Nanoperforated Graphene Research Articles

Graphene Monolayer Nanomesh Structures and Their Applications in Electromagnetic Energy Harvesting for Solving the Matching Conundrum of Rectennas.

In this paper, we investigate various graphene monolayer nanomesh structures (diodes) formed only by nanoholes, with a diameter of just 20 nm and etched from the graphene layer in different shapes (such as rhombus, bow tie, rectangle, trapezoid, and triangle), and their electrical properties targeting electromagnetic energy harvesting applications. In this respect, the main parameters characterizing any nonlinear device for energy harvesting are extracted from tens of measurements performed on a single chip containing the fabricated diodes. The best nano-perforated graphene structure is the triangle nanomesh structure, which exhibits remarkable performance in terms of its characteristic parameters, e.g., a 420 Ω differential resistance for optimal impedance matching to an antenna, a high responsivity greater than 103 V/W, and a low noise equivalent power of 847 pW/√Hz at 0 V.

Dual chemifunctional tin(IV) oxide/nanoperforated graphene interlayer as a polysulfide adsorbent for use in high-performance lithium–sulfur batteries

The development of Li–S batteries is limited by their levels of rapid capacity decay owing to polysulfide dissolution and diffusion in organic electrolytes. We addressed this critical issue by fabricating a dual chemifunctional interlayer comprising SnO2 nanoparticles and nanoperforated graphene (NPG) that could function as a polysulfide adsorbent. The synergistic effects of SnO2 and the high-density functional groups of NPG on polysulfide capture were conceptually confirmed. NPG not only supported the adsorption of Li polysulfides but also provided pathways for simple Li-ion motion within the SnO2/NPG interlayer. A cell assembled with the SnO2/NPG interlayer displayed a high rate capability and initial discharge capacity and good reversible capacity. The excellent high-rate cycling performance of the cathode could mainly be attributed to the strong chemical bonds formed between SnO2/NPG and the polysulfides, rapid electron transfer, and optimized ion diffusion pathways derived from the well-organized structure of the composite. Synthesizing dual chemifunctional interlayers using a composite of metal oxides and NPG may lead to the development of advanced Li–S batteries with numerous practical applications in the near future.

Microspherical assembly of selectively pyridinic N-doped nanoperforated graphene for stable Li-metal anodes: Synergistic coupling of lithiophilic pyridinic N on perforation edges and low tortuosity via graphene nanoperforation

Li-metal is considered the most attractive anode material for high-energy Li batteries. However, its direct use is severely hindered by the uncontrollable growth of Li dendrites and severe volume changes. Herein, the synergistic coupling of lithiophilicity and low-tortuosity channels of a microspherical graphene assembly host for stable Li-metal anodes are achieved via in-plane nanoperforation of graphene and selective doping of high level of pyridinic N on perforation edges. Specifically, 5–10 nm in-plane nanoperforations that are evenly spaced ∼15 nm apart are introduced to graphene. While nanoperforations provide low-tortuosity channels for the Li-ion diffusion throughout the graphene host, perforation edges act as preferential sites to selectively dope pyridinic N. The microspherical assembly of N-doped nanoperforated graphene (N-PSB) possesses the following advantageous features as a Li-metal host: (i) selective pyridinic N-doping on the nanoperforation edges, (ii) high level and uniform distribution of lithiophilic pyridinic N, (iii) electrical conductivity improvement via N-doping, and (iv) low tortuosity to enhance Li-ion transport through in-plane nanoperforations. Owing to the synergistic coupling of these advantageous features, N-PSB guides the uniform deposition of Li and suppresses Li dendrite growth, resulting in excellent cycling stability. Furthermore, pyridinic-N-doped at the nanoperforation edges shows higher lithiophilicity than the carbon radicals at the edges. The reported results are supported by density functional theory calculations. Finally, practical applicability of the N-PSB host is investigated by assembling a full cell with a LiNi0.6Co0.2Mn0.2O2 cathode.

