Electric Dipoles

Shaping light emission with electric dipoles

A group led by Philippe Guyot-Sionnest has produced compelling evidence of an unsuspected structural feature of nanocrystals: a surprisingly large dipole moment. Furthermore, this group exhibited the existence of the dipole moment using two completely different measurements. Research is currently underway to describe the mechanism of this phenomenon, as well as to search for its existence in other families of crystals.

Nanocrystals are materials that exhibit crystalline structures with sizes on the order of 10-9, or one-billionth, meters (nanometer). In addition to their small size these compounds exhibit a relation between their size and the spectrum of light they will absorb and emit. When nanocrystals absorb light of certain wavelengths they will emit extremely intense fluorescent light in response. Researchers can control the size of the nanocrystals and thus fine-tune the color of the emitted light to their liking (see Figure 1). The photo at right shows how drastically color can be changed by altering the size of the nanocrystals. The possible technological applications of these materials are broad; currently they are used in light-emitting diodes (LEDs) and biological markers used in staining and viewing cells. For some time cadmium selenide (CdSe) nanocrystals have been considered one of the best materials for studying nanometer scale semiconductors due to their size-dependent absorption spectra and the widely accepted model that explains them.

Researchers have explained the size-dependent spectra in CdSe nanocrystals by assuming a spherical model of electron distribution. Approximations of the effective mass and Coulomb interactions (potential energy of interaction of two charges separated by a given distance) as well as seven other parameters seemed to adequately explain the size-dependent absorption spectra. Researchers believed this model produced quality measurements and useful theory on nanocrystals. However, recent measurements taken by a team of scientists led by Philippe Guyot-Sionnest at the University of Chicago MRSEC challenges this model. Their data suggests that the near-spherical model may explain the spectral data but in an innacurate fashion. In short, the charge distribution may be unevenly distributed; this new finding complicates measurements but offers a new understanding of electron distribution in nanocrystals.

Guyot-Sionnest and his colleagues discovered in CdSe nanocrystals what chemists call a dipole moment, which is a measure of the separation of charge in a molecule. The dipole moment is calculated simply as the product of the magnitude of the charge and the distance between them. In a diatomic molecule where both atoms are the same (i.e. oxygen or nitrogen gas, O2, N2) neither atom has any net charge and thus no dipole moment exists (symmetric charge distribution). In diatomic molecules that are ionically bonded (one electron transferred from one atom to another), such as potassium fluoride (K+F-) there is an asymmettry of charge distribution, and a strong dipole moment can be measured. Dipole moments in crystals are more complicated than those found in diatomic molecules, but the concept is the same. The CdSe nanocrystals in this study exhibited a relatively large dipole moment (10 times larger than a molecular dipole moment) that increased significantly with the size of the nanocrystals. The assumption of a spherical charge distribution did not take this dipole moment into account when studying nanocrystals

The evidence for this strong dipole moment is the result of two very different methods: low-frequency electrical response and multiphoton excitation. The first technique (also called dielectric dispersion) involves placing a fluid containing the nanocrystals between two capacitor plates and running an oscillating current across the plates. If the oscillation is slow, the dipoles (imagine a + and - charge on opposite sides of the nanocrystal) line up with + charge towards the - plate and the - charge towards the positive plate. The voltage normally measured between the plates is lower than it would be in the absence of dipoles. If the oscillation is fast enough the dipoles cannot line up and the voltage won't be lowered. Thus if one runs a current of low frequency to one of high frequency the voltage measured will go from lower (due to the dipoles) to higher (see Graph 1). The dipole moment of all of the nanocrystals is measured as the magnitude of the change of voltage. Since the number of nanocrystals in the fluid between the capacitor plates is known, one can estimate the dipole moment on each nanocrystal.

The second method of measuring dipole moment involves shining visible light on the nanocrystals and inducing excitation. The excitation causes the crystals to emit fluorescent light. The light used on the sample must be of high frequency and tuned so that the photon energy is equal to the energy needed to excite an electron (see Figure 2). The group led by Guyot-Sionnest found that infrared light with half the frequency can be used.

However, now excitation of the electron uses two photons. Each nanocrystal has several electrons; if the nanocrystal has no dipole moment some excitations can be made with one photon and others by two photons. However, Guyot-Sionnest found excitations that could result from either one or two photons (see Graph 2). That should never happen … unless a dipole moment was already present. By comparing the brightness of fluorescent light emission induced by one and by two photons, one can estimate the magnitude of the dipole moment. The estimate made in this fashion agrees with the measurement taken using the capacitor apparatus above.

Having firmly established the magnitude of the dipole moment, the researchers now hope to explain the origin of this phenomenon. It is known that CdSe nanocrystals have the wurtzite crystal structure, meaning that there is an internal polarization in the structure. Comparing the dipole moments of different sizes of nanocrystals with this internal polarization shows a close agreement. Right now the strongest evidence for a significant dipole moment has been described only in CdSe nanocrystals. CdSenanocrystals have a single polar axis and thus were most likely to exhibit substantial dipole moments. The effect of this research is potentially far-reaching, but more work needs to be completed. Current research seeks the same phenomenon in other crystal types, such as zinc blende. It is possible that significant dipole moments will occur in any small structure when the crystal is cut asymmetrically (i.e. rectangular, not cubic), but more research is needed. Despite the implicit assumption in many textbooks, science rarely progresses in a perfectly linear fashion. Scientists must often retrace their steps when previous findings are suddenly challenged by new data. A recent discovery at the University of Chicago MRSEC presents one such new challenge to the previous way of thinking.

Reference:

  1. "Dielectric Dispersion Measurements of CdSe Nanocrystal Colloids: Observation of a Permanent Dipole Moment" Sean A. Blanton, Robert L. Leheny,- Margaret A. Hines, and Philippe Guyot-Sionnest. Phys. Rev. Lett.. 79:5, 865-868 (4 August 1997).