Spherical CdSe Nanocrystals Research Articles

Surface- and Tip-Enhanced Raman Scattering by CdSe Nanocrystals on Plasmonic Substrates.

This work presents an overview of the latest results and new data on the optical response from spherical CdSe nanocrystals (NCs) obtained using surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS). SERS is based on the enhancement of the phonon response from nanoobjects such as molecules or inorganic nanostructures placed on metal nanostructured substrates with a localized surface plasmon resonance (LSPR). A drastic SERS enhancement for optical phonons in semiconductor nanostructures can be achieved by a proper choice of the plasmonic substrate, for which the LSPR energy coincides with the laser excitation energy. The resonant enhancement of the optical response makes it possible to detect mono- and submonolayer coatings of CdSe NCs. The combination of Raman scattering with atomic force microscopy (AFM) using a metallized probe represents the basis of TERS from semiconductor nanostructures and makes it possible to investigate their phonon properties with nanoscale spatial resolution. Gap-mode TERS provides further enhancement of Raman scattering by optical phonon modes of CdSe NCs with nanometer spatial resolution due to the highly localized electric field in the gap between the metal AFM tip and a plasmonic substrate and opens new pathways for the optical characterization of single semiconductor nanostructures and for revealing details of their phonon spectrum at the nanometer scale.

Green-to-Blue Triplet Fusion Upconversion Sensitized by Anisotropic CdSe Nanoplatelets

Green-to-blue photon upconversion bears great potential in photocatalytic applications. Current hybrid inorganic–organic upconversion schemes commonly utilize spherical CdSe nanocrystals, but size polydispersity could influence efficiencies in future solid-state applications. In this contribution, we introduce anisotropic CdSe nanoplatelets as triplet sensitizers. Here, quantum confinement occurs in only one direction, erasing effects stemming from size polydispersity. Additionally, their high quantum yields, giant oscillator strengths, and large absorption cross sections could prove useful in a triplet sensitization scheme. We investigate the triplet energy transfer process from the CdSe nanoplatelets to the surface-bound triplet acceptor 9-anthracenecarboxylic acid and the resulting upconversion in 9,10-diphenylanthracene. We further investigate the influence of nanoplatelet stacking and singlet back-transfer on the observed upconversion efficiency. We obtain an upconversion quantum yield of 5.4% at a power density of 11 W/cm2 using the annihilator 9,10-diphenylanthracene and a low efficiency threshold Ith of 237 mW/cm2.

Nonlinear optical response of planar and spherical CdSe nanocrystals

The nonlinear optical response of a colloidal solution of planar CdSe semiconductor nanocrystals (nanoplatelets) is studied for the first time. The nonlinear optical response of these nanoparticles is compared to that of spherical CdSe nanocrystals (quantum dots). The photoinduced nonlinearity is attributed to the optical generation of long-lived charge carriers in the nanoobjects under study. It is shown that, upon the exposure of a cell with the solution of nanoparticles to focused continuous-wave (cw) laser radiation with a wavelength of 473 nm, the nonlinear optical responses of CdSe nanoplatelets and quantum dots are somewhat different at identical optical densities at the above-indicated wavelength. The differences are supposedly associated with a higher diffusion rate of spherical nanoparticles in the solution because of their smaller size compared to that of nanoplatelets.

Correlation between CdSe QD Synthesis, Post-Synthetic Treatment, and BHJ Hybrid Solar Cell Performance.

In this publication we show that the procedure to synthesize nanocrystals and the post-synthetic nanocrystal ligand sphere treatment have a great influence not only on the immediate performance of hybrid bulk heterojunction solar cells, but also on their thermal, long-term, and air stability. We herein demonstrate this for the particular case of spherical CdSe nanocrystals, post-synthetically treated with a hexanoic acid based treatment. We observe an influence from the duration of this post-synthetic treatment on the nanocrystal ligand sphere size, and also on the solar cell performance. By tuning the post-synthetic treatment to a certain degree, optimal device performance can be achieved. Moreover, we show how to effectively adapt the post-synthetic nanocrystal treatment protocol to different nanocrystal synthesis batches, hence increasing the reproducibility of hybrid nanocrystal:polymer bulk-heterojunction solar cells, which usually suffers due to the fluctuations in nanocrystal quality of different synthesis batches and synthesis procedures.

Size-Dependent Exciton–Phonon Coupling in CdSe Nanocrystals through Resonance Raman Excitation Profile Analysis

Resonance Raman excitation profiles, including absolute Raman cross sections, have been measured for the optical phonons of organic ligand capped spherical CdSe nanocrystals with average diameters ranging from 2.6 to 5.2 nm. The absorption spectra, fundamental excitation profiles, overtone to fundamental intensity ratios, and depolarization ratio dispersion profiles are simulated using a model for the exciton–phonon coupling strengths in the various excitonic transitions that contribute to the resonance enhancement. The resonance Raman cross sections increase strongly with nanocrystal size as expected from the increased transition dipole moments (oscillator strengths) in the larger particles. However, the exciton–phonon coupling strengths, particularly for the lowest energy excitonic transition (1Se–1S3/2), are relatively independent of nanocrystal size over this range, perhaps decreasing slightly with increasing size. This result is consistent with calculations of exciton–phonon coupling using an atomistic model for the phonon modes and an effective mass approximation-envelope function model for the excitonic states if the electron and hole effective masses are adjusted to fit the experimental excitonic transition energies.

