, 2017, 31(7): 511-517
doi: 10.11901/1005.3093.2016.394
金掺杂降低碳纳米管接触电阻的实验研究
Reducin g Contact Resistance of Carbon Nanotubes by Au Doping
刁加加1, 常春蕊2,, 张志明3, 张好强1, 孙红婵1, 安立宝1,
1 华北理工大学机械工程学院 唐山 063009
2 华北理工大学理学院 唐山 063009
3 华北理工大学材料科学与工程学院 唐山 063009
DIAO Jiajia1, CHANG Chunrui2,, ZHANG Zhiming3, ZHANG Haoqiang1, SUN Hongchan1, AN Libao1,
1 College of Mechanical Engineering, North China University of Science and Technology, Tangshan 063009, China
2 College of Science, North China University of Science and Technology, Tangshan 063009, China
3 College of Materials Science and Engineering, North China University of Science and Technology, Tangshan 063009, China
 Cite this article:
DIAO Jiajia, CHANG Chunrui, ZHANG Zhiming, ZHANG Haoqiang, SUN Hongchan, AN Libao. Reducin g Contact Resistance of Carbon Nanotubes by Au Doping. Chinese Journal of Material Research[J], 2017, 31(7): 511-517 doi:10.11901/1005.3093.2016.394

摘要:

降低与金属之间的接触电阻是碳纳米管在微纳电子领域获得应用的前提,掺杂金纳米粒子可有效降低碳纳米管的接触电阻。本文采用高温焙烧在碳纳米管表面构造缺陷和亲水基团,然后将碳纳米管在氯金酸水溶液中超声浸泡以吸附氯金酸,再在氢气气氛下加热还原氯金酸为金。扫描电子显微(SEM)图片表明碳纳米管被成功地掺杂了金纳米粒子,X射线能量散射谱(EDS)和X射线光电子谱(XPS)表明金是唯一掺杂在碳纳米管上的粒子。掺杂后碳纳米管的拉曼光谱中G带波数降低说明对碳纳米管掺杂金为N型掺杂。N型掺杂剂将电子转移给邻近的碳原子,使碳纳米管中的电子数量增加,进而增强了碳纳米管的导电性。利用介电电泳法将碳纳米管组装到一对金电极之间,并使用精密电感电容电阻(LCR)测试仪实时测量。结果表明碳纳米管与金电极之间的接触电阻较掺杂前得到了有效改善,电阻值最大可降低近50%。

关键词: 复合材料 ; 碳纳米管 ; 金纳米粒子 ; 掺杂 ; 接触电阻

Abstract:

A prerequisite for the application of carbon nanotubes (CNTs) in the industrial sectors of micro- and nano-electronics, it is essential to reduce its contact resistance with metal. Doping Au-nanoparticles can effectively reduce the contact resistance of CNTs. In this paper, a three step process was developed for doping Au-nanoparticles on CNTs, i.e. first, the CNTs are calcinated at high temperature to create defects and hydrophilic groups on their surface, then, the calcinated CNTs are dispersed ultrasonically in chloroauric acid solution to adsorb chloroauric acid, and finally, they are heated in hydrogen atmosphere at high temperature to produce Au-nanoparticles on the surface of CNTs. The produced CNTs are characterized by means of scanning electron microscopy (SEM) X-ray energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Results show that due to the Au-doping, the G-band peak of Raman spectra of the CNTs shifts to a lower frequency, which indicates that the doping is N-type. N-type dopants transfer electrons to adjacent carbon atoms and increase the electron quantity in CNTs, thereby increasing the electrical conductivity of CNTs. Subsequently, CNTs are assembled into the interval of two Au electrodes by dielectrophoresis (DEP), and the results of real-time measurement by using a precision inductance-capacitance-resistance (LCR) show that the contact resistance between the Au-doped CNTs and Au electrodes has been effectively reduced to ca. half of the original values between the bare CNTs and Au electrodes .

