Locally altering the electronic properties of graphene by nanoscopically doping it with Rhodamine 6G

Spatial and Bidirectional Work Function Modulation of Monolayer Graphene with Patterned Polymer "Fluorozwitterists"

Understanding the electronic properties resulting from soft-hard material interfacial contact has elevated the utility of functional polymers in advanced materials and nanoscale structures, such as in work function engineering of two-dimensional (2D) materials to produce new types of high-performance devices. In this paper, we describe the electronic impact of functional polymers, containing both zwitterionic and fluorocarbon components in their side chains, on the work function of monolayer graphene through the preparation of negative-tone photoresists, which we term "fluorozwitterists." The zwitterionic and fluorinated groups each represent dipole-containing moieties capable of producing distinct surface energies as thin films. Kelvin probe force microscopy revealed these polymers to have a p-doping effect on graphene, which contrasts the work function decrease typically associated with polymer-to-graphene contact. Copolymerization of fluorinated zwitterionic monomers with methyl methacrylate and a benzophenone-substituted methacrylate produced copolymers that were amenable to photolithographic fabrication of fluorozwitterist structures. Consequently, spatial alteration of zwitterion coverage across graphene yielded stripes that resemble a lateral p-i-n diode configuration, with local increase or decrease of work function. Overall, this polymeric fluorozwitterist design is suitable for enabling simple, solution-based surface patterning and is anticipated to be useful for spatial work function modulation of 2D materials integrated into electronic devices.

Pulsed Force Kelvin Probe Force Microscopy through Integration of Lock-In Detection

Kelvin probe force microscopy measures surface potential and delivers insights into nanoscale electronic properties, including work function, doping levels, and localized charges. Recently developed pulsed force Kelvin probe force microscopy (PF-KPFM) provides sub-10 nm spatial resolution under ambient conditions, but its original implementation is hampered by instrument complexity and limited operational speed. Here, we introduce a solution for overcoming these two limitations: a lock-in amplifier-based PF-KPFM. Our method involves phase-synchronized switching of a field effect transistor to mediate the Coulombic force between the probe and the sample. We validate its efficacy on two-dimensional material MXene and aged perovskite photovoltaic films. Lock-in-based PF-KPFM successfully identifies the contact potential difference (CPD) of stacked flakes and finds that the CPDs of monoflake MXene are different from those of their multiflake counterparts, which are otherwise similar in value. In perovskite films, we uncover electrical degradation that remains elusive with surface topography.

DNA aptamer selection and construction of an aptasensor based on graphene FETs for Zika virus NS1 protein detection

Zika virus (ZIKV) is a mosquito-borne virus that is phylogenetically close to other medically important flaviviruses with high global public health significance, such as dengue (DENV) and yellow fever (YFV) viruses. Correct diagnosis of a flavivirus infection can be challenging, particularly in world regions where more than one flavivirus co-circulates and YFV vaccination is mandatory. Acid nucleic aptamers are oligonucleotides that bind to a specific target molecule with high affinity and specificity. Because of their unique characteristics, aptamers are promising tools for biosensor development. Aptamers are usually obtained through a procedure called "systematic evolution of ligands by exponential enrichment" (SELEX). In this study, we select an aptamer (termed ZIKV60) by capillary electrophoresis SELEX (CE-SELEX) to the Zika virus non-structural protein 1 (NS1) and counterselection against the NS1 proteins of DENV (serotypes 1, 2, 3, and 4) and YFV. The ZIKV60 dissociation constant (K _d) is determined by enzyme-linked oligonucleotide assay (ELONA) and the aptamer specificity is evaluated by quantitative real-time polymerase chain reaction. ZIKV60 shows a high binding affinity to the ZIKV NS1 protein with a K _d value of 2.28 ± 0.28 nM. The aptamer presents high specificity for ZIKV NS1 compared to NS1 of DENV and YFV. Furthermore, graphene field-effect transistor devices functionalized with ZIKV60 exhibit an evident identification of NS1 protein diluted in human serum. These results point to the applicability of biosensors based on the ZIKV60 aptamer for the differential diagnosis of the Zika virus.

