ClAlPc Molecules Research Articles

Probing the Properties of Quasi‐Isolated ClAlPc by Using Polycrystalline h‐BN on Pt(111) as a Testbed

AbstractThe structural and electronic properties of chloro‐aluminum phthalocyanine (ClAlPc) molecules on insulating hexagonal boron nitride (h‐BN) monolayers (MLs) grown on Pt(111) surfaces are studied via scanning tunneling microscopy and spectroscopy (STM/STS) along with low energy electron diffraction in ultra‐high vacuum conditions. As a reference, the structural properties of the h‐BN/Pt(111) surface are studied, where various moiré patterns associated to different orientations of the h‐BN lattice with respect to the Pt(111) surface below are characterized, for the first time, at the atomic scale. Room temperature deposition of ClAlPc molecules on h‐BN/Pt(111) gives rise to the formation of mainly bilayer molecular structures, where the ClAlPc molecules adopt an alternating Cl‐up and Cl‐down stacking configuration. Upon annealing, bilayer molecular structures become islands comprising only a single layer of molecules adsorbed in a Cl‐up configuration. In the ML regime, the possible influence of the moiré pattern superstructure and the h‐BN lattice on the molecular ordering is also studied. Finally, the electronic properties of ClAlPc on h‐BN/Pt(111) are investigated through high‐resolution STM imaging of the frontier molecular orbitals and differential conductance plots. The results point toward a weak molecule–substrate interaction.

Optomechanical switching of adsorption configurations of polar organic molecules by UV radiation pressure

Using photoemission spectroscopy (PES), we have systematically investigated the behavior of polar organic molecule, chloroaluminum phthalocyanine (ClAlPc), adsorbed in the Cl-down configuration on the Ag(111) substrate at low temperature − 195 °C under UV irradiation with a range of different photon fluxes. Judging from the evolution of photoemission spectral line shapes of molecular energy states, we discovered that the Cl atoms are so robustly anchored at Ag(111) that the impinging photons cannot flip the ClAlPc molecules, but instead they crouch them down due to radiation pressure; we observe that the phthalocyanine (Pc) lobes bend down to interact with Ag atoms on the substrate and induce charge transfer from them. As photon flux is increased, radiation pressure on the Pc plane initiates tunneling of the Cl atom through the molecular plane to turn the adsorption configuration of ClAlPc from Cl-down to an upheld Cl-up configuration, elucidating an optomechanical way of manipulating the dipole direction of polar molecules. Finally, work function measurements provide a distinct signature of the resulting upheld Cl-up configuration as it leads to a large increase in vacuum level (VL), ~ 0.4 eV higher than that of a typical flat-on Cl-up configuration driven by thermal annealing.

Self-assembly of ClAlPc molecules on moiré-patterned graphene grown on Pt(111)

Phthalocyanines are promising molecules for the development of organic electronic devices, for instance, molecular heterojunctions in organic solar cells or organic field-effect transistors. For an optimum performance of these devices, the molecular ordering on the substrate and the molecular electronic level alignment have been shown as crucial factors. In this work, the self-assembled structure and the electronic structure of chloroaluminum phthalocyanines (ClAlPc) on graphene grown on Pt(111) surfaces have been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) and low-temperature conditions. Graphene grown on Pt(111) exhibits multiple moiré patterns with different periodicities, offering a benchmark to investigate the influence of the graphene and the moiré patterns in the ClAlPc ordering. This surface allows to extend previous works performed on graphite and graphene on Cu(100), where no moiré patterns are found. Well-ordered molecular islands exhibiting rotational domains have been observed in the submonolayer regime. The orientation of individual ClAlPc molecules within the structure unit cell has been characterized pointing out to a Cl-Up configuration adopted by the molecules. Our measurements show a correlation between the molecular lattice orientation and the graphene directions, whereas no influence of the underlying moiré patterns has been found. Finally, the ClAlPc electronic structure has been characterized indicating a weak graphene-molecule interaction.

Tuning the band gap of the graphene oxide-chloro aluminum phthalocyanine nanocomposite by reducing the rate of graphene oxide

Abstract RGO−ClAlPc nanocomposite is synthesized by the chemical reduction of graphene oxide, using hydrazine hydrate in the presence of chloro aluminum phthalocyanine (ClAlPc) and sodium dodecyl sulfate (SDS). In this research study the results of FTIR and UV–Vis spectroscopy showed that ClAlPc molecules were attached on the RGO sheets by π−π interactions. The electrical properties of RGO−ClAlPc nanocomposites were investigated over a temperature range [298 K–393 K]. Then, several interdigital planar devices with 30 nm thickness and 200 μm interspacing of the electrodes were fabricated from the RGO−ClAlPc nanocomposite. The variation of current versus temperature at a constant 2 V indicated that the band gap of RGO−ClAlPc nanocomposite was changed as a function of the rate of GO reduction in the nanocomposite.