Key words: composite ; carbon nanotube ; Au nanoparticle ; doping ; contact resistance

自1991年被发现以来,碳纳米管(CNT)在世界范围内引起了科研人员的广泛关注和研究。碳纳米管具有优异的电学、力学、热学性能和化学稳定性,在场发射、传感器和单电子开关等领域的应用成为近年来的研究热点。但碳纳米管与金属电极的接触电阻过高[1, 2],导致电路中能量消散大、发热多,直接影响了基于碳纳米管器件的性能和寿命。因此,降低与金属的接触电阻成为碳纳米管在微纳电子领域获得实际应用的重要前提。

目前,改善碳纳米管接触电阻的方法主要有高温退火法[3]、局部焦耳热法[4]、超声焊接法[5]和电子束沉积法等[6]。但高温退火法容易对器件性能造成损害,局部焦耳热法可重复性不强,超声焊接法在技术上还不够成熟,电子束沉积法因实施条件苛刻而不适合大规模应用。因此,这些方法尚需进一步完善以满足实用化要求。

研究表明,对碳纳米管掺杂也可以降低其与金属间的接触电阻[7-10]。掺杂剂是电子给予体或接受体,掺杂后碳纳米管中的电子或空穴载流子浓度增加,增强了其自身的导电性。当金属性碳纳米管与金属接触时,掺杂可增大其与电极间的接触面积,进而改善其接触电阻[7];当半导体性碳纳米管与金属电极接触时,由于二者费米能级的不同存在肖特基势垒和局部空间电场,掺杂可调节半导体性碳纳米管的能级分布,降低其与金属接触的势垒高度,进而改善接触电阻[8-10]。且掺杂方法可操作性强,对器件无不良影响,不需要苛刻的实施条件,适合于大规模操作。因此,通过掺杂方法降低碳纳米管接触电阻的研究具有重要的应用价值。

金属元素金(Au)化学稳定性好,且与自身金材料电极兼容性好。根据方程式(1),氯金酸(HAuCl44H2O)在氢气气氛下易于被还原为金,还原反应的其它产物中只有氯化氢(HCl)气体和水蒸汽(H2O)。因此,不会有金元素之外的其它杂质残留在碳纳米管上。

2 HAuC l 4 · 4 H 2 O + 3 H 2 [ ] 8 HCl + 2 Au + 8 H 2 O (1)

基于上述原因,本实验使用氯金酸为介质,对碳纳米管进行金元素掺杂。采用介电电泳组装的方法将掺杂前、后的碳纳米管分别组装到金电极上,并使用精密电感电容电阻(LCR)测试仪测量电阻值,借助LabVIEW控制程序实时记录。多次组装并测量表明,掺金后碳纳米管与电极间的接触电阻普遍有所降低,其中最大可减小到将近原来值的50%。

1 实验方法

本实验使用的碳纳米管为直径(140±30) nm、长度(7±2) μm、纯度>95%的多壁碳纳米管(Sigma-Aldrich公司产品),原样标记为MWNT-P。由于表面惰性和疏水性,不含缺陷和亲水基团的碳纳米管很难对其掺杂外来物质,所以需对碳纳米管进行预处理。本实验采用在空气中高温焙烧的方法在碳纳米管表面包括端部构造缺陷和亲水基团,获得的样品标记为MWNT-T。将适量的氯金酸溶于蒸馏水中,然后将定量的高温焙烧后的碳纳米管与其混合,室温下超声振荡1 h使碳纳米管在溶液中分散,并静置一段时间使碳纳米管充分吸附氯金酸,过滤获得的样品标记为MWNT-T-HAuCl44H2O。再将其放入密封管式炉中氢气气氛下加热,吸附在碳纳米管上的氯金酸分子被还原为金单质,获得的样品标记为MWNT-T-Au。

使用高分辨率场发射扫描电镜(Merlin,德国Zeiss公司)、X射线能谱仪(Noran System 7,美国Thermo Fisher公司)、X射线光电子能谱仪(Axis-Ultra DLD-600W,日本岛津/Kratos公司)和激光拉曼光谱仪(DXR,美国Thermo Fisher公司)对上述样品进行表征。

