Innovative mid-infrared detector concepts

Gas sensing is a key technology with applications in various industrial, medical and environmental areas. Optical detection mechanisms allow for a highly selective, contactless and fast detection. For this purpose, rotational-vibrational absorption bands within the mid infrared (MIR) spectral region are exploited and probed with appropriate light sources. During the past years, the development of novel laser concepts such as interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) has driven a continuous optimization of MIR laser sources. On the other hand side, there has been relatively little progress on detectors in this wavelength range. Here, we study two novel and promising GaSb-based detector concepts: Interband cascade detectors (ICD) and resonant tunneling diode (RTD) photodetectors. ICDs are a promising approach towards highly sensitive room temperature detection of MIR radiation. They make use of the cascading scheme that is enabled by the broken gap alignment of the two binaries GaSb and InAs. The interband transition in GaSb/InAs-superlattices (SL) allows for normal incidence detection. The cut-off wavelength, which determines the low energy detection limit, can be engineered via the SL period. RTD photodetectors act as low noise and high speed amplifiers of small optically generated electrical signals. In contrast to avalanche photodiodes, where the gain originates from multiplication due to impact ionization, in RTD photodetectors a large tunneling current is modulated via Coulomb interaction by the presence of photogenerated minority charge carriers. For both detector concepts, first devices operational at room temperature have been realized.Publisher PD

Room temperature operation of GaSb-based resonant tunneling diodes by prewell injection

The authors are grateful for financial support by the state of Bavaria, the German Ministry of Education and Research (BMBF) within the national project HIRT (FKZ 13XP5003B).We present room temperature resonant tunneling of GaSb/AlAsSb double barrier resonant tunneling diodes with pseudomorphically grown prewell emitter structures comprising the ternary compound semiconductors GaInSb and GaAsSb. At room temperature, resonant tunneling is absent for diode structures without prewell emitters. The incorporation of Ga0.84In0.16Sb and GaAs0.05Sb0.95 prewell emitters leads to room temperature resonant tunneling with peak‐to‐valley current ratios of 1.45 and 1.36 , respectively. The room temperature operation is attributed to the enhanced Γ ‐L‐valley energy separation and consequently depopulation of L‐valley states in the conduction band of the ternary compound emitter prewell with respect to bulk GaSb.PostprintPeer reviewe

Alkylidynephosphines : syntheses and reactivity

In the past, different methods have been utilized for the preparation of alkylidynephosphines. Whereas, however, small amounts of thermally instable derivatives might be obtained from reactions in the gas phase, the synthesis of phosphines which are stable under an inert atmosphere, as for instance those with a tert-butyl or a 1-adamantyl substituent at the carbon atom of the PEC group, is best started with tris (trimethylsilyl) phosphine itself or with the more reactive lithium bis (trimethylsilyl) phosphide.2 tetrahydrofuran complex. Treatment of either compound with acyl halides results in the formation of acylbis (trimethylsilyl) phosphines which, at room temperature, rearrange to the corresponding trimethylsilyl [1-(trimethylsiloxy) alkylidene] isomers. As traces of hydrogen halide accelerate the conversion of tris (trimethylsilyl) phosphine to the triacyl derivatives, the use of the lithium phosphide is strongly recommended in all cases where impure acyl halides are used. In the presence of small amounts of solid sodium hydroxide suspended in an etherial solvent, the thus prepared trimethyl[1-(trimethylsiloxy)-alkylidene]phosphines eliminate hexamethyldisiloxane to yield the required alkylidyne compounds. Running the decomposition without a solvent at a higher temperature, Regitz and coworkers were able to improve this method further

