Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy-momentum dispersion relations which cross at the Dirac point [1,2]. However, accessing the physics of the low density region at the Dirac point has been difficult because of the presence of disorder which leaves the graphene with local microscopic electron and hole puddles [3][4][5], resulting in a finite density of carriers even at the charge neutrality point. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult [6,7].Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance [8]. In this letter, we use scanning tunneling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moiré patterns in the topographic images. However, contrary to recent predictions [9,10], this conformation does not lead to a sizable band gap due to the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron-hole charge fluctuations are reduced by two orders of magnitude as compared to those on silicon oxide. This leads to charge fluctuations which are as small as in suspended graphene [6], opening up Dirac point physics to more diverse experiments than are possible on freestanding devices.

We have carried out scanning tunneling spectroscopy measurements on exfoliated monolayer graphene on SiO2 to probe the correlation between its electronic and structural properties. Maps of the local density of states are characterized by electron and hole puddles that arise due to long range intravalley scattering from intrinsic ripples in graphene and random charged impurities. At low energy, we observe short range intervalley scattering which we attribute to lattice defects. Our results demonstrate that the electronic properties of graphene are influenced by intrinsic ripples, defects and the underlying SiO2 substrate.

We report manipulation of a Kondo resonance originated from the spin-electron interactions between a twodimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K using a low temperature scanning tunneling microscope. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a small hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with a decreasing the number of nearest neighbor molecules from six to zero. This Kondo temperature variation is originated from the scattering of surface electrons by the molecules located at the edges of the molecular layer, which reduces spinelectron coupling strength for the molecules inside the layer. Investigations on different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice.

CdS nanoparticles were deposited on a highly stable, two-dimensional (2D) covalent organic framework (COF) matrix and the hybrid was tested for photocatalytic hydrogen production. The efficiency of CdS-COF hybrid was investigated by varying the COF content. On the introduction of just 1 wt% of COF, a dramatic tenfold increase in the overall photocatalytic activity of the hybrid was observed. Among the various hybrids synthesized, that with 10 wt% COF, named CdS-COF (90:10), was found to exhibit a steep H2 production amounting to 3678 μmol h(-1) g(-1), which is significantly higher than that of bulk CdS particles (124 μmol h(-1) g(-1)). The presence of a π-conjugated backbone, high surface area, and occurrence of abundant 2D hetero-interface highlight the usage of COF as an effective support for stabilizing the generated photoelectrons, thereby resulting in an efficient and high photocatalytic activity.

While graphene has attracted significant attention from the research community due to its high charge carrier mobility, important issues remain unresolved that prevent its widespread use in technologically significant applications such as digital electronics. For example, the chemical inertness of graphene hinders integration with other materials, and the lack of a bandgap implies poor switching characteristics in transistors. The formation of ordered organic monolayers on graphene has the potential to address each of these challenges. In particular, functional groups incorporated into the constituent molecules enable tailored chemical reactivity, while molecular-scale ordering within the monolayer provides sub-2 nm templates with the potential to tune the electronic band structure of graphene via quantum confinement effects. Toward these ends, we report here the formation of well-defined one-dimensional organic nanostructures on epitaxial graphene via the self-assembly of 10,12-pentacosadiynoic acid (PCDA) in ultrahigh vacuum (UHV). Molecular resolution UHV scanning tunneling microscopy (STM) images confirm the one-dimensional ordering of the as-deposited PCDA monolayer and show domain boundaries with symmetry consistent with the underlying graphene lattice. In an effort to further stabilize the monolayer, in situ ultraviolet photopolymerization induces covalent bonding between neighboring PCDA molecules in a manner that maintains one-dimensional ordering as verified by UHV STM and ambient atomic force microscopy (AFM). Further quantitative insights into these experimental observations are provided by semiempirical quantum chemistry calculations that compare the molecular structure before and after photopolymerization.

