Small CdSe Nanoparticles Research Articles

GW in the Gaussian and Plane Waves Scheme with Application to Linear Acenes.

We present an implementation of G0W0 and eigenvalue-self-consistent GW (evGW) in the Gaussian and plane waves scheme for molecules. We calculate the correlation self-energy for imaginary frequencies employing the resolution of the identity. The correlation self-energy for real frequencies is then evaluated by analytic continuation. This technique allows an efficient parallel implementation and application to systems with several hundreds of atoms. Various benchmark calculations are presented. In particular, the convergence with respect to the most important numerical parameters is assessed for the benzene molecule. Comparisons with respect to other G0W0 implementations are reported for a set of molecules, while the performance of the method has been measured for water clusters containing up to 480 atoms in a cc-TZVP basis. Additionally, G0W0 has been applied for studying the influence of the ligands on the gap of small CdSe nanoparticles. evGW has been employed to calculate the HOMO-LUMO gaps of linear acenes, linear chains formed of connected benzene rings. Distinct differences between the closed and the open-shell (broken-symmetry) evGW HOMO-LUMO gaps for long acenes are found. In future experiments, a comparison of measured HOMO-LUMO gaps and our calculated evGW values may be helpful to determine the electronic ground state of long acenes.

Size-Selective Growth and Stabilization of Small CdSe Nanoparticles in Aqueous Solution

Using cysteine and its derivatives as capping molecules, we investigated the influence of the physical structure and chemical nature of capping molecules on the selective growth and stabilization of small CdSe nanoparticles (NPs) in aqueous solution at room temperature. Our investigations revealed specific roles for each functional group of cysteine, and we could correlate this structure and nature of the capping molecules with the size, size restriction, size distribution, and stability of the NPs. For selective growth and stabilization of the NPs in aqueous solution, their capping molecules should have at least one functional group with strong nucleophilicity as well as another free, charged functional group. Capping molecules acting as a monodentate ligand were more effective than those acting as a bidentate ligand for restricting the NPs to a smaller size, whereas the former was less effective than the latter for getting a narrower NP size distribution. Capping molecules with relatively bulky spatial geometry near the ligand-NP interface resulted in the formation of NPs with poor short- and long-term stabilities, whereas those having relatively compact spatial geometry near the interface led to NPs with at least moderate short-term stability. We saw that capping molecules having relatively compact outermost spatial geometry led to NPs with excellent long-term stability, whereas those having relatively bulky outermost spatial geometry produced NPs with at most only moderate long-term stability. Our results clearly showed general trends for the possibility of selective growth of stable semiconductor NPs with particular sizes in aqueous solution.

Spectral features above LO phonon frequency in resonant Raman scattering spectra of small CdSe nanoparticles

We report unusual spectral features in the resonant Raman scattering spectra of colloidal CdSe nanoparticles as small as 2–3 nm. High-frequency shoulders of the longitudinal optical phonon peak and its overtones were observed and their dependence on the excitation wavelength, temperature, nanoparticle size, and surface passivation with ZnS shell studied. As the probable origin of the uncommon spectral feature the participation of acoustic phonons and manifestation of the density of surface-related vibrational states is discussed.

Size effects on Raman spectra of small CdSe nanoparticles in polymer films

The results of a resonant Raman scattering (RRS) study of polymer-stabilized colloidalCdSe nanoparticles (NPs) are reported. The size-selective nature of the RRS isdemonstrated by analysing the NP ensembles with different average size and size distribution Δd using a set of excitation wavelengths. The effect of size selection on the estimation of and Δd values from the RRS spectra is discussed, as well as some peculiarities of RRS onsurface optical phonons. From the experimentally observed small variation of theI2LO/ILO ratio for 2–5 nm NPs a minor effect of on the electron–phonon coupling strength in this range is supposed.

A one-step aqueous synthetic route to extremely small CdSe nanoparticles

By employing Na 2Se as a selenium source, we demonstrate that extremely small (∼1 nm) mercapto acid-stabilized CdSe nanoparticles can be conveniently prepared in water. The as-prepared nanoparticles start to show dominant near band-gap photoluminescence in the blue spectral range and show high photoluminescence in the green spectral range.

