Article

Photopatternable Quantum Dots Forming Quasi-Ordered Arrays

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Abstract

We have functionalized core-shell CdSe/ZnS quantum dots (QDs) with a photosensitive monolayer, rendering them solution processable and photopatternable. Upon exposure to ultraviolet radiation, films composed of this material were found to polymerize, forming interconnected arrays of QDs. The photoluminescence properties of the nanocrystal films increased with photocuring. The material was found to be suitable for spin casting and was used as the active layer in a green electroluminescent device. The electroluminescence efficiency of devices containing a photocured active layer was found to be largely enhanced when compared to devices containing nonphotocured active layers. The material also showed excellent adhesion to both organic and inorganic substrates because of the unique combination of a siloxane and a photopatternable layer as ligands. The pristine functionalized nanocrystals could easily be used for two-dimensional patterning on organic and inorganic substrates. The photopatternable quantum dots were uniformly dispersed into a photopolymerizable resin to fabricate QD embedded three-dimensional microstructures.

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... Poor compatibility with the host matrix can lead to poor QD passivation and aggregation in QDpolymer composites, which severely reduces photostability. 259,268,277,278 Phase separation and ligand detachment may be triggered by photopolymerisation. [277][278][279] Furthermore, it is possible that radicals etched the exposed surface of the QDs in samples containing photoinitiator: photoinitiator concentration was and has been linked to QD quenching in QD-photopolymers, 280 as particularly noted with nitroxyl radicals. 281 QD fluorescence was unstable in uncured inks: the amine ligand shell was insufficient to disperse the QDs long-term, resulting in aggregation and quenching. ...
... 280 Poor compatibility with the host matrix is known to lead to QD aggregation and loss of passivation from stabilising ligands. 259,268,277,278 Fluid inks without photoinitiator lost fluorescence over a longer period of time; it is likely that the amine ligands gradually detach, leading to aggregation and quenching. 110,122,249 As primary amines enhance PL of CdSe-based QDs, reduced coverage leads to an immediate, significant reduction in emission intensity. ...
... [277][278][279] Dispersion of QDs in polymers is complex and agglomeration occurs where interaction between particles is poorly shielded, leading to interdigitalisation of alkyl chain capping ligands instead of interaction with the host polymer. 122,268,269 Furthermore, amine ligands are labile and temperature-dependant ligand detachment can occur, [277][278][279] which is problematic as the crosslinking of acrylates is exothermic. This can cause stabilising ligands to be disrupted during photopolymerisation, leading to degradation of QD passivation and loss of QY from exposed surface defects. ...
Article
There is a need for high-resolution and high-sensitivity temperature sensing in fields such as micro/nanoelectronics, integrated photonics, and biomedicine; however, non-invasive integrated sensing is difficult and expensive to achieve in miniaturised devices, as fabrication is greatly complicated by multi-step processes, heat treatments, and material compatibility. Inkjet printing (IJP) is a direct writing technique in the material jetting AM category that is effective for maskless multi-material printing with <50 µm resolution, which enables production of end-use devices and could simplify sensor integration. Existing inkjet-printed temperature sensors comprise simple circuit devices, which use the change in the electrical resistance of a sensing area to measure temperature. While current examples are well-suited to wearable sensors, they do not achieve the spatial and thermal resolutions desired for printed devices such as microfluidics. Development of inks for luminescence nanothermometry would enable inkjet-printable sensing geometries for planar and 3D thermal imaging with submicron and subdegree resolutions. Silicones are polymers suitable for optical sensing due to their ultraviolet (UV) and thermal stability, optical transparency, and high refractive indices. Composite inks for luminescence nanothermometry can be formulated with quantum dots (QDs), fluorescent semiconductor nanocrystals with intrinsic, reversible temperature quenching. Printable optical sensing materials would enable in situ temperature monitoring for applications and geometries that are otherwise impossible to monitor by conventional means. This thesis describes the development of the first inkjet-printable QD-silicone composite, and the first ink for luminescence thermometry, for integrated optical sensing; these may also have use in lighting applications . 2-part addition cure silicone inks and 1-part UV cure silicone inks were explored and QD-silicone composites were synthesised; inkjet printing of an addition cure QD-composite was demonstrated. Printing of reactive addition cure inks, where Ink A contained crosslinker and Ink B contained catalyst, was demonstrated using drop-on-drop IJP with the smallest average drop diameters reported for silicone IJP to date (33 36 µm). To overcome poor contact pinning, a pinned grid strategy was used for single printhead IJP and a line-by-line strategy for dual printhead IJP. Curing was the greatest challenge in reactive inkjet of QD-silicone composites, as labile ligands on the QDs poisoned the platinum catalyst despite low QD loading (0.005 wt% QD-Ink A). PtCl2 catalyst was added at low loading to enable curing and to explore the interactions between QDs and the catalyst. However, quenching was observed, with 70% decrease in emission intensity as PtCl2 concentration doubled; it was theorised that the QDs and catalyst competed for ligands, leading to metal-induced aggregation. Printing of fluorescent QD-silicone composites was demonstrated on a single printhead system using a pinned grid strategy; inks with no PtCl2 had stronger fluorescence but did not cure, highlighting their greater vulnerability to delays or fluctuations in heating. Novel UV curable silicone inks were formulated for inkjet using a high throughput screening method. Two photoinitiators (PIs) were trialled: DMPA (2,2-dimethoxy-2-phenylacetophenone) and TPO (phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide). DMPA was associated with rapid loss of fluorescence in QD-silicones, whereas quenching was not observed with TPO. Detachment of passivating ligands followed by photo-oxidation was suggested as a mechanism: TPO radicals are more susceptible to recombination with oxygen radicals than DMPA derived radicals, which might result in better shielding of the QD surface. Printing of 1 wt% TPO silicone inks without quantum dots was carried out under nitrogen to prevent oxygen inhibition. Jetting was demonstrated with 34-42 µm average drop diameter on silanised glass slides, while printing of continuous films was demonstrated on glass slides coated in a release agent. The temperature sensing performance of novel QD-silicone composites was assessed via fluorescence spectroscopy and imaging. 100 nm diameter QD clusters were observed in transmission electron microscopy and micron-scale QD aggregates in optical microscopy. QD emission appeared to be largely unchanged by immobilisation in silicone, although QD aggregation was expected to reduce photostability of the composite. Intensity- and spectral shift-based optical thermometry was demonstrated using well-plate reading and confocal laser scanning microscopy. Emission sensitivity at 627 nm was found to be approximately -0.7 to -1.2 % °C-1 between 30 50 °C and spectral sensitivity 0.07 to 0.08 nm °C-1, in agreement with other values in QD-sensing literature. Intensity decreased between thermal cycles of the same sample, although values at 60 °C were unchanged, while spectral shift appeared repeatable without redshift. Overall, fluorescent QD-silicone composites were produced via IJP for the first time and were shown to have temperature-sensitive emission. These materials are suitable for inkjet-printable devices with embedded optical temperature sensors using luminescence nanothermometry.
... In addition, practical ultrafast all-optical switches, modulators, flexible emitters, and room temperature singlephoton sources using these QDs can be realized if they can be patterned into waveguide or microcavity structures. Recently there have been several attempts to achieve this goal by embedding QDs in a variety of photonic structures and hosts [86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][127][128][129][130]. A significant challenge here is the incorporation of QDs into transparent host matrices without affecting their optical properties [101][102][103][127][128][129][130]. ...
... Recently there have been several attempts to achieve this goal by embedding QDs in a variety of photonic structures and hosts [86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][127][128][129][130]. A significant challenge here is the incorporation of QDs into transparent host matrices without affecting their optical properties [101][102][103][127][128][129][130]. The achievement of monodispersity, high fill factor, and efficient charge injection is highly desired. ...
... Hence covalent immobilization of QDs on the surface of solids requires the introduction of moieties that carry respective anchor groups which can be achieved either by inclusion of hydrophobic QDs into amphiphilic micelles leading to an interdigitated bilayer or by ligand replacement [104]. This type of cellulosebased gels is envisaged that can be made suitable to produce quantum dot-doped electrospun nanofibers using proper adjustment of chemistry, rheology, and electrospinning methods [127][128][129][130]. Moreover, cellulose-based quantum dot-doped electrospun nanofibers can be used in particular type of composite hydrogel fabrications which are useful for light-emitting purposes with potential use in lab-on-achip devices and also in optical sensing applications [6,. ...
... Quantum dots (QDs) are inorganic semiconductor nanocrystals that can be applied to solar cells, photodetectors, and bioimaging devices. [38][39][40][41] Functionalization of QDs plays an important role in making them compatible with interactions with functional materials. Incorporating inorganic nanomaterials into polymer precursors is a common strategy to improve the refractive index of eventual photocured structures. ...
... This technique was used to functionalize red, green, and blue cadmium containing (II-VI) QDs. [41,43] The in-situ photopatternable functionalization allows patterning of QDs onto glass and flexible substrates. The UV-induced densification of films of photopatternable QDs showed increased photoluminescence compared with QDs without functionalization. ...
... The UV-induced densification of films of photopatternable QDs showed increased photoluminescence compared with QDs without functionalization. [41] Such functionalization improves the uniform mixing of QDs into photoresists, facilitating easy fabrication of QD-doped 3D structures. ...
Article
Full-text available
In this topical review of two-photon stereolithography (TPS), we discuss novel materials and demonstrate applications of this technology. Two-photon-initiated chemical processes are used to fabricate arbitrary three-dimensional structures in TPS. In the first part of this article, the development of novel photoactive materials to fabricate pure inorganic or organic-inorganic hybrid microstructures is discussed. The second part discusses the fabrication of functional microstructures for highly specific applications to demonstrate the importance of TPS in different fields of science.
