Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers

Lin Xiao, et al., Nano Lett., Article ASAP (2008)

Purpose

Carbon nanotube (CNT) films exhibit attractive properties, such as high flexibility, transparency, and an ultra small heat capacity per unit area.  Combined with the thermoacoustic effect and a wide ranging response to sound frequency electric currents, CNT films are herein examined for their use as thin film loudspeakers. 

Method

CNT thin films are drawn from superaligned CNT arrays on a 4-inch silicon wafer.  These films are comprised of sparsely parallel aligned nanotubes with diameters ~10nm, and are grown to 10 centimeters in length.  The resulting thickness of the ultralight transparent film is on the order of tens of nanometers.  The film is then placed on two electrodes under an AC driving current.  Both planar and cylindrical loudspeakers were fabricated and tested under a variety of conditions.  A microphone was placed 5cm away from the loudspeaker and the frequency response was measured by an audio analyzer.  Multilayer film loudspeakers were also tested.

Mechanism

The application of alternating current periodically heats the CNT film thereby inducing pressure oscillations in the surrounding air particles.  The process can be modified by tuning the sheet resistance or specific heat capacity per unit area.

Key Demonstrations

• CNT thin film loudspeakers exhibited a wide ranging frequency response, high sound pressure level (SPL), and low total harmonic distortion (THD).
• Severe low frequency environmental noise was observed.
• A functioning loude speaker attached to the fabric of a waving flag was demonstrated (movie).
• A video iPod was surrounded by a transparent thin film speaker which played the audio for the movie being shown through it (movie).
• The response and output power and can be tuned by stacking or stretching the CNT films.
• Fabrication is noted to be relatively easy and highly scalable.  The authors note that a single 4 inch wafer could draw a 10cm wide CNT sheet up to 60 meters in length, thereby allowing for the fabrication of over five-hundred, 10×10cm speakers in a single process.

Definitions

• Thermoacoustic effect – The transduction of temperature gradients into acoustic vibrations.
• Specific heat capacity – The energy required to raise the temperature of a substance by 1 degree K.  (heat capacity per unit area = HCPUA).

Power generation with laterally packaged piezoelectric fine wires

Rusen Yang, Yong Qin, Liming Dai and Zhong Lin Wang

Nature Nanotechnology Advance online publication, 9 November 2008

Purpose of the Study

Demonstrate a flexible power generator based on cyclic stretching–releasing of a piezoelectric fine wire that has a number of advantages over generators based on vertically aligned nanowire arrays.

Methods

The generator was fabricated by bonding a ZnO piezoelectric fine wire (PFW) laterally on a Kapton polyimide film. Both ends of the PFW is fixed to electrodes. A current/voltage measurement meter was connected to two ends of the PFW without introducing any external power source in the circuit.

Key Findings

1.   When the substrate bends and stretches the wire, a tensile strain of 0.05–0.1% is induced in the wire, and forcing electrons to flow along an external circuit to charge the wire. And when the substrate is released, electrons flow back in the opposite direction.

2.    Periodically bending and releasing the PFW therefore generates an alternating current, which can be up to ~50 mV, and the energy conversion efficiency of the wire can be as high as 6.8%.

3.    Generators based on multiple PFWs can be integrated to raise the output voltage.

Important Definitions

1.    Piezoelectricity: Piezoelectricity is the ability of some materials to generate an electric potential in response to applied mechanical stress.

2.    Kapton: Kapton is a polyimide film developed by DuPont which can remain stable in a wide range of temperatures, from -269 °C to +400 °C (4 K-673 K).

Balasubramanian, K., Burghard, M., Electrochemically functionalized carbon nanotubes for device applications. Journal of Materials Chemistry. 18 (2008) 3071-3083.

Purpose of the study

To examine the many applications of carbon nanotubes and look at how electrochemical functionalization can expand these applications.  Using electrochemistry-based approaches, carbon nanotubes can be functionalized to serve a wide range of applications such as reinforced composites, field-emission displays, scanning probe tips, and molecular-scale electronic devices.

