The energy band structure of single-layer graphene under one-dimensional electric and magnetic field modulation is theoretically investigated. The criterion for bandgap opening at the Dirac point is analytically deriv...The energy band structure of single-layer graphene under one-dimensional electric and magnetic field modulation is theoretically investigated. The criterion for bandgap opening at the Dirac point is analytically derived with a two-fold degeneracy second-order perturbation method. It is shown that a direct or an indirect bandgap semiconductor could be realized in a single-layer graphene under some specific configurations of the electric and magnetic field arrangement. Due to the bandgap generated in the single-layer graphene, the Klein tunneling observed in pristine graphene is completely suppressed.展开更多
Zn1-xMnxO （x = 0.0005, 0.001, 0.005, 0.01, 0.02） nanocrystals are synthesized by using a wet chemical process. The coordination environment of Mn is characterized by X-ray photoelectron spectroscopy, Raman spectrosc...Zn1-xMnxO （x = 0.0005, 0.001, 0.005, 0.01, 0.02） nanocrystals are synthesized by using a wet chemical process. The coordination environment of Mn is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and its X-ray absorption fine structure. It is found that the solubility of substitutional Mn in a ZnO lattice is very low, which is less than 0.4%. Mn ions first dissolve into the substitutional sites in the ZnO lattice, thereby forming Mn2＋O4 tetrahedral coordination when x ≤ 0.001, then entering into the interstitial sites and forming Mn3＋O6 octahedral coordination when x ≥ 0.005. All the samples exhibit paramagnetic behaviors at room temperature, and antiferromagnetic coupling can be observed below 100 K.展开更多
Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H2/Ar atmosphere with different H2/Ar ratios. It is found that hydrogen annealing could change the coordination environment of Mn in ZnO...Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H2/Ar atmosphere with different H2/Ar ratios. It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO. Mn ions initially enter into interstitial sites and a Mn3＋ 06 octahedral coordination is produced in the prepared Mn-doped ZnO sample, in which the nearest neighbor Mn3＋ and 02 ions could form a Mn3＋-O2--Mn3＋ complex. After H2 annealing, interstitial Mn ions can substitute for Zn to generate the Mn2＋O4 tetrahedral coordination in the nanocrystals, in which neighboring Mn2＋ ions and H atoms could form a Mn2＋-O2--Mn2＋ complex and Mn-H-Mn bridge structure. The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn3＋-O2--Mn3＋ complex, while the annealed samples exhibit their ferromagnetism, which originates from the Mn-H-Mn bridge structure and the Mn-Mn exchange interaction in the Mn2＋-O2--Mn2＋ complex.展开更多
Red-emitting at about 640nm from self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy are demonstrated.A doublepeak structure of photoluminescence(PL)spectra from q...Red-emitting at about 640nm from self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy are demonstrated.A doublepeak structure of photoluminescence(PL)spectra from quantum dots was observed,and a bimodal distribution of dot sizes was also confirmed by an atomic force micrograph(AFM)image for uncapped sample.From the temperature and excitation intensity dependence of PL spectra,it is found that the double-peak structure of PL spectra from quantum dots is strongly correlated to the two predominant quantum dot families.Taking into account the quantum-size effect on the peak energy,it is proposed that the high(low)energy peak results from a smaller(larger)dot family,and this result is identical to the statistical distribution of dot lateral size from the AFM image.展开更多
The third-order nonlinear optical properties of water-soluble Cu Se nanocrystals are studied in the near infrared range of 700-980 nm using a femtosecond pulsed laser by the Z-scan technique. It is observed that the n...The third-order nonlinear optical properties of water-soluble Cu Se nanocrystals are studied in the near infrared range of 700-980 nm using a femtosecond pulsed laser by the Z-scan technique. It is observed that the nonlinear optical response of Cu Se nanocrystals is sensitively dependent on the excitation wavelength and exhibits the enhanced nonlinearity compared with other selenides such as ZnSe and CdSe. The W-shaped Z-scan trace, a mixture of the reversed saturated absorption and saturated absorption, is observed near the plasmon resonance band of Cu Se nanocrystals, which is attributed to the state-filling of free carriers generated by copper vacancies (self-doping effect) of Cu Se nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation. The large nonlinear optical response and tunable plasmonic band make Cu Se nanocrystals promising materials for applications in ultra-fast all-optical switching devices as well as nonlinear nanosensors.展开更多
Self-assembled In_(x)Ga_(l-x)As quantum dots(QDs)on(311)and(100)GaAs surfaces have been grown by conventional solid source molecular beam epitaxy.Spontaneously ordering alignment of In_(x)Ga_(l-x)As QDs with lower In ...Self-assembled In_(x)Ga_(l-x)As quantum dots(QDs)on(311)and(100)GaAs surfaces have been grown by conventional solid source molecular beam epitaxy.Spontaneously ordering alignment of In_(x)Ga_(l-x)As QDs with lower In content around 0.3 has been observed on As-terminated(B type)surfaces.The direction of alignment orientation of the QDs formation differs from the direction of misorientation of the(311)B surface,and is strongly dependent upon the In content x.The ordering alignment becomes significantly deteriorated as the In content is increased to above 0.5 or as the QDs are formed on(100)and(311)Ga-terminated(A type)substrates.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 60776067 and 10974011)
文摘The energy band structure of single-layer graphene under one-dimensional electric and magnetic field modulation is theoretically investigated. The criterion for bandgap opening at the Dirac point is analytically derived with a two-fold degeneracy second-order perturbation method. It is shown that a direct or an indirect bandgap semiconductor could be realized in a single-layer graphene under some specific configurations of the electric and magnetic field arrangement. Due to the bandgap generated in the single-layer graphene, the Klein tunneling observed in pristine graphene is completely suppressed.
