High Quantum Capacitance Research Articles

2D Graphene-Based Macroscopic Assemblies for Micro-Supercapacitors.

Rapid development of portable and wearable electronic devices has triggered increased research interest in small-scale power sources, especially in micro-supercapacitors (MSCs) because of their high power densities, long service life, and ability to be charged and discharged quickly. Graphene, an ideal two-dimensional energy-storage electrode material with good conductivity, high quantum capacitance, and large specific surface area, can be used as a building block for MSCs with multi-dimensional architectures. Considerable efforts have been devoted to constructing structures with different dimensions for advanced graphene-based MSCs (GMSCS). In this Review, we summarize the recent progress of graphene-based macroscopic assemblies in MSCs, including 1D fiber GMSCs, 2D planar GMSCs; and 3D in-plane or stacked GMSCs, and discuss the relationship between the structures and applications of the devices. In addition, future prospects and challenges in the MSCs are also discussed.

DFT calculation for stability and quantum capacitance of MoS2 monolayer-based electrode materials

The stability, electrical properties and quantum capacitance of Ti, Au, Ag, Cu, Al, B, N, and P doping pristine and single-vacancy VS MoS2 monolayer have been studied by using the first-principles methods. These doped atoms form strong bonds with MoS2, and the optimized structures exhibit metallicity. MoS2 monolayer has a high double layer capacitance due to its high specific surface area. However, it has low quantum capacitance hindering its application in supercapacitors. This is attributed to the confinement of the electron density of states near Fermi level of pristine MoS2 monolayer. Our calculation of quantum capacitance confirms the advantage of Al substituting S atom in single-vacancy VS MoS2 systems, and it is suitable for symmetric supercapacitors. We also find that B substituting S atom in pristine MoS2 monolayer has a high quantum capacitance, and it is available as potential electrode materials for asymmetric supercapacitors. We demonstrate that the increase of quantum capacitance is due to the formation of local states near Fermi level and/or the shift of Fermi level caused by defects and doping.

Enhancement of quantum capacitance by chemical modification of graphene supercapacitor electrodes: a study by first principles

In this paper, we specify a powerful way to boost quantum capacitance of graphene-based electrode materials by density functional theory calculations. We performed functionalization of graphene to manifest high-quantum capacitance. A marked quantum capacitance of above $$420\,\upmu \mathrm{F}\,\mathrm{cm}^{-2}$$ has been observed. Our calculations show that quantum capacitance of graphene enhances with nitrogen concentration. We have also scrutinized effect on the increase of graphene quantum capacitance due to the variation of doping concentration, configuration change as well as co-doping with nitrogen and oxygen ad-atoms in pristine graphene sheets. A significant increase in quantum capacitance was theoretically detected in functionalized graphene, mainly because of the generation of new electronic states near the Dirac point and the shift of Fermi level caused by ad-atom adsorption.

Route to achieving enhanced quantum capacitance in functionalized graphene based supercapacitor electrodes

We have investigated the quantum capacitance () in functionalized graphene modified with ad-atoms from different groups in the periodic table. Changes in the electronic band structure of graphene upon functionalization and subsequently the of the modified graphene were systematically analyzed using density functional theory (DFT) calculations. We observed that the can be enhanced significantly by means of controlled doping of N, Cl and P ad-atoms in the pristine graphene surface. These ad-atoms are behaving as magnetic impurities in the system, generating a localized density of states near the Fermi energy which, in turn, increases charge (electron/hole) carrier density in the system. As a result, a very high quantum capacitance was observed. Finally, the temperature dependent study of for Cl and N functionalized graphene shows that the remains very high in a wide range of temperatures near room temperature.

Improving the Quantum Capacitance of Graphene-Based Supercapacitors by the Doping and Co-Doping: First-Principles Calculations.

We explore the stability, electronic properties, and quantum capacitance of doped/co-doped graphene with B, N, P, and S atoms based on first-principles methods. B, N, P, and S atoms are strongly bonded with graphene, and all of the relaxed systems exhibit metallic behavior. While graphene with high surface area can enhance the double-layer capacitance, its low quantum capacitance limits its application in supercapacitors. This is a direct result of the limited density of states near the Dirac point in pristine graphene. We find that the triple N and S doping with single vacancy exhibits a relatively stable structure and high quantum capacitance. It is proposed that they could be used as ideal electrode materials for symmetry supercapacitors. The advantages of some co-doped graphene systems have been demonstrated by calculating quantum capacitance. We find that the N/S and N/P co-doped graphene with single vacancy is suitable for asymmetric supercapacitors. The enhanced quantum capacitance contributes to the formation of localized states near the Dirac point and/or Fermi-level shifts by introducing the dopant and vacancy complex.

