Iron(II) phthalocyanine (FePc) has potential applications in the fields of electrocatalysis, magnetic switching, electrochemical sensing, and phototheranostics. Despite it a reliable determination of its ground-state is still challenging. Using state of the art computational methods (CASPT2 and DMRG) and experimental approaches (Mӧssbauer spectroscopy and SQUID magnetic measurements) we identified the ground-state of isolated FePc as quintet [1]. High-resolution atomic force microscopy in combination with the above mentioned computational techniques showed that the FePc ground state is changed to triplet and singlet when the molecule is adsorbed at pristine graphene and N-doped graphene, respectively [2]. To conclude we demonstrated that the spin state transition of organometallic molecules and tuning of their electronic properties is possible not only by strong external stimuli like temperature, pressure or UV radiation but also through weak non-covalent interactions.
REFERENCES: [1] Dana Nachtigallová et al, Chem. Eur. J. doi.org/10.1002/chem.201803380.; [2] Bruno de la Torre et al, Nat. Comm. 2018, 9, 2831.
The concept of graphene-enhanced Raman scattering (GERS) was proposed and experimentally demonstrated in 2010. An enhancement of the Raman signal of molecules in very low concentrations in vicinity of graphene was observed. In addition, the photoluminescence of molecules placed on graphene is quenched.
In our work we showed that the photoluminescence of R6G was quenched also with functionalized graphene. Furthermore, we used functionalized graphene to experimentally probe the theoretical GERS conditions with Rhodamine 6G (R6G) molecules by assuming the change of the Fermi level of graphene due to its functionalization. We were able to qualitatively validate the theoretical GERS conditions by comparing several R6G Raman modes on differently functionalized graphene layers. Moreover, we studied the effect of the degree of fluorination on the GERS effect. The fluorination induces sp3 defects in graphene. We found that high fluorination degree leads to loss of the GERS enhancement, however it can be restored upon fluorine removal.
REFERENCES: [1] V. Valeš, P. Kovaříček, X. Ji, X. Ling, J. Kong, M. S. Dresselhaus, and M. Kalbáč, “Quenching of photoluminescence of Rhodamine 6G molecules on functionalized graphene,” Phys. status solidi B 253, 2347-2350 (2016).; [2] V. Valeš, P. Kovaříček, M. Fridrichová, X. Ji, X. Ling, J. Kong, M. S. Dresselhaus, and M. Kalbáč, “Enhanced Raman scattering on functionalized graphene substrates,” 2D Mater. 4, 25087 (2017).; [3] V. Valeš, Z. Melníková, T. Verhagen, J. Vejpravová, and M. Kalbáč, “Reversibility of Graphene-Enhanced Raman Scattering with Fluorinated Graphene,” Phys. status solidi B, 1700177 (2017).
The deterioration of electronic properties of graphene during conventional transfer in device fabrication requires new methods for direct growth on insulating substrates. Diamond is a semiconductor with a large band gap and high electrical resistance and therefore a natural candidate for dielectric substrates in graphene-based devices. Graphene formation by graphitizing the diamond surface is a promising way for device fabrication and can be promoted by metal catalysts. In this work, we investigate the mechanisms of the Ni-catalyzed graphitization of diamond. Single-crystal diamonds SCD(100), (110) and (111) with flat surfaces were coated with thin Ni layers (20 nm) and heated to 800 °C under high vacuum. During heating, the Ni film undergoes dewetting, breaks into particles and reacts catalytically with the diamond, whereby clear differences for the three surface orientations are observed: In the case of SCD(111), the surface seems rather stable against the catalytic action of Ni. The C atoms are removed only from the top layers of diamond and incorporated into the moving Ni particles. These C atoms migrate through the Ni by surface diffusion and are released either behind the particles or surrounding them entirely. On the SCD(110) and (100) surfaces the catalytic reaction is more advanced and the Ni particles penetrate the SCD rather then moving on their surfaces. However, in these cases the catalytic reaction also leads to the formation of thick graphite rather than graphene. Yet important conclusions on the mechanisms of graphitization of diamond surfaces are drawn.
