Graphene-based Devices Research Articles

Thermal conductivity of quasi-bilayer graphene without and with perfluorophenylazide molecules

The obstacles in further improving the heat-transport performance of the graphene-based films root in the random stacking configuration of the graphene flakes and their agglomeration in solvents. In order to overcome these obstacles, we design one kind of quasi-bilayer graphene device composed of one upper narrow nanoribbon and one lower wide nanoribbon with absorbed perfluorophenylazide (PFPA) molecules. By investigating the thermal conductivity (TC) of the quasi-bilayer graphene without absorbed PFPA molecules, we find the TC will increase as the length and width increase, while the TC will decrease with the increase of the layer number. This indicates that a quasi-bilayer graphene flake of a large area and a small layer number will be beneficial to increasing its TC. Furthermore, in the presence of the PFPA molecules, the TC of the quasi-bilayer graphene, albeit suppressed by the PFPA molecules, is found to decrease slowly with the increase of the PFPA molecule number. This demonstrates a feasible way to fabricate the high-quality graphene-based films based on the technology of absorbing PFPA molecules. This research should be an important reference for designing the graphene-based thermal functional devices.

Transmission and Reflection Spectra of a Bragg Microcavity Filled with a Periodic Graphene-Containing Structure

The transmission and reflection spectra of a one-dimensional microresonator structure with dielectric Bragg mirrors, the working cavity of which is filled with several “dielectric-graphene” or “semiconductor-graphene” periods with controlled material parameters, were obtained using transfer matrices and numerical methods. Carrier drift in graphene monolayers is created to achieve amplification, which makes it possible to use the hydrodynamic approximation to represent graphene conductivity in the terahertz range. The transformation of spectra is achieved both by changing the energy state of the graphene monolayers and by changing the external magnetic field. It is shown that amplification is observed in the region where the real part of the conductivity is negative as the chemical potential (Fermi energy) increases, and the coefficients T and R become substantially greater than unity. The results of the work may be of interest to developers of graphene-based controlled photonic devices.

Ionic Liquid–Graphene Interface: Effect of Anions on the Fermi Level

Since energy conversion and storage processes take place at the electrolyte–electrode interface, it is important to develop experimental and theoretical procedures to understand the interfacial nanostructure in graphene-based electrochemical storage devices where ionic liquids (ILs) are used as electrolytes. In this contribution, the impact of the anions of imidazolium-based ILs on the IL–graphene interface as well as on the electronic structure of graphene is investigated. Raman spectroscopy unveils that 1-butyl-3-methylimidazolium ILs having smaller anions induce n-type doping, while ILs with larger anions have a negligible effect on the doping. Molecular modeling simulations reveal that changes in the electrostatic potential at the IL–graphene interface are responsible for the n-type doping.

Investigating the Device Performance Variation of a Buried Locally Gated Al/Al2O3 Graphene Field-Effect Transistor Process

In this study, a process is developed for the fabrication of buried top-gated graphene transistors with Al2O3 as a gate dielectric, yielding devices that can be suitable for not only flexible electronics but also laser-induced graphene (LIG)-based technology implementations. A new processing option is presented with the use of tetraethyl-orthosilicate (TEOS) as an etch stop for contact via etching of Al2O3. Buried locally gated Al/Al2O3 graphene field-effect transistors (GFETs) are fabricated with Dirac points as low as 4 V, with a metal-to-graphene contact resistance as low as ∼1.7 kΩ·µm, and an average hole mobility of 457.97 cm2/V·s with a non-uniformity of 93%. Large device variation and non-uniformity in electrical performance are not uncommon for graphene-based devices, as process-induced defects play a major role in such variation. AFM, SEM, Raman spectroscopy, and model fitting indicated that the rough Al/Al2O3 surface was the main factor for the observed device variation. AFM analysis indicated a graphene surface roughness Ra of 16.19 nm on top of the buried Al/Al2O3 gate in contrast to a Ra of 4.06 nm over Al2O3/SiO2. The results presented indicate the need to reduce device variability and non-uniformity by improving transfer methods, as well as the use of smoother surfaces and compatible materials. The presented analyses provide a framework with which other researchers can analyze and correlate device variation and non-uniformities while methods to reduce variability are investigated.

