Antiope Lotsari
@chalmers.se
Researcher
Chalmers University of Technology
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Gunnar Símonarson, Antiope Lotsari, and Anders E. C. Palmqvist
MDPI AG
A low-temperature spray deposition synthesis was developed to prepare locally hexagonally ordered mesoporous titania films with polycrystalline anatase pore walls in an evaporation-induced self-assembly process. The titania film preparation procedure is conducted completely at temperatures below 50 °C. The effects of spray time, film thickness, synthesis time prior to spray deposition, and aging time at high relative humidity after deposition on the atomic arrangement and the mesoorder of the mesoporous titania were studied. We find the crystallite size to depend on both the synthesis time and aging time of the films, where longer times result in larger crystallites. Using the photocatalytic activity of titania, the structure-directing agent is removed with UV radiation at 43–46 °C. The capability of the prepared films to remove the polymer template increased with longer synthesis and aging times due to the increased crystallinity, which increases the photocatalytic efficiency of the titania films. However, with increasingly longer times, the crystallites grow too large for the mesoorder of the pores to be maintained. This work shows that a scalable spray coating method can be used to prepare locally ordered mesoporous polycrystalline titania films by judiciously tuning the synthesis parameters.
Mohamed Ben Hassine, Hans-Olof Andrén, Anand H.S. Iyer, Antiope Lotsari, Olof Bäcke, Dirk Stiens, Wiebke Janssen, Thorsten Manns, Johannes Kümmel, and Mats Halvarsson
Elsevier BV
G. Calcagno, A. Lotsari, A. Dang, S. Lindberg, A.E.C. Palmqvist, A. Matic, and C. Cavallo
Elsevier BV
Abstract Hybrid energy storage systems aim to achieve both high power and energy densities by combining supercapacitor-type and battery-type electrodes in tandem. The challenge is to find sustainable materials as fast charging negative electrodes, which are characterized by high capacity retention. In this study, mesoporous anatase beads are synthetized with tailored morphology to exploit fast surface redox reactions. The TiO2-based electrodes are properly paired with a commercial activated carbon cathode to form a Li-ion capacitor. The titania electrode exhibits high capacity and rate performance. The device shows extremely stable performance with an energy density of 27 mWh g-1 at a specific current of 2.5 A g−1 for 10,000 cycles. The remarkable stability is associated with a gradual shift of the potential during cycling as result of the formation of cubic LiTiO2 on the surface of the beads. This phenomenon renews the interest in using TiO2 as negative electrode for Li-ion capacitors.
Gunnar Símonarson, Giulio Calcagno, Antiope Lotsari, and Anders E. C. Palmqvist
Royal Society of Chemistry (RSC)
A combination of electrochemical and structural characterization techniques were used to study the lithiation in spray deposited amorphous mesoporous titania.
Szilvia Vavra, Neus Vilà, Antiope Lotsari, Alain Walcarius, and Anna Martinelli
Elsevier BV
Abstract In parallel to the increasing variety of ionic liquids that show different kinds of nanometer-scale structuration in their pure and solved forms, there is a raising interest in exploring the possibility of using ionic liquids as soft-templates for the synthesis of mesoporous materials. We report the case of 1-hexadecyl-3-methylimidazolium chloride (C16MIMCl), a surface active ionic liquid (SAIL), here used as an excellent soft-template for the formation of vertically aligned, uniform mesochannels, with a well defined pore width of 2.5 nm in silica thin films deposited with the electrochemically assisted self-assembly (EASA) method. The obtained mesochannels run through the entire thickness of the films and after removal of the ionic liquid the emptied mesochannels ensure a thorough mass transport to the substrate, here monitored by the redox-active electrochemical probe Ru(II)/Ru(III) during cyclic voltammetry (CV). Moreover, the mechanism of pore formation is explained; unlike the mechanisms reported for short chain imidazolium ionic liquid silica templates, in the case of C16MIMCl the dominating so-called cooperative interaction is the electrostatic attraction between the C16MIM+ and the network-forming negatively charged silicate oligomers. Therefore, this study provides a better understanding of the templating behavior of long chain imidazolium ionic liquids and motivates further research on the synthesis of ionic liquid-based functional hybrid materials.