Among the porous materials, metal-organic frameworks (MOFs) have particularly attracted attention due to their inherent properties such as high surface area, large pore volume, active metal sites with ordered and tunable pores. Unfortunately, some of the most promising MOFs are limited because an instability with respect to moisture, moderate conducting behavior and micro pores. However, non-porous functional graphene sheets are becoming an inexpensive material, have been proven an outstanding matrix to support various materials, leading to advanced materials for various application in the field of energy and environment. So, making hybrid composites based on MOFs and functional graphene materials enable integration of the unique properties of these two fascinating materials, thus allowing one to design materials possessing properties not possessed by either component. these GO-MOF composites exhibit high conductivity, hierarchical porous behavior, and excellent catalytic activity. In this presentation, we would like to summarize recent results focused on synthesis of GO-MOF by various covalent and noncovalent approaches and exploited for various application in the field energy and environmental applications gas storage/separation/sensing, oil spills separation, bifunctional electrochemical water splitting, thrombin and supercapacitor applications
Mechanical deformation plays an important role in 2D materials because it causes drastic changes of their properties, ranging from (opto)electronic to magnetic, chemical, or mechanical ones. In-plane stress represents well documented and straightforward means of exerting not-so-severe, but reasonably controlled level of deformation. In contrast, direct out-of-plane deformation (compression) can offer very interesting and more effective route to modify the properties of 2D materials thicker than 1 atom, e.g. bilayer graphene or monolayer transition metal dichalcogenides, which will be briefly discussed in the first part of the talk. More importantly, from a different view point, out-of-plane deformation comprises also all kinds of topographical distortions like ripples, wrinkles, folds etc. These corrugations are almost omnipresent because of the low bending rigidity of 2D materials and they form and change readily upon even miniscule stress. The nanometer-sized ripples cause dramatic decrease of mobility. The larger wrinkles and folds can generate a plethora of effects like gauge fields, energy funneling, or quantum confinement. There is a dire need to control the appearance and dimensions of such structures. The second part of the talk will thus summarize our recent efforts of manipulating this “third dimension” of 2D materials, with the focus on (i) the interplay of graphene and a compliant supporting polymer, and (ii) locally supported graphene by nano-pillar arrays.
Molecular recognition among chiral molecules on surfaces is of paramount importance in biomineralization, enantioselective heterogeneous catalysis, and for the separation of chiral molecules into their two mirror-image isomers (enantiomers) via crystallization or chromatography. Understanding the principles of molecular recognition in general, however, is a difficult task and calls for investigation of appropriate model systems. One popular approach is thereby studying intermolecular interactions on well-defined solid surfaces, which allows in particular the use of scanning tunneling microscopy (STM). Examples of chiral amplification via the so-called ‘sergeant-and-soldiers’ effect as well as manipulation of chiral adsorbates via inelastic electron tunneling will be presented. In a Pasteur-type experiment at the nanoscale, molecules that constitute a dimer are spatially separated with a molecular STM tip and their absolute handedness is determined with submolecular resolution STM. Moreover, we report spin-dependent filtering of electrons by monolayers of these helical molecules. Finally the first successful electrical current-driven, unidirectional motion of a synthetic molecule will be presented.
Self-assembly of nanoparticles under the action of laser field can be an universal method for the formation of nanostructures with specific properties for application in sensorics and nanophotonics. For prognosis of the self-assembly processes, the model of movement of an ensemble of quantum dots in a viscous media under the action of laser radiation with the account for interaction of laser-induced polarizations and Brownian dynamics is developed. This model is applied to the investigation of the self-assembly process of a triple of nanoparticles into three-particle structure with a predetermined geometry. Two specific cases of formation of nanostructure from a preliminarily formed pair of particles are studied: either for the pair fixed in space or from the unfixed pair of nanoparticles. The geometry of resulting nanostructures is shown to be determined by the polarization direction of laser radiation and the laser wavelength. Under proper choice of these parameters the formation of structures is shown to be highly efficient. E. g., maximum probability of structures formation is as hig as 36 - 46% per single laser pulse of 10 ns duration.
