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New OLETs emit light more efficiently than equivalent OLEDs
The new OLET, which is 10 times more efficient than any other reported OLET, has a trilayer structure. Electrons from the green layer and holes from the blue layer move to the middle red layer, where excitons are formed and light is emitted. Image copyright: Nature Publishing Group.
Already, organic light-emitting diodes (OLEDs) are becoming commercialized for light display applications due to their advantages such as low fabrication costs and large-area emission. But OLEDs also have intrinsic efficiency limitations due to their structure, which might limit their future development in terms of brightness. Now, a team of researchers has found that another organic semiconductor-based device, the organic light-emitting transistor (OLET), can dramatically increase the efficiency of OLEDs since OLETs have the structure of a transistor rather than a diode. In their recent study, the researchers have created OLETs that are 10 times more efficient than any previously reported OLET, as well as more than twice as efficient as an optimized OLED made with the same materials.
The researchers, Raffaella Capelli, et al., from the Institute for Nanostructured Materials (ISMN) in Bologna, Italy, and the Polyera Corporation in Skokie, Illinois, USA, have published their results in a recent issue of Nature Materials.
As the researchers explain, OLED technology is by far the most developed of the two organic semiconductor-based devices. But the biggest drawback to using OLEDs for light display applications is that they intrinsically suffer from photon loss and exciton quenching. Both effects are a direct result of the structure of OLEDs: The close spatial proximity of the electrical contacts and the light-generation region causes some emitted photons to be absorbed, resulting in photon loss. Similarly, the largest quenching effect in OLEDs, called exciton-charge quenching, reduces the number of excitons, and occurs due to a spatial overlap of excitons and charges.
Because OLETs have a transistor-based structure, researchers have recently been looking for ways to suppress these deleterious effects inherent in the OLED architecture. So far, they have only managed to prevent one type of quenching called exciton-metal quenching, which was done by moving the light-emitting area further away from the electrodes. However, the other effects still remained, so that the best OLETs only achieved an efficiency of no more than 0.6%.
In the new study, the researchers designed an OLET that could avoid photon losses and the two types of quenching. In demonstrations, the new OLETs achieved efficiencies of 5%. In comparison, equivalent OLEDs had efficiencies of just 0.01%, while optimized OLEDs with the same emitting layer as the OLETs achieved efficiencies of 2.2%, with the difference being due to their diode structure. (Although 2.2% is the highest reported efficiency for OLEDs based on fluorescent emitters, researchers have recently reported OLEDs based on phosphorescent emitting material with an efficiency on the order of 20%.)
The researchers call their novel device a tri-layer field-effect OLET due to its three organic semiconducting layers: a top 15-nm-thick p-channel layer that transports holes, a 40-nm-thick middle layer that emits light (the “exciton formation zone”), and a bottom 7-nm-thick n-channel layer that transports electrons. In this set-up, electrons and holes move from their respective layers to the middle layer, where excitons are formed and light is emitted. The three semiconductor layers are positioned on a three-layer substrate of glass, indium tin oxide, and PMMA, and two gold electrodes on top complete the design.
The trilayer architecture offers several advantages. For one, the light-formation and light-emitting regions are located far enough away from the electrodes so that photon losses at the electrodes and exciton-metal quenching are prevented. Also, the light-emitting region is physically separated from the charge flows, which prevents exciton-charge quenching. For these reasons, the researchers describe the tri-layer OLET as a “contactless OLED,” where these deleterious effects are intrinsically prevented. In addition to these improvements, the researchers predict that the efficiency of the new OLET should be able to be increased even further with further adjustments, such as decreasing the operating voltage and carefully tuning every part of the structure.
“Despite the necessary technical improvements, we believe that our tri-layer OLETs represent a viable route to increase even further the device efficiency,” Capelli, a researcher at ISMN, told PhysOrg.com.
Overall, the scientists hope that the OLET represents a route toward developing practical organic light-emitting devices with unprecedented efficiency. The device could offer the potential for many applications, such as intense nanoscale light sources and optoelectronic systems.
“The OLET is a new light emission concept, providing planar light sources that can be easily integrated in substrates of different natures (silicon, glass, plastic, paper, etc.) using standard microelectronic techniques,” said Michele Muccini, a researcher at ISMN. “Our devices provide planar micrometer-size light sources that might enable organic photonic applications like integrated on-chip bio-sensing and high resolution display technology with embedded electronics. Moreover, a long term perspective for OLETs could be related to the realization of an electrically pumped organic laser.”
Entangled-Light-Emitting Diode
Toshiba Research Europe Ltd., Cambridge Research Laboratory
Quantum Information Group, Entangled-Light-Emitting Diode
For some important applications, quantum computers have potentially massive processing power, due to the way data is encoded upon quantum bits (qubits). One of the resources required to operate an optical quantum computer, is entangled light. At Toshiba, our research on entangled light sources has resulted in many important achievements. These include realisation of the first semiconductor source of triggered entangled photons, creation of time-evolving entangled light states, and recently the first electrically driven source of entangled light.
Entangled light possesses the unusual feature that its constituent particles (photons) have inter-related properties, in this case polarisation. Measurement of one photon affects the polarisation of the other, even if they are separated by huge distances. This curious phenomenon was famously declared by Einstein to be “spukhafte Fernwirkung” or “spooky action at a distance”. These properties of entangled light derive from the fact that according to quantum mechanics, the photon pair exist in a superposition state, and the polarisation of the pair is uncertain until measurement of one photon.
We create photon pairs using nanometer-scale regions of semiconductor known as quantum dots. Their small size means quantum dots can capture a maximum of two negative and positive charges (electrons and holes respectively). The electrons and holes recombine to emit a pair of photons.
However, photon pairs emitted by conventional quantum dots are not entangled, as the energies of the emitted photons are polarisation dependent. This means the polarisation of a photon can be determined by measurement of it's energy, providing the dreaded ‘which-path‘ information that is well know to destroy entanglement. We have solved this problem by pioneering a technique to optimise the size and shape of the quantum dot so that the energies of the emitted photons are equal, and entangled light can be emitted. This led to realisation of the first semiconductor source of triggered entangled photon pairs, which we achieved by driving a single quantum dot with a laser.
We have subsequently made many advances in the performance and operation of the device, which include enhanced resolution quantum interferometry, creation of time-evolving entangled states, and improvement of the fidelity, or purity, of the entangled light to 91%. However, entangled light produced previously by us and others requires a laser beam as a power source. For applications such as optical quantum computing that require many entangled photons, the practical advantages of creating entangled light by electrical current are very significant. In collaboration with the University of Cambridge, we now report in the journal Nature, realisation of the first electrically driven source of entangled photons.
Our device is based on a conventional light-emitting-diode (LED) structure, but additionally contains a specially optimised quantum dot. A voltage applied to the LED causes a current to flow, and the quantum dot captures the charge required to emit a pair of photon. In addition, the thickness of the semiconductor material surrounding the quantum dot was optimised to regulate the rate charge is transferred to the dot. Without this feature, entanglement is destroyed by extra charge. We demonstrate that the device works well in both d.c. and a.c. mode, with fidelities up to 82%.
An additional fundamental advantage of the entangled LED is that it has the potential to operate on demand, supplying one entangled pair nearly every cycle. When combined with the practical advantage offered by electrical excitation, the entangled LED will allow simultaneous operation of many entangled light sources on a single chip, opening the path to ultra-powerful semiconductor processors based on quantum computation.
Determination of Surface Energy of Nanomaterials Using Inverse Gas Chromatography by Surface Measurement Systems
Abstract
Ultimate composite strength is highly dependent on interfacial interactions between the filler material and matrix. Nanomaterials are getting continued interest as reinforcement materials in composite systems. The surface energy values for several nanomaterials were measured by Inverse Gas Chromatography (IGC). In this study, the surface energy values were used to determine the thermodynamic works of adhesion and compared with the mechanical performance of the different nanofiller-polyurethane composites.