Anna Peterson, Ida Östergren, Antiope Lotsari, Abhijit Venkatesh, Johannes Thunberg, Anna Ström, Ramiro Rojas, Martin Andersson, Lars A. Berglund, Antal Boldizar,et al.
American Chemical Society (ACS)
Many polymers, including polyethylene, feature a relatively low melting point and hence must be cross-linked to make them viable for applications that demand a high stiffness and creep resistance at elevated temperatures. The resulting thermoset plastics cannot be recycled, and therefore alternative materials with a reconfigurable internal network structure are in high demand. Here, we establish that such a thermoset-like yet recyclable material can be realized through the addition of a nanocellulose reinforcing agent. A network consisting of cellulose nanocrystals, nano- or microfibrils imparts many of the characteristics that are usually achieved through chemical cross-linking. For instance, the addition of only 7.5 wt % of either nanocellulose material significantly enhances the melt stiffness of an otherwise molten ethylene-acrylate copolymer by at least 1 order of magnitude. At the same time, the nanocellulose network reduces the melt creep elongation to less than 10%, whereas the neat molten matrix would rupture. At high shear rates, however, the molten composites do not display a significantly higher viscosity than the copolymer matrix, and therefore retain the processability of a thermoplastic material. Repeated re-extrusion at 140 °C does not compromise the thermomechanical properties, which indicates a high degree of recyclability. The versatility of dynamic nanocellulose networks is illustrated by 3D printing of a cellulose composite, where the high melt stiffness improves the printability of the resin.
Maria Pihl, Krzysztof Kolman, Antiope Lotsari, Marie Ivarsson, Erich Schüster, Niklas Lorén, and Romain Bordes
Informa UK Limited
Abstract Development of multi-purpose probes for mass transport measurements is of importance to gain knowledge in diffusional behaviour in heterogeneous structures such as food, hygiene or pharamceuticals. By combining different techniques, such as Fluorescence Recovery After Photobleaching (FRAP) and Nuclear Magnetic Resonance Diffusometry (NMR-d), information of both local and global diffusion can be collected and used to gain insights on for example material heterogeneities and probe-material interactions. To obtain a FRAP-responsive probe, fluorescent silica particles were produced using fluorescent preconjugates added in a modified Stöber process. A NMR-d responsive moiety was introduced by derivatizing the fluorescent silica particles with polyethylene glycol. The particle size distributions were determined by dynamic light scattering and transmission electron microscopy and these measurements were compared to value extrapolated from diffusion measurements using FRAP and NMR-d. The good agreement between the FRAP and NMR-d measurements demonstrates the potential of multi-purpose probes for future applications concerning mass transport at local and global scale simultaneously. GRAPHICAL ABSTRACT
Gunnar Símonarson, Sanna Sommer, Antiope Lotsari, Björn Elgh, Bo B. Iversen, and Anders E.C. Palmqvist
American Chemical Society (ACS)
Evolution of the polymorph selectivity of titanium dioxide was studied under acidic and low-temperature synthesis conditions. Short synthesis times resulted in high relative amounts of the rutile phase, and long synthesis times resulted in high relative amounts of the brookite and anatase phases. The effect of titania precursor concentration was investigated and found to have a large impact on the polymorph selectivity. As the reaction proceeds with time, changes in the chemical environment, caused in particular by the gradually decreasing titania precursor concentration, are therefore likely the cause of the change in polymorph selectivity observed.
