Hasselt University, Hasselt, Flemish, Belgium

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Publications (258)477.8 Total impact

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    ABSTRACT: A detailed investigation of electron emission from a set of chemical vapour deposited (CVD) diamond films is reported using high-resolution PeakForce-controlled tunnelling atomic force microscopy (PF-TUNA). Electron field emission originates preferentially from the grain boundaries in low-conductivity polycrystalline diamond samples, and not from the top of features or sharp edges. Samples with smaller grains and more grain boundaries, such as nanocrystalline diamond, produce a higher emission current over a more uniform area than diamond samples with larger grain size. Light doping with N, B or P increases the grain conductivity, with the result that the emitting grain-boundary sites become broader as the emission begins to creep up the grain sidewalls. For heavy B doping, where the grains are now more conducting than the grain boundaries, emission comes from both the grain boundaries and the grains almost equally. Lightly P-doped diamond samples show emission from step-edges on the (1 1 1) surfaces. Emission intensity was time dependent, with the measured current dropping to ∼10% of its initial value ∼30 h after removal from the CVD chamber. This decrease is ascribed to the build-up of adsorbates on the surface along with an increase in the surface conductivity due to surface transfer doping.
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    ABSTRACT: Surface pretreatment by gas discharge plasmas of N2, O2 and CF4 is studied for enhanced diamond nucleation on Cr surfaces. The seeding density following the interaction of water-dispersed nanodiamonds (NDs) and the Cr surface is enhanced due to chemical modification of a surface. The surface that is untreated or pretreated with N2 plasma possesses a suppressed electrostatic attraction of NDs, while the pretreatment with O2 or CF4 plasmas render a strong electrostatic attraction and high seeding density. Finally, by this method thin nanocrystalline diamond films are achieved on Cr surfaces after O2 and CF4 plasma pretreatments.
    Chemical Physics Letters 11/2015; 640:50-54. DOI:10.1016/j.cplett.2015.10.002 · 1.90 Impact Factor
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    ABSTRACT: Diamond foams composed of hollow spheres of polycrystalline boron-doped diamond are chemically modified with two donor-acceptor type molecular dyes, BT-Rho and CPDT-Fur, and tested as electrode materials for p-type dye-sensitized solar cells with an aqueous electrolyte solution containing methyl viologen as a redox mediator. Reference experiments with flat polycrystalline diamond electrodes evidence full blocking of the methyl viologen redox reaction by these dyes, whereas only partial blocking is observed for the diamond foams. This is ascribed to sp2-carbon impurities in the foam, viz. trans-polyacetylene and graphite-like carbon. Cathodic photocurrents under solar light illumination are about 3 times larger on foam electrodes compared to flat diamond. Long-term (1-2 days) illumination of the sensitized foam electrodes with chopped light at 1 sun intensity causes an increase of the cathodic photocurrent density to ca. 15-22 μA cm-2. These photocurrent densities represent the largest values reported so far for dye-sensitized diamond electrodes. The photoelectrochemical activation of the sensitized diamond electrodes is accompanied with characteristic changes of the dark voltammogram of the MV2+/MV+ redox couple and with gradual changes of the IPCE spectra. This journal is
    RSC Advances 09/2015; 5(99):81069-81077. DOI:10.1039/c5ra12413a · 3.84 Impact Factor
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    ABSTRACT: Results are presented on the thermal and elastic properties of a thin, 1.5 μm, nanocrystalline diamond coating (NCD), deposited on a silicon substrate by microwave plasma enhanced chemical vapor deposition. A combination of two all-optical measurement techniques, impulsive stimulated thermal scattering and grating induced laser beam deflection, was employed to launch and detect surface acoustic waves (SAWs). The relation between the dispersive propagation velocity of SAWs to the coating-substrate geometry is exploited to determine the elastic properties of the NCD coating. The elastic properties are found to be consistent with literature values. The thermal properties of the coating were determined by monitoring the thermal diffusion induced washing away of the laser induced transient surface temperature grating. The transient thermal grating signals were fitted by the low-frequency limit of a thermoelastic model for a multilayer configuration. Similar to the dispersion of the surface acoustic wave velocity, the characteristic time of the thermal diffusion driven grating decay evolves from a coating-dominated value at short grating spacings towards a substrate-dominated value at grating spacings well exceeding the coating thickness. The grating spacing dependence of the corresponding effective thermal diffusivity was experimentally determined and fitted, leading to a value for the thermal diffusivity of the NCD coating αNCD = 8.4- 0.1+ 2.7 mm2·s- 1, which is an order of magnitude lower than that of the silicon substrate. The low value of the thermal diffusivity is interpreted with a simple touching model.
