Stress, structure, and stoichiometry effects on the properties of nanomaterials

Resume : CNTs are a type of novel material attracting great attention due to their outstanding thermal, electrical, and mechanical properties. These properties make them a promising alternative to interconnect materials for electronic packaging applications. As the performance of semiconductor products increases, the technical challenges increase in areas of power delivery, heat removal, input/output density, and thermomechanical reliability. Down-scaling the traditional interconnects does not satisfy the requirements and the development, process, and design of interconnect materials are driven into entirely new directions. In the presented study the different growth kinetics of several kinds of catalyst/substrate couple are investigated by analyzing the CNT carpets at different times during the PECVD growth, by NEXAFS ex situ and Raman in situ for different experimental conditions. Electrical, thermal and mechanical properties are determined on CNT carpets and interconnect tests are achieved for industrial applications.

Resume : Synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The nanoporous Al2O3 – most established material of this group – has been prepared by anodic oxidation of Al under suitable electrochemical conditions into perfectly ordered, honeycomb-like porous structures (Masuda & Fukuda, Science, 1995). It is the TiO2 that has received the next highest attention motivated by its range of applications. Very significant research efforts have led to reproducible synthesis of self-organized TiO2 nanotube layers by means of anodic oxidation, during which the Ti substrate is converted into highly-ordered nanotubular layer in suitable electrolytes. Although advancements in the anodic synthesis of TiO2 nanotube layers have been presented over past years, the degree of ordering has not reached so far the level known from porous Al2O3. Numerous factors influence the ordering and the homogeneity of TiO2 nanotube layers. In the presentation, we aim to demonstrate considerably significant advancements in the ordering of anodic TiO2 nanotubes and its implications. We will show how to obtain via tailored anodization protocols a very decent degree of uniformity and homogeneity of the nanotube layers. Moreover, based on SEM, EBSD and TEM measurements, we will demonstrate how the Ti grain structure influences the lateral uniformity of the nanotube layers.

Resume : Zero temperature coefficient of resistance (TCR) is essential for the precise control of temperature in heating element and sensor applications. Many studies have focused on developing zero-TCR systems with inorganic compound; however, very few have dealt with developing zero-TCR systems with polymeric materials. Composite systems with a polymer matrix and a conducting filler show either negative (NTC) or positive temperature coefficient (PTC) of resistance, depending on several factors, e.g., the polymer nature and the filler shape. In this study, we have demonstrated a new hybrid bi-layer zero-TCR composite consisting of stacked layers showing NTC and PTC of resistance. The bi-layer composites consisted of a carbon nanotube (CNT)-based layer having a NTC of resistance and a carbon black (CB)-based layer having a PTC of resistance which was in direct contact with electrodes to stabilize the electrical resistance change during electric Joule heating. The composite showed nearly constant resistance values with less than 2 % deviation of the normalized resistance until 200 °C. The CB layer worked both as a buffer and as a distributor layer against the current flow from an applied voltage. This behavior, which was confirmed both experimentally and theoretically, has been rarely reported for polymer-based composite systems.

Resume : The large variety of defects arising from the constraints imposed by the nanoscale is at the origin of the huge emissivity observed in SiO2 nanoparticles. Generally, broad and structureless emission bands characterize the UV-Visible spectral region, reflecting the amorphous matrix in which the defects are embedded. An exception is observed in a vacuum and consists of a structured photoluminescence, between 3.0 eV and 3.5 eV, whose sharp spectral features resemble those of a single molecule. This emission is due to the coupling of an electronic transition with two localized modes of frequency 1370 cm-1 and 360 cm-1. Its quenching in air points out that the defects are allocated on the surface of the nanoparticles and strongly interact with the molecular species of the environment. The pronounced sensitivity to the atmosphere, in combination with the advantageous spectral features, is promising for the use of SiO2 nanoparticles as luminescent sensors of small molecules. To this aim, the understanding of the fundamental mechanisms of the quenching is mandatory. By using time resolved photoluminescence technique, we have carried out a detailed investigation of the effects on the intensity and lifetime of the structured emission on varying the interaction with specific molecular species (O2, N2). The findings indicate that the quenching mechanisms is controlled by collisional- and reaction-limited processes thus evidencing the sensing property of SiO2 nanoparticles.