Introduction
The use of nanomaterials as composite reinforcing materials has shown significant interest in recent years. Both carbon nanotubes and clay nanoparticles have been studied as a means to improve composite properties. The quality and performance of nanocomposites depend strongly on the interaction of the components at their interface. To enhance the adhesion properties at the interface, nanomaterials are often exposed to various surface functionalisation processes.
Full Article
Nanotechnology and Risk Assessment
by Dr. Lang Tran
Background
The management of health risk is a complicated process. The method for risk management consists of two fundamental elements:
1. The steps to be taken (to achieve the specific objectives); and
2. the rationale which justifies the choice of the steps in (1).
In this short article, we will outline the method for managing the potential health risks arising from exposure to engineered nanoparticles (ENP).
Risk to health is a product of both the intrinsic Hazard of a material, and the level of Exposure. We will describe the processes involved in Hazard and Exposure assessment in order to undertake an assessment of risk. Finally, we will outline one possible approach for managing risk.
Hazard Assessment
To assess the hazard of a substance is to evaluate its inherent toxicity. Fundamental to the science of toxicology is the dose-response relationship. For particles in general, exposure tends to be via inhalation. However, because of their extensive use in industrial processes and commercial products, for engineered nanoparticles (ENP), exposure can be via inhalation, ingestion or dermal contact.
Once internalised, it has been shown that ENP may be translocated from the primary organ of entry into secondary organs. This evidence has presented a real challenge to toxicologists, because risk assessment must now be focused on the dose-response relationship of the most sensitive body organ rather than solely on the organ through which ENP enter the body.
Ultimately the assessment of hazard must be related to humans. In toxicology, the most frequently available models for toxicity testing are animal based and may be either in vitro or in vivo. Typically, initial investigation of the dose response relationship between ENP and potential toxicity is undertaken using a body of in vitro tests - selected to be valid (i.e. relevant) to the target organs.
Of primary importance is the range of doses which do not cause a significant effect (i.e. statistically significant response level in comparison to the control). The data generated by these tests can then be used to establish a quantitative relationship between the different physico-chemical characteristics of the ENP and the respone elicited. This process is the basis of Quantitative-Structure-Activity-Relationship (QSAR) modelling.
Those tests undertaken within dose-response assessment of the ENP must undergo two further steps:
1. validation; and
2. verification
Validation is undertaken to ensure that the tests are reproducible, assuming that the test protocols are followed exactly - usually achieved via a round robin aproach between investigators. Verification on the other hand, is to ensure that the results of those in vitro tests carried out correspond to actual observations obtained from animal experiments (or human clinical situations).
The verification of in vitro results therefore usually requires the use of limited in vivo animal experimentation, with which there may be associated ethical issues. As a result, it is essential that these in vivo tests are well designed and focused, in order to satisfy the ethically considerations laid out by the 3Rs principle of refinement, reduction and replacement.
Extrapolation between in vitro and in vivo results requires a judicious choice of both dose administered and dose rate. Recent findings from Oberdörster et. al. demonstrate good concordance between in vitro and in vivo results in the pulmonary system when response is described as response per cm2. Another important issue is distribution of ENP in different target organs, as this is essential to understanding target organ dose and choice of reference ENP material in toxicity tests. Indeed, choice of a suitable reference nanomaterial is of key importance to benchmarking and comparative toxicity between ENPs.
In summary, for hazard assessment it is essential to study the dose-response relationship in context of the most sensitive organ/system the ENP is likely to reach. Ascertaining the physico-chemical properties of ENPs which drive their toxicity, and the range of dose with no observed adverse effects are essential to undertake a meaningful hazard assessment. Figure 1 presents these and other key aspects of hazard assessment.
Exposure Assessment
Regardless of how hazardous a material is, without exposure there is no risk. Assessment of exposure is therefore of equal importance to understanding of hazard in the risk assessment process. Exposure to humans is possible - both directly and indirectly - throughout the entire life cycle of an ENP, from handling in the workplace during production, consumer use and final disposal. At each stage there is a potential for direct exposure to both humans (as either workers or consumers) and the environment (e.g. soil, water and air). Within the environment, the properties of the ENP are likely to be altered by their surroundings, and hence their fate and behaviour in these media is difficult to predict. In addition, ENP may come into contact with different species in the environment, enter the food chain and thus eventually provide an additional indirect source of exposure to humans.
Therefore, important considerations in assessing exposure to ENP are the likelihood of major accidental exposure scenarios (e.g. explosion or major spillage into the environment) and methods for exposure monitoring (including personal sampling and the use of novel biomarkers of exposure which can detect ENP in blood, urine and sputum).
Risk Assessment and Management
Although hazard assessment may yield useful in vitro and/or in vivo results, it is risk assessment which places these findings in the human context. The risk assessment process extrapolates these in vitro / in vivo results to humans, achieved by application of a range of uncertainty factors which attempt to compensate for inter-animal variations and inter-species differences. This approach may lead to over-estimation of risk and as a result setting of unrealistic exposure limits.
A more promising approach may be mathematical modelling of the exposure-dose-response using the available experimental data. Once established, such models can be extrapolated to a human context and used to estimate the level of exposure which does not initiate an adverse effect for a chosen endpoint. This is known as the Derived No Effect Level (DNEL). The advantage of this mathematical modelling approach is that uncertainty can be included readily, most frequently achieved via use of the Monte Carlo simulation. Assessment of risk involves comparison of the calculated DNEL with the total exposure in humans estimated through the exposure assessment process, if the total human exposure is found to be greater than the DNEL then there is a risk of a development of the adverse effect. Figure 1 summarises the risk assessment process.
The next step in Risk Assessment is Risk Decision - the rational decision to accept or reject risk which must be made following the risk assessment. This decision will be based on the impact of calculated health risk on both the social and economic infrastructure. If the risk is small in comparison to the social-economics trade-offs, then the risk may be acceptable. If not, the risk is perceived as too great and the risk rejected.
If the risk is rejected, then it must be suitably managed - this is the process of risk management. The two fundamental processes in risk management are Risk Control and Risk Transfer. Risk Control involves exposure monitoring, the use of protective clothing, and communication to stakeholders via for example standard operating procedures, guidance and dialogue using different media such as TV, internet and open forums..
Another important process is identification of human exposed cohort for a health surveillance exercise. Risk Transfer involves adoption of appropriate insurance for the calculated risk. The main challenge, in this instance, is being how to price appropriately insurance to cover for those health risks arising.
Conclusion
In this short article, the method for risk assessment and management of exposure to ENP has been outlined. One main limitation of the exemplar approach is that it relies heavily on the control of exposure; a step which may be difficult as for example in practice the mass airborne concentration of ENP can be very low and thus difficult to control. However, since ENP are man-made, it will be possible to re-design and produce them without those physico-chemical properties identified as having adverse effects. Needless to say, these redesigned ENP must still fulfil their original industrial needs. Ultimately, it is this hazard reduction method which is key to a responsible development of sustainable nanotechnologies.
Copyright AZoNano.com, Dr. Lang Tran (Institute of Occupational Medicine (IOM))
Date Added: Jun 7, 2010
Nanotechnology for a Brighter and More Sustainable Future
by Professor Javier Garcia-Martínez
Nanotechnology, with its unprecedented control over the structure of materials, can provide us with superior materials that will unlock tremendous potential of many energy technologies currently at the discovery phase. The quest for more sustainable energy technologies is not only a scientific endeavor that can inspire a whole generation of scientists, but the best way to establish a new economy based on innovation, better paid jobs, and care for the environment.
