Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 2 / Section A(i) / 12 / i / Solution

Read the displayed axes as horizontal $[001]$ and vertical $[010]$; the viewing direction is therefore consistent with $[100]$. crystal facets edge-on in this projection have normals in the $yz$ plane. The flat upper/lower boundaries are approximately horizontal, the end face vertical, and the bevels approximately $45^{\circ }$ to both.

A probable low-index assignment is upper face $(010)$, end face $(001)$ and lower-right bevel $(0\bar{1}1)$, with the upper-right bevel $(011)$ if it is also indexed. Thus the flat faces belong to $\{100\}$ and the diagonal faces to $\{110\}$.

For example,giving $90^{\circ }$ and $45^{\circ }$ between normals. The upper $(010)$ normal and lower bevel $(0\bar{1}1)$ normal instead make $135^{\circ }$; their acute unoriented plane angle is $45^{\circ }$. Adjacent polygon corners between a flat edge and bevel have internal angles near $135^{\circ }$.

The distinction between a normal angle, an acute plane angle and an internal polygon angle avoids a supplementary-angle error. This is a probable projected indexation, not a unique full three-dimensional crystal facet reconstruction. The associated work uses tomography precisely to recover information absent from one projection.^[ref] The original facet-normal diagram in the growth-and-LED figure shows the $45^{\circ }$ relation independently of the source micrograph.