Threaded aluminum nitride （A1N） whiskers are grown by a physical vapor transport method in a radio-frequency induction heating furnace. The resultant whiskers are characterized by X-ray diffraction, Raman scattering...Threaded aluminum nitride （A1N） whiskers are grown by a physical vapor transport method in a radio-frequency induction heating furnace. The resultant whiskers are characterized by X-ray diffraction, Raman scattering, scanning electron microscopy, transmission electron microscopy and photoluminescence. The analysis shows that the whiskers are single-crystalline, wurtzite AIN. The threaded A1N whiskers are 0.5 μm^100 μm in diameter and several millimeters in length in the fiber direction, and have lots of tiny sawteeth on the surface. The morphology of this threaded A1N whisker is beneficial for bonding when the whisker is used in composite. The growth of the whiskers is dominated by the vapor-solid （VS） mechanism, and the particular morphology might result from an oscillating condition produced in the radio-frequency induction heating furnace.展开更多
Electrochemical method has been used to insert K/Na into FeSe lattice to prepare alkali-intercalated iron selenides at room temperature. Magnetization measurement reveals that KxFe2Se2 and NaxFe2Se2 are superconductiv...Electrochemical method has been used to insert K/Na into FeSe lattice to prepare alkali-intercalated iron selenides at room temperature. Magnetization measurement reveals that KxFe2Se2 and NaxFe2Se2 are superconductive at 31 K and 46 K, respectively. This is the first successful report of obtaining metal-intercalated FeSe-based high-temperature superconductors using electrochemical method. It provides an effective route to synthesize metal-intercalated layered compounds for new superconductor exploration.展开更多
A self-powered graphene-based photodetector with high performance is particularly useful for device miniaturization and to save energy.Here,we report a graphene/silicon carbide（SiC）-based self-powered ultraviolet ph...A self-powered graphene-based photodetector with high performance is particularly useful for device miniaturization and to save energy.Here,we report a graphene/silicon carbide（SiC）-based self-powered ultraviolet photodetector that exhibits a current responsivity of 7.4 m A/W with a response frequency of over a megahertz under 325-nm laser irradiation.The built-in photovoltage of the photodetector is about four orders of magnitude higher than previously reported results for similar devices.These favorable properties are ascribed to the ingenious device design using the combined advantages of graphene and SiC,two terminal electrodes,and asymmetric light irradiation on one of the electrodes.Importantly,the photon energy is larger than the band gap of SiC.This self-powered photodetector is compatible with modern semiconductor technology and shows potential for applications in ultraviolet imaging and graphene-based integrated circuits.展开更多
The field emission （FE） properties of vertically aligned graphene sheets （VAGSs） grown on different SiC substrates are reported. The VAGSs grown on nonpolar SiC （10-10） substrate show an ordered alignment with t...The field emission （FE） properties of vertically aligned graphene sheets （VAGSs） grown on different SiC substrates are reported. The VAGSs grown on nonpolar SiC （10-10） substrate show an ordered alignment with the graphene basal plane-parallel to each other, and show better FE features, with a lower turn-on field and a larger field enhancement factor. The VAGSs grown on polar SiC （000-1 ） substrate reveal a random petaloid-shaped arrangement and stable current emission over 8 hours with a maximum emission current fluctuation of only 4%. The reasons behind the differing FE characteristics of the VAGSs on different SiC substrates are analyzed and discussed.展开更多
（Received 8 June 2013） Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liqu...（Received 8 June 2013） Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K^46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2（NH3）y （x 0.3 and 0.6）, determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.展开更多
Ferromagnetism is investigated in high-quality Cu-doped A1 N single crystal whiskers.The whiskers exhibit roomtemperature ferromagnetism with a magnetic moment close to the results from first-principles calculations.H...Ferromagnetism is investigated in high-quality Cu-doped A1 N single crystal whiskers.The whiskers exhibit roomtemperature ferromagnetism with a magnetic moment close to the results from first-principles calculations.High crystallinity and low Cu concentrations are found to be indispensable for high magnetic moments.The difference between the experimental and theoretical moment values is explored in terms of the influence of nitrogen vacancies.The calculated results demonstrate that nitrogen vacancies can reduce the magnetic moments of Cu atom.展开更多
A new layered Cu-based oxychalcogenide Ba_3Fe_2O_5Cu_2S_2 has been synthesized and its magnetic and electronic properties were revealed. Ba_3Fe_2O_5Cu_2S_2 is built up by alternatively stacking [Cu_2S_2]^（2-） layers...A new layered Cu-based oxychalcogenide Ba_3Fe_2O_5Cu_2S_2 has been synthesized and its magnetic and electronic properties were revealed. Ba_3Fe_2O_5Cu_2S_2 is built up by alternatively stacking [Cu_2S_2]^（2-） layers and iron perovskite oxide[（FeO_2）（BaO）（FeO_2）]^（2-）layers along the c axis that are separated by barium ions with Fe^（3＋） fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuC h-based（Ch =S, Se, Te） compounds the ＋3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba_3Fe_2O_5Cu_2S_2 is an antiferromagnetic semiconductor with a Ne′el temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3 d states of Fe ions that antiferromagnetically arranged in FeO_2 layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr_3Sc_2O_5Cu_2S_2.展开更多