最后利用介电电泳法将碳纳米管组装到两金电极之间。其过程为:取适量MWNT-P和MWNT-T-Au样品分别放入异丙醇中超声分散,将获得的碳纳米管悬浮液滴于间隙为2 μm的金电极对上,然后施加频率为1 kHz、幅值为3V的交流电压进行组装。由于极化作用,介电电泳力驱使碳纳米管向电场强度大的电极间隙运动,最终搭接在两电极间完成组装。组装过程中,利用LabVIEW控制程序实时记录LCR测试仪测量的电阻值。大量重复组装,并记录每次组装完成后与电极接触的碳纳米管根数及接触电阻值。

需要指出,多壁碳纳米管自身亦有传导电阻[11],但是和它与金属电极的接触电阻相比可以忽略不计[2]。因此,我们认为实验中LCR测试仪测得的电阻即为碳纳米管与金属电极之间的接触电阻。

2 实验结果与讨论
2.1 表征与分析

不同实验阶段碳纳米管样品的形貌如图1所示。从图1a中可以看出碳纳米管原样表面较光滑,但在端部存在异质结构。由于碳纳米管在生长过程中会形成一些碳五元环、碳七元环和不完全键等缺陷,在空气中高温加热时氧原子将首先从这些稳定性较差的缺陷处开始侵蚀碳纳米管。当氧化到一定程度时,如图1b所示,碳纳米管管壁和端部变粗糙,甚至局部位置由于石墨烯层缺失而出现凹陷。高温处理还会在碳纳米管表面产生含氧官能团[12],缺陷位和活性基团的存在使碳纳米管更容易吸附金属组分,从而有利于碳纳米管掺杂。

在氯金酸溶液中浸泡并过滤后,碳纳米管表面掺杂了较多由氯金酸团聚而成的球形纳米粒子,粒径大约在20~30 nm范围内(图1c)。氢气还原后,纳米粒子粒径减小到10~20 nm范围(图1d),这主要是由于氯金酸被还原,纳米粒子中的氯离子转化为HCl溢出所致。


图1

碳纳米管样品的SEM图片

Fig.1

SEM images of the samples of (a) MWNT-P, (b) MWNT-T, (c) MWNT-T-HAuCl44H2O, (d) MWNT-T-Au

X射线能谱仪(EDS)可以用来分析样品中所含元素的种类及含量。图2为各碳纳米管样品的EDS谱图。由于各样品是将碳纳米管置于异丙醇溶液中超声分散后滴于硅基片上制备而成的,所以能谱分析结果中每个样品都明显含有大量的硅和一定量的氧(硅被氧化所致)。比较各EDS谱图可以发现,标记为MWNT-T的样品中不含有金而标记为MWNT-T-HAuCl44H2O的样品中含有金和氯元素,说明碳纳米管原样中不含有金,碳纳米管在氯金酸溶液中浸泡并过滤后掺杂上了金和氯。标记为MWNT-T-Au的最终掺杂样品中只含有金而不含氯,说明经氢气还原后,金是唯一掺杂在碳纳米管上的物质。这一结果也解释了图1c和d中还原前、后碳纳米管表面纳米粒子直径的变化。


