Quantum Size Regime Research Articles

Quenching Fluorescence of Quantum Dots by Using Gold Nanocrystals in Quantum and Classical Size Regime

This paper investigates the factors which can influence the quenching effect from the perspective of energy transfer. The quenched fluorescence of the CdSe semiconductor quantum dots (SQDs) by using plasmonic Au nanocrystals (p-AuNCs) and molecule-like Au nanocrystals (m-AuNCs) in aqueous suspensions and spin-coated films is comparatively investigated. In the aqueous suspensions, the p-AuNCs have larger quenching effect than the m-AuNCs. In the spin-coated films, the p-AuNCs and m-AuNCs have comparable quenching factor. Furthermore, the experiments show that the p-AuNCs simultaneously enhance the radiative and nonradiative rates. But the m-AuNCs only enhance the nonradiative rate of the SQDs, which reveals the difference of quenching process between the p-AuNCs and m-AuNCs. This result of the research has guiding significance for the detection technique based on the fluorescence quenching.

Magnetism of Atomically Precise Gold and Doped Nanoclusters: Delocalized Spin and Interparticle Coupling

The magnetic properties of Au nanoparticles (NPs) have long been an intriguing topic in fundamental research. In the quantum size regime (1–3 nm), Au NPs indeed exhibit distinct magnetism in contrast to diamagnetic gold in bulk or relatively large plasmonic NPs. However, previous studies often give controversial results due to imprecise NPs used in magnetic studies, making it difficult to understand the origin of magnetism in Au NPs. Recent progress has led to atomically precise Au nanoclusters (NCs for differentiation with regular NPs), which can serve as a unique model for studying the delocalized spin in isolated NCs and the spin–spin exchange interaction between NCs in assembled solids. Moreover, such mechanisms are correlated to the atomic structures of the magnetic NCs. The precise formulas of the NCs serve as a clear indicator for magnetism. So far, the Au-thiolate NCs reported to be magnetic, including the doped ones, all contain icosahedral kernel structures, which are facile compared to other structures in adding or removing one electron for endowing magnetism to the NCs. Heteroatom doping in the NCs is an effective method to probe the magnetic mechanism in NCs, such as the origin of magnetic anisotropy, and the counterion tailoring for those charged NCs can also impart magnetism to the initially diamagnetic NCs via stabilizing a particular charge state of the NCs. While only a few cases of NCs have been reported with magnetism thus far, future research on metal NCs (especially in the critical regime between molecular and metallic state) will reveal more fundamentals of magnetism, and the control of spin–spin exchange coupling in nanocluster-assembled solids is particularly promising for tailoring the magnetic functionality of NCs and ultimately finding versatile applications.

Tunable Size Dependence of Quantum Plasmon of Charged Gold Nanoparticles.

The quantum behavior of surface plasmons has received extensive attention, benefiting from the development of exquisite nanotechnology and the diverse applications. Blueshift, redshift, and nonshift of localized surface plasmon resonances (LSPRs) have all been reported as the particle size decreases and enters the quantum size regime, but the underlying physical mechanism to induce these controversial size dependences is not clear. Herein, we propose an improved semiclassical model for modifying the dielectric function of metal nanospheres by combining the intrinsic quantized electron transitions and surface electron injection or extraction to investigate the plasmon shift and LSPR size dependence of the charged Au nanoparticles. We experimentally observe that the nonmonotonic blueshift of LSPRs with size for Au nanoparticles is turned into an approximately monotonic blueshift by increasing the electron donor concentration in the reduction solution, and it can also be transformed to an approximately monotonic redshift after surface passivation by ligand molecules. Moreover, we demonstrate controlled blueshift and redshift for the electron and hole plasmons in Cu_{2-x}S@Au core-shell nanoparticles by injecting electrons. The experimental observations and the theoretical calculations clarify the controversial size dependences of LSPR reported in the literature, reveal the critical role of surface electron injection or extraction in the transformation between the different size dependences of LSPRs, and are helpful for understanding the nature of surface plasmons in the quantum size regime.

