Bright Visible Photoluminescence Research Articles

Atomically Precise Distorted Nanographenes: The Effect of Different Edge Functionalization on the Photophysical Properties down to the Femtosecond Scale.

Nanographenes (NGs) have been attracting widespread interest since they combine peculiar properties of graphene with molecular features, such as bright visible photoluminescence. However, our understanding of the fundamental properties of NGs is still hampered by the high degree of heterogeneity usually characterizing most of these materials. In this context, NGs obtained by atomically precise synthesis routes represent optimal benchmarks to unambiguously relate their properties to well-defined structures. Here we investigate in deep detail the optical response of three curved hexa-peri-hexabenzocoronene (HBC) derivatives obtained by atomically precise synthesis routes. They are constituted by the same graphenic core, characterized by the presence of a heptagon ring determining a saddle distortion of their sp2 network, and differ from each other for slightly different edge functionalization. The quite similar structure allows for performing a direct comparison of their spectroscopic features, from steady-state down to the femtosecond scale, and precisely disentangling the role played by the different edge chemistry.

Highly biocompatible graphene quantum dots: green synthesis, toxicity comparison and fluorescence imaging

Graphene quantum dots (GQDs) have tremendous potential in biological imaging due to their bright visible photoluminescence mission. However, the tedious preparation procedures and potential toxicity of GQDs greatly limit their application in biological field. Here, highly biocompatible GQDs (HGQDs) have been successfully prepared only by glucose in aqueous solution. Compared with GQDs prepared from conventional methods (CGQDs), the cytotoxicity of HGQDs reduced by more than 60%, and the flow cytometric analysis of the normal cells treated with HGQDs showed that the early and late apoptotic rate reduced by more than 72% and 40%, respectively. In vitro fluorescence imaging showed that both cells and bacteria could be imaged by HGQDs, and the morphology of cells and bacteria could be kept to a maximum extent. A long-term in vivo study revealed that no obvious organ (heart, liver, spleen, lung and kidney) damage or lesions were observed, and the blood–brain barrier (BBB) could be overcome, which provides the possibility for treatment and diagnosis of brain-related diseases. With adequate studies of biocompatibility, both in vitro and in vivo, HGQDs may be considered for further biological application.

Superlinear Photoluminescence by Ultrafast Laser Pulses in Dielectric Matrices with Metal Nanoclusters

An intense photoluminescence emission was observed from noble metal nanoclusters (Pt, Ag or Au) embedded in sapphire plates, nucleated by MeV ion-implantation and assisted by an annealing process. In particular, the spectral photoluminescence characteristics, such as range and peak emission, were compared to the behavior observed from Pt nanoclusters embedded in a silica matrix and excited by UV irradiation. Correlation between emission energy, nanoclusters size and metal composition were analyzed by using the scaling energy relation EFermi/N1/3 from the spherical Jellium model. The metal nanocluster luminescent spectra were numerically simulated and correctly fitted using the bulk Fermi energy for each metal and a Gaussian nanoclusters size distribution for the samples. Our results suggest protoplasmonics photoluminescence from metal nanoclusters free of surface state or strain effects at the nanoclusters-matrix interface that can influence over their optical properties. These metal nanoclusters present very promising optical features such as bright visible photoluminescence and photostability under strong picosecond laser excitations. Besides superlinear photoluminescence from metal nanoclusters were also observed under UV high power excitation showing a quadratic dependence on the pump power fluence.

Size Matters: From Two-Dimensional AuI -TlI Metallopolymers to Molecular Complexes by Simple Variation of the Steric Demand.

