Lida Pan

Graphyne- and graphdiyne-based nanoribbons: Density functional theory calculations of electronic structures

We report on the configurations and electronic properties of graphyne and graphdiyne nanoribbons with armchair and zigzag edges investigated with first principles calculations. Our results show that all the nanoribbons are semiconductors with suitable band gaps similar to silicon. And their band gaps decrease as widths of nanoribbons increase. We also find that the band gap is at the Γ point for all graphdiyne ribbons and it is at the X point for all graphyne ribbons. Of particular interest, the band gap of zigzag graphyne nanoribbons show a unique “step effect” as the width increases. This property is good for tuning of the energy band gap, as in a certain range of the ribbon width, the energy gap remains constant and in reality the edge cannot be as neat as that in a theoretic model.

Intercalation of metal islands and films at the interface of epitaxially grown graphene and Ru(0001) surfaces

We report on intercalation of seven kinds of metals—Pt, Pd, Ni, Co, Au, In, and Ce—at the interface between an epitaxially grown graphene layer and a Ru(0001) substrate. Atomic resolution scanning tunneling microscopy images of perfect graphene lattice are obtained on top of these intercalated metals, showing that the high quality of the original graphene is, in the end, undisturbed by the intercalation. A model based theoretical calculation is proposed for the intercalation mechanism: metal atom-aided defect formation and self-healing of C–C bonds at high temperature. These intercalated materials include noble metals, magnetic metals, a IIIA group metal, and a rare earth metal, which indicates that intercalation through epitaxial graphene on Ru(0001) is a universal approach for metals.

Intraband absorption in the 8–12 μm band from Si-doped vertically aligned InGaAs/GaAs quantum-dot superlattice

Normal-incident infrared absorption in the 8–12-μm-atmospheric spectral window in the InGaAs/GaAs quantum-dot superlattice is observed. Using cross-sectional transmission electron microscopy, we find that the InGaAs quantum dots are perfectly vertically aligned in the growth direction (100). Under the normal incident radiation, a distinct absorption peaked at 9.9 μm is observed. This work indicates the potential of this quantum-dot superlattice structure for use as normal-incident infrared imaging focal arrays application without fabricating grating structures.

Self-assembly of C60 monolayer on epitaxially grown, nanostructured graphene on Ru(0001) surface

C60 molecules adsorbed on graphene/Ru(0001) substrate were investigated by scanning tunneling microscopy (STM) at 5 K. On high quality substrates, C60 molecules adopt a commensurate growth mode, leading to formation of a supramolecular structure with perfect periodicity and few defects. On under-annealed substrates with imperfections and domains, the molecules form the same closely packed hexagonal structures in spite of underlying corrugations, disorders or steps, indicating a weak molecule-substrate interaction—a conclusion that is also supported by DFT calculations. This system may be beneficial to the fabrication of carbon based devices and of other types of organic functional overlayers.

Role of Cooperative Interactions in the Intercalation of Heteroatoms between Graphene and a Metal Substrate.

The intercalation of heteroatoms between graphene and a metal substrate has been studied intensively over the past few years, due to its effect on the graphene properties, and as a method to create vertical heterostructures. Various intercalation processes have been reported with different combinations of heteroatoms and substrates. Here we study Si intercalation between graphene and Ru(0001). We elucidate the role of cooperative interactions between hetero-atoms, graphene, and substrate. By combining scanning tunneling microscopy with density functional theory, the intercalation process is confirmed to consist of four key steps, involving creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of an intercalated monolayer. Both theory and experiments indicate that this mechanism applies also to other combinations of hetero-atoms and substrates.

