Disorder-tuned conductivity in amorphous monolayer carbon.

Correlating atomic configurations-specifically, degree of disorder (DOD)-of an amorphous solid with properties is a long-standing riddle in materials science and condensed matter physics, owing to difficulties in determining precise atomic positions in 3D structures1-5. To this end, 2D systems provide insight to the puzzle by allowing straightforward imaging of all atoms6,7. Direct imaging of amorphous monolayer carbon (AMC) grown by laser-assisted depositions has resolved atomic configurations, supporting the modern crystallite view of vitreous solids over random network theory8. Nevertheless, a causal link between atomic-scale structures and macroscopic properties remains elusive. Here we report facile tuning of DOD and electrical conductivity in AMC films by varying growth temperatures. Specifically, the pyrolysis threshold temperature is the key to growing variable-range-hopping conductive AMC with medium-range order (MRO), whereas increasing the temperature by 25 °C results in AMC losing MRO and becoming electrically insulating, with an increase in sheet resistance of 109 times. Beyond visualizing highly distorted nanocrystallites embedded in a continuous random network, atomic-resolution electron microscopy shows the absence/presence of MRO and temperature-dependent densities of nanocrystallites, two order parameters proposed to fully describe DOD. Numerical calculations establish the conductivity diagram as a function of these two parameters, directly linking microstructures to electrical properties. Our work represents an important step towards understanding the structure-property relationship of amorphous materials at the fundamental level and paves the way to electronic devices using 2D amorphous materials.

Engineering Interlayer Electron-Phonon Coupling in WS2/BN Heterostructures.

In van der Waals (vdW) heterostructures, the interlayer electron-phonon coupling (EPC) provides one unique channel to nonlocally engineer these elementary particles. However, limited by the stringent occurrence conditions, the efficient engineering of interlayer EPC remains elusive. Here we report a multitier engineering of interlayer EPC in WS2/boron nitride (BN) heterostructures, including isotope enrichments of BN substrates, temperature, and high-pressure tuning. The hyperfine isotope dependence of Raman intensities was unambiguously revealed. In combination with theoretical calculations, we anticipate that WS2/BN supercells could induce Brillouin-zone-folded phonons that contribute to the interlayer coupling, leading to a complex nature of broad Raman peaks. We further demonstrate the significance of a previously unexplored parameter, the interlayer spacing. By varying the temperature and high pressure, we effectively manipulated the strengths of EPC with on/off capabilities, indicating critical thresholds of the layer-layer spacing for activating and strengthening interlayer EPC. Our findings provide new opportunities to engineer vdW heterostructures with controlled interlayer coupling.

Laser annealing towards high-performance monolayer MoS2 and WSe2 field effect transistors.

The transition metal dichalcogenides (TMDCs) have been intensively investigated as promising nanoelectronic and optoelectronic materials. However, the pervasive adsorbates on the surfaces of monolayer TMDCs, including oxygen and water molecules from the ambient environment, tend to degrade the device performance, thus hindering specific applications. In this work, we report the effect of laser irradiation on the transport and photoresponse of monolayer MoS2 and WSe2 devices, and this laser annealing process is demonstrated as a straightforward approach to remove physically adsorbed contaminants. Compared to vacuum pumping and in situ thermal annealing treatments, the field-effect transistors after laser annealing show a more than one order of magnitude higher on-state current, and no apparent degradation of device performance at low temperatures. The mobility of the monolayer WSe2 devices can be enhanced by three to four times, and for single-layered MoS2 devices with the commonly used SiO2 as the back-gate, the mobility increases by 20 times, reaching [Formula: see text]. The efficient cleaning effect of laser annealing is also supported by the reduction of channel and contact resistance revealed by a transmission line experiment. Further, an enhanced photocurrent, by a factor of ten, has been obtained in the laser annealed device. These findings pave the way for high-performance monolayer TMDC-based electronic and optoelectronic devices with a clean surface and intrinsic properties.

Degree of disorder-regulated ion transport through amorphous monolayer carbon.

