Nanocalorimetry

For over a hundred years, calorimetry has been the method of choice for the investigation on thermodynamic properties. Conventional calorimetry, however, is limited to sample sizes of a few hundred milligrams and greater. To investigate the properties of the smallest of systems - on the order of nanometers - our group has developed a calorimetric technique with energy sensitivity of less than a nanojoule. MEMS fabrication methods allow us to create calorimeters that are sensitive to less than one nanogram of a deposited metal film, ten picograms of polymers and nanoliters of aqueous solutions.

Using our calorimetric technique, we have observed that nanoparticles melt at temperatures far below (ΔT > 100 K) the melting temperature of a bulk sample. Indium particles on the order of 1 nm in diameter can be fully liquid at room temperature! We have also observed that ensembles of "magic" nanostructures have discrete melting points which correlate well to increments in size of one atomic layer. Our group is also investigating the melting of isolated single crystals of polyethylene and liquid-phase chemistry in nanoliter sample sizes.

1. First, low stress silicon nitride is deposited on both sides of a double polished silicon wafer.
2. One face of the wafer is patterned and the nitride is etched away using the PR as an etch mask.
3. The wafer is etched in how potassium hydroxide, which has a high selectivity to silicon. This etches the silicon away and leaves just the nitride on the opposite side, held by a silicon frame. This ability to create large-area free-standing membranes is absolutely critical to nanocalorimetry.
4. Apply photoresist, which is reversed before developing.
5. Evaporate metal, we have used Pt, Ni, Au, Co, with Ti as adhesion layer.
6. Because of the image reversal, we can then liftoff the excess.

• MEMS calorimeter with better than nanojoule sensitivity
• Nanostructures: observed T_mand ΔH_m depression with size, plus discrete melting events
• Polymers: have measured melting of as few as 3 isolated single crystals
• Future applications: biological systems and semiconductors / silicides.

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