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There is an intensive effort to control the nature of attractive interactions between ultrathin semiconductors and metals and to understand its impact on the electronic properties at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS2 films and gold, with a focus on the occupied electronic states near the Brillouin zone center (i.e., the Γ point). To delineate the spectra of WS2 supported on crystalline Au from the suspended WS2, we employ a microscopy approach and a tailored sample structure, in which the WS2/Au junction forms a semi-epitaxial relationship and is adjacent to suspended WS2 regions. The photoelectron spectra, as a function of WS2 thickness, display the expected splitting of the highest occupied states at the Γ point. In multilayer WS2, we discovered variations in the electronic states that spatially align with the crystalline grains of underlying Au. Corroborated by density functional theory calculations, we attribute the electronic structure variations to stacking variations within the WS2 films. We propose that strong interactions exerted by Au grains cause slippage of the interfacing WS2 layer with respect to the rest of the WS2 film. Our findings illustrate that the electronic properties of transition metal dichalcogenides, and more generally 2D layered materials, are physically altered by the interactions with the interfacing materials, in addition to the electron screening and defects that have been widely considered.

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb

As the need for ever greater transistor density increases, the commensurate decrease in device size approaches the atomic limit, leading to increased energy loss and leakage currents, reducing energy efficiencies. Alternative state variables, such as electronic spin rather than electronic charge, have the potential to enable more energy-efficient and higher performance devices. These spintronic devices require materials capable of efficiently harnessing the electron spin. Here we show robust spin transport in Cd3As2 films up to room temperature. We demonstrate a nonlocal spin valve switch from this material, as well as inverse spin Hall effect measurements yielding spin Hall angles as high as θSH = 1.5 and spin diffusion lengths of 10-40 μm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd3As2, are enabling steps toward realizing actual spintronic devices.

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb0.24Sn0.76Te (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20,000 cm2/Vs and a magnetoresistance approaching 100 percent, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/non-polar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of remarkable behaviors in these novel materials.

X-ray and ultraviolet photoelectron spectroscopy are performed on transition metal dichalcogenides (TMDs) MoS2, MoSe2, and MoTe2 monolayers on Au surfaces, to identify charge transfer processes and...

Near-infrared-to-visible second harmonic generation from air-stable two-dimensional polar gallium and indium metals is described. The photonic properties of 2D metals, including the largest second-order susceptibilities reported for metals (approaching 10 nm/V), are determined by the atomic-level structure and bonding of two-to-three-atom-thick crystalline films. The bond character evolved from covalent to metallic over a few atomic layers, changing the out-of-plane metal-metal bond distances by approximately ten percent (0.2 Å), resulting in symmetry breaking and an axial electrostatic dipole that mediated the large nonlinear response. Two different orientations of the crystalline metal atoms, corresponding to lateral displacements2 Å, persisted in separate micrometer-scale terraces to generate distinct harmonic polarizations. This strong atomic-level structure-property interplay suggests metal photonic properties can be controlled with atomic precision.

We demonstrate that unstained DNA nanostructures can be chemically mapped by using aberration-corrected scanning transmission electron microscopy (STEM) with energy-dispersive spectroscopy (EDS). Key to this measurement is the use of graphene supports whose reduced background scattering compared to thicker carbon supports allows EDS to image phosphorus and other elements in the DNA (or in attachments to the DNA) as well as to see the divalent cations used to stabilize the nanostructure. In addition, unlike in other EDS analyses, the chemical maps we obtain can be made quantitative in an absolute sense because each individual DNA nanostructure is in effect a very large macromolecule (∼4.7 MDa) with a known chemical composition. In this way, not only can STEM/EDS serve DNA nanotechnology as a characterization tool but also the DNA structures can function as molecular standards for improving mesoscale EDS techniques.

Scanning tunneling microscopy and spectroscopy (STM/STS) are used to electronically switch atomically-thin memristors, referred to as “atomristors”, based on a graphene/molybdenum disulfide (MoS2)/Au heterostructure. A gold-assisted exfoliation method was used to produce near-millimeter (mm) scale MoS2 on Au thin-film substrates, followed by transfer of a separately exfoliated graphene top layer. Our results reveal that it is possible to switch the conductivity of a graphene/MoS2/Au memristor stack using an STM tip. These results provide a path to further studies of atomically-thin memristors fabricated from heterostructures of two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs).

