Nonlinear alternating-current tunneling microscopy

A new method of scanning tunneling microscopy is proposed that should allow atomic resolution imaging on insulators. A nonlinear ac technique is used that is shown to allow stable control of a microscope tip above insulating surfaces where dc tunneling is not possible, including aluminum oxide and liquid-crystal layers. Images of copper oxide and ${\mathrm{WSe}}_2$ have been obtained using this technique. Nanometer features have been observed.