... In addition, practical ultrafast all-optical switches, modulators, flexible emitters, and room temperature singlephoton sources using these QDs can be realized if they can be patterned into waveguide or microcavity structures. Recently there have been several attempts to achieve this goal by embedding QDs in a variety of photonic structures and hosts [86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][127][128][129][130]. A significant challenge here is the incorporation of QDs into transparent host matrices without affecting their optical properties [101][102][103][127][128][129][130]. ...
... Recently there have been several attempts to achieve this goal by embedding QDs in a variety of photonic structures and hosts [86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][127][128][129][130]. A significant challenge here is the incorporation of QDs into transparent host matrices without affecting their optical properties [101][102][103][127][128][129][130]. The achievement of monodispersity, high fill factor, and efficient charge injection is highly desired. ...
... Hence covalent immobilization of QDs on the surface of solids requires the introduction of moieties that carry respective anchor groups which can be achieved either by inclusion of hydrophobic QDs into amphiphilic micelles leading to an interdigitated bilayer or by ligand replacement [104]. This type of cellulosebased gels is envisaged that can be made suitable to produce quantum dot-doped electrospun nanofibers using proper adjustment of chemistry, rheology, and electrospinning methods [127][128][129][130]. Moreover, cellulose-based quantum dot-doped electrospun nanofibers can be used in particular type of composite hydrogel fabrications which are useful for light-emitting purposes with potential use in lab-on-achip devices and also in optical sensing applications [6,. ...
... Such densification leads to change in optical properties of the films leading to increased photoluminescence. This was found to be beneficial for electroluminescent devices that feature QDs as active layers[9]. Such functionalization also improves the uniform mixing of QDs into photoresists facilitating fabrication of quantum dot doped threedimensional structures[9][10][11]. ...
... This was found to be beneficial for electroluminescent devices that feature QDs as active layers[9]. Such functionalization also improves the uniform mixing of QDs into photoresists facilitating fabrication of quantum dot doped threedimensional structures[9][10][11]. In the current work, we prepare a photopatternable cadmium-free QD. ...
... Doping polymeric films with QDs can be used for engineering the refractive index of polymerized films and hence their optical properties. UV irradiation of films containing polymerizable ligands has been shown to densify the films and improve their luminescent properties[9]. Polymer films and structures incorporating QDs have been studied for their properties such as photonic bandgaps, and as emissive layers in microelectronic devices[14][15][16]. ...
Article
Full-text available
Photopatternable nanoparticles can be easily dispersed into polymeric matrices and used to fabricate optoelectronic devices for display, sensing and quantum information processing applications. Here we report the first instance of a cadmium-free photopatternable quantum dot. A ligand containing dithiolane group at one end and an ene-functionalization at the other end were synthesized for this purpose. The myristic acid ligands on as synthesized red indium zinc phosphide-zinc sulfide (In(Zn)P/ZnS) quantum dots were easily replaced by the newly developed ligand by a simple sonication procedure. The functionalized quantum dots could be easily incorporated into a commercially available photoresist. The quantum dot doped photoresist was used to fabricate three-dimensional quantum dot doped hierarchical microstructures by two-photon lithography. Confocal imaging microscopy was used to verify the uniform incorporation of the nanoparticles in the hybrid microstructure.
... Therefore, the well dispersion of QDs in the photo resins plays a key role in keeping the resin transparent. Apart from the above-mentioned method, there is another routine to functionalization of QDs-polymer resin [137]. As reported, core-shell CdSe/ZnS ...
... Introduction to direct laser writing based on two-photon polymerization of functional photopolymers and related applications 29 microstructures with uniform quantum dot dispersion by TPP-DLW, which shows the potential of these functionalized nanomaterials for application in full color display devices. [137]. ...
Thesis
Luminescent nanostructures with high emission efficiency and spatial resolution are of great interest for optics and photonics. Specifically in quantum optics, remarked as an attractive candidate for single photon sources, quantum dots (QDs) have shown promising quantum yield and stability. In this issue, the crucial point is to integrate QDs emitters with a spatially controlled manner into optical and plasmonic devices. In this thesis, photopolymer containing QDs with 3 different emission colors have been formulated and used for the fabrication of 1D, 2D and 3D nanostructures with sub-100 nm size by direct laser writing based on two-photon polymerization (TPP-DLW). The improvement of the spatial resolution of writing is realized through a strong confinement of the polymerization volume based on the use of free radical inhibitors. The results show that the smallest feature size of 3D polymer woodpiles could reach 60 nm with a period of 350 nm, which has never been reported in the fabrication via TPP-DLW using single laser emitting at 780 nm. Investigations showed that spatial resolution of writing is significantly improved in the presence of Qds. Photochemical and kinetics studies were performed to understand the effect of both inhibitors and Qds on the nanostructuring by TPP. Finally, the local integration of QDs into Ag nanowires, ion exchange-glass optical waveguides, Gold nanocubes and fiber tips was achieved. We believe that our results and approaches of nanofabrication will stimulated further promising works in nano-optics
... Recently, combining nanomaterials such as metallic nanoparticles, [13] quantum dots, [14][15][16] graphene, [17] fullerenes, [18] carbon nanotubes, [13] and nanofibers [19] with AM to create novel composites and applications has gained significant popularity. Among them, mesoporous silica nanoparticles have emerged as a prominent alternative because of their large pore volumes, high surface areas, and versatile properties. ...
Article
Full-text available
In this study, we present the synthesis and a versatile way to incorporate photoresponsive organic luminophores into polymeric materials using mesoporous silica nanoparticles (MSNs). The encapsulated thioethers within the MSNs were employed in polyvinyl alcohol (PVA) films, resin‐based stereolithography, and electrospinning. Due to light‐induced cyclisation to dibenzothiophenes (DBTs), mmOC12 loaded materials were used to inscribe images using UV light. The DBTs formed from mmOC12 (mmDBTA/B) exhibit a long phosphorescence afterglow, which was investigated by steady‐state and time‐resolved photoluminescence spectroscopy. In addition, scanning electron microscopy (SEM) imaging, including energy dispersive X‐ray spectroscopy (EDX), revealed the well‐dispersed and intact MSNs in the polymeric materials. This approach shows a general way to incorporate non‐polar organic luminophores into polymeric materials while retaining their unique emission properties.
... The versatility of these multilayer structures showed great promise for the fabrication of color-tunable and flexible emitting patterns for integration in optical devices such as display panels. Park et al. used luminescent NCPRs based on CdSe@ZnS QDs to construct high-resolution 2D and 3D photonic structures using TPL (Park et al., 2010). Through EBL, Persano et al. induced the in-situ synthesis of ∼3 nm CdS QDs in a modified PMMA PR containing Cd-complexes formed by cadmiumbis(benzylthiol) and 1-methylimidazole in chloroform (Persano et al., 2012). ...
Article
Full-text available
Nanocomposites formed by a phase-dispersed nanomaterial and a polymeric host matrix are highly attractive for nano- and micro-fabrication. The combination of nanoscale and bulk materials aims at achieving an effective interplay between extensive and intensive physical properties. Nanofillers display size-dependent effects, paving the way for the design of tunable functional composites. The matrix, on the other hand, can facilitate or even enhance the applicability of nanomaterials by allowing their easy processing for device manufacturing. In this article, we review the field of polymer-based nanocomposites acting as resist materials, i.e. being patternable through radiation-based lithographic methods. A comprehensive explanation of the synthesis of nanofillers, their functionalization and the physicochemical concepts behind the formulation of nanocomposites resists will be given. We will consider nanocomposites containing different types of fillers, such as metallic, magnetic, ceramic, luminescent and carbon-based nanomaterials. We will outline the role of nanofillers in modifying various properties of the polymer matrix, such as the mechanical strength, the refractive index and their performance during lithography. Also, we will discuss the lithographic techniques employed for transferring 2D patterns and 3D shapes with high spatial resolution. The capabilities of nanocomposites to act as structural and functional materials in novel devices and selected applications in photonics, electronics, magnetism and bioscience will be presented. Finally, we will conclude with a discussion of the current trends in this field and perspectives for its development in the near future.
... Due to a two-photon absorption (TPA) process [25], photo-polymerization (crosslinking of photopolymer) is triggered to form arbitrary hollow 3D polymeric structures by scanning the laser. Recently, embedding nanoemitters into a photopolymer host matrix has been exploited to produce QD-polymer composites by photo-polymerization, which joins the active light-emitting property of QDs together with the technological feasibilities of the polymer matrix [26][27][28][29]. Au et al. and Shi et al. fabricated photonic structures based on QD-polymer nanocomposites [30,31]; however, integrating these QD-polymer nanocomposites onto the pre-existing or preselected waveguides is still a challenge. ...
Article
Full-text available
A major challenge towards nanophotonics is the integration of nanoemitters on optical chips. Combining the optical properties of nanoemitters with the benefits of integration and scalability of integrated optics is still a major issue to overcome. In this work, we demonstrate the integration of nanoemitters positioned in a controlled manner onto a substrate and onto an optical ion-exchanged glass waveguide via direct laser writing based on two-photon polymerization. Our nanoemitters are colloidal CdSe/ZnS quantum dots (QDs) embedded in polymeric nanostructures. By varying the laser parameters during the patterning process, we make size-controlled QD-polymer nanostructures that were systematically characterized using optical and structural methods. Structures as small as 17 nm in height were fabricated. The well-controlled QD-polymer nanostructure systems were then successfully integrated onto a new photonic platform for nanophotonics made of an ion-exchanged waveguide. We show that our QDs maintain their light emitting quality after integration as verified by photoluminescence (PL) measurements. Ultimately, QD emission coupled to our waveguides is detected through a home-built fiber-edge coupling PL measurement setup. Our results show the potential for future integration of nanoemitters onto complex photonic chips.