Plasmon-enhanced photocatalytic activity of Pt@Au and Pt@Cu nanoparticles in quantum size regime

We have synthesized Pt@Au and Pt@Cu bimetallic nanoparticles and investigated their surface plasmon resonance (SPR) and plasmon-enhanced photocatalytic activity in the quantum size regime. The SPR wavelength of Pt@Au bimetallic nanoparticles monotonously blue-shifts as the thickness of Pt shell increases comparing with the quantum-sized Au core nanoparticles with diameter dAu = 3.6 ± 0.4 nm. On the other hand, the SPR of Pt@Cu bimetallic nanoparticles blue-shifts at first and then red-shifts as the Pt shell thickness increases comparing to the quantum-sized Cu core nanoparticles with diameter dCu = 3.7 ± 0.5 nm. Then, we comparatively investigated the photocatalytic activities of the Pt@Au nanoparticles with quantum and classical sizes. The photocatalytic rate for 4-NP:NaBH4 is enhanced 2 times by the strong plasmon resonance and the large surface area/volume ratio of quantum-sized Pt@Au nanoparticles. Additionally, the increasing photocatalytic activity of the quantum-sized Pt@Cu with the increase of Pt shell thickness is also observed. These findings reveal the significance of the quantum plasmon resonance on the enhancement of photocatalytic activity.

Atomistic Characterization of Plasmonic Dimers in the Quantum Size Regime

Plasmonic dimer systems show great promise in a wide range of applications because of their unique optical and electronic properties that arise from the coupling of monomer plasmons. To determine t...

Enhancement of linear and nonlinear optical absorption coefficients in spherical dome semiconductor nanoshells by surface plasmon resonances

Nonlinear optical effects in semiconductor nanostructures with quantum size regime have aroused great interest due to their giant nonlinearities in contrast to those of bulk counterparts, which is of great importance for developing nonlinear optical nano-devices. Here, using quantum theory, we reveal that surface plasmon resonances of metal nanoparticles greatly enhance the linear and nonlinear optical absorption coefficients of spherical dome semiconductor nanoshells with quantum sizes. These enhanced linear and nonlinear optical effects can be flexibly tuned and amplified by manipulating both the structural parameters of the nanoshells and the distance between nanoshells and metal nanoparticles. Furthermore, the bleaching effects are more clearly observed through the surface plasmon resonances. Our founding make spherical dome semiconductor nanoshells a promising candidate for nonlinear optical nano-devices with the potential for integrated nonlinear optical applications.

Electron Tomography of Plasmonic Au Nanoparticles Dispersed in a TiO2 Dielectric Matrix

Plasmonic Au nanoparticles (AuNPs) embedded into a TiO2 dielectric matrix were analyzed by combining two-dimensional and three-dimensional electron microscopy techniques. The preparation method was reactive magnetron sputtering, followed by thermal annealing treatments at 400 and 600 °C. The goal was to assess the nanostructural characteristics and correlate them with the optical properties of the AuNPs, particularly the localized surface plasmon resonance (LSPR) behavior. High-angle annular dark field-scanning transmission electron microscopy results showed the presence of small-sized AuNPs (quantum size regime) in the as-deposited Au-TiO2 film, resulting in a negligible LSPR response. The in-vacuum thermal annealing at 400 °C induced the formation of intermediate-sized nanoparticles (NPs), in the range of 10-40 nm, which led to the appearance of a well-defined LSPR band, positioned at 636 nm. Electron tomography revealed that most of the NPs are small-sized and are embedded into the TiO2 matrix, whereas the larger NPs are located at the surface. Annealing at 600 °C promotes a bimodal size distribution with intermediate-sized NPs embedded in the matrix and big-sized NPs, up to 100 nm, appearing at the surface. The latter are responsible for a broadening and a redshift, to 645 nm, in the LSPR band because of increase of scattering-to-absorption ratio. Beyond differentiating and quantifying the surface and embedded NPs, electron tomography also provided the identification of "hot-spots". The presence of NPs at the surface, individual or in dimers, permits adsorption sites for LSPR sensing and for surface-enhanced spectroscopies, such as surface-enhanced Raman scattering.

