Graphene Wrinkles Enable Spatially Defined Chemistry.

This paper reports a scalable approach to achieve spatially selective graphene functionalization using multiscale wrinkles. Graphene wrinkles were formed by relieving the strain in thermoplastic polystyrene substrates conformally coated with fluoropolymer and graphene skin layers. Chemical reactivity of a fluorination process could be tuned by changing the local curvature of the graphene nanostructures. Patterned areas of graphene nanowrinkles and crumples followed by a single-process plasma reaction resulted in substrates with regions having different fluorination levels. Notably, conductivity of the functionalized graphene nanostructures could be locally tuned as a function of feature size without affecting the mechanical properties.

Charged Layered Boron Nitride-Nanoflake Membranes for Efficient Ion Separation and Water Purification.

2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h-BNF) for highly efficient ion separation is reported. The ion-rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h-BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, ≈1.0 × 10-3 L m m-2 h-1 bar-1 ). Experiments show that the ion-rejection performance of the membrane can be tuned by changing the solution pH. The results also suggest that the rejection is influenced by the ionic size and the electrostatic repulsion between fixed negative charges on the BN surface and the mobile ions, and is consistent with the Donnan equilibrium model. These simple-to-fabricate h-BNF membranes show a unique combination of excellent ion selectivity and high permeability compared to other 2D membranes.

Retained Carrier-Mobility and Enhanced Plasmonic-Photovoltaics of Graphene via ring-centered η6 Functionalization and Nanointerfacing.

Binding graphene with auxiliary nanoparticles for plasmonics, photovoltaics, and/or optoelectronics, while retaining the trigonal-planar bonding of sp2 hybridized carbons to maintain its carrier-mobility, has remained a challenge. The conventional nanoparticle-incorporation route for graphene is to create nucleation/attachment sites via "carbon-centered" covalent functionalization, which changes the local hybridization of carbon atoms from trigonal-planar sp2 to tetrahedral sp3. This disrupts the lattice planarity of graphene, thus dramatically deteriorating its mobility and innate superior properties. Here, we show large-area, vapor-phase, "ring-centered" hexahapto (η6) functionalization of graphene to create nucleation-sites for silver nanoparticles (AgNPs) without disrupting its sp2 character. This is achieved by the grafting of chromium tricarbonyl [Cr(CO)3] with all six carbon atoms (sigma-bonding) in the benzenoid ring on graphene to form an (η6-graphene)Cr(CO)3 complex. This nondestructive functionalization preserves the lattice continuum with a retention in charge carrier mobility (9% increase at 10 K); with AgNPs attached on graphene/n-Si solar cells, we report an ∼11-fold plasmonic-enhancement in the power conversion efficiency (1.24%).

Large-Area, Transfer-Free, Oxide-Assisted Synthesis of Hexagonal Boron Nitride Films and Their Heterostructures with MoS2 and WS2.

Ultrasmooth hexagonal boron nitride (h-BN) can dramatically enhance the carrier/phonon transport in interfaced transition metal dichalcogenides (TMDs), and amplify the effect of quantum capacitance in field-effect gating. All of the current processes to realize h-BN-based heterostructures involve transfer or exfoliation. Rational chemistries and process techniques are still required to produce large-area, transfer-free, directly grown TMDs/BN heterostructures. Here, we demonstrate a novel boron-oxygen chemistry route for oxide-assisted nucleation and growth of large-area, uniform, and ultrathin h-BN directly on oxidized substrates (B/N atomic ratio = 1:1.16 ± 0.03 and optical band gap = 5.51 eV). These intimately interfaced, van der Waals heterostructures of MoS2/h-BN and WS2/h-BN benefit from 6.27-fold reduced roughness of h-BN in comparison to SiO2. This leads to reduction in scattering from roughness and charged impurities, and enhanced carrier mobility verified by an increase in electrical conductivity (5 times for MoS2/h-BN and 2 times for WS2/h-BN). Further, the heterostructures are devoid of wrinkles and adsorbates, which is critical for 2D nanoelectronics. The versatile process can potentially be extrapolated to realize a variety of heterostructures with complex sandwiched 2D electronic circuitry.

