Thanks to these properties, our nonlinear waveguides are able to produce a π nonlinear phase shift, paving the way for the development of practical devices for future optical communication applications.Nonlinear silicon photonic devices have attracted considerable attention thanks to their ability to show large third-order nonlinear effects at moderate power levels allowing for all-optical signal processing functionalities in miniaturized components. Although significant efforts have been made and many nonlinear optical functions have already been demonstrated in this platform, the performance of nonlinear silicon photonic devices remains fundamentally limited at the telecom wavelength region due to the two photon absorption (TPA) and related effects. In this work, we propose an alternative CMOS-compatible platform, based on silicon-rich silicon nitride that can overcome this limitation. By carefully selecting the material deposition parameters, we show that both of the device linear and nonlinear properties can be tuned in order to exhibit the desired behaviour at the selected wavelength region. A rigorous and systematic fabrication and characterization campaign of different material compositions is presented, enabling us to demonstrate TPA-free CMOS-compatible waveguides with low linear loss (~1.5 dB/cm) and enhanced Kerr nonlinear response (Re{ Silicon-On-Insulator (SOI) has been widely accepted as the ideal fabrication platform for silicon photonics components, allowing the implementation of high-index contrast waveguides using CMOS-compatible processes. A wide range of highly performing SOI-devices aimed at applications for next-generation optical networking have already been demonstrated, such as ultra-low loss waveguides and optical filters1, high-speed optical transceivers2, as well as components for all-optical signal processing4. Nonlinear optical processing applications have also been demonstrated, including all-optical wavelength converters6, signal regeneration7, parametric gain8 as well as high-speed switching3. These applications benefit both from the high nonlinear refractive index of silicon and the tight field guidance achieved within SOI waveguides. However, they suffer at a fundamental level from the very large two-photon absorption (TPA) coefficient of silicon at telecommunication wavelengths12. As a result, TPA-related effects become significant in typical strip waveguides in silicon, even at power levels of a few mWs, thereby preventing the widespread adoption of silicon as a nonlinear medium at these wavelengths (unless some active means for suppressing TPA is adopted5).Over the last decade, Silicon Photonics has established itself as a mature technology for the fabrication of low-cost, scalable integrated optical components17 and Hydex ®18 which exhibits exceptional low linear loss at the 1550-nm wavelength region (α < 0.06 dB/cm) and high Kerr nonlinearity19, showing no signs of TPA even at relatively high power levels20. AlGaAs25, Ta2O527 and SiGe29 have also been studied recently and proposed as CMOS-compatible alternative platforms for the development of integrated nonlinear components. Compatibility with existing SOI technologies is still a primary consideration in these studies. Among the various alternatives, silicon nitride Si3N4 (where Si3N4 replaces silicon in the core of the waveguide) has recently been investigated by several research groups33, showing both remarkable linear and nonlinear performance. Silicon nitride also exhibits low absorption in the visible wavelength range (down to 500 nm), adding functionalities that are not possible to achieve with SOI, such as integration with light sources emitting in the visible range, spectroscopic functions and signal up-conversion from the telecom wavelength range30. On the other hand, as a material platform, silicon nitride also suffers from certain limitations, such as a reduced waveguide core/ cladding index contrast  that may require the use of Watt-level optical powers in order to give rise to high enough nonlinear phase shifts for nonlinear optical applications, as well as fabrication challenges when relatively thick wafer layers are required (thicker than 400 nm)35. In order to enhance the nonlinear response of the material, and thus reduce the optical power required to achieve the desired nonlinear phase-shift, modified compositions of silicon nitride have been recently proposed38, leading to silicon-rich silicon nitride, which exhibits enhanced Kerr response with respect to the standard, stoichiometric Si3N4 material composition. However, very high propagation losses (10 dB/cm) were observed in refs xNy employing exclusively low temperature (<350 °C) Back End of Line (BEOL) CMOS compatible processes. The proposed engineered platform provides a high index contrast, low-stress material, facilitating TPA-free operation that can be integrated with the existing SOI technology to allow for the implementation of crucial functionalities.A number of works have emerged in recent years that explore alternative materials, suitable for the implementation of nonlinear integrated optical devices. These materials include amorphous silicon that possesses enhanced nonlinear performance with respect to its crystalline counterpart exhibiting an increased Kerr coefficient and a reduced TPA effect at telecoms wavelengthsxNy materials with different stoichiometric ratios were developed and deposited on a thermally-grown SiO2 layer (2 μm thickness) on a 150-mm diameter Si substrate wafer using Plasma Enhanced Chemical Vapour Deposition (PECVD). The three PECVD processes were optimized in order to provide a repeatable deposition thickness, refractive index and uniformity levels across the wafers. A standard SOI wafer was also used to compare the achieved results to the state of the art, SOI technology. A schematic of the layer composition of each wafer is shown in Fig. xNy were assessed using different characterization techniques: ellipsometry (carried out by means of a M2000DI ellipsometer) was employed to determine the thickness uniformity and refractive index of the layers at 632.8 nm, 1310 nm and 1550 nm and Fourier Transform Infrared Spectroscopy FTIR spectroscopy was used to determine the compositional structure of each fabricated layer, including their bond concentration, atom concentration and hydrogen content. The results collected by ellipsometry (at a wavelength of 1550 nm) are outlined in Table 40:n is the measured layer refractive index and nSi is the refractive index of silicon. Results are presented in Fig. 3N4. Knowledge of the bond compositions for each different SixNy layer is useful in order to understand the optical behaviour of the fabricated material, since they strongly influence the material quality, and subsequently the optical material loss. High concentration of Si-H and Si-Si bonds have been shown to be responsible for the formation of undesired grains, pores and columnar microstructures that can cause additional scattering and absorption losses42. The bond compositions were extracted using the method described by Tin and Smith41 and the following proportionality factors: K(N-H) = 8.2 × 1016 cm−1, K(Si-H) = 5.9 × 1016 cm−1 and K(Si-N) = 2.4 × 1016 cm−1  while the hydrogen, nitrogen and silicon content  were extracted using the following equations41:3N4. This is due to the fact that back bonded Si atoms are dominant when N/Si is decreased, while they tend to decrease in favour of N bonds (Si-N or N-H) when N/Si approaches the value of 1.33 (stoichiometric Si3N4)42.In order to assess the linear and nonlinear optical properties of silicon nitride waveguides, a set of three Si2 layer, thus forming the desired strip waveguide structures. In order to access each photonic structure, both grating couplers and butt coupling tapers were fabricated. Photonic structures incorporating grating couplers were employed in experiments involving continuous-wave (CW) beams, whereas structures with wide- bandwidth spot size converters for butt-coupling were utilized for pulsed-light experiments, thus avoiding any spectral shaping on the input pulse trains. Every waveguide was patterned 100 times across each wafer and all waveguides were tested, thus enabling statistically consistent optical characterization measurements.A number of strip waveguides were patterned on each of the three wafers. The widths of the waveguides varied from 500 nm to 1500 nm (with a 100 nm step) and their lengths from 0.1 cm to 1.1 cm (with a 0.1 cm step). Inductively Coupled Plasma (ICP) etching was used to form waveguide patterns, by performing a single full-etch step down to the underlying SiOLeff that contributes to its nonlinear response is a function of its propagation loss α. In the extreme of a high-loss waveguide, the effective length approaches 1/α, i.e. it is the propagation loss rather than the physical device length L that determines Leff44. In high index contrast waveguides (e.g. Δn > 0.5) it is typically influenced by three main factors: a) the quality of the optical material, b) the material absorption and c) surface roughness on the sidewalls and its interaction with the optical mode. Experimental measurements of the propagation loss were carried out by employing the cut-back technique, using the experimental set-up shown in Fig. The waveguide propagation loss is an important factor in determining the nonlinear efficiency of a photonic device. This is because the effective device length xNy material. The results of these measurements are presented in Fig. Loss measurements were carried out on each sample, for each silicon nitride composition, thus assessing the effect of different silicon contents, within the SiBoth Wafers 01 and 02, exhibiting the lowest refractive indices, show the typical exponential dependence of loss versus waveguide width, with narrower waveguides experiencing higher losses due to a greater interaction between the optical