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  • 1.
    Dev, Apurba
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Dev Choudhury, Bikash
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Abedin, Ahmad
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Fabrication of Periodic Nanostructure Assemblies by Interfacial Energy Driven Colloidal Lithography2014In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 24, no 29, p. 4577-4583Article in journal (Refereed)
    Abstract [en]

    A novel interfacial energy driven colloidal lithography technique to fabricate periodic patterns from solution-phase is presented and the feasibility and versatility of the technique is demonstrated by fabricating periodically arranged ZnO nanowire ensembles on Si substrates. The pattern fabrication method exploits different interfaces formed by sol-gel derived ZnO seed solution on a hydrophobic Si surface covered by a monolayer of colloidal silica spheres. While the hydrophobic Si surface prevents wetting by the seed solution, the wedge shaped regions surrounding the contact point between the colloidal particles and the Si substrate trap the solution due to interfacial forces. This technique allows fabrication of uniform 2D micropatterns of ZnO seed particles on the Si substrate. A hydrothermal technique is then used to grow well-defined periodic assemblies of ZnO nanowires. Tunability is demonstrated in the dimensions of the patterns by using silica spheres with different diameters. The experimental data show that the periodic ZnO nanowire assembly suppresses the total reflectivity of bare Si by more than a factor of 2 in the wavelength range 400-1300 nm. Finite-difference time-domain simulations of the wavelength-dependent reflectivity show good qualitative agreement with the experiments. The demonstrated method is also applicable for other materials synthesized by solution chemistry.

  • 2.
    Dev Choudhury, Bikash
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Deterministic Silicon Pillar Assemblies and their Photonic Applications2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    It is of paramount importance to our society that the environment, life style, science and amusement flourish together in a balanced way. Some trends in this direction are the increased utilization of renewable energy, like solar photovoltaics; better health care products, for example advanced biosensors; high definition TV or high resolution cameras; and novel scientific tools for better understanding of scientific observations. Advancement of micro and nanotechnologies has directly and positively impacted our stance in these application domains; one example is that of vertical periodic or aperiodic nano or micro pillar assemblies which have attracted significant research and industrial interest in recent years. In particular, Si pillars are very attractive due to the versatility of silicon. There are many potential applications of Si nanopillar/nanowire assemblies ranging from light emission, solar cells, antireflection, sensing and nonlinear optical effects. Compared to bulk, Si pillars or their assemblies have several unique properties, such as high surface to volume ratios, light localization, efficient light guiding, better light absorption, selective band of light propagation etc.

         The focus of the thesis is on the fabrication of Si pillar assemblies and hierarchical ZnO nanowires on Si micro structures in top-down and bottom-up approaches and their optical properties and different applications. Here, we have investigated periodic and aperiodic Si nano and micro structure assemblies and their properties, such as light propagation, localization, and selective guiding and light-matter interaction. These properties are exploited in a few important optoelectronic/photonic applications, such as optical biosensors, broad-band anti-reflection, radial-junction solar cells, second harmonic generation and color filters.  

          We achieved a low average reflectivity of ~ 2.5 % with the periodic Si micropyramid-ZnO NWs hierarchical arrays. Tenfold enhancement in Raman intensity is also observed in these structures compared to planar Si. These Si microstructure-ZnO NW hierarchical structures can enhance the performance and versatility of photovoltaic devices and optical sensors. A convenient top-down fabrication of radial junction nanopillar solar cell using spin-on doping and rapid thermal annealing process is presented. Broad band suppressed reflection, on average 5%, in 300- 850 nm wavelength range and an un-optimized cell efficiency of 6.2 % are achieved. Our method can lead to a simple and low cost process for high efficiency radial junction nanopillar solar cell fabrication.     

          Silicon dioxide (SiO2) coated silicon nanopillar (NP) arrays are demonstrated for surface sensitive optical biosensing. Bovine serum albumin (BSA)/anti-BSA model system is used for biosensing trials by photo-spectrometry in reflection mode. Best sensitivity in terms of limit of detection of 5.2 ng/ml is determined for our nanopillar biosensor. These results are promising for surface sensitive biosensors and the technology allows integration in the CMOS platform.  

