Ab initio calculations and modeling contribute to the discovery of a new way to fabricate striped nanorods
Project: Large Scale Nanostructure Electronic Structure Calculations
PI: Lin-Wang Wang, Lawrence
Berkeley National Laboratory
Senior investigators: Byounghak Lee, Joshua Schrier, Denis Demchenko, Nenad Vukmirovic, Sefa Dag, Lawrence Berkeley National Laboratory
Funding: BES, ASCR
Superlatticed or “striped” nanorods — crystalline materials only a few molecules in thickness and made up of two or more semiconductors — are highly valued for their potential to serve in a variety of nanodevices, including transistors, biochemical sensors, and light-emitting diodes (LEDs). Until now the potential of superlatticed nanorods has been limited by the relatively expensive and exacting process required to make them. That paradigm may be shifting.
A team of researchers with Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley, has found a way to make striped nanorods in a colloid — a suspension of particles in solution. Previously, striped nanorods were made through epitaxial processes, in which the rods were attached to or embedded within a solid medium.
“We have demonstrated the application of strain engineering in a colloidal quantum-dot system by introducing a method that spontaneously creates a regularly spaced arrangement of quantum dots within a colloidal quantum rod,” said chemist Paul Alivisatos, who led this research. “A linear array of quantum dots within a nanorod effectively creates a one-dimensional superlattice, or striped nanorod.”
Alivisatos, an internationally recognized authority on colloidal nanocrystal research, is the Director of the Materials Sciences Division and Associate Laboratory Director for Physical Sciences at Berkeley Lab, and is the Larry and Diane Bock Professor of Nanotechnology at UC Berkeley. Collaborators on this project, which culminated in a paper published in the journal Science1, were Richard Robinson of Berkeley Lab’s Materials Sciences Division (lead author), Denis Demchenko and Lin-Wang Wang of Berkeley Lab’s Computational Research Division; and Bryce Sadtler and Can Erdonmez, of the UC Berkeley Department of Chemistry.
One-dimensional fabrication
Today’s electronics industry is built on two-dimensional semiconductor materials that feature carefully controlled doping and interfaces. Tomorrow’s industry will be built on one-dimensional materials, in which controlled doping and interfaces are achieved through superlatticed structures. Formed from alternating layers of semiconductor materials with wide and narrow band gaps, superlatticed structures, such as striped nanorods, can display not only outstanding electronic properties, but photonic properties as well.
“A target of colloidal nanocrystal research has been to create superlatticed structures while leveraging the advantages of solution-phase fabrication, such as low-cost synthesis and compatibility in disparate environments,” Alivisatos said. “A colloidal approach to making striped nanorods opens up the possibility of using them in biological labeling, and in solution-processed LEDs and solar cells.”
Previous research by Alivisatos and his group had shown that the exchange of cations could be used to vary the proportion of two semiconductors within a single nanocrystal without changing the crystal’s size and shape, so long as the crystal’s minimum dimension exceeded four nanometers. This led the group to investigate the possibility of using a partial exchange of cations between two semiconductors in a colloid to form a superlattice. Working with previously formed cadmium-sulfide (CdS) nanorods, they engineered a cation exchange with free-standing quantum dots of the semiconductor silver sulfide (Ag2S) (Figure 1).
“We found that a linear arrangement of regularly spaced silver sulfide contained within a cadmium-sulfide nanorod forms spontaneously at a cation exchange rate of approximately 36 percent,” said Alivisatos. “The resulting striped nanorods display properties expected of an epitaxially prepared array of silver-sulfide quantum dots separated by confining regions of cadmium sulfide. This includes the ability to emit near-infrared light, which opens up potential applications such as nanometer-scale optoelectronic devices.”
Strain engineering
One of the key difference between quantum dots epitaxially grown on a substrate and freestanding colloidal quantum dots is the presence of strain. The use of temperature, pressure, and other forms of stress to place a strain on material structures that can alter certain properties is called “strain engineering.” This technique is used to enhance the performance of today’s electronic devices, and has recently been used to spatially pattern epitaxially grown striped nanorods.
However, strain engineering in epitaxially produced striped nanorods requires clever tricks, whereas Demchenko and Wang discovered — through ab initio calculations of the interfacial energy and computer modeling of strain energies — that naturally occurring strain in the colloidal process would be the driving force that induced the spontaneous formation of the superlattice structures (Figure 2). This is the first time that the elastic energy has been shown to be responsible for pattern formation in a colloidal nanostructure.
Figure 2. Theoretical modeling and experimental optical characterization. (A) Cubic-cutout representation of cells used for ab initio energy calculations. A distorted monoclinic Ag2S (100) plane connects with the wurtzite CdS (001) plane. (B) Elastic energy of the rod as a function of segment separation (center-to-center). (C) Z-axis strain for the case of two mismatched segments at a center-to-center separation distance of 14.1 nm (top) and 12.1 nm (bottom). The elastic interaction between segments is greatly reduced for separations >12.1 nm. Arrows show the placement of mismatched segments. (D) Visible and (E) near-infrared photoluminescence spectra at l = 400- and 550-nm excitation, respectively. Coupling between the CdS and Ag2S is evident by the complete quenching of the visible photoluminescence (D) in the heterostructures. The shift in near-infrared photoluminescence (E) is due to quantum confinement of the Ag2S.
Demchenko and Wang performed the ab initio calculations of the electronic structure of Ag2S and CdS on Seaborg and Bassi using the Vienna Ab-Initio Simulation Package (VASP) and Parallel Total Energy (PEtot) codes, utilizing the local density approximation and generalized gradient approximation to the density functional theory. These techniques were used to estimate stability of various Ag2S phases, find the optimal geometry for the epitaxial attachment, calculate the formation energies of the CdS–Ag2S interfaces, and calculate the corresponding band alignment. Elastic energies and strains were estimated using the valence force field (VFF) method, which is an atomistic bond stretching and bending model. The researchers received assistance with code installation and testing from Zhengji Zhao, a materials science and chemistry specialist in NERSC’s User Services Group.
“This project has involved tight coordination between computer simulations and experiment, and the results obtained here would not have been possible to achieve without the contributions of our computational scientists, Demchenko and Wang,” Alivisatos said. “It is another clear example where we see that theoretical simulations are not just being used to explain materials growth after the fact, but are now an integral part of the materials design and creation process from the very start.”
Even though the colloidal striped nanorods form spontaneously, Alivisatos said it should be possible to control their superlatticed pattern — hence their properties — by adjusting the length, width, composition, etc., of the original nanocrystals. However, much more work remains to be done before the colloidal method of fabricating striped nanorods can match some of the “spectacular results” that have been obtained from epitaxial fabrication.
“For now, the value of our work lies in the unification of concepts between epitaxial and colloidal fabrication methods,” he said.
