Organized nanostructures and nano-objects: fabrication, characterization and applications

Resume : The continuous advance in technology predicted by Moore's law can be mainly attributed to the excellent scalability of the metal oxide semiconductor field effect transistor. However, down-scaling faces an insurmountable limitation when characteristic lengths reach the sub-10 nm regime. The increased leakage at such a scale and the fundamental thermal limit of the subthreshold swing encourage research on new device geometries and transport mechanisms. Non-planar geometries like silicon nanowires are potential successors to conventional planar structures as they feature increased electrostatic control and further potential of miniaturisation as they are able to supress short channel effects. Alternative transport mechanisms like band-to-band tunnelling or impact ionisation enable faster switching and decreased leakage. Similarly, Schottky barrier tunnelling is expected to beneficially affect the thermal dependency of the subthreshold swing. The combined capabilities of both a non-planar design and non-conventional carrier injection mechanisms are subject to recent scientific investigations. In the scope of this work, experiments with silicon nanowires that feature a significant axial n-type/intrinsic doping junction have been conducted. The nanowires with diameters ranging from 60 to 120 nm were synthesised via the vapour-liquid-solid method using gold colloids, and the doping profile was obtained via in-situ doping with phosphine. The nanowires were removed from their growth substrate and drop-casted onto a silicon substrate with a 100 nm layer of aluminium oxide. A heterostructural device design was achieved by employing nickel-silicide as a source contact material at the intrinsic side of the nanowire. This allows for the fabrication of a self-aligned Schottky contact after the evaporation of nickel in an annealing step at a low thermal budget of 510°C. The one-dimensional nature of the silicide contact is markedly beneficial concerning the electrostatic control of the intrinsic channel close to the contact. An ohmic contact was fabricated at the highly doped drain side also by evaporating nickel. In a subsequent step the nanowires were passivated with a 10 nm layer of aluminium oxide. A reconfigurable device is shown which allows to control whether the device acts as p- or n-type transistor. The functionality of the device is based on a dual-gate structure namely the contact-gate just in the vicinity of the silicide-to-silicon junction, and the channel-gate which overlaps the contact-gate and the doping junction. With this approach not only the type but also the carrier injection mode can be altered by applying a suitable voltage at the two gate terminals or by inverting the drain bias. With a combined band-to-band and Schottky tunnelling mechanism, in p-type mode a subthreshold swing as low as 143 mV/dec and an ON/OFF ratio of up to 10^4 is found. As the device operates in forward bias, a non-conventional tunnelling transistor is identified, enabling an effective suppression of ambipolarity. Depending on the drain bias, two different n-type configurations are distinguishable. On the one hand, the carrier injection is dominated by thermionic emission in forward bias. The resulting device operates as a conventional nanowire transistor and exhibits a subthreshold swing of 216 mV/dec at the highest ON/OFF ratio of up to 10^7. On the other hand, in reverse bias a Schottky tunnelling mechanism allows for a normally-off n-type characteristic with a moderate subthreshold swing of 813 mV/dec at an ON/OFF ratio of up to 10^4.

Resume : Tunnel Field-Effect Transistor (TFET) offers the potential to overcome the subthreshold swing (SS) thermal limit of 60mV/dec at 300K inherent to metal-oxide-semiconductor field-effect transistors (MOSFET), since TFET conduction is governed by band-to-band tunnelling (BTBT) rather than thermionic injection. Moreover, the use of nanowire as channel of transistor is an adequate technological solution enabling a high integrated device density and a good electrostatic control [1]. In addition, heterostructure TFET with small band gap material in the source region, such as SiGe [2], is one of the most promising candidates, with theoretically expected on-state current and subthreshold swing improved as compared to Si homostructure TFET. In this context, we have integrated Si/Si/SiGe heterostructure nanowires with an in situ p(Si)-i(Si)-n(SiGe) doping profile on TFET device. These nanowires were elaborated by Chemical-Vapor-Deposition using Vapor-Liquid-Solid mechanism with gold as catalyst. Thanks to the reduction of the band gap of the source material, device optimisation resulted in increased band-to-band tunnelling with an ION current of about 3 µA/µm, an ION/IOFF ratio up to 105 and an average SS less than 135mV/decade. Electrical measurements as a function of temperature have also been performed and will be presented. [1] A.M. Ionescu et al., Nature, vol. 479, pp.329-337, (2011). [2] A.S.Verhulst et al., Appl. Phys. Lett. 104, 064514, (2008).

