Ligand-engineered bandgap stability in mixed-halide perovskite LEDs.

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

In Situ Measurement of Evolving Excited-State Dynamics During Deposition and Processing of Organic Films by Single-Shot Transient Absorption.

A significant advantage of organic semiconductors over many of their inorganic counterparts is solution processability. However, solution processing commonly yields heterogeneous films with properties that are highly sensitive to the conditions and parameters of casting and processing. Measuring the key properties of these materials in situ, during film production, can provide new insight into the mechanism of these processing steps and how they lead to the emergence of the final organic film properties. The excited-state dynamics is often of import in photovoltaic, electronic, and light-emitting devices. This review focuses on single-shot transient absorption, which measures a transient spectrum in a single shot, enabling the rapid measurement of unstable chemical systems such as organic films during their casting and processing. We review the principles of instrument design and provide examples of the utility of this spectroscopy for measuring organic films during their production.

Psychometric Validation of the Chinese Version of the Atrial Fibrillation Knowledge Scale in Chinese Patients With Atrial Fibrillation.

BACKGROUND: Assessment of patients' knowledge on atrial fibrillation (AF) and its management is important for evaluating their learning needs. However, a validated and comprehensive instrument to be used among Chinese patients is yet to be developed. OBJECTIVES: The aim of this study was to develop and validate the Chinese version of the Atrial Fibrillation Knowledge Scale (AFKS-C) in Chinese patients. METHODS: The 11-item AFKS was translated, and then content and face validations were conducted by an expert panel and patients with AF. A sample of 255 patients with AF was recruited from a university-affiliated hospital to evaluate its psychometric properties. The internal consistency and test-retest reliability were evaluated using the Kuder-Richardson formula 20 and κ statistics. Item analysis determined the item difficulty index and item discrimination coefficients. Factorial and discriminant validity were evaluated using exploratory factor analysis and the known-groups method. RESULTS: The content validity index of the AFKS-C was 0.94, and the Kuder-Richardson formula 20 value was 0.60. The difficulty indices of the items ranged from 0.36 to 0.89, and the point-biserial coefficients of the items ranged from 0.122 to 0.255, indicating sufficient discriminatory ability. The test-retest reliability was acceptable, because the κ values ranged from 0.234 to 0.710. The principal axis factoring analysis indicated a 3-factor structure that explained 50.4% of the total variance. The AFKS-C also demonstrated satisfactory discriminant validity, having yielded significantly different scores between patients with newly diagnosed and established AF. CONCLUSION: The AFKS-C has acceptable psychometric properties and can be used to measure the knowledge of patients and evaluate the effects of patient education programs.

An empowerment-based cognitive behavioural therapy for persons with mild cognitive impairment and insomnia: Protocol for a mixed-method pilot study.

AIM: This study aims to examine the feasibility and effects of an empowerment-based cognitive behavioural therapy for insomnia on sleep, cognitive outcomes, and health-related quality of life in persons with mild cognitive impairment and insomnia. STUDY DESIGN: This mixed-methods study comprises a pilot randomized controlled trial and an exploratory qualitative study. METHODS: A total of 60 community-dwelling patients aged ≥50 years with mild cognitive impairment and self-reported sleep complaints will be recruited from the community centres for older people operated by two non-governmental organizations in Hong Kong. The participants will be randomly allocated to intervention or control groups, which will receive the empowerment-based cognitive behavioural therapy for insomnia and usual care respectively. We hypothesize that the cognitive behavioural therapy for insomnia intervention featuring an empowerment-based approach can improve sleep and cognitive function among patients with mild cognitive impairment. A subsample of 10 participants from the intervention group will be invited to take part in a qualitative interview to obtain more in-depth comments about the feasibility and acceptability of the intervention. Ethical approval was obtained on 2 November 2020. This study is supported by the Seed Fund for Basic Research from the University of Hong Kong on 4 September 2020. DISCUSSION: This study will address a neglected risk factor for cognitive decline in persons with mild cognitive impairment. The theoretical integration of empowerment and cognitive model of behavioural changes may inform a wider and more successful application of cognitive behavioural therapy techniques for people with compromised cognitive ability and insomnia. IMPACT: This study will also advance our knowledge on the role of sleep on persons with mild cognitive impairment and generate relevant empirical evidence to inform the care of this vulnerable cohort to affect a worldwide reduction in social, economic and healthcare burdens associated with cognitive impairment. CLINICAL TRIAL REGISTRATION: This study has been registered at ClinicalTrials.gov (NCT04635085).

Broadband single-shot transient absorption spectroscopy.

