In the OSTI Collections: Quantum Dots

Dr. Watson computer sleuthing scientist.

Article Acknowledgement:

Dr. William N. Watson, Physicist

DOE Office of Scientific and Technical Information

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Quantum dots and how they carry electric charge

The term “quantum dot” provides an interesting example of how terms get invented and circulated to express ideas that, at first, are more clearly represented by prototypical examples than in terms of formal definitions in the minds of their users.  Besides this, “quantum dot” also resembles many shorthand expressions whose apparent literal meanings are somewhat different from what their users actually mean by them.  Taken rather literally, “quantum dot” would mean “a ‘dot’—a tiny piece of matter—whose quantum-physical properties are prominent”.  Atoms, most of which are a few tenths of a nanometer wide in our ordinary environment, clearly exhibit quantum-physical behavior.  Since such behavior remains generally prominent in nanometer-sized objects but is obscured in objects much larger than 1,000 nanometers, any particle normally measured in nanometers would qualify as a quantum dot by that literal definition, and some people do use the term that way. 

Yet the term “quantum dot” was coined to refer to nanoparticles of semiconducting material.[APS]  Electric current is carried through semiconductors and metals in somewhat different ways.  Current-carrying electrons in a metal number about one per metal atom, while the number of current-carrying entities in a semiconductor is much smaller than the number of atoms in it.  The quantum-physical behaviors of the current electrons are also quite different in semiconductors and metals.  In some ways, current carriers in semiconductors act more like their negligibly few counterparts in insulators.  Because of those differences, the term “quantum dot” is used by many people to refer to nanoparticles of insulating or semiconducting material[Wikipedia] but not metals; the quite different quantum-physical behavior of metallic “dots” is also important but not what the original users of the term “quantum dot” were referring to. 

Any nonuniformity in the distribution of a metal’s current-carrying electrons results in an imbalance of the electrons’ mutual repulsions and their attractions to the positively charged cores of the metal atoms they move among.  Where the metal’s current-carrying electrons happen to be crowded together more than average or less than average, the unbalanced attractions and repulsions tend to redistribute them toward the average density; however, the momentum they gain as this redistribution occurs makes them overshoot it, so places that had an above- or below-average density of mobile electrons before will have, respectively, a below- or above-average density afterwards with new unbalanced attractions and repulsions forcing the electrons back the other way—which means that the electron distribution will keep oscillating back and forth unless something disrupts the oscillation.  The oscillation can exist with only certain amounts of energy, meaning that the energy can only change by discrete quanta.  Each quantum is known as a plasmon[Wikipedia]

Plasmons aren’t the only kind of quasiparticle quanta that exist in solid matter.  A different kind of quasiparticle found in semiconductors is a kind of bubble.  At the lowest possible energy that a solid could have, all the lowest-energy states available to its electrons would be occupied by one electron each, with no “empty” states between any occupied ones.  Every solid at a temperature above absolute zero, though, will share enough random thermal motion among its constituent particles for some of the electrons to be excited to higher energies, leaving some of the lowest-energy electron states empty.  These empty states in a semiconductor are generally referred to as “holes”[Wikipedia], but the holes don’t necessarily stay in one place:  they act more like bubbles in a liquid that can move around.  The hole “moves” when one of the electrons occupying a nearby energy state changes its motion to that of the unoccupied hole state; this leaves that electron’s former state of motion unoccupied, so the hole has now “relocated”.  The hole will make multiple steps along one path as successive electrons make single steps along the same path in the opposite direction. 

The absence of a negatively-charged electron gives each hole one unit of unbalanced positive charge from the atomic cores of the semiconducting material.  So charge can flow in a semiconductor in the form of current-carrying electrons or as holes.  But these motions needn’t be independent.  The negative charges of electrons excited to states of higher energy and the positive charges of holes attract each other, so that electrons and holes can be bound together as a single comoving unit—not unlike an atom, in which electrons are bound to the positive charge of the atom’s nucleus.  “Atoms” of holes bound to excited electrons are called excitons[Wikipedia].  Thus semiconductors can conduct electric current in the form of electrons, holes, or excitons—among other ways. 

Whether the name “quantum dot” is applied only to nanometer-sized semiconducting crystals or also to metallic ones, both have proven to be as important as their having a special name suggests, as can be seen by reports of several recent investigations of their properties and ways to use them.  