Resume : Doping Si nanostructures by using standard methods presents some critical issues like conformality, control on dopant distribution and reduced structural damage. An effective method to dope nano-patterned Si and create controlled and conformal junction profiles has been recently proposed in literature [Nature Mater. 7, 62 (2008), Mat. Sci. & Eng. B 178 (2013) 686 10.1016/j.mseb.2012.11.019, Phys. Stat. Sol. A 210(8) (2013) 1564 10.1002/pssa.201200949]. It consists in forming a layer of dopant containing molecules by immersion of the samples in a chemical bath and successive deposition of a cap layer and annealing, during which the dopant atom is released by the molecule and diffuses into the Si substrate or in the nanostructures. Here we present a systematic study of this doping technique by varying the important processing parameters, and demonstrate that the method allows for the precise control of the crucial junction's features (i.e. dopant dose and junction's depth) without structural damage. We applied the doping procedure to arrays of nanopores obtained by diblock-copolymer lithography. Due to their optical absorption properties the nanopores provide a light trapping effect on the surface of the solar cell. The doped layers are used as active materials in solar cells prototypes based on the nanostructured Si. The results show improved photoconversion performance respect to the reference flat diodes and pave the way to create efficient nanostructured cells.

Resume : A lack of consensus persists regarding the electronic impurity doping of ultrasmall Silicon nanocrystals in the range from 2 to 5 nm in a dielectric host matrix and the efficiency to provide free electrons [1,2]. Here we apply complementary 3D atomprobe [3] and synchrotron X-ray absorption spectroscopy to study phosphorus (P) incorporation into silicon nanocrystals embedded in SiO2. It is demonstrated that a large amount of P is trapped at the Si/SiO2 interface, although also significant amounts of P are proven to be incorporated into the nanocrystals. However, optical and electrical measurements indicate that the vast majority of these introduced P atoms is inactive suggesting the absence of significant amounts of substitutional P sites. In addition the dopant activation energy is found to be too high to be ionized at room temperature. Contrasting numerous literature reports, it is concluded that P does not appear to be a suitable dopant for Si nanocrystals. [1] Stegner et al., Phys. Rev. B 80, 165326 (2009) [2] Gutsch et al., Appl. Phys. Lett. 100, 233115 (2012) [3] Gnaser et al., J. Appl. Phys. 115, 034304 (2014)

Resume : A monosilane (SiH4) and oxygen (O2) based plasma-enhanced chemical vapor deposition process (PECVD) for the growth of silicon-rich oxide / silicon dioxide superlattices [1] was developed. In contrast to the conventional nitrous oxide (N2O) based oxynitride-process [2], we achieved thereby PECVD-grown size-controlled silicon nanocrystals in pure, N-free silicon dioxide matrix. We present a detailed study based on optical (PL) and electrical (C-V, I-V) measurements that reveal the different properties of nominally identical Si nanocrystals in oxynitride and N-free oxide matrix. Most strikingly we find negligible differences in the optical performance (PL quantum yield), whereas substantial differences in the current transport and transient charging behaviour persist. The role of the pure oxide vs. oxinitride matrix on the properties of Si quantum dots is discussed in the context of potential applications in photovoltaics and optoelectronics. [1] M. Zacharias et al., APL 80, 2002 [2] A.M. Hartel et al., TSF 520, 2011 [3] S. Gutsch et al., JAP 113, 2013

Resume : We present a systematic study of the photoluminescence (PL) mechanisms of size controlled Si nanocrystals (Si NCs) embedded in silicon nitride matrix (Si3N4) [1]. In contrast to numerous papers we found manifold evidence that the origin of PL cannot be related to quantum confined excitons in Si quantum dots. The true PL mechanism is discussed in the context of band tail states and radiative defects. Our argumentation is based on the following observations: - Si3N4 reference samples show similar PL emission as Si NC samples; - the PL decay is 10^5 times faster than normal for Si NCs [2]; - the PL peak shift with NC size is identified as optical interference artifact via PL transfer matrix simulations considering Fabry-Pérot resonances [3]; - PL-excitation measurements prove an identical excitation mechanism irrespective of the presence of Si NCs. Using ESR data of Si/Si3N4 interface defect densities, we calculate infinitesimal probabilities to find defect free and luminescent Si NCs. Also hole trapping and PL quenching Si3N4-VB-tail states are discussed. In terms of application (Si NC based all-Si tandem solar cell [4]), Si3N4 was formerly favored due to its band offsets compared to SiO2 and SiC. However, our results suggest that it has to be rather classified as “subprime” matrix material. [1] Hiller et al., JAP 115, 204301 (2014) [2] Hartel et al., PRB 87, 035428 (2013) [3] Dyakov et al., APL 100, 061908 (2012) [4] Green, Third Generation Photovoltaics (Springer, 2003)