... This modification makes it easy to mix the QD into a polymerizable photoresist. 107 Radical polymerizable methacrylates containing a ligand layer on QDs were accomplished by stepwise reactions on the nanoparticle surface. The oleic acid ligands on the nanoparticle surface were replaced by 11-mercapto-1-undecanol; this was followed by a reaction between a 3-(trimethoxysilyl)propyl methacrylate and the polymerizable QDs. ...
Chapter
Nanomaterials and nanoscale interactions span an important and wide-ranging sector of modern science and industry. Championing developments in these fields, SPIE stages an annual forum to stimulate and support their growth: the Nanoscience and Engineering Symposium. This book represents a selection of compelling contributions from some of those innovators closely involved since the launch of this symposium. Among the key advances included are accomplishments with nanowaveguides, silicon photonics, solar energy conversion, lighting, nanofabrication and structure-determining methods for polymeric, organic, inorganic and composite materials, as well as biomaterials that can frequently achieve first-class response characteristics and that are of low-cost, and are readily available and environmentally responsible. Together, these contributions give a fascinating portrayal of the state of the art in the shifting landscape of current nanoscience and engineering.
... Since the phenomenon of two-photon absorption (TPA) was first experimentally observed in 1961 1 , it has attracted much research interest because of its potential application in three-dimensional optical data storage 2-5 , two-photon fluorescence imaging 6-8 , and microfabrication 9-13 . The femtosecond laser two-photon fabrication technique, which utilizes TPA, has become widely used in various fields 10,12,14,15 , among which the TPA induced reduction of metal ions 15-18 is of great significance due to their unique properties of metal nanostructures in various applications such as surface enhanced Raman scattering 19 , catalytic reactions 20 , optical sensing 21 , and metal nanowiring for electronic interconnections 22 . Up to now, however, the two-photon absorption cross section (TPACS) of metal ions, which characterizes the capability of TPA, still remains unknown because of the difficulty to measure it, since the existing methods for studying TPA are not applicable to metal ions. ...
Article
Full-text available
The photo-reduction of metal ions in solution induced by femtosecond laser is an important and novel method for fabricating three-dimensional metal microstructures. However, the nonlinear absorption cross section of metal ions remains unknown because its measurement is difficult. In the present study, a method based on Two-Photon Excited Sedimentation (TPES) is proposed to measure the two-photon absorption cross section (TPACS) of metal ions in solution. The power-squared dependence of the amount of sediment on the excitation intensity was confirmed, revealing that 800 nm femtosecond laser induced reduction of metal ions was a two photon absorption process. We believe that the proposed method may be applied to measure the TPACS of several metal ions, thereby opening a new avenue towards future analysis of two-photon absorption materials.
... Another approach has been to functionalize the ligand in the quantum dots. Park et al. [87] functionalized CdSe quantum dots with a photosensitive monolayer. Upon exposure to ultraviolet (UV) radiation , films composed of this material were found to polymerize, forming interconnected arrays of dots, rendering them photopatternable. ...
Article
In this thesis, we study charge transport in nanopatterned arrays of PbS colloidal quantum dots using conventional two-probe measurements and an integrated charge sensor. PbS dots are synthesized in solution with an organic ligand or cap that serves to passivate the surface of the dot, provide a tunnel barrier as well as colloidal stability. These dots can self assemble into an array as the solvent evaporates from a drop of solution on a surface. The self-assembled arrays can be thought of as tunable artificial solids, where the coupling between the dots can be tuned by changing the ligand. Using electron beam lithography followed by a lift-off process, we develop a novel technique to nanopattern these arrays and present the first colloidal quantum dot arrays of nanoscale dimensions. Nanopatterning makes it possible to study the electrical properties intrinsic to the dots unimpeded by macroscopic defects, such as cracking and clustering that typically exist in larger-scale arrays. We find that the electrical conductivity of the nanoscale films is higher than that of drop-cast, microscopic films made of the same type of dot. We achieve unprecedented versatility in integrating the patterned films into device structures, which will be valuable both for studying the intrinsic electrical properties of the dots and for nanoscale optoelectronic applications. From two-probe measurements on the nanopatterned arrays that are approximately 15 dots wide, we observe large noise in the current as a function of time. The noise is proportional to the current when the latter is varied by applying source-drain or gate voltage in a field-effect structure or when changing temperature. Owing to the small number of current paths in the system, we often observe telegraph switching, and find that the off times follow non-poissonian statistics. We show that the results can be understood in terms of a model in which a quasi-one-dimensional percolation path is turned on and off, by charging of a dot along the path. Long organic ligands lead to highly resistive colloidal quantum dot arrays, making the low bias regime inaccessible with conventional two-probe measurements. We use an integrated charge sensor to study transport in the low bias regime as a function of the coupling between the dots. We present transport measurements on butylamine and oleic acid capped PbS dots. The resistances measured are the highest measured for colloidal quantum dots. For the native oleic acid ligand, and weak coupling between the dots, the conduction mechanism is nearest neighbor hopping, and the conductance is simply activated. At low source-drain bias voltages, the activation energy is given by the energy required to release a carrier from a trap state plus the activation over barriers resulting from site disorder. The barriers from site disorder are eliminated with a sufficiently high source-drain bias. For the shorter ligand, which gives stronger coupling, the data are consistent with Mott's variable range hopping as the conduction mechanism.
... 0.1 wt. % TP-MOSF-TP (2,7-dibromo-3,6-bis(3,7-dimethyloctyloxy)-9,90-spirobifluoren) was mixed as a photosensitizer in the SCR-500 resin [30]. The TPIF emitted from the TPP resin was collected by the objective lens, then passed through the dichroic mirror and the tube lens, and finally captured by a CCD camera. ...
Article
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The autofocus method has been investigated to improve the precise positioning of a substrate surface at the center of the laser focal spot in two-photon polymerization (TPP) nanofabrication. For this purpose, we developed a novel autofocus method using normalized image size, which was calculated with the second momentum radius (SMR) of two-photon induced fluorescence (TPIF). The SMR of TPIF was theoretically analyzed and experimentally compared with the average intensity of TPIF for various input laser powers. The results show that the proposed method enhanced the precision and robustness of autofocus in TPP. Specifically, the experimental creation of ascending voxel arrays demonstrated both the method’s immunity to input laser power change, and a high precision of ± 0.045 μm . To test the practical feasibility of the proposed autofocus method, 300 μm × 260 μm single-layer honey-comb structures were successfully fabricated with precompensation and dynamic compensation using the proposed autofocus method.
... We surface-functionalized cadmium selenide/zinc sulfide (CdSe@ZnS) QDs to have a spacer ligand followed by an inner siloxane layer and a photopatternable methacrylate outer layer: see Figure 3(a). 7 We achieved this through an initial ligand exchange reaction followed by a series of condensation reactions. ...
... Owing to their specific wavelength emission feature, QDs are useful for applications in particle encoding, imaging, sensing, laser, and solar cells [103]- [106]. Park et al. presented photopatternable QDs that were composed of a CdSe/ZnS core and surrounding photosensitive monolayer [107]. They uniformly dispersed these QDs into photopolymerizable resin, and fabricated 2D patterns and 3D structures with it by TPP. ...
Article
“Intelligent” polymer nanocomposites have been an intensely popular research topic for several decades. These materials are based on combinations of two technologies: 1) the bottom-up synthesis of nanoparticles to assign a novel functionality and 2) the top-down photolithography of polymers to create desirable microstructures. Such intelligent nanocomposites are comprised of functional nanoparticles embedded in polymer mesh, and these nanoparticles enable polymers to display functions and properties that natural polymers cannot possibly show. Herein, we introduce and categorize intelligent polymer nanocomposites according to their functional applications. Three main categories are covered in this review: polymer nanocomposites with magnetic, optical, and biological functions. The statuses of this research field are covered, and the future directions of polymer nanocomposite research are discussed.
... 20À22 Such nanofabricated materials have been widely used for many applications such as photonic crystals, metamaterials, microfluidics, and biomedical implants. 21 Furthermore, there has been a continuous effort in creating 3D, 23 fluorescent, 24 and metal 25 microstructures using this technique. Clearly, this two-photon-based absorption technique outperforms other optical lithographic techniques in terms of 3D spatial resolution and flexibility. ...
Article
Metal nanoclusters consist of a few to few hundreds of atoms, and exhibit attractive molecular properties such as ultrasmall size, discrete energy levels and strong fluorescence. Although patterning of these clusters down to the micro- or nanoscale could lead to applications such as high-density data storage, it has been reported only for inorganic matrices. Here we present submicron-scale mask-free patterning of fluorescent silver nanoclusters in an organic matrix. The nanoclusters were produced by direct laser writing in poly(methacrylic acid) thin films, and exhibit a broadband emission at visible wavelengths with photostability that is superior to Rhodamine 6G dye. This fabrication method could open new opportunities for applications in nanophotonics like imaging, labeling, and metal ion sensing. We foresee that this method can be further applied to prepare other metal nanoclusters embedded in compositionally different polymer matrices.
