Size-tunable Photoluminescence Research Articles

Nonlinear Optical Activity of a Chiral Organic-Inorganic ([(NH3CH2CH2)3NH])2[MnBr5]Br5 Photoluminescent and Piezoelectric Crystal.

The family of Mn-based organic-inorganic hybrids has greatly expanded due to their advantages in applications. They also show superior bright and size-tunable photoluminescence and can be considered a perfect alternative to toxic lead-based compounds. In this work, we present the detailed structural, optical, and electrical characterization of ([(NH3CH2CH2)3NH])2[MnBr5]Br5. The title compound exhibits a unique type of inorganic arrangement created by the trigonal bipyramids. It crystallizes in noncentrosymmetric space group R32, indicating its optical activity, piezoelectricity, and second-order optical nonlinearity proven by the second harmonic of light measurements. The studied crystals exhibit intense photoluminescence originating from the Mn(II) ion 4T1(G) → 6A1 transition. The measured lifetime of the photoluminescence emission is ≤1.5 ms, while the measured quantum yield for both powder and crystal samples reaches ∼70%.

High Energy Limit of the Size-Tunable Photoluminescence of Hydrogen-Terminated Porous Silicon Nanostructures in HF

The photoluminescence (PL) of various porous silicon (PSi) layers was studied during chemical dissolution in HF. The relative PL quantum efficiency of some layers was also monitored. Typically, the PL increased, reached a maximum and then dropped down to complete extinction, accompanied with a PL blueshift. During PL fall, both the PL intensity and layer quantum efficiency fell sharply, accompanied by a decrease in full width at half maximum and a slowing blueshift. In the final stage, the PL intensity decreased without any further blueshift, the saturated PL peak wavelength being ∼515 nm (∼2.4 eV) for most layers, identifying a high energy limit for the achievable PL of hydrogen-terminated Si nanostructures. Our results show that sudden catastrophic mechanical failure of nanostructure cannot explain the sharp PL drop and saturation of PL blueshift. Rather, they support the idea of a critical size (∼1.5–2 nm) below which the PL quantum efficiency vanishes. The possible reasons were discussed, privileging the emergence of structural non-radiative defects below a certain size, though the decreasing intrinsic quantum efficiency of Si nanocrystals with decreasing size could also play an important role. Maximum PL intensity was generally obtained for a peak wavelength of ∼565 nm (∼2.2 eV).

Evaluation of the biocompatibility of the GSH-coated Ag2S quantum dots in vitro: a perfect example for the non-toxic optical probes

Near-infrared quantum dots (NIR QDs) are promising candidate for the fluorescent probes due to their better penetration depth, long-lived luminescence with size-tunable photoluminescence wavelengths. Glutathione-coated silver sulfide quantum dots (GSH-Ag2S QDs) were synthesized using AgNO3 and Na2S in the aqueous media and they can give reaction with glutathione reductase (GR) and glutathione-s transferase (GST) enzymes as acting substrate analogue in vitro. Investigation of the toxicity of the nanomaterials are necessary to use them in the medical field and biomedical applications. Thus, in this study we investigated biocompatibility of the GSH-Ag2S QDs in vitro using 293T and CFPAC-1 cell lines. Cell viability by MTT assay, light microscopy, fluorescence microscopy, oxidative stress enzyme activities and ICP-MS analysis were performed to evaluate the cytotoxicity and internalization of the GSH-Ag2S QDs. GSH-Ag2S QDs showed great biocompatibility with both cell lines and did not cause imbalance in the oxidative stress metabolism. The ultralow solubility product constant of Ag2S QDs (Ksp = 6.3 × 10-50) prevents release of Ag ions into the biological systems that is in agreement with data obtained by ICP-MS. In conclusion, this data prove potential of GSH-Ag2S QDs as a biocompatible optical probe to be used for the detection and/or targeting of GSH impaired diseases including cancer.

Cd-Cu-Fe-S quaternary nanocrystals exhibiting excellent optical/optoelectronic properties.

