Conduction tuning of graphene based on defect-induced localization

Controllable defects in monolayer graphene induced by hydrogen and argon plasma

Graphene has attracted wide attentions since its successfully exfoliation. Honeycombsp2carbon lattice and Dirac semi-metal band structure make graphene a promising material with excellent mechanical strength, thermal conductivity, and carrier mobility. However, the absence of intrinsic bandgap limits its application in semiconductor. Defects in graphene is supposed to modify its band structure and lead to an opened bandgap. Many methods have been demonstrated to introduce defects into graphene, such as chemical reaction, plasma, electron beam, and laser. However, the species of defects are mostly uncontrollable in most treatment processes. In this study, we report three kinds of defects can be controllably induced in graphene via hydrogen (H2) and argon (Ar) plasma. With different parameter and feeding gas, hydrogenated graphene, graphene nanomesh and graphene with vacancies can be well obtained. The defect density can be precisely controlled by tuning plasma power and irradiation time. Morphological, spectroscopic, and electrical characterizations are performed to systematically investigate the defect evolution. Graphene nanomesh and graphene with vacancies show obvious difference for roughness and coverage, whereas the morphology of hydrogenated graphene remains similar with that of as-prepared graphene. For hydrogenated graphene, an opened bandgap of ∼20 meV is detected. For graphene nanomesh and graphene with vacancies, the semiconductive on/off behaviors are observed. We believe this work can provide more details of plasma-induced defects and assist the application of graphene in semiconductor industry.

The effect of charged particle irradiation on the transport properties of bismuth chalcogenide topological insulators: a brief review

Topological insulators (TIs) offer a novel platform for achieving exciting applications, such as low-power electronics, spintronics, and quantum computation. Hence, the spin-momentum locked and topologically nontrivial surface state of TIs is highly coveted by the research and development industry. Particle irradiation in TIs is a fast-growing field of research owing to the industrial scalability of the particle irradiation technique. Unfortunately, real three-dimensional TI materials, such as bismuth selenide, invariably host a significant population of charged native defects, which cause the ideally insulating bulk to behave like a metal, masking the relatively weak signatures of metallic topological surface states. Particle irradiation has emerged as an effective technique for Fermi energy tuning to achieve an insulating bulk in TI along with other popularly practiced methods, such as substitution doping and electrical gating. Irradiation methods have been used for many years to enhance the thermoelectric properties of bismuth chalcogenides, predominantly by increasing carrier density. In contrast, uncovering the surface states in bismuth-based TI requires the suppression of carrier density via particle irradiation. Hence, the literature on the effect of irradiation on bismuth chalcogenides extends widely to both ends of the spectrum (thermoelectric and topological properties). This review attempts to collate the available literature on particle irradiation-driven Fermi energy tuning and the modification of topological surface states in TI. Recent studies on particle irradiation in TI have focused on precise local modifications in the TI system to induce magnetic topological ordering and surface selective topological superconductivity. Promising proposals for TI-integrated circuits have also been put forth. The eclectic range of irradiation-based studies on TI has been reviewed in this manuscript.

Recent progresses on ion beam irradiation induced structure and performance modulation of two-dimensional materials

Two-dimensional (2D) materials are receiving significant attention for both fundamental research and industrial applications due to their unparalleled properties and wide application potential. In this case, the controllable modulation of their structures and properties is essential for the realization and further expansion of their applications. Accordingly, ion beam irradiation techniques, with large scope to adjust parameters, high manufacturing resolution, and a series of advanced equipment being developed, have been demonstrated to have obvious advantages in manipulating the structure and performance of 2D materials. In recent years, many research efforts have been devoted to uncovering the underlying mechanism and control rules regarding ion irradiation induced phenomena in 2D materials, aiming at fulfilling their application potential as soon as possible. Herein, we review the research progress in the interaction between energetic ions and 2D materials based on the energy transfer model, type of ion source, structural modulation, performance modification of 2D materials, and then their application status, aiming to provide useful information for researchers in this field and stimulating more research advances.