Control of the dipole layer of polar organic molecules adsorbed on metal surfaces via different charge-transfer channels

Organic molecules with a permanent electric dipole moment have been widely used as a template for further growth of molecular layers in device structures. Key properties of the resulting organic films such as energy level alignment (ELA), work function, and injection/collection barrier are linked to the magnitude and direction of the dipole moment at the interface. Using angle-resolved photoemission spectroscopy (ARPES), we have systematically investigated the coverage-dependent work function and spectral line shapes of occupied molecular energy states (MESs) of chloroaluminium-phthalocyanine (ClAlPc) grown on Ag(111). We demonstrate that the dipole orientation of the first ClAlPc layer can be controlled by adjusting the deposition rate and postannealing conditions, and we find that the ELA at the interface differs by $\ensuremath{\sim}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ between the Cl up and down configurations of the adsorbed ClAlPc molecules. These observations are rationalized by density functional theory (DFT) calculations based on a realistic model of the ClAlPc/Ag(111) interface, which reveal that the different orientations of the ClAlPc dipole layer lead to different charge-transfer channels between the adsorbed ClAlPc and Ag(111) substrate. Our findings provide a useful framework toward method development for ELA tuning.

Influence of deposition substrate temperature on the morphology and molecular orientation of chloroaluminum phthalocyanine films as well the performance of organic photovoltaic cells

The dependence of the morphology of neat chloroaluminum phthalocyanine (ClAlPc) films on substrate temperature (Tsub) during deposition is investigated by variable angle spectroscopic ellipsometry (VASE), x-ray diffraction (XRD), and atomic force microscopy (AFM) to obtain detailed information about the molecular orientation, phase separation, and crystallinity. AFM images indicate that both grain size and root mean square (RMS) roughness noticeably increase with Tsub both in neat and blend films. Increasing Tsub from room temperature to 420 K increases the horizontal orientation of the ClAlPc molecules with an increase of the mean molecular tilt angle from 60.13° (300 K) to 65.86° (420 K). The UV–vis absorption band of the corresponding films increases and the peak wavelength slightly red shifts with the Tsub increase. XRD patterns show a clear diffraction peak at Tsub over 390 K, implying the π-stacking of interconnected ClAlPc molecules at high Tsub. Planar and bulk heterojunction (BHJ) photovoltaic cells containing pristine ClAlPc films and ClAlPc:C60 blend films fabricated at Tsub of 390 K show increases in the power conversion efficiency (ηPCE) of 28% (ηPCE = 3.12%) and 36% (ηPCE = 3.58%), respectively, relative to devices as-deposited at room temperature. The maximum short circuit current in BHJs is obtained at 390 K in the Tsub range from 300 K to 450 K.

Electric-Field-Induced Molecular Switch of Single Dipolar Phthalocyanine on Cu(111): A Scanning Tunneling Microscopy Study

We demonstrate a molecular switch of a single Cl-down adsorbed choloraluminum phthalocyanine (ClAlPc) molecule on a Cu(111) substrate by scanning tunneling microscopy (STM) at liquid nitrogen temperature (77 K). The Cl-down adsorbed ClAlPc molecule exhibits an in-plane quadruple moment on the four-lobe which stems from the inequivalent charge distribution after anchoring on Cu(111). The quadruple moment under the nonuniform electric field between the STM tip and the sample surface is responsible for the rotation of these Cl-down adsorbed ClAlPc molecules within three metastable states. In response to the local nonuniform electric field, the switching of these Cl-down adsorbed ClAlPc molecules can be controlled by varying the applied biases.

Surface strain mediated dipole alignment of ClAlPc on Au(111)

In this letter, we demonstrate the control of dipole alignment of monolayer chloroaluminum phthalocyanine (ClAlPc) molecules via the strain in Au(111) substrates. Local ordering of ClAlPc dipole comprising alternate Cl-up and Cl-down configurations is found on the tensile-strained Au(111)/mica as a result of strain-enhanced absorption of Cl-down ClAlPc's and the dipole-dipole interaction. In contrast, the strain-released single crystal Au(111) substrate shows negligible coupling to Cl-down ClAlPc, therefore, facilitating the formation of unidirectionally aligned Cl-up ClAlPc dipole array. The dipole-dipole interaction becomes less prominent at low ClAlPc coverage where ClAlPc molecules can find their favorable absorption sites more easily according to their inherent dipole orientation. Our results emphasize the superior role of molecule-substrate interaction in functional molecular engineering on metal surface hence provide fundamental insight into the potential applications in molecular nanodevices with tunable and controllable properties.

