【作者】 吴江红；

【导师】 胡俊青；

【作者基本信息】 东华大学 ， 材料科学与工程， 2013， 博士

【摘要】 在金属、半导体纳米材料中,铜、锗纳米材料在电子领域应用中有着举足轻重的作用。在平板显示器、触屏感应手机及电脑等电子产品中,铜纳米线因其与银等高导电材料有近似的导电性,但造价仅是银以及用在平板显示器中最广泛的电极材料——ITO的百分之一,被越来越多的研究学者看作是十分有潜力的下一代电极材料,可以制作成柔性(或可拉伸)导电薄膜应用在柔性(或可拉伸)显示器中。而对于锗纳米材料来说,相比于同族元素的硅而言有较高的载流子迁移率、较大的激子波尔半径,使得锗纳米材料在场效应晶体管应用中可以实现更快的开关和更高频率的器件；在锂离子电池应用中,因具备更大的蓄电容量和更高的电流密度,被认为是最具有潜力的、可替代石墨电极的下一代锂离子电极材料。另外,相对较大的波尔半径(24.3m)赋予了锗纳米材料更为显著的量子限制效应,加上相比于硫化铬等量子点来说,锗量子点呈低毒性,被看作是另一种有潜力的、可用在生物领域作生物标记的材料。因此本论文发展了几种简易、批量的合成方法制备了一些铜、锗纳米材料并分别研究了它们在电学、光学等领域的性能以及应用。1)超长Cu纳米线作为可拉伸导电薄膜电极为了实现一种柔性(或可拉伸),成本低廉的电子器件,我们采用Cu这种地壳存储量多、廉价、易合成的材料来制取可拉伸、透明导电薄膜以构建柔性电子器件。我们用普通还原法,以Cu(NO3)2为原料,水合肼为还原剂,乙二胺为配体,制备出长度为38±11.2μm的超长Cu纳米线,并制成纳米线薄膜——作为可拉伸电极用在透明薄膜致动器中。经研究发现,我们制得的可拉伸Cu纳米线薄膜电极可以使致动器在外加电压下达到220%的形变,透光率增加44%(从起始的10%T增加到54%T),该形变与前人用碳纳米管做薄膜电极时得到的数值相当,但是透光率增加量却是前人所做的3倍；同时,研究表明该透明致动器显示了良好的形变可逆性,在3.2kV外加电压下,对致动器循环开、关电压150次,其形变一直维持在25%左右,没有逐次递减,同时透光率保持在62±2.2%；另外,该透明致动器有非常快的响应速度,计算表明,在外加电压下,致动器从2.2%的形变增加到20%仅需0.5s,当电压撤掉后致动器恢复仅需0.4s。从而得出致动器的驱动速率为32%s-1,恢复速率为36%s-1,该响应速率与用碳油脂作薄膜电极的致动器数值相当；为了形象的演示这种透明致动器透光率的可调性,我们将该致动器设计成一种电压控制型的光调节器,并且实现了广角度的光强度的逐步调节。研究表明,Cu纳米线光调节器相比于传统的液晶显示器能实现更广角度的透光性。最后,我们证明了纳米线薄膜是通过线与线之间的滑移同时又不断开接触来实现可拉伸性和高度导电性。2)低温液相法制取抗氧化Ge纳米线及其电学性能由于Ge纳米线有较高的载流子迁移率、较大的激子波尔半径,使得锗纳米材料在场效应晶体管、锂离子电池等领域中越来越受到人们的关注。然而,现阶段合成Ge纳米线的方法还是停留在CVD这种能耗高、设备复杂、产率低的传统制备方法上,尽管越来越多的学者开始将目光转移到液相合成法上,所得到的纳米线质量与产率也不尽人意。更为重要的是,用这些方法得到的纳米线表面通常会有一层难以避免的氧化层,这层氧化层对Ge纳米线在电学领域里的应用是不利的。因此,发展一种简易、批量、能耗低且又能抗氧化的Ge纳米线合成方法显得尤为重要。我们采用GeI4作为原料,以油胺为反应媒介,在相对低温(320℃)的液相体系中合成了表面光滑、尺寸均一的Ge纳米线,其长度为8±1.3μm。由于该纳米线在液相中获得,非常容易分散在正已烷、乙醇等有机溶剂中,十分有利于进一步的器件制备。经分析高分辨透射电镜(HRTEM)和X射线衍射图可知,我们制得的Ge纳米线并没有如前人所报道的那样存在明显的氧化层,即使在空气中存放两周也没有出现明显的氧化层；通过探索不同的反应条件和反应时间、温度以及反应物浓度的影响,发现这些条件对能否形成纳米线至关重要：反应时间低于30分钟或温度低于320℃时,没有纳米线生成；而提高反应物浓度时,得到的产物是类似于棒球杆形状的纳米棒。为了得到更好的电学性能和抗氧化性,我们在Ge纳米线表面镀上了一层Au,通过分析单根纳米线的I-V曲线可知,镀Au之后的Ge纳米线其电学性能得到大大的提高,同时又保持着半导体特性,这很可能有助于Ge纳米线在诸如场效应晶体管、锂离子电池等电学领域的应用。3)利用透射电镜观察Ge基纳米材料原位结构变化锗材料是一种很容易被氧化,或者说对外界环境很敏感的材料,特别是对于纳米尺度而言,该种现象尤为明显,往往在材料表面形成一层氧化层或者其它结构的壳层,这层壳层会大大降低材料在器件中的性质,为了了解Ge基材料受外界环境影响时其结构的变化情况,我们利用透射电镜观察Ge基纳米材料的原位结构变化。我们以金属Ge粉为原料,用热氧化法制备了无定形GeO纳米球并在透射电子显微镜产生的电子束辐照下,观察到所合成的GeO纳米球向GeO/(Ge, GeO2)核壳纳米结构,进一步向Ge空心纳米球原位变化的过程。在开始阶段,GeO纳米球逐渐分解为Ge和Ge02复合结构,形成GeO/(Ge, GeO2)核壳纳米结构,当GeO分解完全后,形成均匀的(Ge, GeO2)复合纳米球；在电子束的进一步辐照下,由于Ge02的沸点相对于Ge的低很多,Ge02便从(Ge, GeO2)复合纳米球中挥发出来,Ge被留在球体中,并在表面形成了许多毛刺,当Ge02完全从(Ge, GeO2)复合纳米球中挥发出来后,形成了Ge多晶空心纳米球。这种Ge材料特殊的原位结构变化是第一次报道,这种现象使我们进一步了解Ge材料受到外界条件影响时的变化过程,对于我们理解如Ge纳米线表面通常都会有许多不规则的氧化层或其它壳层是非常有帮助的。此外,该研究进一步证明了透射电子显微镜中的电子束也是一种可以合成或操纵纳米结构非常有用的工具,特别对于那些不能用传统化学和物理方法合成的复杂结构而言更为有利。4)一步法水相条件下制备Ge纳米晶及其光学性能Ge因其较大的激子波尔半径有更为显著的量子限制效应,已被证明有荧光性能,适合用作生物标记。又因Ge纳米晶相对于Ⅱ-Ⅵ、Ⅲ-V族量子点呈低毒性,越来越多的研究学者开始关注Ge纳米晶的合成以及其性能的探索。Boyle与他的合作者将Ge纳米晶用二硝基苯基配体功能化,变成水溶性,能显示较弱的荧光,并证明了其用在RBL-2H3细胞中作生物标记显示低毒性。不过现阶段的合成方法中,合成条件均需要高温并且都用到有毒的Ge前驱体,通常这会带来较高的合成成本,另外,实验中用到的高毒性的有机溶剂也非常有害于人体健康,同时还污染环境。因此发展一种安全、简单、成本低廉、高产、通用且副产物少、对环境友好的Ge纳米晶的绿色合成方法迫在眉睫。我们首次利用了一种简单、方便且环境友好的方法在较低的温度下(60℃),以无毒的Ge02为锗源,在水溶液中合成了高产量、尺寸在3nm左右的Ge纳米晶并探索了其生长机理。经研究发现,在整个合成过程中Ge纳米晶表面并没有明显的氧化现象。良好的分散性与结晶度有可能是在透射电镜下,长时间的电子束辐照所致；通过探索PH值、反应时间、反应温度以及表面活性剂对Ge纳米晶形貌的影响,发现当溶液的PH值为5时,不能形成单个的Ge纳米晶；PH为11时,Ge纳米晶的尺寸增大到6-8nm；温度低于60℃时(如,室温条件下),合成过程中没有生成物出现；采用其它表面活性剂如CTAB, SDS, PEO-PPO-PEO时,并没有观察到Ge纳米晶的生成。因此,要获得尺寸均一、分散性较好的Ge纳米晶,其最佳合成条件为：反应温度为60℃,PH为7,反应时间为3小时。分析荧光光谱可知,当用370nm的光激发纳米晶时可以看到该纳米晶在426nm处产生较强的发射峰,这很有可能是由于Ge纳米晶的小尺寸引起的量子限制效应所致。相对于以前的研究而言,这种方法是用无毒前驱体在大气环境中合成,符合绿色化学的理念,即安全,简单、成本低廉,高产,副产品较少,环境友好,低能源消耗。更多还原

