Combined Resonance Research Articles

Computational technique to free vibration response in a multi-degree of freedom parametric system

The free vibration response is helpful to understand the dynamic characteristics of parametric system, and further could be used as important information source for the failure diagnosis and health monitoring of structure. In this paper, the free vibration response of a multi-degree of freedom (MDOF) parametric system is approached as a closed-form solution, a special trigonometric series with a linear combination of the principal oscillation frequency and the parametric excitation frequency. By algebra operation of harmonic balance, the parametric equation can be transformed into a linear homogeneous algebraic equation, on which the characteristic equation is determined and some important solutions are given. Main contribution of the presented technique is that it not only solves the computational problem of principal oscillation frequencies and modes with high computational accuracy, but also gives the splitting mode, which is the amplitude ratio of the combined harmonic resonance in the parametric system. Besides, the mathematical approximation reflects the physic characteristics in the free vibration response of parametric system. An example of real life is used to illustrate the applicability of the presented computational technique. The more expansion terms in approximation series, the smaller the computational error. Therefore, the given approximation provides a powerful computational tool for the free vibration response in the MDOF parametric system.

Molecular Dynamics Simulations Based on Newly Developed Force Field Parameters for Cu2+ Spin Labels Provide Insights into Double-Histidine-Based Double Electron-Electron Resonance.

Electron paramagnetic resonance (EPR) in combination with the recently developed double-histidine (dHis)-based Cu2+ spin labeling has provided valuable insights into protein structure and conformational dynamics. To relate sparse distance constraints measured by EPR to protein fluctuations in solution, modeling techniques are needed. In this work, we have developed force field parameters for Cu2+-nitrilotriacetic and Cu2+-iminodiacetic acid spin labels. We employed molecular dynamics (MD) simulations to capture the atomic-level details of dHis-labeled protein fluctuations. The interspin distances extracted from 200 ns MD trajectories show good agreement with the experimental results. The MD simulations also illustrate the dramatic rigidity of the Cu2+ labels compared to the standard nitroxide spin label. Further, the relative orientations between spin-labeled sites were measured to provide insight into the use of double electron-electron resonance (DEER) methods for such labels. The relative mean angles, as well as the standard deviations of the relative angles, agree well in general with the spectral simulations published previously. The fluctuations of relative orientations help rationalize why orientation selectivity effects are minimal at X-band frequencies, but observable at the Q-band for such labels. In summary, the results show that by combining the experimental results with MD simulations precise information about protein conformations as well as flexibility can be obtained.

Earliest African evidence of carcass processing and consumption in cave at 700 ka, Casablanca, Morocco

To date, in Africa, evidence for animal processing and consumption in caves routinely used as living spaces is only documented in the late Middle Pleistocene of the North and South of the continent and postdates the Middle Pleistocene in East Africa. Here we report the earliest evidence in a North-African cave (Grotte des Rhinocéros at Casablanca, Morocco) of cut, percussion and human gnawing marks on faunal remains directly associated with lithic knapping activities in the same space and in a well-documented stratified context. Ages for this Acheulean site are provided by the dating of herbivorous teeth to 690–720 ka and 520–550 ka (lower and upper sets) by combined Electron Spin Resonance (ESR) and U-series techniques. Traces of butchery on gazelle, alcelaphin, and zebra bones demonstrate that hominins had primary access to herbivore carcasses. Hominins brought and consumed meat in the cave, as documented by herbivore bones bearing human tooth marks concentrated in a circumscribed area of the excavation. In Africa, this site provides the earliest evidence for in situ carcass processing and meat-eating in cave, directly associated with lithic production and demonstrates the recurrent use by early Middle Pleistocene hominins of a North African cave site 400 000 years before that by Homo sapiens at Jebel Irhoud (Morocco).

