Formation and lithium doping of graphene on the surface of cobalt silicide

The intercalation of silicon under graphene on the Co(0001) surface, which is accompanied by the formation of a silicon solid solution in cobalt and by the formation of a surface crystalline Co₂Si phase, has been investigated using photoelectron spectroscopy. It has been shown that the formation of cobalt silicide leads to a substantial weakening of the hybridization of electronic states of graphene and cobalt and to the recovery of the Dirac spectrum of electronic states of graphene near the Fermi level. This has made it possible to investigate the electron doping of graphene on the cobalt silicide substrate upon deposition of lithium on its surface. It has been found that doping with lithium leads to a significant charge transfer onto graphene, and the electron concentration reaches 3.1 × 10¹⁴ cm^?2. Moreover, the specific form of the Fermi surface creates favorable conditions for the enhancement of the electron-phonon coupling. As a result, the formed system can be considered as a candidate for the creation of superconductivity in single-layer graphene.     

In‐situ Raman spectroscopy of current‐carrying graphene microbridge

In‐situ Raman spectroscopy was performed on chemical vapor deposited graphene microbridge (3 μm × 80 μm) under electrical current density up to 2.58 × 10⁸ A/cm² in ambient conditions. We found that both the G and the G′ peak of the Raman spectra do not restore back to the initial values at zero current, but to slightly higher values after switching off the current through the microbridge. The up‐shift of the G peak and the G′ peak, after switching off the electrical current, is believed to be due to p‐doping by oxygen adsorption, which is confirmed by scanning photoemission microscopy. Both C–O and C=O bond components in the C1s spectra from the microbridge were found to be significantly increased after high electrical current density was flown. The C=O bond is likely the main source of the p‐doping according to our density functional theory calculation of the electronic structure. Copyright © 2014 John Wiley & Sons, Ltd.     

基于密度泛函理论研究掺杂Pd石墨烯吸附O2及CO

基于第一性原理的密度泛函理论研究了单个O₂和CO气体分子吸附于本征石墨烯和掺杂钯(Pd)的石墨烯的体系, 通过石墨烯掺Pd前后气体分子的吸附能、电荷转移及能带和态密度的计算, 发现掺Pd后气体分子吸附能和电荷转移显著增大, 这是由于Pd的掺杂, 在本征石墨烯能带中引入了杂质能级, 增强了石墨烯和吸附气体分子间的相互作用; 氧化性气体O₂和还原性气体CO吸附对石墨烯体系能带结构和态密度的影响明显不同, 本征石墨烯吸附O₂后, 费米能级附近态密度变大, 掺Pd后在一定程度变小; 吸附还原性的CO后, 石墨烯费米能级附近态密度几乎没有改变, 表明掺杂Pd不会影响石墨烯对CO的气体灵敏度, 但由于CO对石墨烯的吸附能增大, 可以提高石墨烯对还原性气体的气敏响应速度.     

Tunable n‐ and p‐type doping of single‐layer graphene by engineering its interaction with the SiO2 support

We report a technique to tune the excess charge concentration in single‐layer graphene from p‐ to n‐type up to densities of |n | ～ 1.2 × 10¹³ cm^–2, corresponding to a displacement electric field of ～2.5 V/nm. The tuning is achieved by engineering the interaction between graphene and the underlying Si/SiO₂ substrate with an amino group‐terminated self‐assembled monolayer, and subsequent rinsing in aqueous solutions at controlled pH. Raman spectroscopy and electrical measurements on treated graphene devices confirm the occurrence of doping. Interestingly, we found the field‐effect mobility not to be significantly affected by the procedure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of chemical solvents on the properties of CVD graphene

The ultra‐clean and defect‐free transfer of chemical vapor deposition (CVD) graphene is essential for its application in electronic devices. Here, we study the influence of commonly used etching solvents during the transfer process, i.e. ammonium persulfate, ferric chloride, and ferric nitrate, on the properties of CVD graphene by Raman spectroscopy. Obvious blue shift and broadening of Raman G and 2D peaks were observed for graphene transferred by ferric chloride and ferric nitrate, as compared to that transferred by ammonium persulfate. These changes are attributed to p‐doping as well as reduction of phonon lifetime of graphene in the presence of residue iron compounds during the transfer process. The latter would also introduce a great reduction of thermal conductivity of graphene, e.g. with 76% reduction for graphene transferred by ferric nitrate as compared to that transferred by ammonium persulfate. This work would provide valuable information for the transfer of high‐quality CVD graphene. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of Annealing on Raman Spectrum of Graphene in Different Gaseous Environments

Graphene grown by a coronene (C-graphene) source is transferred to an SiO₂ surface, and its Raman spectra are investigated in annealing environments of O₂, Ar, and N₂. An irreversible doping effect is observed in all the annealing environments, which is attributed to the enhancement of substrate doping. Compared with the mechanically exfoliated graphene on SiO₂, stronger remnant stress remains in the transferred C-graphene, and wrinkles prevail on the surface. It is found that the defect density increases only after O₂ annealing, and the full width half maximum (FWHM) of the G and 2D bands in the Raman spectrum increases in all the annealing atmospheres. We suggest that the increase of FWHM is caused by the crystalline disorders.     

