Inelastic electron tunneling spectroscopy
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(< 0.5 meV) and high sensitivity (< 1013 molecules are required to provide a spectrum). An additional advantage is the fact that optically forbidden transitions may be observed as well. Within IETS, an oxide layer with molecules adsorbed on it is put between two metal plates. A bias voltage is applied between the two contacts. An energy diagram of the metal-oxide-metal device under bias is shown in the top figure. The metal contacts are characterized by a constant density of states
Density of states
In solid-state and condensed matter physics, the density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not...
, filled up to the Fermi energy
Fermi energy
The Fermi energy is a concept in quantum mechanics usually referring to the energy of the highest occupied quantum state in a system of fermions at absolute zero temperature....
. The metals are assumed to be equal. The adsorbates are situated on the oxide material. They are represented by a single bridge electronic level, which is the upper dashed line. If the insulator is thin enough, there is a finite probability that the incident electron tunnels through the barrier. Since the energy of the electron is not changed by this process, it is an elastic process. This is shown in the right figure.
Some of the tunneling electrons can lose energy by exciting vibrations of the oxide or the adsorbate. These inelastic processes lead to a second tunneling path, which gives an additional current contribution to the tunneling current. Since the incident electron should have enough energy to excite this vibration, there is a minimum energy that is the onset of this (inelastic) process. This is shown in the middle figure, where the lower dashed line is a vibronic state. This minimum energy for the electron corresponds with a minimum bias voltage, which is the onset for the additional contribution. The inelastic contribution to the current is small compared to the elastic tunneling current (~0.1%) and is more clearly seen as a peak in the second derivative
Derivative
In calculus, a branch of mathematics, the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a...
of the current to the bias voltage, as can be seen in the bottom figure.
There is however also an important correction to the elastic component of the tunneling current at the onset. This is a second order effect in electron-vibration coupling, where a vibration is emitted and reabsorbed or vice versa. This is shown in the upper figure on the right. Depending on the energetic parameters of the system, this correction may be negative and it may outweigh the positive contribution of the inelastic current, resulting in a dip in the IETS spectrum. This is experimentally verified in both regular IETS and in STM-IETS and is also predicted theoretically. Not only peaks and dips may be observed, but depending on the energetic parameters also derivative-like features may be observed, both experimentally and theoretically.
STM-IETS
Scanning tunneling microscope
A scanning tunneling microscope is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer , the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and...
(STM) at fixed position over the surface and sweeping the bias voltage, one can record a I-V characteristic. This technique is called scanning tunneling spectroscopy
Scanning tunneling spectroscopy
Scanning Tunneling Spectroscopy is an extension of Scanning Tunneling Microscopy which is used to provide information about the density of electrons in a sample as a function of their energy....
(STS). The first derivative gives information about the local density of states (LDOS) of the substrate, assuming that the tip has a constant density of states. The second derivative gives information on vibrations of the adsorbate, exactly as in IETS. That is why this technique is commonly called STM-IETS. The role of the insulating oxide layer is played by the gap between the tip and the adsorbate.
Nowadays molecular transport junctions have been produced with one single molecule between two electrodes, possibly with an additional, gate electrode near the molecule. The advantage of this method in comparison with STM-IETS is that there is contact between both electrodes and the adsorbate, whereas in STM-IETS there is always a tunneling gap between the tip and the adsorbate. The disadvantage of this method is that it is experimentally very challenging to create and identify a junction with exactly one molecule between the electrodes.