石 墨 烯 在 室 温 和 普 通 光 照 下 可 产 生 电 流, 能 广 泛 用 于 太 阳 能 电 池 和 半 导 体 传 感 器 等 领 域:

石 墨 烯 再 次 给 人 们 带 来 惊 喜。美 国 麻 省 理 工 学 院 及 哈 佛 大 学 的 研 究 人 员 发 现，石墨烯可以对光产生不同寻常的反应，在室温和普通光照射下，就可以发生热载流子效应，产生电流。这一发现不仅为石墨烯再添新奇属性，更有希望使其在太阳能电池、夜视系统、天文望远镜及半导体传感器等应用领域发挥作用。该研究发表在近期出版的《科学》杂志上。

研究人员在实验室制造了复杂的石墨烯纳米P-N结，利用850纳米的激光照射石墨烯P-N结介面，并测量激光照射点产生的光电流。结果发现，随着激光强度的增加，特别是在低温的条件下，可取得最大为5 毫安/瓦（mA/W）的光电流，这一数值比以前的石墨光电器件高6倍。

热载流子效应并不新奇，但通常情况下，需要在接近绝对零度或在极强的激光照射下才会发生，但石墨烯却表现出在室温和普通光下就可以产生热载流子效应的性能，这让人们对石墨烯未来的应用产生了巨大的想像空间。

此前曾发现过石墨烯在光照下产生电流的现象，但研究人员错误地认为是光伏效应。而麻省理工学院的研究人员发现，当光照在石墨烯上时，可以产生两个具有不同电气特性的区域，进而出现温差，产生电流。石墨烯在激光照射加热不一致时，携带电流的电子被加热，而晶格中的碳原子核保持低温。正是由于石墨烯内部的温差，产生了电流。这种不同寻常的机制就称为热载流子效应。（所谓热载流子就是具有高能量的载流子，即其动能高于平均热运动能量。当载流子从外界获得了很大能量时，便可成为热载流子。由于热载流子所造成的一些影响，就称为热载流子效应。）

研究人员认为，石墨烯之所以会产生上述现象，是由于大多数材料的过热电子可将能量传递到周围晶格，而石墨烯则需要很高的能量才能振动其晶格的碳原子核，因此只有很少的电子能将热能转移到晶格。

研究人员表示，该研究成果是光电及能量采集方面十分重要的进展。由于这种现象十分新颖，还需要进一步深入的研究，才能清楚地了解其重要意义。但可以肯定的是，这种特性将会有非常广泛的应用前景。

首先，它有可能在利用太阳能方面产生重大的突破。典型的光伏材料仅对特定频率或颜色的光发生反应，而石墨烯对光发生反应的范围非常宽。研究结果还表明，石墨烯能十分有效地收集太阳能。因此太阳能电池的最大突破有可能会来自于石墨烯。

其次，由于石墨烯以不同的方式产生电流，因此可以利用石墨烯来制造超高速光子探测器。石墨烯对红外光的反应更有其独到之处，有望成为夜视系统及高级天文望远镜的重要组成部分。

此外，石墨烯也可应用于检测重要的生物分子。毒素、病菌或食品污染物等物质在光照射下，会发出红外光。此前所使用的半导体传感器，通常包含一些十分昂贵的稀有元素，而利用石墨烯生产传感器，成本将会大大降低。

石墨烯的发现很晚，有关它的特性人们还不完全清楚，石墨烯还能带给人们什么样的惊喜和奇迹，能否掀起一场石墨烯革命，还要拭目以待。

Oh graphene! The cheap, easy-to-manufacture one-atom-thick sheet of carbon can add yet another weird, fantastical, and possibly life-changing ability to its list of characteristics: it has an incredibly sensitive thermoelectric response to light. In layman’s terms: graphene, when struck by light of almost any wavelength, can produce an electric current.

The discovery was made by a team of researchers from MIT in the US and the National Institute for Materials Science in Japan. A sheet of graphene was treated so that it had two regions with different electrical properties (a p-n junction). Then, by shining an 850nm infrared laser on the material a temperature difference between the two regions is created, and an electrical current flows. This effect is caused by a hot carrier response, where the electrons gain enough energy to move, but the underlying lattice of carbon stays cool.

Now, this is significant because a hot carrier response has only ever been observed in materials that are reduced to (almost) absolute zero, or when an intensely powerful laser is used to heat the material. Graphene’s hot carrier response occurs at room temperature and across a wide range of frequencies, and — most importantly — it only requires a very weak source of light to trigger the effect.

Since its discovery in 2004 (by pulling a thin layer off a piece of pencil graphite using sticky tape) a host of unique electronic, mechanic, and optic properties have led graphene to be dubbed a “wonder material” — and in this new discovery, all three of these spheres overlap to produce an effect that could revolutionize solar power, medicine, astronomy, photography, defense, and more.

It should be stressed that this is just a preliminary discovery, and so it’s hard to divine what the ultimate applications might be, but we can definitely do some educated guesstimating. First, this thermoelectric effect could be used in solar panels. Current photovoltaic panels based on silicon technology are only sensitive to a fraction of the sun’s emitted wavelengths — a graphene solar panel could be cheaper to make and more efficient. In biology, this effect could be used to detect disease and toxin molecules, many of which emit a small amount of light when illuminated from an outside source.

In astronomy and photography, graphene could be used as a highly-sensitive photodetector that works across a huge swath of the spectrum, including infrared which many sensors struggle with. The same tech could be applied to night-vision goggles for military use — and of course, in almost every case listed above, graphene would be a cheaper alternative to the current solutions.

All this, from a material that was only isolated a few years ago! In the long run, we really don’t know the full extent of what graphene-based technologies will enable us to do; it will probably be all of the above and then some.