Ole Hansen

Technical University of Denmark, København, Capital Region, Denmark

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Publications (205)438.22 Total impact

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    ABSTRACT: The electrical performance of graphene synthesized by chemical vapour deposition and transferred to insulating surfaces may be compromised by extended defects, including for instance grain boundaries, cracks, wrinkles and tears. In this study we experimentally investigate and compare the nano- and micro-scale electrical continuity of single layer graphene grown on cm-size single crystal copper with that of previously studied graphene films, grown on commercially available copper foil. The electrical continuity of the graphene films is analysed using two non-invasive conductance characterization methods: ultra-broadband terahertz time-domain spectroscopy and micro four-point probe, which probe the electrical properties of the graphene film on different length scales; 100 nm and 10 µm, respectively. Ultra-broadband terahertz time-domain spectroscopy allows for measurement of the complex conductance response in the frequency range 1-15 terahertz, covering the entire intraband conductance spectrum, and reveals that the conductance response for the graphene grown on single crystalline copper intimately follows the Drude model for a barrier-free conductor. In contrast, the graphene grown on commercial foil copper shows a distinctly non-Drude conductance spectrum that is better described by the Drude-Smith model, which incorporates the effect of preferential carrier backscattering associated with extended, electronic barriers with a typical separation on the order of 100 nm. Micro four-point probe resistance values measured on graphene grown on single crystalline copper in two different voltage-current configurations show close agreement with the expected distributions for a continuous 2D conductor, in contrast with previous observations on graphene grown on commercial copper foil. The terahertz and micro four-point probe conductance values of the graphene grown on single crystalline copper shows a close to unity correlation, in contrast with those of the graphene grown on commercial copper foil, which we explain by the absence of extended defects on the microscale in CVD graphene grown on single crystalline copper. The presented results demonstrate that the graphene grown on single crystal copper is electrically continuous on the nano-, micro-, as well as intermediate scales.
    Nano letters. 10/2014;
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    ABSTRACT: Sputter deposition of 50 nm thick NiO films on p+-n-Si and subsequent treatment in Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe-treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm2 (requirement for >10% efficient devices) at 1.15 V vs. RHE (Reversible Hydrogen Electrode) under red-light (38.6 mW/cm2) irradiation. Thus the photoanodes were harvesting ~80 mV of free energy (voltage), which places them among the best performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V vs. RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes.
    Journal of Physical Chemistry Letters 09/2014; 5:3456–3461. · 6.59 Impact Factor
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    ABSTRACT: A novel molecular beacon (a nanomachine) is constructed that can be actuated by a radio frequency (RF) field. The nanomachine consists of the following elements arranged in molecular beacon configuration: a gold nanoparticle that acts both as quencher for fluorescence and a localized heat source; one reporter fluorochrome, and; a piece of DNA as a hinge and recognition sequence. When the nanomachines are irradiated with a 3 GHz RF field the fluorescence signal increases due to melting of the stem of the molecular beacon. A control experiment, performed using molecular beacons synthesized by substituting the gold nanoparticle by an organic quencher, shows no increase in fluorescence signal when exposed to the RF field. It may therefore be concluded that the increased fluorescence for the gold nanoparticle-conjugated nanomachines is not due to bulk heating of the solution, but is caused by the presence of the gold nanoparticles and their interaction with the RF field; however, existing models for heating of gold nanoparticles in a RF field are unable to explain the experimental results. Due to the biocompatibility of the construct and RF treatment, the nanomachines may possibly be used inside living cells. In a separate experiment a substantial increase in the dielectric losses can be detected in a RF waveguide setup coupled to a microfluidic channel when gold nanoparticles are added to a low RF loss liquid. This work sheds some light on RF heating of gold nanoparticles, which is a subject of significant controversy in the literature.
