| 1. |
- Piscitelli, Francesco, et al.
(författare)
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Neutron reflectometry on highly absorbing films and its application to (B4C)-B-10-based neutron detectors
- 2016
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Ingår i: Royal Society of London. Proceedings A. Mathematical, Physical and Engineering Sciences. - : The Royal Society. - 1364-5021 .- 1471-2946. ; 472:2185
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Tidskriftsartikel (refereegranskat)abstract
- Neutron reflectometry is a powerful tool used for studies of surfaces and interfaces. The absorption in the typical studied materials is neglected and this technique is limited only to the reflectivity measurement. For strongly absorbing nuclei, the absorption can be directly measured by using the neutron-induced fluorescence technique which exploits the prompt particle emission of absorbing isotopes. This technique is emerging from soft matter and biology where highly absorbing nuclei, in very small quantities, are used as a label for buried layers. Nowadays, the importance of absorbing layers is rapidly increasing, partially because of their application in neutron detection; a field that has become more active also due to the He-3-shortage. We extend the neutron-induced fluorescence technique to the study of layers of highly absorbing materials, in particular (B4C)-B-10. The theory of neutron reflectometry is a commonly studied topic; however, when a strong absorption is present the subtle relationship between the reflection and the absorption of neutrons is not widely known. The theory for a general stack of absorbing layers has been developed and compared to measurements. We also report on the requirements that a (B4C)-B-10 layer must fulfil in order to be employed as a converter in neutron detection.
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| 2. |
- Ali, M., et al.
(författare)
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Reduced photoluminescence from InGaN/GaN multiple quantum well structures following 40 MeV iodine ion irradiation
- 2009
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Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 404:23-24, s. 4925-4928
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Tidskriftsartikel (refereegranskat)abstract
- The effects following ion irradiation of GaN-based devices are still limited. Here we present data on the photoluminescence (PL) emitted from InGaN/GaN multiple quantum well (MQW) structures, which have been exposed to 40 MeV I ion irradiation. The PL is reduced as a function of applied ion fluence, with essentially no PL signal left above 1011 ions/cm2. It is observed that even the ion fluences in the 109 ions/cm2 range have a pronounced effect on the photoluminescence properties of the MQW structures. This may have consequences concerning application of InGaN/GaN MQW’s in radiation-rich environments, in addition to defect build-up during ion beam analysis.
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| 3. |
- Fan, Qunping, 1989, et al.
(författare)
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Over 14% efficiency all-polymer solar cells enabled by a low bandgap polymer acceptor with low energy loss and efficient charge separation
- 2020
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Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 13:12, s. 5017-5027
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Tidskriftsartikel (refereegranskat)abstract
- Obtaining both high open-circuit voltage (V-oc) and short-circuit current density (J(sc)) has been a major challenge for efficient all-polymer solar cells (all-PSCs). Herein, we developed a polymer acceptor PF5-Y5 with excellent optical absorption capability (onset extending to similar to 880 nm and maximum absorption coefficient exceeding 105 cm(-1) in a film), high electron mobility (3.18 x 10(3) cm(2) V-1 s(-1)) and high LUMO level (-3.84 eV) to address such a challenge. As a result, the PBDB-T:PF5-Y5-based all-PSCs achieved a high power conversion efficiency of up to 14.45% with both a high Voc (0.946 V) and a high Jsc (20.65 mA cm(-2)), due to the high and broad absorption coverage, small energy loss (0.57 eV) and efficient charge separation and transport in the device, which are among the best values in the all-PSC field. In addition, the all-PSC shows a similar to 15% improvement in PCE compared to its counterpart small molecule acceptor (Y5)-based device. Our results suggest that PF5-Y5 is a very promising polymer acceptor candidate for applications in efficient all-PSCs.
