2011
Nanowires offer opportunities for improved LEDs.
We have made special nanostructures that could be used as light-emitting diodes (LEDs).
In a collaboration with colleagues from Philips Research, Eindhoven University of Technology and Delft University of Technology, we have made special nanostructures that could be used as light-emitting diodes (LEDs). These nanostructures can be used to control the direction of the emission. Controlling the direction of the light is vitally important for increasing the efficiency of LEDs.It is also a step towards a new generation of LEDs that are based on semiconducting nanowires. The results of this research are recently published in the prestigious journal ACS Nano. The direction in which a LED emits light is mainly determined by the surface between the LED and the surrounding air. As light can only escape from the LED at small angles, the direction of emission is usually straight on (perpendicular to the surface). However this can be influenced by nanostructures in the surface of the LED. Inspired by these nanostructures, the researchers have developed a new technology with which the direction of the light can be changed.
Photonic crystal
The new method consists of growing partially-emitting nanowires in an ordered pattern. This pattern forms a ‘photonic crystal’ that sends the light in specific directions. Furthermore, the researchers have shown that the emission can be optimised by a smart positioning of the emitting part within the nanowire. This knowledge could lead to an increased efficiency of LEDs. Moreover it provides opportunities for a next generation of LEDs, based on semiconducting nanowires.
Reference: Controlling the directional emission of light by periodic arrays of heterostructured semiconductor nanowires, Silke L. Diedenhofen, Olaf T.A. Janssen, Moïra Hocevar, Aurélie Pierret, Erik P.A.M. Bakkers, H. Paul Urbach, and Jaime Gómez Rivas, ACS Nano (2011), 5 (7), pp 5830–5837
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 timedomain 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. Please find the article here.
New position as post-doctoral researcher at ICFO-Institute of Photonic Sciences
In June 2011 I moved to ICFO-Institute of Photonic Sciences as a post-doctoral researcher. My research is focused on plasmonically enhanced quantum dot photodetectors.
2010
PhD-defense
I have successfully defended my PhD-thesis “Propagation of Light in Ensembles of Semiconductor Nanowires” on 20th December 2010 at Eindhoven University of Technology.
Nanoscale Free-Carrier Profiling of Individual Semiconductor Nanowires by Infrared Near-Field Nanoscopy.
In collaboration with the groups of Rainer Hillenbrand (CIC Nanogune Consolider, San Sebastian, Spain) and Erik Bakkers (Philips Research, Eindhoven), we have utilized infrared near-field nanoscopy to probe the free carrier concentration in semiconductor nanowires. With a spatial resolution of 25 nm, we determined the gradient of the free-carrier concentration at the interface between a non-doped and a doped segment and we quantified the free-carrier concentration in the doped region. Controlling the free-carrier concentration and the sharpness of the interface between regions with different doping concentrations allows for fabrication of novel nanowire diodes, transistors or light emitting devices. Please read the article here.
Generic nano-imprint process for fabrication of nanowire arrays.
We have developed a generic process for growing defect-free ordered arrays of nanowire in collaboration with the group of Erik Bakkers from Philips Research. Substrate conformal imprint lithography has been used to pattern gold particle arrays on full 2 inch substrates, that catalyze the growth of the nanowires. From the imprint process, organic residues remain on the surface, which induce the growth of additional undesired nanowires. Cleaning the substrates before growth with a wet chemical etch in combination with a thermal anneal results in uniform nanowire arrays. The cleaning procedure is applicable to other lithographic techniques, and therefore represents a generic process. Please download the article for more information.
2009
Efficient light scattering by nanowires for random lasers and next-generation solar cells.
One-dimensional nanowires from semiconductors like InP, GaP, and Si are promising nanomaterials for next-generation inorganic solar cell devices. Understanding their optical response is extremely important for achieving increased performance in solar energy collection. Researchers at the FOM Institute for Atomic and Molecular Physics (AMOLF) and Philips Research in the Netherlands have investigated strong light scattering in layers of nanowires. Examples of nanowire layers are shown in Figure 1. Using a specially developed broadband optical technique, they show that trapping of light by multiple scattering is important in the design of nanowire devices (Figure 2).