Resume : Noble metal nanoclusters are confined by grazing angle Au deposition on rippled glass templates and polymer nanosphere(200-400nm diameter) Photonic Crystals. The nanosphere templates are prepared by soft lithography, while the rippled glasses by a self-organised approach exploiting Ion Beam Sputtering(IBS).Metal deposition on the ripples titled facets allows the confinement of highly anisotropic 1D arrays of metal nanoclusters, characterised by a strong optical dichroism. At low metal doses, as a consequence of Localised Plasmon Resonances (LSPRs) excitation, well-isolated nanoclusters exhibit dichroic optical properties both in the linear and non-linear regime[1]. Increasing the metal dose deposited on rippled templates we are able to form arrays of connected Nanowires which are endowed with elevated electrical conductivity and high optical transparency. Such properties open the possibility to exploit these samples as semi-transparent plasmonic nanoelectrodes, in analogy to the Au nanowire arrays prepared by direct IBS of a polycrystalline metallic film[2,3]. Instead, on the closed-packed nanosphere layer, Au evaporation leads to ordered 2D arrays of isolated nanostructures, shaped as half-moon crescents[4] and characterized by anisotropic optical response due to the excitation of LSPRs[5]. [1]A.Belardini et al. Phys.Rev.Lett, 2011 [2]D.Chiappe et al. Small, 2013 [3]B.Fazio et al. ACSNano 2011 [4]V.Robbiano et al. Adv.Opt.Mater. 2013 [5]A.Belardini et al. Adv.Opt.Mater. 2014

Resume : III-V semiconductor nanowires or pillars have attracted considerable attention due to their outstanding properties such as large surface-to-volume ratio, tunable band gap, high carrier mobility, carrier confinement and enhanced elastic relaxation of misfit strains in heterostructures. Thus, they have a great potential for realizing advanced electronic and optoelectronic devices e.g. high-performance field-effect transistors, light-emitting diodes, photovoltaic cells [1] or sensors. Owing to its direct bandgap GaAs represents an important III-V semiconductor for optoelectronic applications. In the present contribution we analyze the structure and surface chemistry of ultra-thin (diameter in the range of 20 to 50 nm) vertical-sidewall GaAs (111) nanopillars. As demonstrated in a previous paper the pillars are fabricated by deposition of nanosphere lithographically defined Ni mask particles followed by selective SiCl4 reactive ion etching of the GaAs substrate [2]. We use transmission electron microscopy (TEM) imaging to reveal the crystalline structure of the pillars, and analytic TEM to identify conditions allowing for a complete mask removal by hydrofluoric wet chemical etching. References [1] J.A. Czaban, D.A. Thompson, R.R. LaPierre, Nano Lett. 9 (2009) 148. [2] T. Riedl, J.K.N. Lindner, submitted (2014).

Resume : Copper-based materials are widely employed with applications ranging from nano-optoelectronics to gas sensors. However, enhanced performance is demanded and in general fine-grained materials result in property improvements. The present work reports the characterization of Cu nanowires, as well as its further oxidation through microwave radiation or furnace annealing in atmospheric conditions resulting in Cu2O and CuO nanowires, respectively. The nanowires were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). An extensive study of localized charge transport mechanism with simultaneous surface topography of different Cu-based nanowires has been shown by atomic force microscopy (AFM) studies using metal coated conducting probes. Remarkable structural differences were detected between both oxidation processes: nanowires annealed in air consisted of nanocrystalline aggregates displaying hollow structure, whereas microwave radiation resulted in bulk twinned nanowires.

