Magnetotransport Properties of Graphene Nanoribbons with Zigzag Edges.

The determination of the electronic structure by edge geometry is unique to graphene. In theory, an evanescent nonchiral edge state is predicted at the zigzag edges of graphene. Up to now, the approach used to study zigzag-edged graphene has mostly been limited to scanning tunneling microscopy. The transport properties have not been revealed. Recent advances in hydrogen plasma-assisted "top-down" fabrication of zigzag-edged graphene nanoribbons (Z-GNRs) have allowed us to investigate edge-related transport properties. In this Letter, we report the magnetotransport properties of Z-GNRs down to ∼70 nm wide on an h-BN substrate. In the quantum Hall effect regime, a prominent conductance peak is observed at Landau ν=0, which is absent in GNRs with nonzigzag edges. The conductance peak persists under perpendicular magnetic fields and low temperatures. At a zero magnetic field, a nonlocal voltage signal, evidenced by edge conduction, is detected. These prominent transport features are closely related to the observable density of states at the hydrogen-etched zigzag edge of graphene probed by scanning tunneling spectroscopy, which qualitatively matches the theoretically predicted electronic structure for zigzag-edged graphene. Our study gives important insights for the design of new edge-related electronic devices.

Emergence of Tertiary Dirac Points in Graphene Moiré Superlattices.

The electronic structure of a crystalline solid is largely determined by its lattice structure. Recent advances in van der Waals solids, artificial crystals with controlled stacking of two-dimensional (2D) atomic films, have enabled the creation of materials with novel electronic structures. In particular, stacking graphene on hexagonal boron nitride (hBN) introduces a moiré superlattice that fundamentally modifies graphene's band structure and gives rise to secondary Dirac points (SDPs). Here we find that the formation of a moiré superlattice in graphene on hBN yields new, unexpected consequences: a set of tertiary Dirac points (TDPs) emerge, which give rise to additional sets of Landau levels when the sample is subjected to an external magnetic field. Our observations hint at the formation of a hidden Kekulé superstructure on top of the moiré superlattice under appropriate carrier doping and magnetic fields.

Hofstadter Butterfly and Many-Body Effects in Epitaxial Graphene Superlattice.

Graphene placed on hexagonal boron nitride (h-BN) has received a wide range of interest due to the improved electrical performance and rich physics from the interface, especially the emergence of superlattice Dirac points as well as Hofstadter butterfly in high magnetic field. Instead of transferring graphene onto h-BN, epitaxial growth of graphene directly on a single-crystal h-BN provides an alternative and promising way to study these interesting superlattice effects due to their precise lattice alignment. Here we report an electrical transport study on epitaxial graphene superlattice on h-BN with a period of ∼15.6 nm. The epitaxial graphene superlattice is clean, intrinsic, and of high quality with a carrier mobility of ∼27 000 cm(2) V(-1) s(-1), which enables the observation of Hofstadter butterfly features originated from the superlattice at a magnetic field as low as 6.4 T. A metal-insulator transition and magnetic field dependent Fermi velocity were also observed, suggesting prominent electron-electron interaction-induced many-body effects.

Thermally Induced Graphene Rotation on Hexagonal Boron Nitride.

In this Letter, we report the observation of thermally induced rotation of graphene on hexagonal boron nitride (h-BN). After the rotation, two thermally stable configurations of graphene on h-BN with a relative lattice twisting angle of 0° (most stable) and 30° (metastable), respectively, were found. Graphene on h-BN with a twisting angle below (above) a critical angle of ∼12±2° tends to rotate towards 0° (30°) at a temperature of >100 °C, which is in line with our theoretical simulations. In addition, by manipulating the annealing temperature and the flake sizes of graphene, moiré superlattices with large spatial periods of graphene on h-BN are achieved. Our studies provide a detailed understanding of the thermodynamic properties of graphene on h-BN and a feasible approach to obtaining van der Waals heterostructures with aligned lattices.

A general route towards defect and pore engineering in graphene.

Defect engineering in graphene is important for tailoring graphene's properties thus applicable in various applications such as porous membranes and ultra-capacitors. In this paper, we report a general route towards defect- and pore- engineering in graphene through remote plasma treatments. Oxygen plasma irradiation was employed to create homogenous defects in graphene with controllable density from a few to ≈10(3) (µm(-2)). The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well-defined pore size of a few nm or above. The achieved smallest nanopores are ≈2 nm in size, showing the potential for ultra-small graphene nanopores fabrication.

Epitaxial growth of single-domain graphene on hexagonal boron nitride.

