Graphene Devices: Structured Graphene Devices for Mass Transport (Small 6/2011)

The reversible atomic‐mass transport along graphene devices has been achieved. The motion of Al and Au in the form of atoms or clusters is driven by applying an electric field between the metal electrodes that contact the graphene sheet. It is shown that Al moves in the direction of the applied electric field whereas Au tends to diffuse in all directions. The control of the motion of Al is further demonstrated by achieving a 90° turn, using a graphene device patterned in a crossroads configuration. The controlled motion of Al is attributed to the charge transfer from Al onto the graphene so that the Al is effectively charged and can be accelerated by the applied electric field. To get further insight into the actuation mechanism, theoretical simulations of individual Al and Au impurities on a perfect graphene sheet were performed. The direct (electrostatic) force was found to be ～1 pN and dominant over the wind force. These findings hold promise for practical use in future mass transport in complex circuits.     

Ion transport through a graphene nanopore

Molecular dynamics simulation is utilized to investigate the ionic transport of NaCl in solution through a graphene nanopore under an applied electric field. Results show the formation of concentration polarization layers in the vicinity of the graphene sheet. The nonuniformity of the ion distribution gives rise to an electric pressure which drives vortical motions in the fluid if the electric field is sufficiently strong to overcome the influence of viscosity and thermal fluctuations. The relative importance of hydrodynamic transport and thermal fluctuations in determining the pore conductivity is investigated. A second important effect that is observed is the mass transport of water through the nanopore, with an average velocity proportional to the applied voltage and independent of the pore diameter. The flux arises as a consequence of the asymmetry in the ion distribution which can be attributed to differing mobilities of the sodium and chlorine ions and to the polarity of water molecules. The accumulation of liquid molecules in the vicinity of the nanopore due to re-orientation of the water dipoles by the local electric field is seen to result in a local increase in the liquid density. Results confirm that the electric conductance is proportional to the nanopore diameter for the parameter regimes that we simulated. The occurrence of fluid vortices is found to result in an increase in the effective electrical conductance.     

Wafer scale homogeneous bilayer graphene films by chemical vapor deposition

The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm(2) size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices.     

Nanoscale vibration analysis of embedded multi-layered graphene sheets under various boundary conditions

The present article deals with the vibrational analysis of multi-layered graphene sheets with different boundary conditions amongst sheets. An elastic multiple-plate model is utilized in which the nested plates are coupled with each other through the van der Waals interlayer force. The interaction of van der Waals forces between adjacent and non-adjacent layers and the reaction from the surrounding media are included in the Reissner–Mindlin-type field equations on which the theoretical formulation is based. The set of coupled equations of motion for the multi-layered graphene sheets is then solved by the generalized differential quadrature method. The numerical analysis presented herein provides the possibility of considering various combinations of layerwise boundary conditions in a multi-layered graphene sheet. Based on exact solution, explicit formulas for the frequencies of a double-layered graphene sheet with all edges simply supported are also obtained. The results of the present numerical solution are shown to be in excellent agreement with those of exact solution for simply supported graphene sheets.     

Application of scanning probe lithography to graphite patterning

We applied the scanning probe lithographic technique to a graphite patterning in air and analyzed the patterned sample with the lateral force microscopy and Raman spectroscopy. The local electric field generated from a tip caused either etching or oxidization depending on the electric field intensity in air. We have found that the frictional force between the tip and local oxidized graphite surface was increased remarkably from lateral force analysis. Also, it was found that the graphene layer was peeled from the graphite surface in the etching process, which could be a potential tool as a top-down nano-fabrication process for the graphene nano device without contamination.     

Synthesis and characterization of plasmon resonant gold nanoparticles and graphene for photovoltaics

