While charge transport in highly ordered system is generally well understood, correct modelling of conductivity in highly disordered system quite often presents an important theoretical but also experimental challenge. Slow conductance relaxation has been studied in many disordered insulators using field effect measurements. After a quench at low temperatures, a change in the gate voltage is accompanied by a sudden increase in the conductivity, slowly decreases with a roughly logarithmic dependence on time. Memory effects and aging are often seen in the same type of experiments. These glassy behaviour has been interpreted in different ways, but there is a growing tendency to explain them in terms of electron glasses, i.e., systems with states localized by the disorder and long-range Coulomb interactions between carriers.

The use of local probe techniques, such as scanning Kelvin probe microscopy (SKPM), presents two advantages as compared with the conductance measurements performed so far: i) it allows a study of the phenomena at the nanometer scale, and ii) samples with larger resistances can be measured. In the present work Dynamic AFM is used to characterize the electronic properties of two quite different nanoscale highly disordered and low conductivity systems. On the one hand highly resistive granular metal grains[1], and on the other Graphene Oxide islands[2].

We apply Electrostatic and Kelvin Force Microscopy to these samples. In addition, their time evolution is studied using “movies” where topography and surface potential are acquired simultaneously. Our AFM studies suggest evidence of the formation of an electron glass on the materials studied. This evidence includes the presence of domains on the surface potential, uncorrelated with topograph. The fluctuations of the surface potential are compatible with variations of the Coulomb energy of a single charge over the distance between domains. At the same time, the fact that the fluctuations are larger than kT and that time correlations are dominated by a broad distribution of characteristic times can be naturally explained within the electron glass model. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses.

[1] M. Ortuño, E. Escasaín, E. López-Elvira, A. Somoza, J. Colchero and E. Palacios-Lidón. Scientific Reports, Scientific Reports 6, article #21647 (2016).

[2]M.F. Orihuela, A.M. Somoza, J. Colchero, M. Ortuño, E. Palacions-Lidón, Physical Review B 95(20) 205427 (2017).