[diss] Perustieteiden korkeakoulu / SCI

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Adiabatic control in circuit quantum electrodynamics
(Aalto University, 2018) Vepsäläinen, Antti; Paraoanu, Sorin, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Kvantti group; Perustieteiden korkeakoulu; School of Science; Pekola, Jukka, Prof., Aalto University, Department of Applied Physics, Finland
In circuit quantum electrodynamics the coherence of Cooper pairs in superconductors is employed to create macroscopic electric circuits with quantized energy levels. Such circuits can be coupled with each other and exploited as building blocks of a quantum computer. Accurate and robust control of the quantum state in the circuits is a central condition for the operation of the quantum computer and one of the prerequisites for the implementation of complex algorithms used in quantum information processing, such as quantum error correction. In this thesis adiabatic control in circuit quantum electrodynamics is investigated with the focus on manipulating three-level systems. In adiabatic control the eigenstates of the system are slowly modified by changing the external control parameters, which govern the evolution of the system. If the changes in the parameters are slow enough, the state of the system follows the eigenstates in the adiabatic basis, thereby realizing the intended operation. The advantage of adiabatic control is its inherent robustness to small errors or noise in the control parameters; the result of the state manipulation only depends on the asymptotic values of the control parameters, but not on their exact values during the process. Shortcuts to adiabaticity can be used to speed up the otherwise slow adiabatic control by introducing a correction pulse that compensates the diabatic losses during the state manipulation. This allows one to overcome the limitation on the speed of the protocol but simultaneously reduces the method's robustness to the variations in the control parameters. If the level of noise in the control parameters is known, using the shortcut it is possible to find the optimal level of robustness which is required to mitigate the noise. Circuit quantum electrodynamics offers a perfect experimental platform for investigating quantum control due to the possibility of realizing complicated control schemes using microwave electronics. With commercially available digital-to-analog converters the control signals can be digitally created, which enables accurate and coherent control of the quantum circuit. In this thesis both theoretical and experimental results on adiabatic control applied to superconducting transmon circuits are presented. It is shown that stimulated Raman adiabatic passage can be used for population transfer in a three-level transmon, which can be further improved using shortcuts to adiabaticity. Furthermore, a scheme for implementing robust superadiabatic rotation gates in transmon is proposed. Finally, it is demonstrated that superconducting qubit can be used as an ultra-sensitive detector of magnetic flux.
Adiabatic melting experiment: ultra-low temperatures in helium mixtures
(Aalto University, 2020) Riekki, Tapio S.; Tuoriniemi, Juha, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; µKI group; Perustieteiden korkeakoulu; School of Science; Hakonen, Pertti, Prof., Aalto University, Department of Applied Physics, Finland
Mixture of the two stable helium isotopes, 3He and 4He, is a versatile system to study at low temperatures. It is a mixture of two fundamentally different quantum mechanical particles: fermions and bosons. Bosonic 4He component of the dilute mixture is known to become superfluid at about 2 K, while superfluidity of the dilute fermionic 3He component has not yet been observed. The transition is anticipated to occur at temperatures below 0.0001 K (i.e. 100 uK). To reach such ultra-low temperatures, new cooling methods need to be developed, one of which is the main subject of this thesis. Current, well-established, cooling methods rely on external cooling, where a metallic coolant is used to decrease temperature in a liquid helium sample. Their performance is limited by rapidly increasing thermal boundary resistance. Our novel adiabatic melting method relies on internal cooling process, where both the coolant and the sample are same helium. First, we create a phase-separation in the mixture by increasing its pressure to about 25 times the atmospheric pressure. This solidifies the 4He component, and we ideally end up with a system of pure solid 4He and pure liquid 3He. The phase-separated system is then precooled by conventional methods, after which the solid is melted. This allows 4He to mix with 3He again in heat absorbing process, resulting in a saturated mixture with about 8% molar 3He concentration. In theory, the mixing can reduce temperature by more than a factor 1000, but external heat leaks and imperfect phase-separation reduced this to the factor 5-7 in this work. We study the performance of the melting method under various conditions, such as different melting rates, various total amount of 3He, and alternate configurations of the setup. We also developed a computational model of the system, which was needed to evaluate the lowest achieved temperatures, as the mechanical oscillators used for thermometry had already become insensitive. For it, we studied the thermal coupling parameters of our system, including thermal boundary resistances and 3He thermal conductivity. The lowest resolved temperature was (90 +- 20) uK, still above the superfluid transition of the 3He component of the mixture. We also present suggestions for future improvements for the setup.