Solar Energy: Nanotechnology to Capture the Energy of the Sun
According to the IEA Energy Statistics3, the renewable energy accounted for around 13.1% of the fuel share of world's total primary energy supply energy in 2004, where photovoltaic technology represented only the 0.04%. Thus even if solar energy is free and abundant, we are still far away of an energetic system based on this technology.
Besides, the Alternative Policy Scenario presented in 2006 World Energy Outlook4 has predicted an increase of photovoltaics of around 60 times from 2004 to 2030 year. In fact, the evolution of photovoltaic technology has provoked that its price has fallen down to a tenth in the last 20 years (from 2.00 $/kWh in 1980 to 0.20-0.30 $/kWh in 2003). Independent studies suggest that the costs will continue to fall and that it is plausible to envisage costs of around 0.06 $/kWh by 2020.
The application of nanotechnology in PV cells is already producing some significant advantages to increase the efficiency/cost ratio by using materials with different bandgaps, i.e., multilayers of ultra-thin nanocrystalline materials, new dyes or quantum dots, among others. For example, the ability to control the energy bandgap provides flexibility and inter-changeability. Also, nanostructured materials enhance the effective optical path and significantly decrease the probability of charge recombination. Quantum well devices such as quantum dots and quantum wires, as well as devices incorporating carbon nanotubes, are being studied for space applications with a potential efficiency up to 45%.
Nanocrystal quantum dots (NQDs)5 are nanometre-scale single crystalline particles of semiconductors. Due to the quantum confinement effect, their light absorption and emission wavelengths can be controlled by tailoring the size of NQDs. Nowadays, conventional solar cells are mostly built on silicon (Figure 1). Because the cost of silicon keeps growing, this technology will not be the one to bring down the cost of solar generated electricity below1 $/kWh. In contrast, as an example of their attractive future as more efficient solar cells, analogous nanocrystalline quantum dots have close to 40% efficiency.
Figure 1. Evolution of PV technology: from conventional (silicon-based solar cells) to nanostructured solar cells (quantum-based and dye-sensitized solar cells)1
The use of nanocrystalline materials in thin-film multilayered cells also help achieve a regular crystalline structure, which further enhances the energy conversion efficiency. An example of nanostructured layers in thin-film solar cells has been recently reported by Singh et al.6 Nanocrystalline CdTe and CdS films on ITO-coated glass (indium tin oxide) substrates have been synthesized as potential n-type window layers in p-n homo(hetero)junction thin-film CdTe solar cells. CdTe nanocrystals of around 12 nm in diameter exhibit an effective band gap of 2.8 eV, an obvious blue shift from the 1.5 eV of bulk CdTe (Figure 2).
Figure 2. Example of nanomaterials for photovoltaic cells fabrication. Left part: FE-SEM image of a nanocrystalline CdTe film on ITO-coated glass substrate. The inset shows the absorption spectrum of a nanocrystalline CdTe film on ITO-coated glass substrate. Right part: Device configuration of a Glass/ITO/n-Nano-CdTe/p-bulk CdTe/graphite solar cell. Adapted with permission from ref.6. Copyright 2004, Elsevier
Another alternative offered by nanotechnology to conventional silicon-based solar cells is the use of dye-sensitized solar cells. Dye-sensitized photoelectrochemical solar cells (PES or Grätzel cells) represent a relatively new class of low-cost thin-film solar cells7. Nano-structured TiO2, CeO2, CdS and CsTe are of great interests as the windowing and light absorbing layers8,9. These dye-sensitized nanostructured solar cells, which comprise devices such as nanocrystal solar cells, photoelectrochemical cells and polymer solar cells, are being studied for terrestrial applications and represent the third generation of photovoltaics.
The last advances in photovoltaic technology are based on the preparation of nanocomposites based on the mix of nanoparticles with conductive polymers or mesoporous metal oxides with high surface areas thus increasing internal reflections and, consequently, having a single multispectrum layer.
Advanced Nanomaterials for Fast and Efficient Energy Storage
Many of the clean energy alternatives produce (e.g. PV solar cells, wind) or require (e.g. hydrogen production, water splitting) electricity. Therefore, a more novel and efficient way to store electricity is needed. Energy storage systems include batteries, and among them Li-ion batteries are specially attractive because they lead to an increase of 100-150% on storage capability of energy per unit weight and volume as compared with the more traditional aqueous batteries. Nevertheless, some disadvantages arise, related to low energy and power density, large volume change on reaction, safety and costs.
Nanotechnology is already producing some very specific solutions to the field of rechargeable batteries. Electrolyte conductivity increases up to six times by introducing nanoparticles of alumina, silicon or zirconium to non-aqueous liquid electrolytes. Most efforts have been focused on solid state electrolytes, solid polymer electrolytes (SPE).
Poly(ethylene oxide)-based (PEO-based) SPE received most attention since PEO is safe, green and lead to flexible films. Nevertheless, polymers usually have low conductivity at room temperature and, depending on SPE compositions, their interfacial activity and mechanical stability are not high enough.
In this sense, nanocomposite polymer electrolytes could aid in the fabrication of highly efficient, safe and green batteries. For example, the introduction of ceramic nanomaterials as separators in polymer electrolytes increases the electrical conductivity of these materials at room temperature from 10 to 100 times compared with the corresponding undispersed SPE system. TiO2, Al2O3 and SiO2 and S-ZrO2 (sulphate-promoted superacid zirconia) have been used for this purpose and results reveal that the introduction of S-ZrO2 led to the best performance.
Other Opportunities for a Brighter Future
There are many other examples of the use of nanotechnology to make energy production, storage and use more efficient, like the use of nanostructured electrodes in supercapacitors10, novel hierarchical porous catalysts for advanced chemical processing or nanostructured catalytic electrodes for fuel cell applications. But there are many other opportunities, like light nanocomposites for more energy efficient transportantion, the use of nanomaterials in construction and nanoporous adsorbents for CO2 capture11.
Nanotechnology unprecedented control over the size, structure, and organization of matter is providing very tangible examples of how better materials are contributing to the well-being of present and future generations by proving alternative cleaner ways to produce and use energy.
Quantum Dots Are Finding Their Place in the World
Lynn Savage, Features Editor, lynn.savage@photonics.com
There has been a panoply of research into the next big thing in quantum dots – those semiconducting artificial atoms that are ubiquitous in fluorescence imaging, biological and chemical sensing, and display applications. Quantum dots of more (or less) exotic materials and with more (or less) interesting shapes are demonstrated on almost a weekly basis. It is the heyday for the field. But getting less attention are the practical issues of handling quantum dots in such a way that their functionality can be maximized and their use more broadly commercialized.
What follows is a look at some important steps in that direction.
Dropping through the grate
Negative-refractive-index materials, superlenses and Raman scattering-based spectroscopy are just a few applications that have come about – or that have been improved – through research developments involving surface plasmons. Quasiparticles analogous to photons, surface plasmons are the faint residue of electronic oscillations that occur naturally at the interface between metal and dielectric materials, such as silver and air, respectively. Integrate photons into the situation, and you obtain surface plasmon polaritons, another form of energetic quasiparticle that moves across the metal surface until reabsorbed or radiated away.
One way to improve the signal quality in a sensor is to use surface plasmon polaritons to enhance the fluorescence signal of nearby dye particles. For example, using a metallic surface rather than glass can improve the emission intensity of a fluorescent dye by as much as 20 times, perhaps more. Furthermore, if the metal surface is grated, more enhancement can be coaxed from the system because the coupling of photons to surface plasmons is boosted.
To further increase the applicability of fluorescence enhancement to sensor technology is to use quantum dots instead of dyes. Quantum dots have several advantages over fluorescent dyes, including size-dependent emission spectra, wide absorption bands, insusceptibility to photobleaching and higher quantum yields.
