| 1. |
- Aghaeipour, Mahtab, et al.
(author)
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Comparative study of absorption efficiency of inclined and vertical InP nanowires
- 2017
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In: Physics, Simulation, and Photonic Engineering of Photovoltaic Devices VI. - Bellingham, WA : SPIE - International Society for Optical Engineering. - 9781510606401 ; 10099
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Conference paper (peer-reviewed)abstract
- Geometrically designed III-V nanowire arrays are promising candidates for optoelectronics due to their possibility to excite nanophotonic resonances in absorption spectra. Strong absorption resonances can be obtained by proper tailoring of nanowire diameter, length and pitch. Such enhancement of the light absorption is, however, accompanied by undesired resonance dips at specific wavelengths. In this work, we theoretically show that tilting of the nanowires mitigates the absorption dips by exciting strong Mie resonances. In particular, we derive a theoretical optimum inclination angle of about 30 degrees at which the inclined nanowires gain 8% in absorption efficiency compared to vertically standing nanowires in a spectral region matching the intensity distribution of the sun. The enhancement is due to engineering the excited modes inside the nanowires regarding the symmetry properties of the nanowire/light system without increasing the absorbing material. We expect our results to be important for nanowire-based photovoltaic applications. © 2017 SPIE.
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| 2. |
- Aghaeipour, Mahtab, et al.
(author)
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Considering symmetry properties of inp nanowire/light incidence systems to gain broadband absorption
- 2017
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In: IEEE Photonics Journal. - Piscataway : IEEE. - 1943-0655. ; 9:3
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Journal article (peer-reviewed)abstract
- Geometrically designed III-V nanowire arrays are promising candidates for disruptive optoelectronics due to the possibility of obtaining a strongly enhanced absorption resulting from nanophotonic resonance effects. With normally incident light on such vertical nanowire arrays, the absorption spectra exhibit peaks that originate from excitation of HE1m waveguide modes in the constituent nanowires. However, the absorption spectra typically show dips between the absorption peaks. Conventionally, such weak absorption has been counteracted by either making the nanowires longer or by decreasing the pitch of the array, both alternatives effectively increasing the volume of absorbing material in the array. Here, we first study two approaches for compensating the absorption dips by exciting additional Mie resonances: 1) oblique light incidence on vertical InP nanowire arrays and 2) normal light incidence on inclined InP nanowire arrays. We then show that branched nanowires offer a novel route to achieve broadband absorption by taking advantage of simultaneous excitations of Mie resonances in the branches and guided HE1m modes in the stem. Finite element method calculations show that the absorption efficiency is enhanced from 0.72 for vertical nanowires to 0.78 for branched nanowires under normal light incidence. Our work provides new insight for the development of novel efficient photovoltaics with high efficiency and reduced active material volume.
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| 3. |
- Aghaeipour, Mahtab, et al.
(author)
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Enhanced broadband absorption in nanowire arrays with integrated Bragg reflectors
- 2018
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In: Nanophotonics. - Berlin : Walter de Gruyter GmbH. - 2192-8614. ; 7:5, s. 819-825
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Journal article (peer-reviewed)abstract
- A near-unity unselective absorption spectrum is desirable for high-performance photovoltaics. Nanowire (NW) arrays are promising candidates for efficient solar cells due to nanophotonic absorption resonances in the solar spectrum. The absorption spectra, however, display undesired dips between the resonance peaks. To achieve improved unselective broadband absorption, we propose to enclose distributed Bragg reflectors (DBRs) in the bottom and top parts of indium phosphide (InP) NWs, respectively. We theoretically show that by enclosing only two periods of In0.56Ga0.44As/InP DBRs, an unselective 78% absorption efficiency (72% for NWs without DBRs) is obtained at normal incidence in the spectral range from 300 nm to 920 nm. Under oblique light incidence, the absorption efficiency is enhanced up to about 85% at an incidence angle of 50°. By increasing the number of DBR periods from two to five, the absorption efficiency is further enhanced up to 95% at normal incidence. In this work, we calculated optical spectra for InP NWs, but the results are expected to be valid for other direct band gap III-V semiconductor materials. We believe that our proposed idea of integrating DBRs in NWs offers great potential for high-performance photovoltaic applications.
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| 4. |
- Aghaeipour, Mahtab, et al.