Bis(acetylido) aurates(I) and thallium(I) trifluoromethylsulfonates were used to synthesize AuI -TlI metallopolymers, displaying novel and unusual structural motifs of the metal-metal backbones in the solid state: a discrete molecular cluster, 1D chains of interconnected dimers, tetramers, or dodecamers of Au-Tl units, and a 2D-plane network, consisting of alternating edge-linked (AuTl)6 and (AuTl)4 cycles. The formation of the different architectures was primarily controlled by the steric demand of the acetylide-substituent groups. Thus, the bulkiest 2,6-diisopropylphenyl derivative yielded a molecular cluster [Tl2 Au3 ]. Most compounds showed bright visible photoluminescence with quantum yields of up to 25 % at ambient temperature. The color of the emitted light significantly differs with the network structure. Furthermore, theoretical studies of singlet excitations in the molecular cluster, as well as NMR and mass-spectrometric investigations of the fragmentation of the metallopolymers in solution are described in detail.

Bright Visible Photoluminescence from Nanotube Titania Grown by Soft Chemical Process.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Bright Visible Photoluminescence from Nanotube Titania Grown by Soft Chemical Process

Bright visible photoluminescence was observed from nanotube titania (or dehydrated nanotube titanic acid) prepared by a soft chemical process. Its photoluminescence comes from the Ti−OH complex within the tubular structure. With the improvement of the crystalline structure and the desorption of physically adsorbed water on the inner and outer surfaces of the nanotube titania by annealing below 662 K, the photoluminescence intensity is enhanced. However, the intensity decreases dramatically if the tubular structure is destroyed at higher annealing temperature, and simultaneously Ti−OH bonds within the nanotube titania are broken. Therefore, the one-dimensional tubular structure is a very important factor for nanotube titania to obtain strong photoluminescence.

Surface Functionalization of Silicon Nanoparticles Produced by Laser-Driven Pyrolysis of Silane followed by HF−HNO3 Etching

CO2 laser induced pyrolysis of silane was used to produce silicon nanoparticles with an average diameter as small as 5 nm at high rates (up to 200 mg/h). Etching these particles with a mixture of hydrofluoric acid (HF) and nitric acid (HNO3) reduces their size and passivates their surface such that they exhibit bright visible photoluminescence (PL). This paper describes the attachment of organic molecules to hydrogen-terminated and hydroxyl-terminated surfaces of these nanoparticles. Stable particle dispersions in various solvents were obtained by treatment of hydrogen-terminated surfaces with octadecene or undecylenic acid and by treatment of hydroxyl-terminated surfaces with octadecyltrimethoxysilane. Transmission electron microscopy showed that the surface-functionalized particles were well dispersed and crystalline. FTIR spectroscopy confirmed the expected reactions of the organic molecules with the particle surfaces. Photoluminescence measurements showed that surface treatment significantly stabilized the PL properties of the nanoparticles against degradation. Size selective precipitation was applied to particle dispersions and allowed some narrowing and tuning of the PL spectrum.

Hot carriers and excitation of Si/SiOx interface defect photoluminescence in Si nanocrystallites

In low-dimensional silicon wires and dots, as supposed earlier (Phys. Rev. B 65 (2002) 115313), hot carrier ballistic transport towards the Si/SiOx interface can enhance the excitation of oxide defect-related photoluminescence (PL) bands. This article presents new experimental results supporting the role of ballistic transport in bright visible photoluminescence of silicon nano-crystallites. The intensity dependences of the “red” and “orange” photoluminescence bands on excitation light wavelengths, diameters of Si nano-crystallites and surface area of porous layers have been analysed. The models of optical transitions at the excitation of both PL bands are discussed as well.

Process for Preparing Macroscopic Quantities of Brightly Photoluminescent Silicon Nanoparticles with Emission Spanning the Visible Spectrum

