Focal slowing (<4 Hz) of brain waves is often associated with focal cerebral dysfunction and is assumed to be increased closest to the location of dysfunction. Prior work suggests that slowing may be comprised of at least two distinct neural mechanisms: slow oscillation activity (<1 Hz) may reflect primarily inhibitory cortical mechanisms while power in the delta frequency (1–4 Hz) may correlate with local synaptic strength. In focal epilepsy patients, we examined slow wave activity near and far from the seizure onset zone (SOZ) during wake, sleep, and postictal states using intracranial electroencephalography. We found that slow oscillation (0.3–1 Hz) activity was decreased near the SOZ, while delta activity (2–4 Hz) activity was increased. This finding was most prominent during sleep, and accompanied by a loss of long-range intra-hemispheric synchrony. In contrast to sleep, postictal slowing was characterized by a broadband increase of spectral power, and showed a reduced modulatory effect of slow oscillations on higher frequencies. These results suggest slow oscillation focal slowing is reduced near the seizure onset zone, perhaps reflecting reduced inhibitory activity. Dissociation between slow oscillation and delta slowing could help localize the seizure onset zone from interictal intracranial recordings. However, slow oscillations (<1 Hz) have been shown to modulate cortical excitability3 and during the pre-ictal period can localize the seizure onset zone4. Despite evidence linking slow oscillations to the regulation of neuronal excitability, slow frequency changes related to epileptic cortex remain less well studied.Oscillatory brain waves recorded by electroencephalography (EEG) have been categorized into frequency bands spanning 1–50 Hz. More recently, it has been appreciated that broadband EEG (~0.01–500 Hz) contains richer cortical information5. Diffusely increased slow wave activity (SWA), approximately 0.1–4 Hz, is a hallmark of sleep6, while focal or global SWA changes can also be a sign of cerebral dysfunction7. The mechanisms underlying physiologic and pathophysiologic SWA remain unclear. Activity less than approximately 1 Hz may represent activated and quiescent neuronal states8 and function as the default activity of cortical circuits9. Increased power in frequency bands less than 1 Hz may reflect intrinsic neuronal mechanisms that dampen excitability12. On the other hand, activity that includes delta frequencies (1–4 Hz) during sleep is correlated with local synaptic strength and is regulated both globally, constituting a marker of sleep need, and locally15. Relative to slow oscillations, activity that includes delta power may be influenced to a greater extent by thalamic circuits17. Due to these potential differences in underlying mechanisms, we examine SWA with frequencies <1 Hz and SWA limited to higher frequencies in the delta power range.The power spectral density quantifies the contribution of each frequency band to a signal, and for EEG signals lower frequency bands contribute much more power than higher frequencies18. The spatial distribution of SWA during sleep and postictal states is generally considered diffuse, symmetric, and frontally predominant19, although iEEG data suggests a more heterogenous distribution20. Recent work in focal epilepsy suggests the location of SWA is related to the seizure onset zone (SOZ)14. During non-REM sleep, delta activity was increased for patients with focal epilepsy compared to controls both diffusely as well as locally with a maximal increase at the SOZ14. One difficulty in differentiating pathological from physiological slowing is that the much prior work comes from scalp EEG, which has a spatial resolution of about 10 cm2 of cortex21. Although iEEG offers the benefit of increased spatial resolution, usually coverage is predominantly or exclusively over one brain region, and, for example, the ability to compare the ipsilateral hemisphere to the contralateral hemisphere is absent. For the present analysis we selected a rare dataset of patients with extensive bilateral subdural EEG recordings.Distinguishing physiologic from pathologic SWA remains difficult. In patients with seizures, delta activity in frontal and parietal areas is increased during seizures that impair awareness22 and activity that includes delta frequencies17, we hypothesized that these two power bands would differ with distance from the SOZ and between postictal slowing compared to sleep.Despite evidence suggesting distinct physiologic mechanisms underlying activity <1 Hz from faster SWA, it remains unclear whether these differences are evident at the macroscopic level relevant for clinical EEG. Given data supporting mechanistic differences in slow oscillation activity (<1 Hz)21 and coupling with higher frequency activity23. There appears to be increased synchrony near the SOZ25 with overall decreased spatial synchrony for subjects with epilepsy compared to normal subjects26. Increased phase amplitude coupling between low frequencies (<4 Hz) and faster frequencies has been associated with increased cognitive performance in normal subjects27 and can be increased interictally near the SOZ28, suggesting cross-frequency coupling may be increased when cortical excitability is increased.Changes in low frequency power are expected to alter spatial correlations29, and which allows us to sample an extensive region of cortex from ipsilateral and contralateral hemispheres. We also compare sleep and postictal states. We focus on postictal slowing immediately following focal seizure activity that did not lead to bilateral convulsive seizures, with the intent of comparing transient and prolonged slowing of focal origin, which is clinically often indistinguishable. Finally, we examine spatial correlations and cross frequency coupling relative to both the SOZ and behavioral state.Here, we examine how interictal slow oscillation and delta activity is spatially distributed relative to the SOZ. We take advantage of a cohort of patients that had extensive subdural electrode coverage over both hemispheres, data becoming rarer with the increasing use of stereotactic depth electrode techniquesn = 3), right frontal (n = 1), left temporal (n = 1), right temporal (n = 4), and right parietal/occipital (n = 1) lobes. One patient had two right-sided SOZs in the temporal as well as parietal/occipital regions. Another patient likely had bilateral mesial temporal seizures, although seizure activity was arguably more prominent from the left side. For this patient, no definite interictal or ictal epileptiform activity was recorded from right-sided subdural contacts, and all seizure activity considered for this study (as detailed below) was left-sided. Thus, for this patient the left and right hemispheres were considered to be ipsilateral and contralateral to the SOZ, respectively. Patient data are summarized in Table This retrospective analysis with consent waiver was approved by the Institutional Review Board of Yale University. Data from patients undergoing intracranial EEG evaluation at Yale University from January 2014 to August 2017 were examined. Eight patients met the following inclusion criteria: (1) extensive bilateral subdural electrode coverage defined as at least 30 contacts per hemisphere, (2) no widespread abnormalities on MRI, and (3) well-defined seizure onset zones (SOZs). These criteria were chosen to ensure broad bilateral cortical coverage and to exclude cortex with any known widespread abnormalities that could affect slow wave activity. SOZs were determined at a multi-disciplinary epilepsy conference and were present in the left frontal  was used to record intracranial EEG (iEEG) data from eight patients with 196, 172, 203, 201, 199, 213, 154, and 223 subdural contacts. 2, 4, 5, 4, 29, 12, 1, and 4 contacts for these patients were clinically determined to include the SOZ, respectively. To analyze contacts near the SOZ we used all contacts within two cm of these clinically determined SOZ contacts, which gave 7, 22, 11, 12, 66, 31, 11, and 16 contacts for the eight patients, respectively. Note that 97 (55%) contacts come from two of the eight patients. 1061 contacts were ipsilateral to the SOZ and 500 contralateral. Five patients had at least three focal seizures that did not progress to generalized convulsive activity. For these patients, 15-minute EEG epochs were taken during the awake, sleep, and postictal states following each of the first 3 recorded non-generalized seizures, for a total of nine epochs per patient. Awake and sleep epochs were visually inspected and chosen to avoid artifacts. Epochs for awake and sleep states were taken at least two hours following the seizure, while postictal state epochs were commenced immediately following the clinical determination of seizure offset. Sleep epochs were taken from approximately 11 pm to 3 am when there was widespread high amplitude (greater than 100 microvolts) slow wave activity over the frontal lobe intracranial contacts. Scalp electrode data were not sufficient for sleep scoring, and video was not available. Thus, for each patient we measured the ratio of delta (1–4 Hz) to beta (13–25 Hz) power, which is a widely accepted signature of non-REM sleep activitynts Fig. . If two th order Chebyshev Type I filter (cutoff frequency 102.4 Hz) prior to downsampling to 256 Hz for data manipulation. Post-implantation imaging was coregistered to the standard MNI space using BioImage Suite. Locations were determined for each contact and manually segmented into six regions: frontal, temporal and parietal/occipital regions for each side. Contacts involved in seizure onset were determined from clinical reports. Data for each contact were filtered with zero-phase high and low pass fourth-order butterworth filters with cutoff frequencies of 0.1 and 100 Hz, respectively. Frequencies between 0.3 Hz and 50 Hz were considered following computation of the power spectral density by Welch’s method. Spectra were normalized by mean total power from 0.3 to 50 Hz to avoid undue influence specific to experimental conditions of the contact . Power spectra for each contact were calculated and normalized. These estimates were averaged across contacts for three groups: (1) contacts that were ipsilateral to the SOZ and within 2 cm of clinically defined SOZ contacts, (2) contacts that were ipsilateral and more than 2 cm away, and (3) contacts that were contralateral to the SOZ. Power spectra from each contact and epoch across patients were averaged according to these three groups, except as otherwise noted.Data were low pass filtered with an 87 and prior work26 to include slow oscillation (0.3–1 Hz), delta (2–4 Hz), theta and alpha (4–13 Hz), and high beta and gamma (20–50 Hz) frequencies. When comparing slow oscillation and delta powers, we limited delta power to 2–4 Hz given the arbitrary nature of frequency band definitions and evidence of 1–2 Hz as a transition zone.Power bands were chosen according to typical clinical definitionsCorrelations can be assessed spatially between contacts and between frequency bands within a single contact, also known as cross-frequency coupling. For spatial correlations, the Euclidean distance was calculated between all contacts. To assess the strength of the linear relation relationship between contacts, a single Pearson correlation coefficient without any time lag was calculated for each contact pair for all contact combinations. The Fisher z-transformation was used to stabilize the variances of the Pearson coefficients prior to averaging.Data from the 15-minute epochs for Awake, Sleep, and Postictal states were bandpass filtered with fourth-order Butterworth filters, and the correlation coefficients calculated.31 to measure the synchrony between the phase of a slower frequency band (0.3–1 Hz) and the power envelope of a higher frequency band (20–50 Hz). Phases θL and θH were obtained for the slow frequency band (0.3–1 Hz) and for the envelope of higher frequency activity, respectively. To