ZnO nanostructures are subject of interest in many research groups. Particularly the ZnO nanorods have a considerable potential to be applied in gas sensors, piezoelectric nanogenerators, photovoltaic devices or pressure sensors. Most of the applications require position-controlled ordered arrays of physically identical ZnO nanorods. Position-controlled nanorod arrays can be obtained by selective-area epitaxy on a suitable seed layer. The arrangement of the nanorods in hexagonal arrays with equal distances between the nearest neighbors is fundamental to understand their growth mechanisms.
The ZnO arrays were prepared by chemical bath deposition (CBD) through a poly(methyl methacrylate) (PMMA) mask on a sol-gel deposited ZnO seed layer. The most important benefits of CBD are its low cost, reproducibility, and low growth temperature. The PMMA on the ZnO seed layer was patterned by electron beam lithography (EBL). The EBL is a well-known method with a high resolution, which enables to prepare ZnO nanorods with different sizes and precise positioning. The nanorod arrays were characterized by scanning electron microscopy (SEM) and the interface between the nanorods and the nucleation layers was investigated by transmission electron microscopy (TEM).
We present a detailed TEM study of the interfaces between the ZnO nanorods and the ZnO seed layer. We answer the question how the parameters of the growth process, such as the growth temperature, the concentration of precursors, the time, and the solution flow rate, affect the growth of ZnO nanorods. We further demonstrate essential role of both the solution supersaturation and the distance between the nanorods in the array in control of their growth mechanisms.
The study was supported by the Charles University, project GA UK No. 1501318.
It is known that the materials based on tetragonal zirconia nanopowders (3Y-TZP) have a unique combination of physical and mechanical properties such as: high strength and fracture toughness, chemical and radiation resistance, ionic conductivity, biocompatibility, etc. Therefore, these ceramic materials have used in various fields of technology: for the production of extrusion matrices, cutting tools, guide valves, as well as for prosthetics in surgical orthopedics, dentistry, and others. For the realization of these all properties of materials based on zirconia it is necessary to learn scientific bases of synthesis and consolidation processes of nanoparticles, understanding and control of all stages of the ceramic materials production. Today the actual problem in their manufacture is the optimization of conditions, methods of obtaining nanopowders, and especially their consolidation at the sintering process.
The impact of a small amount of the GeO2 additive on the kinetics of the initial sintering stage of 3Y-TZP was investigated. It has been shown the significant influence of different amount (0.2-5 wt%) additive on the sintering behavior of 3Y-TZP. The shrinkage behavior was investigated using the dilatometric data and constant rate of heating (CRH) method for determining the sintering mechanism at the initial sintering stage. It was found the sintering parameters and sintering diffusion mechanisms was changed with GeO2 addition in depends on the synthesis method (two synthesis methods were used to obtain the nanopowder system 3Y-TZP-GeO2 - co-precipitation and mechanical mixing methods). It was shown the sintering mechanism changed from volume diffusion to the grain boundary diffusion in case mixed and milled during 8 hours nanopowders PMM8-3Y-TZP-0.2 wt% GeO2 that leads to the activation energy increasing. With increasing GeO2 addition the sintering mechanism changed again (GBD→VD) which is accompanied by a sharp decrease in activation energy and activation of the sintering process. In case of co-precipitated 3Y-TZP-GeO2, GeO2 addition didn’t change the sintering mechanism but accelerated the sintering process because the viscous flow mechanism takes part in mass transfer process due that led to the activation energy decreasing.
Thus, the modification of the structure of the 3Y-TZP by the GeO2 led to a change in the characteristics of the powders, accelerated the sintering process due to the diffusion mechanism changing and led to a change in the structure of the resulting ceramics, which would entail a change in its properties.
Gold nanoparticles (Au NPs) dispersed on a porous support such as titania nanotubes (TNTs) can be utilized as catalysts in numerous chemical reactions, including oxygen reduction reaction (ORR). Improvement of the reaction kinetics requires an understanding of the catalytic activity-structure relation of Au-TNT systems. The main goal of the present work is to define the effect of titania semiconducting properties as well as Au NPs size and their amount on the efficiency of Au-loaded TNTs for ORR in an alkaline medium.
TNTs were produced by Ti anodization and annealed at 350, 450 and 550 °C. Au NPs with a diameter of 5, 15 and 25 nm were synthesized by the chemical methods and then deposited onto TNTs from colloidal solutions. Electrocatalytic activity of the prepared electrodes toward ORR was examined by cyclic voltammetry.