Advanced synthesis of single-walled carbon nanotube films by aerosol CVD method for electro-optical applications
(Aalto University, 2019) Iakovlev, Vsevolod Ya.; Krasnikov, Dmitry V., Dr., Skolkovo Institute of Science and Technology, Russia; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland; Nasibulin, Albert G., Prof., Skolkovo Institute of Science and Technology, Russia
Single-walled carbon nanotubes (SWCNTs) are a unique family of materials emerging towards high performance applications in electronics and optoelectronics. However, despite significant progress over the last 25 years, the problem of SWCNT production with tailored characteristics, the absence of the growth models and postsynthesis methods to improve specific SWCNT properties are still the main barriers towards a wide range of applications. The methods of the SWCNT synthesis, data processing and SWCNT treatment developed are still not fully optimized. In the current thesis, the abovementioned problems were addressed with the following demonstration of advanced applications of SWNT films: A new reactor for the aerosol CVD growth of SWNTs equipped with spark discharge generator of catalyst nanoparticles was developed. The design proposed resulted in a robust apparatus when compared with the ferrocene CVD reactor. The stability and scalability of the SWCNT synthesis are the main benefits of the spark discharge generator. An advanced control of diameter distribution, defectiveness, and the yield for the first time was achieved through the use of artificial neural networks (ANN). This allowed us to achieve precise control towards mutual relation between the reactor parameters and key SWCNT characteristics. The methodology proposed can help with adjusting the diameter distribution, yield and quality of SWCNTs with prediction error of 4%, 14%,23% respectively for very limited data set. A novel technique for the post-synthesis improvement of electro-optical characteristics of SWCNT films by a laser treatment was proposed. In this process by a short pulse laser irradiation, the transparency of SWCNT increases without any changes in conductivity presumably due to the oxidation of the catalyst particles. We improved transparency by 4% and decreased equivalent sheet resistance by 21%. All developed techniques and methods contributed to the synthesis of SWCNT with defined characteristics open a new possibility for their applications. Thesis work includes three different electro-optical devices with advanced performance: i) a bolometer based on freestanding SWCNT film showing response time of 2.6 ms at room temperature 1mbar (several times faster than the corresponding industrially applied devices); ii) an SWCNT-based heating element of fiber Bragg grid for smooth tuning of the resonant wavelength and a stable laser signal; iii) a saturable absorber based on SWCNT films showing femtosecond pulse generation a low degradation rate.
Aerosol CVD synthesis and applications of single-walled carbon nanotube thin films using spark-discharged produced catalyst
(Aalto University, 2020) Ahmad, Saeed; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
Structural controlled synthesis of single-walled carbon nanotubes (SWCNTs) have attracted a great deal of attention due to their widespread potential applications in electronics and photonics. Floating catalyst chemical vapor deposition (FC-CVD) being a dry and continuous method, is a highly promising technique for the scalable synthesis of SWCNTs. However, due to in-situ catalyst formation in all the conventional FC-CVD approaches, it is hard to get full control of number concentration, composition and size of nanoparticles. Hence, it hinders to investigate the effects of catalyst composition on morphology, yield and structure of SWCNTs. In this thesis, firstly we designed a novel rod-to-tube type spark discharge generator (R-T SDG) to produce ex-situ catalyst nanoparticles for the FC-CVD growth of SWCNTs. We utilized highly time-stable and uniform number size distributions of monometallic (Fe, Ni, Co) and bimetallic (Co-Fe, Co-Ni) catalyst particles for the synthesis of SWCNTs using ethylene as carbon source and 1050 deg C temperature. Optical characterizations revealed that as-grown SWCNTs have high-quality and their mean diameter is around 1 nm. The highest SWCNTs yield was obtained with Fe as a catalyst. From electron diffraction analysis, we observed that Co-Ni can produce comparatively narrower chirality and diameter distribution of SWCNTs. Secondly, we introduced sulfur in the FC-CVD reactor as a growth promoter for the fabrication of SWCNTs based transparent conducting films (TCFs). We systematically investigated the roles of sulfur on yield, morphology, and structure of SWCNTs. It was found that the yield of SWCNTs is largely dependent on amount of sulfur introduced into the FC-CVD reactor and catalyst composition. More importantly, the addition of an optimized amount of sulfur has enhanced approximately three times, the opto-electronic performance of SWCNT-TCFs, by increasing diameter and bundle length along with improving the quality of SWCNTs. The mean diameter of SWCNTs increased from 1 nm to 1.2 nm while the ratio of metallic nanotubes slightly increased from 39 % to 41 % with sulfur addition. Surprisingly, chirality determination of as-grown SWCNTs indicated that sulfur promotor has little influence on modulating the chirality of SWCNTs. Finally, we demonstrated that FC-CVD is a unique and versatile technique for the simultaneous growth of substrate-free 0D-fullerene, 1D-CNT and 2D-graphene. The formation of 0D-1D-2D carbon nanostructures were directly evidenced by lattice-resolved (scanning) transmission electron microscopy (STEM). We showed that the relative number density of graphene-nanoflakes can be tuned by optimizing the synthesis conditions. In addition, the as-synthesized hybrid films can be directly deposited on any surface at ambient temperature with an arbitrary thickness which offers a new route towards ultra-fast manufacturing and dry deposition of the hybrid structures.
Aerosol synthesis of carbon nanotubes and nanobuds
(Aalto-yliopiston teknillinen korkeakoulu, 2010) Anisimov, Anton; Nasibulin, Albert G., Dr.; Teknillisen fysiikan laitos; Department of Applied Physics; Aalto-yliopiston teknillinen korkeakoulu; Kauppinen, Esko I., Prof.
