With the number of communicating sensors linked to the Internet of Things (IoT) ecosystem increasing dramatically, well-designed indoor light energy harvesting solutions are needed. A first step in ...this direction would be to be able to accurately estimate the harvestable energy in a specific light environment. However, inside, this energy varies in spectral composition and intensity, depending on the emission source as well as the time of day. These challenging conditions mean that it has become necessary to obtain accurate information about these variations and determine their impact on energy recovery performance. In this context, this manuscript presented a method to apply an innovative energy harvesting estimation method to obtain practical and accurate insight for the design of energy harvesting systems in indoor environments. It used a very low-cost device to obtain spectral information and fed it to supervised machine learning classification methods to recognize light sources. From the recognized light source, a model developed for flexible GaAs solar cells was able to estimate the harvestable energy. To validate this method in real indoor conditions, the estimates were compared to the energy harvested by an energy harvesting prototype. The mean absolute error percentage between estimates and the experimental measurements was less than 5% after more than 2 weeks of observation. This demonstrated the potential of this low-cost estimation system to obtain reliable information to design energetically autonomous devices.
Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC ...sublimation process results in the growth of long and isolated graphene ribbons (up to 600
μ
m) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping.
4H-SiC Junction Barrier Schottky (JBS) diodes (1.2 and 3.5 kV) have been processed using the same technology with two different layouts. From 4 A and for the whole temperature range, the 3.5-kV ...diodes exhibit a bipolar conduction independent of the layout. However, the behavior of the 1.2-kV diodes depends on the design. At 500 V-300 ^{\circ}\hbox{C} , the leakage current is only 100 nA and 10 \mu\hbox{A} for the 3.5- and 1.2-kV diodes, respectively. The switch-off performances show a reverse peak current of only 50% of the nominal current at 300 ^{\circ}\hbox{C} for all JBS diodes. The JBS diodes have a surge current capability of around 80 A, two times higher than the Schottky diodes. DC electrical stresses are performed during 50 h, and all the 1.2-kV diodes exhibit no bipolar degradation. Nevertheless, some slight bipolar degradation is observed in 3.5-kV JBS diodes. Electroluminescence measurements exhibit the expansion of stacking faults in 3.5-kV diodes unlike in 1.2-kV diodes.
•Energy harvesting calculation model aiming towards autonomous consumer devices.•Model using two intrinsic solar cell characteristics and light environment spectra.•Method applied to standard ...artificial lights and natural indoor light environments.•Results with relatively low errors in real indoor environments.•Calculations validated using an instrumented energy harvesting prototype.
Indoor light can be used as a new energy source to power µW low consumption wireless sensor networks (WSNs), but for wireless electronic devices consuming tens of mW, it is still challenging. The challenge comes from the low level of irradiance and from the several kinds of source combinations varying in time (multi-spectral direct, reflective, and scattered mix of artificial and natural light). This article describes a simple and reliable method that provides a model-based evaluation of the harvestable energy from any real indoor light environment. This method uses ‘real condition’ indoor light spectral measurements with a spectrometer as well as ‘controlled condition’ optoelectrical characteristics of the photovoltaic solar cells. The model-based evaluation of the harvestable energy has been compared with real microsource prototypes based on commercial photovoltaic cells powering commercial wireless e-ink display (more than 10 mW consumption averaged on a day). In this article, we show that it is possible to evaluate the harvestable energy, for several days of indoor light exposure, with an error lower than 6%. Our method, with such an accuracy range, will be a helpful tool to assist engineers and researchers in designing light energy harvesting systems and more generally could find a wide application in the growing IoT ecosystem.
The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, ...because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.
Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8° off-axis 4H-SiC samples, large and homogeneous single epitaxial graphene layers have been ...grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. On the best samples, we find a moderate
p
-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.
