Browsing by Author "Hirvonen, Petri"

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Datasiirtopalveluiden kustannusten vertailu automaattista tietojenkäsittelyä hyväksi käyttäen
(1990) Hirvonen, Petri
Helsinki University of Technology | Master's thesis
Energetics and structure of grain boundary triple junctions in graphene
(2017-07-06) Hirvonen, Petri; Fan, Zheyong; Ervasti, Mikko M.; Harju, Ari; Elder, Ken R.; Ala-Nissilä, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Grain boundary triple junctions are a key structural element in polycrystalline materials. They are involved in the formation of microstructures and can influence the mechanical and electronic properties of materials. In this work we study the structure and energetics of triple junctions in graphene using a multiscale modelling approach based on combining the phase field crystal approach with classical molecular dynamics simulations and quantum-mechanical density functional theory calculations. We focus on the atomic structure and formation energy of the triple junctions as a function of the misorientation between the adjacent grains. We find that the triple junctions in graphene consist mostly of five-fold and seven-fold carbon rings. Most importantly, in addition to positive triple junction formation energies we also find a significant number of orientations for which the formation energy is negative.
Grain extraction and microstructural analysis method for two-dimensional poly and quasicrystalline solids
(2018-10-16) Hirvonen, Petri; La Boissonière, Gabriel Martine; Fan, Zheyong; Achim, Cristian; Provatas, Nikolas; Elder, Ken R.; Ala-Nissilä, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
While the microscopic structure of defected solid crystalline materials has significant impact on their physical properties, efficient and accurate determination of a given polycrystalline microstructure remains a challenge. In this paper, we present a highly generalizable and reliable variational method to achieve this goal for two-dimensional crystalline and quasicrystalline materials. The method is benchmarked and optimized successfully using a variety of large-scale systems of defected solids, including periodic structures and quasicrystalline symmetries to quantify their microstructural characteristics, e.g., grain size and lattice misorientation distributions. We find that many microstructural properties show universal features independent of the underlying symmetries.
Heat transport across graphene/hexagonal-BN tilted grain boundaries from phase-field crystal model and molecular dynamics simulations
(2021-12-21) Dong, Haikuan; Hirvonen, Petri; Fan, Zheyong; Qian, Ping; Su, Yanjing; Ala-Nissila, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the interfacial thermal conductance of grain boundaries (GBs) between monolayer graphene and hexagonal boron nitride (h-BN) sheets using a combined atomistic approach. First, realistic samples containing graphene/h-BN GBs with different tilt angles are generated using the phase-field crystal model developed recently [P. Hirvonen et al., Phys. Rev. B 100, 165412 (2019)] that captures slow diffusive relaxation inaccessible to molecular dynamics (MD) simulations. Then, large-scale MD simulations using the efficient GPUMD package are performed to assess heat transport and rectification properties across the GBs. We find that lattice mismatch between the graphene and h-BN sheets plays a less important role in determining the interfacial thermal conductance as compared to the tilt angle. In addition, we find no significant thermal rectification effects for these GBs.
Honeycomb and triangular domain wall networks in heteroepitaxial systems
(2016-05-07) Elder, Ken R.; Chen, Z.; Elder, K. L M; Hirvonen, Petri; Mkhonta, S. K.; Ying, S. C.; Granato, E.; Huang, Zhi Feng; Ala-Nissilä, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A comprehensive study is presented for the influence of misfit strain, adhesion strength, and lattice symmetry on the complex Moiré patterns that form in ultrathin films of honeycomb symmetry adsorbed on compact triangular or honeycomb substrates. The method used is based on a complex Ginzburg-Landau model of the film that incorporates elastic strain energy and dislocations. The results indicate that different symmetries of the heteroepitaxial systems lead to distinct types of domain wall networks and phase transitions among various surface Moiré patterns and superstructures. More specifically, the results show a dramatic difference between the phase diagrams that emerge when a honeycomb film is adsorbed on substrates of honeycomb versus triangular symmetry. It is also shown that in the small deformation limit, the complex Ginzburg-Landau model reduces to a two-dimensional sine-Gordon free energy form. This free energy can be solved exactly for one dimensional patterns and reveals the role of domains walls and their crossings in determining the nature of the phase diagrams.
