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张珠峰
, 任银拴, 韩璐
ZHANG Zhufeng, REN Yinshuan, HAN Lu
中图分类号: O649.1
文献标识码: A
文章编号: 1006-7167(2017)05-0036-04
通讯作者:
收稿日期: 2016-09-1
网络出版日期: 2017-05-20
版权声明: 2017 《实验室研究与探索》编辑部 《实验室研究与探索》编辑部 所有
基金资助:
作者简介:
作者简介:张珠峰(1976-),男,重庆人,硕士,讲师,研究方向:半导体纳米材料。Tel.: 15922723224; E-mail: zzf221@126.com
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摘要
以ZnO、 S粉和六水氯化铬为原料 (CrCl3·6H2O),乙二胺、乙醇胺为有机溶剂,在190 ℃下,用溶剂热法成功地合成室温铁磁性铬掺杂ZnS 纳米颗粒半导体。X-射线衍仪测试表明,Cr掺杂ZnS为纤锌矿结构。透射电镜表征不同Cr掺杂浓度ZnS的形貌为纳米颗粒,Cr掺杂浓度为5.27%的ZnS纳米颗粒平均尺寸在30~40 nm。电子能量散射谱表明产物由Zn、S、 Cr元素组成。光致发光测试表明,Cr掺杂ZnS相对未掺杂ZnS吸收带边向短波方向微小移动。 振动样品磁强计测试表明未掺杂的ZnS为抗磁性,而Cr掺ZnS为室温强铁磁性,Zn1-xCrxS (x = 0.052 7) 纳米颗粒的饱和磁化强度为4.275 A/m。Cr掺ZnS纳米颗粒具有室温铁磁性的实验结果与通过第一性原理预言Cr掺ZnS纳米片具有室温铁磁性相一致。Cr掺杂ZnS纳米颗粒的铁磁性是稀磁半导体的固有属性。
关键词：
Abstract
Cr-doped ZnS semiconductor nanoparticles of magnetic property was successfully prepared via a solvothermal reaction of ZnO and S with CrCl3·6H2O in mixed solvents of ethylenediamine and ethanolamine at 190 ℃. X-ray diffraction (XRD) measurement demonstrated that the undoped and Cr-doped nanoparticles had a hexagonal structure. Transmission electron microscopy (TEM) images revealed that the morphologies of ZnS with different amounts of chromium consisted of nanoparticles. The average particle size of Zn1-xCrxS (x = 0.052 7) nanoparticles was in the range of 30-40 nm. This product, composed of Cr, Zn, and S, was observed by an energy dispersive spectrometer (EDS). The photoluminescence (PL) spectra of Cr doped ZnS nanoparticles showed that the absorption edge was slightly shifted towards the short wavelength compared with undoped ZnS. A vibrating sample magnetometer (VSM) showed that the Cr-doped ZnS nanoparticles exhibited increased ferromagnetism at room temperature, while the pure ZnS nanoparticles exhibited diamagnetism. The saturation magnetization of the Zn1-xCrxS (x = 0.052 7) nanoparticles was 4.275 A/m. The experimental results confirmed that Cr-doped ZnS nanosheets exhibited ferromagnetism at room temperature, in good agreement with the ferromagnetic properties of Cr-doped ZnS that were predicted by first-principle computations. The origin of the ferromagnetism in the Zn1-xCrxS nanoparticles is intrinsic to the DMS.
