Advanced Topics in Nanotechnology - ATOMKIw3.atomki.hu/PhD/tamop/2_felev/20120924/nanotechno... · XPS Analysis of Thiols on Au Effect of Impurities: The thiol used in this study

Advanced Topics in Nanotechnology�

Lecture 6

Gábor L. Hornyák University of Debrecen, Department of Physics

Invited Lecture

GOLD-THIOL MONOLAYERS

2R(CH2)nSH + 2Au → 2R(CH2)nS + 2Au + H2(g)

Copyright: CRC Press 2009

Thiol Monolayers Self-Assembled on Gold • Monolayers were prepared on Au(111) surfaces under ultrahigh vacuum (UHV) conditions.

• Typical thiol is n-octanethiol X-CH3(CH2)7-SH where X is a functional group or the disulfide versions:

[ X-CH3 (CH2)n -S ]2


Substrate made of smooth silicon, glass, mica, plastic, or another metal Interstitial metal layer that provides better adhesion between the substrate and the noble metal Noble metal layer, usually an fcc metal with a (111) crystal face, deposited via thermal evaporation, RF sputtering, electrodeposition, physical vapor deposition, or electroless deposition

Organo-sulfur compound that consists of (a) the head group (the sulfur, which has an affinity for metal), (b) the n-alkane shaft (–CH2)–n, and (c) the tail group (ranging in reactivity from inert to very reactive). The organosulfur compound is applied via solution or vapor deposition methods


Thiols on Au • Thiol typically forms an angle of 30° from the surface normal.

• The S-Au bond interaction is on the order of 188 kJ·mol-1

(compared to 350 kJ·mol-1 for a C—C bond).

• S has an affinity to bind to Au. The spacer chain is made of methylene (–CH2–) groups. The S and the methylene groups are the drivers to make the monolayer.

• usually a 1 mM solution of ethanol. Monolayers form quickly but the “thermodynamic product (well-ordered and completely packed) takes 12 hours to 2 days.

• Any impurities tend to disrupt perfectly ordered layers. Copyright: CRC Press 2009

XPS Analysis of Thiols on Au Effect of Impurities: The thiol used in this study was PEG4-thiol or PEGT or (1-mercapto-11-undecyl)tetra(ethylene glycol). The impurity was Thioacetic acid (or TAA). All monolayers were made from 1 mM solutions in absolute ethanol. XPS Analysis. The data below represents atomic percentage.

PEG/TAA% C 1s O 1s S 2p Au 4f

100/0% 65 20 1 14

99/1% 56 17 3 24

90/10% 46 13 4 37

Source: M. Boeckl and D. Graham; Material Matters, Vol. 1. No. 2.. 3-19 (2006)

There is an increase in the atomic percentage of gold (decrease in signal attenuation)… the addition of even 1% TAA causes a decrease in the thickness of the monolayer.


Source: Image redrawn with permission from Giacinto Scoles, Department of Chemistry, Princeton University.

A temperature programmed desorption profile of hexanethiol layer deposited at 208°C after 2460 seconds. The lower peak represents physisorbed component of the monolayer. The higher energy peak represents the chemisorbed component. Chain length only affected the position of the lower energy peak.

Nano Phenomenon #123


Even small impurities can affect the thickness of a SAM made of thiol-Au

interface.

How does this compare to the assertion that nanomaterials are more pure than

bulk materials claimed earlier.

ORGANOSILANES


Organosilanes The chemisorption of alkysilanes to an activated (hydroxylated) silicon surface results in robust monolayers for a couple of reasons: (1) the covalent attachment to the surface and (2) lateral cross-linking with other silica groups as the monolayer grows.

The cross-linking is accomplished through a –Si–O–Si– covalent network. The reaction of the silyl ester groups proceeds in a manner similar to a standard esterification reaction between a carboxylic ester and a hydroxyl group but with more facility.


These kinds of reactions, like the one for acetic acid + ethanol in the presence of an acid catalyst, have a relatively low equilibrium constant and therefore are reversible.

Organosilanes (Continued)

€

KEq =R1COOCR2[ ] H2O[ ]R1COOH[ ] R2COH[ ]

= 3.38


Counterclockwise from top left. Hydroxyl-terminated silane esters are added to an activated silicon surface to form the first monolayer. In the next step, 3-aminopropyl-dimethyl- ethoxysilane is reacted with the first monomer by an ester substitution reaction. The red dot represents the amine group. A thin layer of styrene monomers is applied by spin-coating onto the surface of the bilayer. The system is exposed to ultraviolet radiation, thereby initiating cross-linking of the polymer to form a thin robust coating.