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Properties

One report, which mentions how semiconducting quantum dots within matrices of other solid materials are being investigated for use in light-emitting diodes, solar cells, field-effect transistors, photodetectors, and biosensors, describes a particular set of investigations into how the quantum dots’ interactions relate to energy flow and electric-charge transport in the solids that contain them.  The experiments described in “Tunable Quantum Dot Solids: Impact of Interparticle Interactions on Bulk Properties”[SciTech Connect] involved placing cadmium selenide dots into a mixture of toluene and polystyrene, drop-casting the mixture onto a silicon wafer, and peeling the resulting film off and squeezing the film in a diamond anvil.  The pressure on the film, at thousands of times atmospheric pressure, not only changed the quantum dots’ regular spacing in the film, but caused rearrangement of the atoms within the dots, thus affecting how electrons and sound vibrations could move within and between them.  Sets of materials whose interdot spacings differ from each other at ambient pressure have also been studied, but since the spacings were made different by using different structures within the solid, it was unclear which effects were due to different spacings and which were due to the different structures used to produce them. 

Figure 1.  Left:  Low-resolution (A) and high-resolution (B) transmission electron microscope[Wikipedia] images of an ordered array of 5-nanometer particles of cadmium selenide.  Right:  reversible tuning of the interparticle spacing in a quantum-dot solid, with pressure on the solid regulated by a diamond anvil.  Electrons with sufficiently low energy have a very low probability of moving from one quantum dot to another dot very far from it, but the transfer probability is significantly higher as the quantum dots are squeezed closer together.  (After “Tunable Quantum Dot Solids: Impact of Interparticle Interactions on Bulk Properties”[SciTech Connect], pp. 13 and 10.) 

The squeezing reduced the spacing between planes of quantum dots from its original 5.7 nanometers to about 5.3 nanometers, but then increased once the pressure exceeded 8 gigapascals (about 8,000 atmopsheres) to about 6.2 nanometers past 15 gigapascals.  X-ray and optical measurements indicated that the dots’ arrangement also changed, from a face-centered cubic structure to a hexagonal structure.[Wikipedia]  Dropping the pressure below 15 gigapascals didn’t reverse the changes in spacing of quantum-dot planes, which indicated that the dots had sintered together into bundles of nanowires.  The arrangement of atoms within the dots also changed.  The dots, made of cadmium selenide, changed from having their atoms arranged like the atoms of wurtzite[Wikipedia] below 7 or 8 gigapascals to that of rock salt (sodium chloride)[Wikipedia] up to a pressure above 15 gigapascals; releasing the pressure resulted in the structure’s eventually transforming, not to the wurtzite arrangement, but to that of zincblende[Wikipedia].  Since these rearrangements affect the ways electrons can move within and between the dots, the response of the electrons to waves in the electromagnetic field—to light—also differ.  With the wurtzite-like atomic arrangement, the solid can absorb light and emit most of the absorbed energy as light itself; with the rock-salt-like arrangement, much of the energy of absorbed light gets changed into some other form, and little of it is reemitted as light waves. 

Understanding and optimizing the transport of electric charge in quantum-dot solids, and developing their magnetoelectric properties, was the goal of a research program entitled “Conductivity and Magnetism in Strongly Coupled Quantum Dot Solids”[SciTech Connect], in which electrical noise, and the effects on electrical conductivity of magnetic fields and chemical modifications, were investigated by researchers at the University of Chicago.  The chemical modifications of quantum-dot films that were attempted during the project were intended to increase the films’ conductivity while maintaining other properties, but the various attempts ran into complications that remained unresolved when the project’s funding ended.  The other aspects of the project, however, were more successful. 

The electrical noise referred to is the fluctuating random motion of electrons found in any electronic circuit or component that can distort electronic signals, so it’s useful to minimize it.  Within quantum-dot solids, the noise’s intensity decreases with frequency in such manner that it’s equally intense in every octave of signal frequency range, making the noise what’s known as “1/f noise”, or “pink noise” after the color of its visible-light equivalent, or “flicker noise”[Wikipedia].  The noise intensity for all frequencies combined varies with the quantum-dot material.  The researchers expected at first that this noise could be reduced by appropriate choices of dot and matrix materials and film fabrication process.  However, they found that reducing cracks in the films only gets rid of some of the noise, and that a “universal” minimum noise remains which scales with the number of quantum dots in the film and is approximately proportional to the electrical conductance between quantum dots.  The materials used were not found to affect the intensity of this minimum.  The researchers concluded that “the noise is an intrinsic effect of the granularity in conduction, where charges moving in [the] weakly connected part of the network modify the transport in the higher-conductivity regions”. 

