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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

WS2-induced enhanced optical absorption and efficiency in graphene/silicon heterojunction photovoltaic cells.

Van Hove singularity (VHS) induced enhancement of visible-frequency absorption in atomically-thin two-dimensional (2D) crystals provides an opportunity for improved light management in photovoltaics; however, it requires the 2D nanomaterial to be in close vicinity of a photojunction. In this report, we design a Schottky junction-based photovoltaic system with single-layer graphene atop n-type silicon (n-Si), which is interfaced directly with a few layers of tungsten disulfide (WS2) via a bottom-up CVD synthesis strategy. An enhanced power conversion efficiency in the architecture of WS2-graphene/n-Si is observed compared to graphene/n-Si. Here, the WS2 induced photon absorption, with only three atoms above the photo-junction, enhanced the short-circuit current density (Jsc), and the reconfiguration of the energy band structure led to effective built-in electric field induced charge carrier transport (enhanced open-circuit voltage (Voc)). Similar to a graphene/n-Si Schottky junction, the WS2-graphene/n-Si double junction exhibited non-linear current density-voltage (J-V) characteristics with a 4-fold increase in Jsc (2.28 mA cm-2 in comparison with 0.52 mA cm-2 for graphene/n-Si) and 40% increase in the Voc (184 mV compared to 130 mV for graphene/n-Si) with a 6-fold increase in the photovoltaic power conversion efficiency. Futuristically, we envision an evolution in 2D heterojunctions with sharp transitions in properties within a few nanometers enabling control on optical absorption, carrier distribution, and band structure for applications including tandem photovoltaic cells and 2D optoelectronic circuit-switches.

Introduction of Protonated Sites on Exfoliated, Large-Area Sheets of Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) sheets possess an exclusive set of properties, including wide energy band gap, high optical transparency, high dielectric breakdown strength, high thermal conductivity, UV cathodoluminescence, and pronounced thermochemical stability. However, functionalization of large h-BN layers has remained a challenge due to their chemical resistance and unavailable molecular-binding sites. Here we report on the protonation of h-BN via treatment with chlorosulfonic acid that not only exfoliates "large" h-BNs (up to 10 000 µm2) at high yields (∼23%) but also results in their covalent functionalization by introducing four forms of aminated nitrogen (N) sites within the h-BN lattice: sp2-delocalized and sp3-quaternary protonation on internal N sites (>N+═ and >NH+-) and pyridinic-like protonation on the edge N sites (═NH+- and -NH-). The presence of these groups transforms the chemically passive h-BN sheets to their chemically active form, which as demonstrated here can be used as scaffolds for forming composites with plasmonic gold nanoparticles and organic dye molecules. The dispersion of h-BNs exhibits an optical energy band gap of 5.74 eV and a zeta potential of ζ = +36.25 mV at pH = 6.1 (ζmax = +150 mV), confirming high dispersion stability. We envision that these two-dimensional nanomaterials with an atomically packed honeycomb lattice and high-energy band gap will evolve next-generation applications in controlled-UV emission, atomic-tunneling-barrier devices, ultrathin controlled-permeability membranes, and thermochemically resistive transparent coatings.

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.

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%).

Chemical Interaction-Guided, Metal-Free Growth of Large-Area Hexagonal Boron Nitride on Silicon-Based Substrates.

Hexagonal boron nitride (h-BN) is an ideal platform for interfacing with two-dimensional (2D) nanomaterials to reduce carrier scattering for high-quality 2D electronics. However, scalable, transfer-free growth of hexagonal boron nitride (h-BN) remains a challenge. Currently, h-BN-based 2D heterostructures require exfoliation or chemical transfer of h-BN grown on metals resulting in small areas or significant interfacial impurities. Here, we demonstrate a surface-chemistry-influenced transfer-free growth of large-area, uniform, and smooth h-BN directly on silicon (Si)-based substrates, including Si, silicon nitride (Si3N4), and silicon dioxide (SiO2), via low-pressure chemical vapor deposition. The growth rates increase with substrate electronegativity, Si < Si3N4 < SiO2, consistent with the adsorption rates calculated for the precursor molecules via atomistic molecular dynamics simulations. Under graphene with high grain density, this h-BN film acts as a polymer-free, planar-dielectric interface increasing carrier mobility by 3.5-fold attributed to reduced surface roughness and charged impurities. This single-step, chemical interaction guided, metal-free growth mechanism of h-BN for graphene heterostructures establishes a potential pathway for the design of complex and integrated 2D-heterostructured circuitry.

Adhesion Energy of MoS2 Thin Films on Silicon-Based Substrates Determined via the Attributes of a Single MoS2 Wrinkle.

