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

Rapid Electrochemical Flow Analysis of Urinary Creatinine on Paper: Unleashing the Potential of Two-Electrode Detection

The development of low-cost, disposable electrochemical sensors is an essential step in moving traditionally inaccessible quantitative diagnostic assays toward the point of need. However, a major remaining limitation of current technologies is the reliance on standardized reference electrode materials. Integrating these reference electrodes considerably restricts the choice of the electrode substrate and drastically increases the fabrication costs. Herein, we demonstrate that adoption of two-electrode detection systems can circumvent these limitations and allow for the development of low-cost, paper-based devices. We showcase the power of this approach by developing a continuous flow assay for urinary creatinine enabled by an embedded graphenic two-electrode detector. The detection system not only simplifies sensor fabrication and readout hardware but also provides a robust sensing performance with high detection efficiencies. In addition to enabling high-throughput analysis of clinical urine samples, our two-electrode sensors provide unprecedented insights into the fundamental mechanism of the ferricyanide-mediated creatinine reaction. Finally, we developed a simplified circuitry to drive the detector. This forms the basis of a smart reader that guides the user through the measurement process. This study showcases the potential of affordable capillary-driven cartridges for clinical analysis within primary care settings

Rapid Electrochemical Flow Analysis of Urinary Creatinine on Paper: Unleashing the Potential of Two-Electrode Detection

The development of low-cost, disposable electrochemical sensors is an essential step in moving traditionally inaccessible quantitative diagnostic assays toward the point of need. However, a major remaining limitation of current technologies is the reliance on standardized reference electrode materials. Integrating these reference electrodes considerably restricts the choice of the electrode substrate and drastically increases the fabrication costs. Herein, we demonstrate that adoption of two-electrode detection systems can circumvent these limitations and allow for the development of low-cost, paper-based devices. We showcase the power of this approach by developing a continuous flow assay for urinary creatinine enabled by an embedded graphenic two-electrode detector. The detection system not only simplifies sensor fabrication and readout hardware but also provides a robust sensing performance with high detection efficiencies. In addition to enabling high-throughput analysis of clinical urine samples, our two-electrode sensors provide unprecedented insights into the fundamental mechanism of the ferricyanide-mediated creatinine reaction. Finally, we developed a simplified circuitry to drive the detector. This forms the basis of a smart reader that guides the user through the measurement process. This study showcases the potential of affordable capillary-driven cartridges for clinical analysis within primary care settings

Doping-Driven Wettability of Two-Dimensional Materials: A Multiscale Theory

Engineering molecular interactions at two-dimensional (2D) materials interfaces enables new technological opportunities in functional surfaces and molecular epitaxy. Understanding the wettability of 2D materials represents the crucial first step toward quantifying the interplay between the interfacial forces and electric potential of 2D materials interfaces. Here we develop the first theoretical framework to model the wettability of the doped 2D materials by properly bridging the multiscale physical phenomena at the 2D interfaces, including (i) the change of 2D materials surface energy (atomistic scale, several angstroms), (ii) the molecular reorientation of liquid molecules adjacent to the interface (molecular scale, 10⁰–10¹ nm), and (iii) the electrical double layer (EDL) formed in the liquid phase (mesoscopic scales, 10⁰–10⁴ nm). The latter two effects are found to be the major mechanisms responsible for the contact angle change upon doping, while the surface energy change of a pure 2D material has no net effect on the wetting property. When the doping level is electrostatically tuned, we demonstrate that 2D materials with high quantum capacitances (e.g., transition metal dichalcogenides, TMDCs) possess a wider range of tunability in the interfacial tension, under the same applied gate voltage. Furthermore, practical considerations such as defects and airborne contamination are also quantitatively discussed. Our analysis implies that the doping level can be another variable to modulate the wettability at 2D materials interfaces, as well as the molecular packing behavior on a 2D material-coated surface, essentially facilitating the interfacial engineering of 2D materials

