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1.
2.
ACS Omega ; 7(24): 21183-21191, 2022 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-35755348

RESUMO

High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure-unknown before-was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.

3.
Nat Commun ; 13(1): 2875, 2022 May 24.
Artigo em Inglês | MEDLINE | ID: mdl-35610214

RESUMO

Molecular devices constructed by sub-10 nm thick molecular layers are promising candidates for a new generation of integratable nanoelectronic applications. Here, we report integrated molecular devices based on ultrathin copper phthalocyanine/fullerene hybrid layers with microtubular soft-contacts, which exhibit process-programmable functionality switching between photomultiplication and memristive behaviour. The local electric field at the interface between the polymer bottom electrode and the enclosed molecular channels modulates the ionic-electronic charge interaction and hence determines the transition of the device function. When ions are not driven into the molecular channels at a low interface electric field, photogenerated holes are trapped as electronic space charges, resulting in photomultiplication with a high external quantum efficiency. Once mobile ions are polarized and accumulated as ionic space charges in the molecular channels at a high interface electric field, the molecular devices show ferroelectric-like memristive switching with remarkable resistive ON/OFF and rectification ratios.

4.
Inorg Chem ; 59(23): 16913-16923, 2020 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-33205960

RESUMO

We present a systematic study of the evolution of structural parameters and electronic correlations as a function of 3d band filling in a single crystal series of BaT2As2 (T = Cr-Cu). The structure trends are discussed in relation to the orbital occupation of the corresponding d elements supported by calculations of the charge density and electron localization function. Analysis of our specific heat data yields the mass enhancement (m*/mb) throughout the series. By combining the structural data with the mass enhancement values, we find that the decrease in m*/mb for n > 5 follows an increase of the crystal field splitting, determined by the progressive distortion of the As-T-As angle from the ideal tetrahedral environment. This study finds a strong interplay between crystal structure, bonding behavior, band filling, and electronic properties.

5.
Nat Commun ; 11(1): 3592, 2020 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-32680989

RESUMO

Considerable efforts have been made to realize nanoscale diodes based on single molecules or molecular ensembles for implementing the concept of molecular electronics. However, so far, functional molecular diodes have only been demonstrated in the very low alternating current frequency regime, which is partially due to their extremely low conductance and the poor degree of device integration. Here, we report about fully integrated rectifiers with microtubular soft-contacts, which are based on a molecularly thin organic heterojunction and are able to convert alternating current with a frequency of up to 10 MHz. The unidirectional current behavior of our devices originates mainly from the intrinsically different surfaces of the bottom planar and top microtubular Au electrodes while the excellent high frequency response benefits from the charge accumulation in the phthalocyanine molecular heterojunction, which not only improves the charge injection but also increases the carrier density.

6.
RSC Adv ; 10(70): 43242-43247, 2020 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-35514917

RESUMO

A novel ruthenium-acetylide complex was synthesised and characterised in solid state and solution. Thin films of the complex were evaporated on silver and gold foils in ultra high vacuum in order to probe the electronic properties with photoemission spectroscopy. The charge transfer characteristics of the complex with the strong acceptor F6TCNNQ were investigated by UV-vis absorption in solution as well as at an interface with photoemission spectroscopy. A new excitation in the former optical gap of the pristine materials was probed in solution. Moreover, it was possible to identify the oxidised complex as well as the reduced acceptor by X-ray photoemission spectroscopy. In particular, our data reveal that oxidation of the complex mainly occurs at the Ru centre. The charge transfer can be characterised as localised and mainly ionic although signs of a reaction of the acceptors aminogroups with the ruthenium-acetylide complex were found.

7.
Dalton Trans ; 48(46): 17252-17257, 2019 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-31660555

RESUMO

The interaction of manganese(ii) phthalocyanine with 7,7,8,8-tetracyanoquinodimethane and its perfluoro derivative proceeds with the oxidation of Mn and the reduction of the acceptor molecules to give the first mixed single crystals of manganese(iii) phthalocyanine with TCNQ/F4-TCNQ radical anions. The crystals have unusual structures with C-Hπ interactions between the ions and their orthogonal arrangement, as well as remarkable redox properties. The charge transfer was proved by spectroscopic and magnetic studies.

8.
J Chem Phys ; 149(8): 084704, 2018 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-30193487

RESUMO

The electronic excitation spectrum of ß-CuPc has been investigated using electron energy-loss spectroscopy in transmission. Our results demonstrate a rather strong momentum dependence of the lowest exciton features. Both main components show a negative dispersion, and the momentum dependence indicates that this negative dispersion is parallel to the molecular stacks in ß-CuPc. In addition, the spectral shape also varies upon increasing momentum transfer indicating a particular momentum dependence of the inter-molecular interactions.

