Probing Confinement Effects on Fluid Behavior in Nanotubes

在越来越小的空间中，流体的行为发生了显著的变化。一个常见后果的例子是熔点的持续下降，这是由于界面能的重要性日益增加造成的。然而，当约束达到纳米尺度时，一个完全不同的画面出现了，因为它不能再忽视流体的组成——即原子、分子和/或离子。在这种尺度下，流体的行为仍然无法完全理解，即使是像水这样普通的物质也是如此。例如，在碳纳米管（CNTs）的内部，外来冰相可以稳定，其熔点远远超过大块水的熔点。研究发现，通过碳纳米管的水传输是无摩擦的，而通过氮化硼纳米管的水传输则不是。目前，还没有一致的、普遍适用的理论来令人满意地描述纳米流体的这种行为并作出可靠的预测。但是，在提供系统的数据集方面，实验方面的努力也落后于提供系统的数据集，这些数据集来自于被限制在单个的、具有良好特征的纳米孔或纳米通道中的流体。该项目的主要目标是通过实验确定隔离纳米管内流体的相行为。所有的研究将首先集中在水上，然后是其他流体，包括环己烷、正己烷和离子液体。碳纳米管（CNTs）和氮化硼纳米管（bnnt）都将被部署。尽管它们的结构相似，但由于它们不同的电子性质，限制流体的行为是不同的。实验结果将与现有文献进行比较，目的是衡量不同的建模方法，并激发新的理论来推进该领域。具体来说，在碳纳米管的情况下，我将使用拉曼光谱和光致发光结合电测量来探测单个、高质量、自由悬浮的纳米管的液相行为。透射电子显微镜将作为一个补充的表征方法。除了系统地评估作为填充状态、温度和压力函数的光学性质的直径依赖性外，一个目标是测量单个填充与未填充CNTs的温度依赖性比热。bnnt将通过从市售材料开始制造分散体来纯化和分离。随后，将研究这些bnnt的光学性质，特别是关于缺陷态的光致发光。在平面六方氮化硼中，缺陷态量子发射目前受到广泛关注，但在bnnt中研究较少。这个项目的目标是增加我们对这些缺陷状态的理解，并将它们部署为纳米管流体界面的传感器。

The behavior of a fluid changes significantly upon confinement within smaller and smaller space. An example for a common consequence is the continuous depression in melting point, which results from the increasing importance of the interfacial energy. However, when the confinement reaches the nanometer scale, a radically different picture emerges as it can no longer be ignored what the fluid consists of—namely atoms, molecules, and/or ions. The behavior of fluids at this scale still defies our full understanding, and this is true even for a substance as mundane as water. For example, at the inside of carbon nanotubes (CNTs), exotic ice phases can be stabilized with melting points far exceeding the one of bulk water. Water transport through CNTs is found to be frictionless, while it is not through nanotubes made of boron-nitride. Currently, no consistent and universally applicable theory is available to satisfactorily describe such behavior of nanoconfined fluids and to make reliable predictions. But also experimental efforts lag behind in providing systematic data sets from fluids confined in individual, well-characterized nanopores or -channels.The main goal of this project is to experimentally determine the phase behavior of fluids confined within isolated nanotubes. All studies will first focus on water, and subsequently on other fluids including cyclohexane, n-hexane, and ionic liquids. Both carbon nanotubes (CNTs) as well as boron-nitride nanotubes (BNNTs) will be deployed. Despite their structural similarity, differences in the behavior of confined fluids are expected, resulting from their distinct electronic properties. Experimental results will be compared to the existing literature, with the intention to gauge different modelling approaches and to motivate new theory to advance the field.Specifically, in the case of CNTs, I will use Raman spectroscopy and photoluminescence in conjunction with electrical measurements to probe fluid phase behavior in individual, high-quality, freely suspended nanotubes. Transmission electron microscopy will serve as a complementary characterization method. Besides systematically assessing the diameter-dependence of optical properties as a function of filling state, temperature, and pressure, an objective is to measure the temperature-dependent specific heat of individual filled vs. unfilled CNTs.BNNTs will be purified and isolated by fabricating dispersions starting from commercially available material. Subsequently, the optical properties of these BNNTs will be studied, especially regarding the photoluminescence of defect states. In planar hexagonal boron-nitride, quantum emission from defect states currently receives wide attention but is less studied in BNNTs. It is an objective of this project to increase our understanding of these defects states and to deploy them as sensors for a fluid interfacing with the nanotube.