Antiope Lotsari, Anand K. Rajasekharan, Mats Halvarsson, and Martin Andersson
Springer Science and Business Media LLC
Mineralisation of calcium phosphates in bone has been proposed to proceed via an initial amorphous precursor phase which transforms into nanocrystalline, carbonated hydroxyapatite. While calcium phosphates have been under intense investigation, the exact steps during the crystallisation of spherical amorphous particles to platelet-like bone apatite are unclear. Herein, we demonstrate a detailed transformation mechanism of amorphous calcium phosphate spherical particles to apatite platelet-like crystals, within the confined nanodomains of a bone-inspired nanocomposite. The transformation is initiated under the presence of humidity, where nanocrystalline areas are formed and crystallisation advances via migration of nanometre sized clusters by forming steps at the growth front. We propose that such transformation is a possible crystallisation mechanism and is characteristic of calcium phosphates from a thermodynamic perspective and might be unrelated to the environment. Our observations provide insight into a crucial but unclear stage in bone mineralisation, the origins of the nanostructured, platelet-like bone apatite crystals.The growth of apatite crystals from amorphous calcium phosphate is an area of intense study. Here, the authors report on the use of high resolution TEM imaging to provide evidence of nucleation clusters in the transformation process
Anand K. Rajasekharan, Antiope Lotsari, Viviane Lutz-Bueno, Marianne Liebi, and Martin Andersson
Wiley
A key challenge in developing bioinspired composites is the fabrication of well-defined 3D hierarchical structures ranging from nano to the macroscale. Herein, the development of a synthetic polymer-apatite composite realized by integrating bottom-up self-assembly and additive manufacturing (AM) is described. The resulting composite exhibits a bioinspired hierarchical structure over its 3D macroscopic volume. The composite is assembled in a bottom-up manner, where periodic nanoscale assemblies of organic micellar fibrils and inorganic apatite nanocrystals are organized as bundles of mineralized microstructures. These microstructural bundles are preferentially oriented throughout the macroscopic volume of the material via extrusion based AM. The obtained structural hierarchy is investigated in 3D using electron microscopy and small angle X-ray scattering tensor tomography and correlated to the structural hierarchy and anisotropy observed in biological tissues such as bone and the bone-cartilage interface. This work demonstrates the possibility to form polymer-apatite composites with a well-defined hierarchical structure throughout its macroscopic volume, which is crucial for the development of mechanically optimized materials for applications such as bone and osteochondral implants.
Ehsan Hashemi, Filip Hjort, Martin Stattin, Tommy Ive, Olof Bäcke, Antiope Lotsari, Mats Halvarsson, David Adolph, Vincent Desmaris, Denis Meledin,et al.
IOP Publishing
We have investigated the effect of strain-compensating interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors (DBRs). Samples with 10.5 mirror pairs were grown through plasma-assisted molecular beam epitaxy on SiC. Room-temperature current–voltage characteristics were measured vertically in mesas through 8 of the 10.5 pairs. The sample with no interlayers yields a mean specific series resistance of 0.044 Ω cm2 at low current densities, while three samples with 5/5-A-thick, 2/2-nm-thick, and graded interlayers have resistivities between 0.16 and 0.34 Ω cm2. Thus, interlayers impair vertical current transport, and they must be designed carefully when developing conductive DBRs.
T. D. Young, G. Jurczak, A. Lotsari, G. P. Dimitrakopulos, Ph. Komninou, and P. Dłużewski
Wiley
GaN quantum dots grown in ()’orientated AlN are studied. The -nucleated quantum dots exhibit rectangular- or trapezoid-based truncated pyramidal morphology. Another quantum dot type orientated on is reported. Based on high-resolution transmission microscopy and crystal symmetry, the geometry of -orientated quantum dots is proposed. A piezoelectric model is used within a finite element method to determine and compare the strain-state and electrostatic potential associated with the quantum dot morphology and an estimation of the band-edge energy is made. We report on some novel properties of the -orientated quantum dot, including mixed strain-states and strain-state bowing.