SYNTHESIS and MODIFICATION of ZERO-DIMENSIONAL MATERIALS


Au-55 Clusters. Synthesis of uniformly small particles proceeds best from the bottom up. One of the most popular methods to form metal clusters and colloids is through the reduction of metal cations. Common reducing agents include the base complements of organic acids such as sodium citrate, reducing alcohols, Na2S, borohydrides [B2H6], sodium borohydride [NaBH4], and even hydrogen gas. We understand why small clusters wish to agglomerate to form larger clusters via Ostwald ripening.

In order to fabricate metal clusters of pre- determined size (and we assume that nanoscale clusters are desired rather than colloids), special steps need to be taken. First of all, addition of a potential ligand species to the reaction mixture is required. The ligand serves to bind reduced metals and thereby modulate the growth of the embryonic clusters.

Depending on relative concentrations of reactants (the metal salt, reducing agent and ligand), growth of clusters that are monodisperse with desired dimensions is possible.


Reduction of metal cation → agglomeration prevention → ligand stabilization or ligand exchange → extraction from solvent → further surface modification

General Synthesis Reaction

Source: Prof. Günter Schmid, Institut für Anorganische Chemie, Universität Essen

Reaction scheme of formation of Au55 ligand-stabilized cluster is depicted. At the top left, a solution containing dissolved metal cations is shown.

The cations are converted into gold atoms after addition of a reducing agent like citrate. Once formed, the atoms nucleate and grow into aggregates that eventually stop at the cluster phase (depending on the reaction conditions).

Ligands attach to the vertices of the Au55 cluster; there are 12 vertices in this structure. Not shown are the counter-anions, the chloride atoms of Au55[P(Ph)3]12Cl6.

Au-55 Synthesis Reaction

HAuCl4 + (C6H5O 7)Na3 → Auo + oxidized products


AuCl[P(C6H5)3] + B2H6 → Au55[P(C6H5)3]12Cl6 + H3B–P(C6H5)3

Gaseous diborane is passed through a warm 150-mL solution of benzene containing 3.94 g of AuCl[P(C6H5)3]. Diborane is the best reducing agent, but it also acts as a Lewis acid that binds phosphines. This process limits the amount of free ligand available at any time during the course of the reaction. Excess ligand concentration leads to smaller complexes and clusters, an undesirable outcome.

The temperature is raised to 50°C. After 40 min, the colorless solution turns dark brown. Upon cooling, a dark precipitate settles to the bottom of a now colorless solution. The precipitate is filtered, rinsed with dichloromethane, and filtered again through Celite to remove unwanted solids (e.g., colloidal gold).

The product, Au55[P(C6H5)3]12Cl6, is re-precipitated slowly in 250 mL pentane to ensure that phosphine ligands saturate the Au55 cluster. The overall yield of the process is 29%. The ligand cluster is 2.1 nm in diameter (the gold cluster is actually 1.4 nm).


Ligand Exchange Reactions

The triphenylphosphine ligands of the cluster are labile and undergo ligand exchange readily in phase transfer reactions:

Au55[P(Ph)3]12Cl6 (in dichloromethane) +

P(Ph)2(C6H4SO3)Na (in water)

→ Au55[P(Ph)2(C6H4SO3)Na]12Cl6

→ Au55[P(Ph)2(C6H4SO3)]12(Cl6)12– + 12Na+


Au55[P(Ph)2(C6H4SO3)Na]12Cl6 + 12 T8–OSS–SH → Au55[T8–OSS–SH]12Cl6 + 12 PPh3

Synthesis of T8–OSS–SH is shown. The ultimate Au–T8–OSS–SH complex is soluble in pentane, which is not possible for the triphenylphosphine derivative that is soluble only in dichloromethane type solvents. In addition to providing an impressive visual, the utility of these ligand-stabilized clusters in two-dimensional arrays shows promise. The ligand-stabilized cluster increases in dimension from 2.1 to 4.4 nm upon exchange of the ligands. It is soluble in pentane as well as dichloromethane and is considerably more stable (due to its strong Au–S covalent bond) than its phosphine- substituted counterpart.

A thiol derivative of an organo-silsesquioxanes (OSS) is one of the most intriguing ligands synthesized by G. Schmid et al. of the University of Essen in Germany [27]. Its synthesis is shown in Figure 12.15. Several changes in chemical reactivity and physical properties occur in substituted Au55 clusters. The

Source: Prof. Günter Schmid, Institut für Anorganische Chemie, Universität Essen.

Ligand exchange rxns with Au55[P(Ph)3]12Cl6 are graphically depicted. Both the closododeca-borate and the OSS ligands are able to displace the triphenyl-phosphine. The utility of the thiol group in binding gold-based materials is demonstrated. [Au55(B12H11SH)12]2– is soluble in water and [Au55(T8–OSS–SH)12] is soluble in pentane. Au55[P(Ph)3]12Cl6 is soluble in Ch2Cl2: three ligand-stabilized clusters with three different solubility properties.


A colorized TEM image of an array of ligand-stabilized Au55[P(Ph)3]12Cl6 clusters.

