Nanocrystalline Quantum Dots Research Articles

Graphene Oxide Decorated Tin Sulphide Quantum Dots for Electrochemical Detection of Dopamine and Tyrosine

The current study highlights the design and construction of a sensitive and selective sensor for detection of dopamine and tyrosine using a GO-SnS2 quantum dots by a drop casting method on glassy carbon electrode. Highly porous nano-crystalline GO-SnS2 quantum dots were synthesized by using ultrasonication followed by hydrothermal method in a facile manner. XRD, SEM, XPS, TEM, and pore size distribution techniques were used to characterize the quantum dots that were produced. The newly fabricated electrode was evaluated for EIS (Electrochemical impedance spectroscopy), CV (cyclic voltammetry) and chronoamperometric methods. The observed limit of detection of dopamine was observed to be 26 nM. High selectivity and sensitivity were observed for electrochemical detection of dopamine and tyrosine.

Effect of anti-reflection coating on the performance of silicon solar cells with nanocrystalline quantum dots downshifting film

This experiment explores the effect of lithium fluoride (LiF) antireflection coating (ARC) on the performance of commercial silicon solar cells with mist deposited nanocrystalline quantum dots (NQD) downshifting film. The effect of blanket deposited and patterned NQD and ARC films is investigated. Significant improvement of the performance of cells with ARC is observed. Specifically, 67% increase in the average external quantum efficiency of silicon solar cells covered with AR coating in the UV range of 300–400 nm is noted. The micropatterning of NQD/LiF films improves light trapping inside the cell and enhances power conversion efficiency (PCE) of the cell by 19.5%. Overall, this experiment demonstrates that the ARC formation on top of the mist deposited NQD downshifting film results in measurable improvement in the performance of the commercial silicon solar cells.

Evaluation of the optical absorption/emission spectral behavior of the Cd1-xDyxSe quantum dots synthesized via cation exchange approach

Cd1-xDyxSe nanocrystalline quantum dots were synthesized via cation exchange approach for the first time. The as-synthesized Cd1-xDyxSe nanocrystals showed a cubic phase at all Dy ion contents. The formed nanoparticles of the as-synthesized Cd1-xDyxSe nanocrystals were spherical like shape and their diameters increased from 2.1–7.3 nm with increasing the amount of Dy ions. The doping of CdSe with Dy atoms comprises a read shift of the optical absorption spectral curves. Therefore, a decrease of the optical band gap from 2.58 eV to 2.15 eV owing to the increase of Dy atoms took place. The partial substitute of the Cd atoms by Dy atoms instigated remarkable enhancing of the of luminescent intensities by eight order of magnitudes, made the PL peaks more sharper by four orders and achieved enlargement of the quantum yield to reach to value of 93 %. A decrease of the Stokes shift due to the doping by the Dy atoms was reasoned to the passivation of the defects. The enlargement of luminescence intensity, the creation of narrow emission band and the high quantum yield value empowered the as-synthesized Cd1-xDyxSe nanocrystals to be employed as laser materials for the manufacturing of multicolor laser diodes.

Synthesis and characterization of Zn1-xAcxS (0≤x≤0.1) nanocrystalline quantum dots prepared via soft mechanochemical approach

Here, we report a first example of nanocrystalline Zn1-xAcxS (0 ≤ x ≤ 0.1) quantum dots prepared via soft mechanochemical approach. The incorporation of up to 10 at.% of Ac ions into the ZnS quantum dots (QDs) allows preservation of the cubic crystal structure as was confirmed by X-ray diffraction and the selected area electron diffraction. The transmission electron microscopy demonstrated that the increase of the Ac ion doping give rise to the increase of the particle size from 4.8 ± 0.3 nm to 17.9 ± 0.3 nm. The energy dispersive X-ray spectroscopy measurements indicated that the Zn1-xAcxS powders are stoichiometric at all Ac ion substitutions. The substitution of Zn ions by Ac ions in the Zn1-xAcxS nanocrystals was also verified by the Raman and FTIR spectroscopy measurements. The incorporation of the Ac ions into the ZnS QDs showed a red shift of the optical absorption and photoluminescence (PL) spectra. An enhancement of the PL intensity and a decrease of the PL spectrum width were observed due to the partial substitution by the Ac ions in the Zn sites of the ZnS nanocrystals. These properties make the Zn1-xAcxS QDs promising candidates for both light emitting and laser diodes applications.

