Visualizing local doping effects of individual water clusters on gold(111)-supported graphene

The local charge carrier density of graphene can exhibit significant and highly localized variations that arise from the interaction between graphene and the local environment, such as adsorbed water, or a supporting substrate. However, it has been difficult to correlate such spatial variations with individual impurity sites. By trapping (under graphene) nanometer-sized water clusters on the atomically well-defined Au(111) substrate, we utilize scanning tunneling microscopy and spectroscopy to characterize the local doping influence of individual water clusters on graphene. We find that water clusters, predominantly nucleated at the atomic steps of Au(111), induce strong and highly localized electron doping in graphene. A positive correlation is observed between the water cluster size and the local doping level, in support of the recently proposed electrostatic-field-mediated doping mechanism. Our findings quantitatively demonstrate the importance of substrate-adsorbed water on the electronic properties of graphene.     

Toward intrinsic graphene surfaces: a systematic study on thermal annealing and wet-chemical treatment of SiO2-supported graphene devices

By combining atomic force microscopy and trans-port measurements, we systematically investigated effects of thermal annealing on surface morphologies and electrical properties of single-layer graphene devices fabricated by electron beam lithography on silicon oxide (SiO(2)) substrates. Thermal treatment above 300 °C in vacuum was required to effectively remove resist residues on graphene surfaces. However, annealing at high temperature was found to concomitantly bring graphene in close contact with SiO(2) substrates and induce increased coupling between them, which leads to heavy hole doping and severe degradation of mobilities in graphene devices. To address this problem, a wet-chemical approach employing chloroform was developed in our study, which was shown to enable both intrinsic surfaces and enhanced electrical properties of graphene devices. Upon the recovery of intrinsic surfaces of graphene, the adsorption and assisted fibrillation of amyloid β-peptide (Aβ1-42) on graphene were electrically measured in real time.     

Modification of the structural and electrical properties of graphene layers by Pt adsorbates

The properties of graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of platinum (Pt) metal on exfoliated single, bi- and trilayer graphene and on chemical vapor deposition-grown single-layer graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that Pt affects the structure as well as the electronic properties of graphene. The shift of peak frequencies, intensities and widths of the Raman bands were analyzed after the deposition of Pt with different thicknesses (1, 3, 5 nm) on the graphene. The shifts in the G and 2D peak positions of the Raman spectra indicate the n-type doping effect by the Pt metal. The doping effect was also confirmed by gate-voltage dependent resistivity measurements. The doping effect by the Pt metal is stable under ambient conditions, and the doping intensity increases with the increasing Pt deposition without inducing a severe degradation of the charge carrier mobility.     

Effect of heavy doping in SnO2:F films

Thin films of fluorine-doped tin dioxide (SnO₂:F) were deposited by a spray pyrolysis technique on soda lime glass substrates. Structural and electronic transport properties of the films deposited with different doping levels of fluorine (zero to 350 at %) were investigated. X-ray diffraction technique and Hall effect measurements were used for this work. Growth rate of the films was considerably affected by doping, specially at higher doping levels. The films were polycrystalline and preferentially oriented along [200]. This preferred growth played a dominant role in determining the transport properties. Notably the charge carrier mobility was directly governed by this preferred growth. The electrical conductivity was totally governed by the carrier concentration. The respective changes in carrier concentration were used to suggest the site selection of the fluorine dopant in the SnO₂ lattice.     

Light-driven reversible modulation of doping in graphene

We report a route to noncovalently latch dipolar molecules on graphene to create stable chromophore/graphene hybrids where molecular transformation can be used as an additional handle to reversibly modulate doping while retaining high mobilities. A light switchable azobenzene chromophore was tethered to the surface of graphene via π-π interactions, leading to p-doping of graphene with an hole concentration of ~5 × 10(12) cm(-2). As the molecules switch reversibly from trans to cis form the dipole moment changes, and hence the extent of doping, resulting in the modulation of hole concentration up to ~18% by alternative illumination of UV and white light. Light-driven conductance modulation and control experiments under vacuum clearly attribute the doping modulation to molecular transformations in the organic molecules. With improved sensitivities these "light-gated" transistors open up new ways to enable optical interconnects.     