The recent development and commercialization of microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS) has brought the related challenge of independently powering such systems. ...The concept of the nanogenerator (NG) has shown potential for harvesting energy from the ambient environment to power MEMS/NEMS. Kinetic energy harvesting NGs based on the piezoelectric properties of ZnO nanowires have attracted much interest. In this paper, we have fabricated hydrothermally synthesized ZnO-based NGs following the procedures standardized in the published literature. Likewise, reference NGs without ZnO piezoelectric material were fabricated in parallel with the ZnO NGs. The voltage output of both the ZnO NG and the reference NG was measured given a 10-Hz cyclic vertical load. Unexpectedly, both the ZnO and the reference NG were found to produce 3 mV with 0.451 N of load. A finite-element model was created to determine that the voltage potential of the NG should be about 1 mV with the given load. A possible explanation for this unexpected behavior is that the measured signals are not entirely piezoelectric in nature. Rather, the signals recorded from the NGs may be some alternate phenomenon, such as the triboelectric, flexoelectric, or electret effect.
Due to their fabrication simplicity, fully compatible with low-cost large-area device assembly strategies, source-gated transistors (SGTs) have received significant research attention in the area of ...high-performance electronics over large area low-cost substrates. While usually based on either amorphous or polycrystalline silicon (α-Si and poly-Si, respectively) thin-film technologies, the present work demonstrate the assembly of SGTs based on single-crystalline ZnO sheet (ZS) with asymmetric ohmic drain and Schottky source contacts. Electrical transport studies of the fabricated devices show excellent field-effect transport behaviour with abrupt drain current saturation (IDS(SAT)) at low drain voltages well below 2 V, even at very large gate voltages. The performance of a ZS based SGT is compared with a similar device with ohmic source contacts. The ZS SGT is found to exhibit much higher intrinsic gain, comparable on/off ratio and low off currents in the sub-picoamp range. This approach of device assembly may form the technological basis for highly efficient low-power analog and digital electronics using ZnO and/or other semiconducting nanomaterial.
The production of high-quality semiconducting nanostructures with optimized electrical, optical, and electromechanical properties is important for the advancement of next-generation technologies. In ...this context, we herein report on highly obliquely aligned single-crystalline zinc oxide nanosheets (ZnO NSs) grown via the vapor–liquid–solid approach using r-plane (01–12) sapphire as the template surface. The high structural and optical quality of as-grown ZnO NSs has been confirmed using high-resolution transmission electron microscopy and temperature-dependent photoluminescence, respectively. To assess the potential of our NSs as effective building materials in high-performance flexible electronics, we fabricate organic (parylene C)/inorganic (ZnO NS) hybrid field-effect transistor (FET) devices on flexible substrates using room-temperature assembly processes. Extraction of key FET performance parameters suggests that as-grown ZnO NSs can successfully function as excellent n-type semiconducting modules. Such devices are found to consistently show very high on-state currents (I on) > 40 μA, high field-effect mobility (μeff) > 200 cm2/(V s), exceptionally high on/off current modulation ratio (I on/off) of around 109, steep subthreshold swing (s-s) < 200 mV/decade, very low hysteresis, and negligible threshold voltage shifts with prolonged electrical stressing (up to 340 min). The present study delivers a concept of integrating high-quality ZnO NS as active semiconducting elements in flexible electronic circuits.
In the present work, we report the high performance of zinc oxide (ZnO) nanosheet (NS) based source-gated transistors (SGTs) with asymmetric Schottky source and ohmic drain contacts: low saturation ...drain-source voltages (~2V) in the output scans (even at high gate voltages), high current on/off ratio (>107) and low off-currents (0.1pA). For a deeper understanding of the device mechanism and charge transport at metal–semiconductor contact interface, temperature dependent current–voltage studies have been performed. They revealed that the device operation can be ascribed to 3 main processes: i) the reverse biased Schottky source contact, which essentially controls charge carrier injection in to the NS channel, ii) effective manipulation of the source barrier by the gate field and iii) modulation of the depletion region beneath the source contact. These results are likely to improve the future generations of the ZnO based SGTs which offer several advantages for thin-film transistor design, including low power dissipation, small signal amplification, and as active load for the electronic circuits.
•Single-crystalline ZnO Nanosheets on r-plane Saphire•High performance FET using single ZnO Nanosheet (Mobility=50cm2/Vs)•High performance source-gated transistors: low saturation voltages (~2VD)•Low temperature processes for the device fabrication
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