RESEARCH NEWS

November 2005 13

Researchers at Ohio State University

(OSU) and Proctor & Gamble have

studied the nanomechanical properties

of human hair with the aim of

improving cosmetic hair products

[Wei et al., Ultramicroscopy (2005),

doi:10.1016/j.ultramic.2005.06.033].

The OSU team, led by Bharat Bhushan,

investigated African, Asian, and

Caucasian hair that was untreated,

damaged, or treated with conditioner.

They used nanoindentation to study the

hardness, creep behavior, and elastic

modulus of the various human hair

samples. The morphology of the hair

was also examined using a scanning

electron microscope.

Hair morphology changes from root to

tip, and hair thickness varies with

ethnic origin. Asian hair is the thickest,

followed by African. The hardness and

elastic modulus of hair is dependent on

indentation depth, measured direction,

and ethnic origin, with African hair

having the lowest hardness and elastic

modulus. Treated African hair also

yields the highest creep displacement.

While Asian and Caucasian hair behave

similarly, Caucasian hair maintains a

slightly higher modulus and hardness

after being treated.

“The nanomechanical characterization

of hair not only helps evaluate the

effect of cosmetic products on hair

surfaces, but also provides a better

understanding of the physiochemical

properties of a wide variety of

composite biological systems,” the

scientists write.

Since Bhushan’s results show

conditioners typically do not cover the

entire hair shaft, Proctor & Gamble is

now developing a new conditioner that

coats hair more evenly. The reduction

in friction from nearby strands of hair

should result in less wear and tear. Patrick Cain

Nanomechanicshas cosmeticbenefitCHARACTERIZATION

The ability of electron tomography to characterizeembedded nanostructures in three dimensions hasbeen demonstrated by researchers at theUniversities of Cambridge and California, Davis[Arslan et al., Science (2005) 309, 2195].Many applications involve nanostructures that areembedded in other materials. Conventionaltransmission electron microscopy (TEM) onlyprovides two-dimensional projection images of suchsystems, which can be misleading. In contrast, tomography in the scanning transmission electronmicroscope (STEM) builds complete three-dimensional reconstructions of the objects withresolutions of ~1 nm3 from a series of two-dimensional images taken at different tilt angles. The researchers studied Sn quantum dots in a Si matrix. They are able to determine directly the

size, shape, structure, and location of the quantum dots in three dimensions using STEMtomography. The three-dimensional reconstructions reveal twolayers of Sn quantum dots in the Si matrix. Most ofthe dots can be seen to be uniform in width withthe truncated octahedron shapes expected for α-Snquantum dots. However, quantum dots that reach acritical size of ~8 nm are elongated in one direction.It appears that a different phase is present, and α-Sn quantum dots transform into β-Sn. The teamalso observed a small quantum dot outside the twoquantum dot layers. This had formed throughdiffusion of Sn atoms into a Si void. From theirobservations, the researchers are able to proposethat this dot formed by Stranski-Krastonov growth. Jonathan Wood

Viewing embedded nanostructures in 3DCHARACTERIZATION

Making structures in single-crystal diamondFABRICATION & PROCESSING

A new method for fabricating freestandingmicrostructures in single-crystal diamondhas been developed by a team from theUniversity of Melbourne, Australia, Technionin Israel, and the University of Cambridge,UK [Olivero et al., Adv. Mater. (2005),doi: 10.1002/adma.200500752]. StevenPrawer and colleagues have demonstratedthe potential of this technique byconstructing an optical waveguide, the firstmicrophotonic device to be made in single-crystal diamond.Diamond has very desirable mechanicalproperties (hardness, high Young’s modulus,and low friction coefficient) and opticalproperties (wide transparency and highrefractive index). “Furthermore, single-crystal diamond displays a vast repertoire ofluminescent centers that have verypromising attributes for quantum informationprocessing,” explains Paolo Olivero. However,difficulties in micromachining diamond havemeant that previous work has largely beenlimited to polycrystalline films created bychemical vapor deposition.The first step of the new method involvescreating a highly damaged, buried layer inlocalized regions of the single-crystaldiamond by He ion implantation. Next, afocused ion beam of submicrometer size isused to define patterns and mill trenchesdeep enough to expose the damaged layer to

the surface. After an annealing step, thedamaged layer can be selectively removed inan etch step. Lift off of the patterned areasleaves three-dimensional microstructures inthe bulk crystal.The group has fabricated freestandingcantilevers, bridges, and micron-sizedwaveguides and mirrors. They are nowworking to improve performance and deviceintegration.“We believe that the technique representsthe ideal toolkit for the production of fully integrated micromechanical assembliesand nano-optics devices in diamond,” saysOlivero.Jonathan Wood

Scanning electron micrograph of a waveguide structure in single-

crystal diamond. (© 2005 Wiley-VCH.)