The value of Mr DWI and MRS imaging in the diagnosis of prostate cancer

Abstract: objective: to investigate the diagnostic value of combined diffusion-weighted magnetic resonance imaging (DWI) and magnetic resonance spectroscopy (MRS) in prostate cancer (PCa).Materials and methods: A total of 25 patients in group A, ranging in age from 67 to 85 years, had diffuse lesions in 3 of them and bone metastases (ilium, pubis, etc.) in 5 of them were confirmed by final examination. All the patients in group A had higher TPSA levels than the reference standard of 4ng/ml within 7 days before mri examination, and the lowest concentration in one patient was 5.02ng/ml. Results greater than 100ng/ml were measured in 5 patients. The sensitivity, specificity and accuracy of DWI, MRS and DWI combined with MRS for PCa diagnosis were analyzed.Results: the mean ADC values of PCa area, peripheral band and central gland in patients with BPH were (0.83±0.12)× 10-3mm /s, (1.82±0.26)× 10-3mm /s and (1.46±0.16)× 10-3mm /s(F=31.1, P < 0.05), respectively.222The mean (Cho+Cr)/Cit values in PCa region, peripheral zone and central gland of BPH patients were 1.55±0.11, 0.53±0.16 and 0.64±0.13, respectively (F=18.2, P < 0.05).There was a statistically significant difference between PCa and BPH patients' peripheral zone and central gland (Cho+Cr)/Cit (P < 0.05), while there was no statistically significant difference between BPH patients' peripheral zone and central gland (Cho+Cr)/Cit (P >, 0.05).The sensitivity, specificity and accuracy of PCa diagnosed by DWI were 79.17%, 80% and 79.63% respectively.The sensitivity, specificity and accuracy of PCa were 87.5%, 86.67% and 87.03% respectively.The sensitivity, specificity and accuracy of DWI combined with MRS in diagnosing PCa were 91.67%, 93.33% and 92.59%, respectively.Conclusion: DWI combined with MRS is better than DWI and MRS alone in the diagnosis of PCa.

Comparing dynamic causal models of neurovascular coupling with fMRI and EEG/MEG

This technical note presents a dynamic causal modelling (DCM) procedure for evaluating different models of neurovascular coupling in the human brain – using combined electromagnetic (M/EEG) and functional magnetic resonance imaging (fMRI) data. This procedure compares the evidence for biologically informed models of neurovascular coupling using Bayesian model comparison. First, fMRI data are used to localise regionally specific neuronal responses. The coordinates of these responses are then used as the location priors in a DCM of electrophysiological responses elicited by the same paradigm. The ensuing estimates of model parameters are then used to generate neuronal drive functions, which model pre- or post-synaptic activity for each experimental condition. These functions form the input to a model of neurovascular coupling, whose parameters are estimated from the fMRI data. Crucially, this enables one to evaluate different models of neurovascular coupling, using Bayesian model comparison – asking, for example, whether instantaneous or delayed, pre- or post-synaptic signals mediate haemodynamic responses. We provide an illustrative application of the procedure using a single-subject auditory fMRI and MEG dataset. The code and exemplar data accompanying this technical note are available through the statistical parametric mapping (SPM) software.

The Role of Combined Diffusion-Weighted Imaging and Dynamic Contrast-Enhanced MRI for Differentiating Malignant From Benign Breast Lesions Presenting Washout Curve.