Effect of Anion on the Quantum Capacitance of Graphene Cathode in Lithium Ion Capacitor: A DFT Study

Lithium ion capacitor (LIC) is a kind of electrochemical energy storage device that can combine the power property of a supercapacitor and the energy property of a lithium ion battery simultaneously [1]. Graphene is widely employed as cathode because it can provide excellent capacitance, as well as constructing an effective conducting network [2, 3]. However, the detail understandings of electrode/electrolyte interface in graphene-based LIC is still limited. Based on our previous results, graphene with single vacant defect or pyridinic and pyrrolic doped N atom show much higher quantum capacitance (QC) than the pristine one [4]. This difference can be attributed to the presence of reactive σ state near the Fermi level, which is from the C or N atoms around the defect region. However, when it comes to the real LIC system, reactions may occur between the electrolyte and those reactive atoms in graphene cathode. Therefore, the presence of electrolyte will disturb the density of states (DOS) of graphene-based cathode, and hence the QC and the energy storage ability will vary accordingly. In this work, the interaction between the anion in the electrolyte and graphene cathode with high QC is investigated using First Principle calculation. The results suggest that the defect states of graphene cathode can enhance the adsorption energy towards anion, and hence the QC and electrical double layer (EDL) structure is different with that of pristine graphene cathode. Furthermore, this interaction also decrease the stability of the anion in electrolyte. The results from this work would help to further develop high energy graphene-based LIC and shed some light on its capacity fading mechanisms. Reference: [1] Cericola, et al., Electrochim. Acta, 72(2012), 1. [2] Chen, et al., Energ Environ Sci, 6(2013), 1623. [3] Yu, et al., Nano Energy, 15(2015), 43. [4] Su, et al., Catalysts 8(2018,) 444

Theoretical Study on the Quantum Capacitance Origin of Graphene Cathodes in Lithium Ion Capacitors

Quantum capacitance (QC) is a very important character of the graphene cathode in lithium ion capacitors (LIC), which is a novel kind of electrochemical energy conversion and storage device. However, the QC electronic origin of the graphene cathode, which will affect the electrochemical reaction at the electrode/electrolyte interface, is still unclear. In this article, the QC of various kinds of graphene cathode is investigated systematically by DFT calculation. It was found that the value and origin of QC strongly depend on the defects and alien atoms of graphene. Graphene with pentagon defects possesses a higher QC than pristine graphene due to the contribution from the electronic states localized at the carbon pentagon. The introduction of graphitic B can contribute to QC, while graphitic N and P does not work in the voltage range of the LIC cathode. Single vacant defect graphene and pyrrolic N-doped graphene demonstrate very high QC due to the presence of states associated with the σ orbital of unbonded carbon atoms. However, pyridinic graphene shows an even higher QC because of the states from the N atom. For the residual O in graphene, its QC mainly originated from the pz states of carbon atoms and the effect of O, especially the O in bridged oxygen functional group (–COC–), is very limited. These results provide new insight into further study of the catalytic behavior and the design of a high performance graphene cathode for LIC.

Intergrain Diffusion of Carbon Radical for Wafer-Scale, Direct Growth of Graphene on Silicon-Based Dielectrics.

Graphene intrinsically hosts charge-carriers with ultrahigh mobility and possesses a high quantum capacitance, which are attractive attributes for nanoelectronic applications requiring graphene-on-substrate base architecture. Most of the current techniques for graphene production rely on the growth on metal catalyst surfaces, followed by a contamination-prone transfer process to put graphene on a desired dielectric substrate. Therefore, a direct graphene deposition process on dielectric surfaces is crucial to avoid polymer-adsorption-related contamination from the transfer process. Here, we present a chemical-diffusion mechanism of a process for transfer-free growth of graphene on silicon-based gate-dielectric substrates via low-pressure chemical vapor deposition. The process relies on the diffusion of catalytically produced carbon radicals through polycrystalline copper (Cu) grain boundaries and their crystallization at the interface of Cu and underneath silicon-based gate-dielectric substrates. The graphene produced exhibits low-defect multilayer domains ( La ∼ 140 nm) with turbostratic orientations as revealed by selected area electron diffraction. Further, graphene growth between Cu and the substrate was 2-fold faster on SiO2/Si(111) substrate than on SiO2/Si(100). The process parameters such as growth temperature and gas compositions (hydrogen (H2)/methane (CH4) flow rate ratio) play critical roles in the formation of high-quality graphene films. The low-temperature back-gating charge transport measurements of the interfacial graphene show density-independent mobility for holes and electrons. Consequently, the analysis of electronic transport at various temperatures reveals a dominant Coulombic scattering, a thermal activation energy (2.0 ± 0.2 meV), and two-dimensional hopping conduction in the graphene field-effect transistor. A band overlapping energy of 2.3 ± 0.4 meV is estimated by employing the simple two-band model.

Density Functional Theory Calculations of the Quantum Capacitance of Graphene Oxide as a Supercapacitor Electrode

Graphene oxide has become an attractive electrode-material candidate for supercapacitors thanks to its higher specific capacitance compared to graphene. The quantum capacitance makes relative contributions to the specific capacitance, which is considered as the major limitation of graphene electrodes, while the quantum capacitance of graphene oxide is rarely concerned. This study explores the quantum capacitance of graphene oxide, which bears epoxy and hydroxyl groups on its basal plane, by employing density functional theory (DFT) calculations. The results demonstrate that the total density of states near the Fermi level is significantly enhanced by introducing oxygen-containing groups, which is beneficial for the improvement of the quantum capacitance. Moreover, the quantum capacitances of the graphene oxide with different concentrations of these two oxygen-containing groups are compared, revealing that more epoxy and hydroxyl groups result in a higher quantum capacitance. Notably, the hydroxyl concentration has a considerable effect on the capacitive behavior.