Two-photon lithography allows writing of arbitrary nanoarchitectures in photopolymers. This design flexibility opens almost limitless possibilities for biological studies, but the acrylate-based polymers frequently used do not allow for adhesion and growth of some types of cells. Indeed, we found that lithographically defined structures made from OrmoComp do not support E18 murine cortical neurons. We reacted OrmoComp structures with several diamines, thereby rendering the surfaces directly permissive for neuron attachment and growth by presenting a surface coating similar to the traditional cell biology coating achieved with poly-d-lysine (PDL) and laminin. However, in contrast to PDL-laminin coatings that cover the entire surface, the amine-terminated OrmoComp structures are orthogonally modified in deference to the surrounding glass or plastic substrate, adding yet another design element for advanced biological studies.

2D heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper we present a method to tune the interlayer coupling in Bi2Se3/MoS2 2D heterostructures by regulating the oxygen presence in the atmosphere, while applying laser or thermal energy. Our data suggests the interlayer coupling is tuned through the diffusive intercalation and de-intercalation of oxygen molecules. When one layer of Bi2Se3 is grown on monolayer MoS2, an influential interlayer coupling is formed that quenches the signature photoluminescence (PL) peaks. However, thermally annealing in the presence of oxygen disrupts the interlayer coupling, facilitating the emergence of the MoS2 PL peak. DFT calculations predict intercalated oxygen increases the interlayer separation ~17%, disrupting the interlayer coupling and inducing the layers to behave more electronically independent. The interlayer coupling can then be restored by thermally annealing in N2 or Ar, where the peaks will re-quench. Hence, this is an interesting oxygen-induced switching between "non-radiative" and "radiative" exciton recombination. This switching can also be accomplished locally, controllably, and reversibly using a low-power focused laser, while changing the environment from pure N2 to air. This allows for the interlayer coupling to be precisely manipulated with submicron spatial resolution, facilitating site-programmable 2D light-emitting pixels whose emission intensity could be precisely varied by a factor exceeding 200x. Our results show that these atomically-thin 2D heterostructures may be excellent candidates for oxygen sensing.

We propose the combination of k-means clustering with Gaussian Process (GP) regression in the analysis and exploration of 4D angle-resolved photoemission spectroscopy (ARPES) data. Using cluster labels as the driving metric on which the GP is trained, this method allows us to reconstruct the experimental phase diagram from as low as 12% of the original dataset size. In addition to the phase diagram, the GP is able to reconstruct spectra in energy-momentum space from this minimal set of data points. These findings suggest that this methodology can be used to improve the efficiency of ARPES data collection strategies for unknown samples. The practical feasibility of implementing this technology at a synchrotron beamline and the overall efficiency implications of this method are discussed with a view on enabling the collection of more samples or rapid identification of regions of interest.

A major challenge for biomedical science is developing materials and processing methods to enable creation of devices that can meaningfully translate signals between biology and electronics. Protonics-based devices—devices analogous to electronic devices but which use protons as charge carriers instead of electrons—are a promising strategy for such translation. Proton-conductive materials (PCMs)—the media through which protons are transported in such devices—are an element ripe for improvement, since they dominate the performance of protonic devices. We investigate protonic devices comprising sub-micrometer diameter, chitosan-fiber PCMs and palladium hydride (PdHx) protodes, the proton-injecting contact, to analyze how proton transport depends on the chemistry and ordering of the PCM. Current–voltage (I–V) measurements of single fiber-based devices under hydrogen atmospheres show that fibers electrospun from trifluoroacetic acid (TFA) solutions feature substantially higher proton conductivity, up to two orders of magnitude, compared to chitosan PCM films cast from acetic acid solutions. We further used digital simulation of the I–V data to elucidate the electrochemical and electrical processes that control device operation. The hydrogen oxidation reaction kinetics of the protode interfaces with the electrospun chitosan fibers agree well with those reported previously. Using X-ray photoelectron spectroscopy (XPS), we observed that single chitosan fibers spun from TFA solutions are more highly proton-doped than chitosan cast from acetic acid. Furthermore, I–V measurements on electrospun chitosan fibers vs. spin-cast chitosan films, both derived from TFA solutions, reveal that electrospun chitosan fibers yield ∼10-fold higher proton conductivity, suggesting that local polymer ordering within the electrospun fibers further enhances proton transport for chitosan PCMs. Finally, devices fabricated from single doped chitosan fibers behave as ionic field effect transistors (IFETs) when under a contact gate bias established with a conducting probe AFM tip. Switching characteristics are tuned by the gate bias, with proton conduction of a single fiber increasing by over an order of magnitude under negative bias. The switchable ion currents and enhanced conductivity of the chitosan fiber-based PCMs comprise a means of establishing spatiotemporal control over ionic communication between protonic devices and adjacent biological cells and membranes.

