Size-Dependent Color Tuning of Efficiently Luminescent Germanium Nanoparticles

It is revealed that rigorous control of the size and surface of germanium nanoparticles allows fine color tuning of efficient fluorescence emission in the visible region. The spectral line widths of each emission were very narrow (<500 meV). Furthermore, the absolute fluorescence quantum yields of each emission were estimated to be 4–15%, which are high enough to be used as fluorescent labeling tags. In this study, a violet-light-emitting nanoparticle is demonstrated to be a new family of luminescent Ge. Such superior properties of fluorescence were observed from the fractions separated from one mother Ge nanoparticle sample by the fluorescent color using our developed combinatorial column technique. It is commonly believed that a broad spectral line width frequently observed from Ge nanoparticle appears because of an indirect band gap nature inherited even in nanostructures, but the present study argues that such a broad luminescence spectrum is expressed as an ensemble of different spectral lines and can be separated into the fractions emitting light in each wavelength region by the appropriate postsynthesis process

Solution-Processed InSb Quantum Dot Photodiodes for Short-Wave Infrared Sensing

Short-wave infrared (SWIR) photodiodes (PDs) based on colloidal semiconductor quantum dots (QDs) are characterized by the great possibility of device operation at a voltage bias of 0 V, spectral tunability, possible multiple-exciton generation, and high compatibility with printable technology, showing significant benefits toward medical applications. However, the light-absorbing layers of those PDs are hampered by a reliance on RoHS-restricted elements, such as Pb and Hg. Here, we report the SWIR PDs with light-absorbing layers of InSb QDs synthesized by a hot-injection approach using a combination of precursors, InBr3 and SbBr3. Impurity-free and secondary phase-free synthesis was realized by optimized reaction temperature and time, precursor ratio, and quenching of reaction. The diameters of the QDs were controlled in the 5.1–7.8 nm range for strengthened confinement of photogenerated carriers and tuning of bandgaps between 0.64 and 0.98 eV. These QDs were processed to terminate their surfaces with small molecular ligands, giving a narrow interparticle distance between neighboring QDs in a light-absorbing layer sandwiched by carrier transportation layers. The resulting PDs achieve a photoresponse of ∼550 ms at 0 V, with combining the best values of responsivity and external quantum efficiency of 0.098 A/W and 10.1% under a bias voltage of −1 V at room temperature even in ambient air

Formation and Optical Properties of Fluorescent Gold Nanoparticles Obtained by Matrix Sputtering Method with Volatile Mercaptan Molecules in the Vacuum Chamber and Consideration of Their Structures

This paper proposes a novel methodology to synthesize highly fluorescent gold nanoparticles (NPs) with a maximum quantum yield of 16%, in the near-infrared (IR) region. This work discusses the results of using our (previously developed) matrix sputtering method to introduce mercaptan molecules, α-thioglycerol, inside the vacuum sputtering chamber, during the synthesis of metal NPs. The evaporation of α-thioglycerol inside the chamber enables to coordinate to the “nucleation stage” very small gold nanoclusters in the gas phase, thus retaining their photophysical characteristics. As observed through transmission electron microscopy, the size of the Au NPs obtained with the addition of α-thioglycerol varied from approximately 2–3 nm to approximately 5 nm. Plasmon absorption varied with the size of the resultant nanoparticles. Thus, plasmon absorption was observed at 2.4 eV in the larger NPs. However, it was not observed, and instead a new peak was found at approximately 3.4 eV, in the smaller NPs that resulted from the introduction of α-thioglycerol. The Au NPs stabilized by the α-thioglycerol fluoresced at approximately 1.8 eV, and the maximum wavelength shifted toward the red, in accordance with the size of the NPs. A maximum fluorescent quantum yield of 16% was realized under the optimum conditions, and this value is extremely high compared to values previously reported on gold NPs and clusters (generally ∼1%). To our knowledge, however, Au NPs of size >2 nm usually do not show strong fluorescence. By comparison with results reported in previous literature, it was concluded that these highly fluorescent Au NPs consist of gold–mercaptan complexes. The novel method presented in this paper therefore opens a new door for the effective control of size, photophysical characteristics, and structure of metal NPs. It is hoped that this research contributes significantly to the science in this field