Subatomic particles that carry electric charge are also magnets whose interactions can affect each other’s orientation.  An electron whose magnetic field points one way in a solid may induce other electrons in neighboring atoms to orient their own magnetic fields the opposite way.  If the first electron passes through the solid as part of an electric current, its magnetism will induce opposite orientations in atomic electrons as it passes by.  The pattern of opposite orientation follows the current-carrying electron while the atomic electrons themselves remain with their atoms.  Such patterns, known as magnetic polarons[Wikipedia], are expected to form in quantum dots that contain some magnetic atoms as impurities when the dots are electrically charged with an odd number of extra electrons.  (Ordinary polarons[Wikipedia] are a similar pattern that involves the redistribution of atoms’ electric charges rather than the reorientation of their magnetic fields—yet another way for materials to carry electric current.)  Magnetic polarons in a quantum dot solid are in turn expected to influence how the solid’s resistance to electric current is affected by magnetic fields applied to the solid from outside.  The Chicago researchers found signs of this effect when quantum-dot solid films with concentrations of manganese impurity atoms near 10% were exposed to low magnetic fields, but when the dots were highly charged, the resistance varied with magnetic field in a manner unexplained by theory. 

The fabrication method of “Tunable Quantum Dot Solids: Impact of Interparticle Interactions on Bulk Properties” described above, in which a liquid suspension of quantum dots is spread on a substrate, is a typical starting point for making a film of dots in a solid matrix.  More desirable electronic properties result from having the quantum dots closer together.  Squeezing the film was an informative way to change the spacing while minimizing additional influences, but since devices in which the dots are closer together at ambient pressure are more practical, a common next step for device synthesis involves changing the ligands[Wikipedia] attached to the dots—i.e., the molecules that attach themselves to the dots’ metal atoms.  Specifically, the next step involves exchanging longer suspension molecules for shorter groups of atoms to lessen the dots’ separation.  One popular synthesis method begins with a suspension of oleic acid molecules[Wikipedia], whose replacement by shorter ligands takes a long time and tends to severely aggregate the quantum dots.  An improvement on this method was the subject of a slide presentation for the Materials Research Society’s 2016 conference in Phoenix (“A One-Step Method for Depositing Highly Conductive Lead Chalcogenide Quantum Dot Films”[SciTech Connect]).  The newer method uses oleylamine molecules[Wikipedia] in place of oleic acid, which bind more weakly to the quantum dots and are easier to remove in the exchange step, which takes only seconds.  The method works on quantum dots of lead sulfide, lead selenide, and lead telluride.  Solar cells made from the resulting films have efficiencies that depend on what kind of atomic groups replace the oleylamine. 

 

Figure 2.  Results of producing films of lead selenide quantum dots using the process described in “A One-Step Method for Depositing Highly Conductive Lead Chalcogenide Quantum Dot Films”[SciTech Connect]with various ligands. 

Solar cells generate electric current by absorbing energy from photons of sunlight, or any other light with appropriate frequencies, and transferring some of the energy to one or more electrons.  Since electrons in a material can only move in certain ways, they can only absorb this energy if there’s at least enough of it to make up the difference between the electrons’ initial and final states.  But this minimal amount of light energy is less likely to be absorbed by electrons if the electrons’ initial and final energies are both lower than their energy during the transition.  The transition energies can be higher for materials in bulk form than for the same materials in the form of quantum dots.  Also, processes that compete with the transfer of photon energy to electrons are more likely in bulk materials than in quantum dots.  Materials containing quantum dots could thus potentially outperform bulk materials in devices that turn photon energy into electrical energy, particularly for devices in which each absorbed photon would excite multiple electrons. 