In oxygen-saturated solutions, ORR on the bare TNT electrodes is observed as an irreversible peak/wave at potentials more negative than -0.65 V. For the Au-TNT electrodes, an additional wave related to ORR on the gold surface appears at less negative potentials. The overpotential for O2 reduction on the Au surface grows with increasing the temperature of TNT annealing and the Au NPs size. Moreover, the electrocatalytic activity of Au-TNTs is found to be dependent on the surface concentration of Au NPs. Optimal conditions for the preparation of gold-modified TNT electrodes were established. The nature of the observed effects was explained by peculiarities of the electron transport through Schottky barrier formed at the Au NPs - TiO2 interface.
Quantum dots (QDs) are of great interest for creating optoelectronic devices due to their tunable bandgap, high stability and simple solution processability for device fabrication. One of the ways to increase the efficiency of such devices is to combine QDs with different carbon nanostructures. Lead sulfide PbS QDs with a ligand shell of oleic acid (OA), methylammonium iodide (MAI) and PbI2 were employed to study the process of charge transfer process between QDs and sheets of reduced graphene oxide (rGO) and graphene oxide (GO). QDs have been linked to graphene sheets by the (3-mercaptopropyl) trimethoxysilane in colloidal solution. As a result, we observed a change in intensity and kinetics of QDs PL, that allowed to estimate the efficiency of the charge transfer. The transfer efficiency for the QD-rGO system was found to be higher than that of the QD-GO system, ~ 80% vs. 66% respectively. Modification of the QDs surface with MAI and PbI22 allowed to achieve a charge transfer efficiency of up to 83% and 89%, respectively.
GaN/AlGaN-based high electron mobility transistors (HEMTs) attain better performance than their state-of-the-art silicon-based counterparts do, offering higher power, higher frequency as well as higher temperature of operation and stability [1, 2], although their voltage and current limits are somewhat lower than for the SiC-based HEMTs. GaN/AlGaN-based HEMTs are a potential choice for electric-powered vehicles, for which they are approved not only for their power parameters, but also for their good temperature stability, lifetime and reliability. It is important to optimize HEMT structures and their growth parameters to reach the optimum function for the real-world applications.
HEMT structures described and discussed here were grown by MOVPE technology in AIXTRON apparatus on (111)-oriented single-surface polished 6" Si substrates.
Structural, optical and transport properties of the structures were measured with X-ray diffraction, optical reflectivity, photoluminescence, and time-resolved and micro-Raman spectroscopy. Special effort was devoted to the electrical contacts preparation, to the measurements of resistance and sample characterization using the transmission-line matrix (TLM) method.
REFERENCES:
[1] M. L. Yang, Ch.-W. Cheng, M. T. Bulsara, and E. A. Fitzgerald, J. Appl. Phys. 111, 104511 (2012).
[2] B. Brennan, D. M. Zhernokletov, H. Dong, C. L. Hinkle, J. Kim, and R. M. Wallace, Appl. Phys. Lett. 100, 151603 (2012).
This presentation reviews briefly electron beam lithography and focused ion beam (FIB) nano fabrication principles. Links to Raith’s dedicated machines as installed at about 400 places around the world will be given.
Ultra-high resolution nano lithography has enabled “quantum devices”, i.e. devices made to study quantum phenomena or to exploit such phenomena for computing, sensors, cryptography and metrology.
The presentation reviews a couple of selected examples from different fields. All shown examples have been made using Raith instrumentation.
Due to low energy bandgap, high absorption coefficient in near infrared region, size-tunable properties and suitability for wet processing colloidal lead sulfide quantum dots are widely studied material for applications in the NIR. This, combined with tunable non-linear absorption and reflection, make PbS quantum dots possible alternative material for use in the non-linear optics.
Previous Z-scan studies on the third-order nonlinear susceptibility of PbS quantum dots report either saturable or reverse saturable absorption, another issue is that the values of non-linear refractive index are varying from work to work. Both of the mentioned parameters depending on sample preparation method and probing pulse properties. One of the reasons of such controversial results might be thermo-optical effects due to sample overheating, occurring upon exposure to high-frequency repetition rate of a probing pulse. If present, thermo-optical contribution to the refractive index are extremely hard to separate from the purely non-linear contribution.
