Negative ion formation through dissociative electron attachment to Germanium tetrachloride

The interaction of low-energy electrons with neutral gaseous molecules can provide important information about various processes observed in atmospheric chemistry, plasma industry, fiber optics and many more. Low-energy electron interaction with molecules is governed by the formation of a transient negative ion (TNI) which is bound to relax subsequently. One of the relaxation pathways leads to the dissociation of the molecule and formation of a negative fragment. This process is termed Dissociative Electron Attachment (DEA) and is the main focus of the present study. GeCl4 is used extensively in the production of fiber optic materials as well as in other industries where low-energy electrons can play an important role. As a part of an effort to characterize the interaction of low-energy electrons with the group IV tetrahalides XY4 (X = C, Si, Ge and Y = F, Cl, Br), dissociative electron attachment to GeCl4 in the incident electron energy range from ~0 to 10 eV is presented and discussed in context with previous studies. The Appearance Energy (AE) of contributions in the ion yield is determined by fitting the onset of ion yield curve with functions that reproduce the onset. In this study, two independent fitting functions were used, a Wannier type function and a linear function. The observed fragments from the current DEA measurements were GeCl3-, GeCl2-, Cl2- and Cl-. These fragments were formed through three resonances which appeared in the ion yields at ~0, 1.4 and 5-6 eV and have been assigned to A1, T2 and E}symmetries, respectively. The presented measurements provide ion yields with higher resolution than has been obtained before and whereas the results were mostly in agreement with earlier studies, a few features were observed for the first time. In the GeCl3- yield, all three resonances were observed together in one spectra. At 0 eV, the formation of GeCl3- dominates the ion yield due to the high electron affinity of GeCl3-, making it the energetically favorable pathway. However, through the higher energy resonance at 5-6 eV, the formation of Cl- is the preferred pathway. This might be due to increased probability density of the electron around the chlorine atom in the transient negative ion.Víxlverkun lágorkurafeinda við óhlaðnar sameindir á gasformi gegnir veigamiklu hlutverki á ýmsum sviðum, til að mynda í efnafræði andrúmsloftsins, í rafgösum, við framleiðslu ljósleiðara og á fleiri sviðum. Þessi víxlverkun einkennist af myndun neikvætt hlaðinna jóna (móðurjóna) sem alla jafna eru í örvuðu ástandi og leitast því við að losna við þá umframorku sem þær tók upp í myndunarferlinu. Móðurjónirnar geta aförvast með rofi efnatengja, við það myndast eitt eða fleiri stöðug óhlaðin sameindabrot og eitt stöðugt hlaðið sameindabrot. Þetta ferli nefnist rjúfandi rafeindarálagning (e. Dissociative Electron Attachment; DEA) og er meginviðfangsefni þessarrar ritgerðar. Þessi rannsókn er hluti af stærra verkefni sem miðar að því að kortleggja víxlverkun lágorkurafeinda við tetrahalíð frumefna úr IV flokki lotukerfisins XY4 (X = C, Si, Ge og Y = F, Cl, Br). Þessi efni eru mikið notuð í iðnaði og framleiðsluferlum þar sem lágorkurafeindir gegna mikilvægu hlutverki. Í þessarri rannsókn var rjúfandi rafeindarálagning á Germaníum tetraklóríð (GeCl4) á bilinu ~0 til 10 eV skoðuð og niðurstöður ræddar í samhengi við aðrar rannsóknir sem gerðar hafa verið á sama efni. Auk þess var það orkugildi, þar sem fyrst gætir myndunar stöðugrar jónar (e. Appearance Energy), ákvarðað með notkun sniðfalla. Notaðar voru tvær ólíkar gerðir sniðfalla, annarsvegar veldisfall af Wannier gerð og hinsvegar línulegt fall. Þau hlöðnu sameindabrot sem mynduðust við rjúfandi rafeindarálagningu á GeCl4 voru GeCl3-, GeCl2-, Cl2- og Cl-. Þessi sameindbrot urðu til við niðurbrot móðurjóna sem mynduðust við ~0, 1.4 og 5-6 eV. Sýnt hefur verið að samhverfu móðurjónanna sé best lýst með ókjúfanlegu framsetningunum A1, T2 og E. Hægt var í þessu verkefni að ákvarða nakvæmar þau orkugildi þar sem þessi sameindabrott myndast og þótt flestar niðurstöður séu í samræmi við fyrri rannsóknir, sáust nokkur einkenni sem ekki hafa sést áður. Til að mynda sást GeCl3- jónin myndast við öll þrjú orkugildin. Auk þess sást Cl- jónin myndast við 0 eV, en möguleiki á myndun hennar við 0 eV hafði áður verið dregin í efa. Við leiðum líkur að því að ríkjandi myndun GeCl3- við ~0 eV sé vegna hárrar rafeindasækni jónarinnar. Hins vegar er ríkjandi myndun Cl- jónarinnar við 5-6 eV líklega sökum þess að við þessa orku er rafeindin staðsett meira í námunda við klóratómið

Quantitative Electrochemical Control over Optical Gain in Quantum-Dot Solids

Solution-processed quantum dot (QD) lasers are one of the holy grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs &gt;1 exciton per QD, which is difficult to achieve because of fast nonradiative Auger recombination. The threshold can, however, be reduced by electronic doping of the QDs, which decreases the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs, we achieve quantitative control over the gain threshold. We obtain stable and reversible doping of more than two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory, including a vanishingly low gain threshold for doubly doped QDs. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ∼1 × 10-5 excitons per QD. These results demonstrate a high level of control over the gain threshold in doped QD solids, opening a new route for the creation of cheap, solution-processable, low-threshold QD lasers. </p

