Nanomaterials by Design

A dream of strong-field chemistry is to realize deterministic control over chemical reactions for directing synthesis. In our lab we have taken a significant leap toward realizing this goal by developing the "nanomaterial by design" program for bottom-up nanomaterials synthesis using tailored femtosecond laser pulses as "photonic reagents." Femtosecond laser pulses can deposit energy into molecules in a way that is far from thermodynamic equilibrium. This non-equilibrium energy deposition provides a route to developing new paradigms for chemistry in which strong reagents can be created instantaneously and in precise doses. Our initial work has focused on studying femtosecond laser-assisted synthesis of gold nanoparticles as a benchmark for the method, as gold the most well known and understood plasmonic material.

Controlling the size of AuNPs in strong laser fields

Simultaneous spatial and temporal focusing (SSTF) of femtosecond laser radiation is used to produce gold nanoparticles from aqueous KAuCl4 solutions in the presence of poly ethyleneglycol (PEG) as a surfactant molecule. Varying the concentration of PEG45 tunes the diameter and size distribution of the Au nanoparticles from 3.9±0.7 nm to 11±2.4 nm (Figure 1). These PEG capped AuNPs are biocompatible, and stable in aqueous solutions for periods of at least several months (J. Phys. Chem. C, 2013, 117, 18719-18727).

Figure 1. Size control of AuNPs in strong laser fields by increasing the concentration of added polymer surfactant (PEG45).

Mechanistic studies on AuNP formation using shaped laser pulses

Formation of AuNPs by irradiation of KAuCl4 solution with femtosecond laser pulses is investigated using SSTF and compared to the results of conventional geometric focusing (GF). Exploring the effects of the reaction conditions including the capping agent, laser power, and laser chirp shows that SSTF always produces smaller particles with fewer irregular structures than GF at any given experimental condition (Figure 2). The difference is primarily ascribed to the intrinsic plasma properties of the two geometries where SSTF produces a plasma that is more homogeneous and spatially symmetric than that of GF, promoting efficient intrinsic mixing of the solution (J. Phys. Chem. C, 2014, 118, 23986-23995).

Figure 2. SSTF irradiation always produces smaller and more uniform AuNPs compared to the conventional geometric focusing (GF).

Laser-assisted synthesis of Au nanotriangles

Synthesis of surfactant-free Au nanoplates is desirable for the development of biocompatible therapeutics/diagnostics. Rapid Δ-function energy deposition by irradiation of aqueous KAuCl4 solution with a 5 second burst of intense shaped laser pulses, followed by slow addition of H2O2, results in selective formation of nanoplates with no additional reagents or surfactant molecule (Figure 3). The primary mechanism of nanotriangle formation is found to be oriented-attachment of laser-generated Au seeds, which self-recrystallize to form crystalline Au nanoplates upon H2O2 addition for 48 hours. The concept of Δ-function energy deposition-induced nucleation followed by in situ chemical reduction represents a general methodology for high-yield production of shape-selected nanomaterials and provides a new means to control the nucleation event in nanoparticle formation (Nano Lett. 2015, 15, 3377-3382).

Figure 3. Oriented-attachment of laser-generated Au seeds produces surfactant-free Au nanoplates.

 

Levis Group, Department of Chemistry, Temple University, Beury Hall 244, 1901 N. 13th Street, Philadelphia, PA 19122    Tel: 215-204-5241     Fax: 215-204-6179
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