Controlled, defect-guided, metal-nanoparticle incorporation onto MoS2 via chemical and microwave routes: electrical, thermal, and structural properties.

Ultrathin (0.3-3 nm) metal dichalcogenides exhibit confinement of carriers, evolution of band-structure and photophysical properties with thickness, high on/off rectification (in MoS2, WS2, and so forth) and high thermal absorption. Here, we leverage the stable sulfur/nobel-metal binding to incorporate highly capacitive gold nanoparticles (Au NPs) onto MoS2 to raise the effective gate-voltage by an order of magnitude. Functionalization is achieved via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves) with selective deposition on crystallographic edges (with 60° displacement). The electrical, thermal, and Raman studies show a highly capacitive interaction between Au NP and MoS2 flakes (CAu-MoS2 = 2.17 µF/cm(2)), a low Schottky barrier (14.52 meV), a reduced carrier-transport thermal-barrier (253 to 44.18 meV after Au NP functionalization), and increased thermal conductivity (from 15 to 23 W/mK post NP deposition). The process could be employed to attach electrodes to heterostructures of graphene and MoS2, where a gold film could be grown to act as an electron-tunneling gate-electrode connected to MoS2.

Electron-tunneling modulation in percolating network of graphene quantum dots: fabrication, phenomenological understanding, and humidity/pressure sensing applications.

The two-dimensional (2D) electron cloud, flexible carbon-carbon bonds, chemical modifiability, and size-dependent quantum-confinement and capacitance makes graphene nanostructures (GN) a widely tunable material for electronics. Here we report the oxidation-led edge-roughening and cleavage of long graphene nanoribbons (GNRs) (150 nm wide) synthesized via nanotomy (nanoscale cutting) of graphite (with 2 nm edged diamond knife) to produce graphene quantum dots (GQD). These GQDs (~100-200 nm) selectively interfaced with polyelectrolyte microfiber (diameter = 2-20 µm) form an electrically percolating-network exhibiting a characteristic Coulomb blockade signature with a dry tunneling distance of 0.58 nm and conduction activation energy of 3 meV. We implement this construct to demonstrate the functioning of humidity and pressure sensors and outline their governing model. Here, a 0.36 nm decrease in the average tunneling-barrier-width between GQDs (tunneling barrier = 5.11 eV) increases the conductivity of the device by 43-fold. These devices leverage the modulation in electron tunneling distances caused by pressure and humidity induced water transport across the hygroscopic polymer microfiber (Henry's constant = 0.215 Torr(-1)). This is the foremost example of GQD-based electronic sensors. We envision that this polymer-interfaced GQD percolating network will evolve a new class of sensors leveraging the low mass, low capacitance, high conductivity, and high sensitivity of GQD and the interfacial or dielectric properties of the polymer fiber.

Graphene Fermi Level-Guided Attachment of Single Exoelectrogens and Induced Interfacial Doping.

Graphene's exceptional electronic and mechanical properties make it a promising material for bioelectronic applications; however, understanding its interaction with electrogenic bacteria is crucial to harness its full potential. This study investigates the interface between electrogenic bacteria and graphene with Raman spectroscopy by analyzing the distinctive spectral fingerprints to understand electron energy and distribution via this non-destructive and label-free method. We find that the presence of bacteria induces a distinct red-shift in the G peak positions of graphene, indicating electron doping. Correspondingly, the bacteria demonstrate a predilection for attachment on hole-rich sites on the graphene sheet, evidenced by the comparative analysis of pre- and post-spatial Raman mapping, revealing their consistent presence within the hole-doped 2D peak position range of 2673.89-2675.43 cm-1. This affinity of bacteria is due to the overall higher Fermi level (∼4.9 ± 0.2 eV) of these regions, which favors electron transfer. These findings demonstrate the potential of leveraging the graphene's electronic properties in engineering graphene-based biosensors. Tuning graphene's charge carrier concentration would enable the promotion or prevention of bacterial attachment, facilitating capture of specific bacteria or development of antimicrobial surfaces. This approach enables clean, efficient, and accurate study of graphene-based bacterial systems, driving significant advancements and enhancing their performance.

