Recently, I began a series of experiments with the goal of producing nanometer-size crystals of cadmium-based semiconductors. These nanocrystals (NCs) are also referred to as quantum dots (QDs). Due to their incredibly small size (ranging from <1.5 to >8 nanometers), these usually non-fluorescing compounds develop intense, long-lasting colors excitable by UV and visible light LEDs, lasers, etc. The colors produced are a function of the particle size (blue to red and infrared, depending on the diameter of the nanocrystals ) with the smallest NCs fluorescing in the blue and green. The fluorescence is due to a phenomenon called quantum confinement.

When electrons and holes (exciton pairs) generated by photons are confined within a space, or quantum box, smaller than the Bohr exciton radius (i.e., the normal, off-the-shelf, preferred, bulk material electron-to-hole distance of an exciton pair), the semiconductor's band gap widens and its fluorescence shifts towards the blue. The smaller that box, the bluer the fluorescence. Conversely, the looser the confinement, the bigger the quantum box, the redder the fluorescence. Eventually, one enters the infrared and the invisible. The fluorescence is generated when electron and hole recombine. The restricted confinement requires additional energy and this causes the wavelength shift. At this preliminary stage, I am anxious to report my early attempts and any successes, however modest they may be.

I was led down the path to these colloidal and near-perfect crystalline assemblages by a desire to find fluorescent compounds and biological markers better matched to LED excitation than traditional fluorochromes. (Be sure to see the LED Fluorescence pages on this site for additional background.) The smallest nanocrystals are built of only a hundred atoms or so, forming hexagonal crystal structures akin to wurtzite or tetrahedral constructs similar to zinc blende. Their unique behavior has caused some to regard NCs as super-atoms! Quantum dots are regarded as zero dimensional (0D) points. Extended into a single dimension (1D) by, for example, self-assembling, quantum wires are formed. Extension into the second dimension (2D) leads to nanosurfaces referred to as quantum wells, etc. Films of NCs in polymers or coatings on indium tin oxide (ITO) glass can produce electroluminescent devices. NCs are being used in solar cell research. Some manufacturers are talking about adding these crystals to fabrics for brighteners and intense colorants. Despite the existence of ongoing work in the area of quantum dots for over a decade, the field remains one of the most interesting and cutting-edge areas of science!

As will be shown in upcoming, future pages, the major weaknesses of LED Fluorescence (i.e., not quite narrow enough emission bands and reduced brightness in comparison with xenon and mercury-arc sources) are well tolerated by suitably fabricated semiconductor NCs. Quantum dots can be efficiently excited by broad swathes of light below (i.e., at wavelengths shorter than) the emitted fluorescence peak. Multi-colored fluorescent ensembles of NCs can be excited to fluoresce in their respective colors through the excitation of just one bright LED! The characteristics of NCs and LEDs mesh almost hand-in-glove. Both are quantum devices. The NCs are photon driven while LEDs are electrically driven. In fact, research is being done worldwide on producing LEDs from NCs. More recently, layer-by-layer (LBL) techniques and sol-gel developments are helping propel nanocrystal research forward in this major subdivision of nanotechnology.

This photo shows the results of several of my recent experiments where cadmium selenide crystals were produced, collected, and dispersed in toluene. The excitation sources are long-wave UV lamps operating at 365 nm. The spectral range peaks represented here span from 514 nm at right to 610 nm at left. (In this photo, some colors needed adjustment to better match what the eye actually saw.) The crystals are all CdSe core crystals stabilized or passivated with oleic acid, differing only in size. While apparently bright, they are near the bottom of the heap in comparison to nanocrystals capped with several other select organic or inorganic shells!

There is room here for improvement in color range, monodisperse-size spread, and quantum yield. These graphs were produced during one experiment using the fluorimeter-spectrophotometer instrument referenced below. The absorption curves' peaks and valleys (shown in white) tell a lot about the nanocrystals, beginning with the approximate crystal sizes. They indicate how monodisperse (i.e., how close in size with a small standard deviation, for example) the products are. By integrating the fluorescence curves (in color), the quantum yield can be determined relative to standard dyes. And, of course, they indicate the useful range for exciting the NCs as well as the fluorescence emission peak.

To help characterize the nanocrystals, I constructed a combined fluorimeter and absorption spectrometer cell. This allows easy measurement of the fluorescence produced by the fiber-optic piped super-bright blue LED light seen at the upper right. Alternately, the absorption of white light from an intense tungsten, fiber-optic white light source can be used to study the size and behavior of the crystals and is seen coming in at upper left. The fiber-optic at lower right is connected to a PC-plugin spectrometer. The green square in the center is a glass fluorimeter cuvette filled with a "solution" of NCs in toluene. The white light source is shut off when a fluorescence measurement is being run. The aluminum plug seen at lower left is simply blocking off the unused port.

Although I do not have access to a high-resolution TEM, the approximate size of the nanocrystals can be determined by evaluating the absorption spectrum, as mentioned above, and comparing the major peaks to those cited in the literature. The individual fluorescence peaks reveal similar information along with giving an indication of how monodisperse the batch is, based on the FWHM (full width at half-maximum) of the curve. It really is exciting producing and working with these submicroscopic particles that inhabit the quantum realm and therefore adopt such strange and useful behavior.

Next up, looking at the process of preparing nanocrystals.