Modification of electronic properties of graphene with self-assembled monolayers

Electronic Devices Based on Heterostructures of 2D Materials and Self-Assembled Monolayers

2D materials (2DMs), known for their atomically ultrathin structure, exhibit remarkable electrical and optical properties. Similarly, molecular self-assembled monolayers (SAMs) with comparable atomic thickness show an abundance of designable structures and properties. The strategy of constructing electronic devices through unique heterostructures formed by van der Waals assembly between 2DMs and molecular SAMs not only enables device miniaturization, but also allows for convenient adjustment of their structures and functions. In this review, the fundamental structures and fabrication methods of three different types of electronic devices dominated by 2DM-SAM heterojunctions with varying architectures are timely elaborated. Based on these heterojunctions, their fundamental functionalities and characteristics, as well as the regulation of their performance by external stimuli, are further discussed.

Selective Laser-Assisted Direct Synthesis of MoS2 for Graphene/MoS2 Schottky Junction

Implementing a heterostructure by vertically stacking two-dimensional semiconductors is necessary for responding to various requirements in the future of semiconductor technology. However, the chemical-vapor deposition method, which is an existing two-dimensional (2D) material-processing method, inevitably causes heat damage to surrounding materials essential for functionality because of its high synthesis temperature. Therefore, the heterojunction of a 2D material that directly synthesized MoS2 on graphene using a laser-based photothermal reaction at room temperature was studied. The key to the photothermal-reaction mechanism is the difference in the photothermal absorption coefficients of the materials. The device in which graphene and MoS2 were vertically stacked using a laser-based photothermal reaction demonstrated its potential application as a photodetector that responds to light and its stability against cycling. The laser-based photothermal-reaction method for 2D materials will be further applied to various fields, such as transparent display electrodes, photodetectors, and solar cells, in the future.

Fermi-Level Modulation of Chemical Vapor Deposition-Grown Monolayer Graphene via Nanoparticles to Macromolecular Dopants

It is critical to modulate the Fermi level of graphene for the development of high-performance electronic and optoelectronic devices. Here, we have demonstrated the modulation of the Fermi level of chemical vapor deposition (CVD)-grown monolayer graphene (MLG) via doping with nanoparticles to macromolecules such as titanium dioxide nanoparticles (TiO2 NPs), nitric acid (HNO3), octadecyltrimethoxysilane (OTS) self-assembled monolayer (SAM), and poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS). The electronic properties of pristine and doped graphene samples were investigated by Raman spectroscopy and electrical transport measurements. The right shifting of G and 2D peaks and reduction in 2D to G peak intensity ratio (I 2D/I G) assured that the dopants induced a p-type doping effect. Upon doping, the shifting of the Dirac point towards positive voltage validates the increment of the hole concentration in graphene and thus downward shift of the Fermi level. More importantly, the combination of HNO3/TiO2 NP doping on graphene yields a substantially larger change in the Fermi level of MLG. Our study may be useful for the development of graphene-based high-performance flexible electronic devices.

Nanoscale self-assembly: concepts, applications and challenges

Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapour or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Asymmetric Dressing of WSe2 with (Macro)molecular Switches: Fabrication of Quaternary-Responsive Transistors

The forthcoming saturation of Moore's law has led to a strong demand for integrating analogue functionalities within semiconductor-based devices. As a step toward this goal, we fabricate quaternary-responsive WSe₂-based field-effect transistors (FETs) whose output current can be remotely and reversibly controlled by light, heat, and electric field. A photochromic silane-terminated spiropyran (SP) is chemisorbed on SiO₂ forming a self-assembled monolayer (SAM) that can switch from the SP to the merocyanine (MC) form in response to UV illumination and switch back by either heat or visible illumination. Such a SAM is incorporated at the dielectric-semiconductor interface in WSe₂-based FETs. Upon UV irradiation, a drastic decrease in the output current of 82% is observed and ascribed to the zwitterionic MC isomer acting as charge scattering site. To provide an additional functionality, the WSe₂ top surface is coated with a ferroelectric co-polymer layer based on poly(vinylidene fluoride-co-trifluoroethylene). Because of its switchable inherent electrical polarization, it can promote either the accumulation or depletion of charge carriers in the WSe₂ channel, thereby inducing a current modulation with 99% efficiency. Thanks to the efficient tuning induced by the two components and their synergistic effects, the device polarity could be modulated from n-type to p-type. Such a control over the carrier concentration and device polarity is key to develop 2D advanced electronics. Moreover, the integration strategy of multiple stimuli-responsive elements into a single FET allows us to greatly enrich its functionality, thereby promoting the development for More-than-Moore technology.

