Scanning Tunneling Microscopy (STM)

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1 Page 1 of 8 Scanning Tunneling Microscopy (STM) This is the fastest growing surface analytical technique, which is replacing LEED as the surface imaging tool (certainly in UHV, air and liquid). STM has also recently been developed into a tool for nano-fabricating (nano-machining) single-atom-type quantum structures, which are important for the next generation (the ultimate) molecular electronics and nanoelectronics devices. The basic principle of STM is simple:

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3 Page 3 of 8 If we can somehow position a metal tip close enough to a surface atom, quantum mechanics says that the wavefunctions of the tip and the surface atom will join, leading to the so-called quantum mechanical tunneling phenomenon. The magnitude of this tunneling current depends exponentially on the separation s. I Surface 2 2 2ks ψtip e ψ where the inverse decay length of the wavefunction, k, is given by (8π 2 mφ/h 2 ) 1/2, with φ being the work function in ev. (e.g. φ = 4 ev means that k ~ 1Å -1 ). Now, if we can somehow measure this very tiny tunneling current (due to charge transfer from the surface atom to the tip atom or vice versa), then we can obtain information regarding the local electron density of states at the tip position r, i.e. I Σ ψ Surface (r) 2 δ(e Surface, E F ) By keeping I constant while scanning the tip of the surface, we get electronic information of the surface ψ Surface (r) 2 and the topography of the surface (i.e. s).

4 Page 4 of 8 Bias: 0.3 V Taken as red component Bias: 0.8 V Taken as green component Bias: 1.2 V Taken as blue component We can also create a potential difference between the tip and the sample (e.g. a bias on the sample). If the sample bias is negative, tunneling goes from the occupied states (valence band) of the sample to the empty states of the tip. If the sample bias is positive, the reverse occurs, i.e. tunneling goes from the tip to the conduction band states of the sample. This bias potential however may distort the wavefunction itself and the corresponding local density may not correspond to the real atomic positions of the unit cell. NOTE: The topograph gives information on the local electron density of states and not the positions of the

5 Page 5 of 8 surface atoms (c.f. LEED and SEXAFS after appropriate analysis). In general, an STM topograph therefore does not reveal the positions of the surface atoms if the electronic structure is dominated by the presence of surface states. A second limitation of STM is that the STM topograph is not chemical specific (i.e. all chemical species have electron densities and electrons are indistinguishable) and must be combined with information obtained by other surface science techniques to get the full adsorption picture. However, if we know what we put on the surface, it is generally possible to track what happens to these surface species. Other applications of STM to Chemistry can be found in the review article by P. Avouris (J. Phys. Chem. 94 (1990) ).

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7 Page 7 of 8 Step on Si(100) with typical dimer rows note buckled dimer row in upper terrace Dimer row reconstruction undergoes a second-order phase transition from (2x1) to c(2x4) below 150 K [20nm x 20 nm]

8 Page 8 of 8 HOPG at 3.1 K Au(111) at RT Standing waves of a surface state on Cu(111) at T=165K [40nm x 40 nm] REF: IAP/TU Wien NIST LBL - See movies! Atomic Force Microscopy (STM) Question 5 in Assignment #2 Give a 1-2 page description about atomic force microscopy: (1) how does it work; (2) its optimal resolution (and why is it worse than that obtained by STM); and (3) some of its applications to nanotechnology. Be sure to indicate the source (references) of your information.