| 1. |
- Alvi, Sajid, et al.
(författare)
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Enhanced mechanical, thermal and electrical properties of high‐entropy HfMoNbTaTiVWZr thin film metallic glass and its nitrides
- 2022
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Ingår i: Advanced Engineering Materials. - : John Wiley & Sons. - 1438-1656 .- 1527-2648. ; 24:9
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Tidskriftsartikel (refereegranskat)abstract
- The inception of high-entropy alloy promises to push the boundaries for new alloy design with unprecedented properties. This work reports entropy stabilisation of an octonary refractory, HfMoNbTaTiVWZr, high-entropy thin film metallic glass, and derived nitride films. The thin film metallic glass exhibited exceptional ductility of ≈60% strain without fracture and compression strength of 3 GPa in micro-compression, due to the presence of high density and strength of bonds. The thin film metallic glass shows thermal stability up to 750 °C and resistance to Ar-ion irradiation. Nitriding during film deposition of HfMoNbTaTiVWZr thin film of strong nitride forming refractory elements results in deposition of nanocrystalline nitride films with compressive strength, hardness, and thermal stability of up to 10 GPa, 18.7 GPa, and 950 °C, respectively. The high amount of lattice distortion in the nitride films leads to its insulating behaviour with electrical conductivity as low as 200 S cm−1 in the as-deposited film. The design and exceptional properties of the thin film metallic glass and derived nitride films may open up new avenues of development of bulk metallic glasses and the application of refractory-based high entropy thin films in structural and functional applications.
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| 2. |
- Alvi, Sajid, et al.
(författare)
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Synthesis and Mechanical Characterization of a CuMoTaWV High-Entropy Film by Magnetron Sputtering
- 2020
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:18, s. 21070-21079
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Tidskriftsartikel (refereegranskat)abstract
- Development of high-entropy alloy (HEA) films is a promising and cost-effective way to incorporate these materials of superior properties in harsh environments. In this work, a refractory high-entropy alloy (RHEA) film of equimolar CuMoTaWV was deposited on silicon and 304 stainless-steel substrates using DC-magnetron sputtering. A sputtering target was developed by partial sintering of an equimolar powder mixture of Cu, Mo, Ta, W, and V using spark plasma sintering. The target was used to sputter a nanocrystalline RHEA film with a thickness of ∼900 nm and an average grain size of 18 nm. X-ray diffraction of the film revealed a body-centered cubic solid solution with preferred orientation in the (110) directional plane. The nanocrystalline nature of the RHEA film resulted in a hardness of 19 ± 2.3 GPa and an elastic modulus of 259 ± 19.2 GPa. A high compressive strength of 10 ± 0.8 GPa was obtained in nanopillar compression due to solid solution hardening and grain boundary strengthening. The adhesion between the RHEA film and 304 stainless-steel substrates was increased on annealing. For the wear test against the E52100 alloy steel (Grade 25, 700–880 HV) at 1 N load, the RHEA film showed an average coefficient of friction (COF) and wear rate of 0.25 (RT) and 1.5 (300 °C), and 6.4 × 10–6 mm3/N m (RT) and 2.5 × 10–5 mm3/N m (300 °C), respectively. The COF was found to be 2 times lower at RT and wear rate 102 times lower at RT and 300 °C than those of 304 stainless steel. This study may lead to the processing of high-entropy alloy films for large-scale industrial applications.
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| 3. |
- Borgani, Riccardo, 1989-, et al.
(författare)
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Fast multifrequency measurement of nonlinear conductance
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Low noise measurement of small currents in nanometer-scale junctions is of central importance to the characterization of novel high-performance devices and materials for applications ranging from energy harvesting and energy conversion to topological materials for quantum computers. The high resistance of these junctions and the stray capacitance of their measurement leads impose speed limitations (tens of seconds) on the traditional methods of measuring their nonlinear conductance, making detailed investigations of change with external fields or maps of variation over a surface impractical, if not impossible. Here we demonstrate fast (milliseconds) reconstruction of nonlinear current-voltage characteristics from phase-coherent multifrequency lock-in data using the inverse Fourier transform. The measurement technique allows for separation of the galvanic and displacement currents in the junction and easy cancellation of parasitic displacement current due to the measurement leads. We use the method to reveal nanometer-scale variations in the electrical transport properties of organic photovoltaic and semiconducting thin films. The method has broad applicability and its wide-spread implementation promises advancement in high-speed and high-resolution characterization for nanotechnology.
