Low-Temperature Emission Dynamics of Methylammonium Lead Bromide Hybrid Perovskite Thin Films at the Sub-Micrometer Scale

We study the low-temperature (T = 4.7 K) emission dynamics of a thin film of methylammonium lead bromide (MAPbBr3), prepared via the anti-solvent method. Using intensity-dependent (over 5 decades) hyperspectral microscopy under quasi-resonant (532 nm) continuous wave excitation, we revealed spatial inhomogeneities in the thin film emission. This was drastically different at the band-edge (∼550 nm, sharp peaks) than in the emission tail (∼568 nm, continuum of emission). We are able to observe regions of the film at the micrometer scale where emission is dominated by excitons, in between regions of trap emission. Varying the density of absorbed photons by the MAPbBr3 thin films, two-color fluorescence lifetime imaging microscopy unraveled the emission dynamics: a fast, resolution-limited (∼200 ps) monoexponential tangled with a stretched exponential decay. We associate the first to the relaxation of excitons and the latter to trap emission dynamics. The obtained stretching exponents can be interpreted as the result of a two-dimensional electron diffusion process: Förster resonant transfer mechanism. Furthermore, the non-vanishing fast monoexponential component even in the tail of the MAPbBr3 emission indicates the subsistence of localized excitons. Finally, we estimate the density of traps in MAPbBr3 thin films prepared using the anti-solvent method at n∼1017 cm-3.

Doping MAPbBr3 hybrid perovskites with CdSe/CdZnS quantum dots: from emissive thin films to hybrid single-photon sources

We report the first doping of crystalline methyl-ammonium lead bromide perovskite (MAPbBr₃) films with CdSe/CdZnS core/shell quantum dots (QDs), using a soft-chemistry approach that preserves their high quantum yield and other remarkable luminescence properties. Our approach produces MAPbBr₃ films of around 100 nm thickness, doped at volume ratios between 0.01 and 1% with colloidal CdSe/CdZnS QDs whose organic ligands were exchanged with halide ions to allow for close contact between the QDs and the perovskite matrix. Ensemble photoluminescence (PL) measurements demonstrate the retained emission of the QDs after incorporation into the MAPbBr₃ matrix. Photoluminescence excitation (PLE) spectra exhibit signatures of wavelength-dependent coupling between the CdSe/CdZnS QDs and the MAPbBr₃ matrix, i.e., a transfer of charges from matrix to QD, which increases the QD luminescence by up to 150%, or from QD to matrix. Spatially-resolved PL experiments reveal a strong correlation between the positions of QDs and an enhancement of the PL signal of the matrix. Lifetime imaging of the doped films furthermore shows that the emission lifetime of MAPbBr₃ is slower in the vicinity of QDs, which, in combination with the increased PL signal of the matrix, suggests that QDs can act as local nucleation seeds that improve the crystallinity of MAPbBr₃, thus boosting its emission quantum yield. Luminescence antibunching measurements provide clear evidence of single-photon emission from individual QDs in perovskite. Finally, the analysis of blinking statistics indicates an improvement of the photostability of individual QDs in perovskite as compared to bare CdSe/CdZnS QDs. At high CdSe/CdZnS QD doping levels, this work thus opens a route to hybrid solar concentrators for visible-light harvesting and hybrid-based LEDs, while a low degree of doping could yield hybrid single-photon sources than can be embedded in field-effect devices for single-charge control, which would allow the construction of nanophotonic devices via low-cost solution-processing techniques as an alternative to solid-state quantum dots.

Optical properties of individual CdS/CdSe/CdS nanocrystals: spherical quantum wells as single-photon sources

We have synthesized CdS(1.3 nm)/CdSe(1.7 nm)/CdS(3.4 nm) spherical quantum wells (SQWs) with a diameter of 13 nm and demonstrated the first photon-antibunching from their emission, labelling them as single-photon sources. Antibunching survives even at high excitation intensities, ruling-out strong emission from the bi-exciton. For the largest intensities, antibunching coupled to spectral measurements reveal the signature of a blue-shifted emission, associated to an irreversible photo-aging effect. A statistical analysis over 26 SQWs demonstrates a moderate correlation between the energy of the main and the blue-shifted emission. Intensity-timetraces recorded on 28 single SQWs show weak blinking, with a median time spent in the bright state of 89%. Their emission decay reveals a complex dynamic with either three or four exponential components. We assigned three of them to the neutral and singly-charged excitons and the slowest to defect emission. While SQWs have been initially designed for laser-oriented applications, we demonstrate that they can serve as efficient single-photon sources.

