低维锗基纳米材料掺杂改性的第一性原理研究
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摘要
锗(Ge)是最早获得应用的一种半导体材料,具有优异的物理、化学特性以及与硅基材料良好的兼容性,一直以来都是制造红外探测器、光导纤维、高速微电子器件、集成电路和热电设备的首选材料之一。随着科技的飞速发展,电子器件逐步转向集成化、多功能化和微型化,具有低维度、小尺寸的纳米器件逐渐成为微电子学和光电子学发展的新趋势。Ge的激子玻尔半径远大于硅,表明其具有更为显著的量子尺寸效应。因此,低维度和小尺寸的Ge基纳米材料将会展现出更多奇异的特性。
一维的Ge基纳米材料具有优异的光电特性,在制备高性能的场效应晶体管、生化传感器、逻辑门和太阳能电池等方面有良好的应用前景。近年来,大量的理论和实验工作已经致力于一维Ge基纳米材料的制备、微结构表征以及物化特性分析,并取得了许多突破性的进展。尽管如此,对一维Ge基纳米材料的异质改性问题,目前仍缺乏系统的研究和深入的认识。针对这一问题,本文应用基于密度泛函理论框架的第一性原理投影缀加波方法,对完整、缺陷、表面吸附和替换掺杂的一维Ge基纳米材料(纳米线和纳米带)的微结构、稳定性、电子特性和磁性进行了系统和深入研究,得到了一些重要结论。
(1)Al、P吸附Ge纳米线的研究。采用第一性原理投影缀加波方法,重点讨论了不同吸附构型和浓度下,Al和P表面吸附对Ge纳米线电子结构的影响,并与Al、P替换掺杂Ge纳米线的相关性质进行比较。结果表明：Al原子优先吸附于纳米线的表面八边形洞位(MO),吸附后并不破坏邻近的Ge-Ge键;而P原子则优先吸附于纳米线的表面五边形共用桥位(Bb),使其下方Ge-Ge键断裂的同时,形成了新的Ge-P-Ge键。高吸附浓度下,Al和P吸附都在纳米线能带结构中产生一条半填充的杂质诱导电子带,导致Ge纳米线出现“半导体～金属”转变；随着Al(P)吸附浓度的减小,杂质诱导电子带逐渐变平,并最终位于费米能级的下方(上方),这导致Ge纳米线又出现“金属～半导体”转变。在Al、P吸附的Ge纳米线中,杂质诱导电子带分别位于未占据电子带底部和占据电子带顶部,这与传统p型(A1)和n型(P)半导体掺杂遵循的受主和施主机制完全不同。此反常的能带调制行为产生原因可能为：杂质原子只简单的吸附于Ge纳米线表面,故主要与表面Ge原子未完全填充的悬挂键电子态相作用。此外,形成的Al-Ge键主要表现为共价键合特征,而P-Ge键则表现出共价键合和离子键合的双重特征。
(2)过渡金属原子吸附Ge纳米线的研究。采用自旋极化的第一性原理投影缀加波方法,系统研究了10种不同3d过渡金属原子吸附的Ge纳米线的微结构、稳定性、电子特性和磁性。结果表明：Ti、V、Cr、Mn、Fe、Co、Ni、Cu和Zn原子都最优先吸附于纳米线的表面六边形洞位(HH),而Sc原子则优先吸附于纳米线表面五边形与六边形共用Ge原子上方(Top)。最稳定构型下,过渡金属吸附原子的结合能随‘d’电子数的变化趋势与3d过渡金属吸附的(8,0)碳纳米管一致,且与碳纳米管相比,过渡金属原子更容易吸附于Ge纳米线表面。具有较好导电性的金属,如Cu和Zn,与纳米线结合较弱,而Ti、V、Fe、Co和Ni则与纳米线产生较强的键合。不同种类的过渡金属原子吸附可使Ge纳米线展现出各种各样的电子特性和磁性,如非磁金属(Sc或Cu吸附),非磁半导体(Ni或Zn吸附),弱铁磁金属(Ti或V吸附),铁磁半导体(Cr吸附),以及在自旋电子学领域更具有实用价值的半金属铁磁体(Mn、Fe或Co吸附)。对于过渡金属吸附的磁性Ge纳米线而言,其磁机制主要起源于过渡金属3d轨道的自旋劈裂,与此同时,过渡金属还诱导邻近Ge原子产生少量反平行的磁化密度。对具有半金属铁磁性的Mn、Fe和Co吸附的Ge纳米线,继续采用DFT+U方法深入研究了其半金属态的稳定性范围,在考虑原子占据位库仑排斥作用的影响后,发现Mn和Co吸附纳米线的半金属态较Fe吸附纳米线的要更为充沛。
