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1.
ACS Nano ; 13(10): 11613-11622, 2019 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-31509382

RESUMO

WSe2 is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suitable active component for logic circuitry. However, solid-state devices based on single- and bilayer WSe2 typically exhibit unipolar transport and poor electrical performance when conventional SiO2 dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered monolayers on the top of the WSe2 surface, thereby boosting its electrical performance in single- and bilayer field-effect transistors. In particular, by employing SiO2 dielectric substrates and top Au electrodes, we measure unipolar mobility as high as µh = 150 cm2 V-1 s-1 and µe = 17.9 cm2 V-1 s-1 in WSe2 single-layer devices when ad hoc molecular monolayers are chosen. Additionally, by asymmetric double-side functionalization with two different molecules, we provide opposite polarity to the top and bottom layer of bilayer WSe2, demonstrating nearly balanced ambipolarity at the bilayer limit. Our results indicate that the controlled functionalization of the two sides of the WSe2 mono- and bilayer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functionalization, representing a path toward deterministic control over charge transport in 2D materials.

2.
Nat Commun ; 9(1): 3689, 2018 09 06.
Artigo em Inglês | MEDLINE | ID: mdl-30190476

RESUMO

The original version of this article incorrectly listed an affiliation of Sara Bonacchi as 'Present address: Institut National de la Recherche Scientifique (INRS), EMT Center, Boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, 1650, Canada', instead of the correct 'Present address: Department of Chemical Sciences - University of Padua - Via Francesco Marzolo 1 - 35131 Padova - Italy'. And an affiliation of Emanuele Orgiu was incorrectly listed as 'Present address: Department of Chemical Sciences, University of Padua, Via Francesco Marzolo 1, Padova, 35131, Italy', instead of the correct 'Present address: Institut National de la Recherche Scientifique (INRS), EMT Center, Boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, 1650, Canada'. This has been corrected in both the PDF and HTML versions of the article.

3.
ACS Appl Mater Interfaces ; 10(40): 34392-34400, 2018 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-30221920

RESUMO

Transition metal dichalcogenides, such as molybdenum disulfide (MoS2), show peculiar chemical/physical properties that enable their use in applications ranging from micro- and nano-optoelectronics to surface catalysis, gas and light detection, and energy harvesting/production. One main limitation to fully harness the potential of MoS2 is given by the lack of scalable and low environmental impact synthesis of MoS2 films with high uniformity, hence setting a significant challenge for industrial applications. In this work, we develop a versatile and scalable sol-gel-derived MoS2 film fabrication by spin coating deposition of an aqueous sol on different technologically relevant, flexible substrates with annealing at low temperatures (300 °C) and without the need of sulfurization and/or supply of hydrogen as compared to cutting-edge techniques. The electronic and physical properties of the MoS2 thin films were extensively investigated by means of surface spectroscopy and structural characterization techniques. Spatially homogenous nanocrystalline 2H-MoS2 thin films were obtained exhibiting high chemical purity and excellent electronic properties such as an energy band gap of 1.35 eV in agreement with the 2H phase of the MoS2, and a density of states that corresponds to the n-type character expected for high-quality 2H-MoS2. The potential use of sol-gel-grown MoS2 as the candidate material for electronic applications was tested via electrical characterization and demonstrated via the reversible switching in resistivity typical for memristors with a measured ON-OFF ratio ≥102. The obtained results highlight that the novel low-cost fabrication method has a great potential to promote the use of high-quality MoS2 in technological and industrial-relevant scalable applications.

4.
Nat Commun ; 9(1): 2661, 2018 07 09.
Artigo em Inglês | MEDLINE | ID: mdl-29985413

RESUMO

Molecular switches enable the fabrication of multifunctional devices in which an electrical output can be modulated by external stimuli. The working mechanism of these devices is often hard to prove, since the molecular switching events are only indirectly confirmed through electrical characterization, without real-space visualization. Here, we show how photochromic molecules self-assembled on graphene and MoS2 generate atomically precise superlattices in which a light-induced structural reorganization enables precise control over local charge carrier density in high-performance devices. By combining different experimental and theoretical approaches, we achieve exquisite control over events taking place from the molecular level to the device scale. Unique device functionalities are demonstrated, including the use of spatially confined light irradiation to define reversible lateral heterojunctions between areas possessing different doping levels. Molecular assembly and light-induced doping are analogous for graphene and MoS2, demonstrating the generality of our approach to optically manipulate the electrical output of multi-responsive hybrid devices.

5.
Nanoscale ; 10(28): 13449-13461, 2018 Jul 19.
Artigo em Inglês | MEDLINE | ID: mdl-29972180

RESUMO

In this work we propose a realistic model of nanometer-thick SiC/SiOx core/shell nanowires (NWs) using a combined first-principles and experimental approach. SiC/SiOx core/shell NWs were first synthesised by a low-cost carbothermal method and their chemical-physical experimental analysis was accomplished by recording X-ray absorption near-edge spectra. In particular, the K-edge absorption lineshapes of C, O, and Si are used to validate our computational model of the SiC/SiOx core/shell NW architectures, obtained by a multiscale approach, including molecular dynamics, tight-binding and density functional simulations. Moreover, we present ab initio calculations of the electronic structure of hydrogenated SiC and SiC/SiOx core/shell NWs, studying the modification induced by several different substitutional defects and impurities into both the surface and the interfacial region between the SiC core and the SiOx shell. We find that on the one hand the electron quantum confinement results in a broadening of the band gap, while hydroxyl surface terminations decrease it. This computational investigation shows that our model of SiC/SiOx core/shell NWs is capable to deliver an accurate interpretation of the recorded X-ray absorption near-edge spectra and proves to be a valuable tool towards the optimal design and application of these nanosystems in actual devices.

