Chapter 3 Imaging with a STM. Chapter 3 Imaging with a STM. 3.1: Imaging principle and techniques. Constant current imaging. Important parameters

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1 Objective: to learn the different STM imaging techniques. 3.1: Imaging principle and techniques Important parameters Voltage bias determines the energy range of probed electronic states V tip - V sample > 0 : e- from sample to tip, occupied states are imaged. V tip - V sample < 0 : e- from tip to sample, empty states are imaged. Scanning frequency: below the resonance frequency of tubes, below the bandwidth of the tunnel current regulation if active. Constant current imaging Tunnel current regulation is active and effective at every time. I t = Cst During the x,y scan, the regulation output V z is : - amplified and sent to the piezo - measured. V z (x,y) scaled by the Z-piezo sensitivity gives the topography information. Tunnel resistance R t determines the tip-sample distance. Usually, the current is the parameter: R t = V I t = 5 MΩ to 5GΩ V z I t time

2 Constant height imaging Tunnel current regulation is inactive (or active with a long time constant so that mean slope is followed). During scan, the regulation output V z is close to constant, the tunnel current I t is measured. I t gives the topography information. Possible only on atomically-flat surfaces, highly-sensitive but non-linear information. Much less used. 3.2: The spatial resolution V z I t time The corrugation Spatial resolution Corrugation d is by definition: the topography variation amplitude as measured by the microscope. It is a fraction of Angtröm on an atomically flat surface, and can be larger on rougher surfaces. Depends on every experimental conditions. Can be decreased by a blunt tip, an inefficient regulation : not an intrinsic quantity. Binnig (1978); hypothesis of a continuous media. ( ) = I 0 exp' α Δx2 I Δx The tunnel current is concentrated on a scale: R α << R % & R ( * where α = 2mW ) R = 10 nm, α = 1 Å -1 : current flows on a scale x = 1 nm. Spatial resolution is not limited by the tip radius. x Tip radius R

3 tip state is proportional to the z derivatiue of the sample ioaue function at the center of the apex atom Using the expressions for other tip wave functions in terms of Green s functions in the previous section, we im- p, Trying to model the atomic resolution Model of a continuous media surface: homogenous electron gas. a d d h mediately obtain the tunneling matrix elements for all the tip states listed in Table III. Table IV is a summary of the results. The tunneling matrix elements listed in Table IV can be summarized in terms of an extremely simple derivative The origin of atomic resolution Tunnel matrix element value depends on TABLE IU. electronic orbital symmetry: State p and d states give increased modulation and thus increased sensitivity. p [z] Tunneling matrix elements. Misvalue at ro wave function of the sample th an tu d di fin fo B Bz Bq X Calculated corrugation dtheo:, &R + d)/ Δdtheo = exp. π 2 ' *1 h ( αa (2011) Bg, π 2/.if a >> and d >> 1 α α0 - By Stoll (1984) For a metal : a = 2.5 to 3 Å, k = 1 Å-1, h = 3 Å. Tip: R = d = 3 Å calculated dtheo = 0.01 Å: too small compared to experiment! This model fails to describe the atomic resolution: the hypothesis of a continuous media is incorrect. PHYSICAL REVIEW LETTERS Bz Bx Q2p d [zy] dz By B2$ d [xy] Bx By Contrast depends strongly on the nature of d [z' 'r'] BZ2 the atoms at the tip apex: better resolution $2$ Q2f Bx on Si after a controlled collision. W known to have a dz2 surface state. C.J. Chen, Phys. Rev. B 42, 8841 (1990)., week ending 19 AUGUST 2011 nge also in the sample wave function, as shown lly in Fig. 3(b). With this argument, we can the local maxima measured at the position of nodal planes [Fig. 2(b)]. 3(c), we examine the tip position above the f two nodal planes. In this case, the orbital hibits! character and opposing orbital lobes hase change. The phase change in the tip px;y oss these lobes then gives rise to vanishing matrix elements and scurrents. Cu tip: states With this third we can nowcu understand contrast of the tip with atheco molecule g. 2(a)] showing depressions at the crossings adsorbed: p or s+p states anes. It is interesting to note that a dxy tip state to a maximum in this case and to minima in evious cases.contrast enhanced and modified ces betweenwith experimental images using a CO a CO tip. 2(a) and 2(b)] and calculated images using a [Figs. 2(e) and 2(f)] mainly show up above the molecule, at the positions of the outer lobes MO and HOMO, where calculations L. Gross et al,only Phys.the Rev. Lett. 107, al minima. Contributions from other tip states, (2011). e, from the pz -wave character 5" orbital of CO, uled out [17] and can enhance tunneling at the molecule. Note that a pz -wave tip, due to the FIG. 4. Naphthalocyanine on NaCl(2 ML) on Cu(111) meanodal planes perpendicular to the surface, will sured using a Cu tip (a) and a CO tip (b) in constant-current ontrast that is qualitatively similar to that of an m tu se el m is Q2p d [zx] Atomic resolution depends on tip s electronic states 3.3: The STM benchmark Si 7x7 3 co th sp th

