Ole Hansen

Technical University of Denmark, Copenhagen, Capital Region, Denmark

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Publications (183)369.04 Total impact

  • 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; · 3.83 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: 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; · 5.70 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;
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    ABSTRACT: High-quality mass spectrometry data of the oscillatory behavior of CO oxidation on SiO(2) supported Pt-nanoparticles at atmospheric pressure have been acquired as a function of pressure, coverage, gas composition and nanoparticle size. The oscillations are self-sustained for several days at constant temperature, pressure and CO/O(2) ratio. The frequency of the oscillations is very well defined and increases over time. The oscillation frequency is furthermore strongly temperature dependent with increasing temperature resulting in increasing frequency. A plausible mechanism for the oscillations is proposed based on an oxidation-reduction cycle of the nanoparticles which change the rate of CO oxidation on the particles.
    Physical Chemistry Chemical Physics 01/2013; · 3.83 Impact Factor
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    ABSTRACT: We experimentally demonstrate graphene-plasmon polariton excitation in a continuous graphene monolayer resting on a two-dimensional subwavelength silicon grating. The subwavelength silicon grating is fabricated by a nanosphere lithography technique with a self-assembled nanosphere array as a template. Measured transmission spectra illustrate the excitation of graphene-plasmon polaritons, which is further supported by numerical simulations and theoretical prediction of plasmonband diagrams. Our grating-assisted coupling to graphene-plasmon polaritons forms an important platform for graphene-based opto-electronics applications.
    Applied Physics Letters 01/2013; 102(13). · 3.79 Impact Factor
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    ABSTRACT: Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor-electrolyte interface. In this work, we show that a sputtered 100 nm TiO2 layer on top of a thin Ti metal layer may be used to protect an n+p Si photocathode during photocatalytic H2 evolution. Although TiO2 is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H2 evolution conditions. This behavior is due to the fortunate alignment of the TiO2 conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H2 evolution onset of 520 mV vs. NHE and a Tafel slope of 30 mV when illuminated by the red part (lamda > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H2 evolution) was also significantly enhanced by the anti-reflective properties of the TiO2 layer. It was shown that with proper annealing conditions these electrodes could run 24 hours without significant degradation. An Fe2+/Fe3+ redox couple was used to help elucidate details of the band diagram.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
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    ABSTRACT: We demonstrate how resonant excitation of a microelectromechanical system can be used to increase the tuning range of a vertical-cavity surface-emitting laser twofold by enabling both blue- and red-shifting of the wavelength. In this way, a short-cavity design enabling wide tuning range can be realized. A high-index-contrast subwavelength grating vertical-cavity surface-emitting laser with a monolithically integrated antireflection coating is presented. By incorporating an antireflection coating into the air cavity, higher tuning efficiency can be achieved at low threshold current. The first result shows 24-nm continuous resonant tuning range around an emission wavelength of 1060 nm with 0.9 mW output power.
    IEEE Journal of Selected Topics in Quantum Electronics 01/2013; 19(4):1702306-1702306. · 4.08 Impact Factor
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    ABSTRACT: We derive exact, analytic expressions for the sensitivity of resistive and Hall measurements to local inhomogeneities in a specimen's material properties in the combined linear limit of a weak perturbation over an infinitesimal area in a small magnetic field. We apply these expressions both to four-point probe measurements on an infinite plane and to symmetric, circular van der Pauw discs, obtaining functions consistent with published results. These new expressions speed up calculation of the sensitivity for a specimen of arbitrary shape to little more than the solution of two Laplace equation boundary-value problems of the order of N3 calculations, rather than N2 problems of total order N5, and in a few cases produces an analytic expression for the sensitivity. These functions provide an intuitive, visual explanation of how, for example, measurements can predict the wrong carrier type in n-type ZnO.
    Journal of Applied Physics 01/2013; 114(16):163710-163710-10. · 2.21 Impact Factor
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    ABSTRACT: The authors investigated the use of InAlP as a sacrificial layer lattice-matched to GaAs when diluted hydrochloric acid is used for sacrificial etching. They show that InAlP can be used to fabricate submicrometer air gaps in micro-opto-electro-mechanical systems and that a selectivity toward GaAs larger than 500 is achieved. This selectivity enables fabrication control of the nanometer-size structures required in photonic crystal and high-index contrast subwavelength grating structures. The crystallographic dependence of the lateral etch rate in InAlP is shown to be symmetric around the directions where an etch rate of 0.5 μm/min is obtained at 22 °C in HCl:2H2O. Since the etch rate in the directions exceeds by ten times that of the directions, InAlP may be used in sacrificial release of high-aspect ratio structures. Free-hanging structures with length to air-gap aspect ratios above 600 are demonstrated by use of critical point drying following the sacrificial etch.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2013; 31(1):1209-.
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    ABSTRACT: The present work demonstrates that tuning the donor density of protective TiO<sub>2</sub> layers on a photocathode has dramatic consequences for electronic conduction through TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 th
    RSC Advances 01/2013; 1(47):15089-15094. · 2.56 Impact Factor