“In our work, we were able to take advantage of several of the special properties of quantum dots relative to traditional fluorescent dyes,” said Ehren Hwang of the University of Maryland, College Park. “The broadband absorption allowed us to use a pump wavelength widely separated from the quantum dot emission wavelength during the earlier phases of [our] experiment. We also were able to take advantage of the intrinsic resistance of quantum dots to photobleaching, [which] allowed us to interrogate our samples for extended periods of time – several hours – with repeated use over the course of months.”
Hwang, his adviser, professor Christopher C. Davis, and Igor I. Smolyaninov, now with BAE Systems in Washington, recently reported the effects of using quantum dots instead of fluorescent dyes in these systems. They used the polymer PMMA to create gratings over a 50-nm-thick layer of gold on glass. PMMA, the researchers noted, is inexpensive and easy to scribe with electron beam lithography. The polymer tends to absorb dye particles, making measurement of their positions and emissions less than certain, but quantum dots are much bigger, don’t get absorbed into the PMMA and, thus, are more certain when it comes to measurements.
“The physical positioning of the quantum dots was of paramount importance,” Hwang said. “The propensity of quantum dots to remain at the surface of the film was instrumental in enabling us to study closely the relationship between the grating and fluorescence enhancement.” He cautioned, however, that the size of quantum dots may be problematic for biosensing, especially if the particles are expected to be taken up by cells. In addition, the particles also tend to be made of cadmium or other materials that can be toxic to cells.
For some tests, the investigators directly deposited CdSe/ZnS quantum dots onto PMMA; for others, they mixed the PMMA with the quantum dots prior to coating the glass/gold base. The particles had an emission peak of 640 to 660 nm and quantum efficiency of 40 to 50 percent. They also tested grated and ungrated PMMA samples and, to determine whether the surface plasmon polaritons were the sole source of fluorescence enhancement, they made still other samples using chromium or indium tin oxide instead of gold.
After irradiating each sample with a filtered mercury discharge lamp, the investigators found that the gold layer clearly enhanced fluorescence compared with the alternate materials. Interestingly, quantum dots deposited prior to carving out the grating via electron beam lithography remained on the surface afterward.
Quantum dots that were mixed in with the PMMA and that drifted close to the gold thin-film layer were quenched, which was expected.
Overall, Hwang and his colleagues reported in the Dec. 30, 2009, issue of NanoLetters, the thickness of the PMMA layer, the periodicity of the grating and the grating geometry all had an effect on quantum dot-based outcomes.
According to Hwang, he and Davis now are performing experiments to investigate the influence of the substrate material thickness on the quantum dot enhancement effect. They also are drilling down into the nature of fluorescence enhancement in other ways.
“If possible, we would also like to probe our substrates using a more standard Kretschmann prism-coupling setup and to study the dependence of the system on azimuthal rotation of the gratings relative to an off-axis illumination source – that is, spinning the gratings around the Z-axis,” Hwang said. Additionally, “we are interested in probing our structures using a near-field scanning optical microscope in order to examine the near-field response of the interaction.”
Taking a firm position
The quantum dot deposition method used by Hwang and his colleagues is typical but, ultimately, random. You can get a layer of fairly uniform thickness using spin-coating techniques, for example, but it is no way to get a finely arranged array of particles with submicron resolution between particles.
The ability to precisely lay down individual quantum dots could be the next step toward improved biosensors, but also to better LEDs, organic LEDs, solar panels and other optoelectronics.
But it’s not easy, by any stretch of the imagination. First, one must create the particles, then move them around individually to the desired location. Doing this with optical tweezers would be direct, but besides other challenges, would be dreadfully slow when spacing out an array of particles on a commercial scale.
Instead, placing arrangements of quantum dots has typically been done through photochemical means. One such method is to deposit quantum dots with molecular tags, or ligands, that adhere the particles onto a substrate, then, with lasers, chemicals or both, strip the bonds of select adherents, leaving only the quantum dots required.
To researchers at Texas A&M University’s chemistry department in College Station, however, this and other methods are too labor-intensive. To address the issues, they developed a method, dubbed “lithosynthesis,” that takes advantage of the photo-oxidation process.
According to James D. Batteas, associate professor of chemistry and materials science and engineering, lithosynthesis is a process wherein CdSe quantum dots are capped with a photo-oxidizable molecule, then spread onto a positively charged glass, silicon or other substrate. When a laser scans the layered surface, a portion of the caps are broken, leaving behind patterned arrays of quantum dots that have various emission intensities and wavelengths.
Batteas and his colleagues report in the March 10, 2010, Journal of the American Chemical Society that they used 16-mercaptohexadecanoic acid (16-MHA) to cap 4-nm-diameter CdSe quantum dots and positively charged poly(diallyldimethylammonium chloride) to enable the photo-oxidation effect in their experiments.
Using a 488-nm argon-ion laser, they raster-scanned the landscape of capped particles, photo-oxidizing the tandems, breaking their bonds and leaving behind a composition of quantum dots with various intensities and wavelengths.
“This approach allows for the high spatial resolution patterning of quantum dots in which both emission intensity and wavelength can be spatially modified on a single layer of quantum dots,” Batteas said. “With regards to chemical sensing, this has many advantages over carrying out these measurements in solution.”
In particular, he noted, using quantum dots in solution often results in their untimely aggregation and, ultimately, their precipitation. “Since our platform is on a surface, this cannot occur.”
The group also considered ligands besides 16-MHA as caps, but despite the prospect of lower quantum yields, found that quantum dots capped with 16-MHA exhibited a dramatic increase in luminescence after laser exposure. The researchers also noted that the increased luminescence following photo-oxidation is reversible by dipping the quantum dot array into a 16-MHA bath, where reconnections can occur.
An additional advantage to the technique is that the photo-oxidized quantum dots become more capable of binding to newly introduced molecules than the other, unoxidized material.
“The ability to bind new molecules to the photo-oxidized quantum dots allows us to turn their emission on and off, allowing for data storage capabilities,” Batteas said. “They also show the propensity to selectively bind new molecules, allowing the modified quantum dots to be readily adapted for chemical sensing applications.”
Quantum Dots and Their Multimodal Applications.
84 pages PDF file; Published: 24 March 2010
Abstract: Semiconducting quantum dots, whose particle sizes are in the nanometer range,
have very unusual properties. The quantum dots have band gaps that depend in a
complicated fashion upon a number of factors, described in the article. Processingstructure-
properties-performance relationships are reviewed for compound semiconducting
quantum dots. Various methods for synthesizing these quantum dots are discussed, as well
as their resulting properties. Quantum states and confinement of their excitons may shift
their optical absorption and emission energies. Such effects are important for tuning their
luminescence stimulated by photons (photoluminescence) or electric field
(electroluminescence). In this article, decoupling of quantum effects on excitation and
emission are described, along with the use of quantum dots as sensitizers in phosphors. In
addition, we reviewed the multimodal applications of quantum dots, including in
electroluminescence device, solar cell and biological imaging.
White-Light-Emitting Diodes with Quantum Dot Color Converters for Display Backlights
Materials Research Center, Samsung Advanced Institute of Technology
Abstract
Highly luminescent, multiply passivated green- and red-light-emitting quantum dots are used as color converters in InGaN blue LEDs to achieve external quantum efficiencies of 72% and 34%, respectively. White QD-LEDs prepared for a display backlight are shown to have an efficacy of 41 lm W-1 and color reproducibility of 100% compared to the NTSC standard in CIE 1931. Finally, a 46 inch LCD TV panel (see image) using the QD-LED backlight is successfully demonstrated for the first time.
LG introduces nano materials called quantum dots into displays
June 3, Clifford Bryan
Liquid-crystal displays, or LCDs, found in televisions, computers, and cell phones, are very inefficient: their complex optical layers discard over 90 percent of the light they produce internally, some of it because it's not quite the right color. Displays that will be in products made by Korean electronics company LG at the end of the year will have a better color gamut and save battery life by using more of the light that normally gets tossed out.