Enhancement of Photodetective Properties on Multilayered MoS2 Thin Film Transistors via Self-Assembled Poly-L-Lysine Treatment and Their Potential Application in Optical Sensors

Photodetectors and display backplane transistors based on molybdenum disulfide (MoS2) have been regarded as promising topics. However, most studies have focused on the improvement in the performances of the MoS2 photodetector itself or emerging applications. In this study, to suggest a better insight into the photodetector performances of MoS2 thin film transistors (TFTs), as photosensors for possible integrated system, we performed a comparative study on the photoresponse of MoS2 and hydrogenated amorphous silicon (a-Si:H) TFTs. As a result, in the various wavelengths and optical power ranges, MoS2 TFTs exhibit 2~4 orders larger photo responsivities and detectivities. The overall quantitative comparison of photoresponse in single device and inverters confirms a much better performance by the MoS2 photodetectors. Furthermore, as a strategy to improve the field effect mobility and photoresponse of the MoS2 TFTs, molecular doping via poly-L-lysine (PLL) treatment was applied to the MoS2 TFTs. Transfer and output characteristics of the MoS2 TFTs clearly show improved photocurrent generation under a wide range of illuminations (740~365 nm). These results provide useful insights for considering MoS2 as a next-generation photodetector in flat panel displays and makes it more attractive due to the fact of its potential as a high-performance photodetector enabled by a novel doping technique.

Tailoring Superconductivity in Large-Area Single-Layer NbSe2 via Self-Assembled Molecular Adlayers

Two-dimensional transition metal dichalcogenides (TMDs) represent an ideal testbench for the search of materials by design, because their optoelectronic properties can be manipulated through surface engineering and molecular functionalization. However, the impact of molecules on intrinsic physical properties of TMDs, such as superconductivity, remains largely unexplored. In this work, the critical temperature (T_C) of large-area NbSe₂ monolayers is manipulated, employing ultrathin molecular adlayers. Spectroscopic evidence indicates that aligned molecular dipoles within the self-assembled layers act as a fixed gate terminal, collectively generating a macroscopic electrostatic field on NbSe₂. This results in an ∼55% increase and a 70% decrease in T_C depending on the electric field polarity, which is controlled via molecular selection. The reported functionalization, which improves the air stability of NbSe₂, is efficient, practical, up-scalable, and suited to functionalize large-area TMDs. Our results indicate the potential of hybrid 2D materials as a novel platform for tunable superconductivity.

Density functional study of gallium clusters on graphene: electronic doping and diffusion

Motivated by experimental results on transport properties of graphene covered by gallium atoms, the density functional theory study of clustering of gallium atoms on graphene (up to a size of 8 atoms) is presented. The paper explains a rapid initial increase of graphene electron doping by individual Ga atoms with Ga coverage, which is continually reduced to zero, when bigger multiple-atom clusters have been formed. According to density functional theory calculations with and without the van der Waals correction, gallium atoms start to form a three-dimensional cluster from five and three atoms, respectively. The results also explain an easy diffusion of Ga atoms while forming clusters caused by a small diffusion barrier of 0.11 eV. Moreover, the calculations show this barrier can be additionally reduced by the application of an external electric field, which was simulated by the ionization of graphene. This effect offers a unique possibility to control the cluster size in experiments only by applying a gate-voltage to the graphene in a field-effect transistor geometry and thereby without growth temperature assistance.

Boosting and Balancing Electron and Hole Mobility in Single- and Bilayer WSe2 Devices via Tailored Molecular Functionalization

WSe₂ is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suitable active component for logic circuitry. However, solid-state devices based on single- and bilayer WSe₂ typically exhibit unipolar transport and poor electrical performance when conventional SiO₂ dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered monolayers on the top of the WSe₂ surface, thereby boosting its electrical performance in single- and bilayer field-effect transistors. In particular, by employing SiO₂ dielectric substrates and top Au electrodes, we measure unipolar mobility as high as μ_h = 150 cm² V^-1 s^-1 and μ_e = 17.9 cm² V^-1 s^-1 in WSe₂ single-layer devices when ad hoc molecular monolayers are chosen. Additionally, by asymmetric double-side functionalization with two different molecules, we provide opposite polarity to the top and bottom layer of bilayer WSe₂, demonstrating nearly balanced ambipolarity at the bilayer limit. Our results indicate that the controlled functionalization of the two sides of the WSe₂ mono- and bilayer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functionalization, representing a path toward deterministic control over charge transport in 2D materials.