find the phase θL, data were bandpass filtered between 0.3–1 Hz. We used the Hilbert transform to obtain the discrete-time analytic signal. The instantaneous phase of the dominant frequency within the passband of the signal was determined from the analytic signal. To find the phase θH, data were bandpass filtered, and the time-varying envelope of the signal power was obtained as the squared magnitude of the discrete-time analytic signal. The instantaneous phase θH is then found as the phase of the Hilbert transform as above. SI is defined to be N is number of time points and θ is the instantaneous phase. The magnitude of SI reflects the extent to which θL and θH are synchronized. It is calculated for each contact, is equivalent to one minus the circular variance of the phase difference θL − θH32, and is a measure of the concentration of the phase differences. As |SI| increases, the dispersion and circular variance of the phase difference decreases. The phase of SI reflects the “preferred synchronization phase”, the phase of θL for which coupling is maximal. A circular average is used to find the mean phase.To assess correlations between frequency bands, i.e. cross-frequency coupling, we used the Synchronization Index (SI)33. To obtain confidence intervals, the variance for distribution N with n values is estimated by creating k bootstrapped distributions from distribution N. Each of the k distributions is created by randomly drawing n samples from distribution N with replacement. Thus, more frequent values in distribution N have a greater representation in the k bootstrapped distributions, allowing for a non-parametric estimate of variance to be calculated using accepted formulas, such as via the “bootci” function in Matlab . To minimize false positive results due to multiple comparisons, we limited our analyses to those motivated by initial hypotheses and evaluated pooled data prior to individual data. Correction for multiple comparisons was not performed for non-pooled data. Non-overlapping error bars indicate that the population means may be considered significantly different with an error of less than 5%, or p < 0.05, as is typical for hypothesis testing. A two-sample t-test was used for hypothesis testing as reported in Results.95% confidence intervals (bias corrected and accelerated) were obtained by bootstrapping , which is a widely accepted statistical resampling technique that relies on random sampling with replacement of the distribution in question to provide a non-parametric and robust confidence interval estimateAll data and analyses are available upon request.p < 0.05, Fig. 30. Average power spectra showed decreased slow oscillation power, a transition band from approximately 1–2 Hz, and increased higher frequency power for contacts near the SOZ , compared with decrements of 14% for the awake state (t(1939) = −4.6, p < 1e-5) and 11% for the postictal state (t(1255) = −3.6, p < 1e-3). In contrast, relative power in the 2–4 Hz frequency band was significantly increased by 10–18% for contacts closest to the SOZ. Specifically, power was increased by 10% for the awake state (t(1939) = 5.2, p < 1e-6), by 18% for the sleep state (t(1939) = 10.2, p < 1e-23), and 11% for the postictal state (t(1255) = 4.6, p < 1e-6). Similarly, there was increased relative activity in the 20–50 Hz frequency band for the sleep (t(1939) = 11.7, p < 1e-29) and postictal states (t(1255) = 4.4, p < 1e-4) postictal states. Increased high frequency power is also seen within the SOZ in the 4–13 Hz band during sleep but not awake and postictal states. Increased power in the 20–50 Hz band was not present in the SOZ during the awake state compared to sleep, perhaps representing cortical dysfunction.The locations of 1561 electrodes from 8 patients  were determined in standard MNI space and manually segmented into frontal, temporal and parietal/occipital head regions Fig. . Given iSOZ Fig. . To quanSOZ Fig. . For theSOZ Fig. . Specifip < 1e-7), 32% for the sleep state (t(3454) = 8.8, p < 1e-17), and 35% for the postictal state (t(2131) = 6.8, P < 1e-10). In contrast, the same contacts showed relatively decreased delta activity by 11% for the awake state (t(3454) = −5.0, p < 1e-6), 15% for the sleep state (t(3454) = −6.7, p < 1e-10), and 16% for the postictal state (t(2131) = −6.5, p < 1e-10).We expected slow wave power to vary as a function of distance from the SOZ and wondered whether slow oscillation power would vary differently than delta power given the possibly different mechanisms underlying activity in these bands. Given the apparent transition band for 1–2 Hz power in Figs We compared postictal slowing to sleep, given that they represent different clinical states but can appear similar on EEG during visual inspection. Contacts near the SOZ show a broadband increase during postictal slowing compared to the awake state Fig. . In contWe examined spatial correlations and synchrony for SWA with the hypotheses that spatial synchrony would be reduced ipsilateral to the SOZ and that cross-frequency coupling would be increased within the SOZ. Spatial correlations for SWA were reduced for contacts ipsilateral to the SOZ, shown for 0.3–1 Hz activity in the awake, sleep and postictal states Fig. . Similar31, and focused on SWA as the underlying rhythm16. Maximal coupling to 20–50 Hz activity occurs around the positive peak of the 0.3–1 Hz activity  has long been associated with sleep and postictal states. Here, we find that slow oscillation activity (0.3–1 Hz) is decreased near the seizure onset zone (SOZ), while delta activity (2–4 Hz) is increased, suggesting that these frequency bands may result from different underlying mechanisms. We also show that postictal slowing is comprised of broadband power increases and has distinct characteristics from sleep.36 with periods of activity alternating between “up” and “down” states6. Layer 5 cortical neurons probably initiate up states, which persist due to recurrent excitatory synaptic activity8; up states are terminated either due to increased inhibitory inputs22 or as a result of activity-dependent potassium conductances8. Activity-dependent potassium conductances are associated with regulation of neuronal activity and action potential firing rate adaptation37. When inhibitory inputs are decreased, it appears that activity-dependent potassium conductances exert a relatively stronger dampening effect and decrease slow oscillation frequency12. These potassium conductances may be reduced near the SOZ, which could lead to decreased slow oscillation power11. Many antiepileptic medications effectively increase firing rate adaptation via sodium-38 or potassium-related39 mechanisms, suggesting that these medications could alter low frequency power spectra of patients. Other data show that slow wave power including delta activity may reflect the strength of synaptic inputs14, where delta activity has been associated with thalamocortical inputs36 and lower frequency activity persisted without thalamic input17. Interictal epileptiform activity could contribute to SWA, as spike shape and frequency changes near the SOZ40, and interictal epileptiform activity contains a wide range of frequencies including delta and beta activity42. Overall, recent data support the notion that activity in the 0.3 to 4 Hz range likely represents at least two distinct processes43.Slow oscillations at approximately 0.3–1 Hz modulate faster frequencies6. Sleep deprivation results in a decreased number of slow waves below 1 Hz and increased EEG power greater than 1 Hz44. Recent work has compared delta activity during slow wave sleep from normal individuals and those with epilepsy14, finding increased activity near the SOZ. Increased 1–4 Hz activity was also correlated with increased seizure activity over the previous 3–5 days and increased interictal epileptiform activity the hour prior to bedtime. One interpretation is that the SOZ and epileptiform activity represent increased neuronal activity that requires increased synaptic renormalization, which leads to increased delta activity during sleep13. These results are consistent with the idea that the SOZ represents a region of increased neuronal activity reflected in reduced 0.3–1 Hz activity, which could reflect reduced potassium conductances11. Postictal slowing may represent increased excitability relative to the slowing of sleep, given its reduced ratio of slow oscillation to delta power. The broadband increases seen during postictal states are consistent with an overall increased number of neural units contributing to the spectral power5. Thus, what is referred to as postictal slowing may be an increased number of active asynchronous neurons, rather than a smaller group of synchronous neurons as for sleep.Non-rapid eye movement sleep especially occurring during the first portion of the night have long been associated with increased slow wave activity15 and could be useful for the localization of the SOZ. Prior work focusing on lower frequencies (<0.5 Hz) described the utility of DC shifts recorded just prior to seizures to localize the SOZ45. This infraslow activity is often present prior to seizures46 and may have non-neuronal physiological mechanisms47. The reduced slow activity described here is somewhat faster and appears to be a static property of the SOZ, most notable during sleep and in conjunction with increased activity in the beta and low gamma frequency bands. Similar decreases in slow activity and increases in fast activity during event-related tasks have been associated with activated cortex48, and may be related to local desynchronization with a relative increase of excitatory drive, which may also be found in epileptogenic cortex.Changes in the composition of slow wave power can appear on the centimeter spatial scale25 and decreased between electrodes that bridge the SOZ and non-SOZ24, consistent with the idea that the SOZ represents an area of increased local synchrony. Despite this, long-range spatial correlations remain25. Here, spatial coupling is reduced in the hemisphere ipsilateral to the SOZ, consistent with the notion that the SOZ disrupts long-range spatial synchrony in the ipsilateral hemisphere. Phase amplitude coupling has been noted to be increased within the SOZ28. Specifically, during sleep the preferred coupling phase for 0.3–4 Hz to 30–80 Hz activity was noted to occur roughly at the down to up state transition (about 270 degrees)28. Here, slow oscillation activity had a preferred coupling phase of approximately 0 degrees, while delta activity had a phase of approximately 260. Increased coupling and an earlier phase may be related to more excitable cortex50. Especially during the postictal state, delta rather than slow oscillation activity may have an increased modulatory role near the SOZ: power in the slow oscillation frequency range (0.3–1 Hz) is decreased in the SOZ, cross-frequency coupling is increased near the SOZ for the 2–4 Hz power band, and the preferred coupling phase for 2–4 Hz activity occurs earlier than the 0.3–1 Hz activity.Prior work has examined synchrony from subdural iEEG data from epileptic cortex. Linear spatial correlations were increased within the SOZ30.This study is limited by the inclusion of eight patients, five of whom had repeated focal seizures that allowed for the analysis of the postictal state. Further, patient specific electrode coverage limits subject level conclusions. For example, differences in SWA may be more prominent for certain head regions or epilepsy types. More than 1500 subdural electrodes were implanted allowing for conclusions to be drawn from group analysis of contacts. Nonetheless conclusions from our results are limited by the non-uniformity of the electrode coverage between patients. For example, 55% of the contacts that were less than 2 cm from the SOZ arose from two of the eight patients. To address this non-uniformity, we analyzed our results in two ways: by averaging data for each patient to assess inter-patient differences (e.g. Fig.


