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8/13/2019 QDs White Light Semiconductor Materials Sandra

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Quantum dots brighten the future of lighting

byDavid Salisbury| Posted on Tuesday, May. 8, 2012 2:56 PM

(iStock)

With the age of the incandescent light bulb fading rapidly, the holy grail of the lighting

industry is to develop a highly efficient form of solid-state lighting that produces high quality

white light.

One of the few alternative technologies that produce pure white light is white-light quantum

dots. These are ultra-small fluorescent beads of cadmium selenide that can convert the blue

light produced by an LED into a warm white light with a spectrum similar to that of

incandescent light. (By contrast, compact fluorescent tubes and most white-light LEDs emit a

combination of monochromatic colors that simulate white light).

Vial holding original white light quantum dots on the left and the enhanced quantum dots on

the right. (Rosenthal Lab)
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Seven years ago, when white-light quantum dots were discovered accidentally in a Vanderbilt

chemistry lab, their efficiency was too low for commercial applications and several experts

predicted that it would be impossible to raise it to practical levels. Today, however,

Vanderbilt researchers have proven those predictions wrong by reporting that they have

successfully boosted the fluorescent efficiency of these nanocrystals from an original level of

three percent to as high as 45 percent.

Potential commercial applications

Forty-five percent is as high as the efficiency of some commercial phosphors which

suggests that white-light quantum dots can now be used in some special lighting

applications, saidSandra Rosenthal,the Jack and Pamela Egan Chair of Chemistry, who

directed the research which isdescribed onlinein theJournal of the American Chemical

Society.The fact that we have successfully boosted their efficiency by more than 10 times

also means that it should be possible to improve their efficiency even further.

Sandra Rosenthal holding a model of a white-light quantum dot. (Daniel Dubois / Vanderbilt)

The general measure for the overall efficiency of lighting devices is called luminous

efficiency and it measures the amount of visible light (lumens) a device produces per watt.

An incandescent light bulb produces about 15 lumens/watt, while a fluorescent tubes put out

about 100 lumens/watt. White light LEDs currently on the market range from 28 to 93

lumens/watt.

We calculate that if you combine our enhanced quantum dots with the most efficient

ultraviolet LED, the hybrid device would have a luminous efficiency of about 40

lumens/watt, reportedJames McBride,research assistant professor of chemistry who has

been involved in the research from its inception. There is lots of room to improve the

efficiency of UV LEDS and the improvements would translate directly into a higher

efficiencies in the hybrid.

An accidental discovery

Quantum dots were discovered in 1980. They are beads of semiconductor materialthe stufffrom which transistors are madethat are so small that they have unique electronic
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properties, intermediate between those of bulk semiconductors and individual molecules. One

of their useful properties is fluorescence that produces distinctive colors determined by the

size of the particles. As the nanocrystals size shrinks the light it emits shifts from red to blue.

The Vanderbilt discovery was that ultra-small quantum dots, containing only 60 to 70 atoms,

emit white instead of monochromatic light.

James McBride (Susan Urmy / Vanderbilt)

These quantum dots are so small that almost all of the atoms are on the surface, so the

white-light emission is intrinsically a surface phenomena, said Rosenthal.

One of the first methods various groups used in the attempt to brighten the nanocrystals was

shelling growing a shell around them made of a different material, like zinc sulfide.Unfortunately, the shells extinguished the white light effect and the shelled quantum dots

produced only colored light.

Chemists followed their noses

Following a lead from some research done at the University of North Carolina, the

researchers decided to see if treating the quantum dots with metal salts would have a

brightening effect. They noticed that some of the salts seemed to produce a small10 to 20

percentbut noticeable improvement.

They were acetate salts and they smelled a bit like acetic acid, said McBride. We knew

that acetic acid binds to the quantum dots so we decided to give it a try.

The decision to follow their nose proved to be fortunate. The acetic acid treatment bumped up

the quantum dots fluorescent efficiency from eight percent to 20 percent!

Acetic acid is a member of the carbocyclic acid family. So the researchers tried the other

members in the family. They found that the simplest and most acidic memberformic acid,

the chemical that ants use to mark their pathsworked the best, pushing the efficiency as

high as 45 percent.
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The brightness boost had an unexpected side effect. It shifted the peak of the color spectrum

of the quantum dots slightly into the blue. This is ironic because the major complaint of

white-light LEDs is that the light they produce has an unpleasant blue tint. However, the

researchers maintain that they know how to correct the color-balance of the boosted light.

Scanning electron microscope image of a quantum dot that shows the individual atoms.

(Rosenthal Lab)

The researchers next step is to test different methods for encapsulating the enhancedquantum dots.

Other contributors to the study include graduate studentsTeresa E. Rosson,Sarah M.

Claiborne and undergraduate research studentBenjamin Stratton,who is now atColumbia

University.

The work was supported by a grant from theNational Science Foundation.

Read the 2005 story about the original discovery:Quantum dots that produce white

light could be the light bulbs successor

Contact:David Salisbury, (615) 322-NEWS

[email protected]

Semiconductor Materials: Trends and Status

R. Muralidharan
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Solid State Physics Laboratory, Defence Research and Development Organisation, Delhi-110054,

India

Functional materials cover a wide range of organic and inorganic materials and their physical and

chemical properties are sensitive to a change in the environment such as temperature, pressure

electric field, magnetic field, electromagnetic radiation adsorbed gas molecules etc. SemiconductorMaterials are a subset of functional materials and have revolutionised electronics, opto-electronics

and photonics. In this talk a brief overview of some trends in semiconductor materials would be

presented. The IT revolution triggered by silicon based electronics is well known. However, compound

semiconductors have played a key role in the area of defence, space and communication electronics.

In particular the trends in III-V semiconductors would be discussed in detail. The enabling properties

compound semiconductors would be reviewed. This talk would cover the research and development

work being carried out at Solid State Physics laboratory.

Molecular Beam Epitaxy (MBE) and Metallorganic Chemical vapour deposition (MOCVD) techniques

for deposition of epitaxial layers have evolved along with the availability of ultra high purity materials.

This has enabled preparation of epitaxial layer of desired composition, thickness with atomically

smooth interfaces and doping. Further band gap engineering to tailor the electronic properties was

rendered possible .The methods of preparation, characterisation techniques and the results obtained

in Ga-Al-In-As and Ga-Al-N based epitaxial layers and devices would be described. Low dimensional

structures like quantum wells and quantum dots have been grown and characterised by High

Resolution X ray Diffraction and Transmission Electron Microscopy. Pseudomorphic and Metamorphic

High Electron Mobility Transistor structures have been grown and devices with gate length of

~0.25um have been fabricated. High power laser diode and arrays have been fabricated. AlGaN/GaN

HEMT structures have been grown by MBE and MOCVD and devices have been fabricated.

HgCdTe is an important material for the fabrication of Infra red detectors for thermal imaging

applications. HgCdTe epitaxial layers are grown on CdZnTe substrates because of close lattice

matching between them. Preparation of high quality substrates and epitaxial layers is extremely

difficult. This talk would describe the challenges and also the progress made in this area.

CNT and grapheme are emerging as futuristic materials for electronics. Some results on vertically as

well as horizontally aligned CNT would also be presented.

qds white light semiconductor materials sandra

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