Resume : One of the most critical and challenging issues facing the electronics industry is the development of alternatives to silicon-based materials so as to enable large improvement in device performance. Considerable efforts have been devoted to 2D materials, such as graphene, which despite promising charge transport properties is unable to function as a switch in transistor devices due to the lack of an intrinsic bandgap. The recent exfoliation of black phosphorous, which consists of weakly stacked layers of a quasi-planar corrugated half-honeycomb structure, dubbed phosphorene, has garnered huge experimental and theoretical interest due to its relatively large and direct band gap and good charge carrier mobilities, We have used Density Functional Theory to investigate the properties of passivated phosphorene nanoribbons, denoted armchair (a-PNR), diagonal (d-PNR) and zigzag (z-PNR). We found that z-PNRs demonstrate the greatest quantum size effect, tuning the band-gap from 1.4 to 2.6 eV when the width is reduced from 26 to 6 ?. Strain effectively tunes charge carrier transport, in particular leading to a sudden increase in electron effective mass around 8% strain for a-PNRs or hole effective mass around 3% strain in z-PNRs ? differentiating the (m_h^*/m_e^*) ratio by two orders of magnitude in each case. Straining of d-PNRs results in a direct to indirect band-gap transition at either ?7% or 5% strain, and therein creates degenerate energy valleys, with potential applications for valleytronic applications. Finally, we present results on the optical properties of phosphorene-derived nanostructures, with a particular focus on the exciton dynamics and plasmon resonance.

Resume : In recent years, metal-oxide semiconductor nanomaterials such as nanotubes, wires, particles and hollow nanostructures are attractive candidates as active elements for advanced nanoscale devices due to their unique electronic and optical properties, low effective density, high specific surface area, and shell permeability that can be of importance to many areas of technology. Most of the metal-oxide semiconductor hollow nanostructures were prepared using template-based fabrication methods. Especially, fabrication using carbon spheres as the templates offer more advantages when compared with other templates. Recently, ZnO and Ga2O3 nanowires were used as hard templates to fabricate a ZnGa2O4 nanotubes, in which synthesized ZnO@Ga2O3 and Ga2O3@ZnO core-shell nanowires were followed by solid state reaction at high temperatures. Highly uniformed single crystalline ZnGa2O4 hollow nanostructures with cubic spinel structure were successfully fabricated, in which synthesized carbon@Ga(OH)CO3@Zn(OH)2 core-sell-shell structures with carbon spheres as the templates and then calcined process. We report the synthesis and characterization of ZnGa2O4 hollow nanostructures under controlled several experiment condition

Resume : In this work, we present the results of study of porous silicon (PS) fabricated by two-step metal-assisted chemical etching. This method is used to form PS with different structures, including Si nanowires. On the first step, Ag nanoparticles (NPs) were formed by immersion deposition on the surface of Si wafer. Ag was selected because of its high catalytic activity. Variation of AgNO3 concentration in the solution for Ag deposition from 1 to 3 mM, immersion time from 5 to 60 min and type of Si wafer allowed to form Ag NPs with dimensions from 30 to 100 nm. On the second step, Si wafer with Ag NPs was immersed in aqueous solution of HF and H2O2 for 5 – 120 min. The concentrations of HF and H2O2 varied in the ranges of 4,5 – 9,5 M and 0,3 – 3 M, respectively. The morphology and structure of the obtained samples were studied by SEM and XRD analysis. It was shown that PS can have ordered or disordered structure depending on the formation conditions. Thickness of PS varied from 5 to 15 µm. Porosity of ordered and disordered PS with thickness of 10 µm was about 60 - 65 % and 65 - 75 %, respectively. Possible application of such PS in surface enhanced Raman spectroscopy for biomedical sensing is discussed. This research was financially supported by Belarussian Republican foundation for fundamental research in the range of Grant T13M-161.

Resume : We report first studies on temperature dependent (70-450K) Raman spectra of single-walled carbon nanotubes (SWCNTs), high density thin films (40nm). We show that the position of the main Raman mode (G) is soften as the temperature increases. Both phonon shifts and width exhibits a nonlinear behavior in the range of 70-270 K. This effect is explained by the optical phonon decay into two acoustic phonons. The first-order temperature coefficient (χT) was calculated to be -0.02 cm-1/K, which is lower than for any other CNTs. Importantly, we found that laser-induced local temperature change versus global temperature shows nonlinear trend with the minimum at 270 K. Our results give a new contribution to the understanding of the thermal properties of carbon nanotube thin films that could find application e.g. in photovoltaic or thermoelectric devices.

