Experimental I - V data of HRS at different temperatures. (a) Linear fitting for the I-V curve at higher temperatures (80 to 130 K) using a log-log scale. The slope of the fitting curve is between 1 and 2, indicating that the conducting mechanism is electron hopping. (b) Experimental I-V data of HRS at higher temperatures (140 to 200 K). The good linear relationship between ln(I/V) and √V indicates that the electronic behavior of HRS can be predicted by utilizing Poole-Frenkel effect. Y coordinates of line were added with a constant to separate each line. The V1/2 in x-axis means √V in the (b), and it shows the good linear relationship between ln(I/V) and V1/2 in the temperature range 140 to 200 K obviously.