Current trends in optical and X-ray metrology of advanced materials for nanoscale devices IV

Resume : Gas sensors are an important tool in various areas, for example in industrial process control as well as security applications or in research itself. In the present research, a noninvasive gas detection technique, based on the SPR effect with ellipsometric readout (Surface Plasmon Resonance Enhanced Ellipsometry-SPREE), is investigated. The gas sensors consist of a 40 nm SPR-active gold layer and a top-coated 5 nm doped SnOx layer. In the past, it could be shown that, without the top-coating, these type of sensors can detect various gases with sensitivities down to the ppm range (in air). The goal of the present study is to characterize additional top-coating materials (mainly SnOx) in dependence of the coating conditions. With the help of the doped-metal oxide, the sensitivity increases dramatically by a factor of 100. Additionally, a selectivity for specific gases (e.g. CO for Fe-doping) was observed which depends on the doping conditions of the coating. Changing the properties of the plasma coating process gives access to a big variety of different layers and enables us to find the best conditions for the determined gas in selectivity and sensitivity. The result of the sputtering progress is analyzed and characterized by using optical methods, mainly spectroscopic ellipsometry (SE), and X-Ray metrology, mainly XPS, to find a correlation between the doping concentration, the optical properties and the resulting sensing ability.

Resume : Spectroscopic ellipsometry is a widely used technique for thin film analysis because of its extreme sensitivity to surface and thin film features. On the technical side it is non-destructive, uses mostly visible light, and can be applied without the use of vacuum which makes it an ideal in situ quality testing tool. With modern thin-film electronics, the microscopic structure of industrially relevant thin films has drastically increased. To keep up with this development, the metrology of thin films with optical methods has to be improved, as the samples under investigation become more non-ideal. In this paper, we present results of a multi-laboratory study investigating difficult-to-analyse samples with goniospectral ellipsometry. The samples investigated are materials used in organic photovoltaics coated in thin layers with and without lateral structuring. The dielectric function as well as the film thickness is investigated by ellipsometric analysis, in the case of the structured samples including imaging of these properties across the sample with a resolution of several micrometres. We performed the analysis of these samples with a focus on traceability, accuracy, and relevance to measurement requirements of the thin layer photovoltaics production. This paper contains a report on this study evaluating imaged gradients in the sample properties and quantities difficult to determine like mixing ratios of material mixtures and residual solvent concentrations.

Resume : We probe the electronic structure and chemical environment of P doped silicon nanocrystal/silicon oxide multilayers by X-ray absorption fine structure spectroscopy of the P-K edge. The samples were prepared by plasma-enhanced chemical vapor deposition of P doped silicon rich oxide / silicon dioxide multilayers that were subjected to thermal annealing to form oxide embedded silicon nanocrystals. By analyzing the energetic position of the P-K absorption edge and comparing them with reference samples, we demonstrate that the majority of P atoms are found within the nanocrystals. However, a detailed comparison with DFT calculations suggests that these P atoms are rather located at interstitial lattice sites than on substitutional ones [1]. Furthermore, we find that P in silicon oxide leads to midgap electronic states that may be detrimental for the realization of optoelectronic devices. The implications of the results are discussed with respect to optical and electronic properties of doped silicon nanocrystal ensembles. [1] König et al., Nature Scientific Reports (accepted)

Resume : Doped TiO2 thin films have drawn much attention recently as novel transparent conducting oxide materials in view of replacing critical materials, as Indium for example. In this work, Nb or V doped multilayered TiO2 transparent films with a molar ratio TiO2:Nb2O5 and TiO2:V2O5 of 98:2 with 1-10 layers were obtained by sol-gel dipping on low-cost, microscope glass substrate. XPS analysis was used to confirm the doped nature of films in both cases. XRD analysis has shown that only anatase phase was obtained starting with the 2-layered film, regardless of the dopant used. Morphologically, SEM and AFM displayed continuous and homogenous films consisting of small, quasi-spherical particles and some shallow pores of less than 20 nm depth. Ellipsometry in the UV-Vis-NIR range revealed film thickness linearly increases with deposition number allowing easy tailoring of films’ properties for required applications. Transmission data showed a red-shift of the absorption edge with the number of layers for both types of doped films. The C-V and I-V characteristics of MIS structures formed by the films deposited on Si substrates were investigated in order to assess the carrier concentration and deep trapping levels.

