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Otakaari 1 grandhall. Photo: Esa Kapila
 

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Fabrication and characterization of two-dimensional material based devices for photonics and electronics
(Aalto University, 2024) Uddin, MD Gius; Ahmed, Faisal, Dr., Aalto University, Department of Electronics and Nanoengineering, Finland; Elektroniikan ja nanotekniikan laitos; Department of Electronics and Nanoengineering; Photonics Group; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Sun, Zhipei, Prof., Aalto University, Department of Electronics and Nanoengineering, Finland
This thesis explores the potential of two-dimensional (2D) materials in different practical applications and presents the results divided in three parts. The first part focuses on the miniaturized spectrometers. Unlike conventional tabletop spectrometers, we demonstrate miniaturized (~22×8 μm2) computational spectrometers that rely on the electrically tunable spectral response of 2D materials-based single-junction for spectral reconstruction. We achieve high peak wavelength accuracy (~3 nm) and a broad operation window covering the visible and the near-infrared regions, indicating the great potential of the spectrometers to enable numerous portable applications.    The second part of this thesis examines different strategies for tuning the optical and electrical properties of 2D materials. We demonstrate that morphological manipulation of 2D indium selenide (InSe) facilitates enhanced light-matter interaction in InSe. Our 2D InSe/1D nanowire heterostructures, exhibit more than 5 times enhanced optical responses compared to that from bare InSe. Moreover, significant optical anisotropy is observed that makes our mixed-dimensional heterostructures a good candidate for diverse polarization-dependent optoelectronic applications such as photodetectors. Further, in this thesis, we explore a strain engineering approach to increase the charge carrier mobility of molybdenum ditelluride (MoTe₂) field-effect transistors (FETs). It involves the creation of hole arrays in the substrate, transfer of MoTe2 flakes on the hole arrays, and subsequent deposition of ALD Al2O3 passivation layer on top of the MoTe2 flakes. We achieve ~6 times higher charge carrier mobility in the strained MoTe2 FETs than those MoTe2 FETs without strain. The results offer a bright prospect to realize 2D materials-based high-performance devices for future electronics.  In the final part of this thesis, we experimentally demonstrate a novel concept for the miniaturization of broadband light sources. Coherent broadband light is generated (via difference-frequency generation) for the first time with gallium selenide and niobium oxide diiodide crystals at the deep-subwavelength thickness (<100 nm). The broadband spectrum spans more than an octave (from ~565 to 1906 nm) without the need for dispersion engineering. Compared with conventional methods, our demonstration is ~5 orders of magnitude thinner and requires ~3 orders of magnitude lower excitation power. The results open a new path to create ultra-compact, on-chip broadband light sources.
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Photogrammetry for characterizing rock fracture roughness, physical aperture, and hydromechanical properties
(Aalto University, 2024) Torkan, Masoud; Uotinen, Lauri, Staff Scientist, Aalto University, Department of Civil Engineering, Finland; Baghbanan, Alireza, Prof., Isfahan University of Technology, Iran; Rakennustekniikan laitos; Department of Civil Engineering; Mineral-based materials and mechanics; Insinööritieteiden korkeakoulu; School of Engineering; Rinne, Mikael, Prof., Aalto University, Department of Civil Engineering, Finland
Understanding rock mass behavior is vital for various applications, including nuclear waste disposal and civil projects. Geometrical properties of single rock fractures, like roughness and physical aperture, significantly affect shear strength and fluid flow. This research aimed to characterize single rock fracture properties such as roughness, physical aperture, or hydromechanical attributes using photogrammetry of Kuru granite. Push shear tests were conducted on two sample sizes (200 cm × 100 cm and 50 cm × 25 cm), revealing a reduction of peak shear strength and friction angle for the larger size. Roughness back-calculated from shear tests for the larger sample was lower than the estimates from profilometer or photogrammetry. Scale adjustment was necessary for the correction of roughness estimation for the larger sample. Experimental differences may also stem from matedness. Using low-cost cameras in photogrammetry was investigated for a sample size of 50 cm × 50 cm. While smartphones show promise, caution is advised due to potential accuracy issues. Notably, the sampling intervals of 3D point clouds could affect roughness and physical aperture measurement results. A