Abstract
Silicon nanowires are considered promising future biomedical sensors. However, their limited stability under physiological conditions poses a challenge in sensor development and necessitates a significantly improved knowledge of underlying effects as well as new solutions to enhance silicon nanowire durability. In the present study, we deduced the dissolution rates of silicon nanowires under simulated physiological conditions from atomic force microscopy measurements. We correlated the relevant change in nanowire diameter to changes in the electronic properties by examining the I-V characteristics of kinked silicon nanowire p–n junctions. Contact potential difference measurements and ambient pressure photoemission spectroscopy additionally gave insights into the electronic surface band structure. During the first week of immersion, the Fermi level of n-type silicon nanowires shifted considerably to higher energies, partly even above the conduction band edge, which manifested in an increased conductivity. After about a week, the Fermi level stabilized and the conductivity decreased consistently with the decreasing diameter caused by continuous nanowire dissolution. Our results show that a physiological environment can substantially affect the surface band structure of silicon nanowire devices, and with it, their electronic properties. Therefore, it is necessary to study these effects and find strategies to gain reliable biomedical sensors.
Highlights
Silicon nanowires (SiNWs) have emerged as promising nanoscale transducers for biomedical sensor applications [1,2]
When the surface area was increased by decorating the bare silicon surface with SiNWs (Figure 1a), all of which were covered with native oxide of roughly 1.5 nm in thickness [21], the release of Si drastically increased by up to 4.5 folds in the surrounding Phosphate buffered saline (PBS) (Figure 1b, gray triangles compared to dark gray dots)
To characterize the dissolution rate quantitatively and elucidate the relevant changes in SiNWs for biochemical sensor applications, we monitored the diameter of single SiNWs employing atomic force microscopy (AFM)
Summary
Silicon nanowires (SiNWs) have emerged as promising nanoscale transducers for biomedical sensor applications [1,2]. Sci. 2019, 9, 804 ion-sensitive and ion-selective nanowire field-effect-transistors (FETs) combined with the overall compatibility with conventional metal-oxide-semiconductor (CMOS) technologies [3,4,5,6,7]. The physiological environment that is prevalent for said biomedical sensing applications presents challenges to materials scientists. The insulating silicon oxide that typically encapsulates silicon nanowires and that is in principle required for a FET configuration dissolves slowly in aqueous solution as siloxane bonds are hydrolyzed [8]. This effect has long been known for silica nanomaterials [9]
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