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Single-electron transistors

Spintronics at the molecule level
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We aim at investigating the transport properties of single-molecule magnets (SMMs) using single-electron transistor (SET) devices. FOr this, the molecules are attached to nanometer-gapped metal electrodes (source and drain) and gated electrically to form a SET. The interplay between the high-spin states of the molecule and the conduction electrons from the electrodes will be studied  both  statically

(dc transport) and dynamically (transport in the presence of microwave radiation). These studies will allow the investigation of excited molecular states, the effect of different ligands on the transport through SMMs, the Kondo effect, spin-polarized transport, the Berry-phase blockade, quantum oscillations of the magnetization, decoherence, and other topics

A molecular SET

In the figure below is represented the SETs (A) and the energy landscape (B,C) of an ideal molecular SET. The black regions on the sides represent the electron Fermi seas in the source and drain electrodes, with μS and μD being the Fermi levels of the leads. The grey blocks represent the tunnel barriers between the molecule and the source/drain leads (note that these barriers can be quite asymmetric since the disposition of the molecule with respect to the electrodes may vary). The charge states of the molecule are represented by the horizontal lines in between the barriers. The highest of all occupied states (solid lines) represents the molecule with N electrons and an electrochemical potential μN. The first (empty) excited state is separated by an energy Ec + E, where E is the molecular electronic level spacing and Ec is the energy necessary to add one electron into the molecule (charging energy or redox potential). Conduction through a molecular SET only occurs when a molecular electronic level lies between the Fermi energies of the leads. A bias voltage V applied between the source and the drain moves the Fermi level of one of the leads by |eV|. For small bias voltages, |eV| < Ec + E, no current flows though the device because the excited molecular levels are not available to accept conduction electrons (Fig. 3b). This regime is known as Coulomb blockade. As the bias voltage is further increased, excited states open new conduction channels through the device. Abrupt and discrete changes in the current through the SET will be obtained every time a new molecular level becomes energetically accessible. The voltage values at which these current steps occur can be tuned by a potential applied to the gate electrode, VG, which moves the molecular states with respect to the Fermi levels of the electrodes
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Fabrication of nanogapped single-electron transistors for transport studies of individual single-molecule magnets

Three-terminal single electron transistor devices utilizing Al/Al2O3 gate electrodes were developed for the study of electron transport through individual single-molecule magnets SMs.. The devices were patterned via multiple layers of optical and electron beam lithography. Electromigration induced breaking of the nanowires reliably produces 1–3nm gaps between which the SMM can be situated. Conductance through a single Mn12 3-thiophene carboxylate displays the Coulom bblockade effect with several excitations within ±40meV

J. Appl. Phys. 101, 09E102 (2007). pdf_logo04

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Coulomb blockade and the characteristic conduction behavior of an SET are illustrated in the figure below, where numerical calculations of the current flowing through an ideal SET are presented. In A it is shown the typical I-V curves observed in a SET at different gate voltages. Discrete steps are observed whenever a new excited state is accessible for conduction. Contour plots of the current (B) and the current derivative (C) as functions of the bias and gate voltages show the characteristic diamond structure representative of Coulomb blockade. C illustrates how two consecutive charge states of the molecule (N and N+1) are separated by excitation lines. These lines intersect at the point where the gate voltage is equal to the charging energy of the molecule. The dI/dV plot of an SET also reveals the level structure of the molecule and thus constitutes a powerful spectroscopy technique to study the energy landscape. Typically, the transition region between two charge states of the molecule contains multiple excitations that can be traced to the molecule’s energy levels. For example, D shows the differential conductance of a molecule where the current flows through two levels of the first excited charge state. As in other spectroscopic techniques, the position, the shape, the magnitude, and the slope (among others) of the conduction excitations unveil characteristics that are both intrinsic (i.e., the electronic nature of the molecule) and extrinsic (i.e., the disposition of the molecule with respect to the electrodes) of the system under study
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The potential of single-electron transport spectroscopy for the understanding of the fundamental physics and chemistry of individual molecules has been recently demonstrated by various breakthrough experimental discoveries. For example, transport excitations associated with fundamental vibrational modes of an individual C60 molecule in a SET have been reported. More recently, groups at Cornell and Harvard have observed the Kondo effect in individual paramagnetic molecules. Along this line, SMMs will provide a unique venue for probing novel aspects of the interplay between conduction electrons and molecular spin levels.
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