In organic electronics, the interactions at interfaces between different organic and inorganic layers play a decisive role for device functionality and performance. Therefore, more detailed, quantitative studies of charge transfer (CT) at such interfaces are needed to improve the understanding of the underlying mechanisms.
In this study we show that in-situ infrared spectroscopy can be used to investigate CT effects at organic/organic as well as inorganic/organic interfaces quantitatively. For different combinations of commonly used organic semiconductors such as 4,4´-bis(N-carbazolyl)-1,1´-biphenyl (CBP) or fluorinated zinc phthalocyanine (F4ZnPc) and inorganic contact materials such as molybdenum oxide (MoO3) or indium tin oxide (ITO) the CT at the interface was investigated using in-situ IR spectroscopy. The measurements were carried out under UHV conditions during film growth what enables a careful study of the influence of different parameters such as substrate temperature and layer thickness in a controlled way even on a nanometer scale. When the organic molecules are deposited onto the underlying layer charged and non-charged species form which can be identified and quantitatively analyzed in the IR spectra. It was also found that the deposition sequence can strongly influence the interface properties what might have strong implications on the layer stack design. For example, when MoO3 is deposited onto CBP, the CBP layer is strongly doped, due to diffusion of the deposited transition metal oxide clusters into the organic layer.
Financial support by BMBF (project INTERPHASE) is gratefully acknowledged.
A detailed knowledge of optical properties of tissue is important for the development of non-invasive monitoring systems for clinical practice. However, tissue optical properties are rarely known in the mid-infrared wavelength range. Goniometry offers an opportunity to determine in particular the scattering properties of tissue. Here, a custom-built setup is presented for goniometric measurements in the mid-infrared based on quantum cascade lasers.
Infrared microscopy can be performed to observe dynamic processes on a microscopic scale. Fourier-transform infrared spectroscopy-based microscopes are bound to limitations regarding time resolution, which hampers their potential for imaging fast moving systems. In this manuscript we present a quantum cascade laser-based infrared microscope which overcomes these limitations and readily achieves standard video frame rates. The capabilities of our setup are demonstrated by observing dynamical processes at their specific time scales: fermentation, slow moving Amoeba Proteus and fast moving Caenorhabditis elegans. Mid-infrared sampling rates between 30 min and 20 ms are demonstrated.
The spectroscopy of analyte-specific molecular vibrations in tissue thin sections has opened up a path toward histopathology without the need for tissue staining. However, biomedical vibrational imaging has not yet advanced from academic research to routine histopathology due to long acquisition times for the microscopic hyperspectral images and/or cost and availability of the necessary equipment. Here we show that the combination of a fast-tuning quantum cascade laser with a microbolometer array detector allows for a rapid image acquisition and bares the potential for substantial cost reduction. A 3.1×2.8 mm2 unstained thin section of mouse jejunum has been imaged in the 9.2 to 9.7 μm wavelength range (spectral resolution ∼1 cm−1) within 5 min with diffraction limited spatial resolution. The comparison of this hyperspectral imaging approach with standard Fourier transform infrared imaging or mapping of the identical sample shows a reduction in acquisition time per wavenumber interval and image area by more than one or three orders of magnitude, respectively.
Charge injection barriers caused by a misalignment of energy levels are of major concern in organic semiconductor devices. One possibility to improve charge carrier injection is the application of an additional layer at the interface between the contact and the organic semiconductor. Self-assembled monolayers (SAMs) have been proven to form stable and well defined layers on various contact materials. Depending on their molecular dipole they can lower or raise the work function of a material and are therefore very well suited as injection layers. Since SAMs can be processed from solution they form a relevant material for printed organic electronics. The orientation of the SAM and thus important interface properties like the interface dipole and the work-function shift are influenced by various parameters such as concentration of the molecule in solution, immersion time and cleanliness of the solution and of the substrate. Infrared-reflection-absorption-spectroscopy (IRRAS) is a very sensitive tool to measure changes in the orientation of SAMs on metal substrates. We performed IRRAS measurements on SAMs consisting of perfluorinated decanethiol (PFDT) on evaporated gold films in order to probe the orientation, ordering and quality of the SAMs. By systematic variation of immersion time and concentration, we were able to conclude on the process steps of layer formation. Taking into account realistic printing circumstances, we also investigated the impact of oxygen in the solvent and the gold substrate on the layer formation process.
Charge transfer (CT) mechanisms are crucial for device performance in organic electronics, but they are still not understood on a fundamental level. Here we want to show that in situ IR spectroscopy is very well suited to investigate CT effects in organic semiconductors in a qualitative and quantitative way. We study the ambipolar transport material 4,4´-bis(N-carbazolyl)-1,1´-biphenyl (CBP) as matrix and cesium carbonate (Cs2CO3) as n-dopant. To achieve doped layers, both materials were evaporated simultaneously. The system is one of the rare ones for n-doping of organic layers. In the spectra of the doped layers, additional absorption bands appear in the mid IR range. These can be assigned to the negatively charged matrix molecules that indicate electron transfer. The charged molecules exhibit these different absorption bands, as the charge transfer leads to a change in bond length and bond strength of the molecules. Our results very well agree with density functional theory calculations of the vibrational spectra of both, charged and non-charged molecules. By fitting the spectra of the doped layers as a superposition of the vibrational oscillators of neutral and charged species, we were able to quantify the amount of charged matrix molecules and to determine the doping efficiency of the investigated systems. For CBP n-doped with Cs2CO3 a hindrance of the CT due to air exposure could be observed.