Aqueous-Phase Synthesis of Ultra-Stable Small CdSe Nanoparticles

Very stable and small CdSe nanoparticles (NPs) were synthesized from the aqueous solutions containing L-cysteine (Cys) at room temperature. The Cys-capped CdSe NPs showed a very sharp excitonic peak at 420 nm. Its very small full width at half maximum (18 nm) indicates very high quality of the CdSe NPs. Their absorption features experienced little change over a month, implying an excellent stability of the CdSe NPs. The synthesis conditions were very critical to the optical property and stability of the CdSe NPs: only those prepared at specific conditions (nSe-precursor/nCd-precursor = 0.25–0.5, nCys/nCd-precursor = ∼9, pH = ∼12) showed very sharp absorption peaks and maintained an excellent stability against time. Under these conditions, the peaks always appear at nearly the same wavelength, indicating that these NPs are selectively stable and grow at a particular size and structure.

Aqueous-Phase Synthesis of Ultra-Stable Small CdSe Nanoparticles

Very stable and small CdSe nanoparticles (NPs) were synthesized from the aqueous solutions containing L-cysteine (Cys) at room temperature. The Cys-capped CdSe NPs showed a very sharp excitonic peak at 420 nm. Its very small full width at half maximum (18 nm) indicates very high quality of the CdSe NPs. Their absorption features experienced little change over a month, implying an excellent stability of the CdSe NPs. The synthesis conditions were very critical to the optical property and stability of the CdSe NPs: only those prepared at specific conditions (n(Se-precursor)/n(Cd-precursor) = 0.25-0.5, n(Cys)/n(Cd-precursor) = approximately 9, pH = approximately 12) showed very sharp absorption peaks and maintained an excellent stability against time. Under these conditions, the peaks always appear at nearly the same wavelength, indicating that these NPs are selectively stable and grow at a particular size and structure.

Investigation of the Crystallization Process in 2 nm CdSe Quantum Dots

Investigation of the growth of CdSe nanocrystals ( approximately 160 atoms) to the uniquely stable size of 2 nm allows the monitoring of the crystallization process in semiconductor quantum dots. By using a combination of optical techniques, high-resolution transmission electron microscopy (HRTEM), and powder X-ray diffractometry (XRD), new phenomena were explored during the CdSe nanocrystal growth process, which involved significant morphological reconstruction and crystallization of the initially formed amorphous nanoparticles. During the crystallization, the absorption onset of the CdSe quantum dots blue shifted toward higher energies at 3 eV (414 nm), while the photoluminescence red shifted to lower energies. Furthermore, an apparent increasing Stokes shift was observed during the formation of small CdSe nanoparticles. On the other hand, the photoluminescence excitation spectra showed constant features over the reaction time. Additionally, results from HRTEM and XRD studies show that the CdSe nanoparticles were amorphous at early reaction stages and became better crystallized after longer reaction times, while the particle size remained the same during the crystallization process. These observations demonstrate the important role of the surface on the optical properties of small CdSe quantum dots and facilitated the spectroscopic monitoring of the crystallization process in quantum dots.

The effect of surface adsorption on the hyper-Rayleigh scattering of large and small CdSe nanoparticles

Hyper-Rayleigh scattering is used to compare the effect of the addition of n-butylamine to large (3.2 nm) and small (1.6 nm) CdSe nanoparticles (NPs). It is found that although the adsorption of n-butylamine on large NPs enhances their nonlinear optical response, it has the opposite effect when adsorbed on the small nanoparticles for which the amine adsorption was found previously to induce structural changes. This observation is consistent with an increase in the symmetry of the nanocrystal structure such as would be observed in a phase change from a 4-coordinate wurtzite crystal structure to that of a 6-coordinate zinc-blend form.

Observation of Large Changes in the Band Gap Absorption Energy of Small CdSe Nanoparticles Induced by the Adsorption of a Strong Hole Acceptor

When butylamine in relatively high concentrations (0.1−0.25 M) is added to a colloidal solution of CdSe nanoparticles (NPs) that are approximately 1.6 nm in diameter, the band gap absorption changes from a broad, relatively weak absorption centered at 445 nm to a narrow, relatively strong absorption centered at 414 nm. The effect of concentration on the observed spectrum shows an isobestic point, suggesting an equilibrium between two species. The possible mechanisms for such a transformation are discussed.