Resume : Nanosphere lithography (NSL) is attracting increasing interest as a low cost method for the nanopatterning of large area substrates. First colloidal nanoparticle arrays are deposited on planar surfaces by a self-assembly process. The hexagonal particle layers act as shadow mask for subsequent site-controlled surface modifications such as material deposition or removal [1]. Highly regular surface patterns are frequently required for optoelectronic and photonic applications, but “crystal defects” in the self-organized mask typically limit the pattern regularity. We use the doctor blade technique based on convective self-assembly to form monolayers of polystyrene beads on silicon substrates from aqueous suspensions. In this method the bead arrangement is influenced by various parameters such as particle concentration, particle size distribution, blade velocity relative to the substrate and temperature. In order to determine the mask quality a computer program based on Delaunay triangulation was developed which analyses automatically visible light microscopy or scanning electron microscopy images. By this it is possible to measure the areal density of typical mask defects such as grain boundaries, dislocations and point defects. The local sphere coordination can be determined at any position of the mask. Thus, optimum mask deposition conditions can be identified to minimize the density of mask defects. [1] J. C. Hulteen and R. P. van Duyne, J. Vac. Sci. Technol. A 13 (1995)

Resume : Nanosphere lithography (NSL) is a powerful low-cost technique to pattern large area planar substrate surfaces with 2D arrays of nanoobjects. It is based on the self-organization of nanobeads from a colloidal suspension forming a hexagonally close packed mono- or bilayer upon controlled drying of the suspension. The free interstices between neighboring beads are used as mask openings through which material can be either deposited on or removed from the substrate. Typically the nanopatterns on the substrate surface exhibit the triangular or hexagonal shape (for mono- or bilayers) of the projected mask openings. Here we demonstrate that using double-angle resolved deposition through a modified mask a large variety of new motives can be reproducibly formed at each mask opening. By thermal annealing of polystyrene nanosphere masks, the mask openings are contracted to reduced diameter circular apertures located in the equator plain of the spheres. Using a home-built electron beam deposition system with two-axis sample goniometer (inclination ϴ, azimuth φ) allows us to write complex motives at each mask opening in parallel. A ray trace type of algorithm was developed to calculate the route ϴ(t),φ(t) to obtain any desired pattern. Mask clogging effects can be taken into account to tailor the three dimensional shape of nanoobjects. Atomic force microscopy is used to compare the shape of 200 nm diameter optical split ring resonators and other nanoobjects with their layouts.

Resume : Influence of Cu intermediate layer thickness in nanoscaled Fe50Pt50(15nm)/Cu(x, nm)/Fe50Pt50(15nm) (where х = 0;7.5;15;30nm) film composition on SiO2(100 nm)/Si(001) substrates on the transformation of chemically disordered А1 phase into chemically ordered L10 phase at annealing in vacuum was investigated by methods of X-ray diffraction; atomic-force and magnetic-force microscopy; Rutherford backscattering, magnetic properties were measured with magneto-optic Kerra effect method; resistometry. It was established that during deposition the А1 phase forms in all films. The thickness of Cu layer effects on temperature of А1 L10 phase transformation onset in film compositions under investigation. The formation of L10 phase in film with small Cu interlayer thickness of 7.5nm occurs during annealing at 700?С. Increase in thickness of Cu interlayer to 15nm raises the ordering temperature by 100?С tо 800?С due to appearance of big compression stresses and deformations. Coercivity of film increases from 0.05 to 6.7 kOe. In film with big thickness Cu interlayer of 30nm after annealing at 800?С it is fixed formation of FeCuPt ternary compound but passing of ordering process with formation of L10 phase is not established. The authors would like to thank Prof. M. Albrecht and Dr. G. Beddies and workers from Chemnitz University of Technology (Germany) for sample preparation, assistance in conduction of investigations and discussion of results.

Resume : By the discovery of CNT, it has been revealed that CNT has excellent electrical characteristics. Depending on its chirality, CNT can be classified into metallic and semiconducting one and is applied to various fields. In most cases, CNT is used in the form of networks because an individual CNT is hard to be controlled and unsuitable for further processes. The presence of electrical path is crucial when CNT is used in the form of networks. And, percolation theory can be useful to analyze it systematically. Although percolation theory has been applied to diverse CNT networks, theoretical and experimental works considered only the case that both metallic and semiconducting CNT co-exist. Recently, due to the advance of CNT separation, it is available to use highly pure metallic or semiconducting CNT. In spite of this situation, less effort was given to study electrical percolation threshold of semiconducting or metallic CNT networks. In this work, we investigated the percolation threshold of networks required in constituting electrical paths by using semiconducting single-walled carbon nanotube (SWCNT) on field-effect transistor (FET). By theoretical calculation, we obtained the relationship between density of semiconducting SWCNT and percolation probability by changing the channel length defined by source-drain electrodes of FET.