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| 4. |
- Armakavicius, Nerijus
(författare)
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Study of novel electronic materials by mid-infrared and terahertz optical Hall effect
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Development of silicon based electronics have revolutionized our every day life during the last three decades. Nowadays Si based devices operate close to their theoretical limits that is becoming a bottleneck for further progress. In particular, for the growing field of high frequency and high power electronics, Si cannot offer the required properties. Development of materials capable of providing high current densities, carrier mobilities and high breakdown fields is crucial for a progress in state of the art electronics.Epitaxial graphene grown on semi-insulating silicon carbide substrates has a high potential to be integrated in the current planar device technologies. High electron mobilities and sheet carrier densities make graphene extremely attractive for high frequency analog applications. One of the remaining challenges is the interaction of epitaxial graphene with the substrate. Typically, much lower free charge carrier mobilities, compared to free standing graphene, and doping, due to charge transfer from the substrate, is reported. Thus, a good understanding of the intrinsic free charge carriers properties and the factors affecting them is very important for further development of epitaxial graphene.III-group nitrides have been extensively studied and already have proven their high efficiency as light sources for short wavelengths. High carrier mobilities and breakdown electric fields were demonstrated for III-group nitrides, making them attractive for high frequency and high power applications. Currently, In-rich InGaN alloys and AlGaN/GaN high electron mobility structures are of high interest for the research community due to open fundamental questions.Electrical characterization techniques, commonly used for the determination of free charge carrier properties, require good ohmic and Schottky contacts, which in certain cases can be difficult to achieve. Access to electrical properties of buried conductive channels in multilayered structures requires modification of samples and good knowledge of the electrical properties of all electrical contact within the structure. Moreover, the use of electrical contacts to electrically characterize two-dimensional electronic materials, such as graphene, can alter their intrinsic properties. Furthermore, the determination of effective mass parameters commonly employs cyclotron resonance and Shubnikov-de Haas oscillations measurements, which require long scattering times of free charge carriers, high magnetic fields and low temperatures.The optical Hall effect is an external magnetic field induced optical anisotropy in conductive layers due to the motion of the free charge carriers under the influence of the Lorentz force, and is equivalent to the electrical Hall effect at optical frequencies. The optical Hall effect can be measured by generalized ellipsometry and provides a powerful method for the determination of free charge carrier properties in a non-destructive and contactless manner. In principle, a single optical Hall effect measurement can provide quantitative information about free charge carrier types, concentrations, mobilities and effective mass parameters at temperatures ranging from few kelvins to room temperature and above. Further, it was demonstrated that for transparent samples, a backside cavity can be employed to enhance the optical Hall effect.Measurement of the optical Hall effect by generalized ellipsometry is an indirect technique requiring subsequent data analysis. Parameterized optical models are fitted to match experimentally measured ellipsometric data by varying physically significant parameters. Analysis of the optical response of samples, containing free charge carriers, employing optical models based on the classical Drude model, which is augmented with an external magnetic field contribution, provide access to the free charge carrier properties.The main research results of the graduate studies presented in this licentiate thesis are summarized in the five scientific papers.Paper I. Description of the custom-built terahertz frequency-domain spectroscopic ellipsometer at Linköping University. The terahertz ellipsometer capabilities are demonstrated by an accurate determination of the isotropic and anisotropic refractive indices of silicon and m-plane sapphire, respectively. Further, terahertz optical Hall effect measurements of an AlGaN/GaN high electron mobility structures were employed to extract the two-dimensional electron gas sheet density, mobility and effective mass parameters. Last, in-situ optical Hall effect measurement on epitaxial graphene in a gas cell with controllable environment, were used to study the effects of environmental doping on the mobility and carrier concentration.Paper II. Presents terahertz cavity-enhanced optical Hall measurements of the monolayer and multilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed p-type doping for monolayer graphene with a carrier density in the low 1012 cm−2 range and a carrier mobility of 1550 cm2/V·s. For the multilayer epitaxial graphene, n-type doping with a carrier density in the low 1013 cm−2 range, a mobility of 470 cm2/V·s and an effective mass of (0.14 ± 0.03) m0 were extracted. The measurements demonstrate that cavity-enhanced optical Hall effect measurements can be applied to study electronic properties of two-dimensional materials.Paper III. Terahertz cavity-enhanced optical Hall effect measurements are employed to study anisotropic transport in as-grown monolayer, quasi free-standing monolayer and quasi free-standing bilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed a strong anisotropy in the carrier mobilities of the quasi freestanding bilayer graphene. The anisotropy is demonstrated to be induced by carriers scattering at the step edges of the SiC, by showing that the mobility is higher along the step than across them. The scattering mechanism is discussed based on the results of the optical Hall effect, low-energy electron microscopy, low-energy electron diffraction and Raman measurements.Paper IV. Mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect measurements are employed to determine the electron effective mass in an In0.33Ga0.67N epitaxial layer. The data analysis reveals slightly anisotropic effective mass and carrier mobility parameters together with the optical phonon frequencies and broadenings.Paper V. Terahertz cavity-enhanced optical Hall measurements are employed to study the free charge carrier properties in a set of AlGaN/GaN high electron mobility structures with modified interfaces. The results show that the interface structure has a significant effect on the free charge carrier mobility and that the sample with a sharp interface between an AlGaN barrier and a GaN buffer layers exhibits a record mobility of 2332±73 cm2/V·s. The determined effective mass parameters showed an increase compared to the GaN value, that is attributed the the penetration of the electron wavefunction into the AlGaN barrier layer.