Resume : Well-ordered and freestanding bilayer silica  a new member of two-dimensional (2D) family  have great advantage in versatile modern technologies and applications that range from insulating layers in integrated circuits to supports for sensors and catalysts, as well as protective films against corrosion etc. Motivated by this, a lot of work on mechanical properties has been done, while thermal transport properties received less attention. By performing DFT calculation a new ground state of bilayer h-silica was found. Remarkably, it has negative Poisson’s ratio at small strains, while the Poisson’s ratio has negative to positive transition at some critical strain. The underlying mechanism of the negative Poisson’s ratio stems from the intrinsic rotational angle of SiO3 triangular pyramids. Moreover, we studied the thermal conductivity of O-silica, another type of silica layer, under biaxial strain using DFT based Boltzmann transport equation. An unexpected drastic decrease in lattice thermal conductivity of O-silica with tensile strain was observed, in contrast to the behavior of graphene, a representative of 2D materials. The relationship between the unusual strain dependent thermal conductivity and the silica layer structure is further studied. The current research adds scientific value to the fundamentals of 2D materials and we expect that it will arouse great interest of experimentalists to carry forward the application of silica as new electronic materials.

Resume : Top-down fabricated quantum nanodisks (QNDs) lasers are one of the most promising light sources because of their theoretically improved performances compared to quantum well and bulk lasers. The bio-template is used to create a high density etching mask and low-damage etching is performed by using neutral beam (NB). The bio-template is realized by cage-shaped proteins called ferritins of 12 nm outside diameter with a 7 nm iron oxide core, and functionalized with poly-ethylene glycol (PEG) to control the gap between cores to avoid any QNDs coupling after fabrication. After removing the protein shell by oxygen annealing, a high-density nano-pattern of cores is realized as etching mask. The NB etching system consists of an inductively coupled plasma chamber separated from the process chamber by a carbon electrode with a high aspect-ratio aperture array, therefore, the charged particles are efficiently neutralized and the UV photons from plasma almost completely screened. InGaAs/GaAs multiple quantum wells were grown by metalorganic vapor phase epitaxy (MOVPE), with a few nanometers thick InGaAs cap layer. Ferritins were self-assembled by spin-coater. After removing protein shell by oxygen annealing in vacuum, a hydrogen radical treatment was performed to remove the oxide layer. Etching was then realized by pure chlorine NB. Regrowth of GaAs barrier was done by MOVPE. Finally, InGaAs QND based light emitting diode were realized.

Resume : Selective synthesis for integrated nanomaterials with controllable morphology and composition represents an emerging research area in nanoscience and nanotechnology because the intrinsic properties behind nanostructures are generally phase-, shape-, and size- dependent. In this direction the present work shows the capabilities of nanoporous polymeric template systems directly supported on different substrates for the confined growth of epitaxial ferromagnetic complex oxides nanostructures. In particular, we describe the versatility and potentiality of sol-gel precursor solutions combined with track-etched polymers to synthesize i) vertical polycrystalline La0.7Sr0.3MnO3 nanorods on top of single crystal perovskites [1,2], ii) single crystalline manganese based octahedral molecular sieves (OMS) nanowires on silicon substrates [3-5], and iii) the epitaxial directional growth of single crystal OMS nanowires when grown on top of fluorite-type substrates [6]. The influence of the distinct growth parameters on the nanostructural evolution of the resulting nanostructures and their magnetic properties are studied in detail. Therefore, we demonstrate that the combination of soft-chemistry and epitaxial growth opens new opportunities for the effective integration of novel technological functional complex oxides nanomaterials on different substrates. [1] A. Carretero-Genevrier et al. Chem.Soc.Rev., 43, 2042-2054 (2014) [2] A. Carretero-Genevrier et al. Adv.Funct.Mater., 20, 892-897. (2010). [3] A. Carretero-Genevrier et al. Chem.Mater., 26 (2), 1019?1028 (2014) [4] A. Carretero-Genevrier et al. JACS., 133 (11), 4053?4061 (2011) [5] J. Gazquez et al. M&M., 20 (03) 760-766 (2014) [6] A. Carretero-Genevrier et al. Chem.Comm., 48, 6223-6225 (2012)

