Scanning tunneling microscopy
Scanning tunnelling microscopy is a real space method that can be used on conducting samples. The first scanning tunnelling microscope (STM) came into operation in 1981 built by G. Binnig and H. Rohrer. A year later the first atomic resolution images were recorded. Today, STMs obtain routinely a resolution of 10 pm in lateral and of 1 pm in vertical resolution.
The only requirement is a conducting substrate material in order to establish a tunnelling current. The systems studied by STM include metallic surfaces, semiconductor surfaces, oxide surfaces with a narrow band gap, superconductor surfaces, and quasi-crystalline surfaces. On top of this substrate, molecules or thin insulating films were imaged to a thickness of a few layers.
The main applications of STM are to clarify the surface topography and to study electronic and vibrational properties. STM provides atomic resolution offering the possibility to study solitary surface defects and single adsorbates. It is able to resolve electronic and vibrational properties locally. Chemical bonding and adsorption, cluster formation, film growth, magnetic properties, and other surface relevant phenomena were studied by STM.
Basically, STM is based on the quantum mechanical phenomenon of tunnelling, in which an electron travels through a barrier that it can not surmount in classical physics, because its energy is smaller than the barrier’s height (see Figure). As tunnelling is based on the wave nature of electrons, the tunnelling probability depends not only on the width of the barrier but also on the electron’s wave function.
In STM, the electrons tunnel through the vacuum between two conducting electrodes. One of them has ideally only a few atoms at its end. This tip is approached with the aid of piezoelectric elements (called piezos) to the other electrode to a distance of only about a tenth of a nanometer or less. At such a distance, the wave functions of tip and sample start to overlap. Such an overlap is equivalent to a finite tunnelling probability of the electrons between the two electrodes. Application of a bias voltage between tip and sample directs the tunnelling into one direction, from the occupied electronic states at one of the electrodes to the unoccupied states at the other one (see figure). An applied bias voltage of typically a few mV to a few V leads to a tunnelling current of typically pA to nA. This tunnelling current depends exponentially on the distance between tip and sample because wave functions decay exponentially into the vacuum. The current is thus a sensitive measure of the distance between the electrodes.
Spectroscopy with the STM
Two types of spectroscopy are possible in STM. By measuring the first derivative of the tunnelling current versus voltage electronic features of the sample are measured. This spectroscopy is called STS (scanning tunnelling spectroscopy). The measurement of the second derivative reveals vibrational excitations of the substrate and adsorbed molecules. This spectroscopy is called IETS (inelastic electron tunnelling spectroscopy).
STM is often not capable of identifying adsorbates via STS because of significant level broadening and shifting of electronic levels that occur upon chemisorption. It was first predicted by theory that IETS should be used to probe the vibrational properties of individual adsorbates in their local bonding environment and later proven in a multitude of experiments to be possible in particular by the pioneer of IETS, Wilson Ho.
Scanning tunneling spectroscopy
In the scanning tunnelling spectroscopy (STS) mode the STM probes the local density-of-states around the Fermi level (see Figure). Such measurements were first presented in 1986 and are 20 years later standard in many laboratories.
Inelastic electron tunneling spectrosopy
Inelastic electron tunnelling spectroscopy (IETS) probes the vibrational properties of single molecules by measuring those electrons that excite molecular vibrations by tunnelling inelastically. The Figure sketches the general idea behind IETS. For STM imaging as well as dI/dV spectroscopy, we considered only those electrons that retain their energy in the tunnelling process, i.e. electrons that tunnel elastically through the tip-sample gap (blue path in Figure). Most tunnelling electrons follow indeed this path. However, a small part of the electrons (of the order of 10-4 or less) loose energy during the tunnelling current flow by exciting vibrations of adsorbates.
For adsorbed molecules, the so called inelastic channel (red path in Figure) opens when the electron energy matches hw of a molecular vibration. Above hw the inelastic tunnelling electron can continue into a different state with a proportionate smaller energy. IETS uses the abrupt changes in conductance at hw/e to measure the vibrational threshold energy. In first approximation, opening of the inelastic channel increases the conductivity, since the sum over all final states is larger. However, theory showed that the elastic channel is reduced at the same energy because of the many-body character of the inelastic contribution to the conductance and thus the total conductivity might also decrease.
Femtochemistry on the nanoscale
We investigate femtochemistry on the molecular scale by using a combination of low-temperature scanning tunneling microscope with a Femtosecond laser source. The working principle of our combined fs-laser low temperature STM is illustrated in the Figure. First of all molecules adsorbed on a metal surface are scanned by the STM, to indentify the molecules and their absorption sites (a). In the second step ultrashort laser pulses are given to generate hot electrons in the metal substrate. During the illumination the STM tip is retracted avoiding near and far field tip effects (b). The energy of the hot electrons is transferred to molecular vibrations due to non-adiabatic coupling. Because of the high density of hot electrons, the molecule gains enough energy to overcome energy barriers for surface processes such as diffusion or dissociation. These processes are verified by scanning the same sample region again with the STM (c). The measured reaction yield depending on absorbed fluence opens a fascinating look inside mechanisms and dynamics of elementary steps of surface reactions. In our set-up the STM is used to resolve processes on a single molecule scale in real space for example single molecules on terraces, at step edges, at close-by defects or in the proximity of other molecules.
For technical implementation see Mehlhorn et. al, Rev. Sci. Instr. 78 (2007) 033908.IET manipulation
When a current flows through a system, where vibrations are possible, inelastic effects take place. In STM, when an electron tunnels through an adsorbate, inelastic electron scattering may induce elementary electronic, vibrational, or vibronic (= vibrational and electronic) excitations of the adsorbate. The inelastic electrons, which represent a small fraction of the tunnelling current, transfer energy from the electrons to the adsorbate. The energy stored in the vibrational excitation of the molecule is transferred to the reaction coordinate modes corresponding to the relevant nuclear motions and chemical reactions; eventually triggering chemical transformation on the nanoscale. Reactions involving the IET process use low STM bias voltages to achieve more control and a much higher spatial resolution.
IET might be based on two different excitations depending on the applied bias voltage. At high voltages an electron may transiently occupy an electronic orbital of the adsorbate; at low voltages vibrational modes may be excited directly. Most often electronic excitations were found to be operative on semiconductors, while on metals vibrational excitations were found to be operative.
At voltages below electronic resonances, vibrations can be excited directly, as soon as the energy of the tunnelling electrons exceeds the vibrational energy. The internal energy distribution of the adsorbate can be perturbed far away from the initial equilibrium by successive electron scattering. Such a vibrational heating by tunnelling electrons enhances substantially the probability for the adsorbate to pass the reaction barrier.