6.
Phys Chem Chem Phys ; 18(36): 24890-904, 2016 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-27412494

RESUMO

The unoccupied electronic structure of thick films of tetraphenylporphyrin and tetrakis(pentafluorophenyl)porphyrin Cu(ii) complexes (hereafter, CuTPP and CuTPP(F)) deposited on Au(111) has been studied by combining the outcomes of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with those of spin-unrestricted time-dependent density functional (TD-DFT) calculations carried out either within the scalar relativistic zeroth order regular approximation (ZORA) framework (C, N and F K-edges) or by using the Tamm-Dancoff approximation coupled to ZORA and including spin-orbit effects (Cu L2,3-edges). Similarly to the modelling of NEXAFS outcomes pertaining to other Cu(ii) complexes, the agreement between theory and experiment is more than satisfactory, thus confirming the open-shell TD-DFT to be a useful tool to look into NEXAFS results pertinent to Cu(ii) compounds. The combined effect of metalation and phenyl (Ph) fluorine decoration is found to favour an extensive mixing between (Ph)σ* and pristine porphyrin macrocyle (pmc) (pmc)π* virtual levels. The lowest lying excitation in the C and N K-edge spectra of both CuTPP and CuTPP(F) is associated with a ligand-to-metal-charge-transfer transition, unambiguously revealed in the (CuTPP)N K-edge spectral pattern. Moreover, the comparison with literature data pertaining to the modelling of the (Cu(II))L2,3 features in the phthalocyanine-Cu(ii) (CuPc) complex provided further insights into how metal-to-ligand-charge-transfer transitions associated with excitations from 2p(Cu(II)) AOs to low-lying, ligand-based π* MOs may contribute to the Cu(ii) L2,3-edge intensity and thus weaken its believed relationship with the Cu(ii)-ligand symmetry-restricted covalency. Despite the coordinative pocket of CuTPP/CuTPP(F) mirroring CuPc, the ligand-field strength exerted by the phthalocyanine ligand on the Cu(ii) centre is experimentally found and theoretically confirmed to be slightly stronger than that experienced by Cu in CuTPP and CuTPP(F). On the whole, the obtained results complement those published in the near past by the same group on the occupied and empty states of the H2TPP and H2TPP(F) free ligands as well as on the occupied states of both CuTPP and CuTPP(F), thus providing the final piece to get a thorough description of electronic perturbations associated with the metalation and the Ph halogen decoration of H2TPP.

7.
Phys Chem Chem Phys ; 18(28): 18727-38, 2016 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-27166746

RESUMO

Copper complexes of tetraphenylporphyrin (H2TPP) and tetrakis(pentafluorophenyl)porphyrin (H2TPP(F)) deposited as thin films on Au(111) have been studied experimentally and theoretically. Core level emissions from C 1s, N 1s, F 1s and Cu 2p as well as valence states of CuTPP and CuTPP(F) have been investigated using surface photoelectron spectroscopy. The interpretation of experimental results has been guided by theoretical calculations carried out on isolated species in the habit of the density functional theory. Reference to experimental and theoretical outcomes pertaining to H2TPP and H2TPP(F) allowed a confident and detailed assignment of the title molecules' X-ray and ultraviolet photoemission data. With specific reference to the latter, similar to copper phthalocyanine (CuPc), whose coordinative pocket mirrors the CuTPP/CuTPP(F) ones, the lowest ionization energy of the title compounds implies electron ejection from a ring orbital rather than from the Cu 3d-based singly occupied molecular orbital. Moreover, analogous to CuPc, the ionic contribution appears to play an important role in the Cu-N bonding. Nevertheless, differences in the number, symmetry, nature and relative position of CuTPP/CuTPP(F) occupied frontier orbitals compared to CuPc may be stated only by considering in great detail the Cu-ligand covalent interactions.

8.
ACS Appl Mater Interfaces ; 7(34): 19134-44, 2015 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-26280572

RESUMO

A combination of ultraviolet and X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge redistribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F6TCNNQ on the quartz support (p-doping of graphene), the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is n-doped and remains n-doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ monolayer atop, the effect being more pronounced (∼1.3 eV) on Cu compared to quartz (∼1.0 eV) because of the larger electrostatic potential drop associated with the long-distance graphene-mediated Cu-F6TCNNQ electron transfer. We thus provide a means to realize high work function surfaces for both p- and n-type doped graphene.

9.
ACS Appl Mater Interfaces ; 7(22): 11900-7, 2015 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-25986080

RESUMO

We used aromatic phosphonates with substituted phenyl rings with different molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface in order to engineer the energy-level alignment at hybrid inorganic/organic semiconductor interfaces, with an oligophenylene as organic component. The work function of ZnO was tuned over a wide range of more than 1.7 eV by different SAMs. The difference in the morphology and polarity of the SAM-modified ZnO surfaces led to different oligophenylene orientation, which resulted in an orientation-dependent ionization energy that varied by 0.7 eV. The interplay of SAM-induced work function modification and oligophenylene orientation changes allowed tuning of the offsets between the molecular frontier energy levels and the semiconductor band edges over a wide range. Our results demonstrate the versatile use of appropriate SAMs to tune the energy levels of ZnO-based hybrid semiconductor heterojunctions, which is important to optimize its function, e.g., targeting either interfacial energy- or charge-transfer.

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