| 1. |
- Lidbaum, Hans, 1981-
(författare)
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Transmission Electron Microscopy for Characterization of Structures, Interfaces and Magnetic Moments in Magnetic Thin Films and Multilayers
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Structural characterization is essential for the understanding of the magnetic properties of thin films and multilayers. In this thesis, both crystalline and amorphous thin films and multilayers were analyzed utilizing transmission electron microscopy (TEM). High resolution TEM and electron diffraction studies emphasize on the growth of amorphous Fe91Zr9 and Co68Fe24Zr8 on both Al2O3 and Al70Zr30 in multilayer structures by magnetron sputtering. The properties of the growth surfaces were found to strongly influence the formation of nano-crystallites of the magnetic material at interfaces. Field induced uniaxial magnetic anisotropy was found to be possible to imprint into both fully amorphous and partially crystallized Co68Fe24Zr8 layers, yielding similar magnetic characteristics regardless of the structure. These findings are important for the understanding of both growth and magnetic properties of these amorphous thin films. As magnetic systems become smaller, new analysis techniques need to be developed. One such important step was the realization of electron energy-loss magnetic circular dichroism (EMCD) in the TEM, where information about the ratio of the orbital to spin magnetic moment (mL/mS) of a sample can be obtained. EMCD makes use of angular dependent inelastic scattering, which is characterized using electron energy-loss spectroscopy. The work of this thesis contributes to the development of EMCD by performing quantitative measurements of the mL/mS ratio. Especially, methods for obtaining energy filtered diffraction patterns in the TEM together with analysis tools of the data were developed. It was found that plural inelastic scattering events modify the determination of the mL/mS ratio, wherefore a procedure to compensate for it was derived. Additionally, utilizing special settings of the electron gun it was shown that EMCD measurements becomes feasible on the nanometer level through real space maps of the EMCD signal.
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| 2. |
- Duan, Tianbo, 1992-
(författare)
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Functionalized Graphene as Superlattice and Gas Sensor
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene, an atomic-thin carbon sheet with carbon atoms tightly packed honeycomb-like lattice, has attracted enormous interest due to its unique chemical and physical properties. However, the intrinsic zero bandgap characteristic of graphene has so far prevented graphene from building effective electronic and optoelectronic devices. To address this concern, different functionalization methods have been proposed to modify the electronic properties of graphene. This thesis focuses on different graphene surface functionalizations and their applications in gas detections and superlattices.First of all, the surface cleanness of graphene plays a crucial role in the performance of graphene devices. To achieve a controlled removal of polymer residues on graphene surface, a facile solvent based method has been proposed, which can drastically improve the charge carrier mobility of graphene devices by a factor of 3, indicating a potential ballistic transport of graphene under ambient condition. In addition, an electron beam induced fluorination cycle is proposed to eliminate the airborne hydrocarbon contamination related to aging effects on the graphene surface. Subsequent spectroscopic analysis confirms the long-term preservation of graphene using such technique. A similar technique, ion beam induced covalent functionalization has been used to locally fluorinate graphene, which could enhance the sensitivity of NH3 sensing as compared to a pristine graphene gas sensor by a factor of 8. The use of non-covalent, π-π stacking interactions for the functionalization of graphene opens a pathway to bind the functionalizing groups from nearly unlimited variety of p conjugated molecules. Here, we demonstrate that the use of BP2T molecules functionalizing graphene leads to an enhanced sensitivity to NH3 by a factor of 3 comparing with that of pristine graphene. This particle beam induced functionalization technique can be used for the fabrication of graphene superlattices. Here, a direct nanostructuring technique by employing electron beam induced etching with different precursor gases has been proposed to achieve localized structuring of graphene/hBN structures. Suspended fluorinated graphene can be obtained by using this dual-beam process, suggesting the capability of printing antidot superlattices where graphene would be suspended in a controllable way. When functionalizing a graphene bilayer by electron beam activated fluorination, a new type of moiré superlattice with rectangular periodicity can be formed due to the crystalline mismatch between the topmost fluorographene and underneath pristine graphene. Recently, rotational moiré superlattices of graphene were shown to be superconducting. We believe that this unique structure has the potential to equally reveal novel properties of 2D materials.