Keywords：
纳米结构半导体材料在光、电方面有特殊的性能,可以作为重要的光电材料和功能材料,具有潜在的应用价值,从而引起了很多研究者的关注[1]。ZnS作为重要的半导体材料,是直隙半导体,禁带宽度为3.2 eV,由于禁带宽度大,在很长一段时间内被认为是磷光体、光敏电阻、光学涂膜、光催化和传感器等方面的首先材料[2]。研究者已经合成了各种形貌的ZnS纳米结构,如纳米线、纳米球和纳米管等[3],随着合成技术不断发展,研究者发现在半导体材料中掺杂,能提高半导体性能。Soni等[4]研究了Ni掺杂ZnS簇的光电性能;Yuan等[5]用ZnS和MnCl2为原料,在Si衬底以Cu为催化剂,用真空热蒸发的方法合成Mn掺杂ZnS纳米线;Liu等[6]用化学气相沉积法合成Mn掺杂ZnS一维纳米结构;Yanget等[7] 用化学气相沉积法在Si衬底上合成Mn、Cu、Co掺杂ZnS一维纳米构;Yanget等[8]研究了Cu掺杂ZnS纳米棒的相位变化和光性能;Cao等[9]用溶剂热法合成Mn掺杂ZnS纳米棒并研究了发光性能。在半导体材料ZnS中掺杂常有Ti、V、Mn、Fe、Co、Ni和Cu等。
半导体材料中掺入过渡金属元素能产生磁性,这种磁性较弱,通常称为稀磁半导体。稀磁半导体材料在纳米自旋电子学、磁学、光电子和激光设备上都有广泛的应用[10]。
近年来,研究者对Cr掺CdS和Cr掺ZnS 纳米体系的结构及磁性能进行研究。Zhang等[11]用溶剂热法合成了30 nm Cr掺杂CdS纳米颗粒,并研究了紫外吸收光谱和磁性能,未掺杂的CdS纳米颗粒为弱铁磁性,Cr掺杂后室温铁磁性急剧增强。Zhang等[12]用溶剂热法合成了Cr掺杂ZnS纳米片,晶体结构为六角形,并研究了磁性能,未掺杂的ZnS纳米片为抗磁性,掺杂后具有室温铁磁性能,分析了室温铁磁性来源,但是铁磁性较弱,纳米片形貌杂乱,对其光学性能没有报到。Li等[13-14]报道了ZnS是抗磁性半导体,通过第一性原理预言Cr掺杂ZnS纳米颗粒具有室温铁磁性。
本文用溶剂热法成功地合成了Cr掺杂ZnS纳米颗粒(30 ~ 40 nm),纳米颗粒具有优良的光性能,通过振动样品磁强计对Cr掺杂ZnS纳米颗粒进行了测试,ZnS纳米颗粒是抗磁性,而Cr掺杂ZnS纳米颗粒具有室温铁磁性,最大饱和磁化强度可达4.275 A/m,分析了产生铁磁根源,Cr掺杂ZnS纳米颗粒室温铁磁性是稀磁半导体本质属性。
以高纯度ZnO、S粉和CrCl3·6H2O为原料,用溶剂热法制备Cr掺杂ZnS纳米颗粒。实验过程如下:将(5 mmol)ZnO 、(5 mmol)S粉分别和 ( 0、0.50、0.75 mmol) 六水氯化铬(CrCl3·6H2O)加入容量为100 mL盛有15 mL乙二胺的烧杯中,磁力搅拌15 min,得到一澄清溶液,再加入15 mL乙醇胺,继续磁力搅拌15 min,将混合溶液移入容量为40 mL聚四氟乙烯内衬的不锈钢反应釜中,在190 ℃反应24 h,将所得乳白色沉淀抽虑,依次用无水乙醇和去离子水洗涤后,在60 ℃真空干燥箱中干燥6 h,取出自然冷却至室温,收集沉积物。
样品的晶体结构用X射线衍射仪( XRD,Mac Scienc M18XHF22-SRA Cu Kα 靶) 表征;形貌分析利用日本Hitachi-600透射电镜 (TEM) 采集;利用电子能量散射谱(Philips XL-30,EDS)测量样品化学成分;样品的光学特性采用Hitachi F-4500荧光谱仪对样品的光致发光 (PL)测试 (激发波长为250 nm);磁学性能采用美国振动样品磁强计( VSM ) LDJ29600测试。
由图1可见,所有衍射峰都可以标定为立方ZnS [JCPDS, Card No.05-056 6,a = 5.406 2×10-10](111),(200),(220),(311),(222),(400)和(331)晶面,没有出现其他杂质衍射峰,如Cr、Cr2S3和ZnO,说明所得产物为纯相。从图2可以明显看出,Cr掺杂ZnS相对未掺杂ZnS,向低角度发生微小偏移,证明一定量Cr掺入ZnS晶格中,这与随后的电子能量散射谱(EDS)分析结果一致。由图3可知,在x = 0~5.27%范围内,随着Cr掺杂浓度增加,ZnS的晶格常数增加,这是由于较大离子替代较小离子产生替代效应的结果。一般情况下,掺杂有两种方式,一种是间隙式;另一种是替代式。从元素周期表可知,Zn2+ 半径0.074 nm,Cr3+ 半径0.078 nm,Cr3+替代了Zn2+后会增大晶胞的体积,晶格变大,由布拉格公式