    Small 09/2014; · 7.82 Impact Factor
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    ABSTRACT: Within the field of photocatalytic water splitting there are several strategies to achieve the goal of efficient and cheap photocatalytic water splitting. This work examines one particular strategy by focusing on monolithically stacked, two-photon photoelectrochemical cells. The overall aim of the analysis is to compare the relative merits of two fundamentally different designs: one, where the photoanode is the large bandgap material (light-facing side), and the other, where the photocathode is the large bandgap material. Even though the former design is often shown in the literature, the present analysis shows that the latter design has several advantages. This is particularly true when considering designs that incorporate protection layers to protect the photoabsorbers. A high throughput computational screening was used to filter materials databases in search of candidates with the correct properties. These results show that without protective layers there are scarcely any materials which seem viable as photoabsorbers whereas with protection layers there are significantly more candidates. Since the protection layer (and redox catalysts) on the light facing side should not interfere with light absorption, this is the more difficult side to optimize. Nevertheless, by using TiO2 as a transparent cathode protection layer in conjunction with known H2 evolution catalysts, protection is clearly feasible for a large bandgap photocathode. This suggests that there may be promising strategies for photocatalytic water splitting by using a large bandgap photocathode and a low bandgap photoanode with attached protection layers.
    Energy & Environmental Science 07/2014; 7(8). · 11.65 Impact Factor
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    ABSTRACT: Sputter deposition of Ir/IrOx on p+-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H2SO4). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm2 simulated sunlight irradiation (λ > 635 nm, AM 1.5G) and measurements with quartz crystal microbalances. Films exceeding a thickness of 4 nm were shown to be highly active though metastable due to an amorphous character. By contrast, 2 nm IrOx films were stable, enabling OER at a current density of 1 mA/cm2 at 1.05 V vs. RHE. Further improvement by heat treatment resulted in a cathodic shift of 40 mV and enabled a current density of 10 mA/cm2 (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrOx/Ir/p+-n-Si structures not only provide the necessary overpotential for OER at realistic device current, but also harvest 100 mV of free energy (voltage) which makes them among the best-performing Si-based photoanodes in low-pH media.
    Journal of Physical Chemistry Letters 05/2014; · 6.59 Impact Factor
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    ABSTRACT: The far field velocity potential and radiation pattern of baffled circular plates and membranes are found analytically using the full set of modal velocity profiles derived from the corresponding equation of motion. The derivation is valid for a plate or membrane subjected to an external excitation force, which is used as a sound receiver in any medium or as a sound transmitter in a gaseous medium. A general, concise expression is given for the radiation pattern of any mode of the membrane and the plate with arbitrary boundary conditions. Specific solutions are given for the four special cases of a plate with clamped, simply supported, and free edge boundary conditions as well as for the membrane. For all non-axisymmetric modes, the velocity potential along the axis of the radiator is found to be strictly zero. In the long wavelength limit, the radiation pattern of all axisymmetric modes approaches that of a monopole, while the non-axisymmetric modes exhibit multipole behavior. Numerical results are also given, demonstrating the implications of having non-axisymmetric excitation using both a point excitation with varying eccentricity and a homogeneous excitation acting on half of the circular radiator.
    The Journal of the Acoustical Society of America 05/2014; 135(5):2523. · 1.65 Impact Factor
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    EMRS 2014, Lille; 05/2014
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    ABSTRACT: The topography of thermally oxidized, structured silicon dioxide is investigated through simulations, atomic force microscopy, and a proposed analytical model. A 357 nm thick oxide is structured by removing regions of the oxide in a masked etch with either reactive ion etching or hydrofluoric acid. Subsequent thermal oxidation is performed in both dry and wet ambients in the temperature range 950 C to 1100 C growing a 205 ± 12 nm thick oxide in the etched mask windows. Lifting of the original oxide near the edge of the mask in the range 6 nm to 37 nm is seen with increased lifting for increasing processing temperatures. Oxides structured by reactive ion etching are lifted on average a factor of four more than oxides etched in hydrofluoric acid. Both simulations and the analytical model successfully predict the oxide topography qualitatively, showing that the mask lifting phenomenon is governed mainly by diffusion and the geometry of the oxide. Simulations also predict the oxide topography quantitatively, with an average root mean square deviation of 1.2 nm and a maximum deviation of 13 nm (39%) from the mean of the measured values. © 2014 The Electrochemical Society. [DOI: 10.1149/2.003405jss] All rights reserved. Manuscript submitted December 13, 2013; revised manuscript received February 20, 2014. Published March 5, 2014.