Phosphane und Arsane mit Heteroatomen niederer Koordinationszahl

In der anorganischen und elementorganischen Chemie steht gegenwärtig die Frage nach der Existenz, der Synthese und der Reaktivität von Verbindungen, die unter Beteiligung eines Hauptgruppenelementes aus der dritten oder einer höheren Periode eine homo- oder heteronukleare (np-mp)π-Bindung ausbilden und die entgegen der allgemeinen Erwartung bei Zimmertemperatur thermisch stabil sind, im Blickpunkt des Interesses.Während aber unsere Kenntnisse über entsprechende Systeme aus der dritten und vierten Hauptgruppe nur langsam zunehmen, war bei den Alkylidenphosphanen und teilweise auch -arsanen eine überaus rasche Entwicklung zu verzeichnen [2-4]. Nachdem jedoch diese beiden Substanzklassen in den letzten Jahren intensivst bearbeitet worden sind, werden jetzt auch die von verschiedenen Seiten Phosphaalkine genannten Alkylidinphosphane R-C≡P eingehender untersucht. Dabei kommt einer Klärung der Frage, ob sie aufgrund ihrer Reaktivität eher als Homologe der Nitrile anzusehen sind oder ob sie im Sinne der Schrägbeziehungen im Periodensystem Ähnlichkeiten mit Alkinen aufweisen, entscheidende Bedeutung zu

GaSb/AlAsSb resonant tunneling diodes with GaAsSb emitter prewells

The authors are grateful for financial support by the state of Bavaria, and the German Ministry of Education and Research (BMBF) within the national project HIRT (FKZ 13XP5003B).We investigate the electronic transport properties of GaSb/AlAsSb double barrier resonant tunneling diodes with pseudomorphically grown ternary GaAsxSb1-x emitter prewells over a broad temperature range. At room temperature, resonant tunneling is observed and the peak to valley current ratio (PVCR) is enhanced with increasing As mole fraction from 1.88 (GaAs0.07Sb0.93 prewell), to 2.08 (GaAs0.09Sb0.91 prewell) up to 2.36 (GaAs0.11Sb0.89 prewell). The rise in PVCR is attributed to an enhanced carrier density at the Γ-valley within the emitter prewell. On the contrary at cryogenic temperatures, increasing the As mole fractions reduces the PVCR. At a temperature of T = 4.2 K, reference samples without incorporation of an emitter prewell containing As show PVCRs up to 20.4. We attribute the reduced PVCR to a degraded crystal quality of the resonant tunneling structure caused by As incorporation and subsequently an enhanced defect scattering at the interfaces.PostprintPeer reviewe

Effect of age on performance of task with spatial conflict

We investigated the effect of aging during a dual task paradigm involving a postural and cognitive task. We measured postural sway and performance in a virtual environment. Older adults had increased response times and decreased accuracy compared to younger adults. Compared to young adults, older adults displayed a tighter control of the center of pressure, decreased sway in standing, while performing the spatial conflict task. This suggests that older adults either prioritize postural control over the cognitive task or they adopt a strategy of intentionally decreasing sway to facilitate performance. Results support balance-retraining protocols that start with single tasks and progress to more difficult multiple tasks. © 2013 IEEE

Hydrogen and C2–C6 Alkane Sensing in Complex Fuel Gas Mixtures with Fiber-Enhanced Raman Spectroscopy

Power-to-gas is a heavily discussed option to store surplus electricity from renewable sources. Part of the generated hydrogen could be fed into the gas grid and lead to fluctuations in the composition of the fuel gas. Consequently, both operators of transmission networks and end users would need to frequently monitor the gas to ensure safety as well as optimal and stable operation. Currently, gas chromatography-based analysis methods are the state of the art. However, these methods have several downsides for time-resolved and distributed application and Raman gas spectroscopy is favorable for future point-of-use monitoring. Here, we demonstrate that fiber-enhanced Raman gas spectroscopy (FERS) enables the simultaneous detection of all relevant gases, from major (methane, CH4; hydrogen, H2) to minor (C2–C6 alkanes) fuel gas components. The characteristic peaks of H2 (585 cm–1), CH4 (2917 cm–1), isopentane (765 cm–1), i-butane (798 cm–1), n-butane (830 cm–1), n-pentane (840 cm–1), propane (869 cm–1), ethane (993 cm–1), and n-hexane (1038 cm–1) are well resolved in the broadband spectra acquired with a compact spectrometer. The fiber enhancement achieved in a hollow-core antiresonant fiber enables highly sensitive measurements with limits of detection between 90 and 180 ppm for different hydrocarbons. Both methane and hydrogen were quantified with high accuracy with average relative errors of 1.1% for CH4 and 1.5% for H2 over a wide concentration range. These results show that FERS is ideally suited for comprehensive fuel gas analysis in a future, where regenerative sources lead to fluctuations in the composition of gas