Hexagonal boron nitride (h-BN) has recently emerged as an excellent substrate for graphene nanodevices, owing to its atomically flat surface and its potential to engineer graphene's electronic structure. Thus far, graphene/h-BN heterostructures have been obtained only through a transfer process, which introduces structural uncertainties due to the random stacking between graphene and h-BN substrate. Here we report the epitaxial growth of single-domain graphene on h-BN by a plasma-assisted deposition method. Large-area graphene single crystals were successfully grown for the first time on h-BN with a fixed stacking orientation. A two-dimensional (2D) superlattice of trigonal moiré pattern was observed on graphene by atomic force microscopy. Extra sets of Dirac points are produced as a result of the trigonal superlattice potential and the quantum Hall effect is observed with the 2D-superlattice-related feature developed in the fan diagram of longitudinal and Hall resistance, and the Dirac fermion physics near the original Dirac point is unperturbed. The macroscopic epitaxial graphene is in principle limited only by the size of the h-BN substrate and our synthesis method is potentially applicable on other flat surfaces. Our growth approach could thus open new ways of graphene band engineering through epitaxy on different substrates.

[Relationship between human papilloma virus infection and expression of p16 and EGFR in head and neck squamous cell carcinoma and their prognostic significance].

OBJECTIVE: To detect the infection of human papillomavirus (HPV) 6/11, 16/18, 31/33 in patients with head and neck squamous cell carcinoma and explore the relationship between HPV infection and expression of p16 and EGFR in the tumor tissue and their clinical significance. METHODS: The infection of HPV6/11, 16/18, 31/33 was detected by in situ hybridization (ISH), and expression of p16 and EGFR was assessed by immunohistochemistry in biopsy or surgical specimens of 43 cases of head and neck squamous cell carcinoma, and analyzed its impact on the prognosis. Spearman rank correlation method was used for analysis of the relationship. Overall survival rate of the patients was estimated by Kaplan-Meier analysis. Cox regression model was used for multivariate analysis. RESULTS: HPV6/11, 16/18, 31/33 were detected in 25.6% (11/43) of this group of patients, among them, HPV16/18 accounted for 63.6%, HPV31/33 accounted for 27.3%, and HPV6/11 accounted for 0. EGFR was expressed in 69.8% and p16 was expressed in 53.5% of the patients. The difference was statistically significant between the HPV-positive and HPV-negative groups in ethnicity, smoking, alcohol consumption (P = 0.045, 0.040, 0.011, respectively). HPV infection was found to be positively correlated with p16 expression and inversely correlated with EGFR expression (P = 0.029, P = 0.009). The expression of p16 protein was negatively correlated with EGFR protein expression (r = -0.447, P = 0.003). The 3-year overall survival rate was 60.0% in the HPV-positive group and 59.7% in the negative group (P = 0.789); 72.2% in the p16-positive patients and 43.9% in the p16-negative patients (P = 0.012); 48.8% in the EGFR-positive patients and 81.8% in the EGFR-negative patients (P = 0.037). CONCLUSIONS: The results of our study suggest that the HPV infection rate, HPV subtypes and clinicopathological features of HPV-positive SCCHN are in accordance with those reported in Western literatures. There may be differences between the HPV infections in Uygur and Han nationalities. HPV infection is positively correlated with p16 and negatively correlated with EGFR expressions. The prognosis of p16-positive patients is significantly better than that of negative cases, and p16 is an independent prognostic factor for head and neck squamous cell carcinoma.

Graphene edge lithography.

Fabrication of graphene nanostructures is of importance for both investigating their intrinsic physical properties and applying them into various functional devices. In this paper, we report a scalable fabrication approach for graphene nanostructures. Compared with conventional lithographic fabrication techniques, this new approach uses graphene edges as the templates or masks and offers advantage in technological simplicity and capability of creating small features below 10 nm scale. Moreover, mask layers used in the fabrication process could be simultaneously used as the dielectric layers for top-gated devices. The as-fabricated graphene nanoribbons (GNRs) are of high quality with the carrier mobility ∼400 cm(2)/(V s) for typical 15 nm wide ribbons. Our technique allows easy and reproducible fabrication of various graphene nanostructures, such as ribbons and rings, and can be potentially extended to other materials and systems by use of their edges or facets as templates.

Growth, characterization, and properties of nanographene.

A systematic study on nanographene grown directly on silicon dioxide substrates is reported. The growth is carried out in a remote plasma-enhanced chemical vapor deposition system at a low temperature of around 550 °C with methane gas as the carbon source. Atomic force microscopy is used to characterize the nanographene morphology, and Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy are exploited to identify the in-plane sp(2) bonding structures of nanographene samples. Electrical transport properties are measured at various temperatures down to 4 K. Tunneling effects, minimal conductance at the charge-neutral point, sheet resistances, and Dirac point position at different channel lengths are investigated. In addition, nanographene film possesses high transmittance properties, as indicated by transmittance spectra. Nanographene devices are fabricated from rippled structures, and show great promise for strain-gauge sensor applications.

Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity.

Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~10(5) to 10(6) cycles for stochastic switching).

Association analysis between HLA-A, -B, -C, -DRB1, and -DQB1 with nasopharyngeal carcinoma among a Han population in Northwestern China.

BACKGROUND: Most HLA association studies with NPC focus on Southern part of China. Thus, little is known about the genetic association of HLA with NPC in people from Northern China where the genetic background, environmental factors, and lifestyle are very different. METHODS: 132 NPC patients and 168 normal controls of Han ethnic in Xinjiang Province were genotyped for HLA-A, -B, -C, -DRB1, and -DQB1 using the PCR-sequence specific primer technique. RESULTS: Our results confirm previous findings that the HLA-B∗46 and HLA-A(∗)02B(∗)46 haplotypes are strongly associated with NPC, but we did not observe HLA-A(∗)11 and -B(∗)35 association with resistance to NPC. Instead, we found that HLA-B(∗)51 and -A(∗)30 are strongly associated with resistance to NPC. In addition, HLA-A(∗)24 was also strongly associated with susceptibility to NPC. CONCLUSION: A unique pattern of HLA association with NPC susceptibility and resistance was found in patients from Xinjiang Province compared to the published data based on patients from Southern China and Taiwan. HLA-B(∗)46 is most significantly associated with NPC, and its frequency in endemic areas such as Guangdong Province, Taiwan, and Thailand is much higher than in Xinjiang Province and other areas with much less NPC occurrence. Interestingly, HLA-A(∗)30 is in contrast with the resistant allele, whose frequency is much lower in endemic areas but higher in Xinjiang and other areas with little NPC.

Prognoses and long-term outcomes of nasopharyngeal carcinoma in Han and Uyghur patients treated with intensity-modulated radiotherapy in the Xinjiang Autonomous Region of China.

OBJECTIVES: The objective of this study was to investigate the long-term outcomes and prognostic factors for nasopharyngeal carcinoma (NPC) in Han and Uyghur patients treated with intensity-modulated radiotherapy (IMRT) in the Xinjiang region of China. MATERIALS AND METHODS: One hundred twenty-one Han and 60 Uyghur patients with newly diagnosed NPC without distant metastasis received IMRT at the Affiliated Tumor Hospital of Xinjiang Medical University between 2005 and 2008. The Kaplan-Meier method was used to estimate survival rates, and the log-rank test was used to evaluate differences in survival. RESULTS: Comparing Han and Uyghur patients, the 5-year overall survival (OS), disease-free survival (DFS), local control (LC), regional control (RC), and distant metastasis-free survival (DMFS) rates were 81.9% vs 77.6% (P = 0.297), 72.1% vs 65.6% (P = 0.493), 88.3% vs 86.5% (P = 0.759), 95.0% vs 94.6% (P = 0.929), and 79.1% vs 75.2% (P = 0.613), respectively. Multivariate Cox proportional hazards regression identified the following independent prognostic factors in Han patients: N stage (P = 0.007) and age (P = 0.028) for OS, and age (P = 0.028) for DFS. OS differed significantly between Han and Uyghur patients >60 years old group (P = 0.036). Among Uyghur patients, the independent prognostic factors were age for OS (P = 0.033), as well as N stage (P = 0.037) and age (P = 0.021) for DFS. Additionally, Uyghur patients were less likely to experience mucositis and dermatitis than Han patients. CONCLUSION: Han and Uyghur patients with NPC had statistically significant differences in age, smoking history, and N staging. There was no significant difference in overall treatment outcomes with IMRT between these 2 ethnic populations in Xinjiang, China.

A route toward digital manipulation of water nanodroplets on surfaces.

Manipulation of an isolated water nanodroplet (WN) on certain surfaces is important to various nanofluidic applications but challenging. Here we present a digital nanofluidic system based on a graphene/water/mica sandwich structure. In this architecture, graphene provides a flexible protection layer to isolate WNs from the outside environment, and a monolayer ice-like layer formed on the mica surface acts as a lubricant layer to allow these trapped WNs to move on it freely. In combination with scanning probe microscope techniques, we are able to move, merge, and separate individual water nanodroplets in a controlled manner. The smallest manipulatable water nanodroplet has a volume down to yoctoliter (10(-24) L) scale.

Tunable electroluminescence in planar graphene/SiO(2) memristors.

Electroluminescence and resistive switching are first realized simultaneously in graphene/SiO2 memristor devices. The electroluminescence peaks can be tuned between 550 nm and 770 nm reliably via setting the device to different resistance states by applying different voltages. The combination of resistive switching and electroluminescence may bring new functionalities for these memristor devices which are fully compatible with silicon-based electronics.