Here we discuss the use in solar cells of graphene grown by chemical vapor deposition (CVD) and of plasmonic gold nanoparticles (Au NPs) deposited by sputtering. The Au NPs have been coupled with a-Si heterojunction solar cells, with an organic active layer used in organic photovoltaics, and with graphene. Extensive characterization of those three systems by the optical technique of spectroscopic ellipsometry, which is suitable to monitor and analyze the plasmon resonance of the Au NPs, by the microstructural technique of Raman spectroscopy, which is suitable to analyze graphene properties and doping, and by atomic force microscopy has been carried out. Those techniques highlighted interactions between Au NPs and silicon, polymer and graphene, which lead to variation in the plasmon resonance of Au NPs and consequently in the characteristics of the Au NPs/Si, Au NPs/polymer and Au NPs/graphene hybrids. Specifically, we found that an optimal size and density of Au NPs are able to enhance the efficiency of c-Si/a-Si heterojunction solar cells and that exceeding with Au NPs size and density causes device shortcut because of interface interdiffusion between silicon and gold. To discuss organic photovoltaics, Au NPs have been combined with an electro-donating conjugated polymer, the poly[1,4bis(2-thienyl)-2,5-bis-(2-ethyl-hexyloxyphenylenes)]. We found that there is a strong correlation between the thickness and morphology of the organic active layer, which affects the energy and amplitude of the Au NPs plasmon resonance. Finally, Au NPs have been deposited on graphene. We found that Au NPs show the plasmon resonance in the region where graphene is transparent and also yield p-type doping of graphene decreasing its sheet resistance.     

Carrier transport mechanisms and photovoltaic characteristics of Au/toluidine blue/n-Si/Al heterojunction solar cell

Toluidine blue (TB)/n-silicon heterojunction solar cell was fabricated by depositing TB film on n-silicon wafer using thermal deposition technique. X-ray diffraction patterns of the TB film show presence of crystals with size 30 nm dispersed in amorphous matrix. The current–voltage–temperature performance of Au/TB/n-Si/Al device was studied in dark and under illumination conditions. The device showed diode behavior. The diode parameters such as ideality factor, barrier height, series and shunt resistance were determined using a conventional I–V–T characteristics. The analysis of the diode characteristics in forward bias direction confirmed that the transport mechanisms of the Au/TB/n-Si/Al solar cell at applied potential?<?0.1 V is thermionic emission and at high electric field?>?0.1 V is Ohmic conduction. The operating conduction mechanisms in reverse bias direction are Pool–Frenkel effect followed by Schootky field lowering mechanism. The small value of activation energy in reverse bias direction indicates that the conduction process is expected to be by tunneling of electrons between nearest-neighbor sites and it is temperature independent. The photo conduction characteristics of the diode suggests its application as a solar cell.     

Metal electroreflectance measurements on MIM structures: Theoretical and experimental results

MIM electroreflectance data obtained with the symmetrical structures Al/Al₂O₃/Al, Au/Al₂O₃/Au and Au/ZnS/Au afford information about the metal-insulator interfaces. Al/Al₂O₃/Al gives no electroreflectance signal. Interpretation in terms of a change in the surface conductivity of gold when the electric field is applied is in good agreement with experimental data for Au/Al₂O₃/Au but the agreement is only qualitative for Au/ZnS/Au.     

Aspects of the theory of graphene

Following a brief review of the device-friendly features of graphene, recent work on its Green's functions with and without a normal magnetic field are discussed, for an infinite graphene sheet and also for a quantum dot, with analyses of the Landau-quantized energy spectra of the sheet and dot. The random phase approximation dielectric response of graphene is reviewed and discussed in connection with the van der Waals interactions of a graphene sheet with atoms/molecules and with a second graphene sheet in a double layer. Energy-loss spectroscopy for a graphene sheet subject to both parallel and perpendicular particle probes of its dynamic, non-local response properties are also treated. Furthermore, we discuss recent work on the coupling of a graphene plasmon and a surface plasmon, yielding a collective plasma mode that is linear in wavenumber. Finally, we discuss the unusual aspects of graphene conduction and recent work on diffusive charge transport in graphene, in both the DC and AC regimes.     

Manipulating thermal conductance at metal-graphene contacts via chemical functionalization

Graphene-based devices have garnered tremendous attention due to the unique physical properties arising from this purely two-dimensional carbon sheet leading to tremendous efficiency in the transport of thermal carriers (i.e., phonons). However, it is necessary for this two-dimensional material to be able to efficiently transport heat into the surrounding 3D device architecture in order to fully capitalize on its intrinsic transport capabilities. Therefore, the thermal boundary conductance at graphene interfaces is a critical parameter in the realization of graphene electronics and thermal solutions. In this work, we examine the role of chemical functionalization on the thermal boundary conductance across metal/graphene interfaces. Specifically, we metalize graphene that has been plasma functionalized and then measure the thermal boundary conductance at Al/graphene/SiO(2) contacts with time domain thermoreflectance. The addition of adsorbates to the graphene surfaces are shown to influence the cross plane thermal conductance; this behavior is attributed to changes in the bonding between the metal and the graphene, as both the phonon flux and the vibrational mismatch between the materials are each subject to the interfacial bond strength. These results demonstrate plasma-based functionalization of graphene surfaces is a viable approach to manipulate the thermal boundary conductance.     