(author)
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Enhanced optical absorption in nanowires over a desire range of wavelengths
- 2017
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In: MOC2017 : technical digest of the Twenty-Second Microoptics Conference. - : IEEE. - 9784863486096 - 9781509049240 ; , s. 360-361
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Conference paper (peer-reviewed)abstract
- Engineering optical absorption in nanowires, over a desire range of wavelengths is of importance to design high-performance nanowire-based photovoltaics. To this end, we integrate the nanowires with distributed Bragg reflectors to enhance absorption spectra of the nanowires and relate the consequent enhancement to increasing the optical path lengths of the modes. © 2017 The Japan Society of Applied Physics.
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| 5. |
- Aghaeipour, Mahtab
(author)
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Tailoring the Optical Response of III-V Nanowire Arrays
- 2017
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Doctoral thesis (other academic/artistic)abstract
- Semiconductor nanowires show a great deal of promise for applications in a wide range of important fields, including photovoltaics, biomedicine, and information technology. Developing these exciting applications is strongly dependent on understanding the fundamental properties of nanowires, such as their optical resonances and absorption spectra. In this thesis we explore optical absorption spectra of arrays of vertical III-V nanowires with a special emphasis on structures optimized to enhance absorption in the solar spectrum. First, we analyze experimentally determined absorption spectra of both indium phosphide (InP) and gallium phosphide (GaP) nanowire arrays. The study provides an intuitive understanding of how the observed absorption resonances in the nanowires may be tuned as a function of their geometrical parameters and crystal structure. As a consequence, the spectral position of absorption resonances can be precisely controlled through the nanowire diameter. However, the results highlight how the blue-shift in the optical absorption resonances as the diameter of the nanowires decreases comes to a halt at low diameters. The stop point is related to the behavior of the refractive indices of the nanowires. The wavelength of the stop is different for nanowire polytypes of similar dimensions due to differences in their refractive indices. We then present a theoretical argument that it is important to consider symmetry properties when tailoring the optical modes excited in the nanowires for enhanced absorption. We show that absorption spectra may be enhanced compared to vertical nanowires at normal incidence by tilting the nanowires with normal incidence light, or by using off-normal incidence with vertical nanowires. This is because additional optical modes inside the nanowires are excited when the symmetry is broken. Looking forward to omnidirectional applications, we consider branched nanowires as a way to enhance the absorption spectra at normal incidence by taking advantage of simultaneous excitation of the spectrally different optical modes in the branches and the stems. Third, we describe in theoretical terms how integrating distributed Bragg reflectors (DBRs) with the nanowires can improve absorption spectra compared to conventional nanowires. DBRs provide a way to employ light trapping mechanisms which increases the optical path length of the excited modes and thereby improves the absorption of the excited modes. At normal incidence, DBR-nanowires improve the absorption efficiency to 78%, compared to 72% for conventional nanowires. We show that the efficiency is increased to 85% for an off-normal incident angle of 50˚. Overall, our results show that studies of optical resonances in nanowires that take the light-matter interaction into account provide opportunities to develop novel optical and optoelectronic functionalities in nanoscience and nanotechnology.
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| 6. |
- Bordag, Michael, et al.
(author)
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Shear stress measurements on InAs nanowires by AFM manipulation
- 2007
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In: Small. - Weinheim : Wiley-VCH Verlagsgesellschaft. - 1613-6810 .- 1613-6829. ; 3:8, s. 1398-1401
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Journal article (peer-reviewed)abstract
- On an upward curve? The curvature of an elastically deformed nanowire pinned to a flat surface contains information about the maximum static friction force, and hence the shear stress, between the nanowire and the surface. Here, InAs nanowires are bent in a controlled manner using the tip of an atomic force microscope (see image). The shear stress can be obtained from a simple analysis according to the standard theory of elasticity.
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| 7. |
- Borschel, Christian, et al.
(author)
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A New Route toward Semiconductor Nanospintronics : Highly Mn-Doped GaAs Nanowires Realized by Ion-Implantation under Dynamic Annealing Conditions
- 2011
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In: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:9, s. 3935-3940
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Journal article (peer-reviewed)abstract
- We report on highly Mn-doped GaAs nanowires (NWs) of high crystalline quality fabricated by ion beam implantation, a technique that allows doping concentrations beyond the equilibrium solubility limit. We studied two approaches for the preparation of Mn-doped GaAs NWs: First, ion implantation at room temperature with subsequent annealing resulted in polycrystalline NWs and phase segregation of MnAs and GaAs. The second approach was ion implantation at elevated temperatures. In this case, the single-crystallinity of the GaAs NWs was maintained, and crystalline, highly Mn-doped GaAs NWs were obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV). Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our findings suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic material system.