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| 4. |
- Borgani, Riccardo, et al.
(författare)
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Fast Multifrequency Measurement of Nonlinear Conductance
- 2019
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Ingår i: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- We describe a phase-coherent multifrequency lock-in measurement technique that uses the inverse Fourier transform to reconstruct the nonlinear current-voltage characteristic of a nanoscale junction. The method provides separation of the galvanic and displacement currents in the junction and easy cancellation of the parasitic displacement current from the measurement leads. These two features allow us to overcome traditional limitations imposed by the low conductance of the junction and the high capacitance of the leads, thus providing an increase in measurement speed of several orders of magnitude. We demonstrate the method in the context of conductive atomic force microscopy, acquiring current-voltage characteristics at every pixel while scanning at standard imaging speed.
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| 5. |
- Ghamgosar, Pedram, 1979-, et al.
(författare)
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Self-Powered Photodetectors Based on Core-Shell ZnO-Co3O4 Nanowire Heterojunctions
- 2019
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 11:26, s. 23454-23462
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Tidskriftsartikel (refereegranskat)abstract
- Self-powered photodetectors operating in the UV–visible–NIR window made of environmentally friendly, earth abundant, and cheap materials are appealing systems to exploit natural solar radiation without external power sources. In this study, we propose a new p–n junction nanostructure, based on a ZnO–Co3O4 core–shell nanowire (NW) system, with a suitable electronic band structure and improved light absorption, charge transport, and charge collection, to build an efficient UV–visible–NIR p–n heterojunction photodetector. Ultrathin Co3O4 films (in the range 1–15 nm) were sputter-deposited on hydrothermally grown ZnO NW arrays. The effect of a thin layer of the Al2O3 buffer layer between ZnO and Co3O4 was investigated, which may inhibit charge recombination, boosting device performance. The photoresponse of the ZnO–Al2O3–Co3O4 system at zero bias is 6 times higher compared to that of ZnO–Co3O4. The responsivity (R) and specific detectivity (D*) of the best device were 21.80 mA W–1and 4.12 × 1012 Jones, respectively. These results suggest a novel p–n junction structure to develop all-oxide UV–vis photodetectors based on stable, nontoxic, low-cost materials.
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| 6. |
- Ghamgosar, Pedram, 1979-, et al.
(författare)
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ZnO-Cu2O core-shell nanowires as stable and fast response photodetectors
- 2018
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Ingår i: Nano Energy. - : Elsevier. - 2211-2855 .- 2211-3282. ; 51, s. 308-316
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present all-oxide p-n junction core-shell nanowires (NWs) as fast and stable self-powered photodetectors. Hydrothermally grown n-type ZnO NWs were conformal covered by different thicknesses (up to 420 nm) of p-type copper oxide layers through metalorganic chemical vapor deposition (MOCVD). The ZnO NWs exhibit a single crystalline Wurtzite structure, preferentially grown along the [002] direction, and energy gap Eg=3.24 eV. Depending on the deposition temperature, the copper oxide shell exhibits either a crystalline cubic structure of pure Cu2O phase (MOCVD at 250 °C) or a cubic structure of Cu2O with the presence of CuO phase impurities (MOCVD at 300 °C), with energy gap of 2.48 eV. The electrical measurements indicate the formation of a p-n junction after the deposition of the copper oxide layer. The core-shell photodetectors present a photoresponsivity at 0 V bias voltage up to 7.7 µA/W and time response ≤0.09 s, the fastest ever reported for oxide photodetectors in the visible range, and among the fastest including photodetectors with response limited to the UV region. The bare ZnO NWs have slow photoresponsivity, without recovery after the end of photo-stimulation. The fast time response for the core-shell structures is due to the presence of the p-n junctions, which enables fast exciton separation and charge extraction. Additionally, the suitable electronic structure of the ZnO-Cu2O heterojunction enables self-powering of the device at 0 V bias voltage. These results represent a significant advancement in the development of low-cost, high efficiency and self-powered photodetectors, highlighting the need of fine tuning the morphology, composition and electronic properties of p-n junctions to maximize device performances.
