| 1. |
- Palisaitis, Justinas, 1983-, et al.
(författare)
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On the Structural Stability of MXene and the Role of Transition Metal Adatoms
- 2018
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 10:23, s. 10850-10855
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Tidskriftsartikel (refereegranskat)abstract
- In the present communication, the atomic structure and coordination of surface adsorbed species on Nb2C MXene is investigated over time. In particular, the influence of the Nb adatoms on the structural stability and oxidation behavior of the MXene is addressed. This investigation is based on plan-view geometry observations of single Nb2C MXene sheets by a combination of atomic-resolution scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and STEM image simulations.
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| 2. |
- Persson, Ingemar, 1985-
(författare)
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Surface characterization of 2D transition metal carbides (MXenes)
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Research on two-dimensional (2D) materials is a rapidly growing field owing to the wide range of new interesting properties found in 2D structures that are vastly different from their three-dimensional (3D) analogues. In addition, 2D materials embodies a significant surface area that facilitates a high degree of surface reactions per unit volume or mass, that is imperative in many applications such as catalysis, energy storage, energy conversion, filtration, and single molecule sensing. MXenes constitute a family of 2D materials consisting of transition metal carbides and/or nitrides, which are typically formed after selective etching of their 3D parent MAX phases. The latter, are a family of nanolaminated compounds that typically follow the formula Mn+1AXn (n=1-3), where M is a transition metal, A is a group 13 or 14 element, and X is C and or N. Selective etching by aqueous F- containing acids removes the A layer leaving 2D Mn+1Xn slabs instantly terminated by a mix of O-, OH- and F-groups. The first and most investigated MXene is Ti3C2TX, where TX stands for surface termination, which has shown record properties in a range of applications (eg. electrode in Li-batteries, supercapacitors, sieving membrane, electromagnetic interference shielding, and carbon capture). Adding to that, over 30 different MXenes have been discovered since 2011, exhibiting alternative or superior properties. Most importantly, elegant routes for property design in the MXene family has been demonstrated, by means of either varying the chemistry in the Mn+1Xn compound, by alloying two M elements, or by changing the structure of the MXene by introducing vacancies.The present work has a led to an additional route for post synthesis property tuning in MXenes by manipulation of surface termination elements. This enables a unique toolbox for property tuning which is not available to other 2D materials and is highly beneficial for applications that is dependent on surface reactions. Furthermore, chemical and structural characterization of terminations on single sheets is essential to rule out the influence of intercalants or contamination that is typically present in multilayer MXene samples or thin films. For that purpose, a method for preparing isolated contamination free single sheets of MXene samples for transmission electron microscopy (TEM) characterization was established. In order to determine vacancy and termination sites, atomically resolved scanning (S)TEM imaging and image simulations was carried out. Two main processes were employed to substitute the termination elements.1) An initial thermal treatment in vacuum facilitates F desorption and it was shown that O-terminations rearranges on the evacuated sites. H2 gas exposure in a controlled environment demonstrated a removal of the remaining O-terminations. As a result, termination-free MXene is possible to realize under vacuum conditions.2) CO2 was introduced as a first non-inherent termination on MXene by in situ CO2 gas exposure at low temperatures. That was a first demonstration of Ti3C2TX as promising material for carbon capture. Additionally, O-saturated surfaces were demonstrated after introduction of O2 gas on the F-depleted Ti3C2TX MXene, which is highly relevant for hydrogen evolution reactions where fully O-terminated Ti3C2TX are predicted to improve efficiency.A Lewis acid melt synthesis method was used to realize the first MXene exclusively terminated with Cl. Moreover, this was the first report of a MXene directly synthesised with terminations other than O, OH, and F.Furthermore, we have expanded the space of property tuning by introduction of chemical ordering, by selective etching of Y in an alloyed (Mo2/3Y1/3)2CTX MXene. This either produced chemical ordering with one M (Mo) element and vacancies, or ordering between two M (Mo and Y) elements. This was further reported to significantly increase volumetric capacitance because of the increased number of active sites around vacancies, leading to an increasing charge density. As a final note, the stability of Nb2CTX MXene under ambient conditions was investigated. It was found that the surface Nb adatoms, present after etching, got oxidized over time which resulted in local clustering and effectively degraded the MXene.This work has demonstrated reproducible surface characterization methods for determining termination elements and sites in 2D MXenes, that is ultimately governing MXene properties. Most importantly, we report on a new approach for MXene property tuning as well as contributing to several existing property tuning approaches.