Graphene with nanoperforation for high-capacity potassium-ion storage: Decoupling structural defect and doping effects of N-doped graphene

Heteroatom-doping has emerged as an effective strategy for enhancing the charge storage of graphene. N-doping treatment of graphene involves doping of N heteroatoms as well as structural defect formation, and it remains unclear which one is more responsible for the enhanced K-ion storage. Therefore, to design high-capacity graphene, their effects must be decoupled to understand the individual contributions to the K-ion storage properties. To decouple these effects, four graphene samples are prepared: nanoperforated graphene (NPG) with pre-formed structural defects, and N-doped NPG (N-NPG) with a high fraction of pyridinic N preferentially doped at the nanoperforation edges, along with two control samples of reduced graphene oxide (RGO) and N-doped RGO (N-RGO). Specific capacity increased in the order of RGO < N-RGO < N-NPG < NPG. Differential capacity analyses prove to be a very effective tool in the decoupling. Notably, the four materials show distinct differential capacity plots, which can be divided into two potential regions. Differential capacity at each region varied systematically in magnitude upon N-doping and structural defects, which enabled decoupling the effects of structural defects and N heteroatoms. According to the decoupling, the capacity enhancement of N-RGO is mainly due to the N heteroatoms rather than structural defects. As for the N-NPG, however, the capacity decrease is due to the elimination of carbon radicals and oxygen functional groups at the edges by N-doping treatment, thereby, reducing the reactivity of edges of the nanoperforations. Density functional theory calculations support the electrochemical performance of NPG from the nanoperforations.

Hydrogen peroxide electrochemical sensor based on gold nanoparticles modified with nitrogen-doped and nanoperforated graphene nanozymes

Gold nanoparticles (AuNPs) modified with nitrogen-doped and nanoperforated graphene (DNPG) were successfully fabricated on a glassy carbon electrode (GCE). The composite showed excellent electrochemical response for hydrogen peroxide (H2O[Formula: see text] determination. Nitrogen-doped graphene has peroxidase-like activities. DNPG provided abundant surface area to support AuNPs, as well as ion transport shortcuts through the pores on DNPG sheets. DNPG and one-step electrode position of gold nanostructures can greatly improve conductivity and catalytic performance. The electroactive surface area of AuNPs/DNPG/GCE is almost 2.07 times higher than that of bare GCE. When AuNPs/DNPG/GCE were employed for non-enzymatic H2O2sensing, the modified electrode presented an ultrahigh sensitivity of 5.3678 × 10[Formula: see text] mA[Formula: see text]mol L[Formula: see text] cm[Formula: see text] in the range of 0.5–10 mmol L[Formula: see text], as well as excellent selectivity, reproducibility and stability. AuNPs/DNPG/GCE could serve as an electrochemical biosensor for the detection of disease biomarkers in clinical practice.

Synergistic effect of graphene nanoperforation on the reversibility of the conversion reaction of a SnO2/nanoperforated graphene composite

Metal oxide (MOx)-based anodes suffer from large capacity loss and low Coulombic efficiency due to the irreversible formation of Li2O during the conversion reaction. Despite numerous studies addressing this issue, the development of MOx-based anode materials with high cycle reversibility remains a critical challenge. In this study, the reversibility of the conversion reaction of Sn and Li2O to SnO2 is significantly improved through the innovative design of a SnO2/nanoperforated graphene composite as an anode material. Nanoperforations are introduced at the contact points of SnO2 with graphene to increase the interfacial area of Sn/Li2O, which resulted in improved reversibility of the conversion reaction. Ex-situ high-resolution transmission electron microscopy imaging coupled with selected area electron diffraction pattern, ex-situ XRD and XPS analyses corroborate the improved reversibility of the conversion reaction. The specific charge capacity of SnO2 in the SnO2/nanoperforated graphene composite is 1446 mAh g−1 at the current density of 100 mA g−1, which is very close to the theoretical capacity of SnO2 (1494 mAh g−1 based on the fully reversible conversion reaction and alloying/de-alloying reaction). Furthermore, the maintenance of the initial differential capacity plots after 800 cycles demonstrates the improved reversibility of the conversion reaction of the SnO2/nanoperforated graphene composite over extended cycles. These results provide important insights into the rational design of MOx-based anode materials using nanoperforated graphene with improved reversibility of the conversion reaction for Li-ion batteries.

