Herman van Kempen

Tip for scanning tunneling microscopy made of monocrystalline, semiconducting, chemical vapor deposited diamond

A tip for scanning tunneling microscopy (STM) was fabricated from semiconducting, chemical vapor deposited (CVD) diamond, epitaxially grown on a natural diamond substrate. Extremely high, p‐type conductivity was realized by heavy boron doping. A sharp tip was obtained by conventional diamond polishing in such a way that the ultimate tip (radius<12 nm) is situated in the electrically conductive CVD layer. Atomic resolution on graphite surfaces could easily be obtained under normal operating conditions for STM in air. The feasibility of using the diamond tip to create nanostructures on surfaces was also investigated.

STM measurements on graphite using correlation averaging of the data

Images of the (0001) surface of highly oriented pyrolytic graphite observed using scanning tunnelling microscopy, in both constant distance and current imaging modes, show a strong asymmetry of the unit cell. This asymmetry depends on the tunnelling current and voltage. We have also found clear evidence that multiple‐tip tunnelling occurs. To improve the resolution of the imaged unit cells, a method of correlation averaging is introduced.

Origin of Magnetic Contrast in Spin-Polarized Scanning Tunneling Spectroscopy: Experiments on Ultra-Thin Mn Films

Normalized differential tunneling conductivities obtained with Fe-coated W tips show a spin-polarized peak around +0.8 V on ultrathin bct Mn films grown on Fe(001)-whiskers. This spin-polarized peak results in a clear magnetic contrast in spectroscopic images. Our normalization removes the influence of the tunneling probability and makes the spectroscopic curves most reliable for a derivation of the spin-resolved sample density of states (DOS) at positive voltages. From this analysis we conclude that the magnetic contrast in our spectroscopic maps is caused by a highly polarized DOS. Furthermore, a tip polarization of about 15% is found.

Single-electron effects observed with a low-temperature STM

We have used a low‐temperature scanning tunnelling microscope to study the effect of the Coulomb charging energy on the tunnelling behaviour of low capacitance point‐contact junctions. The tunnelling I‐V characteristics between a tungsten tip and various materials, such as stainless steel, aluminium, carbon and YBa2Cu3O7‐δ, show a quadratic behaviour at low voltages and a displaced asymptotic behaviour at high voltages. The I‐V characteristics can be quantitatively understood using the model of single‐electron tunnelling induced by the Coulomb blockade. The capacitances of this type of point‐contact tunnel junctions are in the 10−18 F range, and are adjustable by varying the distance between tip and surface. These capacitances are at least two orders of magnitude lower than can presently be achieved by electron lithography.

STM Studies on Nanosized Porphyrin Wheels at the Solid‐Liquid Interface

Monolayers of nanometer‐sized porphyrin ‘wheels’ composed of circular hexameric and dodecameric porphyrin macromolecules, can be visualized at the 1‐phenyloctane‐HOPG interface with the help of ambient STM. By applying a very low tunneling current (1 pA), it is possible to tunnel through layers of molecules of more than 4 nm thick.

Microscopy breaks the speed limit

Solid-state physicists, like many other scientists, enjoy exploring experimental limits. One example is the study of dynamical phenomena at surfaces, which must be done on the atomic scale and on timescales of a few femtoseconds. More applied research, such as the development of new electronic devices, must also probe structures with smaller sizes and on faster timescales than ever before.