High-Quality van der Waals Epitaxial CsPbBr_(3)Film Grown on Monolayer Graphene Covered TiO_(2)for High-Performance Solar Cells

Two-dimensional materials have been widely used to tune the growth and energy-level alignment of perovskites.However,their incomplete passivation and chaotic usage amounts are not conducive to the preparation of highquality perovskite films.Herein,we succeeded in obtaining higher-quality CsPbBr_(3)films by introducing large-area monolayer graphene as a stable physical overlay on top of TiO_(2)substrates.Benefiting from the inert and atomic smooth graphene surface,the CsPbBr_(3)film grown on top by the van der Waal epitaxy has higher crystallinity,improved(100)orientation,and an average domain size of up to 1.22μm.Meanwhile,a strong downward band bending is observed at the graphene/perovskite interface,improving the electron extraction to the electron transport layers(ETL).As a result,perovskite film grown on graphene has lower photoluminescence(PL)intensity,shorter carrier lifetime,and fewer defects.Finally,a photovoltaic device based on epitaxy CsPbBr_(3)film is fabricated,exhibiting power conversion efficiency(PCE)of up to 10.64%and stability over 2000 h in the air.

Self-assembly to monolayer graphene film with high electrical conductivity

Monolayer chemically converted graphene （CCG） nanosheets can be homogeneously self-assembled onto silicon wafer modified by 3-aminopr- opyl triethoxysilane （APTES） to form very thin graphene film. The CCG film was characterized by FT-IR, XRD, SEM, TEM and AFM. Results show that CCG sheets formed monolayer film after assembled onto silicon wafer and there is a very tight chemical bond between sheets and wafer. Furthermore, the electrical measurements revealed that the monolayer graphene film has an excellent electrical conductivity.

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.

A comparison of the transport properties of bilayer graphene,monolayer graphene,and two-dimensional electron gas

We studied and compared the transport properties of charge carriers in bilayer graphene, monolayer graphene, and the conventional semiconductors （the two-dimensional electron gas （2DEG））. It is elucidated that the normal incidence transmission in the bilayer graphene is identical to that in the 2DEG but totally different from that in the monolayer graphene. However, resonant peaks appear in the non-normal incidence transmission profile for a high barrier in the bilayer graphene, which do not occur in the 2DEG. Furthermore, there are tunneling and forbidden regions in the transmission spectrum for each material, and the division of the two regions has been given in the work. The tunneling region covers a wide range of the incident energy for the two graphene systems, but only exists under specific conditions for the 2DEG. The counterparts of the transmission in the conductance profile are also given for the three materials, which may be used as high-performance devices based on the bilayer graphene.

Transport properties in monolayer–bilayer–monolayer graphene planar junctions

The transport study of graphene based junctions has become one of the focuses in graphene research. There are two stacking configurations for monolayer–bilayer–monolayer graphene planar junctions. One is the two monolayer graphene contacting the same side of the bilayer graphene, and the other is the two-monolayer graphene contacting the different layers of the bilayer graphene. In this paper, according to the Landauer–Büttiker formula, we study the transport properties of these two configurations. The influences of the local gate potential in each part, the bias potential in bilayer graphene,the disorder and external magnetic field on conductance are obtained. We find the conductances of the two configurations can be manipulated by all of these effects. Especially, one can distinguish the two stacking configurations by introducing the bias potential into the bilayer graphene. The strong disorder and the external magnetic field will make the two stacking configurations indistinguishable in the transport experiment.

Nanoindentation Models of Monolayer Graphene and Graphyne under Point Load Pattern Studied by Molecular Dynamics

Molecular dynamics simulations are performed to study the nanoindentation models of monolayer suspended graphene and graphyne. Fullerenes are selected as indenters. Our results show that Young＇s modulus of monolayer-thick graphyne is almost half of that of graphene, which is estimated to be 0.50 TPa. The mechanical properties of graphene and graphyne are different in the presence of strain. A pre-tension has an important effect on the mechanical properties of a membrane. Both the pre-tension and Young＇s modulus plots demonstrate index behavior. The toughness of graphyne is stronger than that of graphene due to Young＇s modulus magnitude. Young＇s moduli of graphene and graphyne are almost independent of the size ratio of indenter to membrane.