Perspectives for CdSe/CdS spherical quantum wells as rapid-response nano-scintillators

We explore the effect of the shell thickness on the time response of CdS/CdSe/CdS spherical quantum wells (SQWs) nanoscintillators under X-ray excitation. We first compare the spectral and timing properties under low and intense optical excitation, which allows us to identify the complex temporal and spectral response of the highly excited species. We find that a defect-induced delayed luminescence appears at large sizes. Under pulsed X-ray excitation, an analysis of the scintillation decay time reveals that multiexcitons are generated, similarly to the intense optical excitation and that the shell thickness does not change the fraction of fast component to a large extent. We performed a two-step simulation of the energy relaxation in the SQWs which reveals that large-size SQWs favor a very high number of excitations per particle, which, however, is counterbalanced by increased Auger quenching, rendering large SQWs less effective regarding the timing performance.

Optical properties of fully inorganic core/gradient-shell CdSe/CdZnS nanocrystals at the ensemble and single-nanocrystal levels

We report the synthesis and optical characterization of fully inorganic gradient-shell CdSe/CdZnS nanocrystals (NCs) with high luminescence quantum yield (QY, 50%), which were obtained by replacing native oleic-acid (OA) ligands with halide ions (Cl^-and Br^-). Absorption, photoluminescence excitation (PLE) and photoluminescence (PL) spectra in solution were unaffected by the ligand-exchange procedure. The halide-capped NCs were stable in solution for several weeks without modification of their PL spectra; once deposited as unprotected thin films and exposed to air, however, they did show signs of aging which we attribute to increasing heterogeneity of (effective) NC size. Time-resolved PL measurements point to the existence of four distinct emissive states, which we attribute to neutral, singly-charged and multi-excitonic entities. We found that the relative contribution of these four components to the overall PL decay is modified by the OA-to-halide ligand exchange, while the excited-state lifetimes themselves, surprisingly, remain largely unaffected. The high PL quantum yield of the halide-capped NCs allowed observation of single particle blinking and photon-antibunching; one surprising result was that aging processes that occurs during the first few days after deposition on glass seemed to offer a certain increased protection against photobleaching. These results suggest that halide-capped CdSe/CdZnS NCs are promising candidates for incorporation into opto-electronic devices, based on, for example, hybrid perovskite matrices, which require eliminating the steric hindrance and electronic barrier of bulky organic ligands to ensure efficient coupling.

Decoupling a hole spin qubit from the nuclear spins

A huge effort is underway to develop semiconductor nanostructures as low-noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath. A hole spin may circumvent the nuclear spin noise. In principle, the nuclear spins can be switched off for a pure heavy-hole spin. In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark-state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is <1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to nanoelectronvolt energies; equivalently dephasing times up to a microsecond. The combination of large and strong optical dipole makes the single hole spin in a GaAs-based device an attractive quantum platform.

Nondestructive Encapsulation of CdSe/CdS Quantum Dots in an Inorganic Matrix by Pulsed Laser Deposition

We report the successful encapsulation of colloidal quantum dots in an inorganic matrix by pulsed laser deposition. Our technique is nondestructive and thus permits the incorporation of CdSe/CdS core/shell colloidal quantum dots in an amorphous yttrium oxide matrix (Y2O3) under full preservation of the advantageous optical properties of the nanocrystals. We find that controlling the kinetic energy of the matrix precursors by means of the oxygen pressure in the deposition chamber facilitates the survival of the encapsulated species, whose well-conserved optical properties such as emission intensity, luminescence spectrum, fluorescence lifetime, and efficiency as single-photon emitters we document in detail. Our method can be extended to different types of nanoemitters (e.g., nanorods, dots-in-rods, nanoplatelets) as well as to other matrices (oxides, semiconductors, metals), opening up new vistas for the realization of fully inorganic multilayered active devices based on colloidal nano-objects.