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| 3. |
- Sher, Omer
(författare)
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Nanoparticles based molecular electronic devices with tunable molecular functionalization shell and gas sensing measurements
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The idea to use molecules as a basic building block in electronic circuits was developed about 50 years ago when a molecular rectifier was developed, but it has been a challenge for this field to make its way to real-world application. Now, due to the advancement in technologies, the properties of single molecules are better understandable and controllable. Some of the main motivations to build molecular electronics devices are that the conductive molecules can be as small as about 1 nm, that they are stable objects and can be tailor-made with desired electronic properties. This small size of molecule poses a challenge in their usage, one solution is to develop the hybrid devices whose properties are based on single and few molecules.In this study, a portable hybrid device is used and further developed called a nanoMoED device, a nano-molecular electronic device. These devices consist of gold nanoparticles (AuNPs), gold nanoelectrodes and conjugated organic molecules. The electrical resistance of the device depends on the molecules functionalizing it and, in this work, they contain phenyl rings such as 4,4’-biphenyldithiol (BPDT), p-ter-phenyl-4,4''-dithiol and oligo phenylene-ethynylene.The 20 nm wide nanogaps are fabricated by a focused ion beam (FIB) creating thus true nanodevices. The molecular nanojunctions are formed by dielectrophoretic trapping of molecule functionalized AuNPs into a nanogap. The distance between the NPs, measured from transmission electron microscopy images is similar to the size of the targeted functionalizing organic molecule that shall bridge the NP-NP gap. We have reported that the primary molecular ligand shell of the AuNPs can be tuned in the synthesis process by the secondary molecular functionalization process. The experimental results showed that this process depends on the interparticle spacing and the structure of the primary functionalizing molecules. The nanoMoED devices showed a successful cyclic molecular place exchange process where alternately BPDT and octanethiol (OT) were moved into the devices. This is confirmed by a change in the electrical resistance of devices showing higher conductance for BPDT than OT.The nanoMoED devices when tested in NO2, ethanol, and NH3 gas atmosphere showed a significant change in device electrical resistance. Density functional theory calculations explain this observation. The analyte molecules bind with the aromatic conjugated molecule and induce additional charge transport channels near the Fermi level of the sensing molecule. In graphene-based, i.e., 2D, micron-sized devices, we could show that the non-covalent molecular functionalization of graphene improves its NH3 gas sensing response by 3 times as compared to pristine graphene. Further experiments are required to understand the device properties under different working conditions as well as to evidence different functionalities for example as a switch.
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| 4. |
- Ali, Hasan, 1985-
(författare)
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Towards atomically resolved magnetic measurements in the transmission electron microscope : A study of structure and magnetic moments in thin films
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The magnetic properties of thin metallic films are significantly different from the bulk properties due to the presence of interfaces. The properties shown by such thin films are influenced by the atomic level structure of the films and the interfaces. Transmission electron microscope (TEM) has the potential to analyse the structure and the magnetic properties of such systems with atomic resolution. In this work, the TEM is employed to characterize the structure of the Fe/V and Fe/Ni multilayers and the technique of electron magnetic circular dichroism (EMCD) is developed to obtain the quantitative magnetic measurements with high spatial resolution.From TEM analysis of short period Fe/V multilayers, a coherent superlattice structure is found. In short period Fe/Ni multilayer samples with different repeat frequency, only the TEM technique could verify the existence of the multilayer structure in the thinnest layers. The methods of scanning TEM imaging and electron energy loss spectroscopy (EELS) results were used and refined to determine interdiffusion at the interfaces. The confirmation of the multilayer structure helped to explain the saturation magnetization of these samples.Electron magnetic circular dichroism (EMCD) has the potential to quantitatively measure the magnetic moments of the materials with atomic resolution, but the technique presents several challenges. First, the EMCD measurements need to acquire two EELS spectra at two different scattering angles. These spectra are mostly acquired one after the other which makes it difficult to guaranty the identical experimental conditions and the spatial registration between the two acquisitions. We have developed a technique to simultaneously acquire the two angle-resolved EELS spectra in a single acquisition. This not only ensures the accuracy of the measurements but also improves the signal to noise ratio as compared to the previously used methods. The second important question is the effect of crystal orientations on the measured EMCD signals, considering the fact that the crystal orientation of a real crystal does not remain the same in the measured area. We developed the methodology to simultaneously acquire the EMCD signals and the local crystal orientations with high precision and experimentally showed that the crystal tilt significantly changes the magnetic signal. The third challenge is to obtain EMCD measurements with atomic resolution which is hampered by the need of high beam convergence angles. We further developed the simultaneous acquisition technique to obtain the quantitative EMCD measurements with beam convergence angles corresponding to atomic size electron probes.
