Mechanism of melting in submonolayer films of nitrogen molecules adsorbed on the basal planes of graphite

URL:http://link.aps.org/doi/10.1103/PhysRevB.52.8515 DOI:10.1103/PhysRevB.52.8515The melting mechanism in submonolayer films of N2 molecules adsorbed on the basal planes of graphite is studied using molecular-dynamics simulations. The melting is strongly correlated with the formation of vacancies in the films. As the temperature increases, the edges of the submonolayer patch become atomically rough and vacancies are first created there. Then there is an onset temperature at which the vacancies penetrate into the patch. At an intermediate region of coverages ∼0.3-0.8 commensurate layers, there is sufficient free volume for the film to melt at that temperature. At higher coverages, ∼0.8-1.0 layers, a solid with defects is formed, and additional free volume must be created by higher energy mechanisms such as layer promotion for melting to occur; thus, the melting temperature rises with coverage. In contrast, for very small patches, the atomically rough zone penetrates the entire patch at a lower temperature where the film melts. The calculated melting temperatures are significantly lower than observed experimentally, indicating a severe fault in the potential model. A possible source of the discrepancy is identified.Our work was partially supported by the National Science Foundation under Grants Nos. DMR-9314235 (H.T.) and DMR-9120199 9L.W.B.) and by the Danish Natural Science Foundation (F.Y.H.)

GHz AlN-based multiple mode SAW temperature sensor fabricated on PEN substrate

Polymeric based surface acoustic wave (SAW) devices represent one of the most interesting platform for the development of wireless passive sensors for IoT and smart packaging applications. An important feature to be addressed in order to develop a compact and conformable wireless SAW sensor is the increase of the working frequency which allows an easy antenna integration and miniaturization. In this work, we present a disposable polymeric SAW temperature sensor device working in the GHz range, based on aluminum nitride (AlN) and built on a 125 μm thick film of polyethylene naphthalate. The flexible device has been compared to the same SAW device fabricated onto silicon substrates. The polymeric based SAW device shows three operating wave modes corresponding to the Rayleigh, Love, and Lamb, highlighting that the Lamb mode exhibits a resonance frequency as high as 1.325 GHz, corresponding to a phase wave velocity of 10,600 m/s, an electromechanical coupling of 2.91 % and Q factor of 109. Temperature coefficient of frequency (TCF) values of 149, 109, 53 ppm/°C have been calculated for the Rayleigh, Love and Lamb waves, respectively. The different behavior of the three SAW modes let us envision the development of a multiple sensing platforms based on different modes in the same device (e.g. temperature, microbiological contamination, light exposure)

Conformable surface acoustic wave biosensor for E-coli fabricated on PEN plastic film

Over the last decades, great effort has gone into developing new biosensor technologies for applications in different fields such as disease diagnosis and detection of pollutants in water and food. Global developments in robotic, IoT technologies and in healthcare sensors require new flexible sensor technologies that are low cost and built from sustainable and reusable or recyclable materials. One of the most promising technologies is based on the development of surface acoustic wave (SAW) flexible biosensors, which are highly reproducible, reliable and wirelessly controllable. This work presents for the first time a novel aluminum nitride (AlN)-based conformable SAW immunosensor fabricated on recyclable polyethylene naphthalate. We apply it to the detection of E.Coli using a faster and innovative functionalization method that exploit Protein-A/antibody affinity. A higher sensitivity (Limit of detection-LoD, 6.54*105 CFU/ml) of the Lamb wave traveling on the polymeric device has been obtained in comparison with SAWs traveling on AlN on silicon substrate (LoD, 1.04*106 CFU/ml). Implementation of a finite element method allowed for the estimation of the single E.Coli mass of approximately 9*10−13 g. This work demonstrates the high biosensing potential of flexible polymeric SAW devices for bacteria contamination control in food chain, water and smart packaging

Photoresponse of the AlN-Based SAW Device on Polymeric and Silicon Substrates

This paper shows the optical photoresponse in the IR-Vis-UV range of a AlN-based piezoelectric surface acoustic wave (SAW) delay-line device. The piezoelectric aluminum nitride (AlN) thin film has been sputtered on both silicon rigid substrate and flexible polyethylene naphthalate (PEN) substrate. Both devices have been investigated in their electroacoustic response, by measuring the transfer function S21 and by laser Doppler vibrometer characterization. The silicon based SAW devices, stimulated by the IR-Vis-UV light, are strongly affected in the out-of-band insertion loss due to the photovoltaic effect. A mathematical model has been implemented to correlate the out-of-band loss with the material's electrical admittance change. In contrast PEN based SAW devices, due to the polymeric nature of the substrate, did not show any variation in the out-of-band loss. Moreover, when exposed to UV light, a frequency downshift of the Rayleigh and Lamb resonances modes have been observed in all the devices, due screening of the photoinduced electrons in the AlN piezoelectric layer which induces an acoustic wave velocity reduction. To the best of our knowledge, this is the first photoresponse study exploiting SAW in the range IR-Vis-UV, suggesting a new detection mode of UV light by a flexible AlN based SAW device. Further development of these devices can lead to a new class of light sensors from UV to IR, based on remote SAW devices