Jem Jos Haverkort

Direct Band Gap Wurtzite Gallium Phosphide Nanowires

The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555–690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality.

Efficiency Enhancement of InP Nanowire Solar Cells by Surface Cleaning

We demonstrate an efficiency enhancement of an InP nanowire (NW) axial p-n junction solar cell by cleaning the NW surface. NW arrays were grown with in situ HCl etching on an InP substrate patterned by nanoimprint lithography, and the NWs surfaces were cleaned after growth by piranha etching. We find that the postgrowth piranha etching is critical for obtaining a good solar cell performance. With this procedure, a high diode rectification factor of 10(7) is obtained at ±1 V. The resulting NW solar cell exhibits an open-circuit voltage (Voc) of 0.73 V, a short-circuit current density (Jsc) of 21 mA/cm(2), and a fill factor (FF) of 0.73 at 1 sun. This yields a power conversion efficiency of up to 11.1% at 1 sun and 10.3% at 12 suns.

Photoelectrochemical Hydrogen Production on InP Nanowire Arrays with Molybdenum Sulfide Electrocatalysts

Semiconductor nanowire arrays are expected to be advantageous for photoelectrochemical energy conversion due to their reduced materials consumption. In addition, with the nanowire geometry the length scales for light absorption and carrier separation are decoupled, which should suppress bulk recombination. Here, we use vertically aligned p-type InP nanowire arrays, coated with noble-metal-free MoS3 nanoparticles, as the cathode for photoelectrochemical hydrogen production from water. We demonstrate a photocathode efficiency of 6.4% under Air Mass 1.5G illumination with only 3% of the surface area covered by nanowires.

All-optical switching due to state filling in quantum dots

We report all-optical switching due to state filling in quantum dots (QDs) within a Mach–Zehnder interferometric switch (MZI). The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530–1570 nm probe beam is switched by optical excitation of one MZI arm. By exciting below the InGaAsP band gap, we prove that the refractive index nonlinearity is entirely due to the QDs. The switching efficiency is 5 rad/(μW absorbed power), corresponding to a 6 fJ switching energy. Probe wavelength insensitivity was obtained using a broad size distribution of QDs.

High optical quality single crystal phase wurtzite and zincblende InP nanowires

We report single crystal phase and non-tapered wurtzite (WZ) and zincblende twinning superlattice (ZB TSL) InP nanowires (NWs). The NWs are grown in a metalorganic vapor phase epitaxy (MOVPE) reactor using the vapor-liquid-solid (VLS) mechanism and in situ etching with HCl at a high growth temperature. Our stacking fault-free WZ and ZB TSL NWs allow access to the fundamental properties of both NW crystal structures, whose optical and electronic behaviors are often screened by polytypism or incorporated impurities. The WZ NWs show no acceptor-related emission, implying that the VLS-grown NW is almost free of impurities due to sidewall removal by HCl. They only emit light at the free exciton (1.491 eV) and the donor bound exciton transition (1.4855 eV). The ZB NWs exhibit a photoluminescence spectrum being unaffected by the twinning planes. Surprisingly, the acceptor-related emission in the ZB NWs can be almost completely removed by etching away the impurity-contaminated sidewall grown via a vapor-solid mechanism.

Relaxation of photo-excitations in films of oligo- and poly-(para-phenylene vinylene) derivatives

Abstract The photo-luminescence from solid films of poly(para-phenylene vinylene) polymers and an oligomeric model system, consisting of seven repeat units, are investigated at low temperature (8 K) using time-resolved spectroscopic techniques. Results are compared to those for the materials in solution. In the case of the oligomer, the shape of the visible absorption band observed for the film is quite different from the band shape for the polymer in frozen solution and is characteristic of H-type aggregates. Theoretical models are presented describing the dependence of the band shape of absorption and emission spectra on intermolecular excited state interactions, electron-vibration coupling and disorder represented by distributions of the molecular excitation and intermolecular interaction energies. Using these models, it is concluded that intermolecular interactions in the film of the oligomer are strong (⩾1400 cm−1), and the disorder low, implying delocalization of the excitation over several molecules. In accordance with these models the fluorescence lifetime for the film (∼2 ns) is considerably longer than for isolated molecules in solution (0.45 ns). The emission spectra of the film, taken early after excitation, are consistent with delocalization of the excitation over several molecules. A time-dependent red shift of the fluorescence band is observed and interpreted in terms of migration of localized excitations between disorder induced trap sites, which exist in the low energy tail of the density of excited states. For the polymers, differences between the shape of the absorption bands of solid film and frozen solution are smaller than for the oligomer indicating that interchain interactions that are, on average, weaker than for the oligomer. For the polymer films, a time-dependent red shift of the emission is observed and fluorescence depolarization measurements provide direct evidence for migration of the photo-excitations between trap sites. For one polymer, a time dependent change in the band shape of the fluorescence after pulsed excitation is observed with the band shape of the long-lived emission being compatible with that expected for an excitation delocalized over at least two, nearly parallel aligned, chains. For a second polymer, the emission band shape and its time evolution indicate that the major part of the fluorescence originates from disorder induced luminescent sites. These results indicate that the spectroscopic properties of films of π-conjugated polymer critically depend on parameters such as density of defects and excited state interchain interaction energy.

