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Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challenge. Here we introduce spectroscopic ellipsometric contrast microscopy (SECM) as a tool for mapping twist angle disorder in optically resonant twisted bilayer graphene. We optimize the ellipsometric angles to enhance the image contrast based on measured and calculated reflection coefficients of incident light. The optical resonances associated with van Hove singularities correlate well to Raman and angle-resolved photoelectron emission spectroscopy, confirming the accuracy of SECM. The results highlight the advantages of SECM, which proves to be a fast, nondestructive method for characterization of twisted bilayer graphene over large areas, unlocking process, material, and device screening and cross-correlative measurement potential for bilayer and multilayer materials.

Moiré superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes1–7, the confinement of excitons in artificial moiré lattices8–13and the formation of exotic quantum phases14–18. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure19,20. To exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton–phonon scattering and subsequent charge transfer at the interlayer-hybridized Σ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moiré modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moiré superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.

Comprehending far-from-equilibrium many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equilibrium functions. In this work, we use time- and angle-resolved photoelectron spectroscopy to study such far-from-equilibrium many-body interactions for the prototypical material graphene. In accordance with theoretical simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequilibrium conditions. These observations can be understood by ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequilibrium dynamics on many-body interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equilibrium states of matter.

Tip-induced hopping of the ethyl fragment (−C2H5) of diethyl ether molecules reacted on Si(001) was shown to be a field-driven process. Although the hopping rate increases continuously with increasing bias voltage, it remains constant when varying the tunneling current. No hopping events are observed at 50 K. The process is thus concluded to be thermally activated with the respective energy barrier being reduced by the applied electric field. At a positive sample bias, the field in the tunneling gap is strong enough to effectively depolarize and thus weaken the covalent Si–C bond. The effect of this depolarization on the hopping barrier is quantified and compared to the strength of the electric field.

Adsorption of methanol on Si(001) was studied by means of X-ray photoelectron spectroscopy (XPS), molecular beam techniques, and scanning tunneling microscopy (STM) at surface temperatures between 50 and 800 K. Even at lowest temperatures, only the final reaction products, i.e., a silicon bound methoxy group and a Si–H entity, were observed in the XPS and STM experiments. However, the initial sticking probability drops with increasing surface temperature, indicating that the reaction proceeds via an intermediate state. Two final configurations with the dissociation products adsorbed either on one or two silicon dimers were observed; their branching ratio does not change with temperature, indicating very similar, low conversion barriers for the two pathways.

Scanning tunneling microscopy gives access to well-controlled manipulation of surface adsorbates by means of activating reaction pathways beyond thermal excitation schemes. For the conversion of diethyl ether on Si(001) from a datively bound intermediate into the covalently bound final state, we show that tip-induced electronic excitation leads to new products when compared to thermal excitation. We emphasize the molecular structure as a key parameter in creating such specific reaction products via electronic excitation by comparing the results for diethyl ether and tetrahydrofuran, the cyclic analogue of diethyl ether. The final configurations of the two systems differ in terms of the reacted surface atoms and their electronic structure; moreover, a new reaction product which indicates the desorption of one of the two molecular fragments induced by tip-induced ether cleavage is dominant in the case of diethyl ether.

Controlled organic functionalization of silicon surfaces as an integral part of semiconductor technology offers new perspectives for a wide range of applications. The high reactivity of the silicon dangling bonds, however, presents a major hindrance for the first basic reaction step of such a functionalization, that is, the chemoselective attachment of bifunctional organic molecules on the pristine silicon surface. We overcome this problem by employing cyclooctyne as the major building block of our strategy. Functionalized cyclooctynes are shown to react on Si(001) selectively via the strained cyclooctyne triple bond while leaving the side groups intact. The achieved selectivity originates from the distinctly different adsorption dynamics of the separate functionalities: A direct adsorption pathway is demonstrated for cyclooctyne as opposed to the vast majority of other organic functional groups. The latter ones react on Si(001) via a metastable intermediate, which makes them effectively unreactive in competition with the direct pathway of cyclooctyne’s strained triple bond.

The adsorption of diethyl ether (Et2O) on Si(001) was studied by means of scanning tunneling microscopy (STM) and photoelectron spectroscopy. Et2O reacts on Si(001) via a datively bonded intermediate, which was isolated at surface temperatures below 100 K. At higher surface temperature, Et2O converts dissociatively into the final state by cleaving one O–C bond; the resulting −O–C2H5and −C2H5fragments are found to attach on two Si dimers of neighboring dimer rows. Tip-induced hopping of the −C2H5fragment on one dimer was observed at positive sample bias. The results are discussed in the context of recent experiments on the reaction of tetrahydrofuran (THF) on Si(001) (Mette et al. ChemPhysChem2014, 15, 3725) and allow a more general description of the reaction of ethers on Si(001).