Graphene oxide (GO)
has shown great potential as a component in
various devices due to its excellent solution processability and two-dimensional
structure. However, the oxygenated form of graphene has a moderate
charge-transport capability. The latter parameter may be enhanced
through controlled deoxygenation of GO with subsequent tuning of its
work function (WF). Various reduction approaches were employed to
investigate the effect of the oxygen content on the work function
of GO derivatives as thin films on an indium tin oxide substrate.
Such films were reduced by stepwise thermal annealing in ultrahigh
vacuum up to 650 °C, by chemical reduction with hydrazine, or
by a combination of chemical and thermal reduction processes. The
effect of the GO film thickness and the flake size on the WF was also
investigated. UV photoelectron spectroscopy and X-ray photoelectron
spectroscopy were used to correlate the WF of GO derivatives with
their oxygen content. The results showed that the WF is strongly dependent
on the oxygen content, reaching a ∼1 eV difference between
GO and highly reduced GO, under the specific reduction conditions.
The film thickness affects the work function, since in thin films
interaction with the substrate is pronounced. Finally, the WF of reduced
GO after combination of chemical and thermal reduction reaches its
lowest value of 4.20 eV, due to the presence of heteroatoms which
doped the surface.
Physical phenomena such as energy quantization have to-date been overlooked in solutionprocessed inorganic semiconducting layers, owing to heterogeneity in layer thickness uniformity unlike some of their vacuum-deposited counterparts. Recent reports of the growth of uniform, ultra-thin (<5 nm) metal-oxide semiconductors from solution, however, have potentially opened the door to such phenomena manifesting themselves. Here, we develop a theoretical framework for energy quantization in inorganic semiconductor layers with appreciable surface roughness, as compared to the mean layer thickness, and present experimental evidence of the existence of quantized energy states in spin-cast layers of zinc oxide (ZnO). As-grown ZnO layers are found to be remarkably continuous and uniform with controllable thicknesses in the range 2-20 nm and exhibit a characteristic widening of the energy band gap with reducing thickness in agreement with theoretical predictions. Using sequentially spin-casted layers of ZnO as the bulk semiconductor and quantum well materials, and gallium oxide or organic self-assembled monolayers as the barrier materials, we demonstrate two terminal electronic devices the current-voltage characteristics of which resemble closely those of double-barrier resonant-tunneling diodes. As-fabricated alloxide/hybrid devices exhibit a characteristic negative-differential conductance region with peak-to-valley ratios in the range 2 -7.
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