Engineering high quality graphene superlattices via ion milled ultra-thin etching masks

Nanofabrication research pursues the miniaturization of patterned feature size. In the current state of the art, micron scale areas can be patterned with features down to ~30 nm pitch using electron beam lithography. Here, we demonstrate a nanofabrication technique which allows patterning periodic structures with a pitch down to 16 nm. It is based on focused ion beam milling of suspended membranes, with minimal proximity effects typical to standard electron beam lithography. The membranes are then transferred and used as hard etching masks. We benchmark our technique by electrostatically inducing a superlattice potential in graphene and observe bandstructure modification in electronic transport. Our technique opens the path towards the realization of very short period superlattices in 2D materials, but with the ability to control lattice symmetries and strength. This can pave the way for a versatile solid-state quantum simulator platform and the study of correlated electron phases.

Electron Transport Tuning of Graphene by Helium Ion Irradiation

This article reviews charge carrier transport phenomena in single-layer graphene, which generates crystalline defects through helium-ion-beam irradiation using a helium-ion microscope. Crystalline defects act as electron scatterers, and the conductivity decays drastically with increase in ion dose. Real-time conductivity monitoring during ion beam scans over the graphene surface is also presented. Negative magnetoresistance is observed in defective graphene in cryogenic measurements under magnetic fields, suggesting that strong localisation occurred in this two-dimensional electron system, which survived even at room temperature. The localised state also contributes to inducing a transport gap around the Dirac point, where the density of states is at its minimum, and it enables field-effect control of the carrier transport by tuning the carrier density. The fabrication and operation of field-effect transistors with defective graphene channels are also demonstrated.

In Situ Defect Engineering Route to Optimize the Cationic Redox Activity of Layered Double Hydroxide Nanosheet via Strong Electronic Coupling with Holey Substrate

A defect engineering of inorganic solids garners great deal of research activities because of its high efficacy to optimize diverse energy-related functionalities of nanostructured materials. In this study, a novel in situ defect engineering route to maximize electrocatalytic redox activity of inorganic nanosheet is developed by using holey nanostructured substrate with strong interfacial electronic coupling. Density functional theory calculations and in situ spectroscopic analyses confirm that efficient interfacial charge transfer takes place between holey TiN and Ni-Fe-layered double hydroxide (LDH), leading to the feedback formation of nitrogen vacancies and a maximization of cation redox activity. The holey TiN-LDH nanohybrid is found to exhibit a superior functionality as an oxygen electrocatalyst and electrode for Li-O2 batteries compared to its non-holey homologues. The great impact of hybridization-driven vacancy introduction on the electrochemical performance originates from an efficient electrochemical activation of both Fe and Ni ions during electrocatalytic process, a reinforcement of interfacial electronic coupling, an increase in electrochemical active sites, and an improvement in electrocatalysis/charge-transfer kinetics.

Localization to delocalization probed by magnetotransport of hBN/graphene/hBN stacks in the ultra-clean regime

We report on magnetotransport in a high-quality graphene device, which is based on monolayer graphene (Gr) encapsulated by hexagonal boron nitride (hBN) layers, i.e., hBN/Gr/hBN stacks. In the vicinity of the Dirac point, a negative magnetoconductance is observed for high temperatures >  ~ 40 K, whereas it becomes positive for low temperatures ≤  ~ 40 K, which implies an interplay of quantum interferences in Dirac materials. The elastic scattering mechanism in hBN/Gr/hBN stacks contrasts with that of conventional graphene on SiO₂, and our ultra-clean graphene device shows nonzero magnetoconductance for high temperatures of up to 300 K.

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

The helium ion microscope has emerged as a multifaceted instrument enabling a broad range of applications beyond imaging in which the finely focused helium ion beam is used for a variety of defect engineering, ion implantation, and nanofabrication tasks. Operation of the ion source with neon has extended the reach of this technology even further. This paper reviews the materials modification research that has been enabled by the helium ion microscope since its commercialization in 2007, ranging from fundamental studies of beam-sample effects, to the prototyping of new devices with features in the sub-10 nm domain.

Controlling the macroscopic electrical properties of reduced graphene oxide by nanoscale writing of electronic channels

The allure of all-carbon electronics stems from the spread of its physical properties across all its allotropes. The scheme also harbours unique challenges, such as tunability of band gap, variability of doping and defect control. Here, we explore the technique of scanning probe tip-induced nanoscale reduction of graphene oxide (GO), which nucleates conducting, [Formula: see text] rich graphitic regions on the insulating GO background. The flexibility of direct writing is supplemented with control over the degree of reduction and tunability of band gap through macroscopic control parameters. The fabricated reduced GO channels and ensuing devices are investigated via spectroscopy and temperature and bias-dependent electrical transport and correlated with spatially resolved electronic properties, using surface potentiometry. The presence of carrier localization effects, induced by the phase-separated [Formula: see text] domains, and large local electric field fluctuations are reflected in the non-linear transport across the channels. Together, the results indicate a complex transport phenomenon, which may be variously dominated by tunnelling or variable range hopping or activated depending on the electronic state of the material.