(3)二维蜂巢Ge、完整和缺陷Ge纳米带的研究。采用第一性原理投影缀加波方法,系统研究了二维蜂巢Ge、完整和缺陷的AGeNRs和ZGeNRs的微结构、稳定性、电子特性和磁性。结果表明：稳定的二维蜂巢Ge是具有皱褶的,并表现出类似石墨烯的零带隙半导体特征,其电子和空穴能带线性地交叉于费米能级,在靠近布里渊区的K点区域,电荷载流子类似“无质量”的狄拉克费米子。完整的AGeNRs是非磁的半导体,其能隙随带宽的增加表现出一个周期性的振荡衰减花样,这使得AGeNRs按其带宽分为不同三类；完整的ZGeNRs具有稳定的反铁磁半导体基态,其能隙随带宽的增加单调减小,净自旋电荷密度主要由纳米带边缘Ge原子的π/π*电子态贡献,且相对边缘的Ge原子具有反平行的磁排列。原子缺陷(空位或空位对)的引入尽管并不能在AGeNRs中引发磁性,但仍可有效地调整纳米带的能隙。对ZGeNRs而言,原子缺陷的引入则可使其由原有的反铁磁半导体转变为反铁磁金属或铁磁金属,从而能更好用于电子传导和自旋存储设备。
(4)过渡金属(Cr、Mn、Fe和Co)吸附的二维蜂巢Ge及其扶手椅型Ge纳米带的研究。第一性原理计算结果表明：不管是单边吸附还是双边吸附,Cr、Mn、Fe和Co都优先结合于二维蜂巢Ge的六边形洞位。根据吸附原子种类和吸附密度的不同,非磁零带隙的二维蜂巢Ge可转变为铁磁金属或反铁磁金属。对AGeNRs的吸附研究中,Cr、Mn、Fe和Co总是优先吸附于靠近纳米带边缘的六边形洞位。除Co吸附的纳米带始终保持非磁基态外,根据带宽、吸附原子种类和吸附密度的不同,Cr、Mn和Fe吸附的Ge纳米带则具有稳定的铁磁或反铁磁态。此外,Cr和Mn吸附还使一些纳米带转变为铁磁或亚铁磁的半金属,在考虑原子占据位库仑排斥作用影响后,我们发现Cr吸附的纳米带的半金属基态与Mn吸附的纳米带相比要更加充沛,故更适合用于自旋电子学设备。
(5)N、B单掺杂和共掺杂不同形状和宽度Ge纳米带的稳定性、电子特性和磁性研究。我们的第一性原理计算结果表明：无论是AGeNRs还是ZGeNRs,边缘Ge原子总是最容易被杂质原子替代。单原子N掺杂或单原子B掺杂都可在AGeNRs中诱发“半导体～金属”过渡；然而,N和B共掺杂于AGeNRs边缘时,由于有效的电荷补偿,AGeNRs仍可保持原有的半导体性。单原子N掺杂或单原子B掺杂通常可使具有反铁磁半导体性质的ZGeNRs转变为铁磁半导体,此“反铁磁～铁磁”转变主要由局域于纳米带掺杂边缘的π/π*电子态的扰动引起；一些单原子掺杂的ZGeNRs还可以表现出半金属铁磁性质。双原子掺杂(无论N-N, B-B和N-B构型)于ZGeNRs的两个边缘时,ZGeNRs原有的反铁磁简并被破坏,最终转变为非磁半导体。总体而言,N掺杂、B掺杂以及N-B共掺杂的AGeNRs和ZGeNRs在Ge基的纳米电子学器件方面,如场效应晶体管,负微分电阻和自旋过滤器等,具有潜在的实际应用。
Germanium (Ge), a tranditional semiconductor material, has been used for various applications, such as infrared detector, optical fiber, high-speed microelectronic devices, integrated circuits and thermoelectric material etc., due to its excellent physics and chemistry properties as well as good compatibility with silicon-based materials. With the rapid development of science and technology, the electronic devices need to be high-integrated, multi-functionalized and miniaturized. At the same time, nanoscale devices with low dimensions and small sizes have gradually become the new trend for the development in fields of microelectronics and optoelectronics. The exciton Bohr radius of Ge is far greater than that of silicon, indicating that obvious quantum size effect will appear in Ge. Therefore, Ge-based nanomaterials should posses much more unique properties.