Defects in silicon carbide （SIC） substrate are crucial to the properties of the epitaxial graphene （EG） grown on it. Here we report the effect of defects in SiC on the crystalline quality of EGs through comparativ...Defects in silicon carbide （SIC） substrate are crucial to the properties of the epitaxial graphene （EG） grown on it. Here we report the effect of defects in SiC on the crystalline quality of EGs through comparative studies of the characteristics of the EGs grown on SiC （0001） substrates with different defect densities. It is found that EGs on high quality SiC possess regular steps on the surface of the SiC and there is no discernible D peak in its Raman spectrum. Conversely, the EG on the SiC with a high density of defects has a strong D peak, irregular stepped morphology and poor uniformity in graphene layer numbers. It is the defects in the SiC that are responsible for the irregular stepped morphology and lead to the small domain size in the EG.展开更多
A nonpolar SiC（1120） substrate has been used to fabricate epitaxial graphene （EG）. Two EGs with layer numbers of 8-10 （referred to as MLG） and 2-3 （referred to as FLG） were used as representative to study the ...A nonpolar SiC（1120） substrate has been used to fabricate epitaxial graphene （EG）. Two EGs with layer numbers of 8-10 （referred to as MLG） and 2-3 （referred to as FLG） were used as representative to study the substrate effect on EG through temperature-dependent Raman scattering. It is found that Raman lineshifts of G and 2D peaks of the MLG with temperature are consistent with that of free graphene, as predicted by theory calculation, indicating that the substrate influence on the MLG is undetectable. While Raman G peak lineshifts of the FLG to that of the free graphene are obvious, however, its lineshift rate （-0.016 cm-1/K） is almost one third of that （-0.043 cm-1/K） of an EG on 6H-SiC （0001） in the temperature range from 300 K to 400 K, indicating a weak substrate effect from SiC （1120） on the FLG. This renders the FLG with a high mobility around 1812 cm2.V-1 .s -1 at room temperature even with a very high carrier concentration about 2.95 × 10 ^13 cm-2 （p-type）. These suggest SiC （1120） is more suitable for fabricating EG with high performance.展开更多
基金Project supported by the National Basic Research Program of China(Grant No.2013CB932901)the National Natural Science Foundation of China(Grant Nos.51210105026 and 51172270)the Funds from the Chinese Academy of Sciences
文摘Threaded aluminum nitride （A1N） whiskers are grown by a physical vapor transport method in a radio-frequency induction heating furnace. The resultant whiskers are characterized by X-ray diffraction, Raman scattering, scanning electron microscopy, transmission electron microscopy and photoluminescence. The analysis shows that the whiskers are single-crystalline, wurtzite AIN. The threaded A1N whiskers are 0.5 μm^100 μm in diameter and several millimeters in length in the fiber direction, and have lots of tiny sawteeth on the surface. The morphology of this threaded A1N whisker is beneficial for bonding when the whisker is used in composite. The growth of the whiskers is dominated by the vapor-solid （VS） mechanism, and the particular morphology might result from an oscillating condition produced in the radio-frequency induction heating furnace.
基金supported by the National Natural Science Foundation of China(Grant Nos.51322211and 91422303)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB07020100)+1 种基金Beijing Nova Program of China(Grant No.2011096)K.C.Wong Education Foundation,Hong Kong,China
文摘Electrochemical method has been used to insert K/Na into FeSe lattice to prepare alkali-intercalated iron selenides at room temperature. Magnetization measurement reveals that KxFe2Se2 and NaxFe2Se2 are superconductive at 31 K and 46 K, respectively. This is the first successful report of obtaining metal-intercalated FeSe-based high-temperature superconductors using electrochemical method. It provides an effective route to synthesize metal-intercalated layered compounds for new superconductor exploration.
基金Project supported by the National Key Basic Research Program of China(Grant Nos.2011CB932700 and 2013CBA01603)the National Natural Science Foundation of China(Grant Nos.51472265 and 51272279)
文摘A self-powered graphene-based photodetector with high performance is particularly useful for device miniaturization and to save energy.Here,we report a graphene/silicon carbide（SiC）-based self-powered ultraviolet photodetector that exhibits a current responsivity of 7.4 m A/W with a response frequency of over a megahertz under 325-nm laser irradiation.The built-in photovoltage of the photodetector is about four orders of magnitude higher than previously reported results for similar devices.These favorable properties are ascribed to the ingenious device design using the combined advantages of graphene and SiC,two terminal electrodes,and asymmetric light irradiation on one of the electrodes.Importantly,the photon energy is larger than the band gap of SiC.This self-powered photodetector is compatible with modern semiconductor technology and shows potential for applications in ultraviolet imaging and graphene-based integrated circuits.
