Irradiated Graphene Research Articles

High-energy heavy ions as a tool for production of nanoporous graphene

Single-layered supported graphene sheets were irradiated by iodine, copper, silicon, and oxygen ions in the MeV energy range. The selected ion beams cover a wide range of nuclear and electronic stopping powers. Raman spectroscopy used for the analysis of irradiated graphene samples reveals similar, nanometer-sized, pores due to the ion impacts, but with varying probability of their formation. We observed an inverse relationship between ion velocity and the size of the nanopores. Also, we observed damage accumulation is influenced by electronic stopping in all cases when electronic stopping is greater than nuclear stopping by two orders of magnitude. The systematic investigation presented in this work enables custom-made production of nanoporous graphene using high-energy heavy ion beams.

In-depth investigation into defect-induced Raman lines in irradiated graphene

Identifying the type of structural defects and determining their concentration is crucial for effective defect engineering strategies since they govern various physical, chemical, and optoelectronic properties of graphene. Here, we study the effects of Ga ion irradiation on freestanding monolayer graphene, specifically focusing on the behavior of three defect-induced Raman lines (D, D' and (D+ D')). By employing a modified approach of the local activation model, we determine the key defect parameters of each line and show their dependence on different vibrational configurations of the iTO and iLO phonons emitted during scattering. The redshift of the lines and the broadening of their width, observed with an increase in the concentration of radiation defects over Nd ≈ 1013cm−2, are explained by the tensile stress of the graphene film and a decrease in the phonon lifetime, respectively. The resulting intensity ratio I(D)/I(D') of 9.7 in samples with different defect densities is consistent with theoretical predictions for vacancy-like defects with a predominance of double vacancies. Our findings provide valuable insights into the underlying mechanisms driving defect-induced Raman scattering in graphene, thereby contributing to an enhanced understanding of defect engineering for diverse applications, such as next-generation electronics, sensing devices, and energy storage systems.

A light-controllable topological transistor based on quantum tunneling of anomalous topological edge states

Motivated by the recent observation of anomalous Hall effects in graphene [Nat. Phys. 16, 38–41 (2020)], we study the tunneling transport properties of topological edge states in irradiated graphene and graphene-like materials. We investigate the quantum tunneling transport in a structure: laser-irradiated graphene/gapped graphene/laser-irradiated graphene. We find that electrons cannot transport through the device because of the band gap, but electrons will tunnel through the device when anomalous topological edge states are induced by a laser. We predict a topological transistor based on the tunneling transport of topological edge states in irradiated graphene-like materials.

Effect of topological non-hexagonal rings and Stone Wale defects on the vibrational response of single and multi-layer ion irradiated graphene

Present study explores the observation of topological non-hexagonal rings (NHR) and Stone Wale (SW) defects by Raman experiments in both single (SLG) and multi-layer graphene (MLG) after they are irradiated with 100–300 eV Ar ions. Although predicted by theoretical studies, here it is experimentally shown that graphene SW/NHR defects have a signature in Raman. Broad bandwidth of the pertinent Raman features suggests the presence of more than one SW/NHR defect mode, in agreement with the DFT studies. Variations in the SW/NHR related Raman mode intensities demonstrate the annihilation of these topological defects at higher energies. Behavior of Raman allowed G and 2D excitations, as well as the disorder-activated D, D’ and G* lines, has also been investigated in SLG and MLG. These indicate an evolution of defects in graphene with ion irradiation, as well as presence of a transition state beyond which the Raman modes are dominated by a rise in sp3 content. Correlation of these aspects with the SW/NHR Raman provide significant insight into ion induced evolution of graphene. The direct observation of SW/NHR defects by Raman spectroscopy could be important in promoting exploration of rich topological aspects of Graphene in various fields.

