Authors: Aaron T. Fafarman, Weon-kyu Koh, Benjamin T. Diroll, David K. Kim, Dong-Kyun Ko, Soong Ju Oh, Xingchen Ye, Vicky Doan-Nguyen, Michael R. Crump, Danielle C. Reifsnyder, Christopher B. Murray, and Cherie R. Kagan
Journal: Journal of the American Chemical Society
Affiliation: University of Pennsylvania- Department of Electrical and Systems Engineering, Department of Chemistry, Department of Materials Science and Engineering
Colloidal nanocrystals are a super interesting material set because the size of the particle controls its band gap. As you can imagine, this is particularly handy for making one material have a variety of different colors, but it also turns out to be useful for electronic applications like solar cells, photodetectors, LEDs, and night-vision goggles (to name a few). One of the challenges in making these materials better is that the molecules bound to the surface can have a big influence on the electronic and optical behavior of the nanocrystal (in some cases, one third of the atoms in the nanocrystal are on the surface!), and probing the surface chemistry is no small feat.
In this article, Fafarman et al. perform a ligand exchange to change which molecule is bound to the surface; they replace the original molecule with a short “vibrational reporter” molecule (thiocyanate) and use FTIR (Fourier Transform Infrared) spectroscopy to learn about the surface of the nanocrystal, and how the molecule is binding to it. The nanocrystals can get pretty close together when the molecule on the surface is short; closer spacing allows better electronic coupling between them, which results in some pretty respectable electronic properties!
The authors looked at the CN stretching region of the FTIR spectra for various cadmium chalcogenide and lead chalcogenide nanocrystals; they found that the CN stretching frequency was a function of the metal (lead vs. cadmium), but not a function of the chalcogen (sulfur vs. selenium vs. tellurium). The interpretation of this trend is that the thiocyanate molecule is binding to the metal atoms (not the chalcogen atoms) on the surface of the nanocrystal. Furthermore, the difference between the CN stretching frequencies observed for the nanocrystals is comparable to the difference between the CN stretching frequencies between the small molecules Pb(NCS)2 and Cd(SCN)2.
This same region of the spectrum (the CN stretch) for ZnSe nanocrystals in particular can differentiate between an ion that is bound to the surface and one that is the compensating charge for an existing surface charge. The authors note that there is a dynamic equilibrium between the two species because isotope-labeled versions of the ligand replace both peaks in solution. After carefully washing the particles to remove any thiocyanate molecules that are compensating the charge of the ammonium counterion they came in with (instead of the charge on the nanocrystal, which is the interesting bit), the authors comparing the integrated area of the nanocrystal CN stretch peak to that of a known molarity of ammonium thiocyanate, to find the minimum number of SCN– anions necessary to compensate the surface charge. They find this number to be ~30 ions per nanocrystal.
The authors find that a film of thiocyanate-covered nanocrystals has a lower-energy excitonic peak than the analogous film with tetradecylphosphonic acid (a more conventional ligand) on the surface. This coupling also allows electrons to move more easily through the nanocrystal film; the authors observe the resistivity of lead telluride films to be many orders of magnitude lower when thiocyanate ligands are used instead of the long, organic ligands that attach to the nanocrystal during synthesis.
This paper uses a short spectroscopically informative ligand on the surface of nanocrystals to elucidate some details of nanocrystal surface chemistry as well as to achieve good solid state electrical properties. The authors intend to further use this type of analysis to understand nanocrystal materials and use them in optoelectronic devices.