mode and the sidewalls. The higher refractive index contrast Wafer 03 does not show this behaviour, and we believe this to be because the optical mode is well confined within the waveguide core even at waveguide widths as small as 500 nm.xNy layer losses, at λ = 1550 nm, are typically influenced by three main factors: (a) the hydrogen content that significantly increases the number of both N-H bonds, which cause an absorption peak at a wavenumber of 3350 cm−1 (first overtone corresponds to λ ~ 1500 nm with low energy tails to 1545–1560 nm) and Si-H bonds, with an absorption peaking at a wavenumber of 2300 cm−1 (second overtone corresponding to λ ~ 1490 nm with low energy tails extending to the C-band wavelength region)47; (b) Si-Si and Si-H bond concentrations that have been identified as a possible cause for the generation of additional silicon dangling bonds which are responsible for additional absorption; (c) material microstructure that may show grains, pores, defects, cluster and voids generated by high concentrations of [H]48, [Si-Si] and [Si-H] bonds42. Wafer_03 material composition showed the highest concentration of both Si-Si and Si-H bonds  the third order susceptibility, neff is the modal effective refractive index at frequency ω and Aeff is the mode area of the considered waveguide. It is important to note that, in general, χ(3) is a complex number and, therefore, γ also has a real and an imaginary part and can be written as:n2 represents the nonlinear refractive index and βTPA is the TPA coefficient defining the strength of the nonlinear loss of the waveguiding device.The waveguide nonlinear response can be generally described by the nonlinear parameter γ which is defined by the following expression22. As presented in the experimental set-up of Fig. p = 1550.11 nm, power ranging from 20 mW to 2 W before the waveguide input grating coupler) was amplified by a polarization maintaining Erbium-doped fibre amplifier (PM-EDFA) and was sent to an optical band pass filter to remove the amplified spontaneous emission (ASE), originated by the EDFA. The band pass filter (BPF) showed a 3 dB bandwidth of 0.078 nm and a 10 dB bandwidth of 0.145 nm. A fibre coupler (50:50) was used to combine the pump signal with a weaker CW light beam originated from a tuneable external cavity laser (ECL) and set at a wavelength of λs = 1549.99 nm. The optical power of the signal beam (λs) was always kept at least 10 dB lower than the pump signal (λp) power level. The two waves were coupled to the waveguide under test by means of grating coupler devices, as depicted in Fig. γ} coefficients of the fabricated devices to be extracted, according to the following formula:Pi(L) and Ps(L) are the idler and signal power levels, measured at the output of the waveguide under test, respectively; Pp(0) represents the pump power measured at the input of the waveguide, Leff is the nonlinear effective length44 and η accounts for the phase-mismatch induced by chromatic dispersion. By placing the signal and pump beams relatively close to each other in wavelength (Δλ < 0.15 nm) the effect of dispersion can be neglected, thus allowing to consider η = 1. The experimental scheme was calibrated and validated by measuring a set of standard silicon waveguides fabricated in our cleanroom starting from a commercial SOI wafer . Figure Pp = 320 mW) while an example of a FWM efficiency-vs-pump power curve  is shown Fig. 44.Re{γ} can be assessed by adopting a CW-FWM-based schemeγ} for the waveguides in the three different wafers. The error bars were calculated through measurements on the 100 different waveguides of the same dimension that were written on each wafer.The graph in Fig. γ} coefficients obtained from the waveguides fabricated on Wafer 01, thus showing results for the standard, stoichiometric Si3N4. Green and purple dots represent data from Wafer 02 and Wafer 03 compositions, respectively. Re{γ} is influenced by the optical mode confinement achieved in each waveguide, which in turn depends on the waveguide dimensions. We calculated the effective area (Aeff) for each waveguide configuration using the following equation51:z component of the Poynting vector, z is the unit vector along the waveguide axis, aNL is the core cross section area and NL denotes the integration over the nonlinear region. A Finite Difference Method (FDM) solver was used to calculate the electrical and magnetic field distributions. Using the following equation:Aeff, the nonlinear refractive index of each wafer configuration can be extracted from the experimental data. Results are presented in Fig. n2 values, for each SixNy configuration, are shown in Table Aeff values (star symbols connected by red lines) show that the optical confinement is maximized at W = 1100 nm, W = 700 nm and W = 500 nm for Wafer 01, Wafer 02 and Wafer 03, respectively. These trends are reflected in the experimental data , confirming that the large enhancement observed in terms of Re{γ} −1. Finally, It is worth noting that the n2 value obtained for the crystalline silicon  is in good agreement with data reported by Lin et al.12.The value of {γ} Fig.  is mainlγ(ω) can be assessed by means of pulsed laser experiments, by following the procedure detailed in ref. The imaginary part of the nonlinear coefficient 14:By measuring the average power at the output of the sample under test as a function of the input peak power, the TPA-coefficients of the various waveguides were assessed. By assuming a hyperbolic-secant pulse temporal profile , the average output power can be related to the input peak power through the following equationγ}) were assessed by using equations (10.TPA-coefficients (thus Im{quations  and 12)γ}) were xNy layer compositions. No TPA power saturation can be observed in waveguides fabricated on Wafer 01, suggesting negligible αTPA. Wafer 02 waveguides also do not exhibit any TPA (second column), while relatively small TPA-induced losses are seen in the waveguides fabricated on Wafer 03, suggesting that the TPA threshold has been exceeded and that a further increase in the Si percentage in the material might cause additional nonlinear losses.Fig. αTPA coefficient of each SixNy composition was determined and these results are shown in Table By combining results obtained for each waveguide-width configuration, the 50:P0 represents the peak optical power at the input of the waveguide.The nonlinear phase shift caused by the interaction between a light beam and a third-order nonlinear device (which determines the efficiency of the device as a nonlinear medium) can be expressed as follows52. In order to evaluate the overall performance of our SixNy nonlinear waveguides we have numerically calculated equation (L = 0.1 m) for the three SixNy different material compositions studied here. The results are shown in Fig. 3N4); thanks to the absence of TPA, no saturation effect is observed on the achievable phase shift, even at high power levels (>10 W). On the other hand, the relatively low Kerr coefficient, strongly limits the achievable amount of nonlinear phase shift in these waveguides, making this configuration not suitable for practical nonlinear applications. By greatly increasing the silicon content in the SixNy composition, the Kerr response can be enhanced as shown in Fig. Leff, thus reducing the overall nonlinear efficiency; a maximum, non-saturated, nonlinear phase shift of π/2 can be achieved in 0.1-m-long waveguides of this configuration. The green line shows results obtained for the Wafer 02 waveguide. Thanks to the absence of TPA, no phase shift saturation effect is observed; moreover, due to the enhanced Kerr response  are required to achieve a π-phase shift thus increasing the total device footprint. Silicon-based nonlinear devices have been widely studied during the last 10 years. As already discussed previously, silicon exhibits a very large Kerr coefficient that makes it attractive for the implementation of nonlinear devices; unfortunately, this is also typically accompanied by a relatively high nonlinear loss (TPA and FCA) that can prevent the operation of such waveguides in the high power regime (>500 mW). Amorphous silicon has also been intensively investigated by researchers, showing reduced TPA compared to c-Si. The main issues regarding this material seem to be the increased linear loss value  and material instability and degradation reported after high intensity light exposure53. A number of Si-based devices are reported in Table 29, however TPA and FCA are increased at telecom wavelength due to the presence of germanium, making this material a better choice for longer wavelengths. AlGaAs and Ta2O5 are also reported in Table et al.33 (high aspect ratio waveguides) and several nonlinear waveguides have been shown in the literature37. However they are usually fabricated using LPCVD that requires high temperature steps. This reason has pushed research towards the development of PECVD SixNy devices that may offer comparable performance without the need for high temperature treatment. A very large Re{γ} has been demonstrated in ref. γ} of the waveguides presented herein has increased by about 3 times with respect to38, while at the same time, low loss and BEOL-CMOS-compatibility has been ensured, providing a novel platform, suitable for nonlinear optical processing experimentation.Table xNy waveguides. We showed that the material can be engineered such that it can exhibit properties that are well-suited for operation at either low of high power levels. The fabrication of Si-rich silicon nitride waveguides based on the PECVD technique at relatively low temperatures (350 °C), with a propagation loss of approximately 1.5 dB/cm was reported. We also demonstrated that the nonlinear Kerr response can be engineered and increased by an order of magnitude increase, in terms of n2, by optimizing the fabrication parameters. TPA can be kept negligible at 1550 nm, enabling high power operation, which