Evolution of Physical and Electronic Structures of Bilayer Graphene upon Chemical Functionalization

The chemical behavior of bilayer graphene under strong covalent and noncovalent functionalization is relatively unknown compared to monolayer graphene, which has been far more widely studied. Bilayer graphene is significantly less chemically reactive than monolayer graphene, making it more challenging to study its chemistry in detail. However, bilayer graphene is increasingly attractive for electronic applications rather than monolayer graphene because of its electric-field-controllable band gap, and there is a need for a greater understanding of its chemical functionalization. In this paper, we study the covalent and noncovalent functionalization of bilayer graphene using an electrochemical process with aryl diazonium salts in the high conversion regime (D/G ratio >1), and we use Raman spectroscopic mapping and conductive atomic force microscopy (cAFM) to study the resulting changes in the physical and electronic structures. Covalent functionalization at high chemical conversion induces distinct changes in the Raman spectrum of bilayer graphene including the broadening and shift in position of the split G peak. Also, the D peak becomes active with four components. We report for the first time that the broadening of the 2D₂₂ and 2D₂₁ components is a distinct indicator of covalent functionalization, whereas the decrease in intensity of the 2D₁₁ and 2D₁₂ peaks corresponds to doping. Conductive AFM imaging shows physisorbed species from noncovalent functionalization can be removed by mechanical and electrical influence from the AFM tip, and that changes in conductivity due to functionalization are inhomogeneous. These results allow one to distinguish covalent from noncovalent chemistry as a guide for further studies of the chemistry of bilayer graphene

Tuning On–Off Current Ratio and Field-Effect Mobility in a MoS₂–Graphene Heterostructure <i>via</i> Schottky Barrier Modulation

Field-effect transistor (FET) devices composed of a MoS₂–graphene heterostructure can combine the advantages of high carrier mobility in graphene with the permanent band gap of MoS₂ for digital applications. Herein, we investigate the electron transfer, photoluminescence, and gate-controlled carrier transport in such a heterostructure. We show that the junction is a Schottky barrier, whose height can be artificially controlled by gating or doping graphene. When the applied gate voltage (or the doping level) is zero, the photoexcited electron–hole pairs in monolayer MoS₂ can be split by the heterojunction, significantly reducing the photoluminescence. By applying negative gate voltage (or <i>p</i>-doping) in graphene, the interlayer impedance formed between MoS₂ and graphene exhibits an 100-fold increase. For the first time, we show that the gate-controlled interlayer Schottky impedance can be utilized to modulate carrier transport in graphene, significantly depleting the hole transport, but preserving the electron transport. Accordingly, we demonstrate a new type of FET device, which enables a controllable transition from NMOS digital to bipolar characteristics. In the NMOS digital regime, we report a very high room temperature on/off current ratio (<i>I</i>_ON/<i>I</i>_OFF ∼ 36) in comparison to graphene-based FET devices without sacrificing the field-effect electron mobilities in graphene. By engineering the source/drain contact area, we further estimate that a higher value of <i>I</i>_ON/<i>I</i>_OFF up to 100 can be obtained in the device architecture considered. The device architecture presented here may enable semiconducting behavior in graphene for digital and analogue electronics

Stochastic Pore Blocking and Gating in PDMS–Glass Nanopores from Vapor–Liquid Phase Transitions

Polydimethylsiloxane (PDMS) is commonly used in research for microfluidic devices and for making elastomeric stamps for soft lithography. Its biocompatibility and nontoxicitiy also allow it to be used in personal care, food, and medical products. Herein we report a phenomenon observed when patch clamp, a technique normally used to study biological ion channels, is performed on both grooved and planar PDMS surfaces, resulting in stochastic current fluctuations that are due to a nanopore being formed at the interface of the PDMS and glass surfaces and being randomly blocked. Deformable pores between 1.9 ± 0.7 and 7.4 ± 2.1 nm in diameter, depending on the calculation method, form upon patching to the surface. Coulter blocking and nanoprecipitation are ruled out, and we instead propose a mechanism of stochastic current fluctuations arising from transitions between vapor and liquid phases, consistent with similar observations and theory from statistical mechanics literature. Interestingly, we find that [Ru­(bpy)₃]²⁺, a common probe molecule employed in nanopore research, physisorbs inside these hydrophobic nanopores blocking all ionic current flow at concentrations higher than 1 × 10^–4 M, despite the considerably larger pore diameter relative to the molecule. Patch clamp methods are promising for the study of stochastic current fluctuations and other transport phenomenon in synthetic nanopore systems

