Molecular materials - towards quantum properties (MOLMAT-Q)

Resume : One of the critical challenges in the field of two-dimensional (2D) supramolecular self-assemblies at surfaces is: how to exploit the room above the substrate, to provide to the physisorbed adlayer potential functionalities in view in view of solving scientific problems and applications in nanoscience? This is why we developed the Janus tecton concept. This is a versatile molecular platform based on the design of three-dimensional (3D) building blocks consisting of two faces linked by a pillar: one face is designed to steer 2D self-assembly on sp2 carbon flat surfaces, the other one exposes the desired functionality (either a chemical or functional unit) above the substrate. Scanning tunneling microscopy and molecular mechanics calculations show effectively that we offer a generic platform exposing a functionality in the third dimension with a well-controlled lateral order, paving the way for the noncovalent functionalization of sp2-carbon materials, opening interesting perspectives for applications in nanoscience as we will demonstrate in this presentation. Moreover, the successful self-assembly on graphene, together with the possibility to transfer the graphene monolayer onto various substrates, considerably expends the domains of application of our functionalization strategy,

Resume : Organic microcavities offer fascinating opportunities to study the interactions of light and organic matter. These observations are made possible by the large oscillator strengths and exciton binding energies provided by organic semiconductors. For electrically driven devices, highly conductive electrodes have to be integrated. For this purpose metallic electrodes are under study, but the large absorption properties of metals are generally to prevent optical coherence. In this talk, we will focus on the impact of metallic structures implemented into organic microcavities. Optimizing the structural design and introducing a higher order confinement are beneficial to maintain optical coherence at low thresholds. First, a homogeneous silver layer of 40nm thickness is implemented into a high quality organic microcavity. By exciting this structure non-resonantly the transition from a single photonic cavity mode to two coupled Tamm states is observed Optimizing the design reduces the overall optical losses and increases the Q factor to 650 allowing for the observation of coherent emission from cavity photons and Tamm plasmons at room temperature. Furthermore, lateral confinement of the optical modes can be introduced by generating an optical gain grating or by structuring the metal layer by means of photo-lithography. Structuring the metal layer into periodically placed stripes leads to the observation of surface plasmon polaritons and Bloch-like photonic bands. Above the lasing threshold, macroscopic coherence on different Bloch states is observed.

Resume : A facile and robust approach for fabricating structured, blue-emitting polymer hybrids is explored by grafting poly(styrene) incorporating π-conjugated terfluorene side-chains, to fluoromica silicate layers through surface-initiated nitroxide-mediated polymerization [1]. The hybrids exhibit blue photoluminescence quantum yield as high as 0.90 in the solid, and significantly enhanced thermal and chemical stabilities with respect to the organic precursors. The successful assessment of the hybrid material as efficient emitter in a light-emitting device is accomplished, and we shed light onto the not straightforward mechanism responsible for its emission. The layered hybrid approach allows us to achieve, at the same time, planarization and chemical and photo- stability of short oligo(fluorene)s whose emissive properties are enhanced thanks to exciton localization. Lying-flat intercalated terfluorene moieties are sensitized by non planar conformers grafted onto the outer polymer chains, that envelop the silicates, through a resonant energy transfer mechanism. Single layer solution processable device displays deep blue electroluminescence with an external quantum efficiency of 1.2 %, maximum luminance close to 1000 cd/m2, chromaticity coordinates of (0.16;0.11) and low efficiency roll-off thanks to the separation of the emissive region from the charge transport one. [1] Leone et al. J. Mater. Chem. C, 2013, 1, 6585