The displays incorporate nanomaterials called quantum dots that convert light from the backlight into narrowly defined bands of color that are matched to the display's filters. Depending on the design of the display, the addition of quantum dots made by Palo Alto, CA-based company Nanosys improves power efficiency by more than 10 percent and significantly improves the color gamut of the display. LG demonstrated a cell-phone-sized display incorporating the quantum-dot technology last week at the Society for Information Display's annual meeting in Seattle. The company has not yet announced what particular product the quantum-dot backlight will be used in first.
"LCDs are very inefficient, and there has not been much improvement in them over decades," says Paul Semenza, a senior analyst at research firm Display Search. All of the major display manufacturers are working on technologies for improving the efficiency of LCDs, particularly for portable electronics like e-readers and cell phones, where battery life is paramount.
One source of inefficiency in these displays is the backlight itself. Because the optics inside LCDs toss out so much light, the backlight has to be very bright to create a good picture. "You go to the trouble to create white light," says Semenza, "but then you have color filters that block most of it out." Some displays get around this problem by using red, blue, and yellow light-emitting diodes (LEDs) rather than a white fluorescent lightbulb. But this is expensive, and not all LEDs are created equal: blue LEDs are much more efficient at converting electricity into light. Coating blue LEDs with a phosphorescent material that converts some of the light into yellow, red, and green, however, has the same drawback as using a white light source: most of that light is tossed out by the filters.
A Quantum Leap in Lighting and Display Technology
WATERTOWN, Mass., Dec. 5, 2008
The display and solid-state lighting industries are about to experience another game-changing advance in light-emitting technology that will result in performance that far exceeds that of today’s products.
Quantum dot technology and the Quantum Light product platform, developed by the MIT-incubated nanotechnology company QD Vision Inc., is already being designed into new products by consumer electronics, flat panel display, electronic signage and solid-state lighting manufacturers. It promises to out perform liquid crystal displays (LCDs), plasma displays, LEDs and organic LEDs (OLEDs), across the key performance categories of brightness, color purity, power conservation, and design flexibility.
“QD Vision is helping to create a new generation of lighting solutions for major industries where color, power and design matter,” said Dan Button, the company’s CEO. “Quantum dot technology and the Quantum Light product platform represent both a step change paving the way toward better displays and solid-state lighting products, and a long-term, enabling technology for a broad range of new commercial and national security applications.”
The Quantum Dot Difference
Quantum dots are nanometer-sized, inorganic crystals that create light when stimulated with photons or electrons. Harnessing the light emitting qualities of quantum dots, QD Vision’s Quantum Light product platform is an environmentally friendly, inorganic nanocrystal-based technology for displays and solid-state lighting.
Leveraging an expanding portfolio of patents from QD Vision and from MIT, the Quantum Light platform combines advanced material and device technologies to deliver three substantial benefits:
• Color: QD Vision has engineered quantum-dot-based nanocrystal solutions to emit light tuned to frequencies across the visible and infrared spectrum. Because quantum dot materials emit pure, finely tuned colors, the Quantum Light product platform provides better color saturation and color rendering than other technologies, without power consumption trade-offs. It is the only materials technology that can be designed to emit any color of light across the entire visible spectrum.
• Power: The Quantum Light platform can directly emit light and color, using less power than other solutions and making it an attractive option for clean-tech applications. In contrast, LCDs utilize white backlights that are filtered to achieve the desired colors, and consume up to ten times the power of quantum dot-based displays.
• Design: QD Vision's quantum dot inks can be applied to virtually any substrate using a wide range of well-known printing and coating techniques. The Quantum LightTM product platform therefore opens up new possibilities in the design and manufacture of consumer and industrial products, including the potential for displays, lighting fixtures, and signage with very large areas, thin and contoured forms, or transparent backgrounds.
Making an Immediate Impact on Consumer Electronics
The high efficiency, color purity, materials stability and reliability, and design flexibility of the Quantum Light product platform make it ideal for multiple consumer electronics applications. It will be used in next-generation user interfaces in mobile phones, mobile devices and personal computers, as well as in flat panel televisions. These initial applications alone represent an addressable market of more than $1 billion by 2012 for quantum dot-based components.
Future applications for QD Vision technology include architectural lighting and general white lighting, digital signage and logos, and solar cells, as well as emitters and detectors for national security applications.
A Company with Deep Roots at MIT
QD Vision was founded in 2004, but its genesis began almost five years earlier when company co-founder and chief technology officer Seth Coe-Sullivan first arrived at the Massachusetts Institute of Technology as a PhD candidate in electrical engineering.
“I’ve always been fascinated by the transformation of scientific discoveries into practical, real-world applications that can make people’s lives richer and easier, and create wealth not just for investors but for society as a whole,” Coe-Sullivan explains. “I guess you could say entrepreneurship was genetic. My grandfather was a successful entrepreneur and engineer who started, owned and operated a custom transformer company for decades; my father is an entrepreneur and engineer who sold the medical x-ray business he was part owner of after 15 years. As I was pursuing my own studies and research, I always felt that if the right opportunity presented itself, I might start a company.”
At MIT, he met Vladimir Bulovic´, a professor and pioneer in OLED research who earlier had closely worked with a leading OLED technology company. Coe-Sullivan saw tremendous commercial potential for quantum dots to transform markets for flat-panel displays, including a new generation of thin-film flexible displays, and other major markets.
He and Bulovic´ secured an “Ignition Grant” from MIT’s Deshpande Center for Technological Innovation, a group funded by serial entrepreneur Desh Deshpande to help MIT researchers realize the commercial potential of their new technologies. The grant supported the team’s research that ultimately resulted in a portfolio of patents and early proofs-of-concept that helped underpin an initial round of funding from two leading venture capital firms in 2005.
With Coe-Sullivan, Jonathan Steckel (who holds a PhD in chemistry from MIT) and Greg Moeller (who earned an MBA at MIT’s Sloan School) as co-founders, a solid team of investors, and Bulovic´ leading its scientific advisory board, QD Vision was on its way.
Unsurpassed Scientific Background
In Professor Bulovic´ and Moungi Bawendi, QD Vision’s scientific advisory board features two of the world’s foremost experts on quantum materials.
Bulovic´ has research interests that include studies of physical properties of organic and organic/inorganic nanocrystal composite thin films and structures, and development of novel optoelectronic organic and hybrid nano-scale devices. By 2008, he had authored over 40 research papers and had more than 30 issued patents in the areas of organic LEDs, lasers, photodetectors, memories, and nanostructured devices.
Bawendi is a tenured faculty member in chemistry at MIT who focuses on creating zero dimensional semiconductor and magnetic quantum materials, and understanding the physical characteristics of molecular devices, including the chemistry, physics, applications and assembly of nanostructures. A pioneer in quantum dot research during his tenure at Bell Laboratories in the 1980s, Bawendi has also served as a scientific advisory board member at Nanosys and a scientific founder of Quantum Dot Corp., focusing on utilizing quantum dots as biochemical labels for research and diagnostic purposes.
Coe-Sullivan’s work spans quantum dot materials, new fabrication techniques (including thin film deposition equipment design), and device architectures for efficient QD-LED light emission. In 2006, he was awarded Technology Review magazine’s TR35 Award, naming him one of the top 35 innovators under age 35, and in 2007 he was selected by BusinessWeek as one of the top entrepreneurs under the age of 30. By 2008, Coe-Sullivan’s work had resulted in more than 20 published papers and patents pending in his fields of expertise.
Executives and Board with Rich Domain Experience
The company recruited uniquely talented executives. QD Vision’s management team is led by Dan Button, who holds a Ph.D. in materials science and engineering from MIT, and has more than two decades of experience in the launching and building of advanced materials companies in global, high-growth markets – including both Fortune 500 organizations (DuPont Electronics, Corning Displays and Rohm & Haas) and three MIT start-ups.
