Unique Constant Phase Element Behavior of the Electrolyte–Graphene Interface

• Main Authors: Jianbo Sun, Yuxin Liu
• Format: Article
• Language: English
• Published: MDPI AG 2019-06-01
• Series: Nanomaterials
• Subjects: electrolyte–graphene interface; constant phase element; frequency response
• Online Access: https://www.mdpi.com/2079-4991/9/7/923

We report a unique constant phase element (CPE) behavior (<inline-formula> <math display="inline"> <semantics> <mrow> <mfrac> <mn>1</mn> <mi>Z</mi> </mfrac> <mo>=</mo> <msub> <mi>Q</mi> <mn>0</mn> </msub> <msup> <mrow> <mrow> <mo>(</mo> <mrow> <mi>j</mi> <mi>&#969;</mi> </mrow> <mo>)</mo> </mrow> </mrow> <mi>&#945;</mi> </msup> </mrow> </semantics> </math> </inline-formula>) of the electrolyte&#8722;graphene interface with both <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>Q</mi> <mn>0</mn> </msub> </mrow> </semantics> </math> </inline-formula> and <inline-formula> <math display="inline"> <semantics> <mi>&#945;</mi> </semantics> </math> </inline-formula> showing dependence on the gate voltage. The frequency response of the electrolyte&#8722;graphene interface was studied using electrochemical impedance spectroscopy (EIS). The result suggests that (1) the electrolyte&#8722;graphene interface should be characterized as a CPE (<inline-formula> <math display="inline"> <semantics> <mi>&#945;</mi> </semantics> </math> </inline-formula> &lt; 1), rather than an ideal capacitor; and (2) both <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>Q</mi> <mn>0</mn> </msub> </mrow> </semantics> </math> </inline-formula> and <inline-formula> <math display="inline"> <semantics> <mi>&#945;</mi> </semantics> </math> </inline-formula> show ambipolar dependence on the applied voltage. We speculate that the CPE behavior of the electrolyte&#8722;graphene interface arises from the charged impurities on the substrate and the defects in the graphene lattice, which could introduce inhomogeneity of local density of states (DOS). The low density of states of graphene makes <inline-formula> <math display="inline"> <semantics> <mi>&#945;</mi> </semantics> </math> </inline-formula> sensitive to these local DOS near the Dirac point, and thus showing dependence on the gate voltage. Measurement of the electrolyte&#8722;graphene interface capacitance based on multi-frequency capacitance-voltage (CV) profiling was demonstrated, and the extraction of the carrier mobility was performed. The study could lead to a more accurate understanding of the capacitive behavior of the electrolyte&#8722;graphene interface, which is instructive for the design and analysis of devices involving the electrolyte&#8722;graphene interface for nanoelectronics and bioelectronics applications.