Cancer Cell Hyperactivity and Membrane Dipolarity Monitoring via Raman Mapping of Interfaced Graphene: Toward Non-Invasive Cancer Diagnostics.

Ultrasensitive detection, mapping, and monitoring of the activity of cancer cells is critical for treatment evaluation and patient care. Here, we demonstrate that a cancer cell's glycolysis-induced hyperactivity and enhanced electronegative membrane (from sialic acid) can sensitively modify the second-order overtone of in-plane phonon vibration energies (2D) of interfaced graphene via a hole-doping mechanism. By leveraging ultrathin graphene's high quantum capacitance and responsive phononics, we sensitively differentiated the activity of interfaced Glioblastoma Multiforme (GBM) cells, a malignant brain tumor, from that of human astrocytes at a single-cell resolution. GBM cell's high surface electronegativity (potential ∼310 mV) and hyperacidic-release induces hole-doping in graphene with a 3-fold higher 2D vibration energy shift of approximately 6 ± 0.5 cm-1 than astrocytes. From molecular dipole-induced quantum coupling, we estimate that the sialic acid density on the cell membrane increases from one molecule per ∼17 nm2 to one molecule per ∼7 nm2. Furthermore, graphene phononic response also identified enhanced acidity of cancer cell's growth medium. Graphene's phonon-sensitive platform to determine interfaced cell's activity/chemistry will potentially open avenues for studying activity of other cancer cell types, including metastatic tumors, and characterizing different grades of their malignancy.

Electronic structures and quantum capacitance of monolayer and multilayer graphenes influenced by Al, B, N and P doping, and monovacancy: Theoretical study

Graphene has been extensively explored as an electrode material in supercapacitors because of its large surface area and high electronic conductivity. By designing diverse morphologies and doping graphene with certain elements, the properties of graphene can be efficiently modified. We present the influence of Al, B, N and P doping, monovacancy and multilayer graphene structures on stability, electronic structures and quantum capacitance, by applying density functional theory calculations. The electrode quantum capacitances are substantially modified due to doping, the presence of monovacancy, and interaction between layers occurred in multilayer structures. Our calculations suggest that the monolayer graphene with monovacancy, the monolayer and multilayer graphene structures with nitrogen doped around the monovacancy, and multilayer graphene structure with aluminum doped could provide substantial change of quantum capacitance. However, the structure stability could be challenging. The interaction between layers could lower quantum capacitances compared to those of the monolayer structures with the same dopant elements. Moreover, the association of monovacancy and nitrogen doping of a single layer structure could lead to as high quantum capacitance as ∼80 μF/cm2. This work suggests the possibility to enhance quantum capacitance of the graphene-based electrodes using the combination effect of doping, vacancy defect and stacking layers.

Interfacial nondegenerate doping of MoS2 and other two-dimensional semiconductors.

Controlled nondegenerate doping of two-dimensional semiconductors (2DSs) with their ultraconfined carriers, high quantum capacitance, and surface-sensitive electronics can enable tuning their Fermi levels for rational device design. However, doping techniques for three-dimensional semiconductors, such as ion implantation, cannot be directly applied to 2DSs because they inflict high defect density. In this issue of ACS Nano, Park et al. demonstrate that interfacing 2DSs with substrates having dopants can controllably inject carriers to achieve nondegenerate doping, thus significantly broadening 2DSs' functionality and applications. Futuristically, this can enable complex spatial patterning/contouring of energy levels in 2DSs to form p-n junctions, integrated logic, and opto/electronic devices. The process is also extendable to biocellular-interfaced devices, band-continuum structures, and intricate 2D circuitry.

Electronic Properties and Orientation-Dependent Performance of InAs Nanowire Transistors

The electronic properties, namely, the band structures, the band gaps, and the electron effective masses of hydrogen-passivated InAs nanowires grown in 〈100〉 , 〈110〉, and 〈111〉 crystallographic directions are studied using sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s* orbital-basis tight-binding model. We then parameterize the band gaps and electron effective masses to facilitate device simulation and to study the orientation-dependent performance of n-channel InAs nanowire transistors using a top-of-the-barrier model. The 〈111〉 and 〈110〉 wire transistors have better performance metrics. The quantum-confinement effect is largest in the 〈100〉 wire, which results in a higher band gap and a heavier effective mass for relatively smaller diameter wires. The consequence is lower current, higher density of states, higher quantum capacitance, and longer delay in the 〈100〉 wire transistors. The 〈110〉 and 〈111〉 wires have a very similar quantum-confinement effect, even for the smaller diameters, which results in similar band gaps, similar effective masses, and similar performance metrics.