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| 5. |
- Bian, Qingzhen, 1988-, et al.
(författare)
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Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes
- 2020
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Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial 200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.
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| 6. |
- Du, Siying, et al.
(författare)
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Nonfullerene acceptors from thieno[3,2-b]thiophene-fused naphthalene donor core with six-member-ring connection for efficient organic solar cells
- 2021
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Ingår i: Dyes and Pigments. - : Elsevier BV. - 0143-7208 .- 1873-3743. ; 185
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Tidskriftsartikel (refereegranskat)abstract
- Comprehensive design ideas on the fused-ring donor-core in state-of-the-art acceptor-donor-acceptor (A-D-A) nonfullerene acceptors (NFAs) are still of great importance for regulating the electron push-pull effect for the sake of optimal light-harvesting, frontier molecular orbital levels, and finally their photovoltaic properties. Herein, thieno[3,2-b]thiophenes were fused in bay-area of naphthalene via six-member-ring connection, resulting in the formation of dihydropyrenobisthieno[3,2-b]thiophene based octacyclic ladder-type donor core, which was flanked by two 1,1-dicyanomethylene-3-indanone (IC) acceptor motifs with and without 5,6-diflourination, namely PTT-IC and PTT-2FIC, respectively, as novel efficient A-D-A fused-ring electron acceptors (FREAs). Compared with PTT-IC, fluorinated PTT-2FIC possesses narrower optical bandgap of 1.48 eV, better π-π stacking, and its PBDB-T:PTT-2FIC blend film exhibited better morphology, and better hole and electron mobility. As a result, nonfullerene solar cells using PBDB-T:PTT-2FIC as the active layer achieved a decent PCE of 10.40%, with an open-circuit voltage (VOC) of 0.87 V, a fill factor (FF) of 0.65, and a much higher short-circuit current (JSC) of 18.26 mA/cm2. Meanwhile, the PBDB-T:PTT-IC cells delivered a lower JSC of 12.58 mA/cm2 but a higher VOC of 0.99 V, thus resulting in a PCE of 7.39% due to its wider optical bandgap of 1.58 eV and higher LUMO energy level. These results demonstrated that NFAs based on fused-ring donor core from fusing thieno[3,2-b]thiophenes with naphthalene via six-member-ring connection are promising for organic photovoltaic applications.
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| 7. |
- Elofsson, Viktor, et al.
(författare)
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Structure formation in Ag-X (X = Au, Cu) alloys synthesized far-from-equilibrium
- 2018
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Ingår i: Journal of Applied Physics. - New York : A I P Publishing LLC. - 0021-8979 .- 1089-7550. ; 123:16
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Tidskriftsartikel (refereegranskat)abstract
- We employ sub-monolayer, pulsed Ag and Au vapor fluxes, along with deterministic growth simulations, and nanoscale probes to study structure formation in miscible Ag-Au films synthesized under far-from-equilibrium conditions. Our results show that nanoscale atomic arrangement is primarily determined by roughness build up at the film growth front, whereby larger roughness leads to increased intermixing between Ag and Au. These findings suggest a different structure formation pathway as compared to the immiscible Ag-Cu system for which the present study, in combination with previously published data, reveals that no significant roughness is developed, and the local atomic structure is predominantly determined by the tendency of Ag and Cu to phase-separate.