Nanocrystal Self-Assembly with Rod-Rod Block Copolymers

Two distinct morphologies of hexylselenophene-hexylthiophene rod-rod block copolymer films can be prepared depending on the molecular weight of the sample (see picture: left M(n) =12.9, right M(n) =3.9 kg mol(-1)). These polymers can be used to organize spherical CdSe nanocrystals (yellow) into either dispersed or aligned hierarchical structures. Scale bars: 200 nm.

Experimental investigation of exciton-LO-phonon couplings in CdSe/ZnS core/shell nanorods

We investigate the size dependence of the exciton-LO-phonon coupling strength in colloidal CdSe nanorods coated with an epitaxial ZnS shell. We find an increase in the coupling strength with decreasing nanorod diameter. The growth of a ZnS shell on the nanorod surface much more strongly reduces the exciton-phonon coupling strength than expected from geometry considerations. The determined radius dependence of the Huang-Rhys factor is similar to that observed for spherical CdSe nanocrystals.

Pressure-induced structural transformation of CdSe nanocrystals studied with molecular dynamics

We have studied the pressure-induced structural transformation of CdSe nanocrystals using constant pressure molecular dynamics simulations for finite system. We have observed the transformation from wurtzite to rocksalt structure, the process of transformation is strongly dependent on the shape and size of the nanocrystals, and transformation pressure decreases with increasing nanocrystal size. The spherical CdSe nanocrystals studied undergo nonuniform deformation, while the faceted ones undergo uniform deformation. The reverse transformation from rocksalt backward to wurtzite structure of the nanocrystals happens below $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature, and the width of the transformation decreases as the temperature increases.

Monitoring surface charge migration in the spectral dynamics of singleCdSe∕CdSnanodot/nanorod heterostructures

Spherical CdSe nanocrystals capped by a CdS rod-like shell exhibit interesting spectral dynamics on the single particle level. Spectral boxcar averaging reveals a high degree of correlation between the emission energy, spectral linewidth, phonon coupling strength, and emission intensity of the single nanocrystal. The results can be described in terms of a spatially varying surface charge density in the vicinity of the exciton localized in the CdSe core, leading to a quantum confined Stark effect which modifies the transition energy and the radiative rate. Whereas internal charging of the particle results in a change in the nonradiative rate, surface charges primarily influence the radiative rate. Additionally, we observe characteristic spectral dynamics in frequency space, the magnitude of which depends slightly on temperature and strongly on excitation density. By distinguishing between continuous spectral jitter and discrete spectral jumping associated with a reversible particle ionization event, we can attribute the spectral dynamics to either a slowly varying surface charge density or a rapidly occurring polarization change due to a reversible expulsion of a charge carrier from the semiconductor nanostructure. Whereas the former exhibits universal Gaussian statistics, the latter is best characterized by a Lorentzian noise spectrum. The Gaussian spectral noise increases with spectral redshift of the emission and with increasing proximity of the surface charge to the localized exciton. The observation of a high degree of correlation between peak position and linewidth right up to room temperature suggests applications of the nanocrystals as extremely sensitive single charge detectors in both solid state devices and in biomolecular labeling, where highly local measurements of the dielectric environment are required. Nanoscale control of the physical shape of nanocrystals provides a versatile test bed for studying electronic noise, making the approach relevant to a wide range of conducting and emissive solid state systems.

Electron (hole) paramagnetic resonance of spherical CdSe nanocrystals

A new mechanism of electron paramagnetic resonance in spherical zinc-blende semiconductor nanocrystals, based on the extended orbital motion of electrons in the entire nanocrystal, is presented. Quantum confinement plays a crucial role in making the resonance signal observable. The mechanism remains operative in nanocrystals with uniaxially distorted shape. A theoretical model based on the proposed mechanism is in good quantitative agreement with unusual ODMR spectra observed in nearly spherical CdSe nanocrystals.

Breaking the phonon bottleneck in nanometer quantum dots: Role of Auger-like processes

An Auger-like mechanism is described for the rapid transfer of the electron energy to the hole in the valence band via their Coulomb interaction in the nanometer size quantum dots. The relaxation time and its dependence on the nanocrystal energy band parameters and radius is obtained. A value of ∼ 2 ps has been calculated for the electron thermalization from the first excited to the ground state of spherical CdSe nanocrystals. Our results show that Auger-like processes remove the thermalization “phonon bottleneck” problem in nanometer quantum dots with level spacings greater than typical phonon energies.