Maakaasu- ja öljykattiloiden muuttaminen puupellettien pölypoltolle - tekninen toteutus ja markkinatilanne Suomessa
(2014-08-25) Hirvonen, Petri
Insinööritieteiden korkeakoulu | Master's thesis
Multiscale modeling of polycrystalline graphene: A comparison of structure and defect energies of realistic samples from phase field crystal models
(2016-07-11) Hirvonen, Petri; Ervasti, Mikko M.; Fan, Zheyong; Jalalvand, Morteza; Seymour, Matthew; Vaez Allaei, S. Mehdi; Provatas, Nikolas; Harju, Ari; Elder, Ken R.; Ala-Nissilä, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We extend the phase field crystal (PFC) framework to quantitative modeling of polycrystalline graphene. PFC modeling is a powerful multiscale method for finding the ground state configurations of large realistic samples that can be further used to study their mechanical, thermal, or electronic properties. By fitting to quantum-mechanical density functional theory (DFT) calculations, we show that the PFC approach is able to predict realistic formation energies and defect structures of grain boundaries. We provide an in-depth comparison of the formation energies between PFC, DFT, and molecular dynamics (MD) calculations. The DFT and MD calculations are initialized using atomic configurations extracted from PFC ground states. Finally, we use the PFC approach to explicitly construct large realistic polycrystalline samples and characterize their properties using MD relaxation to demonstrate their quality.
Ohutkalvon mekaanisen kuormituksen mallintaminen
(2012) Hirvonen, Petri
Kemian tekniikan korkeakoulu | Bachelor's thesis
Phase field crystal modeling of grain boundaries in graphene
(2015) Hirvonen, Petri
Kemian tekniikan korkeakoulu | Master's thesis
Graphene is a two-dimensional allotrope of carbon that exhibits extraordinary crystalline, mechanical, electrical, thermal and optical properties. The material has been studied for over a decade, but the details of its atomic ordering and microstructural evolution are still incomplete. The phase field crystal model for modeling crystalline systems at atomic length and diffusive time scales is suited for studying such complex phenomena. The phase field crystal model has not previously been applied to quantitatively model two-dimensional materials. In the present work, three variants of the original phase field crystal model are assessed to find a good description for microstructural evolution in graphene. Performance of the three models is evaluated by investigating the symmetric tilt grain boundary structures and related energies they predict in free-standing, planar graphene. Two of the three models produce realistic grain boundary structures, namely those comprised of pentagon-heptagon, or 5|7 dislocations. First of these two models gives a rich variety of other structures as well. Some such structures are presumably metastable or even unphysical and tools to better control the relaxation of the modeled grain boundary structures are presented. This first model yields grain boundary energy as a function of the tilt angle in the approximate range of 0 - 5 eV/nm which is in quantitative agreement with other numerical works. The second model captures qualitatively the essential features of the grain boundary energy curve (ca. 0 - 11 eV/nm). The third model gives a similar range for the grain boundary energy (ca. 0 - 10 eV/nm) but captures its features poorly as a function of the tilt angle. The phase field crystal technique is shown applicable to quantitative modeling of graphene microstructures. This approach benefits the study of atomic structures in real world graphene and may help unlock novel applications.
Phase field crystal modeling of graphene/hexagonal boron nitride interfaces
(2019-06-18) Channe, Shrikant
Perustieteiden korkeakoulu | Master's thesis
Two-dimensional materials such as graphene and hexagonal boron nitride(h-BN) are an important class of materials that have enhanced structural and electronic properties in comparison to their bulk counterparts. However, the limited length and time scales of the traditional modeling methods, such as the molecular dynamics (MD) and the quantum mechanical density functional theory (QMDFT) methods poses a severe challenge to study the underlying mechanism of various properties of these materials and their heterostructures. The phase field crystal (PFC) model can reach diffusive time scales (relevant e.g. in nucleation and growth of crystallites, relaxation of strain-driven 2D monolayers, and thermal conduction) that are much larger in comparison to MD and QMDFT methods while retaining atomic resolution. The model also incorporates an atomic length scale and elastic and plastic deformations in a natural manner. Various PFC models have been used to study topological defects, such as pentagon-heptagon (5|7) defects and inversion grain boundaries formed in the graphene and h-BN monolayers, respectively. In this work, we generalize the one-mode PFC model to study graphene/h-BN interface heterostructure by using the conserved dynamics to describe the dynamics of the model. The model was used to determine the elastic constants of the graphene and h-BN monolayers by uniaxial and biaxial stretching of the respective single crystals. The model highlighted the formation of topological defects such as pentagon-heptagon (5|7) defects at the interface of the in-plane graphene/h-BN heterostructure. Lastly we used the model to find the equilibrium shape of crystal of the h-BN crystal embedded in a graphene monolayer.