One-dimensional (1D) Ge-based nanomaterials have excellent photoelectric characteristics thus show a well prospect in applications of high-performance field-effect transistors (FETs), biochemical sensors, logic gates and efficient solar batteries. Recently, a lot of theoretical and experimental researches have been devoted to the synthesis, microstructure characterization and physicochemical characteristics analysis of1D Ge nanomaterials and finally achieved many breakthroughs. Nevertheless, the property-modification for1D Ge-based nanomaterials through hetero atom remains to be less exploited, thus needing to be systematically investigated and deeply understood. In this study, using first-principles projected-augmented wave (PAW) method, we pay special attention to the microstructure, stability, electronic and magnetic properties of the perfect, defected, adsorbed and doped Ge nanowires/nanoribbons. The principal conclusions are shown as follows:
(1) Al and P adsorption on Ge nanowires. Using first-principles calculations, the electronic properties of Al and P adsorbed Ge nanowires with different configurations and concentrations have been extensively studied and the corresponding results been compared with those of Al and P doped nanowires. Al adatom prefers to bind on the hollow site surrounded by a surface octagon ring (MO) of Ge nanowire and does not break the adjacent Ge-Ge bond after adsorption, while P adatom prefers to bind on the bridge site shared by two surface pentagons (Bb) of Ge nanowire, and breaks the beneath Ge-Ge bond to form a new Ge-P-Ge bond. At higher adsorption concentration, an impurity-induced electronic band appears and crosses the Fermi level, thus resulting in a "semiconductor-metal" transition in Ge nanowires. With decreasing of Al (P) adsorption concentration, the impurity-induced electronic band becomes flat and eventually locates below (above) the Fermi level, thus leading a "metal-semiconductor" transition in Ge nanowires. In Al (P) adsorbed wire, the impurity-induced electronic band is located closer to conduction band (valence band), which does not follow the traditional acceptor (donor) mechanism in p-type (n-type) doped semiconductors. Such a reverse behavior is explained by the fact that the adatom is simply adsorbed on the surface of the wire, thus only interacts with the unoccupied electronic states from the dangling bonds of surface Ge atoms. In addition, the formed Al-Ge bonds mainly display a covalent bonding character, while the formed P-Ge bonds display both covalent bonding and ionic bonding characters.