is normally prevented in typical Si waveguides. A nonlinear phase shift of π can be obtained by using the developed SixNy material composition, making it suitable for the realization of telecom devices for advanced nonlinear applications in next generation optical networks.We presented a complete characterization of the linear and nonlinear properties of BEOL-CMOS-compatible SixNy layers were fabricated starting from a 150 mm diameter SiO2-on-Si substrate wafer. Nitride layers were deposited using a PECVD tool (model OIPT SYS 100), by employing the parameters specified in Table 2 cladding layer was finally deposited by PECVD using a standard recipe for the deposition of stoichiometric SiO2. Finally, the samples were cleaved to provide access to the butt-coupling devices. Standard Si- waveguides were also used as a reference. The standard silicon waveguide fabrication started from a 150mm SOI commercial wafer (220 nm top Si layer thickness). Waveguides were patterned and fabricated using the same processing steps as described above.The SiIn order to provide statistically significant results, each measurement presented in the paper was performed on a minimum number of 100 nominally identical waveguides. Error bars have been included in the graphs presented in the paper, which have taken into account both systematic errors and curve fitting standard deviations. The highest and lowest measured values were excluded from the calculation of each value showed in the manuscript. The detailed procedures adopted to evaluate the mean value of each measurement and its associated error are described below.xNy wafer compositions, for different waveguide widths . Each propagation loss value was estimated using cut back measurements, performed on a set of 11 waveguides of different lengths . The result of each cut-back measurement provided as output a fitted value (the propagation loss of a single waveguide width) and its associated standard deviation (αi ± σi). One hundred different cut back measurements were performed on one hundred, nominally identical, waveguide sets, thus providing a set of one hundred measurement results as follows: . The final, averaged, value was calculated as follows:The propagation losses were evaluated over the three SixNy material wafers. According to equation , providing a set of values: . The averaged values, for each single waveguide, along with their standard deviation values were calculated as follows:Kerr coefficients were evaluated for each waveguide width (500 nm to 1500 nm) across the three different Siequation , Re{γ} cγ} coefficient was assessed by employing an optical pulse transmission set-up, for each waveguide (for each different width), across the three SixNy wafers. For a single measurement we used a minimum of 40 input power levels. Each measurement session provided a single value of Im{γ} and its standard deviation σ. We repeated the same measurement session on one hundred nominally identical waveguides (L = 1 cm), producing a set of values: . The averaged Im{γ} values, for each single waveguide, along their standard deviation values were calculated as follows:The Im{


Ambrosia spp.) pollen grains, after being released from anthers, can be dispersed by air masses far from their source. However, the action of air temperature, humidity and solar radiation on pollen grains in the atmosphere could impact on the ability of long distance transported (LDT) pollen to maintain allergenic potency. Here, we report that the major allergen of Ambrosia artemisiifolia pollen (Amb a 1) collected in ambient air during episodes of LDT still have immunoreactive properties. The amount of Amb a 1 found in LDT ragweed pollen grains was not constant and varied between episodes. In addition to allergens in pollen sized particles, we detected reactive Amb a 1 in subpollen sized respirable particles. These findings suggest that ragweed pollen grains have the potential to cause allergic reactions, not only in the heavily infested areas but, due to LDT episodes, also in the regions unaffected by ragweed populations.Allergenic ragweed (The online version of this article (doi:10.1007/s00484-016-1139-6) contains supplementary material, which is available to authorized users. Ambrosia) have been introduced to Europe from North America. The foremost of these is common ragweed (Ambrosia artemisiifolia L.), which is an important weed in agriculture and a source of highly allergenic pollen. It poses a threat to human health, not only in its native North America but also in many parts of the world where it has been introduced. The plant has now become naturalised in Europe, where the largest sources of ragweed pollen are considered to be the Rhône Valley (France), parts of northern Italy, the Pannonian Plain and Ukraine  , but a later survey found that this area was no longer populated  and west (Berlin area)  of The combined analysis of aerobiological data, meteorological data and air mass trajectory calculations has shown that the majority of ragweed pollen grains recorded in Poznań arrived from south-easterly directions, particularly from the Pannonian Plain  could be present in ambient air collected during LDT episodes of ragweed pollen.The allergenic capacity of LDT ragweed pollen remains unclear. Pollen allergenicity could decrease or be lost altogether during flight in the higher layers of the atmosphere, where the action of factors such as air temperature, humidity and solar radiation on the pollen grains could impact on their ability to maintain allergenic potency  between the 11 August and the 28 September 2011, which is within the main flowering period of ragweed in Europe. Poznań is the largest city of western Poland with a population of ∼550,000 people CSO . It is aAmbrosia pollen grains in ambient air (Table 3 (P m−3) , thereby confirming their suitability for airborne allergen monitoring  HCO3 pH 8.1 and lyophilized. Lyophilized material was dissolved in 1/10 of the original volume in phosphate-buffered saline (PBS). Determination of allergen concentrations was performed using enzyme-linked immunosorbent assay (ELISA) following the protocol supplied by Indoor Biotechnologies company , with two exceptions: (i) the streptavidin-peroxidase (Sigma-Aldrich S5512) was used as an enzyme; (ii) the 3,3′5,5′-tetramethylobenzidine (Sigma-Aldrich T0440) was used as a substrate. The reaction was stopped by adding 2.5 M H2SO4 (5 N). The concentration of the Amb a 1 in the sample was determined by reading the absorbance at 450 nm. All samples were determined in duplicate. Each assay was calibrated with a standard curve and included with two controls. Interassay variability was 14 % (n = 14) in the higher range and 13.2 % (n = 14) in the lower range of the calibration curve. All extracts were diluted so that the values were within the linear range of the calibration curve. Reported values for each day are the mean of two filter parts. If the coefficient of variance between values from two filter segments were >25 %, the concentration of Amb a 1 in the third filter segment was determined and included into analysis. Daily average concentrations of Amb a 1 are expressed as pg m−3.This study focused on Amb a 1 because this protein is the major allergen of A. artemisiifolia distribution  model , relative humidity (%), sum of rainfall (mm), wind speed (m/s) and dew point (°C). The meteorological station was located between the Hirst (pollen) and Chemvol® (Amb a 1) sampling devices. As with the pollen and allergen sampling, meteorological parameters included in the statistical analysis were recorded from midday to midday and described as “daily averages” throughout Table .−3 and/or Amb a 1 > 20 pg m−3 were taken into account in statistical analysis (n = 9). Nonparametric Spearman’s rank correlation analysis was used to examine relationships between daily average pollen concentrations, daily average Amb a 1 levels in both air fractions, pollen potency and meteorological parameters. Differences between the level of Amb a 1 in PM > 10 μm and 2.5 > PM > 10 μm air fractions were analysed by the Wilcoxon signed-rank test. The statistical significance of the results was determined by comparing the p value with the significance level (α = 0.05). The amount of Amb a 1 in one ragweed pollen grain  was calculated by dividing the daily average Amb a 1 concentrations (detected in both air fractions) by the daily average ragweed pollen level. All calculations were carried out with the Statistica 10 software package.In order to obtain more robust results , only days with daily average ragweed pollen levels >3 P m−3)  23–27 of August; (II) 4–5 September; and (III) 17–18 September  than the Amb a 1 in pollen grain sized particles. On average, seven times more Amb a 1 was found in stage 1 compared to that found in stage 2. The ratio between the level of Amb a 1 in both air fractions (expressed in %) varied between 0 (no Amb a 1 in stage 2) to 17.2 %. The mean seasonal ragweed pollen potency was 4.3 pg Amb a 1/pollen grain, which is similar to the results obtained for the major allergens of birch and olive recorded at sites belonging to the HIALINE network  , usually arrived from a southerly direction after passing over southern Poland, Czech Republic, central and eastern Austria and Hungary. On the other hand, the amount of ragweed pollen recorded in Poznań dropped to very low levels (bi-hourly ragweed pollen concentrations ≤5 P m−3) when air masses approached from areas not considered to be centres of ragweed infestation like Germany to the west.In order to determine whether the observed differences in ragweed pollen potency were the result of the ragweed pollen originating from different source areas, we have examined air mass trajectories based on meteorological calculations from the GDAS reanalyzed meteorological dataset in combination with the HYSPLIT trajectory model Below is the link to the electronic supplementary material.