Self-assembly in this case resulted in a square planar array of ligand-stabilized gold nanoparticles.


Higher Order

Structures Ligands interlinking clusters do so by several mechanisms ranging from covalent bonding (–R–S–R–) or disulfide (–S–R–S–S–R–S–) linkages that are covalent to aryl group interdigitation, and scores of other intermolecular interactions.


SEMICONDUCTOR QUANTUM DOTS


There are several challenges facing the synthesis of quantum dots: (1) synthesis of dots that are monodisperse with regard to size, shape, and orientation and (2) fabrication of those dots into a uniform array. The inverse micelle-emulsion method of fabrication of semiconductor quantum dots has become increasingly popular.


Formation of CdSe semiconductor quantum dots by an emulsion procedure. A very versatile surfactant known as NaAOT (aerosol-OT) is often used in these reactions. Reactants are sequestered within the micellar structures and react when mixed.

Hexagonal array of semiconductor quantum dots decorates a surface. Differing surface arrangements can be achieved depending on the size of the dot and its ligands (or if there are mixed ligand systems).


SOL GEL SYNTHESIS


Sol Gel Reaction

One of the most celebrated workhorses of sol–gel synthesis is tetraethylorthosilicate (TEOS). In water (or alcohol–water mixtures), TEOS undergoes dimerization and eventual polymerization. The main processes involved in sol–gel synthesis techniques are hydrolysis and condensation.



Bottom-up synthesis of gold-55 clusters, semiconductor quantum dots and the

amazing array of silica-based structures lend a vast arsenal of nanomaterials to

the forefront of the science.


SYNTHESIS and MODIFICATION of ONE-DIMENSIONAL NANOMATERIALS


Bottom-Up Nanowire Synthesis



Nanowires can be made biologically— DNA is in essence a nanowire and is used as such in

technological applications


Synthesis of Pt Nanowires

Block copolymers were first assembled to form within “lithographically defined” channels on a silicon substrate. The block copolymers, 40 nm across, were introduced into the channels and self-assembled into long cylindrical molds after heat treatment at 230°C.

Aqueous solutions of metal ions then were made to infiltrate the lines. The polymer was removed by application of oxygen plasma etching leaving the metal nano wire. The wires were 10 nm in diameter and over 50 µm in length. Buriak’s group fabricated 25 parallel Pt wires

Source: J. Chai, D. Wang, X. Fang, and J. M. Buriak, Assembly of aligned linear metallic patterns on silicon,

Nature Nanotechnology, 2, 500–506 (2007).

Synthesis of Fe-Pt Nanorods

Brown University researchers have found a straightforward way to synthesize Fe–Pt nanorods and nanowires by a technique that offers diameter, length, and composition control. In their solution-based process, varying the ratio of the solvent (octadecene) and surfactant (oleylamine), nanowires ranging from 20 to 200 nm have been produced.

Adding more surfactant resulted in longer nanowires, whereas more solvent yielded shorter rods. A 1:1 ratio produced 20-nm rods.

Source: C. Wang, Y. Hou, J. Kim, and S. Sun, A general strategy for synthesizing FePt nanowires and nanorods, Angewandte Chemie International Edition, 46, 6333–6335 (2007).

Solution Synthesis of Ge Nanowires

Solution synthesis of germanium nanowires was accomplished at 1 atm and less than 400°C. The precursor molecule germanium 2,6,-dibutylphenoxide [Ge(DBP)2] was dissolved in oleylamine surfactant and then immediately transferred into a 1-octadecene solution at 300°C.

The resulting nanowires were single crystal of cubic phase and coated with oleylamine. The length of the wires ranged from 0.1 to 10 µm. A self-seeding mechanism and an aggregation mechanism are proposed to explain the growth process.

Source: H. Gerung, T. J. Boyle, L. J. Tribby, S. D. Bunge, C. J. Brinker, and S. M. Han, Solution synthesis of germanium nanowires using a Ge2+ alkoxide precursor, Journal of the American Chemical Society, 128, 5244–5250 (2006).

DNA Template Synthesis of Organic Nanowires

Organic nanowires are also produced by bottom-up methods. P. Nickels et al. showed that polyaniline (PAN) nanowires can be grown within a DNA template.

Polyaniline was grown on DNA templates immobilized on a surface by (1) oxidative polymerization with ammonium persulfate; (2)enzymatic oxidation with hydrogen peroxide (horseradish peroxidase); or (3) photo-oxidation using a Ru complex as the photo-oxidant.

Source: P. Nickels, W. U. Dittmer, S. Beyer, J. P. Kotthaus, and F. C. Simmel, Polyaniline nanowire synthesis templated by DNA, Nanotechnology, 15, 1524–1529 (2004).