          Si pillar arrays used for surface second harmonic generation (SHG) experiments are shown to have a strong dependence of the SHG intensity on the pillar geometry. The surface SHG can be suitable for nonlinear silicon photonics, surface/interface studies and optical sensing.  

          Aperiodic Si nanopillar assemblies in PDMS matrix are demonstrated for efficient color filtering in transmission mode. These assemblies are designed using the ‘‘molecular dynamics-collision between hard sphere’’ algorithm. The designed structure is modeled in a 3D finite difference time domain (FDTD) simulation tool for optimization of color filtering properties. Transverse localization effect of light in our nanopillar color filter structures is investigated theoretically and the results are very promising to achieve image sensors with high pixel densities (~1 µm) and low crosstalk. The developed color filter is applicable as a stand-alone filter for visible color in its present form and can be adapted for displays, imaging, smart windows and aesthetic applications.

  • 3.
    Dev Choudhury, Bikash
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Abedin, Ahmad
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dev, Apurba
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Sanatinia, Reza
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Silicon micro-structure and ZnO nanowire hierarchical assortments for light management2013In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 3, no 8, p. 1039-1048Article in journal (Refereed)
    Abstract [en]

    We present fabrication and optical characterization of Si microstructure-ZnO nanowire (NWs) hierarchical structures for light management. Random and periodic hierarchical structures constituting Si micro pillar or micro pyramid arrays with overgrown ZnO NWs have been fabricated. Inexpensive colloidal lithography in combination with dry and wet chemical etching is used to fabricate Si microstructures, and ZnO NWs are grown by hydrothermal synthesis. The periodic Si micro pyramid-ZnO NWs hierarchical structure shows broadband antireflection with average reflectance as low as 2.5% in the 300-1000 nm wavelength range. A tenfold enhancement in Raman intensity is observed in this structure compared to planar Si sample. These hierarchical structures with enriched optical properties and high surface to volume ratio are promising for photovoltaic (PV) and sensor applications.

  • 4.
    Dev Choudhury, Bikash
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Anand, Srinivasan
    Rapid thermal annealing treated spin-on doped antireflective radial junction Si nanopillar solar cell2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 8, p. A200-A207Article in journal (Refereed)
    Abstract [en]

    Radial junction nanopillar Si solar cells are interesting for cost effective efficiency improvement. Here, we address a convenient top-down fabrication of Si nanopillar solar cells using spin-on doping and rapid thermal annealing (RTA) for conformal PN junction formation. Broadband suppressed reflection as low as an average of 5% in the 300-1100 nm wavelength range and un-optimized cell efficiency of 7.3% are achieved. The solar cell performance can be improved by optimization of spin-on-doping and suitable surface passivation. Overall, the all RTA processed, spin-on doped nanopillar radial junction solar cell shows a very promising route for low cost and high efficiency thin film solar cell perspectives.

  • 5.
    Dev Choudhury, Bikash
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    RTA treated spin–on doped antireflective radial junction Si nanopillar solar cellManuscript (preprint) (Other academic)
  • 6.
    Dev Choudhury, Bikash
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Casquel, R.
    Banuls, M. J.
    Sanza, F. J.
    Laguna, M. F.
    Holgado, M.
    Puchades, R.
    Maquieira, A.
    Barrios, C. A.
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Silicon nanopillar arrays with SiO2 overlayer for biosensing application2014In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 4, no 7, p. 1345-1354Article in journal (Refereed)
    Abstract [en]

    We present the fabrication of silicon dioxide (SiO2) coated silicon nanopillar array structures and demonstrate their application as sensitive optical biosensors. Colloidal lithography, plasma dry etching and deposition processes are used to fabricate SiO2 coated Si nanopillar arrays with two different diameters and periods. Proof of concept bio recognition experiments are carried out with the bovine serum albumin (BSA)/antiBSA model system using Fourier transform visible and IR spectrometry (FT-VIS-IR) in reflection mode. A limit of detection (LoD) value of 5.2 ng/ml is estimated taking in to account the wavenumber uncertainty in the measurements.