电场极化增强的AlN薄片的储氢特性

采用基于密度泛函理论(DFT)的第一原理方法,研究了外加电场对单层AlN薄片储氢性能的影响。通过几何优化得到AlN薄片最稳定的吸氢位置为N原子顶位。研究结果表明:在一定范围内,随着外加电场强度的增加,H2分子在AlN薄片上的吸附能逐渐增大,N—H键长越来越小,H—H键长越来越大。态密度分析表明,加上外电场之后,H-1s轨道与N-2p轨道杂化导致N与H间的交互作用增强。说明电场极化使AlN薄片与H2分子结合得更加紧密,大大提高了AlN薄片的储氢稳定性。而一旦撤销外加电场,H2分子又能恢复到在AlN薄片上的物理吸附状态,使得吸放氢可逆。研究还发现在电场作用下,可同时在AlN薄片的上下表面各吸附一层H2分子,储氢容量显著提高。     

Tuning the Doping Type and Level of Graphene with Different Gold Configurations

Au nanoparticles and films are deposited onto clean graphene surfaces to study the doping effect of different Au configurations. Micro‐Raman spectra show that both the doping type and level of graphene can be tuned by fine control of the Au deposition. The morphological structures of Au on graphene are imaged by transmission electron microscopy, which indicate a size‐dependent electrical characteristic: isolated Au nanoparticles produce n‐type doping of graphene, while continuous Au films produce p‐type doping. Accordingly, graphene field‐effect transistors are fabricated, with the in situ measurements suggesting the tunable conductivity type and level by contacting with different Au configurations. For interpreting the experimental observations, the first‐principles approach is used to simulate the interaction within graphene–Au systems. The results suggest that, different doping properties of Au–graphene systems are induced by the chemical interactions between graphene and the different Au configurations (isolated nanoparticle and continuous film).     

Synthesis of graphene-gold nanocomposites via sonochemical reduction

A Reduced reduced graphene oxide (RGO)-gold (Au) nanoparticle (NP) nanocomposite was synthesized by simultaneously reducing the Au ions and depositing Au NPs on onto the surface surface of the RGOsRGO simultaneously. To facilitate the reduction of Au ions and the generation of oxygen functionalities for anchoring the Au NPs on the RGOsRGO, ultrasound irradiation was applied to the mixture of reactants. The functional groups were investigated with FT-IR spectra. From the Raman and XPS spectra, the oxygen groups were identified as hydroxyl, epoxy, and carboxyl groups, the same as the one from graphene oxide (GO). As a result, the dense and uniform deposition of nanometer-sized Au NPs with nanometer size was observed on the RGO sheets sheet was observed with from the TEM imagesimage. The Oxygen oxygen functional groups that formed on the surface surface of the RGOs RGO seemed to have served serve as links for Au NPs NP attachment, through the electrostatic attraction of Au ions. Hybrid materials could thus be produced in a short time, with a high yield, by via ultrasound application. Besides, it ultrasound application could can readily take goldAu- binding- peptide (GBP)-modified biomolecules, readily implying its possibility in possible biological applications.     

Effects of external electric field on recrystallization texture and microstructure of 08Al killed steel sheet

The effects of electric field annealing on recovery and recrystallization in a cold-rolled 08Al killed steel sheet were investigated. Results show that, with the application of a DC electric field, the recovery and recrystallization processes are retarded and the recrystallization -fiber texture is strengthened. Those retardation effects are attributed to a decrease of the driving force for recrystallization caused by electric field, which would hinder grain nucleation and growth on the whole. Possible reason for the intensification of the recrystallization -fiber texture through electrical annealing is briefly discussed.     

Facile coordinating decoration of salophen Al on graphene nanosheet: Salophen Al complex functionalized graphene nanocomposite with electrochemical properties

Salophen Al complex functionalized graphene (SCFG) nanocomposite was synthesized by simple coordination of phenol functionalized graphene (PFG) and as-prepared salophen Al complex. This process is facile, convenient and high efficient. We also investigated the structure, optical properties and electrochemical properties of the obtained SCFG composites. The results showed that salophen Al conjugated and incorporated onto the graphene sheet surface and the composites maintained the micro-structure of graphene sheets without agglomeration. The photoluminescence of salophen Al was completely quenched by graphene, due to the charge transfer between the salophen Al and the graphene nanosheets. Moreover, a significant electrochemical signal emerged on the SCFG modified electrodes compared with those of the graphene sheets or salophen Al complex. Owing to preliminary results of the excellent electrochemical property, SCFG nanocomposite is a promising electrochemical redox probe material.     

