Publications Search Results

Now showing 1 - 10 of 188
  • Publication
    Giant Piezoelectricity of Deformed Aluminum Nitride Stabilized through Noble Gas Interstitials for Energy Efficient Resonators
    ( 2021)
    Fiedler, H.
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    Fuchs, F.
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    Leveneur, J.
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    Nancarrow, M.
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    Mitchell, D.R.G.
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    Schuster, J.
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    Kennedy, J.
    Aluminum nitride (AlN) is a material for a wide range of microwave-frequency electronics devices, because of its piezoelectric properties and high chemical stability. To improve the performance of AlN-based devices, such as acoustic wave filters and energy harvesters, an increased piezoelectric modulus is desirable. Here, an increase of the piezoelectric modulus d33 of this material is achieved by ion implantation of noble gases. For a fluence of 3 × 1016 at cm−2 Ar+, a 30% increase of d33 of AlN is obtained. The improvement is attributed to noble gas atoms implanted into interstitial sites of the wurtzite structure, causing a strong deformation of wurtzite AlN. Density functional theory calculations reveal the formation of deformed, metastable AlN with a 350% increase of the longitudinal piezoelectric coefficient. The ion implantation conditions to prepare AlN with a high piezoelectric coefficient are discussed and verified by X-ray diffraction, Raman spectroscopy, and scanning transmission electron microscopy. Heavier elements, larger fluences, and an implantation angle not aligned to the wurtzite crystal are preferred since those conditions generate tetrahedrally coordinated interstitials. In contrast, the opposite conditions lead to octahedrally coordinated interstitials prior to relaxation, which activates the silent B1high phonon vibration and results in a reduced piezoelectric coefficient.
  • Publication
    Electrical Characterization of Germanium Nanowires Using a Symmetric Hall Bar Configuration: Size and Shape Dependence
    ( 2021)
    Echresh, A.
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    Arora, H.
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    Fuchs, F.
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    Li, Z.
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    Hübner, R.
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    Prucnal, S.
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    Schuster, J.
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    Zahn, P.
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    Helm, M.
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    Zhou, S.
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    Erbe, A.
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    Rebohle, L.
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    Georgiev, Y.M.
    The fabrication of individual nanowire-based devices and their comprehensive electrical characterization remains a major challenge. Here, we present a symmetric Hall bar configuration for highly p-type germanium nanowires (GeNWs), fabricated by a top-down approach using electron beam lithography and inductively coupled plasma reactive ion etching. The configuration allows two equivalent measurement sets to check the homogeneity of GeNWs in terms of resistivity and the Hall coefficient. The highest Hall mobility and carrier concentration of GeNWs at 5 K were in the order of 100 cm2/(Vs) and 4×1019cm−3, respectively. With a decreasing nanowire width, the resistivity increases and the carrier concentration decreases, which is attributed to carrier scattering in the region near the surface. By comparing the measured data with simulations, one can conclude the existence of a depletion region, which decreases the effective cross-section of GeNWs. Moreover, the resistivity of thin GeNWs is strongly influenced by the cross-sectional shape.
  • Publication
    Biology-inspired microphysiological systems to advance patient benefit and animal welfare in drug development
    ( 2020)
    Marx, U.
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    Akabane, T.
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    Andersson, T.B.
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    Baker, E.
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    Beilmann, M.
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    Beken, S.
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    Brendler-Schwaab, S.
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    Cirit, M.
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    David, R.
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    Dehne, E.-M.
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    Durieux, I.
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    Ewart, L.
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    Fitzpatrick, S.C.
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    Frey, O.
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    Fuchs, F.
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    Griffith, L.G.
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    Hamilton, G.A.
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    Hartung, T.
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    Hoeng, J.
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    Hogberg, H.
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    Hughes, D.J.
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    Ingber, D.E.
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    Iskandar, A.
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    Kanamori, T.
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    Kojima, H.
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    Kuehnl, J.
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    Leist, M.
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    Li, B.
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    Loskill, P.
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    Mendrick, D.L.
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    Neumann, T.
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    Pallocca, G.
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    Rusyn, I.
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    Smirnova, L.
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    Steger-Hartmann, T.
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    Tagle, D.A.
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    Tonevitsky, A.
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    Tsyb, S.
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    Trapecar, M.
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    Water, B. van de
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    Eijnden-van Raaij, J. van den
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    Vulto, P.
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    Watanabe, K.
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    Wolf, A.
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    Zhou, X.
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    Roth, A.