Performance of Single-Crystal Silicon Solar Cells With Mist-Deposited Nanocrystalline Quantum Dot Downshifting Films

The paper is concerned with an investigation of the effect of the mist-deposited downshifting nanocrystalline quantum dots (NQD) films both continuous and patterned on the performance of single-crystal commercial silicon solar cells. Different thicknesses of the NQD films are explored in an attempt to improve cell's efficiency in the ultraviolet (UV) range (below 400 nm) of the solar spectrum. The results obtained indicate that the maximum UV absorption is observed with a 50-nm-thick NQD film as reflected by on the average 45% increase in UV external quantum efficiency as compared with the solar cell without the NQD film. The patterned NQD films are showing improved performance in the visible range as compared with unpatterned NQD films without compromising the performance of the cell in the UV range of the solar spectrum. Also, an increase in power conversion efficiency by 12.5% is observed. Overall, an improvement of silicon solar cells performance using mist deposition of downshifting NQD films instead of spin-on coating as previously explored in other studies is demonstrated.

Mesoscopic Oxide Double Layer as Electron Specific Contact for Highly Efficient and UV Stable Perovskite Photovoltaics.

The solar to electric power conversion efficiency (PCE) of perovskite solar cells (PSCs) has recently reached 22.7%, exceeding that of competing thin film photovoltaics and the market leader polycrystalline silicon. Further augmentation of the PCE toward the Shockley-Queisser limit of 33.5% warrants suppression of radiationless carrier recombination by judicious engineering of the interface between the light harvesting perovskite and the charge carrier extraction layers. Here, we introduce a mesoscopic oxide double layer as electron selective contact consisting of a scaffold of TiO2 nanoparticles covered by a thin film of SnO2, either in amorphous (a-SnO2), crystalline (c-SnO2), or nanocrystalline (quantum dot) form (SnO2-NC). We find that the band gap of a-SnO2 is larger than that of the crystalline (tetragonal) polymorph leading to a corresponding lift in its conduction band edge energy which aligns it perfectly with the conduction band edge of both the triple cation perovskite and the TiO2 scaffold. This enables very fast electron extraction from the light perovskite, suppressing the notorious hysteresis in the current-voltage ( J-V) curves and retarding nonradiative charge carrier recombination. As a result, we gain a remarkable 170 mV in open circuit photovoltage ( V oc) by replacing the crystalline SnO2 by an amorphous phase. Because of the quantum size effect, the band gap of our SnO2-NC particles is larger than that of bulk SnO2 causing their conduction band edge to shift also to a higher energy thereby increasing the V oc. However, for SnO2-NC there remains a barrier for electron injection into the TiO2 scaffold decreasing the fill factor of the device and lowering the PCE. Introducing the a-SnO2 coated mp-TiO2 scaffold as electron extraction layer not only increases the V oc and PEC of the solar cells but also render them resistant to UV light which forebodes well for outdoor deployment of these new PSC architectures.

Effect of lift-off conditions on micropatterning of nanocrystalline quantum dot films

A micropatterning of nanocrystalline quantum dots (NQD) films using lift-off process has been proposed and experimentally demonstrated earlier. In this paper, the conditions of patterning process are investigated in order to define process parameters window for the lift-off procedure. The patterning of NQD CdSe/ZnS films using lift-off is studied in terms of photoresist thickness, solvent composition, and substrate material. The outcome of the process is monitored by means of fluorescence microscopy, atomic force microscope, and optical microscopy. The results show that patterns less than 1 μm can be resolved in NQD films with minimal alteration of the film characteristics, provided the photoresist layer is thicker than about 100 nm and acetone used as a solvent is spiked with toluene up to 10% of the solvent volume. The performance of the process in terms of resolution and film quality was found to be independent of the substrate used.