The aim of this study is to evaluate the diagnostic performance of combined breast magnetic resonance imaging (MRI) protocol including dynamic contrast-enhanced MRI (DCE-MRI) and diffusion-weighted imaging (DWI) in patients with enhancing lesions that demonstrated washout curve and to determine whether applying apparent diffusion coefficient (ADC) cutoff value could improve the diagnostic value of breast MRI. The retrospective study included 116 patients with 116 suspicious breast lesions, which showed washout curve on DCE-MRI, who underwent subsequent biopsy. Morphologic characteristics on DCE-MRI and ADC values on DWI were evaluated. Apparent diffusion coefficient values and morphologic features of benign and malignant lesions were compared. Diagnostic values of DCE-MRI and combined MRI, including DCE-MRI and DWI (applying an ADC cutoff value) for distinguishing malignancy from benign lesions, were calculated. Of the 116 breast lesions, 79 were malignant and 37 were benign. The ADC value of malignant tumors (median ADC, 0.72 × 10-3 mm2/s) was significantly lower than that of benign lesions (median ADC, 1.03 × 10-3 mm2/s; P < .000). The sensitivity and specificity of an ADC cutoff value of 0.89 × 10-3 mm2/s were 92% and 95%, respectively. Dynamic contrast-enhanced MRI alone presented 100% sensitivity and 59.4% specificity. Adding an ADC cutoff value of 0.89 × 10-3 mm2/s provided 100% sensitivity and 81% specificity, which would have prevented biopsy for 21.6% of benign lesions without missing any malignancies. Applying an ADC cutoff value to DCE-MRI provides an improvement in the diagnostic value of breast MRI for differentiating among lesions presenting washout curve.

γ-aminobutyric acid and glutamate/glutamine alterations of the left prefrontal cortex in individuals with methamphetamine use disorder: a combined transcranial magnetic stimulation-magnetic resonance spectroscopy study.

BackgroundGABAergic and glutamatergic neurotransmitter systems are critical in the pathophysiology of addiction and represent potential targets for repetitive transcranial magnetic stimulation (rTMS). This study aims to investigate changes in γ-aminobutyric acid (GABA) levels, the combined resonance of glutamate and glutamine (Glx) in the left dorsolateral prefrontal cortex (DLPFC), and cognitive function of patients with methamphetamine dependence following rTMS intervention, using proton magnetic resonance spectroscopy (1H MRS).MethodsFifty methamphetamine-dependent patients were randomized to a 4-week course of active or sham rTMS, with 1H MRS measurement of DLPFC GABA and Glx levels relative to n-acetyl-aspartate (NAA) and craving and cognitive function measured at baseline and post-intervention.ResultsWe observed significant reductions of GABA/NAA concentration in the active group and Glx/NAA concentration in the group receiving sham rTMS. There was a significant association between changes in GABA concentration and problem solving/error monitoring.ConclusionsThe effect of rTMS on cognitive function in individuals with methamphetamine dependence may be related to changes in GABA levels in the prefrontal cortex, and warrants further investigation.

Insular and occipital changes in visual snow syndrome: a BOLD fMRI and MRS study.

ObjectiveTo investigate the pathophysiology of visual snow (VS), through a combined functional neuroimaging and magnetic resonance spectroscopy (1H‐MRS) approach.MethodsWe applied a functional MRI block‐design protocol studying the responses to a visual stimulation mimicking VS, in combination with 1H‐MRS over the right lingual gyrus, in 24 patients with VS compared to an equal number of age‐ and gender‐matched healthy controls.ResultsWe found reduced BOLD responses to the visual stimulus with respect to baseline in VS patients compared to controls, in the left (k = 291; P = 0.025; peak MNI coordinate [‐34 12 ‐6]) and right (k = 100; P = 0.003; peak MNI coordinate [44 14 ‐2]) anterior insula. Our spectroscopy analysis revealed a significant increase in lactate concentrations in patients with respect to controls (0.66 ± 0.9 mmol/L vs. 0.07 ± 0.2 mmol/L; P < 0.001) in the right lingual gyrus. In this area, there was a significant negative correlation between lactate concentrations and BOLD responses to visual stimulation (P = 0.004; r = −0.42), which was dependent on belonging to the patient group.InterpretationAs shown by our BOLD analysis, VS is characterized by a difference in bilateral insular responses to a visual stimulus mimicking VS itself, which could be due to disruptions within the salience network. Our results also suggest that patients with VS have a localized disturbance in extrastriate anaerobic metabolism, which may in turn cause a decreased metabolic reserve for the regular processing of visual stimuli.