    The first microfluidic microphysiological systems (MPS) entered the academic scene more than 15 years ago and were considered an enabling technology to human (patho)biology in vitro and, therefore, provide alternative approaches to laboratory animals in pharmaceutical drug development and academic research. Nowadays, the field generates more than a thousand scientific publications per year. Despite the MPS hype in academia and by platform providers, which says this technology is about to reshape the entire in vitro culture landscape in basic and applied research, MPS approaches have neither been widely adopted by the pharmaceutical industry yet nor reached regulated drug authorization processes at all. Here, 46 leading experts from all stakeholders - academia, MPS supplier industry, pharmaceutical and consumer products industries, and leading regulatory agencies - worldwide have analyzed existing challenges and hurdles along the MPS-based assay life cycle in a second workshop of this kind in June 2019. They identified that the level of qualification of MPS-based assays for a given context of use and a communication gap between stakeholders are the major challenges for industrial adoption by end-users. Finally, a regulatory acceptance dilemma exists against that background. This t4 report elaborates on these findings in detail and summarizes solutions how to overcome the roadblocks. It provides recommendations and a roadmap towards regulatory accepted MPS-based models and assays for patients' benefit and further laboratory animal reduction in drug development. Finally, experts highlighted the potential of MPS-based human disease models to feedback into laboratory animal replacement in basic life science research.
  • Publication
    Carbon Nanotubes for Mechanical Sensor Applications
    ( 2019)
    Wagner, C.
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    Blaudeck, T.
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    Meszmer, P.
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    Böttger, S.
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    Fuchs, F.
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    Hermann, S.
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    Schuster, J.
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    Wunderle, B.
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    Schulz, S.E.
    Herein, the evolution of carbon nanotubes (CNTs) as functional material in nano‐ and microelectromechanical systems (N/MEMS) is featured. Introducing material morphologies for the CNTs in a homologue series (single CNTs-bundles, fibers, yarns-networks and thin films), different concepts for mechanical sensors based on the intrinsic and extrinsic properties of the CNT materials are introduced (piezoresistive effect, strain‐induced band bending, charge tunneling). In a rigorous theoretical treatment, the limits of the achievable sensor performance (i.e., gauge factor) are derived and discussed in the context of applications. A careful literature survey shows that highest sensitivity is reached for devices exploiting the intrinsic transport properties of single CNTs. For reliability tests of such sensor systems made from nanomaterials and classical MEMS, the specimen‐centered approach (SCA) is introduced to give viable insights into the structure property relationships and failure modes of CNT mechanical sensors. CNT actuation occurs on the macro‐, micro‐, and nanoscales via atomic force microscopy, electrostatic gating, integration in N/MEMS systems, or through substrate bending.
  • Publication
    Electron transport through NiSi2-Si contacts and their role in reconfigurable field-effect transistors
    ( 2019)
    Fuchs, F.
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    Gemming, S.
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    Schuster, J.
    A model is presented which describes reconfigurable field-effect transistors (RFETs) with metal contacts, whose switching is controlled by manipulating the Schottky barriers at the contacts. The proposed modeling approach is able to bridge the gap between quantum effects on the atomic scale and the transistor switching. We apply the model to transistors with a silicon channel and NiSi2 contacts. All relevant crystal orientations are compared, focusing on the differences between electron and hole current, which can be as large as four orders of magnitude. Best symmetry is found for the orientation, which makes this orientation most advantageous for RFETs. The observed differences are analyzed in terms of the Schottky barrier height at the interface. Our study indicates that the precise orientation of the interface relative to a given transport direction, perpendicular or tilted, is an important technology parameter, which has been underestimated during the previous development of RFETs. Most of the conclusions regarding the studied metal-semiconductor interface are also valid for other device architectures.
  • Publication
    Radially resolved electronic structure and charge carrier transport in silicon nanowires
    ( 2019)
    Fuchs, F.
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    Gemming, S.
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    Schuster, J.
    The electronic structure of silicon nanowires is studied using density functional theory. A radially resolved density of states is discussed for different nanowire diameters and crystal orientations. This approach allows the investigation of spatially varying electronic properties in the radial direction and extends previous studies, which are usually driven by a one-dimensional band structure analysis. We demonstrate strong differences in the electronic structure between the surface and the center of the nanowire, indicating that the carrier transport will mainly take place in the center. For increasing diameters, the density of states in the center approaches the bulk density of states. We find that bulk properties, such as the indirect nature of the band gap, become significant at a nanowire diameter of approximately 5 nm and beyond. Finally, the spatial characteristic of the current is visualized in terms of transmission pathways on the atomic scale. Electron transport is found to be more localized in the nanowire center than the hole transport. It also depends on the crystal orientation of the wire. For the growing demand of silicon nanowires, for example in the field of sensors or field-effect transistors, multiple conclusions can be drawn from the present work, which we discuss towards the end of the publication.
  • Publication
    Feasible device architectures for ultrascaled CNTFETs
    ( 2018)
    Pacheco-Sanchez, A.
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    Fuchs, F.
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    Mothes, S.
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    Zienert, A.
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    Schuster, J.
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    Gemming, S.
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    Claus, M.
