E SC 412 Nanotechnology: Materials, Infrastructure, and Safety Wook Jun Nam

Lecture 17 Outline Colloids and Colloidal Chemistry What is Colloids? Properties of Colloids Examples of Colloids Synthesis of Colloids (e.g. Au, CdSe, Liposome) Vapor-Liquid-Solid (VLS) Approach Nano-elements Integration

Colloids and colloidal chemistry

What is Colloids? The term colloidal refers to a state of subdivision, implying that the molecules or polymolecular particles dispersed in a medium have at least one dimension roughly between 1 nm and 1 um. Comparative size scale http://goldbook.iupac.org/

Properties of Colloids The particles are not molecularly dissolved in the medium (solvent). Colloidal suspensions are like very stable dispersions. The colloidal particles do not aggregate or settle out over time. Colloids can be any combination of the three states of matter, but the most common colloidal mixtures consist of solid (or liquid) particles suspended in a liquid medium.

Properties of Colloids (continued) 2-phase systems: have a dispersed (internal) phase and a continuous (external) phase Dispersed Phase Continuous Phase Large interfacial area between the two phases, due to small dimensions of the dispersed phase Surface effects dominate volume effects

Properties of Colloids (continued) The small size of colloidal particles lends them interesting properties, including: They scatter light (solutions do not) The particles are subjected to Brownian motion The surfaces of particles may become charged, depending on the medium Charged colloidal particles can be moved (separated) by an electric field (e.g., electrophoresis of DNA and proteins)

Examples of Colloids Dispersed Phase Continuous Phase Type Examples Liquid Gas Aerosol Fog, Hairspray Liquid Liquid Emulsion Salad Dressing Liquid Solid Solid Emulsion Pearl, Opal Solid Solid Solid Suspension Pigmented Plastics, Stained Glass Solid Liquid Sol or Paste Ink, Toothpaste Solid Gas Aerosol Inhalers, Smoke Gas Liquid Foam Fire Extinguisher, Soap Suds Gas Solid Solid Foam Pumice, Styrofoam Copyright 2014 by http://www.rsc.org/ Wook Jun Nam chemistryworld/issues/2003/february

Creating Colloids Synthesis of Colloids Condensation Method: pre-cursor molecules coalesce in a controlled manner to form colloidal particles Dispersion Method: larger pieces of material are pulverized until colloidal dimensions are attained Separating Colloids Electrophoresis: external electric field Dialysis: osmotic pressure

Synthesis of Gold Colloids: Chemical The general procedure involves oxidation/reduction reactions in aqueous or nonaqueous sol ns containing soluble or suspended salts. After introduction of reducer, the sol n becomes supersaturated with the product, the precipitate (nano-elements) start to form by nucleation and growth. precursor Reaction precursor Supersaturation of sol n and precipitation of the product in terms of nano-elements solvent reaction

Before the addition of reducer, there are 100% gold ions in sol n. The ordinate of the graph indicates the progress from gold ions to gold atoms as the reducer is added. Immediately after the reducer is added, there is a sharp rise in gold atom content in the solution until this level reaches supersaturation. Aggregation of Au atoms then occurs in the form of nucleation, to form central icosahedral gold cores of 11 atoms at nucleation sites. The formation of nucleation sites, in order to reduce the supersaturation of gold atoms in sol n, occurs extremely quickly. Once this is achieved, the remaining gold atoms in solution continue to bind to the nucleation sites under an energy-reducing gradient until all atoms are removed from sol n. Modified from www.devicelink.com/ivdt/archive/00/03/004.html Au 11 The number of nuclei formed initially determines how many particles finally grow in the sol n. This number, in turn, depends on the amount of reducer added. A large amount of reducer produces a large number of nucleation sites and hence a large number of gold particles. Clearly, the larger the number of nucleation sites for a given amount of gold chloride in solution, the smaller will be the final size of each gold particle. Particle size is thus carefully controlled by the amount of reducer added. If manufacturing conditions are optimized, then all nucleation sites will be formed instantaneously and simultaneously, resulting in all gold particles growing to exactly the same size (monodispersal). This is very difficult to do. Most manufacturing methods do not achieve instantaneous reduction and formation of nucleation sites, resulting in uneven growth and a multidisperse colloid that is virtually unreproducible.