Quaternary Cd-Cu-Fe-S nanocrystals (NCs) exhibiting a strong size tunable photoluminescence were synthesized for the first time by tuning the reaction temperature from 120 °C to 210 °C. The preparation procedure involved cadmium acetate, copper acetate, iron chloride, and sulfur powder dissolved in oleylamine as precursors. The wavelength of the emission can be tuned from 640 nm to nearly 1000 nm by only changing the size of the as-prepared NCs from 3.0 nm to 15 nm. Interestingly, these NCs possess a relatively high quantum yield of over 57% without coating any wide band-gap shell materials. The study on the optoelectronic properties of Cd-Cu-Fe-S NCs, where an order of photocurrent was enhanced under AM1.5 illumination, demonstrated their suitability as optically active components to fabricate optoelectronic devices.

Size-tunable photoluminescence from WS2 nanostructures

Variable size crystalline nanostructures of WS2 have been successfully synthesized in a single-step process using liquid exfoliation in different liquid media. Out of three liquid media chosen in the work, we found that the mixture of de-ionized water and ethanol is most efficient for preparing the nanostructures. The average size of obtained nanostructures (quantum dots) range from 4 nm to 5 nm and that of few layer sheets range from 20 nm to 100 nm or even larger depending upon the choice of liquid media used for exfoilation process. The present work illustrates that choice of liquid media is critical for efficient production of the highly luminescent nanostructures in lesser time. Further, when compared to their bulk counterpart these nanostructures exhibit size dependent photoluminescence in ultraviolet and visible region. Quantum dots displayed absorption features at shorter wavelength and might originate from excitonic transitions in these scaled nanostructures. Highly luminescent nature together with blue shifted absorption feature suggests the presence of quantum confinement effects in the WS2 nanostructures.

Temperature-dependent photoluminescence of size-tunable ZnAgInSe quaternary quantum dots**Project supported by the National High Technology Research and Development Program of China (Grant No. 2013AA014201), the National Key Foundation for Exploring Scientific Instrument of China (Grant No. 2014YQ120351), and the Natural Science Foundation of Tianjin (Grant No. 11JCYBJC00300, 4JCZDJC31200, 15JCYBJC16700, and 15JCYBJC16800).

Colloidal ZnAgInSe (ZAISe) quantum dots (QDs) with different particle sizes were obtained by accommodating the reaction time. In the previous research, photoluminescence (PL) of ZAISe QDs only could be tuned by changing the composition. In this work the size-tunable photoluminescence was observed successfully. The red shift in the photoluminescence spectra was caused by the quantum confinement effect. The time-resolved photoluminescence indicated that the luminescence mechanisms of the ZAISe QDs were contributed by three recombination processes. Furthermore, the temperature-dependent PL spectra were investigated. We verified the regular change of temperature-dependent PL intensity, peak energy, and the emission linewidth of broadening for ZAISe QDs. According to these fitting data, the activation energy () of ZAISe QDs with different nanocrystal sizes was obtained and the stability of luminescence was discussed.

Origin and control of blinking in quantum dots.

Semiconductor nanocrystals offer an enormous diversity of potential device applications, based on their size-tunable photoluminescence, high optical stability and 'bottom-up' chemical approaches to self-assembly. However, the promise of such applications can be seriously limited by photoluminescence intermittency in nanocrystal emission, that is, 'blinking', arising from the escape of either one or both of the photoexcited carriers to the nanocrystal surface. In the first scenario, the remaining nanocrystal charge quenches photoluminescence via non-radiative Auger recombination, whereas for the other, the exciton is thought to be intercepted before thermalization and does not contribute to the photoluminescence. This Review summarizes the current understanding of the mechanisms responsible for nanocrystal blinking kinetics as well as core-shell engineering efforts to control such phenomena. In particular, 'softening' of the core-shell confinement potential strongly suppresses non-radiative Auger processes in charged nanocrystals, with successful non-blinking implementations demonstrated in CdSe-CdS core-thick-shell nanocrystals and their modifications.

Silicon Nanocrystals: Cationic Silicon Nanocrystals with Colloidal Stability, pH‐Independent Positive Surface Charge and Size Tunable Photoluminescence in the Near‐Infrared to Red Spectral Range (Adv. Sci. 2/2016)

Silicon nanocrystals (ncSi) displaying size‐dependent optoelectronic properties functionalized with cationic surface groups are synthesized through a versatile alkyl‐halide surface intermediate. In article number 1500263, together with anionic functionalized ncSi developed by the same group, G. A. Ozin and co‐workers envision numerous combinations to observe the interplay between large and small, cationic and anionic silicon nanocrystals.