This thesis presents the results of experimental investigations of single-walled carbon nanotube (SWCNT) and nanobud synthesis. These carbon nanomaterials were synthesized by two different methods based on CO disproportionation on the surface of iron particles produced by hot-wire generator and ferrocene decomposition methods. Studies of CO disproportionation in the presence of etching molecules (H2O and CO2) led to the discovery of a novel hybrid carbon material — SWCNTs covered by covalently bonded fullerenes — carbon nanobuds. The reagent concentrations required for nanobud synthesis were found to be between 45 and 245 ppm for H2O and between 2000 and 6000 ppm for CO2. The growth mechanism of the nanobuds and their properties were examined. On the basis of in situ sampling investigations, the kinetics of the SWCNT growth was studied. For temperatures of 804, 836, 851, and 915 °C, the average growth rates were found to be 0.67, 1.11, 1.01, and 2.70 µm/s, respectively. It was found that the growth rate constant complies with the Arrhenius dependence with an activation energy of Ea = 1.39 eV, which can be attributed to the diffusion of carbon atoms in the solid iron catalyst. A new method for separating bundles and individual SWCNTs is proposed. This method is based on the fact that bundled SWCNTs coming from the reactor are charged, while individual SWCNTs remain electrically neutral. Studies of the charging phenomenon revealed that SWCNT bundles were charged (up to 99%) and could carry up to 5 elementary charges. It is proposed that SWCNT bundles were positively charged due to electron emissions and negatively charged due to the emission of impurities from the surface. As a potential SWCNT application, a simple and direct thermo-compression method for integrating SWCNT films with adjustable thicknesses, transparency, and conductivity into polymer films is demonstrated. The produced SWCNT/polyethylene composite films exhibited good optical transparency and conductivity as well as high mechanical flexibility. SWCNT/polyethylene thin films demonstrated excellent cold electron field emission properties.
Analysis of single-layer and three-layer nanocomposite fuel cells
(Aalto University, 2020) Jouttijärvi, Sami; Asghar, Muhammad Imran, Prof., Aalto University, Finland and Hubei University, China; Teknillisen fysiikan laitos; Department of Applied Physics; New Energy Technologies; Perustieteiden korkeakoulu; School of Science; Lund, Peter D., Prof., Aalto University, Department of Applied Physics, Finland
Fuel cells (FCs) convert the chemical energy of fuel directly to electricity. FCs are potential canditates for clean electricity sources in the future, provided that the main challenges halting their commercialization can be solved. Several different FC subtypes exist. This Thesis is focused on ceramic nanocomposite FCs (CNFCs) and single-layer FCs (SLFCs). Both of these FCs operate at intermediate temperatures, at around 500-600 °C. CNFC utilizes the traditional three-layer structure: anode, electrolyte, and cathode. The key component is the electrolyte, that consists of a composite of a solid oxide and a salt, here doped ceria and alkali carbonate mixture respectively. This composite electrolyte allows an efficient multi-ion conduction, reducing the ohmic losses in the cell. Excellent power densities, exceeding 1 W per square centimeter, were achieved with two different CNFCs in this Thesis. SLFC is a ground-breaking innovation where all FC functions are compressed into one single layer, consisting of a mixture of a semiconductor (here lithium nicke zinc oxide or copper iron oxide) and an ionic conductor (here doped ceria or doped ceria – alkali carbonate mixture). The SLFC desing allows to elimintate the challenges originating from the three-layer structure and to simplify the manufacturing procedure. In this Thesis, the working principle and performance-affecting factors of SLFCs were studied. The key findings include that the proton is dominating over the oxygen-ion in ionic conduction with the studied SLFC configuration and that applying the composite ionic conductor of CNFC to SLFC improves vastly the cell performance. Since both CNFCs and SLFCs are complex nanoscale structures, studying the microstructure of these devices with electron microscopy and X-ray spectroscopy are identified as crucial procedures to understand the macroscopic output. Systematic studies combined with modern microscopic methods are suggested as a pathway to push both SLFCs and CNFCs towards commercialization.
Anodic TiO2 nanotube arrays for photoconversion based hydrogen production
(Aalto University, 2022) Hou, Xuelan; Lund, Peter D., Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; New Energy Technologies; Perustieteiden korkeakoulu; School of Science; Lund, Peter D., Prof., Aalto University, Department of Applied Physics, Finland
Solar energy conversion and storage are potential technologies to improve energy security and achieve carbon neutrality. Among these technologies, the solar-to-chemicals conversion using photoelectrochemical cells to produce hydrogen fuel is a viable pathway. This work focuses on using highly active, stable, and flexible anodic TiO2 nanotube (TNT) electrodes in photoconversion cells for hydrogen production, such as the application of anode electrodes and cathode electrodes in water splitting (WS) cells. To improve solar energy capture, the two-anode reduction method and the stepwise cathodic reduction method were designed and verified as effective ways to modify pure anodic TNT. The performance of the reduced anodic TNT as a cathode in WS cell was -221.1 mA, which was 17,000-fold higher than that of TNT, and 5-fold that of commercial Ti. A stability test at CP@-100 mA for 24 h showed a decay of 1.3%. In the photoconversion cell, the onset potential of the reduced anodic TNT (TNT-C) has an anodic shift from -0.79 to 0.19 VRHE, corresponding to a performance increase from -110.06 to -210.66 mA at -1.0 VRHE. Self-improving performance was also found during the long-term tests, which were deduced to the self-formed composite structures, such as an n-p-n junction. When used as the anode, the reduced anodic TNT (TAR-2) gave a 3-fold enhancement over the pure one at 1.23 VRHE, 2.05 mA/cm2. The multi-size effects of the anodic TNT were investigated in a range of six orders of magnitude length scales ranging from 10-8 to 10-2 m. Increasing the nm-scale and cm-scale length size, the current density at 1.23 VRHE decreased. However, increasing the μm-scale length size, the performance at 1.23 VRHE increased. The light absorption intensity for reduced anodic TNT was enhanced, the light absorption range was broadened and the absorption edge had a redshift. The TNT-C showed six absorption peaks in the incident photon-to-electron conversion efficiency in the range of 365-1020 nm, while TNT showed an absorption edge at 430 nm. The electrical conductivity of the reduced samples has two arcs representing two electroactive interfaces with different kinetics in the Nyquist plot. The semiconductive arc was ten times smaller than that of pure anodic TNT, and the metallic arc was newly introduced indicating high electrical conductivity. The growth order of anatase TiO2 peaks was firstly reported and the growth rate of the anodic film was confirmed. The thesis shows that cathodic reduction methods, such as stepwise cathodic reduction, are promising for reducing the manufacturing costs of TNT-based electrodes.
