic structures of the 2D/3D heterojunction between MoS2 and Si(100) using density functional theory calculations; especially, we focus on the contact behavior dependence on the interfacial structures of heterojunctions by considering two types of surface termination of Si(100) surfaces. Calculations show that MoS2 and clean Si(100) form an almost n-type ohmic contact with a very small Schottky barrier height (SBH) due to strong covalent bonds between them, and that the contact between MoS2 and H-covered Si(100) makes a p-n heterojunction with weak van der Waals interactions. Such a difference in contact behaviors can be explained by different electric dipole formation at the heterojunction interfaces. Overall, it is concluded that contact properties can be varied depending on the interfacial structures of 2D(MoS2)/3D(Si) semiconductor heterojunctions.

9.
Sci Rep ; 8(1): 12966, 2018 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-30154432

RESUMO

Black Phosphorus (BP) is an excellent material from the post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge for its practical and long-term usage. The optical visualization of the oxidized BP is the key and the foremost step for its successful passivation from the ambience. Here, we have conducted a systematic study of the oxidation of the BP and developed a technique to optically identify the oxidation of the BP using Liquid Crystal (LC). It is interesting to note that we found that the rapid oxidation of the thin layers of the BP makes them disappear and can be envisaged by using the alignment of the LC. The molecular dynamics simulations also proved the preferential alignment of the LC on the oxidized BP. We believe that this simple technique will be effective in passivation efforts of the BP, and will enable it for exploitation of its properties in the field of electronics.

10.
Sci Rep ; 8(1): 10081, 2018 07 04.
Artigo em Inglês | MEDLINE | ID: mdl-29973666

RESUMO

Graphene oxide (GO) modulates the functions of antigen-presenting cells including dendritic cells (DCs). Although carbon nanotubes affect expression of the MHC class I-like CD1d molecule, whether GO can influence immune responses of CD1d-dependent invariant natural killer T (iNKT) cells remains unclear. Here, we investigated the impact of GO on inflammatory responses mediated by α-galactosylceramide (α-GalCer), an iNKT cell agonist. We found that in vivo GO treatment substantially inhibited the capacity of α-GalCer to induce the iNKT cell-mediated trans-activation of and cytokine production by innate and innate-like cells, including DCs, macrophages, NK cells, and γδ T cells. Such effects of GO on α-GalCer-induced inflammatory responses closely correlated with iNKT cell polarization towards TGFß production, which also explains the capacity of GO to expand regulatory T cells. Interestingly, the absence of TLR4, a receptor for GO, failed to downregulate, and instead partially enhanced the anti-inflammatory activity of GO against α-GalCer-elicited responses, implying negative effects of TLR4 signaling on the anti-inflammatory properties of GO. By employing an α-GalCer-induced sepsis model, we further demonstrated that GO treatment significantly protected mice from α-GalCer-induced lethality. Taken together, we provide strong evidence that GO holds promise as an adjuvant to modulate iNKT cell responses for immunotherapy.


Assuntos
Grafite/administração & dosagem , Inflamação/tratamento farmacológico , Sepse/tratamento farmacológico , Receptor 4 Toll-Like/genética , Fator de Crescimento Transformador beta/genética , Animais , Antígenos CD1d/genética , Antígenos CD1d/imunologia , Polaridade Celular/efeitos dos fármacos , Células Dendríticas/efeitos dos fármacos , Células Dendríticas/imunologia , Modelos Animais de Doenças , Galactosilceramidas/administração & dosagem , Humanos , Inflamação/imunologia , Inflamação/patologia , Linfócitos Intraepiteliais/efeitos dos fármacos , Linfócitos Intraepiteliais/imunologia , Ativação Linfocitária/efeitos dos fármacos , Camundongos , Nanotubos de Carbono/química , Células T Matadoras Naturais/efeitos dos fármacos , Células T Matadoras Naturais/imunologia , Sepse/imunologia , Sepse/patologia
11.
J Phys Chem Lett ; 9(9): 2293-2297, 2018 May 03.
Artigo em Inglês | MEDLINE | ID: mdl-29667412

RESUMO

To understand the instability of Sn-based perovskite, CH3NH3SnI3, photoelectron spectroscopy with synchrotron radiation and theoretical calculations, such as density functional theory and ab initio molecular dynamics, were performed. Findings from this experimental and theoretical study highlight the crucial changes of surface-chemical states, the broken chemical bondings in Sn-I, and the depletion of a CH3-NH3+ cation on the surface region. The material instability origin of Sn-based perovskite can be explained by the chemical state instability in the surface.

