C. A. Bobisch

Visualization of Fermi’s Golden Rule Through Imaging of Light Emission from Atomic Silver Chains

Silver Emission Follows Golden Rule Electrons emitted from the tip of a scanning tunneling microscope (STM) can be used in certain cases to excite optical emission from surface structure. Such methods can be used to characterize the “particle-in-a-box” states created in chains of varying lengths of metal atoms on surfaces. Chen et al. (p. 981) were able to spatially resolve photon emission for silver chains up to 10 atoms long, assembled on a nickel-aluminum alloy surface. The emission maxima correlated with the nodes seen in the derivative of STM current with voltage. This correlation follows from Fermis golden rule, which connects the initial and final states of the radiation transition through the momentum operator and leads to a better understanding of the light-emission process. A correlation of photon emission and scanning tunneling microscopy images illustrates a fundamental quantum principle. Atomic-scale spatial imaging of one-dimensional chains of silver atoms allows Fermi’s golden rule, a fundamental principle governing optical transitions, to be visualized. We used a scanning tunneling microscope (STM) to assemble a silver atom chain on a nickel-aluminum alloy surface. Photon emission was induced with electrons from the tip of the STM. The emission was spatially resolved with subnanometer resolution by changing the tip position along the chain. The number and positions of the emission maxima in the photon images match those of the nodes in the differential conductance images of particle-in-a-box states. This surprising correlation between the emission maxima and nodes in the density of states is a manifestation of Fermi’s golden rule in real space for radiative transitions and provides an understanding of the mechanism of STM-induced light emission.

Imaging the dynamics of individually adsorbed molecules

Although noise is observed in many experiments, it is rarely used as a source of information. However, valuable information can be extracted from noisy signals. The motion of particles on a surface induced, for example, by thermal activation or by the interaction with the tip of a scanning tunnelling microscope may lead to fluctuations or switching of the tunnelling current. The analysis of these processes gives insight into dynamics on a single atomic or molecular level. Unfortunately, scanning tunnelling microscopy (STM) is not a useful tool to study dynamics in detail, as it is an intrinsically slow technique. Here, we show that this problem can be solved by providing a full real-time characterization of random telegraph noise in the current signal. The hopping rate, the noise amplitude and the relative occupation of the involved states are measured as a function of the tunnelling parameters, providing spatially resolved maps. In contrast to standard STM, our technique gives access to transiently populated states revealing an electron-driven hindered rotation between the equilibrium and two metastable positions of an individually adsorbed molecule. The new approach yields a complete characterization of copper phthalocyanine molecules on Cu(111), ranging from dynamical processes on surfaces to the underlying electronic structure on the single-molecule level.

Electronic Transport on the Nanoscale: Ballistic Transmission and Ohm's Law

If a current of electrons flows through a normal conductor (in contrast to a superconductor), it is impeded by local scattering at defects as well as phonon scattering. Both effects contribute to the voltage drop observed for a macroscopic complex system as described by Ohms law. Although this concept is well established, it has not yet been measured around individual defects on the atomic scale. We have measured the voltage drop at a monatomic step in real space by restricting the current to a surface layer. For the Si(111)-( [see text]3 x [see text]3)-Ag surface a monotonous transition with a width below 1 nm was found. A numerical analysis of the data maps the current flow through the complex network and the interplay between defect-free terraces and monatomic steps.

Ordered binary monolayer composed of two organic molecules: Copper-phthalocyanine and 3,4,9,10-perylene-tetra-carboxylic-dianhydride on Cu(111)

The two planar organic molecules copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) are found to form an ordered mixed monolayer on Cu(111). The layers have been prepared by exposing the surface to an equivalent of a little bit more than half of a monolayer of CuPc and the same amount of PTCDA followed by thermal annealing. The investigations by scanning tunneling microscopy reveal regular patterns with a commensurate unit cell which contains one CuPc and two PTCDA molecules.

The initial growth of PTCDA on Cu(111) studied by STM

The initial growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) was analysed. Ultrathin films with coverages of up to two layers were prepared on a (111) orientated copper single crystal by means of vapour deposition in an ultrahigh-vacuum chamber. The films were characterized in situ by scanning tunnelling microscopy (STM). Within the first layer, two different structures were found. Both exhibit a herringbone-like arrangement of the molecules, which is also found in the (102) plane of the α and β bulk phases. The two-dimensional unit cell is given by two molecules which are rotated by about 90°. As an effect of the interaction with the substrate, a voltage-dependent moire pattern was observed for one of these phases. For the second layer, a herringbone phase was found that is denser than the phases of the first layer but less dense than the bulk phases.

Ballistic Electron Microscopy of Individual Molecules

We analyzed the transport of ballistic electrons through organic molecules on uniformly flat surfaces of bismuth grown on silicon. For the fullerene C60 and for a planar organic molecule (3,4,9,10-perylene-tetracarboxylic acid dianhydride), the signals revealed characteristic submolecular patterns that indicated where ballistic transport was enhanced or attenuated. The transport was associated to specific electronic molecular states. At electron energies of a few electron volts, this “scanning near-field electron transmission microscopy” method could be applied to various adsorbates or thin layers.

Local potentiometry using a multiprobe scanning tunneling microscope

Scanning tunneling potentiometry (STP) is a powerful tool to analyze the conductance through thin conducting layers with lateral resolution in the nanometer range. In this work, we show how a commercial ultrahigh vacuum multiprobe system, equipped with four independent tips, can be used to perform STP experiments. Two tips are gently pushed into the surface applying a lateral current through the layer of interest. Simultaneously, the topography and the potential distribution across the metal film are measured with a third tip. The signal-to-noise ratio of the potentiometry signal may be enhanced by using a fourth tip, providing a reference potential in close vicinity of the studied area. Two different examples are presented. For epitaxial (111) oriented Bi films, grown on a Si(100)-(2 x 1) surface, an almost constant gradient of the potential as well as potential drops at individual Bi-domain boundaries were observed. On the surface of the Si(111)(3 x 3)-Ag superstructure the potential variation at individual monoatomic steps could be precisely resolved.

Nanoscale electron transport at the surface of a topological insulator.

The use of three-dimensional topological insulators for disruptive technologies critically depends on the dissipationless transport of electrons at the surface, because of the suppression of backscattering at defects. However, in real devices, defects are unavoidable and scattering at angles other than 180° is allowed for such materials. Until now, this has been studied indirectly by bulk measurements and by the analysis of the local density of states in close vicinity to defect sites. Here, we directly measure the nanoscale voltage drop caused by the scattering at step edges, which occurs if a lateral current flows along a three-dimensional topological insulator. The experiments were performed using scanning tunnelling potentiometry for thin Bi2Se3 films. So far, the observed voltage drops are small because of large contributions of the bulk to the electronic transport. However, for the use of ideal topological insulating thin films in devices, these contributions would play a significant role.

Anisotropic scattering of surface state electrons at a point defect on Bi(111)

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry.

Ultrathin Bi films on Si(100)

In this work we report on the growth of high quality bismuth films with a layer thickness of 3–4 nm on a (100)-oriented silicon surface. We present a combined STM and LEED study to determine the best possible growth conditions regarding, for example, film flatness, grain size and low surface roughness. The deposition of bismuth was performed at a low temperature of about 130 K followed by moderate annealing to an ambient temperature or to temperatures slightly above. The result is an epitaxial Bi film with low surface roughness.