A T J van Helvoort

Norwegian University of Science and Technology, Nidaros, Sør-Trøndelag, Norway

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Publications (48)144.59 Total impact

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    ABSTRACT: Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticle's 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.
    Nano Letters 10/2014; · 12.94 Impact Factor
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    ABSTRACT: A method for probing the electrical and structural characteristics of individual as-grown III-V nanowires was studied. In-situ electrical characterization was performed in a focused ion beam / scanning electron microscopy system by using a fine nano-manipulator and ion beam assisted deposition. Transmission electron microscopy specimens of probed nanowires are prepared afterwards. This method would potentially allow the correlation of electrical and structural characteristics (e.g. crystal faults such as twinning) of the nanowire-substrate system. The challenge is in contacting the nanowires so that the electrical characteristics of the nanowire-substrate system can be extracted correctly.
    Journal of Physics Conference Series 06/2014; 522(1):012080.
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    ABSTRACT: We report on the epitaxial growth of large-area position controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2-inch Si wafers using nanoimprint lithography (NIL), for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs, the MBE growth parameter space is tuned, including Ga pre-deposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e. As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position controlled NW arrays for various optoelectronic device applications.
    Nano Letters 01/2014; · 13.03 Impact Factor
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    ABSTRACT: We demonstrate a method for compositional mapping of AlxGa1-xAs heterostructures with high accuracy and unit cell spatial resolution using quantitative high angle annular dark field scanning transmission electron microscopy. The method is low dose relative to spectroscopic methods and insensitive to the effective source size and higher order lens aberrations. We apply the method to study the spatial variation in Al concentration in cross-sectioned GaAs/AlGaAs core-shell nanowires and quantify the concentration in the Al-rich radial band and the AlGaAs shell segments.
    Applied Physics Letters 12/2013; 103(23):2111-. · 3.52 Impact Factor
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    ABSTRACT: In this work we have investigated the variation of Sb concentration among and within zinc blende (ZB) GaAsSb inserts in wurtzite (WZ) GaAs bare-core and WZ GaAs/AlGaAs core–shell nanowires (NWs) grown by Au-assisted molecular beam epitaxy. The Sb concentration variation was related to the optical properties as determined by photoluminescence (PL). The NW structure and the Sb concentration were studied by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX) and quantitative high angle annular dark field scanning TEM (HAADF STEM). A clear trend relating the maximum Sb concentration with the insert length was observed: the longer the insert, the higher the Sb concentration. In addition, there are graded Sb concentration gradients both along and across the GaAsSb inserts. The influence of the Sb concentration variation on the PL emission from the GaAsSb inserts was investigated with correlated micro-PL and TEM-EDX on the same single NWs. Based on the PL results and the observed Sb concentration profiles, we propose a qualitative energy band diagram for a typical ZB GaAsSb insert in a WZ GaAs NW for the heterostructured NWs studied here. Type I transitions within the central region of the ZB GaAsSb inserts were found to dominate the insert-related PL emission. Weak type II transitions within the inserts due to the graded Sb concentration were observed as well. Using an existing empirical model, the Sb concentrations were additionally determined from the ground state PL energies (type I transition). For the average Sb concentration, the concentrations based on PL were in agreement with EDX and quantitative HAADF STEM results. (Some figures may appear in colour only in the online journal)
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    ABSTRACT: Accomplishing control of the crystal phases in III–V semiconductor nanowires (NWs) is important for applications in future advanced nano-devices. In this work, we report on the growth of both zinc blende (ZB) and wurtzite (WZ) GaAs in self-catalyzed GaAs and GaAs/GaAsSb axial heterostructured NWs on Si(111) substrates by the vapor–liquid–solid technique using molecular beam epitaxy. The self-catalyzed GaAs NWs usually adopt the ZB phase. However, by growing GaAs NWs with short GaAsSb axial inserts, the crystal phase of GaAs can be changed from ZB below the GaAsSb insert, to WZ above. This crystal phase change in GaAs can be explained in terms of a change in the contact angle of the Ga droplet which changes the fraction of the triple-phase-line in contact with the edge of the NW top facet and therefore affects the probability of nucleation for ZB and WZ phases. In addition, by growing ZB GaAs after the insert using a flux interruption, we demonstrate the growth of all combinations of crystal phases of the GaAs segments on both sides of the GaAsSb insert. This understanding has also enabled us to achieve the growth of WZ phase in GaAs NWs as well as a ZB–WZ–ZB GaAs NW heterostructure, by changing the Ga droplet contact angle, without growing a GaAsSb insert. The contact angle was controlled by introducing different flux interruptions and tuning the V/III flux ratio.