Resume : Superhard Ti-Si-N coatings were fabricated using vacuum arc evaporation of cathode. Such methods of analysis as slow positron beam (SPB), RBS, µ-PIXE (proton microbeam), XRD, SEM with EDS, XPS, nanohardness and elastic modulus measurements were applied to investigation of such coatings, before and after annealing at the temperature of 600 degrees C for 30 minutes. Redistribution of N and Si occurred on the borders of nanograins after annealing, amorphous phase α-SiNx (Si3N4) was created, defects segregated on interfaces and formed vacancy-type clusters with rather high concentration from 5х10^16 cm^(-3) to 7.5x10^17 cm^(-3) due to thermodiffusion. Solid solution (Ti,Si)N and small concentration of α-SiN (close to XRD detection limits) were formed in the coatings. Deformation reduced after thermal annealing to a value of –2.3%. Size of nanograins of (Ti,Si)N solid solution increased from 12.5 nm to (13.2÷13.4) nm, and 25 nm grains increased to 28.5 nm due to annealing under another deposition regime. Fabricated coatings were implanted by high dose Cu- ions 2x10^17 см^(-2), kinetic energy was 60 keV in order to investigate the limits of irradiation resistance.

Resume : Among various materials being explored in search for silicon replacement in nanoelectronics, SnS2 seems to be especially valuable. Having intrinsic layered structure of crystallites and relatively high energy gap offers multiple sophisticated applications, e.g. in high on/off current field effect transistors. In this study, structural and electron properties of SnS2 nanopowders have been examined. The syntheses have been carried out in aqueous solutions, as well as in organic solvents, using metal salts, sulfur, thioacetamide etc. The obtained materials have been characterized with X-ray powder diffraction, Raman scattering spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and spectrophotometry UV/Vis/NIR.

Resume : In this paper formation process of nano-heterostructures Ag2O-Hg1–xCdxTe(x=0.2) on the surface of solid solution Hg1–xCdxTe (х~0.22) has been investigated. Modification of the ternary chalcogenide semiconductor compound was performed using the method of doping the heterostructure samples with silver ions which was followed by low-temperature treatment. The energy and dose of implanted ions were 100 keV and 4.8x1013сm-2, respectively, the duration of thermal treatment was 5 hours at the temperature of 75ºС. The silver ion migration in the ion-disordered HgCdTe layer and local deformation defects are lead to the topological instability of the irradiated surfaces. The results of topometry based on AFM measurements show the network of quasi-pores with depth from 3.5 to 10 nm and diameter 50 to 160 nm, as well as grains with size 40 to 80 nm densely packed in the surface plane. After implantation with silver ions on the background of insignificant smearing of grain boundaries, with maintenance of initial surface porosity, a uniform array of cone-like spikes was formed, with height h from 5 to 25 nm and base diameter d between 13 and 35 nm. The magnitude of mechanical stresses created in the CdHgTe film after its implantation can be determined (σmax= 2.2x105 Pa). The coefficient of crystal lattice contraction by the introduced implant β was determined using the results of X-ray diffraction studies of specimens ~ 3.51x10-31m3. Deformations arising after embedding silver ions prevent the process of mercury diffusion in the implanted target. Apparently, transformation of the defect structure in semiconductor film reflects elastic relaxation “compression – extension – compression” in the region of MCT loosened by ion implantation.

