Nanoscale, Phonon-Coupled Calorimetry with Sub-Attojoule/Kelvin Resolution

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Abstract
We have developed an ultrasensitive nanoscale calorimeter that enables heat capacity measurements upon minute, externally affixed (phonon- coupled) samples at low temperatures. Fo ra5s measurement at 2 K, we demonstrate an unprecedented resolution of ¢C 0.5 aJ/K (36 000 kB). This sensitivity is sufficient to enable heat capacity measurements upon zeptomole-scale samples or upon adsorbates with sub-monolayer coverage across the minute cross sections of these devices. We describe the fabrication and operation of these devices and demonstrate their sensitivity by measuring an adsorbed 4He film with optimum resolution of 3 × 10-5 monolayers upon an active surface area of only 1.2 × 10-9 m2. Calorimetry is a powerful technique that is widely employed to measure the enthalpy of chemical reactions1 and the heat capacity2 and other physical properties of solid-state systems. Currently, microcalorimeters produced by semiconductor processing techniques are routinely used for measurement of the heat capacity of thin films and the heat of reaction for processes such as catalytic dissociation of carbon monoxide molecules on nickel.3 Current state-of-the-art microcalorimeters have a resolution that is typically of order 1 fJ/K, limited by the addendum (the heat capacity of the calorimeter itself) and the sensitivity of thermometry em- ployed.3,4 The quest to improve this sensitivity is not simply to improve the accuracy of measurement but, more impor- tantly, to enable heat capacity measurement of nanoscale objects and samples such as very thin films (e.g., epitaxial layers of magnetic semiconductors), fullerenes, nanoparticles and nanoclusters, biological macromolecules, and the heat of reaction of individual molecules. In this paper, we demonstrate a suspended silicon nitride (SiN) calorimeter integrated with AuGe resistance thermometry. The device exhibits an optimum resolution of 0.5 aJ/K at 2 K. We discuss the fabrication, operation, and resolution of such devices and demonstrate their use by measuring the low- temperature heat capacity of an adsorbed helium gas film at sub-monolayer coverage over the device area A 1.2