  • 7.
    Dev Choudhury, Bikash
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Sahoo, Pankaj K.
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Nanopillar Assemblies with Deterministic Correlated Disorder for Color FilteringManuscript (preprint) (Other academic)
  • 8.
    Dev Choudhury, Bikash
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Sahoo, Pankaj Kumar
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA. Indian Institute of Technology, India.
    Sanatinia, Reza
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Andler, G.
    Stockholm University.
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Swillo, Marcin
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Electronics and Quantum Optics, QEO.
    Surface second harmonic generation from silicon pillar arrays with strong geometrical dependence2015In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 9, p. 2072-2075Article in journal (Refereed)
    Abstract [en]

    We present experimental demonstration and analysis of enhanced surface second harmonic generation (SHG) from hexagonal arrays of silicon pillars. Three sets of Si pillar samples with truncated cone-shaped pillar arrays having periods of 500, 1000, and 2000 nm, and corresponding average diameters of 200, 585 and 1550 nm, respectively, are fabricated by colloidal lithography and plasma dry etching. We have observed strong dependence of SHG intensity on the pillar geometry. Pillar arrays with a 1000 nm period and a 585 nm average diameter give more than a one order of magnitude higher SHG signal compared to the other two samples. We theoretically verified the dependence of SHG intensity on pillar geometry by finite difference time domain simulations in terms of the surface normal E-field component. The enhanced surface SHG light can be useful for nonlinear silicon photonics, surface/interface characterization, and optical biosensing.

  • 9. Sahoo, Pankaj K.
    et al.
    Choudhury, Bikash Dev
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Joseph, Joby
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    ZnO nanowire-enabled light funneling effect for antireflection and light convergence applications2017In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 42, no 1, p. 45-48Article in journal (Refereed)
    Abstract [en]

    We present a new light trapping technique to reduce reflection loss, as well as for light, focusing at submicron scales for solar cell and image sensing applications. We have fabricated hexagonal arrays of ZnO funnel-like structures on Si substrate by the patterned growth of ZnO nanowires in a hydrothermal growth process. The funnels are optimized so that the effective refractive index along the vertical direction decreases gradually from the Si surface to the top of funnel to reduce Fresnel reflection at a device-air interface. Finite difference time domain simulation is used for optimization of the minimum reflectivity and to analyze optical properties such as angle dependency, polarization dependency, and funneling effect. The structures function similar to a GRIN lens in light trapping and convergence. An optimized structure reduces the average reflectivity close to 3% in the wavelength range of 300-1200 nm with the possibility of confining incident light to a few hundreds of nano-meters.

  • 10.
    Visser, Dennis
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Dev Choudhury, Bikash
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Semiconductor Materials, HMA.
    Krasovska, Inese
    Anand, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Optical RI Sensing in the Visible/NIR Spectrum by Silicon Nanopillar ArraysManuscript (preprint) (Other academic)
  • 11.
    Visser, Dennis
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Dev Choudhury, Bikash
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Krasovska, Inese
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Anand, Srinivasan
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Refractive index sensing in the visible/NIR spectrum using silicon nanopillar arrays2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 11, p. 12171-12181Article in journal (Refereed)
    Abstract [en]

    Si nanopillar (NP) arrays are investigated as refractive index sensors in the visible/NIR wavelength range, suitable for Si photodetector responsivity. The NP arrays are fabricated by nanoimprint lithography and dry etching, and coated with thin dielectric layers. The reflectivity peaks obtained by finite-difference time-domain (FDTD) simulations show a linear shift with coating layer thickness. At 730 nm wavelength, sensitivities of ∼0.3 and ∼0.9 nm/nm of SiO2 and Si3N4, respectively, are obtained; and the optical thicknesses of the deposited surface coatings are determined by comparing the experimental and simulated data. The results show that NP arrays can be used for sensing surface bio-layers. The proposed method could be useful to determine the optical thickness of surface coatings, conformal and non-conformal, in NP-based optical devices.

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