School of Engineering, The University of Western Australia, Perth, WA 6009, Australia; School of Molecular Sciences, The University of Western Australia, Perth, WA 6009, Australia.
School of Molecular Sciences, The University of Western Australia, Perth, WA 6009, Australia; Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, WA 6009, Australia.
J Colloid Interface Sci. 2021 Dec;603:604-614. doi: 10.1016/j.jcis.2021.06.126. Epub 2021 Jun 24.
The properties of the oxidized surface for common materials, such as silicon and titanium, are known to be markedly different from the reduced surface. We hypothesize that surface-oxidized aluminum gallium nitride ((oxidized-AlGaN)/GaN) surface charge behavior is different to unoxidized AlGaN (with ultrathin native oxide only), which can be validated via surfactant adsorption. Understanding these differences will explain why (oxidized-AlGaN)/GaN-based sensors are better performing than AlGaN ones, which has been previously demonstrated but not understood.
The surface of an AlGaN/GaN structure was oxidized with hot piranha solution and oxygen plasma. AFM force measurements and imaging were performed to probe the charge properties of the surface in aqueous solutions of varying pH containing only an acid or base, or with an added ionic surfactant: cationic cetyltrimethylammonium bromide (CTAB) or anionic sodium dodecylsulfate (SDS).
The (oxidized-AlGaN)/GaN surface is positively charged at pH 4 and pH 5.5, although pH 5.5 should be close to the isoelectric point of the surface. The surface is negatively charged at pH 10 and pH 12, and sufficiently charged to attract cooperative adsorption of CTAB aggregates at pH 12. At pH 2, the evidence is inconclusive, but the surface is most likely positively charged. Compared to unoxidized AlGaN, the (oxidized-AlGaN)/GaN surface shows a wider range of surface charge magnitude over pH values between 2 and 12. This suggests that the (oxidized-AlGaN)/GaN surface has a higher surface hydroxyl group density than unoxidized AlGaN, which explains the higher sensitivity for pH sensors based on (oxidized-AlGaN)/GaN structures.
众所周知，常见材料（如硅和钛）的氧化表面的性质明显不同于还原表面。我们假设，经过表面氧化的铝镓氮（（氧化-AlGaN）/GaN）表面电荷行为与未经氧化的 AlGaN（仅具有超薄的本征氧化物）不同，这可以通过表面活性剂吸附来验证。理解这些差异将解释为什么（氧化-AlGaN）/GaN 基传感器的性能优于 AlGaN 传感器，这一点已经得到了以前的证明，但尚未得到理解。
用热过氧酸溶液和氧气等离子体对 AlGaN/GaN 结构的表面进行氧化。进行原子力显微镜（AFM）力测量和成像，以探测表面在含有仅酸或碱的不同 pH 值的水溶液中的电荷特性，或者在添加离子表面活性剂时：阳离子十六烷基三甲基溴化铵（CTAB）或阴离子十二烷基硫酸钠（SDS）。
（氧化-AlGaN）/GaN 表面在 pH 值为 4 和 pH 值为 5.5 时带正电荷，尽管 pH 值 5.5 应该接近表面的等电点。在 pH 值为 10 和 pH 值为 12 时，表面带负电荷，并且在 pH 值为 12 时，电荷足以吸引 CTAB 聚集物的协同吸附。在 pH 值为 2 时，证据不明确，但表面很可能带正电荷。与未经氧化的 AlGaN 相比，（氧化-AlGaN）/GaN 表面在 pH 值为 2 至 12 之间的表面电荷幅度范围更大。这表明（氧化-AlGaN）/GaN 表面的表面羟基密度高于未经氧化的 AlGaN，这解释了基于（氧化-AlGaN）/GaN 结构的 pH 传感器的更高灵敏度。
