Session F37: Focus Session: Functionalization and Atomic Engineering of Graphene

Atomic Scale Visualization of Dopant-Induced Unconventional Kondo Effect in Boron-doped graphene

We describe the synthesis of large-area, highly-crystalline monolayer Boron-doped graphene (BG) sheets via atmospheric-pressure chemical vapor deposition, yielding unique and diverse B-doping site composed of substitutional Boron atoms and carbon vacancies. Scanning tunneling microscopy and spectroscopy (STM and STS) of BG reveal the presence of localized states in both the conduction and valence bands induced by Boron pz orbitals, confirmed by ab initio calculations. Furthermore, we demonstrate for the first time that atomic-resolved spin-polarization in a graphene sublattice via spectroscopic imaging of zero-energy states, induced by Boron incorporation. BG acts as a Kondo system with magnetic dopants embedded in C lattices, fully described by using the non-equilibrium Green's function method within the slave-boson mean-field approximation.   

STM Observation of Molecular Adsorption on Graphene and Nitrogen Doped Graphene

Carbon alloy catalyst (CAC) shows catalytic activity to oxygen reduction reaction (ORR) and it is expected as a substitution of Pt in fuel cells due to its catalytic property. At present CAC are synthesized by burning organic compounds which contain nitrogen atoms such as phthalocyanine. The catalytic activity of CAC is lower than Pt. Since catalytic sites and oxygen reduction process is still unknown, elucidation of catalytic sites of CAC helps to synthesize high performance CAC. STM is a useful tool to investigate adsorption and reaction at atomic level. However, disordered structure of CAC makes it difficult to use STM for catalytic site observation. To overcome this difficulty, we synthesized nitrogen doped graphene (NG) and and pristine graphene (PG) on Pt (111) and used it as model catalyst to study the catalytic property of CAC. Oxygen adsorption is the first step of oxygen reduction reaction. Therefore we investigated the oxygen adsorption to NG and PG by STM. Oxygen adsorbed at domain boundary (DB) of NG?According to XPS measurement nitrogen atoms exist at edge site preferably. These results indicate that nitrogen atom enhances oxygen adsorption activity. In addition, actual reaction process occurs in H$_2$O. Thus we also investigated H$_2$O adsorption on NG.   

Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Chromium Clusters Deposited on Moir\'e Patterns on HOPG

Moir\'e patterns (MP) formed by twisted graphene layers, present great potential for use as periodic substrates to facilitate the growth of nanostructures to obtain useful electronic and/or magnetic properties. The growth of Chromium (Cr) deposited on MPs on the surface of highly ordered pyrolitic graphite (HOPG) and its effects on the electronic structure in the MPs were studied by scanning tunneling microscopy/spectroscopy (STM/STS). Without Cr, two van Hove singularities (VHSs) were observed by STS on the MPs. With low coverage of Cr, atoms deposited on graphite Moir\'e form small clusters randomly distributed over the surface. With the presence of Cr clusters, the energy difference between the two VHS peaks enlarged while its linear dependence on the twisting angle remains. Compare to the situation before deposition, the graphite's Fermi velocity increased while the interlayer interaction decreased. The electronic structure modification caused by a Cr cluster as a function of distance from the cluster was studied with extremely low coverage. The effective distance can reach about 10 lattice cells of the Moir\'e pattern.   

Local Probe Measurement of Atomically-Engineered Graphene Nanostructures

Graphene's unique electronic properties give rise to novel defect behavior at impurities and at edges. This can be seen in graphene's response to charged impurities, where graphene's ultra-relativistic nature leads to impurity states that are unlike those found in any other material. We have explored such impurity states across different impurity-charge regimes by building artificial charge centers (i.e., ``artificial nuclei'') atom-by-atom at the surface of graphene devices and probing them via scanning tunneling microscopy. New results on this topic, including the observation of ``atomic collapse'', will be discussed. The properties of graphene's edges become increasingly important when graphene is cut into nanoscale structures having sharp boundaries. While such structures are difficult to fabricate via traditional ``top-down'' lithography, new ``bottom-up'' synthesis techniques utilizing molecular self-assembly show great promise for creating flexible, atomically-engineered networks. We have recently made progress at fabricating new graphene nanostructures in this way from chemically engineered precursor molecules. New measurements on these systems will be discussed.   