Low Cytotoxicity and Genotoxicity of Two-Dimensional MoS₂ and WS₂

Atomically thin transition-metal dichalcogenides (TMDs) have attracted considerable interest because of their unique combination of properties, including photoluminescence, high lubricity, flexibility, and catalytic activity. These unique properties suggest future uses for TMDs in medical applications such as orthodontics, endoscopy, and optogenetics. However, few studies thus far have investigated the biocompatibility of mechanically exfoliated and chemical vapor deposition (CVD)-grown pristine two-dimensional TMDs. Here, we evaluate pristine molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) in a series of biocompatibility tests, including live–dead cell assays, reactive oxygen species (ROS) generation assays, and direct assessment of cellular morphology of TMD-exposed human epithelial kidney cells (HEK293f). Genotoxicity and genetic mutagenesis were also evaluated for these materials via the Ames Fluctuation test with the bacterial strain <i>S. typhimurium</i> TA100. Scanning electron microscopy of cultured HEK293f cells in direct contact with MoS₂ and WS₂ showed no impact on cell morphology. HEK293f cell viability, evaluated by both live–dead fluorescence labeling to detect acute toxicity and ROS to monitor for apoptosis, was unaffected by these materials. Exposure of bacterial cells to these TMDs failed to generate genetic mutation. Together, these findings demonstrate that neither mechanically exfoliated nor CVD-grown TMDs are deleterious to cellular viability or induce genetic defects. Thus, these TMDs appear biocompatible for future application in medical devices

Formation of High-Aspect-Ratio Helical Nanorods via Chiral Self-Assembly of Fullerodendrimers

Two novel, asymmetric methanofullerenes are presented, which self-assemble in cyclohexane upon thermal cycling to 80 °C. We show that, through the introduction of a dipeptide sequence to one terminus of the dendritic methanofullerene, it is possible to transform the assembly behavior of these molecules from poorly formed aggregates to high-aspect-ratio nanorods. These nanorods have diameters of 3.76 ± 0.52 nm and appear to be composed of interwoven helices of dendritic fullerenes. As evidenced by circular dichroism, the helicity is characterized by a preferential handedness of assembly, which is imparted by the dipeptide moiety

Disorder Imposed Limits of Mono- and Bilayer Graphene Electronic Modification Using Covalent Chemistry

A central question in graphene chemistry is to what extent chemical modification can control an electronically accessible band gap in monolayer and bilayer graphene (MLG and BLG). Density functional theory predicts gaps in covalently functionalized graphene as high as 2 eV, while this approach neglects the fact that lattice symmetry breaking occurs over only a prescribed radius of nanometer dimension, which we label the S-region. Therefore, high chemical conversion is central to observing this band gap in transport. We use an electrochemical approach involving phenyl-diazonium salts to systematically probe electronic modification in MLG and BLG with increasing functionalization for the first time, obtaining the highest conversion values to date. We find that both MLG and BLG retain their relatively high conductivity after functionalization even at high conversion, as mobility losses are offset by increases in carrier concentration. For MLG, we find that band gap opening as measured during transport is linearly increased with respect to the <i>I</i>_<i>D</i>/<i>I</i>_<i>G</i> ratio but remains below 0.1 meV in magnitude for SiO₂ supported graphene. The largest transport band gap obtained in a suspended, highly functionalized (<i>I</i>_<i>D</i>/<i>I</i>_<i>G</i> = 4.5) graphene is about 1 meV, lower than our theoretical predictions considering the quantum interference effect between two neighboring S-regions and attributed to its population with midgap states. On the other hand, heavily functionalized BLG (<i>I</i>_<i>D</i>/<i>I</i>_<i>G</i> = 1.8) still retains its signature dual-gated band gap opening due to electric-field symmetry breaking. We find a notable asymmetric deflection of the charge neutrality point (CNP) under positive bias which increases the apparent on/off current ratio by 50%, suggesting that synergy between symmetry breaking, disorder, and quantum interference may allow the observation of new transistor phenomena. These important observations set definitive limits on the extent to which chemical modification can control graphene electronically

