Today's supercapacitor energy storages are typically discrete devices aimed for printed boards and power applications. The development of autonomous sensor networks and wearable electronics and the miniaturisation of mobile devices would benefit substantially from solutions in which the energy storage is integrated with the active device. Nanostructures based on porous silicon (PS) provide a route towards integration due to the very high inherent surface area to volume ratio and compatibility with microelectronics fabrication processes. Unfortunately, pristine PS has limited wettability and poor chemical stability in electrolytes and the high resistance of the PS matrix severely limits the power efficiency. In this work, we demonstrate that excellent wettability and electro-chemical properties in aqueous and organic electrolytes can be obtained by coating the PS matrix with an ultra-thin layer of titanium nitride by atomic layer deposition. Our approach leads to very high specific capacitance (15 Fcm -3 ), energy density (1.3 mWhcm -3 ) , p o w e r d e n s i t y ( u p t o 2 1 4 W c m -3 ) and excellent stability (more than 13,000 cycles). Furthermore, we show that the PS-TiN nanomaterial can be integrated inside a silicon chip monolithically by combining MEMS and nanofabrication techniques. This leads to realisation of in-chip supercapacitor, i.e., it opens a new way to exploit the otherwise inactive volume of a silicon chip to store energy.
Vibration energy harvesters scavenge energy from mechanical vibrations to energise low power electronic devices. In this work, we report on vibration energy harvesting scheme based on the charging phenomenon occurring naturally between two bodies with different work functions. Such work function energy harvester (WFEH) is similar to electrostatic energy harvester with the fundamental distinction that neither external power supplies nor electrets are needed. A theoretical model and description of different operation modes of WFEHs are presented. The WFEH concept is tested with macroscopic experiments, which agree well with the model. The feasibility of miniaturizing WFEHs is shown by simulating a realistic MEMS device. The WFEH can be operated as a charge pump that pushes charge and energy into an energy storage element. We show that such an operation mode is highly desirable for applications and that it can be realised with either a charge shuttle or with switches. The WFEH is shown to give equal or better output power in comparison to traditional electrostatic harvesters. Our findings indicate that WFEH has great potential in energy harvesting applications.
We investigate carrier transport in a single 22 nm-thick double-gated Si quantum well device, which has independent contacts to electrons and holes. Conductance, Hall density and Hall mobility are mapped in a broad double-gate voltage window. When the gate voltage asymmetry is not too large only either electrons or holes o c c u p y t h e S i w e l l a n d t h e H a l l m o b i l i t y s h o w s t h e f i n g e r p r i n t s o f v o l u m e inversion/accumulation. At strongly asymmetric double-gate voltage an electric field induced electron-hole (EH) bi-layer is formed inside the well. The EH drag resistance R he is explored at balanced carrier densities: R he decreases monotonically from 860 to 37 when the electron and hole density is varied betweeñ 0.4 1.7x10 16 m -2 . *mika.prunnila@vtt.fi
We study theoretically how energy and heat are transferred between the two-dimensional layers of bilayer carrier systems due to the near-field interlayer carrier interaction. We derive the general expressions for interlayer heat transfer and thermal conductance. Approximation formulae and detailed calculations for semiconductor-and graphene-based bilayers are presented. Our calculations for GaAs, Si and graphene bilayers show that the interlayer heat transfer can exceed the electron-phonon heat transfer below the (systemdependent) finite crossover temperature. We show that disorder strongly enhances the interlayer heat transport and pushes the threshold toward higher temperatures.
Abstract. The work function energy harvester (WFEH) is a variable capacitance vibration energy harvester where the charging of the capacitor electrodes is driven by the work function difference of the electrode materials. In this work, we investigate operation modes of the WFEH by utilizing a macroscopic parallel plate capacitor with Cu and Al electrodes and varying plate distance. We show that by charging the electrodes of the WFEH by letting the electrode plates touch during the operation a significant output power enhancement can be achieved in comparison to the case where the electrodes are charged and discharged only through a load resistor. IntroductionElectrostatic energy harvesters are based on electrically charged capacitors the capacitance of which is varied by mechanical motion [1-3]. The energy of the mechanical motion is converted into electric energy of the capacitor. The electrostatic energy harvesters need either electret materials or an external power supply, such as battery, for charging of the variable capacitor. This drawback poses challenges in the fabrication and possibly lifetime of these devices. The work-function energy harvester (WFEH) is similar to the electrostatic energy harvester as it utilizes a variable capacitor. However, this energy harvester, illustrated in figure 1, is not dependent on external power supplies or electret materials [4][5][6][7][8]. Instead, the WFEH employs the fundamental difference in the work functions of two dissimilar materials in the charging of the variable capacitor. In this sense the WFEH is closer to a piezoelectric harvester that relies also on the fundamental properties of solid materials. Furthermore, WFEHs can generate more power than the electrostatic harvesters in many operating conditions [8].The charging effect between two materials with different work functions was thoroughly investigated by Lord Kelvin more than 150 years ago [9,10], but the use of work function difference in vibration energy harvesting has been, remarkably, proposed only quite recently in 2006 [4]. Since then the work has been concentrated on the modelling and design of WFEHs [5][6][7]. Comprehensive theory and its comparison with experiments has been performed only very recently in Ref. [8], where, for example, non-contacting operation mode of figure 1c and d was realized with a resistor load. In this work, we extend the experiments of Ref. [8] and realize an operation mode where the WFEH plates touch during the operation cycle. This type of contacting mode of operation can produce an extremely large maximum capacitance which is limited only by the native oxide layers of the capacitor plates, screening length, and parallelism of the electrode plates. As the maximum capacitance defines largely the power output of the WFEH, contacting mode is expected to lead to performance enhancements and, indeed, our experiments show that this operation mode produces significantly more
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