Despite the successes of mid-infrared hyperspectral imaging in a research environment, progress in the migration of technology into the day-to-day clinical application is slow. Clinical acceptance may be improved if the spectroscopy would be faster and the infrared microscopes available at lower cost. Here we present first results of a fast, multi-scale mid-infrared microscopy setup which allows for the investigation of 10.6×11.7 mm2 and 2.8×3.1mm2 fields of view with a resolution of 23.0±3.5 μm and 9.4±1.8 μm, respectively. Tunable quantum cascade lasers in the wavenumber ranges of 1030-1090 cm-1 and 1160-1320 cm-1 serve as light sources. A vapor cell is used as a frequency reference during the rapid scanning. As far as the imaging is concerned, it is the high spectral power density of the quantum cascade laser which enables the use of a microbolometer array while still obtaining reasonable signal-to-noise ratios on each pixel. Hyperspectral images are taken in times which can be as low as 52s for the overall image acquisition including referencing.
Diabetes mellitus is a disorder of glucose metabolism and it is one of the most challenging diseases, both from a
medical and economic perspective. People with diabetes can benefit from a frequent or even continuous monitoring
of their blood glucose concentrations. The approach presented here takes advantage of the observational
nature of biomedical vibrational spectroscopy in contrast to chemical reactions which consume glucose. The
particular technique employed here is based on the high sensitivity of mid-infrared transmission spectroscopy
where strong vibrational bands of glucose can be monitored at wavelengths around 10 μm. The strong absorption
of water in this spectral region was mitigated by the use of quantum cascade lasers and very short interaction
path lengths below 50 μm. Various sensor concepts have been explored. In one of the concepts, the interaction
of mid-infrared radiation with glucose is established within a miniature measurement cavity, formed by a gap
between two silver halide fibers. In recent experiments, an additional quantum cascade laser was used for reference
purposes. The long-term drift could significantly be reduced for time intervals > 1000 s, e. g., by more than
60% for a 3 hour interval. This extension for the compensation of long-term drifts of the measurement system
in vitro is an important contribution towards the applicability in vivo.
We summarize recent results from in-situ infrared spectroscopic studies of nanofilm growth. These studies, performed
under ultra-high vacuum conditions with sub-monolayer sensitivity, exploited the relationship between morphology and
structure on the one side and, on the other side, vibrational excitations and plasmonic ones. The studies were performed
within various projects ranging from astronomy and high-energy physics to organic electronics and plasmonics. The
results represent examples the description of which needs theoretical models beyond the use of Fresnel's formulae, the
assumption of abrupt interfaces, and the use optical databases for bulk materials. For example, at the SiO-Si interface Si-
O-Si bridges with Si-O bonds longer than in the bulk are formed which can be identified via their special vibration
signals at unusually low vibration frequencies. From a thickness of about 1 nm on, the infrared spectra show typical SiObulk
features. The lowered vibrational frequencies are attributed to changes in the average distribution of Si-O bond
length close to the interface. On the diamond (100) surface, during Cr deposition, we observed the formation of a
conducting nanocrystalline fcc phase of chromium. At a certain thickness, the nanocrystalline phase makes a phase
transition to the typical bulk chromium phase. Cr is a preferred material for electric contacts in single-crystal diamond
detectors the performance of which sensitively depends on the conductivity of the deposited Cr contact. On organic
semiconductor layers metallization may be accompanied by an intermixing at the metal-semiconductor interface. Such
intermixing can be observed as the appearance of new excitation features.
Corresponding to their relative low work function, Ca and Mg are interesting metals for cathodes in organic light emitting devices. In this study, the interaction of these metals with the blue phosphorescent emitter material Ir(cnpmbic)3 is investigated by in-situ infrared spectroscopy. Thin films of the organic material are deposited by vapour sublimation under ultra-high vacuum conditions. Further deposition of Ca on the organic layer at room temperature gives rise to new features in the infrared spectrum of the sample. The new features indicate diffusion of Ca into the organic layer and do not appear at much lower temperature (110 K). They are attributed to dynamic charge transfer processes
that may occur at rough metal surfaces. On the other hand, Mg deposited at room temperature does not stick on the organic material.
The continuous monitoring of the concentration of glucose provides an essential tool for the improved glycemic
control for people with diabetes. Most of the present approaches of transcutaneous, continuous glucose monitors
are based on chemical detection and require the insertion of reagents into the body. In contrast, we aim at the
reagent-free monitoring of glucose by means of mid-infrared spectroscopy.
A quantum cascade laser provides narrow band radiation at wavelengths around the absorption bands of glucose
(≈ 10 μm). At the same time it yields sufficient energy to allow for a good signal-to-noise ratio in transmission
measurements despite the strong background absorption of water.
We investigated various concepts for the sensor head based on the light-guiding properties and handling of
materials such as AgCl/AgBr or silicon. In-vitro experiments were performed using a custom-made, temperaturestabilized
measurement flow chamber. In preparation for future in-vivo applications first results of biotoxicity
tests of the fiber sensors are given.
Metal nanowires with proper length give strong antenna-like plasmonic resonances in the infrared. Their resonance
spectrum is a sensitive measure not only of their geometry but also of their conductivity as we will show for lead
nanoantennae here.
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