Editorial pubs.acs.org/journal/apchd5
New Discoveries and Opportunities from Two-Dimensional Materials Victor W. Brar,† Andrew R. Koltonow,‡ and Jiaxing Huang‡ †
Department of Physics, University of Wisconsin−Madison, Madison, Wisconsin 53711, United States Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
‡
The first gate-dependent measurements of few-layer MoS2 came from exfoliated samples, which provided a direct measurement of the MoS2 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 MoS2 grown on SiO2 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 WSe2 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 MoS2 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 MoS2
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−property− function 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 materialsin which the surface is the bulkare 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. © 2017 American Chemical Society
Received: February 28, 2017 Accepted: February 28, 2017 Published: March 15, 2017 407
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and Nano Letters. Perhaps best known are the highly confined optical modes that can be supported in extremely thin materials, including both plasmonic and polaritonic waves. In the case of graphene, the plasmonic modes have wavelengths that can be more that 102 smaller than free space, a factor thatalong with oscillator strengthhas been shown to display a strong carrier dependence. These effects enable graphene plasmons to be extremely sensitive to their environment and also to potentially be useful for optoelectronic applications. In his ACS Photonics review14 Garcia de Abajo discusses the fundamental physics that make these effects possible and describes many of the challenges the field must address to increase the wavelength range and efficiencies at which graphene plasmonic devices can operate. Most notably, he highlights the possibility of observing quantum effects in extremely small graphene devices that could be created via bottom-up molecular techniques or through the use of advanced fabrication methods. The extreme confinement of polariton modes in h-BN, meanwhile, has been displayed experimentally through a scattering near-field scanning optical microscope measurement, which showed dramatic focusing effects in a naturally occurring h-BN wedge.15 In addition to supporting novel and tunable optical modes in the near field, 2D materials offer opportunities to manipulate light via their strong excitonic effects and interband and intraband absorption processes. These effects offer new opportunities to create media with dynamic optical properties, and the inherently small optical cross sections due to atomic thinness can often be overcome by the ability to incorporate 2D materials into light-trapping structures, which can greatly enhance the strength of light−matter interactions. For example, excitons in TMDs have strong binding energies and spectral signatures that are highly dependent on their environment, local structure, and layer thickness. One recent study showing these behaviors was performed on ReS2, a highly anisotropic TMD: reflection and photoluminescence measurements of varying sample thicknesses showed excitonic features that shifted several hundred meV as the sample thickness was decreased from 7 layers to 1 layer. The samples also displayed strongly polarized emission, which varied between exciton states and was representative of the local atomic structure of the ReS2.16 In another study, in order to enhance the effective optical cross section of excitons in WS2, gold plasmonic antennas were placed on the surface, and when the antenna frequency was closely matched to the exciton energy, it enhanced both the emission and absorption efficiency of the TMD, leading to a 10× increase in photoemission intensity.17 An alternative method to enhance absorption in 2D materials is to couple them to the guided modes photonic crystal structures. This technique was recently demonstrated theoretically for the case of MoS2 on a 100 nm thick photonic crystal with a gold backreflector, where calculations showed nearly total absorption at certain resonant frequencies and more than 50% absorption averaged above the MoS2 band gap, raising hope for the possibility of creating a highly efficientand extremely lightweightTMD-based solar cell.18 In the case of graphene, it is the inter- and intraband absorption processes that provide outstanding opportunities for index tunabilitiy, and several key experiments have recently demonstrated that through clever chemistry and engineering these effects can be leveraged to create useful optoelectronic devices. For example, multilayer graphene placed on a flexible paper substrate soaked in ionic liquid displayed a significant
samples grown on a Au(111) surface, STM methods have been used to find freestanding monolayer patches, which luminesce upon electron injection from the STM tip.7 Just as 2D crystals provide new opportunities for measurement, several key papers in Nano Letters have shown they can also be stacked, integrated, and layered to create both lateral and vertical p−n junctions with atomic precision. Such junctions are often not restricted by lattice matching criterion, and the constituent 2D materials can be gated over wide ranges, switching between p- and n-type behavior and displaying an array of band alignments within the same device. This variation has led to an assortment of interesting electronic and optoelectronic effects to be realized. For example, MoS2/ WSe2 stacks have been shown to display a lateral, type II band alignment8,9 whereby an electrostatic