Response to Comment on Hydrogen and C2–C6 Alkane Sensing in Complex Fuel Gas Mixtures with Fiber-Enhanced Raman Spectroscopy

We recently introduced fiber-enhanced Raman gas spectroscopy (FERS) as a potential tool for comprehensive fuel gas analysis. (1) The presented results demonstrate that FERS enables the identification of methane (CH4) and C2–C6 alkanes, which are typical components of natural gas. Additionally, hydrogen (H2), which might be an important fuel gas component in the future, can be sensed simultaneously. We identified characteristic peaks for the different fuel gas components and determined the limits of detection for the different alkanes using a (diluted) gas mixture containing 1% each of different C2–C6 alkanes in nitrogen. For every major and minor Raman peak, we assigned the vibration that most likely contributes to the signal using calculations based on density functional theory (DFT). Thanks to this approach, we were able to newly assign specific vibrations to several peaks. Finally, we showed that precise and accurate point-of-use fuel gas analysis with FERS is possible. Employing a portable FERS instrument, the signal variability of CH4 measurements was determined as 0.3% and the median relative errors of quantification for CH4 and H2 were below 1.5% in a CH4–H2 mixture. In a comment on our original article, (2) Petrov provides separate Raman spectra for each of the different C2–C6 alkanes. These pure-gas spectra as well as the accompanying tables, which contain relevant information about the signal intensities of characteristic bands of the main natural gas components together with the intensities of other components at the same spectral position, add to the existing literature on the subject. Using this information, Petrov points out that our choice for a characteristic peak of n-hexane at 1038 cm–1 is not ideal as there is marked overlap with other signals, most notably n-pentane. He states that the peaks at 815 or 732 cm–1 would be a better choice, depending on the H2 content of the sample. In the light of the presented information, we agree with Petrov that the peak at 1038 cm–1 is not the best choice to unambiguously identify n-hexane. Looking at our data (Figure 1), however, we would not instead use the peaks at 815 or 732 cm–1 but the one we already suggested for unambiguous identification in the original publication at 892 cm–1. (1) This peak is discernible in our spectra and has only minor overlap with other alkanes (2) and no overlap with H2. The limit of detection for this weak peak is between 0.2 and 0.5

Fiber-Enhanced Raman Gas Spectroscopy for the Study of Microbial Methanogenesis

Microbial methanogenesis is a key biogeochemical process in the carbon cycle that is responsible for 70% of global emissions of the potent greenhouse gas methane (CH4). Further knowledge about microbial methanogenesis is crucial to mitigate emissions, increase climate model accuracy, or advance methanogenic biogas production. The current understanding of the substrate use of methanogenic microbes is limited, especially regarding the methylotrophic pathway. Here, we present fiber-enhanced Raman spectroscopy (FERS) of headspace gases as an alternate tool to study methanogenesis and substrate use in particular. The optical technique is nondestructive and sensitive to CH4, hydrogen (H2), and carbon dioxide with a large dynamic range from trace levels (demonstrated LoDs: CH4, 3 ppm; H2, 49 ppm) to pure gases. In addition, the portable FERS system can provide quantitative information about methanol concentration in the liquid phase of microbial cultures through headspace gas sampling (LoD 25 ppm). We demonstrate how FERS gas sensing could enable us to track substrate and product levels of microbial methanogenesis with just one instrument. The versatility of Raman gas spectroscopy could moreover help us to elucidate links between nitrogen and carbon cycle in microbial communities in the near future