Dobutamine stress testing in patients with Fontan circulation augmented by biomechanical modeling.

Understanding (patho)physiological phenomena and mechanisms of failure in patients with Fontan circulation—a surgically established circulation for patients born with a functionally single ventricle—remains challenging due to the complex hemodynamics and high inter-patient variations in anatomy and function. In this work, we present a biomechanical model of the heart and circulation to augment the diagnostic evaluation of Fontan patients with early-stage heart failure. The proposed framework employs a reduced-order model of heart coupled with a simplified circulation including venous return, creating a closed-loop system. We deploy this framework to augment the information from data obtained during combined cardiac catheterization and magnetic resonance exams (XMR), performed at rest and during dobutamine stress in 9 children with Fontan circulation and 2 biventricular controls. We demonstrate that our modeling framework enables patient-specific investigation of myocardial stiffness, contractility at rest, contractile reserve during stress and changes in vascular resistance. Hereby, the model allows to identify key factors underlying the pathophysiological response to stress in these patients. In addition, the rapid personalization of the model to patient data and fast simulation of cardiac cycles make our framework directly applicable in a clinical workflow. We conclude that the proposed modeling framework is a valuable addition to the current clinical diagnostic XMR exam that helps to explain patient-specific stress hemodynamics and can identify potential mechanisms of failure in patients with Fontan circulation.

Parametric vibrations and instabilities of an elliptical gear pair

Parametric vibrations and instabilities can be induced in an elliptical gear pair. This study examined the effects of basic parameters on vibration instability. Geometric parameters and time-variant mesh stiffness are calculated. A torsional dynamic model is established by introducing the lumped parameter assumption. Based on the model, the parametric vibrations caused by the excitations of eccentricity and time-variant mesh stiffness are investigated, and the additive and difference combination resonances are identified. Comparison between the elliptical and circular gears implies that the vibration amplitude at mesh frequency of the elliptical gears is lower than that of the regular circular gears. The vibrations caused by load torque are also examined. The results show that the vibrations with different frequencies are coupled with each other. The inherent reason behind the coupling is the interaction between the eccentricity, time-variant stiffness, and load.

Analysis of multiple frequency excitations of a cracked beam using nonlinear vibrations

In this study, under harmonic multiple frequency excitations, the dynamic response of a cracked cantilever beam is investigated. The breathing crack model is assumed to show the nonlinear behaviour of a transverse crack. The first mode of vibration and the single degree freedom lumped system is considered to simplify the case study. Because of applying the multiple frequency excitations, the analysis is applied in a combinational resonance. Multiple time scales method is employed to solve the motion equation of the crack, and the nonlinear vibrational responses are obtained. Then, by changing the crack parameters and frequency of the excitations, the different dynamic responses of the crack are demonstrated. The proposed model shows that the crack parameters analysis in nonlinear vibration of multiple excitations could be an appropriate method to recognise the crack and the depth of the damage. Results indicate that the beam analysis under multiple frequency excitations is more sensitive than the single frequency excitation to illustrate the impacts of the crack parameters on its vibrational nonlinearity responses.

Contribution of the Mesencephalon Indices to Differential Diagnosis of Parkinsonian Disorders.