backgate can be used to switch the WSe2 from p-type to n-type, which creates a gatedependent p−n junction between the two layers. For the appropriate backgate voltages, this junction can be operated as a photovoltaic device, with maximum external quantum efficiencies in the range 1.5−12%, depending on the device geometry and contacts. Similar effects can be realized by integrating 2D materials with other semiconducting systems, such as organic semiconductors, which further broaden the range of band alignments that can be realized. For example, by evaporating pentacene onto MoS2, a type II device was created that displays full antiambipolar behavior with either material switching from intrinsic to doped, which enables current through the p−n junction to be highly gate-dependent.10 Such devices can also be structured to act as optical emitters that display strong, gate-dependent electroluminescence to create dynamic light-emitting diode (LED) devices. In even basic geometries, electroluminescence has been observed from carriers moving across a MoS2/WSe2 junction and recombining.8 However, the effect can be maximized and optimized through fabrication of unique tunneling devices, as was shown with WSe2 monolayers sandwiched between thin h-BN sheets, with transparent graphene electrodes on either side.11 When the correct bias was applied across those devices, electrons (holes) could tunnel into the conduction (valence) band of the WSe2 and recombine, emitting light in the process at frequencies that are representative of the unique, tightly bound exciton states that have been shown to exist in 2D crystals. Unlike typical LEDs, this process actually becomes more efficient at high temperature due to the spin−orbit splitting of the WSe2 bands, which creates a low-energy dark exciton state in WSe2. Interestingly, some of the observed emission features in such devices are due to defects in the WSe2, which localize the excitons and create sharp, energetically shifted emission peaks. With careful control of device geometry and biasing, these defects have been shown to act as singlephoton sources that can be electrically controlled and that display fine spectral signatures that are representative of quantum mechanically derived energy splittings.12 These quantized phenomena can also be observed directly in the electrical tunneling signal of stacked 2D materials. For example, a simple Au/h-BN/Au tunneling device has been shown to display Coloumb blockade type effects, indicative of a defectmediated transport process whereby the discrete charging of defects leads to steps in the I−V curve of the device.13 The atomic thinness of 2D materials also enables a wide array of interesting and useful optical properties beyond those observed via electroluminescence, and several of these phenomena have recently been reported in ACS Photonics 408
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change in transmittance when an electric potential was applied, which led to ion intercalation of the graphene and subsequent blocking of interband transitions.19 The effect is fully reversible, with a time scale of ∼1 s, and a flexible, room-temperature device was demonstrated that behaved similarly to an e-ink or liquid crystal display (LCD). In addition, the interband transitions in monolayer graphene are a powerful way to tune the optical resonances of nearby metal plasmonic structures that are engineered to have resonant modes in the mid-IR. This method was used to create a perfect absorber device at 7 μm, with 1 μm of bandwidth tunability that could operate at 20 GHz.20 A similar device was used to demonstrate dynamic phase tunability, where the phase of a 7.7 μm reflected beam could be varied by 55° at speeds that could potentially reach tens of GHz.21 The bandwidth and resolution advantages of such devices over conventional LCD or microelectromechanical systems technology present a significant opportunity for the creation of a spatial light modulator or holographic display based on 2D materials that could outperform current devices. Finally, graphene has recently been incorporated into sensing devices that take advantage of in-plane potential gradients to collect excited carriers with high efficiency and speed. In one such device, large-area CVD graphene was draped over a silicon waveguide tuned to the 1550 nm C-band. The waveguide was constructed such that there was significant spatial overlap between the waveguide mode and the graphene, and optical signal pulses could be detected by collecting excited electrons in the graphene at a nearby electrode. By using this scheme, 50 Gbit/s data rates could be detected, and the device demonstrated −3 dBm of bandwidth at 40 GHz.22 Another graphene-based detector was tuned to operate at 2 THz by placing the graphene inside a metallic bowtie antenna composed of metals with different work functions. This placement created a sloped potential landscape, which led to a strong photovoltaic effect in the graphene and a sensor responsivity of 34 μA/W.23 As demonstrated in these works, the ability to create highly efficient optoelectronic devices from the visible to the THz is a key advantage of graphene. In recent months and years, ACS journals have shown us considerable advancements in the processing of well-known 2D materials and the synthesis of new ones. Advancement in the chemical vapor deposition of MoS2 enabled the growth of largearea monolayers with control over the lattice orientations of grains. This control was achieved by epitaxial growth on a sapphire substrate. MoS2 crystals were mostly limited to orientations of 0° and ±60°, and electrical resistance