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Register a new userSpectroscopic size and thickness metrics for liquid-exfoliated h-BN
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For many 2D materials, optical and Raman spectra are richly structured, and convey information on a range of parameters including nanosheet size and defect content. By contrast, the equivalent spectra for h-BN are relatively simple, with both the absorption and Raman spectra consisting of a single feature each, disclosing relatively little information. Here, the ability to size-select liquid-exfoliated h-BN nanosheets has allowed us to comprehensively study the dependence of h-BN optical spectra on nanosheet dimensions. We find the optical extinction coefficient spectrum to vary systematically with nanosheet lateral size due to the presence of light scattering. Conversely, once light scattering has been decoupled to give the optical absorbance spectra, we find the size dependence to be mostly removed save for a weak but well-defined variation in energy of peak absorbance with nanosheet thickness. This finding is corroborated by our ab initio GW and Bethe-Salpeter equation calculations, which include electron correlations and quasiparticle self-consistency (QSGW). In addition, while we find the position of the sole h-BN Raman line to be invariant with nanosheet dimensions, the linewidth appears to vary weakly with nanosheet thickness. These size-dependent spectroscopic properties can be used as metrics to estimate nanosheet thickness from spectroscopic data.
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Liquid phase exfoliation is a commonly used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues that limit nanosheet application potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width, and thickness distributions for each fraction. The resultant data show a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple nonequilibrium thermodynamics-based model predicting that the power-law prefactor is proportional to both the ratios of in-plane-tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first-principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation.
Liquid-phase-exfoliation is a technique capable of producing large quantities of two-dimensional material in suspension. Despite many efforts in the optimization of the exfoliation process itself not much has been done towards the integration of liquid-phase-exfoliated materials in working solid-state devices. In this article, we use dielectrophoresis to direct the assembly of liquid-phase-exfoliated TiS3 nanoribbons between two gold electrodes to produce photodetectors working in the visible. Through electrical and optical measurements we characterize the responsivity of the device and we find values as large as 3.8 mA/W, which improve of more than one order of magnitude on the state-of-the-art for devices based on liquid-phase-exfoliated two-dimensional materials assembled by drop-casting or ink-jet methods.
The application of the chiral decomposition procedure to hybrid graphene h-BN systems revealed rules for the partition of the system into effective subsystems being bilayers plus monolayer in case the number of layers is odd. Three types of subsystems have been detected namely purely graphene bilayers and monolayers, mixed bilayers and pure h-BN monolayers depending on the hybrid composition. The effective parameters characterizing these chiral subsystems consist of the interlayer couplings and on-site potentials which shows the mechanism of compensation of the asymmetry introduced into the system by h-BN layers. For illustration, we provide a pedagogical overview about chiral tunneling in graphene subsystems (MLG, BLG) present in hybrid with one h-BN layer. We have established the parameter ranges for which the characteristic features in the spectrum are observed, such as Fabry-Perot resonances in the case of MLG and magic angles in the case of effective BLG. We also consider different hybrid stacking in order to indicate effective systems with the desired properties required in the electronic and spintronic applications.
Transition metal dichalcogenides hold promise for applications in novel optoelectronic devices. There is therefore a need for materials that can be obtained in large quantities and with well understood optical properties. In this report, we present a thorough photoluminescence (PL) investigations of monolayer tungsten disulphide obtained via liquid phase exfoliation. As shown by microscopy studies, the exfoliated nanosheets have dimensions of tens of nanometers and thickness of 2.5 monolayers on average. The monolayer content is about 20%. Our studies show that at low temperature the photoluminescence is dominated by excitons localized on nanosheet edges. As a consequence, the PL is strongly sensitive to environment and exhibits an enhanced splitting in magnetic field. As the temperature is increased, the excitons are thermally excited out of the defect states and the dominant transition is that of the negatively charged exciton. Furthermore, upon excitation with a circularly polarized light, the PL retains a degree of polarization reaching 50% and inherited from the valley polarized photoexcited excitons. The studies of PL dynamics reveal that the PL lifetime is on the order of 10 ps, probably limited by non-radiative processes. Our results underline the potential of liquid exfoliated TMD monolayers in large scale optoelectronic devices.
High mobility single and few-layer graphene sheets are in many ways attractive as nanoelectronic circuit hosts but lack energy gaps, which are essential to the operation of field-effect transistors. One of the methods used to create gaps in the spectrum of graphene systems is to form long period moire patterns by aligning the graphene and hexagonal boron nitride (h-BN) substrate lattices. Here, we use planar tunneling devices with thin h-BN barriers to obtain direct and accurate tunneling spectroscopy measurements of the energy gaps in single- and bi-layer graphene-h-BN superlattice structures at charge neutrality (first Dirac point) and at integer moire band occupancies (second Dirac point, SDP) as a function of external electric and magnetic fields and the interface twist angle. In single-layer graphene we find, in agreement with previous work, that gaps are formed at neutrality and at the hole-doped SDP, but not at the electron-doped SDP. Both primary and secondary gaps can be determined accurately by extrapolating Landau fan patterns to zero magnetic field and are as large as $simeq$ 17 meV for devices in near perfect alignment. For bilayer graphene, we find that gaps occur only at charge neutrality where they can be modified by an external electric field. Tunneling signatures of in-gap states around neutrality suggest the development of edge modes related to topologically non-trivial valley projected bands due to the combination of an external electric field and moire superlattice patterns.
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