How do the electrical properties of graphene change with its functionalization?

Functionalization of graphene is essential to interface it with other moieties to expand the scope of its electrical/electronic applications. However, chemical functionalization and/or molecular interactions on graphene sensitively modulate its electrical properties. To evaluate and take advantage of the properties of functionalized graphene, it is important to understand how its electrical attributes (such as carrier scattering, carrier concentration, charge polarity, quantum-capacitance enhanced doping, energy levels, transport mechanisms, and orbital hybridization of energy-bands) are influenced by a change in carbon's structural conformation, hybridization state, chemical potential, local energy levels, and dopant/interface coupling induced via functionalization or molecular interactions. Here, a detailed and integrated model describes factors influencing these electrical characteristics of functionalized graphene (covalent bonds, adsorption, π-π bonds, and lattice incorporation). The electrical properties are governed via three mechanisms: (a) conversion of carbon's hybridized state, (b) dipole interactions enhanced via quantum capacitance, and (c) orbital hybridization with an interfacing molecule. A few graphenic materials are also identified where further studies are essential to understand the effect of their functionalization.

Covalent functionalization of dipole-modulating molecules on trilayer graphene: an avenue for graphene-interfaced molecular machines.

The molecular dipole moment plays a significant role in governing important phenomena like molecular interactions, molecular configuration, and charge transfer, which are important in several electronic, electrochemical, and optoelectronic systems. Here, the effect of the change in the dipole moment of a tethered molecule on the carrier properties of (functionalized) trilayer graphene--a stack of three layers of sp(2)-hybridized carbon atoms--is demonstrated. It is shown that, due to the high carrier confinement and large quantum capacitance, the trans-to-cis isomerisation of 'covalently attached' azobenzene molecules, with a change in dipole moment of 3D, leads to the generation of a high effective gating voltage. Consequently, 6 units of holes are produced per azobenzene molecule (hole density increases by 440 000 holes µm(-2)). Based on Raman and X-ray photoelectron spectroscopy data, a model is outlined for outer-layer, azobenzene-functionalized trilayer graphene with current modulation in the inner sp(2) matrix. Here, 0.097 V are applied by the isomerisation of the functionalized azobenzene. Further, the large measured quantum capacitance of 72.5 µF cm(-2) justifies the large Dirac point in the heavily doped system. The mechanism defining the effect of dipole modulation of covalently tethered molecules on graphene will enable future sensors and molecular-machine interfaces with graphene.

Thermodynamics and Kinetics in Anisotropic Growth of One-Dimensional Midentropy Nanoribbons.

One-dimensional (1D) materials demonstrate anisotropic in-plane physical properties that enable a wide range of functionalities in electronics, photonics, valleytronics, optoelectronics, and catalysis. Here, we undertake an in-depth study of the growth mechanism for equimolar midentropy alloy of (NbTaTi)0.33S3 nanoribbons as a model system for 1D transition metal trichalcogenide structures. To understand the thermodynamic and kinetic effects in the growth process, the energetically preferred phases at different synthesis temperatures and times are investigated, and the phase evolution is inspected at a sequence of growth steps. It is uncovered that the dynamics of the growth process occurs at four different stages via preferential incorporation of chemical species at high-surface-energy facets. Also, a sequence of temperature and time dependent nonuniform to uniform phase evolutions has emerged in the composition and structure of (NbTaTi)0.33S3 which is described based on an anisotropic vapor-solid (V-S) mechanism. Furthermore, direct evidence for the 3D structure of the charge density wave (CDW) phase (width less than 100 nm) is provided by three-dimensional electron diffraction (3DED) in individual nanoribbons at cryogenic temperature, and detailed comparisons are made between the phases obtained before and after CDW transformation. This study provides important fundamental information for the design and synthesis of future 1D alloy structures.

