[@Lafkioti; @Wang2010; @Sabri2009; @Levesque]. The interpretation of experimental works differs; and the mechanism behind the appearance of hysteresis and the role of charge transfer between graphene and its environment are not clarified yet.

In an ideal case of grounded graphene its charge neutrality point (CNP) is located at zero back gate voltage. However, in ambient conditions most of the graphene based FETs show initial p-doping (CNP is positioned at positive Vg) and hysteresis. We point out that these two effects can be related but do not necessarily have the same nature. The doping of graphene can be caused either by the adsorbates on top or underneath the graphene surface[@Schedin2007; @Lafkioti; @Wang2010] or by the electrochemical processes involving graphene[@Sabri2009; @Levesque; @Sidorov2011]. Depending on the nature of the dopant or the electrochemical environment, the initial doping can be either p or n, which introduces a shift of the graphene CNP to positive or negative gate voltages respectively. One should keep in mind that even in the absence of a net doping the dynamic response of the graphene resistance, namely hysteresis, can be different.

There are two types of directions defined for hysteresis; positive and negative [@Wang2010]. The positive direction of hysteresis corresponds to the CNP shifting towards negative voltages while the gate voltage is swept further into the negative regime. In case of negative hysteresis the shift of the resistance with respect to the gate voltage is in the opposite direction: the CNP shifts toward more positive values while sweeping the gate into the negative regime. Wang et al.[@Wang2010] proposed that negative and positive hysteresis directions can be attributed to two competing mechanisms: capacitive coupling and charge trapping from/to graphene, respectively.

Capacitive coupling enhances the local electrical field near graphene, inducing more charge carriers and causing a negative direction of hysteresis. An example of a mechanism for capacitive coupl