New material for flat semiconductors (usable bandgap)

Scientists have long worked to harness the unusual properties of graphene, a two-dimensional sheet of carbon atoms. However, graphene lacks a single critical characteristic that would make it even more useful: a property known as a bandgap, which is essential for designing devices like computer chips and solar cells.

As such, researchers at MIT and Harvard University are currently experimenting with a two-dimensional material whose properties are very similar to graphene, albeit with certain distinct advantages – including the fact that this material naturally boasts a usable bandgap.

Photo Credit: MIT

The research, just published online in the Journal of the American Chemical Society, was conducted by MIT assistant professor of chemistry Mircea Dincă and 7 co-authors.

The new material, essentially a combination of nickel and an organic compound known as HITP, also has the advantage of self-assembly. Indeed, its constituents naturally assemble themselves, a “bottom-up” approach that could lend itself to easier manufacturing and tuning of desired properties by adjusting relative amounts of the ingredients.

According to Dincă, two-dimensional materials that possess extraordinary properties is “all the rage these days, and for good reason.”

 To be sure, graphene offers optimized electrical and thermal conductivity, as well as considerable strength. However, lack of a bandgap forces researchers to modify it for certain uses, which tends to degrade the properties that made the material desirable in the first place.

The new compound, Ni3(HITP)2, shares graphene’s perfectly hexagonal honeycomb structure. In addition, multiple layers of the material naturally form perfectly aligned stacks, with the openings at the centers of the hexagons all of precisely the same size, approximately two nanometers (billionths of a meter) across.

During a series of initial experiments, researchers studied the material in bulk form, rather than as flat sheets. As Dincă notes, this makes the current results – including excellent electrical conductivity – even more impressive, as these properties should be better yet in a 2-D version of the material.

“There’s every reason to believe that the properties of the particles are worse than those of a sheet,” he explains. “[However], they’re still impressive.”

Photo Credit: MIT

Perhaps most importantly, this is just the first example of what could eventually be a diverse family of similar materials built from different metals or organic compounds.

“Now we have an entire arsenal of organic synthesis and inorganic synthesis [that could be harnessed] to tune the properties, with atom-like precision and virtually infinite tunability,” he adds.

Such materials might ultimately lend themselves to solar cells whose ability to capture different wavelengths of light could be matched to the solar spectrum, or improve supercapacitors used to store electrical energy.

 Last, but certainly not least, the new material could lend itself to use in basic research on the properties of matter, the creation of exotic materials such as magnetic topological insulators, or materials that exhibit quantum Hall effects.

“They’re in the same class of materials that have been predicted to have exotic new electronic states. These would be the first examples of these effects in materials made out of organic molecules. People are excited about that,” Dincă concludes.

ATmega328 powers paper-thin Printoo board

Printoo – powered by Atmel’s ATmega328 microcontroller (MCU) – is a lineup of paper-thin, low-power boards and modules that offer Makers and devs new levels of creative flexibility.

The open source platform, created by the Ynvisible crew, made its Kickstarter debut this week.

“Printoo is the first development board that is flexible and light enough to bring any of your 3D printed objects to life – no matter what shape it is. Add Internet and Bluetooth connectivity, input, output, motorization, light and motion sensing and power. Even solar, to almost any configuration or weird shape you print,” a Ynvisible rep explained.

“Plug the modules together, tinker with the Arduino sketches we are making available, and use the apps to connect and control Printoo – bringing your ideas to life. [Plus], we built the apps you need to connect Printoo to the Internet. You’ll be able to remotely control your Printoo creations or use them to trigger or perform action on the Web – from your smartphone, tablet or laptop, from anywhere in the world.”

As we’ve previously discussed on Bits & Pieces, the core Printoo module is powered by Atmel’s ATmega328 microcontroller (MCU).

Additional hardware modules include a display driver, battery connector, batteries (soft and ultra-thin), battery holder, sensor module, solar cell connector, conductive ink adapter, DC motor drivers, electrochromic display, organic photodetector slider, polymer solar cell and LED strip.

The Ynvisible crew has also created a number of Printoo-powered demos such as a Bluetooth fan, 3D printed watercraft, solar powered 3D printed hovercraft, “girlfriend communicator,” electronic voter and the Printoo Man.

Interested in learning more? You can check out the project’s official Kickstarter page here.

Lucid Stead illuminates the desert with Arduino

Phillip K Smith III has created a rather unique work of art in the California desert using an old shack tricked out with an Atmel-based Arduino board and LEDs. According to Gizmag’s Adam Williams, the solar-powered structure is designed to change color throughout the day like a chameleon.

“A curious blend of architecture and art project, Lucid Stead is located on a sizable plot of land owned by the artist himself. The [renovation] process involved adding mirrored strips to the exterior of the shack, and installing a custom Arduino-controlled electronics setup inside,” Williams explained.

