石墨烯：能源收集的另一种选择？

Is graphene turning out to be one of the amazingly versatile elemental substances—somewhat like silicon—that shows up in many guises and potentially solves many unique problems? Only time will tell, but the signs right now are favorable.

首先，关于石墨烯的快速背景。这是一层厚的碳原子，该碳原子排列在二维六边形晶格中。因此，石墨烯是已知的最薄的材料，但强大（比钢强约200倍）。它是热量和电力的出色指挥，具有有趣的吸光度。该材料在2004年由曼彻斯特大学的安德烈·盖姆（Andre Geim）和康斯坦丁·诺维洛夫（Konstantin Novoselov）隔离并进行了特征，他们于2010年因对材料的研究而获得诺贝尔物理奖。

Now, two unrelated research efforts are showing how it may be useful in energy harvesting. First, a team of University of Arkansas physicists has successfully developed a circuit capable of capturing graphene's thermal motion and converting it into an electrical current. The idea of harvesting energy from graphene is controversial because it counters legendary physicist Richard Feynman’s assertion that the thermal motion of atoms its—known as Brownian motion – cannot do work.

然而，根据物理学和主要研究人员教授保罗·蒂巴多（Paul Thibado）的说法，石墨烯的热运动实际上确实在室温下在电路中诱导了交替的电流（AC），这一成就认为是不可能的。他的小组用两个二极管构建了他们的电路，用于将交流转换为直流，从而使电流两种方式流动并通过电路提供单独的路径，以产生在载荷电阻器上进行工作的脉冲直流电流（图。1）。

数值模拟表明，该系统达到热平衡，随机热力学提供的热量和工作的平均热量和工作往往迅速达到零。但是，功率被载荷电阻耗散，其时间平均值完全等于热浴中提供的功率。确切的功率公式类似于Nyquist的噪声功率公式，只是二极管电阻的变化速率显着提高了输出功率，而石墨烯的运动将功率谱转移到较低的频率上。

在其布置中，将石墨烯纤维安装在支架上，以便扫描隧道显微镜（STM）尖端可以接近它，而STM尖端样本连接已纳入所示的电路（图2）。The sample is isolated from ground and connected to two diodes; the tip-sample junction acts as a variable capacitor. The tunneling current, diode 1 current (D1C), and diode 2 current (D2C) are monitored simultaneously.

这个二极管装置用于能源harvesting, but here it’s used to isolate the graphene-induced current from the battery powering the electrically isolated STM. At a tip-sample distance of 2 nm or less, tunneling electrons dominate the current; for larger distances, displacement current dominates.

显然，这项工作涉及强烈而深层的物理学，并在论文中解释了“波动引起的电流来自独立石墨烯“ 出版于APS Physical Review E。它是付费墙的后面，但也发布了here（还有一个简短的简单视频动画here）。

Frankly, there’s some leap of faith required here despite the full analysis in published paper, especially since Prof. Thibado also promotes their research with optimistic statements such as “An energy-harvesting circuit based on graphene could be incorporated into a chip to provide clean, limitless, low-voltage power for small devices or sensors.” But you never know—and should “never say never” when it comes to physics and technology advances.

麻省理工学院的石墨烯发现

在不相关的发展中，马萨诸塞州理工学院（MIT）的材料研究实验室的团队已经想到了一种收获RF能源的方法，从微波到Terahertz乐队。该分析着眼于石墨烯量子力学行为的物理和推测的局限性，以及克服它们的方法。他们发现，通过将石墨烯与另一种材料（在这种情况下为氮化硼）相结合，石墨烯中的电子应将其运动朝向共同方向，从而产生电流流动。

While previous experimental technologies have been able to convert terahertz waves into dc, they could do so only at ultracold temperatures, which obviously limits their practical applications. Instead, lead researcher Hiroki Isobe began an investigation to see if a material’s own electrons could be induced at a quantum-mechanical level to flow in one direction, in order to steer incoming electromagnetic-energy waves into a dc current. The material used would have to be free of impurities so that the electrons in it would flow without scattering off irregularities in the material, and graphene was an attractive material.

但这只是起点。要指导石墨烯的电子向一个方向流动，需要“破坏”材料固有的对称性。因此，电子在各个方向都会感觉到平等的力，这意味着任何传入能量都会随机散射。其他人则通过将石墨烯放置在一层氮化硼的顶部，以使石墨烯电子之间的力被敲除平衡中 - 靠近硼的电源感觉到一种力，而这些电子更接近氮气，经历了不同的拉力。

This “skew scattering” can result in useful current flow. The research team envisioned a terahertz rectifier consisting of a small square of graphene sitting on top of a layer of boron nitride. It would be sandwiched within an antenna that collects and concentrates ambient terahertz radiation, boosting its signal enough to convert it into a dc current（图3）。

The team has filed a patent for their new “high-frequency rectification” design, which is described in the theirScience Advances纸 ”High-frequency rectification via chiral Bloch electrons” along withSupplementary Material。两者都需要大量阅读才能意识到这是一个极为深入的理论分析（我的意思是深入，因为模型，方程式，部分导数和积分的数量令人震惊）。实际上没有构建设备。但不用担心 - 研究人员正在与麻省理工学院的实验物理学家合作，根据他们的洞察力和分析开发物理设备。