PTCDA Research Articles

Isotropic thin PTCDA films on GaN(0 0 0 1)

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

Van Der Waals Heterostructures between Small Organic Molecules and Layered Substrates

Two dimensional atomic crystals, like grapheme (G) and molybdenum disulfide (MoS2), exhibit great interest in electronic and optoelectronic applications. The excellent physical properties, such as transparency, semiconductivity, and flexibility, make them compatible with current organic electronics. Here, we review recent progress in the understanding of the interfaces of van der Waals (vdW) heterostructures between small organic molecules (pentacene, copper phthalocyanine (CuPc), perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and dioctylbenzothienobenzothiophene (C8-BTBT)) and layered substrates (G, MoS2 and hexagonal boron nitride (h-BN)). The influences of the underlying layered substrates on the molecular arrangement, electronic and vibrational properties will be addressed.

Photophysical and electrical properties of organic waveguide nanorods of perylene-3,4,9,10-tetracarboxylic dianhydride

The single crystalline nanostructure of organic semiconductors provides a very promising class of materials for applications in modern optoelectronic devices. However, morphology control and optoelectronic property modulation of high quality single crystalline samples remain a challenge. Here, we report the morphology-controlled growth of single crystalline nanorod arrays of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). We demonstrate that, unlike PTCDA film, PTCDA nanorods exhibits optical waveguide features, enhanced absorption, and Frenkel excitation emission in the visible region. Additionally, we measured the electrical properties of PTCDA nanorods, including the conductivity along the growth direction of the nanorod, which is roughly 0.61 S·m–1 (much higher than that of pure crystalline PTCDA films).

STM study of PTCDA on Sn/Si(111)-2√3×2√3.

The electronic structures of perylene tetracarboxylic dianhydride on Sn/Si(111)-2√3×2√3 have been studied by scanning tunneling microscopy and spectroscopy. Individual molecules have been investigated at 0.15 ML, while at 0.3 ML molecules formed short rods. At 0.6 ML, the molecular rods interacted with each other, coupling with the substrate and forming a new 4√3×2√3 super structure. At 0.9 ML, the surface was further reconstructed and consisted of strips with two and three rods of molecules. We found that these surface structures are strongly modified by the molecule/substrate and the intermolecular interactions. As a result, the HOMO-LUMO gaps of these molecules change with respect to the phases and the thickness. For a single molecular layer of the 4√3×2√3 phase, the HOMO-LUMO levels were split with a gap of approximately 2.1 eV, which is caused by charge transfer from the substrate to the molecules.

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (EF) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO2)Ge). The concentration of oxide on the (GeO2)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO2)Ge surface to 6 nm was studied. At energies below 9 eV above EF, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO2)Ge surface is accompanied by an increase in the work function of the surface, Evac–EF, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.

Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides.

The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics etc. The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in "van der Waals (vdW) heterostructures" and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe2), bare graphite, and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO-LUMO gap on the electronic screening effects from the substrate. The monolayer WSe2 interlayer provides substantial, but not complete, screening at the organic/inorganic interface. Our results lay a foundation for the exploitation of the complex interfacial properties of hybrid systems based on TMD materials.

Highly-oriented molecular arrangements and enhanced magnetic interactions in thin films of CoTTDPz using PTCDA templates.

In the present work, the templating effect of thin layers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on the growth of cobalt tetrakis(thiadiazole)porphyrazine (CoTTDPz) thin films was examined. X-ray diffraction and optical absorption spectra indicate that while CoTTDPz forms amorphous thin films on the bare substrates, it forms crystalline thin films on the PTCDA templates, in which the molecular planes of CoTTDPz are considered to be parallel to the substrates. Magnetic measurements reveal a significantly enhanced antiferromagnetic interaction of CoTTDPz in the templated thin films, with values reaching over 13 K. The ability to generate crystalline films and to control their orientation using molecular templates is an important strategy in the fields of organic electronics and spintronics in order to tailor the physical properties of organic thin films to suit their intended application.

Tuning molecule-substrate coupling via deposition of metal adatoms.