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

Nanomolecular OLED Pixelization Enabling Electroluminescent Metasurfaces

Miniaturization of light-emitting diodes (LEDs) can enable high-resolution augmented and virtual reality displays and on-chip light sources for ultra-broadband chiplet communication. However, unlike silicon scaling in electronic integrated circuits, patterning of inorganic III-V semiconductors in LEDs considerably compromises device efficiencies at submicrometer scales. Here, we present the scalable fabrication of nanoscale organic LEDs (nano-OLEDs), with the highest array density (>84,000 pixels per inch) and the smallest pixel size (~100 nm) ever reported to date. Direct nanomolecular patterning of organic semiconductors is realized by self-aligned evaporation through nanoapertures fabricated on a free-standing silicon nitride film adhering to the substrate. The average external quantum efficiencies (EQEs) extracted from a nano-OLED device of more than 4 megapixels reach up to 10%. At the subwavelength scale, individual pixels act as electroluminescent meta-atoms forming metasurfaces that directly convert electricity into modulated light. The diffractive coupling between nano-pixels enables control over the far-field emission properties, including directionality and polarization. The results presented here lay the foundation for bright surface light sources of dimension smaller than the Abbe diffraction limit, offering new technological platforms for super-resolution imaging, spectroscopy, sensing, and hybrid integrated photonics

Design and Synthesis of Heteroleptic Iridium(III) Phosphors for Efficient Organic Light-Emitting Devices

The phosphorescent emitters are essential to realize energy-efficient display and lighting panels. The solution processability is of particular interest for large-scale and low-cost production. Here, we present a series of the heteroleptic iridium (Ir) complexes, Ir­(ppy)₂L1, Ir­(ppy)₂L2, and Ir­(ppy)₂L3, using the new ancillary ligands, including 1-(2-chlorophenyl)-5-hydroxy-3-methyl-1<i>H</i>-pyrazole-4-carbaldehyde (L1), 5-hydroxy-3-methyl-1-(p-tolyl)-1<i>H</i>-pyrazole-4-carbaldehyde (L2), and 5-hydroxy-3-methyl-1-phenyl-1<i>H</i>-pyrazole-4-carbaldehyde (L3). Their photophysical and electrochemical properties were systematically characterized, followed by comparing with those predicted by density functional theory simulations using hybrid functionals. Among the three phosphors synthesized, Ir­(ppy)₂L1 exhibits the highest photoluminescence quantum yield (Φ_PL = 89%), with an exciton lifetime of 0.34 μs. By using 4,4′-bis­(carbazole-9-yl)­biphenyl as the host material, we demonstrate high current efficiencies of 64 and 40 cd A^–1 at 100 cd m^–2 in its vacuum-evaporated and solution-processed organic light-emitting devices, respectively, revealing the promise for large-area light sources

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

Partially-Screened Field Effect and Selective Carrier Injection at Organic Semiconductor/Graphene Heterointerface

Due to the lack of a bandgap, applications of graphene require special device structures and engineering strategies to enable semiconducting characteristics at room temperature. To this end, graphene-based vertical field-effect transistors (VFETs) are emerging as one of the most promising candidates. Previous work attributed the current modulation primarily to gate-modulated graphene–semiconductor Schottky barrier. Here, we report the first experimental evidence that the partially screened field effect and selective carrier injection through graphene dominate the electronic transport at the organic semiconductor/graphene heterointerface. The new mechanistic insight allows us to rationally design graphene VFETs. Flexible organic/graphene VFETs with bending radius <1 mm and the output current per unit layout area equivalent to that of the best oxide planar FETs can be achieved. We suggest driving organic light emitting diodes with such VFETs as a promising application

Layer Number Dependence of MoS₂ Photoconductivity Using Photocurrent Spectral Atomic Force Microscopic Imaging

Atomically thin MoS₂ is of great interest for electronic and optoelectronic applications because of its unique two-dimensional (2D) quantum confinement; however, the scaling of optoelectronic properties of MoS₂ and its junctions with metals as a function of layer number as well the spatial variation of these properties remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy (PCS-AFM) to image the current (in the dark) and photocurrent (under illumination) generated between a biased PtIr tip and MoS₂ nanosheets with thickness ranging between <i>n</i> = 1 to 20 layers. Dark current measurements in both forward and reverse bias reveal characteristic diode behavior well-described by Fowler–Nordheim tunneling with a monolayer barrier energy of 0.61 eV and an effective barrier scaling linearly with layer number. Under illumination at 600 nm, the photocurrent response shows a marked decrease for layers up to <i>n</i> = 4 but increasing thereafter, which we describe using a model that accounts for the linear barrier increase at low <i>n</i>, but increased light absorption at larger <i>n</i> creating a minimum at <i>n</i> = 4. Comparative 2D Fourier analysis of physical height and photocurrent images shows high spatial frequency spatial variations in substrate/MoS₂ contact that exceed the frequencies imposed by the underlying substrates. These results should aid in the design and understanding of optoelectronic devices based on quantum confined atomically thin MoS₂