Resume : Quantum dots (QDs) are uniquely suited for use as lumophores in light emitting devices for large-area planar lighting. We use the inorganic semiconductors as a charge transport layers and demonstrate devices with light emission. The low luminescence was attributed to luminescence quenching of the QDs in conductive electrodes, QD charging, and the imbalance in the polarity of charges injected into the device. Mechanically smooth films were chosen to prevent electrical shorts between the semiconductor and the QDs. We grow semiconducting oxide films with low free-carrier concentrations to minimize the quenching of the QD EL. The hole transport layer (HTL) and the electron transport layer (ETL) having similar carrier concentrations and energy-band offsets to the QDs were used because the electron and the hole injection into the QD layer should be balanced. Metal oxides offer a deposition-dependent morphologies and conductivities. For the ETL and the HTL, we selected a 40-nm-thck ZnO:SnO2(ZTO) and InGaZnOx(IGZO) with a resistivity of 10 cm. To improve the emission intensities, we observed the effects of the spin coating parameters and the concentrations of QD colloidal solutions. As the spin coating speeds were decreased from 2,000 to 500 rounds per minute (rpm), QD layer uniformities were improved. To reduce the morphologically induced electrical shorts in the device, we control the deposition parameters of ZTO or IGZO sputtering. We deposit transport layer onto the ZnSe:ZnS QDs layers formed on the transport layers. We use the Indium Tin Oxide (ITO) electrode on the glass substrate. Adhesion between the QDs layers and the ZTO layer was improved by using the molecular linker, 2-carboylehtylphosphonic acid (CAPO).We demonstrate the role of the QDs concentration on the luminescence properties. Our devices were measured without environmental packaging and in atmospheric conditions. We obtain the bright and uniform pixel at 10V applied bias. Light emitting uniformity was improved by reducing the rpm of QD spin coating. In the case of QD concentration of 15 mg/mL, we observe the bright electroluminescence at 12V applied bias. Much of the decrease in QD luminescence can be attributed to additional quenching by the free carriers in the ETL.

Resume : Chiral magnetic structures have been recently the focus of intense research because they offer the possibility to store information in topological protected structures like skyrmions. Chirality and magnetism are also directly connected in the interaction between matter and electromagnetic radiation through the magneto-chiral dichroism and birefringence. Magneto-chiral dischroism (MCD), a non-reciprocal effect with different absorption of unpolarized light by systems with opposite chirality in the presence of a magnetic field, is a fascinating phenomenon that has been suggested to be at the origin of homochirality of life on the earth and observed only recently. It is in general very weak, being assumed in first approximation to be related to the product of natural (NCD) and magnetic circular (MCD) dichroism, and only few examples are available in literature with limited information on the factors that originate the phenomenon. We report here a detailed synchrotron investigation of the magneto-chiral effect detected at the 3d-metal K edge in two isostructural molecular helices comprising either isotropic Manganese(II) or anisotropic Cobalt(II) bridged by stable organic radicals. The experiments have revealed a strong magneto-chiral dichroism associated with the non-collinear spin structure of the CoII derivative, which is also the archetype of Single-Chain Magnets, i.e. one-dimensional structures showing magnetic bistability due to short-range magnetic correlation. Interestingly, the chiral nature of these molecular helices allows the presence of novel magneto-electric effects. Density Functional Theory calculations have in fact revealed that the electric polarization of the molecular helix is affected by the relative alignment of neighbouring metal and radical spins, opening the perspective of controlling the magnetization dynamics of these one-dimensional structures through the application of an electric field. The authors acknowledge the financial support of the European Research Council through the AdG MolNanoMaS and of Italian MIUR through FIRB project n° RBAP117RWN.

Resume : Two-dimensional (2D) materials such as graphene, MoS2, Bi2Se3 possess outstanding electrical, thermal, and mechanical properties. While electronic or optical properties are the main focus of research, their nanoscale mechanical and thermal properties of these materials are extremely important for the fundamental understanding and applications of these 2D nanostructures. We use a combination of scanning probe microscopies that combines low frequency and ultrasonic vibrations to map a wide dynamic range of stiffnesses from 0.02 to 2000 N/m with the lateral resolution of few nanometres. That allowed us to investigate results of residual stresses in supported graphene layers that revealed themselves as broken mechanical contact at the interface between graphene layer and the substrate, as well as to explore nanomechanical behaviour of few layer suspended graphene, MoS2 and Bi2Se3 films. We directly observed the transition of graphene layer deformation from plate to stretched membrane behaviour, and created nanoscale maps of shell instability for few layer graphene sheets, providing insight to the stresses in the free standing graphene films. We also addressed the challenge of exploration of thermal phenomena in graphene nanostructures by scanning thermal microscopy in high vacuum environment that allowed us to directly map thermal transport in suspended and suppored graphene layers with nanoscale resolution, and to explore both ballistic and diffusive regimes of heat transfer.