Here, we unveil the fundamental properties of FAPbI3 by spectroscopic investigations of nanocrystals of this material at the single-particle level. We show that these nanocrystals deliver near-infrared single photons suitable for quantum communication. Moreover, the low temperature photoluminescence spectra of FAPbI3 nanocrystals reveal the optical phonon modes responsible for the emission line broadening with temperature and a vanishing exciton–acoustic phonon interaction in these soft materials. The photoluminescence decays are governed by thermal mixing between fine structure states, with a two-optical phonon Raman scattering process. These results point to a strong Frölich interaction and to a phonon glass character that weakens the interactions of charge carriers with acoustic phonons and thus impacts their relaxation and mobility in these perovskites.Formamidinium lead iodide (FAPbI Here, the authors perform spectroscopic investigations of individual Formamidinium lead iodide nanocrystals, demonstrate single-photon emission and unravel the mechanisms of carrier–phonon interactions. These results will guide the development of next-generation devices for photovoltaics and quantum technologies. Their outstanding optical and electronic properties, together with facile and cost-effective production, make these materials useful not only for photovoltaic applications5, but also for light-emitting devices9. Recent advances in the colloidal synthesis of strongly emitting perovskite nanocrystals (NCs) open up new possibilities for the fabrication of tunable light sources based on the composition and quantum size effect tuning, such as light-emitting diodes and lasers, and for the exploration of their potential use as quantum light sources16. Previous works demonstrating single-photon emission of perovskites are mainly focused on fully inorganic cesium lead halide perovskites NCs (CsPbX3), which emit in the visible wavelength region19. It was also shown recently that hybrid organic–inorganic lead halide FAPbBr3 NCs behave as efficient single-photon sources in the visible domain20. Yet, the potential of perovskite NCs as quantum light sources in the near-infrared remains unexplored, in spite of important benefits concerning the tunability of the emission lines to the maximal sensitivity of silicon avalanche photodiodes and their suitability for propagation in telecommunication fibers. So far, red-emissive CsPbI3 NCs have been shown to suffer from a delayed phase transformation into a non-luminescent wide-band-gap 1D polymorph, and MAPbI3 exhibits very limited chemical durability and eventually decompose to PbI2. This so-called “perovskite red wall” has only recently been overcome with the development of a colloidal synthesis method for obtaining phase-stable FA lead iodide (FAPbI3) and FA1-xCsxPbI3 NCs with a bright photoluminescence (PL) expanding to 800 nm in the near-infrared21.Metal halide perovskites, in particular hybrid organic–inorganic lead halides APbX22. Those based on solid-state quantum emitters require cryogenic temperatures to reduce phonon dephasing of the transition dipole and to obtain a sharp zero-phonon emission line (ZPL). Indeed, indistinguishable photons stem from the ZPL when its linewidth reaches its lower bound set by the lifetime of the emitting state. Therefore, quantum emitters that exhibit a weak coupling between the exciton and phonons are appealing for the development of those applications. Besides governing the emission line broadening of hybrid perovskites, phonon scattering is among the factors setting a fundamental intrinsic limit to the mobility of charge carriers in these materials23. The study of electron–phonon coupling in perovskite NCs is thus important for photovoltaic applications. Yet, the strong inhomogeneous line broadening, inherent to PL spectra of bulk samples25 or ensembles of NCs26, obscure the spectral features of phonons and lead to disparities in the extracted values of phonon energies and coupling strengths.Bright light sources that deliver on demand single indistinguishable photons are needed for a variety of quantum information processing schemes27. Moreover, the selection rules for carrier–phonon interactions are specific and similar for both electrons and holes29. In particular, polar acoustic phonon mechanisms related to piezoelectricity and non-polar optical phonon mechanisms related to deformation potentials vanish in the cubic Pm-3m reference phase. Carrier–phonon interactions are thus related to acoustic phonons via deformation potentials and to polar optical phonons. The Fröhlich coupling is expected to dominate in these strongly ionic materials at room temperature30. Furthermore, inorganic halide perovskites display three triply degenerated optical active lattice modes instead of one in classical semiconductors31. These modes are expected to undergo longitudinal optical (LO)–transverse optical (TO) phonon splittings, which strongly depend on the mode polarity. For example, in inorganic CsPbCl3 perovskites, the highest-energy optical mode exhibits a significant LO–TO splitting (25 meV–15 meV), which is almost entirely at the origin of the difference between the static and optical dielectric constants32. In the well-known iodine-based hybrid halide perovskite MAPbI3, the same difference is explained at room temperature by a LO–TO splitting at lower frequencies (5 meV–4 meV)33. Recent Terahertz emission spectroscopy experiments indeed show that the photogenerated carriers drive an ultrafast coherent lattice response around 5 meV with a somewhat broader feature at about 11.6 meV34. There is scarce information about the optical phonons in FAPbI3. An optical mode at ~ 17 meV is observed at room temperature in the bulk α-phase by Raman spectroscopy36, and terahertz emission spectroscopy shows two coherent emission signatures at about 5 meV and 10 meV in FAPbI3-based alloys34.Halide perovskite semiconductors exhibit fundamental differences with classical semiconductors with zinc-blende or würtzite crystal structures. The spin-orbit coupling effect, which is very strong for lead-based compounds, is indeed present in the conduction band rather than in the valence band, leading to doubly spin-degenerated band-edge electronic states37. Recent inelastic neutron-scattering measurements38 indicate that FAPbI3 is an ultra-soft halide perovskite material with a very low elastic stiffness. These measurements are consistent with the prediction of a strong acoustic-optical phonon up-conversion and “hot-phonon bottleneck” in hybrid organic–inorganic lead halide perovskites39, especially in FAPbI3. Indeed, the blocking of acoustic phonon propagation, as well as the presence of organic cations introducing overlapping acoustic and optical phonon branches, is expected to facilitate the up-transition (depopulation) of low-energy acoustic modes to optical modes. An alternative explanation on the role of low-energy acoustic vibrational density of states is connected to a glassy state, where the strong lattice anharmonicity impedes the scattering of electron by acoustic phonons. This latter phenomenon is also expected to dominate the processes of band-edge electron coupling to acoustic phonons and thus charge carrier transport40. These processes prolonging the cooling period of hot charge carriers and affecting charge transport are promising for the next-generation photovoltaics devices, and motivate further investigations of the role of acoustic phonons in the electron–phonon interactions in FAPbI3 materials.Lead halide perovskites are structurally soft semiconductors having their whole acoustic vibrational density of states located at low energies (in the meV range)In this article, we present a spectroscopic study of these near-infrared perovskites at the single NC level and demonstrate the strong photon antibunching of their emission at room temperature. We also investigate their temperature-dependent PL spectra and PL decay at cryogenic temperatures. We find that the homogeneous broadening of the PL spectrum with increasing temperature is negligible up to 30 K, revealing a weak exciton–acoustic phonon coupling in these materials. We demonstrate that the emission line broadening is governed by the coupling between excitons and optical phonon modes whose spectral signatures are identified in the low temperature PL spectra. Moreover, the temperature dependence of the PL decay unveils a thermal mixing mechanism between the lowest two levels of the band-edge exciton fine structure, which is based on a two-phonon Raman process with optical phonons.3 NCs with a schematic crystal structure presented in Fig. 21, refined for improved size and shape uniformity (see Methods for details). High-resolution transmission electron microscopy (HR-TEM) images and high-angle annular dark-field scanning transmission electron microscopy images (HAADF-STEM) reveal that these NCs have cuboid shapes with average sizes of 10–15 nm  matrix and excited at 488 nm. The disparity between PL intensities among these NCs is attributed to the dispersion of their quantum yields around the ensemble-averaged value42. A typical PL spectrum of a single NC is shown in Fig. 18. Although negligible in bulk semiconductors, the Auger effect is strong in NCs due to the enhanced Coulomb interaction between charge carriers and the reduced kinematic restriction on momentum conservation43. In spite of the weak quantum confinement in these FAPbI3 NCs, the Auger processes also provide an efficient channel for nonradiative recombination of multiple excitons, which prevents the emission of multiple photons. Indeed, a remarkable feature of the PL from these single NCs is the strong photon antibunching, which manifests as a dip in its normalized intensity autocorrelation function g(2)(τ) at zero delay time (τ = 0), as exemplified in Fig. g(2)(0)~0.1, close to the ideal signature g(2)(0)=0 of a pure single-photon stream. As single NCs are detected with a high signal-to-background ratio, the correlation of the PL signal with background events only contributes as ~0.02 to g(2)(τ). The main contribution to g(2)(0) is therefore due to residual biexciton radiative recombinations. Similar values of g(2)(0) were reported in the case of individual perovskite NCs of CsPbX318 or FAPbBr320 emitting in the visible domain. The recovery of the coincidence signal with time is well fitted by a monoexponential rise function, which leads to an exciton lifetime of 25 ns at room temperature for this NC.We use single NC spectroscopy to get rid of inhomogeneities due to size and morphological variations among these nanostructures and thus explore their intrinsic optical properties, as well as to probe limiting carrier scattering processes at low temperature. An inverted fluorescence microscope equipped with a 1.45 numerical aperture oil-immersion objective is used to investigate the room temperature properties of single FAPbI3 NCs, we have investigated the PL spectrum of more than a hundred of single NCs at cryogenic temperatures. At the lowest temperatures (~3.6 K), thermal dephasing is reduced and the PL spectra of single NCs mainly present a sharp and intense line attributed to the exciton recombination ZPL, as exemplified in Fig. 45. A possible origin of the spectral diffusion can be explained as follows. Contrary to cations in fully inorganic perovskites, organic cations in hybrid lead halide perovskites are known to have electric dipole and quadrupole moments46. Photogenerated charge carriers cause the neighboring cations to reorient47 and locally align their dipoles to form ferroelectric domains50 and maximize the screening effect. This results in a change of the electrostatic potential in the NC, leading to a shift of the transition energy of the exciton. It is consistent with the recently reported locally disordered low temperature phase of FAPbI351. During the acquisition of the PL spectra, random configurations of cation dipole organizations (domain size and orientation) will be sampled and lead to spectral diffusion of the exciton recombination line. The line shape is thus essentially set by the distribution of emission line frequencies sampled