Resume : Over the past two decades, a significant amount of progress has been achieved in the epitaxial growth of (multi-) functional oxide films. By applying epitaxial strain in thin films, the ferroic transition temperatures can be increased by hundreds of degrees, new phases can be induced, the coupling between different ferroic orders can be modified etc. However, due to the low film thickness and the often only subtle structural modifications, the structural characterization of (multi-) functional oxide thin films remains challenging. Following a short introduction to Raman scattering and its main characteristics, we will illustrate that Raman spectroscopy is a versatile technique for the characterization and understanding of such thin and ultrathin film oxides. Two case studies will be particularly discussed. (i) Probing strain of teh individual components in BaTiO3/LaNiO3/CeO2/YSZ thin film structures on silicon by wavelength-dependent Raman scattering [1]. (ii) Multiple strain-induced phase transitions in thin and ultrathin LaNiO3 films on different substrates. [1] J. Kreisel, M. C. Weber, N. Dix, F. Sánchez, P. A. Thomas, and J. Fontcuberta, Adv. Funct. Mat. 22, 5044 (2012).

Resume : Silicon-Germanium (SiGe) epitaxial nanostructures are well suited materials for integration into the silicon platform. Among them, nanowires (NWs) are increasingly attracting attention both from a fundamental point of view as well as from the multiple specialized applications that their development may empower in nanoelectronics, thermoelectrics, photovoltaics, energy storage, information and communication technologies. Epitaxially grown SiGe NWs on Si are particularly interesting for device integration and horizontal, in-plane geometries, the optimum choice for manufacturing using planar microfabrication technology. In this work, we employ confocal Raman microspectroscopy and imaging to study in-plane gold-seeded SiGe alloy nanowires grown by molecular beam epitaxy (MBE) on Si (001)-oriented wafers which are the usual substrates of standard complementary-metal-oxide-semiconductor (CMOS) technology. The spatial resolution of the Raman images allows us to study individual nanowires. We observe differences in Raman scattering intensity when the light polarization is parallel or perpendicular to the nanowire axis. These variations are correlated to the anisotropic absorption in thin nanowires. Quantitative analysis of the Raman spectra yields the composition and strain variations in the sample, in particular within each nanowire in which we resolve vertical gradients from the base to the top surfaces. These Raman results provide unique insights into the growth processes.

Resume : The effects of 7-MeV electron irradiation at two fluences, 1017 and 1018 electrons/cm2, on room-temperature photoluminescence from a 1.3-μm laser-like strain-compensated GaInNAs/GaNAs/GaAs quantum well structure were investigated. Electron irradiation at the lower fluence is found to cause a large blueshift of photoluminescence when annealed after irradiation. The line shift is assigned to the presence of complex defect centers in the quantum wells. The defects are very unstable. The peak position and line shape of the spectra are restored to their original values by an ageing effect, which occurs even at room temperature. No such behaviour of photoluminescence was observed for the sample irradiated at the higher fluence. The mechanism causing the blueshift/redshift of photoluminescence in the lower-fluence irradiated sample is qualitatively described.

Resume : The traditional measurement of the thermoelectric Seebeck coefficient gives a global value for a given material. In view of the combinatorial approach to discovery of new thermoelectric materials, it is highly desirable to have fast measurement techniques if possible with capabilities to access local fluctuations or gradients in material properties. The optical characterization of semiconducting materials by photoluminescence is the most convenient technique to access the quasi Fermi level splitting and the temperature of the carriers by fitting the spectra with the generalized Planck's law of radiation [1]. These two parameters are directly related to the Seebeck coefficient of the material [2]. We have developed a unique contactless optical measurement technique to determine Seebeck coefficients of materials by recording spectrally resolved photoluminescence images using an absolutely calibrated hyperspectral imager setup [3] with a spatial resolution down to a few micrometers. This contactless setup yields spatial gradients of quasi Fermi level splitting and temperature fluctuations, leading to the local Seebeck coefficient. This method was applied on a multi quantum well structure based on InGaAsP, and on a bulk chalcogenide AgInS2 sample. We will also show the perspectives offered for the research of new thermoelectric materials. [1]Würfel, J. Phys. C : Solid State Phys. (1982) [2]Tauc, Czech J Phys (1955) [3]Delamarre, Appl. Phys. Lett. (2012)