“The Arduino is programmed to slowly change the color of several LEDs, also placed inside, which shine light out of the four windows and door as the day progresses. It’s a simple enough concept, but the effect is striking and makes the building seem to almost disappear, or glow, depending on its state.”

Interestingly, the solar panels were installed some distance away from the shack on a temporary frame, hidden behind already existing desert plants. A battery array, which provides power at night, is also located on the same unobtrusive framework, with wires buried underground to avoid spoiling the minimalist style of the project.

Clearly, Lucid Stead is all about tapping into the quiet and the pace of change of the desert.

“Like the enveloping vista that changes hue as time passes, Lucid Stead transforms. In daylight the 70 year old homesteader shack, that serves as the armature of the piece, reflects and refracts the surrounding terrain like a mirage or an hallucination,” said Smith.

“As the sun tucks behind the mountains, slowly shifting, geometric color fields emerge until they hover in the desolate darkness. When you slow down and align yourself with the desert, the project begins to unfold before you. It reveals that it is about light and shadow, reflected light, projected light and change.”

Current sensing for smart meters and solar panels

In the recent edition of Electronic Products there was a fantastic I/V (current / voltage) diagram of a solar panel. It may have originated at Allegro, where, the authors of the article work. It confirmed something I suspected for a long time. The power output of a solar panel falls as it gets hotter. I will put a low-res version of the graph below but you really need to look at the EP article.

This diagram shows how you get less power out of a hot solar cell. Dotted lines are power out, equivalent to the area under the I/V operating point.

This connects with my realization that a solar cell is like any other photodiode. The forward voltage goes down as it gets hotter. But I was not sure what happened in reverse mode I/V. But with what we call a photodiode, you are usually trying to measure light, not draw power from it. So with many photodiode amplifiers, you short the diode into a virtual ground. With no voltage across it, it is not making any power. But the current output is very linear with respect to the light falling on the diode. And note that this current is a reverse current in the diode. You can think of it as a reverse leakage current that gets way worse when light hits the diode. Indeed, the baseline leakage is called dark current.

A photodiode I/V curve.

So here is the diode I/V curve you might see published in a photodiode amplifier book. Note that you can short the diode and its output has to fall on the –I axis. If you put a negative bias on the diode, and still keep it working into a virtual node so there is no voltage generated across it, then it is like the leftmost response. The negative (aka reverse) bias does not materially change the output, but it does greatly lower the diodes capacitance, since a photodiode is also a varactor. If you hook a photodiode, which is pretty much any diode there is, to a resistor, it will make current but the current into the resistor will also make a voltage. That gives you the output of a resistive load in the chart. The value of the resistor sets the slope of that load line. Note that the output is no longer linear. Doubling the light does not double the output current.

Realize the Allegro solar cell curve is showing you the bottom right quadrant of the generalized photodiode curve above. What the solar cell folks do is re-define positive current as what really comes out of the cell, as opposed to having positive current be defined as a forward diode current. So if you can image flipping photodiode curve up around its x-axis, and then tossing out the left side and the whole bottom half as well, you get the Allegro curve. Note that shining light on a solar cell or photodiode will never make forward diode current, but it will affect the operating point if you are putting forward current into the diode.

And note that you can’t get any power out of a cell unless you get both current and voltage at the same time. You short the solar cell and you will get the most current, but no power. If you leave the cell open circuit, you get the most voltage, but with no current flowing you are not getting power. So what you want to do is change the load on the cell until its operating point on the I/V curve has the most area under it.

The red rectangle is smaller since it does not have enough voltage. The blue rectangle is smaller because it does not have enough current. The green rectangle has the maximum area and hence is the MPP (maximum power point) of the solar cell.

So that is what the MPP (maximum power point) or MPPT (maximum power point tracking) concepts are all about. You get no power if you short the cell or leave it unconnected. What you are trying to do is maximize the area under the operation point. That is because power is current times voltage, just like area is X times Y. So the MPP chart I hacked up above shows three different operating points. You can see that the big dot corresponds to the rectangle with the greatest area. If your magnificent Atmel microcontroller multiplies out the voltage and current in real time, it can dither the operating point by changing the operating point of the dc-dc converter that is taking the solar cell power and putting into a battery or onto the ac line. This is the “T” in MPPT. By tracking the maximum power point, you get the most power you can for any particular solar cell, at any particular temperature, at any give illumination.

Now please read that Electronic Products article about measuring current, since you may want to use those Allegro current sensors in your MPPT inverter, or smart meter or other application. Atmel makes the microcontrollers with security and some have integrated power line communications (PLC) modems. We also have parts that integrate the AFE, so you don’t need these external parts in your smart meter. So if you need to measure and log and report and control current, keep Atmel in mind.

Oh and in case somebody hasn’t thought of it yet, it seems obvious to one skilled in the arts that you can combine the shaded evaporative cooling systems that spray water on your roof beneath shutters, with solar panels as the shutters, so now you are cooling both the roof and your panels. Step C, more power.