New Discoveries and Opportunities from Two-Dimensional Materials

S ince the isolation of graphene in 2004, work on atomically thin two-dimensional (2D) materials has progressed rapidly across a diversity of scientific and engineering subfields.The types of 2D materials available has been ever-growing and now include insulators (e.g., hexagonal boron nitride), semiconductors (e.g., transition metal dichalcogenides, TMDs), and additional semimetals (e.g., black phosphorus).From these new materials, a wide array of optical, mechanical, chemical, and electric phenomena has been realized in 2D crystals produced using top-down exfoliation or bottom-up synthesis (e.g., by chemical vapor deposition, CVD).As 2D material science has become a mature field in-and-of itself, several key advantages have emerged that can be leveraged for new experimentation and device creation.These advantages include the amenability of 2D materials toward top-down and bottom-up lithography methods; their pliability and ability to be mechanically strained to create new structure-propertyfunction relationships; and their unique chemistry, with large surface areas that lead to properties that are highly environmentally dependent.In this virtual issue (http://pubs.acs.org/page/vi/ 2Dmaterials.html),we have compiled a collection of articles from Nano Letters, ACS Photonics, ACS Nano, and Chemistry of Materials that highlight some unique properties of 2D materials including their flexibility toward new synthesis methods and doping techniques, their compatibility with scanned probe measurement techniques, the electronic and optoelectronic properties of their p-n junctions, and their potentially useful optical properties.These articles summarize studies by ACS authors that display the manner in which 2D systems are being grown, modeled, measured, and used in new ways that are made possible by their atomic thinness and particular electronic structures and that could potentially be transformative in creating new areas of research.From a perspective of offering new measurement opportunities, 2D materials provide a unique platform for scanned probe measurements such as scanning tunneling microscopy (STM) as well as scanned gate techniques, and several key papers in this area have recently appeared in Nano Letters.Unlike conventional three-dimensional (3D) materials or interfacial GaAs-GaAlAs 2D systems, 2D materialsin which the surface is the bulkare entirely accessible to such probes, and, since there are no bulk states, the information provided by scanned probe methods can potentially provide complete characterization of the environmentally dependent structural, electronic, and defect properties.Furthermore, these measurements can be conducted while the sample is being gated in situ, thus providing unprecedented information on how quasiparticle behavior changes with carrier density and how the local electronic structure changes under transport conditions.The first gate-dependent measurements of few-layer MoS 2 came from exfoliated samples, which provided a direct measurement of the MoS 2 band gap and the carrier density dependent Fermi level, as well as a direct measurement of deep intergap states that were found in thicker samples. 1 More recently, metal-organic chemical vapor deposition (MOCVD) bilayer and monolayer MoS 2 grown on SiO 2 were measured under STM with in situ gating.This new growth technique enabled larger samples with more contact area and less surface contamination, and those measurements could directly extract the gate-dependent conduction band edge position as well as the gap energy; these data could then be compared to photoluminescence data to determine the exciton binding energy. 2 Local measurements of the grain boundaries in those samples, meanwhile, probed the angle dependence of their atomic structure and found that all angles were electronically inert in MOCVD-grown samples. 2 By comparison, STM measurements of CVD-transferred WSe 2 samples on graphite revealed deep intergap states that formed along the grain boundaries with different periodicities, revealing different edge chemistries in comparison to MOCVD samples.3 Monolayer MoS 2 crystals grown via CVD on graphite substrates, meanwhile, have enabled direct STM measurements of metallic edge states, which were shown to locally pin the Fermi level and to create lateral band bending near the crystal edge with a 5 nm depletion width.4 Scanned probe methods can also be used to modify or induce local properties in 2D materials, both reversibly and irreversibly.For example, for graphene samples exfoliated onto boron nitride, an STM tip has been shown to ionize defects in the h-BN and create local, 50 nm scale doped regions in the graphene, with a polarity that can be controlled via the applied backgate voltage.These doping patterns can control the macroscopic transport properties of the device and, remarkably, can also be "erased" via optical exposure or by using another controlled gate pulse, which effectively deionizes the h-BN dopants.5 A charged tip can also be used as a local gate, which impedes electron flow and can be used to directly probe the motion of quasiparticles in 2D materials.This phenomenon was shown most dramatically for h-BN/graphene/h-BN samples in a Hall bar geometry, where an applied field from a scanned probe tip could locally block the cyclotron motion in high magnetic fields and, thus, provide an effective image of the cyclotron orbits by preventing transport from one electrode to another.6 Finally, scanned probe methods have also been used to extract the local optical properties of 2D materials.For MoS 2