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We have experimentally determined the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS2, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the non-local nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.

However, the conductance variation from junction to junction has made it difficult to verify even the simplest predictions about how molecules should behave in unimolecular devices. Here, using amine link groups 13 to form single molecule junctions, we show a clear correlation between molecule conformation and junction conductance in a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine squared relation predicted theoretically for transport through π-conjugated systems 14 .We recently demonstrated that metal-molecule-metal junctions, formed by breaking Au point contacts in a solution of molecules, exhibit more reliable and reproducible conductance values when amine groups rather than thiols or isonitriles are used to attach the molecules to the junction contacts 13 . Because of this reduced variability, we can 2 determine statistically meaningful average conductance values for specific singlemolecule junctions; this capability in turn allows us to study the impact of molecular properties on junction conductance.We present our experimental results as conductance histograms, where peaks indicate the most prevalent molecular junction conductances while the width of the conductance distributions reflects the microscopic variations from junction to junction.(For details on experimental and data analysis procedures, see SupplementaryInformation.) Figure 1A shows the histograms for 1,4-diaminobenzene (1) and 2,7-diaminofluorene (2), each constructed from over 10000 conductance traces without resorting to any data selection or processing. Many of our conductance traces reveal stepwise changes in conductance not only at conductance values that are multiples of the fundamental quantum of conductance G 0 (2e 2 /h), but also below G 0 (see Fig. 1C and Supplementary Information Figure S1). These steps are due to conduction through a single molecule bridging the gap between the two Au point-contacts. As seen in the histograms of 1 and 2 (Fig. 1A), for each junction type the additional step occurs within a narrow distribution of conductance values. The most prevalent value is determined by a fit to the peak below G 0 in the conductance histograms using a Lorentzian line shape, which we find to fit our peaks more accurately than a Gaussian line shape (see inset of Figure 1A and also Supplementary Information).When the experiment is repeated using a solution containing an equimolar mixture of 1 and 2 (as indicated in Figure 1B), the resulting histogram (red curve in Figure 1A) shows two distinct peaks below G 0 at nearly the same conductance values as those of the peaks seen in the histograms for the individual molecules (green and yellow curves).Individual traces using the solution mixture show conductance plateaus corresponding to either molecule 1 or 2, and sometimes a plateau corresponding to 1 followed by a plateau 3 corresponding to 2 (see Figure 1C). But ...

The electronic structure of benzene on graphite (0001) is computed using the GW approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.