【Abstract】 Copper (Cu) and Germanium (Ge) nanomaterials play important roles in electronic applications among all metal and semiconductor materials. Cu nanowire has been considered as another promissing candidate for transparent electrodes utilized in flat panel displays, touch screens etc. due to its high conductivity and optical transmittance, large earth abundance and low cost (1000times more abundant than indium or silver but100times less expensive). For Ge materials, several advantages, e.g. larger higher intrinsic carriers and larger excitonic Bohr radius over Si materials make it easier to realize on/off current ratio faster switching and higher frequency devices in field effect transistor and a promising candidate for graphite used in Lithium ion battery, which exhibit higher Li storage capacity and high power density. In addition, Ge nanocrystals exhibit more prominent quantum confinement effects derived from its larger Bohr radius, moreover Ge materials are relatively nontoxic compared with Ⅱ-Ⅵ、Ⅲ-Ⅴ quantum dots used as diagnostic and therapeutic agents and would be another promising biologic label.1) Ultra long Cu nanowires as stretchable electrodesUltra long Cu nanowires with length of38±11.2μm were synthesized by using Cu(NO3)2as the Cu precursor, hydrazine as the reductant and EDA as the capping agent through redox method and were made into a transparent stretchable electrode for transparent actuator. Cu nanowires based actuator could be actuated to a maximum area strain of220%and a transmittance of54%. During actuation, the transmittance of the nanowires network increased3.5times, from13%to58%, which could be tuned over a range three times that of carbon nanotube networks. Repetitive cycling of the actuator to an area strain of25%over150times was demonstrated. Moreover, the actuator performs fast response, which took0.5seconds to increase from an area strain of2.2%to20%under an applied voltage, and0.4seconds for the actuator to relax once the voltage was withdrawn. This corresponds to a rate of actuation of32%s-1for increasing strain, and36%s-1for decreasing strain. This response rate is comparable to that of VHB films actuated with carbon grease electrodes. The ability to electrically tune the transmittance of the actuators over a range of up to44%enable the actuator used as a light valve with a widely tunable transmittance across a broader range of optical transmittance. At last we demonstrated that reversible sliding nanowires network allows the actuator with reversible stretchability and high conductivity.2) One-step synthesis of oxidation resistant Ge nanowires and electrical propertyGe nanowires have attracted more and more attentions in terms of field effect transistor and Lithium ion battery due to its higher intrinsic carriers and larger excitonic Bohr radius. Chemical vapor deposition (CVD) based on vapor-liquid-solid (VLS) mechanism has been adopted as an effective way to prepare highly crystalized Ge nanowire. However, low yield and high energy consumption are still its weak points which could not be taken as an idea way for commercial production. Last but not least, usually there is an inevitable layer of oxidate on the surface of CVD synthesized Ge nanowire, which will limit its applications in electronics. Therefore, developing a robust, large-scale and low energy consumption routine to synthesize oxidation resistant Ge nanowires is of fundamental importance. We use GeI4dissolved in oleyamine as the Ge precursor, thermal decomposing at320℃to synthesize Ge nanowires with smooth surface and uniform length (8±1.3μm). Solution synthesized nanowires are easily dispersed in hexane, ethanol etc. organic solvent, which make the fabrication of electronic devices easier. From the observation of HRTEM and X-ray diffraction, there is not obvious oxidate layer even after exploded in the air for two weeks. We explore the reaction parameters and found out that the reaction time, temperature and the concentration of precursor play key roles in the formation of Ge nanowires. No nanowires were observed when the reaction time is lower than30min or the temperature lower than320℃, and low concentration of the precursor will bring baseball club-like nanorods. In order to obtain nanowires with higher conductivity, we coated a layer of Au on the surface of the as-synthesized Ge nanowires. By measuring the conductivity of single nanowire, the Au coated one performs better but remains semiconduct. The Au-coated Ge nanowires would be potentially used in field effect transistor and Lithium ion battery.3) In situ structural evolution of Ge based materials under TEMGe materials easily forms unstable oxides on the surfaces and are sensitive to ambitious environment, especially for the materials on nanoscale. Usually there is an inevitable layer of oxidate on the surface of CVD synthesized Ge nanowire, which will limit its applications in electronics. Motivated by this property of Ge materials, we use TEM to study the in situ structural evolution of Ge based materials in hope of unveiling how this material responded to the variation of ambitious environment. We found that Ge based materials are also sensitive to electron beam irradiation (EBI). We synthesized amorphous GeO spherical nanoparticles via a simple thermal oxidation of Ge powder and observe it under the electron beam irradiation derived from TEM. At the beginning stage, the amorphous GeO nanospheres under the EBI undergoes a reduction oxidation (or disproportionation:2GeO→GeO2+Ge) reaction gradually from the surface to the centre of the nanoshpere, producing a Ge and GeO2composite on the surface domain, forming a GeO/(Ge, GeO2) core-shell nanoparticle. With further irradiation, the interface between the GeO core and (Ge, GeO2) shell becomes blurry, forming a homogeneous (Ge, GeO2) composite nanosphere once the GeO decomposed completely. Owing to the lower boiling point of GeO2, GeO2began to evaporate from (Ge, GeO2) composite