(2)3d transition-metal (TM) atoms adsorption on Ge nanowires. The microstructure, stability, electronic and magnetic properties of ten kinds of3d TM atoms adsorbed Ge nanowires have been investigated by spin-polarized first-principles PAW method. Except that Sc prefers to bind on the top site of the mutual Ge atom which belongs to the surface pentagons and hexagons of the wire, other TM atoms all prefer to bind on the hollow site of surface hexagon (HH) of the wrie. The variation trend of binding energies with'd'electron number agrees well with that of3d TM atoms adsorbed (8,0) carbon nanotubes, and by comparison, all TM atoms also form stronger bonding on Ge nanowires than on carbon nanotubes. Good conducting metals, such as Cu and Zn, can form weak bonding with the wire, whereas those such as Ti, V, Fe, Co and Ni have relative larger binding energies. Various types of wires can be obtained depending on the adatom species, including nonmagnetic (NM) metals (Sc or Cu adsorption) and semiconductors (Ni or Zn adsorption), weak ferromagnetic (FM) metals (Ti or V adsorption), FM semiconductors (Cr adsorption) and more interesting the FM half-metals (Mn, Fe or Co adsorption) which have potential application in spintronics. The magnetism of these wires originates mainly from spin-split of the TM-3d states, and the TM atom also induces some anti-parallel charge density around its adjacent Ge atoms. Furthermore, using DFT+U method, we also considered the effect of on-site Coulomb interaction on the stability of the three FM half-metallic wires and found the half-metallic ground state of Mn-or Co-adsorbed wire is more robust than that of Fe-adsorbed one.
(3) The study of two-dimensional (2D) honeycomb Ge, perfect and defected Ge nanoribbons. The microstructure, stability, electronic and magnetic properties of2D honeycomb Ge, perfect and defected armchair Ge nanoribbons (AGeNRs) and zigzag Ge nanoribbons (ZGeNRs) have been studied in detail by using first-principles PAW calculations. The stable2D honeycomb Ge sheet is slightly buckled and shows semi-metallic character. Its electron and hole bands linearly across at the Fermi level thus the carriers behave like "massless" Dirac fermion near the K point in the Brillouin zone (BZ). The perfect AGeNRs are NM semiconductors with their band gaps exhibit a periodically oscillatory damping as the ribbon width increases, thus making AGeNRs to be classified into three types. The perfect ZGeNRs have stable antimagnetic (AFM) semiconducting ground state with their band gaps monotonously decrease as ribbon width increases. Their net spin charge densities are mainly localized at the edge Ge atoms and contributed by π/π*electronic states, and the spin states at opposite edges have different spin orientations. The band gaps of AGeNRs can be efficiently tuned by atomic defect (vacancy or di-vacancy) at different positions though no magnetism is introduced. The ZGeNRs can become AFM or FM metals by introducing atomic defect, thus can be well used in electronic conduction and spin storage.
(4) TM (Cr, Mn, Fe and Co) atoms adsorption on2D honeycomb Ge and AGeNRs. The results indicate that, all TM atoms considered prefer to adsorb on the hollow site of hexagon of2D Ge whether in single-sided or double-sided adsorption cases, and NM semi-metallic2D Ge finally changes to be either FM or AFM metals depending on both TM species and coverage. For AGeNRs, the most preferential adsorption site is the hollow site of hexagon at the ribbon edge. Except for Co adsorption remaining NM state, Cr-, Mn-and Fe-adsorbed AGeNRs all possess FM state or AFM state according to ribbon width, TM species and coverage. Through Cr or Mn adsorption, some AGeNRs can also become FM or ferrimagnetic (FIM) half-metals. Moreover, considering the effect of on-site Coulomb interaction, we found the half-metallic ground state of Cr-adsorbed ones is more robust than that of Mn-adsorbed one thus can be suitable for spintronic devices.
(5) The stability, electronic and magnetic properties of N, B doped and co-doped AGeNRs and ZGeNRs. Our first-principles calculation results show that, for both AGeNRs and ZGeNRs, edge Ge atoms are always easy to be substituted. Single N-doping and single B-doping can introduce a "semiconductor-metal" transition in AGeNRs, while N and B co-doped AGeNRs also remain its semiconducting character due to the effective charge compensation. Single N-doping or single B-doping usually makes AFM ZGeNRs to be FM semiconductors, and the "AFM-FM" transition originates from the perturbation of π/π*electronic states which localized at the ribbon edges. Some single impurity doped ZGeNRs also exhibit half-metallic properties. Double atom substitution (regardless of N-N, B-B, and N-B configurations) at the edges of ZGeNRs removes the spin-polarization at both edges and transforms them into NM semiconductors. Overall, N, B doped and co-doped AGeNRs and ZGeNRs have potential applications in Ge-based nanoelectronic devices, such as FETs, negative differential resistances (NDR) and spin filters (SF) etc.