... Different from the bilayer approach where the supramolecular assembly is mainly maintained by local hydrophobic interactions, ligand replacementfor example by mercapto-functional groups-allows for establishing much stronger linkages between the QDs and the bridging ligands used for covalent grafting (Dubois et al. 2007). Replacement of ligands has been reported for different types of QDs, such as those composed of CdSe/ZnS (Dubois et al. 2007;Park et al. 2010;Yang and Zhou 2011) or CuInS/ZnS (Kim et al. 2011). For CuInS/ZnS (core/shell) QDs that were obtained by thermolysis of respective salts and 1-mercaptododecane in octadecene at 210°C, a 60 % replacement of the original mercaptododecyl by a-mercapto-x-hydroxyundecyl ligands was obtained when the exchange was performed immediately after completion of the growth of the core particles (Kim et al. 2011). ...
Article
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Photoluminiscent (PL) cellulose aerogels of variable shape containing homogeneously dispersed and surface-immobilized alloyed (ZnS)x(CuInS2)1−x/ZnS (core/shell) quantum dots (QD) have been obtained by (1) dissolution of hardwood prehydrolysis kraft pulp in the ionic liquid 1-hexyl-3-methyl-1H-imidazolium chloride, (2) addition of a homogenous dispersion of quantum dots in the same solvent, (3) molding, (4) coagulation of cellulose using ethanol as antisolvent, and (5) scCO2 drying of the resulting composite aerogels. Both compatibilization with the cellulose solvent and covalent attachment of the quantum dots onto the cellulose surface was achieved through replacement of 1-mercaptododecyl ligands typically used in synthesis of (ZnS)x(CuInS2)1−x/ZnS (core–shell) QDs by 1-mercapto-3-(trimethoxysilyl)-propyl ligands. The obtained cellulose—quantum dot hybrid aerogels have apparent densities of 37.9–57.2 mg cm−3. Their BET surface areas range from 296 to 686 m2 g−1 comparable with non-luminiscent cellulose aerogels obtained via the NMMO, TBAF/DMSO or Ca(SCN)2 route. Depending mainly on the ratio of QD core constituents and to a minor extent on the cellulose/QD ratio, the emission wavelength of the novel aerogels can be controlled within a wide range of the visible light spectrum. Whereas higher QD contents lead to bathochromic PL shifts, hypsochromism is observed when increasing the amount of cellulose at constant QD content. Reinforcement of the cellulose aerogels and hence significantly reduced shrinkage during scCO2 drying is a beneficial side effect when using α-mercapto-ω-(trialkoxysilyl) alkyl ligands for QD capping and covalent QD immobilization onto the cellulose surface.
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A fabrication process of polycarbonate (PC) light guide plate (LGP) incorporating fluorescent quantum dots (QDs) is presented in this study. QD-PC composite pellets with dimension of 500 to 2500 μm were first prepared from a mixture of polyethylene-treated QDs and PC resin powder, followed by injection moulding to form a QD-LGP between 340 and 295 °C. The fabricated QD-LGPs demonstrated dual functions of light guiding and QD colour conversion, which is conventionally provided by a LGP covered with an additional QD diffuser film. By directly embedding QDs into a LGP, it was found that a rather low cadmium content around 41 ppm was sufficient to provide a uniform backlighting for a liquid-crystal display with a colour gamut of ~115% NTSC standard.
Chapter
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Since their inception, quantum dots have proven to be advantageous for light management applications due to their high brightness and well‐controlled absorption, scattering, and emission properties. As quantum dots become commercially available at large scale, the need for robust, stable, and flexible optical components continues to drive the development of robust and flexible quantum dot composite materials. In this review, after a thorough introduction to quantum dots, discussion delves into methods for fabricating quantum dot loaded composite optical elements such as thin films, microfabricated patterns, and microstructures. The importance of surface chemistry and ligand engineering, host matrixes, wet processing, and unique patterning methodologies is presented by considering photostability, aggregation, and phase separation of quantum dots in corresponding composites. With regard to prospective optical applications of quantum dot materials, emphasis is placed on light emitting and guiding composite materials for lasing applications, specifically whispering gallery mode‐based photonic microsystems. These developments will enable novel flexible, portable, and miniaturized optoelectronic devices such as light‐emitting diodes, flexible pixelated displays, solar cells, large‐area microwaveguides, omnidirectional micromirrors, optical metasurfaces, and directional microlasers.
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Quantum dot (QD) light-emitting diodes have been intensively investigated as a future display technology owing to their outstanding optoelectronic properties such as narrow spectral bandwidths and high quantum efficiencies. Significant efforts have been made to achieve full color QD light-emitting diodes (QLEDs) by applying various fine-patterning technologies to active QD layers. However, the reported patterning methods generally require high processing cost and complex facilities which have limited their wide adoption in industrial-scale display applications. In this study, a fine patterning method is presented by employing spatial light-assisted and locally controlled surface tailoring. The chemical functionality of an interfacial polyethyleneimine (PEI) layer between ZnO and the QD layers is locally controlled by spatial light patterning (SLP), which is simultaneously used as a charge transfer layer and an anchoring agent for the selected QD nanoparticles. The versatility of this approach is demonstrated by patterning crossed stripes and multicolor QLED devices on selectively patterned PEI layers with a maximum luminescence of 1950 cd m−2 and a current efficiency of 2.9 cd A−1. The SLP process described herein is a general approach for fabricating full color QLEDs with marginal toxicity. This process is compatible with the standard complementary metal-oxide semiconductor (CMOS) processing technology.
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3D optical security features manufactured by multistep 3D optical laser lithography are presented. These microstructures are composed of a nonfluorescent 3D cross-grid scaffold and fluorescent markers, realized by an acrylate-based resist containing CdSSe-based core–shell semiconductor quantum dots, arranged onto this scaffold at will. The readout of these 3D microstructures can be (exclusively) achieved by optical sectioning methods, for example, 3D confocal fluorescence laser scanning microscopy. As examples, structures with five different layers of markers and one emission color and two different colors, respectively, are shown. This class of deterministic 3D microstructures can be embedded into thin polymer films, can be individualized, and allows for plenty of future variations and extensions. It therefore represents an interesting avenue beyond established 2D holographic or 2D fluorescent optical security features.
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This paper demonstrates femtosecond laser direct writing mediated flexible integration of plasmonic Ag/Pd alloy nanostructures that can be potentially used as a robust surface-enhanced Raman spectroscopy substrate inside the microfluidic chip. Silver–palladium alloy with controllable composition ratio can be patterned into plasmonic nanostructures due to two-photon absorption induced coreduction of silver/palladium double metal ions. Since the alloy structures can effectively protect the silver from oxidation, thus they can facilitate the stable on-chip detection devices with long lifetime. The as-fabricated silver–palladium alloy substrate with 18% content of palladium maintains a relatively high enhancement factor of about 2.62 × 108 while at the same time demonstrating the best stability against aerobic oxidation, as it is stable for up to 20 d under ambient aerobic conditions, exhibiting a significant improvement compared to those unprotected silver substrates which have a limited lifetime of only 3 or 4 d.
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Recently, many studies have been conducted on the nano-scale fabrication technology using twophoton- absorbed polymerization induced by a femtosecond laser. The nano-stereolithography process has many advantages as a technique for direct fabrication of true three-dimensional shapes in the range over several microns with sub-100 nm resolution, which might be difficult to obtain by using general nano/microscale fabrication technologies. Therefore, two-photon induced nano-stereolithography has been recently recognized as a promising candidate technology to fabricate arbitrary 3D structures with sub-100 nm resolution. Many research works for fabricating novel 3D nano/micro devices using the two-photon nano-stereolithography process, which can be utilized in the NT/BT/IT fields, are rapidly advancing.
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A novel and versatile approach to the preparation of multitype polymers brushes on the quantum dots surface is reported. In this method, an azo initiator is first immobilized covalently onto CdSe/CdS particle surface by reacting with the 4,4′-Azobis (4-cyanovaleric acid) through an ester linkage. CdSe/CdS nanoparticles with polymer brushes (CdSe/CdS-polymers) of different properties, including hydrophilic polyacrylamide, poly(acrylic acid), and poly(vinyl pyrrolidone); hydrophobic oleophilic poly(vinyl acetate), poly(methyl acrylate), and poly(methyl methacrylate); polystyrene with conjugate group; and polyacrylonitrile with strong polarity group, are synthesized by one approach (the surface-initiated radical poly­merization technique), respectively. The structures and properties of CdSe/CdS-polymers reveal that the CdSe/CdS-azo initiator has excellent ability to initiate various kinds of vinyl monomers and these synthetic CdSe/CdS-polymers exhibit good stability and can emit multiple colors from the emission 405 to 569 nm. Moreover, CdSe/CdS-polymers brushes show characteristic micromorphologies. Such strategy provides a platform to achieve nanocomposites with different properties by a new CdSe/CdS-azo initiator through the simple conventional radical polymerization technique. Besides, the synthetic approach gives access to multitype modification of different nanoparticles with polymers.
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Recent advancements in nanotechnology have continued to stimulate the development of functional devices based on nanomaterials. However, the controllable assembly of these tiny nanomaterials into functional structures is still a big challenge for further applications; nowhere is this more obvious than in the field of nanodevices. Currently, despite the fact that self-assembly technologies have revealed great potential to reach this end, serious problems with respect to morphology control, designable assembly and even flexible patterning set huge obstacles to the fabrication of functional devices. Nowadays, in addition to self-assembly technologies that make use of interaction forces between different objects, photodynamic assembly (PDA) technology has emerged as a promising route to architect functional materials with the help of optical driving forces towards device fabrication. In this review, we summarize the recent developments in PDA technology for the designable patterning of nanoparticles (NPs). The basic fundamentals of PDA that resort to optical trapping (OT) and typical examples regarding far-field/near-field OT for the PDA of various NPs have been reviewed. In particular, femtosecond laser induced photodynamic assembly (FsL-PDA), which enables the designable patterning of NPs through a direct writing manner, has been introduced. Finally, the current challenges and future prospects of this dynamic field are discussed based on our own opinions.