We have demonstrated that nanowires actually can be grown to form one of the most strongly scattering materials available today. Next to its technological relevance, this property opens exciting new prospects in fundamental research on random lasers and Anderson localization of light. By matching the nanowire diameter to the optical wavelength, light can be trapped for several periods inside the nanowire, leading to a resonant enhancement of their scattering efficiency. The high tunability of nanowire properties and alignment, and the general applicability to groups III-V, II-VI, and IV semiconductors, enable new possibilities for harvesting of the solar spectrum. The results of this research are published in the prestigious journal Nano Letters.
Figure 1 High densities of GaP nanowires were grown with controlled sizes and alignment using vapor-phase epitaxy at Philips Research. (left to right) Cross-sectional Scanning Electron Microscopy images of nanowire layers with increasing nanowire diameters.
Figure 2 Using a new technique of broadband enhanced backscattering, the transport mean free path of light in the nanowire medium was determined over a wide spectral range in the visible and infrared. The result, presented in the right panel, shows a strong variation of the mean free path from the weak to strong scattering regimes with characteristic oscillations corresponding to the guided modes of the wires.
2008
Harvesting light using nanostructured surfaces.
Researchers from the FOM Institute for Atomic and Molecular Physics (AMOLF), located at the High Tech Campus in Eindhoven, and Philips Research developed a method to drastically reduce the reflection of light at the interface between a high refractive index semiconductor and air. The researchers were inspired by the eyes of moths that are covered by tapered nanostructures. Thanks to these nanostructures, night moths are capable of seeing very well in the dark. The results of the research have recently been published in the prestigious journal Advanced Materials.
Light travels as a straight beam in homogeneous media. The direction of the light beam changes when it arrives at an interface between two different media. A fraction of light is reflected into the first medium and the rest is refracted into the second medium. At large angles of incidence at an interface between air and a solid material such as a semiconductor, nearly 100 % of the light is reflected. Interfaces which reduce this reflection exist in nature. For example, the eyes of a moth are covered with tapered nanostructures, which increase the eye sight of the moth in the dark by allowing more light to enter the eyes.
Inspired by these biostructures, researchers from AMOLF and Philips Research developed a method which drastically reduces the reflection between air and a semiconductor. This method consists of the growth of nanowires with different lengths or with the same length, but conically shaped. Using this method, a gradual change from air to semiconductor is achieved, which leads to an efficient coupling of light into the semiconductor and minimizes the reflection. These layers show a large reduction of the reflection over a broad range of colors and angles of incidence. The reduction of the reflection is of importance for different applications. A low reflection can not only increase the sensitivity of a light detector, it can also increase the efficiency of solar cells and LEDs. The results of this research are published in the prestigious journal Advanced Materials.
2008
Semiconductor nanowire metamaterials for photonic applications.
A novel class of optical metamaterials is presented consisting of high densities of aligned gallium phosphide (GaP) nanowires fabricated using metal-organic vapor phase-epitaxy. Starting from a gold island film as a catalyst for nanowire growth, a sequential combination of vapor–liquid–solid and lateralgrowth modes is employed to obtain a continuous tunability of the nanowire volume fraction from 7% to over 35%. By choosing different crystallographic orientations of the GaP substrate, metamaterials are designed with different nanowire orientations. The anisotropy of the nanowire building blocks results in strong optical birefringence. Polarization interferometry demonstrates a very large polarization extinction contrast combined with a sharp angular resonance which holds promise for optical sensing. Nanowire metamaterials may find applications in photonics, optoelectronics, non-linear and quantumoptics, microfluidics, bio-, and gas sensing.You can read more in our recently published Advanced Functional Materials.