    Journal of Crystal Growth 06/2013; 372:163–169. · 1.69 Impact Factor
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    ABSTRACT: Accomplishing control of the crystal phases in III–V semiconductor nanowires (NWs) is important for applications in future advanced nano-devices. In this work, we report on the growth of both zinc blende (ZB) and wurtzite (WZ) GaAs in self-catalyzed GaAs and GaAs/GaAsSb axial heterostructured NWs on Si (111) substrates by the vapor–liquid–solid technique using molecular beam epitaxy. The self-catalyzed GaAs NWs usually adopt the ZB phase. However, by growing GaAs NWs with short GaAsSb axial inserts, the crystal phase of GaAs can be changed from ZB below the GaAsSb insert, to WZ above. This crystal phase change in GaAs can be explained in terms of a change in the contact angle of the Ga droplet which changes the fraction of the triple-phase-line in contact with the edge of the NW top facet and therefore affects the probability of nucleation for ZB and WZ phases. In addition, by growing ZB GaAs after the insert using a flux interruption, we demonstrate the growth of all combinations of crystal phases of the GaAs segments on both sides of the GaAsSb insert. This understanding has also enabled us to achieve the growth of WZ phase in GaAs NWs as well as a ZB–WZ–ZB GaAs NW heterostructure, by changing the Ga droplet contact angle, without growing a GaAsSb insert. The contact angle was controlled by introducing different flux interruptions and tuning the V/III flux ratio. & 2013 Elsevier B.V. All rights reserved.
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    ABSTRACT: Control of the crystal phases of GaAs nanowires (NWs) is essential to eliminate the formation of stacking faults which deteriorate the optical and electronic properties of the NWs. In addition, the ability to control the crystal phase of NWs provides an opportunity to engineer the band gap without changing the crystal material. We show that the crystal phase of GaAs NWs grown on GaAs(111)B substrates by molecular beam epitaxy using the Au-assisted vapor-liquid-solid growth mechanism can be tuned between wurtzite (WZ) and zinc blende (ZB) by changing the V/III flux ratio. As an example we demonstrate the realization of WZ GaAs NWs with a ZB GaAs insert that has been grown without changing the substrate temperature.
    Nanotechnology 01/2013; 24(1):015601. · 3.67 Impact Factor
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    ABSTRACT: We report on the growth, structural and electrical characterizations of Be-doped GaAs nanowires (NWs) grown by the Au- and Ga-assisted vapour-liquid-solid techniques using molecular beam epitaxy. The growth rate of Be-doped GaAs NWs grown by the Au-assisted technique is observed to be lower as compared to the growth rate of undoped GaAs NWs grown under identical conditions. However, no effect on either the growth rate or the morphology of NWs has been observed for Be-doped GaAs NWs grown by the Ga-assisted technique with the same Be flux as used for the Au-assisted technique. Electrical characterization reveals that the Ga-assisted grown NWs show more consistent, symmetric current-voltage (I-V) characteristics with higher electrical current than the Au-assisted grown NWs. Finally, we show that ohmic contacts to Be-doped Ga-assisted NWs can be achieved either by post-annealing the metal-contacted NW or increasing the doping concentration during the NW growth.
    Journal of Crystal Growth 01/2013; · 1.69 Impact Factor
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    ABSTRACT: By utilizing the reduced contact area of nanowires, we show that epitaxial growth of a broad range of semiconductors on graphene can in principle be achieved. A generic atomic model is presented which describes the epitaxial growth configurations applicable to all conventional semiconductor materials. The model is experimentally verified by demonstrating the growth of vertically aligned GaAs nanowires on graphite and few-layer graphene by the self-catalyzed vapor-liquid-solid technique using molecular beam epitaxy. A two-temperature growth strategy was used to increase the nanowire density. Due to the self-catalyzed growth technique used, the nanowires were found to have a regular hexagonal cross-sectional shape, and are uniform in length and diameter. Electron microscopy studies reveal an epitaxial relationship of the grown nanowires with the underlying graphitic substrates. Two relative orientations of the nanowire side-facets were observed, which is well explained by the proposed atomic model. A prototype of a single GaAs nanowire photodetector demonstrates a high-quality material. With GaAs being a model system, as well as a very useful material for various optoelectronic applications, we anticipate this particular GaAs nanowire/graphene hybrid to be promising for flexible and low-cost solar cells.