Local heat generated by a focused He+ ion beam

A focused He⁺ beam with a beam diameter less than 1 nm is heating up the targeted surface for a He⁺ dose larger than 1 × 10²⁰ ions cm^-2. The temperature can reach 1000 °C locally, resulting in surface decomposition or chemical reactions. This temperature was measured by fabricating gold nanodisks down to 20 nm in diameter and 10 nm in thickness on mica and sapphire surfaces. The melting and vaporization of these nanodisks were used to determine this temperature according to the known gold nanocluster melting temperature variations as a function of their volume. This local heat production is very negative for precise He⁺ nanolithography resist processes but advantageous without a resist when used to directly nano-sculpture a nanomaterial when a very thermally conductive support like sapphire is selected.

Conductance Tunable Suspended Graphene Nanomesh by Helium Ion Beam Milling

This paper demonstrates that the electrical properties of suspended graphene nanomesh (GNM) can be tuned by systematically changing the porosity with helium ion beam milling (HIBM). The porosity of the GNM is well-controlled by defining the pitch of the periodic nanopores. The defective region surrounding the individual nanopores after HIBM, which limits the minimum pitch achievable between nanopores for a certain dose, is investigated and reported. The exponential relationship between the thermal activation energy (E_A) and the porosity is found in the GNM devices. Good E_A tuneability observed from the GNMs provides a new approach to the transport gap engineering beyond the conventional nanoribbon method.

Near-field imaging of graphene triangles patterned by helium ion lithography

Plasmon nanoresonators in graphene have many applications in biosensing, photodetectors and modulators. As a result, an efficient and precise patterning technique for graphene is required. Helium ion lithography (HIL) emerges as a promising tool for direct writing fabrication because it owns improved fabrication precision compared to electron beam lithography and conventional gallium focused ion beam technique. In this paper, utilizing HIL, a set of graphene triangles are patterned and excellent plasmon response is detected. Particularly, the evolution of breathing mode in these structures is unveiled by scattering-type scanning near-field optical microscopy. Besides, the plasmon response of graphene structures can be efficiently tuned by adjusting the irradiated ion dose during the etching process, which can be explained by the phenomenal simulation model. Our work demonstrates that HIL is a feasible way for precise plasmonic nanostructure fabrication, and can be applied to graphene plasmon control at the nanoscale as well.

Mobility gap and quantum transport in a functionalized graphene bilayer

In a Bernal graphene bilayer, carbon atoms belong to two inequivalent sublattices A and B, with atoms that are coupled to the other layer by [Formula: see text] bonds belonging to sublattice A and the other atoms belonging to sublattice B. We analyze the density of states and the conductivity of Bernal graphene bilayers when atoms of sublattice A or B only are randomly functionalized. We find that for a selective functionalization on sublattice B only, a mobility gap of the order of 0.5 eV is formed close to the Dirac energy at concentration of adatoms [Formula: see text]. In addition, at some other energies conductivity presents anomalous behaviors. We show that these properties are related to the bipartite structure of the graphene layer.

Self-assembling diacetylene molecules on atomically flat insulators

Single crystal sapphire and diamond surfaces are used as planar, atomically flat insulating surfaces, for the deposition of the diacetylene compound 10,12-nonacosadiynoic acid. The surface assembly is compared with results on hexagonal boron nitride (h-BN), highly oriented pyrolytic graphite (HOPG) and MoS₂ surfaces. A perfectly flat-lying monolayer of 10,12-nonacosadiynoic acid self-assembles on h-BN like on HOPG and MoS₂. On sapphire and oxidized diamond surfaces, we observed assemblies of standing-up molecular layers. Surface assembly is driven by surface electrostatic dipoles. Surface polarity is partially controlled using a hydrogenated diamond surface or totally screened by the deposition of a graphene layer on the sapphire surface. This results in a perfectly flat and organized SAM on graphene, which is ready for on-surface polymerization of long and isolated molecular wires under ambient conditions.