This study aimed to assess the sensitivity and specificity of brain stem morphologic changes to differentiate the progressive supranuclear palsy (PSP) from Parkinson disease (PD) and multiple system atrophy (MSA), by single and combined magnetic resonance imaging (MRI) measurements. Peduncle angle (PA), pons area (P), mesencephalon area (M), middle cerebellar peduncles (MCPs), and superior cerebellar peduncles (SCPs) were measured in 14 PSP, 43 PD, 8 MSA patients, and 45 age-matched control participants on T1-weighted MRI. Neurologists clinically diagnosed all patients. Additionally, P/M ratio, MCPs/SCPs ratio, the previously defined Magnetic Resonance Parkinsonism Index, MRPI: (P/M) · (MCP/SCP), and also the Akdeniz Index (AKI) that we termed were calculated, AKI: (P/M) · (PA/180). Two blinded radiologists evaluated all MR images and inter-/intraobserver variations were measured. Both M and SCPs were significantly lower and P/M, MCPs/SCPs, and PA were significantly higher in PSP patients than the other groups (P < .001). This significance was related to patients with PSP and PD. But all single measurements showed some overlapping values. Therefore, previously defined MRPI was calculated and shown to distinguish patients (negative predictive values: 92%, sensitivity: 78%, specificity: 82%). In this study, interobserver correlation (0.68) was found low for MRPI. Therefore, we identified a more practical index: the Akdeniz Index, which has same diagnostic power with MRPI and higher interobserver correlation (0.91). The Akdeniz Index identified in our study is a practical index with high diagnostic power and can reinforce radiological distinguishing of PSP and PD, which are clinically difficult to distinguish.

Planar and non-planar vibrations of a fluid-conveying cantilevered pipe subjected to axial base excitation

The main aim of the present study is to explore the nonlinear vibrations of a fluid-conveying cantilevered pipe under an axial base excitation, by considering both two-dimensional (2-D) and three-dimensional (3-D) responses of the pipe. For this purpose, the extended Hamilton’s principle is applied to derive the 3-D nonlinear governing equations of the pipe with an axial base excitation. The resulting partial differential equations are then discretized by employing a Galerkin method. The Floquet theory is adopted to determine the stability regions of the pipe system. A linear analysis shows that the internal flow velocity, excitation amplitude and excitation frequency have great influences on the stability regions of the pipe system, indicating that both subharmonic and combination resonances can occur. The planar and non-planar nonlinear responses of the pipe are calculated by using a fourth-order Runge–Kutta integration algorithm. These nonlinear responses of the pipe are displayed in the form of bifurcation diagrams, phase-plane plots, power spectrum diagrams, Poincare maps, oscillating shapes and oscillation trajectories of the pipe tip. Our numerical results demonstrate that, for a pipe conveying fluid with a subcritical flow velocity, the presence of an axial excitation is able to generate resonant responses, while for a pipe conveying fluid with a supercritical flow velocity, the axial excitation can even make the pipe stable in some specific cases. Furthermore, it is interesting that a self-excited non-planar periodic oscillation can evolve to a planar quasi-periodic or periodic oscillation by adding the axial excitation. In such a fluid–structure interaction system, however, it is shown that a self-excited planar motion cannot be shifted to a non-planar one by adding the axial excitation, at least for the system parameters considered in this work.

Neurochemical and brain functional changes in the ventromedial prefrontal cortex of first-episode psychosis patients: A combined functional magnetic resonance imaging-proton magnetic resonance spectroscopy study.

Previous studies showed alterations of brain function in the ventromedial prefrontal cortex of schizophrenia patients. Also, neurochemical changes, especially GABA level alteration, have been found in the medial prefrontal cortex of schizophrenia patients. However, the relationship between GABA level in the ventromedial prefrontal cortex and brain functional activity in schizophrenia patients remains unexplored. In total, 23 drug-naïve, first-episode psychosis patients and 26 matched healthy controls completed the study. The single voxel proton magnetic resonance spectroscopy data were acquired in ventromedial prefrontal cortex region, which was used as the seed region for resting-state functional connectivity analysis. The proton magnetic resonance spectroscopy data were processed to quantify the concentrations of GABA+, glutamine and glutamate, and N-acetylaspartate in ventromedial prefrontal cortex. Spearman correlation analysis was used to examine the relationship between metabolite concentration, functional connectivity and clinical variables. Pearson correlation analysis was used to examine the relationship between GABA+ concentration and functional connectivity value. In first-episode psychosis patients, GABA+ level in ventromedial prefrontal cortex was higher and was positively correlated with ventromedial prefrontal cortex-left middle orbital frontal cortex functional connectivity. N-acetylaspartate level was positively correlated with positive symptoms, and the functional connectivity between ventromedial prefrontal cortex and left precuneus was negatively associated with negative symptoms of first-episode psychosis patients. Our results indicated that ventromedial prefrontal cortex functional connectivity changes were positively correlated with higher local GABA+ level in first-episode psychosis patients. The altered neurochemical concentration and functional connectivity provide insights into the pathology of schizophrenia.

