An energy-filtered

Scanning tunneling microscope (EF-STM). A typical STM yields atomic-scale landscapes of electrically conducting surfaces by mapping the electronic states closest to the Fermi energy, ε_F. Now, physicists at the Colorado School of Mines have used a semiconductor tip (indium arsenide) on an STM to selectively image electronic states of various energies. Their EF-STM works by effectively suppressing tunneling in a range of energies within the “projected bandgap” along the tip’s axis. Changing the bias voltage on the tip shifts the gap relative to the sample’s states and allows electrons with different energies to tunnel. On a silicon surface, the researchers separately mapped dangling bonds from both the silicon adatoms, which have electron energy close to ε_F and are seen by conventional STMs, and the silicon atoms in the second layer, which have electron energies further below ε_F. The group foresees the ability to map the local composition of semiconductor alloys. (P. Sutter et al., Phys. Rev. Lett. 90 , 166101, 2003 http://dx.doi.org/10.1103/PhysRevLett.90.166101 .)