Danny Haberer

Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties

A novel strategy for efficient growth of nitrogen-doped graphene (N-graphene) on a large scale from s-triazine molecules is presented. The growth process has been unveiled in situ using time-dependent photoemission. It has been established that a postannealing of N-graphene after gold intercalation causes a conversion of the N environment from pyridinic to graphitic, allowing to obtain more than 80% of all embedded nitrogen in graphitic form, which is essential for the electron doping in graphene. A band gap, a doping level of 300 meV, and a charge-carrier concentration of ∼8×10(12) electrons per cm2, induced by 0.4 atom % of graphitic nitrogen, have been detected by angle-resolved photoemission spectroscopy, which offers great promise for implementation of this system in next generation electronic devices.

Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions

Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling and solar energy conversion. However, the performance of such devices degrades as their size is reduced. Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale. Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry. It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon. Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.

Evidence for a New Two-Dimensional C4H-Type Polymer Based on Hydrogenated Graphene

www.MaterialsViews.com C O M M U N IC A IO N Danny Haberer , * Cristina E. Giusca , Ying Wang , Hermann Sachdev , * Alexander V. Fedorov , Mani Farjam , S. Akbar Jafari , Denis V. Vyalikh , Dmitry Usachov , Xianjie Liu , Uwe Treske , Mandy Grobosch , Oleg Vilkov , Vera K. Adamchuk , Stephan Irle , * S. Ravi P. Silva , Martin Knupfer , Bernd Büchner , and Alexander Grüneis * Evidence for a New Two-Dimensional C 4 H-Type Polymer Based on Hydrogenated Graphene

Quasifreestanding single-layer hexagonal boron nitride as a substrate for graphene synthesis

We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride (h-BN) from tight chemical bonding to a Ni(111) thin film grown on a W(110) substrate can be achieved by intercalation of Au atoms into the interface. This process has been systematically investigated using angle-resolved photoemission spectroscopy, X-ray photoemission and absorption techniques. It has been demonstrated that the transition of the h-BN layer from the “rigid” into the “quasi-freestanding” state is accompanied by a change of its lattice constant. Using chemical vapor deposition, graphene has been successfully synthesized on the insulating, quasi-freestanding h-BN monolayer. We anticipate that the in situ synthesized weakly interacting graphene/h-BN double layered system could be further developed for technological applications and may provide perspectives for further inquiry into the unusual electronic properties of graphene.

Observation of a universal donor-dependent vibrational mode in graphene

Electron-phonon coupling and the emergence of superconductivity in intercalated graphite have been studied extensively. Yet, phonon-mediated superconductivity has never been observed in the 2D equivalent of these materials, doped monolayer graphene. Here we perform angle-resolved photoemission spectroscopy to try to find an electron donor for graphene that is capable of inducing strong electron-phonon coupling and superconductivity. We examine the electron donor species Cs, Rb, K, Na, Li, Ca and for each we determine the full electronic band structure, the Eliashberg function and the superconducting critical temperature Tc from the spectral function. An unexpected low-energy peak appears for all dopants with an energy and intensity that depend on the dopant atom. We show that this peak is the result of a dopant-related vibration. The low energy and high intensity of this peak are crucially important for achieving superconductivity, with Ca being the most promising candidate for realizing superconductivity in graphene.

Probing Local Hydrogen Impurities in Quasi-Free-Standing Graphene

We report high-resolution scanning tunneling microscopy and spectroscopy of hydrogenated, quasi-free-standing graphene. For this material, theory has predicted the appearance of a midgap state at the Fermi level, and first angle-resolved photoemission spectroscopy (ARPES) studies have provided evidence for the existence of this state in the long-range electronic structure. However, the spatial extension of H defects, their preferential adsorption patterns on graphene, or local electronic structure are experimentally still largely unexplored. Here, we investigate the shapes and local electronic structure of H impurities that go with the aforementioned midgap state observed in ARPES. Our measurements of the local density of states at hydrogenated patches of graphene reveal a hydrogen impurity state near the Fermi level whose shape depends on the tip position with respect to the center of a patch. In the low H concentration regime, we further observe predominantly single hydrogenation sites as well as extended multiple C-H sites in parallel orientation to the lattice vectors, indicating an adsorption at the same graphene sublattice. This is corroborated by ARPES measurements showing the formation of a dispersionless hydrogen impurity state which is extended over the whole Brillouin zone.

Heterostructures through Divergent Edge Reconstruction in Nitrogen-Doped Segmented Graphene Nanoribbons

Atomically precise engineering of defined segments within individual graphene nanoribbons (GNRs) represents a key enabling technology for the development of advanced functional device architectures. Here, the bottom-up synthesis of chevron GNRs decorated with reactive functional groups derived from 9-methyl-9H-carbazole is reported. Scanning tunneling and non-contact atomic force microscopy reveal that a thermal activation of GNRs induces the rearrangement of the electron-rich carbazole into an electron-deficient phenanthridine. The selective chemical edge-reconstruction of carbazole-substituted chevron GNRs represents a practical strategy for the controlled fabrication of spatially defined GNR heterostructures from a single molecular precursor.

Making graphene nanoribbons photoluminescent

We demonstrate the alignment-preserving transfer of parallel graphene nanoribbons (GNRs) onto insulating substrates. The photophysics of such samples is characterized by polarized Raman and photoluminescence (PL) spectroscopies. The Raman scattered light and the PL are polarized along the GNR axis. The Raman cross section as a function of excitation energy has distinct excitonic peaks associated with transitions between the one-dimensional parabolic subbands. We find that the PL of GNRs is intrinsically low but can be strongly enhanced by blue laser irradiation in ambient conditions or hydrogenation in ultrahigh vacuum. These functionalization routes cause the formation of sp3 defects in GNRs. We demonstrate the laser writing of luminescent patterns in GNR films for maskless lithography by the controlled generation of defects. Our findings set the stage for further exploration of the optical properties of GNRs on insulating substrates and in device geometries.

Effect of hydrogen adsorption on the quasiparticle spectra of graphene

We use the non-interacting tight-binding model to study the effect of isolated hydrogen adsorbates on the quasiparticle spectra of single-layer graphene. Using the Greens function approach, we obtain analytic expressions for the local density of states and the spectral function of hydrogen-doped graphene, which are also numerically evaluated and plotted. Our results are relevant for the interpretation of scanning tunneling microscopy and angle-resolved photoemission spectroscopy data of functionalized graphene.

Orbitally Matched Edge-Doping in Graphene Nanoribbons

A series of trigonal planar N-, O-, and S-dopant atoms incorporated along the convex protrusion lining the edges of bottom-up synthesized chevron graphene nanoribbons (cGNRs) induce a characteristic shift in the energy of conduction and valence band edge states along with a significant reduction of the band gap of up to 0.3 eV per dopant atom per monomer. A combination of scanning probe spectroscopy and density functional theory calculations reveals that the direction and the magnitude of charge transfer between the dopant atoms and the cGNR backbone are dominated by inductive effects and follow the expected trend in electronegativity. The introduction of heteroatom dopants with trigonal planar geometry ensures an efficient overlap of a p-orbital lone-pair centered on the dopant atom with the extended π-system of the cGNR backbone effectively extending the conjugation length. Our work demonstrates a widely tunable method for band gap engineering of graphene nanostructures for advanced electronic applications.