We show the first direct observation of minibands in the electronic structure of a twisted heterobilayer using microfocused ARPES., Stacking two-dimensional (2D) van der Waals materials with different interlayer atomic registry in a heterobilayer causes the formation of a long-range periodic superlattice that may bestow the heterostructure with properties such as new quantum fractal states or superconductivity. Recent optical measurements of transition metal dichalcogenide (TMD) heterobilayers have revealed the presence of hybridized interlayer electron-hole pair excitations at energies defined by the superlattice potential. The corresponding quasiparticle band structures, so-called minibands, have remained elusive, and no such features have been reported for heterobilayers composed of a TMD and another type of 2D material. We introduce a new x-ray capillary technology for performing microfocused angle-resolved photoemission spectroscopy with a spatial resolution of ~1 μm, and directly observe minibands at certain twist angles in mini Brillouin zones (mBZs). We discuss their origin in terms of initial and final state effects by analyzing their dispersion in distinct mBZs.

The mechanisms leading to spin relaxation in graphene and its heterostructures continue to be debated. Control of the spin relaxation in graphene-based structures is necessary to achieve the envisioned utility of graphene in future spintronic devices beyond Moore's law. Proximity induced spin relaxation caused by contact to a high spin-orbit material, such as WS2, offers a promising avenue to manipulate the spin lifetime. We demonstrate the operation of WS2/graphene/fluorographene non-local spin valves and extract the spin lifetimes for a range of carrier concentrations by Hanle effect measurements. Four-terminal charge transport measurements allow us to calculate the momentum relaxation time as a function of carrier concentration and compare it to the spin lifetime. These data show that the D'yakonov-Perel’ mechanism is the dominant spin relaxation mechanism for WS2/graphene/fluorographene devices, while, for reference graphene/fluorographene devices, linear scaling between the spin and momentum lifetimes points to spin-flip scattering during strong elastic scattering events where the scattering event is strongly coupled to the electron spin. We attribute the change in spin relaxation type in part with the inclusion of WS2 as a substrate to proximity induced spin-orbit coupling due to the adjacent WS2 layer, and we compare our data to the literature.

We show that dehydrogenation of hydrogenated graphene proceeds much more slowly for bilayer systems than for single layer systems. We observe that an underlayer of either pristine or hydrogenated graphene will protect an overlayer of hydrogenated graphene against a number of chemical oxidants, thermal dehydrogenation, and degradation in an ambient environment over extended periods of time. Chemical protection depends on the ease of oxidant intercalation, with good intercalants such as Br2 demonstrating much higher reactivity than poor intercalants such as 1,2-dichloro-4,5-dicyanonbenzoquinone (DDQ). Additionally, the rate of dehydrogenation of hydrogenated graphene at 300 °C in H2/Ar was reduced by a factor of roughly 10 in the presence of a protective underlayer of graphene or hydrogenated graphene. Finally, the slow dehydrogenation of hydrogenated graphene in air at room temperature, which is normally apparent after a week, could be completely eliminated in samples with protective underlayers over the c...