    Nano Letters 08/2012; 12(9):4570-6. · 13.03 Impact Factor
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    ABSTRACT: The Sb concentration in axial GaAs(1-x)Sb(x) inserts of otherwise pure GaAs nanowires has been investigated with quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The Sb concentration was quantified by comparing the experimental image intensities normalized to the incident beam intensity with intensities simulated with a frozen lattice multislice approach. Including static atomic displacements in the simulations was found to be crucial for correct compositional analysis of GaAs(1-x)Sb(x). HAADF intensities of individual nanowires were analysed both across the nanowires, exploiting their hexagonal cross-sectional shape, and along the evenly thick central part of the nanowires. From the cross-sectional intensity profiles, a decrease in the Sb concentration towards the nanowire outer surfaces was found. The longitudinal intensity profiles revealed a gradual build-up of Sb in the insert. The decrease of the Sb concentration towards the upper interface was either gradual or abrupt, depending on the growth routine chosen. The compositional analysis with quantitative HAADF-STEM was verified by energy dispersive X-ray spectroscopy.
    Micron 07/2012; · 2.06 Impact Factor
  • H Kauko, R Bjørge, R Holmestad, A T J van Helvoort
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    ABSTRACT: GaAs nanowires are a highly promising candidate for future optoelectronic applications. Here quantitative high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) has been used for studying the Sb content x in zinc blende (ZB) GaAs1−xSbx axial inserts in otherwise pure wurtzite (WZ) GaAs nanowires. Direct comparison of the normalized experimental HAADF-STEM image intensities with simulated intensities displayed a good match for the pure structure. It was discovered that the Sb content in a 20 nm long insert is not uniform but increases from 13 ± 3 at.% at the lower to 21 ± 3 at.% at the upper WZ-ZB interface. Non-compositional effects on HAADF intensity have been identified.
    Journal of Physics Conference Series 07/2012; 371(1).
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    ABSTRACT: Low temperature (10 K) micro‐photoluminescence (μ‐PL) of single GaAs∕AlGaAs core‐shell nanowires with single GaAsSb inserts were measured. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the PL emission from the wurtzite GaAs nanowire is perpendiculary polarized to the nanowire axis. The result indicates that the crystal phase, through the optical selection rules, has significant effect on the polarization of the PL from NWs besides the dielectric mismatch. The analysis of the PL results based on the electronic structure of these nanowires supports the correlation between the crystal phase and the PL emission.
    AIP Conference Proceedings. 12/2011; 1399(1):559-560.
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    ABSTRACT: Correlation between the optical and the structural properties of an individual heterostructured nanowire (NW) is crucial for optimising the NW synthesis and device design. In this work, low temperature (10 K) micro-photoluminescence (μ-PL), low and high voltage scanning transmission electron microscopy and conventional transmission electron microscopy are applied to the same individual NWs. The studied NWs, grown by Au-assisted molecular beam epitaxy, have a wurtzite GaAs core with a zincblende GaAsSb axial insert, enclosed with an AlGaAs radial shell and a GaAs capping layer. By subsequent analysis of one and the same NW by μ-PL and several electron microscopy techniques in different microscopes, we can relate the spectral features of a single NW to the structural features, such as different crystal phases, lattice defects and composition.
    Journal of Physics Conference Series 11/2011; 326(1):012043.
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    ABSTRACT: To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (μ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to μ-PL or in NW device processing. The correlation between the three main μ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.
    Nanotechnology 08/2011; 22(32):325707. · 3.67 Impact Factor
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    ABSTRACT: Studies of oxide interfaces, and how they affect physical properties, have recently gained large attention. For ferroelectric materials, the evolution of the order parameter close to an interface is important to understand regarding the stability of the ferroelectric phase, and how to optimize devices taking advantage of the polarization at the interface. Here we employ electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy to compare the electronic and structural properties in both bulk and interface regions of epitaxial PbTiO3 thin films grown on SrTiO3 substrates. At the interface, changes in EELS spectra of the Ti-L3,2 and O-K edges, as compared to the bulk of the thin film, reveal a reduction in the hybridization of Ti 3d and Pb 6sp states with O 2p, and thus tetragonal distortion of the TiO6 octahedron. Real-space multiple-scattering calculations of the O-K edge support the experimental results. Moreover, the analysis of the Ti-valence reveals that the change is gradual over ~2-3 nm. The data implies a decreasing ferroelectric order parameter over ~2-3 nm close to the PbTiO3/SrTiO3 interface with a nonzero value at the interface with an additional screening of the polarization over ~1-2 nm into the SrTiO3 substrate from the Ti atoms.