Source: M. Ye, H. Zhong, W. Zheng, R. Li, and Y. Li, Ultralong cadmium hydroxide nanowires: Synthesis,

characterization, and transformation in CdO nanostrands, Langmuir, 23, 9064–9068 (2007).

Ultra-Long Cadmium Hydroxide Nanowire Synthesis

Ultralong Cd(OH)2 nanowires were synthesized according to a hydrothermal methods from Cd(CH3COO)2·2H2O and CH12N4 aqueous solution at 95°C without the use of templates.

Wires with aspect ratio of several thousands were formed. The formation mechanism is attributed to the oriented attachment of small particles. Transformation into semiconductor material occurred by calcination at 350°C for 3 h.

ELECTRODEPOSITION of NANOWIRES


DC electrodeposition of metals into templates is an effective method to form nanowires that are monodisperse with regard to diameter. The length of the nanowire is dependent on the duration of the electrodeposition process. Both metal and semiconductor nanowires can be formed by electrodeposition.

An effective template is porous anodically formed alumina. The pore channels are parallel and the diameter is determined by the applied voltage during anodizing. Therefore, there is built-in tunability with regard to diameter and length of the wires (e.g. by control of electroplating time).

Electrodeposition of Nanowires


Example 41.1 Q: Electrodeposition in porous alumina membranes. Calculate the number of coulombs required to form nanowires of Ag that are lcyl = 300 nm in length. The density of Ag is ρ = 10.49 g·cm-3, its atomic mass is MAg = 107.868 g·mol-1, the Faraday constant is F = 96,485 C·mol-1; area of the alumina is A = 3.14 cm2 and porosity ε = 30%

€

lcyl =CMAg

FAερ

x107

C = lcylFAερ

MAg ⋅107

= 300nm 96,485C ⋅mol−1 ⋅ 3.14cm2 ⋅ 0.30 ⋅10.49g ⋅ cm−3

107.868g ⋅mol−1 ⋅107 nm ⋅ cm−1

= 0.265 C

Example 41.1 (Continued)

From this problem and solution, we are not able to calculate the diameter of the Ag cylinders. According to this problem, the final form of the silver could be in the shape of a disc that is d = 0.942 cm and h = 300 nm.

If the pores have diameter of 50 nm, then the aspect ratio is simply:


€

Aspect Ratio =lcyl

dpores

= 300nm50nm

= 6

Example 41.2 Q: How many 50 nm diameter pores are to be found in a 3.14 cm2 area of alumina (e.g. what is the pore density?)

In this case, we can assume 100% packing of the 30% porous space available for pore channels because in reality, the pores are separated in the alumina so we use up all available area.

€

Number of Pore Channels, N pores = Aεπ d 2( )2

= 3.14cm2 ⋅ 0.30

π 25nm( )2⋅ cm /107 nm( )2

= 1.51x1011

Pore Density =N pores

A= 4.81x1010 pores ⋅ cm2

In reality, plating metals through narrow pore channels is affected by the length and width of the pore channel by increasing electrical resistance. This factor was not considered in the solution of this problem.


Electroless Methods Electroless deposition plating is the autocatalytic, continuous, and chemical reduction of metal ions onto a substrate. The substrate does not have to be conducting.

The required components of an electroless plating solution include an aqueous solution of metal ions of interest, a catalyst (usually a minute amount of the metal in reduced form), reducing agents, complexing agents (help monitor pH and control free metal ion concentration), and solution stabilizers (catalytic inhibitors to prevent out of control reactions that result in poorly structured products) operating in a specific range of pH, metal ion concentration, and temperature.

No electrical current is required in this form of deposition—hence the name electroless. Whether forming wires or plating on a two-dimensional surface, the final metal layer or wire conforms to the contour of the template or the surface.


An Example of the Electroless Deposition of Copper:

Cu2+ + EDTA4– → Cu(EDTA)2–

2H2C=O + 4OH– → 2HCO2– + 2H2O + H2 + 2e–

Cu2+ + 2e– → Cuo

Cu(EDTA)2– + 2H2C=O + 4OH– → Cuo + 2H2O + H2 + 2HCO2– + EDTA4–


CHEMICAL MODIFICATION of CARBON NANOTUBES


Carbon Nanotubes Revisited SWNTS are all surface, both inside and outside. Every single atom is a surface atom. There are no reactive dangling bonds. Carbon nanotubes are supermolecules that are held together by some of the strongest bonds known to nature, the carbon–carbon bond; there are no dangling bonds since the ends are technically capped by hemispherical fullerenes.

One would expect that SWNTs in particular would be extremely reactive due to highly strained bonds. However, even though carbon nanotubes have no volume atoms, they are relatively inert—not because of thermodynamic considerations but rather because they have no chemical functional groups.


Carbon Nanotubes (Continued) It is possible, however, to disturb the conjugated system of CNTs by attack of strong acids such as HNO3 and/or H2SO4 to form carboxylate appendages. Applications of heat and/or ultra-sonication along with acid treatment result in the formation of dangling bonds that render the tubes reactive. The apparent stability of SWNTs is demonstrated by the low degree of functionalization, even under such harsh conditions.