    ECS Journal of Solid State Science and Technology. 03/2014; 3(3):63-68.
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    Andrea Crovetto, Fei Wang, Ole Hansen
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    ABSTRACT: Modeling of energy harvesting devices is complicated by the coupling between electrical and mechanical domains. In this paper, we present a coupled electromechanical model for electret-based resonant energy harvesters where the two output pads are placed on the same device side (single-sided). An analytical analysis is complemented by 2-D finite element method simulations, where the fringing field effect on a plane capacitor is studied and accounted for by an effective area that is well fitted by a sinusoidal function of the displacement of the proof mass. From analytical calculations, we prove that the electrostatic transducer force is related to the voltage output and cannot be approximated by viscous damping or a Coulomb force as reported previously. The coupled model with two simultaneous differential equations is numerically solved for the voltage output and transduction force with given parameters. The model was verified both by practical measurements from our own fabricated device and results from a reference. An optimization study is carried out using this model to achieve the maximum output power by tuning the allowable movement (X M) of the proof mass. Finally, the effect of a standard power-conditioning circuit is investigated for both continuous and burst power supply applications. [2013-0207]
    Journal of Microelectromechanical Systems 01/2014; · 2.13 Impact Factor
  • Journal of Microelectromechanical Systems 01/2014; 23(4):842-854. · 2.13 Impact Factor
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    ABSTRACT: While silicon is an anisotropic material it is often in literature treated as an isotropic material when it comes to plate calculations. This leads to considerable errors in the calculated deflection. To overcome this problem, we present an in-depth analysis of the bending behavior of thin crystalline plates. An analysis of the compliance tensor for the 32 different crystal classes shows, that for thin plates, only 5 different types of plates exist. An anisotropic plate equation valid for crystalline thin plates is derived and solved for circular, elliptic, rectangular and square plates using both exact analytical expressions and approximate expressions calculated by the Galerkin method. The results are applied to plates made on silicon (001), (011) and (111) substrates, respectively, and analytical equations for the deflection, strain energy and resonance frequency of such plates are presented. These expressions are in excellent agreement with anisotropic finite element calculations. The calculated deflection differs less than 0.1%, for both circular and rectangular plates, compared to finite element calculations. The results are presented as ready-to-use facilitating accurate analytical models involving crystalline plates, such as those often found in the field of micro electro mechanical systems. The effect of elastic boundary conditions is taken into account by using an effective radius of the plate.
    Sensors and Actuators A Physical 01/2014; · 1.84 Impact Factor
  • Kasper Reck, Ole Hansen
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    ABSTRACT: Photon-enhanced thermionic emission (PETE) cells in which direct photon energy as well as thermal energy can be harvested have recently been suggested as a new candidate for high efficiency solar cells. Here, we present an analytic thermodynamical model for evaluation of the efficiency of PETE solar cells including an analysis of the entropy production due to thermionic emission of general validity. The model is applied to find the maximum efficiency of a PETE cell for given cathode and anode work functions and temperatures.
    Applied Physics Letters 12/2013; 104(2). · 3.52 Impact Factor
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    ABSTRACT: Monitoring gene expression is an important tool for elucidating mechanisms of cellular function. In order to monitor gene expression during nerve cell development, molecular beacon (MB) probes targeting markers representing different stages of neuronal differentiation were designed and synthesized as 2'-O-methyl RNA backbone oligonucleotides. MBs were transfected into human mesencephalic cells (LUHMES) using streptolysin-O-based membrane permeabilization. Mathematical modeling, simulations and experiments indicated that MB concentration was equal to the MB in the transfection medium after 10 min transfection. The cells will then each contain about 60,000 MBs. Gene expression was detected at different time points using fluorescence microscopy. Nestin and NeuN mRNA were expressed in approximately 35% of the LUHMES cells grown in growth medium, and in 80-90% of cells after differentiation. MAP2 and tyrosine hydroxylase mRNAs were expressed 2 and 3 days post induction of differentiation, respectively. Oct 4 was not detected with MB in these cells and signal was not increased over time suggesting that MB are generally stable inside the cells. The gene expression changes measured using MBs were confirmed using qRT-PCR. These results suggest that MBs are simple to use sensors inside living cell, and particularly useful for studying dynamic gene expression in heterogeneous cell populations.
    Frontiers in Cellular Neuroscience 12/2013; 7:266. · 4.47 Impact Factor
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    ABSTRACT: The present work demonstrates that tuning the donor density of protective TiO2 layers on a photocathode has dramatic consequences for electronic conduction through TiO2 with implications for the stabilization of oxidation-sensitive catalysts on the surface. Vacuum annealing at 400 °C for 1 hour of atomic layer deposited TiO2 increased the donor density from an as-deposited value of 1.3 × 1019 cm−3 to 2.2 × 1020 cm−3 following the annealing step. Using an Fe(II)/Fe(III) redox couple it was shown that the lower dopant density only allows electron transfer through TiO2 under conditions of weak band bending. However it was shown that increasing the dopant density to 2.2 × 1020 cm−3 allows tunneling through the surface region of TiO2 to occur at significant band bending. An important implication of this result is that the less doped material is unsuitable for electron transfer across the TiO2/electrolyte interface if the potential is significantly more anodic than the TiO2 conduction band due to moderate to large band bending. This means that the lesser doped TiO2 can be used to prevent the inadvertent oxidation of sensitive species on the surface (e.g. H2 evolution catalysts) as long as the redox potential of the material is significantly more anodic than the TiO2 conduction band. Conversely, for situations where an oxidative process on the surface is desired, highly doped TiO2 may be used to enable current flow via tunneling.
    J. Mater. Chem. A. 11/2013; 1(47).
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    Andrea Crovetto, Fei Wang, Ole Hansen
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    ABSTRACT: This paper presents a MEMS energy harvesting device which is able to generate power from two perpendicular ambient vibration directions. A CYTOP polymer is used both as the electret material for electrostatic transduction and as a bonding interface for low-temperature wafer bonding. The device consists of a four-wafer stack, and the fabrication process for each wafer layer is described in detail. All the processes are performed at wafer scale, so that overall 44 devices can be fabricated simultaneously on one 4-inch wafer. The effect of fabrication issues on the resonant frequency of the device is also discussed. With a final chip size of about 1 cm2, a power output of 32.5 nW is successfully harvested with an external load of 17 MΩ, when a harmonic vibration source with an RMS acceleration amplitude of 0.03 g (˜0.3 m s-2) and a resonant frequency of 179 Hz is applied. These results can be improved in an optimized design.
    Journal of Micromechanics and Microengineering 11/2013; 23(11):4010-. · 1.79 Impact Factor
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    ABSTRACT: A new MoS2 protected n(+)p-junction Si photocathode for the renewable H2 evolution is presented here. MoS2 acts as both a protective and an electrocatalytic layer, allowing H2 evolution at 0 V vs. RHE for more than 5 days. Using a MoSx surface layer decreases the overpotential for H2 evolution by 200 mV.