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| 3. |
- Persson, Ingemar, 1985-, et al.
(författare)
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Sub-4 nm mapping of donor-acceptor organic semiconductor nanoparticle composition
- 2023
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Ingår i: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 15:13, s. 6126-6142
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Tidskriftsartikel (refereegranskat)abstract
- We report, for the first time, sub-4 nm mapping of donor : acceptor nanoparticle composition in eco-friendly colloidal dispersions for organic electronics. Low energy scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX) mapping has revealed the internal morphology of organic semiconductor donor : acceptor blend nanoparticles at the sub-4 nm level. A unique element was available for utilisation as a fingerprint element to differentiate donor from acceptor material in each blend system. Si was used to map the location of donor polymer PTzBI-Si in PTzBI-Si:N2200 nanoparticles, and S (in addition to N) was used to map donor polymer TQ1 in TQ1:PC71BM nanoparticles. For select material blends, synchrotron-based scanning transmission X-ray microscopy (STXM), was demonstrated to remain as the superior chemical contrast technique for mapping organic donor : acceptor morphology, including for material combinations lacking a unique fingerprint element (e.g. PTQ10:Y6), or systems where the unique element is in a terminal functional group (unsaturated, dangling bonds) and can hence be easily damaged under the electron beam, e.g. F on PTQ10 donor polymer in the PTQ10:IDIC donor : acceptor blend. We provide both qualitative and quantitative compositional mapping of organic semiconductor nanoparticles with STEM EDX, with sub-domains resolved in nanoparticles as small as 30 nm in diameter. The sub-4 nm mapping technology reported here shows great promise for the optimisation of organic semiconductor blends for applications in organic electronics (solar cells and bioelectronics) and photocatalysis, and has further applications in organic core-shell nanomedicines.
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| 4. |
- Zhang, Hengfang, 1986-
(författare)
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Hot-wall MOCVD of N-polar group-III nitride materials
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Group III-Nitride semiconductors: indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN) and their alloys continue to attract significant scientific interest due to their unique properties and diverse applications in photonic and electronic applications. Group-III nitrides have direct bandgaps which cover the entire spectral range from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN). This makes III-nitride materials suitable for high-efficient and energy-saving optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). The Nobel Prize in Physics 2014 was awarded for the invention of efficient GaN blue LEDs, which further accelerated the research in the field of group III-nitride materials. GaN and related alloys are also suitable for high-temperature, high-power and high-frequency electronic devices with performance that cannot be delivered by other semiconductor technologies such as silicon (Si) and gallium arsenide (GaAs). For example, GaN-based high electron mobility transistors (HEMTs) have been widely adopted for radio frequency (RF) communication and power amplifiers, high-voltage power switches in radars, satellites, and wireless base stations for 5G. Recently, nitrogen (N)-polar group-III nitrides have drawn much attention due to their advantages over their metal-polar counterparts in e.g. HEMTs. These include feasibility to fabricate ohmic contacts with low resistance, an enhanced carrier confinement with a natural back barrier, and improved device scalability. Despite intensive research, the growth of micrometer-thick high-quality N-polar GaN based materials remains challenging. One of the major problems to develop device-quality N-polar nitrides is the high surface roughness, which results from the formation of hexagonal hillocks or step-bunching. Another significant hurdle is the unintentional polarity inversion, which reduces the crystalline quality and prohibits device fabrication. This licentiate thesis focuses on the development of N-polar AlN and GaN heterostructures on SiC substrates for HEMT RF applications. The overall aim is to exploit the advantages of the hot-wall MOCVD concept to grow high-quality N-polar HEMT structures for higher operational frequencies and improved device performance. In order to achieve this goal, special effort is dedicated to understanding the effects of growth conditions and substrate orientation on the structural properties and polarity of AlN, GaN and AlGaN grown by hot-wall MOCVD. N-polar AlN nucleation layers (NLs) with layer by layer growth mode and step-flow growth mode can be achieved on on-axis and 4_ offaxis SiC (000¯1), respectively, by carefully controlling V/III ratio and growth temperature. Utilizing scanning transmission electron microscopy (STEM) we have established a comprehensive picture of the atomic arrangements, local polarity and polarity evolution in AlN, GaN/AlN and AlGaN/GaN/AlN in the cases of low-temperature and high-temperature AlN NLs both for on-axis and off-axis substrates. We have shown that typically employed methods for polarity determination using potassium hydroxide wet etching could not provide conclusive results in the case of mixed-polar AlN as Al-polar domains may be easily over-etched and remain undetected. Atomic scale electron microscopy is therefore needed to accurately determine the polarity. We further have developed growth strategy and have optimized the epitaxial process for N-polar GaN, and have demonstrated high quality N-polar AlGaN/GaN/AlN heterostructures.