Although plastic slip in MgSiO3 post-perovskite has attracted considerable attention, the twinning mechanism has not been addressed, despite some experimental evidence from low-pressure analogues. On the basis of a numerical mechanical model, we present a twin nucleation model for post-perovskite involving the emission of 1/6 <110> partial dislocations. Relying on first-principles calculations with no adjustable parameters, we show that {110} twin wall formation resulting from the interaction of multiple twin dislocations occurs at a twinning stress comparable in magnitude to the most readily occurring slip system in post-perovskite. Because dislocation activities and twinning are competitive strain-producing mechanisms, twinning should be considered in future models of crystallographic preferred orientations in post-perovskite to better interpret seismic anisotropy in the lowermost lower mantle.The plastic properties of MgSiO The discovery in 2004 that bridgmanite, the magnesium silicate with perovskite structure which is the main constituent of the lower mantle, is not stable at pressures comparable to those of the D” layer and transforms at c.a. 120 GPa into a distinct structurally anisotropic phase, named post-perovskite3, has attracted considerable attention. The potential link between this new phase and the seismic anisotropy in the lowermost lower mantle received more support when it was established that the post-perovskite phase can form only in a relatively cold mantle4. The high-pressure magnesium silicate post-perovskite phase exhibits an orthorhombic layered crystal structure of space group Cmcm with strongly different lattice parameters3. Exhibiting layers of SiO6 octahedrons parallel to {010}, the structure is thus highly anisotropic with such a structural characteristic being potentially related to the strong seismic anisotropy of D”. To further establish the role of post-perovskite and to ultimately decipher the flow patterns at the base of the mantle, it is necessary to understand how crystal preferred orientations (CPOs) develop in this phase during plastic flow5. Given the very high-pressure required to stabilize the magnesium silicate post-perovskite, only a few set of experiments have been conducted on this phase8 and most experiments have been performed on analogue materials with the same crystal structure11, but stable at lower pressures. This includes calcium iridate (CaIrO3), which is stable at ambient pressure15. Unfortunately, all these experiments have led to conflicting results, possibly because of textures inherited from phase transformations16 and differences in the crystal chemistry of the analogue materials18. Given the formidable difficulties of deformation experiments under very high pressures, numerical modelling currently represents a very attractive alternative. Using atomic-scale modelling of dislocations21, we have shown that shearing the post-perovskite structure occurs easily along the shortest [100] lattice repeat in the Mg-O layer (010) plane, with a lattice friction of 2 GPa20. The other dense direction in this plane, [001], is the second easiest21 (ca. 3 GPa). However, shearing the Si-bearing layers appears to be much more difficult, because of the breaking of the strong Si-O bonds. Indeed, the lattice friction opposed to [100](001) is on the order of 17 GPa19. On the basis of these results, strong CPO along (010) is thus expected in post-perovskite. However, this cannot be the end of the story because a crystalline aggregate must sustain some strain components along the three directions of space to satisfy strain compatibility. Therefore, to provide reliable models of crystal preferred orientations and hence of seismic properties, it is necessary to understand which deformation mechanisms are active in this structure.Seismic anisotropy is one of our major sources of information about the dynamic processes and flow in the Earth’s mantle. In contrast to the bulk of the lower mantle, which appears to be mostly isotropic, the lowermost lower mantle exhibits strong seismic anisotropy and major heterogeneities. In particular, distinct anisotropy signatures are found in regions thought to be associated with cold downwelling3 post-perovskite14. In this paper, we show that [010] dislocations are not stable in MgSiO3 post-perovskite, leading to partial dislocations that may be linked to mechanical twinning. Hence we present a hierarchical numerical model of the mechanical twinning in MgSiO3 post-perovskite at 120 GPa3, which is compared with the dislocation activity to assess its possible relevance in plastic flow and CPO development in post-perovskite in the lowermost lower mantle.Mechanical twinning is a deformation mechanism that has received little attention despite microscopic observations of its occurrence in deformed CaIrO3 post-perovskite phase exhibits an orthorhombic structure. The computed lattice parameters at a pressure of 120 GPa are a = 2.47 Å, b = 8.11 Å and c = 6.14 Å. The very strong anisotropy of the unit cell and the characteristic layering of the structure have led to a focus on plastic shear along [100]. Not surprisingly, this shear is very easy to produce along the magnesium layer in the (010) plane20. The second easiest slip system also corresponds to shear in (010) but along the [001] direction21. Shear along [010] raises some questions, because it would require activating dislocations with a very large Burgers vector. This is usually unfavourable since the elastic energy of a dislocation scales to the square of the modulus of the Burgers vector. The classical Frank criteria show that [010] dislocations are not stable and decompose into two ½<110> dislocations because b2[010] > 2xb2½<110>. Indeed, in the orthorhombic C-lattice, ½<110>, with a length 4.2 Å, is one of the shortest lattice repeats of the structure and consequently represents another potential Burger vectors. Therefore, in this work, we start by addressing the properties of ½<110> dislocation cores.MgSiObp = 1/6[110] and 1/3[110]  is on the order of a few nanometres  and also respond to an applied stress with significantly different behaviours. Here, we describe the case of a dislocation with an initial edge character . For a typical simulation cell containing approximately 40,000 atoms , we calculate that a shear stress in the range of 2.5–2.8 GPa triggers the expansion or the closure of the stacking fault, but this process is due to the sole displacement of the 1/6[110] partial dislocation . The onset of motion of the 1/6[110] partial dislocation is fairly insensitive to the initial stacking fault width or the investigated atomic system size. The 1/3[110] partial dislocation does not glide unless a high stress is applied. However, the 1/3[110] partial dislocation never glides as a partial dislocation, indeed we compute that the full dislocation first recombines into the compact ½[110] Burgers vector dislocation core  indicate their strong tendency to spread, and hence to glide, in {110} planes. Indeed, regardless of the dislocation character, a full ½[110] Burgers vector spontaneously dissociates in {110} into two partial dislocations bounding a stacking fault with the typical perovskite-like octahedral interconnections by corners Fig. . The mea10] Fig. . The equres Fig. . Resulti10 Burger3 post-perovskite. Instead, the only dislocation that is mobile is the 1/6[110] partial dislocation, which expands a stacking fault in its wake. Activation of partial dislocation is a deformation mechanism, known to occur in some materials , but that may also lead to another mechanism: mechanical twinning. Indeed, because dislocation glide and deformation twinning are complementary mechanisms potentially contributing to plastic deformation, the occurrence of the highly mobile 1/6<110> partial dislocation raises the possibility of a mechanical twinning mechanism associated with the ½<110> {110} slip system in MgSiO3 post-perovskite at 120 GPa. Figure 3 post-perovskite, although, interestingly, {110} twin domains have been detected in deformed CaIrO3 post-perovskite samples quenched to ambient pressure14. These experimental observations in this analogue material, which is stable at ambient pressure, provide additional motivation for investigating ½<110> {110} deformation twinning in MgSiO3 post-perovskite under pressure conditions corresponding to the D” layer. Because great care must be taken with plasticity mechanism interpretations based on analogues24, we conducted calculations for both phases (CaIrO3 and MgSiO3 post-perovskites) to assess a possible bias .The conclusions of this study are that neither the [010] nor the ½[110] dislocations are stable or lead to active deformation mechanisms in MgSiObp = 1/6[110] shear displacement in (isf), forming a one-octahedron-thick perovskite-like lamella  in the , i.e., the barrier preventing a N-layer partial fault from becoming a N + 1-layer partial fault, is denoted γut, and the energy minimum, 2γtsf, is twice the energy of the twin stacking fault . The energy difference between γut and 2γtsf defines the so-called twin migration energy γTM26. As previously discussed by Kibey et al.27 and Wang et al.28, the pathway barriers γus, γut and γTM  through successive nucleation of new dislocation loops. This thickening (growth) stage is generally easier than nucleation30. The total energy associated with the process of twin nucleation31 can be defined as follows:EGPFE is the twin boundary energy28; Eint is the energy term resulting from the interaction of twinning dislocations; El is the twinning dislocation line energy; and W is the work of the applied stress. In this equation, the energy term El is independent of the spacing d between the twinning dislocation, and it consequently does not contribute to the critical twin nucleation stress σcr and will be therefore disregarded in the following.Considering that deformation twins usually form as individual thin plates embedded in the matrix or in contact with a free surface or a grain boundaryons Fig.  belongins = bp/Δh) close to 0.6, twin domains in MgSiO3 and