Multi-Wave Vibration Caused by Flutter Instability and Nonlinear Tip-Shroud Friction

Abstract Measurements revealed the contribution of multiple traveling waves to the flutter vibrations of bladed disks. Saturated flutter vibration, whether in this multi-wave or in its better-understood single-wave form, is a nonlinear phenomenon. However, it is still not understood of what physical origin the relevant nonlinearities are, and under what conditions single-wave or multi-wave flutter vibration occurs. Recent theoretical work suggests that multi-wave flutter vibration can be explained by strongly nonlinear frictional interblade coupling. The verity of this hypothesis is strictly limited by the simplicity of the considered model, namely, a cyclic chain of seven oscillators with frictional coupling and rather unrealistic aeroelastic behavior. In this work, it is demonstrated that nonlinear dynamical contact interactions at tip-shrouds are a likely cause for the observed multi-wave flutter vibration. To this end, a more realistic structural turbine blade row model with a more realistic aeroelastic behavior is considered. Some insight into its intriguing dynamics, dependence of limit states on initial conditions, and eigenvalue placement is provided. For instance, it is shown that there is an intimate relation between internal combination resonance conditions of certain traveling wave modes and the spectral content of single- and multi-wave flutter oscillations.

Investigation of the favorable conditions to apply the combination resonances approach for crack detection purposes

Various structural health monitoring methods were recently applied to fault diagnosis of rotating machinery, in which vibration response–based techniques demonstrated to be well adapted for different scenarios. However, only severe cracks are detected when most of such techniques are applied. Thus, innovative methods are proposed to identify the existence of incipient cracks in rotating shafts. In a previous contribution, the combination resonance approach for crack identification purposes was presented. This method is based on the application of additional forces at combination frequencies along with a heuristic optimization method to identify crack signatures in the vibration responses of the rotor. In the present work, the best conditions to apply the combination resonance approach are investigated in terms of the amplitude and frequency of the so-called diagnostic force. The vibration responses and phase angles of a cracked rotor were determined by using the harmonic balance technique. The FLEX approach is used to model the crack. Numerical simulations were performed using the finite element model of a horizontal rotor.

Nonlinear Dynamics of Adaptive Gun Head Jet System of Fire-Fighting Monitor

The working medium of the adaptive gun head jet system of fire-fighting monitor is generally water containing a little bit of air. During the operation, the pressure pulsation of the fluid will cause the fluctuation of the equivalent stiffness of the gas-liquid mixed fluid, so that the motion of the fluid in the jet system has obvious nonlinear characteristics. In this paper, the nonlinear dynamic model of the jet system is established. The analytical expressions of the nonlinear vibration response of the jet system are derived via the multi-scale method. The main resonance and combined resonance of the jet system are determined. The results show that the external excitation frequency is the dominant frequency of the main resonance response of the jet system, and the combined frequency between the natural frequency of each order and the equivalent stiffness fluctuation frequency of the fluid unit has a small effect on the main resonance, and the maximum amplitude is 0.2592mm; the dominant frequency of the combined resonance response of the jet system is the combined frequency between the natural frequency of each order and the equivalent stiffness fluctuation frequency of the fluid unit, the system amplitude in combined resonance is smaller than that in the main resonance, and the maximum amplitude is 0.002532mm; the main resonance and the combined resonance will adversely affect the dynamic characteristics of the jet system. This research can provide a theoretical basis for the dynamic optimization of the adaptive gun head jet system of the fire-fighting monitor.