Power Generation by Selective Self-Assembly of Biocatalysts

Through a careful chemical and bioelectronic design we have created a system that uses self-assembly of enzyme-nanoparticle hybrids to yield bioelectrocatalytic functionality and to enable the harnessing of electrical power from biomass. Here we show that mixed populations of hybrids acting as catalyst carriers for clean energy production can be efficiently stored, self-assembled on functionalized stationary surfaces, and magnetically re-collected to make the binding sites on the surfaces available again. The methodology is based on selective interactions occurring between chemically modified surfaces and ligand-functionalized hybrids. The design of a system with minimal cross-talk between the particles, outstanding selective binding of the hybrids at the electrode surfaces, and direct anodic and cathodic electron transfer pathways leads to mediator-less bioelectrocatalytic transformations which are implemented in the construction of a fast self-assembling, membrane-less fructose/O₂ biofuel cell.

Threshold Voltage Control of Multilayered MoS2 Field-Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement-Mode Logic Gates

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS₂ ) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS₂ FETs by using self-assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS₂ FETs in an enhancement mode with preservation of electrical parameters such as field-effect mobility, subthreshold swing, and current on-off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature-dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS₂ FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V_DD . More impressively, quantum dot light-emitting diodes, driven by enhancement mode MoS₂ FETs, stably demonstrate 120 cd m^-2 at the gate-to-source voltage of 5 V, exhibiting promising opportunities for future display application.

Threshold Voltage Control in Organic Field-Effect Transistors by Surface Doping with a Fluorinated Alkylsilane

Doping is a powerful tool to control the majority charge carrier density in organic field-effect transistors and the threshold voltage of these devices. Here, a surface doping approach is shown, where the dopant is deposited on the prefabricated polycrystalline semiconducting layer. In this study, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTCS), a fluorinated alkylsilane is used as a dopant, which is solution processable and much cheaper than conventional p-type dopants, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). In this work, the depositions from the gas phase and from solution are compared. Both deposition approaches led to an increased conductivity and to a shift in the threshold voltage to more positive values, both of which indicate a p-type doping effect. The magnitude of the threshold voltage shift could be controlled by the FTCS deposition time (from vapor) or FTCS concentration (from solution); for short deposition times and low concentrations, the off current stayed constant and the mobility decreased only slightly. In the low doping concentration regime, both approaches resulted in similar transistor characteristics, i.e., similar values of shift in the threshold and turn-on voltage as well as mobility, I_ON/ I_OFF ratio and amount of introduced free charge carriers. In comparison with vapor deposition, the solution-based approach can be conducted with less material and in a shorter time, which is critical for industrial applications.

Thionine Self-Assembled Structures on Graphene: Formation, Organization, and Doping

The association of organic molecules with two-dimensional (2D) materials, creating hybrid systems with mutual influences, constitutes an important testbed for both basic science self-assembly studies and perspective applications. Following this concept, in this work, we show a rich phenomenology that is involved in the interaction of thionine with graphene, leading to a hybrid material formed by well-organized self-assembled structures atop graphene. This composite system is investigated by atomic force microscopy, electric transport measurements, Raman spectroscopy, and first principles calculations, which show (1) an interesting time evolution of thionine self-assembled structures atop graphene; (2) a highly oriented final molecular assembly (in accordance with the underlying graphene surface symmetry); and (3) a strong n-type doping effect introduced in graphene by thionine. The nature of the thionine-substrate interaction is further analyzed in experiments using mica as a polar substrate. The present results may help pave the way to achieve tailored 2D material hybrid devices via properly chosen molecular self-assembly processes.

Nanopatched Graphene with Molecular Self-Assembly Toward Graphene-Organic Hybrid Soft Electronics

Increasing the mechanical durability of large-area polycrystalline single-atom-thick materials is a necessary step toward the development of practical and reliable soft electronics based on these materials. Here, it is shown that the surface assembly of organosilane by weak epitaxy forms nanometer-thick organic patches on a monolayer graphene surface and dramatically increases the material's resistance to harsh postprocessing environments, thereby increasing the number of ways in which graphene can be processed. The nanopatched graphene with the improved mechanical durability enables stable operation when used as transparent electrodes of wearable strain sensors. Also, the nanopatched graphene applied as an electrode modulates the molecular orientation of deposited organic semiconductor layers, and yields favorable nominal charge injection for organic transistors. These results demonstrate the potential for use of self-assembled organic nanopatches in graphene-based soft electronics.