Resume : Polyhedral oligomeric silsesquioxanes (POSS) are organosilicon compounds with the empirical chemical formula RSiO3/2. Their three dimensionality, high symmetry, and nanometer size make them promising building blocks for nanomaterials. Combining the robust core with the functionalities of the attached organic groups can also change the physical properties of the POSS. A route to a large variety of functionalized POSS compounds by a two-step procedure has been developed. In the first step, an aminofunctionalized alkoxo silane such as 3-aminopropyltriethoxy silane is converted to amino-functionalized POSS (amine-POSS) by a sol–gel process. In the second step, the amine groups of amine-POSS are converted by state-of-the-art amine chemistry. A large number of reactants, including acids halides, esters, anhydrides or acrylates as well as reactants suitable for nucleophilic substitution can be applied. Conversion of an amine-POSS with acids halides leads to an amide-POSS. Amine chemistry can usually be performed under mild conditions, which suppresses degradation and/or cross-linking reactions of the amine-POSS during the conversion. Here we report syntheses and characterization of a new amido-POSS with alkyl and aryl groups. ESI-TOF mass spectrometry and nuclear magnetic resonance spectroscopy (1H, 13C, 29Si, and correlation spectroscopy) have been applied to prove the conversion of an amine-POSS to an amide-POSS.

Resume : Research on smaller and more powerful devices for data storage involves the design of new materials at the nanoscale. Our aim is to develop magnetic nanoparticles with high magnetic anisotropy (oxides or alloys) localized in well-organized nanocraters of a non-magnetic thin film.[1-2] Prussian blue analogues (PBA) were chosen as precursors of the magnetic particles owing to their particularly well-defined structure and chemical composition combined to their versatile chemistry. By tuning PBA chemistry, it should be possible to tune the chemical composition, the structure and therefore the magnetic properties of the derived oxides and alloys. The nanoperforated oxyde thin films were performed using sol-gel chemistry in addition with surfactant micelle templating. PBA growth into the nanocraters has been made layer by layer after a functionalization of the surfaces. The thermal treatment under controlled atmospheres allows the transformation of PBA into oxides or alloys. [3-4] Here we present our first results on the thermal treatment under oxidizing and reductive atmospheres of Co4[Fe(CN)]2.7 PBA confined into the nanocraters of nanoperforated TiO2 thin film. [1] S. Lepoutre, D. Grosso, C. Sanchez, G. Fornasieri, E. Riviere, A. Bleuzen, Advanced Materials 22 (2010) 3992. [2] J. Allouche, D. Lantiat, M. Kuemmel, M. Faustini, C. Laberty, C. Chaneac, E. Tronc, C. Boissiere, L. Nicole, C. Sanchez, D. Grosso, J. Sol-Gel Sci. Technol. 53 (2010) 551. [3] M. Yamada, M. Maesaka, M. Kurihara, M. Sakamoto, M. Miyake, Chem. Commun. 2005) 4851. [4] R. Zboril, L. Machala, M. Mashlan, V. Sharma, Crystal Growth & Design 4 (2004) 1317.

Resume : Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires (i) adjustable electrical resistance of the capping molecules, (ii) a reliable contact of the capping molecules to the electrode material[1] and (iii) immobilization of a defined number AuNP into electrode structures. In order to accomplish the latter we investigated the adsorption characteristics of 1,8-mercaptooctanoic acid and 8-amino-1-octanethiol stabilized AuNP (12.3±1.1 nm) on gold/palladium and platinum surfaces. Here, we present how AuNP differentially adsorb to the respective metals, directed by pH, ionic strength, and the composition of the electrolyte chosen[2]. In summary, AuNP with acidic end groups adsorb exclusively on platinum at pH 9 but on gold/palladium at pH 5. AuNP with amine end groups adsorb preferentially on platinum at pH 3. With these findings the integration of single, molecule capped AuNP and even the directed immobilization in nanoelectronic devices gets feasible. [1] S. Karthäuser, J. Phys. Cond. Mat. 23, 013001 (2011) [2] C. Kaulen et al., Langmuir (2014) dx.doi.org/10.1021/la404110y

