In-Situ vs. Ex-Situ Nanocrystallization of Mn-Doped ZnO

Following the current interest in transition metal doped dielectrics for spintronic applications, Mn-doped ZnO is being made by thermal decomposition of Mn-doped hydrozincite, 
(MnxZn1–x)5(OH)6–2n (CO3)1+n, precursors that have been prepared by a novel synthesis method. While the aim is to make solely (MnxZn1–x)O phase, experiments show that the Mn-Zn-O system is very susceptible to the formation of “contaminant” Mn-rich phases such as zinc manganate. For instance, TEM analysis shows that thermal decomposition in air of the Mn-doped hydrozincite at either 200°C or 700°C leads to the formation of Mn-rich phases around the edge of ZnO grains. On the other hand, decomposition in air at 400°C produces purely Mn-doped ZnO phase with grains around 20 nm in diameter. While ex-situ thermal decomposition of the hydrozincite precursor displays this tendency towards the formation of second phases, in-situ decomposition of the precursor using an intense electron beam in the TEM rapidly generates nanocrystals, around 5 nm in diameter, of only the wurtzite ZnO structure. The orientation of these nanocrystals is textured by the structural relationship of the ZnO to the hydrozincite. Similar transformation rates with the sample at –170°C suggest that this transformation is not simply a result of ionization-induced heating. To understand better the thermal decomposition of the hydrozincite, in-situ heating experiments are being conducted in the TEM. While the experimental results give a temperature for completed decomposition to be the same 400°C as found with ex-situ experiments, the product is quite different to the usual wurtzite-structured nanocrystals. It instead appears to be more single crystalline, with a structure inherited from the hydrozincite precursor. It is suggested that the vacuum/low pO2 conditions within the TEM column stabilize a defected form of ZnO that is structurally based on the hydrozincite lattice, not wurtzite. Thus, the results of the in-situ heating do not directly correlate with those from ex-situ decomposition in air. This currently leaves interpretation open as to the optimum method for precursor decomposition to Mn-doped ZnO.