was negligibly low at the boundaries between these oriented grains. The mobilities of a single-crystal-length (4 μm) device and an 80-μm-long device were not discernibly different, and the films could readily be transferred to other substrates by etching the sapphire. The uncompromised electrical and photovoltaic performance of this film over such large areas may make it suitable for application in electronics and photovoltaics.24 Similarly, large oriented films of MoO3 have also been reported, by epitaxial growth on mica. Despite existing as few-layer crystals, the MoO3 is found to have a monolayer-like band structure by density functional theory (DFT). MoO3 crystals of up to a millimeter can be grown by remarkably simple means: rather than using a conventional CVD chamber, the film is prepared in open air by using a hot plate to sublime molybdenum foil onto a mica target. The oriented crystals can be transferred to other substrates and are an effective photodetector material.25
Recent work in the synthesis of graphene nanoribbons highlights the potential of surface-assisted molecular assembly strategies. Atomically precise chiral graphene ribbons could be formed by polymerization of an anthracene-based molecule adsorbed to a copper surface. The strategic placement of bromine atoms on the molecule blocked off undesired connection geometries, ensuring the desired edge structure was maintained, and when kinks in the wires did occur, πconjugation was uninterrupted through the kink. By these means, graphene nanoribbons could be grown between prepatterned electrodes.26 Another unusual graphene geometry was seen during the van der Waals epitaxy synthesis of graphene on a boron nitride monolayer. The nonepitaxial interaction between h-BN and its nickel substrate can be so weak that the graphene layer can grow on the underside of the h-BN, sandwiched between the h-BN and the substrate. This may prove an effective way to create precise stacks of differing 2D materials. Similarly a second h-BN layer can be grown on the bottom of the first.27 Recent characterization studies of substrate-bound 2D materials have revealed properties that may be useful for patterning. Both ReS2 and sulfur-deficient MoS2 were observed to offer tunable atomic-scale electron transport paths. In the case of ReS2, the diamond-shaped chain-like structural motif known to be present on the sheet surface was found to be directly correlated to electronic mobility, with transport favored along the length of the chains. Furthermore, the strain induced by electron beam irradiation could be used to alter the directions of the diamond-shaped chains, thereby patterning conductive pathways into the sheet.28 Meanwhile, periodic buckled stripes were seen in MoS2 on Au(111), owing to the slight lattice mismatch. The strain in the sheet induces a shift in the band energies, effectively creating a periodic doping of the MoS2 sheet.29 Separately, aberration-corrected transmission electron microscopy has also shown that sulfur vacancies in monolayer MoS2 tend to arrange themselves into line defects as a way of mitigating accumulated strain energy. By DFT these line defects are predicted to be metallic, and this finding suggests that focused irradiation of the nanosheet could be used to pattern atomic wires into the otherwise semiconducting sheet.30 Another use for irradiation may be to weld nanoscopic components together. The use of electron beam irradiation to create defects at a graphene−metal junction was shown to improve electrical contact at the junction, evidently by helping the system overcome the barrier to helpful atomic rearrangements.31 Meanwhile, exfoliation continues to attract intense research interest as a scalable synthesis strategy for 2D materials. Phosphorene, first isolated in 2014 by mechanical exfoliation, has recently been synthesized at a much larger scale by liquid exfoliation. A combination of shear mixing and sonication in a water- and oxygen-free environment led to the successful exfoliation in N-methyl-2-pyrrolidone at up to a 6 g scale. Centrifugal size separation enabled the isolation of different products ranging from nanometers-wide monolayers to micrometer-wide multilayers, and band absorption experiments reveal that the band gap of this semiconducting 2D material is markedly dependent on the number of layers in the particle, ranging continuously from the monolayer value of 1.8 eV to the bulk value of 0.3 eV.32 Tin sulfur selenide and gallium sulfide were recently added to the list of materials that can be exfoliated to form liquid dispersions of atom-thin sheets. SnS2−xSex sheets can be synthesized over the full range of 409
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polymers may yet make this an attractive strategy for future work.41 The articles featured in this virtual issue represent large subfields of 2D material science that are uniquely possible in 2D systems and provide broad opportunities for device applications. On the other hand, since a diverse array of wellstudied 2D materials are already available, we are now much better prepared to pursue a new research direction using bottom-up assembly to create bulk-sized, engineering materials based on 2D building blocks. Compared to nanostructures with other shapes, 2D sheets have a unique advantage to address the stability and scalability challenges for creating such new bulk nanostructured materials with emerging material properties or scalable materials performances.42 We look forward to further developments in 2D material research, as new chemistry, physics, and device integration methods continue to be discovered by ACS authors.