一维的Ge基纳米材料具有优异的光电特性,在制备高性能的场效应晶体管、生化传感器、逻辑门和太阳能电池等方面有良好的应用前景。近年来,大量的理论和实验工作已经致力于一维Ge基纳米材料的制备、微结构表征以及物化特性分析,并取得了许多突破性的进展。尽管如此,对一维Ge基纳米材料的异质改性问题,目前仍缺乏系统的研究和深入的认识。针对这一问题,本文应用基于密度泛函理论框架的第一性原理投影缀加波方法,对完整、缺陷、表面吸附和替换掺杂的一维Ge基纳米材料(纳米线和纳米带)的微结构、稳定性、电子特性和磁性进行了系统和深入研究,得到了一些重要结论。
(1)Al、P吸附Ge纳米线的研究。采用第一性原理投影缀加波方法,重点讨论了不同吸附构型和浓度下,Al和P表面吸附对Ge纳米线电子结构的影响,并与Al、P替换掺杂Ge纳米线的相关性质进行比较。结果表明：Al原子优先吸附于纳米线的表面八边形洞位(MO),吸附后并不破坏邻近的Ge-Ge键;而P原子则优先吸附于纳米线的表面五边形共用桥位(Bb),使其下方Ge-Ge键断裂的同时,形成了新的Ge-P-Ge键。高吸附浓度下,Al和P吸附都在纳米线能带结构中产生一条半填充的杂质诱导电子带,导致Ge纳米线出现“半导体～金属”转变；随着Al(P)吸附浓度的减小,杂质诱导电子带逐渐变平,并最终位于费米能级的下方(上方),这导致Ge纳米线又出现“金属～半导体”转变。在Al、P吸附的Ge纳米线中,杂质诱导电子带分别位于未占据电子带底部和占据电子带顶部,这与传统p型(A1)和n型(P)半导体掺杂遵循的受主和施主机制完全不同。此反常的能带调制行为产生原因可能为：杂质原子只简单的吸附于Ge纳米线表面,故主要与表面Ge原子未完全填充的悬挂键电子态相作用。此外,形成的Al-Ge键主要表现为共价键合特征,而P-Ge键则表现出共价键合和离子键合的双重特征。
(2)过渡金属原子吸附Ge纳米线的研究。采用自旋极化的第一性原理投影缀加波方法,系统研究了10种不同3d过渡金属原子吸附的Ge纳米线的微结构、稳定性、电子特性和磁性。结果表明：Ti、V、Cr、Mn、Fe、Co、Ni、Cu和Zn原子都最优先吸附于纳米线的表面六边形洞位(HH),而Sc原子则优先吸附于纳米线表面五边形与六边形共用Ge原子上方(Top)。最稳定构型下,过渡金属吸附原子的结合能随‘d’电子数的变化趋势与3d过渡金属吸附的(8,0)碳纳米管一致,且与碳纳米管相比,过渡金属原子更容易吸附于Ge纳米线表面。具有较好导电性的金属,如Cu和Zn,与纳米线结合较弱,而Ti、V、Fe、Co和Ni则与纳米线产生较强的键合。不同种类的过渡金属原子吸附可使Ge纳米线展现出各种各样的电子特性和磁性,如非磁金属(Sc或Cu吸附),非磁半导体(Ni或Zn吸附),弱铁磁金属(Ti或V吸附),铁磁半导体(Cr吸附),以及在自旋电子学领域更具有实用价值的半金属铁磁体(Mn、Fe或Co吸附)。对于过渡金属吸附的磁性Ge纳米线而言,其磁机制主要起源于过渡金属3d轨道的自旋劈裂,与此同时,过渡金属还诱导邻近Ge原子产生少量反平行的磁化密度。对具有半金属铁磁性的Mn、Fe和Co吸附的Ge纳米线,继续采用DFT+U方法深入研究了其半金属态的稳定性范围,在考虑原子占据位库仑排斥作用的影响后,发现Mn和Co吸附纳米线的半金属态较Fe吸附纳米线的要更为充沛。
(3)二维蜂巢Ge、完整和缺陷Ge纳米带的研究。采用第一性原理投影缀加波方法,系统研究了二维蜂巢Ge、完整和缺陷的AGeNRs和ZGeNRs的微结构、稳定性、电子特性和磁性。结果表明：稳定的二维蜂巢Ge是具有皱褶的,并表现出类似石墨烯的零带隙半导体特征,其电子和空穴能带线性地交叉于费米能级,在靠近布里渊区的K点区域,电荷载流子类似“无质量”的狄拉克费米子。完整的AGeNRs是非磁的半导体,其能隙随带宽的增加表现出一个周期性的振荡衰减花样,这使得AGeNRs按其带宽分为不同三类；完整的ZGeNRs具有稳定的反铁磁半导体基态,其能隙随带宽的增加单调减小,净自旋电荷密度主要由纳米带边缘Ge原子的π/π*电子态贡献,且相对边缘的Ge原子具有反平行的磁排列。原子缺陷(空位或空位对)的引入尽管并不能在AGeNRs中引发磁性,但仍可有效地调整纳米带的能隙。对ZGeNRs而言,原子缺陷的引入则可使其由原有的反铁磁半导体转变为反铁磁金属或铁磁金属,从而能更好用于电子传导和自旋存储设备。