We present a lithography-free technique for fabrication of clean, high quality graphene devices. This technique is based on evaporation through hard Si shadow masks, and eliminates contaminants introduced by lithographical processes. We demonstrate that devices fabricated by this technique have significantly higher mobility values than those obtained by standard electron beam lithography. To obtain ultra-high mobility devices, we extend this technique to fabricate suspended graphene samples with mobilities as high as 120 000 cm 2 /(V·s).

We have performed low temperature scanning tunneling spectroscopy measurements on exfoliated bilayer graphene on SiO 2 . By varying the back gate voltage we observed a linear shift of the Dirac point and an opening of a band gap due to the perpendicular electric field. In addition to observing a shift in the Dirac point, we also measured its spatial dependence using spatially resolved scanning tunneling spectroscopy. The spatial variation of the Dirac point was not correlated with topographic features and therefore we attribute its shift to random charged impurities.Monolayer graphene (MLG), which is just a single sheet of carbon atoms thick, has novel electronic properties as a consequence of its linear band structure. Stacking one more layer on top of the monolayer gives rise to bilayer graphene (BLG) that is an exciting system with a different set of tunable properties 1,2 . The bilayer structure is characterized by a quadratic dispersion relation E = ± 2 k 2 /2m with the conduction band and valence bands touching making BLG a zero band gap semiconductor. When an electric field is applied perpendicular to the plane of carbon atoms, it is possible to open up a band gap between the conduction band and valence band [3][4][5] . Recent experiments with techniques like angle resolved photoemission spectroscopy 6 , infrared spectroscopy 7-9 and transport measurements with a double-gate 10 have confirmed this band gap opening. These techniques are non-local and only provide information about the average properties of the BLG. However, from a device application perspective it is important to get details about how the spatial extent and morphology of the layers affect the electronic properties. Scanning tunneling microscopy (STM) is a powerful tool for this purpose. Previous STM studies have shown that impurites in MLG 11,12 and phonons 13 influence the chargecarrier scattering mechanisms in graphene. In this letter, we present scanning tunneling spectroscopy results for BLG on a SiO 2 substrate. These results show the spatial variation of the Dirac point as well as the control of the Dirac point and band gap due to the application of an electric field from the back gate.The BLG was prepared using the mechanical exfoliation technique 14,15 . Degenerately doped Si with 300 nm thick SiO 2 on top was used as a back gate. Bilayer areas were identified using an optical microscope and then Ti/Au electrodes were deposited using a shadow mask technique described elsewhere 16 . The device was then cooled to 4.6 K using an Omicron low temperature STM operating in ultrahigh vacuum (p ≤ 10 −11 mbar). Electrochemically etched tungsten tips that exhibited a constant density of states on a Au surface were used for a) Electronic mail: leroy@physics.arizona.edu imaging and spectroscopy to avoid unwanted tip effects. Due to the cleaner fabrication procedure, no PMMA is used, it is possible to obtain atomic resolution images over large areas of the BLG without any additional cleaning procedure unlike in previous STM measurements on exfol...

A range of artificial molecular systems has been created that can exhibit controlled linear and rotational motion. In the further development of such systems, a key step is the addition of communication between molecules in a network. Here, we show that a two-dimensional array of dipolar molecular rotors can undergo simultaneous rotational switching when applying an electric field from the tip of a scanning tunnelling microscope. Several hundred rotors made from porphyrin-based double-decker complexes can be simultaneously rotated when in a hexagonal rotor network on a Cu(111) surface by applying biases above 1 V at 80 K. The phenomenon is observed only in a hexagonal rotor network due to the degeneracy of the ground-state dipole rotational energy barrier of the system. Defects are essential to increase electric torque on the rotor network and to stabilize the switched rotor domains. At low biases and low initial rotator angles, slight reorientations of individual rotors can occur, resulting in the rotator arms pointing in different directions. Analysis reveals that the rotator arm directions are not random, but are coordinated to minimize energy via crosstalk among the rotors through dipolar interactions.