Quantitative Electrochemical Control over Optical Gain in Quantum-Dot Solids

Solution-processed quantum dot (QD) lasers are one of the holy grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs &gt;1 exciton per QD, which is difficult to achieve because of fast nonradiative Auger recombination. The threshold can, however, be reduced by electronic doping of the QDs, which decreases the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs, we achieve quantitative control over the gain threshold. We obtain stable and reversible doping of more than two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory, including a vanishingly low gain threshold for doubly doped QDs. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ∼1 × 10-5 excitons per QD. These results demonstrate a high level of control over the gain threshold in doped QD solids, opening a new route for the creation of cheap, solution-processable, low-threshold QD lasers. ChemE/Opto-electronic MaterialsBN/Technici en Analiste

Spectroscopic Evidence for the Contribution of Holes to the Bleach of Cd-Chalcogenide Quantum Dots

In transient absorption (TA) measurements on Cd-chalcogenide quantum dots (QDs), the presence of a band-edge (BE) bleach signal is commonly attributed entirely to conduction-band electrons in the 1S(e) state, neglecting contributions from BE holes. While this has been the accepted view for more than 20 years, and has often been used to distinguish electron and hole kinetics, the reason for the absence of a hole contribution to the BE-bleach has remained unclear. Here, we show with three independent experiments that holes do in fact have a significant impact on the BE-bleach of well-passivated Cd-chalcogenide QD samples. Transient absorption experiments on high photoluminescence quantum yield CdSe/CdS/ZnS core-shell-shell QDs clearly show an increase of the band-edge bleach as holes cool down to the band edge. The relative contribution of electron-to-hole bleach is 2:1, as predicted by theory. The same measurements on core-only CdSe QDs with a lower quantum yield do not show a contribution of holes to the band-edge bleach. We assign the lack of hole bleach to the presence of ultrafast hole trapping in samples with insufficient passivation of the QD surface. In addition, we show measurements of optical gain in core-shell-shell QD solutions, providing clear evidence of a significant hole contribution to the BE transient absorption signal. Finally, we present spectroelectrochemical measurements on CdTe QDs films, showing the presence of a BE-bleach for both electron and hole injections. The presence of a contribution of holes to the bleach in passivated Cd-chalcogenides QDs bears important implications for quantitative studies on optical gain as well as for TA determinations of carrier dynamics.ChemE/Opto-electronic Material

Electrochemical Modulation of the Photophysics of Surface-Localized Trap States in Core/Shell/(Shell) Quantum Dot Films

In this work, we systematically study the spectroelectrochemical response of CdSe quantum dots (QDs), CdSe/CdS core/shell QDs with varying CdS shell thicknesses, and CdSe/CdS/ZnS core/shell/shell QDs in order to elucidate the influence of localized surface trap states on the optoelectronic properties. By correlating the differential absorbance and the photoluminescence upon electrochemically raising the Fermi level, we reveal that trap states near the conduction band (CB) edge give rise to nonradiative recombination pathways regardless of the CdS shell thickness, evidenced by quenching of the photoluminescence before the CB edge is populated with electrons. This points in the direction of shallow trap states localized on the CdS shell surface that give rise to nonradiative recombination pathways. We suggest that these shallow trap states reduce the quantum yield because of enhanced hole trapping when the Fermi level is raised electrochemically. We show that these shallow trap states are removed when additional wide band gap ZnS shells are grown around the CdSe/CdS core/shell QDs.ChemE/Opto-electronic Material

Imaging CO₂ reinjection into basalts at the CarbFix2 reinjection reservoir (Hellisheiði, Iceland) with body‐wave seismic interferometry

As part of the Synergetic Utilisation of CO₂ storage Coupled with geothermal EnErgy Deployment project, investigating CO₂ reinjection with different seismic methods, both passive and active seismic surveys have been conducted at the geothermal power plant at Hellisheiði, Iceland. During the 2021 survey, two geophone lines recorded noise for a week. We process the passive-source data with seismic interferometry to image the subsurface structure around the CarbFix2 reinjection reservoir. To improve image quality, we perform an illumination analysis to select only noise panels dominated by body-wave energy. The results show that most noise panels are dominated by air-wave energy arriving from the direction of the power plant. We use panels with a near-vertical incidence to create a zero-offset image and a larger selection of body-wave-dominated panels to create virtual common-shot gathers. We process the gathers with a simple reflection seismology processing workflow to obtain stacked images. The zero-offset images show a relatively lower signal-to-noise ratio and only horizontal reflectors. The stacked images show slightly dipping reflectors and possibly lateral amplitude variations around the expected injection region. This could indicate a region of interest for future research into the reinjection reservoir.ISSN:0016-8025ISSN:1365-247

Combined Large-N Seismic Arrays and DAS Fiber Optic Cables across the Hengill Geothermal Field, Iceland

From June to August 2021, we deployed a dense seismic nodal network across the Hengill geothermal area in southwest Iceland to image and characterize faults and high-temperature zones at high resolution. The nodal network comprised 498 geophone nodes spread across the northern Nesjavellir and southern Hverahlíð geothermal fields and was complemented by an existing permanent and temporary backbone seismic network of a total of 44 short-period and broadband stations. In addition, we recorded distributed acoustic sensing data along two fiber optic telecommunication cables near the Nesjavellir geothermal power plant with commercial interrogators. During the time of deployment, a vibroseis survey took place around the Nesjavellir power plant. Here, we describe the network and the recorded datasets. Furthermore, we show some initial results that indicate a high data quality and highlight the potential of the seismic records for various follow up studies, such as high-resolution event location to delineate faults and body- and surface-wave tomographies to image the subsurface velocity structure in great detail.ISSN:0895-0695ISSN:1938-205