Resume : One-dimensional nitride heterostructures have demonstrated novel optical and electronic properties making use of quantum confinement effects and strain engineering. The development of disruptive functionalities is related to the growth and technology controls, but also to the availability of advanced high spatial resolution techniques. - Micro-Laue diffraction with focused polychromatic beam (5-22 keV, French BM32 beamline) is used to study epitaxial relationships of single wires on sapphire substrates, presence of single defects, relative orientation of core/branches in tripods and strain along core-shell InGaN/GaN multiple quantum well (MQWs). - X-ray coherent diffraction imaging (ESRF, ID01 beamline) is used to analyse the 3D structure of Inversion Domain Boundaries (IDB) inside n-doped gallium nitride core [1] in terms of lattice displacements along/across the wire length (measurement of several Bragg peaks and phase retrieval analysis [2]). - X-ray excited optical luminescence (XEOL, ESRF, ID16 beamline) is used to study GaN/InGaN MQWs grown on the m-plane sidewalls of c-axis GaN wires. The (polarized) local blue light emission can be also analysed in the spatiotemporal domain with 50 ps resolution to relate the electron confinement/local composition to the wire geometry. Extension of this technique will be shown on contacted wires [3] and on nitride planar photovoltaic devices. Granted by French ANR [1] MECANIX, [2] XDISPE [3] and GANEX projects.

Resume : Strain and texture analysis in thin films and nanostructures is becoming more and more important with the increasing complexity of microelectronic devices where complex epitaxies between fundamentally different materials come into play. X-ray diffraction has ever played an important role in the analysis of strain, texture and epitaxial relation. Traditionally x-ray diffraction is considered as a method with poor spatial resolution yielding only spatial averages as useful results. Very recent developments in the use of highly focused beams produced on the most advanced synchrotron sources show however a great and rapidly developing potential of diffraction imaging techniques. With the just accomplished upgrade of the ESRF beamlines, ID01 is the world leading facility specialized on nanodiffraction imaging of device like structures using scanning probe- and full-field techniques. Brought to maturity these techniques allow for strain and texture imaging in thin films with a spatial resolution 100 nm and below in range and a strain sensitivity of 1exp-5. With the capacity of imaging buried layers, the enormous gain in data recording speed (comparable to other scanning probe this strain imaging technique offers a highly promising method for the characterization of advanced materials. The talk will give an overview on recent results and an outlook on the potentials of this rapidly developing imaging techniques offered to the European material science community.

Resume : Graphene grown by sublimation of SiC holds great promise for large-scale production of next generation fast electronic devices. The key point to enable technological advances is to identify and control the substrate effects on graphene uniformity, thickness and electronic properties. In this work we demonstrate the use of spectroscopic ellipsometry and optical Hall effect techniques to shed light on the nature of graphene grown on SiC and to provide vital information for growth optimization. First, we discuss the mapping of the dielectric function and thickness of graphene on cubic 3C-SiC(111) substrates. Correlations between the number of graphene monolayers on one hand and the main transition associated with an exciton enhanced van Hove singularity at 4.5 eV and the free-charge carrier scattering time, on the other are established. We show that the interface structure on the Si- and C-polarity of the 3C-SiC(111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene. Second, we present results from mid-IR optical Hall effect measurements of graphene on cubic and hexagonal SiC substrates, which allow for Landau level spectroscopy. The polarization selection rules of Landau level transitions in decoupled and AB stacked graphene layers are determined. Different Fermi velocities are deduced for the stacks of decoupled graphene layers depending on the substrate polytypes, and the results are discussed in view of strain effects. The mobility parameters in the decoupled layers are determined and their dependence on magnetic field is studied in order to draw conclusions about the predominant scattering mechanism. Our results clearly show that graphene grown by high-temperature sublimation on C-face SiC consists of decoupled layers while AB stacked graphene can be associated with stacking defects whose density can be controlled by growth conditions, substrate polytype and surface status.