A. Adikimenakis, A. Lotsari, G. P. Dimitrakopulos, Th. Kehagias, K. E. Aretouli, K. Tsagaraki, M. Androulidaki, Ph. Komninou, and A. Georgakilas
AIP Publishing
The spontaneous growth of GaN nanopillars (NPs) by direct plasma-assisted molecular beam epitaxy on nitridated r-plane sapphire substrates has been studied. The emanation of metal-polarity NPs from inside an a-plane nonpolar GaN film was found to depend on both the substrate nitridation and the growth conditions. The density of NPs increased with increasing the duration of the nitridation process and the power applied on the radio-frequency plasma source, as well as the III/V flux ratio, while variation of the first two parameters enhanced the roughness of the substrate's surface. Transmission electron microscopy (TEM) techniques were employed to reveal the structural characteristics of the NPs and their nucleation mechanism from steps on the sapphire surface and/or interfacial semipolar GaN nanocrystals. Lattice strain measurements showed a possible Al enrichment of the first 5–6 monolayers of the NPs. By combining cross-sectional and plan-view TEM observations, the three-dimensional model of the NPs was constructed. The orientation relationship and interfacial accommodation between the NPs and the nonpolar a-plane GaN film were also elucidated. The NPs exhibited strong and narrow excitonic emission, suggesting an excellent structural quality.
K. Filintoglou, M. Katsikini, J. Arvanitidis, D. Christofilos, A. Lotsari, G. P. Dimitrakopulos, N. Vouroutzis, A. O. Ajagunna, A. Georgakilas, N. Zoumakis,et al.
AIP Publishing
Angular-dependent polarized Raman spectroscopy was utilized to study nonpolar a-plane ( 11¯20) and semipolar s-plane ( 101¯1) InN epilayers. The intensity dependence of the Raman peaks assigned to the vibrational modes A1(TO), E1(TO), and E2h on the angle ψ that corresponds to rotation around the growth axis, is very well reproduced by using expressions taking into account the corresponding Raman tensors and the experimental geometry, providing thus a reliable technique towards assessing the sample quality. The s- and a-plane InN epilayers grown on nitridated r-plane sapphire (Al2O3) exhibit good crystalline quality as deduced from the excellent fitting of the experimental angle-dependent peak intensities to the theoretical expressions as well as from the small width of the Raman peaks. On the contrary, in the case of the s-plane epilayer grown on non-nitridated r-plane sapphire, fitting of the angular dependence is much worse and can be modeled only by considering the presence of two structural modificat...
A. Lotsari, Th. Kehagias, G. Tsiakatouras, K. Tsagaraki, M. Katsikini, J. Arvanitidis, D. Christofilos, S. Ves, Ph. Komninou, A. Georgakilas,et al.
AIP Publishing
Heteroepitaxial non-polar III-Nitride layers may exhibit extensive anisotropy in the surface morphology and the epilayer microstructure along distinct in-plane directions. The structural anisotropy, evidenced by the “M”-shape dependence of the (112¯0) x-ray rocking curve widths on the beam azimuth angle, was studied by combining transmission electron microscopy observations, Raman spectroscopy, high resolution x-ray diffraction, and atomic force microscopy in a-plane GaN epilayers grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). The structural anisotropic behavior was attributed quantitatively to the high dislocation densities, particularly the Frank-Shockley partial dislocations that delimit the I1 intrinsic basal stacking faults, and to the concomitant plastic strain relaxation. On the other hand, isotropic samples exhibited lower dislocation densities and a biaxial residual stress state. For PAMBE growth, the anisotropy was correlated to N-rich (or Ga-poor) condit...
G. P. Dimitrakopulos, A. Lotsari, Th. Kehagias, A. Ajagunna, A. Georgakilas, Th. Karakostas, and Ph. Komninou
Wiley
Based on bicrystal symmetry and transmission electron microscopy observations, we elaborate on the coexistence of the two orientation variants of semipolar s-plane () InN epilayers grown on r-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE). It is shown that variant coexistence is favored by a high order of coincident symmetry ensuring significant lattice continuity. The (0002) || () low-energy grain boundary was identified to principally delimit the two InN variants. Aside of the variant coexistence, the InN/sapphire interface was observed to comprise protrusions attributed to the InN buffer layer growth. Rapid thermal annealing was employed in order to improve the epilayer quality and it was found to induce defect reduction attributed to dislocation glide. However, the InN/sapphire interface was adversely affected by this process.