    Thin Solid Films 09/2015; 590:284-292. DOI:10.1016/j.tsf.2015.08.007 · 1.76 Impact Factor
  • Ken Haenen · Jose Antonio Garrido · John Robertson ·

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    ABSTRACT: In this study some affinity and dielectric properties of molecularly imprinted (MIP) conducting polymer – polypyrrole (Ppy) based thin films were evaluated. Films of polypyrrole molecularly imprinted with theophylline (MIP-Ppy) and non-imprinted polypyrrole (NIP-Ppy) were formed on boron doped silicon (Si) substrates in order to evaluate the efficiency of Ppy to bind theophylline. The substrates were modified with boron-doped oxygen terminated nanocrystalline diamond (B:NCD:O) The dielectric properties of B:NCD:O/Ppy-based multi-layered structures were analyzed using spectroscopic ellipsometry and spectrophotometric techniques. Electrochemical impedance spectroscopy was applied for the investigation of kinetics of theophylline interaction with MIP-Ppy and NIP-Ppy. The sensitivity of molecularly imprinted and non-imprinted polymer films was analyzed by injection of different theophylline concentrations. Assuming that Ppy film electrical capacitance change is a result of Ppy dielectric constant change induced by absorbed theophylline molecules, the electrical capacitance change (ΔC) kinetics at different concentrations of theophylline was analyzed using first pseudo order kinetic equation. The dissociation equilibrium constant KD of MIP-Ppy/theophylline complex at room temperature was calculated as 1.7·10-8 M, and Gibbs free energy change (ΔG) of MIP-Ppy/theophylline complex formation was calculated as-43.5 kJ/mol. It was concluded that molecularly imprinted polypyrrole thin film could be used for the detection of theophylline.
    Synthetic Metals 08/2015; 209:206–211. DOI:10.1016/j.synthmet.2015.07.021 · 2.25 Impact Factor
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    ABSTRACT: Photoactive reaction centers (RCs) are protein complexes in bacteria able to convert sunlight into other forms of energy with a high quantum yield. The photo-stimulation of immobilized RCs on inorganic electrodes result in the generation of photocurrent that is of interest for bio-solar cell applications. This paper reports on the use of novel electrodes based on functional conductive nanocrystalline diamond onto which bacterial RCs are immobilized. A 3-dimensional conductive polymer scaffold grafted to the diamond electrodes enables an efficient entrapment of photoreactive proteins. The electron transfer in these functional diamond electrodes is optimized through the use of a ferrocene-based electron mediator, which provides significant advantages such as a rapid electron transfer as well as high generated photocurrent. A detailed discussion of the generated photocurrent as a function of time, bias voltage and mediators in solution unveils the mechanisms limiting the electron transfer in these functional electrodes. This work featuring diamond-based electrodes in bio-photovoltaics, offers general guidelines that can serve to improve the performance of similar devices based on different materials and geometries.