Organic-inorganic hybrids constitute an important class of functional materials. The fundamentals at the molecular levels are, however, relatively unexplored. PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) is a colorant and extensively applied in organic-based optoelectronic devices. PTCDA/Cu(111) and Fe-PTCDA/Cu(111) metal-organic hybrid monolayers are studied by low temperature scanning tunneling microscopy and spectroscopy (STS) and density functional theory (DFT). The former exhibits Moiré pattern-modulated molecular density of states while the latter adapts a commensurate adlattice. Both imaging and spectroscopic results suggest a strong hybridization between PTCDA molecules and Cu(111) substrate. Weak PTCDA-Cu(111) coupling can be obtained by the introduction of Fe adatoms. Compared to PTCDA/Cu(111), STS spectra of Fe-PTCDA/Cu(111) exhibit a higher energy and sharper features of the frontier orbitals. Together with the DFT results, we found that the PTCDA-Cu(111) coupling is attenuated by the presence of Fe adatoms and Fe-PTCDA coordination.

Indirect coupling of an organic semiconductor to ad-orbital surface state

Adsorption of the organic semiconductor perylene tetracarboxylic dianhydride onto $\mathrm{Cr}(001)$ decreases the metal $d$-derived surface-state lifetime without causing a shift in its energy. This suggests an indirect electronic interaction that contrasts sharply with expectations of p-d electronic coupling based on direct chemisorption. Lifetime changes are measured with scanning tunneling spectroscopy as a function of temperature and quantified as arising from a molecule-induced increase in electron-electron scattering rate into bulk bands within Fermi-liquid theory. Adsorbate-induced effects extend far beyond the adsorption site of the molecule, decaying exponentially away with a characteristic length scale of $\ensuremath{\sim}2.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, similar to the carrier mean free path in Cr.

Organic Monolayer Protected Topological Surface State.

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)/Bi2Se3 and Fe/PTCDA/Bi2Se3 heterointerfaces are investigated using scanning tunneling microscopy and spectroscopy. The close-packed self-assembled PTCDA monolayer possesses big molecular band gap and weak molecule-substrate interactions, which leaves the Bi2Se3 topological surface state intact under PTCDA. Formation of Fe-PTCDA hybrids removes interactions between the Fe dopant and the Bi2Se3 surface, such as doping effects and Coulomb scattering. Our findings reveal the functionality of PTCDA to prevent dopant disturbances in the TSS and provide an effective alternative for interface designs of realistic TI devices.

Bias and humidity effects on the ammonia sensing of perylene derivative/lutetium bisphthalocyanine MSDI heterojunctions

In this paper, we prepared and studied sensors based on Molecular Semiconductor-Doped Insulator (MSDI) heterojunctions. These original devices are built with two stacked layers of molecular materials and exhibit very specific electrical and sensing properties. We studied the properties of a MSDI composed of the perylenetetracarboxylic dianhydride, PTCDA, or the fluorinated perylenebisimine derivative, C4F7-PTCDI, as n-type molecular material sublayers, and LuPc2 as a p-type semiconductor top layer. Their response to ammonia was compared to that of a resistor formed of only the top layer of the MSDI (LuPc2). Ammonia increases the current in the MSDIs whereas it causes a decrease in the case of LuPc2 resistors, as previously observed for other MSDIs with a n-type sublayer. At first, we showed the significant effect of the applied potential on the response of these sensors. There is an optimum operating point for which the response to ammonia is maximized, which depends on the nature of the samples. The influence of the thickness of the layers was studied, showing a strong influence of this parameter on the sensing properties. MSDIs prepared with fluorinated PTCDI exhibit a higher energy barrier and a lower current. Finally, in general, PTCDA/LuPc2 MSDIs show a strong response to ammonia with a response up to 250% to 30ppm NH3 at 50% of relative humidity for the MSDI PTCDA/LuPc2 50nm/50nm. A discrimination better than 10ppm between the ammonia concentrations was observed, regardless of relative humidity between 10 and 70% rh.

Photoinduced decomposition of PTCDA molecules and desorption of their fragments from the films formed on the GaAs(110) surface

Photoinduced fragmentation and desorption of thin films of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) vacuum-deposited on the GaAs(110) surface were studied by time-of-flight (TOF) mass spectroscopy under ultrahigh vacuum conditions. The main effects under pulsed laser light irradiation (pulse duration 10 ns, photon energy 2.34 eV, laser fluence 0.5–7 mJ/cm2) were the fragmentation of the PTCDA molecule and desorption of the resulting fragments in vacuum, whereas no desorption of the intact PTCDA molecule was recorded. The fragments included the perylene core C20H8, its half C10H4, carbon dioxide and monooxide, and atomic oxygen. The desorbed fragments of different types have significantly different kinetic energies of translational motion in the gas phase because of the nonthermal mechanism of photofragmentation and photodesorption.