9.
J Phys Condens Matter ; 30(20): 205502, 2018 May 23.
Artigo em Inglês | MEDLINE | ID: mdl-29633954

RESUMO

We have investigated indirect excitons in bulk 2H-MoS2 using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the [Formula: see text] and [Formula: see text] directions of the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K v and conduction band Λc points, the Γv and K c points as well as adjacent K v and [Formula: see text] points. The energy-momentum dispersions for the K v-Λc, Γv-K c and K v1-[Formula: see text] excitons along the [Formula: see text] line are presented. The former two transitions exhibit a quadratic dispersion which allowed calculating their effective exciton masses based on the effective mass approximation. The K v1-[Formula: see text] transition follows a more linear dispersion relationship.

10.
J Chem Phys ; 148(4): 044701, 2018 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-29390825

RESUMO

We have investigated the electronic excitation spectrum of individual manganese phthalocyanine (MnPc) molecules in a coronene matrix. Our results corroborate that the observed excitation spectrum is rather sensitive to oxygen contamination of the respective samples. Annealing in vacuum allowed the determination of the electronic excitations of individual MnPc molecules. These results confirm previous data from the literature and, in addition, we have identified a low energy excitation at about 0.8 eV, which had not been reported before.

11.
Beilstein J Nanotechnol ; 8: 1601-1615, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-28884064

RESUMO

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard.

12.
Beilstein J Nanotechnol ; 8: 927-932, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-28546887

RESUMO

We have used photoelectron spectroscopy to determine the energy-level alignment at organic heterojunctions made of manganese phthalocyanine (MnPc) and the fullerene C60. We show that this energy-level alignment depends upon the preparation sequence, which is explained by different molecular orientations. Moreover, our results demonstrate that MnPc/C60 interfaces are hardly suited for application in organic photovoltaic devices, since the energy difference of the two lowest unoccupied molecular orbitals (LUMOs) is rather small.

13.
J Phys Condens Matter ; 29(16): 165502, 2017 Apr 26.
Artigo em Inglês | MEDLINE | ID: mdl-28247854

RESUMO

We present electron energy-loss spectroscopic measurements of potassium (K) intercalated tungsten diselenide (WSe2). After exposure of pristine WSe2 films to potassium, we observe a charge carrier plasmon excitation at about 0.97 eV, which indicates a semiconductor-to-metal transition. Our data reveal the formation of one particular doped K-WSe2 phase. A Kramers-Kronig analysis allows the determination of the dielectric function and the estimation of the composition of K0.6WSe2. Momentum dependent measurements reveal a substantial plasmon dispersion to higher energies.

14.
Langmuir ; 32(18): 4464-71, 2016 05 10.
Artigo em Inglês | MEDLINE | ID: mdl-27093097

RESUMO

Molecular anchoring and electronic properties of macrocyclic complexes fixed on gold surfaces have been investigated mainly by using scanning tunnelling microscopy (STM) and complemented with X-ray photoelectron spectroscopy (XPS). Exchange-coupled macrocyclic complexes [Ni2L(Hmba)](+) were deposited via 4-mercaptobenzoate ligands on the surface of a Au(111) single crystal from a mM solution of the perchlorate salt [Ni2L(Hmba)]ClO4 in dichloromethane. The combined results from STM and XPS show the formation of large monolayers anchored via Au-S bonds with a height of about 1.5 nm. Two apparent granular structures are visible: one related to the dinickel molecular complexes (cationic structures) and a second one related to the counterions ClO4(-) which stabilize the monolayer. No type of short and long-range order is observed. STM tip-interaction with the monolayer reveals higher degradation after 8 h of measurement. Spectroscopy measurements suggest a gap of about 2.5 eV between HOMO and LUMO of the cationic structures and smaller gap in the areas related to the anionic structures.

15.
J Chem Phys ; 143(15): 154708, 2015 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-26493923

RESUMO

We report the doping induced changes of the electronic structure of tetracene and pentacene probed by electron energy-loss spectroscopy in transmission. A comparison between the dynamic response of undoped and potassium-intercalated tetracene and pentacene emphasizes the appearance of a new excitation feature in the former gap upon potassium addition. Interestingly, the momentum dependency of this new excitation shows a negative dispersion. Moreover, the analysis of the C 1s and K 2p core-level excitation results in a significantly lower doping level compared to potassium doped picene, a recently discovered superconductor. Therefore, the present electronic structure investigations open a new pathway to better understand the exceptional differences between acenes and phenacene and their divergent behavior upon alkali doping.