John Ritter, executive vice president and head of product development, holds BS and MA degrees in chemical engineering from MIT, and has more than 20 years’ experience in all aspects of materials, process and product development and engineering at both entrepreneurial companies and global enterprises.
The company is guided by a board of directors that includes non-executive chair Willy Shih, a senior lecturer in technology and operations management at Harvard Business School and the former leader of Eastman Kodak’s Display and Components Group; Edward H. Braun, the chairman of Veeco, a provider of metrology and process equipment solutions used by manufacturers in multiple industries, including semiconductors and lighting; Sean Dalton, managing general partner of Highland Capital Partners; and Jamie Goldstein, general partner of North Bridge Venture Partners.
Building a Foundation for Leadership
Multiple rounds of venture investment have validated QD Vision’s technology leadership and business strategy. In April 2008, the company announced that it had closed a $9 million round of investment from existing investors Highland Capital Partners and North Bridge Venture Partners, and a new investor, In-Q-Tel, the strategic investment firm launched in 1999 by the US Central Intelligence Agency. Since its founding, QD Vision has raised $20 million in venture capital and has booked several million dollars in development contracts from commercial and government customers.
During the first half of 2008, the company also increased its headquarters office space by 80 percent – with most of the expansion devoted to research and development, testing and manufacturing. It expects to double its workforce to thirty-five people by the end of the year.
“QD Vision has built upon research breakthroughs that enable quantum-dot materials to serve as a platform for the next generation of lighting and display applications,” Button said. “And as the Quantum Light product platform is being designed into new products by our strategic partners, we’re building out our infrastructure to accommodate their commercial production needs. We’re positioning QD Vision to create and seize upon an exceptional opportunity.”
For more information, visit: www.qdvision.com
Quantum Leap in Lighting
QD Vision is using its quantum dots in LED lighting to produce more pleasing white light.
Seth Coe-Sullivan flicks the switches on two desk lamps, and even from across the conference room, it's immediately obvious which light the chief technology officer of QD Vision is there to brag about. The light coming from the lamp on the left is a harsh bluish white. The lamp on the right casts a warmer, more yellow glow. Coe-Sullivan holds a hand under each lamp. The hand under the bluish light looks pale and sickly; the other looks darker and healthier. The harsher light lacks wavelengths in the red end of the spectrum, so there's no light to illuminate the reddish tinge that blood provides to human skin.
QD Vision, based in Watertown, MA, is promoting a new LED-based lamp that it made with Nexxus Lighting of Charlotte, NC. Nexxus makes a lamp designed to screw into standard sockets used in recessed ceiling lighting. It consists of an array of white-light LEDs encircled by fins that remove excess heat. QD Vision adds an optic--a plastic cover with a special coating that snaps into place over the LEDs.
It's that coating that makes the difference in the quality of the light. It consists of quantum dots--tiny bits of semiconductor material just a few nanometers in diameter. When excited by a light source--in this case, the LEDs--quantum dots radiate light in a wavelength that varies according to the size of the dot: a two-nanometer dot gives off blue light, a four-nanometer dot emits green, and a six-nanometer dot produces red. The company makes the dots in controlled sizes, then mixes them in the right ratio to get the desired color.
This color-tailoring ability solves one of the major problems with using LEDs for general lighting applications. LEDs are appealing because they last for years, use perhaps 20 percent of the electricity of a standard incandescent bulb, and are highly efficient at converting electricity into visible light instead of into heat. But to make white light, you either have to mix together LEDs of different colors or use a blue LED coated with a phosphor that emits yellow light to produce a whitish mix. The problem with the phosphors is that they don't emit evenly across the visible spectrum. They tend to have gaps in the green section and even more so in the red, leading to the harsher, bluish light. "You can't precisely tailor phosphors anywhere in the visible spectrum," says Dan Button, QD Vision's CEO.
The QD Vision optics absorb the energy from the phosphor-coated LEDs and reemit it at a new mix of wavelengths. Coe-Sullivan says that the color rendering index--a measure of what colors look like under the light--of the company's optic is 90, compared with the 70s for the LEDs without the optic. Sunlight has a color rendering index of 100, and standard incandescent lightbulbs about 99. The QD Vision light is far more efficient than an incandescent, producing 65 lumens per watt of electricity, whereas an incandescent produces about 15 lumens per watt. A compact fluorescent lightbulb produces about 30 lumens per watt; has a cooler, harsher light than the QD Vision lamp; and has the added environmental problem of containing mercury.
"These are good numbers," says Nadarajah Narendran, director of the Lighting Research Center at Rensselaer Polytechnic Institute. "This may be the first market-ready product [based on quantum dots], so in that sense they might be unique," he says. "You have to have all the ingredients right." Those include not only lifetime, energy efficiency, and color: "Price is the most important one at the moment."
Companies are struggling to get the cost of their LED-based bulb replacements down to $65 and below. The Nexxus lamp could cost about $100. Button says that if you consider how much a user will save in electricity, the cost of replacement bulbs, and the labor that would otherwise be needed to replace multiple ceiling fixtures over the projected seven-and-a-half-year lifetime, the lamp will be competitive.
The two companies displayed their lamp last week at the Lightfair International trade show in New York, and they say that they'll have it available for sale by the end of the year. Meanwhile, Button says that there's nothing exclusive about QD Vision's deal with Nexxus, and that he hopes to work with other companies to produce more products for general lighting. QD Vision had initially focused on using quantum dots to make better display screens for computers and other devices. It's still in that business, but given that most of the display makers are in Asia, the company felt as though lighting would give the company a quicker entry into the market, Coe-Sullivan says.
The beauty of QD Vision's technology, Button says, is that it's just a small addition to existing LEDs designed for standard light fixtures. "They're getting a market without changing anything," he says. "You give us a light input, and we're going to improve the color without diminishing the efficiency."
Quantum light: The plastic lamp containing an array of LEDs screws into a standard recessed ceiling socket. The yellow-tinted optic on the surface contains quantum dots that convert the light into a better white hue.
Colorful Quantum-Dot Displays Coming to Market
The nanomaterials give the displays a color and efficiency boost.
Liquid-crystal displays, or LCDs, found in televisions, computers, and cell phones, are very inefficient: their complex optical layers discard over 90 percent of the light they produce internally, some of it because it's not quite the right color. Displays that will be in products made by Korean electronics company LG at the end of the year will have a better color gamut and save battery life by using more of the light that normally gets tossed out.
The displays incorporate nanomaterials called quantum dots that convert light from the backlight into narrowly defined bands of color that are matched to the display's filters. Depending on the design of the display, the addition of quantum dots made by Palo Alto, CA-based company Nanosys improves power efficiency by more than 10 percent and significantly improves the color gamut of the display. LG demonstrated a cell-phone-sized display incorporating the quantum-dot technology last week at the Society for Information Display's annual meeting in Seattle. The company has not yet announced what particular product the quantum-dot backlight will be used in first.
"LCDs are very inefficient, and there has not been much improvement in them over decades," says Paul Semenza, a senior analyst at research firm Display Search. All of the major display manufacturers are working on technologies for improving the efficiency of LCDs, particularly for portable electronics like e-readers and cell phones, where battery life is paramount.
One source of inefficiency in these displays is the backlight itself. Because the optics inside LCDs toss out so much light, the backlight has to be very bright to create a good picture. "You go to the trouble to create white light," says Semenza, "but then you have color filters that block most of it out." Some displays get around this problem by using red, blue, and yellow light-emitting diodes (LEDs) rather than a white fluorescent lightbulb. But this is expensive, and not all LEDs are created equal: blue LEDs are much more efficient at converting electricity into light. Coating blue LEDs with a phosphorescent material that converts some of the light into yellow, red, and green, however, has the same drawback as using a white light source: most of that light is tossed out by the filters.