CdSe/ZnS quantum dots as sensors for the local refractive index

We explore the potential of CdSe/ZnS colloidal quantum dots (QDs) as probes for their immediate dielectric environment, based on the influence of the local refractive index on the fluorescence dynamics of these nanoemitters. We first compare ensembles of quantum dots in homogeneous solutions with single quantum dots dispersed on various dielectric substrates, which allows us to test the viability of a conceptual framework based on a hard-sphere region-of-influence and the Bruggeman effective-medium approach. We find that all our measurements can be integrated into a coherent description, provided that the conceptualized point-dipole emitter is positioned at a distance from the substrate that corresponds to the geometry of the QD. Three theoretical models for the evolution of the fluorescence decay rate as a function of the local refractive index are compared, showing that the classical Lorentz approach (virtual cavity) is the most appropriate for describing the data. Finally, we use the observed sensitivity of the QDs to their environment to estimate the detection limit, expressed as the minimum number of traceable streptavidin molecules, of a potential QD-nanosensor based on fluorescence lifetime.

Autocorrelation analysis for the unbiased determination of power-law exponents in single-quantum-dot blinking

We present an unbiased and robust analysis method for power-law blinking statistics in the photoluminescence of single nanoemitters, allowing us to extract both the bright- and dark-state power-law exponents from the emitters' intensity autocorrelation functions. As opposed to the widely used threshold method, our technique therefore does not require discriminating the emission levels of bright and dark states in the experimental intensity timetraces. We rely on the simultaneous recording of 450 emission timetraces of single CdSe/CdS core/shell quantum dots at a frame rate of 250 Hz with single photon sensitivity. Under these conditions, our approach can determine ON and OFF power-law exponents with a precision of 3% from a comparison to numerical simulations, even for shot-noise-dominated emission signals with an average intensity below 1 photon per frame and per quantum dot. These capabilities pave the way for the unbiased, threshold-free determination of blinking power-law exponents at the microsecond time scale.

High resolution coherent population trapping on a single hole spin in a semiconductor quantum dot

We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physicslike dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing. We identify the cause of this process as charge noise by, first, demonstrating that the hole spin g factor in this configuration (in-plane magnetic field) is strongly dependent on the vertical electric field, and second, by characterizing the charge noise through its effects on the optical transition frequency. An important conclusion is that charge noise is an important hole spin dephasing process.

A dark-field microscope for background-free detection of resonance fluorescence from single semiconductor quantum dots operating in a set-and-forget mode

Optically active quantum dots, for instance self-assembled InGaAs quantum dots, are potentially excellent single photon sources. The fidelity of the single photons is much improved using resonant rather than non-resonant excitation. With resonant excitation, the challenge is to distinguish between resonance fluorescence and scattered laser light. We have met this challenge by creating a polarization-based dark-field microscope to measure the resonance fluorescence from a single quantum dot at low temperature. We achieve a suppression of the scattered laser exceeding a factor of 10(7) and background-free detection of resonance fluorescence. The same optical setup operates over the entire quantum dot emission range (920-980 nm) and also in high magnetic fields. The major development is the outstanding long-term stability: once the dark-field point has been established, the microscope operates for days without alignment. The mechanical and optical designs of the microscope are presented, as well as exemplary resonance fluorescence spectroscopy results on individual quantum dots to underline the microscope's excellent performance.

Self-assembled quantum dots in a nanowire system for quantum photonics

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Probing single-charge fluctuations at a GaAs/AlAs interface using laser spectroscopy on a nearby InGaAs quantum dot

We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5  nm resolution. The results identify and quantify the main source of charge noise in the commonly used optical field-effect devices.

Midinfrared absorption measured at a lambda/400 resolution with an atomic force microscope

Midinfrared absorption can be locally measured using a detection combining an atomic force microscope and a pulsed excitation. This is illustrated for the midinfrared bulk GaAs phonon absorption and for the midinfrared absorption of thin SiO(2) microdisks. We show that the signal given by the cantilever oscillation amplitude of the atomic force microscope follows the spectral dependence of the bulk material absorption. The absorption spatial resolution achieved with microdisks is around 50 nanometer for an optical excitation around 22 micrometer wavelength.