Understanding the energetics of adhesion between two-dimensional nanomaterials and their supporting substrates is crucial for the design and fabrication of corrersponding structures with controlled interfacial effects that influence phononics, charge-carrier distribution, and electronic response. Here, we show a mechanical energy model that equates the adhesion energy of MoS2 on rigid and flat substrates (SiO2 and Si3N4) to the attributes of a single wrinkle in a MoS2 flake. The amplitude of the observed wrinkles was normalized for thickness (A/t) to select the wrinkles valid for the model. The adhesion energy values of 0.170 ± 0.033 J m-2 for MoS2 on SiO2 and 0.252 ± 0.041 J m-2 for MoS2 on Si3N4 were determined. This mechanical energy model is consistent with the model based on the local equilibrium at the contact point in the Young's equation. We also propose a method to measure the plane-strain in wrinkled MoS2. The geometrical properties (symmetry and normalized dimensions) of wrinkles and substrate effects are also discussed.

Cancer Cell Hyperactivity and Membrane Dipolarity Monitoring via Raman Mapping of Interfaced Graphene: Toward Non-Invasive Cancer Diagnostics.

Ultrasensitive detection, mapping, and monitoring of the activity of cancer cells is critical for treatment evaluation and patient care. Here, we demonstrate that a cancer cell's glycolysis-induced hyperactivity and enhanced electronegative membrane (from sialic acid) can sensitively modify the second-order overtone of in-plane phonon vibration energies (2D) of interfaced graphene via a hole-doping mechanism. By leveraging ultrathin graphene's high quantum capacitance and responsive phononics, we sensitively differentiated the activity of interfaced Glioblastoma Multiforme (GBM) cells, a malignant brain tumor, from that of human astrocytes at a single-cell resolution. GBM cell's high surface electronegativity (potential ∼310 mV) and hyperacidic-release induces hole-doping in graphene with a 3-fold higher 2D vibration energy shift of approximately 6 ± 0.5 cm-1 than astrocytes. From molecular dipole-induced quantum coupling, we estimate that the sialic acid density on the cell membrane increases from one molecule per ∼17 nm2 to one molecule per ∼7 nm2. Furthermore, graphene phononic response also identified enhanced acidity of cancer cell's growth medium. Graphene's phonon-sensitive platform to determine interfaced cell's activity/chemistry will potentially open avenues for studying activity of other cancer cell types, including metastatic tumors, and characterizing different grades of their malignancy.

Increased Hierarchical Wrinklons on Stiff Metal Thin Film on a Liquid Meniscus.

Wrinklons-the hierarchical merging of wrinkles-are observed on several surfaces including thin films, curtains, graphene sheets, and skin. Wrinklons are a consequence of the interplay between bending, stretching, and gravitational energies and generally exhibit 1 to 2 hierarchical transitions (λn+1 = 2λn). Here we show that parallel and self-similar wrinklons on ultrathin cobalt/chromium film atop a contracting silicone oil meniscus can produce up to 5 hierarchical wrinklon transitions near the fluid-solid boundary. Further, these wrinklons do not follow the standard von-Kármán wrinklon scaling near the edge, attributed to the added surface energy (L/λ ∝ (A/t)(0.31)). A model developed via scale analysis shows (a) the relationship between wavelength and length of the wrinkles and (b) a linear relation between the amplitude and the length of wrinkles at all observed hierarchic levels (L ∝ A), fitted well with previous literature results. This work provides a mechanism for thin-film metal wrinkling on liquids and shows that surface stretching effects can allow increased hierarchical levels in wrinklons.

Confined, Oriented, and Electrically Anisotropic Graphene Wrinkles on Bacteria.

Curvature-induced dipole moment and orbital rehybridization in graphene wrinkles modify its electrical properties and induces transport anisotropy. Current wrinkling processes are based on contraction of the entire substrate and do not produce confined or directed wrinkles. Here we show that selective desiccation of a bacterium under impermeable and flexible graphene via a flap-valve operation produces axially aligned graphene wrinkles of wavelength 32.4-34.3 nm, consistent with modified Föppl-von Kármán mechanics (confinement ∼0.7 × 4 µm(2)). Further, an electrophoretically oriented bacterial device with confined wrinkles aligned with van der Pauw electrodes was fabricated and exhibited an anisotropic transport barrier (ΔE = 1.69 meV). Theoretical models were developed to describe the wrinkle formation mechanism. The results obtained show bio-induced production of confined, well-oriented, and electrically anisotropic graphene wrinkles, which can be applied in electronics, bioelectromechanics, and strain patterning.