CaIrO3 post-perovskites  versus the distance d between twinning dislocations can be neglected. In such a twin lamella, the twinning dislocations are assumed to belong to two flat surfaces and the total interaction energy of all twinning dislocation31 is given by:N is the number of twinning dislocations of Burgers vector bp; L is the arbitrary size of a crystal grain th twinning dislocations  describes the disregistry function of N twinning dislocations characterized by the uniform half-width ζ and separated from each other by the invariant distance d /bp(MgSiO3). For γSF, f(y) is considered to be in the range 0 ≤ f(y) ≤ bp, corresponding to the intrinsic stacking fault region, while γtwin is computed for bp < f(y) ≤ Nbp with N > 1, describing the disregistry function of the twin domain nucleated on the existing stacking fault configuration.The twin boundary energy ape Fig.  and on te d Fig. . For MgSmentclass2pt{minimγSF and γtwin  plane remains an open question which is fundamental to address in order to model CPO fabrics and the resulting seismic properties. The fact that [010] and ½[110] dislocations spontaneously decompose into partial dislocations and that only one of these is able to move raises a fundamental question: which mechanism mediates plastic strain out of the (010) plane? Is it plastic slip of either perfect or partial dislocations, or is it mechanical twinning? These three mechanisms involve the emission of a leading 1/6[110] partial dislocation as the first step. In case of full dislocation slip, this first event  must be followed by the nucleation of the associated trailing 1/3[110] partial dislocation. In case of twinning, the nucleation of the first 1/6[110] partial dislocation is followed by the nucleation of a second 1/6[110] adjacent twinning dislocation. This process may or may not occur, because it may be more favourable to nucleate a second partial dislocation at a completely different location in the crystal rather than next to the first one, thus suggesting that extended partial slip may be a third possible strain-producing mechanism.At the microscopic scale, plastic deformation can result from several mechanisms. In this study we focus on intracrystalline mechanisms. Owing to the very peculiar crystal structure of post-perovskite, dislocation glide appears easy only in the plane of structural layering32, the emission of a partial dislocation can be associated with the energy barrier γus . For full dislocation slip, by analogy, an energy barrier can also be associated with the nucleation of the trailing partial dislocation by considering the difference between γisf and the unstable energy corresponding to the trailing partial dislocation. Classically, in fcc metals, the emission of the trailing partial dislocation  involves thus the sole term (γus - γisf). The situation is different here for post-perovskite compounds, in which the trailing partial dislocation corresponds to a different Burgers vector. The emission of the trailing partial dislocation of partial Burgers vector 1/3<110>, which is basically not mobile, involves a second unstable energy γus/3[110]1 that is higher than γus   Table , which iγut - γisf) and γus  have never been reported in TEM observations of deformed post-perovskite analogues. The situation is not so straightforward in MgSiO3 post-perovskite, because (γut - γisf) is comparable to γus. In that case, the simple consideration of γut/γisf and γus/γisf ratios is not sufficient for robust conclusions34. The twinning mechanism efficiency is known to result from a complex interplay between the various energies associated with the GPFE energy landscape, as pointed out by Kibey et al.27 in fcc Cu-Al alloys. Going beyond these simple energetic considerations is the goal of the hierarchical model described above, which computes the twinning stress on the basis of the entire GPFE landscape. For the MgSiO3 post-perovskite phase, the critical twinning stress σcr is approximately 880 MPa, computed according to the GPFE calculated at a confining pressure of 120 GPa. By itself, σcr already appears to be a rather low value. This value can be further compared to the lattice friction opposed to the motion of a 1/6<110> partial dislocation . By computing the ½<110> dislocation core structure and the evolution of the core under applied shear stress, we find that the motion of the leading partial dislocation can occur only at a stress level of 2.7 GPa, which is 3 times higher than the critical twinning stress. In a deforming grain of MgSiO3 post-perovskite, the twinning stress will be reached well before the lattice friction of partial slip is overcome. This suggests a high ability for MgSiO3 post-perovskite (as in CaIrO3) to readily exhibit mechanical twinning, according to the present mechanism involving <110>{110} slip system.The competition between the slip of individual partial dislocations and twinning can be assessed by comparing 20 and [001](010)21 are 2 and 3 GPa, respectively. Thus, twinning appears to be the best candidate to account for the, so far, missing mechanism responsible for shear along the [010] direction. Up to now, such twinning mechanism has never been considered to explain CPO development in post-perovskite deformation, although the contribution of deformation twinning is especially important in low-symmetry crystals in which information on five independent slip systems is needed to describe a general deformation of the material.Finally, the twinning stress, computed here at 0 K and supposed to be mostly athermal35 and the all-electron projector augment-wave (PAW) method as implemented in VASP37. To achieve computational convergence, a plane-wave cut-off Ecut of 600 eV was applied. The first Brillouin zone was sampled using the Monkhorst-Pack scheme38 with an 8 × 6 × 1 k-point grid. The convergence energy is 10−3 meV/atom. Simulations were performed at constant volume, corresponding to a bulk volume under confining pressure of 120 GPa and 0 GPa for MgSiO3 and CaIrO3, respectively. All calculations were performed using fully periodic atomic arrays containing 180 atoms. The simulation cells were built on vectors a1 = ½[110], a2 = [001] and a3 = [x axis and that the  to the twinning plane. Along the z axis, the simulation cells were as thick as 18 octahedral SiO6/IrO6 layers, i.e., 43.34 Å for MgSiO3 and 54.99 Å for CaIrO3. The interplanar distance Δh between the subsequent {110} stacking planes was equal the size of one SiO6/IrO6 octahedron along z.Generalized planar fault energy (GPFE) calculations were performed based on the density-functional theory (DFT) within the generalized gradient approximation (GGA), with the PW91 parameterisationbp. For the second layer fault, a new shear level is defined one octahedron above the already existing first layer fault. Atoms above this new level are rigidly shifted by the same incremental amount to reach a total displacement of 2 bp. A similar procedure is repeated to create the third, fourth, etc layers. In order to keep the periodicity of the atomic array, the a3 vector is tilted by the displacement vector. Practically, the path was sampled using eleven shear steps per twinning Burgers vector bp. The full GPFE curve was initially computed using the classical molecular statics simulations described below using pairwise potential of Buckingham form. After this first minimization, we selected four shear steps per twinning Burgers vector with increment 0.25 bp. These configurations were relaxed using the VASP code. For the minimum energy configurations on GPFE curves , full atomic relaxation, including the directions normal and parallel to the shear plane, was allowed. For the high-energy configurations, the degrees of freedom along the  simulations. The force field used corresponds to the Buckingham form of a pairwise potential parameterized by Oganov et al.39. Calculations were performed using the LAMMPS package, relying on Ewald summation methods for Coulombic interactions40. Optimization of the dislocation core configurations was performed using a conjugate gradient minimizer on a so-called periodic slab geometry containing a single dislocation core, as described by Hirel et al.41. The simulation cells were designed to be fully periodic along the dislocation line direction  and the dislocation glide direction . Along the direction normal to the {110} glide plane (Cartesian z), atoms at the bottom and at the top of the supercell were kept fixed to their regular positions given by the dislocation long-range elastic field. The total width  of the layer with fixed atoms was equal to the potential short range cut-off distance of 12 Å. For screw dislocations, the supercell parameter aligned with y was tilted by ½b along x to maintain periodicity along the dislocation glide direction. The simulation cell size was equal to one unit cell repeat along x, 160 Å along z and varies from 300 to 450 Å along y. Such simulation systems contained approximately 36,000–70,000 atoms. To initiate a dislocation glide, εxz and εxy strain components were gradually increased for screw and edge dislocations, respectively. To ensure quasi-static loading, the sheared atomic configuration was optimized every shear strain increment of 10−5–10−4.For MgSiOSupplementary material