nanosphere, forming many fine channels or glochids around the surface while leaving Ge in the nanospheres. After the GeO2evaporated thoroughly, there are a large amount of intercrystallite spaces present in these spheres, contained in plenty of Ge nanocrystallites, which would aggregate to be compact under prolonged heating by EB. Correspondingly, the intercrystallite spaces are supposed to combine into a single void. Moreover, based on the Ostwald ripening mechanism, the Ge nanocrystallites located in the inner part of the spheres are likely to diffuse into those in the outer parts, leaving a spacious void in the centre, as a result, polycrystalline hollow Ge nanospheres were formed. This study has further proved that the electron beam inside TEM is a powerfully useful tool for the fabrication and manipulation of nanostructures.4) One-step aqueous solution synthesis of Ge nanocrystalsGermanium (Ge) nanocrystals (NCs) have attracted much attention owing to its larger excitonic Bohr radius than Si, which will exhibit more prominent quantum confinement effects. Free-standing Ge NCs are relatively nontoxic compared with Ⅱ-Ⅵ、Ⅲ-Ⅴ quantum dots. Combined with their photoluminescence properties, Ge NCs with surface-modification have been proven to act as biological labels in biological applications Boyle et al. prepared the carboxyfluorescein (CF)-labeled dinitrophenyl (DNP)-functionalized Ge NCs which shows photoluminescence (PL) property and are proved sufficiently nontoxic to serve as biomarkers for cell signaling in RBL-2H3cells in vitro. So far, considerable efforts have been devoted to synthesis of Ge NCs, but those methods rely on high temperatures and toxic precursors, which will bring greater pollution to our living circumstances. So, a green synthetic route, which is safe, simple, inexpensive, reproducible, with few by-products, and user environment friendly to Ge NCs remains a challenge. We developed a facile, one-step routine useing GeO2as the precursor to synthesize Ge NCs at60℃in aqueous solution under an ambient atmosphere and explored its growth mechanism. The as-synthesized Ge NCs are around3nm in diameter and there is on oxidation observed during the whole process. Good monodispersity with high crystallinity may result from long time electron beam irradiation. By exploring the effect of PH, the reaction time and temperature, we found out that no mono-disperse Ge NCs formed when PH is5but larger diameter (6-8nm) when PH is11. And no Ge NCs were observed when the temperature is lower than60℃or other surfactants used (i.e. CTAB, SDS, PEO-PPO-PEO). It was found that the most suitable reaction conditions for the Ge NCs forming are60℃,3hours and PH=7. The emission spectrum shows a relatively narrow region of intensive luminescence with a peak centered at426nm, with excitation at370nm (dashed curve), which is attributed to the quantum confinement effects of very small sized Ge NCs. Our research meets the growing requirements of green chemistry, i.e., safe, simple, inexpensive, reproducible, few by-products, user environment friendly and low energy consumption.更多还原