Electro-optical interfacial effects on a graphene/π-conjugated organic semiconductor hybrid system

The influence of graphene and retinoic acid (RA) - a π-conjugated organic semiconductor - interface on their hybrid system is investigated. The physical properties of the interface are assessed via scanning probe microscopy, optical spectroscopy (photoluminescence and Raman) and ab initio calculations. The graphene/RA interaction induces the formation of a well-organized π-conjugated self-assembled monolayer (SAM) at the interface. Such structural organization leads to the high optical emission efficiency of the RA SAM, even at room temperature. Additionally, photo-assisted electrical force microscopy, photo-assisted scanning Kelvin probe microscopy and Raman spectroscopy indicate a RA-induced graphene doping and photo-charge generation. Finally, the optical excitation of the RA monolayer generates surface potential changes on the hybrid system. In summary, interface-induced organized structures atop 2D materials may have an important impact on both design and operation of π-conjugated nanomaterial-based hybrid systems.

Electronic transport properties of graphene doped by gallium

In this work we present the effect of low dose gallium (Ga) deposition (<4 ML) performed in UHV (10^-7 Pa) on the electronic doping and charge carrier scattering in graphene grown by chemical vapor deposition. In situ graphene transport measurements performed with a graphene field-effect transistor structure show that at low Ga coverages a graphene layer tends to be strongly n-doped with an efficiency of 0.64 electrons per one Ga atom, while the further deposition and Ga cluster formation results in removing electrons from graphene (less n-doping). The experimental results are supported by the density functional theory calculations and explained as a consequence of distinct interaction between graphene and Ga atoms in case of individual atoms, layers, or clusters.

Laser-Induced Functionalization of Organo/Carbon Interfaces for Selective Adsorption of Au Nanoparticles in Microsized Domains

Laser microprocessing of highly oriented pyrolytic graphite (HOPG) in conjunction with chemical functionalization routines is used to fabricate functional microsized domains. Infrared and Auger electron spectroscopy, contact angle measurements, and electron microscopy are used for characterization of laser-fabricated structures. HOPG samples are coated with alkylsiloxane monolayers. Laser-induced bromination of coated HOPG samples in gaseous bromine is carried out using a microfocused laser beam at a wavelength of 514 nm and 1/e² laser spot diameter of about 2 μm. Subsequent azidation and amination results in functional domains with sizes in the range of 1.2 to 40 μm and more. At low laser powers and irradiation times fully functionalized circular-shaped structures are formed. At high laser powers and irradiation times laser processing results in decomposition of the organic monolayer and substrate in the center of the structures yielding donut-shaped structures. After laser processing and chemical transformation Au nanoparticles are selectively adsorbed onto the functional domains. This provides an opportunity to build up functional nanoparticle microarrays on carbon-based materials, e.g., for applications in sensing and electrocatalysis.

Progress of Large-Scale Synthesis and Electronic Device Application of Two-Dimensional Transition Metal Dichalcogenides

The recent exploration of semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) with atomic thickness has taken both the scientific and technological communities by storm. Extensively investigated TMD that are accessible by large-scale synthetic methods materials are remarkably stable, such as MoS₂ and WSe₂ . They allow superior gate control due to their 2D nature and favorable electronic transport properties, thus suggesting a bright future for digital and RF electronics. In this review, the latest developments in the controlled synthesis of large scale TMDs are firstly introduced by discussing various approaches. The major obstacles that must be overcome to achieve wafer-scale, uniform, and high-quality TMD films for practical electronic applications are included. Advances in the electronic transport studies of TMDs are presented, such as doping, contact engineering, and mobility improvement, which contribute to overall device performance. A perspective and a look at the future for this field is provided in closing.

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High-Performance Solar Cells

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

AC-Impedance Spectroscopic Analysis on the Charge Transport in CVD-Grown Graphene Devices with Chemically Modified Substrates

A comprehensive study for the effect of interfacial buffer layers on the electrical transport behavior in CVD-grown graphene based devices has been performed by ac-impedance spectroscopy (IS) analysis. We examine the effects of the trap charges at graphene/SiO₂ interface on the total capacitance by introducing self-assembled monolayers (SAMs). Furthermore, the charge transports in the polycrystalline graphene are characterized through the temperature-dependent IS measurement, which can be explained by the potential barrier model. The frequency-dependent conduction reveals that the conductivity of graphene is related with the mobility, which is limited by the scattering caused by charged adsorbates on SiO₂ surface.