First-Principles Studies of Ordered Potassium Monolayers on Graphite and Graphene

We investigate the structural and electronic properties of ordered monolayers of potassium adosorbates on graphite and graphene using first-principles methods based on density-functional theory, pseudopotentials and periodic supercells. Several ordered structures are investigated and their total energies are mapped onto an effective Ising-like hamiltonian. Monte-Carlo simulations using this hamiltonian are performed in order to construct the phase diagram for this system, which is then compared to experimental results on graphite surfaces. In agreement with experiments, we find that structures with potassium concentrations larger than 1/3 ($\sqrt{3}\times \sqrt{3}$) are unstable with respect to metallic potassium segregation at the surface.   

The stability, energetics, and magnetic states of Co adsorption on graphene

The adsorption of transition metal adatoms on graphene has attracted significant research interest due to their possible use to induce magnetism on graphene for spintronic applications. Single Co atoms on graphene have been extensively studied both theoretically and experimentally. In our previous work, we used auxiliary-field quantum Monte Carlo (AFQMC) and a size-correction embedding scheme to calculate the binding energy of Co/graphene for the six-fold hollow site.\footnote{Y. Virgus, W. Purwanto, H. Krakauer, and S. Zhang, Phys. Rev. B, \textbf{86}, 241406(R) (2012).} Recent experimental results show that single Co atoms can be adsorbed on graphene at both the hollow and the top sites.\footnote{T. Eelbo, M. Wasniowska, M. Gyamfi, S. Forti, U. Starke, and R. Wiesendanger, Phys. Rev. B, \textbf{87}, 205443 (2013).} We use AFQMC to investigate Co/graphene for the three high-symmetry adsorption sites; six-fold hollow site, two-fold bridge site, and top site. Highly accurate binding energy curves for the three sites are obtained. The stabilities of the different magnetic states and adsorption sites will be examined and discussed in relation to the experimental observations.   

Surface fluorination on graphene field effect transistors

Graphene, a zero-gap semiconductor with massless charge carriers, has attracted tremendous interest because of its outstanding electronic properties. One very special property of graphene is the fact that graphene is an all-surface material, so every atom has access to the surface, which has a direct impact on its electronic and chemical performance. Therefore, surface functionalizations provide very effective methods to engineer its electronic properties and make it even more suitable for electronic device applications. Here, we demonstrate that controlled exposure of graphene devices to XeF2 is an effective way to open a bandgap in graphene. High on/off ratio fluorograhene-based field effect transistors are fabricated and analyzed. Raman characterization is also carried out to investigate their structural changes as a result of fluorination treatment.   

Infrared Spectroscopy of Functionalized Graphene Sheets from First Principle Calculations

Detailed characterization of the structure of functionalized graphene sheets (FGSs) is an important and challenging task which could help to improve the performance of FGS materials for technological applications. We present here first principles calculations for the infrared (IR) spectra of different FGS models aimed at identifying the IR signatures of different functional groups and defect sites on FGSs. We found that vacancies and edges have significant effects on the IR frequencies of the functional groups on FGSs. In particular, hydroxyl groups close to vacancies have higher stretching and lower bending frequencies in comparison to hydroxyls in defect free regions of FGSs. More interestingly, the OH vibrations of carboxyl groups at edges exhibit unique features in the high frequency IR bands, which originate from the interactions with neighboring groups and the relative orientation of the carboxyl with respect to the FGS plane. Our results are supported by experimental IR measurements on FGS powders.   

