An-Ping Li

Hexagonal pore arrays with a 50-420 nm interpore distance formed by self-organization in anodic alumina

Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.

Exceptional ballistic transport in epitaxial graphene nanoribbons

Graphene nanoribbons will be essential components in future graphene nanoelectronics. However, in typical nanoribbons produced from lithographically patterned exfoliated graphene, the charge carriers travel only about ten nanometres between scattering events, resulting in minimum sheet resistances of about one kilohm per square. Here we show that 40-nanometre-wide graphene nanoribbons epitaxially grown on silicon carbide are single-channel room-temperature ballistic conductors on a length scale greater than ten micrometres, which is similar to the performance of metallic carbon nanotubes. This is equivalent to sheet resistances below 1 ohm per square, surpassing theoretical predictions for perfect graphene by at least an order of magnitude. In neutral graphene ribbons, we show that transport is dominated by two modes. One is ballistic and temperature independent; the other is thermally activated. Transport is protected from back-scattering, possibly reflecting ground-state properties of neutral graphene. At room temperature, the resistance of both modes is found to increase abruptly at a particular length—the ballistic mode at 16 micrometres and the other at 160 nanometres. Our epitaxial graphene nanoribbons will be important not only in fundamental science, but also—because they can be readily produced in thousands—in advanced nanoelectronics, which can make use of their room-temperature ballistic transport properties.

Fabrication and Microstructuring of Hexagonally Ordered Two-Dimensional Nanopore Arrays in Anodic Alumina**

washed three times with water, and dried over MgSO4. After removing the solvent, chromatography is carried out on the residue on neutral alox. First, unreacted compound 4 and monoadduct 8 are eluted with pentane, and then bis-adduct 5 with petrolether/ethyl acetate. (54 %, m.p.: 191 C). H NMR (CDCl3, ppm) 3.64 (s, 8 H, CH2), 4.33 (dd, 2 H, CH), 6.99 (dd, 2 H, CH), 7.14 (s, 8 H, aromatic); C NMR (CDCl3, ppm) 33.72, 54.79, 126.37, 129.22, 134.8, 139.74, 140.72. Compound 2: Compound 5 (0.8 g, 2.64 mmol) and chloranil (1.28 g, 5.28 mmol) are dissolved in toluene (150 mL), and heated for 1.5 h under reflux. After removing the solvent, the residue is purified via chromatography (neutral alox/toluene) (75 %, m.p.: 239 C). H NMR (CDCl3, ppm) 5.33 (dd, 2 H, CH), 7.06 (dd, 2 H, CH), 7.37 (AA¢BB¢, 4 H, aromatic), 7.71 (AA¢BB¢, 4 H, aromatic), 7.73(s, 4 H, aromatic); C NMR (CDCl3, ppm) 50.74, 121.71, 126, 127.92, 132.25, 138.73, 142.66. Compound 6a: A solution of compound 2 (100 mg, 0.32 mmol) and 2,3,4,5-tetrachlorothiophenedioxide (167 mg, 0.64 mmol) in CH2Cl2 (4 mL) is heated for 3 days at 70 C under a pressure of 6 kbar. After the reaction is complete, the solvent is removed under vacuum. Chromatography is carried out on the residue on silica gel with hexane/toluene 4/1 as eluent. The first fraction gives the desired product (75 %). The material can be further purified by recrystallization from dichloroethane/ethyl acetate. H NMR (CD2Cl2, ppm) 3.54 (s, 2 H, CH), 5.13 (s, 2 H, CH), 7.45 (AA¢BB¢, 4 H, aromatic), 7.80 (s, 2 H, aromatic), 7.82 (AA¢BB¢, 4 H, aromatic), 7.86 (s, 2 H, aromatic); C NMR (CD2Cl2, ppm) 48.08, 48.86, 122.91, 123.93, 124.29, 126.24, 126.35, 128.02, 128.07, 131.38, 132.91, 133.07, 137.47, 139.58. Compound 6b: Using the procedure described for the synthesis of compound 6a, compound 6b is obtained in a yield of 70 %. H NMR (CD2Cl2, ppm) 3.58 (s, 2 H, CH), 5.19 (s, 2 H, CH), 7.46 (AA¢¢BB¢, 4 H, aromatic), 7.82 (s, 2 H, aromatic), 7.84 (AA¢BB¢, 4 H, aromatic), 7.86 (s, 2 H, aromatic); C NMR (CD2Cl2, ppm) 49.74, 51.89, 120.49, 122.87, 123.96, 126.22, 126.33, 128.02, 128.08, 128.72, 132.90, 133.06, 137.51, 139.73.

