Strain Effect on the Electronic and Optical Properties of CdSe Nanowires
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First-principles density functional theory (DFT) simulations were carried out to study the strain dependence on the electronic and optical properties of cadmium selenide (CdSe) nanowires (NWs). The band structures, effective masses of electron and holes, dielectric properties, and other optical properties (such as extinction coefficient, optical reflectivity, and absorption coefficient) were calculated under both compressive and tensile uniaxial strains. Size-dependence was also discussed by comparing results among CdSe wires with various diameters. Simulation results show that an interesting band-switch behavior occurs at the valence bands regardless of size. The cause and the consequences of such band-switch behavior were also studied. Further strain dependence on corresponding electronic and optical properties were examined as well. Our results provide insights to possible mechanical tuning via strain on the electronic and optical properties of CdSe NWs.
KeywordsStrain Cadmium selenide Nanowire Electronic properties Optical properties
Conduction band minimum
Generalized gradient approximation
Tran and Blaha
Valence band maximum
Cadmium selenide (CdSe) nanostructures have attracted much attention due to their potential applications in micro- and nano-optoelectronics [1, 2, 3, 4, 5]. Recently, a great variety of CdSe nanostructures including nanowires (NWs) , nanospheres , nanorods , nanosheets , and nanocrystals  have been successfully synthesized. The structural, electronic, and optical properties of various CdSe nanostructures were also widely studied [1, 11, 12, 13, 14]. Among all CdSe nanostructures, CdSe NWs are by far the most attractive to researchers and scientists due to their unique opto-electrical properties, high length-diameter aspect ratio, and high surface-to-volume ratio. Especially CdSe NWs has been shown as good field-effect transistor  similar to carbon nanotube bundles . Experimentally, CdSe NWs have been successfully synthesized by various methods [6, 17, 18, 19], such as γ-irradiation, electrochemistry, and solution–liquid–solid (SLS). Depending on the synthesis conditions, researchers were able to get two structures of CdSe nanowires: the zinc blende (ZB) structure and wurtzite (WZ) structure [6, 20, 21, 22, 23]. Most WZ wires grew along  crystallographic direction [23, 24]. Since then, many efforts have been put into the study of the electronic, thermal, and optical properties of CdSe NWs as well [25, 26]. However, semiconductor NWs, as basic units of microelectronic and optoelectronic nanodevices, often work under the existence of strain. Therefore, it is essential to also study the influence of strain on various structural, electronic, and optical properties of NWs. Indeed, strain effect has been widely studied on many nano-systems such as GaAs  and ZnO . Interesting results were published to enrich our understanding. In 2013, Peng et al. found that GaAs nanowires undergo a direct–indirect bandgap transition under compressive strain. The studies of Lu group showed that the applied strain also affects the dielectric function peaks of ZnO nanowires. However, very limited information is available on how strain affects the electronic and optical properties of CdSe NWs. Previous studies have shown that first-principles DFT calculations well performs when it comes to the simulations of CdSe nano-systems [26, 29, 30]. Thus, in order to achieve a better understanding of the stain effect on CdSe NWs, we have carried out DFT calculations to investigate the effect of strain on the band structures and optical properties of CdSe NWs of various diameters.
The nanowire structures were optimized using Atomistix Toolkit (ATK) package [31, 32, 33] under generalized gradient approximation (GGA). Perdew–Burke–Ernzerhof exchange-correlation functional  and L-BFGS optimizer  were used. The convergence criteria were set to be 1 × 10−5 eV/supercell for energy and 0.05 eV/Å for forces. The energy cutoff was set to be 75 Ha. The electronic and optical properties were calculated via Meta-GGA (MGGA) method with Tran and Blaha (TB09) functional  within the ATK package. Troullier–Martins  pseudopotentials were used, and the relativistic core corrections were also included. In both GGA and MGGA calculations, the Brillouin zone of CdSe NW is sampled using a (1 × 1 × 21) Monkhorst-Pack  special k-point mesh, which yields well-converged results. A vacuum region of at least 15 Å is used to avoid possible interactions between adjacent NWs that may be introduced due to periodic boundary conditions.
Results and Discussion
First-principles DFT calculations were carried out to study the effect of strain on electronic and optical properties of CdSe NWs. The bandgap of CdSe wires generally reduces under increasing tensile strain. This is due to the fact that the conduction band moves down towards the Fermi level as tensile strain is applied. An interesting band-shift behavior was also found at the valence bands, where the top three valence bands in turns define VBM. The cause roots in the relative orientations between the bonds and strain. This band-shift behavior further also leads to sudden decrease in the effective mass of the holes and affects the corresponding optical properties of the wires as well. Our simulation results provide novel understandings and valuable insights on the strain effect of CdSe nanowires, which may also help the investigations of other CdSe nanomaterials.
This work is supported by Science and Technology Commission of Shanghai Municipality (No. 14ZR1431100).
HH performed the computational work and drafted the manuscript. LC drafted the manuscript. XY carried out the coordination of the research, drafted the manuscript, and had given the final approval of the version of the manuscript to be published. To publish the work, all authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- 31.Atomistix ToolKit version 2014.3, (QuantumWise A/S) www.quantumwise.com.
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