Fracture strength and failure mechanism of graphene-containing grain boundaries and pores

Grain boundaries and pores commonly manifest in graphene sheets during experimental preparation. Additionally, pores have been intentionally incorporated into graphene to fulfill specific functions for various applications. However, how does the simultaneous presence of pores and grain boundaries impact the mechanical properties of graphene? This paper establishes uniaxial tension models of single-layer graphene-containing pores and three types of experimentally observed. The effect of interaction between pores and grain boundaries on the fracture strength of graphene was studied respectively for three types of grain boundaries by employing molecular dynamics simulations and considering factors such as pore size, the distance between pores and grain boundaries, and loading angle. A competitive mechanism between the intrinsic strength of pristine graphene with grain boundaries (referred to as pristine GGBs), which varies with the loading angle and the fracture strength of graphene sheets with pores that changes with the size of the pores, governs the fracture strength and failure modes of GGBs with pores. When the former exceeds the latter, the fracture strength of GGBs with pores primarily depends on the size of the pores, and fractures occur at the edges of the pores. Conversely, when the former is lower, the fracture strength of GGBs with pores relies on the loading angle and the distance between pores and grain boundaries, leading to grain boundary rupture. If the two strengths are comparable, the failure modes are influenced by the distance between pores and grain boundaries as well as the loading angle. The findings further elucidate the impact of coexisting grain boundaries and pores on the fracture behavior of graphene, providing valuable guidance for the precise design of graphene-based devices in the future.

Analysis of electromagnetic scattering from array of time-modulated graphene ribbons.

An accurate and fast method is presented for the analysis of scattering of electromagnetic waves from an array of time-modulated graphene ribbons. We derive a time-domain integral equation for induced surface currents under subwavelength approximation. Using the method of harmonic balance, this equation is solved for a sinusoidal modulation. The solution of the integral equation is then used to obtain the transmission and reflection coefficients of time-modulated graphene ribbon array. The accuracy of the method was verified through comparison with results of full-wave simulations. In contrast with previously reported analysis techniques, our method is extremely fast and can analyze structures with a much higher modulation frequency. The proposed method also provides interesting physical insights useful for designing novel applications and opens up new vistas in the fast design of time-modulated graphene-based devices.

Enhanced polarization conversion and giant Faraday rotation in patterned terahertz graphene metamaterials with combined electrical and magnetic tuning

Graphene metamaterials (MMs) have the potential to reconfigure and dynamically control terahertz (THz) waves. In this study, we conducted numerical investigations to explore the effects of externally applied magnetic fields up to 20 Tesla on the transmission properties of graphene patterned split ring resonator (GSRR) MMs in the THz region. We quantitatively compared the tunability of resonance amplitude and frequency in the co-polarized transmission component between the magnetic method and the traditional electrical approach. Our results demonstrate that magnetic tuning can effectively modulate the resonant properties of GSRR MMs. Furthermore, when combining electrical and magnetic tuning, we observed an enhancement in the polarization conversion ratio, as well as the achievement of a significant Faraday rotation angle of nearly 90 degrees in GSRR MMs. These findings indicate the potential of functional graphene-based THz devices, including switches, modulators, polarization converters, and sensors.

Planar phonon anisotropy, and a way to detect local equilibrium temperature in graphene

The effect of inclusion of the planar phonon anisotropy on thermo-electrical behavior of graphene is analyzed. Charge transport is simulated by means of Direct Simulation Monte Carlo technique coupled with numerical solution of the phonon Boltzmann equations based on deterministic methods.The definition of the crystal lattice local equilibrium temperature is investigated as well and the results furnish possible alternative approaches to identify it starting from measurements of electric current density, with relevant experimental advantages, which could help to overcome the present difficulties regarding thermal investigation of graphene.Positive implications are expected for many applications, as the field of electronic devices, which needs a coherent tool for simulation of charge and hot phonon transport; the correct definition of the local equilibrium temperature is in turn fundamental for the study, design and prototyping of cooling mechanisms for graphene-based devices.

Oxygen doping modulating thermal-activated charge transport of reduced graphene oxide for high performance temperature sensors

Graphene and its derivatives have sparked intense research interest in wearable temperature sensing due to their excellent electric properties, mechanical flexibility, and good biocompatibility. Despite these advantages, the weak temperature dependence of charge transport makes them difficult to achieve a highly sensitive temperature response, which is one of the remaining bottlenecks in the progress towards practical applications. Unfortunately, detailed knowledge about the key factors of the charge transport temperature dependence in this material that determines the critical performance of electrical sensors is very limited up to now. Here, we reveal that oxygen absorption on the ultrathin reduced graphene oxide (RGO) films (∼3 nm) can significantly increase their conductance activation energy over 200% and thus greatly improve the temperature dependence of thermal-activated charge transport. Further investigations suggest that oxygen introduces the deep acceptor states, distributed at an energy level ∼0.175 eV from the valence-band maximum, which allows a highly temperature-dependent impurity ionization process and the resulting vast holes release in a wide temperature range. Remarkably, our temperature sensors based on oxygen-doped ultrathin RGO films show a high sensitivity with temperature conductive coefficient of 14.58% K−1, which is one order of magnitude higher than the reported CNT or graphene-based devices. Moreover, the ultrathin thickness and high thermal conductivity of RGO film allow an ultrafast response time of ∼86 ms, which represents the best level of temperature sensors based on soft materials. Profiting from these advantages, our sensors show good capacity to identify the slight temperature difference of human body, monitor respiratory rate, and detect the environmental temperature. This work not only represents substantial performance advances in temperature sensing, but also provides a new approach to modulate the charge transport temperature dependence, which could be benefited to both device design and fundamental research.

Structural, Electronic and Optical Properties of Some New Trilayer Van de Waals Heterostructures.

Constructing two-dimensional (2D) van der Waals (vdW) heterostructures is an effective strategy for tuning and improving the characters of 2D-material-based devices. Four trilayer vdW heterostructures, BP/BP/MoS2, BlueP/BlueP/MoS2, BP/graphene/MoS2 and BlueP/graphene/MoS2, were designed and simulated using the first-principles calculation. Structural stabilities were confirmed for all these heterostructures, indicating their feasibility in fabrication. BP/BP/MoS2 and BlueP/BlueP/MoS2 lowered the bandgaps further, making them suitable for a greater range of applications, with respect to the bilayers BP/MoS2 and BlueP/MoS2, respectively. Their absorption coefficients were remarkably improved in a wide spectrum, suggesting the better performance of photodetectors working in a wide spectrum from mid-wave (short-wave) infrared to violet. In contrast, the bandgaps in BP/graphene/MoS2 and BlueP/graphene/MoS2 were mostly enlarged, with a specific opening of the graphene bandgap in BP/graphene/MoS2, 0.051 eV, which is much larger than usual and beneficial for optoelectronic applications. Accompanying these bandgap increases, BP/graphene/MoS2 and BlueP/graphene/MoS2 exhibit absorption enhancement in the whole infrared, visible to deep ultraviolet or solar blind ultraviolet ranges, implying that these asymmetrically graphene-sandwiched heterostructures are more suitable as graphene-based 2D optoelectronic devices. The proposed 2D trilayer vdW heterostructures are prospective new optoelectronic devices, possessing higher performance than currently available devices.

Method of Autonomous Blocks with Floquet Channels for Solving Linear and Nonlinear Problems of Diffraction for Graphene-based THz Devices

Method of Autonomous Blocks with Floquet Channels for Solving Linear and Nonlinear Problems of Diffraction for Graphene-based THz Devices

Intercalation of hafnium oxide between epitaxially-grown monolayer graphene and Ir(111) substrate

Intercalation of insulating materials between epitaxial graphene and the metal substrates is highly demanded to restore the intrinsic properties of graphene, and thus essential for the graphene-based devices. Here we demonstrate a successful solution for the intercalation of hafnium oxide into the interface between full-layer graphene and Ir(111) substrate. We first intercalate hafnium atoms beneath the epitaxial graphene. The intercalation of the hafnium atoms leads to the variation of the graphene moiré superstructure periodicity, which is characterized by low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (LT-STM). Subsequently, we introduce oxygen into the interface, resulting in oxidization of the intercalated hafnium. STM and Raman’s characterizations reveal that the intercalated hafnium oxide layer could effectively decouple the graphene from the metallic substrate, while the graphene maintains its high quality. Our work suggests a high-k dielectric layer has been successfully intercalated between high-quality epitaxial graphene and metal substrate, providing a platform for applications of large-scale, high-quality graphene for electronic devices.

Bandwidth-tunable absorption enhancement of visible and near-infrared light in monolayer graphene by localized plasmon resonances and their diffraction coupling

Bandwidth-tunable light absorption enhancement in monolayer graphene is practically important for graphene-based photoelectric devices. Especially, it is still a huge challenge to achieve high-efficiency graphene absorption with extremely narrow sub-nanometer bandwidth much smaller than one nanometer. In this work, both broadband and narrowband absorption peaks of monolayer graphene are numerically achieved in visible and near-infrared wavelength ranges. The broadband absorption peak is ascribed to localized dipolar plasmon resonances in individual silver nanodisks, and the narrowband absorption peak arises from collective first-order diffraction coupling effect in periodic array of silver nanodisks. By changing the array period, the full width at half maximum (FWHM) of the broadband absorption peak can vary from 100 nm to 50 nm. Correspondingly, the FWHM of the narrowband absorption peak is tuned from about 6.4 nm to only 0.25 nm, thus realizing an ultra-narrow sub-nanometer bandwidth.

Defect and doping concentration study with series and shunt resistance influence on graphene modified perovskite solar cell: A numerical investigation in SCAPS-1D framework

Perovskite solar cells (PSCs) have the potential to be highly efficient, low-cost next-generation solar cells. By raising open circuit voltage (Voc), the interfacial recombination kinetics can further improve device performance. In this study, we used simulation concept to elucidate the influence of using graphene as a surface passivation material in perovskite solar cells. Graphene works well as an interlayer to promote hole extraction and reduce interfacial recombination. In order to evaluate the effect of graphene in PSCs, the simulation was done in the SCAPS-1D framework to compare the performance of a device with and without graphene. Three interface layers were included to the model: TiO2/MAPbI3, MAPbI3/Graphene, and Graphene/Spiro-OMeTAD, in order to account for the impacts of interface defect density on device performance. The impacts of absorber doping concentration, absorber defect density, ETL doping concentration, HTL doping concentration, series resistance, and shunt resistance were also evaluated for the modelled PSC. Without any optimization, the control device with power conversion efficiency (PCE) of 20.677% was outperformed by the graphene-modified device with PCE of 20.911%. This difference is mostly due to the lower recombination losses and more effective suppression of interfacial non-radiative recombination. With optimization, the modified graphene-based device has a PCE of 26.667%. This result shows an enhancement of ∼1.28 times over that of the pristine graphene-based device. The outcomes have opened the way for the development of cost-effective and comparable state-of-the-art, high-efficiency perovskite solar cells with graphene interlayer by eliminating defects and managing non-radiative recombination.

In Situ Growth of Graphene on Polyimide for High-Responsivity Flexible PbS-Graphene Photodetectors.

Graphene is an ideal material for flexible optoelectronic devices due to its excellent electrical and optical properties. However, the extremely high growth temperature of graphene has greatly limited the direct fabrication of graphene-based devices on flexible substrates. Here, we have realized in situ growth of graphene on a flexible polyimide substrate. Based on the multi-temperature-zone chemical vapor deposition cooperated with bonding a Cu-foil catalyst onto the substrate, the growth temperature of graphene was controlled at only 300 °C, enabling the structural stability of polyimide during growth. Thus, large-area high-quality monolayer graphene film was successfully in situ grown on polyimide. Furthermore, a PbS-graphene flexible photodetector was fabricated using the graphene. The responsivity of the device reached 105 A/W with 792 nm laser illumination. The in-situ growth ensures good contact between graphene and substrate; therefore, the device performance can remain stable after multiple bending. Our results provide a highly reliable and mass-producible path for graphene-based flexible devices.

Designed Graphane-based spin filters by tuning the sp2/sp3 configuration

The electronic transport properties of H-vacancy-defected graphane are investigated by using the density functional theory (DFT) and Non-equilibrium Green’s Function (NEGF) methods. Results show that the band gap of graphane depends on the relative positions of the defect bands introduced by H vacancies and the valence or conduction bands, suggesting the band gap controllability. The defect bands are obviously spin polarized and the ratio is almost 100%. And the magnetization of the system can be tuned by defect concentration and location. Moreover, in the two-probe systems, the presence of H-vacancy-defect bands near the Ef plays an important role in controlling spin-dependent transport of graphane. A perfect spin-filtering effect with 100% spin polarization could be realized by H-vacancy defects. The spin properties indicate the potential of graphane as an option for future graphene-based spin filters and spintronic devices.