(4)过渡金属(Cr、Mn、Fe和Co)吸附的二维蜂巢Ge及其扶手椅型Ge纳米带的研究。第一性原理计算结果表明：不管是单边吸附还是双边吸附,Cr、Mn、Fe和Co都优先结合于二维蜂巢Ge的六边形洞位。根据吸附原子种类和吸附密度的不同,非磁零带隙的二维蜂巢Ge可转变为铁磁金属或反铁磁金属。对AGeNRs的吸附研究中,Cr、Mn、Fe和Co总是优先吸附于靠近纳米带边缘的六边形洞位。除Co吸附的纳米带始终保持非磁基态外,根据带宽、吸附原子种类和吸附密度的不同,Cr、Mn和Fe吸附的Ge纳米带则具有稳定的铁磁或反铁磁态。此外,Cr和Mn吸附还使一些纳米带转变为铁磁或亚铁磁的半金属,在考虑原子占据位库仑排斥作用影响后,我们发现Cr吸附的纳米带的半金属基态与Mn吸附的纳米带相比要更加充沛,故更适合用于自旋电子学设备。
(5)N、B单掺杂和共掺杂不同形状和宽度Ge纳米带的稳定性、电子特性和磁性研究。我们的第一性原理计算结果表明：无论是AGeNRs还是ZGeNRs,边缘Ge原子总是最容易被杂质原子替代。单原子N掺杂或单原子B掺杂都可在AGeNRs中诱发“半导体～金属”过渡；然而,N和B共掺杂于AGeNRs边缘时,由于有效的电荷补偿,AGeNRs仍可保持原有的半导体性。单原子N掺杂或单原子B掺杂通常可使具有反铁磁半导体性质的ZGeNRs转变为铁磁半导体,此“反铁磁～铁磁”转变主要由局域于纳米带掺杂边缘的π/π*电子态的扰动引起；一些单原子掺杂的ZGeNRs还可以表现出半金属铁磁性质。双原子掺杂(无论N-N, B-B和N-B构型)于ZGeNRs的两个边缘时,ZGeNRs原有的反铁磁简并被破坏,最终转变为非磁半导体。总体而言,N掺杂、B掺杂以及N-B共掺杂的AGeNRs和ZGeNRs在Ge基的纳米电子学器件方面,如场效应晶体管,负微分电阻和自旋过滤器等,具有潜在的实际应用。
Germanium (Ge), a tranditional semiconductor material, has been used for various applications, such as infrared detector, optical fiber, high-speed microelectronic devices, integrated circuits and thermoelectric material etc., due to its excellent physics and chemistry properties as well as good compatibility with silicon-based materials. With the rapid development of science and technology, the electronic devices need to be high-integrated, multi-functionalized and miniaturized. At the same time, nanoscale devices with low dimensions and small sizes have gradually become the new trend for the development in fields of microelectronics and optoelectronics. The exciton Bohr radius of Ge is far greater than that of silicon, indicating that obvious quantum size effect will appear in Ge. Therefore, Ge-based nanomaterials should posses much more unique properties.