Impurity states and indirect exchange interaction in irradiated graphene

We theoretically investigate the impurity levels and exchange interaction between magnetic impurities in graphene driven by an off-resonant circularly polarized light field. Our analysis captures the non-perturbative effects resulting from scattering with magnetic impurities with a strong onsite potential. Under irradiation, a dynamical band gap opens up at the Dirac point, allowing impurity levels to exist inside the gap. These impurity levels are shown to give rise to a resonance feature in the exchange energy for impurities located either at the same or different sublattices. The exchange interaction also shows a wider spatial range of antiferromagnetic behavior due to irradiation. Our work demonstrates that the exchange energy of magnetic impurities in graphene is extensively tunable by light irradiation in the presence of strong potential scattering.

Nondiagonal disorder enhanced topological properties of graphene with laser irradiation

Laser irradiation, as a versatile tool to tune topological properties of electronic systems, is under intensive studies. Experimentally, laser irradiation induced anomalous Hall effect in graphene has been observed (McIver et al., Nat. Phys. 16, 38 (2020)). Disorder is ubiquitous in real materials, and it has been shown that diagonal disorders, i.e., onsite disorder, can enhance topological properties of time-periodically driven quantum materials (Titum et al., Phys. Rev. Lett. 114, 056801 (2015)). Here, we investigate circularly polarized laser irradiated graphene with non-diagonal disorders, i.e., disordered tunneling, and find that disorder can induce nontrivial topological properties, characterized by Bott index and the real-space Chern number. Moreover, we show that one can turn on the laser irradiation non-adiabatically to drive the disordered graphene into non-trivial topological phase. It is a scheme which is especially interesting for experimental implementations.

Cu ions irradiation-induced defects in graphene and their effects on optical properties

The radiation-induced damage in graphene irradiated with 8-MeV Cu2+ ions at fluences varying from 1 × 1015 ions/cm2 to 1 × 1016 ions/cm2 was studied. The samples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectra analysis. The number of point defects was observed to increase with the ion fluence until the adjacent point defects started to merge, forming individual amorphous regions. Then a critical stage, when the metallic nature of graphene was transformed into semiconducting, was reached for the first time in an ion beam irradiated graphene. It was noticed that further increase of the ion fluence results in a total conversion of the crystalline graphene structure into the massively disordered layer with decreased optical absorption. Density function theory calculations supported obtained experimental results.

Gamma-ray irradiated graphene nanosheets/polydopamine hybrids as a superior anode material for lithium-ion batteries

Gamma-ray irradiated graphene nanosheets/polydopamine hybrids as a superior anode material for lithium-ion batteries

Investigating the impact of UV irradiated graphene oxide on human breast cancer cells

Graphene oxide (GO) is well known for its photo-reactivity with potential reduction under ultraviolet (UV) radiation. GO induces cytotoxicity in cancer cells even though the mechanism involved has not been firmly established when exposed to cancer cells. Therefore, to investigate the potential interaction of GO with UV and their impact on cancer cells, MCF-7 human breast cancer cells were exposed to GO, UV, or in combination, and the cellular impacts associated with these exposures have been identified. UV-irradiated GO showed differences in appearance, UV-absorbance, FTIR, and Raman spectra, indicating substantial changes in UV-irradiated GO structure compared to the non-irradiated GO. Furthermore, UV-irradiated GO reduced the cell viability of MCF-7 cells and increased the cellular level of ROS in the cancer cells that were dependent on the UV exposure time and GO concentration, respectively. Moreover, a synergistic increase in cellular ROS was observed when MCF-7 cells were treated with different GO concentrations followed by UV-irradiation. The synergistic interaction of GO and UV induced apoptotic cell death in MCF-7 cells as evidenced by the increases in MCF-7 cells exhibiting round cell morphology, nuclei condensation, activation of BAX and Caspase as well as the release of Cyt. C from mitochondria, suggesting the potential of UV-irradiated GO in cancer treatment.