Reversibility Enhancement of Conversion Reaction for Lithium Storage in SnO2/Graphene Composite Enabled By Graphene Nanoperforation at the Contacts of SnO2

Lithium-ion batteries (LIBs) have been widely used in hybrid electric vehicles and portable electronic devices because of their high energy density and light weight. However, the energy density of the batteries is required to improve for practical applications. For LIB anode materials, commercially used graphite based on intercalation reaction is far from satisfying the requirements for high energy density due to its limited theoretical capacity (372 mAh g-1). Searching for alternative electrode materials with high specific capacity is vital for the development of next-generation Li-ion batteries with high energy densities. In the case of anode materials, electrode materials based on conversion reaction has attracted extensive attention due to its high specific capacities. For example, SnO2 have been studied as one of the promising candidates for anode materials of LIBs owing to their high theoretical capacities (783 mAh g-1) and natural abundance. However, SnO2 anodes suffer from three main drawbacks: 1) irreversible conversion reaction of Sn and Li2O to SnO2 during initial cycles; 2) large volume change (~300 %) during lithiation/delithiation process, which causes severe electrode pulverization and fast capacity fading; 3) low electrical conductivity, thereby low rate capability. Numerous studies tried to solve the issues 2) and 3) through the design of nanostructures and incorporation of conductive carbon materials to mitigate the internal stress from the volume expansion of the SnO2.Previous studies show that SnO2 goes through an irreversible conversion reaction during the initial cycle, which leads to formation of metallic Sn and Li2O, and each Sn atom reversibly reacts with 4.4 Li atoms, corresponding to a reversible capacity of 783 mAh g-1. Therefore, the theoretical capacity of SnO2 was considered to be 783 mAh g-1. If the conversion reaction of Li2O is made reversible, the theoretical specific capacity of SnO2 would increase to 1494 mAh g-1. Accordingly, the irreversibility of conversion reaction of Li2O in SnO2-based anodes has been a critical challenge for achieving high specific capacity.On the other hand, few works were reported about the causes of the irreversible conversion reaction and detailed mechanism of the reversible conversion reaction still remains unclear.Recently, other researchers have reported that SnO2 shows a capacity beyond the above-mentioned theoretical value owing to the reversible conversion reaction between Sn and Li2O during the delithiation process. The increased capacity was mainly attributable to partial reversible conversion of Sn and Li2O to SnO2.Design of nanostructured SnO2 is regarded as one of the effective ways to enhance reversibility of conversion reaction between Sn and Li2O during the delithiation process. The conversion of Sn and Li2O to SnO2 is known to be irreversible, but could be reversible in the case of nanostructured SnO2 through high fraction of Sn/Li2O interface. Thus, there have been various researches ranging from fabricating SnO2 nanomaterials to incorporating SnO2 with conductive carbon materials.Furthermore, nano-sized transition metals (Co, Ni, Fe) can act as the catalyst to promote the decomposition of Li2O which is beneficial for the conversion reaction. Although partially reversible conversion reaction has occurred in these works, but they reported limited ICE and capacity decay upon cycling. Thus, to enhance the reversibility of the conversion reaction in SnO2-based anodes, increasing interface area between Sn and Li2O is of great importance.In this work, we synthesized SnO2/nanoperforated graphene (NPG) microballs with nanoperforations at the contact of SnO2 and graphene which are novel design of SnO2/graphene structure to increase Sn/Li2O interface. SnO2 can act as nanocatalysts inducing catalytic carbon gasification, which is essential to introduce nanoperforations at the contact of SnO2 and graphene. Catalytic carbon gasification induces the selective decomposition of the carbon in contact with the nanocatalysts at a temperature lower than the normal combustion temperature of carbon. The SnO2/nanoperforated graphene (NPG) microballs with nanoperforations at the contact of SnO2 and graphene have several advantages in Li-ion storage performance. 1) Enhancement in reversibility of conversion reaction of Li2O through an increase in Sn/Li2O interface area by increasing interfacial area of SnO2 in contact with electrolyte, 2) improvement in rate capability by increasing interfacial area of SnO2 in contact with the electrolyte, 3) improved cross-plane diffusion of Li-ions through nanoperforations, 4) increased capacity through Li-ion insertion/extraction from the edge sites of nanoperforated graphene, and 5) improved cycle performance by accommodating the volume change of SnO2.