Scanning Probe Lithography: State-of-the-Art and Future Perspectives

High-throughput and high-accuracy nanofabrication methods are required for the ever-increasing demand for nanoelectronics, high-density data storage devices, nanophotonics, quantum computing, molecular circuitry, and scaffolds in bioengineering used for cell proliferation applications. The scanning probe lithography (SPL) nanofabrication technique is a critical nanofabrication method with great potential to evolve into a disruptive atomic-scale fabrication technology to meet these demands. Through this timely review, we aspire to provide an overview of the SPL fabrication mechanism and the state-the-art research in this area, and detail the applications and characteristics of this technique, including the effects of thermal aspects and chemical aspects, and the influence of electric and magnetic fields in governing the mechanics of the functionalized tip interacting with the substrate during SPL. Alongside this, the review also sheds light on comparing various fabrication capabilities, throughput, and attainable resolution. Finally, the paper alludes to the fact that a majority of the reported literature suggests that SPL has yet to achieve its full commercial potential and is currently largely a laboratory-based nanofabrication technique used for prototyping of nanostructures and nanodevices.

Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides

We investigated the nature of graphene surface doping by zwitterionic polymers and the implications of weak in-plane and strong through-plane screening using a novel sample geometry that allows direct access to either the graphene or the polymer side of a graphene/polymer interface. Using both Kelvin probe and electrostatic force microscopies, we observed a significant upshift in the Fermi level in graphene of ∼260 meV that was dominated by a change in polarizability rather than pure charge transfer with the organic overlayer. This physical picture is supported by density functional theory (DFT) calculations, which describe a redistribution of charge in graphene in response to the dipoles of the adsorbed zwitterionic moieties, analogous to a local DC Stark effect. Strong metallic-like screening of the adsorbed dipoles was observed by employing an inverted geometry, an effect identified by DFT to arise from a strongly asymmetric redistribution of charge confined to the side of graphene proximal to the zwitterion dipoles. Transport measurements confirm n-type doping with no significant impact on carrier mobility, thus demonstrating a route to desirable electronic properties in devices that combine graphene with lithographically patterned polymers.

Photogating in the Graphene-Dye-Graphene Sandwich Heterostructure

In this work, we developed an atomically thin (∼2.5 nm) heterostructure consisting of a monolayer rhodamine 6G (R6G) film as a photoactive layer that was sandwiched between graphene films functioning as channels (graphene-R6G-graphene, G-R-G). Through a comparison of results of both photocurrent measurements and chemically enhanced Raman scattering (CERS) experiments, we found that our G-R-G heterostructure exhibited ∼7 and ∼30 times better performance than R6G-attached single-graphene (R6G-graphene, R-G) and MoS₂ devices, respectively; here, the CERS enhancement factor was highly correlated with the relative photoinduced Dirac voltage change. Furthermore, the photocurrent of the G-R-G device was found to be ∼40 times better than that of the R-G photodetector. The top graphene was highly operative in the monolayer, of which the performance is significantly deteriorated by fluorescence and tailored charge transfer efficiency with the increment of R6G film thickness. Overall, the responsivity of the G-R-G photodetector was ∼40 times higher than that of the R-G photodetector because of the more efficient carrier transfer between the organic dye and graphene induced by weaker π-π interactions between the top and bottom graphene channels in the former device. This atomically thin (∼2.5 nm) and highly photosensitive photodetector can be employed for post-Si-photodiode (PD) image sensors, single-photon detection devices, and optical communications.

Modulating the Surface State of SiC to Control Carrier Transport in Graphene/SiC

Silicon carbide (SiC) with epitaxial graphene (EG/SiC) shows a great potential in the applications of electronic and photoelectric devices. The performance of devices is primarily dependent on the interfacial heterojunction between graphene and SiC. Here, the band structure of the EG/SiC heterojunction is experimentally investigated by Kelvin probe force microscopy. The dependence of the barrier height at the EG/SiC heterojunction to the initial surface state of SiC is revealed. Both the barrier height and band bending tendency of the heterojunction can be modulated by controlling the surface state of SiC, leading to the tuned carrier transport behavior at the EG/SiC interface. The barrier height at the EG/SiC(000-1) interface is almost ten times that of the EG/SiC(0001) interface. As a result, the amount of carrier transport at the EG/SiC(000-1) interface is about ten times that of the EG/SiC(0001) interface. These results offer insights into the carrier transport behavior at the EG/SiC heterojunction by controlling the initial surface state of SiC, and this strategy can be extended in all devices with graphene as the top layer.