    ACS Applied Materials & Interfaces 04/2015; 7(15). DOI:10.1021/acsami.5b00711 · 6.72 Impact Factor
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    ABSTRACT: Lead phthalocyanine (PbPc) thin films of 5 and 50 nm have been deposited on hydrogen and oxygen terminated single crystal diamond (SCD) using organic molecular beam deposition. Atomic force microscopy and X-ray diffraction (XRD) studies showed that PbPc grown on the hydrogen terminated SCD forms layers with a high degree of crystallinity, dominated by the monoclinic (320) orientation parallel to the diamond surface. The oxygen terminated diamond led to a randomly oriented PbPc film. Absorption and photocurrent measurements indicated the presence of both polymorphs of PbPc, however, the ratio differed depending on the termination of the SCD. Finally, polarized Raman spectroscopy was used to determine the orientation of the molecules of the thin film. The results confirmed the random orientation on the O-terminated diamond. On SCD:H, the PbPc molecules are lying down in accordance with the XRD results.
    Physical Chemistry Chemical Physics 03/2015; 17(15). DOI:10.1039/c5cp00174a · 4.49 Impact Factor
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    ABSTRACT: Nanocrystalline diamond (NCD) is a promising material for electronic and mechanical micro- and nanodevices. Here we introduce a versatile pick-up and drop technique that makes it possible to investigate the electrical, optical and mechanical properties of as-grown NCD films. Using this technique, NCD nanosheets, as thin as 55 nm, can be picked-up from a growth substrate and positioned on another substrate. As a proof of concept, electronic devices and mechanical resonators are fabricated and their properties are characterized. In addition, the versatility of the method is further explored by transferring NCD nanosheets onto an optical fibre, which allows measuring its optical absorption. Finally, we show that NCD nanosheets can also be transferred onto 2D crystals, such as MoS2, to fabricate heterostructures. Pick-up and drop transfer enables the fabrication of a variety of NCD-based devices without requiring lithography or wet processing.
    Nanotechnology 03/2015; 26(12). DOI:10.1088/0957-4484/26/12/125706 · 3.82 Impact Factor
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    ABSTRACT: In this study we show an optical biosensor concept, based on elastic light scattering from sapphire microspheres. Transmitted and elastic scattering intensity of the microspheres (radius 500 μm, refractive index 1.77) on an optical fiber half coupler is analyzed at 1510 nm. The 0.43 nm angular mode spacing of the resonances is comparable to the angular mode spacing value estimated using the optical size of the microsphere. The spectral linewidths of the resonances are in the order of 0.01 nm, which corresponds to quality factors of approximately 105. A polydopamine layer is used as a functionalizing agent on sapphire microspherical resonators in view of biosensor implementation. The varying layer thickness on the microsphere is determined as a function of the resonance wavelength shift. It is shown that polymer functionalization has a minor effect on the quality factor. This is a promising step toward the development of an optical biosensor.
    Sensors and Actuators A Physical 02/2015; 222. DOI:10.1016/j.sna.2014.11.024 · 1.90 Impact Factor
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    ABSTRACT: Boron doped diamond layers have been grown on (110) single crystal diamond substrates with B/C ratios up to 20 ppm in the gas phase. The surface of the diamond layers observed by scanning electron microscopy consists of (100) and (113) micro-facets. Fourier Transform Photocurrent Spectroscopy indicates substitutional boron incorporation. Electrical properties were measured using Hall effect from 150 to 1000 K. Secondary ion mass spectrometry analyses are consistent with the high incorporation of boron determined by electrical measurements. A maximum mobility of 528 cm2.V- 1.s- 1 was measured at room temperature for a charge carrier concentration of 1.1 1013 cm- 3. Finally, properties of boron doped (110) diamond layers are compared with layers on (100) and (111) orientated substrates.
    Diamond and Related Materials 01/2015; 53. DOI:10.1016/j.diamond.2015.01.006 · 1.92 Impact Factor
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    ABSTRACT: Graphene has potential for applications in solar cells. We show that the short circuit current density of P3HT (Poly(3-hexylthiophene-2,5-diyl):PCBM((6,6)-Phenyl C61 butyric acid methyl ester) solar cells is enhanced by 10% upon the addition of graphene, with a 15% increase in the photon to electric conversion efficiency. We discuss the performance enhancement by studying the crystallization of P3HT, as well as the electrical transport properties. We show that graphene improves the balance between electron and hole mobilities with respect to a standard P3HT:PCBM solar cell. (C) 2014 AIP Publishing LLC.