Information needed to fully realize this potential is discussed in a Nature Communications paper entitled “Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots”[DOE PAGES].  Its authors note, first, that “the majority of quantitative insights” into the generation of multiple electron-hole pairs per photon have been obtained by studying solutions of isolated quantum dots instead of the quantum-dot films used in devices, in which the dots are not isolated, but electronically coupled.   Second, existing quantum-dot film studies had not clarified how the electronic coupling affected the production of multiple excitons.  The effects of electric fields, and the role of certain multicarrier deexcitation processes, were also unknown, and rapid and reliable means to evaluate charge-multiplying nanomaterials’ performance were lacking.  The authors addressed these problems by applying an experimental method previously used with organic materials to quantum-dot films, and discovered ways to chemically treat the films to enhance the electric current they generated from light.  

A project conducted from 2002 to 2015 examined what happened to electrons in quantum dots that were excited by exposure to successive laser pulses.  The first pulse would drive electrons into new states of motion, which would affect their interactions with the laser pulses that followed.  The electrons’ responses were inferred from plotting their intensity with respect to two dimensions:  the light-wave frequencies of the initial and later laser pulses.  The report “Optical Two-Dimensional Spectroscopy of Disordered Semiconductor Quantum Wells and Quantum Dots”[SciTech Connect] describes the project’s various experimental setups and results in some detail.  The “quantum wells”[Wikipedia] of the title were semiconductor sections of very little thickness throughout which electrons could move quite freely, being very confined in their motion only along the wells’ thicknesses, much as the electrons in a quantum dot are confined in their motion along every direction.  The quantum dots studied first and most extensively in this project were dots that formed “naturally” as variations in the thickness of quantum wells; later experiments involved quantum dots suspended in colloids. 

 

Figure 3.  (a):  Schematic diagram of “natural” gallium arsenide quantum dots, formed as variations in the thickness of a gallium arsenide quantum well in an aluminum gallium arsenide solid.  (b) and (c):  Spectrum of quantum-well and quantum-dot ensemble at temperatures of 6 kelvins (b) and 50 kelvins (c).  Features shift to lower frequencies but cover wider frequency range with increase in temperature.  (After “Optical Two-Dimensional Spectroscopy of Disordered Semiconductor Quantum Wells and Quantum Dots”[SciTech Connect], p. 10.)

The quantum dots’ responses provided data about how electrical charges move within and between them, and how they gain or lose energy, in response to photons from outside and to the thermal vibrations of the dots’ own atoms.  As the report notes, such data may inform designers of electronic devices that have quantum dots as working elements.  The dots’ behavior, having prominent quantum-physical features, may even enable them to store and manipulate qubits (quantum bits) in a quantum computer[OSTI], each of which represents not only a 1 or a 0, but different combinations of both. 

In passing, the report’s author notes a fact associated with the atomic vibrations that occur in quantum dots.  Single atoms don’t vibrate, but atoms bound together in molecules do.  Quantum dots are often described as “artificial atoms” because their electrons, unlike the electrons in bulk solids[Wikipedia], have an atomlike discreteness in their possible energy levels[Wikipedia].  But given their ability to vibrate like molecules (which also have an atomlike discreteness in their possible energy levels[Wikipedia]), quantum dots are more like molecules than atoms. 

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Purposes

While the aforementioned reports described investigations of quantum-dot properties with the idea of informing eventual practical applications generally, other projects have aimed more directly at applying quantum dots to particular purposes. 

A quantum-dot based device that produces a “white” laser beam—actually a combination of red, green, and blue laser beams—is described in U.S. Patent 9,373,931 B2, “Red, green, and blue lasing enabled by single-exciton gain in colloidal quantum dot films”[DOepatents].  Light, or any other electromagnetic wave, is emitted when electric charges vibrate and thus produce waves in the electromagnetic field around them.  If the charges can be made to vibrate in step with each other[Wikipedia], the waves will also be coherent[Wikipedia] instead of vibrating with random phases like ordinary nonlaser light. 

Lasers made of solid layers of semiconducting material have not been able to cover the visible spectrum the same way to make a beam that would appear white to human vision.  Other devices made with semiconducting quantum dots have been limited by competing internal processes from getting their electric charges to vibrate coherently.  The type of device described in the patent uses a film of densely packed quantum dots, each of which consists of a cadmium selenide core in a zinc cadmium sulfide shell, to absorb energy from a “pump” light source[Wikipedia] and turn it into coherent laser beams.  The dots are shaped more like pyramids than spheres, and have different sizes, the larger ones emitting the longer wavelengths.  The patent, somewhat unusually, describes not only the device itself, but also experiments with the kind of quantum dots it contains that indicate what internal processes they undergo to produce laser light by the vibrations of the electrons and holes in single excitons. 