基金Project supported by the National Basic Research Program of China (Grant No.013CB934001)the National Natural Science Foundation of China (Grant No.51272015)
文摘Zn1-xMnxO （x = 0.0005, 0.001, 0.005, 0.01, 0.02） nanocrystals are synthesized by using a wet chemical process. The coordination environment of Mn is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and its X-ray absorption fine structure. It is found that the solubility of substitutional Mn in a ZnO lattice is very low, which is less than 0.4%. Mn ions first dissolve into the substitutional sites in the ZnO lattice, thereby forming Mn2＋O4 tetrahedral coordination when x ≤ 0.001, then entering into the interstitial sites and forming Mn3＋O6 octahedral coordination when x ≥ 0.005. All the samples exhibit paramagnetic behaviors at room temperature, and antiferromagnetic coupling can be observed below 100 K.
基金supported by the National Basic Research Program of China(Grant No.2013CB934001)the National Natural Science Foundation of China(Grant Nos.51072012 and 51272015)
文摘Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H2/Ar atmosphere with different H2/Ar ratios. It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO. Mn ions initially enter into interstitial sites and a Mn3＋ 06 octahedral coordination is produced in the prepared Mn-doped ZnO sample, in which the nearest neighbor Mn3＋ and 02 ions could form a Mn3＋-O2--Mn3＋ complex. After H2 annealing, interstitial Mn ions can substitute for Zn to generate the Mn2＋O4 tetrahedral coordination in the nanocrystals, in which neighboring Mn2＋ ions and H atoms could form a Mn2＋-O2--Mn2＋ complex and Mn-H-Mn bridge structure. The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn3＋-O2--Mn3＋ complex, while the annealed samples exhibit their ferromagnetism, which originates from the Mn-H-Mn bridge structure and the Mn-Mn exchange interaction in the Mn2＋-O2--Mn2＋ complex.
基金Supported by the National Natural Science Foundation of China under Grant No.69736010the National Advanced Materials Committee of China(715-014-0040).
文摘Red-emitting at about 640nm from self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on GaAs substrate by molecular beam epitaxy are demonstrated.A doublepeak structure of photoluminescence(PL)spectra from quantum dots was observed,and a bimodal distribution of dot sizes was also confirmed by an atomic force micrograph(AFM)image for uncapped sample.From the temperature and excitation intensity dependence of PL spectra,it is found that the double-peak structure of PL spectra from quantum dots is strongly correlated to the two predominant quantum dot families.Taking into account the quantum-size effect on the peak energy,it is proposed that the high(low)energy peak results from a smaller(larger)dot family,and this result is identical to the statistical distribution of dot lateral size from the AFM image.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11274302,11474276 and 11674240
文摘The third-order nonlinear optical properties of water-soluble Cu Se nanocrystals are studied in the near infrared range of 700-980 nm using a femtosecond pulsed laser by the Z-scan technique. It is observed that the nonlinear optical response of Cu Se nanocrystals is sensitively dependent on the excitation wavelength and exhibits the enhanced nonlinearity compared with other selenides such as ZnSe and CdSe. The W-shaped Z-scan trace, a mixture of the reversed saturated absorption and saturated absorption, is observed near the plasmon resonance band of Cu Se nanocrystals, which is attributed to the state-filling of free carriers generated by copper vacancies (self-doping effect) of Cu Se nanocrystals as well as the hot carrier thermal effect upon intense femtosecond laser excitation. The large nonlinear optical response and tunable plasmonic band make Cu Se nanocrystals promising materials for applications in ultra-fast all-optical switching devices as well as nonlinear nanosensors.
基金Supported by the National Natural Science Foundation of China under Grant No.69736010the National Advanced Materials Committee of China under Grant No.715-014-0040.
文摘Self-assembled In_(x)Ga_(l-x)As quantum dots(QDs)on(311)and(100)GaAs surfaces have been grown by conventional solid source molecular beam epitaxy.Spontaneously ordering alignment of In_(x)Ga_(l-x)As QDs with lower In content around 0.3 has been observed on As-terminated(B type)surfaces.The direction of alignment orientation of the QDs formation differs from the direction of misorientation of the(311)B surface,and is strongly dependent upon the In content x.The ordering alignment becomes significantly deteriorated as the In content is increased to above 0.5 or as the QDs are formed on(100)and(311)Ga-terminated(A type)substrates.