Density functional theory calculation of IR spectra of Zn-phthalocyanine and Zn-phthalocyanine-d 16

Normal modes of Zn-phthalocyanine (ZnPc), ZnPc-d 16, and ClAlPc molecules were calculated by the density-functional method. All out-of-plane modes were assigned. Assignments for in-plane modes of E u symmetry were refined. An explanation was given for the differences in IR spectra of the α and β crystalline modifications arising during the formation of rare-earth dimers and certain intermolecular interactions.

Investigation of Interface Properties for ClAlPc/C60 Heterojunction-Based Inverted Organic Solar Cell

The interface properties of organic donor–acceptor heterojunction comprising chloroaluminium phthalocyanine (ClAlPc) and C60 in an inverted organic solar cell configuration have been investigated, with particular emphasis on the molecular orientation and interfacial energy level alignment through the combination of in situ near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet photoelectron spectroscopy (UPS) measurements. In situ NEXAFS measurements reveal that ClAlPc molecules on C60 film on indium tin oxide (ITO) electrode adopt a lying configuration with their molecular π-plane nearly parallel to the substrate surface. Such lying configuration of ClAlPc films can facilitate the efficient charge transport in heterojunction based solar cell devices. As confirmed by in situ UPS measurements, the large ionization potential of ClAlPc leads to a large energy offset between the highest-occupied-molecular-orbital (HOMO) of ClAlPc and the lowest-unoccupied-molecular-orbital (LUMO) of C60 at the ClA...

Tunable two-dimensional molecular dipole dot arrays on graphite

We report the construction of two dimensional molecular dipole dot arrays with tunable dipole density via self-assembly of binary molecular system of dipolar molecule of chloroaluminium phthalocyanine (ClAlPc) with copper hexadecafluorophthalocyanine (F16CuPc) on graphite surface, as revealed by in situ low-temperature scanning tunneling microscopy experiments. The formation of multiple intermolecular C−H…F−C hydrogen bondings between the periphery F atoms on F16CuPc and H atoms on neighboring ClAlPc molecules can ensure structural rigidity and stability for potential applications in molecular nanodevices.

Chemical vapor deposition graphene as structural template to control interfacial molecular orientation of chloroaluminium phthalocyanine

Chemical vapor deposition (CVD) graphene has been used as an effective structural template to manipulate molecular orientation of organic thin film of chloroaluminium phthalocyanine (ClAlPc) on indium tin oxide (ITO) electrode. As revealed by in-situ near-edge x-ray adsorption fine structure measurement, ClAlPc molecules on the CVD graphene modified ITO electrode adopt a well-aligned lying-down configuration with their molecular π-plane nearly parallel to the electrode surface, in contrast to the random orientation of ClAlPc molecules on the bare ITO electrode. This lying-down configuration results in an optimized stacking of the molecular π-plane perpendicular to the electrode surface and hence facilitates efficient charge transport along this direction.

Controlling Molecular Packing for Charge Transport in Organic Thin Films

Tuning of the packing mode, including the film morphology, molecular orientation, and phase state of chloroaluminum phthalocyanine (ClAlPc), is demonstrated, and yields optimal parallel and vertical channels for charge transport. On a pure SiO2 surface, the ClAlPc molecules lie flat and grow in the Volmer–Weber mode, resulting in a cone-array structure that is highly desirable for heterojunction organic solar cells and organic field emission. On an OTS-modified surface, the ClAlPc molecules stand obliquely and Stranski–Krastanov growth of a continuous and flat film occurs, which is suitable for organic transistors.

High-resolution electron energy loss spectroscopy of chloroaluminum phthalocyanine ultrathin films

High-resolution electron energy loss spectra (HREELS) were measured for ultrathin films of chloroaluminum phthalocyanine (ClAlPc) deposited on a cleaved HOPG surface in order to investigate the molecular orientation and its thickness dependence. It was found that for the monolayer film ClAlPc molecules lie flat on the HOPG surface without heat treatment. With increase in the film thickness, the molecules become oriented at a tilt. The result show a marked difference from the orientation of ClAlPc molecules on MoS 2, where the molecules lie tilt on MoS 2 in the as-grown monolayer.