In our work, we present results of a Z-scan measured non-linear refraction properties of colloidal lead sulfide quantum dots of different sizes in different non-polar solvents. In order to obtain purely non-linear change in refractive index and to avoid thermal effects we modify classic Z-scan method to operate with extremely low probing laser pulse repetition rate of 1 Hz.
Extracellular biosynthesis of nanoparticles have many important advantages such as well dispersed in aqueous solutions, low energy requirements, ecofriendly, non-toxic, low-costs and non-flocculate. This technique have shown significant promise as targeted drug delivery applications. In this investigation, for the first time, we examine the efficacy of targeted therapeutic delivery with t-PA and SK immobilized to biosynthesis of nanoparticles (CuNP) by using Streptococcus equi strains isolated from the horses of Iran and their ability to produce metallic nanoparticles. Also we compared them with their chemical synthesis. The S. equi was screened for its ability to produce MNPs. The different sizes and shapes (23 to 89 nm) are presented in the formation with good dispersion and high stability. Response Surface methodology was applied for the optimized production of biological CuNPs. The growth factors like pH, temperature and incubation time was changed. The optimum conditions to obtain CuNPs were found with the culture conditions of pH 7.5 in 120h at 35 ºC. To determine some of MNPs structural properties UV-vis absorption spectrophotometer, FTIR, XRD and SEM has characterized. The results provided some parameters may impact on the formation of biological MNPs. Lastly, these MNPs were conjugated with t-PA and SK, as a drug carrier. In addition, effective thrombolysis with magnet-guided SiO2-CuNPs-tPA-SK is demonstrated in rat embolism model where 18.59% of the regular t-PA dose and 15.78% of SK dose restored and 15-25 min reductions in blood clot lysis time were observed compared with runs with free t-PA and without magnet-guided and using the same drug dosage. The comparison between CuNPs with MNPs shows that thrombolysis had not been directed to the type of magnetic carrier under the magnetic guide.
Until two years ago, the smallest permanent magnets were single molecular magnets. They are promising candidates for magnetic information storage, molecular spintronics, and quantum bits (qubits) [1, 2]. Surface Science has opened up an alternative approach to this field in studying the magnetic properties of single atoms adsorbed onto surfaces. Spectacular properties such as very large magnetic anisotropies were reported, but until two years ago, all atoms were paramagnetic. Very recently a major breakthrough was achieved by identifying systems where a single surface adsorbed atom can indeed be a stable magnet. Ho atoms on two monolayer thick MgO(100) films grown on Ag(100) where found to exhibit magnetic remanence up to 30 K and relaxation times of at least one hour at 2 K [3]. This result was obtained from ensemble measurements using X-ray magnetic circular dichroism
(XMCD). Spin-polarized scanning tunneling microscopy (STM) measurements demonstrated reading and writing of individual Ho atoms and confirmed their long magnetic lifetimes [4]. Very recent STM experiments show stable magnetization over two hours in external fields of 8 T that are applied opposite to the magnetization of the atoms and at temperatures of 30 K; the first spontaneous switching is observed at 45 K [5]. We have identified two more systems that exhibit permanent magnetism in single adatoms, namely Dy atoms graphene on Ir(111) [6] and Tb/MgO/Ag(100) [7]. These single atom magnets are all candidates for magnetic information storage; the combined coercitive field and thermal stability of Ho/MgO(100) outperforms the
best single molecular magnets. Electron spin resonance (ESR) has been performed on individual Fe and Ti atoms with the STM [8 - 10]. This technique gives access to the coherence time, which is one of the benchmarks of qubits. We will elaborate on the possibility of single atom qubits and coherent manipulation of their magnetic quantum states.
