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1.
Nanoscale ; 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32080701

RESUMO

Self-assembled monolayers (SAMs) deposited on bottom electrodes are commonly used to tune charge carrier injection or blocking in optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. In this work, we report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a diarylethene derivative designed for optically control the electronic properties. Following the demonstration of dense SAM formation and its photochromic activity, as a proof-of-principle, an organic light-emitting diode (OLED) embedding the light-responsive SAM-covered electrode was fabricated and characterized. Optically addressing the two-terminal device by irradiation with ultraviolet light doubles the electroluminescence. The original value can be restored reversibly by irradiation with visible light. This expanded functionality is based on the photoinduced modulation of the electronic structure of the diarylethene isomers, which impact the charge carriers' confinement within the emissive layer. This approach could be successfully exploited in the field of opto-communication technology, for example to fabricate opto-electronic logic circuits.

2.
Sci Rep ; 9(1): 16682, 2019 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-31723200

RESUMO

Near-infrared sensitization of monolayer MoS2 is here achieved via the covalent attachment of a novel heteroleptic nickel bis-dithiolene complex into sulfur vacancies in the MoS2 structure. Photocurrent action spectroscopy of the sensitized films reveals a discreet contribution from the sensitizer dye centred around 1300 nm (0.95 eV), well below the bandgap of MoS2 (2.1 eV), corresponding to the excitation of the monoanionic dithiolene complex. A mechanism of conductivity enhancement is proposed based on a photo-induced flattening of the corrugated energy landscape present at sulfur vacancy defect sites within the MoS2 due to a dipole change within the dye molecule upon photoexcitation. This method of sensitization might be readily extended to other functional molecules that can impart a change to the dielectric environment at the MoS2 surface under stimulation, thereby extending the breadth of detector applications for MoS2 and other transition metal dichalcogenides.

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.
Nano Lett ; 17(2): 1149-1153, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28098466

RESUMO

The scaling of nonvolatile memory (NVM) devices based on resistive filament switching to below a 100 nm2 footprint area without employing cumbersome lithography is demonstrated. Nanocolumns of the organic semiconductor 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]diphenyl (α-NPD) were grown by glancing angle deposition on a silver electrode. Individual NVM devices were electrically characterized by conductive atomic force microscopy with the tip of a conductive cantilever serving as second electrode. The resistive switching mechanism is unambiguously attributed to Ag filament formation between the electrodes. This sets the upper limit for the filament diameter to well below 100 nm. Full functionality of these NVM nanodevices is evidenced, revealing a potential memory density of >1 GB/cm2 in appropriate architectures.

5.
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.

6.
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.

7.
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.

8.
Chemphyschem ; 16(12): 2602-8, 2015 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-26137864

RESUMO

The interaction of gold nanoparticles (AuNPs) with prototypical organic semiconductors used in optoelectronics, namely, tris(8-hydroxyquinoline)aluminium (Alq3 ) and 4,4-bis[N-(1-naphthyl)-N-phenylamino]diphenyl (α-NPD), is investigated in situ by X-ray photoelectron spectroscopy (XPS). These AuNPs-on-molecule experiments are compared with the reversed molecule-on-Au cases. The molecules-on-Au systems show only weak interactions, and the evolution of the XP spectra is dominated by final-state effects. In contrast, in the AuNPs-on-molecules cases, both initial-state effects and final-state effects occur. Spectral features arising for both molecules and metal indicate charge transfer and the formation of organometallic complexes (initial-state effects). The energy shift in the metal emission underlines the size-induced nanometric nature of the molecule/Au interaction (final-state effects). Consequently, the chemical interaction between metals and organic semiconductors likely depends strongly on the deposition sequence in general.

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.

10.
Angew Chem Int Ed Engl ; 53(39): 10355-61, 2014 Sep 22.
Artigo em Inglês | MEDLINE | ID: mdl-25044532

RESUMO

The technological exploitation of the extraordinary properties of graphene relies on the ability to achieve full control over the production of a high-quality material and its processing by up-scalable approaches in order to fabricate large-area films with single-layer or a few atomic-layer thickness, which might be integrated in working devices. A simple method is reported for producing homogenous dispersions of unfunctionalized and non-oxidized graphene nanosheets in N-methyl-2-pyrrolidone (NMP) by using simple molecular modules, which act as dispersion-stabilizing compounds during the liquid-phase exfoliation (LPE) process, leading to an increase in the concentration of graphene in dispersions. The LPE-processed graphene dispersion was shown to be a conductive ink. This approach opens up new avenues for the technological applications of this graphene ink as low-cost electrodes and conducting nanocomposite for electronics.

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