Chapter
This chapter provides introductory background information and state-of-the-art progress in the field of nanocomposite materials, films, and patterns realized by the exploitation of in situ methodologies based on thermal, photon, and electron-beam-assisted synthesis. It introduces the most widely used precursor molecules and processes accounting for precursor decomposition and nanoparticles (NPs) nucleation, mainly during thermal-assisted experiments. Several pathways can be used to induce NPs nucleation inside a polymer matrix, such as chemical reduction, photoreduction, or thermal decomposition. One of the most extensively used energy source for the in situ synthesis of NPs is the heat. An overview on the most extensive techniques used for the nanocomposite microstructural characterization (X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM)) and optical spectroscopy is reported. The chapter defines the most promising in situ synthesis and patterning methods also in combined approaches, based on photon- and electron-beam-assisted procedures.
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Hybrid materials composed of colloidal semiconductor quantum dots and π-conjugated organic molecules and polymers have attracted continuous interest in recent years, because they may find applications in bio-sensing, photodetection, and photovoltaics. Fundamental processes occurring in these nanohybrids are light absorption and emission as well as energy and/or charge transfer between the components. For future applications it is mandatory to understand, control, and optimize the wide parameter space with respect to chemical assembly and the desired photophysical properties. Accordingly, different approaches to tackle this issue are described here. Simple organic dye molecules (Dye)/quantum dot (QD) conjugates are studied with stationary and time-resolved spectroscopy to address the dynamics of energy and ultra-fast charge transfer. Micellar as well as lamellar nanostructures derived from diblock copolymers are employed to fine-tune the energy transfer efficiency of QD donor/dye acceptor couples. Finally, the transport of charges through organic components coupled to the quantum dot surface is discussed with an emphasis on functional devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Various bicolored fluorescent micropatterns are fabricated on a single polymer film by concurrent photobleaching and a photobase generation process. A polymer, bearing anthracene and oxime-urethane groups, is dyed with rhodamine B isothiocyanate after irradiation with 310-nm UV light. The photochemical reaction of the polymer is monitored by UV, IR absorption, and fluorescence emission spectroscopy. Differently colored fluorescent micropatterns are obtained by selectively exciting each dye moiety. Various bicolored fluorescent micropatterns are observed through varying the excitation wavelength and observation wavelength ranges using a confocal microscope. This bicolored fluorescence patterning method will be useful to apply in photonic/electronic devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Semiconductor quantum dots have enormous potential as essential components in various optoelectronic devices. Patterned quantum dot nanocomposites such as quantum dot-polymers and quantum dot-photonic crystals are fabricated by Y. Song and co-workers on page 1649 through a reactive inkjet printing technique. Straightforward and efficient, this technique is capable of generating large-area patterns economically. It is anticipated that this concept will be broadened to the fabrication of other functional nanomaterials and optoelectronic devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Highly luminescent carbon dots with a fluorescence quantum yield of up to 45% were synthesized by the one step pyrolysis of anhydrous citric acid in diamino-silane. Thanks to surface methoxysilyl groups, the carbon dots are easily polymerized to three dimensional networks in the presence of trace water and under the catalysis of surface secondary amine groups. A remarkable enhancement in mechanical strength is observed after further reaction of the secondary amine with tri-isocyanate or copolymerization with silane. The size of the network mesh of the reinforced carbon dots monolith is around 1.2 μm, with the variation possibility through controlling the gelation step. Due to the advantages of strong luminescence, low cost, thermal stability and macroporous properties, the resulting monolith is expected to hold great technological promise for wide applications.
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Nanometer-scale optical waveguides are attractive due to their potential applicability in photonic integration, optoelectronic communication, and optical sensors. Nanoscale white light-emitting and/or polychromatic optical waveguides are desired for miniature white-light generators in microphotonic circuits. Here, polychromatic (i.e., blue, green, and red) optical waveguiding characteristics are presented using a novel hybrid composite of highly crystalline blue light-emitting organic nanowires (NWs) combined with blue, green, and red CdSe/ZnS quantum dots (QDs). Near white-color waveguiding is achieved for organic NWs hybridized with green and red QDs. Light, emitted from QDs, can be transferred to the organic NW and then optically waveguided through highly packed π-conjugated organic molecules in the NW with different decay characteristics. Remote biosensing using dye-attached biomaterials is presented by adapting the transportation of QD-emitted light through the organic NW.
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We found that Cd0.5Zn0.5S-ZnS core (4.2 nm in diameter)-shell (1.2 nm in thickness) quantum dots (QDs) demonstrated a typical energy-down-shift (2.76-4.96 → 2.81 eV), which absorb ultra-violet (UV) light (250-450 nm in wavelength) and emit blue visible light (∼442 nm in wavelength). They showed the quantum yield of ∼80% and their coating on the SiNX film textured p-type silicon solar-cells enhanced the external-quantum-efficiency (EQE) of ∼30% at 300-450 nm in wavelength, thereby enhancing the short-circuit-current-density (JSC) of ∼2.23 mA cm(-2) and the power-conversion-efficiency (PCE) of ∼1.08% (relatively ∼6.04% increase compared with the reference without QDs for p-type silicon solar-cells). In particular, the PCE peaked at a specific coating thickness of the Cd0.5Zn0.5S-ZnS core-shell QD layer; i.e., the 1.08% PCE enhancement at the 8.8 nm thick QD layer.
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Here we review our work on incorporating high refractive index inorganic materials into microstructures by two-photon lithography. We describe the direct writing of silver microstructures from dispersions of silver salt solutions, as well as the fabrication of silver nanoparticle embedded microstructures by a combination of UV exposure and thermal curing. The last section summarizes the functionalization of semiconductor quantum dots for their incorporation into microstructures. These materials pave the way for new investigations into photonic properties of hybrid inorganic nanomaterial incorporated microstructures.
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Hybrid nanowires (NWs) of light-emitting poly(3-hexylthiophene) (P3HT) blended with gold nanoparticles (Au-NPs) were fabricated by a wetting method. The functionalized CdSe/ZnS quantum dots (QDs) were attached to the surfaces of P3HT/Au-NPs NWs. The nanoscale photoluminescence (PL) characteristics of the P3HT, P3HT/Au-NPs, and QDs/P3HT/Au-NPs single NWs were investigated using a high resolution laser confocal microscope (LCM). For a P3HT/Au-NPs single NW, the LCM PL intensity of the P3HT NW decreased due to the luminescence quenching effect by the blending with Au-NPs. However, the LCM PL intensity of the P3HT/Au-NPs NW drastically increased when the QDs were attached to the surface of the NW. The PL enhancement of the P3HT NW part in the hybrid QDs/P3HT/Au-NPs single NW originated from the Förster resonance energy transfer (FRET) effect between the QDs and the P3HT NW, which was assisted by the surface plasmon (SP) coupling of Au-NPs with the QDs. Based on the analysis of time-resolved PL spectra, the exciton lifetimes of the QDs for the QDs/P3HT/Au-NPs NW were found to decrease considerably in comparison with those of the QDs/P3HT NWs without Au-NPs. We also found that the energy transfer rate of the QDs/P3HT NW increased from 0.76 to 0.93 with the Au-NPs. These observations support the notion of SP assisted FRET effect in hybrid nanosystems.
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The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.
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We demonstrate that the optical and photoresponsive electrical properties of a single nanowire (NW) consisting of a p-type poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were changed by hybridization with functionalized CdSe/ZnS QDs. Surface decorating and bulky infiltrating methods were employed for the hybridization of the QDs with the NWs. For the QD-infiltrated P3HT/PCBM single NW, the current density in both dark and light conditions was clearly enhanced and the nanoscale photoluminescence (PL) intensity was reduced due to the charge transfer effect. For the P3HT/PCBM NWs decorated with QDs, the current density was not changed much, however, the PL characteristics of both the QDs and the NWs were simultaneously changed by the energy transfer effect. From time-resolved PL spectra, the exciton lifetimes of the QDs in the hybrid NWs drastically decreased through the hybridization with the NWs, supporting the charge and/or energy transfer effects.
Article
Reported here is the programmable assembly of CdTe quantum dots (QDs) into various pre-designed microstructures by using a femtosecond laser direct writing (FsLDW) technique. As nanobuilding blocks, CdTe QDs could be driven by a focused femtosecond laser beam to construct arbitrary-shape micropatterns with high resolution (~170 nm). The optical properties of pristine CdTe QDs have been well inherited after the FsLDW induced programmable assembly, which has been confirmed by the luminescence spectrum and the high resolution transmission electron microscope (HR-TEM) image of the assemblies. By using this technique, the CdTe QDs microstructures have been integrated within a microfluidic devices, and it shows the capability of on-chip detection of heavy metal ions qualitatively. The FsLDW induced assembly of QDs may open up a new way for designable assembly of QDs towards flexible fabrication and integration of QDs-based microdevices.