Transformations of PTCDA structures on rutile TiO2 induced by thermal annealing and intermolecular forces.

Transformations of molecular structures formed by perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules on a rutile TiO2(110) surface are studied with low-temperature scanning tunnelling microscopy. We demonstrate that metastable molecular assemblies transform into differently ordered structures either due to additional energy provided by thermal annealing or when the influence of intermolecular forces is increased by the enlarged amount of deposited molecules. Proper adjustment of molecular coverage and substrate temperature during deposition allows for fabrication of desired assemblies. Differences between PTCDA/TiO2(110) and PTCDA/TiO2(011) systems obtained through identical experimental procedures are discussed.

Topographic and Electronic Properties of 3,4,9,10-Perylene Tetra Carboxylic Dianhydride (PTCDA) on Indium Tin Oxide (ITO) Surface

We present the surface characterization and the local electronic properties of archetypical p-type perylene-based semiconductor organic molecule of Perylene Tetra Carboxylic Dianhydride (PTCDA) thermally evaporated on a transparent conducting metal oxide surface. A modified indium tin oxide (ITO) surface was successfully obtained by employing a subsequent chemical and physical treatment. Physisorbed PTCDA molecules exhibited a stacked-grain structure covering completely ITO surface. Scanning tunneling spectroscopy (STS) spectra of physisorbed PTCDA molecules were performed. In contrast to the previous studies of the homolog n-type perylene derivative thin films, here we successfully extracted both of the outmost frontier energy levels by measuring the current-voltage characteristics of PTCDA molecules in an estimated tunneling resistance from 4.17 to 100 GΩ at room temperature. Using numerical derivative of the I-V spectra, we extracted the series of transport gap of PTCDA molecule are lies in the region of 4.70-4.87 eV.

Patterned growth of single-crystal 3, 4, 9, 10-perylenetetracarboxylic dianhydride nanowire arrays for field-emission and optoelectronic devices

3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) organic nanostructures possess extraordinary electronic and optoelectronic properties. However, it remains a challenge to achieve patterned growth of PTCDA nanowire (NW) arrays for integrated device applications. Here, we demonstrated the high-density, large-area, uniform, and cross-aligned growth of single-crystalline PTCDA NW arrays by using Au nanoparticles (NPs) as the growth templates. The high surface energy of Au NPs led to the cross-aligned growth of organic NWs, enabling the growth of PTCDA NW arrays with any desirable patterns by pre-patterning the Au films on a Si substrate. The PTCDA NW arrays as field emitters show good performance with a large emission current density and high emission stability. Furthermore, photodetectors based on PTCDA NW arrays were constructed via a simple in-situ growth approach, which exhibited high sensitivity to a wideband light ranging from 400–800 nm and surpassed the individual NW-based photodetectors in terms of higher photocurrent and faster response speed. Successful applications of PTCDA NW arrays in field emission and photodetectors show a great potential application of organic NW arrays in future efficient electronic and optoelectronic devices.

Formation of the conduction band electronic structure during deposition of ultrathin dicarboximide-substituted perylene films on the oxidized silicon surface

The results of the investigation of the conduction band electronic structure and the interfacial potential barrier during deposition of ultrathin dicarboximide-substituted perylene films (PTCBI-C8) on the oxidized silicon surface have been presented. The measurements have been performed using the very low energy electron diffraction (VLEED) technique implemented in the total current spectroscopy (TCS) mode with a variation in the incident electron energy from 0 to 25 eV. Changes in the intensities of the maxima from the deposited PTCBI-C8 film and from the substrate with an increase in the organic coating thickness to 7 nm have been analyzed using TCS measurements. A comparison of the structure of the maxima of PTCBI-C8 and perylene-tetracarboxylic-dianhydride (PTCDA) films has made it possible to distinguish the energy range (8–13 eV above EF) in which distinct differences in the structures of maxima for PTCDA and PTCBI-C8 films are observed. This energy range corresponds to low-lying σ*-states of the conduction band of the films studied. The formation of the interfacial region of the PTCBI-C8 film and (SiO2)n-Si substrate is accompanied by an increase in the surface work function by 0.6 eV, which corresponds to the electron density charge transfer from the (SiO2)n-Si substrate to the PTCBI-C8 film.

