Serkan Zorba

Thermodynamic equilibrium and metal-organic interface dipole

We determined the interface dipoles at a number of metal-organic interfaces using ultraviolet and x-ray photoelectron spectroscopy. A linear dependence of the dipole on the metal work function is observed. This is consistent with the theory based on the charge transfer and thermodynamic equilibrium across the interface. The agreement suggests that charge transfer is one major factor in the formation of interface dipole. In addition, we find that the pushing back of the electron cloud tail that extends out of the metal surface and the permanent dipole moment of the organic molecule affect the interface dipole.

Direct observation of Fermi-level pinning in Cs-doped CuPc film

The electronic structures of pristine and Cs-doped CuPc films are investigated using photoemission spectroscopy and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital can be directly observed by IPES and ultraviolet photoemission spectroscopy simultaneously. We found that the Fermi-level position in organic film can be modified by Cs doping. The observed onset of the LUMO of the CuPc film is shifted by Cs doping to less than 0.2 eV above the Fermi level. The result indicates that the energy alignment and charge injection properties of the organic materials can be modified by a simple doping process.

Magnetically Actuated Liquid Crystals

Ferrimagnetic inorganic nanorods have been used as building blocks to construct liquid crystals with optical properties that can be instantly and reversibly controlled by manipulating the nanorod orientation using considerably weak external magnetic fields (1 mT). Under an alternating magnetic field, they exhibit an optical switching frequency above 100 Hz, which is comparable to the performance of commercial liquid crystals based on electrical switching. By combining magnetic alignment and lithography processes, it is also possible to create patterns of different polarizations in a thin composite film and control over the transmittance of light in particular areas. Developing such magnetically responsive liquid crystals opens the door toward various applications, which may benefit from the instantaneous and contactless nature of magnetic manipulation.

Self-assembly of superparamagnetic magnetite particles into peapod-like structures and their application in optical modulation

Superparamagnetic Fe3O4/SiO2/TiO2 peapod-like nanostructures have been successfully synthesized by using Fe3O4/SiO2 core/shell particles as building blocks and TiO2 as the adhesive without the need of any hard or soft templates. The fabrication process involves chaining the Fe3O4/SiO2 cores during magnetic stirring and subsequent fixing of the chain structure during TiO2 coating. The number of Fe3O4/SiO2 cores arranged linearly in the chains could be effectively controlled by tuning the amount of titanium precursor or the magnetic stirring rate. The double layer coating of SiO2 and TiO2 enhances thermal and chemical stability of the nanopeapods, and the one-dimensional chain structure produces interesting properties that enable applications not possible with conventional magnetite materials. As a demonstration, we show here the use of these superparamagnetic peapod-like nanostructures for low-frequency optical modulation.

Feasibility of static induction transistor with organic semiconductors

We have fabricated and studied static induction transistors (SITs) with various organic semiconductor materials, such as pentacene, perylene, tris(quinoline-8-hydroxylate)aluminum (Alq), and N,N′-di(4-methylphenyl)-N,N′-diphenylbenzidine (TPD), used as the active element. The former two resulted in unsuccessful operation due to a short caused by pinholes formed in the films. Modification of the deposition rates did not change the outcome. The latter two provided successful operation. No fatal pinholes were observed in the thin films of the latter two. It is thus shown that the morphology has crucial effect in the operation of SITs. A transition from linear to nonlinear behavior has been observed in the drain-source I–V characteristics of Alq and TPD.

Magnetic Tuning of Plasmonic Excitation of Gold Nanorods

By using gold nanorods as an example, we report the dynamic and reversible tuning of the plasmonic property of anisotropically shaped colloidal metal nanostructures by controlling their orientation using external magnetic fields. The magnetic orientational control enables instant and selective excitation of the plasmon modes of AuNRs through the manipulation of the field direction relative to the directions of incidence and polarization of light.

Fractal-mound growth of pentacene thin films

The growth mechanism of pentacene film formation on

Interface formation of pentacene on Al2O3

\mathrm{Si}{\mathrm{O}}_{2}

Kinetic roughening study of perylene on glass and Au substrates

substrate was investigated with a combination of atomic force microscopy measurements and numerical modeling. In addition to the diffusion-limited aggregation (DLA) that has already been shown to govern the growth of the ordered pentacene thin films, it is shown here that the Schwoebel barrier effect steps in and disrupts the desired epitaxial growth for the subsequent layers, leading to mound growth. The terraces of the growing mounds have a fractal dimension of 1.6, indicating a lateral DLA shape. This growth morphology thus combines horizontal DLA-like growth with vertical mound growth.

Photoemission characterization of interfaces between Au and pentacene

We examined the interface formed by pentacene deposition onto an Al2O3 substrate. We found that upon pentacene deposition onto Al2O3 the pentacene vacuum level aligns with that of Al2O3. We observe the immediate appearance of a measurable pentacene molecular orbital near the Fermi level upon deposition of as little as 2 A of pentacene onto the Al2O3 surface. This suggests that there are no chemical bonds at this interface. The vacuum level change at the interface dipole is found to be less than 0.25 eV, and at least part of it can be attributed to the oxygen adsorbates at the in situ prepared aluminum oxide surface.