Smaller Than Stippling : Quantum Dots

With an article out in UDaily, we are connecting the last of the dots in putting Art in Science together.

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Sleeping Girl, Roy Lichtenstein, Legion of Andy

Dot Techniques in Art

Dots are featured in many different art techniques.The dot technique that may be the most familiar is the Ben-Day dots most frequently seen in comic books. Ben-Day dots are characterized as a series of large unicolor dots with of uniform size and spacing.

Although they were created by and named after Benjamin Henry Day, Ben-Day dots are most iconic in the work of Roy Lichtenstein during the pop art movement in 1950-1960. Lichtenstein used the dots to create different tones in his pieces while still largely using only primary colors. The close up of Lichtenstein’s sleeping girl to the right demonstrates the use of red dots to develop the peach tone of skin and the deeper red of the lip.

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Femmes au bord de l’eau, Georges Seurat, Wikimedia

Pointillism is another dot related art technique. Developed by Georges Seurat and Paul Signac in the 1880s as a derivative of impressionism, pointillism overlays small colored dots.  Similarly to impressionism, pointillism does not blend any colors together but uses the overlaid colors to create the impression of a new color.

This concept of overlaying colors to create the impression of a new color is used on a much smaller scale in color printers. Color printers follow four color model, also known as the CMYK model. The CMYK model overlays dots of cyan (C), magenta (M), yellow (Y), and black/key (K) to create the full spectrum of colors.

Example of Stippling

While there are more art techniques that feature dots, the last technique we will discuss is stippling. Stippling differs from Ben-Day dots and pointillism in that the dots are small, uniform in size, but shading is created by altering the spacing between dots. The denser the dots, the darker the color appears. Because stippling relies only on the changes in spacing between dots, it is often found in monochromatic works.

Quantum Dots

Quantum dots are spherical nanosized semiconductors. A semiconductor is characterized by a material with a specific set of electrical properties. Semiconductors have a resistance between that of conductive materials (metals, low resistance) and insulators (high resistance). Hence, the material is somewhat or “semi-” conducive.

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Cadmium Telluride Quantum Dots, PlasmaChem GmbH, Wikimedia

Quantum dots absorb white light and emit colored light. The color of quantum dots depends on the size of the dot. 2-3 nanometer quantum dots emit short wavelengths colors such as blue while larger quantum dots (5-6 nanometers in radius) emit longer wavelength colors such as red. The control over quantum dot size and the resulting optical effect is useful in many technologies.

Electronic Displays

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LCD vs Quantum dot display, Engadget

Similar to organic light-emitting diode (OLED) displays, using quantum dots would create flexible electronic displays. However, large quantum dot displays will be easier to create than large OLED displays and also have longer lifetimes. Additionally, quantum dot-based LEDs have very narrow bandwidths, which means that the colors emitted by the quantum dot based LEDs are very bright, pure, and efficient. Compared to current liquid crystal displays (LCD), quantum dot displays can display 50% more color. Using the optical properties of quantum dots, quantum dot displays would allow for brighter, more colorful, and more efficient electronic displays.

Although quantum dot displays can improve on current electronic display technology, there are a few obstacles hindering its widespread use.These obstacles include, but are not limited to: difficulties manufacturing the very small blue quantum dots, and unequal quantum yields across different quantum dots. Quantum yield is a ratio that compares how much light is emitted for each unit of light used for excitation. Different quantum yields among quantum dots means that although the light absorbed by individual quantum dots are the same intensity, the light emitted by the quantum dots will differ in intensity. However, quantum dot displays are already available in selected devices, and there will likely be more to come.

Cancer Imaging

One of the difficulties in tumor removal is that there is no clear boundary between the tumor and the surrounding healthy tissue. Additionally, there may be satellite tumors hidden in the surrounding tissue several centimeters away from the primary tumor. It is difficult for surgeons to completely remove all cancerous tissue while leaving all of the healthy tissue. Quantum dots can be modified to target cancerous tissue allowing surgeons to visualize both the boundary of the primary tumor and also any satellite tumors. For images and more information, please read this ActaNaturae paper.

Gene Detection

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Schematic of resonance energy transfer

The emission of quantum dots, similar to fluorophores, can be used to excite other quantum dots of a different size and color. This effect, called resonance energy transfer, only occurs when the two quantum dots are in close proximity.

To detect specific genes, a specific sequence of DNA is attached to one size of quantum dot. Called a complementary strand, this sequence of DNA will pair perfectly with DNA from a specific gene and reveal whether that specific gene is expressed. Then, DNA from a cell or person is attached to a second size of quantum dot. If the specific gene is expressed, it will bind to the complementary strand. The binding of the specific gene DNA and complementary strand DNA brings the two quantum dots into close proximity. Analyzing the quantity and location of the colors emitted reveals any resonance energy transfer, and therefore, whether the specific gene is expressed.

From art to science, dots are used to create beautiful and useful images. In the end, dots all we need to know!

Until Next Time

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