P. Kavouras, A. Lotsari, Th. Kehagias, A. Georgakilas, Ph. Komninou, and G. P. Dimitrakopoulos
Wiley
a-plane () GaN thin films grown under various conditions on r-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE) were characterized by nanoindentation and transmission electron microscopy (TEM) techniques. It was found that the increase of the built-in threading dislocation (TD) density induces an increase on the values of both the nanohardness (H) and the reduced elastic modulus (E*). Characteristic single ‘pop-in’ discontinuities were detected on the loading segments of the load–unload cycle. The TDs appear to cause a decrease of the load for the onset of the pop-in discontinuity. The dependence of the elastoplastic behaviour on the TD density is attributed to their sessile character, causing them to become obstacles for basal plane slip.
T. Koukoula, A. Lotsari, Th. Kehagias, G.P. Dimitrakopulos, I. Häusler, A. Das, E. Monroy, Th. Karakostas, and Ph. Komninou
Elsevier BV
Abstract The nanoscale structural properties of ultrathin (2 nm high) self-assembled (0 0 0 1) polar and ( 1 1 2 ¯ 2 ) semipolar InGaN/GaN quantum dot (QD) superlattices, grown by plasma-assisted molecular beam epitaxy, were investigated using transmission electron microscopy (TEM) techniques. Samples grown under two sets of temperature ranges were compared. The higher-temperature uncapped polar QDs were well-defined and exhibited a truncated pyramidal morphology. Similar morphology was observed for the embedded QDs, albeit faintly diffused. On the other hand, the polar superlattices grown at lower temperatures were heavily distorted due to a large stacking fault density. Semipolar QDs exhibited lenticular morphology. The QD superlattices were found to be elastically strained using geometrical phase analysis, and their strain state was well-described by a biaxial approximation. The extrapolated indium content was consistent with reduced indium incorporation efficiency for the semipolar case compared with the polar one.
A. Lotsari, G. P. Dimitrakopulos, Th. Kehagias, A. O. Ajagunna, E. Iliopoulos, A. Georgakilas, and Ph. Komninou
Wiley
The structural properties of InN films grown on r -plane (102) sapphire by plasma-assisted molecular beam epitaxy (PAMBE) using low temperature buffer layers are studied. Nonpolar a -plane (120) and semipolar s -plane (101) films were deposited depending on buffer layer and growth conditions. Single crystalline a -plane InN was grown using a GaN buffer layer following sapphire nitridation. Transmission electron microscopy (TEM) observations showed three-dimensional growth and interactions of inclined threading dislocations emanating from the buffer layer. The defect interactions gradually reduced the threading dislocation density. Semipolar s -plane was grown when a thin InN buffer layer was employed. The epilayer was found to comprise two s -plane variants. Semipolar nucleation directly on the sapphire was observed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
J. Kioseoglou, A. Lotsari, E. Kalesaki, and G. P. Dimitrakopulos
AIP Publishing
Observations of easy transition between nonpolar and semipolar orientations during III-Nitride heteroepitaxy identify the 90o 〈1¯21¯0〉 rotation relationship as being very important in defining this coexistence. A rigorous analysis of this relationship using the topological theory of interfaces showed that it leads to a high order of coincident symmetry and makes energetically favorable the appearance of the intergranular boundaries. Principal low-energy boundaries, that could also be technologically exploited, have been identified by high-resolution transmission electron microscopy (HRTEM) observations and have been studied energetically using empirical potential calculations. It is also shown that these boundaries can change their average orientation by incorporating disconnections. The pertinent strain relaxation mechanisms can cause such boundaries to act as sources of threading dislocations and stacking faults. The energetically favorable (101¯0) || (0001) boundary was frequently observed to delimit m...