图2

碳纳米管样品的EDS谱图

Fig.2

EDS spectra of the samples of (a) MWNT-T, (b) MWNT-T-HAuCl4·4H2O, (c) MWNT-T-Au

X射线光电子谱(XPS)可以用来更精确地分析样品中所含元素的种类、含量以及各原子价态。图3为掺金碳纳米管样品(MWNT-T-Au)表面的XPS谱。从图3a中可以看出,在相应结合能处分别出现了对应Au4f、C1s和O1s的峰。检测到的O1s是由于样品吸附水分或高温焙烧碳纳米管时在其表面形成含氧官能团所致,所以EDS能谱分析中的氧除硅基片被氧化所致还包含样品中携带的氧。在Au4f XPS谱中(图3b),呈现金的两个特征峰,其中结合能84 eV处的峰对应Au04f7/2,而结合能87.7 eV处的峰对应 Au04f5/2,说明在H2还原反应之后得到了碳纳米管掺金样品。进一步地,根据XPS分析出掺金碳纳米管样品中只含有碳、氧和金三种元素,且碳、氧、金元素含量分别为93.75%、6.2%、0.05%。还原反应之后,没有检测到氯元素,说明氯金酸已被氢气完全还原,金是唯一掺杂在碳纳米管上的元素,这一点也与上述EDS分析的结果相一致。需要指出,掺金碳纳米管样品中金的含量并不高,但这种方法掺杂的金纳米颗粒粒径较小、较均匀。


图3

MWNT-T-Au样品的XPS谱

Fig.3

XPS spectra of the samples of MWNT-T-Au, (a) the spectral region from 50 to 550 eV, (b) Au 4f

图4为各碳纳米管样品的拉曼光谱图,图中位于1350 cm-1和1580 cm-1附近的峰分别被称为D峰和G峰。D峰是由于碳纳米管中存在的混乱结构、缺陷和杂质而形成的,而G峰对应于碳原子SP2杂化的面内伸缩振动,反映了碳纳米管管壁的石墨化程度。D峰与G峰强度比值I1350/I1580宏观反映了碳纳米管表面的破损程度,比值越大说明含缺陷越多。图4中4个样品的I1350/I1580值分别为0.13、0.31、0.28和0.26。高温焙烧后,I1350/I1580值从0.13增大到0.31,说明在空气中高温热处理降低了碳纳米管表面的石墨化程度,在碳纳米管上产生了缺陷。而掺杂后碳纳米管样品的I1350/I1580值有所减小说明在缺陷处掺杂了氯金酸或金。此外,掺金碳纳米管的G峰从波数1587 cm-1降低到1576 cm-1,说明对碳纳米管掺杂金为N型掺杂 [13-14]。经氢气还原,电子由掺杂物传给了相邻的碳原子,从而使碳纳米管中的电子浓度增加,导电性增强。由于电荷转移,掺金碳纳米管的费米能级水平有所升高,使得功函数降低,有利于其与金属电极之间的结合。


图4

各碳纳米管样品的拉曼光谱图

Fig.4

Raman spectra of CNT samples

2.2 金掺杂碳纳米管的电接触特性研究

本实验采用介电电泳法分别将原样碳纳米管和掺金碳纳米管组装到金电极上。实验中分别统计了10组2根、10组4根、10组6根、10组8根、10组10根和10组15根共60组原样碳纳米管与电极间的接触电阻,且统计了相同分组条件下掺金碳纳米管与电极间的接触电阻,将统计的数据取平均后如图5所示。从图中可以看出,掺金后碳纳米管与电极间的接触电阻普遍降低,最大可减小到将近原来值的50%。其中,在掺杂前的统计数据中20 kΩ以下占13%,30 kΩ以下占24%,而在掺杂后20 kΩ以下占到23%,30 kΩ以下则占到了38%。


图5

掺杂前后碳纳米管与电极间接触电阻的变化

Fig.5

Changes of contact resistance between CNTs and electrodes before and after doping

分析掺杂后碳纳米管接触电阻降低的原因,当金属性多壁碳纳米管与金属电极接触时会在两者之间存在真空间隙,从而影响并劣化碳纳米管与金属间的电接触[15];而N型掺杂使碳纳米管掺杂处局部电子浓度增大,当其掺杂处与金属接触时,增强的局部电荷会增加金属对碳纳米管的浸润性[7],使碳纳米管与金属间的接触面积增加、真空间隙减小[9]。这相当于增加了导电通道,从而降低了碳纳米管与金属间的接触电阻。同时,掺金碳纳米管降低的功函数会降低它与金电极之间的接触势垒,从而成为掺杂后碳纳米管接触电阻降低的另一个原因。

