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突破!科学家找到让“太赫兹光谱”进入单分子状态的新方法

2018-11-22

光与物质的相互作用是光谱学的基础,光谱学是一套位于物理和化学核心的技术。从红外光到X射线,广泛的波长扫描用于刺激振动,电子跃迁和其他过程,从而探测原子和分子的世界。

然而,一种较少使用的光形式是太赫兹(THz)区域。在太阳光和微波之间的电磁波谱上,太赫兹辐射确实具有正确的频率(大约1012赫兹)来激发分子振动。不幸的是,它的长波长(数百微米)是典型分子尺寸的约100,000倍,因此不可能通过传统光学器件将THz光束聚焦到单个分子上。只能研究大量的分子。

最近,由东京大学工业科学研究所(IIS)领导的团队找到了解决这个问题的方法。在Nature Photonics的一项研究中,他们发现THz辐射确实可以检测到单个分子的运动,克服了聚焦光束的经典衍射极限。事实上,该方法足够灵敏,可以测量单个电子的隧道效应。

IIS团队展示了一种称为单分子晶体管的纳米级设计。两个相邻的金属电极,即晶体管的源极和漏极,以“蝴蝶结”形状放置在薄硅晶片上。然后,单个分子 - 在这种情况下为C60,又名富勒烯 - 沉积在源极和漏极之间的亚纳米间隙中。电极充当天线,将THz光束紧密地聚焦到隔离的富勒烯上。

“富勒烯吸收聚焦的太赫兹辐射,使它们围绕质心振荡,”研究第一作者杜少青解释道。 “除了固有电导率外,超快分子振荡会提高晶体管中的电流。”尽管这种电流变化微不足道 - 在毫微微安培(fA)的数量级上 - 可以用用于捕获分子的相同电极精确测量。以这种方式,绘制了在约0.5和1THz处的两个振动峰。

事实上,测量足够灵敏,可以测量吸收峰的轻微分裂,这是由于只加一个电子而引起的。当C60在金属表面上振荡时,其振动量子(振子)可被金属电极中的电子吸收。因此受到刺激,电子隧穿进入C60分子。得到的带负电的C60-分子以比中性C60略低的频率振动,从而吸收不同频率的THz辐射。

除了提供隧道的一瞥之外,该研究还展示了一种实用的方法,可以获得仅微弱吸收太赫兹光子的分子的电子和电子信息。这可以开辟太赫兹光谱学的广泛应用,这是一种与可见光和X射线光谱学互补的欠发达方法,与纳米电子学和量子计算高度相关。


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Terahertz spectroscopy enters the single-molecule regime
原文 2018-09-05   来源:PHYS

Illustration of a Single molecule transistor (SMT) with a bowtie antenna structure. S, D, and G denote the source, drain, and gate electrodes of the SMT, respectively. A single molecule is captured in the created nanogap. Credit: 2018 Kazuhiko Hirakawa, Institute of Industrial Science, The University of Tokyo


The interaction of light with matter is the basis of spectroscopy, a set of techniques lying at the heart of physics and chemistry. From infrared light to X-rays, a broad sweep of wavelengths is used to stimulate vibrations, electron transitions, and other processes, thus probing the world of atoms and molecules.


However, one lesser-used form of light is the terahertz (THz) region. Lying on the electromagnetic spectrum between infrared and microwaves, THz radiation does have the right frequency (around 1012 Hz) to excite molecular vibrations. Unfortunately, its long wavelength (hundreds of micrometers) is around 100,000 times a typical molecular size, making it impossible to focus THz beams onto a single molecule by conventional optics. Only large ensembles of molecules can be studied.

Recently, a team led by The University of Tokyo's Institute of Industrial Science (IIS) found a way around this problem. In a study in Nature Photonics, they showed that THz radiation can indeed detect the motion of individual molecules, overcoming the classical diffraction limit for focusing light beams. In fact, the method was sensitive enough to measure the tunneling of a single electron.

The IIS team showcased a nanoscale design known as a single-molecule transistor. Two adjacent metal electrodes, the source and the drain of the transistor, are placed on a thin silicon wafer in a "bowtie" shape. Then, single molecules—in this case C60, aka fullerene—are deposited in the sub-nanometer gaps between the source and drain. The electrodes act as antennas to tightly focus the THz beam onto the isolated fullerenes.

"The fullerenes absorb the focused THz radiation, making them oscillate around their center-of-mass," explains study first-author Shaoqing Du. "The ultrafast molecular oscillation raises the electric current in the transistor, on top of its inherent conductivity." Although this current change is minuscule—on the order of femto-amps (fA)—it can be precisely measured with the same electrodes used to trap the molecules. In this way, two vibrational peaks at around 0.5 and 1 THz were plotted.

In fact, the measurement is sensitive enough to measure a slight splitting of the absorption peaks, caused by adding or subtracting only one electron. When C60 oscillates on a metal surface, its vibrational quantum (vibron) can be absorbed by an electron in the metal electrode. Thus stimulated, the electron tunnels into the C60 molecule. The resulting negatively charged C60- molecule vibrates at a slightly lower frequency than neutral C60, thus absorbing a different frequency of THz radiation.

Apart from providing a glimpse of tunneling, the study demonstrates a practical method to obtain electronic and vibronic information on molecules that only weakly absorb THz photons. This could open up the wider use of THz spectroscopy, an under-developed method that is complementary to visible-light and X-ray spectroscopy, and highly relevant to nanoelectronics and quantum computing.

(来源:明日情报



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