Band gap opening and surface morphology of monolayer graphene induced by single ion impacts of argon monomer and dimer ions

Craters have been observed upon irradiation by single ion impacts of Ar monomer and Ar dimer ions with 35 keV/atom in monolayer graphene on copper at a fluence of 1 × 1012 atoms/cm2. The observed craters at the ion impacts are in the underlying copper substrate, which pull down the graphene layer conformally towards them, introducing disorder into graphene. The number density of craters per atom produced by Ar-monomers is higher than that of Ar-dimer ions. In the ion impact region, disorder is high, and graphene is metallic in both Ar-monomer and dimer irradiation. In the ion impact region of dimer irradiated graphene, local density of states is higher and there is a shift in Dirac point position by +0.07 eV and a bandgap opening of 0.25 eV in the ordered region ∼15 nm away from the ion impact. Microscopic ripples in graphene are not observed in the case of monomer irradiation while they are present in the case of dimer irradiation with a reduced wavelength. Isotropic compressive strain is introduced by the ion impacts and it is larger for Ar-dimer irradiation. The combined effects of compressive strain along with breaking the equivalence between A and B sublattice of graphene is attributed to the opening of the band gap in graphene.

Topological metal phases in irradiated graphene sandwiched by asymmetric ferromagnets

The ill-defined Chern number and disappearing topological edge states make it difficult to characterize the topological properties of a metal. In this work, we investigate the abundant topological phases of monolayer graphene, which is sandwiched by asymmetric ferromagnets and irradiated by off-resonant light. In this system, there exist some rarely noticed metal phases, which are spin-valley polarized metal, topological spin metal (TSM), spin half metal, and topological spin half metal (TSHM). Particularly, for the TSM, the subband with spin up or spin down is topological, but the whole state becomes a metal. For the TSHM, a subband with one spin is topological, while the other subband with the opposite spin is insulated, and the whole state is a spin half metal. As a consequence, one topological protected spin current flows on the edge and the other spin propagates in the bulk. Further calculations indicate that the Berry curvatures for the metal phases are nonzero. We propose to probe the topological properties of the metal states with the anomalous Nernst effect. For these topological phases, spin and valley splitting and flip can be obtained by modulating the Fermi level. An electrically or magnetically controlled switch is designed by a two-terminal TSHM junction. It is expected that these topological metal phases can broaden the band engineering in monolayer graphene and support a promising platform for the studies of spin and valley caloritronics.

UV-Ozone Treated Modified Laser-Induced Graphene Based Electrochemical Sweat Sensor

Recent studies shows laser-induced graphene (LIG) based electrodes can be very effective for electrochemical sensing applications. In this work, we aim to develop a stable ion selective membrane (ISM) based sweat sensor using laser-induced graphene (LIG) as the electrode material. However, the surface of pristine LIG is hydrophobic. Hence, coating these hydrophobic LIG films with ISM produces low sensitivity in sensing applications. In this study, UV-Ozone irradiated laser-induced graphene (LIG) based electrodes were explored for sensing biomarkers like ion in sweat. Solid-state ion-selective electrodes (ISEs) sensitive to ions were fabricated by a two-step drop-coating process of polyvinyl chloride solution containing plasticizer, ionophore, and ion-exchanger on the LIG electrode. We found the ozone treatment process significantly increased the electrochemical active surface area (EASA) and porosity of the pristine LIG. Hydrophobic to hydrophilic transition induced by UV-Ozone treatment reduced the contact angle (CA), resulting in better permeation of the ion-selective membrane (ISM) into the ozonized LIG film. Raman spectroscopy and IR absorption studies indicate physisorption of ozone on LIG. The concentration of the ISM solution has a great influence on attachment to the O-LIG film. Here, we report the applicability of O-LIG as an electrochemical sensor towards the detection of sodium ion () using the open circuit potential (OCP) method. The performance of ozonized LIG (O-LIG) electrodes was much better than pristine LIG and screen printed carbon electrodes. The sensitivity of 60.2 ± 0.9 mV/ decade to Na+ ions and a lower limit of detection of 1 × 10-6 M was achieved using O-LIG based ISEs. Response time of the O-LIG based electrodes were around 1 min. The current study demonstrates that the LIG-based electrodes can be used as a flexible electrochemical sensing platform and is suitable for future wearable sensing devices. Keywords Laser-induced graphene; ion-selective electrode; wearable sensors; electrochemical sensor; sweat sensor.

