Sebastian Beck

Optical phonons in methylammonium lead halide perovskites and implications for charge transport

Lead-halide perovskites are promising materials for opto-electronic applications. Recent reports indicated that their mechanical and electronic properties are strongly affected by the lattice vibrations. Herein we report far-infrared spectroscopy measurements of CH3NH3Pb(I/Br/Cl)3 thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine the frequencies of the transversal and longitudinal optical phonons, and use them to calculate, via appropriate models, the static dielectric constants, polaron masses, electron–phonon coupling constants, and upper limits for the phonon-scattering limited charge carrier mobilities. Within the limitations of the model used, we can place an upper limit of 200 cm2 V−1 s−1 for the room temperature charge carrier mobility in MAPbI3 single crystals. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

Safety analysis of endoscopist‐directed propofol sedation: A prospective, national multicenter study of 24 441 patients in German outpatient practices

Since 2008, there exists a German S3‐guideline allowing non‐anesthesiological administration of propofol for gastrointestinal endoscopy. In this prospective, national, multicenter study, we evaluated the safety of endoscopist‐administered propofol sedation (EDP) in German outpatient practices of Gastroenterology.

Spatial extent of plasmonic enhancement of vibrational signals in the infrared.

Infrared vibrations of molecular species can be enhanced by several orders of magnitude with plasmonic nanoantennas. Based on the confined electromagnetic near-fields of resonantly excited metal nanoparticles, this antenna-assisted surface-enhanced infrared spectroscopy enables the detection of minute amounts of analytes localized in the nanometer-scale vicinity of the structure. Among other important parameters, the distance of the vibrational oscillator of the analyte to the nanoantenna surface determines the signal enhancement. For sensing applications, this is a particularly important issue since the vibrating dipoles of interest may be located far away from the antenna surface because of functional layers and the large size of biomolecules, proteins, or bacteria. The relation between distance and signal enhancement is thus of paramount importance and measured here with in situ infrared spectroscopy during the growth of a probe layer. Our results indicate a diminishing signal enhancement and the effective saturation of the plasmonic resonance shift beyond 100 nm. The experiments carried out under ultra-high-vacuum conditions are supported by numerical calculations.

Respiratory complications in outpatient endoscopy with endoscopist-directed sedation.

Background & Aims: Respiratory complications represent an important adverse event of endoscopic procedures. We screened for respiratory complications aer endoscopic procedures using a questionnaire and followed-up patients suggestive of respiratory infection. Method: In this prospective observational, multicenter study performed in Outpatient practices of gastroenterology we investigated 15,690 patients by questionnaires administered 24 hours aer the endoscopic procedure. Results: 832 of the 15,690 patients stated at least one respiratory symptom aer the endoscopic procedure: 829 patients reported coughing (5.28%), 23 fever (0.15%) and 116 shortness of breath (SOB, 0.74%); 130 of the 832 patients showed at least two concomitant respiratory symptoms (107 coughing + SOB, 17 coughing + fever, 6 coughing + coexisting fever + SOB) and 126 patients were followed-up to assess their respiratory complaints. Twenty-nine patients (follow-up: 22.31%, whole sample: 0.18%) reported signs of clinically evident respiratory infection and 15 patients (follow-up: 11.54%; whole sample: 0.1%) received therefore antibiotic treatment. Coughing or vomiting during the endoscopic procedure resulted in a 156.12-fold increased risk of respiratory complications (95% CI: 67.44 - 361.40) and 520.87-fold increased risk of requiring antibiotic treatment (95% CI: 178.01 - 1524.05). All patients of the follow-up sample who coughed or vomited during endoscopy developed clinically evident signs of respiratory infection and required antibiotic treatment while this occurred in a signicantly lower proportion of patients without these symptoms (17.1% and 5.1%, respectively). Conclusions: We demonstrated that respiratory complications following endoscopic sedation are of comparably high incidence and we identied major predictors of aspiration pneumonia which could inuence future surveillance strategies aer endoscopic procedures.

Alumina films as gas barrier layers grown by spatial atomic layer deposition with trimethylaluminum and different oxygen sources

Alumina layers were grown from trimethylaluminum (TMA) and water, ozone as well as an oxygen plasma as co-reactants in low temperature spatial atomic layer deposition (ALD). The influence of the amount of precursor, the precursor exposure duration, and substrate temperature were investigated with respect to the growth rate while employing different oxygen sources. The TMA/water process provided alumina (AlOx) films with superb film quality as shown by infrared measurements. Ozone-based processes allowed lower substrate temperatures. Nevertheless, carbon residuals in different binding states were found within the bulk material. However, the carbon impurities have no impact on the barrier performance, since 50 nm AlOx grown by TMA either with water or ozone exhibited a water vapor transition rate in the range of 10−6 g/m2/day. However, when our home-built microwave plasma source was applied in a remote configuration, the water vapor transition rate was one order of magnitude higher due to a reduction in fil...

Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface.

Raman spectroscopy and microscopy of electrochemically and chemically doped high-mobility semiconducting polymers

The polaronic nature of two high-mobility hole-conducting polymers (PBTTT and DPPT-TT) is investigated by Raman spectroscopy and density functional theory (DFT) calculations. Chemical and electrochemical hole doping of these polymers leads to characteristic changes in the intensity ratios of the Raman active CC stretching modes but no significant frequency shifts. The data indicate a localization of positive polarons on the electron-rich thienothiophene cores that are present in both polymers. DFT calculations show that the Raman intensity ratio variations are most likely caused by the local electric field that originates from negatively charged dopant molecules or electrolyte anions and the positive polaron on the polymer chain. The characteristic changes in the Raman mode intensity ratios with the degree of doping enable in situ mapping of charge carrier concentration in the channel of electrolyte-gated polymer transistors with high spatial resolution.

Structure–Property Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide

Studying the structure-property relations of tailored dipolar phenyl and biphenylphosphonic acids, we report self-assembled monolayers with a significant decrease in the work function (WF) of indium-tin oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating N, N-dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease in ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid.

Electrical and optical properties of reduced graphene oxide thin film deposited onto polyethylene terephthalate by spin coating technique

We present the reduction of solution processed graphene oxide films by hydrogen iodide vapor. The films were studied by Raman spectroscopy and Fourier-transform infrared spectroscopy and its optoelectronic properties characterized. We obtained reduced graphene oxide films on polyethylene terephthalate flexible substrates with good electrical properties, 3.74×10-6  Ω·m, and high optical transmittance of 70% in the visible range. The fabricated layers contain graphene sheets with sizes up to ∼10  μm long and ∼6  μm wide. The presented solution, with highly concentrated processed graphene oxide, could be used as printing ink for manufacturing transparent and conductive electrodes on plastic substrates without the requirement of elevated temperatures.

IR spectroscopic investigation of charge transfer at interfaces of organic semiconductors(Conference Presentation)

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.