Optimization of barrier thickness for efficient carrier capture in graded-index and separate-confinement multiple quantum well lasers

We present results from numerical calculations on the carrier capture efficiency in separate‐ confinement and graded‐index separate‐confinement multiple quantum well (MQW) lasers. We find that the capture time oscillates as a function of the well width as well as the barrier width between the wells, due to a changing overlap of the barrier wave functions with the bound states in the wells. We show that one order of magnitude improvement in the carrier capture efficiency can be accomplished by properly choosing the dimensions of the layers in the active region of the MQW laser.

Ultrafast carrier capture at room temperature in InAs∕InP quantum dots emitting in the 1.55μm wavelength region

The energy and excitation density dependence of the carrier dynamics in self-assembled InAs∕InP quantum dots (QDs), emitting in the 1.55μm wavelength region, is investigated by means of time-resolved pump-probe differential reflection spectroscopy at room temperature. We observe ultrafast carrier capture and subsequential carrier relaxation into the QD ground state within 2.5 ps. The carrier lifetime in the QDs strongly depends on the QD optical transition energy within the QD ensemble as well as the carrier density, and ranges from 560 up to 2600 ps.

High yield transfer of ordered nanowire arrays into transparent flexible polymer films

The factors affecting transfer of nanowire arrays from their substrates into flexible PDMS films have been systematically investigated. Experiments were carried out on gallium phosphide nanowires with a standard length of 10 μm with varying pitch (0.2-1.5 μm). The important factors were found to be penetration of the PDMS within the nanowire arrays and the strength/rigidity of the PDMS film. The PDMS penetration between wires in the arrays is affected by both the viscosity of the PDMS solution and the presence of air pockets trapped within nanowire arrays, particularly at small pitches. Dilution with hexane and curing in a vacuum desiccator solve the wire penetration problem, and an increase in cure/base ratio increases the rigidity and strength of the PDMS. The procedures for preparation and deposition of the PDMS solution are optimized and a high yield, up to 95%, of wire transfer across a range of nanowire pitches has been obtained.

Quantifying losses and thermodynamic limits in nanophotonic solar cells

Nanophotonic engineering shows great potential for photovoltaics: the record conversion efficiencies of nanowire solar cells are increasing rapidly and the record open-circuit voltages are becoming comparable to the records for planar equivalents. Furthermore, it has been suggested that certain nanophotonic effects can reduce costs and increase efficiencies with respect to planar solar cells. These effects are particularly pronounced in single-nanowire devices, where two out of the three dimensions are subwavelength. Single-nanowire devices thus provide an ideal platform to study how nanophotonics affects photovoltaics. However, for these devices the standard definition of power conversion efficiency no longer applies, because the nanowire can absorb light from an area much larger than its own size. Additionally, the thermodynamic limit on the photovoltage is unknown a priori and may be very different from that of a planar solar cell. This complicates the characterization and optimization of these devices. Here, we analyse an InP single-nanowire solar cell using intrinsic metrics to place its performance on an absolute thermodynamic scale and pinpoint performance loss mechanisms. To determine these metrics we have developed an integrating sphere microscopy set-up that enables simultaneous and spatially resolved quantitative absorption, internal quantum efficiency (IQE) and photoluminescence quantum yield (PLQY) measurements. For our record single-nanowire solar cell, we measure a photocurrent collection efficiency of >90% and an open-circuit voltage of 850 mV, which is 73% of the thermodynamic limit (1.16 V).