16.
Sci Rep ; 5: 14506, 2015 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-26411304

RESUMO

The electronic properties of NdGaO3/SrTiO3, LaGaO3/SrTiO3, and LaAlO3/SrTiO3 interfaces, all showing an insulator-to-metal transition as a function of the overlayer-thickness, are addressed in a comparative study based on x-ray absorption, x-ray photoemission and resonant photoemission spectroscopy. The nature of the charge carriers, their concentration and spatial distribution as well as the interface band alignments and the overall interface band diagrams are studied and quantitatively evaluated. The behavior of the three analyzed heterostructures is found to be remarkably similar. The valence band edge of all the three overlayers aligns to that of bulk SrTiO3. The near-interface SrTiO3 layer is affected, at increasing overlayer thickness, by the building-up of a confining potential. This potential bends both the valence and the conduction band downwards. The latter one crossing the Fermi energy in the proximity of the interface and determines the formation of an interfacial band offset growing as a function of thickness. Quite remarkably, but in agreement with previous reports for LaAlO3/SrTiO3, no electric field is detected inside any of the polar overlayers. The essential phenomenology emerging from our findings is discussed on the base of different alternative scenarios regarding the origin of interface carriers and their interaction with an intense photon beam.

17.
J Chem Phys ; 142(18): 184702, 2015 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-25978900

RESUMO

The electronic excitation spectra of charge transfer compounds built from the hydrocarbons picene and chrysene, and the strong electron acceptors F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and TCNQ (7,7,8,8-tetracyanoquinodimethan) have been investigated using electron energy-loss spectroscopy. The corresponding charge transfer compounds have been prepared by co-evaporation of the pristine constituents. We demonstrate that all investigated combinations support charge transfer, which results in new electronic excitation features at low energy. This might represent a way to synthesize low band gap organic semiconductors.

18.
Inorg Chem ; 53(20): 10825-34, 2014 Oct 20.
Artigo em Inglês | MEDLINE | ID: mdl-25299133

RESUMO

Complexation of the ambidentate ligand 4-mercaptobenzoate (4-SH-C6H4CO2H, H2mba) by the macrocyclic complex [Ni2L(µ-Cl)]ClO4 (L(2-) represents a 24-membered macrocyclic hexaazadithiophenolate ligand) has been examined. The monodeprotonated Hmba(-) ligand reacts with the Ni2 complex in a selective manner by substitution of the bridging chlorido ligand to produce µ1,3-carboxylato-bridged complex [Ni2L(Hmba)](+) (2(+)), which can be isolated as an air-sensitive perchlorate (2ClO4) or tetraphenylborate (2BPh4) salt. The reactivity of the new mercaptobenzoate complex is reminiscent of that of a "free" thiophenolate ligand. In the presence of air, 2ClO4 dimerizes via a disulfide bond to generate tetranuclear complex [{Ni2L}2(O2CC6H4S)2](2+) (3(2+)). The auration of 2ClO4 with [AuCl(PPh3)], on the other hand, leads to monoaurated complex [Ni(II)2L(mba)Au(I)PPh3](+) (4(+)). The bridging thiolate functions of the N6S2 macrocycle are deeply buried and are unaffected/unreactive under these conditions. The complexes were fully characterized by electrospray ionization mass spectrometry, IR and UV/vis spectroscopy, density functional theory, cyclic voltammetry, and X-ray crystallography [for 3(BPh4)2 and 4BPh4]. Temperature-dependent magnetization and susceptibility measurements reveal an S = 2 ground state that is attained by ferromagnetic coupling between the spins of the Ni(II) ions in 2ClO4 (J = +22.3 cm(-1)) and 4BPh4 (J = +20.8 cm(-1); H = -2JS1S2). Preliminary contact-angle and X-ray photoelectron spectroscopy measurements indicate that 2ClO4 interacts with gold surfaces.

19.
J Chem Phys ; 141(9): 094706, 2014 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-25194385

RESUMO

We have prepared phthalocyanine heterojunctions out of MnPc and F16CoPc, which were studied by means of X-ray absorption spectroscopy. This heterojunction is characterized by a charge transfer at the interface, resulting in charged MnPc(δ +) and F16CoPc(δ -) species. Our data reveal that the molecules are well ordered and oriented parallel to the substrate surface. Furthermore, we demonstrate the filling of the Co 3d(z(2)) orbital due to the charge transfer, which supports the explanation of the density functional theory, that the charge transfer is local and affects the metal centers only.

20.
Chemistry ; 19(24): 7787-801, 2013 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-23595564

RESUMO

A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2L(Cl)]ClO4 (1ClO4), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2L(dppba)](+), which can be isolated as an air-stable perchlorate [Ni2L(dppba)]ClO4 (2ClO4) or tetraphenylborate [Ni2L(dppba)]BPh4 (2BPh4) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II)2Au(I) complex [Ni(II)2L(dppba)Au(I)Cl]ClO4 (3ClO4). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II)2L(dppba)Au(I)Ph]BPh4 (4BPh4), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J = +15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H = -2 JS1S2). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative Au-P bonds in a monolayer.

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