Nanopore technology, re-fueled by two-dimensional (2D) materials such as graphene and MoS2, controls mass transport by allowing certain species while denying others at the nanoscale and has a wide application range in DNA sequencing, nano-power generation, and others. With their low transmembrane transport resistance and high permeability stemming from their ultrathin nature, crystalline 2D materials do not possess nanoscale holes naturally, thus requiring additional fabrication to create nanopores. Herein, we demonstrate that nanopores exist in amorphous monolayer carbon (AMC) grown at low temperatures. The size and density of nanopores can be tuned by the growth temperature, which was experimentally verified by atomic images and further corroborated by kinetic Monte Carlo simulation. Furthermore, AMC films with varied degrees of disorder (DOD) exhibit tunable transmembrane ionic conductance over two orders of magnitude when serving as nanopore membranes. This work demonstrates the DOD-tuned property in amorphous monolayer carbon and provides a new candidate for modern membrane science and technology.

Unlocking More Potentials in Two-Dimensional Space: Disorder Engineering in Two-Dimensional Amorphous Carbon.

The theory of the nature of glass has been described as the deepest but unsolved problem in solid state theory. The fundamental understanding of the structural characteristics of glassy materials and disorder-property correspondence remains incomplete due to difficulties in fully characterizing disordered structures in three-dimensional materials. Recently, two-dimensional amorphous materials were treated as an atomic-level playground to uncover previously unknown structure-property relationships in vitreous materials. Here, we summarize recent research on one prototypical material, two-dimensional amorphous carbon, including atomic structural characterizations, controllable synthesis, exotic properties, and application potentials. Fundamental discrepancies only induced by the amorphous nature, when compared with crystalline materials, will be highlighted. Finally, we discuss the restricted definition of two-dimensional amorphous carbon, existing challenges, and future research directions.

Synthesis of centimeter-scale high-quality polycrystalline hexagonal boron nitride films from Fe fluxes.

High-quality hexagonal BN (hBN) crystals, owing to their irreplaceable roles in new functional devices such as universal substrates and excellent layered insulators are exceedingly required in the field of two-dimensional (2D) materials. Although large-scale monolayer hBN crystals have been successfully grown on catalytic metals, the synthesis of large-area continuous hBN films with thickness in microns is challenging, hindering their applications at the mesoscopic level. Herein, we report the single-metal flux growth of centimeter-large, micron-thick, and high-quality continuous hBN films by balancing the grain size and coverage. The as-grown films can be readily exfoliated and transferred onto arbitrary substrates. Isotopically engineered hBN crystals can be obtained as well by the method. The narrow Raman line widths of the intralayer E2g mode peak (2.9 cm-1 for h11BN, 3.3 cm-1 for h10BN, and 7.9 cm-1 for hNaBN) and ultrahigh thermal conductivity (830 W m-1 K-1 for 4L h11BN) demonstrate high crystal quality and low defect density. Our results provide the foundation for the cost-efficient and lab-achievable synthesis of high-quality hBN films aimed at its mesoscopic applications.

Direct imaging of the nitrogen-rich edge in monolayer hexagonal boron nitride and its band structure tuning.

Identification of edge atoms and tracking the edge structure evolution of two-dimensional (2D) crystals at the scale of individual atoms is critical for understanding the edge-dominated properties and behavioral responses to external field stimuli. Here, direct imaging of the edge configuration of monolayer hexagonal boron nitride (h-BN) is demonstrated at the atomic scale, by using aberration-corrected transmission electron microscopy. Tracking of the edge atoms revealed that a nitrogen-terminated zigzag arrangement dominates along the edge, naturally leading to nitrogen rich (N-rich) characteristics in this area, while the stoichiometric interior of the h-BN monolayer is maintained. Both top-down fabrication and bottom-up growth were proposed to obtain novel h-BN flakes with an N-rich ratio larger than 1% when the size is reduced to the threshold of 25 nm. Furthermore, density functional theory calculations revealed that a new bandgap of ∼3 eV is created by the N-rich characteristics, and h-BN transforms into an n-type semiconductor by self-doping. The results call for the development of ultra-small h-BN islands to be used in intriguing 2D electronic devices with a photoresponse function to visible light.