Resume : Fourier scatterometry measures the scattered light from a focused spot. The same microscope objective is used for illumination and measurement. At the back focal plane of the objective each point corresponds to a given scattering angle. Consequently, by imaging the back focal plane to a CCD, the scattered intensities can simultaneously and quickly be measured in a wide range of angles (azimuth angles of 0-360° and, using a numerical aperture of 0.9, reflection angles of 0-64°). Several methods have been demonstrated to involve the phase information in the measurement and to increase the sensitivity of Fourier scatterometry utilizing a scanning spot [N. Kumar et al., Optics Express 22 (2014) 24678] or interferometry [S. Roy et al., Journal of Optics 15 (2013) 075707]. Recently, we proposed an ellipsometric approach, revealing a significant gain in the sensitivity, and emphasizing numerous advantages like incoherent light source or the fact that there is no need to measure the input wavefront [P. Petrik et al., J. Europ. Opt. Soc. Rap. Public. (2015) accepted for publication]. In the present study, examples will be shown for the application of Fourier ellipsometry to periodic photonic structures.

Resume : Nanostructured superhydrophobic films [1], which are fabricated from spinodally phase-separated glass films, have some interesting and potentially useful characteristics. First of all, they are non-wetting, in that water balls up upon interaction with the surface. Secondly, the film surface is mechanically and environmentally robust, resisting scratches and abrasion. Thirdly, the superhydrophobic characteristic of the films allows them to self-clean. Furthermore, these surfaces have been shown to be very good anti-reflection coatings for glass based materials in that they form a naturally graded refractive index profile. [2] These films can easily be manufactured, offering significant potential for many optical applications. In this paper, we will discuss spectroscopic ellipsometry and other optical measurements on these superhydrophobic surfaces fabricated on glass. We will show that the nanostructured film is well-modeled with a graded surface, where the refractive index goes incrementally from the surface to the substrate. The refractive index of the central part of the film is significantly lower than MgF2 (~1.21 versus 1.38). These two factors reduce the optical impedance between air and the glass resulting in a significant reduction in the film/glass reflectivity. 1. T. Aytug, et. al, Nanotechnology 24, 315602 (2013) 2. T. Aytug, et. al., submitted to Advanced Energy Materials.

Resume : ZnO is a well-known n-type TCO material with many applications: lasers, LEDs, solar cells, sensors or photocatalysts. Lately p-type ZnO has been investigated for optoelectronic applications. Co-doping with a donor-acceptor pair, such as In-N, is reported to lead to stable, transparent p-ZnO and the use of chemical deposition methods has the advantages of low cost, large area coverage and easy control of the doping process. In the present study, In-N co-doped ZnO thin films were deposited on n-Si, p-Si, Al2O3 and soda-lime glass substrates by a two step process: an undoped ZnO “seed” layer was deposited by sol-gel method, followed by the hydrothermal deposition of the co-doped ZnO and annealing at 500oC for 1h. XPS analysis confirmed the co-doped nature of the films. SEM evidenced the presence of interconnected, randomly oriented, hexagonal nanorods (NR) with a width of 70-100 nm and a length of 300-400 nm function of the substrate used. Spectroellispometry determined a high transmission (~ 75%) and a low band gap value of 2.55 eV for the films deposited on glass substrate. Due to the interconnected nature of the NR in the film, Hall Effect measurements were possible and confirmed p-type character for films deposited on resistive substrates, with carrier concentration of ~1017 cm-3. The Hall measurements are discussed and compared with the results from C-V and I-V characteristics of MIS structures formed by the films deposited on Si substrates. Promising results were obtained from CO sensing tests, which need further investigation.

Resume : The heterogeneous epitaxy of III-V compounds onto Si substrate has been widely studied for decades in the context of low cost monolithic integration of III-V photonics and PVs on silicon. However, most III-V semiconductors on Si lead to a very high dislocation density (due to a large lattice mismatch). An alternative solution has been proposed with GaP quasi-lattice matched growth on Si substrate to realize III-V/Si dislocation-free pseudosubstrates. However, Antiphase boundaries (APB) and microtwins (MT) are still difficult to avoid. In this study, thorough XRD analyses (combining both synchrotron and lab set-up X-ray) of GaP nanolayers on Si substrates are presented to quantitatively evaluate both defects generated at the GaP/Si interface. Lateral correlation length associated with mean APB distances and absolute quantification of MT fractional volumes are used for optimisation of the growth procedure in order to limit the defect density. Complementary AFM and TEM&STEM technics are employed to give a complete structural characterization. Most recent optimized growth conditions lead to a quasi elimination of microtwins and a APB annihilation within the first 10 nm. Finally, ab-initio simulation is on the way in order to comprehend the mechanism of defects generation at the GaP/Si interface. Support: ANR projects MENHIRS 2011-PRGE-007-01, OPTOSI 12-BS03-0002 , ANTIPODE 14-CE26-0014-01