Methods

Several methods used to extend or enhance the applications of nanotubes based on electrochemical functionalization are surveyed in this paper.  Electrochemical functionalization involves the creation of an active species from a precursor in the vicinity of a working electron.  Methods to electrochemically functionalize nanotubes can be broken into covalent functionalization and non-covalent functionalization including electropolymerization, electrodeposition of inorganic compounds, electrodeposition of mixed carbon nanotube-polymer films, and electrophoretic deposition.

Key Points

1.    Electrochemically functionalized carbon nanotubes have a wide range of applications and this paper primarily focuses on their application as a sensing device, specifically biosensing.
2.    Due to their high surface area and chemical stability, carbon nanotubes make very good nanoscale sensors.
3.    The small size of nanotubes makes the ideal for accessing the interior of redox enzymes and other small areas needed in chemical sensing
4.    Several different biosensing methods can be carried out with functionalized nanotubes
a.    Electrochemical sensors
b.    Chemiresistors
c.    Electrochemical field-effect sensors
5.    Possible applications of future development with electrochemically functionalized carbon nanotubes include energy storage such as super-capacitors or rechargeable batteries, advanced membranes, and nanoscale probes for fluid flow

Functionalized Nanoporous Gold Leaf Electrode Films for the Immobilization of Photosystem I

Ciesielski, P. et al. ACS Nano, Article ASAP (2008)

Purpose of the Study
The purpose of this study was to develop a technique for fabricating nanoporous gold leaf electrode films, and functionalize the surface of the electrode with PSI for solar energy conversion.

Methods
Nanoporous gold leaf was fabricated from an initial gold/silver alloy film. Immersion in concentrated nitric acid dissolved the silver, dealloying the film. The porous film was then placed on a gold/silicon support substrate. Characterization was performed using cyclic voltammetry and SEM. Self-assembled monolayers were formed by exposing the film to a variety of ω-terminated alkyl thiols. Presence of SAMs was determined using electrochemical impedance spectroscopy (EIS). Photosystem I was directly attached to the NPGL by functionalizing the electrode surface with terephthaldialdahyde (TPDA). Measurements were made of photocurrent enhancement with PSI adsorption.

Key Findings

• Maximum surface area enhancement occurs for short dealloying times, but maximum photocurrent enhancement occurs for longer dealloying times. This is due to the small pore size associated with short dealloying times.
• Immobilization of PSI on NPGL electrodes provides an increase in PSI-catalyzed photocurrent, compared to planar electrodes.

Definitions

• Cyclic voltammetry – A potentiodynamic electrochemical measurement which sweeps the working electrode potential linearly, and reverses direction when reaching a set potential. This can cycle several times during an experiment, and gives information about the oxidation and reduction of mediator species in solution.
• Electrochemical impedance spectroscopy – Probes energy storage in a device by measuring the impedance of a system over a range of AC frequencies.

Characterization of Defects-Location in Hydrogenated Microcrystalline Silicon Thin Films and Its Influence on Solar Cell Performance

Shuichi HiZA, Akira YAMADA, and Makoto KONAGAI

 

Purpose of the study

By using both experiment and numerical analysis to study hydrogenated microcrystalline silicon (Si:H) based solar cells. And discuss the effect of grain-growth on the defect-fromation and on the performance of solar cells.

 

Methods

Intrinsic Si:H thin films of varying of thickness were prepared by HW-CVD. Then surfaces of the prepared films were observed by scanning electron microscopy (SEM) and crystallinity was estimated from Raman spectra. Numerical study was carried out using the AMPS-1D device simulator, which was based on Poison’s equation and electron and hole continuity equations.

 

Key findings

1. Each large grain in Si:H grewithout collisions with neighbors to a thickness of 50nm assuming a typical cone-shaped growth of the grain from a nucleus. In this region, the grain size increased almost linearly with the thickness.  
2. The calculated open circuit voltage Voc function of the assumed density of the Gaussian state as the gap state in numerical analysis was showed. The linear relations between Voc and the logarithm of thesumed density of the gap staes were found independent of the assumed capture cross-sections.
3. The correlation of the surface area of large grains can be greatly affected by the shape of the grains in terms of the total surface area of the large grains.  
4. Defects in Si:H material are located mainly at the boundaries between the large grains in which hundreds of nano-sized crystallites are contained. Moreover, each nano-sized crystallite which forms a large grain seems to have no defects at the surface.