Potentiometric ion-selective sensors based on UV-ozone irradiated laser-induced graphene electrode

Disposable devices for sensing biomarkers like Na+ ion in sweat was developed using UV-Ozone irradiated laser-induced graphene (LIG) based electrodes. Solid-state ion-selective electrodes (ISEs) sensitive to Na+ ions were fabricated by a two-step drop-coating process of polyvinyl chloride solution containing plasticizer, ionophore and ion-exchanger on LIG electrode. Hydrophobic to hydrophilic transition induced by UV-Ozone treatment reduced the contact angle, resulting in better permeation of the ion-selective membrane into the ozonized LIG film. Raman spectroscopy and IR absorption studies indicate physisorption of ozone on LIG. Performance of ozonized LIG (O-LIG) electrodes was much better than pristine LIG and screen-printed carbon electrodes. Sensitivity of 60.2 ± 0.9 mV/ decade to Na+ ions and a lower limit of detection of 1 × 10−6 M was achieved using O-LIG based ISEs. Response time of the devices were around 1 min.

Modulating thermal conductance across the metal/graphene/SiO2 interface with ion irradiation.

Optimizing the efficiency of heat dissipation across an interface is a great challenge with the continuously increasing integration of microelectronic devices. In this work, an effective method in tuning the heat conduction across the Al/graphene/SiO2 interface is reported. It was found that the interfacial thermal conductance of Al/irradiated graphene/SiO2 can be increased by a factor of 3, as compared with that of Al/pristine graphene/SiO2. The X-ray photoelectron spectroscopy (XPS) analysis indicates that ion irradiation may promote the formation of CO bonds on the irradiated graphene surface, which is beneficial to the enhancement of interfacial thermal conductance. The density functional theory (DFT) calculations reveal that in addition to the formed bonds between O atoms and Al atoms, the adsorption strength between Al and irradiated graphene is intensified, which plays a dominant role in enhancing the interfacial thermal conductance of Al/graphene/SiO2.

Helium-bubble-assisted deformation twinning in irradiated graphene (reduced graphene oxide)-aluminum composite with a nanolaminated structure

Nanolaminated graphene (reduced graphene oxide)-aluminum composite irradiated with high energy helium ions at room temperature was shown to form deformation twins upon nanoindentation. This is in stark contrast to the dislocation-mediated plasticity in the as-fabricated composite, and the elongated helium bubbles that flowed with the soft Al matrix in the deformed composite irradiated at high temperature. A phenomenological model for helium-bubble-assisted twin formation was developed to rationalize the unique deformation microstructure observed in the composite irradiated at room temperature. The presence of deformation twinning mechanism in the irradiated composite may provide a potential measure to mitigate the irradiation-induced embrittlement.

Floquet-engineered topological flat bands in irradiated twisted bilayer graphene

We propose a tunable optical setup to engineer topologically nontrivial flat bands in twisted bilayer graphene under circularly polarized light. Using both analytical and numerical calculations, we demonstrate that nearly flat bands can be engineered at small twist angles near the magic angles of the static system. The flatness and the gaps between these bands can be tuned optically by varying laser frequency and amplitude. We study the effects of interlayer hopping variations on Floquet flat bands and find that changes associated with lattice relaxation favor their formation. Furthermore, we find that, once formed, the flat bands carry nonzero Chern numbers. We show that at currently known values of parameters, such topological flat bands can be realized using circularly polarized UV laser light. Thus, our work opens the way to creating optically tunable, strongly correlated topological phases of electrons in moiré superlattices.Received 21 October 2019Revised 5 May 2020Accepted 2 November 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043275Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasChern insulatorsFractional quantum Hall effectPhysical SystemsFloquet systemsGrapheneTopological materialsCondensed Matter, Materials & Applied Physics

Near-perfect microlenses based on graphene microbubbles

Microbubbles acting as lenses are interesting for optical and photonic applications such as volumetric displays, optical resonators, integration of photonic components onto chips, high-resolution spectroscopy, lithography, and imaging. However, stable, rationally designed, and uniform microbubbles on substrates such as silicon chips are challenging because of the random nature of microbubble formation. We describe the fabrication of elastic microbubbles with a precise control of volume and curvature based on femtosecond laser irradiated graphene oxide. We demonstrate that the graphene microbubbles possess a near-perfect curvature that allows them to function as reflective microlenses for focusing broadband white light into an ultrahigh aspect ratio diffraction-limited photonic jet without chromatic aberration. Our results provide a pathway for integration of graphene microbubbles as lenses for nanophotonic components for miniaturized lab-on-a-chip devices along with applications in high-resolution spectroscopy and imaging.

