| Recently,great achievements have been made on optical / electronic and electronic/ optical transform of one-dimensional(1D)semiconductors.For example,plasmonic laser,nanosensors,nanophotovoltaic devices,and nano imaging,etc have achieved a striking progress.More importantly,1D nanomaterials exhibit a great appliction prospect in the field of flexible photovoltaic devices.However,stress and strain must exist in the flexible photovoltaic devices,which inversely has an influence on 1D nanomaterials’ properties.Therfore,we experimentally investigate optical and photoelectronic properties of a few II-VI compounds in this dissertation.In the meantime,we will calculate electronic structures and mechanic properties of CdSe with different structures((B1(rocksalt)-,B2(cesium chloride)-,B3(zincblende)-,and B4(wurtzite))under different static pressures by the first principle pseudopotential method.The main contents are as follows:Ultrawide ZnSe and CdSe nanoribbons have been synthesized at 1100 oC and 900 oC by simple thermal evaporation.The two kinds of nanoribbons have smooth surfaces with uniform widths in length.For ZnSe,the width and thickness of the nanoribbons are in the range of 5–10 μm and 40–100 nm,respectively.The length of some ribbons is up to several micrometers.At low power densities,we observed a broad peak centered at 470 nm with a full width at full maximum of 10 nm,corresponding to the near band gap emission of ZnSe.The PL peak becomes narrower and shows superlinear intensity dependence on the excitation power density as the latter increases above the threshold value of 65 kW/cm2.Both features are characteristic of optical gain in the ZnSe nanoribbons.For CdSe,the width and thickness of the nanoribbons are in the range of 1–20 μm and 60 nm,respectively.Its length of some ribbons is up to several centimeters.Room-temperature photoluminescence indicated the lasing emission at 710 nm was observed under optical pumping(266 nm)at power densities of 25-153 kW/cm2.The full width half maximum(FWHM)of lasing mode is 0.67 nm with a threshold value of 43 kW/cm2.Er doped CdS nanobelts have been obtained by adjusting experimental condition.The width and thickness of Er-CdS nanobelts are 3–20 μm and 30-80 nm,respectively.The length of many belts is 100–200 μm.The device based on a single Er-CdS nanobelt was fabricated and measured its photoconductivity.It is found that Er-CdS device has the ability of detecting multicolor light with wavelengths of 457.5,620,and 955 nm and it has high Ion/Ioff ratios as large as 103,102,and 10 at the wavelengths of 457.5,620,and 955 nm,corresponding to power densities of 30,27,and 15 μW/cm2,respectively.The corresponding EQE values calculated are 93800,16280 and 11930,indicating that the Er-CdS nanobelt offers an avenue to develop highly sensitive and excellent multicolor photodetector applications.Based on the density functional theory,the structure stability for different structures(B1(rocksalt)-,B2(cesium chloride)-,B3(zincblende)-,and B4(wurtzite))of CdSe was investigated by the first principle pseudopotential method.The dependence of bulk(B),shear(G)moduli,Young’s modulus E and the Poisson’s ratio for the CdSe structures are obtained from the Hill’s the relationships.Based on the average elastic wave velocity,the Debye temperature is estimated.Moreover,the total energy,band structures,electronic density of states,mechanical and optical properties were calculated for different atomic configuration.This research provides an effective way for designing and optimizing applications for CdSe.The main calculated results are as follows:(1)For B1,B2,B3,and B4 structures,the stable order was determined as B4>B3>B1>B2,but B3 and B4 phase stability are very similar.(2)In terms of the mechanical stability criteria,that B4,B3,and B1 are all possible stable structures for CdSe,and while the B2 is an instable one,which agrees with the experimental results.(3)It is found that CdSe transforms from the B3、B4 structure to the B1 structure at about 4.3 GPa,and when the pressure increases up to about 89 GPa,it transforms from the B1 structure to the B2 structure.And the enthalpy of B3 and B4 structures increased with the increasing pressure,but no trend of phase transformation observed.(4)Our results also show that B3 structures CdSe exhibit good ductility,and the ductility of B3 structures is better than that of B4,indicating that B3 structure CdSe is more suitable as a flexible substrate solar cell materials.The dispersion curves of band structures reveal that theB3-and B4-type CdSe are semiconductors with a direct band gap,and B1 with an indirect band gap,while B2 structures CdSe are metal.(5)Our results indicate that pressure changes the dispersion relations of band edge for the B3 and B4,B1 phase CdSe,which leads to the change of the carrier transport properties and optical properties.We find that the top of valence band moves to the direction of low energy and there is almost no change for the bottom of the conduction band,which leads to the increase of the band gap.However,the change trend of band gap for B1 phase is contrast to the structure of B3 and B4 with the increasing pressure.Finally we studied the external pressure modulation effect on the optical properties of B3 and B4 structure.Calculations show that dielectric spectrum characteristic peaks and absorption peaks shift to the direction of higher energy with the increase of the pressure. |