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| 5. |
- Zhang, Hengfang, 1986-
(författare)
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Hot-wall MOCVD of N-polar group-III nitride materials and high electron mobility transistor structures
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Group III-Nitride semiconductors: indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN) and their alloys continue to attract significant scientific interest due to their unique properties and diverse applications in photonic and electronic applications. Group-III nitrides have direct bandgaps which cover the entire spectral range from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN). This makes III-nitride materials suitable for high-efficient and energy-saving optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). The Nobel Prize in Physics 2014 was awarded for the invention of efficient GaN blue LEDs, which further accelerated the research in the field of group III-nitride materials. GaN and related alloys are also suitable for high-temperature, high-power and high-frequency electronic devices with performance that cannot be delivered by other semiconductor technologies such as silicon (Si) and gallium arsenide (GaAs). For example, GaN-based high electron mobility transistors (HEMTs) have been widely adopted for radio frequency (RF) communication and power amplifiers, high-voltage power switches in radars, satellites, and wireless base stations for 5G.Recently, nitrogen (N)-polar group-III nitrides have drawn much attention due to their advantages over their metal-polar counterparts in e.g. HEMTs. These include feasibility to fabricate ohmic contacts with low resistance, an enhanced carrier confinement with a natural back barrier, and improved device scalability. Despite intensive research, the growth of micrometer-thick high-quality N-polar GaN based materials remains challenging. One of the major problems to develop device-quality N-polar nitrides is the high surface roughness, which results from the formation of hexagonal hillocks or step-bunching. Another significant hurdle is the unintentional polarity inversion, which reduces the crystalline quality and prohibits device fabrication. This thesis focuses on the development of N-polar AlN and GaN heterostructures on SiC substrates for HEMT RF applications. The overall aim is to exploit the advantages of the hot-wall MOCVD concept to grow high-quality N-polar HEMT structures for higher operational frequencies and improved device performance. In order to achieve this goal, special effort is dedicated to understanding the effects of growth conditions and substrate orientation on the structural properties and polarity of AlN, GaN and AlGaN grown by hot-wall MOCVD. N-polar AlN nucleation layers (NLs) with layer by layer growth mode and step-flow growth mode can be achieved on on-axis and 4 off-axis SiC (000¯1), respectively, by carefully controlling V/III ratio and growth temperature. Utilizing scanning transmission electron microscopy (STEM) we have established a comprehensive picture of the atomic arrangements, local polarity and polarity evolution in AlN, GaN/AlN and AlGaN/GaN/AlN in the cases of low-temperature and high-temperature AlN NLs both for on-axis and off-axis substrates. We have shown that the typically employed methods for polarity determination using potassium hydroxide wet etching could not provide conclusive results in the case of mixed-polar AlN as Al-polar domains may be easily over-etched and remain undetected. Atomic scale electron microscopy is therefore needed to accurately determine the polarity. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrates offcut angles in order to achieve desired growth mode and polarity. We further have developed growth strategy and have optimized the epitaxial process for N-polar GaN by multiple-step temperature processes on three types of C-face SiC substrates with different offcut angles. A highquality N-polar AlGaN/GaN/AlN heterostructure has been demonstrated. A 2DEG carrier density up to 1013 cm−2 have been demonstrated for N-polar HEMT structure.
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