Resume : Cerium oxide (CeO2) thin films have been deposited on Si (100) and steel substrates using pulsed laser deposition technique at different substrate temperatures from room temperature (RT) up to 500C in a controlled atmosphere of oxygen (0.1 mbar). Structural, morphological and optical properties have been investigated using X-ray diffraction (XRD), Raman, scanning electron microscopy and ellipsometry techniques. The refractive index is increasing (1.53 - 2.2) with the increasing of the substrate temperature. XRD results showed that the deposited films are polycrystalline with cubic structure. The Raman peak appeared at 460 cm−1 due to the F2g active mode. Nanostructured “pyramids” like features were observed for high temperature substrate.

Resume : Aging Cu-Mn-Al alloys with original magnetic characteristics undergo thermo-induced martensitic transformation (MT). Nowadays, such type of MTs which occur after solid solution decomposition with ferromagnetic nanoparticles precipitation in nonferromagnetic matrix attract interest. By thermal treatment, the system of ferromagnetic nano-dispersed particles in nonferromagnetic matrix can be formed. Herewith, coherent nanoparticles precipitated during decomposition of high-temperature Cu-Al-Mn betta1-phase are coherently connected with matrix and do not undergo spontaneous MT at cooling . The influence of aging regimes of high-temperature phase on subsequent martensitic transformation in Cu-Al-Mn alloy was studied. The morphology of martensitic transformation behavior as a result of alloy aging under an annealing in a constant magnetic field with different sample orientation relatively to the field and without the field was investigated for directly control of the process of martensite induction at cooling. The temperature dependences of electrical resistance, magnetic susceptibility, and the temperature and field dependences of magnetization, phase composition were found. The tendency of oriented growth of the precipitation-phase particles in a direction of applied field and the increase of volume fraction of these particles under thermal magnetic treatment of material what favors a reversibility of induced martensitic transformation.

Resume : We report on the transfer of novel polymer-antibiotic-bioactive glass composites by matrix assisted pulsed laser evaporation to uniform thin layers onto stainless steel implant. Influence of the deposition process on the structure of nanomaterials was studied. The targets were prepared by freezing in liquid nitrogen of mixtures containing polymer and antibiotic reinforced with bioglass powders. The cryogenic targets were submitted to multipulse ablation with an UV KrF* (λ=248 nm, τ ~ 25 ns) excimer laser source. The main advantages with this coating are multiple: stopping any leakage of metal and metal oxides to the biological fluids and finally to inner organs (by polymer use), speeding up osteointegration (by bioactive glass use), antimicrobial effect (by antibiotics use) and decreasing of the implant price (by cheaper stainless steel use). The behaviour of polymer-antibiotic-glass/stainless steel structure in condition which simulates the physiological environment was evaluated in vitro by complementary techniques. The bioactivity and the release of the antibiotic were assessed by immersion into simulated body fluid and monitoring by FTIR and UV-VIS spectrometry and electrochemical measurements involving corrosion and EIS studies were carried out in order to investigate the corrosion resistance. The biological properties were tested including the microbial viability using Gram - and Gram + bacterial strains, the microbial adherence and the cytotoxicity on eukariotic cell.

Resume : Statical and dynamical aspects of a class of macromolecules based on the architecture of the well known fullerenes is theoretically investigated. The building blocks used to geometrically construct these molecules are the two dimensional structures: porous graphene and biphenylene-carbon. Density Functional-based tight binding (DFTB) methods as well as reactive molecular dynamics (ReaxFF) methods are applied to study the electronic and structural properties of these molecules. Our calculations predict that these structures can be stable up to temperatures of 2500K. The atomization energies of carbon structures is predicted to be in the range 0.45 eV/atom to 12.11 eV/atom (values relative to the C60 fullerene), while the BN analogues have atomization energies between -0.17 eV/atom and 12.01 eV/atom (compared to the B12N12 fullerene). Due to their high porosity, these structures may be good candidates for gas storage and/or molecular encapsulation. This possibility of encapsulation is explored using DFTB methodology. For this end, several DFTB based molecular dynamics simulations were carried in order to obtain new insights regarding mechanisms involved in the possible encapsulation of small molecules by these compounds.