Cationic Silicon Nanocrystals with Colloidal Stability, pH-Independent Positive Surface Charge and Size Tunable Photoluminescence in the Near-Infrared to Red Spectral Range.

In this report, the synthesis of a novel class of cationic quaternary ammonium‐surface‐functionalized silicon nanocrystals (ncSi) using a novel and highly versatile terminal alkyl halide‐surface‐functionalized ncSi synthon is described. The distinctive features of these cationic ncSi include colloidal stability, pH‐independent positive surface charge, and size‐tunable photoluminescence (PL) in the biologically relevant near‐infrared‐to‐red spectral region. These cationic ncSi are characterized via a combination of high‐resolution scanning transmission electron microscopy with energy‐dispersive X‐ray analysis, Fourier transform infrared, X‐ray photoelectron, and photoluminescence spectroscopies, and zeta potential measurements.

Size tunable fluorescent nano-graphite oxides: preparation and cell imaging applications

Fluorescent nano-graphite oxides (NGO) with different size distribution were prepared via a one-pot hydrothermal route using ultrasmall graphite powder as starting material and subsequently separated using dialysis tubes with different molecular weight cutoff. The biomedical applications of these NGO for cell imaging were further investigated. Fourier transform infrared spectra demonstrated that many functional groups including the hydroxyl group, carboxyl group and epoxy group were present on NGO, which endowed them with good water solubility. These NGO showed size-dependent photoluminescence and excellent biocompatibility with A549 cells. As evidenced by laser scanning confocal microscopy images, NGO could be internalized by A549 cells and located in the cytoplasm. Given their good water solubility, size tunable photoluminescence and excellent biocompatibility, these NGO should be promising for bioimaging and various biomedical applications.

Evolution of biofunctional semiconductor nanocrystals: a calorimetric investigation

Semiconductor nanomaterials have found numerous applications in optoelectronic device fabrication and in platforms for drug delivery and hyperthermia cancer treatment, and in various other biomedical fields because of their high photochemical stability and size-tunable photoluminescence (PL). However, little attention has been paid to exploring the energetics of formation of these semiconductor nanoparticles. We demonstrate that formation of nanocrystals with biofunctionalization supported by widely used groups, BSA and cysteine, is an exothermic spontaneous process driven by enthalpy. The whole energetics of the reaction shows that formation of smaller particles is favored with lower synthesis temperature. Further, it is shown that the thermodynamics of nanoparticle formation is strongly influenced by the conformation of the protein matrix. We also demonstrate that protein supported formation of nanocrystals is thermodynamically more favorable compared to that involving smaller organic thiol groups. The favorable enthalpy of formation compensates unfavorable entropy, resulting in favorable Gibbs free energy. Thus, this study can open up new avenues for establishing a thermodynamic basis for the design of nanosystems with new and tunable properties.

Colloidal CuInSe2 Nanocrystals in the Quantum Confinement Regime: Synthesis, Optical Properties, and Electroluminescence

Photoluminescent, near-stoichiometric colloidal CuInSe2 nanocrystals are synthesized in large batches and with good colloidal quality. An organometallic method is used with a moderate reaction temperature (≤200 °C) to produce CuInSe2 nanocrystals with size-tunable photoluminescence spectra ranging from ∼600 to ∼850 nm. Two-dimensional photoluminescence excitation–emission maps are reported for the CuInSe2 nanocrystals, highlighting the size-tunable excitonic features. Type I heterostructured CuInSe2/ZnS nanocrystals are prepared and purified. They are found to have absolute photoluminescence quantum yields up to ∼26%. The potential to use CuInSe2/ZnS core/shell nanocrystals as a potential low toxicity active layer in light-emitting diodes is demonstrated by fabricating electroluminescent devices.

