Observation of the Direct Energy Band Gaps of Defect Tolerant Cu3N by Ultrafast Pump-Probe Spectroscopy

02 Dec 201912:00
Prof. Matthew Zervos
Nanostructured Materials and Devices Laboratory,
School of Engineering University of Cyprus,
Nicosia, Cyprus


Cu3N, Zn3N2 and Sn3N4 are earth abundant, metal nitrides, which are interesting from a fundamental and technological point of view but Cu3N has been investigated in greater detail due to its unique properties that are attractive for energy conversion and storage.  Cu3N has a cubic a-ReO3 crystal structure with a lattice constant of 3.8 Å and space group Pm3m, number 221. It is similar to the ABX3 anti-perovskite structure with a vacant body-center position, so it can readily accommodate impurities and is a defect tolerant semiconductor. Here we show that Cu3N with a cubic crystal structure can be prepared from Cu on fused SiO2 under a flow of NH3: O2 between 400°C and 600°C. All Cu3N layers exhibited distinct maxima in differential transmission at  500 nm, 550 nm, and 630 nm, 670 nm with the same spectral structure and shape on an fs time scale as shown by ultrafast pump-probe spectroscopy. We show that the maxima at 630 nm (= 1.97 eV) and 670 nm (= 1.85 eV) correspond to the M and R direct energy band gaps of Cu3N in excellent agreement with density functional theory calculations of the electronic band structure. In addition we suggest that the peaks at 500 nm (=2.48 eV) and 550 nm (=2.25 eV) most likely correspond to the M and R direct gap of certain regions of Cu3N under strain that changes the lattice constant and band gap. We discuss the charge carrier generation and recombination mechanisms in terms of Cu interstitials and vacancies that are known to be energetically located near the band edges thus allowing the observation of the direct energy band gaps in this defect tolerant semiconductor.  Finally we describe ongoing work on the synthesis and properties of Cu3N nanoparticles prepared from Cu2O nanoparticles