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).

Photonic metamaterials by direct laser writing and silver chemical vapour deposition

Rill, Michael S.. “Photonic metamaterials by direct laser writing and silver chemical vapour deposition.” Nature materials 7.7 (2008):543-546.

Purpose

Photonic metamaterials exhibit fascinating magnetic properties at optical frequencies.  Functional devices will ultimately require large-scale three-dimensional systems.  However, current 3D device fabrication is usually limited to pattering two-dimensional systems via electron-beam lithography and metal film evaporation followed by repeated stacking.  This work investigates a faster and more direct method of laser writing and metallic chemical vapour deposition (CVD) for the purpose of rapidly prototyping 3D photonic metamaterials. 

Method

A 3D polymeric template is fabricated by standard methods of direct laser writing (DLW) a thin film of SU-8 photoresist (on glass substrate).  Post-bake and developing are performed after exposure.  Next, a thin layer of SiO2 is applied in order to increase mechanical stability and to serve as a backbone for the subsequent deposition of silver.  Following activation of the oxide layer with O2 plasma, a metal-organic precursor is added and CVD cycles are applied.  Ten cycles of silver CVD generate a silver thickness of 50nm.  Electron micrographs are used to examine the quality of the structure.  Optical properties are determined via normal-incidence transmittance measurements in a Fourier-transform microscope spectrometer.

Key findings

1. Electron imaging revealed a uniform silver coating.  This was noted to be in sharp contrast to the typical 2D evaporation method.
2. Optical transmittance measurements corresponded well with theory.
3. Overall, this method was successfully demonstrated to be a quick and reliable way of achieving 3D photonic metamaterials.  The authors note that they have approached a bottleneck in the theory of such devices.  They hope that this work spurs further theoretical investigation and research in this emerging field.

Key terms

CVD – chemical vapor deposition; DLW – direct laser writing.

Dip-pen lithography using aqueous metal nanocrystal dispersions

John Thomas, P.. “Dip-pen lithography using aqueous metal nanocrystal dispersions.” Journal of materials chemistry 14.4 (2004):625-628.

Purpose of Study

This work seeks to fabricate patterns of chemically prepared nanocrystals using dip-pen nanolithography (DPN).  Special interest is found in the use of metallic nanocrystals due to their ability to self-assemble.  Further applications are expected in the realm of nanoelectronics; however, realizing future nanodevices will require accurate positioning of nanocrystals on a surface.

Method

Pd and Au nanocrystals were fabricated by standardized methods reported elsewhere.  Notably, each nanocrystal synthesis procedure involved a surface stabilization step.  In that manner, Pd nanocrystals were capped with polyvinylpyrrolidone (PVP) and the Au nanocrystals were stabilized with tetrakishydroxymethyl phosphonium (THP).  DPN was performed under ambient conditions using a Nanoscope IV controller attached to a Digital Instruments multimode-head set in contact mode.  Silicon-nitride cantilevers were dipped in nanocrystal dispersions and allowed to dry in air.  Patterning was carried out on mica surfaces using scan speeds of one micro-meter per second.

Key Findings

1.) Aqueous dispersions of Au and Pd nanocrystals were successfully demonstrated as nanopatterning inks using DPN.

2.) High aspect-ratio lines and rectangles of nanocrystals were fabricated. 

3.) No evidence for lateral diffusion was found when the cantilever was held to the mica surface for extended time-periods.  Instead, a small vertical height increase is observed with time.  This was shown to occur regardless of the applied force.  These results are attributed to interactions with the substrate.  Thus, this process is noted as being highly substrate dependent.

Key words:  DPN – dip pen lithography, Aspect Ratio - the proportion of an image size given in terms of the horizontal length vs. the vertical height.

Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity

Na Tian, et al. Science 316, 732 (2007);

Purpose:

A high yield method for fabricating noble metal nanocrystals with high-index facets is demonstrated.  Specifically, tetrahexahedral (THH) platinum nanocrystals are fabricated and shown to have enhanced catalytic properties which arise from an increased surface density of atomic steps and dangling bonds.

Method:

THH Pt NCs were fabricated via an eletrochemical treatment of Pt nanospheres electrodeposited on a glassy carbon substrate.  A 10Hz square-wave potential was applied using a standard three-electrode electrochemical cell and sulfuric and ascorbic acids at room temperature.  When starting with a typical dispersion of 750nm nanospheres, an example 60 minute run demonstrates the generation of 217nm diameter THH Pt NCs with greater than a 90% yield.  Tuning of experimental conditions allows for the production of THH Pt NCs down to 20nm in diameter.  The size and shape of the THH Pt NCs are characterized by scanning electron microscopy and transmission electron microscopy.

Key Findings:

• The catalytic activity of THH Pt NCs is shown to be 160 – 400% enhanced over that of Pt nanospheres and 200-310% over commercial Pt/C catalysts.
• The enhanced catalytic activity is shown to arise from high-index facets which exhibit a greater density of atomic steps and dangling bonds.
• Despite the large surface energy of the high-index planes, THH Pt NCs are demonstrated to be stable up to 800C.
• Lastly, the THH Pt NCs are applied in promoting the electro-oxidation of both formic acid and ethanol (which are promising alternative fuels for direct fuel cells) and are shown to yield similar improvements to output current density.

Definitions:

THH: tetrahexahedral, NC: nanocrystal, Catalyst: substance that increases the rate of a reaction