possible compositions and exfoliated by sonication in ethanol. The resulting semiconducting sheets are a single-unit-cell thick and micrometers in area; when assembled into chemiresistor gas sensors, the exfoliated sheets responded rapidly to analytes, owing to their large surface area.33 GaS exfoliation did not proceed so easily, and the optimized GaS sheets were multilayered with a sheet size of less than one micrometer. Analysis of the exfoliated sheets’ stability in various solvents enabled an estimate of the sheets’ solubility parameter and pointed to ideal solvents for exfoliation. GaS sheets were applied as hydrogen evolution catalysts, where once again the best performance was shown by the smallest, thinnest sheets with the most active sites.34 As in years before, graphene has been the subject of numerous exfoliation studies. By use of a new combination of chemical exfoliants, a dispersion of graphene platelets was obtained in quantitative yield. The micrometers-wide platelets were 20−100 atomic layers thick, but the interlayer registry between them was broken.35 Meanwhile, a stable aqueous dispersion of graphene sheets could be generated in good yield using graphene quantum dots (GQDs) as a dispersing agent. Graphene quantum dots are surfactants in a sense, since their oxidized regions stabilize them in water, while their graphitic regions interact favorably with graphene. Addition of a small amount of GQDs enabled an outsize amount of graphene to be exfoliated into 2−3 layer sheets at high yield. After the graphene sheets were processed into films, the stabilizing particles could be removed by washing.36 Also recently demonstrated was an alternative synthesis strategy for GQDs. A monolayer GQD sample was prepared in high yield by cage-opening of fullerene molecules. As might be expected, the GQDs obtained this way are especially monodisperse and strongly fluorescent.37 A unique subset of 2D materials named “MXenes” can be made by selectively etching layered carbides or carbonitrides, followed by exfoliation. Recently two new groups of 2D double transition metal carbides have been experimentally verified: M′2M″C2 and M′2M″2C3, where M′ and M″ represent two different transition metals (e.g., Mo or Cr and Ti). In such double transition metal MXenes, the inner M″ atoms stabilize the backbone lattice and the outer M′ atoms control the surface chemical properties. This discovery suggests that the scope of MXene materials can be greatly broadened.38 Graphene quantum dots were observed to buckle into chiral configurations when they were decorated with chiral moieties around their perimeter. However, GQDs of different chirality displayed different biotoxicity, and molecular dynamics simulations suggest that this differing biotoxicity may be caused by a difference in the GQDs’ ability to enter the lipid bilayer membrane.39 Beyond buckling, 2D nanosheets have continued to find use in larger 3D architectures. Graphene oxide (GO) sheets, for instance, were assembled into highly porous spherical particles by a process akin to deep-frying. The fluffy, low-density particle preserved the high surface area of the 2D nanosheets and buffered against the mechanical expansion of encapsulated silicon particles, enabling the material to function effectively in a battery electrode application.40 Meanwhile, a family of 2D material composites was synthesized by using GO as the oxidant in the synthesis of conducting polymers. Graphene oxide was reduced in the process, yielding rGO flakes coated with globules of conducting polymer. Although the composites were not shown to be conductive or responsive, the ease of synthesis and versatility toward several conducting
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AUTHOR INFORMATION
ORCID
Jiaxing Huang: 0000-0001-9176-8901 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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ACKNOWLEDGMENTS J.H. thanks the support of the Office of Naval Research (ONRN000141310556). V.W.B. thanks the support of the Wisconsin Alumni Research Foundation.
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REFERENCES
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Editorial
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DOI: 10.1021/acsphotonics.7b00194 ACS Photonics 2017, 4, 407−411