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| 5. |
- Fu, Le, et al.
(författare)
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Far from equilibrium ultrafast high-temperature sintering of ZrO2-SiO2 nanocrystalline glass-ceramics
- 2023
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Ingår i: Journal of The American Ceramic Society. - : John Wiley & Sons. - 0002-7820 .- 1551-2916. ; 106:7, s. 4005-4012
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast high-temperature sintering (UHS) is a novel sintering technique with ultrashort firing cycles (e.g., a few tens of seconds). The feasibility of UHS has been validated on several ceramics and metals; however, its potential in consolidating glass-ceramics has not yet been demonstrated. In this work, an optimized carbon-free UHS was utilized to prepare ZrO2-SiO2 nanocrystalline glass-ceramics (NCGCs). The phase composition, grain size, densification behavior, and microstructures of NCGCs prepared by UHS were investigated and compared with those of samples sintered by pressureless sintering. Results showed that NCGCs with a high relative density (similar to 95%) can be obtained within similar to 50 s discharge time by UHS. The UHS processing not only hindered the formation of ZrSiO4 and cristobalite but also enhanced the stabilization of t-ZrO2. Meanwhile, owing to the ultrashort firing cycles, the UHS technology allowed the NCGCs to be consolidated in a far from equilibrium state. The NCGCs showed a microstructure of spherical monocrystalline ZrO2 nanocrystallites embedded in an amorphous SiO2 matrix.
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| 6. |
- Fu, Le, et al.
(författare)
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Microstructure of rapidly-quenched ZrO2-SiO2 glass-ceramics fabricated by container-less aerodynamic levitation technology
- 2023
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Ingår i: Journal of The American Ceramic Society. - : John Wiley & Sons. - 0002-7820 .- 1551-2916. ; 106:4, s. 2635-2651
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Tidskriftsartikel (refereegranskat)abstract
- In this work, an aerodynamic levitation technology (ALT) was utilized to prepare ZrO2-SiO2 glass-ceramics with two different ZrO2 contents, that is, 35 mol% and 50 mol%. The glass-ceramics were partially melted at similar to 2000 degrees C or fully melted at similar to 3000 degrees C by ALT, followed by rapid quenching to obtain spherical glass-ceramic beads. The phase compositions and microstructures of the glass-ceramics were characterized. Crystallization of ZrO2 occurred during the solidification process and ZrO2 content, processing temperature, and the addition of yttrium (3 mol%) affected the crystalline phase of ZrO2. No ZrSiO4 or crystalline SiO2 were formed during the solidification process and the glass-ceramics were away from thermodynamic equilibrium due to rapid quenching. The glass-ceramics showed a microstructure of irregular-shaped ZrO2 micro-aggregates embedded in an amorphous SiO2 matrix, with lamellar twins and lattice defects formed within ZrO2 crystals. For samples prepared at similar to 3000 degrees C, a liquid-liquid phase separation occurred in the melt, which eventually resulted in the formation of large and irregular-shaped ZrO2 aggregates. In comparison, for samples prepared at similar to 2000 degrees C, pre-existed ZrO2 crystals formed during heating acted as nucleation sites during the cooling process, followed by grain growth to form large ZrO2 aggregates. Solidification and microstructure formation mechanisms were proposed to elucidate the solidification process during rapid cooling and the microstructure of the glass-ceramics obtained.
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| 7. |
- Fu, Le, et al.