Applications of hybrid single-electron turnstiles: To current standards and beyond
(Aalto University, 2023) Marín Suárez, Marco; Peltonen, Joonas, Dr., Aalto University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Pico group; Perustieteiden korkeakoulu; School of Science; Pekola, Jukka, Prof., Aalto University, Department of Applied Physics, Finland
Hybrid single-electron transistors formed of a normal-metal island and superconducting leads (SINIS) have been used for generating single-electron currents. These single-electron turnstiles (ST) are voltage biased while a periodical voltage modulation is applied to its gate electrode. Accuracy in current generation in this device is limited, however, investigation of the physics behind its operation has allowed applications beyond metrological purposes. For example, they serve as a probe of the concentration of superconducting excitations in its leads. In this thesis, these applications are used first by probing extraction of superconducting excitations generated in the leads of the device by its bare operation. These quasiparticles (QPs) are unpaired electrons in the superconducting condensate of Cooper-pairs. The extraction is done by voltage biased Josephson junctions which share one lead with the SINIS ST and have other with higher energy gap. A reduction of one order of magnitude in QP density is observed by using the deviation of the generated current as an accurate probe. Then, an extension of the SINIS ST applications is presented. By driving this device with a signal of frequency f, two QPs are created in the leads close to the energy gap Δ so that a power 2Δf is generated in total. This enables to envision the development of the SINIS ST into a standard for the unit of power. It is also shown that such power generation is possible even in the absence of net particle current at zero bias. Furthermore, an analysis about the ultimate possible accuracy of power generation in a simplified version of this device is presented in this thesis. It is seen that errors increase with increasing operation frequency, tunnel resistance, temperature and presence of sub-gap states. Additionally, it is shown that detection efficiency of QP energy can be >99% at typical cryogenic temperatures. Following this, it is shown that by injecting an extra modulation to the source electrode of the transistor, different driving trajectories can be drawn in its stability diagram. With this, a new driving method with twice the frequency applied to the drain-source bias compared to the one applied to the gate is proposed. By doing so, tunneling events occurring against the biased direction are suppressed. These tunneling events lower the current below the expected outcome. Accuracy of current generated by SINIS ST is increased by one order of magnitude using the new driving method. Furthermore, it is shown that a similar driving method can be used for generating single-electron currents at zero-average bias, which had not been investigated until now in SINIS STs.
Applications of positron annihilation spectroscopy in nuclear materials research
(Aalto University, 2017) Heikinheimo, Janne; Teknillisen fysiikan laitos; Department of Applied Physics; Antimatter and Nuclear Engineering group; Perustieteiden korkeakoulu; School of Science; Tuomisto, Filip, Prof., Aalto University, Department of Applied Physics, Finland
In this work, positron annihilation spectroscopy was used in studying lattice point defects in some of the technologically relevant nuclear materials. In addition, the behaviour of the detectors were studied to deepen understanding of their influential properties in lifetime and Doppler broadening spectroscopy. Detectors play a significant role in the performance of a measurement setup and in the quality of the obtained results, both in lifetime and Doppler broadening spectroscopy. Lifetime spectroscopy is very sensitive to the excess activity of studied samples, which limits its use in the study of nuclear materials. The only way of improving the tolerance of lifetime spectrometry is to add an extra coincidence detector to the setup. In Doppler broadening spectroscopy a detector energy resolution function directly affects the obtained results. The sensitivity of two-detector lifetime spectroscopy to false coincidence events was studied by adding a cobalt-60 source next to the studied Si reference samples. The improvement of the radiation tolerance in a three-detector setup was estimated based on simple theoretical models. To study energy resolution function in Doppler broadening spectroscopy, a sodium-22 positron source was directly attached to the studied samples enabling simultaneous recordings of Doppler broadening and discrete photoabsorption spectrum. The impact of the source-detector geometry on the obtained results was studied and the reason behind the changing results is discussed based on systematic experiments supported by Monte Carlo simulations. Microstructural defects play an important role in the properties of nuclear materials, such as corrosion and irradiation resistivity. Zircaloy-4 is currently the most used cladding alloy in nuclear power plants, but its oxidation in reactor conditions is a complex process including cyclic oxidation rates. In fusion power plants, materials close to the plasma are exposed to a very hostile environment including high temperatures and high particle irradiation fluxes. Tungsten is considered a promising candidate material for this purpose. The fundamental oxidation properties of zirconium are discussed based on microstructural defect evolution in the studied Zircaloy-4 samples that were oxidized in pressurized water reactor -type conditions. Doppler broadening spectroscopy supported by theoretical modelling was harnessed to characterize lattice point defect behaviour in the sample oxide layers. Positron lifetime spectroscopy was applied in the study of mono-vacancies in pre-annealed proton-irradiated tungsten samples. Migration barriers for interstitial atoms and mono-vacancies were directly detected with a positron lifetime spectrometer connected to the cold temperature irradiation facility.