The duration of transient absorption spectroscopy measurements typically limits the types of systems for which the excited state dynamics can be measured. We present a single-shot transient absorption (SSTA) instrument with a spatially encoded 60 ps time delay range and a 100 nm spectral range that is capable of acquiring a transient spectrum in 20 s. We describe methods to spatially overlap the flat-top pump and probe beams at the sample plane, calibrate the spatially encoded time delay, and correct for non-uniform excitation density. SSTA measurements of organic materials in solution and film demonstrate this technique.

Revealing the evolving mixture of molecular aggregates during organic film formation using simulations of in situ absorbance.

In this work, we introduce a method for modeling the evolving absorbance spectrum of an organic molecule, pseudoisocyanine (PIC), measured during the process of molecular aggregation. Despite being historically considered a J-aggregate, we find that the absorbance spectrum of PIC cannot be adequately modeled using solely J-aggregates either during molecular aggregation or in the final dry film. The collection of absorbance spectra during solution-casting is particularly difficult since a distribution of aggregates with various sizes and structures can coexist. Here, spectra measured during film formation are fit to a weighted sum of simulated spectra of two aggregate species, revealing the combinations of Coulombic coupling values, Huang-Rhys parameters, and aggregate sizes that provide good fits to measured spectra. The peak intensity ratios and relative peak positions are highly sensitive to the aggregate structure, and fitting only these features enables the rapid comparison of aggregate combinations. We find that the spectra of PIC aggregates cannot be modeled using the Huang-Rhys factor of the PIC monomer, as is typically assumed, leading us to consider models that utilize independent Huang-Rhys factors for each aggregate species. This method of fitting only the key spectral features allows an experimental spectrum to be modeled within 1 h-2 h when using a single Huang-Rhys factor, making the simulation of a series of in situ measurements during aggregation computationally feasible.

Single-shot transient absorption spectroscopy with a 45 ps pump-probe time delay range.

We report a single-shot transient absorption apparatus that successfully uses a tilted pump pulse to spatially encode a 45 ps pump-probe time delay. The time delay range is significantly improved over other reported instruments by using a spatial light modulator to flatten the intensity of the excitation field at the sample position. The full time delay range of the instrument is demonstrated by measuring a long-lived dye. A signal-to-noise ratio of >35 is attained in 8 s. This advance will enable the measurement of excited state dynamics of systems that are not at structural equilibrium.

In Situ Measurement of Exciton Dynamics During Thin-Film Formation Using Single-Shot Transient Absorption.

The exciton dynamics of pseudoisocyanine (PIC) is reported during the formation of a thin film dropcast from solution. Tilted pump pulses are used to spatially encode a pump-probe time delay, enabling the collection of a transient in a single shot. We demonstrate that a spatially encoded delay can be used to accurately measure exciton dynamics in thin-film samples, with a signal-to-noise ratio above 20 attained in 2 s. We report in situ linear absorption, fluorescence, and transient absorption measurements during the molecular aggregation of PIC. These measurements reveal a highly fluorescent intermediate stage during thin-film formation that we ascribe to J-aggregates, in contrast to the final, less fluorescent, dry thin film that exhibits photophysics indicative of disordered J-aggregates. The ability to measure exciton dynamics in situ during materials formation will provide a deeper understanding of how functional materials properties evolve, and will enable direct feedback for rational materials design.

Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature.

Photosynthesis makes use of sunlight to convert carbon dioxide into useful biomass and is vital for life on Earth. Crucial components for the photosynthetic process are antenna proteins, which absorb light and transmit the resultant excitation energy between molecules to a reaction centre. The efficiency of these electronic energy transfers has inspired much work on antenna proteins isolated from photosynthetic organisms to uncover the basic mechanisms at play. Intriguingly, recent work has documented that light-absorbing molecules in some photosynthetic proteins capture and transfer energy according to quantum-mechanical probability laws instead of classical laws at temperatures up to 180 K. This contrasts with the long-held view that long-range quantum coherence between molecules cannot be sustained in complex biological systems, even at low temperatures. Here we present two-dimensional photon echo spectroscopy measurements on two evolutionarily related light-harvesting proteins isolated from marine cryptophyte algae, which reveal exceptionally long-lasting excitation oscillations with distinct correlations and anti-correlations even at ambient temperature. These observations provide compelling evidence for quantum-coherent sharing of electronic excitation across the 5-nm-wide proteins under biologically relevant conditions, suggesting that distant molecules within the photosynthetic proteins are 'wired' together by quantum coherence for more efficient light-harvesting in cryptophyte marine algae.