Observation of Switchable Photoresponse of a Monolayer WSe₂–MoS₂ Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging

In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p–n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe₂–MoS₂ lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe₂ and MoS₂, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS₂ (or WSe₂) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures

Direct Covalent Chemical Functionalization of Unmodified Two-Dimensional Molybdenum Disulfide

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS₂) are generating significant excitement due to their unique electronic, chemical, and optical properties. Covalent chemical functionalization represents a critical tool for tuning the properties of TMDCs for use in many applications. However, the chemical inertness of semiconducting TMDCs has thus far hindered the robust chemical functionalization of these materials. Previous reports have required harsh chemical treatments or converting TMDCs into metallic phases prior to covalent attachment. Here, we demonstrate the direct covalent functionalization of the basal planes of unmodified semiconducting MoS₂ using aryl diazonium salts without any pretreatments. Our approach preserves the semiconducting properties of MoS₂, results in covalent C–S bonds, is applicable to MoS₂ derived from a range of different synthesis methods, and enables a range of different functional groups to be tethered directly to the MoS₂ surface. Using density functional theory calculations including van der Waals interactions and atomic-scale scanning probe microscopy studies, we demonstrate a novel reaction mechanism in which cooperative interactions enable the functionalization to propagate along the MoS₂ basal plane. The flexibility of this covalent chemistry employing the diverse aryl diazonium salt family is further exploited to tether active proteins to MoS₂, suggesting future biological applications and demonstrating its use as a versatile and powerful chemical platform for enhancing the utility of semiconducting TMDCs

Direct Covalent Chemical Functionalization of Unmodified Two-Dimensional Molybdenum Disulfide

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS₂) are generating significant excitement due to their unique electronic, chemical, and optical properties. Covalent chemical functionalization represents a critical tool for tuning the properties of TMDCs for use in many applications. However, the chemical inertness of semiconducting TMDCs has thus far hindered the robust chemical functionalization of these materials. Previous reports have required harsh chemical treatments or converting TMDCs into metallic phases prior to covalent attachment. Here, we demonstrate the direct covalent functionalization of the basal planes of unmodified semiconducting MoS₂ using aryl diazonium salts without any pretreatments. Our approach preserves the semiconducting properties of MoS₂, results in covalent C–S bonds, is applicable to MoS₂ derived from a range of different synthesis methods, and enables a range of different functional groups to be tethered directly to the MoS₂ surface. Using density functional theory calculations including van der Waals interactions and atomic-scale scanning probe microscopy studies, we demonstrate a novel reaction mechanism in which cooperative interactions enable the functionalization to propagate along the MoS₂ basal plane. The flexibility of this covalent chemistry employing the diverse aryl diazonium salt family is further exploited to tether active proteins to MoS₂, suggesting future biological applications and demonstrating its use as a versatile and powerful chemical platform for enhancing the utility of semiconducting TMDCs

Direct Covalent Chemical Functionalization of Unmodified Two-Dimensional Molybdenum Disulfide

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS₂) are generating significant excitement due to their unique electronic, chemical, and optical properties. Covalent chemical functionalization represents a critical tool for tuning the properties of TMDCs for use in many applications. However, the chemical inertness of semiconducting TMDCs has thus far hindered the robust chemical functionalization of these materials. Previous reports have required harsh chemical treatments or converting TMDCs into metallic phases prior to covalent attachment. Here, we demonstrate the direct covalent functionalization of the basal planes of unmodified semiconducting MoS₂ using aryl diazonium salts without any pretreatments. Our approach preserves the semiconducting properties of MoS₂, results in covalent C–S bonds, is applicable to MoS₂ derived from a range of different synthesis methods, and enables a range of different functional groups to be tethered directly to the MoS₂ surface. Using density functional theory calculations including van der Waals interactions and atomic-scale scanning probe microscopy studies, we demonstrate a novel reaction mechanism in which cooperative interactions enable the functionalization to propagate along the MoS₂ basal plane. The flexibility of this covalent chemistry employing the diverse aryl diazonium salt family is further exploited to tether active proteins to MoS₂, suggesting future biological applications and demonstrating its use as a versatile and powerful chemical platform for enhancing the utility of semiconducting TMDCs