during the integration time of the spectra. The random process at the origin of spectral diffusion leads to a Gaussian-shaped emission line. The sharpest lines are obtained when reducing the integration time, as well as the excess energy between the excitation photons and the emission photons. Linewidths as narrow as 0.8 meV can be obtained when recording the spectra over a very short integration time (0.2 s) and by using an excitation wavelength in the near-infrared (~730 nm)  to λexc = 488 nm (far above resonance). Moreover, its relative spectral position with respect to the ZPL undergoes significant fluctuations over time under blue excitation  = Γ0+σAcT+ΓLO/[exp(ELO/kBT)−1]70, where Γ0 is the zero-temperature ZPL linewidth, which is temperature independent and mainly set by spectral diffusion, σAc and ΓLO are, respectively, the exciton–acoustic phonon and the exciton–optical phonon coupling coefficients, and ELO is the optical phonon energy. The contribution of optical phonons to the line broadening is indeed proportional to their Bose–Einstein occupation number, whereas for acoustic phonons a linear dependence on temperature is generally assumed. We find that coupling to a single optical phonon mode is enough to reproduce the temperature evolution of the linewidth, whereas more than one polar optical modes are present in halide perovskites. We thus assume that the ELO parameter is an effective parameter representing the carrier–LO phonon interaction. In the example of Fig. 0 = 1.5 meV, σAc = 0, γLO = 27 meV and ELO = 10.7 meV. The values of ELO and γLO are in agreement with previous measurements on bulk FAPbI325. However, this value of ELO differs from that deduced from other measurements on bulk FAPbI3 and FAPbI3 NC ensembles26, providing values ELO ~18 meV. Our single NC studies are self-consistent as the LO-phonon modes participating to the homogeneous broadening are directly observed in the low temperature PL spectra, while buried in the inhomogeneously broadened spectrum of the bulk material or NC ensembles (larger by one order of magnitude). It is worth noting that the energies of LO1 and LO2 phonon sidebands are very stable with temperature , is found negligible for all the studied NCs. We find an upper bound for 3 NCs. Figure 78. For FAPbI3 NCs, no spectroscopic study of the band-edge exciton fine structure is reported. Nevertheless, this fine structure should be weakly influenced by the nature of the cation, and thus be similar to that of CsPbI3. It comprises a bright triplet, whose degeneracy may be lifted by crystal field depending on the crystal structure and a dark single state53. Fine structure splitting measurements, based on PL spectra of single CsPbI3 NCs, led to a statistical distribution with an average splitting of ~300 µeV and an upper value of ~500 µeV45. To unveil a dark state emission line via magnetic brightening, we recorded PL spectra of individual FAPbI3 NCs under external magnetic fields up to 7 T. We did not observe any emerging peak but a slight emission line broadening  and ΓB (mainly radiative). Thermal mixing of the excitonic states in NCs is commonly attributed to the emission and absorption of acoustic phonons from a mode whose energy matches the energy splitting between bright and dark excitons79. However, as shown in Supplementary Fig. 3 NC PL decays cannot be reproduced by a one-phonon thermal mixing model. In fact, the temperature evolution of the long decay component  whose energy difference matches the bright−dark splitting  and γ↓ = γ0NLO1A(NLO1B + 1), where NLO1i = 1/[exp(ELO1i/kBT) − 1] are the Bose–Einstein phonon numbers , and γ0 is a characteristic two-phonon mixing rate, which is discussed in the Supplementary Note 80 The decay rates ΓS and ΓL of the short and the long time components are deduced from the solutions of rate equations for the excitonic states populations:S and ΓL for the NC displayed in Fig. 1 sideband (centered around 3.5 meV). Their difference is explored between 200 and 500 µeV to match the dark-bright energy splitting. The determination of ΓD and ΓB is robust as these parameters are highly constrained by the values of the long and the short decay rates in the limits of low and high temperature regimes. Indeed, for kBT ≪ ELO1i , we have ΓS→ΓB and ΓL→ΓD, whereas for kBT ≫ ELO1i , we have ΓS→2γ0(kBT)2/(ELO1AELO1B) and ΓL→(ΓB + ΓD)/2. The parameter γ0 is accurately adjusted to reproduce the evolutions with temperature of both ΓS and ΓL. In Figs. B = 0.53 ns−1, ΓD = 0.015 ns−1, γ0 = 0.16 ns−1, ELO1A = 3.3 meV, ELO1B = 3.6 meV. Furthermore, Fig. 3 NCs support our two-phonon thermal mixing model and lead to relaxation rates similar to those extracted from Fig. S(T) and ΓL(T) remain very similar and reproduce as well the experimental data  was loaded into a 50 mL three-neck flask along with octadecene  and oleic acid . The reaction mixture was degassed three times at room temperature, heated to 100 °C under nitrogen until the reaction was completed and cooled down to room temperature. The obtained solution was stored in a glovebox.In a 25 mL three-necked flask, lead (II) iodide  was suspended in ODE (4.6 mL), heated to 60 °C and then dried under vacuum for 30 min. Subsequently, the reaction mixture was heated to 110 °C under nitrogen, followed by the syringe addition of dried solvents—oleylamine  and oleic acid (1.0 mL). Once the lead (II) iodide was dissolved, the reaction mixture was cooled to 80 °C. At this point, the mixture of FA oleate stock solution (5.0 mL) and ODE (1.0 mL) was injected into the reaction flask. After another 15 s, the reaction mixture was cooled by a water-ice bath.The crude solution was centrifuged at 12.1 krpm for 5 min (Centrifuge: Eppendorf 5804) and the supernatant was discarded. The precipitate was dissolved in hexane (0.3 mL) and the solution was centrifuged again . The supernatant, containing monodisperse NCs, was retained for the subsequent purification, whereas the precipitated NCs were discarded. In order to remove the excess of the organic ligands, hexane (0.9 mL), toluene (0.6 mL) and methyl acetate (1.95 mL) were added to the colloidal solution of NCs and the resulting solution was centrifuged at 13.4 rpm for 3 min. The supernatant was discarded and the precipitate was dissolved in toluene (1 mL), yielding concentration of ca. 10–15 mg/mL. This solution was additionally filtered through a 0.45 µm PTFE-filter prior to optical measurements.Ensemble ultraviolet-visible absorption spectra were collected using a Jasco V770 spectrometer operated in transmission mode. Fluoromax iHR 320 Horiba Jobin Yvon spectrofluorimeter equipped with a photomultiplier was used to acquire steady-state PL spectra from solution. The excitation wavelength was 400 nm, provided by a 450 W Xenon lamp dispersed with a monochromator. Measured intensities were corrected to take into account the spectral response of the detector. Transmission electron microscopy images were collected using a JEOL JEM-2200FS microscope operated at 200 kV. Powder X-ray diffraction patterns were collected with a STOE STADI P powder diffractometer, operating in transmission mode. A germanium monochromator, Cu Kα1 irradiation and a silicon strip detector (Dectris Mythen), were used.2 atmosphere for further analysis. For single NC measurements, a dilute solution of FAPbI3 NCs was mixed with a 1 wt% poly(methyl methacrylate) solution and spin-coated onto clean glass or sapphire coverslips with a rotation speed of 2000 rpm. Single NC measurements at room temperature were conducted using a customized confocal scanning fluorescence microscope. It is based on a commercial microscope (Nikon Eclipse Ti) and a 1.45 numerical aperture oil-immersion objective. The emitted photon stream is filtered by a long-pass filter (593 nm blocking edge), then split by a 50/50 beam splitter and sent to two avalanche photodiodes. For wide-field imaging, an additional lens is used to focus the excitation beam on the back focal plane of the objective and the emitted photons are sent to a CCD camera . For the low temperature measurements, a home-built scanning confocal microscope based on a 0.95 numerical aperture objective is placed in a cryostat and used to image single NCs by raster scanning the sample. The NCs are excited at 488 nm or 727 nm with a laser passed through a clean-up filter. The emitted photons are filtered by a long-pass filter (776 nm blocking edge) and sent to a single-photon counting avalanche photodiode and a spectrograph. PL decays are recorded with a conventional time correlated single-photon-counting setup using a pulsed laser source .The NCs in toluene were kept under NAll relevant data that support our experimental findings are available from the corresponding author upon reasonable request.Supplementary InformationPeer Review File


Biological motors are marvels of nature that inspire creation of their synthetic counterparts with comparable nanoscale dimensions, high efficiency and diverse functions. Molecular motors have been synthesized, but obtaining nanomotors through self-assembly remains challenging. Here we describe a self-assembled colloidal motor with a repetitive light-driven rotation of transparent micro-particles immersed in a liquid crystal and powered by a continuous exposure to unstructured ~1 nW light. A monolayer of azobenzene molecules defines how the liquid crystal’s optical axis mechanically couples to the particle’s surface, as well as how they jointly rotate as the light’s polarization changes. The rotating particle twists the liquid crystal, which changes polarization of traversing light. The resulting feedback mechanism yields a continuous opto-mechanical cycle and drives the unidirectional particle spinning, with handedness and frequency robustly controlled by polarization and intensity of light. Our findings may lead to opto-mechanical devices and colloidal machines compatible with liquid crystal display technology. Biological motors which convert energy into mechanical work inspire the fabrication of synthetic motors. Here the authors demonstrate self-assembled colloidal motors which are driven to a range of responses controlled by the feedback between light polarization and deformation of a liquid crystal. Man-made nanomotors were first envisaged by Feynman2 and are key to many future applications7. Feringa chemically synthesized light-driven molecular motors8, which were later used in synthetic nanocars9 and in controlling chiral liquid crystals (LCs) with colloidal inclusions10. Much like in a four-stroke cycle combustion engine, the directional rotation of Feringa’s motors stems from driving the molecule between a series of four conformations selected by cycling temperature and exposure to light9. Rotation of shape-anisotropic or birefringent particles can be also achieved by harnessing optical angular momentum transfer owing to the change in polarization of transmitted light11, but this requires high-power laser traps. Many organic molecules, such as azobenzenes12, exhibit facile polarization-sensitive responses to light, but their utility in the designs of artificial motors is limited1. Similarly, LCs are known for their fascinating ability of controlling polarization and intensity of light, with plethora of technological applications, such as the flat panel displays13, but LC-enabled motors or machines capable of converting external sources of energy into mechanical work are rarely demonstrated10.Miniaturization of motors and machines down to nanometer and micrometer scales could literally lead to a new industrial revolution, but remains a challenge14. This is achieved by many self-assembled biomolecules that cooperatively drive essential biological processes like flagellum rotation in bacteria, DNA packing in capsids of viruses and crawling of cells14. However, development of such self-assembled motors in purely synthetic systems is a grand challenge in colloidal science and nanotechnology.Biological motors consume energy and convert it into motion or mechanical work, often with efficiency superior to that of the best combustion engines and electric motors12 mechanically couple the LC’s optical axis to the particle’s surface, mediating