Molecular charge-transfer interaction with single-layer graphene

While the effect of electrochemical doping on single-layer graphene (SG) with holes and electrons has been investigated, the effect of charge-transfer doping on SG has not been examined hitherto. Effects of varying the concentration of electron donor and acceptor molecules such as tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) on SG produced by mechanical exfoliation as well as by the reduction of single-layer graphene oxide have been investigated. TTF softens the G-band in the Raman spectrum, whereas TCNE stiffens the G-band. The full-width-at-half-maximum of the G-band increases on interaction with both TTF and TCNE. These effects are similar to those found with few-layer graphene, but in contrast to those found with electrochemical doping. A common feature between the two types of doping is found in the case of the 2-D band, which shows softening and stiffening on electron and hole doping, respectively. The experimental results are explained on the basis of the frequency shifts, electron–phonon coupling and structural inhomogeneities that are relevant to molecule–graphene interaction.     

Graphene as transparent conducting electrodes in organic photovoltaics: studies in graphene morphology, hole transporting layers, and counter electrodes

In this work, organic photovoltaics (OPV) with graphene electrodes are constructed where the effect of graphene morphology, hole transporting layers (HTL), and counter electrodes are presented. Instead of the conventional poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) PEDOT:PSS HTL, an alternative transition metal oxide HTL (molybdenum oxide (MoO(3))) is investigated to address the issue of surface immiscibility between graphene and PEDOT:PSS. Graphene films considered here are synthesized via low-pressure chemical vapor deposition (LPCVD) using a copper catalyst and experimental issues concerning the transfer of synthesized graphene onto the substrates of OPV are discussed. The morphology of the graphene electrode and HTL wettability on the graphene surface are shown to play important roles in the successful integration of graphene films into the OPV devices. The effect of various cathodes on the device performance is also studied. These factors (i.e., suitable HTL, graphene surface morphology and residues, and the choice of well-matching counter electrodes) will provide better understanding in utilizing graphene films as transparent conducting electrodes in future solar cell applications.     

喷雾热解法生长N掺杂ZnO薄膜机理分析

通过超声喷雾热解工艺,以醋酸锌和醋酸铵的混合水溶液为前驱溶液,在单晶Si(100) 衬底上制备了N掺杂ZnO薄膜,采用热质联用分析(TG—DSC—MS)、X射线衍射(XRD)、场发射扫描电镜(FESEM)和霍耳效应(Hall-effect)测试等手段研究了喷雾热解工艺下N掺杂ZnO薄膜的生长机理、晶体结构和电学性能.结果表明,随衬底温度的不同,薄膜呈现出不同的生长机理,从而影响薄膜的晶体结构和电学性能.在优化的衬底温度下,实现了ZnO薄膜的p型掺杂,得到的p型ZnO薄膜具有优异的电学性能,载流子浓度为3.21×10¹⁸cm^-3,霍耳迁移率为110cm²·V^-1s^-1,电阻率为1.76×10^-2Ω·cm.     

Doping of graphene exfoliated on SrTiO3

We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO(3) substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be Φ(SLG) = 4.409 ± 0.039 eV and Φ(BLG) = 4.516 ± 0.035 eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons.     