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    ABSTRACT: The semiconducting materials used for photoelectrochemical (PEC) water splitting must withstand the corrosive nature of the aqueous electrolyte over long time scales in order to be a viable option for large scale solar energy conversion. Here we demonstrate that atomic layer deposited titanium dioxide (TiO2) overlayers on silicon-based photocathodes generate extremely stable electrodes. These electrodes can produce an onset potential of +0.510 V vs. RHE and a hydrogen evolution saturation current of 22 mA cm−2 using the red part of the AM1.5 solar spectrum (λ > 635 nm, 38.6 mW cm−2). A PEC chronoamperometry experiment was carried out for 2 weeks under constant illumination at +0.300 V vs. RHE with negligible degradation (<5%). Further testing showed slight degradation, but the re-addition of catalyst recovered the activity. These results show that properly processed TiO2 overlayers may have the potential to be stable for the long time frames that will be necessary for commercial devices.
    RSC Advances 01/2013; 3(48):25902-25907. · 2.56 Impact Factor
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    ABSTRACT: We demonstrate a light-extraction approach using a whispering gallery resonators array. The wavelength-scale resonant dielectric nanospheres support whispering gallery modes, which can be coupled with the confined waveguide modes inside the bulk material, thus dramatically improving light extraction. Broadband light-extraction enhancement across the entire visible spectral range is achieved by exciting three low-order and low-quality-factor resonances. As an example, the broadband extraction enhancement of about 50% is obtained for the emission of fluorescent SiC at all the tested angles. The experimental results are supported by numerical simulations. Our light-extraction strategy could enable the manufacturing of high-throughput, nondestructive, and affordable optical coating in a variety of optical devices.
    Applied Physics Letters 12/2012; 101(24). · 3.79 Impact Factor
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    ABSTRACT: The accurate determination of the sheet resistance and carrier depth profile, i.e. active dopant profile, of shallow junction isolated structures involving new high mobility materials, such as germanium, is a crucial topic for future CMOS development. In this work, we discuss the capabilities of new concepts based on micro machined, closely spaced contact probes (10 μm pitch). When using four probes to perform sheet resistance measurements, a quantitative carrier profile extraction based on the evolution of the sheet resistance versus depth along a beveled surface is obtained. Considering the use of only two probes, a spreading resistance like setup is obtained with small spacing and drastically reduced electrical contact radii (~10 nm) leading to a substantial reduction of the correction factors which are normally required for converting spreading resistance profiles. We demonstrate the properties of both approaches on Al+ implants in germanium with different anneal treatments.
    11/2012;
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    ABSTRACT: We present a new, preparation-free method for measuring the leakage current density on ultra-shallow junctions. The junction leakage is found by making a series of four-point sheet resistance measurements on blanket wafers with variable electrode spacings. The leakage current density is calculated using a fit of the measured four-point resistances to an analytical two-sheet model. The validity of the approximation involved in the two-sheet model is verified by a comparison to finite element model calculations.
    11/2012;
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    ABSTRACT: We propose a polymer electret based energy harvesting device in order to extract energy from vibration sources with low acceleration. With MEMS technology, a silicon structure is fabricated which can resonate in 2D directions. Thanks to the excellent mechanical properties of the silicon material, the proof mass could be successfully driven by an external vibrations with acceleration as low as 0.014 g (~0.14 m/s 2). A root mean square (RMS) power output of 1.17 µW under 0.014 g RMS acceleration at 75 Hz is measured when an optimal load of 20.3 MΩ is applied. The frequency response of the device is also studied and a high Q factor of 250 is achieved.
    26th European Conference on Solid-State Transducers, EUROSENSOR 2012; 09/2012
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    ABSTRACT: We demonstrate a combination of micro four-point probe (M4PP) and non-contact terahertz time-domain spectroscopy (THz-TDS) measurements for centimeter scale quantitative mapping of the sheet conductance of large area chemical vapor deposited graphene films. Dual configuration M4PP measurements, demonstrated on graphene for the first time, provide valuable statistical insight into the influence of microscale defects on the conductance, while THz-TDS has potential as a fast, non-contact metrology method for mapping of the spatially averaged nanoscopic conductance on wafer-scale graphene with scan times of less than a minute for a 4-in. wafer. The combination of M4PP and THz-TDS conductance measurements, supported by micro Raman spectroscopy and optical imaging, reveals that the film is electrically continuous on the nanoscopic scale with microscopic defects likely originating from the transfer process, dominating the microscale conductance of the investigated graphene film.
    Nano Letters 09/2012; 12(10):5074-81. · 13.03 Impact Factor
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    ABSTRACT: A low-cost substitute: A titanium protection layer on silicon made it possible to use silicon under highly oxidizing conditions without oxidation of the silicon. Molybdenum sulfide was electrodeposited on the Ti-protected n(+)p-silicon electrode. This electrode was applied as a photocathode for water splitting and showed a greatly enhanced efficiency.
    Angewandte Chemie International Edition 08/2012; 51(36):9128-31. · 13.73 Impact Factor

Publication Stats

966 Citations
369.04 Total Impact Points

Institutions

  • 1995–2013
    • Technical University of Denmark
      • • Department of Physics
      • • Department of Micro- and Nanotechnology
      Copenhagen, 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