Atomic and electronic transport on surfaces and interfaces
(Aalto University, 2013) Hakala, Mikko; Teknillisen fysiikan laitos; Department of Applied Physics; COMP/SIN; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Distinguished Prof., Aalto University, Department of Applied Physics, Finland; Foster, Adam, Prof., Aalto University, Department of Applied Physics, Finland
The properties of interfaces and surfaces play a key role in the functional design of many technologies, particularly in the development of next generation micro-electronic devices and nanocatalysts. In micro-electronics, hafnia is seen as a reliable replacement for silica in modern transistors, yet little is known about its interface with silicon and probable defects formed. Similarly, the formation of metallic nanoparticles on insulators is a promising route to new catalytically active materials, but much work is needed to understand the dynamical growth of these particles on surfaces from deposited metal atoms. In this thesis we have modeled the properties of defects in silicon-hafnia interfaces and metal adatoms on alkali halide surfaces. The calculations have been performed within the density-functional theory (DFT), supported by electron transport calculations for the interface studies. Although these results have been successful in building our understanding, we have identified the need to enhance the accuracy of the standard DFT approach without sacrificing computational speed. For this we have implemented efficient hybrid functionals into the SIESTA code and shown that it indeed improves our description of critical materials' properties.
Atomic-scale defects in solar cell material CuInSe₂ from hybrid-functional calculations
(Aalto University, 2013) Oikkonen, Laura; Ganchenkova, Maria, Dr., National Research Nuclear University MEPHI, Russia; Seitsonen, Ari, Dr., University of Zurich, Switzerland; Teknillisen fysiikan laitos; Department of Applied Physics; Electronic Properties of Materials; Perustieteiden korkeakoulu; School of Science; Nieminen, Risto, Prof., Aalto University, Finland
The operation and properties of any semiconductor device rely on its defect microstructure. In thin-film solar cells, point defects are a determining factor for the conversion efficiency of the device by controlling doping but also by degrading device performance. Additionally, point defects play a role in material growth and processing by mediating diffusion. Knowledge of point defects and defect-related processes is therefore essential in optimizing solar cell performance. In this thesis, point defects in the solar cell absorber material CuInSe₂ (CIS) have been investigated with computational methods. Starting from the thermodynamics of individual point defects and extending to diffusion kinetics and clustering, the aim of the thesis is to gain a comprehensive understanding of the defect microstructural features of the material and their effect on its electronic properties. The calculations have been performed with density-functional theory (DFT) employing a hybrid exchange-correlation functional, which has been demonstrated to describe semiconductor properties better than previously used (semi)local-density functionals. The calculations presented in this thesis show that point defects in CIS participate in a variety of competing atomic-scale processes, which affect their distribution within the material. By taking into account defect interactions, it is demonstrated that there exists a thermodynamic driving force towards the creation of defect complexes such as InCu-2VCu and VSe-VCu. Interaction of intrinsic defects with impurities can also lead to surprising effects: it is found that by introducing sodium atoms into CIS, copper mass transport is reduced due to the capture of copper vacancies. The effect of the prevalent point defects and defect complexes on the electronic properties is found to essentially depend on whether the defect is of cationic or anionic type. Only selenium-related anionic defects are observed to induce deep defect levels within the CIS band gap, implying that they may act as recombination centers. The results presented in this thesis help to explain and interpret experimental observations of atomic-scale phenomena occurring in CIS. Further, they provide computational insight on defect-related mechanisms that may sometimes remain out of reach in experiments. The findings can be employed to gain better control over film quality and device operation in CIS-based solar cells.
Atomistic Simulations of Solid-Liquid Interfaces
(Aalto University, 2013) Reischl, Bernhard; Foster, Adam S., Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; COMP/Surfaces and Interfaces at the Nanoscale (SIN); Perustieteiden korkeakoulu; School of Science; Foster, Adam S., Prof., Aalto University, Department of Applied Physics, Finland
Solid-liquid interfaces can be encountered in systems and processes ranging from biomineralization to fuel cell technology, and play an important role in growth or dissolution mechanisms of particles or surfaces in solution. The surface-induced changes of material properties not only affect the solid, but also the liquid itself: the structure of the liquid at the interface is very different from bulk. Understanding these processes occurring at solid-liquid interfaces at the atomistic scale is fundamental to a wide range of disciplines. New insight can be gained by combining cutting edge experimental techniques and computer simulations. The atomic force microscope (AFM) can be used to study solid-liquid interfaces in high resolution. We have developed new simulation methods, based on atomistic molecular dynamics and free energy calculations in order to model the complex imaging mechanism. In addition to the direct interactions between AFM tip and surface, our approach takes into account entropic contributions from interactions with water molecules in hydration layers on top of the surface as well as in the solvation shell of the AFM tip. For the Calcite (10-14) surface in water, we find good agreement between our simulations and recent 3D AFM data. We have also developed and tested a simple model to calculate AFM images only from differences in equilibrium local water density in hydration layers, reducing the computational cost by up to three orders of magnitude compared to free energy calculations including an explicit AFM tip. We have further studied the hydration layer structure and dissociation kinetics of the NaCl (100) surface in water from ab initio molecular dynamics, as well as the role of surface premelting of ice in the context of atomic scale friction at the ice-ice interface.