从图4可以看出,样品全部为纳米颗粒,产量较大,未掺杂ZnS颗粒约为70~80 nm,Cr掺杂2.18%ZnS颗粒约为50~60 nm,Cr掺杂5.27% ZnS颗粒约为30~40 nm。从以上结果可以得出,ZnS纳米颗粒大小与Cr掺杂量有关,随着Cr掺杂量增加,ZnS纳米颗粒尺寸在减小,
其中:Dhkl为沿垂直于晶面(hkl)方向的晶粒直径;k为Scherrer常数(通常为0.89); λ为入射X射线波长(Cuka 波长为0.154 06 nm,Cuka1 波长为0.154 18 nm);θ为布拉格衍射角(°);β为衍射峰的半高峰宽(rad)。可以估算出样品的晶粒粒径是40~70 nm。
表1是未掺杂和Cr掺杂ZnS纳米颗粒成分表。从图5可以看出,只有Zn、S和Cr 3种元素的衍射峰,没有出现其他元素衍射峰,这证明了Cr元素已经掺入ZnS中。通过计算,可以得到Cr在ZnS纳米颗粒中的原子含量分别为 2.18%和 5.27%,这与前面的XRD检测结果相一致。
表1 未掺杂和Cr掺杂ZnS纳米颗粒成分表
| 样品编号 | ZnO/mmol | CrCl3·6H2O/mmol | 最初(Zn2+/Cr3+)/mol | 最终(Zn2+/Cr3+)/mol | Zn1-xCrxS掺杂成分比 |
|---|---|---|---|---|---|
| a | 5 | 0 | 1∶0 | 1∶0 | ZnS |
| b | 5 | 0.25 | 20∶1 | 97.82∶2.18 | Zn97.82Cr0.218S |
| c | 5 | 0.50 | 10∶1 | 94.73∶5.27 | Zn0.947 3Cr0.052 7S |
从图6可以看出,未掺杂和掺杂 2.18 %、5.27%的Cr3+时, ZnS 的发射峰由 475 nm 蓝移到 470 nm 处,在ZnS材料中,S空位是深电子陷阱很容易从价带俘获电子而形成空位,同时留一个空穴在价带。因此 475 nm 的发射峰对应着施主能级和受主能级之间的复合。纯的 ZnS 在475 nm 的发射峰可以认为是 S空位缺陷发光,这与Li等[15]的结论相一致。根据Henglein经验公式:
其中:λe 为入射波的波长;R为纳米颗粒半径(nm)。可以计算出Cr掺杂ZnS纳米颗粒直径约为40~50 nm,该结果与TEM测试的结果相一致。
在室温条件下,用振动样品磁强计(VSM)测试纳米颗粒的磁性,测试精度可达10-5 A/m。未掺杂ZnS展现出抗磁性,而Cr杂ZnS纳米颗粒有明显的磁滞回线,说明Cr杂ZnS纳米颗粒具有室温铁磁性。Cr杂量为2.18%和5.27% 时,纳米颗粒的饱和磁化强度分别为1.624 3和4.275 A/m,矫顽力分别为7.89和4.44 kA/m(见图7)。
从图8可以看出,随着晶格常数的增大磁化强度也在增大,说明了Cr原子掺入ZnS晶格。
对于Cr掺杂ZnS纳米颗粒铁磁性来源,首先排除了样品的磁性来源于单质Cr的团藏,因为Cr本身是抗磁的,XRD谱图上未观察到任何与Cr相关的化合物如Cr2O3存在,所以可以排除Cr掺杂ZnS纳米颗粒磁性来源于与Cr相关的磁杂相。晶体缺陷,双交换机制用来解释铁磁性的来源,Elavarthi等认为,铁磁性来源不是Cd缺陷和S缺陷,而铁磁性来源与Cr置换了CdS晶格有关。
Li等[13]理论上预言了Cr掺杂ZnS纳米颗粒具有室温铁磁性,用第一性原理解释了铁磁性的来源,未掺杂ZnS是抗磁性半导体,带隙宽度3.150 eV。Cr掺入ZnS产生很多杂质态,这些杂质态主要为Cr 3d54s1和S 3s23p4态,Cr 3d54s1和S 3s23p4态形成很强的杂化,2个Cr原子通过1个S原子间接形成相互作用,2个Cr原子通过1个S原子耦合形成耦合磁性,根据Hund法则,处在3d态的2个相邻的Cr原子与处在3p态的S原子形成反铁磁耦合,从而产生2个Cr原子间低能量的铁磁耦合,根据以上的分析,Cr掺杂ZnS的铁磁性来源是双交换机制,因此Cr掺杂ZnS纳米颗粒的铁磁性是稀磁半导体的固有属性。
本文用溶剂热法,以ZnO、S粉和六水氯化铬为原料(CrCl3·6H2O)、乙二胺、乙醇胺为有机溶剂,成功地合成Cr掺杂ZnS纳米颗粒。通过XRD表征Cr掺杂ZnS纳米颗粒相对未掺杂ZnS衍射峰向低角度偏移。TEM表征时样品是颗粒状,约30~ 40 nm。通过EDS测试得知Cr:Zn原子比可达到5.27%。通过VSM测试表明Zn1-xCrxS纳米颗粒在室温为铁磁性,最大饱和磁化强度可达4.275 A/m,矫顽力4.44 kA/m。Cr掺杂ZnS纳米颗粒的室温铁磁性是稀磁半导体的固有属性。
The authors have declared that no competing interests exist.