Reversible sulfuric acid doping of graphene probed by in-situ multi-wavelength Raman spectroscopy

Since lattice strain and charge density affect various material properties of graphene, a reliable and efficient method is required for quantification of the two variables. While Raman spectroscopy is sensitive and non-destructive, its validity towards precise quantification of chemical charge doping has not been tested. In this work, we quantified in-situ the fractional frequency change of 2D and G peaks in response of charge density induced by sulfuric acid solution as well as native lattice strain. Based on the experimental data and theoretical corroboration, we presented an optical method that simultaneously determines strain and chemically-induced charge density for three popular excitation wavelengths of 457, 514 and 633 nm. In order to expedite intercalation of dopant species through the graphene-SiO2 substrates, dense arrays of nanopores were precisely generated in graphene by thermal oxidation. The nano-perforated graphene membrane system was robust for multiple cycles of doping and undoping processes, and will be useful in studying various types of chemical interactions with graphene.

Rational design of oxide/carbon composites to achieve superior rate-capability via enhanced lithium-ion transport across carbon to oxide

The superior rate-capability of nano-perforated graphene wrapped Li4Ti5O12 composite indicate that lithium-ion transport across the carbon coating layer is critical to the rate capability of carbon-coated oxides.

Nanoperforated graphene with alternating gap switching for optical applications

Complete configurations of monolayer graphene with zigzag-edged hexagonal nanohole are systematically built using three indices. The three integers lead to an atomistic precisely gap opening criteria, which steered via density functional theory based calculations. The guiding rule reveals that the semimetallic and semiconducting variation is consistent with a peculiar sequence with a period of 3 for odd and even indices differently. According to the cyclic sequence, there are one-third nanoporous graphene in our framework open a gap. The physical correspondence of Dirac point matching in reciprocal space and charge distribution in real space for the gap opening/closing is revealed. The periodic nanoperforation induced gap sizes were fitted as functions of three key indices. Furthermore, the coexistence of Dirac and flat bands is observed for some unique structures, which is sensitive to the atomic patterns. Our results indicate that the energy gap tunability of nano-perforated graphene can be used for flexible optoelectronic, photocatalytic and electrochemical applications. In particular, the atomically-thin electrically-connected geometry of nanoporous graphene suggests potential application in functional conductive coatings. The clear advantage of our system is in the combination of two properties: the possibility of applying electric voltage to the perforated sheet and rich chemically active atomic sites.

Resonance absorption of terahertz radiation in nanoperforated graphene

Recent measurements of the conductivity of nanoperforated graphene are interpreted in terms of edges states existing near the edge of each nanohole. The perimetric quantization of edge states should result in the formation of a quasi-equidistant ladder of quasistationary energy levels. Dirac fermions filling this ladder rotate about each nanohole in the direction determined by the valley index. It is shown that the irradiation of this system by circularly polarized terahertz radiation leads to a resonance in absorption in one of the valleys. The magnitude of absorption at the resonance frequency can be controlled by means of gate voltage.

Transfer of pre-assembled block copolymer thin film to nanopattern unconventional substrates.

In this work, we demonstrate that a preassembled block copolymer (BCP) thin film can be floated, transferred, and utilized to effectively nanopattern unconventional substrates. As target substrates, we chose Cu foil and graphene/Cu foil since they cannot be nanopatterned via conventional processes due to the high surface roughness and susceptibility to harsh processing chemicals and etchants. Perpendicular hexagonal PMMA cylinder arrays in diblock copolymer poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] thin films were preassembled on sacrificial SiO2/Si substrates. The BCP thin film was floated at the air/water interface off of a SiO2/Si substrate and then collected with the target substrate, leading to well-defined nanoporous PS templates on these uneven surfaces. We further show that the nanoporous template can be used for a subtractive process to fabricate nanoperforated graphene on Cu foil in sub-20 nm dimension, and for an additive process to create aluminum oxide nanodot arrays without any polymeric residues or use of harsh chemicals and etchants.