The psychophysiological effects of Tai-chi and exercise in residential schizophrenic patients: a 3-arm randomized controlled trial.

BACKGROUND: Patients with schizophrenia are characterized by high prevalence rates and chronicity that often leads to long-term institutionalization. Under the traditional medical model, treatment usually emphasizes the management of psychotic symptoms through medication, even though anti-psychotic drugs are associated with severe side effects, which can diminish patients' physical and psychological well-being. Tai-chi, a mind-body exercise rooted in Eastern health philosophy, emphasizes the motor coordination and relaxation. With these potential benefits, a randomized controlled trial (RCT) is planned to investigate the effects of Tai-chi intervention on the cognitive and motor deficits characteristic of patients with schizophrenia. METHODS/DESIGN: A 3-arm RCT with waitlist control design will be used in this study. One hundred and fifty three participants will be randomized into (i) Tai-chi, (ii) exercise or (iii) waitlist control groups. Participants in both the Tai-chi and exercise groups will receive 12-weeks of specific intervention, in addition to the standard medication and care received by the waitlist control group. The exercise group will serve as a comparison, to delineate any unique benefits of Tai-chi that are independent of moderate aerobic exercise. All three groups will undergo three assessment phases: (i) at baseline, (ii) at 12 weeks (post-intervention), and (iii) at 24 weeks (maintenance). All participants will be assessed in terms of symptom management, motor coordination, memory, daily living function, and stress levels based on self-perceived responses and a physiological marker. DISCUSSION: Based on a promising pilot study conducted prior to this RCT, subjects in the Tai-chi intervention group are expected to be protected against deterioration of motor coordination and interpersonal functioning. They are also expected to have better symptoms management and lower stress level than the other treatment groups. TRIAL REGISTRATION: The trail has been registered in the Clinical Trials Center of the University of Hong Kong (HKCTR-1453).

Estimated prevalence of frailty and prefrailty in patients undergoing coronary artery or valvular surgeries/procedures: A systematic review and proportional meta-analysis.

BACKGROUND: The aging population has led to an increasing number of older patients undergoing cardiac surgeries/procedures. Frailty and prefrailty have emerged as important prognostic indicators among these patients. This proportional meta-analysis estimated the prevalence of frailty and prefrailty among patients undergoing cardiac surgery. METHODS: We searched seven electronic databases for observational studies that used validated measure(s) of frailty and reported prevalence data on frailty and/or prefrailty in older patients undergoing coronary artery or valvular surgeries or transcatheter procedures. Meta-analyses were performed using a random-effects model. RESULTS: One hundred and one articles involving 626,863 patients were included. The pooled prevalence rates of frailty and prefrailty were 28% (95% confidence interval [CI]: 23%-33%) and 40% (95% CI: 31%-50%), respectively, for patients scheduled for open-heart surgeries and 40% (95% CI: 36%-45%) and 43% (95% CI: 34%-53%), respectively, for patients undergoing transcatheter procedures. Frailty measured using a multidimensional approach identified a higher proportion of frail patients when compared with measures solely focused on physical frailty. Older age, female sex, and lower body mass index and hemoglobin concentrations were significantly associated with higher frailty prevalence. Moreover, countries with higher gross domestic product spent on healthcare exhibited a higher frailty prevalence. CONCLUSION: Frailty represents a considerable health challenge among patients undergoing cardiac surgeries/procedures. Routine screening for frailty should be considered during perioperative care planning.

Formation of Iodide-Rich Domains During Halide Segregation in Lead-Halide Perovskite Nanocrystals.

Mixed iodide-bromide methylammonium lead perovskite (MAPbIxBr3-x) nanocrystals (NCs) hold promise for use in light-emitting applications owing to the size- and composition-tunability of their bandgap. However, the segregation of halides during light exposure causes their band gaps to become unstable and narrow. Here, we use transient absorption spectroscopy to track excited-state dynamics during photoinduced halide segregation. The Auger recombination dynamics are observed to accelerate as the bandgap narrows, suggesting enhanced electron-hole overlap. We simulate the motion of iodide within the NC and estimate the evolving bandgap and electron-hole overlap during two possible mechanisms of halide segregation. Our results support a segregation mechanism in which iodide anions form a domain within the NC, rather than a mechanism in which iodide anions independently segregate toward the NC surface. Such mechanistic insight will contribute to future NC bandgap stabilization strategies.

The prognostic impacts of frailty on clinical and patient-reported outcomes in patients undergoing coronary artery or valvular surgeries/procedures: A systematic review and meta-analysis.

