Tuning negative differential resistance in a molecular film

Abstract

We have observed variable negative differential resistance (NDR) in scanning tunneling spectroscopy measurements of a double layer of C 60 molecules on a metallic surface.Minimum to maximum current ratios in the NDR region are tuned by changing the tunneling barrier width. The multi-layer geometry is critical, as NDR is not observed when tunneling into a C 60 monolayer. Using a simple model we show that the observed NDR behavior is explained by voltage-dependent changes in the tunneling barrier height. 2Negative differential resistance (NDR) is a crucial property of several important electronic components [1,2]. Originally observed in highly doped tunneling diodes [3], NDR has been seen in a variety of systems and caused by several different mechanisms [4,5,6,7,8]. Here we present a scanning tunneling spectroscopy (STS) study showing the appearance of NDR in the tunneling signature of thin molecular C 60 films deposited on Au(111). NDR is completely absent for tunneling into a single C 60 monolayer, but emerges when tunneling into second and higher layers of C 60 . In previous STS studies of molecular systems NDR has been commonly attributed to the convolution of energetically localized tip states with the molecular density of states [7]. The NDR observed in our study is inconsistent with this interpretation, but instead stems from the voltage dependence of the tunneling barrier height [4]. We further find that the relative decrease in current, induced by the NDR, increases with increasing tunneling barrier width, allowing for tunability of the NDR behavior. This behavior is explained by using a simple tunneling model. Our experiments were conducted using a homebuilt ultrahigh vacuum (UHV) STM with a PtIr tip. The single-crystal Au(111) substrate was cleaned in UHV and dosed with C 60 using a calibrated Knudsen cell evaporator before being cooled to 7K in the STM stage. dI/dV spectra and images were measured through lock-in detection of the ac tunneling current driven by a 451Hz, 10mV (rms) signal added to the junction bias under open-loop conditions (bias voltage here is defined as the sample potential referenced to the tip). All data were acquired at 7K.Figure 1(a) shows the topographic structure of a single layer of C 60 (monolayer), a second layer of C 60 (bilayer), and a third layer of C 60 (trilayer). Each layer is well ordered 3 and has a topographic structure consistent with previous measurements performed on similar monolayer and layered C 60 systems [9,10]. The step height of each C 60 layer is ~8.0Å.Step edges in the underlying Au(111) substrate lead to 2Å steps that run through the C 60 layers. dI/dV spectra performed on the C 60 monolayer and bilayer are shown in Fig. 1 (b).These spectra exhibit several common features: a shoulder in the filled density of states (V<0) and two peaks in the empty density of states (V>0) that arise from tunneling into the C 60 highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital LUMO, and LUMO+1, respectively [11]. In the monolayer (bilayer) s...

Heat flow in nonlinear molecular junctions: Master equation analysis

“…The negative differential resistance observed in molecular films of C 60 could be explained due to a voltage dependent tunneling barrier [29]. Rectification of electron current was theoretically exhibited in one-dimensional asymmetric electronic conductors with screened electron-electron interactions [30].…”

Tuning fulleride electronic structure and molecular ordering via variable layer index

“…Islands of second (blue) and third layer (red) K x C 60 can be seen residing on top of the first K x C 60 layer (green). The average layer thickness is ~9.9 Å, greater than the 8 Å spacing found in undoped C 60 films 11 .…”

Site-Specific Polarization Screening in Organic Thin Films

“…The observed line shape of the A molecule reveals a molecule-substrate interaction beyond image charge attraction: The broadening is comparable to the phase and thus indicates wave function overlap between the system of the flat-lying molecule and the metal, leading to hybridization and lifetime broadening. In contrast, the spectra of all other molecules consist of sharp resonances, broadened only by additional satellite peaks due to vibronic progressions [24,25] at the high energy side of the main peak, and regions of negative differential conductance [5,6,26].…”

“…Because of their importance for device functions, the electronic properties of molecules close to interfaces have been studied in particular [4]. For example, it has been shown that the chemical bonding to a metal surface may change both the electronic levels and geometric structure of organic semiconductor molecules profoundly [5][6][7][8][9]. During charge transport, the electronic states of a molecular material may also be influenced by polarization screening, i.e., the stabilization of a locally injected charge through the polarization of the surrounding molecular environment [10,11].…”