Robustness of quantized transport through edge states of finite length: Imaging current density in Floquet topological versus quantum spin and anomalous Hall insulators

The theoretical analysis of topological insulators (TIs) has been traditionally focused on infinite homogeneous crystals with band gap in the bulk and nontrivial topology of their wavefunctions, or infinite wires whose boundaries host surface or edge metallic states. However, experimental devices contain finite-size topological region attached to normal metal (NM) leads, which poses a question about how precise is quantization of longitudinal conductance and how electrons transition from topologically trivial NM leads into the edge states. This is particularly pressing issues for recently conjectured two-dimensional (2D) Floquet TI where electrons flow from time-independent NM leads into time-dependent edge states---the very recent experimental realization of Floquet TI using graphene irradiated by circularly polarized light did not exhibit either quantized longitudinal or Hall conductance. Here we employ charge conserving solution for Floquet-nonequlibrium Green functions (NEGFs) of irradiated graphene nanoribbon to compute longitudinal two-terminal conductance, as well as spatial profiles of local current density as electrons propagate from NM leads into the Floquet TI. In the case of Floquet TI both bulk and edge local current densities contribute equally to total current, which leads to longitudinal conductance below the expected quantized plateau that is slightly reduced by edge vacancies. We propose two experimental schemes to detect coexistence of bulk and edge current densities within Floquet TI: (i) drilling a nanopore in the interior of irradiated region of graphene will induce backscattering of bulk current density, thereby reducing longitudinal conductance by $\sim 28$%; (ii) imaging of magnetic field produced by local current density using diamond NV centers.

Electrical Behavior of Graphene/SiO<sub>2</sub>/Silicon Material Irradiated by Electron for Field Effect Transistor (FET) Applications

In this paper, the detail study of electrical conductivity of single layer graphene (SLG) on silicon dioxide (SiO2)/Silicon substrate irradiated by high energy (MeV) electron is presented. The SLG samples prepared by Chemical Vapor Deposition (CVD) were irradiated by 50 kGy, 100 kGy and 200 kGy doses of electron radiation at energy voltage of 3 MeV. Current-Voltage (I-V) characteristics and conductivity of the pristine and irradiated graphene samples were measured and analysed using I-V measurement at room temperature. The non-linear I-V curves were clearly observed as the voltage reach to 2.0 V for non-irradiated and irradiated samples. This may be attributed to the non-uniform charges by high energy electron irradiation and poor metal contact of the sample. Hysteresis loop form at 2.0 V probably due to the to the charge trapping occurs at the interface of the graphene and SiO2. The reaction of high energy particles lead to creation of more carrier charges that contribute to the increment of conductivity compare to the small number of atom displacement of knock-on collisions with the nuclei of carbon atoms at higher dose. This study provides significant findings on the graphene electrical characteristics when irradiated with high energy (MeV) electron.

Nonequilibrium RKKY interaction in irradiated graphene

We demonstrate that the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in graphene can be strongly modified by a time-periodic driving field even in the weak drive regime. This effect is due to the opening of a dynamical band gap at the Dirac points when graphene is exposed to circularly polarized light. Using Keldysh-Floquet Green's functions, we develop a theoretical framework to calculate the time-averaged RKKY coupling under weak periodic drives and show that its magnitude in undoped graphene can be decreased controllably by increasing the driving strength, while mostly maintaining its ferromagnetic or antiferromagnetic character. In doped graphene, we find RKKY oscillations with a period that is tunable by the driving field. When a sufficiently strong drive is turned on that brings the Fermi level completely within the dynamically opened gap, the behavior of the RKKY coupling changes qualitatively from that of doped to undoped irradiated graphene.