Article
Hybrid nanosystems comprising functionalized CdSe/ZnS core–shell quantum dots (QDs) on the surface of light-emitting poly(3-hexylthiophene) (P3HT), metallic copper (Cu), and insulating polystyrene (PS) nanowires (NWs) are fabricated. Using high-resolution scanning transmission electron microscopy, we observe that the QDs are attached to the surface of the NWs. The nanoscale photoluminescence (PL) characteristics of the hybrid QD/P3HT, QD/Cu, and QD/PS single NWs are investigated using laser confocal microscopy (LCM) with high spatial resolution. For the hybrid QD/P3HT single NW, the LCM PL intensity from the P3HT NW increases considerably, while that of the QDs decreases due to Förster resonance energy transfer. Hybridization affects the nanoscale PL characteristics of both the P3HT NW and the QDs. The LCM PL intensity of the hybrid QD/Cu NW is three times higher than that of the QD/PS NW, because of surface plasmon resonance coupling energy transfer between the QDs and the Cu NW. Time-resolved PL spectra reveal that the exciton lifetimes of the QDs drastically decrease after the hybridization with P3HT or Cu NWs, due to energy transfer effects. The nanoscale PL efficiency of the QDs can be controlled by hybridization with NWs having distinct properties.
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A facile method for the fabrication of Quantum Dot/poly(N-isopropylacrylamide) (QD/PNIPAM) micropatterns on glass is presented. Water-soluble QDs functionalized with polymerizable methacrylic groups effectively copolymerize with N-isopropylacrylamide monomer resulting in stable hybrid organic–inorganic networks. The patterns display temperature and pH responsive luminescence behaviour.
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We report on the first demonstration of the coupling of fully confined electrons and photons using a combination of three-dimensional photonic crystal nanocavities and quantum dots. The three dimensional photonic crystals were assembled by stacking planar components using a sophisticated micromanipulation technique. Point defects, containing embedded quantum dots, were introduced into the photonic crystals as active sites. By measuring the photoluminescence spectra of the assembly, the process by which light emitted from the quantum dots is coupled to the defect modes of a three dimensional photonic crystal was demonstrated for the first time. The characteristics of the sharp emission peaks agreed well with numerical simulations, and these were confirmed to be resonant modes by polarization measurements. The highest quality factor (Q-factor) for three dimensional photonic crystals (2,300) was achieved.
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The investigation of the photodetection efficiency of a polymeric nanocomposite containing IR-active PbSe quantum dots (QD) and conducting single walled carbon nanotube (SWNT) is discussed. A stable SWNT PbSe conjugate is efficient for the realization of efficient optoelectronic behavior. Efficient dissociation and charge transfer depends on the differences in the potential energy and electron affinity between the photoactive species and the other components. The transfer of photogenerated holes to the polymer also proceeds without the requirement for any activation energy, as the ionization potential of the polymer lies closer to value for a vacuum than that of the QDs. Transmission electron microscopy (TEM) images of the SWNT-PbSe conjugate shows that a large number of PbSe QDs bind to the terminal thiol groups of the SWNTs. Efficient charge separation and transport is achieved by incorporating SWNT-PbSe conjugates, in which the QDs are attached to the nanotubes.
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Fluorescent semiconductor nanocrystals (quantum dots) have the potential to revolutionize biological imaging, but their use has been limited by difficulties in obtaining nanocrystals that are biocompatible. To address this problem, we encapsulated individual nanocrystals in phospholipid block-copolymer micelles and demonstrated both in vitro and in vivo imaging. When conjugated to DNA, the nanocrystal-micelles acted as in vitro fluorescent probes to hybridize to specific complementary sequences. Moreover, when injected into Xenopus embryos, the nanocrystal-micelles were stable, nontoxic (<5 x 10(9) nanocrystals per cell), cell autonomous, and slow to photobleach. Nanocrystal fluorescence could be followed to the tadpole stage, allowing lineage-tracing experiments in embryogenesis.
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Quantum dots (QDs) have been increasingly used in biolabeling recently as their advantages over molecular fluorophores have become clear. For bioapplications QDs must be water-soluble and buffer stable, making their synthesis challenging and time-consuming. A simple aqueous synthesis of silica-capped, highly fluorescent CdTe quantum dots has been developed. CdTe QDs are advantageous as the emission can be tuned to the near-infrared where tissue absorption is at a minimum, while the silica shell can prevent the leakage of toxic Cd(2+) and provide a surface for easy conjugation to biomolecules such as proteins. The presence of a silica shell of 2-5 nm in thickness has been confirmed by transmission electron microscopy and atomic force microscopy measurements. Photoluminescence studies show that the silica shell results in greatly increased photostability in Tris-borate-ethylenediaminetetraacetate and phosphate-buffered saline buffers. To further improve their biocompatibility, the silica-capped QDs have been functionalized with poly(ethylene glycol) and thiol-terminated biolinkers. Through the use of these linkers, antibody proteins were successfully conjugated as confirmed by agarose gel electrophoresis. Streptavidin-maleimide and biotinylated polystyrene microbeads confirmed the bioactivity and conjugation specificity of the thiolated QDs. These functionalized, silica-capped QDs are ideal labels, easily synthesized, robust, safe, and readily conjugated to biomolecules while maintaining bioactivity. They are potentially useful for a number of applications in biolabeling and imaging.
Article
This comprehensive, accessible resource abridges the ``Polymeric Materials Encyclopedia'', presenting more than 1,100 articles and featuring contributions from more than 1,800 scientists from all over the world. The text discusses a vast array of subjects related to the: (1) synthesis, properties, and applications of polymeric materials; (2) development of modern catalysts in preparing new or modified polymers; (3) modification of existing polymers by chemical and physical processes; and (4) biologically oriented polymers.
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The polymer electrolyte distribution in PEMFC electrodes plays an important role for the catalyst utilization and various transport processes in the electrode. Moreover, its influence on the transport processes is not only limited to proton transport but it may also affect gas transport, electron conductivity and water management of the cell. However, experimental techniques to study the polymer electrolyte distribution are scarce. In this paper we present various approaches based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to characterize the polymer electrolyte distribution. The methods presented include staining of the polymer electrolyte with heavy metal ions, energy dispersive X-ray (EDX) mapping and energy filtered imaging (EFI). Their use for the analysis of the polymer electrolyte distribution and electrode structure will be presented and current limitations of these techniques will be discussed.
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In recent years considerable efforts have been put forth in two-photon initiated polymerization (TPP)-based two- and three-dimensional (2D and 3D) nano/microfabrication for the development of novel nano/microdevices. In less than two decades of research, TPP has been employed in the fabrication of a great number of diverse micro-objects through the use of a wide variety of effective two-photon absorbing chromophores. In TPP, when a near-infrared ultrashort-pulsed laser is closely focused into a volume of photocurable resins, real 3D microstructures can be fabricated using a layer-by-layer accumulating technique making it a promising technique for 3D nano/microfabrication. More recently, a spatial resolution of sub-100nm scale was achieved with TPP by employing a radical quenching mechanism. There also have been many studies that aimed at improving the fabrication efficiency and precision of TPP. Focus of ongoing research is the development of efficient two-photon absorbing chromophores. In this article, we discuss recent efforts in developing two-photon absorbing chromophores with focus on their structure property relationship and some recent outstanding attempts at improving the fabrication efficiency of 3D nano/microfabrications based on TPP.
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ELECTROLUMINESCENT devices have been developed recently that are based on new materials such as porous silicon1 and semiconducting polymers2,3. By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals4-6, and of electroluminescent polymers7, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV)8-10 with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage11 of only 4V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect19-24 the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.
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The effects of thermal annealing in Si base p-n diode with self-assembled Ge dots stacked in eight layers structure are investigated. The effects of annealing are discussed based on the photovoltage spectra, the PL spectra and the Raman spectra. Three main effects occur after thermal annealing: the reduction of point defects, the intermixing of Si-Ge and the strain relaxation. The experimental result shows that 800 degrees C might be a suitable annealing temperature for photovoltaic applications. (c) 2007 Elsevier B. V. All rights reserved.
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A simple synthetic route for the preparation of high-quality CdSe/ZnSe core/shell nanocrystals without the use of any pyrophoric organometallic precursors is presented. Effective surface passivation of monodisperse CdSe nanocrystals is achieved by overcoating them with a ZnSe shell, applying zinc stearate as a zinc source. The resulting core/shell nanocrystals exhibit high room temperature photoluminescence efficiencies (60−85%) in organic solvents as well as in water after functionalization with mercaptoundecanoic acid (MUA).
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A novel method for effective incorporation of Quantum Dots (QD) into poly(N-isopropylacrylamide) (PNIPAM) polymeric matrix by water-based copolymerization was investigated. The method used was a inverse miniemulsion photopolymerization using a modified sorbitol surfactant Span 80, which provided high quality composite microspheres. QDs were obtained by pyrolysis of the organometallic precursors at high temperature, yielding nanoparticles with hydrophobic ligands coatings. The availability of precisely engineered designer nanocomposites with stimulus responsive polymers is going to support the ultimate integration of QDs in future. A novel, designer polymeric coating for hydrophobic QDs was prepared and used to transfer the QDs from nonpolar to aqueous medium. The analysis has also explored the thermoresponsive PNIPAM/QD assemblies in context with their applications in thermometry, biosensing, temperature modulated bioimaging, and as temperature-controlled drug delivery systems.
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We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 Å. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the growth of ZnS on CdSe cores as locally epitaxial and determine how the structure of the ZnS shell influences the photoluminescence properties.
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Sum frequency generation (SFG) vibrational spectroscopy is applied to study the surface characteristics of a film composed of nanoscale TiO2 particles, in which methanol is employed as a molecular probe. The SFG signal from methanol adsorbed on the TiO2 film surface in the CH region consists of four primary peaks located at 2828, 2855, 2935, and 2968 cm-1. The peaks at 2855 and 2968 cm-1 arise from molecular methanol; the other two are attributed to methoxy produced by dissociative chemisorption of methanol on TiO2. Changes in the SFG features with methanol pressure and with surface temperature provide the first direct proof of methoxy generation on nanoparticulate films. In comparison with molecular methanol, methoxy sits more perpendicularly on the surface. The chemisorption is barrierless, and a Langmuir constant of (2.13 ± 0.14) × 103 is derived from the adsorption isotherm. Moreover, UV pretreatment of the TiO2 film substantially increases the methoxy signal. Changes in SFG intensity are in good agreement with the current models for surface active sites and face specificities on TiO2.