REFERENCES: [1] S. Thiele, F. Balestro, R. Ballou, S. Klyatskaya, M. Ruben, and W. Wernsdorfer, Science 344, 1135 (2014).; [2] R. J. Blagg et al., Nat. Chem. 5, 673 (2013).; [3] F. Donati et al., Science 352, 318 (2016).; [4] F. D. Natterer et al. Nature 543, 226 (2017).; [5] F. D. Natterer, F. Donati, F. Patthey and H. Brune, Phys. Rev. Lett. in press (2018).; [6] R. Baltic et al. Nanolett. 16, 7610 (2016).; [7] L. Persichetti et al., to be published.; [8] S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan, and A. J. Heinrich, Science 350, 417 (2015).; [9] K. Yang et al., Phys. Rev. Lett. 119, 227206 (2017).; [10] P. Willke et al., Sci. Adv. 4, eaaq1543 (2018).
We use quantum-mechanical calculations to test a hypothesis of Grover et al. (J. Mag. Mag. Mater. 15 (1980) 699) that Co atoms in the Fe2AlCo compound have on average 3 Fe and 3 Co atoms in their second nearest neighbor shell. We have simulated four structural configurations of Fe2AlCo including the full Heusler structure, inverse Heusler polymorph and two other phases matching this idea. The highest thermodynamic stability at T = 0 K is indeed predicted for one of the phases with the distribution of atoms according to Grover and et al. However, small energy differences among three of the studied polymorphs lead to a disordered CsCl-structure-like (B2-like) phase at elevated temperatures. The fourth variant, the full Heusler phase, is predicted to be mechanically unstable. The global magnetic states are predicted to be ferromagnetic but local magnetic moments of Fe and Co atoms sensitively depend on the composition of the first and second coordination shells (for details see our recent publication Friák et al., Materials 11 (2018) 1543).
The phase diagram of the alloy nanoparticles (nanoalloys) were investigated by CALPHAD method. The binary nanoalloys Ag-Au, Ag-Ni and Au-Ni were investigated with respect to nanoparticle size as independent variable. The phase diagram of the ternary Ag-Au-Ni nanoalloy was predicted also with CALPHAD. The thermodynamic approach was developed by surface contribution for different shapes of particles. There is presented combination of fully miscible system Au-Ni with two fully immiscible systems Ag-Ni and Au-Ni. The calculation was completed by theoretical values of surfaces energy from density functional theory (DFT).
Iron aluminides have a potential to substitute stainless steels in high temperature applications. They offer excellent chemical resistance to corrosion, reasonable strength and low material cost. These Fe-Al-based superalloys constitute a class of lightweight materials that may be applied in automotive, wind energy and aviation applications to lower fuel consumption, to produce electricity more efficiently and to reduce CO2 emissions. We perform quantum-mechanical calculations to examine the magnetic and electronic properties of interfaces between the disordered α-phase Fe-18.75at.%Al and the ordered intermetallic compound Fe3Al using density functional theory (DFT) implemented in the Vienna Ab-initio Simulation Package (VASP). In order to reveal the relations between magnetic and structural properties of a disordered α-phase Fe-18.75at.%Al and its impact on the interfaces, three different models of special quasi-random structure (SQS) are used. It turns out that the properties of interfaces between these two phases sensitively depend on a distribution of Al atoms in the disordered phase. The financial support from the Czech Science Foundation received under the Project No. 17-22139S is gratefully acknowledged.
Nanoparticles of metal alloys (nanoalloys) have been in the focus of considerable attention for displaying many interesting properties, such as depression of melting point, surface plasmon resonance, enhanced catalytic activity, and variable morphology. A challenging task is to calculate nanoalloy phase diagrams in dependence on the particles size for complex systems. Nanoalloys could be synthesized by the solvothermal method with the hot-injection into oleylamine. We synthesized binary metal alloy nanoparticles AgNi and AgCu by injection of suitable molecular metal precursors, such as [Cu(PPh3)2(bea)] (bea = benzoate) and [Cu(PPh3)3(Hphta)] (phta = phthalate) into a mixture of oleylamine and octadecene at 230 °C. The type and nature of the ligands have a decisive effect on nanoparticle formation and their morphology and properties. We demonstrated the influence on the final nanoalloy elemental distribution, phase composition, particle size distribution, shape, morphology and optical properties. Dynamic Light Scattering (DLS), Transmission Electron Microscopy (STEM), Elemental analyses (ICP OES), and Small-Angle X-ray Scattering (SAXS) analyses were performed for determination of the chemical composition, average size, size distribution, and shape of the prepared nanoparticles. Plasmon resonances were also observed. The combination of CALPHAD approach with the ab initio calculations of surface stresses was used for modelling of size-dependent phase diagrams of binary systems. Temperatures of invariant reactions were obtained by DSC measurements and experimental results were in good agreement with the calculated values.