Heteroepitaxial Growth of Two-Dimensional Hexagonal Boron Nitride Templated by Graphene Edges

Heteroepitaxy Writ Thin A common method for creating a thin single-crystal layer of a semiconductor for use in an electronic device is heteroepitaxy—growing the layer on the face of a single crystal of a different material that acts as a template for assembly. Liu et al. (p. 163) now describe a similar process in which the edge of a graphene layer that was grown on a copper surface directs the assembly of a monolayer of hexagonal boron nitride. The boron nitride grew from inside edge of holes created in the graphene layer. The interface and the relative orientation of the two layers were determined by a variety of scanning microscopy and surface diffraction techniques. The orientation of a growing boron nitride film is determined by its graphene template and not the underlying substrate. By adapting the concept of epitaxy to two-dimensional space, we show the growth of a single-atomic-layer, in-plane heterostructure of a prototypical material system—graphene and hexagonal boron nitride (h-BN). Monolayer crystalline h-BN grew from fresh edges of monolayer graphene with atomic lattice coherence, forming an abrupt one-dimensional interface, or boundary. More important, the h-BN lattice orientation is solely determined by the graphene, forgoing configurations favored by the supporting copper substrate.

Polycrystalline and Monocrystalline Pore Arrays with Large Interpore Distance in Anodic Alumina

We have prepared hexagonally arranged pore arrays with large interpore distance by using two kinds of techniques: a self‐organization process and a prepattern guided anodization on aluminum. The self‐organized pore arrays were produced by anodizing aluminum in an aqueous phosphoric acid solution , and show a polycrystalline, i.e., poly‐domain, feature. Within each domain of several micrometers, the arrays are hexagonally arranged with an interpore distance of , while the domains are oriented randomly in plane. A monocrystalline pore array structure with variable interpore distance was prepared based on electron‐beam lithography and a subsequent anodization in oxalic acid solution. With a well‐designed set of anodization parameters, the prepattern can act as the initiation points and guide the pore growth in the anodic film. Very high aspect ratios (~500) can be achieved. ©2000 The Electrochemical Society

Polycrystalline nanopore arrays with hexagonal ordering on aluminum

Nanopore arrays with 6×108–5×1010 cm−2 pore densities were fabricated by self-organized anodization on aluminum. A two-step anodization process was used to oxidize aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays were obtained within domains of a few micrometers, which are separated from neighboring domains with different orientation of the pore lattice by domain boundaries, i.e., the nanopore arrays show characteristics analogous to two-dimensional polycrystalline structure. The interpore distance can be controlled by changing the electrolyte and/or the applied voltage.

VISIBLE ELECTROLUMINESCENCE FROM SEMITRANSPARENT AU FILM EXTRA THIN SI-RICH SILICON-OXIDE FILM P-SI STRUCTURE

Visible electroluminescence (EL) has been reported from semitransparent Au film/extra thin Si-rich silicon oxide film/p-Si diodes at room temperature. The Si-rich silicon oxide films, with thickness of about 40 Angstrom, were grown using the magnetron sputtering technique. At forward bias of 4 V, EL spectra with peak energy of 1.9 eV and full width at half maximum of 0.5 eV can be observed from diodes with such extra thin Si-rich oxide films having not been annealed. EL peak energy shows a small red shift under low forward bias but does not shift again when increasing the bias further. Annealing at 800 degrees C, EL spectra widen and show several shoulders at about 1.5, 2.2, and 2.4 eV, and the EL peak energy shows blue shift with increasing forward bias. These results are shown to be consistent with light emission at several types of luminescence centers in the Si-rich silicon oxide films

Epitaxial ferromagnetic Mn5Ge3 on Ge(111)

Ferromagnetic Mn5Ge3 thin films were grown on Ge(111) with solid-phase epitaxy. The epitaxial relationship between the alloy film and substrate is Mn5Ge3(001)//Ge(111) with [100]Mn5Ge3//[110]Ge. The alloy films exhibit metallic conductivity and strong ferromagnetism up to the Curie temperature, TC=296 K. These epitaxial alloy films are promising candidates for germanium-based spintronics.

Ferromagnetic percolation in MnxGe1−x dilute magnetic semiconductor

We have studied the magnetic and magnetotransport properties of Mn-doped Ge grown by molecular-beam epitaxy. This group-IV dilute ferromagnetic semiconductor exhibits two magnetic transitions. An upper critical temperature TC* (∼112K for x∼0.05) is evident from the extrapolated Curie–Weiss susceptibility and from the Arrott plot analysis of anomalous Hall effect data. The existence of a lower critical temperature TC (∼12K for x∼0.05) is established from ac susceptibility and magnetotransport data. The data are fully compatible with the existence of bound magnetic polarons or clusters below TC* which percolate at TC⪡TC*.

Surface-Induced Orientation Control of CuPc Molecules for the Epitaxial Growth of Highly Ordered Organic Crystals on Graphene

The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation, and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc molecules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micrometer sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable improvement in the device performance of organic thin films, wherein charge transport, exciton diffusion, and dissociation are currently limited by grain size effects and molecular orientation.