Site-Specific Chemical Surface Functionalization and Electronic Patterning of Graphene by Electrooxidative Lithography

The combination of different properties being manipulated on nanomaterials is one of the challenges in nanotechnology research. In particular, the possibility to tailor the electronic and chemical properties offers promising possibilities for the design of functional nanostructures. Herein, we report an approach that permits control of these properties on the basis of electrooxidative lithography to structure reduced graphene oxide functionalized with a self-assembled monolayer of n-octadecyltrichlorosilane. The electrochemical oxidation process first induces the formation of polar acid groups on the monolayer, which can be used to covalently bind nanoparticles and molecules and, secondly, also allows the reoxidation of the underlying reduced graphene oxide. As such, the chemical signature as well as the electronic properties of the substrate can be tailored on the micro- and nanometer scale. Details on the oxidation of the monolayer as well as thorough characterization of the electronic properties will be presented. Finally, the approach is used to demonstrate the fabrication of a sensitive glucose sensor device.

Photo-reduction of CO2 Using a Rhenium Complex Covalently Supported on a Graphene/TiO2 Composite

One of the promising solutions for decreasing atmospheric CO2 is artificial photosynthesis, in which CO2 can be photoconverted into solar fuels. In this study, a rhenium complex Re(PyBn)(CO)3 Cl (PyBn=1-(2-picolyl)-4-phenyl-1H-1,2,3-triazole) was covalently grafted onto the surface of reduced graphene oxide (rGO). This was further combined with TiO2 to fabricate a novel catalyst composite TiO2 -rGO-Re(PyBn)(CO)3 Cl for CO2 photo-reduction. This hybrid composite demonstrated high selectivity conversion of CO2 into CO under xenon-lamp irradiation. Compared with the unsupported homogeneous catalyst Re(PyBn)(CO)3 Cl, the covalent immobilized catalyst composite TiO2 -rGO-Re(PyBn)(CO)3 Cl enhanced the turnover number six times and significantly improved catalyst stability. During the process of CO2 photo-reduction, intermediate species with lifetimes longer than hundreds of microseconds were observed and the formation of CO products was revealed using timeresolved infrared spectroscopy. A plausible mechanism for CO2 photo-reduction by the TiO2 -rGO-Re(PyBn)(CO)3 Cl catalyst composite has been suggested. The obtained results have implications for the future design of efficient catalyst composites for CO2 photo-conversion.

Gate-Tunable Dirac Point of Molecular Doped Graphene

Control of the type and density of charge carriers in graphene is essential for its implementation into various practical applications. Here, we demonstrate the gate-tunable doping effect of adsorbed piperidine on graphene. By gradually increasing the amount of adsorbed piperidine, the graphene doping level can be varied from p- to n-type, with the formation of p-n junctions for intermediate coverages. Moreover, the doping effect of the piperidine can be further tuned by the application of large negative back-gate voltages, which increase the doping level of graphene. In addition, the electronic properties of graphene are well preserved due to the noncovalent nature of the interaction between piperidine and graphene. This gate-tunable doping offers an easy, controllable, and nonintrusive method to alter the electronic structure of graphene.

Moving beyond flexible to stretchable conductive electrodes using metal nanowires and graphenes

Stretchable and/or flexible electrodes and their associated electronic devices have attracted great interest because of their possible applications in high-end technologies such as lightweight, large area, wearable, and biointegrated devices. In particular, metal nanowires and graphene derivatives are chosen for electrodes because they show low resistance and high mechanical stability. Here, we review stretchable and flexible soft electrodes by discussing in depth the intrinsic properties of metal NWs and graphenes that are driven by their dimensionality. We investigate these properties with respect to electronics, optics, and mechanics from a chemistry perspective and discuss currently unsolved issues, such as how to maintain high conductivity and simultaneous high mechanical stability. Possible applications of stretchable and/or flexible electrodes using these nanodimensional materials are summarized at the end of this review.

Novel NiCo2S4@reduced graphene oxide@carbon nanotube nanocomposites for high performance supercapacitors

Novel three-dimensional NiCo₂S₄@rGO@CNT nanocomposite electrode material was synthesized.

Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption

MoS2 is considered a promising two-dimensional active channel material for future nanoelectronics. However, the development of a facile, reliable, and controllable doping methodology is still critical for extending the applicability of MoS2. Here, we report surface charge transfer doping via thiol-based binding chemistry for modulating the electrical properties of vacancy-containing MoS2 (v-MoS2). Although vacancies present in 2D materials are generally regarded as undesirable components, we show that the electrical properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol group and sulfur vacancies and by choosing different functional groups. For example, we demonstrate that NH2-containing thiol molecules with lone electron pairs can serve as an n-dopant and achieve a substantial increase of electron density (Δn = 3.7 × 10(12) cm(-2)). On the other hand, fluorine-rich molecules can provide a p-doping effect (Δn = -7.0 × 10(11) cm(-2)) due to its high electronegativity. Moreover, the n- and p-doping effects were systematically evaluated by photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electrical measurement results. The excellent binding stability of thiol molecules and recovery properties by thermal annealing will enable broader applicability of ultrathin MoS2 to various devices.

Engineering Graphene Conductivity for Flexible and High-Frequency Applications

Advances in lightweight, flexible, and conformal electronic devices depend on materials that exhibit high electrical conductivity coupled with high mechanical strength. Defect-free graphene is one such material that satisfies both these requirements and which offers a range of attractive and tunable electrical, optoelectronic, and plasmonic characteristics for devices that operate at microwave, terahertz, infrared, or optical frequencies. Essential to the future success of such devices is therefore the ability to control the frequency-dependent conductivity of graphene. Looking to accelerate the development of high-frequency applications of graphene, here we demonstrate how readily accessible and processable organic and organometallic molecules can efficiently dope graphene to carrier densities in excess of 10(13) cm(-2) with conductivities at gigahertz frequencies in excess of 60 mS. In using the molecule 3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane (F2-HCNQ), a high charge transfer (CT) of 0.5 electrons per adsorbed molecule is calculated, resulting in p-type doping of graphene. n-Type doping is achieved using cobaltocene and the sulfur-containing molecule tetrathiafulvalene (TTF) with a CT of 0.41 and 0.24 electrons donated per adsorbed molecule, respectively. Efficient CT is associated with the interaction between the π electrons present in the molecule and in graphene. Calculation of the high-frequency conductivity shows dispersion-less behavior of the real component of the conductivity over a wide range of gigahertz frequencies. Potential high-frequency applications in graphene antennas and communications that can exploit these properties and the broader impacts of using molecular doping to modify functional materials that possess a low-energy Dirac cone are also discussed.

Graphene Anodes and Cathodes: Tuning the Work Function of Graphene by Nearly 2 eV with an Aqueous Intercalation Process

To expand the applications of graphene in optoelectronics and microelectronics, simple and effective doping processes need to be developed. In this paper, we demonstrate an aqueous process that can simultaneously transfer chemical vapor deposition grown graphene from Cu to other substrates and produce stacked graphene/dopant intercalation films with tunable work functions, which differs significantly from conventional doping methods using vacuum evaporation or spin-coating processes. The work function of graphene layers can be tuned from 3.25 to 5.10 eV, which practically covers the wide range of the anode and cathode applications. Doped graphene films in intercalation structures also exhibit excellent transparency and low resistance. The polymer-based solar cells with either low work function graphene as cathodes or high work function graphene as anodes are demonstrated.

Highly efficient plasmonic enhancement of graphene absorption at telecommunication wavelengths

A hybrid graphene system consisting of graphene and silica layers coated on a metal film with groove rings is proposed to strongly enhance light absorption in the graphene layer. Our results indicate that the excited localized plasmon resonance in groove rings can effectively improve the graphene absorption from 2.3% to 43.1%, even to a maximum value of 87.0% in five-layer graphene at telecommunication wavelengths. In addition, the absorption peak is strongly dependent on the groove depth and ring radius as well as the number of graphene layers, enabling the flexible selectivity of both the operating spectral position and bandwidth. This favorable enhancement and tunability of graphene absorption could provide a path toward high-performance graphene opto-electronic components, such as photodetectors.

Facile fabrication of silica–polymer–graphene collaborative nanostructure-based hybrid materials with high conductivity and robust mechanical performance

SiO₂@poly(methyl methacrylate)–reduced graphene oxide composites with outstanding thermal stability, robust mechanical performance and excellent conductivity have been prepared by dispersion polymerization and electrostatic assembly.