A. Lotsari, G.P. Dimitrakopulos, Th. Kehagias, A. Das, E. Monroy, and Ph. Komninou
Elsevier BV
Graphical abstractDisplay Omitted Highlights? InGaN/GaN quantum dot superlattices grown on (11-22) GaN were studied by HRTEM. ? Nominal (11-22) InGaN dots were lenticular-shaped. ? QD size and faceting increased when nucleation occurred on inclined planes. ? Strain and interaction with threading dislocations introduced indium fluctuations. ? Lattice strain analysis was correlated to the indium content. The nanoscale properties of self-assembled semipolar InGaN/GaN quantum dot (QD) superlattices, grown by plasma-assisted molecular beam epitaxy (PAMBE) on ( 1 1 2 ? 2 ) GaN template, were investigated by transmission electron microscopy (TEM) techniques. Preferential QD nucleation on crystal planes inclined at small angles relative to the ( 1 1 2 ? 2 ) plane was observed. Nominal ( 1 1 2 ? 2 ) QDs were lenticular-shaped but the QD size and faceting increased when nucleation occurred on the inclined planes. Strain and interaction with the threading dislocations introduced fluctuations in the indium concentration. Lattice strain analysis along the growth direction was correlated to the average indium content.
A. Lotsari, A. Das, Th. Kehagias, Y. Kotsar, E. Monroy, Th. Karakostas, P. Gladkov, Ph. Komninou, and G.P. Dimitrakopulos
Elsevier BV
Abstract V-shaped pits (V-defects) were observed in semipolar (1122)-oriented InGaN/GaN layers grown by plasma-assisted molecular beam epitaxy (PA-MBE). The pit morphology was that of inverted rhombic or trigonal pyramids aligned along the in-plane [1100] direction. Pit facet orientations were identified. The surface pits were found to be connected at their apex to mixed type a + c threading dislocations with large screw components. Such dislocations exhibited [1120] average line directions with zig-zag 〈1010〉 local lines, and they also induced V-defects at the InGaN/GaN interface.
A. Das, G. P. Dimitrakopulos, Y. Kotsar, A. Lotsari, Th. Kehagias, Ph. Komninou, and E. Monroy
AIP Publishing
Semipolar (11-22)-oriented InGaN/GaN quantum dots (QDs) emitting in the 380–620 nm spectral range were synthesized by plasma-assisted molecular-beam epitaxy. The influence of the growth temperature on the properties of InGaN QDs has been investigated by photoluminescence and transmission electron microscopy. Growth temperatures low enough to prevent indium desorption provide a favorable environment to semipolar plane (11-22) to enhance the internal quantum efficiency of InGaN/GaN nanostructures.
G. P. Dimitrakopulos, E. Kalesaki, J. Kioseoglou, Th. Kehagias, A. Lotsari, L. Lahourcade, E. Monroy, I. Häusler, H. Kirmse, W. Neumann,et al.
AIP Publishing
GaN quantum dots (QDs) grown in semipolar (112¯2) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (112¯2)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (112¯2) surface, QDs nucleated at depressions comprising {101¯1} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (112¯2). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces.
Th. Kehagias, L. Lahourcade, A. Lotsari, E. Monroy, G. P. Dimitrakopulos, and Ph. Komninou
Wiley
The interfacial properties of (1122) AlN epilayers grown on m-plane sapphire by plasma-assisted molecular-beam epitaxy (PAMBE) were investigated in the context of achieving high-quality semipolar GaN/AlN heterostructures with reduced internal polarization. Epilayers grown under metal-rich conditions with and without nitridation pre-treatment of the sapphire surface were compared using transmission electron microscopy (TEM). In addition to the dominant semipolar orientation, the films exhibited an interfacial zone comprising AlN nanocrystals with the m-plane orientation. The size of the nonpolar nanocrystals increased significantly with sapphire nitridation. A good epitaxial relationship with the substrate was attained by both the semipolar as well as the nonpolar AlN. However, the interfacial structure was perturbed by interfacial roughness and pit formation. The surface nanofacets were correlated to the increased introduction of nanocrystals with m-plane orientation.