Diindenoperylene Molecules Research Articles

Anomalous photoluminescence quenching in DIP/MoS2 van der Waals heterostructure: Strong charge transfer and a modified interface

Due to the fascinating optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) and the flexible nature, vast library, and easy fabrication process of organic molecules, the combination of 2D TMD materials and organic molecules have been gaining enormous attention in recent years. The organic semiconductors can enhance the optoelectronic and improved transport properties by interfacing with atomically thin TMDs through vdW interaction. Here, we investigate the photoluminescence (PL) properties of type-II heterostructure made of monolayer molybdenum disulfide (ML MoS2) and a non-planar Diindenoperylene(DIP) molecules. A drastic quenching of PL intensity nearly 300% is observed with slightly redshift in DIP/MoS2as compared to the PL of isolated MoS2. The observed quenching behavior in DIP/MoS2heterostructure can be explained by the appeared ‘trap – like’ states in density of states (DOS), and suppression of exciton to trion ratio due to electron transfer from the lowest unoccupied molecular orbital (LUMO) level of DIP to the conduction band minima (CBM) of MoS2. Furthermore, the redshift in the PL of heterostructure could be due to weak coupling strength which is ~8 times smaller than that of the energy of A-exciton resonance of individual layers and dielectric screening effect induced by the non-planar DIP based organic layer. The expected strong interaction between the DIP molecule and MoS2 layer due to the permanent dipole moments (significant molecular quadruple moment 8.25D) of DIP molecules is another reason, which may slightly reduce the bandgap and results in red-shift of PL. We observed Raman (A1g phonon mode) is blue-shifted, which is supposed to stiffening due to the non-planar DIP molecule onto ML MoS2. The strong PL quenching behavior of DIP/MoS2heterostructure can be a potential candidate for tailoring the optoelectronic properties of TMDs that may demonstrate for light-harvesting and canserve as the next generation TMD/organic-based photovoltaic applications.

Reorientation of π-conjugated molecules on few-layer MoS2 films.

Small π-conjugated organic molecules have attracted substantial attention in the past decade as they are considered as candidates for future organic-based (opto-)electronic applications. The molecular arrangement in the organic layer is one of the crucial parameters that determine the efficiency of a given device. The desired orientation of the molecules is achieved by a proper choice of the underlying substrate and growth conditions. Typically, one underlying material supports only one inherent molecular orientation at its interface. Here, we report on two different orientations of diindenoperylene (DIP) molecules on the same underlayer, i.e. on a few-layer MoS2 substrate. We show that DIP molecules adopt a lying-down orientation when deposited on few-layer MoS2 with horizontally oriented layers. In contrast, for vertically aligned MoS2 layers, DIP molecules are arranged in a standing-up manner. Employing in situ and real-time grazing-incidence wide-angle X-ray scattering (GIWAXS), we monitored the stress evolution within the thin DIP layer from the early stages of the growth, revealing different substrate-induced phases for the two molecular orientations. Our study opens up new possibilities for the next-generation of flexible electronics, which might benefit from the combination of MoS2 layers with unique optical and electronic properties and an extensive reservoir of small organic molecules.

Structure formation in diindenoperylene thin films on copper(111).

First-principles calculations were combined with scanning tunneling microscopy (STM) measurements to analyze the adsorption of diindenoperylene (DIP) molecules on Cu(111) surfaces. The influence of the substrate on the geometry of single adsorbed molecules, their diffusion barriers, as well as the role of step-edges and intermolecular interactions for molecular self-assembly and structure growth are studied. Long-range ordered arrangements of DIP molecules are found to be most favorable irrespective of the terrace width. Energetically less favored short-range order structures, however, are observed as well.

Diindenoperylene derivatives: A model to investigate the path from molecular structure via morphology to solar cell performance