Phase field crystal modeling of two-dimensional materials
(2019) Hirvonen, Petri
School of Science | Doctoral dissertation (article-based)
Pristine two-dimensional (2D) materials display many exceptional properties unseen in conventional bulk materials. However, techniques scalable to large-scale production yield polycrystalline microstructures with grains in different orientations and grain boundary line defects between them. Grain boundaries can impair the unique properties of these materials. Better control of the growth process is needed to avoid defects or at least to control their distribution. Unfortunately, the formation of 2D microstructures is poorly understood due to the large span of length and time scales involved. Phase field crystal (PFC) models are a recent approach to multiscale modeling of defected materials. PFC holds great promise for studying complex large-scale microstructures and their slow evolution over long, diffusive time scales. In the work reported in this thesis, we have extended the PFC framework to quantitative modeling of real 2D materials. We first assessed the suitability of PFC for modeling the grain boundaries and triple junctions found in polycrystalline graphene. After detailed comparisons with experimental evidence and atomistic calculations, we found realistic defect structures and formation energies. Finally, we studied the coarsening and characteristics of different crystalline and quasicrystalline microstructures using PFC. We observed that many microstructural properties are universal and independent of the underlying lattice symmetry. Having identified a PFC model capable of yielding realistic graphene structures, we exploited it in a multiscale approach to produce sample model systems for heat transport molecular dynamics studies. These realistic, highly-relaxed samples allowed multiple new discoveries. For example, we showed that the in- and out-of-plane phonon modes in graphene are scattered very differently by grain boundaries. We repeated these analyses for PFC samples of hexagonal boron nitride and observed qualitatively similar behavior. In addition to new knowledge about 2D microstructures and heat transport through them, we developed various computational techniques to facilitate our research. These methods are related to sampling low-energy defect configurations, converting PFC density fields to discrete atomic coordinates and analyzing microstructures. As an example, we developed a method for detecting the local lattice orientation and defects in crystalline and quasicrystalline microstructures, and for extracting the grain structures in them. This was the first method of this kind applicable to quasicrystals. The work in this thesis has laid a solid foundation for application of the PCF methodology to study other 2D materials or further physical properties of them, such as their mechanical or electronic transport properties.
Phase-field crystal model for heterostructures
(2019-10-16) Hirvonen, Petri; Heinonen, Vili; Dong, Haikuan; Fan, Zheyong; Elder, Ken R.; Ala-Nissila, Tapio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Atomically thin two-dimensional heterostructures are a promising, novel class of materials with ground-breaking properties. The possibility of choosing many constituent components and their proportions allows optimization of these materials to specific requirements. The wide adaptability comes with a cost of large parameter space making it hard to experimentally test all the possibilities. Instead, efficient computational modeling is needed. However, large range of relevant time and length scales related to physics of polycrystalline materials poses a challenge for computational studies. To this end, we present an efficient and flexible phase-field crystal model to describe the atomic configurations of multiple atomic species and phases coexisting in the same physical domain. We extensively benchmark the model for two-dimensional binary systems in terms of their elastic properties and phase boundary configurations and their energetics. As a concrete example, we demonstrate modeling lateral heterostructures of graphene and hexagonal boron nitride. We consider both idealized bicrystals and large-scale systems with random phase distributions. We find consistent relative elastic moduli and lattice constants, as well as realistic continuous interfaces and faceted crystal shapes. Zigzag-oriented interfaces are observed to display the lowest formation energy.
Suurten lauhdevoimalaitosten hyötysuhteen parantaminen lisäämällä välitulistusvaiheita
(2012-04-21) Hirvonen, Petri
Insinööritieteiden korkeakoulu | Bachelor's thesis
Thermal conductivity decomposition in two-dimensional materials: Application to graphene
(2017-04-19) Fan, Zheyong; Pereira, Luiz Felipe C; Hirvonen, Petri; Ervasti, Mikko M.; Elder, Ken R.; Donadio, Davide; Ala-Nissilä, Tapio; Harju, Ari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Two-dimensional materials have unusual phonon spectra due to the presence of flexural (out-of-plane) modes. Although molecular dynamics simulations have been extensively used to study heat transport in such materials, conventional formalisms treat the phonon dynamics isotropically. Here, we decompose the microscopic heat current in atomistic simulations into in-plane and out-of-plane components, corresponding to in-plane and out-of-plane phonon dynamics, respectively. This decomposition allows for direct computation of the corresponding thermal conductivity components in two-dimensional materials. We apply this decomposition to study heat transport in suspended graphene, using both equilibrium and nonequilibrium molecular dynamics simulations. We show that the flexural component is responsible for about two-thirds of the total thermal conductivity in unstrained graphene, and the acoustic flexural component is responsible for the logarithmic divergence of the conductivity when a sufficiently large tensile strain is applied.
Triple Junctions in Hexagonal Boron Nitride: A Phase Field Crystal Approach
(2019-10-22) Suviranta, Jaarli
Perustieteiden korkeakoulu | Master's thesis