The unique characteristics of graphene have generated much excitement due to its utility across a variety of applications. One of the principal issues inhibiting the development of graphene technologies pertains to difficulties in engineering high-quality metal contacts on graphene. In this regard, the thermal transport at the metal/graphene interface plays a significant role in the overall performance of devices. Here, we demonstrate the use of electron beam produced plasmas to chemically modify graphene and how these modifications can be used to tune the thermal transport across metal-graphene interfaces. We show that the operating conditions of the plasma can be adjusted to control the character and quantity of chemical moieties at the interface. Typically, when changes in the surface chemistry favor an increase in adhesion between the graphene and metal, the thermal boundary conductance can be notably improved. Interestingly, the conventional approach of adding a titanium “wetting layer” to improve metal adhesion did not improve the thermal boundary conductance at gold contacts.

Two-dimensional (2D) heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper, we present a method to tune the interlayer coupling in Bi

The thermal stability of WS2 flakes is studied after integration into a device structure with a Si/SiO2 substrate, a Ti adhesion layer and a Au-contact layer. WS2 /Au/Ti is commonly used in catalytic and electronic devices, and its thermal stability is important for extended operation and mitigation of breakdown at local hot-spots during operation. The thermal stability is severely impacted by rapid diffusion of Ti into and through the polycrystalline Au. Thin film samples of Au/Ti, with and without an exfoliated WS2 monolayer, were annealed sequentially from 275 to 625 °C in vacuum. The near-surface chemical information was extracted with X-ray photoelectron spectroscopy for each annealing step. Ti from the sticking layer, which is buried under 25 nm of Au, diffuses into the near surface region, and initiates a wide range of reactions forming: Ti oxide, carbide, sulfide and a Ti-Au alloy, mainly Au2Ti. WS2 experiences S loss for T > 350 °C and forms Ti-sulfide for T > 600 °C. Higher thermal loads spanning the same temperature range lead to coalescence within the nanocrystalline Au layer, limiting grain boundary diffusion of Ti. The results provide insight on the stability of the WS2/Au/Ti material system which impacts its performance in catalytic and electronic applications.

We describe a method for phase separating and transferring block copolymer (BCP) nanoscale patterns to arbitrary substrates for surface-independent nanolithography. The enabling technology is a hydrogenated or oxidized graphene thin film that only weakly adheres to its substrate. BCPs are applied to these graphene-based materials and solvent annealed to effect nanoscale phase separation. Then, taking advantage of the weak interaction of the graphene film and its substrate, the BCP/graphene stack is delaminated easily in water. A target substrate is then used to retrieve the stack, which can then serve as a lithographic mask. The use of water as a lift-off agent allows for chemically mild retrieval of the phase-separated BCP, extending the BCP lithography technique to essentially arbitrary substrates. We demonstrate this graphene-enabled BCP lithography on silicon nitride and polyethylene. We also show that using reduced graphene oxide (RGO) as a thin film enables the transfer of wafer-scale BCP films and lithography on SiOx and Si. We use an RGO support to produce phase-separated BCP solvent-annealed patterns on polystyrene, a result which is not possible using standard BCP solvent annealing and which shows the utility of this technique. Finally, we demonstrate the ability to create nanopatterns of higher complexity by stacking multiple BCP masks, a capability that is not possible using conventional BCP lithography. This technique may have applications in fabricating nanoporous membranes and photonically active coatings.

In bioelectronic medicine and electroceuticals, electronic devices that conduct electrons are used to monitor and control ion-based biochemical reactions in order to detect and treat medical conditions. Advancing these devices will require a thorough understanding of the electrochemical pathways that transduce electronic currents into the ionic currents that interact with the natural system. Here, we analyze the tranduction of electronic current into a protonic current (H+) using Pd/PdHx contacts and a model proton conductor, chitosan. Linear sweep voltammetry and electrochemical impedance spectroscopy data indicate that, for Pd, limited proton injection occurs at the interface aided by water oxidation. For PdHx, H desorption and electrochemical oxidation to H+ lead to sustainable proton injection and transfer to the chitosan protonic conductor. We have developed electroanalytical expressions and predictive digital simulations that match the experimental results. This work confirms that PdHx contacts integrated in DC devices, in parallel with electrochemical impedance spectroscopy, comprise a suitable means for measuring H+ currents and interrogating the proton conductivity in biomaterials.