Combination resonances of imperfect SSFG cylindrical shells rested on viscoelastic foundations

The present paper investigates the combination resonance behavior of imperfect spiral stiffened functionally graded (SSFG) cylindrical shells with internal and external functionally graded stiffeners under two-term large amplitude excitations. The structure is embedded within a generalized nonlinear viscoelastic foundation, which is composed of a two-parameter Winkler-Pasternak foundation augmented by a Kelvin-Voigt viscoelastic model with a nonlinear cubic stiffness, to account for the vibration hardening/softening phenomena and damping considerations. With regard to classical plate theory of shells, von-Karman equation and Hook law, the relations of stress-strain are derived for shell and stiffeners. The spiral stiffeners of the cylindrical shell are modeled according to the smeared stiffener technique. According to the Galerkin method, the discretized motion equation is obtained. The combination resonance is obtained by using the multiple scales method. Finally, the influences of the stiffeners angles, foundation type, the nonlinear elastic foundation coefficients, material distribution, and excitation amplitude on the system resonances are investigated comprehensively.

Research on Dynamic Modeling and Parameter Influence of Adaptive Gun Head Jet System of Fire-Fighting Monitor

The jet medium of the adaptive gun head jet system of the fire-fighting monitor is generally a mixture of air and water. During the operation, the equivalent stiffness of the jet fluid fluctuates periodically or randomly due to the pressure pulsation of the jet system and the external excitation such as wind, thus the dynamic behavior of the adaptive gun head jet system of the fire-fighting monitor has typical nonlinear characteristics. In this paper, based on the assumption that the equivalent stiffness of the fluid fluctuates in type of simple harmonic motion, a nonlinear dynamic model of the jet system is established, and the model is solved by the multi-scale method. The influence of design parameters such as fluid pulsation frequency, air inlet rate and fluid pressure on the amplitude of the main resonance and combined resonance of the jet system is analyzed. Results show that the existence of the fluid pulsation frequency increases the resonance frequency range of the jet system. As to the influence of design parameters on the dynamic characteristics of the jet system, from the point of view of amplitude change, the air inlet rate has the greatest impact on the main resonance, and the fluid pulsation frequency has the dominant influence on the combined resonance. This research can provide theoretical support for the dynamic optimization of the adaptive gun head jet system of the fire-fighting monitor.

Light-induced charge transfer at the CH3NH3PbI3/TiO2 interface—a low-temperature photo-electron paramagnetic resonance assay

The performance of organic–inorganic metal halide perovskites-based (MHPs) photovoltaic devices critically depends on the design and material properties of the interface between the light-harvesting MHP layer and the electron transport layer (ETL). Therefore, the detailed insight into the transfer mechanisms of photogenerated carriers at the ETL/MHP interface is of utmost importance. Owing to its high charge mobilities and well-matched band structure with MHPs, titanium dioxide (TiO2) has emerged as the most widely used ETL material in MHPs-based photovoltaic devices. Here, we report a contactless method to directly track the photo-carriers at the ETL/MHP interface using the technique of low-temperature electron paramagnetic resonance (EPR) in combination with in situ illuminations (Photo-EPR). Specifically, we focus on a model nanohybrid material consisting of TiO2-based nanowires (TiO2NWs) dispersed in the polycrystalline methylammonium lead triiodide (MAPbI3) matrix. Our approach is based on observation of the light-induced decrease in intensity of the EPR signal of paramagnetic Ti3+ () in non-stoichiometric TiO2NWs. We associate the diminishment of the EPR signal with the photo-excited electrons that cross the ETL/MHP interface and contribute to the conversion of Ti3+ states to EPR-silent Ti2+ states. Overall, we infer that the technique of low-temperature Photo-EPR is an effective strategy to study the transfer mechanisms of photogenerated carriers at the ETL/MHP interface in MAPbI3-based photovoltaic and photoelectronic systems.