Orbital quantization in a system of edge Dirac fermions in nanoperforated graphene

The dependence of the electric resistance R of nanoperforated graphene samples on the position of the Fermi level E F, which is varied by the gate voltage V g, has been studied. Nanoperforation has been performed by irradiating graphene samples on a Si/SiO2 substrate by heavy (xenon) or light (helium) ions. A series of regular peaks have been revealed on the R(V g) dependence at low temperatures in zero magnetic field. These peaks are attributed to the passage of E F through an equidistant set of levels formed by orbitally quantized states of edge Dirac fermions rotating around each nanohole. The results are in agreement with the theory of edge states for massless Dirac fermions.

Electronic Transport and Raman Scattering in Size-Controlled Nanoperforated Graphene

We demonstrate the fabrication and study of the structure-property relationships of large-area (>1 cm(2)) semiconducting nanoperforated (NP) graphene with tunable constriction width (w = 7.5-14 nm), derived from CVD graphene using block copolymer lithography. Size-tunable constrictions were created while minimizing unintentional doping by using a dual buffer layer pattern-transfer method. An easily removable polymeric layer was sandwiched between an overlying silicon oxide layer and the underlying graphene. Perforation-size was controlled by overetching holes in the oxide prior to pattern transfer into graphene while the polymer protected the graphene from harsh conditions during oxide etching and lift off. The processing materials were removed using relatively mild solvents yielding the clean isolation of NP graphene and thereby facilitating Raman and electrical characterization. We correlate the D to G ratio as a function of w and show three regimes depending on w relative to the characteristic Raman relaxation length. Edge phonon peaks were also observed at 1450 and 1530 cm(-1) in the spectra, without the use of enhancement methods, due to high density of nanoconstricted graphene in the probe area. The resulting NP graphene exhibited semiconducting behavior with increasing ON/OFF conductance modulation with decreasing w at room temperature. The charge transport mobility decreases with increasing top-down reactive ion etching. From these comprehensive studies, we show that both electronic transport and Raman characteristics change in a concerted manner as w shrinks.

CHARACTERIZATION OF CONDUCTION MECHANISMS RELEVANT TO DEVICE PERFORMANCE IN NANOPERFORATED GRAPHENE

We have recently reported on the synthesis and characterization of a new form of nanostructured graphene that we call "nanoperforated graphene". Nanoperforated graphene is fabricated by etching a periodic array of nanoscale holes into atomic membranes of graphene to create an ultrathin superlattice-like structure. Nanoperforated graphene demonstrates semiconductor-like behavior and we have realized room-temperature field-induced conductance modulation as high as 450 (compared with &lt; 10 for unpatterned graphene) with field-effect mobilities of ~ 1 cm2V-1s-1. Here, we discuss the conduction mechanisms in nanoperforated graphene and the relevance of this new material for field-effect transistor devices. In nanoperforated graphene with 15 nm nanoconstrictions, we observe that the low-bias mobility is independent of temperature, consistent with elastic scattering-limited conduction. At low temperatures, a transport gap limits conduction in the sub-threshold regime and affects the threshold voltage for band conduction. We show that the high-bias electrical characteristics of nanoperforated graphene are similar to "artificial solids," a class of materials made of 2D arrays of Coulomb islands, consistent with observed Coulomb Blockade features in the sub-threshold regime. Currently, the device characteristics of the nanopatterned graphene material are found to be suitable for large-area, thin-film transistor applications. Future higher-performance applications are expected.

Fabrication and Characterization of Large-Area, Semiconducting Nanoperforated Graphene Materials

We demonstrate the fabrication of nanoperforated graphene materials with sub-20-nm features using cylinder-forming diblock copolymer templates across >1 mm(2) areas. Hexagonal arrays of holes are etched into graphene membranes, and the remaining constrictions between holes interconnect forming a honeycomb structure. Quantum confinement, disorder, and localization effects modulate the electronic structure, opening an effective energy gap of 100 meV in the nanopatterned material. The field-effect conductivity can be modulated by 40x (200x) at room temperature (T = 105 K) as a result. A room temperature hole mobility of 1 cm(2) V(-1) s(-1) was measured in the fabricated nanoperforated graphene field effect transistors. This scalable strategy for modulating the electronic structure of graphene is expected to facilitate applications of graphene in electronics, optoelectronics, and sensing.