Low-Temperature Sputtered Ultralow-Loss Silicon Nitride for Hybrid Photonic Integration

Silicon-nitride-on-insulator (Si₃N₄) photonic circuits have seen tremendous advances in many applications, such as on-chip frequency combs, Lidar, telecommunications, and spectroscopy. So far, the best film quality has been achieved with low pressure chemical vapor deposition (LPCVD) and high-temperature annealing (1200°C). However, high processing temperatures pose challenges to the cointegration of Si₃N₄ with pre-processed silicon electronic and photonic devices, lithium niobate on insulator (LNOI), and Ge-on-Si photodiodes. This limits LPCVD as a front-end-of-line process. Here, ultralow-loss Si₃N₄ photonics based on room-temperature reactive sputtering is demonstrated. Propagation losses as low as 5.4 dB m⁻¹ after 400°C annealing and 3.5 dB m⁻¹ after 800°C annealing are achieved, enabling ring resonators with highest optical quality factors of > 10 million and an average quality factor of 7.5 million. To the best of the knowledge, these are the lowest propagation losses achieved with low temperature Si₃N₄. This ultralow loss enables the generation of microresonator soliton frequency combs with threshold powers of 1.1 mW. The introduced sputtering process offers full complementary metal oxide semiconductor (CMOS) compatibility with front-end silicon electronics and photonics. This could enable hybrid 3D integration of low loss waveguides with integrated lasers and lithium niobate on insulator.     

First-principles study of the formation mechanisms of nitrogen molecule in annealed ZnO

Recently, N₂ molecule was reported to induce localized states in the band gap and trap two holes in ZnO. In this Letter, the detailed mechanism for the formation of N₂ molecule in high temperature annealing process in ZnO was investigated based on density-functional theory. By analyzing the interactions between N-related defects, we found that the nitrogen molecule would form by the binding of two interstitial nitrogen atoms. Interstitial oxygen facilitated the formation of N₂ by kicking out N_O to interstitial site. The formation of nitrogen molecule in ZnO would cause low doping efficiency and degeneration of the p-type in annealing process. Our results could explain the recently reported formation of N₂ molecule in high temperature annealing process in N-doped ZnO. Appropriate annealing conditions were suggested in order to get p-type ZnO.     

Supercapacitor performances of rich nitrogen-doped mesoporous graphene fabricated by a facile template-free copyrolysis process

Rich nitrogen-doped mesoporous graphene (NDMG) with a large specific surface area of 496.8 m² g^?1 and high electrical conductivity of 327.2 S cm^?1, and suitable pore size was synthesized by a facile co-thermal annealing of pre-prepared phenolic polymer and dicyandiamide. The NDMG has a high nitrogen content (7.9 wt%) and can act as promising electroactive materials for two-electrode symmetric supercapacitors. The NDMG cells displayed a high specific capacitance of ca. 316 F g^?1 at 0.5 A g^?1, which is much higher than that of the pristine graphene devices (ca. 123 F g^?1). Moreover, compared with the capacitance drop rate of pristine graphene devices (8.9 %), the specific capacitance of NDMG cells was decreased by only 3.2 % after 2000 cycles, exhibiting a good cycling performance and reversibility. In addition, the specific capacitance of the NDMG cells can reach 251 F g^?1 at 5.0 A g^?1, revealing an excellent rate capability and implying the ability to deliver a high energy density at a high power density. The good electrochemical performances of NDMG can be attributed to its high surface area, suitable mesopore size, and high electrical conductivity.     