Fluorescent Microporous Polyimides Based on Perylene and Triazine for Highly CO2-Selective Carbon Materials

Carbon dioxide (CO2) capture from point sources like coal-fired power plants is a potential solution for stabilizing atmospheric CO2 content to avoid global warming. Sorbents with high and reversible CO2 uptake, high CO2 selectivity, good chemical and thermal stability, and low cost are desired for the separation of CO2 from N2 in flue or natural gas. We report here, for the first time, on the synthesis of new microporous polyimide (PI) networks from the condensation of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and 1,3,5-triazine-2,4,6-triamine (melamine) using a Lewis acid catalyst zinc acetate/imidazole complex. These PI network materials, prepared in the absence and presence of dimethyl sulfoxide (DMSO) as weak solvent template, exhibit strong fluorescence. Nitrogen-containing carbons can be accessed from our PI networks via a simple thermal pyrolysis route. The successful construction of new microporous PI networks and derived N-containing carbons is shown here to provide promising CO2 sor...

Electrical conductivity, optical properties and mechanical stability of 3, 4, 9, 10-perylenetetracarboxylic dianhidride based organic semiconductor

The 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) doped polymer films were prepared with Polypyrrole (PPy) and Polyvinyl alcohol (PVA) polymers by solution-casting. The change in structure and chemical composition of samples was identified by XRD and FTIR respectively. The UV–visible spectroscopy demonstrates the optical characteristics and band gap properties of sample. The homogeneous morphology of sample for higher wt% of PTCDA was examined by atomic force microscopy (AFM). The differential scanning calorimetry (DSC) results demonstrate the decrease in melting temperature (Tm) and degree of crystallinity (χc%) of polymeric organic semiconductor. The mechanical property demonstrates the high tensile strength and improved plasticity nature. Impedance spectroscopy was evaluated to determine the conductivity response of polymeric organic semiconductor. The highest DC conductivity (2.08×10−3S/m) was obtained for 10wt% of PTCDA at 140°C. The decrease in activation energy (Ea) represents the non-Debye process and was evaluated from the slope of lnσdc vs. 103/T plot.

Nonlinear spectroscopy in the near-field: time resolved spectroscopy and subwavelength resolution non-invasive imaging

AbstractThe combination of near-field microscopy along with nonlinear optical spectroscopic techniques is presented here. The scanning near-field imaging technique can be integrated with nonlinear spectroscopic techniques to improve spatial and axial resolution of the images. Additionally, ultrafast dynamics can be probed down to nano-scale dimension. The review shows some examples for this combination, which resulted in an exciton map and vibrational contrast images with sub-wavelength resolution. Results of two-color femtosecond time-resolved pump-probe experiments using scanning near-field optical microscopy (SNOM) on thin films of the organic semiconductor 3,4,9,10 Perylenetetracarboxylic dianhydride (PTCDA) are presented. While nonlinear Raman techniques have been used to obtain highly resolved images in combination with near-field microscopy, the use of femtosecond laser pulses in electronic resonance still constitutes a big challenge. Here, we present our first results on coherent anti-Stokes Raman scattering (fs-CARS) with femtosecond laser pulses detected in the near-field using SNOM. We demonstrate that highly spatially resolved images can be obtained from poly(3-hexylthiophene) (P3HT) nano-structures where the fs-CARS process was in resonance with the P3HT absorption and with characteristic P3HT vibrational modes without destruction of the samples. Sub-diffraction limited lateral resolution is achieved. Especially the height resolution clearly surpasses that obtained with standard microCARS. These results will be the basis for future investigations of mode-selective dynamics in the near-field.

Influence of the adsorption geometry of PTCDA on Ag(111) on the tip–molecule forces in non-contact atomic force microscopy

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) adsorbed on a metal surface is a prototypical organic–anorganic interface. In the past, scanning tunneling microscopy and scanning tunneling spectroscopy studies of PTCDA adsorbed on Ag(111) have revealed differences in the electronic structure of the molecules depending on their adsorption geometry. In the work presented here, high-resolution 3D force spectroscopy measurements at cryogenic temperatures were performed on a surface area that contained a complete PTCDA unit cell with the two possible geometries. At small tip-molecule separations, deviations in the tip-sample forces were found between the two molecule orientations. These deviations can be explained by a different electron density in both cases. This result demonstrates the capability of 3D force spectroscopy to detect even small effects in the electronic properties of organic adsorbates.