Under the right conditions, the intrinsic curvature of chemical bonds (thermodynamic properties) making up nanotubes provides the basis for chemical modification. Modification of the outer surface of SWNTs reduces intertube attraction because the perfect Van der Waals interactions no longer overlap exactly. Chemical functionalization also enhances interaction between the SWNT or MWNT and a potential polymer matrix material.


Covalent Modification of Carbon Nanotubes Covalent chemical modification involves breaking old covalent bonds and forming new covalent bonds. Processes such as oxidation and fluorination are examples of covalent modification.

Covalent modification perturbs the structure of the nanotube and introduces defects into the carbon nanotube that disrupt the aromatic structure. Defects alter the mechanical strength, electrical conductivity, and thermal conductivity of the nanotube.

Depending on the chemical nature of the modifying group, the nanotube is capable of linking to its host polymeric matrix. Such cross-linking, always an important aspect of polymer design, serves to strengthen the polymer as a whole.


Functionalization of SWNTs to generate –CO2H decorations takes place in a rather severe chemical environment. SWNTs are first refluxed in 2–3 M nitric acid for 12–48 h at 115°C. Chemical modification can cease at this point if carboxylic acid groups are the desired decoration. The product is washed and dried.

If further modification is required, carboxylated SWNTs suspended in dimethylformamide (DMF) are exposed to thionylchloride (SOCl2). This process transforms the –CO2H group into a –COCl chloric acid derivative. This transformation renders the SWNTs reactive to further modification by long-chain amines such as dodecylamine (C12H27N, a.k.a. f12) and octadecylamine (C18H39N, a.k.a. f18). A surfactant, NaDDBS (sodium dodecylbenzenesulfonic acid), was used to disperse the mixtures.

R. Haggenmueller, F. Du, J. E. Fischer, and K. I. Winey, Interfacial in situ polymerization of single wall carbon nanotube/nylon 6,6 nanocomposites, Polymer, 47, 2381–2388 (2006).

Di-Epoxide Terminated SWNTs In another application, SWNTs were first carboxylated in a sulfuric-nitric acid (3:1%-vol) and then sonicated for 3 h at ambient temperature [74].

Following dilution with distilled water (1:5), the mixture was forced through a PTFE filter (10-µm pore size) and washed until no residual acid remained. The success of carboxylation was verified by FTIR [74]. Di-epoxide-terminated molecules were then attached to the CO2H-modified SWNTs by adding EPON 828 and subsequent sonication for 1 h. KOH was added as a catalyst at 70°C.

After washing and filtering, the derivatized SWNTs were analyzed by TGA (to quantify the amount of attached molecules) and FTIR (to verify the presence of epoxides).

A. Eitan, K. Jiang, D. Dukes, R. Andrews, and L. S. Schadler, Surface modification of multi-walled carbon nanotubes: Toward tailoring of the interface in polymer composites, Chemistry of Materials, 15, 3198–3201 (2003).c

A. Eitan, K. Jiang, D. Dukes, R. Andrews, and L. S. Schadler, Surface modification of multi-walled carbon nanotubes: Toward tailoring of the interface in polymer composites, Chemistry of Materials, 15, 3198–3201 (2003).

Covalent surface modification involves making bonds between the carbon framework of the nanotube and the modifying moiety. A multiwalled carbon nanotube underwent carboxylation by application of a sulfuric-nitric acid (3:1) mixture while under sonication.

Following rinsing and purification, the carboxylated MWNTs were reacted with di-epoxide- terminated molecules (Epon 828) in the presence of KOH catalyst.

The terminal epoxide is available to undergo further chemical reactions. If the derivatized nanotubes are immersed into a bulk epoxy polymer matrix, strong links with the bulk polymeric material are formed.

Such linking allows for efficient load transfer from the bulk polymer to the MWNTs. In this way, the MWNTs are able to enhance the mechanical properties of the polymer composite material.

Di-Epoxide Terminated SWNTs (Continued)

A. Eitan, K. Jiang, D. Dukes, R. Andrews, and L. S. Schadler, Surface modification of multi-walled carbon nanotubes: Toward tailoring of the interface in polymer composites, Chemistry of Materials, 15, 3198–3201 (2003).

Left: Chemical modification of SWNT: (1) oxidation with strong acids, reflux, temperature > 100°C, (2) chemical reaction with acetyl amide to form reactive terminal group, (3) and (4) addition of free radical initiator and styrene monomer to initiate polymerization process, (5) propagation and subsequent termination of the polymerization process to form a polystyrene.

Right: Chemically modified SWNTs aligned in an electric field are cross-linked by polystyrene chains. Anisotropic mechanical properties result from such alignment [43].

