Fabrication and characterization of size-controlled CuTCNQ charge-transfer complex nanocrystals

We report the selective fabrication of phase I CuTCNQ (TCNQ=7,7′,8,8′-tetracyanoquinodimethane) nanocrystals (NCs) in the size range from 20 nm to several μm. The NCs were fabricated by chemical reduction and/or cation exchange reprecipitation methods. Scanning electron microscopy, transmission electron microscopy and powder X-ray diffraction provide evidence that the CuTCNQ NCs have the phase I structure, with crystal growth along the [1 0 0] direction. Phase I is characterized by high electric conductivity and switching behavior. Powder X-ray diffraction patterns, Fourier transform infrared spectra and Raman spectra of the CuTCNQ NCs are the same as those of bulk crystals, regardless of NC size and fabrication method. The absorption around the near infrared region increases with increasing crystal size, due to inter-band transition. The size effect is discussed in relation to band structure (e.g., band gap and density of states) and lattice softening as a function of crystal size. This interesting phenomenon is important for novel electronic device applications.