Effects of rapid thermal annealing on self-assembled InGaAs/GaAs quantum dots superlattice

Postgrowth rapid thermal annealing was used to study the relaxation mechanism and optical properties of InGaAs/GaAs self-assembled quantum dots superlattice grown by molecular beam epitaxy. It is found that a significant narrowing of the luminescence linewidth (from 80 to 42 meV) occurs together with about 86 meV blue shift at annealing temperature up to 950°C. Double crystal X-ray diffraction measurements show that the intensity of the satellite diffraction peak, which corresponds to the quantum dots superlattice, decreased with the increasing annealing temperature and disappeared at 750°C, but recovered and increased again at higher annealing temperatures. This behavior can be explained by two competing relaxation mechanisms; interdiffusion and favored migration. The study indicates that a suitable annealing treatment can improve the structural properties of the quantum dots superlattice.

Structural characterization of InGaAs/GaAs quantum dots superlattice infrared photodetector structures

InGaAs/GaAs quantum dots (QDs) superlattice grown by molecular beam epitaxy (MBE) at different substrate temperatures for fabricating 8–12 μm infrared photodetector were characterized by transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL). High-quality QDs superlattice can be achieved by higher growth temperature. Cross-sectional TEM shows the QDs in the successive layers are vertically aligned along growth direction. Interaction of partial vertically aligned columns leads to a perfect vertical ordering. With increasing number of bilayers, the average QDs size becomes larger in height and rapidly saturates at a certain value, while average lateral length nearly preserves initial size. This change leads to the formation of QDs homogeneous in size and of a particular shape. The observed self-organizations are attributed to the effect of strain distribution at QDs on the kinetic growth process. DCXRD measurement shows two sets of satellite peaks which corresponds to QDs superlattice and multi quantum wells formed by the wetting layers. Kinematical simulations of the wetting layers indicate that the formation of QDs is associated with a decrease of the effective indium content in the wetting layers.

Room-Temperature, Low-Barrier Boron Doping of Graphene.

Doping graphene with boron has been difficult because of high reaction barriers. Here, we describe a low-energy reaction route derived from first-principles calculations and validated by experiments. We find that a boron atom on graphene on a ruthenium(0001) substrate can replace a carbon by pushing it through, with substrate attraction helping to reduce the barrier to only 0.1 eV, implying that the doping can take place at room temperature. High-quality graphene is grown on a Ru(0001) surface and exposed to B2H6. Scanning tunneling microscopy/spectroscopy and X-ray photoelectron spectroscopy confirmed that boron is indeed incorporated substitutionally without disturbing the graphene lattice.

The influence of annealing temperature on the morphology of graphene islands

We report on temperature-programmed growth of graphene islands on Ru (0001) at annealing temperatures of 700 °C, 800 °C, and 900 °C. The sizes of the islands each show a nonlinear increase with the annealing temperature. In 700 °C and 800 °C annealings, the islands have nearly the same sizes and their ascending edges are embedded in the upper steps of the ruthenium substrate, which is in accordance with the etching growth mode. In 900 °C annealing, the islands are much larger and of lower quality, which represents the early stage of Smoluchowski ripening. A longer time annealing at 900 °C brings the islands to final equilibrium with an ordered moire pattern. Our work provides new details about graphene early growth stages that could facilitate the better control of such a growth to obtain graphene with ideal size and high quality.

The origin of half-metallicity in conjugated electron systems—a study on transition-metal-doped graphyne

We studied the mechanism of half-metallicity (HM) formation in transition-metal-doped conjugated carbon based structures by first-principles electronic structure simulations. It is found that the HM is a rather complex phenomenon, determined by the ligand field splitting of d-orbitals of the transition metal atoms, the exchange splitting and the number of valence electrons. Since most of the conjugated carbon based structures possess ligands with intermediate strength, the ordering of the d-orbital splitting is similar in all structures, and the HM properties evolve according to the number of valence electrons. Based on this insight we predict that Cr-, Fe- and Co-doped graphyne will show HM, while Mn- and Ni-doped graphyne will not. By tuning the number of valence electrons, we are thus able to control the emergence of HM and control the energy gaps evolving in the majority or minority spin channels.