Scalable and High-Quality Monolayer Graphene Transfer onto Polymer Membranes Assisted by Camphor

The quest for scalable integration of monolayer graphene into functional composites confronts the bottleneck of high-fidelity transfer onto substrates,crucial for unlocking graphene’s full potential in advanced applications.Addressing this,our research introduces the camphor-assisted transfer(CAT)method,a novel approach that surmounts common issues of residue and structural deformation endemic to existing techniques.Grounded in the sublimation dynamics of camphor,the CAT method achieves a clean,contiguous transfer of centimeter-scale monolayer graphene onto an array of polymer films,including ultra-thin polyethylene films.The resultant ultrathin graphene-polyethylene(gPE)films,characterized by their exceptional transparency and conductivity,reveal the CAT method’s unique ability to preserve the pristine quality of graphene,underscoring its practicality for preparing flexible transparent electrodes by monolayer graphene.In-depth mechanism investigation into the camphor sublimation during CAT has led to a pivotal realization:the porosity of the target polymer substrate is a determinant in achieving high-quality graphene transfer.Ensuring that camphor sublimates initially from the polymer side is crucial to prevent the formation of wrinkles or delamination of graphene.By extensive examination of CAT on a spectrum of commonly used polymer films,including PE,PP,PTFE,PI and PET,we have confirmed this important conclusion.This discovery offers crucial guidance for fabricating monolayer graphene-polymer composite films using methods akin to CAT,underscoring the significance of substrate selection in the transfer process.

A general synthetic strategy to monolayer graphene

The emergence and establishment of new techniques for material fabrication are of fundamental importance in the development of materials science. Thus, we herein report a general synthetic strategy for the preparation of monolayer graphene. This novel synthetic method is based on the direct solid-state pyrolytic conversion of a sodium carboxylate, such as sodium gluconate or sodium citrate, into monolayer graphene in the presence of Na2CO3. In addition, gram-scale quantities of the graphene product can be readily prepared in several minutes. Analysis using Raman spectroscopy and atomic force microscopy clearly demonstrates that the pyrolytic graphene is composed of a monolayer with an average thickness of - 0.50 nm. Thus, the present pyrolytic conversion can overcome the issue of the low monolayer contents （i.e., 1 wt.%-12 wt.%） obtained using exfoliation methods in addition to the low yields of chemical vapor deposition methods. We expect that this novel technique may be suitable for application in the preparation of monolayer graphene materials for batteries, supercapacitors, catalysts, and sensors.

Triple plasmon-induced transparency and outstanding slow-light in quasi-continuous monolayer graphene structure

We propose a simple quasi-continuous monolayer graphene structure and achieve a dynamically tunable triple plasmon-induced transparency(PIT)effect in the proposed structure.Additional analyses indicate that the proposed structure contains a selfconstructed bright-dark-dark mode system.A uniform theoretical model is introduced to investigate the spectral response characteristics and slow light-effects in the proposed system,and the theoretical and the simulated results exhibit high consistency.In addition,the influences of the Fermi level and the carrier mobility of graphene on transmission spectra are discussed.It is found that each PIT window exhibits an independent dynamical adjustability owing to the quasi-continuity of the proposed structure.Finally,the slow-light effects are investigated based on the calculation of the group refractive index and phase shift.It is found that the structure displays excellent slow-light effects near the PIT windows with high-group indices,and the maximum group index of each PIT window exceeds 1000 when the carrier mobility of graphene increases to 3.5 m^2 V^-1s^-1.The proposed structure has potential to be used in multichannel filters,optical switches,modulators,and slow-light devices.Additionally,the established theoretical model lays a theoretical basis for research on multimode coupling effects.

Wettability and Surface Free Energy Analyses of Monolayer Graphene

Recently Rafiee et al. experimentally demonstrated the wetting transparency of graphene, but there is still no comprehensive theoretical explanation of this physical phenomenon. Since surface free energy is one of the most important parameters characterizing material surfaces and is closely related to the wetting behavior, the surface free energy of suspended monolayer graphene is analyzed based on its microscopic formation mechanism. The surface free energy of suspended monolayer graphene is shown to be zero, which suggests its super-hydrophobicity. Neumann's equation of state is applied to further illustrate the contact angle, θ, of any liquid droplet on a suspended monolayer graphene is 180 o. This indicates that the van der Waals(vd W) interactions between the monolayer graphene and any liquid droplet are negligible; thus the monolayer graphene coatings exhibit wetting transparency to the underlying substrate. Moreover, molecular dynamics(MD) simulations are employed to further confirm the wetting transparency of graphene in comparison with experimental results of Rafiee et al. These findings provide a fundamental picture of wetting on ideal single atomic layer materials, including monolayer graphene. Thus, these results provide a useful guide for the design and manufacture of biomaterials, medical instruments, and renewable energy devices with monolayer graphene layers.

