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1.
Nature ; 577(7789): 204-208, 2020 01.
Artigo em Inglês | MEDLINE | ID: mdl-31915394

RESUMO

Graphene films grown by chemical vapour deposition have unusual physical and chemical properties that offer promise for applications such as flexible electronics and high-frequency transistors1-10. However, wrinkles invariably form during growth because of the strong coupling to the substrate, and these limit the large-scale homogeneity of the film1-4,11,12. Here we develop a proton-assisted method of chemical vapour deposition to grow ultra-flat graphene films that are wrinkle-free. Our method of proton penetration13-17 and recombination to form hydrogen can also reduce the wrinkles formed during traditional chemical vapour deposition of graphene. Some of the wrinkles disappear entirely, owing to the decoupling of van der Waals interactions and possibly an increase in distance from the growth surface. The electronic band structure of the as-grown graphene films shows a V-shaped Dirac cone and a linear dispersion relation within the atomic plane or across an atomic step, confirming the decoupling from the substrate. The ultra-flat nature of the graphene films ensures that their surfaces are easy to clean after a wet transfer process. A robust quantum Hall effect appears even at room temperature in a device with a linewidth of 100 micrometres. Graphene films grown by proton-assisted chemical vapour deposition should largely retain their intrinsic performance, and our method should be easily generalizable to other nanomaterials for strain and doping engineering.

2.
Histol Histopathol ; : 18183, 2019 Nov 08.
Artigo em Inglês | MEDLINE | ID: mdl-31701509

RESUMO

Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer death worldwide. Lung adenocarcinoma is the main tumor type of NSCLC. Recent advances in the molecular characterization and personalized therapies have improved NSCLC patient prognosis. Previous studies showed that protein tyrosine kinase 7 (PTK7) plays an important role in human cancers. However, the role of PTK7 has not been investigated. PTK7 expression was assessed by immunohistochemistry in 95 patients with lung adenocarcinoma. Correlations of PTK7 expression levels with clinicopathological parameters, EGFR mutation and EML4-ALK fusion were examined. Positive PTK7 expression was detected in 47.4% of lung adenocarcinoma. PTK7 expression was associated with gender (P=0.024), lymph node metastasis (P<0.001), ALK mutation (P=0.050), and EGFR mutations (P=0.014). No significant association was found between PTK7 expression and age (P=0.831), differentiation (P=0.494), adenocarcinoma subtype (P=0.098) and Ki67 (P=0.473). Our data suggest that PTK7 plays an oncogenic role in lung adenocarcinoma and may be a molecular marker for lymph node metastasis.

3.
Org Biomol Chem ; 17(17): 4220-4224, 2019 04 24.
Artigo em Inglês | MEDLINE | ID: mdl-30946414

RESUMO

A metal-free and radical-free synthesis of heteroaromatic aldehydes was developed through aerobic oxidation of methyl groups in an I2/DMSO/O2 catalytic system. Under the reaction conditions, various functional groups such as methoxy, aldehyde, ester, nitro, amide, and halo (F, Cl, Br) groups were well tolerated. The bioactive compounds like chlorchinaldin derivative and papaverine were also oxidized to the corresponding aldehydes and ketones. This reaction provided an efficient method for preparing the valuable heteroaromatic aldehydes.

4.
Nat Mater ; 18(6): 602-607, 2019 06.
Artigo em Inglês | MEDLINE | ID: mdl-30858568

RESUMO

Two-dimensional transition metal selenides (TMSs) possess fascinating physical properties. However, many as-prepared TMSs are environmentally unstable and limited in sample size, which greatly hinder their wide applications in high-performance electrical devices. Here we develop a general two-step vapour deposition method and successfully grow different TMS films with controllable thickness, wafer size and high quality. The superconductivity of the grown NbSe2 film is comparable with sheets exfoliated from bulk materials, and can maintain stability after a variety of harsh treatments, which are ascribed to the absence of oxygen during the whole growth process. Such environmental stability can greatly simplify the fabrication procedure for device applications, and should be of both fundamental and technological significance in developing TMS-based devices.

5.
Sci Rep ; 9(1): 2685, 2019 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-30804450

RESUMO

Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices. The 1T' crystalline phase transition can drive the monolayer MX2 to be a 2D topological insulator. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T' and 2H phase monolayer WSe2 on bilayer graphene (BLG) substrate. The crystalline structures of these two phases were characterized using scanning tunneling microscopy. The monolayer 1T'-WSe2 was found to be metastable, and can transform into 2H phase under post-annealing procedure. The phase transition temperature of 1T'-WSe2 grown on BLG is lower than that of 1T' phase grown on 2H-WSe2 layers. This thermo-driven crystalline phase transition makes the monolayer WSe2 to be an ideal platform for the controlling of topological phase transitions in 2D materials family.