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    ABSTRACT: In this study development of impedimetric sensor based on oxygen terminated boron-doped nanocrystalline diamond (B:NCD:O) modified with theophylline imprinted polypyrrole is described. Hydrogen peroxide induced chemical formation of polypyrrole molecularly imprinted by theophylline was applied for the modification of conducting silicon substrate covered by B:NCD:O film. Non-imprinted polypyrrole layer was formed on similar substrate in order to prove efficiency of imprinted polypyrrole. Electrochemical impedance spectroscopy was applied for the evaluation of analyte-induced changes in electrochemical capacitance/resistance. The impact of polymerization duration on the capacitance of impedimetric sensor was estimated. A different impedance behavior was observed at different ratio of polymerized monomer and template molecule in the polymerization media. The influence of ethanol as additive to polymerization media on registered changes in capacitance/resistance was evaluated. Degradation of sensor stored in buffer solution was evaluated.
    Electrochimica Acta 12/2014; 130:361-367. DOI:10.1016/j.electacta.2014.03.035 · 4.50 Impact Factor
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    ABSTRACT: Donor-acceptor type light-harvesting molecular wires are covalently attached to a boron-doped diamond surface via a combination of diazonium electrografting and Suzuki cross-coupling. For the Suzuki reaction, various catalytic systems are compared with respect to their imposed surface coverage. Combining 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (SPhos) and Pd(0), the diamond coverage improves considerably (by 98%) as compared to the standard tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) catalyst. As the energy levels between the molecular chromophores and the diamond film align well, the sophisticated functionalized diamond surfaces present a first step towards the development of fully carbon-based devices for light to electricity conversion.
    RSC Advances 10/2014; 4(92). DOI:10.1039/C4RA90027H · 3.84 Impact Factor
  • Bohuslav Rezek · Miloš Nesládek · Ken Haenen ·

    Physica Status Solidi (A) Applications and Materials 10/2014; 211(10). DOI:10.1002/pssa.201470267 · 1.62 Impact Factor
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    ABSTRACT: A microcavity-based deoxyribonucleic acid (DNA) optical biosensor is demonstrated for the first time using synthetic sapphire for the optical cavity. Transmitted and elastic scattering intensity at 1510 nm are analyzed from a sapphire microsphere (radius 500 μm, refractive index 1.77) on an optical fiber half coupler. The 0.43 nm angular mode spacing of the resonances correlates well with the optical size of the sapphire sphere. Probe DNA consisting of a 36-mer fragment was covalently immobilized on a sapphire microsphere and hybridized with a 29-mer target DNA. Whispering gallery modes (WGMs) were monitored before the sapphire was functionalized with DNA and after it was functionalized with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). The shift in WGMs from the surface modification with DNA was measured and correlated well with the estimated thickness of the add-on DNA layer. It is shown that ssDNA is more uniformly oriented on the sapphire surface than dsDNA. In addition, it is shown that functionalization of the sapphire spherical surface with DNA does not affect the quality factor (Q ≈ 10 4) of the sapphire microspheres. The use of sapphire is especially interesting because this material is chemically resilient, biocompatible, and widely used for medical implants. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JBO.19.9.097006]
    Journal of Biomedical Optics 09/2014; 19(9):097006. DOI:10.1117/1.JBO.19.9.097006 · 2.86 Impact Factor
  • S.D. Janssens · Sien DRIJKONINGEN · Ken HAENEN ·
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    ABSTRACT: Surface conductivity in hydrogen-terminated single crystal diamond is an intriguing phenomenon for fundamental reasons as well as for application driven research. Surface conductivity is also observed in hydrogen-terminated nanocrystalline diamond although the electronic transport mechanisms remain unclear. In this work, the piezoresistive properties of intrinsic surface conductive nanocrystalline diamond are investigated. A gauge factor of 35 is calculated from bulging a diamond membrane of 350 nm thick, with a diameter of 656 μm and a sheet resistance of 1.45 MΩ/sq. The large piezoresistive effect is reasoned to originate directly from strain-induced changes in the resistivity of the grain boundaries. Additionally, we ascribe a small time-dependent fraction of the piezoresistive effect to charge trapping of charge carriers at grain boundaries. In conclusion, time-dependent piezoresistive effect measurements act as a tool for deeper understanding the complex electronic transport mechanisms induced by grain boundaries in a polycrystalline material or nanocomposite.
