Imaging crystalline surfaces

One class of large molecules that was investigated relatively early was liquid crystals [37, 38], and in particular the group 4-n-alkyl-4 -cyanbiphenyl (mCB). These molecules fonu a highly crystalline surface adlayer, and STM images clearly show the characteristic shape of the molecule (figure B 1.19.8). 

In this chapter, we discuss the images of perfect crystalline surfaces. First, wc present the analytic method for handling surface wavefunctions —... 

For a solid surface with two-dimensional periodicity, such as a defect-free crystalline surface, all the measurable quantities have the same two-dimensional periodicity, for example, the surface charge distribution, the force between a crystalline surface and an inert-gas atom (Steele, 1974 Goodman and Wachman, 1976 Sakai, Cardino, and Hamann, 1986), tunneling current distribution, and STM topographic images (Chen, 1991). These quantities can be expanded into two-dimensional Fourier series. Usually, only the few lowest Fourier components are enough for describing the physical phenomenon, which requires a set of Fourier coefficients. If the surface exhibits an additional symmetry, then the number of independent Fourier coefficients can be further reduced. 

Figure 10.12 (a) HRTEM images of YBa2Cu307 6, viewed down the [110] direction. The initial image showing a thin layer of disordered coating, and the image recorded from the same area after electron beam irradiation for 3h. The disordered layer recrystallized into a clean crystalline surface. 

The unique resolution capabilities of SPM are based on the combined use of two elements an ultrasharp tip, which probes the sample surface and enables their mutual interaction (and therefore the measured property) to be highly localized, and a piezoceramic scanner, which controls the tip-sample relative position in the three spatial directions with subangstrom precision. This approach has allowed the imaging of surfaces with atomic resolution in STM/AFM, as many examples have demonstrated [5]. To attain such resolution level, the sample under study is mainly required to be atomically flat and highly crystalline. 

The static contact mode is the simplest mode to use for obtaining basic topographic information of solid surfaces. Figure 5.17 is an image of surface crystalline polyethylene obtained with the contact mode. The contact mode was able to detect the nano-scale roughness of crystalline polyethylene, which is believed to be chain folds of polyethylene. The scale of the topography in Figure 5.17 implies that the primary function of an AFM is not the same as a surface profiler... 

Fig.2. Gold crystalline surface imaged at atomic resolution by in-situ STM in 0.01 M CUSO4 and 0.05 M H2SO4 in Millipore water with E = 200 mV and = -110 mV (filtered), d = 0.02 nm, It = -3 nA.

Fig. 3. Gold crystalline surface oxidized ex-situ at ambient pressure for 24 h. Two layers of gold oxide are seen to the upper left, and to the lower right the gold was only partially oxidized. Along the mid-diagonal, rows of individual atoms are observed. In-situ STM images, electrolyte 0.01 M CUSO4 and 0.05 H2SO4 in MiUipore water, E = 400 mV, Et =-91 mV, d = 0.5 mn, /, = 1.2 nA.

One successful application of STM beyond imaging has been the STM break junction (STM-BJ) technique [56] and modification thereof [13, 57, 58], which are becoming very popular experimental platforms for nanoelectronics and molecular electronics [59-61]. In this technique. Figure 9.5a, a tip (usually Au) is brought into mechanical contact to a defined depth (crash-to-contact) with a single-crystalline surface of the same material. The tip is then withdrawn at a suitable rate such that a metal nanoconstruction is formed, elongated, and eventually broken (break-of-contact). During tip withdrawal, the conductance is recorded as a function of piezo displacement. The procedure can be repeated many thousands of... 

If the above scheme indeed holds in crystal nucleation, the conclusion is that a mother solution does not contain crystalline nuclei of all possible crystal polymorphs, but contains the mother of all polymorphs, a fluxional dropletready to evolve in several different crystalline directions. The mechanism of this process could be a mirror image of surface melting while in surface melting the isotropic surface layer rushes in toward complete melting, in crystallization within the nanoparticle the molecular ordering of the inner core successively templates the growing outer layer. 

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