6.
Nat Mater ; 17(6): 504-508, 2018 06.
Artigo em Inglês | MEDLINE | ID: mdl-29713039

RESUMO

Time reversal and spatial inversion are two key symmetries for conventional Bardeen-Cooper-Schrieffer (BCS) superconductivity 1 . Breaking inversion symmetry can lead to mixed-parity Cooper pairing and unconventional superconducting properties1-5. Two-dimensional (2D) NbSe2 has emerged as a new non-centrosymmetric superconductor with the unique out-of-plane or Ising spin-orbit coupling (SOC)6-9. Here we report the observation of an unusual continuous paramagnetic-limited superconductor-normal metal transition in 2D NbSe2. Using tunelling spectroscopy under high in-plane magnetic fields, we observe a continuous closing of the superconducting gap at the upper critical field at low temperatures, in stark contrast to the abrupt first-order transition observed in BCS thin-film superconductors10-12. The paramagnetic-limited continuous transition arises from a large spin susceptibility of the superconducting phase due to the Ising SOC. The result is further supported by self-consistent mean-field calculations based on the ab initio band structure of 2D NbSe2. Our findings establish 2D NbSe2 as a promising platform to explore novel spin-dependent superconducting phenomena and device concepts 1 , such as equal-spin Andreev reflection 13 and topological superconductivity14-16.

7.
Oncol Lett ; 15(4): 5071-5080, 2018 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-29552140

RESUMO

Esophageal cancer is one of the most common types of malignant tumors located within the digestive system, with >50% of esophageal cancer cases worldwide occurring in China. Recent studies have demonstrated that long non-coding RNAs (lncRNAs) are frequently dysregulated in cancer; however, few lncRNAs have been characterized in esophageal squamous cell carcinoma (ESCC). In the present study, a novel lncRNA, SET-binding factor 2 (SBF2) antisense RNA1 (SBF2-AS1) was exhibited in ESCC. Expression levels of SBF2-AS1 in ESCC and adjacent non-cancerous tissues were detected using the reverse transcription-quantitative polymerase chain reaction. SBF2-AS1 was knocked down, and proliferation, migration, invasion, apoptosis and the cell cycle were examined in ESCC cells. Results identified that SBF2-AS1 was significantly upregulated in ESCC compared with adjacent non-cancerous tissues (fold increase, 8.82; P=0.023). The SBF2-AS1 expression level was significantly increased in patients who had a smoking (9.927 vs. 4.507; P=0.030) and/or drinking (10.938 vs. 4.232; P=0.032) history. Patients with a large tumor size exhibited increased SBF2-AS1 expression (≥4 vs. <4 cm, 14.898 vs. 5.435; P=0.018). Patients with advanced ESCC exhibited increased upregulation of SBF2-AS1 [tumor-node-metastasis (TNM) I-II vs. TNM III-IV, 1.302 vs. 15.475; P<0.01]. SBF2-AS1 was also silenced using small interfering RNA. Cell proliferative and invasive ability were significantly inhibited (P<0.05) following SBF2-AS1 silencing, the cell cycle was arrested in the G2 phase; however, there was no significant difference in the proportion of apoptotic cells. Gene Set Enrichment Analysis revealed that genes associated with cell cycle biological processes, including the cancer suppressor gene cyclin-dependent kinase 1A (CDKN1A), were significantly associated with SBF2-AS1 in ESCC tissues. Further validation confirmed that CDKN1A expression levels were increased in ECA-109 cells following SBF2-AS1 silencing. The results of the present study demonstrate that SBF2-AS1 is significantly upregulated in ESCC, and that silencing of SBF2-AS1 inhibits the proliferative and invasive ability of ESCC cells. SBF2-AS1 may be a novel biomarker and therefore a potential therapeutic target for ESCC.

8.
Chem Commun (Camb) ; 54(12): 1521-1524, 2018 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-29367968

RESUMO

A Ni-catalyzed hydrogenation of C-O compounds with sodium formate is developed. Various esters, i.e. aryl, alkenyl, benzyl pivalates, and even the aryl ethers, were efficiently reduced with a loading of nickel catalysts down to 0.5 mol%. Reactive functional groups such as C-C double bonds, carbonyl, CN, MeS and halogen groups are tolerable. This reaction can be used for the modification of complex molecules and carried out at a large scale.