Efficient organic electronic devices require a detailed understanding of the relation between molecular structure, thin film growth, and device performance, which is only partially understood at present. Here, we show that small changes in molecular structure of a donor absorber material lead to significant changes in the intermolecular arrangement within organic solar cells. For this purpose, phenyl rings and propyl side chains are fused to the diindenoperylene (DIP) molecule. Grazing incidence X-ray diffraction and variable angle spectroscopic ellipsometry turned out to be a powerful combination to gain detailed information about the thin film growth. Planar and bulk heterojunction solar cells with C60 as acceptor and the DIP derivatives as donor are fabricated to investigate the influence of film morphology on the device performance. Due to its planar structure, DIP is found to be highly crystalline in pristine and DIP:C60 blend films while its derivatives grow liquid-like crystalline. This indicates that the molecular arrangement is strongly disturbed by the steric hindrance induced by the phenyl rings. The high fill factor (FF) of more than 75% in planar heterojunction solar cells of the DIP derivatives indicates excellent charge transport in the pristine liquid-like crystalline absorber layers. However, bulk heterojunctions of these materials surprisingly result in a low FF of only 54% caused by a weak phase separation and thus poor charge carrier percolation paths due to the lower ordered thin film growth. In contrast, crystalline DIP:C60 heterojunctions lead to high FF of up to 65% as the crystalline growth induces better percolation for the charge carriers. However, the major drawback of this crystalline growth mode is the nearly upright standing orientation of the DIP molecules in both pristine and blend films. This arrangement results in low absorption and thus a photocurrent which is significantly lower than in the DIP derivative devices, where the liquid-like crystalline growth leads to a more horizontal molecular alignment. Our results underline the complexity of the molecular structure-device performance relation in organic semiconductor devices.

Effect of Gap States on the Orientation-Dependent Energy Level Alignment at the DIP/F16CuPc Donor–Acceptor Heterojunction Interfaces

The interface properties of organic–organic heterojunctions (OOHs) between diindenoperylene (DIP) and copper hexadecafluorophthalocyanine (F16CuPc), including interface morphology, molecular orientation, and energy level alignment, have been investigated systematically by atomic force microscopy, in situ ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and synchrotron-based near-edge X-ray adsorption fine structure (NEXAFS) measurements. As revealed by NEXAFS measurements, DIP molecules adopt the standing orientation on the standing F16CuPc on the SiO2 substrate, while they have the lying-down configuration on the flat-lying F16CuPc on the highly oriented pyrolytic graphite substrate. From the UPS and XPS studies, there is a large interfacial charge transfer and a band-bending-like behavior in the standing DIP/F16CuPc OOHs, while the vacuum level is almost aligned at the lying-down DIP/F16CuPc OOH interface. We propose that the defects-induced gap states have a crucial...

Templating Effect for Organic Heterostructure Film Growth: Perfluoropentacene on Diindenoperylene

We present a systematic study of templating effects in organic heterostructure growth, that is, where the growth behavior of the top layer is significantly influenced by the structural and morphological properties of the bottom layer. We prepared thin films of the donor material diindenoperylene (DIP) differing in crystal structure, grain size, and grain orientation on indium tin oxide (ITO). The acceptor material perfluoropentacene (PFP) was chosen to be evaporated on DIP to form the heterostructure. We employed X-ray reflectivity and grazing incidence X-ray diffraction as well as atomic force microscopy to investigate the heterostructures. The structure and morphology of the PFP layers depend on the properties of the DIP layer underneath. Two main effects were observed: PFP molecules in the top layer tend to adopt the orientation of the DIP molecules in the bottom layer and the crystalline quality of the PFP layer correlates with the crystalline quality of the DIP layer underneath in terms of the in-pla...

Hyperthermal organic thin film growth on surfaces terminated with self-assembled monolayers. I. The dynamics of trapping

We have examined the initial stages of growth of a crystalline small molecule organic thin film, diindenoperylene (DIP), on SiO(2) surfaces terminated with a series of self-assembled monolayers (SAMs). In this study we make use of supersonic molecular beam techniques to vary the incident kinetic energy of the DIP molecules, and we use in situ, real time synchrotron x-ray scattering to monitor the buildup of each molecular layer in the growing thin film. We find that the effects of the SAMs are most apparent concerning growth in the sub-monolayer regime, before the substrate is entirely covered by the DIP thin film. In this coverage regime on bare SiO(2), and SiO(2) terminated with either hexamethyldisilazane or perflurooctyltrichlorosilane the adsorption dynamics are consistent with trapping-mediated adsorption as observed in more simple systems, where the probability of adsorption decreases significantly with increasing kinetic energy. Once these surfaces are covered with DIP, however, the adsorption probability increases, particularly at the highest incident kinetic energy, and the probability of adsorption exhibits only a weak dependence on the incident kinetic energy. In contrast, on surfaces terminated by octyl- (OTS) and octadecyltrichlorosilane (ODTS) the trapping probability is high and exhibits little dependence on the incident kinetic energy, essentially the same as what is observed on these same surfaces covered by DIP. We postulate, which is backed by the results of molecular dynamics simulations, that direct molecular insertion into the OTS and ODTS layers is a primary explanation for efficient trapping on these surfaces.