Colloidal TlI and TlBr nanocrystals are prepared, which show violet-blue luminescence, high carrier mobility and long diffusion lengths thus suggesting the suppression of deep-trap states. 2+ and Tl+ exhibit a 6s2 inert pair of electrons and strong spin–orbit coupling. Although the crystal structure of TlX is not perovskite, the similarities of its electronic structure with lead halide perovskites motivated us to prepare colloidal TlX NCs. These TlX NCs exhibit a wide bandgap (>2.5 eV or <500 nm) and the potential to exhibit a reduced density of deep defect states. Optical pump terahertz (THz) probe spectroscopy with excitation fluence in the range of 0.85–5.86 × 1013 photons per cm2 on NC films shows that the TlBr NCs possess high effective carrier mobility (∼220 to 329 cm2 V–1 s–1), long diffusion length (∼0.77 to 0.98 μm), and reasonably high photoluminescence efficiency (∼10%). This combination of properties is remarkable compared to other wide-bandgap (>2.5 eV) semiconductor NCs, which suggests a reduction in the deep-defect states in the TlX NCs. Furthermore, the ultrafast carrier dynamics and temperature-dependent reversible structural phase transition together with its influence on the optical properties of the TlX NCs are studied.Colloidal lead halide based perovskite nanocrystals (NCs) have been recently established as an interesting class of defect-tolerant NCs with potential for superior optoelectronic applications. The electronic band structure of thallium halides  show a strong resemblance to lead halide perovskites, where both Pb Herein, we present a new variety of colloidal semiconductor NCs in the form of TlX (X = I and Br), with optical band gaps <500 nm, which exhibit photoconductivity with high effective carrier mobilities (∼330 cm2 V–1 s–1), long diffusion lengths (∼1 μm), and reasonably high photoluminescence (PL) efficiency (∼10%). These results suggest the suppression of defect mediated charge trapping in TlX NCs compared to many other semiconductors with band gaps >2.5 eV.Defect tolerance signifies the tendency of a semiconductor to retain its electronic, optical and optoelectronic properties even in the presence of defects. One of the major drawbacks of colloidal semiconductor nanocrystals (NCs) such as CdSe is surface defects, which trap charge carriers and thus severely limit their electronic, optical, and optoelectronic properties.et al.2 reported CsPbX3  perovskite NCs with exceptionally high (∼90% for X = Br) PL efficiency. Subsequently, we reported the suppression of PL blinking and terahertz (THz) conductivity with high carrier mobility (∼4500 cm2 V–1 s–1) within a CsPbBr3 NC.4 These results suggest that CsPbBr3 NCs do not exhibit mid-gap deep trap states, despite their high surface-to-volume ratio. Therefore, CsPbX3 NCs hold great potential as new type of practically trap-free NCs with superior optical and optoelectronic properties,5 as has been already reported in terms of achieving high-efficiency light emitting diodes (LED),7 solar cells,8 low threshold for lasing,9 strong nonlinear absorption together with multi-photon pumped lasing,11 and single photon emission.12 There has been significant progress in designing and exploring the properties of CsPbX3 and MAPbX3 (MA = methylamine) NCs with various shapes, sizes and compositions.23In early 2015, Protesescu 3 perovskite NCs motivated us to search for other types of trap-free colloidal semiconductor NCs, particularly with high optical gaps corresponding to the UV-blue region, where the probability of the formation of deep-trap states increases. In this regard, TlX NCs have great potential since Tl+ possesses a number of common characteristics with Pb2+, such as Tl+ is isoelectronic with Pb2+ and they both possess a lone pair of outermost s-electrons. The anti-bonding interactions of the filled 6s orbital of Tl+ with the 4p/5p orbital of halide forms the valence band maximum (VBM), as shown in 24 On the other hand, Tl+ 6p orbitals mainly contribute to the conduction band minimum (CBM). Another anticipated common feature between Tl+ and Pb2+ is strong spin–orbit coupling in the 6p band.27 Thus, both 6s2 electrons and spin–orbit coupling yield the desired electronic band structure for TlX and CsPbX3, where the anti-bonding states form the VBM and spin–orbit coupling stabilizes the CBM, as depicted in 28 Such electronic band structure is expected to reduce the deep-trap states in semiconductor nanocrystals. Therefore, metal halides with different compositions and structures, but with a valence orbital band structure similar to 31The successful applications of CsPbX32 whereas, TlBr exhibits a cubic structure with a band gap of 3.05 eV (407 nm).33 The crystal structures of both TlI and TlBr are different from that of ABX3 perovskite ((A): monovalent metal/organic ion, (B): divalent metal ion, and X: halide ion), but they are analogous to ABX3 in terms of the valence orbital band structure. Bulk and thin films of TlX and its derivatives have already been explored as potential candidates for room temperature X-ray and gamma-ray detection.36 Herein, we report colloidal TlX NCs, where their optical properties and ultrafast optical pump THz probe (OPTP) spectroscopy suggest the suppression of deep-defect mediated charge trapping. Furthermore, temperature dependent structural phase transition is observed using variable-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC). To the best of our knowledge, this is the first report on colloidal TlX NCs, which show interesting properties suitable for optoelectronic applications in the UV-blue region. However, the toxicity of Tl based compounds is well-known,38 which will be a major hindrance for the many real life applications of TlX NCs.At room temperature, TlI exhibits an orthorhombic structure with a band gap of 2.86 eV (434 nm),+ and X– ions. The fast growth of NCs makes it difficult to isolate smaller NCs and study the growth kinetics. Another intrinsic behaviour of TlI is that the crystal phase transition occurs at around the reaction temperatures (170–300 °C). Therefore, controlling the crystal structure of TlI NCs becomes more challenging. The growth of TlX NCs during our synthesis was stopped immediately after the precursors were injected by applying a bath containing dry ice in acetone. Lowering the reaction temperature is beneficial to achieve both crystal structure homogeneity for the TlX product NCs and suppression of NC growth. The product NCs exhibit colloidal stability for about a week under ambient conditions. Further modification of the reaction methodology is required to achieve better separation of the nucleation and growth processes in order to control the size and shape of the nanocrystals over a wider range. In this regard, exploring various surface ligands and washing procedures might help to achieve better quality nanocrystals with improved colloidal stability and optical properties.We employed a hot-injection route for the colloidal synthesis of TlX NCs, as discussed in the ESI. Colloid1H NMR) spectra given in Fig. S4 of the ESIElemental analysis by energy dispersive X-ray spectroscopy (EDS), which is shown in Fig. S1 of the ESI,39 TlI. The inset in + and I– ion. Thus the coordination environment of TlI is different from that in CsPbI3 cubic perovskite.2 Samara et al. reported polymorphism in bulk TlI with ∼1% difference in lattice energy between the cubic and orthorhombic phases.39 This prompted us to study the possible crystal phase transition in our organic capped colloidal NCs, where surface energy plays a crucial role in contrast to their bulk counterpart. The powder XRD patterns of the TlI NCs in the temperature range of 303 to 543 K together with the DSC data  as a function of temperature (T) for the TlI NCs is shown in  for both phases of the TlI NCs. For the orthorhombic phase, αV = 1.47 × 10–4 K–1 and for the cubic phase αV = 1.5 × 10–4 K–1. These αv values are in accordance with the previously suggested values for bulk39 TlI.The powder XRD pattern in 32 orthorhombic TlI. According to prior reports, the Bohr excitonic diameter of TlI NCs is expected to be in the range of 9–12 nm for the orthorhombic phase  and shallow defect states.42 Unlike the mid-gap deep trap states, shallow defects are almost delocalized and optically active. The absence of long (∼100 ns) lifetimes shows the absence of deep-trap related radiative recombination in the NCs. The qualitative nature of the PL decay profiles is similar for all three emission wavelengths shown in To understand the PL mechanism, the PL decay dynamics of the TlI NCs shown in 43 TlBr, at room temperature. The inset in + and Br– ion in cubic symmetry. Furthermore, the cubic phase of the TlBr NCs is stable in the temperature range of 303 K to 573 K with αV = 3.2 × 10–5 K–1  of the TlBr NCs is more than the estimated values (Table S1 in the ESIThe UV-visible absorption spectrum of the TlBr NCs in σ(tp)) of NC films on a non-conducting substrate (polystyrene in the present work) is proportional to the photo-induced THz transmission (–ΔE(tp)/E0(tp)), which is given by the following equation,44ε0 is the permittivity of free space, c is the speed of light in vacuum, d is the sample thickness, and na and nb are the refractive indices of the media on both sides of the film. In our experiment, na = 1, nb = 1.525 are the refractive indices of air and the polystyrene substrate, respectively. The thickness of the TlBr and TlI NC films are 11.2 μm and 8.1 μm, respectively, as obtained from the optical surface profilometry scans shown in Fig. S16 in the ESI.N0). Based on the previous reports45 on metal halide perovskites, we assume that the carrier mobility does not change significantly within the temporal window of our experiments. Thus, the THz transients essentially represent the temporal evolution of the free carrier density. Here, we argue that the THz probe responds only to the free carriers, not to the bound electron–hole pairs (excitons).46 This is evident from the positive real conductivities observed in the complex conductivity spectra shown in Fig. S17 in the ESI.47ai is the contribution of the ith process with a time constant τi towards the overall dynamics. The fits are shown as solid lines in –2. In this case, a single exponential with a time constant of 71.4 ps could fit the decay profile reasonably well. The details of the fits are given in Tables S3 and S4 in ESI.