Electrical field tuning of magneto-Raman scattering in monolayer graphene

In this work, we report the electrical field tuning of magneto-phonon resonance in monolayer graphene under magnetic fields up to 9 T. It is found that the carrier concentration can drastically affect the G （E2g） phonon response to a varying magnetic field through a pronounced magneto-phonon resonance （MPR）. In charge neutral or slightly doped monolayer graphene, both the energy and the line width of the E2g phonon show clear variation with magnetic fields. This is attributed to magneto-phonon resonance between magnetoexcitations and the E2g phonons. In contrast, when the Fermi level of the monolayer graphene is far away from the Dirac point, the G band shows weak magnetic dependence and exhibits a symmetric line-shape. This suggests that the magneto-phonon coupling around 4 T has been switched off due to the Pauli blocking of the inter-Landau level excitations. Moreover, the G band asymmetry caused by Fano resonance between excitonic many-body states and the E2g phonons is observed. This work offers a way to study the magnetoexcitation phonon interaction of materials through magneto-Raman spectroscopy with an external electrical field.

Bound states of Dirac fermions in monolayer gapped graphene in the presence of local perturbations

In graphene,conductance electrons behave as massless relativistic particles and obey an analogue of the Dirac equation in two dimensions with a chiral nature.For this reason,the bounding of electrons in graphene in the form of geometries of quantum dots is impossible.In gapless graphene,due to its unique electronic band structure,there is a minimal conductivity at Dirac points,that is,in the limit of zero doping.This creates a problem for using such a highly motivated new material in electronic devices.One of the ways to overcome this problem is the creation of a band gap in the graphene band structure,which is made by inversion symmetry breaking（symmetry of sublattices）.We investigate the confined states of the massless Dirac fermions in an impured graphene by the short-range perturbations for ＂local chemical potential＂ and ＂local gap＂.The calculated energy spectrum exhibits quite different features with and without the perturbations.A characteristic equation for bound states（BSs） has been obtained.It is surprisingly found that the relation between the radial functions of sublattices wave functions,i.e.,f_m~＋（r）,g_m~＋（r）,and f_m^-（r）,g_m^-（r）,can be established by SO（2） group.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors （GFETs） fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density I DS, improved transconductance g m, reduced sheet resistance lion, and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Orbital electronic heat capacity of hydrogenated monolayer and bilayer graphene

The tight-binding Harrison model and Green＇s function approach have been utilized in order to investigate the contribution of hybridized orbitals in the electronic density of states（DOS） and electronic heat capacity（EHC） for four hydrogenated structures, including monolayer chair-like, table-like, bilayer AA- and finally AB-stacked graphene. After hydrogenation, monolayer graphene and bilayer graphene are behave as semiconducting systems owning a wide direct band gap and this means that all orbitals have several states around the Fermi level. The energy gap in DOS and Schottky anomaly in EHC curves of these structures are compared together illustrating the maximum and minimum band gaps are appear for monolayer chair-like and bilayer AA-stacked graphane, respectively. In spite of these, our findings show that the maximum and minimum values of Schottky anomaly appear for hydrogenated bilayer AA-stacked and monolayer table-like configurations, respectively.

Redistribution of carbon atoms in Pt substrate for high quality monolayer graphene synthesis

The two-dimensional material graphene shows its extraordinary potential in many application fields.As the most effective method to synthesize large-area monolayer graphene, chemical vapor deposition has been well developed; however, it still faces the challenge of a high occurrence of multilayer graphene, which causes the small effective area of monolayer graphene. This phenomenon limits its applications in which only a big size of monolayer graphene is needed. In this paper, by introducing a redistribution stage after the decomposition of carbon source gas to redistribute the carbon atoms dissolved in Pt foils, the number of multilayer flakes on the monolayer graphene decreases. The mean area of monolayer graphene can be extended to about 16 000μ m^2, which is eight times larger than that of the graphene grown without the redistribution stage. A Raman spectrograph is used to demonstrate the high quality of the monolayer graphene grown by the improved process.

Ion and water transport in charge-modified graphene nanopores

Porous graphene has a high mechanical strength and an atomic-layer thickness that makes it a promising material for material separation and biomolecule sensing. Electrostatic interactions between charges in aqueous solutions are a type of strong long-range interaction that may greatly influence fluid transport through nanopores. In this study, molecular dynamic simulations were conducted to investigate ion and water transport through 1.05-nm diameter monolayer graphene nanopores, with their edges charge-modified. Our results indicated that these nanopores are selective to counterions when they are charged. As the charge amount increases, the total ionic currents show an increase-decrease profile while the coion currents monotonically decrease. The co-ion rejection can reach 76.5% and 90.2% when the nanopores are negatively and positively charged, respectively. The Cl-ion current increases and reaches a plateau, and the Na＋current decreases as the charge amount increases in systems in which Na＋ions act as counterions. In addition, charge modification can enhance water transport through nanopores. This is mainly due to the ion selectivity of the nanopores. Notably, positive charges on the pore edges facilitate water transport much more strongly than negative charges.