Spectral Properties of GaS Nanoparticles Obtained by Laser Ablation

Bulk single crystals of GaS were grown by Bridgman-Stockbarger method in an evacuated (10-6 Torr) quartz ampoules of optical quality. Nanoparticles of quasi 2-D GaS crystals were obtained by laser ablation (LA) technique (KrF Excimer laser at 248 nm (COHERENT COMPex 201), and characterized by: XRF, GDOES, SEM, TEM and UV-VIS absorption spectroscopy. Nanoparticles obtained for the first time were ablated by using of excimer KrF laser operated at f = 50 Hz with pulse duration t = 4 ns and maximum energy 200 mJ. Obtained results are analyzed with respect to particle size. Absorption spectra of particles with diameter less than 18 nm turned out to be shifted in the blue range of spectra. Blue shift in the optical absorption spectra of GaS nanoparticles with decreasing the particle sizes was explained by presence of nanocrystals in the quantum size regime. These results are consistent with a perturbation of GaS band structure due to carrier confinement, resulting in a widening of the forbidden gap.

An atomistic-electrodynamics theory for the optical response of periodic lattices of metallic nanoparticles in the quantum size regime

We present a new theoretical method for calculating the optical properties of periodic arrays of metallic nanoparticles whose dimensions are in the quantum-size limit and cannot, therefore, be described by macroscopic electrodynamic theory based on Maxwell’s equations. In the first stage, the method calculates the scattering matrix for a single metal nanoparticle described as a polyhedral cluster of atoms, via a discrete-dipole approximation/point-matching technique. The resulting scattering matrix is incorporated into a layer-multiple-scattering technique which allows the modeling of two- and three-dimensional nanostructures containing very small metal nanoparticles. The method is demonstrated for square arrays of silver nanoparticles with radii below 2 nm and the corresponding results are compared against classical local and nonlocal electrodynamic approaches.

Quantum plasmonics: longitudinal quantum plasmons in copper, gold, and silver

The propagation of plasmonic waves in various metallic quantum nanostructures has received considerable attention for its applications in technology. The quantum plasmonic properties of metallic nanostructures in the quantum size regime have been difficult to describe using an appropriate model. Here nonlocal quantum plasmons are investigated in the most important metals of copper, gold, and silver. The dispersion properties of these metals and the propagation of longitudinal quantum plasmons in the high photon energy regime are studied using a new model of nonlocal quantum dielectric permittivity. The epsilon-near-zero properties are investigated and the spectrum and the damping rate of the longitudinal quantum plasmons are obtained in these metals. The quantum plasmons’ wave function is shown for both the classical and quantum limits. It is shown that silver is the most appropriate for quantum metallic structures in the development of next-generation quantum optical and sensing technologies, due to its low intrinsic loss.

From Well-Defined Alkylzinc Phosphinates to Quantum-Sized ZnO Nanocrystals.

Despite various applications of alkylzinc carboxylates in chemistry and materials science, the corresponding organozinc derivatives of organophosphorus compounds still represent an insufficiently explored area. To fill this gap, we report on the synthesis of alkylzinc phosphinates and their use as efficient precursors of phosphinate-coated ZnO nanocrystals in the quantum size regime. Examples of a series of alkylzinc phosphinates with the general formula [RZn(O2 PR'2 )] (R=tBu or Et) have been prepared through equimolar reactions between ZnR2 and a selected phosphinic acid, namely dimethylphosphinic acid (dmpha-H), methylphenylphosphinic acid (mppha-H), diphenylphosphinic acid (dppha-H), or bis(4-methoxyphenyl)phosphinic acid (dmppha-H). The reactivities of alkylzinc phosphinate complexes toward H2 O and O2 have also been investigated, which resulted in the isolation of two oxo-zinc phosphinate clusters, that is, [Zn4 (μ4 -O)(dppha)6 ] and [Zn4 (μ4 -O)(dmppha)6 ], as well as the unique alkoxy(oxo)zinc cluster [Zn4 (μ4 -O)(μ2 -OtBu)(dppha)5 ]. Analysis of the crystal structures has revealed that organozinc complexes incorporating phosphinate ligands exhibit a unique capacity for shape-driven self-assembly to produce extended networks, including noncovalent quasi-porous materials. Finally, monodispersed and quantum-sized ZnO NCs coated with phosphinate ligands have been prepared using a non-external-surfactant-assisted wet-chemical organometallic approach based on well-defined [RZn(O2 PR'2 )]-type compounds. The resulting brightly luminescent ZnO NCs exhibit average core sizes and hydrodynamic diameters in the ranges 2-4.5 nm and 5-8 nm, respectively. The size of the inorganic core is slightly affected by the character of the incorporated phosphinate ligand, being smallest for ZnO NCs coated by asymmetrically substituted mppha ligands. Regardless of whether or not various phosphinate coating ligands could be controllably applied on the ZnO NC surface, no significant differences were found in the luminescence profiles of the analyzed nanosystems.