    Journal of Applied Physics 01/2011; 109. · 2.19 Impact Factor
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    ABSTRACT: In this work we demonstrate by transmission electron microscopy and piezoresponse force spectroscopy that the polarization direction in hydrothermally synthesized lead titanate (PbTiO3) nanorods can be changed from parallel to the nanorod axis to perpendicular to it by a simple heat treatment above the Curie temperature. The heat treatment also introduced 90° domains, caused a rearrangement of the surface and a reduction in the amounts of defects. The polarization of the heat-treated nanorods could be successively switched in the direction perpendicular to the nanorod axis. This control of the polarization in PbTiO3 nanorods opens up possibilities of tailoring the ferroelectric properties and is therefore highly relevant for the use of ferroelectric nanorods in devices.
    Journal of Applied Physics 12/2010; 108(12):124320-124320-6. · 2.19 Impact Factor
  • Brit Graver, Antonius T. J. van Helvoort, Kemal Nisancioglu
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    ABSTRACT: The presence of trace elements in Group IIIA–VA is known to activate aluminium anodically in chloride environment. The purpose of this paper is to investigate the surface segregation of trace element In by heat treatment and resulting surface activation. Model binary AlIn alloys, containing 20 and 1000ppm by weight of In, were characterized after heat treatment at various temperatures by use of glow discharge optical emission spectroscopy, electron microscopy and electrochemical polarization. Heat treatment for 1h at 300°C gave significant segregation of discrete In particles (thermal segregation), which activated the surface. Indium in solid solution with aluminium, obtained by 1h heat treatment at 600°C, also activated by surface segregation of In on alloy containing 1000ppm In, resulting from the selective dissolution of the aluminium component during anodic oxidation (anodic segregation). The effect of anodic segregation was reduced by decreasing indium concentration in solid solution; it had negligible effect at the 20ppm level. The segregated particles were thought to form a liquid phase alloy with aluminium during anodic polarization, which in turn, together with the chloride in the solution destabilized the oxide.
    Corrosion Science 11/2010; 52(11):3774-3781. · 3.69 Impact Factor
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    ABSTRACT: We report on a crystal phase-dependent photoluminescence (PL) polarization effect in individual wurtzite GaAs nanowires with a zinc blende GaAsSb insert grown by Au-assisted molecular beam epitaxy. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the emission from the wurtzite GaAs nanowire is perpendicularly polarized. The results indicate that the crystal phases, through optical selection rules, are playing an important role in the alignment of the PL polarization in nanowires besides the linear polarization induced by the dielectric mismatch. The strong excitation power dependence and long recombination lifetimes ( approximately 4 ns) from the wurtzite GaAs and zinc blende GaAsSb-related PL emission strongly indicate the existence of type II band alignments in the nanowire due to the presence of nanometer thin zinc blende segments and stacking faults in the wurtzite GaAs barrier.
    Nano Letters 08/2010; 10(8):2927-33. · 13.03 Impact Factor
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    ABSTRACT: Synthesis and application of semiconductor nanowires (NWs) require detailed understanding of their structures. Transmission electron microscopy (TEM) has proven to be the most used structural characterisation tool for these nanostructures. For the characterisation of more complex heterostructured NWs, in this study axial (i.e. with "insert") or radial (i.e. "core-shell") III-V NWs, we use dark field TEM and high angle annular dark field scanning TEM (HAADF-STEM) as imaging modes. For the GaAs/GaAsSb/GaAs axial heterostructured NWs, dark field TEM exploits the structural difference between wurtzite GaAs NW and zinc blende GaAsSb insert, resulting in an excellent contrast between the GaAs part and the GaAsSb insert of the NW. In addition, twins, stacking faults and polytypes of the GaAs phase can easily be identified. For the radial GaAs/AlGaAs core-shell NWs where both core and shell have the wurtzite structure, HAADF-STEM is found to be a very useful technique.
    Journal of Physics Conference Series 08/2010; 241(1):012084.

Publication Stats

374 Citations
144.59 Total Impact Points

Institutions

  • 2004–2014
    • Norwegian University of Science and Technology
      • • Department of Physics
      • • Department of Electronics and Telecommunications (IET)
      • • Department of Materials Science and Engineering
      Nidaros, Sør-Trøndelag, Norway
  • 2011
    • NTNU Samfunnsforskning
      Nidaros, Sør-Trøndelag, Norway
  • 2003
    • University of Cambridge
      • Department of Materials Science and Metallurgy
      Cambridge, England, United Kingdom