Lower-frequency lasers based on quantum dots are the subject of the paper “Mid-Infrared Quantum-Dot Quantum Cascade Laser: A Theoretical Feasibility Study”[DOE PAGES] published in the MDPI journal Photonics.  Quantum cascade lasers[Wikipedia] normally consist of a stack of closely spaced quantum wells.  If the wells were widely separated, an electron with sufficiently low energy in any one well would be practically confined to that well, since the only stable electron states outside the well are states of higher energy.  But when the wells are as close together as they are in a quantum cascade laser, electrons have a nonnegligible probability of crossing the gaps between wells, so they are only somewhat confined.  Electrons somewhat confined to the wells on one side of the laser have higher energies than electrons that are similarly somewhat confined to the wells on the other side.  An electron starting out on the high-energy side can thus undergo a cascade-like process in which it is first energized, then emits a photon as its energy decreases, moves into the adjoining lower-energy well, emits another photon there, moves to the next lower-energy well, emits another photon, and so on until it reaches the last quantum well. 

The feasibility study published in Photonics examined a variant of this design, in which quantum dots were placed between the quantum wells.  The authors had previously calculated that such devices could operate with lower densities of electric current while also achieving an amplification comparable to that of similar devices based on quantum wells alone.  Their paper describes calculations of how the performance of one proposed design is affected by the density of its quantum dots and differences between their electrons’ environments, which affect the frequency of the light they emit.  The authors found that a high quantum-dot density could compensate for the adverse effects of moderately high variation in their electrons’ environments, but would require a higher current density to function.  However, the variation in the electrons’ environments has been made smaller with present fabrication techniques, and thus allows for both a lower quantum-dot density and a significantly lower current density than present quantum-well cascade lasers. 

The light from quantum dots needn’t be coherent to be useful.  The report “Science-based design of stable quantum dots for energy-efficient lighting”[SciTech Connect] notes that dots made of cadmium selenide, cadmium telluride, or similar materials can be manufactured to emit light at chosen narrow frequency ranges, but would convert their input energy into light too inefficiently to be practical unless holes were confined to them by being coated with a suitable shell material.  Shells and cores are necessarily made of different materials, though, and their different-size atoms line up differently.  For the quantum dots described in this report, this results in the cores being compressed along their radii and the shells being stretched along tangents to their surfaces.  Different treatments can relieve these stresses but reduce the efficiency of the dots’ light output.  The authors thus developed a mathematical model[Wikipedia] of such dots to explore how suitable ones might be fabricated. 

Figure 4.  Left:  Electron microscope image of quantum dots having cores of one material and shells of a different material.  Middle:  Higher-resolution image.  Right:  Image of simulated cubic nanoparticle approximately 3.83 nanometers wide of cadmium tellurium selenide (formula CdTe1-xSex, x=0.2) used in a molecular-dynamic mathematical model.  Cadmium atoms are shown in red, tellurium in blue, and selenium in yellow; three of the cube’s faces are terminated by cadmium atoms while the opposite faces are terminated by a mixture of tellurium and selenium atoms.  (After “Science-based design of stable quantum dots for energy-efficient lighting”[SciTech Connect], pp. 100 and 66.) 

“Final Progress Report for Project Entitled: Quantum Dot Tracers for Use in Engineered Geothermal Systems”[SciTech Connect], explored the use of the same kinds of quantum dots for a specific lighting use:  characterizing fracture networks in underground reservoirs that are engineered for extracting geothermal energy[OSTI].   The object was to eventually produce tracers that would sorb[Wikipedia] and diffuse in predictable ways with fracture surfaces so that the areas of those surfaces could be determined.  The project used not only computer models, but actual dots made in the lab and tested there and in the field.  The dots’ core/shell structure served not only to enhance their light output but to resist their oxidation in water, in which they would have to function in a geothermal reservoir.  These dots were also further contained in silica, with several dots inside each silica capsule.  The encapsulation, after heat treatment in a simulated geothermal environment, enhanced the dots’ luminescence even more.  The report describes experiments with different preparation methods and their resulting beneficial or detrimental effects, showing how the researchers arrived at a tracer design.  The last efforts described involved investigating how to scale up the researchers’ quantum-dot synthesis method from producing 50 milliliters per run to liter-scale and, eventually, hectoliter scale per run for field testing.  The researchers demonstrated a tenfold increase in synthesis volume without loss of the dots’ quality and stability under conditions in a geothermal reservoir. 