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We report our systematic studies of the environmental effects on the photoluminescence (PL) from colloidal CdSe nanocrystals (NCs) embedded in polymer thin films. The highly luminescent bare-core CdSe NCs were significantly more robust than the core/shell CdSe/ZnS NCs against photoannealing under inert environments, while the core/shell NCs are more resistant to photooxidation. For the case of bare-core NCs, we show the importance of the initial photoactivation as the proper treatment for subsequent studies, a step that is basically irrelevant in the case of core/shell NCs. By measuring the PL wavelength shift, PL line width, and PL intensity, we investigated the dynamic changes of the emission properties of these NCs under different environments, including argon, oxygen, air, water vapor, and wet oxygen, and under different excitation conductions to reveal the photoinduced nature of the interactions between the nanocrystal surface and the environment. Mechanisms related to photoactivation, photooxidation, and PL enhancement are briefly discussed.
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The linear absorption cross section is a crucial parameter to the design of nanocrystal quantum dot devices and to the interpretation of spectroscopic data. We measure and report the size-dependent absorption cross section of CdSe nanocrystal quantum dots. We compare the results for absorption far above the band edge, where the quantum dot density of states may be approximated as a continuum, to simple theoretical models of light scattering from light-absorbing small particles. Excellent agreement with theory is found for dilute dispersions in hexane. We find that for absorption at 350 nm the per particle absorption cross section Cabs (in cm2) for CdSe is Cabs = (5.501 × 105)a3 cm-1, where a is the particle radius in cm. The absorption cross section is observed to be largely insensitive to the solvent refractive index. Detailed modeling of the effect of the ligand shell may be necessary to understand the lack of sensitivity of the absorption properties of nanocrystal quantum dots to the refractive index of the medium.
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Colloidal platinum (diameter 1−2 nm) in styrene was prepared in situ by reduction of a platinum(II) compound followed by partial evaporation of styrene. The resulting dispersions were purified from the reaction side-products, and ammonium O,O‘-dialkyldithiophosphates of different chain length or octadecanethiol was added. Polymerization was started with a radical initiator. Several parameters were varied and, particularly in the presence of long-chain dialkyldithiophosphates, self-assembled superstructures of metal colloids appeared at certain conditions in the resulting poly(styrene)-platinum nanocomposites. Unusual hollow shell structures of typical diameters of 50−300 nm were found, which phenomenologically resembled bilayer vesicles in aqueous solutions, although the formation mechanism of the bilayers and the superstructures in the nanocomposites is considered to differ. It is supposed that the formation of the superstructures in the nanocomposites is induced by crystallization of alkyl chains adsorbed at the platinum surface below monolayer coverage.
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We show that the luminescence from CdSe quantum dot monolayers can be strongly influenced by the interaction of water molecules adsorbed on the surface. Light-induced alterations in the surface states following adsorption of water, results in quasi-reversible luminescence changes in the quantum dot. The excitonic QY increases by a factor of 20 during the first 200 s of illumination in air (post vacuum) and then steadily decreases to a level 6 times that of the vacuum reference after 5000 s. The exciton emission exhibits an exponential blue shift of nearly 16 nm (60 meV) over 1 h of illumination. During this time, the line width decreases by 10% during the first 100 s and then slowly increases to 96% of the vacuum reference line width after 5000 s. Our model suggests that water molecules adsorbed on the surface of the quantum dot act to passivate surface traps, which results in increased luminescence, similar to an effect well-known for bulk CdSe surfaces. In addition, adsorbed water molecules act to oxidize the surface of the quantum dot, which results in the blue shift of the exciton emission and eventually introduces new surface defects that lower the luminescence. It is the competition between these two processes that is responsible for the complex kinetics of the luminescence QY.
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A method is presented for determining the residual unsaturations in copolymers of methyl methacrylate with glycol dimethacrylates, and the fraction of dimethacrylate molecules which have reacted at both ends (the cross-linking efficiency) is calculated from the result. Residual unsaturations are determined from a comparison of the observed volume contractions per mole of double bond with the corresponding contractions for complete conversion of monomer to polymer. In the several instances where comparisons were made, the results are shown to be in satisfactory agreement with residual unsaturations determined by infrared analyses. The efficiencies are found to depend markedly on the concentration and structure of the dimethacrylate and are generally less than one. For copolymers containing dimethacrylates of ethylene glycol, decamethylene glycol or 2-ethyl-2-butylpropanediol-1,3, the efficiency is limited by geometrical restrictions on the motions of unsaturated vinyl groups attached to the cross-linked network. An approximate theory is presented for this case which satisfactorily accounts for the observed dependence of the efficiency on the degree of cross-linking and on the length of the cross-linker. For the dimethacrylate of 2,2-dimethylpropanediol the efficiency is limited by the occurrence of glass formation in the polymerizing system. Only a qualitative interpretation of the data was possible in this case. It is indicated that high efficiency in a cross-linking reaction may be obtained by employing (1) a divinyl molecule with a very long flexible chain between vinyl- groups and (2) a polymerization temperature which exceeds the second-order transition temperature of the cross-linked product.
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The extinction coefficient per mole of nanocrystals at the first exitonic absorption peak, ε, for high-quality CdTe, CdSe, and CdS nanocrystals was found to be strongly dependent on the size of the nanocrystals, between a square and a cubic dependence. The measurements were carried out using either nanocrystals purified with monitored purification procedures or nanocrystals prepared through controlled etching methods. The nature of the surface ligands, the refractive index of the solvents, the PL quantum yield of the nanocrystals, the methods used for the synthesis of the nanocrystals, and the temperature for the measurements all did not show detectable influence on the extinction coefficient for a given sized nanocrystal within experimental error.
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An approach based on orthogonal self-assembly strategies for obtaining crosslinked arrays of nanoparticles with controlled morphologies was presented. In this approach double-stranded DNA served as a template for the interaction of psoralene functionalized gold nanoparticles, which were subsequently crosslinked to the template via a photochemical transformation. The CdSe nanoparticles modified with reactive vinylbenzene functionalities assembled at the oil/water interface and subsequent heating afforded robust, extended nanoparticle sheets. The approach was basically used for producing complex one and two dimensional (2D) nanoparticle morphologies in a controlled fashion and achieving mild crosslinking conditions.
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A facile strategy for the in situ synthesis and fabrication of 3D semiconductor-polymer nanocomposite microstructures was presented. The size of the in situ synthesized CdS nanoparticles (NP) within the polymer matrix is confined by the space available within the crosslinking polymer network. The polymer network density has a significant effect on the CdS nanoparticle size, which is verified by designing a series of photopolymerizable resins by changing the crosslinker content. It is also found that the emission band arises from the near-band-edge emission of CdS NPs and the blue-shift of emission peaks occurs due to quantum size effect in CdS NPs. The decrease in the average size of in situ synthesized NPs from 6 to 2.6 nm upon increasing the crosslinker content from 0 to 48.7 wt% is also confirmed. The presence of a crosslinker with multifunctional end groups strongly affects the size of CdS NPs by making tight polymer networks.
Article
We describe the synthesis of water-soluble semiconductor nanoparticles and discuss and characterize their properties. Hydrophobic CdSe/ZnS core/shell nanocrystals with a core size between 2 and 5 nm are embedded in a siloxane shell and functionalized with thiol and/or amine groups. Structural characterization by AFM indicates that the siloxane shell is 1-5 nm thick, yielding final particle sizes of 6-17 nm, depending on the initial CdSe core size. The silica coating does not significantly modify the optical properties of the nanocrystals. Their fluorescence emission is about 32-35 nm fwhm and can be tuned from blue to red with quantum yields up to 18%, mainly determined by the quantum yield of the underlying CdSe/ZnS nanocrystals. Silanized nanocrystals exhibit enhanced photochemical stability over organic fluorophores. They also display high stability in buffers at physiological conditions (>150 mM NaCl). The introduction of functionalized groups onto the siloxane surface would permit the conjugation of the nanocrystals to biological entities.
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We demonstrate a single-step synthetic method for highly luminescent (i.e., quantum yield up to 80%) and stable quantum dots (QDs) by using the reactivity difference between Cd and Zn precursors and that between Se and S precursors. A wide range of emission wavelengths (500-610 nm) with a narrow fwhm (<35 nm) is obtained by changing the ratios of the precursors. Under the reaction conditions selected, Cd-and Se (with a bit of S)-based cores are formed first and Zn-and S-based shells are formed successively; therefore, the QDs have a core/shell structure with composition gradients, which relieve the lattice mismatch between core and shells. The QDs are characterized using the combined techniques of HR-TEM, UV–vis, PL spectroscopy, and ICP-AES. The QDs also have energy gradients depending on their compositions in a radial direction, which energetically confine carriers (electrons and holes) to the cores. This leads to the stability of QDs during their surface passivation from oleic acid to mercaptopropionic acid and ensures their processibility for further purposes such as optoelectronic and biological applications.