their joint rotation in response to changes of the light’s polarization. Therefore, when the particle rotates, it causes LC twisting and further changes of light polarization13, resulting in a feedback mechanism that leads to a continuous opto-mechanical cycle and unidirectional particle spinning. We characterize and discuss the physical underpinnings behind this fascinating behavior.To achieve such cooperative light-driven work, we let hundreds of millions of azobenzene molecules self-assemble into monolayers on surfaces of thin colloidal platelets, which are then dispersed in a nematic LC. This yields a colloidal light-driven motor with a highly controllable repetitive rotation of the micro-particle powered by low-intensity unstructured light (~1 nW). Self-assembled azobenzene molecules15, nematic colloidal platelets define surface boundary conditions for the surrounding molecules, spontaneously align with the large-area faces parallel to the sample plane and perturb the long-range orientational order of the LC described by a director n, the average local direction of rod-like molecular ordering16. These distortions manifest themselves through appearance of bright brushes around the platelet edges when viewed under a microscope between crossed polarizers  around these particles results in formation of surface point defects, called “boojums”, at the particle’s poles along n0  light, and when white or blue imaging or excitation light has linear polarization orthogonal to n0  across the cell above and below the platelet  through surface anchoring, where Pp is the orientation of the long axis of polarization ellipse of light traversing the platelet. Excitation light with linear polarization Pe ⊥ n0 traverses the sample in an ordinary mode without changing its polarization state13, allowing for an equilibrium “stationary” configuration of a colloidal platelet having large-area faces parallel to n0 and azobenzene moieties orthogonal to Pp || Pe  and at the platelet-LC interfaces together with the platelet itself. To meet the boundary conditions on the rotating particle surfaces and the confining substrates, n(r) twists across the cell  to yield an opto-mechanical cycle. This nonlinear feedback can be tuned to obtain desired dynamical responses of particles, ranging from pendulum-like orientational oscillations to a continuous unidirectional rotation  both across the cell thickness and in lateral directions. Operation of our light-driven motor would be impossible without the LC host medium. In fact, the platelet rotation stops completely when LC is heated to the isotropic phase (>35 °C for the used LC host). Key to enabling the repetitive particle rotation are optical, surface anchoring, and viscoelastic properties of the LC host medium. These properties allow for smooth and dynamic spatial deformation of the optical axis directly related to the dynamic polarization changes of light traversing the platelet13, cooperative mutual alignment of LC and azobenzene dye molecules at surfaces that mechanically couple n(r) to the platelet through surface anchoring and orientational elasticity12, as well as the strong response of azobenzene moieties to weak external stimuli17. Below we characterize and model details of this behavior.Remarkably, low-intensity ~1 nW blue excitation light with polarization states on Figs.  to perioon Figs. , 5. Whenon Figs. , indicatlet Fig. . At the  Pe Fig. . Light well Fig.  and modi12, though ambient white light causes out-of-equilibrium colloidal dynamics too. Rotation of azobenzene moieties to be orthogonal to Pp in the trans-state can be understood in simple terms as driven by their tendency to avoid the excited state18. Continuous particle rotation exhibits a threshold-like behavior that depends on polarization of incident light, with the onset of full rotation at typical optical powers of ~1 nW per particle  coupled to the easy axis set by the average orientation of azobenzene moieties at the LC interface with a rotating platelet  at the large-area faces, the two boojums exhibit the stick-slip dynamics at the six vertices of a hexagonal platelet -distortions around platelets depend on θ and slowly evolve with time   Fig. , also dets) Fig. . Once th. θ Fig. , though let Fig. . Intereslet Fig. . This isPeriodic rotation and other types of dynamics can be achieved not only for the hexagonal platelets, but also for cogwheel-like Fig.  and manyn(r) twist  at platelet-LC interfaces. As the net twist of n(r) from the confining substrate to platelet across the cell increases, the ellipticity of light’s polarization increases  at the LC-platelet interfaces. This dynamics of n(r) establishes the feedback mechanism responsible for the light-driven colloidal motor. Fine details of this dynamics are revealed by monitoring temporal evolution of patterns of polarization state and intensity of light after passing the LC with oppositely twisted regions above and below the platelet  above and below the platelet, which is very specific to LCs17 and in fact similar to that found in LC displays13, where electric fields drive rotation of n(r) and where these rotations are resisted by LC-specific torques due to orientational elasticity (related to the fact that full rotation of our light-driven motor requires intensity above a certain threshold) and director’s rotational viscosity γn. When the studied LC is heated to isotropic phase, only the viscous torque associated with γeff survives and resists particle rotations, though it also changes as compared to that in the nematic phase. Without particle present, dynamic rotations of n(r) are resisted by a viscous torque associated with γn. In our case of spinning particle accompanied by twisting of n(r), however, both these torques are present and comparable because γeff and γn are of the same order of magnitude17. The torque associated with γeff can be characterized by probing rotational diffusion of the platelets in response to thermal fluctuations29 when such platelets are immersed in the LC at no optical torques and for uniform director across the LC cell. On the other hand, the torque associated with the dynamics of n(r) can be estimated using known value γn = 0.08 Pa s of the rotational viscosity of the used LC30. The power of mechanical work associated with light-driven rotational dynamics of the platelet and n(r) around it is then estimated as Wm ≈ ω2(kBT/Dφ + γndAp), where ω is the angular frequency of particle rotation, d is the sample thickness, Ap is the area of the platelet’s hexagonal face, kB = 1.38 × 10−23 J K−1 is the Boltzmann constant, T = 298 K is temperature and Dφ = 3.8 × 10−4 rad2 s−1 is the rotational diffusion coefficient of particles obtained from probing their Brownian motion (see Methods). About 0.5% of the excitation power is absorbed by azobenzene molecular monolayers, which yields an estimate of the optical-to-mechanical power conversion efficiency of ≈10−3% at 200 nW of excitation light and the corresponding rotation frequency of ≈1 Hz 1/2, is qualitatively consistent with experimental measurements  and prompting the stick-slip dynamics of boojums. Ambient sunlight intensity ~50 mW cm−2 on a sunny day is significantly higher than the intensity of 8 mW cm−2 corresponding to Woth measured in our experiments, indicating that such colloidal motors can be driven simply by harnessing solar energy and ambient white light .Light-driven rotation of our platelets is resisted by a viscous torque associated with an effective flow viscosity  Hz Fig. . The scants Fig. . The thr26 and that they can be self-assembled into colloidal micromachines, of interest for many technological applications7.To conclude, self-assembly on molecular and colloidal scales yields a light-driven rotary motor within which cooperative responses of hundreds of millions of dye molecules and trillions of LC molecules interact with a colloidal platelet to convert optical power into mechanical work. We have uncovered the LC-based feedback mechanism enabling realization of different forms of colloidal dynamics, ranging from periodic oscillations to continuous unidirectional rotation, as well as demonstrated that light intensity and geometric shape of particles can be used in controlling angular speed of particles. We envisage that collective behavior of our light-driven motors can yield new forms of active matter33. In 20 mL of dichloromethane, 1.35 g of methyl red and 1.11 g of 1,3-dicyclohexylcarbodiimide were dissolved and then combined with 1.19 mL of (3-aminopropyl) triethoxysilane under nitrogen. After reacting overnight under stirring, the solution was purified by column chromatography  and dried to yield dark red crystals of the desired azobenzene-containing dye with chemical structure shown in Fig. 34. First, plasma-enhanced chemical vapor deposition was used to deposit a layer of silica of 0.5 μm or more in thickness on a silicon wafer followed by spin-coating an additional layer of photoresist AZ5214 (from Clariant AG). Geometric shapes of particles were defined in the photoresist layer with a direct laser-writing system DWL 66FS (Heidelberg Instruments) operating at 405 nm and subsequently in the silica layer by inductively coupled plasma etching. The photoresist mask was then removed with acetone and the silicon substrate was further etched using selective inductively coupled plasma to detach micro-platelets from the substrate. To facilitate surface binding of azobenzene-containing dye molecules, the platelets were immersed in a piranha solution  for 1 h to produce hydroxylated surfaces. They were then submerged in 20 mL toluene solution of 5 mg of the azobenzene-containing dye and 20 μL n-butylamine at elevated temperature of 45 °C overnight, followed by rinsing with toluene and curing at 120 °C for 2 h. Ultrasonic bath of the resulting substrate yielded a suspension of micro-platelets in isopropanol with self-assembled monolayers of azobenzene-containing dye atop of particles, which was then mixed with a nematic LC pentylcyanobiphenyl . After isopropanol evaporated at room temperature, the LC colloidal dispersion was infiltrated between glass plates, with gap defined by glass spacers and boundary condition by unidirectional rubbing of polyimide PI2555 (HD Microsystem) spin-coated on the glass plates. Unless noted otherwise, all chemicals were purchased through Sigma-Aldrich and used as received.The details of our experimental system are presented in Fig. n(r) were imaged and characterized with polarizing optical microscopy based on an upright microscope   Fig. . To avoius) Fig. . Opticalus) Fig.  to excitus) Fig.  and anisus) Fig.  under co35. To quantitatively probe spatial patterns of polarization ellipse for initially linearly polarized light after it passes through the LC with a rotating platelet, an additional broadband quarter-wave plate  was inserted before the analyzer and rotated to obtain images at different angles. Polarizing optical micrographs were obtained for different waveplate orientations every 22.5° , starting with the orientation of the waveplate’s fast axis parallel to n0 and the analyzer. These images were then numerically processed to produce visualizations of polarization ellipses36 on the pixel-by-pixel basis that characterize light’s polarization state after it passes through the sample  in the reflection channel of the BX51 Olympus microscope Fig.  was used13. The polarization state of light traversing through the twisted LC slab can be then expressed as:n(r). Taking into account that the blue excitation light traversing the platelet passes through a slab of LC twist, which is approximately half of the cell thickness, we define the following variables describing the light-LC interaction at the moment of reaching the colloidal platelet in the cell midplane:ϕ is the director twist angle, d is the thickness of the LC cell (defining the thickness of the twisted LC slab through which light propagates before reaching the platelet as ≈d/2), λ is the wavelength of the light and ne − no is the difference between extraordinary and ordinary indices (birefringence) of the LC. When the polarization of the incident light is parallel to the