Impermeable graphenic encasement of bacteria.

Transmission electron microscopy (TEM) of hygroscopic, permeable, and electron-absorbing biological cells has been an important challenge due to the volumetric shrinkage, electrostatic charging, and structural degradation of cells under high vacuum and fixed electron beam.(1-3) Here we show that bacterial cells can be encased within a graphenic chamber to preserve their dimensional and topological characteristics under high vacuum (10(-5) Torr) and beam current (150 A/cm(2)). The strongly repelling π clouds in the interstitial sites of graphene's lattice(4) reduces the graphene-encased-cell's permeability(5) from 7.6-20 nm/s to 0 nm/s. The C-C bond flexibility(5,6) enables conformal encasement of cells. Additionally, graphene's high Young's modulus(6,7) retains cell's structural integrity under TEM conditions, while its high electrical(8) and thermal conductivity(9) significantly abates electrostatic charging. We envision that the graphenic encasement approach will facilitate real-time TEM imaging of fluidic samples and potentially biochemical activity.

Modulation of electron tunneling in a nanoparticle array by sound waves: an avenue to high-speed, high-sensitivity sensors.

Using an electrostatic self-assembly process, metal nanoparticles are deposited on polyelectrolyte fibers such that the interparticle distance between the nanoparticles is comparable to the polyelectrolyte's molecular width. By modulating the dielectric properties of the interparticle polymer layer, a highly sensitive, reversible humidity sensor with an ultrafast response time of ≈3 ms is demonstrated. The higher sensitivity at low humidity shows a conductivity increase by over two orders of magnitude in response to a change in relative humidity from 21 to 1%.

Phononics of Graphene Interfaced with Flowing Ionic Fluid: An Avenue for High Spatial Resolution Flow Sensor Applications.

While ionic flow over graphenic structures creates electromotive potential, there is a need to understand the local carrier density induced in graphene without any electrode-induced Fermi-level pinning. Here, we show the electrolyte-flow induced localized doping in graphene via inspecting its Raman phononic energy. Graphene's Fermi energy level has a logarithmic dependence to the flow velocity over 2 orders of magnitude of velocity (∼100 µm s-1 to 10 mm s-1). A theoretical model of the electric double layer (EDL) during ionic transport is used to correlate the Fermi level of graphene with the flow rate and the electronic structure (HOMO-LUMO levels) of the ionic species. This correlation can allow us to use graphene as a reliable, non-invasive, optical flow-sensor, where the flow rates can be measured at high spatial resolution for several lab-on-a-chip applications.

COVID-19 Spike Protein Induced Phononic Modification in Antibody-Coupled Graphene for Viral Detection Application.

With an incubation time of about 5 days, early diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is critical to control the spread of the coronavirus disease 2019 (COVID-19) that killed more than 3 million people in its first 1.5 years. Here, we report on the modification of the dopant density and the phononic energy of antibody-coupled graphene when it interfaces with SARS-CoV-2 spike protein. This graphene chemeo-phononic system was able to detect SARS-CoV-2 spike protein at the limit of detection of ∼3.75 and ∼1 fg/mL in artificial saliva and phosphate-buffered saline, respectively. It also exhibited selectivity over proteins in saliva and MERS-CoV spike protein. Since the change in graphene phononics is monitored instead of the phononic signature of the analyte, this optical platform can be replicated for other COVID variants and specific-binding-based biodetection applications.

3D-printed graphene/polymer structures for electron-tunneling based devices.

