Highlights Photochemical oxidation (UV/ozone treatment) of self-assembled solution-processed graphene films is demonstrated for the first time  The effect that photochemical oxidation has on self-assembled graphene films is a reduction of sheet resistance, the opposite of the earlier reported effect on CVD or mechanically exfoliated graphene  Edges as the dominant defect type in self-assembled graphene films play a crucial role in the presented defect density decrease  Photochemical oxidation is proposed as a method of increasing the carrier mean free path, doping, and lowering sheet resistance of solution-processed graphene films Abstract Liquid phase exfoliation followed by Langmuir-Blodgett self-assembly (LBSA) is a promising method for scalable production of thin graphene films for transparent conductor applications.However, monolayer assembly into thin films often induces a high density of defects, resulting in a large sheet resistance that hinders practical use. We introduce UV/ozone as a novel photochemical treatment that reduces sheet resistance of LBSA graphene threefold, while preserving the high optical transparency. The effect of such treatment on our films is opposite to the effect it has on mechanically exfoliated or CVD films, where UV/ozone creates additional defects in the graphene plane, increasing sheet resistance. Raman scattering shows that exposure to UV/ozone reduces the defect density in LBSA graphene, where edges are the dominant defect type. FTIR spectroscopy indicates binding of oxygen to the graphene lattice during exposure to ozone. In addition, work function measurements reveal that the treatment dopes the LBSA film, making it more conductive. Such defect patching paired with doping leads to an accessible way of improving the transparent conductor performance of LBSA graphene, making solution-processed thin films a candidate for industrial use.

Surface plasmon polaritons in graphene couple strongly to surface phonons in polar substrates leading to hybridized surface plasmon-phonon polaritons (SPPPs). We demonstrate that a surface acoustic wave (SAW) can be used to launch propagating SPPPs in graphene/h-BN heterostructures on a piezoelectric substrate like AlN, where the SAW-induced surface modulation acts as a dynamic diffraction grating. The efficiency of the light coupling is greatly enhanced by the introduction of the h-BN film as compared to the bare graphene/AlN system. The h-BN interlayer not only significantly changes the dispersion of the SPPPs but also enhances their lifetime. The strengthening of the SPPPs is shown to be related to both the higher carrier mobility induced in graphene and the coupling with h-BN and AlN surface phonons. In addition to surface phonons, hyperbolic phonons appear in the case of multilayer h-BN films leading to hybridized hyperbolic plasmon-phonon polaritons (HPPPs) that are also mediated by the SAW. These results pave the way for engineering SAW-based graphene/h-BN plasmonic devices and metamaterials covering the mid-IR to THz range.

We experimentally demonstrate that the Raman active optical phonon modes of single layer graphene can be modulated by the dynamic local strain created by surface acoustic waves (SAWs). In particular, the dynamic strain field of the SAW is shown to induce a Raman scattering intensity variation as large as 15 % and a phonon frequency shift of up to 10 cm −1 for the G band, for instance, for an effective hydrostatic strain of 0.24 % generated in a single layer graphene atop a LiNbO3 piezoelectric substrate with a SAW resonator operating at a frequency of ∼ 400 MHz. Thus, we demonstrate that SAWs are powerful tools to modulate the optical and vibrational properties of supported graphene by means of the high-frequency localized deformations tailored by the acoustic transducers, which can also be extended to other 2D systems.

New architectures of transparent conductive electrodes (TCEs) incorporating graphene monolayers in different configurations have been explored with the aim to improve the performance of silicon-heterojunction (SHJ) cell front transparent contacts. In SHJ technology, front electrodes play an important additional role as anti-reflectance (AR) coatings. In this work, different transparent-conductive-oxide (TCO) thin films have been combined with graphene monolayers in different configurations, yielding advanced transparent electrodes specifically designed to minimize surface reflection over a wide range of wavelengths and angles of incidence and to improve electrical performance. A preliminary analysis reveals a strong dependence of the optoelectronic properties of the TCEs on (i) the order in which the different thin films are deposited or the graphene is transferred and (ii) the specific TCO material used. The results shows a clear electrical improvement when three graphene monolayers are placed on top on 80-nm-thick ITO thin film. This optimum TCE presents sheet resistances as low as 55 Ω/sq and an average conductance as high as 13.12 mS. In addition, the spectral reflectance of this TCE also shows an important reduction in its weighted reflectance value of 2–3%. Hence, the work undergone so far clearly suggests the possibility to noticeably improve transparent electrodes with this approach and therefore to further enhance silicon-heterojunction cell performance. These results achieved so far clearly open the possibility to noticeably improve TCEs and therefore to further enhance SHJ contact-technology performance.

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We theoretically demonstrate the strong coupling between excitons in 2D semiconductors and surface plasmons in a thin metal film forming exciton−plasmon polaritons (or plexcitons) that can be optically accessed by means of a surface acoustic wave (SAW). The strain field of the SAW creates a dynamic diffraction grating, providing the momentum match for the surface plasmons, whereas the piezoelectric field that could dissociate the excitons is canceled out by the metal. This is exemplified for monolayer MoS 2 and mono-and few-layer black phosphorus on top of a thin silver layer on a LiNbO 3 piezoelectric substrate, providing Rabi splittings of 100−150 meV. Thus, we demonstrate that SAWs are powerful tools to modulate the optical properties of supported 2D semiconductors by means of the high-frequency localized deformations tailored by the acoustic transducers, that can serve as electrically switchable launchers of propagating plexcitons suitable for active high-speed nanophotonic applications.

As demands for data storage capability continue to increase, nonvolatile memory devices with discrete nanocrystals as the charge-storage nodes are being extensively investigated. To alleviate scaling issues, use of metal-nanocrystal-based ultrahigh-bitdensity memory devices, capable of multilevel cell operations, have been proposed and studied widely. Here we propose a nonvolatile floating gate memory, utilizing nanocrystals of the group-IV metal, β-Tin (β-Sn), which spontaneously self-assemble on a variety of high-k dielectric oxides and silicon during molecular beam epitaxy at low temperatures. In metal-oxide-semiconductor memory devices, we demonstrate a large memory window (∼3 V) at moderate operating voltages of ±6 V and investigate the retention and endurance characteristics. The observed results are promising for realization of memory devices, compatible with the silicon complementary-metal-oxide-semiconductor technology.

Coulomb drag in double layer graphene systems separated by an h-BN interlayer allows probing of the electron-electron interactions in the effective limit of zero layer separation. Although these interactions can be influenced by plasmons, phonons and exchange and correlation effects, these excitations have never been studied altogether, missing the effects of their coupling on the drag physics. Here we study theoretically the effects of these quasiparticles and their coupling, including also the effects of the electronic exchange and correlation, and demonstrate that the drag resistivity can attain a maximum value at room temperature and beyond, where hybridized plasmon-phonon modes contribute significantly. In particular, the hybridization of the plasmons with the hyperbolic phonons of h-BN, confined within the reststrahlen bands, enhance the drag resistivity. This study paves the way for the exploration of novel many-body physics phenomena in systems coupled through emerging 2D hyperbolic materials.