The accompanying image shows photoluminescence from four different samples of CdTe nanocrystals, each embedded in a polymer matrix. The photoluminescence occurs because the samples are being irradiated by a UV light source. The nanocrystals in each vial have different average sizes. The sizes are 4.0, 3.5, 3.2, and 2.8 nm. (a) Which vial contains the 4.0-nm nanocrystals?

Verified step by step guidance

1. Understand that photoluminescence is the emission of light from a material after it absorbs photons, typically from a UV light source.
2. Recognize that the size of nanocrystals affects the wavelength of light they emit due to the quantum confinement effect.
3. Recall that larger nanocrystals tend to emit light at longer wavelengths (redder light), while smaller nanocrystals emit at shorter wavelengths (bluer light).
4. Identify the color of light emitted by each sample in the image, noting that the sample emitting the reddest light likely contains the largest nanocrystals.
5. Determine which vial corresponds to the 4.0-nm nanocrystals by matching the color of emitted light to the expected wavelength for larger nanocrystals.

Key Concepts

Photoluminescence

Photoluminescence is the emission of light from a material after it absorbs photons, typically from a UV light source. In nanocrystals, this process is influenced by their size and composition, leading to variations in emitted light color and intensity. Understanding this phenomenon is crucial for analyzing the behavior of CdTe nanocrystals when exposed to UV radiation.

Quantum Confinement

Quantum confinement occurs when the dimensions of a semiconductor nanocrystal are reduced to the nanoscale, leading to discrete energy levels. This effect significantly alters the electronic and optical properties of the material, including its photoluminescence characteristics. The size of the nanocrystals directly affects the energy gap, which in turn influences the color of light emitted.

Nanocrystal Size and Emission Wavelength

The size of nanocrystals, such as those made from CdTe, directly correlates with the wavelength of light they emit. Larger nanocrystals typically emit light at longer wavelengths (red-shifted), while smaller ones emit at shorter wavelengths (blue-shifted). This relationship is essential for identifying which vial contains the 4.0-nm nanocrystals based on the observed photoluminescence.