far-field director n0, θ = 0°, its Jones vector isKey to the opto-mechanical feedback mechanism described in this work is the propagation of light through a twisted structure of the director field, as depicted in Fig. and a simple matrix calculation yieldsλ = 490 nm, ne − no = 0.2, and d/2 = 2.4 µm, we characterize ellipticity e and azimuthal orientation θ = ψ + ϕ versus the director twist angle ·M(ϕ), so that the polarization state of outgoing light can be found asIn our polarimetric imaging experiments Fig. , the pollet Fig. . We assuGiven the same linear polarization of incident light as above, the Jones vector of the outgoing imaging light isThe corresponding ellipticity and azimuthal orientation angle can be expressed asAdditionally, the light transmission coefficient for each pixel of the polarizing optical micrograph obtained under crossed polarizers readsT for parallel polarizers. Using experimental parameters and λ = 640 nm, we characterize the polarization states of light passing through the LC cell with a rotating platelet  is the surface anchoring coefficient (an anisotropic part of surface tension) and np defines the orientation of n(r) at the platelet’s surface. This free energy term is always minimized when Pp ⊥ np, as defined by the response of azobenzene-containing monolayers, but the strength of such coupling through light-directed boundary conditions depends on both intensity and polarization state of light traversing the platelet, as determined by σ. When polarization state changes so that the long axis of ellipse PP rotates, this prompts rotation of nP and rotation of both the platelet and n(r) in the LC bulk, though slipping occurs at large e and low I due to weak coupling defined by the ensuing small σ. The rotation-driven n(r) distortions in the LC bulk are resisted by the elastic free energy describing the energetic cost of these distortions at fixed boundary conditions at the cell’s confining substratesK11, K22, K33, and K24 are Frank elastic constants for splay, twist, bend and saddle splay director deformations, respectively. The competition of corresponding light-dependent surface anchoring free energy term and bulk elastic free energy defines conditions for slipping of n(r) at the platelet-LC interfaces, which, in turn, defines the dynamics of our system.To model interaction of our LC colloidal motors with light, we assume infinitely strong (fixed) surface boundary conditions at the confining substrates of the LC cell. The free energy potential describing the generally soft surface boundary conditions at the LC-platelet interfaces is dependent on light’s intensity Description of Additional Supplementary FilesSupplementary Movie 1Supplementary Movie 2Supplementary Movie 3Supplementary Movie 4Supplementary Movie 5Supplementary Movie 6Supplementary Movie 7Supplementary Movie 8Supplementary Movie 9


Manipulating liquids with tunable shape and optical functionalities in real time is important for electroactive flow devices and optoelectronic devices, but remains a great challenge. Here, we demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell. We observe electrowetting and merging of sulfur droplets under different potentiostatic conditions, and successfully control these processes via selective design of sulfiphilic/sulfiphobic substrates. Moreover, we employ the electrowetting phenomena to create a microlens based on the liquid sulfur microdroplets and tune its characteristics in real time through changing the shape of the liquid microdroplets in a fast, repeatable, and controlled manner. These studies demonstrate a powerful in situ optical battery platform for unraveling the complex reaction mechanism of sulfur chemistries and for exploring the rich material properties of the liquid sulfur, which shed light on the applications of liquid sulfur droplets in devices such as microlenses, and potentially other electrotunable and optoelectronic devices. Manipulating liquids with tunable shape and optical functionalities in real time remains a great challenge. Here, the authors demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell and tune its characteristics in a fast, repeatable, and controlled manner. Significant advances have been achieved through engineering micro/nanostructured surfaces to enhance liquid-repelling or wetting capabilities, such as designing ordered arrays of nanopits/nanopillars4, asymmetric nanostructured surfaces5, and tapered conical nanotextures6 to adjust the hydrophilic/hydrophobic properties. Other approaches to dynamically tune or actuate liquids such as pH-responsive microstructured surfaces7, thermal stimulated homeostasis system8, and magnetically-tunable structures9 have been designed to allow for spatial control of liquid movement under acid/base, temperature oscillation, and magnetic field conditions. However, the necessity for complex surface structure design/modification or irreversible transformation and slow response time are still challenges that need to be addressed.There is a great demand to manipulate liquids with tunable shape, size, and motion for applications in electronic displays, optical switching, and electroactive flow devices11. Normally, a minimum of several tens of volts is required to apply across a dielectric layer to drive these devices12. A reduction in the operating voltage could greatly decrease the energy consumption of the whole system due to its voltage-squared dependence. Reducing the thickness of the dielectric layer leads to a lower required voltage; e.g., reversible electrowetting was achieved at < 50 V on an amorphous fluoropolymer dielectric layer thinner than 1 μm13, while an applied voltage of 15 V was sufficient to electrowet a thin (70 nm) dielectric layer with a fluoropolymer coating14. Recently, ultra-low-voltage electrowetting systems based on the interface between two immiscible electrolytic solutions15 or using graphite surfaces17 have been reported, reducing the voltage requirement to < 2.0 V, showing their great promise. However, despite these efforts, realization of a low activating voltage requires an extremely thin dielectric layer, which is challenging. In addition, previous studies have mainly investigated the wettability of water/oil on various insulating substrates (dielectric layers)11. To the best of our knowledge, active control of liquid wettability on a conducting surface has rarely been investigated so far.Electrowetting is another facile and effective strategy to manipulate liquid by controlling the liquid wettability on a surface via applying an external electric field with the advantages of fast response and simple implementation without the need of complex surface structuring and patterning  in electrochemical lithium sulfur (Li–S) battery cells22. Furthermore, combining computational and experimental efforts, we achieved selective growth of sulfur droplets with controllable shape and distribution. Finally, we established an approach to create adjustable liquid microlenses and present electrotunable optical imaging. Our findings enable active control of the wettability of sulfur on conducting surfaces, open a variety of possibilities beyond common wettability control architectures such as wetting of water/oil on insulating substrates, and broaden the applications of supercooled liquid materials.Here, we propose a new mechanism to tune the wetting of a liquid droplet directly on a conducting substrate without a dielectric layer, and the system is operated with extremely low voltages  through a deposition mask. Li metal was laminated onto a copper foil and used as the counter electrode. The Au electrodes, Li metal, and the liquid electrolyte  with 1 M LiTFSI salt and 1 wt% LiNO3 additive) were assembled in an Ar-filled glovebox and sealed by a thermoplastic film  of 2.3–2.4 V with respect to Li/Li+. According to classical nucleation theory, the transformation of polysulfides to heterogeneously-nucleated elemental sulfur needs to overcome the free energy barrier related to the kinetics of the critical sulfur clusters26, which can be overcome by increasing the overpotential of the polysulfide oxidation processes , consistent with classical nucleation and growth theory26 . The voltage window can be divided into the following three regions , but became noncircular in shape due to the electrowetting phenomenon when the overpotential was increased, and finally fused together to form island-like morphologies  due to the coalescence of multiple sulfur droplets. Supplementary Fig. Interestingly, we also observed that some spherical droplets deformed into elliptic shapes Fig. , the detres Fig.  to ellipres Fig. . The numres Fig. , indicatres Fig. . The eleons Fig. . At low 27. Analogously, a surface with sulfiphilic (strong affinity for sulfur) and/or sulfiphobic (weak/no affinity for sulfur) properties can be designed and find potential applications in Li–S batteries. Here, an “SU” pattern with alternate gold and titanium coverage has been designed to show the successful selective wetting of liquid sulfur on the substrate  and gold (111) were compared using density functional theory (DFT) calculations  covered by one layer of oxygen was constructed to simulate the adsorption behavior of a S8 ring. Anatase TiO2 was used to obtain the conductivity properties of the passive oxide coating. The weak adsorption energy (−0.99 eV) that mainly attributes to the van der Waals interactions between S8 and the Ti–O layer indicates low affinity of S8 to the Ti–O layer. Furthermore, the passive oxide coating with a large band gap of 3.21 eV  is as high as −2.94 eV  surface and will decompose into two more stable S4 chains on Au (111)  after relaxing  before the adsorption of S8 rings, which weakens the interaction between S8 and the gold surface  applied to the gold disk, well-defined sulfur microdroplets are formed15. The combination of induced dipole forces and electrostatic interactions of Sx molecules with a gold substrate allow the wetting angle, and hence the optical lens properties, to be tuned. The droplet-electrolyte interface refracts the incident light twice before it is focused to a single spot, with light paths shown in Fig. 30. The focusing ability of a microlens is measured by taking a series of optical images at different height and matches well with ray tracing calculations  and the different solvents (dimethyl sulfoxide (DMSO) and tetraethylene glycol dimethyl ether (TEGDME)) on the wetting behavior and lens performance were also investigated. Similar wetting phenomenon was observed for 3 M Li2S8 when applying a voltage of 4.0 V . By tuning the focal length f from 52 µm to 59 µm to 67 µm, we change the value of Z2 from 94 µm to 122 µm to 162 µm, corresponding to a reduced (0.77×), equal (1.00×), and magnified (1.34×) image, respectively -based microlenses, which generally have a high operating voltage of 10–100 V22. We demonstrate electrowetting and rapid de-wetting of sulfur droplets under different overpotentials and show different overpotential regimes with signatures of no sulfur, sulfur nucleation, and sulfur wetting. We further realize selective wetting of sulfur droplets on different substrates and explain these phenomena by DFT theoretical calculations. Finally, we demonstrate in situ modulation of liquid sulfur microdroplets and show dynamically tunable microlenses based on liquid sulfur droplets. This platform provides a path to tune the properties of such supercooled liquid materials in real time as well as opens up new approaches for investigating the mechanisms in complex electrochemical reactions.In summary, we have developed a platform to directly visualize liquid dynamics and wetting behaviors in real time. We have investigated the nucleation and growth behaviors of sulfur droplets and revealed the linear relationship between radius and inverse overpotential, and between the number density of nuclei and the cubic power of the overpotential, in good agreement with classical nucleation and growth theory2/Si substrates through a deposition mask. For the microlens devices, the gold pads and the “S” patterns on the cover glass were defined by photolithography followed by Ti/Au (3/50 nm) depositions.Gold electrodes were patterned by depositing Ti/Au (3/50 nm) onto SiO2/Si substrates was used as the working electrode. Li metal was laminated onto the copper foil and used as the counter electrode. A cover glass slide was placed on top of the electrodes and the cell was then sealed by a thermoplastic ionomer , leaving two small openings for filling liquid electrolyte. Here, Li2S8 dissolved in DOL/DME (1:1 v/v) with 1 M LiTFSI and 1 wt% LiNO3 additive was used as the catholyte. 