τ) are calculated as τ = ∑fiτi, where, fi (fi = aiτi/∑ajτj) signifies the contribution of the ith process towards the entire recombination dynamics.48We studied films of the TlBr and TlI NCs with average diameters of 28.7 nm and 8.4 nm, respectively. The NCs were coated on polystyrene substrates for OPTP measurements. The pump-induced change in THz transmissions were recorded by measuring the peak THz field  towards the overall decay (τ) as a function of fluence in f1τ1) is mostly independent of fluence. However, the contribution of the second process with intermediate time constants (f2τ2) increases with an increase in fluence (becomes slower), whereas the slowest process (f3τ3) becomes faster. The overall dynamics represented by the average life time (τ) becomes faster with an increase in carrier density, which follows the trend of f3τ3. We assign τ1 to the carrier trapping process (monomolecular or single particle) which is independent of pump fluence, and τ3 to the bimolecular (two particle) electron–hole recombination. The process with the time constant τ2 could possibly be a different type of carrier trapping that becomes slower with an increase in pump fluence because of the reduction in the density of trap states. A possible kinetic scheme for this process is: , where, ne/h is the carrier density and nt is the density of trap states which does not become saturated even at the highest pump fluence used here. This process is different from the other carrier trapping (τ1), already mentioned for the TlBr NCs and TlI NCs, where the trap states are saturated even at the lowest fluence of this study. Alternatively, the τ2 process could be an Auger recombination that leads to electronic heating, which slows the system down with an increase in fluence.49 Clearly, the recombination mechanism depends strongly on the excitation fluence within our experimental range. Measuring carrier dynamics at fluences lower than our experimental range will perhaps provide more insight into the carrier recombination mechanism. Certainly, further studies are required to understand the recombination mechanism, which will be investigated in our future effort.To understand how different components of the recombination mechanism respond to the change in pump fluence, we plotted the contribution of individual processes (σ) for both samples at different pump fluences. The effective carrier mobilities were calculated using equation, where, q is the elementary charge and N0 is the total carrier density. N0 = 2Nphφ, where, Nph is the density of absorbed photons and φ is the photon to free carrier conversion ratio. Our approach evaluates the total carrier mobility (μ) which has contribution from both electron mobility (μe) and hole mobility (μh). Here, we assume that each absorbed photon is converted into two charge carriers (electron and hole). Depending on the pump fluence, we find that the initial carrier density is in the range of 1.1 × 1015 cm–3 to 7.5 × 1015 cm–3 for the TlBr NCs and 1.8 × 1015 cm–3 to 9.8 × 1015 cm–3 for the TlI NCs. We emphasize that these carrier densities (pump fluences) are quite modest for evaluating mobilities and carrier recombination dynamics.46 Since the actual value of φ is not known, these measurements yield effective initial mobilities (φμ). It should be noted that this analysis considers the upper limit of Nph since we ignore the reflection and scattering losses if any. The initial mobilities  (εr = 30.6 for TlBr and εr = 21.6 for TlI at 20 °C)67 compared to materials such as CdS (εr = 8.9), CdSe (εr = 10.2), and ZnSe (εr = 9.2). This high dielectric constant in TlX is expected to promote excitonic dissociation which is undesirable for PL, but desirable for photoconductivity. A high dielectric constant not only reinforces decoupling of photo-generated excitons, but also screens charge carriers from ionic trap-states thereby improving charge transport.2813 photons per cm2, similarly to We measured the effective carrier mobility and diffusion length using contact-less THz AC photoconductivity with excitation fluence in the range of 0.85–5.86 × 102 V–1 s–1 and long diffusion lengths of 0.98 and 0.25 μm for the TlBr and TlI NCs, respectively. In general, wide bandgap (UV-blue region) semiconductor NCs often exhibit a high density of deep-defect states trapping the charge carriers. On the contrary, the combination of reasonably high PL QY, high carrier mobility and long diffusion lengths suggest a decrease in deep-defect states in our TlX NCs. These results suggest the possibility that the TlX NCs possess a defect-tolerant nature owing to their interesting electronic band structure  and TlI NCs (8.4 nm and 4 nm diameters) as new wide-bandgap materials with interesting optical properties in the near-UV to blue spectral region. The TlI NCs display an orthorhombic structure at room temperature and then undergo a reversible structural phase transition at 443 K (170 °C). On the other hand, the TlBr NCs exhibit a stable cubic phase in the studied temperature range of 303 K to 573 K. Both types of NCs exhibit excitonic absorption features and PL with a reasonably high QY of 6–10% at room temperature in the violet-blue region, in contrast to their bulk counterpart. Ultrafast OPTP spectroscopy was employed to study the carrier dynamics of the TlX NC films at different pump fluences. The TlBr NCs exhibit fluence dependent carrier dynamics which are different from the TlI NCs. Subsequently, this THz photoconductivity reveals high effective carrier mobilities of 329 and 220 cmtructure , which i


Legionella pneumophila is, by far, the species most frequently associated with Legionnaires’ disease (LD). Human infection occurs almost exclusively by aerosol inhalation which places the bacteria in juxtaposition with alveolar macrophages. LD risk management is based on controlling water quality by applying standardized procedures. However, to gain a better understanding of the real risk of exposure, there is a need (i) to investigate under which conditions Legionella may be aerosolized and (ii) to quantify bacterial deposition into the respiratory tract upon nebulization. In this study, we used an original experimental set-up that enables the generation of aerosol particles containing L. pneumophila under various conditions. Using flow cytometry in combination with qPCR and culture, we determined (i) the size of the aerosols and (ii) the concentration of viable Legionella forms that may reach the thoracic region. We determined that the 0.26–2.5 μm aerosol size range represents 7% of initial bacterial suspension. Among the viable forms, 0.7% of initial viable bacterial suspension may reach the pulmonary alveoli. In conclusion, these deposition profiles can be used to standardize the size of inoculum injected in any type of respiratory tract model to obtain new insights into the dose response for LD. Legionella are facultative intracellular pathogens that cause Legionnaires’ disease (LD), although some species are strictly intracellularLegionella. Man-made ecosystems  have been the source of several outbreaks of LD23467LegionellaA bioaerosol is defined as a collection of biological particles (including bacteria and fungi in addition to viruses and fragments of organisms) dispersed in the air1112Legionella to cause disease is associated with their ability to enter the respiratory system through respirable-size droplets as transporting particles. The conditions under which Legionella may be aerosolized and how they enter the respiratory tract upon aerosol generation remain poorly documented10The ability of Legionella4Droplet generation is thought to be a key step in the transmission of L. pneumophila transfer from the water in the reservoir to the air and then to the human respiratory tract, we evaluated the viability of the inoculum within the generated droplets. At least 14 physiological forms of L. pneumophila have been described according to the various environments they may colonizeLegionella detected by standard methods. Our preliminary studies on anthropic reservoirs have demonstrated the presence of VBNC forms of Legionella after water disinfection treatments (heatLegionella intracellular forms (intra-amoebic) has been little investigated in environmental samples21In addition to investigating the effectiveness of the ents heat. FinallyLegionella and size of Legionella-containing droplets25Legionella cells in airborne droplets of a given size. More specifically, new insights are provided into the size characterization of Legionella bioaerosols, including assessment of the number of viable (culturable (VC) or not (VBNC)) bacterial cells in respirable droplet size fractions. The secondary objectives were (i) to improve our knowledge of the role of aerosols in the transmission of Legionella using vibrating-mesh nebulization and (ii) to fully characterize an experimental set-up that enables determination of aerosol deposition profiles under laboratory conditions within an animal respiratory tract.Very few studies have sought to quantify the presence of all viable forms of ® vibrating-mesh nebulizer was performed in a glove box  by an artificial throat. A pump-simulating inspiration was used during nebulization to collect aerosols on the 13-DLPI stages. The size classification in DLPI was set at up to 10 μm, with evenly distributed impactor stages.Aerosol generation by an e-Baselove box . BrieflyLp1 008 suspensions of different concentrations and stages: exponential phase forms (EPFs) and stationary phase forms (SPFs) according to a procedure described by Faulkner and GardunoBacterial detection and viability before and after aerosolization were assessed using culture, qPCR and FCA on Lp1 008 EPF cells were composed mostly of cells approximately 1 μm long whereas Lp1 008 SPF cells consisted of polymorphic bacilli ranging from 1 μm to >5 μm. It is known that Legionella become highly polymorphic with age28Lp1 008 EPF cell suspensions seemed more pertinent for further testing.Legionella DC were observed in Lp1 008 SPF cell than in Lp1 008 EPF cell suspensions at the two concentrations tested (2 × 106 and 2 × 107/mL). Because the detection limit of FCA is 103 cells/mL, the use of a calibrated suspension at 2 × 106 CFU/mL was not appropriate. These results validated our choice to use Lp1 008 EPF cell suspensions at 2 × 107 CFU/mL for the characterization of Legionella aerosols.After FCA, more Legionella nebulization generate distinctive aerosols, they were compared with respect to cumulative distribution (NaF cumulative mass distribution versus Legionella cumulative distribution in % of