MRS Bulletin / Volume35 / Issue09 / September Copyright 2010, pp 641-641 2014 by Wook Jun Nam Synthesis of Gold Colloids: Chemical Reaction (movie) http://www.youtube.com/watch?v=nqkwm9o1 s-w Surface of Au nanoparticles was coated by dodecanethiols (DDT): Steric repulsion DDT coated Au nanoparticles can be dried into a powder, and be re-dispersed in solvents without size change. DDT coated Au nanoparticles were transferred to the hexane with all reaction byproducts remaining in the aqueous-acetone phase.

Synthesis of Gold Colloids: Laser Ablation (movie) http://www.youtube.com/watch?v=pxaochcl 6kM&feature=related

Synthesis of CdSe Colloids (movie) http://www.youtube.com/watch?v=bnuoym7s u4o&feature=related

Synthesis of Liposome (movie) http://www.youtube.com/watch?v=04sp8tw3 hte&feature=relmfu http://www.youtube.com/watch?v=vgzqde3go4&feature=relmfu

Stabilization of Colloids Remember: An important aspect of colloidal engineering is the suspension of the particle in a medium often water. Colloidal particles can be hydrophobic or hydrophilic. Hydrophilic groups generally contain oxygen and nitrogen. They are water loving. Hydrophobic colloids can be prepared in water only if they are stabilized in some way. The lack of affinity for water will cause them to settle or float. More general terms are lyophilic (likes the external phase) and lyophobic (dislikes the external phase). These terms are used when the medium is not water.

Stabilization of Colloids (continued) How do the particles remain suspended in solution? For such small particles, the forces of Brownian motion exceed the force of gravity, which otherwise would cause the particles to settle out. Particles suspended in water often acquire a negative surface charge. Particles with charged surfaces repel each other at short distances. Steric repulsion can also be used to keep particles from aggregating. This is useful for suspending neutral particles in non-polar continuous phases. Electrostatic Repulsion - - - - - - - - - - - - - - - - - - - - - -- - - - - - Steric Repulsion

Functionalization of Colloids Wolf, Edward L., Introduction, Nanophysics and Nanotechnology: An Introduction to Modern Concepts in Nanoscience, 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Vapor-Liquid-Solid (VLS) Approach

Synthesis of Semiconductor Nanowires Vapor-liquid-solid process (VLS) Chemical vapor deposition (CVD) Pulsed laser ablation Molecular beam epitaxy Thermal evaporation/vapor transport evaporation, mostly used in metal oxide nanowires (e.g. ZnO, SnO 2 )

VLS Si Nanowire Growth: Step 1 Step 1: Metallic nanoparticles are formed on the substrate. These metal nanoparticles act as a catalyst for subsequent steps For SiNW growth, Au is commonly used as catalyst Other metal: Al, Cu, Ag, Fe, Ni, Pt, Zn, Ga, In, Sn

VLS Si Nanowire Growth: Step 1 (continued) Methods to form these nanoparticles on substrate Annealing a thin film at higher temperature to form discrete particles Use colloidal nanoparticles and transfer them onto substrate Use conventional photolithography and metal deposition techniques to define nanoparticles on a substrate

VLS Si Nanowire Growth: Step 2 Step 2: A silicon-containing source gas is introduced over the substrate. SiH 4 or SiCl 4 for SiNW growth by chemical vapor deposition (CVD) process The catalyst causes the gas to decompose to form silicon vapor

VLS Si Nanowire Growth: Step 3 Step 3: The silicon vapor diffuses into gold catalyst nanoparticle to form a gold-silicon eutectic alloy The temperature is maintained above the eutectic temperature so that the catalyst is maintained in the liquid state

VLS Si Nanowire Growth: Step 3 Gold Silicon Phase Diagram (continued)

VLS Si Nanowire Growth: Step 4 Step 4: As the process is continued, More and more silicon diffuses into the alloy. The eutectic becomes supersaturated. Silicon precipitates at the liquid-solid interface forming the nanowire The metal catalyst determines the diameter and location of the grown nanowire, length of the wire is controlled by the growth time, temperature and vapor pressure. Si precipitation at liquid/solid interface Growth

Example of VLS Grown Si Nanowire (J. Appl. Phys. 103, 2008, p. 024304)

(www-drfmc.cea.fr/images/astimg/291_1.jpg) Example of VLS Grown Si Nanowire Gold catalyst Grown SiNW

Example of Template Assisted VLS Grown Si Nanowire commercially available porous membrane remove silver deposit silver grow silicon nanowire deposit gold dissolve porous Membrane then place K.K.Lew, et al, J. Vac. Sci. Technol. B (2002)

( Lew et al., Adv. Mater. Vol.15(24), pp. 2073) Example of Template Assisted VLS Grown Si Nanowire SiGe nanowires grown by VLS technique using a commercially available alumina template.

