Lasers ♦ news digest
of band gaps between graphene, which is a semimetal, and the boron nitride insulator.”
Once the Ajayan and Lou teams were able to grow such large MDS arrays, the ORNL team imaged the atomic structures using aberration-corrected scanning transmission electron microscopy. The atomic array can clearly be seen in the images and, more importantly, so can the defects that alter the material’s electronic properties.
“In order to improve the properties of 2-D materials, it’s important to first understand how they’re put together at a fundamental scale,” Idrobo rematks. “Our microscopy facility at ORNL allows us to see materials in a way they’ve never been seen before - down to the level of individual atoms.”
MDS is distinct from graphene and hBN because it isn’t exactly flat. Graphene and hBN are flat, with arrays of hexagons formed by their constituent atoms. But while MDS looks hexagonal when viewed from above, it is actually a stack, with a layer of molybdenum atoms between two layers of sulphur atoms.
Co-author Zheng Liu, a joint research scientist in Lou’s and Ajayan’s labs, notes the Yakobson group predicted that MDS and carbon atoms would bind. “We’re working on it,” he says. “We would like to stick graphene and MDS together (with hBN) into what would be a novel, 2-D semiconductor component.”
“The question now is how to bring all the 2-D materials together,” adds co-author Sina Najmaei, a Rice graduate student. “They’re very different species and they’re being grown in very different environments.”
Until recently, growing MDS in a usable form has been difficult. The “Scotch tape” method of pulling layers from a bulk sample has been tried, but the resulting materials were inconsistent, Lou said. Early CVD experiments produced MDS with grains that were too tiny to be of use for their electrical properties.
But in the process, the researchers noticed “islands” of MDS tended to form in the furnace where defects or even pieces of dust appeared on the substrate. “The material is difficult to nucleate, unlike hBN or graphene,” Najmaei points out. “We started learning that we could control that nucleation by adding artificial edges to the substrate, and now it’s growing a lot better between these structures.”
“Now we can grow grain sizes as large as 100 microns,” Lou continues. That’s still only about the width of a human hair, but in the nanoscale realm, it’s big enough to work with, he says.
Yakobson, a theoretical physicist, and his team specialise in analysing the interplay of energy at the atomic scale. With ORNL’s images in hand, they were not only able to calculate the energies of a much more complex set of defects than are found in graphene or BN but could also match their numbers to the images.
Among the Yakobson team’s interesting finds was the existence, reported last year, of conductive subnano “wires” along grain boundaries in MDS. According to their calculations, the effect only occurred when grains met at precise 60-degree angles. The ORNL electron microscopy images make it possible to view these grain boundaries directly.
The Rice researchers see many possible ways to combine the materials, not only in two-dimensional layers but also as three-dimensional stacks. “Natural crystals are made of structures bound by the van der Waals force, but they’re all of the same composition,” Lou maintains. “Now we have the opportunity to build 3-D crystals with different compositions.”
“These are very different materials, with different electronic properties and band gaps. Putting one on top of the other would give us a new type of material that we call van der Waals solids,” Ajayan adds. “We could put them together in whatever stacking order we need, which would be an interesting new approach in materials science.
Computations were performed on Rice’s DAVinCI system and at the Cyberinfrastructure for Computational Research, both funded by NSF.
The Welch Foundation, the National Science Foundation (NSF), the U.S. Army Research Office, the U.S. Office of Naval Research, the Nanoelectronics Research Corporation and the Department of Energy supported the work.
This work is described in detail in the paper, Vapour phase growth and grain boundary structure of
July 2013
www.compoundsemiconductor.net 119
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179