Nanoscale Curvature-Induced Hydrogen Adsorption in Alkali Metal Doped Carbon Nanomaterials

K.R.S. Chandrakumar, K. Srinivasu, S. K. Ghosh

Journal of Physical Chemistry, 2008, 112 (40), 15670-15679

Purpose of Study:

The article investigated the reactivity of concave and convex carbon nanomaterials by simulating the effect on hydrogen adsorption of systematically varying the dihedral angle of the carbon atoms in the materials.

 

Methods:

The authors divided the paper into two parts focusing on simulations of increasing difficulty; initially, they modeled the interactions of hydrogen with a six-membered carbon ring, then entire carbon nanomaterials were constructed and the interactions of hydrogen with these larger systems were explored.  For the initial section, the dihedral angles of the six-membered carbon ring were systematically varied from 0 – 35° to simulate molecular curvature in common carbon-based nanomaterials such as fullerenes and carbon nanotubes.  In the more complex analyses, entire carbon-based nanomolecules were manipulated into different angles and modeled with hydrogen adsorption.

 

Results:

1.       The modeling results demonstrate the importance of curvature of various carbon materials when measuring hydrogen adsorption.

2.       Surfaces with maximum curvature were found to demonstrate the highest reactivity which is due to π-conjugation weakening.  This is illustrated when considering the differences in reactivity between the curved carbon nanotubes and the two-dimensional sheets of graphene.

3.       Hydrogen adsorption interactions of a carbon-based nanomaterial  bound to a sodium atom showed that the binding energy of the sodium atom and the curvature of the nanomaterial are related in a quadratic manner.

4.       The simulations predict that metal-doped concave nanotubes with smaller radii will be the best candidates for hydrogen adsorption.

 

XPS study of the surface chemistry of Ag-covered L-CVD SnO2 thin films

M. Kwoka, L. Ottaviano, M. Passacantando, G. Czempik, S. Santucci, J. Szuber

Applied Surface Science 254 (2008) 8089-8092

 

Purpose of Study:

To produce and characterize the surface chemistry of silver covered SnO₂ films prepared by laser chemical vapor deposition and characterized by x-ray photoelectron spectroscopy. 

 

Methods:

Tin dioxide thin films were prepared using laser chemical vapor deposition (L-CVD).  Si (1 0 0) wafers were used as substrates to grow the thin films of SnO₂, which were grown from a mixture of TMT and O₂.  The resulting films had an average thickness of 20 nm after 1 hour.   One monolayer of silver was then deposited in a thin layer on the SnO₂ films using a thermal source under UHV in the same CVD chamber.  Some of the resulting samples were then equilibrated with dry air and then UHV annealed.  The control samples were UHV annealed.   All samples were then characterized using an XPS spectrometer (PHI 5700 model with an Al Kα 1486.6 eV x-ray source).  Notably, depth profiling was performed using the XPS to determine how far into the SnO₂ film the Ag penetrated.  Atomic force microscopy was also used to determine the surface topography and morphology of the samples.

 

Results:

1.    XPS analysis of the control showed that the SnO2 films were mixtures of SnO and SnO₂.  The [Ag]/[Sn] ratio was found to be 0.50 ± 0.05, which corresponds to approximately a 0.5 nm layer of Ag on the SnO₂ films (1 monolayer coverage).

2.    The samples aged in dry air prior to annealing oxidized (their [O]/[Sn] ratio reached 1.7 ± 0.05).  These samples also showed carbon contamination via an C 1s XPS peak (>2 monolayers of carbon contamination).

3.    UHV annealed samples showed less carbon contamination and the XPS depth profile analysis showed that Ag migrated throughout the SnO₂ films.  Specifically, the depth profile analysis confirmed that the Ag atoms diffused up to a depth of 20 nm, which corresponds to the SnO₂ film thickness.

4.    This analysis showed that the different methods for producing Ag-doped SnO₂ thin films produce films with different physical characteristics.  Specifically, exposing the films to dry air causes oxidation and carbon contamination to occure.  Annealing the films with UHV at 400C decreased the carbon contamination and oxidation, but causes the silver atoms to diffuse throughout the SnO₂ film.

Adhesion detachment and movement of gold nanoclusters induced by dynamic atomic force microscopy

G. Paolicelli, K. Mougin, A. Vanossi, S. Valeri

J. Phys.: Condens. Matter 20 (2008) 354011

Purpose of Study:

This study used dynamic atomic force microscopy in tapping mode to investigate the adhesion and frictional properties of nanoclusters deposited on a surface. 

 

Methods:

Atomic force microscopy was used in tapping mode with amplitude feedback (AM-AFM), meaning that the AFM was performed by measuring and controlling the energy released by the AFM tip to the system while moving across a surface.  In this case, the amplitude oscillation was made larger than that optimal for imaging purposes in order to measure the energy dissipated when the nanoclusters bound to the surface detach as a result of the extra energy.  In this way, the group was able to measure the adhesion properties of gold nanoclusters on a Si (100) surface.

Experiments were performed in air at room temperature using a VEECO Enviroscope and Nanoscope IV.  Two standard silicon cantilevers were used with nominal frequencies of 75 and 350 kHz, and spring constants of 3 and 40 N/m. 

 

Key Findings:

1.       The AM-AFM technique provides the ability to measure the energy detachment threshold for Au nanoclusters down to the 10 nm scale.

2.       The AM-AFM technique provides information on different types of detachment techniques, including collisions between moving and pinned particles, and a detachment track of sets of nanoclusters.

3.       This analysis proves that the adhesion energy of nanoclusters depend on their size down to the contact area of 10² nm².

Size-Dependent Theoretical Tensile Strength and other Mechanical Properties of [001] Oriented Au, Ag and Cu Nanowires

by F. Ma and K.W. Xu

Journal of Materials Research, 21, No. 11 (2006)

Purpose of Study:

To use the modified embedded atom method simulation technique to develop stress-strain curves for Au, Ag and Cu nanowires, and use the curves to determine mechanical properties at the nanoscale (including tensile strength, elastic modulus and yield strength.)

Methods:

The modified embedded atom method was used to perform the nanoscale simulations.  This is an atomic scale modeling technique that was used to monitor rectangular [001] oriented defect-free Au, Ag and Cu nanowires with uniaxial tensile loading applied in the wire axial direction.  A periodic boundary condition was applied in the wire axial direction during modeling.  This simulation allowed for an estimation of the total energy of the optimized structures at each strain (energy (E)/strain (Є) relationships.)  Stress-strain curves were then calculated using equation (1) in the text.  The following mechanical properties were calculated using the stress-strain curve:

·      Theoretical tensile strength = peak curve value

·      Elastic modulus = slope coefficient

·      Yield strength = maximum value of the linear portion of curve

Results:

1.   The calculated stress-strain curves vary based on the width of the nanowires.  This implies that the mechanical properties of nanoscale materials depend heavily upon size.

2.   The theoretical tensile strength, yield strength and elastic modulus all increase as wire width decreases for the three simulated wires (Au, Ag and Cu).

3.   Notably, for all wire types, the calculated tensile strength was found to be one order of magnitude higher than the tensile strength of the corresponding bulk material.

4.   The authors concluded that the increase in mechanical properties at smaller sizes are due to electrons redistributing at the surface of the nanowires, which causes an enhanced attraction between the atoms of the nanowires.

Useful Definitions (adapted from www.wikipedia.com):

1.   Tensile strength – the stress at which a material breaks or permanently deforms.

2.   Yield strength – the stress at which a material begins to plastically deform.

3.   Elastic modulus – the tendency of an object to elastically deform.