Coexistence of Co doping and strain on arsenene and antimonene: tunable magnetism and half-metallic behavior

Effectively modulating the magnetism of two-dimensional (2D) systems is critical for the application of magnetic nanostructures in quantum information devices. In this work, by employing density functional theory calculations, we found the coexistence of Co doping and strain can effectively control the spin states of arsenene and antimonene structures. Unstrained Co-doped arsenene (arsenene–Co) and Co-doped antimonene (antimonene–Co) structures are nonmagnetic while under a strain, the magnetic moments of both cases were abruptly increased to about 2 μ_B. The emergence of magnetism can be reflected by the reduction of the interactions between Co and its neighboring atoms by the strain, which leads to the spin-splitting of Co-3d states. More importantly, we found that the transition of magnetism accompanies the modifications of the electronic properties of arsenene and antimonene so that under strain both structures can exhibit a novel half-metallic behavior. These results provide an effective pathway for the development of arsenene- and antimonene-based electronic devices by applying Co doping and strain.

Effectively modulating the magnetism of two-dimensional (2D) systems is critical for the application of magnetic nanostructures in quantum information devices.

Defect-engineered graphene chemical sensors with ultrahigh sensitivity

We report defect-engineered graphene chemical sensors with ultrahigh sensitivity (e.g., 33% improvement in NO2 sensing and 614% improvement in NH3 sensing). A conventional reactive ion etching system was used to introduce the defects in a controlled manner. The sensitivity of graphene-based chemical sensors increased with increasing defect density until the vacancy-dominant region was reached. In addition, the mechanism of gas sensing was systematically investigated via experiments and density functional theory calculations, which indicated that the vacancy defect is a major contributing factor to the enhanced sensitivity. This study revealed that defect engineering in graphene has significant potential for fabricating ultra-sensitive graphene chemical sensors.

Nitrogen Gas Field Ion Source (GFIS) Focused Ion Beam (FIB) Secondary Electron Imaging: A First Look

The recent technological advance of the gas field ion source (GFIS) and its successful integration into systems has renewed the interest in the focused ion beam (FIB) technology. Due to the atomically small source size and the use of light ions, the limitations of the liquid metal ion source are solved as device dimensions are pushed further towards the single-digit nanometer size. Helium and neon ions are the most widely used, but a large portfolio of available ion species is desirable, to allow a wide range of applications. Among argon and hydrogen, $${\rm N}_{2}^{{\plus}} $$ ions offer unique characteristics due to their covalent bond and their use as dopant for various carbon-based materials including diamond. Here, we provide a first look at the $${\rm N}_{2}^{{\plus}} $$ GFIS-FIB enabled imaging of a large selection of microscopic structures, including gold on carbon test specimen, thin metal films on insulator and nanostructured carbon-based devices, which are among the most actively researched materials in the field of nanoelectronics. The results are compared with images acquired by He+ ions, and we show that $${\rm N}_{2}^{{\plus}} $$ GFIS-FIB can offer improved material contrast even at very low imaging dose and is more sensitive to the surface roughness.

Tuning the electrical property of a single layer graphene nanoribbon by adsorption of planar molecular nanoparticles

In this study, a simple and fast approach of band gap formation in a single layer graphene nanoribbon (sGNR) is demonstrated by using hexaazatriphenylenehexacarbonitrile (HAT-CN₆) as an adsorbate molecule. sGNRs were successfully synthesized through the unzipping of double-walled carbon nanotubes followed by casting HAT-CN₆ in acetone solution to alter the electronic properties of the sGNRs. Then, the electrical property of a sGNR was measured using a field effect transistor structure and also by point-contact current imaging atomic force microscopy. The results demonstrate the formation of electron trapping sites with the nanoparticles and the neck structure of the sGNR near the adsorbed region of the molecule. Therefore, the charge carriers on the sGNR can only pass through the neck region, which works similarly to a narrow sGNR. Such a narrow sGNR has a lateral confinement of charge carriers around the neck region; hence, the device becomes semiconducting. The fabricated semiconducting sGNR could be widely used in electronic devices.

Solid-state nanopore localization by controlled breakdown of selectively thinned membranes

We demonstrate precise positioning of nanopores fabricated by controlled breakdown (CBD) on solid-state membranes by spatially varying the electric field strength with localized membrane thinning. We show 100 × 100 nm² precision in standard SiN _x membranes (30-100 nm thick) after selective thinning by as little as 25% with a helium ion beam. Control over nanopore position is achieved through the strong dependence of the electric field-driven CBD mechanism on membrane thickness. Confinement of pore formation to the thinned region of the membrane is confirmed by TEM imaging and by analysis of DNA translocations. These results enhance the functionality of CBD as a fabrication approach and enable the production of advanced nanopore devices for single-molecule sensing applications.