Highly Efficient Light-Emitting Diodes of Colloidal Metal-Halide Perovskite Nanocrystals beyond Quantum Size.

Colloidal metal-halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete film coverage and prevents exciton quenching at the PEDOT:PSS/perovskite particle film interface. With these strategies, we achieved a very high PLQE (∼60.5%) in compact perovskite particle films without any complex post-treatments and multilayers and a high current efficiency of 15.5 cd/A in the LEDs of colloidal perovskite NCs, even in a simplified structure, which is the highest efficiency to date in green LEDs that use colloidal organic-inorganic metal-halide perovskite nanoparticles including perovskite QDs and NCs. These results can help to guide development of various light-emitting optoelectronic applications based on perovskite NCs.

PH-dependent optical properties of N-acetyl-L-cysteine-capped ZnSe(S) nanocrystals with intense/stable emissions

In the present study, a series of aqueous-based ZnSe(S) nanocrystals (NCs) was prepared at different solution pH ranging from 8 to 11.9, and using N-acetyl-L-cysteine (NAC) as capping agent. In addition to zinc blende structure, the X-ray diffraction studies demonstrated the quantum size regime of the ZnSe(S) NCs. To gain further insight toward the influence of the quantum confinement and pH values on optical properties of the as-prepared NCs, their UV-visible absorption and photoluminescence spectra were systematically analyzed. The absorption spectra experienced a red shift from ~340 to ~382 nm as the pH increased from 8.0 to 11.9, indicating the growth of the as-prepared ZnSe(S) NCs. The emission spectra also show the obvious red shift and the relative area of excitonic to trap emission, firstly increases from pH = 8.0 to 10.7, and then decreases by further increasing of the solution pH. The initial behavior might be due to the improved surface passivation of the trap dangling states by better deprotonation of thiol groups in NAC, whereas at pH >10.7, the faster growth rate of the ZnSe(s) NCs may lead to the formation of many defect sites. All of these phenomena were combined in the scheme which displays the effect of quantum confinement and solution pH on variation of the excitonic and trap-related emissions.

Hierarchical Self-Assembly of Cu7Te5 Nanorods into Superstructures with Enhanced SERS Performance.

This paper reports a strategy to get self-assembly of Cu7Te5 nanorods into hierarchical superstructures: the side-by-side self-assembly of nanorods into microscale one-dimensional (1D) nanowires (primary structure), the side-by-side alignments of the 1D nanowires into two-dimensional (2D) nanowire bundles (secondary structure), and the further rolling up of the 2D bundles into three-dimensional (3D) microtubes (tertiary structure). It was found that the oleylamine (OLA)/n-dodecanethiol (DDT) mixture as a binary capping agent was key to produce Cu7Te5 nanorods in the quantum size regime with high monodispersity, and this was a prerequisite for their hierarchical self-assembly based on elaborate control of the solvent evaporation process. The obtained Cu7Te5 microtube superstructures were used as SERS substrate and showed much stronger SERS enhancement than the as-prepared Cu7Te5 nanorods before assembly. This was probably ascribed to the remarkably enhanced local electromagnetic field arising from the plasmon coupling of Cu7Te5 nanorods in the well-assembled superstructures.

Crossover from quantum to classical electron transport in ultrathin metal films

Using the Boltzmann approach we have developed one dimensional (1D) model of metal films conductivity for the quantum electron transport. Thickness fluctuation of film has an effect on the electron spectra via carriers scattering in quantum size regime. In the framework of the developed model size dependent conductivity of metal films has been calculated. Here, we employed above mentioned model to explain experimental data on single crystalline CoSi2 films and fine-grained gold films. Wide film thickness ranges applicable by the described model cover also the crossover from quantum to quasiclassical electron transport.

Atomistic electrodynamics simulations of bare and ligand-coated nanoparticles in the quantum size regime.