Designing 3D printed micro-architectures using electronic materials with well-understood electronic transport within such structures will potentially lead to accessible device fabrication for 'on-demand' applications. Here we show controlled nozzle-extrusion based 3D printing of a commercially available nano-composite of graphene/polylactic acid, enabling the fabrication of a tensile gauge functioning via the readjustment of the electron-tunneling barrier width between conductive graphene-centers. The electronic transport in the graphene/polymer 3D printed structure exhibited the Fowler Nordheim mechanism with a tunneling width of 0.79-0.95 nm and graphene centers having a carrier concentration of 2.66 × 1012/cm2. Furthermore, a mechanical strain that increases the electron-tunneling width between graphene nanostructures (~ 38 nm) by only 0.19 Ǻ reduces the electron flux by 1e/s/nm2 (from 18.51 to 19.51 e/s/nm2) through the polylactic acid junctions in the 3D-printed heterostructure. This corresponds to a sensitivity of 2.59 Ω/Ω%, which compares well with other tensile gauges. We envision that the proposed electron-tunneling model for conductive 3D-printed structures with thermal expansion and external strain will lead to an evolution in the design of next-generation of 'on-demand' printed electronic and electromechanical devices.

Organophilicity of Graphene Oxide for Enhanced Wettability of ZnO Nanorods.

Interfacing two-dimensional graphene oxide (GO) platelets with one-dimensional zinc oxide nanorods (ZnO) would create mixed-dimensional heterostructures suitable for modern optoelectronic devices. However, there remains a lack in understanding of interfacial chemistry and wettability in GO-coated ZnO nanorods heterostructures. Here, we propose a hydroxyl-based dissociation-exchange mechanism to understand interfacial interactions responsible for GO adsorption onto ZnO nanorods hydrophobic substrates. The proposed mechanism initiated from mixing GO suspensions with various organics would allow us to overcome the poor wettability (θ ∼ 140.5°) of the superhydrophobic ZnO nanorods to the drop-casted GO. The addition of different classes of organics into the relatively high pH GO suspension with a volumetric ratio of 1:3 (organic-to-GO) is believed to introduce free radicals (-OH and -COOH), which consequently result in enhancing adhesion (chemisorption) between ZnO nanorods and GO platelets. The wettability study shows as high as 75% reduction in the contact angle (θ = 35.5°) when the GO suspension is mixed with alcohols (e.g., ethanol) prior to interfacing with ZnO nanorods. The interfacial chemistry developed here brings forth a scalable tool for designing graphene-coated ZnO heterojunctions for photovoltaics, photocatalysis, biosensors, and UV detectors.

Photo-organometallic, Nanoparticle Nucleation on Graphene for Cascaded Doping.

Controlling the doping levels in graphene by modifying the electric potential of interfaced nanostructures is important to understand "cascaded-doping"-based applications of graphene. However, graphene does not have active sites for nanoparticle attachment, and covalently adding functional groups on graphene disrupts its planar sp2-hybridization, affecting its cascaded doping. Here we show a hexahepto (η6) photo-organometallic chemistry to interface nanoparticles on graphene while retaining the sp2-hybridized state of carbon atoms. For testing cascaded doping with ethanol interaction, transition metal oxide nanoparticles (TMONs) (Cr2O3/CrO3, MoO3, and WO3) are attached on graphene. Here, the transition metal forms six σ-bonds and π-back-bonds with the benzenoid rings of graphene, while its opposite face binds to three carbonyl groups, which enable nucleation and growth of TMONs. With a radius size ranging from 50 to 100 nm, the TMONs downshift the Fermi level of graphene (-250 mV; p-doping) via interfacial charge transfer. This is consistent with the blue shift of graphene's G and 2D Raman modes with a hole density of 3.78 × 1012 cm-2. With susceptibility to ethanol, CrxO3 nanoparticles on graphene enable cascaded doping from ethanol that adsorbs on CrxO3, leading to doping of graphene to increase the electrical resistance of the TMONs-graphene hybrid. This nanoparticle-on-graphene construct can have several applications in gas/vapor sensing, electrochemical catalysis, and high-energy-density supercapacitors.

Strain in a single wrinkle on an MoS2 flake for in-plane realignment of band structure for enhanced photo-response.