One-dimensional (1D) Ge-based nanomaterials have excellent photoelectric characteristics thus show a well prospect in applications of high-performance field-effect transistors (FETs), biochemical sensors, logic gates and efficient solar batteries. Recently, a lot of theoretical and experimental researches have been devoted to the synthesis, microstructure characterization and physicochemical characteristics analysis of1D Ge nanomaterials and finally achieved many breakthroughs. Nevertheless, the property-modification for1D Ge-based nanomaterials through hetero atom remains to be less exploited, thus needing to be systematically investigated and deeply understood. In this study, using first-principles projected-augmented wave (PAW) method, we pay special attention to the microstructure, stability, electronic and magnetic properties of the perfect, defected, adsorbed and doped Ge nanowires/nanoribbons. The principal conclusions are shown as follows:
(1) Al and P adsorption on Ge nanowires. Using first-principles calculations, the electronic properties of Al and P adsorbed Ge nanowires with different configurations and concentrations have been extensively studied and the corresponding results been compared with those of Al and P doped nanowires. Al adatom prefers to bind on the hollow site surrounded by a surface octagon ring (MO) of Ge nanowire and does not break the adjacent Ge-Ge bond after adsorption, while P adatom prefers to bind on the bridge site shared by two surface pentagons (Bb) of Ge nanowire, and breaks the beneath Ge-Ge bond to form a new Ge-P-Ge bond. At higher adsorption concentration, an impurity-induced electronic band appears and crosses the Fermi level, thus resulting in a "semiconductor-metal" transition in Ge nanowires. With decreasing of Al (P) adsorption concentration, the impurity-induced electronic band becomes flat and eventually locates below (above) the Fermi level, thus leading a "metal-semiconductor" transition in Ge nanowires. In Al (P) adsorbed wire, the impurity-induced electronic band is located closer to conduction band (valence band), which does not follow the traditional acceptor (donor) mechanism in p-type (n-type) doped semiconductors. Such a reverse behavior is explained by the fact that the adatom is simply adsorbed on the surface of the wire, thus only interacts with the unoccupied electronic states from the dangling bonds of surface Ge atoms. In addition, the formed Al-Ge bonds mainly display a covalent bonding character, while the formed P-Ge bonds display both covalent bonding and ionic bonding characters.
(2)3d transition-metal (TM) atoms adsorption on Ge nanowires. The microstructure, stability, electronic and magnetic properties of ten kinds of3d TM atoms adsorbed Ge nanowires have been investigated by spin-polarized first-principles PAW method. Except that Sc prefers to bind on the top site of the mutual Ge atom which belongs to the surface pentagons and hexagons of the wire, other TM atoms all prefer to bind on the hollow site of surface hexagon (HH) of the wrie. The variation trend of binding energies with'd'electron number agrees well with that of3d TM atoms adsorbed (8,0) carbon nanotubes, and by comparison, all TM atoms also form stronger bonding on Ge nanowires than on carbon nanotubes. Good conducting metals, such as Cu and Zn, can form weak bonding with the wire, whereas those such as Ti, V, Fe, Co and Ni have relative larger binding energies. Various types of wires can be obtained depending on the adatom species, including nonmagnetic (NM) metals (Sc or Cu adsorption) and semiconductors (Ni or Zn adsorption), weak ferromagnetic (FM) metals (Ti or V adsorption), FM semiconductors (Cr adsorption) and more interesting the FM half-metals (Mn, Fe or Co adsorption) which have potential application in spintronics. The magnetism of these wires originates mainly from spin-split of the TM-3d states, and the TM atom also induces some anti-parallel charge density around its adjacent Ge atoms. Furthermore, using DFT+U method, we also considered the effect of on-site Coulomb interaction on the stability of the three FM half-metallic wires and found the half-metallic ground state of Mn-or Co-adsorbed wire is more robust than that of Fe-adsorbed one.