E. V. Barrera, M. L. Shofner, and E. L. Corral, Applications: Composites, In Carbon nanotubes: Science and applications, M. Meyyappan, ed., pp. 253–275, CRC Press, Boca Raton, FL (2005).

An illustration of the partial hydrogenation of single-wall carbon nanotubes. Hydrogenation proceeds by exposing the nanotubes to atomic hydrogen and the hydrogen radicals add to the double bond of the SWNT. The hydrogenation process is reversible at 500–600°C.

Others claim reversibility at lower temperatures. Smaller SWNTs are less stable against hydrogenation than larger tubes. Following hydrogenation, SWNTs undergo structural deformations, reduced electrical conductance, and increased semiconductor behavior. The diameter of SWNTs increased from 1.0 to 1.3 nm or from 1.8 to 2.1 nm, depending on the starting material, following hydrogenation.

Hydrogen Storage

A. Nikitin, H. Ogasawara, D. Mann, R. Denecke, Z. Zhang, H. Dai, K. Cho, and A. Nilsson, Hydrogenation of Singlewalled Carbon Nanotubes, Physical Review Letters, 95, 225507 (2005).

Non-Covalent Modification of CNTs Noncovalent types of modification are governed by thermodynamic parameters. This form of chemical functionalization is accomplished by means of intermolecular attractions between an adduct molecule and the nanotube.

In this way the backbone structure of the nanotube is not altered and properties such as mechanical strength, electrical conductivity, and thermal conductivity are compromised, relatively speaking.

The problems of solubility, dispersion, and separation of native tubes are addressed by noncovalent chemical functionalization. As a result, subsequent processing of nanotubes with the desired outcome is more likely.


The planar pyrene molecule reacts with nanotube surfaces by π-π interactions—intermolecular interactions that are not as strong as covalent bonds. They serve as nonspecific anchoring constituents. Reactive groups attached to the pyrene enable further chemistry to take place.

A. B. Artyukhin, O. Bakajin, P. Stroeve, and A. Noy, Layer-by-layer electrostatic self-assembly of polyelectrolyte nanoshells on individual carbon nanotube templates, Langmuir, 20, 1442–1448 (2004).

Noy et al. accomplished layer-by-layer electrostatic assembly of polyelectrolyte nanoshells on individual suspended carbon nanotubes.

The goal of the study was to develop a robust strategy for noncovalent modification. The purpose of the nanotube was to serve as a bridge and template.

The first step involved exposing the CNTs to pyrene derivative (1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt and 1-pyrenepropylamine hydrochloride or PyrNH3) followed by layer-by-layer deposition of polyelectrolyte macro-ions.

The polymers used in the study were poly(diallyldimethylammonium chloride (PDDA) and polystyrene sulfonate sodium salt (PSS). The entire process was based on stepwise self-assembly among the constituents.

A generic version of carbon nanotube decorated with pyrene groups is shown in the previous slide.

A. B. Artyukhin, O. Bakajin, P. Stroeve, and A. Noy, Layer-by-layer electrostatic self-assembly of polyelectrolyte nanoshells on individual carbon nanotube templates, Langmuir, 20, 1442–1448 (2004).

More Non-Covalent Modification of CNTs R. E. Smalley et al. reported in 2001 about the derivatization of SWNTs by a process called polymer wrapping. The process entailed association of the SWNT with water-soluble linear polymers such as polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS).

The process was successful in disrupting the hydrophobicity of the nanotube as well as the intertube attraction to form bundles. Unwrapping of the nanotubes occurred by changing the supporting solvent.

M. J. O’Connell, P. Boul, L. M. Ericson, C. Huffman, Y. Wang, E. Haroz, C. Kuper, J. Tour, K. D. Ausman, and R. E. Smalley, Reversible water-solubilization of singlewalled carbon nanotubes by polymer wrapping,

Chemical Physics Letters, 342, 265–271 (2001).

SWNT–polycarbonate component is depicted. The sliding occurs at the interface between the two materials of the nanocomposite.

J. Suhr, W. Zhang, P. Ajayan, and N. Koratkar, Nano Letters, 6, 219–223 (2006).

Polymer Wrapping


Carbon nanotube fillers via chemical modification enhance

mechanical properties of nanocomposite materials.


MODIFICATION of THIN FILMS


GW’s General Functionalization Rules


First: Functionalized alkane–thiols will form ordered SAMs with the terminal capping group pointed away from the substrate;

Second: The above applies best for n-alkane thiols with small cap groups (e.g., –OH, –CN, –CO2H, etc.);

Third: The first rule does not take into account steric, molecular free volume, lateral segregation (multi-component assemblies), and neighbor and surface interactions.

The development of guidelines for designing SAMs that predict the organization and composition of the monolayer and incorporate all these elements remains to be worked out.

Supramolecular Design— More Sophisticated But….