Graphene Nanoribbon Based Thermoelectrics: Controllable Self- Doping and Long-Range Disorder

Control of both the regularity of a material ensemble and nanoscale architecture provides unique opportunities to develop novel thermoelectric applications based on 2D materials. As an example, the authors explore the electronic and thermal properties of functionalized graphene nanoribbons (GNRs) in the single-sheet and helical architectures using multiscale simulations. The results suggest that appropriate functionalization enables precise tuning of the doping density in a planar donor/acceptor GNR ensemble without the need to introduce an explicit dopant, which is critical to the optimization of power factor. In addition, the self-interaction between turns of a GNR may induce long-range disorder along the helical axis, which suppresses the thermal contribution from phonons with long wavelengths, leading to anomalous length independent phonon thermal transport in the quasi-1D system.

Attoliter Chemistry for Nanoscale Functionalization of Graphene

The nanoscale, multiplexed functionalization of graphene in a device array is a critical step to realize graphene-based chemical and biosensors. We demonstrate that graphene can be functionalized with submicron resolution and in well-defined locations and patterns using reaction agents in attoliter quantities, utilizing dip-pen nanolithography or microchannel cantilever spotting. Specifically, we functionalize graphene with a biotin azide using click-chemistry and demonstrate the subsequent binding of fluorescently tagged streptavidin. The technique can be scaled up to multiplex functionalize graphene devices on a wafer-scale for sensor and biomedical applications.

Organic Dye-Sensitized CH3NH3PbI3 Hybrid Flexible Photodetector with Bulk Heterojunction Architectures

A flexible photodetector based on the bulk heterojunction of an organometallic halide perovskites CH₃NH₃PbI₃ and an organic dye Rhodamine B (RhB) has been fabricated via a solution casting process. It showed a high responsivity (R_max = 43.6 mA/W) to visible lights, short response time (t_r ≈ 60 ms, t_d ≈ 40 ms), high on-off ratio (I_on/I_off ≈ 287) and satisfactory stability because of its Schottky barrier structure and the dye enhanced light absorption.

Reactions in ultra-small droplets by tip-assisted chemistry

The confinement of chemical reactions within small droplets has received much attention in the last few years. This approach has been proved successful for the in-depth study of naturally occurring chemical processes as well as for the synthesis of different sets of nanomaterials with control over their size, shape and properties. Different approaches such as the use of self-contained structures or microfluidic generated droplets have been followed over the years with success. However, novel approaches have emerged during the last years based on the deposition of femtolitre-sized droplets on surfaces using tip-assisted lithographic methods. In this feature article, we review the advances made towards the use of these ultra-small droplets patterned on surfaces as confined nano-reactors.

Clean Transfer of Wafer-Scale Graphene via Liquid Phase Removal of Polycyclic Aromatic Hydrocarbons

Pentacene (C22H14), a polycyclic aromatic hydrocarbon, was used as both supporting and sacrificing layers for the clean and doping-free graphene transfer. After successful transfer of graphene to a target substrate, the pentacene layer was physically removed from the graphene surface by using intercalating organic solvent. This solvent-mediated removal of pentacene from graphene surface was investigated by both theoretical calculation and experimental studies with various solvents. The uses of pentacene and appropriate intercalation solvent enabled graphene transfer without forming a residue from the supporting layer. Such residues tend to cause charged impurity scattering and unintentional graphene doping effects. As a result, this clean graphene exhibited extremely homogeneous surface potential profiles over a large area. A field-effect transistor fabricated using this graphene displayed a high hole (electron) mobility of 8050 cm(2)/V·s (9940 cm(2)/V·s) with a nearly zero Dirac point voltage.

Electrically configurable graphene field-effect transistors with a graded-potential gate

A device architecture for electrically configurable graphene field-effect transistor (GFET) using a graded-potential gate is present. The gating scheme enables a linearly varying electric field that modulates the electronic structure of graphene and causes a continuous shift of the Dirac points along the channel of GFET. This spatially varying electrostatic modulation produces a pseudobandgap observed as a suppressed conductance of graphene within a controllable energy range. By tuning the electrical gradient of the gate, a GFET device is reversibly transformed between ambipolar and n- and p-type unipolar characteristics. We further demonstrate an electrically programmable complementary inverter, showing the extensibility of the proposed architecture in constructing logic devices based on graphene and other Dirac materials. The electrical configurable GFET might be explored for novel functionalities in smart electronics.