Inverse transfer method using polymers with various functional groups for controllable graphene doping

The polymer-supported transfer of chemical vapor deposition (CVD)-grown graphene provides large-area and high-quality graphene on a target substrate; however, the polymer and organic solvent residues left by the transfer process hinder the application of CVD-grown graphene in electronic and photonic devices. Here, we describe an inverse transfer method (ITM) that permits the simultaneous transfer and doping of graphene without generating undesirable residues by using polymers with different functional groups. Unlike conventional wet transfer methods, the polymer supporting layer used in the ITM serves as a graphene doping layer placed at the interface between the graphene and the substrate. Polymers bearing functional groups can induce n-doping or p-doping into the graphene depending on the electron-donating or -withdrawing characteristics of functional groups. Theoretical models of dipole layer-induced graphene doping offered insights into the experimentally measured change in the work function and the Dirac point of the graphene. Finally, the electrical properties of pentacene field effect transistors prepared using graphene electrodes could be enhanced by employing the ITM to introduce a polymer layer that tuned the work function of graphene. The versatility of polymer functional groups suggests that the method developed here will provide valuable routes to the development of applications of CVD-grown graphene in organic electronic devices.

Imaging and tuning molecular levels at the surface of a gated graphene device

Gate-controlled tuning of the charge carrier density in graphene devices provides new opportunities to control the behavior of molecular adsorbates. We have used scanning tunneling microscopy (STM) and spectroscopy (STS) to show how the vibronic electronic levels of 1,3,5-tris(2,2-dicyanovinyl)benzene molecules adsorbed onto a graphene/BN/SiO2 device can be tuned via application of a backgate voltage. The molecules are observed to electronically decouple from the graphene layer, giving rise to well-resolved vibronic states in dI/dV spectroscopy at the single-molecule level. Density functional theory (DFT) and many-body spectral function calculations show that these states arise from molecular orbitals coupled strongly to carbon-hydrogen rocking modes. Application of a back-gate voltage allows switching between different electronic states of the molecules for fixed sample bias.

Microscopic signature of metallic state in semicrystalline conjugated polymers doped with fluoroalkylsilane molecules

Fluoroalkylsilane (FTS) acts as an efficient p-type dopant for organic semiconductors. FTS-doped films of the semicrystalline PBTTT polymer exhibit relatively high conductivities. We demonstrate that highly doped PBTTT films exhibit a metallic nature with clear Pauli paramagnetism as observed microscopically using electron spin resonance spectroscopy. The metallic state is realized within crystalline grains, as confirmed from the anisotropy of the ESR signal.

Tailoring the physical properties of molybdenum disulfide monolayers by control of interfacial chemistry

We demonstrate how substrate interfacial chemistry can be utilized to tailor the physical properties of single-crystalline molybdenum disulfide (MoS2) atomic-layers. Semiconducting, two-dimensional MoS2 possesses unique properties that are promising for future optical and electrical applications for which the ability to tune its physical properties is essential. We use self-assembled monolayers with a variety of end termination chemistries to functionalize substrates and systematically study their influence on the physical properties of MoS2. Using electrical transport measurements, temperature-dependent photoluminescence spectroscopy, and empirical and first-principles calculations, we explore the possible mechanisms involved. Our data shows that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS2. These findings can be used to effectively enhance or modulate the conductivity, field-effect mobility, and photoluminescence in MoS2 monolayers, illustrating an approach for local and universal property modulations in two-dimensional atomic-layers.

Work-function-tuned reduced graphene oxide via direct surface functionalization as source/drain electrodes in bottom-contact organic transistors

RGO electrodes with work functions that can be widely tuned using direct surface functionalization are demonstrated by self-assembled monolayers anchored onto the surfaces of the RGO electrodes, which can remarkably enhance the device performance of organic field-effect transistors.

Carrier control of MoS2 nanoflakes by functional self-assembled monolayers

Carrier doping of MoS2 nanoflakes was achieved by functional self-assembled monolayers (SAMs) with different dipole moments. The effect of SAMs on the charge transfer between the substrates and MoS2 nanoflakes was studied by Raman spectroscopy, field-effect transistor (FET) measurements, and Kelvin probe microscope (KFM). Raman data and FET results verified that fluoroalkyltrichlorosilane-SAM with a large positive dipole moment, acting as hole donors, significantly reduced the intrinsic n-doping characteristic of MoS2 nanoflakes, while 3-(trimethoxysilyl)-1-propanamine-SAMs, acting as electron donors, enhanced the n-doping characteristic. The additional built-in electric field at the interface between SiO2 substrates and MoS2 nanoflakes induced by SAMs with molecular dipole moments determined the charge transfer process. KFM results clearly demonstrated the charge transfer between MoS2 and SAMs and the obvious interlayer screening effect of the pristine and SAM-modified MoS2 nanoflakes. However, the KFM results were not fully consistent with the Raman and FET results since the externally absorbed water molecules were shown to partially shield the actual surface potential measurement. By eliminating the contribution of the water molecules, the Fermi level of monolayer MoS2 could be estimated to modulate in a range of more than 0.45-0.47 eV. This work manifests that the work function of MoS2 nanoflakes can be significantly tuned by SAMs by virtue of affecting the electrostatic potential between the substrates and MoS2 nanoflakes.