BACKGROUND: Frailty is emerging as an important prognostic indicator for patients undergoing cardiac surgeries/procedures. We sought to evaluate the prognostic and differential impacts of frailty on patients undergoing coronary artery or valvular surgical procedures of different levels of invasiveness, and to explore the differential predictability of various frailty measurement models. METHODS: Eight databases were searched for prospective cohort studies that have adopted validated measure(s) of frailty and reported clinical, healthcare service utilization, or patient-reported outcomes in patients undergoing coronary artery or valvular surgeries/procedures. RESULTS: Sixty-two articles were included (N = 16,679). Frailty significantly predicted mortality (short-term [≤ 30 days]: odds ratio [OR]: 2.33, 95% confidence interval [CI]: 1.28-4.26; midterm [6 months to 1 year]: OR: 3.93, 95%CI: 2.65-5.83; long-term [>1 year]: HR: 2.23, 95%CI: 1.60-3.11), postoperative complications (ORs: 2.54-3.57), discharge to care facilities (OR: 5.52, 95%CI: 3.84-7.94), hospital readmission (OR: 2.00, 95%CI: 1.15-3.50), and reduced health-related quality of life (HRQoL; standardized mean difference: -0.74, 95%CI: -1.30 to -0.18). Subgroup analyses showed that frailty exerted a greater impact on short-term mortality in patients undergoing open-heart surgeries than those receiving transcatheter procedures. Multidimensional and physical-aspect-focused frailty measurements performed equally in predicting mortality, but multidimensional measurements were more predictive of hospital readmission than physical-aspect-focused measurements. CONCLUSION: Frailty was predictive of postoperative mortality, complications, increased healthcare service utilization, and reduced HRQoL. The impact of frailty on short-term mortality was more prominent in patients undergoing open-heart surgeries than those receiving transcatheter procedures. Multidimensional measures of frailty enhanced prognostic risk estimation, especially for hospital readmission.

Three-pulse photon-echo peak shift spectroscopy and its application for the study of solvation and nanoscale excitons.

An understanding of chemical reactivity begins with an understanding of the dynamics involved regarding system-bath interactions. Spectroscopic studies of these interactions in condensed-phase multichromophoric systems are intricate and contain much information. Photon-echo spectroscopy has proven to be a useful tool for probing these interactions. A description of three-pulse photon-echo peak shift spectroscopy (3PEPS)--theory, experiment, and application--for the study of solvation and dynamics of nanoscale excitons is presented. Also, we discuss how two-dimensional photon-echo spectroscopy (2DPE) relates to 3PEPS and show how 3PEPS data can be extracted from 2D photon-echo data.

Biexcitonic fine structure of CdSe nanocrystals probed by polarization-dependent two-dimensional photon echo spectroscopy.

The spectroscopy of colloidal CdSe nanocrystals is investigated using two-dimensional photon echo (2DPE) spectroscopy with copolarized and cross-polarized pulse sequences. Clearly resolved excited state absorption features are observed to beat at the frequency of the longitudinal-optical phonon, and the phase of this beating is found to be polarization-dependent. A simulation is performed using the excitonic and biexcitionic fine structure states predicted by theory, and the polarization-dependent beating allows each feature to be assigned to a particular excited state absorption pathway. Owing to their circularly polarized selection rules, the polarization-dependent 2DPE technique provides valuable insights into the spectroscopy of quantum dots. In particular, transient absorption features observed in pump-probe studies of CdSe quantum dots can now be assigned to specific fine structure transitions to the ground state biexciton.

Single-shot transient absorption spectroscopy techniques and design principles.

Single-shot transient absorption (SSTA) spectroscopy is fundamentally identical to transient absorption (TA) spectroscopy but differs in its implementation to enable the measurement of sample response at a range of pump-probe time delays in a single laser shot. As in TA, a pump pulse in SSTA photoexcites a sample, inducing a change in the absorption of a probe pulse. Both commercial and home-built TA instruments typically execute serial measurements at a range of pump-probe time delays to yield transients that report on the dynamics of the photoexcited species, with the sample returning to the same relaxed state between each measurement. SSTA instruments acquire a range of pump-probe time delays simultaneously by somehow encoding the time delay into the profile of the probe beam. This dramatically reduces the time required for SSTA measurements, enabling the measurement of unstable systems undergoing irreversible processes that cannot be accurately characterized using typical TA instruments. The implementation of the encoded time delay must be appropriately designed and carefully calibrated to suit the targeted system and ensure accurate measurements. This review describes techniques used to encode the time delay and design principles for SSTA instruments. Strategies are presented to implement a broadband probe, account for spatial variations in pump and probe beam profiles that influence the intensity and noise of the spatially encoded signal, optimize detection, and correct for dynamic background signals. With these design principles in place, SSTA is capable of measuring an array of unstable and evolving systems that cannot be addressed using typical TA instruments.