0.2 M Li2S8 was used for the sulfur nucleation experiment in Fig. 2S8 was used for all other experiments. After filling the electrolyte, epoxy was used to finally seal the remaining two openings.The entire cell was assembled inside an Ar-filled glovebox. Patterned gold electrode on the SiOThe in situ electrochemical reaction was performed with a MTI 8-channel battery tester, while being imaged at the same time using light microscope equipped with air-immersion objective , broadband Xenon lamp, and CMOS detector. The image series were taken with a frame rate of 1 frame/second and the spatial resolution of the microscope is ~500 nm. Constant voltage charging was used to produce sulfur droplets and wetting phenomena, with galvanostatic discharging to de-wet and dissolve the liquid sulfur.z direction of 1 µm (± 50 nm).Imaging and measurements of the sample were implemented with an optical microscope coupled to a CCD camera . The intensity profile of lens focusing was measured with a motor stage with a resolution in the 34. The exchange-correlation interactions were treated by the generalized gradient approximation35 in the form of the Perdew–Burke–Ernzerhof functional36. The van der Waals interactions were described by using the empirical correction in Grimme’s scheme, i.e., DFT + D337. To get the exact band gap of TiO2, the Hubbard U (DFT + U) treatment was applied and the U was set to 5.3 eV.All calculations were performed using DFT as implemented in the Vienna Ab-initio Simulation Package (VASP) codeSupplementary InformationDescription of Additional Supplementary FilesSupplementary Movie 1Supplementary Movie 2Supplementary Movie 3Supplementary Movie 4Supplementary Movie 5


The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm Two-dimensional hybrid perovskites are promising excitonic materials; however, there currently lacks understanding on exciton diffusion and annihilation. Here Deng et al. employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton transport and annihilation in these materials. The variable n corresponds to the number of the inorganic quantum-well layers sandwiched between the long organic chains RNH3. Because dielectric screening is low from the surrounding organic ligands, excitons with binding energy as high as 0.5 eV dominate the optical properties of these materials20. The exciton binding energy for the n = 1 structure is comparable with those found in monolayer transition metal dichalcogenides (TMDCs)21. The structure of 2D perovskites is extremely programmable; for instance, continuous tuning of the exciton emission over a wide range of energy can be achieved by either changing n or the halide in the composition22. In addition, both the R and A site cations can be modified to influence electron-phonon coupling, dielectric environment, and lattice disorder23. The highly tunable structure coupled with emission quantum yield as high as 79%3 represents a significant advantage of 2D perovskites over other 2D semiconductors.Two-dimensional (2D) Ruddlesden-Popper phase organic–inorganic metal halide perovskites have emerged as an exciting class of optoelectronic materials. Efficient light emitting diodes (LED), lasers, and solar cells with superior stability over their 3D counterparts have been demonstrated26, representing a key challenge in realizing their optoelectronic applications.To harvest excitons in a device, a thorough understanding of exciton transport and annihilation in 2D perovskites is necessary. For example, the ability to transport excitons over long distances is necessary for solar cell performance. On the other hand, many-body exciton–exciton annihilation (EEA), a form of Auger recombination, is an important loss mechanism that limits the density of excitons and ultimately determines the efficiency of lasers and LEDs. EEA occurs when an exciton recombines nonradiatively by transferring its kinetic energy to another exciton. Rapid EEA rates are usually associated with nanostructures with large exciton binding energies because of the enhanced electron-hole interactions in these systems11, the understanding of exciton transport is extremely limited and currently there is no direct measurement on exciton diffusion. Because of the requirement of two excitons in proximity, exciton diffusion typically proceeds annihilation in 1D or 2D materials. EEA can occur either in the diffusion-limited or the reaction-limited regime27. To differentiate these two regimes, measurements of exciton population dynamics in both spatial and temporal domains are required. However, the majority of studies on exciton annihilation so far are based on time-resolved photoluminescence (PL) or transient absorption (TA) spectroscopy that offers no spatial resolution, leading to annihilation rates convoluted with exciton diffusion constants30, making it difficult to elucidate factors that control these two processes independently.Despite the impressive progress made in the optoelectronic devices based on 2D perovskites31. Distinct from the results in 3D perovskites, the measurements on 2D perovskites elucidated the critical role of electron-hole interactions in controlling exciton dynamics and transport. These results showcase the unique ability of 2D perovskites in achieving a combination of large exciton binding energy, long-range exciton transport, and slow annihilation, suggesting their large potential in optoelectronic applications.To address this challenge, here we employ transient absorption microscopy (TAM) as a direct means to image exciton population in space and in time to disentangle exciton diffusion and annihilation in 2D perovskites. We have demonstrated in a previous work the capability of TAM in imaging free carrier diffusion and extracting Auger recombination constants in bulk 3D perovskites4H9NH3)2(CH3NH3)n−1PbnIn+13 ((BA)2(MA)n−1PbnIn+13) were investigated in this study, as schematically illustrated in Fig. n ranging from 1 to 5 were mechanically exfoliated on fused silica for the optical measurements as shown in Supplementary Fig. n. In these samples, the [PbI6]4− octahedral layers are parallel to the substrate surface, as shown in the X-ray diffraction profiles . α Fig.  and the α on the order of 105 cm−1 , shifts to higher energy as n decreases  instead of two-dimensional imaging N was performed for the following TAM measurements. 1D imaging also allowed more measurement cycles to improve the signal-to-noise ratio. Although the pump and probe beam sizes were diffraction-limited (100 s of nm in visible light range), the accuracy to determine the width of exciton population profile was determined by the signal-to-noise ratio and it was better than 50 nm31  in 3D, we depicted T at the center  as a function of pump-probe delay time, which conveyed the same information. The fitting results are shown in Fig. n = 5. The experimental and simulated results for n ranging from 1 to 4 quantum wells are illustrated in Fig. n. The initial exciton densities were determined experimentally using absorption coefficient shown in Fig. n = 1, which is likely due to defect trapping of the excitons that is not included in the modeling. Defect density was the highest in n = 1, and probably not all the defects were passivated by the pump photons under our experimental conditions30. For n ranging from 2 to 5, such deviation was not observed because defect density decreased in thicker quantum wells and the pump fluences used were sufficiently high to passivate the defects30. The extracted D, k1, k2, and k3 from the fitting are summarized in Table To understand the spatial-temporal behavior, we modeled the exciton population as function of pump-probe delay time and probe position using Eq. les Fig.  as well mentclass2pt{minimD and diffusion length L0 exhibit strong dependence on n. As shown in Table D increases as n increases, from 0.06 ± 0.03 (n = 1) to 0.34 ± 0.03 cm2 s−1 (n = 5). Within the band transport picture36, the diffusion constant is determined by τs is the momentum relaxation time corresponding to the inverse of the sum of all scattering rates, k is the Boltzmann constant, T is temperature, and m is the reduced mass of the exciton. m was reported to be only weakly dependent on n18, and therefore the n dependence of D is more likely resulting from the difference in τs. The scattering sources include carriers, phonons, defects, and impurities. Defect density has been found to be higher for n = 1 than the thicker quantum wells30, which contributes to a more frequent defect scattering (a shorter τs) and a lower D. Dielectric screening could also play a role. As the n value increases from 1 to 5, the thickness of inorganic layer grows from 0.6 to 3 nm leading to the effective optical-frequency dielectric constant enlarging from 4 to 5.537. Because the scattering by charge carriers, polar optical phonons, and charge impurities are coulombic in nature, the reduced Coulomb interactions in the thicker quantum wells with higher dielectric constant results in a longer τs. The exciton diffusion constants measured for the 2D perovskites are consistent with the carrier diffusion constant of around 1 cm2 s−1 at the 3D limit of n = ∞38.Both exciton diffusion constant L0 as a function of n is presented in Fig. L0 increases from 160 to 670 nm as n increases from 1 to 5 . Therefore, EEA is reaction-limited instead of diffusion-limited, i.e., only a fraction of exciton encounters result in annihilation. We use the ratio of k2/D to describe the annihilation probability per encounter as given in Table n = 5 and they encounter at the highest rate, only about 0.4% of the encounters result in annihilation. Annihilation probability increases by two orders of magnitudes to 30% for n = 1. The reaction-limited EEA in 2D perovskites is in contrast to the diffusion-limited EEA in other low-dimensional nanostructures, such as single-walled carbon nanotubes42, which suggests that exciton diffusion and annihilation can be separately optimized.EEA takes place at densities of around 10n dependence of k2 underscores the importance of electron-hole interactions and dielectric screening in exciton annihilation26. EEA is mediated by the coupling between the transition dipole for promoting exciton 1 to the higher excited state and that for exciton 2 to decay to the ground state26. As n increases, the exciton wavefunction becomes located only in the inorganic layer with higher dielectric constant  similar to EEA, the Auger recombination probability for free carriers per encounter decreases with increasing n due to enhanced screening; and (2) the fraction of free carriers increases with as n increases.The strong ant Fig. , which e43. We note that there are a few previous studies on EEA and Auger recombination in 2D perovskites; however, an explicit n dependence of EEA rate has not been reported. Delport et al.30 reported EEA rate was nearly independent on n using time-resolved PL spectroscopy. The discrepancy between our results and those reported in Delport et al.30 could be due to that PL measurements only monitor a subset of the exciton population, namely the emissive species. In contrast, the photoinduced bleach TA signal reflects to all exciton population. Using TA spectroscopy, Chen et al.29 reported both k2 and k3 increases as n decreases; however, they interpreted the k2 in terms of bimolecular recombination of free carriers instead of EEA. Auger recombination in mixed-phase 2D-3D perovskites has been studied by time-resolved THz measurements by Milot et al.;28 however, THz measurements are sensitive to free carriers instead of excitons.The observation of enhanced EEA rates at larger exciton binding energies in 2D perovskites are consistent with