注入Ar+的蓝宝石晶体退火前后光致发光谱的分析

对注入Ar⁺后不同晶面取向的蓝宝石晶体在不同退火条件下的光致发光谱进行了分析.分析结果表明:三种晶面取向的蓝宝石样品经Ar⁺注入后,其光致发光谱中均出现了新的位于506nm处的发光峰;真空和空气气氛下的退火均对样品在506nm处的发光有增强作用,不同晶面取向的样品发光增强程度不同,且发光增强至最大时的退火温度也不同,空气气氛下的退火使样品发光增强程度更为显著.由此可以看出,退火气氛、退火温度和晶面取向均对样品发光峰强度有影响. 关键词： 2O₃')" href="#">Al₂O₃ 离子注入 退火 光致发光谱     

Fabrication of suspended graphene devices and their electronic properties

Suspended graphene devices are successfully fabricated by using a novel PMMA/MMA/PMMA tri-layer resist technique. The gap between graphene and dielectric substrate can be easily controlled by the thickness of the bottom PMMA layer, and no wet-etching with hazardous hydrofluoric acid is involved in our fabrication process. Electrical characterizations on suspended graphene devices are performed in vacuum when in-situ current annealing directly leads to a significant improvement on transport properties of graphene, i.e., the increase of carrier mobility with the reduction of width of Dirac peak. Our results make a new opportunity to study intrinsic properties of graphene.     

Adsorption and desorption of fullerene on graphene/SiC(0001)

Adsorption and desorption of fullerene on a single layer of graphene grown on SiC(0001) were investigated by photoemission spectroscopy (PES). No significant change in the band structure of graphene was observed after fullerene deposition on the graphene layer under vacuum conditions, and subsequent exposure to the air. After annealing the fullerene layer at 275 °C in a vacuum, complete desorption of fullerene was observed without any resulting damage to the graphene structure. The desorption temperature of fullerene was significantly higher than that of pentacene, indicating that fullerene layers show higher stability than pentacene as protection layers of graphene-based devices.     

Molecular adsorption of NO on free-standing and on graphene-supported Mo<Subscript>3</Subscript>W<Subscript>5</Subscript> cluster: a density functional theory investigation

The adsorption of molecular NO on the free-standing and graphene-supported Mo₃W₅ cluster is studied using methods from the gradient-corrected density functional theory. Before, the effect of the graphene support on the properties of the metal cluster was investigated. The interaction between the metal cluster and the graphene sheet takes place mainly through W atoms, which form up to three bonds with the support. Interaction energies are in the range from 0.6 to 1.5 eV. An amount of charge of about 0.4–0.5 e\(^-\) is transferred from the cluster to the support. Geometric distortions in the metal aggregate are negligible. An important decrease in the magnetic moment of Mo₃W₅ with respect to its free-standing value is observed after the interaction with the support. Molecular NO adsorbs on sites involving W atoms only, both for the free-standing and the supported metal cluster. Adsorption energies are in a range from 2 to 4 eV. A parallel mode is the preferred mode from an energetic point of view. Moreover, for that parallel adsorption mode, the N–O bond is more effectively activated. Magnetic moments change largely after adsorption indicating important rearrangement in the electronic configuration of the metal cluster. An important amount of electronic charge is transferred both from the free-standing and from the supported metal cluster to NO. The amount of charge transferred seems to be closely related to the activation of the N–O bond. The effect of the graphene sheet on the catalytic properties of Mo₃W₅ seems to be negligible, with the exception of some changes in the electronic configuration of the cluster.     

Ab initio study of the (2 × 2) phase of barium on graphene

We present a first-principles density functional theory study on the structural, electronic and dynamical properties of a novel barium doped graphene phase. Low energy electron diffraction of barium doped graphene presents clear evidence of (2 × 2) spots induced by barium adatoms with BaC₈ stoichiometry. First principles calculations reveals that the phase is thermodynamically stable but unstable to segregation towards the competitive BaC₆ monolayer phase. The calculation of phonon spectrum confirms the dynamical stability of the BaC₈ phase indicating its metastability, probably stabilized by doping and strain conditions due to the substrate. Barium induces a relevant doping of the graphene π states and new barium-derived hole Fermi surface at the M-point of the (2 × 2) Brillouin zone. In view of possible superconducting phase induced by foreign dopants in graphene, we studied the electron–phonon coupling of this novel (2 × 2) obtaining λ = 0.26, which excludes the stabilization of a superconducting phase.     

Magnetic properties of CuCr2Se4 single crystals

The magnetic properties of CuCr₂Se₄ single crystals, which were grown by chemical transport reactions using iodine as a carrier, have been investigated. The magnetic moment at 0 K is found to be 5.07 μ_B per mole. The susceptibility at high temperatures follows a Curie-Weiss law with an asymptotic Curie temperature 430 K and a Curie constant 2.55 emu · $\text{deg}\text{mol}$. The magnetocrystalline anisotropy constants K₁ and K₂ are ? 6.9x 10⁵ and ? 0.9x 10⁵$\text{erg}\text{cm}{}^3$, respectively, at 5.1 K.In order to examine the effects of annealing on the magnetocrystalline anisotropy, the ferromagnetic resonance at room temperature was measured after annealing in vacuum and subsequently in an atmosphere of Se. It is found that the absolute values of K₁ and K₂ decrease after annealing in vacuum and increase to the initial values after annealing in an atmosphere of Se.     