“……… it is exceedingly more difficult to predict the physical and chemical properties of SAMs and we must rely on semi-empirical studies for at least a little while longer. However, modification of SAMs occurs by both covalent and intermolecular mechanisms.

There is no limit to the variety of derivatization that can occur to receptive monolayers. Monolayer surfaces, especially, require modification if future design involves biological or biochemical applications.”

Source: J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Buzzo, and G. M. Whitesides, Self- assembled monolayers of thiolates on metals as a form of nanotechnology, Chemical Reviews, 105, 1103–1169 (2005).

The chemistry of covalent chemical modification is influenced strongly by nucleophilic substitution reactions—e.g., basic organic chemistry. Bromine is a good leaving group and is used in many chemical modification procedures in nucleophilic substitution reactions.

A basic SN2 nucleophilic substitution is shown. Bromine is a good leaving group in the presence of nucleophilic moieties. In SAM systems, due consideration is always allotted to designing the chemistry of the end group.

The strength of leaving groups depends on several parameters: size of the atom, electronegativity, electric charge, steric properties, and the solvent, to name a few. For example, F is not a good leaving group, whereas Br is a good leaving group. Inspection of some of the most basic SAM chemical procedures reveals that –OH is a good nucleophile and that –Cl and various alkoxides serve as good leaving groups. The conjugate bases of strong acids are good leaving groups. The sulfonate ion is an excellent leaving group and is employed in many kinds of substitution-chemical modification reactions of monolayer end groups

The solvent, just like in supramolecular chemical reactions, is also an important consideration in chemical modification reactions and plays an important role in the rate of displacement reactions.

For example, the displacement of iodine in methyl iodide by an acetate anion takes place 10 million times faster in dimethylformamide (DMF) than it does in methanol [64]. If an SN2 nucleophilic displacement reaction involves ions, then polar solvents are required.

Hydroxylic solvents, for example, promote hydrogen bonding of reactants and products. Hydroxylic solvents include water and alcohols. Polar aprotic solvents include acetonitrile, DMF, dimethyl sulfoxide, and acetone. Nonpolar solvents consist of the family of low molecular weight alkanes, toluene, benzene, and other familiar solvents.

The free energy of activation ΔG‡ of reaction of nucleophiles with methyl iodide varies significantly from solvent to solvent. For example, the free energy barrier of CN– + CH3I substitution is 59 kJ·mol–1 in DMF but as high as 92 kJ·mol–1 in methanol.


More like a non-nano-phenomenon. Standard organic

and metallorganic chemical techniques are used to modify 2-D

nanomaterials to produce films with superior properties.


TEMPLATE SYNTHESIS


Chemistry in Confined Spaces Molecularly confined spaces are defined loosely as three-dimensional spaces less than 1 nm in diameter. In essence, we are talking about nanosized chemical reactors.

EFFECTS:

• Exhibit accelerated rates and alterations in product selectivity—both kinetic effects

• Chirality of cations can be controlled in confined spaces \

• Surface-confined metal species are amenable to small-ligand chemistry of alkoxide metal centers



Confined spaces allow for kinetic alteration of kinetic properties—

very much like the active site pocket in a enzyme— yes, a

nanomaterial!


Template Synthesis in General Template synthesis is the fabrication of nanomaterials within porous materials and interstitial spaces. We focus primarily on porous materials formed by inorganic and organic–inorganic hybrid materials.

ALTHOUGH non-porous materials can also serve as templates. The example of K+ earlier is an example or of nanowires that are able to direct synthesis.

According to the IUPAC definition (as before), there exist three classifications of porous materials: macroporous (d > 50 nm), mesoporous (2 < d < 50 nm), and microporous (d < 2 nm)


The conversion of the nitro group on the benzene ring into amines is accomplished with an electron beam. By this way, specific sectors are created on an otherwise homogenous monolayer coating. Further chemical modification is facilitated with the amine terminus of the molecule.

Such patterning is possible by direct writing with the electron beam. Adjustments in pH convert the amine into an ammonium moiety that is receptive to electro static interaction with citrate-stabilized gold colloids that have negative charges.

Macroscopic Template Materials The most popular method of forming macroscopic porous materials is from templates consisting of close-packed spherical polymer beads (a.k.a. latex beads) by an infiltration-sintering process. The process to form them is as follows:

• Synthesis of polymeric latex beads by emulsion polymerization;

• Packing of the beads into arrays by several techniques, such as evaporation, filtration, settling, and dip-coating;

• Infiltration of the interstitial volume with silicate, zirconia, titania, or aluminate sol–gel precursor solutions by capillary action or vacuum induction of the solution or by infiltration by solutions containing metal–anion components; and

• Treatment at high temperature (~500°C or more) to remove polymeric components and simultaneously sinter to form ceramic matrix. Filling scaffolds by electrochemical deposition and chemical vapor deposition are also effective methods of creating porous materials.


