A more reliable measurement method for metal/graphene contact resistance.

The contact resistance of metal/graphene is becoming a major limiting factor for graphene devices. Among various kinds of contact resistance test methods, the transmission line model is the most common approach to extract contact resistance in graphene devices. However, experiments show that in some cases there exists large inaccuracy and instability using this method. In this study, we added a cross-bridge structure at the terminal of the transmission line as a supporting test. This modified transmission line measurement structure can easily compare not only the transmission line and Kelvin contact resistance, getting a more reliable value, but also the other contact-related parameters, such as specific contact resistivity, transfer length and the graphene sheet resistance under and outside contact metal at the same time. The new measurement test is very helpful in enabling us to study the contact property accurately. The specific contact resistivity in our experiment is in the range of 2.0 × 10(-6) Ω · cm(2) and 3.0 × 10(-6) Ω · cm(2) at room temperature. With the temperature decreasing from 290 K to 60 K, the transfer length fluctuates around 1.7 µm, and the specific contact resistivity reduces to less than 2.0 × 10(-6) Ω · cm(2).

Author Correction: Spectrally filtered passive Si photodiode array for on-chip fluorescence imaging of intracellular calcium dynamics.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Single-Cell Optogenetic Control of Calcium Signaling with a High-Density Micro-LED Array.

Precise optogenetic control, ideally down to single cells in dense cell populations, is essential in understanding the heterogeneity of cell networks. Devices with such capability, if built in a chip scale, will advance optogenetic studies at cellular levels in a variety of experimental settings. Here we demonstrate optogenetic control of intracellular Ca2+ dynamics at the single cell level using a 16-µm pitched micro-light emitting diode (LED) array that features high brightness, small spot size, fast response, and low voltage operation. Individual LED pixels are able to reliably trigger intracellular Ca2+ transients, confirmed by fluorescence microscopy and control experiments and cross-checked by two genetically coded Ca2+ indicators. Importantly, our array can optogenetically address individual cells that are sub-10 µm apart in densely packed cell populations. These results suggest the possible use of the micro-LED array toward a lab-on-a-chip for single-cell optogenetics, which may allow for pharmaceutical screening and fundamental studies on a variety of cell networks.

Spectrally filtered passive Si photodiode array for on-chip fluorescence imaging of intracellular calcium dynamics.

On-chip fluorescence imaging devices are recognized for their miniaturized and implantable nature that can benefit the study of intracellular dynamics at a variety of settings. However, it is challenging to integrate a spectral filter onto such devices (to block the excitation light) that has similar performance to the state-of-the-art emission filters used in fluorescence microscopes. In this work, we report a 100%-yield, spectrally filtered passive Si photodiode array designed for on-chip fluorescence imaging of intracellular Ca2+ dynamics. Coated with a spectral filter layer that has a high extinction ratio (>103), our array features high wavelength selectivity (>102), high linearity (R2 > 0.98), and low detection limit (45.1 µW 640/30 nm light). Employing fluorescence microscopy as the reference, we demonstrate that our array can conduct on-chip Ca2+ imaging in C2C12 cells that were chemically triggered to increase their intracellular Ca2+ levels. Importantly, our array-level data qualitatively captured the static fluorescence image of the cells and the intracellular Ca2+ dynamics, both of which are correlated with the microscope-collected data. Our results suggest the possible use of the spectrally filtered array towards a miniaturized on-chip fluorescence imaging device, which may open up new opportunities in tissue-level pharmaceutical screening and fundamental studies on cell networks.

Carrier-Number-Fluctuation Induced Ultralow 1/f Noise Level in Top-Gated Graphene Field Effect Transistor.

A top-gated graphene FET with an ultralow 1/f noise level of 1.8 × 10-12 µm2Hz1- (f = 10 Hz) has been fabricated. The noise has the least value at Dirac point, it then increases fast when the current deviates from that at Dirac point, the noise slightly decreases at large current. The phenomenon can be understood by the carrier-number-fluctuation induced low frequency noise, which caused by the trapping-detrapping processes of the carriers. Further analysis suggests that the effect trap density depends on the location of Fermi level in graphene channel. The study has provided guidance for suppressing the 1/f noise in graphene-based applications.