Resume : Sn4Sb6S13 thin films have been deposited by single source vacuum thermal evaporation onto glass substrates heated at various substrate temperatures in the range 30-200 °C. Some of structural, optical and morphological characteristics have been studied. The structural investigation performed by means of X-ray diffraction confirmed that the films crystallized with the preferential orientation plane. The optical constants were obtained from the analysis of the experimental recorded transmission and reflectance spectral data over the wavelength range 300-1800 nm. The absorption coefficient is larger than 105 cm-1 in the photon energy range of 1.5-2.5 eV. We found that by increasing the substrate temperature from 30 to 200 °C, the direct optical band gap decreased from 2.03 to 1.69 eV. The dispersion of the refractive index, electric free carrier susceptibility and the carrier concentration on the effective mass ratio were explained in terms of the single oscillator Wemple-DiDomenico and Cauchy models, as well as using the model of Spitzer-Fan.

Resume : TXRF spectrometry is characterized by very low detection limits for pollutions analysis of studied materials in near-surface layer with thickness 3-5 nm and absence of the matrix effect influence. TXRF inverse method (grazing-exit X-ray spectrometry) demonstrates low detection limits too, but it requires a matrix correction [1]. At the same time both TXRF methods are characterized by an reduced sensitivity at light elements diagnostics. These elements diagnostics improvement can be achieved by change the exciting agent from X-ray to ion beams [2]. The work describes the schematic solution using the waveguide-resonator cell for TXRF spectrometry at ion beam excitation of X-ray fluorescence. There is presented the direct experimental proof of X-ray fluorescence yield from near surface layers in the modified PIXE measurements and executed TXRF spectra comparison at X-ray and ion beam excitation of the same target. We showed by experimentally that the ion beam excitation of light elements X-ray fluorescence is more effective in comparison with X-ray one for thin surface layer of materials. [1] X-ray spectrometry: Recent technological advances / Eds. by K. Tsuji, J. Injuk, R. Van Grieken. Wiley: Chichester. 2004. 603 p. [2] Particle-induced X-ray emission spectrometry / Eds. By S.A.E. Johansson, J.L. Campbell, K.G. Malmquist. Wiley: Chichester. 1995. 451 p.

Resume : We present in this work a new beamline of Synchrotron SOLEIL dedicated to the study of thin films, nanostructures, and advanced materials via x-ray diffraction and spectroscopy in the energy range 1.4–12 keV. This range covers most of the absorption edges of interest in the field of semiconductors and functional oxides. In order to meet the increasing demand of advanced real-time characterization of nanoscale materials, the SIRIUS beamline has been designed with remarkable dynamic characteristics. Mounted on a high-flux undularor source, it uses a fast monochromator featuring the new “direct drive” technology and a very fast and precise 7-circle diffractometer. SIRIUS hosts two end-stations used for in situ characterization. The first is a small chamber, mounted on the kappa head of the diffractometer and equipped with motors for surface alignment, an oven and gas inlets. This chamber allows us to perform GI-XRD at energy ≥ 4 keV, and XAFS/XRR down to 1.4 keV. The second end-station is an ALD/MOCVD reactor dedicated to in situ studies of oxides. This reactor is mounted on a six-axis tower, which substitutes the kappa head of the diffractometer. The station allows us to perform GI-XRD, GI-XAFS, XRR and GI-SAXS, and has been recently used to monitor the incipient growth of ZnO thin films. A third station planned for the future is an in-vacuum diffractometer for resonant studies at energies ≤ 4 keV. We show here selected examples of the first in situ results obtained at SIRIUS.