high-precision photogrammetric method was developed for measuring the physical aperture of three 25 cm × 25 cm samples. Markers at predefined distances used as scale bars were attached to each sample. The Root Mean Square Error (RMSE) between actual and calculated distances ranged from 20 to 30 µm. This method showed high accuracy compared to linear variable displacement transducers (LVDTs) for measuring fracture closure under normal stresses (0, 0.1, 0.3, and 0.5 MPa), with differences ranging from 1 to 8 µm. Achieving this level of accuracy required using at least 200 scale bars. Hydromechanical tests were conducted with fluid pressure gradients from 20 to 200 kPa/m and under the abovementioned normal stresses. The relationship between fluid pressure gradient and flow rate followed the nonlinear Forchheimer equation. Roughness displayed anisotropy, with greater roughness resulting in lower conductivity. Simulations were performed under different conditions and compared with laboratory fluid flow tests for validation. Scale effects study revealed significant variations in roughness and permeability with sample size changes. Three 100 cm × 100 cm surfaces were extracted from the 3D model of the bottom half of the 200 cm × 100 cm sample. Then, square subsample sizes ranging from 5 to 100 cm were extracted to estimate roughness and permeability. The surfaces were duplicated and shifted 350 µm to match the initial physical aperture of the 25 cm × 25 cm samples. Square subsample sizes below 30 cm showed variations in roughness and permeability, while these properties tended to be relatively stable states beyond this sample size. In conclusion, the study showed the feasibility of using photogrammetry to accurately characterize different rough fracture sizes for different applications.
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Accelerating the energy transition toward zero-emission district heating systems through policy codesign
(Aalto University, 2024) Auvinen, Karoliina; Juntunen, Jouni K., Assoc. Prof., University of Vaasa, Finland; Muotoilun laitos; Department of Design; Taiteiden ja suunnittelun korkeakoulu; School of Arts, Design and Architecture; Hyysalo, Sampsa, Prof., Aalto University, Department of Design, Finland
Achieving zero-emission energy systems is necessary for mitigating climate change. This requires replacing fossil fuels with energy-saving measures, low lifecycle-emission primary energy sources, energy storage, and smart control systems. A significant portion of fossil fuels is consumed in district heating systems in cold climate regions worldwide. The main research question in this thesis is: "How can the energy transition toward zero-emission district heating systems be accelerated with policy codesign?" My research intersects with literature on socio-technical transitions, energy system decarbonization, transition management, and codesign. My research was conducted in collaboration with four research groups in Finland. The research methods included a triangulation of qualitative and quantitative approaches, such as interviews, energy system modelling, and prototyping. In the context of transition management, we designed and developed a mid-range pathway creation toolset and a transition arena process, which we then experimented with high-level influencers. Furthermore, by engaging with investors and other key stakeholders, we investigated socio-technical barriers and formulated policy proposals aimed at decarbonizing district heating systems. Finally, we proposed a transition pathway model for Helsinki, incorporating heat auctions to promote third-party access to the local district heating network. Our research in Finland confirmed the presence of numerous barriers to energy system decarbonization. Our research experiments indicated that mid-range transition arena processes, along with other policy codesign events, have the potential to produce effective policy suggestions for accelerating zero-emission energy transitions. Achieving energy system transformation requires wide-range policy interventions. However, implementing these in formal policy decision-making processes is contested and challenging. Transitions produce uneven costs and benefits across society. Transforming energy systems requires destabilizing the existing regime, and incumbent actors often resist this change due to path dependency. In conclusion, I propose a transition management model to accelerate the zero-emission energy transition, aiming to achieve emission reductions within district heating systems that are in line with the Paris Climate Agreement's timeline. Transition management and codesign approaches need to evolve toward institutionalization in order to create societal impact, and they must develop further in order to handle the related tensions and conflicts. However, given the current paradigm and system complexities, achieving a rapid energy transition appears improbable.