Size-Tunable Phosphorescence in Colloidal Metastable γ-Ga2O3 Nanocrystals

We report a colloidal synthesis of gallium oxide (Ga(2)O(3)) nanocrystals having metastable cubic crystal structure (gamma phase) and uniform size distribution. Using the synthesized nanocrystal size series we demonstrate for the first time a size-tunable photoluminescence in Ga(2)O(3) from ultraviolet to blue, with the emission shifting to lower energies with increasing nanocrystal size. The observed photoluminescence is dominated by defect-based donor-acceptor pair recombination and has a lifetime of several milliseconds. Importantly, the decay of this phosphorescence is also size dependent. The phosphorescence energy and the decay rate increase with decreasing nanocrystal size, owing to a reduced donor-acceptor separation. These results allow for a rational and predictable tuning of the optical properties of this technologically important material and demonstrate the possibility of manipulating the localized defect interactions via nanocrystal size. Furthermore, the same defect states, particularly donors, are also implicated in electrical conductivity rendering monodispersed Ga(2)O(3) nanocrystals a promising material for multifunctional optoelectronic structures and devices.

ChemInform Abstract: Hybrid Electrochemical/Chemical Synthesis of Quantum Dots.

The electrochemical/chemical (or method) is a new wet chemical method for synthesizing semiconductor quantum dots on graphite surfaces. The E/C synthesis of quantum dots composed of the generic semiconducting salt, MX, typically involves three steps: (1) electrochemical deposition of nanoparticles of the metal, M degrees, from a solution of metal ions, M(n)(+); (2) electrochemical oxidation of these metal particles to MO(n)()(/2), and; (3) displacement of the oxygen from MO(n)()(/2) using HX (for example) to yield nanoparticles of MX. This conversion from metal to metal oxide to metal salt occurs on a particle-by-particle basis; that is, each metal nanoparticle is converted into a semiconductor nanoparticle. E/C-synthesized beta-CuI and CdS quantum dots possess many of the attributes of quantum dots synthesized using molecular beam epitaxy, including epitaxial orientation on the graphite surface, a narrow size dispersion, and strong, particle size-tunable photoluminescence. However, the E/C method is faster, cheaper, and applicable to a greater number of materials.

Controlled organic/inorganic interface leading to the size-tunable luminescence from Si nanoparticles

Highly luminescent Si nanoparticles (NPs) terminated with alkoxy monolayers have been synthesized via sodium biphenylide reduction of SiCl4 encapsulated with inverse micelles of dimethyldioctylammonium bromide in a mixture of toluene and glyme, and subsequent alkoxylation of surface Si–Cl bonds. The optical absorption and emission spectra are blueshifted with reduction in size of the Si NPs (d ≤ 5 nm), relatively. Interestingly, it is found that the monolayers, which serve as a shell to protect the Si core from surface oxidation, play an important role in the appearance of the quantum size effect which generates a size-tunable photoluminescence (PL) property with a high PL QY. Decrease in molecular coverage of an alkoxy shell causes the partial surface oxidation of a Si NP, leading to the appearance of the interfacial-related PL feature. Time-resolved study of the fluorescence in Si NPs with and without oxide is performed to understand the essential difference of the electron–hole recombination process between each NP systems. The synthesis of highly luminescent NPs with tailored PL properties in the ultraviolet–visible region required full control of the NP size distribution and the chemical property at the heterogeneous interface between the Si cores and the organic shells.

Hybrid electrochemical/chemical synthesis of quantum dots.

The "electrochemical/chemical method" (or "E/C method") is a new wet chemical method for synthesizing semiconductor quantum dots on graphite surfaces. The E/C synthesis of quantum dots composed of the generic semiconducting salt, MX, typically involves three steps: (1) electrochemical deposition of nanoparticles of the metal, M degrees, from a solution of metal ions, M(n)(+); (2) electrochemical oxidation of these metal particles to MO(n)()(/2), and; (3) displacement of the oxygen from MO(n)()(/2) using HX (for example) to yield nanoparticles of MX. This conversion from metal to metal oxide to metal salt occurs on a particle-by-particle basis; that is, each metal nanoparticle is converted into a semiconductor nanoparticle. E/C-synthesized beta-CuI and CdS quantum dots possess many of the attributes of quantum dots synthesized using molecular beam epitaxy, including epitaxial orientation on the graphite surface, a narrow size dispersion, and strong, particle size-tunable photoluminescence. However, the E/C method is faster, cheaper, and applicable to a greater number of materials.