Role of applied bias and tip electronic structure in the scanning tunneling microscopy imaging of highly oriented pyrolytic graphite

Controlled scanning tunneling microscopy (STM) experiments and first-principles simulations show that applied bias can significantly affect the topographic STM contrast of highly oriented pyrolytic graphite (HOPG) measured with W tips in the pure tunneling regime. Depending on the magnitude and polarity of the bias, both the hexagonal and triangular structures were imaged with the same stable tip. Statistical analysis of the experimental data reveals an enhancement of the corrugation amplitude for small negative biases and the occurrence of different contrast reversals at positive biases, whereby the relative brightness of primary and secondary image features is inverted with respect to that for negative biases. Simulations of HOPG imaging with three different W-tip models explain these findings on the basis of tip-convolution effects governed by the subtle interplay of the tip electronic states with different angular and magnetic moments and vacuum decay lengths. Finally, the calculated library of image contrasts and corrugation amplitudes allows us to rationalize the large variety of, sometimes contrasting, STM data on the basis of effects that different tip terminations, compositions, and sharpness have on the STM imaging of HOPG.