Synthesis and physical properties of ZnO.ppt

,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Synthesis and physical properties of,ZnO/CdTe,core shell,nanowires,grown by low-cost deposition methods,V.Consonni,1,2,a G.Rey,1,2,J.Bonaim,3,N.Karst,2,B.Doisneau,4,H.Roussel,2,S.Renet,3,and D.Bellet,2,1,Laboratoire des Technologies de la,Microlectronique,CNRSUJF,17 rue des Martyrs 38054,Grenoble,France,2,Laboratoire des,Matriaux,et du,Gnie,Physique,CNRSGrenoble INP,3,parvis,Louis,Nel,38016 Grenoble,France,APPLIED PHYSICS LETTERS,98,111906 2011,Abstract,竖直排列的,ZnO/CdTe,core/shell,纳米线阵列通过,low-cost deposition,技术生长得到，,ZnO,nanowires,为纤锌矿结构，并沿,c,轴生长。,CdTe,shell,由,nanograins,组成，并完全把氧化锌纳米线覆盖。,和二维的薄膜相比，纳米线由于其较高的晶格质量，其内的载流子传输和收集被认为应该得到改善，纳米线阵列很有可能被用作光子晶体，来提高光的吸收和捕获。为了有效地诱导电荷载体的分离，,type II band alignment,被认为一个很好的选择，它既不需要掺杂 也不需要加入,quasi-electronic field,就能把电子和空穴分离。由于,CdTe,具有较高的光吸收系数（,10,4,cm,1,）和室温下较窄的禁带宽度（,1.5eV),，是一种有效的吸收材料，所以,ZnO/CdTe,异质结构目前受到了广泛的关注。,Experiment,1,.,在,导电玻璃上,90,生长,ZnO,纳米线,;,2.,CdTe,nanograins,通过,close space sublimation,（,CSS,）的方法在,480,沉积在,ZnO,纳米线上。,FIG.1.FESEM images of a bare,ZnO,NWs,and bd,ZnO/CdTe,core/shell,NWs,showing the uniformity of the,CdTe,shell deposition.a and c are tilted views with a tilt angle of 23 while b and d are top views,Results,ZnO,NWs,wurtzite,structure,diameter 40 to 80 nm,length 600 to 800 nm,CdTe,shell.,zinc blend structure,mean diameter about 40 nm,but widely spread in the range of 30 to 100 nm,一般包覆几个,ZnO,纳米线,HRTEM image of a three-dimensional,CdTe,island with a spherical cap shape on a,ZnO,NW vertical sidewall,Results,Results,CdTe,没有外延生长在纳米线上的原因：,较高的生长速率，大约,20nm/min,然而相比于无序生长的,CdTe,，它具有闪锌矿的结构，并稍微沿,方向择优生长。（,according to the Harris method described in Ref.16,the texture coefficient is 1.91,corresponding to a degree of preferred orientation of about 0.48 by taking into account,seven reflections in the XRD patterns.,）,T,he driving force for,grain growth is governed by thermodynamic,considerations:,The,CdTe,111 planes have the lowest surface energy in the zinc-,blende,structure,revealing that grain growth is driven by surface energy minimization as expected in very thin films.,Results,Results,Results,Interestingly,the growth of,CdTe,NGs,on,ZnO,NWs,follows the,Volmer,Weber mechanism:16,17,threedimensional,islands with a spherical-cap shape having a contact angle smaller than 90 nucleate both on the NW sidewalls and top facet and then coarsen until eventually coalescing.The coalescence of,CdTe,NGs,is expected to induce very high biaxial tensile stress,especially for grains with,nanoscale,dimensions.,Results,in which,E,is the Youngs modulus,is the Poissons coefficient,is the surface energy,is the grain boundary energy and r is the grain radius.This equation yields a biaxial tensile stress,Such a strong biaxial tensile stress is liable to drastically tune the,CdTe,bandgap,.,As a consequence,a biaxial tensile stress of about 1.52,GPa,should result in a decrease in the optical band gap from 1.5,eV,at room temperature for bulk,CdTe,down to 1.47+-0.005,eV,for,CdTe,NGs,.It is remarkable that the ultraviolet/visible absorption measurements in,FiG,4 yield the optical band gap of about 1.46,eV,for,CdTe,NGs,suggesting that stress effects strongly operate.,FIG.4.Color online CL spectra recorded at 5 K of bare,ZnO,NWs,and of,ZnO/CdTe,core/shell,NWs,.,Results,For bare,ZnO,NWs,the CL spectra present three main types of emissions located at 3.36,3.14,and 1.9,eV,.In addition,the 1.9,eV,red-orange emission band undergoes a,redshift,toward an energy of 1.68,eV,.More importantly,CdTe,NGs,contribute to the appearance of an intense emission band located at about 1.56,eV,.This emission band is likely to correspond to,excitonic,radiative,transitions and is slightly shifted toward lower energy in agreement with the decrease in the optical band gap for,CdTe,NGs,.Similarly,the overall CL intensity does not quench for,ZnO/CdTe,core/shell,NWs,with respect to bare,ZnO,NWs,:this may be due to a significant increase in the total,radiative,efficiency correlated with a decrease in the density of,nonradiative,recombination centers.This decrease may originate from the beneficial,passivation,process of the free surfaces of,ZnO,NWs,by the,CdTe,shell.,Conclusions,In conclusion,the synthesis of,ZnO/CdTe,core/shell NW,arrays has been achieved with similar growth methods as,compared to,CdS/CdTe,thin film based-solar cells.The,CdTe,shell is composed of,NGs,and completely covers vertically,aligned c-axis-oriented,ZnO,NWs,:its growth follows the,Volmer,Weber mechanism.Both,ZnO,and,CdTe,have,excitonic,emission bands around 3.36,eV,and 1.56,eV,respectively.,