3 结论

高温焙烧在碳纳米管表面构造了缺陷位和亲水基团,在氯金酸溶液中浸泡对含缺陷的碳纳米管掺杂了氯和金,氢气还原将掺杂在碳纳米管上的氧化态金还原为金,氯则以HCl气体的形式散发出去,最终金成为唯一掺杂在碳纳米管表面的物质。掺金后碳纳米管的拉曼光谱中G峰波数降低说明金为N型掺杂剂,为碳纳米管注入了更多的电子,使碳纳米管导电性增强,且使碳纳米管与金属电极间的接触面积增大,从而降低了碳纳米管与电极间的接触电阻。进一步的电接触特性研究表明,掺金使碳纳米管与电极间的接触电阻普遍降低,最大可减小至接近原阻值的50%。因此,对碳纳米管掺杂金纳米粒子可以有效降低碳纳米管与金属电极间的接触电阻,进而改进碳纳米管器件的性能与使用寿命。

The authors have declared that no competing interests exist.

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A simple ultrasonic nanowelding technique has been developed to reliably bond single-wall carbon nanotubes (SWCNTs) onto metal electrodes, by pressing SWCNTs against electrodes under a vibrating force at ultrasonic frequency. The bonds formed have been demonstrated to be mechanically robust. Using this technique, a stable low-Ohmic contact between SWCNTs and metal electrodes was achieved, with resistances in the range of 8-24 kΩ for a 1 08m long metallic SWCNT at room temperature. The performance of carbon nanotube field-effect transistors (FETs) fabricated using this ultrasonic nanowelding method has also been greatly improved. Transconductance as high as 3.6 08S among the solid-state back-gate individual nanotube FETs has been achieved.
[本文引用: 1]
[6]
Madsen D N, Mølhave K, Mateiu R, et al.Soldering of nanotubes onto microelectrodes[J]. Nano Letters, 2003, 3(1): 47
DOI:10.1021/nl0257972 URL
ABSTRACT Suspended bridges of individual multiwalled carbon nanotubes were fabricated inside a scanning electron microscope by soldering the nanotube onto microelectrodes with highly conducting gold61carbon material. By the decomposition of organometallic vapor with the electron beam, metal-containing solder bonds were formed at the intersection of the nanotube and the electrodes. Current61voltage curves indicated metallic conduction of the nanotubes, with resistances in the range of 96129 kΩ. Bridges made entirely of the soldering material exhibited resistances on the order of 100 Ω, and the solder bonds were consistently found to be mechanically stronger than the carbon nanotubes.
[本文引用: 1]
[7]
(宋晓辉. 碳纳米管/金属界面键合机制及其相关技术研究 [D]. 上海: 上海交通大学, 2010)
Song X.Study on mechanism of carbon nanotube/metal interfacial bonding and related technology [D].Shanghai: Shanghai Jiao Tong University, 2010
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[8]
(刘恩科, 朱秉升, 罗晋生. 半导体物理学(第七版) [M]. 北京: 电子工业出版社, 2011)
Liu E K, Zhu B S, Luo J S, The Physics of Semiconductors (7th Edition) [M]. Beijing: Publishing House of Electronics Industry, 2011
[本文引用: 1]
[9]
Kong B S, Jung D H, Oh S K, et al.Single-walled carbon nanotube gold nanohybrids: application in highly effective transparent and conductive films[J]. The Journal of Physical Chemistry C, 2007, 111(23): 8377
DOI:10.1021/jp071297r URL
We report a new technique to enhance the electrical conductivity of transparent single-walled carbon nanotube (SWNT) films with a negligible loss of their optical transmittance. Hybridization of the SWNT films with gold nanoparticles increased the electrical conductivity 2-fold to a maximum of 2.0 x 10(5) S/m while maintaining the transmittance of the initial value. The same trends are shown for other SWNT films with various initial conductivities and transparency levels. Functional changes in the gold-nanoparticle-coated SWNT films suggest that the electrical conductivity change is due to an electron depletion mechanism as a result of a doping effect.
[本文引用: 1]
[10]
Kim K K, Bae J J, Park H K, et al.Fermi level engineering of single-walled carbon nanotubes by AuCl3 doping[J]. Journal of the American Chemical Society, 2008, 130(38): 12757
DOI:10.1021/ja8038689 PMID:18729358 URL