Silicon nanoparticles with bright visible photoluminescence have been prepared by a new combined vapor phase and solution phase process, using only inexpensive commodity chemicals. CO2 laser induced pyrolysis of silane was used to produce Si nanoparticles at high rates (20−200 mg/h). Particles with an average diameter as small as 5 nm were prepared directly by this vapor phase (aerosol) synthesis. Etching these particles with mixtures of hydrofluoric acid (HF) and nitric acid (HNO3) reduced the size and passivated the surface of these particles such that after etching they exhibited bright visible luminescence at room temperature. The wavelength of maximum photoluminescence (PL) intensity was controlled from above 800 nm to below 500 nm by controlling the etching time and conditions. Particles with blue emission (maximum PL intensity at 420 nm) were prepared by rapid thermal oxidation of orange-emitting particles. The particle synthesis methods; steady-state photoluminescence spectra; results of their characterization using TEM, XRD, FTIR absorption spectroscopy, and XPS; and preliminary assessments of the stability of the photoluminescence properties with time are presented here. Preparation of macroscopic quantities by the methods described here opens the door to chemical studies of free silicon nanoparticles that could previously be carried out only on porous silicon wafers, as well as to potential commercial applications of silicon nanoparticles.

Bright visible photoluminescence from silica nanotube flakes prepared by the sol–gel template method

We report macroscopic synthesis of silica nanotubes by the sol–gel template method. A large number of silica nanotubes with small diameters (30–50 nm) were produced and were shaped into flakes successfully. Strong photoluminescence (PL) was observed in both as-grown and annealed nanotube flakes. The PL spectra have maxima at 2.55 and 2.30 eV for the as-grown and annealed samples, respectively; the PL intensity of annealed nanotubes is much higher than that of as-grown nanotubes. The strong emission may be due to the Si–OH complex located on both the inner and outer surfaces of the nanotubes. The nanotube flakes we prepared may have potential applications in future integrated optical devices.

Bright visible photoluminescence of spark-processed Ge, GaAs, and Si

High-frequency spark discharges were applied to single-crystalline wafers of Ge, GaAs, and Si. The spark-processed (sp-) samples were characterized by photoluminescence (PL) and Raman measurements. Strong and stable luminescence with wavelengths centered at 416 and 525 nm was observed in sp-Ge and sp-Si layers, respectively, when excited with a 325 nm laser beam. A considerable blue shift of the PL (compared to the unsparked specimen) was also detected for sp-GaAs with an average peak wavelength around 500 nm. The Raman shifts of the spark-processed materials indicate that nanocrystals were formed, having diameters of 3.5–4 nm for Si and about 6 nm for Ge. A correlation between the PL wavelengths, the nanocrystal sizes, and the different semiconductor materials has been established based on the effective-mass approximation. Making use of this model the nanocrystallite sizes have been found to range between ∼3 nm for Si and ∼5 nm for Ge. The related wavelengths for optical transitions confirm the PL results. The findings support the quantum confinement model.

Microstructure and composition of an anodically etched (111) Si surface

Scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy data for the topmost part of an anodically etched (111) p-Si wafer exhibiting bright visible photoluminescence have been analysed with regard to microstructure and chemical composition. It is concluded that this region consists of two layers. The first topmost layer contains six compounds: Si-(O2H2), SiO2, SiO, Si6O3H6, Si and SiH. In this layer the silicon species depth profile and temperature dependence were investigated. We find that Si-(O2H2) and SiH decomposed completely at 650 K and at 730 K respectively. The amount of siloxene is independent of temperature up to 650 K and then begins to decrease. This was accomplished with desorption of hydrogen from Si6O3H6 surface species and irreversible oxidation of the samples.

Comparison of light emission from stain-etch and anodic-etch silicon films

Bright visible photoluminescence (PL) has been measured in stain-etch silicon films prepared by chemical etching in a dilute hydrofluoric and nitric acid solution. The PL emission is observed to degrade exponentially when the stain-etch films are illuminated in air (intensity decreases by 1/e over 22.45 min). Anodic-etch silicon films, prepared using a novel electrochemical cell, show similar strong visible PL but a degradation rate an order of magnitude smaller. The wavelength of the PL peak for anodic-etch silicon (650–710 nm) shifts toward the blue with decreasing electrolyte HF concentration while the PL peak position of stain-etch silicon (∼650 nm) does not vary with process conditions investigated.