    Feasible device architectures for ultrascaled carbon nanotubes field-effect transistors (CNTFETs) are studied down to 5.9 nm using a multiscale simulation approach covering electronic quantum transport simulations and numerical device simulations. Schottky-like and ohmiclike contacts are considered. The simplified approach employed in the numerical device simulator is critically evaluated and verified by means of comparing the results with electronic quantum simulation results of an identical device. Different performance indicators, such as the switching speed, switching energy, the subthreshold slope, Ion/Ioff -ratio, among others, are extracted for different device architectures. These values guide the evaluation of the technology for different application scenarios. For high-performance logic applications, the buried gate CNTFET is claimed to be the most suitable structure.
  • Publication
    Hyperspectral imaging for standoff trace detection of explosives using quantum cascade lasers
    ( 2017)
    Fuchs, F.
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    Hugger, S.
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    Jarvis, J.P.
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    Härtelt, M.
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    Yang, Q.K.K.
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    Rattunde, M.
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    Ostendorf, R.
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    Schilling, C.
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    Driad, R.
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    Aidam, R.
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    Wagner, J.
    Reliable standoff detection of traces of explosives is still a challenging task. Imaging MIR backscattering spectroscopy has been shown to be a promising technique for non-contact detection of traces of explosives on various surfaces. This technique, which is eye-safe, relies on active imaging with MIR laser illumination at various wavelengths. Recording the backscattered light with a MIR camera at each illumination wavelength, the MIR backscattering spectrum can be extracted from the three-dimensional data set recorded for each point within the laser illuminated area. Applying appropriate image analysis algorithms to this hyper-spectral data set, chemically sensitive and selective images of the surface of almost any object can be generated. This way, residues of explosives can be clearly identified on the basis of characteristic finger print backscattering spectra and separated from the corresponding spectra of the underlying material. To achieve a high selectivity, a large spectral coverage is a key requirement. Using a MIR EC-QCL with a tuning range from 7.5 mm to 9.5 mm, different explosives such as TNT, PETN and RDX residing on different background materials, such as painted metal sheets, cloth and polyamide, could be clearly detected and identified. For short stand-off detection distances (<3 m), residues of explosives at an amount of just a few 10 mg, i .e. traces corresponding to a single fingerprint, could be detected. For larger concentration of explosives, stand-off detection over distances of up to 20 m has already been demonstrated. During the European FP7 projects EMPHASIS and HYPERION several field tests were performed at the test site of FOI in Sweden. During these tests realistic scenarios were established comprising test detonations of IEDs. We could demonstrate the potential of QCL-based imaging backscattering spectroscopy for the detection of trace amounts of hazardous substances in such scenarios.
  • Publication
    Real-time spectroscopic sensing using a widely tunable external cavity-QCL with MOEMS diffraction grating
    ( 2016)
    Ostendorf, R.
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    Butschek, L.
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    Merten, A.
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    Grahmann, J.
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    Jarvis, J.P.
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    Hugger, S.
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    Fuchs, F.
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    Wagner, J.
    We present spectroscopic measurements performed with an EC-QCL combining a broadly tunable quantum cascade laser chip with a tuning range of more than 300 cm-1 and a resonantly driven MOEMS scanner with an integrated diffraction grating for wavelength selection in Littrow configuration. The grating geometry was optimized to provide high diffraction efficiency over the wide tuning range of the QCL, thus assuring high power density and high spectral resolution in the MIR range. The MOEMS scanner has a resonance frequency of 1 kHz, hence allowing for two full wavelength scans, one up and the other downwards, within 1 ms. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets as well as on gaseous samples of carbon monoxide. For the latter one, a large portion of the characteristic CO absorption band containing several absorption lines in the range of 2070 cm-1 to 2280 cm-1 can be monitored in real-time.
  • Publication
    Recent advances and applications of external Cavity-QCLs towards hyperspectral imaging for standoff detection and real-time spectroscopic sensing of chemicals
    ( 2016)
    Ostendorf, R.
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    Butschek, L.
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    Hugger, S.
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    Fuchs, F.
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    Yang, Q.
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    Jarvis, J.
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    Schilling, C.
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    Rattunde, M.
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    Merten, A.
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    Grahmann, J.
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    Boskovic, D.
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    Tybussek, T.
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    Rieblinger, K.
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    Wagner, J.
    External-cavity quantum cascade lasers (EC-QCL) are now established as versatile wavelength-tunable light sources for analytical spectroscopy in the mid-infrared (MIR) spectral range. We report on the realization of rapid broadband spectral tuning with kHz scan rates by combining a QCL chip with a broad gain spectrum and a resonantly driven micro-opto-electro-mechanical (MOEMS) scanner with an integrated diffraction grating in Littrow configuration. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets, as well as on hazardous substances, such as explosives. Furthermore, different applications for the EC-QCL technology in spectroscopic sensing are presented. These include the fields of process analysis with on- or even inline capability and imaging backscattering spectroscopy for contactless identification of solid and liquid contaminations on surfaces. Recent progress in trace detection of explosives and related precursors in relevant environments as well as advances in food quality monitoring by discriminating fresh and mold contaminated peanuts based on their MIR backscattering spectrum is shown.