Bottom-up synthesis of mesoscale nanomeshes of graphene nanoribbons on germanium

The synthesis of functional graphene nanostructures on Ge(001) provides an attractive route toward integrating graphene-based electronic devices onto complementary metal oxide semiconductor-compatible platforms. In this study, we leverage the phenomenon of the anisotropic growth of graphene nanoribbons from rationally placed graphene nanoseeds and their rotational self-alignment during chemical vapor deposition to synthesize mesoscale graphene nanomeshes over areas spanning several hundred square micrometers. Lithographically patterned nanoseeds are defined on a Ge(001) surface at pitches ranging from 50 to 100 nm, which serve as starting sites for subsequent nanoribbon growth. Rotational self-alignment of the nanoseeds followed by anisotropic growth kinetics causes the resulting nanoribbons to be oriented along each of the equivalent, orthogonal Ge⟨110⟩ directions with equal probability. As the nanoribbons grow, they fuse, creating a continuous nanomesh. In contrast to nanomesh synthesis via top-down approaches, this technique yields nanomeshes with atomically faceted edges and covalently bonded junctions, which are important for maximizing charge transport properties. Additionally, we simulate the electrical characteristics of nanomeshes synthesized from different initial nanoseed-sizes, size-polydispersities, pitches, and device channel lengths to identify a parameter-space for acceptable on/off ratios and on-conductance in semiconductor electronics. The simulations show that decreasing seed diameter and pitch are critical to increasing nanomesh on/off ratio and on-conductance, respectively. With further refinements in lithography, nanomeshes obtained via seeded synthesis and anisotropic growth are likely to have superior electronic properties with tremendous potential in a multitude of applications, such as radio frequency communications, sensing, thin-film electronics, and plasmonics.

Review on infrared nanospectroscopy of natural 2D phyllosilicates.

Phyllosilicates have emerged as a promising class of large bandgap lamellar insulators. Their applications have been explored from the fabrication of graphene-based devices to 2D heterostructures based on transition metal dichalcogenides with enhanced optical and polaritonics properties. In this review, we provide an overview of the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) for studying nano-optics and local chemistry of a variety of 2D natural phyllosilicates. Finally, we bring a brief update on applications that combine natural lamellar minerals into multifunctional nanophotonic devices driven by electrical control.

Graphene Strain-Effect Transistor with Colossal ON/OFF Current Ratio Enabled by Reversible Nanocrack Formation in Metal Electrodes on Piezoelectric Substrates

Extraordinarily high carrier mobility in graphene has led to many remarkable discoveries in physics and at the same time invoked great interest in graphene-based electronic devices and sensors. However, the poor ON/OFF current ratio observed in graphene field-effect transistors has stymied its use in many applications. Here, we introduce a graphene strain-effect transistor (GSET) with a colossal ON/OFF current ratio in excess of 107 by exploiting strain-induced reversible nanocrack formation in the source/drain metal contacts with the help of a piezoelectric gate stack. GSETs also exhibit steep switching with a subthreshold swing (SS) < 1 mV/decade averaged over ∼6 orders of magnitude change in the source-to-drain current for both electron and hole branch amidst a finite hysteresis window. We also demonstrate high device yield and strain endurance for GSETs. We believe that GSETs can significantly expand the application space for graphene-based technologies beyond what is currently envisioned.

Graphene nanoparticles as data generating digital materials in industry 4.0

One of the potential applications of 2D materials is to enhance multi-functionality of structures and components used in aerospace, automotive, civil and defense industries. These multi-functional attributes include sensing, energy storage, EMI shielding and property enhancement. In this article, we have explored the potential of using graphene and its variants as data generating sensory elements in Industry 4.0. We have presented a complete roadmap to cover three emerging technologies i.e. advance materials, artificial intelligence and block-chain technology. The utility of 2D materials such as graphene nanoparticles is yet to be explored as an interface for digitalization of a modern smart factory i.e. “factory-of-the-future”. In this article, we have explored how 2D material enhanced composites can act as an interface between physical and cyber spaces. An overview of employing graphene-based smart embedded sensors at various stages of composites manufacturing processes and their application in real-time structural health monitoring is presented. The technical challenges associated with interfacing graphene-based sensing networks with digital space are discussed. Additionally, an overview of the integration of associated tools such as artificial intelligence, machine learning and block-chain technology with graphene-based devices and structures is also presented.