 

Figure 5.  Photo of setup for producing quantum dots in a batch-reaction volume of about 4 liters.  (After “Final Progress Report for Project Entitled: Quantum Dot Tracers for Use in Engineered Geothermal Systems”[SciTech Connect], p. 9.)  

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As components

The research just discussed involved quantum dots that are intended to function as the primary or sole component of some device or system.  Other investigations of quantum-dot uses envision them as playing one significant role in a system alongside other nanoscale components. 

One 2015 paper[DOE PAGES] in the journal Scientific Reports deals with a way to couple a type of nanometer-scale antenna to quantum dots or dot clusters much larger than 10 nanometers to enhance or shape the dots’ electromagnetic-wave output.  Here, the electromagnetic waves processed by the antenna are not the long-wavelength radio waves of a broadcast antenna, but the much higher-frequency, few-hundred-nanometer waves of visible light that the quantum dots produce.  A typical antenna for such systems contains a nonconducting dielectric film, about 14 to 30 nanometers thick, sandwiched in the gap between a nanoscale metal patch and a metal substrate.  Quantum dots with diameters smaller than 10 nanometers can be manufactured to precisely align with these gaps, but uneven dielectric layers would result from trying to build in quantum dots or dot clusters that were much larger.  The paper describes a way to solve this problem, by making the antennas first and then placing large, thick-shelled, single-nanocrystal quantum dots along their edges instead of within the dielectric gaps.  The lowest-energy vibrational mode of the antenna’s electromagnetic field has a nonnegligible strength beyond its edges, so the quantum dots there can interact electromagnetically with the antenna.  A proper combination of dot size, metal patch size, and dielectric layer thickness would result in the dots’ resonating with the antenna’s lowest-energy vibrational mode.  The paper describes how such nanoscale devices were made and functioned, along with a mathematical analysis of how they enhanced the quantum dots’ output of electromagnetic waves. 

 

Figure 6.  Comparison between quantum dots coupled to patch nanoantennas with diameters of 142 nanometers and 104 nanometers.  (a) and (b) show scanning electron microscope[Wikipedia] pictures of the patch nanoantenna arrays and the structures with “giant quantum dots” coupled to individual nanoantennas.  (c) shows a transmission electron microscope image of the giant quantum dots with 4-nanometer cores of cadmium selenide with thin shells composed of 16 monolayers[Wikipedia] of cadmium sulfide, surrounded in turn by shells of silicon dioxide 10 to 12 nanometers thick; (d) shows the spectrum of light emitted by the dots.  (e) and (f) are intensity distributions for the dot-antenna nanostructures’ light output in response to stimulation by an outside light source; (g) shows fluorescence of giant quantum dots on arrays of patch nanoantennas with different array spacings and antenna diameters.  (After “Coupling single giant nanocrystal quantum dots to the fundamental mode of patch nanoantennas through fringe field[DOE PAGES], p. 2.) 

Another paper, published later in Nature Communications by a different group, describes the coupling of quantum dots to a different kind of antenna—nanoscale diamonds containing a certain type of light-producing impurity.[DOE PAGES]  Pure diamond consists of a particular regular array of carbon atoms.  Similar arrays that replace two neighboring carbon atoms, one with a nitrogen atom and the other with either nothing or a single electron (almost nothing), can produce light when the replacement sites are stimulated by various means.  Such “nitrogen-vacancy centers”[Wikipedia] in nanodiamonds are inevitably near the nanodiamonds’ surfaces, where they could interact strongly with other nanoscale structures, like (semiconducting) quantum dots or metallic nanoparticles of similar size.  The Nature Communications paper describes “a general facile bottom-up synthetic approach” to making nanostructures that couple metallic or semiconducting quantum dots to nanodiamonds with nitrogen-vacancy centers “in a highly controlled manner”, which improves on the typically complicated, time-consuming processes that involve top-down lithography or individual particle manipulation and can be hard to scale up. 