Composite Supraparticles with Tunable Light Emission.

Robust luminophores emitting light with broadly tunable colors are desirable in many applications such as light-emitting diode (LED)-based lighting, displays, integrated optoelectronics and biology. Nanocrystalline quantum dots with multicolor emission, from core- and shell-localized excitons, as well as solid layers of mixed quantum dots that emit different colors have been proposed. Here, we report on colloidal supraparticles that are composed of three types of Cd(Se,ZnS) core/(Cd,Zn)S shell nanocrystals with emission in the red, green, and blue. The emission of the supraparticles can be varied from pure to composite colors over the entire visible region and fine-tuned into variable shades of white light by mixing the nanocrystals in controlled proportions. Our approach results in supraparticles with sizes spanning the colloidal domain and beyond that combine versatility and processability with a broad, stable, and tunable emission, promising applications in lighting devices and biological research.

Fluorescence Microscopy Characterization of Patterned Nano-Crystalline Quantum Dot Films

This experiment is a continuation of the study concerned with the lift-off based patterning of nanocrystaline quantum dot (NQD) films [1]. The ultra-thin films formed using colloidal solutions of nanocrystalline quantum dots show good promise in display/LED, as well as nano-labeling applications between others. Regardless of application, to become a part of the functional device the NQD film must be patterned accordingly. Following on the experiments demonstrating effectiveness of the lift-off process in patterning NQD films with resolution down to 1 µm [1] further investigation of the performance of the proposed patterning scheme needed to be carried out. The goal of this contribution is to study uniformity of surface coverage with lift-off patterned NQD films by means of fluorescence microscopy. When exposed to UV light the NQDs feature high photoluminescence with intensity depending on the dots size. The fluorescence microscopy allows capturing the intensity to obtain information about the NQD film uniformity. As a result, using images obtained by means of fluorescence microscopy and employing dedicated image processing tools information regarding film uniformity, pattern definition accuracy, and average intensity can be derived. Also, the fluorescence image can be transferred into grey scale to study its histogram plot and get information about intensity distribution across the NQD patterns. In this study silicon wafers used as substrates are first covered by spin coating with a layer of positive 1805 photoresist to the thickness of about 420 nm and then cured at 100oC. The resist covered wafers were subsequently exposed through the standard Cr-photomask in an aligner/exposure and developed in photoresist developer (351) to create the required patterns [1]. After that, NQDs were blanked deposited using mist deposition technique. The NQD deposited were CdSe/ZnS core/shell featuring average diameter of 6.5 nm and corresponding red (620 nm) emission wavelength. Then, the photoresist removal performed using either dry or wet lift-off. The dry lift-off applied using O2 plasma system and the wet lift-off used acetone dissolution [2]. Figure 1 demonstrates the fluorescence image for NQD patterned film and its intensity distribution. The intensity analysis of the patterns after the lift-off process is considered in terms of an average intensity. The results obtained in this study give further insights into the process of lift-off patterning of nano-crytalline quantum dot films and demonstrate feasibility of the proposed approach. In the full account of this work a complete set of experimental results obtained in the course of this investigation is presented and more detailed analysis of fluorescence images provided.

Fluorescence Microscopy Characterization of Patterned Nano-Crystalline Quantum Dot Films

This experiment is a continuation of the study concerned with the lift-off based patterning of nanocrystaline quantum dot (NQD) films. The NQD films patterned by lift-off process are analyzed by means of fluorescence microscopy to obtain information about film uniformity, pattern definition accuracy, and average fluorescence intensity. The lift-off process is implemented using wet (acetone immersion) and dry (oxygen plasma) photoresist removing ambient. Methods under investigation were found to perform similarly in terms of pattern resolution accuracy. However, in terms of the uniformity of fluorescence intensity distribution, acetone dissolution of photoresist was found to produce better results.

Engineering of Semiconductor Nanocrystals for Light Emitting Applications.