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We report a newly synthesized inorganic polymer photoresist with a high ceramic yield by the functionalization of polyvinyl-silazane (KiON VL20) with 2-isocyanatoethyl methacrylate via linkage or insertion reaction routes. The chemistry of the synthesis and the pyrolytic conversion as well as the mechanical evaluation were investigated by using various analytical instruments. We show for the first time that this photosensitive resin is a novel precursor for the fabrication of complex 3D SiCN ceramic microstructures with a 210 nm resolution via a two-photon absorbed crosslinking process and subsequent pyrolysis at 600 degrees C under a nitrogen atmosphere. Moreover, the dimensional deformation during pyrolysis was significantly reduced by adding silica nanoparticles as a filler. In particular, the ceramic microstructures containing 40 wt % silica nanoparticles exhibited a relatively isotropic shrinkage owing to its sliding free from the substrate during pyrolysis.
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Electron energy-loss spectroscopy (EELS) and elemental imaging under the energy-filtered transmission electron microscope are powerful tools for the characterization of iron-rich particles present in natural waters. Features present in EEL spectra (Fe-M2,3 Fe-L2,3 and O-K ionization edges) of goethite (α-FeOOH) have been studied with an energy filter operated at 80 keV to determine optimal quantification and elemental imaging of Fe-rich natural aquatic particles in the 30–200 nm range of thickness. For quantitative aims, the Fe-M2,3 ionization edge cannot be used easily, but the Fe-L2,3 edge provides more accurate results owing to a better background extrapolation. The partial cross-section of the Fe(III) M shell has been determined for iron oxide. The use of two-windows (jump-ratio) and three-windows (background stripping) imaging methods is discussed in relation to the specimen thickness.
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Uniaxially oriented composites of high-density polyethylene and silver nanoparticles were prepared using solution-casting, melt-extrusion and solid-state drawing techniques. The absorption spectrum in the visible wavelength range of the drawn nanocomposites was observed to strongly depend on the polarisation direction of the incident light. For instance, the nanocomposites appear bright yellow or red when the vibration direction of linearly polarised light is perpendicular or parallel, respectively, to the drawing axis. The optical anisotropy of the drawn nanocomposites originates from uniaxially oriented, pearl-necklace type of arrays of nanoparticles of high aspect ratios. The absorption spectrum of the nanocomposites can be shifted to higher wavelengths using appropriate annealing procedures. The annealing results in an increased size of the primary silver particles, due to Ostwald ripening, and consequently a range of polarisation-dependent colours can be generated in the drawn nanocomposites.
Article
We have designed and synthesized a multidentate polymer ligand used for water-solubilization of luminescent quantum dots (QDs). The synthesis of the multidentate ligand (PAA-g-MEA) was based on several thiol groups grafted to a linear polymer chain through a simple carboxy-amine coupling reaction between poly(acryl acid) (PAA) and mercaptoethylamine (MEA). Water-soluble QDs capped with these PAA-g-MEA ligands were prepared via ligand exchange from the original hydrophobic ones. The resulting PAA-g-MEA capped water-soluble QDs with relatively small hydrodynamic diameters possess higher photoluminescence quantum yields than the initial hydrophobic QDs, extraordinary stability over extended periods of time and over a broad pH range (3-14), salt concentrations (up to saturated NaCl solution), and thermal treatment at 100 degrees C.
Article
To combine the optical properties of CdSe@ZnS quantum dots (QDs) with the electrical properties of semiconducting polymers, we prepared QD/polymer hybrids by grafting a block copolymer (BCP) containing thiol-anchoring moieties (poly(para-methyl triphenylamine-b-cysteamine acrylamide)) onto the surfaces of QDs through the ligand exchange procedure. The prepared QD/polymer hybrids possess improved processability such as enhanced solubility in various organic solvents as well as the film formation properties along with the improved colloidal stability derived from the grafted polymer shells. We also demonstrated light-emitting diodes based on QD/polymer hybrids, exhibiting the improved device performance (i.e., 3-fold increase in the external quantum efficiency) compared with the devices prepared by pristine (unmodified) QDs.
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An epoxy-based negative-tone photoresist, which is known as a suitable material for high-aspect-ratio surface micromachining, is functionalized with red-light-emitting CdSe@ZnS nanocrystals (NCs). The proper selection of a common solvent for the NCs and the resist is found to be critical for the efficient incorporation of the NCs in the epoxy matrix. The NC-modified resist can be patterned by standard UV lithography down to micrometer-scale resolution, and high-aspect-ratio structures have been successfully fabricated on a 100 mm scaled wafer. The “as-fabricated”, 3D, epoxy-based surface microstructures show the characteristic luminescent properties of the embedded NCs, as verified by fluorescence microscopy. This issue demonstrates that the NC emission properties can be conveniently conveyed into the polymer matrix without deteriorating the lithographic performance of the latter. The dimensions, the resolution, and the surface morphology of the NC-modified-epoxy microstructures exhibit only minor deviations with respect to that of the unmodified reference material, as examined by means of microscopic and metrologic investigations. The proposed approach of the incorporation of emitting and non-bleachable NCs into a photoresist opens novel routes for surface patterning of integrated microsystems with inherent photonic functionality at the micro- and nanometer-scale for light sensing and emitting applications.
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Quantum dots (QDs) are a novel class of inorganic fluorophores, which are gaining widespread recognition as a result of their exceptional photophysical properties. They are rapidly being integrated into existing and emerging technologies, and could play an important role in many areas in the future. Significant phenomena, such as photoactivation, are still unknown and must be understood and more fully defined before they can be widely validated. This review provides an overview of the photoactivation process of quantum dots in a systematic way, covering QD characteristics, solubilisation strategies, and a description of different photoactivation mechanisms, depending on the type of QDs and their surrounding environment.
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The polymerization shrinkage of a range of poly(n-alkyl methacrylates) in the range C1 to C16, some of their isomers and the polymers of cyclic and heterocyclic methacrylates have been measured using densitometry. The percentage volume shrinkage decreases with the size of the substituent side group. This proved to reflect the fact that the change in molar volume on polymerizing a methacrylate ester is reasonably constant at 22 cc/mol irrespective of the geometry of the substituent group. However, the glass transition temperature of the polymer depends very critically on the geometry of the side group. Hence one method for the development of low shrinkage glassy polymers is to investigate methacrylate esters of large molar volume, but with side group geometry that results in a high glass transition temperature. Polymerization shrinkage can be used to determine the degree of conversion of a polymer, using the value of 22 cc/mol as the change in molar volume.
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The aim of this study was to investigate the polymerization contraction and the conversion of light-curing methacrylate resins based on bisphenol-A bis(2-hydroxypropyl)methacrylate (BisGMA) diluted with triethylene glycol dimethyacrylate (TEGDMA), methyl methacrylate (MMA), hydroxypropyl methacrylate (HPMA) or (+/-)-2-ethylhexyl methacrylate (EHMA). The contraction measurements were carried out with a linometer, a simple device to determine true linear polymerization contraction of liquid monomers at ambient temperature. The contraction increased with the amount of diluting monomer. The estimated conversion of the BisGMA-TEGDMA, calculated using the contraction, is consistent with literature values. The BisGMA-HPMA mixtures showed high conversions at moderate contraction.
Article
CdSe nanocrystals in solution and films can enter a metastable state in which the highly luminescent nanocrystals become dark. This change, which we attribute to a surface transformation, can be caused by heating or by changing the environment of the nanocrystals at room temperature. The metastable transformation is reversed upon illumination of above-band-gap light, at which point the nanocrystals are again highly luminescent.
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The analytical sensitivity in terms of the signal-to-noise ratio (SNR) was investigated for elemental mapping by a transmission electron microscope equipped with an energy filter (EF-TEM) and a scanning transmission electron microscope with an X-ray energy dispersive spectrometer (STEM-XEDS). To compare the detectability limits of the elemental maps by the two techniques, homogeneous Cu-0.98+/-0.34 wt% Mn and Cu-4.93+/-0.49 wt% Mn thin specimens were used. Elemental maps can be considered as either an image or a spectrum. Therefore, the detectability limits of the elemental maps were characterized by the spectral SNR. To evaluate the detectability limits of the elemental maps with statistical confidence limits such as 1 sigma, 2 sigma and 3 sigma, the SNR values were reviewed from the statistical point of view. In STEM-XEDS mapping, the spectral SNR values improve as the specimen thickness increases since the signal intensity increases. Conversely, the spectral SNR in EF-TEM mapping is maximized at a certain thickness and then reduces as the thickness increases. To compare the two mapping techniques with regard to the analytical sensitivity, a method to estimate the minimum mass fraction (MMF) from measured signal and background intensities was developed. In this experimental approach, the MMF value can be evaluated by selecting the appropriate SNR value corresponding to the statistical confidence limits. In comparing the estimated MMF values from the two mapping approaches, EF-TEM mapping can be more sensitive than STEM-XEDS mapping up to specimen thicknesses <20-30 nm in the 1 sigma confidence limit and < approximately 50 nm in the 3 sigma limits. However, as the specimen thickness increases, the XEDS maps provide better detectability limits in the Cu-Mn dilute alloy specimens.
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Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240 degrees C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy, TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed, some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells.
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We report on the preparation and structural characterization of CdSe nanocrystals, which are covered by a multishell structure from CdS and ZnS. By using the newly developed successive ion layer adhesion and reaction (SILAR) technique, we could gradually change the shell composition from CdS to ZnS in the radial direction. Because of the stepwise adjustment of the lattice parameters in the radial direction, the resulting nanocrystals show a high crystallinity and are almost perfectly spherical, as was investigated by X-ray diffraction and electron microscopy. Also, due to the radial increase of the respective valence- and conduction-band offsets, the nanocrystals are well electronically passivated. This leads to a high fluorescence quantum yield of 70-85% for the amine terminated multishell particles in organic solvents and a quantum yield of up to 50% for mercapto propionic acid-covered particles in water. Finally, we present experimental results that substantiate the superior photochemical and colloidal stability of the multishell particles.