This research has been financially supported by the MEYS CR under the project CEITEC 2020 (LQ1601) and by the Czech Science Foundation (GA 17-15405S). CIISB project LM2015043 is gratefully acknowledged for the financial support of the measurements at the X-ray diffraction and BioSAXS Core Facility.
In the current era of nanotechnology size control of materials down to molecular level (≤ 2 nm) is of major importance with respect to fundamental science as well as to their potential applications ranging from biomedicine to photovoltaics. Surface chemistry, reactivity, structure, photoluminescence may, on such extreme size scale, significantly deviate from the bulk, including appearance of quantum phenomena, phase changes etc.
Due to a specific nanodiamonds (NDs) preparation routes (detonation or high-pressure high-temperature (HPHT) synthesis), NDs with mean size of 2 nm or lower have not been available so far in a reasonable amount. In fact, it was not clear if such molecular-sized NDs can at all be stable, keeping the diamond crystalline structure.
By Raman spectroscopy and scanning transmission electron microscopy (STEM) we demonstrate that nanodiamonds can indeed stably exists down to 1 nm (S. Stehlik et al., J. Phys. Chem. C. 119 (2015) 27708-27720), and we introduce a high-yield technique for preparation of molecular-sized nanodiamonds with the volumetric mean size around 2 nm. This is achieved by controllable size reduction of conventional 4-6 nm detonation nanodiamonds (DNDs) via oxidative etching in air (S. Stehlik et al., Sci. Rep. 6 (2016) 38419). Analytical ultracentrifuge together with atomic force microscopy (AFM) serve here as in particular useful and accurate tools for analyses and adjustment of size distribution of NDs in colloidal dispersions on nanometer scale in contrast to conventional dynamic light scattering (DLS) method. We show that molecular-sized DNDs keep the specific DND structure and variable surface chemistry (e.g. hydrogenated to oxidized) and corresponding zeta potentials (positive to negative) down to 2 nm or below (S. Stehlik et al., ACS Appl. Mater. Interfaces 9 (2017) 38842-38853). Several applications of molecular-sized NDs in photonics and biomedicine will be proposed and discussed.
ACKNOWLEDGEMENTS: This study was supported by the Czech Science Foundation (grant No. 18-11711Y).
One-dimensional materials represent an attractive class of nanostructures, mainly because of their geometry which inherently implies applications such as electrodes for sensing purposes or conduction channels in nanoscale electronics. Different mechanisms may be utilized to prepare nanowires, e.g. stress-driven one-dimensional diffusion, metal-catalyzed growth (VLS) etc. The most important role in identifying and description of the growth mechanisms was played by real-time microscopies, mostly TEM. However, although very powerfull in terms of image resolution, TEM is also limited in use, especially because of very strict sample geometry requirements.
In this contribution, I will present our real-time in-situ scanning electron microscopy experiments of nanowire growth. Two different material systems will be presented - germanium nanowires catalyzed by Au nanoparticles and WOx nanowires. As for the latter case, our experiments reveal a very unusual oxidation mechanism of tungsten disulfide nanotubes, resulting in tungsten oxide nanowire formation. Although the nanotube oxidation and nanowire growth occur spontaneously at high temperatures, they are significantly accelerated by the electron beam. This finding allows on-demand formation of tungsten oxide nanowires utilizing the focused electron beam. Additionally, in agreement with recent reports on tungsten oxide, we have identified quantum emitters within the nanowires. This result is a very promising step towards deterministic placement of quantum emitters with high spatial resolution.
The paper discusses the influence of the molecular structure of small-molecule organic semiconductors on charge transport in molecular crystals. Using examples of diketopyrrolopyrroles derivatives, we will discuss the key properties leading to high charge carrier mobility. Furthermore, the possibility of preparing soluble molecular semiconductors with high charge carrier mobility will be discussed. By introducing solubilizing groups, spatial obstruction and resulting greater intermolecular distances discouraging efficient charge transfer usually occur. A solution to this unwanted effect may be using solubilizing sub-units derived from molecules of high ability to produce rigid and stable crystals, e.g. adamantane [1,2]. Successful realization of such approach leading to mobility in the range of 10-1 cm2/V/s will be demonstrated.