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| 8. |
- Eriksson, Martin
(författare)
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Photoluminescence Characteristics of III-Nitride Quantum Dots and Films
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- III-Nitride semiconductors are very promising in both electronics and optical devices. The ability of the III-Nitride semiconductors as light emitters to span the electromagnetic spectrum from deep ultraviolet light, through the entire visible region, and into the infrared part of the spectrum, is a very important feature, making this material very important in the field of light emitting devices. In fact, the blue emission from Indium Gallium Nitride (InGaN), which was awarded the 2014 Nobel Prize in Physics, is the basis of the common and important white light emitting diode (LED).Quantum dots (QDs) have properties that make them very interesting for light emitting devices for a range of different applications, such as the possibility of increasing device efficiency. The spectrally well-defined emission from QDs also allows accurate color reproduction and high-performance communication devices. The small size of QDs, combined with selective area growth allows for an improved display resolution. By control of the polarization direction of QDs, they can be used in more efficient displays as well as in traditional communication devices. The possibility of sending out entangled photon pairs is another QD property of importance for quantum key distribution used for secure communication.QDs can hold different exciton complexes, such as the neutral single exciton, consisting of one electron and one hole, and the biexciton, consisting of two excitons. The integrated PL intensity of the biexciton exhibits a quadratic dependence with respect to the excitation power, as compared to the linear power dependence of the neutral single exciton. The lifetime of the neutral exciton is 880 ps, whereas the biexciton, consisting of twice the number of charge carriers and lacks a dark state, has a considerably shorter lifetime of only 500 ps. The ratio of the lifetimes is an indication that the size of the QD is in the order of the exciton Bohr radius of the InGaN crystal making up these QDs in the InGaN QW.A large part of the studies of this thesis has been focused on InGaN QDs on top of hexagonal Gallium Nitride (GaN) pyramids, selectively grown by Metal Organic Chemical Vapor Deposition (MOCVD). On top of the GaN pyramids, an InGaN layer and a GaN capping layer were grown. From structural and optical investigations, InGaN QDs have been characterized as growing on (0001) facets on truncated GaN pyramids. These QDs exhibit both narrow photoluminescence linewidths and are linearly polarized in directions following the symmetry of the pyramids.In this work, the neutral single exciton, and the more rare negatively charged exciton, have been investigated. At low excitation power, the integrated intensity of the PL peak of the neutral exciton increases linearly with the excitation power. The negatively charged exciton, on the other hand, exhibits a quadratic power dependence, just like that of the biexciton. Upon increasing the temperature, the power dependence of the negatively charged exciton changes to linear, just like the neutral exciton. This change in power dependence is explained in terms of electrons in potential traps close to the QD escaping by thermal excitation, leading to a surplus of electrons in the vicinity of the QD. Consequently, only a single exciton needs to be created by photoexcitation in order to form a negatively charged exciton, while the extra electron is supplied to the QD by thermal excitation.Upon a close inspection of the PL of the neutral exciton, a splitting of the peak of just below 0.4 meV is revealed. There is an observed competition in the integrated intensity between these two peaks, similar to that between an exciton and a biexciton. The high energy peak of this split exciton emission is explained in terms of a remotely charged exciton. This exciton state consists of a neutral single exciton in the QD with an extra electron or hole in close vicinity of the QD, which screens the built-in field in the QD.The InGaN QDs are very small; estimated to be on the order of the exciton Bohr radius of the InGaN crystal, or even smaller. The lifetimes of the neutral exciton and the negatively charged exciton are approximately 320 ps and 130 ps, respectively. The ratio of the lifetimes supports the claim of the QD size being on the order of the exciton Bohr radius or smaller, as is further supported by power dependence results. Under the assumption of a spherical QD, theoretical calculations predict an emission energy shift of 0.7 meV, for a peak at 3.09 eV, due to the built-in field for a QD with a diameter of 1.3 nm, in agreement with the experimental observations.Studying the InGaN QD PL from neutral and charged excitons at elevated temperatures (4 K to 166 K) has revealed that the QDs are surrounded by potential fluctuations that trap charge carriers with an energy of around 20 meV, to be compared with the exciton trapping energy in the QDs of approximately 50 meV. The confinement of electrons close to the QD is predicted to be smaller than for holes, which accounts for the negative charge of the charged exciton, and for the higher probability of capturing free electrons. We have estimated the lifetimes of free electrons and holes in the GaN barrier to be 45 ps and 60 ps, in consistence with excitons forming quickly in the barrier upon photoexcitation and that free electrons and holes get trapped quickly in local potential traps close to the QDs. This analysis also indicates that there is a probability of 35 % to have an electron in the QD between the photoexcitation pulses, in agreement with a lower than quadratic power dependence of the negatively charged exciton.InN is an attractive material due to its infrared emission, for applications such as light emitters for communication purposes, but it is more difficult to grow with high quality and low doping concentration as compared to GaN. QDs with a higher In-composition or even pure InN is an interesting prospect as being a route towards increased quantum confinement and room temperature device operation. For all optical devices, p-type doping is needed. Even nominally undoped InN samples tend to be heavily n-type doped, causing problems to make pn-junctions as needed for LEDs. In our work, we present Mg-doped p-type InN films, which when further increasing the Mg-concentration revert to n-type conductivity. We have focused on the effect of the Mg-doping on the light emission properties of these films. The low Mg doped InN film is inhomogeneous and is observed to contain areas with n-type conductivity, so called n-type pockets in the otherwise p-type InN film. A higher concentration of Mg results in a higher crystalline quality and the disappearance of the n-type pockets. The high crystalline quality has enabled us to determine the binding energy of the Mg dopants to 64 meV. Upon further increase of the Mg concentration, the film reverts to ntype conductivity. The highly Mg doped sample also exhibits a red-shifted emission with features that are interpreted as originating from Zinc-Blende inclusions in the Wurtzite InN crystal, acting as quantum wells. The Mg doping is an important factor in controlling the conductivity of InN, as well as its light emission properties, and ultimately construct InN-based devices.In summary, in this thesis, both pyramidal InGaN QDs and InGaN QDs in a QW have been investigated. Novel discoveries of exciton complexes in these QD systems have been reported. Knowledge has also been gained about the challenging material InN, including a study of the effect of the Mg-doping concentration on the semiconductor crystalline quality and its light emission properties. The outcome of this thesis enriches the knowledge of the III-Nitride semiconductor community, with the long-term objective to improve the device performance of III-Nitride based light emitting devices.