    Physical Chemistry Chemical Physics 10/2013; · 4.20 Impact Factor
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    ABSTRACT: Micromechanical photothermal infrared spectroscopy is a promising technique, where absorption related heating is detected by frequency detuning of microstring resonators. We present photothermal infrared spectroscopy with mechanical string resonators providing rapid identification of femtogram-scale airborne samples. Airborne sample material is directly collected on the microstring with an efficient non-diffusion limited sampling method based on inertial impaction. Resonance frequency shifts, proportional to the absorbed heat in the microstring, are recorded as monochromatic IR light is scanned over the mid-infrared range. As a proof-of-concept we sample and analyze polyvinylpyrrolidone (PVP) and the measured photothermal spectrum matches the reference FTIR spectrum. We further identify the organic surface coating of airborne TiO2 nanoparticles with a total mass of 4 pg. With an estimated detection limit of 44 fg, the presented sensor demonstrates a new paradigm in ultrasensitive vibrational spectroscopy for identification of airborne species.
    Analytical Chemistry 10/2013; · 5.82 Impact Factor
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    ABSTRACT: A new Pyrex-based μ-reactor for photocatalytic and optical characterization experiments is presented. The reactor chamber and gas channels are microfabricated in a thin poly-silicon coated Pyrex chip that is sealed with a Pyrex lid by anodic bonding. The device is transparent to light in the UV-vis-near infrared range of wavelengths (photon energies between ∼0.4 and ∼4.1 eV). The absorbance of a photocatalytic film obtained with a light transmission measurement during a photocatalytic reaction is presented as a proof of concept of a photocatalytic reactivity measurement combined with in situ optical characterization. Diffuse reflectance measurements of highly scattering photocatalytic nanopowders in a sealed Pyrex μ-reactor are also possible using an integrating sphere as shown in this work. These experiments prove that a photocatalyst can be characterized with optical techniques after a photocatalytic reaction without removing the material from the reactor. The catalyst deposited in the cylindrical reactor chamber can be illuminated from both top and bottom sides and an example of application of top and bottom illumination is presented.
    The Review of scientific instruments 10/2013; 84(10):103910. · 1.52 Impact Factor
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    ABSTRACT: The combination of graphene with noble-metal nanostructures is currently being explored for strong light-graphene interaction enhanced by plasmons. We introduce a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances in the visible range. Strong coupling of graphene to the plasmon modes of the nanovoid arrays results in significant frequency shifts of the underlying plasmon resonances, enabling more than 30% absolute light absorption in a single layer of graphene and up to 700-fold enhancement of the Raman response of the graphene. These new perspectives enable us to verify the presence of graphene on gold-void arrays and the enhancement even allows us to accurately quantify the number of layers. Experimental observations are further supported by numerical simulations and perturbation-theory analysis. The graphene gold-void platform is beneficial for sensing of molecules and placing R6G dye molecules on top of the graphene, we observe a strong enhancement of the R6G Raman fingerprints. These results pave the way toward advanced substrates for surface-enhanced Raman scattering (SERS) with potential for unambiguous single-molecule detection on the atomically well-defined layer of graphene.
    Nano Letters 09/2013; · 13.03 Impact Factor
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    ABSTRACT: We theoretically investigate the propagation of graphene plasmon polaritons in graphene nanoribbon waveguides and experimentally observe the excitation of the graphene plasmon polaritons in a continuous graphene monolayer. We show that graphene nanoribbon bends do not induce any additional loss and nanofocusing occurs in a tapered graphene nanoriboon, and we experimentally demonstrate the excitation of graphene plasmon polaritonss in a continuous graphene monolayer assisted by a two-dimensional subwavelength silicon grating.
    Proc SPIE 09/2013;

Publication Stats

1k Citations
438.22 Total Impact Points

Institutions

  • 1995–2014
    • Technical University of Denmark
      • • Department of Micro- and Nanotechnology
      • • Department of Physics
      København, Capital Region, Denmark
  • 2011
    • FEI Company
      Hillsboro, Oregon, United States
    • Pennsylvania State University
      • Department of Electrical Engineering
      University Park, MD, United States
  • 2007
    • Koc University
      • College of Engineering
      İstanbul, Istanbul, Turkey
  • 2005
    • New York State
      New York City, New York, United States
  • 2002
    • Peking University
      Peping, Beijing, China