Chemically modified graphenes (CMGs) offer a means to tune a wide variety of graphene's exceptional properties. Critically, CMGs can be transferred onto a variety of substrates, thereby imparting functionalities to those substrates that would not be obtainable through conventional functionalization. One such application of CMGs is enabling and controlling the subsequent growth of inorganic thin films. In the current study, we demonstrated that CMGs enhance the growth of inorganic films on inert surfaces with poor growth properties. Fluorinated graphene transferred onto polyethylene enabled the dense and homogeneous deposition of a cadmium sulfide (CdS) film grown via chemical bath deposition. We showed that the coverage of the CdS film can be controlled by the degree of fluorination from less than 20% to complete coverage of the film. The approach can also be applied to other technologically important materials such as ZnO. Finally, we demonstrated that electron beam-generated plasma in a SF6-containing background could pattern fluorine onto a graphene/PE sample to selectively grow CdS films on the fluorinated region. Therefore, CMG coatings can tailor the surface properties of polymers and control the growth of inorganic thin films on polymers for the development of flexible electronics.

We investigate hyperthermal ion implantation (HyTII) as a means for substitutionally doping layered materials such as graphene. In particular, this systematic study characterizes the efficacy of substitutional N-doping of graphene using HyTII over an N(+) energy range of 25-100 eV. Scanning tunneling microscopy results establish the incorporation of N substituents into the graphene lattice during HyTII processing. We illustrate the differences in evolution of the characteristic Raman peaks following incremental doses of N(+). We use the ratios of the integrated D and D' peaks, I(D)/I(D') to assess the N(+) energy-dependent doping efficacy, which shows a strong correlation with previously reported molecular dynamics (MD) simulation results and a peak doping efficiency regime ranging between approximately 30 and 50 eV. We also demonstrate the inherent monolayer depth control of the HyTII process, thereby establishing a unique advantage over other less-specific methods for doping. We achieve this by implementing twisted bilayer graphene (TBG), with one layer of isotopically enriched (13)C and one layer of natural (12)C graphene, and modify only the top layer of the TBG sample. By assessing the effects of N-HyTII processing, we uncover dose-dependent shifts in the transfer characteristics consistent with electron doping and we find dose-dependent electronic localization that manifests in low-temperature magnetotransport measurements.

The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue towards engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. While the twist angle-dependence of these properties has been explored, direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines a functional twisted bilayer graphene device using in-operando angle-resolved photoemission with a nano-focused light spot. A twist angle of 12.2$^{\circ}$ is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene., Comment: 16 pages, 4 figures. A revised version of the manuscript has been accepted for publication in Advanced Materials

We report field emission energy distributions from planar graphene edges. Field effect transistors with integrated gate and drain electrodes and vacuum transport parallel to the substrate surface are fabricated using graphene edge emission sources are prepared from two types of graphene. The emission distributions produced by reduced graphene oxide are roughly symmetric, often contain multiple peaks, and shift over time. These characteristics suggest the emission originates in local edge states. Emission produced by high quality graphene grown on Cu foil more closely resembles emission from a linear density of states, suggesting the emission starts from the delocalized band as well as discrete states.

Functional surfaces find application in a number of areas, such as designing flexible electronic devices and integrating electronic systems with biological ones. However, the preparation of functional surfaces entails processing that is destructive to fragile polymer or biological substrates. A benign transfer method is thus needed to move pre-functionalized surfaces from a stable substrate to a fragile one. Chemical hydrogenation of graphene weakens the adhesion force between the graphene and its substrate. We exploit this phenomenon to construct a method for transferring graphene with pre-formed chemical, physical, and electronic functionalities from a heat-, vacuum-, and chemical-stable substrate such as silicon to several less robust ones, including polymers and living cells. We also discuss reversibility of graphene hydrogenation and the implications for re-adhering graphene securely to new substrates.

The integration of graphene and 2D materials into device technologies requires a detailed understanding of how intrinsic and extrinsic forces impact their properties, as well as the development of engineering strategies to vary their properties for a specific response. In this paper we describe and review our efforts for hybridizing graphene in different ways so as to modify or enhance a range of properties. This hybridization comes in the form of chemical or electronic modification for use in applications ranging from chem/bio sensors to nanoelectronics. We discuss results on exploiting chemistry and defects in graphene for chemical vapor sensing, on hybridizing graphene with fluorine atoms for potential use in nanoelectronics, and on electronically hybridizing graphene in multilayer stacks that give rise to new optical and surface properties.