Resume : Based on the results of experimental studies using scanning electron microscopy, electron and electron-probe microanalysis of structural and phase composition, and also computing modeling of the porous silicon layer formed by anodization on different ways treatment Si-wafer surface, its electronic and optical properties of the proposed generalized model of the peculiarities of the finishing chemical treatment (FCT) on the formation and physical and chemical properties of nanostructured porous silicon. Pretreatment of HNO3: HF: CN3COOH = 5:3:3 when creating Si-structures used as a polishing etching. During FCT in ammoniac-peroxide (AmPS) and the acid-peroxide solution (AcPS) in addition to effectively clean the surface of organic and inorganic impurities formed surface hydride (-Si-H), hydroxide (-Si-OH) or oxide layers (-Si -SixOy,-Si-SiO2). The surface layer may have a complex (multi-layered) structure at successive stages of the application FCT - AmPS and AcPS. The conditions and methods of FCT, in which the layers of macro-, micro-or mezoporous silicon are preferably formed are proposed. For samples p-Si is optimal anode current density between 50 and 70 mA/sm2. Indirectly finishing chemical treatment affects the electronic and optical properties of structures based on porous silicon. Oxidation of nanostructured layer of the porous Si and the presence of carbon leads to the formation of structures of nc-Si-SiO2-c-Si and nc-SihSu-SiO2-c-Si with a system of quantum dots in dielectric SiO2. As a result, there are quantum size effects, which manifest themselves in the form of resonance tunneling and hopping diffusion-drift mechanism of charge transfer in the detector structures with Schottky barrier Au-pc-p-Si-Cu and photoluminescence bands of nc-Si-SiO2 and nc-SihSu-SiO2 in the visible wavelength range. The direct impact of the method of finishing chemical treatment on the optical properties caused by the formation nanocrystals nc-Si and nc-Si(x)C(y) with different sizes. In the photoluminescence spectra is evident shift of the main emission band in the long-or short-wave region.

Resume : Due to many advantageous properties of nanosized silicon carbide SiC, there has been ever growing interest in its improved and affordable synthesis. In this regard, there are several routes known to produce nano-SiC materials. In the most popular and, arguably, the oldest method SiC is produced from silica/coke mixtures by electric heating. In that route, large microcrystals of α-SiC are obtained which can, subsequently, be crushed to grains of smaller sizes. Herein, we present the results of a study on the preparation of nanosized β-SiC by a two-stage pyrolysis process of several weeds containing native silica incorporated in organic parts of the plants. In the first stage of pyrolysis, the biomass was deprived of volatiles to yield a solid carbonizate composed, mostly, of silica and carbon. In the follow-up heat treatment at 1650 C under neutral gas atmosphere, silicon carbide in a composite system with an excess free carbon was produced due to carbothermal reduction of silica. All products were investigated with powder XRD and FT-IR spectroscopy supplemented with scanning electron microscopy SEM. The products were shown to be composite nanopowders made of SiC and free/excess C. By mild oxidation of excess carbon, it was possible to obtain pure bio-originated nanosized β-SiC powders. Acknowledgment. This work was supported by AGH University of Science and Technology Grant No. 11.11.210.213.

Resume : Core-shell GaAs/AlGaAs nanowires (NWs) for photovoltaic and photonic applications were grown on Si(111) by plasma assisted molecular beam epitaxy (PAMBE) via the vapor-liquid-solid mechanism, using Ga droplets as a catalyst. First, GaAs NWs were grown and then, growth continued adjusting the fluxes of Al, Ga, and As, to form the AlGaAs shell surrounding the initial GaAs core. The nanostructure of core-shell NWs was explored by high-resolution and scanning transmission electron microscopy (HRTEM-STEM). NWs are zinc-blende (ZB) single crystals grown epitaxially along the [111] direction, despite the presence of the thin amorphous native oxide. They originate on small heavily twinned GaAs crystals and then, they grow by a constant sequence of (111) mirror twins. The core-shell structure of the NWs was revealed by both diffraction contrast TEM and annular dark-field (ADF) STEM imaging, showing that the AlGaAs shell occupies at least one half of the projected diameter of the NWs, ranging from 80 nm to 200 nm. The Al content of the shell was estimated at ~35% by energy dispersive X-ray (EDX) analysis. Finally, molecular dynamics (MD) simulations of plan-view slices of the NWs were applied to calculate the variation of the energy, the stress tensors, the displacement field and the strain components of the core-shell configuration. Acknowledgments: Research co-financed by the EU (ESF) and Greek national funds - Research Funding Program: ARISTEIA II, project NILES.

