2D vibrational properties of epitaxial silicene on Ag(111)

The two-dimensional silicon allotrope, silicene, could spur the development of new and original concepts in Si-based nanotechnology. Up to now silicene can only be epitaxially synthesized on a supporting substrate such as Ag(111). Even though the structural and electronic properties of these epitaxial silicene layers have been intensively studied, very little is known about its vibrational characteristics. Here, we present a detailed study of epitaxial silicene on Ag(111) using in situ Raman spectroscopy, which is one of the most extensively employed experimental techniques to characterize 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous. The vibrational fingerprint of epitaxial silicene, in contrast to all previous interpretations, is characterized by three distinct phonon modes with A and E symmetries. Both, energies and symmetries of theses modes are confirmed by ab initio theory calculations. The temperature dependent spectral evolution of these modes demonstrates unique thermal properties of epitaxial silicene and a significant electron-phonon coupling. These results unambiguously support the purely two-dimensional character of epitaxial silicene up to about 300°C, whereupon a 2D-to-3D phase transition takes place. The detailed fingerprint of epitaxial silicene will allow us to identify it in different environments or to study its modifications. © 2016 IOP Publishing Ltd

Optical properties of the interfaces in organic/organic multilayered heterostructures

The optical response of the organic superstructures consisting of alternative layers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)$/$copper phthalocyanine (CuPc) and tris-(8-hydroxyquinoline)-aluminum(III) (Alq$_{3})/$N,N'-Di-[(1-naphthyl)-N,N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine ($\alpha$-NPD) is compared. The optical response of the multilayer is influenced by the optical properties of each individual layer and also by optical interferences. As a result the optical properties of the superstructure can be tuned within a large range using different layer thicknesses and different periods. The superstructures were prepared by organic molecular beam deposition (OMBD) in high vacuum (HV) on hydrogen passivated, (111) oriented silicon. The substrates were kept at room temperature during the deposition. The optical response of the superstructure was investigated by means of spectroscopic ellipsometry in spectral range of 0.73–5 eV. While for PTCDA and CuPc the molecular interaction gives rise to strong anisotropic crystalline layers the amorphous growth of Alq$_{3}$ and $\alpha$-NPD results in isotropic layers. Taking into account the dielectric function of the single layers the optical response of the Alq$_{3}/\alpha$-NPD superstructure can be modeled assuming sharp optical interfaces. However, for the PTCDA/CuPc superstructure the optical response requires a more sophisticated approach than simply superimposing the responses of the individual layers. The deviation between simulated and experimental data is assigned to the electronic interaction at the interfaces between the $\pi$ orbitals of PCDA and CuPc

Temperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes

Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated

New insights into colloidal gold flakes structural investigation, micro ellipsometry and thinning procedure towards ultrathin monocrystalline layers

High quality fabrication of plasmonic devices often relies on wet chemically grown ultraflat, presumably single crystalline gold flakes due to their superior materials properties. However, important details about their intrinsic structure are not well understood yet. In this study, we present a new synthesis routine for large flakes with diameters of up to 100 m and an in depth investigation of their structural and optical properties. The flakes were precisely analyzed by transmission electron microscopy, electron backscatter diffraction and micro ellipsometry. We found new evidence for the existence of multiple planar twin defects inside the flakes parallel to their surface in contrast to earlier studies. Micro Ellipsometry was carried out to determine the complex dielectric function and to compare it to previous measurements of bulk single crystalline gold. Finally, we used focused ion beam milling to prepare smooth crystalline layers and high quality nanostructures with desired thicknessdown to 10 nm. Our findings support the plasmonics and nano optics, community with a better understanding of this material which is ideally suited for superior plasmonic nano structures

Effect of a plasma polymerised linalyl acetate dielectric on the optical and morphological properties of an n-type organic semiconductor

Thin films of the n-type, organic semiconductor PDI-8CN2 were thermally evaporated on two different dielectric surfaces and their optical and morphological properties investigated using Variable Angle Spectroscopic Ellipsometry (VASE) and Atomic Force Microscopy (AFM), respectively. The two dielectric surfaces used were SiO2 and a plasma polymer derived from the non-synthetic monomer linalyl acetate. The characterisations were performed in order to assess the viability of plasma polymerised linalyl acetate (PLA) thin films as dielectric layers in future Organic Field-Effect Transistor (OFET) devices. These studies resulted in determination of the optical profiles (refractive index and extinction coefficient) in the UV-Vis band of PDI-8CN2 grown on SiO2 and an observation of uniaxial anisotropy in the organic semiconductor. This information is useful for the design of opto-electronic devices using PDI-8CN2 layers. Variations in morphological properties and small variations optical properties were found when the PDI-8CN2 films were grown on PLA layers, and attributed to the change in surface chemistry between dielectrics