We report electron field emission from planar graphene edge sources with integrated gate and drain electrodes, forming a field effect transistor with vacuum transport parallel to the substrate surface. The drain-source voltage required to produce a given current changes with the potential applied to the integrated gate in a manner consistent with models. The energy distribution of the field emitted beam peaks near the Fermi energy.

2D materials play a special role in the race to make smaller and smaller devices. Their unique and strong in-plane bonding makes them impervious to diffusion into other layers and provides excellent thickness control. Their van der Waal’s bonding with other monolayers or substrates allows for heterostructures unattainable by any other technique. This is reflected by the abundant popularity of research into graphene and other 2D materials. In this review article, we will describe the out-of-plane properties of graphene and functionalized graphene. We will use three specific examples to illustrate how these out-of-plane properties can be used in spintronic devices, in section “Graphene as a Tunnel Barrier” we will describe a magnetic tunnel junction (MTJ) based on graphene. Section “Graphene Based MTJs” will describe the spin injecting properties of a graphene tunnel barrier on silicon. Section “Graphene in Semiconductor Spintronic Devices” describes how you can use functionalized graphene to make a homoepitaxial graphene device. The second part of this article reviews monolayer transition-metal dichalcogenides (TMDs). First, we will show how TMDs are grown and specifically how we can grow large-area TMDs by chemical vapor deposition. Secondly, we will describe the optical properties of several TMDs and compare the results from several authors. Finally, we choose a chemical sensor as a specific example to show how TMDs can be used in a device.

We report several methods of chemical dehydrogenation of hydrogenated graphene (HG), characterizing the results using Raman, X-ray photoelectron spectroscopy, and electrical conductivity measurements. Notably, the hydrogen–graphene bonds appear to activate the graphene toward subsequent reaction such that, in several cases, the addition of the dehydrogenating agent to the graphene accompanies the removal of hydrogen. We compare the uptake of chemical groups on HG to those on pristine graphene and find that HG reacts more readily than pristine graphene with radical generators such as chlorine and AIBN.

We report systematically tuning the mechanical properties of vertical graphene (VG) sheets through fluorination. VG sheets were synthesized using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique and were functionalized through exposure to xenon difluoride (XeF2) gas. An atomic force microscopy technique, PeakForce Quantitative Nanomechanical Mapping (QNM®), was used to measure the mechanical properties of the VG sheets. We show that fluorination can significantly enhance the reduced modulus of surfaces comprised of VG sheets. Samples with only ∼3.5% fluorine had a reduced modulus approximately eight times higher than unfunctionalized VG sheets, which is attributed to sp2 to sp3 conversion and a change of the C–C bond length after functionalization. Fluorination also decreased the energy dissipation of the VG sheets and reduced their adhesion to the AFM tip. This method represents a unique approach towards modification of the mechanical properties of nanostructures without a significant increase in weight or change of the VG sheet morphology.

Layered two-dimensional crystal systems can exhibit complex interlayer interactions, which are influenced by local crystal structure and/or electronic variations. Here, we study the influence of defects in twisted bilayer graphene (TBG) using Raman spectroscopy. We explore the varied influence of defects on three characteristic Raman modes of both fully-defected TBG, with defects introduced in both layers, and half-defected TBG, with defects introduced in only a single layer. The resonance condition responsible for a strong enhancement of the G peak is sensitive to structural disorder and is quenched within a radius ∼3 nm of defects, while the twist-angle dependence of the 2D peak is influenced only at the site of structural disorder (∼1 nm radius).