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| 8. |
- Chalangar, Ebrahim, 1984-, et al.
(author)
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Influence of morphology on electrical and optical properties of graphene/Al-doped ZnO-nanorod composites
- 2018
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In: Nanotechnology. - Bristol : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 29:41
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Journal article (peer-reviewed)abstract
- The development of future 3D-printed electronics relies on the access to highly conductive inexpensive materials that are printable at low temperatures (<100 C). The implementation of available materials for these applications are, however, still limited by issues related to cost and printing quality. Here, we report on the simple hydrothermal growth of novel nanocomposites that are well suited for conductive printing applications. The nanocomposites comprise highly Al-doped ZnO nanorods grown on graphene nanoplatelets (GNPs). The ZnO nanorods play the two major roles of (i) preventing GNPs from agglomerating and (ii) promoting electrical conduction paths between the graphene platelets. The effect of two different ZnO-nanorod morphologies with varying Al-doping concentration on the nanocomposite conductivity and the graphenedispersity are investigated. Time-dependent absorption, photoluminescence and photoconductivity measurements show that growth in high pH solutions promotes a better graphene dispersity, higher doping levels and enhanced bonding between the graphene and the ZnO nanorods. Growth in low pH solutions yields samples characterized by a higher conductivity and a reduced number of surface defects. These samples also exhibit a large persistent photoconductivity attributed to an effective charge separation and transfer from the nanorods to the graphene platelets. Our findings can be used to tailor the conductivity of novel printable composites, or for fabrication of large volumes of inexpensive porous conjugated graphene-semiconductor composites. © 2018 IOP Publishing Ltd.
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| 9. |
- Chalangar, Ebrahim, PhD student, 1984-, et al.
(author)
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Nanopatterned rGO/ZnO : Al seed layer for vertical growth of single ZnO nanorods
- 2023
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In: Nanotechnology. - Bristol : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 34:25, s. 1-7
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Journal article (peer-reviewed)abstract
- In this work, we demonstrate a novel low-cost template-assisted route to synthesize vertical ZnO nanorod arrays on Si (100). The nanorods were grown on a patterned double seed layer comprised of reduced graphene oxide (rGO) and Al-doped ZnO nanoparticles. The seed layer was fabricated by spray-coating the substrate with graphene and then dip-coating it into a Al-doped ZnO sol-gel solution. The growth template was fabricated from a double-layer resist, spin-coated on top of the rGO/ZnO:Al seed layer, and patterned by colloidal lithography. The results show a successful chemical bath deposition of vertically aligned ZnO nanorods with controllable diameter and density in the nanoholes in the patterned resist mask. Our novel method can presumably be used to fabricate electronic devices on virtually any smooth substrate with a thermal budget of 1 min at 300 °C with the seed layer acting as a conductive strain-relieving back contact. The top contact can simply be made by depositing a suitable transparent conductive oxide or metal, depending on the specific application. © 2023 The Author(s). Published by IOP Publishing Ltd.
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| 10. |
- Chalangar, Ebrahim, PhD student, 1984-, et al.
(author)
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Synthesis of Vertically Aligned ZnO Nanorods Using Sol-gel Seeding and Colloidal Lithography Patterning
- 2021
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In: Nanoscale Research Letters. - Heidelberg : Springer Science and Business Media LLC. - 1931-7573 .- 1556-276X. ; 16:1
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Journal article (peer-reviewed)abstract
- Different ZnO nanostructures can be grown using low-cost chemical bath deposition. Although this technique is cost-efficient and flexible, the final structures are usually randomly oriented and hardly controllable in terms of homogeneity and surface density. In this work, we use colloidal lithography to pattern (100) silicon substrates to fully control the nanorods' morphology and density. Moreover, a sol-gel prepared ZnO seed layer was employed to compensate for the lattice mismatch between the silicon substrate and ZnO nanorods. The results show a successful growth of vertically aligned ZnO nanorods with controllable diameter and density in the designated openings in the patterned resist mask deposited on the seed layer. Our method can be used to fabricate optimized devices where vertically ordered ZnO nanorods of high crystalline quality are crucial for the device performance.
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