Edge effect on resistance scaling rules in graphene nanostructures

We report an experimental investigation of the edge effect on the room-temperature transport in graphene nanoribbon and graphene sheet (both single-layer and bilayer). By measuring the resistance scaling behaviors at both low- and high-carrier densities, we show that the transport of single-layer nanoribbons lies in a strong localization regime, which can be attributed to an edge effect. We find that this edge effect can be weakened by enlarging the width, decreasing the carrier densities, or adding an extra layer. From graphene nanoribbon to graphene sheet, the data show a dimensional crossover of the transport regimes possibly due to the drastic change of the edge effect.     

Limitations of Au Particle Nanoassembly Using Dielectrophoretic Force—A Parametric Experimental and Theoretical Study

When a gold colloidal suspension is subjected to ac electric field, “gold pearl chains” will form due to the dielectrophoretic (DEP) force. Our latest experiments show that the formation rate of gold pearl chains, which tends to zero at high and low frequency limits and has a maximum at a narrow mid range of frequency, is dependent on the applied field frequency. This letter analyzes the frequency-dependent DEP manipulation of gold colloid suspensions using the protoplast model. Simulated results show that the relationship curve between the frequency of applied field and the velocity of gold colloids motion due to DEP agrees with our experimental observations. In addition, the orders of magnitude of the velocity due to various effects in our experimental system, such as DEP force, Brownian motion, gravity, and fluid flows induced by electric field, were also estimated. The result implies that the DEP-based manipulation of less than 2 nm gold colloids is extremely difficult to be controlled.      

Vibration characteristics of embedded multi-layered graphene sheets with different boundary conditions via nonlocal elasticity

A nonlocal elastic plate model accounting for the small scale effects is developed to investigate the vibrational behavior of multi-layered graphene sheets under various boundary conditions. Based upon the constitutive equations of nonlocal elasticity, derived are the Reissner–Mindlin-type field equations which include the interaction of van der Waals forces between adjacent and non-adjacent layers and the reaction from the surrounding media. The set of coupled governing equations of motion for the multi-layered graphene sheets are then numerically solved by the generalized differential quadrature method. The present analysis provides the possibility of considering different combinations of layerwise boundary conditions in a multi-layered graphene sheet. Based on exact solution, explicit expressions for the nonlocal frequencies of a double-layered graphene sheet with all edges simply supported are also obtained. The results from the present numerical solution, where possible, are indicated to be in excellent agreement with the existing data from the literature.     

Ion detection by Fourier transform ion cyclotron resonance: the effect of initial radial velocity on the coherent ion packet

Ion detection by Fourier transform ion cyclotron resonance (FT-ICR) is accomplished by observing a coherent ion packet produced from an initially random ensemble of ions. The coherent packet is formed by excitation with a resonant oscillating electric field. Ions that are out of phase with the applied radio frequency (rf) electric field experience a continuous misalignment of the electric field vector. The misalignment creates a net force of the electric field perpendicular to ion motion. The perpendicular component of the rf electric field creates a frequency shift resulting in phase synchronization of the ion ensemble. The phase coherence of the ion packet affects both the sensitivity and the resolution of FT-ICR.     

电场对Au/SiO_2及Au-Ag/SiO_2表面结构与原子迁移特性的影响

利用扫描俄歇微探针（SAM）和原子力显微镜（AFM）研究了SiO2衬底上在外加直流电场作用下沉积的Au薄膜及Au－Ag复层薄膜的表面形貌、结构变化及电迁移扩散行为。结果表明：①在衬底表面施加水平方向电场辅助沉积制备的Au薄膜其表面显示出平整的椭球形晶粒，并沿外电场方向呈织构取向。与未加电场的热蒸发沉积膜相比，具有较为均匀、有序的表面微观结构。②SiO2表面Au-Ag复层薄膜在直流电场作用下，Au，Ag物种同时向负极方向作走向迁移扩散，这与Au－Ag复层薄膜在Si（111）表面电迁移时Au，Ag分别向两极扩散的特点不同，反映了衬底性质对表面原子电迁移的影响。③Au－Ag复膜在电迁移过程中还发生了表面原子聚集状态的变化，原来沉积排布的细小晶粒在电迁移扩散过程中出现不均匀长大，导致薄膜表面粗糙度显著增加。