Resume : Tailoring surface energies is the key factor to control the orientation of nanoscopic structures in block copolymer (BCP) thin films. When the BCP thin film is confined between substrate/polymer and air/polymer interfaces, preferential wetting of either the air or substrate interfaces by one of the blocks leads to a parallel orientation of the microdomain morphology with respect to these interfaces. One of the most popular methods to control the BCP morphology orientation is the chemical modification of substrate surface by hydroxyl terminated random copolymers composed of the same monomers as the BCP. As the random copolymer chemical composition can be precisely controlled, a fine-tuning of the surface characteristics can be obtained. The grafting occurs by a thermally activated reaction of HO-Si substrates with the hydroxyl groups of the functional (co)polymers. In this work, we prepared a series of hydroxyl terminated poly(styrene-r-methylmethacrylate) random copolymers with different molar masses, ranging from 1500 to 70000 g/mol. Then the Rapid Thermal Processing (RTP) technology was employed to perform flash grafting reactions of the random copolymers to the activated silicon wafer surface. The use of RTP technology allows the grafting reaction to be obtained on the timescale of few seconds, thus opening new possibilities in several fields of surfaces and interfaces in science and technology.

Resume : Here we report on the optical functionalization of glass and semiconductor (Si and GaAs) substrates in view of light trapping and photon harvesting in photovoltaic devices [1-2]. Defocused Ion Beam Sputtering, assisted by a sacrificial self-organised Au stencil mask, is exploited in order to project in the underlying substrate a pattern of high aspect ratio nanoscale features. We describe the formation of a quasi-periodic array of 1-D nanostructures with RMS roughness in the 80-130 nm range as a function of the total ion dose (vertical dynamic range roughly 500 nm). The characteristic lateral size of the nanostructures, estimated from the height-height correlation function, is in the range of 200 nm. The tailored optical properties of the substrates are characterised in terms of total reflectivity and haze by means of integrating sphere measurements as a function of the morphological modification at increasing ion fluence. A broad-band anti-reflection effect (10-20% reduction with respect to a flat reference) is found in the VIS-NIR range. At the same time the patterned substrates have shown enhanced broadband light scattering functionalities as quantitatively highlighted by the Haze functions which can be tailored in the 30%-60% range[1-2].The first encouraging results demonstrate that a:Si solar cells grown on patterned glasses exhibit a 15% relative enhancement in photocurrent due to light trapping. [1] [2] C.Martella et al.Nanotechnology 2013, J. Appl. Phys. 2014.

Resume : Hybrid polymeric materials are the perspective direction of the modern material science. Such materials consist of nano-scale organic and inorganic fragments. They have advantages of both material types due to the such feature. The mechanical properties investigation of them can be the purposeful development basis of instrumental and functional materials. The free volume of hybrid polymers play the particular role in their mechanical properties. There is a possibility to regulate the parameters of the free volume. In turn changes of these parameters can influence on mechanical properties of hybrid polymers. Metal-containing polyphenyles were used as hybrid polymers. The free volume of them was studied. In particular, free volume fraction was investigated. It was studied with the help of mass-spectrometry data. The influence of the free-volume fraction on the Young modulus, wear resistant, and other properties were confirmed. The mechanism of the such influence was proposed. There is an ability to control free volume fraction by filler introduction, polymerization temperature and pressure. Optimal parameters of the free volume fraction size were found.

Resume : Self-organized nanostructures appear on metal surfaces exposed to ultra-short laser pulses. Periodic stripes (also referred to as LIPSS), cells and localized structures with characteristic lengths in the range from 100s to 1000 nm can be observed. Here we discuss the characteristics of the self-organized patterns and the experimental possibilities, how these characteristics can be controlled. New physical model of the periodic pattern formation based on the commonly-used two-temperature model (TTM) is proposed. It is shown that the temperature profile on the surface may be unstable with respect to a periodic perturbation, which can give rise to the periodic modulation of the height profile. Characteristic wavelength of the instability and the growth rate of the periodic pattern are estimated from the material constants of the target. Possible influence of hydrodynamic instabilities induced by the periodic temperature profile modulation on the surface are discussed.