(författare)
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Ultrastrong translucent glass ceramic with nanocrystalline, biomimetic structure
- 2018
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:11, s. 7146-7154
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Tidskriftsartikel (refereegranskat)abstract
- Transparent/translucent glass ceramics (GCs) have broad applications in biomedicine, armor, energy, and constructions. However, GCs with improved optical properties typically suffer from impaired mechanical properties, compared to traditional sintered full-ceramics. We present a method of obtaining high-strength, translucent GCs by preparing ZrO2-SiO2 nanocrystalline glass ceramics (NCGCs), with a microstructure of monocrystalline ZrO2 nanoparticles (NPs), embedded in an amorphous SiO2 matrix. The ZrO2-SiO2 NCGC with a composition of 65%ZrO2-35%SiO2 (molar ratio, 65Zr) achieved an average flexural strength of 1 GPa. This is one of the highest flexural strength values ever reported for GCs. ZrO2 NPs have a core-shell structure, and the shell is a thin (2–3 nm) amorphous Zr/Si interfacial layer that provides strong bonding between the ZrO2 NPs and SiO2 matrix. The diffusion of Si atoms into the ZrO2 NPs forms a Zr-O-Si superlattice. Electron tomography results show that some of the ZrO2 NPs are connected in one direction, forming in situ ZrO2 nanofibers (with length of ~500 nm), and that the ZrO2 nanofibers are stacked in an ordered way in all three dimensions. The nano-architecture of the ZrO2 nanofibers mimics the architecture of mineralized collagen fibril in cortical bone. Strong interface bonding enables efficient load transfer from the SiO2 matrix to the 3D nano-architecture built by ZrO2 nanofibers and NPs, and the 3D nano-architecture carries the majority of the external load. These two factors synergistically contribute to the high strength of the 65Zr NCGC. This study deepens our fundamental understanding of the microstructure-mechanical strength relationship, which could guide the design and manufacture of other high-strength, translucent GCs.
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| 8. |
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| 9. |
- Ghajeri, Farnaz, et al.
(författare)
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The Influence of Residuals Combining Temperature and Reaction Time on Calcium Phosphate Transformation in a Precipitation Process
- 2022
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Ingår i: Journal of Functional Biomaterials. - : MDPI. - 2079-4983. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation is one of the most common processes to synthesize hydroxyapatite, which is the human body’s mineral forming bone and teeth, and the golden bioceramic material for bone repair. Generally, the washing step is important in the precipitation method to remove the residuals in solution and to stabilize the phase transformation. However, the influence of residuals in combination with the reaction temperature and time, on calcium phosphate formation, is not well studied. This could help us with a better understanding of the typical synthesis process. We used a fixed starting ion concentration and pH in our study and did not adjust it during the reaction. XRD, FTIR, ICP-OES, and SEM have been used to analyze the samples. The results showed that combining residuals with both reaction temperature and time can significantly influence calcium phosphate formation and transformation. Dicalcium phosphate dihydrate formation and transformation are sensitive to temperature. Increasing temperature (60◦C) can inhibit the formation of acidic calcium phosphate or transform it to other phases, and further the particle size. It was also observed that high reaction temperature (60◦C) results in higher precipitation efficiency than room temperature. A low ion concentration combining reaction temperature and time could still significantly influence the calcium phosphate transformation during the drying. © 2022 by the authors.
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| 10. |
- Jafri, Syed Hassan Mujtaba, 1979-
(författare)
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Building Systems for Electronic Probing of Single Low Dimensional Nano-objects : Application to Molecular Electronics and Defect Induced Graphene
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nano-objects have unique properties due to their sizes, shapes and structure. When electronic properties of such nano-objects are used to build devices, the control of interfaces at atomic level is required. In this thesis, systems were built that can not only electrically characterize nano-objects, but also allow to analyze a large number of individual nano-objects statistically at the example of graphene and nanoparticle-molecule-nanoelectrode junctions. An in-situ electrical characterization system was developed for the analysis of free standing graphene sheets containing defects created by an acid treatment. The electrical characterization of several hundred sheets revealed that the resistance in acid treated graphene sheets decreased by 50 times as compared to pristine graphene and is explained by the presence of di-vacancy defects. However, the mechanism of defect insertion into graphene is different when graphene is bombarded with a focused ion beam and in this case, the resistance of graphene increases upon defect insertion. The defect insertion becomes even stronger at liquid N2 temperature. A molecular electronics platform with excellent junction properties was fabricated where nanoparticle-molecule chains bridge 15-30nm nanoelectrodes. This approach enabled a systematic evaluation of junctions that were assembled by functionalizing electrode surfaces with alkanethiols and biphenyldithiol. The variations in the molecular device resistance were several orders of magnitude and explained by variations in attachment geometries of molecules. The spread of resistance values of different devices was drastically reduced by using a new functionalization technique that relies on coating of gold nanoparticles with trityl protected alkanedithiols, where the trityl group was removed after trapping of nanoparticles in the electrode gap. This establishment of a reproducible molecular electronics platform enabled the observation of vibrations of a few molecules by inelastic tunneling spectroscopy. Thus this system can be used extensively to characterize molecules as well as build devices based on molecules and nanoparticles.
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