Resume : There is a current interest in producing micro/nano structures on surfaces in order to enhance their optical properties or to create new ones which can mimic nature due to their high potential for several applications such as catalysis, photovoltaics or sensing. Most of the commercial instrumentation for optically characterizing these structures focuses on the spectral capabilities rather than on the spatial ones, while the ones having spatial resolution involve imaging for which the incorporation of spectral information is generally limited to colour ranges or otherwise need heavy data processing. In this work, we present a low cost 1D spatial resolution spectroscopic technique that is easy-to-use and has the potential to be extended for 2D analysis by sequential acquisition. The technique is based on hybrid images (1D spectral /1D spatial) acquired with a camera connected to a spectrograph. As a proof of concept, we have analysed fringed patterns produced by laser interference in metal films. The patterns have periods in the range 6.7-1.7 um and the optical response becomes modulated as one moves across the pattern since the fringes have different plasmonic response. The results are compared to the ones obtained with commercial spectrophotometers without spatial resolution and correlated to the structural properties through scanning electron and optical microscopy images. The spatial and spectral resolutions as well as ranges of use finally are discussed.

Resume : Amorphous In-Ga-Zn oxide (IGZO) is successfully used as channel semiconductor in transparent field effect transistors (TFET) [1]. The electrical properties depend on thin film density, cation stoichiometry and oxygen concentration. X-ray reflectivity (XRR) is the only technique which provides direct information about thin film density, thickness and roughness. The working parameters (field effect mobility, Ion/Ioff, Von) of IGZO-based TFET and their electrical stability are significantly improved after soft thermal treatment of the devices [2]. Although IGZO thin films remain amorphous after typical annealing procedure, variations of their electrical conductivity are recorded. XRR measurements and analyses are performed to examine the influence of the thermal treatment on the in-depth uniformity, the interface roughness and the density of the IGZO films. The XRR results are discussed with respect to the electrical characteristics of the TFET devices. References: 1. Transparent Oxide Electronics: From Materials to Devices, Rodrigo Martins, Elvira Fortunato, Pedro Barquinha, Luis Pereira, ed. John Wiley & Sons, 2012. 2. Y.J. Tak, D.H. Yoon, S. Yoon, U.H. Choi, M.M. Sabri, B.D. Ahn, H.J. Kim, Enhanced Electrical Characteristics and Stability via Simultaneous Ultraviolet and Thermal Treatment of Passivated Amorphous In−Ga−Zn−O Thin-Film Transistors, ACS Appl. Mater. Interfaces 6 (2014) 6399-6405.

Resume : The scattering in the light emission wavelength of semiconductor nano-emitters assigned to nanoscale variations in strain, thickness, and composition is critical in current and novel nanotechnologies from high efficient light sources to photovoltaics. Here, we present a correlated experimental and theoretical study of single nanorod light emitting diodes (nano-LEDs) based on InGaN/GaN multi quantum wells to separate the contributions of these intrinsic fluctuations. Cathodoluminescence measurements show that nano-LEDs with identical strain states probed by non-resonant micro-Raman spectroscopy can radiate light at different wavelengths. It was possible to differentiate between the Raman signals originating from a single, diffraction-limited nanorod LED and the supporting substrate based on a distinct two-peak structure for the E2(high) phonon mode of GaN. The deviations in the measured optical transitions agree very well with band profile calculations for quantum well thicknesses of 2.07 – 2.72 nm and In fractions of 17.5 - 19.5 % tightly enclosing the growth values. The nanorod surface roughness controls the appearance of surface optical phonon modes with direct implications on the design of phonon assisted nano-LED devices. This work establishes a new, simple, and powerful approach for fundamental understanding as well as quantitative analysis of the strain - light emission relationship and surface-related phenomena in the emerging field of nano-emitters.

Resume : Upon addition of perylene bisimide (PBI2+) to ruthenium polyoxometalate (RuPOM) in aqueous solution a spontaneous self-assembly into a nano-scaffold occurs. In both TEM and SEM images of the hybrid material various morphologies were observed from which no detailed information on the intermolecular structure could be gained. Subsequent solution SAXS measurements reveal the formation of plate-like superstructures in the nanometer regime where a disordered lateral arrangement of the molecules is suggested. The detailed formation under various conditions will be shown point out a link between the precursors’ structure and the final hybrid material aggregate. AFM measurements confirmed the predicted nano-plate morphology as building blocks with potential application as photocatalysts