Automating high-resolution atomic force microscopy image interpretation
(Aalto University, 2023) Oinonen, Niko; Urtev, Fedor, Dr., Aalto University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Surfaces and Interfaces at the Nanoscale (SIN); Perustieteiden korkeakoulu; School of Science; Foster, Adam S., Prof., Aalto University, Department of Applied Physics, Finland
Atomic force microscopy (AFM) has become an important tool in nanoscale studies of matter, imaging and characterizing its properties, reaching from micrometer scale down to the sub-nanometer scale. In particular, high-resolution AFM performed in ultra-high vacuum with functionalized tips has achieved a resolution capable of identifying individual atoms in single molecules adsorbed on surfaces. However, applications so far have been mostly limited to simple planar molecules due to the difficult interpretability of the obtained images for more complicated sample molecules. At the same time, fast and accurate simulations of high-resolution AFM images have become available in the form of the Probe Particle Model (PPM). The PPM simulations make it easy to go from an atomic structure into an AFM image, but the reverse process is not as easy, often requiring a lot of manual labour with testing different candidate molecule geometries. An automated approach for solving this inverse problem would be a major benefit for the wider applicability AFM into imaging atomic scale systems. To this end, one can note the recent emergence of large-scale machine learning (ML) models, especially deep neural networks, enabling major progress in many fields of science. Utilizing a very large dataset, it is possible to train an ML model to perform a task where no known algorithmic approach works. Given the rapid data-generation capability of the PPM simulations, it should be possible to train an ML model to perform the inverse-imaging task in AFM, going from an AFM image into a molecule geometry or some other property of interest. The work in this thesis utilizes the PPM to generate simulations for a large database of molecules, and uses those simulations to train neural networks for predicting the molecule geometry as well as the electrostatic field from AFM images. The trained models are subsequently tested also on experimental AFM images, where generally good results are found, but in some cases differences in the details between the simulation and the real experiment make the predictions incorrect or ambiguous. At this stage full general automation of the AFM image interpretation process is not yet possible, but the results here present the first steps in this direction. Additionally, an ML method for automatizing the tip functionalization part of the AFM measurement preparation is presented.
Avalanches in plastic deformation of materials
(Aalto University, 2016) Ovaska, Markus; Alava, Mikko, Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex Systems and Materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics, Finland
Materials often do not deform continuously, but in an intermittent manner with sudden bursts of activity separated by periods of low activity. The sizes of these bursts, or avalanches, typically follow power law distributions, and spatial and temporal correlations are observed between avalanches. Experimental evidence of such behaviour in material deformation ranges from compressed single crystals on the micrometer scale to the movement of tectonic plates. In the first part of this work (Publication I-III) we study avalanches and other collective phenomena in the deformation of crystalline materials with simple numerical models. We extend a standard 2d discrete dislocation dynamics model to include point defects, which interact with dislocations and act as obstacles for their movement. With immobile defects, we find two new phases different from the pure 2d system. For a moderate pinning strength, the critical properties match those seen in depinning transitions, with critical exponents different from mean field theory. At a very high pinning strength, critical avalanche dynamics ceases. With mobile solute atoms, dislocations and solute clouds form growing structures, which we characterize through spatial correlations. The structure formation leads to Andrade creep in an extended range of external stress for creep simulations. The effect of mobile solute atoms in avalanches is seen as a linear size-duration relationship, and a stationary cutoff regime for avalanche distributions. In a pure 2d system, we show that the avalanche statistics depend on the avalanche triggering mechanism, and that critical avalanche dynamics is seen even at zero external stress. In the second part (Publication IV) we study avalanches in wood compression by analyzing the acoustic emission recorded during compression experiments. The acoustic event time series displays several similarities to earthquakes and experiments on brittle porous materials. These include power law distributions of event energies and waiting times between avalanches, as well as the Omori law for aftershocks. By using digital image correlation, we show that the peaks of highest acoustic activity are related to the sudden collapse of softwood layers.
Bio-inspired functional materials
(Aalto University, 2012) Jin, Hua; Ras, Robin H. A., Dr.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Ikkala, Olli, Acad. Prof.
The thesis shows strategies how to learn from Mother Nature to make functional materials. Firstly, inspired by lotus leaf and water strider, superhydrophobic and superoleophobic surfaces are prepared from nanofibrillated cellulose aerogels. Furthermore, we explore potential applications of the superhydrophobic and superoleophobic materials for carrying cargo on liquid surfaces and continuous propulsion. Interestingly, the self-propelled locomotion has constant velocity and can last for prolonged time. This allows transduction of chemical energy into motility and could open doors for new generation of autonomous miniaturized soft devices. Subsequently, superhydrophobic and superoleophobic surfaces are made from silica aerogel, and the emphasis is on the damage resistance of superhydrophobicity and superoleophobicity. After mechanical abrasion with sandpaper, the superhydrophobicity and superoleophobicity retain. More interestingly, the contact angle hysteresis for water and oil decreases after abrasion with sandpaper. The last part of the thesis is about bio-inspired tough materials from nanofibrillated cellulose and nanoclay. By a simple method of centrifugation, bulk nanocomposites are achieved that have a high work to fracture of 23.1 MJ/m3 with high strain to failure of 36% under compression. Considering the simple preparation methods and bio-based origins of nanocellulose and clay, the tough material shows potential in applications for sustainable and environmentally friendly materials in construction and transportation.