The optical properties of metallic nanoparticles with nanometre dimensions exhibit features that cannot be described by classical electrodynamics. In this quantum size regime, the near-field properties are significantly modified and depend strongly on the geometric arrangements. However, simulating realistically sized systems while retaining the atomistic description remains computationally intractable for fully quantum mechanical approaches. Here we introduce an atomistic electrodynamics model where the traditional description of nanoparticles in terms of a macroscopic homogenous dielectric constant is replaced by an atomic representation with dielectric properties that depend on the local chemical environment. This model provides a unified description of bare and ligand-coated nanoparticles, as well as strongly interacting nanoparticle dimer systems. The non-local screening owing to an inhomogeneous ligand layer is shown to drastically modify the near-field properties. This will be important to consider in optimization of plasmonic nanostructures for near-field spectroscopy and sensing applications.

Oxidation catalysis by large trisoctahedral gold nanoparticles: Mind the step!

To understand the origin of catalysis by gold and to probe the role of defects and interfaces, benchmark reactions namely CO oxidation and benzyl alcohol oxidation were carried out on supported trisoctahedral gold nanoparticles and on the inverse catalyst system. The sizes of these particles which are between 45nm and 110nm, are well beyond the quantum size regime and well above the optimum size range where gold is considered to be catalytically active. The periodic steps sites in contact with oxide support interface were able to promote the reaction.

Facile green extracellular biosynthesis of CdS quantum dots by white rot fungus Phanerochaete chrysosporium

This study details a novel method for the extracellular microbial synthesis of cadmium sulfide (CdS) quantum dots (QDs) by the white rot fungus Phanerochaete chrysosporium. P. chrysosporium was incubated in a solution containing cadmium nitrate tetrahydrate, which became yellow from 12h onwards, indicating the formation of CdS nanocrystals. The purified solution showed a maximum absorbance peak between 296 and 298nm due to CdS particles in the quantum size regime. The fluorescence emission at 458nm showed the blue fluorescence of the nanoparticles. X-ray analysis of the nanoparticles confirmed the production of CdS with a face-centered cubic (fcc) crystal structure. The average grain size of the nanoparticles was approximately 2.56nm, as determined from the full width at half-maximum (FWHM) measurement of the most intense peak using Scherer's equation. Transmission electron microscopic analysis showed the nanoparticles to be of a uniform size with good crystallinity. The changes to the functional groups on the biomass surface were investigated through Fourier transform infrared spectroscopy. Furthermore, the secretion of cysteine and proteins was found to play an important role in the formation and stabilization of CdS QDs. In conclusion, our study outlines a chemical process for the molecular synthesis of CdS nanoparticles.

Interfaced Metal Heterodimers in the Quantum Size Regime

Synthesis of nanoparticle dimers made of asymmetric compositions is very challenging because of the difficulty in manipulating the nanoparticles' surface chemistries in order to control the assembly and/or growth of different nanoparticles. In this Letter, we report a seed-mediated, surface-confined epitaxial overgrowth strategy that enables the synthesis of high-quality interfaced Au-Ag heterodimers in the quantum size regime (diameters <10 nm). Au and Ag share a common face-centered cubic lattice and have nearly identical lattice constants, which facilitates epitaxial overgrowth and allows direct contact between the Au and Ag domains. Quantum size effects, formation of the Au/Ag interfaces, and chemical interactions with surfactant molecules strongly influence the optical properties of the dimers and lead to the observation of unique surface plasmon resonances. In particular, we find an unusual enhancement of the characteristic Au surface plasmon resonance and the emergence of a charge transfer plasmon across the Au/Ag domains, which together lead to broad-band absorption spanning visible to near-infrared wavelengths. A model that captures the changes in optical behavior due to chemical interactions and quantum size effects is used to calculate the absorption spectra of the interfaced heterodimers, resulting in good agreement with experimental measurements.

Role of nano in catalysis: Palladium catalyzed hydrogen desorption from nanosized magnesium hydride

Chemical reactions involve competing pathways. Here, two pathways for H desorption from MgH2 are identified by first-principles calculations: One involves H diffusion in bulk Mg, while the other involves H vacancy diffusion at the MgH2 surface. The latter is sensitive to the size of the reactant MgH2 and self-terminates in bulk material as dehydrogenation eventually eliminates exposed MgH2. However, this surface vacancy pathway can maximize the catalyst effect of Pd by decoupling the kinetics of H desorption from that of H diffusion. When the surface-to-bulk ratio is large as in the case of MgH2 nanostructures, H desorption will take place primarily via the low-barrier surface vacancy pathway. Our picture attributes the experimentally observed size effect of MgH2 on H desorption, beyond the quantum size regime, to a synergy between the nanosize of the reactant and the catalyst.