Methods and Tehniques in Surface Science Prof. Dumitru LUCA “Alexandru Ion Cuza” University, Iasi, Romania

Scanning Tunneling Microscopy (STM)  Allows for surface mapping by evidencing atomic-scale surface features  Characterization of surface topology.  Evidencing surface growth and interatomic bridging sites. Resolution: lateral < 1.0 Å vertical < 0.1Å Remark: What is evidenced is the electron density at surface, not atom positions! Images are frequently interpreted as “atoms”, but this is not necessarily true under any circumstance. X Z

The tunneling effect Useful reference:

STM schematics The Inventors of the STM, Gerd Binnig and Heinrich Rohrer have been awarded Nobel prize in physics in Dedicated only to investigate conductive surfaces…

STM basics Bulk EFEF EVEV Potential Barrier 11 STM Tip Bulk EFEF EVEV Potential Barrier 22 Sample Bulk EFEF EVEV Potential Barrier STM Tip Bulk EFEF EVEV Potential Barrier Sample d Useful reference: Electrons flow in both directions with equal probability!

Bulk EFEF EVEV Potential barrier Sample Bulk EFEF EVEV Potential barrier STM tip d By applying a biasing voltage V: The physical background of STM j –current density; a 1, a 2 – constants; V – bias voltage; s – tip-to-sample distance  av – the average value of tip and surface work function;

STM operation A constant value of approx. I tunnel = ( nA) is currently used. To keep I tunnel = constant, z should be automatically adjusted by a feedback circuit. Two modes: constant current and constant inaltime height The most frequently mode is the constant curent. Useful reference:

Constant current mode Tunneling currents up to 0.03 nA are recorded, which are small enough to investigate even low-conductivity surfaces, including biological samples..

Constant height mode Here, a certain value is set for z, and I tunnel is directly measured. No feedback! Utilized only for very flat surfaces!  A periodic variation of the tip-to- sample distance occurs.  There, where tip will be positioned exactly on top of a sample atom, the tunneling current will be maxim  When the tip is above a valley, the tunneling current will be much smaller.

Constant Height Mode STM Drawbacks: Complexity in data processing, for certain surfaces: The surface image is not straightforwardly determined by its topography, but also by: - density of states, - the sign and value of the bias, - current value etc.

What information can we get from STM imagining 1. Atom seggregation at surface… …including seggregtion of impurities at grain boundaries. This image depicts the (110) surface of the Fe-3.5at%Si alloy. Aproximately 1/3 of surface atoms are Si (dark color in the picture) which substitute the Fe atoms! Carbon atoms cannot be detected, but they mask the Fe atoms in the central rows of the ladder-type structure. Images taken from the paper by Biedermann, M. Schmid, P. Varga, Surf. Sci (1995)

1.Pb and Cu are non-miscibile metals: r Pb = 1.37 r Cu. E s Pb = 0.50 J/m 2, while E s Cu (1.96 J/m 2 ). According to the classical theory of growth phenomena, Cu should grow as small islands on Pb surface. 2. Pb atomii are very mobile.  A Cu island grown on Pb has, in its turn, a surface at its lateral edges, therefore its surface energy increases.  The most favorable configuration (minimal energy) occurs when these side wings are covered with Pb. 2. Surface growth processes What information can we get from STM images? A compromise between the seggregation tendency of Cu and the large mobility of Pb atoms.

Other STM applications Nanolithography  The most direct way for surface manufacturing.  The specimen’s surface under the STM tip can be melted and evaporated. An example of STM lithography: an STM image of 3 ML of a film during 3 electric pulses exposure.

Nano-anodization  A voltage is applied between the conductive cantilever tip and the metal surface to be anodized. Electrochemical processes occur that lead to the formation of oxide nanostructures.  By using the electro-lithography, the local geometric and composition properties of the sample surface can be altered..

Fe on Cu (111) Nano-manipulation