Discovering Amorphous Indium Phosphide Nanostructures with High-Temperature ab Initio Molecular Dynamics

Abstract

We employ high-temperature ab initio molecular dynamics (AIMD) as a sampling approach to discover low-energy, semiconducting, indium phosphide nanostructures. Starting from under-coordinated models of InP (e.g. a single layer of InP (111)), rapid rearrangement into a stabilized, higher-coordinate but amorphous cluster is observed across the size range considered (In 3 P 3 to In 22 P 22 ). These clusters exhibit exponential decrease in energy per atom with system size as effective coordination increases, which we define through distance-cutoff coordination number assignment and partial charge analysis. The sampling approach is robust to initial configuration choice as consistent results are obtained when alternative crystal models or computationally efficient generation of structures from sequential addition and removal of atoms are employed. This consistency is observed across the 66 structures compared here, and even when as many as five approaches are compared, the average difference in energy per pair of atoms in these structures is only 1.5 kcal/mol at a given system size. Interestingly, the energies of these amorphous clusters are lower than geometry optimized spherical models of bulk InP typically used for simulations of quantum dots. Favorable energetics appear correlated to highlycoordinated indium and phosphorus with coordination numbers up to five and seven, respectively, as well as formation of phosphorus-phosphorus bonds.2

Direct Observation of Early-Stage Quantum Dot Growth Mechanisms with High-Temperature Ab Initio Molecular Dynamics

“…Rearrangement of the cluster geometry is then observed after the dissociation of the third P–H bond until the end of the 40 ps AIMD simulation, with the phosphorus CN dynamically changing between four (∼41% probability, approximate tetrahedral geometry) and five (∼59% probability, approximate pyramidal or trigonal bipyramidal geometry). The average CN on the phosphorus atom is calculated to be around 4.6, slightly higher than in the bulk, zinc blende InP crystals (CN = 4) but consistent with previous observations of coordination preference in other InP clusters …”

Predicting the Stability of Fullerene Allotropes Throughout the Periodic Table

“…The BN, AlN, AlP, AlAs, and GaN materials penalize the five-membered ring structures more than fourmembered rings, whereas all of the other III−V materials show the reverse trend. In previous work, 75 we identified P−P bonds to be favorable in amorphous InP clusters, consistent with a lower five-membered ring penalty than four-membered ring penalty in this material (see Table 2). The more ionic nature and larger size of the substituents in II−VI materials result in a much larger penalty for the nonpolar bond-containing fivemembered ring structures and relatively small penalties for the four-membered rings.…”

Atomistic Modeling of Quantum Dots at Experimentally Relevant Scales Using Charge Equilibration

“…A useful measure of general surface structure is the fraction of surface unsaturation: where N 1– coord , N 2– coord , and N 3– coord are the numbers of singly, doubly, and triply coordinated surfaces atoms within a given QD structure, respectively. Relatively unsaturated surface atoms (singly and doubly coordinated atoms) have been shown (with ab initio methods) to be higher energy in comparison with more saturated (triply coordinated) surface atoms for both CdSe (ab initio DFT) and InP (ab initio high temperature molecular dynamics) nanocrystals. Fraction of surface unsaturation was determined for each structure within the Θ s = −15 l Δ structure-set, and the correlation between f unsat and energy is shown in Figure A.…”