9.
Sci Rep ; 7(1): 6710, 2017 07 27.
Artigo em Inglês | MEDLINE | ID: mdl-28751726

RESUMO

We report a shockwave method for altering the properties of the superconductor material Bi2Sr2CaCu2O8+δ (Bi2212). We find that the superconducting transition temperature (T c) increases from 84 K for the pristine sample to 94 K for the sample treated at a temperature and pressure of ~1200 K and ~31 GPa, respectively. X-ray diffraction and transmission electron microscopy characterizations indicate that this T c enhancement arises from a phase transition from pristine Bi2212 to a mixture of superconducting Bi2212 and semiconducting Bi2Sr2CuO6+δ (Bi2201) during the shockwave treatment. The shockwave-treated sample exhibits n-type semiconductor properties (with an on-off ratio ~5), in contrast to the pure metallic pristine sample. Our study offers an alternative route for modifying the superconducting properties via a shockwave treatment. Furthermore, this method may provide a new approach for studying other temperature- and pressure-sensitive materials.

10.
ACS Nano ; 10(10): 9500-9508, 2016 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-27700035

RESUMO

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = group 1 or 2 element, Tt = group 14 tetrel element, and Pn = group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman-active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant toward oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy, density functional theory, and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 Ω·m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics.

11.
Phys Rev Lett ; 117(10): 106801, 2016 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-27636485

RESUMO

Recent experimental advances in atomically thin transition metal dichalcogenide (TMD) metals have unveiled a range of interesting phenomena including the coexistence of charge-density-wave (CDW) order and superconductivity down to the monolayer limit. The atomic thickness of two-dimensional (2D) TMD metals also opens up the possibility for control of these electronic phase transitions by electrostatic gating. Here, we demonstrate reversible tuning of superconductivity and CDW order in model 2D TMD metal NbSe_{2} by an ionic liquid gate. A variation up to ∼50% in the superconducting transition temperature has been observed. Both superconductivity and CDW order can be strengthened (weakened) by increasing (reducing) the carrier density in 2D NbSe_{2}. The doping dependence of these phase transitions can be understood as driven by a varying electron-phonon coupling strength induced by the gate-modulated carrier density and the electronic density of states near the Fermi surface.

12.
Nat Nanotechnol ; 10(9): 765-9, 2015 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-26192206

RESUMO

Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated extensively, modifications in the many-body collective phenomena in the exact two-dimensional limit remain relatively unexplored. Here, we report a combined optical and electrical transport study on the many-body collective-order phase diagram of NbSe2 down to a thickness of one monolayer. Both the charge density wave and the superconducting phase have been observed down to the monolayer limit. The superconducting transition temperature decreases on lowering the layer thickness, but the newly observed charge-density-wave transition temperature increases from 33 K in the bulk to 145 K in the monolayer. Such highly unusual enhancement of charge density waves in atomically thin samples can be understood to be a result of significantly enhanced electron-phonon interactions in two-dimensional NbSe2 (ref. 4) and is supported by the large blueshift of the collective amplitude vibration observed in our experiment. Our results open up a new window for search and control of collective phases of two-dimensional matter, as well as expanding the functionalities of these materials for electronic applications.

13.
Phys Rev Lett ; 113(9): 096401, 2014 Aug 29.
Artigo em Inglês | MEDLINE | ID: mdl-25215996

RESUMO

The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressure-induced band inversion and topological phase transitions in Pb(1-x)Sn(x)Se (x=0.00, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the band gap as well as a maximum in the free carrier spectral weight. The enhanced density of states near the band gap in the topological phase gives rise to a steep interband absorption edge. The change of density of states also yields a maximum in the pressure dependence of the Fermi level. Thus, our conclusive results provide a consistent picture of pressure-induced topological phase transitions and highlight the bulk origin of the novel properties in topological insulators.

14.
Phys Rev Lett ; 111(15): 155701, 2013 Oct 11.
Artigo em Inglês | MEDLINE | ID: mdl-24160613

RESUMO

We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis bonding through p(z) band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition and are consistent with a recent theoretical proposal.

15.
Phys Rev Lett ; 105(25): 257006, 2010 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-21231618

RESUMO

We report the complex optical conductivity of a superconducting thin film of Nb 0.5 Ti 0.5 N in an external magnetic field. The field was applied parallel to the film surface and the conductivity extracted from far-infrared transmission and reflection measurements. The real part shows the superconducting gap, which we observe to be suppressed by the applied magnetic field. We compare our results with the pair-breaking theory of Abrikosov and Gor'kov and confirm directly the theory's validity for the optical conductivity.

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