Oxidation and Electrochemical Behavior of MonolayerGraphene-Coated Copper in Simulated Primary Water

In high-temperature and high-pressure water, traditional anticorrosion approaches are not suitable to be used to protect structural materials from oxidation and corrosion. In this study, monolayer graphene was explored as a barrier to protect the materials from degradation. The oxidation and corrosion rate of the monolayer-graphene-coated copper is much lower than that of the bare copper, suggesting that the monolayer graphene can effectively protect the copper from oxidation and corrosion in the simulated primary water of pressurized water reactors.

Tuning of the graphene surface plasmon by the monolayer MoS_(2)

We proposed a graphene based active plasmonic device by the introduction of graphene-MoS_(2) heterostructures. The device was composed of a monolayer MoS_(2) layer between the silicon substrate and periodically arranged graphene nanoribbon arrays. The finite-difference time domain(FDTD) method was used to analyze and compare the changes of the surface plasmon resonant wavelength and modulation depth(MD) in the two cases with and without MoS_(2). It was found that all the parameters of the width, period and Fermi level of the graphene nanoribbons affect the surface plasmon resonant wavelength of the plasmonic device. The introduction of the monolayer MoS_(2) can produce a redshift about 3 μm of the surface plasmon resonant wavelength, while the MD is basically unchanged. The redshift of the graphene surface plasmon resonant wavelength will provide application prospects for new active graphene plasmonic devices.

High-performance self-powered ultraviolet photodetector in SnO_(2)microwire/p-GaN heterojunction using graphene as charge collection medium

Graphene monolayer has been extensively applied as a transparency electrode material in photoelectronic devices due to its high transmittance,high carrier mobility,and ultrafast carrier dynamics.In this study,a high-performance self-powered photodetector,which is made of a SnO_(2)microwire,p-type GaN film,and monolayer graphene transparent electrode,was proposed and fabricated.The detector is sensitive to ultraviolet light signals and illustrates pronounced detection performances,including a peak respon-sivity∼223.7 mA W^(-1),a detectivity∼6.9×10^(12)Jones,fast response speed(rising/decaying times∼18/580μs),and excellent external quantum efficiency∼77%at 360 nm light illumination without exter-nal power supply.Compared with the pristine SnO_(2)/GaN photodetector using ITO electrode,the device performances of responsivity and detectivity are significantly increased over 6×10^(3)%and 3×10^(3)%,respectively.The performance-enhanced characteristics are mainly attributed to the high-quality het-erointerface of n-SnO_(2)/p-GaN,the highly conductive capacity,and the unique transparency of graphene electrodes.Particularly,the built-in potential formed at the SnO_(2)/GaN heterojunction interface could be strengthened by the Schottky potential barrier derived from the graphene electrode and SnO_(2)wire,en-hancing the carrier collection efficiency through graphene as a charge collection medium.This work is of great importance and significance to developing excellent-performance ultraviolet photodetectors for photovoltaic and optoelectronic applications in a self-powered operation manner.

Electron transition manipulation under graphene-mediated plasmonic engineering nanostructure

Monolayer graphene has attracted enormous attention owing to its unique electronic and optical properties.However,achieving an effective approach without applying electrical bias for manipulating the charge transfer based on graphene is elusive to date.Herein,we realized the manipulation of excitons’transition from emitter to the graphene surface with plasmonic engineering nanostructures and firstly obtained large enhancements for photon emission on the graphene surface.The localized plasmons generated from the plasmonic nanostructures of shell-isolated nanoparticle coupling to ultra-flat Au substrate would dictate a consistent junction geometry while enhancing the optical field and dominating the electron transition pathways,which may cause obvious perturbations for molecular radiation behaviors.Additionally,the three-dimensional finite-difference time-domain and time-dependent density functional theory were also carried out to simulate the distributions of electromagnetic field and energy levels of hybrid nanostructure respectively and the results agreed well with the experimental data.Therefore,this work paves a novel approach for tunning graphene charge/energy transfer processes,which may hold great potential for applications in photonic devices based on graphene.