Suppressing Klein tunneling in graphene using a one-dimensional array of localized scatterers

Graphene's unique physical and chemical properties make it an attractive platform for use in micro- and nanoelectronic devices. However, electrostatically controlling the flow of electrons in graphene can be challenging as a result of Klein tunneling, where electrons normally incident to a one-dimensional potential barrier of height V are perfectly transmitted even as V → ∞. In this study, theoretical and numerical calculations predict that the transmission probability for an electron wave normally incident to a one-dimensional array of localized scatterers can be significantly less than unity when the electron wavelength is smaller than the spacing between scatterers. In effect, placing periodic openings throughout a potential barrier can, somewhat counterintuitively, decrease transmission in graphene. Our results suggest that electrostatic potentials with spatial variations on the order of the electron wavelength can suppress Klein tunneling and could find applications in developing graphene electronic devices.

Nanostructuring graphene for controlled and reproducible functionalization

The 'graphene rush' that started almost a decade ago is far from over. The dazzling properties of graphene have long warranted a number of applications in various domains of science and technology. Harnessing the exceptional properties of graphene for practical applications however has proved to be a massive task. Apart from the challenges associated with the large-scale production of the material, the intrinsic zero band gap, the inherently low reactivity and solubility of pristine graphene preclude its use in several high- as well as low-end applications. One of the potential solutions to these problems is the surface functionalization of graphene using organic building blocks. The 'surface-only' nature of graphene allows the manipulation of its properties not only by covalent chemical modification but also via non-covalent interactions with organic molecules. Significant amount of research efforts have been directed towards the development of functionalization protocols for modifying the structural, electronic, and chemical properties of graphene. This feature article provides a glimpse of recent progress in the molecular functionalization of surface supported graphene using non-covalent as well as covalent chemistry.

Patterning of Quantum Dots by Dip-Pen and Polymer Pen Nanolithography

We present a direct way of patterning CdSe/ ZnS quantum dots by dip-pen nanolithography and polymer pen lithography. Mixtures of cholesterol and phospholipid 1,2-dioleoyl-sn-glycero-3 phosphocholine serve as biocompatible carrier inks to facilitate the transfer of quantum dots from the tips to the surface during lithography. While dip-pen nanolithography of quantum dots can be used to achieve higher resolution and smaller pattern features (approximately 1 μm), polymer pen lithography is able to address intermediate pattern scales in the low micrometre range. This allows us to combine the advantages of micro contact printing in large area and massive parallel patterning, with the added flexibility in pattern design inherent in the DPN technique.

Doping graphene with an atomically thin two dimensional molecular layer

Atomically thin and chemically versatile GO sheets are used as p-type dopants of CVD-graphene. This method enables the strong, stable, large-scale, low-temperature, and controllable p-doping of graphene with preserved charge mobility, intrinsic roughness, and transmittance.

Classic, liquid, and matrix-assisted dip-pen nanolithography for materials research

As a powerful atomic force microscopy-based nanotechnological tool, dip-pen nanolithography (DPN) has provided an ideal direct-write "constructive" lithographic tool that allows materials to be patterned from DPN tips onto a surface with high registration and sub-15 nm resolution. In the past few decades, DPN has been enormously developed for studying the patterning of inorganic, organic, and biological materials onto a variety of substrates. The focus of this review is on the development of three types of DPN: classic, liquid, and matrix-assisted DPN. Such development mainly includes the following aspects: the comparisons of three types of DPN, the effect factors and basic mechanisms of three types of DPN, and the application progress of three types of DPN.

Multiplexed biomimetic lipid membranes on graphene by dip-pen nanolithography

The application of graphene in sensor devices depends on the ability to appropriately functionalize the pristine graphene. Here we show the direct writing of tailored phospholipid membranes on graphene using dip-pen nanolithography. Phospholipids exhibit higher mobility on graphene compared with the commonly used silicon dioxide substrate, leading to well-spread uniform membranes. Dip-pen nanolithography allows for multiplexed assembly of phospholipid membranes of different functionalities in close proximity to each other. The membranes are stable in aqueous environments and we observe electronic doping of graphene by charged phospholipids. On the basis of these results, we propose phospholipid membranes as a route for non-covalent immobilization of various functional groups on graphene for applications in biosensing and biocatalysis. As a proof of principle, we demonstrate the specific binding of streptavidin to biotin-functionalized membranes. The combination of atomic force microscopy and binding experiments yields a consistent model for the layer organization within phospholipid stacks on graphene.

The sensitivity and selectivity of graphene-based biosensors depends on attaching various functional groups to graphene. Hirtz et al. use dip-pen nanolithography to directly write phospholipid membranes on graphene, which enables multiplexed and heterogeneous non-covalent functionalization.