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Modeling and Using Biographical Linked Data for Prosopographical Data Analysis
(Aalto University, 2024) Leskinen, Petri; Tuominen, Jouni, Dr., University of Helsinki & Aalto University, Finland; Tietotekniikan laitos; Department of Computer Science; Semantic Computing Research Group; Perustieteiden korkeakoulu; School of Science; Hyvönen, Eero, Prof., Aalto University, Department of Computer Science, Finland
Biographical data is used for identifying people, groups, and organizations and for conveying information about them. Biographical data describes life stories of people with the aim of getting a better understanding of their personality, actions, and interperson relations. The underlying texts can also be used for data analysis and distant reading once the documents are provided in a machine-readable format. Prosopographical analysis delves into the life stories of individuals within a defined group to identify shared characteristics and patterns. This dissertation presents and utilizes a comprehensive framework for managing and analyzing biographical data in Digital Humanities research. It includes data models, methods, and applications that enrich biographical content with links and reasoning to enhance the findability, accessibility, interoperability, and re-usability following the FAIR principles. Furthermore, the framework includes versatile tools for both individual biographical research and prosopographical research on groups of people. Linked Data together with event-based data model schemas are used in the published datasets to achieve the interoperability of heterogeneous data regarding historical people. Events are used as the glue combining information from various sources. The event-based modeling enables depicting historical narratives as data, which can be further enriched with the events of individual people and organizations. The research included in this dissertation follows the principles of the design science and action research. The research has been carried out in multiple research projects concentrating on biographical data: WarSampo (2015–), BiographySampo (2018–2021), Norssi High School Alumni (2017), AcademySampo (2019–2021), LetterSampo (2020–2022), and ParliamentSampo (2021–). The data publications and services, online portals, and published articles with analysis are represented as the results of the work accomplished for this thesis. Besides, this thesis tackles the practices of creating, modeling, and publishing Linked Data, as well as analyzing this biographical and prosopographical data by the means of network and data analysis.
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An exploration of e-scooter injuries and severity: Impact of restriction policies in Helsinki, Finland
(Elsevier Ltd, 2024-12) Dibaj, Samira; Vosough, Shaghayegh; Kazemzadeh, Khashayar; O'Hern, Steve; Mladenović, Miloš; Department of Built Environment; Planning and Transportation; University of Cambridge; University of Leeds
Introduction: The emergence of shared electric scooter (e-scooter) services has introduced a new mobility option in numerous urban areas worldwide. Safety concerns surrounding e-scooter riding have prompted some cities to impose bans or restrictions on shared e-scooters. This study aims to assess the impact of e-scooter restriction policies, on the spatiotemporal distribution of e-scooter injuries and factors influencing injury severity in Helsinki, Finland, in 2021 and 2022. These restrictions include banning shared e-scooter use from midnight to 5 a.m. on weekends and reducing speeds during certain hours. Method: This study employed an ordered logit model, heatmap analysis of crash locations, and temporal analysis across different time frames to achieve these objectives. Results: The findings indicate a 64% reduction in the number of e-scooter injuries after the restrictions. However, the severity of injuries experienced only a slight decrease. Notably, the trend of injury severity appeared smoother in 2022 compared to 2021, with spikes occurring from Friday to Sunday. The spatial distribution of crashes revealed that, in 2021, most crashes were concentrated in the city center, while in 2022, the crash locations were more scattered, partly due to the increased area serviced by e-scooters. The results also underscored the substantial impact of alcohol intoxication, as it significantly increased the probability of more severe injuries. Furthermore, higher age groups and people using e-scooters from 4 p.m. to 9 p.m. are more likely to experience higher injury severity after the restrictions were implemented. These research outcomes offer valuable insights for other cities, providing lessons on how to tailor policies to effectively reduce the number of e-scooter-related injuries.