4 Si (111) 7x7 Si (111) annealed at 1000 C, slow cooling-down. Single vacancies or adsorbates are visible: true atomic resolution. Si (111) 7x7 : the model 7x7 reconstruction minimizes nb of pending bonds : 49->19 Miller index refer to the number (= 7) of atomic cells involved. In STM, the adatoms only are visible, a well as the corner vacancies. Omicron website: Surface dynamics studies Real-time dynamics of Pb atoms on Si J.-M. Rofriguez-Campos et al, Phys. Rev. Lett. 76, 799 (1996), Institut Néel, Grenoble.

5 The chemical contrast: (110) 3.4: Imaging at different bias Two superposed images: occupied states: V sample < 0, red level:. + empty states: V sample > 0, green level: Images are taken simultaneously to avoid hysteresis effects between two succesive images. J. Stroscio et al, Phys. Rev. Lett. 58, 1192 (1987). Occupied / empty states: a naive picture Occupied / empty states: a naive picture potential more attractive than. Close to Fermi level, occupied states are on, empty ones on. potential more attractive than. Close to Fermi level, occupied states are on, empty ones on. E F B = tip ev > 0: atoms are imaged. ev < 0: atoms are imaged. Depending on bias, occupied or empty states participate to tunneling: complementary information can be accessed. E F ev > 0 B = tip ev > 0: atoms are imaged. ev < 0: atoms are imaged. Depending on bias, occupied or empty states participate to tunneling: complementary information can be accessed. N A (E) N B (E) N A (E) N B (E)

6 Occupied / empty states: a naive picture Real-time dynamics of Pb atoms on Si potential more attractive than. Close to Fermi level, occupied states are on, empty ones on. E F ev < 0 B = tip ev > 0: atoms are imaged. ev < 0: atoms are imaged. Depending on bias, occupied or empty states participate to tunneling: complementary information can be accessed. A different contrast is obtained, depending on bias. N A (E) N B (E) J.-M. Rofriguez-Campos et al, Phys. Rev. Lett. 76, 799 (1996), Institut Néel, Grenoble. Three / six-fold symmetry in graphite 3.5: HOPG, Highly Oriented Pyrolitic Graphite Clean surface thanks to the layered structure and scotch technique. STM images display a triangular lattice, not a hexagonal one: coupling with the second layer make every other two atom different. STM images not the atoms but the electronic clouds.

7 Moiré in graphene UHV annealing of SiC, epitaxy on Re: formation of a graphene sheet on a crystalline substrate. Atomic lattice visible, 6-fold periodicity. Conclusion STM imaging usually carries other information than pure topography, that is related to electronic properties. Images shows electronic interference effects with the buffer layer: moiré. P. Mallet, J.Y. Veuillen et al, Phys. Rev. B 76, (R) (2007), Institut Néel. C. Tonnoir et al, Phys. Rev. Lett. 111, (2013), INAC.