Bioinspired materials: Non-covalent modification of nanofibrillated cellulose and chitin via genetically engineered proteins and multilayered graphene
(VTT Technical Research Centre of Finland, 2015) Malho, Jani-Markus; Linder, Markus, Prof., Aalto University, Department of Biotechnology and Chemical Technology, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Ikkala, Olli, Academy Prof., Aalto University, Department of Applied Physics, Finland
Biological nanocomposites such as nacre, bone and wood synergistically combine strength, stiffness and toughness with lightweight structure, whereas most man-made engineering materials with higher densities follow the rule-of-mixtures, according to which strength and toughness are mutually exclusive properties. Biomimetic approaches study and mimic nature’s concepts and material structures with the aim of developing high-performance bioinspired materials. Recent studies have shown that many of the properties of natural nanocomposites arise from their hierarchical structures from multiple length scales. Molecular level control and design are known to be crucial for the performance of the natural materials especially at the interfaces of the softer matrix and the harder reinforcing elements. In this work, examples of biopolymer matrices were studied from the mechanical perspective in order to understand how biological components, such as genetically engineered proteins and graphene flakes, could be used to design an organic matrix at the molecular level and to control its macroscopic material properties. The results indicated that the biopolymer networks can be functionalized non-covalently in aqueous and mild conditions directly via self-assembly in order to influence the mechanical properties. In Publications I and II, genetically engineered fusion proteins, incorporating hydrophobin - double cellulose binding domain or plain double cellulose binding domain, were used to tune the nanofibrillar cellulose network under conditions of controlled humidity. In Publication III, another genetically engineered fusion protein, chitin binding domain - aspein, was used to modify nanofibrillated chitin matrix through ionic interactions and biomimetic mineralization of calcium carbonate. In Publication IV, multilayered graphene flakes were exfoliated directly into native nanofibrillated cellulose networks in order to create nanocomposites with improved mechanical properties. Non-covalent modification of the colloidal biopolymer matrices is an efficient route to construct and study multifunctional nanocomposite materials by engineering the interfaces between the soft and hard phases. Importantly, genetically engineered proteins could pave the way towards new functional components for biomimetic structural nanocomposite materials while Nature’s materials continue to provide the constructing principles and inspiration for the development of biomimetic materials.
Biomimetic Designs by Supramolecular Constructs
(Aalto University, 2018) Myllymäki, Teemu T. T.; Nonappa, Dr., Aalto University, Department of Bioproducts and Biosystems, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Molecular Materials; Perustieteiden korkeakoulu; School of Science; Ikkala, Olli, Prof., Aalto University, Department of Applied Physics, Finland
Structurally important biological materials generically show hierarchical structures to allow functional properties. They exploit self-assemblies over the length scales by competing interactions and combining tailored supramolecular interactions. Examples are provided by spider silk, nacre, and bone, which show extraordinary mechanical properties, regardless of the weak individual building blocks. In these materials, the strength and toughness arise from nanoscale toughening mechanisms, where hard and soft domains are connected covalently and by weak interactions. They work in synergy to transfer stress from bulk to the reinforcing parts via sacrificial bonds and hidden lengths through hierarchical design. In this thesis, an overview of the important aspects in biomimetic material design, such as supramolecular chemistry, self-assembly, hierarchical structures, and sacrificial bonding is first given. In the later chapters, four articles and their important findings towards novel biomimetic materials are highlighted. In the publications I and II, self-assemblies of asymmetric bile acid -based amphiphilic polymers were studied. The results suggest designing complex amphiphilic self-assembling systems to create hierarchical materials from nanoscale to bulk upon "switching-on" supramolecular interactions. In the publication III, a nanocomposite between multi-walled carbon nanotubes and a polymer was synthesized. The adhesion between the polymer and the carbon nanotubes was supramolecularly enhanced to control their relative slipping. Due to supramolecular reinforcements and hierarchical structure, the resulting nanocomposite showed slow crack propagation upon fracturing, reminiscent of natural materials. In the publication IV, well-defined oligomeric oligosaccharide-based molecules with end-groups capable of supramolecular hydrogen bonds were studied. Polarized optical microscopy suggested columnar liquid crystallinity in a specific hydrogen bonding solvent medium and upon complete solvent removal hydrogen bonds between the oligosaccharides were formed allowing supramolecular polymers and fiber spinning. This work paves ways to understand switching-on of hydrogen bonds "on-demand" in the processing, mimicking silk-spinning. In summary, the present work shows ways to incorporate supramolecular interactions of different strengths for functional materials, inspired by biological materials.
Biomimetic materials design towards tough nanocomposites and strain-stiffening hydrogels
(Aalto University, 2021) Martikainen, Lahja; Nonappa, Assoc. Prof., Tampere University, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Molecular Materials; Perustieteiden korkeakoulu; School of Science; Ikkala, Olli, Prof., Aalto University, Department of Applied Physics, Finland
Biological organisms use only a few chemical elements to construct materials, such as proteins, polysaccharides, and minerals, which are efficiently processed into hierarchical structures across length scales. These clever multi-component designs produce materials and structure with remarkably improved mechanical properties and functionalities relative to the original components. The field of biomimetics aims to mimic these well-adapted design strategies, structures and functions to solve material engineering problems. This thesis focuses on two nature-inspired systems: (i) tough and strong nacre-mimetic nanocomposites, and (ii) strain-stiffening biopolymer hydrogels and their application for cell culturing. Both systems consist of nanoscale components and their mechanical properties and structure are studied. The design strategy for tough and strong composites is inspired by the nacre, a biomaterial with outstanding mechanical properties. Here the structural organization of nacre is mimicked via self-assembling core-shell structured colloidal platelets, i.e. nanoclay coated with a polymer, via vacuum filtration. In publications I and II, we alter the interactions between the colloidal platelets with DNA-based monophosphates and demonstrate a simple way to modify intermolecular interactions resulting in increased stiffness, strength, and toughness. Like natural nacre, these nanocomposites are sensitive to humidity. In publication III, we study the effect of water in polymer-clay nanocomposites and find that the glass transition temperature of the nanoconfined polymer is lowered due to residual water. The second part is inspired by typical extracellular matrix-based protein gels, which show strain-stiffening. In publication IV, we show that agarose hydrogels are strain-stiffening, consisting of helically twisted semiflexible fibrillar networks. In publication V, we analyze this strain-stiffening response more closely and simultaneously show, for the first time, that agarose gels also contract when sheared, which is seen as negative normal force and normal stress difference. Our main findings indicate that the mechanical response of agarose networks is enthalpic and that connectivity dictates their strain-stiffening response similarly as in collagen gels. Finally, in publication VI, we present an application for agarose hydrogels as a luminal cell identity and estrogen receptor α+-preserving scaffold for breast cancer tissue explant culture. Base on a biomimetic materials design approach, the first part of this thesis illustrates a simple method to control the mechanical properties of layered clay-polymer nanocomposites. The second part presents insights into the fibril network mechanics of agarose hydrogels. The final publication introduces a reliable agarose-based preclinical model, which can be used as a platform for breast cancer drug development and personalized cancer therapy.
