Negative differential resistance in scanning tunneling microscopy: simulations on C60-based molecular overlayers
We determine the conditions in which negative differential resistance (NDR) appears in the C60based molecular device of [Phys. Rev. Lett. 100, 036807 (2008)] by means of ab-initio electrontransport simulations. Our calculations grant access to bias-dependent intrinsic properties of the molecular device, such as electronic levels and their partial widths. We show that these quantities depend on the molecule-molecule and molecule-electrode interactions of the device. Hence, NDR can be tuned by modifying the bias behavior of levels and widths using both types of interactions. Since the creation of the first tunnel diode [1], a nonlinear I–V characteristic, particularly showing negative differential resistance (NDR) is the cornerstone in twoterminal devices [2, 3]. NDR permits the electronic current of these devices to be modified by the applied bias in a complex way. This active control of the current motivates a long search for NDR in molecular devices [4–10] where the scanning tunneling microscope (STM) plays an important role [5, 6]. The STM can vary applied bias and tunneling current in an independent manner, yield atomically precise data and still be a two-terminal device. More recently, STM studies of C60 molecules have spurred a lot of interest because of the NDR capabilities of C60-based devices [8, 10–13]. As early as the first NDR cases were obtained in STM junctions, the existence of sharp resonances on both electrodes were used to explain the decrease of current as the bias increased [5, 6, 14]. However, Grobis and coworkers [11] explained their own NDR data by uncovering yet another mechanism: the voltage-dependent increase of the tunneling barrier height. Recently, one more mechanism has been advanced in order to explain the dependence of NDR on the tip’s material, namely orbital matching between molecule and tip [15]. This last mechanism has been thoroughly studied and validated, while showing the need of sharp STM tips [16]. However, in most molecular devices, the accurate evaluation of the voltage drop across the device is crucial. As a matter of fact, Tu and co-workers [17] showed the paramount importance of the actual voltage-dependent barrier for the presence or absence of NDR. This spread of mechanisms shows that many parameters change among different molecular junctions and it is of fundamental importance to understand the key ingredients in NDR and thus the final functionality of possible molecular devices. In this letter, we study the molecular device of Ref. [12] unraveling the mechanisms leading to the measured NDR [12]. The simulated molecular device consists of C60 molecules partially decoupled from a Au (111) substrate by spectator 1,3,5,7-tetraphenyladamantane (TPA) molecules. Despite the complexity of the simulations, we can extract the molecular levels and their coupling with the electrodes while describing their bias dependence. This allows us to have unprecedented insight in the actual physics of a molecular device with a quantitative description. We show that we can tune the size of the NDR signal by modulating the interplay of the bias dependence of the levels with the bias dependence of their partial widths which ultimately depend on molecule-molecule and molecule-electrode interactions. In the Landauer formalism [18], the electronic current, I, is given in terms of the electron transmission function T(E,V ) with E the electron energy and V the applied bias (e and h being the electron charge and Planck’s constant, respectively), by: I = 2e