Ultraweak-absorption microscopy of a single semiconductor quantum dot in the midinfrared range

We show that we can measure the room temperature ultraweak absorption of a single buried semiconductor quantum dot. This is achieved by monitoring the deformation field induced by the absorption of midinfrared laser pulses and locally detected with an atomic force microscope tip. The absorption is spectrally and spatially resolved around lambda approximately 10 microm wavelength with 60 nm lateral resolution (lambda/150). The electronic S-D intersublevel absorption of a single quantum dot is identified around 120 meV and exhibits a homogeneous linewidth of approximately 10 meV at room temperature.

[Results of cardiac surgery in patients over 70 years of age. Apropos of a retrospective study of 170 patients]

The ever increasing age of the general population and the frequency of coronary and aortic lesions in patients aged 70 or older make cardiac surgery in the elderly a topic of current interest. In a retrospective study of 170 cases the overall mortality rate was 14 p. 100 (4.7 p. 100 in 1986). During the past two years (1985 and 1986), the mortality rates in patients with coronary disease who underwent elective surgery were 10 p. 100 and 4 p. 100 respectively. Emergency surgery (left coronary trunk and unstable angina excluded) and associated operations on the heart made the prognosis worse with overall mortality rates of 13 p. 100 and 15 p. 100 respectively (9 p. 100 in 1985, 10 p. 100 in 1986). In aortic valve surgery, hospital mortality rates were 28 p. 100 in 1985 and 6 p. 100 in 1986. Post-operative morbidity was high (52 p. 100 of patients) and consisted mostly of respiratory and neurological complications (14.7 p. 100 and 9.4 p. 100 respectively of all patients). Only 4 p. 100 of the patients developed peri-operative myocardial infarction. Functional results were satisfactory in both coronary disease and heart valve disease patients. The survival rate at 4 years was 74.5 +/- 10 p. 100 globally and 87.3 +/- 9 p. 100 in coronary patients operated upon electively. Thus, the considerable advances in surgical techniques and post-operative intensive care achieved during the last few years have significantly improved the results obtained in this population.(ABSTRACT TRUNCATED AT 250 WORDS)

[Disobstruction of the inferior vena cava under extracorporeal circulation]

An original technique used to remove obstructions of the inferior vena cava under cardiopulmonary bypass is described. This technique is indicated in pulmonary embolectomy or to dispose of a vena cava clot prolapsed into the right cardiac cavities. Through a rigid and blunt thoracic drain introduced via the right atrium, a Fogarthy venous catheter is pushed into the inferior vena cava, thus avoiding the risks of accidental passage into the supra-hepatic or renal veins and blockage in a thebesian valve. The catheter is inflated, then removed together with the drain, thereby freeing the caval lumen. The technique is reliable and reproducible, and it has proved very effective.

[Peculiar indications and results of the use of a blood cell saver apparatus in cardiac surgery]

Results are reported of the use of autotransfusion by means of the blood cell saver in a group of patients at high risk for hemorrhage in Heart Surgery Department. Methods employed are described, and results are discussed: these were encouraging both from the clinical and biological viewpoints. The cost benefits of this procedure are emphasized and technical perspectives outlined with respect to recovery of plasma and platelets.

[Symptomatology of hydatid cyst of the heart. Study of a continuous series of 13 cases and value of x-ray computed tomography]