Resume : Current trends regarding design and engineering of highly efficient miniaturized devices and machines require innovative materials to satisfy requirements such as high strength at low density. The purpose of the present study was to investigate mechanical properties and deformation behavior of nanoporous Au and its ultra-fine grained bulk counterpart, both fabricated from the same base material. Microstructural investigations of the foam showed a ligament size of ~100 nm consisting of ~60 nm diameter grains in average, while the ultra-fine grained Au exhibits an average grain size of 250 nm. Nanoindentation lends itself as the method of choice to obtain local materials properties, at room temperature as well as at elevated temperatures, respectively. We performed a series of indentation tests and nanoindentation relaxation experiments to determine hardness, Young’s modulus, strain-rate sensitivity, and activation volume from room temperature to elevated temperatures up to 300 °C for the porous and bulk materials. Due to the small characteristic dimensions in terms of grain size or ligament diameter, high hardness values were determined for both materials, which rapidly drop at elevated temperatures. In addition, an enhanced strain-rate sensitivity accompanied by low activation volumes was determined, increasing with temperature for both states. We associate this mechanical behavior with interactions between dislocations and grain boundaries or free surfaces, respectively.

Resume : Superlattices (SPLs) consisting of ferroelectric/paraelectric oxide layers present a large interest due to their flexibility for tuning of ferroelectric properties. In such SPLs a large number of interfaces are present, and additionally the lattice strain in the SPLs can differ notably with respect to single phase films. Discerning between strain and interface effects on the ferroelectric properties constitutes a bottleneck. We have fabricated BaTiO3/SrTiO3 (BTO/STO) SPLs by pulsed laser deposition (PLD) assisted by high pressure reflection high energy electron diffraction (RHEED). RHEED and atomic force microscopy confirm persistent layer-by-layer and atomically flat surfaces in complex M x (n-BTO / n-STO) SPLs, with period n from 1 to 10 and M adjusted to have total of 120 monolayers. X-ray reflectivity and X-ray diffractometry (XRD) patterns show Kiessig and Laue fringes signaling the high quality of the samples. XRD reciprocal space maps and high resolution transmission electron microscopy confirmed coherent growth. Polarization loops were measured, with remnant polarization (Pr) from a few μC/cm2 to more than 20 μC/cm2, increasing (decreasing) with the number of interfaces (STO thickness in each period). We present a thin film growth strategy that permits changing lattice strain for a fixed M x (n-BTO / n-STO) SPL architecture, and we show that Pr is much less sensitive to lattice strain than to the number of interfaces.

Resume : Vanadium dioxide (VO2) with its reversible metal-insulator transition (MIT) close to room temperature (341 K in single crystals) is attracting more and more interest due to the increasing number of applications in which it can be used. The constant and technologically relevant need of lowering its transition temperature (TMI) has resulted in an impressive number of theoretical and experimental studies investigating the influence of the substrate nature and crystallographic orientation, layer thickness, growing temperature, effect of doping, hydrogenation, interface quality, usage of an buffer layer or interfacial strain. Among them, the interfacial strain is considered as the most straightforward way to modulate the TMI. In this talk, we will present our recent results on VO2 thin films grown over a wide temperature range, and discuss the effect of interfacial strain on the TMI determined by electrical measurements. The films have been grown on low-miscut (0 0 1) TiO2 substrates by Pulsed Laser Deposition (PLD). Their crystal structure has been studied by high resolution X-ray diffraction (HR-XRD) and high resolution transmission electron microscopy (HR-TEM), while the quality of the interfaces (microstructure and stoichiometry) has been characterized by HR-TEM and Electron Energy Loss Spectroscopy (EELS). The strain nature of the films has been evaluated from HR-XRD and Geometrical Phase Analysis (GPA) measurements.