ACKNOWLEDGEMENTS: This work was supported by Czech Science Foundation, Czech Republic via project No. GA17-21105S; research infrastructure was supported by project MŠMT, Czech Republic No. LO1211.
REFERENCES: [1] Kovalenko et al. J. Mater. Chem. C, 2017, 5, 4716-4723.; [2] Krajčovič et al. J. Lumines., 2016, 175, 94-99.
Rare earth-doped yttrium aluminium garnets (YAG) represent key photonic materials. However, their crystallization temperature above 800 °C strongly limits their applications. The nanocomposite based on YAG nanocrystals encapsulated inside a sol-gel matrix is a promising way allowing the preparation of thin films that keep optical properties of YAG nanocrystals.
We present a versatile sol-gel route to obtain europium-doped yttrium aluminium garnet (Eu:YAG) nanocrystals encapsulated inside a silica glass matrix. Defect-free Eu:YAG nanocrystals with tailored size were prepared by a radiation induced synthesis. The nanocrystals were dispersed in a sol prepared by a controlled hydrolysis of tetramethylorthosilicate.
The structural evolution of Eu:YAG nanocrystals inside a silica matrix during the thermal treatment was analyzed by structural and spectroscopic techniques, such as X-ray diffraction, as well as absorption and emission spectroscopy. Optical and luminescence properties of formed materials were related to their structural properties.
The sol-gel approach resulted in the formation of highly transparent monoliths. The Eu:YAG nanocrystals were homogenously distributed inside a silica glass matrix and they were thermally stable up to 1250 °C. The xerogels exhibited absorption spectra typical for Eu3+ ions localized inside a crystalline lattice of YAG. The thermal treatment above 1300 °C resulted to the solid-state reaction between the nanoparticles and the silica matrix. The promising application of prepared nanocomposites in the field of radioluminescent phosphors and high-power laser technology is widely discussed in the contribution. ACKNOWLEDGEMENTS: The financial support of Grant Agency of the Czech Republic, project N° 17-06479S is gratefully acknowledged.
The L21-structure Fe2AlTi intermetallic compound appears as one of the two phases in Fe-Al-Ti superalloy nanocomposites [1]. Experimental data related to low-temperature magnetic properties of this Heusler compound indicate that the total magnetic moment is about 0.1 Bohr magneton per formula unit [2]. In contrast, previous quantum-mechanical calculations [2] predicted the total magnetic moment in this compound nearly an order of magnitude higher. We thus performed first-principles calculations which were both unconstrained and fix-spin-moment constrained when searching for the ground-state electronic structure. In attempt to explain discrepancy between theory and experiment we simulated different conditions which may reduce the magnetic moment. The existence of non-magnetic states is unlikely to provide the explanation similarly as trigonal and tetragonal deformations but deviations from the stoichiometric state - iron and titanium rich states - seem very likely to explain the discrepancy. The financial support from the Czech Science Foundation received under the Project No. 17-22139S is gratefully acknowledged.
The increase of nonradiative rate with decreasing emission energy reduces the luminescence quantum yield (QY) of near-infrared photoluminescent (NIR PL) metal nanoclusters. Efficient surface ligand chemistry can significantly improve the PL QY of NIR emitting metal nanoclusters. In contrast to the widely reported but modestly effective thiolate ligand-to-metal core charge transfer, we show that metal-to-ligand charge transfer (MLCT) can be used to greatly enhance the luminescence QY of NIR PL gold nanoclusters (AuNCs). We synthesized water-soluble and stable colloidal suspensions of AuNCs with high QY (∼25%) upon introduction of triphenylphosphonium moieties into the surface capping layer. By using a combination of spectroscopic and theoretical methods, we provide evidence for gold core-to-ligand charge transfer occurring in AuNCs. This work could stimulate the development of very bright AuNCs for promising optoelectronic, bioimaging, and other applications.