Biomimetics - Architectural Considerations for Functional Nanocomposites
(Aalto University, 2015) McKee, Jason R.; Ikkala, Olli, Academy Prof., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Molecular Materials; Perustieteiden korkeakoulu; School of Science; Ikkala, Olli, Academy Prof., Aalto University, Department of Applied Physics, Finland
This thesis will focus on synthetic nanomaterials that combine hard nanocellulose reinforcement with soft molecularly engineered synthetic polymeric components. The architectural designs have all been inspired by natural nanocomposites, such as silk, animal bone and plant fibres. Typically, the design of natural structures are built around hard reinforcing nanodomains bound together by energy dissipating sacrificial networks. These natural materials consist of proteins, carbohydrates or brittle minerals. Separately each component is usually mechanically weak; however, when combined in the balanced and hierarchical ways, each component will synergistically contribute towards mechanically excellent networks, by combining strength, toughness and stiffness. Hence, one of the main focuses in this thesis will be the structural design of the constituting components and how they relate to the mechanical properties of the overall composites. Another key feature is the compatibility issues between the reinforcing nanocellulose with the synthetic supramolecular networks. Each material has been designed in a way that yields a homogeneous dispersion of all colloidal components. This allows more efficient reinforcment as well as allows the components to more effectively together. And finally, specific functionalities were engineered into the nanocomposite materials either through the supramolecular binding or by the use of functional polymers. In Publication I, highly dynamic supramolecular nanocomposite hydrogels were developed that uniquely combine: high stiffness, approaching those of solid elastic networks; self-healing within seconds; and temporal stability, allowing for self-healing of the exposed surface areas even after prolonged storage. In Publication II, biomimetic sacrificial bonds were chemically engineered into one-component nanocomposites. This resulted in engineered fracture energy dissipation, which considerably increased the toughness of the glassy nanocomposite. In Publications III and IV, functional nanocomposite hydrogels were formed by linking colloidal nanocellulose by adsorbing functional polysaccharides onto the surface. In the first example, a thermoresponsive nanocomposite with switchable modulus was shown. In the second example, both the modulus and yield-strain were enhanced by adding an interconnected sacrificial network into the nanofibrillar network. These nanocellulose-based nanocomposites demonstrate promising new concepts for material design as well as never before seen combinations of mechanical properties and functionalities.
Bloch line dynamics within magnetic domain walls
(Aalto University, 2018) Herranen, Touko; Laurson, Lasse, Dr., Aalto Unversity, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex Systems and Materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics, Finland
Magnetic domains are uniformly magnetized regions within a ferromagnet separated by magnetic domain walls. The internal degrees of freedom of a domain wall can be excited by applying a magnetic field greater than so called Walker field. As a result the domain wall velocity experiences the Walker breakdown, an abrupt drop of the average velocity, and the magnetization of the domain wall starts a cyclic rotation. If this mechanism is triggered in a domain wall with a dimension greater than a material characteristic Bloch line width, the excitations become non-uniform, which results in nucleation of domain walls within the domain wall called Bloch lines. The dynamics of domain walls in disordered media have been studied extensively using various computational methods as well as experimentally. In this doctoral dissertation we use a micromagnetics software to simulate the Bloch line dynamics and the effects of Bloch lines on domain wall dynamics in samples with perpendicular magnetic anisotropy. In Publication I we study the domain wall dynamics in CoPtCr thin films with different widths. We observe nucleation of Bloch lines within domain walls in disordered and perfect samples when driven with a magnetic field higher than the Walker field. We construct a geometry to confine the domain wall between two notches, and use it to study Bloch line dynamics under an in-plane field. Finally we demonstrate the performance potential of an electrical current operated Bloch line memory. In Publication II we study the effects of boundary conditions and thickness effects on the domain wall dynamics in a magnetic garnet, and we find they determine the internal dynamics allowed for the magnetization of the domain wall. The sample thickness limits the maximum achievable stable velocity before the breakdown. The velocity limit is also found to be related to the spatial width of Bloch lines. In Publication III we use a micromagnetics approach to study the Barkhausen effect and avalanche statistics in a thin Pt/Co/Pt multilayer. The domain wall is driven using a quasistatic constant velocity. The novel approach enables us to determine magnetization of the domain wall segment where avalanches are triggered. Internal magnetization dynamics show that during avalanches the activity of in-plane magnetization, i.e. the Bloch line motion, is higher than the activity related to the domain wall motion. Avalanche size and duration distributions obtained from the activity signals follow power law scaling, and the corresponding features extracted from the domain wall velocity show no significant difference. The analysis also shows that the results obtained using micromagnetic simulations are close to the values expected from a simpler model describing a short-range elastic string in a random medium.

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