Synthesis, Properties And Devices Of Copper And Germanium Nanomaterials

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from an area strain of2.2%to20%under an applied voltage, and0.4seconds for the actuator to relax once the voltage was withdrawn.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.