G. Hong, et. al., Carbon, 50, 2067 (2012) VLS Carbon Nanowire Growth (a) Si nanowire VLS (b) TEM image of Si nanowire (c) Carbon nanotube growth mechanism (d) TEM image of carbon nanotube

VLS Carbon Nanowire Growth: Catalyst G. Hong, et. al., Carbon, 50, 2067 (2012)

Nano-elements Integration

Integration of Nanowire into Devices Grow-and-Place: Grow nanowires Harvest them Place them and make devices Step-and-Grow Grow-in-Place

Examples of Grow-and-Place Example 1: SEMFIB Example 2: Pick and Place Y. Long et. al, Appl. Phys. Lett. (2003) L. Roschier et. al, Appl. Phys. Lett. (1999)

Examples of Grow-and-Place (continued) Example 3 : Microfluidic Assisted Example 4 : Electric Field Induced Y. Huang et. al, Science (2001) Z. Chen et. al, J. Vac. Sci. Technol. B (2004)

Limitations of Grow-and-Place Nanowires may break randomly from the substrate; there is no control on the nanowires length. No control on the number of nanowires. Not suitable for single nanowire electronics.

Step-and-Grow Approach A Unit Polyaniline (PANI) Nanowire Synthesis Process Flow electrical pads on substrate positioning of template separation synthesis W. J. Nam, et. al., the 210th ECS Meeting, Cancun, Mexico, (2006)

Step-and-Grow Approach Multipule Polyaniline (PANI) Nanowire Synthesis Process Flow : the approach can produce many such resistor structures in a substrate W. J. Nam, et. al., the 210th ECS Meeting, Cancun, Mexico, (2006)

http://ma.ecsdl.org/content/ma2010-01/2/124.full.pdf Grow-in-Place Approach Capping layer Gold slug A B Nano channel SiNW/SiNR Capping layer Gold cap Gold caps SiNW 5% SiH4 in H2 C Gold cap D Nano channel Nanochannel confines the catalyst and the Si nanowire (SiNW) is grown inside Growing nanowires follow the shape and the size of the nanochannels.

http://ma.ecsdl.org/content/ma2010-01/2/124.full.pdf Si Nanowire Grown by Grow-in-Place Approach I Gold cap SiNW/SiNR (a) SiNW/SiNR Remaining Au slug Remaini ng Gold SiNW/SiNR Gold caps Gold cap 5% SiH 4 in H 2 For long Au catalyst slug, SiNWs grow at each end of the Au slug Si absorption and diffusion coexist and compete each other. The top ends of Au slug get supersaturated before Si can diffuse to the center. Silicon locally saturates the tips of the gold slugs. Two SiNWs or SiNRs, each grow from Au slug ends, leaving a central slug

Si Nanowire Grown by Grow-in-Place Approach II Gold cap (b) SiNW/SiNR SiNW/SiNR Gold cap Silicon substrate Gold caps 5% SiH 4 in H 2 For short Au catalyst slug, SiNW/Rs grow from the center of Au slug Si absorption and diffusion coexist and compete each other. The whole Au slug gets supersaturated SiNW grows from the center and Au slug is split into two caps One SiNW or SiNR grow with two Au caps, leaving no Au central slug http://ma.ecsdl.org/content/ma2010-01/2/124.full.pdf

Grow-in-Place SINW AMOSFET Accumulated mode MOSFET SiNW after Au etching and cleaning SiNW oxidation and gate contact patterning Source/drain contact deposition. http://ma.ecsdl.org/content/ma2010-01/2/124.full.pdf Source/drain region patterning and oxide removal

Grow-in-Place SINW AMOSFET 1 um FESEM top view image of reaal SiNW AMOSFET On-off current ratios=10 6 http://ma.ecsdl.org/content/ma2010-01/2/124.full.pdf