Preparation Of Rare-earth Hexaboride (RB₆;R=Nd?La_1-xPr_x?Sm?Yb?Gd) One-dimensional Nanostructures And Their Electronic Transport Properties

Rare-earth hexaborides?RB₆?are the best thermionic electron sources due to their low work function,low volatility at high temperature,high conductivity,high chemical resistance,and high mechanical strength.They have received renewed research interest in recent years because these materials have a variety of unique physical properties including superconductivity,semiconductivity,fluctuating valence,and topological insulator.Recently,one-dimensional?1-D?nanowire systems have attracted much attention because of their novel structural and electronic properties as compared to bulk counterparts.The main work of this thesis is to synthesize a series of rare-earth hexaboride 1D nanomaterials by chemical vapor deposition and high pressure solid-state method.The composition,morphology and structure of the nanomaterials are described in detail.The growth mechanism of 1D nanomaterials is discussed.We have tested the electronic transport properties of the single SmB₆ and YbB₆ nanowires.Neodymium hexaboride?NdB₆?submicroawls have been fabricated via a simple flux-controlled selfcatalyzed method using neodymium?Nd?powders and boron trichloride?BCl3?as starting materials at 1000?.Scanning electron microscopy?SEM?reveals that the submicroawls are tapered,with a length of 2–5 ?m and a diameter ranging from approximately 0.1–0.3 ?m at the roots and 5–50 nm at the tips.Transmission electron microscopy?TEM?shows that the submicroawls are single crystalline with the preferred growth direction along [001].For systematic research,we have discussed the morphological change of Nd B₆ submicron structures by varying reaction temperatures,catalysts and duration.Moreover,a multistage growth model of the NdB₆ submicroawls is proposed.Three kinds of one-dimensional?1D?neodymium hexaboride?NdB₆?nanostructures,including nanobelts,nanoawls,and nanotubes,have been synthesized through a chemical vapor deposition?CVD?process with a self-catalyzed mechanism.For the first time,we report the preparation of NdB₆ nanotubes.Transmission electron microscopy images show that they have different growth directions: [111],[001],and [110],respectively.In addition,detailed growth mechanisms of the nanobelts,nanoawls,and nanotubes are presented.A droplet induced self-catalyzed mechanism,self-catalyzed with a vapor–solid mechanism,and diffusion limited self-catalyzed mechanism are proposed to explain the growth of nanobelts,nanoawls,and nanotubes,respectively.Lanthanum–praseodymium hexaboride?LaxPr1-xB₆?nanoawls have been fabricated via a simple flux-controlled method using lanthanum?La?powders,praseodymium?Pr?powders and boron trichloride?BCl3?gas as starting materials at 1050?.Scanning electron microscopy?SEM?shows that the tapered nanoawls have a length of 2–10 ?m and a diameter ranging from 50 to 300 nm at the roots and 10–80 nm at the tips.Transmission electron microscopy?TEM?reveals that the nanoawls are single crystalline with the preferred growth direction along [110].Raman spectra indicate that the T1 u mode splitting effect occurs in this ternary rare-earth hexaboride.In addition,a concentration gradient is necessary to create these awl-like structures where the combination of self-catalyzed and vapor–solid growth is responsible for the growth of tapered LaxPr1-x B₆ nanoawls.We report a novel solid state method to prepare samarium hexaboride?SmB₆?nanowires at low temperatures of 220–240 ?.Scanning electron microscopy?SEM?images show that the Sm B₆ nanowires have diameters from 50 to 120 nm and lengths from 1 to 8 ?m.Transmission electron microscopy?TEM?images reveal that the SmB₆ nanowires are single crystalline with a preferred [001] growth direction.Resistivity saturated at low temperature indicate the existence of surface states.Electronic transport shows that a SmB₆ nanowire has a high saturation temperature of 6 K and an enhanced surface conduction.For the first time we have successfully synthesized high-quality single crystalline YbB₆ nanowires at very low temperatures of 200-240 ? by a high pressure solid state?HPSS?method with a new chemical reaction using Yb,H₃BO₃,Mg and I2 as raw materials.HRTEM images and SAED patterns reveal that the YbB₆ nanowires are single crystalline with a preferred growth direction along [001].The XPS spectrum suggests that the valence of Yb ion in YbB₆ is divalent.We report the electronic transport measurement in an individual free-standing single crystal YbB₆ nanowire.The temperature dependence of resistivity shows a typical metallic behaviour in the temperature range 20–300 K and a quasi plateau below 20 K.The linearly positive magnetoresistance?LPMR?effect is observed at 4.2 K,20 K and 50 K.We have successfully synthesized high-quality single crystalline GdB₆ nanowires at very low temperatures of 220-240 ? by a high pressure solid state?HPSS?method with a new chemical reaction using Gd,H₃BO₃,Mg and I2 as raw materials.HRTEM images and SAED patterns reveal that the GdB₆ nanowires are single crystalline with a preferred growth direction along [001].The magnetic properties of GdB₆ show that ??T?follows the CurieWeiss law with an effective magnetic moment of 6.26 ?B.The optical properties exhibit relatively high absorbance in NIR range?760-1000 nm?and relatively low absorbance in UV range?around 350 nm?.In a summary,RB₆ nanostructures with different morphologies have a low-index crystal faces,and most of them are?001?.The growth mechanism can be attributed to the formation of rare earth metal as a self-catalyst and boron atom selective orientation orientation to obtain one-dimensional nanostructures.