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| 9. |
- Kang, Evan S. H., et al.
(författare)
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Organic Anisotropic Excitonic Optical Nanoantennas
- 2022
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Ingår i: Advanced Science. - : Wiley. - 2198-3844 .- 2198-3844. ; 9:23
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Tidskriftsartikel (refereegranskat)abstract
- Optical nanoantennas provide control of light at the nanoscale, which makes them important for diverse areas ranging from photocatalysis and flat metaoptics to sensors and biomolecular tweezing. They have traditionally been limited to metallic and dielectric nanostructures that sustain plasmonic and Mie resonances, respectively. More recently, nanostructures of organic J-aggregate excitonic materials have been proposed capable of also supporting nanooptical resonances, although their advance has been hampered from difficulty in nanostructuring. Here, the authors present the realization of organic J-aggregate excitonic nanostructures, using nanocylinder arrays as model system. Extinction spectra show that they can sustain both plasmon-like resonances and dielectric resonances, owing to the material providing negative and large positive permittivity regions at the different sides of its exciton resonance. Furthermore, it is found that the material is highly anisotropic, leading to hyperbolic and elliptic permittivity regions. Nearfield analysis using optical simulation reveals that the nanostructures therefore support hyperbolic localized surface exciton resonances and elliptic Mie resonances, neither of which has been previously demonstrated for this type of material. The anisotropic nanostructures form a new type of optical nanoantennas, which combined with the presented fabrication process opens up for applications such as fully organic excitonic metasurfaces.
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| 10. |
- Kang, Evan S. H., et al.
(författare)
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Strong Plasmon-Exciton Coupling with Directional Absorption Features in Optically Thin Hybrid Nanohole Metasurfaces
- 2018
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 5:10, s. 4046-4064
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Tidskriftsartikel (refereegranskat)abstract
- Plasmons and excitons can interact to form new hybridized light-matter states, with a multitude of potential applications including optical logic circuits and single-photon switches. Here, we report the first observation of strong coupling based on optically thin plasmonic nanohole films. The absorptive plasmon resonances of these nanohole films lead to suppressed transmission and Fano-shaped extinction peaks. We prepared silver nanohole films by colloidal lithography, which enables large-scale fabrication of nanoholes distributed in a short-range order. When coated with J-aggregate molecules, both extinction and absorption spectra show clear formation of two separated polariton resonances, with vacuum Rabi splitting on the order of 300 meV determined from anticrossing experiments. In accordance with strong coupling theory, the splitting magnitude increases linearly with the square root of molecular concentration. The extinction peak positions are blue-shifted from the absorption polariton positions, as explained by additional Fano interference between the hybridized states and the metal film. This highlights that absorption measurements are important not only to prove strong coupling but also to correctly determine hybridized polariton positions and splitting magnitudes in hybrid plasmonic nanohole systems. The polariton absorption peaks also show strong dependence on illumination direction, as found related to inherent directionality of the plasmonic nanohole metasurface and differences in light interaction with nonhybridized molecules. Importantly, optical simulations could successfully reproduce the experimental results and all coupling features. Furthermore, simulated spatial distribution of the absorption provides additional evidence of strong coupling in the hybrid nanohole system. The work paves the way toward strong coupling applications based on optically thin nanohole systems, as further promoted by the scalable fabrication.
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