Exciton fine structure and spin relaxation in semiconductor colloidal quantum dots.

Quantum dots (QDs) have discrete quantum states isolated from the environment, making QDs well suited for quantum information processing. In semiconductor QDs, the electron spins can be coherently oriented by photoexcitation using circularly polarized light, creating optical orientation. The optically induced spin orientation could serve as a unit for data storage and processing. Carrier spin orientation is also envisioned to be a key component in a related, though parallel, field of semiconductor spintronics. However, the oriented spin population rapidly loses its coherence by interaction with the environment, thereby erasing the prepared information. Since long-lasting spin orientation is desirable in both areas of investigation, spin relaxation is the central focus of investigation for optimization of device performance. In this Account, we discuss a topic peripherally related to these emerging areas of investigation: exciton fine structure relaxation (EFSR). The radiationless transition occurring in the exciton fine structure not only highlights a novel aspect of QD exciton relaxation but also has implications for carrier spin relaxation in QDs. We focus on examining the EFSR in connection with optical spin orientation and subsequent ultrafast relaxation of electron and hole spin densities in the framework of the exciton fine structure basis. Despite its significance, the study of exciton fine structure in colloidal QDs has been hampered by the experimental challenge arising from inhomogeneous line broadening that obscures the details of closely spaced fine structure states in the frequency domain. In this Account, we show that spin relaxation occurring in the fine structure of CdSe QDs can be probed by a time-domain nonlinear polarization spectroscopy, circumventing the obstacles confronted in the frequency-domain spectroscopy. In particular, by combining polarization sequences of multiple optical pulses with the unique optical selection rules of semiconductors, fast energy relaxation among the QD exciton fine structure states is selectively measured. The measured exciton fine structure relaxation, which is a nanoscale analogue of molecular radiationless transitions, contains direct information on the relaxation of spin densities of electron and hole carriers, that is, spin relaxation in QDs. From the exciton fine structure relaxation rates measured for CdSe nanorods and complex-shaped nanocrystals using nonlinear polarization spectroscopy, we elucidated the implications of QD size and shape on the QD exciton properties as well, for example, size- and shape-scaling laws governing exciton spin flips and how an exciton is delocalized in a QD. We envision that the experimental development and the discoveries of QD exciton properties presented in this Account will inspire further studies toward revealing the characteristics of QD excitons and spin relaxation therein, for example, spin relaxation in QDs made of various materials with different electronic structures, spin relaxation under an external perturbation of QD electronic states using magnetic fields, and spin relaxation of separated electrons and holes in type-II QD heterostructures.

Evolving Stark Effect During Growth of Perovskite Nanocrystals Measured Using Transient Absorption.

Methylammonium lead triiodide (MAPbI3) nanocrystals (NCs) are emerging materials for a range of optoelectronic applications. Photophysical characterization is typically limited to structurally stable NCs owing to the long timescales required for many spectroscopies, preventing the accurate measurement of NCs during growth. This is a particular challenge for non-linear spectroscopies such as transient absorption. Here we report on the use of a novel single-shot transient absorption (SSTA) spectrometer to study MAPbI3 NCs as they grow. Comparing the transient spectra to derivatives of the linear absorbance reveals that photogenerated charge carriers become localized at surface trap states during NC growth, inducing a TA lineshape characteristic of the Stark effect. Observation of this Stark signal shows that the contribution of trapped carriers to the TA signal declines as growth continues, supporting a growth mechanism with increased surface ligation toward the end of NC growth. This work opens the door to the application of time-resolved spectroscopies to NCs in situ, during their synthesis, to provide greater insight into their growth mechanisms and the evolution of their photophysical properties.

Ideal dipole approximation fails to predict electronic coupling and energy transfer between semiconducting single-wall carbon nanotubes.

The electronic coupling values and approximate energy transfer rates between semiconductor single-wall carbon nanotubes are calculated using two different approximations, the point dipole approximation and the distributed transition monopole approximation, and the results are compared. It is shown that the point dipole approximation fails dramatically at tube separations typically found in nanotube bundles ( approximately 12-16 A) and that the disagreement persists at large tube separations (>100 A, over ten nanotube diameters). When used in Forster resonance energy transfer theory, the coupling between two point transition dipoles is found to overestimate energy transfer rates. It is concluded that the point dipole approximation is inappropriate for use with elongated systems such as carbon nanotubes and that methods which can account for the shape of the particle are more suitable.