genomic units). To determine whether sodium fluoride (NaF) and Legionella showed an MMAD of 3.4 μm ± 0.4 with a GSD of 4.1 μm ± 1.2. The particle size distribution of Legionella generated by the nebulizer is highly polydispersed in comparison with the transporting droplets. This difference in the polydispersity index should be due to different compositions of the aerosols , as measured during nebulization using the vibrating mesh technology.The aerosols produced using NaF as a chemical marker for size distribution of transporting droplets demonstrated a mass median aerodynamic diameter (MMAD) of 6.1 μm ± 0.4 with a geometric standard deviation (GSD) of 1.7 μm ± 0.1. Thus, the size distribution of transporting droplets generated by the vibrating-mesh nebulizer can be considered to be monodispersed. In addition, the distribution of airborne Legionella bacteria, which are several times larger than the water-transporting particles. Moreover, a liquid environment appears to be necessary for the bacteria to remain viable while airborne. Thus, we assumed that the bacterial spread that may cause LD requires the generation of water droplets small enough to be deposited in the thoracic region but large enough to contain viable and virulent Legionella. The size distribution of airborne droplets is critical for delivering viable bacteria to the respiratory tract, and the mass of droplets of this given size is related to the number of bacteria contained within. A specific size range of droplets (2–10 μm) is required for the bacteria to be delivered to the thoracic region14versus nose inhalation of aerosols), results may vary. Indeed, several studies have shown that nose-inhalation aerosols of more than 2.5 μm in diameter cannot reach the pulmonary alveoli31Particles with aerodynamic diameters in the submicron range cannot transport Legionella DNA was in a particle size range of 1.6–9.92 μm . Indeed, the gravitational settling speed of a particle is determined by the opposing effects of the gravitational force and the aerodynamic resistance. The aerodynamic drag force depends on the shape and orientation of the particle with respect to its direction of motion. Thus, a cylindrical particle settles under gravity with an orientation parallel to its direction of motion. This phenomenon was observed with inhalation of fibrous particles such as asbestosWe suggest two possible explanations why the particle size of the remaining 6.6%  of the total airborne m-1.6 μm , Table 1detected  and 2. T0.949 μm  and 2. TLegionella DNA that were detected in the size range of 0.949–1.6 μm can be attributed to airborne Legionella in transporting droplets (entrapped or not) because the droplet diameter that may contain these bacteria is between 1 and 2 μm14The detection of culturable cells from the 262 nm cut-off diameter of the DLPI cascade impactor clearly shows that viable bacteria that were not entrapped in droplets were aerosolized in the 262–949 nm range. Specifically, the detection of culturable cells of 262 nm is to be expected if we consider the orientation of the bacteria with respect to their direction of motion in the cascade impactor, given the bacteria’s rod-like shape and their size (approximately 1–2 μm long and 0.3–0.9 μm wide). Finally, the 3.8% of the total airborne Legionella (Legionella (entrapped or not) in droplets (28–262 nm), airborne bacteria not entrapped in droplets and oriented by the air flow direction during the respiration process (0.262–0.949 μm), airborne bacteria (entrapped or not) in droplets (0.949–1.6 μm), and airborne bacteria packaged in droplets (1.6–9.92 μm).Thus, we can identify 4 modes of transportation during the aerosolization of gionella : cellulaLegionella DNA or 7% of initial bacterial suspension in the 0.26–2.5 μm range reached pulmonary alveoli. Among viable forms (VC and VBNC) as determined by FCA and cultivability, 43% of total airborne viable Legionella or 0.7% of initial viable bacterial suspension reached the pulmonary alveoli  only3637Few attempts have been made to investigate the sizes of liquid aerosols that have disease potential. Insights into the human dose response for LD are determined by quantitative microbial risk assessment (QMRA) techniques that animal model data may provideLegionella by vibrating-mesh technology, which enables delivery of Legionella aerosol to a laboratory animal model.Animals are infected via intratracheal, nasal, intraperitoneal injections or tracheal instillation. This study is the first investigation of the aerosolization of 7 CFU/mL (2–3 days with Lp1 008 EPF cells). The aerosol experiments were performed in triplicate . The biological analysis of each 13-DLPI stage was performed in duplicate .Results presented in the tables and figures are for a bacterial concentration of 2.10Legionella pneumophila serogroup 1 GFP (green fluorescent protein)-modified strain was used39LP1 008-GFP strain was stored at −80 °C in Cryobank tubes . After thawing, the strain was plated onto BCYE agar  supplemented with chloramphenicol  at 8 mg/mL for 72 h at 37 °C (for plasmid selection) and then re-plated onto the same medium and incubated at 37 °C for another 2–3 days to obtain Lp1 008 EPF cells or for 5–6 days to obtain Lp1 008 SPF cells. These cultures were then used to achieve 5 mL suspensions in sterile normal saline (0.9% NaCl) water at a final concentration of 2.106 or 2.107 CFU/mL. An optical density of 0.2 at 600 nm  was taken as a reference for a suspension containing 108 CFU/mL. The 5 mL suspensions were immediately placed in the nebulizer for cascade impactor experiments to determine the size distribution of airborne particles.A ® Nebulizer System . The mechanism of aerosol generation was based on a perforated oscillating membrane technology. It consists of the high-frequency vibration of a very thin membrane with a multitude of micrometric holes. The holes allow very fine nebulization from the liquid in the device’s reservoir eservoir .−1, particles were impacted depending on their inertia-related aerodynamic diameter in one of the 13 size fraction stages. The size classification in DLPI was set at up to 10 μm, with evenly distributed impactor stages. In each size fraction, the particles were collected on collection substrates to obtain their gravimetric size distribution. Specifically, the mass median aerodynamic diameter (MMAD) and the geometric standard deviation (GSD) were calculated. The MMAD was defined as the median of the distribution of the airborne particle mass with respect to the aerodynamic diameter. The GSD was used to characterize the dispersion of aerosols. When the GSD was lower than 2, aerosols were considered to be monodispersed; when it was higher than 2, the particle size distribution was polydispersed3241The airborne particle size distribution was assessed using a 13-stage cascade low-pressure impactor . At an airflow of 10 L minWe used NaF as a molecular tracer for aerosols in order to determine the aerodynamic size distribution of the droplets according to the NF EN 13544-1 standard. For this purpose, 4 mL of NaF (2.5 wt%) was introduced into the nebulizer reservoir. The nebulizer was connected to the DLPI via an artificial throat  . The NaF3241Legionella in viable conditions. Thus, the concentration of Legionella bacteria depending on the water droplet inertia-related aerodynamic diameter should also be determined. These experiments were conducted inside a Glove Box  to prevent infection of the experimenters. The same DLPI protocol as that used for the NaF aerosol experiments was employed, except that each stage of the cascade impactor was covered with a specific membrane for Legionella collection . After nebulization, each membrane potentially retaining bacteria was collected in 5 mL of physiological water. After 30 sec of sonication , the bacterial suspension of each 13-DLPI stage was analyzed by culture (viable and culturable cells—VC), flow cytometry (percentage of VBNC cells) and qPCR .A bacterial analysis of the size-classified particles was performed when a bacterial suspension was introduced into the nebulizer reservoir. Indeed, transporting water droplets must be large enough to contain Lp1 008-GFP cells. The forward and reverse primers were 5′-AGAGTGCCATGCCCGAAGGT-3′ and 5′-AAGGACAGGGCCATCGCCAA-3′, respectively. Plasmid DNA was extracted from all samples with the NuCleoSpin Plasmid kit (Macherey-Nagel) according to a procedure described by Cormack et al. Real-time quantitative PCR (qPCR) was carried out on an ABI Prism 7500 automate (Applied Biosystems), using the 2X Power SYBR® Green PCR Master Mix (Life Technologies) under the following conditions: initial denaturation for 15 min at 95 °C, 50 cycles consisting of 15 s denaturation at 95 °C, and 1 min annealing and elongation at 60 °C. At the end of every elongation step, the fluorescence of the incorporated SYBR green dye was measured. After 50 cycles of amplification, a melting curve program was incorporated to check for any primer dimers or other non-specific amplifications. For each experiment, three independent extractions, on the initial calibrated suspension and on each 13-DLPI stage, were performed, and each extraction was run in duplicate. The results were analyzed using Sequence Detection Software version 1.4 . The results were expressed in genomic units (GU) of amplified GFP sequence.The bacteria were quantified using a GFP mut2 sequenceLegionella colonies (CFU) expressing GFP.Cultivability assessment (determination of the number of viable and culturable cells—VC) was performed on initial bacterial suspensions and on bacterial suspensions collected from each 13-DLPI stage. Experiments were performed in duplicate. Each bacterial suspension (100 μL) was plated on two BCYE agar samples with or without chloramphenicol supplementation and incubated at 37 °C for 72 h. All BCYE plates were observed under UV at 366 nm to enumerate As previously described2023How to cite this article: Allegra, S. et al. Characterization of aerosols containing Legionella generated upon nebulization. Sci. Rep. 6, 33998; doi: 10.1038/srep33998 (2016).