Since 2D transition metal dichalcogenides (TMDs) exhibit strain-tunable bandgaps, locally confining strain can allow lateral manipulation of their band structure, in-plane carrier transport and optical transitions. Herein, we show that a single wrinkle (width = 10 nm-10 µm) on an MoS2 flake can induce confined uniaxial strain to reduce the local bandgap (40-60 meV per % deformation), producing a microscopic exciton funnel with an enhancement in photocurrent over flat MoS2 devices. This study also shows that wrinkles can spatially reconfigure the distribution of dopants and enhance the light absorption in the MoS2 layer via Fabry-Perot interference in its nanocavity. In the field-effect transistor studies on the MoS2 flat-wrinkle-flat device-structure, a higher carrier mobility and an improvement in the on/off ratio were exhibited in the devices with a single wrinkle. This phenomenon is attributed to the built-in potential induced by the bandgap reduction at the wrinkle site and the change in doping of the suspended wrinkle. The wrinkle-induced tunability of the local bandgap and manipulation of the spatial transport barriers, and the enhanced light absorption can enable development of next-generation electronic and optoelectronic devices guided by in-plane deformation of 2D nanomaterials.

Quantum Capacitance Based Amplified Graphene Phononics for Studying Neurodegenerative Diseases.

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset motor neuron disease (MND) characterized by a rapid loss of upper and lower motor neurons resulting in patient death from respiratory failure within 3-5 years of initial symptom onset. Although at least 30 genes of major effect have been reported, the pathobiology of ALS is not well understood. Compounding this is the lack of a reliable laboratory test which can accurately diagnose this rapidly deteriorating disease. Herein, we report on the phonon vibration energies of graphene as a sensitive measure of the composite dipole moment of the interfaced cerebrospinal fluid (CSF) that includes a signature-composition specific to the patients with ALS disease. The second-order overtone of in-plane phonon vibration energy (2D peak) of graphene shifts by 3.2 ± 0.5 cm-1 for all ALS patients studied in this work. Further, the amount of n-doping-induced shift in the phonon energy of graphene, interfaced with CSF, is specific to the investigated neurodegenerative disease (ALS, multiple sclerosis, and MND). By removing a severe roadblock in disease detection, this technology can be applied to study diagnostic biomarkers for researchers developing therapeutics and clinicians initiating treatments for neurodegenerative diseases.

Intergrain Diffusion of Carbon Radical for Wafer-Scale, Direct Growth of Graphene on Silicon-Based Dielectrics.

Graphene intrinsically hosts charge-carriers with ultrahigh mobility and possesses a high quantum capacitance, which are attractive attributes for nanoelectronic applications requiring graphene-on-substrate base architecture. Most of the current techniques for graphene production rely on the growth on metal catalyst surfaces, followed by a contamination-prone transfer process to put graphene on a desired dielectric substrate. Therefore, a direct graphene deposition process on dielectric surfaces is crucial to avoid polymer-adsorption-related contamination from the transfer process. Here, we present a chemical-diffusion mechanism of a process for transfer-free growth of graphene on silicon-based gate-dielectric substrates via low-pressure chemical vapor deposition. The process relies on the diffusion of catalytically produced carbon radicals through polycrystalline copper (Cu) grain boundaries and their crystallization at the interface of Cu and underneath silicon-based gate-dielectric substrates. The graphene produced exhibits low-defect multilayer domains ( La ∼ 140 nm) with turbostratic orientations as revealed by selected area electron diffraction. Further, graphene growth between Cu and the substrate was 2-fold faster on SiO2/Si(111) substrate than on SiO2/Si(100). The process parameters such as growth temperature and gas compositions (hydrogen (H2)/methane (CH4) flow rate ratio) play critical roles in the formation of high-quality graphene films. The low-temperature back-gating charge transport measurements of the interfacial graphene show density-independent mobility for holes and electrons. Consequently, the analysis of electronic transport at various temperatures reveals a dominant Coulombic scattering, a thermal activation energy (2.0 ± 0.2 meV), and two-dimensional hopping conduction in the graphene field-effect transistor. A band overlapping energy of 2.3 ± 0.4 meV is estimated by employing the simple two-band model.