12.
ACS Appl Mater Interfaces ; 10(15): 13150-13157, 2018 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-29578329

RESUMO

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS2. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS2 flakes in our BP/WS2 van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 104, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS2 van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS2 van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

13.
Nanotechnology ; 29(4): 045201, 2018 Jan 26.
Artigo em Inglês | MEDLINE | ID: mdl-29192890

RESUMO

P-N junctions represent the fundamental building blocks of most semiconductors for optoelectronic functions. This work demonstrates a technique for forming a WS2/Si van der Waals junction based on mechanical exfoliation. Multilayered WS2 nanoflakes were exfoliated on the surface of bulk p-type Si substrates using a polydimethylsiloxane stamp. We found that the fabricated WS2/Si p-n junctions exhibited rectifying characteristics. We studied the effect of annealing processes on the performance of the WS2/Si van der Waals p-n junction and demonstrated that annealing improved its electrical characteristics. However, devices with vacuum annealing have an enhanced forward-bias current compared to those annealed in a gaseous environment. We also studied the top-gate-tunable rectification characteristics across the p-n junction interface in experiments as well as density functional theory calculations. Under various temperatures, Zener breakdown occurred at low reverse-bias voltages, and its breakdown voltage exhibited a negative coefficient of temperature. Another breakdown voltage was observed, which increased with temperature, suggesting a positive coefficient of temperature. Therefore, such a breakdown can be assigned to avalanche breakdown. This work demonstrates a promising application of two-dimensional materials placed directly on conventional bulk Si substrates.

14.
Nano Lett ; 18(1): 460-466, 2018 01 10.
Artigo em Inglês | MEDLINE | ID: mdl-29268017

RESUMO

Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electrochemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new heterointerface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density >5 × 1013 cm2 with mobility >103 cm2/(V s) in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures.

16.
Nanoscale ; 9(36): 13725-13730, 2017 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-28884182

RESUMO

The special properties of graphene can be largely influenced by point defects in the lattice. However, TEM studies of topological defects in few-layered graphene have rarely been reported. In this work, the two simplest forms of point defects monovacancy and divacancy in twisted bilayer graphene are characterized using aberration-corrected transmission electron microscopy (AC-TEM) at 80 kV. A convenient approach by using a negative mask in the fast Fourier transform (FFT) has been applied to separate the image signal of the two graphene layers. In the study combined with density functional theory (DFT) calculations and tight-binding molecular dynamics simulations, the analysis of the defect structure and movement shows the stability and migration behavior of both defects. DFT calculations indicate that the migration of monovacancy in bilayer graphene needs to overcome a higher energy barrier.

17.
Nanoscale Res Lett ; 12(1): 445, 2017 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-28683541

RESUMO

The atomic, electronic, and magnetic properties of vacancy structures with triangular shape related to their growth in single hexagonal boron nitride (h-BN) sheet are investigated using density functional theory calculations. We find that the optimized structures of triangular vacancies depend on the vacancy sizes with N-terminated zigzag edge. Then, vacancy structures obtained during the vacancy evolution in h-BN sheet are considered by removing a boron-nitrogen pair (BN pair) from edges of triangular vacancies. The magnetic properties of those vacancy structures are investigated by local density of states and spin densities. It is found that the stability of the optimized structures with a BN missing pair depends on the BN-pair missing position: the most stable structure is a BN-pair missing structure at the edge face region with the smallest magnetic moment.

18.
Phys Chem Chem Phys ; 19(25): 16881-16887, 2017 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-28627535

RESUMO

We have performed density functional theory (DFT) calculations of the atomic and electronic structures of ethylenediamine on Ge(100). The two amine groups in ethylenediamine can interact with germanium surface atoms through a N-H dissociative nucleophilic reaction and/or N-dative bonding with an electron-deficient down Ge atom. Of the monodentate and row-bridged bidentate structures that formed, the dative-bonded configurations were found to be more stable than the NH dissociative adsorption structures. The formation of row-bridged bidentate, structures is more favorable than that of on-top or end-bridged structures. In simulated STM images, the three types of row-bridged adsorption structure have characteristic features, and the row-bridged dative-bonded configuration gives rise to features due to both adsorbed ethylenediamine molecules and the underlying Ge atoms.

19.
J Phys Condens Matter ; 29(24): 245301, 2017 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-28443604

RESUMO

Using density functional theory calculations, we have studied the edge-functionalization of armchair graphene nanoribbons (AGNRs) with pentagonal-hexagonal edge structures. While the AGNRs with pentagonal-hexagonal edge structures (labeled (5,6)-AGNRs) are metallic, the edge-functionalized (5,6)-AGNRs with substitutional atoms opens a band gap. We find that the band structures of edge-functionalized (5,6)-N-AGNRs by substitution resemble those of defect-free (N-1)-AGNR at the Γ point, whereas those at the X point show the original ones of the defect-free N-AGNR. The overall electronic structures of edge-functionalized (5,6)-AGNRs depend on the number of electrons, supplied by substitutional atoms, at the edges of functionalized (5,6)-AGNRs.

20.
J Phys Condens Matter ; 29(8): 085001, 2017 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-28081016

RESUMO

Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.

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