  

Figure 7.  Universal synthetic route for “hybrid nanodot-metal nanoparticles”.  (a)  Schematic synthetic paradigm illustrating different growth stages (S1-S6).  Stage S1:  pure quantum dot.  Stage S2:  acid-treated dot with carboxylic groups[Wikipedia].  Stage S3:  dot with functionalized Poly(vinylpyridine) molecules[Sigma-Aldrich].  Stage S4:  anchoring metal ions onto the dot surface.  Stage S5:  nucleation of metal nanoparticles on the dot surface.  Stage S6:  growth of metal nanoparticles on the dot surface.  (b)  A typical large-scale transmission electron microscope image showing “excellent dispersion and uniformity” of hybrid dot-gold nanostructures made by following the synthesis scheme shown in (a).  Scale bar in (b) represents 200 nanometers.  (After “Nanodiamond-based nanostructures for coupling nitrogen-vacancy centres to metal nanoparticles and semiconductor quantum dots”[DOE PAGES], p. 3.) 

Some of the reasons for interest in such structures are evident from the authors’ citation of earlier papers demonstrating their use in measuring the magnetic fields of individual electrons and temperatures in living cells; another point of interest is that the authors’ coupled quantum dot/nanodiamond structures “can serve as a structural scaffold for self-assembling bottom-up hybrid quantum devices”.  The precise control demonstrated by the authors’ new fabrication method “is a pre-requisite for investigating the underlying physics and further engineering-related optical properties” of the nanostructures, which “offer a toolset capable of tailoring properties of [nitrogen-vacancy centers] via various coupling interactions and strength, and are fundamentally different from those structures in prior study”. 

One other report describes Northwestern University researchers’ experimental examination of how organic molecules, adsorbed onto the surfaces of semiconducting quantum dots, control how the dots’ electric charges are confined, and their mathematical modeling of how electrons are transferred between the dots and different adsorbed molecules.  Their report, “Chemical Control of Charge Trapping and Charge Transfer Processes at the Organic-Inorganic Interface within Quantum Dot-Organic Complexes”[SciTech Connect], describes “exciton delocalizing ligands”—ligands that alter the confinement of excitons in quantum dots—as well as how a quantum dot’s surface chemistry dictates the probability that the dot will exchange electrons with a small molecule.  The researchers believed that the fundamental advances they made would enable them to design quantum dots to be “the next great class of visible-light photocatalysts”—substances that can accelerate chemical reactions that involve light.[Wikipedia] 

Plans for advancing our understanding of things, being necessarily based on the incomplete knowledge that we start with, always have some probability of requiring modification as the investigation proceeds; and the investigation may conclude with more or less information than one intended.  The Northwestern project’s report shows that even successful projects like this one can still illustrate all of these common situations.  While the project’s original plan called for using three different major experimental techniques, the researchers ended up using only two of them as the third proved too technically difficult to apply to the systems they were studying; however, they did find themselves making major use of a different third analysis technique.  Also, the report’s author, while indicating that their results didn’t address some of the questions they originally proposed to address, did say that the project “was extremely successful in that our achievements … addressed the great majority of questions we outlined in the original proposal and answered questions I did not think to ask in that original proposal”—a reminder that, in attempting anything new, success may consist in achieving something different from one’s original aim. 

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References

Wikipedia

Reports available through OSTI’s SciTech Connect

  • “Chemical Control of Charge Trapping and Charge Transfer Processes at the Organic-Inorganic Interface within Quantum Dot-Organic Complexes” [Metadata]
    Northwestern University
    November 6, 2015; 6 pp.

Journal articles available through DOE PAGES

Patents available through DOepatents

  • “Red, green, and blue lasing enabled by single-exciton gain in colloidal quantum dot films” [Metadata]
    U.S. Patent 9,373,931 B2
    Assignee:  Brown University
    June 21, 2016; 22 pp. 

Previous In the OSTI Collections articles

  • December 2012 (“From ‘1 or 0’ to ‘1 or 0 and Both’—Toward Real Quantum Computers”)
  • September 2016 (“Geothermal Energy”)

Additional references

  

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Last updated on Tuesday 21 March 2017