Tuning the electronic and magnetic properties of germanene by surface adsorption of small nitrogen-based molecules

The structural, energetic and electronic properties of germanene adsorbed with small nitrogen-based molecules, including N₂, NH₃, NO₂ and NO, have been investigated by using first-principles calculations. The results show that all nitrogen-based molecules considered bind much stronger to germanene than to graphene due to the hybridized sp²-sp³ bonding of Ge atoms. The N₂, NO and NO₂ molecules all act as an acceptor, while the NH₃ molecule donates electrons to germanene. We also found sizable band gaps (2–158 meV) are opened at the Dirac point of germanene through N₂, NH₃, and NO₂ adsorptions, but with only slightly destroying its Dirac cone shape. The NO₂ molecule also shows a heavy p-type doping character and makes germanene to be metallic. Moreover, when adsorbed by NO molecule, the germanene can change to be a ferromagnetic half-metal with 100% spin-polarization at the Fermi level. Overall, the different adsorption behaviors of small nitrogen-based gas molecules on germanene provide a feasible way to exploit chemically modified germanene for a wide range of practical applications, such as field-effect transistors, gas sensors and spintronic devices.     

Photoluminescence of focused ion beam implanted Er3+:Y2SiO5 crystals

Erbium‐doped low symmetry Y₂SiO₅ crystals attract a lot of attention in perspective of quantum information applications. However, only doping of the samples during growth is available up to now, which yields a quite homogeneous doping density. In the present work, we deposit Er³⁺‐ions by the focused ion beam technique at yttrium sites with several fluences in one sample. With a photoluminescence study of these locally doped Er³⁺:Y₂SiO₅ crystals, we are able to evaluate the efficiency of the implantation process and develop it for the highest efficiency possible. We observe the dependence of ion activation after the post‐implantation annealing on the fluence value. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Fe,Co,Ni掺杂石墨烯表面吸附C_2H_4的第一性原理研究

基于密度泛函理论(DFT)的广义梯度近似(GGA),本文对本征石墨烯以及掺杂Fe,Co,Ni石墨烯的几何结构和电子性质进行了优化计算,并计算了C_2H_4在本征石墨烯以及掺杂石墨烯表面的吸附过程,讨论了体系的吸附能、稳定性、DOS及掺杂对键长的影响.结果表明C_2H_4在本征石墨烯B位的吸附和掺杂石墨烯的吸附为化学吸附,在本征石墨烯T和H位的吸附为物理吸附;掺杂后石墨烯的比表面积增大,与本征石墨烯相比,掺杂使费米能级附近的态密度积分显著提高,表明掺杂石墨烯的电导性会发生变化,从而影响对C_2H_4的气敏度..C_2H_4在Fe、Co、Ni分别掺石墨烯的最佳吸附位为T位、H位和B位;掺杂Fe,Ni后体系的吸附能力显著提高,且掺杂Ni时体系的吸附能力最好.     

Lithium and antimony adsorbed on graphene studied by first-principles calculations

We have investigated the Sb and Sb₂ doping of graphene and the effect of Li_n (n = 2-4) atoms in detail. We find chemisorption only when we replace C with Sb and Sb₂ and distort the lattice. The additional adsorption of Li atoms changes the electronic band structure of the system.     

Behavior of oxygen in zinc oxide films through thermal annealing and its effect on sheet resistance

Behavior of oxygen in sputtering deposited ZnO films through thermal annealing and its effect on sheet resistance of the films were investigated. The crystallinities of the ZnO film were improved by post-deposition annealing in vacuum. However, the sheet resistance of ZnO film was dramatically decreased after post-deposition annealing in vacuum at more than 300 °C, while O₂ desorbed from the film. The oxygen vacancies which acted as donors were formed by the thermal annealing in vacuum. The sheet resistance of the films was recovered by annealing in oxygen ambient. In this paper, ¹⁸O₂ gas as an oxygen isotope was used as the annealing ambient in order to distinguish from ¹⁶O, which was constituent atom of the ZnO films. SIMS analysis revealed that ¹⁸O diffused into the ZnO film from the top surface by ¹⁸O₂ annealing. Therefore oxygen vacancies formed by the post-deposition annealing in vacuum could be compensated by the annealing in oxygen ambient.