Semiconductor nanocrystals are rapidly spreading into the display and lighting markets. Compared with liquid crystal and organic LED displays, nanocrystalline quantum dots (QDs) provide highly saturated colors, wide color gamut, resolution, rapid response time, optical efficiency, durability and low cost. This remarkable progress has been made possible by the rapid advances in the synthesis of colloidal QDs and by the progress in understanding the intriguing new physics exhibited by these nanoparticles. In this review, we provide support to the idea that suitably engineered core/graded-shell QDs exhibit exceptionally favorable optical properties, photoluminescence and optical gain, while keeping the synthesis facile and producing QDs well suited for light emitting applications. Solid-state laser emitters can greatly profit from QDs as efficient gain materials. Progress towards fabricating low threshold, solution processed DFB lasers that are optically pumped using one- and two-photon absorption is reviewed. In the field of display technologies, the exploitation of the exceptional photoluminescence properties of QDs for LCD backlighting has already advanced to commercial levels. The next big challenge is to develop the electroluminescence properties of QD to a similar state. We present an overview of QLED devices and of the great perspectives for next generation display and lighting technologies.

ChemInform Abstract: Interfacial Engineering for Quantum‐Dot‐Sensitized Solar Cells

AbstractReview: [109 refs.

Interfacial Engineering for Quantum-Dot-Sensitized Solar Cells.

Quantum-dot-sensitized solar cells (QDSCs) are promising solar-energy-conversion devices, as low-cost alternatives to the prevailing photovoltaic technologies. Compared with molecular dyes, nanocrystalline quantum dot (QD) light absorbers exhibit higher molar extinction coefficients and a tunable photoresponse. However, the power-conversion efficiencies (PCEs) of QDSCs are generally below 9.5 %, far behind their molecular sensitizer counterparts (up to 13 %). These low PCEs have been attributed to a large free-energy loss during sensitizer regeneration, energy loss during the charge-carrier transport and transfer processes, and inefficient charge separation at the QD/electrolyte interfaces, and various interfacial engineering strategies for enhancing the PCE and cell stability have been reported. Herein, we review recent progress in the interfacial engineering of QDSCs and discuss future prospects for the development of highly efficient and stable QDSCs.

Lift-Off Patterning of Nano-Crystalline Quantum Dot Films

The paper is concerned with the effectiveness of the lift-off process in pattern definition of ultra-thin nanocrystalline quantum dot (NQD) films. The lift-off process is implemented and investigated in conjunction with mist deposition and spin coating of NQD films. The lift-off process is preformed in gas-phase (oxygen plasma exposure) and liquid-phase (acetone immersion). The results of NQD patterned films using lift-off process are presented and discussed in terms of performance of the lift-off medium (dry and wet), resolution of the pattern transfer process, as well as photoresist thickness. It demonstrates that while fully compatible with the mist deposition, the patterning scheme proposed is not compatible with spin coating of NQDs films.

Quantum dots/silica/polymer nanocomposite films with high visible light transmission and UV shielding properties

The dispersion of light-absorbing inorganic nanomaterials in transparent plastics such as poly(ethylene-co-vinyl acetate) (PEVA) is of enormous current interest in emerging solar materials, including photovoltaic (PV) modules and commercial greenhouse films. Nanocrystalline semiconductor or quantum dots (QDs) have the potential to absorb UV light and selectively emit visible light, which can control plant growth in greenhouses or enhance PV panel efficiencies. This work provides a new and simple approach for loading mesoporous silica–encapsulated QDs into PEVA. Highly luminescent CdS and CdS-ZnS core–shell QDs with 5 nm size were synthesized using a modified facile approach based on pyrolysis of the single-molecule precursors and capping the CdS QDs with a thin layer of ZnS. To make both the bare and core–shell structure QDs more resistant against photochemical reactions, a mesoporous silica layer was grown on the QDs through a reverse microemulsion technique based on hydrophobic interactions. By careful experimental tuning, this encapsulation technique enhanced the quantum yield (∼65%) and photostability compared to the bare QDs. Both the encapsulated bare and core–shell QDs were then melt-mixed with EVA pellets using a mini twin-screw extruder and pressed into thin films with controlled thickness. The results demonstrated for the first time that mesoporous silica not only enhanced the quantum yield and photostability of the QDs but also improved the compatibility and dispersibility of QDs throughout the PEVA films. The novel light selective films show high visible light transmission (∼90%) and decreased UV transmission (∼75%).

