Plasmon-enhanced light emission from hybrid nanowires: towards electrically driven nanowire lasers

半导体纳米线是一种重要的量子系统，它可以有效地在宽波长范围内发射光，其中由于其小尺寸而产生的量子效应大大增强了发射。使用这些微型设备非常具有挑战性的一个问题是，如何有效地向它们注入电流，并通过电流产生光。我们打算探索一种新的方法来生长这些纳米线，使用微小的液氦液滴，使我们能够精确地控制这些纳米线的结构。我们可以自底向上构建这些系统，也就是说，通过将原子/分子一个接一个地添加到氦滴中，使我们能够生产出用任何其他方法都难以制造的纳米线。这些氦滴是氦原子的集合，其大小从几十个氦原子到超过1000亿个氦原子不等。由于温度非常低（0.37 K），液滴具有一些显著的物理性质，这使得每个液滴都是超流体。任何加入到氦液滴中的原子或分子都会迅速冷却到这个温度，因为超流氦具有极高的导热性，而且通过弱束缚的氦原子的蒸发损失，可以迅速从液滴中除去多余的能量。当许多原子和/或分子被添加到液滴中时，它们可以聚集并形成纳米级尺寸的物体。例如，最近有研究表明，由几百到几百万个原子组成的金属纳米粒子可以通过这种途径制造出来，而纳米线可以通过向大氦滴中存在的一维量子化漩涡中添加原子/分子来生长。此外，这些纳米物体可以通过与固体表面的碰撞从液滴中移除，从而实现软着陆。这些初步研究，其中有几项来自我们的团队，意义重大，因为它们为氦滴作为合成纳米科学工具的使用铺平了道路。氦气液滴带来的巨大前景是，可以添加几乎无限的材料组合，这些材料将在高度控制下聚集成纳米颗粒或纳米线。利用氦滴技术，我们可以生产出以金属细丝为核心，包覆一系列半导体的纳米线，反之亦然，这是由于金属和半导体之间的不润湿，用其他合成方法很难制造的。这些纳米线将利用表面等离子体大大增强光发射，在生长过程中光滑的金属涂层将使我们能够有效地接触纳米线；因此，它们是开发由电流驱动的非常小的激光器的理想选择，并最终可用于构建纳米尺寸的光电器件。在这方面，我们将探索的纳米线将具有显著的优势：它们是独立的和可移动的，允许它们被操纵、传输和集成到纳米光子电路中。我们有一系列最先进的激光器，我们可以用来研究这些系统的光学特性，这些系统的光是由高分辨率的显微镜系统收集的，这些显微镜系统可以在温度低至4 K的样品下操作。我们还将使用电子束和电子显微镜技术在这些纳米线上以受控的模式写入接触，这样我们就可以在电和光学上激发单个纳米线，并收集和分析它们的发射。我们设想的应用范围从传感器、量子光源和光伏设备到纳米激光器，我们可以在很大范围内控制发射波长。该项目旨在在基础纳米科学和应用纳米技术方面取得突破性进展。

Semiconductor nanowires are important quantum systems that can emit light at a broad range of wavelengths efficiently, where quantum effects resulting from their small size enhance the emission considerably. One issue that makes working with these tiny devices very challenging is the ability to inject current into them effectively and generate light by an electric current. We intend to explore a new means of growing these nanowires using tiny droplets of liquid helium that enables us to control the structure of these nanowires with exquisite precision. We can build these systems from the bottom up, i.e., by the addition of atoms/molecules one by one to helium droplets, allowing us to produce nanowires that are extremely difficult to make by any other means. These helium droplets are collections of helium atoms which may range in size from as few as several dozen helium atoms all the way to in excess of 100 billion atoms. The droplets possess some remarkable physical properties owing to the very low temperature, 0.37 K, which makes each droplet a superfluid. Any atoms or molecules added to a helium droplet cool rapidly to this temperature because of the exceptionally high thermal conductivity of superfluid helium and the fact that excess energy can be removed rapidly from the droplet by evaporative loss of the weakly bound helium atoms. When many atoms and/or molecules are added to the droplets they can aggregate and form objects that have nanoscale dimensions. For example, very recently it has been shown that metal nanoparticles composed of a few hundred to several million atoms can be made via this route, and nanowires can be grown by adding atoms/molecules to one-dimensional quantised vortices present in large helium droplets. Furthermore, these nano-objects can be removed from the droplets by collision with a solid surface, delivering a soft-landing. These preliminary studies, several of which originated from our team, are highly significant because they pave the way for the use of helium droplets as a tool in synthetic nanoscience. The great promise offered by helium droplets is the almost unlimited combination of materials that can be added, which will aggregate into nanoparticles or nanowires with a high degree of control.With the helium droplet technology, we can produce nanowires which have a thin filament of metal at their core, clad with a range of semiconductors, or vice versa, which are very difficult to make with other synthetic methods due to the non-wetting between metals and semiconductors. These nanowires will take advantage of surface plasmons to enhance the optical emission greatly, and smooth metallic coatings during growth would enable us to contact the nanowires efficiently; hence they are ideal to develop very small lasers driven by electric currents, and ultimately can be used to construct nano-sized electro-optical devices. In this regard, the nanowires that we will explore will have significant advantages: they are standalone and moveable, allowing them to be manipulated, transported and integrated into nanophotonic circuits. We have a range of state-of-the-art lasers that we can use to investigate the optical properties of these systems where the light is collected by high-resolution microscope systems that can operate with samples cooled to temperatures as low as 4 K. We will also use e-beam and electron microscopy techniques to write contacts onto these nanowires in controlled patterns so that we can excite individual nanowires both electrically and optically, and collect and analyse their emission. Applications we envisage range from sensors, quantum light sources and photovoltaic devices to nanolasers where we can control the emission wavelength over a large range. The project aims to deliver radical advances in both fundamental nanoscience and applied nanotechnology.