Fundamental Study of Ordered MXenes and Their Defects

非技术描述：2004年，一个单原子层的石墨，称为石墨烯，被分离出来，其显着的特性进行了表征。因此，人们对横向尺寸远大于厚度的二维（2D）固体的兴趣激增。2010年石墨烯获得诺贝尔奖，进一步激发了人们的兴趣。通常，二维实体的特征在于非常大的表面积。此外，通过将电子（即电荷载流子）限制在2D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MXene的单层被隔离在衬底上，并进行表征，以更好地了解化学，温度，表面终端，秩序和磁场对电传输的影响。这些结果被用于更好地理解MXene中电荷的流动方式，这反过来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，仅举这个令人兴奋的2D固体新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。本科生每年夏天都通过德雷克塞尔大学的星星计划（该计划支持完成第一年学业、GPA高且对研究感兴趣的学生）参与研究。这些学生中的许多人后来完成了研究生学位。技术说明：最近，已经发现了两类新的四元有序MAX相（M '，M "）n +1AXn：分别为面外和面内有序的o-和i-MAX。o-MAX相是一种过渡金属夹在另一种过渡金属之间的相。i-MAX相位是M平面本身被排序的211（n = 1）个相位。通过明智地选择蚀刻剂，两个M元素中的任何一个或仅一个都不能与A层一起被蚀刻，以在后一种情况下产生具有有序双空位的MXene。拟议工作的首要科学目标是在基本层面上了解M层和点缺陷的有序化对单个2D薄片和由相同薄片制成的旋转浇铸薄膜的磁输运和光学特性的影响。提出的一些问题涉及有序/无序MXene中的主要电子散射机制;影响载流子密度的因素;终端及其有序/无序对这些值的影响;以及是否可以在这些材料中诱导带隙。在影响方面，MXene可能会在“简单”作为欧姆接触的异质结的“乐高”方法中发挥关键/使能作用。此外，尽管它们相对年轻，但MXenes已经显示出作为储能材料的巨大潜力-无论是在超级电容器和锂离子电池/电容器中，还是在电磁屏蔽，水净化和脱盐，催化，透明导电电极，聚合物复合材料中的增强材料等。这项工作有助于为任何给定的应用选择最佳的MXene化学。

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.