Nanostructured silver substrates with stable and universal sers properties: Application to organic molecules and semiconductor nanoparticles

Nanostructured silver films have been prepared by thermal deposition on silicon, and their properties as SERS substrates investigated. The optimal conditions of the post-growth annealing of the substrates were established. Atomic force microscopy study revealed that the silver films with relatively dense and homogeneous arrays of 60-80-nm high pyramidal nanoislands are the most efficient for SERS of both organic dye and inorganic nanoparticles analytes. The noticeable enhancement of the Raman signal from colloidal nanoparticles with the help of silver island films is reported for the first time. © 2009 The Author(s)

Hydrogen-induced sp2-sp3 rehybridization in epitaxial silicene

We report on the hydrogenation of (3×3)/(4×4) silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of (3×3)/(4×4) silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed sp2-sp3 to a dominating sp3 state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries. © 2017 American Physical Society

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

Surface Enhanced Raman Scattering of Light by ZnO Nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E-2(high) and A(1)(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 10(3)) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model

Density of occupied and unoccupied states monitored during metal deposition onto phthalocyanine layers

Phthalocyanine (Pc) materials find applications in electronic devices such as organic light emitting devices (OLEDs), solar cells, thin film transistors or gas sensors. The electronic properties at interfaces with different materials e.g. inorganic semiconductors or metals, usually dominate the device performance. Therefore the knowledge of the interface electronic properties is required. In the present work valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the density of occupied and unoccupied states upon silver deposition onto layers of two phthalocyanines (H$_{2}$Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already in the initial stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H$_{2}$Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e. the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB-PES spectra the H$_{2}$Pc and CuPc characteristic features are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge transfer and chemical reactions at the interfaces

In situ reflectance anisotropy spectroscopy monitoring of wide bandgap biomolecules on vicinal silicon surfaces

DNA base molecules, adenine, thymine, guanine, and cytosine may be employed as charge transport molecules in biomolecular electronic devices. Their electronic properties compete with those of inorganic wide bandgap materials, e.g. GaN, with the absorption onset in the near ultra-violet (UV) range. A recent field effect transistor study based on a modified DNA base revealed that the prototype bio-transistor gives rise to a better voltage gain compared to a carbon nanotube one (CNT) [1]. Reflectance Anisotropy Spectroscopy (RDS$/$RAS) measures the difference in reflection for normally incident light linearly polarized along two orthogonal directions in the sample surface as a function of photon energy. In situ RDS$/$RAS is employed under ultra-high vacuum (UHV) conditions for the first time for the characterization of DNA base molecules on vicinal hydrogen passivated Si(111) surfaces. Such vicinal substrates consisting of steps and terraces can provide a versatile template for molecular ordering. Indeed, the RDS$/$RAS measurements reveal information about molecular ordering of DNA bases induced by the density of steps on silicon surfaces. All four molecules behave differently on the vicinal substrates. The orientation of the transition dipole moments of the molecules with respect to the substrate directions can be evaluated from the RDS$/$RAS spectra. For adenine and thymine the transition dipole moments align mainly perpendicular to the step edge direction while for guanine and cytosine they align parallel to this direction, however, only in very thin layers. The RDS$/$RAS signal of the guanine and cytosine layers with thicknesses above 20 nm saturates due to the loss of ordering

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