Charge versus energy transfer in atomically-thin graphene-transition metal dichalcogenide van der Waals heterostructures

Van der Waals heterostuctures, made from stacks of two-dimensional materials, exhibit unique light-matter interactions and are promising for novel optoelectronic devices. The performance of such devices is governed by near-field coupling through, e.g., interlayer charge and/or energy transfer. New concepts and experimental methodologies are needed to properly describe two-dimensional heterointerfaces. Here, we report on interlayer charge and energy transfer in atomically thin metal (graphene)/semiconductor (transition metal dichalcogenide (TMD, here MoSe$_2$)) heterostructures using a combination of photoluminescence and Raman scattering spectroscopies. The photoluminescence intensity in graphene/MoSe$_2$ is quenched by more than two orders of magnitude and rises linearly with the photon flux, demonstrating a drastically shortened (\sim 1~\tr{ps}) room temperature MoSe$_2$ exciton lifetime. Key complementary insights are provided from analysis of the graphene and MoSe$_2$ Raman modes, which reveals net photoinduced electron transfer from MoSe$_2$ to graphene and hole accumulation in MoSe$_2$. Remarkably, the steady state Fermi energy of graphene saturates at 290\pm 15~\tr{meV} above the Dirac point. This behavior is observed both in ambient air and in vacuum and is discussed in terms of band offsets and environmental effects. In this saturation regime, balanced photoinduced flows of electrons and holes may transfer to graphene, a mechanism that effectively leads to energy transfer. Using a broad range of photon fluxes and diverse environmental conditions, we find that the presence of net photoinduced charge transfer has no measurable impact on the near-unity photoluminescence quenching efficiency in graphene/MoSe$_2$. This absence of correlation strongly suggests that energy transfer to graphene is the dominant interlayer coupling mechanism between atomically-thin TMDs and graphene.Comment: Physical Review X, in press. 14 pages, 7 figures, with supplemental materia

Probing electronic excitations in mono- to pentalayer graphene by micro-magneto-Raman spectroscopy

We probe electronic excitations between Landau levels in freestanding $N-$layer graphene over a broad energy range, with unprecedented spectral and spatial resolution, using micro-magneto Raman scattering spectroscopy. A characteristic evolution of electronic bands in up to five Bernal-stacked graphene layers is evidenced and shown to remarkably follow a simple theoretical approach, based on an effective bilayer model. $(N>3)$-layer graphene appear as appealing candidates in the quest for novel phenomena, particularly in the quantum Hall effect regime. Our work paves the way towards minimally-invasive investigations of magneto-excitons in other emerging low-dimensional systems, with a spatial resolution down to 1$~\mu$m.Comment: to appear in Nano Letter

Photothermal Single Particle Microscopy

Photothermal microscopy has recently complemented single molecule fluorescence microscopy by the detection of individual nano-objects in absorption. Photothermal techniques gain their superior sensitivity by exploiting a heat induced refractive index change around the absorbing nano-object. Numerous new applications to nanoparticles, nanorods and even single molecules have been reported all refering to the fact that photothermal microscopy is an extinction measurement on a heat induced refractive index profile. Here, we show that the actual physical mechanism generating a photothermal signal from a single molecule/particle is fundamentally different from the assumed extinction measurement. Combining photothermal microscopy, light scattering microscopy as well as accurate Mie scattering calculations to single gold nanoparticles, we reveal that the detection mechanism is quantitatively explained by a nanolensing effect of the long range refractive index profile. Our results lay the foundation for future developments and quantitative applications of single molecule absorption microscopy.Comment: main manuscript (5 figures), 1 supplement (3 figures