Electric Field Tunable Spin-Flip Scattering in Dilute Fluorinated Bilayer Graphene

In earlier work, we showed that a dilute coverage of fluorine adatoms covalently bonded to single-layer graphene leads to intriguing and striking phenomena including metal-insulator transition, very large negative magneto-resistance and enhanced spin-flip scattering. By fluorinating only the top layer of a bilayer graphene sheet, this work investigates the possibility of tuning the spin-flip scattering rate $in$ $situ$ via a perpendicular electric field $D$. Dual HfO$_2$ gated field effect transistors of dilute fluorinated bilayer graphene (DFBG) (F:C ~ 0.03 \%) are used, in which we independently control $D$ and the carrier density $n$. The $n$-dependence of the conductance exhibits signatures of midgap state scattering. The midgap states also lead to increased conduction in the band gap of biased DFBG. Magneto-resistance measurements and weak localization analyses over a wide range of $n$, temperatures, and $D$-fields indicate the presence of spin-flip scattering, similar to what is observed in dilute fluorinated single-layer graphene. Most strikingly, the spin-flip rate can be tuned by over a factor of 2 via controlling the direction and magnitude of the $D$-field. These results demonstrate the potential of DFBG in spintronic applications.   

Coupled Chemisorption and Physisorption of Oxygen on Single Layer Graphene Devices

We investigate adsorption of molecular oxygen on single layer graphene devices and demonstrate that chemisorption of molecular oxygen at low temperatures is strongly coupled to the physisorption process. Through low temperature adsorption and variable-temperature desorption studies, we establish the ability to use electrical measurements to separately identify the physisorption and chemisorption of oxygen on graphene: chemisorption is identified by a change in Dirac point voltage, while physisorption is identified through its increase of the mobility. By utilizing the electrostatic gate controlled chemisorption, we demonstrate that the chemisorption at low temperatures is driven by a two-step process in which free oxygen molecules are first captured onto graphene by physisorption, and then the oxygen undergoes a physisorption-to-chemisorption conversion. Our study provides a better understanding of the effect of gas adsorbates on graphene and could be useful in future applications of graphene-based gas sensors.   

Lithium-Intercalated Few Layer Graphene: Approaching the Limits of Transparency and Conductivity in Graphene-based Materials

We measure simultaneous \textit{in situ} optical transmittance spectra and electrical transport properties of few-layer graphene (FLG) nanostructures upon electrochemical lithiation/delithiation. Reversible Li-intercalation stages and a two-phase boundary are observed optically. Due to the unusual electronic structure of FLG, upon intercalation we observe a simultaneous increase of both optical transmittance and DC conductivity, strikingly different from other materials. Transmission as high as 91.7{\%} for sheet resistance of 3.0 $\Omega $/square is achieved for 19 layer LiC$_{6}$, corresponding to a figure of merit (FOM) $\sigma_{dc}$/$\sigma_{opt\, \, }=$ 1400, five times higher than any previously demonstrated for a continuous transparent electrode. The unconventional modification of FLG optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques can enable investigation of other aspects of intercalation in nanostructures, for example intercalation dynamics and solid-electrolyte interface formation.   

Achieving robust n-type nitrogen-doped graphene via a binary-doping approach

Among various dopant candidates, nitrogen (N) atoms are considered as the most effective dopants to improve the diverse properties of graphene. Unfortunately, recent experimental and theoretical studies have revealed that different N-doped graphene (NGR) conformations can result in both p- and n-type characters depending on the bonding nature of N atoms (substitutional, pyridinic, pyrrolic, and nitrilic). To overcome this obstacle in achieving reliable graphene doping, we have carried out density functional theory calculations and explored the feasibility of converting p-type NGRs into n-type by introducing additional dopant candidates atoms (B, C, O, F, Al, Si, P, S, and Cl). Evaluating the relative formation energies of various binary-doped NGRs and the change in their electronic structure, we conclude that B and P atoms are promising candidates to achieve robust n-type NGRs. The origin of such p- to n-type change is analyzed based on the crystal orbital Hamiltonian population analysis. Implications of our findings in the context of electronic and energy device applications will be also discussed. This work was supported by the Basic Science Research Grant (No. 2012R1A1A2044793), Global Frontier Program (No. 2013-073298), and Nano-Material Technology Development Program (2012M3A7B4049888) of the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea.