Integrated colloidal quantum dot photodetectors with color-tunable plasmonic nanofocusing lenses

High-sensitivity photodetection is at the heart of many optoelectronic applications, including spectroscopy, imaging, surveillance, remote sensing and medical diagnostics. Achieving the highest possible sensitivity for a given photodetector technology requires the development of ultra-small-footprint detectors, as the noise sources scale with the area of the detector. This must be accomplished while sacrificing neither the optically active area of the detector nor its responsivity. Currently, such designs are based on diffraction-limited approaches using optical lenses. Here, we employ a plasmonic flat-lens bull’s eye structure (BES) to concentrate and focus light into a nanoscale colloidal quantum dot (CQD) photodetector. The plasmonic lenses function as nanofocusing resonant structures that simultaneously offer color selectivity and enhanced sensitivity. Herein, we demonstrate the first CQD photodetector with a nanoscale footprint, the optically active area of which is determined by the BES; this detector represents an exciting opportunity for high-sensitivity sensing.Peer ReviewedPostprint (published version

Determination of carrier lifetime and mobility in colloidal quantum dot films via impedance spectroscopy

Impedance Spectroscopy (IS) proves to be a powerful tool for the determination of carrier lifetime and majority carrier mobility in colloidal quantum dot films. We employ IS to determine the carrier lifetime in PbS quantum dot Schottky solar cells with Al and we verify the validity of the technique via transient photovoltage. We also present a simple approach based on an RC model that allows the determination of carrier mobility in PbS quantum dot films and we corroborate the results via comparison with space charge limited measurements. In summary, we demonstrate the potential of IS to characterize key-to-photovoltaics optoelectronic properties, carrier lifetime, and mobility, in a facile way

Generic nano-imprint process for fabrication of nanowire arrays

A generic process has been developed to grow nearly defect free arrays of (heterostructured) InP and GaP nanowires. Soft nanoimprint lithography has been used to pattern gold particle arrays on full 2 inch substrates. After lift-off organic residues remain on the surface, which induce the growth of additional undesired nanowires. We show that cleaning of the samples before growth with piranha solution in combination with a thermal anneal at 550 C for InP and 700 C for GaP results in uniform nanowire arrays with 1% variation in nanowire length, and without undesired extra nanowires. Our chemical cleaning procedure is applicable to other lithographic techniques such as e-beam lithography, and therefore represents a generic process.Comment: 12 pages, 4 figures, 2 table

Propagation of light in ensembles of semiconductor nanowires

Semiconductor nanowires have received a lot of interest during the last years because of their extreme optical properties. Giant polarization anisotropy in the absorption and emission of light fromsingle nanowires, extreme light confinement assisted by exciton polaritons, and enhanced detection sensitivity of analytes are some examples. These properties are leading to novel applications such as nano-light sources, electrical sensors, or quantum emitters. Many applications, such as nanowire solar cells or LEDs, will rely on large areas covered by ensembles of nanowires. Therefore, for devices based on nanowires, the knowledge on how light propagates in these layers is of utmost importance. In this thesis, an experimental study on light propagation in ensembles of GaP nanowires and arrays of InP nanowires is given. First, the growth of GaP and InP nanowires using the vapor-liquid-solid growth mechanism by metal-organic vapor phase epitaxy is introduced in Chapter 2. The vapor-liquid-solid growth mechanism requires a metal catalyst particle. Depending on the catalyst, ordered or disordered ensembles of nanowires can be grown. We describe the growth of disordered ensembles of nanowires from a thin gold film and the growth of arrays of nanowires from nanoparticles patterned by substrate conformal imprint lithography. Further,we discuss the nanowire morphology depending on the growth parameters and the substrate. We show that the nanowires grow vertical on (111) substrates. Nanowires grown on (100) substrates preferentially form an angle of 35¿ with respect to the substrate surface. While the initial diameter of the nanowires is defined by the gold catalyst, increasing the growth temperature allows for growing shells around the nanowires. The growth at a specific temperature results in conically shaped nanowires. Depending on the morphology of the nanowires, light propagates differently in ensembles of nanowires. While layers of thin nanowires form an effective medium that is strongly birefringent, i.e., the refractive index is different for different polarizations, layers of thick nanowires form a strongly scattering medium with a short scattering mean free path. Conically shaped nanowires form a graded refractive index layer that guides the light into the substrate and therewith reduces the reflection at the interface. In Chapter 3, the birefringence of thin layers of nanowires is described. The refractive index of these layers of nanowires is different for light polarized along or perpendicular to the nanowire elongation. The difference between the two refractive indices defines the birefringence parameter. We determine the birefringence by measuring the reflection contrast, i.e., the ratio of reflected light passing through crossed and parallel aligned polarizer and analyzer. We obtain the birefringence parameter of the nanowire layer from fits to the measurements with a transfer-matrix formalism based on Jones calculus. The experimentally determined birefringence is compared to the birefringence parameter determined from Maxwell-Garnett effective medium theory. The birefringence parameter is slightly lower than expected from theory due to bending of the nanowires. We find that the birefringence parameter of layers of nanowires is constant over a broad range of wavelengths. The large birefringence of ensembles of vertically aligned GaP nanowires can be significantly modified by adding a shell as thin as 10 nm of SiO2 around the nanowires. In Chapter 4, the modification of the birefringence is determined experimentally by polarization-dependent reflection measurements. This modification is modeled with Maxwell-Garnett effective medium theory and Jones calculus for anisotropic layers. We show that s-polarized light is more sensitive to changes in the surrounding of the nanowires than p-polarized light. The reflection contrast exhibits large and narrow peaks that shift strongly due to the presence of the thin shell. In contrast to Chapters 3 and 4, where we have determined the birefringence of vertically aligned nanowires, we have investigated the birefringence of nanowire layers that are grown on (100) GaP substrates in Chapter 5. These nanowires are oriented such that they form an angle of 35¿ with respect to the substrate surface. Due to this alignment, dense ensembles of GaP nanowires formbiaxial media. We determine the in-plane birefringence of layers of nanowires with different nanowire diameter by measuring the transmission contrast. We find that a nanowire layer with a certain nanowire diameter forms a ¿/4-waveplate. In Chapter 6, we describe the propagation of light in layers of thick nanowires. Scattering of light influences the propagation of light depending on the nanowire diameter. We determine the scattering mean free path of light, i.e., the mean distance between two scattering events, in layers of vertically aligned nanowires. We show that the scattering is anisotropic and that the scattering mean free path varies with the angle of incidence due to the alignment of the nanowires. GaP nanowires grown on (100) substrates form a stronger scattering medium than vertically aligned nanowires. We find that ensembles of nanowires belong to the strongest scattering media to date. In Chapter 7, we describe that graded refractive index layers reduce the reflection and increase the coupling of light into a substrate by matching the refractive index at the interfaces. For obtaining a graded refractive index layer based on GaP nanowires, the GaP filling fraction needs to be gradually increased from the top to the bottom of the layer. We show that ensembles of GaP nanorods form graded refractive index layers when they are conically shaped. Alternatively, a graded refractive index can be obtained using cylindrically shaped nanorods with a distribution of lengths, which also leads to an increased GaP filling fraction at the bottom of the layer. We model the graded index layers using a transfer-matrix method for isotropic layered media. We find that the coupling of light into a GaP substrate is increased for a broad range of wavelengths and angles. In Chapter 8, we demonstrate experimentally that arrays of base-tapered InP nanowires on top of an InP substrate form a strongly broadband and omnidirectional absorbing medium due to their specific geometry. Almost perfect absorption of light (higher than 97 %) occurs in the system. We explain the strong optical absorption by finite-difference time-domain simulations and we find that the base-tapered geometry of the nanowires strongly enhances the absorption for wavelengths below the electronic bandgap energy of InP. Above the electronic bandgap energy of InP, the light is efficiently coupled into the underlying substrate due to guided optical modes in the nanowires. Based on the findings of Chapters 4, 7, and 8, we propose in Chapter 9 possible applications of ensembles of nanowires. The high sensitivity of layers of nanowires to thin shells around the nanowires that is described in Chapter 4, inspired us to propose a very sensitive gas and bio-sensor. Tapered nanowires form a graded refractive index layer, which we describe in Chapter 7. We propose using graded refractive index layers based on GaP nanowires for increasing light coupling into III/Vmulti-junction solar cells. From the strong absorption of light in arrays of base-tapered InP nanowires (Chapter 8), we propose a novel solar cell concept based on base-tapered nanowire arrays

Integrated colloidal quantum dot photodetectors with color-tunable plasmonic nanofocusing lenses

High-sensitivity photodetection is at the heart of many optoelectronic applications, including spectroscopy, imaging, surveillance, remote sensing and medical diagnostics. Achieving the highest possible sensitivity for a given photodetector technology requires the development of ultra-small-footprint detectors, as the noise sources scale with the area of the detector. This must be accomplished while sacrificing neither the optically active area of the detector nor its responsivity. Currently, such designs are based on diffraction-limited approaches using optical lenses. Here, we employ a plasmonic flat-lens bull’s eye structure (BES) to concentrate and focus light into a nanoscale colloidal quantum dot (CQD) photodetector. The plasmonic lenses function as nanofocusing resonant structures that simultaneously offer color selectivity and enhanced sensitivity. Herein, we demonstrate the first CQD photodetector with a nanoscale footprint, the optically active area of which is determined by the BES; this detector represents an exciting opportunity for high-sensitivity sensing.Peer Reviewe

Integrated colloidal quantum dot photodetectors with color-tunable plasmonic nanofocusing lenses

High-sensitivity photodetection is at the heart of many optoelectronic applications, including spectroscopy, imaging, surveillance, remote sensing and medical diagnostics. Achieving the highest possible sensitivity for a given photodetector technology requires the development of ultra-small-footprint detectors, as the noise sources scale with the area of the detector. This must be accomplished while sacrificing neither the optically active area of the detector nor its responsivity. Currently, such designs are based on diffraction-limited approaches using optical lenses. Here, we employ a plasmonic flat-lens bull’s eye structure (BES) to concentrate and focus light into a nanoscale colloidal quantum dot (CQD) photodetector. The plasmonic lenses function as nanofocusing resonant structures that simultaneously offer color selectivity and enhanced sensitivity. Herein, we demonstrate the first CQD photodetector with a nanoscale footprint, the optically active area of which is determined by the BES; this detector represents an exciting opportunity for high-sensitivity sensing.Peer Reviewe

Bio-inspired broadband and omni-directional antireflective surface based on semiconductor nanorods

Bio-inspired layers of semiconductor nanorods increase light coupling into a high refractive index substrate. Reflection and transmission measurements show unambiguously, that the reduced reflection is due to optical impedance matching at the interfaces

Mimicking moth's eyes for photovoltaic applications with tapered GaP nanorods

We demonstrate experimentally that ensembles of conically shaped GaP nanorods form layers of graded refractive index due to the increased filling fraction of GaP from the top to the bottom of the layer. Graded refractive index layers reduce the reflection and increase the coupling of light into the substrate, leading to broadband and omnidirectional antireflection surfaces. This reduced reflection is the result of matching the refractive index at the interface between the substrate and air by the graded index layer. The layers can be modeled using a transfer-matrix method for isotropic layered media. We show theoretically that the light coupling efficiency into silicon can be higher than 95% over a broad wavelength range and for angles up to 60° by employing a layer with a refractive index that increases parabolically. Broadband and omnidirectional antireflection layers are specially interesting for enhancing harvesting of light in photovoltaics

Broadband and omnidirectional anti-reflection layer for iii/v multi-junction solar cells

Contains fulltext : 103915.pdf (preprint version ) (Open Access

Strong geometrical dependence of the absorption of light in arrays of semiconductor nanowires

We demonstrate experimentally that arrays of base-tapered InP nanowires on top of an InP substrate form a broad band and omnidirectional absorbing medium. These characteristics are due to the specific geometry of the nanowires. Almost perfect absorption of light (higher than 97%) occurs in the system. We describe the strong optical absorption by finite-difference time-domain simulations and present the first study of the influence of the geometry of the nanowires on the enhancement of the optical absorption by arrays. Cylindrical nanowires present the highest absorption normalized to the volume fraction of the semiconductor. The absolute absorption in layers of conical nanowires is higher than that in cylindrical nanowires but requires a larger volume fraction of semiconducting material. Base-tapered nanowires, with a cylindrical top and a conical base, represent an intermediate geometry. These results set the basis for an optimized optical design of nanowire solar cells