Trapping dynamics of diindenoperylene (DIP) in self-assembled monolayers using molecular simulation

All-atom Molecular Dynamics simulation methods employing a well-tested intermolecular potential model, MM3 (Molecular Mechanics 3), demonstrate the propensity for diindenoperylene (DIP) molecules to insert between molecules of a self-assembled monolayer (SAM) during a deposition process intended to grow a thin film of this organic semiconductor molecule onto the surface of self-assembled monolayers. The tendency to insert between SAM molecules is fairly prevalent at normal growth temperatures and conditions, but is most strongly dependent on the density and the nature of the SAM. We posit the existence of an optimal density to favor surface adsorption over insertion for this system. DIP is less likely to insert in fluorinated SAMs, like FOTS (fluorooctatrichlorosilane), than its unfluorinated analog, OTS (octatrichlorosilane). It is also less likely to insert between shorter SAMs (e.g., less insertion in OTS than ODTS (octadecyltrichlorosilane)). Very short length, surface-coating molecules, like HDMS (hexamethyldisilazane), are more likely to scatter energetic incoming DIP molecules with little insertion on first impact (depending on the incident energy of the DIP molecule). Grazing angles of incidence of the depositing molecules generally favor surface adsorption, at least in the limit of low coverage, but are shown to be dependent on the nature of the SAM. The validity of these predictions is confirmed by comparison of the predicted sticking coefficients of DIP at a variety of incident energies on OTS, ODTS, and FOTS SAMs with results obtained experimentally by Desai et al. (2010) [23]. The simulation predictions of the tendency of DIP to insert can be explained, in large part, in terms of binding energies between SAM and DIP molecules. However, we note that entropic and stochastic events play a role in the deposition outcomes. Preliminary studies of multiple deposition events, emulating growth, show an unexpected diffusion of DIP molecules inserted within the SAM matrix in a clear attempt of the DIP molecules to aggregate together.

Epitaxial growth of diindenoperylene ultrathin films on Ag(111) investigated by LT-STM and LEED

The epitaxial growth of the 1st and 2nd monolayer (ML) diindenoperylene (DIP) on Ag(111) has been systematically investigated using low temperature scanning tunneling microscopy, low energy electron diffraction and first-principles calculations. At one ML regime, DIP molecules commensurately arrange in either herringbone or brick-wall superstructures, depending on the deposition rate. Tip-induced structural transformation from herringbone to brick-wall is observed. Calculations based on density functional theory reveal that the top site of Ag(111) is energetically favorable for both superstructures. The 2nd ML DIP aggregate in either herringbone or brick-wall superstructures depending on the arrangements of the 1st ML DIP, indicating that the structural properties of DIP thin films on Ag(111) are sensitive to growth conditions. The observed variation in DIP ultrathin film structure may result in different electronic properties and have implications for DIP-based organic electronic devices, such as organic field-effect transistors or organic photovoltaic cells.

Raman polarization studies of highly oriented organic thin films

AbstractIn this work we report on the capability of polarized Raman spectroscopy to investigate the structure of thin organic films. Diindenoperylene (DIP) thin films on (1 × 1)‐rutile(110) were prepared via organic molecular beam deposition (OMBD). Raman spectra of DIP thin films showed several strong Raman modes in the wavenumber region from 1200 to 1650 cm−1. The Ag mode at 1284 cm−1 shows two contributions, thereby indicating the coexistence of at least two DIP film structures. Polarized Raman spectroscopy was applied to characterize the molecular orientation and the dominance of the σ‐configuration (i.e. upright standing DIP molecules) was found. Copyright © 2009 John Wiley & Sons, Ltd.

From interfaces to surfaces: soft x-ray spectromicroscopy investigations of diindenoperylenethin films on gold

We present the results of photoemission electron microscopy investigationson diindenoperylene (DIP) thin films deposited on polycrystalline gold,prepared in order to have a roughness much larger than the molecular size. Ourinvestigations revealed the ability of the DIP molecule to form well-organized films,exhibiting a different molecular orientation with respect to the already knownλ andσ phases. In locally thicker film regions, the energy of the films is minimizedby a molecular arrangement that has an asymptotic tendency to theσ phase.

Ultrathin Films of Diindenoperylene on Graphite and SiO2

In situ low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based high-resolution photoemission spectroscopy, and near-edge X-ray absorption fine structure measurements were used to study the supramolecular packing, molecular orientation, and electronic structures of ultrathin films of diindenoperylene (DIP) on graphite and SiO2. LT-STM measurements reveal that monolayer DIP on highly ordered pyrolytic graphite (HOPG) adopts a long-range ordered “brick-wall” arrangement with their molecular π-planes orientated parallel to the graphite surface, arising from the DIP−graphite interfacial π−π interaction. In contrast, DIP molecules stand upright on inert SiO2 due to the weak interfacial interaction. The ionization potential (IP) of DIP films largely depends on their molecular orientation, i.e., the IP for the standing-up DIP film on SiO2 is 0.40 ± 0.05 eV lower than that of the lying-down film on graphite. This is attributed to the intrinsic surface dipoles that originate from the intramolecular dipolar C−H bonds exposed at the surfaces of the standing-up thin film.