Nano- and microstructuring of graphene using UV-NIL

In this work we demonstrate for the first time the micro-?and nanostructuring of graphene by means of UV-nanoimprint lithography. Exfoliated graphene on SiO(2) substrates, as well as graphene deposited by chemical vapor deposition (CVD) on polycrystalline nickel and copper, and transferred CVD graphene on dielectric substrates, were used to demonstrate that our technique is suitable for large-area patterning (2?×?2?cm(2)) of graphene on various types of substrates. The demonstrated fabrication procedure of micrometer as well as nanometer-sized graphene structures with feature sizes down to 20?nm by a wafer-scale process opens up an avenue for the low-cost and high-throughput manufacturing of graphene-based optical and electronic applications. The processed graphene films show electron mobilities of up to 4.6?×?10(3)?cm(2)?V (-1)?s(-1), which confirms them to exhibit state-of-the-art electronic quality with respect to the current literature.     

Local electronic properties of graphene on a BN substrate via scanning tunneling microscopy

The use of boron nitride (BN) as a substrate for graphene nanodevices has attracted much interest since the recent report that BN greatly improves the mobility of charge carriers in graphene compared to standard SiO(2) substrates. We have explored the local microscopic properties of graphene on a BN substrate using scanning tunneling microscopy. We find that BN substrates result in extraordinarily flat graphene layers that display microscopic Moire? patterns arising from the relative orientation of the graphene and BN lattices. Gate-dependent dI/dV spectra of graphene on BN exhibit spectroscopic features that are sharper than those obtained for graphene on SiO(2). We observe a significant reduction in local microscopic charge inhomogeneity for graphene on BN compared to graphene on SiO(2).     

Electrochemically metal-doped reduced graphene oxide films:Properties and applications

The fine control of doping levels in graphene materials such as reduced graphene oxide(RGO) is important to properly manipulate their ambipolar transport characteristics for various device applications. However, conventional doping methods involve complex chemical reactions, large-scale doping processes,and poor stability. Herein, a simple and controllable electrochemical doping treatment(EDT), performed via the conductive channels created at the RGO surface by the application of an electric field, is introduced to tailor the electrical properties of RGO films. X-ray photoelectron spectroscopy and Raman spectroscopy measurements are performed to detect the presence of Ni atoms in RGO films after the EDT(EDT-RGO).Then, EDT-RGO field-effect transistors(FETs) are fabricated with different doping areas(0 to 100% fractional area) on the RGO active channel to investigate the effect and selective-area doping capability of the EDT. Owing to p-type doping compensation by the intercalated Ni atoms, the electron mobility of the EDT-RGO FET decreases from 1.40 to 0.12 cm~2 V~(-1)s~(-1) compared with that of the undoped RGO-FET,leading to the conversion from ambipolar to unipolar p-type transfer characteristics.     

Copper/silver nanoparticle incorporated graphene films prepared by a low-temperature solution method for transparent conductive electrodes

A facile one-step co-reduction and low-temperature solution process was developed to prepare Cu–graphene (Cu–G), Ag–graphene (Ag–G), and Cu–Ag–graphene (Cu–Ag–G) composite films on glass substrates. Scanning electron microscope and transmission electron microscope images show that Cu/Ag nanoparticles are either distributed on the surface of graphene nanosheets or covered by graphene. The conductivity and transparency of these films were studied, and the results show that incorporation of Cu and Ag nanoparticles into graphene films can improve film conductivity. Ag nanoparticles are more effective in improving film conductivity. The conductivity and transparency of the composite films can be balanced by introducing the optimum amount of Cu or Ag nanoparticles. The conductivity and transparency of Cu–Ag–G films with optimum metal nanoparticle concentration are as good as those of Ag–G composite films. The Cu–Ag–G films meet the requirements of low-cost, high-conductivity, and transparent films that can be used as electrode materials. Thus, the proposed low-temperature solution process is a new route to preparing low-cost transparent and conductive electrodes on various substrates, including glass and flexible polymer substrates.     