Nanosys has developed an add-on for blue LED display backlights that converts some of the blue light into red and green light of narrowly defined wavelengths selected to match the LCD's filters. The company's "quantumrail," a thin capillary that can be attached to a backlight, contains a suspension of quantum dots that convert the light.
Quantum dots get their name from their unusual properties: when structured at the nanoscale, the optical and electronic properties of certain semiconducting materials such as cadmium are dictated by their dimensions. Conventional semiconducting materials emit light of a particular color when they're bombarded with electrons or photons--this is how light-emitting diodes work. By carefully controlling the dimensions of quantum dots at the nanoscale, it's possible to precisely tune what color light they emit.
Researchers have been making quantum dots since the 1980s, but it's only this year that these nanomaterials have been incorporated into consumer products. MIT spinoff QD Vision was the first to market with a consumer product. Its quantum dots are incorporated into energy-efficient LED lighting made by Nexus. They convert light from an LED into a mixture of colors that's more pleasing to the eye. Company CTO Seth Coe-Sullivan says QD Vision is also working with major display companies to incorporate quantum dots into LCD backlights. Coe-Sullivan says these products will launch next year.
The displays incorporating Nanosys's quantumrail that were exhibited at the conference in Seattle had a better color gamut than traditional LCDs. A good notebook display can generate 72 percent of the colors dictated by a commonly used measurement of color gamut called the National Television System Committee standard, but the LG display rates 103 percent--that is, it can show colors that aren't included in this standard. Running the display at lower power to create a 72 percent color gamut can add 10 percent to the battery life, according to Nanosys.
Quantum color: This prototype display (top) made by Korean company LG is about 4.6 centimeters by 8 centimeters. It has a better color gamut than other LCDs due to the integration of nanomaterials called quantum dots into the display’s backlight. The quantum dots are contained in capillaries (bottom) made by California company Nanosys.
LG Display Exhibits LCD Panel With Quantum-dot LED Backlight
LG Display Co Ltd exhibited an LCD panel whose color gamut was expanded by using quantum dots at SID 2010, the largest international conference on display technologies.
The color gamut of the panel was expanded by 30% by using quantum-dot LEDs for the backlight source.
A quantum dot is a semiconductor grain with a diameter of several nanometers to about 20nm. Because a band gap appears where a quantum dot exists, it can be used for luminescence materials, etc.
When a quantum-dot LED is used for a backlight source, it can expand the color gamut of the LCD panel. It is because the colorimetric purity of a quantum-dot LED is higher than that of a normal LED.
The screen's diagonal dimension is 3.2 inches. The pixel count, pixel pitch and luminance of the LCD panel are 480 x 800, 0.087mm and 450cd/m2, respectively.
In general, to expand the color gamut of an LCD panel with a normal pixel structure (RGB colors), its power consumption has to be compromised. It is because the light transmission rate of the color filter is lowered by heightening the color of the color filter to expand the color gamut. Therefore, it becomes necessary to increase the luminance of the backlight, resulting in an increase in power consumption.
On the other hand, when quantum-dot LEDs with a high colorimetric purity are used as a backlight source, it becomes possible to expand the color gamut without heightening the color of the color filter, which makes it necessary to increase the power consumption of the LCD panel (or lower the luminance of the panel).
With quantum-dot LEDs, the luminous wavelength can be controlled by changing their size. This time, LG Display used quantum-dot LEDs as a light source that has peak areas for three colors. Moreover, the company combined it with a yellow YAG phosphors.
LG Display did not reveal the materials for the quantum-dot LED or when it will start commercial production of the LCD panel.
Thanks chessmite.
All help in building up this new board is appreciated!
FLAT-PANEL DISPLAYS: QD display pixels are inkjet-printed
Sep 1, 2009
Inkjet printing is so good at positioning and meting out the right quantity of material that it can be used not just for producing documents, but in industry as well–for example, to fabricate solar cells or to create etch patterns for photodiode arrays. Now, scientists at Arizona State University (ASU; Tempe, AZ) and the University of Oulu (Oulun Yliopisto, Finland) are using the process to print quantum-dot LEDs (QDLEDs). Not only does inkjet printing conserve expensive QD-containing fluid; it also allows for the possibility of patterning full-color pixelated displays (although the printer that the scientists are using now can print only a single color at a time).
Quantum-dot ink
In one example, quantum dots with a cadmium selenide core, zinc sulfide shell, and a capping organic ligand were obtained from Evident Technologies (Troy, NY) and used as received; the QDs were suspended in chlorobenzene to create ink. The substrate, a piece of glass precoated with the transparent conductor indium tin oxide (ITO), was coated with a 20 nm layer of poly-TPD, which was then cross-linked and served as the hole-transport layer.
Pixelated patterns of QDs with three different emission wavelengths–red, green, and blue–were printed in monochrome arrays on three different substrates, then coated with an electron-transport layer (TPBi) and a lithium fluoride/aluminum electrode (see figure). The ink fluid and the jetting properties of the printer (a Dimatix DMP 2800) were chosen and controlled to reduce variations in the thicknesses of the printed features.
The printed pixels were L-shaped, with a total area each of 19,600 µm2 (about 160 × 160 µm); spacing between each pixel was 80 µm. “The number of drops used per pixel, or subpixel, depends on the ink itself. Normally we use 5 to 25 drops per pixel, depending on ink,” notes Ghassan Jabbour, a professor at the School of Materials at ASU and the director of the ASU Advanced Photovoltaics Center.
Refining the process
To print the red, green, and blue pixels, the researchers had to optimize the drop registration, given the fact that the inkjet system could print only one color at a time. “Due to this limitation, the cartridge-change process creates registration misalignment, which needs to be addressed and checked each time a cartridge is changed,” says Jabbour. “We have done this beautifully.”
The resulting operating voltage is 10 to 15 V and the brightness is on the order of 300 cd/m2. “We are working on decreasing the drive voltage and increasing the brightness further,” says Jabbour.
Although the peak external quantum efficiency (EQE) of the first prototype reached only about 0.19% at 14.2 V, this low performance was not due to the inkjet process; for comparison, the researchers spin-coated a layer of QDs and measured its EQE to be even lower–about 0.15% at 20.7 V. Boosting the performance of the inkjet-printed QDLEDs could be achieved by washing the QDs repeatedly before they are incorporated into the ink, say the researchers.
Next: active-matrix
In addition to optimizing the device performance, the group is working on integration of the inkjet-printed pixels with an active-matrix backplane, according to Jabbour. If achieved, this would be a major step toward the appearance of pure red-green-blue full-color QD displays with an active layer manufactured as easily as one prints out a document.
–John Wallace
Renewable Energy, Solar Energy, and Nanotechnology Research
By Michael S. Wong, Ph.D
Houston, TX
April 30, 2010
PDF presentation
Low temperature synthesis of ZnS and CdZnS shells on CdSe quantum dots
Full text PDF (2.66 MB)
Rice University, Houston, TX, USA
Published 2 June 2010
Abstract
Methods for synthesizing quantum dots generally rely on very high temperatures to both nucleate and grow core and core–shell semiconductor nanocrystals. In this work, we generate highly monodisperse ZnS and CdZnS shells on CdSe semiconductor nanocrystals at temperatures as low as 65 °C by enhancing the precursor solubility. Relatively small amounts of trioctylphosphine and trioctylphosphine oxide have marked effects on the solubility of the metal salts used to form shells; their inclusion in the precursor solutions, which use thiourea as a sulfur source, can lead to homogeneous and fully dissolved solutions. Upon addition to suspensions of quantum dot cores, these precursors deposit as uniform shells; the lowest temperature for shell growth (65 °C) yields the thinnest shells (d < 1 nm) while the same process at higher temperatures (180 °C) forms thicker shells (d ~ 1–2 nm). The growth of the shell structures, average particle size, size distribution, and shape were examined using optical spectroscopy, transmission electron microscopy, x-ray diffraction, and transmittance small angle x-ray scattering. The photoluminescence quantum yield (QY) of the as-prepared CdSe/ZnS quantum dots ranged from 26% to 46% as compared to 10% for the CdSe cores. This method was further generalized to CdZnS shells by mixing cadmium and zinc acetate precursors. The CdSe/CdZnS nanocrystals have a thicker shell and higher QY (40% versus 36%) as compared to the CdSe/ZnS prepared under similar conditions. These low temperature methods for shell growth are readily amenable to scale-up and can provide a route for economical and less energy intensive production of quantum dots.
