Analysis of assembling ZnO nanoparticles into nanogap electrodes for nanoscale electronic device applications

We report the assembly of semiconducting ZnO nanoparticles into nanogap electrodes using ac dielectrophoresis (DEP) process. ZnO nanoparticles were grown using sol-gel method. As grown ZnO nanoparticles (diameter approximately 4 nm) were in a quasi-crystalline state and with increase in annealing temperature (in air) upto 400 degrees C the crystallinity was found to increase as revealed by X-ray diffraction and transmission electron microscopy. Absorption spectroscopy revealed that absorption edge of the as grown and annealed samples exhibited a blue-shift and could be attributed to the size quantization. A strong band tailing in the absorption spectra indicates the presence of shallow defects in the nanoparticles. The annealed ZnO nanoparticles (diameter approximately 9 nm) were assembled into nanogap electrodes (60 nm) using DEP process. DEP parameters such as frequency, applied voltage, and time were optimized to assemble ZnO nanoparticles into nanogap electrodes. Frequency variation study revealed that positive DEP is active at frequencies less than 500 kHz, and the negative DEP starts dominating at 1 MHz. It was observed that when DEP force overcomes the Brownian motion, the nanoparticles start chaining together in the region between the electrodes tip due to particle-field and particle-particle interactions. Under the optimized DEP parameters such as frequency of 150 kHz, peak-to-peak voltage of 3 V, and trapping time of 30 s, a minimum number of ZnO nanoparticles were assembled into nanogap electrodes. The ZnO nanoparticles based device was studied for field effect transistor (FET) characteristics and the typical current-voltage (I(D)-V(DS)) characteristic curves as a function of gate voltage (V(G)) were obtained. The curves indicate that ZnO nanoparticles based FET exhibits a significant gate effect as the conductance of the ZnO nanoparticles increases with gate voltage suggesting n-type semiconducting behavior of ZnO nanoparticles. Results show the myriad potential of dielectrophoretically assembled ZnO nanoparticles into nanogap electrodes for designing nanoscale devices for electronic and optoelectronic applications.