Graphene (Gr)–polystyrene (PS) and graphene (Gr)–ultra-high molecular weight polyethylene (UHMW PE) laminates were fabricated using a transfer print approach that relies on differential adhesion to remove graphene from Cu foil without chemical etching. The polymer surfaces were prepared using plasma functionalization followed by N-ethylamino-4-azidotetrafluorobenzoate (TFPA) deposition. Then, the graphene on Cu foil and the TFPA coated polymers were pressed at elevated temperature and mild pressure. Finally, they were separated by mechanical peeling. No additional processing was applied. Detailed chemical, structural, and morphological characterization of PS and UHMW PE before and after graphene transfer print was performed using a suite of complementary surface analysis techniques including X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), Raman Spectroscopy, and Atomic Force Microscopy (AFM). The charge carrier density and charge carrier mobility of the transferred graphene were determined using Hall effect measurements. We found that graphene’s electrical properties were preserved and comparable to those of graphene on SiO2/Si. Furthermore, modulation of TFPA attachment to PS and UHMW PE led to different TFPA-layer microstructure and therefore to a different amount of functional azide groups available to form carbene bonds with graphene causing changes in graphene’s compressive strain, doping and mobility.

Fluorinated graphene obtained by exposing single-layer CVD-grown graphene to xenon difluoride was reacted with a series of amine-, alcohol-, and sulfur-bearing nucleophiles, and the progress and nature of the reactions were monitored using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. The results of these experiments indicate that amine and alcohol nucleophiles can displace the fluorine groups to form covalent bonds to the graphene. For nucleophiles with more than one possible reactive site, close examination of XPS features reveals the orientation of these groups on the graphene. Sulfur nucleophiles act preferentially as reducing agents, removing fluorine rather than replacing it. Finally, a proof-of-principle nucleophilic substitution is performed on bulk graphite fluoride, showing that the chemical functionality of graphite can be extended through nucleophilic substitution in an analogous manner to that of single-layer graphene.

Although robust chemical vapor detection by chemielectric point sensors remains as a largely unmet challenge at present, the best performance to date and the most likely avenue for future progress is with sensor designs in which the transductive element is a disordered nanostructured material. We here review the evidence for this claim, with illustrations drawn from recent work on sensors made from gold nanoparticles, carbon nanotubes, and reduced graphene oxide nanoplatelets. These examples can be regarded as being prototypical of disordered nanostructured films formed of primitive objects that are nanoscopic in 3-D, 2-D, and 1-D, respectively.

Partially hydrogenated graphene is ferromagnetic and may be patterned by electron-beam irradiation. Sequential patterning produces a patterned magnetic array. Removal of the hydrogen atoms also can convert electrically insulating fully hydrogenated graphene back into conductive graphene, enabling the writing of chemically isolated, dehydrogenated graphene nanoribbons as narrow as 100 nm.

By combining optical imaging, Raman spectroscopy, kelvin probe force microscopy (KFPM), and photoemission electron microscopy (PEEM), we show that graphene’s layer orientation, as well as layer thickness, measurably changes the surface potential (Φ). Detailed mapping of variable-thickness, rotationally-faulted graphene films allows us to correlate Φ with specific morphological features. Using KPFM and PEEM we measure ΔΦ up to 39 mV for layers with different twist angles, while ΔΦ ranges from 36–129 mV for different layer thicknesses. The surface potential between different twist angles or layer thicknesses is measured at the KPFM instrument resolution of ≤ 200 nm. The PEEM measured work function of 4.4 eV for graphene is consistent with doping levels on the order of 1012cm−2. We find that Φ scales linearly with Raman G-peak wavenumber shift (slope = 22.2 mV/cm−1) for all layers and twist angles, which is consistent with doping-dependent changes to graphene’s Fermi energy in the ‘high’ doping limit. Our results here emphasize that layer orientation is equally important as layer thickness when designing multilayer two-dimensional systems where surface potential is considered.

Using graphene nanomechanical resonators we demonstrate the extent to which the mechanical properties of multilayer graphene films are controllable, in real time, through introduction and rearrangement of defects. We show both static and re-entrant (cyclical) changes in the tensile stress using a combination of ion implantation, chemical functionalization, and thermal treatment. While the dramatic increase in static tensile stress achievable through laser annealing can be of importance for various MEMS applications, we view the direct observation of a time-variable stress as even more significant. We find that defect-rich films exhibit a slow relaxation component of the tensile stress that remains in the resonator long after the laser exposure is finished (trelax ≈ 100 s ≫ tcooling), analogous to a wind-up toy. We attribute this persistent component of the time-variable stress to a set of metastable, multivacancy structures formed during the laser anneal. Our results indicate that significant stress field...

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and nonequilibrium Green's function (NEGF).