Formation of Air Stable Graphene p–n–p Junctions Using an Amine‐Containing Polymer Coating

An ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), is deposited on a back‐gated field effect graphene device to form graphene p–n–p junctions. Characteristic I–V curves indicate the superposition of two separate Dirac points, which confirms an energy separation of neutrality points within the complementary regions. This is a simple approach for making graphene p–n–p junctions without a need for multiple lithography steps or electrostatic gates and, unlike, the destructive techniques such as substitutional doping or covalent functionalization, it induces a minor defect, if any, as there is no discernible D peak in the Raman spectra of the graphene films after creating junctions and degradation in the charge carrier mobilities of the graphene devices. This method can be easily processed from dilute solutions in environmentally‐friendly solvents such as water or methoxyethanol and does not suffer any change upon exposure to air or heating at temperatures below 100 °C.     

Enhanced Peltier Effect in Wrinkled Graphene Constriction by Nano‐Bubble Engineering

Inspired by the promising applications in thermopower generation from waste heat and active on‐chip cooling, the thermoelectric and electrothermal properties of graphene have been extensively pursued by seeking ingeniously designed structures with thermoelectric conversion capability. The graphene wrinkle is a ubiquitous structure formed inevitably during the synthesis of large‐scale graphene films but the corresponding properties for thermoelectric and electrothermal applications are rarely investigated. Here, the electrothermal Peltier effect from the graphene wrinkle fabricated on a germanium substrate is reported. Peltier cooling and heating across the wrinkle are visualized unambiguously with polarities consistent with p‐type doping and in accordance with the wrinkle spatial distribution. By direct patterning of the nano‐bubble structure, the current density across the wrinkle can be boosted by current crowding to enhance the Peltier effect. The observed Peltier effect can be attributed to the nonequilibrium charge transport by interlayer tunneling across the van der Waals barrier of the graphene wrinkle. The graphene wrinkle in combination with nano‐bubble engineering constitutes an innovative and agile platform to design graphene and other more general two‐dimensional (2D) thermoelectrics and opens the possibility for realizing active on‐chip cooling for 2D nanoelectronics with van der Waals junctions.     

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).     

Effect of oxidation on thickness dependencies of thermoelectric properties in PbTe/mica thin films

The dependencies of the Hall coefficient R_H and Seebeck coefficient S at room temperature on the thickness (d=10-550 nm) of thin PbTe films prepared by the thermal evaporation in vacuum of n-type PbTe crystals with various charge carrier concentrations (10¹⁷-10¹⁹ cm⁻³) and their deposition on mica substrates were obtained. It was established that, with decreasing thickness of PbTe films, a transition from an electron to a hole conductivity occurs, and the inversion point shifts to smaller d values as the electron concentration in the target material increases. The experimental R_H(d) and S(d) dependencies are interpreted in terms of the acceptor states created by oxygen on the film surface. These dependencies were also calculated theoretically, taking into account the existence of two types of charge carriers (electrons and holes). The theoretical curves are found to be in good agreement with the experimental data.     

Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor

The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO(2) back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5x1013 cm-2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO(2) back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.     

Photoconductivity of Bulk‐Film‐Based Graphene Sheets

Time‐resolved photoconductivity measurements are carried out on graphene films prepared by using soluble graphene oxide. High photocurrent generation efficiency is observed for these graphene‐based films, and the relationships between their photoconductivity and different preparation methods, incident light intensity, external electric field, and photon energies are investigated. Higher photoconductivity is observed with higher photon energy at same incident light intensity. By fitting the experimental data to the Onsager model, the primary quantum yields for charge separation to generate bound electron–hole pairs and the initial ion‐pair thermalization separation distance are calculated.     

Thin tellurium films prepared by partially ionized vapour deposition

The effects of partially ionized vapour deposition on the morphology of tellurium films on KBr substrates were investigated. Compared with films formed by conventional neutral vapour deposition, films prepared by partially ionized vapour deposition show several pronounced structural features: (1) enhanced surface coverage on the substrate; (2) disappearance of the separate tips in the shape of a swallow's tail which grow on the end of the crystallites formed by neutral vapour deposition; (3) almost no decoration effect along the step line of the cleavage face of KBr substrates. Enhanced surface coverage of the tellurium film also occurred on glass and mica substrates.