Nanoconfinement effects in the self-assembly of diindenoperylene (DIP) on Cu(111) surfaces

The structure of monolayer films of diindenoperylene (DIP) on Cu(111) single crystals has been studied by means of scanning tunneling microscopy (STM). Pronounced film confinement effects are observed, resulting in distinctly different film structures depending upon the substrate terrace width. On wide terraces (exceeding approximately 15 nm in width) a short range ordered structure forms with the DIP molecules aligned along three discrete directions related by 60 degrees rotations. On the contrary, on narrow terraces (<15 nm), the confinement enhances the role of the step edges and leads to the formation of a long range ordered structure with co-directionally oriented molecules, tentatively ascribed to an energy minimization by optimization of the surface coverage.

Exciton-phonon coupling in diindenoperylene thin films

We investigate exciton-phonon coupling and exciton transfer in diindenoperylene (DIP) thin films on oxidized Si substrates by analyzing the dielectric function determined by variable-angle spectroscopic ellipsometry. Since the molecules in the thin-film phase form crystallites that are randomly oriented azimuthally and highly oriented along the surface normal, DIP films exhibit strongly anisotropic optical properties with uniaxial symmetry. This anisotropy can be determined by multiple sample analysis. The thin-film spectrum is compared with a monomer spectrum in solution, which reveals similar vibronic subbands and a Huang-Rhys parameter of $S\ensuremath{\approx}0.87$ for an effective internal vibration at $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{eff}}=0.17\text{ }\text{eV}$. However, employing these parameters the observed dielectric function of the DIP films cannot be described by a pure Frenkel exciton model, and the inclusion of charge-transfer (CT) states becomes mandatory. A model Hamiltonian is parametrized with density-functional theory calculations of single DIP molecules and molecule pairs in the stacking geometry of the thin-film phase, revealing the vibronic coupling constants of DIP in its excited and charged states together with electron and hole transfer integrals along the stack. From a fit of the model calculation to the observed dielectric tensor, we find the lowest CT transition ${E}_{00}^{\text{CT}}$ at $0.26\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{eV}$ above the neutral molecular excitation energy ${E}_{00}^{F}$, which is an important parameter for device applications.

Enantiotropic Polymorphism in Di-indenoperylene

The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP) at 403 K has been structurally characterized, using single-crystal X-ray diffraction. Both the low temperature α- and the high-temperature β-phase have a herringbone-type structure, with the α-phase being triclinic with doubled unit cell volume compared to the monoclinic β-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from β- to α-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules.

Observation of competing modes in the growth of diindenoperylene on SiO2

Diindenoperylene (DIP) thin films deposited on SiO2-surfaces by organic molecular beam deposition in ultra-high vacuum at room temperature have been investigated by atomic-force-microscopy and X-ray scattering as a function of film thickness. We observe the evolution of two distinct molecular orientations; the long axis of the DIP molecule is pointing either along the surface normal (σ-orientation) or along the surface (λ-orientation). Initially, the films exhibit predominantly σ-orientated islands. Increasing the film thickness, elongated, fibrous λ-orientated islands nucleate on top of the σ-oriented islands and dominate the growth front due to their fast 3D growth mode. Upon annealing at 150 °C, the fibers disappear. We argue that these observations can be understood as a competition between surface energy (favoring the σ-orientation) and growth kinetics (favoring the λ-orientation).

High structural order in thin films of the organic semiconductor diindenoperylene

We report extraordinary structural order along the surface normal in thin films of the organic semiconductor diindenoperylene (DIP) deposited on silicon–dioxide surfaces. Cross-sectional transmission electron microscopy (TEM), noncontact atomic force microscopy (NC–AFM), as well as specular and diffuse x-ray scattering measurements were performed to characterize thin films of DIP. Individual monolayers of essentially upright-standing DIP molecules could be observed in the TEM images indicative of high structural order. NC–AFM images showed large terraces with monomolecular steps of ≈16.5 Å height. Specular DIP Bragg reflections up to high order with Laue oscillations confirmed the high structural order. A semi-kinematic fit to the data allowed a precise determination of the oscillatory DIP electron density ρel.,DIP(z). The mosaicity of the DIP thin films was obtained to be smaller than 0.01°.