Resume : Investigation of the geometric and electronic structure of the polyacetylene with substituent’s of different strengths of donor and acceptor functional groups was carried out with 3-21G basis sets, by incorporating The rates of this geometrical isomerization and Arrhenius parameters. In the case of unsubstituted polyacetylene as the reference.. The values of the equilibrium constant for the reaction have also been determined at various temperatures between 300 and 500 K and the value of the energies change calculated. The results demonstrate that the isomerization energies are positive and the barrier heights ∆Ebarrier are expected to be sensitive for the magnitude of substituents effects.

Resume : This work reports the colloidal synthesis of silver telluride (Ag2Te) nanoparticles by adapting a predefined procedure. As synthesized materials were rinsed several times with deionized water and anhydrous ethanol, and preserved in toluene. All the characterizations were performed on the drop-casts inks on suitable substrate. The crystallinity and phase group were assessed by x-ray diffraction (XRD) technique. Field emission scanning electron microscope (FESEM-EDX) analyses reveal nearly stoichiometric composition and self-assembled nature of the nanocrystals. Size, shape, crystallinity and dispersity were studied by high resolution transmission electron microscopy (HRTEM), which reveals that the nanoparticles are of nearly spherical and cubicle in morphology and in the size range of 5-10 nm resulted in different batches. Results will be discussed in detail.

Resume : Colloidal synthesis of lead telluride nanocrystals have been carried out by adapting a predefined procedure. The crystallinity and phase group were assessed by x- ray diffraction (XRD) technique. Qualitative analyses of the sample were performed using Raman spectroscopy. The interaction between oleic acid and PbTe nanocrystals are analyzed by subjecting the samples to Fourier transform infrared spectrometry. Field emission scanning electron microscopic analyses reveal lead rich stoichiometric composition and self-assembled nature of the nanocrystals both on silicon substrate as well as on carbon tape. High resolution transmission electron microscopy analyses reveal the high crystallinity and monodispersity of synthesized nanoparticles. Size ranging from 6.5 to 16 nm were obtained in different synthesis conditions. The long term environmental stability of a selected batch of nanocrystals in the ambient condition was also studied by subjecting the samples to XRD, Raman and HRTEM at different intervals from the date of synthesis. It is inferred that the nanocrystals retained its crystallinity, morphology and composition even after 20 months from the date of synthesis. Results will be discussed in detail.

Resume : Green synthesis and characterization of silver telluride (Ag2Te) nanoparticles have been carried out for the first time to best of our knowledge. We present the synthesis process, structural, morphological and compositional characterizations of the nanoparticles. A predefined process was adapted by replacing 1-dodecanethiol with powders of curcuma longa, a natural reducing agent. The as-synthesized materials were washed several times with deionized water and anhydrous ethanol, and re-dispersed in toluene. Further investigations have been carried out on drop-cast films deposited onto a chosen substrate. Field emission scanning electron microscope (FESEM-EDX) analyses reveal nearly stoichiometric Ag2Te nanomaterials. Transmission electron microscopy (TEM) reveals that most of the prepared Ag2Te nanoparticles are within a size range of 6-10 nm, along with some large nanoparticles of about 15-20 nm range. Optical properties and interaction of natural agent with the nanoparticle were also studied by subjecting the Ag2Te to Absorption, FTIR and Raman spectroscopy techniques. Results will be discussed in detail.

Resume : The single molecule magnet (SMM) terbium(III) bis(phthalocyanine) is considered as a promising candidate for spintronic applications. In this work, films with thicknesses below 100 nm were deposited by organic molecular beam deposition on ferromagnetic, in-plane magnetized Co films. The (magneto-)optical properties were investigated by means of variable angle spectroscopic ellipsometry (VASE) and magneto-optical Kerr effect (MOKE) spectroscopy at room temperature. The molecular orientation was determined from the degree of uniaxial anisotropy of the optical constants and from the amplitude of the magneto-optical Voigt constant. The magnetic coupling of the molecules with the substrate is investigated by MOKE magnetometry in the temperature range from 4 K to 300 K.