Transport behavior of SMM-based SETs

Very recently we have obtained results that, while still very preliminary, prove the feasibility of the SMM-based SET devices. The figure above (A) shows an AFM image of Mn₁₂-3-thiophenecarboxylate (Mn₁₂-3tpc) SMMs covering completely an atomically flat Au surface. SMM solutions of different molarities (0.1 mM in A) were deposited on Au for different times (1hour in A) until an optimal coverage of the surface was obtained. XPS data confirmed the presence of Mn on the surface by peaks in both the Mn 3p and Mn 4p regions. To deposit the SMMs on the SET devices, the chip is first cleaned with oxygen plasma and, then, immediately immersed into a SMM solution for a specific period of time, and finally removed and blown dry with N_2. We have tried using two types of SMM samples so far: i) Samples containing sulfur functionalized ligands designed for control of the assembly of the molecules onto Au, namely thiophene ring (this ligand has been previously shown to adhere to Au both independently and as part of a SMM); and, ii) raw SMMs not functionalized, more specifically Mn₁₂-bromoacetate. We have observed conductance behaviors characteristic of a molecular SET with both types of SMMs.

The experiment was done at T = 4 K in an SET filled with Mn₁₂-3tpc molecules. B shows the stepwise I-Vcurves measured at different gate voltages. Peaks in the current derivatives (C) clearly reveal the discrete nature of the conductance through our SMM-based SET. The conduction excitations (current steps) can be easily followed in D, where the differential conductance (dI/dV) is contour-plotted as a function of bias and gate voltages (white lines are to guide the eyes along the linear conductance excitations). The presence of multiple parallel excitations forming the characteristic diamond shape of a molecular SET indicates the crossing between two charge states (N and N+1) of the molecule at V_g ~0.3V and reveals the complex nature of the excited states of this particular molecule. The excitations are found in an energy range (0-40 meV) similar to what had been observed by other groups for Mn₁₂-based SETs. A similar behavior has been observed for other molecules trapped in the same set of chips. Experiments are currently being run at lower temperatures and in the presence of magnetic fields with arbitrary orientations.