3. Conclusions
A versatile low temperature synthesis method was successfully
developed to obtain highly monodisperse core/shell
semiconductor nanocrystals of CdSe/ZnS and CdSe/CdZnS.
The thickness of the ZnS shell can be well controlled by
injection of Zn(OAc)2 and thiourea solutions at various
reaction temperatures ranging from 65 to 180 ?C. The gradual
growth of highly monodisperse core/shell structures, average
particle size, size distribution, and shape were verified by
optical spectroscopy, TEM, transmittance SAXS, and XRD
measurements. The photoluminescence quantum yield (QY)
of the as-prepared semiconductor nanocrystals increased from
9.9% for the CdSe plain cores to between 26% and 46% for the
CdSe/ZnS quantum dots. In addition, Cd(OAc)2 can be added
into Zn(OAc) 2 to synthesize CdZnS shell on CdSe cores,
with thicker shell and higher QY than CdSe/ZnS nanocrystals.
This synthesis is amenable to scale-up as demonstrated in the
production of gram quantities of high quantum yield materials.
Tunable-white-light-emitting nanowire sources
Full text PDF (1.74 MB)
Abstract
Tunable-white-light-emitting materials are developed by combining two single-crystal oxide nanowire materials—ZnO and SnO2—having different light emissions. The tuning of white-light emission from cool white to warm white is achieved for the first time by adjusting the growth sequence and growth time of the ZnO and SnO2 nanowires. Combined ZnO:SnO2 nanowire arrays yield a desired emission color from (0.30, 0.31) to (0.35, 0.37) and a white luminescence of ~ 100 cd m - 2, whose reproducibility can be controlled accurately. These results pave a new way to understand and generate a desired white-light emission, which is a key technology in large-area planar display devices, including flexible and/or transparent display devices.
4. Conclusion
In summary, we have developed tunable-white-light-emitting
nanowire sources that have a controllable white balance. The
CIE values of cool-white- and warm-white-light emissions
from nanowire arrays are (0.30, 0.32) and (0.35, 0.37),
respectively, with a peak luminescence of ~100 cd m-2.
These results indicate the possibility of tuning the whitelight
emission from cool white to warm white by the tuning
of the mixing ratio of ZnO and SnO2 nanowires on a
planar substrate. The results also show that the CIE values
of white-light emission can be optimized to meet stringent
requirements for practical applications such as surface whitelight-
emitting displays, mobile phones, PDAs, and hand-held
personal computers. Further, the developed sources are also
promising for use in flexible and/or transparent light-emitting
displays.
QD Vision Acquires Motorola Patent Portfolio Covering the use of Quantum Dots in Display and Lighting Applications
May 24, 2010
WATERTOWN, Mass.--(BUSINESS WIRE)--QD Vision, (www.qdvision.com), developer of Quantum Light™ nanotechnology-based products for solid state lighting and displays, today announced it has purchased from Motorola a patent portfolio pertaining to the use of quantum dot technology in display and lighting products.
The addition of this portfolio augments QD Vision's position in quantum dot intellectual property. Included in the acquisition is U.S. Patent No. 5,442,254 of Jaskie, one of the earliest patents on the use of photoluminescent quantum dots in product applications, and pending applications relating to the use of quantum dots to a wide variety of display applications, including LCD backlight units. Prior to this acquisition, QD Vision's position in quantum dot intellectual property already included nine issued patents and more than 130 patents pending originating at MIT and QD Vision.
QD Vision is developing Quantum Light™ solutions for display backlight units (BLUs) used in several markets, including cell phones, laptops, and TVs, and plans to ship its first products in 2011. The company currently sells its Quantum LightTM optic product to the solid state lighting industry. All QD Vision products integrate cutting-edge technologies in nanotech materials, spanning chemistry, manufacturing methods, device designs and applications.
"As we continue to develop new products for solid state lighting and displays based on quantum dot technology, we are building one of the strongest patent portfolios in the industry," said Dan Button, QD Vision President and CEO. "This Motorola patent portfolio will be added to the vast wealth of intellectual property we already possess, which will enable us to maintain our market leading positions with our existing and future products."
China also in the race
Don't know how the CdTe compares to the CdSe
ASOE seems to have a sweet gig on the source of Tellurium.
" Apollo Solar Energy, Inc., through its wholly owned subsidiary, Sichuan Apollo Solar Science and Technology Co., Ltd, is primarily engaged in refining and producing tellurium (Te) and high-purity tellurium-based metals for specific segments of the electronic materials market. The Company's products include CdTe thin-film compounds, CIGS thin-film compounds, ultra-high purity metals and commercial-purity metals. Apollo Solar also expects to be a constructor and operator in future government-funded solar farm projects in China, including a possible 10 GW solar community in Anhui province, China.
About China Energy Conservation Solar Energy Technologies, Inc.
China Energy Conservation Solar Energy Technologies, Inc. (CECS), the only Chinese state-owned solar energy oriented company focusing on the power conservation industry, is leading the new renewable energy industry by directing development throughout all of China. CECS has been charged with the construction and operation of scaled solar power stations in China, and is currently the largest investor and systematic operator of solar photovoltaic technology in China. CECS plans to develop and build photovoltaic power stations using its technology internationally, and also seeks opportunities to invest in valuable photovoltaic grid-connected system projects in the future. At present, CECS's installed electric capacities are almost 1,400 MW, including solar power station construction projects that are still in the development and construction stages."
good luck with the board, I'll be checking in
The idea behind this board is to have a place where companies involved in nanotechnology can be discussed and promoted, as well as to create a database where all past, present and future developments in nanotechnology can be gathered and stored as a reference for people who are interested in investing in nanotechnology companies.
Nanotechnology Portal
Nanotechnology is the design, characterization, production and application of structures, devices and systems by controlling shape and size at the nanoscale.
Nanotechnology has been put to practical use for a wide range of applications, including stain resistant pants, enhanced tire reinforcement and improved suntan lotion.
There is no single field of nanotechnology. The term broadly refers to such fields as biology, physics or chemistry, any scientific field, or a combination thereof, that deals with the deliberate and controlled manufacturing of nanostructures.
Nanoscience (in its traditional version) is the study of phenomena and manipulation of material at the nanoscale. The traditionally educated scientists hope that this nanoscience could be in essence an extension of existing sciences into the nanoscale.
Nanoscience is not the world of atoms and molecules. It is the world of nanoparticles, macromolecules, quantum dots, and macromolecular assemblies, and is dominated by surface effects such as Van der Waals force attraction, hydrogen bonding, electronic charge, ionic bonding, covalent bonding, hydrophobicity, hydrophilicity, and quantum mechanical tunneling, to the virtual exclusion of macro-scale effects such as turbulence and inertia.
With their High-Quality Tetrapod Quantum Dots Quantum Materials Corporation (QTMM) will probably be the dominant player in this industry for the future. So enjoy the ride and let us make this the place to be for the next technological revolution.
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Nanotechnology - Wikipedia, the free encyclopedia
Nanotechnology basics, news, and general information
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