those reported in single-walled carbon nanotubes and colloidal quantum dots2 monolayers34 to be around 2 cm2 s−1, more than one order higher than the n = 1 quantum-well. A possible reason is the formation of exciton polarons44 that have larger effective mass leading to slower diffusion. Another feasible explanation is that structural disorder is much larger in hybrid perovskites than in TMDCs due to the ionic nature. Structural disorder results in fluctuations in the energy landscape, slowing down exciton transport45. We suggest that exciton transport in 2D perovskites can be further enhanced through structural tuning to increase rigidity and reduce structural disorder, for instance, by employing phenyl group motifs at the R site3.We compare exciton transport and annihilation properties of 2D perovskites to those of TMDCs. We have measured the exciton diffusion constant in exfoliated WS2 s−1 for monolayer WS246, similar to other TMDCs47, which is more than one order of magnitude higher than that of n = 1. This large difference in exciton annihilation rates given similar exciton binding energies in two materials is intriguing and could be related to the unique dielectric function of hybrid perovskites, for instance, as the result of the motion of the polar organic cations48. Further, an advantage brought by the highly tunable structure of 2D perovskites is the ability to modulate Coulomb interactions to suppress annihilation through composition tuning. Different from TMDCs that undergo direct to indirect bandgap transition at bilayer49, 2D perovskites are direct bandgap semiconductors with high PL quantum yields regardless of the value of n.Notably, exciton annihilation is significantly reduced in 2D perovskites in comparison to TMDCs. In a previous work, we determined the exciton–exciton annihilation rate to be 0.41 cmn ranging from 1 to 5 in Fig. n. k2 becomes larger than k1 at exciton density as low as 2 × 1010 cm−2 for n = 1, indicating that EEA turns into the dominant recombination pathway. This is probably one of the key reasons why high-performance solar cell and LED based on n = 1 2D perovskites have not been successfully demonstrated so far. For n = 5, the switch over occurs at higher exciton density around 1011 cm−2. The benefit of the reduction in annihilation is most significant for increasing from n = 1 to n ranging from 2 to 4, suggesting that these structures with Eb ranging from 150 to 250 meV might be optimal for light emitting devices with the combined benefits of large exciton binding energy and slow annihilation. This is consistent with a recent study50 in which van-der-Waals-coupled 2D perovskite multi-quantum-wells were shown to have better light emitting performance than 3D perovskites due to strongly bound excitons. For solar cells, quantum wells with large n are suitable because of the combined benefits of the small exciton binding energy and long-range diffusion. Further suppression of EEA should be possible through modulating R and A site organic cations to modify dielectric screening. Such promise has been demonstrated recently, for instance, a study has showed that Auger recombination rate in the hybrid perovskite nanocrystals containing polar organic cations is significantly lower than in the fully inorganic Cs-based ones51.As a guide, we plot the total annihilation rates n−1PbnIn+13 (n ranging from 1 to 5) were synthesized via a solution precipitation reaction. PbO powders, hydroiodic acid , hypophosphorous acid  were purchased from Sigma–Aldrich. Methylammonium iodide (MAI), formamidinium iodide (FAI), n-Butylammonium bromide (BABr), and n-Butylammonium iodide (BAI) were purchased from Greatcell Solar Ltd. All chemicals were used as received. Commercial Scotch tapes (3 M) were used to exfoliate 2D halide perovskite crystals. The mechanical exfoliation was performed in N2-filled glovebox. The thicknesses of the flakes were measured by AFM (Veeco Dimension 3100). More details on the synthesis and sample preparation can be found in Supplementary Note The 2D Ruddlesden-Popper phase halide perovskite crystals (BA)A home-built microscope was used for the micro-absorption and PL spectroscopy measurements. For the micro-absorption measurements, a halogen tungsten lamp was used as white light source, which was focused by objective on sample to form light spot with size of around 2 µm. The reflective or transmitted light was collected by an objective , and detected by a spectrometer (Andor Shamrock 303i) and CCD (Andor Newton 920) for spectral analysis. For the PL measurements, a picosecond pulse laser with wavelength of 447 nm was used as excitation light and an objective  was employed to focus the excitation light on the sample and collect the epic scattered PL light. The same spectrometer and CCD combo as described above were used for PL spectral analysis.31. Briefly, the output from two independent optical parametric amplifiers  served as pump and probe beam respectively, which were focused on sample with an objective . An acousto-optic modulator  triggered by a lock-in amplifier  was used to modulate the pump beam. A mechanical translation stage  was used to delay the probe with respect to the pump. A 2D Galvo mirror (Thorlabs GVS012) was employed to scan the probe beam relative to the pump beam in space  was extracted by the lock-in amplifier.The home-built TAM setup used in this study has been described in our previous publicationSupplementary InformationPeer Review File


The current study evaluated the presence of CMV on surfaces in homes of the children.Caring for young children is a known risk factor for cytomegalovirus (CMV) infection mainly through exposure to their saliva and urine. In a previous study, 36 CMV-seropositive children 2 mo. to 4 years old were categorized as CMV shedders  and two in contact with children’s urine . Samples were tested by PCR and viral culture to quantify the presence of CMV DNA and viable virus.p < 0.001). Saliva–associated samples were more often CMV positive with higher viral loads than urine-associated samples. The higher the CMV viral load of the child in the home, the more home surfaces that were PCR positive (p = 0.01) and viral culture positive (p = 0.05).A total of 654 surface samples from 36 homes were tested; 136 were CMV DNA positive, 122 of which (90%) were in homes of the children shedding CMV (The main source for CMV on surfaces in homes was saliva from the child in the home. Higher CMV viral loads shed by children correlated with more viable virus on surfaces which could potentially contribute to viral transmission. Cytomegalovirus (CMV) infection is the most common cause of congenital viral infection in developed countries. Primary infection and reinfection of adults occurs at higher rates among those in contact with young children in the home, daycare centers and schools –3. ChildIn this study, we tested for the presence of CMV by quantitative PCR and viral culture on samples taken from surfaces in homes where young CMV-seropositive children lived to better understand potential routes of CMV transmission beyond direct contact with the saliva and urine of shedding children.In a previous study we enrolled 36 healthy, CMV-seropositive children 2 mo. to 4 years old (mean age 17 months) that were still in diapers from regional daycare centers and pediatrician’s offices in the metropolitan Atlanta area. None of the children had chronic medical conditions or had been diagnosed with congenital CMV infection . Saliva Surface specimens were collected at 4 times by study staff at the same visits where saliva and urine were collected from children. At each visit, 5 surface specimens were collected using sterile cotton-tipped swabs  pre-moistened with phosphate buffered saline (PBS): (1) Spoon; spoon shortly after removal from child’s mouth, (2) Cheek; child’s cheek area where there was visible moisture, (3) Washcloth; washcloth shortly after the mother used it to wipe the child’s mouth, (4) Hands; the palm of the mother’s hand immediately after a diaper change, (5) Changing area; changing table immediately after a diaper change. Swabs were immediately immersed in 1 ml viral transport media  in 15 ml tubes, placed on ice and transported to the CDC laboratory for processing, the same day in most cases.4 copies/ml, medium viral load as 104–106 copies/ml, and high viral load as > 106 copies/ml.Swabs in VTM were removed to a syringe and centrifuged 10 min at 2000 rpm to extract liquid which was mixed back with the VTM. For PCR testing, 100 μl was removed and mixed with 100 μl of Quick Extract buffer , incubated 1 h at 56 °C, then 100 °C for 3 min, then cooled on ice. The extract was used for Taqman-based PCR targeting the CMV gB gene  with comFor viral culture, the remaining VTM (about 0.9 ml) was added to a T-25 tissue culture flask containing human lung fibroblast cells  from which media had been removed. Following 1 h adsorption, 5 ml of fresh culture medium with antibiotics was added to the flask. Cultures were observed for cytopathic effect twice per week for 3 weeks. Urine samples were dried on filter paper and thus not usable for culture.P-values for proportions by using the chi-square or Fisher’s exact tests as appropriate. When comparing viral loads or antibody titers, we used the Cochran-Mantel-Haenszel test and the Wilcoxon rank-sum test.Associations were examined using SAS version 9.3 . We calculated p < 0.001) and 10% from the homes of non-shedding children which was mainly from one home where the child began shedding CMV after enrollment.A total of 913 specimens were collected over 4 visits from 36 children in 35 households; 259 specimens were from children and 654 from surfaces. Figure p = 0.01) as evidence that the shedding child in the home was the source of CMV in the home. The same three viral load categories shown in Table 4, 104–106, > 106).Specimens were CMV PCR positive at a rate of 12–56% depending on the type of specimen (Table p = 0.05). The higher viral load specimens were saliva and saliva-associated surfaces and were more often positive by viral culture than lower viral load specimens associated with urine. Urine was not amenable to viral culture testing because it was dried on filter paper. High PCR viral load was strongly associated with positive viral culture  of specimens processed for culture; 36% failed for technical reasons including poor quality cell sheets (20%), or heavy bacterial contamination 16%) despite the use of antibiotics in culture media. Culture results by specimen type are displayed in Table 6% despit6 of specOur study evaluated CMV on various home surfaces exposed to saliva and urine of young CMV-seropositive children to better understand potential routes of transmission. The same children were enrolled in a related study that took weekly measurements of their saliva and urine over 100 days to measure CMV shedding , 16. ResUnique aspects of our study include the focus on surfaces in homes where there are few potential sources of CMV, measurement of wild-type virus, and known CMV sero-status of children. Several studies since the 1980’s have documented high rates of CMV shedding and transmission showing that young children are a main source of CMV transmission, and peak rates of shedding occur at 1–2 years of age , 17–21. Another useful result of our study is demonstration of the effective use of 903 filter paper to collect and test urine for CMV. The U.S. newborn screening program collects blood on 903 filter paper from a heel stick from virtually all infants. Blood spots are then used to screen for a wide range of birth defects. Our laboratory has previously used newborn blood spots to test for CMV , and in Several studies to date indicate prevention education can reduce CMV infection –28. Our The main source for CMV on surfaces in homes was young children living in the home. Surfaces associated with saliva had higher CMV viral loads and more viable virus than surfaces associated urine, therefore saliva is likely more important vehicle for transmission than urine. Better understanding of the specific routes of transmission can lead to prevention messages that are more evidence-based and effective.