Lift-Off Patterning of Nano-Crystalline Quantum Dot Films

The ultra-thin films formed using colloidal solutions of nanocrystalline quantum dots (NQD) show good promise in display/LED, as well as nano-labeling applications between others (e.g. [1]). Regardless of application, to become a part of the functional device the NQD film must be patterned accordingly. The conventional top-down patterning sequence involving photolithography and etching cannot be used in this case as the process would results in the critical damage to the NQD film. Hence, alternative patterning technologies suitable for NQD films patterning need to be pursued. The goal of this contribution is to continue exploration of an approach to NQD film patterning based on the lift-off principle [2]. In electronic/photonic device manufacturing the lift-off process is a method allowing patterning of the thin-film using a sacrificial material, most commonly a photoresist. The lift-off patterning is employed to created desired geometries in the films which are either very difficult to etch (e.g. gold) or are not sufficiently structurally coherent to be subject to a conventional photolithographic process without enduring permanent damage. The thin-films comprised of nano-scale materials such as nandots, nanowires and nanotubes fall into this last category. The principle of the NQD film pattering using lift-off process is demonstrated in Fig.1. In this study NQDs deposited were CdSe/ZnS core/shell NQDs featuring average diameter of 6.5 nm and corresponding red (620 nm) emission wavelength. Two deposition methods used in this study was a mist deposition and a conventional spin coating. The method of mist deposition converts colloidal solution, in this case CdSe/ZnS NQDs suspended in toluene, into a fine mist of sub-micrometer sized particles using an atomizer. The mist droplets are carried from the atomizer to the deposition chamber by ultrapure N2carrier gas and then coalesce on the substrate in the presence of an electric field. The same colloidal solution was used in the preparation of the samples by spin coating. Silicon wafers used as substrates are first covered by spin coating with a layer of positive 1811 photoresist to the thickness of about 1 µm and then cured at 100oC. The resist covered wafers were subsequently exposed through the standard Cr-photomask in an aligner/exposure tool following which photoresist was develop using in 351 developer (1:4) diluted with Di water for 1 minute (Fig.1a). Then, CdSe/ZnS QD film was blanked deposited during 20 min. long mist deposition process and thermally cured (Fig. 1b). Then, the samples were exposed for 20 min. to 20% O2in nitrogen plasma at the pressure of 1 Torr at the power of 200 watts (Fig. 1c). In another experimental run instead of plasma ashing resulting in the partial NQD film lift-off, the photoresist removal was carried out by immersion of the wafer in acetone. The results obtained in this study demonstrate effectiveness of the lift-off process in pattern definition of NQD films. Figure 2 shows patterns created in NQD films using proposed technique. In the full account of this work a complete set of experimental results obtained in the course of this investigation is presented and possible applications of the processes under investigation is considered.

Methods of Patterned Mist Deposition of Nano-Crystalline Quantum Dot Films

This paper is concerned with patterning of ultra-thin films formed by means of mist deposition technique using colloidal solutions of nanocrystalline quantum dots (NQDs). It discusses patterns created using mechanical masks as well as area selective bottom-up growth of quantum dot films which follows patterns created by means of pre-mist deposition surface functionalization. A new method of nanocrystalline quantum dot film patterning by means of a lift-off process is proposed and investigated. The results obtained show effectiveness of both gas-phase (oxygen plasma) and liquid-phase (acetone) based lift-off steps in forming NQD patterns with the resolution defined by the resolution of the photolithographic process used to pattern photoresist.