(3) The study of two-dimensional (2D) honeycomb Ge, perfect and defected Ge nanoribbons. The microstructure, stability, electronic and magnetic properties of2D honeycomb Ge, perfect and defected armchair Ge nanoribbons (AGeNRs) and zigzag Ge nanoribbons (ZGeNRs) have been studied in detail by using first-principles PAW calculations. The stable2D honeycomb Ge sheet is slightly buckled and shows semi-metallic character. Its electron and hole bands linearly across at the Fermi level thus the carriers behave like "massless" Dirac fermion near the K point in the Brillouin zone (BZ). The perfect AGeNRs are NM semiconductors with their band gaps exhibit a periodically oscillatory damping as the ribbon width increases, thus making AGeNRs to be classified into three types. The perfect ZGeNRs have stable antimagnetic (AFM) semiconducting ground state with their band gaps monotonously decrease as ribbon width increases. Their net spin charge densities are mainly localized at the edge Ge atoms and contributed by π/π*electronic states, and the spin states at opposite edges have different spin orientations. The band gaps of AGeNRs can be efficiently tuned by atomic defect (vacancy or di-vacancy) at different positions though no magnetism is introduced. The ZGeNRs can become AFM or FM metals by introducing atomic defect, thus can be well used in electronic conduction and spin storage.
(4) TM (Cr, Mn, Fe and Co) atoms adsorption on2D honeycomb Ge and AGeNRs. The results indicate that, all TM atoms considered prefer to adsorb on the hollow site of hexagon of2D Ge whether in single-sided or double-sided adsorption cases, and NM semi-metallic2D Ge finally changes to be either FM or AFM metals depending on both TM species and coverage. For AGeNRs, the most preferential adsorption site is the hollow site of hexagon at the ribbon edge. Except for Co adsorption remaining NM state, Cr-, Mn-and Fe-adsorbed AGeNRs all possess FM state or AFM state according to ribbon width, TM species and coverage. Through Cr or Mn adsorption, some AGeNRs can also become FM or ferrimagnetic (FIM) half-metals. Moreover, considering the effect of on-site Coulomb interaction, we found the half-metallic ground state of Cr-adsorbed ones is more robust than that of Mn-adsorbed one thus can be suitable for spintronic devices.
(5) The stability, electronic and magnetic properties of N, B doped and co-doped AGeNRs and ZGeNRs. Our first-principles calculation results show that, for both AGeNRs and ZGeNRs, edge Ge atoms are always easy to be substituted. Single N-doping and single B-doping can introduce a "semiconductor-metal" transition in AGeNRs, while N and B co-doped AGeNRs also remain its semiconducting character due to the effective charge compensation. Single N-doping or single B-doping usually makes AFM ZGeNRs to be FM semiconductors, and the "AFM-FM" transition originates from the perturbation of π/π*electronic states which localized at the ribbon edges. Some single impurity doped ZGeNRs also exhibit half-metallic properties. Double atom substitution (regardless of N-N, B-B, and N-B configurations) at the edges of ZGeNRs removes the spin-polarization at both edges and transforms them into NM semiconductors. Overall, N, B doped and co-doped AGeNRs and ZGeNRs have potential applications in Ge-based nanoelectronic devices, such as FETs, negative differential resistances (NDR) and spin filters (SF) etc.
引文
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