Resume : Tip Enhanced Raman Scattering (TERS) has been a subject of great scientific interest for 15 years. We previously showed TERS experimental studies of strain measurement in semiconducting nano-stripes [1], of carbon-based nanomaterials [2] and organic molecules imaging [3]. But regardless of these recent achievements, the application of TERS as an analytical method has been hampered by extremely long acquisition times, measured in hours, required for collection of reasonably high pixel density TERS maps.We present here a stimulated TERS technique that offers the possibility for a substantially faster imaging of the surface with respect to normal TERS. With this stimulation, a billion-fold increase in the Raman signal over conventional tip-enhanced Raman spectroscopy/microscopy is shown. It is achieved by introducing a stimulating beam confocal with the pump beam into a conventional TERS setup. A stimulated TERS spectrum, closely corresponding to its spontaneous TERS counterpart, is obtained by plotting the signal intensity of the strongest Raman peak of an azobenzene thiol self-assembled monolayer versus the stimulating laser frequency. The stimulated TERS image of azobenzene thiol molecules grafted onto Au ⟨111⟩ clearly shows the surface distribution of the molecules, whereas, when compared to the simultaneously recorded surface topography, it presents an image contrast of different nature. The experimentally obtained stimulated gain is estimated at 1.0 × 109, which is in reasonable agreement with the theoretically predicted value. In addition to the signal increase, the signal-to-noise ratio was 3 orders of magnitude higher than in conventional spontaneous TERS [4].[1] G. M. Vanacore, M Chaigneau, et al, Physical Review B 88 (11), 115309 (2013).[2] G. Picardi, M. Chaigneau, R. Ossikovski, Chemical Physics Letters 469 (1), 161-165 (2009).[3] G. Picardi, M. Chaigneau, R. Ossikovski, C. Licitra, G. Delapierre, Journal of Raman Spectroscopy 40 (10), 1407-1412 (2009); M. Chaigneau, G. Picardi, R. Ossikovski, International journal of molecular sciences 12 (2), 1245-1258 (2011).[4] H. K. Wickramasinghe, M. Chaigneau, R. Yasukuni, G. Picardi, R. Ossikovski, ACS Nano, 8 (4), 3421–3426 (2014).

Resume : Organic solar cells are being extensively studied owing to the potential for low cost production and high mechanical robustness. Particularly, bulk heterojunction solar cells, consisting of conjugated polymers and fullerene derivatives, receive increasing attention due to their relative ease of fabrication and high power conversion efficiency, recently surpassing 10%. In the present work we study a blend of the semiconducting polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) with the fullerene derivative [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM). The blends were spin-coated on thermally oxidized Si substrates that were coated with thermally evaporated MoO3. The absorption coefficient in the visible spectral range, an indicator of photocurrent generation in organic solar cells, is analyzed in the current study using spectroscopic ellipsometry in the visible and near IR spectral range. The influence of the blend composition on the optical properties of the mixed films is examined. We present the results obtained for blends with the concentration of PC70BM in the blend ranging from 50 to 95 wt.%. The thickness of the studied layers, the concentration of the two constituents, and the dielectric functions were determined by employing a BEMA (Bruggeman effective medium approximation) model. Surface topography from atomic force microscopy was used as input for the analysis of the investigated layers.

Resume : Atomic layer deposition (ALD) processes for metals are often characterized by a nucleation controlled growth mode. Islands are initially formed that coalesce in a continuous film after a sufficient number of ALD cycles. This work demonstrates the unique suitability of synchrotron-based grazing incidence small angle x-ray scattering (GISAXS) for in situ monitoring of the nanoscale surface morphology during ALD of Pt. GISAXS allows to quantify the evolution in size, shape and spacing of the Pt nuclei. Here, we have studied the influence of the reactant type (O2 gas, O2 plasma, N2 plasma) on the nucleation of Pt ALD using the MeCpPtMe3 precursor. GISAXS indicated that, for the O2-based processes, the spacing between the particles increases with progressing Pt deposition, suggesting that atom and cluster surface diffusion phenomena play a major role during particle nucleation and coalescence. On the other hand, for the N2 plasma process, the average particle distance increases during the initial nucleation phase, but remains constant during the particle growth process. The particle dimension analysis revealed that the N2 plasma process results in elongated particles, while the O2-based processes give rise to flattened particles. Based on this analysis, it is concluded that O2 induces atom and cluster surface diffusion and promotes the ripening of the Pt nanoparticles, while diffusion phenomena seem to be suppressed during N2 plasma based ALD leading to smoother Pt coatings.