Thirteen consecutive cases of cardiac hydatic cyst were studied. The cysts were usually multiple, predominantly left ventricular. There were two cases of rupture of left ventricular cysts with systemic embolism, and three ruptures of right ventricular cysts causing pulmonary embolism. The average age of the patients was 31 years: 12 patients lived in regions where hydatid disease was endemic. Other localisations of hydatic disease were found in 7 cases. Two patients were completely asymptomatic. Three patients presented with a tumoral syndrome. The presentation was atypical simulating coronary artery disease in 2 cases, valvular heart disease in 2 cases, pericarditis in 2 cases and bronchopneumonia in 3 cases. The electrocardiogramme showed sinus rhythm in all cases and changes of subepicardial ischaemia in 8 cases and acute cor pulmonale in 3 cases. Chest X-ray showed a localised deformation of the cardiac silhouette in 5 cases and calcification in 4 cases. Direct angiocardiographic signs were present in 2 out of 8 cases; usually there was an indirect image of endocavitary filling defect. The tumour was demonstrated by 2D echocardiography in the three most recent cases. The value of CAT was assessed in the last 2 cases: it was absolutely diagnostic in 1 case of septal localisation. The results were poor in the second case due to the presence of mitral calcification. Twelve patients underwent surgery; there were 3 deaths due to rupture and 1 death before surgery. The clinical presentation of cardiac hydatid disease is very variable and the diagnosis is difficult.(ABSTRACT TRUNCATED AT 250 WORDS)

[Value of the aminoglycoside-fosfomycin combination. Apropos of a case of bacterial endocarditis]

The authors report a case of Staphylococcus epidermidis prosthetic valve endocarditis. After two unsuccessful treatments, the association Gentamicin-Fosfomycin provided adequate serum bactericidal titers and had a good efficacy.

[Suspension of the heart during aortocoronary bypass. A simplified technic]

A technique for suspending the heart, thereby facilitating exposure of the coronary arteries, is described. In this perfectly simple and safe technique, the heart is suspended by a gauze compress passed behind the inferior vena cava and forming a sling.

[Resistant ventricular tachycardia caused by right ventricular dysplasia. A case of surgical recovery reported 6 years after intervention]

A 52 year old patient with no previous medical history had an attack of ventricular tachycardia, the configuration of which showed left sided delay. Electrical reduction was followed by multiple recurrences. There were ST-T wave changes over the right precordium on the basal ECG. The hydro-electrolytic equilibrium was normal. Coronary angiography and selective left ventricular cineangiography were also normal. Right angiocardiography showed an aneurysmal deformation of the pulmonary infundibulum. The recurrence of attacks over a two year period led to several hospital admissions, demonstrating the failure of antiarrhytmic therapy. The attacks became so frequent in the last three months that surgery was undertaken. Epicardial mapping showed delayed potentials over the pulmonary infundibulum and surgery consisted in resection of the abnormal infundibular zone. The macro- and microscopic pathological findings were of wall thinning and muscular degeneration with fibrosis and fatty infiltration. The authors discuss the relationship between right ventricular dysplasia and a localised form of Uhl's anomaly. Six years after surgery the patient has had no recurrence of the arrhythmia or shown signs of cardiac failure in the absence of any drug therapy.

[Application of echocardiography to the diagnosis of tricuspid valve diseases. Apropos of 103 cases]

In a series of 103 consecutive surgical patients, the normal and pathological echocardiographic appearances of the tricuspid valve were analysed and compared with the anatomical findings. It was possible to define the nature of the lesions by echocardiography, which was superior to angiography in this field. The right ventricular index (right ventricular dimension body surface area) correlated well with the mean pulmonary artery pressure (r = 0.75; p less than 0.001). The index may be useful in the long term follow-up of post-operative patients.

[A rare cause of continuous thoracic murmur. A fistula between the internal mammary artery and the pulmonary circulation]

The authors present a case of a continuous right lateral sternal murmur which led to the discovery of a systemic-to-pulmonary fistula at the origin of the internal mammary artery. The fact that the patient had a right pulmonary abscess with pyopneumothorax drained several years previously has led them to discuss the possible etiology of this fistula. The murmur disappeared after surgical treatment.

[Inidications for multivalve prostheses]

The authors consider that the indications for surgery should take into account three unknowns: the natural history of the valvulopathies, our incomplete state of knowledge about the outcome in operated cases, and surgical progress, which may transform the whole subject in a short time. They emphasize the seriousness of the disorders in which mitral incompetence is associated with aortic stenosis and/or incompetence; such cases justify earlier surgery. By contrast, some cases of aortic incompetence, when associated with a predominant mitral valve lesion, may not constitute an indication for valve replacement. Tricuspid annuloplasty is a noteworthy advance which provides a simple answer to some of the problems.