Resume : Porous silicon (PS) has proved itself as a versatile template for a manufacturing nanostructure arrays and nanocomposites. However PS lacks reproducibility and regularity for certain applications such as cold cathode arrays or defect-free waveguides with low losses. Up to now different attempts have been made to form a highly ordered PS with a regular pore structure. In this work we present results of our study of the PS formation by electrochemical anodization at a pulse mode. The PS layers were formed in n+-type silicon wafers using an aqueous 9 % solution of HF as an electrolyte. A single cycle of the pulse mode consisted of two current density peaks of 100 mA/cm2 for 1 s in a forward direction and 5 mA/cm2 for 0.5 s in a reverse direction divided by 4 s intervals at a zero current density. PS pores grew perpendicularly to the silicon wafer surface. No branching, typical to PS formed at a constant mode, was observed. An average pore diameter was 80 nm. A dispersion of the pore diameters was less than 20 %. Thickness of silicon crystallites forming porous silicon skeleton was 20 nm. A distance between pore centers was constant across the whole PS. The highest aspect ratio of the pores achieved in the present study was 1:600. It was shown that PS formed by the developed technique is a promising template for the fabrication of nanocomposites with a high anisotropy. The work has been supported by the Belarus Government Research Program grants 1.1.14 and 2.4.16.

Resume : Self-assembling (SA) diblock copolymers (DBCs) generate nanostructured patterns that could be useful for advanced lithographic applications. The ordering dynamics in polystyrene-b-polymethylmethacrylate (PS-b-PMMA) DBCs thin films under conventional thermal treatments in oven/furnace is extremely slow, resulting in limited correlation length values even after prolonged annealing at relatively high temperatures. In this study, we describe the pattern coarsening dynamics of both asymmetric PS-b-PMMA DBCs with molecular weight of 67 kg/mol treated in the Rapid Thermal Processing (RTP) system, as a function of the annealing parameters (i.e. annealing temperature Ta and time ta) and the film thickness. The proposed methodology allows obtaining in few seconds high correlation length values in PS-b-PMMA films of variable thickness, taking advantage of the amount of solvent naturally trapped within the film during the spinning process. Irrespective of the Ta (chosen between 230 and 270 °C), the correlation length increases steeply at first with a growth exponent φ = 0.38 and then more gradually (φ = 0.09) as a function of ta, with a well-defined threshold, which depends on the annealing temperature, thus suggesting the occurrence of two distinct growth regimes. The obtained results are in good agreement with behavior previously reported by our group for symmetric PS-b-PMMA DBCs.

Resume : Perfectly ordered microstructures with nanometrically defined periodicity offer promising opportunities in microelectronics wherein the pitch reduction and the component density increase are two of the main challenges. To produce long-range ordered arrays inherent to such technologies, the combination of the “bottom-up” self-assembly with “top-down” guiding patterned templates has been successfully introduced leading to new technological breakthroughs wherein block copolymers are the cornerstone of the “bottom-up” nanofabrication processes through their self-assembly in periodic mesostructures. Well-ordered 2D-arrays of block copolymers can be produced by solvent or thermal annealing thin films cast onto grooved substrates. Consequently, directed self-assembly (DSA) of block copolymers stands out as a promising methodology to supplement conventional lithography. We have developed a first generation of lithographic materials based on the DSA of PMMA-b-PS and PMMA-s-PS copolymers synthesized by Arkema following the ITRS for the 22 nm node. Results summarizing this approach will be exposed during this talk. Nevertheless this system suffers from deficiencies such as a low Flory Huggins parameter and a low chemical contrast as regards to the etching processes which could be problematic for targeting sub-16 nm features. Consequently we have developed new systems which are characterized by high segregation strength as well as strong chemical etching contrast.

Resume : Diblock Copolymers (DBCs) have attracted a wide interest over the last decades due to their ability to self-assemble (SA) into well-ordered nanometric size structures. In this work, we studied the SA process of cylinder-forming PS-b-PMMA block copolymers (BCPs) having different molecular weight (between 39 and 205 kg/mol), by means of a Rapid Thermal Processing (RTP) machine. With this annealing method, we were able to fabricate, in a relatively short amount of time, hexagonally packed cylindrical structures of different diameters ranging between 12 and 30 nm and center-to-center distance L0 from 24 to 73 nm. The study was performed on both flat pre-patterned surfaces. The SA process of BCPs inside densely packed and nanometer wide trenches was investigated as a function of the RTP annealing parameters (temperature and time). In particular, the ordering dynamic of the RTP treated BCP deposited inside the trenches showed an unexpected and irreversible flipping of the cylindrical nanodomains from parallel to perpendicular orientation with respect to the surface.