Mesoporous Template Materials The most popular method of forming macroscopic porous materials is from templates consisting of close-packed spherical polymer beads (a.k.a. latex beads) by an infiltration-sintering process. The process to form them is as follows:

• Synthesis of polymeric latex beads by emulsion polymerization;

• Packing of the beads into arrays by several techniques, such as evaporation, filtration, settling, and dip-coating;

• Infiltration of the interstitial volume with silicate, zirconia, titania, or aluminate sol–gel precursor solutions by capillary action or vacuum induction of the solution or by infiltration by solutions containing metal–anion components; and

• Treatment at high temperature (~500°C or more) to remove polymeric components and simultaneously sinter to form ceramic matrix. Filling scaffolds by electrochemical deposition and chemical vapor deposition are also effective methods of creating porous materials.


Macroscopic and mesoscopic materials are compared. Mesoscopic materials have pores that are “officially” under 50 nm.

Macroporous template synthesis: porous ceramic (titania or silicate) ordered arrays formed from spherical latex templates.

The structure on the bottom is interconnected with channels similar to those found in zeolites. The size of the spheres can be several hundred nanometers.

Anodic Alumina Revisited


We have discussed this material in some detail in the text and in some of the lectures already. It is a premier template material that is easy to fabricate. Its utility is huge (just check to find how many research papers are written about the template).

We go into some detail about its fabrication and then some about its utility.

PAA or AAO are some of the acronyms applied to this amazing material.

Anodize Parameters Anodizing aluminum is an easy way to make nanoporous materials. Placing a plate of high purity aluminum in one of the acids below results in parallel pore channels of various diameter.

Anodically formed porous structures have pores in the range of 20–50 nm or larger. The porous layer is formed on top of the barrier layer. The size of the scallop depends on the applied voltage. By double-anodize method, a perfect honeycombed structure can be formed. During the first anodization (e.g., 24 h), equilibrium positioning of the pore channels is achieved. All the pore channels have the same diameter with the same barrier layer thickness. After dissolution of the primary layer, re-anodization produces a new film that sprouts from the preexisting scalloped structure—already with the equilibrium size and distribution.

There are several ways to remove the AAO membrane from its aluminum metal substrate. The most efficient means is to apply the procedure of stepwise voltage reduction. We know that pore size is a function of applied potential. If we were to reduce the potential stepwise, then a network of smaller and smaller pore channels would form in the barrier layer.

The barrier layer is a highly permeable ionic membrane during anodizing. If an instantaneous 10% voltage reduction is applied, the current undergoes depletion and then recovers to the new steady- state condition dictated by the newly applied potential. Essentially, a new set of pores, proportionally smaller than the parent pore, are formed in the barrier layer. This process is repeated until the pore diameter is reduced along with a concomitant thinning of the barrier layer.

Membrane Detachment

Each step represents a mini-anodize process, with new voltage, current and new pore diameter.

The process is continued until a network of really small pores infiltrates the barrier layer.

Once achieved, the plate with the voltage-reduced membrane is placed in an acid detachment solution.

Membrane Dissolution When 1 V is achieved (pore diameter ≈1.5 nm, barrier layer thickness ≈1 nm), the process is stopped and the aluminum and oxide still attached to the metal plate is placed in an acid detachment solution (25% H2SO4 for sulfuric

2Al2O3 + 12H+ → 4Al3+ + 6H2O + 3H2(g)

2Alo + 6H+ → 2Al3+ + 3H2(g)

Hydrogen gas is produced between the alumina film and the metal substrate.

Chemical modification of the surfaces of the pore channels is quite straightforward. The alumina surface is a natural source of hydroxyl groups that are able to act as nucleophiles in ester exchange reactions.

Alkylphosphonic acids and diethlybutylphosphonate are used to change the wetting character of the surface from hydrophilic to hydro- phobic. Silanes, carboxylates, phosphonates, and numerous other species are able to react with the pore channel walls of AAO membranes.

Chemical Derivatization of Anodic Surfaces

Chemical modification of the pore walls of porous materials affects both physical and chemical properties. Additional layers (or just the presence of one layer if the pore diameter is small enough) constrict the orifice and thereby enhance the size-exclusion ability of the material. Chemical modification of the surface allows for separation based on inter-molecular chemical interactions. Chemical modification of two surfaces of porous alumina formed by an anodize process. The shape, size, charge, and chemical affinity of the diffusing molecule play more important roles in smaller pore channels (also confinement effects). Gas permeation and separation, dialysis, electrodialysis, osmosis, and reverse osmosis are all membrane-based phenomena.

Documents

Advanced Topics in Nanotechnology - ATOMKIw3.atomki.hu/PhD/tamop/2_felev/20120924/nanotechno... · XPS Analysis of Thiols on Au Effect of Impurities: The thiol used in this study