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| 7. |
- Gilzad Kohan, Mojtaba, et al.
(författare)
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All-oxide solar cells
- 2020
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Ingår i: Solar Cells and Light Management. - : Elsevier. ; , s. 229-246
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- One of the most intensively investigated directions in the field of photovoltaics is the development of technologies able to provide vacuum-free and low-cost solar cells with decent efficiency, based on earth-abundant and environmentally friendly materials. Solar cells based on oxide materials are a promising candidate for the purpose, being most of the investigated oxides comparatively more stable than most of solar cell technologies alternative to silicon, and composed of harmless materials. While oxides can exhibit high extinction coefficient in the visible and near-infrared spectral region, guaranteeing full absorption of sunlight, the main factor limiting efficiency in such kind of p–n junction devices is the low hole mobility in the p-type oxide, which represents the main challenge to be overcome to make this technology competitive. This chapter illustrates the latest results in the field, including integration of nanowire geometries as viable solution toward fast charge transport and collection.
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| 8. |
- Gilzad Kohan, Mojtaba, 1991-
(författare)
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Alternative Energy Harvesting and Conversion Systems Based on Nanostructured Heterostructures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Conversion and storage of the solar radiation into applicable forms of energy, using ubiquitous materials is of central importance that quests several disciplinary fields in both applied technology and fundamental science. Harnessing the solar energy received by the earth has the potential to replace the current sources of energy and it is imperative for sustainable development. Since the early development of modern photovoltaics (PVs), based on silicon wafers, a rational step was the substantial development of the new generation PV technologies that can provide lower-cost and higher efficiency than their predecessors. Deliberate solutions involved employing different semiconducting materials that are indispensable, non-toxic and compatible with large-scale fabricating technologies. Exploiting metal oxide (MOx) semiconductors, a broad class of non-toxic, cheap and abundant materials, is already promoted as a key component for high-performance optoelectronic devices and can be an ideal solution for inexpensive harnessing of sustainable energy resources like Sun light. The favorable band gap and high absorption cross-section of some MOx semiconductors permit utilizing different spectral region of the solar spectrum. However, at this present, the implication of MOx in high-throughput optoelectronic devices remained on the low side. Some of the main drawbacks that attain to poor performance of the MOx are associated with their poor intrinsic carrier mobility especially in p-type light absorbers and insufficient visible light absorption notably in n-type semiconductors. The main aim of this thesis is to further contribute to the development and exploitation of this class of materials with the main focus on their role in optoelectronic devices and energy storage systems. The content of this thesis considers two main aspect of the research. Substantially, this work analyses the vital role of the interface engineering using nanostructured MOx, where we exploit unique phenomena such as intense electric field confinement in 1dimensional (1D) structures resulting in ample light trapping in the fabricated heterojunctions. Unfortunately, this fact comes at the cost of introducing space charge region (SCR) limits in the fabricated devices attaining for poor derived currents. Here I would probably spend couple of words for introducing the Co3O4 NR as the basis for p-n inverted nanorod junction…Plasmonic metal nanoparticles (NPs) were conventionally used to extend the spectral response of the wide-bandgap semiconductors. Within the scheme of this thesis, we employ the silver plasmonic NPs in a 1D light harvesting structure of zinc oxide (ZnO), where we mediate hot-carrier collection of the charges via controlled illuminations. Even further, we provide a comprehensive analysis on the hot-carrier redistribution mechanisms of the plasmonic NPs to semiconductor, providing direct experimental proof using transient pump-probe spectroscopy and time-resolved photoluminescence analysis. Our work resulted in a distinct understanding of the radiative and non-radiative carrier transfer between the active constituents of the system, which have not been corroborated