Quantum transport localization through graphene

Localization of atomic defect-induced electronic transport through a single graphene layer is calculated using a full-valence electronic structure description as a function of the defect density and taking into account the atomic-scale deformations of the layer. The elementary electronic destructive interferences leading to Anderson localization are analyzed. The low-voltage current intensity decreases with increasing length and defect density, with a calculated localization length ζ = 3.5 nm for a defect density of 5%. The difference from the experimental defect density of 0.5% required for an oxide surface-supported graphene to obtain the same ζ is discussed, pointing out how interactions of the graphene supporting surface and surface chemical modifications also control electronic transport localization.

Precise milling of nano-gap chains in graphene with a focused helium ion beam

A focused helium ion beam was used to introduce nano-sized gap chains in graphene. The effect of beam scanning strategies in the fabrication of the nano-gap chains was investigated. The tuning of graphene conductivity has been achieved by modulating the magnitude and uniformity of the ion dose and hence the morphology of the nano-gap chains. A model based on the site-specific and dose-dependent conductivity was built to understand the tuning of the conductivity, taking into account the nanoscale non-uniformity of irradiation.

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

This review presents recent progress on charge transport properties, carrier scattering mechanisms, and carrier mobility engineering of two-dimensional transition metal chalcogenides.

Self-Passivation of Defects: Effects of High-Energy Particle Irradiation on the Elastic Modulus of Multilayer Graphene

The elastic modulus of multilayer graphene is found to be more robust to damage created by high-energy α-particle irradiation as compared to monolayer graphene. Theoretical analysis indicates that irradiation of multilayer graphene generates interlayer links that potentially increase the stiffness of the multilayer by passivating local defects.

Patterning, characterization, and chemical sensing applications of graphene nanoribbon arrays down to 5 nm using helium ion beam lithography

Bandgap engineering of graphene is an essential step toward employing graphene in electronic and sensing applications. Recently, graphene nanoribbons (GNRs) were used to create a bandgap in graphene and function as a semiconducting switch. Although GNRs with widths of <10 nm have been achieved, problems like GNR alignment, width control, uniformity, high aspect ratios, and edge roughness must be resolved in order to introduce GNRs as a robust alternative technology. Here we report patterning, characterization, and superior chemical sensing of ultranarrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time. The patterned GNR arrays possess narrow and adjustable widths, high aspect ratios, and relatively high quality. Field-effect transistors were fabricated on such GNR arrays and temperature-dependent transport measurements show the thermally activated carrier transport in the GNR array structure. Furthermore, we have demonstrated exceptional NO2 gas sensitivity of the 5 nm GNR array devices down to parts per billion (ppb) levels. The results show the potential of HIBL fabricated GNRs for the electronic and sensing applications.

Tunable Anderson localization in hydrogenated graphene based on the electric field effect

Effective control of hydrogenation of graphene is of great scientific and technological importance. However, the reversible control of H density (n(H)) on graphene is difficult due to the irreversible H2 formation of the detached H adatoms. Here we present a novel mechanism for controlling n(H) by using the unique proton transfer reaction between NH3 gas and hydrogenated graphene, which can be tuned by applying perpendicular electric fields. Using first-principles calculations, we show that n(H) can be reversibly tuned by the applied electric fields around the critical density for the Anderson localization in hydrogenated graphene. The proposed field-induced control of H adsorption or desorption on graphene opens a path toward the development of new graphene transistors based on the tunable degree of disorder.

Conductivity of graphene with resonant and nonresonant adsorbates

We propose a unified description of transport in graphene with adsorbates that fully takes into account localization effects and loss of electronic coherence due to inelastic processes. We focus in particular on the role of the scattering properties of the adsorbates and analyze in detail cases with resonant or nonresonant scattering. For both models, we identify several regimes of conduction, depending on the value of the Fermi energy. Sufficiently far from the Dirac energy and at sufficiently small concentrations, the semiclassical theory can be a good approximation. Near the Dirac energy, we identify different quantum regimes, where the conductivity presents universal behaviors.