NMR, ESR, and Luminescence Characterization of Bismuth Embedded Zeolites Y

Thermal treatment of bismuth-embedded zeolite Y yields luminescent Bi⁺ substructures without the formation of metallic nanoparticles. The structural and photophysical features of the resulting zeolite Y have been thoroughly characterized by using extensive experimental techniques including nuclear magnetic resonance (NMR), electron spin resonance (ESR), 2-dimentional excitation–emission and absorption spectra. NMR and ESR results indicate that some Al and oxygen are expelled from the zeolite Y framework after undergoing thermal treatment. The detailed analyses of luminescence and absorption spectra, coupled with TDDFT calculations, suggest that all Bi⁺ substructures (i.e., Bi₄⁴⁺, Bi₃³⁺, Bi₂²⁺, and Bi⁺) are optically active in the near-infrared (NIR) spectral range. It is found that Bi⁺, Bi₂²⁺, Bi₃³⁺, and Bi₄⁴⁺ units result in NIR emissions peaking at ca. 1050, 1135, 1145, and 1240/1285 nm, respectively. The emission lineshapes under diverse excitation wavelengths greatly depend on the Bi concentration and annealing temperature, as a result of the change in the relative concentration and the spatial distribution, as well as local structural features of Bi active species. Specifically, the above analyses imply that the reducing agents for Bi³⁺ are water molecules as well as framework oxygen. These findings represent an important contribution to the understanding of the processes involved in the formation of Bi⁺ and of the luminescence mechanisms of Bi⁺ substructures in zeolite Y frameworks, which are not only helpful for the in-depth understanding of experimentally observed photophysical properties in other Bi-doped materials but also important for the development of novel photonic material systems activated by other p-block elements

Synchrotron X-ray, Photoluminescence, and Quantum Chemistry Studies of Bismuth-Embedded Dehydrated Zeolite Y

For the first time, direct experimental evidence of the formation of monovalent Bi (i.e., Bi⁺) in zeolite Y is provided based on the analysis of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence results as well as quantum chemistry calculations suggest that the substructures of Bi⁺ in the sodalite cages contribute to the ultrabroad near-infrared emission. These results not only enrich the well-established spectrum of optically active zeolites and deepen the understanding of bismuth related photophysical behaviors, but also may raise new possibilities for the design and synthesis of novel hybrid nanoporous photonic materials activated by other heavier p-block elements

Synchrotron X-ray, Photoluminescence, and Quantum Chemistry Studies of Bismuth-Embedded Dehydrated Zeolite Y

For the first time, direct experimental evidence of the formation of monovalent Bi (i.e., Bi⁺) in zeolite Y is provided based on the analysis of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence results as well as quantum chemistry calculations suggest that the substructures of Bi⁺ in the sodalite cages contribute to the ultrabroad near-infrared emission. These results not only enrich the well-established spectrum of optically active zeolites and deepen the understanding of bismuth related photophysical behaviors, but also may raise new possibilities for the design and synthesis of novel hybrid nanoporous photonic materials activated by other heavier p-block elements

Efficient Dual-Modal NIR-to-NIR Emission of Rare Earth Ions Co-doped Nanocrystals for Biological Fluorescence Imaging

A novel approach has been developed for the realization of efficient near-infrared to near-infrared (NIR-to-NIR) upconversion and down-shifting emission in nanophosphors. The efficient dual-modal NIR-to-NIR emission is realized in a β-NaGdF₄/Nd³⁺@NaGdF₄/Tm³⁺–Yb³⁺ core–shell nanocrystal by careful control of the identity and concentration of the doped rare earth (RE) ion species and by manipulation of the spatial distributions of these RE ions. The photoluminescence results reveal that the emission efficiency increases at least 2-fold when comparing the materials synthesized in this study with those synthesized through traditional approaches. Hence, these core–shell structured nanocrystals with novel excitation and emission behaviors enable us to obtain tissue fluorescence imaging by detecting the upconverted and down-shifted photoluminescence from Tm³⁺ and Nd³⁺ ions, respectively. The reported approach thus provides a new route for the realization of high-yield emission from RE ion doped nanocrystals, which could prove to be useful for the design of optical materials containing other optically active centers