基金Project supported by the National Key Basic Research Program of China (Grant No.2011CB932700)the National Natural Science Foundation of China (Grant Nos.51272279,51072223,and 50972162)
文摘The field emission （FE） properties of vertically aligned graphene sheets （VAGSs） grown on different SiC substrates are reported. The VAGSs grown on nonpolar SiC （10-10） substrate show an ordered alignment with the graphene basal plane-parallel to each other, and show better FE features, with a lower turn-on field and a larger field enhancement factor. The VAGSs grown on polar SiC （000-1 ） substrate reveal a random petaloid-shaped arrangement and stable current emission over 8 hours with a maximum emission current fluctuation of only 4%. The reasons behind the differing FE characteristics of the VAGSs on different SiC substrates are analyzed and discussed.
基金supported by the National Natural Science Foundation of China(Grant Nos.90922037,51072226,and51202286)the Chinese Academy of Sciences+1 种基金the International Center for Diffraction Data(ICDD)the Beijing Nova Program,China(Grant No.2011096)
文摘（Received 8 June 2013） Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K^46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2（NH3）y （x 0.3 and 0.6）, determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.
基金supported by the National Basic Research Program of China(Grant No.2013CB932901)the National Natural Science Foundation of China(Grant Nos.51372267,51210105026,and 51172270)+1 种基金the Funds from the Chinese Academy of Sciences,the International Centre for Diffraction Data,USA(2013 Ludo Frevel Crystallography Scholarship Award)the Funds from the Ministry of Education of China(2012 Academic Scholarship Award for Doctoral Candidates)
文摘Ferromagnetism is investigated in high-quality Cu-doped A1 N single crystal whiskers.The whiskers exhibit roomtemperature ferromagnetism with a magnetic moment close to the results from first-principles calculations.High crystallinity and low Cu concentrations are found to be indispensable for high magnetic moments.The difference between the experimental and theoretical moment values is explored in terms of the influence of nitrogen vacancies.The calculated results demonstrate that nitrogen vacancies can reduce the magnetic moments of Cu atom.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51472266,51202286,and 91422303)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grant No.XDB07020100)the ICDD
文摘A new layered Cu-based oxychalcogenide Ba_3Fe_2O_5Cu_2S_2 has been synthesized and its magnetic and electronic properties were revealed. Ba_3Fe_2O_5Cu_2S_2 is built up by alternatively stacking [Cu_2S_2]^（2-） layers and iron perovskite oxide[（FeO_2）（BaO）（FeO_2）]^（2-）layers along the c axis that are separated by barium ions with Fe^（3＋） fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuC h-based（Ch =S, Se, Te） compounds the ＋3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba_3Fe_2O_5Cu_2S_2 is an antiferromagnetic semiconductor with a Ne′el temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3 d states of Fe ions that antiferromagnetically arranged in FeO_2 layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr_3Sc_2O_5Cu_2S_2.
基金Project supported by the National Key Basic Research Program of China(Grant No.2011CB932700)the Knowledge Innovation Project of the Chinese Academy of Sciences(Grant No.KJCX2-YW-W22)the National Natural Science Foundation of China(Grant Nos.51272279 and 51072223)
文摘Defects in silicon carbide （SIC） substrate are crucial to the properties of the epitaxial graphene （EG） grown on it. Here we report the effect of defects in SiC on the crystalline quality of EGs through comparative studies of the characteristics of the EGs grown on SiC （0001） substrates with different defect densities. It is found that EGs on high quality SiC possess regular steps on the surface of the SiC and there is no discernible D peak in its Raman spectrum. Conversely, the EG on the SiC with a high density of defects has a strong D peak, irregular stepped morphology and poor uniformity in graphene layer numbers. It is the defects in the SiC that are responsible for the irregular stepped morphology and lead to the small domain size in the EG.
基金Project supported by the National Basic Research Program of China (Grant No. 2011CB932700)the Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KJCX2-YW-W22)the National Natural Science Foundation of China (Grant Nos. 51072223 and 50972162)
文摘A nonpolar SiC（1120） substrate has been used to fabricate epitaxial graphene （EG）. Two EGs with layer numbers of 8-10 （referred to as MLG） and 2-3 （referred to as FLG） were used as representative to study the substrate effect on EG through temperature-dependent Raman scattering. It is found that Raman lineshifts of G and 2D peaks of the MLG with temperature are consistent with that of free graphene, as predicted by theory calculation, indicating that the substrate influence on the MLG is undetectable. While Raman G peak lineshifts of the FLG to that of the free graphene are obvious, however, its lineshift rate （-0.016 cm-1/K） is almost one third of that （-0.043 cm-1/K） of an EG on 6H-SiC （0001） in the temperature range from 300 K to 400 K, indicating a weak substrate effect from SiC （1120） on the FLG. This renders the FLG with a high mobility around 1812 cm2.V-1 .s -1 at room temperature even with a very high carrier concentration about 2.95 × 10 ^13 cm-2 （p-type）. These suggest SiC （1120） is more suitable for fabricating EG with high performance.