    Applied Physics Letters 09/2014; 105(10):101601-101601-4. DOI:10.1063/1.4895458 · 3.30 Impact Factor
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    ABSTRACT: In this article, we report on a label-free real-time method based on heat transfer resistivity for thermal monitoring of DNA denaturation and its potential to quantify DNA fragments with a specific sequence of interest. Probe DNA, consisting of a 36-mer fragment was covalently immobilized on a nanocrystalline diamond surface, created by chemical vapor deposition on a silicon substrate. Various concentrations of full matched 29-mer target DNA fragments were hybridized with this probe DNA. We observed that the change in heat transfer resistance upon denaturation depends on the concentration of target DNA used during the hybridization, which allowed to determine the dose response curve. Therefore, these results illustrate the potential of this technique to quantify the concentration of a specific DNA fragment and to quantify the hybridization efficiency to its probe.
    Diamond and Related Materials 09/2014; 48. DOI:10.1016/j.diamond.2014.06.008 · 1.92 Impact Factor
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    ABSTRACT: Graphene has potential for applications in solar cells. We show that the short circuit current density of P3HT (Poly(3-hexylthiophene-2,5-diyl):PCBM((6,6)-Phenyl C61 butyric acid methyl ester) solar cells is enhanced by 10% upon the addition of graphene, with a 15% increase in the photon to electric conversion efficiency. We discuss the performance enhancement by studying the crystallization of P3HT, as well as the electrical transport properties. We show that graphene improves the balance between electron and hole mobilities with respect to a standard P3HT:PCBM solar cell.
    Applied Physics Letters 08/2014; 105(8):083306-083306-5. DOI:10.1063/1.4893777 · 3.30 Impact Factor
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    ABSTRACT: Nanocrystalline diamond (NCD) thin films were produced by chemical vapor deposition (CVD) and doped by the addition of phosphine to the gas mixture. The characterization of the films focused on probing the incorporation and distribution of the phosphorus (P) dopants. Electron microscopy evaluated the overall film morphology and revealed the interior structure of the nanosized grains. The homogeneous films with distinct diamond grains featured a notably low sp2:sp3-ratio as confirmed by Raman spectroscopy. High resolution spectroscopy methods demonstrated a homogeneous P-incorporation, both in-depth and in-plane. The P concentration in the films was determined to be in the order of 1019 cm–3 with a significant fraction integrated at substitutional donor sites. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
    physica status solidi (RRL) - Rapid Research Letters 08/2014; 9999(8-9999):n/a-n/a. DOI:10.1002/pssr.201409235 · 2.14 Impact Factor

Publication Stats

3k Citations
477.80 Total Impact Points


  • 2005-2015
    • Hasselt University
      • Institute for Materials Research (IMO)
      Hasselt, Flemish, Belgium
  • 2012
    • Friedrich-Schiller-University Jena
      • Department of Solid State Physics
      Jena, Thuringia, Germany
  • 2006-2009
    • Transnationale Universiteit Limburg
      Belgium, Wisconsin, United States
  • 2008
    • imec Belgium
      Louvain, Flemish, Belgium
  • 2002
    • University of Leuven
      Louvain, Flanders, Belgium