Graphene-related nanomaterials: tuning properties by functionalization

In this review, we discuss the most recent progress on graphene-related nanomaterials, including doped graphene and derived graphene nanoribbons, graphene oxide, graphane, fluorographene, graphyne, graphdiyne, and porous graphene, from both experimental and theoretical perspectives, and emphasize tuning their stability, electronic and magnetic properties by chemical functionalization.

Films of Carbon Nanomaterials for Transparent Conductors

The demand for transparent conductors is expected to grow rapidly as electronic devices, such as touch screens, displays, solid state lighting and photovoltaics become ubiquitous in our lives. Doped metal oxides, especially indium tin oxide, are the commonly used materials for transparent conductors. As there are some drawbacks to this class of materials, exploration of alternative materials has been conducted. There is an interest in films of carbon nanomaterials such as, carbon nanotubes and graphene as they exhibit outstanding properties. This article reviews the synthesis and assembly of these films and their post-treatment. These processes determine the film performance and understanding of this platform will be useful for future work to improve the film performance.

Graphene Films for Flexible Organic and Energy Storage Devices

Graphene and its derivatives have been the subject of extensive research in fundamental science and have viable applications in current and future technology. The exceptionally high electronic and thermal conductivity, optical transparency, and high specific surface area, combined with excellent mechanical flexibility and environmental stability leave graphene poised to be a material of the future. This perspective introduces the importance of graphene electrodes, discusses the synthesis of graphene and transfer onto desired substrates and the role of graphene in electrodes for a broad range of flexible devices such as photovoltaic, electronic, and electrochemical energy storage.

Anisotropic crystalline organic step-flow growth on deactivated Si surfaces

We report the first demonstration of anisotropic step-flow growth of organic molecules on a semiconducting substrate using metal phthalocyanine thermally deposited on the deactivated Si(111)-B sqrt[3]×sqrt[3] R30° surface. With scanning probe microscopy and geometric modeling, we prove the quasiepitaxial nature of this step-flow growth that exhibits no true commensurism, despite a single dominant long-range ordered relationship between the organic crystalline film and the substrate, uniquely distinct from inorganic epitaxial growth. This growth mode can likely be generalized for a range of organic molecules on deactivated Si surfaces and access to it offers new potential for the integration of ordered organic thin films in silicon-based electronics.

Transformation of the electrical characteristics of graphene field-effect transistors with fluoropolymer

We report on the improvement of the electronic characteristics of monolayer graphene field-effect transistors (FETs) by an interacting capping layer of a suitable fluoropolymer. Capping of monolayer graphene FETs with CYTOP improved the on-off current ratio from 5 to 10 as well as increased the field-effect mobility by as much as a factor of 2 compared to plain graphene FETs. Favorable shifts in the Dirac voltage toward zero with shift magnitudes in excess of 60 V are observed. The residual carrier concentration is reduced to ~2.8 × 10(11) cm(-2). Removal of the fluoropolymer from graphene FETs results in a return to the initial electronic properties before depositing CYTOP. This suggests that weak, reversible electronic perturbation of graphene by the fluoropolymer favorably tune the electrical characteristics of graphene, and we hypothesize that the origin of this improvement is in the strongly polar nature of the C-F chemical bonds that self-organize upon heat treatment. We demonstrate a general method to favorably restore or transform the electrical characteristics of graphene FETs, which will open up new applications.

Graphene-PAMAM dendrimer-gold nanoparticle composite for electrochemical DNA hybridization detection

Graphene oxide is chemically functionalized using planar structured first generation polyamidoamine dendrimer (G1PAMAM) to form graphene core GG1PAMAM. The monolayer of GG1PAMAM is anchored on the 3-mercapto propionic acid monolayer pre-immobilized onto a gold transducer. The GG1PAMAM is decorated using gold nanoparticles for the covalent attachment of single-stranded DNA through simple gold-thiol chemistry. The single- and double-stranded DNAs are discriminated electrochemically in the presence of redox probe K3[Fe(CN)6]. Double-stranded-specific intercalator methylene blue is used to enhance the lower detection limit. The use of linear and planar G1PAMAM along with the graphene core has enhanced the detection limit 100 times higher than the G1PAMAM with the conventional ethylene core. This chapter presents the details of GG1PAMAM preparation and application to DNA sensing by electrochemical methods.