Fundamental Study of Ordered MXenes and Their Defects

非技术描述：2004年，一种被称为石墨烯的单原子石墨层被分离出来，并对其显著的性能进行了表征。因此，对于横向尺寸远大于厚度的二维、二维、固体的兴趣已经爆发。2010年授予石墨烯诺贝尔奖进一步激发了人们的兴趣。一般来说，二维固体的特点是表面积非常大。此外，通过将电子（即电荷载流子）限制在二维也可以产生新的物理学。最近发现的二维固体家族被称为MXenes，其中M是过渡金属，X是碳和/或氮。在这个项目中，选择单层的MXenes被隔离在衬底上并进行表征，以便更好地理解化学、温度、表面末端、顺序和磁场对电输运的影响。这些结果被用来更好地理解MXenes中的电荷流动，这反过来又对设计更好的锂电池、超级电容器、催化剂和透明导电电极至关重要，这只是这一令人兴奋的新型二维固体家族的几个潜在应用。参与该项目的研究生通过参与欧洲和亚洲合作实验室的研究活动获得国际经验。每年夏天，本科生都可以通过德雷塞尔大学的STAR项目参与研究（该项目为完成第一年学业、gpa较高且对研究感兴趣的学生提供支持）。这些学生中的许多人随后完成了研究生学位。技术描述：最近，发现了两类新的四阶有序MAX相（M′，M′）n+1AXn：分别为面外有序和面内有序的o-和i-MAX。o-MAX相是一种过渡金属夹在另一种过渡金属中间的相。i-MAX相是211 （n = 1）相，其中m平面本身是有序的。通过明智地选择蚀刻剂，两个M元素中的任何一个或只有一个可以与a层一起蚀刻，从而在后一种情况下产生具有有序空位的MXenes。所提出的工作的首要科学目标是在基本层面上理解M层和点缺陷的排序对单个2D薄片和由相同薄片制成的旋铸薄膜的磁输运和光学性质的影响。提出的一些问题涉及有序/无序MXenes中的主要电子散射机制；哪些因素影响载流子密度；终止及其顺序/无序对这些值的影响；以及能否在这些材料中产生带隙。就影响而言，MXenes可能会在“乐高”方法中发挥关键/使能作用，将异质结“简单地”作为欧姆接触。此外，尽管MXenes的出现时间相对较短，但它已经在储能材料方面显示出了巨大的前景——无论是超级电容器、锂离子电池/电容器还是其他领域——电磁屏蔽、水净化和海水淡化、催化、透明导电电极、聚合物复合材料的增强等。这项工作有助于为任何给定的应用选择最佳的MXene化学物质。

NON-TECHNICAL DESCRIPTION: In 2004, a single atomic layer of graphite, called graphene, was isolated and its remarkable properties were characterized. Consequently, interest in two-dimensional, 2D, solids, where the lateral dimensions are much greater than the thickness has exploded. The awarding of the Nobel prize for graphene in 2010 fueled further interest. In general, 2D solids are characterized by very large surface areas. Furthermore, by confining the electrons, viz. the electric charge carriers, to 2D can also result in new physics. The most recent family of 2D solid discovered is called MXenes where M is a transition metal and X is carbon and/or nitrogen. In this project, single layers of select MXenes are being isolated on a substrate and characterized in order to better understand the effects of chemistry, temperature, surface terminations, order, and magnetic fields on electrical transport. These results are being used to better understand how charge flows in MXenes which, in turn, is of vital importance to designing better lithium batteries, supercapacitors, catalysts, and transparent conductive electrodes to name only a few potential applications of this new and exciting family of 2D solids. Graduate students involved in this project obtain international experience through participation in research activities in collaborating laboratories in Europe and Asia. Undergraduate students are involved in research every summer through Drexel University's STAR program (which supports students who completed their first year of studies, have high GPAs and are interested in research). Many of these students subsequently complete graduate degrees. TECHNICAL DESCRIPTION: Quite recently, two new classes of quaternary ordered MAX phases (M',M")n+1AXn, have been discovered: o- and i-MAX for out-of-plane and in-plane order, respectively. The o-MAX phases are ones where one transition metal sandwiches the other. The i-MAX phases are 211 (n = 1) phases in which the M-planes are themselves ordered. With the judicious choice of etchants, neither, or only one, of the two M elements can be etched - together with the A-layers - to yield MXenes with ordered divacancies in the latter case. The overarching scientific goal of the proposed work is to understand, at a fundamental level, the effects of ordering, both of the M layers and point defects, on the magneto-transport and optical properties of single 2D flakes and spincast films made of the same flakes. Some of the questions posed involve the main electron scattering mechanisms in the ordered/disordered MXenes; what factors influence carrier densities; the effect of terminations and their order/disorder on these values; and whether a band gap can be induced in these materials. In terms of impact, MXenes will likely play a critical/enabling role in the "lego" approach to heterojunctions 'simply' as ohmic contacts. Furthermore, and despite their relatively young age, MXenes have already shown great promise as materials in energy storage - both in supercapacitors and ion batteries/capacitors of lithium and beyond - electro-magnetic shielding, water purification and desalination, catalysis, transparent conductive electrodes, reinforcements in polymer composites among many others. This work facilitates choosing the best MXene chemistry for any given application.