previously.In a parallel approach, the research activities in this work, take a few steps ahead and investigates the issues related to the disparities in the PV plants. A common prerequisite after conversion of the solar light using PV devices is the electrochemical storage of the energy where it can answer the needs for far-reaching energy requirements. Fostered by the intrinsic capacitance characteristic of the MOx, we interplay the role of the interfacial engineering in Co3O4 porous films and investigate the effect of their lateral architecture on Li+ ion adsorption and desorption properties.Finally, our findings resulted in the fabrication of a hybrid device with dual functionality as an all-oxide PV system that can directly store the converted Sunlight as in a supercapacitor device. The prospect of this device can provide the over-potential required for direct storage of the converted solar energy into larger high storage systems.In summary, the results presented in this thesis highlights the potential of the MOx semiconductors for photovoltaic and storage applications. We identify the various step-forward routes, which can provide the possibility of large-scale deployment of this novel class of materials.
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| 9. |
- Gilzad Kohan, Mojtaba, et al.
(författare)
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In-depth Carrier Transport in a Barrier Variable Iron-oxide and Vertically Aligned Reduced-Graphene Oxide Composite.
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A key requirement for semiconductors operating in light harvesting devices, is to efficiently convert the absorbed photons to electronic excitations while accommodating low loss pathways for the photogenerated carrier’s transport. The quality of this process corresponds to different relaxation phenomena, yet primarily it corresponds to minimized thermalization of photoexcited carriers and maximum transfer of electron-hole pairs in the bulk of semiconductor through carrier-carrier scattering process. However, several semiconductors, while providing a suitable platform for light harvesting applications, pose intrinsic low carrier diffusion length of photoexcited carriers. Here we report a system based on a vertical network of reduced graphene oxide (rGO) embedded in a thin-film structure of iron oxide semiconductor, intended to employ carrier-carrier scattering properties of rGO to increase the photoexcited carrier transfer in the bulk of the semiconductor. Using intermodulation conductive force microscopy, we locally monitored the fluctuation of current output, which is the prime indication of the prevailing carrier-carrier scattering mechanism in the system. We reveal the fundamental properties of vertical rGO and semiconductor junction in light harvesting systems that enable the design of new promising materials with broad-band optical applications.
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| 10. |
- Gilzad Kohan, Mojtaba, et al.
(författare)
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In-depth photocarrier dynamics in a barrier variable iron-oxide and vertically aligned reduced-graphene oxide composite
- 2022
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Ingår i: NPJ 2D MATERIALS AND APPLICATIONS. - : Springer Nature. - 2397-7132. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- A key requirement for semiconductors operating in light-harvesting devices, is to efficiently convert the absorbed photons to electronic excitations while accommodating low loss pathways for the photogenerated carrier’s transport. The quality of this process corresponds to different relaxation phenomena, yet primarily it corresponds to minimized thermalization of photoexcited carriers and maximum transfer of electron-hole pairs in the bulk of semiconductor. However, several semiconductors, while providing a suitable platform for light-harvesting applications, pose intrinsic low carrier diffusion length of photoexcited carriers. Here we report a system based on a vertical network of reduced graphene oxide (rGO) embedded in a thin-film structure of iron oxide semiconductor, intended to exploit fast electron transport in rGO to increase the photoexcited carrier transfer from the bulk of the semiconductor to rGO and then to the external circuit. Using intermodulation conductive force microscopy, we locally monitored the fluctuation of current output, which is the prime indication of successful charge transfer from photoexcited semiconductor to rGO and efficient charge collection from the bulk of the semiconductor. We reveal the fundamental properties of vertical rGO and semiconductor junction in light-harvesting systems that enable the design of new promising materials for broad-band optical applications.
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