Instructor: Dr. Marinella Sandros 1 Nanochemistry NAN 601 Lecture 13: Synthesis

 Typical reaction for formation of a nanomaterial is best described as almost like setting off a bomb.  You take a reaction in an organic solvent, such as a phosphine, at 330 degrees and inject varied intermediates to produce nanomaterials.  The result is a large distribution in material quality and little control.

 Offers control the nanomaterial growth directly.  No more high-temperature injections; no more toxic solvents.  By use of the microwave cavity, what used to take hours (or even days), we can do in seconds.

 Nucleation is controlling the size of the initial particle that’s formed.  This is going to depend on several things: ◦ How high or how fast can you get the reaction to temperature? ◦ How uniform is the energy field that this reaction is being done in? ◦ Thermal gradients? MICROWAVE CAN CONTROL ALL OF THE ABOVE!!

 Microwave energy has been used for making materials such as: Quantum Dots Zeolites Metal Oxides Metal Organic Frameworks Gold, silver, and other metal nanomaterials Nanocomposite materials Nitride powders Lithium ion materials

CdSe NPs Heating rates for the reactions with CdSe/HDA/IL (IL ) ionic liquid; 12 °C/s) and with TOPO/HDA/CdO (9 °C/s) are enhanced relative to heating pure HDA (3 °C/s) or the reaction CdSe/HDA (4 °C/s). The effect of the ionic liquid and TOPO to rapidly heat the bulk solution can be traced back to their selective ability to couple with the microwaves and efficiently convert electromagnetic energy into heat.

III−V materials show no time, temperature, or power dependent growth in the microwave; however when the typical high-boiling noncoordinating solvent, octadecene (ODE), is replaced with a low boiling solvent, decane, the colloidal size distribution is significantly narrowed and the quantum efficiencies are increased (QY = 15%) presumably due to increased reaction pressures that may anneal out vacancy or defects in the forming nanocrystals.

 Ag 2 Te exhibits high electron mobility and low thermal conductivity, which are desirable for thermoelectric applications.  The optical and optoelectronic properties of monodisperse 3.1 nm Ag 2 Te NCs which uniquely exhibit a sharp optical absorption feature at 1154 nm. This wavelength is of particular interest as it falls in a window of transparency for biological tissue. Chem. Mater., 2011, 23, 4657.

 Ag 2 Te NCs were synthesized via rapid injection of trioctylphosphine-telluride (TOP-Te) into a mixture of silver-dodecanethiol and 4- tert-butyltoluene.  TEM images and the absorption spectra, respectively, of Ag 2 Te NCs as a function of growth time.  After 1 h the growth solution shows a bimodal distribution of large (6–10 nm) and small 3.1 nm NCs (Figure 1b) and a distinct absorption peak arising from the small NCs starts to emerge (B in Figure 1g).1 Chem. Mater., 2011, 23, 4657.

10 min after TOP-Te injection show a wide distribution in NC size and shape (Figure 1a) and a broad absorption feature (A in Figure 1g). After 1 h the growth solution shows a bimodal distribution of large (610 nm) and small 3.1 nm NCs (Figure 1b) and a distinct absorption peak arising from the small NCs starts to emerge (B in Figure 1g). Large NCs transform to a barrel shaped NCs (Figure 1c) with no distinct absorption peaks (C in Figure 1g), whereas a population of small NCs does not change size further with time. After 12 h large NCs grow and precipitate leaving behind small monodisperse NCs in solution (Figure 1d and D in g). After 24 h the majority of the large NCs have precipitated and the sharp absorption features arising from the small NCs (E in Figure 1g) predominate in solution. Figure 1f are mixtures of tabular and prismatic shapes common in monoclinic minerals

 TEM images of the small (3.1 nm), large NCs (6–10 nm), and the precipitates (prismatic shapes larger than 15 nm in width and tabular shape larger than 35 nm in width).  Bulk Ag 2 Te with the stoichiometric excess of Ag shows three polymorphs. The low temperature monoclinic β- phase (optical energy gap 0.67 eV) transforms to the face-centered cubic α-phase at 145 °C and to the body- centered cubic γ-phase at 802 °C.  Small NCs the XRD pattern deviates from the β-phase better matching the simulated mixtures of 67% γ (BCC) and 33% α (FCC) Chem. Mater., 2011, 23, 4657.

 A dramatic blue shift is seen as the temperature is lowered.  For a comparison, silver NCs show negligible shift as a function of temperature (Fig 3c).3  Ag 2 Te NCs also do not show spectral shift when the dielectric constants of the solvent are changed. Chem. Mater., 2011, 23, 4657.

 (a)time-resolved photocurrent with 1 V applied between gold electrodes in the dark and under 750 nm illumination at an intensity of 486 mW/cm 2.  The distinct photoconductive response supports the inherent semiconducting nature of constituent Ag 2 Te NCs in the device.  The current of the device increases from 2.43 to 4.01 nA under illumination in 25.6 s followed by a slow saturating behavior.  (b) Two characteristic time constants t 1 and t 2 represent bulk-like rapid decay process and the slow decay process which is more likely related to surface-traps. I(t) = Aexp(−t/t 1 ) + Bexp(−t/t 2 ) Chem. Mater., 2011, 23, 4657.

 These robust spectral responses could enable the use of Ag 2 Te NCs as: ◦ a temperature reporter ◦ a transducer of infrared energy in biological tissues and chemical environments free of complications stemming from spectral shifts due to the surrounding dielectric constant.

1. What benefits does microwave synthesis offer over conventional synthesis? 2. What applications could use Ag 2 Te nanoparticles?

1) Offers control the nanomaterial growth directly. No more high temperature injections; no more toxic solvents. By use of the microwave cavity, what used to take hours (or even days), we can do in seconds. 2) thermoelectric applications Temperature reporter a transducer of infrared energy in biological tissues and chemical environments free of complications stemming from spectral shifts due to the surrounding dielectric constant