We investigated the modulation of optical properties of (SWCNTs) by AuCl 3 doping. The van Hove singularity transitions (E 11 (S), E 22 (S), E 11 (M)) in absorption spectroscopy disappeared gradually with an increasing doping concentration and a new peak appeared at a high doping concentration. The work function was downshifted up to 0.42 eV by a strong charge transfer from the SWCNTs to AuCl 3 by a high level of p-doping. We propose that this large work function shift forces the Fermi level of the SWCNTs to be located deep in the valence band, i.e., highly degenerate, creating empty van Hove singularity states, and hence the work function shift invokes a new asymmetric transition in the absorption spectroscopy from a deeper level to newly generated empty states.
[本文引用: 2]
[11]
Kim S, Kulkarni D D, Rykaczewski K, et al.Fabrication of an ultralow-resistance ohmic contact to MWCNT - metal interconnect using graphitic carbon by electron beam-induced deposition (EBID)[J]. IEEE Transactions on Nanotechnology, 2012, 11(6):1223
DOI:10.1109/TNANO.2012.2220377 Magsci URL
Reduction in contact resistance is one of the foremost challenges for carbon nanotube/graphene nanodevices. In this study, we present a novel fabrication process for low-temperature, Ohmic contact between open-ended multiwalled carbon nanotubes (MWCNTs) and metal interconnects using graphitic carbon deposited via electron beam-induced deposition (EBID). The electrical and structural properties of the contact interface are characterized for making connection to the single (outermost) shell only, as well as to multiple conducting shells of MWCNTs. In addition to establishing the scaling relationship between the carbon contact length and the resulting contact resistance, the magnitude of the contact resistance has been quantified with and without post-deposition thermal annealing. The results indicate that the contact is Ohmic in nature, and ranges from 26.5 k for the connection made to the outermost shell of an MWCNT down to just 116 for the multiple-shell connection performed via a process suggested through the EBID process simulations. These results provide a significant advance in application of MWCNTs to future interconnect technologies.
[本文引用: 1]
[12]
(田春华. 多壁碳纳米管改性负载铂锡在丙烷脱氢反应中催化性能的研究 [D]. 上海: 上海师范大学, 2014)
Tian C H.Catalyst performance of Pt-Sn/MWCNTs (modified) on the dehydrogenation of propane [D]. Shanghai: Shanghai Normal University, 2014
[本文引用: 1]
[13]
Skákalová V, Kaiser A B, Dettlaff-Weglikowska U, et al.Effect of chemical treatment on electrical conductivity, infrared absorption, and Raman spectra of single-walled carbon nanotubes[J]. The Journal of Physical Chemistry B, 2005, 109(15): 7174
URL
[本文引用: 1]
[14]
Rao A M, Eklund P C, Bandow S, et al.Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering[J]. Nature, 1997, 388(6639): 257
DOI:10.1038/40827 URL
Single-walled carbon nanotubes (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br) frequencies. Little change is seen for Idoping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements, suggests that doping does increase the carrier concentration of the SWNT bundles.
[本文引用: 1]
[15]
Lim S C, Jiang J H, Bae D J, et al.Contact resistance between metal an d carbon nanotube interconnects: effect of work function and wettability[J]. Applied Physics Letters, 2009, 95(26): 264103-1
DOI:10.1063/1.3255016 URL
The contact resistance of 14 different electrode metals with the work function between 3.9 and 5.7 eV has been investigated for carbon nanotube (CNT) interconnects. We observed that the contact resistance was mainly influenced by the two following parameters: the wettability and the work function difference of electrode metal to CNT. Ti, Cr, and Fe with good wettability showed lower resistance than other metals. Furthermore, no dependence of the contact resistance on the work function difference has been observed. However, the contact resistance of Au, Pd, and Pt with poor wettability increased as the work function difference became larger.
[本文引用: 1]
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