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Room-temperature ferromagnetic and optical properties of Cr-doped CdS nanoparticles via a solvothermal preparation [J].DOI:10.1007/s10854-016-5432-2 URL [本文引用: 1] 摘要
Abstract Undoped and Cr doped CdS nanoparticles have been successfully prepared via a solvothermal route. X-ray diffraction measurements demonstrated that the undoped and Cr doped CdS nanoparticles had a hexagonal structure. Transmission electron microscopy images revealed that the morphologies of CdS with different amounts of chromium consisted of nanoparticles. The average particle size of Cd161xCrxS (x = 0.0394) nanoparticles was in the range of 30–60 nm. This product, composed of Cr, Cd, and S, was observed by an energy dispersive spectrometer. UV–Vis absorption spectra was used to investigate the optical properties of Cr-doped CdS nanoparticles. Absorption spectra of Cr doped CdS nanoparticles showed that the absorption edge was slightly shifted towards the short wavelength when compared to undoped CdS. A vibrating sample magnetometer showed that the Cr-doped CdS nanoparticles exhibited increased ferromagnetism at room temperature, while pure CdS nanorods exhibited weak ferromagnetism. The saturation magnetization of the Cr-doped CdS nanoparticles increased with Cr concentration over the range of x = 0.0000–0394. The saturation magnetization of the Cd161xCrxS (x = 0.0394) nanoparticles was 9.258 (10613 emu/g). The origin of the ferromagnetism in the Cd161xCrxS nanoparticles is attributed to be related to the Cr substitution in CdS lattice.
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| [13] |
Achieving ferromagnetism in single-crystalline ZnS wurtzite nanowires via chromium doping [J].DOI:10.1021/jp102875p URL [本文引用: 2] 摘要
The magnetic properties of Cr-doped wurtzite ZnS nanowires were studied through first-principles computations. At a low doping concentration of 2.08% (one Cr atom in a 96-atom supercell), the Cr atom prefers to substitute for the surface four-coordinated Zn atom in the groove of the ZnS nanowire with a diameter of 1.2 nm; at a higher doping concentration of 4.17% (two Cr atoms in the 96-atom supercell), the Cr atoms preferably substitute for the surface four-coordinated Zn atom and its neighboring surface three-coordinated Zn atom. Irrespective of the sites that Cr atoms occupy, Cr atoms in ZnS nanowires are always ferromagnetically coupled to each other and antiferromagnetically coupled to the mediating S atom; moreover, the robust ferromagnetism is not sensitive to surface passivation. The ferromagnetism in Cr-doped ZnS nanowires is attributed to a double-exchange mechanism.
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| [14] |
Structures and magnetic properties of ZnS nanotubes doped with Cr atom [J]. |
| [15] |
沉淀法制备 ZnS∶Cr 纳米晶及其光学性能研究 [J].
以十二烷基苯磺酸钠和六偏磷酸钠作为分散剂,采用沉淀法制备了ZnS及不同掺杂浓度的 ZnS∶Cr纳米晶。利用XRD和TEM对纳米晶物相和形貌进行了分析。结果表明,ZnS和ZnS∶Cr纳米晶均为立方闪锌矿结构,利用谢乐公式估算 ZnS和ZnS∶Cr纳米晶平均粒径分别为2.1和2.2nm。TEM观察到纳米晶近似为球形,平均粒度为3nm左右,具有较好的单分散性且分布均匀。荧 光光谱（PL）表明,纳米晶在420、440和495nm处有发射谱带,前两者被认为是S空位深陷阱发光,后者被认为是表面态或中心辐射复合发光。
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