Resume : Ultrasmall zinc sulfide nanocrystals with cubic (blende) structure, doped with Mn2+ ions, have been prepared by colloidal synthesis [1] at pH = 8. According to XRD and HRTEM measurements they exhibit a core-shell structure, with a crystalline cZnS core of 2 nm average diameter and a shell of disordered, orthorhombic e-Zn(OH)2 of 0.3 to 1.9 nm thickness [2]. The analysis of the electron paramagnetic resonance spectra of the low concentration Mn2+ ions used as local atomic probes during pulse annealing experiments [3] resulted in the observation of a three-steps thermal decomposition of the e-Zn(OH)2 shell into nanostructured ZnO, in the 80-450 oC range, instead of a direct decomposition into crystalline ZnO at 120 oC, observed for the crystalline bulk e-Zn(OH)2 shell. This unusual behavior is explained by the sequential decomposition by dehydration of the disordered Zn(OH)2 shell into two new intermediate oxyhydrated Zn nanocompounds, which are not stable in the bulk form. Our results demonstrate that the chemistry of nanosized compounds can be different from the chemistry of the bulk counterparts. [1] S. V. Nistor et al., Superlattices Microstruc. 46 (2009) 306 [2] S. V. Nistor et al, J. Chem. Phys. C 42 (2013) 2217 [3] S. V. Nistor et al., J. Therm. Analys. Calorim. 118 (2014) 1021

Resume : Nowadays there is a strong belief that thermoelectric thin film-based devices represent a suitable route to mitigate thermal management problems in micro- and nano-electronics. Bi0.5Sb1.5Te3 (BST) is considered to be a state-of-the-art p-type thermoelectric material, at temperatures near room temperature, due to its high power factor value. Nevertheless, the deposition of BST thin films with bulk-like thermoelectric properties remains a challenge because of issues related to stoichiometry and antisite defects. There is still a need for investigating the effect of chemical compositions on the thermoelectric properties of BST thin films. We have grown p-type Bi0.5-xSb1.5Te3+x thin films onto different types of substrates using pulsed laser deposition at 248nm and home-made targets with different Bi concentrations, and investigated their structural, electrical and thermoelectrical properties. In this talk, we will present our recent results on Seebeck coefficient, electrical resistivity and Hall carrier concentration as a function of temperature for a series of Bi0.5-xSb1.5Te3+x thin films. We will discuss how their thermoelectric properties are affected by the substrate type and Bi content. Also, we will address the effect of post-annealing treatment on their structural and thermoelectric properties.

Resume : One of the main difficulties in understanding and predicting frictional response is the intrinsic complexity of highly non-equilibrium processes in any tribological contact, which include breaking and formation of multiple interatomic bonds between surfaces in relative motion. To understand the physical nature of the microscopic mechanism of friction and design new tribologic materials, we conducted a systematic quantum mechanic investigation at the atomic scale on the geometric, electronic and dynamic properties of prototipical MX2 (M=Mo, W; X=S, Se, Te) Transition Metal Dichalcogenides under variable load. We have been able to disentangle the electro-vibronic contribution to the frictional response by adopting a new investigation protocol: we combined the structural and dynamic information from group theoretical analysis and phonon band structure calculations with the characterisation of the electronic features using non-standard methods like orbital polarization and the recently formulated bond covalency and cophonicity analyses. The vibrational modes relevant during tribological conditions are individuated, quantified and put in relation with the atomic types and the electronic features that the latter determine. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase. The formulated protocol can be promptly applied to the design of new materials with diverse applications other than tribology.