RESUMEN
The photoluminescence from a variety of individual molecules and nanometre-sized crystallites is defined by large intensity fluctuations, known as 'blinking', whereby their photoluminescence turns 'on' and 'off' intermittently, even under continuous photoexcitation. For semiconductor nanocrystals, it was originally proposed that these 'off' periods corresponded to a nanocrystal with an extra charge. A charged nanocrystal could have its photoluminescence temporarily quenched owing to the high efficiency of non-radiative (for example, Auger) recombination processes between the extra charge and a subsequently excited electron-hole pair; photoluminescence would resume only after the nanocrystal becomes neutralized again. Despite over a decade of research, completely non-blinking nanocrystals have not been synthesized and an understanding of the blinking phenomenon remains elusive. Here we report ternary core/shell CdZnSe/ZnSe semiconductor nanocrystals that individually exhibit continuous, non-blinking photoluminescence. Unexpectedly, these nanocrystals strongly photoluminesce despite being charged, as indicated by a multi-peaked photoluminescence spectral shape and short lifetime. To model the unusual photoluminescence properties of the CdZnSe/ZnSe nanocrystals, we softened the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe. As photoluminescence blinking severely limits the usefulness of nanocrystals in applications requiring a continuous output of single photons, these non-blinking nanocrystals may enable substantial advances in fields ranging from single-molecule biological labelling to low-threshold lasers.
RESUMEN
We elucidate the atomic and electronic structure of graphene oxide (GO) using annular dark field imaging of single and multilayer sheets and electron energy loss spectroscopy for measuring the fine structure of C and O K-edges in a scanning transmission electron microscope. Partial density of states and electronic plasma excitations are also measured for these GO sheets showing unusual pi* + sigma* excitation at 19 eV. The results of this detailed analysis reveal that the GO is rough with an average surface roughness of 0.6 nm and the structure is predominantly amorphous due to distortions from sp3 C-O bonds. Around 40% sp3 bonding was found to be present in these sheets with measured O/C ratio of 1:5. These sp2 to sp3 bond modifications due to oxidation are also supported by ab initio calculations
Asunto(s)
Grafito/química , Nanotecnología/métodos , Óxidos/química , Carbono/química , Electrones , Microscopía de Fuerza Atómica/métodos , Modelos Moleculares , Conformación Molecular , Nanopartículas/química , Oxígeno/químicaRESUMEN
CdSe quantum rods (QRs) are very promising novel materials with unique electronic and optical properties. In this paper, we utilize a broad spectrum of techniques including high-resolution annular dark field scanning transmission electron microscope imaging, electron nanodiffraction, and computer simulations to study the internal structure of individual QRs. Random small-angle rotations are commonly found between various sections within individual QRs which can be resolved into twists around the c-axis and bends. The possible origins of these small-angle rotations are further studied. We propose that imperfect oriented attachment coupled with electrostatic interactions between smaller nanoparticles during the growth process results in such small-angle rotations. These small-angle rotations may significantly affect the electronic and mechanical properties of CdSe QRs.
Asunto(s)
Compuestos de Cadmio/química , Cristalización/métodos , Modelos Químicos , Modelos Moleculares , Nanoestructuras/química , Nanoestructuras/ultraestructura , Nanotecnología/métodos , Compuestos de Selenio/química , Simulación por Computador , Sustancias Macromoleculares/química , Ensayo de Materiales , Conformación Molecular , Tamaño de la Partícula , Rotación , Propiedades de SuperficieRESUMEN
The luminescence efficiency of individual single-walled carbon nanotubes was determined by comparing the fluorescence from individual nanotubes to single CdTe/ZnS quantum dots with a well-defined fluorescence quantum yield (QY). The single carbon nanotube QY was determined to be 3 +/- 1%, nearly 100 times greater than values previously reported for ensembles. The intrinsic nanotube QY is potentially much higher than previously believed and appears lower in ensembles due to defective nanotubes and residual bundles.
Asunto(s)
Nanotubos de Carbono/química , Microscopía Electrónica de Transmisión de Rastreo , Teoría Cuántica , Espectrometría de Fluorescencia , EspectrofotometríaRESUMEN
Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) was used to determine the chemical distribution of semiconductor shell material around colloidal core-shell CdSe/ZnS quantum dots (QDs). EELS signals from positions around the QD indicate a well-defined shell of ZnS surrounding the CdSe core, but the distribution of the shell material is highly anisotropic. This nonuniformity may reflect the differences in chemical activity of the crystal faces of the core QD and implies a nonoptimal QD surface passivation.
RESUMEN
Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped particles. The surface charge results from a slight angle between the QR sides and the direction of internal electric polarization. By contrast, despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.
RESUMEN
The measured dispersion of a low-loss, hollow-core photonic band-gap fiber is anomalous throughout most of the transmission band, and its variation with wavelength is large compared with that of a conventional step-index fiber. For an air-filled fiber, femtosecond self-frequency--shifted fundamental solitons with peak powers greater than 2megawatts can be supported. For Xe-filled fibers, nonfrequency-shifted temporal solitons with peak powers greater than 5.5 megawatts can be generated, representing an increase in the power that can be propagated in an optical fiber of two orders of magnitude. The results demonstrate a unique capability to deliver high-power pulses in a single spatial mode over distances exceeding 200 meters.
Asunto(s)
Tecnología de Fibra Óptica , Fotones , Aire , Matemática , Fibras Ópticas , Dióxido de Silicio , Espectrometría Raman , XenónRESUMEN
The introduction of an experimental black level may introduce unintended artifactual details into high-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) lattice images. This article presents the multislice simulation results of such possible situations. Three simulated scanning transmission electron microscopy (STEM) probes of sizes 0.8, 1.2 and 2.0 A are scanned on the surface of a <1;10> oriented Si/Ge crystal. The simulation results suggest that high-frequency artifact peaks will appear in the power spectra when an artificial black level clips the lowest (background) signal. The lowest signal in an ADF-STEM image decreases as the incident probe shrinks in size. Therefore, care must be taken when interpreting the resolution limit of the microscope from images taken with nonzero black level setting, especially in case of sub-A microscope. The simulation result is compared with an experimental image and they agree with each other. The analysis suggests that aberration corrected STEM provide sensitive low level detail.