Methods of Patterned Mist Deposition of Nano-Crystalline Quantum Dot Films

This study extends an investigation concerned with patterning of ultra-thin films formed by means of mist deposition technique using colloidal solutions of nanocrystalline CdSe nanodots with ZnS shell suspended in organic solvent. The CdSe/ZnS nanocrystalline quantum dots (NQD) show very good promise in display/LED and variety of nano-labeling and counterfeiting applications between others.Regardless of application, to become a part of the functional device the NQD film must be patterned accordingly. The conventional top-down patterning sequence involving photolithography and etching cannot be used in this case as the process would results in the critical damage to the NQD film. Hence, alternative patterning technologies must be pursued. The goal of this contribution is to review past experiments and to introduce new approach to NQD film patterning in conjunction with mist deposition technology.In this study NQDs deposited were CdSe/ZnS core/shell NQDs featuring average diameter of 6.5 nm and corresponding red (620 nm) emission wavelength. The method of mist deposition converts colloidal solution, in this case CdSe/ZnS NQDs suspended in toluene, into a fine mist of sub-micrometer sized particles using an atomizer. The mist droplets are carried from the atomizer to the deposition chamber by ultrapure N2 carrier gas and then coalesce on the substrate in the presence of an electric field.Patterned deposition using mechanical masks of nanocrystalline quantum dot (NQD) films by means of mist deposition on various substrates was demonstrated earlier (e.g. [1]). The process can be used for the variety of purposes including security labeling, barcode printing as well as in photonic detection/conversion device fabrication. In the continuation of this investigation the feasibility of patterned mist deposition of NQD films by surface functionalization applied prior to mist deposition instead of the use of mechanical masks during deposition was explored. With this approach desired pattern can be created through the bottom-up process by properly functionalizing SAM material prior to NQD film formation by mist deposition. Surfaces of substrates used in this experiment, including Si wafers and laboratory glass were functionalized by immersion in trichloro (1H,1H,2H,2H-perfluorooctyl) silane (FOTS) and let dry in an ambient air. UV exposure was carried out in ambient air and at room temperature using apparatus equipped with low-pressure mercury lamp featuring intense 185 and 254 nm wavelengths. Further modifications of the SAM material were accomplished by means of the lamp-based rapid optical surface treatment (ROST) at the temperatures up to 300 oC. In terms of selectivity of deposition the results obtained in this study were strongly affected by the modification of wetting properties of the surface by the presence of electric field accelerating process of mist deposition. This effect, known as electrowetting, was influencing FOTS surface wettability and preventing selectivity of the deposition process. The effect can be partial reversed with the low-temperature thermal treatment in ambient air. The manipulation of the surface wettability of FOTS using UV light, electric field and temperature process is employed to alter selectivity of the NQD film deposition process.In the extension of this investigation, approach to NQD CdSe/ZnS film patterning based on the principles of a lift-off process is investigated. In this case a negative image of the final NQD pattern is created by means of photolithography in the layer of photoresist and blanket mist deposition of NQD film follows. Subsequently, patterned photoresist is oxygen plasma ashed lifting off NQDs from the resist covered areas and leaving them in the areas not covered with photoresist.The overall results obtained in this study demonstrate that, unlike other thin film deposition methods using colloidal solutions such as spin-on or micro-spray, mist deposition is compatible with a range of film patterning scenarios. In the full account of this work a complete set of experimental results obtained in the course of this investigation is presented and possible applications of the processes under investigation considered.

Photochemical release of carbon disulfide from 1,1‐dithiooxalate and the analogous esters (780.10)

Carbon disulfide (CS2) is thought to be a biologically relevant molecule that, upon reacting with amines, forms dithiocarbamates, which disrupt nuclear factor κB mediated tumor growth. To insure careful control we have designed a photocatalytic system whereby CS2 uncaged from the surface of binary chalcogenide cadmium selenide (CdSe) nanocrystalline quantum dots (QD). This is achieved through a hole transfer process to 1,1-dithiooxalate (DTO) adsorbed to the nanoparticle surface after exciting the QD with visible and near UV light. An analogous photoreaction occurs with the t-butyl- DTO ester. We have also synthesized an octyl-DTO ester through an acetylation and a subsequent sulfur replacement reaction with chloride substituents. These esters provide a modular system where the photochemical breakdown of the DTO compound could release additional therapeutics. Currently, we are working to synthesize a dye-functionalized DTO ester to further understand this photochemical process and product distribution. Th...

Surface Processing for Area Selective Mist Deposition of Nanocrystalline Quantum Dot Films

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