We have studied the structural and electronic properties of tetracyanoethylene (TCNE) molecules on different noble-metal surfaces using scanning tunneling spectroscopy and density functional theory. Striking differences are observed in the TCNE behavior on Au, Ag, and Cu substrates in the submonolayer limit. We explain our findings by a combination of charge-transfer and lattice-matching properties for TCNE across substrates that results in a strong variation of molecule-molecule and molecule-substrate interactions. These results have significant implications for future organic/inorganic nanoscopic devices incorporating molecule-based magnetism.

We have measured the elastic and inelastic tunneling properties of isolated Gd@C(82) molecules on Ag(001) using cryogenic scanning tunneling spectroscopy. We find that the dominant inelastic channel is spatially well localized to a particular region of the molecule. Ab initio pseudopotential density-functional theory calculations indicate that this channel arises from a vibrational cage mode. We further show that the observed inelastic tunneling localization is explained by strong localization in the molecular electron-phonon coupling to this mode.

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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...

We present a low-temperature scanning tunneling microscopy (STM) study of K(x)C60 monolayers on Au(111) for 3 < or = x < or = 4. The STM spectrum evolves from one that is characteristic of a metal at x = 3 to one that is characteristic of an insulator at x = 4. This electronic transition is accompanied by a dramatic structural rearrangement of the C60 molecules. The Jahn-Teller effect, a charge-induced mechanical deformation of molecular structure, is directly visualized in the K4C60 monolayer at the single-molecule level. These results, along with theoretical analyses, provide strong evidence that the transition from metal to insulator in K(x)C60 monolayers is caused by the Jahn-Teller effect.

The ability to tune competing interactions in the fullerides arises from advances in our ability to grow well-controlled heterogeneous molecular films. Here we describe measurements on potassium doped C 60 (K x C 60 ) ultra-thin films having variable thickness from one to three layers (layer index i = 1, 2, and 3) for three specific doping concentrations (x = 3, 4, and 5). Fig. 1a displays a scanning tunneling microscope (STM) topograph of a representative K x C 60 multilayer on Au(111), where the color scale highlights the plateau structure. Narrow slivers of C 60 -free voids containing only K atoms 3 (brown) exist between continuous patches of K x C 60 . 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 .We begin by describing our results for a multilayer of the x = 3 metallic system.Layer-dependent electronic structure in K 3 C 60 can be seen in Fig. 2a, which shows spatially-averaged dI/dV spectra measured at three different layer levels. Within each layer the spectrum is highly uniform with no sign of spatial inhomogeneity such as that found in the surface of bulk fullerides 12 . The first layer dI/dV displays a wide peak at the Fermi energy (E F ), reflecting the large electronic density of states (DOS) of a metallic LUMO-derived band (LUMO = Lowest Unoccupied Molecular Level). In contrast, the second layer spectrum shows a sharp dip at E F , indicating the emergence of an energy gap that tends to split the band into two halves. A similar gap-like feature persists in the third layer. The width of the gap-like feature (measured between adjacent local maxima) is ~ 0.2 eV, a much larger value than the superconducting gap 2∆ sc ~ 6 meV found in bulk K 3 C 60 13.The spatial arrangement of C 60 molecules also changes dramatically with layer index. The first layer of K 3 C 60 (Fig. 2c) exhibits a complex 3 3 × superstructure of bright molecules having different orientation from their dimmed nearest neighbors 8 . In the second layer ( Fig. 2d), however, C 60 molecules form a very simple hexagonal lattice (lattice constant a ~10.5 Å) with long-range orientational ordering. The tri-star-like topography of each molecule suggests that C 60 in the second layer is oriented with a hexagon pointing up 14 . The third layer topograph is the same as the second layer. 4The insulating x = 4 multilayer system displays a similar trend. Fig. 3a shows dI/dV spectra measured on a K 4 C 60 plateau structure where the number of layers is varied from i = 1 to 3. First layer spectra (i = 1) exhibit an insulating energy gap ∆ ~ 0.2 eV that is induced by molecular Jahn-Teller (JT) distortion 8 . As the layer index increases from i = 1 to 3, the energy gap opens continuously (by layer 3 the gap has well-defined edges and a flat bottom). The gap amplitudes observed here are estimated to be ∆ ~ 0.6 eV and 0.8 eV for layer 2 and 3 respectively. As seen in the metallic x = 3 ...

We report a method for controllably attaching an arbitrary number of charge dopant atoms directly to a single, isolated molecule. Charge-donating K atoms adsorbed on a silver surface were reversibly attached to a C60 molecule by moving it over K atoms with a scanning tunneling microscope tip. Spectroscopic measurements reveal that each attached K atom donates a constant amount of charge (approximately 0.6 electron charge) to the C60 host, thereby enabling its molecular electronic structure to be precisely and reversibly tuned.

We have fabricated hybrid magnetic complexes from V atoms and tetracyanoethylene ligands via atomic manipulation with a cryogenic scanning tunneling microscope. Using tunneling spectroscopy we observe spin-polarized molecular orbitals as well as Kondo behavior. For complexes having two V atoms, the Kondo behavior can be quenched for different molecular arrangements, even as the spin-polarized orbitals remain unchanged. This is explained by variable spin-spin (i.e., V-V) ferromagnetic coupling through a single tetracyanoethylene (TCNE) molecule, as supported by density functional calculations.